H₂ and O₂ Evolution from Water Half-Splitting Reactions by Graphitic Carbon Nitride Materials

Abstract

Graphitic carbon nitride compounds were prepared by thermal treatment of C−N−H precursor mixtures (melamine C 3 N 6 H 9 , dicyandiamide C 2 N 4 H 4 ). This led to solids based on polymerized heptazine or triazine ring units linked by −N or −NH− groups. The H content decreased, and the C/ N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 °C due to increased layer condensation. The UV−vis spectra exhibited a strong π−π* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600−650 °C showed an additional absorption near 500 nm that is assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H 2 evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h −1 H 2 evolution with UV−vis irradiation, but this dropped to ∼0.23 μmol h −1 when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H 2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H 2 production rates (31.5 μmol h −1 ) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O 2 evolution was determined for RuO 2 -coated samples using the 300 W lamp with aqueous AgNO 3 solution as the sacrificial agent. The materials all showed production rates ∼9 μmol h −1 . A highly crystalline compound containing polytriazine structural units ((C 3 N 3 ) 2 (NH) 3 •LiCl) prepared in this study did not show measurable photocatalytic activity.

Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

“…S1). 44 Lower synthesis temperatures of 450 and 390 • C result in two crystalline materials (Fig. S2), mainly composed of melem and its oligomers, such as dimelem and trimelem, as suggested by elemental analysis (Table S1) …”

Carbon nitrides: synthesis and characterization of a new class of functional materials

“…6), 77 with that of a sample prepared from urea (CH 4 N 2 O) heated to 550 1C in air 78 ( Fig. 7 and 8d-f).…”

“…130 The semiconducting bandgaps of gCNs are determined by p-p* transitions of the heterocyclic aromatic constituents between around 2.5-2.8 eV, leading to optical absorption beginning in the violet-blue range of the visible spectrum. 2,77 This causes the yellow/brown coloration typically exhibited by gCN materials, 77 while also resulting in their poor electronic conductivity. 14 The optical absorption color evolves from pale yellow to deep brown as the degree of polymer condensation increases with cross-linking and loss of terminal -NH 2 components.…”

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

“…However, such papers generally suggest ways of improving the optical gap without considering the effect on the thermodynamic driving force for solar fuel production. Calculation of the optical gap and the overall optical absorption spectrum can also provide insight in the dominant structural elements of an (amorphous) photocatalyst, for example, to understand [36] the difference between differently prepared carbon nitride photocatalysts [21,26].…”