Materials Science and Engineering B 151 (2008) 133–139 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Photocatalytic activity of pulsed laser deposited TiO2 thin films H. Lin a,c , Abdul K. Rumaiz b , Meghan Schulz c , Demin Wang a , Reza Rock d , C.P. Huang a , S. Ismat Shah b,c,∗ a Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA Department of Astronomy and Physics, University of Delaware, Newark, DE 19716, USA c Department of Material Science Engineering, University of Delaware, Newark, DE 19716, USA d Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA b a r t i c l e i n f o Article history: Received 16 December 2007 Received in revised form 20 May 2008 Accepted 22 May 2008 Keywords: Pulsed laser deposition Titania Thin films Photocatalysis a b s t r a c t Nanostructured TiO2 thin films were prepared by pulsed laser deposition (PLD) on indium-doped tin oxide (ITO) substrates. Results from X-ray photoelectron spectroscopy (XPS) show that Ti 2p core level peaks shift toward the lower binding energy with decrease in the buffer gas pressure (O2 :Ar = 1:1). This suggests that oxygen vacancies are created under insufficient oxygen conditions. Anatase-to-rutile ratio is also found to be system pressure dependent. Under deposition pressure of 100 Pa, only anatase phase was observed even at 1073 K substrate temperature which is much higher that the bulk anatase-to-rutile phase transformation temperature. The deposited TiO2 thin films were fabricated as photoanodes for photoelectrochemical (PEC) studies. PEC measurements on TiO2 photoanodes show that the flatband potential (Vfb ) increases by 0.088 eV on absolute vacuum energy scale (AVS) with decrease in the deposition pressure from 100 to 33 Pa at 873 K. The highest incident photon to current conversion efficiency [IPCE()] of 2.5 to 6% at  = 320 nm was obtained from the thin films prepared at substrate temperature of 873 K. Combining the results from XPS and PEC studies, we conclude that the deposition pressure affects the concentration of the oxygen vacancies which changes the electronic structure of the TiO2 . With reference to photoelectrochemical catalytic performance, our results suggest that it is possible to adjust the Fermi energy level and structure of TiO2 thin films by controlling the buffer gas pressure and temperature to align the energy of the flatband potential (Vfb ) with respect to specific redox species in the electrolyte. © 2008 Published by Elsevier B.V. 1. Introduction Titanium dioxide (TiO2 ) has been proven to be an effective material for applications such as photocatalysis [1,2], dye sensitized solar cells [3,4], heterogeneous catalysis [2,5], self-cleaning/antifogging surface coatings [6], etc. Typically, TiO2 is used in two main forms: powder and thin film. The powder form of crystalline TiO2 is commonly used for gas and liquid phase catalysis. Its photocatalytic activity is normally determined by the particle size [2], phase composition [7], and the position of the conduction and valance bands in the energy scale [8]. In the thin film form, TiO2 is usually used for photon harvesting in photovoltaic applications such as photoelectrochemical cells (PECs) and dye sensitized solar cells (DSSCs) [4,9]. Moreover, thin films offer the additional advantage of being able to optimize the alignment between the energy positions of the valance band edge and the redox species, by applying a potential bias. This helps to optimize the quantum efficiency. ∗ Corresponding author at: Department of Material Science Engineering, University of Delaware, Newark, DE 19716, USA. E-mail address: ismat@UDel.Edu (S.I. Shah). 0921-5107/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.mseb.2008.05.016 Several TiO2 thin film deposition techniques have been reported which include metalorganic chemical vapor deposition (MOCVD) [10,11], sol–gel [12], electrophoretic deposition [13], reactive rf sputtering [14], and pulsed laser deposition (PLD) [15–21]. Among available techniques, PLD is a high-energy process which provides a well adherent thin film with good mechanical rigidity [22] and surfaces with high specific surface area [19]. In addition, PLD also offers advantages such as stoichiometrically transferring material from target to substrate [22], capability of inert and reactive gas deposition, wide range of operational pressure and temperature, and variety in options for substrate materials. TiO2 thin film prepared by pulsed laser deposition has been studied by various research groups [9,15–17,20,21,23,24]. However, different type of targets (i.e. Ti and TiO2 ), variation in substrate materials, wide range of operation pressures (i.e. from ultra high vacuum to 80 Pa), and the differences in synthesis temperature (i.e. from room temperature to 1273 K) make it difficult to compare and understand the differences in properties of the thin films in a consistent manner. The anatase and rutile multi-phase structures in TiO2 thin films were observed by several groups [9,15,21]. Only Luca et al. [21] have addressed the increase in anatase phase composition with increase in oxygen partial pressure. However, the possible 134 H. Lin et al. / Materials Science and Engineering B 151 (2008) 133–139 Fig. 1. Schematic diagram of the pulsed KrF gas excimer laser PLD system. mechanism was not proposed. The distinctive difference of PLD process from chemical-based synthesis (i.e. sol–gel and MOCVD) is that multi-valance Ti species are usually created during the PLD deposition process. This is unlikely to occur in chemical synthesis methods due to the presence of molecular oxygen in both the precursor and the synthesis environment [25]. Even with a TiO2 target, Kitazawa [26] observed the presence of neutral and ionized Ti and TiO species, based on the optical measurement of the laser ablation plume. The reduced TiO2 surface (oxygen vacancy) is of particular interest for its photocatalysis applications such as dehydration of formic acid and dissociation of molecular water [27–29]. Very few studies have discussed the relationship between the valance states of Ti and the deposition condition during PLD synthesis of TiOx thin films [21]. Luca et al. [21] reported that O:Ti ratio ranges from 1.78 to 2.0 at different combinations of temperatures and pressures. They also reported that TiO2 and its suboxides (i.e. TiO and Ti2 O3 ) co-exist when deposition was performed at 423 K. Pulsed laser deposition of TiO2 is a simple process yet involves a complicated physical phenomenon. Variation in deposition parameters such as pressure and temperature results in different chemical and structural compositions [15,18,21]. Among the reports on pure TiO2 thin films prepared by PLD methods, only few have tested the photocatalytic activities of the films [19,30]. In this study we prepared TiO2 thin films by PLD at substrate temperatures of 873 and 1073 K to avoid the formation of amorphous TiO2 [18]. Buffer gas (O2 :Ar = 1:1) pressure, from 33 to 100 Pa, was used to prevent the formation of suboxide (i.e. TiO or Ti2 O3 ) [21]. By operating the PLD system within this temperature and pressure regime, we were able to prepare TiO2 films with compositions ranging from slightly reduced to stoichiometric with a resulting variation in the rutile/anatase phase composition. For photovoltaic applications, a conductive substrate is required. Quartz substrate coated with indium-doped tin oxide (ITO), which is one of the most widely used transparent conductive oxides (TCOs), were used as the substrates in our study. The surface topography, crystalline structure, binding energy of Ti, quantum efficiency spectra, and photocurrent onset of TiO2 /ITO thin films prepared under different synthesis conditions of temperature and pressure are reported in this paper. 2. Experimental procedure system base pressure at 1.3 × 10−7 Pa. Buffer gas of 50:50 by volume Ar:O2 mixture was used for the reactive PLD process. Two 500 W halogen lamps were used as irradiative heating source to control the substrate temperature from room temperature up to 1073 K. The rotating target disc was composed of rutile-phased TiO2 , prepared by sintering pure TiO2 powder (Aldrich) at 1273 K. Target rotation speed was kept at 15 rpm. The incident laser beam maintained a 45◦ angle to the target surface. Laser pulse frequency was set at 15 Hz with a calculated laser beam fluence of 1.8 J/cm2 . The deposition rate of TiO2 thin films was about 0.09 Å per laser pulse. ITO-coated quartz with a sheet resistance of 10 ± 1 / (SPI supplies Inc., PA, USA) was used as the substrate for deposition. Prior to the deposition, all ITO-coated quartz substrates were cleaned ultrasonically in pure acetone solution and triple rinsed with deionized water (18.0 M). Three series of TiO2 thin films were deposited corresponding to substrate temperatures of (1) room temperature, (2) 873 K, and (3) 1073 K. The temperatures 873 and 1073 K correspond to below and above the anatase-to-rutile phase transition temperature (∼973 K), respectively [31,32]. Depositions were carried out at buffer gas pressures of 33 and 100 Pa for each substrate temperature. 2.2. Film characterization The surface topography of TiO2 thin films was observed by using atomic force microscopy (AFM), which was equipped with a piezoelectric tube scanner. Images were taken in contact mode with a scan rate of 1.11 Hz (J-scanner, multimode AFM/SPM, Veeco). The crystalline structure of TiO2 thin films was determined by using Rigaku D-Max B diffractometer which was equipped with a graphite crystal monochromator. –2 scans were recorded using Cu Ka radiation of wavelength 1.5405 Å from 20◦ to 80◦ with a step size of 0.05◦ . The oxidation state of Ti was determined by X-ray photoelectron spectroscopy (XPS). An SSI-M probe XPS was used employing Al Ka (h = 1486.6 eV) excitation source. High resolution XPS spectra were collected at 26 eV pass energy with a dwell time of 100 ms per point. Sample peak positions were referenced to the C 1s peak at 284.6 eV. The instrument was calibrated with respect to the Au 4f7/2 peak; however, when studying semiconducting samples, the C 1s spectrum provides a better reference of peak shift due to the charging effect. This is due to the fact that carbon impurities experience a similar potential to the sample material, in contrast to Au. 2.3. Photocurrent measurement Deposited TiO2 /ITO thin films were used to fabricate photoanodes for the PEC studies. The photoelectrochemical system was equipped with a three-electrode potentiostat (model AFRDE 4, Pine Instrument Inc., USA), a custom made PEC system with a fused silica window, and a monochromatic excitation source (Model RF-5301, Shimadzu, Japan). Platinum wire was used as the counter electrode. A saturated calomel electrode (SCE) was selected as the reference electrode. Sweeping voltage was within the range of ± 0.6 V (vs. SCE). Potassium iodide [KI = 0.05 M (pH∼9)] was used as the electrolyte for photocurrent measurements. In order to calculate incident photon to current conversion efficiency [IPCE(l)] as a function of wavelength (), photocurrent was recorded under single wavelength irradiation ranging from 300 to 500 nm. 2.1. Film preparation 3. Results and discussion The schematic diagram of the experimental setup is shown in Fig. 1. A KrF excimer pulsed laser system (wavelength  = 248 nm) was used for the deposition of TiO2 thin films. A turbo molecular pump and a mechanical pump were employed in series to maintain Fig. 2 shows AFM images of the TiO2 thin films deposited at substrate temperatures of 873 and 1073 K, and deposition pressures of 33 and 100 Pa. From the topographic images it can be seen that, H. Lin et al. / Materials Science and Engineering B 151 (2008) 133–139 135 Fig. 2. AFM images of the TiO2 thin films deposited at temperature of 873 and 1073 K under system pressure of 33 and 100 Pa. (a) 873 K, 33 Pa; (b) 873 K, 100 Pa; (c) 1073 K, 33 Pa and (d) 1073 K, 100 Pa. for the same pressure, the topography of the films deposited at 873 K appears to be more uniform than the topography of the sample deposited at 1073 K. At 873 K, the section analysis shows that RMS roughness values are 8.4 and 11.2 nm for thin films deposited under 33 and 100 Pa, respectively. In contrast, the depositions at 1073 K have much higher RMS roughness values: 47.2 and 50.6 nm for depositions performed at 33 and 100 Pa, respectively. There is a very small pressure dependence of the roughness but temperature certainly changes the topography drastically. A possible explanation for this observation is that surface mobility of the adatoms is higher at higher temperature (1073 K) which results in higher surface diffusion length, island separation, and lateral size. When island separation length is greater than the lateral size of the island, terrace and stairs topography are normally favored. On the other hand, surface mobility of the adatoms are lower at lower temperature (873 K), thus islands are more closely spaced. If island separation is smaller than the island lateral size, more uniform growth of the thin film is preferred [33,34]. The Ti core level electron binding energy for the TiO2 thin films deposited at different temperatures (T = 873 and 1073 K) and buffer gas pressures (P = 33 and 100 Pa) are compared in Fig. 3. At 873 K, the binding energy of Ti 2p3/2 increases from 457.9 to 458.2 eV when system pressure is increased from 33 to 100 Pa (Fig. 3a). The binding energy of Ti 2p3/2 has been reported to be in the range of 458.5–458.9 eV and 456.8–456.9 eV for stoichiometric TiO2 [35,36] and Ti2 O3 [36,37], respectively. Thus, the binding energies 136 H. Lin et al. / Materials Science and Engineering B 151 (2008) 133–139 1073 K, respectively. As it can be seen, at both temperatures, the anatase and rutile phases co-exist in both the 33 and 67 Pa samples. When deposition pressure increased to 100 Pa, there was no clear rutile R(1 1 0) peak from samples deposited at either temper- Fig. 3. High resolution XPS spectra of Ti 2p region. of Ti 2p3/2 in these two samples are much closer to that reported value for TiO2 than for Ti2 O3 . The 0.3 eV increase in binding energy from 457.9 to 458.2 and relatively larger peak separation (∼5.8 eV) between Ti 2p1/2 and Ti 2p3/2 for films deposited at 250 mTorr suggest that higher Ti valance states are created at higher oxygen partial pressures. The same trend was observed for samples prepared at 1073 K (Fig. 3b). The binding energy of Ti 2p3/2 increases from 458.1 to 458.5 eV with the increase in system pressure from 33 to 100 Pa. Note that the binding energy of Ti 2p3/2 at 458.5 eV is within the reported binding energy range for stoichiometric TiO2 . In our previous work at very low oxygen partial pressure (0.003 Pa), we have seen a shoulder in the Ti 2p3/2 peaks related to the formation of vacancies [38]. In the current case, the concentration of vacancies is not high enough to register an effect on the XPS spectra. From repeated XPS measurements, it is consistently shown that samples prepared at 33 Pa have a 0.3–0.4 eV lower binding energy on Ti 2p3/2 for both deposition temperatures. Our results suggest that at the same temperature, higher concentration of oxygen vacancy or less stoichiometric TiO2 is created at lower pressures, which is likely due to insufficient availability of O species under such conditions. Fig. 4(a) and (b) shows the high resolution XRD scans (2 = 22–30◦ ) of the prepared TiO2 thin films deposited at 873 and Fig. 4. XRD results of as-deposited TiO2 thin films. (a) High resolution scan of A(1 0 1) and R(0 1 1) region at substrate temperature of 873 K, (b) high resolution scan of A(1 0 1) and R(1 1 0) region at substrate temperature of 1073 K, and (c) anatase weight percentage and its corresponding operation pressure at different temperatures. H. Lin et al. / Materials Science and Engineering B 151 (2008) 133–139 137 ature. The anatase weight fraction (Fig. 4c) is computed, based on Spurr’s formula (Eq. (1)) [21,39] where the IA and IR are the integrated intensity of anatase A(1 0 1) and rutile R(1 1 0) peaks from the XRD pattern, respectively. WA = IA IA + 1.265IR (1) At constant temperature, our results show that the anatase fraction increases with buffer gas pressure. A possible cause could be the differences in kinetic energy of adatoms due to the changes in deposition pressure, which subsequently contributes to the difference in crystalline growth. In a pulsed laser deposition system, the kinetic energy of the deposited species is greatly affected by the background pressure. The increase in background gas pressure has several effects: (1) fluorescence increases due to the increased collision rate on the leading edge of the expansion; (2) the plume boundary can be slowed by collisions with background gas; (3) materials propagate through the background gas are attenuated dramatically [40]. Based on the blast-wave model, the velocity of the depositing species decreases by half when the O2 pressure increases from 10 to 80 Pa [9]. From the shock and drag models, it has been shown that increasing background pressure not only greatly reduces the velocity of the depositing materials, but also the velocity distribution of the ablated species [40]. The relationship between the speed of charged species and background gas pressure were experimentally examined by ion flux measurements during deposition under different pressures [40,41]. The calculated packing factors for rutile and anatase unit cells are 0.486 and 0.445, respectively. We suspect that titanium species deposited under lower background gas pressure carry higher kinetic energy, which form the denser and thermodynamically stable structure: rutile. In contrast, the kinetic energy of ablated materials is lower under high pressures; therefore, a less dense and metastable structured anatase is favored. Similar trends were also reported by Luca et al. [21] and Kitazawa et al. [18] except under different temperature, pressure, and substrate conditions. Our results show that even when the thin film was prepared at temperatures higher than the generally accepted anatase-to-rutile phase transformation temperature (973 K), the anatase phase is still stable. When a semiconductor electrode is in contact with a solution containing certain redox species, charge carriers will migrate until equilibrium of chemical potentials in both phases at the solid/liquid junction is reached (i.e. Fermi energies of the solids and the redox species in the electrolyte). A space charge region is normally formed near the surface of a semiconductor; “band bending” occurs due to the internal electric field created. The degree of this band bending can be manipulated by the applied potential bias. The flatband potential is the potential at which net current is zero due to the Fermi energy of the solids equaling that of the redox species in the electrolyte. To link the energy between the semiconductor and the electrolyte, the flatband potential (Vfb ) can be expressed as the following equation [8,42]: Vfb (NHE) = A + 1EF + VH + E0 , (2) where A is the electron affinity (energy difference between acceptor states and vacuum level), 1EF is the energy difference between the Fermi level and its majority carrier band edge (i.e. conduction band for n-semiconductor and valance band for p-semiconductor), VH is the potential drop across the Helmholtz layer, and E0 is the scale factor relating the normal hydrogen electrode (NHE) scale to the absolute vacuum scale (AVS) (i.e. −4.5 V for NHE). The flatband potential depends on several parameters, such as crystalline structure [43] and crystal plane [44,45] of the semiconductor, the adsorbed ion species [46], impurity concentration [47], surface states [48], etc. In addition to the Mott–Schottky plots methods Fig. 5. Tafel plots of TiO2 thin films prepared under different buffer gas pressure and temperature. The applied voltage is converted with reference to the standard normal hydrogen electrode (NHE) and the absolute vacuum scale (AVS). [43], the flatband potential of a semiconductor photoelectrode can also be determined based on photocurrent onset potential (Eonset ) [46,49]. In our study, we presume that the main factor affecting the Vfb is the variation in electronic properties of the TiO2 thin films since all experiments were performed under identical conditions. Based on linear scan voltammetry, the Tafel plots obtained from TiO2 thin films deposited at different temperatures and buffer gas pressures are shown in Fig. 5. The photocurrent onset corresponds to the peak on the left-hand side of each curve. To reference the applied potential, we plot the voltage scale in terms of the normal hydrogen electrode (NHE) potential and the absolute vacuum scale (AVS) with respect to log |J| (J is current flux under irradiation, mA/cm2 ). It can be seen that at 873 K, the flatband potentials (Vfb ) of TiO2 electrodes are −0.205 and −0.117 V (vs. NHE) for thin films prepared under 33 and 100 Pa buffer gas pressures, respectively. That is, the flatband potential (Vfb ) is increased by 0.088 eV in absolute vacuum scale when deposition pressure decreased from 100 to 33 Pa. We have earlier described the formation of reduced TiO2 under certain experimental conditions due to the creation of oxygen vacancies during deposition. The valance band mainly has O-derived 2p states separated from the empty Ti-derived 3d states by a bulk bandgap of 3.2 eV [50,51]. By removing one O2− ion from TiO2 lattice, two electrons are freed and they will potentially occupy the Ti 3d orbitals to form a more localized states within the bandgap [52]. This has been predicted in several DFT calculations [51,53] and also observed experimentally [38]. Thus when the Ti-derived conduction band becomes occupied upon reduction, it has been shown that the O 2p valence band moves away from the Fermi level [38]. The easiest way to interpret this would be to assume a rigid band model where the reduction moves the Fermi level into the conduction band of TiO2 . Therefore, it is reasonable to see higher Vfb for more reduced TiO2 due to reasons mentioned earlier. For samples prepared at 1073 K, the Vfb is positioned at +0.022 and +0.011 V (vs. NHE) for thin films prepared under 33 and 100 Pa buffer gas pressures, respectively. Although the Vfb trend at this temperature is reversed compared to 873 K prepared samples, the 0.011 eV difference is too small to surpass the experimental error. At both substrate temperatures, XPS results show 0.3–0.4 eV variation in binding energy of Ti 2p3/2 core level when varying the deposition pressure between 33 and 100 Pa. However, the difference in flatband potential (Vfb ) is only pronounced for thin films prepared at the substrate temperature of 873 K. Since XPS is a surface-sensitive technique, we suspect that the binding energy difference between samples prepared under 33 and 100 Pa at 873 K is greater in the 138 H. Lin et al. / Materials Science and Engineering B 151 (2008) 133–139 bulk than at the surfaces. In other word, our XPS results might have under-estimated the binding energy difference due to possible oxidation of the reduced TiO2 surface during the sample transfer process. To examine the photocatalytic efficiency of prepared TiO2 thin films, the incident photon conversion efficiency [IPCE(l)] test was performed. The [IPCE(l)] is defined as the number of electrons transferred per incident photon (Eq. (3)) [14]: IPCE (%) = Jsc () = eIinc ()  1240Jsc (A/cm2 ) e (nm) Iinc (W/cm2 )  × 100 (3) where the Jsc () is short-circuit photocurrent density under monochromatic light (A/cm2 ), e is the electron charge (Coulomb), Iinc () is the incident photon flux (W/cm2 ). This method provides more information than optical absorption, due to the fact that surface morphology and mass transfer limitation effect are not fully taken into account in the latter method. The [IPCE()] is a quantum efficiency profile measurement of a particular semiconductor photoelectrode as a function of incident wavelength. At a certain photon energy/wavelength, the obtained quantum efficiency represents the energy differences between the local electronic states and the conduction band minimum. If there is no significant change in energy position of the conduction band minimum, the edge of the [IPCE()] spectrum is qualitatively proportional to the density of states for the valance band maximum. Therefore, the onset of the [IPCE()] spectrum is nearly the same as the wavelength of bandgap threshold (bg ) of a particular semiconductor. Fig. 6a shows the [IPCE()] spectra of TiO2 thin films prepared at T = 873 K. The samples prepared at 33 and 100 Pa differ slightly on both [IPCE()] and optical transmittance spectra. The former has [IPCE()] onsets at  = 400–425 (E ∼ 3.0 ± 0.1 eV); the latter has a lower value,  = 375–400 nm (E ∼ 3.2 ± 0.1 eV). These numbers are very close to the commonly reported bandgap values for anatase (∼3.2 eV) and rutile (∼3.0 eV) [54] and also agree with the dominant phases indicated in Fig. 4. Thus, we conclude that the differences in [IPCE()] are mainly contributed by the variation in crystalline structure; rutile and anatase are the predominant phases at 33 and 100 Pa deposition pressure, respectively. At T = 1073 K (Fig. 6b), a more distinct difference in [IPCE()] between the TiO2 thin films prepared at 33 and 100 Pa can be seen. The [IPCE()] onset is identical to that at T = 873 K for films prepared under the same pressure. Again, the differences are due to the change in predominant crystalline phase. At both temperatures, the [IPCE()] spectra of 33 and 100 Pa samples intersect at wavelength ∼370 nm. The TiO2 thin films prepared at 33 Pa (mainly rutile) have higher photon conversion efficiency when incident wavelengths are above 370 nm, and those prepared at 100 Pa (mainly anatase) have greater photon conversion efficiency when incident wavelength is below 370 nm. This is very consistent with the experimental results reported by Karakitsou and Verykios [54]. They proved that anatase has significantly higher light absorption capacity than rutile based on diffuse reflectance measurement using the Schuster–Kubelka–Munk equation. It is worth noting that the films prepared at 873 K have much higher photon conversion efficiencies throughout the entire spectrum (300–500 nm) than those prepared at 1073 K. The highest photon conversion efficiencies obtained are in the range of 5–5.5% and 0.4–1.8% (at  = 320 nm) for films prepared at 873 and 1073 K, respectively. A possible reason is that TiO2 thin films deposited at 1073 K are likely to be more densely deposited, providing limited reactive surface area. In contrast, the films prepared at lower temperatures (T = 873 K) are likely to have relatively more porous structures, which yields more reactive surface area. Fig. 6. Incident photon to current conversion efficiency [IPCE()] between wavelength of 300–500 nm. Inserts are the corresponding transmittance measurements obtained by double beam UV–vis spectroscopy. 4. Conclusion Nanostructured TiO2 thin films were deposited on ITO substrates by PLD under different temperature and pressure conditions (T = 873 and 1073 K; P = 33 and 100 Pa). AFM results show that samples prepared at 873 K have much more uniform surfaces and smaller particle size than that prepared at 1073 K. Our XPS results indicate that the binding energy of the Ti 2p core level is system pressure dependent, which suggests that oxygen vacancies can be created depending on experimental conditions. At 33 and 67 Pa, it was found that both anatase and rutile phases co-exist regardless of the deposition temperature. However, under 100 Pa, only anatase phase was observed even at the temperature higher than the commonly reported anatase-to-rutile phase transition range (∼973 K). Based on photoelectrochemical studies, we found that the flatband potential of TiO2 thin films is increased in absolute vacuum energy scale with decrease in operational pressure at deposition temperature of 873 K. Combining the results from XPS, we conclude that the shift in Vfb is induced by oxygen vacancies, which contribute to the formation of reduced TiO2 . The incident photon to current conversion efficiency [IPCE()] was observed to be much higher for thin films deposited at T = 873 K than T = 1073 K. 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