Hot π-Electron Tunneling of Metal-Insulator-COF Nanostructures for Efficient Hydrogen Production.

A metal-insulator-semiconductor (MIS) photosystem based on covalent organic framework (COF) semiconductors was designed for robust and efficient hydrogen evolution under visible-light irradiation. A maximal H2 evolution rate of 8.42 mmol h-1 g-1 and a turnover frequency of 789.5 h-1 were achieved by using a MIS photosystem prepared by electrostatic self-assembly of polyvinylpyrrolidone (PVP) insulator-capped Pt nanoparticles (NPs) with the hydrophilic imine-linked TP-COFs having =C=O-H-N= hydrogen-bonding groups. The hot π-electrons in the photoexcited n-type TP-COF semiconductors can be efficiently extracted and tunneled to Pt NPs across an ultrathin PVP insulating layer to reduce protons to H2 . Compared to the Schottky-type counterparts, the COF-based MIS photosystems give a 32-fold-enhanced carrier efficiency, attributed to the combined enhancement of photoexcitation rate, charge separation, and oxidation rate of holes accumulated in the valence band of the TP-COF semiconductor.

High-Rate, Tunable Syngas Production with Artificial Photosynthetic Cells.

An artificial photosynthetic (APS) system consisting of a photoanodic semiconductor that harvests solar photons to split H2 O, a Ni-SNG cathodic catalyst for the dark reaction of CO2 reduction in a CO2 -saturated NaHCO3 solution, and a proton-conducting membrane enabled syngas production from CO2 and H2 O with solar-to-syngas energy-conversion efficiency of up to 13.6 %. The syngas CO/H2 ratio was tunable between 1:2 and 5:1. Integration of the APS system with photovoltaic cells led to an impressive overall quantum efficiency of 6.29 % for syngas production. The largest turnover frequency of 529.5 h-1 was recorded with a photoanodic N-TiO2 nanorod array for highly stable CO production. The CO-evolution rate reached a maximum of 154.9 mmol g-1 h-1 in the dark compartment of the APS cell. Scanning electrochemical-atomic force microscopy showed the localization of electrons on the single-nickel-atom sites of the Ni-SNG catalyst, thus confirming that the multielectron reduction of CO2 to CO was kinetically favored.