Design and characterization of g-C3N4/ZnO tetrapod hybrid heterostructures for enhanced UV-activated NO2 gas sensing

Semiconductor gas sensors often suffer from limited sensitivity and slow response at room temperature, especially for NO2 detection. In this paper we design and characterize hybrid heterostructures combining zinc oxide tetrapods (ZnO-T) with graphitic carbon nitride (g-C3N4) to enhance ultraviolet (UV)-activated NO2 sensing. Three-dimensional g-C3N4/ZnO-T networks were synthesized and investigated by field emission scanning electron microscopy coupled with energy-dispersive X-ray mapping, X-ray diffraction analysis, Fourier-transform infrared spectroscopy, UV–visible and photoluminescence spectroscopies, as well as Brunauer–Emmett–Teller surface area and porosity measurements. The hybrids formed interconnected frameworks where ZnO tetrapods were anchored or embedded within the g-C3N4 matrix, resulting in increased surface area and pronounced macro- and mesoporosity. Photoluminescence results indicated that the g-C3N4/ZnO-T heterojunction promoted efficient separation of photogenerated electron–hole pairs and suppressed their recombination. Gas sensors fabricated by spray-coating the hybrids onto gold interdigitated electrodes on SiO2/Si substrates were tested under pulsed and continuous UV illumination at room temperature (23 °C). Compared with pristine ZnO-T, g-C3N4/ZnO-T devices exhibited faster photoresponse and recovery and a significantly larger NO2 response, with gas response ratios of 6.49 and 2.30, respectively, for 10 ppm NO2. These findings demonstrate that engineering three-dimensional g-C3N4/ZnO tetrapod heterostructures is an effective approach to improve UV-driven NO2 sensing performance at room temperature.