We report a kind of gas sensor using ZnO nanowires as the field ionization anode. The sharp tips of nanowires generate very high electric fields at relatively low voltages. The sensors show good sensitivity and selectivity. Moreover, the detection limitation of the field ionization based ZnO nanowire gas sensors is about 5%. More importantly, a sensor with ZnO nanowires as the anode exhibits an impressive performance with respect to stability and anti-oxidation behavior, which are significantly better than those of carbon nanotubes (CNTs) as electrodes. Therefore, the simple, low-cost, sensors described here could be deployed for a variety of applications.
A low-cost and catalyst-free two-step approach has been developed to produce ZnO nanotubes (ZNTs) by simple thermal oxidation of Zn nanowires under 20 Pa at a low temperature of 400 °C. The growth mechanism of ZNTs is discussed in detail. The formation of these tubular structures is closely linked to the oxidation pressure and temperature, which involves a process consisting of the deposition of Zn nanowires, cracking of the Zn nanowires and sublimation of the Zn cores, and subsequent oxidation to ZNTs. The optical properties were studied by using Raman and photoluminescence spectra, where a strong green emission related to the single ionized oxygen vacancy appears. The photocatalytic activity measurement indicates an enhanced photocatalytic activity of the prepared ZNTs due to their high surface-to-volume ratios and abundant oxygen vacancies near the surfaces of the ZNTs. This type of high surface area structural ZNTs could find promising potential for optoelectronic and environmental applications.
We experimentally showed ferromagnetism at above 300 K for ZnO nanowires prepared by reactive vapour deposition and subsequently implanted with 2 at% of Cu. The magnetic moments of the samples annealed in oxygen and in argon at 600 °C for 120 min are about 0.42 µB and 0.33 µB per Cu atom, respectively. No ferromagnetism-related secondary phase existed according to the results of x-ray diffraction, x-ray photoelectron spectroscopy and transmission electron microscopy including HRTEM and EELS mapping. It can be concluded that the d9 configuration in the ground state of Cu2+ plays an important role in ferromagnetism. This controllable and shallow doped ion implantation technique may find potential applications in spintronic, especially nano-spintronic, devices.
The Zn1−xCoxO and Al-doped Zn1−xCoxO nanowires have been synthesized by the water-assistance reactive vapor deposition method. No Co precipitates appeared in the both kinds of ZnO nanowires according to the results observed by high-resolution transmission electron microscope, x-ray diffraction, and electron energy-loss spectroscopy, revealing that the magnetism of the ZnO nanowires was independent of the Co precipitates. The Al-doped Zn1−xCoxO nanowires show much stronger ferromagnetism than Zn1−xCoxO nanowires in the temperature range of 4–350 K. It is believed that the ferromagnetism in Zn1−xCoxO nanowires could be enhanced by Al doping to introduce donor carriers. Moreover, a multiple mechanism for the ferromagnetism of Al-doped Zn1−xCoxO nanowires is discussed.
Using ZnO buffer layers prepared by simply thermal oxidation of ion beam sputtered Zn films, highly oriented and uniformly aligned single-crystalline ZnO micropillars arrays have been synthesized by thermal evaporation of Zn powder with free catalysts at low temperature of 430 ℃. The ZnO micropillars show sharp hexagonal umbrella-like tips with thin ZnO nanowire grown on the tips. The umbrella-like tips grow in a layer-by-layer mode along the direction of [001]. The growth mechanism has been discussed. The formation of the micropillars basically depends on the gradually decreasing Zn vapor pressure and subsequently cooling process. The photoluminescence (PL) spectrum indicates a moderately good crystal quality of the ZnO micropillars. Our results may reinforce the understanding of the formation mechanism of different ZnO nano/microstructures. This kind of complex microstructures may find potential applications in multifunctional microdevices, optoelectronic and field emission devices.One-dimensional wide band-gap nanowires have attracted much interest because of their remarkable physical properties and their potential applications in constructing nanoscale electronic and optoelectronic devices. Hence, the synthesis of semiconducting nanostructures with different controllable dimension and morphology is very important and challenging [1] . ZnO is one of promising semiconducting materials for potential applications due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV). Over the past few years, a number of different nanostructure morphologies have been reported. Obtained morphologies include nanowires, tubes, rings [2] , nails [3] , screws [4] , and so on. Recently, considerable effort has also been made to fabricate well-ordered ZnO nanoarrays. An ordered one-dimensional structure of ZnO nanoarrays are highly interesting in laser emission, gas sensor, and field emission devices. Wang et al and Liu et al fabricated well-aligned ZnO nanowires using porous alumina and porous silica membranes respectively [5,6] , but the applications of nanowire arrays were limited by the difficulties in separating nanowires from the templates introduced in the synthesis. Jie et al also demonstrated that aligned ZnO nanowires could be formed on ZnO substrate [7] . Unfortunately, the prepared nanowires were rather thick and the used substrates were c-oriented ZnO film grown on expensive sapphire.
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