Photocatalytic Water Splitting

Introduction

Hydrogen is the most attractive energy source for its high energy density and environmental benign, which is an ideal strategy to replace the current non-renewable fossil fuels. Currently, hydrogen is produced in industries via coal gasification, cryogenic distillation, steam reforming, and electro or photocatalytic water splitting. Among the available techniques, hydrogen evolution by photochemical water splitting has been regarded as the most promising approach to produce hydrogen, because it can directly convert the free and abundant solar energy into hydrogen energy. Inorganic semiconductor nanostructures often used as photocatalysts for hydrogen production, however, the hydrogen evolution rate, photostability and quantum efficiencies are too low for practical application, limiting the commercialization of the inorganic nanostructure-based water splitting process. Due to the structural diversity, MOFs and COFs have good light absorption ability, adjustable band gap position and good photostability. These advantages make MOFs and COFs expected to become a highly efficient catalytic agent in the field of photocatalytic hydrogen production.

Application of MOFs in Photocatalytic Water Splitting

A variety of MOFs materials are used for photocatalytic hydrogen production, which can be mainly divided into the following several types^[1]:

• Noble metal-based MOFs as photocatalysts: Ru-MOFs as a photocatalyst was taken the lead using in hydrogen evolution, after that, several noble metal-based MOFs such as Rh, Pt, Au and Pa have been developed, which exhibit a high hydrogen evolution rate.
• Nobel metal-free MOFs as photocatalysts: Several noble metal-free MOFs such as Ti, Cd, Cu, Co, Ni, In, and Ln MOFs are used in hydrogen production with the advantages of cheap and accessible.
• MOFs supported nanocomposites as photocatalysts: The effective strategies that fabrication of nanocomposites with the combination of other visible light semiconductors (CdS and TiO₂), metal complexes (Co, Ni, and Fe-Fe complexes), or conductive 2D nanostructures such as graphene, g-C₃N₄, or MoS₂ and the formation of heterojunctions between MOFs and other guest materials are generating a monumental amount of reporting, which exhibit excellent light harvesting capability and charge transportation property.
• MOFs derived nanostructures as photocatalysts: By utilizing MOFs as sacrificial templates to synthesize hollow nanostructures with high surface areas. These nanostructures can improve the transport property of the charge carriers between the semiconductor nanostructures and carbon layers, thereby enhancing the photocatalytic performances.

Fig.1 Brief description of the overall content of MOFs-based photocatalysts

Application of COFs in Photocatalytic Water Splitting

At present, photocatalytic hydrogen production by COFs mainly includes the following three systems: COFs as photocatalyst, COFs heterojunction as photocatalyst and COFs composition as photocatalyst.

• COFs as photocatalyst: COFs with different linkage such as hydrazone linkage, azine linkage, enamine linkage and triazine linkage COFs to create a series of COFs as photocatalysts for visible light driven hydrogen production. All the COFs presented rather high ability for the photocatalysis. And, very recently, the diacetylene and thiophene series COFs with high conjugation, narrow band gap and high charge mobility have been developed, showing efficient photocatalytic hydrogen production.
• COFs heterojunction photocatalyst: Heterojunction catalyst was formed by introducing donor and acceptor into COFs layer or composite COFs with other materials to form heterojunction catalyst. These two strategies can effectively adjust the band structure of photocatalyst and the separation ability of photogenerated electrons and holes, thus improving potocatalytic efficiency.
• COFs composition photocatalyst: Banerjee et al. synthesized ketoenamine linked COFs (TPPA-2 COF) with high conjugation and high specific surface area and high stability. Subsequently, CdS nanoparticles were uniformly dispersed in the synthesized TPPA-2 COF to form a composite photocatalyst. The hydrogen production rate of CdS-COF composite photocatalyst is very high, up to 3678 μmol/h/g^[2].

Mechanism of MOFs and COFs in Photocatalytic Water Splitting

The photocatalytic hydrogen production by MOFs and COFs can be briefly describe as follows: When the MOFs and COFs absorbed visible-light energy, electrons are excited to the conduction band (CB), while holes are present in the valence band (VB). The electrons present in the CB combine with protons to generate hydrogen, and the holes combine with reductive reactants to create oxidative products^[3]. Fig. 2 describes the schematic diagram of photocatalysis of hydrogen generation using MOFs and COFs.

Fig.2 The schematic diagram of photocatalytic hydrogen production using MOFs and COFs

What Can Alfa Chemistry Do

Alfa Chemistry provides various MOFs and COFs with high-activity for use in photocatalytic hydrogen production. And our professional technology team also provides customers with professional, high-quality MOFs and COFs design and customization services, no matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in photocatalytic hydrogen production by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.