Dual enhancement design of charge separation and CO2 adsorption by Na and amination treatment in g-C3N4 for efficient CO2 photoreduction

The high electron-hole recombination rate and low CO2 adsorption ability restrain the CO2 photoreduction performance of graphitic carbon nitride (g-C3N4, CN). This study designed a Na-doped and amino-loaded g-C3N4 (CN-NN) to synergistically improve the charge separation and CO2 adsorption ability of CN. The CN-NN was fabricated by thermal polymerization of melamine with NaCl and NH4Cl. CN-NN presented a CO yield of 7.17 mu mol g-1 h- 1, 4.29 times of CN. The CO yield of CN-NN is also much higher than the sum of those of single Nadoped and amino-loaded g-C3N4. This significant improvement was primarily due to the synergistical effect of Na doping and amino loading. Na doping broadened the light absorbance region and improved the charge separation and migration efficiency. Based on Na doping, amino loading further reduced the charge transfer resistance, and enhanced the adsorption of CO2. In-situ DRIFTS tests showed that CN-NN transferred CO2 to CO through sequential intermediates of CO2*, COOH*, and CO*, and CN-NN presented stronger activation of CO2 and production of COOH* than CN. Further DFT calculations indicated that CN-NN owned a more discrete distribution of the valance band maximum (VBM) and the conduction band minimum (CBM), and the product CO molecules were more easily to desorb from CN-NN than from CN. These further promoted the electron-holes separation and improved the CO production. This study proposes a novel strategy of dual treatment by Na and amination to synergistically enhance the CO2 photoreduction performance of g-C3N4.