K⁺ defect engineering in carbon-based geopolymer microspheres for photocatalytic degradation of tetracycline

Targeted tuning of defect density in carbon-based photocatalysts is a key strategy to improve their photocatalytic performance. However, the current methods for intrinsic carbon defect sites construction are commonly with harsh conditions and costly materials, it is significant to develop more greener and cost-effective methods. In addition, the structure-activity relationship between the structural defects of material and photocatalytic performance was ambiguous up to now. Herein, a K+-assisted synthetic strategy was developed to synthesize porous carbon-based geopolymer micros (rpsBC-GPs2) with abundant intrinsic defects using papermaking sludge as the precursor. And a mechanistic insight was given for explaining why the K+-induced defect density tuning made rpsBC-GPs2 exhibit high photocatalytic activity. The introduction of K+ changed the crystal structure of Quartz, calcite and pyrrhotite on the catalyst surface and promote the generation of lattice defects. The abundant lattice defects and sp(3) carbon defects of rpsBC-GPs2 were evidenced with X-ray Photoelectron Spectroscopy, X-ray Diffraction, and scanning transmission electron microscopy measurements. In the photocatalytic process, the photogenerated electrons was driven to capture dissolved oxygen and form center dot O-2, because the conduction position of rpsBC-GPs2 was higher than the potential of O-2/O-2(-) (-0.33 eV). In addition, the high-conductive biochar play a role of "electron shuttle" in the photocatalytic process, which promoted the electrons transfer from CB to center dot O-2. Therefore, the production efficiency of reactive center dot O-2(-) was improved, and promoted the aromatization reaction process of tetracycline (TC). In this study, an industrial waste paper sludge was utilized to synthesize the active photocatalyst, and a new strategy was proposed for the photocatalytic performance evaluation, through targeted tuning of defect density in carbon-based photocatalysts using K+ as activator.