Photoelectrochemical Properties Of TiO2 Research Articles

Photocathodic protection of 304 stainless steel by coating muscovite/TiO2 heterostructure

The sol-gel method was used to prepare a series of muscovite/TiO2 heterostructures for photocathodic protection of 304 stainless steels (304SS). The effects of different mass ratio of muscovite to TiO2 (1:0.5, 1:1 and 1:1.5, denoted as MT-0.5, MT-1 and MT-1.5, respectively) on the crystal phase, grain size, light absorption properties and photoelectrochemical properties of TiO2 and muscovite/TiO2 heterostructures were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–visible diffuse reflection spectroscopy (UV–vis DRS). The corrosion resistances and photoelectrochemical properties of the 304SS coated with muscovite/TiO2 heterostructures (304SS/MT) were studied through corrosion immersion test and electrochemical workstation in 3.5 wt% NaCl solution, respectively. In addition, Tafel curves and the electrochemical impedance spectroscopy (EIS) measurements were carried out to evaluate the photogenerated electron separation and transfer efficiency, and the interfacial charge transfer resistance of the 304SS and 304SS/MT electrodes, respectively. The time-dependent photocurrent responses of the MT-1 heterostructures demonstrated an enhanced photo-generated cathodic protection performance for 304SS substrates under visible light irradiation compared to TiO2. A photoinduced potential drop value for 304SS coated with MT-1 in 3.5 wt% NaCl solution was −0.54 V (vs. SCE). After immersion test, the corrosion image analyses of the 304SS, 304SS-MT, and 304SS coated with physically mixed muscovite/TiO2 samples showed that the 304SS/MT-1 exhibits the best corrosion resistances. After connecting with 304SS/MT-1 in 3.5 wt% NaCl solution, the corrosion current densities (icorr) of 304SS increased significantly under the white light illumination, indicating more photoinduced electrons were migrated from the muscovite/TiO2 heterostructures to the 304SS compared with that from pure TiO2. Furthermore, the simulated charge transfer resistance (Rct) of 304SS/MT-1 electrode is smaller than that of the 304SS electrode coated with TiO2, indicating that there exists an effective separation of the photogenerated electrons and holes, and a fast interfacial charge transfer after coating muscovite/TiO2 heterostructures.

Influence of annealing atmosphere on photoelectrochemical response of TiO2 nanotubes anodized under controlled hydrodynamic conditions

• Annealing in argon and nitrogen atmospheres increase oxygen vacancies formation. • Nitrogen atoms appear in the matrix of the crystalline TiO 2 annealed in nitrogen. • Band gap of nanostructures decreases annealing in non-oxidizing atmospheres. • Nitrogen or argon atmospheres increases the photocatalytic activity of nanotubes. The influence of three annealing atmospheres (air, nitrogen and argon) and the use of controlled hydrodynamic conditions (from 0 to 5000 rpm) on morphological, structural, chemical and photoelectrochemical properties of TiO 2 nanotubes have been evaluated. For this purpose, different characterization techniques have been used: Field Emission Scanning Electron Microscopy, Raman Confocal Laser Spectroscopy, X-Ray Diffraction, X-Ray Photoelectron Spectroscopy, Incident Photon-to-electron Conversion Efficiency measurements, ultraviolet–visible absorption spectra, Mott-Schottky analysis and photoelectrochemical water splitting tests. According to the results, it can be concluded that both hydrodynamic conditions and annealing in non-oxidizing atmospheres improve the photoelectrochemical response of the TiO 2 nanotubes. This fact has been attributed to the oxygen vacancies formed after annealing in argon and nitrogen atmospheres and also to the presence of nitrogen into the TiO 2 lattice due to the thermal treatment in the nitrogen atmosphere.

Photocathodic Protection Performance of ZnS-CdS-Ag@TiO2 for 403SS Under Visible Light

Up to date, TiO2 nanotubes have attracted extensive interest in photocatalysts [1], dye-sensitized solar cells [2], photocatalytic decomposition of water into hydrogen [3], and sensors [4]. In addition, some researchers have reported that the photoelectrochemical properties of TiO2 could be applied to the cathodic protection of metals [5-7]. Compared with conventional cathodic protection, the photocathodic protection technology has no need to supply current or sacrificial anodes, and can achieve green and nonpolluting protection of metals [5]. However, there are two key drawbacks to the practical application of photocathodic protection of bare TiO2 films. In this work, an all-solid-state Z-scheme type CdS-Ag@TiO2 nanocomposite was fabricated by pulse electrodeposition and the successive ion layer adsorption and reaction (SILAR) technique. To suppress CdS photocorrosion, a ZnS shell was provided by the SILAR method. The CdS was chosen to sensitize the TiO2, and Ag acted as a transporter of electrons. This nanocomposite shows an impressive photocathodic protection performance for the 403 stainless steel (SS). The morphologies were observed using field-emission scanning electron microscope Hitachi FE-SEM S4800. The results showed that the film possessed a porous network morphology and the network consisted of scrolled-up nanowires. It can be observed that CdS and ZnS particles were uniformly distributed on the nanowire surface. The EDX spectrum and XPS measurement revealed that the ZnS-CdS-Ag@TiO2 films were composed of Ti, O, Ag, Cd, Zn and S. The UV-vis absorption spectra showed that the photoabsorption spectrum of the composite film was expanded into the visible light region. The corrosion performances of 403 stainless steel in a 0.5 M NaCl solution coupled to a titanate film photoanode in 1.0 M NaOH solutions were evaluated by photogenerated potential and electrochemical impedance spectroscopy (EIS). The steel was coupled to the composite photoanode under illumination and dark conditions. Under irradiation, the stabilized potential of 403SS coupled to the composite photoanode was recorded at approximately -1174 mV (vs.SCE). Under dark condition, the composite photoanode still provides the certain cathodic protection for the steel. To further study the corrosion process of 403SS under photocathodic protection, EIS was performed under illumination. It was found that the impedance arc of the 403SS coupled to the composite photoanode was markedly smaller. This may be due to a larger number of photogenerated electrons being transferred from the photoanode to the the surface of 403SS, leading to the cathodic current density increase for the steel. The EIS result revealed composite film could provide a good photocathodic protection effect for 403SS. Acknowledgements This work was supported by the the National Natural Science Foundation of China (Nos. 51801013) and the Natural Science Foundation of Shandong Province of China (ZR2016BP11) and A Project of Shandong Province Higher Educational Science (J16LC18).

SILAR preparation of Bi2S3 nanoparticles sensitized TiO2 nanotube arrays for efficient solar cells and photocatalysts

Bi2S3 nanoparticles were deposited on TiO2 nanotube arrays (TiO2 NTs/Bi2S3) by the simple successive ionic layer adsorption and reaction (SILAR) method. The morphology, optical absorption and photoelectrochemical properties of TiO2 NTs/Bi2S3 were investigated by the changing of SILAR cycles. The results indicated that the TiO2 NTs/Bi2S3 (5) prepared with 5 SILAR cycles showed the optimal photoelectrochemical properties. The visible light photocurrent density of TiO2 NTs/Bi2S3 (5) was 5.99 mA/cm2, the photovoltage was −0.29 V/cm2, the flat band was −0.67 V, and the carrier concentration was 1.15 × 1020 cm-3. The TiO2 NTs/Bi2S3 (5) showed the optimal photoelectrocatalytic (PEC) removal efficiencies of rhodamine B (RhB), methyl orange (MO) and Cr(VI). Furthermore, the photoelectrochemical mechanism was tentatively proposed, and the excellent solar absorption and electron transportation were the main reason for the high photoelectric conversion and PEC removal of pollutants.

Effect of Ti3+ on enhancing photocatalytic and photoelectrochemical properties of TiO2 nanorods/nanosheets photoelectrode

Abstract In the study, visible-light active Ti3+ doped TiO2 nanorods/nanosheets (Ti3+/TiO2 NRs/NSs) photoelectrode was successfully fabricated by hydrothermal reaction, followed by sodium borohydride reduction treatment. Moreover, physicochemical properties of the resulting samples were studied by series of techniques. Also, photocatalytic (PC) activity of Ti3+/TiO2 NRs/NSs photoelectrode was measured by degradation of methylene blue (MB). Results suggested that Ti3+ and oxygen vacancies (Ov) were simultaneously formed and induce the formation of impurity energy level in the TiO2 NRs/NSs band gap by solution reduction treatment, which exerted a huge influence on the photocatalytic and photoelectrochemical properties of Ti3+/TiO2 NRs/NSs photoelectrode in the mean time. Furthermore, Ti3+/TiO2 NRs/NSs photoelectrode exhibited higher PC activity (89.61%) than that of pristine TiO2 NRs/NSs (73.56%) within 150 min visible light illumination owing to the enhancement of visible light harvesting and separation efficiency of photoproduced charges. Moreover, the possible enhanced PC mechanism of Ti3+/TiO2 NRs/NSs was proposed and confirmed. Furthermore, Ti3+/TiO2 NRs/NSs photoelectrode displayed good stability and reusability.

Synthesis of α-Bi2Mo3O12/TiO2 Nanotube Arrays for Photoelectrochemical COD Detection Application.

One-dimensional anodic TiO2 nanotube arrays hold great potential as a photoelectrochemical sensor for the determination of chemical oxygen demand (COD). In this work, we report a warm synthesis of modified TiO2 nanotube arrays with enhanced photoelectrochemical determination performance. Herein, a bismuth-based semiconductor (α-Bi2Mo3O12) was introduced into TiO2 nanotube arrays by sequential chemical bath deposition (CBD) at room temperature. Field-emission scanning electron microscopy, X-ray diffraction, Raman, and X-ray photoelectron spectroscopy were used to investigate the morphologies, structures, and elemental analysis of the products. The photoelectrochemical properties of TiO2 and α-Bi2Mo3O12/TiO2 NTAs were measured by amperometry and cyclic votammetry methods. The α-Bi2Mo3O12/TiO2 nanotube arrays decrease the background photocurrent and increase the current response to organics at the same time, both of which are beneficial to enhancing the photoelectrochemical detection performance. The optimized α-Bi2Mo3O12/TiO2 NTAs with enhanced photoelectrochemical detection performance can achieve a detection sensitivity of 2.05 μA·cm-2/(mg·L-1) and a COD detection range of 0.366-208.9 mg/L respectively. With the α-Bi2Mo3O12 modification, the surface electrochemical reactions of TiO2 NTAs were regulated, the mechanisms of which were also further studied.

Thermal Decomposition Fabrication of Fe2O3 Nanoparticle-Sensitized TiO2 Nanotube Arrays and Their Photoelectrochemical Properties.

TiO2 nanotube arrays/Fe2O3 nanoparticles (TiO2 NTs/Fe2O3) with enhanced visible-light activity were synthesized by a two-step approach including an electrochemical anodization technique followed by a thermal decomposition approach. The structural investigation indicated that Fe2O3 nanoparticles grew uniformly on the walls of TiO2 nanotubes. The Fe2O3 modification of TiO2 NTs resulted in an increase in the visible light adsorption, and the increases in photocurrent and photocatalytic efficiency were dependent on the thermal decomposition times. The enhanced photoelectrochemical properties of TiO2 NTs/Fe2O3 could be attributed to the improvement of charge separation derived from the coupling effect of TiO2 NTs and Fe2O3 nanocomposite.

Studies on the preparation and the photoelectrochemical properties of the nanoporous titania films attached with and without photosensitizer TCPP

Fabrication of TiO2 nanoporous films was carried out by anodic oxidation using two-step voltage mode while the subsequent attaching of the photosensitizer 5,10,15,20-tetra(4-carboxyphenyl) porphyrin (TCPP) on the prepared TiO2 nanoporous films was carried out by the electrochemical method. Photoelectrochemical properties of TiO 2 nanoporous films attached with and without photosensitizer TCPP were analysed by fluorescence spectroscopy and electrochemical test. Effects of process parameters on the photoelectrochemical properties of TiO2 nanoporous films were also investigated. The optimized process parameters for the preparation of TiO2 nanoporous films with the best photoelectrochemical property can be concluded as follows: oxidation potentials is 70–140 V, oxidation temperature is 25∘C, H2SO4 electrolyte concentration is 0.5 mol l−1 and oxidation time is 60 min. The results also show that attaching of the photosensitizer TCPP on the TiO2 nanoporous films can indeed improve the properties of TiO2 nanoporous films, and the optimized attaching temperature and attaching voltage are 25∘C and 60 V, respectively.

Enhanced Photoelectrocatalytic Water Splitting at Hierarchical Gd3+:TiO2 Nanostructures through Amplifying Light Reception and Surface States Passivation

The influence of rare earth gadolinium (Gd3+) ion doping on optical and photoelectrochemical properties of TiO2 is studied. The hierarchical clump-type TiO2 nanostructure was fabricated using poly-vinyl acetate as soft-template. The optical absorbance quantity of TiO2 was strikingly promoted at bandgap energy region (380 nm) by Gd3+ doping, as well as it extend a wide absorbance in visible wavelength region (400 – 800 nm) elucidating the sub-bandgap formation. As a result, Gd3+:TiO2 exhibits high photocurrent density than undoped TiO2 in photoelectrocatalytic experiments. Another plausible reason for enhancing the photocurrent density at Gd3+:TiO2 was analyzed through electrochemical impedance spectroscopy. The underlying mechanism of surface states controlled charge transfer at TiO2/electrolyte interfaces affected the photoelectrocatalytic hydrogen fuel generation, and compete with Gd3+ ion doping through bottlenecking of photoelectrons trapping at surface states. The improved charge separation (e−/h+) at Gd3+:TiO2 result effective photoelectron collection and thus yield 180 % higher hydrogen gas (∼ 2.34 mL.h−1.cm−2 ) generation compare to pristine TiO2 (1.28 mL.h−1.cm−2) under UV light irradiation. The improved optical and charge transfer characteristics of hierarchical TiO2 by Gd3+ ions can be implemented to wide range of other metal oxide based photocatalytic fuel generation.

CaCO3/TiO2 Nanoparticles Based Dye Sensitized Solar Cell

In this communication, the synthesis and structural, morphological, optical, and photo-electrochemical properties of TiO2 and CaCO3/TiO2 nanoparticles as well as their applications in dye sensitized solar cells (DSSCs), have been reported. In an X-ray diffraction pattern of CaCO3/TiO2 nanoparticles, the peak at 29.41° of CaCO3 has been detected, demonstrating its coating on the surface of TiO2, which is further verified using high resolution-transmission electron microscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The strong quenching in photoluminescence emission, in the case of CaCO3/TiO2 nanoparticles, has been attributed to the decrease in recombination rate of photo-generated electron–hole pairs. In the case of UV–visible reflectance spectra, the absorption edge for CaCO3/TiO2 nanoparticles has slightly been found to be blue-shifted as compared to bare TiO2 nanoparticles, which corresponds to an increase in energy band gap of the former. The dye desorption studies reveal that CaCO3/TiO2 electrodes adsorbed more dye than the bare TiO2 electrode. CaCO3/TiO2 based DSSC show improved photo-electrochemical properties compared to the bare TiO2 based DSSC as CaCO3 coating on TiO2 forms an energy barrier, and, consequently suppressing the charge recombination, and, thus, improving the overall energy conversion efficiency (η) from 0.46% to 1.44% under the illumination of simulated light of 100 mW/cm2.

Beneficial effect of Nb doping on the photoelectrochemical properties of TiO2 and TiO2-polyheptazine hybrids

Nb-doped TiO2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO2-polyheptazine hybrid photoanodes for water photooxidation under visible (λ>420nm) light. The optimum Nb-doping concentration was 0.1at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO2. Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials.

Photoelectrochemical properties of TiO2/CdS heterostructures

The photoelectrochemical properties of TiO2, CdS, and TiO2/CdS anodes have been studied. The results demonstrate that, under illumination, CdS anodes are subject to photocorrosion, and Cd2+ ions pass into solution. Corrosion-resistant films of TiO2 prevent CdS photocorrosion, and the CdS/TiO2 system exhibits good photosensitivity in the visible range.

Morphology and photoelectrochemical properties of TiO 2 electrodes prepared using functionalized plant oil binders

Chemically functionalized plant oils, viz. acrylated epoxidized soybean oil (AESO) and maleinized acrylated epoxidized soybean oil (MAESO), were used as bio-based binders for the TiO 2 electrodes of dye-sensitized solar cells (DSSC). The surface roughness and number of appropriate pores were increased in the TiO 2 films prepared using the plant oil binders in comparison with the film prepared using polyethylene glycol (PEG), due to the larger number of functionalities. The short circuit photocurrent ( I SC) and open circuit photovoltage ( V OC) were increased, and the conversion efficiency was significantly improved, in the cell using the plant oil binders.

Effect of Composite Modes on Photoelectrochemical Properties of MWCNTs/TiO<SUB>2</SUB> Nanocomposite Films

A series of nanocomposite of TiO2 thin films and multi-walled carbon nanotubes (MWCNTs) in different forms were prepared by sol-gel methods. The surface morphologies of the nanocomposite films were characterized via scanning electron microscopy (SEM). The effects of inducing MWCNTs into TiO2 thin film on the structure and absorption properties of TiO2 were characterized by X-ray diffraction (XRD) and UV-Vis spectrophotometer methods. The nanocomposite films were also characterized by photoelectrochemical methods. The results showed that the MWCNTs/TiO2 nanocomposite films were porous and rough and the absorption edge can extend to visible light region. The photoelectrochemical properties of TiO2 could be evidently enhanced by bottom distribution mode, or otherwise, decreased by surface distribution mode and uniform distribution mode. The effects of composite modes on the photoelectrochemical activities were discussed in terms of separation of charge carriers. MWCNTs distributed in bottom mode within the nanocomposite film could improve electron-collecting efficiency and reduce recombination.

Tuning the optical and photoelectrochemical properties of surface-modified TiO2

Surface-modification of TiO(2) is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO(2). High surface area nanocrystalline TiO(2) was modified by urea pyrolysis products at different temperatures between 300 degrees C and 500 degrees C. Modification occurs through incorporation of nitrogen species containing carbon into the surface structure of titania. The N1s XPS binding energies are 399-400 eV and decrease with increasing modification temperature whereby the Ti2p(3/2) peak is also shifted to lower binding energies by about 0.5 eV. With increasing modification temperature the optical bandgap of surface-modified TiO(2) continuously decreases down to approximately 2.1 eV and the quasi-Fermi level of electrons at pH 7 is gradually shifted from -0.6 V to -0.3 V vs. NHE. The surface-modified materials show enhanced sub-bandgap absorption (Urbach tail) and exhibit photocurrents in the visible down to 750 nm. The maximum incident photon-to-current efficiency (IPCE) was observed for the materials modified at 350 degrees C and 400 degrees C (IPCE approximately 14% at 400 nm, and IPCE approximately 1% at 550 nm, respectively). The efficiency of photocurrent generation is limited by surface recombination, which leads to a significant decrease in IPCE values and significantly changes the shape of the IPCE spectra in dependence on the optical bandgap.

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis

Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO 2 and TiO 2, were deposited sequentially by spray pyrolysis. TiO 2 films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s. The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO 2 films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region. Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained. Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes.

Effect of oxygen nonstoichiometry on photo-electrochemical properties of TiO 2− x

Polycrystalline TiO 2− x pellets were equilibrated at 1220–1420 K in a flow of Ar + 7 vol.% H 2 gas mixture. The resulting deviation x from stoichiometric composition of titanium dioxide was determined gravimetrically. X-ray diffractometry revealed coexistence of rutile with Ti n O 2 n−1 Magnéli phases. Optical reflectance and photocurrent spectroscopies served as experimental methods of the band gap determination. Polycrystalline TiO 2− x were used as photoanodes in a photoelectrochemical cell. The photocurrent response to the applied voltage was studied. It was found that TiO 2− x with x ca. 0.006 exhibited the best photoelectrochemical performance.

Effect of solution pH on the adsorption and photocatalytic reaction behaviors of dyes using TiO 2 and Nafion-coated TiO 2

The purposes of this research were to study the effects of solution pH on the photoelectrochemical properties of TiO 2 photoelectrode and the adsorption behaviors and photocatalytic reactions of two dyestuffs (Reactive Red 22, RR22 and Basic Red 2, BR2) in aqueous solution using various photocatalyst films (TiO 2 and Nafion-coated TiO 2, Nf/TiO 2). The open circuit voltage and the short circuit photocurrent of TiO 2 photoelectrode increased as solution pH decreased and indicated more photoelectrons were generated in acidic solutions. The response times reaching half maximum photovoltage and photocurrent were determined to be faster in acidic solutions than those in alkaline solutions, indicating higher electron diffusion occurred in acidic solutions. The coating of 0.5 g of Nafion per gram of TiO 2 was enough to reduce the zeta potential of Nf/TiO 2 to less than −20 mV in aqueous solution and exhibited a hydrophobic surface that might decrease the adsorption and photocatalytic decomposition of dye. Experimental results on the adsorption and photocatalytic decomposition of RR22 and BR2 indicating that the charges of TiO 2 surface and reactant dye markedly influence the reaction rate. The photocatalytic decompositions of RR22 using TiO 2 were favored to occur in acidic conditions and exhibited a minimum decomposition rate near the isoelectric point of TiO 2. Nevertheless, no obvious RR22 decompositions were found in experiments conducted using Nf/TiO 2. Decompositions of BR2 using both TiO 2 and Nf/TiO 2 were more favorable in alkaline conditions; however, decompositions of BR2 were found to be decreased in experiments conducted using Nf/TiO 2.

Photoelectrochemical application of nanotubular titania photoanode

Titania/titanium (TiO 2/Ti) electrodes with tailored surface structure have been fabricated by galvanostatic–potentiostatic anodization process. Highly ordered titania nanotubes array can be prepared by electrolyzing titanium foil at 20 V for 40 min in HF–H 3PO 4 electrolyte. Comparatively, micro-structured and crystallized TiO 2 multiporous film can be prepared at 20–40 V for 6 h in H 2SO 4–H 3PO 4–H 2O 2–HF electrolyte. The morphological characteristics and crystal behaviors of both nanotubular and micro-structured TiO 2 films are investigated by field emission scanning electron microscopy and X-ray diffraction measurement. Photoelectrochemical properties of TiO 2/Ti film electrodes are examined by anodic photocurrent response and cyclic voltammetry measurement. Photocatalytic and photoelectrocatalytic application are investigated by using either nanotubular TiO 2/Ti thin-film or micro-structured TiO 2/Ti thick-film electrodes as photoanodes for recalcitrant organic pollutant degradation.

Influence of Cr on photoelectrochemical properties of TiO 2 thin films

Both undoped and Cr-doped TiO 2 thin films were investigated. The dependencies of the flat band potential on pH, Cr concentration as well as thermal reduction were determined. On the basis of the photocurrent experiments, the effect of Cr concentration and high-temperature reduction on solar energy conversion efficiency was discussed. It was found that addition of Cr leads to an increase of the recombination process of photoelectronic carriers and shifts the flat band potential towards negative values. Linear changes of the flat band potential with pH of the electrolyte has been observed for both undoped and Cr-doped films. The maximum energy conversion efficiency (above 2%) has been observed for a voltage bias of ca. 0.1– 0.4 V . The highest energy conversion efficiency has been observed for undoped-TiO 2 thin films. It reaches values of 1.9% and 2.2% for as sputtered and reduced samples, respectively.