Correlation between electronic structure and photocatalyticproperties of non-metal doped TiO2/ZrO2 thin films obtained bypulsed laser deposition method
O. Linnik a, N. Shestopal a, N. Smirnova a, A. Eremenko a, O. Korduban b, V. Kandyba a,T. Kryshchuk a, G. Socol c, N. Stefan c, G. Popescu-Pelin c, C. Ristoscu c, I.N. Mihailescu c, *
a Chuiko Institute of Surface Chemistry, National Academy of Science of Ukraine, 17 General Naumov Str., Kyiv, 03164, Ukraineb Kurdyumov Institute of Metallophysics, National Academy of Sciences of Ukraine, Acad. Vernadsky blvd. 36, Kyiv, 03680, Ukrainec National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Str., Magurele, PO Box MG-36, Ilfov, RO-77125 Romania
a r t i c l e i n f o
Article history:Received 15 August 2014Received in revised form12 December 2014Accepted 12 December 2014Available online 26 December 2014
Keywords:TiO2/ZrO2 thin filmsPhotocatalytical activityPLD doping and composite films synthesis
TiO2/ZrO2 thin films doped with N, C and/or O were synthetized by pulsed laser deposition in low-pressure, chemically active gaseous mixtures. Photocatalytical activity of the obtained structures wasstudied against reduction of toxic bichromate ions. A direct correlation was observed between thepresence of ZrO2, the type and level of doping with N, C and/or O, the absorption in UV and visible rangeand the efficiency of the reduction reaction. The best conversion was achieved in case of ZrO2(10%)/TiO2
films deposited in N2/CH4 (5:1) mixture at a total pressure of 1 mbar. An interpretation of the obtainedresults based upon XPS analyses is provided.
In order to enhance the efficiency of TiO2 under solar irradiation,the modification of semiconductors to absorb visible light isrequired. There are many published works on non-metal dopedTiO2 but the origin of the absorption in the visible light region andthe state of the non-metal atoms in the TiO2 lattice are still underdebate [1,2]. Non-metal doping of TiO2 has shown a great potentialin visible light active photocatalysis, with nitrogen being the mostpromising dopant. Nitrogen can be introduced inside TiO2 struc-ture, due to its comparable atomic size with oxygen, small ioniza-tion potential and high stability. Doping of TiO2 by non-metalsproceeds according to the following mechanisms: (i) band-gapnarrowing introduced by Asashi et al. [3] based upon the hybridi-zation of N 2p state with O 2p states in anatase TiO2; (ii) impurityenergy levels formation above the valence band as a result of thesubstitution of oxygen by nitrogen atoms in titania as suggested byIrie et al. [4] and (iii) generation of oxygen vacancies proposed byIhara et al. [5]. It was reported that TiO2 doped with substitutional
ailescu).
nitrogen has shallow acceptor states above the valence level, whiledoping with interstitial nitrogen leads to isolated impurity states inthe middle of the band-gap where energy levels of the impurity aremainly hybridized by N 2p and O 2p states [6]. The alternative pointof view where the visible-light activation of anion-doped TiO2
originates from the defects associated with oxygen vacancies wasalso reported. As a result, the colour centres appeared displayingthese absorption bands, and not to a narrowing of the originalband-gap of TiO2 (EBG ¼ 3.2 eV; anatase) via mixing of dopant andoxygen states [7].
The modification of titania by doping with metal oxides can bean effective method to improve the photocatalytic activity as aresult of the cooperative action of metal ions and anatase. Zirco-nium incorporation into TiO2 lattice by solegel synthesis wasachieved with the formation of Ti1-xZrxO2 solid solution for the filmwith a Zr content up to 10 mol.% as evidenced by XRD and XPS. Itwas demonstrated that the formation of ZreOeTi bonds has aninfluence on the thermal stability during sintering of the meso-porous structure of the films, surface texture and optical propertiesas well as in the changes of number of surface active sites fornanocomposite films [8]. In our previous study, co-doped with ni-trogen TiO2/ZrO2 films were obtained by Pulsed Laser Depositionmethod under different synthesis conditions [9]. The films obtained
in nitrogen atmosphere or their mixtures with methane are char-acterized by the formation of OeTieN bonds in the oxide matrix asshown by XPS analysis. The film synthesized at minimummethanecontent in N2/CH4 mixture and minimum pressure exhibits themaximum reaction rate constant of photodegradation in conjunc-tion with the highest distribution of Ti2p and N1s states connectedto OeTieN bonds. It is suggested that the interaction of formingcarbon atoms and radicals under laser pulse with oxygen atoms ofOeTieO fragments leads to the formation of oxygen vacancies andincrease of OeTieN fragments. The development of new andoptimization of existing synthesis conditions of photocatalyts aretherefore crucially requirements for the improvement of activityunder visible light. To this propose, efforts should be paid to theinvestigation of the electronic, optical and photocatalytic proper-ties of non-ion doped TiO2.
2. Experimental details
2.1. Thin films preparation
PLD depositions were performed using a KrF* laser source(l ¼ 248 nm and tFWHM ¼ 25 ns) model COMPexPro 205 (LambdaPhysics-Coherent) that operated at a repetition rate of 20 Hz.Pristine TiO2 and ZrO2 doped TiO2 targets with 2.5, 5 and 10% wtconcentrations were laser ablated with 20,000 subsequent pulsesto grow coatings on 26� 26 mm2 glass plates. Prior to introductioninside the deposition chamber, the deposition substrates weresuccessively cleaned in an ultrasonic bath with acetone, ethanoland deionized water for 15 min and then dried with pure nitrogen.The laser fluence was of 4.5 J/cm2 whereas the laser spot area wasset to 10 mm2. In order to avoid the drilling, duringmultipulse laserirradiation the target was rotated at a rate of 0.4 Hz. The films weregrown at a substrate temperature of 450 �C while the tar-getesubstrate separation distance was of 5 cm. Two batches ofsamples were deposited at 0.03 or 1 mbar of pure oxygen or mixedN2:CH4 atmospheres with 5:1 or 9:1 ratios. Gas composition insidethe deposition chamber was monitored by means of a MKS50 massflow controller.
Deposition conditions are collected in Table 1(a).
Table 1(a) Targets composition, deposition conditions and (b) band-gap values of synthe-sized films at the pressure of 1 mbar.
2.2. Physical-chemical characterisation of deposited films
Optical spectra were recorded with a double beam spectro-photometer (Lambda 35, PerkinElmer) within the wavelengthrange of (190e1200) nm.
XPS spectra were explored with an electron spectrometer EC-2402 equipped with a PHOIBOS-100_SPECS energy analyzer.Nonmonochromatic X-rays (МgКa 1253.6 eV) were employed at apower of Р¼ 200W. All peaks of XPS spectra were charge correctedusing C 1smaximum position as the reference point. Surface chargeof the tested samples was in the range of 1.5e2.7 eV. Chargeneutralization was achieved using a flood gun FG15/40 andaluminium screen-trap onto the sample. The vacuum in theworking chamber was kept inferior to 2 � 10�7 Pа.
Spectra of Ti2p-versus Zr3d-levels were deconvoluted intoconnected with each other pairs of the components with consid-eration of their spin-orbit splitting and parameters (Ti2p:DЕ¼ 5.76 еV; I1/2/I3/2 ¼ 0.5, FWHM¼ 1.28 еV; Zr3d: DЕ¼ 2.4 еV; I3/2/I5/2 ¼ 0.66, FWHM ¼ 1.3 еV). Spectra of O1s- and N1s-levels weredeconvoluted into the components with FWHM ¼ 1.4 еV. The XPSsignals were fitted using the GaussianeNewton method in themode of bounded parameters. Variation of component intensityand bond energy was conducted. Width of the components andratio contribution of GaussianeLorentzian distribution for certainatoms of the tested samples in the process of spectrum deconvo-lution were fixed. The component area was determined afterbackground subtraction by Shirley method.
2.3. Photocatalytic activity of the films
Reduction of toxic bichromate ions was monitored via thephotocatalytic activity of the films. The detailed description ofphotocatalytic set-up is given elsewhere [9]. The change of Cr(VI)ions concentration was monitored with a Lambda 35 UVevisspectrophotometer (Lambda 35, PerkinElmer) every 20 min atl ¼ 350 nm. The reaction conversion means the percentage ofphotoreduced amount of Cr(VI) ions. The filmwas immersed in thesolution until complete adsorption in the dark occurred, and thenirradiated by 1000 W middle-pressure mercury lamp for 120 min.The distance from lamp to reactor was set at 90 cm. No significantchanges in the absorption spectra of the liquid phase wereobserved for two blanks: dark condition and irradiation withoutfilm. A filter transmitting light with l > 380 nm was introduced inthe photocatalytic set-up under visible light irradiation.
3. Results and discussion
3.1. Spectroscopic studies
UVeVis absorbance spectra were obtained from the samplesgrown on glass substrates. The plots of absorbance versus lightwavelength are shown in Fig. 1. The absorption curves of the filmsobtained in O2 atmosphere in UV region (Fig. 1a) are modified,where the increase of zirconia contents leads to the shift of theabsorption onset to shorter wavelength. The shift in the opticalabsorption edge corresponds to a change in the band-gap calcu-lated by extrapolating the linear parts of the (ahn)1/2 ~ f(hn) curves(the indirect electronic transition) for the tested semiconductivefilms (Table 1(b)).
An absorption tail extending into the visible-light region wasobserved for all non-metal doped films (Fig. 1b). Oppositely to thesamples synthesized in O2, the non-metal doped films are charac-terized by a shift to the longer wavelength from lower to highercontents of ZrO2 and a sharp decrease in 10ZrTiN5C1 band-gapenergy. The band-gap values of the films synthesized in a gas
Fig. 1. Absorption spectra of ZrO2/TiO2 films synthesized in oxygen (a) and N2/CH4 with ratio 5:1 (b) at 1 mbar: 1)�2.5% ZrO2, 2)�5% ZrO2, 3)�10% ZrO2. Inset: samples 5ZrTiN5C1(0.03 mbar) -a, 5ZrTiN9C1 (0.03 mbar) - b, 5ZrTiN5C1 (1 mbar)-c.
O. Linnik et al. / Vacuum 114 (2015) 166e171168
atmosphere with higher ratio of N2 to CH4 (9:1) are lowered whenincreasing zirconia content (Table 1(b)). For doped compositesamples synthesised at 0.03 mbar, the calculation of Eg wasimpossible because of the strong absorption in visible region (Fig1b, inset).
3.2. XPS investigations
Non-metal doping, in particular with nitrogen, of titania andmixed metal oxides is widely studied by XPS, as the new form and/or changed structural peculiarities could be detected. There is nodefinite opinion about the XPS measurements of N1s binding en-ergy, where the values of 396e397 eV assigned to the NeTieN[10,11] or OeTieN bonds [12] were reported. The formation of NeN
Fig. 2. XPS spectra of N1s binding energy for 2.5ZrTiN9C1 (a), 2.5ZrTiN5C1
or NeC, NeO groups or chemisorbed dinitrogen was suggested forhigher energies of 400e402 eV [12e15].
From our previous results [9], the XPS data of ZrO2/TiO2 filmssynthesized in N2 and different ratios of N2/CH4 gases evidencedthe influence of methane and its content on the process of the ni-trogen incorporation in the metal oxide matrix at 600 �C. It wassuggested that the carbon from gas mixture interacts with oxygenfrom TiO2 leading to the formation of an oxygen deficient latticeand, in turn, assists the efficient incorporation of nitrogen atoms.
To quantify the influence of zirconia content, gas ratios and thesynthesis pressure on the efficiency of doping at lower calcinationtemperature (450 �C), the XPS data for 2.5, 5 and 10% wt ZrO2 intitania films are analysed. The N1s line at 395.8 eV is present for allsamples and attributed to OeTieN bonds (Figs. 2 and 3). In the caseof 5ZrTiN9C1 (1 mbar), the peak has higher intensity in comparison
(b), 5ZrTiN9C1 (c) and 5ZrTiN5C1 (d) samples synthesized at 1 mbar.
Fig. 3. XPS spectra of N1s binding energy for TiN5C1 (a and c) and 10ZrTiN5C1 samples (b and d) synthesized at 1 mbar (a and b) and 0.03 mbar (c and d).
Table 2N1s binding energy values in Fig. 2 andtheir relative intensity.
BE, eV I,%
graph a 2.5ZrTiN9C1395.8 7.9397.8 3.8399.2 60.4400.8 27.9graph b 2.5ZrTiN5C1395.8 8.5397.8 14.4399.2 55.6400.8 21.5graph c 5ZrTiN9C1395.8 21.4397.8 11.6399.2 44.5400.8 22.5graph d 5ZrTiN5C1395.8 13.4397.8 18.1399.2 47.0400.8 21.5
O. Linnik et al. / Vacuum 114 (2015) 166e171 169
with 5ZrTiN5C1 (1 mbar) probably due to increasing the dinitrogencontent in gas mixture (Fig. 2c,d). Note that N1s binding energies of399.1 and 400.5 eV measured for modified by urea commerciallyavailable TiO2 powder were assigned to carbon nitrides(399e400 eV, C¼NeC), graphite-like phases (400.6 eV, N-Csp2) andto polycyanogen (399.0, 400.5 eV (eC¼Ne)x) [1,16].
We suppose that the deconvoluted peak at 399.2 eV could berelated to the NeC formation and/or N2 adsorption onto the oxidesurface. The higher energy peak at 400.8 eV is assigned to theformation of N2 or NO species onto the surface. The intensity of thispeak is characteristic for all samples at 1 mbar pressure (Tables 2and 3).
Quite surprisingly, N1s peak for both TiN5C1 and 10ZrTiN5C1obtained at 0.03 mbar contains the single high intensity N1s lineat 395.9 eV assigned to OeTieN bonds (Fig. 3c,d). We suggestthat carbon effectively interact with oxygen atoms forming CO2during PLD. As a result, nitrogen is effectively incorporated inoxide matrix due to the oxygen deficiency and the formation ofnon-stoichiometric TiO2�x is promoted. That can be confirmedby the lower ratio (1.6) of OTi1s/Ti2p for samples TiN6C1 and10ZrN5C1synthesized at 0.03 mbar in comparison with pristineTiO2 where its ratio equals 2. The samples obtained at 0.03 mbarare black coloured and also exhibit the electrical conductivitypointing to the appearance of oxygen vacancies and the absenceof the band-gap. This mechanism could be valid for the syn-thesis of all films with effective incorporation of substitutional N(N1s line at 395.9 eV). The open question is why the lowpressure stimulates carbon atoms to effectively interact withoxygen atoms.
After comparing the intensity of N1s line at 395.8 eV of thesamples 2.5ZrTiN5C1e5ZrTiN5C1e10ZrTiN5C1 and2.5ZrTiN9C1e5ZrTiN9C1 (1mbar) (Tables 2 and 3), it is evident thatthe larger content of zirconia corresponds to the higher peak
intensity. Thus,when the zirconia content is increasing to10w/w%, amaximum of OeTieN bonds is forming (41%).
The peaks of Ti2p3/2 at 458.3 and 458.8 eV that belong to Ti4þ
ions surrounded by oxygen ions are typical to all samples. Thesignificant intensity of peak at 457.5 eV that is related to theOeTieN bonds was observed for 10ZrTiN5C1 sample (not shownhere). This is an evidence of the presence of Ti3þ states on the TieOand TieN bonds. Zr3d5/2 line (Eb ¼ 182.3 eV) (not shown here) wasrecorded for all zirconia containing samples, meaning the forma-tion of Zr4þ ions surrounded by O2� ions [8,9]. It is supposed that
the distortion of Ті4þО2 lattice with the advent of Ti3þ states occursdue to larger size of zirconium ions (RZr4þ ¼ 0.720 Å;RTi4þ ¼ 0.650 Å). The relatively high stability of Ti3þ states of thetested samples is associated to the presence of Zr4þ ions. Note thatthe non-equilibrium character of the PLD process is favourable tothe generation of strains in the lattice. XPS data pointed to thecorrelation between zirconium contents and ratio of number ofnitrogen atoms linked in OeTieN bonds to the number of Ti3þ
states ((Int. N1s 395.8/Int.Ti2p 457.5) � 100), as well as the relativecontents of N1s with Eb ¼ 395.8 eV. This can be indicative for theformation of ОeТіeN fragments in the coordination sphere of Ti3þ
ions. In other words, the N1s-states with Eb ¼ 395.8 eV responsiblefor photocatalytic activity of doped samples [3,16] could be con-nected with Ті3þ states of Ті4þО2 lattice.
3.3. Photocatalytic activity
Photocatalytic properties of films were tested in the reductionprocess of bichromate ions. The mixed oxide films (Fig. 4, column 7,8, 9) exhibited a lower activity under both UV and visible lightcompared with 2.5ZrTiN5C1, 5ZrTiN5C1 and 10ZrTiN5C1 samples(Fig. 5, column 2, 3, 4) synthesized at 1 mbar. The photocatalyticperformance of 2.5ZrTiN5C1 and 5ZrTiN5C1 is slightly higher un-der UV and is lower under visible light than in case of TiN5C1sample (compare in Fig. 4 columns 1, 2, and 3). However,10ZrTiN5C1 sample demonstrated the highest yield (51% reducedions) after 120 min exposure to UV light. For the visible light
Fig. 4. Conversion of chromium (VI) ions by photocatalytic reduction after 120 min und10ZrTiN5C1; 5 e 2.5ZrTiN9C1; 6 e 5ZrTiN9C1; 7 e 2.5ZrTi; 8 e 5ZrTi; 9 e 10ZrTi sample
irradiation, the high activity with a conversion of 9 or 14% is noticedfor TiN5C1 and 10ZrTiN5C1 samples, respectively.
It must be emphasized that all films obtained at 0.03 mbar arealmost inactive under both UV and visible light irradiation (Fig. 5).This is most probably due to the formation of non-stoichiometricTiO2�x possessing the low semiconductive properties. Maximumconversions of 10% for 5ZrTiN5C1 (under UV) and of 6% in case of10ZrTiN5C1 sample (under visible irradiation) were obtained. Itresults that the activity of the samples is affected by zirconia con-tent, as well as by the gas ratio used in PLD synthesis.
In summary, the highest conversion yields are obtained for10ZrTiN5C1 sample under both UV and visible light, as a result ofeffective nitrogen atom substitution into metal oxide lattice asconfirmed by N1s XPS line at 395.8 eV. It follows that the substi-tutional nitrogen (TieN) is basically responsible for the observedphotoactivity, as the peak in the range of 398e401 eV is alsoobservable for the samples with low reactivity, as well as for theZrO2/TiO2 films synthesized in CH4 atmosphere [9] and pristineTiO2 powders [3,17]. In some cases the peak at 400 eV only wasobserved for N-doped TiO2 materials that exhibited photocatalyticactivity under visible illumination [18,19]. The doping of non-metals could narrow the band-gap and might drive the responseto visible light and catalytic activity. In particular, the interstitial Nimpurities give rise to the higher energy states in the gap, andmight behave as stronger hole trapping sites, reducing the directoxidation activity of the sample in the photocatalytic process [20].
As seen from Fig. 6, the percentage of reduced Cr(VI) ions underUV light is correlated with the contribution of N1s at 395.8 eVwhich is in turn dependent on the zirconia content. The ratio of lineintensities of N1s at 395.8 eV to Ti2p at 457.5 eV is pointing to thelarge number of OeTi3þeN bonds with the increase of ZrO2 con-tents. It is to our opinion obvious that photocatalytic activity underUV light strongly depends on the efficiency of substitutional ni-trogen incorporation inside titania matrix.
4. Conclusions
We have observed that the PLD synthesis conditions dictate thetype and level of nitrogen doping which, in turn, essentially in-fluences the photocatalytic activity. For non-metal doped films, theabsorption edge was extended to the longer wavelength region,from lower to higher content of ZrO2, resulting in the band-gapnarrowing.
The best ratio of N2 to CH4 as well the pressure during synthesisprocedure were identified for the optimum photocatalytic activityof semiconductive materials. The films obtained in N2/CH4 atmo-sphere with the ratio 5:1 at 1 mbar were more active than thoseobtained with a ratio of 9:1. When the films were synthesized at a
er UV (a) and visible light (b): 1 e TiN5C1; 2 e 2.5ZrTiN5C1; 3 e 5ZrTiN5C1; 4 e
Fig. 5. Conversion of chromium (VI) ions by photocatalytic reduction after 120 min under UV (a) and visible light (b): 1 e TiN5C1; 2 e 2.5ZrTiN9C1; 3 e 2.5ZrTiN5C1; 4 e
5ZrTiN9C1; 5 e 5ZrTiN5C1; 6 e 10ZrTiN5C1 samples. The films were deposited at 0.03 mbar.
Fig. 6. Influence of zirconia content in ZrTiN5C1 films on the photocatalytic conver-sion under UV (black column), ratio of N1s at 395.8 eV to Ti2p at 457.5 eV (red column)and N1s at 395.8 eV to total N1s (blue column). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)
O. Linnik et al. / Vacuum 114 (2015) 166e171 171
low pressure of 0.03 mbar, photocatalytically inactive non-stoichiometric TiO2�x structures were deposited. As a result, theirphotocatalytic performance remained at the level of blanks.
The correlation between the zirconia content and the efficiencyof substitutional N incorporation was noticed. It was supposed thatthe distortion of Ті4þО2 lattice with the advent of Ti3þ statesoccurred due to the larger radius of zirconium ions (RZr4þ ¼ 0.720 Åas compared to RTi4þ ¼ 0.650 Å). The relative high stability of Ti3þ
states was assigned to the presence of Zr4þ ions. The highest con-version percent of photoreduced Cr(VI) ions under UV (51%) andvisible irradiation (14%) was reached in the case of ZrO2(10%)/TiO2obtained in N2/CH4 (5:1) atmosphere and 1 mbar pressure.
The possible contribution of both substitution of O to N andconcomitant oxygen vacancies in the oxide matrix to additionalvisible-light absorption should also be considered. It is suggestedthat the level of the interstitial N atoms in TiO2 matrix (rather thanTiO2�x) is essential for photocatalytic reduction of bichromate ionsunder both UV and visible light.
Acknowledgements
Romanian authors acknowledge the financial support of thiswork by UEFISCDI under the contract ID304/2011 and CoreProgrammme.
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