Synthesis And Photoelectrocatalytic Properties Of A Metal-Free Material: Ultrathin Two-Dimensional Graphitic Carbon Nitride Nanosheets

Nowadays, energy crisis and environmental pollution are the two major challenges that limits the sustainable development of human society so much that the development of clean new energy technologies have become an urgent event. Since the advantages of solar energy such as rich in resources, renewable, widely available, environmentally friendly etc. and the superiority of hydrogen as clean, efficient, storageable, transportable ideal pollution free renewable energy carrier, hydrogen production through solar-driven water reduction is an ideal way of storage and utilization of solar energy. Due to the key role of semiconductor material in absorption and transformation of solar energy, the development of economic and efficient photocatalyst materials is the main approach among solar energy conversion and utilization technologies.Ti O2 and other traditional semiconductor catalysts still have a series of problems, such as poor visible light utilization efficiency, light corrosion, low catalyst stability, etc. which limit the practical application of semiconductor photocatalysts. Graphitic carbon nitride(g-C3N4), a new type of non-metal polymer semiconductor, which possesses a two-dimensional(2D) graphite-like layered structure and unique physicochemical properties is a perspective photoactive material working with visible light for hydrogen production owing to its small band gap of 2.7e V. However, the photocatalytic performance of bulk g-C3N4 is still unsatisfied due to its small specific surface area, poor liquid phase dispersion, low charge mobility and high recombination of charge carriers. Besides, among all the modified strategies based on bulk g-C3N4, such as nano-structure modification, dopping other elements into the matrix or coupling with other components, copolymerization is the widely used method which lead to ineffective and uncontrollable composites. In this work, surface functionalization by exfoliation and protonation based on bulk g-C3N4 is used to modify the morphology, enlarge the specific surface area, improve the liquid dispersion and band gap electronic properties, and further enhance its photoelectric catalytic properties.The main research content and results of this paper is divided into three parts:① The bulk g-C3N4 powders(B-g-C3N4) were synthesized by pyrolysis condensation of organic precursors melamine and then the 2D g-C3N4 nanosheets(E-g-C3N4) were prepared by the liquid exfoliation of bulk g-C3N4 through sonication treatment. The protonated 2D g-C3N4 nanosheets(P-g-C3N4) can be easily protonated by concentrated hydrochloric acid treatment. Ultrasonic liquid exfoliation and protonation treatment based on bulk g-C3N4 is proved an effective approach to prepare of ultrathin nanosheets, while the as- prepared nanosheets are tend to reagglomeration due to the van der Waals force between layers which reduce the exfoliated availability. Protonated g-C3N4 nanosheets(2≈4 nm in thickness) possessed positive polarity in contrast with the bulk one, which leads to the same charged surface repulsing the intermolecular forces to keep a stable and uniform distribution in liquid.②The effection of exfoliation and protonation on morphology, surface condition, band gap and charge mobility of the photocatalysts were characterized systematically. The photocatalysts electrode were prepared by drop coating, and then its photoelectric catalytic properties were characterized through electrochemical methods. Compared to the bulk g-C3N4, ultrathin nanosheets structure of E-g-C3N4 and P-g-C3N4 can provide larger specific surface area and more reaction active sites. The charge of the P-g-C3N4 surface turn to be positive and the absolute electric potential is increased which is the main reason for the homogeneous dispersion in liquid phase. The band gap is enlarged and simultaneously valence band and conduction band both exhibit positive potential shift after treatment. Ultrathin g-C3N4 nanosheets perform a better charge separation and migration property, which decreases the combination of photoinduced electron-hole pairs in the materials and greatly promotes photoelectric catalytic properties.③The mechanism of relationship between photocatalysts structure affected by different processing treatments and photoelectric catalytic properties of g-C3N4 was investigated. The 2D ultrathin structure of the nanosheets after exfoliation and protonation is beneficial to the migration of the photoinduced carriers, greatly improving the material conductivity, which is the main reason for the enhancement of the photoelectric catalytic activity. In particular, the protonated surface of P-g-C3N4 can not only keeps the stability of the 2D ultrathin structure, but also be favorable for the catalytic reaction of hydrogen production in kinetics.In this paper, the research can provide favorable reference for the work of g-C3N4 photocatalysis materials design, preparation and characterization, especially for the self-assembly of g-C3N4 2D nanosheets and a potential application for preparation of g-C3N4 based composites via counteranion exchange.