1
POLITECNICO DI MILANO
Scuola di Ingegneria Industriale e dell'Informazione
Laurea Magistrale in
MATERIALS ENGINEERING AND NANOTECHNOLOGY
STRUCTURAL AND ELECTROCHEMICAL
CHARACTERIZATION OF MoO3 THIN FILMS
Supervisor: Prof. Guglielmo Lanzani
Gizem ÇALIŞKAN
798366
2014-2015
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Abstract English
As the global warming and its side effects are rising, importance of technologies for
alternative source of clean energies are becoming more important to take an attention. One of
the more interesting fields of modern research in renewable energy is the production of
hydrogen from water in terms of clean energies. Water splitting using a catalyst and solar energy
is an ideal future fuel source for the hydrogen production. Water splitting into molecular
hydrogen and oxygen has been considered to be an urgent subject for daily life by using
photocatalysts. The most innovative approach to achieve this aim is the building
photoelectrochemical hybrid devices which has excellent visible light absorption by some
organic materials accompanied by the inorganic materials that have efficiency of charge
extraction. The most performing architectures involve the use of selective inorganic contacts to
facilitate the extraction of the photogenerated charge in a polymeric layer. One of the brightest
materials for this type of application is molybdenum trioxide (MoO3) thanks to depth of its
valence band that behaves as a hole selective layer. Suitably intercalated MoO3 allows the
transition of electrons through the polymeric layer of the hybrid devices. Electrochemically, the
intercalation process in liquid phase has a great improvement in the studies.
This work includes the analysis of the different process of intercalation on deposited and
thermal treated α-MoO3 thin films. There have been several characterization techniques to
describe the behavior of the selective layer as intercalated in different potentials and time sets.
As a review, reader can get some ideas related to the properties of the molybdenum tri oxide,
and specific usage on the FTO glass as a selective layer of water splitting devices by concerning
the electrochemical techniques.
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CONTENTS
1. Introduction ............................................................................................................................................. 7
1.1. Motivation of the thesis ......................................................................................................................... 7 1.1.1. Phocs Project ................................................................................................................................ 7
1.2. Molybdenum Tri-Oxide Properties ......................................................................................................... 9 1.2.1. Structural Properties ..................................................................................................................... 9 1.2.2. Optical properties and electronic structure................................................................................ 11
1.3. Intercalation of the MoO3 .................................................................................................................... 13
1.4. Molybdenum oxide applications .......................................................................................................... 14 1.4.1. Photochromism and Electrochromism ....................................................................................... 15 1.4.2. Supercapacitors: ......................................................................................................................... 16 1.4.3. MoO3 as a Hole selective layer: Solar Cells ................................................................................. 19
2. Experimental Methods .......................................................................................................................... 24
2.1. Preparation of the Samples ................................................................................................................. 24 2.1.1. Preparing substrate..................................................................................................................... 25 2.1.2. Deposition Technique of the thin films ....................................................................................... 26 2.1.3. Annealing .................................................................................................................................... 27 2.1.4. Intercalation of the samples ....................................................................................................... 28
2.2. Sample Characterization ...................................................................................................................... 31 2.2.1. Raman Spectroscopy ................................................................................................................... 32 2.2.2. UV/Vis/NIR Spectrometry, Absorption Measurements .............................................................. 39 2.2.3. X-Ray Diffraction ......................................................................................................................... 43 2.2.4. Cyclic Voltammetry Measurements in Acetonitrile [CH3CN] .................................................... 46 2.2.5. Mott-Schottky Measurements .................................................................................................... 49 2.2.6. Kelvin Probe Microscope ............................................................................................................ 51 2.2.7. X-Ray Photoelectron Spectroscopy ............................................................................................. 52 2.2.8. Dissolution test ........................................................................................................................... 54
3. Results and discussion ........................................................................................................................... 55
4. Conclusion ............................................................................................................................................. 78
5. References ............................................................................................................................................. 79
6. Appendix ............................................................................................................................................... 82
6.1. Pulsed DC Magnetron Sputtering .............................................................................................................. 82
6.2 Mott-Schottky Analysis ............................................................................................................................... 84
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List of Figures
Figure 1.1.1 : Schematic representation of the hybrid system of PHOCS devices .................................................. 8
Figure 1.2.1.1 : Crystal structure of layered (a) α-MoO3 ..................................................................................... 10
Figure 1. 2.1.2 : (a) Octahedral symmetry of the α-MoO3 is emphasized in (b) ................................................... 11
Figure 1.1.2.1 Band Structure of α-MoO3 and HxMoO3 (x=0.25 and 0.5) obtained using DFT calculations. ......... 12
Figure 1.4.2.1. : The working principle of supercapacitors. a) Electric double layer, b) redox reaction on surface, and c) redox reaction in bulk. ....................................................................................................................... 17
Figure 1.4.2.2 : Illustrative diagram for the working process of MoO2+x(-) //2 M Li2SO4//MnO2(+) micro-device ........................................................................................................................................................... 19
Figure: 1.4.3.1 : In light, J–V characteristics of the FTO/PEDOT : PSS/P3HT : PCBM/Al and the FTO/MoO3/P3HT : PCBM/Al devices corresponding to different substrate temperature deposited MoO3 films. .................... 20
Figure 1.4.3.2 : Energy diagram of the FTO/ MoO3/P3HT:PCBM/Al solar cell ...................................................... 21
Figure 1.4.3.3 : (a) Schematic of TMO/n-type c-Si solar cells. (b) Processflow diagram ....................................... 23
Figure 1.4.3.4 : Current density-voltage (J–V) response of the fabricated TMO/n-Si solar cells .......................... 23
Figure 2.1.2.1 : Metallic Mo deposited samples on Pulsed DC Magnetron Sputtering (ready to be annealed) ... 27
Figure 2.1.3.2 : Deposited and annealed samples: MoO3 100nm thin film on FTO substrate .............................. 28
Figure 2.1.4.1 : The Three-Electrode Pine Electrochemical Cell that is used for intercalation processes and electrochemical characterization of the processed samples ....................................................................... 29
Table 1 Voltage Series .......................................................................................................................................... 29
Table 2 Time Series .............................................................................................................................................. 30
Figure 2.1.4.2 : The samples images of before intercalation & after intercalation holding with a crocodile. ...... 30
Figure 2.1.4.3 : The images of destroyed films after too much intercalation rates. ............................................. 31
Figure 2.2.1.1 : Renishaw inVia Raman microscope/spectrometer ...................................................................... 33
Figure 2.2.1.2 : Schematic of the different light scattering possibilities: Rayleigh, Stokes and anti-Stokes. ........ 34
Figure 2.2.1.3 : Typical Raman spectrum of the alpha phase of molybdenum oxide ........................................... 36
Figure 2.2.1.4 : Raman spectra of MoO3 deposited on quartz (blue line), FTO (green line), glass (red line) and ITO (black line) substrates. The inset figure represents the intensity ratio between the peaks at 821 and 666 (blue line) cm-1 and those at 821 and 995 cm-1 (red line). .................................................................... 36
Figure 2.2.1.5 a), b) 2D layered MoO3, cross section of the FET device c) Raman spectra of intercalated MoO3 38
Figure 2.2.1.6 : Raman spectra of sub-stoichiometric MoO (3-x) acquired from a thick sample, which was adjacent to the Pt electrodes, that was deposited using highly energetic electron beam. ......................... 39
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Figure 2.2.2.1 : Schematic diagram of a spectrophotometer ............................................................................... 40
Figure 2.2.2.2: PerkinElmer LAMBDA 1050 UV/Vis/NIR Spectrophotometer ....................................................... 40
Figure 2.2.2.3: Method for determination of the band gap of the α-MoO3 following various conditions of intercalation, since the absorbance spectra and by the energies of the incident photons. ........................ 42
Figure 2.2.3.1 : The X-Ray diffractometer Bruker ................................................................................................. 43
Figure 2.2.3.2 : Inter-planar spacing (d) of the MoO3 films (Al-Kuhaili, Durrani, & Bakhtiari, 2009) .................... 44
Figure 2.2.3.3 : XRD patterns of the MoO3 deposited on quartz (blue line), FTO (green line), glass (red line), and ITO (black line) substrates. The labeled peaks correspond to α- MoO3 (*, JCPDS 05-0508), substoichiometric MoO3 (□), SiO3 (■), FTO (◊), and ITO (○). ......................................................................... 45
Figure 2.2.3.4 : XRD patterns of (a) HTaMoO6.xH2O,(b) HNbMoO6.yH2O,(c) C3-HTaMoO6, and (d)C3-HNbMoO6 46
Figure 2.2.4.1 : Cyclic voltammograms of the Li+ inserted HMB crystal ............................................................... 47
Figure 2.2.4.2. : Cyclic voltammograms of the Li+ inserted HMB crystal .............................................................. 48
Figure 2.2.5.1 : A masked sample for the Mott-Schottky measurements as a working electrode ...................... 50
Figure 2.2.5.2. : a) The used electrochemical cell with the working electrode (E) in central position (red), reference electrode (RE) connected to the blue crocodile, counter electrode (CE) connected to the black crocodile and pipes for the flow of nitrogen. b) Diagram of operation of an electrochemical cell with three electrodes. .................................................................................................................................................... 51
Figure 2.2.7.1 : Core level spectra of the MoO3 film showing the presence of Mo+6 oxidation state, only. ......... 52
Figure. 2.2.7.2 : XPS core-level spectra of molybdenum (Mo-3d) for MoO3 films: (a) Tsub= RT, (b) Tsub= 100 ˚C, (c) Tsub= 200 ˚C and (d) Tanne= 300 ˚C. ................................................................................................................ 53
Figure 2.2.7.3 : XPS spectra of the core levels of MoOx ........................................................................................ 54
Figure 2.2.8.1. : Images of the dissolution test ..................................................................................................... 55
Figure 3.1 : Typical Raman spectra of α-MoO3 acquired using our parameters: 50x magnification, %1 laser power, and 1000 accumulation. ................................................................................................................... 56
Figure 3.2:Optical microscope image of α-MoO3 obtained by deposition and annealing processes respectively. ...................................................................................................................................................................... 58
Figure 3.3 : Optical microscope image of FTO glass (no baseline created) ........................................................... 58
Figure 3.4 : Raman spectra of potential series exposed 20 seconds (-0.1V, 0.0V, 0.1V and 0.2V vs Ag/agCl) (50x magnification, %0.1 laser power, and 1000 accumulation) ......................................................................... 59
Figure 3.5 : Raman spectra of time series at -0.1V vs Ag/AgCl respect to as deposited α-MoO3 (50x magnification, %1 laser power, and 1000 accumulation) ............................................................................ 60
Figure 3.6 : XRD spectra of the MoO3 on -0,1 V vs Ag/AgCl at different times .................................................... 61
Figure 3.7 : XRD spectra of the MoO3 on -0.1 V vs Ag/AgCl at different applied potentials ................................. 62
Figure 3.8. : Comparison of the XRD spectra between as deposited (untreated) MoO3 and intercalated at -0.1V vs Ag/AgCl for 20 seconds. ........................................................................................................................... 63
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Figure 3.9. : The graph of transmittance spectra between 300 and 800 nm of four significant samples as the deposited and intercalated ones at 20 seconds as a potential series in the electrochemical cell. .............. 64
Figure 3.10. : The graph of absorbance spectra between 400 and 2250 nm of four significant samples intercalated at +0.1V vs Ag/AgCl for 5, 10, 15 and 20 seconds in the electrochemical cell. ........................ 65
Figure 3.11. : The graph of absorbance spectra between 400 and 2250 nm of six significant samples the as-deposited MoO3, FTO as substrate and those intercalated at +0.1V vs Ag/AgCl for 5, 10, 15 and 20 seconds in the electrochemical cell............................................................................................................................ 65
Figure 3.12. : The graph of absorbance spectra between 400 and 2250 nm of four significant samples intercalated at 20 seconds as a potential series in the electrochemical cell. .............................................. 66
Figure 3.13. : The graph of absorbance spectra between 400 and 2250 nm of six significant samples the as-deposited MoO3, FTO as substrate and those intercalated at 20 seconds as a potential series in the electrochemical cell. ..................................................................................................................................... 66
Figure 3.14. : XPS spectra of the core levels of MoO3 ........................................................................................... 67
Figure 3.15. : XPS spectra of the core levels of MoO3 after intercalation processes at +0.1V vs Ag/AgCl for20s. 68
Figure 3.16. : XPS spectra of the core levels of MoOx after intercalation processes at -0.1V vs Ag/AgCl for 20s. 69
Figure 3.17. : XPS spectra of the core levels of MoO3 and intercalated MoO3 based on potential series that is described before. ......................................................................................................................................... 69
Figure 3.18. : Work function of intercalated samples related to potential series performed under 20 seconds. 70
Figure 3.19. : Work function of intercalated samples related to time series at 0.1V vs Ag/AgCl ......................... 71
Figure 3.20. : Work function diagram related to thickness of the film ................................................................. 72
Figure 3.21. : Work Function Diagram ................................................................................................................. 73
Figure 3.22. : Cyclic Voltammograms of the as deposited MoO3 in three scan .................................................... 74
Figure 3.23. : Cyclic voltammograms of the intercalated samples as a potential series at 20 seconds................ 75
Figure 3.24. : Cyclic voltammetry of the intercalated samples as a time series at -0.1V vs Ag/Ag+ ..................... 76
Figure 3.25. : Cyclic voltammetry of the intercalated samples as a time series at -0.1V vs Ag/Ag+ ..................... 77
Figure 6.1.1 : A schematic diagram of an experimental setup. ............................................................................ 82
Figure 6.1.2. : Example of collision cascade promoted by an incident beam. ...................................................... 83
Figure 6.2.1 : Graphs of Nyquist (a) and Bode (b) as an example ......................................................................... 86
Figure 6.2.2 : (a) Equivalent circuit of an electrochemical cell (b) Subdivision of Zf in Rs and Cs, or Rct and Zw .... 86
Figure 6.2.3: graph example Mott-Schottky for powders of zinc oxide synthesized. ........................................... 88
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1. Introduction
1.1. Motivation of the thesis
1.1.1. Phocs Project
Phocs (Photogenerated Hydrogen by Organic Catalytic Systems) is an EU project funded by
the the Future and Emerging Technologies (FET), with contibutors of the different research
centers and companies such as Fondazione Istituto Italiano di Tecnologia (Italy), ENI SPA –
Istituto Eni-Donegani (Italy), Universitas Jaume I de Castellon (Spain), Instituto Superior
Tecnico (Portugal), Ecole Polytechnique Federale de Lausanne (Switzerland),
IMDEANanociencia (Spain), and Technische Universität München (Germany). The aim of the
project is the production of the hydrogen based on hybrid organic / inorganic and / organic
liquid interfaces. It has studied as a device that intends to combine the organic materials that
helps to absorb visible light together with the inorganic semiconducting materials with
enhanced charge transport capabilities.
Metal oxides with wide range of band gap only allows the absorption of the typical wavelengths
of UV while the solar radiation is in the visible region, that is not convenient for using as a
photoactive material. Instead, a photoactive material that absorbs visible light and maximize
the generation of the photocurrent should be in contact with a layer of metal oxide. Recently
metal oxides are selected as a hole selective layer to direct the electron transfer within the hybrid
devices.
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Figure 1.1.1 : Schematic representation of the hybrid system of PHOCS devices
(Taken from: http://www.phocs.eu/project/abstract.html)
In figure 1.1.1 is seen schematic representation of the hybrid system to indicate photo-
electrochemical working principle. On the left diagram shows the layer of the polymer which
absorbs the visible radiation that is in direct contact with the aqueous electrolyte and an
electrode coupled with inorganic semiconductor. Diagram of the energy levels to promote the
transfer of charge carriers. In a particular configuration the electrons must come from the orbital
LUMO (lowest unoccupied molecular orbital) of the polymer, in which process the generation
of couples electrons and holes by photogeneration. The photogenerated e- and h+ in the organic
semiconductor are transferred to the acceptor and the inorganic layer, respectively, if the bands
have energy levels by increasing orbital HOMO (highest occupied molecular orbital) of the
acceptor to the valence band of the semiconductor. By adequate HOMO-LUMO levels and
bands engineering, free carriers are able to promote hydrogen reduction and water oxidation
reactions at the polymer and counter electrode interface with water, respectively. In order to
build properly-working photo-electrochemical cells, issues such as stability, wettability, and
electron transfer processes at the polymer/electrolyte interface should be taken into account.
It is necessary to obtain a correct alignment of the electronic bands of the different materials of
the system, in order to allow to extract effectively the photogenerated charge from organic layer.
As regards the efficiency in the charge transport, instead, they were selected appropriate
inorganic semiconductors such as MoO3 hole blocking layer and TiO2 as electron selective layer
(ESL), taking a cue from existing optimizations for organic photo voltaics (OPV) [28]. In a
modified configuration, however, a layer of molybdenum oxide place between the polymer and
the photoactive transparent conductive oxide (transparent conductive oxide - TCO) functions
as a hole blocking layer. Being an n-type semiconductor and having a work function (work
function, minimum thermodynamic work to extract an electron) very high, it is not able to carry
the gaps; more precisely, the valence band is located much below the level of the orbitals
HOMO of the polymer, and then locks the gaps at the oxide / polymer. The photogenerated
electrons, however, are extracted from the photocathode toward the contact of titanium oxide
(ESL), which also fulfills the role of supporting the platinum electrocatalyst in order to facilitate
the reduction process. Moreover, as a result of intercalation processes, they are formed in the
oxide of molybdenum were not employed at a level slightly lower than the conduction band of
the TCO: the electrons transported from the electrolytic solution, through the external circuit,
are released into the cell through the TCO and, passing through the states of molybdenum oxide
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intercalated just mentioned, can recombine with the gaps locked and close the circuit (for the
mechanism of the states not occupied.
In the present study it will be shown as molybdenum trioxide characterization of structural and
electrochemical ways to obtain the behavior under the operating conditions (pH <2 and the
potential for higher than that of hydrogen evolution) for performing as a hole selective layer
within the hybrid photo-electrochemical devices due to its electronic structure and optical
properties.
1.2. Molybdenum Tri-Oxide Properties
1.2.1. Structural Properties
Molybdenum is known as a transition metal which is used for many applications due to its rich
properties in chemical structure with high melting point, good thermal conductivity, high
thermal resistance and low vapor pressure. However, transition metal oxides, especially
Molybdenum trioxide (MoO3) has been recently attracting interest in research areas due to its
unique layered structure. Amorphous MoO3 can be converted to crystalline form which exists
in two common phases which include metastable β- MoO3 (monoclinic structure) and α- MoO3
(orthorhombic structure). These two phases have very different physical and chemical
properties, such as refractive indices, bandgap energies, and mechanical hardness. α- MoO3 has
an interesting layered structure consisting of corner sharing chains of MoO6 octahedra (Figure
1.2.1.2 b) which are held together by covalent forces. In addition, Van der Waals bond is
observable between the different levels which are stacked in a staggered arrangement. This
unique layered structure is characterized by high chemical stability and electrochemical
activity. Each central Mo atom is surrounded by six oxygen atoms as reported in Figure 1.2.1.1.
In between these octahedra, there are extended tunnels that can serve as conduits and
intercalation sites for mobile ions. The MoO6 octahedron is the building block for both the
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orthorhombic and monoclinic structure. . Lattice constants of α-MoO3 are a = 3.962 Å, b =
13.855 Å, and c = 3.699 Å [2, 14].
Molybdenum oxide in its β-phase has a monoclinic structure, which is a perovskite-like type
(ReO3 structure) and can be regarded as an infinite framework of corner-sharing MoO6
octahedra likely α- MoO3. It shares the corners in all directions not only one direction so, β-
phase is not particularly suitable for the formation of crystalline planes. Moreover, the β phase
is a metastable phase that can be generally transformed into the more stable, layered α-MoO3
phase above 350 °C [2].
Figure 1.2.1.1 : Crystal structure of layered (a) α-MoO3
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Figure 1. 1.1.2 : (a) Octahedral symmetry of the α-MoO3 is emphasized in (b)
1.2.2. Optical properties and electronic structure
Oxidation states of Mo show diversity from -2 to + 6 and it has coordination number ranging
from 0 to 8. This is the reason why it is able to form a wide variety of stable oxides and having
also several metastable phases, such as MoO2, MoO3 (α phases, β or amorphous), Mo4O11,
Mo8O23.
MoO3 exhibits the highest value of work function among the non-soluble transition metal
oxides (5.5 eV) (He & Yao, 2003), high transparency (> 80%) in visible lights and near IR
range and wide band gap (3.0-3.8 eV). Compared to the α-MoO3 phase, β-MoO3 has lower
bandgap values which had been reported previously in study of Simchi et.al [25]. The indirect
wide bandgap of the bulk α-MoO3 material, (>3 eV), is able to tunability via a range of
approaches. One common procedure is extensively intercalation of Li+ or H+ atoms into the
crystal lattice. There are other intercalants such as organic compounds and alkali metals like
Na, K as well as Li and H. These intercalants manipulate the stoichiometry and band structure
a) b)
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[2]. Ion intercalation leads to formation of the energy levels differs with doped material and a
change of the bad gap is observed.
In study of Balendhran, adsorbed H atoms forms hydrogen molybdenum bronzes (HxMoO3)
and the bandgap of such structures shift to metallic upon a hydrogen loading of x > 0.25
(Figures 1.3 a and b). Further information will be discussed in Intercalation section.
Figure 1.1.2.1 Band Structure of α-MoO3 and HxMoO3 (x=0.25 and 0.5) obtained using DFT calculations.
MoO3 films has fast response times, high coloration efficiency and long times. Gesheva et. al.,
states that, especially the mixed MoO3–WO3 films have higher optical absorption with maxima
at a closer position with respect to the human eye sensitivity peak at 2.5 eV [12]. A higher
coloration efficiency is expected, due to increased electron transitions between the two kinds
of metal sites with different valences by changing the structural properties. In the following the
MoOx films with oxygen vacancies as an example of transition metal oxides films has a great
optical transparency with high work function to be a candidate to use as a carrier-selective
contacts in heterojunctions.
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1.3. Intercalation of the MoO3
The process of intercalation consists of the insertion of an atom or an ion that are movable in a
crystal lattice. When the ion occupies an interstitial position within the lattice, therefore it is
stated that the process is an interstitial phenomena. Typically, intercalation reactions are
reversible. In addition, the host lattice tends to partially maintain its integrity both during
intercalation and deintercalation [5]. This phenomena will be irreversible for 2D lattice because,
generally that lattice tends to formation new structure with intercalation compounds, due to the
flexibility of the inter-layer bond deformation so, they can adapt to size of species coming from
outer easily. In the light of literature, we know that some alkali metals such as Li, Na, K as well
as H ion and organic compounds can be used as intercalants. The molybdenum trioxide with its
phase orthorhombic crystalline (α-MoO3) facilitates the permeation between layers. Hydrogen
ions H+ and other ions of alkali metals react to form oxide bronzes of molybdenum, according
to the typical reaction:
𝐌𝐨𝐎𝟑 + 𝐱𝐌+ + 𝐱𝐞− ⇔ 𝐌𝐱𝐌𝐨𝐎𝟑
where M+ represents the hydrogen ion or metal cation [26]. This reaction can be triggered by
different methods consisting of UV radiation due to the high value of the band gap of the MoO3
and electrochemistry. In the context of extensive studies mentioned on literature, we know that
some alkali metals such as Li, Na, K as well as H ion and organic compounds can be used as
intercalants. Electrochemically, the intercalants are supplied from electrolyte by applying a
potential between working electrode and electrolyte containing the cations to be intercalated.
In other words, H+ ions are doped into the conduction band by manipulating the stoichiometry
and band structure. As an example,
In particular, in the processes photochromic following reactions take place:
𝐌𝐨𝐎𝟑 + 𝒉𝝂 → 𝐌𝐨𝐎𝟑∗ + 𝐞− + 𝐡+
𝐌𝐨𝐎𝟑 + 𝒙𝐇+ + 𝒙𝐞− → 𝐇𝒙𝐌𝐨𝒙
𝑽𝐌𝐨𝟏−𝒙𝑽𝑰𝐎𝟑
𝐌𝐨𝑨𝑽𝑰 + 𝐌𝐨𝑩
𝑽 𝒉𝝂→ 𝐌𝐨𝑨
𝑽 + 𝐌𝐨𝑩𝑽𝑰
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where h is Planck's constant and ν is the frequency of the photon. The first excitation of MoO3
is due to a photon with consequent generation of charge (electron-hole pair, respectively e- and
h+); the second is the process of intercalation, where H+ ions originate from the solution in the
liquid phase or a gaseous phase in contact with the oxide. Finally, the third shows the charge
transfer between atoms of Mo in two different oxidation states, induced by light radiation [14].
Ion insertion in the oxide matrix promotes blue coloration of the sample. The density of blue
coloration depends on the oxygen vacancies density. On the other hand, samples with high
oxygen fluxes which is stoichiometric MoO3 in this case, changes from transparent coloration
to blue coloration with low fluxes to substoichiometric MoO3 [13].
(Balendhran, 2013) indicates that with intercalation processes, H atoms are adsorbed. Hydrogen
adsorption can be induced by H+ onto the MoO3 thin film. It forms hydrogen molybdenum
bronzes (HxMoO3). The bandgap of hydrogen molybdenum bronzes shift to metallic with
reference to MoO3 if hydrogen loading is up of x > 0.25. By adding H+ ions to MoO3, the layer
enhances conductivity.
(Dhanasankar, Purushothaman, & Muralidharan, 2010) shows the effect of the cerium doping
to molybdenum oxide films as it enhances the electrochromic performance and stability of the
molybdenum oxide films.
1.4. Molybdenum oxide applications
It is easy to say regarding to literature, molybdenum oxide thin films have potential optical
applications, including chromogenic coatings, display devices, smart windows, optical
switching, and optical recording. In addition, MoO3 catalysts have been used in a variety of
partial oxidation reactions and it is used as a gas sensors in some applications [1]. Molybdenum
oxide has various application areas such as selective oxidation catalysis, bulk-hetero-junction
solar cells etc [25].
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1.4.1. Photochromism and Electrochromism
Photochromism is derived from electromagnetic radiation which can be UV, visible or IR by
changing color irreversibly. Reversibility is able to control as a result of using different
methodology by exposure to the light at a different frequency, by heating in the dark, by
electrochemical polarization or by chemical oxidation [15].
Photochromic effect of molybdenum trioxide has resulted in many applications such as erasable
optical storage media, large-area display, chemical sensor, control of radiation intensity, self-
developing photography, smart windows and so on [14].
In case of MoO3, photochromic effect can be defined as turning color from transparent to blue
due to the charge separation if the energy gap (Eg) of incident light is higher than MoO3.
At this point, it is very important to elucidate another relevant and extremely useful concept,
which is called Electrochromism. Electrochromic (EC) based systems such as smart windows
and optical displays, have been studied for more than three decades [8]. Redox-active transition
metal oxides (i.e., V2O5, MoO3, and WO3) have shown considerable promise for application in
the areas of electrochromics. The transparency of MoO3 can be switched by ions H+, Li+ and
Na+ which are coming from electrolyte. These ions effect the reversibility and persistency of
the shifting color by applying low voltages. The origin of the phenomena is variation of the
valance states as a transition of MoO3 from 6+ (transparent) to 5+ (Prussian blue) upon
positive ion intercalation [2].
More recently, attempts at enhancing these properties have focused on fabrication of materials
with nanoscale dimensions. Researches show that structural modification may cause an
improvement on the switching times due to reduction of diffusion lengths and increased surface
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area. Development of new and improved electrochromic devices centers on increasing
coloration efficiency and cycle life in addition to decreasing coloration response time [22].
1.4.2. Supercapacitors:
Supercapacitors is known as an energy storage devices which are based on electrochemical
reactions. There are two general categories of electrochemical supercapacitors: electric double-
layer capacitors (EDLC) which is consists of a charge separation at the electrode/electrolyte
and pseudocapacitors (PCs) which has a transportation of faradic charging on the surface of the
electroactive material. EDL capacitors are based on carbon materials, PCs are based on either
transition metal oxides or conductive polymers. Besides, hybrid capacitors can be counted as a
third category which is generated by carbon and oxides or polymers. Each of them has a
different electrochemical mechanism in case of charge reactions. EDLCs are non-Faradaic, PCs
are Faradaic, and HCs are a combination of both Faradaic and non-Faradaic [19].
Working principle of a supercapacitor can be explained depending on two immersed electrodes
in an electrolyte (KOH, H2SO4, salt, etc.) with a separator between the electrodes. The energy
stored in supercapacitors is expressed by E=1/2 CV2, where C is the capacitance and V the
potential across the electrodes. Each electrodes has a net capacitance of C/2.
17
Figure 1.4.2.1. : The working principle of supercapacitors. a) Electric double layer, b) redox reaction on surface, and c) redox reaction in bulk.
The working principles of a supercapacitor which includes an electric double layer (EDL)
charge storage mode where polarization/depolarization takes place at the electrode surface and
a pseudo-capacitive charge storage mode is seen on the Figure 1.4.2.1. Figure 1.4.2.1.a
describes an electrostatic process which is similar to physical dielectric capacitor. The major
difference is that the insulating barrier in a dielectric capacitor is replaced by electrolytes in a
supercapacitor Figure 1.4.2.1. b and c represents an electrochemical process with mechanisms
consisting two different ones called surface and bulk processes. Pseudocapacitive charge
storage fundamentally depends on transportation of the faradaic charging between electrode
materials and electrolyte ions. The electric energy is generated by fast faradaic redox reaction,
which produces pseudocapacitance. Redox reactions can take place in different locations and it
allows pseudocapacitive charge storage can be catalogued into surface charge storage and bulk
charge storage.
The performance of supercapacitors is mainly determined by the electrochemical activity and
kinetic feature of the electrodes. To improve the power density of supercapacitors, it is crucial
to enhance the kinetics of ion and electron transport in electrodes and at the electrode/electrolyte
interface. Therefore, electrodes should have proper pore structure and good electrical
18
conductivity. On the other hand, the energy density of supercapacitors can be increased by
optimizing the structure of electrode materials and designing energy storage devices according
to energy storage mechanisms.
In the light of literature, we declare that many metal oxides exhibit pseudocapacitive behavior
in aqueous solution, thus could potentially provide higher specific capacitance due to multi-
electron transfer during fast faradaic reactions. Mo is the one of the metal compounds which
can be used as an electrode materials for supercapacitors. As reported by (Sugimoto, Ohunma,
Murakami, & Takasu, 2000), the results show that MoO3/C composite electrode gives a high
enhancement of the energy density in electrochemical supercapacitors even with poor electrical
conductivity of the oxide material by increasing specific capacitance of high surface area
between carbon and oxide material.
The carbon based electrodes with high capacitance have been fabricated in both macro and
micro scales successfully, however their usable capacitance is limited due to their double layer
capacitance only. The latter, metal oxides exhibit pseudocapacitive behavior in aqueous
solution that can provides higher specific capacitance. It has been revealed in the literature that
molybdenum oxides have been researched as pseudocapacitive materials due to their activity
of ion intercalation. The amorphous MoOx species with oxidation state of +5 to +6 provides
pseudocapacitance through reversible insertion/extraction of H+ and Li+. In study of (Can Liu,
Li, & Zhang, 2014), it is observed that MoO3-x can act as a negative electrode material by
showing high specific capacitance although it has a main drawback which is its poor electronic
conductivity. This drawback is accomplished by using conductive supporting materials with
large surface area as a composite electrode or by considering the variable composition of the
oxides in which both crystalline MoO2 and amorphous MoOx species to exist MoO2+x.
In the device, Li+ ions inside the electrolyte triggers the intercalation process of the electrode
composed by amorphous molybdenum oxide and MoO2 confined grains. Amorphous MoOx
19
with higher oxidation states plays a role as an active substrate providing pseudocapacitance by
insertion/extraction of H+ and Li+. MoO2 grains behave also as a catalyst for water
decomposition on the hydrated surfaces of them. Therefore, an increase of the H atoms leads to
higher capacitance in charge storage processes. The working mechanism of the MoO2+x (-) //
2M Li2SO4 // MnO2 (+) m-EC concluded as Figure 1.4.2.2 below.
Figure 1.4.2.2 : Illustrative diagram for the working process of MoO2+x(-) //2 M Li2SO4//MnO2(+) micro-device
On the other hand, as mentioned before, materials like metal oxides have added interest for
energy storage devices. In the light of this concern, maximizing the stored energy per unit mass
have a great place instead of improving structural stability over numerous charge/discharge
cycles. For cathode materials used in rechargeable batteries (metal chalcogenides or oxides), it
has been found that the component particle size and chemical structure greatly determine the
resulting energy density and power performance.
1.4.3. MoO3 as a Hole selective layer: Solar Cells
In recent years, Molybdenum oxide (MoO3) has been widely used in bulk-heterojunction (BHJ)
solar cells as an electron-blocking and hole-selective layer to replace poly(3,4-
20
ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT : PSS) (Fan et al., 2011). There are
many techniques to prepare MoO3 films for BHJ solar cells. It can be said that the commonly
used one is the thermal evaporation, however RF magnetron sputtering has advantages in case
of optimizing various deposition processes parameters that can effect formation and
performance of the film in different structures. In study of Fan et al., MoO3 layer was deposited
on FTO substrates at different substrate temperatures from 100 to 400˚C to produce BHJ solar
cells. From the J-V characteristics of the solar cells, it can be observed that power conversion
efficiencies vary as 3.27% and 3.12% at 100˚C and 200˚C, respectively, exhibiting a higher
performance which is much higher than that of the devices with a PEDOT: PSS layer and a
thermal evaporated MoO3 layer. Hence, MoO3 prepared by RF magnetron sputtering is quite
suitable for fabricating BHJ solar cells as an electron-blocking and hole-selective layer.
Figure: 1.4.3.1 : In light, J–V characteristics of the FTO/PEDOT : PSS/P3HT : PCBM/Al and the FTO/MoO3/P3HT : PCBM/Al devices corresponding to different substrate temperature deposited MoO3 films.
Figure 1.4.3.2, it is seen that MoO3 is used as a hole-selective and electron- blocking layer
due to comparison of the energy levels of the device.
21
Figure 1.4.3.2 : Energy diagram of the FTO/ MoO3/P3HT:PCBM/Al solar cell
Lowest unoccupied molecular orbital (LUMO) of P3HT is much lower than the energy level of
the conduction band (CB) of MoO3, which significantly blocks the electron transfer to MoO3.
Hole injection has an important role. Since decreasing deposition temperature of the MoO3
layer it leads also a decrease in optical band gap of the film and valance band of the film has a
higher level gradually. Hence, an increase in efficiency is seen easily due to hole injection from
P3HT to FTO by lowering the potential barrier between FTO and P3HT:PCBM. Eventually
Fan et al. stated that the optimized device shows a significant improvement in efficiency of
3.27%. They also suggested a key design principle in BHJ solar cells which can be defined as
the buffer layer should have a suitable valence band between the highest occupied molecular
orbital (HOMO) of P3HT and the Fermi level (Ef) of the electrodes as well as a suitable CB,
which is higher than the LUMO of P3HT.
It is worth noting that MoO3 has an importance for optical devices. It has been extensively used
in organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). MoO3 has been
22
used as a hole selective layer in organic photovoltaics. Regarding to (.Kim et al., 2009), MoO3
was used as an interlayer at the ITO electrode. Dramatically it is seen that MoO3 layer in range
of 10 nm, lowers the resistance of the PVs devices.
According to (Matsushima, Kinoshita, & Murata, 2007) MoO3 are used as a hole transporting
layers by an electron transfer from indium tin oxide (ITO) and α-NPD to MoO3 in word of
Ohmic hole injection.
Transparent substoichiometric molybdenum trioxide (MoOx, x < 3) is a novel material due to
its high work function with a low density of states of Fermi level for organic solar cells. It
originates oxygen vacancies as a defect which are located inside the band gap. In solar cells, it
behaves as a hole-selective [3, 6]. Rarely studied MoOx / silicon solar cell, MoOx is an
interesting candidate to form a hole selective contact for silicon solar cells and it works in
conjuction with n-type silicon absorbers. Bivour, Temmler, Steinkemper, & Hermle had
declared in their study, although MoOx presents excellent properties in terms of the selective
hole extraction before annealing, at 180 °C annealing caused degradation of the properties in
the structure of the device. The bulk and/or interface properties of MoOx are adversely affected
during annealing (more below). Amongst these, structures utilizing sub-stoichiometric
molybdenum oxide MoOx, offer the additional benefits of ease of fabrication and favorable
optical properties for photovoltaic applications. It demonstrates that, although modest in terms
of performance, reasonable passivation and good transport can be simultaneously achieved with
MoOx and MoOx/SiOx structures.
Many reports can be found in the literature regarding the use of TMOs as p-type contacts in n-
type c-Si (n-Si) for MoO3 and WO3 demonstrating a power conversion efficiency (PEC) of
18.8% for this novel solar cell concept.
23
Figure 1.4.3.3 : (a) Schematic of TMO/n-type c-Si solar cells. (b) Processflow diagram
Transition metal oxides (TMO) such as V2O5, WO3 along with MoO3, act as front p-type
contacts for n-type crystalline silicon heterojunction solar cells. The atomic structure of the
MoO3-x creates intrinsic oxygen vacancies as mentioned above. This properties generates n type
semiconductivity. Atomic structure demonstrates a variation form their fully stoichiometric
configuration (MoO3) to metallic-like conductors (MoO2). As we know the occupancy of the
d-states of the metal influences their conductivity and work function value.
Figure 1.4.3.4 : Current density-voltage (J–V) response of the fabricated TMO/n-Si solar cells
Figure 1.4.3.4 shows the current density-voltage (J–V) response of the fabricated TMO/n-Si
solar cells, measured under standard illumination conditions (AM1.5 g solar spectrum, 1000
W/m2) at 25°C. The best performance is achieved for V2Ox with a PCE of 15.7% and a VOC of
606 mV. It is followed by MoOx (13.6%, 581 mV) and WOx (12.5%, 577 mV), whose reduced
24
efficiencies are a result of lower VOC and FF values. The underperformance of WOx could be
partially explained by the absence of oxygen vacancies or merely by its lower passivation
potential [11].
2. Experimental Methods
2.1. Preparation of the Samples
Preparation of the samples in a small scale is one of the most important operations which is the
starting point to understand desirable properties for construction of the multi-layer devices
which are used in Phocs Project. Devices are built with different materials, consisting of the
FTO glass as a substrate, MoO3 deposition, spin coated polymer and catalysts Ti or Pt, with
architecture of FTO/MoO3/BHJ/TiO2/Pt. The samples used in this work is consisting only hole
selective layer of the devices with architecture of FTO/MoO3. The preparation steps were done
by author. Only the deposition step was performed by the stuff of the PLD Lab of Center of
Nano Science and Technology.
Irregular surfaces or the presence of dust and oxides can seriously compromise the
adhesion of the future processes which are intercalation steps or building whole devices.
Amorphous MoO3 thin films have been deposited by the team of PLD lab by magnetron
sputtering technique on different substrates, namely FTO, Silicon and glass. In particular, glass
substrates have been employed for optical measurements while morphological characteristics
have been investigated on silicon substrates. In order to perform the electrochemical tests FTO
conductive substrate need to be used to ensure a proper electrical contact. .
Each sample deposited and thermal treated MoO3 film on FTO is subjected to intercalation
process electrochemically in different conditions. The latter, structural and electrochemical
characterizations are subsequently performed for each sample. Below will be described the
theoretical basis, the setup and operating principles for each of these techniques, and then
discusses the results of the analysis.
These are the steps of preparation of the samples for deposition of the MoO3 film on FTO glass:
25
2.1.1. Preparing substrate
MoO3 film is able to be fabricated in PLD Lab of the IIT-CNST@Polimi on to the Florioune
doped TiN oxide (FTO) coated soda-lime glass substrates with commercial name Dyesol, with
sheet resistance 15 Ω msq-1 and ±10 µm flatness. The preparations of samples is one of the most
important operations. The roughness and the cleanliness of surfaces are mandatory factors for
a successful deposition because irregular surfaces or presence of dust and oxides can seriously
compromise the adhesion of the future processes which are intercalation steps or building whole
devices.
o Cutting
The FTO has 15 Ω/m2 sheet resistance. The FTO glass are cut into pieces by handy cutter. It is
crucial to be careful not to spoil the conducting layer which has a great role for both deposition
processes and the electrochemical cells. Selected size of the substrate is 14x20 mm.
o Etching
According to the usage of the samples etching steps are applied. During the etching process, to
protect the FTO layer, an area which is 17x8mm on the prepared pieces is enclosure by the tape.
Zn powders are sprinkled on the taped FTO homogenously, then HCI 25% solution is dropped
with the pipe on the edge of the tape. After 30 seconds, residuals are cleaned by a tissue. To
stop the chemical reaction between Zn and HCl, all pieces are cleaned by immersion of (Simchi,
McCandless, Meng, Boyle, & Shafarman, 2013) the distilled water. Afterwards, tapes are taken
out and acetone is used as a solvent to get rid of the dirt.
Cleaning (Sonicator) and Plasma Cleaning
After etching, all the samples should clean according to the following protocols inside solvents
by sonicator as sonic cleaning. The ultrasonic bath which is located inside the fumehood was
used for accurate cleaning our FTO samples in determined bath recipe. The bath allows the
ultrasonication of different samples to be cleaned. Ultrasonic waves are generated by the
instrument and propagate in the bath through distilled water, in which containers must be
merged.
The ultrasonic bath is made of three parts: the tank, the removable basket and the
beaker-holder cover. Samples, such as glass, ITO-glass, FTO etc. can be sonicated for cleaning.
26
Sample-holders of different sizes are available to place it inside a beaker of proper size and
fill with the proper solvent which are acetone, iso-propanol (IPA), and distilled water. Each
term takes place 20 minutes in acetone, isopropanol, and distilled water, respectively.
Acetone cleans the organic contamination on the FTO-ITO samples after etching to be ready
for the next step which is deposition of the MoO3 layer. After acetone sonication, beaker should
be filled by isopropanol to remove contamination of the previous acetone solution from the
sample’s surface. Finally, distilled water helps the removal of all sonication traces from the
surfaces. Each sonication steps takes place for 20 minutes with required temperature. After all,
samples were dried by Nitrogen gas gun each by each and stored in petri box coated with
aluminum sheet. If the samples need the plasma cleaning, it is done exactly before the
deposition to keep them in purely condition.
The oxygen plasma cleaning is the last step before deposition. It is done inside the WET-LAB
of CNST. It is necessary to obtain super clean the surface from any possible contaminant such
as any organic dirt and residual of the solvents. Plasma cleaning is performed for 20 min with
a power of 100W RF, it is adjusted to the frequency of 13.56 MHz under 40 Pa of O2 gas
pressure and the background gas pressure was 0.2 Pa inside the plasma environment.
2.1.2. Deposition Technique of the thin films
There are many techniques to prepare Mo films such as thermal evaporation which is widely
used. Another widely used preparation technique is Radio Frequency Magnetron Sputtering
form targets Mo or MoO3 or Reactive Dc planar Magnetron Sputtering from a Mo target.
Other types of deposition varies including pulse laser deposition, sol gel, spray pyrolysis,
chemical vapor deposition, hydrothermal and electrodeposition from solutions (Ou, Campbell,
& Yao, 2011). The preparation techniques also include electron-beam evaporation, reactive
pulsed laser deposition, thermal decomposition of electrodeposited MoS3, and the laser
vaporization-controlled condensation technique for making MoO3 films (Tao He & Yao,
2003).Pulsed DC Magnetron Sputtering was used as a deposition technique with given
parameters as, 50W power, 50 kHz during 4 minute and 20 seconds deposition time. The
vacuum chamber was evacuated to a base vacuum pressure lower than 3.0×10−3 Pa before each
deposition. The operating pressure was selected 1 Pa and Argon gas flow was 110 sccm for Mo
deposition discharge. Mo metallic target was purchased from Mateck Gmbh as a target material
27
with a purity 99.99%. Dyesol FTO glass was selected as a substrate. The target- substrate
distance was adjusted to 50 mm. The final thickness of the deposited films was 100 nm.
Deposited sample by magnetron sputtering is put in petri glass to be heat treated as seen figure
2.1.2.1.
Figure 2.1.2.1 : Metallic Mo deposited samples on Pulsed DC Magnetron Sputtering (ready to be annealed)
2.1.3. Annealing
Heat treatment has done after deposition of metallic Mo on the super cleaned FTO surfaces in
a muffle furnace. As a result of annealing crystallization and grain growth has occurred on the
surface. The 300 ˚C annealed films were β phase which has lower bandgap values compared
with the α-MoO3 phase had been reported by (Simchi et al., 2013). Increasing temperature
creates additional oxygen vacancies which can lead to an increase the intensity of the Mo5+
state. It is mentioned by (Battaglia et al., 2014) the formation of oxygen vacancies is reversible
by annealing the amorphous MoOx network in ambient O2 leading to a suppression of the Mo5+
and Mo4+ states. Annealing at high temperature leads to a partial crystallization of the MoOx
films which can be avoided by reduction or oxidation at room temperature in atomic hydrogen
or ozone environment respectively. In this thesis work, furnace adjusted at 500˚C for 30 min
with 1˚C/min heating ramp to obtain alpha phase stoichiometric by heat treatment. To cook
samples carefully it is better to cover with Al sheet or petri glass to prevent spilling some
impurities on the film. Circulation of the air should be homogenously so placement of the cover
must be done sensitively. After process, petri glass filled samples put on the brick to decrease
28
heat gradually for both bottom and top. At 500˚C, deposited metallic Mo turns into
orthorhombic α phase of the MoO3.
Figure 2.1.3.2 : Deposited and annealed samples: MoO3 100nm thin film on FTO substrate
2.1.4. Intercalation of the samples
Experiments are done with a three-electrode electrochemical cells inside the buffer solution,
which is prepared manually with amount of chemicals that will be mixed of Sulfuric acid H2SO4
(1.38ml in 250ml Hz0) and Sodium sulfate Na2SO4 (4.2617g in 300ml Hz0).
The platinum counter electrode, reference electrode and working electrode are immersed into
buffer solution. The FTO/MoO3 substrates acts as the working electrode, along with an
Ag/AgCl reference electrode and a platinum wire (Aldrich) serving as the counter electrode.
Bubbling has done before each intercalation cycles for a while and the atmosphere is controlled
with a noble gas in experimental set-up. It is a crucial step to keep the atmosphere under control
without any air or foreign ions which can be contributed to intercalation process. pH value of
the electrolyte was 1,37 and it should be stable to be sure that all the intercalation rates are
pursuing equivalently.
29
Figure 2.1.4.1 : The Three-Electrode Pine Electrochemical Cell that is used for intercalation processes and electrochemical characterization of the processed samples
Designed experiment includes 2 different base batch to see how the intercalation effects the
MoO3 layer on top of FTO. These are declared as voltage series and time series. As it appears,
different voltages and different times are picked up as a intercalation parameters which are
respectively -0.1 V, 0.0 V, 0.05 V 0.1 V, 0.15 V, 0.2 V (all the voltages are respect to Ag/AgCl
reference electrode) on fixed time range which is 20 seconds and 5, 10,15,20,30 seconds at a
fixed voltages which are -0.1 V and 0.1 V vs Ag/AgCl in a mildly acidic electrolyte. Blue-
colored thin films were obtained by applying a fixed potential of -0.1 V vs Ag/ AgCl for a
predetermined length of time and a fixed time of 20 seconds for diverse potentials ranging from
-0.1V to 0.2V vs Ag/AgCl as seen on Table 1 and Table 2 as named Voltage series and Time
series, respectively.
Voltage Series in Volt (V vs Ag/Agcl)
20 s 20 s 20 s 20 s 20 s 20 s 20 s
Table 1 Voltage Series
30
Time series in second (s)
-0.1V 5s 10s 15s 20s 30s
0.1V 5s 10s 15s 20s 30s
Table 2 Time Series
Intercalation steps of our samples is observed visually by chancing color from transparent to
blue tones on the FTO-deposited and thermal treated MoO3 thin films. Experimentally expected
fact is the differences on color density depending on the time of exposure of the voltage and
range of the voltage at potentiostat (Metrohm Autolab PGSTAT 302N) on predetermined time
range.
Figure 2.1.4.2 : The samples images of before intercalation & after intercalation holding with a crocodile.
31
It should be stated that, over intercalated MoO3 cannot work properly and split up from the FTO
layer which destroys the usage of the devices as seen on fig 2.1.4.3.
Figure 2.1.4.3 : The images of destroyed films after too much intercalation rates.
The effect of H+ on α-MoO3 was characterized using X-ray photo-emission spectroscopy
(XPS), Raman analysis, and X-ray diffraction (XRD) and electrochemically on the following
chapters it will be seen that the transmittance and potential curves as a function of intercalation
degree are strongly dependent on microstructure and stoichiometry of the samples.
The formation of molybdenum bronzes is a consequence of the intercalation processes. On the
next chapters, structural analysis shows how the characteristic spectrum of MoO3 will be
significantly modified due to deformation of the crystal lattice by presence of Mo+5 which
changes the vibrational modes of the bonds between Mo and O. As a result of the literature
review, rearrangement on the position and detected shape of the peaks depends on the degree
of intercalation [30].
The modification of the electronic structure through this kind of process allows to have a higher
electrical conductivity and a band gap smaller. Therefore it has many advantageous of these
oxides in case of application in sensors and photovoltaic devices, and energy storage systems.
2.2. Sample Characterization
The characterization can be divided into principle topics that describe the aim of this work. On
next chapters there will be several types of measurements that had performed to characterize
32
the intercalated samples to find out how it behaves. These measurements are taken into account
as,
Structural Characterization (Raman Spectroscopy, X-ray diffraction; XRD, X-ray
photo-emission spectroscopy; XPS)
Optical Characterization: UV/VIS/NIR Spectrometry
Work function : Kelvin Probe; KP
Electrochemical Characterization (Cyclic Voltammetry, Mott Schottky)
Bandgap calculations were performed based on the absorbance spectral data acquired
of the H+ exposure
Raman Spectrometry and Electrochemical characterizations were done by the author at PLD
Lab of the CNST, the other characterization of samples were carried out by ENI-Istituto
Donegani.
2.2.1. Raman Spectroscopy
A physicist, C. V. Raman, who won the Nobel Prize on 1928, discovered the wavelength of the
scattered light of the some molecules is different form the wavelength of the incident beam.
These wave scattering is caused by the difference of the chemical structure of the molecules
and it is obtained recently by Raman Spectrometer which is simply a kind of microscope with
lenses that have different magnification ratios and focused laser light.
33
Figure 2.2.1.1 : Renishaw inVia Raman microscope/spectrometer
Raman spectroscopy is a technique that consists of exciting the rotational, vibrational and other
low-frequency energetic levels of a molecule. Typically, a sample is illuminated with
monochromatic laser light and the resulting scattered photons are collected with a lens and
dispersed on a detector.
Monochromatic light (laser), interacts with molecular vibrations and phonons of the sample,
resulting in an energetic shift of the photon upwards and downwards. The measurement is based
on inelastic scattering, called Raman scattering.
34
Figure 2.2.1.2 : Schematic of the different light scattering possibilities: Rayleigh, Stokes and anti-Stokes.
It should be mentioned that, in a process called Rayleigh scattering, almost all the incident
photons are elastically scattered by the interaction with the material. The scattered photons
continue their movement with the same kinetic energy (frequency and wavelength) they had
before. The Raman Effect is described as an inelastic scattering which is seen on the Figure
2.2.1.2 differently from the Rayleigh scattering. This process represents a small fraction of
photons by relaxing to a different state after an excitation and scattering. This different energy
state of a molecule can be less or more, respectively for the Stokes and anti-Stokes scattering
process which are seen on Figure 2.2.1.2. In briefly, incident photons are absorbed and
reemitted with an energy profile depending on the type of interaction witnessed by the light
particle.
To analyze structural information of the intercalated samples that we have, was being performed
with a Renishaw inVia Raman microscope/spectrometer.
Raman spectral measurements were carried out by spectroscope inVia Renishaw Raman
microscope. The samples are irradiated with a green laser (532 Edge, excitation line 532 nm)
in visible range. Measurements are done on the sample surfaces related to different regions and
spots to get more precise results. Experiment was quiet sensible depending on the intercalation
degrees. Laser power should be determined carefully not to destroy the deposited and
intercalated film layer. Total power of the laser was 74mW. During the measurements, the
35
power of the incident radiation was corresponding to small fraction of the total power such as
1%, 5%, or 0.74 mW 3.7 mW respect to condition of the intercalated samples. 50X
magnification was selected to focus on the several spots which are effected from intercalation
ratios. 1000 accumulation was generally done for every cycle of the measurements of Raman
spectral to reduce the noises proceeded by laser. It was noted that increasing the number of
accumulations decrease the background noise of the curve due to the fact that it characterizes
as distribution of the noise vibrations that are randomly lessened by highlighting peaks.
Raman Spectrometer helped to determine the intercalation degree of the H+ ions bounded on
the MoO3 structure, as it is highlighted in literature, we had expected the formation of the
hydrated molybdenum bronzes HxMoO3 (HMB) as a result of the intercalation process of the
deposited MoO3 samples. Recently HMB are interesting materials to make electrodes for the
electro-reduction of organic compounds, for electrochromic devices and for anodes in fuel cells
owing to their high hydrogen content [9].
As mentioned before, Molybdenum tri-oxide is known to exhibit two major crystal phases:
thermodynamically stable α-MoO3 and metastable β-MoO3. Orthorhombic α-MoO3 has a
characteristic layered crystal structure. In light of literature, after H+ intercalation process HBM,
molybdenum bronzes are identified on the Raman spectra. Depending on the preparation
method of intercalation, a series of compounds HxMoO3 (HMB) with 0 < x < 2 molybdenum
bronzes are able to be identified on the Raman spectra [9]. The formation of molybdenum
bronzes is a consequences of variation on both the crystal structure of the film and electronic
structure of the film. The Raman spectroscopic analysis can, in fact, reveal how the spectrum
characteristic of the MoO3 can be significantly modified after intercalation processes. The
deformation of the crystal lattice and the presence of Mo5+ changes the vibrational modes of
the bonds between Mo and O atoms. It can be detected by this analysis technique that is focused
on the characteristic arrangement and shape of the peaks in the spectrum depending on the
degree of intercalation that the film has undergone [30].
Principal Raman peaks belonging to the pure α-MoO3 orthorhombic structure by the occurrence
of the three Mo-O stretching are located at 666 (B2g, B3g) cm-1, 819 (Ag, B1g) cm-1, and 995
(Ag, B1g) cm-1 [2]. McCarron stated that the Raman peaks associated to the monoclinic β phase
has (776 cm-1, 850 cm-1, and 900 cm-1) peaks based on Mo-O stretching at low temperature
[22].
Typical Raman spectrum of the alpha phase of molybdenum oxide is seen on Figure 2.2.1.3.
36
Figure 2.2.1.3 : Typical Raman spectrum of the alpha phase of molybdenum oxide
Taken from, http://rruff.info/molybdite/display=default/
Figure 2.2.1.4 : Raman spectra of MoO3 deposited on quartz (blue line), FTO (green line), glass (red line) and ITO (black line) substrates. The inset figure represents the intensity ratio between the peaks at 821 and 666 (blue line) cm-1 and those at 821 and 995 cm-1 (red line).
If necessary to look deeply literately, in study of Campbell et.al, the MoO3 samples were
deposited thermally form the MoO3 powder on the different substrates that are quartz, glass,
FTO and ITO. The resulting films were analyzed using Raman Spectrometer. The Raman
spectra of four different samples shown in various color are presented in Figure 2.2.1.4 [24].
As can be seen, strong peaks occur at 284, 666, 821, and 995 cm-1 for all these samples. These
peaks are relating to alpha MoO3 that mentioned before. It should be noted that there is another
point of view to be concerned in this paper. It is seen clearly on the figure, intensity of the peaks
vary depending on the substrates. This is due to the formation of the crystalline relying on the
37
surface morphologies of the substrate’s materials. It is proved in the paper as a result of the
XRD analyzes. The change of the substrate from quartz to FTO results in the formation of
layered rectangular Nano-belts rather than hexagonal plates. Similar to the FTO sample, the
glass sample also consists of layered rectangular belts with slightly larger dimensions, lying flat
on the surface of the substrate. For the ITO sample, these layered rectangular belts seem to be
condensed together, forming large and flat surfaces on top of the ITO substrate. The size
distributions of the surface morphology for each sample change the intensity of the Raman
signals. The authors claimed eventually, this indicates the significance of forming low surface-
to-volume ratio crystallites in generating strong Raman signals, in which the planar vibrations
are confined within one axial direction.
In our experiments, only FTO substrate is used for deposition of the MoO3 films, therefore,
there is no this kind of diversity of Raman signal’s intensity.
According to (Ou et al., 2011), strong peak occurred at 284, 666,821 and 995 cm-1 corresponds
to different types of bonding modes that are indicated below in detail,
The 284 cm-1 peak represents the bending mode for double bond (Mo=O) vibration.
The 666 cm-1 peak is assigned to triply coordinated oxygen (Mo3-O) stretching mode,
which results from edge-shared oxygen atoms in common to three adjacent octahedra.
The 821 cm-1 peak is for doubly coordinated oxygen (Mo2-O) stretching mode, which
results from corner-sharing oxygen atoms common to two octahedra.
The peak at 995 cm-1 is assigned to terminal oxygen (Mo6+=O) stretching mode, which
results from an unshared oxygen. Singly coordinated oxygen.
After intercalation processes, shifting on the MoO3 peak are expected and it is obviously a
reason of different bonding vibrations. These are the some examples form the literature view:
38
Figure 2.2.1.5 a), b) 2D layered MoO3, cross section of the FET device c) Raman spectra of intercalated MoO3
In Figure 2.2.1.5 a and b schematically illustrates the reduction process of FET along the cross
section. MoO3 used as a layer in the FET device and formation of the sub stoichiometric states
in MoO3 occurs as a result of electron beam bombardment and intercalation. In figure 4.1.5.c
is the Raman spectra of the MoO3 flake with the broad peak at 780 cm-1. The peak at 780 cm-1
indicates presence of MoO3-x which is absent in background measurement of substrate.
In another experimental studies show that, individual MoO3 layers were rendered with
excess electrons either due to hydrogen intercalation (HyMoO3) or due to the formation of
oxygen vacancies resulting in non-stoichiometric structure (MoO(3–x)). As mentioned earlier,
both these mechanisms lead to similar electronic behavior [16].
On the other hand, another used processes for deposition and annealing, differs from our
experimental way, can also reduce the material and shows a shift in Raman spectra of the MoO3.
One of the examples of the literature states MoO3 undergoes direct electron beam lithography
for fabrication Pt electrodes. The spectral acquisition was taken from near electrode and further
from electrode. Both spectra show partial reduction, similar to those of H+ intercalation.
Additionally there are other methods that can be equally used for obtaining MoO(3–x). To take
into account of the presence of sub-stoichiometric MoO(3–x), the function of the x has
performance to change the properties of the film. For instance the ability to reduce bandgap
hardly depends on a function of x. In brief, by introducing the oxygen vacancies, Mo6+
neighboring the oxygen vacancies in the MoO3 lattice would be reduced to Mo5+, injecting
39
an electron to the conduction band. The additional electron from such Mo5+ is delocalized
within the layers, since it is not tightly bound to any Mo6+ core cation. This will give rise to
a gap state in between the valence and conduction bands of MoO3, hence narrowing its
band gap. In Figure 2.2.1.6 it is seen the effect of the sub-stoichiometric occurrences on the
Raman spectra of the Mo films.
Figure 2.2.1.6 : Raman spectra of sub-stoichiometric MoO (3-x) acquired from a thick sample, which was adjacent to the Pt electrodes, that was deposited using highly energetic electron beam.
2.2.2. UV/Vis/NIR Spectrometry, Absorption Measurements
UV/Vis/NIR Spectrophotometry allows to take into account the study of the interaction between
matter and electromagnetic radiation. In particular, with the term UV/VIS/NIR is shown the
analysis of the spectra in the ultraviolet field at high energies and low wavelengths (above 250
nm), visible light and near infrared (below 2500 nm). The spectrophotometer is a device for the
measurement of light intensity, equipped with a light source and a detector for measuring the
radiation. By the interaction of the radiation with a solid sample, depending on the combinations
of positioned the emitter and/or detector relatively, it is possible to obtain spectra of
transmittance, total or diffuse, and reflectance, which is also the total or diffuse.
40
With the UV-Vis-NIR, you can measure the absorption of light in a given material. It is not
measured directly, but is calculated as a result of the measures of total transmittance and total
reflectance. The relationship between these three variables is as follows:
A=100-Tt-Rt
Absorption spectrum has the dependence of absorbance from wavelength: A = f(λ). From the
absorption spectrum can be determined the position of the absorption maxima and the intensity
of the absorption maxima. The amount of absorbed radiation may be measured in a number of
ways:
- Transmittance T = I/I0 %T: [0%, 100%]
- Absorbance A = log (1/T) A: [0, ∞)
If all the light is absorbed, then percent transmittance is zero, and absorption is infinite. If all
the light passes through a solution without any absorption, then absorbance is zero, and percent
transmittance is 100%.
Figure 2.2.2.1 : Schematic diagram of a spectrophotometer
Figure 2.2.2.2: PerkinElmer LAMBDA 1050 UV/Vis/NIR Spectrophotometer
41
In order to characterize the optical properties of the intercalated samples, PerkinElmer
LAMBDA 1050 UV/Vis/NIR Spectrophotometer is used, seen on Fig. 2.2.2.2.
The principle of operation of a spectrometer is based on excitation behavior of the illuminated
samples. It should be considered what happens a sample is crashed by a photon of the ultraviolet
spectrum, visible or infrared. The photon can excite an electron and it makes to pass from its
equilibrium energy level to a state electronic excited. While exposure of the light, the photons
absorbed and caused a decrease in the intensity of the radiation that passes through the sample.
This is the phenomenon that is quantified by calculating the absorption, which it joins the
reflection phenomenon. The sum of the two phenomena mentioned causes the decrease of
intensity in the light transmission through a material; the ratio between incident and transmitted
intensity is transmittance. In order to determine the optical properties, measurements carried
out in the present work, the range of wavelengths chosen is that between 250 and 2500 nm,
with a sampling interval of 10 nm.
In this study intercalated samples were measured by UV/VIS/NIR spectrometer (Perkin Elmer
Lambda 1050) operating with a 150 mm integrating sphere to observe the behavior of the
deposited layer of the MoO3. The intercalation process gives an observable electrochromic
effect in the films. In the context of extensive studies of MoO3, (Gesheva, Szekeres, & Ivanova,
2003) states that it has a lower coloration efficiency related to insertion of the ions or charges
but, it has closer position of its optical absorption peak to the human eye sensitivity peak that
makes this material very attractive for many applications. Also, the mixed films of MoO3 with
other oxides were studied by Gesheva et,al. They have higher optical absorption with higher
inserted charge density through the films. Especially after annealing, tungsten and molybdenum
oxide films become more transparent, while transparency of the mixed oxides films decreases
in the range of 400–750 nm and slightly increases towards longer wavelengths. In our
experiments, the modification of films in case of the being transparency is able to detect after
annealing. A higher coloration efficiency is expected, due to increased electron transitions
between the metal sites with different valences (Mo6+, Mo5+, Mo4+) or with different structural
surroundings. Furthermore, the higher optical absorption is observed by naked eyes depending
on higher ion insertion while different intercalation rates applied within time and potential
series of the experiments. In the view of this study, FTO-glass substrates were used in UV–VIS
spectrophotometry measurements. A comparison between different intercalated films was
directly made because of the same film thicknesses that is 100nm for each samples. From the
absorbance spectra also, the optical band gap energy Eg were determined. Interestingly, using
42
intercalation, the bandgap of MoO3 can be reduced to cover a wide range of the visible
and infrared spectrum.
In study of (Fan et al., 2011), the optical band gap energies (Eg) are evaluated from the Tauc
plot. They are found to be 3.67 ± 0.01eV, 3.64 ± 0.01eV, 3.72 ± 0.01eV and 3.82 ±0.01eV for
the films deposited at 100 ◦C, 200 ◦C, 300 ◦C and 400 ◦C respectively, depending on the
structural morphologies and crystal defects. The direct band gap energy increases with an
increase in substrate temperature, which might be attributed to the reduction of oxygen
deficiency and the stoichiometric approach of film composition. At a higher substrate
temperature, the sputtering process may yield more active oxygen species due to the plasma
decomposition of O2 than that at a lower temperature [10].
The optical band gap energy of intercalated α-MoO3, Egap, has been calculated from the
transmittance spectra using a Tauc-plot analysis, by plotting (αhν)2 as a function of hυ in the
proximity of the UV absorption edge. Tauc-plot analysis was performed on the transmittance
data of the two phases to calculate the optical band gap energy. Values of 2.85 and 3.23 e.V.
were obtained for the β and α phases respectively, well in agreement with similar analysis
reported in the literature. Moreover, thanks to the techniques of spectrophotometry, it is possible
to indirectly assess the effect of intercalation on the reduction of the band gap, seen on Figure
2.2.2.3.
Figure 2.2.2.3: Method for determination of the band gap of the α-MoO3 following various conditions of intercalation, since the absorbance spectra and by the energies of the incident photons.
43
2.2.3. X-Ray Diffraction
X-ray diffraction is a method used for identifying the atomic and molecular structure of the
crystal. It is performed to understand in which the crystalline atoms cause a beam of incident X-
rays to diffract into many specific directions. By measuring the angles and intensities of these
diffracted beams, it is able to measure the density of electrons within the crystal. From this
electron density, the mean positions of the atoms in the crystal can be determined, as well as
their chemical bonds, their disorder and various other information. The X-Ray diffractometer
Bruker was used for the XRD analysis of the specimens. The pattern of diffraction peaks is used
as a fingerprint for the identication of a material (based on Bragg’s Law), with the help of
standard reference pattern to be used for comparison.
Figure 2.2.3.1 : The X-Ray diffractometer Bruker
In this work, XRD is the one of the techniques to get structural information. XRD measurement
helps to interpret the effect of the intercalation processes on the molecular structure. There are
many works that is done related the principle MoO3 peaks and effect of the intercalants to the
MoO3.
44
Figure 2.2.3.2 : Inter-planar spacing (d) of the MoO3 films (Al-Kuhaili, Durrani, & Bakhtiari, 2009)
(Li, Jiang, Pang, Peng, & Zhang, 2006) states the XRD pattern of the MoO3 by indicating the
orthorhombic MoO3 with cell parameters: a = 3.962 Å, b = 13.858 Å, c = 3.697 Å). In study of
(Ou et al., 2011), the strong diffraction peaks appear at 12.8, 25.7, and 39.1˚, which correspond
to the (0 2 0), (0 4 0), and (0 6 0) planes, respectively. The strong intensities of the reflection
peaks of (0 k 0) with k = 2, 4, and 6 also prove the existence of the lamellar structure. Other
obvious peaks at 23.4, 52.9, and 67.6˚ can be assigned to (1 1 0), (2 1 1), and (0 10 0) planes,
respectively. It can be observed that a very small amount of substoichiometric MoO3 exists in
all samples because small peaks appear at 11.7, 24.7, and 37.4˚, which is a common
characteristic for most of the metal oxides prepared by thermal evaporation methods.
45
Figure 2.2.3.3 : XRD patterns of the MoO3 deposited on quartz (blue line), FTO (green line), glass (red line), and ITO (black line) substrates. The labeled peaks correspond to α- MoO3 (*, JCPDS 05-0508), substoichiometric MoO3 (□), SiO3 (■), FTO (◊), and ITO (○).
In study of (Xu et al., 2015), The LiTaMoO6 and LiNbMoO6 was subjected to proton-exchange
reactions in HNO3 solution, yielding hydrated layered HTaMoO6 and HNbMoO6, whose XRD
patterns are presented in Figure 2.2.3.4 (a) and (b), respectively. From both the XRD patterns,
one can see that the basal interlayer distances (d002) were expended to 1.77 and 1.80 nm,
respectively, as the consequence of intercalation of the protonic layered oxides after the
performed intercalation reactions. It is displayed significantly increased interlayer distances on
the XRD patterns. This is a definitive evidence for successfully intercalation of the layered
HTaMoO6 with large Cr (III)-containing species
46
Figure 2.2.3.4 : XRD patterns of (a) HTaMoO6.xH2O,(b) HNbMoO6.yH2O,(c) C3-HTaMoO6, and (d)C3-HNbMoO6
X-ray diffractometer (Philips X'Pert, Cu Kα radiation) was used in reflection mode. The
parameters of the XRD measurement were assessed such as exposure time: 0.1 seconds and
angular step size 0.016°. Chemical state information was obtained using an AXIS Ultra DLD
instrument (Kratos) equipped with an Al Kα X-ray source with a 1486.6 eV, operated at 10 mA
and 15 kV.
2.2.4. Cyclic Voltammetry Measurements in Acetonitrile [CH3CN]
The cyclic voltammetry measurements were performed to identify the range of potential that
do not occur redox reactions. Consequently the depletion zone was determined to do analysis
of Mott-Schottky. On the other hand, by using CV technique the electrochromic nature of the
films can be analyzed by inserting H+ ions from the relevant electrolyte solution [26].
This technique is generally used to study the properties of surfaces or structures of the bulk in
the solution, as well as for the study of redox systems. Upon variation of the potential, for each
sample, which can be reduced or oxidized, there is an exchange of electrons with the counter-
electrode (and ions with the electrolyte), it will correspond to a peak in the current signal of the
47
voltammogram. By applying the potential, it is reached a value which reduces or oxidizes. The
peak is formed in the first part of the measure. If the process is reversible, when the potential is
reversed, a current of the opposite polarity is produced; then appears a new peak in the
voltammogram that is not being necessarily mirror. From this, it is derived the potential of
redox. Another consideration is the potential at open circuit (OCP) which is obtained when the
two electrodes are not connected, and provides the potential of the cell (tendency to reach
equilibrium). This parameter helps to be taken into account to determine the potential departure
of cyclical scans.
Intercalated films show well defined redox peaks. To set a light on cyclic voltamograms, the
study of Dunn et.al can be examined in the case of Li ion insertion similar to H insertion that
will be our case in this work. The cathodic peak at 2.7 V vs Li/Li+ can be attributed to an
irreversible phase transition of the MoO3 into Li+ intercalated seen on figure 4.4.1. This cause
an expansion of the interlayer spacing [5]. Also, these redox peaks are characterized as
reduction and oxidation pairs. The 2/2’ and 3/3’ reversible redox peaks, seen on figure 2.2.4.1
occurrence of the Li+ ion insertion called as intralayer and interlayer, respectively [7]. There is
another broad peak shown on the diagram. This response can be occurred due to the slow ion
insertion/extraction kinetics in case of diffusion of ions through the bulk of the film.
Figure 2.2.4.1 : Cyclic voltammograms of the Li+ inserted HMB crystal
In study of (Endres & Schwitzgebel, 1996), the voltammograms is shown that by repeated
scans, the height of the peaks tend to be higher. The reason is the penetration of the electrolyte
into the working active area. Therefore oxidative and reductive range become distinguishable.
If the net charges is considered, negative net charge indicates the hydrogen diffuses from the
surface into the bulk of each HMB crystal.
48
Figure 2.2.4.2. : Cyclic voltammograms of the Li+ inserted HMB crystal
A rising cathodic peak is attributed to hydrogen evolution and it can be converted the peak in
the positive scan as the reoxidation of adsorbed hydrogen indicated in the figure 4.4.2.
Electrochemical characterization was carried out by using an Autolab potentiostat/galvanostat
(PGSTAT 302N), in a three electrode configuration. Ag/AgNO3 in a saturated 0.1 M TBAPF6
in CH3CN solution and a Pt wire are used as reference and counter electrode respectively.
Cyclic Voltammetry (CV) is performed with a scan rate of 50 mV/s.
Acetonitrile Measurements were done with 3-electrode setup inside the prepared solution
described as, 0.1M Acetonitrile [ACN or CH3CN] + Tetrabutylammoniumhexafluorophosphate
[TBAPF6]. Reference Electrode is used as Ag/Ag+ [Ag/AgNO3].
Reference electrode tube is filled with ACN+AgNO3 solution and Ag wire is immersed into the
solution and then sealed with the parafilm which is flexible plastic paraffin film to prevent
evaporation during measurements. A three-electrode potentiostatic setup was used with an
Ag/Ag+ as the reference electrode and a large Pt as the counter electrode. The cell solution was
unstirred and consists of 0.1 M TBAPF6 in CH3CN, in the absence of a redox-active species.
Cyclic voltammograms were measured by using an Autolab potentiostat system. Samples were
sealed with the Teflon tape to create significant contact area which has 0.5 cm diameter. The
data were collected from -0.9 to 0.2 V vs. SHE with a scan rate of 50 mVs -1. After each scan,
the scan range was increased step by step according to formation of the cyclic voltammograms
picks to find the double layer.
49
Different scan rates changes the shape of the curves. At high scan rates, curves will begin to
shrink and peaks will be shown out of the potential windows. In study of Can Liu et al., 2014,
different scan rates leads to have different specific capacitance. At low scan rates as a 20 mVs-
1 with 860 nm thick film of MoO2+x reaches 33 mFcm-2 as a real capacitances and 385 Fcm-3 as
a volumetric capacitance. While an increase occurs on the scan rate, 500 mVs-1, film shows a
capacitance of 181 Fcm-3 which is %47 higher rate relative to that at 20 mVs-1. This implies
that MoO2+x film is comparable to electrochemical double layer capacitive materials [20].
The tests of CV (cyclic voltammetry) were carried out on a sample of α-MoO3 (made with
annealing in air at 500 ° C for 30 minutes) and on samples of intercalated MoO3 (as performed
under different potentials and time). All tests were carried out with reference electrode
Ag/AgNO3. Applied potentials was shifted to the SHE (+0.54 V vs SHE, standard hydrogen
electrode). Current was normalize compared to the active area of the sample that has a diameter
of 0.5 cm.
In the present work the cyclic voltammetry is used to analyze the electrochemical behavior of
the samples allowing to identify the reactions at the electrode-electrolyte.
2.2.5. Mott-Schottky Measurements
To investigate the presence of redox reactions and the properties of transportation of the
fabricated and intercalated samples (such as resistance to charge transport and Specific
capacity), electrochemical measurements were conducted using different techniques such as
cyclic voltammetry, electrochemical impedance measurements and analysis Mott-Schottky.
These measurements were carried out in a standard three-electrode electrochemical cell
provided by Pine Instruments. The electrochemical cell is equipped with channels for intake
and outflow of a purge gas, Nitrogen to remove oxygen from inside the cell atmosphere and the
electrolyte solution.
The reference electrode was used as an Ag/AgNO3 (+ 0.54 vs SHE, standard hydrogen
electrode). A platinum wire was used as a counter electrode. Applied potential and current
monitored and recorded by a potentiostat (Metrohm Autolab PGSTAT 302N). The
measurements were obtained using the software of NOVA (Metrohm) version 1.8.
50
The electrolytic solution used is composed of acetonitrile (ACN) and tetra-n-butyl-ammonium
hexafluorophosphate (TBAPF6) as a supporting electrolyte that has the task to prevent the
migration of charged species. The solution and 0.1M was obtained by adding 9.686g of TBAPF6
to 250 ml of ACN. The reason of the choosing this solution is to be avoided additional
intercalation of the species that would have altered reactivity of the sample when the applied
bias to electrode is conducting in the aqueous solution. The samples were masked with a Teflon
tape having a hole diameter of 5 mm to define the active surface of the working electrode.
Determined active area on the electrode helps to normalize the density of the current on
impedance and capacity.
Figure 2.2.5.1 : A masked sample for the Mott-Schottky measurements as a working electrode
51
Figure 2.2.5.2. : a) The used electrochemical cell with the working electrode (E) in central position (red), reference electrode (RE) connected to the blue crocodile, counter electrode (CE) connected to the black crocodile and pipes for the flow of nitrogen. b) Diagram of operation of an electrochemical cell with three electrodes.
2.2.6. Kelvin Probe Microscope
Kelvin Probe Microscope is one of the used measurement in order to verify the surface
properties of the material and its potential for charge selectivity. The Kelvin Probe is a
non-contact, non-destructive vibrating capacitor device used to measure the work function
(WF) of conducting materials or surface potential of semiconductor or insulating surfaces.
Kelvin probe measurements at ENI laboratory were performed by using a Kelvin Probe
Microscope (KPM Bruker, Dimension Icon) in air and at room temperature. A reference sample
of graphite (HOPG, Highly Ordered Pyrolytic Graphite) is used for calibration, as determined
work function value of it is (4.6 eV). The results will be discussed in Chapter 3.
a) b)
52
2.2.7. X-Ray Photoelectron Spectroscopy
The compositional stoichiometry of the films was studied by X-ray photoelectron spectroscopy
(XPS) (Sivakumar et al., 2007). Every intercalated samples were analyzed to examine the effect
of intercalation on Mo oxidation state. X-ray photoelectron spectroscopy (XPS) spectra were
obtained for film deposited on a FTO substrate both before and after intercalation at different
potential series and time series. The XPS spectra was determined for the Mo 3d3/2, Mo 3d5/2.
Figure 2.2.7.1 : Core level spectra of the MoO3 film showing the presence of Mo+6 oxidation state, only.
In the light of the literature, on Figure 2.2.7.1 , it is seen that the core level spectrum consists
of two peaks located at 232.6±0.2 eV and 235.8±0.2 eV, which corresponds to the Mo 3d3/2 and
Mo 3d5/2 orbitals, respectively [10].
The core-level spectra of MoO3 films exhibit the characteristic Mo 3d3/2 and Mo 3d5/2 doublet
caused by spin–orbit coupling. Based on these literatures, it can be said that the electrochromic
mechanism of MoO3 films depends on the existence of different final states such as Mo6+ and
Mo5+.
The characteristic doublet peaks are observed at binding energies of 232.32 and 235.44 eV from
the Mo-3d core-level spectra (Fig. 4.7.2. (a)–(d)). These doublet core-level binding energies
i.e., 232.32 eV and 235.44 eV correspond to Mo-3d5/2 and Mo-3d3/2, respectively, which are
53
due to the spin–orbit splitting (Mo-3d) and are in good agreement with the values reported by
the other works for MoO3 films.
Figure. 2.2.7.2 : XPS core-level spectra of molybdenum (Mo-3d) for MoO3 films: (a) Tsub= RT, (b) Tsub= 100 ˚C, (c) Tsub= 200 ˚C and (d) Tanne= 300 ˚C.
The Mo 3d core level spectrum is seen on Figure 2.2.7.2. , as deposited MoO3 films prepared
at different temperature. It is obviously seen that the intensity and broadening of the peaks
points out varieties due to the difference in electrical environment around Mo site and also the
amorphous nature of the films. Films prepared at low temperature (a) have less intensity than
those of the as- deposited films prepared at higher substrate temperatures (b, c) and annealed
54
films (d). Increased temperature during preparation of the films indicates an improvement in
the crystallinity by obtaining sharper peaks [10, 26].
Stoichiometric configuration of deposited MoO3 films specify electrical property as an
insulator. On the other hand, MoO2 is named as metallic-like conductors. Semiconductivity can
be another electrical property of the MoO3 films in terms of variation of the oxygen vacancies
in their atomic structure that can be symbolized as MoO3-x. The conductivity and work function
values can be determined according to their occupancy of the d-states. In work of the Gerling
et al., oxidation state transitions and generation of states within the Egap have been reported as
characteristic features of oxygen loss during TMO deposition. In order to identify such vacancy
related effects, the XPS photoemission spectra were analyzed [11].
Figure 2.2.7.3 : XPS spectra of the core levels of MoOx
Similarly, the Mo 3d core level was deconvoluted into two doublets centered at 233.4 and 231.8
eV (ΔBE=3.1 eV, 3:2 area ratio), representative of Mo+6 and Mo+5 respectively as shown in
Figure 2.2.7.3.
XPS measurements performed by ENI-Istituto Donegani to obtain chemical state information
by using an AXIS Ultra DLDinstrument (Kratos) equipped with an AlKa X-ray source (1486.6
eV) operated at 10 mA, 15 kV.
2.2.8. Dissolution test
Dissolution test has done at -0.1 V vs Ag/AgCl for 9000 seconds with settled parameters of
Chorono Amprometry at Potentiostat. Sample FTO/MoOx (amorphous MoOx) is used as a
55
working electrode in 3 electrode cell. The pH of the solution is the 1.37 like previous
experiments. The open circuit potential (OCP) is registered as 0.02 V. During measurement, a
decrease in current was observed. The current at 9000sec was 5nA while it was 0.21µA at
180sec. After measurement, it is controlled by tester. The part without immersion and coupled
with FTO/MoOx showed the value 180 which is seen on the tester screen. FTO approximately
had 26.5 conductivity. The part which is immersed in the solution had the same conductivity
value with FTO which means it is dissolved. The film was on the top, however, it is no more
conductive.
Figure 2.2.8.1. : Images of the dissolution test
3. Results and discussion
The first objective of this research is the identification of the effect by intercalation in which
are formed the different stages of the oxide of molybdenum. The effect of the intercalation
depends on the applied time range at -0.1 V vs Ag/AgCl is shown on the photos as a 5s, 10s,
20s and 30s, respectively.
56
First of all, we tried to get the phase α, already described in the deposition techniques section.
Figure 5.1 shows the Raman spectra of the film that is deposited and annealed. Sharp peaks at
819 and 996 cm-1 corresponding to the dominant M-O stretching modes as well as other
vibrational modes are observed in the MoO3 spectrum.
Figure 3.1 : Typical Raman spectra of α-MoO3 acquired using our parameters: 50x magnification, %1 laser power, and 1000 accumulation.
Raman spectrometry was carried out with 50x magnification, %1 laser power and 1000
accumulation. The spectrum of α-MoO3 was obtained and the peaks were suited with the values
typically indicated in literature. Deposition techniques and annealed conditions are based on
previous studies of Nanostructuring and Nanomanufacturing Lab (N2E-Lab) of the Italian
Institute of Technology (IIT), an as-deposited sample was heat treated at 500 ° C in air for 30
minutes in a muffle furnace. In these conditions, characterizing the sample through Raman
spectroscopy it has been observed as the film had become uniformly constituted by the phase α
of the oxide of molybdenum. According to the Raman spectra of MoO3 seen Figure 3.1, it is
57
obviously clear that deposited and annealed films has totally phase α. Subsequently,
intercalation steps were carried out at different potentials and time variation.
In this present work, as deposited MoO3 films are subjected to intercalation process with an
electrochemical reduction in aqueous acidic media in order to prepare HxMoO3. The layered
structure of MoO3 allows the incorporation of hydrogens into lattice, forming the so called
molybdenum bronzes and alkali metal cations intercalated into the van der Waals gap,
accompanied by the partial reduction of Mo oxidation states. The implantation of H ions effects
the vibrational properties of MoO3 that might be determined by Raman spectra. The
molybdenum bronzes exhibit various structures and physical properties of interest depending
on the kind and/or the concentration of an incorporated element during the intercalation. In the
context of extensive studies of the HMB, they can be formed over different phases as follows,
HxMoO3; (0 < x ≤ 2): nonstoichiometric phase I (H0.23–0.40MoO3, blue, orthorhombic), phase II
(H0.85–1.04MoO3, blue, monoclinic), phase III (H1.55–1.72MoO3, red, monoclinic), phase IV, x=2
(green, monoclinic [16, 17]. The hydrogen molybdenum bronze is of particular interest in the
electron/proton mixed conductance, and has been investigated for possible applications in
hydrogen-transfer catalysts, electrochromic displays, fuel cells, hydrogen storage, and gas
sensors, as mentioned on previous chapters.
In order to identify the parameters of the Raman spectrometer respect to sample properties,
many pre-testing had done. Laser power was selected starting from the low values 0.1% and
0.5% to higher ones. These laser power rates provided a low signal/noise ratio compared to 1%
and %5 ones. Subsequently, it was decided not to increase the power of the laser to a value
greater than 5% of the maximum power to avoid damaging the samples and mostly %1 laser
power is used to get clear signals on the Raman spectra. All accumulations and magnification
were applied equivalently to all measurements (1000 accumulation, 50x magnification). Finally
it has been optimized observed optical magnification through focusing the laser beam on the
sample. The samples were irradiated with the 532 nm line of laser.
58
Figure 3.2:Optical microscope image of α-MoO3 obtained by deposition and annealing processes respectively.
As seen on Figure 3.1. , the principle peaks of the alpha MoO3 are 663, 819 and 996 cm-1
obtaining with Raman spectra. The optical image of the focused microscope is seen on figure
3.2.
200 400 600 800 1000 1200 1400
Ra
ma
n In
ten
sit
y/a
rb.u
nit
s
Raman Shift (cm-1)
0.1% FTO
Figure 3.3 : Optical microscope image of FTO glass (no baseline created)
59
FTO glass surface without ant deposited film layer was scanned by Raman spectrometer,
however any peaks was not determined, seen on Figure 3.3.
Figure 3.4 : Raman spectra of potential series exposed 20 seconds (-0.1V, 0.0V, 0.1V and 0.2V vs Ag/agCl) (50x magnification, %0.1 laser power, and 1000 accumulation)
60
Figure 3.5 : Raman spectra of time series at -0.1V vs Ag/AgCl respect to as deposited α-MoO3 (50x magnification, %1 laser power, and 1000 accumulation)
A sample spectra seen on Figure 3.4 is the HxMoO3 with low and high hydrogen content: The
sharpness of peaks in black curve represents the high crystalline state and excellent structural
order in α-MoO3 film. Other sequent curves give the Raman spectrum of intercalated samples
at -0.1V vs Ag/AgCl with different time intervals, and all peaks easily corresponding to those
in curve as deposited. However, some differences are present between the Raman spectrum of
as deposited and intercalated ones. First, the intensity is obviously high in time series curves.
Second, all peaks of intercalated samples shift toward low wavenumber, slightly and
unanimously. Third, most peaks in intercalated ones broaden with respect to as deposited peaks.
The peak shifts in the Raman spectrum are related to changes in the force constants of the bonds,
positive and negative shifts correspond to larger and smaller force constants, respectively.
Because the lattice is enlarged due to the intercalation, the force constants of bonds become
small and the peaks shift to lower wavenumbers.
61
On Figure 3.5, the curve of 30s at -0.1V exhibits the Raman spectrum of sample that has high
hydrogen content. The Raman peaks increase greatly in intensity, as compared to those of
sample untreated and those treated at low time intervals. Because of the large lattice expansion
and structural distortion, some peaks are not observed. In addition, the delocalized electrons
present in intercalated samples may play a role in the intensity reduction because of their
screening effect on phonons [17].
The samples were analyzed using the X-ray diffraction (XRD) technique in the 2θ
configuration.
The XRD analysis of the deposited-annealed and intercalated films show dominant peaks of
layered orthorhombic α-MoO3 and minor contributions of hydrated MoO3. The dominant peaks
observed at 23.3, 25.8, and 27.4˚ correspond to (110), (040), and (130) planes. The principle
peaks of the MoO3 are represented on figure 3.6 as a black line.
Figure 3.6 : XRD spectra of the MoO3 on -0,1 V vs Ag/AgCl at different times
62
On the basis of the above XRD analysis, the crystalline lattice of intercalated ones expands
slightly relative to sample of as deposited. Also small structural distortions likely happen on
those intercalated as well, as a result of hydrogen intercalation. Therefore, the structural order
is lowered in treated ones, which can lead to the decreased Raman vibrational intensity and
peak broadening. Generally, the Raman spectra are more sensitive than XRD detection to the
structure change.
Figure 3.7 : XRD spectra of the MoO3 on -0.1 V vs Ag/AgCl at different applied potentials
63
Figure 3.8. : Comparison of the XRD spectra between as deposited (untreated) MoO3 and intercalated at -0.1V vs Ag/AgCl for 20 seconds.
The results of the performed XRD coincide highly with the XRD results reported in the
literature.
The broad peak appearing at 971 cm–1, and the broadening of the 819 cm–1 and 663 cm–1
peaks, indicate the presence of hydrated MoO3, which is in agreement with the XRD
analysis [2]. The loss of sharpness and intensity in the dominant MoO3 peaks can be attributed
to further hydration of the active MoO3 surface during the baselining and immobilization
process [23].
This Raman spectra indicates the comparison of the different samples subjected to time interval
under the same potential value given by potentiostat in the electrochemical environment. Some
small peaks shifted but most of the characteristic peaks are remained similar. It is important to
point out that, intensity and broadening of the peaks shows different behavior due to the
intercalation of the H+ ions that are coming from the electrolyte.
64
A Perkin Elmer Lambda 1050 spectrophotometer seen on Fig 4.2.2 is used to record optical
properties in transmission and reflectance mode of the samples of series time and potential
exposed to intercalation processes. The results are discussed below relying on literature view.
300 400 500 600 700 800
0
20
40
60
80
100
MoO3 a.d.
HMoO3 +0,22V vs RHE
TT
(%
)
Wavelength (nm)
HMoO3 +0,12V vs RHE
HMoO3 +0,32V vs RHE
FTO
300 320 340 360 380 400
0
20
40
HMoO3 +0,22V vs RHE
TT
(%
)
Wavelength (nm)
HMoO3 +0,12V vs RHE
HMoO3 +0,32V vs RHE
FTO MoO3 a.d.
Figure 3.9. : The graph of transmittance spectra between 300 and 800 nm of four significant samples as the deposited and intercalated ones at 20 seconds as a potential series in the
electrochemical cell.
Compared to the red colored line, already reported, the spectrum of α-MoO3 obtained through
deposition and thermal treatment at 500 ° C, which Raman analysis results to have a uniform
layer of crystalline phase, show the lower transmittance value above 880 nm (NIR), and
between 500 and 740 nm (in visible range). This is in agreement with the crystalline nature of
the material, which hinders further the crossing of the light due to the crystalline planes which
give rise to reflection and diffraction in the lattice. On the other hand, intercalation at different
potential at a constant time interval have a tendency to behave differently. Intensity of the
transmittance is lowering by intercalation depending to applied potential.
65
Figure 3.10. : The graph of absorbance spectra between 400 and 2250 nm of four significant samples intercalated at +0.1V vs Ag/AgCl for 5, 10, 15 and 20 seconds in the electrochemical
cell.
Figure 3.11. : The graph of absorbance spectra between 400 and 2250 nm of six significant samples the as-deposited MoO3, FTO as substrate and those intercalated at +0.1V vs Ag/AgCl
for 5, 10, 15 and 20 seconds in the electrochemical cell.
As regards the optical properties of the different phases of the oxide of molybdenum after
intercalation, in Figure 3.10 is a graph of absorbance spectra between 400 and 2250 nm of four
significant samples, the as-deposited and those intercalated at +0.1V vs Ag/AgCl for 5, 10, 15
and 20 seconds in the electrochemical cell.
in Figure 3.11 is a graph of absorbance spectra between 400 and 2250 nm of six significant
samples, the as-deposited, FTO substrate and those intercalated at +0.1V vs Ag/AgCl for 5, 10,
66
15 and 20 seconds in the electrochemical cell. Comparison between intercalated ones and as
deposited is shown in Fig. 3.11, intercalated samples have more absorbance than the as
deposited one.
Figure 3.12. : The graph of absorbance spectra between 400 and 2250 nm of four significant samples intercalated at 20 seconds as a potential series in the electrochemical cell.
Figure 3.13. : The graph of absorbance spectra between 400 and 2250 nm of six significant samples the as-deposited MoO3, FTO as substrate and those intercalated at 20 seconds as a
potential series in the electrochemical cell.
On Figure 3.13 is seen that there is a markedly increased absorption in the NIR (wavelengths
greater than 1250 nm) upon intercalation. This behavior is consistent with previous works in
the literature. Upon H atom intercalation, the transmission spectra in molybdenum trioxides
indicate a coloration response significantly enhanced in the NIR region.
67
With the intercalation of ions and electrons in the crystalline oxides, the excess electrons render
the valance band partially filled, causing the intercalated oxide to exhibit metallic behavior [4].
It is possible that the crossover to metallic behavior occurs at a much higher wavelength in
MoO3 films by intercalation.
The literature review mentioned above states that position of the XPS peaks indicates the film
composed of Mo+6 and/or a molybdenum species of lower oxidation states presumably Mo+5 by
regarding the peaks widening. As inditaced on behalf of Hu X, et.al, lattice unit expand after H
atoms are introduced into MoO3 and slight structural adjustment occurs when the hydrogen
content is high.
According to our XPS results, some graphs will be shown down in case of deposited MoO3 and
intercalated ones in terms of potential series. Most revealed figures are selected as deposited
MoO3, intercalated MoO3 at +0.1V vs Ag/AgCl and at -0.1V Ag/AgCl during 20 seconds, it is
classified as a potential series in this work.
Figure 3.14. : XPS spectra of the core levels of MoO3
The characteristic doublet peaks are observed at binding energies of 232.32 and 235.44 eV from
the Mo-3d core-level spectra in literature view. These doublet core-level binding energies i.e.,
232.32 eV and 235.44 eV correspond to Mo-3d5/2 and Mo-3d3/2, respectively, which are due to
the spin–orbit splitting (Mo-3d) and are in good agreement with the values reported by the other
works on MoO3 films.
68
Figure 3.15. : XPS spectra of the core levels of MoO3 after intercalation processes at +0.1V vs Ag/AgCl for20s.
The intercalated sample within electrochemical cell during 20 seconds at +0.1 V vs Ag/AgCl
has both oxidation states of the Mo, named Mo+5 and Mo+6. Percentage of the presence of the
oxygen vacancies increased from 7 % to 26 % after the intercalation process on particular
parameters and it obviously can be said that probability of the location of H atoms from the
electrolyte to the film is increased. H atoms easily can be positioned in the atomic stoichiometry
of the MoO3 films, and it is observed by changing color to blue from transparent during the
intercalation.
Figure 3.16 displays the XPS results obtained on the surface of an intercalated MoO3 at -0.1 V
vs Ag/AgCl during 20 seconds. The Mo 3d peak in progression with the intercalation process,
displays a shift towards the left. The observed shift towards to the left implies a decrease in
binding energy and designates partial reduction. It is not seen sharp as much as previous ones.
Percentage of the presence of the oxygen vacancies increased from 26 % to 42 % after the
intercalation process by decreasing the applied potential level during the electrochemical
experiment to generate potential series at fixed time interval.
69
Figure 3.16. : XPS spectra of the core levels of MoOx after intercalation processes at -0.1V vs Ag/AgCl for 20s.
Figure 3.17. : XPS spectra of the core levels of MoO3 and intercalated MoO3 based on potential series that is described before.
70
Figure 3.17 shows a comparison between previous graphs. With lower potential oxidation states
of the Mo has occurred as Mo+5 with a decrease of the binding energy and a broadening of the
new peaks. XPS had performed in order to identify vacancy related effects of the intercalation
process. It is reasonable to support that, intercalation converts the atomic structure which is
observed by naked eyes in color and tone changing in different potential at fixed time interval.
Work function data were obtained by using a Kelvin Probe Microscope (Bruker, Dimension
Icon) in air and at room temperature. A reference sample of graphite (HOPG, highly ordered
pyrolytic graphite) was used for calibration, as its work function value (4.6 eV) is well
determined. FTO has the 4.8 eV value of the work function. Value of 5.0 eV of the as deposited
MoO3 is seen on the Figure 3.18 and Figure 3.19. On figure 3.18., values of 4.7, 4.85, 4.55,
4.62, and 4.8 eV were recorded for the samples intercalated in time of 20 seconds at 0.0V,
0.05V, 0.1V, 0.15V and 0.2V respectively.
Kelvin Probe Measurement were carried out the changes in Work function of the as deposited
fils after intercalation processes in order to verify the its potential for charge selectivity.
FTO Mo MoOx MoO3
4,5
4,6
4,7
4,8
4,9
5,0
5,1
Wo
rk F
un
ctio
n (
eV
)
0,00 0,05 0,10 0,15 0,20
4,5
4,6
4,7
4,8
4,9
5,0
5,1
V vs Ag/AgCl
Figure 3.18. : Work function of intercalated samples related to potential series performed under 20 seconds.
71
FTO Mo MoOx
4,5
4,6
4,7
4,8
4,9
5,0
5,1
Wo
rk F
un
ctio
n (
eV
)
0 5 10 15 20 25 30
4,5
4,6
4,7
4,8
4,9
5,0
5,1
Intercalation time (sec)
Figure 3.19. : Work function of intercalated samples related to time series at 0.1V vs Ag/AgCl
Values of 4.6, 4.55, 4.75, 4.55, and 4.62 eV were recorded for the samples intercalated at
0.1V vs Ag/agCl as a time series at 5, 10, 15, 20 and 30 seconds, respectively.
In both graphs of the kelvin probe measurements seen above, shows a tendency of a decrease
in work function on the film after intercalation processes in terms of time and potential series.
72
FTO Mo MoOx
4,60
4,65
4,70
4,75
4,80
4,85
4,90
4,95
5,00
5,05
Wo
rk fu
nctio
n (
eV
)
0 20 40 60 80 100
4,90
4,92
4,94
4,96
4,98
5,00
5,02
5,04
5,06
Thickness (nm)
Figure 3.20. : Work function diagram related to thickness of the film
As seen on the figure 3.20, the thickness of the films indicates different value of the work
function. It was measured that nearly 50nm thickness and more shows constant value for the
work function. From these graphs it is easily to say, intercalation has a great effect on the
property of film as charge selective by lowering the work function. This is an improvement for
the multi layered devices as hybrid devices to construct the band engineering inside the devices.
73
Ar/H2 Ar O2
4,5
4,6
4,7
4,8
4,9
5,0
Wo
rk fu
nctio
n (
eV
)
WF
Figure 3.21. : Work Function Diagram
Cyclic voltammetry experiments of the as deposited MoO3 was performed in a potential range
between -0.4 V and 0.7V vs SHE to avoid irreversible reduction of MoO3 as well as to prevent
solvent reduction/degradation. CV of the as deposited MoO3 was enabled to show a capacitive
trend. In the region of low potential, reduction of the cathodic peak is not seen, an oxidation
peak at high potential range has occurred. This type of response indicates characteristic of dense
MoO3 materials and it has importance to have an idea about the stability of the film. The
voltammetric response is rather capacitive and exhibits no defined reduction or oxidation peaks
due to the bulk disorder in the film.
74
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8
-0,00015
-0,00010
-0,00005
0,00000
0,00005
0,00010
Cu
rre
nt D
en
sity (
A/c
m2)
V vs SHE (V)
As deposited
Figure 3.22. : Cyclic Voltammograms of the as deposited MoO3 in three scan
The trend of the cyclic voltammetry of the sample intercalated shows several peaks
corresponding to the phenomena of oxidation and reduction at the electrode-electrolyte.
The applied potential range was enlarged until exploring the redox peaks. It can be seen that by
increasing the applied potential range, more oxidation and reduction peaks can be
observed in the curves as a voltammetric response over a broad potential range. This is more
significant in the case of reduction peaks.
The interval of depletion is consequently limited to a range of potential according to the
intercalation rates, and it can be stated, it is significantly lower than that of the sample not
intercalated.
On figure 3.22 the voltammetric response is rather capacitive and exhibits no defined reduction
or oxidation peaks due to the bulk disorder in the as deposited film.
75
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0C
urr
en
t D
en
sity (
mA
/cm
2)
V vs SHE (V)
0.0V 20s
0.1V 20s
0.2V 20s
-0.1V 20s
Figure 3.23. : Cyclic voltammograms of the intercalated samples as a potential series at 20 seconds
76
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8C
urr
en
t D
en
sity (
A/c
m2)
V vs SHE (V)
10s
20s
30s
Figure 3.24. : Cyclic voltammetry of the intercalated samples as a time series at -0.1V vs Ag/Ag+
The observed voltammetric response exhibits two very broad, ill-defined peaks, one centered
at -0.2 V and the other at 0.3 V within the time series voltagrammes. Increasing in scan of
cycling, the small peaks observed during the first insertion cycle appear and the voltammetric
response becomes even resolved. A slight decrease in insertion capacity is also observed, but
subsequent cycles are relatively constant after the 6 voltammetric cycle. In graphs seen on
figure 3.23 and 3.24, has only one cycle for each condition of the intercalation to indicate the
depletion zone where there is no redox peaks in order to guide for mott-shottky measurements.
77
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0C
urr
en
t D
en
sity (
A/c
m2)
V vs SHE (V)
5s
10s
15s
20s
30s
MoO3
Figure 3.25. : Cyclic voltammetry of the intercalated samples as a time series at -0.1V vs Ag/Ag+
Add Mott-Schottky here!
78
4. Conclusion
The objective of this work was to characterize crystalline films intercalated of molybdenum
oxide (MoO3) for the use of the same in photo-electrochemical devices for the production of
hydrogen. The requirements for a good performance of the films within the device are
crystallinity, good conductivity, homogeneity and stability.
The samples, deposited on substrates of FTO by the team N2E Lab IIT through the technique
of magnetron sputtering, were heat-treated in the muffle furnaces. The experiments for the
characterization performed on the intercalated films have allowed the identification of hydrogen
molybdenum bronzes and a crystalline alpha phase and, therefore, of the respective zones of
stability. In parallel, it was analyzed electrochemical behavior as a selective layer.
The films deposited and thermal treated to get crystalline phase has intercalated in the
electrochemical cell in acidic medium. There have been series of treatments alternately by
varying potential and time on samples of alpha phases of molybdenum oxide. It is investigated
the electrochromic response of molybdenum trioxide films prepared with deposition technique
annealing and intercalation It is established that degradation during the processes of the film
does not occur by manufacturing for electrochemical intercalation. It is observed a strong
coloration response in these films. Upon intercalation of ions, films show a broad coloration
response. The films is much more wavelength selective and mainly limited to the visible range.
Annealed films of molybdenum oxide do not show a strong increase in NIR reflection upon
intercalation. The strong electrochromic response of the films suggests the suitability of these
films for device fabrication, in which a strong and efficient coloration response is desired. By
increasing the H insertion by changing the potential and time on samples, has definitely effects
on the optical, structural and electrochemical properties. In addition, the electrochemical tests
have shown the validity of the approach used, confirming the good electrical conductivity of
the oxides intercalated in the liquid phase.
The results obtained in this work suggest many future opportunities for experimentation, which
predict excellent prospects for improvement of the devices studied.
The next step will be the realization of devices using films optimized in this study to assess the
actual performance. Performance improvements in terms of stability, combined with the
79
optimization and energy savings, could open up new avenues for the realization of devices for
photocatalytic hydrogen production as a selective layer.
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6. Appendix
6.1. Pulsed DC Magnetron Sputtering Pulsed DC Magnetron sputtering deposition (MS) is a deposition technique for thin and
nanostructured film growth which has started to blossom in the past few years especially it is
used for Mo and Pt deposition. Sputtering processes are employed for different purposes, but
they are widely used for film depositions on substrate.
It consists of a target holder and a substrate holder housed in a vacuum chamber. Film growth
can be carried out in a reactive environment containing any kind of gas with or without plasma
excitation.
Figure 6.1.1 : A schematic diagram of an experimental setup.
Magnetron Sputtering is a process based on the emissions of ions, neural atoms or cluster from
a solid surface, called target. The mechanism of the sputtering depends on the hitting of the
incident ion to the first atom at the surface of the target. As a consequence of the bombarding
of an incident beam, the collision has occurred. The required energy of ions to leave the surface
of atoms should be higher than binding energy. Otherwise the particles will be remain on the
surface of the target due to the recoil energy. The interaction between the incident beam and
the atoms of the target turns into kinetic energy and momentum that are distributed among the
ejected atoms near to the surface of the target. The collision cascade is seen figure 3.2.2.
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Figure 6.1.2. : Example of collision cascade promoted by an incident beam.
Briefly it can be described that bombarding the target with ions consequently let the sputtered
particles travel and deposit on the substrate surface by atomic diffusion. The substrate to be
deposited takes place inside the vacuum chamber and acts as an anode. The target behaves as a
cathode. The potential difference between the two allows the plasma generation, which is
created by ionizing the sputtering gas (typically Ar). In this way, the ions bombard the target
and the eroded particles travel to the substrate. In order to obtain films with certain
properties, some parameters should be controlled during the deposition process such as,
sputtering gas, gas pressure, target-substrate distance, and power. Determining the sputtering
gas relies on the type of deposition techniques that is to be reactive sputtering or non-reactive
sputtering. Non-reactive sputtering do not allow any chemical reaction on the eroded particles
due to the inertness of the used gas such as Ar. Gas pressure is a critical parameter in the
chamber because it governs the mean free path of the traveling particles before collision and,
consequently, the mean energy of the sputtered atoms that impact the substrate to coat.
With the low pressure, the free path will be high and the particles conserve high energy that
they need. Moreover, if the pressure is too high, collisions occurs too often and the electrons
can’t gather enough energy in order to be able to ionize the gas. The mean free path λ can be
estimated from kinetic theory of molecular collisions:
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Theoretically target-substrate distance is adjustable maintaining the same pressure. It can cause
same effect on the mean free path of the travel before deposited on the substrate surface. Since
a higher distance at constant pressure increases the probability that a collision occurs and
vice versa. The power is provided by the generator and it affects the density of the plasma. If a
high power is set, high kinetic energy is expected. Direct current sputtering is the one of the
thin film deposition techniques of sputtering. In DC sputtering, the plasma is enabled by a
direct current input and a relatively high voltage, ranging from 0.3 kV to 5 kV. When the
plasma is generated, the Ar+ ions are accelerated by the potential gradient to the cathode, which
is the target, and the erosion occurs (De Paola, 2013). Finally, the eroded particles are deposited
on substrate surface by atomic diffusion and the film starts growing. Because of its own nature,
this technique is only available whenever the target is a conductor, otherwise the direct current
won’t flow and the plasma can’t turn on.
6.2 Mott-Schottky Analysis
In particular, the electrochemical impedance measurements (EIS) allow to obtain the main
quantitative parameters such as specific capacity and resistance to charge transport.
Experimentally, a measure of EIS provides for the application of a disturbance of the sinusoidal
type of small amplitude to a stationary value of the potential. The system response will be a
current signal suitably damped sinusoidal having out of phase depending on the linear
frequency response of the system.
The ratio between the two sinusoids of current and voltage will give an impedance value, which
will be evaluated in a wide range of frequencies so as to obtain a spectrum of impedances,
indicative of the various physical phenomena that occur in the system.
𝒁(𝜔) =𝑽(𝜔)
𝑰(𝜔)
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The analysis of the spectra of the EIS is carried out by the construction of a circuit model
equivalent to the system under analysis, to interpret the equivalent resistance and capacitance
values in terms of interfacial phenomena.
It leverages the relationship between potential and current date by the carrier impedance:
𝒁(𝜔) = 𝑍𝑅𝑒 − 𝑗𝑍𝐼𝑚
where SI (ω) is the impedance function of the frequency ω, and ZRe ZIm respectively its real and
imaginary parts. From this one can derive the magnitude and phase of the impedance:
|𝒁|𝟐 = 𝑹𝟐 + 𝑿𝑪𝟐 = (𝒁𝑹𝒆)
𝟐 + (𝒁𝑰𝒎)𝟐
𝒕𝒂𝒏𝚽 =𝒁𝑰𝒎𝒁𝑹𝒆
=𝑿𝑪𝑹=
𝟏
𝝎𝑹𝑪
where XC is the capacitive reactance, and the resistance and C the capacity.
The variation of the impedance with the frequency can be represented by two types of graphs:
o In a Nyquist type graph is displayed ZIm vs ZRe for different values of ω
o In a chart type Bode, log | X | and Ф are displayed both vs logω
a b
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Figure 6.2.1 : Graphs of Nyquist (a) and Bode (b) as an example
In general, an electrochemical cell can be considered as an impedance, when subjected to a
small sinusoidal excitation; it can therefore be represented as an equivalent circuit of resistors
and capacitors in which circulates a current with the same amplitude and the same phase of the
current that circulates in the real cell. The most used model is that of Randles, which represents
the resistance to the flow of the current imposed by the electrolyte with a resistance R (Ω) and
the film itself with a capacitor (capacitance Cd) in parallel with an impedance Zf.
The main elements for the modeling of the system to simplify the properties of the
electrochemical system based on electrode / electrolyte, consisting of a resistance Rs (series
resistance) in series to a capacitor Cs or of a resistance Rct (resistance to charge transfer) which
may be connected in series to a second impedance Zw (Warburg impedance) or to a CPE
(Constant Phase Element).
This circuit element used in the model is not ideal as behavior of capacitors. In general, the
considered model allows to distinguish between the phenomena of charge transfer (Rct) and
diffusive phenomena and mass transport (Zw or CPE) [31].
Figure 6.2.2 : (a) Equivalent circuit of an electrochemical cell (b) Subdivision of Zf in Rs and Cs, or Rct and Zw
The circuit elements described may be combined suitably to model the transport phenomena at
the base of complex systems.
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The graph Nyquist provides additional information through a study of the yield curve at low
and high frequencies. The first intersection of the spectrum impedance with the real axis at high
frequencies (the left side of the graph) corresponds to the resistance offered by the electrolyte
while, when a first semicircle evident in the spectrum, its intercept with the abscissa axis takes
into account of the internal resistance (ohmic resistance) of the system. In this term includes
both the contact resistances between the various elements of the cell is the electrolyte resistance.
The portion of the curve at the low frequencies (right side of graph) is instead indicative of
diffusive phenomena electrode.
Qualitative information on the properties of a semiconductive or metallic material may be
obtained through the analysis of Mott-Schottky. Technique is based the following relationship
Mott-Schottky:
𝑪𝒔𝒄−𝟐 =
𝟐
𝒆𝜺𝜺𝟎𝑵(𝑬 − 𝑬𝑭𝑩 −
𝒌𝑻
𝒆)
Where CSC = capacity, ε = dielectric constant of the semiconductor, ε0 = dielectric constant in
vacuum, N = density of carriers, k = Boltzmann's constant, T = temperature, E = applied
potential and EFB = flat band potential [31].
The latter is a key variable in the study of interfacial behavior between semiconductor and
electrolyte and it can be graphically determined by the value of the intercept of the straight line
representing the linear regression of the curve with the x-axis. By varying the potential of the
semiconductor artificially through the use of a potentiostat there is a separation of the Fermi
level of the semiconductor and the redox couple of the electrolyte, and then a phenomenon of
band bending due to the process of charge depletion on the semiconductor. When the potential
applied is such that you do not experience the phenomenon of band bending and therefore
charge depletion, the semiconductor is its potential for flat band [32].
The slope of the regression line to the nonlinear part in a diagram of the Mott Schottky (Csc-2
vs. E) instead provides information about the type of density of charge carriers of a
semiconductor: in particular an angular coefficient in positive features a p-type semiconductor
while a negative slope, and the opposite characteristics to the n-type semiconductors.
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Particular attention should be paid to the quantitative determination of the parameters involved
in the equation of Mott-Schottky because it describes a model referring to single crystals. For
this reason, the analysis of the Mott-Schottky is used more often for the qualitative comparison
of the electronic behavior of samples morphologically similar.
Figure 6.2.3: graph example Mott-Schottky for powders of zinc oxide synthesized.
Taken from: http://pubs.rsc.org/en/content/articlehtml/2014/cp/c3cp55136a.
