Carbon-Induced Crystallization of SiO ₂ at Low Temperature: N-Doped Carbon-Modified α-Cristobalite as a Photocatalyst for CO ₂ Conversion

The crystallization of amorphous silica into ordered polymorphs generally requires harsh temperature and pressure conditions, which limit its functional applications. Here, we describe a low-temperature crystallization method of α-cristobalite nanostructures induced by N-doped carbon, which is an effective visible-light photocatalyst for CO2 conversion. The incorporation of thiourea into an acidic sodium trisilicate precursor enables simultaneous nitrogen doping and carbon integration, thereby triggering phase transformation at significantly reduced temperatures. The structural and physicochemical analyses confirm the formation of α-cristobalite nanostructure and elucidate the role of carbon during the crystallization and defect generation process. In contrast to amorphous SiO2, the presence of N-doped carbon reduces the band gap, favorably uplifts the conduction and valence band, suppresses deep trap states and improves charge separation through the introduction of conductive Si–C/N states. As a result, the catalyst exhibits photocatalytic activity for CO2-epoxide cycloaddition, affording cyclic carbonate yields of 69–73% under blue-light irradiation (455 nm) with a cocatalyst loading of 5 mol %, outperforming both dark and thermally driven reactions. Investigations of the electronic properties of α-cristobalite surface and the carbon-containing α-cristobalite surface using periodic density functional theory (DFT) calculations explained the experimental observation of band gap reduction triggered by the presence of carbon material. In addition, a plausible reaction mechanism was shown using optoelectronic characteristics and a sacrificial agent. Fundamentally, this work suggests the great potential of N-doped carbon as an inducer in the synthesis of crystalline SiO2 at low temperature, as well as enhanced photocatalytic properties.