New materials with high photocatalytic activity in the visible range are essential for the development of commercially viable technologies. Of particular interests are metal organic framework (MOF) compounds, mesoporous materials (aluminosilicates), and polyoxometa – lates (POM). Despite being in an early stage of research, these materials already demonstrate comparable or greater efficiency in the degradation of organic compounds than does TiO2 [88].
Metal organic frameworks are two-dimensional or three-dimensional crystalline porous materials consisting of metal ions or clusters coordinated to rigid organic ligands. The photocatalytic activity of MOF compounds is due to their semiconductor-like properties with band gaps between 1.0 and 5.5 eV [89]. Compared to classical photocatalysts such as TiO2, MOFs preserve a variety of pore sizes with a maximum surface area of approximately 6000 m2 g-1 [90]. Such a large surface area is highly desirable, as this increases the probability for adsorption of reactants and carrier trapping on the catalyst surface. Another advantage of MOFs is that their electronic structures can be manipulated by bridging distinct chemical constituents with unique organic linkers to achieve synergistic photocatalytic activities.
In addition to MOFs, another structural motif known as polyoxometalates (POMs) represents an emerging class of materials that appears promising for photocatalytic applications. POMs are a group of molecular clusters, which consists of three or more transition metal oxyanions (usually group 5 or group 6 transition metals) linked together by shared oxygen atoms to form a large, yet enclosed, three-dimensional framework. Some typical POM structures include:
i. Keggin [XM12O40]"-
ii. Wells-Dawson [X2M18O62]”-
iii. Anderson [XM6O24]”-
iv. Lindqvist [M6O19]”- structures, where X is the heteroatom (P5+,Si4+, B3+), and M is usually Mo, V, or W [88].
POMs have well-defined structures with a typical size of a few nanometers. They are very stable and can be easily deposited onto organic substrates for catalytic reaction via the formation of a pre-associated complex. POMs represent an attractive class of photoactive materials due to their highly tunable band gap, which can be tailored by adjusting the size or composition of the metal oxide particulates [91]. In general, decreasing the size of the particulates will increase the band gap, and allow for precise modulation of electronic structure to achieve tunable photocatalytic properties across the UV to near-IR spectrum.
Two other good candidates filling the above requirement are graphitic carbon nitride sheets and graphene nanoribbons [92]. They both have a two-dimensional rigid structure which is slightly different from the traditional conjugated polymers. They both exhibit significant stability under acidic, basic, and high temperature conditions, and their band structures can be tuned by adjusting their size and edge structures. Although studies on both are still in early stages due to the fact that they are still relatively new materials, this field is growing rapidly because of the interest generated by these favorable properties. Polyoxometalate/ carbon nanotubes show remarkably improved efficiency and operational stability [93]. This result opens new avenues for innovative materials designed for efficiency and cost optimization of photocatalysis.
