Size Stability Study of Catalytically Active Sub-2 nm Diameter Gold Nanoparticles Synthesized with Weak Stabilizers

Abstract

Here we report on the very low size stability of electrocatalytically active 1.5 to 2.0 nm diameter tetrakis­(hydroxymethyl)­phosphonium chloride-stabilized Au nanoparticles (THPC Au2nm NPs) chemically attached to glass/indium tin oxide electrodes. The potential for oxidative dissolution of THPC Au2nm NPs in the presence of bromide is about 250 mV negative of 4 nm diameter citrate-stabilized Au NPs (Cit Au4nm NPs) and 450 mV negative of bulk Au, which provides us with an easy method to assess the size stability using anodic stripping voltammetry. The THPC Au2nm NPs show a strong CO2 reduction wave at about −0.40 V (vs RHE), which is nonexistent for the Cit Au4nm NPs or bulk Au. The THPC Au2nm NPs are also comparatively more electroactive for the hydrogen evolution reaction. In acid electrolyte, however, the potential for surface Au2O3 formation on THPC Au2nm NPs is significantly negative relative to bulk Au, and a single cycle through the surface oxide and reduction waves leads to an increase in the NP size to about 4 nm. Similarly, the THPC Au2nm NPs undergo Ostwald ripening in the presence of bromide within 5 min at potentials well before oxidation, which increases their size to 4–10 nm in diameter by 35 min. Exposure to ozone for only 1–2 min also causes the THPC Au2nm NPs to increase in size to about 4 nm. In comparison, Cit Au4nm NPs are stable under all of these conditions, requiring much longer times to change in size. These differences in reactivity and size stability are due to the different Au NP size. Sub-2 nm diameter NPs with weak stabilizers are potentially very useful for electrocatalysis, but their low oxidation potential and poor size stability are major issues of concern.

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

“…TPPS Au 0.9nm and THPC Au 1.6nm NPs do not display a distinct LSPR band in the UV‐Vis region, consistent with them being below 2 nm in diameter. We showed this for Au 1.6nm NCs previously and again here to compare to Au 0.9nm NCs. Figures B and 1 C show transmission electron microscopy (TEM) images and scanning transmission electron microscopy (STEM) images (see the insets) for TPPS Au 0.9nm and THPC Au 1.6nm NCs, respectively.…”

“…We next monitored the oxidation behavior of the various‐sized Au NCs and NPs attached to glass/ITO/APTES electrodes by performing ASV in Br − containing electrolyte solution as described by our group previously . The peaks in the ASVs in Figure G are due to current produced primarily by the 3 electron oxidation of Au to form AuBr 4 − in 0.01 M KBr plus 0.1 M KClO 4 .…”

“…Au NCs in this size range exhibit quantized double layer charging, the emergence of discrete electronic energy levels, and photoluminescence . They have also found use in electrocatalysis . While they have desirable properties, small‐sized NCs are also more prone to oxidation and transformation to larger sizes by sintering, which causes rapid deactivation of their desirable properties and reactivity during applications.…”

“…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 . NCs may also change their size during the application of interest.…”

“…These size changes make it difficult to correlate the initial size with their reactivity . Recently, we reported that 1.6 nm diameter Au NCs coated with weak stabilizers are more reactive for the CO 2 reduction reaction and hydrogen evolution reaction (HER) than thiol‐passivated Au NCs of similar size . The same Au NCs converted to 4–10 nm diameter Au NPs after one electrochemical surface oxidation‐reduction cycle in acid, 2 min of ozone treatment, and after holding the potential at 0.30 V versus Ag/AgCl (3 M KCl) for various times in acidic KBr solution.…”