Fabrication of vanadium dioxide thin films and application of its thermochromic and photochromic nature in self-cleaning: A review

Vanadium dioxide (VO2) and many binary compounds of vanadium can transform from a semiconducting state to a metallic state under the influence of temperature or light. The transformation is triggered by modifications in the oxide's crystalline structure, often initiated by changes in its light-sensing and electrical attributes. This ability makes them suitable for smart coatings, electronic switches, self-cleaning surfaces, window glass, and many more. These potential applications have prompted tremendous interest in VO2thin films, focusing on their fabrication and application. This article reviews current fabrication methods for VO2thin films, the potential application of their thermochromic and photochromic properties in self-cleaning coatings, the challenges facing their application, and some remedies devised to address the challenges. The various challenges facing its different synthesis and fabrication methods are also highlighted. First, the different transitional phases of VO2and routes to transform from one stage to the other are presented.


Introduction
The emergence of the Industry 4.0 revolution emphasizes the need to use smart materials in many engineering applications.Smart materials are those that manifest self-change once subjected to external stimuli.Vanadium dioxide (VO 2 ) is an inorganic substance that exhibits phase change at very low temperatures. 1and is thus considered a smart material.The compound has good absorbance in the average infrared spectrum, making it suitable for photocatalytic application.It can continually transform under the influence of temperature and lighting from an insulator or semiconductor to a conductor and vice versa, making it a thermochromic and photochromic smart material. 2This phase transformation usually occurs with VO 2 at around 68°C, and in a thin film, this can assist in controlling the sun's radiation. 3Vanadium itself is polyvalent, existing in several oxidative states.These include V 2+ , V 3+ , V 4+ , V 5+ , , and more. 4These vanadium's outstanding properties allow it to exhibit several binary compounds of vanadium and oxygen, such as VO, VO 2 , V 2 O 3 , and V 2 O 5 .A reversible semiconductor-to-metal transition, popularly referred to as a metal-to-insulator transition (MIT), often occurs when vanadium oxides express an abrupt change in their intrinsic electrical conductivity under externally induced conditions of stress or temperature. 5This is why VO 2 has been among the most widely recognized oxides in the past six decades for high-sensitivity switching elements and smart devices, including window glass.Besides their catalytic, electronic, and thermochromic applications, self-cleaning and photocatalytic degradation are essential applications of VO 2 thin films. 6VO 2 has also been doped with other metals or metal composites to improve its self-cleaning functionality. 4,7,8The doping resulted in better optical properties, improved photodegradation characteristics, and excellent hydrophilicity.These made them more suitable for energy saving and smart window uses.Studies on the influence of hydrogenation, 9 optical phase transition properties, [10][11][12] effect on deposition temperatures, 13,14 and thermal properties of VO 2 thin films [15][16][17] have also been carried out While VO 2 has attracted significant interest as a diverse inorganic material for energy-saving and innovative appliance applications, the experimental construction of pure thermochromic VO 2 is very difficult.It is still complicated to prepare a stable thermochromic VO 2 structure free of impurities in bulk quantities due to their low redox stability, multivalent character, and many oxidation states. 18The growing of thermochromic VO 2 nanostructures on particular substrates frequently results in structures with confined size and output, suggesting the need for a straightforward and well-ordered large-scale technique for fabricating stable VO 2 monocrystals.
However, some authors have researched to mitigate thermochromic deterioration and improve the stability of VO 2 thin films. 19,20or more than six decades of existence, research work on VO 2 has produced only a few review publications.Chain presented a article highlighting the optical characteristics of vanadium oxide coatings, linking them to specimen fashioning and the resulting film structure to better understand film properties. 3Eyert reviewed the band theory of VO 2 and presented the first fundamental calculation to determine the electronic composition of metalliferous rutile and the nonconducting monoclinic (M1) form of VO 2 . 21Nag and Haglund reviewed the different synthesis methods of VO 2 thin film, emphasizing how VO 2 films can be doped to alter their switching characteristics and transition temperatures. 22Gao et al. offered a fresh look at polymer-aided deposition methods of nanoceramic thermochromic VO 2 films and compared them to gas-phase methods. 23Wang et al. discussed the latest progress in VO 2 smart coatings, presenting techniques to ameliorate its thermochromic features. 24Liu et al. summarized the properties and significant application of VO 2 , giving insight into future studies and highlighting opportunities and challenges. 25Ko et al. reported on the most current advances in VO 2 technology.Their review discussed the physical sources of optical properties of VO 2 , developments in switchable color filters, daylight radiative cooling, and highlighted recent discoveries in VO 2 photonic applications and more. 26urrently, the investigation into the properties of self-cleaning films is attracting interest because of their low cost, high durability, and the fact that there is no downtime for maintenance. 7One form of VO 2 can be photocatalytic, thus demonstrating its ability to self-clean. 27,28This article presents a detailed review of advances in fabrication methods for VO 2 thin films, challenges facing its application, and suggested remedies to address them.First, the different transitional phases of VO 2 and routes to transform from one stage to the other are presented.Brief historical background of each fabrication technique and current innovations to improve it are reviewed.The potential application of VO 2 thin film, challenges facing its applications, and proposed solutions are also examined.The manuscript concludes with the current state of VO 2 thermochromism and photochromism properties in self-cleaning technology and suggestions for future research perspectives.

Background of the different transitional phases and phase transformation routes of VO 2
Changes from one phase to another in the VO 2 binary structure influenced by external stimuli such as magnetic field, strain energy, heat, and surface energy bring about corresponding changes in its physical properties known as a phase transition.This resonates with crystal changes from the lowtemperature monoclinic phase to the rutile phase at higher temperatures due to anatomic variance in electron orbitals.It is, therefore, critical to understanding the physical and chemical characteristics of the different VO 2 binary phases as it will facilitate the mode of application of each phase.The most common phases recently reported are shown in Figure 1, together with conditions that enable their transformation from one to another.VO 2 (M) can either appear as VO 2 (M1) or VO 2 (M2).Both polymorphs display the desired MIT property.The difference between the two is the space grouping of their unit cell properties. 29VO 2 (R) is the high-temperature rutile phase of VO 2 , often obtained in the presence of a surfactant. 30They possess lesser unit cell values than the VO 2 (M) polymorphs.
VO 2 (A) is another phase of VO 2 having unit cell values higher than those of VO 2 (R).They display MIT property at a transition temperature of 68°C.VO 2 (B) is active between 150 and Figure 1.Vanadium dioxide (VO 2 ) phases and conditions of transformation. 31,3260°C.It has found application as a suitable material for the electrode of thermal sensitive batteries due to its layer structure. 33VO 2 (C) can span from a few hours to a few days.It has a format that is made of square pyramids of VO 5 binary compounds sharing its four square edges with four close squares of the VO 5 pyramid.VO 2 (C) was first prepared by Hagrman et al. in a process termed the soft chemical method by dehydrating layered VO 2 hydrate (VO 2 .H 2 O). 34VO 2 (D) possesses more remarkable magnetic properties and quickly transforms into VO 2 (M) at 300°C.Its typical reaction conditions are still under establishment. 35VO 2 (P) was first synthesized by Wu and his collaborators using an elementary chemical reaction path.It quickly transforms into VO 2 (M) once under rapid annealing. 36f all the polymorphs, VO 2 (M), VO 2 (R), VO 2 (A), and VO 2 (B) display greater similarity in structures, which is a VO 6 octahedron.The six oxygen atoms surround the one vanadium atom at the center.The shapes inform the different spatial groups of VO 2 of the octahedron and the different connection models of the atomic coordinates.VO 2 (M) contains the transitional and monoclinous phases of VO 2 (M1) and VO 2 (M2).After exceeding the MIT temperature, VO 2 (M1) transits to VO 2 (R).While transiting from the nonconducting to the conducting phase, the six-sided V-O structure changes from a partial six-sided configuration at low to normal octahedron anatomy at high temperatures.The octahedron V-O bond angle also varies from 90°C to between 78 and 99°C. 30hese anatomic changes correspondingly impact the mechanical, magnetic, optical, and electrical properties of VO 2 (M1) and VO 2 (M2), which may be harnessed for various applications.VO 2 (M2) and VO 2 (M1) nonconducting phases with the monoclinic structure are identified separately by the alignment of V atoms along the c-coordinate.In VO 2 (M1), the pairs of V-V atoms chain along the c-coordinates are curvy, while in VO 2 (M2) phase, the V atom chains are either paired and uninclined or are inclined but unpaired along the c-coordinate.VO 2 (M2) can be regarded as an intermediary state of VO 2 (M1) because it is formed when VO 2 (M1) is strained along a particular axis or is doped with three positive valence ions. 37VO 2 (A) is made of two blocks of the two-sided octahedral shape formed along the c-coordinate and also possesses MIT property at a higher transition temperature of 162°C.Unlike in VO 2 (M), monoclinic VO 2 (A) and VO 2 (B) do not display visible MIT transition properties that may encourage noticeable reforms in their ocular and electrical stance.The temperature coefficient of resistance of VO 2 (B) is outstanding at −7%/K near ambient temperature, making it suitable for application as electrode materials. 38Below 440MPa, VO 2 (B) transforms into VO 2 (A).Horrocks et al. demonstrated that in the presence of argon gas, VO 2 (A) and VO 2 (B) calcined at 475°C could be transformed into VO 2 (R).Various phases of VO 2 may still emerge for thin film and nanostructural applications with the advancement of material manufacturing technology.Table 1 lists crystallographic information on different VO 2 polymorphs, including their space data.

Methods to synthesize and deposit VO 2 thin films
In this section, the various VO 2 thin-film fabrication methods are described.It has been reported that the morphology of VO 2 thin films depends on the synthesis and deposition method. 42Here, the merits and demerits of each approach relative to the surface texture and possible influence on self-cleaning ability are reviewed

Sol-gel
Sol-gel is one of the wet chemical synthesis procedures that involves dissolving atomic particles of a precursor (generally a metal alkoxide) into water or alcohol and then heating and stirring it to form a gel.The prepared precursor solution may be hydrolyzed or condensed, aged and dried, or heat treated.The metal alkoxide activity, the water-to-alkoxide ratio, solution pH, redox conditions, solvent properties, and additives affect the hydrolysis and condensation reactions in sol-gel synthesis.The initial precursor state will significantly impact the prepared films' chemical, physical, and stoichiometric properties.Catalysts are often employed to regulate hydrolysis and condensation in sol-gel processes.Figure 2 shows the sol-gel process and possible deposition routes following the gel formation.The first sol-gel synthesis of VO 2 dates back to 1983 when Greenberg used Vanadium(V) oxytripropoxide as a precursor to synthesize VO 2 . 43Table 2 summarizes recent efforts in advancing the fabrication of VO 2 thin films using the sol-gel process.
Many researchers are interested in sol-gel technology because it is a cheap, uncomplicated, and low-temperature process.Key advantages and disadvantages of the sol-gel process are presented in Table 3, together with those of the other deposition methods.Lukong et al.

The dip-coating method
Dip-coating is a common technique for applying a thin, homogenous coating on substrates of different geometries.It involves immersing the substrate in a gel of the material, usually less dense, to allow excess material to flow back into the gel solution while the substrate is lifted out, as shown in Figure 3.A nip roller squeezes the surplus material off the substrate surface to create a smooth thin film. 48he dip-coating technique is handy for parts with complex geometries and curved surfaces, which other spraying methods cannot coat.Although automatic machines are the most convenient, hand dipping works well and speeds up the bonding of the coating onto the substrate. 78Before dipping, the coating solution is first synthesized, just as in the sol-gel process.The substrate is then immersed consistently in the coating solution.
Usually, the substrate is left in the solution for a short period and then pulled out vertically at a steady speed. 79As the substrate is lifted from the solution, the material hangs onto it, forming a film.Research works to enhance the deposition of VO 2 thin film using the dip-coating procedure are being carried out using different precursor types.Costals et al. used vanadium tetrakis (t-butoxide) as a precursor to synthesize and dip coat VO 2 thin films on glass slides.The coated film exhibited alternate semiconductor-to-metal transition around 67°C. 53 The method was also employed to doped VO 2 thin films with molybdenum and coated on a glass substrate.
The thin film was heat-treated in a low-pressure carbon monoxide atmosphere to investigate the impact of the dopants on the semiconductor-to-metal phase change transition temperature.The dopant lowered the transition temperature and improved optical reflectance. 80Recently, Outón et al. dip-coated VO 2 thin films by hydrolyzing vanadyl acetylacetonate in a polymer-aided deposition process to study the impact of process parameters on their use in smart window glasses.The thermochromic characteristic of the VO 2 thin films was investigated by altering reduction temperatures and drying conditions.The optical stability of the film relative to its thermochromic action at the microlevel in the material could be aligned to macro changes in its structure. 49he dip-coating approach allows the film to be deposited on both substrate surfaces, easy doping of different metal oxides on each other, excellent layer uniformity, and the capacity to coat large substrate surfaces.It has the disadvantage of being a lengthy procedure and allows delicate parts to slip and drop off the conveyor, creating inconsistency in film thickness.

Atomic layer deposition
With atomic layer deposition (ALD), thin and precisely patterned films can be created on various surfaces.This deposition technology is well progressed.It addresses the pressing requirement in industries for better-quality thin films with thicknesses stretching from less than a nanometer to a few tens over a tangible substrate surface.Because of the inherent self-limiting capability of the ALD technique, submicron proportionate surfaces, including rounded solids and nanostructured porous materials, can be coated to conformity. 81These capabilities have made the ALD one of the most preferred thin-film deposition methods.
In the ALD operation, two precursors or more are pumped changeably into a chamber holding a substrate at a set temperature and pressure.This allows materials to be deposited layer by layer on the substrate's surface. 50In each ALD deposition cycle, the first precursor is exposed in the reactor chamber to create the primary layer on the substrate surface.Excess of the first precursor and all the by-products are then purged out.The second precursor is exposed in the chamber.After which excess of the second precursor and any products are also purged out.The procedure keeps recurring until the desired film thickness is obtained. 51he ALD's capability to fabricate complex and three-dimensional nanoscale materials has attracted great interest in using the technique to develop thin films.Several factors have been considered for the ALD of VO 2 thin films from using novel precursors to considering several parameters governing the process.Willinger et al. used the ALD as a new technique to coat VO 2 films as sensing layers on multi-walled carbon nanotube using vanadium n-propoxide as a precursor and acetic acid at 200°C.A consistent and well-controlled coating of vanadium oxide on the inside and outside walls of the carbon nanotubes (CNTs) was achieved. 52,82The technology provided a new method for fabricating non-sticky resilient composite electrode fabric with considerable thickness, controlled porosity, increased electrical conductivity, and cyclic stability in supercapacitors.Blanquart et al. employed the ALD to fabricate VO 2 thin films from vanadyl acetylacetonate precursor.They studied the impact of process parameters and post-deposition calcination on the film structure.They reported that it was feasible to preferentially generate either VO 2 or V 2 O 5 by changing the environment in which the films were annealed. 83Prasadam et al. demonstrated the highly conformal nature of the ALD by creating vanadium oxide films on CNTs. Figure 4 shows images from a scanning electron microscope (SEM) of the CNT layers before and after the ALD of a conformal vanadium oxide film with a thickness of 15 nm.The SEM examination of the cross-section demonstrated the conformal character of ALD deposits.The vanadium oxide film coated on the CNT and the silicon substrates had the same thickness. 84O 2 thin films grown by ALD have also been adapted to address some critical issues, such as complacent absorption and durable instability accompanying the extensively employed spiro-OMeTAD and MoOx hole transport and barrier layers in lucid and n-i-p self-assembled solar cells for two-end sequential devices.Raiford and collaborators presented an alternate hole contact bilayer of a 30 nm thick spiro-Tetra(N,N-di-p-toly)amino-9,9-spirobifluorene (TTB) undoped layer and 9 nm of air-stable vanadium oxide produced through ALD, using vanadium oxytriisopropoxide as the precursor and water as an oxygen source.This led to optimized optical functionality of the perovskite monolithic cell on a heterostructured silicon substrate. 54Yang et al. used the ALD process to address the limiting effect of power conversion efficiency caused by excessive carrier recombination losses at the contact points in crystalline silicon solar cells. 55Costals et al. published results of vanadium oxide films produced by ALD, performing as a hole-selective junction for n-type transparent silicon solar cells without needing an extra stabilizing sheet.The obtained bothsided contact solar cells had more than an 18% efficiency increase.This revealed the potential of transition metal oxides deposited using the ALD process. 53Figure 5(a) shows a scanning transmission electron microscopic image of the deposited vanadium oxide thin film on a crystalline silicon substrate, (d) and (e) revealing the interlayer between Indium tin oxide and Al 2 O 3 solar cell structures, while Figure 5(b), (c) and (f) depicts XPS analysis for vanadium 2p 3/2 , silicon 2p orbits, and carrier life span against charge density, respectively.Due to its atomic-level dominance in film thickness and high conformance on nanostructured surfaces, the ALD technique deposits various metal-based compounds.ALD methods that extend beyond binary material deposition to permit the synthesis of alloyed, doped, ternary, or quaternary materials are becoming more popular. 85he main merits of the ALD process-which at the moment is an excellent deposition method compared to other deposition techniques-is that it can produce pin-hole-free high-density ultrathin films with exceptional film thickness conformity.The method's drawbacks include significant material and energy waste, toxic nanoparticle emissions, and prolonged deposition times. 56omparatively, VO 2 thin films generated by the ALD process are well suited for microelectronics, holograms, planar lenses, rectifiers, and wave plates.

Pulse laser deposition
Pulsed laser deposition (PLD) is a fast-growing and widely used growth process for thin films and submicron structures, primarily in research settings but with commercial applications. 86It has drawn significant interest in the past decade due to its capability to manufacture stoichiometric films of complicated substances, such as superconductors with accurate crystal structures.With the PLD technique, a high-energy laser source is pointed at a target surface in a highly vacuumed chamber.The target surface is rubbed to create a plasma plume.The plasma plume contains the  target molecules, which are then coated on the substrate surface to obtain a thin film.Using a highenergy laser, rapid melting and evaporation of a target cause all the elements in the target to evaporate simultaneously.This approach can make multicomponent coatings with stoichiometry proportionate to target substances quickly. 87Figure 6 shows a schematic depiction of the most recent PLD process used to create VO 2 thin films on Silicon wafers to study their electrical and optical properties at different temperatures. 88t is now possible to control PLD-generated thin film's properties to meet particular requirements for a specific application by adjusting deposition parameters such as laser flux, background gas, substrate temperature, and distance from the target. 89Several kinds of materials are now being deposited by PLD, including ferromagnetic materials. 90, chalcogenides, 91 metals, 92 oxides of metals, 93 and laser host materials, like garnets, which are often trichotomous or quaternion in composition.
Lee et al. established the possibility of reducing the order of magnitude of resistivity of VO 2 hin films by oxidation in a well-controlled environment using the PLD technique. 94Shibuya and Sawa employed the PLD method to explore the influence of substrate temperature and oxygen pressure on the optical and anatomical properties of VO 2 thin films.Their findings created a phase diagram for oxygen pressure against the inverse of substrate temperature for vanadium oxide films, which now serve as a reference for optimizing the conditions for VO 2 film formation on silicon substrates. 95he impact of gamma rays on PLD-grown VO 2 thin films to understand the implications of gamma rays on the properties of the thin films on a silicon substrate studied by Madiba et al.Substantial gamma irradiations were focused on the thin film in amounts similar to those experienced by space mission jets.The gamma rays created disorders in the delicate film structures pushing its orientation to lower contact angles.Tetra hertz transmission evaluations demonstrated the preservation of VO 2 's integrated design after gamma irradiation; gamma-ray exposure caused a reduction in coating resistivity and transition temperature. 96The findings encourage the possibility for VO 2 films to be used as a shielding layer for spacecraft.
PLD is one of the most reliable methods for depositing single-phase VO 2 thin films without thermodynamic instability issues.Though this is possible, the process is affected by parameters such as target-substrate distance, oxygen flow rate, laser energy, substrate temperature, and more.Bukhari et al. looked into the impacts of oxygen flow rate on the quality of VO 2 thin films under constant pressure. 97The study established the effect of the gas flow rate on the MIT transition property of VO 2 thin film using the PLD process.In the experiments, the resistivity variance between the metallic and insulator phases, the temperature span of the transition, and the width of lagging were all seen to be changing by controlling the flow rate.The results of this research set a benchmark for producing thin VO 2 films that are of high quality.Figure 7(a) to (g) shows the morphologies and structures of the obtained VO 2 thin films under different oxygen flow rates, (r) depicts XRD, (s) XPS scan, while (t) and (u) are the HR-XPS of V 2p and 01s regions, respectively of VO 2 thin film from PLD method.
The application of VO 2 thin films in metamaterials and smart window glass has also been made possible with the PLD method.Bhardwaj et al. grew absolute VO 2 (M1) phase thin films on silicon and silicon dioxide substrate in the presence of heat.Despite their lower heat loss, the thin-film transition rate increased resistance and improved reflectivity by 60%, indicating their potential for smart window use. 98Although limited in terms of surface-covered area, PLD as a thin-film synthesis method continues to pique researchers' interest due to several key advantages.The PLD method makes it simple to make multicomponent films with the desired stoichiometric ratio.It has a high deposition rate, a short test period, and requires a low substrate temperature.
The advantages of the PLD procedure include adaptability to create consistent films from multi-target with multilayers.There are no restrictions on the types of PLD targets that can be used, making it easy to create a wide range of thin-film materials and heterojunction films.All PLD plasma-generated films have lower porosity, increased adherence to the substrate, improved morphology and chemical composition uniformity, and controlled degree of phase, crystallinity, and thickness of deposited coatings.The whole setup is very affordable, nonpolluting, and easy to manage.However, the process is sluggish compared to other deposition methods.In addition, it is not yet possible to use the PLD for vast area deposition.Because of the high cost and large deposition volume of laser equipment for film preparation, the PLD is best suited for areas like optical technology, sensor technology, microelectronics, and new material development. 57e spin coating method Spin coating is a thin-film coating technique that uses a spinning process to produce uniformly adherent films on flat surfaces.A spin coater is the name of the device used to spin a coat.Danglad-Flores et al. were among the first researchers to utilize and expand on this method. 58he centrifugal force produced by the spinning object serves as the basis for how the spin coater functions.A small amount of the coating material is carefully placed onto the center of a flat substrate while either motionless or rotating at a plodding speed.The flat substrate then spins from a slow to a fast rate through the machine's spinning action along with the material that has been applied.The high-speed spinning of the flat substrate caused the capillary action and centrifugal force to drag the viscous solution, forcing it to spread into an even film.All moisture evaporates during the spreading process, leaving the inhaled material on the glass surface as a thin film.Figure 8 shows the spin coating procedure can be broken down into five stages.Post-spin coating procedures such as drying and annealing may follow the deposition phase.Van Zele et al. used the spin coating method to create thermochromic VO 2 thin films.They analyzed the relationship between these thermochromic characteristics and coating thickness to create an ideal thermochromic film that balances a high degree of optical transparency and strong thermochromic switching capabilities. 59The definition of IR switching was determined to be the difference in transmittance between 25°C and 100°C, taken at wavelength λ = 2000 nm, as shown in equation (1)   (%)ΔTr IR = Tr IR (25%) − Tr IR (100 Equation ( 2) was used to evaluate the rate of solar modulation (Tm sol ) The change in solar modulation between the two boundary temperatures (ΔTmsol) was given by equation ( 3) Then equations ( 4) and ( 5) were used to determine the critical temperature Tcr and hysteresis loop width ΔT cr, respectively.Tcr= T1 + T2 2 (4) where ϕ is the air mass spectrum of solar irradiance, taken as 1.5, when the sunrise is 37°above the skyline. 100T 1 and T 2 were the transition temperatures during heating and cooling, respectively.Their report found a VO 2 film thickness of 35 nm to be the ideal coating thickness for achieving  99 excellent visual transparency and high thermochromic switching.Since the spin coating process is cheap and easy to operate, it has been employed by several researchers to study the structural, optical, and morphological properties of VO 2 thin film. 60Also, in combination with the sol-gel technique, it has been employed for high-performing VO 2 thin films and to dope VO 2 thin films with other metallic elements or oxides. 101,102The spin coating method is still widely preferred because of these lucrative advantages.

Other VO 2 thin-film fabrication techniques
One most common VO 2 deposition methods are the magnetron sputtering method.With magnetron sputtering, a gaseous plasma is created and contained within the area where the substance to be deposited-known as the "target"-is located.Highly energized ions hit down the target's surface in the plasma, and the released atoms pass through the vacuum conditions and deposit onto a substrate to form a thin layer.The earliest magnetron-sputtering experiments were conducted in the late 1930s, while the term magnetron first appeared in print in 1921. 66There are numerous source configurations and control options for magnetron sputtering.Successful magnetron sputter deposition methods investigated recently include high-power impulse magnetron sputtering (HiPIMS), pulsed direct current (DC) sputtering, reactive gas sputtering, radio frequency (RF) magnetron sputtering, DC magnetron sputtering, and facing target sputtering.DC sputtering and RF magnetron sputtering are the most often used in the deposition of VO 2 thin films. 67Some innovations have been made to improve these popular methods by making them dual ion assisted or RF-inverted cylindrical magnetron sputtering, producing high-quality VO 2 thin films. 103Recently research interest has been picked in using the HiPIMS sputtering technique to deposit VO 2 thin films. 68,104In reality, the production of high-quality VO 2 films requires high temperatures exceeding 400°C, thereby restricting their use on a broad scale and their compatibility with substrates that are sensitive to temperature.Due to the increased ionization level, high plasma density, and consequently high ion flux of the HiPIMS technique toward the substrate, high crystalline thermochromic VO 2 thin films can be deposited at lower temperatures. 69nother approach for synthesizing crystalline compounds from a hot aqueous solution at high vapor pressure is the hydrothermal method.In this method, crystals are formed in an autoclave that contains both materials and water.Since the starting materials for hydrothermal synthesis are high-purity precursors, the powdered materials have higher chemical purity. 70Gao et al. used hydrothermal synthesis to prepare well-crystallized, highly oriented, and free-standing VO 2 (B) thin films. 71The thermal properties of the produced VO 2 (B) thin films were examined to increase their suitability for Li-ion batteries.Another work on hydrothermal synthesis of VO 2 thin films was carried out by Xygkis et al.In the experiment, a set quantity of VO 2 grains was distributed in differing amounts of polyvinylpyrrolidone, resulting in hybrid materials with different VO 2 /polyvinylpyrrolidone concentrations when dropped on amalgamated silica glass substrates.Varying the molar ratio of VO 2 /polyvinylpyrrolidone influenced a shift in the films' critical transition temperatures, affirming its suitability for energy-saving surfaces. 105he ratio of the reducing agents is critical in hydrothermal synthesis as it influences the outcome morphology of the thin films.Considering the film's high sensitivity to temperature changes, the hydrothermal synthesis reaction is seemingly governed by thermodynamics and agitation. 72This technique allows high-vapor pressure materials to grow near or at the material's melting point. 106Crystals of high quality can be synthesized using this method.Its simplicity makes it an excellent candidate for industrial application.
Photosensitive sol-gel is another innovation recently introduced by Wu et al. to fabricate VO 2 thin films at a microscale.The new microfabrication method involved the exposition of coated films' solution prepared from photosensitive precursors to ultraviolet irradiation, as shown in Figure 9. Then after, the patterned films are heat treated in the presence of nitrogen at 550°C for an hour.It is a prospective substitute technique for VO 2 thin-film-based optics, electronics, and magnetic devices.It has the benefit of producing a variety of pleasing patterns, is inexpensive, and is straightforward. 73Also successfully used in the deposition of VO 2 thin films are methods such as electron beam deposition, 74,107 multilevel absorption, 75 thermal oxidation, 76,108 and variable emittance. 109y way of electrochemical or photochemical electron transfer reactions through optical excitation of electron-hole pairs, vanadium can be changed from one valence metal state to another.For this reason, it can now address a range of construction needs, including store window displays, tomographic screens, conversion of solar energy, and concealment. 110n oblique angle deposition technique has also been developed to create VO 2 thin films with different porosities and microstructures, referred to as sculptured thin films.Porosity and microstructure-controlled thin films with increased optical constants and unique properties are helpful in several applications, such as wrinkled filters, sensors, and phase retarders. 25Sun et al. prepared VO 2 sculptured thin film by reducing V 2 O 5 through thermal evaporation and annealing.In examining how oblique incidence angles affect its potential for application, they found that VO 2 -sculptured thin films are more practical for use as smart windows than uniform VO 2 films. 111

Possible application of VO 2 thin films
Thin films of VO 2 and many of its composites have found application in microelectronics, solar cells, self-cleaning, temperature control, switches, memory devices, actuators, and smart windows manufacturing.Monoclinic VO 2 has received much attention for applications not only as smart solar control coatings, with the potential to reduce the need for heating and air conditioning loads within building infrastructure, but also as self-cleaning surfaces.Its phase transition characteristics also involve changes in electrical conductivity.The monoclinic phase semiconductors with a distinct band gap (∼0.7 eV), and the rutile phase shows semi-metallic properties.These characteristic features make VO 2 a unique material for thermochromic applications. 112VO 2 thermochromic coatings on composite structures have been successfully applied as antifogging, energy-saving, and self-cleaning coatings on buildings, window glass, and car bodies. 113These multifunctional Figure 9. Fabrication of patterned-vanadium dioxide (VO 2 ) thin films using the photosensitive sol-gel method. 73haracteristics can be integrated into the different layers of the film and make it applicable to headway communication, defense, and space networks. 114The thermochromic property of VO 2 has also made it suitable for thermoregulating applications such as heat sensors and s solar energy control surfaces.These properties applied on textile surfaces have shown better durability, great super-hydrophobicity, and self-cleaning. 115The ability of VO 2 thin films to modify solar energy helps to optimize the optical performance of other self-cleaning catalysts such as TiO 2, giving it the capability of withstanding extreme weather changes and multifunction advantages in self-cleaning, especially the TiO 2 -VO 2 composites. 116Another exciting application of thermochromic VO 2 property is in the antibacterial enhancement of paints when double-doped with ZnO and WO 3 .Prommin and his collaborators demonstrated that white color paint with ZnO and WO 3 co-doped VO 2 nano-pigment exhibited self-cleaning properties and almost a 100% antibacterial success under UV irradiation. 117he reversible photochromic property of VO 2 enhances the hydrophilic and photocatalytic properties of surfaces, making it suitable for developing environmentally friendly smart windows.Though no literature is currently available, the photochromic VO 2 property can be used in designing and regulating the coloration degree of surfaces exposed to sunlight. 4lexible VO 2 films could be used in contemporary, flexible electronics because of their unique phase-transition property.However, flexible substrates must be used to generate flexible VO 2 films for practical applications.To fabricate thin films of VO 2 that are flexible, Kim et al. adopted a transfer method whereby graphene was produced on Copper foils to support already created radiofrequency magnetron-sputtered VO 2 films.Copper sheets were then etched, and a graphene-VO 2 hybrid was applied to the polyethylene terephthalate film.The resulting flexible VO 2 films bonded to the glass using appropriate adhesives.The strategy was innovative but complex, raising production costs. 118 VO 2 has also been investigated for metasurface device applications such as biosensing, 120 planar lenses, 121 absorbers surfaces, 122,123 radiative thermal rectifiers, 124 holograms, 125 wave plates, 126 and more.In the past, adjustable metasurfaces were made to control only a single variable, such as phase or amplitude.However, by adjusting the device temperature, it is now possible to create devices that can change both amplitude and phase at a specific wavelength using VO 2

Thermochromic property of VO 2 and self-cleaning
Thermochromic coatings based on VO 2 have been studied extensively for over four decades, with today being a pivotal time for VO 2 applications. 24Changes from the monoclinic phase at low temperatures to the rutile phase at higher temperatures result in structural modifications.In contrast to the monoclinic phase, which is semiconducting and transmissive, the rutile is metallic and reflective in the infrared spectrum.The optical characteristics of the VO 2 in the visible part of the spectrum remain unchanged in both the rutile and monoclinic phases. 127owering the transition temperature for VO 2 from 68°C to more closely to ambient temperature makes it suitable as a self-cleaning coating. 128The drop in transition temperature can be achieved by doping the VO 2 lattice with metal ions.It has been established that at the low transition temperature, the absorption band edge of VO 2 films shifts into the UV region of the spectrum, leaving films transparent in the visible region and improving solar absorption and photocatalytic activity. 129Based on its exceptional thermochromism, self-cleaning characteristics and photocatalytic features are combined into VO 2 material to degrade pollutants in the presence of UV irradiation. 20or thermochromic VO 2 to be effective in self-cleaning coatings, issues of transition efficiency maximization, hysteresis reduction, and infrared reflectivity must be considered appropriately. 130he Ti 4 + ion could dope thermochromic VO 2 because it has a comparable ionic radius and electronic structure to the V 4 + ion. 131In awareness of this characteristic, Chen et al. investigated the effect of Ti doping on VO 2 polymer composites by evaluating the optical characteristics of films made of pure VO 2 and films with various Ti doping concentrations.Their analysis showed that flexible VO 2 films with Ti-doping had better optical elements desired for good self-cleaning functioning, as shown in Figure 11(a) to (d). 132The considerably smaller particle sizes were thought to be responsible for improving the optical characteristics of doped VO 2 films.
Thermochromic VO 2 films are desired in self-cleaning applications, but equity between their luminous transmittance and solar modulation needs to be achieved.In self-cleaning, thin films with high luminous transmittance are preferred because they allow enough solar rays to reach the solar cells beneath the coatings, thus leading to increased power conversion efficiency.VO 2 alone has not been proven to deliver this desired characteristic.About 7%, 50%, and 43% of solar energy are included in the solar spectrum's ultraviolet, visible, and infrared portions, correspondingly. 133Increasing the disparity between the semiconducting and metallic states in the visible light spectrum makes it possible to significantly boost the corresponding solar modulation ability of VO 2 -based self-cleaning coatings.This indicates that the luminous transmittance of the VO 2 smart layer in its semiconducting form should be higher than that in the visible light area, which should be maintained at least 50%.According to several studies, combining VO2's solar transmittance with specific materials produces strong reflection in the visible light area at various temperatures. 134tilizing SiO 2 , TiO 2 , and ZrO 2, among other materials, enhanced VO 2 luminous transmittance. 135he optimized VO 2 /TiO 2 structure showed a tremendous improvement in the luminous transmittance of VO 2 , making it appropriate for use in self-cleaning systems.These composite films demonstrated self-cleaning qualities with a low contact angle and breakdown of organic pollutants due to crystallized anatase TiO 2 . 116ptimization is needed on the refractive index (n) and film thickness (d) for a maximum integrated luminous transmittance.A study by Xu et al. showed that the superior value of n = 2.2 is obtained at a thickness of 50 nm for VO 2 . 136Before a straightforward and economical solution can be created, more studies will be needed to determine how well VO 2 -based self-cleaning ability can balance their luminous transmittances and solar modulation capacity.The best structure at the moment, taking advantage of the thermochromic property of VO 2 , striking a parity amid its solar modulation and luminous transmittance, and simultaneously delivering good self-cleaning characteristics is the proposed Cr 2 O 3 /VO 2 /SiO 2 by Chang et al. 20 The bottom Cr 2 O 3 layer improves VO 2 crystallinity and the luminous transmittance of the structure.The top SiO 2 layer enhances resistance against environmental hazards, serves as a reflection-reducing layer, and provides an excellent self-cleaning capability.To prevent VO 2 from environmental deterioration when used as self-cleaning overlays, some chemically resilient oxide coatings, like WO 3, Al 2 O 3 , and CeO 2 , were explored but posed a challenge whereby some of these substances have the propensity to impair VO 2 's optical characteristics. 137,138otochromic property of VO 2 and self-cleaning Light-induced reversible color shift is known as photochromism.The photochromism of VO 2 is equally an extensive area of research interest.It is expressed by VO 2 s' ability to display quick coloring and bleaching capabilities under sunlight or UV irradiation.It is induced by modifying the valency of vanadium by photolysis or an electrochemical electron transport reaction that occurs when a light-excited electron-hole pair is present.Once stimulated by light, VO 2 automatically starts modulating the transmittance of light in the natural environment. 139Poor transmittance Photochrominisn of VO 2 in isolation has not been openly reported to contribute enormously to the self-cleaning of surfaces because single-phase VO 2 photocatalysts are uncommon. 140Minimal research has recently been conducted to investigate VO 2 's ability to do self-cleaning. 27,141It has been shown to degrade methylene blue, toluidine blue O, RhB aqueous solution, and naproxen.Figure 12(a) to (d) shows the self-cleaning ability of VO 2 of different thicknesses using MB as a pollutant.The graph clearly shows that the degradation rate was fastest for VO 2 film of 300 nm thickness.The photochromic property of VO 2 produces photo-generated holes that aid in creating oxygen vacancies, which adsorb water molecules and encourage hydrophilic action, which photocatalytically destroys organic pollutants.The electron-hole pairs are consumed when the breakdown is complete, resulting in more oxygen vacancies.This is because good photochromic VO 2 in high-intensity light permits visible light transmission through coatings of lower thickness. 142hinen et al. reported that thin films with a high thickness absorb more light reducing the film's transmittance due to increased surface roughness.This led to increased absorbance and selfcleaning activities. 143Work to improve photochromism and self-cleaning of VO 2 by combining it with other materials is already promising. 144hallenges facing the application of VO 2 as self-cleaning thin films and possible remedies The semiconducting and metallic phases of VO 2 exhibit strong absorption in the visible region, demonstrating high photocatalytic and self-cleaning potential.However, the VO 2 films with the best solar modulation are translucent, limiting their practical application in energy-efficient casements.Due to the release of V-ion from VO 2 -rooted thermochromic coatings as they age, many crystalline VO 2 nanoparticles on layers turn out to be hazardous.According to a study conducted by Wu et al., as VO 2 nanoparticles age, more V ions are produced from them than from the original films, resulting in high bacterial toxicity.The degradation reaction leading to the release of V-ions that form toxic bacteria may be characterized in two steps, as illustrated in equations ( 6) and ( 7). 145 This shows why the products of these reactions can easily flow off the surface more than pure VO 2 films.Apart from the toxicity of VO 2 , biofilms of microorganisms form naturally on anthropogenic glass surfaces posing health concerns from the hazardous bacteria it generates.This has to be prevented by shelling VO 2 thin films on window glass.Common shell materials include chemically firm compounds such as TiO 2 , Al 2 O 3 , SiO 2 , MgF 2 , and ZnO. 146Chang et al. thoroughly examined the effect of various interfaces influencing the deteriorating process of VO 2 films. 19They fabricated a hydrophobic HfO 2 encapsulation structure and achieved a prolonged life span in addition to outstanding optical properties for VO 2 film, as shown in Figure 13(a) to (c).Their efforts produced a fresh perspective on the VO 2 thin films' deteriorating process, a successful method to stop it, and a materially improved likelihood of commercialization.Figure 13(b) shows that the VO 2 surface exhibited significant hydrophilicity and wettability because the unsaturated surface oxygen in VO 2 formed hydrogen bonds with the water hydrogen cations.Figures 13 (d) and (e) show the result of the deterioration process of HfO 2 encapsulation.The relative solar modulation capabilities and optical variance at 2500 nm, which are crucial indices to indicate VO 2 's phase transition feature, were computed together with the periodic transmittance measurements to assess the stability of VO 2 films.The life span of the fabricated VO 2 /HfO 2 composite film was estimated using the Hallberg and Peck acceleration factor defined by equation (8). 147

AAF = exp
where AAF is the aging acceleration factor, Ae is the activation energy in (eV), k = 8.617 × 10 −5 eV/K and is known as Boltzmann's constant, T 0 and T t are the operation and the testing temperature (K), respectively, while AH and OH are the speeding up the test and operating condition humidity, respectively, t is an exponential value recommended to be 3 for fully encapsulated HfO 2 in VO 2 thin films.The thermochromic performance of the plain sample denoted V-1 is observed to degrade, along with the phase transition properties rapidly, and completely disappear after 15 days, as shown in Figure 13(d).At the same time, the sample with complete encapsulation by the HfO 2 layer denoted VH-2 exhibits strong stability during the expedited test, while the matching transmittance spectra show no visible change, as shown in Figure 13(e).The assured life of the suggested VO 2 /HfO 2 structure was estimated for complete deterioration to be about 16 years after 100 days of testing time at 60°C temperature and 90% relative humidity, satisfying the condition for practical applications.
Depositing crystalline VO 2 without a buffer layer necessitates soaring temperatures to achieve better lattice anatomy and stoichiometric on flexible substrates.Therefore, doping VO 2 with buffer materials such as Cr 2 O 3 , ZnO, V 2 O 3 , and SnO 2 ; can reduce the temperature at which VO 2 films can be deposited.Hu et al. carried out such a study and, as shown in Figure 14(a) to (d), observed a distinct decrease in the width of temperature hysteresis at the highest Ti dopant concentration. 148he main challenges facing the application of VO 2 thin films and proposed remedies to make them suitable for industrial applications in self-cleaning and smart windows are presented in Table 4.

Conclusion
This article has reviewed the different methods of fabricating VO 2 thin films.The different transitional phases of VO 2 and the different routes of obtaining one phase from the other were examined.Advances in the various techniques to fabricate VO 2 thin films and the challenges facing each method were also reviewed, followed by suggestions that have been made to address those challenges.The article looked at the different sectors for possible application of the different phases of VO 2 films based on the individual properties of each transitional phase.The review concludes with efforts and the feasibility of harnessing the thermochromic and photochromic characteristics of VO 2 in better self-cleaning technology.It is observed that much research is needed to understand how the photochromism of VO 2 can be harnessed for self-cleaning action.This research suggests that the stability issue may be solved by doping VO 2 with other metal oxides and producing multilayered thin films with sandwich architectures made of various polymers and metal oxides.This will assist in preparing VO 2 thin films and their composite for photocatalytic and self-cleaning surfaces.

Future research outlook
Most VO 2 thin-film deposition techniques are only viable at the laboratory level.The chemical vapor deposition method is the only technology currently appropriate for industrial processing.However, it has a high operational cost.The creation of straightforward but effective scale-up techniques for synthesis and film deposition will be crucial in the future.The production of VO 2 thin films on an industrial scale is also considered viable using a two-step process.Future studies should be concerned about how colors like yellow, brown, and deep blue will improve the self-cleaning viability of photochromic VO 2 .Although adding titanium, magnesium, and fluorine has improved its optical and self-cleaning qualities, there is still a need to develop methods of managing VO 2 .
Once exposed to moist acidic environments, VO 2 's thermochromic characteristics appear to deteriorate, making its commercial applications, like self-cleaning film, difficult.here has not been much research done to prove how photochromism works or how useful it is for self-cleaning.More theoretical research should be conducted to comprehend further the connection between photocatalytic activities and VO 2 color changes caused by visible light exposure.

Figure 2 .
Figure 2. Illustration of the sol-gel process.

Figure 3 .
Figure 3. Annotated depiction of the dip-coating technique adapted from Taşdemir et al.77

Figure 5 .
Figure 5. Atomic layer deposition (ALD) deposited vanadium oxides film adapted from Adamatzky and Schubert.85 ; (a) STEM images of vanadium oxide on

Figure 6 .
Figure 6.Illustration of the pulsed laser process used to deposit vanadium dioxide (VO 2 ) thin films on silicon substrate reproduced from Ahmed et al. 88 .

Figure 8 .
Figure 8. Principal stages of the spin coating process.99 Figure 10(a) shows the detailed steps of the transfer technique and (b) the anatomy of the produced graphene-reinforced VO 2 film on polyethylene terephthalate.

Figure 10 .
Figure 10.(a) Steps in the creation of graphene-reinforced flexible vanadium dioxide (VO 2 ) film and (b) different layers of the graphene-reinforced VO 2 coating on polyethylene terephthalate substrate adapted from Cao et al.119

Figure 11 .
Figure 11.The differences in optical characteristics of pure vanadium dioxide (VO 2 ) and Ti-doped VO 2 films.(a) The plot of transmittance against wavelength for 25°C and 90°C deposition conditions, (b) Transmittance of films near-infrared spectrum.(c) The film's ability to transmit and modulate visible light.(d) The 1.1% Ti-doped VO 2 film's ability to modulate the sun's light and its ability to send visible light at different VO 2 concentrations.132

Figure 13 .
Figure 13.Influence of HfO 2 layer on the deterioration of vanadium dioxide (VO 2 ) films in the presence of moisture reproduced from Chang et al. 19 (a) Relationship of water molecules, VO 2 , and hafnium dioxide surfaces.(b) A single layer of VO 2 that was hydrophilic changed to a hydrophobic surface when HfO 2 was added.(c) The different samples of the composite film with HfO 2 protection layer encapsulated.(d) The transmission spectrum of a plain VO 2 sample reveals a quick decline in thermochromic ability.(e) The transmittance spectra of the VO 2 sample with a complete HfO 2 layer encapsulation exhibit strong stability.

Table 1 .
Unit cell properties and space group information of current vanadium dioxide (VO 2 ) polymorphs.

Table 2 .
Key outcomes of some advances in the sol-gel processing of vanadium dioxide (VO 2 )thin films.

Table 3 .
The advantages and disadvantages of the various vanadium dioxide (VO 2 ) thin-film deposition methods, some applications, and references.
-film instability due to its ability to react with variables in the air, such as acid, moisture, and oxygen.Develop multilayer thin films with a protective layer that will improve the service life of the thin film 153Low luminous transmittance for practical applicationHybridize with metal ligands or create nanocomposite of VO 2 doped with Zinc that can improve its response to temperature and color change for better luminous transmittance