Liejin Guo

Semiconductor-based Photocatalytic Hydrogen Generation

2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting

We report on a novel heterojunction WO(3)/BiVO(4) photoanode for photoelectrochemical water splitting. The heterojunction films are prepared by solvothermal deposition of a WO(3) nanorod-array film onto fluorine-doped tin oxide (FTO) coated glass, with subsequent deposition of a low bandgap, 2.4 eV, visible light responding BiVO(4) layer by spin-coating. The heterojunction structure offers enhanced photoconversion efficiency and increased photocorrosion stability. Compared to planar WO(3)/BiVO(4) heterojunction films, the nanorod-array films show significantly improved photoelectrochemical properties due, we believe, to the high surface area and improved separation of the photogenerated charge at the WO(3)/BiVO(4) interface. Synthesis details are discussed, with film morphologies and structures characterized by field emission scanning electron microscopy and X-ray diffraction.

Vertically Aligned WO3 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties

Photocorrosion stable WO(3) nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO(3) morphologies of hexagonal and monoclinic structure, ranging from nanowire to nanoflake arrays, are tailored by adjusting solution composition with growth along the (001) direction. Photoelectrochemical measurements of illustrative films show incident photon-to-current conversion efficiencies higher than 60% at 400 nm with a photocurrent of 1.43 mA/cm(2) under AM 1.5G illumination. Our solvothermal film growth technique offers an exciting opportunity for growth of one-dimensional metal oxide nanostructures with practical application in photoelectrochemical energy conversion.

Hydrogen production from glucose used as a model compound of biomass gasified in supercritical water

Abstract Thermochemical gasification of biomass has been identified as a possible system for producing renewable hydrogen. A continuous tubular supercritical water gasification system is under development that can be used for solution or slurry materials gasification without drying. A unique feature of this system is its ability to realize the overall high-pressure continuous reaction by operating the valves. By the use of this system, designed for temperatures up to 923.15 K and pressures up to 35 MPa , glucose, as a model compound of biomass, was gasified in supercritical water at a series of temperature and pressure during different resident times to form a product gas composed of H2, CO, CH4, CO2, and a small amount of C2H4 and C2H6. Glucose at low concentrations (ca. 0.1 M ) can be completely gasified in 923.15 K , 25 MPa , and 3.6 min resident time and no char or tar was observed. Consequently, we adopted these conditions as baseline reaction conditions for the following glucose concentration, alkali addition and reactor tube diameter effect studies. The raw biomass feedstock of sawdust with some CMC was also gasified in this system, the gasification efficiency in excess of 95% was reached.

Metal sulphide semiconductors for photocatalytic hydrogen production

Photocatalytic hydrogen production using semiconductor materials is one of the ideal processes for direct solar energy conversion. Sulphide semiconductor photocatalysts have attracted much attention due to their excellent solar spectrum responses and high photocatalytic activities. This article presents recent research progress in the development of visible light driven sulphide photocatalysts, focusing on the expansion of solar spectrum response and enhancement of charge separation efficiency. As is known, the ultimate goal of photocatalytic hydrogen production is to meet the practical energy demand of human beings. Thus, design of highly efficient and low cost sulphide photocatalysts with excellent sunlight response is highly desired. So we also highlight the crucial issues in the development of highly efficient sulphide photocatalysts without noble metal cocatalysts. The present paper is expected to provide important scientific reference for future works. Finally, the challenges and perspectives in this area are also discussed.

Twins in Cd1−xZnxS solid solution: Highly efficient photocatalyst for hydrogen generation from water

Cd1−xZnxS solid solution with nano-twin structures are synthesized and exhibit superior photocatalytic activities for H2 evolution from water under visible light irradiation (λ ≥ 430 nm) without noble metal co-catalysts. Such Cd0.5Zn0.5S nanocrystals show the highest activity for hydrogen evolution with an extremely high apparent quantum yield (AQY = 43%) at 425 nm, achieving a hydrogen evolution rate of 1.79 mmol h−1 without noble metals. The hydrogen evolution rate of 1.70 mmol h−1 was achieved under simulated sunlight conditions (without infrared light). The “back to back” potential formed by parallel nano-twins in the Cd1−xZnxS crystals can significantly improve the separation of the photo-generated electrons/holes (preventing their recombination) thus enhancing the photocatalytic activity. Photodeposition experiments of noble metals strongly support such a mechanism. It is found that noble metals were selectively photo-deposited at central regions between the twin boundaries. The concentration of free electrons at the central region of twins was markedly higher and the twins can effectively separate the H2 evolution sites (electrons) from oxidation reaction sites (holes).

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.

Twin-induced one-dimensional homojunctions yield high quantum efficiency for solar hydrogen generation

Efficient charge separation is of crucial importance for the improvement of photocatalytic activity for solar hydrogen evolution. Here we report efficient photo-generated charge separation by twin-induced one-dimensional homojunctions with type-II staggered band alignment, using a ternary chalcogenate, i.e. Cd0.5Zn0.5S nanorod as a model material. The quantum efficiency of solar hydrogen evolution over this photocatalyst, without noble metal loading, reaches 62%. Unlike traditional heterojunctions, doping or combination of additional elements are not needed for the formation of this junction, which permits us to tune the band structures of semiconductors to the specific application in a more precise way. Our results highlight the power of forming long-range ordered homojunctions at the nanoscale for photocatalytic and photoelectrochemical applications.

Structure defects in g-C3N4 limit visible light driven hydrogen evolution and photovoltage

Graphitic carbon nitride (g-C3N4) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C3N4 is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520–640 °C). The highest activity of 0.301 mmol h−1 g−1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron–hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and −0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C3N4 lattice.

Nanoparticles enwrapped with nanotubes: A unique architecture of CdS/titanate nanotubes for efficient photocatalytic hydrogen production from water

CdS/titanate nanotubes (CdS/TNTs) photocatalysts with a unique morphology were successfully synthesized via a simple one-step hydrothermal method. Compared with traditional CdS@TNTs composite photocatalysts prepared by the common two-step method, CdS/TNTs exhibited much higher activity for photocatalytic hydrogen evolution under visible light irradiation. Transmission electron microscopy (TEM) revealed that the CdS nanoparticle was intimately enwrapped by the surrounding TNTs. This unique architecture resulted in the appropriate dispersion of CdS nanoparticles and the intimate multipoint contacts between the CdS nanoparticle and TNTs, which led to significant enhancement of charge separation in CdS/TNTs. Accordingly, the photoactivity was improved. Meanwhile, X-ray powder diffraction (XRD) demonstrated that the highly crystalline hexagonal CdS was obtained in CdS/TNTs, which was also essential for the enhanced photocatalytic performance. The unique morphology and photocatalytic activity of CdS/TNTs were influenced by the Cd/Ti molar ratio with an optimal value of 0.05. Under this condition, the CdS amount was only 6 wt% of the total photocatalyst, which was important from an environmental point of view. The influence of loaded Pt on the activity of CdS/TNTs had also been investigated. The hydrogen production rate of 2.0 wt% Pt-loaded CdS/TNTs reached 353.4 μmol h−1, with the apparent quantum yield of 25.5% at 420 nm. This study provides a potential way to synthesize highly efficient composite photocatalysts with a novel architecture.