White-light-controlled resistive switching and photovoltaic effects in TiO2/ZnO composite nanorods array at room temperature

White-light-controlled resistance switching and photovoltaic effects in TiO₂/ZnO composite nanorods array grown on fluorine-doped tin oxide (FTO) substrate by hydrothermal process were investigated. The average length of TiO₂/ZnO nanorods is about 3 μm, and the average diameter is about 200 nm. ZnO nanoparticles with size 5–10 nm are embedded in TiO₂ base material. The current–voltage characteristics of Ag/[TiO₂/ZnO]/FTO device demonstrate an outstanding rectifying property and bipolar resistive switching behavior. Specially, the resistive switching behavior can be regulated by white-light illuminating. In addition, this structure also exhibits a substantial white-light photovoltaic effect. This study is helpful for exploring the multifunctional materials and their applications in nonvolatile multistate memory devices and solar cells.     

Synthesis of 3D micro- and nano-hierarchical structure InVO4 porous microspheres with improved visible-light photocatalytic activities

Three-dimensional (3D) hierarchical structure InVO₄ porous microspheres were fabricated by a hydrothermal method with the assistance of cetyltrimethyl-ammonium bromide (CTAB). X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, adsorption analyzer, diffuse refection spectroscopy, UV–Vis spectroscopy, and photoluminescence spectroscopy were adopted to analyze the structure–property relationship of samples, and the formation mechanism was also discussed. The results show that the concentration of CTAB solution has important influence on the grain size, crystal structure, and morphology of as-synthesized InVO₄ samples, thus affecting the optical property and photocatalytic activity. When the concentration of CTAB solution is 1 wt%, we can obtain perfect 3D hierarchical porous microspheres. Such microspheres with diameter size about 2–5 μm are assembled by numerous nanocrystals, and exhibit an enhanced photocatalytic activity in the degradation of Rhodamine B under visible light irradiation. Moreover, Oswald ripening and self-assembly aggregation are presumed to play an important role in the formation of these 3D hierarchical structure InVO₄ porous microspheres.     

Hydrothermal synthesis,characterization, and luminescence of BaWO4 nanorods

Barium tungstate (BaWO₄) nanorods were synthesized by a hydrothermal process with the assistant of cetyltrimethyl ammonium bromide (CTAB). The samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence measurements. The XRD and FT-IR results show that BaWO₄ samples can be indexed as a pure tetragonal scheelite structure. The SEM results show that the morphologies are nanorods with a diameter about 45 nm and a length exceeding 1 μm. The CTAB and “oriented attachment” play key roles in the growth of BaWO₄ nanorods in the hydrothermal process. When excited at 265 nm, BaWO₄ nanorods show the intrinsic emission band centered at 467 nm. The calculated luminescence lifetime of WO₄ ^2? in BaWO₄ is 8.9 μs.     

Effect of Eu doping concentration on the morphologies and optical properties of ZnO film prepared by ultrasonic spray pyrolysis

In this paper, we report a new and simple method to prepare different concentrations in molarities Eu-doped ZnO films on the ITO glass substrates by ultrasonic spray pyrolysis. The morphologies, crystal structures and optical properties were investigated by using scanning electro microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The SEM images show that the morphologies of Eu doping concentrations 3 and 9 at.% of ZnO films are lamellae. When the Eu doping concentration in molarities is 6 at.%, the morphology of films are graininess and dense, particle diameter is about 200–250 nm. The XRD results indicate that when the Eu doping concentration is 6 at.%, the structure of Eu-doped ZnO films have better hexagonal polycrystalline structure, and characteristic diffraction peak of Eu₂O₃ was appeared at 2θ = 50.47°. The PL spectra of different concentrations Eu-doped ZnO films show that for the Eu doping concentration 6 at.%, ZnO film has a stronger red emission at 613 nm with excitation wavelength at 280 nm.     

水热法一步合成氧化锌纳米棒及其场发射特性的研究

利用极其简单的方法,即将氯化锌溶液和氨水按一定配比混合进行水浴加热制备了ZnO纳米棒,其直径约为100nm.利用扫描电子显微镜和能谱仪对其结构和成分进行了分析.场发射测试结果表明,所制备的ZnO纳米棒有较低的开启场和阈值场,分别为2.7V/μm和4.95V/μm,其场增强因子为5390.这样高的场增强因子来源于氧化锌参差的层状结构.     

Chemical vapor deposition-based growth of aligned ZnO nanowires on polycrystalline Zn<Subscript>2</Subscript>GeO<Subscript>4</Subscript>:Mn substrates

ZnO nanowires have been grown on polycrystalline Zn₂GeO₄:Mn substrates for the first time using a chemical vapor deposition method. Both Zn and ZnO sources were used to supply Zn vapor in the growth process of ZnO nanowires. The Zn₂GeO₄:Mn substrates were prepared using solid-state ceramic synthesis methods, and average grain sizes of ~1 μm were achieved. The nanowires of diameters in the range of 100–200 nm and length of ~30 μm were observed. In addition to nanowires, other morphologies of ZnO nanostructures, such as ZnO tetrapods, were observed when Zn powder was used as the source for the CVD growth. The results reveal that polycrystalline substrates are also promising as novel alternative substrates for growth of ZnO nanostructures. The as-synthesized ZnO nanowire/Zn₂GeO₄:Mn composites are being developed for future electroluminescent devices.     

Enhanced photocatalytic activity of BiFeO3 particles by surface decoration with Ag nanoparticles

BiFeO₃ particles with different morphologies and sizes were synthesized via a hydrothermal process, where the morphology and size was tailored by using different KOH concentrations in precursor solution. The samples prepared at n(KOH) = 3, 4.5, 6, and 7.5 M are composed, respectively, of octahedron-shaped particles (500–600 nm), cube-like particles (200–500 nm), irregular spherical agglomerates (9–16 μm) formed from disk-like grains with diameter of 1.4–2.8 μm and thickness of 0.2 μm, and cuboid-shaped particles with length-to-width ratio of 1.4:1–3.5:1 and width size ranging from 80 to 280 nm. Ag nanoparticles were deposited on the surface of BiFeO₃ particles by a chemical reduction method to produce Ag@BiFeO₃ nanocomposites. The photocatalytic activity of prepared samples was evaluated by degrading rhodamine B under simulated sunlight irradiation. It is demonstrated that Ag-decorated BiFeO₃ particles exhibit an enhanced photocatalytic activity compared to bare BiFeO₃ particles. This can be explained by the effective transfer of photogenerated electrons from the conduction band of BiFeO₃ to Ag nanoparticles and hence increased availability of holes for the photocatalytic reaction. Hydroxyl radicals were detected by the photoluminescence technique using terephthalic acid as a probe molecule and are found to be produced over the irradiated BiFeO₃ and Ag@BiFeO₃ photocatalysts; especially, an enhanced yield is observed for the latter.     

Facile hydrothermal synthesis and optical properties of flower-like CeO<Subscript>2</Subscript> composed of numerous porous nanorods

Flower-like CeO₂ hierarchical structures have been successfully synthesized by a facile template-free hydrothermal method using ethanol/water mixtures as solvent. X-ray diffraction shown that the synthesized flower-like CeO₂ nanostructures exhibited a fluorite cubic structure. It was found that the flower-like CeO₂ hierarchical structures with a diameter of 2–5 µm are composed of numerous porous nanorods as the petals with an average diameter of about 15 nm, which connect at one end and diverge at the other end to form an open hierarchical architecture. The synthesized CeO₂ samples show excellent room temperature optical properties, which is likely associated with Ce³⁺ ions and oxygen vacancies in the samples as supported by the XPS and Raman scattering data.     

Oriented growth of urchin-like zinc oxide micro/nano-scale structures in aqueous solution

The urchin-like ZnO microcrystals with high crystallinity decomposed from [Zn(OH)₄]^2? directly were obtained via a hydrothermal method. The morphology, particle size, crystalline structure and fluorescence of the as-prepared ZnO were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) analyses. The results demonstrated that the urchin-like ZnO crystals with wurtzite structure had a narrow distribution in size, which could be adjusted in the range of 30–80 μm by varying reaction time. Broad visible light emission peak was also observed in the PL spectra of the synthesized ZnO products. A multistep growth process about how to form such a structure was proposed.     

分级微纳结构氧化锌的制备及其光致发光

采用低温热蒸发法研制出新型的ZnO分级微纳结构,利用场发射扫描电子显微镜和X射线衍射仪对其形貌与结构进行了表征,结果表明,所制备的分级结构为纯六方纤锌矿结构,由主干直径为1-3μm的ZnO微米线和表面的宽度为1μm、厚度约为100nm的晶片组成。用气-固（VS）机制阐明了ZnO分级结构的生长机理。在室温下,用近场光学显微镜测量了ZnO分级结构的光致发光谱,结果显示,在380nm处存在很强的近带隙发光峰,而508nm左右的缺陷发光很弱。     

Hydrothermal growth of symmetrical ZnO nanorod arrays on nanosheets for gas sensing applications

The hierarchical ZnO nanostructures with 2-fold symmetrical nanorod arrays on zinc aluminum carbonate (ZnAl-CO₃) nanosheets have been successfully synthesized through a two-step hydrothermal process. The primary nanosheets, which serve as the lattice-matched substrate for the self-assembly nanorod arrays at the second-step of the hydrothermal route, have been synthesized by using a template of anodic aluminum oxide (AAO). The as-prepared samples were characterized by XRD, FESEM, TEM and SAED. The nanorods have a diameter of about 100 nm and a length of about 2 μm. A growth mechanism was proposed according to the experimental results. The gas sensor fabricated from ZnO nanorod arrays showed a high sensitivity to ethanol at 230°C. In addition, the response mechanism of the sensors has also been discussed according to the transient response of the gas sensors.     

Hierarchical CuCo<Subscript>2</Subscript>O<Subscript>4</Subscript>/C microspheres assembled with nanoparticle-stacked nanosheets for sensitive non-enzymatic glucose detection

The hierarchical CuCo₂O₄/C microspheres have been fabricated via a two-step method involving hydrothermal and calcination processes. SEM, TEM, HRTEM, XRD and XPS were used to characterize the morphology, structure, and composition of the materials. The CuCo₂O₄/C microspheres have a hierarchically flower-like structure composed of nanoparticles-stacked nanosheets. Moreover, the as-prepared double-metal oxide hierarchical microsphere composites exhibit greatly improved electrochemical performance than that of pure CuCo₂O₄, owing to the synergistic effect of CuCo₂O₄ and carbon spheres. The CuCo₂O₄/C 5:1 modified electrode exhibited high sensitivity of 707.71 μA mM^?1cm^?2 in a wide linear range from 5 to 8000 μM with detection limit of about 1.5 μM. The outstanding glucose sensing performance of CuCo₂O₄/C 5:1 demonstrated that this kind of spinel bimetallic oxides composites can be favorable candidates for the development of non-enzymatic sensor.     

Morphological syntheses of ZnO nanostructures under microwave irradiation

ZnO nanostructures with flower-, rod-, and flake-like morphologies have been controllably synthesized using Zn(acac)₂·H₂O (acac = acetylacetonate) as a single-source precursor through a facile and fast microwave-assisted method. The morphologies of ZnO nanostructures can be systematically adjusted by using various surfactants. The ZnO products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction. The results show that all ZnO nanostructures are of single-crystalline nature with hexagonal wurtzite structure. The possible formation mechanism for these ZnO nanostructures is proposed and their photoluminescence properties are also investigated.     

Effect of dimethyl borate composition on the performance of boron doped ZnO dye-sensitized solar cell (DSSC)

Effect of dimethyl borate (C₃H₉BO₃) composition on the structure, morphology, thickness, elemental composition, optical absorption, photoluminescence of ZnO nanotubes and the performance of the DSSC has been studied. It was found that the structure, diameter, thickness, elemental composition, optical absorption and morphology of ZnO nanostructure are significantly influenced by the composition of dimethyl borate. The diameter and thickness of ZnO nanotubes decrease with the increase in the composition of C₃H₉BO₃. The photoluminescence increases with the composition of C₃H₉BO₃. The DSSC utilizing ZnO nanotubes synthesized at 1 wt% C₃H₉BO₃ yields the J _SC and η of 1.9 mA cm^?2 and 0.222 %, respectively.     

Solvothermal synthesis of Ag/ZnO micro/nanostructures with different precursors for advanced photocatalytic applications

Micro/nanostructured systems based on metallic oxide (ZnO) with noble metal (Ag) on the surface (Ag/ZnO) are synthesized by solvothermal method from zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O), zinc acetylacetonate hydrate (Zn(C₅H₇O₂)₂·xH₂O) and silver nitrate (Ag(NO₃)) as precursors. In these systems, polyvinylpyrrolidone (PVP) is used as surfactant for controlling particle morphology, size and dispersion. The obtained materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV–vis diffuse reflectance spectroscopy (DRS), N₂ gas adsorption–desorption (BET) and Raman spectroscopy (RS). By XRD results, all major peaks are indexed to the hexagonal wurtzite-type structure of the ZnO and samples with noble metal, extra diffraction peaks are detected which correspond to the face-centered-cubic (fcc) structure of the metallic Ag. Depending on used precursor, different morphologies have been obtained. Mainly, ZnO prims-like rods – NRs (with 0.8 ? aspect ratio ? 3.4) – have been observed. Quasi-spherical particles of metallic Ag (with diameters between 558 ± 111 μm and 22 ± 1 nm) have been detected on the ZnO surface. Photocatalytic results (all samples studied >30% MB degradation) verify the important effect of surfactant and the viability of synthesized Ag/ZnO micro/nanocomposites for environmental applications.     

Synthesis and characterization of SiO2 nano- and microwires by a non-catalytic technique

In this paper, we report on the synthesis of pure silica (SiO₂) nano- and microwires by a non-catalytic process. The structure, morphology and properties of the products were examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The microstructural analysis shows that the SiO₂ wires are high purity silica and are completely amorphous; their diameter ranges from 70 nm to 2 μm. The fiber length varies from 100 μm to several mm. The wire manufacturing process involves reduced temperatures and times (1000 °C, 30 min) compared to other processes, without needing any catalyst and has been carried out on different kinds of substrates (SiC and Si). Successful growth of SiO₂ wires on the surface of silicon microcantilevers has been demonstrated; they could be used as an alternatively way to enhance the sensor performances.     

Synthesis of two-dimensional ZnO nanosheet-structures for the application in dye-sensitized solar cells

A large amount of nano-ZnO with a thin sheet-like morphology has been manufactured by a simple citric acid assisted hydrothermal route. The influences on the morphologies and structures of ZnO products for the introduction of citric acid in the reaction system have been investigated. A dye-sensitized solar cell assembled by the ZnO nanosheets as photoanode shows an efficiency of 1.82 % with a short-circuit current density of 7.48 mA cm^?2 which is over 1.5 times higher than the one without adding citric acid during the synthetic process. This result is mainly connected to the unique superiority of the two-dimensional sheet-like ZnO nanostructures for light scattering and dye loading.     

Preparation of nano-sized flower-like ZnO bunches by a direct precipitation method

In this work, nano-sized ZnO particles were prepared by a direct precipitation method with Zn(NO₃)₂·6H₂O and NH₃·H₂O as raw materials, and the impact of the synthesis process was studied. The optimal thermal calcined temperature of precursor precipitates of ZnO was obtained from the differential thermal analysis (DTA) and the thermal gravimetric analysis (TGA) curves. The purity, microstructure, morphology of the calcined ZnO powders were studied by X-ray diffraction (XRD), energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The synthesized ZnO powders had a wurtzite structure with high purity. The final products were of flower-like shape and the nanorods which consisted of the flower-like ZnO bunches were 20–100 nm in diameter and 0.5–1 μm in length. The effect of process conditions on the morphology of ZnO was discussed.     

Influence of NH4 + on the preparation of carbonaceous spheres by a hydrothermal process

The influence of NH₄ ⁺ on the preparation of carbonaceous spheres by a hydrothermal process was studied. A scanning electronic microscope and laser particle size analyzer were used to observe and measure the morphology and size distribution of the spheres. Fourier transform infrared spectroscopy was used to analyze the functional groups of the spheres. The results show that the addition of NH₄ ⁺ has a significant influence on the morphology, size, and yield of the spheres. The slight addition of NH₄ ⁺ greatly increases the diameter of the spheres and simultaneously improves their dispersion. With the change of NH₄ ⁺ concentrations, the as-received spheres have sizes from 0.2 to 20 μm in diameter. The Fourier transform infrared spectroscopy results indicate that the C = O group disappears in the samples with NH₄ ⁺ addition, different from the obvious peak at 1700.2 and 1305.6 cm^?1 in the samples with no NH₄ ⁺ addition. A possible mechanism of the formation and growth of the spheres with the addition of NH₄ ⁺ is also proposed in the paper.     

A simple one-step solvothermal synthesis of hierarchically structured ZnO hollow spheres for enhanced selective ethanol sensing properties

A simple one-step solvothermal method, using ethanolamine as solvent without any additives except zinc source, has been employed to synthesize hierarchically structured ZnO hollow spheres consisting of numerous orderly and radical nanorods with diameter of several tens nanometers and length of 2–3 μm. The ethanolamine and the solvothermal process play the critical role in the synthesis of the ZnO hollow spheres by the primary formation of ZnO crystal nucleus and subsequent transformation into nanorods, which self-assemble into hollow spheres. The morphology and structure of the spheres have been characterized by transmission electron microscopy, field emission scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and Brunauer–Emmett–Teller N₂ adsorption–desorption analyses. The results also indicate that the sensor based on the prepared ZnO hollow spheres exhibit good ethanol sensing performance, which can be attributed to its structural defects and high surface-to-volume ratio that significantly facilitate the absorption of oxygen species and diffusion of target gas. Besides, the sensor shows high selectivity to ethanol because ZnO as a basic oxide is favored for dehydrogenation of ethanol.