Showing papers by "Gagik G. Gurzadyan published in 2022"

Plasmonic Active “Hot Spots”‐Confined Photocatalytic CO2 Reduction with High Selectivity for CH4 Production

Abstract: Plasmonic nanostructures have tremendous potential to be applied in photocatalytic CO2 reduction, since their localized surface plasmon resonance can collect low‐energy‐photons to derive energetic “hot electrons” for reducing the CO2 activation‐barrier. However, the hot electron‐driven CO2 reduction is usually limited by poor efficiency and low selectivity for producing kinetically unfavorable hydrocarbons. Here, a new idea of plasmonic active “hot spot”‐confined photocatalysis is proposed to overcome this drawback. W18O49 nanowires on the outer surface of Au nanoparticles‐embedded TiO2 electrospun nanofibers are assembled to obtain lots of Au/TiO2/W18O49 sandwich‐like substructures in the formed plasmonic heterostructure. The short distance (< 10 nm) between Au and adjacent W18O49 can induce an intense plasmon‐coupling to form the active “hot spots” in the substructures. These active “hot spots” are capable of not only gathering the incident light to enhance “hot electrons” generation and migration, but also capturing protons and CO through the dual‐hetero‐active‐sites (Au‐O‐Ti and W‐O‐Ti) at the Au/TiO2/W18O49 interface, as evidenced by systematic experiments and simulation analyses. Thus, during photocatalytic CO2 reduction at 43± 2 °C, these active “hot spots” enriched in the well‐designed Au/TiO2/W18O49 plasmonic heterostructure can synergistically confine the hot‐electron, proton, and CO intermediates for resulting in the CH4 and CO production‐rates at ≈35.55 and ≈2.57 µmol g−1 h−1, respectively, and the CH4‐product selectivity at ≈93.3%.

Plasmonic Active “Hot Spots”‐Confined Photocatalytic CO 2 Reduction with High Selectivity for CH 4 Production (Adv. Mater. 14/2022)

Abstract: Photocatalytic CO2 Reduction In article number 2109330, Zhenyi Zhang and co-workers report plasmonic active “hot spot”-confined photocatalytic CO2 reduction over a well-designed Au/TiO2/W18O49 plasmonic heterostructure. The active “hot spot” can synergistically confine hot electrons, CO, and protons, thereby leading to high photocatalytic activity and selectivity for CH4 production.

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads.

Abstract: Linking stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced intersystem crossing (REISC). Compared with a previously reported perylene fused nitroxide radical compound (triplet lifetime = 0.1 µs), these new adducts show a longer-lived triplet excited state (9.5 µs). Based on the singlet oxygen quantum yield (7%), we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA, we observed the interconversion of the two states (D1/Q1) at ~80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate , we observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. DFT computations showed that the interaction between radical and chromophore is ferromagnetic ( J >0, 0.05~0.10 eV).

Singlet Fission, Polaron Generation and Intersystem Crossing in Hexaphenyl Film

Abstract: The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells.

Plenty of Room on the Top: Pathways and Spectroscopic Signatures of Singlet Fission from Upper Singlet States.

Abstract: We investigate dynamic signatures of the singlet fission (SF) process triggered by the excitation of a molecular system to an upper singlet state SN (N > 1) and develop a computational methodology for the simulation of nonlinear spectroscopic signals revealing the SN → TT1 SF in real time. We demonstrate that SF can proceed directly from the upper state SN, bypassing the lowest excited state, S1. We determine the main SN → TT1 reaction pathways and show by computer simulation and spectroscopic measurements that the SN-initiated SF can be faster and more efficient than the traditionally studied S1 → TT1 SF. We claim that the SN → TT1 SF offers novel promising opportunities for engineering SF systems and enhancing SF yields.