Persistent organic pollutants (POPs) are carbon-based chemical substances that are resistant to environmental degradation and may not be completely removed through treatment processes. Their persistence can contribute to adverse health impacts on wild-life and human beings. Thus, the solar photocatalysis process has received increasing attention due to its great potential as a green and eco-friendly process for the elimination of POPs to increase the security of clean water. In this context, ZnO nanostructures have been shown to be prominent photocatalyst candidates to be used in photodegradation owing to the facts that they are low-cost, non-toxic and more efficient in the absorption across a large fraction of the solar spectrum compared to TiO 2 . There are several aspects, however, need to be taken into consideration for further development. The purpose of this paper is to review the photo-degradation mechanisms of POPs and the recent progress in ZnO nanostructured fabrication methods including doping, heterojunction and modification techniques as well as improvements of ZnO as a photocatalyst. The second objective of this review is to evaluate the immobilization of photocatalyst and suspension systems while looking into their future challenges and prospects.

A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW þ lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral VO DFEs from the DFT calculations along the ZnO [0001] and [10 � directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [10 � directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations. V C 2013 Author(s). All article

Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations

Many methods based on spectrophotometric, fluorometric, piezoresistive, amperometric or conductive measurements have been proposed for detecting the concentration of formaldehyde in air. However, conventional formaldehyde measurement systems are bulky and expensive and require the services of highly-trained operators. Accordingly, the emergence of sophisticated technologies in recent years has prompted the development of many microscale gaseous formaldehyde detection systems. Besides their compact size, such devices have many other advantages over their macroscale counterparts, including a real-time response, a more straightforward operation, lower power consumption, and the potential for low-cost batch production. This paper commences by providing a high level overview of the formaldehyde gas sensing field and then describes some of the more significant real-time sensors presented in the literature over the past 10 years or so.

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Formaldehyde Gas Sensors: A Review

Gas sensing applications of 1D-nanostructured zinc oxide: Insights from density functional theory calculations

Gas sensing property of ZnO nanorods prepared by plasma-enhanced chemical vapor deposition (CVD) method is studied using formaldehyde as the probe gas, and the intrinsic defects are investigated by photoluminescence (PL). The results show that high ratio of visible to ultra-violet luminescence cannot account for high gas response. The PL spectra are Gaussian decomposed to subpeaks according to their origination, which are separated into donor-(DL) and acceptor-related (AL) ones. A conclusion is derived that where the content of DL is high and that of AL is low, the gas response is high. This conclusion is further confirmed by tuning the PL spectra and gas sensing property through annealing in different atmospheres. (C) 2009 Elsevier B.V. All rights reserved.

Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method

One of the challenges in realizing metal oxide semiconductor gas sensors is to enhance the sensitivity of active materials in order to respond to the low concentration of detecting gases effectively and efficiently. In this report, transition metals such as Mn. Ni, Cu, and Co are used as dopants for the synthesis of highly formaldehyde-sensitive ZnO nanorods prepared by plasma enhanced chemical vapor deposition (PECVD) method. All the doped ZnO nanorods show improved formaldehyde-sensitivity as compared to undoped ZnO nanorods, and a gas sensitivity maximum of similar to 25/ppm was obtained by using 10 mol% CdO activated 1.0 mol% Mn doped ZnO nanorods. Moreover, the ZnO nanorods have a higher sensitivity as compared to ZnO nanomaterials prepared by other methods such as precipitation and hydrothermal, which can be attributed to the abundant crystal defects induced by the dopants in a short crystallization process in this PECVD method. (C) 2012 Elsevier B.V. All rights reserved.

Highly formaldehyde-sensitive, transition-metal doped ZnO nanorods prepared by plasma-enhanced chemical vapor deposition

Sn-, Ni-, Fe- and Al-doped ZnO and pure ZnO are prepared by coprecipitation method, and characterized by scanning electron microscope (SEM), energy diffraction spectra (EDS) and X-ray diffraction (XRD). Their formaldehyde gas sensing properties are evaluated and the results show that 2.2 mol% Sn dopant can increase the response of ZnO by more than 2 folds, while other dopants increase little response or even decrease response. Further, CdO is used to activate ZnO based formaldehyde sensing material. It is demonstrated that 10 mol% CdO activated 2.2 mol% Sn-doped ZnO has the highest formaldehyde gas response, with a linear sensitivity of similar to 10/ppm at lowered work temperature of 200 degrees C than 400 degrees C of pure ZnO, and high selectivity over toluene, CO and NH(3), as well as good stability tested in 1 month. (C) 2011 Published by Elsevier B.V.

CdO activated Sn-doped ZnO for highly sensitive, selective and stable formaldehyde sensor

Nanostructured ZnO materials including Zn@ZnO core–shell structure (CS), ZnO hollow sphere (HS), and hierarchically structured ZnO hollow microsphere with nanorods (HSNR) grown on the outer surface were prepared by a simple and versatile chemical vapor deposition process. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and room temperature photoluminescence spectroscopy were used to characterize the morphology and crystalline structure of the samples. It was found that the gas-sensing property of the three structured ZnO had different responses in resistive mode using formaldehyde gas as the probe. In contrast with CS and HS samples, the HSNR exhibits better sensing property because it has more oxygen vacancies and more surface sites, which produce a higher resistance. However, the higher work function of Zn core than that of the ZnO sheath forms the ohmic contact, producing negative charge accumulating layers within ZnO side of Zn–ZnO junction, thus contributing a lower sensing response of CS model when exposed to the targeted formaldehyde gas.

Chemical vapor deposition preparation of nanostructured ZnO particles and their gas-sensing properties

We demonstrate the first example of efficient and cost-effective graphene on silicon solar cells prepared using spray coating. The spray coating process is optimized by investigating the effects of substrate temperature and graphene film thickness on device performance. With the aid of a simple hydroquinone/methanol surface passivation method, spray-coated G/Si solar cells with a power conversion efficiency of 4.41% can be obtained. Our method is faster and simpler than conventional fabrication methods, and is easy to scale up, highlighting its great potential in the mass production of efficient and low-cost G/Si solar cells.

Dangwen Zhang

Papers

Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method

Highly formaldehyde-sensitive, transition-metal doped ZnO nanorods prepared by plasma-enhanced chemical vapor deposition

CdO activated Sn-doped ZnO for highly sensitive, selective and stable formaldehyde sensor

Chemical vapor deposition preparation of nanostructured ZnO particles and their gas-sensing properties

Efficient and cost-effective graphene on silicon solar cells prepared by spray coating