Z-scheme SnO2−x/g-C3N4 composite as an efficient photocatalyst for dye degradation and photocatalytic CO2 reduction

• Novel Z-scheme SnO2−x/g-C3N4 composites are prepared and tested.
• SnO2−x/g-C3N4 composite degrades RhB 8.8 times faster than g-C3N4 under visible light.
• SnO2−x/g-C3N4 composites also show high activity in photocatalytic CO2 reduction.
• The Z-scheme mechanism is verified by reactive species trapping experiment.

Highly efficient SnO2−x/g-C3N4 composite photocatalysts were synthesized using simple calcination of g-C3N4 and Sn6O4(OH)4. The synthesized composite exhibited excellent photocatalytic performance for rhodamine B (RhB) degradation under visible light irradiation. The optimal RhB degradation rate of the composite was 0.088 min−1, which was 8.8 times higher than that of g-C3N4. The SnO2−x/g-C3N4 composite also showed high photocatalytic activity for CO2 reduction and photodegradation of other organic compounds. Various techniques including Brunauer–Emmett–Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL) and an electrochemical method were applied to determine the origin of the enhanced photoactivity of SnO2−x/g-C3N4. Results indicated that the introduction of SnO2−x on g-C3N4 increased its surface area and enhanced light absorption performance. More importantly, a hetero-junction structure was formed between SnO2−x and g-C3N4, which efficiently promoted the separation of electron–hole pairs by a direct Z-scheme mechanism to enhance the photocatalytic activity. This study might represent an important step for the conversion of solar energy using cost-efficient materials.