Xiaoqiang An

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.

Cu2ZnSnS4-Pt and Cu2ZnSnS4-Au Heterostructured Nanoparticles for Photocatalytic Water Splitting and Pollutant Degradation

Cu2ZnSnS4, based on abundant and environmental friendly elements and with a direct band gap of 1.5 eV, is a main candidate material for solar energy conversion through both photovoltaics and photocatalysis. We detail here the synthesis of quasi-spherical Cu2ZnSnS4 nanoparticles with unprecedented narrow size distributions. We further detail their use as seeds to produce CZTS-Au and CZTS-Pt heterostructured nanoparticles. Such heterostructured nanoparticles are shown to have excellent photocatalytic properties toward degradation of Rhodamine B and hydrogen generation by water splitting.

Cu2O/Reduced Graphene Oxide Composites for the Photocatalytic Conversion of CO2

A facile one-step microwave-assisted chemical method has been successfully used for the synthesis of Cu2O/reduced graphene oxide (RGO) composites. Photocatalytic CO2 reduction was then investigated on the junction under ambient conditions. The RGO coating dramatically increases Cu2O activity for CO2 photoreduction to result in a nearly six times higher activity than the optimized Cu2O and 50 times higher activity than the Cu2O/RuOx junction in the 20th hour. Furthermore, an apparent initial quantum yield of approximately 0.34 % at 400 nm has been achieved by the Cu2O/RGO junction for CO2 photoreduction. The photocurrent of the junction is nearly double that of the blank Cu2O photocathode. The improved activity together with the enhanced stability of Cu2O is attributed to the efficient charge separation and transfer to RGO as well as the protection function of RGO, which was proved by XRD, SEM, TEM, X-ray photoelectron spectroscopy, photo-electrochemical, photoluminescence, and impedance characterizations. This study further presents useful information for other photocatalyst modification for efficient CO2 reduction without the need for a noble-metal co-catalyst.

Biomolecule-assisted fabrication of copper doped SnS2 nanosheet–reduced graphene oxide junctions with enhanced visible-light photocatalytic activity

Novel copper (3 at%) doped SnS2 nanosheet–reduced graphene oxide (RGO) junctions were fabricated by a facile cysteine-assisted hydrothermal method. Superior visible-light-driven activity for methyl orange decomposition has been achieved. The average apparent rate of Cu-doped SnS2 nanosheet–RGO composites is more than 7 times higher than that of SnS2, and even 2 times higher than CdS nanoparticles, the benchmark material. Further characterization indicates that facilitating charge separation, fast electron transport and a large surface area together play key roles in the materials enhanced photoactivity. Due to their chemical stability, low cost and lower toxicity, Cu-doped SnS2–RGO composites show great potential as high-efficiency visible-light-driven photocatalysts for environmental remediation and energy conversion.

Fe2O3–TiO2 Nanocomposites for Enhanced Charge Separation and Photocatalytic Activity

Photocatalysis provides a cost effective method for both renewable energy synthesis and environmental purification. Photocatalytic activity is dominated by the material design strategy and synthesis methods. Here, for the first time, we report very mild and effective photo-deposition procedures for the synthesis of novel Fe2 O3 -TiO2 nanocomposites. Their photocatalytic activities have been found to be dramatically enhanced for both contaminant decomposition and photoelectrochemical water splitting. When used to decompose a model contaminant herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), monitored by both UV/Vis and total organic carbon (TOC) analysis, 10% Fe-TiO2 -H2 O displayed a remarkable enhancement of more than 200 % in the kinetics of complete mineralisation in comparison to the commercial material P25 TiO2 photocatalyst. Furthermore, the photocurrent is nearly double that of P25. The mechanism for this improvement in activity was determined using density functional theory (DFT) and photoluminescence. These approaches ultimately reveal that the photoelectron transfer is from TiO2 to Fe2 O3 . This favours O2 reduction which is the rate-determining step in photocatalytic environmental purification. This in situ charge separation also allows for facile migration of holes from the valence band of TiO2 to the surface for the expected oxidation reactions, leading to higher photocurrent and better photocatalytic activity.

CdS nanorods/reduced graphene oxide nanocomposites for photocatalysis and electrochemical sensing

CdS nanorods/reduced graphene oxide (RGO) composites were fabricated through a facile one-pot hydrothermal synthesis. The sample CdS/RGO-1% possesses excellent photocatalytic properties under visible light for the degradation of methyl orange with a rate constant of 1.76 × 10−2 min−1, which is about three times higher than that of pure CdS nanorods. The novel composite material can also be used as an enzyme-free biosensor for H2O2 based on its electrocatalytic behaviour. The low-cost sensor shows rapid response and high sensitivity. This method opens up a facile route for preparing graphene-based nanocomposites with excellent photocatalytic and electrocatalytic properties.

The effect of the Ga content on the photocatalytic hydrogen evolution of CuIn1−xGaxS2 nanocrystals

We report on the photocatalytic hydrogen evolution under full-arc light irradiation of CuIn1−xGaxS2 wurtzite nanocrystals in the presence of SO32− and S2− as sacrificial reagents. We analyzed the hydrogen generation rate as a function of the Ga content and associated it with the energy band positions. For photocatalytic water splitting, the CuInS2 bandgap is slightly too low to efficiently overcome the reaction over-potential. The presence of Ga shifts up the CuInS2 conduction band edge providing a larger driving force for photogenerated carriers to activate the water splitting reduction reaction. The larger the Ga content, the more energetically favorable the electron injection, and thus a more efficient use of the photogenerated carriers is reached. However, the band gap increase associated with the Ga incorporation reduces the concentration of photogenerated carriers available for water splitting, and consequently a lower hydrogen conversion rate is obtained for very high Ga contents. The optimum Ga concentration was experimentally found at CuIn0.3Ga0.7S2.

Transformation of humic acid and halogenated byproduct formation in UV-chlorine processes

The synergistic effect of ultraviolet light (UV) and chlorine on the structural transformation of Humic Acid (HA) and formation of chloro-disinfection byproducts (DBPs) in water were investigated, with chlorination as a reference. The transformation and mineralization of HA were enhanced upon co-exposure to UV and chlorine. Electron spin resonance (ESR) studies revealed that hydroxyl radical (OH) and chlorine radical (Cl) were predominant active species in a pH range from 4 to 7, while Cl dominated at pH 2 and pH higher than 7. The impact of different radicals on the transformation of HA was investigated by UV254, fluorescence and TOC measurements. OH were found to be responsible for the removal of chromophoric groups and mineralization of HA, while Cl mainly reacted with HA and intermediates from HA degradation. Due to the competitive and synergistic reaction of OH and Cl with HA, higher removal of HA and lower formation of chloro-DBPs appeared in UV-chlorine than chlorination, thus the combined UV-chlorine processes should be a promising method for water purification.

Photocatalytic mineralisation of herbicide 2,4,5-trichlorophenoxyacetic acid: enhanced performance by triple junction Cu-TiO2-Cu2O and the underlying reaction mechanism

A mild and facile photodeposition method was used to fabricate novel Cu–TiO2–Cu2O composite photocatalysts. Due to the in situ rectifying charge carrier separation and enhanced conductivity, the composites present superior photocatalytic activity, leading to more than 90% mineralisation of the toxic 2,4,5-trichlorophenoxyacetic acid herbicide. This result was confirmed by both TOC and UV-vis absorption measurements. The effect of active radicals on the photodegradation of the herbicide was further investigated in order to clarify the underlying mechanism, based on which a hole-dominated photooxidation mechanism was proposed. These results not only offer a green and economical method for constructing triple junction photocatalyst materials, but also shed new insight on the rational design of a low cost and high-efficiency photocatalyst for environmental remediation.