Morphology-Controllable Synthesis Of Noble Metal-Based Colloidal Nanocrystals And Their Applications In Photo/Electrocatalysis

The development of green and sustainable approachs to drive chemical reactions is a promising way in response to the energy shortage and environmental issues.Among various solutions,the conversions of greenhouse gas carbon dioxide（CO₂）and nitrogen（N₂）into the value-added fuels and feedstocks by use of solar or electric energy are two feasible ways.Although substantial endeavors have been devoted to this field,it remains challenging to design and fabricate effective catalysts due to the thermodynamically stable and chemically inert property of the two molecules.Recently,noble metal nanocrystals have been attracting increasing interest owing to their unique catalytic property and excellent adjustable morphology.In this thesis,we developed two types of noble metal-based colloidal nanocrystals and studied their applications in photo/electrocatalytic reduction of CO₂ and N₂.The main results are as follows:（1）Construction of Janus silver/ternary silver halide nanostructures for plasmon-driven photocatalytic CO₂ reduction to acetaldehyde.Solar-driven CO₂ redution to chemical fuels and feedstocks represents a sustainable way to address energy and environmental challenges.During the past decades,semiconductor materials have been widely employed as the photocatalysts for photocatalytic CO₂ redution because of their excellent stability and high reactivity.However,the photocatalytic products of the CO₂reduction are mainly limited to C₁ compounds,including CO,CH₄,and CH₃OH.The generation of C₂ products,especially those liquid products with high volumetric energy densities,is still a significant chanllenge.Plasmonic metal nanocrystals possess localized surface plasmon resonance（LSRP）properties.The integration of plasmonic metals with semiconductors to obtain plasmonic photocatalysts can not only promote the photocatalytic activity but also unlock new reaction pathways towards the production of C₂ products.In this thesis,we developed a preparation method to fabricate the Janus silver/ternary silver halide（Ag/Ag Cl Br）nanostructures by precisely manipulating the growth kinetics.The composition and light-harvesting property of the Janus nanostructures can be facilely adjusted by changing the molar ratios of feeding reagents in the precursors.The obtained Janus nanostructures featuring both spatially separated architecture and broad light-harvesting capability facilitate the photocatalytic reduction of CO₂under solar illumination.The photocatalytic CO₂ reduction with the characteristics of high activity and good selectivity can generate a 10-electron reduction product CH₃CHO with a generation rate of 209.3±9.5μmol h^-1 g^-1 and a selectivity of 96.9%,which are rarely achieved in previously reported photocatalytic CO₂ reduction systems.We further employed in situ Fourier transform infrared spectroscopy and the theoretical calculation to track the reaction mechanism.On the basis of the experimental and theoretical results,a photocatalytic CO₂reduction mechanism on Janus Ag/Ag Cl Br was prosposed.（2）Synthesis of porous Pd Ru alloy nanosheets for electrochemical N₂ reduction reaction.N₂ fixation,the reduction of N₂ to ammonia（NH₃）,is an important chemical reaction in nature and industry.Industrially,N₂ fixation is achieved through the classical Haber-Bosch process,which is conducted under high-temperature and high-pressure conditions.Electrochemical N₂ redution reaction（NRR）driven by electrical energy is a clean and renewable process for NH₃ synthesis.Metal nanocrystals have shown great potential in the field of electrochemical NRR,while the low catalytic activity of monometallic catalyst is still a significant challenge.In this thesis,we developed a synthetic method to prepare Pd Ru alloy nanosheet electrocatalyst for the application of electrochemical NRR process.The obtained Pd Ru alloy nanosheets featured with ultrathin and porous properties possess a thickness of about（16）（13）（23）（21）±0.27 nm.More importantly,the size and number of the holes in Pd Ru alloy nanosheets can be simply adjusted by manupluting the reation conditions.The ultrathin and porous properties make Pd Ru nanosheets a good candidate for the electrochemical applications.Electrochemical experiments showed that the Pd Ru₅₀ sample exhibited the best electrocatalytic performance towards the electrochemical NRR,with an NH₃ generation rate(YR_NH3)of 28.04±6.06μg h^-1 mg^-1_cat.and a Faradaic efficiency（FE）of 10.2%.The excellent electrocatalytic performance is attributed to the unique architecture and the synergistic effect of Pd and Ru in the alloy nanosheets.A possible mechanism for electrochemical NRR on Pd Ru alloy nanosheets was finally proposed.