Mechanism of Charge Accumulation of Poly(heptazine imide)

In recent years, carbon nitride polymers have attracted enormous interest studied as inexpensive and easy-to-synthesize photocatalysts. Moreover, carbon nitride polymers has been actively studied because it can generate hydrogen through photocatalytic activity in visible light region around 450 nm unlike TiO₂, which can be activated in ultraviolet light. Through many pioneering work on carbon nitride polymers, several types of new carbon nitrides have been discovered. In particular, poly(heptazine imide) (PHI) is one of the newly carbon nitride polymers, which structure has two-dimensionally polymerized carbon nitride skeleton. Unlike other carbon nitride polymers, PHI has attracted attention not only as a materials that shows high photocatalytic performance in visible light but also shows dark photocatalytic activity. Dark photocatalysis is a function in which the charges generated by light irradiation are stored inside the material and consumed in the dark state, and Lau <i>et al.</i> were firstly reported about dark photocatalytic performance of PHI in 2017. Combining the photocatalytic and dark photocatalytic activities of PHI makes it possible to continue generating hydrogen day and night, which cycle is called day-night photocatalysis. If day-night photocatalysis has realized, it is expected to significantly improve the efficiency of hydrogen collection, which has been an important issue for photocatalysts.<br/>Dark photocatalytic activity has also been reported for materials such as TiO₂+WO₃, and it is generally believed that carriers generated by light irradiation accumulate on the surface and exhibit catalytic activity even in the dark state through a state called charge accumulation. In short, it is important to investigate the charge accumulation in detail in order to improve the function of dark photocatalytic activity. For example, studying the charge accumulation phenomenon and improving the amount of stored charge will lead to increase the evolution of hydrogen by dark photocatalytic activity, and also allow the activity to be maintained for longer periods of time in the dark. However, the dark photocatalytic activity of PHI occurs only in solution, making it difficult to analyze changes in electronic and chemical structure, for this reason the mechanism of charge accumulation in PHI has not been discussed from a microscopic perspective.<br/>In this study, we succeeded in solving such a problem by making it possible to take PHI out of the test tube in the form of a gel. The PHI gel (PHIG) can be obtained by mixing PHI and a substance called an ionic liquid, which has an extremely low vapor pressure and a very high viscosity. PHIG has made it possible to carry out many experiments to explore the properties of PHI, including experiments in vacuum such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/ energy dispersive X-ray spectroscopy (SEM/EDX). In our study, we found two basic features of PHI, photochromism, which is reversibly changes of molecular structure by light irradiation, and light-driven ad/absorption of potassium ions, which is a component of PHI. On the other hand, as mentioned above, the charge accumulation phenomenon of PHI can be observed by photoirradiation in PHI. Therefore, we hypothesized that the charge accumulation may be caused by the two characteristics of PHI, and devised a model to explain the phenomenon. More specifically, we considered that the charge accumulation in PHI is caused by three steps: (1) light irradiation generates photocarriers in PHI, (2) photochromism causes narrowing of the energy gap, and (3) the narrowed energy gap acts as a trap and photocarriers are trapped in it. In this presentation, we will discuss the relationship between photochromism, ab/adsorption of potassium ions, and charge accumulation, both of which originate from photoirradiation in PHI and we propose a new model of charge accumulation in PHI based on the results.