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abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high quality substrates and reports of p type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935) studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966) detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954) and its growth by chemical vapor transport in 1970 by Galli and Coker [Appl. Phys.
author_names: Umit Ozgur

Yahya Alivov

C Liu

Ali Teke

Michael A Reshchikov

Seydi Dogan

Vitaliy Avrutin

Sang-Jun Cho

Hadis Morkoc
title: A COMPREHENSIVE REVIEW OF ZNO MATERIALS AND DEVICES
year: 2005

abstract: 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 TiO2. 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.
author_names: Chin Boon Ong

Law Yong Ng

Abdul Wahab Mohammad
title: A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications
year: 2018

abstract: Abstract Among the popular photocatalysts, ZnO is one of the most potent photocatalysts considering its green properties, cheap price, and durability. However, the practical application of ZnO is limited because of its large band gap energy and rapid recombination of the photoinduced electron hole pairs. This paper reviews the main advancements in overcoming the barriers accompanied by pure ZnO and the criteria for fabrication of effective visible light responsive ZnO based photocatalysts. Herein, the binary ZnO based nanocomposites with p n heterojunctions, n n heterojunctions, and ternary ZnO based nanocomposites based on different heterostructures, and their mechanism for enhanced light harvesting and charge separation/transfer were thoroughly discussed.
author_names: Mahsa Pirhashemi

Aziz Habibi-Yangjeh

Shima Rahim Pouran
title: Review on the criteria anticipated for the fabrication of highly efficient ZnO based visible light driven photocatalysts
year: 2018

abstract: Abstract Among the most challenging problems that human beings appear to face are depleting energy sources and increasing environmental pollutions. Heterogeneous photocatalytic processes are the most rewarding technology to generate renewable energy and degrade environmental pollutants. In these processes, semiconductors are used as photocatalysts. ZnO is a widely used photocatalyst, because of its strong oxidation ability, cost effectiveness, non toxicity, versatility in synthesis, abundance in nature, and ease of crystallization. However, pure ZnO has some drawbacks, due to its wide band gap, poor solar light utilization, and rapid recombination of the photoinduced charge carriers. Modification of ZnO using different strategies including coupling with narrow band gap semiconductors, noble metal deposition, surface sensitization by organic dyes, and elemental doping can easily address these shortcomings. In addition, separation of photocatalysts from the treated systems limits their broad applications. Incorporation of photocatalysts in magnetic materials will help their recycling using external magnetic field. This combination leads to a new generation of photocatalysts, known as magnetically separable photocatalysts. The present review provides helpful insights into preparation of magnetically separable photocatalysts based on ZnO and their applications for degradations of different pollutants.
author_names: Maryam Shekofteh-Gohari

Aziz Habibi-Yangjeh

M Abitorabi

A Maureen Rouhi
title: Magnetically separable nanocomposites based on ZnO and their applications in photocatalytic processes: A review
year: 2018

abstract: Abstract Different photocatalysts based on commercial ZnO modified by silver photodeposition were prepared in this work. The samples were characterized by X ray fluorescence spectrometry (XRF) specific surface area (SSA) transmission electron microscopy (TEM) X ray photoelectron spectroscopy (XPS) X ray diffraction (XRD) and UV vis diffuse reflectance (UV vis DRS) XRD and XPS showed that Ag/ZnO samples are composed of metallic Ag (Ag0) and ZnO structure was identified. Furthermore, TEM analysis evidenced that the number of silver particles increased with the Ag content. At last, UV vis DRS results revealed a reflectance band for Ag/ZnO samples, ascribed to the surface plasmon resonance (SPR) absorption of metal silver particles. Commercial ZnO and Ag/ZnO samples were evaluated in the phenol removal under UV light irradiation. It was observed an enhancement of photocatalytic phenol removal from aqueous solutions by silver addition in comparison to commercial ZnO. In particular, the phenol removal increased with the silver content from 0.14 to 0.88 wt% after this content (i.e 1.28 wt% the phenol degradation significantly decreased indicating that the optimal Ag content was equal to 0.88 wt% The influence of the best photocatalyst dosage and the change of the initial phenol concentration in solution were also investigated in this work and the best photocatalytic performance was obtained by using 50 mg L 1 of phenol initial concentration and 0.15 g L 1 of photocatalyst dosage. Finally, the optimized Ag/ZnO photocatalyst was employed for the treatment of a real drinking wastewater containing phenol in which the almost total phenol removal was achieved after 180 min of UV irradiation time.
author_names: Vincenzo Vaiano

Mariantonietta Matarangolo

J J Murcia

Hugo A Rojas

Jose Antonio Navio

M C Hidalgo
title: Enhanced photocatalytic removal of phenol from aqueous solutions using ZnO modified with Ag
year: 2018

abstract: Synergy between copper and zinc oxide on a catalyst surface facilitates methanol synthesis via CO2 hydrogenation. Metal oxide synergy The hydrogenation of carbon dioxide is a key step in the industrial production of methanol. Catalysts made from copper (Cu) and zinc oxide (ZnO) on alumina supports are often used. However, the actual active sites for this reaction Zn Cu bimetallic sites or ZnO Cu interfacial sites are debated. Kattel et al. studied model catalysts and found that ZnCu became as active as ZnO/Cu only after surface oxidation formed ZnO. Theoretical studies favor a formate intermediate pathway at a ZnO Cu interface active site. Science, this issue p. 1296 The active sites over commercial copper/zinc oxide/aluminum oxide (Cu/ZnO/Al2O3) catalysts for carbon dioxide (CO2) hydrogenation to methanol, the Zn Cu bimetallic sites or ZnO Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x ray photoemission spectroscopy, density functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both experimental and theoretical results agree that ZnCu undergoes surface oxidation under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.
author_names: Shyam Kattel

Pedro J Ramirez

Jingguang G Chen

Jose Antonio Rodriguez

Ping Liu
title: Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts
venue: Science
year: 2017

abstract: Abstract Photocatalytic degradation is an effective method to alleviate environmental pollution caused by organic pollutants. In this work, research progress on the application of photocatalytic degradation and the antibacterial properties of zinc oxide (ZnO) nanomaterials is reviewed. The visible light photo response of ZnO has been expanded by employing various strategies, such as enhancing the photocatalytic activity of ZnO through modification of its electronic and optical properties, doping metal/nonmetal atoms, depositing noble metals, constructing heterojunctions, and coupling carbon materials, because the wide band gap of ZnO likely restricts its applications in photocatalysis. Although ZnO nanomaterials are commonly used for antibacterial applications, our understanding on the toxicity mechanisms of ZnO is limited. Some of the main toxicity mechanisms of this compound include reactive oxygen species generation, Zn2+ release, membrane dysfunction, and nanoparticle internalization into cells. Some of the main methods that improve antibacterial activities are coating inorganic or organic antimicrobial agents, doping ZnO, and tuning the size, morphological characteristics, and concentration of ZnO nanomaterials. This review aims to examine the current research progress on ZnO based nanomaterials developed for the photocatalysis of organic contaminant degradation and antibacterial applications.
author_names: Kezhen Qi

Bei Cheng

Jiaguo Yu

Wingkei Ho
title: Review on the improvement of the photocatalytic and antibacterial activities of ZnO
year: 2017

abstract: Abstract In the last 15 years, more than 50,000 papers with zinc oxide (ZnO) in the title are listed within ISI database. The outstanding popularity of ZnO has many reasons; the most important one appears to be its multi functionality, resulting in applications in physics, chemistry, electrical engineering, material science, energy, textile, rubber, additive manufacturing, cosmetics, and pharmaceutical or medicine, as well as the ease to grow all kinds of nano and microstructures. A key structure is the tetrapod shaped ZnO (T ZnO) which we want to focus on in this mini review to demonstrate the remarkable properties and multifunctionality of ZnO and motivate why even much more research and applications are likely to come in near future. As T ZnO came into focus again mainly during the last 10 years, the big data problem in T ZnO is not as severe as in ZnO; nevertheless, a complete overview is impossible. However, this brief T ZnO overview attempts to cover the scopes toward advanced technologies; nanoelectronics/optoelectronics sensing devices; multifunctional composites/coatings; novel biomedical engineering materials; versatile energy harvesting candidates; and unique structures for applications in chemistry, cosmetics, pharmaceuticals, food, agriculture, engineering technologies, and many others. The 3D nanotechnology is a current mainstream in materials science/nanotechnology research, and T ZnO contributes to this field by its simple synthesis of porous networks as sacrificial templates for any desired new cellular materials.
author_names: Yogendra Kumar Mishra

Rainer Adelung
title: ZnO tetrapod materials for functional applications
year: 2017

abstract: The power conversion efficiency of perovskite solar cells (PSCs) has ascended from 3.8% to 22.1% in recent years. ZnO has been well documented as an excellent electron transport material. However, the poor chemical compatibility between ZnO and organo metal halide perovskite makes it highly challenging to obtain highly efficient and stable PSCs using ZnO as the electron transport layer. It is demonstrated in this work that the surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA) readily makes ZnO as a very promising electron transporting material for creating hysteresis free, efficient, and stable PSCs. Systematic studies in this work reveal several important roles of the modification: (i) MgO inhibits the interfacial charge recombination, and thus enhances cell performance and stability; (ii) the protonated EA promotes the effective electron transport from perovskite to ZnO, further fully eliminating PSCs hysteresis; (iii) the modification makes ZnO compatible with perovskite, nicely resolving the instability of ZnO/perovskite interface. With all these findings, PSCs with the best efficiency up to 21.1% and no hysteresis are successfully fabricated. PSCs stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells.
author_names: Jing Cao

Binghui Wu

Ruihao Chen

Youyunqi Wu

Yong Hui

Bingwei Mao

Nanfeng Zheng
title: Efficient, Hysteresis Free, and Stable Perovskite Solar Cells with ZnO as Electron Transport Layer: Effect of Surface Passivation.
venue: Advanced materials
year: 2018

abstract: Perovskite solar cells (PSCs) have developed rapidly over the past few years, and the power conversion efficiency of PSCs has exceeded 20% Such high performance can be attributed to the unique properties of perovskite materials, such as high absorption over the visible range and long diffusion length. Due to the different diffusion lengths of holes and electrons, electron transporting materials (ETMs) used in PSCs play a critical role in PSCs performance. As an alternative to TiO2 ETM, ZnO materials have similar physical properties to TiO2 but with much higher electron mobility. In addition, there are many simple and facile methods to fabricate ZnO nanomaterials with low cost and energy consumption. This review focuses on recent developments in the use of ZnO ETM for PSCs. The fabrication methods of ZnO materials are briefly introduced. The influence of different ZnO ETMs on performance of PSCs is then reviewed. The limitations of ZnO ETM based PSCs and some solutions to these challenges are also discussed. The review provides a systematic and comprehensive understanding of the influence of different ZnO ETMs on PSCs performance and potentially motivates further development of PSCs by extending the knowledge of ZnO based PSCs to TiO2 based PSCs.
author_names: Peng Zhang

Jiang Wu

Ting Zhang

Yafei Wang

Detao Liu

Hao Chen

Long Ji

Chunhua Liu

Waseem Ahmad

Zhi David Chen

Shibin Li
title: Perovskite Solar Cells with ZnO Electron Transporting Materials.
venue: Advanced materials
year: 2018