Investigation of p-n sensing transition and related highly sensitive NH3 gas sensing behavior of SnPx/rGO composites

Tin phosphides (SnPx) with alternating layers of P and Sn atoms possess abundant acidic P vacancy sites and high electronegativity, which can be potential candidates for gas sensing at room temperatures (RT). Herein, SnPx/rGO composites were fabricated for NH3 gas sensing at RT. Structural characterizations indicated the uniform distribution of SnPx on the rGO surface with small sizes and the existence of abundant acidic P vacancy sites. Gas sensing results showed that the response of SnPx/rGO composites with the phosphating ratio of 1:5 (SG-5) to 40 ppm NH3 at RT can be calculated as 117.5%. Moreover, SG-5 also exhibited a high response to NH3 gas with low concentrations (Response: 3% to 500 ppb NH3) and a LOD of 43.6 ppb. Besides, the NH3-TPD and in situ DRIFTs analysis were conducted to reveal the physical adsorption and complete desorption of NH3 gas on the material surface. More interestingly, due to different adsorption sites and conductive channels, the SnPx/rGO composites exhibited different sensing types upon exposure to reducing and oxidizing gases. Sensing response under different background gas atmospheres indicated that NH3 gases were mainly adsorbed on the heterojunctions to react with the surface oxygen species, exhibiting an n-type sensing behavior. However, NO2 gases were mainly adsorbed on the rGO surface to attract electrons, exhibiting a p-type sensing behavior. Density functional theory calculation results also confirmed the preferred adsorption of NH3 on SnPx and NO2 on rGO surface. This work provides an insight into gas adsorptions and desorptions on the SnPx surface and will promote the applications of metal phosphides in gas sensing.