Post-Calcined Carbon Nitride Nanosheets as an Efficient Photocatalyst for Hydrogen Production under Visible Light Irradiation Mohammad Reza Gholipour, † Francois Be ́ land, ‡ and Trong-On Do* ,† † Department of Chemical Engineering, Laval University, Que ́ bec City, Qué bec G1V 0A8, Canada ‡ SiliCycle Inc., 2500, Boul. du Parc-Technologique, Que ́ bec City, Que ́ bec G1P 4S6, Canada ABSTRACT: Hydrogen production via photocatalytic water splitting using sunlight has enormous potential to solve the worldwide energy and environmental crisis. The key challenge in this process is to develop efficient photocatalysts which must satisfy several criteria such as strong sunlight absorption, effective charge separation, and high photochemical stability. Graphitic carbon nitride is one of the best semiconductors for hydrogen evolution because of its conduction band edge, narrow band gap, and high chemical stability. However, it produces a small amount of hydrogen under visible light irradiation due to its small surface area and high recombination rates. In this work, nanosheets of graphitic carbon nitride with carbon vacancies and nanoholes were synthesized by a two-step treatment process (argon treatment followed by air calcination). These post-calcined carbon nitride nanosheets exhibited much higher photocatalytic activity compared to common graphitic carbon nitride. By depositing platinum as a cocatalyst via a photodeposition method, this semiconductor showed noticeable improvement in hydrogen production rate at 10 times that of graphitic carbon nitride. Its hydrogen evolution rate was 5261 μmol h -1 g -1 under visible light illumination with a quantum efficiency of 29.2% at 400 nm and 21.3% at 420 nm. This high amount of hydrogen production rate could be due to large specific surface area, an extension of visible light absorption tail-end, and lower charge recombination centers throughout the semiconductor. In addition, by a recalcination step in air, some defects are introduced into the structure of carbon nitride nanosheets owing to carbon vacancies. These defects are considered to be highly active photocatalytic sites for hydrogen production. KEYWORDS: Photocatalyst hydrogen production, Visible light photocatalyst, Graphitic carbon nitride, High quantum efficiency photocatalyst ■ INTRODUCTION Development of technology requires a cheap and accessible source of energy. Although fossil fuels are the most well-known sources of energy for their low cost and availability, they have some important issues for humans such as emission of a high amount of carbon dioxide into the atmosphere which is believed to be the main reason for the greenhouse effect and climate change. Others claim that fossil fuel resources are limited and cannot be recovered once they are used. Therefore, researchers have tried to find other alternatives for fossil fuels, and the best option is solar energy. Solar energy is abundant, and only a very small amount of it can provide all the energy demands of humanity around the world for one year. 1,2 Moreover, this source of energy is renewable and sustainable, which means there is no way to overconsume it in the present or future. Interestingly, sunlight energy is environmentally friendly, and it does not produce any harmful gases; as a result, we will not have any climate crisis in the future. 2 However, using this source of energy is quite expensive, and so it is very difficult to utilize it in large scale applications. Hydrogen molecules can act as an energy carrier in order to store solar energy and use it later. One of the most promising ways to produce hydrogen energy is to split water into Received: June 10, 2016 Revised: November 8, 2016 Published: November 14, 2016 Research Article pubs.acs.org/journal/ascecg © 2016 American Chemical Society 213 DOI: 10.1021/acssuschemeng.6b01282 ACS Sustainable Chem. Eng. 2017, 5, 213-220