J. Phys. Chem. C 2007, 111, 13163-13169
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Surface Tailoring for Controlled Photoelectrochemical Properties: Effect of Patterned TiO2 Microarrays Da Chen,†,‡ Yanfang Gao,† Geng Wang,† Hao Zhang,† Wu Lu,† and Jinghong Li*,† Departments of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, and UniVersity of Science and Technology of China, Hefei, Anhui 230026, China ReceiVed: May 23, 2007; In Final Form: July 3, 2007
In efforts to make use of unprecedented physical and chemical characteristics of titania (TiO2) nanomaterials and investigate their innovative developments in microsystem applications, it is essential to affix them on surfaces or arrange them in an organized network. In this work, a simple, fast, and cheap patterning technique for the fabrication of patterned TiO2 microarrays with different features is presented. As an alternative to typical pattern transfer techniques for microfabrication, this work employed a standard microcontact printing (µCP) process for the fabrication of patterned titania microarrays onto F-doping SnO2 (FTO) conductive glass substrates. During the µCP process, the titania precursor was used as the “ink” and transferred from a pattern-featured poly(dimethylsiloxane) “stamp” onto the pretreated FTO substrate. Following the subsequent thermal oxidation, patterned TiO2 microarrays with different features (100, 200, and 400 µm) were successfully achieved. The surface properties and the photoelectrochemical properties of as-prepared patterned TiO2 microarrays were investigated by scanning electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, electrochemical impedance spectroscopy, Mott-Schottky spectroscopy, photocurrent action spectroscopy, and photocatalytic degradation. It was demonstrated that these properties were dependent on the feature size of the TiO2 patterns. For the patterned TiO2 thin film photoelectrodes with 100, 200, and 400 µm patterns, the generated peak photocurrent was ca. 5, 2, and 1 nA, and the photodegradation rate constant of methylene blue was found to be 1.747%, 1.415%, and 0.96% min-1, respectively. Clearly, with the decrease of the feature size, the photocurrent action and photocatalytic ability of the patterned TiO2 thin film increased, which was due to the increased TiO2 surface area as well as the increased optical path length within the patterned TiO2 thin film, resulting from multiple reflection of incident light. This work indicates that patterned TiO2 thin films are attractive systems for surface tailoring and also provide a novel method to effectively control the photoelectrochemical properties of nanostructured TiO2 thin films with promising applications in microsystem devices for solar energy conversion, photocatalysis, sensing, and so on.
Introduction Titania (TiO2) nanostructures have attracted extensive research interests due to their excellent physicochemical properties and photoinduced activities, which play an important role in applied fields such as photovoltaics, photocatalysis, and photoinduced superhydrophilicity.1-5 Usually, these physicochemical properties and photoinduced activities of TiO2 nanostructures depend on their crystal phase, specific surface areas, pore structures, and so on, which are mostly dependent on the TiO2 preparation procedures and conditions. In addition, over the past 20 years, studies of phenomena such as surface interactions and miniaturization of technical devices have created the need for spatially controlled chemical modification of surfaces on reduced scales. Thus, in efforts to make use of unprecedented physical and chemical characteristics of TiO2 nanomaterials and investigate their innovative developments in microelectronic, optoelectronic, and photonic microdevices, it may be necessary to affix them on surfaces or arrange them in an organized network. * To whom correspondence should be addressed. Phone: +86 10 6279 5290. Fax: +86 10 6279 5290. E-mail: jhli@ mail.tsinghua.edu.cn. † Tsinghua University. ‡ University of Science and Technology of China.
Through controlling the spatial regulation of discrete titania domains within an organized network, their surface properties as well as chemical properties would be accordingly well controlled. There have been many momentous breakthroughs in achieving one- to three-dimensional arrangements on the microscale or nanoscale, in light of the favored patterning approach.6-8 A number of techniques, such as template-assisted atomic layer deposition (ALD),9,10 femtosecond laser pulses,11 excimer laser irradiation,12 oxidation,13 top-down contact lithography,14 and liquid-phase deposition (LPD),15,16 have been successfully developed to fabricate patterned TiO2 thin films on various substrates. For example, Passinger et al.11 demonstrated a direct 3D patterning of TiO2 structures using a photosensitive solgel-based spin-coatable TiO2 resist by means of femtosecond laser pulses. Using aqueous hydrogen peroxide solution as an oxidant, Zuruzi et al.13 reported the formation of a patterned nanostructured TiO2 layer from the prepatterned Ti film. Francioso et al.14 fabricated a TiO2 nanowire array on a silica mesa using 365 nm UV lithography and fluorine plasma etching, while Yang et al.15 fabricated positive and negative TiO2 micropatterns on a wettability-patterned polymer surface using the LPD technique. Although the aforementioned techniques
10.1021/jp074003a CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
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Chen et al. TiO2. On the basis of this developed technique, patterned TiO2 thin films with different features (100, 200, and 400 µm) were easily and simply fabricated, and their surface properties and photoelectrochemical properties were subsequently investigated. It was demonstrated that the surface properties as well as photoelectrochemical properties could be effectively regulated by tailoring the feature of patterned TiO2 thin films. Hence, the patterning technique described in this paper represents a simple, speedy, cheap, and practical route for fabricating patterned TiO2 microarrays with different features, which can be used to effectively regulate the surface properties as well as photoelectrochemical properties of TiO2 thin films. This work is expected to provide new strategies for the fabrication and applications of titania-based microsystem devices with controlled photoelectrochemical properties. Experimental Section
Figure 1. Optical images of patterned TiO2 microarrays on an FTO conductive glass surface with different features: (a) 100 µm, (b) 200 µm, (c) 400 µm.
are satisfactory for some applications, these techniques are far from optimum and still require specific facilities and several complicated fabrication steps to complete the patterning. Thus, it is still a big challenge to devise an efficient approach for fabricating patterned TiO2 thin films. Furthermore, despite the great body of work about the patterned TiO2 thin films until now, papers devoted solely to the systematic investigation of their photoelectrochemical properties have seldom been published. In the present work, we report a simple direct patterning technique for the fabrication of a patterned TiO2 thin film onto the F-doping SnO2 (FTO) surface by combining the microcontact printing (µCP) process with a thermal oxidation process. The technique involves microcontact printing of the titania precursor directly onto a pretreated FTO substrate using a pattern-featured poly(dimethylsiloxane) (PDMS) stamp, followed by the thermal oxidation of the titania precursor to obtain
Materials and Reagents. Silicon wafers, PDMS prepolymer (RTV615, components A and B), and photoresist (SU8) were purchased from Silicon Co. (Tianjin, China), GE Silicone (Wilton, CT), and MicroChem (Newton, MA), respectively. Unless otherwise specified, all other reagents and materials involved were obtained commercially from the Beijing Chemical Reagent Plant (Beijing, China) and used as received without further purification. Ultrapure water (resistivity g 18 MΩ cm) was used during the experimental process. The experiments were carried out at room temperature and humidity. Fabrication of TiO2 Microarray Patterns. Micropattern arrays were fabricated on silicon wafers using standard photolithographic techniques.17,18 From a silicon master, complementary PDMS replicas were prepared and used as stamps in subsequent microcontact printing steps for the fabrication of patterned TiO2 microarrays on FTO conductive glass. Prior to the patterning, the FTO glass substrates (1 cm × 2.5 cm, 15 Ω/square) were pretreated by being immersed in a mixture of 1 g of potassium hydroxide, 60 mL of ethanol, and 40 mL of water with ultrasonication for ca. 2 min, followed by being rinsed three times in pure water with ultrasonication for 1 min and dried with N2. After this pretreatment, the FTO glass substrates were completely hydrophilic and anionic.19 A titania precursor solution (1:10 (v/v) mixture of tetrabutyl titanate (TBT, (CH3(CH2)3O)4Ti) and ethanol) was used to ink the PDMS stamps. After evaporation of the solvent, the PDMS stamp was briefly dried and then brought into contact with the pretreated FTO substrate for 2 min at room temperature. The stamped titania precursor was bound to the negatively charged OH groups of the pretreated FTO substrate surface, resulting in a transferred layer. Following the subsequent calcination procedure at 450 °C for 2 h in the air, the patterned TiO2 electrode was obtained. Moreover, the patterned TiO2 electrodes with different features could be produced by using different PDMS stamps with feature sizes of 100, 200, and 400 µm, respectively. Characterization. A DP30BW microscope digital camera (Olympus) was used to capture the optical images. Scanning electron microscopy (SEM) images were acquired on a field emission (FE) scanning electron microscope (JEOL JSM-6700F) operated at 1.0 kV. Powder X-ray diffraction (XRD) was performed on a Bruker D8-Advance X-ray powder diffractometer with monochromatized Cu KR radiation (λ ) 1.5418 Å). The 2θ range used in the measurements was from 20° to 70°. UV-vis absorption spectra using the diffuse reflection method were obtained with a Cary-500 UV-vis spectrophotometer (Varian). Electrochemical impedance spectroscopy (EIS) and
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Figure 2. FE-SEM images of square-patterned TiO2 microarrays on the FTO glass surface with different features: 100 µm (a), 200 µm (b), 400 µm (c). Magnified image of the 100 µm patterned TiO2 microarrays (d), corresponding enlarged micrographs of the square-patterned TiO2 boundary (e) (inset: profile image), and EDS spectra (f).
Mott-Schottky (MS) spectroscopy were carried out on a PARSTAT 2273 potentiostat/galvanostat (Advanced Measurement
Technology Inc.) by using one three-electrode cell with the patterned TiO2 photoanode as the working electrode, a platinum
13166 J. Phys. Chem. C, Vol. 111, No. 35, 2007 wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. Impedance measurements were performed in the presence of a 2.5 mM K3[Fe(CN)6]/ K4[Fe(CN)6] (1:1) mixture as a redox probe in 0.1 M KCl aqueous solution. The results were presented as Nyquist plots, which were recorded potentiostatically by applying an ac voltage of 5 mV amplitude in the 100 kHz to 0.01 Hz frequency range. The MS tests were performed in 0.1 M KCl aqueous solution. Photocurrent Measurements. Photocurrent action spectra were measured in a two-electrode configuration home-built experimental system, where the patterned TiO2 photoanode served as the working electrode and a platinum wire was used as the counter electrode.20 A 500 W Xe lamp with a monochromator was used as the light source. The photoelectrochemical cell was illuminated from the FTO side of the TiO2 electrode by incident light. The generated photocurrent signal was collected by using a lock-in amplifier (Stanford SR830 DSP instrument) synchronized with a light chopper (Stanford SR540 instrument). The monochromatic illuminating light intensity was about 15 µW/cm2, estimated with a radiometer (Photoelectronic Instrument Co. IPAS). The illumination area of the TiO2 electrode was about 0.12 cm2. All measurements were done after N2 was bubbled for 20 min and controlled automatically by a computer. Photocatalytic Measurements. Aqueous suspensions of methylene blue (MB) (1 × 10-5 M) and the patterned TiO2 electrodes were placed in a 3 mL quartz-glass vessel. The photoreaction vessel was exposed to UV-vis irradiation under ambient conditions with an average intensity of 35 mW cm-2 produced by a 100 W high-pressure mercury lamp, which was positioned 12 cm away from the vessel. At given time intervals, the photoreacted solution was analyzed with a UV-vis spectrophotometer by recording variations of the absorption band maximum (660 nm) in the UV-vis spectrum of MB. Results and Discussion TiO2 Microarray Patterns. µCP seems to be a general method for forming patterns of a variety of materials on surfaces of solid substrates through contact pattern transfer using an elastomeric PDMS stamp.17,21-24 Herein, this process was adapted to enable the stamping of an FTO substrate surface with a titania precursor (TBT) for the fabrication of a microarraypatterned TiO2 electrode. A TBT/ethanol mixture (1:10, v/v) was used as an “ink” for the microcontact printing of the FTO surface with a PDMS stamp. After 2 min of stamp time on the pretreated FTO surface at room temperature, a transferred layer of titania precursor on the FTO surface came into being. The patterned TiO2 electrode was subsequently obtained with the calcination procedure at 450 °C for 2 h in the air. The optical images (shown in Figure 1) portray typical patterned TiO2 electrodes with different features, which show that the microscopic structures of the TiO2 layer consisted of dense square microarrays with different features (100, 200, and 400 µm). SEM images (Figure 2a-c) further indicated a consistently high quality of the pattern geometry and definition. The µCP process seems to work well since the squared microarrays were well defined, with sharp edges demarcating the boundaries between the patterned TiO2 region and the FTO background region. The dark ring at the pattern border (Figure 2d) was probably due to a topographical effect. The enlarged micrographs of the square-patterned TiO2 boundary (Figure 2e) clearly show that a strong adhesive nature and clear patterning of the TiO2 thin film were achieved on the FTO surfaces, and
Chen et al.
Figure 3. XRD pattern of the TiO2 film on the FTO conducting glass surface: solid squares, anatase TiO2; hollow squares, FTO substrate.
the thickness of the patterned TiO2 film was estimated from the SEM profile image to be ca. 600 nm. It was proposed that the reaction between a Ti sol-gel monomer and OH-terminated self-assembled monolayer (SAM) occurred via a ligand exchange reaction of the SAM’s OH sites with the Ti sol-gel monomer.25-27 In our case, the same mechanism might apply, because OH groups exist on the pretreated FTO surfaces. Corresponding energy-dispersive spectroscopy (EDS) during SEM measurement was used to quantitatively monitor the surface composition that would provide optimum chemical contrast (in terms of TiO2 patterns) and high surface cleanliness. Two different regions of the patterned surface (one on the TiO2 pattern and another on the FTO background) were typically recorded (Figure 2f). It should be noted that two main metal elements (Ti and Sn) in the spectra were only taken into account for stoichiometries, while another two metal elements (Au and Fe) were excluded from stoichiometries since they were induced during the course of the SEM measurement. As expected, the TiO2 region and the FTO region showed chemical compositions with different levels of titanium: approximately 78 atom % for the former and approximately 3 atom % for the latter. These results (including optical and SEM measurements) reveal that patterned TiO2 thin films could be obtained by using the µCP process, and the pattern feature could be conveniently adjusted by using different PDMS stamps for surface tailoring. The crystallization character of patterned TiO2 was analyzed by small-incidence XRD spectroscopy (Figure 3). It can be seen that all the peaks of patterned TiO2 could be assigned to the anatase phase [JCPDS 73-1764], and other peaks could be indexed to SnO2 [JCPDS 46-1088] from the FTO substrate. This result indicates that pure anatase crystalline TiO2 was obtained after the thermal oxidation of the patterned titania precursor on the FTO substrate. It is well-known that the titania thin film has a very strong absorption in the ultraviolet range.15 The patterned TiO2 thin films with different features on the FTO surface should have different absorbance intensities due to their different ordered arrangements of discrete TiO2 domains. Figure 4 shows the UV-vis spectra of an FTO substrate (a control sample) and three as-patterned TiO2 thin films with different features (100, 200, and 400 µm) on FTO substrates. As expected, the patterned TiO2 thin films with different features had different absorptions in the ultraviolet range, and the absorbance intensity increased with the increase of the density of discrete TiO2 domains. This enhancement in the absorbance intensities could be attributable to the following two reasons: (i) the surface area of TiO2
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Figure 4. UV-vis absorbance spectra of different square-patterned TiO2 microarrays on FTO conducting glass surfaces: (a) FTO (control sample), (b) 400 µm square pattern, (c) 200 µm square pattern, (d) 100 µm square pattern.
increases with an increase of the density of TiO2 domains, which leads to higher absorption; (ii) the total optical path length within the patterned TiO2 thin film would also increase with an increase of the density of TiO2 domains, which in turn enhances UVvis absorption. These results indicate that the feature size of the TiO2 patterns can effectively affect the UV-vis absorption performance of the patterned TiO2 thin films. EIS and MS Studies. It is well-known that EIS is an effective tool for probing the features of surface-modified electrodes. In EIS, the semicircle diameter equals the electron-transfer resistance (Ret), which controls the electron-transfer kinetics of the redox probe at the electrode interface.28-30 Thus, the insulating modifier on the electrode is expected to retard the interfacial electron-transfer kinetics and to increase the electron-transfer resistance. The electron-transfer resistance at the electrode can be given by eq 1, where R0 and Rmod are the constant electron-
Ret ) R0 + Rmod
(1)
transfer resistance of the unmodified electrode and the variable electron-transfer resistance introduced by the modifier, respectively. In the present case, to a much greater extent, Ret reflects the restricted diffusion of the redox probe through the TiO2 thin film and relates directly to the accessibility of the underlying electrode or the film permeability.31,32 Figure 5A shows the typical EIS spectra for patterned TiO2 thin film electrodes with different features in aqueous 0.1 M KCl solution containing the redox probe of [Fe(CN)6]3-/4-. It is reasonable to believe that only [Fe(CN)6]3-/4- reacted at the FTO electrode surface at the formal potential of redox couples. Significant changes in the impedance spectra are observed for the different patterned TiO2 thin film electrodes. The impedance spectra follow the theoretical shapes and include a semicircle portion, observed at higher frequencies, that corresponds to the kinetic control of the charge-transfer process, followed by a linear part characteristic of the lower frequency attributable to the diffusion process. For a bare FTO electrode surface, the impedance spectra exhibit a small semicircle line probably due to the resistance of the thin oxidized layer on the glass substrate, which can be assigned to R0 in eq 1. After the deposition of the patterned TiO2 thin film, the diameters of the semicircle parts increased significantly. This increase in diameter indicates that the charge-transfer rate of [Fe(CN)6]3-/4- was reduced, which
Figure 5. (A) EIS spectra of patterned TiO2 thin film electrodes with different structures: FTO (0), 100 µm patterned TiO2 film (4), 200 µm patterned TiO2 film (1), 400 µm patterned TiO2 film (]). Inset: enlarged EIS data of the FTO substrate. (B) MS plots of the synthetic TiO2 thin film electrode with no pattern (0), a 100 µm pattern (O), a 200 µm pattern (4), and a 400 µm pattern (]). The MS plots were obtained at a frequency of 1 kHz in an aqueous solution of KCl (0.1 M).
was attributed to the hindrance effect to the redox couples, caused by the deposition of the TiO2 insulating thin film on the electrode surface. That is to say, the Ret value increased significantly due to the introduction of Rmod, which came from the modifier (the patterned TiO2 insulating thin film). Because of the same FTO substrate and the supporting electrolyte, the Ret value is supposed to be varied with the Rmod value, which could be determined from the deposited TiO2 thin film. With a decrease of the feature size of the TiO2 pattern, the surface area of the patterned TiO2 increased, which in turn led to an increase of the ohmic resistance of the patterned TiO2 thin film. This was demonstrated by the EIS results in Figure 5A, which showed that Ret increased with a decrease of the feature size (i.e., an increase of the density of discrete TiO2 domains). These results indicate that the electron-transfer resistances of patterned TiO2 thin films were dependent on the feature size of the TiO2 patterns. To understand the difference in the electronic properties of the patterned TiO2 thin film electrodes, MS measurements were performed by using the impedance technique.20,33,34 Figure 5B shows the MS plots of the TiO2 thin film electrodes with different features. Reversed sigmoidal plots were observed with
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Figure 7. Absorption changes (λ ) 660 nm) for the photocatalytic degradation of MB over TiO2 electrodes with different patterns: no catalyst (0), 100 µm square patterns (4), 200 µm square patterns (3), 400 µm square patterns (]). C/C0 is the normalized concentration of the solution.
Figure 6. (A) Photocurrent action spectra of patterned TiO2 thin film electrodes with different features: (a) 100 µm square patterns, (b) 200 µm square patterns, (c) 400 µm square patterns. (B) Photocurrent response of patterned TiO2 thin film electrodes with different features under the on-off illumination at 345 nm: (a) 100 µm square patterns, (c) 200 µm square patterns, (d) 400 µm square patterns.
an overall shape consistent with that typical for n-type semiconductors, and the reproducible flat-band potentials, that is, the potentials corresponding to the situation in which there is no charge accumulation in the semiconductor so that the energy bands show no bending, could be obtained from the x intercepts of the linear region. Compared to the TiO2 thin film electrode with no pattern, the three patterned TiO2 thin film electrodes showed a positive shift of the conduction band. It is well-known that the presence of a large number of surface states can lead to a considerable change of the band position.20,35 Also, similar surface states might exist in the three patterned TiO2 thin film electrodes with 100, 200, and 400 µm features, since these samples have similar apparent flat-band potentials. In addition, the slopes of the linear region for all the TiO2 thin film electrodes were approximately identical, clearly indicating the same donor density for all the TiO2 thin film electrodes. Photocurrent Actions. Since TiO2 is the most commonly used electrode material in photoelectrochemical cells and optoelectronics, for understanding the photoelectrochemical properties of the synthesized patterned TiO2 thin films, photocurrent measurements have been performed. The photocurrent action spectra of the patterned TiO2 thin films with different features (100, 200, and 400 µm) are shown in Figure 6A. As
shown, all three patterned TiO2 thin film electrodes with different features showed photocurrent spectra with the same maximum wavelength at 345 nm corresponding to the band gap of nanocrystalline TiO2, which was blue-shifted from the band gap of bulk TiO2 (387 nm, 3.2 eV) due to the quantum confinement effect.36,37 Moreover, the photocurrent decreased with an increase of the feature size of the TiO2 patterns. To further investigate the photoinduced behavior of the generated photocurrent, the photocurrent responses of patterned TiO2 thin film electrodes illuminated with the intensity of the monochromatic incident light (λ ) 345 nm) are shown in Figure 6B. When the light was regularly switched on and off, a series of almost identical electric signals could be obtained. For the patterned TiO2 thin films with different features (100, 200, and 400 µm), the generated photocurrents were ca. 5, 2, and 1 nA, respectively. The above results indicate that photocurrent generation depended on the feature size of the TiO2 patterns. The peak photocurrent of the patterned TiO2 thin film electrodes undergoes a gradual increase with a decrease of the feature size. This enhancement not only is due to an increase of the surface area of the patterned TiO2, but also may be due to photonic enhancement or an increased optical path length within the patterned TiO2 thin film with a smaller feature size, resulting from multiple reflection of incident light,35 which might lead to enhanced photocurrent generation. Photocatalytic Activities. The photocatalytic activities of asprepared TiO2 thin film electrodes with different features were measured with photocatalytic degradation of active MB as a model reaction. The MB dye initially showed a major absorption band at 660 nm, while a gradual decrease in absorption was observed with light irradiation through an aqueous MB solution containing the TiO2 thin film electrode. During the degradation process, the dye was photodegraded in a stepwise manner, with the color of the solution changing from an initial deep blue to nearly transparent. Figure 7 shows the temporal concentration changes of MB over the patterned TiO2 thin film electrodes with 100, 200, and 400 µm features. The normalized concentration of the solution (C/C0) is proportional to the normalized maximum absorbance (A/A0), and therefore, we used C/C0 instead of A/A0. As shown, the photodegradation of MB catalyzed by the TiO2 thin film electrodes follows a first-order rate law, -[ln(C/C0)] ) Kt, where K is the apparent rate constant of the degradation. In the present work, K was found to be
Controlled Photoelectrochemical Properties 1.747%, 1.415%, and 0.96% min-1 for the patterned TiO2 thin film electrode with 100, 200, and 400 µm patterns, respectively. Clearly, the photoactivities increased with a decrease of the feature size of the TiO2 patterns. This enhancement in photoactivity performance was, on one hand, due to an increase of the TiO2 surface area, which can enhance the adsorption ability of the catalytic surface toward target molecules and the ability of generating photoinduced electron-hole pairs of active sites39-41 and, on the other hand, due to an increased optical path length or photonic enhancement within the patterned TiO2 thin film electrode with a smaller feature size. Conclusions Patterned TiO2 thin films with pure anatase phase on FTO substrates were successfully fabricated via µCP in this work. To the best of our knowledge, this is the first time that the inorganic precursor TBT was directly used as an “ink” for the microcontact printing of a TiO2 precursor on the FTO substrate with a pattern-featured PDMS stamp, which is very similar to the polymer-on-polymer stamping process. Following the subsequent thermal oxidation process (450 °C, 2 h), patterned TiO2 with pure anatase was achieved. Compared to the conventional patterning techniques, this is a very promising method for fabrication of patterned TiO2 thin films with different features, which opens up the possibility of forming twodimensional high-featured inorganic thin films on large-area substrates using a simple, speedy, and low-cost process in an atmospheric environment. In addition, the photoelectrochemical behaviors of the patterned TiO2 thin films with different features were systematically investigated. It was demonstrated that the photocurrent actions and the photocatalytic activities depended on the feature size of the TiO2 patterns. The generated photocurrent and the photocatalytic activities increased with a decrease of the pattern size of the patterned TiO2 thin film. This enhancement was due to the increased TiO2 surface area and also due to the increased optical path length within the patterned TiO2 thin film, resulting from multiple reflection of incident light. These results demonstrate that patterned TiO2 thin films are attractive systems for the effective control of photoelectrochemical properties, which can be easily achieved by regulating the feature size of the TiO2 patterns. Therefore, this study is of importance for developing a novel method to control the photoelectrochemical properties of semiconductor materials with promising applications in microsystem devices for solar energy conversion, photocatalysis, sensing, and so on. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 20435010 and 20675044) and 863 Project (Grant 2006AA05Z123). References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2002, 106, 2967.
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