Hongchao Ma

Synthesis of visible-light responsive Sn-SnO2/C photocatalyst by simple carbothermal reduction

Sn-SnO2/C photocatalysts with visible light activity have been successfully synthesized by a simple heat-treating process using SnCl4 as precursor. The Sn-SnO2 heterojunction is built by depositing of metallic Sn on crystal lattice oxygen of as-formed SnO2, which obviously increase the amount of adsorbed oxygen on the surface of catalyst. The as-synthesized catalysts display higher photocatalytic activity than SnO2 and SnO2/C for degradation of reactive brilliant blue KN-R under visible light irradiation. The enhancement of photocatalytic performance of Sn-SnO2/C can be attributed to the formation of metallic Sn on the surface of catalysts. The metallic Sn and adsorbed oxygen as the sinks of photoinduced electron and electronic scavenges, respectively, hinder the recombination of photoexcitated electron-hole pairs, sequentially enhance the photocatalytic activity. Furthermore, a possible growth mechanism of the Sn-SnO2/C photocatalysts was proposed.

Synthesis, characterization and photocatalytic activity of Cu-doped Zn/ZnO photocatalyst with carbon modification

Herein, we report the preparation of Cu-doped Zn/ZnO composites with carbon modification via a simple replacement–hydrothermal method using Zn powder and CuSO4·5H2O as raw materials. The as-synthesized composites were characterized by powder X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). The results showed that Cu doping promotes crystal growth of ZnO, inhibits phase transfer of metallic Zn to ZnO, enhances the optical absorption in the visible region and reduces the recombination of photogenerated electrons and holes. Interestingly, CO2 dissolved in solution was converted to carbon adhered onto the composite surface (supported by the XPS data). The abundant carbon species present on the composite surface is favorable for surface chemical reactions because it can trap or adsorb reactants which facilitate the transfer of reactants to active sites. The photocatalytic efficiency of the as-synthesized catalysts was evaluated by the degradation of anthraquinone dye (reactive brilliant blue KN-R) in solution under sunlight irradiation. The degradation results revealed that the Cu-doped Zn/ZnO composites have better photocatalytic activities than those of ZnO and Zn/ZnO. The enhancement of the photocatalytic activity of the composites can be attributed to the existence of Cu doping, the Zn/ZnO hetero-structure and covered carbon on the surface of the photocatalysts, which causes electrons to be easily excited from the valence band to the conduction band and efficient separation of electron–hole pairs, as well as quick surface reactions in doped ZnO. Furthermore, a possible growth mechanism of the Cu-doped Zn/ZnO composites with carbon modification was proposed.

Controllable electrostatic self-assembly of sub-3 nm graphene quantum dots incorporated into mesoporous Bi2MoO6 frameworks: efficient physical and chemical simultaneous co-catalysis for photocatalytic oxidation

Over the past few years, the direct assembly of co-catalyst/modification materials into mesoporous photocatalysts has been considered a great challenge. Additionally, for photooxidation, the simultaneous achievement of fast charge separation, broad spectrum photocatalytic activity and higher carrier utilization efficiency (generating more active oxidizing groups) is quite necessary but has never been studied. To this end, for the first time, using sub-3 nm GQDs as co-catalyst, we have successfully achieved uniform modification for a mesoporous photocatalyst (mesoporous Bi2MoO6) using a novel electrostatic self-assembly method. The sub-3 nm GQDs, which were prepared from graphene nanosheets by a modified chemical oxide method, exhibit many unique physical and chemical properties, such as small size, electronic capture, up-conversion, and in particular, peroxidase-like activity. After the GQDs were modified, the resulting mesoporous hybrid photocatalyst (GQDs–BM) exhibited excellent charge separation efficiency and broad spectrum photocatalytic activity from UV to NIR light. More importantly, we found that a certain amount of H2O2 was produced through a photoreduction effect during the photocatalytic process. Unfavorably, for bare Bi2MoO6, the continuously-accumulating H2O2 could not efficiently convert into ·OH by a one-photoelectron reduction, which results in the indirect waste of photo-excited electrons. However, the chemical co-catalysis of GQDs could make this process (H2O2 → ·OH) more quick and efficient and moreover, did not need any additional photoelectrons, which means the effective enhancement of the utilization efficiency of photo-excited electrons (generating more ·OH). Additionally, for the as-prepared GQDs–BM, a sharp increase in photo-degradation activity for different target pollutants, such as BPA, MB, TC, CIP and phenol further confirmed that the simultaneous physical and chemical co-catalysis of GQDs can efficiently enhance the photocatalytic activity of mesoporous Bi2MoO6.

Ultra-thin C3N4 nanosheets for rapid charge transfer in the core–shell heterojunction of α-sulfur@C3N4 for superior metal-free photocatalysis under visible light

The core–shell heterojunctions of ultra-thin C3N4 nanosheet enwrapping spherical α-S composites (α-S@C3N4) were fabricated via a self-assembly process by electrostatic force to realize enhanced photocatalytic ability under visible light. The photocatalytic ability can be adjusted by tuning the amount of the ultra-thin C3N4 nanosheet. The α-S@C3N4 composite with 35% composition of C3N4 nanosheet has the highest photocatalytic ability. The degradation rate of Rhodamine B (RhB) with α-S@C3N4 (35% C3N4) is 6.72 times faster compared to α-S as photocatalyst. This increase could be attributed to the efficient photogenerated holes and electrons separation by the heterojunction, which has excellent charge transfer ability arising from the ultra-thin C3N4 nanosheet. The stability of the α-S is also largely improved by the heterojunction construction.

Preparation of β-Bi2O3/g-C3N4 nanosheet p–n junction for enhanced photocatalytic ability under visible light illumination

Abstractβ-Bi2O3/g-C3N4 nanosheet (NS) p–n junction of g-C3N4 NS-wrapped β-Bi2O3 was fabricated via self-assembly process by electrostatic force. By the analyses of scanning electron microscopy images and Fourier transform infrared spectra, the g-C3N4 NS has been wrapped on the spherical β-Bi2O3 particles. Constructing p–n junction with g-C3N4 NS can enhance the separation efficiency of photogenerated carriers and light absorption ability of β-Bi2O3 particles obtained by the characterization of fluorescence spectra and diffuse reflectance spectra. The photocatalytic removal rate of RhB is largely enhanced by construction of β-Bi2O3/g-C3N4 NS p–n junctions. The photocatalytic ability of β-Bi2O3/g-C3N4 NS p–n junctions can be adjusted by tuning the loading amount of g-C3N4 NS. The enhanced photocatalytic performance is firstly attributed to the p–n junction effect of efficient separation of photogenerated charge carriers. Furthermore, the increased light absorbance of the composites also contributes to the enhanced photocatalytic ability.

Photonic crystal coupled porous BiVO4 hybrid for efficient photocatalysis under visible light irradiation

A bilayer TiO2 photonic crystal (PC)/porous BiVO4 was constructed by using liquid-phase deposition (LPD) and spin coating method for the enhancement of visible-light-driven photocatalytic ability by the improvement of light absorbance. Crystal form, morphology, film thickness, and light absorption performance were investigated. The photocatalytic activity of the bilayer TiO2 PC/porous BiVO4 film was evaluated by the degradation of MB and was compared with that of porous BiVO4 film. The effect of the film thickness of the porous BiVO4 on its photocatalytic ability was determined. It was found that the film thickness significantly affected the light absorption, and therefore its photocatalytic ability. The bilayer film with 1.03 μm of thickness of porous BiVO4 film exhibited the highest photocatalytic performance. The photocatalytic ability of porous BiVO4 was enhanced by combination with a TiO2 PC layer as the back reflector. This increase could be attributed to the intensified light absorbance produced by reflection from the TiO2 PC layer. The effects of porous structure and the application of photoelectrocatalytic process on the photocatalytic performance were also discussed.

Hydrogenated Bismuth Molybdate Nanoframe for Efficient Sunlight-Driven Nitrogen Fixation from Air.

Sunlight-driven dinitrogen fixation can lead to a novel concept for the production of ammonia under mild conditions. However, the efficient artificial photosynthesis of ammonia from ordinary air (instead of high pure N2 ) has never been implemented. Here, we report for the first time the intrinsic catalytic activity of Bi2 MoO6 catalyst for direct ammonia synthesis under light irradiation. The edge-exposed coordinatively unsaturated Mo atoms in an Mo-O coordination polyhedron can act as activation centers to achieve the chemisorption, activation, and photoreduction of dinitrogen efficiently. Using that insight as a starting point, through rational structure and defect engineering, the optimized Bi2 MoO6 sunlight-driven nitrogen fixation system, which simultaneously possesses robust nitrogen activation ability, excellent light-harvesting performance, and efficient charge transmission was successfully constructed. As a surprising achievement, this photocatalytic system demonstrated for the first time ultra-efficient (1.3 mmol g-1  h-1 ) and stable sunlight-driven nitrogen fixation from air in the absence of any organic scavengers.

Preparation of Ni Doped ZnO-TiO2 Composites and Their Enhanced Photocatalytic Activity

Herein, Ni doped ZnO-TiO2 composites were prepared by facile sol-gel approach and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The results indicated that the Ni ions can be incorporated into the lattice of TiO2 structure and replace Ti. The introduction of Ni expanded light absorption of TiO2 to visible region, increased amount of surface hydroxyl groups and physically adsorbed oxygen (as the electronic scavenges), and then enhanced separation rate of photogenerated carriers. The photodegradation test of reactive brilliant blue (KN-R) under simulated solar light indicated that Ni doped ZnO-TiO2 composites have better photocatalytic activities, as compared to those of TiO2 and ZnO-TiO2.

Towards understanding the photocatalytic activity enhancement of ordered mesoporous Bi2MoO6 crystals prepared via a novel vacuum-assisted nanocasting method

Bi2MoO6 is an outstanding semiconductor photocatalyst and has been extensively studied; the construction of ordered mesoporous Bi2MoO6, however, remains a great challenge. Herein, for the first time, ordered mesoporous Bi2MoO6 crystals were synthesized employing a novel vacuum-assisted nanocasting method. More interestingly, this nanocasting method creatively utilized the amorphous complex as a precursor instead of metal salt ions, by which a new kind of single phase Bi2MoO6 (M-Bi2MoO6) could be successfully prepared. The as-prepared M-Bi2MoO6 exhibited much enhanced photocatalytic activity, which is about 5 times higher than that of conventional Bi2MoO6 for the photodegradation of RhB, BPA, CBZ or the photocatalytic reduction of Cr(VI). More importantly, in this paper, through some quantitative results, the mechanism of the photocatalytic activity enhancement of ordered mesoporous Bi2MoO6 was thoroughly investigated. Compared to conventional Bi2MoO6, the nanometer-sized, ordered mesoporous structure can make the as-prepared M-Bi2MoO6 possess excellent light harvesting ability, higher charge separation efficiency and larger catalysis activity areas, which are beneficial for photocatalytic oxidation and reduction. In summary, this work not only constructed a new kind of ordered mesoporous nanomaterial and provided a new vacuum-assisted nanocasting method, but also offered scientific insights into the photo-electron conversion property and carrier transport mechanism.

Improved Photocatalytic Activity of Copper Heterostructure Composites (Cu–Cu2O–CuO/AC) Prepared by Simple Carbothermal Reduction

Cu–Cu2O–CuO/activated carbon heterostructure composites with visible-light activity have been successfully synthesized by a simple carbothermal reduction procedure using CuSO4 as a single precursor. The resultant samples were characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements. The results showed that the Cu–Cu2O–CuO composites with size less than 10 nm dispersed well on the surface of activated carbon. Activated carbon played both a reducing agent and support role in the formation of Cu–Cu2O–CuO/activated carbon heterostructure composites. X-ray photoelectron spectroscopy analysis suggests that the outside of the nanoparticles is CuO and the inside of the nanoparticles is Cu metal and Cu2O. Moreover, the composition of Cu–Cu2O–CuO/activated carbon composites can be tailored by varying the Cu loading, heat-treatment temperature, and heat-treatment time. The photocatalytic activities of the catalysts were investigated by degrading reactive brilliant blue KN-R under visible-light irradiation. The Cu–Cu2O–CuO/activated carbon heterostructure composites showed excellent photocatalytic activity compared with other catalysts (pure CuO, Cu2O, Cu2O/activated carbon, CuO/activated carbon, and Cu2O–CuO/activated carbon), which is ascribed to synergistic action between the activated carbon support and photoactive copper species, and the presence of interfacial structures such as a Cu2O/CuO heterostructure, Cu/Cu2O (or CuO) Schottky barrier, and Cu2O/Cu/CuO ohmic heterojunction.