Polyoxometalate-based molecular/nano composites: Advances in environmental remediation by photocatalysis and biomimetic approaches to solar energy conversion

Abstract

Polyoxometalates (POMs) have peculiar optoelectronic properties and high reduction potential, playing as excellent electron pools. Thus, during the recent decade, POMs have been increasingly recognized as important building blocks for highly efficient photocatalysts and photoelectrochemical devices by hybridizing with photofunctional semiconductor nanostructures and organic/inorganic molecules. In this review, development of various molecular and nano composites derived from POMs are discussed with regard to photocatalytic environmental remediation, hydrogen production, carbon dioxide reduction and photoelectrochemical solar cells. The mechanisms involved in photo-induced interfacial electron transfer processes and subsequent photochemical reactions are explored along with a brief description about their advances in emerging solar application areas. More fundamental information of the photocatalytic activities of the POM-based composites would be very useful in constructing next generation artificial photosynthetic systems with higher spectral response in visible region for efficient solar energy conversion into electricity and fuels.

Introduction

The earth has been supported by nature with most fundamental requirements of life such as food, water, clean air and energy. These natural features are the subjects of many fields of scientific research for deliberate improvement of life on the earth. Particularly, for the past few decades, environment pollution and global energy demand have been more persistent due to the increased rate of human population. Hence much attention has been devoted to development of new technologies for both environmental remediation and clean energy production in an economically viable way. Among the many technologies for the environmentally clean energy production, solar energy utilization techniques have been attracting the greatest attention because of their low cost and easy fabrication techniques. In the development of solar energy utilization technologies, efficient solar energy conversion systems in which solar energy is efficiently used to generate sustainable electrons and holes triggering the reduction and oxidation (redox) reactions [1], [2], are vitally prerequisite. More particularly, an artificial photosynthetic system mimicking the plant photosynthetic system has been recognized as the most promising solar energy conversion system to generate high energy fuels such as hydrogen and hydrocarbons through photocatalytic water splitting and CO₂ reduction [3], [4]. This is due to the well sustained photoinduced electrons from two photosystems due to their cooperative electron transfer process (Z-scheme process), and it becomes increasingly evident that construction of the two-color photoreaction system composites would be the most useful in a wide range of environmental applications such as photocatalysis, solar cells and generation of solar fuels.

In this context, polyoxometalates (POMs), complex early-transition metal–oxygen clusters have been attracting much attention as excellent candidates as one of two photosystem because they have photoelctrochemical activities as electron pools originated from their unique structures [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Examples of some fundamental polyoxometalate structures are shown in Fig. 1, exhibiting Lindqvist, Keggin, Dawson and Anderson structures. The Lindqvist ion is an iso-polyoxometalate, and the other threes are hetero-polyoxometalates. The Keggin and Dawson structures have tetrahedrally coordinated hetero-atoms, such as phosphorous or silicon while Anderson structure has an octahedral central atom, such as aluminum.

Hence their other fundamental properties like elemental composition, solubility, redox potential, charge density, size and shape can be systematically altered to a considerable degree. Particularly, it is interesting that the reduction potential (or oxidizing ability) of the POMs decreases linearly with decreasing valence of the central atom or increasing negative charge of the heteropolyanions, as shown in the following series, PW₁₂³⁻ > GeW₁₂⁴⁻ > SiW₁₂⁴⁻ > FeW₁₂⁵⁻ > BW₁₂⁵⁻ > CoW₁₂⁶⁻ > CuW₁₂⁷⁻ [16], [17]. Therefore, various types of POMs have been designed for their application in advanced materials such as photo and electrochromic devices, analytical chemistry, medicine and materials science [18], [19] with their peculiar optoelectronic properties. For an example, the photocatalytic reactions by POMs have been applied in organic transformations such as the oxidation of alcohol to aldehydes or ketones [20], [21], the functionalization of alkanes to form alkenes or ketones [22] and dimerization of alkenes [23] with their mechanism studies catalytic behaviours of POMs [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34].

As for the application in material science, the POM-photoassisted preparations of metal nanoparticles have been attracting much attention in the field of nanotechnology [35] as Li et al. [36], [37] and Bamoharram [38] reported the fabrication of ZnO nanostructures with various morphologies in the presence of polyoxometalate. The preparation of organoimido-substituted derivatives of POM was also reported to offer valuable building blocks for the construction of nanostructured organic–inorganic hybrid materials [39] useful for fascinating developments of new advanced functional materials. In this regard, porphyrin and perylene monoimide covalently attached to POM demonstrated formation of relatively long-lived charge-transfer state under light illumination [40], [41], [42], and the POM composites can be a key feature for developing artificial photosynthetic system. The composites derived from the POM and cationic metallo-porphyrins were reported to reduce metal ions into metal nanoparticles under visible light irradiation, suggesting that photoinduced electron transfer takes place from porphyrin units to POM [43], [44], [45].

Intrinsically, the POMs alone can behave as photocatalysts by the photoexcitation of the oxygen-to-metal charge transfer (O → M CT) bands of POM to separate the electron–hole pair to be used for the reductive and oxidative reactions with surrounding molecules [46]. Although POMs exhibit good photocatalytic activity in homogenous systems [47], [48], [49], [50], [51], [52], the main disadvantage for practical application lies in their low surface area and difficulty of separation from the reaction mixture due to its high solubility in water. Thus, for practical applications, it is desirable to develop a heterogeneous photocatalytic system by immobilizing POM in various supports to make it in a more recoverable form. However, in solid state, the catalytic activity of POM is low owing to their low specific surface area (<10 m²/g) [53]. Hence, considerable interests have been focused on the coupling of POM with many supports such as carbon, silica, titania, zirconia and mixed oxides for the past few decades in diverse application areas. Also precipitation with large counter cations like cesium results in insoluble POMs which has high surface area compared to their parents [54], [55], [56], [57]. These high surface area-supports facilitate the stabilization of the Keggin units of the POMs, providing the acid sites to result in the improved catalytic performances of the POMs with thermal stabilities. Therefore, nowadays, fabrications of novel solid POM-based hybrid materials became interesting targets to develop highly efficient photocatalysts. Further, the photofunctional inorganic/organic nanocomposites incorporated with POMs have been reported to display some of the key feature characteristics of the artificial photosynthetic systems.

Hence, in the light of the interesting photochemical activities of POM-based molecular/nano composites, we would like to give an overview of their contribution in environmental remediation and solar energy conversion processes.