Hybrid photocatalysts composed of titania modified with plasmonic nanoparticles and ruthenium complexes for decomposition of organic compounds

Abstract

Plasmonic photocatalysts were prepared by deposition of 2 wt% of gold or silver nanoparticles (NPs) on commercial titania particles with different structural properties. Ruthenium(II) complexes with phosphonic and carboxylic acid binding groups were synthesized and adsorbed on bare titania and noble metal-modified titania. The structural properties of the samples were characterized by DRS, XPS, XRD, STEM, TLC, 1 H-NMR and 2D-COSY. Large surface area, small crystallite sizes, low pH value, nature of the deprotected phosphonate binding groups, and pre-deposited nanoparticles of noble metals 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 NPs of gold, NPs of silver and Ru(II) complex. 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 NPs of noble metals, and therefore decreasing the recombination between charge carriers. The photocatalytic activity of hybrid photocatalysts under UV/vis was influenced by the nature of the plasmonic metal and © 2014. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 structural properties of the metal and titania, 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. Therefore, it is thought that the Ru(II) complex bound also to the surface of positively charged silver. Under visible light irradiation, fine titania modified with an Ru(II) complex exhibited the highest level of photocatalytic activity. The presence of an Ru(II) complex highly enhanced photocatalytic activity of titania modified with plasmonic NPs. However, NPs of plasmonic metals hindered the photoactivity of Ru(II)-titania.

Mono- and Dual-modified Titania with a Ruthenium(II) Complex and Silver Nanoparticles for Photocatalytic Degradation of Organic Compounds

“…368.2, 367.2 and 366.2 eV, respectively. Silver on the surface existed predominantly in the positively charged form Ag + (more than 94%), which is consistent with our former result for silver deposition on bare titania of P25 and ruthenium(II) pre-modified titania of P25 (26), and it is likely to accommodate the negatively charged anchoring group of ruthenium(II). Existence of ruthenium(II) resulted in slight change in distributions of the oxidation states of silver.…”

“…8.5 nm was practically the same in all samples. The crystallite sizes of silver were smaller in this work than in previous report using various kinds of titania as supporting material, e.g., 33.8 nm, 19.8 nm and 34.6 nm of Ag NPs on titania of ST41, P25, and STG1, respectively (26). The metallic NPs were proposed to be formed on the crystalline defect, hence the titania of ST01 which possesses fine particles and large amount of such defect sites could induce the formation of fine metal nanoparticles (37,38), while titania of ST41, P25 and STG1 possessing larger particles and therefore aggregation of Ag NPs resulted in formation of larger metal deposits.…”

“…Similar results were obtained with [Ru(bpy) 2 (4,4′-(CH 2 PO 3 H 2 ) 2 bpy)] 2+ which has exactly the same binding motif of 4,4′-(CH 2 PO 3 H 2 ) 2 bpy main ligand, but only differs with the ancillary ligands of bpy (2,2′-bipyridine), and even after irradiation with strong energy of UV/Vis light, negligible amount of complex was detached from titania (27). In addition, the chosen titania of ST01 possesses fine crystallite size and large specific surface area, which is also the reason of the fast and stable immobilization, since for titania of larger crystallite sizes, slower adsorption processes were observed, e.g., for anatase ST41 with 208-nm crystallites (26).…”

“…Generally, phosphonate groups are excellent candidates in the application of immobilization on titania. Many studies have proved their efficiency and strong affinity (29,30), while other anchoring groups, for example, carboxylate groups turned out to be unstable and easily detached from titania (26). As the detachment of the dyes would be harmful or decrease the efficiency of the photocatalytic reactions, stability of the modified titania is required.…”

“…performance (21,24,25). In our previous investigation, two kinds of modifiers were adopted to fabricate titania, e.g., silver nanoparticles (Ag NPs) plus gold nanoparticles (Au NPs), and ruthenium(II) complex plus Au NPs (26,27). Notably, the sequences to introduce modifiers have significant influence on the properties of the prepared photocatalysts, which in consequence could influence the photocatalytic activities.…”