Controlled Synthesis Of Gold-based Nanoparticles With High-index Facets And Study Of Their Catalytic And SERS Performance

Gold nanoparticles(Au NPs)bear a variety of potential applications in many fields,such as catalysis,surface enhanced Raman scattering(SERS),biomedicine and biosensor due to their peculiar physical-chemical properties.However,the physical-chemical properties of Au NPs are morphology and size-dependent.For instance,Au NPs with high-index facets generally exhibit much higher catalytic activity than those with low-index facets,because the high-index facets have a large density of atomic steps,ledges,and kinks,which usually serve as active sites.Additionally,the sharp corners and edges from high-index faceted Au NPs can also result in significant electromagnetic field enhancements,which can greatly improve the SERS performance.Thus,controlled synthesis of Au NPs bounded by high-index facets can simultaneously enhance their performance in catalysis and SERS.Core-shell(CS)structured bimetallic NPs can bear multi-functionality and also exhibit enhanced performance due to the synergistic effects among metals,in comparison with their monometallic counterparts.For instance,CS Au@Pd NPs can be obtained by directly deposition of ultrathin Pd shells on the surfaces of Au NPs with high-index facets.The resulting CS Au@Pd NPs can not only improve the catalysis performance but also achieve the incorporation of catalysis and in situ SERS monitoring.However,thick Pd shells were usually formed on the resulting Au@Pd NPs,which resulted in the decrease in catalytic performance due to the large amount of Pd precursor and lower Pd utilization efficiency,and a great reduction in their SERS performance due to a large damping effect of thick Pd shells on SPR of Au cores.Therefore,it's desirable to develop one new method to fabricate high-index faceted Au@Pd NPs with ultrathin Pd shells.Additionally,due to the high cost,insufficient activity,and poor durability of Pt as a catalyst in proton electrolyte membrane fuel cells,it is still urgent to explore an effective strategy to obtain Pt-containing catalysts with high electrocatalytic activity and stability.The Pt-containing catalysts need to not only reduce the amount of Pt used,but also improve their electrocatalytic performance.Moreover,the incorporation of Au into Pt-containing catalysts can improve the stability of Pt-containing catalysts.However,the direct deposition of Pt on the surfaces of Au NPs to obtain Au-Pt catalysts with high electrocatalytic performance is relatively hard because of the restriction from their surface energy.Since Pd and Pt have good miscibility,the restriction from surface energy between Au and Pt may be solved by the assistance of Pd usage.Accordingly,for trimetallic CS structured catalysts,the quality of transition layers would be of paramount importance,which would affect the charge transfer between the core and the shell,and further impact the electrocatalytic performance of CS catalysts.Thus,in order to enhance the catalytic performance,it would be of great interest to select Au cores with suitable facets,which can be beneficial to form one perfect transition layer between the Au core and PdPt shell.On the basis of the current issues mentioned above,the synthesis and application of Au-based NPs with high-index facets as main goal were studied in the thesis.Firstly,70 nm trisoctahedral(TOH)Au NPs were obtained through the seed-mediated growth method,and then they were further used as seeds to successfully synthesize {331}-faceted TOH Au NPs with sizes ranging from 60 to 255 nm.Next,the epitaxy growth of ultrathin Pd shells of a few atomic layer on the surfaces of Au NPs of different morphologies({331}-faceted TOH,cubic,and spherical shapes)in the presence of cetyltrimethylammonium chloride(CTAC)by regulating the pH value of the growth solution were successfully achieved.Lastly,hyperbranched,TOH-shaped core-shell(HTCS)Au@PdPt NPs were successfully fabricated by facet-dependent growth of a high quality transition Pd layer and uniform PdPt alloy shell with nanobranches,respectively,by two batches of addition of AA solution,on {331}-faceted TOH AuNPs.The main contents of this thesis are as follows:In Chapter 2,we investigate the effect of Au seeds prepared in CTAC solution on the index facets of TOH Au NPs.Using Au NPs coated with cetyltrimethylammonium bromide(CTAB)ligands as seeds,{221}-faceted TOH Au NPs can be obtained.{331}-faceted TOH Au NPs with controllable sizes(from 60 to 255 nm)can be successfully prepared in high yield by using Au NPs coated with CTAC ligands as seeds.We find that the electrocatalytic performance on methanol oxidation and SERS activity of{331}-faceted TOH Au NPs is size-dependent.In comparison with well-known nanoporous Au NPs(0.088 mA cm-2),{331}-faceted TOH Au NPs with sizes of 110 nm exhibit fairly high catalytic activity(0.178 mA cm-2)on methanol oxidation in alkaline media.Their current density is reduced by less than 7%after 500 cycling tests.{331}-Faceted TOH Au NPs with sizes of 175 nm exhibit the highest SERS activity for 4-aminothiophenol(4-ATP)molecules.The enhancement factors of ai modes of 4-ATP molecules can reach the order of 109 when the 4-ATP concentration is 3 × 10-6 M.This is because the SERS properties of the TOH Au NPs is higher than spherical Au NPs of comparable size due to sharp extremities.In Chapter 3,the epitaxy growth of ultrathin Pd shells of a few atomic layer on the surfaces of AuNPs of different morphologies({331}-faceted TOH,cubic,and spherical shapes)as the cores are achieved by regulating the pH value of the aqueous CTAC solution and finely tuning the amount of the Pd precursors.It is found that the critical shell thickness for epitaxy Pd growth under optimal pH value is determined to be 4 atomic layers in our case,taking {331}-faceted TOH Au@PdnL NPs as example.Moreover,the resulting TOH Au@Pd1L NPs(100.9 m2 g-1,13.2 AmgPd-1,and 13.1 mA cm-2)exhibit excellent electrocatalytic performance and a long-term electrocatalytic activity for ethanol oxidation,which are about 4.8-fold,66-fold,and 21.8-fold better than commercial Pd/C catalysts(31 m2 g-1,0.2 AmgPd-1,and 0.6 mAcm-2).Furthermore,the resulting TOH Au@Pd1L NPs can not only markedly enhance chemical catalytic activity for the reduction of 4-nitrophenol(4-NP),but also achieve in situ SERS monitoring of reaction process of Pd-catalyzed reduction of 4-nitrotiophenol(4-NTP).In Chapter 4,we report the production of hyperbranched,TOH-shaped core-shell Au@PdPt nanoparticles(HTCS Au@PdPt NPs)by facet-dependent growth of transition Pd layers and uniform PdPt alloy shells,respectively,by two batches of addition of the reducing agent,ascorbic acid(AA)solution,on preformed {331}-faceted TOH NPs.It is found that the thickness and composition of the PdPt alloy shell of HTCS Au@PdPt NPs can be accurately controlled by varying the amount of the Pd and Pt precursors.Moreover,the ultrasmall PdPt branches on the TOH Au cores can lead to a significantly increased electrochemically active surface area(up to 55.4 m2 gPt-1).Taking electrooxidation of methanol and formic acid by Au@PdPt catalysts as an example,three types of Au cores enclosed with different facets were selected to investigate the effect of the resulting transition Pd layer on the performance of CS catalysts with similar structures.By comparison of their electrocatalytic performance,the enhanced electrocatalytic performance of the HTCS Au100@Pd20Pt20 NPs can be mainly attributed to improved charge transfer between the well-defined TOH Au cores and the highly crystalline PdPt alloy shells besides the unique dendritic PdPt alloyed shell and CS structure.The as-prepared HTCS Au100@Pd20Pt20 NPs have excellent electrocatalytic performances in electro-oxidation of methanol and formic acid,which are significantly higher than those of commercial Pt black and other Au@PdPt NPs with Au cores enclosed with different facets.In addition,HTCS Au100@Pd20Pt20 NPs also exhibit the best durability among them possibly due to their superior anti-CO poisoning capability.Chapter 5 is a summary and prospect of current research work.In summary,in this thesis,we prepared uniform Au NPs with high-index facets,Au@Pd CS bimetallic NPs with ultrathin Pd shells and hyperbranched,TOH-shaped CS Au@PdPt trimetallic NPs,which greatly expand the application of Au-based NPs in catalytic and SERS.