On-demand Design And Controllable Construction Of Carbon-based Photocatalysts For NIR Light Induced Water Splitting

In order to cope with the growing energy crisis and environmental pollution problems, gaseous hydrogen produced from photocatalytic water splitting under solar irradiation has great potential to be clean and renewable energy resources. Photocatalytic water splitting using semiconductors can transform the low density solar energy into high density chemical energy, which plays an important role in the sustainable development of human beings. Therefore, research on this issue has attracted many attentions and become one of the hot forefronts of science owing to its great significance to human society.To date, for most photocatalytic H2 production systems it is difficult to use near-infrared light(NIR) which accounts for more than 40% of solar energy. This has become one of the main limitations to solar energy utilization. Reported photocatalysts with infrared response are mostly depended on materials containing precious metals or organic dyes. These materials often suffer from various defects such as toxicity, high cost and photocorrosion susceptibility, which largely hinder their practical applications. In view of the current situation, my thesis focuses on composite materials with low cost, long term stability, environmental friendliness and high catalytic activity under near-infrared light. Motivated by the demand of "Developing photocatalytic systems with full solar spectrum response for hydrogen production using earth-abundant elements" this scientific key topic, we are in a planned way to design the composition and structure of target photocatalytic systems, orderly construct required materials as building blocks with fined control of synthesis routes and conditions. Based on the rich content in nature, low toxicity and easy processing, carbonaceous materials(carbon quantum dots, graphite carbon nitride and graphene) can be involved in synthesis of the structure controllable composite photocatalytic materials. Taking advantage of developed mechanics such as upconversion emission and multistep excitation Z-scheme, the goal of this thesis is to realize photocatalytic water splitting driven by sunlight from UV to NIR, thereby offer new approaches and inspirations to the effective chemical utilization of solar energy.First section: Carbon quantum dots(CQDs) has many advantages, such as simple synthesis, environment friendly, large cross-section of absorption and upconversion emission with tuned wavelengths. UV-Vis absorption spectra of graphite carbon nitride nanosheets(CNNS) with good activity of photocatalytic hydrogen production can partially overlap the upconversion emission spectrum of CQDs. This spectral coupling can expand the responsive spectra of wide bandgap CNNS into longer wavelenghes to obtain broadband photocatalytic responses from the UV to NIR region. Furthermore, CQDs anchored onto CNNS can strikingly facilitate energy and charge transfer by forming electronic coupling. Based on this, an innovative metal-free photocatalyst CNNS/CQDs with NIR light activity for H2 evolution was prepared via a simple one-step hydrothermal method. The unusual photocatalytic performances originate from the interaction of spectral and electronic coupling between CNNS and CQDs that synergistically improve the wide range absorption and photoinduced charge separation. Besides the large promotions to both UV-Vis and Vis photocatalytic activities, real metal-free NIR driven H2 photocatalytic evolution is achieved for first time.Second section: Artificial Z-scheme water splitting system, mimicking the natural photosynthesis process, is based on the principle of a two-step excitation process. In this system, two kinds of photocatalytic materials serves as photo oxidation and reduction agent of water, respectively, which could overcome the restriction on single semiconductor to directly use low energy photon for overall water splitting. Recently, most of the reported Z-scheme water splitting photocatalysts is UV- or Vis-active materials with inefficiency of producing H2 in the NIR light region. This thesis creatively integrated partially reducted graphene(PRG) with W12PO38.5(WPO) composite oxide into a tightly combined heterostructure via a one-step calcination process. PRG and WPO, depending on their appropriate band positions, played the role of reduction site and oxidation site of water, respectively. Moreover, oxygen substituent groups of graphene oxide were removed during calcination and a part of W6+ was then reducted to W5+ which can act as the electron shuttle of sol-id-state Z-scheme photocatalytic water splitting system. As a result, for first time, gaseous hydrogen produced from Z-scheme photocatalytic water splitting under NIR light was realized in this work.