Precise nanoparticle–reactant interaction outplays ligand poisoning in visible-light photocatalysis

Abstract

The ability to move electrons under the influence of visible-light in an efficient manner is one of the most fundamental challenges in photocatalysis. (1−6) The “hot” charge carriers in metal nanoparticles (NPs) have been shown to participate effectively in various reductive and oxidative photocatalytic chemical transformations. (7−14) During this process, the electrons and holes, oftentimes, have to encounter the “insulating” organic ligands capped on the NPs. (15−19) Generally, the surface ligand plays a crucial role in stabilizing the NPs as well as dictating its physicochemical properties. (20−23) However, for applications in photocatalysis, where the stability as well as the surface accessibility of NPs is desirable, the role of surface ligands is conflicting. In principle, the surface ligands can “poison” a photocatalyst by hindering the (i) movement of electrons/holes (due to its insulating nature) (15,16) and (ii) accessibility of the NP surface to the reactants (due to steric effect). (17−19) The alternative is to deposit NPs onto a support or use “ligand free” NPs for catalysis. (24−27) However, the available surface area and stability of NPs are compromised during the course of catalysis. (24) Thus, metal NPs and surface ligands are two inseparable entities, and strategies have to be developed to accomplish photocatalysis by retaining and taking advantage of the ligands on the NP surface. We address this challenge by using ligands that can enhance the NP catalyst–reactant interactions, which in turn can facilitate the electron transfer process.

Our hypothesis was tested in the model photocatalytic reaction of ferricyanide reduction by gold nanoparticles (AuNPs) in the presence of ethanol as the hole scavenger. (6,28,29) A favorable interaction between NP catalyst and ferricyanide reactant was created through precise surface engineering, which resulted in the enhancement of the photocatalytic activities (both in terms of hot electron transfer rate constant and conversion yield). Cationic ([+]) and anionic ([−]) organic ligands were functionalized on AuNP surface to generate favorable and unfavorable interactions with [−] ferricyanide, respectively. Our studies show that the favorable interaction, arising from the strong electrostatic attraction, increases the local concentration of [−] ferricyanide around the [+] AuNP catalyst. Consequently, the NP accessibility and probability of hot electron injection from [+] AuNP to [−] ferricyanide was enhanced. On the other hand, the local concentration of the reactants and catalytic activities were lower when standard [−] AuNP was used as the catalyst. For instance, the rate constant increased from ∼8 × 10–4 to ∼4 × 10–3 min–1 (∼5-fold increment in reaction rate) when the NP–reactant interaction was made favorable, along with an appreciable increase in the ferricyanide conversion yield (from ∼10% for [−] AuNP to ∼60% for [+] AuNP). The dependence of catalytic activities on the NP surface potential ascertained the potency of electrostatics in enhancing the visible-light photocatalysis. Thermodynamic analysis based on Marcus model of outer sphere electron transfer revealed a higher pre-exponential factor (Φ) for [+] AuNP catalyst, a parameter directly related to the local concentration of reactants. Thus, the introduction of favorable interaction improves the NP accessibility to the reactants and the probability of hot electron transfer, thereby suppressing the “poisoning” effect of the “insulating” organic ligands.