Influence of High Temperature Synthesis on the Structure of Graphitic Carbon Nitride and Its Hydrogen Generation Ability.

Emilia Alwin,Kamila Kočí,Michał Zieliński,Robert Wojcieszak,Mariusz Pietrowski,Miroslava Edelmannová

doi:10.3390/ma13122756

Abstract

Graphitic carbon nitride (g-C3N4) was obtained by thermal polymerization of dicyandiamide, thiourea or melamine at high temperatures (550 and 600 °C), using different heating rates (2 or 10 °C min−1) and synthesis times (0 or 4 h). The effects of the synthesis conditions and type of the precursor on the efficiency of g-C3N4 were studied. The most efficient was the synthesis from dicyandiamide, 53%, while the efficiency in the process of synthesis from melamine and thiourea were much smaller, 26% and 11%, respectively. On the basis of the results provided by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–vis), thermogravimetric analysis (TGA), elemental analysis (EA), the best precursor and the optimum conditions of synthesis of g-C3N4 were identified to get the product of the most stable structure, the highest degree of ordering and condensation of structure and finally the highest photocatalytic activity. It was found that as the proton concentration decreased and the degree of condensation increased, the hydrogen yields during the photocatalytic decomposition of water–methanol solution were significantly enhanced. The generation of hydrogen was 1200 µmol g−1 and the selectivity towards hydrogen of more than 98%.

Highlights

IntroductionGraphitic carbon nitride (g-C3 N4 ) is the organic polymer, based on heptazine (tri-s-triazine) building blocks
Graphitic carbon nitride (g-C3 N4 ) is the organic polymer, based on heptazine building blocks
Preparation of g-C3 N4 g-C3 N4 was prepared by the pyrolysis of dicyandiamide (Sigma-Aldrich, Darmstadt, Germany, 99%), thiourea (Sigma-Aldrich, 99%), and melamine (Sigma-Aldrich, 99%) in a semi-closed system according to a defined procedure

Summary

Introduction

Graphitic carbon nitride (g-C3 N4 ) is the organic polymer, based on heptazine (tri-s-triazine) building blocks. Heptazine tectons are linked into chains of the melon type through the C−NH−C bonds or into layers of fully condensed carbon nitride through the C−N(C)−N bonds (Figure 1). Both forms, melon and fully condensed g-C3 N4 , make graphite-like layered structures. G-C3 N4 is a semiconductor characterized by the band gap of ~2.7 eV [1,2,3] Due to this property, g-C3 N4 can be active in many photocatalytic reactions with the use of visible light [3,4,5,6,7], and as a heterogeneous catalyst [8,9] and due to its layered structure and high nitrogen content as a flame retardant [10,11,12]. Attempts have been made to increase its specific surface area (SSA) using the hard template method [11,16,17,18,19,20,21] and to reduce the energy

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Conclusion