Boosting Gas-Phase TiO2 Photocatalysis with Weak Electric Field Strengths of Volt/Centimeter.

Among semiconductor nanomaterials, titanium dioxide is at the forefront of heterogeneous photocatalysis, but its catalytic activity greatly suffers from the loss of photoexcited charge carriers through deleterious recombination processes. Here, we investigate the impact of an external electric field (EEF) applied to conventional P25 TiO2 nanopowder with or without Au nanoparticles (NPs) to circumvent this issue. The study of two redox reactions in the gas phase, water splitting and toluene degradation, reveals an enhancement of the photocatalytic activity with rather modest electric fields of a few volt/centimeters only. Such an improvement arises from the electric-field-induced quenching of the green emission in anatase, allowing the photoexcited charge carriers to be transferred to the adsorbed reactants instead of pointless radiative recombinations. Applying an EEF across a trap-rich metal oxide material, such as TiO2, which, when impregnated with Au NPs, leads, respectively, to 12- and 6-fold enhancements in the production of hydrogen and the oxidation of toluene for an electric field of 8 V/cm, without any electrolysis, is a simple and elegant strategy to meet higher photocatalytic efficiencies.

Hot-Electron Photodynamics in Silver-Containing BEA-Type Nanozeolite Studied by Femtosecond Transient Absorption Spectroscopy.

Silver cations were introduced in nanosized BEA-type zeolite containing organic template by ion-exchange followed by chemical reduction towards preparation of photoactive materials (Ag0 -BEA). The stabilization of highly dispersed Ag0 nanoparticles with a size of 1-2 nm in the BEA zeolite was revealed. The transient optical response of the Ag-BEA samples upon photoexcitation at 400 nm was studied by femtosecond absorption. The photodynamic of the hot electrons was found to depend on the sample preparation. The lifetime of the hot electrons in the Ag-BEA samples containing small Ag nanoparticles (1-2 nm) is significantly shortened in comparison to bear Ag nanoparticles with a size of 10 nm. While for the larger Ag nanoparticles, the energy absorbed in the conduction band is decaying by electron-phonon coupling into the metal lattice, the high surface-to-volume ratio of the small Ag nanoparticles favors the dissipation of the energy of the hot electrons from the metal nanoparticles (Ag0 ) towards the zeolitic micro-environment. This finding is encouraging for further applications of Ag-containing zeolites in photocatalysis and plasmonic chemistry.

Effect of zeolite morphology on charge separated states: ZSM-5-type nanocrystals, nanosheets and nanosponges.

In the present work, we investigate the electron transfer occurring in the porous void of three MFI-type zeolite (ZSM-5) nanomaterials (nanocrystals, nanosheets and nanosponges) after adsorption and photoexcitation of t-stilbene (t-St). ZSM-5 nanosheets are constituted of lamellar stacking of several nanosheets (20-40 nm) where each nanosheet has a thickness of 2 nm. Nanosponges are composed of ZSM-5 nanocrystals (2-3 nm) separated by mesoporous holes of 5.8 nm facilitating the synthesis of hierachical materials. While the nanosheets show microporosity similar to that observed for the ZSM-5 nanocrystals, the absorption isotherms of the nanosponges show the existence of secondary micropores. After photoirradiation of t-St, UV-vis absorption spectroscopy shows the formation of charge separated states (radical cation and charge transfer complex) in the nanocrystals and in the nanosheets whereas no ionized species is detected in the nanosponges. The radical cation (RC) is stabilized in the nanosheets while it evolves very rapidly towards a Charge Transfer Complex (CTC) in the nanocrocrystals. The particular morphology of the nanosheets and nanosponges is put forward to explain this result since all host materials are of the MFI-type. To investigate ultra-short phenomena in the three nanomaterials, the UV-vis transient spectra were recorded between 2 and 450 µs after photoexcitation by nanosecond laser pulses. In the nanocrystals and nanosheets only the RC is detected whereas CTC formation is not observed. Photoexcitation of t-St in the nanosponges also leads to the formation of a RC but it recombines completely within 70 µs. This suggests the preferential location of t-St in the secondary micropores with pores larger than the micropores of the MFI-type framework and possibly in the mesopores of the nanosponges.

Molecular Porous Photosystems Tailored for Long-Term Photocatalytic CO2 Reduction.

The molecular-level structuration of two full photosystems into conjugated porous organic polymers is reported. The strategy of heterogenization gives rise to photosystems which are still fully active after 4 days of continuous illumination. Those materials catalyze the carbon dioxide photoreduction driven by visible light to produce up to three grams of formate per gram of catalyst. The covalent tethering of the two active sites into a single framework is shown to play a key role in the visible light activation of the catalyst. The unprecedented long-term efficiency arises from an optimal photoinduced electron transfer from the light harvesting moiety to the catalytic site as anticipated by quantum mechanical calculations and evidenced by in situ ultrafast time-resolved spectroscopy.

Selective photocatalytic conversion of methane into carbon monoxide over zinc-heteropolyacid-titania nanocomposites.

Chemical utilization of vast fossil and renewable feedstocks of methane remains one of the most important challenges of modern chemistry. Herein, we report direct and selective methane photocatalytic oxidation at ambient conditions into carbon monoxide, which is an important chemical intermediate and a platform molecule. The composite catalysts on the basis of zinc, tungstophosphoric acid and titania exhibit exceptional performance in this reaction, high carbon monoxide selectivity and quantum efficiency of 7.1% at 362 nm. In-situ Fourier transform infrared and X-ray photoelectron spectroscopy suggest that the catalytic performance can be attributed to zinc species highly dispersed on tungstophosphoric acid /titania, which undergo reduction and oxidation cycles during the reaction according to the Mars-van Krevelen sequence. The reaction proceeds via intermediate formation of surface methyl carbonates.

Spatially close azo dyes with sub-nanosecond switching speeds and exceptional temporal resolution.

Photoswitchable bis-azo dyes with an outstanding temporal resolution of 10(15) times between the thermal relaxation rates of their two constituting photochromes are reported. Remarkably, the close spatial proximity of both azo photochromes in these molecular assemblies translates in an unprecedented 10(3) -fold acceleration of the thermal isomerization rate of their faster azo unit compared to the one displayed by the isolated counterpart. Indeed, the relaxation time of the fast-isomerizing platform of the herein reported bis-azobenzenes is as low as 200 ps under ambient conditions. In the wake of these results, the bis-azo dyes described herein are invaluable chromophores for the design of multifunctional light-addressable materials in which simultaneous switching in two very different timescales might be essential.

In situ infrared molecular detection using palladium-containing zeolite films.

In situ IR detection of carbon monoxide in the presence of hydrocarbons (methanol and pentane) using Pd-containing zeolite thin films is reported. The thin films are prepared by spin coating deposition of nanosized LTL and BEA type zeolites suspensions; the palladium clusters are introduced in the nanosized zeolites by ion exchange followed by γ radiolysis of the coating suspensions. The Pd-containing zeolite films with a thickness of 200 nm are exposed to a single gas (either CO or hydrocarbons) or gas mixtures in the presence of water (100 ppm), and the IR spectra are collected continuously at 25, 75, and 100 °C. The fast recognition of very low concentrations of CO (2-100 ppm) in the presence of highly concentrated vapors of methanol or pentane (400-4000 ppm) with the Pd-containing zeolite films is demonstrated. The detection of CO and hydrocarbons is instant, which is a function of the low thickness of the films, small size of the individual zeolite crystals, and regular size and high stability of the Pd clusters in the zeolite films. The heat of adsorption for all experiments is similar (15 kJ.mol(-1)), which is explained with weak interactions between the carbon monoxide and palladium clusters in the zeolite films at temperatures below 100 °C. The nanosized zeolites with homogeneously distributed Pd clusters deposited in thin films demonstrate high molecular recognition capacity toward low concentrations of carbon monoxide under real environmental conditions, i.e., in the presence of water and hydrocarbons.

Distance dependence of the reaction rate for the reduction of metal cations by solvated electrons: a picosecond pulse radiolysis study.

The decay of the solvated electron generated by picosecond electron pulse radiolysis is studied by broad-band transient absorption measurements in ethylene glycol solutions containing decimolar concentrations of Cu(2+), Ni(2+), and Pb(2+) metal cations. Analysis of the nonexponential kinetics of the decays reveals molecular parameters of the electron transfer reaction. It is found that the reaction occurs at long distance for Cu(2+) solutions and is only limited to contact distance in the case of Ni(2+) solutions. The distribution of reaction distance strongly depends on the free enthalpy change of the reactions.

Subnanometer CdS clusters self-confined in MFI-type zeolite nanoparticles and thin films.

One-step colloidal synthesis of subnanometer CdS clusters in hydrophobic MFI-type zeolite crystals in the presence of 3-mercaptopropyl-trimethoxysilane (MPS), cadmium precursor, and tetrapropylammonium hydroxide (TPAOH) is performed. MPS is used as the bifunctional agent, as it hydrolyzes fast, cross-links with the silica framework, and provides thiol groups to anchor Cd(2+), and subsequently forms CdS clusters. The MFI crystals with the thiol groups not only function as a nanochamber for the formation of CdS but also prevent further moisture-induced agglomeration of the clusters. Direct evidence for the presence of asymmetric shaped subnanometer CdS clusters aligned in the channels of MFI crystals stabilized in suspensions and films is provided by high resolution transmission electron microscopy (HRTEM), grazing incidence X-ray diffraction (GI-XRD), and photoluminescence spectra (emission < 350 nm). The CdS clusters are stable for months in colloidal suspensions and films without any particular precaution against water. The hydrophobicity of the MFI zeolite and the presence of the organic template in the channels favor the stabilization of small CdS clusters, which are available for further applications.

Pulse radiolysis studies on the temperature-dependent spectrum and the time-dependent yield of solvated electron in propane-1,2,3-triol.

With a revisit of the absorption coefficient of the solvated electron in propane-1,2,3-triol, the temperature-dependent behavior of the absorption spectrum of solvated electron was studied from room temperature to 573 K by pulse radiolysis techniques. The change in the absorption spectrum of solvated electron in propane-1,2,3-triol observed by cooling down from a high temperature to 333 K is compared with that occurring during the electron solvation process at 333 K. The effect of the specific molecular structure of propane-1,2,3-triol compared to other alcohols is discussed.

Ultrafast bidirectional dihydroazulene/vinylheptafulvene (DHA/VHF) molecular switches: photochemical ring closure of vinylheptafulvene proven by a two-pulse experiment.

The photochromic couple dihydroazulene/vinylheptafulvene (DHA/VHF) is an established system for molecular switching. The photoinduced ring opening of the thermally stable dihydroazulene proceeds in the picosecond time regime with subsequent rapid relaxation to the electronic ground state. So far it was not possible to reverse the reaction photochemically. It was always found that the back reaction proceeds thermally on a longer time scale. In the case of the cyanophenyl-DHA derivative utilized in the present study, this is accounted to s-cis/s-trans isomerization of the primarily formed s-cis VHF. Since this particular isomerization takes much longer than nanoseconds, we now successfully applied a second visible femtosecond pulse subsequent to the initial UV pulse (25 ps delay) achieving ring closure of the primarily formed s-cis VHF. The now bidirectional photoswitching was monitored by the changes in the cw spectrum. As a result, the DHA/VHF system is found to be a multifold switchable system by itself: it is both a very fast photoreversible switch and a photochemical/thermal switch with a thermal lock mode.