Piezoelectric materials for catalytic/photocatalytic removal of pollutants: Recent advances and outlook

Highlights

• photocatalytic oxidation of organic contaminants is reviewed in the present study.

• Ferroelectric, pyroelectric and piezoelectric effects of photocatalytic materials was exploited.

• Due to non-centrosymmetric nature, the piezoelectric materials showed catalytic properties.

• Applications of piezoelectric materials in environmental remediation are reviewed.

Abstract

The accumulation of various contaminants in air, soil and water is threatening the natural environment. The remediation of the environmental contaminations is today an urge. Among the remediation methods employed, advanced oxidation processes (AOPs) are a class of techniques based on the in situ generation of highly reactive and oxidizing radical species which can destroy most of the organic pollutants. AOPs driven by light are found to be the most popular for wastewater treatment due to the abundance of solar light in some regions. The removal of organic contaminants using semiconductor-based photocatalysts has been extensively investigated. However, low charge carrier mobility and rapid electron-hole pair recombination are the common problems that limit the semiconductor-based photocatalysis. Although a large number of alternative systems have been investigated, electron–hole pair separation is still too low in photocatalytic systems. A new concept was introduced recently in which the built-in electric field by ferroelectric, pyroelectric and piezoelectric effects in photocatalytic particles was exploited to enhance the separation of photoinduced charge carriers. Among these new systems that are still under investigation, the use of piezoelectric materials in the photodegradation of pollutants recently drew a lot of attention for environmental remediation. Due to the non-centrosymmetric nature, the piezoelectric materials demonstrate unique catalytic properties as a result of the creation of the built-in electric field by the dipole polarization. This latter provides a driving force for the transport of the photoinduced charge carriers enabling their separation. This review covers the use of piezoelectric materials in photocatalysis and catalysis, especially piezoelectric-catalysis, for environmental remediation. The paper details the fundamentals and basic properties of ferroelectric, pyroelectric and piezoelectric materials. The effect of the built-in electric field in these materials on the photocatalysis/catalysis charge carrier separation is discussed. Possible applications of piezoelectric materials in environmental remediation are reviewed and discussed taking into account several different aspects such as the kinetics of the degradation of the organic pollutants and water splitting. Finally, the current research trends and future prospects of piezocatalysis and piezophotocatalysis are discussed.

Introduction

the contemporary world is facing the environmental contamination due to the rapid global industrialization leading to the deterioration of the quality of water, air and soil by the accumulated organic and inorganic toxic compounds [1,2]. These toxic compounds present hazardous properties that affect the ecosystem affecting, in turn, the human health [3,4]. Today, clean water, air and soil are of great importance for the betterment of the humankind. Among the various environmental remediation methods, the degradation of pollutants by means of photocatalytic semiconductor materials and renewable solar energy has been considered as a promising green technique for the environmental remediation [5,6].

A semiconductor to be used as a photocatalyst, it should have suitable properties such as appropriate energy positions and gap [7], excellent photostability, low recombination of electron-hole pairs, low cost, and a nontoxic nature [8]. Up to now, a vast number of photocatalytic active semiconductors such as TiO₂, ZnO, ZnS, Fe₂O₃, SnO₂, WO_3, CdS, CdSe, CuO, Nb₂O₅ and SrTiO₃ [7,[9], [10], [11], [12]] have been used as photocatalysts and excellent reviews on the environmental remediation by photocatalytic systems have been reported [[13], [14], [15], [16], [17]]. The photocatalytic reactions immensely depend on the generation as well as the separation of e-h⁺ pairs. These charge carriers are triggered by the radiation and their efficient separation is vital to participate in the reduction and/or oxidation reactions [18]. Despite many methods have been proposed to enhance the photocatalytic activities further by the band gap tuning, formation of composite semiconductors, anion doping and metal and metal ion implantation, nanostructured morphology maneuvering and introducing defects in the lattice, etc., it is still a challenging task to achieve an efficient charge separation process in photocatalytic systems. In the recent past, there is a keen interest on the use of ferroelectric and piezoelectric materials in photocatalysis to achieve better charge separation. When the piezoelectric materials with ferroelectric properties are employed or coupled with the other photocatalysts, the in-built electric field in the vicinity of piezoelectric material assists the charge separation [[19], [20], [21], [22]]. On the other hand, the “piezophotocatalysis” based on piezoelectric materials without ferroelectric or pyroelectric properties were developed recently and are yet to be explored widely [[23], [24], [25], [26]]. The charge separation process of piezophotocatalysis is resemblance to that of ferroelectric materials wherein the former, the charge separation is achieved by the in-situ generated electric field by the stress while in the latter by the permanent in-built electric field due to the polarization of the dipole [27,28]. In this review, we report and discuss the recent developments of piezophotocatalysis, especially piezophotocatalysis used for environmental remediation. In the first part, we will discuss the piezoelectric materials including pyroelectric and ferroelectric and their involvement and influence on the photocatalysis/catalysis. In the second part, we will detail the applications and recent progress of piezophotocatalysis/catalysis.

The phenomenon of piezoelectricity or the production of electricity by applying mechanical stress on to a variety of different materials was first observed by the Curie brothers in 1880 [29]. The term “piezoelectricity” is derived from the two Greek words: “Piezo” which means to squeeze or press and ‘electron’ that refers to amber, a source of electric charge. The piezoelectricity refers to the electricity created when a piezoelectric substance is deformed and hence the piezoelectric effect is a molecular phenomenon which is observed at the macroscopic level [30]. Both organic and inorganic piezoelectric materials can convert mechanical stress into electrical charges, and vice versa. The piezoelectricity origins from the non-centrosymmetric nature of the material, (i.e. crystals not having a center of symmetry in their structure), leads to electric dipoles within the material. In most crystals, the unit cell is symmetrical but not in piezoelectric crystals. Despite their asymmetric arrangement of atoms in the lattice, piezoelectric crystals are electrically neutral as a result of canceling out a positive charge by a neighboring negative charge. However, if a piezoelectric crystal is subjected to stress by squeezing or stretching, the atoms are displaced from their original position gaining a net electrical charge in the whole crystal and hence net positive and negative charges appear on the opposite and outer faces of the crystal i.e wurtzite ZnO [31,32]. The hexagonal wurtzite ZnO crystal structure lacks the center symmetry in which Zn²⁺ cations and O²⁻ anions are tetrahedrally coordinated and the centers of the positive ions and negatives ions overlap with each other and do not show spontaneous polarization. When a stress is exerted on the unit cell, the original atomic positions of O²⁻ anions and Zn²⁺ cations are relatively displaced and that would lead to gaining of a dipole moment in the unit cell (Fig. 1a). Hence, a crystal gains a piezoelectric potential due to the collective induced polarization of charges in whole unit cells in the crystal (Fig. 1b) which is the piezoelectric potential (piezopotential) [33].

The materials that possess piezoelectric properties are subcategorized into ferroelectrics, pyroelectrics and piezoelectrics [35,36]. The understanding of the symmetry of crystalline structure is important for the understanding of the origin of the piezoelectricity and hence the symmetry of element is briefly discussed in the following section. Out of the 32 crystal point groups, 21 are non-centrosymmetric and among them, 20 point groups exhibit the piezoelectric effect. Again, among these 20 non-centrosymmetric point groups, 10 belong to polar crystals i.e. crystals that possess a unique polar axis (an axis showing different properties at the two ends) (Fig. 2) [37,38]. The spontaneous polarization can occur only in materials that possess a unique polar axis and these polar crystals can be spontaneously polarized. If an electric charge is developed on the faces of the crystal perpendicular to the polar axis upon the change in temperature on these crystals, they are pyroelectric e.g., ZnO, (CH₂CF₂)_n and Pb(Zr, Ti)O₃. If the polarization along the polar axis of the polar crystals can be reversed by reversing the polarity of the electric field, such crystals are called ferroelectric (spontaneously polarized materials with reversible polarization). Hence, by default, all ferroelectric materials are simultaneously pyroelectric and piezoelectric (Fig. 2). Similarly, all pyroelectric materials are piezoelectric, but only some piezoelectric materials (those whose symmetry belongs to polar groups) are pyroelectric such as AlN and GaN [39,40] (Fig. 2).

No matter the origin (mechanical, thermal or spontaneously), the in-built electric field in materials affects the reactions taking place on their surfaces [[42], [43], [44], [45], [46], [47]]. In a non-ferroelectric/piezoelectric semiconductor (i.e. non-polar domain, e.g. TiO₂, Fig. 3a,i), free carriers are moved to the interface due to differences in local chemical potential that result in band bending when in contact with an ionic solution. Fig. 3a(ii) shows that in a ferroelectric/piezoelectric semiconductor, a spontaneous internal electric field can be generated due to the polarization of charges. These polarized charges are neutralized producing a depolarization field either by the flow of free charges within the crystal or by the absorption of ions on the polarized crystal surface by the surrounding medium [[48], [49], [50]]. The surface at which the polarization produces a positive potential is labeled as the C⁺ domain and the surface where the polarization produces a negative potential is labeled as the C⁻ domain. (Fig. 3a,iii) As illustrated in Fig. 3b, in a positive surface (C⁺ domain), a potential is dropped across the domain, such that the surface is at a lower potential than the bulk causing downward bending at the C⁺ domain. Similarly, in a negative surface (C⁻ domain), a potential is gained across the domain, such that the surface is at a higher potential than the bulk causing upward bending at the C⁻ domain leading to the accumulation of electrons and holes at C⁺ and C⁻ faces respectively. Thus, the internal electric field of a ferroelectric material exhibits a spontaneous polarization and acts like an internal p–n junction. The strength of the internal electric field determines the band bending at the interface. Henceforth, with the appropriate control of the built-in-fields, photogenerated electrons and holes can be driven to the different interfaces of the crystal to achieve efficient charge separation.

Fig. 4 shows that in the ferroelectric materials the electric field is generated spontaneously while in pyroelectric and piezoelectric materials it is generated under the thermal effect and mechanical energy respectively. One of the major challenges in photocatalysis is to achieve an efficient charge separation hindering their recombination. Therefore, the in-built electric field in piezo/pyro/ferroelectric semiconductors provides a practical strategy to improve the photocatalytic activity by effective separation of the photo-excited carriers by the internal polarization. In the following section, the piezoelectric field and its influence on the catalytic and photocatalytic environmental remediation are discussed.

In general, under the influence of the piezoelectric effect, the free carriers in the materials are separated as shown in Fig. 1, Fig. 3a (ii) and a (iii). Once these charges are accumulated at the interface, they can generate free radicals reacting with the adsorbed molecules.