Synthesis of vacant graphitic carbon nitride in argon atmosphere and its utilization for photocatalytic hydrogen generation

Abstract

Graphitic carbon nitride (C₃N₄) was synthesised from melamine at 550 °C for 4 h in the argon atmosphere and then was reheated for 1-3 h at 500 °C in argon. Two band gaps of 2.04 eV and 2.47 eV were observed in all the synthetized materials. Based on the results of elemental and photoluminescence analyses, the lower band gap was found to be caused by the formation of vacancies. Specific surface areas of the synthetized materials were 15-18 m²g^-1 indicating that no thermal exfoliation occurred. The photocatalytic activity of these materials was tested for hydrogen generation. The best photocatalyst showed 3 times higher performance (1547 μmol/g) than bulk C₃N₄ synthetized in the air (547 μmol/g). This higher activity was explained by the presence of carbon (V_C) and nitrogen (V_N) vacancies grouped in their big complexes 2V_C + 2V_N (observed by positron annihilation spectroscopy). The effect of an inert gas on the synthesis of C₃N₄ was demonstrated using Graham´s law of ammonia diffusion. This study showed that the synthesis of C₃N₄ from nitrogen-rich precursors in the argon atmosphere led to the formation of vacancy complexes beneficial for hydrogen generation, which was not referred so far.

Figure 1

UV–Vis reflectance spectra of CN and CN-Ar materials.

Figure 2

PL spectra of CN and CN-Ar materials.

Figure 3

Photocurrents of CN and CN-Ar materials recorded at 1 V versus Ag/AgCl in deoxygenated 0.1 mol L⁻¹ KNO₃.

Figure 4

XRD patterns of CN and CN-Ar materials.

Figure 5

FTIR spectra of CN and CN-Ar materials (left) and spectrum of CN-Ar0 (right).

Figure 6

XPS spectra of CN and CN-Ar0 materials.

Figure 7

XPS N 1s spectra of C₃N₄ synthesised in air and argon. (A) CN, (B) CN-3, (C) CN-Ar0, and (D) CN-Ar3.

Figure 8

Pore distribution plots of CN and CN-Ar materials.

Figure 9

SME (BSE + SE) micrographs of CN (left) and CN-Ar0 (right).

Figure 10

Energy diagram of conduction and valence band potentials of CN and CN-Ar materials at pH = 7.

Figure 11

Time dependence of hydrogen yields during the photocatalytic hydrogen generation from water–methanol mixture in the presence of CN, CN-Ar materials and TiO₂ at irradiation of 254 nm.

Figure 12

Amount of produced H₂, CH₄* and CO* yields after 4 h of irradiation in the presence CN, CN-Ar materials and TiO₂ (*multiplied by 10).

Figure 13

Calculated positron lifetimes for perfect C₃N₄ lattice (labelled C₃N₄) and for different types of point defects: carbon vacancies (V_C), nitrogen vacancies (V_N) and their complexes (nV_C + mV_N).

Figure 14

Dependence of lifetimes and intensities of individual components on annealing time in argon. Horizontal dashed lines show calculated lifetimes for different types of defects.

Figure 15

Dependence of lifetimes and intensities of individual components on annealing time in air. Horizontal dashed lines show calculated lifetimes for different types of defects.