Synchrotron-based X-ray Spectro- and microscopic techniques are used in the present study to understand the origin of enhancement of photoelectrochemical (PEC) properties with nanocomposite BiVO4 (BVO) coated on ZnO nanodendrites, named as BVO/ZnO. This high PEC nanodendrites core-shell BVO/ZnO heterojunction is successfully grown and well-characterized for morphological and structural details [1]. Although the band alignment at BVO/ZnO heterojunction is likely to type I, the charge transport behavior is belonging in type II with the strong charge transfer (CT) with forming the high PEC heterojunction [2]. The strongly CT behavior from the V 3d (at shell-BVO) to Zn 4s/p (core-ZnO) in core-shell BVO/ZnO with the high number of O 2p unpair derived states at the interface is caused by the increasing the oxygen defects at the interface to construct interfacial band gap at 2.6 eV in core-shell BVO/ZnO. The interfacial band gap enhances the PEC performance with an increase in the efficiency of visible light-absorption and electron-hole separation. In addition, the distortion in the interface of core-shell BVO/ZnO with the high interfacial oxygen defects affects the O 2p -V 3d hybridization by decreasing the crystal field energy 10Dq ~2.2 eV, resulting the high electron-hole separation at the interface to improve PEC performance [3]. This study provides the evidence that the high PEC properties in nano-structure core-shell BVO/ZnO heterostructures are developed by the strongly CT, high electron-hole separation, and large visible light-absorption at the interface due to the increase in interfacial oxygen defects in the core-shell interface.
These insights from the local electronic and atomic structures in BVO layer coated ZnO nanodentrites may guide the fabrication of semiconductor heterojunctions with optimal compositions and interface that are highly desired to maximize the solar light utilization for PEC water splitting and their applications.
[1] Yang, J. S.; Wu, J. J., Low-potential driven fully-depleted BiVO4/ZnO heterojunction nanodendrite array photoanodes for photoelectrochemical water splitting. Nano Energy 2017, 32, 232-240.
[2] Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z.-X.; Tang, J., Visible-light driven heterojunction photocatalysts for water splitting – a critical review. Energy Environ. Sci. 2015, 8, 731-759.
[3] Chen, Z.; Fan, T.; Shao, M.; Yu, X.; Wu, Q.; Li, J.; Fang, W.; Yi, X., Simultaneously enhanced photon absorption and charge transport on a distorted graphitic carbon nitride toward visible light photocatalytic activity. Appl. Catal. B 2019, 242, 40-50.
To reduce greenhouse gas emissions in response to globalization and increasingly strict carbon emission policies, green energy technologies must be developed. Improving energy conversion/generation/storage efficiency of energy materials has always been a great challenge. Monitoring the atomic/electronic structures close the interface in many important energy materials, such as nanostructured catalysts, artificially photosynthesizing materials, smart materials, and energy storage devices, is of great importance. Designing such a material with improved performance without understanding its atomic/electronic structures, and their changes under operating conditions, is difficult. Understanding and controlling the interfacial electronic structures of energy materials require in-situ characterizations, of which synchrotron x-ray spectroscopy is the one with many unique features. The last decade has witnessed a golden age of in situ synchrotron x-ray spectroscopy for energy materials. X-ray absorption spectroscopy can be used to determine unoccupied electronic structures while X-ray emission spectroscopy can be utilized to examine occupied electronic structure. The additional use of resonant inelastic X-ray scattering reveals inter-electric d-d excitation or intra-electric charge transfer excitation that reflects the chemical and physical properties of the material. An emerging technique, scanning transmission x-ray microscopy is a spectro-microscopic approach, providing regional x-ray absorption spectroscopy, is also gearing up for energy science. This presentation will report recent studies and perspectives of the application of in situ/operando synchrotron x-ray spectroscopy to energy materials. Tamkang University (TKU) end-stations constructed at the Taiwan Photon Source (TPS) 45A & 27A beamlines for the x-ray spectroscopic investigation of energy materials will be also introduced.
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