Highly efficient Photochemical Synthesis of H2O2 heterostructure of hematite
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First author: Zhujun Zhang Corresponding author: Takashi Tachikawahttps://doi.org/10.1038/s41467-022-28944-y full-text quick reference as we all know, often observed in the ion oxide dopant segregation can be used for engineering materials and devices.However, dopants are mostly confined to nanoscale regions due to poor driving forces for ion migration and/or the presence of large grain boundaries.In this paper, metal-doped hematite mesocrystals were annealed in one step at relatively low temperature and in air to form core-shell heterostructures by directional self-segregation.The sintering of highly ordered interfaces between nanocrystalline supramaterials within the mesocrystals removes grain boundaries and leaves many oxygen vacancies in the bulk.This causes the dopant (~90%) to be effectively separated on the outer surface, forming an oxide overlay.The high activity (~0.8 μmol min-1 cm-2) and selectivity (~90%) of photochemical H2O2 production were achieved by hematite mesocrystalline photoanodes containing Sn and Ti dopant oxide coating, which opened a new way for photochemical H2O2 production.Ionic oxide heterostructures with adjustable optical, electrical and magnetic properties by precisely controlling the concentration and position of elements have attracted wide attention in various fields from catalysis to (magnetic) photoelectronics.Most of these heterogeneous structures are created by vacuum techniques such as atomic layer deposition and chemical vapor deposition.They rely on high-precision equipment and specialized precursor reagents, which limits their large-scale application in industrial production.Dopant segregation is another method.It usually occurs in ionic solids containing heterovalently doped ions and is driven by elastic and/or electrostatic interactions (Figure 1A).However, in many cases, due to limited ion migration due to driving force or poor grain size, only a small amount of the doping agent can reach the outer surface boundary (GB) of the polycrystalline or nanocrystalline material at high temperatures (1300°C for Sn-doped hematite) (Figure 1B).External or intrinsic defects in a crystal (e.g., vacancies and interstitial atoms) give rise to regions of space charge that change the local electrostatic potential, but usually result in an uneven distribution of properties and inevitably reduce its performance.Therefore, it is challenging to construct heterostructures by dopant segregation.Currently, researchers can promote external segregation by removing GB from the doped material and adding extra space charges;However, these strategies are incompatible.The mesomorphic (MC) concept of directionally connected ordered nanocrystals provides a solution to this problem.The authors have recently found that heat treatment at relatively low temperatures (e.g., hematite (α-Fe2O3) MCs of 700°C) results in interfacial sintering (i.e., GB elimination) and the generation of a large number of interfacial oxygen vacancies (VO), which contributes to the directional migration of dopant ions and photogenerated charges (Figure 1B).The charge transfer efficiency and catalytic activity of semiconductor materials are greatly affected by their bulk electronic structure and surface structure.For example, hematite MC with thin rutile TiO2 coating shows excellent performance in photochemical (PEC) water oxidation to obtain O2 due to inhibiting the surface recombination of photogenerated electrons and holes.H2O2 is another product of water oxidation, which can be used as a green oxidizer for industrial chemistry and environmental purification, as well as a clean energy source for fuel cells.The production of PEC H2O2 is mainly achieved by using BiVO4 photoanode based water oxidation double-electron pathway.However, these photoanodes remain unstable in practical use due to the dissolution of V5+ due to anodic photocorrosion.1. Space charge-induced dopant segregation.A Distribution of dopant and electrostatic potential (donor doping case) in ionic oxides based on space charge theory.B Schematic diagram of dopant segregation in different types of ionic oxide crystals: (I) Crystals with large amounts of GBs: dopants tend to segregate at the surface and at GBs.(II) Disordered nanocrystals: even at high temperatures, small amounts of dopants tend to segregate at the surface due to lack of driving force.(III) Ordered nanocrystals with highly aligned interfaces: due to interfacial sintering (GB elimination), a large amount of dopant segregation on the outer surface generates a large amount of interfacial VO and shrinks the space charge layer to drive charge migration.Figure 2. Characterization of MC derived heterostructures of hematite.A TEM and B HRTEM images of the synthesized SnTi-Fe2O3 MCs.C HRTEM image of Annealed SnTi-Fe2O3 MC.The illustration represents a fast Inverse Fourier transform (FT) image of the selected region in the same color as shown in the dotted box.D HaADF-STEM images and corresponding EDX element mapping images of SnTi-Fe2O3 MCs after annealing.E-haadf-stem images of annealed SNTi-Fe2O3MC (left) and EELS composition of Ti (451.7 — 469.7eV) (middle) and Sn (507.5 — 525.5eV) (right) signals.F XPS depth analysis of synthesized and annealed SnTi-Fe2O3 samples.G Annealed Sn 3dXPS spectra of SnTi-Fe2O3 and Sn-Fe2O3 MCs.Non-in-situ Sn K-Edge XANES spectra of h-annealed Sn containing samples were measured in CEY mode.I The Sn K-Edge FT-EXAFS spectra corresponding to the sample.Figure 3. Crystallographic analysis of dopant segregation.A Powder XRD patterns of synthesized (solid line) and annealed (dotted line) samples were measured at a scanning rate of 10° min-1.B d104 lattice spacing of samples before and after annealing.C Powder XRD patterns of annealed samples measured at a scanning rate of 1.0° min-1.D PDF analysis of samples.E Driving force diagram of dopant directional segregation in hematite during heat treatment.Figure 4. In situ observation of dopant segregation.A Non-in-situ Ti K-Edge XANES spectra of annealed Titanium samples and reference samples.In-situ Ti K-Edge XANES spectra of SnTi-Fe2O3 synthesized by B were measured using a heating procedure similar to that of electrode preparation.In-situ FT-EXAFS spectra of Synthesized SnTi-Fe2O3 were measured using a heating procedure similar to that used for electrode preparation.D During heat treatment of the synthesized SnTi-Fe2O3 MC, the binary dopant segregation diagram of heterogeneous structure is realized.Figure 5. Synthesis and DFT calculation of PEC H2O2.A schematic diagram of PEC water decomposition system using hematite based photoanode.B Current densive-voltage curves of Fe2O3, Sn-Fe2O3, Ti-Fe2O3 and SnTi-Fe2O3 photoanodes in 1.0 M NaHCO3 under dark and backlit AM 1.5G simulated sunlight.C The amount of H2O2 produced by the photoanode at 1.23V vs RHE with increasing illumination time.D The photocurrent density and FEs of H2O2 obtained by different electrodes under 1.23V vs RHE illumination.E Structural models of hematite (110) (left) and sno2-X (110) (right) for DFT calculations.F active volcanic diagram of the calculated limit potential energy as a function of δ GOH*.Summary and Outlook Based on the above results, MC based binary dopant segregation was developed to construct heterostructures with high activity of photoelectrochemical synthesis of H2O2.Sn4+ on the oxidized surface of disordered SnTiOx coating is the active site for effective H2O2 generation.By regulating the composite overlay on hematite, the PEC performance for actual use can be further improved and applied to specific other sustainable reactions, such as CO2RR.In addition, other types of coatings, such as nitrides and hydrides, can be prepared by varying the conditions of synthesis (e.g., annealing in N2 or H2 atmosphere).