Spontaneous Carbon-Support-Induced Metal Deposition

Recent investigations have demonstrated the possibility for spontaneous deposition of noble metal nanoparticles from corresponding metal ion solutions on carbon supports, in the absence of additional reductants in the experimental systems. This process is a result of a direct redox reaction between the solute metal species and the carbon materials and differs from the typical electroless plating, the latter requiring additional reducing agents or catalysts to drive the reduction reaction. Due to the diversity of the used carbon materials including both dispersed nanostructured carbons and supported electrode-like carbon materials and the different approaches to follow the process and characterize the products, these studies are scattered in the scientific literature. The aim of the present review is to put these diverse investigations in a common context and focus on the existing experimental findings, the ways to monitor and control the process, and the general concept. Some aspects that need to be further corroborated are outlined in view of the involvement of the spontaneous redox process as a practical tool for the development of new catalysts.

Investigating the Effects of Surface Adsorbates on Gold and Palladium Deposition on Carbon

Surface functional groups have a strong influence on the deposition and final state of nanoparticles adsorbed on to the surface, a role discussed by Professor Spencer in his work. This tribute to Spencer explores the formation of hydroxyls, thiosulfates, sulfites and sulfur atoms on carbon (HOPG) surfaces and their effect on the deposition of gold and palladium from aqueous solutions. Hydroxyls formed from ammonium hydroxide treatment have identical behaviour to those formed by acid treatment, and gold adsorption from Au3+ solutions gives Au0 initially, with Au3+ formed at higher concentrations on these surfaces. In contrast, palladium adsorption is hindered by the presence of the hydroxyls and there is no indication of any reduction to the metallic state. Ammonium thiosulfate adsorbs dissociatively from aqueous solutions on HOPG if the surface is pre-activated by the presence of surface hydroxyls. At low concentrations of ammonium thiosulfate, adsorbed sulfite and sulfur are formed in equimolar concentrations whereas adsorption of high concentrations of ammonium thiosulfate gives some degree of molecular adsorption, with evidence in XP spectra for an ammonium ion and a sulfur 2p peak at 282.9 eV attributed to the undissociated thiosulfate ion. Both sulfur and the sulfite are stable at the surface in neutral solutions but the sulfite desorbs when treated with acidified solutions (~ pH ≤ 6). These two groups are also stable at 373 K but begin to desorb by 473 K. Exposure to a weak chloroauric acid solution causes the desorption of the sulfite and formation of a gold species with an XP binding energy of 84.6 eV; we cannot determine from the present data whether this peak is due to a Au(I) state or very small nanoparticles of Au(0).

Graphic Abstract

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

Iron oxide nanomaterials participate in redox processes that give them ideal properties for their use as earth-abundant catalysts. Fabricating nanocatalysts for such applications requires detailed knowledge of the deposition and growth. We report the spontaneous deposition of iron oxide nanoparticles on HOPG in defect areas and on step edges from a metal precursor solution. To study the nucleation and growth of iron oxide nanoparticles, tailored defects were created on the surface of HOPG using various ion sources that serve as the target sites for iron oxide nucleation. After solution deposition and annealing, the iron oxide nanoparticles were found to nucleate and coalesce at 400 °C. AFM revealed that the particles on the sp³ carbon sites enabled the nanoparticles to aggregate into larger particles. The iron oxide nanoparticles were characterized as having an Fe³⁺ oxidation state and two different oxygen species, Fe-O and Fe-OH/Fe-OOH, as determined by XPS. STEM imaging and EDS mapping confirmed that the majority of the nanoparticles grown were converted to hematite after annealing at 400 °C. A mechanism of spontaneous and selective deposition on the HOPG surface and transformation of the iron oxide nanoparticles is proposed. These results suggest a simple method for growing nanoparticles as a model catalyst.

Eco-Friendly, Direct Deposition of Metal Nanoparticles on Graphite for Electrochemical Energy Conversion and Storage

Simple, green, and energy-efficient methods for preparing electroactive materials used to generate and store renewable energy are important for a sustainable future. In this study, we showed that noble and certain non-noble metal nanoparticles can be deposited on graphite without the aid of any reducing agent. This method of reducing metal ions to metal nanoparticles by graphite involves only one step (i.e., immersion into a solution) and one chemical (i.e., a metal salt). Hence, the method is exceedingly simple, green, and does not require any energy input. Large amounts of metal nanoparticles are generated both on the surface and deep into the bulk of graphite (∼100 μm). Despite the simplicity of this method, the metal deposited on graphite showed good electrocatalytic performance for ethanol oxidation and oxygen evolution reactions and also functioned as electrodes for supercapacitors. This method is thus ideal for preparing electrocatalytic materials and electrochemical energy storage devices due to its simplicity and environmental sustainability. The simplicity of the method is due to the inherent reducing potential of graphite (i.e., a material that is generally perceived as inert). Results from analyses showed that functionalization of the reactive edges in the regions of defects allowed the graphite to serve as a reducing agent. Increasing the amount of defects (e.g., via chemical or simple mechanical treatments) is shown to be the fundamental principle for increasing the reactivity of graphite.

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

We discuss the electrodeposition of two-dimensional (2D) Pt-nanostructures on Highly Oriented Pyrolytic Graphite (HOPG) achieved under constant applied potential versus a Pt counter electrode (Eappl = ca. -2.2 V vs. NHE, normal hydrogen electrode). The deposition conditions are discussed in terms of the electrochemical behavior of the electrodeposition precursor (H₂PtCl₆). We performed cyclic voltammetry (CV) of the electrochemical Pt deposit on HOPG and on Pt substrates to study the relevant phenomena that affect the morphology of Pt deposition. Under conditions where the Pt deposition occurs and H₂ evolution is occurring at the diffusion-limited rate (-0.3 V vs. NHE), Pt forms larger structures on the surface of HOPG, and the electrodeposition of Pt is not limited by diffusion. This indicates the need for large overpotentials to direct the 2D growth of Pt. Investigation of the possible effect of Cl- showed that Cl- deposits on the surface of Pt at low overpotentials, but strips from the surface at potentials more positive than the electrodeposition potential. The CV of Pt on HOPG is a strong function of the nature of the surface. We propose that during immersion of HOPG in the electrodeposition solution (3 mM H₂PtCl₆, 0.5 M NaCl, pH 2.3) Pt islands are formed spontaneously, and these islands drive the growth of the 2D nanostructures. The reducing agents for the spontaneous deposition of Pt from solution are proposed as step edges that get oxidized in the solution. We discuss the possible oxidation reactions for the edge sites.

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

We discuss the electrodeposition of two-dimensional (2D) Pt-nanostructures on Highly Oriented Pyrolytic Graphite (HOPG) achieved under constant applied potential versus a Pt counter electrode (E_appl = ca. -2.2 V vs. NHE, normal hydrogen electrode). The deposition conditions are discussed in terms of the electrochemical behavior of the electrodeposition precursor (H₂PtCl₆). We performed cyclic voltammetry (CV) of the electrochemical Pt deposit on HOPG and on Pt substrates to study the relevant phenomena that affect the morphology of Pt deposition. Under conditions where the Pt deposition occurs and H₂ evolution is occurring at the diffusion-limited rate (-0.3 V vs. NHE), Pt forms larger structures on the surface of HOPG, and the electrodeposition of Pt is not limited by diffusion. This indicates the need for large overpotentials to direct the 2D growth of Pt. Investigation of the possible effect of Cl^- showed that Cl^- deposits on the surface of Pt at low overpotentials, but strips from the surface at potentials more positive than the electrodeposition potential. The CV of Pt on HOPG is a strong function of the nature of the surface. We propose that during immersion of HOPG in the electrodeposition solution (3 mM H₂PtCl₆, 0.5 M NaCl, pH 2.3) Pt islands are formed spontaneously, and these islands drive the growth of the 2D nanostructures. The reducing agents for the spontaneous deposition of Pt from solution are proposed as step edges that get oxidized in the solution. We discuss the possible oxidation reactions for the edge sites.