Morphology-Driven Photocatalytic Behavior of Bare Cu₂O for Efficient CO₂-to-Methanol Conversion under Solar Illumination

The continued accumulation of atmospheric carbon dioxide (CO₂) remains a critical driver of climate change, necessitating the urgent development of technologies that not only capture CO₂ but also convert it into value-added chemicals. One promising strategy is the direct photocatalytic transformation of CO₂ into solar fuels and platform chemicals using abundant semiconductor materials. However, the success of this approach hinges not only on catalytic activity but also on selectivity, favoring the formation of specific products to eliminate costly downstream separation. The pursuit of efficient photocatalytic materials for carbon dioxide (CO₂) reduction is central to sustainable energy innovation. Copper(I) oxide (Cu₂O), a visible-light-responsive semiconductor, is attracting renewed interest for its ability to drive the selective production of C₂+ products—particularly methanol, a key energy carrier and feedstock for green fuels. In this study, we report the photocatalytic performance of structurally distinct, bare Cu₂O particles synthesized via a glucose-assisted reductive precipitation method. By modulating reaction conditions, we obtained Cu₂O with varied morphologies and crystallite sizes, which were systematically characterized to establish structure–property relationships. Photocatalytic experiments conducted under simulated sunlight revealed that spherical Cu₂O structures exhibited enhanced methanol selectivity, while smaller particles consistently delivered higher overall activity. These trends were attributed to improved charge carrier separation, increased surface area, and favorable facet exposure. This work underscores the potential of unmodified Cu₂O as a cost-effective photocatalyst for CO₂ valorization. Importantly, the findings demonstrate how rational control over catalyst morphology can be leveraged to guide selectivity toward methanol under mild, solar-driven conditions, supporting the development of low-carbon pathways to liquid solar fuels.