Noble metal nanomaterials with novel crystal phase : syntheses and catalytic applications

• Main Author: Chen, Ye
• Other Authors: Zhang Hua
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Nanotechnology; DRNTU::Science::Chemistry::Physical chemistry::Catalysis; DRNTU::Science::Chemistry::Crystallography::Crystal structure and growth
• Online Access: https://hdl.handle.net/10356/83263; http://hdl.handle.net/10220/48007
• Institution: Nanyang Technological University

Noble metal nanomaterials possess outstanding catalytic properties in a wide range of reactions. In the past decades, controlled synthesis to fine tune the size, shape, architecture, and composition of noble metal nanocrystals to boost their catalytic performances has been intensively investigated. The crystal structure represents another important parameter of nanocrystals. However, to date, most studies on crystal phase engineering of noble metal nanocrystals reply on extreme conditions such as high pressure and high temperature. It is still relatively difficult to prepare large-scale, free-standing metal nanostructures with unusual crystal structures under ambient condition, hindering a comprehensive study on their phase-dependent physicochemical properties. Moreover, the effects of crystal structure on the catalytic properties of noble metal nanomaterials and the strategies to further improve the catalytic performances via crystal phase control are yet to be explored. The first project introduces the epitaxial growth of a series of metals, namely Ru, Rh, Ir, Os, Cu, with novel 4H crystal phase under mild conditions. By simply reducing the metal precursors in the solution of 4H Au nanoribbons, free standing Au@metal nanostructures are obtained under room temperature. Importantly, Ru, Rh, Ir, Os, Cu shells with 4H crystal phase are obtained for the first time. The general approach can be readily applied to a wide range of other metals. Compared to the Au NRBs with pure 4H crystal phase, Au nanostructures with a mixture of different crystal phases may be more suitable for the template of nanocatalysts, due to the high density of active sites generated at the phase boundaries. To explore the possibility of preparing new Au substrates, the second project first introduces the synthesis of crystal-phase-heterostrucured 4H/fcc Au nanorods with high yield. Then, the subsequent epitaxial growth of 4H/fcc Pd shell for electrochemical oxidation of ethanol is carried out. For the first time, crystal-phase-heterostrucured 4H/fcc Au nanorods with alternating 4H and fcc crystal phases grown on each single nanorod are prepared with yield approaching 100%. The 4H/fcc Au nanorods serves as excellent template to grow other metals with 4H/fcc crystal phase heterostrucures. As a proof of concept, a Pd shell is growth epitaxially on the Au nanorods. The resultant Au@Pd nanorods demonstrate impressive performance in the ethanol oxidation reaction with mass activity 6.2 times those of commercial Pd catalyst. Based on the general method developed by the first project and the crystal-phase-heterostructured 4H/fcc Au NRs discovered in the second project, it is possible to further extend the crystal phase library of metals by preparing crystal-phase-heterostructured 4H/fcc non-noble metals. The third work introduces the epitaxial growth of a transition metal, Cu, on the crystal-phase-heterostructured 4H/fcc Au nanorods and the superior performance of the crystal-phase-heterostructured 4H/fcc Au@Cu nanorods in electrocatalytic CO2 reduction reaction. The high yield of the 4H/fcc Au@Cu nanorods allows a relatively fair comparison between heterophased 4H/fcc Au@Cu and fcc Cu nanoparticles, which suggests that the 4H/fcc heterophased Cu exhibit better selectivity of C2H4 and activity than fcc Cu CPS in the CO2RR catalysis.