Photocatalytic activity enhancement of nanostructured metal-oxides photocatalyst: a review

Nanostructured metal oxide semiconductors have emerged as promising nanoscale photocatalysts due to their excellent photosensitivity, chemical stability, non-toxicity, and biocompatibility. Enhancing the photocatalytic activity of metal oxide is critical in improving their efficiency in radical ion production upon optical exposure for various applications. Therefore, this review paper provides an in-depth analysis of the photocatalytic activity of nanostructured metal oxides, including the photocatalytic mechanism, factors affecting the photocatalytic efficiency, and approaches taken to boost the photocatalytic performance through structure or material modifications. This paper also highlights an overview of the recent applications and discusses the recent advancement of ZnO-based nanocomposite as a promising photocatalytic material for environmental remediation, energy conversion, and biomedical applications.


Introduction
The application of nanostructured metal oxide semiconductors in environmental remediation and clean energy production has gained substantial attention in recent years.Among environmentally friendly chemical methods, semiconductor-based heterogeneous photocatalysts are a focal point for solar or indoor light in various environmental applications like water splitting, hazardous waste remediation, and water purification [1][2][3].Semiconductor heterogenous photocatalysis is the highlight of this review, due to its versatile, and low-cost.Heterogeneous photocatalysis is a procedure in which a catalyst, typically composed of semiconductor material, accelerates a photochemical reaction through the absorption of light energy.Heterogeneous photocatalysis operates on solar energy, making it a sustainable and energy-efficient process [4,5].
Metal oxide in nanoscale material dimension has high surface reactivity and photosensitivity due to the high surfaceto-volume ratio.Generally, they can effectively generate reactive oxygen species (ROS) under photon irradiation [16,17].Through the generation of transient hydroxyl radicals (OH•), they enable the efficient breakdown of organic pollutants, pesticides, heavy ions, and other chemical bonds of organic compounds [7], as well as the biological cell structures [18].These photocatalytic reactions lead to the formation of simpler and harmless byproducts [19].
Nanostructured metal oxides are also known for their chemical and photochemical stability.They can maintain their photocatalytic activity over extended periods, making them suitable for continuous applications [20].Moreover, metal oxides are typically abundant and cost-effective materials.The synthesis of metal oxide-based nanostructures is simple and low-cost [16].This makes them economically viable for large-scale applications [21].Photocatalytic processes using metal oxides are environmentally friendly since they do not require the use of chemical reagents or produce harmful byproducts [22].They are a green and sustainable technology.
Although many potential applications of nanostructured metal oxides as photocatalysts have been demonstrated.But attention needs to be paid because some drawbacks cause the performance of the photocatalyst to be low.One of the significant disadvantages of metal oxides is that they mainly operate under ultraviolet (UV) light, which makes them less efficient for applications in natural sunlight or under visible light [23].In some cases, recombination of photogenerated electrons and holes can occur, reducing the photocatalytic efficiency.Strategies to prevent recombination, such as using co-catalysts or modifying the surface, may be necessary [24].The photocatalytic activity of metal oxides can vary depending on the type of pollutant.However, some organic compounds or pollutants may not be effectively degraded using metal oxide photocatalysts [25].
As mentioned earlier, TiO 2 and some other nanostructured metal oxides require UV light to initiate photocatalysis.This limitation can make them less effective in indoor environments or cloudy conditions where UV light is limited.In addition, some metal oxides, such as ZnO, can undergo photo corrosion when exposed to UV light, leading to a loss of material and a reduction in photocatalytic activity over time [26].
The photocatalytic process also can sometimes be timeconsuming, requiring extended exposure to light for effective pollutant degradation, which may not be suitable for rapid treatment in some applications [27].
These reports have provided a foundation for understanding the strengths of nanostructured metal oxides as photocatalysts in various applications.The field of photocatalysis is continuously evolving, and ongoing research continues to uncover new possibilities and improve the efficiency of these nanomaterials for environmental, biomedical, and energy-related applications.

Nanostructured metal oxide photocatalytic mechanism
The main factor of photodegradation of organic compounds is the formation of reactive oxygen species, (ROS) such as hydroxyl radicals or superoxide when photocatalysts are irradiated [28].The electron (e − ) in the valence band, the VB of ZnO becomes excited to the conduction band, CB, and leaves a hole (h + ) in the VB when irradiated with UV and the light appears to be shown in figure 1 Photogenerated electrons engage in a reaction with oxygen, resulting in the creation of superoxide radicals.Subsequently, the superoxide radicals combine with hydrogen ions (H + ) to generate the initial HO 2 • radical.This HO  Photo-generated radicals oxidize structured pollution in the oxidation process with CO 2 and H 2 O as its by-products [14] When the excitation of light with energy is greater than Eg, pairs of electron holes can be produced.Whether spontaneously or under the action of an electric field, the resulting electrons and holes move in the opposite direction, either further inward or on the photocatalyst surface, where the electron receiver can be reduced, and where the donor can oxidize.Otherwise, the recombination of the electron-hole pairs can occur and lead to a loss of opportunity for their use in any redox reaction.This is important because the recombination of electron holes is a limitation in photocatalysts [19].
The energy absorbed by the substance produces the formation of a pair of electron holes but if the pair does not migrate far away or recombination before reaching the surface of the semiconductor, energy is released only as heat.Semiconductor photocatalysts are generally found to have low quantum efficiency due to the recombination of electron holes shortly after they are produced which dominates the activity of photocatalytic.A typical semiconductor redox response has an efficiency of about 30%, which means that 70% of the electron-hole pairs produced recombination before any reaction occurs.Therefore, for the optimal design and functionality of the semiconductor photocatalyst, the recombination should not be ignored [29].
Research in the field of nanostructured metal oxide semiconductor photocatalyst has focused on the design of materials with high photocatalytic activity.The photocatalytic activity depends on the surface area and the size of the semiconductor powder particles, the crystalline structure of the material, and the adjustment of its bulk properties.The reason for the significant impact of these properties on photocatalytic activity is that they serve to isolate electrons and holes or reduce the probability of recombination.

Nanostructured metal oxide as photocatalyst
This review exclusively emphasizes the single nanostructured metal oxide photocatalyst such as TiO 2 , ZnO, WO 3 , and others, attributing its selection to factors such as chemical stability, biocompatibility, resilience in the face of oxidation, satisfactory photocatalytic performance, remarkable photosensitivity, as well as favorable pyroelectric and piezoelectric properties resulting from its specific shape and size.Additionally, its potential for straightforward synthesis using diverse scalable methods is highlighted.These qualities make them a viable choice for a sustainable approach to environmental remediation [30].

Zinc oxide
Some examples of nanostructured metal oxides are iron (III) oxide, zinc oxide (ZnO), niobium pentoxide, titanium oxide (TiO 2 ), tungsten trioxide, vanadium oxide, and zirconia [7].Among all, ZnO has become the most well-known and commonly used material as a photocatalyst.This is because of its special qualities that have shown much promise in recent years [31].These include their affordable price, exceptional oxidation capacity, exceptional photosensitivity, bioconsistency, respectable photocatalytic performance, high chemical stability, and exceptional pyroelectric and piezoelectric properties.ZnO nanostructures has a larger specific surface area (i.e. a highly acute area) compared to ZnO in bulk formed, that plays an important role in the production of reactive oxygen species (ROS) such as hydroxyl radicals and photogenerated charge carriers that, when exposed to UV light, allow the adsorption of materials pollutants and mineralization.Some drawbacks, such as fast recombination and poor corrosion resistance that prevent the practical use of electron-hole pairs, and the relatively low band gap (about 3.37 eV) are among the challenges to be faced in the future.
The band gap of nanostructured ZnO, when calculated solely through DFT, experienced a significant underestimation, necessitating a transition to DFT+U.This approach incorporates an extra Hubbard U potential for each element to rectify discrepancies in band overlap shown in figure 2 [32].The optical band gaps of engineered ZnO nanostructures range from 3.10 eV to 3.37 eV.All the mentioned strengths and weaknesses have made ZnO one of the promising materials as an efficient photocatalyst.
Presently, ZnO-based photocatalysts in the nanoparticle form have been extensively researched and applied, especially for the degradation of organic dyes [33].ZnO nanomaterials have also found utility in drug degradation [34], and the removal of various toxins [35,36].However, ZnO nanoparticles tend to aggregate, a phenomenon that can hinder the photocatalytic activity of ZnO.This aggregation, along with the complexities involved in filtering nano-sized inorganic particles in murky wastewater, limits the efficiency of largescale photocatalytic processes employed for degrading aqueous-phase pollutants [27].Moreover, the released nanoparticles can transform into other carcinogenic, mutagenic, and toxic intermediates, inadvertently leading to the creation of secondary pollutants [37].Another drawback of ZnO nanorods (NRs) as photocatalysts is their limited absorption of photons in the UV range and high recombination rates, which result in low photocatalytic activity [38].
To date, several methods have been explored to modify ZnO's bandgap to mitigate charge carrier recombination losses, enhance its responsiveness to visible light, and improve its resistance to photo-erosion.Numerous approaches, including cationic, anionic, earth-abundant, or co-precipitation methods, as well as thin-film deposition, ion implantation, incorporation of nanoparticles with noble metals (such as Pt, Pd, Au, and Ag), or the coupling of semiconductors with other metal oxides, have been investigated [39].It is important to note that nanomaterials, due to their nano-size and enhanced generation of radicals and reactive oxygen species (ROS) when exposed to radiation, can potentially pose increased toxicity and cytotoxicity risks.This is particularly concerning when considering the potential penetration of photocatalysts into the human skin.However, heterojunction oxide materials can also have adverse effects on human health.
Nonetheless, by supporting and dispersing ZnO nanomaterials to create larger structures and reduce their interaction with or penetration into the human body or other organisms, without sacrificing the effective surface area, the development of heterostructure composites based on nanosemiconductor materials not only boosts catalytic activity but can also mitigate potential health concerns [40].
ZnO nanorod has an optical bandgap of 3.25 eV indicating their high sensitivity at UV light wavelengths [41].For photocatalytic applications employing visible light, the synthesis of heterojunctions and nanocomposites with nanocarbon-based materials such as fullerenes, graphene oxide (GO), carbon nanotubes, and graphene has proven to be an effective strategy [42].Through rGO coating onto the ZnO nanorods, the bandgap can be reduced down from 3.25 to 3.17 eV meaning that its sensitivity can be shifted towards the visible light spectrum [41].The ZnO NRs/rGO had a greater absorbance intensity than pure ZnO NRs in the UV to the visible range.In addition to enhancing structural and electrical properties, nanocomposites enable the utilization of straightforward photocatalysts.
We compared dye photodegradation performance using metal oxide photocatalysts that have been doped and modified as shown in table 1.

Coupling with carbon-based material (Hybrid system).
A heterojunction photocatalyst is also effective in improving the photocatalyst properties of ZnO.It has been reported by Chang et al [50] that photocatalyst heterostructures, combined with the virtues of different compounds including light absorption, charge separation, and charge transfer between different types of semiconductor channelers can lead to rapid separation of photo-generated charges.The carbon nanostructure plays an important role in the development of nanocomposite sites.In the past decade, it has been reported that hetero-junction ZnO with carbon nanotubes can improve the performance of nanocomposites by acting as electron sensitization agents [51].
The formation of nanocomposites with carbon-based nanomaterials (e.g.graphene, graphene oxide) can be used as an alternative to improve photo-electrochemical reactions and energy alteration.Photocatalytic is enhanced using nanocomposite because structural modifications prevent the recombination of the charge carrier [42].The conjugated π system is decentralized by rGO due to the merger of ZnO nanostructures has been shown to enable high charge carrier mobility and relatively low graphene oxide recombination rate [52].The performance of the photocatalyst is predicted to be enhanced by a combination of ZnO and rGO nanostructures [53].Coupling rGO with ZnO NRs has been found to reduce the band gap of ZnO, which can decrease as the grain size decreases due to increased tension [54].
The high aspect ratio, high thermal and electrical conductivity, exceptional strength, exceptional flexibility, and ease of functioning and nano-modification of graphenebased materials have driven their demand for use in various applications in electronics, catalysis, photocatalytic, sensing, and medical [55,56].For example, the development of graphene-semiconductor nanocomposites with chemical properties and surfaces and their potential for use in environmental and energy applications has drawn constant attention to the production of high-quality graphene derivatives such as GO and rGO due to their simple and realistic scaling, and their biocompatibility.Since the surface functions of GO and rGO have carboxylic, hydroxyl, and epoxy groups, which offer useful functions when combined with compounds of metal or alloy materials such as Pd, Au, Pt, Ag, and CoPt, as well as polymers and metal oxides such as ZnO, MnO 2 , Fe 2 O 3 , and TiO 2 , all of which can improve catalytic or photocatalytic performance [57].

2.1.2.
ZnO-rGO photocatalyst.ZnO's photocatalyst capabilities can also be enhanced with photocatalyst heterojunction.According to Chang et al [50] photocatalyst heterostructures, in conjunction with the benefits of various compounds, such as light absorption, charge separation, and charge transfer between different types of semiconductors, can result in rapid separation of photogenerated charges.
Due to its excellent electrical conductivity, high surface area, high electron mobility, and chemical stability, graphene (a member of the carbon family) has recently attracted significant interest in several applications including photocatalysts, gas sensing, photovoltaic devices, and fuel cells [58].The thick sheet of a single atom of graphene, which is a nanomaterial made of carbon sp 2 and arranged in a scattering of hexagonal honeycomb lattice, is made of carbon.Compared to other materials, single-layer graphene has a fairly large specific surface area (2630 m 2 g −1 ), Young modulus (1.0 tera pascals),

Rz dye
The pseudo-first-order reaction rate constant for 15% Cu-doped ZnO is equal to 10.17 × 10 −2 min −1 Cu doping causes an extrinsic defect and intrinsic oxygen vacancies because of the high surface-to-volume ratio in nanorods ZnO: Ag/rGO 100 ml MO solution 0.01746 min −1 Photo-generated electron-hole pairs can be separated by Ag doping and rGO incorporation, which can suppress the recombination probability in ZnO [48] (10 mg l −1 ) Uv 50 mg of photocatalyst 30 min to degrade 40.6% of MO ZnO@GO 50 ml of 1 mM MB 98.5% after 15 min Improved efficiency as a result of the addition of GO, which lowers charge recombination and increases light absorption [49] 10 mg of synthesized ZnO@GO composite 0.254 min −1 thermal conductivity (5000 Wm −1 g −1 ), and mobility charge carrier (200 000 cm 2 V −1 s −1 ) [59][60][61][62].The functional group includes hydroxyl, epoxide, carboxylic, and carbonyl groups primarily responsible for this ideal quality.The Hummer approach is used to increase the interplanar space between graphite through the oxidation process.Excellent electrical insulation is produced when graphene oxide (GO), interferes with electrical conductivity [63].
By adjusting the pH of the liquid in which GO is present and then subjecting it to thermal processes, GO can be reduced to graphene oxide (rGO).Due to the capping of graphene, the specific surface area, and the ability of electric transport, recent studies have shown that the ZnO/GO heterostructure has a higher carrier transport efficiency [64,65] synthesized ZnO-decorated graphene using the solgel method, and they found that higher graphene loading increased dye adsorption during the photocatalytic process, suggesting that graphene can also be used as an absorbent in water treatment.According to research work [66], пthe conjugation between organic pollutants and sp 2 rGO areas is most likely the cause of the increased adsorption of organic pollutants.To achieve the highest removal efficiency, more research on graphene-adsorption behavior in various conditions is needed.The breakdown of organic contaminants by photocatalyst should be enhanced when ZnO and rGO are combined.However, loading rGO above the recommended dose will result in a decrease in degradation rate, which may be due to strong photo absorption and spread [67].
Moreover, the composite also has proven to work effectively as an antibacterial agent.The inactivation of Escherichia coli K-12 was assessed using the visible-lightdriven photocatalytic activity of graphene oxide-zinc oxide (GO-ZnO) composite, which was made using a straightforward hydrothermal method.Under natural solar light irradiation, GO-ZnO composite demonstrated noticeably better photocatalytic bacterial inactivation within 10 min, suggesting that GO-ZnO composite has great potential in wastewater treatment and environmental protection [68].The antibacterial activity of modified metal oxide incorporated with graphene toward yeast, Gram-positive, and Gramnegative bacteria.
For photo-oxidation of organic matter, scientists have worked hard to produce GO-based ZnO nanocomposite in the past 20 years.The development of ZnO-rGO and ZnO-GO heterostructures reduces the rate of recombination of charge carriers and prolongs the light response to visible light, which causes an increase in photocatalytic performance, especially when exposed to visible light.The main ZnO-rGO and ZnO-GO nanocomposite photocatalysts for color photo-oxidation and as antibacterial agents are listed in table 2 along with their main characteristics, including synthesis, morphology, and photocatalytic conditions and performance.

Iron (III) oxide
Iron (III) oxide, also known as ferric oxide or hematite (Fe 2 O 3 ), has been explored as a photocatalyst in various research studies.Photocatalysis involves the use of a semiconductor material to absorb light and generate electronhole pairs, which can then participate in redox reactions to catalyse certain chemical reactions.
Iron (III) oxide has a wide band gap (around 2.2 eV), which makes it responsive to visible light.It absorbs light in the ultraviolet and visible regions of the electromagnetic spectrum.Iron (III) oxide has been investigated for its ability to break down organic compounds in water and air under light irradiation and has been explored for photocatalytic water splitting to generate hydrogen gas, which is a clean and renewable energy source [9].

Niobium pentoxide
Niobium pentoxide (Nb 2 O 5 ) is another material that has been investigated for its photocatalytic properties.Niobium pentoxide typically has a band gap in the range of 3.0-3.5 eV, which places it in the ultraviolet region.It absorbs light in the UV region and extends into the visible range, depending on its specific form and modifications.
Similar to other photocatalysts, researchers have explored strategies such as doping niobium pentoxide with other elements and nanostructuring to enhance its photocatalytic activity [12].Doping with different elements can modify the electronic band structure, expanding the absorption range into the visible spectrum.

Titanium dioxide
Titanium dioxide (TiO 2 ) is one of the most widely studied and utilized photocatalysts due to its excellent photocatalytic properties.Titanium dioxide has a relatively wide band gap of around 3.2 eV, placing it in the ultraviolet (UV) range.It primarily absorbs UV light, limiting its efficiency in utilizing visible light for photocatalysis.
TiO 2 is commonly used for the degradation of organic pollutants, such as dyes, pesticides, and contaminants in air and water [7].TiO 2 -coated surfaces are used for their selfcleaning properties, as they can break down organic substances when exposed to light [76].

Tungsten trioxide
Tungsten trioxide typically has a relatively large band gap, which can vary depending on its specific crystalline structure.The band gap is generally in the range of 2.5-2.8eV.Similar to other metal oxides, WO 3 primarily absorbs light in the UV region, and extending its absorption into the visible range is a focus of research.
Tungsten trioxide exhibits different crystal structures, with the most common phases being monoclinic and orthorhombic.The arrangement of atoms in the crystal lattice influences its physical and chemical properties.Tungsten trioxide has catalytic properties and is used as a catalyst or catalyst support in various chemical reactions, including oxidation and hydrogenation reactions.
Tungsten trioxide nanoparticles and nanostructures have unique characteristics, and their properties can differ from those of the bulk material.Nanoscale tungsten trioxide exhibits enhanced surface area and reactivity, impacting its performance in various applications.Understanding these properties allows scientists and engineers to tailor the use of tungsten trioxide in different technological applications, ranging from electrochromic devices to catalysis and gas sensing [13].

Vanadium oxide
Vanadium oxide (V 2 O 5 ) has attracted attention as a photocatalyst due to its unique properties and potential applications in various fields, particularly in environmental remediation and solar energy conversion.Vanadium(V) oxide (V 2 O 5 ) typically has a band gap in the range of 2.2-2.5 eV.It absorbs light in the UV and part of the visible spectrum.
Vanadium oxide exhibits photocatalytic activity, allowing it to facilitate chemical reactions under the influence of light, typically ultraviolet (UV) light.This property is harnessed for various applications, including the degradation of pollutants in air and water [11].
The electronic structure of vanadium oxide plays a crucial role in its photocatalytic behaviour.Vanadium exists in multiple oxidation states, and the transition between these states during photocatalysis contributes to the generation of electron-hole pairs, which drive redox reactions.

Metal oxide photocatalytic activity enhancements
Metal oxide is a wide-band gap semiconductor with excellent photocatalytic properties.When exposed to ultraviolet (UV) or visible light, metal oxide generates electron-hole pairs, which can participate in redox reactions, thereby facilitating the degradation of organic pollutants, such as dyes, in wastewater.However, several challenges limit the efficiency of metal oxide in photocatalysis, including rapid charge recombination and limited light absorption in the visible spectrum.
Enhancing the photocatalytic activity of metal oxides is a critical aspect of improving their efficiency for various applications.Several methods and strategies have been developed to boost the photocatalytic performance of metal oxide materials.In this subtopic, we will explore some of these methods based on recent literature reviews to support their effectiveness summarized in figure 3.

Doping with nonmetals or metals
Doping metal oxides with specific non-metals (e.g.nitrogen, sulfur) or metals (e.g.transition metals) is a well-established method to enhance photocatalytic activity.These dopants modify the electronic structure of metal oxide, narrowing its bandgap, and creating new energy levels that trap charge carriers, thereby reducing recombination.Doped metal oxide exhibits improved photocatalytic activity under visible light [77].
As previously mentioned, the photocatalytic reaction primarily relies on the energy band gaps and the presence of hydroxyl radicals.A notable drawback of metal oxide semiconductors as photocatalysts is their relatively low efficiency in separating charges.To address this limitation, strategies have been employed to modify metal oxide's physical and chemical properties by introducing metallic and non-metallic impurities.These impurities serve to shift the energy levels of the valence band upwards and narrow the bandgap towards the ultraviolet-visible spectrum [78].
Recent investigations have revealed that non-metallic dopants like nitrogen, carbon, sulfur, and fluorine can modify the metal oxide bandgap by substituting oxygen vacancies (V O ), thereby introducing additional oxygen vacancy defects on the surface of nanoparticles.Specifically, elements such as carbon, fluorine, oxygen, and nitrogen can penetrate lattice vacancies and form bonds with atoms through oxidation processes due to their exceptionally small sizes [79].
Among non-metallic dopants, carbon stands out as a leading candidate for semiconductor doping due to its high mechanical strength, excellent chemical resistance, and unique electronic properties [80].Furthermore, doping can lead to an increased generation of OH• radicals, resulting in enhanced efficiency in the degradation of organic pollutants [81].This phenomenon can be explained by the fact that dopants can act as electron sensitizers, preventing the recombination of electron-hole pairs and thus releasing a positive hole (H + ) within the photocatalyst, which is crucial for the formation of OH• radicals [82].
Additionally, compounds such as transition metals, rare earth metals, noble metals, and other metallic elements have demonstrated advantages in tuning metal oxide morphology for photocatalytic applications and specialized purposes.The introduction of metal dopants into metal oxide can elevate photocatalytic activity by creating more trap sites for photo-induced charge carriers, thereby reducing the recombination rate of photo-induced electron-hole pairs [83].In the context of applications such as dye degradation and gas sensing, metal dopants like Ce, Nd, Cu, and Al have been used to decrease the energy band gap of the photocatalyst [83].
For instance, Ag-doped ZnO NRs embedded with graphene oxide have been synthesized from previous studies for photocatalytic degradation of MO.The degradation rate indicates that ZnO: Ag/rGO is more effective than pure ZnO.The merger of the two nanomaterials can slow down the recombination with charge isolation, thus leading to enhanced photocurrent generation and photocatalytic activity [47].

Surface modification
Surface modification of metal oxide photocatalysts involves the deposition of co-catalysts (e.g.noble metals like Pt, Au) or semiconductors (e.g.quantum dots) onto the metal oxide surface.These co-catalysts act as electron sinks, facilitating the efficient transfer of photogenerated electrons and holes, thus improving catalytic activity [84].
Additionally, Pt NPs, which are made up of many nanodots, can trap light in their immediate surroundings, improving light absorption and scattering as well as increasing the surface area available for the adsorption of oxygen and hydroxyl ions [45].
Coating metal oxide with organic or inorganic materials, such as polymers, graphene, or noble metals like platinum or gold, enhances their stability, reduces photo corrosion, and promotes charge separation.These coatings also enable the absorption of a broader range of light wavelengths.This sensitization allows metal oxide to harness sunlight, making them more effective in real-world applications.

Hetero-junction formation
Creating heterojunctions by coupling two different metal oxide materials can lead to synergistic effects that enhance photocatalytic activity.Heterojunctions between ZnO and other semiconductors like titanium dioxide (TiO 2 ) or bismuth oxide (Bi 2 O 3 ) enhance charge separation and overall photocatalytic activity [85].These heterojunctions act as sinks for photoinduced electrons and holes.
A strategy involving the coupling of two semiconductors is an approach that incorporates metal oxides, typically in the form of MxOy/MezOt (where M and Me represent different metal types and x, y, z, and t denote their respective oxidation states within these metal oxides) [86].This combination of materials, known as a nanocomposite, offers certain advantages, particularly in photocatalytic applications, due to its enhanced light absorption, effective suppression of the recombination of photo-induced electron-hole pairs, and improved charge separation.Lin and Chiang's research [87] has revealed that the extended lifespan of charge carriers, achieved through the transfer of electrons between particles within the nanocomposite conduction pathways, leads to a greater number of electrons participating in the photodegradation process.This advantageous property is attributed to the energy level structure within the composite materials [88].
Another study conducted by Nur et al [89] has also demonstrated that highly active photocatalysts can be developed by combining two semiconductors with differing band gaps.According to the proposed mechanism, efficient charge separation occurs as photo-induced electrons are transferred between the photocatalysts.Therefore, nanocomposite heterostructures offer an intriguing alternative for enhancing photocatalyst activity.In the context of ZnO combined with other semiconductors, materials such as TiO 2 /ZnO, SnO 2 /ZnO, SnO 2 /ZnO/TiO 2 , and Co 3 O 4 /ZnO have been the subject of extensive investigation for photocatalytic processes.However, it is worth noting that heterojunction oxide materials have raised concerns regarding the increased toxicity and potential cytotoxicity of synthesized nanomaterials [86].

Nanostructuring
Enhancing volume-to-surface ratios and the impact of quantum properties on particle size are two ways that nanomaterials can change their physical characteristics [90].Likewise, nanostructures differ greatly from conventional materials in terms of their magnetic, optical, and electrical properties.Furthermore, nanomaterials are linked to properties like high adsorption, catalytic activity, and reactivity.
Nanosized particles provide a high surface area, allowing more active sites for reactions and improving light absorption due to quantum size effects [91].Smaller metal oxide structures possess a larger bandgap, enabling them to absorb visible light more effectively and enhance photocatalytic efficiency [92].Currently, nanostructured metal oxides stand out as captivating functional materials, making them a subject of active research.
Additionally, nanowires with lower crystallinity and increased defect levels have been reported as advantageous in photocatalytic applications.This advantage may be attributed to the presence of hydroxyl groups associated with defects, such as oxygen and surface defects, which facilitate the trapping of electron-hole pairs generated by light and consequently enhance their separation.
Table 3 outlines the merits and drawbacks associated with various nanostructures in the context of photocatalytic applications [93].Nanoparticles are favored in solar catalysis due to their substantial surface area, which enables efficient pollutant absorption and the attainment of higher degradation rates.

Improving crystallinity
Controlling the crystallinity of metal oxide photocatalysts is crucial.Highly crystalline materials exhibit fewer defects and grain boundaries, which can hinder charge carrier mobility.Precise control of synthesis conditions can lead to welldefined crystal structures.
Tailoring the exposed crystal facets of ZnO nanorods, such as the (001) or (101) facets, can significantly influence photocatalytic performance.Certain facets exhibit higher reactivity, and controlling facet exposure can enhance dye degradation [94].

Advanced characterization techniques for photocatalytic material
Advanced characterization techniques play a crucial role in understanding the effects of structural, optical, and surface properties of metal oxide photocatalysts on its performance.These techniques provide valuable insights into the photocatalytic material's behaviours during photocatalytic processes, aiding in the optimization of performance.Here are some advanced characterization methods commonly used for metal oxide photocatalysts: XRD are used to determine the crystalline structure and phase composition of metal oxide photocatalysts [95].Next, scanning/transmission electron microscopy (SEM/TEM) provides microstructure, morphology, and elemental composition of photocatalysts at the nanoscale of metal oxide photocatalysts [96].These techniques can reveal how the design and architecture of nanomaterials impacts performance.X-ray photoelectron spectroscopy (XPS) offers information about the elemental composition and chemical state of the surface [97].This helps identify active sites and chemical transformations occurring during photocatalysis.UV-visible spectroscopy investigates the optical properties, band gap, and absorption characteristics which determines light harvesting ability [75].Raman Spectroscopy provides information about molecular vibrations and crystal structures.Photoelectrochemical measurements analyse charge transfer processes and band energies of semiconductor photocatalyst [48].Photoluminescence spectroscopy (PL) used to study charge carrier trapping, immigration, and recombination processes in semiconductor photocatalysts [98].These processes significantly impact photocatalytic efficiency.
These advanced characterization techniques offer a comprehensive understanding of metal oxide photocatalysts, aiding researchers and engineers in optimizing their properties for various applications, including chemical photocatalytic degradation and hydrogen production.

Nanostructured metal oxide photocatalyst application
In general, the enhanced physicochemical attributes of metal oxide nanostructures, encompassing optical, magnetic, electrical, and catalytic characteristics, enable their utilization across diverse technological and industrial domains.These applications paint pigments, cosmetics, pharmaceuticals, catalysis, and supports, as well as medical diagnostics, magnetic and optical devices, flat panel displays, batteries, fuel cells, electronic and magnetic devices, biomaterials, structured materials, and protective coatings as summarized in figure 4.
Nanostructured metal oxide semiconductors, known for their exceptional stability and eco-friendly nature, find extensive application in photovoltaics.They are employed either as photoelectrodes in dye-sensitized solar cells (DSSCs) or for constructing metal oxide p-n junctions [99].
Metal based catalysts also have gained broad acceptance as practical materials for lowering the activation energy in nearly all chemical reactions.Extensive research has been conducted on metal oxides, including 2 , 2 , ZnO, WO 3 , Fe 2 O 3 , Cu 2 O, and SrTiO 3 , in the field of photocatalysis [100].
Moreover, considerable research efforts have been dedicated to the creation of gas sensors using nanostructured metal oxides that exhibit elevated sensitivity, selectivity, and rapid response [100].Recent advancements in this area have been extensively examined in other sources.The sensing proficiency of nanoscale metal oxide materials originates from their highly responsive charge transfer interaction with the surrounding gas composition.
More precisely, magnetic nanoparticles are utilized in the realms of biology and medicine, encompassing applications like magnetic targeting, magnetic resonance imaging, diagnostics, immunoassays, RNA and DNA purification, cell separation and purification, and the generation of hyperthermia [101].
These optical properties have opened a wide range of potential applications that take advantage of their tremendous photo-absorption capabilities.Table 4 summarizes the recent applications of nanostructured metal oxide semiconductors.

Water purification
One of the widely used applications of photocatalysis is as an alternative method in the water treatment process.Especially the wastewater treatment of the food industry, printing, pharmaceuticals, cosmetics, or textiles.It is estimated that as many as 800 000 tons of dyes are produced annually, and about 10%-15% of synthetic dyes are discarded during the textile industry process.Most of these industries use synthetic  dyes [112].Azo dyes, such as methylene blue and methylene orange, with their complex structures, account for approximately 70% of the dyes utilized in textile processing.Notably, the textile industry generates a substantial volume of liquid waste, encompassing both organic and inorganic compounds [113].After processing 12-20 tons of textiles, the daily discharge of effluents ranges from 1000 to 3000 cubic meters [114].These effluents contain elevated levels of dyes and trace metals like chromium (Cr), arsenic (As), copper (Cu), and zinc (Zn).These substances can pose health risks to humans, including issues such as bleeding, skin irritation, nausea, and skin ulcers [115].
Several substances have been studied to develop efficient photocatalysts to allow the decomposition of chemicals.At the same time, semiconductors such as ZnO, TiO 2 , SnO 2 , In 2 O 3 , and Fe 2 O 3 are promising photocatalysts used in water treatments [116].Due to photo-erosion, however, metal sulfide semiconductors and α-Fe2O3 are unstable.TiO 2 and ZnO semiconductors have better pollution degradation efficiency than other metal oxides [31].When irradiated with ultraviolet light, metal oxide generates highly reactive oxygen species (ROS) that break down contaminants [77].

Antimicrobial agents
Gram-positive and Gram-negative bacteria can be effectively inhibited from growing by metal oxide nanoparticles, which are the most commonly used type.Nanostructured metal oxide challenges antibacterial resistance in addition to preventing bacterial growth [117].The most well-known and extensively studied MO-NPs are zinc oxide (ZnO), magnesium oxide (MgO), titanium oxide (T i2 O), iron oxide (Fe 2 O 3 ), copper oxide (CuO), and silver oxide (Ag 2 O) [118,119].Because of their durability, great stability, and reduced mammalian cell cytotoxicity in comparison to organic NPs, these metal oxides are sought-after as antimicrobial pharmaceuticals [120].
Through a variety of mechanisms, including lipid peroxidation, oxidative stress, cell membrane lysis, enzyme inhibition, and proteolysis, MO-NPs can cause bacterial toxicity due to their nanoscale and variable surface chemistry [121].For example, titanium dioxide is often used as a photocatalyst.When exposed to UV light, TiO 2 can generate ROS, which have antimicrobial effects.TiO 2 coatings are applied to various surfaces and materials to help reduce bacterial contamination [122].The inactivation of Escherichia coli K-12 was assessed using the visible-light-driven photocatalytic activity of graphene oxide-zinc oxide (GO-ZnO) composite as shown in figure 5.

Air purifications
Researchers have recently become concerned about volatile organic compounds (VOCs), such as benzene, toluene, ethylbenzene, and xylene which are air pollutants that can harm both the environment and human health due to their toxicity, difficulty in removal, and widespread source.The majority of researchers successfully treat them using low-temperature plasma biological treatment, catalytic oxidation, adsorption, condensation, and recovery methods.Among these, catalytic oxidation technology is currently widely utilized for VOC degradation due to its advantages, which include minimal secondary pollution, high safety factor, high treatment efficiency, and low energy consumption [123].
Nanostructured metal oxide photocatalysts are effective in removing airborne pollutants, including VOCs and nitrogen oxides (NOx).Photocatalytic coatings and materials integrated into air purification systems can help mitigate air pollution and improve indoor air quality.The ability to chemically adsorb such organic compounds has been demonstrated by nanocrystalline MgO, CaO, and Al 2 O 3 as a result of their enhanced surface reactivity [124].For this application, composite metal oxide catalysts for the removal of VOCs are suitable candidates.
Air purification units typically employ plate and annular reactors [125].Because of concerns about cost and safety, UVA light sources are used in the majority of commercial air purification systems.Extending the absorption range is the primary objective of current photocatalysis research, as this will enable solar radiation to efficiently activate the photocatalyst and produce additional benefits for the environment and economy.
Carbon dioxide reduction photocatalysis involves using nanostructured photocatalysts to convert carbon dioxide (CO 2 ) into valuable chemicals or fuels energy-rich carbon products, such as CO and CH 4 through the absorption of light energy.The process typically utilizes semiconductor materials as photocatalysts, which, when exposed to light, generate electron-hole pairs that can participate in redox reactions.In the past few years, scientists have explored different metal oxide semiconductors like titanium dioxide (TiO2), zinc oxide (ZnO), cuprous oxide (Cu 2 O), bismuth vanadate (BiVO 4 ), and strontium titanate (SrTiO 3 ) as potential candidates for photocatalyzing carbon dioxide reduction [126,127].Ongoing research aims to improve the efficiency, selectivity, and stability of photocatalysts for CO 2 reduction, with the goal of mitigating climate change and producing valuable chemicals [128][129][130][131][132].

Hydrogen productions
In 1973, the price of crude oil soared suddenly due to sanctions by the Organization of the Arab Petroleum Exporting Countries (OAPEC), and for the first time, the shortage of future oil became a serious concern.This oil crisis is driving research into alternative energy sources.Fujishima and Honda [133] later reported in the Nature article, about electrochemical photolysis of water using rutile electrodes exposed to light near UV and connected to platinum counter electrodes through electrical loads.
These works provide new potential for the production of clean hydrogen using abundant and cheap water and sunlight.[134] at that time worked for the Allied Chemical Corporation, proving that external potential is not needed to separate water molecules and photo-activity can be enhanced if noble metals (for example, Pt) are implemented.The subsequent investigation [135] compared photogeneration H 2 by a single crystal SrTiO 3 .
Currently, hydrogen production using metal oxides as catalysts is an area of active research and development.There are several methods and reactions through which metal oxides can facilitate hydrogen generation.Here are a few common approaches: I. Water splitting (water electrolysis): Metal oxides, particularly transition metal oxides like ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ), and manganese oxide (MnO 2 ), can be used as catalysts in the water-splitting reaction.Water splitting involves the decomposition of water (H 2 O) into hydrogen gas (H 2 ) and oxygen gas (O 2 ) using electrical energy.Metal oxide catalysts are employed at the anode and cathode to facilitate the electrochemical reactions.These catalysts help accelerate the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, thus improving the overall efficiency of the process [136].

II. Thermochemical water splitting:
Metal oxides can also be used in thermochemical water-splitting processes.These processes involve the use of heat (usually from concentrated solar energy) to drive chemical reactions that result in hydrogen production.Metal oxides, such as cerium oxide (CeO 2 ), are used as oxygen carriers in two-step thermochemical cycles like the cerium-based cycle.In this process, metal oxides release oxygen from water at high temperatures and later reabsorb it in a closed loop, producing hydrogen as a byproduct [137].III.Steam methane reforming (SMR) and dry reforming of methane (DRM): Metal oxides can serve as catalysts in methanereforming reactions.In steam methane reforming, metal oxides can be used to promote the reaction of methane (CH 4 ) with steam (H 2 O) to produce hydrogen gas (H 2 ) and carbon monoxide (CO) [138].Similarly, in dry reforming of methane, metal oxides can facilitate the reaction of methane with carbon dioxide (CO 2 ) to produce hydrogen and carbon monoxide [139].Metal oxides like nickel oxide (NiO) and cerium oxide (CeO 2 ) are commonly used in these processes.IV.Ammonia decomposition: Metal oxides can also be employed as catalysts in the decomposition of ammonia (NH 3 ) to produce hydrogen.This reaction is known as ammonia decomposition, and metal oxides have been studied for this purpose.Ammonia can act as a hydrogen carrier, and by decomposing it using metal oxide catalysts, hydrogen can be generated on-site for various applications [140].
The choice of the metal oxide catalyst depends on the specific reaction conditions, temperature, and the desired outcome.Researchers continue to explore new catalyst materials and optimize existing ones to make hydrogen production more efficient and sustainable.These methods are of great interest for green hydrogen production, which is a key component of efforts to transition to a low-carbon and renewable energy future.
All these applications, advantages, and disadvantages highlight the challenges associated with the use of metal oxides as photocatalysts and the need for ongoing research to address these limitations and enhance their performance in various applications.
Therefore, in this paper, an in-depth analysis of several strategies is provided that are aimed at boosting the photocatalytic activity performance of metal oxide.The discussion starts with the mechanisms involved in photocatalytic activity followed by the approaches in the material and structure manipulations and finally highlights recent advancements in this field.

Important factors affecting metal oxide photocatalytic efficiency
The efficiency of photocatalytic processes involving metal oxides is influenced by several crucial factors.This subtopic explores the key factors that impact the photocatalytic efficiency of metal oxide and their implications summarized in figure 6.

Bandgap energy
One of the fundamental factors affecting the photocatalytic efficiency of metal oxides is their band gap energy.The band gap represents the energy difference between the valence band (VB) and the conduction band (CB) and determines the range of light wavelengths that a photocatalyst can absorb.Metal oxides with narrower band gaps, typically in the visible or UV range, are more efficient in harnessing solar energy for photocatalysis.Titanium dioxide (TiO 2 ) and zinc oxide (ZnO) are examples of metal oxides with band gaps in the UV region, making them effective for UV-driven photocatalytic reactions.
Therefore, the designation was adopted to modify the physical and chemical properties of ZnO by introducing impurities such as metal or non-metal, to divert the energy of the ZnO valence band upwards and shrink the energy of the strip interval to the ultraviolet-visible area [78].For example, the ZnO photocatalyst's absorption edge band shifted to the visible region as a result of doping [141].This shift was attributed to the decreased energy gap of ZnO due to improved absorption in the direction toward the visible region [142].To reduce the interval of the photocatalyst energy band, metal dopants such as Ce, Nd, Cu, and Al have been used in applications in dye degradation and gas sensing [143][144][145].
Recent studies have shown that non-metallic dopants such as nitrogen, carbon, sulfur, and fluorine can divert the metal oxide band gap by replacing V O (oxygen vacancy), thus introducing a greater oxygen vacancy defect on the surface of the nanoparticles.More specifically, C, F, O, and N can seep through lattice crevices and bind atoms through oxidation processes due to their very small size [79].Carbon is the leading candidate for non-metallic dopants in semiconductors due to its high mechanical strength, good chemical resistance, and special electronic properties [80].

Surface area and morphology
The surface area and morphology of metal oxide photocatalysts significantly impact their efficiency.The high surface area provides more active sites for photocatalytic reactions, while specific morphologies like nanowires, nanorods, and nanoparticles offer enhanced light absorption and charge carrier separation.It has also been reported that lower crystallinity and more defects in nanowires are advantages in photocatalytic applications.This may be due to hydroxyl groups tied to defects (i.e.oxygen and surface defects) that promote the capture of pairs of electron holes caused by the photo and thus increase their separation.
Therefore, the synthesis method plays a crucial role in tailoring the surface area and morphology of metal oxides, allowing for the optimization of photocatalytic efficiency.Enhanced photocatalytic activity of metal oxide nanorods contributed by intrinsic oxygen vacancies due to the high surface-to-volume ratio in nanorods [43].

Charge carrier dynamics
Efficient charge carrier dynamics are vital for successful photocatalysis.Photogenerated electron-hole pairs should be effectively separated and transported to the surface to participate in redox reactions.Metal oxides with intrinsic properties that minimize charge carrier recombination, such as ZnO, are preferred for photocatalytic applications.
Metal doping can increase photocatalytic activity by increasing the trap site of the photo-induced charge carrier and thus reducing the recombination rate of photo-induced electron-hole pairs [83].
In addition, doping can also contribute to greater production of OH• radicals, thus, leading to higher degradation efficiency of organic pollutants [81].This is explained by the fact that dopants can act as electron sensitization and prevent the recombination of electron-hole pairs, thus, freeing a positive hole (H + ) of photocatalyst (which is important for the formation of the OH•) radical [82].
Additionally, the introduction of co-catalysts like noble metals or graphene-based materials can enhance charge carrier separation and overall efficiency.Ag doping and incorporation of rGO could suppress the recombination probability in ZnO by the separation of photo-generated electron-hole pairs [48].

Crystal structure and defects
The crystal structure and defects within metal oxide photocatalysts play a significant role in determining their efficiency.Crystal defects, such as oxygen vacancies, can act as active sites for photocatalytic reactions.Moreover, the choice of crystal structure (e.g.anatase, rutile for TiO 2 ) can influence photocatalytic performance [146].Engineering crystal defects and optimizing crystal structures are among the strategies to improve efficiency.

Surface functionalization
Surface functionalization involves modifying the surface of metal oxide photocatalysts with organic or inorganic species.Functionalization can enhance light absorption, facilitate charge separation, and introduce catalytic sites.Strategies like doping with non-metals (e.g.nitrogen), coupling with other semiconductors, or attaching sensitizing molecules can lead to improved photocatalytic efficiency.ZnO: TiO2 nanocomposite powder exhibits better absorption of visible light and generates more electron-hole pairs [147].This is because creating heterojunctions with other semiconductors or other metal oxides can promote charge separation and improve overall photocatalytic.

Environmental conditions
Photocatalysis is sensitive to environmental conditions, including temperature, pH, irradiation, and the presence of specific ions or pollutants [148].These factors can influence the rate and selectivity of photocatalytic reactions.Understanding the optimal conditions for a given metal oxide photocatalyst is crucial for maximizing its efficiency in realworld applications [149].
For example, the effect of various parameters, such as pH of the solution, Nd dosage, initial concentration of MB, and UV light intensity on the photodegradation of a di-azo dye by UV/Nd-TiO 2 was investigated to obtain the highest efficiency of MB photodegradation [150].

Catalyst loading and reactor design
The amount of metal oxide catalyst loaded onto a substrate and the design of the photocatalytic reactor can affect overall efficiency.Higher catalyst loading can lead to increased photocatalytic activity but may also result in light scattering [151].Proper reactor design ensures efficient light penetration and effective mass transfer, both critical for achieving high photocatalytic efficiency [152,153].
There are two primary types of bulk reactors based on how the photocatalysts form: slurry reactors, where the nanoparticles of the catalyst are suspended in water samples to form a slurry, and immobilized reactors, where the catalyst is immobilized on the substrates as a film coating.With a large SA: V and fast mass transfer, the former has low photon transfer due to the non-uniform light distribution caused by the suspended photocatalyst nanoparticles' absorption and scattering.Further complicating operations is the requirement to filter out the suspended nanoparticles following purification.On the other hand, the latter type requires no post-filtration and has good photon transfer, but the low SA: V results in a slow mass transfer [27].
Current research implementing a microfluidic platform has shown that higher degradation of dye using photocatalysts can be achieved, as the nanostructured photocatalyst is brought into very close proximity to the water flow channel [154].
Metal oxides have shown immense promise as photocatalysts for various applications, but their efficiency depends on a multitude of factors.The band gap energy, surface area, morphology, charge carrier dynamics, crystal structure, defects, surface functionalization, environmental conditions, catalyst loading, and reactor design all contribute to the photocatalytic performance of metal oxides.Researchers continue to explore novel strategies to optimize these factors and enhance the efficiency of metal oxide-based photocatalysis, driving progress in clean energy production and environmental remediation.

Challenges and future direction of nanostructured metal oxide as photocatalyst
Metal oxides, such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), and iron oxide (Fe 2 O 3 ), have gained prominence as nanophotocatalysts due to their unique properties and applications in environmental remediation, energy conversion, and more.Here, we explore some of the current hurdles and future directions in the utilization of metal oxides as nanophotocatalysts.
Many metal oxides, including ZnO, primarily absorb ultraviolet (UV) light, limiting their efficiency in utilizing visible light, which constitutes a significant portion of solar radiation.Rapid recombination of photogenerated electronhole pairs is also a common challenge, leading to reduced photocatalytic efficiency.Strategies to minimize this recombination are crucial for enhancing performance.Moreover, nanoparticles tend to aggregate, reducing their effective surface area and catalytic activity.Ensuring the stability and dispersion of metal oxide nanoparticles is vital for long-term performance.
Achieving controlled synthesis of metal oxide nanostructures with desired properties is challenging.Additionally, scalable and cost-effective synthesis methods are necessary for practical applications.In addition, chieving selectivity in the photocatalytic process to target specific pollutants or reactions is a challenge.Enhancing the selectivity while avoiding unintended byproducts is crucial for practical applications.Some metal oxides may exhibit toxicity, raising concerns about their environmental and biological impact.Understanding and mitigating potential toxicity is essential for safe and sustainable applications.
Research efforts are focused on developing metal oxide photocatalysts that can efficiently absorb visible light.This involves designing new materials or modifying existing ones to extend their light absorption range.Innovative strategies, such as doping metal oxides with other elements or designing heterojunctions, aim to reduce the recombination of photogenerated charge carriers, thereby improving overall photocatalytic efficiency.Tailoring the size, shape, and morphology of metal oxide nanostructures can enhance their surface area and, consequently, their catalytic activity.
Advanced nanostructure engineering techniques are being explored for better performance.Continued research into novel synthesis techniques, including template-assisted methods, sol-gel processes, and hydrothermal methods, seeks to achieve better control over nanoparticle properties and improve scalability.Developing metal oxide photocatalysts with enhanced selectivity for specific pollutants or reactions is a crucial future direction.This involves a deeper understanding of reaction mechanisms and the design of catalysts with tailored properties.Rigorous studies on the environmental and biological impact of metal oxide nanoparticles are necessary to address concerns related to toxicity.Strategies to minimize any adverse effects will be essential for the responsible use of these materials.
In conclusion, the challenges and future directions of metal oxides as nanophotocatalysts highlight the need for multidisciplinary research.Addressing these challenges and pursuing innovative solutions will unlock the full potential of metal oxide nanoparticles for a wide range of applications, contributing to sustainable and effective solutions for environmental and energy-related challenges.

Conclusion
In conclusion, the enhancement of metal oxide photocatalytic activity involves various strategies, including doping to optimize band structure, surface modification, heterojunction formation, nanostructuring, and improving crystallinity.These methods have been successfully applied to various metal oxides, and their effectiveness depends on the specific application and desired outcomes.Modified ZnO-based photocatalyst enhanced ZnO's capacity to degrade a wide range of organic and inorganic pollutants, including heavy metal ions, dyes, phenolic compounds, antibiotics, and bacteria.Improved adsorption and charge separation capacity with high stability, extended light absorption range, and enhanced ZnO/graphene-based nanocomposites are among the additional functionalities that graphene-based nanomaterial persuasively suggests.To improve the performance of multifunctional nanocomposites, however, more sophisticated mechanisms must be used in real-world field applications to apply progressive nanocomposites for the effective degradation of pollutants.Continued research and innovation in these areas are essential to unlock the full potential of metal oxide photocatalysts in addressing environmental and energy challenges.

[ 43 ] 44 ]
1 ml of 0.1 g l −1 ZnO NW solution Cu-doped ZnO nanorods 5 ml 0.05 mM MB 97% The addition of Cu doping enlarged the light absorption in the visible light spectrum [Pt Nanoparticle decorated ZnO nanorods 20 ml 10 μM MB 54% UV Pt NPs are made up of numerous nanodots that have the ability to trap light in their surroundings.This increases the surface area available for the adsorption of oxygen and hydroxyl ions and improves light scattering and absorption [45] 120 min 32% visible light ZnO microsphere/Au NPs 40 ml 3.0 × 10−2 mM MB solution ∼98% ZMS/Au composites have higher photoinduced charge carrier transfer efficiency and a higher charge separation 5 ppm of methylene dye 3 mg of the obtained specimens ∼99% efficiency within 30 min Enhanced photon absorption in the visible region and lowered rGO/ZnO/Ag band gap values [47]

Figure 3 .
Figure 3. Methods to enhance the photocatalytic activity of metal oxide semiconductors.

Figure 6 .
Figure 6.Factors affecting photocatalytic efficiency of metal oxide.
2• radical, in turn, reacts with more H + molecules, yielding hydrogen peroxide (H 2 O 2 ) molecules.The presence of hydrogen peroxide molecules then catalyzes the generation of extra hydroxyl radicals.Meanwhile, holes generated in the process interact with water and hydroxide ions to form additional hydroxyl radicals

Table 1 .
Comparison of dye degradation.

Table 2 .
Comparison of photocatalytic performance of photocatalysts ZnO-rGO and ZnO-GO nanocomposite in photo-oxidation of organic dyes and as antibacterial agents.