  Hydrogen as the cleanest energy carrier is a promising alternative renewable
resource to fossil fuels. There is an ever-increasing interest in exploring
efficient and cost-effective approaches of hydrogen production. Recent
experiments have shown that single platinum atom immobilized on the metal
vacancies of MXenes allows a high-efficient hydrogen evolution reaction (HER).
Here using ab initio calculations, we design a series of substitutional
Pt-doped Tin+1CnTx (Tin+1CnTx-PtSA) with different thicknesses and terminations
(n = 1, 2 and 3, Tx = O, F and OH), and investigate the quantum-confinement
effect on the HER catalytic performance. Surprisingly, we reveal a strong
thickness effect of the MXene layer on the HER performance. Amongst the various
surface-terminated derivatives, Ti2CF2-PtSA and Ti2CH2O2-PtSA are found to be
the best HER catalysts with the change of Gibbs free energy {\Delta}G*H ~ 0 eV,
complying with the thermoneutral condition. The ab initio molecular dynamics
simulations reveal that Ti2CF2-PtSA and Ti2CH2O2-PtSA possess a good
thermodynamic stability. The present work shows that the HER catalytic activity
of the MXene is not solely governed by the local environment of the surface
such as Pt single atom. We point out the critical role of thickness control and
surface decoration of substrate in achieving a high-performance HER catalytical
activity.
