Here we have used Raman spectroscopy to investigate molecular level changes in the zeolitic imidazolate framework ZIF-8 (a prototypical zeolite-like porous metal organic framework) as a function of temperature. Temperature dependent Raman spectra suggest that at low temperature the softening of the C-H stretching frequencies is due to the decrease in steric hindrance between the methyl groups of methyl imidazole. The larger separation between the methyl groups opens the window for increased nitrogen and methane uptake at temperatures below 153 K. The appearance of Raman bands at 2323 cm(-1) and 2904 cm(-1) at or below 153 K in ZIF-8 are characteristic signatures of the adsorbed nitrogen and methane gases respectively. Nanoscale ZIF-8 uptakes more molecules than bulk ZIF-8, and as a result we could provide evidence for encaged CO2 at 203 K yielding its Raman mode at 1379 cm(-1).

Photocatalytic reduction of carbon dioxide (CO) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires an understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-nanoparticle spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under conditions of visible light-driven CO reduction. In situ surface-enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic nanoparticles, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions.

The ability of plasmonic nanoparticles to harness visible light can be being combined with their catalytic activity to drive photocatalytic transformations. This Review introduces the promise of this new class of photocatalysts for fulfilling the quest for sunlight-driven recycling of CO 2 into transportable liquid fuels. We discuss the prospects and challenges of such an approach. Despite considerable advances, a selective, stable, and efficient CO 2 reduction reaction (CO 2 RR) catalyst has been elusive. These open challenges may be addressable by the strategic utilization of plasmonic light excitation. Plasmonic catalysts have exhibited the ability to drive a rich milieu of CO 2 RR processes under visible light excitation. At this stage, improved mechanistic understanding and reaction control are needed. To motivate rational design of photocatalytic materials and processes by a future generation of researchers, we suggest potential pathways by which plasmonic-assisted CO 2 RR can take place. We describe unique physical and chemical aspects of plasmonic catalysis, some of which may allow modulation of CO 2 RR product selectivity in favor of higher hydrocarbons. The intertwining of the photophysics of plasmon resonances and chemistry of CO 2 RR creates a wide-open space for fundamental inquiry and technological development. Whether the future of artificial photosynthesis is "plasmonic" will be dictated by scientific understanding and engineering advances accomplished in the coming decade.

We herein report an unusual CO(2) adsorption behavior in a fluoro-functionalized MOF {[Zn(SiF(6))(pyz)(2)]·2MeOH}(n) (1) with a 1D channel system, which is made up of pyrazine and SiF(6)(2-) moieties. Surprisingly, desolvated 1 (1') adsorbs higher amounts of CO(2) at 298 K than at 195 K, which is in contrast to the usual trend. Combined Raman spectroscopic and theoretical studies reveal that slanted pyrazine rings in 1' with an angle of 17.2° with respect to the (200) Zn(II)-Si plane at low temperature block the channel windows and thus reduce the uptake amount.

Surface-enhanced Raman spectroscopy (SERS) has gained paramount importance in the recent past due to its widespread applications in biodetection, monitoring chemical reactions, small molecule protein interactions, etc. It is believed that SERS is a distance-dependent phenomenon and is effective within 1 nm from the nanoparticle surface. In this work, we have investigated this distance dependence of SERS as a function of nanoparticle size. Earlier attempts have made use of flexible separators, like DNA and chemical molecules, between nanoparticle and analyte to vary the distance. We have used silica coating to vary the distance, without ambiguity, of the analyte from the silver nanoparticle surface. Our results suggest that SERS is observed up to a distance of 1 nm for 20 nm silver nanoparticles juxtaposed to 5 nm in the case of 90 nm silver nanoparticles. This is due to large scattering cross sections and increased radiative damping in the case of the larger nanoparticles. This study gives direct correlation between the size of nanoparticles and distance probed through SERS which would aid in designing nanoparticle system for various applications and analytes in the future.

Most syntheses of advanced materials require accurate control of the operating temperature. Plasmon resonances in metal nanoparticles generate nanoscale temperature gradients at their surface that can be exploited to control the growth of functional nanomaterials, including bimetallic and core@shell particles. However, in typical ensemble plasmonic experiments these local gradients vanish due to collective heating effects. Here, we demonstrate how localized plasmonic photothermal effects can generate spatially confined nanoreactors by activating, controlling, and spectroscopically following the growth of individual metal@semiconductor core@shell nanoparticles. By tailoring the illumination geometry and the surrounding chemical environment, we demonstrate the conformal growth of semiconducting shells of CeO 2 , ZnO, and ZnS, around plasmonic nanoparticles of different morphologies. The shell growth rate scales with the nanoparticle temperature and the process is followed in situ via the inelastic light scattering of the growing nanoparticle. Plasmonic control of chemical reactions can lead to the synthesis of functional nanomaterials otherwise inaccessible with classical colloidal methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices.

Ethylene epoxidation is used to produce 2 × 107 ton per year of ethylene oxide, a major feedstock for commodity chemicals and plastics. While high pressures and temperatures are required for the reaction, plasmonic photoexcitation of the Ag catalyst enables epoxidation at near-ambient conditions. Here, we use surface-enhanced Raman scattering to monitor the plasmon excitation-assisted reaction on individual sites of a Ag nanoparticle catalyst. We uncover an unconventional mechanism, wherein the primary step is the photosynthesis of graphene on the Ag surface. Epoxidation of ethylene is then promoted by this photogenerated graphene. Density functional theory simulations point to edge defects on the graphene as the sites for epoxidation. Guided by this insight, we synthesize a composite graphene/Ag/α-Al2O3 catalyst, which accomplishes ethylene photo-epoxidation under ambient conditions at which the conventional Ag/α-Al2O3 catalyst shows negligible activity. Our finding of in situ photogeneration of catalytically active graphene may apply to other photocatalytic hydrocarbon transformations.

Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non-porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4′-Bipy) 0.5 · DMF(H 2 O) 0.5 } was selected for study. We have investigated structural transformation in MOF-508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF-508 framework is driven by ligand conformation change and inter-linker interactions. The presence of Raman signatures of adsorbed CO 2 and its librational mode at 149 cm À1 suggests cooperative adsorption of CO 2 in the MOF-508 framework, which is also confirmed from DFT calculations that give a binding energy of 34 kJ/mol.Additional supporting information may be found in the online version of this article at the publisher's web site.Understanding guest and pressure-induced porosity