Preparation of functionalized platinum nanoparticles: a comparison of different methods and reagents

Au–Pd core–shell nanocubes and triangular nanoparticles were systematically synthesized from a few Pd layers up to fully grown morphologies by a modified seed-mediated growth method. The shape evolution of Au–Pd core–shell nanoparticles from single crystal and singly twinned seed to final concave nanocube and triangular plates are presented at atomic level by Cs-corrected scanning transmission electron microscopy (STEM). The growth mechanism of both morphologies was studied throughout different sizes. It was found that the concave nanocubes grew from octahedral Au seeds due to fast growth along 〈111〉 directions; while the triangular nanoparticles grew from singly twinned Au seeds, growing twice as fast in 〈110〉 directions along the twin boundary; compared to the 〈111〉 direction perpendicular to the twin boundary. Both the concave nanocubes and triangular nanoparticles presented high index facet (HIF) surfaces that will increase the catalytic activity of different reactions.     

Probing reaction mechanisms in mixed phase TiO2 by EPR

Charge separation processes in mixed phase TiO₂ photocatalysts are investigated by electron paramagnetic resonance (EPR) spectroscopy. The mechanisms of interfacial electron transfer, subsequent charge migration and recombination at surface sites, and other interfacial effects on chlorophenol/TiO₂ chemistry have been probed. Distorted interfacial sites have been observed and are proposed as catalytically reactive hot spots. This detailed knowledge of charge transfer processes is critical to the nanoscale design of catalysts and subsequent improvement of catalytic efficiency.     

Narrow-band irradiation of a homologous series of chlorophenols on TiO2: charge-transfer complex formation and reactivity

The goal of this research was to investigate the formation and reactivity of charge-transfer complexes (CTCs) among a homologous series of chlorophenols on TiO2. We previously showed that 2,4,5-trichlorophenol (245TCP) forms a CTC with Degussa P25, a commercial preparation of TiO2. Here, we probe how light energy influences reactivity and product formation. Slurries of P25 containing 245TCP were irradiated at 360, 400, 430, 480, and 550 nm. At each wavelength, the amount of transformation of 245TCP correlates to the diffuse-reflectance absorbance of a 245TCP/P25 system, supporting the CTC as the cause of reaction. In addition, polymeric products are formed only under wavelengths that excite the CTC, indicating a different reaction mechanism for the CTC than for bandgap excitation of TiO2. We also found a higher quantum efficiency for CTC reactivity than for bandgap activation of the catalyst, suggesting that the photocatalytic efficiency and selectivity can be improved for certain compounds by designing catalytic materials that form CTCs with them. Furthermore, to determine how chlorine substitution patterns affected adsorption and sub-bandgap reactivity, P25 slurries containing phenol, 4-chlorophenol, 2,4-dichlorophenol, or 2,4,6-trichlorophenol were probed following dark contact or irradiation at 360, 430, or 550 nm. With respect to the extent of adsorption, complexation, reaction, and polymerization on P25, the behavior of 245TCP far exceeded that of the other chlorophenols. Among these chlorophenols, only 2,4-dichlorophenol produced a polymeric product. 245TCP is unique among this family of chlorophenols, which we attribute to a chlorine arrangement that leads to a favorable orbital overlap with TiO2 and sterically permits coupling reactions. Our results demonstrate the critical role that charge-transfer complexation can play in determining the rates and products of photocatalytic reactions.     

Synthesis of bifunctional Ru complexes with 1,2-dithiolane and carboxylate-substituted ligands

An N3-type, Ru heteroleptic complex, AK1, having one bipyridyl ligand modified with COOH groups for tethering to TiO₂ and a second bipyridyl ligand modified with two lipoic acid units for binding to platinum, was synthesized. The photophysical and spectroelectrochemical properties were studied in solution, on TiO₂, in a dye-sensitized solar cell and on a Pt wire electrode. The results showed that AK1 can produce a photocurrent on TiO₂. Furthermore, AK1 binds to Pt via the lipoic acid ligand but not via the carboxylic acid group, and can be electrochemically addressed by the Pt via the lipoic acid linkage.