Synthesis Of Au-based Noble Metal Nanomaterials And Study Of Their SERS And Electrocatalvtic Performances

Due to the unique physical and chemical properties, noble metal nanomaterials have shown considerable interest and great potentials in energy, catalysis, biotechnology, sensors, medical and many other applications. With the development of science and technology, single component nanomaterials cannot meet the demands of daily work and production. Thus, the hybrid nanomaterials emerge and have aroused great concern. Noble metal nanocomposites could combine the advantages and overcome the disadvantages of each composite. Gold nanoparticles (Au NPs), known since the times of ancient Romans, are concerned to be one of the most stable metal nanoparticles. Au NPs have aroused great concern and have been extensively studied because of their special properties such as biocompatibility, stability, CO oxidation catalysis in low temperature and electrical properties, etc. They are widely used in the fields of biology, energy, catalysis, photovoltaic and so on. Au-based NPs possessed amazing properties due to their excellent performance stemming from the existing of Au component.Among Au NPs, quasi-spherical Au NPs are still the workhorse in many technical applications mainly because of the fact that their synthesis protocols are easy and accessible to researchers from different disciplines such as chemistry, physics, and biology. However, there need Au NPs with large size range in the applications while the synthetic procedures are rather complicated. Thus, it is still a big challenge to develop a simple protocol to synthesize monodisperse, quasi-spherical, high yield Au NPs with controlled size. Au NPs and Ag NPs are widely used in SERS (surface enhanced Ranman scattering) as substrates for the reason that they can multiple enhance the Raman signals, thus improving the sensitivity and accuracy of Raman detection and facilitating the Raman testing in trace amounts. Due to the superior plasmonic property and especially electromagnetic enhancement in the visible range, Ag-based NPs have been the major workhorse in SERS studies. However, compared with the preparation of Au NPs, it remains an experimental challenge to synthesis Ag NPs with uniform sizes and shapes and in turn to correlate the SERS performance of the Ag NPs with their structural features. This has strongly encouraged the preparation of core@shell Au@Ag NPs (CS Au@Ag NPs) by using pre-formed Au NPs as seeds and the utilization of the Ag shells instead of Ag NPs for SERS studies. Their SERS properties can be better than those of pure Ag NPs with comparable sizes. However, two imporatnt technical issues are not completely addressed yet:1) what the critical thickness of Ag shells is needed for the best SERS performance of Au@Ag NPs and 2) if and how the critical Ag shell thickness is dependent on the the Au core sizes. Pt catalysts are the key parts of the fuel cell for their super catalytic activities. However, the stability and the anti-CO ability of Pt are relative poor. Au-based NPs decorated with Pt show super catalytic activities due to their higher chemical stability and durability stemming from the Au component and electronic effects between Au and Pt. Therefore, preparation of Au@Pt NPs with superior catalysis activity and stability by a simple method has a great significant effect in the applications of fuel cell.On the basis of the current issues mentioned above and the research results on Au NPs by our group, this thesis mainly focuses on the preparation of Au@M (M= Au, Ag, and Pt) NPs and their applications in SERS and electrocatalysis. Here, the main research content of this thesis is divided into three parts:In Chapter 2, quasi-spherical, monodisperse Au NPs with sizes of 17 to 183 nm were synthesized by seeded growth method under ambient conditions. In this strategy, 17 nm quasi-spherical, monodisperse Au NPs obtained by Turkevich method were used as Au seeds, the bio-comparable Tris-base (Tris(hydroxymethyl)methyl aminomethane) was used as stabilizer, and H2O2 was used for reduction agent. The Au NPs of different sizes were then prepared by adjusting the amount of the Au seeds at fixed amount of precursor (HAuCl4). The results of characterizations by Transmission electron microscopy (TEM) and UV-vis spectroscopy demonstrate that Au NPs prepared by this strategy show uniform morphology and size; there are few triangle, rod and other by-products. The diameters of the Au NPs obtained by calculation are fairly comparable to those experimentally obtained via the present seeded growth approach, underlining the quantitative control of the resulting Au NP sizes by the amount of Au seeds. This result also confirms that no secondary nucleation occur and further underlines that the resulting Au NPs are monodisperse and quasi-spherical. The diameters of Au NPs can be figured out by the amount of Au seeds. On the other hand, the formular can aslo be used to guide the synthesis of Au NPs with defined size. Furthermore, Au NPs can be yielded on large scale by expanding the amount of the precursors.In Chapter 3, a series of Au2r@Agt NPs were synthesized via overgrowth of Ag on pre-formed Au NPs at room temperature, in which the diameters of Au cores (2r) are ranged from 12 to 20,30,45, and 55 nm and the thickness of the Ag shells (t) were properly controlled by the amount of Ag+ used for overgrowth. The SERS activities of the resulting Au2r@Agt NPs were systematically investigated by using 4-ATP (4-aminothiophenol) molecules as the SERS probe. Taken the result of Au20@Agt NPs as an example, the intensities of the Raman bands of 4-ATE molecules increase with the Ag shell thickness and it reaches the plateau value when the Au shell thickness is above 3.7 nm. This suggests that the critical Ag shell thickness (tc) for Au20@Agt NPs to reach the close-maximum SERS activity is 3.7 nm. We also studied the SERS spectra of 4-ATP molecules absorbed on the films of Au2r@Agt NPs with different Au core diameters coated on the glass substrate. The values of tc are 2.5, 5.0,2.9, and 2.7 nm for Aui2@Agt NPs, Au30@Agt NPs, Au45@Agt NPs, and Au55@Agt NPs, respectivly. We found that the SERS activities of the resulting Au2r@Agt NPs reached the maximal value when t was above a critical value (tc). The tc value could be empirically correlated with r by tc= 0.301 t+0.695 when 2r was below the mean free path of bulk gold (42 nm), while tc was fixed about 3 nm when 2r> 42 nm. Our results also demonstrate that Au@Ag NPs with the Ag shell thickness above tc can have the SERS activity comparable to that of pure Ag NPs with comparable total sizes at the excitation laser wavelength of 633 nm and 785 nm.In Chapter 4, we present a simple and effective strategy for high yield synthesis of well-dispersed, core-shell Au@Pt nanodendrites (CS Au@Pt NDs) via overgrowth of platinum on in situ 5.5 nm gold nanoparticles in water at room temperature. The sizes of the resulting CS Au@Pt NDs are about 14 nm, which should be the smallest so far, to the best of our knowledge. The average dimensions of the small Pt branches on the Au nanoparticle surfaces are about 2.6 nm×4.2 nm, which lead to a significantly increased electrochemically active surface area (up to 35.2 m2 g-1). It is found that the morphology of CS Au@Pt NDs is dependent on the reaction conditions such as the incubation time of citrate-HAuCl4 solution, the mixing time of citrate-HAuCl4-K2PtCl4 solution before AA addition, and Pt-to-Au and AA-to-Pt molar ratios.In comparison with commercial Pt black (0.12 A mgpt-1 under acidic condition; 0.3 A mgpt-1 under alkaline conditions), the resulting Au@Pt5 NDs show a superior catalytic activity towards methanol oxidation (0.45 A mgpt-1 under acidic condition; 2.1 A mgpt-1 under alkaline conditions) due to the electronic interaction between the Au cores and Pt branches in bimetallic Au@Pt NDs and the high fraction of atomic steps, kinks, and corner atoms on the surfaces of the Pt branches.In summary, in this paper, Au@M (M= Au, Ag, and Pt) NPs were prepared and their applications in SERS and electrocatalysis were investigated.1) Quasi-spherical, monodisperse Au NPs with sizes of 17 to 183 nm were synthesized by seeded growth method in one-pot manner under ambient conditions.2) One empirical formula on the relationship between the thickness of Ag shell and the diamters of Au core is obtained, which can be used for the design of Au@Ag NPs in SERS applications.3) Well-dispersed CS Au@Pt NDs with superior catalysis activities were prepared by a simple strategy at room temperature, which are the smallest ones so far.