https://chemistry.beloit.edu/classes/nanotech/ZnO/mattoday10_05_40.pdf

ZnO - nanostructures, defects, and devices

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

/pdf/one-dimensional-zno-nanostructures-solution-growth-and-30xs4d6be7.pdf

One-dimensional ZnO nanostructures: Solution growth and functional properties

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

Hydrothermal technology for nanotechnology

Different shapes of ZnO microcrystals have been achieved controllably by a capping-molecule-assisted hydrothermal process. The flowerlike, disklike, and dumbbell-like ZnO microcrystals of hexagonal phase have been obtained respectively using ammonia, citric acid (CA), and poly(vinyl alcohol) (PVA) as the capping molecules. Only a very strong UV emission at ∼380 nm is observed in the photoluminescence (PL) spectra of the three kinds of ZnO microcrystals, indicative of their high crystal quality. The formation mechanisms for the hydrothermally synthesized microcrystals in different morphologies have been phenomenologically presented.

Controllable growth of ZnO microcrystals by a capping-molecule-assisted hydrothermal process

Gas sensors based on the Fe 2 O 3 –ZnO nanocomposites with different compositions of Fe:Zn was prepared by a sol–gel and spin-coating method. Morphology of the Fe 2 O 3 –ZnO nanocomposites was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD, D/max-rA) and energy dispersive X-ray analysis (EDX). The results of electrical and sensing measurement indicated that the sensor with Fe:Zn = 2% exhibited fairly excellent sensitivity and selectivity to NH 3 at room temperature. The response and recovery time of the sensor were both less than 20 s. Finally, the mechanism for the improvement in the gas sensing properties was discussed.

A selective NH3 gas sensor based on Fe2O3–ZnO nanocomposites at room temperature

Controllable growth of ZnO nanostructures by citric acid assisted hydrothermal process

CuO nanodendrites have been synthesized by a simple and novel hydrothermal method. Their morphology, structure, and composition have been also characterized. The results show that an as-prepared nanodendrite is composed of a main branch-like nanorod near upon a micrometre in length and about 100 nm in diameter and the sub-branch nanorods several hundred nanometres in length and 20–60 nm in diameter. The dendrite-like CuO nanostructures are of monoclinic phase and single crystalline in nature. The investigation of the hydrothermal process assisted by different quantities of ethylene glycol (EG) indicates that EG plays a critical role in the formation of dendrite-like CuO nanostructures. A possible mechanism for the formation of such CuO nanostructures is discussed. Furthermore, the correlation between the optical spectrum and the different morphologies of copper oxides nanomaterials is also discussed.

https://iopscience.iop.org/article/10.1088/0957-4484/15/11/007/pdf

CuO nanodendrites synthesized by a novel hydrothermal route

Airplane-like FeO(OH) nanostructures have been first synthesized by an ethylene glycol (EG) assisted hydrothermal process and sequential annealing at 600 °C for 1 h. The as-prepared products were investigated by X-ray diffraction (XRD), UV−vis spectroscopy (UV), Raman spectroscopy, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). It is revealed that the single crystalline orthorhombic FeO(OH) nanostructures prepared by the EG-assisted hydrothermal process with airplane-like morphology can be easily transformed to single-crystalline hexagonal Fe2O3 nanostructures with airplane-like morphology. The critical role of EG in the hydrothermal synthesis of airplane-like nanostructures is discussed.

Shenzhong Li

Papers

Controllable growth of ZnO microcrystals by a capping-molecule-assisted hydrothermal process

A selective NH3 gas sensor based on Fe2O3–ZnO nanocomposites at room temperature

Controllable growth of ZnO nanostructures by citric acid assisted hydrothermal process

CuO nanodendrites synthesized by a novel hydrothermal route

Shape-Control Fabrication and Characterization of the Airplane-like FeO(OH) and Fe2O3 Nanostructures