Here we describe the oxidation of <4 nm diameter Au nanoparticles (NPs) attached to indium tin oxide-coated glass electrodes in Br(-) and Cl(-) solution. Borohydride reduction of AuCl(4)(-) in the presence of hexanethiol or trisodium citrate (15 min) led to Au NPs <4 nm in diameter. After electrochemical and ozone removal of the hexanthiolate ligands from the thiol-coated Au NPs, Au oxidation peaks appeared in the range 0-400 mV vs Ag/AgCl (1 M KCl), which is 850-450 mV negative of the bulk Au oxidation peak near 850 mV. The oxidation potential of citrate-coated Au NPs is in the 300-500 mV range and those of 4 and 12 nm diameter Au NPs in the 660-780 mV range. The large negative shift in potential agrees with theory for NPs in the 1-2 nm diameter range. The oxidation potential of Au in Cl(-) solution is positive of that in Br(-) solution, but the difference decreases dramatically as the NP size decreases, showing less dependence on the halide for smaller NPs.

Here we describe size-dependent electrophoretic deposition (EPD) of citrate-stabilized Au nanoparticles (NPs) onto indium-tin-oxide-coated glass (glass/ITO) electrodes as studied by linear sweep stripping voltammetry (LSSV) and scanning electron microscopy (SEM). LSSV allows both the determination of the Au NP coverage and NP size from the peak area and the peak potential, respectively. Two-electrode EPD in aqueous solutions of Au NPs plus HO reveal that a minimum potential of 1.5 V is needed for significant deposition of 4 nm diameter Au NPs as opposed to 2.0 V for 33 nm diameter Au NPs. EPD at 0.4 V in a solution of Au NPs prepared with a short 5 min reaction time led to the successful capture of 1-2 nm diameter Au NPs with appreciable coverage. In all cases, deposition did not occur in the absence of HO. Three-electrode experiments with a real reference electrode revealed the same size selective deposition with potential and that the amount of Au deposited depends on the deposition time and HO concentration. The deposition occurs indirectly by oxidation of HO, which liberates protons and neutralizes the citrate stabilizer, leading to precipitation of the Au NPs onto the glass/ITO electrode. Studies on pH stability show that larger Au NPs aggregate at lower pH compared to smaller Au NPs. More importantly, though, 4 nm diameter Au NPs are much more catalytic for HO oxidation, which is the main reason for the size selective deposition.

Here we report on the size-dependent oxidation of Au nanoparticles (NPs) electrodeposited directly on indium tin oxide-coated glass (glass/ITO) electrodes as compared to those chemically synthesized and electrostatically or drop-cast deposited onto aminopropyltriethoxysilane (APTES)-modified, mercaptopropyltrimethoxysilane (MPTMS)-modified, or unmodified glass/ITO electrodes. The peak oxidation potential (Ep) of 54 nm diameter Au NPs shifts by as much as 155 mV negative when deposited electrostatically on the highly positively charged glass/ITO/APTES surface and oxidized at low pH as compared to their oxidation on more neutral glass/ITO or glass/ITO/MPTMS surfaces at all pH's and on glass/ITO/APTES at neutral pH. Electrodeposited Au NPs on glass/ITO of similar size also oxidize at more positive potentials due to the neutral electrode surface charge. Ag NPs show a similar charge dependence on their Ep. Interestingly, the Ep value of Au and Ag NPs smaller than about 10 nm in diameter is independent of surface charge. The Ep of 9 nm diameter citrate-capped Ag NPs attached to Au, Pt, glassy carbon (GC), and glass/ITO electrodes electrostatically through short amine-terminated organic linkers depends on the electrode material, following the order (vs Ag/AgCl) of Au (384 ± 7 mV) ≈ Pt (373 ± 12 mV) > GC (351 ± 2 mV) > glass/ITO (339 ± 1 mV). The underlying electrode material affects the Ag NP Ep even though the NPs are not directly interacting with it. In addition to size, the electrode material and its surface charge have a strong influence on the oxidation potential of surface-confined metallic nanostructures.

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.

The photoelectrocatalytic reduction of CO 2 uses sunlight to reduce the external energy needed to convert this greenhouse gas into value-added products. We report the deposition of a thin film of copper oxide onto a large-surface-area plasmonic silver structure, which generates an efficient photoelectrocatalyst for CO 2 reduction. Using incoherent visible light illumination and applying −0.4 V versus Ag/AgCl(KCl 1M), CO 2 was reduced to acetate with a faradaic efficiency of 54%. Rather than adding plasmonic nanoparticles as sensitizers onto semiconductors, here we electrodeposit a thin uniform layer of Cu 2 O/CuO over a plasmonic silver structure. The formation of acetate at this low potential has not been reported before and appears to arise from synergistic effects in this hybrid plasmonic-semiconductor material. In this work, we investigate changes in the photophysics under different preparation conditions. Varying the deposition time of Cu 2 O/CuO deposited onto the Ag to form the Ag/ Cu 2 O/CuO electrodes alters electron−hole recombination. The Ag/Cu 2 O/CuO electrodes show the highest photocurrent density when a minimal Cu 2 O/CuO film covers the Ag structure. Synergistic effects between the localized surface plasmon resonance of silver and semiconductor properties of Cu 2 O/CuO decrease the necessary overpotential required for CO 2 reduction, reduce charge recombination processes, and stabilize the Cu 2 O/ CuO semiconductor on the photoelectrode. The stabilization of Cu 2 O/CuO in the presence of energetic charge carriers is believed to be key to producing acetate with high efficiency. These properties suggest an interesting approach to photoelectrocatalytic materials.

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# equal contribution SUPPORTING INFORMATION Table S1. Localized surface plasmon resonance (LSPR) frequencies and scanning electron microscopy (SEM) images of 16 representative constructs consisting of a Au nanoplate (NP) with a Au nanosphere (NS) attached on the edge.

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