Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO₂, WO₃ and ZnO)

Highlights

• TiO₂, WO₃ and ZnO based photocatalysis is reviewed.

• Advances to improve the efficiency are emphasized.

• Differences and similarities in the modifications are highlighted.

• Charge carrier dynamics for each strategy are discussed.

Abstract

Metal oxide semiconductors (TiO₂, WO₃ and ZnO) finds unparalleled opportunity in wastewater purification under UV/visible light, largely encouraged by their divergent admirable features like stability, non-toxicity, ease of preparation, suitable band edge positions and facile generation of active oxygen species in the aqueous medium. However, the perennial failings of these photocatalysts emanates from the stumbling blocks like rapid charge carrier recombination and meager visible light response. In this review, tailoring the surface-bulk electronic structure through the calibrated and veritable approaches such as impurity doping, deposition with noble metals, sensitizing with other compounds (dyes, polymers, inorganic complexes and simple chelating ligands), hydrogenation process (annealing under hydrogen atmosphere), electronic integration with other semiconductors, modifying with carbon nanostructures, designing with exposed facets and tailoring with hierarchical morphologies to overcome their critical drawbacks are summarized. Taking into account the materials intrinsic properties, the pros and cons together with similarities and striking differences for each strategy in specific to TiO₂, WO₃ & ZnO are highlighted. These subtlety enunciates the primacy for improving the structure-electronic properties of metal oxides and credence to its fore in the practical applications. Future research must focus on comparing the performances of ZnO, TiO₂ and WO₃ in parallel to get insight into their photocatalytic behaviors. Such comparisons not only reveal the changed surface-electronic structure upon various modifications, but also shed light on charge carrier dynamics, free radical generation, structural stability and compatibility for photocatalytic reactions. It is envisioned that these cardinal tactics have profound implications and can be replicated to other semiconductor photocatalysts like CeO₂, In₂O₃, Bi₂O₃, Fe₂O₃, BiVO₄, AgX, BiOX (X = Cl, Br & I), Bi₂WO₆, Bi₂MoO₆, etc., to improve their competence for various environmental applications.

Introduction

Semiconductor spirited heterogeneous photocatalysis using various functional nanomaterials is the extensively investigated technique for the degradation of several recalcitrant compounds in the aqueous medium and gaseous phase under UV/visible light. Along with the energetically featured quintessential TiO₂ [1], [2], [3], [4], ZnO and WO₃ also brags some prospect in wastewater purification, as the former absorbs more photons from the incident light source and latter gesticulated with visible light absorption respectively [5], [6], [7]. The important features like non-toxicity, chemical stability, redox potential of charge carriers, compatible growth over various supports and suitable electronic band structures boost its relevance in the photocatalytic process. These metal oxides can be obtained with diverse morphologies using variety of precursors like metal foils, metal acetates, metal salts, metal alkoxides, etc. Unfortunately, scaling the wastewater treatment process using these nanomaterials towards industrialization is still a debacle as they are conflated with their own adversaries; (i) high band gap of TiO₂ and ZnO cannot be activated under solar light; (ii) ZnO is vulnerable to dissolution and corrosion at acidic and alkaline pH respectively; (iii) ZnO and WO₃ undergoes photocorrosion in the course of extended light illuminating conditions; (iv) absorption edge of WO₃ is very narrow and fails to utilize the photons from the major portion of solar spectrum. In addition, ineffective charge carrier separation as a consequence of shorter carrier lifetime is the ultimate crisis for these oxides, thus constituting an impasse in achieving the desired performance.

In a quest to develop these oxides for many green energy applications, research is intensified to overcome these bottlenecks and momentous advancements are attained over the years. The critical analysis of the literature from the recent past perhaps indicated that selective blue-print approaches like doping with impurities, noble metal deposition, sensitizing with narrow bandgap absorption materials and hydrogenation process (annealing under hydrogen atmosphere) are bestowed for enabling visible light absorption, while other strategies such as heterstructuring with other semiconductors, integrating with carbon nanostructures, designing with exposed reactive facets and hierarchical morphologies are mainly concentrated to improve structural stability and charge carrier separation kinetics. Fortunately, these are the breakthrough amendments that can be implanted to promote the photocatalytic performance of every semiconducting photocatalysts. Many insightful review articles related to these materials are currently available in the open literature [1], [2], [3], [4], [5], [6], [7], but are sparse and does not enlighten for the comprehensive information for the comparison among TiO₂, WO₃ and ZnO. In this review, generalized approach and relevant modifications for advancing the performance of these metal oxides are articulated to have a broader knowledge on the area of materials chemistry interfacing with the photocatalysis. The similarities and salient differences observed in the materials behavior of these metal oxides in specific to the modification adopted are emphasized, besides outlining the associated charge carrier dynamics and subsequent effect on photocatalytic reactions.