ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

ZnO纳米棒水热法生长与表征

采用两步法在FTO导电玻璃衬底上制备ZnO纳米棒,首先利用浸渍-提拉法在FTO导电玻璃衬底上制备ZnO晶种层,然后把有ZnO晶种层的FTO衬底放入盛有生长溶液的反应釜中利用水热法制备ZnO纳米棒.研究了生长溶液的浓度、生长温度和生长时间对所制备的对ZnO纳米棒阵列的微结构和光致发光性能的影响,利用X射线衍射(XRD)、扫描电子显微镜(SEM)和光致发光谱(PL)研究了ZnO样品的结构、形貌和光学性质.实验结果表明:所制备的ZnO纳米棒呈现六方纤锌矿结构,沿(002)晶面择优取向生长,纳米棒的平均直径约为100 nm,长度约为2.5 μm.所制备的ZnO纳米棒在390 nm附近具有很强的紫外发光峰和在550 nm附近有较弱的宽绿光发光峰.     

Growth temperature dependent properties of ZnO nanorod arrays on glass substrate prepared by wet chemical method

In this work, ZnO nanorod arrays were grown on glass substrate by the wet chemical method, and the effect of synthesis temperature on the properties was investigated. The grown nanorods were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Raman and Photoluminescence (PL) measurements. XRD pattern showed that nanorod prepared at 80 °C and 90 °C has high crystallinity with wurtzite structure and orientated along the c-axis. However, nanorods were not formed at 60 °C and 70 °C due to less energy supply for the growth of the ZnO. FE-SEM results showed that the morphology and the size of ZnO can be effectively controlled. In particular, as the temperature increased, diameter of the nanorod was increased while length decreased. Raman scattering spectra of ZnO nanorod arrays revealed the characteristic E₂^high mode that is related to the vibration of oxygen atoms in the wurtzite ZnO. Room-temperature PL spectra of the ZnO nanorods revealed a near-band-edge (NBE) emission peak. The NBE (UV light emission) band at ~383 nm might be attributed to the recombination of free exciton. The narrow full-width at half-maximum (FWHM) of the UV emission indicated that ZnO nanorods had high crystallinity.     

ZnO nanorods surface-decorated by WO3 nanoparticles for photocatalytic degradation of endocrine disruptors under a compact fluorescent lamp

Nanoscaled tungsten oxide (WO₃) particles coated on ZnO nanorods (ZNRs) were newly synthesized by combining a hydrothermal technique with a chemical solution process. The structure, morphologies and compositions of the as-prepared WO₃–ZNR nanocomposites were characterized through XRD, FESEM, TEM and Raman measurements. The results revealed that pure monoclinic WO₃ nanoparticles with an average size range of 18–26 nm were distributed on the surfaces of ZNRs and attached strongly. Particularly, the optical properties as well as photocatalytic characteristics of pure ZNRs and WO₃–ZNR nanocomposites with different loadings of WO₃ were also examined. The absorption of WO₃–ZNR nanocomposites was redshifted due to effective immobilization of WO₃ on ZNRs. Under irradiation of a 55 W compact fluorescence lamp, the photocatalytic activities of the WO₃–ZNR nanocomposites were superior to those of pure ZNRs and P25 in the degradation of resorcinol (ReOH). Furthermore, WO₃–ZNR nanocomposites showed very favorable recycle use potential and high sedimentation rate. Other endocrine disrupting chemicals (EDCs) such as phenol, bisphenol A (BPA) and methylparaben were also successfully photodegraded under identical conditions. These characteristics showed the practical applications of the WO₃–ZNR nanocomposites in indoor environmental remediation.     

Hydrothermally grown ZnO nanorods arrays for selective NO2 gas sensing: Effect of anion generating agents

Vertically aligned ZnO nanorods (ZNRs) arrays with various aspect ratios were deposited by using a simple and inexpensive hydrothermal route at relatively low temperature of 90 °C. The influence of hydroxide anion generating agents in the solution on the growth of ZNRs arrays was studied. Hexamethylenetetramine (HMTA) and ammonia were used as hydroxide anion generating agents while polyethyleneimine (PEI) as structure directing agent. The combined effect of these three agents plays a crucial role in the growth of ZNRs arrays with respect to their rod length and diameter, which controls the aspect ratio. The deposited ZNRs exhibited hexagonal wurtize crystal structure with preferred orientation along (002) plane. The highly crystalline nature and pure phase formation of ZNRs was confirmed from FT-Raman studies. The maximum gas response (R_g/R_a) of 67.5 was observed for high aspect ratio ZNRs, deposited with combination of HMTA, ammonia as well as PEI. The enhancement in gas response can be attributed to high surface area (45 cm²/g) and desirable surface accessibility in high aspect ratio ZNRs. Fast response–recovery characteristics, especially a much quicker gas response time of 32 s and recovery time of 530 s were observed at 100 ppm NO₂ gas concentration.     

Polystyrene sphere monolayer assisted electrochemical deposition of ZnO nanorods with controlable surface density

In this paper we report the zinc oxide nanorods (ZnO NRs) growth by electrochemical deposition onto polycrystalline gold electrodes modified with assemblies of polystyrene sphere monolayers (PSSMs). Growth occurs through the interstitial spaces between the hexagonally close packed spheres. ZnO NRs nucleate in the region where three adjacent spheres leave a space, being able to grow and projected over the PSSMs. The nanorod surface density (N_NR) shows a linear dependence with respect to a PS sphere diameter selected. XRD analysis shows these ZnO NRs are highly oriented along the (0 0 2) plane (c-axis). This open the possibility to have electronic devices with mechanically supported nanometric materials.     

Electrochemical deposition of branched hierarchical ZnO nanowire arrays and its photoelectrochemical properties

Branched hierarchical ZnO nanowire arrays are synthesized on fluorine-doped tin oxide (FTO) substrate via a two-step electrochemical deposition process, which involves the electrodeposition of ZnO nanowire arrays on conductive glass substrate, followed by the electrochemical growth of ZnO nanorod branches on the backbones of the primary ZnO nanowires. The formation mechanism of the branched hierarchical nanostructure is discussed. It is demonstrated that coating the primary nanowire arrays with ZnO nanoparticles seed layer plays a key role in synthesising the branched hierarchical ZnO nanostructure. By adjusting the concentration of Zn(CH₃COO)₂ colloid in coating process and the reaction time of the second-step deposition, the density and the length of the secondary nanorod branches in the hierarchical nanostructures can be both varied. Moreover, the photoelectrochemical properties of the dye-sensitized solar cell (DSSC) based on branched hierarchical ZnO nanowire arrays are investigated. Due to the enlargement of the internal surface area within the branched nanostructure photoelectrode, the DSSC consisting of branched hierarchical ZnO nanowire arrays yields a power conversion efficiency of 0.88%, which is almost twice higher than that of the DSSC fabricated using bare ZnO nanowire arrays.     

Effect of Metal Doped Zinc Oxide Nanorods on Photoelectrical Characteristics of ZnO/Polyaniline Heterojunction

The n-type vertically aligned metal doped ZnO nanorods (NRs) and p-type proton acid doped polyaniline (PANI) inorganic/organic heterojunction diodes have been fabricated. Aluminium (Al) and iron (Fe) doped ZnO NRs were grown on seed ZnO layer on fluorine doped tin oxide coated glass substrates by high temperature chemical bath deposition method. The elemental analysis using EDAX confirm doping of Al and Fe in ZnO. The morphology of doped ZnO nanorods and ZnO/PANI heterojunction exhibit well defined uniform nanorod arrays and interface between nanorods and polyaniline matrix respectively. The dark current–voltage curves confirmed the rectifying diode like behaviors of the heterojunctions, whereas under illumination, the junction revealed good sensitivity to UV and visible range with increased current densities. The highest ideality factor and lowest barrier height was found for FeZnO/PANI heterojunction under dark and under light compared to that of ZnO/PANI, AlZnO/PANI. This research is innovative with respect to low cost synthesis of efficient and sensitive hybrid p–n junction diodes and possibly serves as the building blocks for future optoelectronic applications.     

The preparation,characterization and photocatalytic activity of radical-shaped CeO2/ZnO microstructures

This paper uses a wet-chemical precipitation route to prepare radical-shaped ZnO microprisms and to deposit Cerium oxide (CeO₂) on the surface of ZnO, to form CeO₂/ZnO microstructures. The samples are characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy. Their catalytic activity is also evaluated using methylene blue (MB) as a detection reagent. CeO₂/ZnO systems exhibit higher UV absorption and transparency in the visible region. The experimental results show that the deposition of CeO₂ nanospecies is successful and that the radical-shaped microstructures of ZnO are well maintained. The CeO₂/ZnO microstructures exhibit a much greater intensity of UV-light absorptivity and much higher photocatalytic activity than those of radical-shaped ZnO microprisms.     

Growth of vertically aligned ZnO nanorods using textured ZnO films

A hydrothermal method to grow vertical-aligned ZnO nanorod arrays on ZnO films obtained by atomic layer deposition (ALD) is presented. The growth of ZnO nanorods is studied as function of the crystallographic orientation of the ZnO films deposited on silicon (100) substrates. Different thicknesses of ZnO films around 40 to 180 nm were obtained and characterized before carrying out the growth process by hydrothermal methods. A textured ZnO layer with preferential direction in the normal c-axes is formed on substrates by the decomposition of diethylzinc to provide nucleation sites for vertical nanorod growth. Crystallographic orientation of the ZnO nanorods and ZnO-ALD films was determined by X-ray diffraction analysis. Composition, morphologies, length, size, and diameter of the nanorods were studied using a scanning electron microscope and energy dispersed x-ray spectroscopy analyses. In this work, it is demonstrated that crystallinity of the ZnO-ALD films plays an important role in the vertical-aligned ZnO nanorod growth. The nanorod arrays synthesized in solution had a diameter, length, density, and orientation desirable for a potential application as photosensitive materials in the manufacture of semiconductor-polymer solar cells.

PACS

61.46.Hk, Nanocrystals; 61.46.Km, Structure of nanowires and nanorods; 81.07.Gf, Nanowires; 81.15.Gh, Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)     

Growth of three-dimensional ZnO nanorods by electrochemical method for quantum dots-sensitized solar cells

There-dimensional (3D) superstructure was expected to fabricate high performance photoelectrodes of quantum dot-sensitized solar cells (QDSCs). In this paper, the ZnO 3D superstructure with multi-layer structure (3D ZnO nanorods) was grown on ITO glass by a novel electrochemical method at low temperature (60–90 °C). The 3D ZnO nanorods were composed of close-packed ZnO nanorod bundles with wide dimension distribution ranging from hundreds of nanometers to several micrometers. The effects of some important parameters, such as concentration of Zn(NO₃)₂, deposition temperature and concentration of ZnO nanoparticles (served as growth seeds for ZnO nanorods), on the morphology of 3D ZnO nanorods were also studied by scanning electron microscopy. Once being applied in QDSCs, the 3D ZnO nanorods showed more superior photoelectrochemical performance to ZnO nanorod array. The conversion efficiency of 1.42% achieved by the QDSC based on 3D ZnO nanorods was a very promising value for the QDSCs based on ZnO electrodes.     

Room-temperature nonequilibrium growth of controllable ZnO nanorod arrays

In this study, controllable ZnO nanorod arrays were successfully synthesized on Si substrate at room temperature (approx. 25°C). The formation of controllable ZnO nanorod arrays has been investigated using growth media with different concentrations and molar ratios of Zn(NO₃)₂ to NaOH. Under such a nonequilibrium growth condition, the density and dimension of ZnO nanorod arrays were successfully adjusted through controlling the supersaturation degree, i.e., volume of growth medium. It was found that the wettability and electrowetting behaviors of ZnO nanorod arrays could be tuned through variations of nanorods density and length. Moreover, its field emission property was also optimized by changing the nanorods density and dimension.     

Effect of ZnO seed layers on the solution chemical growth of ZnO nanorod arrays

High density ZnO nanorod arrays were grown on Si substrates coated with ZnO seed layers via aqueous solution route. The ZnO seed layers were deposited on the substrate using DC reactive sputtering and RF magnetron sputtering. It was found that ZnO seed layer with (1 0 3) preferred orientation, prepared using DC reactive sputtering, did not facilitate the formation of ZnO nanorods in the solution grown process. Prior seeding of the surface by ZnO layer with (0 0 2) preferred orientation, deposited using RF magnetron sputtering, leads to nucleation sites on which ZnO nanorod arrays can grow in a highly aligned fashion. ZnO nanorods with well-defined hexagonal facets (0 0 2) were grown almost vertically over the entire substrate. The uniformity and alignment of the nanorod arrays are strongly related to the properties of underneath ZnO seed layers.     

Fabrication of TiO2 nanotubes by using electrodeposited ZnO nanorod template and their application to hybrid solar cells

Vertically aligned TiO₂ nanotubes have been fabricated on the indium-doped tin oxide (ITO) by a simple and versatile technique using the electrochemically deposited ZnO nanorods, oriented along the c-axis, as a template in the spin-on based sol-gel reaction of a Ti precursor. The diameter, length, and shape of TiO₂ nanotubes were controlled by changing the initial ZnO nanorod template and the spin conditions during sol-gel process of a Ti precursor. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were used to confirm the successful formation of TiO₂ nanotubes and characterize their structure and morphology. Furthermore, as an application of the TiO₂ nanotubes, hybrid solar cells based on TiO₂ and poly[2-methoxy,5-(2′-ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) were successfully fabricated.     

Selective Growth of Vertical-aligned ZnO Nanorod Arrays on Si Substrate by Catalyst-free Thermal Evaporation

By thermal evaporation of pure ZnO powders, high-density vertical-aligned ZnO nanorod arrays with diameter ranged in 80–250 nm were successfully synthesized on Si substrates covered with ZnO seed layers. It was revealed that the morphology, orientation, crystal, and optical quality of the ZnO nanorod arrays highly depend on the crystal quality of ZnO seed layers, which was confirmed by the characterizations of field-emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and photoluminescence measurements. For ZnO seed layer with wurtzite structure, the ZnO nanorods grew exactly normal to the substrate with perfect wurtzite structure, strong near-band-edge emission, and neglectable deep-level emission. The nanorods synthesized on the polycrystalline ZnO seed layer presented random orientation, wide diameter, and weak deep-level emission. This article provides a C-free and Au-free method for large-scale synthesis of vertical-aligned ZnO nanorod arrays by controlling the crystal quality of the seed layer.     

Highly sensitive C2H2 gas sensor based on Ag modified ZnO nanorods

The sliver (Ag) modified zinc oxide (ZnO) nanorods were successfully obtained with a simplified and environmentally friendly solvothermal method. Materials characterization indicated that the metallic Ag was located on the outside of ZnO nanorods after annealing. In comparison with ZnO nanorods, Ag modified ZnO (Ag–ZnO) nanorods exhibited a considerably enhanced response to C₂H₂. The response of the 3 at% Ag–ZnO based sensor operating at 175 °C is 539 (R_a/R_g), which is the highest value among all the sensors in detecting 100 ppm C₂H₂. The Ag–ZnO based sensors exhibited fast response speed, lower operation temperature and higher selectivity.     

Fabrication and characterization of silicon wire solar cells having ZnO nanorod antireflection coating on Al-doped ZnO seed layer

In this study, we have fabricated and characterized the silicon [Si] wire solar cells with conformal ZnO nanorod antireflection coating [ARC] grown on a Al-doped ZnO [AZO] seed layer. Vertically aligned Si wire arrays were fabricated by electrochemical etching and, the p-n junction was prepared by spin-on dopant diffusion method. Hydrothermal growth of the ZnO nanorods was followed by AZO film deposition on high aspect ratio Si microwire arrays by atomic layer deposition [ALD]. The introduction of an ALD-deposited AZO film on Si wire arrays not only helps to create the ZnO nanorod arrays, but also has a strong impact on the reduction of surface recombination. The reflectance spectra show that ZnO nanorods were used as an efficient ARC to enhance light absorption by multiple scattering. Also, from the current-voltage results, we found that the combination of the AZO film and ZnO nanorods on Si wire solar cells leads to an increased power conversion efficiency by more than 27% compared to the cells without it.     

水热合成法制备ZnO纳米棒有序阵列薄膜

采用水热合成法在预先生长的ZnO种子层的玻璃衬底上制备出ZnO纳米棒有序阵列薄膜。通过X射线衍射、扫描电镜、透射电镜和选区电子衍射分析表明：所制备的薄膜由垂直于ZnO种子层的纳米棒组成,呈单晶六角纤锌矿ZnO结构,且沿[001]方向择优生长,纳米棒的平均直径和长度分别为10．0nm和3．3μm。     

Growth of ZnO Nanorods by the Chemical Solution Method with Assisted Electrical Field

Zinc oxide (ZnO) nanorod arrays on the ZnO-coated seed substrates were prepared by the solution chemical method from Zn(NO₃)₂/NaOH under an assisted electrical field. The influence of the electrical field on ZnO nanorod growth was primarily explored, and the positive effects of the electrical field were demonstrated by adding polyethylene glycol in growth solution. It has been proved that the electrical field enhances ion adsorption to the substrate and lowers the nucleation energy barrier by increasing charge intensity; meanwhile, it produces H⁺ through oxidation of OH⁻ and increases properly the degree of solution supersaturation near the substrate surface. XRD results show that the nanorods grown under the electrical field primarily have a zincite structure. With increasing precursor concentration, the average diameter and length of ZnO nanorods increase. The maximum rod growth rate at a given concentration of Zn²⁺ ion occurs at a specific temperature.     

Optical and electrical properties of p-type Ag-doped ZnO nanostructures

Zn_1−xAg_xO nanoparticles (NPs) (x=0, 0.02, 0.04, and 0.06) were synthesized by a sol–gel method. The synthesized undoped ZnO and Zn_1−xAg_xO-NPs were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and UV–visible spectroscopy. The XRD patterns indicated that undoped and Ag-doped ZnO crystallize in a hexagonal wurtzite structure. The TEM images showed ZnO NPs with nearly spherical shapes, with particle size distributed over the nanometer range. Evidence of dopant incorporation is demonstrated in the XPS measurements of the Ag-doped ZnO NPs. The Raman measurements indicated that the undoped and Ag-doped ZnO-NPs had a high crystalline quality. From the result of UV–vis, the band-gap values of prepared undoped and Ag-doped ZnO were found to decrease with an increase in Ag concentration. The obtained undoped and Ag-doped ZnO nanoparticles were used as a source material to grow undoped and Ag-doped ZnO nanowires on n-type Si substrates, using a thermal evaporation set-up. Two probe method results indicated that the Ag-doped ZnO nanowires exhibit p-type properties.