Zizhong Zhang

Monolayered Bi2WO6 nanosheets mimicking heterojunction interface with open surfaces for photocatalysis.

Two-dimensional-layered heterojunctions have attracted extensive interest recently due to their exciting behaviours in electronic/optoelectronic devices as well as solar energy conversion systems. However, layered heterojunction materials, especially those made by stacking different monolayers together by strong chemical bonds rather than by weak van der Waal interactions, are still challenging to fabricate. Here the monolayer Bi2WO6 with a sandwich substructure of [BiO]+–[WO4]2−–[BiO]+ is reported. This material may be characterized as a layered heterojunction with different monolayer oxides held together by chemical bonds. Coordinatively unsaturated Bi atoms are present as active sites on the surface. On irradiation, holes are generated directly on the active surface layer and electrons in the middle layer, which leads to the outstanding performances of the monolayer material in solar energy conversion. Our work provides a general bottom-up route for designing and preparing novel monolayer materials with ultrafast charge separation and active surface.

Organic semiconductor for artificial photosynthesis: water splitting into hydrogen by a bioinspired C3N3S3polymer under visible light irradiation

A novel organic semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (–S–S–) bridged C3N3S3polymer, was designed and developed to generate hydrogen from water under visible light irradiation. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on organic semiconductors.

Copolymerization with 2,4,6-triaminopyrimidine for the rolling-up the layer structure, tunable electronic properties, and photocatalysis of g-C3N4.

Copolymerization with 2,4,6-triaminopyrimidine (TAP) is developed for precise substitution of one nitrogen with carbon atom in the triazine ring of polymeric g-C3N4. Direct incorporation of C4N2 ring from TAP into the network retains the structural features of g-C3N4, but induces the rolling-up of g-C3N4 sheets into tubular configuration. The band gap energy is narrowed from 2.7 to 2.4 eV by a negative shift of valence band of the g-C3N4 photocatalyst, which enhances charge-carrier migration and separation, leading to higher photocatalytic activity for NO gas pollutant removal. It is attributed to the decrease of the π-deficiency and the generation of imbalanced electron density in π-electron conjugated units of g-C3N4 by TAP incorporation. This work provides a significant technique for precise control of heteroatom in the framework of g-C3N4 to finely adjust its intrinsic electronic properties and its photocatalytic properties.

Catalytic Role of Cu Sites of Cu/MCM-41 in Phenol Hydroxylation

Four types of copper-containing MCM-41 mesoporous silicas were synthesized by the surface organometallic chemistry (SOMC) procedure (Cu/MCM-41-S), mechanical mixing (Cu/MCM-41-M), impregnation (Cu/MCM-41-I), and the hydrothermal technique (Cu/MCM-41-H). The resultant samples were characterized in detail by X-ray diffraction (XRD), N(2) physical adsorption, transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), temperature-programmed reduction (TPR), and infrared spectroscopy (IR) of NO adsorption. Catalytic behaviors of these samples for hydroxylation of phenol with H(2)O(2) were evaluated. The results revealed that depending on the preparation methods the samples contain different copper-oxo species and thus show different catalytic behaviors. Among these samples, the one prepared by SOMC contains a predominant amount of isolated Cu(2+) and exhibits the most excellent catalytic activity and selectivity. The amount of isolated copper species decreases in the order of Cu/MCM-41-S > Cu/MCM-41-H > Cu/MCM-41-I > Cu/MCM-41-M, while the amount of copper oxide clusters increases in a reversal order. The difference in the catalytic activity and product selectivity of these four samples could be rationally explained by the distinction of chemical states of copper species. The highly dispersed isolated Cu(2+) species are identified as the active sites in the phenol hydroxylation, while the nonisolated Cu(2+) clusters or oxide are responsible for the deep oxidation of primary product HQ and the decrease of product selectivity. The mechanism of the copper-catalyzed phenol hydroxylation was proposed.

Nitrogen-doped graphene stabilized gold nanoparticles for aerobic selective oxidation of benzylic alcohols

Increasing efforts have been made to fabricate Au/graphene composites due to the fascinating properties of both graphene and gold. Some Au nanoparticles with an average size of tens of nanometers were directly deposited on reduced graphene oxide (RGO), utilizing the residual oxy-functional groups as the “hitching post” of the nanoparticles. Some functional groups, such as amino and thiol, were attached to the surface of the graphene in order to stabilize gold nanoparticles with a smaller particle size (<5 nm in general). Unfortunately, most of these strategies result in Au particles with limited exposed atoms, which is certainly a disadvantage for their application, such as catalysis. Introduction of nitrogen heteroatoms into the framework of graphene can not only modulate the electronic structure, but also change the surface properties of the graphene. In this work, naked Au nanoparticles with an average size of about 2–4 nm were fabricated on nitrogen-doped graphene nanosheets (NG) via the direct simple reduction method. The Au/NG nanocomposites were characterized by XRD, XPS, TEM, AFM and Raman. It was revealed that the nitrogen atoms doped in NG, rather than defects or oxygen moieties, play an essential role in stabilizing Au NPs. It was also found that the initial reaction rate of benzyl alcohol oxidation over the Au/NG catalyst is about ten fold higher than that over Au/graphene catalysts. Our findings may provide a clue of nitrogen incorporation to stabilize uncapped noble metal nanoparticles on graphene or other inorganic oxide supports, such as TiO2.

Vacuum heat-treatment of carbon nitride for enhancing photocatalytic hydrogen evolution

Polymeric carbon nitride prepared by thermal condensation of cyanuric chloride with melamine was post-treated under vacuum conditions at different temperatures in order to study in depth the structure–performance relationship. The structure, composition, photoelectric and photocatalytic properties of the resulting samples were characterized in detail by physicochemical means, such as X-ray diffraction, thermogravimetry, elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photocurrent response, electrochemical impedance spectroscopy photocatalytic hydrogen production, etc. The results revealed that the untreated polymeric carbon nitride was not a single phase but a mixture consisting of unequal-sized particles with different degrees of polymerization and chemical structures containing both s-triazine and tri-s-triazine ring building blocks. This makes it possible to modify polymeric carbon nitride through a post-treatment. Tremendous changes in the C/N ratio, IR spectra, photoresponse, morphology, and photocatalytic activity occurred mainly in two temperature ranges of 300–500 °C and 500–600 °C. Among all samples, the carbon nitride treated at 500 °C showed the highest photocatalytic activity for production of hydrogen from water, as a result of the higher content of the tri-s-triazine phase, better lamellar morphology, wider photoabsorption, and smaller electrochemical impedance. The vacuum heat-treatment at temperature above 500 °C gave rise to the broken structure, and consequently the photoactivity was reduced.

Photochemical synthesis of submicron- and nano-scale Cu2O particles.

Submicron- and nano-scale cuprous oxide particles derived from copper acetate and copper gluconate complexes were synthesized via a photochemical route in polar media without further reducing agents. The morphology, composition, and phase structure of as-prepared Cu(2)O were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). Factors affecting the morphology and size of the Cu(2)O products have been examined in detail to determine the optimum parameters to achieve a controllable synthesis. The results show that solvent is the most key factor in size- and shape-control of the Cu(2)O products. Water induces the formation of submicron particles, while alcohol results in nanoscale particles. The photochemical growth of Cu(2)O particles can be fine tuned by varying the parameters of the reaction procedure, e.g. solvent, precursor ligand, and additive. The IR results indicate that these Cu(2)O particles result from the photoinduced intramolecular electron transfer between metal and ligand. The method can be easily controlled and is expected to be applicable for the preparation of cuprous oxide supported catalysts.

Robust Photocatalytic H2O2 Production by Octahedral Cd3(C3N3S3)2 Coordination Polymer under Visible Light.

Herein, we reported a octahedral Cd3(C3N3S3)2 coordination polymer as a new noble metal-free photocatalyst for robust photocatalytic H2O2 production from methanol/water solution. The coordination polymer can give an unprecedented H2O2 yield of ca. 110.0 mmol • L−1 • g−1 at pH = 2.8 under visible light illumination. The characterization results clearly revealed that the photocatalytic H2O2 production proceeds by a pathway of two-electron reduction of O2 on the catalyst surface. This work showed the potential perspective of Mx(C3N3S3)y (M = transitional metals) coordination polymers as a series of new materials for solar energy storage and conversion.

A Template-Free Solution Route for the Synthesis of Well-Formed One-Dimensional Zn2GeO4 Nanocrystals and Its Photocatalytic Behavior

Well-defined Zn2GeO4 hexagonal nanorods and nanofibers with high aspect ratios have been readily realized in high yield via a simple and general hydrothermal synthesis method free of any surfactant or template. Field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), powder X-ray diffraction (XRD), and ultraviolet-visible light diffuse reflectance spectroscopy (UV-vis DRS) revealed a unique hexagonal-prism-shaped one-dimensional (1-D) structure, surface features, anisotropic crystal growth, and crystal phase of Zn2GeO4. Detailed investigations indicated that the prismatic Zn2GeO4 nanocrystals are uniform single crystal with the longitudinal direction along the [001] and were dominated by (110) and (110) surfaces. The addition of increasing amounts of NaOH was found to facilitate the morphology transition from a hexagonal nanorod shape to a hexagonal fiber shape. As an important wide-band-gap photocatalyst, the products of regular Zn2GeO4 nanocrystals with a hexagonal 1-D structure exhibit superior photocatalytic activities for the photocatalytic decomposition of water-methanol solution to hydrogen under UV irradiation.

Probing the Electronic Structure and Photoactivation Process of Nitrogen‐Doped TiO2 Using DRS, PL, and EPR

The electronic structure and photoactivation process in N-doped TiO(2) is investigated. Diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and electron paramagnetic resonance (EPR) are employed to monitor the change of optical absorption ability and the formation of N species and defects in the heat- and photoinduced N-doped TiO(2) catalyst. Under thermal treatment below 573 K in vacuum, no nitrogen dopant is removed from the doped samples but oxygen vacancies and Ti(3+) states are formed to enhance the optical absorption in the visible-light region, especially at wavelengths above 500 nm with increasing temperature. In the photoactivation processes of N-doped TiO(2), the DRS absorption and PL emission in the visible spectral region of 450-700 nm increase with prolonged irradiation time. The EPR results reveal that paramagnetic nitrogen species (N(s)·, oxygen vacancies with one electron (V(o)·), and Ti(3+) ions are produced with light irradiation and the intensity of N(s)· species is dependent on the excitation light wavelength and power. The combined characterization results confirm that the energy level of doped N species is localized above the valence band of TiO(2) corresponding to the main absorption band at 410 nm of N-doped TiO(2), but oxygen vacancies and Ti(3+) states as defects contribute to the visible-light absorption above 500 nm in the overall absorption of the doped samples. Thus, a detailed picture of the electronic structure of N-doped TiO(2) is proposed and discussed. On the other hand, the transfer of charge carriers between nitrogen species and defects is reversible on the catalyst surface. The presence of oxygen-vacancy-related defects leads to quenching of paramagnetic N(s)· species but they stabilize the active nitrogen species N(s)(-).