A New Si/TiO₂/Pt p-n Junction Semiconductor to Demonstrate Photoelectrochemical CO₂ Conversion

This work presents a new Si/TiO₂/Pt p-n junction semiconductor prepared by sputtering, chemical vapor deposition (CVD), photolithography and lift-off techniques. XRD, EDS, FE-SEM, diffuse reflectance (DRS) and photocurrent vs potential curves had been used for semiconductor characterization. The material was designed for high porosity and uniformity of both TiO₂ and Pt deposits; both TiO₂ anatase phase formation and Pt presence were confirmed. This semiconductor has a characteristic of high light absorption in the ultraviolet and visible regions. A good photocurrent response for the cathodic region was obtained in a CO₂ saturated solution (-1.0 mA under -0.8 V and UV-vis light), confirming electron-hole pair formation and CO₂ electron scavenging. A small Si/TiO₂/Pt electrode (1 × 1 cm) was employed in photoelectrocatalytic CO₂ reduction, forming methanol (0.88 mmol L^-1), ethanol (2.60 mmol L^-1) and acetone (0.049 mmol L^-1) as products reaching a Faradaic efficiency of 96.5%. These results had been obtained under the following optimal experimental conditions: 0.1 mol L^-1 NaHCO_3, pH 8 saturated with CO₂, 125 W UV-vis irradiation (from 250 to 600 nm) and -0.8 V applied potential. Suitable charge transfer mechanisms in the electrode surface, and products formation after CO₂ reduction, are proposed.