Co‐Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light

Abstract

The development of stable systems to generate chemical fuels through water splitting by sunlight is a key challenge of modern materials chemistry, one that is driven by increasing energy demands and climate change. The central problems are to design chemically stable light-harvesting antenna molecules and co-factors, and then to assemble these active components into an integrated photosystem. Various substances have been examined as visible-light converters, including metal-organic dyes, [1] inorganic semiconductors, [2] and conjugated polymers. [3] The last are of particular interest, as they are much closer to biological systems in composition and are potentially sustainable, as well as cheap and easily processable. Principally, properties such as the HOMO and LUMO position and the resulting band gap can be precisely chemically engineered by synthesizing the constituents. To date, in spite of the wide availability of conducting polymers developed for photovoltaics, [4] the impact of these polymers on water splitting technology remains minor, because ordinary conducting polymers are usually unstable to visible light irradiation in conjunction with exposure to oxygen and water.A recent study has explored metal-free, polymeric graphitic carbon nitride (g-C 3 N 4 ) for the generation of hydrogen from a protic solution with visible light. [5] This carbon nitride (CN) polymer was not only found to be a stable semiconductor, but also to be capable of achieving both half reactions of water splitting, meaning that the band-gap covers both the water reduction and water oxidation potentials. This is indeed a rare and lucky case. However, the activity of pristine g-C 3 N 4 remains moderate. [5] Several strategies, such as nanostructuring, [6] doping, [7] cocatalyzing, [5,8] and copolymerization, [9] have been exploited to improve the photocatalytic activity and selectivity of carbon nitride. Indeed, it was shown that by copolymerizing simple barbituric acid with the carbon nitride precursor through a Schiff base reaction, the performance of g-C 3 N 4 could be enhanced, which is an effect resulting from extension of the optical absorption of the polymer to cover more of the visible light range. [9] However, in this case the HOMO was decreased in energy, thus lowering the oxidation potential, which is presumably the most difficult step in achieving overall water splitting with organic semiconductors. This is why the design of appropriate comonomers with diverse chemical composition and structure to allow for modification of the band structure and optoelectronic properties of carbon nitride is still a relevant and promising task.Herein, we advance this strategy by employing a variety of new monomer building blocks with the desired compositions and electronic structures for chemical incorporation into the conjugated polymeric network of g-C 3 N 4 . Most precursors of carbon nitride polymers contain cyano groups, amino groups, or both, with the simplest case being cyanamide, which can undergo multiple thermal condensations to ...

Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles

“…The application of these mono-disperse hollow conjugated semiconductor vesicles has been demonstrated using a photocatalytic hydrogen-evolution assay, and their advantages from the inner optical reflection and improved structure condensation provide a remarkably increased photochemical activity, reaching an overall AQY of approximately 7.5%. Although the blue shift of the band gap, which was associated to either a quantum size effect or enhanced H-type interlayer packing, was unwanted, further chemical control, such as extending the pi system by anchoring aromatic motifs, are already available to expand the visible absorption, for example, through copolymerisation 46,47 . Hybrid nanoarchitectures based on finely tuned polymeric carbon nitride capsules provide a valuable platform for constructing highly organized photosynthetic systems for the efficient and sustained utilization of solar radiation after the controlled deposition of a cocatalyst onto the exterior and/or interior surfaces and the construction of a dyadic layer to promote exciton dissociation.…”

Relations between Structure, Activity and Stability in C3N4 Based Photocatalysts Used for Solar Hydrogen Production

“…All as-prepared CN-containing catalysts show a narrow isotropic singlet with Lorentzian line shape at g = 2.0042 already in the dark (not shown) arising from single electrons trapped at a surface sp 2 -carbon in a typical heptazine unit [31,32]. Note that the conduction band of CN is formed by C 2p orbitals, while the valence band is mainly made up of N 2p orbitals [5].…”

Can environmental pharmaceuticals be photocatalytically degraded and completely mineralized in water using g-C3N4/TiO2 under visible light irradiation?—Implications of persistent toxic intermediates

“…Higher photocatalytic activity of g-C 3 N 4 /TiO 2 occurred because of the existed large number of amino, carboxyl, and hydroxyl groups in the hybrid photocatalyst; these could form cross-linked connections and covalent bonds between g-C 3 N 4 and TiO 2 [37]. This could strengthen the chemical interaction and may be of significance to the transfer carriers, inducing a synergetic effect to enhance visible light absorbance and photocatalytic activity [43][44][45]. For the samples prepared at different hydrothermal temperatures (Fig.…”