Photophysical Characterization of Ru Nanoclusters on Nanostructured TiO2 by Time-Resolved Photoluminescence Spectroscopy

Despite the promising performance of Ru nanoparticles or nanoclusters on nanostructured TiO₂ in photocatalytic and photothermal reactions, a mechanistic understanding of the photophysics is limited. The aim of this study is to uncover the nature of light-induced processes in Ru/TiO₂ and the role of UV versus visible excitation by time-resolved photoluminescence (PL) spectroscopy. The PL at a 267 nm excitation is predominantly due to TiO₂, with a minor contribution of the Ru nanoclusters. Relative to TiO₂, the PL of Ru/TiO₂ following a 267 nm excitation is significantly blue-shifted, and the bathochromic shift with time is smaller. We show by global analysis of the spectrotemporal PL behavior that for both TiO₂ and Ru/TiO₂ the bathochromic shift with time is likely caused by the diffusion of electrons from the TiO₂ bulk toward the surface. During this directional motion, electrons may recombine (non)radiatively with relatively immobile hole polarons, causing the PL spectrum to red-shift with time following excitation. The blue-shifted PL spectra and smaller bathochromic shift with time for Ru/TiO₂ relative to TiO₂ indicate surface PL quenching, likely due to charge transfer from the TiO₂ surface into the Ru nanoclusters. When deposited on SiO₂ and excited at 532 nm, Ru shows a strong emission. The PL of Ru when deposited on TiO₂ is completely quenched, demonstrating interfacial charge separation following photoexcitation of the Ru nanoclusters with a close to unity quantum yield. The nature of the charge-transfer phenomena is discussed, and the obtained insights indicate that Ru nanoclusters should be deposited on semiconducting supports to enable highly effective photo(thermal)catalysis.

Low-Temperature Solution Synthesis of Au-Modified ZnO Nanowires for Highly Efficient Hydrogen Nanosensors

In this research, the low-temperature single-step electrochemical deposition of arrayed ZnO nanowires (NWs) decorated by Au nanoparticles (NPs) with diameters ranging between 10 and 100 nm is successfully demonstrated for the first time. The AuNPs and ZnO NWs were grown simultaneously in the same growth solution in consideration of the HAuCl₄ concentration. Optical, structural, and chemical characterizations were analyzed in detail, proving high crystallinity of the NWs as well as the distribution of Au NPs on the surface of zinc oxide NWs demonstrated by transmission electron microscopy. Individual Au NPs-functionalized ZnO NWs (Au-NP/ZnO-NWs) were incorporated into sensor nanodevices using an focused ion bean/scanning electron microscopy (FIB/SEM) scientific instrument. The gas-sensing investigations demonstrated excellent selectivity to hydrogen gas at room temperature (RT) with a gas response, I_gas/I_air, as high as 7.5-100 ppm for Au-NP/ZnO-NWs, possessing a AuNP surface coverage of ∼6.4%. The concentration of HAuCl₄ in the electrochemical solution was observed to have no significant impact on the gas-sensing parameters in our experiments. This highlights the significant influence of the total Au/ZnO interfacial area establishing Schottky contacts for the achievement of high performances. The most significant performance of H₂ response was observed for gas concentrations higher than 500 ppm of H₂ in the environment, which was attributed to the surface metallization of ZnO NWs during exposure to hydrogen. For this case, an ultrahigh response of about 32.9 and 47 to 1000 and 5000 ppm of H₂ was obtained, respectively. Spin-polarized periodic density functional theory calculations were realized on Au/ZnO bulk and surface-functionalized models, validating the experimental hypothesis. The combination of H₂ gas detection at RT, ultralow power consumption, and reduced dimensions makes these micro-nanodevices excellent candidates for hydrogen gas leakage detection, including hydrogen gas monitoring (less than 1 ppm).

Comparison studies of interfacial energetic and electronic properties of bimetallic AuCu/TiO2 hetero-structures from DFT calculations

The modification in which an Au co-catalyst is replaced with a bimetallic AuCu co-catalyst to build a TiO₂-based composite photocatalyst not only enhances the interaction of the metal layer with the TiO₂ substrate, but also promotes electron transfer and charge separation across the interface.

Adsorption of AuxCuy (x + y = 1, 2, 3) nanoclusters on the anatase TiO2(101) surface and their catalytic activity: a density functional theory study

The adsorption of Au_xCu_y (x + y = 1, 2, 3) nanoclusters on the anatase TiO₂(101) surface were studied in detail using density functional theory. The introduction of Cu atoms in Au clusters not only significantly reduces the cost, but also changes the catalytic process of Au/TiO₂, which may be helpful to some catalytic reactions.

Geometric and electronic properties of ultrathin anatase TiO2(001) films

The electronic structures of TiO₂(001) films exhibit pronounced odd–even oscillations with the number of atomic layers in them.