Photoelectrochemical Characterization and Photocatalytic Properties of Mesoporous TiO 2 / ZrO 2 Films

Optically transparent, crack-free mesoporous titania and zirconia-doped titania thin film photocatalysts were fabricated by sol-gel technique, using nonionic amphiphilic block copolymer Pluronic P123 as template. The structural and optical properties of these films were characterized using SEM, low-angle XRD, and UV/Vis spectroscopy, hexane adsorption investigation. Band gap energy and the position of flatband potentials were estimated by photoelectrochemical measurements. Enhancing of photocatalytic activity of zirconia-doped films relative to pure TiO2 originates from an anodic shift of the valence band edge potential. Catalytic activity of mesoporous TiO2 and TiO2/ZrO2 (5–50% of ZrO2) films in the processes of CrVI to CrIII photoreduction and 2,4dinitroaniline photooxidation correlates with crystalline size and growth with increasing of specific surface area of the samples.


INTRODUCTION
Many organic substrates have been shown to be oxidatively (in some cases reductively) degraded and ultimately mineralized completely under UV irradiation on nanosized titania catalysts [1][2][3].Catalytic activity, i.e. strongly reductive/oxidative effect of titanium dioxide is based on the electron/hole pair formed upon photoexcitation.A great deal of research has been conducted recently to optimise the performance of the TiO 2 photocatalysts, especially mesoporous films, with high surface area [4][5][6][7].
Mixed oxide composite materials can often be more efficient photocatalysts than pure substances.This phenomenon arises through the generation of new active sites due to interactions between titania and dopant oxides, through improved mechanical strength, thermal stability, and surface area of doped titania [8][9][10] and perhaps to the absence of a rutile phase in the supported oxide samples.Mesoporous TiO 2 and TiO 2 /ZrO 2 coatings showed high activity in the gas-phase photooxidation of ethanol and acetone [8,11].
Fu et al. [12] reported the enhancing of photocatalytic activity of mixed metal oxides (TiO 2 /SiO 2 and TiO 2 /ZrO 2 ) compared to TiO 2 for oxidation of ethylene due to increased thermal stability and surface acidity.Also, it was found that the photocatalytic activity depends on the crystalline size and crystalline phases that modify the TiO 2 band gap [3,13].Although the knowledge on the band edge position is particularly useful in photocatalysis, the effect of zirconium content in TiO 2 /ZrO 2 mixed oxides for their photophysical properties has not been much studied.
Herein, we report on mesoporous TiO 2 and TiO 2 /ZrO 2 thin films with 5-30% ZrO 2 content that were prepared via the templating sol-gel route and characterized using SEM, XRD measurements, UV/Vis spectroscopy, and hexane adsorption investigation.
The electronic properties of TiO 2 /ZrO 2 photocatalysts such as band gap energy, flat-band potential, and resistivity were measured and correlated with the photocatalytic activities in Cr VI to Cr III photoreduction and 2,4-dinitroaniline oxidation.

Preparation of the samples
For the synthesis of TiO 2 and TiO 2 /ZrO 2 mesoporous films, titanium and zirconium tetra-iso-propoxides (Aldrich) were used as metal sources.Triblock copolymer (PEO) 20 (PPO) 70 -(PEO) 20 (Pluronic P123, BASF) has been used as templating agent.To control hydrolysis-condensation reaction rates and to prevent oxides precipitation, acetylacetone (acac) was used as a complexing agent.The typical molar ratios TIPT : Zr(0Pr) 4 : P123 : acac : HCl : H 2 O : C 2 H 5 OH of the reacting solution was 1-0.7 : 0-0.3 : 0.05 : 0.5 : 4.6 : 10 : 41.Dip-coated films on glass substrates were aged for 12 hours at room temperature and calcined by heating at the rate of 1.5 • C/min to 500 • C, and kept for 2 h to remove the block copolymer species and to increase cross-linking of the inorganic framework.For comparison, TiO 2 coatings without template were prepared in parallel.

Structural and optical characterizations
X-ray diffraction measurements on sintered coatings were performed with a Cu-K ff DRON-4-07 M system (LOMO St. Petersburg) with the minimum glancing angle θ = 0.4.For the SEM measurements, films were prepared on conducting glass substrate coated with SnO 2 .The samples were imaged by the LEO 1530 electron microscope.
The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the thin film coated on covering slides for microscopy were determined from hexane adsorption-desorption isotherms [11], obtained at 20 • C in thermostatting vacuum setup in the relative pressure (P/P 0 ) range of 0.01-1.00.All samples were degassed at 200 • C prior to measurements.

The photoelectrochemical properties
The photoelectrochemical properties of the TiO 2 /ZrO 2 electrodes were estimated using the spectral dependence of the photoelectrochemical current (i ph ), measured with a commercial spectrometer KSVU-1 (LOMO, Russia) with spectral resolution 1 nm.The experiments were carried out at 22 • C under pure argon bubbling in the temperature-controlled quartz cell with optically transparent planar-parallel windows.The i ph spectra were measured with usage of the mechanical light chopper of 20 Hz frequencies and standard circuit synchronous detection followed by computerassisted spectral analysis.A high-pressure xenon lamp with stabilized discharge current was used as light source.The i ph spectra were expressed in units of quantum efficiency (electron/photon).The resistivity of TiO 2 /ZrO 2 films on Ti substrate was measured by means of common alternating current bridge BM401.In parallel, the resistivity of the TiO 2 /ZrO 2 films on a glass substrate was measured with the help of a digital multimeter P386, that gives the good reproducibility.Ag/AgCl electrode was used as the reference electrode on the pH value of the electrolyte.Running water was circulated through the jacket to ensure constant temperature of the magnetically stirred reaction mixture.During the experiments, concentration of reagents has been controlled with an UV-VIS spectrometer Perkin-Elmer Lambda-35.

Structural characterization
Prepared by the sol-gel process from the Zr and Ti alkoxides using a template agent, TiO 2 /ZrO 2 films on the glass substrates are optically transparent.The thickness of onecoated films calculated ranges between 70-90 nm, refractive indexes 1.9-2.1 [11].The low-angle X-ray diffraction pattern of as-synthesised films displays 3 peaks in the low-angle range typical for ordered lamellar mesophase.The 500 • C calcined films showed only one peak in the low-range pattern due to a partial collapse of the ordered pore structure.No peaks were seen in the wide-angle XRD pattern.This observation witnesses rather about pore walls contained tiny crystallites not resolvable by conventional wide-angle XRD than about noncrystalline pore walls.XRD patterns of TiO 2 /ZrO 2 powders, prepared from film precursors after gelation, were analysed to estimate the structure and crystallite size of samples.When pure TiO 2 and TiO 2 /ZrO 2 (5-10%) showed anatase structure with 80% of crystalline phase and mean crystallite size 9-10 nm, increasing of Zr content leads to drop of crystallinity.Only weak reflex at 2θ = 25.5 corresponding to anatase is observed for TiO 2 /ZrO 2 (30%), while TiO 2 /ZrO 2 (50%) is amorphous.As we reported previously [14], no phases associated with pure zirconia were observed, only tetragonal titania anatase was detected in the range of ZrO 2 concentrations of 5-30% after 600 • C calcination.This means that some titanium atoms are substitutes by zirconium in the anatase structure.
The adsorption-desorption isotherms of hexane on 500 • C calcined TiO 2 and TiO 2 /ZrO 2 samples demonstrated the type IV shape, which indicates the presence of mesoporosity in accordance with [15].The specific surface areas (S BET ) and mean pore sizes of TiO 2 /ZrO 2 films annealed at 500 • C are listed in Table 1.Uniform porosity with r = 8-10 nm is typical for samples with low ZrO 2 content.Pure TiO 2 and 30% TiO 2 /ZrO 2 films show broader pore size distribution shifted to the larger pores with radius around 14 nm.
From the above results, it is clear that the films with low ZrO 2 (5-10%) concentration are homogeneous with stable mesoporous structure.Decreasing of homogeneity with further increasing of zirconia content up to 30% of ZrO 2 leads to the collapse of porous structure after calcinations.

Photoelectrochemical properties
To obtain the value of the band gap energy, spectral dependences of photocurrent were measured for the TiO 2 /ZrO 2 electrodes (TiO 2 /ZrO 2 films were coated on Ti substrate).It is well known that quantum yield of photoelectrochemical current η in semiconductors can be expressed as [16] where hν is the photon energy, m = 1/2 for the direct transition, and m = 2 for the indirect transition.
For the tested TiO 2 /ZrO 2 compositions, we cannot obtain linear dependence in (η • hν) 1/2 = f (hν) coordinates, that were possibly connected with some reasons such as: photocurrent in the longwave spectral region caused by defects in the anatase structure, low intensity of photocurrent corresponded to indirect transition in thin films due to low absorption coefficient.Photocurrent spectra were presented as (η • hν) 2 = f (hν) dependence, which was linear in the wide range of wavelength.As follows from the result presented in Figure 2a, experimental data fit better to a direct transition.Band gap (E g ) values were calculated by extrapolation of straight line of these dependences to the abscissa (Table 2).With growing of zirconium content, band gap increases from 3.17 for TiO 2 to 3.45 eV for TiO 2 /ZrO 2 (50%) was obtained that can been attributable to quantum-size effect [17].Our data agreed with diverse literature (E g = 3.4-3.9eV for the different nanocrystals size and measurement technique).This result indicates that zirconum doping in TiO 2 inhibits crystallite growing [9,10].Crystallite sizes calculated in accordance with [18] are listed in Table 2. To test whether the larger band gap is caused by a shift of the valence (E v ) or conduction (E c ) band edges, the position of the flatband potential (U fb ) of the catalysts was determined by the direct electrochemical measurements of photocurrent as a function of applied potential in aqueous 0.5 NaCl.
Flatband potentials were estimated from i ph changes measured at the photocurrency maximum for TiO 2 and TiO 2 /ZrO 2 films coated on the titanium substrate in aqueous 0.5 M NaCl plotted against applied potential by extrapolation of straight line of these dependences to the abscissa (Figure 2b).Flatband potential values for the samples with different zirconium content differ insignificantly and are comparable with the value of −(0.47-0.49)V, obtained at pH ≈ 7 for nitrogen-doped titanium dioxide [19] and U fb = −0.58V measured for anatase single crystal [20].
Resistivity of films was measured for evaluation of relative shift of the bottom of the conduction band (ΔE c ) and the upper edge of the valence band (ΔE c ) positions relative to pure TiO 2 depending on Zr content.The difference (ΔE fc ) between electrochemical potential of the electrodes (Fermi level potential), that corresponds to flatband potential, and potential of the bottom of the conduction band −ΔE c was calculated using the following equation [21]: (i) ΔE fc = kT/e ln(N c /n 0 ), where n 0 and N c denote concentration of electrons and density of electronic states in the conduction band, respectively.
ΔE c and ΔE v values, calculated in assumption, that mobility of electron and N c do not depend on TiO 2 /ZrO 2 ratio, are presented in Table 2: As followed from Table 2, increase of Zr content leads to anodic shift of the upper edge of the valence band ΔE v positions, when relative shift of the bottom of the conduction band ΔE c was not significant.As the location of the valence band is a measure of the oxidation power of the photogenerated holes, we can predict the enhancing of catalytic activity in photooxidation processes.At the constant electrochemical potential of TiO 2 photocatalytical reactivity of titania coatings can be controlled by electrons concentration in the conductive band, for example, by metal doping.
For high resistive TiO 2 films, we can take into consideration alteration of Fermi quasilevel under illumination, and E fb readjustment as the results of trapping of photogenerated charge carriers on surface electronic states and electronic states in the space charge region of semiconductor [21].Alteration of Fermi quasilevel in semiconductor depends on light intensity and can be significant (up to 1 V, as was experimentally obtained for A II B YI compounds [22]).

Photocatalytic activity of TiO 2 /ZrO 2 coatings in toxic Cr(VI) to nontoxic Cr(III) ions photoreduction
Photocatalytic activity of mesoporous TiO 2 /ZrO 2 films has been tested in the photoreduction of toxic Cr(VI) to nontoxic Cr(III) ions in acid water solutions in the presence of environmentally important substrate EDTA (Table 1).This process has been taken as a model of real wastewaters, where oxidants and reductants are present together, for comparable studies of commercial samples and platinized TiO 2 powders [2,23,24].The mechanism of photocatalytic Cr(VI) reduction in the presence of electron scavenger (EDTA, salicylic acid or other organic molecules) is well described in [2,23,24].
In Figure 3, we show the changes in Cr(VI) concentration followed by decrease of absorption band intensity at 349 nm under irradiation in the presence of TiO 2 /ZrO 2 films; simultaneously the absorption at 550 nm increases due to nontoxic Cr(III) formation.
Photocatalytic activity of mesoporous TiO 2 and TiO 2 /ZrO 2 (5-30%) films, in comparison with films prepared without template, increases in accordance with increasing specific surface area of the samples (Table 1).The conversion of Cr 2 O 7 2− , calculated for mesoporous TiO 2 films, was 4 times higher than that observed for nonporous samples, and 10 times higher than that reported in [24] for the equal amount of TiO 2 , supported on hollow glass microbeads during the same time of irradiation.
Enhanced activity was observed for TiO 2 /ZrO 2 films with low concentration of zirconium that possesses high surface area and composes of nanoparticles with mean size 3-4 nm.Zr doping retards not only anatase to rutile transformation, but also inhibits the crystalline growth [9,10].It can be seen that in general there is increase of E g with the anodic shift of the upper edge of the valence band ΔE v positions as the Zr concentration increases.Processes of EDTA oxidation accelerate, improving charge separation, and Cr(IV) reduction proceeds faster due to synergism between the oxidation and reduction reactions.Further increasing of Zr content leads to low crystallinity of TiO 2 /ZrO 2 (30%) and amorphous structure of TiO 2 /ZrO 2 (50%) samples.This effect explains the drop of activity of TiO 2 /ZrO 2 (30%) samples in comparison with pure TiO 2 .

Photocatalytic activity of TiO 2 /ZrO 2 coatings in 2,4-dinitroaniline photooxidation
The activity of the prepared samples in photooxidation processes was estimated in 2,4-dinitroaniline decomposition.The concentration of 2,4-dinitroaniline was calculated from the intensity of the adsorption band at 346 nm (Figure 4).During 2,4-dinitroaniline irradiation, overall decrease of absorption was observed.2,4-dinitroaniline concentration decreased exponentially with time of irradiation.Process was followed by significant pH changes from 7 at the beginning to 4.8-4.3(depending on the sample activity) after 3 hours of irradiation that can be assumed as completely mineralization of organic substrate accompanied with nitric acid, CO 2 and H 2 O formation.NO 3 − formation was detected by increase of corresponding absorption band (203 nm).Qualitative reaction with diphenylamine gives characteristic for NO 3 − ions blue coloration [25].
On the base of literature [1,2,26], the heterogeneous photocatalytic process of degradation of 2,4-dinitroaniline can be expressed as follows:  with OH − groups on the TiO 2 surface: For comparison of photoactivity of the samples, the rates of photooxidation were calculated in pseudo-first-order reaction approach [2] under equal conditions (Figure 4, inset).Mesoporous samples show enhanced activity (2-3 times) toward the coatings prepared without template.Highest activity in 2,4-dinitroaniline decomposition corresponds to TiO 2 /ZrO 2 films with optimum 5-10% Zr concentration having nanocrystalline structure (crystal size 3-4 nm) with high step of crystallinity and high surface area.

CONCLUSIONS
Optically transparent, mesoporous TiO 2 and TiO 2 /ZrO 2 thin film photocatalysts with stable porous structure were fabricated by sol-gel technique, using nonionic amphiphilic block copolymer Pluronic P123 as template.
Zr doping on the stage of sol-gel process improves thermal stability, retards sintering of the films, and stabilizes the nanocrystalline structure with developed porosity.

International Journal of Photoenergy
The E g values obtained for mesoporous TiO 2 /ZrO 2 films coincide with reported ones for nanostructured TiO 2 (anatase); mean particles size decreases with increasing of Zr content.
Whereas the flatband potentials of −0.52 V and bottom of conducting band edge position, measured for TiO 2 and for TiO 2 /ZrO 2 versus NHE, differ insignificantly, enhancing of catalytic activity of zirconia-doped films originates from an anodic shift of the valence band edge.
Highest activity in photoreduction of Cr(VI) in the presence of EDTA as well as in the photooxidation process of 2,4-dinitroaniline decomposition corresponds to TiO 2 /ZrO 2 films with optimum 5-10% Zr concentration, nanocrystalline structure (crystal sizes 3-4 nm), high degree of crystallinity (80%), and high surface area.
*For film prepared without template, SBET cannot be measured directly.