Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl₄]⁻: Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction

Abstract

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4](-) reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4](-) and (2) radical-mediated reduction of [AuCl4](-) upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4](-) under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4](-) at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.

Photochemical Synthesis of Gold Nanoparticles by Irradiation of Gold Chloride with the 2nd Harmonic of a Nd:YAG Laser

“…As can be seen in the insets of Figure 5, there is an increase of H 2 (at m/z 2) and O 2 (at m/z 32) after irradiation, confirming the formation of these two water splitting species after irradiation: (5) (6) Regarding the interaction between the laser radiation and HAuCl 4 , Tibbets et al 18 proposed a mechanism that involved the photolytic direct split of water, formation of hydrated electrons and autocatalysis due to the formed hydrogen peroxide. The overall reactions corresponding to these processes can be seen below: On the other hand, Shang et al, 16 during their study of the reduction of HAuCl 4 by UV irradiation, proposed a direct photochemical reduction of the Au III complex to the Au II complex, which, in turn, would disproportionate into Au III and Au I , and a final photochemical reduction of the latter.…”

“…The AuNPs average sizes obtained are comparable to the sizes reported in previous studies. 12,21,18,25 This is remarkable considering that the particles obtained by this method do not involve stabilizers or surfactants.…”

“…In their study of the synthesis of gold nanoparticles Tibbetts et al 18 proposed similar reactions, but considered that the laser beam could generate hydrated electrons. Nevertheless, the final products would remain the same.…”

“…By instance, reduction of gold(III) solutions using UV-irradiation 16 or irradiation of similar gold solutions by pulsed lasers. 17,18 In general, these procedures rely on the presence of stabilizers, or even preselected capping species. The laser drop method devised by Scaiano and co-workers 19 involves the laser irradiation of a solution drop containing pre-formed gold particles originated by the mild reduction of a gold chloride solution by hydrogen peroxide.…”

Au Nanoparticle Synthesis Via Femtosecond Laser-Induced Photochemical Reduction of [AuCl4]−

“…The first is bulk-metal ablation, in which metal atoms are ejected from the target material and form nanoparticles in solution [13]. The second is by irradiating a metal-salt solution to produce reducing agents via solvent-molecule photolysis [14][15][16]. Controlling nucleation and growth of the nanoparticles during metal-salt reduction by changing laser parameters (focusing conditions, pulse duration, pulse energy, irradiation time), and chemical parameters (metal-ion concentration, solvent composition, presence of capping agents), determines the size, shape, and stability of the colloidal products [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30].…”