Surface/Interface Design And Mechanism Studies For Hybrid Anocatalysts Supported On Two-Dimensional Materials

The rapid development of two-dimensional (2D) nanomaterials has offered new opportunities for fabricating catalysts with high activity, selectivity and stability. Hybrid nanocatalysts supported on2D materials have been widely used in photocatalysis and electrocatalysis, which show potential applications for renewable energy production and environmental remediation. The2D components greatly promote the catalytic performance of hybrid catalysts owing to their unique structures. Meanwhile, the design of surface and interface has shown its increasing importance to optimizing the performance of hybrid catalysts. However, the synthetic techniques and catalytic mechanism studies have lagged behind the demands for rationally designing the surface and interface of hybrid catalysts.In this dissertation, with the combinative use of controlled synthesis, characterization techniques and theoretical simulations, surface and interface design is used to promote the catalytic activity, selectivity and stability of hybrid catalysts supported on2D materials. In the meantime, the relationships between the performance and surface/interface parameters offer a set of clear and well-defined platforms for further understanding the catalytic mechanisms that remained unexplored in the past. The surface parameters of components including exposed facets, compositions, surface areas and crystal phases have been tailored toward higher surface activation abilities for specific photocatalytic and electrocatalytic reactions. Moreover, the interfaces between two components have been adjusted to favor the photoinduced charge transfer for photocatalytic reactions. Furthermore, the "surface polarization effect" has been brought into electrocatalytic reaction through the simultaneous control of surface and interface. The main results can be summarized as follows:1. Pd co-catalysts exposed with different facets are loaded on2D C3N4nanosheets to form hybrid catalysts, in which the Pd facet-dependent selectivity in photocatalytic CO2reduction in present of H2O is investigated. It is found that the unique structure of C3N4nanosheets enables the equivalent efficiency of photogenerated electrons transferring from C3N4to different Pd facets for reduction of H2O and CO2. As a result, C3N4-Pd nanotetrahedrons with Pd{111} facet prefer to reducing CO2into carbon products with the assistant of H2O. In comparison, Pd nanocubes with Pd{100} facet mainly undergo H2O reduction to produce H2. According to first-principles simulations, we have been able to reliably assess that Pd{111} facets offer higher CO2adsorption energy and lower CO2activation barrier while Pd{100} facets show much higher H2O adsorption energy.2. MoS2nanosheets with different phases are used as co-catalysts to load TiO2and form hybrid catalysts, in which the MoS2phase-dependent activity in photocatalytic water splitting to produce H2is investigated. It is found that the MoS2nanosheets with1T phase (1T-MoS2) greatly promote the photocatalytic efficiency of TiO2, whereas the MoS2nanosheets with2H phase (2H-MoS2) cannot. In the2H-MoS2, the active sites for H2production are only located at the edges of nanosheets, and this semiconducting phase offers relatively low mobility for charge transport. In comparison, the metallic1T-MoS2nanosheets have additional reaction sites on their basal plane, and possess higher charge transport mobility. Thus the higher diffusion rate and shorter diffusion distance enabled by the1T-MoS2allow significantly the electrons photogenerated in TiO2to more easily arrive at the reaction sites and participate in H2evolution reactions before they lose their lives.3. The charge transfer and separation efficiency of Cu2O-Pd Schottky junction in different architectural structures is assessed. It is found that interfacial defects and lack of bulk-to-surface charge-transfer channels limit the charge separation in Pd-decorated Cu2O structure and Pd-Cu2O core-shell structure, respectively. To circumvent this undesirable situation, a novel semiconductor-metal-graphene stack structure has been designed. The ternary structure not only inherits the advantage of core-shell structure in defect elimination, but also offers a new Pd-graphene interface for the holes to migrate out for surface reactions owing to the high charge mobility of graphene. As a result, the stack structure shows significantly higher photocatalytic H2production rate in comparison with other Cu2O-based catalysts.4. The approach to synergetic utilization of plasmonic effect and Schottky junction for full-spectrum photocatalysis is developed by employing BiOCl nanoplates, Ag nanocubes and Pd nanocubes as semiconductor, plasmonic metal and nonplasmonic metal components, respectively. Ag nanocubes and Pd nanocubes are selectively assembled on the top and bottom BiOCl(001) facet and the side BiOCl(110) facet for plasmonic hot carrier injection under visible light and Schottky-junction carrier trapping under UV light, respectively. The synergetic utilization of the two interfaces makes the obtained Ag-(001)BiOCl(110)-Pd structure show dramatically higher photocatalytic oxygen evolution activity in comparison with its component structures.5. Starting from the Pd nanocubes supported on graphene (rGO) nanosheets, the surface parameters of the hybrid catalyst are designed step by step toward electrocatalytic performance enhancement in oxygen reduction reaction (ORR). Firstly, the selective coating of Pt shells on the Pd surface to form Pt-Pd-rGO structures makes the transformation from Pd{100} to Pt{100} surface with higher ORR activity. Secondly, the selective removal of the Pd nanocubes through chemical etching to produce PtPd nanocages-rGO structures almost doubles the surface area of Pt{100}. As a result, the catalytic activity of the samples increases in the order:Pd nanocubes-rGO<Pt-Pd-rGO<PtPd nanocages-rGO.6. Pt-Pd-graphene stack structures with different Pt thickness are fabricated, in which the Pt thickness-dependent activity in electrocatalytic hydrogen evolution reaction (HER) is investigated. It is found that the HER activity increases with a reduction in the Pt thickness, which is well explained by surface polarization mechanism as suggested by first-principles simulations.In this hybrid system, the difference in work functions of Pt and Pd results in surface polarization on the Pt surface, tuning its surface charge state for enhanced HER activities with the reduced Pt thickness.