Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

TiO 2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO 2 photocatalysis. In this review, recent developments in the area of TiO 2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO 2 material can affect its properties and functions, including its photocatalytic performance, and also more specifically its surface area, adsorption, reflectance, adhesion, and carrier transportation properties. We provide a brief introduction to the current situation in TiO 2 photocatalysis, and describe structurally controlled TiO 2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO 2 surfaces for the fabrication of wettability patterns and for printing are discussed.

TiO2 photocatalysis: Design and applications

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

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

ZnO nanorods were fabricated by a simple low-temperature hydrothermal process in high yield (about 85%), starting with Zn(OH) 4 2− aqueous solution in the presence of CTAB, the CTAB serving as a structure director, and no calcination process was needed. The morphology and crystal structure of the prepared ZnO nanorods were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM) and Transmission electron microscope (TEM). The ZnO nanorods were then used to construct a gas sensor for ethanol detection at different operating temperature. The as-prepared ZnO nanorod gas sensor exhibited a high, reversible and fast response to ethanol, indicating its potential application as a gas sensor to detect ethanol.

ZnO nanorod gas sensor for ethanol detection

Hierarchically three-dimensional (3D) porous ZnO architectures were synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. First, a precursor of hierarchical basic zinc carbonate (BZC) nanostructures self-assembled by sheet-like blocks was prepared. Then calcination of the precursor produced hierarchically 3D porous ZnO architectures composed of interconnected ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursor. The products were characterized by X-ray diffraction, Fourier tranform infrared spectroscopy, thermogravimetric−differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer−Emmett−Teller N2 adsorption−desorption analyses. Control experiments with variations in solvent and reaction time respectively revealed that ethanol was responsible for the formation of the BZC precursor, and the self-assembly of BZC nanosheets into hierarchically 3D architectures was h...

Hierarchically Porous ZnO Architectures for Gas Sensor Application

A novel sensing hybrid-material of Au nanoparticles (Au NPs)-functionalized ZnO nanowires (Au–ZnO NWs) was successfully synthesized by a two-stage solution process. First, ZnO NWs were fabricated via a low-temperature one-pot hydrothermal method with SDSN introduced as a structure-directing agent. Afterward, the as-prepared ZnO NWs were used as supports to load Au NPs with small sizes via precipitating HAuCl4 aqueous solution with ammonia. The obtained samples were characterized by means of XRD, SEM, TEM and EDX. Both pristine and Au–ZnO NWs were practically applied as gas sensors to compare the effect of Au NPs on the sensing performances and the obtained results demonstrated that after functionalization by catalytic Au NPs, the hybrid sensor exhibited not only faster response and recovery speeds but also a higher response to benzene and toluene than the pristine ZnO sensor at 340 °C, especially showing high selectivity and long-term stability for low concentration toluene, which is rarely reported with this method, indicating its original sensor application in detecting benzene and toluene. To interpret the enhanced gas sensing mechanism, the strong spillover effect of the Au NPs and the increased Schottky barriers caused by the electronic interaction between Au NPs and ZnO NW support are believed to contribute to the improved sensor performance.

A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene

ZnO hollow spheres were successfully prepared by using carbon microspheres as templates and characterized by X-ray diffraction (XRD) analysis, scanning electron microscope (SEM), transmission electron microscope (TEM) and selected area electron diffraction (SAED). A gas sensor was fabricated from the as-prepared ZnO hollow spheres and tested to different concentrations of NH3 and NO2 at different operating temperatures. The results showed that the ZnO hollow-sphere sensor exhibited extremely different sensing behaviors to NH3 and NO2. The optimum operating temperature of the sensor was 220 °C for NH3 and 240 °C for NO2, respectively. At 220 °C, the responses to 25, 50 and 75 ppm NH3 were 7.9, 11.1 and 20.4 and the response times were as long as several minutes. At 240 °C, the responses to 10, 50 and 100 ppm NO2 were 140.6, 172.8 and 286.8 and the corresponding response times were 31, 19 and 9 s. In addition, it is also shown that the gas sensor exhibited much higher response to NO2 than to other gases at 240 °C, implying the good selectivity and potential application of the sensor for detecting NO2.

ZnO hollow spheres: Preparation, characterization, and gas sensing properties

This work examined the possibility of using SnO 2 hollow spheres mediated by carbon microspheres for the efficient detection of NO 2 gas. The influences of gas concentration, operating temperature, crystallite (grain) size and structure feature on the sensing performance of the sensors based on SnO 2 hollow spheres were comprehensively studied. The SnO 2 hollow spheres had a mean crystallite size of 12.7, 14.1 and 22.8 nm when calcined at 450, 500 and 550 °C, respectively. It was found that the SnO 2 hollow spheres calcined at 450 °C with a crystallite size of 12.7 nm were the most sensitive to NO 2 gas. The sensor also had a better selectivity to NO 2 than to ethanol, methanol, gasoline, acetone, CCl 4 and NH 3 when operated at 160 °C.

Weiping Huang

Papers

ZnO nanorod gas sensor for ethanol detection

Hierarchically Porous ZnO Architectures for Gas Sensor Application

A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene

ZnO hollow spheres: Preparation, characterization, and gas sensing properties

NO2 sensing performance of SnO2 hollow-sphere sensor