Photodriven reduction and oxidation reactions on colloidal semiconductor particles: Implications for prebiotic synthesis

Photoelectrochemical reactions on mineral surfaces may have contributed to the synthesis of prebiotic molecules on early Earth. Laboratory experiments were conducted to develop this idea. Colloidal particles of ZnS were selected as a prototypical semiconducting mineral material. We investigated (1) the capability of the ZnS colloid to reduce aqueous CO2 under ultraviolet irradiation to yield formate in the presence of a sulfur hole scavenger and (2) to oxidize formate to yield CO2 in the absence of a hole scavenger. In the presence of the hole scavenger, the initial quantum efficiency is 10% for formate production from CO2 at pH 6.3. The reaction rate is sensitive to the pH of the solution, increasing with acidity for the pH range of 5–9. The initial formate production rate is proportional to the concentration of aqueous inorganic carbon from 13 to 65 mM, suggesting an absence of surface saturation. Photoproducts other than formate also form, including acetate and propionate, a finding which demonstrates the formation of carbon–carbon coupling products. In the absence of hole scavenger but in the presence of the ZnS colloid, formate is photooxidized with a quantum efficiency of 1.1%. The likely presence of semiconducting colloidal particles in the oceans of the prebiotic Earth suggests that photochemical reactions on their surfaces could have played a significant role in the synthesis of organic molecules.