Abstract
In the last decades photocatalysis has become one of the most employed technologies for the implementation of the so-called Advanced Oxidation Processes (AOPs) for the removal of harmful pollutants from wastewaters. The materials identified as the best photocatalysts are transition metal oxides, in which the band structure allows charge carrier separation upon solar irradiation. The photoinduced charge carrier can thus cause oxidative and reductive redox reactions at the surface, inducing the formation of the radical species able to initiate the AOPs. Despite the great advantages of this process (non-toxic, cheap and environmentally clean), the main drawback lies in the fact that the most efficient semiconductors are only able to absorb UV irradiation, which accounts for only 5% of the total solar irradiation at the Earth’s surface and not enough to generate the required amount of electron-hole pairs. On the other hand, many efforts have been devoted to the sensitization of wide band gap transition metal oxides to visible light, which represents a higher percentage (almost 45%) in the solar electromagnetic spectrum. Among all the strategies to sensitize transition metal oxides to visible irradiation, doping with lanthanides has been less explored. In this regard, lanthanides offer a unique electronic configuration, consisting in 4f orbitals shielded by a 5s5p external shell. This occurrence, coupled with the different occupation of the localized 4f orbitals would provide an astounding opportunity to tune these materials’ properties. In this review we will focus in depth on the modification of two promising photocatalytic transition metal oxides, namely ZnO and ZrO2, with cerium, europium and erbium atoms. The aim of the work is to provide a comprehensive overview of the influence of lanthanides on the structural, optical and electronic properties of the modified materials, emphasizing the effect of the different 4f orbital occupation in the three considered doping atoms. Moreover, a large portion of the discussion will be devoted to the structural-properties relationships evidencing the improved light absorption working mechanism of each system and the resulting enhanced photocatalytic performance in the abatement of contaminants in aqueous environments.
Highlights
Water pollution arising from anthropogenic industrial contamination is one of the major challenges with which humanity will have to cope in the 21st century
In this mini-review we have focused our attention on the wide band gap transition metal oxides ZnO and ZrO2 and their modifications when doped with cerium, europium and erbium, respectively
The introduction of Eu ions, with larger ionic radius, would cause a high lattice strain and large inhomogeneity in the lattice inducing smaller crystal size and higher amount of surface defects, among which oxygen vacancies represent the main ones. These lasts, coupled with the redox coupled of Eu2+/3+ ions would guarantee a higher surface activity for the photocatalytic processes and an improved charge carrier separation, respectively, since the photoexcited electrons would be trapped in the 4f orbitals, located in the forbidden gap of ZnO
Summary
Water pollution arising from anthropogenic industrial contamination is one of the major challenges with which humanity will have to cope in the 21st century. In what concerns the band gap size, ZnO can only absorb in the UV range, with the absorption edge peaking around 380 nm, while ZrO2, being an insulator, can absorb in the ultrahigh UV range, at almost 250 nm In this regard, the UV radiation on the Earth surface accounts for less than the 5% of the total solar irradiation, too little to efficiently photo-activate these materials. Being strong Lewis acids, they can favour the adsorption of oxygen, a vital species as electron scavenger at the material surface and successively for the production of peroxide species that can effectively contribute to the contaminant destruction [61,64,69] In this mini-review we have focused our attention on the wide band gap transition metal oxides ZnO and ZrO2 and their modifications when doped with cerium, europium and erbium, respectively.
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