Accepted Manuscript Title: A Novel Field Ionization Gas Sensor based on Self-Organized CuO Nanowire Arrays Author: Raheleh Mohammadpour Hassan Ahmadvand Azam Iraji zad PII: S0924-4247(14)00221-0 DOI: http://dx.doi.org/doi:10.1016/j.sna.2014.04.038 Reference: SNA 8779 To appear in: Sensors and Actuators A Received date: 23-1-2014 Revised date: 19-4-2014 Accepted date: 26-4-2014 Please cite this article as: R. Mohammadpour, H. Ahmadvand, A.I. zad, A Novel Field Ionization Gas Sensor based on Self-Organized CuO Nanowire Arrays, Sensors and Actuators: A Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.04.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Transcript
Accepted Manuscript
Title: A Novel Field Ionization Gas Sensor based onSelf-Organized CuO Nanowire Arrays
PII: S0924-4247(14)00221-0DOI: http://dx.doi.org/doi:10.1016/j.sna.2014.04.038Reference: SNA 8779
To appear in: Sensors and Actuators A
Received date: 23-1-2014Revised date: 19-4-2014Accepted date: 26-4-2014
Please cite this article as: R. Mohammadpour, H. Ahmadvand, A.I. zad, A Novel FieldIonization Gas Sensor based on Self-Organized CuO Nanowire Arrays, Sensors andActuators: A Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.04.038
This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.
. aInstitute for Nanoscience and Nanotechnology, Sharif University of Technology, Azadi Street, P.O. Box 14588-89694, Tehran, Iran.
bDepartment of Physics, Sharif University of Technology, Azadi Street, Room 436, P.O. Box 11365-9161, Tehran, Iran
* Corresponding author. Department of Physics, Sharif University of Technology, Azadi Street, Room 436, P.O. Box 11365-9161, Tehran 14588-89694, Iran. Tel.: +98 21 66164513; fax: +98 21 66005410. E-mail addresses: [email protected] (A. Iraji zad)
Abstract
In this study, we present fabrication and characterization of a gas ionization sensor based on high aspect
ratio one-dimensional CuO nanowires as the field enhancing medium. Self-organized arrays of CuO
nanowires have been synthesized based on a low-cost thermal oxidation method and integrated into a Gas
Ionization Sensor (GIS). The self-organized arrays of CuO nanowires have been employed to detect the
identity of several gas species such as He, Ar and CO at ambient temperature and pressure. The sharp
nanoscale size of CuO tips provide very high electric fields at moderate voltages (less than 100V) and
provoke the breakdown of different gases. The reduced breakdown current of the metal oxide CuO
nanowire electrodes result in reduced structural deformation. The proposed GIS based on CuO nano-tips
exhibits good repeatability and selectivity for various types of gases and their mixtures.
Keywords: CuO nanowire; Thermal oxidation; Gas ionization sensor
1. Introduction
Gas Ionization sensors (GISs) had been introduced to overcome the typical problems of conventional
chemical gas sensors [1]. Chemical gas sensors usually measure the electrical response of an active
electrode upon adsorption of gas molecules on their surface. Since adsorption of different gases may
induce similar electric response, chemical sensors typically suffer from selectivity issues [2-4]. On the
other hand, chemical gas sensors have potential difficulties in detecting gases with low adsorption
energies such as inert gases. High working temperature of many types of chemical gas sensors is the other
important challenge that should be considered [5]. As a physical type of gas sensor, GISs can fingerprint
Page 2 of 27
Accep
ted
Man
uscr
ipt
the ionization characteristics of distinct gases at room temperature [1]. Since at constant temperature,
every gas displays unique breakdown characteristics, we can expect superior selectivity and low working
temperature. The main drawback of using GIGs, employing bulk electrodes, is the necessity of working at
high voltages [1]. Recently, a novel type of GIS based on one-dimensional nanostructures has been
introduced [1, 6-11]. The nanoscale curvature of one-dimensional nano-tips can generate very high
electric field at relatively low voltages, which can provoke the breakdown of various gases. The electrode
of the first reported miniaturized GIS consists of multi-wall carbon nanotubes [1]. However, carbon
nanotubes are oxidized and degraded easily through high density electric current at breakdown voltages.
In this respect, some other kinds of one-dimensional nanostructures such as gold, ZnO and TiO2
nanowires and nanotubes were employed in GISs to enhance the stability of the sensor [6-11]. Since the
breakdown voltage and current depends on specific parameters such as the electrode’s material and
morphology, gas parameter and inter-electrode spacing, we could expect that by employing novel
materials and morphologies, gas ionization characteristics of the sensors could be enhanced specifically.
In fact, employing novel materials such as CNT arrays results in reduced breakdown voltage, however it
can be easily oxidized and degraded under ambient atmosphere and high electric breakdown current [12].
On the other hand, employing metal oxide nanostructure such as ZnO and TiO2 may result in more
chemical stability; however they typically work in higher range of voltage [3,4].
In the present work, we have utilized arrays of CuO nanowires as the anode of GIS for the first time.
Copper oxide is a p-type semiconductor with narrow band-gap of 1.2 eV that has been employed in
different applications such as high-Tc superconductors, lithium–copper oxide electrochemical cells,
chemical gas sensors, photo-thermal and photoconductive materials [13-18]. Different morphologies of
CuO can be synthesized through various fabrication methods [19-21]. Among different complex chemical
and electrochemical synthesis methods, recently a new simple and fast method based on thermal
oxidation of copper foil has been developed to fabricate the CuO nanowires [22-25]. Thermal annealing
of copper foil provides an appropriate method for synthesizing self-assembled arrays of CuO nanowires.
Very high electric field at low voltages can be achieved at sharp nano-scale curvature of CuO nanowire
tips, synthesized through thermal oxidation. Besides, the metal oxide nature of CuO controls the
breakdown currents thorough them which resulted in reduced structural deformation. The gas ionization
characteristics of CuO nanowires based-electrode for different types of gases and their mixtures were
investigated by measuring current-voltage characterization. The proposed CuO-nanowire based GISs
exhibited superior sensitivity, selectivity and reliability, and could be used in a variety of applications like
environmental monitoring.
Page 3 of 27
Accep
ted
Man
uscr
ipt
2. Experiments
Copper substrates with a thickness of 100 μm were chemically polished in aqueous solution of 0.5% vol
H2SO4 and 25% vol H2O2 for 5 min in 30 oC. The substrates were subsequently cleaned in 5% vol H2SO4,
aceton and ethanol, followed by repeated rinsing with distilled water and dried under a N2 gas flow. The
cleaned copper substrate was placed on an alumina holder and immediately put on electric oven. The
samples were annealed to 400 oC with heating rate of 10 oCmin-1. The surface morphology of the oxidized
samples was examined employing scanning electron microscopy (Philips-XL30). The reflection spectra of
films were recorded using a PerkinElmer UV-Vis-NIR model Lambda 950 double beam
spectrophotometer in wavelength range of 400 nm to 1300 nm. For fabrication of GISs, the electrode was
assembled with Ti foil worked as the anode and CuO nanowire electrode acted as the cathode.
Thermoplastic frames with different thicknesses of 100 μm and 200 μm were kept between the anode and
the tips of CuO nanowires. We biased the CuO nanowire as the negative electrode to enhance the current
and improvement of the sensitivity of the sensor. Fig. 1 shows the schematics of the CuO-nanowire based
GISs employing spacers between electrodes. Since the electrodes composed of CuO nanowire can be very
fragile upon bending or crashing, this design can be very advantageous to typical GIS designs [7], since in
this compact set of nanostructured cathod-spacer-anode design there is a no possibility for bending the
CuO nanowre based electrodes and they can work stable. Fabricated GIS was placed in a small stainless
steel chamber with several electrical feed-through, gas inlet and gas outlet. A mechanical gauge was used
for pressure monitoring and the flow of gas mixtures were controlled by a mass flows controller (MFC)
from Brooks Instrument B.V. The current-voltage characteristics of the sensor were measured using a
Keithly 2400 source/meter. For gas sensing measurement, the sample was loaded into the chamber and
the breakdown voltage was measured in target gas environment under various gas pressure.
3. Results and discussion
Scanning Electron Microscopy (SEM) observations indicated the formation of oxide nanowires over the
entire surface of the Cu substrate. Fig. 2a illustrates images of CuO nanowires after the foil had been
heated in air for 4 hours. It can be seen that the surface is covered by dense CuO nanowires, with diameter
ranging from 50 nm to 100 nm and length up to several micrometers. Fig. 2b is a cross-sectional image of
an oxidized Cu sample, from which a two layered structure of Cu2O bottom layer and top oxide nanowire
layer is evident. It seems that CuO nanowires are formed randomly on the substrate. By applying bending
stress on the initial Cu substrate, the oxide nanowires were formed more aligned, perpendicular to
substrate. Fig. 2c shows the top view and the cross section of CuO nanowires. It is obvious that the tensile
stresses can significantly increase the nanowire growth density and enhanced their alignment. It is
Page 4 of 27
Accep
ted
Man
uscr
ipt
reported that the improved nanowire growth under bending stresses is associated with size reduction of
the oxide grains and an increase in the number of grain boundaries in underlying Cu2O and CuO layers
[26]. Increasing the annealing temperature up to 600 oC caused low density of short nanowires (Fig. 2d).
Fig. 3 shows the XRD spectra of the species after oxidation of the Cu substrate for 4 h at 400 oC and 600 oC. Both Cu2O and CuO phases are formed after oxidation processes, however the dominant Cu phase
suggests that the oxide thickness layer is very thin. With increasing the annealing temperature to 600 oC,
concentration of CuO phase increases in regards to Cu and Cu2O materials.
Formation of the oxide layer has been traced during annealing. In terms of optical characteristics, the
surface of the substrate turned red after oxidation at 200 oC and it gradually turned black upon formation
of CuO nanowires . The under-layer of the CuO nanowire layer remains red. Fig. 4a shows the reflectance
spectra as well as the Scanning Tunneling Spectrum (STS) of the formed layer on Cu after oxidation at
200 oC. The optical absorption edge of around 500 nm and the relative gap energy of 2 eV from STS
diagram indicated formation of the Cu2O layer, since the Cu2O is a p-type material with bulk band gap of
2.1 eV. Fig. 4b illustrates the reflectance spectrum as well as STS curves of the substrate annealed at 400 oC. Existence of absorption edge in 800 nm in the reflection spectrum with relative 1.5 eV band gap
energy in the STS curves indicated the formation of CuO material with bulk band gap of around 1.5 eV.
To examine the ohmic contact of the CuO-Cu2O-Cu layer we have made an upper Cu contact on the CuO
nanowire and measure the current-voltage characteristics. Fig. 5 illustrates the schematic of CuO/Cu2O
interface and its ohmic current-voltage characteristics of this multi-layer oxide layer employing Cu
contacts.
GIS sensors based on CuO nanowire electrode were placed in the vacuum chamber, where gas was
introduced. Each gas ionization test was repeated for three samples. The variance in current-voltage
characteristics for different samples was negligible. The GIS was first tested in air under the condition of
ambient temperature of ~25 oC and relative humidity around 40%. The inter-electrode distance of 100 μm
and 200 μm was kept constant utilizing thermoplastic spacer. Breakdown voltages of 355 V and 88 V
have been achieved by utilizing 200 μm and 100 μm spacers respectively. Fig. 6a shows the current-
voltage curves of several gases including He, Ar, dry air and CO. The anode-cathode distance was
maintained at 200 μm. The minimum breakdown voltage at pressure 1 atmosphere was 237 V for He,
followed by Ar (260 V), air+40% humidity (355 V), dry air (360 V) and CO (380 V). So, each gas
exhibits a distinct breakdown behavior that can show the selective behavior of this gas sensor. Increasing
the pressure in gas chamber to 2 atmospheres did not affect the breakdown voltage, however it caused the
increase in breakdown current. This can be related to the increased number of gas molecules that may
Page 5 of 27
Accep
ted
Man
uscr
ipt
contribute to the conduction. Reducing the anode-cathode separation distance to 100 μm caused
breakdown voltage reduction significantly. Fig. 6b illustrates the current-voltage curves for different
gases at electrode separation of 100 μm. The breakdown voltage of He reaches the value of 65 V followed
by Ar (80 V), air+40% humidity (80 V), air (88 V), dry air (90 V) and CO (135 V). These values of
breakdown voltages are much smaller in comparison to previously reported GISs based on ZnO
nanowires and TiO2 nanotube arrays or even carbon nanotube arrays[ 1,3,4, 6-11]. The nanoscale tip of
CuO nanowire could enhance the breakdown of gas molecules in very small values of voltages. The
breakdown voltage of different gases with different pressure and electrode distances are summarized in
column diagram in Fig. 6c. The distinctive breakdown behavior of these gases indicates the selective
feature of GIS based on CuO nanowires.
To evaluate the selective behavior of various gases we have done pre-breakdown measurements in static
mode. The vacuum chamber was filled with dry air, and the applied voltage maintained at 220 V, near to
pre-breakdown voltages of Ar and He gas. The current was recorded while introducing each gas with a
relative flow of 300 mLit min-1. Fig. 7a illustrates the static current of the GIS. It is obvious that the pre-
breakdown current changes once the target gas is introduced. The measured current of the device
indicates that CuO nanowire based-GISs have short response and recovery times because the
adsorption/desorption process of gas molecules on the CuO nanowires is fast. The current-voltage curves
of the GIS in air atmosphere under flow of Ar and He gases are shown in Figs. 7b-7c respectively. It is
interesting that in the gas mixture of He and Ar with air, two breakdown behaviors appeared. At lower
voltages either Ar or He breaks down, increasing the voltage further causing another breakdown
phenomena relative to air breakdown. This indicates that the GISs based on CuO nanowires can show
various gases distinctively in ambient environment.
Fig. 7d shows the stability experiment of GIS based on CuO nanowire electrodes. The breakdown
phenomena were generated continuously 10 times in air and He ambient. The fluctuation of breakdown
voltages were less than 7% of the initial values for both conditions. Considering the results of current
response under introducing gases in Fig. 7a and also the cyclic test of the GIS in Fig. 7d we can conclude
that sensors based on CuO nanowires have initial requirements for stable working condition.
4. Conclusion
Nanoscale tips of CuO nanowires were successfully fabricated by a low cost thermal oxidation method.
The self-organized wires were integrated into a gas ionization sensor. The characteristic of this novel gas
ionization sensor indicates that self-organized CuO nanowires grown on Cu foils are good a candidate for
utilizing as electrodes of GIS. Low breakdown voltage less that 100 V may achieve, which is lowest
Page 6 of 27
Accep
ted
Man
uscr
ipt
among the previous reported GIS based on metal oxide nanostructures. The metal oxide nature of this
structure caused controlled breakdown current of GIS which results in good stability. These sensors
showed good sensitivity, selectivity and reliability, and could be a promising active electrode in sensing
applications.
Page 7 of 27
Accep
ted
Man
uscr
ipt
References
[1] A. Modi, N. Koratkar, E. Lass, B. Wei, P.M. Ajayan, Miniaturized gas ionization sensors using
carbon nanotubes, Nature, 424 (2003) 171-174.
[2] S. Lakkis, Y. Rafic, A. Yasser Alayli, S. Mohamad, Review of recent trends in gas sensing
technologies and their miniaturization potential, Sens. Rev. 34 (2014) 24-35.
[3] L. Liao, H.B. Lu, M.S. huai, J.C.Li, Y.L.Liu, C. Liu, Z.X. Shen, T Yu, A novel gas sensor based
on field ionization from ZnO nanowires: moderate working voltage and high stability,
Nanotechnology 19 (2008) 175501-175505.
[4] S. Spitsina, M. Kahrizi, ZnO crystalline nanowires array for application in gas ionization sensor,
In IECON 2012-38th Annual Conference on IEEE Industrial Electronics Society (2012) 3997-
4002.
[5] S. Wang, K. Yanfei, W. Liwei, Z. Hongxin, W. Yanshuan, W. Yao, Organic/inorganic hybrid
sensors: A review, Sens. Actuators B 182 (2013) 467–481.
[6] C.T. Chang, C.Y. Huang, Y.R. Li, H.C. Cheng, Improving the stability and reproducibility of the
carbon nanotube gas ionization sensor by Co–Ti/Ti co-deposited catalyst layer, Sens. Actuators A
203 (2013) 137-141.
[7] A. Nikfarjam, A. Iraji zad, F. Razi, S.Z. Mortazavi, Fabrication of gas ionization sensor using
carbon nanotube arrays grown on porous silicon substrate, Sens. Actuators A 162 (2010) 24–28.
[8] A. Nikfarjam, R. Mohammadpour, A. Iraji zad, Fabrication of gas ionization sensor based on
titanium oxide nanotube arrays, Appl. Phys. A (2014) to be published.
[9] B, Gesemann, R, Wehrspohn, A, Hackner, G. M¨uller, Large-Scale Fabrication of Ordered
Silicon Nanotip Arrays Used for Gas Ionization in Ion Mobility Spectrometers, IEEE Trans.
Nanotech. 10 (2011) 50-52.
[10] M. Baghgar, Y. Abdi, E. Arzi, Fabrication of low-pressure field ionization gas sensor using bent
carbon nanotubes, J. Phys. D: Appl. Phys. 42 (2009) 135502-135506.
[11] G. Hui, L. Wu, M. Pan, Y. Chen, T. Li, Z. Zhang, a novel gas-ionization sensor based on aligned
[12] M.S. Wang, L.M. Peng, J.Y. Wang, and Q. Chen, Electron field emissioncharacteristics and field
evaporation of a single carbon nanotube, J. Phys. Chem. B 109 (2005) 110-113.
[13] J. Morales, L. Sa´ nchez, F. Martı´n, J.R. Ramos-Barrado, M. Sa´ nchezUse of low-temperature
nanostructured CuO thin films deposited by spray-pyrolysis in lithium cells, Thin Solid Films 474
(2005) 133-140.
[14] J. Morales, L. Sa´ nchez, F. Martı´n, J.R. Ramos-Barrado, M. Sa´ nchez, Nanostructured CuO
thin film electrodes prepared by spray pyrolysis: a simple method for enhancing the
electrochemical performance of CuO in lithium cells, Electrochim. Acta 49 (2004) 4589-4597.
[15] A. Cruccolini, R. Narducci, R. Palombari, Gas adsorption effects on surface conductivity of
nonstoichiometric CuO, Sens. Actuators B 98 (2004) 227-232.
[16] V.R. Katti, A.K. Debnath, K.P. Muthe, M. Kaur, A.K. Dua, S.C. Gadkari, S.K. Gupta, V.C.
Sahni, Mechanism of drifts in H2S sensing properties of SnO2:CuO composite thin film sensors
prepared by thermal evaporation, Sens. Actuators B 96 (2003) 245-252.
[17] A. Santos, P. Yustos, A. Quintanilla, G. Ruiz, F. Garcia-Ochoa, Study of the copper leaching in
the wet oxidation of phenol with CuO-based catalysts: Causes and effects Appl. Catal. B 61
(2005) 323-333.
[18] G. Larsen, S. Noriega, Dendrimer-mediated formation of Cu–CuOx nanoparticles on silica and
their physical and catalytic characterization, Appl. Catal. A 278 (2004) 73-81.
[19] L.X. Yang, B. Liu, G.H. Li, Morphology-Controlled Fabrication of CuO Architectures, Adv.
Mater. Res. 581 (2012) 544-547.
[20] C. Deng, H. Hanmei, G. Xinqing, H. Chengliang, Z. Difang Zhao, S. Guoqua, One-pot
sonochemical fabrication of hierarchical hollow CuO submicrospheres, Ultrason. sonochem. 18
(2011) 932-937.
[21] J. Wang, L. Yongchang, W. Shuyu, G. Xiaoting, L. Yuping, Facile fabrication of pompon-like
hierarchical CuO hollow microspheres for high-performance lithium-ion batteries, J. Mater.
Chem. A 2 (2014) 1224-1229.
[22] X. Jiang, T. Herricks, Yo. Xia, CuO Nanowires Can Be Synthesized by Heating Copper
Substrates in Air, Nano Lett. 2 (2002) 1333-1338.
Page 9 of 27
Accep
ted
Man
uscr
ipt
[23] L. Yuan, Y. Wang, R. Mema, G. Zhou, Driving force and growth mechanism for spontaneous
oxide nanowire formation during the thermal oxidation of metals, Acta Materialia 59 (2011)
2491–2500.
[24] M. Kaur, K.P. Muthe, S.K. Despande, S. Choudhury, J.B. Singh, N. Verma, S.K. Gupta, J.V.
Yakhmi, Growth and branching of CuO nanowires by thermal oxidation of copper, J.Crystal
Growth 289 (2006) 670–675.
[25] C.H. Xu, C.H. Woo, S.Q. Shi, Formation of CuO nanowires on Cu foil, Chem. Phys. Lett. 399
(2004) 62–66.
[26] R. Mema, L. Yuan, Q. Du, Y. Wang, G. Zhou, Effect of surface stresses on CuO nanowire
growth in the thermal oxidation of copper, Chem. Phys. Lett. 512 (2011) 87–91.
Biographies
Raheleh Mohammadpour received her PhD degree in nanophysics from Sharif University of
Technology University in 2010. She is currently assistant professor in Institute for Nanoscience and
Nanotechnology, Sharif University of Technology. Her main interest is fabrication of novel
nanomaterials, sensors and nanostructured solar cells.
Hassan Ahmadvand received his MSc degree in Physics Department at Sharif University of Technology
2012. He is currently PhD student in Institute for Nanoscience and Nanotechnology, Sharif University of
Technology. His main interest is sensors and nanotechnology.
Azam Iraji zad received her PhD degree in surface physics from Sussex University in 1990. She is
professor in Physics Department at Sharif University of Technology. Currently, She is the chair of
Institute for Nanoscience and Nanotechnology in Sharif University of Technology. Her main interest is
experimental surface physics, thin films and nanotechnology.
Page 10 of 27
Accep
ted
Man
uscr
ipt
Figure captions:
Fig. 1. Schematic of GIS employing CuO nanowire based electrode as the anode and a Ti foil as cathode. The anode-cathode distance was kept constant utilizing thermoplastic spacer.
Fig. 2. SEM images of CuO nanowires prepared by thermal oxidation at 400oC for 4h, (a) on unbent Cu substrate, (b) cross sectional view (a), (c) on bent Cu substrate. Inset: cross sectional SEM image of CuO nanowire on bent Cu foil, and (d) CuO structure on unbent Cu substrate oxidized at 600oC for 4h.
Fig. 3. X-Ray diffraction pattern of CuO nanowire/Cu2O layer formed on Cu foil by thermal oxidation method at (a) 400oC and (b) 600oC.
Fig. 4. Reflectance spectrum and scanning tunneling current of (a) Cu2O layer, (b) CuO nanowire film formed on Cu foil.
Fig. 5. Current-Voltage characteristic of CuO nanowire/Cu2O interface employing Cu contacts.
Fig. 6. The dependence of current discharge at different applied voltage exposed to different inert gases, at room temperature (25◦C) (a) at 100 µm, (b) 200 µm inter-electrode separation. (c) Extracted values of current discharge and breakdown voltage for different gas species.
Fig. 7. (a) Current response of GIS to introducing Ar and He gas on CuO nanowire electrode. Current-breakdown voltages of (b) Ar-air mixture, (c) He-air mixture and (d) Fluctuation of Breakdown voltage for device in air and He.
Page 11 of 27
Accep
ted
Man
uscr
ipt
Highlights:
A novel field ionization gas sensor has been fabricated for the first time based on self‐organized arrays of CuO nanowires.
Nanoscale size of CuO tips make it possible to have gas ionization sensor works at less than 100 V.
The metal oxide nature of CuO nanowires caused controlled breakdown current of gas ionization sensor which results in good stability.
