Lanthanide ions doped ZnO based photocatalysts

Highlights

• Lanthanide ion doped ZnO photocatalyst for dye degradation is reviewed.

• Structure-optical properties of Ln-ZnO is critically discussed.

• Reaction parameters influencing the efficacy is highlighted.

• Co-doped ZnO associated with lanthanide ion is emphasized.

Abstract

ZnO is a versatile semiconductor that finds prominence in the area of photocatalysis due to their compatible crystal structure and band gap edge potentials that are vital for the pollutant degradation reaction. Thus, fabrication of ZnO based nanomaterials with tailored properties remains as the trending topic in recent times. In this context, doping with lanthanide ion finds interesting because of their unique 4f electronic configuration which offers stupendous opportunity to modulate the optical, luminescent and surface acid-base properties of ZnO. This article provides concise overview about the influence of lanthanide ions dopant on the structure-optical properties of ZnO. The performance of Ln-ZnO towards the degradation of organic pollutants is discussed encompassing both the materials properties and photocatalytic reaction conditions under UV/visible and solar light. The co-doping of Ln-ZnO with non-metal/transition metal ion and modification of Ln-ZnO with carbon materials is also emphasized to underscore the recent advancements in this field. We anticipate that our review article provides adequate information into the rational design and development of Ln-ZnO for various applications.

Introduction

Semiconductor mediated photocatalytic degradation of recalcitrant compounds represents an environmental and lucrative approach due to their capacity to function under the diverse reaction conditions. Among the various semiconductors such as TiO₂, SnO₂, WO₃, Fe₂O₃, Nb₂O₅ and Bi₂O₃ that are illustrated for air and water purification, water disinfection, and hazardous waste remediation, ZnO remains as unparalleled and potential substitute for these materials because of its unique functionalities like significant optical transmittance, high exciton binding energy (60 meV), direct band gap, suitable band edge potentials, low toxicity, cheaply available precursors, low temperature for crystallization, high refractive index, plethora of morphological features, thermal stability and unique optoelectronic and piezoelectric properties [1], [2], [3], [4], [5], [6], [7]. The band gap excitation mechanism of both ZnO and TiO₂ follows the similar mechanism and ZnO absorbs superior fraction of UV light accompanied with intense light energy per quanta compared to TiO₂. In addition, ZnO exhibits higher electron mobility and longer lifetime for photoexcited electrons compared to TiO₂, which is an added advantage for many reduction pathways [8]. The ZnO also produces hydroxyl radicals, superoxide radicals, singlet oxygen and H₂O₂ over its surface under the light illuminating conditions [1], [2], [3], [4], [5], [6], [7]. Due to these admirable considerations, ZnO is often reputed as the most reliable material to replace the benchmark TiO₂ for the degradation of various pollutants [9], [10], [11]. Despite of these striking features, the obstacles such as wide band gap, rapid recombination of photogenerated charge carriers, chemical instability under extreme pH environment and photocorrosion susceptibility associated with ZnO have forestalled the achievable efficiency. Thus, research towards the fabrication of visible light active ZnO based nanomaterials has triggered immense interest due to their vast abundance in the solar energy spectrum. The strategies such as dye sensitization, impurity doping, noble metal deposition and coupling with other semiconductors have been extensively investigated over the years to surmount the aforementioned drawbacks [1], [2], [3], [4], [5], [6], [7]. The dye sensitization cannot be practically employed for the pollutant degradation process as complete mineralization is very hard to achieve from this approach. The noble metal deposition requires high cost for its precursor itself and subsequent oxidation of deposited metal by the photogenerated VB holes posture problem to the structural stability [12], [13]. Conversely, designing the ZnO based composites is always multistep approach and performance is highly dependent on the interface properties [14]. In addition, impurity diffusion among the integrated components might severely modify the chemical composition as well as optical properties of the composite which may not always contribute to the required performance [15]. In this direction, extrinsic doping of ZnO with foreign ions at low concentration appears to be the promising avenue as they not only expand the light absorption capacities, but also increase the conductivity and photoconductivity of the doped ZnO. Besides, they also serve as shallow carrier trap states for the excitons and temporarily suppress them from the recombination pathways [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26].

The previously published review articles on the doped ZnO furnish the information on the morphological evolution, defect chemistry, magnetic, luminescent and photocatalytic properties [1], [2], [3], [4], [5], [6], [7], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Only a few reviews have focused on the photocatalytic properties and antibacterial activity of the doped ZnO [1], [2], [3], [4], [5], [6], [7], [16], [18], [20], [25]. However, concise review article dealing with the specific role of lanthanide ions doping on the resultant band gap response and photocatalytic activity of ZnO have received less attention. To fill this gap, this review intends to focus on the degradation of various organic compounds with Ln-ZnO under the UV/visible and solar light. The choice of lanthanide ions dopant and various wet-chemical approaches to obtain Ln-ZnO is discussed under the light of different reaction environment. The doping effects on the structure-optical properties of pristine ZnO encountered for various lanthanide ions and their relationship with the photocatalytic performances is presented. The further advancements in the modification of Ln-ZnO through co-doping and integrating with carbon materials with relevance to photocatalytic applications are also underscored. Finally, gap analysis in this research area and future prospects are presented to pave the way for developing the efficient Ln-ZnO for the treatment of wastewater.