(Invited) Hybrid Photocatalysts Composed of Titania Modified with Plasmonic Nanoparticles and Ruthenium Complexes for Photocatalytic Decomposition of Organic Compounds
Authors: Kowalska Ewa , Zheng Shuazhi , Yoshiiri Kenta , Wei Zhishun , Rau Sven , Ohtani Bunsho 
Published: February 2020

Wide-bandgap semiconductors modified with nanoparticles (NPs) of noble metals have been extensively studied for more than forty years owing to possible enhanced photocatalytic activity under UV irradiation since NPs of noble metals work as an electron sink hindering the recombination of charge carriers. The research has intensified over the last decade for photocatalytic activity under visible-light irradiation due to localized surface-plasmon resonance (LSPR) of noble metals, and those materials have been named plasmonic photocatalysts. The mechanism under visible-light irradiation (electron and/or energy transfer) as well as the enhancement of photocatalytic performance (activity and stability) have been intensively examined. Although the photocatalytic activities of plasmonic photocatalysts have been confirmed for various environmental applications such as environmental purification and solar-energy conversion, the photocatalytic activities under visible-light irradiation are much lower (ca. 1-2 orders in magnitude) than that under UV. Therefore, the present study focuses on improvement of photocatalytic activity for possible commercial applications.

In our studies, plasmonic photocatalysts composed of commercial or self-synthesized titania and NPs of silver, gold and copper have been examined. In general, the broader the LSPR peak was, the higher was the photocatalytic activity under visible-light irradiation. Various strategies for improvement of the photocatalytic performance have been applied, e.g., (i) enlargement of the interface between titania and noble-metal NPs, (ii) preparation of bimetallic plasmonic photocatalysts, (iii) the use of faceted anatase NPs (octahedral and decahedral), and (iv) preparation of hybrid photocatalysts composed of heterogeneous and homogeneous photocatalysts, i.e., plasmonic photocatalysts and ruthenium complexes.

Hybrid photocatalysts were prepared by deposition of gold/silver NPs on commercial titania particles with different structural properties. Ruthenium(II) (Ru(II)) complexes with carboxylic and phosphoric acid-binding groups were synthesized and adsorbed on bare titania and noble metal-modified titania particles. The structural properties of the samples were characterized by diffuse-reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), thin layer chromatography (TLC), proton nuclear magnetic-resonance spectroscopy (1H-NMR) and two-dimensional homonuclear correlation spectroscopy (2D-COSY). Large surface area, small crystallite sizes, low pH value, nature of the deprotected phosphonate binding groups, and pre-deposited noble-metal NPs enhanced the adsorption yield. Modification caused titania activation toward visible light (> 450 nm) for 2-propanol oxidation and for methanol dehydrogenation under UV/vis irradiation. The modified samples exhibited enhanced activity under UV/vis irradiation for acetic-acid oxidation depending on the kind of modifiers and properties of titania, i.e., 2–6, 3–9 and 1–3-fold enhancement was observed after modification with gold NPs and silver NPs with Ru(II) complex, respectively. The time-resolved microwave conductivity (TRMC) method proved that higher photocatalytic activity of modified titania under UV irradiation was caused by scavenging of mobile electrons by noble-metal NPs, and therefore decreasing the recombination of charge carriers. The photocatalytic activity of hybrid photocatalysts under UV/vis irradiation was influenced by the nature of the plasmonic metal and structural properties of the metal and titania particles, e.g., crystallite size and polymorphic form. Different kinds of action were observed for two plasmonic metals in two reaction systems under UV irradiation, e.g., modification with an Ru(II) complex caused 12-times faster dehydrogenation of methanol for silver-modified large anatase titania (ST41) and hardly changed the activity of gold-modified samples, while during acetic acid oxidation, only hybrid photocatalysts composed of gold NPs and Ru(II) complex exhibited a slight increase of photocatalytic activity (1.1-1.3 times). The difference between gold and silver hybrid photocatalysts might have been caused by differences in surface charges of metallic deposits, i.e., the surface of silver NPs was mainly positively charged, while gold was zero-valent. It was proposed that Ru(II) complex bound also to the surface of positively charged silver. Therefore, the sequence of modification (Ru(II) complex adsorption or gold/silver deposition) was investigated to check its influence on the resultant properties and thus photocatalytic performance. Different distributions of gold particle sizes and chemical compositions were obtained for the hybrid photocatalysts prepared with opposite sequence. It was found that photocatalytic activities depended on the range of used irradiation (UV/vis or vis) and the kind of modifiers in different ways. Under visible-light irradiation, fine titania modified with Ru(II) complex exhibited the highest level of photocatalytic activity. The presence of Ru(II) complex highly enhanced photocatalytic activity of titania modified with plasmonic NPs. However, NPs of plasmonic metals hindered the photocatalytic activity of Ru(II)-titania.