Semiconductor-based Photocatalytic Hydrogen Generation

Synthesis of C60-decorated SWCNTs (C60-d-CNTs) and its TiO2-based nanocomposite with enhanced photocatalytic activity for hydrogen production

A novel nanostructured carbon/TiO(2) nanocomposite photocatalyst is firstly fabricated via a facile hydrothermal process by using fullerene (C(60)) decorated single-walled carbon nanotubes (SWCNTs) as carbon source, which is denoted as C(60)-d-CNTs. The obtained nanostructured carbon/TiO(2) nanocomposites are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectra (DRS), Raman spectra, X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL), and then are used as catalysts for photocatalytic hydrogen production. It is found that the kinds and contents of various carbon nanostructured materials (such as SWCNTs, C(60) and C(60)-d-CNTs) coupled with TiO(2) can significantly enhance the photoactivity for hydrogen production, and the 5 wt% C(60)-d-CNTs/TiO(2) nanocomposite exhibits the best performance. Experimental results suggest that the C(60)-d-CNTs as a novel carbon nanostructured material could be more beneficial for the photogenerated carrier separation than SWCNTs and C(60) when these carbon nanostructured materials are coupled with TiO(2).

Visible light-driven α-Fe₂O₃ nanorod/graphene/BiV₁-xMoxO₄ core/shell heterojunction array for efficient photoelectrochemical water splitting

We report the design, synthesis, and characterization of a novel heterojunction array of α-Fe(2)O(3)/graphene/BiV(1-x)Mo(x)O(4) core/shell nanorod for photoelectrochemical water splitting. The heterojunction array was prepared by hydrothermal deposition of α-Fe(2)O(3) nanorods onto Ti substrate, with subsequent coating of graphene interlayer and BiV(1-x)Mo(x)O(4) shell by photocatalytic reduction and a spin-coating approach, respectively. The heterojunction yielded a pronounced photocurrent density of ∼1.97 mA/cm(2) at 1.0 V vs Ag/AgCl and a high photoconversion efficiency of ∼0.53% at -0.04 V vs Ag/AgCl under the irradiation of a Xe lamp. The improved photoelectrochemical properties benefited from (1) the enhanced light absorption due to behavior of the "window effect" between the α-Fe(2)O(3) cores and BiV(1-x)Mo(x)O(4) shells, and (2) the improved separation of photogenerated carriers at the α-Fe(2)O(3) nanorod/graphene/BiV(1-x)Mo(x)O(4) interfaces. Our results demonstrate the advantages of the novel graphene-mediated core/shell heterojunction array and provide a valuable insight for the further development of such materials.

Beet juice-induced green fabrication of plasmonic AgCl/Ag nanoparticles

A simple, green, and fast approach (complete within 5 min) was explored for the fabrication of hybrid AgCl/Ag plasmonic nanoparticles under microwave (MW) irradiation. In this method, beet juice served as a reducing reagent, which is an abundant sugar-rich agricultural produce. Interestingly, the obtained AgCl/Ag samples have a smaller size than the AgCl reaction precursor. This makes it an unusual top-down hydrothermal synthesis. The as-prepared material displayed good photocatalytic activity for the degradation of methyl orange (MO) dye.

Recent Development on Titania Based Mixed Oxide Photocatalysts for Environmental Application under Visible Light

In the recent years most of the studies are confined to the mixing of ZrO2, SiO2, WO4 or ceria with titania in different composition so as to stabilize anatase phase, maintain high surface area and increase visible light absorption for better photocatalytic activity. Method of synthesis also helps in effective doping and enhancing surface area of the resultant materials. Nonmetal doping of oxide semiconductor materials facilitates the visible light application of photocatalysis. Based on the recent literature this review elaborately discuss on the development of titania based mixed oxide catalyst with or without different doping for visible light application. In addition this review deals with critical analysis of these materials towards photocatalytic oxidation of organics and reduction of pollutants like toxic metal ions and nitrates.

Recent Progress in Photocatalysis Mediated by Colloidal II-VI Nanocrystals

The use of photoexcited electrons and holes in semiconductor nanocrystals as reduction and oxidation reagents is an intriguing way of harvesting photon energy to drive chemical reactions. This review focuses on recent research efforts to understand and control the photocatalytic processes mediated by colloidal II-VI nanocrystalline materials, such as cadmium and zinc chalcogenides. First, we highlight how nanocrystal properties govern the rates and efficiencies of charge-transfer processes relevant to photocatalysis. We then describe the use of nanocrystal catalyst heterostructures for fuel-forming reactions, most commonly H2 generation. Finally, we review the use of nanocrystal photocatalysis as a synthetic tool for metal-semiconductor nano-heterostructures.

Hydrogen Production by Photoreforming of Renewable Substrates

This paper focuses on the application of photocatalysis to hydrogen production from organic substrates. This process, usually called photoreforming, makes use of semiconductors to promote redox reactions, namely, the oxidation of organic molecules and the reduction of H+ to H2. This may be an interesting and fully sustainable way to produce this interesting fuel, provided that materials efficiency becomes sufficient and solar light can be effectively harvested. After a first introduction to the key features of the photoreforming process, the attention will be directed to the needs for materials development correlated to the existing knowledge on reaction mechanisms. Examples are then given on the photoreforming of alcohols, the most studied topic, especially in the case of methanol and carbohydrates. Finally, some examples of more complex but more interesting substrates, such as waste solutions, are proposed.

In situ preparation of novel p-n junction photocatalyst BiOI/(BiO)2CO3 with enhanced visible light photocatalytic activity

Novel p-n junction photocatalysts BiOI/(BiO)2CO3 with different contents of BiOI were in situ synthesized by etching (BiO)2CO3 precursor with hydroiodic acid (HI) solution. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectrometry (FT-IR), energy-dispersive spectroscopy (EDS) and UV-vis diffuse reflectance spectroscopy (DRS) were employed to study the structures, morphologies and optical properties of the as-prepared samples. Under visible light (λ>420 nm), BiOI/(BiO)2CO3 hybrid displayed much higher photocatalytic activity than pure (BiO)2CO3 and BiOI for the degradation of methyl orange (MO). The increased photocatalytic activity of BiOI/(BiO)2CO3 could be attributed to the formation of the p-n junction between p-BiOI and n-(BiO)2CO3, which effectively suppresses the recombination of photoinduced electron-hole pairs. Moreover, the tests of radical scavengers confirmed that •O2- and h+ were the main reactive species for the degradation of MO.

In situ loading of ultra-small Cu2O particles on TiO2 nanosheets to enhance the visible-light photoactivity

In this work, ultra-small Cu(2)O nanoparticles have been loaded on TiO(2) nanosheets with {001} facets exposed through a one-pot hydrothermal reaction. These Cu(2)O nanoparticles are well-dispersed on TiO(2) nanosheets with narrow size distributions and controllable sizes from 1.5 to 3.0 nm. Through XRD, TEM, N(2) absorption-desorption isotherms and UV-vis diffuse reflectance spectra, the Cu(2)O/TiO(2) nanosheets show similar phase structures, morphologies, pore structures as compared to pure TiO(2) nanosheets. Due to the loading of ultra-small Cu(2)O nanoparticles, heterojunctions are formed between Cu(2)O and TiO(2), which favors the efficient separation of photo-generated electrons and holes. Caused by the electron transfer from Cu(2)O to TiO(2), Cu(2)O/TiO(2) nanosheets show excellent visible-light activity, about 3 times that of N-doped TiO(2) nanosheets with {001} facets exposed. Furthermore, charge transfer rate across the interface of Cu(2)O and TiO(2) shows great dependence on the size of Cu(2)O particles. The charge transfer across the interface may be more efficient between TiO(2) nanosheets and smaller Cu(2)O nanoparticles. Therefore, the Ti : Cu = 30 : 1(atomic ratio) sample shows the best activity due to its balance in light harvest and electron transfer rate in the degradation of phenol under visible light.

Enhancement of visible-light-driven photoresponse of Mn/ZnO system: photogenerated charge transfer properties and photocatalytic activity

A visible-light-active ZnO photocatalyst system in the presence of manganese ions (Mn/ZnO) was prepared via a simple and rapid approach. XRD, XPS, Raman scattering and UV-Vis DRS confirmed the manganese exists in multivalent forms (Mn(3+)/Mn(2+)) in the ZnO lattice, furthermore, ZnO light absorption is extended to the visible region. The photocatalytic activities of the catalysts were evaluated by measuring the photodegrading efficiency of 2,4-dichlorophenol (DCP) under visible light irradiation. With an optimal molar ratio of 5% in Mn/ZnO the highest rate photodegradation was achieved under the experimental conditions. We have characterized the separation and transfer behavior of the photogenerated charges in the visible region by means of surface photovoltage (SPV), surface photocurrent (SPC) and transient photovoltage (TPV) techniques. Based on the comprehensive investigation of the photovoltaic properties of Mn/ZnO photocatalyst, we illustrate the behavior of photogenerated charges have distinct effects on the photocatalytic activity. It is demonstrated that the incorporation of multivalent Mn in ZnO promoted the separation of photogenerated charges, inhibited the recombination of photogenerated carriers, and thus prolonged the charges lifetime to participate in the photocatalytic reaction, resulting in highly efficient photocatalytic activity, which is attributed to the formation of a strong electronic interaction between the multivalent Mn and ZnO.

Progress in graphene-based photoactive nanocomposites as a promising class of photocatalyst

Carbon nanomaterials based nanocomposites have gained increasing interest owing to their potential applications in many aspects, such as photocatalysis, solar cells, solar fuel, etc. Graphene, as a new type of carbon nanomaterial, has attracted consideration attention due to its unique two-dimensional conjugated structure and electronic properties. In this feature article, we cover recent advances in the applications of graphene-based photoactive nanocomposites in environmental remediation and energy conversion, including photocatalytic degradation of organic contaminants, photoelectrochemical solar cells, and solar fuel production. Finally, the perspectives of the challenges and opportunities in this emerging area are also discussed.

Porous graphitic carbon nitride synthesized via direct polymerization of urea for efficient sunlight-driven photocatalytic hydrogen production

Energy captured directly from sunlight provides an attractive approach towards fulfilling the need for green energy resources on the terawatt scale with minimal environmental impact. Collecting and storing solar energy into fuel through photocatalyzed water splitting to generate hydrogen in a cost-effective way is desirable. To achieve this goal, low cost and environmentally benign urea was used to synthesize the metal-free photocatalyst graphitic carbon nitride (g-C₃N₄). A porous structure is achieved via one-step polymerization of the single precursor. The porous structure with increased BET surface area and pore volume shows a much higher hydrogen production rate under simulated sunlight irradiation than thiourea-derived and dicyanamide-derived g-C₃N₄. The presence of an oxygen atom is presumed to play a key role in adjusting the textural properties. Further improvement of the photocatalytic function can be expected with after-treatment due to its rich chemistry in functionalization.

Hierarchically plasmonic photocatalysts of Ag/AgCl nanocrystals coupled with single-crystalline WO₃ nanoplates

The hierarchical photocatalysts of Ag/AgCl@plate-WO₃ have been synthesized by anchoring Ag/AgCl nanocrystals on the surfaces of single-crystalline WO₃ nanoplates that were obtained via an intercalation and topochemical approach. The heterogeneous precipitation process of the PVP-Ag⁺-WO₃ suspensions with a Cl⁻ solution added drop-wise was developed to synthesize AgCl@WO₃ composites, which were then photoreduced to form Ag/AgCl@WO₃ nanostructures in situ. WO₃ nanocrystals with various shapes (i.e., nanoplates, nanorods, and nanoparticles) were used as the substrates to synthesize Ag/AgCl@WO₃ photocatalysts, and the effects of the WO₃ contents and photoreduction times on their visible-light-driven photocatalytic performance were investigated. The techniques of TEM, SEM, XPS, EDS, XRD, N₂ adsorption-desorption and UV-vis DR spectra were used to characterize the compositions, phases and microstructures of the samples. The RhB aqueous solutions were used as the model system to estimate the photocatalytic performance of the as-obtained Ag/AgCl@WO₃ nanostructures under visible light (λ ≥ 420 nm) and sunlight. The results indicated that the hierarchical Ag/AgCl@plate-WO₃ photocatalyst has a higher photodegradation rate than Ag/AgCl, AgCl, AgCl@WO₃ and TiO₂ (P25). The contents and morphologies of the WO₃ substrates in the Ag/AgCl@plate-WO₃ photocatalysts have important effects on their photocatalytic performance. The related mechanisms for the enhancement in visible-light-driven photodegradation of RhB molecules were analyzed.

H₂ production by renewables photoreforming on Pt-Au/TiO₂ catalysts activated by reduction

Bimetallic Pt-Au nanoparticles supported on reduced anatase nanocrystals represent a new class of promising photocatalysts with high activity in hydrogen production by photoreforming of aqueous solution of renewable feedstock, such as ethanol and glycerol. The catalysts are easily obtained by metal impregnation of commercial TiO₂, followed by a reductive treatment. Remarkably, deeper catalyst pre-reduction results in enhanced photoactivity. When ethanol is used as sacrificial agent, under both UV-A or simulated sunlight irradiation, H₂ is the most abundant product in the gas stream whereas, in the case of glycerol, significant amounts of CO₂ have also been detected, indicating a more efficient oxidation of the organic sacrificial agent. The presence of bimetallic Pt-Au nanoparticles and of Ti³⁺ sites/O²⁻ vacancies in the bulk structure of titania are two key parameters to maximize light absorption and feedstock activation, finally resulting in good photocatalytic performances.

Assembly of CuIn(1-x)Ga(x)S2 nanorods into highly ordered 2D and 3D superstructures

Here, we report self- and directed assembly of CuIn(1-x)Ga(x)S(2) (CIGS) nanorods into highly ordered 2D and 3D superstructures. The assembly protocol is dictated by the ligand environment and is hence chemically tunable. Thiol capped nanorods spontaneously assemble into 3D aligned nanorod clusters over a period of hours with end to end and side to side order. These clusters can be disassembled by ligand exchange with an amine and subsequently reassembled either at a substrate interface or as free floating 2D sheets by directed assembly protocols. This dimensional control of CIGS nanorod assembly, extending over device scale areas with high degrees of order, is highly attractive for applications utilizing these important quaternary photoabsorbers.

Plasmon inducing effects for enhanced photoelectrochemical water splitting: X-ray absorption approach to electronic structures

Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.

Template free synthesis of crystallized nanoporous F-Ta2O5 spheres for effective photocatalytic hydrogen production

Crystallized and fluorinated nanoporous Ta(2)O(5) spheres were prepared by a combination of co-precipitation and hydrothermal methods in a NH(4)F-containing water solution. The as-prepared porous spheres, with a pore-size of around 7 nm and a large specific surface area of 132 m(2) g(-1), show superior photocatalytic hydrogen production activity over P25 and commercial-Ta(2)O(5).