7TH INTL. SYMP. ON SYNTHESIS AND PROPERTIES OF NANOMATERIALS FOR FUTURE ENERGY DEMANDS

Air conditioning, chemical processing, food storage, etc. bring up the need for humidity sensing in various aspects of everyday life [1]. Semiconducting oxides are well known in the area of sensing materials, while recently lowering their constituting domains in the nano-range opened up possibility for added benefit of behalf of greater specific surface area and pore volume. Among semiconducting oxides nanomaterials, ceria has attracted much attention in recent years. Ceria in various thin-film configurations is often prepared by methods such as pulsed laser deposition, spray pyrolysis, magnetron sputtering, chemical vapour deposition etc. [2]. All of these require specific reaction conditions (atmosphere, temperature, etc.) all of which can be avoided by the use of a simple but efficient tape casting method [3].
In this work we chemically derived ceria nanoparticles with solution homogeneity, prepared slurries thereof and tape casted them on conducting glass substrates. We varied thin-film thicknesses to obtain a mechanically and electrically optimized samples which were characterised in detail by XRD, UV-VIS DRS, GIXRD, SEM and AFM. Ceria samples in the form of pellets and thin-films were studied by impedance spectroscopy (IS), under controlled relative humidity (RH) from 30 % up to 85 %, and in a wide temperature and frequency range. Moreover, for thin-film setup, measurements were performed in surface-mode and cross-section-mode. In addition to compositional influence on relative humidity, the role of the configuration and film thickness on electrical properties and derivative humidity-sensing performance was studied in detail.
Structural analysis points to single phase crystalline ceria. Microstructure reveals slightly agglomerated spherical particles. Thin-films exhibit low surface roughness. Under controlled humidity, with an increase in RH, the shape of the conductivity spectrum stays the same; however, a shift to higher conductivity values is present. Relaxation is slow and conductivity values need a long time to return to starting values suggesting thickness of the pellet plays a crucial role in the relaxation process. One can see how the increase in humidity has a positive effect on the total DC conductivity, similarly to the temperature effect with semiconducting behaviour.
For surface measurement setup the film thickness has an impact on the shape of spectra and number of observed processes. We can conclude that surface measurement turns out to be more sensitive to relative humidity changes, emphasized for higher RH, along with an increase in thin-film thickness. We showed that moisture directly affects conductivity spectra in the dispersion part, i.e. on the localised short-range charge carriers. It can be concluded the moisture sensitivity is a reversible process for thin-film samples, in contrast to pellet form samples.

References:
[1] P. Kronenberg, P.K. Rastogi, P. Giaccari, H.G. Limberger, Optics letters 27 (2002) 1385-1387.
[2] Y.J. Acosta-Silva, M. Toledano-Ayala, G. Torres-Delgado, I. Torres-Pacheco, A. Méndez-López, R. Castanedo-Pérez, O. Zelaya-Ángel, Journal of Nanomaterials (2019) 5413134.
[3] R.K. Nishihora, P. L. Rachadel, M.G. Novy Quadri, D. Hotza, Journal of the European Ceramic Society 38 (2018) 988-1001.

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.

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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