Surface Of Zinc Oxide Nanowires Research Articles

Selective sensing performance of UV-activated ZnO nanowires decorated with Ir and Rh nanoparticles

Single-crystal zinc oxide (ZnO) nanowires (NWs) were synthesized through self-assembly hydrothermal reaction and decorated with oxidative and reductive catalytic active sites to tailor their gas sensing selectivity. The sensing performances of the developed materials were investigated under ultraviolet-light emitting diode (UV-LED) irradiation against low concentrations of nitrogen dioxide (NO2) and ammonia (NH3) gases, as oxidizing and reducing gas representatives, respectively. The growth of NWs towards the hexagonal c-axis was demonstrated, where presence of Rh3d and Ir4f active sites on the surface of ZnO NWs was observed. The sensing performance of the surface modified ZnO NWs was enhanced significantly, compared with that of the pristine ZnO NWs. The Rh-decorated ZnO NWs sensor exhibited superior performance in the detection of NO2, while the Ir sites promoted the oxidative reaction in the presence of NH3, attributed to the Fermi level of the metal-decorated films under UV irradiation.

Superhydrophobic and Antibacterial Hierarchical Surface Fabricated by Femtosecond Laser

Superhydrophobic surfaces are important in many applications owing to their special properties such as self-cleaning, anti-icing, antibacterial, and anti-fogging. In this paper, a micro/nano hierarchical superhydrophobic surface with a low roll-off angle was created on 304 stainless steel. The water contact angle was measured to be 152° with a roll-off angle of 7.3°. Firstly, microscale bumps were created by femtosecond laser irradiation. Secondly, zinc oxide (ZnO) nanowires were fabricated on the laser-induced bumps using a hydrothermal synthesis method. Results show that after laser treatment and ZnO nanostructuring, the stainless steel surface became superhydrophobic. However, the roll-off angle of this hierarchical structure surface was larger than 90°. To reduce the surface activity, trimethoxy silane hydrophobic coating was applied. A 7.3° roll-off angle was achieved on the coated surface. The underlying mechanism was discussed. The hydrophobic ZnO structured surface can help prevent bacterial contamination from water, which is important for implants. Thus, for biomedical applications, the antibacterial property of this hierarchical surface was examined. It was found that the antibacterial property of sample surfaces with ZnO nanowires were significantly increased. The optical density (OD) of Escherichia coli (E. coli) attached to the original surface was 0.93. For the micro-structured surface (with bumps), the OD was 0.9, and for the hierarchical surface (with bump & nanowires), it was 0.54. For nanostructured ZnO nanowire surface, the OD was only 0.09. It demonstrates good antibacterial properties of ZnO nanowires.

Light-activation of gas sensitive layers based on zinc oxide nanowires

In this work, a sensor layer based on zinc oxide nanowires was synthesized by low-temperature hydrothermal method. The gas sensitivity of sample to isopropyl alcohol vapor was studied both during heating and under ultraviolet radiation. It was shown that depending on the presence of moisture on the surface of zinc oxide nanowires, isopropyl alcohol vapor can act as a reducing or oxidizing gas upon activation of gas sensitivity by ultraviolet irradiation. It was shown that there is an optimal intensity of irradiation, which determines the maximum response of sample. The gas sensitivity under moist air conditions demonstrates the possibility of practical application of adsorption resistive gas sensors with light-activation at room temperature.

Study of surface chemical composition of oxide nanostructures by X-ray photoelectron spectroscopy

Zinc oxide and zinc stannate nanowires were synthesized by hydrothermal method. Zinc oxide nanowires were doped by iodine. The formation of multicomponent and doped oxides was studied by X-ray photoelectron spectroscopy. It was found that formation of zinc stannate from zinc oxide nanowires occurs in 1 hour. The percentage of OH-groups increases on the surface of zinc oxide nanowires as a result of doping by iodine.

Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties

Synthesis of zinc oxide (ZnO) nanowires (NWs) grown via vapor-liquid-solid (VLS) process using Gold (Au) as a catalyst metal on aluminum-doped zinc oxide (AZO) seed layer is reported in the present work. During the growth procedure, the nucleation process helps us to obtain ZnO nanowires with Au on the tip, confirming the VLS growth mechanism. Different morphologies were obtained after the variation in the growth parameters in the VLS process, and further, their role in the photocatalytic performance was studied. Changes in the structural properties of nanowires allowed us to modify the aspect ratio and surface area of the nanostructures. X-ray diffraction (XRD) showed that the principal orientation of the nanowires was (002) in the present case. Scanning electron microscopy (SEM) showed the structural properties of 1-D nanostructures (nanowires), and statistical analysis revealed that the average diameter in the present case was found to be varied from 57 to 85 nm. Scanning transmission electron microscopy (STEM) technique revealed the different elements present on the surface of ZnO NWs. Further, the compositional profile of nanostructures was cross-verified using Energy dispersive Spectroscopy (EDS). Photoluminescence (PL) and UV Visible studies were employed to study the optical properties of nanowires. UV–Vis measurements showed the role of different structural properties of nanowires on the absorption spectra, especially in the visible region. The ZnO nanowires were tested as photocatalysts for hydrogen production from water splitting reaction, and it was found in particular nanowires with random orientation with optimal diameter distribution show the stable and highest photocatalytic performance.

Molecular Copper(I)-Copper(II) Photosensitizer-Catalyst Photoelectrode for Water Oxidation.

Copper(II)-based electrocatalysts for water oxidation in aqueous solution have been studied previously, but photodriving these systems still remains a challenge. In this work, a bis(diimine)copper(I)-based donor-chromophore-acceptor system is synthesized and applied as the light-harvesting component of a photoanode. This molecular assembly was integrated onto a zinc oxide nanowire surface, and upon photoexcitation, chronoamperometric studies reveal that the integrated triad can inject electrons directly into the conduction band of zinc oxide, generating oxidizing equivalents that are then transferred to a copper(II) water oxidation catalyst in aqueous solution, yielding O2 from water with a Faradaic efficiency of 76%.

Tuning Areal Density and Surface Passivation of ZnO Nanowire Array Enable Efficient PbS QDs Solar Cells with Enhanced Current Density

AbstractColloidal PbS quantum dots (QDs) have provoked a revolution in the field of optoelectronic devices owing to their low‐cost fabrication processing and excellent physical properties. Recently, the fabrication of nanostructured PbS QD photovoltaic (PV) devices based on zinc oxide (ZnO) nanowire array appears as an effective strategy for improving the overall device performance. Despite its potentially strong impact on the device performance, the role of nanowire areal density on photon absorption and exciton dynamics has not yet been studied and still remains unexplored. Here, for the first time, the areal density of ZnO nanowires is tuned through controlling the precursor concentration and its impact on PbS QD PV performance is studied. It is found that the device with optimized ZnO nanowire areal density yields significantly increased power conversion efficiency (PCE) (10.1% vs 8.5% of control nanowire‐based device) due to improved antireflection effect and reduced surface recombination states. To further improve the photovoltaic performance, the ZnO nanowire surface is treated with hydrogen plasma. Transient photovoltage (TPV) measurement reveals that this passivation process noticeably reduces the nonradiative charge recombination yielding a champion device with a remarkable PCE of 10.8%.

Low-Temperature Solution Synthesis of Au-Modified ZnO Nanowires for Highly Efficient Hydrogen Nanosensors.

In this research, the low-temperature single-step electrochemical deposition of arrayed ZnO nanowires (NWs) decorated by Au nanoparticles (NPs) with diameters ranging between 10 and 100 nm is successfully demonstrated for the first time. The AuNPs and ZnO NWs were grown simultaneously in the same growth solution in consideration of the HAuCl4 concentration. Optical, structural, and chemical characterizations were analyzed in detail, proving high crystallinity of the NWs as well as the distribution of Au NPs on the surface of zinc oxide NWs demonstrated by transmission electron microscopy. Individual Au NPs-functionalized ZnO NWs (Au-NP/ZnO-NWs) were incorporated into sensor nanodevices using an focused ion bean/scanning electron microscopy (FIB/SEM) scientific instrument. The gas-sensing investigations demonstrated excellent selectivity to hydrogen gas at room temperature (RT) with a gas response,Igas/Iair, as high as 7.5-100 ppm for Au-NP/ZnO-NWs, possessing a AuNP surface coverage of ∼6.4%. The concentration of HAuCl4 in the electrochemical solution was observed to have no significant impact on the gas-sensing parameters in our experiments. This highlights the significant influence of the total Au/ZnO interfacial area establishing Schottky contacts for the achievement of high performances. The most significant performance of H2 response was observed for gas concentrations higher than 500 ppm of H2 in the environment, which was attributed to the surface metallization of ZnO NWs during exposure to hydrogen. For this case, an ultrahigh response of about 32.9 and 47 to 1000 and 5000 ppm of H2 was obtained, respectively. Spin-polarized periodic density functional theory calculations were realized on Au/ZnO bulk and surface-functionalized models, validating the experimental hypothesis. The combination of H2 gas detection at RT, ultralow power consumption, and reduced dimensions makes these micro-nanodevices excellent candidates for hydrogen gas leakage detection, including hydrogen gas monitoring (less than 1 ppm).

Gas Sensor by Direct Growth and Functionalization of Metal Oxide/Metal Sulfide Core-Shell Nanowires on Flexible Substrates.

We have developed a novel fabrication method for flexible gas sensors for toxic gases based on sequential wet chemical reaction. In specific, zinc oxide (ZnO) nanowires were locally synthesized and directly integrated on a flexible polymer substrate using localized hydrothermal synthesis methods and their surfaces were selectively functionalized with palladium (Pd) nanoparticles using a liquid phase deposition process. Because the entire process is conducted at a low temperature in a mild precursor solution, it can be applied for flexible substrates. Furthermore, the surface of ZnO nanowires was sulfurized by hydrogen sulfide (H2S) gas to form zinc oxide/zinc sulfide (ZnO/ZnS) core-shell nanowires for stable sensing of H2S gas. The locally synthesized ZnO/ZnS core-shell nanowires enable an ultracompact-sized device, and Pd nanoparticles improve the sensing performance and reduce the operating temperature (200 °C). The device shows a high sensitivity [(Ggas - Gair)/Gair × 100% = 4491% to 10 ppm], fast response (response/recovery time <100 s) to hydrogen sulfide, and outstanding selectivity (>100 times) to other toxic gases (e.g., carbon monoxide, acetone, ethanol, and toluene). Moreover, vertically synthesized nanowires provide a long bending path, which reduces the mechanical stresses on the structure. The devices showed stable gas sensing performance under 9 mm positive radius of curvature and 5 mm negative radius of curvature. The mechanical robustness of the device was also verified by numerical simulations which showed dramatic decrease of maximum stress and strain to 4.2 and 5.0%, respectively.

Humidity-dependent piezopotential properties of zinc oxide nanowires: Insights from atomic-scale modelling

The humidity effect is inevitable in the fabrication and operation of zinc oxide (ZnO) nanowires (NWs). In this paper, the piezopotential properties of ZnO NWs under the humidity condition are compressively investigated based on atomic-scale simulations. Through molecular dynamics simulations, we find a mixed molecular and dissociative adsorption of water on the surface of ZnO NWs under the humidity condition. Absorptions of molecular and dissociative water both structurally reconstruct the surface atoms of ZnO NWs and thus modify their surface properties, which finally make the material properties of ZnO NWs under the humidity condition significantly different from those of their counterparts under the vacuum condition. Applying the obtained material properties to the numerical and analytical calculations of the piezopotential of ZnO NWs, we find a significant reduction in the piezopotential induced by the humidity effect. Similar to the results observed in the previous experiments, the reduction in the piezopotential is found to become more significant as the humidity level increases, since in this process more surface atoms of ZnO NWs will be adsorbed by molecular or dissociated water. Moreover, our density functional theory calculations indicate that the humidity effect can also influence the semiconducting properties of ZnO NWs by downward shifting their conduction band edges. However, the change in semiconducting properties induced by the humidity effect is found to have trivial effect on the piezopotential of ZnO NWs.

Immobilization of horseradish peroxidase on ZnO nanowires/macroporous SiO2 composites for the complete decolorization of anthraquinone dyes.

A zinc oxide (ZnO) nanowires/macroporous silicon dioxide composite was used as support to immobilize horseradish peroxidase (HRP) simply by in situ cross-linking method. As cross-linker was adsorbed on the surface of ZnO nanowires, the cross-linked HRP was quite different from the traditional cross-linking enzyme aggregates on both structure and catalytic performance. Among three epoxy compounds, diethylene glycol diglycidyl ether (DDE) was the best cross-linker, with which the loading amount of HRP with pI of 5.3 reached as high as 118.1 mg/g and specific activity was up to 14.9 U/mg-support. The mass loss of HRP cross-linked with DDE was negligible during 50-H leaching at 4 °C, and the thermal stability of the immobilized HRP was also quite good. The catalytic performance of immobilized HRP to decolorize anthraquinone dye was explored by using Reactive Blue 19 (RB 19) and Acid Violet 109 (AV 109) as model substrates. The results indicated that the immobilized HRP exhibited high decolorization efficiency and good reusability. The decolorization efficiency reached 94.3% and 95.9% for AV 109 and RB 19 within the first 30 Min, respectively. A complete decolorization of these two dyes has been realized within 2-3 H by using this new biocatalysis system.

E-beam-induced in situ structural transformation in one-dimensional nanomaterials

Electron beam (e-beam) irradiation is an inevitable, but crucial issue for electron microscopy. Our investigation results show the e-beam-induced in situ structural transformations in silicon (Si) nanowires and zinc oxide (ZnO) nanowires (NWs), respectively. Crystal to amorphous structure transition was revealed in Si NWs utilizing high resolution electron microscopy and electron energy loss spectroscopy. Reconstruction at the (10ī0) surface of ZnO NWs was also observed in the transmission electron microscope (TEM) using aberration-corrected electron microscopy. These e-beam-induced in situ structural transformations prove that the electron beam irradiation effect is able to be used for the local modification of one-dimensional nanomaterials.

Controlling ZnO nanowire surface density during its growth by altering morphological properties of a ZnO buffer layer by UV laser irradiation

Zinc oxide (ZnO) nanocrystals, which are characterized by their configurations and fine structures, are unique oxide semiconductors. In this report, it is demonstrated that the number density of ZnO nanowires can be controlled by proper treatments of the buffer layer with ultraviolet laser irradiation. ZnO nanowires were synthesized on the locally laser-irradiated ZnO buffer layer using nanoparticle-assisted pulsed-laser deposition (NAPLD). The number density of ZnO nanowires decreased in the region laser-irradiated with <300 mJ/cm2, whereas it increased in the region laser-irradiated with more than 400 mJ/cm2. Effects of laser irradiation on ZnO buffer layers were investigated by atomic force microscopy, Kelvin probe force microscopy (KPFM), Raman spectroscopy, and X-ray diffraction analyses. In particular, the effects of laser irradiation on the surface work functions of ZnO buffer layers were investigated by KPFM, which is reported for the first time. Additionally, periodically aligned ZnO sub-microcrystals were fabricated as an application of controlling the number density of ZnO nanowires on micropatterned ZnO buffer layers using the four-beam interfered third harmonic of a Nd:YAG laser followed by NAPLD. ZnO sub-microcrystals can be used to fabricate field emitter arrays and can be developed for the application of ZnO nano/microcrystals due to their high throughput.

Field emission behavior of vertically aligned ZnO nanowire planar cathodes

A field emission (FE) study by scanning anode field emission microscopy was performed to evaluate the FE properties of vertically aligned zinc oxide (ZnO) nanowire arrays electrodeposited on a plane conductive surface. The specific FE behaviors of the cathode observed experimentally are (1) a turn-on macroscopic field of about 6 V/μm for a FE current density JFE = 5 × 10−4 A/cm2, (2) a stable FE characteristics for 5 × 10−4 &amp;lt; JFE &amp;lt; 5 × 10−2 A/cm2, and (3) a brutal shut down of FE when JFE crossed a limiting value of ∼0.05 A/cm2 due to a rapid evolution of the nanowires toward a bulbous tip geometry or a complete melting. A physical process of FE from ZnO nanostructures is proposed from the experimental data analyses. An effective surface barrier of about 1 eV was determined from the experimental Fowler–Nordheim plot and the presence of a Zn enriched surface was assumed in considering the possibility of important modifications of the crystallography and charge transfers at the surface of ZnO nanowires during the application of the strong electric field required for FE.

Nitric oxide gas sensing at room temperature by functionalized single zinc oxide nanowire

We report on room temperature detection of nitric oxide (NO) gas at low-ppm level by Cr nanoparticle decorated single zinc oxide nanowire (ZNW) sensor. In this sensor device Cr nanoparticles, act as catalyst to transform NO into NO2 and results in change in current value through ZNW, corresponding to NO gas concentration. The sensor exhibits average sensitivity of ∼46% for 10 ppm of NO gas. The lowest detection limit of the sensor is ∼1.5 ppm for NO gas. Sensor shows selectivity towards NO gas during a cross-sensitivity test with N2, CO and CO2 gases. Recovery of the sensor is achieved by ultraviolet light induced desorption of gases from ZNW surface.

Au Decorated Zinc Oxide Nanowires for CO Sensing

We present the room temperature detection of carbon monoxide using Au decorated zinc oxide (ZnO) nanowires. Zinc oxide nanowires were grown via the vapor liquid solid method whereas the gold nanoparticles were prepared by the solution growth method. The surface of the ZnO nanowires was decorated by Au nanoparticles. Gas sensing properties of such nanowires were studied at room temperature for various concentrations of CO (100 to 1000 ppm) in synthetic air. Enhancement in gas sensing response by Au decoration on ZnO nanowires was observed. Au nanoparticles act as a catalyst in chemical sensitization and improve the reaction and response time. The improvement in gas sensing behavior is attributed to the change in conductivity of the metal decorated ZnO nanowires on CO exposure due to the transfer of electrons resulting from gas oxidation at the ZnO nanowire surface.