ZrO 2 -Based Photocatalysts for Wastewater Treatment: From Novel Modiﬁcation Strategies to Mechanistic Insights

: Zirconium dioxide (ZrO 2 ) 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 ZrO 2 . Previously, vast research efforts have been made to improve the photoefﬁcacy of ZrO 2 , 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 ZrO 2 , 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 ZrO 2 . Conclu-sively, in light of research advances involving ZrO 2 photocatalyst, this review article may expedite further investigation for enhancing the large-scale photocatalytic applications for environmental and energy concerns.


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 H 2 O [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 O 2 into the superoxide radical •O 2 − (−0.33 V vs. SHE at pH = 7) and oxidize H 2 O 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 10 6 -10 9 M −1 s −1 [17,21].
Water treatment by photocatalysis using inorganic semiconductors like TiO 2 , SnO 2 , 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 TiO 2 , because of its cost-effectiveness, chemical and photochemical stability, and non-toxic nature [23].Currently, ZrO 2 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, ZrO 2 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 ZrO 2 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 ZrO 2 with narrow band semiconductors like MoS 2 , TiO 2 , and g-C 3 N 4 also retards the photocarrier reassembly and augments its photocatalytic performance [26].The optical properties of ZrO 2 can be enhanced by introducing new electronic states by doping with rare earth metals.For example, the introduction of Ce (IV) ions in ZrO 2 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 ZrO 2 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 ZrO 2 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 ZrO 2 for the synergistic improvement in opto-electronic properties still needs further comprehension.Hence, the present review article highlights the features of ZrO 2 semiconducting material for the photocatalytic removal of organic as well as inorganic pollutants present in wastewater.Starting from the optimal crystal structure of ZrO 2 , various phases and the subsequent photocatalytic features are comprehensively reviewed.Significant methods to circumvent the inherent drawbacks of the bare ZrO 2 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 ZrO 2 -based photocatalytic materials.Then, in order to highlight the advancement of ZrO 2 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 ZrO 2 with the purpose of solving the burgeoning environmental crisis in the world.

ZrO 2 as Photocatalyst
ZrO 2 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 ZrO 2 , like thermal stability, high strength, strong chemical resistance, ductility, and low thermal conductivity makes it an alluring photocatalytic material [25].Consequently, ZrO 2 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 ZrO 2 , which endorse its usage as photocatalytic material, are described as follows:

•
ZrO 2 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].
ZrO 2 is rich in OVs, which exist on the surface or in the lattice's interstitial sites, which further promotes the optoelectronic properties of ZrO 2 leading to boosted light harvesting ability and superior isolation of photocarriers [22].

•
The optical characteristics of ZrO 2 are modified by the introduction of midgap states in the elemental doping, resulting in optimal visible light responsive photocatalyst [22].By doping with ZrO 2 , 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 ZrO 2 [33].

Crystal Structure
The three major polymorphs of ZrO 2 , 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 ZrO 2 and further to cubic ZrO 2 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 ZrO 2 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 The main features of monoclinic ZrO 2 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-ZrO 2 consists of oxygen vacancies, unsaturated surface hydroxyl groups (Zr-O pairs), and Lewis's acid sites (Zr 3+ , Zr 4+ ).In comparison to other phases, monoclinic-ZrO 2 has a less symmetrical lattice structure, making it a more adaptable surface for catalytic activity [37].
In the case of tetragonal ZrO 2 , 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-ZrO 2 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].

Bandgap Formation
Compared to the normal hydrogen electrode (NHE), ZrO 2 possesses a strong reduction ability due to its comparatively high CBM, which is roughly 2.1 V less than the O 2 /•O 2 − redox potential, yet its VBM is more positive (approximately 0.7 V) than the H 2 O/•OH = 2.72 V (vs.NHE) potential [28,41].The VB edge of ZrO 2 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 TiO 2 anatase (0.1 V) potential, and the highest VB potential is ca.+4.0 V, which is superior to TiO 2 (+3.1 V), suggesting that the band potentials of ZrO 2 are highly appropriate for various photocatalytic applications [27].

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 ZrO 2 exhibits twelve ionic structures containing four formula units in ZrO 2 , 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 ZrO 2 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].
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 2s 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 5s 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.Interestingly, the DFT calculation studies are further helpful to explore the intricate electronic structures of ZrO 2 .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, ZrO 2 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 ZrO 2 , 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.

Modification Strategies
ZrO 2 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 ZrO 2 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 ZrO 2 photocatalyst, as discussed below.

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 ZrO 2 photocatalyst can also be modified by varying the dopant concentration.In particular, the porosity of ZrO 2 nanoparticles can be enhanced by varying the dopant concentration.For instance, the variation in the morphology of ZrO 2 photocatalyst was observed by varying the concentration of Mg 2+ ion dopant from 0.1 to 5 mol% [45].From scanning electron microscope (SEM) results (Figure 3a), the hollow sphere structure of the ZrO 2 photocatalyst was observed with a large number of cracks at the surface indicating a highly porous nature.The unique interaction between Zr 4+ and Mg 2+ ions resulted in a complex polysaccharides structure with strong bindings.Furthermore, the increase in the Mg 2+ ion concentration in ZrO 2 photocatalyst leading to increased light absorption capability due to the development of mid-gap states.Thus, Mg doping in ZrO 2 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 Ahmed et al. also studied the variation in morphology of the ZrO 2 photocatalyst after doping with cobalt ions [25].The SEM results (Figure 3b) reveal that both the un-doped and Co-doped ZrO 2 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 ZrO 2 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 ZrO 2 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 ZrO 2 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 ZrO 2 nanocrystals takes place at high temperatures (150-240 • C), which leads to the formation of a cube-like morphology.

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 ZrO 2 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 Zr 4+ (0.80 Å) are reported to be an appropriate dopant as it can modify the optical properties and dielectric characteristics of ZrO 2 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 ZrO 2 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 ZrO 2 photocatalyst for the highest degradation of 96.5% of methylene blue (MB).Other than the transition metal dopant, boron doping into oxygen vacancy-rich ZrO 2 (B@ZrO 2 -OV) reportedly resulted in more impurity electronic levels, which significantly reduced the band gap value and boosted the capability of ZrO 2 to harvest light in the visible region [28].Consequently, the B@ZrO 2 -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 ZrO 2 photocatalyst that further facilitated the separation of photocarriers.The DFT calculation was helpful in analyzing the effect of N-doping in the ZrO 2 photocatalyst with its subsequent impact on the band structure and electronic density of states in N-doped ZrO 2 (ZrO 2−x N x , x = 0.125) [46].The results revealed that the band gap of ZrO 2 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 ZrO 2 photocatalyst resulted in the narrowed bandgap and extension of visible light absorptivity giving rise to enhanced photocatalytic degradation of formaldehyde (HCHO) into CO 2 under the exposure of visible light for 50 min.
Doping with Au has also facilitated the reduction in bandgap energy of ZrO 2 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 ZrO 2 photocatalyst.Hence, doping is an effective way to boost the photoactivity of the ZrO 2 photocatalyst with noble metals, non-metals, and transition metals by tuning the bandgap energy to some extent by varying the concentration of impurity atoms.

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].ZrO 2 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 ZrO 2 /ZnO heterojunction was efficiently reported [52].With the formation of optimized ZrO 2 /ZnO (1:2) heterojunction, the bandgap energy of the nanocomposite was decreased from 5.3 to 3.2 eV.Additionally, the ZrO 2 /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 ZrO 2 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 ZrO 2 /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 ZrO 2 and PPy photocatalysts after heterostructure formation improved the charge carrier kinetics and provided sufficient carriers at the surface for photoredox reactions.While ZrO 2 -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-C 3 N 4 /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-C 3 N 4 /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 •O 2 − species, which primarily participated in the degradation process.Consequently, g-C 3 N 4 /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/CeO 2 -ZrO 2 nanocomposite [56].The formation of threedimensionally ordered macroporous Ag/CeO 2 -ZrO 2 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-C 3 N 4 /ZrO 2 heterojunction was achieved via a simple calcination method for improved photocatalytic performance [26].The g-C 3 N 4 /ZrO 2 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-C 3 N 4 /ZrO 2 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).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.

Generation of Oxygen Vacancies
The introduction of OVs in the ZrO 2 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 ZrO 2 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 ZrO 2 photocatalyst.
For instance, OVs in ZrO 2 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-ZrO 2 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 ZrO 2 , which inhibit the charge carrier's reassembly and amend the photocatalytic performance.Wahba et al. studied the formation of OVs in the Zr-doped ZrO 2 /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 ZrO 2 and wurtzite hexagonal ZnO were obtained as delineated in Figure 4d.Additionally, the Zr 4+ ions were replaced by Zn 2+ 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 ZrO 2 photocatalysts.For instance, Reddy et al. reported the formation of OVs in Ni-doped ZrO 2 by analyzing the XPS spectra of O 1 s [59].In detail, the XPS study showed three types of OVs on the ZrO 2 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 ZrO 2 photocatalyst, respectively.
From the above study, it is evident that the modification strategies are significantly effective in incrementing the photocatalytic performance of the ZrO 2 photocatalyst.The change in morphology and optoelectronic structure of ZrO 2 leads to enhanced visible light absorption and superior charge isolation.Doping with suitable dopants and the formation of ZrO 2 -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 ZrO 2 nanomaterials.

Synthesis Methods
ZrO 2 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 ZrO 2 using distinct synthesis methods are discussed as follows.

Hydrothermal/Solvothermal Method
The hydrothermal/solvothermal strategy is a synthesis method for the preparation of the ZrO 2 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 ZrO 2 photocatalyst [53].Typically, the ZrO 2 /PPy composite was fabricated by maintaining the hydrothermal reaction condition (180 • C/24 h) with the adjustment of pH value.The individual PPy and ZrO 2 system showed the planer-structured PPy nanosheets with spherical ZrO 2 particles.TEM results also revealed the wrinkled lamer structure nanosheets of PPy and ZrO 2 samples (Figure 5a).Besides pH, the hydrothermal temperature also affects the morphology of the particles.As Rashid et al. prepared the ZrO 2 doped ZnCo 2 O 4 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 ZrO 2 -ZnCo 2 O 4 was achieved at 350 • C calcined temperature.After calcination, the ZrO 2 -ZnCo 2 O 4 showed the presence of granular particles.Furthermore, the SEM results revealed the appearance of uniform-size nanoparticles of ZrO 2 -ZnCo 2 O 4 composites at higher temperatures (Figure 5b).

Crystal Structure Engineering
The introduction of an impure atom can distort the crystal structure and change the balance of the ZrO 2 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 ZrO 2 /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 ZrO 2 /ZnO nanocomposite.In a different study, Navio et al. studied the distinction in the morphology of ZrO 2 photocatalysts by doping with Fe [62].The Fe/ZrO 2 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 ZrO 2 photocatalysts.Agorku et al. prepared the multi-element doped ZrO 2 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 ZrO 2 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-ZrO 2 exhibited the highest photocatalytic activity due to reduced bandgap energy and improved surfaceactive sites of the photocatalyst as a result of multi-element doping.Chen et al. also studied the morphology of g-C 3 N 4 loaded ZrO 2−x nanotubes [55].ZrO 2 nanotubes are tailored through the anodic oxidation of ZrO 2 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-C 3 N 4 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-C 3 N 4 concentration.The presence of g-C 3 N 4 in the composite was confirmed with the help of Fourier transform infrared (FTIR) spectroscopy as delineated in Figure 5c.The broad band of g-C 3 N 4 in the NH 2 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 ZrO 2 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.

Thermal Treatment
Thermal treatment is a significant method utilized for the introduction of OVs in ZrO 2 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-C 3 N 4 /ZrO 2 heterostructure via a simple calcination method [26].Experimental results explained the morphology changes in g-C 3 N 4 /ZrO 2 heterojunction after variation in calcination temperature.The presence of irregularly arranged nanoparticles on the surface of ZrO 2 with the variation in calcination temperature was confirmed by SEM results.With variation in the g-C 3 N 4 concentration, the surface of ZrO 2 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 ZrO 2 and Bi 2 O 3 via post-thermal treatment rendered a facile morphology control approach by inhibiting the crystal growth of Bi 2 O 3 [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 Bi 2 O 3 -ZrO 2 , Bi 2 O 3 , and ZrO 2 were depicted in Figure 5d where a red shift in the composite was observed due to the electron-electron transition in between bare ZrO 2 and Bi 2 O 3 .The spectra also explained the absorbance intensity shift of Bi 2 O 3 -ZrO 2 towards the longer wavelength than Bi 2 O 3 and ZrO 2 .Hence, thermal treatment can be successfully employed to modulate the surface morphology of ZrO 2 and ZrO 2 -based photocatalysts.

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 photo-

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 ZrO 2 /g-C 3 N 4 heterojunction via mixing of g-C 3 N 4 powder into ZrO 2 colloidal dispersion through ultrasonic treatment at 30 • C for 2 h [65].The integration of ZrO 2 nanoparticles into g-C 3 N 4 increased their surface area to 51.8 m 2 g −1 due to its distinct morphology.The TEM results indicated that g-C 3 N 4 showed planer and sheet-like structures while ZrO 2 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 ZrO 2 photocatalyst.For example, Yu et al. synthesized the TiO 2 -N/ZrO 2−x N x heterojunction by subjecting the precursors such as ZrOCl 2 •8H 2 O and Ti (OC 4 H 9 ) 4 under ultrasonic treatment [46].With the variation in the concentration of N, the transformation of tetragonal ZrO 2 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 ZrO 2 which led to the substitution of O by nitrogen dopants due to high-energy ultrasonic waves.

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@ZrO 2 -OV photocatalyst through the introduction of NaBH 4 as reducing agent [28].In this process, ZrO 2 and NaBH 4 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 ZrO 2 :NaBH 4 was optimized to 1:0.4 because, beyond this, a further increase in the amount of NaBH 4 meant that the crystal phase of ZrO 2 was transformed into ZrB 2 .The electron paramagnetic resonance (EPR) spectroscopy confirmed the existence of OVs in the B@ZrO 2 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.

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].

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 ZrO 2 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 ZrO 2 /ZnFe 2 O 4 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 (•O 2 − ), which further reacted with H 2 O to produce H 2 O 2 .This generated H 2 O 2 reaction with α-Fe 2 O 3 in ZZFO 12 further produced •OH radicals and participated in dye degradation, as shown in Equations ( 1)-( 7) •OH + indigo carmine → Degradation (6)

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 photo- Other than dyes, phenolic compounds have also been treated effectively with ZrO 2 -based nanocomposites.For example, ZrO 2 /g-C 3 N 4 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 •O 2 − radicals which boosted the degradation efficiency.Moreover, from the radical scavenging results, it was confirmed that •O 2 − radicals and h+ primarily participated in the degradation process.

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 ZrO 2 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 Bi 2 O 3 -ZrO 2 nanocomposite [64].Bi 2 O 3 -ZrO 2 has a greater photocatalytic activity (92.30%) after an irradiation period of 180 min.The mechanism for enhanced photocatalytic activity of Bi 2 O 3 -ZrO 2 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 H 2 O to create O 2 (Figure 6c).The detailed photocatalytic processes that occur on the surface of Bi 2 O 3 -ZrO 2 are summarized in Equations ( 8)-( 11

Concluding Remarks
Among various metal-oxide photocatalysts, ZrO 2 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 ZrO 2 , 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 ZrO 2 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 ZrO 2 .Moreover, various synthetic routes involving bottomup and top-down methods can be utilized to modulate the physicochemical features of ZrO 2 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 ZrO 2 for large-scale photocatalytic applications.