Interlayer Palladium-Single-Atom-Coordinated Cyano-Group-Rich Graphitic Carbon Nitride for Enhanced Photocatalytic Hydrogen Production Performance

Abstract

To improve the photocatalytic hydrogen evolution activity of palladium-assisted graphitic carbon nitride (g-C3N4), here, palladium-single-atom-coordinated cyano-group-rich g-C3N4 (Pd/DN-UCN) are synthesized, and the synthesis process includes copolymerization of urea-derived supramolecular aggregates and NH4Cl followed by wet impregnation. By combining powerful characteristic results and theoretical calculations, the formation mechanism of Pd single atoms on the ultrathin, mesoporous cyano-group-rich g-C3N4 nanosheets is proposed, highlighting that the Pd single atoms are firmly stabilized in the interlayers of g-C3N4 nanosheets caused by the combination of the physical confinement effect of ultrathin, mesoporous g-C3N4 nanosheets and coordination bonding of cyano groups with Pd atoms; additionally, Pd–N3 coordination in the Pd/DN-UCN heterojunctions is confirmed, in which one Pd atom coordinates with one N atom of the cyano group and two sp2-hybridized N atoms in the adjacent layer. The presence of cyano groups and Pd–N coordination in the Pd/DN-UCN induces a midgap state in the band structure of g-C3N4. At optimal Pd loading levels (0.16%), the synthesized 0.16%Pd/DN-UCN0.50 exhibits enhanced photocatalytic hydrogen production activity as compared to electrostatically stabilized Pd single atoms on the “sixfold cavities” of g-C3N4, and apparent quantum yield values at the stationary point of the 0.16%Pd/DN-UCN0.50 concentration (1.2 g L–1) can reach up to 14.6, 15.8, 4.69, and 3.05% under monochromatic light irradiation at 365, 400, 450, and 550 nm, respectively. The cooperation of significantly boosted transfer of photoexcited electrons to atomically dispersed Pd sites via as-built interlayer Pd–N coordination delivery channels and the maximal Pd atom utilization efficiency dominates the enhanced photocatalytic hydrogen evolution activity of Pd/DN-UCN.

Cyanamide-defect-induced built-in electric field in crystalline carbon nitride for enhanced visible to near-infrared light photocatalytic activity

“…Compared with KCCN, the KCCN-cya samples exhibit clearly enhanced EPR intensity, which indicates more lone electrons provided by the introduced cyano groups, as well as better delocalization of the electrons on the aromatic heptazine ring. 8,43 Moreover, the enhanced photocurrent intensity of KSCN 1.0 indicates the more efficient photoexcitation response in the KCCN-cya materials (Fig. S15 †).…”

“…Additionally, since the cyanamide groups act as a good ligand for Pt 4+ , the coordination of Pt particles with electron-accepting character would consequently lead to high-efficiency carrier migration and the decreased decay lifetime. 8 Next, linear sweep voltammetry (LSV) measurements were conducted to investigate the charge recombination and evaluate the H 2 generation ability. As shown in Fig.…”

“…This highly accumulated distribution in the cyano-rich region contributes to the generation of potential differences and alteration of the electronic structure, resulting in the generation of BIEF. 8,12,30 As a result, this well-separated distribution could facilitate electron-hole separation and ultimately enhance the hydrogen evolution. In addition, the electron localization function (ELF) result shows clearly localized charge density on the cyano region in KCCN-cya, which further confirms the accumulated distribution in the cyano-introduced structure, giving rise to the generation of the BIEF (Fig.…”

“…Hydrogen evolution photocatalytic water splitting is believed to be a promising way to meet these demands. 1 To achieve a high hydrogen efficiency, several materials and/or structures have been reported, such as superior (in)organic semiconductors, 2 heterostructure constructions, [3][4][5] organic framework materials, 6,7 single-atom-site catalysts, 8,9 ionic 2D materials, 10 materials with micro-structural optimization, [11][12][13][14][15] etc. The reported systems have high efficiency in hydrogen generation from water, but most of them are high-cost, hazar-dous and/or highly operationally demanding; thus, there are still bottlenecks to their practical applications.…”

“…Furthermore, the formed cyanamide terminals could be a better ligand for cocatalysts such as Pt compared to the primary amine and N atoms of CN. 8,30 Crystalline carbon nitride (CCN), a recently reported CN material synthesized in the presence of a molten salt or single salt, showed more efficient carrier transfer kinetics than CN. 16,[30][31][32] CCN usually has obvious crystal plane properties, which makes it more advantageous for BIEF generation than amorphous CN.…”