Zhanfeng Zheng

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

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.

Heterojunctions in g-C3N4/TiO2(B) nanofibres with exposed (001) plane and enhanced visible-light photoactivity

The formation of heterojunctions is an efficient strategy to extend the light response range of TiO2-based catalysts to the visible light region. In addition to the bandgap edge match between the narrow bandgap semiconductors and the TiO2 substrate, a stable phase interface between the sensitiser and TiO2 is crucial for the construction of heterojunctions, since it acts as a tunnel for the efficient transfer of photogenerated charges. Herein, the coincidence site density (1/Σ) of graphite-like carbon nitride (g-C3N4) nanoflakes and two types of TiO2 nanofibres [anatase and TiO2(B)] was calculated by near coincidence site lattice (NCSL) theory. It was found that the coincidence site density of g-C3N4 and TiO2(B) nanofibre with an exposed (001) plane is 3 times of that of the g-C3N4 and anatase nanofibre with exposed (100) plane. This indicated that the g-C3N4 nanoflakes are more favoured to form stable heterojunctions with TiO2(B) nanofibres. As expected, a stable phase interface was formed between the plane of (22–40) of g-C3N4 and the plane (110) of TiO2(B) which had same d-spacing of 0.35 nm and the same orientation. Under visible light irradiation, the photogenerated electrons could efficiently migrate to the TiO2(B) nanofibres from the g-C3N4 through the heterojunctions. So the g-C3N4/TiO2(B) system exhibited better photodegradation ability for sulforhodamine B (SRB) dye than the g-C3N4/anatase system, although the photoactivity of the anatase nanofibres was much better than that of the TiO2(B) nanofibres.

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.

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

Viable photocatalysts under solar-spectrum irradiation: nonplasmonic metal nanoparticles.

Supported nanoparticles (NPs) of nonplasmonic transition metals (Pd, Pt, Rh, and Ir) are widely used as thermally activated catalysts for the synthesis of important organic compounds, but little is known about their photocatalytic capabilities. We discovered that irradiation with light can significantly enhance the intrinsic catalytic performance of these metal NPs at ambient temperatures for several types of reactions. These metal NPs strongly absorb the light mainly through interband electronic transitions. The excited electrons interact with the reactant molecules on the particles to accelerate these reactions. The rate of the catalyzed reaction depends on the concentration and energy of the excited electrons, which can be increased by increasing the light intensity or by reducing the irradiation wavelength. The metal NPs can also effectively couple thermal and light energy sources to more efficiently drive chemical transformations.

Correlation of the catalytic activity for oxidation taking place on various TiO2 surfaces with surface OH groups and surface oxygen vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO(2) nanostructures in water, together with two reactions employing Au/TiO(2) nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO(2) nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au-NP) catalysts were supported on some of these TiO(2) nanostructures (to form Au/TiO(2) catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO(2) nanostructures studied gave the same trend as the catalytic activities of the TiO(2) nanostructures or their respective Au/TiO(2) catalysts for the three oxidation reactions. Both IES and X-ray photoelectron spectroscopy (XPS) proved that anatase TiO(2) had the strongest OH regeneration ability among the four types of TiO(2) phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O(2-) sites on TiO(2) surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O(2) molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O(2). A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO(2) catalysts is also proposed, based on the photocatalytic activity of the Au-NPs under visible light. The Au-NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.

Enhancing Photoactivity of TiO2(B)/Anatase Core–Shell Nanofibers by Selectively Doping Cerium Ions into the TiO2(B) Core

Cerium ions (Ce(3+)) can be selectively doped into the TiO2(B) core of TiO2(B)/anatase core-shell nanofibers by means of a simple one-pot hydrothermal treatment of a starting material of hydrogen trititanate (H2Ti3O7) nanofibers. These Ce(3+) ions (≈0.202 nm) are located on the (110) lattice planes of the TiO2(B) core in tunnels (width≈0.297 nm). The introduction of Ce(3+) ions reduces the defects of the TiO2(B) core by inhibiting the faster growth of (110) lattice planes. More importantly, the redox potential of the Ce(3+)/Ce(4+) couple (E°(Ce(3+)/Ce(4+))=1.715 V versus the normal hydrogen electrode) is more negative than the valence band of TiO2(B). Therefore, once the Ce(3+)-doped nanofibers are irradiated by UV light, the doped Ce(3+) ions--in close vicinity to the interface between the TiO2(B) core and anatase nanoshell--can efficiently trap the photogenerated holes. This facilitates the migration of holes from the anatase shell and leaves more photogenerated electrons in the anatase nanoshell, which results in a highly efficient separation of photogenerated charges in the anatase nanoshell. Hence, this enhanced charge-separation mechanism accelerates dye degradation and alcohol oxidation processes. The one-pot treatment doping strategy is also used to selectively dope other metal ions with variable oxidation states such as Co(2+/3+) and Cu(+/2+) ions. The doping substantially improves the photocatalytic activity of the mixed-phase nanofibers. In contrast, the doping of ions with an invariable oxidation state, such as Zn(2+), Ca(2+), or Mg(2+), does not enhance the photoactivity of the mixed-phase nanofibers as the ions could not trap the photogenerated holes.