Uncovering the Nature of Band Gap Engineering of Adsorption Energy by Elucidating an Adsorbate Bonding Mechanism on Two-Dimensional TiO₂(110)

To unravel the nature of band gap engineering of adsorption energy, we investigated the adsorption of H and CO on two-dimensional two-layer (2L)- and three-layer (3L)-TiO₂(110) semiconductors by density functional theory calculations. Molecular orbital theory was used to develop a new H–O bonding mechanism. We propose that the H 1s orbital combines linearly with the occupied σ_O–Ti bonding orbital to form σ_H–OTi bonding and σ*_H–OTi antibonding orbitals. Two of three electrons (one from H and two from σ_O–Ti) fill the σ_H–OTi bonding orbital, and the other electron fills the pristine lowest unoccupied σ*_O–Ti orbital rather than σ*_H–OTi. Consequently, the H adsorption energy E_H‑ads is decided by the stabilization energies ΔE_AB (occupation of σ_H–OTi) and ΔE_e (lowered energy due to electron filling of σ*_O–Ti) and the destabilization energy ΔE_AB (occupation of σ*_O–Ti). The stabilization energies for H adsorption on the 2L and 3L are similar; therefore, the E_H‑ads difference is predominantly determined by the corresponding ΔE_AB, which is dominated by the 2L and 3L band gaps. The C–O bonding mechanism is similar. This mechanism for surface chemical bonding shows the critical role of the band gap in determining the adsorption energy and provides a theoretical basis for band gap engineering in heterogeneous catalysis.