Ren Su

Promotion of phenol photodecomposition over TiO2 using Au, Pd, and Au-Pd nanoparticles

Noble metal nanoparticles (Au, Pd, Au-Pd alloys) with a narrow size distribution supported on nanocrystalline TiO(2) (M/TiO(2)) have been synthesized via a sol-immobilization route. The effect of metal identity and size on the photocatalytic performance of M/TiO(2) has been systematically investigated using phenol as a probe molecule. A different phenol degradation pathway was observed when using M/TiO(2) catalysts as compared to pristine TiO(2). We propose a mechanism to illustrate how the noble metal nanoparticles enhance the efficiency of phenol decomposition based on photoreduction of p-benzoquinone under anaerobic conditions. Our results suggest that the metal nanoparticles not only play a role in capturing photogenerated electrons, but are strongly involved in the photocatalytic reaction mechanism. The analysis of the reaction intermediates allows us to conclude that on M/TiO(2) undesired redox reactions that consume photogenerated radicals are effectively suppressed. The analysis of the final products shows that the reusability performance of the catalyst is largely dependent on the pretreatment of the catalyst and the identity of the metal nanoparticle. Interestingly, the as-prepared Pd and Au-Pd decorated TiO(2) materials exhibit excellent long-term photoactivity, in which ~90% of the phenol can be fully decomposed to CO(2) in each cycle.

Designer titania-supported Au-Pd nanoparticles for efficient photocatalytic hydrogen production

Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au-Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd(shell)-Au(core) nanoparticles immobilized on TiO2 exhibit extremely high quantum efficiencies for H2 production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO2 is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis.

High-quality Fe-doped TiO2 films with superior visible-light performance

We report on high-quality polycrystalline Fe-doped TiO2 (Fe–TiO2) porous films synthesized via one-step electrochemical oxidation. We demonstrate that delicate properties such as the impurity concentration and the microstructure that strongly influence the performance of the material for photovoltaic and photocatalysis applications can be controlled by adjusting the electrolyte composition. Compared to Fe-doped TiO2 films prepared with traditional phosphate- or silicate-based electrolytes, our newly synthesised Fe–TiO2 films contain solely Fe dopants, which results in excellent photocatalytic and photovoltaic performance under visible light irradiation.

Band gap narrowing of SnS2 superstructures with improved hydrogen production

Transition metal sulfides exhibit chemical and physical properties that are of much scientific and technological interest and can largely be attributed to their covalent bonding of 3d electrons. Hierarchical structures of these materials are suited for a broad range of applications in energy storage, as biological scaffold, and as sensors. In this work, hierarchical SnS2 structures have been synthesized and show excellent photocatalytic performance for the production of H2 under blue light (450 nm) irradiation. A combination of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy indicates the formation of layered SnS2/SnS superstructures with a lattice mismatch between the two alternating layers. This indicates the presence of S vacancies and results in a drastic decrease of the band gap by 0.3 eV compared to bulk SnS2. This strategy of self-narrowing of the band-gap demonstrates its great potential for the design of new materials with visible light reactivity. Finally, we have extended this strategy to the synthesis of other transition metal sulfides (Ni3S4, CuS, CuS@C, and FeS2) with similar hierarchical structures, which have potential applications such as supercapacitors and electrode materials for sodium/lithium ion batteries.

Fluoride concentration controlled TiO2 nanotubes: the interplay of microstructure and photocatalytic performance

Electrochemical anodization has been considered an efficient technique to prepare highly ordered TiO2 nanotubes (NTs) for photocatalysis. Most researchers focus on the influence of preparation parameters on the micromorphology of TiO2 NTs, while the correlation between micro-morphology and the photocatalytic performance of TiO2 NTs has been ignored to some extent. Fluoride ions in the electrolyte are essential for the formation of TiO2 NTs, as the F− concentration influences both the microstructure and the corresponding photocatalytic performance of TiO2 NTs. Here, we prepared a series of TiO2 NTs by varying the [F−] of the electrolyte. The micro-morphology, crystallography information, and chemical compositions of the TiO2 NTs were characterized by SEM, XRD and XPS, respectively. We found that an increase in [F−] in the electrolyte led to an increase in the TiO2 NTs length and tube diameter, and excess [F−] resulted in a decrease in the integrity of the TiO2 NTs surface and a shorter length due to over-etching of the TiO2 NTs surface. The corresponding photoelectrocatalytic and photocatalytic performance were characterized by photocurrent response and photo-decolorization of rhodamine B (RhB). The results showed that longer TiO2 NTs arrays prepared at higher [F−] enhanced both the photoelectrocatalytic and photocatalytic performance due to an improved separation and transportation of photo-generated charge carriers. However, excess [F−] in the electrolyte reduced both the length and integrity of TiO2 NTs, which depressed the photoelectrocatalytic performance as a consequence of a faster recombination rate of the electron–hole pairs. A larger diameter was beneficial for the photo-decolorization of RhB because of the faster mass transfer of RhB molecules.

Light-tuned selective photosynthesis of azo- and azoxy-aromatics using graphitic C3N4

Solar-driven photocatalysis has attracted significant attention in water splitting, CO2 reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C3N4 is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.The synthesis of azo- and azoxy-aromatic dyes via photoreduction of nitroaromatics is hindered by high costs and low catalytic efficiencies and selectivities. Here the authors demonstrate the facile synthesis of these important dyes from their corresponding nitroaromatic precursors by using an inexpensive graphitic C3N4 photocatalyst.

Alternative Materials to TiO2

One of the most significant investigations on heterogeneous photocatalytic process can be dated back to the 1970s, when Fujishima and Honda showed that the TiO2 electrode is capable of water splitting under suitable electromagnetic irradiation. Since then, TiO2-based materials have become the dominant photocatalyst and have been investigated for decades due to their abundance, non-toxicity, and relatively high reactivity. However, the bandgap of pristine TiO2 is larger than 3 eV, which can only absorb light that has a wavelength of less than 400 nm. Unfortunately, this portion of photons only corresponds to 4–5 % of the solar spectrum, which has limited the application of photocatalysis at an industrial scale. Moreover, the conduction band position of TiO2 is only slightly negative relative to that of the proton reduction potential, resulting in a relatively poor reduction power for solar-to-fuel conversion. Therefore, the development of alternative photocatalysts with visible light absorption and tunable properties is essential in the application of photocatalysis techniques.