54 Physical Science Research Frontiers 2022 H y d r o g e n p e r o x i d e ( H 2 O 2 ) h a s a t t r a c t e d considerable interest owing to its usefulness as a green oxidant for applications in industrial chemistry and environmental purification, as well as a clean energy source for fuel cells. Solar-driven photoelectrochemical (PEC) water splitting is one of the ideal strategies for the on-site production of H 2 O 2 and CO 2 -free H 2 gas [1]. PEC H 2 O 2 production has been mostly realized by a two-electron pathway of water oxidation using BiVO 4 -based photoanodes, but these photoanodes are expensive and have low stability. Alternatively, hematite ( a -Fe 2 O 3 ) is naturally abundant with good stability and a suitable bandgap (~2.1 eV) for the absorption of sunlight, and has been extensively studied for use in solar-driven water oxidation to generate O 2 . However, there are no reports on hematite-based photoanodes for water oxidation to obtain H 2 O 2 , probably because of their unfavorable surface properties. A mesocrystal (MC) is an ordered assembly of nanoparticles via oriented attachment and has been applied as a photocatalyst [2]. We have recently discovered that the thermal treatment of hematite MCs at high temperatures (e.g., 700°C) induces the sintering of the interface, which creates numerous interfacial oxygen vacancies (V O ) for the enhanced separation of photogenerated electrons and holes [3,4]. Herein, we present MC-based dopant segregation to effectively modify the hematite MCs surface for highly efficient and selective solar-driven H 2 O 2 production (Fig. 1) [5]. The sintering of highly ordered interfaces between the nanocrystal subunits inside the MC eliminates substantial grain boundaries, leading to the efficient segregation of dopants on the external surface, which results in the formation of deficient SnTiO x overlayers for H 2 O 2 production. The synchrotron-based X-ray total scattering with pair distribution function (PDF) analysis, which is a powerful method of characterizing disordered or amorphous structures, was performed with the incident X-ray energy of 61.4 keV at SPring-8 BL04B2 . Data were collected using the hybrid detectors of Ge and CdTe. The reduced PDF G ( r ) was obtained by the conventional Fourier transform (FT) of the Faber– Ziman structure factor S ( Q ) extracted from the collected data. The as-synthesized Sn- and Ti-doped hematite (SnTi–Fe 2 O 3 ) sample shows a peak shift, along with the broadening of the peaks, towards a larger interatomic distance ( r ) than the undoped Fe 2 O 3 sample (Fig. 2), suggesting the expansion of the hematite lattice due to the replacement of Fe 3+ sites by the larger Sn and Ti atoms. The thermal treatment at 700°C in air for 20 min leads to a lattice rearrangement in the bulk, as indicated by the peak shifts toward the profile of the undoped Fe 2 O 3 sample. Considering that the length of the corner-sharing Ti–Ti (and Sn–Sn) bond of rutile TiO 2 (and SnO 2 ) is 3.55 Å (and 3.68 Å), the increase in amplitude at 3.5–3.7 Å implies the formation of the dopant oxide overlayers. To elucidate the local structures of the overlayers, X-ray absorption spectra (XAS) at Ti and Sn K -edges were collected at SPring-8 BL01B1 . The samples were prepared by pelletizing the uniform mixtures of hematite-based MCs or reference samples with dehydrated boron nitride powders. Figure 3(a) shows the normalized Ti K -edge XANES spectra of the Ti-containing samples. For transition metals in oxides, the threshold energy position of the spectrum is sensitive to their oxidation states, while the shapes of the peaks give information about the local structural environments of the absorbing elements. The peak shapes of the annealed Ti-doped hematite (Ti–Fe 2 O 3 ) Local structures of hematite-mesocrystal-based photoanodes for efficient and selective solar water splitting Fig. 1. Hematite-mesocrystal-based photocatalyst for solar H 2 O 2 production. A hematite mesocrystal is a superstructure of particles, each around 20 nm in size. The segregation of Sn and Ti atoms doped in the mesocrystal was thermally induced to form a composite oxide (SnTiO x ) layer. A transmission electron microscopy image is also shown. Sn,Ti-doped Fe 2 O 3 mesocrystal to Pt for H 2 evolution O 2 H 2 O H 2 O 2 SnTiO x SnTiO x Sn Sn Sn Sn Sn Sn Sn Sn Sn Sn Sn Sn Sn S Sn Sn Sn n Sn Sn Sn n Sn Sn Sn Sn S S T T T Ti Ti Ti Ti T T T T T T T T T T T T T T T T T T T T T T T O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O x x x x x x x x x x x x x x x x x x x x SnTiO x SnTiO x Sn Sn SnTiO x /Fe2 O 3 mesocrystal Fe 2 O 3 Thermally-induced dopant segregation Sunlight h + h + e – e – 10 nm 55 Research Frontiers 2022 MCs in pre- and post-edge regions are similar to those of rutile TiO 2 , suggesting the formation of the rutile overlayer. For the annealed SnTi-Fe 2 O 3 sample, the characteristic peak of rutile TiO 2 in the pre-edge region was not clearly observed. In situ Sn K -edge FT-EXAFS measurements helped us to gain more information on the dynamics of thermally induced dopant segregation. For the annealed SnTi–Fe 2 O 3 sample, as shown in Fig. 3(b), the second main shell has a shorter radial distance than that of SnO 2 owing to the formation of Sn–Ti coordination. The peak intensity of the Sn–Fe coordination shell gradually decreases with increasing temperature from 48 to 700°C because the Sn coordination becomes disordered as a result of the diffusion of Sn atoms from the hematite lattice. On the other hand, the Sn– Ti coordination peak appears at approximately 700°C and becomes stronger during the heat preservation and cooling stages. From these results, we concluded that Sn atoms initially migrate from the hematite lattice owing to their larger radius and segregate on the surface to form the SnO 2 phase. The deficient SnTiO x overlayers are then formed when Ti atoms segregate at the surface and react with SnO 2 at 700°C (Fig. 1). The composite overlayers on the hematite MCs could act as co-catalysts for highly selective solar H 2 O 2 synthesis. Fig. 2. Synchrotron-based X-ray total scattering measurements with PDF analysis. The peak at approximately 2 Å is composed of shorter Fe–O (1.94 Å) and longer Fe–O (2.12 Å) distances. The peak at approximately 3 Å is composed of the first-neighbor edge-sharing (Fe–Fe e , 2.95 Å) and face-sharing (Fe–Fe f , 2.90 Å) Fe–Fe distances. The first- and second-neighbor corner-sharing Fe–Fe pairs are at 3.39 Å (Fe–Fe c1 ) and 3.72 Å (Fe–Fe c2 ), respectively. Fig. 3. X-ray absorption spectroscopy (XAS) measurements. ( a ) Ex situ Ti K -edge XANES spectra of annealed Ti-containing samples and reference samples. ( b ) In situ Sn K -edge FT-EXAFS spectra of as-synthesized SnTi-Fe 2 O 3 measured with a heating procedure similar to that used for electrode preparation. Takashi Tachikawa Molecular Photoscience Research Center, Kobe University Email: tachikawa@port.kobe-u.ac.jp References [1] K. Sayama: ACS Energy Lett. 3 (2018) 1093. [2] T. Tachikawa et al. : NPG Asia Mater. 6 (2014) e100. [3] Z. Zhang et al. : Nat. Commun. 10 (2019) 4832. [4] Z. Zhang et al. : Angew. Chem. Int. Ed. 59 (2020) 9047. [5] Z. Zhang, T. Tsuchimochi, T. Ina, Y. Kumabe1, S. Muto, K. Ohara, H. Yamada, S. L. Ten-no and T. Tachikawa: Nat. Commun. 13 (2022) 1499. 2 3 16 17 18 19 20 –5 0 5 10 r (Å) G ( r ) Fe 2 O 3 Fe–Fe e Fe–Fe c2 Fe–Fe c1 Fe–O SnTi–Fe 2 O 3 (before annealing) SnTi–Fe 2 O 3 (after annealing) Dopant segregation (Lattice rearrangement) Doping (Lattice expansion) r 4960 4970 4980 4990 5000 0 0.5 1 1.5 Normalized Absorbance Energy (eV) (a) 0 1 2 3 4 5 6 41 ° C 48 ° C 700 ° C 700 ° C Radial Distance (Å) (b) Ti–Fe 2 O 3 SnTi–Fe 2 O 3 rutile TiO 2 rutile TiO 2 Ti K -edge 4960 4965 4970 0 0.1 0.2 SnTiO x (700°C)
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