Deposition of triazine-based graphitic carbon nitride via plasma-induced polymerisation of melamine

We present a novel plasma-assisted approach to deposit crystalline triazine-based graphitic carbon nitride directly onto solid substrates by fast and specific polymerisation of 2,4,6-triamino-1,3,5-triazine (melamine) in a pulsed rf plasma. We extensively investigate the molecular structure of the melamine precursor by single-crystal diffraction and bond-energy calculations. Moreover, thermogravimetric analysis is conducted to characterise the vaporisation behaviour of melamine. To get insights into the fragmentation of the monomer upon electron impact in the plasma, in situ optical emission spectroscopy is performed. Accurate assignment of the deposition product and its identification as pure triazine-based graphitic carbon nitride involve powder X-ray diffraction and an extensive study via X-ray photoelectron spectroscopy, which excludes the presence of other carbon nitrides. A band gap of 2.1 eV is identified via photoluminescence spectroscopy. Applying a constant dc bias of -210 V relative to ground to the non-heated substrate yields selectivity and prevents the co-deposition of melamine. We identify millisecond plasma pulses and a low duty cycle to be crucial for gaining a sponge-like morphology of the product, which is beneficial for catalytic purposes. Finally, the deposited polymer is investigated in terms of its photocatalytic behaviour by water-splitting experiments.