Characterizing Structure-Dependent TiS₂/Water Interfaces Using Deep-Neural-Network-Assisted Molecular Dynamics

As a promising layered electrode material, TiS₂-based capacitive deionization (CDI) devices for water desalination have attracted significant attention. However, TiS₂/H₂O interfacial features, potentially important for device optimization, remain unidentified. Using Deep Potential Molecular Dynamics (DPMD), we characterized distinct aqueous interfaces introduced by four TiS₂ terminations expected to be present as water intercalates into TiS₂, namely, Armchair, Zigzag, Zigzag-L, and Zigzag-R. First, we assessed important representative physical properties of the system to validate the deep potentials (DPs). DPMD simulations agree well with experiments and first-principles simulations, suggesting the DPs are accurate and reliable. Subsequent simulations of these TiS₂/water interfaces revealed how TiS₂ surface termination influences the structure of interfacial water. This effect is most evident in the first and second water layers close to the TiS₂ surface, and more pronounced when spontaneous dissociative adsorption of water occurs. The extent of water dissociation on each surface was evaluated using enhanced sampling. Zigzag-L is the only interface where proton transfer from adsorbed water to TiS₂ surface S atoms is thermodynamically and kinetically favored. The coexistence of surface four-fold-coordinated Ti (Ti_4c) and one-fold-coordinated S (S_1c) is found to be essential to making proton transfer feasible on the Zigzag-L surface. Furthermore, remaining unprotonated S_1c atoms can act as good proton acceptors after water dissociation. Thus, TiS₂ with Zigzag-L termination may be a surface to avoid in CDI device construction, given that pH fluctuations adversely affect performance. This work provides new understanding of TiS₂/H₂O interfacial features that could aid future design and optimization of TiS₂-based CDI devices for water desalination.