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

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.