Towards efficient visible-light active photocatalysts: CdS/Au sensitized TiO 2 nanotube arrays

Biosynthesized Quantum Dots as Improved Biocompatible Tools for Biomedical Applications

Quantum Dots (QDs), whose diameters are often limited to 10 nm, have been of interest to researchers for their unique optical characteristics, which are attributed to quantum confinement. Following their early application in the electrical industry as light-emitting diode materials, semiconductor nanocrystals have continued to show great potential in clinical diagnosis and biomedical applications. The conventional physical and chemical pathways for QD syntheses typically require harsh conditions and hazardous reagents, and these products encounter non-hydrophilic problems due to organic capping ligands when they enter the physiological environment. The natural reducing abilities of living organisms, especially microbes, are then exploited to prepare QDs from available metal precursors. Low-cost and eco-friendly biosynthesis approaches have the potential for further biomedical applications which benefit from the good biocompatibility of protein-coated QDs. The surface biomass offers many binding sites to modify substances or target ligands, therefore achieving multiple functions through simple and efficient operations. Biosynthetic QDs could function as bioimaging and biolabeling agents because of their luminescence properties similar to those of chemical QDs. In addition, extensive research has been carried out on the antibacterial activity, metal ion detection and bioremediation. As a result, this review details the advanced progress of biomedical applications of biosynthesized QDs and illustrates these principles as clearly as possible.

High photoelectrochemical performance of reduced graphene oxide wrapped, CdS functionalized, TiO2 multi-leg nanotubes

Absorption of visible light and separation of photogenerated charges are two primary pathways to improve the photocurrent performance of semiconductor photoelectrodes. Here, we present a unique design of tricomponent photocatalyst comprising of TiO₂ multileg nanotubes (MLNTs), reduced graphene oxide (rGO) and CdS nanoparticles. The tricomponent photocatalyst shows a significant red-shift in the optical absorption (∼2.2 eV) compared to that of bare TiO₂ MLNTs (∼3.2 eV). The availability of both inner and outer surfaces areas of MLNTs, the visible light absorption of CdS, and charge separating behavior of reduced graphene oxide layers contribute coherently to yield a photocurrent density of ∼11 mA cm^-2 @ 1 V vs. Ag/AgCl (100 mW cm^-2, AM 1.5 G). Such a high PEC performance from TiO₂/rGO/CdS photoelectrode system has been analyzed using diffused reflectance (DRS) and electrochemical impedance (EIS) spectroscopy techniques. The efficient generation of charge carriers under light irradiation and easy separation because of favourable band alignment, are attributed to the high photoelectrochemical current density in these tricomponent photocatalyst systems.

Efficient Charge Transfer in Heterostructures of CdS/NaTaO3 with Improved Visible-Light-Driven Photocatalytic Activity

Photocatalyst NaTaO₃ with a cube-shaped morphology and an average particle size of 100 nm was synthesized using an effortless hydrothermal method. The composite heterostructures of CdS/NaTaO₃ with variable concentrations of CdS were fabricated after the surface functionalization of bare NaTaO₃ using 3-mercaptopropionic acid. As-synthesized photocatalysts were characterized using powder X-ray diffraction analysis, Raman spectroscopy, field-emission scanning and transmission electron microscopies with energy-dispersive X-ray spectroscopy furnished with elemental mapping, multipoint Brunauer-Emmett-Teller (BET), and UV-vis diffuse reflectance spectroscopy. Average lifetime (τ_avg) of photoexcitons in heterostructures was studied using photoluminescence (PL) empowered with the time-correlated single-photon counting technique. The diminishing PL peak intensity and reduced average lifetime (τ_avg) of photoexcitons in heterostructures indicate the inhibition of photoexciton recombination along with efficient photoexciton exchange between heterostructures. As-synthesized heterostructures demonstrate enhanced visible-light harvesting and appreciably increased the photocatalytic performance toward the degradation of dye rhodamine B. This work highlights the importance of heterostructures with new archetypes which may provide a lead to develop highly capable and reusable photocatalysts to organic dye degradation.

Synthetic strategies and application of gold-based nanocatalysts for nitroaromatics reduction

With the increasing requirement of efficient organic transformations on the basic concept of Green Sustainable Chemistry, the development of highly efficient catalytic reaction system is greatly desired. In this case, gold (Au)-based nanocatalysts are promising candidates for catalytic reaction, especially for the reduction of nitroaromatics. They have attracted wide attention and well developed in the application of nitroaromatics reduction because of the unique properties compared with that of other conventional metal-based catalysts. With this respect, this review proposes recent trends in the application of Au nanocatalysts for efficient reduction process of nitroaromatics. Some typical approaches are compared and discussed to guide the synthesis of highly efficient Au nanocatalysts. The mechanism on the use of H₂ and NaBH₄ solution as the source of hydrogen is compared, and that proposed under light irradiation is discussed. The high and unique catalytic activity of some carriers, such as oxides and carbons-based materials, based on different sizes, structures, and shapes of supported Au nanocatalysts for nitroaromatics reduction are described. The catalytic performance of Au combining with other metal nanoparticles by alloy or doping, like multi-metal nanoparticles system, is further discussed. Finally, a short discussion is introduced to compare the catalysis with other metallic nanocatalysts.

In Situ Shape Change of Au Nanoparticles on TiO2 by CdS Photodeposition: Its Near-Field Enhancement Effect on Photoinduced Electron Injection from CdS to TiO2

Hemisphere-like gold nanoparticles (NPs) were loaded on TiO₂ (Au/TiO₂) by the deposition-precipitation method. Subsequent photodeposition of CdS on the Au surface of Au/TiO₂ at 50 °C yields Au(core)-CdS(shell) hybrid quantum dots with a heteroepitaxial (HEPI) junction on TiO₂ (Au@#CdS/TiO₂), whereas nonHEPI Au@CdS/TiO₂ was formed by CdS photodeposition at 25 °C. In the HEPI system, the shape of the Au core changes to an angular shape, whereas it remains in a hemisphere-like shape in the nonHEPI system. The hot photodeposition technique was applied to the Au NP-loaded mesoporous TiO₂ nanocrystalline film (Au/mp-TiO₂). Using Au@CdS/mp-TiO₂ and Au@#CdS/mp-TiO₂ as the photoanodes, two-electrode quantum dot-sensitized photoelectrochemical cells were fabricated for hydrogen (H₂) generation from water, and the performances of the cells were evaluated under illumination of simulated sunlight. In the photocurrent and the rate of H₂ evolution, the Au@#CdS/mp-TiO₂ photoanode cell surpasses the CdS/mp-TiO₂ and Au@CdS/mp-TiO₂ ones. Three-dimensional finite-difference time-domain calculations for the model systems indicated that the angular shape Au core generates an intense electric field at the corners and edges, extending the electric field distribution over the Au core-CdS shell interface. The striking shape effect on the cell performances can originate from the promotion of the CdS excitation and charge separation due to the near-field enhancement by the deformed Au core.

Enhanced visible light photocatalytic performance of CdS sensitized TiO2 nanorod arrays decorated with Au nanoparticles as electron sinks

In this paper, we propose a nanostructure with Au nanoparticles (NPs), as electron sinks, located at the most outside layer of CdS sensitized TiO₂ nanorod arrays (TiO₂ NRAs/CdS/Au). By the introduction of Au NPs, TiO₂ NRAs/CdS/Au performs higher visible light photocatalytic capacity in the degradation of unsymmetrical dimethylhydrazine wastewater than TiO₂ NRAs/CdS. The optimal deposition time for Au NPs is 30 s. The visible light induced degradation ability of TiO₂ NRAs/CdS/Au (30 s) is 1.4 times that of TiO₂ NRAs/CdS. The cycling stability of TiO₂ NRAs/CdS is greatly enchanced after Au NPs decoration, which can maintain 95.86% after three cycles. Photoluminescence spectra and photoelectrochemical measurements were carried out to reveal the underlying mechanism for the improved visible light photocatalytic capacity of TiO₂ NRAs/CdS/Au. This work demonstrates a promising way for the rational design of metal-semiconductor photocatalysts used in decomposition reaction that can achieve high photocatalytic efficiency.

3D macroporous TiO2inverse opal binary and ternary composite materials and their photocatalytic activity

We have fabricated binary and ternary 3D macroporous composite materials containing inverse opal TiO₂, gold nanoparticles and nanosized cadmium sulfide on glass slides and these materials show excellent photocatalytic activity.