Solar-driven photocatalysis has attracted significant attention in water splitting, CO₂ reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C₃N₄ is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.

Nanodiamond/carbon nitride (ND/CNx) nanoarchitectures with a stacked carbon nitride layer on nanodiamond have been successfully fabricated through a facile pyrolysis approach of pristine nanodiamond and melamine at temperatures of 650, 700, and 750 °C, which challenges the long-held axiom that a CNx layer can only be formed at condensation temperatures of less than 600 °C and it decomposes and inserts into the carbon matrix at temperatures higher than 600 °C. The structure and surface chemical properties of ND/CNx nanomaterials are strongly dependent on pyrolysis temperature and the mass ratio of nanodiamond to melamine. The optimized ND/CNx hybrid carbon nanoarchitecture exhibits synergistically enhanced catalytic performance for the direct dehydrogenation of ethylbenzene to styrene under oxidant- and steam-free conditions. A steady-state styrene rate of 4.0 mmol g⁻¹ h⁻¹ with 99% selectivity for the developed catalyst was achieved, whereas steady-state styrene rates of only 2.7 and 2.0 mmol g⁻¹ h⁻¹ with 95% and 96% selectivity were obtained for pristine nanodiamond and mesoporous carbon nitride, respectively, under the same reaction conditions. This was attributed to the synergistic effect between the nanodiamond and carbon nitride of the hybrid material with an appropriate amount of CNx layer and surface chemical properties. The developed ND/CNx carbon hybrid nanoarchitecture demonstrated 1.48 and 4.15 times the steady-state styrene rate of the established ND and the industrially used K–Fe catalyst, respectively, which allows it to be a potential catalyst for future industrial applications for styrene production through the metal-free dehydrogenation of ethylbenzene under oxidant- and steam-free conditions. This work also presents a facile method to synthesize new carbon nitride layer-containing hybrid nanocarbon materials for diverse applications with excellent properties.