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.

Here we report the electrochemical determination of the surface-area-to-volume ratio (SA/ V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of coulombs of charge passed during the reduction of surface AuO following Au NS oxidation in HClO, whereas V is determined by electrochemically measuring the coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Br to form aqueous soluble AuBr. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/ V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/ V versus 1/radius for five different-sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/ V versus 1/radius is linear with a slope of 3.5, and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of the SA.

Here, we describe the size-dependent, electrochemically controlled Ostwald ripening of 1.6, 4, and 15 nm-diameter Au nanoparticles (NPs) attached to (3-aminopropyl)­triethoxysilane (APTES)-modified glass/indium-tin-oxide electrodes. Holding the Au NP-coated electrodes at a constant negative potential of the dissolution potential in a bromide-containing electrolyte led to electrochemical Ostwald ripening of the different-sized Au NPs. The relative increase in the diameter of the NPs (D final/D initial) during electrochemical Ostwald ripening increases with decreasing NP size, increasing applied potential, increasing NP population size dispersity, and increasing NP coverage on the electrodes. Monitoring the average size of the Au NPs as a function of time at a controlled potential allows the measurement of the Ostwald ripening rate. Anodic stripping voltammetry and electrochemical determination of the surface area-to-volume ratio provide fast and convenient size analysis for many different samples and conditions, with consistent sizes from scanning electron microscopy images for some samples. It is important to better understand electrochemical Ostwald ripening, especially under potential control, since it is a major process that occurs during the synthesis of metal NPs and leads to detrimental size instability during electrochemical applications.

Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10–4 M Ag+ solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10–5 M PtCl4 2–, which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl4 2–, STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E0) for Au oxidation with decreasing NP size.

We present here high sensitivity attenuated total reflectance (ATR) spectroelectrochemical studies of electron injection (reduction) into surface-tethered, submonolayer to monolayer coverages of CdSe quantum dots (QDs) linked to indium–tin oxide (ITO) electrodes using a strong X-type bifunctional phosphonic acid (PA) surface linker, octanediphosphonic acid (ODiPA). Estimates of conduction band energies (E CB) were obtained from the onset of absorbance bleaching as a function of QD diameter (3.2–6.4 nm) and as a function of the supporting electrolyte (LiClO4) concentration. For CdSe QDs created from combinations of moderately strong stearic acid, hexadecylamine, trioctylphosphine oxide, and trioctylphosphine ligands, surface-tethering was accompanied by decreases in QD diameter and loss of up to 25% volume for the largest QDs. For QDs prepared with PA ligands, followed by aggressive (3×) pyridine exchange to produce QDs with weak capping ligands, no size reduction was observed as a result of adsorption to the ODiPA/ITO surface. For both types of tethered CdSe QDs, significant stabilization of the reduction product of the surface-tethered QD was observed with ca. 700 meV lowering of E CB relative to estimates of E CB obtained from our recent in vacuuo UV-photoemission studies of bare CdSe QDs tethered to Au surfaces. A sizeable fraction of that stabilization is proposed to arise from the tethering of these asymmetric QDs to a complex, high dielectric constant interface region. At least 200 meV of the stabilization arises from concentration-dependent charge screening by the solution counter ion (Li+), with no evidence for the incorporation of Li+ as a result of the electron injection process. The overall stabilization in the reduced form of these tethered QDs is larger than seen for previous spectroelectrochemical studies of QD reduction, in solution, tethered at higher coverages to transparent electrodes, or as electrophoretically deposited multilayer QD thin films. This waveguide ATR spectroelectrochemical approach to estimating energetics for QDs tethered to semiconductor or oxide substrates at low surface coverages is likely to be relevant for a wide array of energy conversion and energy storage processes using nanomaterials and may be especially useful for studying the effects of surface coverage, type of surface linker, contacting solvent/electrolytes, and adsorbed molecular reactants.