Bias-driven surface reconstruction of SnSSe into an active SnO/SnSSe interface for photoelectrochemical water oxidation

Photoelectrodes based on two-dimensional (2D) materials can undergo substantial surface reconstruction during photoelectrochemical (PEC) operation, although identifying the surface that actually mediates water oxidation remains challenging. Here, mixed-anion SnSSe nanosheets were used to determine how band-edge engineering and potential-dependent surface chemistry determine the working state of a tin-dichalcogenide photoanode. Comparison with the binary SnS 2 and SnSe 2 end members shows that SnSSe retains the layered CdI 2 -type structure after liquid-phase exfoliation while combining a visible-light optical gap of approximately 1.8 eV with a valence band maximum near −6.0 eV versus vacuum, providing the oxidative driving force required for photoanodic water oxidation. Wavelength-resolved PEC measurements in acidic, neutral, and alkaline electrolytes establish reproducible photoanodic operation with a ratiometric power-saved efficiency for OER of 1.27% in acidic electrolyte. The origin of this activity is resolved by combining density functional theory with four complementary X-ray photoelectron spectroscopy approaches, enabling operando and in situ spatially resolved analysis of individual exfoliated flakes, also under electrochemical polarization. Calculations show that pristine SnSSe is intrinsically limited by *OOH formation, whereas complete oxidation to an SnO 2 -like termination produces an energetically unfavorable OER pathway. In contrast, the formation of an under-oxidized SnO-like layer strongly alters the reaction energetics and generates the most favorable OER surface among the investigated states. Operando XPS directly identifies the corresponding SnO-like component under intermediate anodic polarization, predominantly within approximately 0.8–2.0 V versus RHE, while higher potentials promote Sn(IV)-rich SnO 2 formation and irreversible passivation. The results therefore identify a bias-selected SnO/SnSSe interface as the functional state of the photoanode and establish controlled surface reconstruction, rather than resistance to oxidation alone, as a design parameter for mixed-anion photoelectrodes for solar water splitting.