Cooperative Interactions between Surface Terminations Explain Photocatalytic Water Splitting Activity on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$

${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ is a highly efficient photocatalyst for the overall water splitting reaction under UV irradiation. However, an atomic-level understanding of the active surface sites responsible for the oxidation and reduction reactions is still lacking. Here we present a unified experimental and computational account of the photocatalytic activity at the SrO and ${\mathrm{Ti}\mathrm{O}}_2$ terminations of aqueous solvated [001] ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$. Our experimental findings show that the overall water-splitting reaction proceeds on the ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ surface only when the two terminations are simultaneously exposed to water. Our simulations explain this, showing that the photogenerated hole-driven oxidation primarily occurs at SrO surfaces in a sequence of four single hole transfer reactions, while the ${\mathrm{Ti}\mathrm{O}}_2$ termination effects the crucial band alignment of the photocatalyst relative to the water oxidation potential. The present work elucidates the interdependence of the two chemical terminations of ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ surfaces, and has consequent implications for maximizing sustainable solar-driven water splitting.

Popular Summary

Reducing our dependence on fossil fuels is one of the key challenges of this century. For decades, photocatalytic water splitting has been touted as a promising avenue for producing sustainable alternative fuel from abundant sunlight; however, development in this area has been slow. One factor in the slow progress is the difficulty of characterizing and modeling heterogeneous catalysis, since it requires atomistic knowledge of the local surface morphology and electronic structure during the chemical reaction. In this work, the authors combine experimental observations from scanning probe techniques with first-principles computations, based on density-functional theory (DFT) and molecular dynamics (MD) simulations, to obtain such an atomistic picture of photocatalytic water splitting on the surface of ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$. Though this material's band gap is too large to allow high-efficiency solar photocatalysis, ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ is often used as a model system for understanding the process and developing design principles to be applied to other systems. However, even in this “model” system, some of the basic aspects of the water-splitting reaction were not known.

This work demonstrates that a specific surface structure with a mixture of different atomic surface terminations is required for efficient water splitting. This is a surprising and unexpected result; indeed, it was assumed that a single atomic termination (i.e., either $\mathrm{Sr}\mathrm{O}$ or ${\mathrm{Ti}\mathrm{O}}_2$) fulfilled all of the requirements for catalyzing the water-splitting reaction. However, atomic-force microscopy characterization before and after water-splitting reactions on samples with carefully controlled surface termination clearly shows the necessity for mixed terminations. Using DFT calculations and MD simulations, the authors determine the atomistic model of the reaction, clearly identifying the separate roles that the different surface terminations and catalytic active sites play in the overall efficiency of the reaction.

Figure 1

AFM topography scans of surfaces before (left) and after (right) silver deposition. (a) ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ surface that has been cut and polished only, (b) surface that has been treated with a procedure that only partially segregates the SrO on the surface, (c) surface that has been treated such that SrO is segregated to the step edges, (d) surface that has been treated to obtain SrO termination by extending the duration of the high-temperature anneal. Panel (e) shows (from left to right) artistic representations of the termination arrangement corresponding to (a)–(d).

Figure 2

AFM line profiles perpendicular to step edges. (a) Line profile of a surface that has been treated such that SrO is segregated to the step edges. (b) Line profile of a surface that has been treated to obtain SrO termination. Bottom figures illustrate the distribution of SrO and ${\mathrm{Ti}\mathrm{O}}_2$ planes corresponding to the line profiles.

Figure 3

AFM topography scans of ${\mathrm{Ti}\mathrm{O}}_2$-terminated surfaces after silver deposition on (a) a treated substrate and (b) a deposited ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ film.

Figure 4

An equilibrated molecular dynamics simulation for (a) (001) ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ surfaces—${\mathrm{Ti}\mathrm{O}}_2$ (left, in blue) and SrO (right, in green)—interacting with a box of 64 water molecules. The number density distribution along the vertical $z$ direction for (b) ${\mathrm{H}}_2\mathrm{O}$ molecules and (c) dissociated ${\mathrm{OH}}^{-}$ species. The positions of the ${\mathrm{Ti}\mathrm{O}}_2$ and SrO surfaces with respect to the box of water are shown with blue and green dashed lines, respectively.

Figure 5

Band alignments for symmetric SrO-terminated and symmetric ${\mathrm{Ti}\mathrm{O}}_2$-terminated ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/$vacuum as reported in Ref. [54], and fully solvated ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3{/\mathrm{H}}_2\mathrm{O}$ slabs using DFT HSE06. The red dotted lines indicate the water redox potentials referenced to the vacuum level ($E_{{\mathrm{H}}^{+}/{\mathrm{H}}_2}=-4.44$ eV, $E_{{\mathrm{O}}_2/{\mathrm{H}}_2\mathrm{O}}=-5.67$ eV).

Figure 6

(a) A sequence of four PCET events reveal the pathway for conversion of (i) ${\mathrm{OH}}_{\mathrm{adsorbed}}^{-}$ to (v) ${\mathrm{O}}_2$ at the SrO surface via three reaction intermediates: (ii) oxygen anion radical (${\mathrm{O}}^{\bullet -}$), (iii) hydroperoxyl radical (${\mathrm{OOH}}^{-}$), (iv) superoxide ion (${\mathrm{O}}_2^{\bullet -}$). The proton removed at each stage of the PCET mechanism is highlighted in gray. Spin density (in yellow) corresponding to ${\mathrm{O}}^{\bullet -}$ species formed at (b) SrO-terminated and (c) ${\mathrm{Ti}\mathrm{O}}_2$-terminated aqueous ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ surfaces.