Development of Cu-Doped ZIF-8/g-C3N4 Photocatalyst and Hydrogen Generation from Water Splitting
Authors: Suda Koshiro , Nagata Morio 
Published: December 2020

Energy and environmental issues are serious problems and early solution is desired. In particular, the impact of using fossil fuels is significant. Although it is somewhat affected by technological innovation, their usable years tend to decrease. In addition, greenhouse gases such as CO2 emitted from their use have greatly contributed to environmental problems. Meanwhile, the demand for renewable and alternative energy as a clean energy source is increasing. Hydrogen is used as an energy source for fuel cells, and is a substance that attracts attention because of its portability, no emission of carbon dioxide after use, and its high energy density. However, the current steam supply method, which is a hydrogen supply method, has a problem that a large amount of CO2 is simultaneously emitted when hydrogen is generated. Therefore, it is strongly desired to establish another technology capable of supplying hydrogen stably and cleanly.
Photocatalysts are famous materials that attract attention from a hydrogen supply perspective. They are used as materials for artificial photosynthesis, and artificial photosynthesis is an effective method for energy and environmental issues due to the following advantages; using water that is abundant on the earth as a raw material, using sunlight that infinitely falls on the earth as a supply energy source and emitting no gas that is harmful to the environment. However, photocatalysts have a serious drawback of poor hydrogen generation efficiency, and many researchers are working to solve this problem for practical use. However, researches on highly efficient photocatalysts often use expensive metals such as Rh complexes and Pt cocatalysts, or sulfides containing highly toxic and unstable Cd. From an environmental and biological point of view, these solutions are problematic in the long term use of photocatalysts, and the development of safe and stable photocatalysts while generating hydrogen with high efficiency is desired.
Metal-organic Frameworks (MOFs) are also attracting materials as new porous materials applied to drug delivery, catalysis, sensors, etc. MOFs have unique structures formed by coordinating central metals and organic linkers and have photocatalytic functions because of its band structures. MOFs have been actively studied in the photocatalyst field due to large surface area and property of easily modifying structure. In many MOFs, Zeolitic Imidazole Framework-8 (ZIF-8) has high alcohol adsorption and UV-excited photocatalytic function i.e. band gap energy = 5.1 eV. Since ZIF-8 has properties that photocatalysts should have, such as heat resistance and pH resistance, there is great interest in improving its wide band gap.
In this study, we improve the photocatalytic efficiency by Cu doping and composite with g-C3N4. g-C3N4 is one of the popular visible-light-driven photocatalysts and we introduce this material for the purpose of supplementing the absorption band near 480 nm where ZIF-8 could not absorb. Although simple g-C3N4 has a shortcoming of low hydrogen generation efficiency due to fast charge recombination, we also aim to reduce the amount of bulk g-C3N4 by combining and dispersing it using the porosity of ZIF-8. Cu-doped ZIF-8 carrying Cu is synthesized by partially changing Zn, which is the raw material of ZIF-8, to Cu. g-C3N4 is gained by a simple method of calcining melamine in air. The sample of Cu-doped ZIF-8 and g-C3N4 composite, named XCZGC (X is value of prepared Cu weight percentage) is synthesized by dispersing in water at a weight ratio of 1: 1 and drying overnight. 40CZGC shows 2.5 and 16 times higher efficient (32.2 μmolH2 g-1 h-1 and 213.3 μmolH2 g-1 h-1, respectively) by Cu doping and combining as-Cu-doped ZIF-8 with g-C3N4. The former improves efficiency due to the small amount of Cu2+ dope and the deposition of metallic Cu on the surface of ZIF-8, and the latter improves due to the formation of Z-scheme type charge separation structure with g-C3N4. The structure and absorption light are analyzed by XRD and UV-vis DRS, and the added amount of copper is measured by XPS and ICP-OES. The hydrogen generation experiment is performed in a closed circulation system, and changes over time are observed under 300 W Xe lamp irradiation.
Through this study, we propose a novel Z-scheme-type composite photocatalyst using Cu-doped ZIF-8. The above is just an expectation considered from action spectrum experiment since the transient absorption spectrum could not be measured at the time of submission. Therefore, the near goal is to confirm the Z-Scheme structure with analyzing the transient absorption spectrum. As a long goal, we consider further improvement in efficiency by using alcohol as a sacrificial reagent.

Figure 1