One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Photoluminescence of ZnO Nanowires: A Review.

Zinc oxide (ZnO) is a fascinating wide band gap semiconductor material with many properties that make it widely studied in the material science, physics, chemistry, biochemistry, and solid-state electronics communities. Its transparency, possibility of bandgap engineering, the possibility to dope it into high electron concentrations, or with many transition or rare earth metals, as well as the many structures it can form, all explain the intensive interest and broad applications. This review aims to showcase ZnO as a very versatile material lending itself both to bottom-up and top-down fabrication, with a focus on the many devices it enables, based on epitaxial structures, thin films, thick films, and nanostructures, but also with a significant number of unresolved issues, such as the challenge of efficient p-type doping. The aim of this article is to provide a wide-ranging cross-section of the current state of ZnO structures and technologies, with the main development directions underlined, serving as an introduction, a reference, and an inspiration for future research.

ZnO as a Functional Material, a Review

Review of ZnO-based nanomaterials in gas sensors

Acetone is a well-known volatile organic compound that is widely used in different industrial and domestic areas. However, it can have dangerous effects on human life and health. Thus, the realization of sensitive and selective sensors for recognition of acetone is highly important. Among different gas sensors, resistive gas sensors based on nanostructured metal oxide with high surface area, have been widely reported for successful detection of acetone gas, owing to their high sensitivity, fast dynamics, high stability, and low price. Herein, we discuss different aspects of metal oxide-based acetone gas sensors in pristine, composite, doped, and noble metal functionalized forms. Gas sensing mechanisms are also discussed. This review is an informative document for those who are working in the field of gas sensors.

https://www.mdpi.com/1424-8220/20/11/3096/pdf

Nanostructured Metal Oxide-Based Acetone Gas Sensors: A Review

Rapid detection of low concentration CO using Pt-loaded ZnO nanosheets.

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

The design of hierarchical zinc oxide (ZnO) microstructures modified with silver (Ag) nanoparticles have emerged as an effective approach for improving the hydrogen gas sensing performance. Here, we report a simple, low-cost chemical co-precipitation method to obtain the Ag/ZnO hollow microstructures and morphologically characterized them by field emission scanning electron microscopy (FESEM). FESEM images revealed the clear hollow hexagonal tube-like morphology. The existence of Ag in ZnO was confirmed by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). UV visible absorption and photoluminescence (PL) spectra were also recorded to observe the effect of Ag nanofiller on the optical properties of ZnO. Furthermore, the gas sensing properties of the as-prepared bare ZnO and Ag/ZnO sensor were investigated. The thoroughly sensing experiments demonstrated that after modification with Ag nanoparticles the ZnO sensor shows superior sensitivity 479 % towards 300 ppm hydrogen gas concentration, whereas 101 % response was noted for the pure ZnO sensor at 250 °C working temperature. Meanwhile, Ag/ZnO hybrids exhibited excellent selectivity, fast response, and recovery time and also obtained a good and stable response signal at 5 ppm H2 exposure, which indicated that the lower concentration measurement is also attainable. This enhancement in the sensing performance of Ag/ZnO structures towards hydrogen is due to the chemical and electronic sensitization effect of Ag nanoparticles. As a result, such microstructure is attributed to more oxygen species and active sites, and it enhances the sensitivity of a sensor. Moreover, this type of hybrid opens a new path and supports the next generation of innovative materials to fabricate highly selective and sensitive H2 gas sensing devices.

An efficient hydrogen gas sensor based on hierarchical Ag/ZnO hollow microstructures

Zinc oxide nanostructures with three different morphologies, namely, nanoparticles (NP), nanorods (NR) and nanosheets (NS) were synthesized using chemical co-precipitation method. X-ray diffraction patterns showed that all the ZnO nanostructures had hexagonal wurtzite structures. The average crystallite size of NP, NS, and NR was found to be 25, 27 and 35 nm, respectively. The morphology and size of nanostructures were confirmed by scanning electron microscopy and transmission electron microscopy. The optical properties of zinc oxide nanostructures were investigated using UV–Vis absorption spectroscopy and photoluminescence (PL) spectroscopy. The optical band gap was found to be 3.26, 3.24 and 3.10 eV for NP, NS, and NR nanostructures, respectively. It was clearly observed from the PL spectrums that luminescence intensity was maximum for NP in the UV region and maximum for NS in the visible region. It is evident from the results that by changing the size and morphology of ZnO nanostructures, the optical properties can be tuned according to their desired applications.

Morphology-dependent structural and optical properties of ZnO nanostructures

This article deals with the study of some mechanical properties of ZnS/poly(methyl methacrylate) nanocomposites prepared by solution casting method. The obtained ZnS/PMMA nanocomposites have ZnS nanoparticles in (0, 2, 4, 6, and 8) wt % and characterized through X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and Fourier transform infrared (FTIR) spectroscopy measurements. Mechanical properties of ZnS/PMMA nanocomposites have been determined at different temperatures (30°C, 50°C, 70°C, and 90°C) through their stress–strain behavior using dynamic mechanical analyzer (DMA). The properties have been found to increase upto 6 wt % of ZnS nanoparticles and then decrease for 8 wt % of ZnS nanoparticles. A theoretical model has also been employed to predict the strain softening and strain hardening of the material. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of ZnS nanofiller and temperature on mechanical properties of poly(methyl methacrylate)

Zinc sulphide/poly(methyl methacrylate) (ZnS/PMMA) nanocomposites with different (0, 2, 4, 6 and 8) wt% of ZnS nanoparticles have been prepared by solution casting method. The obtained ZnS/PMMA nanocomposites have been characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Transient plane source (TPS) technique was used to determine the effective thermal conductivity of ZnS/PMMA nanocomposites over the temperature range from room (30° C) to 120° C. The results indicated that the effective thermal conductivity shows increasing behavior up to glass transition temperature beyond which, it becomes constant due to the straightening of chains and vacant site scattering of phonons, respectively. It is also observed that effective thermal conductivity of ZnS/PMMA nanocomposites increases with the increase of concentration of ZnS nanoparticles up to 6 wt% and decreases at 8 wt%. This increase and decrease of thermal conductivity of these nanocomposites are explained on the basis of their structure.

/pdf/study-on-effective-thermal-conductivity-of-zinc-sulphide-k4f42hptto.pdf

Sonalika Agarwal

Papers

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

An efficient hydrogen gas sensor based on hierarchical Ag/ZnO hollow microstructures

Morphology-dependent structural and optical properties of ZnO nanostructures

Effect of ZnS nanofiller and temperature on mechanical properties of poly(methyl methacrylate)

Study on effective thermal conductivity of zinc sulphide/poly(methyl methacrylate) nanocomposites