Huaiyong Zhu

An Efficient Photocatalyst Structure: TiO2(B) Nanofibers with a Shell of Anatase Nanocrystals

A new efficient photocatalyst structure, a shell of anatase nanocrystals on the fibril core of a single TiO(2)(B) crystal, was obtained via two consecutive partial phase transition processes. In the first stage of the process, titanate nanofibers reacted with dilute acid solution under moderate hydrothermal conditions, yielding the anatase nanocrystals on the fiber. In the subsequent heating process, the fibril core of titanate was converted into a TiO(2)(B) single crystal while the anatase crystals in the shell remained unchanged. The anatase nanocrystals do not attach to the TiO(2)(B) core randomly but coherently with a close crystallographic registry to the core to form a stable phase interface. For instance, (001) planes in anatase and (100) planes of TiO(2)(B) join together to form a stable interface. Such a unique structure has several features that enhance the photocatalytic activity of these fibers. First, the differences in the band edges of the two phases promote migration of the photogenerated holes from anatase shell to the TiO(2)(B) core. Second, the well-matched phase interfaces allow photogenerated electrons and holes to readily migrate across the interfaces because the holes migrate much faster than excited electrons, more holes than electrons migrate to TiO(2)(B) and this reduces the recombination of the photogenerated charges in anatase shell. Third, the surface of the anatase shell has both a strong ability to regenerate surface hydroxyl groups and adsorb O(2), the oxidant of the reaction, to yield reactive hydroxyl radicals (OH(.)) through reaction between photogenerated holes and surface hydroxyl groups. The adsorbed O(2) molecules can capture the excited electrons on the surface, forming reactive O(2)(-) species. The more reactive species generated on the external surface, the higher the photocatalytic activity will be, and generation of the reactive species also contributes to reducing recombination of the photogenerated charges. Indeed, the mixed-phase nanofibers exhibited superior photocatalytic activity for degradation of sulforhodamine B under UV light to the nanofibers of either pure phase alone or mechanical mixtures of the pure phase nanofibers with a similar phase composition. Finally, the nanofibril morphology has an additional advantage that they can be separated readily after reaction for reuse by sedimentation. This is very important because the high cost for separating the catalyst nanocrystals has seriously impeded the applications of TiO(2) photocatalysts on an industrial scale.

Visible‐Light‐Driven Oxidation of Organic Contaminants in Air with Gold Nanoparticle Catalysts on Oxide Supports

One of the great challenges in catalysis is to devise new catalysts that have high activity when illuminated by visible light. Solving this challenge will allow us to use sunlight, an abundant and clean low-cost energy source, to drive chemical reactions. Visible light (wavelength l> 400 nm) constitutes around 43% of solar energy, and the energy of sunlight to the Earth is about 10000 times more than the current energy consumption of the world. Many approaches have been proposed to develop visible light photocatalysts, including doping TiO2 with metal ions or metal atom clusters, [4,5] incorporating nitrogen and carbon into TiO2, and employing other metal oxides or polymetallates as catalyst materials. 5,8] Research has been mainly concentrated on semiconductor oxides. Sulfides have also been studied, but they are not suitable catalysts because of their poor chemical stability. However, searching for catalysts that can work under visible light should not be limited to semiconductor materials with band-gap structure, but can be extended to other materials, such as gold nanoparticles. It can be said that glaziers in medieval forges were the first nanotechnologists who produced colors with gold nanoparticles of different sizes, although they had little understanding of the modern day principles which have become a hot topic in the last two decades. In recent years there have been numerous studies on the optical properties of gold nanoparticles. Gold nanoparticles absorb visible light intensely because of the surface plasmon resonance (SPR) effect. The electromagnetic field of incident light couples with the oscillations of conduction electrons in gold particles, resulting in strong-field enhancement of the local electromagnetic fields near the rough surface of gold nanoparticles. The enhanced local field strength can be over 500 times larger than the applied field for structures with sharp apices, edges, or concave curvature (e.g. nanowires, cubes, triangular plates, and nanoparticle junctions). The SPR absorption may cause rapid heating of the nanoparticles. Gold nanoparticles supported on metal oxides are efficient catalysts for important oxidation process, including selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as CO, CH3OH, and HCHO at moderately elevated temperatures. Therefore, the combination of the SPR absorption and the catalytic activity of gold nanoparticles presents an important opportunity: if the heated gold nanoparticles could activate the organic molecules on them to induce oxidation of the organic compounds, then oxidation on gold catalysts can be driven by visible light at ambient temperature. Moreover, the SPR is a local effect, limited to the noble metal particles, so that the light only heats gold nanoparticles, which generally account for a few percent of the overall catalyst mass. This leads to significant saving in energy consumption for catalyzing organic compound oxidation. To verify the possibility of driving the VOC oxidation with visible light at room temperature, we prepared gold particles supported on various oxide powders. ZrO2 and SiO2 powders were first chosen as supports, because their band gaps are circa 5.0 eV and circa 9.0 eV, respectively, which are much larger than the energies of the photons of visible light (less than 3.0 eV). Thus, the light cannot excite electrons from the valence band to the conduction band. It is also impossible for the gold nanoparticles on ZrO2 to reduce the band gaps of ZrO2 enough for visible light photons to be absorbed and excite electrons in ZrO2. Thus, the catalytic activity is not caused by the same mechanism as occurs in semiconductor photocatalysts, but is due to the SPR effect of gold nanoparticles. The changes in the concentrations of the reactant (HCHO, 100 ppm) and product (CO2), when gold supported on ZrO2 was used as the catalyst, are depicted in Figure 1a. The initial concentration of HCHO in the glass vessel was 100 ppm. HCHO content decreased by 64% in two hours under the irradiation of six light tubes of blue light (with wavelength between 400 and 500 nm and the irradiation energy determined to be 0.17 Wcm 2 at the position of glass slides coated with the gold catalysts), and the CO2 content in the vessel increased accordingly. These results confirm that the oxidation of formaldehyde to carbon dioxide proceeds to a large extent at ambient temperature. The turnover frequency was calculated as being about 1.2 = 10 3 molecules of [*] Dr. X. Chen, Prof. H.-Y. Zhu, Z.-F. Zheng School of Physical and Chemical Sciences Queensland University of Technology Brisbane, Qld 4001 (Australia) Fax: (+61)7-3864-1804 E-mail: hy.zhu@qut.edu.au

Reduction of Nitroaromatic Compounds on Supported Gold Nanoparticles by Visible and Ultraviolet Light

Shedding light: Nitroaromatic compounds on gold nanoparticles (3 wt %) supported on ZrO2 can be reduced directly to the corresponding azo compounds when illuminated with visible light or ultraviolet light at 40 °C (see picture). The process occurs with high selectivity and at ambient temperature and pressure, and enables the selection of intermediates that are unstable in thermal reactions.

Porous Clays and Pillared Clays-Based Catalysts. Part 2: A Review of the Catalytic and Molecular Sieve Applications

Metal oxide pillared clay (PILC) possesses several interesting properties, such as large surface area, high pore volume and tunable pore size (from micropore to mesopore), high thermal stability, strong surface acidity and catalytic active substrates/metal oxide pillars. These unique characteristics make PILC an attractive material in catalytic reactions. It can be made either as catalyst support or directly used as catalyst. This paper is a continuous work from Kloprogges review (J.T. Kloprogge, J. Porous Mater. 5, 5 1998) on the synthesis and properties of smectites and related PILCs and will focus on the diverse applications of clay pillared with different types of metal oxides in the heterogeneous catalysis area and adsorption area. The relation between the performance of the PILC and its physico-chemical features will be addressed.

Titanate Nanofibers as Intelligent Absorbents for the Removal of Radioactive Ions from Water

Layered titanate nanofibers can absorb bivalent ions from waste water via an ion exchange process. The sorption induces a considerable deformation of the layered structure, thus trapping the cations in the fibers permanently. Therefore, the fibers are desirable sorbents for the removal of toxic, radioactive Ra(2+) and Sr(2+) ions from water and subsequent safe disposal thereof.

Mechanism of supported gold nanoparticles as photocatalysts under ultraviolet and visible light irradiation

Gold nanoparticles strongly absorb both visible light and ultraviolet light to drive an oxidation reaction for a synthetic dye, as well as phenol degradation and selective oxidation of benzyl alcohol under UV light.

Supported silver nanoparticles as photocatalysts under ultraviolet and visible light irradiation

The significant activity for dye degradation by silver nanoparticles (NPs) on oxide supports was better than popular semiconductor photocatalysts. Moreover, silver photocatalysts can degrade phenol and drive oxidation of benzyl alcohol to benzaldehyde under ultraviolet light. We suggest that surface plasmon resonance (SPR) effect and interband transition of silver NPs can activate organic molecules for oxidation under ultraviolet and visible light irradiation.

Discoloration and mineralization of Reactive Red HE-3B by heterogeneous photo-Fenton reaction.

Discoloration and mineralization of Reactive Red HE-3B were studied by using a laponite clay-based Fe nanocomposite (Fe-Lap-RD) as a heterogeneous catalyst in the presence of H2O2 and UV light. Our experimental results clearly indicate that Fe-Lap-RD mainly consists of Fe2O3 (meghemite) and Fe2Si4O10(OH)2 (iron silicate hydroxide) which have tetragonal and monoclinic structures, respectively, and has a high specific surface area (472 m2/g) as well as a high total pore volume (0.547 cm3/g). It was observed that discoloration of HE-3B undergoes a much faster kinetics than mineralization of HE-3B. It was also found that initial HE-3B concentration, H2O2 concentration, UV light wavelength and power, and Fe-Lap-RD catalyst loading are the four main factors that can significantly influence the mineralization of HE-3B. At optimal conditions, complete discoloration of 100 mg/L HE-3B can be achieved in 30 min and the total organic carbon removal ratio can attain 76% in 120 min, illustrating that Fe-Lap-RD has a high photo-catalytic activity in the photo-assisted discoloration and mineralization of HE-3B in the presence of UV light (254 nm) and H2O2.

Capture of Radioactive Cesium and Iodide Ions from Water by Using Titanate Nanofibers and Nanotubes

Radioactive Cs and I ions are the products of uranium fission, and can be easily dissolved in water during an accident at a nuclear reactor, such as those that occurred at Chernobyl in 1986, at Three Mile Island in Pennsylvania in 1979, and in 2011 at Fukushima, Japan. In 2009, leaks of radioactive materials such as Cs and I isotopes also occurred during minor accidents at nuclear power stations in Britain, Germany, and the U.S. These leaks have raised concerns about exposure levels in the nearby communities because it is feared that these fission products could make their way into the food chain when present in waste water. Radioactive iodine is also used in the treatment of thyroid cancer, and, as a result, radioactive wastewater is discharged by a large number of medical research institutions. The wide use of radioisotopes requires effective methods to manage radioactive waste, and methods currently used are complex and extremely costly. Herein we demonstrate a potentially cost-effective method to remediate Cs and I ions from contaminated water by using the unique chemistry of titanate nanotubes and nanofibers, which can not only chemisorb these ions but efficiently trap them for safe disposal. Inorganic cation exchangers, such as crystalline silicotitanates, zeolites, clay minerals, layered Zr phosphates, and layered sulfide frameworks, have been studied for separation of Cs ions from nuclear wastewater and safe disposal of the exchanged cations because of the ability of these exchangers to withstand intense radiation and elevated temperatures, in addition to their high ion-exchange capacity. Because ion exchange in materials is usually a reversible process, except in micas, the radioactive ions in the exchanger may be released to water. Titanates are refractory mineral substances that are very stable with respect to radiation and chemical, thermal, and mechanical changes. Titanate nanofibers and nanotubes (with chemical formula Na2Ti3O7) can be easily synthesized at low cost under hydrothermal conditions. These materials possess a layered structure in which TiO6 octahedra are the basic structural units (Figure S1 in the Supporting Information). These layers carry negative charges and are approximately two oxygen atoms thick. Na ions are situated between the layers and can be exchanged with other cations. In the present study, we show how trititanate nanofibers (T3NF) and nanotubes (T3NT) can be used to efficiently remove radioactive Cs ions from aqueous solution by cation exchange. Figure 1a shows that the nanotubular T3NT can remove 80% of Cs ions from solutions with Cs concentrations up to 250 ppm. The ions can be completely removed when the Cs ion concentration is below 80 ppm. In contrast, the fibril T3NF has a comparatively lower absorption capacity than

Copper nanoparticles on graphene support: an efficient photocatalyst for coupling of nitroaromatics in visible light.

Copper is a low-cost plasmonic metal. Efficient photocatalysts of copper nanoparticles on graphene support are successfully developed for controllably catalyzing the coupling reactions of aromatic nitro compounds to the corresponding azoxy or azo compounds under visible-light irradiation. The coupling of nitrobenzene produces azoxybenzene with a yield of 90 % at 60 °C, but azobenzene with a yield of 96 % at 90 °C. When irradiated with natural sunlight (mean light intensity of 0.044 W cm(-2) ) at about 35 °C, 70 % of the nitrobenzene is converted and 57 % of the product is azobenzene. The electrons of the copper nanoparticles gain the energy of the incident light through a localized surface plasmon resonance effect and photoexcitation of the bound electrons. The excited energetic electrons at the surface of the copper nanoparticles facilitate the cleavage of the NO bonds in the aromatic nitro compounds. Hence, the catalyzed coupling reaction can proceed under light irradiation and moderate conditions. This study provides a green photocatalytic route for the production of azo compounds and highlights a potential application for graphene.