Synthesis of noble metal nanomaterials with new crystal phases for electrocatalytic reaction

• Main Author: Wang, Jie
• Other Authors: -
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/138247
• Institution: Nanyang Technological University

To prepare noble metal nanomaterials with unconventional crystal phase is of extraordinary interest for the research of new features of noble metal nanomaterials towards a series of applications. Hence, the target of this thesis is to produce noble metal nanomaterials with unconventional crystal phase and then investigate their features in a range of catalytic applications. Firstly, the crystal phase-controlled preparation of PtCu alloy nanostructures on hexagonal 4H phase Au nanoribbons (referred to as 4H-Au NRBs) has been accomplished by tuning the thickness of PtCu shell on the 4H-Au NRB core, forming 4H-Au@PtCu core-shell NRBs. When the shell is thin, 4H-PtCu is obtained because of the epitaxial relationship between PtCu shell and Au substrate; and when the shell is thick, face-centered cubic (fcc) phase PtCu (fcc-PtCu) is prepared as the epitaxial relationship between Au substrate and PtCu film is broken. Significantly, the phase-dependent electrocatalytic features of the as-synthesized PtCu alloy nanostructures in the ethanol oxidation reaction are explored. Note that, the 4H-Au@4H-PtCu NRBs possess a much higher specific activity of 50.2 mA/cm2, which is 2.1 times that of 4H-Au@fcc-PtCu NRBs (24 mA/cm2) and 5.9 times that of Pt black (8.5 mA/cm2). Secondly, by using hexagonal 2H phase Au nanosheets as the templates, high-yield preparation of mixed phase 2H/fcc Au@Ir core-shell nanosheets have been accomplished. Significantly, similar with the case using 4H Au nanoribbons as the templates, the crystal phase of Ir nanostructures is controlled by tuning the layer numbers of the Ir shell, i.e., thin Ir shell epitaxially grown on Au core inherits 2H phase while thick Ir shell crystallize in fcc phase. However, different from the case using 4H Au nanoribbons as the templates, the Au core transforms to fcc phase simultaneously when Ir shell gets thicker as the epitaxial relationship between Au and Ir could be preserved regardless of shell thickness. Thus the resultant 2H/fcc ratio in the Au@Ir nanosheets could be controlled by tuning the layer numbers of shell atoms. Note that, the phase-dependent electrocatalytic features of the as-synthesized Au@Ir nanosheets in the hydrazine electro-oxidation reaction (HzOR) are probed. Note that, the 2H/fcc Au@Ir nanosheets possess a remarkable mass activity of 507.5 mA/mg Au+Ir, which is 1.96 times that of the fcc Au@Ir nanosheets (259 mA/mg (Au+Ir)) and 3.36 times that of the Ir black (151 mA/mg Au+Ir). Thirdly, because of the difference between the cases using 2H Au nanosheets and 4H Au nanoribbons as the templates, respectively, 2H Au and 4H Au should play different role in the seed-mediated synthesis. Inspired by this, 2H Au nanosheets and 4H Au nanoribbons are employed as the seed to deposit a thin layer of PdFe alloy shell, generating 2H Au@PdFe nanosheets and 4H Au@PdFe nanoribbons, respectively. Impressively, the phase-dependent electrocatalytic features of the as-prepared 2H Au@PdFe nanosheets and 4H Au@PdFe nanoribbons in the methanol oxidation reaction (MOR) in alkaline media are studied. Note that, 2H Au@PdFe nanosheets possess the highest mass activity of 4.39 A/mgPd, which is 1.2, 2.8 and 12.5 times those of 4H Au@PdFe nanoribbons, Pt/C and Pd black, respectively.