The deposition of metal nanoparticles on carbon surfaces: the role of specific functional groups

Abstract

The enormous complexity of a typical heterogeneous catalyst makes understanding the development and properties of any active nanoparticles present extremely challenging.In the case of carbon based catalysts that difficulty is compounded by the variability of the carbon powders used. We have previously developed a strategy that addresses these problems by mimicking the catalyst preparation conditions very closely but using highly ordered pyrolytic graphite crystals (HOPG) as a model surface. This enables us to examine the effects of specific functional groups on nanoparticle formation. We report here an extension of our work characterising functional groups on the HOPG surface, using XPS and AFM to explore the deposition of gold from aqueous solution onto HOPG surfaces treated in a variety of ways to alter the surface functionality. The structure and oxidation state of the resulting nanoparticles depend critically on the nature of the functional groups present and offers some insight into the development of catalysts based on these materials. Hydroxyls are identified as key functional species, reducing gold ions to their metallic state whilst being oxidised themselves to carbonyls.Carbonyls meanwhile promote the nucleation of Au 3+ , creating a network of islands at the HOPG surface. The results have relevance not only to catalysts using activated carbons but also the new generation of materials based on graphene and carbon nanotubes.

Electrodeposition of Two-Dimensional Pt Nanostructures on Highly Oriented Pyrolytic Graphite (HOPG): The Effect of Evolved Hydrogen and Chloride Ions

“…We are not aware of reports on the half oxidation reaction during Pt deposition in the literature, in part because the spontaneous deposition of Pt on C is a point that has received limited attention in the literature. Because the electrochemical papers lack XPS studies of the surface oxidation before and after Pt deposition, we get insight into the half reactions of oxidation from the XPS studies of the adsorption and deposition of Au on HOPG from acidic solutions [ 38 , 39 , 40 ]. Because they do not report the redox properties of these surface reactions, we must deduce possible reactions from surface composition studies.…”

“…During heating, the hydroxyl groups oxidize to produce ketone and ester groups on the HOPG surface [ 39 , 40 ]. However, the C–OH groups reduced Au 3+ to Au 0 from solution while the C–OH oxidize to C=O groups [ 38 ]. Therefore, based on the findings by XPS and AFM of Au deposition on HOPG [ 38 , 39 , 40 ] along with the AFM studies in this work ( Figure 6 A,B,D) and previous reports that show preferential deposition of Pt along the step edges [ 32 , 34 , 35 ], we propose the following oxidation reactions in our precursor solution.…”

“…However, the C–OH groups reduced Au 3+ to Au 0 from solution while the C–OH oxidize to C=O groups [ 38 ]. Therefore, based on the findings by XPS and AFM of Au deposition on HOPG [ 38 , 39 , 40 ] along with the AFM studies in this work ( Figure 6 A,B,D) and previous reports that show preferential deposition of Pt along the step edges [ 32 , 34 , 35 ], we propose the following oxidation reactions in our precursor solution. Initially, during immersion of HOPG to the precursor solution (3 mM H 2 PtCl 6 0.5 M NaCl pH 2.3, which corresponds to dilute HCl), the step edges oxidize to C–OH groups as shown in Scheme 1 .…”

“…However, there are no reports on the half oxidation reaction that reduces the Pt precursor. Interestingly, the electrochemical papers lack X-ray photoelectron spectroscopy (XPS) studies of the surface oxidation and, to get insight into the half reactions, we look at the surface energies and composition studies of the spontaneous deposition of Au on HOPG from solutions [ 38 , 39 , 40 ]. Here, we propose that edge sites oxidize sequentially to C–OH and C=O groups, based on analogous studies for the deposition of Au on HOPG to produce gas-phase catalysts and our own results that see preferential deposits on the edges, in agreement with previous reports [ 32 , 34 , 35 ].…”

Spontaneous selective deposition of iron oxide nanoparticles on graphite as model catalysts

“…24,27 Carbon supports are typically utilized due to their high thermal and mechanical stability, without surface metal support interactions. [28][29][30][31][32][33][34] Since the discovery of graphene, 35 various methods have been used to alter the top layers of the surface providing nucleation points on an otherwise chemically stable material. [36][37][38][39][40][41][42] Defects on graphene and graphite (highly oriented pyrolytic graphite) surfaces have been used to create surface sites for further growth of metals, particularly using various sputtering and plasma methods.…”