The photoelectrocatalytic reduction of CO2 to useful fuels via energy from sunlight, has received significant attention as a promising route for generating carbon-neutral fuels and value-added chemicals. Standard electrocatalytic technologies require unfeasibly high overpotentials, rendering them inefficient and impractical. Encouragingly, experimental evidence suggests that pyridine (Py) is an effective co-catalyst during CO2 reduction. Bocarsly and co-workers reported CO2 reduction to methanol over a p-GaP photocathode at near 100% Faradaic efficiency (J. Am. Chem. Soc. 2008; 130: 6342). The experiments demonstrated that pyridinium (PyH+) is essential to achieving selective CO2 reduction. This suggests that PyH+ acts somehow as an electron shuttle, facilitating the transfer of photoexcited electrons from the semiconductor to CO2. However, the mechanism by which Py-catalyzed CO2 reduction occurs is still under debate. A theoretical study by Keith and Carter (J. Am. Chem. Soc. 2012; 134: 7580-7583) demonstrated that the single-electron reduction of PyH+ to a PyH radical is infeasible in homogenous solution, suggesting that the photocatalyst surface itself participates in a heterogeneous mechanism. This is further supported by recent reports of Py co-catalysis over CdTe (RSC Advances 2014; 4: 3016) and CuInS2 (RSC Advances 2014; 4: 39435) photocathodes. Hence, the Py-catalyzed reaction mechanism is likely heterogeneous in nature, involving the active participation of the GaP, CdTe, and CuInS2 surfaces. During catalytic operation, such surfaces are expected to evolve to a steady-state structure, exposing a variety of unique surface sites. As such, verification of stable surfaces is essential, as this will establish plausible models for adsorption sites contributing to the heterogeneous mechanism. This work reports a density functional theory (DFT) investigation of the stability of GaP, CdTe, and CuInS2 surface reconstructions, and the resulting impact such reconstructions may have on overall activity during CO2 photo-reduction. We employ the formalism of ab initio thermodynamics to compute the relative free energy of possible surface terminations, thus identifying reconstructions that are likely to occur. Having identified proper surface models, we then investigate adsorption trends across the GaP, CdTe, and CuInS2 surfaces to determine possible intermediate species along the CO2 reaction path, thus lending insight into the Py-catalyzed reaction mechanism.

How do surface reconstructions affect CO2 reduction over GaP, CdTe, and CuInS2 photoelectrodes?

251st National Meeting (2016)

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