Homogeneous Electron Doping into Nonstoichiometric Strontium

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Homogeneous Electron Doping into Non-Stoichiometric Strontium Titanate Improves Its Photocatalytic Activity for Hydrogen and Oxygen Evolution Shunta Nishioka, Junji Hyodo, Junie Jhon M. Vequizo, Shunsuke Yamashita, Hiromu Kumagai, Koji Kimoto, Akira Yamakata, Yoshihiro Yamazaki, and Kazuhiko Maeda ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01379 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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Homogeneous Electron Doping into Non-Stoichiometric Strontium Titanate Improves Its Photocatalytic Activity for Hydrogen and Oxygen Evolution Shunta Nishioka,a† Junji Hyodo,b† Junie Jhon M. Vequizo,c Shunsuke Yamashita,d Hiromu Kumagai,a Koji Kimoto,d Akira Yamakata,c Yoshihiro Yamazaki,bef* and Kazuhiko Maedaa* a

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.

b

INAMORI Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 8190395, Japan c

Graduate School of Engineering, Toyota Technical Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan

d

Electron Microscopy Group, Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan e

Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan f

Kyushu University Platform of Inter-/Transdisciplinary Energy Research (Q-PIT), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan †

The authors equally contributed to this work.

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ABSTRACT

Water splitting using a semiconductor photocatalyst has been extensively studied as a means of solar-to-hydrogen energy conversion. Powder-based semiconductor photocatalysts, in particular, have tremendous potential in cost mitigation due to system simplicity and scalability. The control and implementation of powder-based photocatalysts are, in reality, quite complex. The identification of the semiconductor-photocatalytic activity relationship and its limiting factor has not been fully solved in any powder-based semiconductor photocatalyst. In this work, we present systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution using a model strontium titanate powder/aqueous solution interface in a half reaction. The electron density was controlled from 1016 to 1020 cm-3 throughout the strontium titanate powder by charge compensation with oxygen non-stoichiometry (the amount of oxygen vacancy) while maintaining its crystallinity, chemical composition, powder morphology, and the crystal and electronic structure of surface. The photocatalytic activity of hydrogen evolution from aqueous methanol solution was stable and enhanced by forty-fold by the electron doping. The enhancement was correlated well with increased Δabsorbance, an indication of prolonged lifetime of photoexcited electrons, observed by transient absorption spectroscopy. Photocatalytic activity of oxygen evolution from aqueous silver nitrate solution was also enhanced by three-fold by the electron doping. Linear correlation was found between the photocatalytic activity and the degree of surface band bending, ΔΦ, above 1.38 V. The band bending, potential downhill for electronic holes, enlarges the total flux of photoexcited holes towards surface, which drives the oxygen evolution reaction.

KEYWORDS


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Artificial photosynthesis, Defect chemistry, Heterogeneous photocatalysis, Solar energy conversion, Water splitting INTRODUCTION Photocatalytic water splitting has drawn significant attention as a potential means of H2 production using solar energy.1,2 Many semiconductors that contain early transition metal cations with d0 electronic configurations (e.g., Ti4+, Nb5+, or Ta5+) or d10 typical metal cations (e.g., Ga3+, Sn4+, or Sb5+) loaded with a suitable cocatalyst have been reported to activate overall water splitting.1–19 Metal oxide photocatalysts, particularly those with the ABO3 perovskite structure, have been widely studied because of their wide variety of possible compositions and high chemical stability under irradiation.1–14,20,21 They have large potential to mitigate fabrication costs and accelerate large-scale hydrogen production because of the system simplicity (i.e., no electrodes and wires required).2,3 Development of high-performance photocatalysts, however, remains challenging over the last half century. High-performance photocatalysts typically requires complex morphology of semiconductor powder loaded with a nanoparticulate cocatalyst, the latter sometimes composed of core shell structure, along with a specific combination of materials, chemical composition, and preparation condition.3,4,6,7,16 This complexity makes it difficult to understand what the most decisive factor for resulting photocatalytic activity is, the characteristics of semiconductor powder or cocatalyst, or the chemical, physical, or morphological aspects of the semiconductor and cocatalyst interface. The photocatalytic activity is also very sensitive to many aspects of the semiconductor characteristics (e.g. chemical composition, crystal structure, crystallinity, defect type and density, surface area, surface electronic structure, and interface structure) of powder, making the practical


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control and implication of powder-based photocatalysts further complex. For example, hightemperature calcination improves the crystallinity of powder, which is advantageous for photocatalytic activity (it reduces the density of structural defects that can act as electron-hole recombination sites).1–7 High-temperature calcination also decreases the surface area (the density of reaction sites) of powder, which is disadvantageous to photocatalytic activity.22 The overall photocatalytic activity is the consequence of photoexcited carrier generation, carrier transfer toward each reaction site, and successful reactions under driving force. Therefore, it is difficult to understand which chemical, physical, morphological aspect of semiconductor powder results in the observed photocatalytic activity, especially in a quantitative manner. As such, decisive factor and step that kinetically limit the photocatalytic activity of semiconductor powders remain unknown. Defects in powder photocatalysts also have a significant impact on photocatalytic activity for overall water splitting (mostly negative),3,9,21 and for the half reactions of hydrogen or oxygen evolution (sometimes positive).23–32 Mao et al. have reported enhanced photocatalytic activity for H2 evolution from an aqueous methanol solution using reduced TiO2 powder in which defects are accumulated on the surface.28 More qualitative study about the degradation of methylene blue using reduced rutile TiO2–δ powder suggested the influence of the Fermi level and surface defects.33 Much less has been known in SrTiO3 system about the defect-photocatalytic activity relationship. The enhancement of photocatalytic activity for H2 evolution from aqueous methanol solution, particularly in UV region, was similarly reported when suitable density of defects (such as Ti3+ and/or oxygen defects) was introduced.29–31 In particular, the oxygen vacancies have accumulated only on the powder surface, making inhomogeneous microstructure.29 Cation nonstoichiometry in Y2Ti2O7 pyrochlore also changed its photocatalytic hydrogen evolution from 10


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vol% methanol solution.34 Although these results ensure that defects in powder photocatalysts influence its photocatalytic activity, no quantitative correlation between defects and photocatalytic activity has been obtained in any powder-based semiconductor since the seminal work by Fujishima and Honda.35 The challenge of rational design in powder-based photocatalysts, in part, lies in the complex morphology of semiconductor/cocatalyst3,4,6,7,16 and the unknown impact of semiconductor characteristics on overall photocatalytic activity. The latter would relate to the difficulty of precise control of defect type and density in the semiconducting powder, the quantitative determination of their semiconducting characteristics (e.g., electron density and Fermi level), and the establishment of quantitative correlation with photocatalytic activity while maintaining other structural and morphological factors. Inhomogeneous microstructure of reduced TiO2 (ref. 28) and SrTiO3 (ref. 29) powders (defect accumulation on the surface), in fact, prevented the estimation of electron density and Fermi level. Lack of such information has hindered a fundamental understanding of which factor limits the photocatalytic activity of semiconducting powders and of how to rationally maximize the utilization of absorbed light defined by its band gap. In this study, we present a systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution in a half reaction using a model powder system, non-stoichiometric strontium titanate (SrTiO3–δ) powder. The material is both technologically relevant (one of the most studied photocatalysts that exhibit high hydrogen evolution activity)12,22,36-38 and experimentally manageable (combined with comprehensive literatures on the non-stoichiometry, electron density, and the mobility of electrons and holes).39-42 To solely evaluate the SrTiO3– δ/aqueous

solution interface, experiments on photocatalytic activity and photogenerated carrier


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dynamics were performed without the loading of cocatalysts. To further avoid complexity in the carrier separation dynamics towards the proton reduction and water oxidation reaction sites, and to assure that the hydrogen or oxygen evolution reaction kinetics become the rate-limiting step, the half reaction was investigated using methanol or AgNO3 aqueous solution. Unlike previous studies,28-29 the oxygen vacancies were homogeneously introduced throughout the powers by high temperature annealing while maintaining its crystallinity, chemical composition, powder morphology, and the surface crystal and electronic structure. It enables to control the electron density of SrTiO3–δ powder from 1016 to 1020 cm-3 and quantitatively assess the sole impact of the electron density and Fermi level on its photocatalytic activity. A possible variety of techniques such as the electrochemical, optical, mass, and X-ray photoelectron spectroscopies, and electron microscopy were employed to confirm physicochemical similarity between the SrTiO3-δ powder. Overall effort makes it possible to evaluate crucial factor that controls the gas evolution activity from the SrTiO3–δ/aqueous solution interface in the half reactions.

METHOD

Oxygen vacancies, electron density and Fermi level of non-stoichiometric SrTiO3-δ

Oxygen vacancies are one of the most common forms of defects that exist in metal oxides, and have been extensively studied in the fields of solid-state chemistry and defect chemistry.39–42 Under a reducing atmosphere (low oxygen partial pressure), the formation of oxygen vacancies is thermodynamically preferred in a metal oxide and their density increases with decreasing oxygen partial pressure, which accompanies with electron doping into the oxide due to charge compensation. Under thermodynamic equilibrium, the electron density is quantitatively defined by thermodynamic quantities, temperature and oxygen partial pressure.


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Two types of oxygen vacancies, that are, doubly42 and singly41 charged oxygen vacancies, have been reported in the SrTiO3–δ system depending on its temperature. At a high temperature, the doubly-charged oxygen vacancies are created in charge compensation with electrons, which are expressed using Kröger–Vink notation,43 𝟏

𝐎𝒙𝐎 = 𝐕𝐎∙∙ + 𝟐𝐞, + 𝟐 𝐎𝟐 ,

(1)

where 𝐎×𝐎 and 𝐕𝐎∙∙ represent oxygen and vacancy in the oxygen site (the latter called as oxygen vacancy), respectively, in the oxide. Singly-charged oxygen vacancies, 𝐕𝐎∙ , are formed at a lower temperature at which electrons are thermodynamically favored to be trapped by the doubly charged oxygen vacancies, 𝐕𝐎∙∙ + 𝒆, = 𝐕𝐎∙ .

(2)

Under an ideal solution limit, the equilibrium constant of reactions in Equations (1) and (2), K1 and K2, are expressed as 𝟏

𝑲𝟏 =

[𝐕𝐎∙∙ ]𝒏𝟐𝑷𝟐𝐎 𝐱5 3𝐎𝐎

𝟐

,

(3)

and 𝑲𝟐 =

[𝐕𝐎∙ ]

3𝐕𝐎∙∙ 5𝒏

,

(4)

respectively, where the brackets denote the concentration of the defect, n represents the electron density, and 𝑷𝐎𝟐 represents the oxygen partial pressure. The electroneutrality of the system is given as 𝒏 = 𝟐[𝐕𝐎∙∙ ] + [𝐕𝐎∙ ].

(5)

When the singly-charged oxygen defect dominates the system, Equation (5) is approximated to be 𝒏 ≈ [𝐕𝐎∙ ].

(6)


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Equations (3), (4) and (6) result in the oxygen partial pressure dependence of -1/4 slope, 𝟏

𝟏

𝐥𝐧 𝒏 = 𝟐 𝐥𝐧([𝐎𝒙𝐎 ]𝑲𝟏 𝑲𝟐 ) − 𝟒 𝐥𝐧 𝑷𝐎𝟐 .

(7)

The –1/4 dependence has been reported in the SrTiO3–δ system at room temperature41 whereas 1/6 dependence has been reported at high temperatures.42 The increased electron density of SrTiO3–δ shifts its Fermi level, EF, towards the conduction band. In an n-type semiconductor in which donor density is larger than the intrinsic carrier density, the Fermi level is expressed as44 𝑬𝐝𝐚𝐫𝐤 = 𝑬𝐝𝐚𝐫𝐤 + 𝐅 𝐂

𝒌𝐁 𝑻 𝒆

𝑵𝐝𝐚𝐫𝐤

𝐂 𝐥𝐧 G 𝒏𝐝𝐚𝐫𝐤 I,

(8)

is the conduction band potential, 𝑵𝐝𝐚𝐫𝐤 is the density of states in the conduction where 𝑬𝐝𝐚𝐫𝐤 𝐂 𝐂 band, the superscript dark explicitly expresses the value without irradiation, and kB is the Boltzmann constant. Inserting the electron density of sample into Equation (8) gives the Fermi level of each SrTiO3–δ when 𝑬𝐝𝐚𝐫𝐤 and 𝑵𝐝𝐚𝐫𝐤 are known. 𝐂 𝐂

Preparation of SrTiO3–δ powder

An oxide precursor of Sr and Ti was prepared by the polymerized complex method reported by Kakihana.45 Ti(O-iPr)4 (Kanto Chemicals, >97.0%), SrCO3 (Kanto Chemicals, >96.0%), ethylene glycol (Kanto Chemicals, >99.5%), and anhydrous citric acid (Wako Pure Chemicals, >98.0%) were dissolved in methanol (Kanto Chemical, ≥99.8%) solution at a molar ratio of 1:1:40:10. The solution was stirred and heated at 573 K on a hot plate, which yielded a glassy resin. The resin was, then, heated at 673 K in a mantle heater. The resultant black powder was subsequently calcined on an Al2O3 plate at 823 K for 5 h in air.


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The obtained white powder was lightly molded into a cylindrical shape and annealed at 1423 K for 2 h under controlled oxygen partial pressure in an infrared furnace (ULVAC, VHT-E44), with a ramp rate of 500 K min–1 for both heating and cooling processes. The temperature was controlled and monitored using an S-type thermocouple located beside the sample. For precise control of the oxygen partial pressure, high-purity certified gases such as 10 ppm O2 balanced with Ar, dry air, dry H2, and humidified H2 diluted with Ar were introduced into the chamber with a flow rate of 300 mL min–1. The oxygen partial pressure of exhaust gas was monitored with an oxygen sensor (SETNAG, MicroPoas) operated at ca. 1073 K. The system is quite tight against leakage, where 11 ppm of oxygen partial pressure was detected using the oxygen sensor when certified 10 ppm O2 gas balanced with Ar was supplied. Each annealing resulted in slightly different amount of oxygen vacancies, thus electron density in SrTiO3–δ powder because the control of oxygen partial pressure was not perfect. Annealing time to introduce defect homogeneously throughout powder was chosen based on powder size and an analysis using a solution of Fick’s second law (Figure S1).

Crystallographic, electronic, and chemical characterizations in bulk and near surface

The as-prepared powder samples were characterized using powder X-ray diffraction (XRD; Bruker AXS, D2 PHASER, Cu Kα), UV-visible diffuse reflectance spectroscopy (DRS; JASCO, V−565), scanning electron microscopy (SEM; Hitachi, S−4700), Cs-corrected scanning transmission electron microscopy (STEM; FEI, Titan3) and X-ray photoelectron spectroscopy (XPS; Shimadzu, ESCA−3400). The accelerating voltage, the probe current, and the convergence angle of STEM observation were 300 kV, 21 pA, and 25 mrad, respectively. XPS spectra were recorded with the pass energy of 75 eV. Peak deconvolutions were conducted using


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Shirley background and gauss function. The binding energies were corrected with respect to the position of C 1s peak (285.0 eV) for each sample. The chemical composition of SrTiO3−δ was determined using inductively coupled plasma mass spectrometry (ICP-MS; Agilent Technologies, 7700x). The Brunauer-Emmett-Teller (BET) surface area was measured using a gas adsorption apparatus (BEL Japan, BELSORP-mini) at liquid nitrogen temperature (77 K).

Electrochemical characterization

AC impedance spectroscopy (Biologic, VSP−300) and the DC four-point method (Keithley, 2182A and 2400) were used to measure the resistance of a single crystal SrTiO3−δ pellet (commercially available from Shinkosha Co., Ltd.). The pellet was cut into a bar form, annealed at 1423 K for 12 h under various oxygen partial pressure, and immediately quenched to room temperature in the infrared furnace. For electrochemical measurements, four Ag electrodes were deposited onto the pellets using DC sputtering system (Sanyu Electron Co., Ltd., SC-701HMC II) and connected to Au wires by Ag paste.

Photocatalytic reactions

Reactions were conducted at room temperature using a top-irradiation type cell connected to a closed gas circulation system. 100 mg of SrTiO3–δ powder was dispersed in 10 vol% methanol aqueous solution (pH = 7) or 50 mM AgNO3 aqueous solution (140 mL, pH = 5). After outgassing the reactant solution with a vacuum pump, Ar gas (ca. 5 kPa) was introduced into the reaction system and the solution was irradiated under a 300 W Xe lamp (Cermax, PE300BF) with an output current of 20 A. The irradiation wavelength was controlled by the combination of a total reflection mirror and water filter (λ >300 nm). A HAL-320 solar simulator (Asahi Spectra)


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with a power density of (100 mW cm–2) was also employed as the light source. The evolved gases were analyzed using gas chromatography (GL Sciences, GC–3200 with a thermal conductivity detector (TCD), argon carrier gas). The apparent quantum yield (AQY) was calculated using the following equation: AQY(%) = (A × R/I) × 100,

(8)

where A, R, and I represent coefficients (2 for H2 evolution and 4 for O2 evolution), the H2 (or O2) evolution rate, and the rate of incident photons, respectively. The total number of incident photons (0.88 mW) from a 300 W xenon lamp (Asahi Spectra, MAX-303) was measured using a power meter. The irradiation wavelength was controlled by a band-pass filter and water filter (λ = 300 nm).

Transient absorption spectroscopy

Transient absorption spectroscopy measurements were performed using laboratory-built spectrometers described previously.46,47 Briefly, SrTiO3–δ samples were excited by 355 nm laser pulses from a Nd:YAG laser (Continuum, Surelite I, 6 ns duration, 0.5 mJ, 1–5 Hz), and the transmittance and reflectance were measured below and above 6000 cm-1, respectively, using mid-IR (6,000–1,000 cm–1; MoSi coil), the near-IR (10,000–6,000 cm–1; halogen lamp, 50 W), and the visible (25,000–10,000 cm–1; halogen lamp, 50 W) light irradiation. The probe light was dispersed by the spectrometer, and the monochromated mid-IR, near-IR, and visible light was detected with an MCT detector (Kolmar), an InGaAs detector, and a Si photodiode, respectively. The time resolution of this spectrometer was limited to 1–2 µs by the AC-coupled amplifier (Stanford Research Systems, SR560) in the mid-IR and the near-IR region, and that was limited


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to ca. 4 µs by the stray light of the pump pulse and/or short-lived strong emission from the sample in the visible region.

Figure 1. a) Lattice constant, b) representative (mode) particle size, c) BET surface area, and d) Sr/Ti cation ratio plotted against calculated electron density for nonstoichiometric SrTiO3-δ powders.

RESULTS AND DISCUSSION

Crystallographic, morphological and chemical characteristics in bulk

Non-stoichiometric SrTiO3–δ powders annealed at 1423 K under various oxygen partial pressure were prepared and characterized using X-ray diffraction, scanning electron microscopy (SEM), BET surface measurement, and inductively coupled plasma mass spectrometry (ICP-MS). X-ray diffraction patterns indicate single perovskite phase for all powder samples without any indication of secondary phase (Figure S2). The Rietveld refinement shows good fitting (GOF


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≤1.84) to the space group of 𝑷𝒎𝟑𝒎 . All parameters obtained by the refinement were summarized in Table S1. Lattice constant (Figure 1a) and crystallite size (Figure S3) were almost identical among the samples. SEM observations showed that particles were agglomerated, with its particle distribution from 200 nm to ~1 µm (Figures S4 and 5). Representative (mode) particle size was found around 430 nm, consistent for all non-stoichiometric powders (Figure 1b). The BET specific surface areas of these powders were quite similar considering the experimental errors for small surface area samples, 2.1 m2 g-1 in average (Figure 1c). The ratios of Sr/Ti were reproducibly determined as approximately unity, 0.98–1.00, using ICP-MS with the experimental errors of 0.01–0.02 (Figure 1d). All results show that the prepared non-stoichiometric SrTiO3–δ powders are quite similar in crystal structure, powder morphology, and chemical composition.

Crystallographic and electronic structure near surface

Near-surface regions of non-stoichiometric SrTiO3–δ powders were observed using scanning transmission electron microscopy (STEM, Figures 2a, 2b, and S6). Figures 2a and 2b show the annular bright field (ABF)- and high-angle annular dark field (HAADF)-STEM images taken from the [001] direction of perovskite for the least and most reduced powders, respectively. White region of ABF-STEM images on the top right (Figure 2a) and on the down left (Figure 2b) is located near power surface. The broad grey spots in the ABF-STEM image correspond to oxygen site whereas the brightest spots in the HAADF-STEM image correspond to Sr site. For the least reduced sample, periodic broad grey spots in the ABF-STEM image were found, which indicates the oxygen sites of TiO6 octahedron in perovskite structure (Figure 2a). Noticeably darker spots of the oxygen sites, which correspond to oxygen vacancies, were not observed in both inner and near-surface regions of ABF-STEM image. It is reasonable considering very few


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oxygen vacancies in the order of 10-7 per perovskite unit were formed in the sample. For the most reduced powder sample, where much larger amount of oxygen vacancies in the order of 10– 3

per perovskite unit was introduced, a few darker spots of oxygen sites (dotted red circles, for

examples) were observed in the ABF-STEM image, reflecting the existence of oxygen vacancies in these powders. Such spots were similarly observed in the deeper region (Figure 2b). STEM images show no obvious difference in the degree of oxygen vacancy formation between nearsurface and inner regions.


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Figure 2. a), b) Near-surface ABF- and HAADF-STEM images and c)–f) XPS spectra of Ti 2p for non-stoichiometric SrTiO3–δ powders annealed under various oxygen partial pressure. Top right and down left of Figure 2a and 2b, respectively, correspond to powder surface. Dotted red circles in the ABF image of b) represent the darker spots of oxygen sites for the high oxygen vacancy sample. Red symbols in XPS spectra show the measured spectra after subtracting the background with Shirley function. Blue lines show fitted spectra after peak deconvolution with gauss function. X-ray photoelectron spectroscopy (XPS) was also applied to the non-stoichiometric SrTiO3–δ powders. The XPS spectra of Ti 2p show negligible difference in the surface electronic states among samples (Figures 2c to f). Peak deconvolution between Ti3+ and Ti4+ based on literatures48,49 also indicates that the annealing condition has negligible influence on their peak positions, full width of half maximum (FWHM), and peak area (Figure S7). These trends can be also observed in the XPS spectra of O 1s and Sr 3d (Figures S8–S11). The XPS spectra, information about the surface electronic structure, show no apparent influence from oxygen


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partial pressures, which support the STEM results, no accumulation of oxygen vacancies on the surface.

Electronic characteristics in bulk

Homogenous existence of oxygen vacancies in bulk samples ensures to determine the electron density from AC and DC electrochemical measurements. Such measurements were performed at room temperature using reduced SrTiO3–δ single crystals. Single crystal was used to determine the electron conductivity, σe, in bulk crystal without any impact of grain boundaries. The results showed identical resistance of 8.25 × 10–2 Ω (σe = 147.3 S cm-1) from both methods (Figure 3a). The electron density can be obtained by inserting measured σe and reported electron mobility, 𝜇M (= 6.02 cm2 V–1 s–1 at room temperature, which is constant against the non-stoichiometry of a SrTiO3–δ single crystal),39,41 into this equation, n = 𝜎M ⁄(𝑒𝜇M ) = 1.53 × 1020 cm-3. Such values are plotted against oxygen partial pressure (𝑝RS ) (Figure 3b). The electron density linearly increases with decreasing oxygen partial pressure with the slope of -1/4, showing that the large amount of electrons in the range of 1018–1020 cm-3 can be doped into the SrTiO3–δ single crystal. The slope of -1/4 further shows that the half of introduced electrons are trapped with doubly-charged oxygen vacancies, forming singly-charged oxygen vacancies.41,42 The other electrons probably locate in the Ti site, forming Ti3+. Those oxygen vacancies and Ti3+ likely act as donors, creating the donor levels in the band gap (~3.2 eV) of strontium titanate.


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Figure 3. a) Bode representation of AC impedance result obtained from SrTiO3–δ single crystal annealed at 1423 K for 12 h under 𝑝RS = (8.30 ± 0.04) × 10–22 atm. b) Electron density obtained for SrTiO3–δ single crystals. Z in a) represents the impedance while the inset shows the results of DC four-point measurements. The blue squares and red line in b) represent the determined and \

fitted values, respectively. The fitting expressed as log 𝑛 = 14.94 − ] log 𝑝RS is consistent with the defect model of electrons associated with oxygen vacancies. The inset images in b) show that the color of powder SrTiO3–δ varies with annealed oxygen partial pressure. The electron densities of non-stoichiometric SrTiO3–δ powders can be estimated from obtained linear dependence (red line in Figure 3b). The electron density in the powders varies 4 orders of magnitude from 1016 to 1020 cm-3 as the oxygen partial pressure varies from 10-5 to 1020

atm. The color of SrTiO3–δ powders shaped into cylindrical form became more grayish when

annealed under more reducing atmosphere (insets of Figure 3b). It shows that the absorption of


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visible light is enhanced by an increase in the donor concentration. The increase in electron density shifts the Fermi level, EF, towards the bottom of conduction band. The Fermi energies calculated by inserting their electron density, n, and literature data of 𝐸_`abc (= −0.42 V vs. SHE)50 and 𝑁_`abc (= 2.1 × 1020 cm–3 at 298 K)42 into Equation (8) range from -0.17 to -0.39 V vs SHE for pH = 0. The estimated Fermi levels are summarized in Table 1 and Figure S12 together with its electron density and oxygen partial pressure. Table 1. Estimated electronic properties of SrTiO3–δ powders. The Fermi level represents the flat band potential of SrTiO3–δ. Oxygen partial pressure, pRS / atm

Density of oxygen vacancy, [𝐕𝐎∙ ] / cm–3

Density of electron, n / cm–3

Estimated Fermi level, EF vs. SHE (pH = 0) / V

Degree of surface band bending, ΔΦ / eV

Depletion layer width / nm

Diffusion length of hole / nm

(2.8 ± 0.1) × 10–20

6.7 × 1019

6.7 × 1019

–0.39

1.51

27

185

(5.1 ± 0.5) × 10–16

5.8 × 1018

5.8 × 1018

–0.33

1.45

91

310

(5.0 ± 0.1) × 10–11

3.3 × 1017

3.3 × 1017

–0.25

1.38

373

310

(1.25 ± 0.03) × 10–5

1.5 × 1016

1.5 × 1016

–0.17

1.30

1715

310

The degree of surface band bending, ΔΦ, and the depletion layer width were calculated against the AgNO3 aqueous solution (pH = 5) interface. Details in calculation and used parameters are given in the supporting information.


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Figure 4. a) UV-visible diffuse reflectance spectra, b) calculated band gap, and c) visible light absorption for nonstoichiometric SrTiO3–δ powders. Corresponding electron densities in SrTiO3–δ are given in the figures. The visible light absorption in c) is obtained at 420 nm.

Light absorption characteristics

Figure 4a shows UV-visible diffuse reflectance spectra for the non-stoichiometric SrTiO3–δ powders. Although all the non-stoichiometric samples show an equivalent absorption band edge


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around 390 nm, they exhibit clear visible light absorption at wavelengths longer than 400 nm. The band gap calculated from the Tauc plots, 3.24 eV (Figure S13), is equivalent to the reported value51 and constant against electron density (Figure 4b). The visible light absorption becomes more pronounced as the electron density increases (Figure 4c), which attributes to the reduced titanium species and/or oxygen vacancies in SrTiO3–δ powder.

Photocatalytic activity

The photocatalytic activities and the stability of reduced SrTiO3–δ powders were evaluated under UV light irradiation (λ >300 nm). Time course of photocatalytic hydrogen evolution shows stable reaction from 10 vol% methanol aqueous solution for 15 h even using the most reduced powder (n = 1.2 × 1020 cm–3, Figure 5a). No noticeable difference in the UV-visible DRS spectra was observed before and after the hydrogen evolution reaction (Figure S15), indicating that, unlike TiO2,52−54 the density of electron and/or Ti3+ was not changed by the photo-irradiation and its hydrogen evolution reaction. The stability of oxygen evolution from water was also confirmed for 20 h using the SrTiO3–δ (n = 2.8 × 1017 cm-3) coloaded with 0.5 wt % Pd and 1.0 wt % Cr (Figure S14).55 The reduced SrTiO3–δ powders were thus stable under these photocatalytic reaction conditions. Hydrogen and oxygen evolution activities were determined using the non-stoichiometric SrTiO3–δ powders (Figure S16). The electron density was not perfectly the same as one used for previous characterization because of the limited amount of powder annealed in the same batch and each annealing resulted in slightly different oxygen partial pressure. The activities were significantly enhanced, almost linearly, with an increase of the electron density in the log-log plot (Figure 5b). Higher hydrogen and oxygen evolution kinetics directly show more


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photoexcited electrons and holes, respectively, available at the reaction sites per unit time. The enhancement of hydrogen and oxygen evolution rate are forty-fold and three-fold, respectively, as the electron density increases 104 times. It indicates that initially doped electrons were not directly utilized for these gas evolutions, as expected. No gas evolution was observed under irradiation with its wavelengths >400 nm, showing that visible light absorption originated from the oxygen vacancies and/or Ti3+ did not induce photocatalytic hydrogen/oxygen evolution and only electrons excited from the valence band to the conduction band activated the photocatalytic gas evolution. We can also rule out the possibility that the three-fold increase in oxygen evolution activity is largely a result of differences in surface area. Although the most left and right samples have similar BET surface area, 2.2 and 3.3 m2 g–1, respectively, considering the experimental error for small surface area samples, they show 2.1 times difference in the activity, which is beyond the experimental errors. The apparent quantum yield for O2 evolution for the most active powder (n = 1.2 × 1020 cm–3) reached 76% at 300 nm. Such hydrogen and oxygen evolutions were also observed under simulated sunlight irradiation (AM1.5G, 100 mW cm-2, Figure S17).


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Figure 5. a) Time course of hydrogen evolution over the SrTiO3–δ (n = 1.2×1020 cm–3) sample under UV irradiation (λ >300 nm). b) Rate of gas evolution from methanol aqueous solution (10 vol%) and AgNO3 aqueous solution (50 mM) using SrTiO3–δ under UV light (λ >300 nm) as a function of the electron density in SrTiO3–δ. Reaction conditions: catalyst, 100 mg; solution, 140 mL; light source, xenon lamp (300 W); reaction vessel, Pyrex top-irradiation type.

Photoexcited charge carrier dynamics

To understand the forty-fold enhancement of hydrogen evolution rate, time-resolved absorption spectroscopy was carried out. We probe light absorption depending on wavelength and its decay while pumping the pulsed light of 355 nm every 200 ms and 1 s, respectively (Figures 6 and S18). The energy of pump light was high enough to excite the electrons from the valence band to the conduction band of SrTiO3–δ and the pumping interval of 1s was long enough to capture the decay of photoexcited carriers. The difference in the absorbance before and after


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the pulse, Δabsorbance, subtracts the influences of probing light. Δabsorbance plotted against wavelength, thus, solely visualizes the energy levels and the dynamics of charge carriers generated by the pulsed light. Two absorption peaks were mainly observed around 11,000 and 20,000 cm–1 for the lower electron density sample (n = 4.0 × 1017 cm–3 in Figures 6a and S18a). The wavenumbers of 11,000 and 20,000 cm–1 reflect the deep levels of photoexited carriers, 1.4 and 2.5 eV apart from the conduction band or valence band in the band gap. Similar results were reported in the literature, where the former and latter were assigned to electrons and holes, respectively, trapped by defects.50 Those peaks significantly decreased with increasing electron density and a new peak appeared lower than 7,000 cm–1 when the electron density reached close to ca. 3 × 1019 cm– 3

(Figures 6a and S18c). The decrease of two peaks suggests that the deep levels have been

already filled with doped electrons before irradiation (electron trapping confirmed in Figure 3b), which resulted in no change in the status by photoexcitation. The appearance of new peak suggests that photoexcited carriers occupy the shallow levels, ~0.25 eV away from the conduction band, consistent with previous assignment, free and/or shallowly trapped electrons.46


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Figure 6. a) Transient absorption spectra at 10 µs and b) decay curves of transient absorption intensity at 2000 cm-1 for SrTiO3–δ powders after photoexcition. c) Rate of gas evolution from methanol aqueous solution (10 vol.%) vs. Δabsorbance. Transmittance and reflectance were measured below and above 6000 cm-1, respectively, after UV (355 nm) laser pulses under vacuum. The pump energy was 0.5 mJ per pulse with a repetition rate of 5 and 1 Hz for a) and b), respectively.


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Figure 6b shows decay curves at 2,000 cm–1 for powders that contain various initial electron densities. 2,000 cm–1 was chosen because it is within the characteristic peak observed for the high electron density powder (Figure 6a) and apart from the characteristic wavenumber for O-H vibration (3200–3600 cm-1). Prolonged lifetimes of photoexcited carriers, perhaps electrons, were observed when electrons were heavily doped into the powder (Figure S19). The correlation between the increased Δabsorbance at 10 µs (the prolonged lifetime of photoexcited charge carriers) and enhanced hydrogen evolution rate was further found (Figure 6c).

Figure 7. a) Schematic representation of the interface between the SrTiO3–δ surface and AgNO3 aqueous solution under dark conditions. b) Rate of gas evolution from AgNO3 aqueous solution (50 mM) using SrTiO3–δ under UV light (λ >300 nm) as a function of ΔΦ for pH = 5. EC is the conduction band, EV is the valence band, EF is the Fermi level, ΔΦ is the difference between the valence (or conduction) band edge on the interfacial surface and the valence band (or conduction


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band) in the bulk, and W is the depletion layer width defined as a distance from the surface to the point at which the potential does not vary with position. Reaction conditions: catalyst, 100 mg; solution, 140 mL; light source, xenon lamp (300 W); reaction vessel, Pyrex top-irradiation type.

Factors that affect photocatalytic activities

The crystallographic and morphological aspects of semiconducting powders (e.g. crystallinity and specific surface area) have been considered as factors that contribute to the photocatalytic activity.1–7 In this study, we prepared the SrTiO3–δ powders that maintain physicochemical characteristics such as crystallinity, morphology, specific surface area, and the crystal and electronic structure of surface (Figures 1, 2, and S2–S10) while dramatically increasing doped electron density. The photocatalytic activities for hydrogen and oxygen evolution were significantly enhanced by electron doping. Here we discuss such enhancement based on two criteria: the photoexcited charge carrier dynamics and surface band bending. In the hydrogen evolution reaction from methanol aqueous solution, the proton reduction reaction, 2H+ + 2e- → H2(g), has been regarded as the rate-determining reaction due to very fast oxidation reaction of methanol.56 Photoexcited electrons and their kinetic behaviors are, thus, important for the hydrogen evolution rate. Higher initial electron density in the SrTiO3–δ powders led to longer lifetimes of photoexcited charge carriers (Figure S19). The correlation between the increased Δabsorbance (the prolonged lifetime of photoexcited charge carriers) and enhanced hydrogen evolution rate was further found (Figure 6c), indicating that the photoexcited carriers observed at 2000 cm-1 are, in fact, electrons, consistent with literature.46 Electron doping has already filled the deep levels with electrons without irradiation. Photoexcited electrons, thus, do not need to fall into the deep levels but the shallow levels, which enlarged the probability of


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photoexcited electrons in the conduction band. As a result, more electrons are available at the reaction sites per unit time, which enhanced the hydrogen evolution kinetics. In the oxygen evolution reaction from AgNO3 aqueous solution, the water oxidation reaction, 2H2O → 4H+ + 4e- + O2(g), would be the rate-limiting reaction because the irreversible reduction of Ag+ ions ensures fast kinetics.57,58 The kinetic behavior of photoexcited holes on the top of valence band is critical for the oxygen evolution rate. We propose that the band bending of SrTiO3–δ facing with AgNO3 aqueous solution is a key factor that influences the photocatalytic activity toward oxygen evolution. Band structure of semiconductor facing to an aqueous solution can be constructed if the Fermi level, the valence band, and the flat band potential of semiconductor, and the Fermi level of aqueous solution are known. Here, it is assumed that the Fermi level of AgNO3 aqueous solution is represented by the middle of potentials for the oxidation and reduction reactions, typically found in a textbook.59 Although the Fermi level pinning has been observed at the non-oxide semiconductor (Si or GaAs) /aqueous solution interfaces,60 the band edge pinning has been reported in the SrTiO3 system (the flat band potential is known).50 Details of constructing procedure are described in the supporting information. Figure 7a schematically shows the constructed electronic potential of SrTiO3–δ semiconductor faced to AgNO3 aqueous solution (pH = 5) under dark. The degree of surface band bending, ΔΦ, is defined as the difference in the valence band (or the conduction band) between bulk and interfacial surface. Just glancing at the schematics, potential downhill for electronic holes on the top of valence band could benefit its transfer to the reaction site. Constructed band diagram reveals that there is a quantitative correlation between the photocatalytic activity of oxygen evolution and ΔΦ. When plotted against ΔΦ, the photocatalytic


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O2 evolution rate shows linear enhancement above 1.38 V (Figure 7b), demonstrating that the flux of photogenerated holes towards the SrTiO3–δ reaction sites (the surface) is a factor that governs the oxidation reaction kinetics. If this occurs, holes need to migrate to the reaction site (the surface) before its recombination. Estimated migration distance of hole before its recombination (10–28 ns in SrTiO3–δ)61 is 185~310 nm (Table 1 and supporting information), which is reasonable and consistent with such understanding. The diffusion length of holes shows an opposite trend against the oxygen evolution activity (higher activity for shorter diffusion length), which also supports our conclusion, the importance of ΔΦ that drives the generated holes to the reaction site. Below 1.38 V, the photocatalytic O2 evolution rate is almost unchanged. Although the meaning of 1.38 V is not fully understood, it may indicate that another rate-limiting step begins to play a role for oxygen evolution kinetics below the voltage. Or it simply suggests that the surface band bending is not significant enough for this sample (n = 1.5 x 1016 cm-3) since the depletion layer width calculated as 1715 nm (Table 1 and supporting information) is much larger than the particle size, ~400 nm. ΔΦ accelerates the flow of photoexcited holes into the reaction sites per unit time, which amplified the photocatalytic activity of oxygen evolution.

CONCLUSION

Systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution was performed using a model SrTiO3–δ powder/aqueous solution interface in a half reaction. The nonstoichiometric SrTiO3–δ powders were prepared to contain similar physicochemical characteristics such as the crystallinity, chemical composition, morphology, specific surface area, and the crystal and electronic structure of surface while dramatically changing electron density from 1016 to 1020 cm-3. The photocatalytic activities for hydrogen and oxygen evolution from


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aqueous methanol and silver nitrate solutions, respectively, were investigated without cocatalyst loading. The hydrogen evolution activity was enhanced by forty-fold when the electron density of SrTiO3–δ powder was increased by four orders of magnitude. The stability of the SrTiO3–δ powders was confirmed by performing repeated photocatalytic hydrogen evolution reactions for 15 h. Transient absorption spectroscopy indicated that the lifetime of photoexcited electrons was prolonged with an increase in the electron density, which correlated well with the improved hydrogen evolution activity. The oxygen evolution activity was also enhanced by three-fold with a linear correlation to the degree of surface band bending defined as the difference in the valence band between bulk and interfacial surface, ΔΦ, above 1.38 eV. In this region, ΔΦ enlarges the total flux of photoexcited holes towards the reaction sites (surface), which drives the oxygen evolution reaction. Complex phenomena take place behind the photocatalysis even in the simplest model interface between non-stoichiometric SrTiO3–δ powder semiconductor and aqueous solution. Fundamental understanding of the model interface would be the first step towards surface band engineering for powder-based semiconductor photocatalysts. Toward improved overall water splitting, electron doping into the semiconductor photocatalyst along with a cleaver choice of cocatalysts that can further enlarge ΔΦ even in water (pH=7) would be a strategy for improved oxygen evolution kinetics. Ideal cocatalyst also accelerates the electronhole separation (longer electron lifetime after irradiation), which would further promote the hydrogen evolution kinetics. Such improvement by the tuning of surface band bending and cocatalyst loading has yet to be explored, but this work provides the fundamental insight necessary towards the rational design of powder-based semiconductor photocatalysts.

ASSOCIATED CONTENT


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Supporting Information

Required time for oxygen defect formation in bulk, XRD analysis, SEM images, particle size distribution, additional STEM images, XPS results, energy diagram, UV-visible spectra, transient absorbance, and photocatalytic reaction data. Detailed procedures for calculation of ΔΦ and diffusion length of hole before its recombination. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

* [email protected], [email protected]

Author Contributions

Y.Y. and K.M. designed the project. S.N. and J.H. conducted most of the experiments and analysis with H.K., Y.Y., and K.M. J.J.M.V. and A.Y. performed transient absorption spectroscopy. S.Y. and K.K. conducted STEM observations. S.N., Y.Y., and K.M. wrote a draft of the manuscript. All authors discussed and provided comments on the experiments and the manuscript during preparation.

ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area “Mixed Anion (Projects JP16H06440, JP16H06441 and JP17H05491)” from the Japan Society


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for the Promotion of Science (JSPS), and was also partially supported by Grants-in-Aid for Young Scientists (A) (Project JP16H06130) and for Challenging Exploratory Research (Project JP15K14220). K.M. acknowledges the Noguchi Institute, the Hosokawa Powder Technology Foundation and the PRESTO/Japan Science and Technology Agency (JST) “Chemical Conversion of Light Energy” program for financial support. S.N. and K.M. acknowledge the Academy for Co-creative Education of Environment and Energy Science (ACEEES) for the Leading Program Educational Research Fund. S.N. acknowledges financial support from Grantin-Aid for JSPS Fellows from JSPS. Y.Y. acknowledges financial support from the PRESTO/JST, Kakenhi Grants-in-Aid (Projects JP15H02287 and JP16H00891) from JSPS, and a Kyushu University research program PROGRESS 100.

REFERENCES

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Homogeneous Electron Doping into Nonstoichiometric Strontium

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