In recent years core/shell metal NPs with various structures, such as nanocubes,106 nanorods,107 and nanotubes,108 have gained tremendous interest in the field of nanoscience due to their potential applications in catalysis and sensing. The formation of these core/shell nanostructure has been mostly performed during
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the chemical reduction process, where ions of one metal were reduced onto the surface of another metal in the presence of some type of stabilizer. However, the synthesis of nanomaterials by this strategy takes on multiple steps, which sometimes makes it difficult to control the shape and structure. Therefore, researchers have been searching for alternative methods to synthesize nanomaterials of well-controlled structure. Recently, galvanic exchange reactions have become a promising alternative approach to make designer, well-engineered nanomaterials. The galvanic exchange reaction is driven by the difference in the standard potential of two different metals. Usually this reaction leads to the deposition of a more stable metal by the oxidation of a less stable metal. This strategy has been used recently for making various uniquely-structured nanomaterials, including hollow nanocubes,109 nanotubes, nanorattles,110 and nanocages.111 For example, Murshid and coworkers performed a galvanic exchange reaction between Ag decahedral NPs and an Au3+ ions to form stable core/shell Ag/Au decahedral alloy NPs.112 The following reaction is one example of a typical galvanic exchange reaction between two metals, Ag(0) and an Au(III) complex in this case.
3Ag(s) + AuCl4-(aq) → Au(s) + 3Ag+(aq) + 4 Cl-(aq) There are examples of non-conventional galvanic exchange between the ion of the less noble metal and the metallic form of the more noble metal, which should not occur thermodynamically. For example, galvanic exchange between thiolate-stabilized Au25 nanoclusters with Ag+ and Cu2+ ions was reported first by Murray and coworkers.113 They believe that addition of metal ions on the Au25
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nanocluster was possible because thiolate ligands gain a partial negative charge after attaching to the Au or Ag surface, which assisted in Ag+ ion reduction on the Au surface. Later, Wu and coworkers reported that galvanic exchange of thiol- stabilized Au nanoclusters with Ag+ ions is only possible if the Au nanocluster size becomes lower than 3 nm in diameter.114 They also believe that this reaction occurs due to the partial negative charge of thiolate ligand, which acts as a reducing agent. In 2014, Wang and coworkers reported the anti-galvanic exchange reaction of naked Au NPs.115 Ligand free naked Au NPs were obtained by the reduction of Au+ ions with NaBH4 and the resultant aggregated NPs were dispersed by the laser ablation method. In this process they were able to get well disperse 1 to 19 nm diameter Au NPs with an average size of 13.5 ± 0.5 nm in diameter. After addition of an aqueous solution of AgNO3 to the Au NPs solution they found that Ag atoms deposited on the Au NPs based on the peak in the X-ray photoelectron spectroscopy (XPS). This finding indicates that anti-galvanic exchange of Au NPs with Ag+ ions is not due to the thiolate ligand but could be due to the thermodynamic negative shift of the oxidation peak potential of naked Au NPs. Yao and coworkers reported that Ag atoms do not replace Au atoms in the precursor particles, but that they instead place side by side of Au atoms by depositing on Au.116 They proved this phenomenon based on the mass spectroscopic analysis and the finding was also supported by theoretical calculations. Recently, Tian and coworkers reported that anti-galvanic exchange between Au nanoclusters and Ag+ ions depends on the structure ions precursor.117 For example, galvanic exchange between aqueous AgNO3 and Au25(PET)18, (PET
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= SC2H4Ph) shows Au25Ag2(PET)18 as a main product, while galvanic exchange between Au25(PET)18 with Ag–DTZ complexes (DTZ is dithizone) shows Au24Ag(PET)18 as a main product in the mass spectrum. Luo and coworkers reported the conversion of a 23 atom [Au23(SR)16]– nanocluster to a 21 atom [Au21(SR)12(Ph2PCH2PPh2)2] + nanocluster by resection of two surface Au atoms first with two Ag+ ions followed by replacement of the two Ag atoms by galvanic exchange with a Au(I)-diphosphine complex, Au2Cl2(P–C–P).118This slight surgery
on the Au clusters provided a 10-fold increase in the luminescent properties of the Au nanoclusters.
Several other groups also studied galvanic exchange reactions with 2-3 nm diameter Au nanoclusters for use of the resulting alloy nanoclusters for catalytic applications.119-120 For example, Young and coworkers reported that the addition of Ag atoms on the surface of Au nanoclusters by galvanic exchange significantly increased the 4-electron oxygen reduction reaction (ORR) in alkaline media as compared to other Au-Ag alloy NPs.121 These studies indicate that galvanic exchange between Au and Ag+ ions is possible, however, the mechanism of this exchange is unknown in the literature. In this dissertation we report that galvanic exchange between Au NPs and Ag+ is thermodynamically favorable when the NP size becomes less than 2 nm in diameter due to the negative shift of the oxidation potential of Au NPs with decreasing size. Details of the size-dependent galvanic exchange between Au NPs with Ag+ ions will be the focus of Chapter VII.
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