Understanding the NO ₂ Sensing Mechanism on CdS Quantum Dots via Experimental and First-Principles Calculations

Nitrogen dioxide (NO 2 ) is a highly toxic atmospheric pollutant, necessitating sensing materials that combine high efficiency, selectivity, and a rapid response. In this work, cadmium sulfide (CdS) quantum dots (QDs) were synthesized via a wet-chemical precipitation route and extensively characterized using XRD, Raman, UV–vis absorption, PL/TRPL, XPS, FESEM–EDX, BET–BJH, and HRTEM. The QDs crystallize in the cubic phase with a particle size of 4–5 nm, exhibiting strong quantum confinement and high surface activity. Time-resolved PL measurements revealed a long decay lifetime of 7.11 μs, indicative of an effective trap-assisted charge retention that favors gas sensing. Experimentally, the CdS QD sensor delivered a notable NO 2 response of 78% at 125 °C (at 40 ppm), with fast response and recovery times of 6 and 24 s, along with excellent selectivity and operational stability. Density functional theory (DFT) calculations using the GGA + U method showed that adsorption is strongly site-dependent: NO 2 binds most strongly at the S site ( E ads = −0.88 eV, charge transfer = +2.595 e ), while N-on-Cd exhibits weak physisorption (−0.14 eV, +0.040 e ). Optical conductivity derived from ε 2 (ω) indicated enhanced σ(ω) for Cd-site adsorption, supporting rapid activation, whereas S-site chemisorption governs sensitivity. These synergistic effects highlight CdS QDs as promising candidates for high-performance NO 2 sensing.