ZnO Nanowire Films Research Articles

Size dependence of photovoltaic properties and surface states modulation in ZnO nanowire/poly(3-hexylthiophene) hybrid nanostructures

An appropriate diameter and wire-to-wire distance is critical for optimizing the performance of hybrid inorganic/organic photovoltaic devices. For a deep understanding of their influences on such hybrid structures, the well-ordered ZnO nanowires with different diameters are fabricated by the versatile hydrothermal growth. The dependence of the photovoltaic performance on the surface states, wire diameter and wire-to-wire distance is investigated. We demonstrate that the pristine thick ZnO nanowires film possess a higher surface photovoltage (SPV) response than the thin one. This is mainly due to the influence of surface states on the thin ZnO nanowires, which can capture the photo-generated carriers. When the two kinds of ZnO nanowires are fabricated into a hybrid inorganic/organic structure, the thin ZnO nanowires/poly(3-hexylthiophene) hybrid film has a higher SPV response than the thick one, which is contrary to the pristine ZnO nanowires. This is benefited from the smaller diameter and wire-to-wire distance of the thin ZnO nanowires owned. The crystallinity, wire diameter and wire-to-wire distance have the crucial influence on the final photovoltaic performance. The results shown here give us insights toward designing efficient hybrid photovoltaic devices.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators.

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

Synthesis of Te-doped ZnO nanowires with promising field emission behavior

In the current study, a successful synthesis of ZnO nanowire films doped with Te was performed through electrodeposition on the zinc foil followed by annealing in air for 4 hours at 400 °C.

Effect of indium on photovoltaic property of n-ZnO/p-Si heterojunction device prepared using solution-synthesized ZnO nanowire film

Preparation of n-ZnO/p-Si heterostructures using solution-synthesized ZnO nanowire films and their photovoltaic characterization is reported. The solution-grown ZnO nanowire film is characterized using scanning electron microscope, electron dispersive x-ray, and optical absorption studies. Electrical and photovoltaic properties of the fabricated heterostructures are studied using e-beam-evaporated aluminum as metal contacts. In order to use transparent contact and to simultaneously collect the photogenerated carriers, sandwich-type solar cells were fabricated using ZnO nanorod films grown on p-silicon and indium tin oxide (ITO) coated glass as ITO/n-ZnO NR/p-Si. The electrical properties of these structures are analyzed from current-voltage (I−V) characteristics. ZnO nanowire film thickness-dependent photovoltaic properties are also studied. Indium metal was also deposited over the ZnO nanowires and its effects on the photovoltaic response of the devices were studied. The results demonstrated that all the samples exhibit a strong rectifying behavior indicating the diode nature of the devices. The sandwich-type ITO/n-ZnO NR/p-Si solar cells exhibit improved photovoltaic performance over the Al-metal-coated n-ZnO/p-Si structures. The indium deposition is found to show enhancement in photovoltaic behavior with a maximum open-circuit voltage (Voc) of 0.3 V and short-circuit current (Isc) of 70×10−6 A under ultraviolet light excitation.

Synthesis of ZnO nanowire array film on Mg doped gallium nitride substrate

ZnO nanowire array films, composed of well aligned ZnO nanowires ∼200 nm in diameter and 1 μm in length, were successfully synthesised on Mg doped gallium nitride by hydrothermal method. In addition, the films possess quite flatten surface. In the synthesised process, there was no catalyst that had been used. Growth conditions were comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled with vertically aligned nanowire may have potential applications as UV light emitting diodes.

Comparative Study of Fe‐Doped ZnO Nanowire Bundle and Their Thin Film for NO2 and CH4 Gas Sensing

SummaryThe free standing Fe‐doped ZnO nanowire bundle is synthesized by employing a typical vapor phase transport method in single step from Fe+Zn powder in O2+Ar flow. Fe‐doped ZnO nanowires are characterized by Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) suggesting formation of hexagonal wurtzite phase of ZnO with 7 at% Fe in ZnO with typical nanowire diameter in the range of 20 nm–70 nm. These nanowire bundles are ultrasonicated and processed into thin film using spin coating technique. Both of the Fe‐ doped ZnO nanowire bundle and thin films are tested for their NO2 and CH4 (20–100 ppm) sensing properties. The thin film exhibited higher operating temperature (300°C) and response time (45 sec) as compared to nanowire bundle (150°C and 15 sec).

In-situ observation of hydrothermal growth of ZnO nanowires on patterned Zn substrate and their photocatalytic performance

ZnO nanowire thin film was synthesized in situ on patterned Zn substrate via a simple and seed-free hydrothermal method. The as-synthesized products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersion X-ray analysis(EDS), UV–visible diffuse reflectance spectroscopy (UV–vis-DRS), and photoluminescence (PL) spectroscopy. The time-, temperature- and ammonia concentration-dependent morphology evolution were observed. The results showed that the Zn MPs, in the presence of the reaction solution, can be gradually dissolved, thus providing the Zn source for successive nucleation and growth of ZnO nanowires. The increase of the light absorption as well as a red shift of the band edge was related to the increase of the quantity and the defects of ZnO nanowires. A growth mechanism was proposed to understand the formation of ZnO nanowires on the plated Zn substrate. Under UV irradiation, the as-synthesized ZnO nanowire film exhibited good photocatalytic activity and high photochemistry stability in a wide pH range of 5–10. Furthermore, the ZnO nanowire film can be easily separated, demonstrating great potential for practical application in the degradation of the organic pollutant while exhibiting high reusability.

Fast Response and High Sensitivity of ZnO Nanowires-Cobalt Phthalocyanine Heterojunction Based H2S Sensor.

The room temperature chemiresistive response of n-type ZnO nanowire (ZnO NWs) films modified with different thicknesses of p-type cobalt phthalocyanine (CoPc) has been studied. With increasing thickness of CoPc (>15 nm), heterojunction films exhibit a transition from n- to p-type conduction due to uniform coating of CoPc on ZnO. The heterojunction films prepared with a 25 nm thick CoPc layer exhibit the highest response (268% at 10 ppm of H2S) and the fastest response (26 s) among all samples. The X-ray photoelectron spectroscopy and work function measurements reveal that electron transfer takes place from ZnO to CoPc, resulting in formation of a p-n junction with a barrier height of 0.4 eV and a depletion layer width of ∼8.9 nm. The detailed XPS analysis suggests that these heterojunction films with 25 nm thick CoPc exhibit the least content of chemisorbed oxygen, enabling the direct interaction of H2S with the CoPc molecule, and therefore exhibit the fastest response. The improved response is attributed to the high susceptibility of the p-n junctions to the H2S gas, which manipulates the depletion layer width and controls the charge transport.

Synthesis of ZnO Nanowire Array Film on Mg-Doped Gallium Nitride Substrate by Simple Hydrothermal Method

ZnO nanowire array films, composed of well-aligned ZnO nanowires with about 200 nm in diameter and 1 μm in length, were successfully synthesized on Mg-doped gallium nitride by hydrothermal method. The films possess quite flattened surface. In the synthesized process, there was no catalyst used. Growth conditions are comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled of vertically aligned nanowire may have potential applications as UV light-emitting diodes.

Highly hydrophobic hierarchical nanomicro roughness polymer surface created by stamping and laser micromachining

ABSTRACTThis article describes the design and fabrication of hierarchical nanomicrostructured polymer surfaces with high hydrophobicity. The nanoscale roughness is achieved by stamping a ZnO nanowire film into PDMS. Subsequently, microstructures with different periodicities are created in the stamped PDMS sample by direct laser writing using femtosecond pulses. With this approach, we were able to produce hierarchical surface morphologies, composed of nano and microscale structures that exhibit water contact angles larger than 160°. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42082.

Plastic Deformation Study of Vertical Zinc Oxide Nanowires for Polymer Cantilever-Based Sensor Applications

In this letter, the mechanical properties of piezoelectric ZnO nanowire (NW) films have been measured using a nanoindentation technique. We demonstrated a fabrication process to embed this NW film inside a polymeric (SU-8) cantilever. Mechanical properties of this SU-8/ZnO NW film and cantilever have been measured. The ZnO NWs have been grown vertically using a low-temperature chemical synthesis. The observed value of Young's modulus for ZnO NW is in the range 72-93 GPa. After embedding this NW film inside a polymer matrix, the Young's modulus value was reduced to the range 5-19 GPa. The mechanical behavior of the NW-embedded cantilever has also been investigated using the nanoindentation technique. These cantilevers have been shown to be suitable for surface stress-sensing applications.

Effects of synthesizing parameters on surface roughness and contact angles of ZnO nanowire films.

Effects of the synthesizing parameters on the surface roughness and the contact angles of ZnO nanowire films were studied in this paper. ZnO nanowire films were synthesized with the hydrothermal method on glass substrates, and the synthesizing parameters include the concentrations of the growth solution and the seed layer solution, the growth time span as well as the temperature. Atomic force microscopy and scanning electron microscopy were employed respectively to characterize the surface and the profile roughness of ZnO nanowire films. The measurement results by atomic force microscopy were in agreement with that by scanning electron microscopy, hence the former was used for the investigation of aforementioned effects. Relationships between the synthesizing parameters, the surface roughness and the contact angles of ZnO nanowire films were established, revealing that the synthesizing parameters affected significantly not only the surface roughness but also the contact angles of ZnO nanowire films. The results can be used for batch fabrication of ZnO nanowire-based structures and these structures-based sensors in a wide variety of applications.

Piezotronic Effect on ZnO Nanowire Film Based Temperature Sensor

In this work, we demonstrated the first study of piezotronic effect as a potential means for measuring temperature by utilizing ZnO nanowire (NW) film. The film was synthesized by the wet chemical deposition method and transferred to a flexible substrate using photoresist. The primary role of piezotronic effect over geometrical and piezoresistive effect in the as-fabricated devices has been confirmed, and piezotronic effect on charge carrier transportation under different strains is subsequently studied. In addition, we also presented that the temperature sensing capability of as-fabricated NW film based piezotronic devices can be tuned by piezopotential, which exhibits dramatically enhanced sensitivity. A theoretical model is proposed to interpret the observed behaviors of the sensor. This study provides an effective method to fabricate temperature sensors with higher performance based on piezotronic effect in the future.

Photodegradation of Orange II by ZnO and TiO2 powders and nanowire ZnO and ZnO/TiO2 thin films

The present study is focused on photocatalytic degradation of the organic azo-dye Orange II in water solutions using TiO2 and ZnO powders or bare and TiO2 doped ZnO nanowire thin films. Commercial anatase TiO2 and ZnO are used for the photocatalytic experiments in slurry. The ZnO seeded substrates are prepared by repetition of four seed deposition cycles from zinc acetate solution using spin coating technique. The ZnO nanowires array films are obtained via wet chemical method. The ZnO and TiO2 doped ZnO nanowire thin films are investigated by SEM and X-ray diffraction. The photocatalytic tests are performed in cylindrical glass reactors under UV or visible light irradiation. The pollutants concentrations are 20ppm Orange II for slurry and 10ppm for film photocatalysis. The dyes photodegradation is observed spectrophotometrically.

Analysis of Temperature-Dependent Electrical Characteristics of n-ZnO Nanowires (NWs)/p-Si Heterojunction Diodes

This paper presents the electrical characteristics of n-zinc oxide (ZnO) nanowires (NWs)/p-Si (100) heterojunction diodes fabricated by the oxidation of thermally deposited metallic Zn on Al:ZnO-coated p-Si 〈1 0 0〉 substrates. The electrical parameters of the n-ZnO NWs/p-Si diodes have been estimated by using the room temperature capacitance-voltage (C-V) and temperature-dependent current-voltage (I-V) characteristics of the heterojunction. The carrier concentration of the ZnO NW film and the barrier height of the diode estimated from the C-V characteristics at room temperature are 1.54 × 10 15 cm -3 and 0.75 eV, respectively. The thermionic emission model was used to analyze the temperature-dependent measured I-V characteristics to estimate the parameters of the diode. The estimated values of the barrier height and ideality factor at room temperature were 0.715 eV and 2.13, respectively. The spatial barrier inhomogeneity was included in the aforementioned analysis by assuming a Gaussian distribution for the barrier height at the n-ZnO NWs/p-Si heterojunction. The Richardson constant A* of ZnO was found to be increased from a relatively low value of 9.75 ×10 - 8 A ·cm - 2 ·K - 2 to a more realistic value of 49A ·cm - 2 ·K - 2 after incorporating the barrier inhomogeneity phenomenon in the aforementioned analysis.

Defect controlled water splitting characteristics of gold nanoparticle functionalized ZnO nanowire films

An ITO/FTO free AuNP functionalized ZnO NW photoanode in a dual role such as photo active centers for catalytic activity and an efficient transport medium for photo-generated charge carriers.

Wetting behavior of triethoxyoctylsilane modified ZnO nanowire films

ZnO nanowire films have been prepared by chemical process at 90°C for different growth time varied from 4h to 12h. The effect of growth time on the structure, morphology, and the hydrophobicity of ZnO nanowire films were investigated. The results indicate that all of the films exhibit the hexagonal wurtzite phase with growth direction perpendicular to the substrate surface. The scanning election microscopy images of ZnO nanowire films show that prolonging the growth time results in longer nanowires with uneven diameter. The influence of the diameter of nanowires on the wettability of the film was analyzed. Typically, the film prepared for 4h exhibits a high water contact angle of about 161.7±0.3° with the advancing contact angle of 157.2±0.5° and receding contact angle of 163.2±0.5° and a small sliding angle of 6° after hydrophobic treatment. Based on the Cassie theory, the analysis results reveal that only about 6% of the water surface contacts with the ZnO films and the remaining 94% contacts with the air cushion, which is reasonable for the hydrophobicity of a ZnO nanowire films.

Hydrogen-incorporated ZnO nanowire films: stable and high electrical conductivity

Post-growth hydrogen annealing treatment of highly oriented ZnO nanowire (NW) films (ZnO : H) results in high electrical conductivity (3.7 × 103 S m−1) and fully suppressed defect emission at room temperature. The formation of hydrogen-related vacancy complexes is responsible for the suppression of vacancies ( and ), leading to a reduction in defect-based emission. ZnO : H NW films show five orders larger stable electrical conductance with a four-fold increment in carrier mobility (7–28 cm2 V−1 s−1). As compared with pristine NWs, the carrier concentration in ZnO : H NW films increases from 1015 to 1019 cm−3, which is in the range of commercial transparent conducting oxides. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analyses reveal stable OH bond formation, which strongly supports the prediction of H doping. These films offer a promising conducting oxide platform for photovoltaic applications.

Physical model for the exotic ultraviolet photo-conductivity of ZnO nanowire films

Employing a simple and efficient method of electro-chemical anodization, ZnO nanowire films are fabricated on Zn foil, and an ultraviolet (UV) sensor prototype is formed for investigating the electronic transport through back-to-back double junctions. The UV (365 nm) responses of surface-contacted ZnO film are provided by I—V measurement, along with the current evolution process by on/off of UV illumination. In this paper, the back-to-back metal—seconductor—metal (M—S—M) model is used to explain the electronic transport of a ZnO nanowire film based structure. A thermionic-field electron emission mechanism is employed to fit and explain the as-observed UV sensitive electronic transport properties of ZnO film with surface-modulation by oxygen and water molecular coverage.

Influence of chemical routes on optical and field emission properties of Au–ZnO nanowire films

The Au–ZnO nanowire films have been synthesized in two steps by combining the chemical routes, i.e. electrochemical deposition (ECD) and chemical bath deposition (CBD) techniques, and annealing process in air at 400 °C on zinc foil. The X-ray diffraction patterns exhibit formation of the ZnO wurtzite structure along with binary phases Au3Zn and AuZn3. The scanning electron microscope image show presence of ZnO nanowires possessing length of several micrometers and diameter less than 120 nm synthesized by ECD and in the range of 70–400 nm using CBD technique. The UV–visible results indicated different band gap absorption edge on the films. In addition, field emission experiments show different behavior for the films that were grown by different routes. Furthermore, the Au–ZnO prepared by ECD technique exhibited excellent emission current stability. These results suggest that the Au–ZnO nanowires could apply as high current sources.