ZnO Nanowire Arrays Research Articles

Thermo-Convective Solution Growth of Vertically Aligned Zinc Oxide Nanowire Arrays for Piezoelectric Energy Harvesting

The search for a synthesis method to create longer ZnO NWAs with high-quality vertical alignment, and the investigation of their electrical properties, have become increasingly important. In this study, a hydrothermal method for growing vertically aligned arrays of ZnO nanowires (NWs) using localized heating was utilized. To produce longer NWs, the temperature environment of the growth system was optimized with a novel reaction container that provided improved thermal insulation. At a process temperature above ~90 °C, ZnO NWs reached a length of ~26.8 µm within 24 h, corresponding to a growth rate of 1.1 µm/h, nearly double the rate of 0.6 µm/h observed in traditional chemical bath growth using a glass reactor. The densely grown NWs (~1.9/µm2), with a diameter of ~0.65 µm, exhibited a preferred hexagonal c-axis orientation and were vertically aligned to the (100) silicon (Si) substrate. These NW structures have multiple applications, e.g., in piezotronic strain sensors, gas sensing, and piezoelectric energy harvesting. As proof of concept, a piezoelectric nanogenerator (PENG) was fabricated by embedding the NWs in an S1818 polymer matrix over a 15 mm × 15 mm area. Under repeated impulse-type compressive forces of 0.9 N, a maximum peak output voltage of ~95.9 mV was recorded, which is higher by a factor of four to five than the peak output voltage of 21.6 mV previously obtained with NWs measuring ~1.8 µm in length.

Improved UV Photoresponse Performance of ZnO Nanowire Array Photodetector via Effective Pt Nanoparticle Coupling.

Ultraviolet (UV) photodetectors (PDs) based on nanowire (NW) hold significant promise for applications in fire detection, optical communication, and environmental monitoring. As optoelectronic devices evolve towards lower dimensionality, multifunctionality, and integrability, multicolor PDs have become a research hotspot in optics and electronic information. This study investigates the enhancement of detection capability in a light-trapping ZnO NW array through modification with Pt nanoparticles (NPs) via magnetron sputtering and hydrothermal synthesis. The optimized PD exhibits superior performance, achieving a responsivity of 12.49 A/W, detectivity of 4.07 × 1012 Jones, and external quantum efficiency (EQE) of 4.19 × 103%, respectively. In addition, the Pt NPs/ZnO NW/ZnO PD maintains spectral selectivity in the UV region. These findings show the pivotal role of Pt NPs in enhancing photodetection performance through their strong light absorption and scattering properties. This improvement is associated with localized surface plasmon resonance induced by the Pt NPs, leading to enhanced incident light and interfacial charge separation for the specialized configurations of the nanodevice. Utilizing metal NPs for device modification represents a breakthrough that positively affects the preparation of high-performance ZnO-based UV PDs.

Hard Magnetic FePt Nanoparticles within Nanostructured Silicon to Improve the Maximum Energy Product

In this work nanostructured silicon, silicon nanotubes (SiNTs) and porous silicon (PSi), with embedded hard magnetic FePt nanoparticles (NPs) is used as platform to create nanomagnet-arrays. The magnetic response of FePt-loaded composite materials is investigated, which have potential in high-performance magnets and as rare earth magnet alternatives. PSi/FePt demonstrates superior hard magnetic behavior with a higher coercivity and remanence compared to SiNTs/FePt. Varying the Fe molar ratio in deposits results in a small coercivity range. FePt-loaded samples consistently show increased coercivity and remanence compared to Co-loaded samples, with PSi exhibiting a stronger effect compared to SiNTs.The fabrication of the PSi samples was performed by anodization of p+ silicon in a 5 wt% HF solution. A current density of 50 mAcm-2 was applied during the etching. The samples offer an average diameter of 50 nm. The SiNTs were produced by using arrays of ZnO nanowires (NWs) as template, followed by Si deposition and finally etching off the ZnO NWs which is described in (1). FePt nanocrystals were formed within the SiNTs by a multistep process, consisting of soaking previously APTES functionalized SiNTs in a solution containing 0.1 mM Citric acid, 0.5 mM of H2PtCl6 6H2O, and depending on the given ratio 0.5-1.5 mM of Fe(NO3)3 9H2O. The samples were dried in a vacuum oven, followed by a heat treatment at 500° C for 1 hr in Ar atmosphere, then the samples were washed several times with water and ethanol. In a similar way FePt NPs were formed in PSi samples, however PSi did not receive the APTES functionalization before the solution soaking.For these samples, the average FePt particle size is 5 (± 2) nm in SiNTs samples and 9 (± 4) nm for the particles inside PSi, however it should be pointed out that the FePt ratios calculated from reactants was different than the ratios determined from EDX experimental data. The calculated stoichiometry of Pt:Fe (based on reactant concentrations) of 1:1, 1:3 and 1:6 was experimentally determined values via EDX to be 10:1, 1.6:1 and 0.5:1 for SiNTs; for the corresponding PSi samples of calculated 1:3 and 1:6 ratios was experimentally 40:1 and 10:1.Magnetization measurements were carried out by VSM in recording the magnetic response dependent on the magnetic field. Samples with different Fe content (Pt:Fe = 1:1, 1:3, 1:6) offer a variation of the coercivity in the range of 5% for both types of matrices (SiNTs, PSi). Furthermore the investigations show that the coercivities of PSi loaded with FePt NPs is about twice the coecivities of SiNTs loaded with FePt NPs. SiNTs/FePt samples with a ratio Pt:Fe 1:6 offer a coercivity of about 240 Oe and PSi/FePt offers a coercivity more than twice as high of about 560 Oe.Comparing the FePt loaded samples with samples loaded with Co NPs, in both sample types an increase of the coercivity is observed for FePt. Also in the case of Co-loading the utilization of PSi as template offers higher coercivities compared to SiNTs as template. From the investigated composite systems the ones consisting of PSi and FePt offer the highest energy product.(1) Huang, X., Gonzalez-Rodriguez, R., Rich, R., Gryczynski, Z., Coffer, J.L., Chem. Comm. 2013, 49, 5760.

High Temperature and FORC Investigations of Semiconducting/Ferromagnetic Nanocomposites

In the frame of this work the temperature-dependency of the magnetization of various composite systems consisting of porous silicon (PSi) and silicon nanotubes (SiNTs) with embedded Ni, Co, NiCo, FePt, and Fe3O4 structures is discussed. The temperature is varied between 80 and 1273 K which allows to determine the Curie Temperature of the composites which is higher than for the corresponding bulk materials. High temperatures result in an alteration of the chemical sample-composition. Depending on the temperature specific metal-silicide formation occurs usually beginning around 600 K. This leads to modified magnetic behavior of the composites which can be observed by repeated measurements. Magnetic cross-talk between metal deposits can be controlled by the morphology of the porous silicon (distance between the pores) or silicon nanotubes (wall thickness) and also by the distribution and size of the deposits within the pores. To get a clear knowledge about the magnetic interactions single hysteresis curves are not sufficient and thus first order reversal curves (FORC) are performed.The employed PSi is produced by anodization of a highly n-doped silicon wafer. The structures offer an average pore-diameter of 50 nm and a mean distance between the pores of 50 nm which ensures a clear separation of the pores, a crucial point to achieve the appropriate magnetic properties. The SiNTs are produced by ZnO seed deposition on a silicon wafer, growing of the ZnO nanowire arrays (NWA), followed by Si deposition and finally etching off the ZnO. Ni and Co are deposited electrochemically in using for Ni formation a Watts electrolyte, consisting of NiCl2, NiSO4 and H3BO3 and for Co deposition a CoSO4 solution is utilized. A further approach is to grow Co particles inside the pores/tubes in using a CoCl2 6H2O and NaBH4 solution. Fe3O4 are synthesized by high temperature decomposition in the presence of an organic precursor (1). The synthesized particles are infiltrated into the pores/tubes with the help of a magnet beneath the samples. FePt nanocrystals are formed within the SiNTs and PSi by a multistep process, using a solution containing Citric acid, H2PtCl6 6H2O, and Fe(NO3)3 9H2O.Considering Ni loaded porous silicon samples, the magnetic response is recorded in dependence on the temperature to figure out the Curie temperature (TC) of the composite system which is determined from hysteresis curves measured at various temperatures between 80 and 950 K in field steps of 50 K. TC is determined at 700 K, which is about 70 K higher than for bulk Ni. Co loaded porous silicon has been measured up to 1200 K which was still too low to determine TC. TC of bulk magnetite is at 858 K. The investigated porous silicon sample with 8 nm Fe3O4 NPs infiltrated into the pores shows TC around 900 K, 42 K higher than for the bulk material. In the case of FePt loading, the system with PSi as template shows a lower TC compared to SiNTs. The investigations show that the TC of the investigated nanostructured composite systems is always higher than for the according bulk materials. FORC diagrams are used to detremine magnetostatic interactions between the metal deposits. Magnetic coupling is present for Ni, Co and NiCo deposits, wheras in the case of FePt NP loading no magnetic cross-talk between the particles is observed.(1) L. Gutierrez, R. Costo, C. Grüttner, F. Westphal, N. Gehrke, D. Heinke, A. Fornata, Q.A. Pankhurst, C, Johansson, S. Veintemillas-Verdaguer, M.P. Morales,Dalton Transactions, 44, 2943, (2015).

Insulated π-Conjugated Azido Scaffolds for Stepwise Functionalization via Huisgen Cycloaddition on Metal Oxide Surfaces.

In organic-inorganic hybrid devices, fine interfacial controls by organic components directly affect the device performance. However, fabrication of uniformed interfaces using π-conjugated molecules remains challenging due to facile aggregation by their strong π-π interaction. In this report, a π-conjugated scaffold insulated by covalently linked permethylated α-cyclodextrin moiety with an azido group is synthesized for surface Huisgen cycloaddition on metal oxides. Fourier-transformed infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy confirm the successful immobilization of the insulated azido scaffold on ZnO nanowire array surfaces. Owing to the highly independent immobilization, the scaffold allows rapid and complete conversion of the surface azido group in Huisgen cycloaddition reactions with ethynyl-terminated molecules, as confirmed by FT-IR spectroscopy monitoring. Cyclic voltammetry analysis of modified indium tin oxide substrates shows the positive effects of cyclic insulation toward suppression of intermolecular interaction between molecules introduced by the surface Huisgen cycloaddition reactions. The utility of the scaffold for heterogeneous catalysis is demonstrated in electrocatalytic selective O2 reduction to H2O2 with cobalt(II) chlorin modified fluorine doped tin oxide electrode and photocatalytic H2 generation with iridium(III) dye-sensitized Pt-loadedTiO2 nanoparticle. These results highlight the potential of the insulated azido scaffold for a stepwise functionalization process, enabling precise and well-defined hybrid interfaces.

Electrocatalytic hydrogen evolution coupled with dye hydrogenation reactions for sustainable wastewater treatment using transition-metal (Fe, Co, Ni, Cu) nanoparticles with ZnO nanowire supports

A water electrolysis coupled with chemical reduction strategy is proposed to realize sustainable wastewater treatment. Correspondingly, a series of transition-metal (e.g., iron, cobalt, nickel and copper) nanoparticles anchored on zinc oxide nanowire array (ZnO NA) with bifunctional catalytic activity for hydrogen evolution reaction together with dye hydrogenation reaction (HER/DHR) are developed, while their catalytic activity are comparatively investigated at the same time. This renewable strategy with environmental and economic benefits exhibits high-efficiency treatment capability towards various dye wastewaters. For example, a typical system based on nickel nanoparticles with ZnO NA supports, which exhibits higher catalytic activity towards the HER/DHR than those of other three counterparts under the same conditions, can transform waste 4-nitrophenol into valuable 4-aminophenol with an apparent rate constant of 36.95 × 10-3 min−1 and a conversion rate of 98.32 %, thereby achieving dye decolorization and resource regeneration simultaneously. Its efficiency is significantly superior to that of chemical method dependent on traditional reducing agents and commercial nickel materials, demonstrating its stronger competitiveness. It is mainly benefited from the interface synergistic effect of nickel nanoparticles and zinc oxide nanowires. This novel system is expected to replace conventional sewage remediation technologies for efficient, inexpensive, and eco-friendly dye degradation, and thus is very important for energy and environmental sustainability.

Atomic force microscopy in mechanical measurements of single nanowires

In this paper, we present the results of mechanical measurement of single nanowires (NWs) in a repeatable manner. Substrates with specifically designed mechanical features were used for NW placement and localization for measurements of properties such as Young's modulus or tensile strength of NW with an atomic force microscopy (AFM) system. Dense arrays of zinc oxide (ZnO) nanowires were obtained by one-step anodic oxidation of metallic Zn foil in a sodium bicarbonate electrolyte and thermal post-treatment. ZnO NWs with a hexagonal wurtzite structure were fixed to the substrates using focused electron beam-induced deposition (FEBID) and were annealed at different temperatures in situ. We show a 10-fold change in the properties of annealed materials as well as a difference in the properties of the NW materials from their bulk values with pre-annealed Young modulus at the level of 20 GPa and annealed reaching 200 GPa. We found the newly developed method to be much more versatile, allowing for in situ operations of NWs, including measurements with different methods of scanning probe microscopy.

Exploration of the optical characteristics in sol-gel synthesized europium-doped zinc oxide thin films for potential optoelectronic use: a comprehensive characterization approach

Abstract In this study, we present our findings on the spin coated pure Zinc-Oxide (ZnO) nanowire arrays and Europium doped ZnO (Eu3+: ZnO) nanostructure thin films. These arrays were grown on soda lime (silica) coated glass substrate. We conducted a comprehensive analysis of the structural and optical properties of the as-synthesized nanostructures. The characterization techniques included x-ray Diffraction, FESEM Analysis, Raman Spectroscopy, Photoluminescence and UV–vis Analysis. The XRD findings indicate the incorporation of Europium (Eu3+) into the ZnO crystalline lattice, potentially replacing Zn2+ ions. This doping with Europium (Eu3+) led to a reduction in crystalline size, as determined by Scherrer’s equation decreasing from 48 nm to 28 nm demonstrates a decrease in defects within the films. Raman Shift analysis revealed changes in the optical properties of films with inclusion of Europium in host matrix. Photoluminescence studies demonstrated a distinctive 5D0 to 7F2 transition arising from the Eu3+ ions, observed at approximately 612 nm. We have thoroughly examined the optical characteristics of both Europium-doped and pure ZnO thin films through a systematic study. The optical properties were assessed by analyzing the absorption spectra (220–600 nm) and transmission spectra within the wavelength range of 200 to 1200 nm. The film exhibited an impressive 80% transparency, particularly noteworthy for window layer application. The refractive index (n), extinction coefficient (k) and all other associated parameters were found to be impacted by the doping of Europium. The refractive index dispersion relation has been explored using a single oscillator model. Furthermore, the non-linear optical susceptibility (χ 3) and non-linear refractive index were computed using semi-empirical relationships based on the linear optical parameters. The Europium (Eu3+) doping in ZnO led to an increase in the χ 3 value elevating it from 4.07 × 10–1° to 5.91 × 10–1° e.s.u. These findings suggest that Europium-doped ZnO nanostructures have the potential to be a promising platform for the development of efficient multispectral light-emitting diodes (LED’s) and optical devices.

Imaging the mechanical properties of nanowire arrays

Abstract Dimensional and contact resonance (CR) images of nanowire (NW) arrays (NWAs) are measured using our newly developed microprobe CR imaging (CRI) setup. Then a reference method is employed to calculate the indentation modulus of NWs (M i,NW ) representing the elasticity of NWs, by measuring NWAs and reference samples at the same static probing force. Furthermore, topography is imaged in combination with CR and M i,NW separately by software, in which the z values indicate the topography of the NWs and the color bars show its CR or M i,NW . Then NWs’ topography relation to M i,NW is visualized. As typical examples, 3D imaging of topography and measurement of M i,NW is performed with Si<111> pillar arrays as well as Cu and ZnO NWAs. The novel method enables fast mechanical performance measurements of large-scale vertically-aligned NWAs without releasing them from their respective substrates. For instance, the diameter and pitch of the Si<111> pillars and the diameter of the Cu NWAs are in good agreement with the values measured by scanning electron microscopy (SEM). The position of ZnO NWs bunches grown at arbitrary sites on silicon can be identified with the help of combined topography and indentation modulus images. Furthermore, M i,NW measured by our homemade CRI setup agrees well with bulk values. Differences between the measured M i,NW and bulk M i values may be related to a size effect in NW elasticity.

ZnO Nanowire Arrays Decorated with Cu Nanoparticles for High-Efficiency Nitrate to Ammonia Conversion

ZnO Nanowire Arrays Decorated with Cu Nanoparticles for High-Efficiency Nitrate to Ammonia Conversion

Structural and optical properties of position-controlled n-ZnO nanowire arrays: Potential applications in optoelectronics

Highly position-controlled ZnO nanowire (NW) arrays and nanowire-cluster (NW–C) arrays are grown on mask-patterned gallium nitride (GaN) substrates by hydrothermal method. Electroluminescence (EL) emissions with blue-violet light at forward-bias(fb) voltage, EL emissions with yellow light at reverse-bias(rb) voltage, and EL emission at both rb voltages and rb voltages were achieved by the adjustment of the size of the heterojunction interface between wire arrays and film. The high rb current is attributed to electron tunneling transport edge effect via a narrow tunnel barrier. The obtained results highlight that the EL characteristics of ZnO NWs can be tuned and controlled through the size of the heterojunction interface between wire arrays and film. Besides, the nonplanar heterogeneous structure of ZnO NW arrays/GaN film also shown a great spectral response characteristic in the ultraviolet(UV) band, which proves that the structure of is worthy of being studied to achieve high performance UV photodetectors.

A Self-Powered Lactate Sensor Based on the Piezoelectric Effect for Assessing Tumor Development.

The build-up of lactate in solid tumors stands as a crucial and early occurrence in malignancy development, and the concentration of lactate in the tumor microenvironment may be a more sensitive indicator for analyzing primary tumors. In this study, we designed a self-powered lactate sensor for the rapid analysis of tumor samples, utilizing the coupling between the piezoelectric effect and enzymatic reaction. This lactate sensor is fabricated using a ZnO nanowire array modified with lactate oxidase (LOx). The sensing process does not require an external power source or batteries. The device can directly output electric signals containing lactate concentration information when subjected to external forces. The lactate concentration detection upper limit of the sensor is at least 27 mM, with a limit of detection (LOD) of approximately 1.3 mM and a response time of around 10 s. This study innovatively applied self-powered technology to the in situ detection of the tumor microenvironment and used the results to estimate the growth period of the primary tumor. The availability of this application has been confirmed through biological experiments. Furthermore, the sensor data generated by the device offer valuable insights for evaluating the likelihood of remote tumor metastasis. This study may expand the research scope of self-powered technology in the field of medical diagnosis and offer a novel perspective on cancer diagnosis.

Bioinspired Integrated Multidimensional Sensor for Adaptive Grasping by Robotic Hands and Physical Movement Guidance

AbstractThe construction of piezoresistive sensors capable of distinguishing multiple mechanical stimuli is important for sensing higher level applications. However, the mutual interference between sensing signals is a technical bottleneck for sensors to recognize multiple mechanical stimuli. Inspired by the structure of human skin and muscle, a multidimensional sensor is designed by integrating three sub‐sensors. Each sub‐sensor is only sensitive to stimulus components in one of the three orthogonal axes, whereas it remains insensitive to others because of its anisotropic sensing properties. Specifically, an omnidirectional gradient wrinkled polyurethane film with MXene‐embedded ZnO nanowire arrays serves as a strain‐insensitive pressure sub‐sensor, while two aligned segmental polyimide/polyurethane films through orthogonal stacking act as two pressure‐insensitive anisotropic strain sub‐sensors. A unique characteristic of multidimensional sensor is its ability to distinguish and measure the type, magnitude, and direction of strain, pressure, and shear. Moreover, multidimensional sensor exhibits maximal gauge factor of 863.7 for strain and highest sensitivity of 187.71 kPa−1 for pressure. Importantly, multidimensional sensor exhibits an unprecedented ability to quantitatively evaluate shear using a library of electrical responses. The adaptive grasping of robotic hands and free‐throw training of players have been demonstrated to initiate the development of robotic object manipulation and physical movement guidance.

Modeling the Temporal Response of Gated ZnO Nanowire Field Emitter by Considering the Charging and Self-Heating Effect for Improving the Response Speed

ZnO nanowire is a promising candidate for large-area gated field emitter arrays. How to improve its temporal response is one of the key problems to be solved for applications. In this work, a device model for a gated ZnO nanowire field emitter with consideration of charging and self-heating effect has been established to investigate its temporal response. It is found that while the charging effect is responsible for the delay at the beginning of the pulse, the self-heating effect which induces delay due to the thermal conduction process can shorten the charging time because of its lowering of nanowire resistance. The response time can be minimized when these two effects are balanced at an optimal field which is below the critical field for thermal runaway. We further investigate the optimal response time of a nanowire with the same resistance but a different length, radius, and electrical properties. The results imply that a lower heat capacity and higher critical temperature for thermal runaway are in favor of a shorter response time, which must be taken into account in the reduction in nanowire resistance for improving response speed. All the above should be useful for the device design of a fast-response gated ZnO nanowire field emitter array.

Density-controlled electrochemical synthesis of ZnO nanowire arrays using nanotextured cathode

Zinc oxide (ZnO) nanowires fabricated via wet chemical synthesis on flexible polymer substrates are inherently unstable against mechanical bending stress because of their high density and weak adhesion to the substrate. We introduce a novel method for controlling the density of such ZnO nanowire arrays using a three-dimensional corrugated metal substrate. These metal substrates, featuring extruded and recessed patterns fabricated via nanoimprint lithography, were employed as cathodes during the electrochemical deposition of ZnO nanowire arrays. The ZnO nanowire arrays synthesized on the patterned metal thin film exhibited smaller diameters and lower densities compared to those on non-patterned metal films. This reduction in density can be attributed to aligned nucleation and limited growth on the patterned metal surface. Crucially, ZnO nanowires synthesized on patterned metal substrates displayed remarkable mechanical robustness against external forces, a direct consequence of their reduced density. In contrast, nanowires synthesized on non-patterned metal substrates were broken under mechanical bending. Detailed morphological analyses performed after mechanical bending tests confirm that ZnO nanowires synthesized on nanoimprinted metal electrodes exhibited enhanced mechanical characteristics compared to those on non-patterned metal electrodes. These findings clearly demonstrate the promise of utilizing density-controlled ZnO nanowires in piezoelectric devices.

Area-Selective Growth of Zinc Oxide Nanowire Arrays for Piezoelectric Energy Harvesting.

In this work, we present the area-selective growth of zinc oxide nanowire (NW) arrays on patterned surfaces of a silicon (Si) substrate for a piezoelectric nanogenerator (PENG). ZnO NW arrays were selectively grown on patterned surfaces of a Si substrate using a devised microelectromechanical system (MEMS)-compatible chemical bath deposition (CBD) method. The fabricated devices measured a maximum peak output voltage of ~7.9 mV when a mass of 91.5 g was repeatedly manually placed on them. Finite element modeling (FEM) of a single NW using COMSOL Multiphysics at an applied axial force of 0.9 nN, which corresponded to the experimental condition, resulted in a voltage potential of -6.5 mV. The process repeated with the same pattern design using a layer of SU-8 polymer on the NWs yielded a much higher maximum peak output voltage of ~21.6 mV and a corresponding peak power density of 0.22 µW/cm3, independent of the size of the NW array. The mean values of the measured output voltage and FEM showed good agreement and a nearly linear dependence on the applied force on a 3 × 3 µm2 NW array area in the range of 20 to 90 nN.

Synergy of ZnO Nanowire Arrays and Electrospun Membrane Gradient Wrinkles in Piezoresistive Materials for Wide-Sensing Range and High-Sensitivity Flexible Pressure Sensor

Synergy of ZnO Nanowire Arrays and Electrospun Membrane Gradient Wrinkles in Piezoresistive Materials for Wide-Sensing Range and High-Sensitivity Flexible Pressure Sensor

Surface energy and stress driven growth of extremely long and high-density ZnO nanowires using a thermal step-oxidation process.

Formation of highly crystalline zinc oxide (ZnO) nanowires with an extremely high aspect ratio (length = 60 μm, diameter = 50 nm) is routinely achieved by introducing an intermediate step-oxidation method during the thermal oxidation process of thin zinc (Zn) films. High-purity Zn was deposited onto clean glass substrates at room temperature using a vacuum-assisted thermal evaporation technique. Afterwards, the as-deposited Zn layers were thermally oxidized under a closed air ambient condition at different temperatures and durations. Structural, morphological, chemical, optical and electrical properties of these oxide layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoemission spectroscopy (XPS). It was noticed that the initial thermal oxidation of Zn films usually starts above 400 °C. Homogeneous and lateral growth of the ZnO layer is usually preferred for oxidation at a lower temperature below 500 °C. One-dimensional (1D) asymmetric growth of ZnO started to dominate thermal oxidation above 600 °C. Highly dense 1D ZnO nanowires were specifically observed after prolonged oxidation at 600 °C for 5 hours, followed by short-step oxidation at 700 °C for 30 minutes. However, direct oxidation of Zn films at 700 °C resulted in ZnO nanorod formation. The formation of ZnO nanowires using step-oxidation is explained in terms of surface free energy and compressive stress-driven Zn adatom kinetics through the grain boundaries of laterally grown ZnO seed layers. This simple thermal oxidation process using intermittent step-oxidation was found to be quite unique and very much useful to routinely grow an array of high-density ZnO nanowires. Moreover, these ZnO nanowires showed very high sensitivity and selectivity towards formaldehyde vapour sensing against few other VOCs.

Controlling by Defects of Switching of ZnO Nanowire Array Surfaces from Hydrophobic to Hydrophilic: First-Principles Calculations

Controlling by Defects of Switching of ZnO Nanowire Array Surfaces from Hydrophobic to Hydrophilic: First-Principles Calculations

Piezoelectric properties of ZnO nanowire arrays/DAST-PDMS flexible nanogenerators

The piezoelectric properties of 4-(4-dimethylaminostyryl) methylpyridine p-toluene sulfonate (DAST)-polydimethylsiloxane (PDMS) composite thin film was investigated. Flexible nanogenerators based on zinc oxide (ZnO) nanowire (NW) arrays/DAST-PDMS were fabricated. DAST holds organic crystal structure, and it can possess piezoelectric properties, and it can enhance the piezoelectric properties of ZnO NW arrays. PDMS can not only mix DAST powder with a liquid to prepare a thin film but also protect ZnO NW arrays and DAST from deterioration. When applying pressure with a balance weight of 100 g, the flexible nanogenerator based on ZnO NW arrays/DAST-PDMS can generate an output voltage of 2.3 V, which is much larger than that of the nanogenerator based on ZnO NW arrays. The effect of the strength of vertical external force, frequency of vertical external force, and degree of bending deformation on the ZnO NW arrays/DAST-PDMS flexible nanogenerators were investigated in this Letter. In our experiments, the output voltage of the ZnO NW arrays/DAST-PDMS flexible nanogenerators can reach 3.3 V. After bending the nanogenerator 20 times, the energy collection device based on the ZnO NW arrays/DAST-PDMS flexible nanogenerator was able to light an LED bulb.