Effect of Etching on Electron–Hole Recombination in Sr-Doped

Article pubs.acs.org/JPCC

Effect of Etching on Electron−Hole Recombination in Sr-Doped NaTaO3 Photocatalysts Longjie An,† Yohan Park,† Youngku Sohn,‡ and Hiroshi Onishi*,† †

Department of Chemistry, School of Science, Kobe University, Nada, Kobe, Hyogo 657-8501 Japan School of Chemistry and Biochemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Sodium tantalate (NaTaO3) photocatalysts doped with Sr2+ produce core− shell-structured NaTaO3−SrSr1/3Ta2/3O3 solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg−Xe lamp irradiation, the steady-state population of electrons in the core−shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed.

1. INTRODUCTION Clean, renewable energy sources are imperative for the sustainable development of our society. Photocatalytic evolution of H2 from water has demonstrated significant potential for the conversion of solar energy to chemical fuel.1 Photocatalysts based on perovskite-structured metal oxide, ABO3, have been frequently observed to split water.2−5 NaTaO3, a representative compound, exhibits the highest quantum efficiencies for water splitting when doped with selected metal elements at optimum concentrations.6,7 These efficiencies are attributed to the recombination of electrons and holes, which is restricted in appropriately doped NaTaO3, as evidenced by transient infrared (IR) absorption studies.8,9 Our latest study10 has shown that Sr-doped NaTaO3 photocatalysts prepared through the solid-state method form NaTaO3−SrSr1/3Ta2/3O3 solid solutions, resulting in segregation of Sr on the particle surface. A Sr-rich solid-solution shell forms in a heteroepitaxial manner over a Sr-poor solid solution core. Regularly separated ten-nanometer-length steps spontaneously appear, thereby correcting the lattice mismatch at the epitaxial interface (Figure 1). The rate of electron−hole recombination decreases in the core−shell-structured solidsolution photocatalysts. The steady-state population of photoexcited electrons accordingly increases by 180 times at most,10 and the rate of photocatalytic Ag deposition increases by 5 times.11 In the present study, core−shell-structured photocatalysts were etched with hydrofluoric acid (HF) solution to investigate the different roles of the core and shell in electron− hole recombination. © XXXX American Chemical Society

Figure 1. Sr-doped NaTaO3 photocatalysts prepared through the solid-state method.10 (A) Cross-sectional composition of a particle. (B) Perovskite-structured lattices mismatched at the heteroepitaxial core−shell interface generating regularly separated steps on the surface. White and gray squares represent unit cells of NaTaO3− SrSr1/3Ta2/3O3 solid solution in the core and shell, respectively.

2. EXPERIMENTAL SECTION 2.1. Photocatalyst Preparation. Strontium-doped NaTaO3 photocatalysts, hereafter referred to as Sr-NTO, were prepared through the solid-state method. First, mixtures of Na2CO3 (99.8%, Kanto), Ta2O5 (99.99%, Rare Metallic), and SrCO3 (99.9%, Kanto) were calcined in an alumina crucible at 1173 K for 1 h and then at 1423 K for 10 h. The Na−Ta molar ratio in the mixtures was adjusted to 1.05 to compensate for the loss of Na during calcination. Second, the calcined products were washed with water to remove excess Na-containing materials. Third, each photocatalyst (1 g) was stirred in an aqueous HF solution (10 wt %, 3 mL, Wako) for a Received: October 2, 2015 Revised: December 1, 2015

A

DOI: 10.1021/acs.jpcc.5b09638 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C predetermined period at room temperature and then washed with water until the pH of the washing fluid was maintained at 7. According to a metallurgical study on Ta-containing minerals, the following stoichiometric reaction for etching the photocatalysts is possible: NaTaO3 + 8HF → NaF + H2TaF7 + 3H2O.12 2.2. Photocatalyst Characterization. The single perovskite-structured phase of Sr-NTO was examined and confirmed by X-ray diffractometry (XRD; Rigaku, SmartLab). The size and shape of the etched particles were observed by scanning electron microscopy (SEM; Hitachi High-Technologies, S4800). Bulk or surface compositions were quantified by energydispersive X-ray fluorescence spectrometry (EDX; Shimadzu, EDX-720) or X-ray photoelectron spectroscopy (XPS; UlvacPhi, PHI X-tool). In XPS, the photocatalysts were fixed on an Ir foil and excited by Al Kα radiation. The binding energies of Ta 4f, Sr 3d, and F 1s emissions were calibrated relative to the oxygen 1s emission at 530.0 eV. IR absorption was examined under a vacuum of 10 Pa by using a Fourier transform IR spectrometer (Jasco, FT/IR610). The steady-state population of photoexcited electrons that had not recombined in Sr-NTO was evaluated from the change in absorbance induced by ultraviolet (UV) light irradiation. First, each photocatalyst was suspended in water to a weight concentration of 3 g L−1. Second, 0.5 mL of the suspension was transferred to a CaF2 plate and dried at 293 K for 20 h in air. The transmission IR absorption spectrum of the dried plate was obtained in the presence and in the absence of UV light. A Hg−Xe lamp (200 W) was used as the UV light source. Light power at wavelengths of less than or equal to 370 nm was 60 mW cm−2 in the full spectrum of radiation. The present study is aimed at simulating the number of electrons and holes available in steady-state water splitting. Hence, absorbance change induced by steady light irradiation was observed. Transient absorption (TA) measurement triggered by pulsed light is more effective in quantifying charge carrier lifetime, though. Excitation to enable TA measurement is much more intense than that in steady-state reactions. Recombination kinetics should be sensitive to the number of electrons and holes controlled by excitation strength. The strength in a TA measurement of NaTaO3 was estimated to be 108 times as intense as the strength in the steady-state water splitting reaction.8 Raman scattering was observed by using a spectrometer fabricated in-house to examine lattice vibrations affected by Sr doping. Photocatalysts on plates dried in air were exposed to He−Cd laser light (wavelength: 442 nm). Raman scattering at this excitation wavelength enabled bulk-sensitive characterization, as Sr-NTO is optically transparent at wavelengths longer than 320 nm.

Figure 2. Scanning electron micrographs of 5.1 mol % Sr-NTO etched for a: 0, b: 5, c: 10, d: 30, e: 120, f: 300, and g: 480 min.

reconstruction on the surface were observed (Figure 2a). As the etching time increased from 5 to 480 min, the particle size decreased, as can be observed in images b−g. With etching times of 5, 10, and 30 min (b, c, and d, respectively), tennanometer-length steps were observed on particles, whereas with etching times of 120, 300, and 480 min (e, f, and g, respectively), these steps were not observed on the particles. Etching should affect the bulk and surface compositions as Sr segregates on the Sr-NTO surface.7 Bulk composition was investigated by EDX. Figure 3 shows The Sr−Ta molar ratio estimated from the intensity of the Sr−Kα emission relative to that of the Ta-Lα emission. Observed Sr-Kα emission spectra are available in Figure S1 of Supporting Information. During etching in the first 10 min, the initial Sr−Ta ratio (5.1 mol %) decreased to 4.6 mol %. This decrease supports the core−shell structure which the authors proposed in ref 10. The Sr-rich shell was removed in the first 10 min. With etching for longer periods, the Sr−Ta ratio gradually decreased. This gradual but finite response to etching time is indicative of the Sr concentration gradient in the 5.1 mol % Sr-NTO core. The two-step response of the Sr−Ta ratio, a fast decrease followed by the gradual reduction, evidenced the interface of Sr-rich shell and Sr-poor core. As can be observed by XPS, the surface of the particles exposed to the etching solution was chemically affected.

3. RESULTS AND DISCUSSION 3.1. Response to Etching Time. Sr-NTOs with different bulk Sr concentrations (1.0, 5.1, and 8.3 mol % relative to the amount of Ta) were prepared. The particle size, shape, bulk, and surface composition, Raman-active lattice vibrations, and UV-induced IR absorbance change of 5.1 mol % Sr-NTO were examined as a function of etching time. Figure 2 shows SEM images of 5.1 mol % Sr-NTO unetched particles as well as those etched for 5, 10, 30, 120, 300, and 480 min. Earlier studies6,7 have demonstrated the generation of ten-nanometer-length steps on metal-doped NaTaO3 particles. For unetched Sr-NTO, elementary particles greater than 0.5 μm in diameter with step B

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appeared at 135−136 eV. Etching slightly affected the peak shape of the Ta 4f emission, but the binding energy of the peak tops remained intact. Tails at higher binding energy for the photocatalysts etched for 5 and 10 min were observed (spectra b and c, respectively). The tails disappeared with longer etching times (spectra d−g); the reason for this disappearance is unclear at this time. The surface concentration of Sr relative to Ta was estimated from the emission intensity ratio. We assumed a common escape depth of 2.4 nm for the Sr 3d and Ta 4f photoelectrons,16 as the kinetic energies of electrons at the two levels were 1.3 and 1.4 keV, respectively. Figure 4(D) shows a plot of surface Sr concentration, featuring a decrease within the first 10 min followed by gradual reduction. These results agree with those obtained from EDX; during etching for the first 10 min, the Sr-rich shell detached, leaving a core with a Sr concentration gradient. Undoped NaTaO3 (NTO) exhibits Raman-active modes of lattice vibrations at 450, 500, and 620 cm−1.17 In our previous study,10 doping with Sr through the solid-state method resulted in additional Raman bands at 760 and 860 cm−1. Two bands for unetched 5.1 mol % Sr-NTO prepared in this study were observed (spectrum a in Figure 5(A)). The intensities of the two bands exhibited different sensitivities to etching time. The band at 760 cm−1 weakened during etching for 5 min (spectrum b) and completely disappeared at 10 min (spectrum c). On the other hand, the intensity of the band at 860 cm−1 relative to that of the band at 620 cm−1 decreased by 30% in the first 10 min and then gradually decreased with etching time, as can be observed in Figure 5(B). Even after etching for 480 min, the band at 860 cm−1 in spectrum g could be clearly observed. The different responses to etching time are attributed to the proposed core−shell structure for Sr-NTO. In the first 10 min, the shell that was removed contained two species; one species resulted in the band at 760 cm−1, and the other resulted in the band at 860 cm−1. The core remaining in particles etched for 10 min or longer contained only the latter. The two Raman bands

Figure 3. Sr−Ta molar ratios of etched 5.1 mol % Sr-NTO estimated in EDX analysis.

Doublet peaks with different spin−orbit couplings corresponding to Ta 4f and Sr 3d emissions for unetched Sr-NTO were observed (Figure 4(A) and 4(B)). The simple, well-separated shapes suggest that the chemical environment around the two metals is uniform. The binding energies of the Ta 4f 7/2 and Sr 3d 5/2 states, 25.6 and 132.6 eV, respectively, are consistent with those reported for Sr2Ta2O7 having oxidation states of Ta5+ and Sr2+.13 As shown in Figure 4(C), the F 1s emission appeared at 684.2 eV with the shortest etching time (5 min). This indicates that fluorine adsorbed on the surface because of etching. The increase in relative intensity of the F 1s emission for the photocatalysts etched for 120, 300, and 480 min (spectra e, f, and g, respectively) is attributed to the increased surface/ volume ratio on the particles of reduced size. As fluorine is more electronegative than is oxygen, the Sr and Ta cations bound to F anions exhibit binding energies higher than those bound to oxygen, as has been reported previously for fluorinated carbon14 and silicon oxyfluorides.15 Doublet peaks of the Sr 3d emission lost their original feature and showed one broad band possibly including signals from Sr bound to several F anions. A tail at higher binding energy also

Figure 4. X-ray photoelectron spectra of 5.1 mol % Sr-NTO etched for a: 0, b: 5, c: 10, d: 30, e: 120, f: 300, and g: 480 min. Panels A, B, and C show Ta 4f, Sr 3d, and F 1s emissions, respectively. Intensities were normalized relative to that of the Ta 4f 7/2 state. Surface Sr concentrations estimated from the XPS results are plotted with error bars in D as a function of etching time. C

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Figure 5. Raman spectra of 5.1 mol % Sr-NTO normalized at 620 cm−1. Panel A shows the spectra of Sr-NTO etched for a: 0, b: 5, c: 10, d: 30, e: 120, f: 300, and g: 480 min. The spectrum of NTO is shown along with curve h as a reference. Panel B shows the relative intensity of the band at 860 cm−1 with error bars as a function of etching time.

SrSr1/3Ta2/3O3 solid solutions were present in 5.1 mol % SrNTO prepared through the solid-state method, as assumed in ref 10. Intense Sr segregation at the surface induced the formation of the core−shell structure. Sr-rich shell produced Raman bands at 760 and 860 cm−1. One Raman band for the Sr-poor core was observed at 860 cm−1, and a discontinuous boundary was observed between the core and shell. The particle surface showed reconstruction with ten-nanometerlength steps, which corrected the lattice mismatch at the interface. During etching for the first 10 min, the shell was removed, and the core was exposed on the surface. The surface Sr concentration determined by XPS decreased accordingly, and the band at 760 cm−1 disappeared in the Raman spectrum. The thickness of the shell has been estimated to be smaller than the escape depth of the Sr 3d and Ta 4f photoelectrons (2.4 nm).10 It is inevitable for such a thin layer of metal oxide to be completely removed in the initial stage of etching. Etching for longer than 120 min was required to eliminate the tennanometer-length steps on the particle surface. The surface and bulk Sr concentrations estimated by EDX and XPS gradually decreased as the etching time increased from 10 to 480 min. The intensity of the Raman band at 860 cm−1 also decreased with etching time. These results indicate that the core contains a NaTaO3−SrSr1/3Ta2/3O3 solid solution with a Sr concentration gradient. The outer core contained more Sr interfacing with the inner core with less Sr. X-ray diffraction results of the etched photocatalyst were consistent with the picture in Figure 6, as described in Supporting Information. The roles of the core and shell in electron−hole recombination were evaluated from the change in the IR absorbance induced by UV light. For photocatalysts and photoelectrodes such as NaTaO 3 , 8−10 K 3 Ta 3 B 2 O 12 , 20

were assigned to the breathing vibrations of TaO6 octahedra in NaTaO3−Sr(Sr1/3Ta2/3)O3 solid solutions. In a number of Bsite-doped perovskites (AB1−xB′xO3), the BO6 breathing vibration results in Raman bands ranging from 780 to 850 cm−1.18,19 Two bands separated by 100 cm−1 are indicative of different environments where TaO6 octahedra are located, while the chemical identities of the two are unknown. The center wavenumber of the breathing vibration may be sensitive to the local concentration of the doped B sites. We prepared in the earlier study10 the Sr-rich extreme of the solid solution, SrSr1/3Ta2/3O3, which produced one Raman band at 810 cm−1. The band of Sr-NTO at 860 cm−1 shifted to lower wavenumbers with increasing Sr concentration, eventually merging with the band at 810 cm−1. We summarize the results obtained from SEM, EDX, XPS, and Raman to depict the manner in which 5.1 mol % Sr-NTO was etched with the HF solution in Figure 6. NaTaO3−

Figure 6. Etching of 5.1 mol % Sr-NTO with HF solution. The spatial distribution of Sr and the shape of particles are shown.

Figure 7. Change in IR absorbance induced by UV light irradiation. (A) Absorbance change spectra of 5.1 mol % Sr-NTO etched for a: 0, b: 5, c: 10, d: 30, e: 120, f: 300, and g: 480 min. The NTO spectrum is shown with curve h as a reference. (B) Integrated absorbance change relative to that of unetched NTO as a function of time of etching with HF. Error bars are superposed. D

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Figure 8. (A) X-ray photoelectron spectra and (B) UV-induced IR absorbance changes of the photocatalyst NTO etched for a: 0, b: 10, and c: 300 min.

TiO2,21−29 SrTiO3,30 LaTiO2N,31 Bi2WO6,32 Ga2O3,33 and GaN,34 UV-induced IR absorption has been utilized to evaluate the amount of excited electrons not yet recombined. Figure 7(A) shows the change in the absorbance spectra of etched and unetched 5.1 mol % Sr-NTO. The spectrum of unetched SrNTO contains two components: one monotonically strengthens from 6000 to 900 cm−1, while the other shows a broad bandlike maximum at 2000−2500 cm−1. The monotonic and bandlike components are attributed to electron transitions in the conduction band (CB) and from shallow midgap trap states to the CB. The absorbance change was integrated within the range of 6000 to 900 cm−1 and plotted as a function of etching time in panel B. During etching for the first 10 min, the integrated absorbance change, which is proportional to the steady-state number of photoexcited electrons, was enhanced by 40%. This enhancement suggests that electrons and holes become excited in the core but still recombined in the shell. When the shell was removed from the core, the recombination rate decreased, and the electron population in the core increased accordingly. Notably, the ten-nanometer-length steps negatively affect surface redox reactions in metal-doped NaTaO3 photocatalysts.9 A particularly high efficiency of conversion to products via water splitting by the photoexcited electrons was observed with NTO and 0.5 mol % Sr-NTO. Surface reconstruction with ten-nanometer-length steps on the two photocatalysts was not observed. The conversion efficiency for the photocatalysts doped with Ca, Sr, Ba, or La to concentrations of 1−5 mol % decreased. The photocatalysts with lower efficiencies exhibited surface reconstruction with ten-nanometer-length steps. In the preceding paragraph, we assume that recombination occurs between electrons and holes in the 5.1 mol % Sr-NTO shell. The decreased electron-toproduct conversion efficiencies are attributed to recombination in the shell. Electrons excited in the core move to the surface during redox reactions. Efficient transport across the shell is required but is blocked on the core−shell-structured NaTaO3 photocatalysts. Surface reconstruction with ten-nanometerlength steps on the solid solutions of NaTaO3 with LaIrO3,35 LaCrO3,36 and LaFeO3 was also observed.37 These solidsolution photocatalysts for visible-light harvesting may not be free from recombination in their shells. Another issue with the effect of etching is the continuous decrease in the integrated absorbance change at etching times of 10−480 min. The shell of each particle was removed, and the core was etched at this stage. The bare and unetched core, which appeared at an etching time of 10 min, exhibited the largest absorbance change. It is essential to investigate the

decrease in the absorbance change caused by the etching of the core. The decreased Sr concentration in the etched core is possibly one reason for the decrease in the absorbance change. However, this was not the case. As depicted in Figure 3, the bulk Sr concentration was 3.5 mol % in 5.1 mol % Sr-NTO etched for 480 min. The integrated absorbance change of unetched Sr-NTO prepared through the solid-state method was measured and plotted as a function of the bulk Sr concentration in Figure 7(C) of ref 10. According to the data reported in that paper, the integrated absorbance change of the unetched asprepared 3.5 mol % Sr-NTO was enhanced by 150 times relative to that of NTO. On the other hand, 5.1 mol % Sr-NTO etched for 480 min exhibited enhancement of only 20 times (Figure 7(B) in the present study). The enhancement of the electron population in the two photocatalysts was different although they had equivalent bulk Sr concentrations. Hence, the bulk Sr concentration is not the key to the enhancement of the electron population. Instead, we propose that the concentration gradient of Sr cations placed in B sites controls electron population in the core. As evidenced by EDX, XPS, and Raman results, radial distribution of Sr at B sites was not uniform with more Sr in the outer core and less Sr in the inner core. The shell even further Sr-rich possibly reserved excess Sr2+ cations distributed in the outer core. Concentration gradient can be large in the outer core and small in the inner core. Electron population should reduce by etching the core on this assumption. This is what we observed in Figure 7 (B). One Sr cation out of four is located at the B site in the NaTaO3−SrSr1/3Ta2/3O3 solid solution. As the CB of NaTaO3 derives from Ta 5d orbitals,38−40 the energy width of the CB is sensitive to the Sr concentration at the B sites. Hence, if there is any Sr concentration gradient, it should generate a potential energy gradient at the bottom or at the shallow trap states just below the bottom. The potential gradient drives the separation of excited electrons from complementary holes, thereby restricting recombination. This assumption aids in the interpretation of electron−hole recombination in Sr-rich solid solutions observed in our earlier study.10 The UV-induced IR absorbance change of Sr-NTO exhibited a maximum at a Sr concentration of 1.8 mol %. Sr concentrations greater than 1.8 mol % led to weaker absorbance changes. The Sr-rich extreme, Sr(Sr1/3Ta2/3)O3, exhibited an absorbance change as weak as that exhibited by NTO with no enhancement caused by the presence of Sr. The concentration gradient increased with absolute concentration in dilute solid solutions, whereas the gradient was naturally limited in Sr-rich solid solutions and disappeared in Sr(Sr1/3Ta2/3)O3. E

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Figure 9. Etching of 8.3 mol % Sr-NTO. (A) Raman spectra of a: unetched and b: etched 8.3 mol % Sr-NTO. The spectrum of NTO is shown as a reference. (B) Spectra showing UV-induced IR absorbance changes for a: unetched and b: etched 8.3 mol % Sr-NTO.

Figure 10. Etching of 1.0 mol % Sr-NTO. (A) The Raman spectrum of unetched 1.0 mol % Sr-NTO. The spectrum of NTO is shown as a reference. (B) Spectra showing UV-induced IR absorbance changes for a: unetched and b: etched 1.0 mol % Sr-NTO.

3.2. Etching NTO. NTO was prepared through the solidstate method and etched with the HF solution for comparison with etched 5.1 mol % Sr-NTO. Fluorine anions adsorbed on 5.1 mol % Sr-NTO were detected. The adsorbed electronegative element might have affected the recombination of electrons and holes in etched Sr-NTOs. The possible contribution of fluorine to recombination was examined and excluded here. As shown by XPS patterns in Figure 8(A), fluorine anions were present on NTO etched for 10 and 300 min. On the other hand, Figure 8(B) shows that the UV-induced IR absorbance change is insensitive to the presence or absence of fluorine anions. Hence, the enhancement of the electron population observed in Figure 7 was an intrinsic property of etched SrNTOs and was independent of the contribution of adsorbed fluorine anions. The undoped NTO presented a small absorbance change, since electron population enhancement by Sr doping was absent. Spectrum h in Figure 7(A) and spectrum a in Figure 8(B) are common. Removal of fluorine anions from the etched Sr-NTO was examined and described in Supporting Information. 3.3. Response to Initial Sr Concentration. Two photocatalysts, 8.3 mol % Sr-NTO and 1.0 mol % Sr-NTO, were prepared and etched with the HF solution for comparison with 5.1 mol % Sr-NTO. The etching time was maintained at 10 min. According to EDX analysis, the bulk Sr concentration of 8.3 mol % Sr-NTO decreased to 7.5 mol % by etching for 10 min. Figure 9(A) shows the Raman spectra of unetched and etched 8.3 mol % Sr-NTO. Unetched Sr-NTO produced a band at 760 cm−1, corresponding to a Sr-rich shell, which disappeared during etching. These results indicate that 8.3 mol % unetched Sr-NTO has a core−shell structure and that etching removes the Sr-rich shell. Figure 9(B) shows that the UV-induced IR absorbance change increased with etching. The integrated absorbance change increased by 20%. These results suggest that electrons and holes become excited in the 8.3 mol

% Sr-NTO core and recombine in the shell, as has been proposed for 5.1 mol % Sr-NTO. On the other hand, the bulk Sr concentration in 1.0 mol % Sr-NTO determined by EDX remained intact at an etching time of 10 min. The band at 760 cm−1 band was not observed in the Raman spectrum of the unetched form, as shown in Figure 10(A). As can be observed, 1.0 mol % Sr-NTO did not have a core−shell structure. The amount of Sr was insufficient to induce the spontaneous formation of the shell. In our earlier study,10 the threshold Sr concentration for induction of the core−shell structure was found to be 0.5 mol %. The core− shell structure was present at 1 mol % in the earlier study, whereas it is absent in the present study. This quantitative disagreement is attributed to uncertainty with regard to the preparation of Sr-NTO in the present study. As shown in Figure 10(B), the UV-induced IR absorbance change for 1.0 mol % Sr-NTO weakened upon etching. The integrated absorbance change decreased by 30%. This response is qualitatively different from that observed for 5.1 mol % SrNTO and 8.3 mol % Sr-NTO. The absence of the shell in unetched 1.0 mol % Sr-NTO causes the qualitatively different response. Unetched 1.0 mol % Sr-NTO contained the bare core, and etching the core resulted in a decreased absorbance change, as can be observed for 5.1 mol % Sr-NTO etched for 10−480 min.

4. CONCLUSION In this study, core−shell structures were formed by doping of NaTaO3 particles with Sr to concentrations of 5.1 and 8.3 mol % through the solid-state method. The Sr-rich shell was removed from the Sr-poor NaTaO3−SrSr1/3Ta2/3O3 solidsolution core by etching with HF solution for 10 min. The steady-state population of photoexcited electrons was enhanced by 40−20% when the shells were removed. The core−shell structure was absent in the photocatalyst doped at 1.0 mol %, the electron population of which decreased upon etching for 10 F

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The Journal of Physical Chemistry C

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min. We assume that electrons were excited in the core and recombined in the shell. Further etching of the photocatalyst core doped with 5.1 mol % Sr resulted in a decrease in electron population. A strontium concentration gradient observed in the core is proposed to control to electron population. Fluorine anions were present on the etched photocatalysts. The possible contribution of this adsorbed fluorine to the electron population was examined and excluded by comparison of the properties of undoped NaTaO3 etched with HF solution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09638. EDX spectra, XRD data, and characterization of a SrNTO photocatalyst etched and then heated at 773 K (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 81-78-803-5657. Fax: 8178-803-5674. Notes

The authors declare no competing financial interests. Y.P. and Y.S. contributed to SEM imaging of the photocatalysts (Figure 2a−e).

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ACKNOWLEDGMENTS Hidenori Saito of the Kanagawa Academy of Science and Technology observed the SEM images of Figure 2f and 2g. This study was supported by the JSPS Grant-in-Aid for Scientific Research (no. 15H01046).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b09638 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b09638 J. Phys. Chem. C XXXX, XXX, XXX−XXX