Interfacial engineering of biochar/SnO2 heterojunctions for enhanced charge transfer and gas-sensing performance

P-doped biomass carbon/SnO 2 heterostructure delivers 32.9k% ethylene glycol response, 0.53 ppm LoD, and sustained performance under 85% RH and 6-month air exposure. • 1D Poisson-based n-n heterojunction model reveals 0.39 eV barrier modulation and carbon-dominated depletion control for signal amplification. • DFT and experiments confirm SnO 2 (110) governs selective adsorption, while P-doped carbon modulates barrier for signal amplification, guiding sensor design. The integration of carbon materials with metal oxide semiconductor (MOS) to engineer heterojunction composites is a proven strategy for enhancing gas-sensing performance. However, the interfacial chemistry and charge transfer mechanisms governing the carbon/MOS interface remain largely elusive. Herein, an n-n heterojunction composite of P-doped biochar/SnO 2 was constructed using cotton stalks and wool as dual-biomass precursors. This system achieves highly efficient detection of ethylene glycol (EG) at room temperature, exhibiting a high response of 32.9k%, a limit of detection of 0.53 ppm, and excellent long-term stability over half a year. Mechanistic studies reveal that the preferentially exposed (110) facets of SnO 2 exhibit a remarkably strong adsorption energy of −2.829 eV for EG, which dictates the gas selectivity of the composite system. More importantly, based on the one-dimensional Poisson's equation, an electrical amplification model coupling heavy-doping and light-doping regions was established. This model clarifies that the biochar side with lower carrier concentration dominates the variation in the depletion layer width. Arrhenius fitting confirms that the introduction of P elements expands the modulation range of the interfacial potential barrier height from 0.03 eV to 0.39 eV. These results define the specific roles of MOS, which govern adsorption selectivity, and biochar, which govern gas-sensing signal amplification, at the physical level. This dual-precursor strategy enables the simultaneous formation of a tubular carbon scaffold and in situ P doping in a single step. The established quantitative physical model demonstrates that the depletion layer is dominated by the lightly doped biochar side. This work provides a mechanistic framework for understanding interfacial engineering in carbon/MOS composites, enabling rational design of advanced heterojunctions through precise control of interface properties and charge transfer kinetics.