Temperature effects on ZnO electrodeposition

Two-dimensional ZnO nanosheet networks composed of many thin and uniform hexagonal-shaped ZnO nanosheets and ZnO nanodiscs were prepared in a large scale on silicon substrate through thermal evaporation using ZnCl2 and O2 as source materials for Zn and oxygen, respectively, without the use of metal catalysts or additives Detailed structural studies indicated that the synthesized products are single crystalline with wurtzite hexagonal structure Raman scattering of the synthesized products confirmed that the as-grown structures have good crystal quality with a hexagonal wurtzite phase Room temperature photoluminescence spectra showed a strong green band with a suppressed UV emission from the ZnO nanosheet networks, but on the other hand a dominant and strong near band edge emission with a much suppressed deep level emission was observed in the nanodiscs The growth mechanism of these structures is also discussed in detail

https://iopscience.iop.org/article/10.1088/0957-4484/17/9/016/pdf

ZnO nanosheet networks and hexagonal nanodiscs grown on silicon substrate: growth mechanism and structural and optical properties

The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the "melt-and-quench" technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the "leaky" TiO2 recently discovered for the photochemical water-splitting applications.

Oxygen vacancy and hole conduction in amorphous TiO2

Since the positive influences of defects on the performance of electroceramics were discovered, investigations concerning on defects and aliovalent doping routes have grown rapidly in the fields of inorganic chemistry and condensed matter physics. In this article, we summarized the types of defects in electroceramics as well as characterization tools of defects and highlighted the effects of intrinsic and extrinsic defects on the material performances with the emphasis on dielectric, ferroelectric, and piezoelectric properties. We mainly introduced defect related theoretical simulation and experimental results in several typical incipient ferroelectrics, ferroelectrics, and antiferroelectrics. Hence, the influences of defects on the crystal lattice were summed up, and then the main physical mechanisms were highlighted. Particularly, the performance enhancements of aliovalently doped electroceramics were also evaluated and reviewed. Finally, the outlook and challenges were discussed on the basis of their current developments. This article covers not only an overview of the state-of-the-art advances of defects and aliovalent doping routes in electroceramics but also the future prospects that may open another window to tune the electrical performance of electroceramics via intentionally introducing certain defects.

Defects and Aliovalent Doping Engineering in Electroceramics.

Abstract Zinc oxide (ZnO) is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of ZnO and the massive recombination of photogenerated charge carriers, especially in its nanosize, limit the overall photocatalytic efficiency. This can be further overcome by modifying the electronic band structure of ZnO by hybridization with a narrow band gap material, including metal, metal oxide, carbon based, and polymeric based. Indeed, ZnO hybridization with the respective materials contributed to its sensitizer by shifting the absorption wavelength to the visible region of the spectrum. This review encompasses several advancements made in the mentioned aspects, and also some of the new physical insights related to the charge transfer events, such as charge carrier generation, trapping, detrapping, and their transfer to surface, are discussed for each strategy of the hybrid ZnO. The synergistic effects in the mixed polymorphs of ZnO and also the theories proposed for their enhanced activity are reported. The review also highlights the potential application of ZnO hybrid for different kinds of pollutants from different wastewater sources.

Review on ZnO hybrid photocatalyst: impact on photocatalytic activities of water pollutant degradation

Zinc oxide (ZnO) crystals joined by using an intergranular glass phase were investigated in order to develop surge filters for low-voltage applications and to clarify the current transport mechanism through the junction. The junctions having a ZnO/glass/ZnO sandwich structure were synthesized by using an interfacial glass phase of the Bi–B–O system. A highly nonlinear current–voltage curve was obtained for the ZnO/glass/ZnO junction with Co-doped ZnO single crystals, and the current through the junction at the breakdown voltage was proportional to the 30th power of the bias voltage. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface, and the observed high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.

Synthesis of zinc oxide varistors with a breakdown voltage of three volts using an intergranular glass phase in the bismuth–boron–oxide system

An anatase TiO2 film was epitaxially grown on a conductive Nb-doped single-crystalline SrTiO3 (100) substrate by metalorganic chemical vapor deposition. RuO2 Schottky electrode was fabricated on the epitaxial anatase film by reactive dc magnetron sputtering. The dark I–V and capacitance–voltage characteristics indicated the good rectification and thermal stability of the RuO2/anatase junction. This RuO2/anatase junction enables the stable measurements of deep level transient spectroscopy in the high-temperature region and is a promising Schottky electrode to examine the origins of deep levels in the band gap of anatase.

Superior Schottky electrode of RuO2 for deep level transient spectroscopy on anatase TiO2

Deep level transient spectroscopy was carried out to investigate the origin of the deep levels in the band gap of anatase TiO2. The epitaxial anatase-TiO2 film grown by metal-organic chemical vapor deposition possessed a deep level whose activation energy was 0.52eV. In contrast, this deep level at 0.5eV was not observed in the films grown by sputtering. However, by adding CH4 or H2 to the sputtering gas, the deep level at 0.5eV was observed in the sputter-grown films. Furthermore, the density of this deep level increased with increasing hydrogen gas, suggesting that this deep level originated from hydrogen doping.

Takahira Miyagi

Papers

Synthesis of zinc oxide varistors with a breakdown voltage of three volts using an intergranular glass phase in the bismuth–boron–oxide system

Superior Schottky electrode of RuO2 for deep level transient spectroscopy on anatase TiO2

Discovery of the deep level related to hydrogen in anatase TiO2