Reconsideration of Intrinsic Band Alignments within Anatase and

Kyoung Chul Ko , Stefan T. Bromley , Jin Yong Lee , and Francesc Illas. The Journal of Physical ... Peter N. O. Gillespie and Natalia Martsinovich. Th...
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Reconsideration of Intrinsic Band Alignments within Anatase and Rutile TiO2

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On the other hand, the theoretical calculation with a local density approximation by Kang et al.6 supported the band alignment of Figure 1A. They concluded that the valence-band maxima (EVB) of anatase and rutile were close to each other, whereas the ECB of anatase was found to be about 0.2 eV higher than that of rutile.6 Recent investigation with transient infrared absorption−excitation energy scanning spectra showed that the relative position of the energy band for anatase and rutile depended on the sizes of the polymorphs and that the electron migration in the mixed-phase crystal powder was controlled by dynamical factors.7 The argument was initiated from the electrochemical experiments,8 which were reported two decades ago to determine the flat-band potentials (UFB) of anatase and rutile TiO2 single crystals, as indicated in the band alignment of Figure 1A. To explain the difference in the results of the electrochemical UFB experiments from those of the theoretical calculation and photoelectron experiments in vacuum, Kullgren et al.9 showed by the theoretical study that the UFB data could be explained by taking into account the adsorption of OH− and H+ in the electrolyte solution because in the case of nanopowders the alignment was affected by the dipole layer created by the surface adsorption.9 However, in the case of electrodes, Nakato suggested that the surface band energies (or UFB) in the presence of various kinds of surface charges should be determined by the band energy that lied deepest from the surface because energy barriers formed by surface charges were thin enough for charge carriers in bands to tunnel.10 In the present report, to clarify the apparent discrepancy stated above, the experimental details reported for determining UFB to obtain the band alignment of rutile and anatase of TiO2 are examined and the reasons why the ECB of anatase has been regarded at higher potential are discussed. How is the position of valence-band top experimentally determined? It was reported based on the experimental measurements11 and largely cited that UFB of metal oxides such as TiO2 was correlated with the band gap energy, Eg, with the relationship of UFB(SHE) = 2.94 V − Eg because their valence band commonly consists of O 2p. This means that the energy position of the valence-band top (EVB) does not depend on the kind of metals but is almost constant at 2.94 V. However, the recent collective data for metal oxides show that the EVB largely changes depending on the kinds of metal oxide semiconductors.12 Hence, one should recall that the top of the energy band depends on the width of the band. Generally, with increasing the density of metal oxides, the interaction among the atomic orbitals in the crystal is promoted and the energy bands become wider. Therefore, the higher position of EVB is expected for rutile TiO2 because the density is higher than that of anatase. Actually, the theoretical calculation showed the

itanium dioxide (TiO2) is one of the most popular photocatalysts. In fact, based on an analysis with SciFinder of the American Chemical Society, 45% of documents concerning photocatalysis in the last two years deal with TiO2. There are two major polymorphs for TiO2: anatase and rutile with the band gap energy of 3.2 and 3.0 eV, respectively. Thus, it has been believed that the potential of the conductionband bottom (ECB) of anatase is 0.2 eV higher (more negative) than that of rutile, as illustrated in Figure 1A, based on which

Figure 1. Two proposed band alignments for anatase and rutile polymorphs of TiO2 and charge movement for mixed-phase crystals. Reprinted by permission from ref 1. Copyright 2013 Macmillan Publishers Ltd.

the charge movement in mixed-phase crystalline TiO2 powders has been discussed.1 However, based on theoretical calculation and X-ray photoelectron experiments, it has been recently suggested that to the contrary the ECB of anatase should be lower than that of rutile, as shown in Figure 1B.1 In the case of the theoretical calculation, Labat et al.2 described that no matter which Hamiltonian and method were used, anatase was found to be more stable than rutile. With a similar calculation, Deák et al.3 later reported that the valence-band top (EVB) and ECB of rutile lied higher than those of anatase. In addition, Ju et al.4 recently reported that the calculated EVB and ECB of rutile lied 0.52 and 0.22 eV above those of anatase, respectively. Not only theoretical work but also experimental application of photoelectron spectroscopy supported the alignment of Figure 1B. The interface formation of anatase and rutile TiO2 with RuO2 and tin-doped indium oxide was studied by Pfeifer et al.5 showing that the EVB of rutile was 0.7 ± 0.1 eV above that of anatase. Thus, most of the theoretical calculations and photoelectron experiments support the alignment of Figure 1B. © 2016 American Chemical Society

Published: February 4, 2016 431

DOI: 10.1021/acs.jpclett.5b02804 J. Phys. Chem. Lett. 2016, 7, 431−434

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The Journal of Physical Chemistry Letters increase of the bandwidth, which is consistent with the experimental data.2 We recall another simple method to reflect the position of EVB. For Cu(II)-grafted TiO2, Irie et al.13 reported the photoexcitation of electrons from the valence band to Cu(II) clusters, which is called an interfacial charge transfer (IFCT). As shown in the Supporting Information, IFCT could be observed only for rutile TiO2. By assuming the redox potential E0 of +0.16 V for the Cu(II)/Cu(I) redox, the EVB of rutile is calculated to be +2.9 V (SHE) because the IFCT absorption was observed at the wavelength shorter than 450 nm. How is the position of conduction-band bottom experimentally determined?The position of conduction-band bottom (ECB) of an n-type semiconductor could be determined experimentally from the Mott−Schottky plot with the electrochemical method.14 In the experiments, the capacitance of the space charge layer, CSC, is measured as a function of the applied potential, U, with an impedance meter. From the intercept of the plot for CSC−2 as a function of U, UFB can be obtained. The UFB obtained from the Mott−Schottky plot could be regarded as equal to ECB because the semiconductors used for the electrochemical measurements are heavily doped. Panels A and B of Figure 2 show the representative Mott− Schottky plots for rutile and anatase single crystals, respectively.

Figure 3. Differential amount of reduced methylviologen (MV+) after storing electrons in TiO2 powder of (A) rutile and (B) anatase as a function of the potential E. Reprinted with permission from ref 19. Copyright 2003 PCCP Owner Societies.

dependence of the plot is caused by the inhomogeneity of the donor density near the surface.18 Even taking into account some scattering, the conclusion that ECB of anatase is higher (more negative) than that of rutile, i.e., the band alignment of Figure 1A, seems to be supported by the electrochemical measurements with the Mott−Schottky plots. Ikeda et al.19 examined the energy level of electrons stored in TiO2 powders suspended in the aqueous solution by monitoring the electron transfer to methylviologen at various pH. Figure 3 shows the differential amount of the reduced viologen at various potential energy, E, where E = 0 V is set to the reported UFB, that is, +0.04 and −0.20 V for rutile and anatase, respectively. Peaks observed near 0 V for both polymorphs seem to indicate that the electrons were stored at the ECB of Figure 1A. However, for anatase, there were electrons stored below the ECB. Even at E = 0.45 V electrons were stored by 1/10 of the maximum population for anatase, while the stored electron at E = 0.25 V became zero for rutile. This difference suggests that a part of the conduction band of anatase should extend bellow the ECB of rutile. The other points of experimental evidence showing a higher reduction potential for rutile are the following: The amount of · O2− formed by the one-electron reduction of O2 was lager for rutile powders.20 The onset reduction potential of various TiO2 powders showed also a deeper trap for anatase.21 The EPR experimental data on the mixed-phase crystallite imply that the excitation of the rutile part causes the electrons trapped at the conduction band of anatase.22 These experimental observations support the alignment of Figure 1B, which contradicts the results of the Mott−Schottky plots described above. How could the direct and indirect CB states be introduced in the energy band alignments? The reported energy-band diagrams for anatase3 show that the wave-vector (Γ) at the minimum energy of the CB is different from the wave-vector (Z) at the maximum energy for the VB (see Figure S2), indicating the indirect transition for the band gap of 3.2 eV as clearly shown in the absorption spectra of anatase at 10 K reported by Tang et al.23 Naturally, the density of the states (DOS) near the bottom of CB for anatase becomes very small, as shown in the reports of theoretical calculations by Scanlon et al.1 and Pfeifer et al.5 (Figure S2). When one recalls the potential distribution of the stored electrons in Figure 3, it is probable that the UFB observed by the Mott−Schottky plot for anatase may be attributed to the minimum direct band gap, which is reported to be 3.8 eV,22 but not to the indirect band gap of 3.2 eV. Band bending with the

Figure 2. Mott−Schottky plots of (A) rutile (110) and (B) anatase (101) TiO2 electrodes measured at various frequencies. Reprinted from refs (A) 16 and (B) 8. Copyright (A) 2005 and (B) 1996 American Chemical Society.

Kavan et al.8 reported that the UFB for rutile (001) and anatase (101) in 1 M H2SO4 solution were −0.20 and −0.4 V vs SCE at pH 0, respectively. Furthermore, the onset potentials of the photocurrent supported those values. Because the potential of SCE is about +0.24 V (vs SHE), the UFB of rutile and anatase are +0.04 and −0.16 V (vs SHE), respectively. Later they reported UFB for anatase (101) and (001) in 0.5 M HCl solution to be −0.28 and −0.34 V (vs Ag/AgCl),15 which correspond to −0.06 and −0.12 V (vs SHE at pH 0), respectively, because the potential of Ag/AgCl electrodes is +0.20 V (vs SHE) and the UFB at pH 0.3 shifts with the change in the proton concentration. For rutile TiO2 crystals, Nakamura et al. reported that the atomically flat (100) and (110) TiO2 electrodes in 0.1 M HClO4 possess UFB = −0.34 and −0.25 V (vs Ag/AgCl), which correspond to −0.08 and +0.01 V (vs SHE at pH 0), respectively. The recent report17 showed that when the surface was not atomically flat, the UFB for these surfaces was almost equal to −0.06 V (vs SHE at pH 0). In the Mott−Schottky plot, the slope depends on the frequency of the measurement for anatase (Figure 2B), while that for rutile is independent from the frequency (Figure 3A). The frequency 432

DOI: 10.1021/acs.jpclett.5b02804 J. Phys. Chem. Lett. 2016, 7, 431−434

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The Journal of Physical Chemistry Letters

the reduction potential of O2. The fact that the reduction rate of O2 by CB electrons is as slow as 10 μs26 is consistent with the electron transfer being slightly uphill from ECB to O2. In conclusion, on the basis of the discussion mainly of the experimental data reported to date, this Viewpoint suggests that the previously reported ECB (−0.2 V vs SHE at pH 0) of anatase TiO2 should be dominated by that for the direct band gap of 3.8 eV, while the ECB for the indirect band gap of 3.2 eV should be 0.4 V lower than the ECB of rutile TiO2, as suggested from the theoretical calculations and photoelectron spectroscopy.1

formation of a space charge layer on the application of electric potential to an anatase electrode would be represented in Figure 4. In the measurements of the Mott−Schottky plot, it is

Yoshio Nosaka* Atsuko Y. Nosaka

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Figure 4. Plausible band bending at the flatband potentials for (A) direct transition state and (B) indirect transition state of the conduction band for anatase TiO2.

Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02804. Figures and descriptions for (1) interfacial charge transfer for Cu(II)-grafted TiO2 powder and (2) energy-band diagram and DOS reported for rutile and anatase TiO2 (PDF)

probable that the small change corresponding to the indirect band was overlooked when the potential was scanned from the minus to plus direction.8 Similarly, the small change in the photocurrent may not be recognized on the scanning from the negative to the positive potential in the measurements of the onset potential.8 Thus, the ECB for anatase that is 0.2 eV higher than that for rutile should be attributed to the direct band gap of 3.8 eV and the energy band alignment can be represented in Figure 5. Upon introducing the direct band gap for anatase, the ECB for the indirect band gap of 3.2 eV for anatase is 0.4 eV lower than that for rutile.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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REFERENCES

(1) 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.; Watson, G. W.; Keal, T. W.; Sherwood, P.; Walsh, A.; Sokol, A. A. Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798−801. (2) Labat, F.; Baranek, P.; Domain, C.; Minot, C.; Adamo, C. Density Functional Theory Analysis of the Structural and Electronic Properties of TiO2 Rutile and Anatase Polytypes: Performances of Different Exchange-Correlation Functionals. J. Chem. Phys. 2007, 126, 154703. (3) Deák, P.; Aradi, B.; Frauenheim, T. Band Lineup and Charge Carrier Separation in Mixed Rutile-Anatase Systems. J. Phys. Chem. C 2011, 115, 3443−3446. (4) Ju, M.-G.; Sun, G.; Wang, J.; Meng, Q.; Liang, W. Z. Origin of High Photocatalytic Properties in the Mixed-Phase TiO2: A FirstPrinciples Theoretical Study. ACS Appl. Mater. Interfaces 2014, 6, 12885−12892. (5) Pfeifer, V.; Erhart, P.; Li, S.; Rachut, K.; Morasch, J.; Brötz, J.; Reckers, P.; Mayer, T.; Rühle, S.; Zaban, A.; Mora Sero, I.; Bisquert, J.; Jaegermann, W.; Klein, A. Klein, Energy Band Alignment between Anatase and Rutile TiO2. J. Phys. Chem. Lett. 2013, 4, 4182−4187. (6) Kang, J.; Wu, F.; Li, S.-S.; Xia, J.-B.; Li, J. Calculating Band Alignment between Materials with Different Structures: The Case of Anatase and Rutile Titanium Dioxide. J. Phys. Chem. C 2012, 116, 20765−20768. (7) Mi, Y.; Weng, Y. Band Alignment and Controllable Electron Migration between Rutile and Anatase TiO2. Sci. Rep. 2015, 5, 11482. (8) Kavan, L.; Grätzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. Electrochemical and Photoelectrochemical Investigation of SingleCrystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716−6723. (9) Kullgren, J.; Aradi, B.; Frauenheim, T.; Kavan, L.; Deák, P. Resolving the Controversy about the Band Alignment between Rutile

Figure 5. Proposed band alignment considering the direct band gap for anatase.

The position of ECB has been discussed along with the potential for the reduction of O2 to ·O2−.21,24 The fact that anatase TiO2 forms ·O2− by the excitation at 365 nm19 seems to contradict with the assumption that the ECB of anatase is +0.3 V, which is lower than the reductive potential of O2/·O2− (−0.33 V at pH > 4.8).25 However, because usually the reaction takes place at pH 7, the ECB of anatase becomes −0.11 V (= +0.3 − 0.059 × 7) at pH 7. Therefore, the electron transfer is an uphill reaction of 0.22 eV. When one assumes a Boltzmann distribution, about 10−4 (≈ 10−0.22/0.059) of CB electrons attain 433

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DOI: 10.1021/acs.jpclett.5b02804 J. Phys. Chem. Lett. 2016, 7, 431−434