ZnO NWs Research Articles

Gas Sensing Study of ZnO Nanowire Heterostructured with NiO for Detection of Pollutant Gases

Abstract The use of metal oxide single crystalline nanowires for chemical sensing of pollutant gases has been proposed as a reliable solution to ensure high sensor performance due to small size of the crystallites and good stability over long term operation at high temperature. To obtain enhanced sensing capabilities of ZnO nws, we prepared composite nanostructures based on p-type NiO and n-type ZnO NWs that combine the different physical and chemical properties of individual components. The superior functional performances are demonstrated in the current work by testing functional properties of chemical sensors towards reducing and oxidizing gases. The ZnO/NiO heterostuctures showed better relative response towards acetone and ethanol, with a decrease in response and recovery times with respect to uncoated ZnO nws. This is especially evident at 400 °C and for a determined thickness of the NiO layer.

Enhanced photovoltaic performance utilizing effective charge transfers and light scattering effects by the combination of mesoporous, hollow 3D-ZnO along with 1D-ZnO in CdS quantum dot sensitized solar cells

A combination of 3-dimensional (3D) hollow mesoporous ZnO microspheres (ZnO HMSP) and vertically grown one-dimensional ZnO nanowires (1D ZnO NWs) on a fluorine doped tin oxide (FTO) coated glass substrate has been investigated as a photoanode for a CdS quantum dot-sensitized solar cell (QSSC). A comparative study of the photovoltaic performance of the solar cell with devices fabricated with pristine ZnO HMSPs and ZnO NWs was carried out. The proposed photovoltaic device exhibits an enhancement in power conversion efficiency (PCE) upto ∼74% and ∼35%, as compared to the 1D ZnO NW and ZnO HMSP based solar cells. The maximum incident photon-to-current conversion efficiency (IPCE) for the solar cell was observed to be ∼40%, whereas for the devices fabricated with bare ZnO HMSP and ZnO NW the IPCE were only ∼32% and ∼19%, respectively. The enhanced photovoltaic performance of the solar cell is attributed to the high Brunauer-Emmett-Teller (BET) surface area, efficient light-scattering effects and facilitated diffusion of the electrolyte for better functioning of the redox couple (S(2-)/Sn(2-)) in the hybrid photoanode. Moreover, a faster electron transport through 1D ZnO NWs provides better charge collection from the photoactive layer, which leads to an increase in the short circuit current density of the device. The present study highlights the design and development of a new hybrid photoanode for solar harvesting.

Effects of Annealing on Structural and Optical Properties of ZnO Nanowires

ABSTRACTWe report, for the first time, effects of annealing of ZnO NWs grown on p-Si substrates. ZnO NWs are grown using metalorganic chemical vapor deposition (MOCVD) and thermal annealing was performed in situ under nitrogen ambient at different stages of the growth process. Increasing the annealing temperature of the ZnO seed epi-layer from 635 °C to 800 °C does not affect the morphology of the grown NWs. In contrast, annealing the NWs themselves at 800 °C results in a 48% decrease of the surface area to volume ratio of the grown NWs. The optical quality can be improved by annealing the seed layer at a higher temperature of 800 °C, although annealing the NWs themselves does not affect the defect density.

A Flexible Sandwich Nanogenerator for Harvesting Piezoelectric Potential from Single Crystalline Zinc Oxide Nanowires

High-quality single crystalline zinc oxide nanowires were grown on silver and gold coated plastic substrates for the fabrication of a sandwich-like nanogenerator using the aqueous chemical growth method. The applicability of this configuration as a nanogenerator is demonstrated by studying the harvested electrical output under mechanical deformation. Three different configurations were fabricated and utilized for harvesting piezoelectric potential by applying an external force. The maximum resulting output open circuit voltage and short circuit current were 2.4 V and 152.2 μA, respectively. The comparison between the different configurations indicates that more piezoelectric potential can be harvested by using two arrays of ZnO NWs placed face-to-face than by using a single nanowire configuration. In addition, the use of a piezoelectric polymer will enhance the amount of generated piezoelectric potential. The obtained results from different configurations of zinc oxide nanowire nanogenerators offer a cost-effective, flexible, long term stable nanogenerator for promising application. The principle of the sandwich nanogenerator demonstrated a new idea for novel self-powering nanotechnology that harvests electricity from the environment for applications such as portable electronics.

Characteristics of light-induced electron transport from P3HT to ZnO-nanowire field-effect transistors

We fabricated ZnO-nanowire (NW) field-effect transistors (FETs) coated with poly(3-hexylthiophene) (P3HT) and characterized the electron-transfer characteristics from the P3HT to the ZnO NWs. Under irradiation by laser light with a wavelength of 532 nm, photo-induced electrons were created in the P3HT and then transported to the ZnO NWs, constituting a source-drain current in the initially enhancement-mode P3HT-coated ZnO-NW FETs. As the intensity of the light increased, the current increased, and its threshold voltage shifted to the negative gate-bias direction. We estimated the photo-induced electron density and the electron-transfer characteristics, which will be helpful for understanding organic-inorganic hybrid optoelectronic devices.

Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity

ZnO nanowires have relatively high sensitivity as ultraviolet (UV) photodetectors, while the bandgap of 3.37 eV is an important limitation for their applications in solar-blind UV (SBUV), visible (VIS) and near infrared (NIR) range. Besides UV response, in this study, we demonstrate the promising applications of individual undoped ZnO NWs as high performance SBUV-VIS-NIR broad-spectral-response photodetectors, strongly depended on applied bias voltage and illumination intensity. The dominant mechanism is attributed to the existence of surface states in nanostructured ZnO. At a negative bias voltage electrons can be injected into surface states from electrode, and moreover, under light illumination photogenerated electron-hole pairs can be separated efficiently by surface built-in electric field, resulting into a decrease of potential barrier height and depletion region width, and simultaneously accompanying a filling of oxygen vacancy and a rise of ZnO Fermi level.

ZnO/CuO Heterojunction Branched Nanowires for Photoelectrochemical Hydrogen Generation

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

Uniform Arrays of ZnO 1D Nanostructures Grown on Al:ZnO Seeds Layers by Hydrothermal Method

In obtaining uniform array of ZnO 1D nanostructures, especially using solution based methods, the thickness and the morphology of the epitaxial seeds layer are very important. The paper presents the effect of the thickness and the morphology of the Al:ZnO seeds layer on the morphology and properties of ZnO nanowires array grown by hydrothermal method. Compact and vertically aligned ZnO 1D nanostructures were obtained. Concentration of 0.02 M of zinc nitrate was found to be optimal for growing nanowires with diameters up to 50 nm and lengths between 1.5 and 2.5 microns. Using 0.04 M solution, nanorods with diameter between 50 and 100 nm were obtained. The correlation between the crystal structure and optical properties of ZnO nanowires is discussed. From electrical measurements on single nanowire, resistivity value of 9 x 10(-2) omega cm was obtained. The I-V curves of single ZnO NWs show quasi diode characteristic when an e-beam is irradiating the NWs, and a typical semiconductive behaviour when the e-beam is turned off.

3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting

We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size) and their photoelectrochemical (PEC) performances (efficiency and stability) under neutral conditions. Specifically, we show that for b-NW electrodes with lightly doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give a higher photocathodic current, while for b-NWs with heavily doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide a higher photoanodic current. Interestingly, the photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production.

Room temperature H2S sensor based on Au modified ZnO nanowires

A room temperature H2S sensor based on pure and Au modified ZnO nanowires has been demonstrated. Modification of ZnO nanowires with Au resulted in a remarkable 16-fold increase in the sensor response over pure ZnO NWs toward 5ppm H2S at room temperature. A sensing mechanism based on the formation of nano-Schottky type barrier junction at the interface between Au and ZnO has been proposed. The enhanced response is attributed to the alteration of barrier properties by the adsorption or desorption of adsorbed species and/or H2S gas molecules. Observance of higher resistivity and a higher work function (0.2eV) for Au modified samples further corroborates the finding. Low temperature resistivity measurements indicate that the charge transport is governed by the thermal conduction and the nearest neighbor hopping mechanism.

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C2H5OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e−) which was higher than the CO gas (2e−). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.

Preparation and Characteristic of Relative-Humidity Sensors Based on Laterally Grown ZnO Nanowires

The use of laterally oriented zinc oxide nanowires (ZnO NWs) grown by a hydrothermal growth (HTG) method for relative-humidity (RH) sensing devices at room temperature (RT) is demonstrated. Sensing response under various RH conditions (12–96%) at RT is presented and discussed. A humidity sensor based on laterally oriented ZnO NWs with a sensing response (R12%/R96%) as high as 2.2 was obtained at RT. The RT-operable humidity-sensing characteristics with comparably good sensitivity of the proposed humidity sensors is attributed to the full utilization of the entire NW surface, because the current path is aligned with the orientation of the bridged lateral ZnO NWs during the humidity sensing application, thus making possible.

Electrochemical growth of ZnO nanowires on atomic layer deposition coated polystyrene sphere templates

We report on the electrochemical growth of n-type ZnO NWs on thin films of atomic layer deposited ZnO on polystyrene spheres. The development of crystal agglomerates during ALD is shown to control the diameter of nanowires obtained by electrochemical deposition. Increasing the thickness of the ALD coating from 20nm to 100nm increases the crystallite size from about 10nm to 22nm on the polystyrene spheres as well as the maximum agglomerate size from 80nm to 190nm of the coatings. Subsequent electrochemical deposition with identical parameters showed that the resulting ZnO nanowire diameter is almost 1:1 related to the agglomerate dimensions of the ALD coating. This effect offers another degree of freedom in synthesizing transparent, high surface, tailored urchin-inspired building blocks having applications in solar cells and low wetting coatings.

Optoelectronic properties of novel amorphous CuAlO2/ZnO NWs based heterojunction

Amorphous p-type CuAlO2 thin films were grown onto n-type crystalline ZnO NWs forming a heterojunction through the combination of sol–gel process and hydrothermal growth method. The effects of temperature on structure and optoelectronic properties of CuAlO2 thin films were investigated through various measurement techniques. It was found that the derived CuAlO2 is Al-rich with thin film. UV–Vis measurements showed that the deposited CuAlO2 films are semi-transparent with maximum transmittance ∼82% at 500nm. Electrical characterization and integration into pn junction confirms that the amorphous CuAlO2 is p-type and exhibited photovoltaic behavior.

Piezotronic Effect in Solution-Grown p-Type ZnO Nanowires and Films

Investigating the piezotronic effect in p-type piezoelectric semiconductor is critical for developing a complete piezotronic theory and designing/fabricating novel piezotronic applications with more complex functionality. Using a low temperature solution method, we were able to produce ultralong (up to 60 μm in length) Sb doped p-type ZnO nanowires on both rigid and flexible substrates. For the p-type nanowire field effect transistor, the on/off ratio, threshold voltage, mobility, and carrier concentration of 0.2% Sb-doped sample are found to be 10(5), 2.1 V, 0.82 cm(2)·V(-1)·s(-1), and 2.6 × 10(17) cm(-3), respectively, and the corresponding values for 1% Sb doped samples are 10(4), 2.0 V, 1.24 cm(2)·V(-1)·s(-1), and 3.8 × 10(17) cm(-3). We further investigated the universality of piezotronic effect in the as-synthesized Sb-doped p-type ZnO NWs and reported for the first time strain-gated piezotronic transistors as well as piezopotential-driven mechanical energy harvesting based on solution-grown p-type ZnO NWs. The results presented here broaden the scope of piezotronics and extend the framework for its potential applications in electronics, optoelectronics, smart MEMS/NEMS, and human-machine interfacing.

Effect of CVD Synthesis Parameters on the Growth of Catalyst-Free ZnO NRs

The main development of ZnO nanorods (NRs) is focused on the gold catalyst and heteroepitaxial approach.However, the presence of Au may generate undesired deep level traps in the ZnO bandgap, which could be very harmful to the performance of transistors. The objective of this study is to synthesize ZnO NRs via homoepitaxial growth without using foreign catalyst by Chemical Vapour Deposition (CVD) technique. The growth of catalyst-free ZnO NWs at different CVD synthesis parameters such as amount of Zn powder, substrate location and synthesis duration on the catalyst-free ZnO NRs were studied systematically. The effect of these parameters on the size and areal density of ZnO NRs provided a better understanding on the growth mechanism of NRs via the Vapour-Solid (VS) mechanism.

Surface free-carrier screening effect on the output of a ZnO nanowire nanogenerator and its potential as a self-powered active gas sensor

The output of a piezoelectric nanogenerator (NG) fabricated using ZnO nanowire arrays is largely influenced by the density of the surface charge carriers at the nanowire surfaces. Adsorption of gas molecules could modify the surface carrier density through a screening effect, thus, the output of the NG is sensitive to the gas concentration. Based on such a mechanism, we first studied the responses of an unpackaged NG to oxygen, H2S and water vapor, and demonstrated its sensitivity to H2S to a level as low as 100 ppm. Therefore, the piezoelectric signal generated by a ZnO NWs NG can act not only as a power source, but also as a response signal to the gas, demonstrating a possible approach as a self-powered active gas sensor.

Novel all solution processed heterojunction using p-type cupric oxide and n-type zinc oxide nanowires for solar cell applications

Nanostructured three-dimensional heterojunctional photovoltaic devices were synthesized through combination of hydrothermal synthesis of zinc oxide nanowires (ZnO NWs) and sol–gel derived cupric oxide. The ZnO NWs were prepared by the hydrothermal method at 90 °C, whereas the CuO thin films were synthesized by the sol–gel method. The derived CuO thin films were thoroughly characterized through scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, UV–vis spectroscopy and electrical measurement. It was found that power conversion efficiency ~0.3% can be obtained through the proposed combination.

A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene

A novel sensing hybrid-material of Au nanoparticles (Au NPs)-functionalized ZnO nanowires (Au-ZnO NWs) was successfully synthesized by a two-stage solution process. First, ZnO NWs were fabricated via a low-temperature one-pot hydrothermal method with SDSN introduced as a structure-directing agent. Afterward, the as-prepared ZnO NWs were used as supports to load Au NPs with small sizes via precipitating HAuCl4 aqueous solution with ammonia. The obtained samples were characterized by means of XRD, SEM, TEM and EDX. Both pristine and Au-ZnO NWs were practically applied as gas sensors to compare the effect of Au NPs on the sensing performances and the obtained results demonstrated that after functionalization by catalytic Au NPs, the hybrid sensor exhibited not only faster response and recovery speeds but also a higher response to benzene and toluene than the pristine ZnO sensor at 340 °C, especially showing high selectivity and long-term stability for low concentration toluene, which is rarely reported with this method, indicating its original sensor application in detecting benzene and toluene. To interpret the enhanced gas sensing mechanism, the strong spillover effect of the Au NPs and the increased Schottky barriers caused by the electronic interaction between Au NPs and ZnO NW support are believed to contribute to the improved sensor performance.

Hierarchically assembled ZnO nanoparticles on high diffusion coefficient ZnO nanowire arrays for high efficiency dye-sensitized solar cells

In this study, ZnO nanoparticles (ZnO NPs) were conformally covered on the surfaces of ZnO nanowires (ZnO NWs) with high diffusion coefficient (1.2 × 10(-2) cm(2) s(-1)) to make a composite photoanode. By using N719 to sensitize the composite photoanode, the conversion efficiency can reach 7.14%.