Photoelectrochemical CO2 reduction by a p-type boron-doped g-C3N4 electrode under visible light

Graphitic carbon nitride (g-C3N4) has attracted much attention as a metal-free semiconductor having visible light absorption and relatively high chemical stability under visible light irradiation. Graphitic carbon nitride (g-C3N4) and boron-doped g-C3N4 (B-doped g-C3N4, BCNx) were prepared by heating melamine and a mixture of dicyanodiamide and BH3NH3, respectively. X-ray diffraction, a Brunauer, Emmett and Teller (BET) apparatus, and UV–vis spectra were used to analyze the physical properties of the prepared samples. Electrodes of these samples were prepared by using the electrophoresis method. X-ray photoelectron spectroscopy analyses confirmed the incorporation of boron atoms in the g-C3N4 framework as well as the amount of boron atoms. Au, Ag or Rh as a co-catalyst was coated on the surface of g-C3N4 and B-doped g-C3N4 by using the magnetron sputtering method. The photocurrent response was observed using a solar simulator as a light source. The photocurrent response of B-doped g-C3N4 was about 5-times larger than that of pure g-C3N4. B-doped g-C3N4 coated with Rh as a co-catalyst showed the highest photocurrent response under solar light irradiation, its photocurrent being about 10-times larger than that of original g-C3N4. Under photoelectrochemical conditions, we also observed the products in gas phase and aqueous phase. C2H5OH was observed as a main product, while small amounts of CO and H2 were observed in gas phase. We also discuss the relationship between co-catalysts and photocurrent responses and the carbon source of C2H5OH as a main product. The source of carbon of C2H5OH obtained by CO2 reduction is discussed on the basis of results of a labeling experiment using 13CO2.