Tannin-mediated assembly of gold–titanium oxide hybrid nanoparticles for plasmonic photochemical applications

Plasmonic Heterostructure Functionalized with a Carbene-Linked Molecular Catalyst for Sustainable and Selective Carbon Dioxide Reduction

Hybridization of homogeneous catalytic sites with a photoelectrode is an attractive approach to highly selective and tunable photocatalysis using heterogeneous platforms. However, weak and unclear surface chemistry often leads to the dissociation and irregular orientation of catalytic centers, restricting long-term usability with high selectivity. Well-defined and robust ligands that can persist under harsh photocatalytic conditions are essential for the success of hybrid-type photocatalysis. Here, we introduce N-heterocyclic carbene as a durable linker for the immobilization of a Rubpy complex-based CO₂ reduction site (cis-dichloro-(4,4'-diphosphonato-Rubpy)(p-cymene) (RuCY)) on a p-type gallium nitride/gold nanoparticle (p-GaN/AuNP) heterostructure. The p-GaN/AuNPs/RuCY photocathode was coupled with a hematite photoanode to drive photoelectrochemical CO₂ reduction along with water oxidation. Highly selective CO₂ reduction into formates, up to 98.2%, was achieved utilizing plasmonic hot electrons accumulated on AuNPs. The turnover frequency was 1.46 min^-1 with a faradic efficiency of 96.8% under visible light illumination (243 mW·cm^-2). This work demonstrates that the N-heterocyclic carbene-mediated surface functionalization with homogeneous catalytic sites is a promising approach to increase the sustainability and usability of hybrid catalysts.

Photochemically Enhanced Selective Adsorption of Gold Ions on Tannin-Coated Porous Polymer Microspheres

Metal recovery from electronic waste and industrial wastewater has attracted increasing attention to recycle precious metals and inhibit the emission of hazardous heavy metals. However, the selective recovery of precious metals with a large quantity is still very challenging because wastewater contains a variety of different cations while precious metal ions are relatively scarce. Here, we introduce a simple method to selectively increase the adsorption of gold ions using tannin-coated porous polymer microspheres through photochemical reduction. Mesoporous poly(ethylene glycol dimethacrylate- co-acrylonitrile) microspheres with an average pore diameter of 13.8 nm were synthesized and used as an adsorbent matrix. Tannic acid (TA) was deposited onto the internal pores of the polymer matrix by simple immersion in an aqueous milieu. TA coatings increased the maximum number of adsorbed gold ions by 1.3 times because of the well-known metal ion chelation of TA. Under light illumination, the maximum number of adsorbed gold ions dramatically increased by 6.1 times. We examined two distinct mechanisms presumably involved in the enhanced adsorption: the photooxidation of TA and plasmon-induced hot electrons. Moreover, TA-coated microspheres exhibited remarkable selectivity for gold ions among competing metal ions commonly found in waste resources. This work suggests that the photochemically activated TA can serve as an excellent adsorbent for the selective and efficient recovery of gold ions from wastewater.