Direct synthesis of ZnO nanowire arrays on Zn foil by a simple thermal evaporation process

Deformation twin traces on gold surfaces: A pathway to tailored epitaxial growth of 1D semiconductors

The field of one-dimensional semiconducting materials holds a wide variety of captivating applications, such as photovoltaic cells, electronic devices, catalysis cells, lasers, and more. The tunability of electrical, mechanical, or optical attributes of a semiconductor crystal relies on the ability to control and pattern the crystal's growth direction, orientation, and dimensions. In this study, we harvest the unique properties of crystallographic defects in Au substrates, specifically twin boundaries, to fabricate selective epitaxial growth of semiconducting nanocrystals. Different crystallographic defects were previously shown to enhance materials properties, such as, screw dislocations providing spiral crystal growth, dislocation outcrops, and vacancies increasing their catalytic activity, dislocation strengthening, and atomic doping changing the crystal's electrical properties. Here, we present a unique phenomenon of directed growth of semiconductor crystals of gold(I)-cyanide (AuCN) on the surface of thin Au layers, using traces of deformation twins on the surface. We show that emergence of deformation twins to the {111} Au surface leads to the formation of ledges, exposing new {001} and {111} facets on the surface. We propose that this phenomenon leads to epitaxial growth of AuCN on the freshly exposed {111} facets of the twin boundary trace ledges. Specific orientations of the twin boundaries with respect to the Au surface allow for patterned growth of AuCN in the <110> orientations. Nano-scale patterning of AuCN semiconductors may provide an avenue for property tuning, particularly the band gap acquired.

The Growth of High-Quality Hexagonal GaTe Nanosheets Induced by ZnO Nanocrystals

The monoclinic and hexagonal gallium tellurides (m-GaTe and h-GaTe) show different applications in optoelectronic devices. Compared to the m-GaTe, the h-GaTe is a metastable phase, which generally exists in ultrathin samples and is difficult to obtain by direct chemical reaction. Herein, a hexagonal ZnO-induced crystal growth strategy was used for the design and fabrication of h-GaTe. The high-quality h-GaTe nanosheets were successfully grown on the (001) surface of hexagonal ZnO by the chemical vapor deposition method under ambient pressure. The SEM, XPS, XRD, and HRTEM characterizations uncovered a flower-like nanosheet morphology and a hexagonal crystal structure for the obtained GaTe samples. Meanwhile, the conductive atomic force microscope measurement indicates that the obtained h-GaTe nanosheet is a p-type semiconductor. Based on the electron localization function simulation, the lattice-induced crystal growth of h-GaTe was demonstrated. The results give an insight into the synthesis of metastable phase crystal and open an avenue for fabricating new two-dimensional devices by p-type h-GaTe.

In situ unraveling of the effect of the dynamic chemical state on selective CO2 reduction upon zinc electrocatalysts

Unraveling the reaction mechanism behind the CO₂ reduction reaction (CO₂RR) is a crucial step for advancing the development of efficient and selective electrocatalysts to yield valuable chemicals. To understand the mechanism of zinc electrocatalysts toward the CO₂RR, a series of thermally oxidized zinc foils is prepared to achieve a direct correlation between the chemical state of the electrocatalyst and product selectivity. The evidence provided by in situ Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray diffraction significantly demonstrates that the Zn(ii) and Zn(0) species on the surface are responsible for the production of carbon monoxide (CO) and formate, respectively. Specifically, the destruction of a dense oxide layer on the surface of zinc foil through a thermal oxidation process results in a 4-fold improvement of faradaic efficiency (FE) of formate toward the CO₂RR. The results from in situ measurements reveal that the chemical state of zinc electrocatalysts could dominate the product profile for the CO₂RR, which provides a promising approach for tuning the product selectivity of zinc electrocatalysts.

Room temperature sintering of polar ZnO nanosheets: II-mechanism

In a previous work by the authors (A. Fernández-Pérez el al., Room temperature sintering of polar ZnO nanosheets: I-evidence, 2017, DOI: 10.1039/C7CP02306E), polar ZnO nanosheets were stored at room temperature under different atmospheres and the evolution of their textural and crystal properties during storage was followed. It was found that the specific surface area of the nanosheets drastically decreased during storage, with a loss of up to 75%. The ZnO crystals increased in size mainly through the partial merging of their polar surfaces at the expense of narrow mesoporosity, in a process triggered by the action of moisture, oxygen and, in their absence, by light. In the present work, a set of spectroscopic techniques (FTIR, Raman and XPS) has been used in an attempt to unravel the mechanism behind this spontaneous sintering process. The mechanism starts with the molecular adsorption of water, which takes place on Zn atoms close to oxygen vacancies on the (100) surface, where H₂O dissociates to form two hydroxyl groups and to heal one oxygen vacancy. This process triggers the room temperature migration of Zn interstitials towards the outer surface of the polar region. What were previously interstitial Zn atoms now gradually occupy the mesopores, with interstitial oxygen being used to build up the O sublattice until total occupancy of the narrow mesoporosity is achieved.

The Influence of Short-Range Correlation on the Phonon Confinement of a Single ZnO Nanowire

Plenty of researches have been performed to probe the diverse properties of ZnO nanowires, but only a few have focused on the physical properties of a single nanowire since to analyze the optical confinement and their correlation lengths along a single nanowire is difficult. In this study, a single ZnO nanowire was synthesized using a Ti-assisted chemical vapor deposition (CVD) method to avoid the appearance of catalytic contamination. Confocal Raman spectroscopy is a powerful tool for probing the phonon confinement effect in a single ZnO nanowire. A confinement model was used to calculate the correlation lengths along the growth direction. The Raman mapping of ZnO nanowires was obtained by a confocal Raman spectrometer. A phonon confinement model was used to fit the Raman curves of the E₂ mode and to obtain the correlation lengths along the growth direction of the ZnO nanowire. The correlation lengths are related to the phonon confined region by boundaries and/or defects.

Self-Organized Growth of Two-Dimensional GaTe Nanosheet on ZnO Nanowires for Heterojunctional Water Splitting Applications

Epitaxial two-dimensional GaTe nanosheets on ZnO nanowires were routinely prepared via a two-step chemical vapor deposition procedure. The epitaxial relationship and growth mechanism of the GaTe/ZnO core/shell structures were explored and attributed to a layer-overlayer model. The hybrid structures increased the surface area and the favorable p-n heterojunction enhanced the charge separation for photoelectrochemical performance in water splitting. The above synergistic effects boosted the photocurrent density from -0.3 mA cm^-2 for the pristine ZnO nanowires to -2.5 mA cm^-2 for the core/shell GaTe/ZnO nanowires at -0.39 V vs RHE under the visible light irradiation. This highlights the promise for utilization of GaTe nanosheet/ZnO nanowires as efficient photoelectrocatalyst for water splitting.

Photoluminescence and electrical properties of bidirectional ZnO nanowires on Zn foils via a thermal oxidation method

ZnO nanowires were directly grown on ductile zinc foils through a two-step process. Zn foils were fabricated from a mixture of Zn and ZnO powders; and ZnO NWs were produced using thermal oxidation at temperatures of 300–600 °C.

Facile synthesis of hierarchical porous WO3 nanofibers having 1D nanoneedles and their functionalization with non-oxidized graphene flakes for selective detection of acetone molecules

Hierarchical and porous one-dimensional (1D) nonwoven WO₃ nanofibers were synthesized by electrospinning and controlled two-step heat-treatment, and are applicable for diabetes diagnostic sensors by selective detection of breath acetone.

ZnO nanowire lasers

The pathway towards the realization of optical solid-state lasers was gradual and slow. After Einstein's paper on absorption and stimulated emission of light in 1917 it took until 1960 for the first solid state laser device to see the light. Not much later, the first semiconductor laser was demonstrated and lasing in the near UV spectral range from ZnO was reported as early as 1966. The research on the optical properties of ZnO showed a remarkable revival since 1995 with the demonstration of room temperature lasing, which was further enhanced by the first report of lasing by a single nanowire in 2001. Since then, the research focussed increasingly on one-dimensional nanowires of ZnO. We start this review with a brief description of the opto-electronic properties of ZnO that are related to the wurtzite crystal structure. How these properties are modified by the nanowire geometry is discussed in the subsequent sections, in which we present the confined photon and/or polariton modes and how these can be investigated experimentally. Next, we review experimental studies of laser emission from single ZnO nanowires under different experimental conditions. We emphasize the special features resulting from the sub-wavelength dimensions by presenting our results on single ZnO nanowires lying on a substrate. At present, the mechanism of lasing in ZnO (nanowires) is the subject of a strong debate that is considered at the end of this review.

Synthesis of BiFeO3/ZnO core-shell hetero-structures using ZnO nanorod positive templates

We report the synthesis, morphology, and magnetization characteristics of BiFeO(3) (BFO)-covered ZnO nanorod arrays (ZNAs). High quality and well-aligned ZNAs were grown by a hydrothermal method. BFO shells were deposited by sputtering at ambient temperature and then annealing in an oxygen atmosphere. The BFO shells crystallized to form a perovskite structure at 450 °C. Scanning electron microscopy and high resolution transmission electron microscopy demonstrated that the BFO shell was polycrystalline and randomly oriented, covering the ZnO nanorods well. The magnetization-magnetic field loops measured at 5 and 300 K indicate that the BFO/ZNA hetero-structure exhibits ferromagnetic order. The BFO/ZNA displays enhanced coercivity and saturated magnetization as compared with BFO thin films.

Preparation and photocatalytic property of a novel dumbbell-shaped ZnO microcrystal photocatalyst

A novel dumbbell-shaped ZnO microcrystal photocatalyst was successfully synthesized by hydrothermal method in the present study. The prepared ZnO photocatalyst was systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TG-DTA), photoluminescence spectrum (PL) and UV-vis absorption spectrum (UV-vis). The characterizations of dumbbell-shaped ZnO were also compared with the commercial ZnO. The results show that the prepared ZnO photocatalyst has a unique dumbbell shape and it belongs to the hexagonal wurtzite family. In addition, the photocatalytic activity of the prepared dumbbell-shaped ZnO microcrystal photocatalyst was evaluated by the degradation of three different kinds of dyes wastewater (Crystal Violet, Methyl Violet and Methylene Blue). After 75 min reaction, the decolourization efficiencies of the three kinds of dyes wastewater achieved 68.0%, 99.0% and 98.5%, the TOC removal efficiencies achieved 43.2%, 59.4% and 70.6%, respectively. Compared to commercial ZnO, 16-22% higher TOC removal efficiency was obtained by the dumbbell-shaped ZnO. The results indicated that the prepared dumbbell-shaped ZnO microcrystal photocatalyst showed good photocatalytic activity and it could be considered as a promising photocatalyst for dyes wastewater treatment.

UV and humidity sensing properties of ZnO nanorods prepared by the arc discharge method

The UV and humidity sensing properties of ZnO nanorods prepared by arc discharge have been studied. Scanning electron microscopy and photoluminescence spectroscopy were carried out to analyze the morphology and optical properties of the as-synthesized ZnO nanorods. Proton induced x-ray emission was used to probe the impurities in the ZnO nanorods. A large quantity of high purity ZnO nanorod structures were obtained with lengths of 0.5-1 microm. The diameters of the as-synthesized ZnO nanorods were found to be between 40 and 400 nm. The nanorods interlace with each other, forming 3D networks which make them suitable for sensing application. The addition of a polymeric film-forming agent (BASF LUVISKOL VA 64) improved the conductivity, as it facilitates the construction of conducting networks. Ultrasonication helped to separate the ZnO nanorods and disperse them evenly through the polymeric agent. Improved photoconductivity was measured for a ZnO nanorod sensor annealed in air at 200 degrees C for 30 min. The ZnO nanorod sensors showed a UV-sensitive photoconduction, where the photocurrent increased by nearly four orders of magnitude from 2.7 x 10(-10) to 1.0 x 10(-6) A at 18 V under 340 nm UV illumination. High humidity sensitivity and good stability were also measured. The resistance of the ZnO nanorod sensor decreased almost linearly with increasing relative humidity (RH). The resistance of the ZnO nanorods changed by approximately five orders of magnitude from 4.35 x 10(11) Omega in dry air (7% RH) to about 4.95 x 10(6) Omega in 95% RH air. It is experimentally demonstrated that ZnO nanorods obtained by the arc discharge method show excellent performance and promise for applications in both UV and humidity sensors.