A density functional study on the formaldehyde recognition by Al-doped ZnO nanosheet

Experimental works have shown that doping ZnO nanosheets by the Al atom meaningfully increases their sensitivity toward HCHO gas. Here, we applied density functional theory calculations to study the Al-doping effect on the sensitivity of a ZnO nanosheet to HCHO gas, explaining the experimental results. We found that the pristine ZnO nanosheet weakly interacts with the HCHO with adsorption energy (E_ad) of -9.3 kcal/mol, and the sensing response (S) value of 2.2 at 623 K. After the Al-doping process, the E_ad and S values increase to -40.7 kcal/mol and 83.9, respectively, indicating an excellent agreement with the experimental results. We showed a relation between the HOMO-LUMO gap and the S value, confirming with the experimental results. Also, the proposed Al-doped ZnO nanosheet shows a good selectivity to HCHO gas at the presence of H₂, NH₃, C₂H₅OH, CO, and CH₄ gases. A short recovery time of 18 s is predicted for Al-ZnO-based sensor which is comparable with the experimental value of 26 s. Finally, we conclude that the Al-doping makes the ZnO nanosheet a promising HCHO gas sensor with a high sensitivity, an excellent selectivity, and short recovery time.