ZnO Nanowire Films Research Articles

Freestanding ZnO Nanowire Films Derived from Carbon Nanotube Template: Implications for High-Performance UV Photodetectors

Freestanding ZnO Nanowire Films Derived from Carbon Nanotube Template: Implications for High-Performance UV Photodetectors

Nanocomposites of ZnO Nanostructures and Reduced Graphene Oxide Nanosheets for NO2 Gas Sensing

A depletion region formed at the junction of two materials depends on the charge carrier concentration and the barrier height. The depletion width can be controlled by the overlayer thickness. Modification of the depletion width after gas exposure is mainly contributing to the sensor response. The maximum effect of gas molecules can be realized when the depletion width becomes comparable to the Debye length. Consequently, the selection of an appropriate overlayer thickness is very crucial. In the present work, chemiresistive gas sensing characteristics of ZnO nanowire (NW) films modified with different thicknesses of reduced graphene oxide (rGO) have been studied. The hybrid films showed selective outstanding NO2 response as compared to pure ZnO NW films. At an optimum rGO thickness of ∼15.3 nm, the response toward NO2 (with a minimum detection limit of 400 ppb) was found to be the highest that is nearly three times higher than that of pure ZnO nanowire films. A synergetic balance of the depletion width at the ZnO/rGO interface with the Debye length is attributed to the improved selective NO2 response of hybrid films.

Optical and Field Emission Properties of Transparent Flexible ZnO Nanowire Films

Optical and Field Emission Properties of Transparent Flexible ZnO Nanowire Films

Self‐Powered, Flexible Ultraviolet Photodetector Based on ZnO/Te All Nanowires Heterojunction Structure

Self‐powered and flexible photodetectors exhibit great potential in the application of optical communication, deep space exploration, and other related fields. Herein, a self‐powered ultraviolet photodetector based on ZnO nanowires/Te nanowires heterojunction has been prepared by two‐step vacuum filtration method, which also has superior flexibility and fast response speed. The obtained photodetector has a high photocurrent (about 1 μA) at 5 V bias under 365 nm UV light illumination and a fast response speed (3.47/0.13 s). Even if the bending angle reaches 90°, its photocurrent still maintains about 0.5 μA under 5 V bias, showing superior flexibility. Under 0 V bias, the photocurrent of the photodetector is 0.18 nA and the response time is 65/140 ms, displaying its good self‐powered characteristic and fast response speed. These superior properties are caused by the flexibility of ZnO nanowire film and Te nanowire film, and the heterojunction is formed between ZnO and Te. The obtained results provide a solid foundation for the further development of flexible optoelectronic devices based on ZnO nanostructures.

Thickness dependent wavelength selective NIR-UV-visible photoresponse in ZnO nanowires and silicon heterojunction

In this paper, we report ZnO nanowires (NWs) and silicon-based type-II PN heterojunction for UV–Visible–Infrared self-powered photodetection. The as-grown ZnO NWs were highly crystalline and aligned along the c-axis in the [002] direction revealed in the HRTEM and XRD measurements. The Hall measurements revealed the n-type behavior for ZnO and p-type for p-Si with carrier concentrations of 4.09 × 1016 cm−3 and 1.38 × 1017 cm−3, respectively. The depletion widths were estimated to be ∼35 nm and ∼120 nm, respectively for p-Si and n-ZnO NWs. The Ag/n-ZnO NWs/p-Si/Ag PN heterojunction showed large photoresponse, even at zero bias, under the illumination of commercially available UV–Visible–NIR LEDs, thus acting as a self-powered photodetector. It was interesting to observe that the photoresponse was dependent on the growth time and hence the thickness of ZnO NWs thin film. A maximum zero bias responsivity of ∼0.1 A/W at green (515 nm) was observed and was large for the junction with thicker ZnO NWs film (5 h growth), compared with thinner (3 h growth) device under IR (950 nm) LED illumination, however, it was observed otherwise for UV (395 nm) LED. This suggests that tuning the thickness of the ZnO NWs thin film results in the wavelength selective photoresponse, consequently, paving the way towards UV blind IR-visible photodetector based on ZnO NWs. The transient short circuit current (Isc vs t) and open circuit voltage (Voc vs t) properties showed fast and large responses under periodic illumination of all LEDs (UV–Vis–NIR). The response was observed to depend on the intensity of light and the maximum Voc comes out to be ∼102 mV and Isc ∼5.58 μA, under the illumination of a red laser diode.

Tracing the degradation pathway of temephos pesticide achieved with photocatalytic ZnO nanostructured films

The degradation pathway of temephos pesticide was elucidated using efficient photocatalytic ZnO nanowire and flower-like films. The monitoring of the subproducts is crucial as they are even more toxic than the original pesticide.

Synthesis of Self-Supporting ZnO Nanowire Array Film and its Optical Property and Room Temperature Ferromagnetism

X-ray diffractometer, field emission scanning electron microscope (SEM, Hitachi S-4800), laser confocal micro-region Raman spectrometer and vibration sample magnetometer were used to systematically study the effects of polyethyleneimine concentrations and exposure time on the morphology and size of ZnO nanowire arrays. The photoelectric property and the relationship between the morphology of nanowire arrays and ferromagnetism at room temperature were also analyzed. Under 15 min exposure time, when the polyethyleneimine concentration is 2.25 g / L, the obtained ZnO nanowire array film exhibits the smallest size, the optimal density and vertical orientation. According to the study of luminescence and room temperature magnetism, it is shown that the optical and ferromagnetic property are related to the variation tendency of oxygen defects and surface defects of the ZnO nanowires.

An unexplored strategy for synthesis of ZnO nanowire films by electrochemical anodization using an organic-based electrolyte. Morphological and optical properties characterization

In this work we report the preparation of ZnO nanowires by an unexplored strategy of electrochemical anodization using an electrolyte medium based mainly on ethylene glycol and fluoride ions, studying the anodization as a function of parameters, such as: temperature and voltage. The results obtained by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) show mainly a nanowire morphology with an average length of 296 nm. Microscopic evidence demonstrate that the size of nanostructures, morphology and homogeneity of resulting surfaces are completely related to the temperature and voltage conditions used during the zinc anodization. The results obtained by Raman spectroscopy and X-ray diffraction (XRD) corroborate the formation of only ZnO, with a crystallographic structure of hexagonal wurtzite. Finally, through diffuse reflectivity spectroscopy, bandgap values of [3.19–3.22] eV were estimated, classifying it as a material with potential characteristics for the fabrication of photoelectrodes for photoelectrochemical and photocatalytic applications.

Vertically Aligned Al-Doped ZnO Nanowire Arrays as Efficient Photoanode for Dye-Sensitized Solar Cells

In this communication, we report on the synthesis of vertically aligned aluminium (Al)-doped ZnO (ZnO:Al) nanowire (NW) thin films on FTO-coated glass substrates and their use as photoanode in dye-sensitized solar cells (DSSC). Very thin Al layers (∼3 nm, ∼6 nm and ∼10 nm) were deposited onto chemically synthesized ZnO nanowire film by electron-beam evaporation. The films were then subjected to rapid thermal annealing to incorporate different amounts of Al (∼0.98 at.%, 1.94 at.% and ∼2.89 at.%) into the ZnO nanowires. Optical, microstructural and compositional study of the films confirmed the growth of highly transparent and well-aligned ZnO:Al nanowires with a hexagonal crystal structure. The basic DSSC structure was fabricated using both undoped ZnO nanowire and ZnO:Al nanowire thin films as photoanode. In both cases, commercially available N3 dye was used as a photosensitizer, iodide/tri-iodide solution as electrolyte and FTO-coated glass as counter electrode. A significant increase in short-circuit current was observed, from 1.3 mA cm−2 for the pristine ZnO nanowire film-based DSSC to 4.4 mA cm−2 for the ZnO:Al (2.89 at.%) nanowire film-based DSSC. The overall power conversion efficiency (PCE) was also found to increase from 0.13% (for pristine ZnO nanowire thin film) to 0.49% for the ZnO:Al thin film-based DSSC.

Structural, optical properties and optical modelling of hydrothermal chemical growth derived ZnO nanowires

Abstract ZnO nanowire films were produced at 90 °C using a hydrothermal chemical deposition method, and were characterised with scanning electron microscopy, optical transmission spectrometry and X-ray diffraction. The results showed that the optical band gap is 3.274–3.347 eV. Film porosity and microstructure can be controlled by adjusting the pH of the growth solution. ZnO nanowire films comprise a 2-layer structure as demonstrated by SEM analysis, showing different porosities for each layer. XRD analysis shows preferential growth in the (002) orientation. A comprehensive optical modelling method for nanostructured ZnO thin films was proposed, consisting of Bruggeman effective medium approximations, rough surface light scattering and O'Leary-Johnson-Lim models. Fitted optical transmission of nanostructured ZnO films agreed well with experimental data.

A Facile and Flexible Approach for Large-Scale Fabrication of ZnO Nanowire Film and Its Photocatalytic Applications.

A novel strategy for large-scale synthesis of ZnO nanowire film is reported, which inherits the advantages of the solution-phase method and seeded growth process, such as low-temperature, efficient, economical, facile and flexible. It is easy to implement on various metals through room-temperature electrodeposition, followed by hydrothermal treatment at 90 °C, and suitable for industrialized production. The ZnO nanowires with an average wire diameter about 40 nm are in situ grown from and on nanocrystalline zinc coating, which forms a strong metallurgical bonding with the substrates. The p-type ZnO nanowire film has a well-preferred orientation along the (100) direction and a wurtzite structure, thereby displaying an effective photocatalytic capability for carcinogenic Cr6+ ions and CO2 greenhouse gas reduction under visible light irradiation. In addition to these features, the ZnO nanowire film is easy to recycle and, therefore, it has broad application prospects in contaminant degradation and renewable energy.

In situ growth of ZnO nanowire film on nanocrystalline zinc electrodeposit via a low-temperature hydrothermal reaction

Abstract This article reports a novel method for producing ZnO nanowire film, achieved through in situ growth on nanocrystalline zinc electrodeposit using a low-temperature hydrothermal process at 90 °C without utilization of catalyst or template. The ZnO nanowire film has an approximate wire diameter of 50 nm, showing strong response to UV illumination, leading to large UV emission and high optoelectronic activity. This alternative process for synthesis of ZnO nanowire film is simple, efficient, and easy to be extended to massive production.

Recent progress in photodetectors based on low-dimensional nanomaterials

Abstract Photodetectors (PDs) have great potential in applications of imaging, telecommunication, and biological sensing. In this article, state-of-the-art achievements on typical low-dimensional nanostructured PDs and hybrid PDs are reviewed. In the 2D nanostructured PDs part, 2D transition metal dichalcogenides have a natural gap, which promise high sensitivity of photodetection. Graphene and black phosphorus can also stand for 2D nanostructured PDs due to their broadband absorption and tunable direct bandgap, respectively. In the 1D nanostructured PDs part, owing to its high photoconductive characteristic, ZnO nanowire film is a promising material for ultraviolet PDs. Carbon nanotubes show potential in infrared (IR) detection due to its unique physical properties. In the 0D nanostructured PDs part, lead sulfide has a small bandgap and large Bohr exciton radius, which collectively give it a wide spectral tunability in the IR. In the hybrid PDs part, electrical and chemical doping is applied to combine different nanomaterials to realize PDs with high performance. In each part, the present situation and major challenges are overviewed. Then, the evolutions of the methods to overcome these challenges and the tremendous research breakthroughs are demonstrated. At last, future directions that could improve the performance of PDs are discussed.

Factors Affecting the Power Conversion Efficiency in ZnO DSSCs: Nanowire vs. Nanoparticles

A comparative assessment of nanowire versus nanoparticle-based ZnO dye-sensitized solar cells (DSSCs) is conducted to investigate the main parameters that affect device performance. Towards this aim, the influence of film morphology, dye adsorption, electron recombination and sensitizer pH on the power conversion efficiency (PCE) of the DSSCs is examined. Nanoparticle-based DSSCs with PCEs of up to 6.2% are developed and their main characteristics are examined. The efficiency of corresponding devices based on nanowire arrays (NW) is considerably lower (0.63%) by comparison, mainly due to low light harvesting ability of ZnO nanowire films. The dye loading of nanowire films is found to be approximately an order of magnitude lower than that of nanoparticle-based ones, regardless of their internal surface area. Inefficient anchoring of dye molecules on the semiconductor surface due to repelling electrostatic forces is identified as the main reason for this low dye loading. We propose a method of modifying the sensitizer solution by altering its pH, thereby enhancing dye adsorption. We report an increase in the PCE of nanowire DSSCs from 0.63% to 1.84% as a direct result of using such a modified dye solution.

A broadband photodetector based on Rhodamine B-sensitized ZnO nanowires film

A broadband photodetector has been developed on the basis of ZnO nanowires (NWs)/Rhodamine B (RhB) hybrid system. The device is fabricated by spraying NWs on to gold interdigital electrodes followed by modifying the NWs via an RhB solution-casting process. Measurements show that the as-fabricated device demonstrates photoresponsivity ranging from 300 nm to 700 nm with a bandwidth as large as 400 nm. The role of the dye sensitizer adsorbed on the surface of NWs is modeled to alter the transportation path of photo-generated carriers. The calculations based on the measurements reveal that the device exhibits a prominent responsivity in the interested band with maximum responsivity of 5.5 A/W for ultraviolet (UV) light and 3 A/W for visible (VIS) light under 8 V bias, respectively. The sensitization not only widens the response spectrum with external quantum efficiency leaping up to 771% at VIS but also improves UV responsivity with maximum 51% enhancement. From the time–dependent photo-current measurement, it is found that the response time (rise and decay times in total) of the device largely reduced from 17.5 s to 3.3 s after sensitization. A comparison of the obtained photodetector with other ZnO-based photodetectors is summarized from the view point of responsivity and bandwidth.

A simple template-free immersion process to fabricate ZnO nanowire films on nanocrystalline zinc substrate at room temperature

A novel approach for synthesizing ZnO nanowire film is reported. The film is produced by immersing a nanocrystalline zinc electrodeposit in a NaCl solution at room temperature without template required. The ZnO nanowire film has an average wire diameter of 40nm, which shows strong light absorption capability and high optoelectronic activity under UV illumination. This is a simple low-cost approach with the promise for large-scale production of ZnO nanowire films.

UV-induced terahertz wave modulation in free-standing ZnO nanowire films

We present terahertz (THz) wave modulation by using free-standing ZnO nanowire (NW) network films. The ZnO NW films were virtually transparent against THz waves without UV illumination. Conversely, the THz waves were attenuated under very low-intensity UV illumination, making the ZnO NW film a promising platform for low-loss, low-power and all-optical THz modulators. The complex dielectric constants measurements reveal that the UV laser induces an enhancement in the ac conductivity while leaving the real part of the dielectric constant unchanged. The relatively slow time response implies that the UV-induced modulation is closely linked to the surface trap states. The THz attenuations showed clear saturation behavior with respect to the UV intensity, from which we extracted the ZnO NW surface trap density.

Vertically aligned and ordered ZnO/CdS nanowire arrays for self-powered UV–visible photosensing

Photodetectors based on photoconductivity effect are usually driven by an external power source. A self-powered photodetector can be powered by incident light using the photovoltaic effect. Here, photoelectrochemical cells with periodically aligned ZnO/CdS nanowire arrays as photoanodes were fabricated and investigated for detecting UV and visible light. At zero bias, this self-powered UV–visible photodetector showed high responsivities of 35.4 and 23.2 mA/W for UV and visible light, a fast rise time of 0.18 s, and a decay time of 0.32 s. The spectral responses of the self-powered photodetectors based on ZnO/CdS nanowire arrays exhibited superior photoresponse in both UV and visible regions in comparison with ZnO nanowire film and ZnO nanowire arrays. The high photosensing performance originates from the excellent light trapping ability at broadband wavelengths and the high charge collection efficiency of the highly ordered ZnO/CdS nanowire arrays. The results indicate that the ZnO/CdS heterojunctions with periodic nanostructures provide a facile frame for UV–visible detecting applications.

Glucose biosensor based on functionalized ZnO nanowire/graphite films dispersed on a Pt electrode

We present a glucose biosensor based on ZnO nanowire self-sustained films grown on compacted graphite flakes by the vapor transport method. Nanowire/graphite films were fragmented in water, filtered to form a colloidal suspension, subsequently functionalized with glucose oxidase and finally transferred to a metal electrode (Pt). The obtained devices were evaluated using scanning electron microscopy, energy-dispersive x-ray spectroscopy, cyclic voltammetry and chronoamperometry. The electrochemical responses of the devices were determined in buffer solutions with successive glucose aggregates using a tripolar electrode system. The nanostructured biosensors showed excellent analytical performance, with linear response to glucose concentrations, high sensitivity of up to ≈17 μA cm−2 mM−1 in the 0.03–1.52 mM glucose concentration range, relatively low Michaelis–Menten constant, excellent reproducibility and a fast response. The detection limits are more than an order of magnitude lower than those achievable in commercial biosensors for glucose control, which is promising for the development of glucose monitoring methods that do not require blood extraction from potentially diabetic patients. The strong detection enhancements provided by the functionalized nanostructures are much larger than the electrode surface-area increase and are discussed in terms of the physical and chemical mechanisms involved in the detection and transduction processes.

Improvement in the Piezoelectric Performance of a ZnO Nanogenerator by a Combination of Chemical Doping and Interfacial Modification

In this work, we investigate the improvement in the piezoelectric performance of a ZnO nanowire film by a chemical doping and interfacial modification strategy. The Cl-doped ZnO nanowire films were synthesized by a modified hydrothermal approach and characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and I–V characterization. The effect of the Cl dopant with different doping concentrations has been demonstrated by an improvement of the piezoelectric output performance due to the induced lattice strain along the ZnO c-axis, which significantly facilitates the piezocharge separation under the applied stress. The experimental results indicate that the existence of lattice strain along the doped ZnO nanowire polar axis resulted from the different ionic sizes between Cl and O. More importantly, by preparing a Cl-doped ZnO nanowire film on a p-type CuO film, our...