Tuning the catalytic activity of Nb2C MXenes via surface functionalization and defects

Herein, we investigated the pathways to modulate the electronic properties and improve the activity of Nb2C MXenes by functionalizing their surface with the terminating groups and creating surface defects. Through an in-depth Density Functional Theory (DFT) study, the activity of Nb2C MXenes terminated with F, Cl, S, and their combinations, denoted as Nb2C-T, were explored, and their activities were tested for H2O, O2, N2, and CO2 adsorption and dissociation. It was found that the work function of Nb2C-T MXenes varies from 3.79 to 5.07 eV, and Nb d-band center ranges between -2.53 to -1.53 eV. This provides the opportunities to tune their activities for specific chemical reactions since most catalytic reactions depend on the electronic properties of materials. To improve the catalytic activity of terminated Nb2C-T further, Nb2C-S and Nb2CCl, having the highest and lowest work functions, respectively, were analyzed with defects created by removing one surface terminating group, representing partially functionalized surfaces (Nb2C-Tx-1). The Climbing Image Nudge Elastic Band (CI-NEB) calculations for pristine and defected Nb2C-S and Nb2CCl surfaces confirmed that defected surfaces have a lower dissociation barrier for O2, H2O, N2, and CO2, and therefore, could prove as better catalysts, as compared to the pristine functionalized Nb2C surfaces. Specifically, Nb2C-Sx-1 showed a dissociation barrier as low as 0 eV for O2 and CO2, and 0.16 and 0.17 eV for H2O and N2, respectively, proving it to be an attractive candidate for reactions involving the dissociation of O2, H2O, N2, and CO2. Overall, this work indicates that there is a potential to improve the catalytic performance of Nb2C MXenes by altering its terminating groups. By choosing the right synthesis method, functionalized Nb2C MXenes with the desired electronic properties could be prepared in the future for critical chemical reactions.