The unique properties of nanostructured carbon nitride materials make them attractive as polymeric semiconductors. With a band gap of ~ 2.7 ev, high thermal and chemical stability and chemical inertness, this material is a promising candidate in different applications including direct methanol fuel cells, catalysis and photocatalysis. Carbon nitrides are typically prepared by simple pyrolysis of nitrogen rich precursors such as melamine or urea. They suffer from low photocatalytic efficiency (due to large band gap) and high recombination rate of electron-hole pairs. To overcome this challenge, doping different heteroatoms (B, S, O, P, etc…) into the carbon nitride structure has been widely studied. Another challenge in synthesizing carbon nitride materials is controlling morphology and porosity of these materials. Introducing porosity into the structure provides facilitated transport in applications such as catalysis, and could alter the band gap of the material.
The conventional synthesis of these materials is conducted through high temperature poly-condensation of nitrogen containing monomers. Hard-templating techniques using mesoporous silica as sacrificial support, has been utilized to control the carbon nitride morphology. However, these approaches usually involve post treatment with harsh solvents to remove the template. Hence, developing a low temperature, solution-based route to pre-organize and fix the polymeric sheets prior to pyrolysis will provide a simple one step method to control the final textural properties.
In this work, we used melamine and cyanuric acid supra-molecular assembly through hydrogen bonding1 in the presence of different surfactants and additives as structure-directing agents to systematically study the pseudo-ternary phase diagram of surfactant/precursor/solvent system. The carbon nitride samples were synthesized by mixing the precursors (melamine and cyanuric acid) with water in the presence of a surfactant (Pluronic F-127, Pluronic P-123 and Triton X-100). The self-assembly was allowed to proceed for few hours followed by evaporation of the solvent and filtration. The obtained white powder was pyrolized in a furnace at 500°C for 4 hours, under nitrogen flow. Different morphologies including sheet-like, interconnected sponge-like, high porosity structures with thin walls and spheres were synthesized by adjusting the surfactant/monomer ratio, controlling the temperature and changing the pH using HCl.
The synthesized nitrogen containing carbons were characterized thoroughly using XPS, SEM, TEM and Nitrogen physisorption, High porosity carbon nitride with a desired carbon to nitrogen ratio of 0.75 and large BET surface area of 160 m2/g (compared to 10-20 m2/g for carbon nitride synthesized directly through pyrolysis of C/N containing monomers) was obtained through controlled self-assembly of melamine and cyanuric acid in the presence of surfactant and HCl.
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