Scanning Probe Microscopic Characterization of Surface-Modified n

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Langmuir 1998, 14, 3405-3410

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Scanning Probe Microscopic Characterization of Surface-Modified n-TiO2 Single-Crystal Electrodes Takeshi Miki and Hisao Yanagi* Faculty of Engineering, Kobe University, Rokkodai, Nada, Kobe 657, Japan Received October 7, 1997. In Final Form: February 5, 1998 Photoelectrochemical and photocatalytic properties of surface-modified rutile (001) surfaces of n-type titanium dioxide (n-TiO2) were investigated by photoassisted scanning probe microscopy. The n-TiO2 surface was modified by lapping and chemical etching which caused linear scratches and pyramidal etch pits on the surface, respectively. The photoelectrochemical efficiency of water oxidation was reduced by donorlike surface states produced on defect sites of the modified surfaces. Scanning photocurrent images of the chemically etched surface, taken with a conductive cantilever controlled by the atomic force feedback under UV illumination, revealed that these surface defects were localized at the bottom of irregularly etched steplike pits, whereas the regular faces of pyramid-shaped pits enhanced the photocurrents via the surface states formed in the band gap of n-TiO2. In situ atomic force microscopy indicated that photocatalytic deposition of platinum clusters in UV-illuminated solution preferably occurred on the scratches of the lapped n-TiO2 surface.

Introduction Titanium dioxide (TiO2) has widely attracted interests in its photoactive potentials such as photocleavage of water1 and photocatalytic decomposition of organics.2-4 In recent years, antibacterial and self-cleaning abilities of TiO2 photocatalysts have been practically used in our living environments,5,6 while photoelectrochemical cells using dye-sensitized TiO2 photoelectrodes have been expected as one promising material for solar energy conversion.7,8 The continuous conversion from ultraviolet (UV) light to currents and chemical reactions is based on a remarkable stability of TiO2. Photoactivity of TiO2 depends on its bulk and surface semiconducting properties.9 Among three crystal phases of TiO2,10,11 photocatalytic activity has been extensively studied for anatase particles while a number of photoelectrochemical experiments with single-crystal TiO2 electrodes have been carried out using rutile. Since intrinsic TiO2 crystals have high electrical resistance due to its wide band-gap energy (e.g., 3.0 eV for rutile),12 they are doped by reduction to have n-type semiconduction for electrochemical applications.13-15 To prepare single-crystal TiO2 electrodes, the surface is then etched to remove contamination and damage caused by polishing and doping.16 (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (3) Kiwi, J.; Gra¨tzel, M. J. Phys. Chem. 1984, 88, 1302. (4) Wong, C.-M.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114, 5230. (5) Hashimoto, K.; Fujishima, A. Shokubai 1994, 36, 524. (6) Hashimoto, K.; Fujishima, A. Chem. Today 1996, 8, 23. (7) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (8) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6383. (9) Finklea, H. O., Ed. Semiconductor Electrodes; Elsevier: Amsterdam, 1988. (10) Horn, M.; Schwerdtfeger, C. F.; Meagher, E. P. Z. Kristallografiya 1972, 136, 273. (11) Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1971, 55, 3206. (12) Cronemeyer, D. C. Phys. Rev. 1952, 87, 876. (13) Noufi, R. N.; Kohl, P. A.; Frank, S. N.; Bard, A. J. J. Electrochem. Soc. 1978, 125, 246. (14) Wilson, R. H.; Harris, L. A.; Gerstner, M. E. J. Electrochem. Soc. 1979, 126, 844. (15) Serwicka, E.; Schindler, R. N.; Schumacher, R. Ber. BunsenGes. Phys. Chem. 1981, 85, 192.

The earlier studies17,18 have revealed that the surface structures and energetics modified by those preparatory processes play an important role in the photocatalytic and photoelectrochemical abilities of TiO2. Recent progress in scanning probe microscopy (SPM) has promoted a better understanding between topographic structures and electronic states.19,20 Fan and Bard21 imaged the (001) surface of an n-type rutile (n-TiO2) with the atomic scale resolution when the sample was biased at negative voltage, using scanning tunneling microscopy (STM). Their tunneling spectroscopy investigations revealed that the currentvoltage characteristics corresponded to electron tunneling from the valence band of n-TiO2 at a negative bias and into the conduction band at a positive bias. Furthermore, Fan and Bard22 extended their STM experiments to study under illumination using n-type tungsten diselenide (nWSe2) crystals. These photoassisted STM images revealed that the photocurrent at defect edges on the n-WSe2 surface were smaller than that on defect-free smooth regions, and they concluded that the defect sites acted as recombination centers. Thus, the photoassisted STM technique is very useful to elucidate important questions concerning spatial electronic structure and local photoactivity of semiconductor electrodes which are affected by their surface nature. One difficulty in these STM measurements is simultaneous imaging of the topography and photocurrent since the site-dependent electronic characteristics and photoconductivity have an influence on the tunneling current by which the sample-tip distance is controlled with the feedback loop. Therefore, the photocurrent image by Fan and Bard22 was recorded by interrupting the feedback loop at each sampling point in topographic imaging. An alternative technique to image such simultaneous surface properties is atomic force microscopy (16) Dare-Edwards, M. P.; Hamnett, A. J. Electroanal. Chem. 1979, 105, 283. (17) Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 303. (18) Henrich, V. E. Appl. Surf. Sci. 1979, 9, 143. (19) Gilbert, S. E.; Kennedy, J. H. J. Electrochem. Soc. 1988, 135, 2385. (20) Itaya, K.; Tomita, E. Chem. Lett. 1989, 285. (21) Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1990, 94, 3761. (22) Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1993, 97, 1431.

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Figure 1. AFM images of (a) untreated, (b) lapped, and (c) etched (001) surfaces of n-TiO2. The x-y-axis is scaled in micrometers.

(AFM) using a conductive cantilever. Since in AFM the sample-cantilever distance is controlled by the atomic force feedback loop, a current image can be simultaneously taken independent of topography. Furthermore, the AFM in solution allows us to observe changes in surface topography caused by photoelectrochemical and photocatalitic reactions in situ. In this study, from these points of view, we used the AFM techniques to characterize local photoactivities of TiO2 single-crystal electrodes. To clarify the relationship between the surface structures and site-dependent properties, the (001) face of n-TiO2 was modified by lapping and etching processes. One-directional lapping caused linear scratches, while chemical etching produced etch pits on the surface. Their morphology and photoelectric properties were simultaneously observed by AFM using a conductive cantilever under UV illumination in air. Photocatalytic activity of the modified n-TiO2 surface was investigated by photodeposition of Pt. A site-dependent heterogeneous growth of photodeposited Pt clusters was examined by in situ AFM in an aqueous solution. Experimental Section Surface Modification of a TiO2 Single Crystal. The (001) polished face (10 × 10 × 1.5 mm3) of a single-crystal TiO2 with the rutile phase, purchased from Earth Chemical Co., Ltd., was provided for surface modification and photoelectrochemical measurements. The TiO2 crystal was doped by reduction in hydrogen flow at 550 °C for 2 h.13,14 The doped n-TiO2 surface was modified by lapping or chemical-etching processes. The (001) face was lapped with a 1 µm diamond slurry while keeping the n-TiO2 crystal fixed in a certain position against a rotating table using a Musashino Denshi MS-2 polishing machine. Chemical etching was carried out by heating the doped n-TiO2 crystals in concentrated sulfuric acid (H2SO4) at 200 °C for 1 h. Surface topography and photoelectronic properties of these modified n-TiO2 (001) faces were examined by SPM using a Seiko SPI3700 scanning probe microscope. A commercially available conductive Si cantilever coated with Au was used for imaging current signals. An ohmic electrical contact between the backside of the n-TiO2 crystal and a sample stage on a piezo scanner was made by painting eutectic In-Ga. Bias voltages were applied to the n-TiO2 crystal against the conductive cantilever. Topographic and current images in the dark as well as under UV illumination in air were simultaneously recorded under the AFM feedback control. UV light was obliquely illuminated onto the specimen surface by setting an optical bundle fiber, which was connected to a 250-W superhigh-pressure Hg lamp (USHIO ML251B/A), to a window on the AFM head.

Photoelectrochemical Measurements. The surface-modified n-TiO2 electrodes were characterized by photoelectrochemical measurements using a three-electrode cell with a coiled Pt counter and an Ag/AgCl (saturated KCl) reference electrodes according to described procedures.23 Photodecomposition of water on the modified n-TiO2 (001) surfaces was examined in an electrolyte solution of 0.5 M Na2SO4, 0.25 M phosphate buffer (pH ) 7.0). An active area of the n-TiO2 electrode surface in contact with the electrolyte solution was 0.5 cm2. Photocurrent-voltage curves were recorded under UV illumination using the above-mentioned superhigh-pressure Hg lamp at the light intensity on the electrode surface of 20 mW/cm2. Photodeposition of Pt Clusters. Photocatalytic reactivity of the modified n-TiO2 surface was examined by in situ AFM observation of photodeposition of Pt in solution using the abovementioned SPM instrument. The lapped n-TiO2 (001) face was provided for observation by attaching it to the bottom of a Teflon cell. This cell was mounted on a piezo scanner and then filled with an aqueous solution of 10 mM H2PtCl6. A commercially available Si3N4 cantilever was mounted on a quartz window holder attached to an AFM head. The cantilever was immersed in the solution and approached to the n-TiO2 surface. A laser alignment on the cantilever was carried out through the quartz window in contact with the solution. Under topographic observations by the AFM feedback control photodeposition reactions were started by illuminating the specimen surface by UV light according to the above-mentioned setting. The photodeposition of Pt on the n-TiO2 surface was also examined by X-ray photoelectron spectroscopy (XPS) using a KRATOS EXAM800 with an Mg KR radiation (hν ) 1254 eV). A Pt plate rubbed with rutile powder (Aldrich) was used as a reference for binding energies.

Results and Discussion Characterization of Modified n-TiO2 (001) Surfaces. Doping of TiO2 crystals under hydrogen reduction resulted in n-type semiconducting TiO2 (n-TiO2) with transparent, blue-black color.24 The (001) surface of the doped n-TiO2 crystal (referred to as untreated n-TiO2, hereafter) is structureless and fairly smooth, having a small corrugation of several angstroms, as shown in an AFM image in Figure 1a. This n-TiO2 (001) surface was modified by lapping and etching. The lapping procedure in which the sample surface was fixed with respect to the rotational polishing table resulted in fine scratches (23) Yanagi, H.; Chen, S.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R.; Fujishima, A. J. Phys. Chem. 1996, 100, 5447. (24) Hippel, A.; Kalnajs, J.; Westphal, W. B. J. Phys. Chem. Solids. 1962, 23, 779.

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Figure 3. Schematic diagrams for energy bands of surfacemodified n-TiO2 surfaces under a photooxidation process relaxed by surface states at anodic potentials. Ef is Fermi energy and Ec and Ev are conduction and valence band edges, respectively. Figure 2. Photocurrent-voltage characteristics for n-TiO2 electrodes having (a) untreated, (b) lapped, and (c) etched (001) surfaces measured under UV illumination in an electrolyte of 0.5 M Na2SO4, 0.25 M phosphate buffer (pH ) 7.0).

aligning in one direction. As shown in Figure 1b, the depth of the linear scratches are 20 nm at the largest. When the n-TiO2 crystal was chemically etched by hot H2SO4, etch pits appeared on the (001) surface. A typical topography shown in Figure 1c (more clearly in Figure 4a) indicates that the etch pit has a pyramidal shape with the size of 1 µm at the largest. This morphology of etch pits coincides with those produced on the (001) face of undoped rutile.25 From the scanning direction with respect to the n-TiO2 crystal, the square contour of the etch pits is known to align in parallel to its [110] directions. Line profiles of the pit measured by AFM show that four internal planes of the pyramid makes average angles of 38°, 43°, 48°, and 52°, respectively, against the basal (001) plane of rutile. Although it requires further detailed measurements to define these internal planes, two of these angles are comparable to those calculated for the (111) face (42.3°) and the plane of the TiO6 octahedral (36.8°). Photoelectrochemical properties of these untreated, lapped, and etched (001) surfaces of n-TiO2 electrodes were compared by experiments of the photodecomposition of water. Figure 2 shows their photocurrent-voltage curves in 0.5 M Na2SO4 (pH ) 7.0) under UV illumination. All the electrodes exhibit typical characteristics for n-type semiconductors having flat-band potential of ca. -0.1 V versus Ag/AgCl, which is consistent with the reported values.6 However, the photocurrents at anodic potentials are higher for the untreated surface and are decreased by the lapping and etching treatments. These photoelectrochemical behaviors are explained in terms of energy band diagrams of the modified n-TiO2 surfaces as schematically shown in Figure 3. The n-TiO2 crystals doped by hydrogen reduction have donor levels below its conduction band. When the n-TiO2 surface is in contact with the electrolyte solution, space charges are formed at the vicinity of the interface to build an upward band bending. Under illumination the electrons excited in the conduction band flow along the built-in potential and the holes in the valence band oxidize water that results in anodic photocurrents.6,26 When the n-TiO2 surface is modified by lapping or etching, surface states would be created at an energy within the band gap as shown in the figure. Our previous photoelectron spectroscopic study on n-TiO2 surfaces etched by ion sputtering23 suggests that these surface states are donorlike caused by surface (25) Hirthe, W. M.; Brittain, J. O. J. Am. Ceram. Soc. 1962, 45, 546. (26) Desplet, J.-L. J. Appl. Phys. 1976, 47, 5102.

defects. From this analogy, the lapping and etching treatments probably produce oxygen defects at the damaged sites that result in unstoichiometric oxidation states of Ti2+, Ti3+. In the presence of these surface states the electrons from the occupied donor state recombine with valence band holes, so that the photocurrents are reduced. Scanning Probe Microscopy of Etched n-TiO2 Surface. Semiconducting characteristics of the surfacemodified n-TiO2 were further investigated by SPM using a conductive cantilever under application of bias voltages and UV illumination in air. Figure 4 shows SPM images of the chemically etched n-TiO2 (001) surface. The topographic image taken under the atomic force feedback control by the contact mode clearly indicates a pyramidal morphology of the etch pit (Figure 4a). Simultaneously, scanning current images were recorded at various bias voltages in the dark and UV illumination. Figure 4b shows a dark-current image taken at -1.5 V that is applied to the n-TiO2 crystal with respect to the Au-coated tip. There is little difference in the dark currents between on the (001) face and on the pyramidal pit, except that the current contour at the crossing ridges traces the pyramidal shape of the pit. Such a morphological correspondence of the pyramidal nature between the topographic and current images of the etch pit suggests that an artifact caused by a tip effect is unlikely to distort the scanning images in the present observations. At this negative voltage scanning photocurrent images taken under UV illumination also indicated no remarkable difference between on the (001) face and on the pyramidal etch pit. When the dark current image is taken at a positive bias of +1.5 V, no difference still appeared between the inside and outside of the pit except that the currents turned to positive. By contrast, photocurrent images exhibited a remarkable sitedependence at positive voltages. Figure 4c shows a representative photocurrent image taken at a bias voltage of 2 V. It indicates that the photocurrents selectively flow on the etch pit whereas the (001) face is less photoactive. These SPM behaviors of the etched n-TiO2 (001) surface are explained in terms of charge transfer at a metal/ insulator/semiconductor (MIS) junction reported for photoassisted scanning tunneling microscopy by Fan and Bard.21,22 Figure 5 shows energy band schemes for the n-TiO2/gap/Au MIS models. At negative forward bias voltages, the electrons flow from the conduction band of the n-TiO2 to the tip as shown in Figure 5a. Since the major electron carriers contribute to the currents, there should be little difference in the currents between in the dark and under illumination. At the forward bias, furthermore, the holes created in the valence band under illumination diffuse into the bulk. Therefore, carrier recombination is predominant in the bulk, and site-

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Figure 4. SPM images of an etch pit on the (001) surface of n-TiO2. (a) Topographic image; (b) scanning current image in the dark at a bias voltage of -1.5 V; (c) scanning current image under UV illumination at a bias voltage of 2.0 V. The x-y-axis is scaled in micrometers.

Figure 5. Energy-band diagrams of n-TiO2/gap/Au MIS models for modified n-TiO2 surfaces under illumination at (a) negative and (b) positive bias voltages.

dependent phenomena on the surface would not be remarkable. At positive bias voltages, on the other hand, the tunneling current flows from the n-TiO2 surface to the Au-coated cantilever (i.e., the electrons from the tip to the sample). In the dark, the electrons cannot flow into the n-TiO2 surface as long as the conduction band is located above the Fermi level (Ef) of Au of the tip, since the concentration of holes in the valence band of the wide band-gap n-TiO2 is very small in the dark. Under illumination, however, the holes created in the valence band are accumulated at the surface. When the sample is positively biased in the way that the surface state formed in the band gap is overlapped to the Ef of Au, the electrons from the tip can tunnelingly flow into the surface states and then fall in the valence band, as shown in Figure 5b. Therefore, photocurrents are observed selectively on the etch pit even if the conduction band is located above the Ef of Au. Fan and Bard21 pointed out that the conductance peak at positive biases in their tunneling spectroscopy were associated with ionized surface donors which were caused by the lower oxygen coordination. Our result (Figure 4c) suggests that these effective surface states are localized at the side faces of regularly etched pyramidal pits. It is noted that the photocurrent image in Figure 4c exhibits a dark spot at the bottom-right part of the etch pit. These dark spots were often observed from irregularly etched pits under illumination, as shown in Figure 6. The photocurrent image at a negative voltage of -1.5 V (Figure

6a) indicates square steplike contours of the etch pits, but no crossing ridges tracing the pyramidal shape are observed. At a positive bias of 1.0 V, steplike contrasts at the etch pits become clear (Figure 6b). At positive biases between 2.0 and 4.0 V, dark spots due to low photocurrents appeared on the bottom of the etch pits, as shown in Figure 6c-e, respectively. At a higher voltage of 5.0 V, then, the dark spot again becomes faint as shown in Figure 6f. Fan and Bard22 observed a similar image on the exposed edges of the layertype n-WSe2 surface using photoassisted STM, and they ascribed this lower photocurrent contrast to recombination centers for charge carriers.27 From the bias voltage dependencies of the photocurrent images in Figure 6, we assume that these dark spots at the bottom of the pits are associated with such recombination centers. At negative biases (Figure 6a) major carrier electrons flow from the conduction band of n-TiO2 to the tip, while electrons are directly injected from the tip to the conduction band when the sample is highly biased at positive voltages (Figure 6f). In these two cases, therefore, current images are less affected by the presence of the recombination centers on the surface. On the other hand, when the Ef of the tip locate below the conduction band of n-TiO2 at positive biases (as in the cases of Figure 6c-e), photoexcited carriers are probably quenched by the recombination centers existing on defects of the irregularly etched pits. The observation of these surface defect states explains the low photocurrent efficiency of the etched n-TiO2 electrodes (Figure 2c). Photodeposition of Pt on Lapped n-TiO2 Surface. Site-dependent photocatalytic reactivity was examined by means of photodeposition of Pt clusters on the lapped n-TiO2 (001) surface. As is shown in Figure 3, photoexcited TiO2 surfaces provide both holes and electrons having oxidizing and reducing powers, respectively. From extensive studies concerning site selectivity of the heterogeneous reactions on n-type semiconductors,28-31 it has been generally recognized that illuminated parts of the semiconductor surfaces serve as oxidation sites, while their (27) Ahmed, S. M.; Gerischer, H. Electrochim. Acta 1972, 76, 3460. (28) Yoneyama, H.; Toyoguch, Y.; Tamura, H. J. Phys. Chem. 1972, 76, 3460. (29) Mo¨llers, F.; Tolle, T. J.; Memming, R. J. Electrochem. Soc. 1974, 121, 1160. (30) Wrighton, M. S.; Wolczanski, P. T.; Ellis, A. B. J. Solid State Chem. 1977, 22, 17. (31) Reich, H.; Dunn, W. W.; Bard, A. J. J. Phys. Chem. 1979, 83, 2248.

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Figure 6. Scanning current images of etch pits on the (001) surface of n-TiO2 under UV illumination at a bias voltages of (a) -1.5, (b) 1.0, (c) 2.0, (d) 3.0, (e) 4.0, and (f) 5.0 V. The x-y-axis is scaled in micrometers.

dark parts serve as reduction sites. Kobayashi et al.32 reported that the reductive deposition of Pt selectively occurred on defect sites irrespective of whether they were illuminated or not, using the rutile surface damaged by lapping. The present study by in situ AFM observations in the reaction solution demonstrates such selective photodeposition behaviors. Figure 7 parts a and b shows in situ AFM images of the lapped n-TiO2 (001) surface in an aqueous solution of 10 mM H2PtCl6 after UV illumination for 90 and 210 min, respectively. In contrast with the lapped surface before deposition (Figure 1b), the photodeposited surface is inhomogeneously covered with small clusters of Pt. It seems that the Pt clusters are preferably deposited on the scratched lines (as marked in Figure 7a) in the initial stage of the reaction, and then they grow along the damaged structure (as marked in Figure 7b). Chemical interactions of the photodeposited Pt clusters with the n-TiO2 (001) surface was examined by XPS measurements as shown in Figure 8. The reference sample (Figure 8a), a Pt plate rubbed with rutile powder, indicates photoemission peaks of the Pt 4f7/2 and Pt 4f5/2 core levels at binding energies of 70.7 and 74.0 eV, respectively, which are close to those values for bulk Pt metal. By contrast, the photodeposited Pt clusters (Figure 8b) show a different spectrum having pronounced peaks (32) Kobayashi, T.; Taniguchi, Y.; Yoneyama, H.; Tamura, H. J. Phys. Chem. 1983, 87, 768.

at the high binding energy side. Such chemical shifts of the Pt 4f core levels have been reported for small Pt particles evaporated on the SiO2 (110) surface and are interpreted by the size effect of the particles having a diameter smaller than 1 nm.33 This is not likely for the present case because the photodeposited Pt clusters grow larger, as seen in Figure 7. Recently, Schierbaum et al.34 reported that chemical interactions were formed between Pt and TiO2 when Pt was vapor-deposited on the TiO2 (110) surface prereduced by an Ar+ bombardment. On the prereduced surface, a localized electronic charge transfer was found from Ti3+ states to deposited Pt atoms. However, they observed no shift of the Pt 4f core level binding energies because this charge transfer would not change the total number of free electrons within the clusters. As mentioned before, the lapping treatment produces oxygen defects that involve unstoichiometric oxidation states of Ti3+ similar to those in the case of Ar+sputtered surfaces.23,34 In the present photodeposition process, these surface states probably act as reduction sites. Under UV illumination the excited electrons reduce the Pt(IV) complex cations selectively at these defect sites, and then Pt atoms are formed on the Ti sites with the lower oxygen coordination. The shift of the Pt 4f core levels to higher binding energies observed in Figure 8b (33) Parmigiani, F.; Kay, E.; Bagus, P.; Nelin, C. J. J. Electron Spectrosc. Relat. Phenom. 1985, 36, 257. (34) Schierbaum, K. D.; Fischer, S.; Torquemada, M. C.; Segovia, J. L.; Roma´n, E.; Martı´n-Gago, J. A. Surf. Sci. 1996, 345, 261.

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Figure 7. In situ AFM images of the photodeposition process of Pt clusters on a lapped n-TiO2 surface in 10 mM H2PtCl6 aqueous solution taken after UV illumination for (a) 90 min and (b) 210 min. The arrows indicate Pt clusters selectively deposited on lapped scratches. The x-y-axis is scaled in micrometers.

Conclusions Surface modifications by lapping and chemical etching affected photoelectrochemical and photocatalytic properties of the n-TiO2 (001) surface. Lapped scratches and etch pits act as defect sites having donorlike surface states. In the photoelectrochemical reactions, the efficiency of photooxidation processes is reduced by these surface states, since they quench holes photogenerated in the valence band of n-TiO2. The photoassisted SPM of the chemically etched n-TiO2 surface revealed that these defects are localized on the bottom of irregularly etched steplike pits, whereas the regular faces of pyramidal pits enhance the photocurrents at positive bias voltages. Photoreduction processes preferably occur at the defect sites where photoexcited electrons in the conduction band of n-TiO2 flow via the surface states and reduce redox species in an electrolyte. The in situ AFM and XPS observations suggested, therefore, that the photoreductive deposition of Pt probably nucleate at the lapped scratches, and chemical interactions are formed between the Pt clusters and Ti cations at the defect sites. Figure 8. X-ray photoelectron spectra of (a) a Pt plate rubbed with rutile powder as reference and (b) phodeposited Pt clusters on a (001) surface of n-TiO2.

suggest the presence of Pt(I) or Pt(II) species. We assume that these oxidized Pt species are formed by the chemical interaction between the defect sites of n-TiO2 and the Pt(IV) ions under the photoreductive deposition.

Acknowledgment. The authors would like to thank the Center for Instrumental Analysis of Kobe University for the XPS measurements. This work was part of the photonics materials program at the Venture Business Laboratory of Kobe University. LA971099Z