Synthesis And Photocatalytic Performance Of Visible-light-driven Graphene/Bi-based-semiconductor Composite Photocatalysts

Energy shortage and environmental pollution are the topic issues in the sustainable development of human society. Photocatalytic reaction has shown its great potential in solar hydrogen production and pollutants photodegradation. Unfortunately, conventional TiO2 suffers from mismatch with solar spectrum and abundant recombination of photogenerated electrons and holes. The developments of new visible-light-responsive photocatalysts together with additional modification are strategies to accquire photocatalysts with desirable perfomances. On the one hand, BiVO4 and BiOI are the typical bismuth-based visible-light photocatalysts with the bandgap of 2.4 eV and 1.9 eV, respectively. On the other hand, the unique 2D structure and superior electronic transport properties of graphene facilitate absorption of pollutants and separation of photogenerated carriers. Nevertheless, a random "hard" integration of graphene nanosheets and semiconductor nanoparticles contribute little to improving their photocatalytic performance due to an inadequate interface contact. To obtain highly active compostite photocatalysts, surface charge modification of semiconductor nanoparticles and nitrogen doping in graphene have been employed in this thesis. Surface charge modification on BiVO4 has converted surface charge from negative to positive. The electrostatic attraction drives the self-assembly of positively charged BiVO4 nanoparticles and negatively charged graphene oxide nanosheets, forming sufficient interfacial contact between them. The well-contacted interface could facilitate carrier separation and enhance photocatalytic activity. Nitrogen-doped graphene shows n-type behavior and intrinsic BiOI is an attractive p-type. The p-n heterojunction composite photocatalysts were obtained by coupling p-type BiOI with n-type nitrogen-doped graphene. The formation of p-n junction enhanced the charge generation and suppressed the charge recombination, thereby improved the photocatalytic performance. Main research works are as follows:1. Surface charge modification on amorphous BiVO4 powders has achieved with the aid of silane coupling agent. The positively charged amorphous BiVO4 powders and negatively charged graphene oxide nanosheets were mixed and self-assembly, followed by a one-step graphene oxide reduction and BiV04 crystallization via hydrothermal treatment. Corresponding characterizations were performed on the as-prepared samples to show the morphology, structure, component and photocatalytic activity, revealing the mechanism for photocatalytic activity enhancement.2. Nitrogen-doped graphene(N-rGO) was synthesized via pyrolysis of graphene oxide and urea. Nanoscale BiOI/N-rGO heterojunctions with highly efficient visible light photocatalytic degradation activities were synthesized via solvothermal reactions. Corresponding characterizations were performed on the as-prepared samples to show the morphology, structure, component and photocatalytic activity, revealing the mechanism for photocatalytic activity enhancement. This simple modification strategy could be employed to construct highly active graphene-based nanocomposite photocatalysts for photocatalytic degradation of pollutants and hydrogen generation from water splitting.