Comparative Studies of g-C3N4 and C3N3S3 Organic Semiconductors—Synthesis, Properties, and Application in the Catalytic Oxygen Reduction

Abstract

Exfoliated g-C3N4 is a well-known semiconductor utilized in heterogenous photocatalysis and water splitting. An improvement in light harvesting and separation of photogenerated charge carriers may be obtained by polymer doping with sulfur. In this work, we incorporate sulfur into the polymer chain by chemical polymerization of trithiocyanuric acid (C3N3S3H3) to obtain C3N3S3. The XRD measurements and TEM images indicated that C3N3S3, in contrast to g-C3N4, does not exist in the form of a graphitic structure and is not exfoliated into thin lamellas. However, both polymers have similar optical properties and positions of the conduction and valence bands. The comparative studies of electrochemical oxygen reduction and hydrogen evolution indicated that the overpotentials for the two processes were smaller for C3N3S3 than for g-C3N4. The RDE experiments in the oxygen-saturated solutions of 0.1 M NaOH have shown that O2 is electrochemically reduced via the serial pathway with two electrons involved in the first step. The spectroscopic experiments using NBT demonstrated that both polymers reveal high activity in the photocatalytic reduction of oxygen to superoxide anion radical by the photogenerated electrons.

The Role of Boron Dopant in the Improvement of Electron Transfer in g-C3N4 Photocatalyst

“…In order to determine the band diagrams of the samples, which are of crucial importance for prediction of their photocatalytic activity, the upper edges of the valence bands of the polymers (vs standard hydrogen electrode, SHE) were determined from VB-XPS spectra. As visible in Figure c, the low binding energy arm of the spectrum of g-C 3 N 4 consists of two segments of different slopes, which lead to two different crossing points with the background line, at 2.1 and 1.5 eV. , The VB energy obtained from the second value, 1.56 V vs SHE, conforms with the values determined by other groups from VB-XPS spectra (1.53 V), , and DFT calculations (1.57 V), ,, and therefore, it was taken by us to construct the band diagram of g-C 3 N 4 , presented in Figure d.…”

“…As visible in Figure 5c, the low binding energy arm of the spectrum of g-C 3 N 4 consists of two segments of different slopes, which lead to two different crossing points with the background line, at 2.1 and 1.5 eV. 31,59 The VB energy obtained from the second value, 1.56 V vs SHE, conforms with the values determined by other groups from VB-XPS spectra (1.53 V), 22,60 and DFT calculations (1.57 V), 1,2,61 and therefore, it was taken by us to construct the band diagram of g-C 3 N 4 , presented in Figure 5d.…”

“…Graphitic carbon nitride was obtained by thermal condensation of urea, according to the procedure described elsewhere . Mixtures of g-C 3 N 4 and NaBH 4 at different weight ratios (g-C 3 N 4 /NaBH 4 , 5:1, 4:1, 3:1 and 2.5:1) were ground in a mortar for 30 min.…”

“…Graphitic carbon nitride was obtained by thermal condensation of urea, according to the procedure described elsewhere. 31 Mixtures of g-C 3 N 4 and NaBH 4 at different weight ratios (g-C 3 N 4 /NaBH 4 , 5:1, 4:1, 3:1 and 2.5:1) were ground in a mortar for 30 min. Then, the mixtures were placed in ceramic combustion boats in a tube furnace, heated to 500 °C with a ramp rate 5 °C/min, and kept for 1 h under Ar atmosphere.…”

“…The electrochemical and EIS measurements were performed with Autolab PGSTAT302N (Metrohm, The Netherlands) equipped with a FRA module in a Teflon electrochemical cell with a quartz window. 31 The impedance measurements were done in the frequency range 100 kHz to 0.1 Hz at the modulation signal amplitude of 10 mV.…”