Heteroepitaxial barium-doped NaTaO3 films on SrTiO3(001) substrate
Introduction
The photocatalytic splitting reaction of water demonstrates promise for the production of hydrogen fuel. Polycrystalline particles of sodium tantalate (NaTaO3) doped with alkaline-earth metal (e.g., Ba, Sr, or Ca) cations exhibit high quantum efficiencies for this reaction [[1], [2], [3], [4]] and also the steam reforming reaction of methane [5] when the bandgap is excited by ultraviolet light. The photocatalytic reduction of CO2 using pristine [6], metal-doped NaTaO3 [7,8] and also KTaO3 [9] has been reported. Steady-state [10] and time-resolved [11] infrared absorption studies revealed that the recombination of photoexcited electrons and holes is restricted by the doping of alkaline-earth metals into NaTaO3. The restricted recombination in the bulk provides an explanation for the observed increase in the quantum efficiency. In addition, the photoexcited electrons and holes efficiently drive redox reactions at the surface to produce hydrogen and oxygen. Therefore, surface reaction centers on NaTaO3 should be investigated for further development. A series of well-defined, crystalline NaTaO3 films, either doped or undoped with foreign metal cations, is required for surface science studies. In this study, Ba-doped and undoped NaTaO3 films were epitaxially grown on centimeter-sized SrTiO3(001) substrates to satisfy this requirement.
Section snippets
Previously reported NaTaO3 films
Thus far, polycrystalline NaTaO3 films have been prepared by hydrothermal reactions [[12], [13], [14], [15]], synthesis under NaNO3 flux [16,17], and magnetron sputtering [18]. In this study, hydrothermal and solvothermal reactions were employed to produce epitaxial films on SrTiO3 substrates. Typically, hydrothermal [[19], [20], [21], [22], [23], [24], [25]] and solvothermal [26,27] reactions have been utilized to produce micrometer-sized undoped NaTaO3 particles. In addition, NaTaO3 particles
Materials and methods
For hydrothermal synthesis, 15 mol L−1 aqueous solutions of NaOH (96%, Wako) containing Ta2O5 (99.99%, Rare Metallic Co.) and BaCO3 (99.99%, Wako) were sealed in a Teflon container and heated at 473 K in an autoclave (OMlab-Tech, MR28) for 12 or 24 h. Previously, high concentrations of KOH have been reported to be favorable for KTaO3 production instead of pyrochlore-structured KTa2O5(OH) [36]. The solutions exhibited a Na/Ta molar ratio of 30 with Ba/Ta ratios of 0, 0.02, or 0.05. A one-side
Film composition
Fig. 3(A) shows the wide-scan XPS spectra of the substrate and films. In the spectrum (a) of the SrTiO3 substrate, major signals corresponding to Sr, Ti, O, and C were observed, while a weak emission was observed at a binding energy of 500 eV, possibly related to contamination by Na. An undoped NaTaO3 film prepared by the hydrothermal reaction for 24 h (hereafter referred to as HTM-NTO) exhibited signals corresponding to Na, Ta, and O with no sign of contamination other than the presence of
Conclusions
Perovskite-structured NaTaO3 films were deposited on centimeter-sized SrTiO3(001) wafers by a hydrothermal or solvothermal reaction. The addition of Ba in the starting solutions afforded Ba-doped NaTaO3 films. The heteroepitaxial relationship of the films and substrates was investigated by XRD and SEM analysis. X-ray fluorescence holography was applied to a Ba-doped film to further provide evidence for the epitaxial relationship suggested by the two other methods.
Acknowledgements
Yoshihiro Ebisu and Takuya Ogura helped the authors in observing X-ray fluorescence holograms. Youngku Sohn of Yeungnam University, Korea afforded facilities for XRD measurements. The X-ray fluorescence measurements were performed at the BL13XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2015B0116, 2016A0116 and 2016B1107). This work was supported by JSPS KAKENHI Grant Number JP15H01046 and JP16H02250.
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Present address: Center for Energy and Environmental Science, Shinshu University, Wakasato, Nagano, 380-8553 Japan.