Xiufang Chen

Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light

We investigated semiconductor characteristics for polymeric carbon nitride as a metal-free photocatalyst working with visible light and have shown that the efficiency of hydrogen production by photochemical water reduction can be improved by approximately 1 order of magnitude by introducing the right type of mesoporosity into polymeric C(3)N(4). We anticipate a wide rang of potential application of C(3)N(4) as energy transducers for artificial photosynthesis in general, especially with a 3D continuous nanoarchitecture. Moreover, the results of finding photoactivity for carbon nitride nanoparticles can enrich the discussion on prebiotic chemistry of the Earth, as HCN polymer clusters are unequivocal in the solar system.

Synthesis of a Carbon Nitride Structure for Visible‐Light Catalysis by Copolymerization

and nonmetallic elements (N, C, B) creates localized/ delocalized states in the band gap and thus extends its optical absorption to the visible region, but doping usually comes with accelerated charge recombination and lower stability of the doped materials. Meanwhile, various other inorganic, non-TiO2-based, visible-light catalysts have been developed (e.g., metal oxides, nitrides, sulfides, phosphides, and their mixed solid solutions), whereby Ga, Ge, In, Ta, Nb, and W are the main metal constituents. However, sustained utilization of solar energy calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far. Recently, a polymeric semiconductor on the basis of a defecteous graphitic carbon nitride (g-C3N4), was introduced as a metal-free photocatalyst which fulfills the basic requirements for a water-splitting catalyst, including being abundant, stable, and responsive to visible light. In the following, we use the notation “g-C3N4” to describe this class of materials rather than the idealized structure. The most active system is in fact presumably an N-bridged “poly(tri-s-triazine)”, already described by Liebig as “melon”. A semiconductor structure with band edges straddling the water redox potential was revealed for melon by DFT calculations, albeit electrochemical analysis is still awaited. g-C3N4 is considered to be the most stable phase of covalent carbon nitride, and facile synthesis of the melon substructure from simple liquid precursors and monomers allows easy engineering of carbon nitride materials to achieve the desired nanostructures via soft-chemical processing routes and methods. For instance, a high surface area (67–400 mg ) can be imparted on g-C3N4 materials by polymerization of cyanamide on a silica template, which results in photocatalytically more active g-C3N4 nanostructures. [8] Metal-doped gC3N4 can also be conveniently obtained by polymerization of dicyandiamine in the presence of metal salts, and thus multifunctionalization of such materials for a variety of applications can be achieved. Most importantly, the electronic and optical properties of carbon nitride, regarded as a polymer semiconductor, are in principle adjustable by organic protocols. Such organic protocols have been widely used to control the performance of traditional p-conjugated polymers, for example, to improve solar-cell efficiencies by constructing copolymerized donor–acceptor structures, or to modify electronic properties by co-blending with p/n-type organic dopants. Our aim was to use such organic modifications to extend the insufficient light absorption of g-C3N4 (a result of its large band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum. Here we demonstrate that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA). The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for hydrogen production from water containing an appropriate sacrificial reagent with visible light. In principle, BA can be directly incorporated into the classical carbon nitride condensation scheme (Scheme 1). New carbon nitride structures were therefore synthesized by dissolving dicyandiamide with different amounts of BA in water, followed by thermally induced copolymerization at 823 K. For simplicity, the resulting samples are denoted CNBx, where x (0.05, 0.1, 0.2, 0.5, 1, 2) refers to the weighedin amount of BA. The structure, texture, and electrochemical properties of these materials were characterized, and their photochemical performance analyzed. Their XRD patterns (Figure S1, Supporting Information) are dominated by the characteristic (002) peak at 27.48 of a graphitic, layered structure with an interlayer distance of d = [*] J. Zhang, X. Chen , Prof. X. Fu, Prof. X. Wang State Key Laboratory Breeding Base of Photocatalysis Fuzhou University, Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

Fe-g-C3N4-Catalyzed Oxidation of Benzene to Phenol Using Hydrogen Peroxide and Visible Light

A bioinspired iron-based catalyst with semiconductor photocatalytic functions in combination with a high surface area holds promise for synthetic chemistry via combining photocatalysis with organosynthesis. Here exemplified for phenol synthesis, Fe-g-C(3)N(4)/SBA-15 is able to oxidize benzene to phenol with H(2)O(2) even without the aid of strong acids or alkaline promoters. By taking advantage of both catalysis and photocatalysis functions of g-C(3)N(4) nanoparticles, the yield of the phenol can be markedly promoted.

One‐Step Solvothermal Synthesis of a Carbon@TiO2 Dyade Structure Effectively Promoting Visible‐Light Photocatalysis

The development of sunlight harvesting chemical systems to catalyze relevant reactions, i.e., water splitting, CO 2 fi xation, and organic mineralization, is the key target in artifi cial photosynthesis but remains a diffi cult challenge. Titanium dioxide (TiO 2 ) has been widely used as a photocatalyst for solar energy conversion and environmental applications because of its low toxicity, abundance, high photostability, and high effi ciency. [ 1–4 ] However, the application of pure TiO 2 is limited, because it requires ultraviolet (UV) light, which makes up only a small fraction ( < 4%) of the total solar spectrum reaching the surface of the earth. Therefore, over the past few years, considerable efforts have been directed towards the improvement of the photocatalytic effi ciency of TiO 2 in the visible (vis)-light region. [ 5–7 ] This has been mainly achieved by introducing various dopants into the TiO 2 structure which can narrow the bandgap. The initial approach to dope TiO 2 materials was achieved using transition metals ions such as V, Cr, or Fe. [ 6 , 8–10 ] However, such metal doped materials lack the necessary thermal stability, exhibit atom diffusion and a remarkably increased electron/hole recombination of defect sites, which results in a low photocatalytic effi ciency. [ 11 ] Non-metal doping has since proved to be far more successful and has been extensively investigated. Thus, numerous reports on TiO 2 doped with B, F, N, C, S, or I have demonstrated a signifi cant improvement of the visible-light photocatalytic effi ciency. [ 4 , 12–16 ]

Synthesis of transition metal-modified carbon nitride polymers for selective hydrocarbon oxidation.

Modification of graphitic carbon nitride (g-C(3)N(4)) photocatalyst with transition metals was achieved with a simple soft-chemical approach using dicyandiamide monomer and metal chloride as precursors, in combination with a thermal-induced polycondensation at 600u2009°C under nitrogen atmosphere. The resultant organic-inorganic hybrid materials were thoroughly characterized by a variety of techniques, including X-ray diffraction (XRD), UV/Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), N(2)-sorption, transmission electron microscopy (TEM), photoluminescence (PL), and FTIR. Benzene hydroxylation and styrene epoxidation reactions were employed to evaluate the catalytic/photocatalytic activity of the synthesized g-C(3)N(4)-based catalysts. Results showed that Fe- and Cu-modified g-C(3)N(4) were active for the hydroxylation of benzene to phenol using H(2)O(2) under mild conditions. It was also found that g-C(3)N(4) could promote the catalytic epoxidation of styrene using molecular oxygen as the primary oxidant; after modification with Co and Fe, the catalytic performance for styrene epoxidation with O(2) could be significantly improved, especially when coupled with visible-light irradiation.

N-Doped SiO2/TiO2 mesoporous nanoparticles with enhanced photocatalytic activity under visible-light irradiation

Mesoporous nanocrystalline N-doped SiO2/TiO2 visible-light photocatalysts were prepared by treating SiO2/TiO2 xerogels in a flow of nitrogen gas bubbled through concentrated ammonia solution. Structural characterization and performance analysis results revealed that the addition of SiO2 remarkably altered the phase composition, specific surface area, microstructure, as well as the photocatalytic activity of N-doped TiO2. The presence of SiO2 in N-doped TiO2 particles suppressed the formation of rutile phase and the crystal growth of N-doped TiO2 particles during thermal calcinations. When weight ratio of SiO2/TiO2 was in 0.05-0.20, the N-doped SiO2/TiO2 exhibited higher photocatalytic activity than the N-doped TiO2, and optimum ratio was found to be 0.05. The enhanced photocatalytic activity could be attributed to the higher specific area, larger pore volume, and more surface hydroxyl groups in the catalyst.

Hierarchical macro/mesoporous TiO2/SiO2 and TiO2/ZrO2 nanocomposites for environmental photocatalysis

Photocatalytic purification methods for polluted air and wastewater show great promise for environmental remediation, allowing the “green” mineralization of organic pollutants under ambient conditions. The creation of macro/mesopores in semiconductor photocatalysts has been found to improve the overall photocatalytic efficiency, but porous systems are generally unstable against thermal sintering, which is indispensable for removing organic templates and enhancing structural crystallization. In this study, we employed nanosized ZrO2 and SiO2 as structural modifiers to improve the structural stability in a macro/mesoporous TiO2 photocatalyst. This was accomplished by soft-chemical synthesis in the presence of surfactants, followed by calcination at high temperatures. The resulting porous TiO2-based nanocomposites not only feature enhanced textural properties and improved thermal stability, but also show an improvement in photocatalytic activity over pure TiO2. The introduction of a secondary phase imparts the additional functions of improved surface acidity and extra binding sites onto the porous structures. The favorable textural properties, along with the improved surface functions contribute to the high photocatalytic activity of catalysts calcined at high temperatures.