Monolayer CuBr-based gas sensor to detect habitat and industry-relevant molecules with high sensitivity and selectivity: a first-principles study

The adsorption characteristics of different environmental gas molecules such as HF, CO, CO₂, SO₂, H₂S, NH₃, NO and NO₂ on the surface of a CuBr monolayer have been studied using DFT+U calculations with Grimme scheme DFT-D2 for accurate description of the long-range interactions (van der Waals). Our findings indicate that the CuBr monolayer (ML) exhibits high sensitivity to CO, SO₂, H₂S, NH₃, NO and NO₂, as evidenced by their strong adsorption energies and significant charge transfer. In contrast, HF and CO₂ molecules show weak adsorption on the CuBr ML, due to their low adsorption energies and minimal charge transfer. High diffusion energy barriers for gas molecules (CO, CO₂, NH₃ and NO₂) indicate that they are less mobile and tend to remain stable at their adsorption sites. Conversely, low diffusion energy barriers (HF, SO₂, H₂S and NO) suggest that a lesser amount of energy needs to be expended and gases can move easily across the surface of the substrate. The band structure and partial density of states calculations reveal that the electronic properties of the CuBr ML are altered due to the contributions of the orbitals of the gas molecules (C-p and O-p of CO, F-p of HF, O-p of CO₂, S-p of H₂S, N-p of NH₃, S-p and O-p of SO₂, N-p and O-p of NO and NO₂) and CuBr ML (Cu-p, Cu-d, Br-p). The charge density difference and Bader charge analysis indicate that the gas molecules (CO, HF, SO₂, CO₂, NO and NO₂) either act as charge acceptors or donors (H₂S and NH₃). The work function variations of the CuBr ML before and after adsorption and significant changes in the conductivity verify the high sensitivity of CO, SO₂, H₂S, NH₃, NO and NO₂ with the CuBr ML. The band gap variations (before and after adsorption) are small for HF, CO, CO₂, H₂S and NH₃ whereas large variations in band gap for SO₂, NO and NO₂ reveal that the CuBr ML is quite selective to these three gases. The recovery time for gas molecules desorption from CuBr ML is reduced to a reasonable recovery time by increasing the temperature from ambient to 500 K with UV exposure. Thus our theoretical results indicate that the CuBr ML is a promising candidate as a gas sensor for sensing applications of CO, SO₂, H₂S, NH₃ NO and NO₂ with high sensitivity and selectivity.