Removal of Thiol Ligands from Surface-Confined Nanoparticles without Particle Growth or Desorption

Abstract

Size-dependent properties of surface-confined inorganic nanostructures are of interest for applications ranging from sensing to catalysis and energy production. Ligand-stabilized nanoparticles are attractive precursors for producing such nanostructures because the stabilizing ligands may be used to direct assembly of thoroughly characterized nanoparticles on the surface. Upon assembly; however, the ligands block the active surface of the nanoparticle. Methods used to remove these ligands typically result in release of nanoparticles from the surface or cause undesired growth of the nanoparticle core. Here, we demonstrate that mild chemical oxidation (50 ppm of ozone in nitrogen) oxidizes the thiolate headgroups, lowering the ligand's affinity for the gold nanoparticle surface and permitting the removal of the ligands at room temperature by rinsing with water. XPS and TEM measurements, performed using a custom planar analysis platform that permits detailed imaging and chemical analysis, provide insight into the mechanism of ligand removal and show that the particles retain their core size and remain tethered on the surface core during treatment. By varying the ozone exposure time, it is possible to control the amount of ligand removed. Catalytic carbon monoxide oxidation was used as a functional assay to demonstrate ligand removal from the gold surface for nanoparticles assembled on a high surface area support (fumed silica).

Au Capping Agent Removal Using Plasma at Mild Temperature

“…Once the capping agent has been removed, the Au nanoparticle interacts directly with the support, leading to decreased mobility and thus decreased sintering. This was also observed by Elliott et al [26], who showed an increase in Au nanoparticle size only when no more S signal from the residual capping agent was observed in XPS. The XPS data (discussed in more detail below) show that the as-prepared Au nanoparticles are metallic, whereas the ozone-treated Au nanoparticles are cationic.…”

“…For a sulphur-containing capping agent, Menard et al showed a tripling of the nanoparticle size (from 0.8 to 2.7 nm) upon calcination at 400 °C, whereas the particle size increase was limited to 1.2 nm after ozone treatment [25]. Elliott et al [26] found that slight sintering only occurred after all capping agent had been removed by the ozone treatment. The particle size stayed at 1.4 nm upon partial capping agent removal and increased to 1.6 nm when the S signal was no longer observed in the XP spectra.…”

“…Menard et al [25] showed decreased sintering after ozone treatment as compared to classical calcination for titania-supported Au nanoparticles, but they did not investigate whether any residue of their S-containing capping agent was still present. Recently, another study on ozone treatment for capping agent removal from Au nanoparticles was published [26]. This study tackled a stronger binding S-containing capping agent, which was partially oxidised by mild ozone treatment and then washed off using water.…”

Electrooxidation, Size Stability, and Electrocatalytic Activity of 0.9 nm Diameter Gold Nanoclusters Coated with a Weak Stabilizer

“…Unfortunately, this goal is not always realized. For example, Elliot and coworkers reported that thiol‐stabilized 1.4 nm diameter Au NCs increased in size after 16 min of ozone cleaning, while the size didn't change significantly for up to 8 min of ozone cleaning . We previously studied the ozone stability of the THPC Au 1.6nm NCs, showing a significant size increase after just 1–2 minutes .…”

“…Whether the ligands are strongly‐attached or weakly‐attached, researchers have developed various treatments to remove stabilizers in order to render the metal surface more active for the application of interest. These treatments include ozone cleaning, thermal treatment, or chemical and electrochemical treatments . Unfortunately, the treatment may change the size of the NCs in addition to removing the stabilizer .…”