Nanostructured ZnO chemical gas sensors

Here, we report the hydrothermal synthesis of flower-shaped ZnO nanostructures and investigated their morphology-dependent gas sensing properties. Scanning electron microscope (SEM) study confirmed the formation of two kinds of floral structures. At short reaction time, flower-like structures (2–3 μm in size) composed of nanoparticles are formed, whereas floral assemblies (˜ 5 μm) of nanorods are formed at long reaction time. X-ray diffraction (XRD) confirmed the formation of the hexagonal wurtzite structure of ZnO. The average crystallite size of prepared nanoflowers and nanorods were found to be 21 nm and 43 nm, respectively. These results are supported by transmission electron microscopy (TEM). The band gap of ZnO nanostructures was calculated from the UV–vis absorption spectrum and found to be 3.0 eV and 3.19 eV for ZnO nanoflowers and nanorods, respectively. Broad absorption peak in the visible region of photoluminescence (PL) spectra confirmed the presence of oxygen vacancies in both specimens. Furthermore, morphology dependent gas sensing property was investigated for ethanol, benzene, carbon monoxide, and nitrogen dioxide at different operating temperatures and concentrations. Although both morphologies have shown good sensitivity and selectivity towards NO2 at ppb, the response of nanoflower was higher than that of nanorods, which was attributed to its relatively higher surface area and amount of surface defects.

Gas sensing properties of ZnO nanostructures (flowers/rods) synthesized by hydrothermal method

Designing composite nanomaterials is one of the most important key techniques for improving gas sensing performance. In this paper, a design of the ethanol sensor based on ZnO–SnO 2 heterostructure is reported. The hollow SnO 2 nanofibers are first synthesized by using the electrospinning method, and then the ZnO shell is subsequently grown on the fibers via the hydrothermal method. The ZnO–SnO 2 core–shell structure is confirmed by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), scanning electron microcopy (SEM), transmission electron microscopy (TEM) and elemental mapping analysis. The gas sensing behaviors of the fabricated sensors are systematically investigated. Under optimum operating temperature (200 °C) at 100 ppm ethanol, the response of ZnO–SnO 2 sensor is 392.29, which is 11 times larger than that of SnO 2 sensor (about 35.02). The response and recovery time of ZnO–SnO 2 sensor are 75 s and 12 s, while that of SnO 2 sensor are 86 s and 14 s, respectively. The results reveal that ZnO–SnO 2 core–shell structure enhances the sensing performance and shortens the response/recovery time, which is attributed to unique hollow structure, oxygen vacancies and n–n heterojunction. In addition, the energy band structure of ZnO–SnO 2 heterojunction and the ethanol sensing mechanism are analyzed.

Enhanced ethanol sensing performance of hollow ZnO–SnO2 core–shell nanofibers

Preparation of Pd nanoparticle-decorated hollow SnO2 nanofibers and their enhanced formaldehyde sensing properties

The pencil-like ZnO hollow tubes with 9–12 μm in length, 350–700 nm in width, 200 nm in wall thickness coating with g-C3N4 have been prepared via a chemical deposition process. As compared with uncoated ZnO or g-C3N4, these g-C3N4/ZnO composites showed the enhanced photocatalytic activity which can be attributed to the heterojunction structure. Furthermore, it is worth pointing out that the weight ratios of g-C3N4 to ZnO (g-C3N4/ZnO) played a significantly influence on the photodegradable properties. With increasing the mass ratio, the photocatalytic activity increased firstly and then decreased after reaching to an optimal photocatalytic performance. It can be inferred that the appreciation of g-C3N4 on the ZnO surface can improve the contact area which resulted in high separation of electrons and holes. However, excessive g-C3N4 may hinder the electrons transferring from the g-C3N4 to ZnO, and thus worse its photocatalytic performance. In our study, the g-C3N4/ZnO sample prepared with 10 wt% of g-C3N4 exhibited the optimal photodegradable efficiency which 94% of Rhodamine B (RhB) has been degraded just in 2 h.

Enhancement photocatalytic activity of the graphite-like C3N4 coated hollow pencil-like ZnO

Effect of annealing process in tuning of defects in ZnO nanorods and their application in UV photodetectors

Encapsulated ZnO nanorod arrays are synthesized on glass substrate by a two-step route, hydrothermal followed by dip coating It was found that the encapsulating process increases the rod diameters from ∼20 to ∼40nm without any significant effects on the microstructure, even with an Fe-doped ZnO encapsulated layer Optical studies indicated a reduction in the band gap of the encapsulated nanorods The electrical measurements illustrated a remarkable reduction in resistance due to the encapsulation process, especially in the encapsulated ZnO nanorod film with an Fe-doped-ZnO shell layer The gas-sensing properties of the sensors were systematically investigated to different concentrations of ethanol vapour at temperatures up to 270 ◦ C An optimum operating temperature was found for each sensor in which high sensitivity and fast recovery were obtained The response was improved from 117 to 19 by Fe doping on the ZnO encapsulated nanorod at a constant 500ppm concentration of ethanol The response threshold decreased also by encapsulating at 165 ◦ C

https://iopscience.iop.org/article/10.1088/0022-3727/47/7/075003/pdf

A. Khayatian

Papers

Effect of annealing process in tuning of defects in ZnO nanorods and their application in UV photodetectors

Enhanced gas-sensing properties of ZnO nanorods encapsulated in an Fe-doped ZnO shell

Enhancing photoresponsivity of ultra violet photodetectors based on Fe doped ZnO/ZnO shell/core nanorods

Diameter-controlled synthesis of ZnO nanorods on Fe-doped ZnO seed layer and enhanced photodetection performance

Improving ultraviolet photodetection of ZnO nanorods by Cr doped ZnO encapsulation process