Jimmy C. Yu

Crystal facet engineering of semiconductor photocatalysts: motivations, advances and unique properties

Crystal facet engineering of semiconductors has become an important strategy for fine-tuning the physicochemical properties and thus optimizing the reactivity and selectivity of photocatalysts. In this review, we present the basic strategies for crystal facet engineering of photocatalysts and describe the recent advances in synthesizing faceted photocatalysts, in particular TiO(2) crystals. The unique properties of faceted photocatalysts are discussed in relation to anisotropic corrosion, interaction dependence of adsorbates, photocatalytic selectivity, photo-reduction and oxidation sites, and photocatalytic reaction order. Ideas for future research on crystal facet engineering for improving the performance of photocatalysts are also proposed.

Effective Photocatalytic Disinfection of E. coli K-12 Using AgBr−Ag−Bi2WO6 Nanojunction System Irradiated by Visible Light: The Role of Diffusing Hydroxyl Radicals

Urgent development of effective and low-cost disinfecting technologies is needed to address the problems caused by an outbreak of harmful microorganisms. In this work, we report an effective photocatalytic disinfection of E. coli K-12 by using a AgBr-Ag-Bi(2)WO(6) nanojunction system as a catalyst under visible light (lambda >or= 400 nm) irradiation. The visible-light-driven (VLD) AgBr-Ag-Bi(2)WO(6) nanojunction could completely inactivate 5 x 10(7) cfu mL(-1) E. coli K-12 within 15 min, which was superior to other VLD photocatalysts such as Bi(2)WO(6) superstructure, Ag-Bi(2)WO(6) and AgBr-Ag-TiO(2) composite. Moreover, the photochemical mechanism of bactericidal action for the AgBr-Ag-Bi(2)WO(6) nanojunction was investigated by using different scavengers. It was found that the diffusing hydroxyl radicals generated both by the oxidative pathway and the reductive pathway play an important role in the photocatalytic disinfection. Moreover, direct contact between the AgBr-Ag-Bi(2)WO(6) nanojunction and bacterial cells was not necessary for the photocatalytic disinfection of E. coli K-12. Finally, the photocatalytic destruction of the bacterial cells was directly observed by TEM images and further confirmed by the determination of potassium ion (K(+)) leakage from the killed bacteria. This work provides a potential effective VLD photocatalyst to disinfect the bacterial cells, even to destruct the biofilm that can provide shelter and substratum for microorganisms and resist to disinfection.

Design, Fabrication, and Modification of Nanostructured Semiconductor Materials for Environmental and Energy Applications

Considerable effort has been made to design, fabricate, and manipulate nanostructured materials by innovative approaches. The precise control of nanoscale structures will pave the way not only for elucidating unique size/shape-dependent physicochemical properties but also for realizing new applications in science and technology. Nanotechnology offers unprecedented opportunities for improving our daily lives and the environment in which we live. This review mainly describes our recent progress in the design, fabrication, and modification of nanostructured semiconductor materials for environmental applications. Their potential applications in the field of energy are briefly introduced. The scope of this article covers a variety of semiconductor materials, focusing particularly on TiO(2)-based nanostructures (e.g., pure, doped, coupled, nanoporous, mesoporous, hierarchically porous, and ordered mesoporous TiO(2)). The preparation of nanoparticles, hierarchical nanoarchitectures, thin films, and single crystals by sol-gel, microemulsion, hydrothermal, sonochemical, microwave, photochemical, and nanocasting methods is discussed.

Graphene-based photocatalytic composites

The use of graphene to enhance the efficiency of photocatalysts has attracted much attention. This is because of the unique optical and electrical properties of the two-dimensional (2-D) material. This review is focused on the recent significant advances in the fabrication and applications of graphene-based hybrid photocatalysts. The synthetic strategies for the composite semiconductor photocatalysts are described. The applications of the new materials in the degradation of pollutants, photocatalytic hydrogen evolution and antibacterial systems are presented. The challenges and opportunities for the future development of graphene-based photocatalysts are also discussed.

WO3 nanorods/graphene nanocomposites for high-efficiency visible-light-driven photocatalysis and NO2 gas sensing

One-dimensional (1-D) nanostructures are of great importance due to their superior charge transport properties. Anchoring 1-D semiconductor nanomaterials on graphene offers potential advantages in photoelectrochemical and sensing applications. This paper presents a systematic investigation on the incorporation of WO3 nanorods and graphene for high-efficiency visible-light-driven photocatalysis and NO2 gas sensing. This novel composite shows remarkably enhanced performance compared to pure WO3 nanorods for these applications. The high photocatalytic activity of the WO3/graphene nanocomposite is found to be related to the increased adsorption toward chemical species, enhanced light absorption and efficient charge separation and transfer. Meanwhile, the improved conductivity, specific electron transfer and increased gas adsorption also contribute to their superior sensitivity and selectivity to NO2 gas.

Degradation of Acid Orange 7 using magnetic AgBr under visible light: the roles of oxidizing species.

AgBr was creatively immobilized on a magnetic substrate (SiO(2)-coated Fe(3)O(4) nanoparticle, SFN) to achieve magnetic separation after visible light-driven photocatalytic oxidation (PCO). The resulted Ag/AgBr/SFN was characterized by TEM, vibrating sample magnetometer and other techniques. It is found that the average diameter of the Ag/AgBr/SFN particle is less than 20 nm. The typical superparamagnetic behavior of Ag/AgBr/SFN implies that the catalyst can be magnetically separated. The physicochemical features of the used Ag/AgBr/SFN after visible light irradiation were not dramatically changed by X-ray diffraction, UV-Vis diffuse reflectance spectra and Fourier transform-infrared analysis. SiO(2) interlayer was proven to slightly increase the degradation efficiency for an azo dye Acid Orange 7. UV-Vis spectra and HPLC analysis indicated that the dye was oxidized and decomposed. The photoactivity of Ag/AgBr/SFN was partly maintained after successive PCO under visible light. In order to evaluate the roles of e(-)-h(+) pairs and reactive oxygen species, the quenching effect was examined by employing Ag/AgBr/SFN and commercial TiO(2) (P-25) under visible light (lambda>400 nm) and UV-A irradiation, respectively. Active h(+) and the resulting (*)OH played the major roles for degradation. The effect of active h(+) and (*)OH were proven to be highly dependent on the concentration of photocatalysts. The effect of (*)OH was more obvious for P-25, while that of active h(+) was more predominant for Ag/AgBr/SFN.

Degradation of azo dye Procion Red MX-5B by photocatalytic oxidation

The photocatalytic oxidation (PCO) of a monoazo dye Procion Red MX-5B under various physico-chemical conditions was investigated. Degradation of the dye by PCO was enhanced by augmentation in UV intensity, titanium dioxide and hydrogen peroxide concentrations but was inhibited by increase in initial dye concentration. The PCO process was affected by pH in a peculiar way. In the presence of 100 mg/l of TiO2 and the absence of H2O2, the highest reaction rate was observed when the initial pH was 10. With 500 mg/l of TiO2 and 10 mM of H2O2, the reaction was the fastest at initial pH of 3-5. The optimal conditions for the degradation of the dye, at an UV intensity of 17 mW/cm2, were determined to be: TiO2 concentration, 500 mg/l; initial H2O2 concentration, 10 mM; initial pH, 5.0. Monitoring of TOC loss showed that the dye was mineralized by 90% within 80 min under these conditions. Nevertheless, the persistence of a low level of TOC indicated that mineralization was not complete and dead-end product(s) which was (were) resistant to PCO might have accumulated.

Characterization of chemical species in PM2.5 and PM10 aerosols in Hong Kong

Aerosol samples for PM10 and PM2.5 were collected in wintertime from November 2000 to February 2001 at three different sampling locations in Hong Kong. PM10 and PM2.5 were collected by high-volume (hi-vol.) samplers and the concentrations of major elements, ions, organic and elemental carbons were quantified. The ratios of PM2.5/PM10 were 0.61 and 0.78 at the PolyU campus and Kwun Tong (KT), respectively. These results indicated that the concentrations of PM2.5 contribute the majority of the PM10 fraction. The concentrations of anthropogenic species (e.g. Pb and Cu) in PM10 and PM2.5measured at urban areas were generally higher than at an urban background site (Hok Tsui, HT). The major fractions of sulfate at three monitoring sites are non-sea-salts (nss) sulfates. Although HT is located in coastal areas, the contribution of sea salts to sulfate in fine particles was small, indicating a substantial anthropogenic origin. The OC/EC ratios were less than 2 in PolyU and KT monitoring stations for PM10 and PM2.5. However, the OC/EC ratios were over 3 at HT for both PM10 and PM2.5. This indicates the presence of secondary organic aerosols. Correlations between OC and EC as well as OC and sulfate in HT during both seasons were used to prove that atmospheric transport and transformation of anthropogenic organic species from northeastern area was the dominant source in winter. The chemical composition of the samples was reconstructed from the observed elemental composition. The contribution of the seven components, namely crustal matter, sea salt, ammonium, sulfate, nitrate, elemental carbon and organic matter accounted for 77–84% of the PM10 and PM2.5 mass in the urban area (PolyU and KT) and 74% for Hok Tsui (HT). Sulfate, organic matter and elemental carbon are the major constituents in particles especially in PM2.5 collected at PolyU and KT. The major constituents of PM10 in HT are sea salt and sulfate.

Preparation, Characterization, and Catalytic Activity of Core/Shell Fe3O4@Polyaniline@Au Nanocomposites

We report a new method to synthesize magnetically responsive Fe3O4@polyaniline@Au nanocomposites. The superparamagnetic Fe3O4@polyaniline with well-defined core/shell nanostructure has been synthesized via an ultrasound-assisted in situ surface polymerization method. The negatively charged Au nanoparticles with a diameter of about 4 nm have been effectively assembled onto the positively charged surface of the as-synthesized Fe3O4@polyaniline core/shell microspheres via electrostatic attraction. The morphology, phase composition, and crystallinity of the as-prepared nanocomposites have been characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The central Fe3O4 cores are superparamagnetic at room temperature with strong magnetic response to externally applied magnetic field, thus providing a convenient means for separating the nanocomposite from solution. As-prepared inorganic/organic nanocomposite can be used as a magnetically recoverable nanocatalyst for the reduction of a selected substrate.

An investigation on photocatalytic activities of mixed TiO2-rare earth oxides for the oxidation of acetone in air

The photocatalytic activities of the mixtures of TiO2 (P25) with three rare earth oxides were investigated. The effects of the rare earth oxide contents and calcination temperature on the photocatalytic activities were studied. The results reveal that the mixtures of TiO2 with La2O3 (0.5 wt.%) or Y2O3 (0.5 wt.%) calcined at 700 °C or 650 °C exhibit higher photoactivity than pure TiO2 (P25) for the oxidation of acetone. On the other hand, the mixtures of TiO2 with CeO2 have lower photoactivity than pure TiO2. Experimental results of polycrystalline X-ray diffraction, photoexcited transient absorption decay and zeta potential measurements show that the presence of these rare earth oxides can inhibit anatase to rutile transformation at elevated temperatures. The lower photocatalytic activity of TiO2/CeO2 mixtures can be explained by the fast recombination rate of photogenerated electron-hole pairs and high isoelectric point in terms of pH value. The preparation method of these mixtures is also described.