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Highly efficient photocatalytic H₂ evolution from water using visible light and structure-controlled graphitic carbon nitride.

Martin DJ, Qiu K, Shevlin SA, Handoko AD, Chen X, Guo Z, Tang J - Angew. Chem. Int. Ed. Engl. (2014)

Bottom Line: Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported.The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5% under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts.Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

View Article: PubMed Central - PubMed

Affiliation: Solar Energy Group, Department of Chemical Engineering, UCL, Torrington Place, London, WC1E 7JE (UK).

No MeSH data available.


Geometric and electronic structure of g-C3N4. a) Supercell model ofsheet carbon nitride; b) supercell model of protonated carbon nitride. Nitrogen is denoted bylight-blue spheres, carbon by red-gray spheres, and hydrogen by white spheres. c) Totaldensity of states for sheet carbon nitride (black line) and protonated carbon nitride (red dashedline). Energy is given with respect to the zero of the simulation for sheet carbon nitride. The DOSof the protonated carbon nitride has been shifted so that the corresponding zero points align.
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fig03: Geometric and electronic structure of g-C3N4. a) Supercell model ofsheet carbon nitride; b) supercell model of protonated carbon nitride. Nitrogen is denoted bylight-blue spheres, carbon by red-gray spheres, and hydrogen by white spheres. c) Totaldensity of states for sheet carbon nitride (black line) and protonated carbon nitride (red dashedline). Energy is given with respect to the zero of the simulation for sheet carbon nitride. The DOSof the protonated carbon nitride has been shifted so that the corresponding zero points align.

Mentions: To determine exactly why polymerization and protonation status influencesH2-production rates, we modeled protonation by DFT simulations using periodic supercells.Time-dependent DFT (TDDFT) simulations were performed on cluster models to determine the effects ofhydrogen on excited-state properties. The density of states (DOS) is shown in Figure 3 c. It can be clearly seen that the conduction-band edge(CBE) of the protonated system is shifted down in energy (towards more positive values with respectto the normal hydrogen electrode, NHE) by 0.34 eV. This shift significantly modifies theelectrochemical properties, as it provides a lower overpotential for reduction reactions, as alsoshown in the UV/Vis absorption spectra (see Figure S2 c). The reason behind the dropin the position of the CBE can be clearly seen in the site-decomposed DOS (see discussion in theSupporting Information and Figure S12). The effects of protonation on excited-stateproperties of a molecular model were also calculated. The lowest energy vibrationally stablestructure involves strong distortions from planarity of all three heptazine rings. The onset ofoptical absorption on protonated g-C3N4 occurs at a lower energy (morepositive with respect to the NHE) than for deprotonated g-C3N4. Indeed, twoabsorption peaks of the C18N28H13 model occur at lower energiesthan that of the initial absorption peak of the C18N28H12 model, inqualitative agreement with the DFT DOS in Figure 3.This result verifies our DFT-based electronic-structure analysis with TDDFT.


Highly efficient photocatalytic H₂ evolution from water using visible light and structure-controlled graphitic carbon nitride.

Martin DJ, Qiu K, Shevlin SA, Handoko AD, Chen X, Guo Z, Tang J - Angew. Chem. Int. Ed. Engl. (2014)

Geometric and electronic structure of g-C3N4. a) Supercell model ofsheet carbon nitride; b) supercell model of protonated carbon nitride. Nitrogen is denoted bylight-blue spheres, carbon by red-gray spheres, and hydrogen by white spheres. c) Totaldensity of states for sheet carbon nitride (black line) and protonated carbon nitride (red dashedline). Energy is given with respect to the zero of the simulation for sheet carbon nitride. The DOSof the protonated carbon nitride has been shifted so that the corresponding zero points align.
© Copyright Policy - open-access
Related In: Results  -  Collection

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fig03: Geometric and electronic structure of g-C3N4. a) Supercell model ofsheet carbon nitride; b) supercell model of protonated carbon nitride. Nitrogen is denoted bylight-blue spheres, carbon by red-gray spheres, and hydrogen by white spheres. c) Totaldensity of states for sheet carbon nitride (black line) and protonated carbon nitride (red dashedline). Energy is given with respect to the zero of the simulation for sheet carbon nitride. The DOSof the protonated carbon nitride has been shifted so that the corresponding zero points align.
Mentions: To determine exactly why polymerization and protonation status influencesH2-production rates, we modeled protonation by DFT simulations using periodic supercells.Time-dependent DFT (TDDFT) simulations were performed on cluster models to determine the effects ofhydrogen on excited-state properties. The density of states (DOS) is shown in Figure 3 c. It can be clearly seen that the conduction-band edge(CBE) of the protonated system is shifted down in energy (towards more positive values with respectto the normal hydrogen electrode, NHE) by 0.34 eV. This shift significantly modifies theelectrochemical properties, as it provides a lower overpotential for reduction reactions, as alsoshown in the UV/Vis absorption spectra (see Figure S2 c). The reason behind the dropin the position of the CBE can be clearly seen in the site-decomposed DOS (see discussion in theSupporting Information and Figure S12). The effects of protonation on excited-stateproperties of a molecular model were also calculated. The lowest energy vibrationally stablestructure involves strong distortions from planarity of all three heptazine rings. The onset ofoptical absorption on protonated g-C3N4 occurs at a lower energy (morepositive with respect to the NHE) than for deprotonated g-C3N4. Indeed, twoabsorption peaks of the C18N28H13 model occur at lower energiesthan that of the initial absorption peak of the C18N28H12 model, inqualitative agreement with the DFT DOS in Figure 3.This result verifies our DFT-based electronic-structure analysis with TDDFT.

Bottom Line: Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported.The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5% under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts.Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

View Article: PubMed Central - PubMed

Affiliation: Solar Energy Group, Department of Chemical Engineering, UCL, Torrington Place, London, WC1E 7JE (UK).

No MeSH data available.