Electrical characterization of ZnO-coated nanospring ensemble by impedance spectroscopy: probing the effect of thermal annealing

Fabrication of a P-Si/ZnO heterojunction based on galvanic cell driven and the complete degradation of RhB via fast charge transfer

Semiconductor heterojunctions can significantly enhance photocatalytic degradation efficiency by facilitating rapid interfacial charge transfer. This article is based on the galvanic-cell driven principle; porous silicon (P-Si) was prepared by the carbon-catalytic etching method, and ZnO was loaded on its surface via electroless chemical deposition technology to form a P-Si/ZnO heterojunction, which was applied to the degradation of Rhodamine B (RhB). At a deposition temperature of 90 °C, a flawless 1D hexagonal prism structure of ZnO was formed, allowing the P-Si/ZnO heterojunction to completely degrade RhB within 2 hours with a degradation rate of 100%. Compared with a single P-Si material, the degradation performance is improved by 1.7 times. The formation of the built-in electric field and the rapid charge transfer at the heterojunction interface realized the complete degradation of RhB organic pollutants. After 20 cycles of use, the photocatalytic degradation rate remains above 70%, demonstrating excellent stability and recyclability.

Fabrication and Elastic Properties of TiO2 Nanohelix Arrays through a Pressure-Induced Hydrothermal Method

TiO₂ nanohelices (NHs) have attracted extensive attention owing to their high aspect ratio, excellent flexibility, elasticity, and optical properties, which endow promising performances in a vast range of vital fields, such as optics, electronics, and micro/nanodevices. However, preparing rigid TiO₂ nanowires (TiO₂ NWs) into spatially anisotropic helical structures remains a challenge. Here, a pressure-induced hydrothermal strategy was designed to assemble individual TiO₂ NWs into a DNA-like helical structure, in which a Teflon block was placed in an autoclave liner to regulate system pressure and simulate a cell-rich environment. The synthesized TiO₂ NHs of 50 nm in diameter and 5-7 mm in length approximately were intertwined into nanohelix bundles (TiO₂ NHBs) with a diameter of 20 μm and then assembled into vertical TiO₂ nanohelix arrays (NHAs). Theoretical calculations further confirmed that straight TiO₂ NWs prefer to convert into helical conformations with minimal entropy (S) and free energy (F) for continuous growth in a confined space. The excellent elastic properties exhibit great potential for applications in flexible devices or buffer materials.

The effects of sub-bandgap transitions and the defect density of states on the photocurrent response of a single ZnO-coated silica nanospring

The electrical and optoelectronic properties of nanometer-sized ZnO structures are highly influenced by its native point defects. Understanding and controlling these defects are essential for the development of high-performance ZnO-based devices. Here, an electrical device consisting of a polycrystalline ZnO-coated silica nanospring was fabricated and used to characterize the electrical and photoconductive properties of the ZnO layer using near-UV (405 nm) and sub-bandgap (532 and 633 nm) excitation sources. We observe a photocurrent response with all three wavelengths and notably with 532 nm green illumination, which is the energy associated with deep oxygen vacancies. The polycrystalline ZnO-coated silica nanospring exhibits a high responsivity of 1740 A W^-1 with the 405 nm excitation source. Physical models are presented to describe the photocurrent rise and decay behavior of each excitation source where we suggest that the rise and decay characteristics are highly dependent on the energy of the excitation source and the trapping of electrons and holes in intermediate defect levels in the bandgap. The energy levels of the trap depths were determined from the photoconductive decay data and are matched to the reported energy levels of singly and doubly ionized oxygen vacancies. A phenomenological model to describe the dependence of the saturation photocurrent on excitation intensity is presented in order to understand the characteristics of the observed breaks in the slopes of the saturation photocurrent versus excitation intensity profile.

High-Temperature Atomic Layer Deposition of GaN on 1D Nanostructures

Silica nanosprings (NS) were coated with gallium nitride (GaN) by high-temperature atomic layer deposition. The deposition temperature was 800 °C using trimethylgallium (TMG) as the Ga source and ammonia (NH₃) as the reactive nitrogen source. The growth of GaN on silica nanosprings was compared with deposition of GaN thin films to elucidate the growth properties. The effects of buffer layers of aluminum nitride (AlN) and aluminum oxide (Al₂O₃) on the stoichiometry, chemical bonding, and morphology of GaN thin films were determined with X-ray photoelectron spectroscopy (XPS), high-resolution x-ray diffraction (HRXRD), and atomic force microscopy (AFM). Scanning and transmission electron microscopy of coated silica nanosprings were compared with corresponding data for the GaN thin films. As grown, GaN on NS is conformal and amorphous. Upon introducing buffer layers of Al₂O₃ or AlN or combinations thereof, GaN is nanocrystalline with an average crystallite size of 11.5 ± 0.5 nm. The electrical properties of the GaN coated NS depends on whether or not a buffer layer is present and the choice of the buffer layer. In addition, the IV curves of GaN coated NS and the thin films (TF) with corresponding buffer layers, or lack thereof, show similar characteristic features, which supports the conclusion that atomic layer deposition (ALD) of GaN thin films with and without buffer layers translates to 1D nanostructures.