Kinetic Control of [AuCl₄]⁻ Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers

Abstract

Laser-induced photochemical reduction of aqueous [AuCl4]− is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq –) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]− reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k 1, dependent on eaq –; and growth rate k 2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k 2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of “naked” gold nanoparticles produced by photochemical [AuCl4]− reduction can be effectively tuned by controlling the reaction kinetics.

Laser-assisted synthesis of gold–graphene oxide nanocomposites: effect of pulse duration

“…Despite extensive investigations into laser synthesis of metal-GO nanocomposites, [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] there are no previous reports comparing these materials produced using different laser sources under otherwise similar conditions to the best of our knowledge. produced during water photolysis, via the reactions [47][48][49] [AuCl 4 ] À + 3e aq À -Au 0 + 4Cl À (1)…”

“…Moreover, the k Au = 0.23 AE 0.01 mmol À1 s À1 value obtained for the 5 nm Au NPs synthesized with the ns laser in the absence of GO is nearly identical to the k Au = 0.22 AE 0.02 mmol À1 s À1 obtained for 4 nm Au NPs synthesized with tightly focused fs laser pulses in our previous work. 49 Hence, the presence of GO enhances the PNP reduction rate beyond what would be expected based solely on Au NP size. The observed rate enhancement is likely due to two previously reported synergistic effects between the Au and graphene: (1) the high adsorption of PNP onto graphene and (2) electron donation from graphene to the Au NPs, which facilitates electron transfer to PNP.…”

“…This down-shifted value has been observed in some previous reports of Au-GO composites 17,19 and in our previous reports of Au NPs synthesized by laser processing. 47,49,56 The down-shift can be attributed to interaction with the GO support, 17,19 the presence of low-coordinated Au atoms, 47,49,56,57 or both. The peaks at 84.7 eV (fs) and 85.0 eV (ns) are slightly lower than the reported for Au + (85.6 eV) and correspond to partially oxidized Au atoms, denoted as Au d+ .…”

“…The efficiency of the Au atoms present in the PNP reduction reaction can be quantified by normalizing the k app value to the moles of Au added to the reaction. 49 500 mL of each Au-prGO solution was used in the catalysis reactions, corresponding to 0.05 mmol of Au added. Fig.…”

“…Photoreduction with fs lasers is driven by reactive species such as hydrated electrons formed by water photolysis. [47][48][49][50] In contrast, ns laser pulses induce thermal decomposition of the [AuCl 4 ] À precursor and photothermal autocatalytic reduction in the presence of AuNPs due to the 532 nm laser wavelength being resonant with the surface plasmon resonance of Au. 47 In this work, we report on the role of GO in determining Au NP size distributions, prGO chemical composition, and catalytic activity of Au-prGO nanocomposites produced using ns and fs lasers.…”