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

Nitrogen dioxide (NO₂) 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 NO₂ 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 + <i>U</i> method showed that adsorption is strongly site-dependent: NO₂ binds most strongly at the S site (<i>E</i>_ads = -0.88 eV, charge transfer = +2.595 <i>e</i>), while N-on-Cd exhibits weak physisorption (-0.14 eV, +0.040 <i>e</i>). Optical conductivity derived from ε₂(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 NO₂ sensing.