Article pubs.acs.org/JPCC
Thickness-Dependent Flat Band Potential of Anatase TiO2(001) Epitaxial Films on Nb:SrTiO3(001) Investigated by UHVElectrochemistry Approach Yuji Matsumoto,*,†,‡ Yoshihiro Miura,‡ and Shintaro Takata‡ †
Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07, Aramaki Aza, Aoba, Aoba-ku, Sendai 980-8579, Japan ‡ Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku Yokohama 226-8503, Japan S Supporting Information *
ABSTRACT: The semiconductor characteristics of anatase TiO2(001) films, which were pulsed laser-deposited on a 0.05 wt % Nb-doped SrTiO3(001) single crystal electrode, as well as the film/electrolyte heterointerfaces were investigated by the ultrahigh-vacuum electrochemistry approach. In addition to the current−voltage measurements, equivalent circuit modeling in electrochemical impedance spectroscopy at different electrode potentials reveals that the flat band potential of anatase TiO2 films shifts by +0.5 V when the film thickness increases with (1 × 4) surface reconstruction more visible in reflection high energy electron diffraction. The possible origins of the observed flat band potential shift will be discussed from the viewpoint of electric double layer effects, mainly of donor-type surface states and surface reconstruction, compared with the results of single crystal rutile TiO2(110).
■
significant positive shift of the flat band potential through the EDL by about 0.5 V for single crystal rutile TiO2(110).7 Considering such a non-negligible EDL effect on the flat band potential, even for a single crystal rutile TiO2, the present study sheds light on the flat band potential of single crystalline anatase TiO2. It is well-known that the anatase TiO2(001) surface undergoes a stable (1 × 4) reconstruction, composed of only Ti4+,8 in contrast to the (1 × n) reconstruction of heavily reduced rutile TiO2(110) surfaces with a sizable amount of Ti3+ included.9,10 In this study, we have prepared anatase TiO2(001)/electrolyte heterointerfaces by pulsed laser depositing (PLD) anatase epitaxial films on a TiO2-terminated Nb:SrTiO3(001) (STO) substrate and have carefully investigated their semiconductor electrochemical characteristics by the ultrahigh-vacuum electrochemistry approach.11−14 As a result, we found a substantial flat band potential shift when the film thickness increased with (1 × 4) surface reconstruction more visible in reflection high energy electron diffraction, approaching very close to that of rutile TiO2.
INTRODUCTION Semiconductor electrochemical characteristics at an electrolyte/ semiconductor interface have a profound effect on total performance in electrochemical devices such as dye-sensitized solar cells1 and photocatalysts.2 Among unsolved issues toward the practical application of these devices is the reproducibility in the flat band potential control, one of the most decisive parameters for charge transfer across the electrolyte/semiconductor interfaces. One of the reasons for this is a complex nature of the electric double layer (EDL), across which the interfacial potential change can dramatically affect the flat band potential. One example is that a general consensus places the flat band potential of TiO2 for anatase to be more negative by 0.1−0.2 V than that for rutile on an electrochemical potential scale,3,4that is, the conduction band minimum (CBM) level of anatase is more close to the vacuum level in electrolyte. In contrast, a careful theoretical and experimental study recently shows that the CBM level of rutile is more close to the vacuum level by 0.22 eV in vacuum.5 The reversal relation between the CBM levels in vacuum and in electrolyte strongly implies that the interfacial potential changes across the EDL can be significantly different between rutile and anatase surfaces, up to ∼0.4 V in this case. For another example, the flat band potentials for atomically smooth rutile (100) and (110) TiO2 surfaces are found to be different by up to 0.1 V6 in air. Furthermore, our in situ ultrahigh-vacuum electrochemical experiments have revealed that a donor type surface state in the band gap induces a © XXXX American Chemical Society
■
EXPERIMENTAL SECTION In Situ PLD−EC system. All experiments were performed on our originally developed in situ pulsed laser deposition− electrochemical cell (PLD−EC) system,7,15,16 which was designed essentially on the basis of the concept of ultrahighReceived: September 7, 2015 Revised: January 3, 2016
A
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. A schematic of in situ pulsed laser deposition−electrochemical cell (PLD−EC) system that was originally developed in our group.
Figure 2. (a) The equivalent circuit model used in analysis of electrochemical impedance spectroscopy without a surface capacitance component. (b) The space charge layer width at the onset potential at which the film resistance abruptly increases plotted against the film thickness. (c) Mott− Schottky plots of anatase films for its thickness d of 6, 9, and 18 nm and of a Nb:STO(001) bulk substrate as a reference. The film resistance Rfilm values are also plotted on the right-hand axis. The inset RHEED patterns illustrate a gradual development of (1 × 4) reconstruction with increasing the film thickness.
Impedance Measurements. All the electrochemical measurements were performed in atmospheric pure Ar gas (6 N, Tokyo Koatsu Yamazaki), using a three-electrode electrochemical cell with 4 N Ag and Pt wires (Nilaco) as reference and counter electrodes, respectively. The electrolyte solution used in this study was 1.0 M HClO4 (Kanto Kagaku) containing 0.005 M FeSO4·7H2O + 0.0025 M Fe2(SO4)3· nH2O, and the dissolved O2 in the electrolyte solution was removed by Ar bubbling before the electrochemical measurements. PURELAB Ultra (Analytic ORGANO) was used as the source of pure water. In addition to the I−V curves, the impedance spectroscopy was measured with an Ivium CompactStat electrochemical and impedance analyzer. For anatase films, it was measured at a frequency range of 0.1−100 kHz at a modulation amplitude of 10 mV with 50 mV steps, and the values of the capacitance C and resistance R were obtained as a fitting parameter in the equivalent circuit model so that the frequency dependent absolute value and phase of the measured complex impedance were both reproduced. For single crystal STO and rutile TiO2, on the other hand, it was measured at only a frequency of 1000 Hz, analyzed using a simple series R−C equivalent circuit. To make an ohmic contact, a piece of Al foil or InGa alloy was inserted between the back side of the substrate crystal and the sample holder plate, the details of which are described elsewhere.7,16
vacuum electrochemistry (UHV−EC) as shown in Figure 1. In this in-situ system, STO single-crystal and film samples can be subjected to repeated electrochemical measurements by transferring them back and forth between the PLD and EC chambers without being exposed to air; after the electrochemical measurements, each sample is washed in pure water in the EC chamber and is transferred back to the PLD chamber. Thin Film Deposition and Oxidation. A TiO2-terminated Nb-doped STO(001) (0.05 wt %: Nb:STO) substrate,17 purchased from Shinkosya Co. Ltd., was cleaned by annealing at 660 °C in 0.1 Torr O2 for 1 h to exhibit a sharp (1 × 1) reflection high energy electron diffraction (RHEED) pattern before deposition. Anatase TiO2 films were deposited by PLD at 650 °C.18 The partial O2 pressure was 1 × 10−6 Torr during the deposition, except that it was in 1 × 10−5 Torr for the 6 nm thick film. The laser fluence and repitition rate were 1.5 J/cm2 and 0.5 Hz, respectively. The oxidation was done by annealing at 400 °C in 1 × 10−3 Torr O2. Redox Treatments of Single Crystal Rutile TiO2(110). Atomically flat-treated Nb-doped TiO2(110) (0.5 wt %: Nb:TiO2) substrates19 were annealed at 400 °C in 1 × 10−5 Torr O2 for 1 h to exhibit a sharp (1 × 1) RHEED pattern. The reduction was done by annealing in vacuum, while the oxidation was done by annealing in 0.1 Torr O2, at 400 °C for 1 h. B
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
■
films is not so obvious, though a quantum size effect was recently discovered for a several nanometer thick SrVO3 film on Nb:STO.27 In fact, high-energy resolution X-ray photoelectron spectroscopy has revealed no distinctive change in the band structure of epitaxial anatase films grown on Nb:STO(001) in this thickness range.20 Second, to discuss the possibility of the EDL effect of donortype surface states, we go back again to the Mott−Schottky plots in Figure 2c. The 1/C2 value is found to take a local minimum (marked by symbol ▲) at a certain potential for all anatase TiO2 films, indicative of the appearance of a surface state.28 In this potential range, as discussed earlier, the front of the SCL in the film has already reached the interface with Nb:STO, and thus, the CSS values for each electrode potential (>the onset potential) are calculated on the basis of the equivalent circuit in the inset of Figure 3b as follows:
RESULTS AND DISCUSSION Across the anatase TiO2/STO interface, their CBM and valence band minimum (VBM) levels are both continuous with negligible band bending,20 and so the equivalent circuit model in impedance analysis can be simplified as shown in Figure 2a, where Rsol is the resistance of the electrolyte solution, Cfilm is the film capacitance, and Rfilm is the film resistance. As a result, Mott−Schottky plots, 1/C2 versus electrode potential U for anatase films with different thicknesses, including the Nb:STO(001) used as a substrate, were obtained as shown in Figure 2c. The film resistance Rfilm values were also plotted on the right-hand axis. The inset reflection high energy electron diffraction (RHEED) patterns illustrate a gradual development of (1 × 4) reconstruction with increasing the film thickness d of 0, 6, 9, and to 18 nm. The films are considerably of leakage along the growing direction, giving small Rfilm values. This is probably due to their inclusion of many grain boundaries that will often result from the large lattice mismatch between anatase TiO2(001) and Nb:STO(001).21,22 The onset potential at which the film resistance abruptly increases indicates that the front of the space charge layer (SCL) in the film has just reached the interface with STO. In fact, the SCL width at this potential, calculated from the film capacitance Cfilm ∼ C using a relative permittivity value εr of 3123 for anatase TiO2, is almost in good agreement with the film thickness calibrated by RHEED intensity oscillation18 as shown in Figure 2b. This agreement also justifies the equivalent circuit model we employed in analysis. Thus, the flat band potential Ufb and donor density ND were deduced from the linear part of the Mott−Schottky plots at more negative electrode potentials than the onset potential and were plotted against the film thickness, along with those of Nb:STO as a reference,24,25 in Figure 3a.
CSS = C − Cfilm Cfilm =
εrε0 (d = 6, 9, 18 nm) d
(1)
where ε0 and εr are vacuum permittivity and relative permittivity, respectively. Here, we may neglect the small contribution of the SCL capacitance of STO in series because it would be relatively much larger than that of the anatase film in a highly doped STO with a relative permittivity of 340.24 Figure 3b is the plots of the surface capacitance component CSS values against the potential relative to Ufb, that is, from the CBM level of anatase TiO2 for different film thicknesses. The donor-type surface state appears in a broad energy range of 0.3−1.2 eV below the CBM, probably originating from oxygen vacancies or oxygen deficit related defects near the surface. In fact, the 18 nm thick anatase TiO2 film that had been subjected to the electrochemical measurements was annealed in 1 × 10−3 Torr O2 at 400 °C after rising and bringing back to UHV. The anatase film exhibited again a very sharp (1 × 4) RHEED pattern as shown in Figure S1a. As shown in Figure S1b, no such remarkable local minimum was found any more in the 1/ C2 plots, while ND decreased by about half (Figure 3a). Furthermore, in response to this, a substantial decrease in the anodic current, also a sign of the reduction of the oxygen vacancies near the surface,16 was observed in the current− voltage measurement (Figure S2). A similar donor-type surface state was found at 0.4 eV below the CBM on rutile TiO2(110), whose origin we recently concluded is not the surface-bridging hydroxyls or oxygen vacancies but rather the interstitial Ti3+ ions in the subsurface region.7 Because of the interstitial nature of such a donor-type surface state in rutile TiO2(110), it cannot respond in a frequency range above 1 kHz.7 In contrast, the present surface state in anatase TiO2(001) can respond in a high-frequency range up to 100 kHz, suggestive of the lattice oxygen vacancy. The overall surface state capacitance decreases, especially in the deeper levels, with increasing the film thickness. According to the first-principle calculations by Cheng and Selloni,30 in anatase TiO2(001) (1 × 4), lattice oxygen vacancies dominantly form at the subsurface region, but not on the topmost surface, which might be due to its surface stability. Considering the gradual development of (1 × 4) reconstruction with the film thickness, as observed in RHEED, the deeper components of surface states would be from surface oxygen vacancies, while those near the CBM level would be from subsurface ones. Such a donor type surface state in the band gap should induce a positive shift of the flat band
Figure 3. (a) The flat band potential and donor density plotted against the film thickness, including those for Nb:STO as a reference. (b) The surface capacitance values plotted against the potential relative to the flat band potential Ufb, i.e., from the CBM of anatase TiO2. The inset is the equivalent circuit model used in analysis of electrochemical impedance spectroscopy with a surface capacitance component.
Surprisingly, the figure clearly displays a considerable shift of the flat band potential to more positive values with the film thickness, gradually approaching the saturation value of about −0.5 V versus Ag, though CBM levels of STO and anatase TiO2 from the vacuum level, determined by photoemission experiments, are almost the same.20 To understand such a flat band potential shift of anatase TiO2 films, we first rule out the possibility of the nanosize effect. Some nanosize effects on the band gap blue shift in TiO2 particles have been reported in a radius range of one to a few tens of nanometers,26 some of which are likely due to the Burstein−Moss effect, but this is still in controversy. In contrast to such 3D-nanoparticles, the nanosize effect in 2D-thin oxide C
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4a−d illustrates a gradual shift of flat band potential with an increase of the partial coverage of (1 × 4) areas on
potential,7 but the fact is opposite, that is, the positive shift is found though the overall surface state capacitance decreases. Therefore, the present flat band potential shift has nothing to do with the observed donor-type surface states, reasonably consistent with the values of the surface state capacitance being much smaller than that of rutile TiO2(110).28 As pointed out in Figure S1a, the recovery of a very sharp (1 × 4) RHEED pattern after the electrochemical measurements suggests that the (1 × 4) reconstruction essentially should remain in solution; otherwise, an annealing temperature much higher than 400 °C would be necessary for recovery of the (1 × 4) reconstruction from (1 × 1).8,21,29 If this is the case, it allows us to discuss the possibility of the EDL effect of surface reconstruction. It is well-known that the flat band potential of TiO2 depends on pH in the electrolyte solution.31 In essence, the lattice Ti4+ ions act as a Lewis acid site for the adsorption of hydroxyl ions, whereas the bridging lattice oxygen acts as a Lewis base site for protons. As a result, depending on the pH, the TiO2 surface becomes charged either positively or negatively. The isoelectric point (pHIEP) is defined as the pH at which the net surface charge is zero. Under the assumption of pH < pHIEP, which would be reasonable in the present case that a strong acid electrolyte such as 1.0 M HClO4 is used, the potential change ΔUEDL due to the additional formation of an EDL by the proton adsorption, that is, the flat band potential shift across the EDL ΔUfb, can be expressed by31,32 ΔUfb = ΔUEDL = 2.3
kT (pH IEP − pH) e
Figure 4. Schematic of interface structures of EDL and SCL for anatase TiO2(001) grown on Nb:STO(001) with a qualitative description of the potential drop across the interface. (a) Electrolyte/STO(001) interface before depositing the film, (b) electrolyte/(1 × 1)-dominant anatase TiO2 on STO(001) interface, (c) electrolyte/ (1 × 1 + 1 × 4) mixture anatase TiO2 on STO(001) interface, and (d) electrolyte/(1 × 4)-completed anatase TiO2 on STO(001) interface. With a gradual development of (1 × 4) surface reconstruction, more protons adsorb on the TiO2 topmost surface to form an additional EDL with a more precipitous potential drop at the interface.
(2)
where k is the Boltzmann constant, T is the absolute temperature, and e is the electron charge. Accordingly, the flat band potentials will be different even among anatase surfaces with different pHIEP values, in other words, different degrees of the acid−base imbalance, even among used electrolytes with the same pH value. A question that may come up here is whether there can be any good reason why (1 × 1) and (1 × 4) anatase surfaces have different pHIEP values. At the moment, one possible answer is their different ability of H2O adsorption between (1 × 1) and (1 × 4) surfaces. The dissociative adsorption of some H2O molecules will take place on a TiO2 surface in an electrolyte solution. H2O is first adsorbed on lattice Ti4+ sites, giving off a H atom onto the neighboring bridging oxygen Ob atoms. In consequence, the Ti4+ site, originally a Lewis acid site, becomes Ti−OH as a Brønsted acid site, while the Ob atom, a Lewis base site, is terminated with H to almost lose its ability.32 According to the theoretical calculations by Gong et al.,33 H2O is dissociatively adsorbed on the (1 × 1) anatase TiO2(001) surface with an adsorption energy of 1.25 eV. In contrast, for the (1 × 4) reconstructed surface, which consists of flat terrace and ridge areas according to the ADM model,33 the dominant molecular adsorption of H2O occurs with an adsorption energy of 1.1 eV on the terrace areas, though it is up to 1.82 eV locally on the ridge areas. The different water adsorption ability may come from the surface stability:34,35 the (1 × 4) surface is much more stable and inactive to the dissociative water adsorption than the (1 × 1) surface. Therefore, the (1 × 4) surface has more remaining Ob atoms, that is, Lewis base sites, and is therefore able to attract more protons than the (1 × 1) surface in electrolyte. As a result, larger pHIEP and thereby more positively shifted flat band potential values are expected for (1 × 4) TiO2 than those for (1 × 1) TiO2, as indicated by eq 2.
anatase TiO2 electrode, where the potential change across the EDL layer becomes larger for more proton adsorption. Considering the similarity of the topmost surface and electronic structures between TiO2-terminated STO(001) and anatase TiO2(001), the most negative flat band potential of −1.1 V versus Ag for TiO2-terminated STO(001) suggests that the proton adsorption is much less on the TiO2-terminated STO(001) than on anatase TiO2(001), but the calculated water dissociative adsorption energy is as small as 0.6−0.8 eV as reported in the literature.36,37 The topmost TiO2 layer in STO may essentially have a weak acid−base interaction, probably because STO is a complex oxide resulting from the reaction between acid TiO2 and basic SrO. Finally, we compare the flat band potentials of anatase and rutile TiO2. The flat band potential for rutile TiO2(110) was also determined in the same electrolyte conditions as in the present experiments. A significant shift of the flat band potential value is found depending on the redox treatments (Figure S3, Table S1), which is probably due to the EDL effect through the donor-type surface state on the flat band potential shift.7 To minimize such an EDL effect, it is reasonable to take the value of the oxidized sample for the comparison; the value is −0.44 V versus Ag (indicated by a dashed line in Figure 3a). Although the flat band potentials are not reversed as found in the previous work,5 the difference is very small between anatase and rutile TiO2 even in the present result.
■
CONCLUSIONS We have found a significant flat band potential shift of epitaxial anatase TiO2(001) films by up to +0.5 V with an increase of the film thickness. The result demonstrates that the flat band D
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
(8) Liang, Y.; Gan, S.; Chambers, S. A. Surface Structure of Anatase TiO2(001): Reconstrction, Atomic Steps, and Domains. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 235402−1−7. (9) Onishi, H.; Iwasawa, Y. Reconstruction of TiO2(110) Surface: STM Study with Atomic-scale Resolution. Surf. Sci. 1994, 313, L783− L789. (10) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (11) Stickney, J. L.; Rosasco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Superlattices Formed by Electrodeposition of Silver on Iodine-Pretreated Pt(111); Studied by LEED, Auger Spectroscopy and Electrochemistry. Surf. Sci. 1983, 130, 326−347. (12) Taniguchi, M.; Kuzembaev, E.; Tanaka, K. A Mimic Model of Pt-Rh Catalyst Prepared by Electrochemical Deposition of Rh on Pt(110) Surface. Surf. Sci. 1993, 290, L711−L717. (13) Yamada, T.; Batina, N.; Itaya, K. Interfacial Structure of Iodine Electrodeposited on Au(111): Studied by LEED and In situ STM. Surf. Sci. 1995, 335, 204−209. (14) Reniers, F. J. The Development of a Transfer Mechanism between UHV and Electrochemistry Environments. J. Phys. D: Appl. Phys. 2002, 35, R169−R188. (15) Takata, S.; Tanaka, R.; Hachiya, A.; Matsumoto, Y. Nanoscale Oxygen Nonstoichiometry in Epitaxial TiO2 Films Grown by Pulsed Laser Deposition. J. Appl. Phys. 2011, 110, 103513−5. (16) Miura, Y.; Takata, S.; Matsumoto, Y. Nondestructive and Repeatable Capacitance-Voltage and Current-voltage Measurements across the Oxide/Electrolyte Interface by UHV-Electrochemistry Approach. Appl. Phys. Express 2014, 7, 095802−4. (17) Kawasaki, M.; Takahashi, K.; Maeda, T.; Tsuchiya, R.; Shinohara, M.; Ishiyama, O.; Yonezawa, T.; Yoshimoto, M.; Koinuma, H. Atomic Control of the SrTiO3 Crystal Surface. Science 1994, 266, 1540−1542. (18) Murakami, M.; Matsumoto, Y.; Nakajima, K.; Makino, T.; Segawa, Y.; Chikyow, T.; Ahmet, P.; Kawasaki, M.; Koinuma, H. Anatase TiO2 Thin Films Grown on Lattice-matched LaAlO3 Substrate by Laser Molecular-Beam Epitaxy. Appl. Phys. Lett. 2001, 78, 2664−2666. (19) Yamamoto, Y.; Nakajima, K.; Ohsawa, T.; Matsumoto, Y.; Koinuma, H. Preparation of Atomically Smooth TiO2 Single Crystal Surfaces and Their Photochemical Property. Jpn. J. Appl. Phys. 2005, 44, L511−L514. (20) Chambers, S. A.; Ohsawa, T.; Wang, C. M.; Lyubinetsky, I.; Jaffe, J. E. Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Inerface. Surf. Sci. 2009, 603, 771−780. (21) Herman, G. S.; Sievers, M. R.; Gao, Y. Structure Determination of the Two-Domain (1 × 4) Anatase TiO2(001) Surface. Phys. Rev. Lett. 2000, 84, 3354−3357. (22) Ohsawa, T.; Yamamoto, Y.; Sumiya, M.; Matsumoto, Y.; Koinuma, H. Combinatorial Scanning Tunneling Microscopy Study of Cr Deposited on Anatase TiO2(001) Surface. Langmuir 2004, 20, 3018−3020. (23) Hengerer, R.; Kavan, L.; Krtil, P.; Grätzel, M. Orientation Dependence of Charge-Transfer Processes on TiO2(Anatase) Single Crystals. J. Electrochem. Soc. 2000, 147, 1467−1472. (24) Suzuki, S.; Yamamoto, T.; Suzuki, H.; Kawaguchi, K.; Takahashi, K.; Yoshisato, Y. Fabrication and Characterization of Ba1‑xKxBiO3/Nbdoped SrTiO3 All-Oxide-Type Schottky Junctions. J. Appl. Phys. 1997, 81, 6830−6836. (25) Matsumoto, Y.; Takata, S.; Tanaka, R.; Hachiya, A. Electrochemical Impedance Analysis of Electric Field Dependence of the Permittivity of SrTiO3 and TiO2 Single Crystals. J. Appl. Phys. 2011, 109, 14112−4. (26) Serpone, N.; Lawless, D.; Khairutdinov, R. Size Effects on the Photophysical Properties of Colloidal Anatase TiO2 Particles: Size Quantization versus Direct Transitions in This Indirect Semiconductor? J. Phys. Chem. 1995, 99, 16646−16654. (27) Yoshimatsu, K.; Horiba, K.; Kumigashira, H.; Yoshida, T.; Fujimori, A.; Oshima, M. Metallic Quantum Well States in Artificial Structures of Strongly Correlated Oxide. Science 2011, 333, 319−322.
potentials of anatase and rutile TiO2 can be sometimes almost the same in contrast to the widely accepted flat band potential relation of rutile and anatase TiO2. The possible EDL effect of the particular surface reconstruction of anatase TiO2(001) on such a flat band potential shift was proposed. Further understanding of the origin of the present flat band potential shift would pave a way to tailor band realignment with respect to the redox levels in electrolyte at the electrolyte/semiconductor interface, leading to both higher efficiencies and better reproducibility in electrochemical device development.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08708. RHEED pattern and 1/C2 plots vs electrode potential for 18 nm thick anatase TiO2 film after oxidation, current− voltage curve measurement for the 18 nm thick anatase TiO2 film before and after oxidation, RHEED patterns and Mott−Schottky plots for a 0.5 wt % Nb:TiO2(110) rutile single crystal with different redox treatments, flat band potential and donor density for Nb-doped rutile TiO2(110) (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI (No. 20360294) and by Grant Program of Metallic Glass Inorganic Materials Joining Technology Development from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The author Y.M. also thanks Tokyo Institute of Technology for financial support of Challenging Research Incentive Award 2008.
■
REFERENCES
(1) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (2) Fujishima, A.; Zhang, X.; Tryk, D. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (3) Di Paola, A.; Bellardita, M.; Palmisano, L. Brookite, the Least Known TiO2 Photocatalyst. Catalysts 2013, 3, 36−73. (4) Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. Electrochemical and Photoelectrochemical Investigation of SingleCrystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716−6723. (5) Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P.; et al. Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798−801. (6) Nakamura, R.; Ohashi, N.; Imanishi, A.; Osawa, T.; Matsumoto, Y.; Koinuma, H.; Nakato, Y. Crystal-Face Dependences of Surface Band Edges and Hole Reactivity, Revealed by Preparation of Essentially Atomically Smooth and Stable (110) and (100) n-TiO2 (Rutile) Surfaces. J. Phys. Chem. B 2005, 109, 1648−1651. (7) Takata, S.; Miura, Y.; Matsumoto, Y. Evidence for Intrinsic Nature of Band-Gap States Electrochemically Observed on Atomically Flat TiO2(110) Surface. Phys. Chem. Chem. Phys. 2014, 16, 24784− 24789. E
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (28) Kobayashi, K.; Aikawa, Y.; Sukigara, M. Surface States at Semiconductorliquid Junction. J. Appl. Phys. 1983, 54, 2526−2532. (29) Durinck, G.; Poelman, H.; Clauws, P.; Fiermans, L.; Vennik, J.; Dalmai, G. Observation of Surface Phonons of the (001) and (100) Surfaces of Anatase Minerals. Solid State Commun. 1991, 80, 579−581. (30) Cheng, H.; Selloni, A. Surface and Subsurface Oxygen Vacancies in Anatase TiO2 and Differences with Rutile. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 092101−4. (31) Gerischer, H. Neglected Problems in the pH Dependence of the Flat Band Potential of Semiconducting Oxides and Semiconductors Covered with Oxide Layers. Electrochim. Acta 1989, 34, 1005−1009. (32) Beranek, R. (Photo) Electrochemical Methods for the Determination of the Band Edge Positions of TiO2‑Based Nanomaterials. Adv. Phys. Chem. 2011, 2011, 1. (33) Gong, X.-Q.; Selloni, A.; Vittadini, A. Density Functional Theory Study of Formic Acid Adsorption on Anatase TiO2(001): Geometries, Energetics, and Effects of Coverage, Hydration, and Reconstruction. J. Phys. Chem. B 2006, 110, 2804−2811. (34) Lazzeri, M.; Selloni, A. Stress-Driven Reconstruction of an Oxide Surface: The Anatase TiO2(001)-(1 × 4) Surface. Phys. Rev. Lett. 2001, 87, 266105−4. (35) Diebold, U.; Ruzycki, N.; Herman, G. S.; Selloni, A. One Step towards Bridging the Materials Gap: Surface Studies of TiO2 Anatase. Catal. Today 2003, 85, 93−100. (36) Evarestov, R. A.; Bandura, A. V.; Blokhin, E. N. The Water Adsorption on the Surfaces of SrMO3 (M= Ti, Zr, and Hf) Crystalline Oxides: Quantum and Classical Modelling. J. Phys.: Conf. Ser. 2007, 93, 012001−10. (37) Becerra-Toledo, A. E.; Enterkin, J. A.; Kienzle, D. M.; Marks, L. D. Water Adsorption on SrTiO3(001): II. Water, Water, Everywhere. Surf. Sci. 2012, 606, 791−802.
F
DOI: 10.1021/acs.jpcc.5b08708 J. Phys. Chem. C XXXX, XXX, XXX−XXX
