2 play vital roles in the hydrogenation of CO₂ to methanol by these composite catalysts. Surface structural reorganization and particle growth during catalysis deleteriously reduce these active interfaces, diminishing both catalytic activities and MeOH selectivities. Here we report the use of preassembled bpy and Zr₆(μ₃-O)₄(μ₃-OH)₄ sites in UiO-bpy metal-organic frameworks (MOFs) to anchor ultrasmall Cu/ZnO_x nanoparticles, thus preventing the agglomeration of Cu NPs and phase separation between Cu and ZnO_x in MOF-cavity-confined Cu/ZnO_x nanoparticles. The resultant Cu/ZnO_x@MOF catalysts show very high activity with a space-time yield of up to 2.59 g_MeOH kg_Cu^-1 h^-1, 100% selectivity for CO₂ hydrogenation to methanol, and high stability over 100 h. These new types of strong metal-support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs.]]>

Titanium dioxide (TiO2) displays photocatalytic behavior under near-ultraviolet (UV) illumination. In another scientific field, it is well understood that the excitation of localized plasmon polaritons on the surface of silver (Ag) nanoparticles (NPs) causes a tremendous increase of the near-field amplitude at well-defined wavelengths in the near UV. The exact resonance wavelength depends on the shape and the dielectric environment of the NPs. We expected that the photocatalytic behavior of TiO2 would be greatly boosted if it gets assisted by the enhanced near-field amplitudes of localized surface plasmon (LSP). Here we show that this is true indeed. We named this new phenomenon "plasmonic photocatalysis". The key to enable plasmonic photocatalysis is to deposit TiO2 on a NP comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. The most appropriate diameter for Ag NPs and thickness for the SiO2 shell giving rise to LSP in the near UV were estimated from Mie scattering theory. Upon implementing a device that took these design considerations into account, the measured photocatalytic activity under near UV illumination of such a plasmonic photocatalyst, monitored by decomposition of methylene blue, was enhanced by a factor of 7. The enhancement of the photocatalytic activity increases with a decreased thickness of the SiO2 shell. The plasmonic photocatalysis will be of use as a high performance photocatalyst in nearly all current applications but will be of particular importance for applications in locations of minimal light exposure.

2 to NH₃ is an essential process for sustaining life. One grand challenge is to develop efficient catalysts to photofix N₂ under ambient conditions. Herein we report an all-inorganic catalyst, Au nanocrystals anchored on ultrathin TiO₂ nanosheets with oxygen vacancies. It can accomplish photodriven N₂ fixation in the "working-in-tandem" pathway at room temperature and atmospheric pressure. The oxygen vacancies on the TiO₂ nanosheets chemisorb and activate N₂ molecules, which are subsequently reduced to NH₃ by hot electrons generated from plasmon excitation of the Au nanocrystals. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to NH₃ is higher than those reported so far. Optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods further enhances the N₂ photofixation rate by 66.2% in comparison with Au nanospheres used alone. This work offers a new approach for the rational design of efficient catalysts toward sustainable N₂ fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.]]>

Photoexcitation of the plasmon band in metallic nanoparticles adsorbed on a TiO2 surface initiates many important photovoltaic and photocatalytic processes. The traditional view on the photoinduced charge separation involves excitation of a surface plasmon, its subsequent dephasing into electron-hole pairs, followed by electron transfer (ET) from the metal nanoparticle into TiO2. We use nonadiabatic molecular dynamics combined with time-domain density functional theory to demonstrate that an electron appears inside TiO2 immediately upon photoexcitation with a high probability (~50%), bypassing the intermediate step of electron-hole thermalization inside the nanoparticle. By providing a detailed, atomistic description of the charge separation, energy relaxation, and electron-hole recombination processes, the simulation rationalizes why the experimentally observed ultrafast photoinduced ET in an Au-TiO2 system is possible in spite of the fast energy relaxation. The simulation shows that the photogenerated plasmon is highly delocalized onto TiO2, and thus, it is shared by the electron donor and acceptor materials. In the 50% of the cases remaining after the instantaneous photogeneration of the charge-separated state, the electron injects into TiO2 on a sub-100 fs time scale by the nonadiabatic mechanism due to high density of acceptor states. The electron-phonon relaxation parallels the injection and is slower, resulting in a transient heating of the TiO2 surface by 40 K. Driven by entropy, the electron moves further into TiO2 bulk. If the electron remains trapped at the TiO2 surface, it recombines with the hole on a picosecond time scale. The obtained ET and recombination times are in excellent agreement with the experiment. The delocalized plasmon state observed in our study establishes a novel concept for plasmonic photosensitization of wide band gap semiconductors, leading to efficient conversion of photons to charge carriers and to hybrid materials with a wide variety of applications in photocatalysis and photovoltaics.

In recent years, enormous attention has been paid to the construction of metal cluster-semiconductor nanocomposites because of the fascinating and unique properties of metal clusters; however, investigations on photoelectrochemical (PEC) and photocatalytic properties of metal cluster-semiconductor systems are still rare. Moreover, to date, intrinsic correlation between metal clusters and bulk metal nanocrystals has yet to be elucidated. In this work, a facile layer-by-layer (LbL) self-assembly strategy has been developed to judiciously and intimately integrate gold nanocrystals (Au) within the interface between gold clusters (Au(x)) and hierarchically ordered TiO2 nanotube arrays framework, by which imperative roles of Au nanocrystals as electron relay mediator and plasmonic sensitizer for Aux clusters were revealed. In addition, it was found that synergistic interaction between Au nanocrystals and Aux clusters contributed to promising visible-light-driven photocatalytical and PEC performances. It is anticipated that our work could provide a general way for rationally constructing metal and metal clusters codecorated semiconductor heterostructures and, more significantly, bridge the gap between metal clusters and metal nanocrystals for a diverse range of applications.

2O upon mild oxidation, whereas fully core-shell Cu@Ag NCs are robust against oxidation up to 100 °C. The plasmonic and interband absorptions of Cu-Ag NCs depend on the composition and the degree of Cu oxidation, which may find application in light-driven catalysis.]]>

2 nanoparticles (Cu-TiO₂) using different methods aiming at the production of highly efficient visible light photocatalysts. Photocatalytic H₂ evolution rates obtained from methanol/water mixtures revealed no significant influence of the presence of copper oxides on the photoreaction upon visible light illumination. The photocatalytic H₂ production rates were evaluated upon illumination with different spectral ranges (≥420 nm or ≥500 nm) and the results evidenced that the visible light induced charge carrier formation on the Cu-TiO₂ photocatalysts consists of two distinct pathways: the direct excitation of TiO₂ and the induced excitation by the so-called surface plasmon resonance (SPR) effect of the Cu nanoparticles on the TiO₂ surface. Both pathways are present when the full visible range of the spectrum is used (≥420 nm), while for illumination at longer wavelengths (≥500 nm), the photocatalytic activity is solely promoted by the Cu-SPR effect. Electron paramagnetic resonance (EPR) and laser flash photolysis measurements were performed to clarify the underlying mechanism of Cu-TiO₂ photocatalysts upon visible light illumination.]]>

2CO₃ (Bi-BOC) nanocomposites and Bi elemental photocatalysts. The Bi nanoparticles were produced via the insitu reduction of (BiO)₂CO₃ by NaBH₄. The catalysts were utilized for the photocatalytic NO removal under visible light and UV illumination. Significantly, the photocatalytic capability of the Bi-BOC was highly enhanced with an unprecedented NO removal of 63.6%. The Bi metal demonstrated a direct plasmonic photocatalytic NO removal ratio of 53.6% under UV irradiation. The significantly enhanced photocatalytic capability of Bi-BOC can be ascribed to the synergistic effects of the SPR effect, enhanced visible-light-harvesting and the efficient electron-hole separation induced by Bi nanoparticles. The Bi nanoparticles can perform as a non-noble metal-based plasmonic cocatalyst for advancing photocatalytic ability. The mechanism of photocatalytic NO oxidation was proposed and compared under both visible light and UV illumination. Furthermore, the Bi-BOC photocatalysts showed good photochemical stability under repeated tests. This work could not only offer new insights into in-situ fine-tune reduction strategy for Bi-based photocatalysts, but also proves the potentials of utilizing low cost Bi cocatalysts as a substitute for noble metals to improve other photocatalysts.]]>

3 nanosheets and nanowires with dominant exposed facets of {001} and {110}, respectively. The lower hole effective mass on {110} (0.94 m₀) than on {001} (1.28 m₀) calculated by density functional theory leads to the higher hole mobility on {110} (4.92 cm² V^-1 s^-1) than on {001} (3.14 cm² V^-1 s^-1). Combined with the Einstein equation and the lifetime of the hole, the calculated hole diffusion length on {110} (74.8 nm) is larger than on {001} (53.4 nm). Overall, the lower hole effective mass, higher hole mobility, and greater hole diffusion length on {110} collectively result in a photocatalytic activity on benzyl alcohol oxidation 2.46 times as high as that on {001}.]]>

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.

Heavily doped semiconductors have recently emerged as a remarkable class of plasmonic alternative to conventional noble metals; however, controlled manipulation of their surface plasmon bands toward short wavelengths, especially in the visible light spectrum, still remains a challenge. Here we demonstrate that hydrogen doped given MoO3 and WO3 via a facile H-spillover approach, namely, hydrogen bronzes, exhibit strong localized surface plasmon resonances in the visible light region. Through variation of their stoichiometric compositions, tunable plasmon resonances could be observed in a wide range, which hinge upon the reduction temperatures, metal species, the nature and the size of metal oxide supports in the synthetic H2 reduction process as well as oxidation treatment in the postsynthetic process. Density functional theory calculations unravel that the intercalation of hydrogen atoms into the given host structures yields appreciable delocalized electrons, enabling their plasmonic properties. The plasmonic hybrids show potentials in heterogeneous catalysis, in which visible light irradiation enhanced catalytic performance toward p-nitrophenol reduction relative to dark condition. Our findings provide direct evidence for achieving plasmon resonances in hydrogen doped metal oxide semiconductors, and may allow large-scale applications with low-price and earth-abundant elements.

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.

2O on desired sites of anisotropic Au nanocrystals. Both the exploitation of structural characteristics of Au nanocrystals and the selective stabilization of their surfaces are keys to the construction of heteronanocrystals with a specific configuration. Our approach can provide an opportunity to precisely explore the link between the solar energy conversion efficiency and the structure of heteronanocrystals as well as to obtain important insights into the underpinning mechanism. Heteronanocrystals produced by Cu₂O overgrowth preferentially on the multiple high-curvature sites of Au nanocrystals exhibited prominent photocatalytic hydrogen production activity due to efficient charge separation by strong plasmon excitation at the Au-Cu₂O interface and subsequent sustainable hot electron transfer from Au to Cu₂O.]]>