Shaoqin Peng

Eosin Y-sensitized graphitic carbon nitride fabricated by heating urea for visible light photocatalytic hydrogen evolution: the effect of the pyrolysis temperature of urea

Graphitic carbon nitride (g-C3N4) was prepared by pyrolysis of urea at different temperatures (450-650 °C), and characterized by thermogravimetric and differential thermal analysis (TG-DTA), elemental analysis (C/H/N), X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), Brunauer-Emmett-Teller (BET) analysis, Fourier transform-infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectra. The samples prepared at low temperatures (450 and 500 °C) are a mixture of g-C3N4 and impurities, whereas the samples prepared at high temperatures (550, 600 and 650 °C) should be g-C3N4 (polymeric carbon nitride). The polymerization degree of g-C3N4 for the prepared samples increases to a maximum at 600 °C with increasing pyrolysis temperature and then decreases, whereas the defect concentration changes conversely, that is, g-C3N4 prepared at 600 °C has the lowest defect concentration. Using Eosin Y (EY) and the prepared sample as the sensitizer and the matrix, respectively, the photocatalytic activity for hydrogen evolution from aqueous triethanolamine solution was investigated. The g-C3N4 prepared at 600 °C exhibits the highest sensitization activity. Under optimum conditions (1.25 × 10(-5) mol L(-1) EY and 7.0 wt% Pt), the maximal apparent quantum yield of EY-sensitized g-C3N4 prepared at 600 °C for hydrogen evolution is 18.8%. The highest activity can be attributed to the pure composition, the higher dye adsorption amount and the lowest defect concentration.

Nitrogen-doped TiO2 modified with NH4F for efficient photocatalytic degradation of formaldehyde under blue light-emitting diodes.

A nitrogen-doped TiO(2) (N-TiO(2)) photocatalyst was prepared by calcination of the hydrolysis precipitate of Ti(SO(4))(2) with aqueous ammonia. The prepared N-TiO(2) was treated with NH(4)F (F-N-TiO(2)) by an impregnation-calcination method. The photocatalyst (F-N-TiO(2)) was characterized by X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR), UV-vis diffusive reflectance spectroscopy (DRS), BET and X-ray photoelectron spectroscopy (XPS). With blue light-emitting diode (LED) as the light source, its photocatalytic activity for the degradation of formaldehyde was investigated. NH(4)F treatment enhances markedly photocatalytic activity of N-TiO(2). The treatment increases the visible absorption of N-TiO(2), decreases its specific surface area and influences the concentration of oxygen vacancies in N-TiO(2). Photocatalytic activity of F-N-TiO(2) depends on the visible absorption, the specific surface area, and the concentration of oxygen vacancies. The preparation conditions, such as the calcination temperature and the initial molar ratio of NH(4)F to N-TiO(2), have a significant influence on the photocatalytic activity. The doping mechanism of NH(4)F was investigated.

Phosphate-assisted hydrothermal synthesis of hexagonal CdS for efficient photocatalytic hydrogen evolution

Cubic nanocrystalline CdS was hydrothermally transformed into hexagonal CdS in the presence of Na3PO4 at 180 °C for 12 h. The as-prepared CdS samples were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), BET, electrophoretic analysis, photoluminescence (PL) spectra and UV-Vis absorption spectra techniques. Effects of phosphate concentration, hydrothermal time and Pt loading content were investigated. Their photoactivity was evaluated by hydrogen evolution from aqueous solution containing formic acid as a hole scavenger under visible light (λ ≥ 420 nm) irradiation. Phosphate markedly promotes the phase transformation of CdS from cubic to hexagonal. With 0.050 mol L−1 PO43−, the formed hexagonal phase content reaches a maximum (82%). The as-prepared CdS with a high percentage of hexagonal phase displays excellent activity for photocatalytic hydrogen evolution. Pt is highly dispersed on CdS so that the Pt content for the effective hydrogen evolution is very low. The CdS loaded with 0.025 wt% Pt shows the maximum activity for the hydrogen evolution. The apparent quantum yield at 420 nm amounts to 21.4%. This work highlights a facile and low-cost method for the preparation of a highly-efficient CdS photocatalyst. The possible mechanisms were discussed.

Synergetic effect of metal nickel and graphene as a cocatalyst for enhanced photocatalytic hydrogen evolution via dye sensitization

Exploiting new, low-cost and efficient electrocatalysts for hydrogen evolution reaction (HER) is important to resolve the energy crisis and environment pollution. In this work, graphene decorated with Ni nanoparticles (NPs) were synthesized via one-pot reduction using graphene oxide (GO, the obtained composite was denoted as GN) as a precursor. The as-prepared composite GN exhibits much better electrocatalytic and dye-sensitized HER activities than single Ni and reduced graphene oxide (RGO), namely, a great synergetic effect of RGO and Ni for HER. The coupling of metal Ni with the defect carbons of RGO plays a key role in the synergetic effect. The structure of GN composites is another key factor to the synergetic effect. The highest apparent quantum yield (AQY) for dye-sensitized photocatalytic hydrogen evolution at 470 nm reaches 30.3% under the optimal conditions.

Enhancement of photocatalytic H2 evolution of eosin Y-sensitized reduced graphene oxide through a simple photoreaction

Summary A graphene oxide (GO) solution was irradiated by a Xenon lamp to form reduced graphene oxide (RGO). After irradiation, the epoxy, the carbonyl and the hydroxy groups are gradually removed from GO, resulting in an increase of sp2 π-conjugated domains and defect carbons with holes for the formed RGO. The RGO conductivity increases due to the restoration of sp2 π-conjugated domains. The photocatalytic activity of EY-RGO/Pt for hydrogen evolution was investigated with eosin Y (EY) as a sensitizer of the RGO and Pt as a co-catalyst. When the irradiation time is increased from 0 to 24 h the activity rises, and then reaches a plateau. Under optimum conditions (pH 10.0, 5.0 × 10−4 mol L−1 EY, 10 μg mL−1 RGO), the maximal apparent quantum yield (AQY) of EY-RGO24/Pt for hydrogen evolution rises up to 12.9% under visible light irradiation (λ ≥ 420 nm), and 23.4% under monochromatic light irradiation at 520 nm. Fluorescence spectra and transient absorption decay spectra of the EY-sensitized RGO confirm that the electron transfer ability of RGO increases with increasing irradiation time. The adsorption quantity of EY on the surface of RGO enhances, too. The two factors ultimately result in an enhancement of the photocatalytic hydrogen evolution over EY-RGO/Pt with increasing irradiation time. A possible mechanism is discussed.

NaCl-assisted low temperature synthesis of layered Zn-In-S photocatalyst with high visible-light activity for hydrogen evolution

Single ZnIn2S4 or Zn-In-S composites were synthesized by a simple low temperature (80 °C) method assisted by the presence of NaCl. The products were characterized by XRD, SEM, TEM, TG/DTA, ICP-AES, BET and UV-Vis absorption spectrometry. The decomposition of thioacetamide (TAA) at low temperature was investigated by UV-Vis absorption spectrometry. The decomposition rate of TAA and NaCl concentration influenced the composition, structure, morphology and grain size of the products. The obtained samples are marigold-like microspheres consisting of nanosheets. Loading 0.10 wt% Pt on the samples by in situ photoreduction, allowed the photocatalytic activity of the prepared samples to be evaluated by hydrogen evolution from aqueous solution containing triethanolamine as the electron donor under visible light (λ ≥ 420 nm) irradiation. The activity of the sample obtained in the presence of 0.50 mol L−1 NaCl is ca. 5 times higher than samples without NaCl. Thus, a photocatalyst with layered structure is beneficial for photoactivity. A possible mechanism is discussed.

Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-Sensitized H2 Evolution

Graphene is an advanced carbon energy material due to its excellent properties. Reduction of graphene oxide (GO) is the most promising mass production route of graphene/reduced graphene oxide (rGO). To maintain graphenes properties and avoid restacking of rGO sheets in bulk, the preparation of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy. This article presents a facile route to synthesize graphene sponge by thermal treating GO powder at low temperature of 250 °C under N2 atmosphere. The sponge possesses macroporous structure (5-200 nm in size) with BET specific surface area of 404 m(2) g(-1) and high conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer Eosin Y (EY) and cocatalyst Pt was investigated. The rGO sponge shows highly efficient dye-sensitized photocatalytic H2 evolution compared to that obtained via a chemical reduction method. The maximum apparent quantum yield (AQY) reaches up to 75.0% at 420 nm. The possible mechanisms are discussed. The synthesis method can be expanded to prepare other graphene-based materials.

Composition, morphology and photocatalytic activity of Zn-In-S composite synthesized by a NaCl-assisted hydrothermal method

Zn-In-S composites were synthesized by a NaCl-assisted hydrothermal method and characterized by an X-ray diffractometer (XRD), Scanning electron microscope (SEM), Energy dispersive spectrometer (EDS), BET, UV-Vis absorption spectrometer and inductively coupled plasma-atomic emission spectrometer (ICP). The products obtained by the hydrothermal method are a single ZnIn2S4 or a composite of ZnIn2S4, ZnmIn2Sm+3, ZnS and In2S3, depending on the NaCl concentration. The samples are consisted of microspheres and sheet-like grains with a marigold-like superstructure. The grain size of the products increases and the specific surface area decreases with an increase of the NaCl concentration. Their photoactivities were evaluated by hydrogen evolution from an aqueous triethanolamine solution under visible light (λ ≥ 420 nm) irradiation. 0.50 wt% Pt was deposited on the samples for the photocatalytic hydrogen evolution by in situ photoreduction. The sample obtained in the presence of 0.20 mol L−1 NaCl exhibits the highest activity for hydrogen evolution, ca. 4 times as high as that of the sample prepared without NaCl. A possible mechanism was discussed.

Modification of TiO2 with sulfate and phosphate for enhanced eosin Y-sensitized hydrogen evolution under visible light illumination

TiO2 was modified with sulfate and phosphate (denoted as S/TiO2 and P/TiO2) through a simple sulfuric or phosphoric acid treatment. A strong coordination bond forms between sulfate or phosphate and Ti(4+) of TiO2. Eosin Y (EY)-sensitized S/TiO2 and P/TiO2 (Pt as a co-catalyst and triethanolamine as a sacrificial electron donor) exhibit enhanced photocatalytic activity for hydrogen evolution under visible light illumination (λ > 420 nm) compared to that of EY-sensitized TiO2. The conduction band (CB) edges of S/TiO2 and P/TiO2 shift toward the negative, and the hydrogen bond interaction between the reduced radical EY˙-H and S/TiO2 or P/TiO2 is enhanced due to the inducing effect of the bound sulfate and phosphate. Thus, the photocatalytic hydrogen evolution is promoted. The effects of the concentration of the sulfuric or phosphoric acid as well as concentration of EY on the sensitization hydrogen evolution were investigated. The possible mechanism was discussed.

Template-free synthesis of hollow Ni/reduced graphene oxide composite for efficient H2 evolution

To resolve the energy crisis and environment pollution, it is urgent to exploit highly efficient and inexpensive catalysts for the hydrogen evolution reaction (HER). Herein, we report a hollow Ni/RGO (reduced graphene oxide) composite (HNG) as an efficient HER catalyst, which was fabricated through a facile template-free method using graphene oxide (GO) sol as the raw material. The explosively released gases from GO under thermal treatment restrained the growth of Ni species towards the formed gas bubbles, resulting in the formation of hollow Ni nanoparticles (NPs). The hollow structure contributes to the diffusion of the reactant H2O to the active sites for HER. The electrocatalytic HER activity of HNG increases with the amount of hollow Ni NPs in the range from 3 to 12 wt% Ni loading. However, the dye-sensitized photocatalytic H2 evolution activity reaches the maximum at 9.0 wt%, due to the interaction between the dye and RGO. Under optimal conditions, the highest apparent quantum yield (AQY) for the hydrogen evolution at 470 nm reaches 47.7%. This study paves the way for the synthesis of various hollow metal–RGO materials.