ZrO₂-Based Photocatalysts for Wastewater Treatment: From Novel Modification Strategies to Mechanistic Insights

Abstract

Zirconium dioxide (ZrO₂) has garnered substantial research interest in the field of photocatalytic water treatment due to its appealing properties, such as thermal stability, considerable physical strength, and strong chemical resistance. However, the wide bandgap energy endorses less photoabsorption and rapid charge carrier recombination kinetics, thus restricting the photoactivity of ZrO₂. Previously, vast research efforts have been made to improve the photoefficacy of ZrO₂, and hence it is worth exploring the potential strategic modifications responsible for incremented photocatalytic efficiency. In this regard, the present review article emphasizes the optical, structural, and electronic features of ZrO₂, which makes it an interesting photocatalytic material. The exceptional modification strategies that help to modulate the crystal structure, morphology, bandgap energy, and charge carrier kinetics are primarily discussed. The potential synthetic routes involving bottom-up and top-down methods are also outlined for understanding the rationale for incorporating these techniques. Moreover, the photocatalytic performance evaluation was done by investigating the photodegradation kinetics of various organic and inorganic pollutants degradation by ZrO₂. Conclusively, in light of research advances involving ZrO₂ photocatalyst, this review article may expedite further investigation for enhancing the large-scale photocatalytic applications for environmental and energy concerns.

1. Introduction

The past several decades have witnessed a tremendous rise in industrialization, agrochemical processes, and excessive exploitation of non-renewable resources, resulting in extreme deterioration of water resources [1,2]. Owing to the exclusion of these multifarious exactor pollutants from contaminated water and to abate their dangerous consequences on the ecosystem, several conventional methods for eliminating organic and inorganic pollutants have been used, including biodegradation, adsorption, coagulation, flocculation, chlorination, reverse osmosis, chemical precipitation, ion exchange, etc. [3,4]. However, these are not proven to be effective enough to eradicate these recalcitrant compounds because of their unreasonable resistance and less biodegradability [5,6,7]. Further, these approaches have several drawbacks, including high maintenance costs, significant sludge formation, limited scale application, excessive energy consumption, long processing period, and incomplete degradation of persistent pollutants [8,9,10]. Currently, numerous research efforts have addressed advanced oxidation processes (AOPs) because of their ability to generate reactive oxidative species for the effective and rapid degradation of these toxic industrial effluents to safe biodegradable mineralized salts, CO_2, and H₂O [11,12,13]. This special class of oxidation methods is environmentally benign, sustainable, and operates at feasible temperature and atmospheric pressure [14,15,16].

Photocatalysis is an interesting AOP that has been given substantial attention due to its features like effective degradation of pollutants without generating harmful by-products, cost-effectiveness, high efficiency, and the ability to operate under the illumination of solar light [17,18]. Heterogeneous photocatalysis is a photo redox reaction that proceeds with the harnessing of light by a semiconductor, specifically including the generation of e⁻/h⁺ pairs, capturing of charges at the surface, and inter-interface charge transfer [19,20]. Typically, in photocatalysis, the initiating event is the photoexcitation of electron-hole pairs (EHP) via absorption of the radiation within the bandgap energy. The photoinduced electrons and holes reduce O₂ into the superoxide radical •O₂⁻ (−0.33 V vs. SHE at pH = 7) and oxidize H₂O and OH⁻ into •OH (2.8 V vs. SCE at pH = 0) radicals for photo-oxidation reactions, respectively. [14]. The hydroxyl radicals have a short life span due to a single unpaired electron which readily attacks a series of organic wastes (e.g., dyes, insecticides, pesticides, antibiotics, etc.) that are often hard to degrade, showing rate constants typically in the order of 10⁶–10⁹ M⁻¹s⁻¹ [17,21].

Water treatment by photocatalysis using inorganic semiconductors like TiO₂, SnO₂, CuO, and ZnO, has been vastly explored owing to its non-toxic nature, optical properties, and reasonable cost [22]. For instance, Fujishima and Honda, in 1972, reported the photoelectrolysis of water using TiO₂, because of its cost-effectiveness, chemical and photochemical stability, and non-toxic nature [23]. Currently, ZrO₂ as broad bandgap (3.25–5.1 eV) semiconductor has been widely explored due to the presence of a large number of oxygen vacancies on the surface, significant ion exchange ability and physiochemical stability [23]. However, ZrO₂ possesses a high value of the dielectric constant, tensile strength, melting and boiling point, however its wide band gap and low separation rate of photogenerated electron and hole hinder the wide-scale applications [24]. To overcome this, various enhancement strategies like doping, heterojunction formation incorporation of cocatalysts, etc., have been potentially explored. For instance, Co doped ZrO₂ synthesized by facile-chemical precipitation showed greater degrading activity against MB dye when exposed to visible light due to the improved dielectric properties [25]. However, coupling ZrO₂ with narrow band semiconductors like MoS₂, TiO₂, and g-C₃N₄ also retards the photocarrier reassembly and augments its photocatalytic performance [26]. The optical properties of ZrO₂ can be enhanced by introducing new electronic states by doping with rare earth metals. For example, the introduction of Ce (IV) ions in ZrO₂ enhanced the photoresponse through the formation of midgap states and the Ce 4f empty states acted as a electron transfer bridge between VB and CB of ZrO₂ by low energy photons [27].

Recent research has manifested that zirconia-based materials show high redox abilities, large surface area, and low thermal conductivity in photocatalytic processes. In this regard, Aldeen et al. comprehensively explored the altered ZrO₂ based photocatalysts for organic pollutant degradation [24]. However, the typical insights of inorganic pollutant degradation, specifically the heavy metal ions present in waste water, are missing. Additionally, the importance of generating oxygen vacancies (OVs) in ZrO₂ for the synergistic improvement in opto-electronic properties still needs further comprehension. Hence, the present review article highlights the features of ZrO₂ semiconducting material for the photocatalytic removal of organic as well as inorganic pollutants present in wastewater. Starting from the optimal crystal structure of ZrO₂, various phases and the subsequent photocatalytic features are comprehensively reviewed. Significant methods to circumvent the inherent drawbacks of the bare ZrO₂ photocatalyst have been inclusively illustrated using different methods such as doping, heterojunction formation, morphology, and structural modulation along with the formation of OVs. After this, the review introduces facile synthesis routes involving bottom-up (solvothermal, Morphology control, and doping) and top-down methods (ultrasonication and chemical reduction) to design ZrO₂-based photocatalytic materials. Then, in order to highlight the advancement of ZrO₂ as a photocatalyst, the mechanistic insights of wastewater treatment via photocatalysis of organic and inorganic pollutants have been broadly reviewed. Lastly, the review summarizes various challenges in the field with a broad perspective to scale up the photocatalytic applications of ZrO₂ with the purpose of solving the burgeoning environmental crisis in the world.

2. ZrO₂ as Photocatalyst

ZrO₂ is an n-type semiconducting material that possesses a bandgap energy of 5.0 eV, which facilitates its application in the heterogeneous catalysis [28,29]. Moreover, the tremendous properties of ZrO₂, like thermal stability, high strength, strong chemical resistance, ductility, and low thermal conductivity makes it an alluring photocatalytic material [25]. Consequently, ZrO₂ has been explored for different applications, such as oxygen sensors, power device electrolytes, colors for sharpened sunlight-based cells, the photocatalytic parting of water for hydrogen creation, the particular amalgamation of natural mixtures, air refinement, and photocatalytic abatement of organic and inorganic pollutants present in wastewater [30]. Certain other features of ZrO₂, which endorse its usage as photocatalytic material, are described as follows:

• ZrO₂ is a transition metal oxide and a prominent candidate for photocatalysis owing to its high refractive index, high corrosion resistance, and good photochemical stability, as well as optical absorption ability [31,32].
• ZrO₂ is rich in OVs, which exist on the surface or in the lattice’s interstitial sites, which further promotes the optoelectronic properties of ZrO₂ leading to boosted light harvesting ability and superior isolation of photocarriers [22].
• The optical characteristics of ZrO₂ are modified by the introduction of midgap states in the elemental doping, resulting in optimal visible light responsive photocatalyst [22]. By doping with ZrO₂, ions diffuse throughout the lattice at interstitial sites, resulting in a new valance band maximum (VBM) or conduction band minimum (CBM), which narrows the optical bandgap of ZrO₂ [33].

2.1. Crystal Structure

The three major polymorphs of ZrO₂, namely monoclinic, tetragonal, and cubic crystal structures are depicted in Figure 1. At ambient temperature, the monoclinic stage remains constant, but when heated to 1170 °C, it transforms to the tetragonal ZrO₂ and further to cubic ZrO₂ by increasing the temperature up to 2370 °C. The transformation is reversible when the temperature is lowered; the shift from tetragonal to monoclinic occurs at a lower temperature (around 950 °C) [34]. Tetragonal ZrO₂ is formed by the elongation of any of the three equal axes of cubic fluorite-type structure and the relocation of oxygen anions from their characteristic places in the same direction [35].

The main features of monoclinic ZrO₂ involve sevenfold coordination of the Zr-atom and alteration of fluorite layers possessing OII ions in the tetrahedral coordination with triangular coordinated layers containing OI. Additionally, the nearest Zr-O distance is observed to be 3.77 Å, which signifies the coordination number to be seven (Figure 1a). The Zr-OII bond length in the OII coordinated tetrahedron varies from 2.16 to 2.26 Å, while the central bond angles at OI range from 100 to 132°. The Zr-OI bond length in the OI coordinated triangle is 2.04 Å, 2.10 Å, and 2.15 Å with 2.10 Å as an average angle, while the central angles at OI are 106°, 109°, and 145° [36]. The surface site of m-ZrO₂ consists of oxygen vacancies, unsaturated surface hydroxyl groups (Zr-O pairs), and Lewis’s acid sites (Zr³⁺, Zr⁴⁺). In comparison to other phases, monoclinic-ZrO₂ has a less symmetrical lattice structure, making it a more adaptable surface for catalytic activity [37].

In the case of tetragonal ZrO₂, the Zr-atoms are surrounded by oxygen atoms (eight), four of which are in a flattened tetrahedron at a distance of 2.065 Å and the remaining four at a distance of 2.455 Å in an extended tetrahedron (Figure 1b). Each oxygen atom has two oxygen neighbors at 2.635 Å and four at 2.655 Å, which are connected to two Zr-atoms at 2.065 Å, and two more Zr-atoms at 2.455 Å [38]. The tetragonal phase is more active than the other two phases due to the presence of defects and its stable nature. t-ZrO₂ is excellent for photocatalytic reactions because of its antioxidant activity for free radical trapping, high redox ability, chemical and optical stability, and environmental affinity [25]. The cubic phase is thermally stable above 2300 °C, so it is rarely produced at low temperatures (Figure 1c). Yttrium doping was used to successfully construct cubic ZrO2 structures with abundant OVs, resulting in cubic yttria-stabilized zirconia [39,40].

2.2. Bandgap Formation

Compared to the normal hydrogen electrode (NHE), ZrO₂ possesses a strong reduction ability due to its comparatively high CBM, which is roughly 2.1 V less than the O₂/•O₂⁻ redox potential, yet its VBM is more positive (approximately 0.7 V) than the H₂O/•OH = 2.72 V (vs. NHE) potential [28,41]. The VB edge of ZrO₂ fundamentally arises from the O 2p orbitals, whereas the CB edge is overwhelmed by the 4d orbitals of Zr. The VB potential is 2.77 V higher than the required potential for water oxidation, whereas the CB edge is 1.0 V lower in potential than the hydrogen reduction level [42]. The lowest CB potential is ca. 1.0 V (vs. NHE, pH 0), which is considerably lower than the TiO₂ anatase (0.1 V) potential, and the highest VB potential is ca. +4.0 V, which is superior to TiO₂ (+3.1 V), suggesting that the band potentials of ZrO₂ are highly appropriate for various photocatalytic applications [27].

2.3. Opto-Electronic Properties

Density Functional Theory (DFT) studies can anticipate the characteristics of a material with less inconsistency and estimate bandgap due to the capability to detect quantum mechanical features of atoms, sub-atomic constituents, and inexact depiction of electronic interactions through generalized density approximation (GGA) and local density approximation (LDA), respectively. In the DFT analysis, the unit-cell structures of the oxide are optimized and determined by the lattice parameters for understanding the structural characteristics. Through DFT, it is found that ZrO₂ exhibits twelve ionic structures containing four formula units in ZrO₂, with seven O atoms surrounding the cation and two oxygen locations OI and OII in Wyckoff points. Each unit cell is thought to be made up of three distinct ions due to the varying atomic locations of the oxygen atom. An average bond length of 2.86 Å and 3.58 Å for Zr-O and Zr-Zr, respectively, was observed [43]. The structure of pure ZrO₂ is found to be cubic, which belongs to the space group “Fm3m” (N 225) with zirconium occupied position at (0, 0, 0), and oxygen atoms at (1/4, 1/4, 1/4) and (3/4, 3/4, 3/4), respectively (Figure 2a) [32].

Interestingly, the DFT calculation studies are further helpful to explore the intricate electronic structures of ZrO₂. The electrical levels of a crystal structure are presented by the bandgap positioning estimated through the Bloch vector (k) and the band index (n). When Fermi energy is set to the zero level, ZrO₂ showed an indirect band gap, with the placement of CBM and VBM at M and N, respectively (Figure 2b). Furthermore, the density of states (DOS) indicates the number of unoccupied levels accessible for the electron to be occupied at a certain energy level. The total density of state (TDOS) and partial density of state (PDOS) charts can be used to show the contribution of distinct atoms. In the case of ZrO₂, four different bands are present with the appearance of a lowest band around 27 eV. The next energy band starts from 19 to 16.5 eV with the upper VB positioning in between 5.3–0 eV and the CB at 4.14 eV. In the lowest VB states, Zr 3p states contribute the most while O 2 s state participates in the lower VB formation. In the VB below the Fermi level, O 2p states are the dominant states, with minor assistance from Zr 4d and 5 s hybridized orbitals. The Zr 4d and O 2p orbitals dominate the formation of CB as shown in Figure 2c [43]. The partial DOS (Figure 2d) further represents the characteristics of ionic bonding between the O 2p and Zr 4d orbitals placed below and above the Fermi energy level, respectively.

3. Modification Strategies

ZrO₂ is a potential photocatalyst due to its advantages of appropriate redox potentials, exceptional oxygen storage capacity, high thermal and chemical strength, etc. However, the limitations of smaller surface area, restricted visible light captivation, and speedy charge carrier recombination rate reduced the efficiency of the ZrO₂ photocatalyst. To overcome these drawbacks, various modification strategies have been incorporated so far, such as doping, heterojunction formation, morphology and structural modulation, and formation of OVs. These strategies are reported to increment the photocatalytic activity of the ZrO₂ photocatalyst, as discussed below.

3.1. Morphology and Structural Modulation

Modulation in the morphology and structure is an effective means to enhance the photocatalytic performance of photocatalysts as it can increase the specific area, amend the surface-active sites for photocatalytic reactions, facilitate the mass transportation and light harvesting along with accelerated charge migration and suppressed charge recombination [44]. Generally, factors like pH, temperature, reducing agents, etc., dominantly influence the morphology of photocatalysts and in turn affect the photocatalytic performance. However, the morphology of ZrO₂ photocatalyst can also be modified by varying the dopant concentration. In particular, the porosity of ZrO₂ nanoparticles can be enhanced by varying the dopant concentration. For instance, the variation in the morphology of ZrO₂ photocatalyst was observed by varying the concentration of Mg²⁺ ion dopant from 0.1 to 5 mol% [45]. From scanning electron microscope (SEM) results (Figure 3a), the hollow sphere structure of the ZrO₂ photocatalyst was observed with a large number of cracks at the surface indicating a highly porous nature. The unique interaction between Zr⁴⁺ and Mg²⁺ ions resulted in a complex polysaccharides structure with strong bindings. Furthermore, the increase in the Mg²⁺ ion concentration in ZrO₂ photocatalyst leading to increased light absorption capability due to the development of mid-gap states. Thus, Mg doping in ZrO₂ could facilitate the enhancement in light harvesting, efficient inhibition in the reassembly of charge carriers, and incremented specific surface area corresponding to improved photocatalytic performance.

Ahmed et al. also studied the variation in morphology of the ZrO₂ photocatalyst after doping with cobalt ions [25]. The SEM results (Figure 3b) reveal that both the un-doped and Co-doped ZrO₂ photocatalyst presented spherical structures having identical size and showed ultrafine morphology. Thus, the parameters like shape and size intensities were influenced by the presence of defects that leads to the lowering of electron-hole pairs reassembly which contributed to boosted photocatalytic activity. In a different study, the cube-like or star-like morphology of the ZrO₂ colloidal nanocrystal clusters (CNCs) was also observed by varying the reaction conditions [47]. The occurrence of Cl⁻ ions in the hydrothermal process resulted in the establishment of cube-like ZrO₂ CNCs with a rough surface and increased particle size from 74 to 170 nm by incrementing the reaction time (six to ten hours). Furthermore, the morphology of ZrO₂ CNCs changed to star-like structures by the combined action of both Cl⁻ and Na⁺ ions in the hydrothermal process due to its highly crystalline nature and systematic aggregation of nanocrystals. Additionally, the clustering of ZrO₂ nanocrystals takes place at high temperatures (150–240 °C), which leads to the formation of a cube-like morphology.

3.2. Bandgap Tuning via Doping

The band gap of semiconductors can be modulated by elemental doping, which triggers the creation of the allowed defective electronic levels within the bandgap energy and promotes the electronic excitation with lesser energy. To date, doping of ZrO₂ photocatalyst has been performed with noble metals [48], non-metals [49], and transition elements to adjust its band gap, amend the photoactivity, and inhibit the electron-hole pairs recombination rate [20]. For instance, Co^2+/3+ ions with radii 0.74 Å comparable to Zr⁴⁺ (0.80 Å) are reported to be an appropriate dopant as it can modify the optical properties and dielectric characteristics of ZrO₂ photocatalyst along with a reinforced charge separation rate leading to higher photocatalytic activity [25]. With the varied dopant concentration of Co from 3%, 5%, 7%, to 10%, the band gap energy of ZrO₂ photocatalyst changed from 3.7 eV to 2.74, 2.40, 2.23, and 2.12 eV, respectively (Figure 3c), due to the presence of OVs and dislocation defects. However, the optimal Co-doping concentration was found to be 10% in ZrO₂ photocatalyst for the highest degradation of 96.5% of methylene blue (MB). Other than the transition metal dopant, boron doping into oxygen vacancy-rich ZrO₂ (B@ZrO₂-OV) reportedly resulted in more impurity electronic levels, which significantly reduced the band gap value and boosted the capability of ZrO₂ to harvest light in the visible region [28]. Consequently, the B@ZrO₂-OV photocatalyst exhibited prompted photocatalytic performance by degrading 87% of RhB and 71% of the colorless phenol owing to enhanced photon harvesting capability.

The DFT study was used to analyze the decrement in bandgap energy of N doped ZrO₂ photocatalyst that further facilitated the separation of photocarriers. The DFT calculation was helpful in analyzing the effect of N-doping in the ZrO₂ photocatalyst with its subsequent impact on the band structure and electronic density of states in N-doped ZrO₂ (ZrO_2−xN_x, x = 0.125) [46]. The results revealed that the band gap of ZrO₂ photocatalyst narrowed down from 5.0 to 2.860 eV by doping with N atoms, as shown in Figure 3d,e. Furthermore, the VBM increased from 0 to 0.473 V, while the CBM decreased from 3.398 to 3.333 V. Consequently, the introduction of N-atoms in a substitutional mode of ZrO₂ photocatalyst resulted in the narrowed bandgap and extension of visible light absorptivity giving rise to enhanced photocatalytic degradation of formaldehyde (HCHO) into CO₂ under the exposure of visible light for 50 min.

Doping with Au has also facilitated the reduction in bandgap energy of ZrO₂ reduced from 5.72 to 5.40 eV [50]. With the increase in Au concentration, interactions occur between charge carriers and ionized impurities leading to a decreased band gap and photoluminescence (PL) intensity of ZrO₂ photocatalyst. Hence, doping is an effective way to boost the photoactivity of the ZrO₂ photocatalyst with noble metals, non-metals, and transition metals by tuning the bandgap energy to some extent by varying the concentration of impurity atoms.

3.3. Heterojunction Formation

Heterojunction formation is one of the most important ways to achieve better photocatalytic activity by reducing the high charge recombination rate, less visible-light absorption, and low electron utilization ratio [51]. ZrO₂ has reportedly formed type-I heterostructure systems for water treatment applications. For instance, the formation of a type-I heterojunction for modulating the photocatalytic efficiency of ZrO₂/ZnO heterojunction was efficiently reported [52]. With the formation of optimized ZrO₂/ZnO (1:2) heterojunction, the bandgap energy of the nanocomposite was decreased from 5.3 to 3.2 eV. Additionally, the ZrO₂/ZnO (1:2) heterojunction exhibited 93% photocatalytic degradation of Acid orange 8 (AO 8) under UV light illumination. In a different study, the photo-absorption ability of ZrO₂ photocatalyst was reportedly enhanced when coupled with Polypyrrole (PPy) in the type-II mechanistic mode [53]. The decreased eh⁺ recombination rate and enhanced isolation of charge carriers were accredited to the type-II system with reduced bandgap energy from 3.26 to 2.45 eV. Moreover, the photocatalytic performance of the ZrO₂/PPy system was assessed by degrading Reactive Black 31 (RB 31; 77%) and 4-chlorophenol (4-CP; 98%) under the exposure of visible light for 60 min. The synergic effect between the ZrO₂ and PPy photocatalysts after heterostructure formation improved the charge carrier kinetics and provided sufficient carriers at the surface for photoredox reactions. While ZrO₂-based type-I and type-II heterojunctions have shown considerable photocatalytic performance, the accumulation of photocarriers on one photocatalyst only and the electrostatic forces of repulsion between charge carriers along with the decreased redox potential in type-II system hampers the overall photocatalytic performance [54]. Thus, the formation of the Z-scheme photocatalytic system is considered to be a prominent approach that has been vastly opted to expedite the photocatalysis process.

For example, the photoactivity of g-C₃N₄/ZrO_2−x Z-scheme heterojunction was analyzed for wastewater treatment [55]. The report illustrated that, due to the involvement of the Z-scheme charge transference, the g-C₃N₄/ZrO_2−x system exhibited extended visible light irradiation and enhanced separation of charge carriers. Additionally, the presence of abundant, spatially separated e- facilitated the formation of •O₂⁻ species, which primarily participated in the degradation process. Consequently, g-C₃N₄/ZrO_2−x system showed 90.6% tetracycline hydrochloride (TC-H) degradation under 1 h exposure to visible light (Figure 4a). Similarly, the photodegradation of Congo Red (CR) dye was reported by Hao et al. by utilizing the Ag/CeO₂-ZrO₂ nanocomposite [56]. The formation of three-dimensionally ordered macroporous Ag/CeO₂-ZrO₂ composite enabled the absorption of reactive molecules on the surface of photocatalysts, which reduced the recombination probability of charge carriers leading to enhanced photocatalytic performance.

Interestingly, the arrangement of the S-scheme heterojunction is quite similar to Type-II but it follows a completely different charge transfer mechanism [57]. Typically, the photoexcited e⁻ and h⁺ are accumulated on the CB of reduction photocatalyst (RP) and VB of oxidation photocatalyst (OP), respectively, which corresponds to a strong redox potential. The remaining (unutilized) photocarriers are eliminated through recombination, leading to better photocatalytic reactions. For instance, the formation of the S-scheme g-C₃N₄/ZrO₂ heterojunction was achieved via a simple calcination method for improved photocatalytic performance [26]. The g-C₃N₄/ZrO₂ S-scheme heterojunction inhibited the recombination rate of photocarriers due to a sufficiently exposed large specific surface area with abundant active sites. Moreover, the optimized 15% g-C₃N₄/ZrO₂ heterojunction exhibited excellent photocatalytic performance by degrading 82% RhB, 50% methyl orange (MO), and 98% acid orange II (AO II) under visible light irradiation for 150 min (Figure 4b).

3.4. Generation of Oxygen Vacancies

The introduction of OVs in the ZrO₂ photocatalyst is a useful strategy to increase the absorption range in the visible light region and to promote the adsorption affinity leading to enhanced photocatalytic degradation efficacy. The formation of vacancies in ZrO₂ crystal acts as a trapping site for one or more electrons, which gives rise to F-centers with the local states in the crystal energy gap and produces the new energy band near the Fermi level [58]. The determination of OVs and surface defects can be done with the help of PL intensity, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) pattern. Over the past few years, various works have reported the successful generation and identification of OVs in the ZrO₂ photocatalyst.

For instance, OVs in ZrO₂ were created by depositing Co on its surface [25]. The XRD pattern identified the presence of OVs due to the distinct ionic radii of Zr and Co ions. The slight shift in the peak position intensity of Raman spectra (Figure 4c) and decreased the band gap energy of Co-ZrO₂ occurred by the presence of OVs and surface defects. Furthermore, the OVs trapped the photogenerated e- leading to the formation of F-centers in the crystal structure of ZrO₂, which inhibit the charge carrier’s reassembly and amend the photocatalytic performance. Wahba et al. studied the formation of OVs in the Zr-doped ZrO₂/ZnO heterojunction through experimental and simulation studies [22]. The XRD pattern revealed that, with the increasing concentration of Zr from 30 to 50 wt%, two diffraction peaks corresponding to monoclinic ZrO₂ and wurtzite hexagonal ZnO were obtained as delineated in Figure 4d. Additionally, the Zr⁴⁺ ions were replaced by Zn²⁺ ions, which induced the OVs in the crystal structure for trapping charge carriers and increasing their separation. XPS analysis can also be employed to evaluate the presence of OVs in ZrO₂ photocatalysts. For instance, Reddy et al. reported the formation of OVs in Ni-doped ZrO₂ by analyzing the XPS spectra of O 1 s [59]. In detail, the XPS study showed three types of OVs on the ZrO₂ surface that is neutral, singly charged and doubly charged oxygen vacancy with different binding energies. The binding energy 530.7 eV, and 532.1 eV corresponded to singly and doubly charged OVs in ZrO₂ photocatalyst, respectively.

From the above study, it is evident that the modification strategies are significantly effective in incrementing the photocatalytic performance of the ZrO₂ photocatalyst. The change in morphology and optoelectronic structure of ZrO₂ leads to enhanced visible light absorption and superior charge isolation. Doping with suitable dopants and the formation of ZrO₂-based heterojunctions resulted in narrowed bandgap energy and improved charge carrier kinetics enabling their effectiveness in photocatalytic performance. Additionally, the presence of OVs not only acts as trapping sites for photoinduced charge carriers, but also renders defective states in between the bandgap energy which synergistically enhances the photoactivity of ZrO₂ nanomaterials.

4. Synthesis Methods

ZrO₂ photocatalysts having different morphologies, sizes, and compositions have been designed by various bottom-up and top-down strategies for efficient photocatalytic activity. Different advantages and challenges in the fabrication of ZrO₂ using distinct synthesis methods are discussed as follows.

4.1. Bottom-Up Strategies

4.1.1. Hydrothermal/Solvothermal Method

The hydrothermal/solvothermal strategy is a synthesis method for the preparation of the ZrO₂ photocatalyst, which involves a sealed vessel with water or any organic solvent as a reaction medium [60]. The crystallization of catalyst occurs at high temperatures and pressure maintained in the vessel. This method leads to the formation of powders with fine size or narrow size distribution with good chemical homogeneity and is dependent on some reaction parameters such as reaction time, pH value, temperature, solvent, etc. Surfactants also inhibit the growth of crystals by forming an ultrathin structure. Parveen et al. showed the pH dependence of hydrothermal reaction involving coupling of PPy with ZrO₂ photocatalyst [53]. Typically, the ZrO₂/PPy composite was fabricated by maintaining the hydrothermal reaction condition (180 °C/24 h) with the adjustment of pH value. The individual PPy and ZrO₂ system showed the planer-structured PPy nanosheets with spherical ZrO₂ particles. TEM results also revealed the wrinkled lamer structure nanosheets of PPy and ZrO₂ samples (Figure 5a). Besides pH, the hydrothermal temperature also affects the morphology of the particles. As Rashid et al. prepared the ZrO₂ doped ZnCo₂O₄ nanocomposites by the hydrothermal process with sulfate as a precursor, ethylene glycol (EG) and water as solvents at 250 to 650 °C calcination temperature [61]. The maximum increase in the surface area of ZrO₂-ZnCo₂O₄ was achieved at 350 °C calcined temperature. After calcination, the ZrO₂-ZnCo₂O₄ showed the presence of granular particles. Furthermore, the SEM results revealed the appearance of uniform-size nanoparticles of ZrO₂-ZnCo₂O₄ composites at higher temperatures (Figure 5b).

4.1.2. Crystal Structure Engineering

The introduction of an impure atom can distort the crystal structure and change the balance of the ZrO₂ photocatalyst by creating the OVs on its surface. Doping with suitable dopants further leads to a change in the morphology of the nanoparticles. For example, Wahba et al. analyzed the variation in morphology of Zr doped ZrO₂/ZnO nanocomposite by preparing it with EG as a complexing agent [22]. The 2.5 wt% of Zr concentration changed the morphology of nanoparticles to elongated non-uniform distribution with large size. By increasing the concentration up to 5 wt%, the particle exhibited uniform distribution with less agglomeration. Moreover, the SEM results also showed the presence of homogeneous particle size and uniform distribution with 30, 40, and 50 wt% of Zr concentration in ZrO₂/ZnO nanocomposite. In a different study, Navio et al. studied the distinction in the morphology of ZrO₂ photocatalysts by doping with Fe [62]. The Fe/ZrO₂ was prepared by maintaining the appropriate room temperature, and pH (9–10) of the sample. The SEM results revealed that the particles have a large shape and dimension distribution. With increasing the Fe content in the sample, the particles exhibited a high degree of wrinkled texture due to surface deposition. TEM descriptions indicated no substantial change in the morphology of the sample. However, in some cases, enlarged and different round-size grain particles were also observed.

Various studies have been reported to date to study the morphology of ZrO₂ photocatalysts. Agorku et al. prepared the multi-element doped ZrO₂ with Gd, C, N, and S elements to study the morphology of the photocatalyst [63]. The TEM results revealed the presence of a small-size multi-element doped ZrO₂ with spherical shape and distinct grain boundaries. However, the SEM monographs showed uniform morphology with a crystalline-shaped photocatalyst. Furthermore, 0.6% Gd doped C, N, and S-ZrO₂ exhibited the highest photocatalytic activity due to reduced bandgap energy and improved surface-active sites of the photocatalyst as a result of multi-element doping. Chen et al. also studied the morphology of g-C₃N₄ loaded ZrO_2−x nanotubes [55]. ZrO₂ nanotubes are tailored through the anodic oxidation of ZrO₂ in a two-electrode system with a 20 mm diameter for each electrode. The prepared ZrO_2−x nanotubes were further annealed at 550 °C for 3 h in the presence of Argon. The SEM results showed the clean surface morphology of ZrO_2−x nanotubes with a 50 nm diameter. By increasing the g-C₃N₄ concentration, the optimal 0.09CN/ZrO_2−x sample exhibited completely covered structure nanotubes. Furthermore, TEM results revealed the regular and clean tubular morphology of ZrO_2−x nanotubes whereas the tubular morphology of 0.06 CN/ZrO_2−x nanotubes with apparent attachments was further observed by increasing the g-C₃N₄ concentration. The presence of g-C₃N₄ in the composite was confirmed with the help of Fourier transform infrared (FTIR) spectroscopy as delineated in Figure 5c. The broad band of g-C₃N₄ in the NH₂ group stretching mode was observed at 3178 cm⁻¹, while for ZrO_2−x, the NTs showed a broad band of OH stretching at 3420 cm⁻¹. The Zr-O bond in ZrO₂ corresponds to 490, 580, and 780 cm⁻¹ bands vibrations and the composite showed a peak at 2046 cm⁻¹. The as-synthesized composite exhibited 90.6% degradation of tetracycline hydrochloride after 1 h of visible light exposure.

4.2. Top-Down Strategies

4.2.1. Thermal Treatment

Thermal treatment is a significant method utilized for the introduction of OVs in ZrO₂ at high temperatures, which leads to the cleavage of bonds and further boosts the escape of atoms from the lattice. At definite temperatures, the metals become decomposed to form nanoparticles by involving a chemical reaction. For instance, Zhang et al. prepared an S-scheme g-C₃N₄/ZrO₂ heterostructure via a simple calcination method [26]. Experimental results explained the morphology changes in g-C₃N₄/ZrO₂ heterojunction after variation in calcination temperature. The presence of irregularly arranged nanoparticles on the surface of ZrO₂ with the variation in calcination temperature was confirmed by SEM results. With variation in the g-C₃N₄ concentration, the surface of ZrO₂ photocatalyst became dimmer by the existence of spindle-shaped nanoparticles. It has been observed that the mixing of two metal oxide phases can also help to modulate the photocatalytic activity by inhibiting the crystal growth during the heterostructure formation via calcination. For instance, the heterojunction formation between ZrO₂ and Bi₂O₃ via post-thermal treatment rendered a facile morphology control approach by inhibiting the crystal growth of Bi₂O₃ [64]. From the TEM images, an optimal particle size of 20 nm was observed and was in good agreement with the XRD results. The UV-Visible diffuse reflectance spectroscopy (UV-vis-DRS) spectra of Bi₂O₃-ZrO₂, Bi₂O₃, and ZrO₂ were depicted in Figure 5d where a red shift in the composite was observed due to the electron–electron transition in between bare ZrO₂ and Bi₂O₃. The spectra also explained the absorbance intensity shift of Bi₂O₃-ZrO₂ towards the longer wavelength than Bi₂O₃ and ZrO₂. Hence, thermal treatment can be successfully employed to modulate the surface morphology of ZrO₂ and ZrO₂-based photocatalysts.

4.2.2. Ultrasonication

Sonication involves the dispersion of the precursor in a solvent followed by further exfoliation of materials. Furthermore, shock waves and high-speed inter-particle collisions also help to improve the surface structure and may sometimes induce the formation of OVs. Zarei et al. synthesized ZrO₂/g-C₃N₄ heterojunction via mixing of g-C₃N₄ powder into ZrO₂ colloidal dispersion through ultrasonic treatment at 30 °C for 2 h [65]. The integration of ZrO₂ nanoparticles into g-C₃N₄ increased their surface area to 51.8 m² g⁻¹ due to its distinct morphology. The TEM results indicated that g-C₃N₄ showed planer and sheet-like structures while ZrO₂ nanoparticles were spherical in shape with a 15 nm particle size. Additionally, the XRD pattern revealed the existence of strong and sharp diffraction peaks, which confirmed a high degree of crystallinity in the sample (Figure 5e). Furthermore, the incorporation of ultrasonic treatment with various dopant concentrations can also disturb the morphology of the ZrO₂ photocatalyst. For example, Yu et al. synthesized the TiO₂-N/ZrO_2−xN_x heterojunction by subjecting the precursors such as ZrOCl₂·8H₂O and Ti (OC₄H₉)₄ under ultrasonic treatment [46]. With the variation in the concentration of N, the transformation of tetragonal ZrO₂ to monoclinic was strictly inhibited with resultantly improved surface-active sites. The XRD patterns helped to understand the interactions between N atoms with O atoms in ZrO₂ which led to the substitution of O by nitrogen dopants due to high-energy ultrasonic waves.

4.2.3. Chemical Reduction

Chemical reduction is a method used for the reduction of ionic salt in the presence of surfactants by using reducing agents such as sodium borohydride, lithium aluminum hydride, etc. Interestingly, OVs can also be introduced with the help of reducing agents and by varying the concentration of reductants, the content of vacancies can be controlled. For instance, Qin et al. produced a B@ZrO₂-OV photocatalyst through the introduction of NaBH₄ as reducing agent [28]. In this process, ZrO₂ and NaBH₄ were taken in different weight ratios and heated at 650 °C for 2 h followed by the annealing of the sample. More importantly, the weight ratios of ZrO₂:NaBH₄ was optimized to 1:0.4 because, beyond this, a further increase in the amount of NaBH₄ meant that the crystal phase of ZrO₂ was transformed into ZrB₂. The electron paramagnetic resonance (EPR) spectroscopy confirmed the existence of OVs in the B@ZrO₂ photocatalyst at 2.003 g-value. Moreover, the XPS analysis showed three deconvoluted peaks at 529.9 eV, 531.4 eV, and 532.8 eV ascribed to the lattice oxygen, oxygen deficient, and chemically adsorbed oxygen species, respectively.

5. Photodegradation and Mechanism

Environmental contamination by organic dyes, pharmaceuticals, and insecticides is posing a serious threat worldwide owing to their toxic, cancerous nature and difficulty in total decomposition causing a shortage in drinking water. Thus far, various traditional procedures, including biological, physical, and chemical methods have been used to remove these pollutants, however the applicability of these technologies is limited due to their high cost, risk of hazardous byproducts, and poor efficiency. As a result, a more direct and effective way to deal with these contaminants is highly essential [55,66]. Water treatment employing a simple method like photocatalysis is in the limelight owing to its low energy requirement, cost-effectiveness, capacity to adsorb a wide range of natural and inorganic substances on the photocatalyst’s surface, and complete degradation without hazardous by-products [67]. Even photocatalysis seemed to be a promising method due to the easy availability of a large amount of solar energy [23]. Semiconductor photocatalytic activity has potential applications in the degradation of environmental contaminants and pollutant transformation. Thus, it has aroused a lot of attention [68].

5.1. Organic Pollutant Degradation

Organic pollutants are identified as harmful for human health as they are toxic, carcinogenic, and teratogenic, so the use of heterogeneous photocatalysis to decontaminate wastewater containing organic compounds is an attractive solution to the effluent treatment problem since it brings about chemical degradation into non-toxic molecules like carbon dioxide and water [29,66]. Charge transfer and electron transfer are involved in the degradation of organic pollutants, and these processes are impacted by the presence of charged species [69]. The alluring features of ZrO₂ involving physicochemical stability and distinctive band structures render optimal photodegradation ability, however the broad bandgap and low separation rate of photocarriers limit its wide-scale practical uses [55]. However, the synergistic coupling of ferrites with zirconia boosts the overall photoactivity by the combined action of inhibited charge separation and integration of photo and Fenton activity. In detail, Figure 6a depicts the suggested photocatalytic process on the ZrO₂/ZnFe₂O₄ nanocomposite (ZZFO 12 NC) for the photodegradation of indigo carmine dye (98%) [31]. After the excitation and isolation of photocarriers, the reaction of photoinduced electrons oxygen yielded superoxide radicals (•O₂⁻), which further reacted with H₂O to produce H₂O₂. This generated H₂O₂ reaction with α-Fe₂O₃ in ZZFO 12 further produced •OH radicals and participated in dye degradation, as shown in Equations (1)–(7)

$$\mathrm{ZZFO}+\mathrm{h}\mathsf{\nu }\to {\mathrm{ZrO}}_2-{\mathrm{h}}^{+}+{\mathrm{ZnFe}}_2{\mathrm{O}}_{4 }-{\mathrm{e}}^{\_}$$

(1)

$${\mathrm{ZnFe}}_2{\mathrm{O}}_{4 }-{\mathrm{e}}^{\_}+{\mathrm{O}}_2^{• }\to •{\mathrm{O}}_2^{-}$$

(2)

$$•{\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2+•{\mathrm{OH}}^{ }$$

(3)

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathsf{\alpha }-{\mathrm{Fe}}_2{\mathrm{O}}_3\to 2 •{\mathrm{OH}}^{ }$$

(4)

$${\mathrm{h}}^{+}+{\mathrm{OH}}^{-}\to 2 •{\mathrm{OH}}^{ }$$

(5)

$$•\mathrm{OH}+\mathrm{indigo} \mathrm{carmine}\to \mathrm{Degradation}$$

(6)

$$•{\mathrm{O}}_2^{-}+\mathrm{indigo} \mathrm{carmine}\to \mathrm{Degradation}$$

(7)

Other than dyes, phenolic compounds have also been treated effectively with ZrO₂-based nanocomposites. For example, ZrO₂/g-C₃N₄ nanocomposite synthesized by ultrasonication method for the efficient degradation of 4-chlorophenol (4-CP) (Figure 6b) in water with the highest degradation rate of 0.0173 min⁻¹ [65]. The synergistic action of higher surface area with exposed active sites facilitates the adsorption of 4-CP and the surface-induced charges accelerated the formation of •O₂⁻ radicals which boosted the degradation efficiency. Moreover, from the radical scavenging results, it was confirmed that •O₂⁻ radicals and h+ primarily participated in the degradation process.

5.2. Inorganic Pollutant Degradation

The higher concentration of inorganic pollutants in the water due to their discharge from various industries like fertilizers, refineries, and pharmaceuticals is causing a major threat to human as well as aquatic lives. Thus the photocatalytic treatment of such toxic effluents using ZrO₂ as a photocatalyst can be a potential solution. In this regard, enhancement in Cr (VI) photoreduction under visible light irradiation is achieved by developing a Bi₂O₃-ZrO₂ nanocomposite [64]. Bi₂O₃-ZrO₂ has a greater photocatalytic activity (92.30%) after an irradiation period of 180 min. The mechanism for enhanced photocatalytic activity of Bi₂O₃-ZrO₂ is explained according to previously published literature on the photocatalytic activity of nanocomposites. Typically, Cr (VI) is photo-reduced by CB electrons captured by ZrO_2, and the VB holes further oxidized H₂O to create O₂ (Figure 6c). The detailed photocatalytic processes that occur on the surface of Bi₂O₃-ZrO₂ are summarized in Equations (8)–(11):

$${\mathrm{Bi}}_2{\mathrm{O}}_3+\mathrm{hv}\to {\mathrm{Bi}}_2{\mathrm{O}}_3\left({\mathrm{h}}_{\mathrm{VB}}^{+}+{ \mathrm{e}}_{\mathrm{CB}}^{-}\right)$$

(8)

$${\mathrm{Bi}}_2{\mathrm{O}}_3\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+{\mathrm{ZrO}}_2\to {\mathrm{Bi}}_2{\mathrm{O}}_3+{\mathrm{ZrO}}_2\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)$$

(9)

$${\mathrm{ZrO}}_2\left(6{\mathrm{e}}_{\mathrm{CB}}^{-}\right)+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}\to 2{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O}+{\mathrm{ZrO}}_2$$

(10)

$$2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{h}}^{+}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}$$

(11)

Various ZrO₂-based semiconductor photocatalysts with incremented photodegradation activity are summarized in

Table 1. Summary of various ZrO₂-based semiconductor photocatalysts for the incremented photocatalytic degradation of aqueous phase pollutants.

Photocatalyst	Enhancement Strategies	Reaction Parameter	Targeted Pollutant	Photocatalyst Efficiency	Ref.
Ni doped ZrO2	Doping	Visible light lamp (>400 nm), 15 mg photocatalyst	Methylene blue	90.2% in 100 min.	[59]
C-doped ZrO2	Doping	PL-L lamp, 0.2 g/L photocatalyst	Methylene blue	75%	[70]
Nd doped ZrO2	Doping	pH = 7	Methylene blue, Rhodamine B, and acetophenone	90%, 77%, and 60%	[71]
Al2O3/ZrO2	Heterojunction	Visible light, 0.04 g catalyst	Reactive blue 222 and Reactive yellow 145	91.4% and 94.6% in 60 min.	[72]
RE/ZrO2 (RE = Sm, Eu)	Heterojunction	350 W Xenon lamp	Methylene blue and Rhodamine B	100% in 30 min. and 96.3% in 90 min.	[73]
ZnO QDs@ZrO2-TiO2	Heterojunction	Ultra violet light	Congo Red	94.62%	[74]
ZrO2/Dy2O3	Heterojunction	Xenon lamp with cut-off UV filter	Rhodamine B, and Methylene blue	100% in 30 min. and 87.79%	[75]
TiO2-ZrO2	Heterojunction	125 W Mercury lamp, pH = 7.6, 10 mg/L photocatalyst	Metformin	92% in 150 min.	[76]
g-C3N4/ZrO2	Heterojunction	50 W LED lamp, 30 mg photocatalyst	Methylene blue, Rhodamine B, Congo Red, and Tetracycline	96%, 98%, 90%, and 83%	[77]
CuCo2O4@ZrO2	Heterojunction	Visible light	Tetracycline	95%	[78]
TiO2/ZrO2	Heterojunction	100 W LED lamp	Rhodamine B	90%	[79]

[59,70,71,72,73,74,75,76,77,78,79].

6. Concluding Remarks

Among various metal-oxide photocatalysts, ZrO₂ is an n-type wide bandgap semiconducting material with substantial features like physicochemical stability, facile synthesis, cost-effectiveness, etc. In spite of various fascinating properties of ZrO₂, there are various factors that limit its practical applications. Therefore, to enhance its photocatalytic activity, suitable modification techniques such as doping, heterojunction formation, and vacancy creations have been opted. Considering that, the present study highlights the recent advances in the photocatalytic activity of ZrO₂ and its nanocomposites. The structural features, formation of band edge potentials, and the characteristics of optical as well as electronic properties are comprehensively discussed by integrating experimental and computational studies. Various strategic modulations, such as morphology and structural control, bandgap engineering, heterostructure construction, and vacancy engineering are the major approaches for synergistically improving the charge carrier’s dynamics and light harvesting ability of ZrO₂. Moreover, various synthetic routes involving bottom-up and top-down methods can be utilized to modulate the physicochemical features of ZrO₂ and attain effective photocatalytic degradation performance. Thus, it is envisaged that, with the aid of the research advances discussed in this review, future research can be focused to explore the photocatalytic performance of modified ZrO₂ for large-scale photocatalytic applications.