Photoelectrochemical Reduction of Carbon Dioxide to Methanol at Hybrid System Composed of Titanium(IV) and Copper(I) Oxides

Due to gradual decline of energy resources, there has been growing interest in new energy systems that include direct transformation of solar energy to chemical energy using oxide semiconductor materials. Furthermore, carbon dioxide (CO₂), as the primary greenhouse gas also emitted through human activities, could be converted using sunlight to organic compounds. The approach permits simultaneous generation of alternative fuels and environmental remediation of carbon emissions from the continued use of conventional fuels.

The aim of this work has been to develop hybrid materials by utilizing combination of metal oxide semiconductors thus capable of effective photoelectrochemical reduction of carbon dioxide. The combination  of titanium (IV) oxide (TiO₂) and copper (I) oxide (Cu₂O) has been explored toward the reduction of carbon (IV) oxide (CO₂) before and after sunlight illumination. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from photoelectrochemical reduction of carbon dioxide mostly to methanol (CH₃OH) as demonstrated upon identification of final products.

 In particular, fabrication of the hybrid system (heterojunction photocathode) of  TiO₂ and Cu₂O (on the conducting glass) has required first the cathodic electrodeposition copper (I) oxide. The latter step has been followed by sputtering n-type titanium (IV) oxide. The spectral responses of the Cu₂O/TiO₂­ electrode is comparable to that of simple Cu₂O, and the photoresponse is enhanced at relatively short wavelengths. It is reasonable to expect that the light absorbed by the inner Cu₂O film produces energetic electron (e^-) – hole (h⁺) pairs. The excited electrons are driven through the conduction band to the external TiO₂/electrolyte interface. The role of TiO₂ is not stabilizing; the oxide is also expected to prevent the recombination of charge carriers (e^- - h⁺ pairs). By using gas chromatography with the flame ionization detector, it was confirmed that methanol predominates as a final product.