Influence of crystal structure of polymorphic cotton cellulose on the adsorption and photocatalysis properties of biochar-TiO₂ composites

Cellulose derived biochar can be used for adsorbent and photocatalyst mainly because of its good compatibility, high hydrophilicity, and the large amount of electron-rich hydroxy groups. Nevertheless, the impact of the crystal structure of cellulose on the absorption and photocatalytic efficiency of biochar-based composites derived from cellulose remains uncertain. Herein, four different types of biochar derived from cotton (cellulose I_β, II, III, and IV) were individually impregnated with a TiO₂ catalyst through hydrothermal and pyrolysis processes, and were analyzed using various characterization methods. Their adsorption behavior and photocatalytic activities were compared using Congo red and methylene blue dye as the model. The analysis and test outcomes suggested that the crystal structure of cotton cellulose impacted the pore structure and TiO₂ content of biochar-TiO₂ composites to different degrees, resulting in variations in the adsorption and photocatalytic capabilities of biochar-TiO₂ composites. In comparison with cellulose II, III and IV, cellulose I derived biochar-TiO₂ composite had a large specific surface area, a more stable structure, a high aromatic carbon content, and a high TiO₂ loading, resulting in the strong adsorption ability and superior photoactivity to organic dyes. The adsorption and photocatalysis mechanisms were also clarified.