Zhi-An Lan

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity

Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π-π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Ultrafine Cobalt Catalysts on Covalent Carbon Nitride Frameworks for Oxygenic Photosynthesis

The rational cooperation of sustainable catalysts with suitable light-harvesting semiconductors to fabricate photosynthetic device/machinery has been regarded as an ideal technique to alleviate the current worldwide energy and environmental issues. Cobalt based species (e.g., Co-Pi, Co3O4, and Co-cubene) have attracted particular attentions because they are earth-abundant, cost-acceptable, and more importantly, it shows comparable water oxidation activities to the noble metal based catalysts (e.g., RuO2, IrO2). In this contribution, we compared two general cocatalysts modification strategies, based on the surface depositing and bulk doping of ultrafine cobalt species into the sustainable graphitic carbon nitride (g-C3N4) polymer networks for oxygenic photosynthesis by splitting water into oxygen, electrons, and protons. The chemical backbone of g-C3N4 does not alter after both engineering modifications; however, in comparison with the bulk doping, the optical and electronic properties of the surface depositing samples are efficiently promoted, and the photocatalytic water oxidation activities are increased owing to much more exposed active sites, reduced overpotential for oxygen evolution and the accelerated interface charge mobility. This paper underlines the advantage of surface engineering to establish efficient advanced polymeric composites for water oxidation, and it opens new insights into the architectural design of binary hybrid photocatalysts with high reactivity and further utilizations in the fields of energy and environment.

Cobalt selenide: a versatile cocatalyst for photocatalytic water oxidation with visible light

Cobalt selenide has been developed as an effective cocatalyst to decorate visible light photocatalysts, including WO3, BiVO4 and g-C3N4, establishing intense surface contact with substrates for the photocatalytic water oxidation reaction. Compared with CoOx and CoSx, CoSe2 is superior for improving the photocatalytic water oxidation activity due to the lower anionic electronegativity, which leads to dense adhesion and enables fast interfacial charge transfer for the water oxidation reaction.

Merging Surface Organometallic Chemistry with Graphitic Carbon Nitride Photocatalysis for CO2 Photofixation

The creation and development of efficient solar-energy conversion systems for artificial photosynthesis is one of the challenges of modern chemistry and materials. Currently, the search for sustainable and stable photocatalytic systems for CO2 reduction by visible light is being actively pursued. Many useful and energy-rich chemicals (e.g. CH3OH and HCOOH) can be obtained by photocatalytic CO2 reduction, which can reduce the emissions of greenhouse gases. Recently, significant improvements have been achieved in the area of CO2 reduction by using a heterogeneous photocatalyst under visible-light irradiation. However, most of the catalysts screened did not exceed the state-of-the-art system with turnover numbers (TONs) of approximately 200 and apparent quantum yields (AQYs) of approximately 2 %. The introduction of melon-based graphitic carbon nitride (gC3N4) polymers as solar-energy transducers has significantly extended the scope of conventional inorganic photocatalysts to polymeric photocatalysts. The latter is even more promising than the conventional photocatalysts because it is metal-free, sustainable, and has demonstrated the ability to induce water splitting, CO2 reduction, and selective organic synthesis by means of visible light. The use of g-C3N4 for CO2 reduction is an emerging research topic that couples organic basic functionality to photocatalytic functionality and allows for activation/adsorption and reduction of CO2. In the previous system for CO2 to CO photocatalytic conversion the quantum yield is less than 1. Recently, Maeda and co-workers have developed a promising heterogeneous system for the reduction of CO2 into formic acid under visible-light irradiation by merging organometallic chemistry with polymer photocatalysis by using g-C3N4 and a ruthenium complex as light-harvesting units and catalytic active sites, respectively. By carefully optimizing the heterogeneous catalyst and the reaction conditions the AQY was remarkably enhanced to 5.7 with a high TON of >1000, which are the highest values for g-C3N4 and are better than other heterogeneous photocatalysts working with visible light. The authors promote surface kinetics for both the reduction reaction and charge transfer by chemically modifying g-C3N4 with a Ru complex by means of a surface-chemistry strategy. The surface Ru-complex catalysis is demonstrated to change reaction pathway to produce the more valuable product, formic acid, rather than carbon monoxide. Four different Ru-based complexes (Scheme 1), trans-(Cl)[Ru(bpyX2)(CO)2Cl2] (bpyX2 = 2,2’-bipyridine with substituent X in the 4-position, X = H, CH3, PO3H2, or CH2PO3H2), were used as kinetic promoters for CO2 reduction, along with a suitable reaction environment to increase the efficiency of the overall process. Mesoporous graphitic carbon nitride (mpg-C3N4) with a specific surface area of 180 m g¢1 and pore volume of 0.7 cm g¢1 was selected as a light-harvesting semiconductor. The surface of the porous organic photocatalyst is covered with amino groups, which can be easily functionalized by surface organic chemistry. This feature is unique and is not available for traditional inorganic photocatalysts. Results indicated that RuP and RuCP could adsorb on the surface of mpg-C3N4 owing to the intense interaction between acid and base functional groups on the two units. However, no adsorption was found to occur for RuH and RuMe, which do not have an anchoring group. The rich amino groups on the surface of the gScheme 1. Photocatalytic CO2 reduction by Ru-complex anchored g-C3N4 photocatalysts. C.B. = Conduction band, V.B. = Valance band, D = electron donor. Reprinted with permission from Ref. [1] .