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

Abstract

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.

Keywords

Vanadium dioxide fabrication thermochromism photochromism self-cleaning review

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 (VO₂) is an inorganic substance that exhibits phase change at very low temperatures.¹ and 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.² This phase transformation usually occurs with VO₂at around 68°C, and in a thin film, this can assist in controlling the sun's radiation.³ Vanadium itself is polyvalent, existing in several oxidative states. These include V²⁺, V³⁺, V⁴⁺, V⁵⁺,^, and more.⁴ These vanadium's outstanding properties allow it to exhibit several binary compounds of vanadium and oxygen, such as VO, VO₂, V₂O₃, and V₂O₅. 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.⁵ This is why VO₂ 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 VO₂thin films.⁶ VO₂ has also been doped with other metals or metal composites to improve its self-cleaning functionality.^4,7,8 The 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,^10–12 effect on deposition temperatures,^13,14 and thermal properties of VO₂ thin films^15–17 have also been carried out

While VO₂ has attracted significant interest as a diverse inorganic material for energy-saving and innovative appliance applications, the experimental construction of pure thermochromic VO₂ is very difficult. It is still complicated to prepare a stable thermochromic VO₂ structure free of impurities in bulk quantities due to their low redox stability, multivalent character, and many oxidation states.¹⁸ The growing of thermochromic VO₂ 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 VO₂ monocrystals.

However, some authors have researched to mitigate thermochromic deterioration and improve the stability of VO₂ thin films.^19,20

For more than six decades of existence, research work on VO₂has 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.³ Eyert reviewed the band theory of VO₂ and presented the first fundamental calculation to determine the electronic composition of metalliferous rutile and the nonconducting monoclinic (M1) form of VO₂.²¹ Nag and Haglund reviewed the different synthesis methods of VO₂ thin film, emphasizing how VO₂ films can be doped to alter their switching characteristics and transition temperatures.²² Gao et al. offered a fresh look at polymer-aided deposition methods of nanoceramic thermochromic VO₂ films and compared them to gas-phase methods.²³ Wang et al. discussed the latest progress in VO₂ smart coatings, presenting techniques to ameliorate its thermochromic features.²⁴ Liu et al. summarized the properties and significant application of VO₂, giving insight into future studies and highlighting opportunities and challenges.²⁵ Ko et al. reported on the most current advances in VO₂ technology. Their review discussed the physical sources of optical properties of VO₂, developments in switchable color filters, daylight radiative cooling, and highlighted recent discoveries in VO₂ photonic applications and more.²⁶

Currently, 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.⁷ One form of VO₂can be photocatalytic, thus demonstrating its ability to self-clean.^27,28 This article presents a detailed review of advances in fabrication methods for VO₂ thin films, challenges facing its application, and suggested remedies to address them. First, the different transitional phases of VO₂and 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 VO₂ thin film, challenges facing its applications, and proposed solutions are also examined. The manuscript concludes with the current state of VO₂ 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 VO₂

Changes from one phase to another in the VO₂ 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 low-temperature 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 VO₂ 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. VO₂ (M) can either appear as VO₂ (M1) or VO₂ (M2). Both polymorphs display the desired MIT property. The difference between the two is the space grouping of their unit cell properties.²⁹ VO₂ (R) is the high-temperature rutile phase of VO₂, often obtained in the presence of a surfactant.³⁰ They possess lesser unit cell values than the VO₂ (M) polymorphs.

Figure 1.

Vanadium dioxide (VO₂) phases and conditions of transformation.^31,32

VO₂ (A) is another phase of VO₂ having unit cell values higher than those of VO₂ (R). They display MIT property at a transition temperature of 68°C. VO₂ (B) is active between 150 and 260°C. It has found application as a suitable material for the electrode of thermal sensitive batteries due to its layer structure.³³ VO₂ (C) can span from a few hours to a few days. It has a format that is made of square pyramids of VO₅ binary compounds sharing its four square edges with four close squares of the VO₅ pyramid. VO₂ (C) was first prepared by Hagrman et al. in a process termed the soft chemical method by dehydrating layered VO₂ hydrate (VO₂.H₂O).³⁴ VO₂ (D) possesses more remarkable magnetic properties and quickly transforms into VO₂ (M) at 300°C. Its typical reaction conditions are still under establishment.³⁵ VO₂ (P) was first synthesized by Wu and his collaborators using an elementary chemical reaction path. It quickly transforms into VO₂ (M) once under rapid annealing.³⁶

Of all the polymorphs, VO₂ (M), VO₂ (R), VO₂ (A), and VO₂ (B) display greater similarity in structures, which is a VO₆ octahedron. The six oxygen atoms surround the one vanadium atom at the center. The shapes inform the different spatial groups of VO₂ of the octahedron and the different connection models of the atomic coordinates. VO₂ (M) contains the transitional and monoclinous phases of VO₂(M1) and VO₂(M2). After exceeding the MIT temperature, VO₂ (M1) transits to VO₂ (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.³⁰ These anatomic changes correspondingly impact the mechanical, magnetic, optical, and electrical properties of VO₂ (M1) and VO₂ (M2), which may be harnessed for various applications. VO₂ (M2) and VO₂ (M1) nonconducting phases with the monoclinic structure are identified separately by the alignment of V atoms along the c-coordinate. In VO₂ (M1), the pairs of V-V atoms chain along the c-coordinates are curvy, while in VO₂ (M2) phase, the V atom chains are either paired and uninclined or are inclined but unpaired along the c-coordinate. VO₂ (M2) can be regarded as an intermediary state of VO₂ (M1) because it is formed when VO₂(M1) is strained along a particular axis or is doped with three positive valence ions.³⁷ VO₂ (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 VO₂ (M), monoclinic VO₂ (A) and VO₂ (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 VO₂ (B) is outstanding at −7%/K near ambient temperature, making it suitable for application as electrode materials.³⁸ Below 440MPa, VO₂ (B) transforms into VO₂ (A). Horrocks et al. demonstrated that in the presence of argon gas, VO₂ (A) and VO₂ (B) calcined at 475°C could be transformed into VO₂(R). Various phases of VO₂ may still emerge for thin film and nanostructural applications with the advancement of material manufacturing technology. Table 1 lists crystallographic information on different VO₂ polymorphs, including their space data.

Table 1.

Unit cell properties and space group information of current vanadium dioxide (VO₂) polymorphs.

Polymorph	Space group	Crystal structure	Lattice constants				References
Polymorph	Space group	Crystal structure	Αβγ (°)	a (Å)	b (Å)	c (Å)	References
VO₂ (A)	138 (P42/(nmc)	Tetragonal	α = β = γ = 90°	8.43	8.43	7.68	³⁹
VO₂ (B)	12 (C2/m)	Tetragonal	β = 106.6°	12.03	3.69	6.42	³³
VO₂ (C)	139 (I4/mmm)	Tetragonal	N/A	3.72	15.42	15.42	³⁴
VO₂ (D)	13 (P2/c)	Monoclinic	β = 89.39°	4.59	5.68	4.91	³⁵
VO₂ (M1)	14 (P21/c)	Monoclinic	β = 122.6°	5.74	4.55	5.38	²⁹
VO₂(M2)	12 (C2/m)	Monoclinic	β = 91.88°	9.08	5.80	4.53	⁴⁰
VO₂ (P)	62 (Pbnm)	Orthorhombic	β = 90°	4.95	9.39	2.89	³⁶
VO₂ (R)	136 (P42/mnm)	Tetragonal	α = β = γ = 90°	4.55	4.55	2.86	⁴¹

Methods to synthesize and deposit VO₂thin films

In this section, the various VO₂ thin-film fabrication methods are described. It has been reported that the morphology of VO₂ thin films depends on the synthesis and deposition method.⁴² Here, 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 VO₂dates back to 1983 when Greenberg used Vanadium(V) oxytripropoxide as a precursor to synthesize VO₂.⁴³ Table 2 summarizes recent efforts in advancing the fabrication of VO₂ thin films using the sol–gel process.

Figure 2.

Illustration of the sol–gel process.

Table 2.

Key outcomes of some advances in the sol–gel processing of vanadium dioxide (VO₂)thin films.

Authors	Year	Key outcome	References
Ding et al.	2013	Used cetyltrimethylammonium vanadate as a precursor for the first time in a sol–gel and dip-coating process to produce hierarchically porous and continuous VO₂ thin films with significantly improved visible light transmittance	⁴⁴
Cao et al.	2014	Prepared nanoporous VO₂ thin films and increased their light transmittance and solar modulating capability by varying precursor concentration and pre-freezing temperature leading to the best thermochromic property of VO₂.	⁴⁵
Alsawafta et al.	2015	They invented an excellent approach for synthesizing vanadium oxide nanostructures by heating a thin layer at 375–500°C for several hours, leading to better near-infrared electrochromic performance.	⁴⁶
Yazid et al. Chotirat et al.	2018 2021	Established the relationship to enhance the thickness and surface morphology of VO₂ and spin speed Advanced the importance of calcination temperature in achieving the monoclinic VO₂ phase by introducing the second calcination step	^13,47

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.

Table 3.

The advantages and disadvantages of the various vanadium dioxide (VO₂) thin-film deposition methods, some applications, and references.

Method	Advantages	Disadvantages	Application	References
Sol–gel	(1) Simple, cheap, and allows precise control of variables at the molecular level (2)Easily synthesize homogeneous composite (3) Can create amorphous materials (4) Have high chemical reactivity of precursors	(1) Understanding the composition of the gelling solutions is a prior necessity (2) Requires post-deposition heat treatment due to high moisture content (3) Creating thicker films may lead to shrinkage or cracking (4) Requires long preparation, aging, and curing times	Wet chemical synthesis of catalysts, thin films of complex structures, doping, and composite oxide films	^44,46–48
Dip-coating	(1) Can produce layers of any thickness with excellent uniformity (2) Can coat large substrate surface of complex geometries (3)Achieves uniform film deposition on both surfaces (4) Different metal oxides can easily be doped	(1) Process is lengthy (2) Wedging effect in film thickness is common (3) Inconsistency in film thickness leads to product view obscurity (4) Mostly suitable for metal oxides only	Thin films of complex geometries back to back, semiconductors, smart window glass, sensors, and switches	^49–52
Atomic layer deposition (ALD)	(1) Deep-rooted film quality due to its self-limiting capability (2) Can deposit on delicate substrates such as Teflon and substrates with complex structures (3) Produces high-quality ultra-thin films with excellent reproducibility and thickness control (4) Can coat multiple layers in a single operation.	(1) Lot of wasted energy is involved (2) By-products may be very toxic, thus requiring careful management (3) ALD is expensive and uses expensive precursor (4) The process requires time for its chemicals to react	The supercapacitor, ultra-thin films, nanotechnology, biotechnology, photovoltaic, power conversion, and various energy-saving processes	^53–56
Pulse laser deposition (PLD)	(1) Easy and adaptable for the growth of compatible and consistent films. (2) No restrictions on the types of PLD targets (3) It is nonpolluting and easy to manage (4) Not expensive	(1) Low film quality when splashing of molten material occurs (2) The PLD is slow compared to other deposition methods (3) Not suitable for vast area deposition. (4) Material loss due to evaporation	The PLD is best suited for areas like the optical industry, sensor technology, microelectronics, and new material development	^57–60
Spin coating	(1) Cheap and easy to operate (2) Fast spin speeds lead to quick film drying (3) Easy superposition of different materials on different layers (4) Easy regulation of substrates and chemicals used	(1) Only one substrate is processed at a time (2) Substrate size is restricted by its holder (3) Not suitable for complex shapes (4) High manufacturing waste	Nanotechnology, solid-state electronics, semiconductors and textile industries, polymer thin films, and membrane development	^61–64
Sputtering	(1) Creates stable and uniform films (2) Easily deposit materials with a high melting point (3) Can deposit a wide variety of metals, composites, alloys, and insulators (4) Produce precise films with excellent quality	(1) Some materials required very low deposition speeds (2) Target area being bombarded is too small (3) Some dielectrics need an RF energy source (4) Equipment is sophisticated and expensive to run	Thin-film research, metal alloys, integrated circuits, optics, solar panels, capacitors, and transistors.	^65–68
Hydrothermal synthesis	(1) Higher chemical purity (2)Environmentally friendly (3) Easily adapted to suit industrial needs (4) Size, shape, and chemical composition are easily regulated	(1) Autoclaves are expensive (2) Longer reaction times needed (3) Requires concentrated solutions (4) Observing the crystal growth process is difficult if steel tubes are employed	Hybrid materials, Li-ion batteries, amalgamated silica glass, and energy-saving surfaces	^69–72
Electron beam deposition	(1) Excellent deposition rate (2) Effective material use compared to other methods (3) Film's structure and morphology are easily regulated (4) High potential for industrial application	(1) Coating the interior surface of complicated geometries is difficult (2) Filament deterioration may lead to nonuniform films (3) It isn't easy to enhance step coverage (4) X-ray damage might result from e-beam evaporation.	Solar panels, semiconductors, and the coating of thermal barriers, hard surfaces, and wear-resistance thin films	^62,63,73,74
Thermal oxidation	(1) Creates oxide films of high quality (2) Produce dense thin films (3) Easily controlled (4) Cleaner than wet oxidation	(1) Slow growth rate (2) Process is costly (3)Sophisticated equipment (4) Not environmentally friendly	Creating semiconducting thin films, sensors, optical switches, and anti-reflective and energy-saving window glass.	^64,65,75,76

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.⁴⁸

Figure 3.

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

The 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.⁷⁸ Before 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.⁷⁹ As the substrate is lifted from the solution, the material hangs onto it, forming a film. Research works to enhance the deposition of VO₂ 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 VO₂ 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 VO₂thin 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.⁸⁰ Recently, Outón et al. dip-coated VO₂ 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 VO₂ 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.⁴⁹

The 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.⁸¹ These 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.⁵⁰ In 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.⁵¹

The 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 VO₂ 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 VO₂ 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.^52,82 The 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 VO₂ 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 VO₂ or V₂O₅ by changing the environment in which the films were annealed.⁸³ Prasadam 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.⁸⁴

Figure 4.

Example of scanning electron microscope (SEM) images for carbon nanotubes (CNTs) prior (upper row) and after (lower row) the atomic layer deposition (ALD) of vanadium dioxide (VO₂) film.⁸⁴

VO₂thin 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.⁵⁴ Yang 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.⁵⁵ Costals 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 both-sided contact solar cells had more than an 18% efficiency increase. This revealed the potential of transition metal oxides deposited using the ALD process.⁵³ Figure 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 Al₂O₃ 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.⁸⁵

Figure 5.

Atomic layer deposition (ALD) deposited vanadium oxides film adapted from Adamatzky and Schubert.⁸⁵; (a) STEM images of vanadium oxide on bare silicon with the interlayer region, (b) XPS for vanadium 2p^3/2 orbitals, (c) XPS for silicon 2p orbits, (d) schematic of film structure with transfer length, (e) quasi state film structure with photoconductance measurement technique, and (f) a plot of effective carrier life span against excess charge carrier density.

The 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 ultra-thin films with exceptional film thickness conformity. The method's drawbacks include significant material and energy waste, toxic nanoparticle emissions, and prolonged deposition times.⁵⁶ Comparatively, VO₂thin 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.⁸⁶ It 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 high-energy 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.⁸⁷ Figure 6 shows a schematic depiction of the most recent PLD process used to create VO₂ thin films on Silicon wafers to study their electrical and optical properties at different temperatures.⁸⁸

Figure 6.

Illustration of the pulsed laser process used to deposit vanadium dioxide (VO₂) thin films on silicon substrate reproduced from Ahmed et al.⁸⁸.

It 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.⁸⁹ Several 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 VO₂hin films by oxidation in a well-controlled environment using the PLD technique.⁹⁴ Shibuya and Sawa employed the PLD method to explore the influence of substrate temperature and oxygen pressure on the optical and anatomical properties of VO₂ 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 VO₂ film formation on silicon substrates.⁹⁵ The impact of gamma rays on PLD-grown VO₂ 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 VO₂'s integrated design after gamma irradiation; gamma-ray exposure caused a reduction in coating resistivity and transition temperature.⁹⁶ The findings encourage the possibility for VO₂ films to be used as a shielding layer for spacecraft.

PLD is one of the most reliable methods for depositing single-phase VO₂ 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 VO₂ thin films under constant pressure.⁹⁷ The study established the effect of the gas flow rate on the MIT transition property of VO₂ 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 VO₂ films that are of high quality. Figure 7(a) to (g) shows the morphologies and structures of the obtained VO₂ 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 VO₂ thin film from PLD method.

Figure 7.

Influence of oxygen flow rate on the thermochromic property of pulsed laser deposition (PLD)-generated vanadium dioxide (VO₂) thin films reproduced from Bukhari et al.⁹⁷ (a) to (g) SEM images at 0.5, 1, 10, 30, 50, and 90 sccm gas flow rate, respectively; (h) to (p) VO₂ thin film's roughness at 0.5, 1, 10,30,50, and 90 sccm gas flow rate; (q) plot of average surface roughness versus flow rate; (r) XRD plot of the VO₂ thin film at 0.5, 1, 10,30,50, and 90 sccm gas flow rate; (s) XPS scan at 1 and 50 sccm gas flow rate; (t) HR-XPS image showing VO₂ thin film at V_2p region and several oxidation states; and (u) HR-XPS spectra of VO₂ thin film in O1s state at 1 and 50 sccm flow rate.

The application of VO₂ thin films in metamaterials and smart window glass has also been made possible with the PLD method. Bhardwaj et al. grew absolute VO₂ (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.⁹⁸ Although 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.⁵⁷

The 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.⁵⁸ The 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.

Figure 8.

Principal stages of the spin coating process.⁹⁹

Van Zele et al. used the spin coating method to create thermochromic VO₂ 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.⁵⁹ The 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)

(%) Δ T r_{IR} = T r_{IR} (25 %) - T r_{IR} (100^{\circ} C)

(1)

Equation (2) was used to evaluate the rate of solar modulation (Tm_sol)

T m_{sol} = \frac{\int^{ϕ} s o l (λ) * T m (λ) d λ}{\int^{ϕ} s o l (λ) d λ}

(2)

The change in solar modulation between the two boundary temperatures

(Δ T m s o l)

was given by equation (3)

Δ T m s o l = Δ T m s o l (25^{\circ} C) - Δ T m s o l (100^{\circ} C)

(3)

Then equations (4) and (5) were used to determine the critical temperature Tcr and hysteresis loop width

Δ T

cr, respectively.

Tcr = \frac{T 1 + T 2}{2}

(4)

Δ T cr = T_{1} - T_{2}

(5)

where $ϕ$ is the air mass spectrum of solar irradiance, taken as 1.5, when the sunrise is 37° above the skyline.¹⁰⁰ T₁ and T₂ were the transition temperatures during heating and cooling, respectively. Their report found a VO₂ film thickness of 35 nm to be the ideal coating thickness for achieving 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 VO₂ thin film.⁶⁰ Also, in combination with the sol–gel technique, it has been employed for high-performing VO₂ thin films and to dope VO₂ thin films with other metallic elements or oxides.^101,102 The spin coating method is still widely preferred because of these lucrative advantages.

Other VO₂ thin-film fabrication techniques

One most common VO₂ 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 1921.⁶⁶ There 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 VO₂ thin films.⁶⁷ Some innovations have been made to improve these popular methods by making them dual ion assisted or RF-inverted cylindrical magnetron sputtering, producing high-quality VO₂ thin films.¹⁰³ Recently research interest has been picked in using the HiPIMS sputtering technique to deposit VO₂ thin films.^68,104 In reality, the production of high-quality VO₂ 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 VO₂ thin films can be deposited at lower temperatures.⁶⁹

Another 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.⁷⁰ Gao et al. used hydrothermal synthesis to prepare well-crystallized, highly oriented, and free-standing VO₂ (B) thin films.⁷¹ The thermal properties of the produced VO₂ (B) thin films were examined to increase their suitability for Li-ion batteries. Another work on hydrothermal synthesis of VO₂ thin films was carried out by Xygkis et al. In the experiment, a set quantity of VO₂ grains was distributed in differing amounts of polyvinylpyrrolidone, resulting in hybrid materials with different VO₂/polyvinylpyrrolidone concentrations when dropped on amalgamated silica glass substrates. Varying the molar ratio of VO₂/polyvinylpyrrolidone influenced a shift in the films’ critical transition temperatures, affirming its suitability for energy-saving surfaces.¹⁰⁵

The 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.⁷² This technique allows high-vapor pressure materials to grow near or at the material's melting point.¹⁰⁶ Crystals 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 VO₂ 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 VO₂ thin-film-based optics, electronics, and magnetic devices. It has the benefit of producing a variety of pleasing patterns, is inexpensive, and is straightforward.⁷³ Also successfully used in the deposition of VO₂ thin films are methods such as electron beam deposition,^74,107 multilevel absorption,⁷⁵ thermal oxidation,^76,108 and variable emittance.¹⁰⁹

Figure 9.

Fabrication of patterned-vanadium dioxide (VO₂) thin films using the photosensitive sol–gel method.⁷³

By 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.¹¹⁰

An oblique angle deposition technique has also been developed to create VO₂ 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.²⁵ Sun et al. prepared VO₂ sculptured thin film by reducing V₂O₅ through thermal evaporation and annealing. In examining how oblique incidence angles affect its potential for application, they found that VO₂-sculptured thin films are more practical for use as smart windows than uniform VO₂ films.¹¹¹

Possible application of VO₂ thin films

Thin films of VO₂ 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 VO₂ 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 VO₂ a unique material for thermochromic applications.¹¹² VO₂ thermochromic coatings on composite structures have been successfully applied as antifogging, energy-saving, and self-cleaning coatings on buildings, window glass, and car bodies.¹¹³ These multifunctional characteristics can be integrated into the different layers of the film and make it applicable to headway communication, defense, and space networks.¹¹⁴ The thermochromic property of VO₂ 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.¹¹⁵ The ability of VO₂ 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 TiO₂–VO₂ composites.¹¹⁶ Another exciting application of thermochromic VO₂ property is in the antibacterial enhancement of paints when double-doped with ZnO and WO₃. Prommin and his collaborators demonstrated that white color paint with ZnO and WO₃ co-doped VO₂ nano-pigment exhibited self-cleaning properties and almost a 100% antibacterial success under UV irradiation.¹¹⁷

The reversible photochromic property of VO₂ 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 VO₂ property can be used in designing and regulating the coloration degree of surfaces exposed to sunlight.⁴

Flexible VO₂ films could be used in contemporary, flexible electronics because of their unique phase-transition property. However, flexible substrates must be used to generate flexible VO₂ films for practical applications. To fabricate thin films of VO₂ that are flexible, Kim et al. adopted a transfer method whereby graphene was produced on Copper foils to support already created radiofrequency magnetron-sputtered VO₂ films. Copper sheets were then etched, and a graphene-VO₂ hybrid was applied to the polyethylene terephthalate film. The resulting flexible VO₂ films bonded to the glass using appropriate adhesives. The strategy was innovative but complex, raising production costs.¹¹⁸ Figure 10(a) shows the detailed steps of the transfer technique and (b) the anatomy of the produced graphene-reinforced VO₂ film on polyethylene terephthalate.

Figure 10.

(a) Steps in the creation of graphene-reinforced flexible vanadium dioxide (VO₂) film and (b) different layers of the graphene-reinforced VO₂ coating on polyethylene terephthalate substrate adapted from Cao et al.¹¹⁹

VO₂ 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 VO₂

Thermochromic property of VO₂ and self-cleaning

Thermochromic coatings based on VO₂ have been studied extensively for over four decades, with today being a pivotal time for VO₂ applications.²⁴ Changes 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 VO₂ in the visible part of the spectrum remain unchanged in both the rutile and monoclinic phases.¹²⁷ Lowering the transition temperature for VO₂from 68°C to more closely to ambient temperature makes it suitable as a self-cleaning coating.¹²⁸ The drop in transition temperature can be achieved by doping the VO₂lattice with metal ions. It has been established that at the low transition temperature, the absorption band edge of VO₂ films shifts into the UV region of the spectrum, leaving films transparent in the visible region and improving solar absorption and photocatalytic activity.¹²⁹ Based on its exceptional thermochromism, self-cleaning characteristics and photocatalytic features are combined into VO₂ material to degrade pollutants in the presence of UV irradiation.²⁰

For thermochromic VO₂to be effective in self-cleaning coatings, issues of transition efficiency maximization, hysteresis reduction, and infrared reflectivity must be considered appropriately.¹³⁰ The Ti₄⁺ ion could dope thermochromic VO₂ because it has a comparable ionic radius and electronic structure to the V₄⁺ ion.¹³¹ In awareness of this characteristic, Chen et al. investigated the effect of Ti doping on VO₂ polymer composites by evaluating the optical characteristics of films made of pure VO₂ and films with various Ti doping concentrations. Their analysis showed that flexible VO₂ films with Ti-doping had better optical elements desired for good self-cleaning functioning, as shown in Figure 11(a) to (d).¹³² The considerably smaller particle sizes were thought to be responsible for improving the optical characteristics of doped VO₂ films.

Figure 11.

The differences in optical characteristics of pure vanadium dioxide (VO₂) and Ti-doped VO₂ 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 VO₂ film's ability to modulate the sun’s light and its ability to send visible light at different VO₂ concentrations.¹³²

Thermochromic VO₂ 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.VO₂ 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.¹³³ Increasing 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 VO₂-based self-cleaning coatings. This indicates that the luminous transmittance of the VO₂ 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.¹³⁴ Utilizing SiO₂, TiO₂, and ZrO_2, among other materials, enhanced VO₂ luminous transmittance.¹³⁵ The optimized VO₂/TiO₂ structure showed a tremendous improvement in the luminous transmittance of VO₂, 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 TiO₂.¹¹⁶

Optimization 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 VO₂.¹³⁶ Before a straightforward and economical solution can be created, more studies will be needed to determine how well VO₂-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 VO₂, striking a parity amid its solar modulation and luminous transmittance, and simultaneously delivering good self-cleaning characteristics is the proposed Cr₂O₃/VO₂/SiO₂ by Chang et al.²⁰ The bottom Cr₂O₃ layer improves VO₂ crystallinity and the luminous transmittance of the structure. The top SiO₂ layer enhances resistance against environmental hazards, serves as a reflection-reducing layer, and provides an excellent self-cleaning capability. To prevent VO₂ from environmental deterioration when used as self-cleaning overlays, some chemically resilient oxide coatings, like WO_3, Al₂O₃, and CeO₂, were explored but posed a challenge whereby some of these substances have the propensity to impair VO₂'s optical characteristics.^137,138

Photochromic property of VO₂ and self-cleaning

Light-induced reversible color shift is known as photochromism. The photochromism of VO₂ is equally an extensive area of research interest. It is expressed by VO₂s’ 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, VO₂ automatically starts modulating the transmittance of light in the natural environment.¹³⁹ Poor transmittance may be due to high absorption or high reflection of the solar rays. It is beneficial for self-cleaning due to increased absorption in the overlay films since high solar absorption leads to more photocatalytic activity.

Photochrominisn of VO₂ in isolation has not been openly reported to contribute enormously to the self-cleaning of surfaces because single-phase VO₂ photocatalysts are uncommon.¹⁴⁰ Minimal research has recently been conducted to investigate VO₂'s ability to do self-cleaning.^27,141 It 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 VO₂ of different thicknesses using MB as a pollutant. The graph clearly shows that the degradation rate was fastest for VO₂ film of 300 nm thickness. The photochromic property of VO₂ 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 VO₂ in high-intensity light permits visible light transmission through coatings of lower thickness.¹⁴² Shinen 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 self-cleaning activities.¹⁴³ Work to improve photochromism and self-cleaning of VO₂ by combining it with other materials is already promising.¹⁴⁴

Figure 12.

Photocatalytic degradation of methylene blue for different thicknesses of vanadium dioxide (VO₂) thin film; (a) 300nm, (b) 400nm, (c) 500 nm, and (d) rate constant for each thickness. Reproduced with permission from Zhang et al.¹⁴²

Challenges facing the application of VO₂ as self-cleaning thin films and possible remedies

The semiconducting and metallic phases of VO₂ exhibit strong absorption in the visible region, demonstrating high photocatalytic and self-cleaning potential. However, the VO₂ films with the best solar modulation are translucent, limiting their practical application in energy-efficient casements. Due to the release of V-ion from VO₂-rooted thermochromic coatings as they age, many crystalline VO₂ nanoparticles on layers turn out to be hazardous. According to a study conducted by Wu et al., as VO₂ 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

V O_{2} + 4 H^{+} \to V^{4 +} + 2 H_{2} O

(6)

4 V O_{2} + O_{2} \to 2 V_{2} O_{5}

(7)

This shows why the products of these reactions can easily flow off the surface more than pure VO₂ films. Apart from the toxicity of VO₂, 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 VO₂ thin films on window glass. Common shell materials include chemically firm compounds such as TiO₂, Al₂O₃, SiO₂, MgF₂, and ZnO.¹⁴⁶ Chang et al. thoroughly examined the effect of various interfaces influencing the deteriorating process of VO₂ films.¹⁹ They fabricated a hydrophobic HfO₂ encapsulation structure and achieved a prolonged life span in addition to outstanding optical properties for VO₂ film, as shown in Figure 13(a) to (c). Their efforts produced a fresh perspective on the VO₂ thin films’ deteriorating process, a successful method to stop it, and a materially improved likelihood of commercialization.

Figure 13.

Influence of HfO₂ layer on the deterioration of vanadium dioxide (VO₂) films in the presence of moisture reproduced from Chang et al.¹⁹ (a) Relationship of water molecules, VO₂, and hafnium dioxide surfaces. (b) A single layer of VO₂ that was hydrophilic changed to a hydrophobic surface when HfO₂ was added. (c) The different samples of the composite film with HfO₂ protection layer encapsulated. (d) The transmission spectrum of a plain VO₂ sample reveals a quick decline in thermochromic ability. (e) The transmittance spectra of the VO₂ sample with a complete HfO₂ layer encapsulation exhibit strong stability.

Figure 13(b) shows that the VO₂ surface exhibited significant hydrophilicity and wettability because the unsaturated surface oxygen in VO₂ formed hydrogen bonds with the water hydrogen cations. Figures 13 (d) and (e) show the result of the deterioration process of HfO₂ encapsulation. The relative solar modulation capabilities and optical variance at 2500 nm, which are crucial indices to indicate VO₂'s phase transition feature, were computed together with the periodic transmittance measurements to assess the stability of VO₂ films. The life span of the fabricated VO₂/HfO₂ composite film was estimated using the Hallberg and Peck acceleration factor defined by equation (8).¹⁴⁷

A A F = \exp [\frac{A e}{k} * (\frac{1}{T_{0}} - \frac{1}{T_{t}})] * {(\frac{A H}{O H})}^{t}

(8)

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, T₀ 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 HfO₂ in VO₂ 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 HfO₂ 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 VO₂/HfO₂ 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 VO₂ without a buffer layer necessitates soaring temperatures to achieve better lattice anatomy and stoichiometric on flexible substrates. Therefore, doping VO₂ with buffer materials such as Cr₂O₃, ZnO, V₂O₃, and SnO₂; can reduce the temperature at which VO₂ 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.¹⁴⁸

Figure 14.

Hysteresis loops of vanadium dioxide (VO₂) and Ti-doped VO₂ transmittances versus temperature at a fixed wavelength for (a) pure VO₂, then VO₂ doped with (b) 1.2, (c) 2.0, and (d) 2.8 percentage concentrations of Ti, respectively.¹³²

The main challenges facing the application of VO₂ thin films and proposed remedies to make them suitable for industrial applications in self-cleaning and smart windows are presented in Table 4.

Table 4.

Some challenges facing vanadium dioxide (VO₂) thin-film application and proposed remedies.

Challenges	Proposed remedies	References
Low adhesion of films to substrate and vulnerability to chemical attack	Develop different multifunctional thin films of VO₂ composites that can combat these weaknesses	¹⁴⁹
Particle aggregation on the surface, which often decrease the photocatalytic efficiency of VO₂ thin films	Add surfactants to the aqueous precursor solutions to ensure the VO₂ nanoparticles are adequately dispersed	¹⁵⁰
VO₂ nanoparticle deterioration due to aging may generate hazardous ions and microorganisms, posing a health hazard.	Create core–shell structures to cover VO₂ nanoparticles in the core. This retards VO₂ deterioration and toxicity	¹⁴⁶
The constraint of deposition temperature in using standard instruments to deposit VO₂ films on flexible polymer substrates.	Use buffer layers or employ high ionization sputtering apparatus with radiofrequency to deposit the thin film	¹⁵¹
Low solar modulation and incompatibility with many organic polymer structures	Create composites that can absorb visible light and alter color in reaction to temperature variations	¹⁵²
VO₂ thin-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	¹⁵³
Low luminous transmittance for practical application	Hybridize with metal ligands or create nanocomposite of VO₂ doped with Zinc that can improve its response to temperature and color change for better luminous transmittance	¹⁵⁴

Conclusion

This article has reviewed the different methods of fabricating VO₂ thin films. The different transitional phases of VO₂ and the different routes of obtaining one phase from the other were examined. Advances in the various techniques to fabricate VO₂ 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 VO₂ 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 VO₂ in better self-cleaning technology. It is observed that much research is needed to understand how the photochromism of VO₂ can be harnessed for self-cleaning action. This research suggests that the stability issue may be solved by doping VO₂ with other metal oxides and producing multilayered thin films with sandwich architectures made of various polymers and metal oxides. This will assist in preparing VO₂ thin films and their composite for photocatalytic and self-cleaning surfaces.

Future research outlook

Most VO₂ 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 VO₂ 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 VO₂. Although adding titanium, magnesium, and fluorine has improved its optical and self-cleaning qualities, there is still a need to develop methods of managing VO₂.

Once exposed to moist acidic environments, VO₂'s thermochromic characteristics appear to deteriorate, making its commercial applications, like self-cleaning film, difficult. More research will be needed to get around this restriction.

There 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 VO₂ color changes caused by visible light exposure.

Footnotes

Acknowledgments

The authors acknowledge support from the National Research Foundation of South Africa, The University Research Council of the University of Johannesburg, South Africa, and the Global Excellence Stature award.

Author’s contribution

All authors contributed to the study's conception and design. Mr Valantine Takwa Lukong performed the originator, data collection, and first draft. Preliminary review and editing was done by Dr Ukoba Kingsley and Prof. Tien-Chien Jen carried out final editing and source of funding. All authors read and approved the final manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Valantine Takwa Lukong

Tien-Chien Jen

Valantine Takwa Lukong is a PhD candidate at the University of Johannesburg. He holds a master's degree in Mechanical Engineering Science. His area of study is renewable energy, focusing on material characterization and self-cleaning technologies for solar panels, smart materials, and thin films.

Kingsley Ukoba holds a PhD in Mechanical Engineering from the University of Kwazulu-Natal, South Africa. His area of interest is in the food-energy-water nexus resilience and sustainable development in relation to climate change and affordability. He serves as a reviewer for high-impact journals and rapporteur for conferences.

Tien-Chien Jen holds a PhD in Mechanical and Aerospace Engineering specializing in the thermal aspect of grinding. He is the head of the Department of Mechanical Engineering Science, the University of Johannesburg. His research interest in atomic layer deposition, cold gas dynamics spraying deposition, hydrogen generation/filtration/storage, solar cells, fuel cells, nano fabrication, nano structure and materials, renewable energies, bio-fuel, etc. He is the Director of the Atomic Layer Deposition Research Center at the University of Johannesburg, Director of the UJ-Nanjing Tech University of China on Sustainable Materials and Manufacturing. He is an ASME Fellow and the Fellow of the Academy of Science and South Africa.

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

Abstract

Keywords

Introduction

Background of the different transitional phases and phase transformation routes of VO2

Methods to synthesize and deposit VO2thin films

Sol–gel

The dip-coating method

Atomic layer deposition

Pulse laser deposition

The spin coating method

Other VO2 thin-film fabrication techniques

Possible application of VO2 thin films

Thermochromic property of VO2 and self-cleaning

Photochromic property of VO2 and self-cleaning

Challenges facing the application of VO2 as self-cleaning thin films and possible remedies

Conclusion

Future research outlook

Footnotes

Acknowledgments

Author’s contribution

Declaration of conflicting interests

Funding

ORCID iDs

References

Background of the different transitional phases and phase transformation routes of VO₂

Methods to synthesize and deposit VO₂thin films

Other VO₂ thin-film fabrication techniques

Possible application of VO₂ thin films

Thermochromic property of VO₂ and self-cleaning

Photochromic property of VO₂ and self-cleaning

Challenges facing the application of VO₂ as self-cleaning thin films and possible remedies