One‐Pot Hydrothermal Synthesis of Heterostructured ZnO/ZnS Nanorod Arrays with High Ethanol‐Sensing Properties

ZnO/ZnS heterostructured nanorod arrays with uniform diameter and length were synthesized from zinc substrates in a one‐pot procedure by using a simple hydrothermal method. Structural characterization by HRTEM indicated that the heterostructured nanorods were composed of parallel segments of wurtzite‐type ZnO and zinc‐blende ZnS, with a distinct interface along the axial direction, which revealed the epitaxial relationship, ZnO (10$\bar 1$ 0) and ZnS ($\bar 1$ 1$\bar 1$ ). The as‐prepared ZnO/ZnS nanorods showed only two green emissions at around 523 nm and 576 nm. We also found that the nanorods exhibited high sensitivity to ethanol at relatively low temperatures, owing to their smaller size and structure.     

CuO/ZnO Nanocomposite Gas Sensors Developed by a Plasma‐Assisted Route

CuO/ZnO nanocomposites were synthesized on Al₂O₃ substrates by a hybrid plasma‐assisted approach, combining the initial growth of ZnO columnar arrays by plasma‐enhanced chemical vapor deposition (PE‐CVD) and subsequent radio frequency (RF) sputtering of copper, followed by final annealing in air. Chemical, morphological, and structural analyses revealed the formation of high‐purity nanosystems, characterized by a controllable dispersion of CuO particles into ZnO matrices. The high surface‐to‐volume ratio of the obtained materials, along with intimate CuO/ZnO intermixing, resulted in the efficient detection of various oxidizing and reducing gases (such as O₃, CH₃CH₂OH, and H₂). The obtained data are critically discussed and interrelated with the chemical and physical properties of the nanocomposites.     

CdS‐Encapsulated TiO2 Nanotube Arrays Lidded with ZnO Nanorod Layers and Their Photoelectrocatalytic Applications

A novel TiO₂ nanotube array/CdS nanoparticle/ZnO nanorod (TiO₂ NT/CdS/ZnO NR) photocatalyst was constructed which exhibited a wide‐absorption (200–535 nm) response in the UV/Vis region and was applied for the photoelectrocatalytic (PEC) degradation of dye wastewater. This was achieved by chemically assembling CdS into the TiO₂ NTs and then constructing a ZnO NR layer on the TiO₂ NT/CdS surface. Scanning electron microscopy (SEM) results showed that a new structure had been obtained. The TiO₂ NTs looked like many “empty bottles” and the ZnO NR layer served as a big lid. Meanwhile the CdS NPs were encapsulated between them with good protection. After being sensitized by the CdS NPs, the absorption‐band edge of the obtained photocatalyst was obviously red‐shifted to the visible region, and the band gap was reduced from its original 3.20 eV to 2.32 eV. Photoelectric‐property tests indicated that the TiO₂ NT/CdS/ZnO NR material maintained a very high PEC activity in both the ultraviolet (UV) and the visible region. The maximum photoelectric conversion efficiencies of TiO₂ NT/CdS/ZnO NR were 31.8 and 5.98 % under UV light (365 nm) and visible light (420–800 nm), respectively. In the PEC oxidation, TiO₂ NT/CdS/ZnO NR exhibited a higher removal ability for methyl orange (MO) and a high stability. The kinetic constants were 1.77×10^?4 s^?1 under UV light, which was almost 5.9 and 2.6 times of those on pure TiO₂ NTs and TiO₂ NT/ZnO NR, and 2.5×10^?4 s^?1 under visible light, 2.4 times those on TiO₂ NT/CdS.     

Incorporating Zn2SnO4 Quantum Dots and Aggregates for Enhanced Performance in Dye‐Sensitized ZnO Solar Cells

The performance of dye‐sensitized ZnO solar cells was improved by a facile surface‐treatment approach through chemical‐bath deposition. After the surface treatment, the quantum dots of Zn₂SnO₄ were deposited onto ZnO nanoparticles accompanied by the aggregations of Zn₂SnO₄ nanoparticles. The ZnO film displayed a better resistance to acidic dye solution on account of the deposited Zn₂SnO₄ nanoparticles. Meanwhile, the open‐circuit photovoltage was greatly enhanced, which can be ascribed to the increased conduction‐band edge of ZnO and inhibited interfacial charge recombination. Although the deposition of Zn₂SnO₄ decreased the adsorption amounts of N719 dye, the aggregates of Zn₂SnO₄ with a size of 350–450 nm acted as the effective light‐scattering layer, thereby resulting in an improved short‐circuit photocurrent. By co‐sensitizing 10 μm‐thick ZnO film with N719 and D131 dyes, a top efficiency of 4.38 % was achieved under the illumination of one sun (AM 1.5, 100 mW cm^?2).     

Large‐Area Plasmonic Substrate of Silver‐Coated Iron Oxide Nanorod Arrays for Plasmon‐Enhanced Spectroscopy

One‐dimensional iron oxide materials fabricated on conducting glass substrates and their unique properties make these nanostructures promising candidates for a wide range of applications. Herein, vertically oriented α‐Fe₂O₃ nanorod arrays synthesized under hydrothermal conditions over a large area are described, as an active platform for surface‐enhanced resonance Raman scattering (SERRS) and surface‐enhanced fluorescence (SEF). From scanning electron microscopy images the formation of a homogeneous distribution of vertically oriented rods in a large area is confirmed. For activating the localized surface plasmon resonances, which are responsible for SERRS and SEF, a 6 nm layer of Ag is deposited onto the α‐Fe₂O₃ nanorod arrays by physical vapor deposition to form Ag islands.     

Effects of temperature on the properties of TiO2 photocatalysts prepared by the chemical vapor deposition (CVD) method

In this study, a TiO₂ film was prepared in an annular reactor by the chemical vapor deposition (CVD) method. Results indicated that anatase crystals were formed, except for at a deposition temperature of 200°C without calcination. At a calcination temperature of 850°C, anatase crystal was the major species formed with a small amount of rutile crystals. After conducting a photocatalytic reaction of toluene, the best activity was found with a preparation temperature of 350°C for the deposition, and 550°C for calcination.     

ZnGa2O4 Nanorod Arrays Decorated with Ag Nanoparticles as Surface‐Enhanced Raman‐Scattering Substrates for Melamine Detection

The availability of sensitive, reproducible, and stable substrates is critically important for surface‐enhanced Raman spectroscopy (SERS)‐based applications, but it presently remains a challenge. In this work, well‐aligned zinc gallate (ZnGa₂O₄) nanorod arrays grown on a Si substrate by chemical vapor deposition were used as templates to fabricate SERS substrates by deposition of Ag nanoparticles onto the ZnGa₂O₄ nanorod surfaces. The coverage of the Ag nanoparticles on the ZnGa₂O₄ nanorod surfaces was easily controlled by varying the amount of AgNO₃. SERS measurements showed that the number density of Ag nanoparticles on the ZnGa₂O₄ nanorod surfaces had a great effect on SERS activity. The SERS signals collected by point‐to‐point and SERS mapping images showed that as‐prepared SERS substrates exhibited good spatial uniformity and reproducibility. Detection of melamine molecules at low concentrations (1.0×10^?7 M ) was used as an example to show the possible application of such a substrate. In addition, the effect of benzoic acid on the detection of melamine was also investigated. It was found that the SERS signal intensity of melamine decreased greatly as the concentration of benzoic acid was increased.     

Band‐Structure Engineering of ZnO by Anion–Cation Co‐Doping for Enhanced Photo‐Electrochemical Activity

To look for efficient visible light‐driven catalysts for photo‐electrochemical (PEC) water‐splitting, the band structure and optical absorption of monodoped, compensated, and noncompensated n–p pairs of co‐doped bulk ZnO are systemically studied by using both general gradient approximation and hybrid density functional theory approaches (PBE and HSE). Calculations show that n–p co‐doping cannot only enhance the stability that stems from the strong electrostatic attraction between the n‐ and p‐type dopants, but also effectively reduce the band‐gap of ZnO. More importantly, compensated (Ti+C) and noncompensated (Sc+C) and (Cr+C) co‐doped ZnO may be compelling candidates for PEC water‐splitting because of their narrowed band‐gaps, potentially reduced electron–hole recombination centers, appropriate band‐edge positions, enhanced optical absorption, and good stability.     

A Study on the Influences of Processing Parameters on the Growth of Oxide Nanorod Arrays by Sol Electrophoretic Deposition

Template-based sol electrophoretic deposition has been demonstrated as an attractive method for the synthesis of oxide nanorod arrays, including simple and complex oxides in the forms of amorphous, polycrystalline, and single crystal. This paper systematically studied a number of processing parameters to control nanorod growth by sol electrophoretic deposition. The influences of particle and template zeta potentials, condensation rate, deposition rate (or externally applied electric field), the presence of organic additives, and sol concentration on the growth of nanorod arrays were studied. It was found that higher zeta potential or electric field resulted in higher growth rates but less dense packing. Templates with charge opposite to that of the sol particles prevented formation of dense nanorods, sometimes resulting in nanotubes, depending on the field strength during electrophoresis. In addition, the pH of the sol and chelating additives were also varied and likely affected the deposition process by affecting the condensation reactions.     

Surface‐Enhanced Raman Scattering Based on Controllable‐Layer Graphene Shells Directly Synthesized on Cu Nanoparticles for Molecular Detection

Graphene shells with a controllable number of layers were directly synthesized on Cu nanoparticles (CuNPs) by chemical vapor deposition (CVD) to fabricate a graphene‐encapsulated CuNPs (G/CuNPs) hybrid system for surface‐enhanced Raman scattering (SERS). The enhanced Raman spectra of adenosine and rhodamine 6G (R6G) showed that the G/CuNPs hybrid system can strongly suppress background fluorescence and increase signal‐to‐noise ratio. In four different types of SERS systems, the G/CuNPs hybrid system exhibits more efficient SERS than a transferred graphene/CuNPs hybrid system and pure CuNPs and graphene substrates. The minimum detectable concentrations of adenosine and R6G by the G/CuNPs hybrid system can be as low as 10^?8 and 10^?10 M , respectively. The excellent linear relationship between Raman intensity and analyte concentration can be used for molecular detection. The graphene shell can also effectively prevent surface oxidation of Cu nanoparticles after exposure to ambient air and thus endow the hybrid system with a long lifetime. This work provides a basis for the fabrication of novel SERS substrates.     

Luminescence properties of Cu‐ion‐implanted and annealed ZnO thin films deposited by chemical vapor deposition and pulsed laser deposition

Ion implantation techniques were used to study the effect of an MgO additive on the luminescence properties induced by Cu in ZnO thin films. Cu ions (accelerating voltage of 75 keV, dose of 4.5 × 10¹⁴ ions/cm²) were implanted at room temperature in nondoped and Mg‐doped ZnO thin films. After annealing, emissions in the visible region originating from Cu phosphor were observed at 510 nm in CVD‐ZnO and at 450 nm in Mg‐doped ZnO (MZO) thin films. The Cu depth profile shows distortion in the low‐concentration region of CVD‐ZnO. After the annealing, the Cu implant was homogenized in thin films, and then the Cu concentration was determined to be 1.5 × 10¹⁹ ions/cm³ in CVD‐ZnO and 5.6 × 10¹⁸ ions/cm³ in MZO thin films. Copyright © 2005 John Wiley & Sons, Ltd.     

The Role of Oxygen Dissociation in Plasma Enhanced Chemical Vapor Deposition of Zinc Oxide from Oxygen and Diethyl Zinc

The influence of electron impact dissociation of oxygen on neutral chemistry was studied for plasma-enhanced chemical vapor deposition (PECVD) of zinc oxide using oxygen and diethyl zinc. Electron conditions in the reactor were estimated based on simulations of well-known Ar-O₂ plasmas, while the majority of the thermal chemistry was abstracted from the combustion literature. A rudimentary model of film growth was developed using the rate of oxygen dissociation as the lone adjustable parameter.n Model results were compared directly with experimental measurements of deposition rates and neutral species densities for a wide range of conditions. Good quantitative agreement between experiments and model were observed as a function of composition and rf power. The system is highly sensitive to the electron impact dissociation of oxygen, which creates the radical pool that drives the majority of the chemistry. The approach detailed here provides a framework for the development of models of oxide PECVD derived from other metalorganic precursors.     

tert‐Butyl(tert‐butoxy)zinc Hydroxides: Hybrid Models for Single‐Source Precursors of ZnO Nanocrystals

Alkylzinc alkoxides, [RZnOR′]₄, have received much attention as efficient precursors of ZnO nanocrystals (NCs), and their “Zn₄O₄” heterocubane core has been regarded as a “preorganized ZnO”. A comprehensive investigation of the synthesis and characterization of a new family of tert‐butyl(tert‐butoxy)zinc hydroxides, [(tBu)₄Zn₄(μ₃‐OtBu)_x(μ₃‐OH)_4?x], as model single‐source precursors of ZnO NCs is reported. The direct reaction between well‐defined [tBuZnOH]₆ ( 1₆ ) and [tBuZnOtBu]₄ ( 2₄ ) in various molar ratios allows the isolation of new mixed cubane aggregates as crystalline solids in a high yield: [(tBu)₄Zn₄(μ₃‐OtBu)₃(μ₃‐OH)] ( 3 ), [(tBu)₄Zn₄(μ₃‐OtBu)₂(μ₃‐OH)₂] ( 4 ), [(tBu)₄Zn₄(μ₃‐OtBu)(μ₃‐OH)₃] ( 5 ). The resulting products were characterized in solution by ¹H NMR and IR spectroscopy, and in the solid state by single‐crystal X‐ray diffraction. The thermal transformations of 2 – 5 were monitored by in situ variable‐temperature powder X‐ray diffraction and thermogravimetric measurements. The investigation showed that the Zn?OH groups appeared to be a desirable feature for the solid‐state synthesis of ZnO NCs that significantly decreased the decomposition temperature of crystalline precursors 3 – 5 .     

Hierarchical Structures of Carbon Nanotubes and Arrays of Chromium‐Capped Silicon Nanopillars: Formation and Electrical Properties

The formation of stochastically oriented carbon‐nanotube networks on top of an array of free‐standing chromium‐capped silicon nanopillars is reported. The combination of nanosphere lithography and chemical vapor deposition enables the construction of nanostructures that exhibit a hierarchical sequence of structural sizes. Metallic chromium serves as an etching mask for Si‐pillar formation and as a nucleation site for the formation of carbon nanotubes through the chemical vapor deposition of ethene, ethanol, and methane, respectively, thereby bridging individual pillars from top to top. Iron and cobalt were applied onto the chromium caps as catalysts for CNT growth and the influence of different carbon sources and different gas‐flow rates were investigated. The carbon nanotubes were structurally characterized and their DC electrical properties were studied by in situ local‐ and ex situ macroscopic measurements, both of which reveal their semiconductor properties. This process demonstrates how carbon nanotubes can be integrated into Si‐based semiconductors and, thus, this process may be used to form high‐surface‐area sensors or new porous catalyst supports with enhanced gas‐permeation properties.     

Hierarchically Self‐Assembled Star‐Shaped ZnO Microparticles for Electrochemical Sensing of Amines

Novel, hierarchically nanostructured, star‐shaped ZnO (SSZ) microparticles are synthesized by a hydrothermal synthetic route. The SSZ microparticles serve as effective platforms for electrochemical detection of amines in solution. The morphology and structure of the materials are characterized by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and UV/Vis spectroscopy. The as‐synthesized SSZ microparticles comprise self‐assembled hexagonal prisms that possess nanometer and micrometer pores in their structure and on their surfaces—structural features that are conducive to sensing applications. An electrode fabricated by using the hierarchically nanostructured SSZ materials serve as a sensitive electrochemical sensor for detection of low concentrations of ethylenediamine, with a sensitivity of 2.98×10^?2 mA cm^?2 mm ^?1, a detection limit of 2.36×10^?2 mm , and a short response time of 8 s.     

Nanostructural properties of zinc oxide thin films grown on non‐planar substrates

The crystallographic structure of zinc oxide thin films grown on optical fibres using single source chemical vapour deposition (SSCVD) was analysed using near edge X‐ray absorption fine structure (NEXAFS). Zinc diethyl carbamate was used as a precursor for the growth of highly conformal films in a one‐step deposition process without substrate rotation and at substrate temperatures of 400–575 °C. It was found that the growth temperatures greatly affected the crystallographic structure of the film with no preferred crystallographic orientation and negligible crystallinity at low temperatures and very high crystallinity with pure c‐axis orientation at high temperatures. Cross‐sectional analysis of the films by scanning electron microscopy (SEM) showed the presence of a film at all points around the fibre. These films generally consisted of densely packed columns that bore a strong resemblance to c‐axis‐oriented films grown on planar substrates. Copyright © 2005 John Wiley & Sons, Ltd.