Highly efficient photocatalytic H<inf>2</inf> evolution from water using visible light and structure-controlled graphitic carbon nitride

Abstract

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4exhibits an extraordinary hydrogen-evolution rate (ca. 20 000μmol h-1g-1under full arc), which leads to a high turnover number (TON) of over 641 after 6h. The reaction proceeds for more than 30h 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-C3N4photocatalysts. 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. A recipe for success: Graphitic carbon nitride exhibited an internal quantum yield of 26.5 at 400nm when prepared by a specific tailored recipe. The activity was shown to be inversely proportional to the protonation status at specific nitrogen sites in heptazine units (see picture; N blue, C gray, H white). Theoretical results indicated that protonation significantly influences reductive power and charge migration to active sites. © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.