Competition between Itineracy and Localization of Electrons Doped

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Competition between Itineracy and Localization of Electrons Doped into the Near-Surface Region of Anatase TiO 2

Yoshihiro Aiura, Kenichi Ozawa, Eike F. Schwier, Kenya Shimada, and Kazuhiko Mase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05955 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry

Competition between Itineracy and Localization of Electrons Doped into the Near-Surface Region of Anatase TiO2 Yoshihiro Aiura,*,† Kenichi Ozawa,**,‡ Eike F. Schwier,§ Kenya Shimada,§ and Kazuhiko Mase∥,⊥ †

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,

Ibaraki 305-8568, Japan ‡

Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

§

Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-0046, Japan

∥

Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

⊥

SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan

ABSTRACT: The competition between itineracy and localization of electrons doped into the near-surface region of anatase TiO2 (a-TiO2) is examined by photoemission spectroscopy. Since a-TiO2 samples used in this study are naturally-grown single crystals, some electrons derived from oxygen vacancies are inherently present within the bulk and result in both a metallic state just below the Fermi level (EF) and a localized state tightly trapped by the oxygen vacancies (deep-trap state) within the energy gap. Additional electrons were doped to the surface by in situ deposition of potassium, and by irradiation with high intensity synchrotron radiation (SR). The potassium deposition does not cause a direct disturbance in the conduction pathway of the doped electrons, whereas the SR radiation produces oxygen vacancies in the pathway. Most of the doped electrons produced by depositing potassium contribute to a metallic state just below EF, and behave as the carriers responsible for the surface conductivity. The remaining fraction of electrons doped at the outermost a-TiO2 surface are loosely trapped by the deposited potassium ions and produce a quasi-localized state in the band gap (shallow-trap state). The shallow-trap state has a finite spectral intensity at EF, so that it is considered to contribute in part to the surface conductivity. In the case where the number of oxygen vacancies generated by SR irradiation remains below a critical threshold, doped electrons are transferred into the nascent metallic state, in addition to the localized state within the band gap. The doped electrons in the vicinity of the oxygen vacancies near the surface are found to be more tightly trapped compared with those trapped by the potassium ions and hardly contribute to the conductivity. As the oxygen vacancies further increase, all the doped electrons near the surface are trapped and localized. Since such microscopic electronic behaviors of the doped electrons are closely related to the macroscopic conduction properties such as the mobility and dopability of electron carriers, our results on the electronic behavior of electrons doped into the near-surface region of a-TiO2 will be expected to contribute to improving the functionality of existing oxide devices and/or developing innovative technologies, that utilizes the change in physical properties near the surface.

metal and oxygen ions that constitute the oxide, doping results in more or less spatial and/or electrical disturbances to the conduction pathway. The disturbance of the conduction pathway for doped electrons/holes can be considered as one of the origins of the localized state. Determining the relationship between the conduction pathway and the electronic behavior of the doped electrons/holes has to be considered an important task in order to elucidate the semiconducting properties, such as the mobility and dopability, of many oxide materials.

1. INTRODUCTION The elucidation of the microscopic electronic behaviors of doped electrons/holes will give an important key in drastically improving the functionality of existing oxide devices and/or newly developing innovative technologies. Based on a simple band picture, the doped electrons and holes will occupy bands in the vicinities of the conduction band minimum and valence band maximum, respectively, so that the oxides will become to show metallic behavior. In reality, however, with the exception of some, many oxides show semiconducting or insulating behavior upon electron/hole doping. From many previous spectroscopic studies, it is well known that the doped electrons/holes often appear as a (dispersion-less) localized state inside the band gap of pristine (insulating) oxides, which is not explained by a simple band structure model. Since the doped charge carriers conduct through a pathway composed of

TiO2 is one of the most important oxide materials used in applications such as catalysis and photocatalysis.1-3 Recently, basic/applied frontier researches aimed at practical use of diverse TiO2-based devices such as dye sensitized solar cells4 and water splitting5 have also progressed actively. Naturally grown TiO2 exists in three different polymorphs of rutile, anatase, and brookite, and it is well known that metastable anatase TiO2 (a-TiO2) shows a higher catalytic activity than the 1

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an insulator based on band calculations,6, 17 the naturally grown a-TiO2 used in this study is a semiconductor, with a mobility of 1.3 cm2V-1s-1 estimated from a Hall measurement. Since no impurity was detected from X-ray fluorescence spectroscopy and energy dispersive X-ray spectroscopy, it is believed that the conduction properties are a result of the electron carriers derived from oxygen vacancies within the bulk. The orientation of the a-TiO2 surface was set to be the naturally (101) cleaving plane.18 The momentum direction shown in the angle-resolved photoemission spectroscopy (ARPES) images was set to the [010] axis. To prevent the composition from deviating from the bulk value, clean surfaces were obtained by fracturing the single crystal in situ below a pressure of 10-8 Pa.19, 20 The observation of clear dispersive valence bands implies that the fractured surfaces are of high-quality and suitable for ARPES measurement (see section 1 of Supporting Information).

stable rutile phase (r-TiO2). Since the a-TiO2 is expected as a promising material for light-energy conversion applications, the physical/chemical properties have been actively studied to elucidate its basic mechanism and improve the function.6 Since the essential characteristics of the TiO2-based devices are closely related to the semiconducting properties near the surface, the microscopic electronic structure has been actively studied by means of various surface-sensitive spectroscopic technique such as photoemission spectroscopy (PES).4, 5, 7 By irradiating ultraviolet (UV) light, it is well known that oxygen vacancies are easily generated on TiO2 surfaces that are closely related to the surface characteristics. Therefore, understanding the microscopic behavior of the electrons doped by the oxygen vacancies near the surface will lead to elucidating/improving its function in the TiO2-based devices. For a-TiO2, a metallic surface state, the so-called two-dimensional electron gas/liquid (2DEG/L), was reported to be formed upon irradiating with UV-light,810 which was not the case with r-TiO2.11-14 Such UV-light irradiation induces oxygen vacancies in the conduction pathway near the surface, and the electronic behavior of the doped electrons is considered to be directly affected by the oxygen vacancies. Recently, such a 2DEG/L was also observed by depositing potassium on the a-TiO2 surface.15 Since the potassium ions are deposited on the surface, the doped electrons are supposed to be affected only by loose and indirect disturbances in the conduction pathway. As inferred from a previous study of cuprates,16 the physical properties of oxides are considered to be sensitive to the conduction pathway.

PES measurements were performed using two different undulator beamlines: BL-1 of Hiroshima Synchrotron Radiation Center (HiSOR)21 and BL-13B of the Photon Factory, High Energy Accelerator Research Organization (KEK)22. In order to simultaneously detect not only the valence bands including the gap states but also the shallow core levels such as O 2s, Ti 3p, and K 3p, the photon energy was set as 100 eV. The photon flux at KEK around 100 eV is too intense and oxygen vacancies near the surface are easily created at and under the surface. This suggests that the SR at KEK is suitable for investigating the change in the electronic structure derived from the oxygen vacancies near the surface generated by high intensity SR, but unsuitable for elucidating the essential change in the electronic structure due to potassium deposition. On the other hand, the photon flux density at HiSOR is less intense compared with that at KEK.23 The appearance of the Ti3+ 3p core state and the reduction of the O 2s core state derived from the oxygen vacancies as a result of SR irradiation at HiSOR were not detectable even after several hours of irradiation (see section 5 of Supporting Information). Although the spectral intensity just below EF increased upon SR irradiation at HiSOR, the change is much smaller than that observed by potassium deposition. Therefore, the spectral change by potassium deposition was studied using the SR at HiSOR, whereas the irradiation dependence of the electronic structure was measured at KEK.

In order to understand the disturbing influence on the conduction pathway for the electron carriers doped into the near-surface region of a-TiO2, the electronic states induced by in situ deposition of potassium and intense synchrotron radiation (SR) were studied by means of PES. Based on a simple band picture,6, 17 the electrons doped into a-TiO2 are supposed to conduct through a pathway of the antibonding bands composed of the Ti 3dxy and oxygen 2p orbitals, which are located at the conduction band minimum. This picture partly holds because a fraction of the doped electron due to potassium deposition and SR irradiation occupies a metallic state just below the Fermi level (EF). These electrons mainly responsible for the surface conductivity of doped a-TiO2. The remaining small number of electrons doped by potassium deposition are loosely trapped near the potassium ions at the surface, and form a (dispersion-less) localized state at -0.83 eV. These trapped electrons also contribute to the conductivity because the state extends towards EF and has a finite spectral intensity at EF. On the other hand, the remaining electrons doped by SR irradiation are trapped in the vicinity of the oxygen vacancies to form a trap state at -0.99 eV and deeper. These electrons hardly are considered to contribute to the surface conductivity. Furthermore, the density of the metallic state is suppressed as the density of the oxygen vacancies are increased. These results indicate a significant disturbance effect on the conduction pathway by the oxygen vacancies rather than deposited potassium. In this paper, we highlight the differences in electronic behavior of a-TiO2 by two different electron doping techniques. We present a microscopic model of disturbance of conduction pathways to explain our findings. Since this model gives the proper electronic picture to the doped electrons, it will be useful to improving the functionality of existing oxide devices and/or developing innovative technologies, that utilizes the change in physical properties near the surface.

Linearly polarized SR in the horizontal plane of incidence, i.e., in ppolarization geometry, was used. The PES spectra measured at KEK and HiSOR were normalized with the ring current and focusing mirror current, respectively. Hemispherical electron energy analyzers with acceptance angles of 30° (Scienta, R4000) and 12° (Scienta, SES200) were used at HiSOR and KEK, respectively. The valence bands and gap states were measured in the angular mode, whereas the shallow core levels such as K 3p, O2s, and Ti3p were recorded in the transmission mode. The energy distribution curves (EDCs) of the valence band and gap states were obtained by integrating the ARPES spectra along the acceptance angle of the energy analyzers. The spectra were taken at 20 K and room temperature for HiSOR and KEK, respectively. The instrumental energy resolution due to the energy analyzer and the SR was about 70 meV for both the systems.

3. RESULTS SR-irradiation dependence Using the SR at KEK, the spectral change in the gap states due to the irradiation was investigated. Twenty three PES spectra of a-TiO2 were successively taken. The acquisition time, i.e., the irradiation time, per spectrum was about 5 min. Therefore, the total irradiation time was about 2 h. Figure 1 shows the change in the PES spectra of a-TiO2 upon

2. EXPERIMENTAL METHODS Naturally grown a-TiO2 single crystals were used (SurfaceNet GmbH, Germany). Although the ideal stoichiometric a-TiO2 should be 2


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SR irradiation. As shown in Figure 1(a), no signal corresponding to the Ti3+ 3p core level was revealed just after irradiating (SR #0, black line), whereas, after irradiating for 2 h, a shoulder structure due to the Ti3+ 3p core spectrum was clearly observed and the intensity of the O 2s core spectrum was slightly reduced (SR #22, red line). This suggests that, upon SR irradiation, oxygen vacancies were generated near the surface, and that the Ti3+ 3p core state appeared due to the charge compensation. As a result, the electrons doped by the oxygen vacancies enhance the spectral intensity in the energy gap (Figure 1(b)). Note that the gap state observed just after irradiation (black line) is not derived from the newly formed oxygen vacancies near the surface, but from those inherently existing in the bulk of naturally grown a-TiO2. The gap states consist of a coherent state (metallic state), which has a remarkable spectral intensity near EF, and a broad in-gap state (IGS), which is located at the binding energy of about -1 eV. As shown by the ARPES image in Figure 1(c), under the experimental conditions of the photon energy (100 eV) and the polarization of SR (p-polarization geometry) used in this study, a metallic state can be observed around the center of the second Brillouin zone, but not around the center of the first zone. Interestingly, a recent ARPES study on a-TiO2 reported that the metallic state was observed around the center of the first zone in the case of using the liner vertical polarization of the SR (s-polarization geometry).15 It is well known that, due to the final-state effect, the spectral intensity strongly depends on the experimental condition, especially, the polarization direction of SR and the photon energy.24 Recently, the electronic behavior/origin of the metallic state has been actively discussed.8, 9, 25-27 On the other hand, the IGS appears across the entire measured momentum space, and shows little dispersion. In previous report, the metallic state and the IGS are well known as a shallow gap state and a deep one, respectively, and understanding the electronic behavior of the shallow gap

state is suggested to be crucial to elucidate the photocatalytic and catalytic reactions.28 The EDCs of the gap states shown in Figure 1(b) were obtained by integrating the ARPES image over the entire momentum space. In order to deconvolute the gap states into metallic states and IGSs, the contribution of the IGS was first extracted by integrating the ARPES image by excluding the momentum region where the metallic state is present. Then, the IGSs were normalized so as to coincide with the high binding energy side of the gap states, which are not affected by the metal states. Finally, the contribution of the metallic state was extracted by subtracting the IGSs from the gap states. The green and red lines in Figure 1(c) show the spectral contributions of the IGS and metallic state, respectively, in the gap state (black line). Figures 1(d) and 1(e) show the spectral changes in the metallic states and IGSs by SR irradiation, respectively. The spectral width of the metallic states was shown to increase with increasing spectral intensity. A finite spectral intensity of the IGSs was observed near EF, as shown in Figure 1(e). The integrated spectral intensity of the gap states (black line), IGSs (green line), and metallic state (red line) are summarized in Figure 1(f). The integrated intensities of the gap states and IGSs increase monotonically with irradiation time, consistent with a previous report.9 On the other hand, the metal states increase monotonically during the initial stage of irradiation (the pink area in Figure 1(f)), but then saturate abruptly and begin to decrease gradually (blue area). The peak positions of the IGSs, indicated by the vertical bars in Figure 1(e), are shown by the green dotted line in Figure 1(f). At the initial stage of the irradiation, the peak position shifts monotonically to lower binding energy, and remains at -0.99 eV when a certain irradiation time is exceeded. The sharp changes in the spectral intensity of the metallic states and the peak positions of the IGSs occur, within the precision of the measurement, at the same irradiation time.

Figure 1. Change in PES spectra by irradiating SR. (a) Ti 3p and O2p core levels before (black line) and after (red line) the irradiation, and the difference spectra (dotted line). (b) Spectral change in the gap states upon SR irradiation. The gap states are deconvoluted into metallic states and IGSs, as shown by the dotted lines. (c) Example of an ARPES image after irradiation, corresponding to data #15 in Figure (b). The ARPES image was recorded along the [010] direction. (d) and (e) SR-irradiation dependence of the metallic states and IGSs, respectively. (f) Irradiation dependence of the spectral intensities of the gap states (black line), IGSs (green line), and metallic states (red line). The spectral intensity of the metallic states is shown by enlarging five times. The binding energies of the IGSs scaled on the right axis are indicated by the green dotted line. 3



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Since oxygen vacancies were hardly formed near the surface by irradiating the SR at HiSOR (see section 5 of Supporting Information), the observed slight reduction in the spectral intensity of the O2s core level is considered to be caused by the deposition of potassium on the surface. On the contrary, the clear changes in the Ti 3p core level (Figure 2(a)) and in the gap states (Figure 2 (b)) are considered to be attributable to the potassium deposition. The deposited amount of potassium was estimated from the ratio of the spectral intensity of the K-3p core-level to that of Ti-3p core level. The coverage is considered to be 0.68 after potassium deposition denoted by K#8 (see section 6 of Supporting Information).

Potassium deposition dependence Using the SR at HiSOR, the spectral change in the gap states upon depositing potassium was investigated. Nine PES spectra of a-TiO2, which are numbered K #0 to K #8 in Figure 2, were successively taken. The PES spectrum numbered K #0 corresponds to the one before potassium deposition. The acquisition time per spectrum was about 15 min, and the total SR-irradiation time was about 140 min. As shown in Figure 2(a), the spectral intensity of the Ti4+ 3p core level monotonically reduces with the deposition of potassium, whereas that of the Ti3+ 3p core level gradually increases. A successive reduction in the spectral intensity of the O 2s core levels is also shown, but this is considerably smaller compared with the spectral enhancement observed in the K 3p core levels.

Figure 2. Change in PES spectra by depositing potassium. (a) Potassium deposition dependence of Ti 3p, O2s, and K 3p core levels, and the difference spectra between those observed after (K #1-#8) and before (K #0) potassium deposition. (b) Potassium deposition dependence of the gap states. The gap states are deconvoluted into metallic states and IGSs, as shown by the dotted lines. (c) Example of an ARPES image after potassium deposition, corresponding to K #3 in Figure (b). The ARPES image was recorded along the [010] direction. (d) and (e) Potassium deposition dependence of the metallic states and IGSs, respectively. (f) Potassium-deposition dependence of the spectral intensities of the gap states (black line), IGSs (blue line), and metallic states (red line) as a function of the ratio of the spectral intensity of K-3p core level (IK3p) and that of Ti-3p core-level (ITi3p). as a result of depositing potassium is weaker. Further, the spectrum of the IGS just after cleaving (K #0) is almost symmetric around the binding energy of -1.60 eV, but that after potassium deposition (K #8) shows an asymmetric shape, consisting of a peak centered at about -1 eV and a tail structure on its high binding energy side. For the irradiation, the IGSs shift gradually to low binding energies with increasing density of oxygen vacancies. Figure 2(f) summarizes changes in the integrated spectral intensities of the gap states (black line), IGSs (blue line), and metallic states (red line) as a function of IK3p/ITi3p. The integrated intensities monotonically increase with the deposition of potassium.

Using the same method as reported in Figure 1, the gap states were deconvoluted into metallic states and IGSs. An example is shown in Figure 2(c) (see Figure S6 of Supporting Information for all the data sets). Figures 2(d) and 2(e) show the spectral contributions of the metallic states and IGSs, respectively. Since the spectra at HiSOR were recorded at low temperature (20K), the clear dip structures of the metallic state, caused by electron-phonon coupling, are clearly visible.8, 9, 25-27 Since the spectra at KEK were recorded at room temperatures, on the other hand, the dip structures of the metallic state were not clearly shown due to the thermal broadening effect, as shown in Figure 1 (d). The spectral width of the metallic states gradually increased with increasing spectral intensity. Comparing Figures 2(d) and 2(e) with the corresponding images of Figure 1, it becomes clear that the changes in the metallic states and IGSs by potassium deposition are different from those due to SR irradiation. First, the spectral intensity of the metallic state is preferentially enhanced by depositing potassium, compared with that by irradiating SR. On the contrary, the enhancement in the spectral intensity of the IGSs

4. DISCUSSION Potassium deposition dependence As shown by the black line in Figure 2(d), the metallic state of the aTiO2 surface before potassium deposition shows a clear dip structure at a binding energy of about 0.1 eV. From recent PES studies of a-TiO2,8, 9, 4


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27

it was suggested that the dip structure of the metallic state is derived from the electron-phonon coupling,30, 31 as discussed considerably for cuprates.29 The dip structure of the metallic state gradually becomes obscure with increasing amount of potassium. The metallic state before potassium deposition is formed by the native electron carriers in the bulk. After potassium deposition, on the other hand, the metallic state is derived from the electron carriers doped near the surface, in addition to the bulk contribution. The vestige of the similar electron-phonon coupling was also shown for the reduced surface of SrTiO3.32 Recently, such electron-phonon coupling strengths were reported to strongly depend on the carrier density near the surface.15, 32 In addition, it was shown that the surface phonon energy of a-TiO233 depends on the surface orientation, and is somewhat different from that of the bulk.30, 31 Therefore, it is considered that the observed successive changes in the spectral shape of the dip structure in the metallic state were caused by the increase in the electron carriers at the surface by depositing potassium, and/or by the change in the surface phonon energy.

also showed the dispersion-less state located near the midpoint of the band gap and discussed its origin in detail.34 In our previous PES studies on electron-doped SrTiO3, it was shown that the spectral intensity of the gap state is closely related with the number of doped electrons, suggesting that the gap state is not derived from the oxygen vacancies at the surface caused by the cleaving process or the short SR irradiation before the first measurement.20, 35-37 A similar IGS in the a-TiO2 was observed in the vicinity of the center of the band gap from resonant inelastic X-ray scattering measurements, and was concluded to be attributed to the oxygen vacancies in the bulk.31, 38 Therefore, it seems reasonable to consider that the IGS located at -1.60 eV in our PES spectra also represents an intrinsic electronic structure derived from the oxygen vacancies in the bulk and is unique to naturally grown a-TiO2. The IGS at -1.60 eV is a localized state, in which a fraction of the doped electron is tightly trapped in the vicinity of the oxygen vacancies. Therefore, the IGS at -1.60 eV is hereafter referred to as a deep-trap state (DTS). Theoretical calculations also predicted such a DTS for a-TiO2 with oxygen vacancies in the bulk.39, 40 Since the DTS has almost no spectral intensity around EF, it is considered that the (electron) carrier responsible for the conductivity in the bulk is derived from the metallic state. On the other hand, the (electron) carrier generation efficiency, or the dopability is defined as the ratio of the number of the electron carriers (that is, the spectral intensity of the metallic state) to the total number of the doped electrons (that is, the spectral intensity of the gap state consisting of not only the metallic state but also the DTS). Therefore, the DTS is closely related with the dopability of the doped electrons.

Just after cleaving, the finite spectral intensity in the vicinity of EF in the IGSs is clearly shown by black line in Figure 2(e). Observing it in detail, the spectral intensity gradually increases from EF, and the shape is somewhat different from a stepped-like feature characteristic of a FermiDirac function. This spectral intensity near EF appears almost uniformly across the entire measured momentum space (see Figure S7(a) of Supporting Information). For r-TiO2 having no metallic state, on the other hand, the IGS has almost no spectral intensity near EF, as shown in Figure S9(a) of Supporting Information. These results suggest that the finite IGS intensity near EF shown for a-TiO2 before potassium deposition is closely related with the metallic state in the bulk, i.e., it is a result of the secondary photoelectrons derived from the inelastic scattering of the metallic state in the bulk.

As shown in Figure 2 (e), the DTSs at -1.60 eV can still be observed as a shoulder structure even after potassium deposition. In the case that the electrons generated upon potassium deposition are doped deep inside the crystal, the DTS intensity is considered to decrease rapidly with increasing the amount of deposited potassium. The insensitivity of the DTS intensity to the deposition of potassium means that the electrons are doped only the vicinity of the surface.15 As a result, the DTS shown in the PES spectra just after cleaving is considered to be also observed even after potassium deposition. In order to extract the essential contribution of potassium deposition, we subtracted the DTS before potassium deposition from the IGSs after potassium deposition. From the difference spectra in Figure 3(b), it is clearly seen that another state gradually appears at -0.83 eV, which is almost constant regardless of the amount of deposited potassium (Figure 3 (c)). As shown in Figure S7 of Supporting Information, the state at -0.83 eV appears over a wide momentum area and shows no clear dispersion. Similar to the case of the DTS, a fraction of the doped electron due to potassium deposition is trapped in the vicinity of the potassium ions, which produces another localized state near the surface. The peak position of the localized state (-0.83 eV) induced by potassium deposition is about half that of the DTS (-1.60 eV) induced by the oxygen vacancies. Therefore, we refer to the localized state at -0.83 eV hereafter as a shallow-trap state (STS). The difference in the peak position of the localized states is considered to be caused by the difference in the valence between the oxygen vacancies (formally divalent) and the potassium ions at the surface (monovalent). The traps of the doped electrons in the vicinity of the monovalent potassium ions seem to be loose and incomplete, so that the STS induced by potassium deposition has a finite spectral intensity at EF. In Figure 3 (d), an example of the IGS spectra after potassium deposition (black solid circles), together with the DTS (blue open circles) and STS (red open circles) contributions, is shown. The DTS could be easily fitted by the Voigt function (blue line) with background (green line). Similarly, the STS could be well reproduced by the Voigt function multiplied

Next, we shall discuss the enhancement in the IGS intensity near EF caused by the deposition of potassium, shown in Figure 2(e). As the IGS intensity near EF increases with the deposition of potassium, the background intensity around the binding energy of -3 eV also increases, but considerably small compared to the increase in the IGS intensity near EF. Interestingly, when potassium is heavily deposited (K #8), the background intensity becomes smaller than that of the IGS near EF. These results suggest that the net increase in the IGS intensity near EF due to potassium deposition is mainly caused by a phenomenon that is different from the inelastic scattering of the metallic state in the bulk. As will be discussed later in detail, another dispersion-less state appears at 0.83e V by potassium deposition. The state at -0.83 eV is close to EF compared to the IGS derived from the oxygen vacancies in the bulk (1.60 eV), so that there is a finite spectral intensity near EF. Assuming that the electrons generated by depositing potassium are doped only into the vicinity of surface,15 the state at -0.83 eV induced by potassium deposition is considered to be less subject to the inelastic scattering. That is, since the inelastic background is in first order only dependent on the inelastic mean free path of electrons, it can be reasoned that electrons emitted from the vicinity of the topmost surface will contribute less to the background compared to those emitted from deeper inside the crystal. Subsequently, we will discuss the origin of the IGS at -1.60 eV and the state at -0.83 eV shown for the potassium-deposited surface. In Figure 3 (a), the IGS of the pristine surface just after cleaving is shown. It consists of a dispersion-less state located at a binding energy of -1.60 eV and the inelastic background. On the basis of the spectral width, the IGS has no spectral intensity near EF, suggesting no contribution to electrical conduction. Previous soft-x-ray PES spectra of electron-doped SrTiO3 5



with a Fermi-Dirac function (red line). Interestingly, the spectral shape of the STS is almost the same as that of the DTS, as shown in Figure 3 (e), except for the intensity cutoff at EF in the STS intensity.

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SR irradiation dependence Next, we will discuss the spectral change in the PES spectra of the aTiO2 surface after SR irradiation. As shown from the red line of Figure 1 (f), the spectral intensity of the metallic state initially increases with irradiation time, then saturates, and finally decreases slowly. This implies that, in the detection region (probing depth) of the photoelectrons, the electrons due to oxygen vacancies are partially doped as carriers during the initial stage of the irradiation, but not at all during the later stage. Interestingly, one finds that the change in the peak position of the IGS (dotted line) is closely related to the spectral intensity of the metallic state (red line). However, the peak shift of the IGS as a result of the irradiation cannot be predicted by the rigid-band model. The background intensity around -3 eV increases slowly but clearly upon irradiation (Figure 1 (b)), whereas it does not increase as much with potassium deposition (Figure 2 (b)). This suggests that the oxygen vacancies as a result of the irradiation were generated not only in the vicinity of the outmost surface but also deep portion under the surface. As a result, the observed increase in the background intensity around -3 eV is considered to be derived from the scattering of photoelectrons by the oxygen vacancies generated at the deep portion under the surface. As shown by the black line in Figure 3 (e), the spectral shape of the IGS induced by the irradiation is considerably asymmetric. The spectral shape of the IGS on the low binding-energy side is similar to that of the DTS and STS, whereas, on the high binding-energy side, the spectral intensity of the IGS is stronger than those of DTS and STS (gray area). Assuming that the peak position of the IGS induced by the irradiation depends on the density of the oxygen vacancies, the observed asymmetric spectral shape of IGS can be explained by the disproportion in the density of oxygen vacancies induced by the irradiation. Considering the probing depth of photoelectron without any scattering, it is considered that the IGS near the peak position is mainly caused by the oxygen vacancies in the vicinity of the surface, and the enhancement in the spectral intensity at the high energy side is derived from the oxygen vacancies at the subsurface. In addition, the successive peak shifts in the IGS with irradiation time, shown in Figures 1 (e) and 1(f), can be easily understood on the basis of this assumption. Disturbing influence on the conduction pathway Figure 4 presents a schematic view of the gap state due to the electrons doped near the surface by (a) potassium deposition and (b) SR irradiation. Understanding the microscopic relationship between the metallic state and the IGS on the surface will lead to elucidating the macroscopic semiconducting properties such as the (electron) carrier generation efficiency and mobility.

Figure 3. (a) IGS spectrum before potassium deposition (blue solid circles). The Shirley background derived from the metallic state is denoted by the green line. An empirical DTS (blue open circles) was estimated by subtracting the background (BG) from the IGS. The DTS was fitted using the Voigt function (DTSVoigt, blue thin line). The peak position of the DTS was estimated at -1.60 eV. (b) Difference spectra before and after potassium deposition. (c) Peak position and integrated intensity of the difference spectra. (d) An example of the IGS spectra after potassium deposition. The empirical shallow-trap state (STS; red open circles) was estimated by subtracting the BG and DTSVoigt from the IGS. The STS could be well fitted with the Voigt function multiplied with a Fermi-Dirac function (red line). The peak position was estimated at 0.83 eV. (e) Comparison of the DTS induced by the oxygen vacancies in the bulk (blue line), the STS induced by potassium deposition on the surface (red line), and the IGS by SR irradiation (black line). In order to compare the spectral shapes, the energy position of each components was aligned, and then normalized with respect to the peak height. The EF positions of the IGSs by potassium deposition and by SR irradiation were denoted by the red and black vertical lines, respectively.

The enhancement in the metallic state as a result of potassium deposition can be interpreted as being simply caused by the doping of the electrons into the surface state called 2DEG/L.9 In other word, many electrons doped by potassium deposition are considered to function as the (electron) carriers responsible for the surface conductivity. In addition to the metallic surface state, a fraction of the doped electron is loosely and incompletely trapped around the monovalent deposited potassium cations (-0.83 eV) to form a STS having a finite spectral intensity at EF. These electrons doped into the STS are considered also to contribute to the surface conductivity. That is, there are two kinds of (electron) carrier sources: the metallic state and the STS induced by potassium deposition. The potassium ions on the surface do not directly or indirectly disturb the (electron) carrier motion in the conduction pathway. The ratio of the spectral intensities of the metallic state and the STS is hardly affected by the amount of deposited potassium. It seems that 6


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the (electron) carriers at the surface are effectively generated by potassium deposition, and that the efficiency is hardly sensitive to the amount of potassium deposited in this study.

the crystallinity near the surface, which might in turn lead to the localization of the itinerant electrons forming the 2DEG. As shown in section 5 of Supporting Information, a few oxygen vacancies are generated on the surface by irradiating with the SR at HiSOR, and preferentially cause the surface metallic states. Assuming that the oxygen vacancies due to less-intense SR irradiation at HiSOR are not generated below the surface but only at the topmost surface (see Figure S12 of Supporting Information), we consider that the doped electrons are partially trapped by the surface Ti ions neighboring the oxygen vacancies at the topmost surface, but most of the remaining electrons are doped into the non-disturbed (or vacancy-free) conduction pathway below the surface and behave as electron carriers. From the viewpoint of electron supply, that is, the topmost surface containing the oxygen vacancies is considered to be similar in the potassium-deposited surface. In other words, from the viewpoint of the disturbing influence on the conduction pathway for electron carriers, we speculate that the doping mechanism by less-intense SR/UV irradiation is similar to that by potassium deposition. On the other hand, in the case of the usage of intense light which can induce the oxygen vacancies not only at the topmost surface but also below the subsurface, the electrons generated by the oxygen vacancies are doped into the conduction pathway disturbed by the oxygen vacancies. When the oxygen vacancies in the conduction pathway below the subsurface are not so much, a fraction of the doped electron is able to function as electron carriers. As the oxygen vacancies in the conduction pathway exceed a certain threshold, however, the doped electrons are hard to behave as electron carriers in the disturbed conduction pathway, as shown in Figure 4(b).

Minohara et al. showed that electrons can be doped onto a-TiO2 by a heterointerface between a-TiO2 and the LaO-terminated polar surface of LaAlO3 (001).41 On the basis of the reported sheet carrier density, most of the electrons generated at the interface are considered to behave as electron carriers. In addition, the mobility observed empirically is high, compared with that of the impurity/vacancy-doped a-TiO2 reported previously. Although such semiconducting properties are thought to be originated from electron carriers doped into ideal conduction pathway without the disturbance, it is hard to explain the temperature dependence of the resistivity merely based on the metallic state. Like the potassium-deposited a-TiO2 surface shown here, a fraction of the electron doped onto the interface is considered to be weakly and indirectly trapped/scattered by the lanthanum/oxygen ions outside the conduction pathway. As a result, it is expected that loosely and incompletely trapped (or weakly and indirectly scattered) electrons also partially contribute to the interface conductivity, along with the coherent electrons. This scenario may explain the peculiar temperature dependent resistivity.

5. CONCLUSIONS In this study, we used naturally-grown a-TiO2 with oxygen vacancies in the bulk. PES spectra showed that electrons generated by the oxygen vacancies result in a metallic state just below EF and a localized state tightly trapped by the oxygen vacancies (deep-trap state) within the energy gap. We doped additional electrons into the conduction pathway near the surfaces by in site deposition of potassium, and by irradiation of intense SR. Like the doped electrons in the bulk, most of the doped electrons generated by depositing potassium contribute to the metallic state just below EF, and function as the carriers responsible for the surface conductivity. The electrons doped at the outermost surface are loosely trapped by the deposited potassium ions and produce a quasi-localized state in the band gap (shallow-trap state). The shallow-trap state has a finite spectral intensity at EF, so that it is considered to contribute in part to the surface conductivity. Therefore, the electron carriers in the conduction pathway below the topmost surface are supplied by the potassium ions outside the conduction pathway. Since there are no vacancies such as oxygen and titanium in the conduction pathway, the motion of the doped electrons is supposed to be only weakly and indirectly influenced by the potassium ions. For the intense SR irradiation, on the other hand, oxygen vacancies are generated not only at the topmost surface but also below the surface. In the case that the number of oxygen vacancies generated by SR irradiation remains below a critical threshold, PES spectra showed that the doped electrons behave as the itinerant metallic state, in addition to the localized state within the band gap. The localized state in the vicinity of the oxygen vacancies near the surface is found to be more tightly trapped compared with the shallow-trap state by the potassium ions and hardly contribute to the conductivity. As the oxygen vacancies further increase, all the doped electrons near the surface are

Figure 4. Schematic view of the gap state of a-TiO2 due to the electrons doped by (a) potassium deposition and (b) SR irradiation. In the case that the oxygen vacancies are generated by irradiating with the intense SR, on the other hand, it is supposed that many doped electrons are more tightly trapped around these vacancies near the surface, as shown in Figure 4(b). The IGS reveals almost no spectral intensity at EF, suggesting a negligible contribution to the conductivity. The oxygen vacancies cause a direct disorder in the conduction pathway, so that the intensity of the metallic state relative to the IGS, or the (electron) carrier generation efficiency, is considered to decrease with increasing oxygen vacancies. This is consistent with the results by Rödel et al., as shown in Figure 3 of Ref. [9]. They concluded that a high concentration of oxygen vacancies induced by intense SR irradiation destroys 7



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their Interaction with Dye and Solvent Molecules. J. Phys. Chem. C 2007, 111, 849-854. (5) Lichterman, M. F.; Hu, S.; Richter, M. H.; Crumlin, E. J.; Axnanda, S.; Favaro, M.; Drisdell, W.; Hussain, Z.; Mayer, T.; Brunschwig, B. S.; et al. Direct Observation of the Energetics at a Semiconductor/Liquid Junction by Operando X-ray Photoelectron Spectroscopy. Energy Environ. Sci. 2015, 8, 2409-2416. (6) Baldini, E.; Chiodo, L.; Dominguez, A.; Palummo, M.; Moser, S.; Yazdi-Rizi, M.; Auböck, G.; Mallett, B. P. P.; Berger, H.; Magrez, A.; et al. Strongly Bound Excitons in Anatase TiO2 Single Crystals and Nanoparticles. Nat. Commun. 2017, 8, 13. (7) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (8) Moser, S.; Moreschini, L.; Jaćimović, J.; Barišić, O. S.; Berger, H.; Magrez, A.; Chang, Y. J.; Kim, K. S.; Bostwick, A.; Rotenberg, E.; et al. Tunable Polaronic Conduction in Anatase TiO2. Phys. Rev. Lett. 2013, 110, 196403. (9) Rödel, T. C.; Fortuna, F.; Bertran, F.; Gabay, M.; Rozenberg, M. J.; Santander-Syro, A. F.; Le Fèvre, P. Engineering Two-Dimensional Electron Gases at the (001) and (101) Surfaces of TiO2 Anatase using Light. Phys. Rev. B 2015, 92, 041106(R). (10) Wang, Z.; Zhong, Z.; McKeown Walker, S.; Ristic, Z.; Ma, J. -Z.; Bruno, F. Y.; Riccò, S.; Sangiovanni, G.; Eres, G.; Plumb, N. C.; et al. Atomically Precise Lateral Modulation of a Two-Dimensional Electron Liquid in Anatase TiO2 Thin Films. Nano Lett. 2017, 17, 2561-2567. (11) Thomas, A. G.; Flavell, W. R.; Mallick, A. K.; Kumarasinghe, A. R.;

trapped and localized. That is, the motion of the doped electrons is supposed to be strongly and directly disturbed by the oxygen vacancies inside the conduction pathway. Since the microscopic electronic behaviors of the doped electrons of a-TiO2 are critically involved in the macroscopic charge transport, catalysis and charge transfer processes, our results on the competition between itineracy and localization of doped electrons based on the disturbance of the conduction pathway will be expected to contribute to improving the functionality of existing TiO2-based devices and/or developing innovate technologies such as water splitting and dye sensitized solar cells, that utilizes the change in physical properties near the surface.

ASSOCIATED CONTENT Supporting Information This material is available free of charge on the ACS Publications website at DOI: ARPES images of pristine a-TiO2 and r-TiO2; PES spectra of rTiO2; Ti 3p core spectra of a-TiO2; Potassium deposition dependence of gap states; SR irradiation damage of gap states at HiSOR; Potassium amount deposited on a-TiO2 surface (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] **E-mail: [email protected]

Tsoutsou, D.; Khan, N.; Chatwin, C.; Rayner, S.; Smith, G. C.; Stockbauer, R. L.; et al. Comparison of the Electronic Structure of Anatase and Rutile TiO2 Single-Crystal Surfaces using Resonant Photoemission and X-ray Absorption Spectroscopy. Phys. Rev. B 2007, 75, 035105. (12) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. Ø.; Matthiesen, J.; Blekinge-Rasmussen, A.; Lægsgaard, E.; Hammer, B.; et al. The Role of Interstitial Sites in the Ti3d Defect State in the Band Gap of Titania. Science 2008, 320, 1755-1759. (13) Yim, C. M.; Pang, C. L.; Thornton, G. Oxygen Vacancy Origin of the Surface Band-Gap State of TiO2(110). Phys. Rev. Lett. 2010, 104, 036806.

ORCID Yoshihiro Aiura: 0000-0002-4478-7680 Kenichi Ozawa: 0000-0003-2157-4671 Eike F. Schwier: 0000-0003-3881-4045 Kenya Shimada: 0000-0002-1945-2352 Kazuhiko Mase: 0000-0002-3613-0021

Notes The authors declare no competing financial interest.

(14) Sánchez-Sánchez, C.; Garnier, M. G.; Aebi, P.; Blanco-Rey, M.; de Andres, P. L.; Martín-Gago, J. A.; López, M. F. Valence Band Electronic Structure Characterization of the Rutile TiO2 (110)-(1×2) Reconstructed Surface. Surf. Sci. 2013, 608, 92-96. (15) Yukawa, R.; Minohara, M.; Shiga, D.; Kitamura, M.; Mitsuhashi, T.; Kobayashi, M.; Horiba, K.; Kumigashira, H. Control of Two-Dimensional Electronic States at Anatase TiO2 (001) Surface by K Adsorption. Phys. Rev. B 2018, 97, 165428. (16) Eisaki, H.; Kaneko, N.; Feng, D. L.; Damascelli, A.; Mang, P. K.; Shen,

ACKNOWLEDGMENT Y.A. acknowledges Dr. Naoto Kikuchi and Ms. Akane Samizo for the composition analysis, and Dr. Makoto Minohara for discussing the semiconducting properties and preparing Figure S12 in the supporting Information. The ARPES measurements were performed under the approval of the Photon Factory Advisory Committee (Proposal No. 2017G694) and the approval from HiSOR (proposal No. 17AG003). This work was supported in part by a Grant-in-Aid for Scientific Research (Grant No. 16H03867) from Ministry of Education, Culture, Sports, Science, and Technology of Japan.

K. M.; Shen, Z.-X.; Greven, M. Effect of Chemical Inhomogeneity in Bismuth-Based Copper Oxide Superconductors. Phys. Rev. B 2004, 69, 064512.

(17) Asahi, R.; Taga, Y.; Mannstadt, W.; Freeman, A. J. Electronic and Optical Properties of Anatase TiO2. Phys. Rev. B 2000, 61, 7459-7465. (18) Dulub, O.; Diebold, U. Preparation of a Pristine TiO2 Anatase (101) Surface by Cleaving. J. Phys. Condens. Matter 2010, 22, 084014. (19) Aiura, Y.; Kawanaka, H.; Bando, H.; Yasue, T. Ultraviolet Photoemission Study of CaVO3. Surf. Sci. 2001, 492, 249-254. (20) Aiura, Y.; Hase, I.; Bando, H.; Yasue, T.; Saitoh, T.; Dessau, D. S. Photoemission Study of the Metallic State of Lightly Electron-Doped SrTiO3. Surf. Sci. 2002, 515, 61-74. (21) Iwasawa, H.; Shimada, K.; Schwier, E. F.; Zheng, M.; Kojima, Y.; Hayashi, H.; Jiang, J.; Higashiguchi, M.; Aiura, Y.; Namatamea, H.; et al. Rotatable High-Resolution ARPES System for Tunable Linear-Polarization Geometry. J. Synchrotron Rad. 2017, 24, 836–841.

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Competition between Itineracy and Localization of Electrons Doped

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