Influence of Atomic Hydrogen, Band Bending, and Defects in the Top Few Nanometers of Hydrothermally Prepared Zinc Oxide Nanorods

Effect of Hydrogen Plasma Treatment on the Sensitivity of ZnO Based Electrochemical Non-Enzymatic Biosensor

Information on vitamin C-ascorbic acid (AA)-content is important as it facilitates the provision of dietary advice and strategies for the prevention and treatment of conditions associated with AA deficiency or excess. The methods of determining AA content include chromatographic techniques, spectrophotometry, and electrochemical methods of analysis. In the present work, an electrochemical enzyme-free ascorbic acid sensor for a neutral medium has been developed. The sensor is based on zinc oxide nanowire (ZnO NW) arrays synthesized via low-temperature chemical deposition (Chemical Bath Deposition) on the surface of an ITO substrate. The sensitivity of the electrochemical enzyme-free sensor was found to be dependent on the process treatments. The AA sensitivity values measured in a neutral PBS electrolyte were found to be 73, 44, and 92 µA mM-1 cm-2 for the ZnO NW-based sensors of the pristine, air-annealed (AT), and air-annealed followed by hydrogen plasma treatment (AT+PT), respectively. The simple H-plasma treatment of ZnO nanowire arrays synthesized via low-temperature chemical deposition has been shown to be an effective process step to produce an enzyme-free sensor for biological molecules in a neutral electrolyte for applications in health care and biomedical safety.

pH Controlled Nanostructure and Optical Properties of ZnO and Al-Doped ZnO Nanorod Arrays Grown by Microwave-Assisted Hydrothermal Method

The low-temperature microwave-assisted hydrothermal method was used to successfully grow pure and Al-doped ZnO (AZO) nanorod (NR) arrays on glass substrates. The combined effects of doping and pH on the structural properties, surface chemistry, and optical properties of all samples were investigated. Thermodynamic-based simulations of the growth solution were performed and a growth mechanism, that considers the effects of both the pH and Al-doping, is proposed, and discussed. Tuning the solution pH is key parameter to grow well-aligned, single crystal, highly packed, and high aspect ratio nanorod arrays. Moreover, the optical absorption in the visible range is enhanced by controlling the pH value. The PL spectra reveal a shift of the main radiative emission from the band-to-band into a transition involving deep defect levels of Zinc interstitial Zni. This shift is caused by an enhancement of the non-radiative components (phonon relaxation) at high pH values. The production of well-ordered ZnO and AZO nanorod arrays with visible-active absorption/emission centers would increase their potential use in various applications.

Some Distinct Attributes of ZnO Nanorods Arrays: Effects of Varying Hydrothermal Growth Time

This study investigates the growth time effect on the structural, morphological, optical, and photoelectrochemical characteristics of highly oriented ZnO nanorod arrays (ZNRAs). The nanorod arrays were grown on ITO substrates using the unified sol-gel spin coating and hydrothermal techniques. ZnO nanoparticles (ZNPs) were synthesized using the sol-gel spin coating method. In contrast, the hydrothermal method was used to grow the ZnO nanorods. The hydrothermal growth time investigated was between 4 and 12 h. The synthesized ZNRAs were used as the photoanode electrodes to investigate their photoelectrochemical (PEC) electrode potency. The as-prepared ZNRAs were characterized using various analytical tools to determine their structures, morphologies, optical, and photoelectrochemical traits. EDX spectra showed the presence of uncontaminated ZnO chemical composition, and FTIR spectra displayed the various functional groups in the samples. A rod-shaped ZnO nanocrystallite with mean lengths and diameters of 300-500 nm and 40-90 nm, respectively, is depicted. HRTEM images indicated the nucleation and growth of ZNRAs with a lattice fringe spacing of 0.26 nm and a growth lattice planer orientation of [002]. The optimum ZNRAs (grown at 8 h) as photoelectrode achieved a photoconversion efficiency of 0.46% and photocurrent density of 0.63 mA/cm², that was 17 times higher than the one shown by ZNPs with Ag/AgCl as the reference electrode. Both values were higher than those reported in the literature, indicating the prospect of these ZNRAs for photoelectrode applications.

Stabilization of the Surface of ZnO Films and Elimination of the Aging Effect

Zinc oxide is a promising multifunctional material. The practical use of nano- and polycrystalline ZnO devices faces a serious problem of instability of electrical and luminescent characteristics, due to the adsorption of oxygen by the surface during aging. In this paper, the aging effect in ZnO films and nanorod arrays was studied. It was found that ZnO samples demonstrate different behavior of the degradation process, which corresponds to at least two different types of adsorbing surface sites for O₂, where O₂ adsorption is of a different nature. The first type of surface sites is rapidly depassivated after hydrogen passivation and the aging effect takes place due to these centers. The second type of surface sites has a stable structure after hydrogen passivation and corresponds to HO-ZnO sites. The XPS components of these sites include the Zn2p_3/2 peak at 1022.2 ± 0.2 eV and Zn2p_1/2 peak at 1045.2 ± 0.2 eV, with a part of the XPS O1s peak at 531.5 ± 0.3 eV. The annealing transforms the first type of site into the second one, and the subsequent short-term plasma treatment in hydrogen results in steady passivation, where the degradation of characteristics is practically reduced to zero.

Visible-Light-Responsive Photocatalytic Activity Significantly Enhanced by Active [VZn+VO+] Defects in Self-Assembled ZnO Nanoparticles

Defect influences on the photoactivity of ZnO nanoparticles prepared by a powdered coconut water (ACP) assisted synthesis have been studied. The crystalline phase and morphology of ZnO nanoparticles were effectively controlled by adjusting the calcination temperature (400-700 °C). An induced transition of hybrid Zn₅(CO₃)₂(OH)₆/ZnO nanoparticles to single-phase ZnO nanoparticles was obtained at 480 °C. The morphological analysis revealed a formation of ZnO nanoparticles with semispherical (∼6.5 nm)- and rod-like (∼96 nm) shapes when the calcination temperatures were 400 and 700 °C, respectively. Photoluminescence characterizations revealed several defects types in the samples with V_Zn and V_O⁺ being in the self-assembly of semispherical- and rod-like ZnO nanoparticles. The photocatalytic activity of the ZnO nanoparticles was examined by assessing the degradation of methylene blue in an aqueous solution under low-intensity visible-light irradiation (∼3 W m^-2). The results point toward the self-assembly of semispherical- and rod-like ZnO nanoparticles that had significantly better photocatalytic activity (∼31%) in comparison to that of spherical-agglomerated- or near-spherical-like species within 120 min of irradiation. The possible photocatalytic mechanism is discussed in detail, and the morphology-driven intrinsic [V_Zn+V_O⁺] defects are proposed to be among the active sites of the ZnO nanoparticles enhancing the photocatalytic activity.

Visible light photocatalytic degradation of polypropylene microplastics in a continuous water flow system

Microplastic pollution of water and ecosystem is attracting continued attention worldwide. Due to their small sizes (≤5 mm) microplastic particles can be discharged to the environment from treated wastewater effluents. As microplastics have polluted most of our aquatic ecosystems, often finding its way into drinking water, there is urgent need to find new solutions for tackling the menace of microplastic pollution. In this work, sustainable green photocatalytic removal of microplastics from water activated by visible light is proposed as a tool for the removal of microplastics from water. We propose a novel strategy for the elimination of microplastics using glass fiber substrates to trap low density microplastic particles such as polypropylene (PP) which in parallel support the photocatalyst material. Photocatalytic degradation of PP microplastics spherical particles suspended in water by visible light irradiation of zinc oxide nanorods (ZnO NRs) immobilized onto glass fibers substrates in a flow through system is demonstrated. Upon irradiation of PP microplastics for two weeks under visible light reduced led to a reduction of the average particle volume by 65%. The major photodegradation by-products were identified using GC/MS and found to be molecules that are considered to be mostly nontoxic in the literature.

T, Amador-Mendez N, Gogneau N, K.G. N, Tchernycheva M, Das S. The elevated colour rendering of white-LEDs by microwave-synthesized red-emitting (Li, Mg)3RbGe8O18:Mn4+ nanophosphors

The bright red emitting (Li, Mg)₃RbGe₈O₁₈:Mn⁴⁺ nanophosphors significantly enhanced the colour rendering of a white-LED based on blue-LED chip and YAG:Ce³⁺ phosphor.

Dye wastewater treatment enabled by piezo-enhanced photocatalysis of single-component ZnO nanoparticles

Conventionally, composite materials are usually employed as a catalyst in piezo-photocatalytic dye wastewater treatment. Here, we report the synthesis of ZnO nanoparticles, as a single-component catalyst, by surfactant-assisted precipitation in which the size of ZnO nanoparticles (20–100 nm) can be simply controlled by the use of Tween80 as a surfactant. Although, ZnO nanoparticles exhibited appreciable photocatalytic activities for the degradation of methylene blue (MB) dye, upon the addition of a mechanical force, the photocatalytic dye degradation efficiency was substantially improved. Furthermore, we postulated that the surface properties of ZnO play an important role in charge transfer phenomena based on photoluminescence results together with functional groups on the surface of ZnO. In addition, application of single-component ZnO in piezo-promoted photocatalytic degradation of cationic and anionic dyes was accomplished. Our results regarding the behaviour of single-component ZnO nanoparticles under vibrational energy in addition to their conventional solar harvesting can provide a promising strategy for developing photocatalysts for practical wastewater treatment.

Single-component ZnO nanoparticles, synthesized by a simple synthetic method, exhibit appreciable piezo-enhanced photocatalytic activities, representing an alternative to other complex systems.

Broadband luminescence in defect-engineered electrochemically produced porous Si/ZnO nanostructures

The fabrication, by an all electrochemical process, of porous Si/ZnO nanostructures with engineered structural defects, leading to strong and broadband deep level emission from ZnO, is presented. Such nanostructures are fabricated by a combination of metal-assisted chemical etching of Si and direct current electrodeposition of ZnO. It makes the whole fabrication process low-cost, compatible with Complementary Metal-Oxide Semiconductor technology, scalable and easily industrialised. The photoluminescence spectra of the porous Si/ZnO nanostructures reveal a correlation between the lineshape, as well as the strength of the emission, with the morphology of the underlying porous Si, that control the induced defects in the ZnO. Appropriate fabrication conditions of the porous Si lead to exceptionally bright Gaussian-type emission that covers almost the entire visible spectrum, indicating that porous Si/ZnO nanostructures could be a cornerstone material towards white-light-emitting devices.