After more than a decade, it is still unknown whether the plasmon-mediated growth of silver nanostructures can be extended to the synthesis of other noble metals, as the molecular mechanisms governing the growth process remain elusive. Herein, we demonstrate the plasmon-driven synthesis of gold nanoprisms and elucidate the details of the photochemical growth mechanism at the single-nanoparticle level. Our investigation reveals that the surfactant polyvinylpyrrolidone preferentially adsorbs along the nanoprism perimeter and serves as a photochemical relay to direct the anisotropic growth of gold nanoprisms. This discovery confers a unique function to polyvinylpyrrolidone that is fundamentally different from its widely accepted role as a crystal-face-blocking ligand. Additionally, we find that nanocrystal twinning exerts a profound influence on the kinetics of this photochemical process by controlling the transport of plasmon-generated hot electrons to polyvinylpyrrolidone. These insights establish a molecular-level description of the underlying mechanisms regulating the plasmon-driven synthesis of gold nanoprisms.
Water reduction under two different visible-light ranges (λ > 400 nm and λ > 435 nm) was investigated in gold-loaded titanium dioxide (Au-TiO2) heterostructures with different sizes of Au nanoparticles (NPs). Our study clearly demonstrates the essential role played by Au NP size in plasmon-driven H2O reduction and reveals two distinct mechanisms to clarify visible-light photocatalytic activity under different excitation conditions. The size of the Au NP governs the efficiency of plasmon-mediated electron transfer and plays a critical role in determining the reduction potentials of the electrons transferred to the TiO2 conduction band. Our discovery provides a facile method of manipulating photocatalytic activity simply by varying the Au NP size and is expected to greatly facilitate the design of suitable plasmonic photocatalysts for solar-to-fuel energy conversion.
The interaction of hydrogen with reduced ceria (CeO2−x) powders and CeO2−x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce3+ species to hydrogen, which results in the formation of Ce4+, and thus in oxidation of ceria.
Strong metal–support interactions (SMSI) is an important concept in heterogeneous catalysis. Herein, we demonstrate that the Au‐TiO2 SMSI of Au/TiO2 catalysts sensitively depends on both Au nanoparticle (NP) sizes and TiO2 facets. Au NPs of ca. 5 nm are more facile undergo Au‐TiO2 SMSI than those of ca. 2 nm, while TiO2 {001} and {100} facets are more facile than TiO2{101} facets. The resulting capsulating TiO2−x overlayers on Au NPs exhibit an average oxidation state between +3 and +4 and a Au‐to‐TiO2−x charge transfer, which, combined with calculations, determines the Ti:O ratio as ca. 6:11. Both TiO2−x overlayers and TiO2−x‐Au interface exhibit easier lattice oxygen activation and higher intrinsic activity in catalyzing low‐temperature CO oxidation than the starting Au‐TiO2 interface. These results advance fundamental understanding of SMSI and demonstrate engineering of metal NP size and oxide facet as an effective strategy to tune the SMSI for efficient catalysis.
The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO2) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO2(111) thin films and CeO2 powders, and theoretical calculations of CeO2(111) surfaces with oxygen vacancies (Ov) at the surface and in the bulk. We show that, on a stoichiometric CeO2(111) surface, H2 dissociates and forms surface hydroxyls (OH). On the pre‐reduced CeO2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce3+ ↔ Ce4+ redox reaction. As the Ov concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H2/CeO2 interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria‐based catalysts in a hydrogen‐rich atmosphere.
Well-dispersed M@TiO 2 (M = Ag, Pd, Au, Pt) nanocomposite particles with a diameter of 200-400 nm can be synthesized on a large scale by a clean photochemical route which does not require any additives using spherical rutile nanoparticles as a support. The sizes of Pt, Au, and Pd nanoparticles formed on the surface of TiO 2 particles are about 1 nm, 5 nm, and 5 nm, respectively, and the diameter of Ag nanoparticles is in the range 2-20 nm. Moreover, the noble metal nanoparticles have good dispersity on the particles of the TiO 2 support, resulting in excellent catalytic activities. Complete conversion in catalytic CO oxidation is reached at temperatures as low as 333 and 363 K, respectively, for Pt@TiO 2 and Pd@TiO 2 catalysts. In addition, the antibacterial effects of the as-synthesized TiO 2 nanoparticles, silver nanoparticles, and Au@TiO 2 and Ag@TiO 2 nanocomposites have been tested against Gram-negative Escherichia coli (E. coli) bacteria. The results demonstrate that the presence of the TiO 2 matrix enhances the antibacterial effect of silver nanoparticles, and the growth of E. coli can be completely inhibited even if the concentration of Ag in Ag@TiO 2 nanocomposite is very low (10 μg/mL).
Cu–ZnO–Al2O3 catalysts are used as the industrial catalysts for water gas shift (WGS) and CO hydrogenation to methanol reactions. Herein, via a comprehensive experimental and theoretical calculation study of a series of ZnO/Cu nanocrystals inverse catalysts with well-defined Cu structures, we report that the ZnO–Cu catalysts undergo Cu structure-dependent and reaction-sensitive in situ restructuring during WGS and CO hydrogenation reactions under typical reaction conditions, forming the active sites of CuCu(100)-hydroxylated ZnO ensemble and CuCu(611)Zn alloy, respectively. These results provide insights into the active sites of Cu–ZnO catalysts for the WGS and CO hydrogenation reactions and reveal the Cu structural effects, and offer the feasible guideline for optimizing the structures of Cu–ZnO–Al2O3 catalysts.
Photocatalytic oxidation of methanol on various anatase TiO2 nanocrystals was studied by in situ and time‐resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk‐to‐surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO2 nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold‐coordinated Ti sites (Ti4c) at which CH3OH molecules dissociate to form the CH3O adsorbate (CH3O(a)Ti4c). CH3O(a)Ti4c localized density of states are almost at the valence band maximum of TiO2 surface, facilitating the interfacial hole transfer process; CH3O(a)Ti4c with a high coverage promotes upward surface band bending, facilitating bulk‐to‐surface hole migration. CH3O(a)Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO2 nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.
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