Review Article | Open Access
Nanotechnological Advances in Catalytic Thin Films for Green Large-Area Surfaces
Large-area catalytic thin films offer great potential for green technology applications in order to save energy, combat pollution, and reduce global warming. These films, either embedded with nanoparticles, shaped with nanostructuring techniques, hybridized with other systems, or functionalized with bionanotechnological methods, can include many different surface properties including photocatalytic, antifouling, abrasion resistant and mechanically resistive, self-cleaning, antibacterial, hydrophobic, and oleophobic features. Thus, surface functionalization with such advanced structuring methods is of significance to increase the performance and wide usage of large-area thin film coatings specifically for environmental remediation. In this review, we focus on methods to increase the efficiency of catalytic reactions in thin film and hence improve the performance in relevant applications while eliminating high cost with the purpose of widespread usage. However, we also include the most recent hybrid architectures, which have potential to make a transformational change in surface applications as soon as high quality and large area production techniques are available. Hence, we present and discuss research studies regarding both organic and inorganic methods that are used to structure thin films that have potential for large-area and eco-friendly coatings.
Structuring thin films at nanometer scale enables distinctive mechanical, chemical, and physical surface functionalities including higher surface area, self-cleaning, photocatalytic, antibacterial, nontoxic, antifouling, hydrophobic, and oleophobic properties [1–6]. In addition to these, abrasion resistant and mechanically resistive features are important for durable and functional applications to save energy and provide clean air and water [7, 8]. Thus, green approaches are essential considering the population increase and growth in industry and their inevitable impact on the world’s natural resources. Such smart and functional coatings can be utilized in different fields including construction industry to enhance material properties and provide energy conservation , flexible surfaces with high hydrophobicity  and high conductivity , large-area solar modules [12, 13], structures with high mechanical strength , bioremediation , textiles , biomedical applications , antifouling , and photocatalytic applications [19, 20]. Hence, with their superior surface properties, the nanostructured coating applications can be extended from self-cleaning processes [21–23] to the protection of cultural heritage assets  and food packaging .
In this review, we specifically focus on catalytic nanostructured large-area thin films that have potential to provide solutions for environmental pollution problems either at present time or in the future after some further advancement. These will include photocatalytic coatings since large-area photocatalytic applications offer potential solutions such as self-cleaning and receive industrial attention also regarding their additional properties (e.g., antibacterial, hydrophobic, and oleophobic properties). However, to examine organic and inorganic methods at once, catalytic enzymatic methods of implementing potentially green and low-cost coatings have been reported considering recent advances in genetic engineering and reduced cost of enzymes [26–29]. With the advances in the last years, incorporating biocatalysts in thin films and coatings has been receiving considerable attention by different research groups due to superior biodegradability, selectivity, and less by-product formation compared to inorganic catalysts [30–32]. To remain in the scope of the topic, only the nanoscale and eco-friendly catalytic process research studies are included. This review excludes prior thin film nanoelectronics research work that cannot be associated with eco-friendly applications and focuses on novel methods to improve solid film catalytic efficiencies preferably at low cost for wide spread usage.
2. Increasing the Performance of Catalytic Thin Films
2.1. Methods for Enhanced Photocatalytic Efficiency
For about 40 years, photocatalysis in the presence of metal oxides [19, 33–37] and for the last decade enhancement of photocatalytic reactions in the presence of nanostructured particles have been investigated [38–43]. Different photocatalytic metal oxide semiconductors have been studied for numerous applications, some of which are water splitting [44, 45], oxidation of organic and inorganic pollutants [46–48], photocatalytic destruction of gaseous organic pollutants [49, 50], and photo disinfection . Additionally, such photocatalytic materials have been used for different applications such as solar cells , chemical sensors , photoelectrochemical cells , and other novel devices . Such a wide range of applications is viable for metal oxides because of their variety of electronic and structural possibilities allowing possible compounds with magnetic, conducting, insulating, or semiconducting behavior . Among the metal oxides, in consequence of its specific properties such as high photocatalytic activity, stability and inertness together with its low cost, and nontoxicity, TiO2 has been exploited most thoroughly by a vast number of researchers and associated fundamental studies have been already discussed by several review articles [19, 20, 57–59].
There are numerous publications and considerable review reports for metal oxide photocatalysis in aqueous environment [48, 60, 61], which clearly shows that using metal oxide photocatalysis is a viable option for decomposing many inorganic contaminants including heavy metals as well as for destroying the microorganisms such as bacteria and viruses [62–65]. In these studies, although complete mineralization in remediation processes can be achieved, the processes are long, expensive, and not practical and lack flexibility to manufacture devices. Thus, photocatalytic reaction in solid form is an important requirement . By this way, utilization of the metal oxides for large-area applications will be enabled.
Apparently when photocatalytic particles are immobilized in thin films instead of being used in water environment, the activation environment is to be regulated cautiously due to the inherent reduction of photocatalytic area  and the emerging recombination problem [68, 69]. Photo-induced carriers are transmitted at the boundary of the photocatalyst and the adsorbed material and reaction efficiency is reduced for solid films because of reduced effective catalyst surface [57, 70]. Photocatalytic activity is found to improve with increasing TiO2 surface area and understandably higher UV light intensity is observed to have a positive effect on photocatalytic reactions . Another important factor is the rather large energy bandgap of the common metal oxides for efficient optical absorption of solar spectrum. The most commonly used metal oxide, TiO2, can absorb less than 5% of the solar energy irradiation if no modification is carried out . Although photocatalyst with large bandgaps can still be applied effectively, obviously improving the light response range will make an important impact on such coatings to become economically viable for large areas. Hence, up to date, different techniques for better light absorption are implemented to overcome the issues related with wide application of such photocatalytic coatings . Figure 1 shows the schematic diagram of methods, mechanisms, and challenges for higher efficiency of photocatalytic reactions in thin films. For photocatalytic functionality to become available for green applications with self-cleaning or antibacterial properties, it is essential to obtain high photocatalytic efficiency at large areas for the visible spectral range. Therefore, first of all, one must choose the photocatalytic material since its structural characteristics such as its crystallinity, geometric, and electronic properties are crucial to determine the overall performance [41, 73]. As it will be reviewed in more detail, size effect, in other words using nanoparticles, is of great importance for increased efficiency in solid films . Another essential criterion for better performance is related to surface properties and, among the techniques to modify the surface features, nanostructuring techniques are promising . Again, as it will be examined with a closer look, synergistic effect with a cocatalyst can improve the activation of redox radicals, decrease recombination of the generated electron-hole pairs, enhance carrier diffusion in bulk, and increase carrier transfer to surface. In general, the utilized techniques can enable changes on the surface or the bulk of the photocatalytic material or they can transform the material’s electronic structure. Thus, there are numerous production, doping, and hybridization techniques to enhance the photocatalytic activity and add other functionalities. Using such techniques, the active spectral range of photocatalytic materials can be controlled and electron-hole generation, recombination, and diffusion reactions can be regulated . In this respect, nanotechnology can potentially present solutions to the current problems related to reaction efficiency and recombination rate in photocatalytic processes such as bandgap engineering, building heterostructured junctions and integration with cocatalysts. Additionally, to utilize the visible light from the sun, and hence improve the photocatalytic performance under solar light, nanostructure design is of great importance .
2.1.1. Nanosize Effect
In the case of working with photocatalytic activation in aqueous environment, typically, photocatalytic particles are added into water and are later recovered from the environment after the photocatalytic process is completed, which increases both operation duration and expenses. For the processes to be less-priced and to take less time, these particles are immobilized in thin films, which also leads to new functionalities for coatings with innovative properties, which is not otherwise possible in aqueous environment. On the other hand, because photo-induced carrier transfer happens at the boundary of the photocatalyst material and the adsorbed material , immobilization is expected to lower the catalyst surface and degrade the photocatalytic effectiveness [68, 78]. For example, when photocatalytic efficiency is compared in different forms of titanium dioxide between the forms of inorganic fiber, sputtering, slurry, and sol-gel, the slurry structure showed the highest performance . The reason suggested for the lower efficiency in solid form is the higher recombination rate because of ionic specimen, relatively smaller active surface, and less chance of ion exchange . Hence, the main problem in immobilization of metal oxides is the relatively low photocatalytic efficiency  for large-area photocatalytic applications to be widely used and provide solutions to pollution and environmental remediation. In this respect, one of the first approaches that aim to increase the photocatalytic efficiency of photocatalytic solids has been via employing nanostructured semiconductors, increasing charge carrier transfer on the reaction surface, or decreasing the recombination rate of electron-hole pairs [81, 82]. Thus, using nanoscale photocatalytic particles is beneficial to overcome the efficiency reduction because of their high surface-to-volume ratio [71, 83] when photocatalytic materials are immobilized in thin films and when it is expected for the photocatalytic activity to take place on the surface of a solid structure . Specifically, the photochemistry of nanocrystalline metal oxides has taken great interest where the photophysical properties have been investigated thoroughly [74, 85, 86]. Transition between molecular and bulk phases occurs when the diameter range is 1–10 nm and consequently different properties are obtained thanks to quantum effects .
For the case of bulk materials, the kinetic energies of photoexcited electrons can take various values as the density of states in the conduction band is rather high . However, when the size of the particles gets as small as the size of the first excited state (or smaller), the generated electron-hole pairs are enforced to move to an upper kinetic energy state . Therefore, reduction of particle sizes to smaller than a threshold brings confined carriers in a potential well, similar to “particle in a box” case, which includes quantum mechanically interacting nanoparticles . Hence, particle size directly affects the redox potential of holes in the valence band and that of electron in the conduction band  where holes and electrons are present on the nanoparticle surface together enabling simultaneous oxidative and reductive reactions .
Up to date, nanoparticles with various geometries and structures have been demonstrated: for example, 3D-ordered films of titania hollow spheres with adjusted sphere diameters have provided higher absorbance of visible light resulting in photocatalytic enhancement . An ideal photocatalytic thin film should be stable, be of low cost, and allow for safe processes with high photocatalytic efficiency. Semiconductor metal oxides that are used as catalysts utilize the energy of incident light that exceeds the bandgap of the metal oxide material and produces electron-hole pairs. Upon the optical excitation of the material system by a photon with energy equal to or higher than bandgap energy of the photocatalytic system, electron makes transition from the valence band to the conduction band and leaves a hole in the valence band. This photo-generated electron-hole pair then diffuses to the surface and initiates a chain of reactions. As a result, these excited states supply H+, OH−, H2O2, and species, which degrade the existing pollutants. In fact, most common metal oxides have large bandgaps, which decrease the efficient utilization of solar light. This is due to the small portion of the UV light in natural sunlight (5%–8% of the solar spectrum at sea level)  reducing the fraction of the generated electron-hole pairs, which limits the degradation of the organic materials. Another problem is related to losing electron-hole pairs in a very short time as recombination can be critically high when the conditions such as pH of the environment, size of the photocatalytic material, and adsorption are not appropriate . Therefore, nanotechnological methods to improve the catalytic efficiency is appealing as surface area for catalytic reactions can be increased; the bandgap of the catalyst can be modified or recombination rate can be lowered.
2.1.2. Hybridizing with Other Systems
There are various surface structuring techniques including porous nanosphere, nanotube, nanorod, and nanowire formation to achieve surface functionality [71, 95]. Also, hybrid nanostructures, which include photocatalysts grown on graphene flakes [96–98], microspheres coated with photocatalysts , polymer coated photocatalysts , hollow spherical hybrid photocatalysts , and other composites , have appealed considerable attention. Among these, as future prospects, 2D material hybridized photocatalysts are promising from perspectives of large-area applications and high efficiency.
Figure 2 shows different hybrid models for increasing the photocatalytic efficiency. It is possible to use the interaction of plasmonic photocatalyst with a graphene sheet, facilitating enhanced absorbance in the visible region as a result of surface plasmon resonance absorption of Ag nanocrystal . Figure 2(a) shows Ag@AgCl/RGO hybrids with rather low charge recombination and enhanced photocatalytic activity . Higher photocatalytic activity is explained by the effective charge transfer from plasmon-excited Ag nanocrystal to reduced graphene oxide  and the reduced charge recombination during photocatalytic process. Similar to graphene hybridization effect, BN-Ag3PO4 composites showed higher photocatalytic activity than the pure Ag3PO4 due to suppression of the electron-hole pair recombination .
ZnO/graphene composites have also been used to increase photocatalytic efficiency  where the coverage of graphene on the surface of ZnO nanoparticles has been found to be crucial for higher photocatalytic activity since the enhancement of photocatalytic activity of the hybrid structure is explained by the higher separation efficiency of electron-hole pairs. Similarly, TiO2 (P25)-graphene nanocomposite photocatalyst is shown to exhibit improved charge separation and enhanced light absorption . In general, graphene in the composites is reported to act as an electron acceptor and transporter . Figure 2(b) describes layered MoS2/graphene hybrid as a high-performance photocatalyst for H2 evolution . Although this study focuses on solar H2 generation system, the research work can be adopted to coatings technology as long as progress on large-area growth of 2D thin films continues [104, 110]. The enhanced photocatalytic activity in the case of MoS2/graphene hybrid is attributed to the synergetic effect between the MoS2 and graphene components acting as an electron collector and a source of active adsorption sites, respectively.
2.1.3. Techniques for Surface Modification
The methods related to surface modification include a wide range of techniques which involve surface modification of metal oxide particles before embedding them in thin films and/or alteration of metal oxide-hybridized thin film surface [72, 111–113]. We will briefly examine the advancements of such surface techniques enhancing the photocatalytic activity. The extent of electron-hole pair diffusion into interfacial charge-transfer reactions determines photocatalytic efficiency directly . Such charge-transfer reactions can be increased by different surface methods such as surface derivatization , chemical solution deposition with metal oxides such as Fe2O3 and Al2O3 , complexation-chelation , platinization , grafting polymer , using polymeric g-C3N4 photocatalysts loaded with Ag nanoparticles , and depositing metals . Adsorption of complexants is suggested to enhance the electron transfer from the conduction band to the acceptors . Complexation of titanium surface ions is found to shift the redox level of Ti(IV)/Ti(III) electronic state to the conduction band avoiding electron trapping. Chelation relies on metal ions that react with organic molecules. Film derivation by using the suspension of nanoparticles takes interest due to low-temperature process enabling economic and large-area deposition. However nonuniform film surface is formed because of agglomerations . Surface coverage by dyes is found to enhance optical absorption in the visible range . Surface platinization of TiO2 has been investigated in the literature  as the platinized TiO2 shows high photocatalytic activity due to lower recombination rate under the presence of Pt deposits on TiO2. It has been demonstrated that Pt provides electron trapping and enables interfacial electron transfer to acceptors . Therefore, noble metals including Rh, Ni, Pt, and Pd can increase the electron transfer to O2 and hence the quantum yield [126, 127]. Electron-hole pair separation and reduction rates are improved by depositing these catalytic metals on semiconductor catalysts [111, 120].
2.1.4. Combination of Photocatalysts
When cocatalysts are used to enhance each other’s catalytic activity, this leads to important benefits including higher photocatalytic efficiency and higher absorption of solar irradiation especially in the visible range. In the case of using composites, electron and hole transfer from one side to the other can decrease electron-hole pair recombination rate. For the TiO2-ZnO composite example, the hole transfer occurs from the valance band of ZnO to the valance band of TiO2 and the electron transfer occurs from the conduction band of TiO2 to the conduction band of ZnO . In this process, the narrower indirect bandgap of TiO2 provides charge transfer to ZnO, whose conduction band is energetically equal to that of TiO2 . There are other reports showing photocatalytic synergy effect and enhanced photocatalytic activity due to charge transfer [43, 129]. Also, such higher photocatalytic activity of composites is explained by the increased electron-hole transfer between the conduction and valence bands due to the high binding energy of ZnO together with the high reactivity of TiO2 . The mechanisms of combined semiconductor systems and the obtained benefits such as higher electron-hole pair separation and holding reduction and oxidation reactions at two different reaction sites are demonstrated; when two different metal oxides establish a heterojunction, their work functions cause negatively charged carriers to transfer from the semiconductor with lower work function to the other semiconductor up to the state of thermal equilibrium [76, 131].
As an example for composite photocatalysts, g-C3N4/BiOBr material system is shown in Figure 3, indicating that the obtained bandgap is lower than that for the pure BiOBr and an enhanced photocatalytic activity is obtained .
The most extensively considered photocatalyst, TiO2, is in the form of either anatase, rutile, or brookite where these might also be mixed . In the case of using mixed forms of TiO2, electron transfer from the conduction band of anatase to that of rutile provides higher photodegradation. The cocatalysts stimulate operational transfer of photo-induced carriers by acting as an antenna to collect the carriers and contribute to increasing the efficiency of the charge-transfer process .
Bandgap engineering is another important approach for improving photocatalytic activity where conduction and valance bands are modified using the elements of the composite photocatalysts . Regarding this method, it particularly focuses on water splitting processes  but considering its potential for large-area surfaces, recent advancements are included to this review. In this respect, modified materials system is shown to have an extended absorption edge into the visible region and high mobility of holes in the valence band upon visible-light illumination .
2.1.5. Doping Effect
To change the properties of photocatalytic materials, doping is commonly used and doping of various elements including transition metals and nonmetals together with their effect on the photocatalytic activity has been studied [76, 138–140]. Doping is found to be useful to enhance absorption properties of the large bandgap photocatalyst materials. Among the studied transition metal ion dopants, Fe3+ was shown to contribute to reduction in recombination of carriers [141, 142]. In fact, there are controversial results in literature because the way that the dopant is added to the system is crucial for the operation. For example, Cr3+ both can have a positive effect  and a negative effect  on photocatalytic performance. Mesoporous Au/TiO2 nanocomposites have also been found to show enhanced light absorption and relatively higher quantum efficiency resulting in superior photocatalytic efficiency . Introduction of Zn and S has provided red shift of absorption edge for TiO2-based nanomaterials .
In addition to metals, nonmetals are also used for the modification of the electronic structure and hence photocatalytic efficiency and absorption regime. One of the possible elements is nitrogen [147, 148], which shows effect of bandgap narrowing down behavior. Another effective material can be boron, enhancing visible-light absorption and improving photocatalytic activity . Using combination of boron and nitrogen as dopants  also exhibits higher visible-light reactions whereas nitrogen might reduce surface charge transfer . For the case of both carbon and sulphur doping, again improved photocatalytic activity under indoor daylight is observed [150, 151]. Liu et al. suggest that dopant locations and their chemical states allow for the intended surface conditions for efficient photocatalysis . Figure 4 shows the possible mechanisms of nonmetal doping in TiO2, which is found to be more beneficial than metal doping causing deterioration of surface structure due to the clusters. Although Figures 4(a)–4(e) show the possible reasons of visible-light absorption, the doping influence determining the electronic network is not totally clear.
2.1.6. Effect of Plasmonics
Plasmonic photocatalysis has been another alternative for increasing photocatalytic efficiency under visible-light irradiation [152–155]. This enhancement is explained by the excitation of localized plasmon polaritons on the surface of silver nanoparticles that provides a significant increase in the near-field intensities in the near UV supporting the photocatalytic behavior of TiO2 . The schematics in Figure 5 show that the Au nanoparticle can absorb visible light and can excite more holes and electrons when compared to TiO2 itself . The first intention of using plasmonic effect arises from the fact that the effective thickness of a TiO2 photocatalytic layer is limited due to its opaqueness. The structures consisting of noble metal nanoparticles distributed on semiconductor photocatalysts can also behave like a Schottky junction in addition to localized surface plasmonic resonance [154, 156]. Such Schottky diode structures are found to be useful because of their contribution to charge separation and transfer performance where localized surface plasmonic resonance is important for absorption of visible light.
2.2. Methods for Enhanced Biocatalytic Efficiency
In the recent years, an increasing interest has developed towards environment-friendly paints and coatings. In this respect, enzyme-added films receive a considerable attention [31, 157]. Biocatalysts are favored in green process applications  due to their low chemical consumption requirements and absence of toxic byproducts. In addition, they are safe, easy to use, effective in low doses, applicable for a specific- or wide-range use, environmentally benign, and cost-effective . These biocatalytic coatings are used for bioremediation applications, such as easy-cleaning , detoxifying, and self-decontaminating surfaces [22, 158, 159], demonstrating strong antiviral, bactericidal, and sporicidal effects [23, 160, 161]. In this review, we discuss the methods used for incorporation of enzymes into and onto surface coatings specifically for large-area applications, namely, enzyme entrapment and using nanostructures for enzyme immobilization such as nanoparticles , single-walled , and multiwalled nanotubes . Figure 6 shows different methods, challenges, and effective parameters to be considered for generation of high-performance biocatalytic thin films.
Incorporation of enzymes into or onto solid materials is mostly achieved by either physical adsorption, covalent attachment, or matrix entrapment . However, the relatively fragile nature of enzymes requires additional stabilization protocols before incorporation of the biocatalysts into the thin film coatings. Noncompatibility of the enzymes with hydrophobic polymers is another challenging aspect of incorporation of biocatalysts into plastic materials . In this respect, nanostructured materials have been recognized as highly effective immobilization supports providing promising stabilization and performance solutions [32, 164, 165] to overcome the major catalytic, stability, leaching, and dispersibility problems in functional large-area coatings. Although the techniques for enzyme immobilization have already been exploited commonly , there are still critical drawbacks for practical applications including stability and catalytic functionality issues. In general, although storage stability is significantly enhanced [160, 167], the activity of biocatalyst is substantially reduced upon immobilization [161, 168]. The major challenges related to enzyme immobilization are identifying new matrix materials with suitable structural characteristics such as morphology and surface functionality and compositions as well as understanding the interactions between enzymes and the matrix to enhance the biocatalytic efficiency. Another point of consideration is the dispersibility of the enzymes in the bioreactive coatings. In other words, it is required to construct a film with uniform activity per unit area throughout the polymeric and paint composites [163, 169].
2.2.1. Polymer-Enzyme Coatings
Implementing enzymes into biocatalytic coatings requires finding a suitable matrix for immobilization, which will not compromise their activity and stability. Physical entrapment within and covalent attachment to polymer matrixes are the two strategies used for immobilization. Covalent attachment usually requires multistep protocol involving chemical modification of the enzyme. This type of immobilization may result in loading of solid matrixes with enzymes possessing unfavorable conformational changes which leads to lower activity and weaker substrate binding . Physical entrapment on the other hand provides means to incorporate enzymes in polymer matrixes by retaining their native conformation . Physical retention of enzymes into porous solid matrixes or films of polymers is defined as entrapment. Entrapment is considered as the easiest immobilization technique that generally results in no significant conformational alterations of the enzyme. However, this method is characterized by low enzyme loading and noticeable mass transfer limitations. Enzymes can be incorporated into functional films by mixing the biocatalysts with polymer precursors and through emulsion-polymerization procedure coating metal plates by a draw-down method [22, 171], by standard solution coating technique , or using a paint spreader [169, 172]. Another entrapment method is generation of sol-gel coatings. A sol is a suspension of dispersed colloids with diameters of around 1–100 nm. A rigid porous gel is generated as the sol is interconnected by cross-linking and the solution viscosity is increased by evaporation of water or alcohol in the suspension . The main advantage of generating biocatalytic coatings via sol-gel entrapment is that the coatings retain a significant amount of water , which is crucial for enzyme structural, operational, and storage stability. In addition, sol-gel processes involve low-temperature hydrolysis of monomeric precursors and are thus highly suitable for the microencapsulation of fragile biomolecules .
The main problem reported for physically entrapped enzymes within a coating matrix is the decrease in biocatalytic efficiency of the film and loss of biocatalyst due to leaching upon use [174–176]. To reduce the rate and extent of leaching, one strategy is to further treat the polymer film composites with a cross-linking agent, such as glutaraldehyde, to bound the protein to the polymer network  or covalently immobilize the enzymes to the two-component system prior to polymerization . The latter method also results in uniform distribution of enzymes within the polymer coating . Another method is to produce the polymer film and then surface-attach the enzymes to the solid support via various strategies such as physical adsorption , covalent binding, and intermolecular cross-linking. Kim et al. (2001) investigated the effect of incorporation of proteolytic enzymes (α-chymotrypsin and pronase) into films for applications as fouling-resistant coatings . They compared the effectiveness of sol-gel entrapment technique, covalent attachment of enzymes to the polydimethylsiloxane (PDMS) matrix, and direct surface attachment of the enzymes to the film. Stability-wise, the enzymatic films and paints showed higher thermal stability compared to the enzymes that are free in solution and the ones that are surface-attached to the PDMS support. However, while the covalently attached films exhibited no apparent enzyme leakage from the films, they observed that some sol-gel particles were released into solution [167, 170] during application studies.
2.2.2. Enzyme-Nanostructure Conjugate Containing Coatings
In particular, oil- and water-based paints represent cheap and readily available materials that can easily be applied to various surfaces. Incorporation of enzyme-nanostructural conjugates into these paints can create the biocatalytic surfaces with desired functional properties [167, 174]. However, the paint should not contain volatile organic solvents that could inactivate the enzyme [159, 172].
The use of nanostructural materials such as nanoparticles, nanotubes, and nanofibers, offers high surface-to-volume ratio, resulting in higher enzyme loadings and improved thermal, operational, and storage stability . Moreover, addition of enzyme-nanostructural conjugates within paint matrix produces more porous composites that are not diffusionally limited, compared to polymer coatings, even at high thicknesses of 400 nm . Nevertheless, the immobilization efficiency, including optimal protein loading, favorable conformation, and high activity of the enzymes, is greatly affected by the properties of a nanomaterial such as its surface chemistry, morphology, and size . Table 1 summarizes different immobilization techniques applied in the generation of biocatalytic thin film coatings and points out observations regarding their performances.
|% Residual activity with respect to free enzyme.|
Covalent attachment of listeria bacteriophage endolysin Ply500 onto functionalized silica nanoparticles and incorporation of the conjugates into a thin poly-hydroxymethyl methacrylate (pHEMA) films resulted in highly stable (95% killing efficiency after 40-day storage) and effective biocatalytic films capable of completely killing present L. innocua cells and preventing further growth at 4°C . Upon continuous washing of the films, no leaching of the enzyme conjugates was observed. The study illustrates that the SNP-Ply500-containing pHEMA films are promising candidates for applications as antilisteria coatings of cold storage rooms or food processing equipment .
Single-walled carbon nanotubes (SWNTs) are fine structures having diameter of 1.3–2.0 nm and length of tenths of microns, hence having a very high surface area to volume ratios . Incorporation of these nanostructures into the polymeric matrix offers a high surface for interaction with biocatalysts or polymer chains that comprise the film. This enhanced interaction results in profound retention of the enzymes in the biocatalytic coatings and films . In addition, owing to their surface curvature, SWNTs and nanoparticles provide means for enzyme attachment by ensuring their structural and functional stability keeping them in more native-like conformation than when attached to flatter surfaces [174, 175, 179]. Enzymes bound to SWNTs or silica nanoparticles retained higher catalytic activity compared to MWNTs at similar loading .
Biocatalytic surfaces for large-area applications are also generated by immobilization of enzymes onto multiwalled carbon nanotubes (MWNTs) and subsequently mixing the resulting conjugates with eco-friendly paints [23, 161, 163, 176]. MWNTs are preferred support materials for enzyme immobilization due to their ease of surface functionalization, very high surface-area-to-volume ratios that afford high enzyme loading without diffusional limitations . Moreover, their light weight and high mechanical strength make them excellent filling materials to reinforce polymers and ensure entrapment of the biocatalyst and thus retain the enzyme within the composite (with no enzyme leaching) . However, pristine carbon nanotubes tend to aggregate in both aqueous and organic solutions due to surface-surface van der Waals interactions. This, however, reduces the available area for protein attachment as well as the nanotube dispersion within polymer matrix or paint . Similar studies with carbon nanoparticles showed that aggregation of nanoparticles created agglomerates ranging from several hundred nm to several μm . This agglomeration influenced enzyme loading and activity by reducing the available adsorption surface. To improve dispersibility, it has been shown that functionalization of nanotubes with flexible poly ethylene glycol (PEG) spacer arms increases nanotube dispersion in solution and prevents clumping of the nanostructures by blocking hydrophobic side walls of the carbon nanotubes . Moreover, it is crucial to optimize spacer lengths, since long ones (>2000) might potentially create porous composites with altered mechanical properties . In addition, oxidizing MWNT generates highly hydrophilic structures (carboxyl acid “handles” on the surface) that enables them to become water soluble/dispersible [161, 175].
2.2.3. Enzyme Loading
Enzyme-carrier materials with reduced size can generally improve the efficiency of enzyme immobilization by providing larger surface area for attachment, leading to higher protein loading per unit mass of particles . The amount of enzyme molecules immobilized per unit area (i.e., protein loading) may increase or decrease the performance of biocatalytic film. For some enzymes, too much loading on the surface may restrict the spatial orientation of the protein chain, limit the accessibility of the enzyme’s active site, or denature the enzymes, thereby, reducing their activity . Rather than the enzyme loading, it is critical to obtain optimum surface accessibility and functional stability for the biocatalyst, so that the reactions take place. However, it has been shown that, upon active site titration of enzyme-containing polymeric films, the observed high activity is mainly due to enzymes embedded within the polymeric coating rather than on the surface only .
3. Growth Techniques towards Large-Area Catalytic Coatings
Growth techniques are critical in determining size, orientation, and shape of catalytic material systems in addition to the characteristics of thin film that these particles or enzymes are embedded. Naturally crystallinity and surface features significantly affect catalytic reactions in thin films. Hence, various process methods have been used to form the thin film and the photocatalytic metal oxides and determine the crystal, surface, and electronic structure of materials, which affects bulk diffusion, surface transfer, and redox potentials . Liquid based processes can be advantageous because they start forming the structures in aqueous solution and they allow for forming homogeneous films and enabling control of growth parameters and hence film properties . However, for wide utilization of photocatalytic effect and to get rid of recycling problems, thin film forming needs to be performed while the photocatalytic activity is still kept at high levels. Up to now, there have been significant efforts to obtain high photocatalytic efficiency on large areas. We will present the major film formation methods in this section.
Using hydrothermal and solvo methods photocatalytic particles are crystallized at high temperature and high vapor pressure conditions, utilizing organic solvents including methanol  and toluene . With the ability to tune synthesis conditions, grain size, morphology, and crystalline phase can be controlled. gels are treated with hydrothermal methods in mineralizing additives including fluorides, hydroxides, and chloride and also in pure water [57, 183]. TiSO4, powder of Ti, TiCl4, H2, and TiO(C2O4) can be used to obtain TiO2  where inorganic materials are realized at low temperatures . Also different from coprecipitation, this method does not need extra thermal processing. Chemical solution decomposition , two-step wet chemical method , and chemical vapor decomposition  techniques have also been used to obtain photocatalytic particles.
In the case of sol-gel technique, there are important advantages such as the ability to control the structure of a nanosized material starting from the first steps of the process and also homogenous coatings that can be formed at rather low temperatures . These show its potential for coating large and complex areas at a relatively low cost  with multifunctionality . The films commonly show mesoporous structures by ionic and neutral surfactants . Hydrolysis and polycondensation occur during the formation of solid thin film where either alkoxide or nonalkoxide techniques can be used depending on the intended physical and chemical properties of the end products. In the case of commonly used alkoxide route, metal alkoxides are the raw materials  and for the nonalkoxide technique, inorganic salts are needed to be used . The sol-gel method exhibits another advantage where different metals such as Ca2+ , Pb2+ , Ag+ , Fe3+ , V5+ , Co2+ , Au3+ , and La3+  can be added to films to increase photocatalytic effect.
In many cases, chemical vapor deposition (CVD) is beneficial because of its versatility and fast processing abilities , which enables large-area processing. According to the activation method, pressure, and precursors, compounds are formed either by a chemical reaction or from decomposition of gas phase precursors . Physical vapor deposition (PVD) deposits compounds physically from gas phase to coat substrate instead of chemical reactions. As thermal evaporation is the most commonly used technique in PVD systems, to realize sufficiently conductive TiO2, it is required to heat up to 900°C in hydrogen atmosphere ambient .
Spray pyrolysis uses aerosol precursor solution and directs this aerosol to the target, contrary to chemical vapour deposition, which needs diffusion [206, 207]. This method is also beneficial with its low cost, high reproducibility, and speed to coat large areas with moderately good structural properties whereas uniformity is still a problem. There are some other techniques for growing metal oxide particles; although they may not be proved to be used for large-area surfaces at low cost, they are still worth being mentioned. Despite the fact that microemulsions were observed to be encouraging in early experimental works, the synthesis of controlled titania with microemulsions has not been straightforward up to date .
Advanced film properties of structures such as nanoporosity, epitaxial layers, superlattice, and quantum dot structures can be controlled by using electrochemical synthesis. The desired properties are obtained by altering the parameters including temperature, potential value, pH, and current density where acidic and oxygen-free environment is required for electrolysis to be successful . It has been reported that overcoming this problem is possible by using nonaqueous solutions . In addition to these techniques, magnetron sputtering  and atomic layer deposition  are of significance considering their potential to realize large-area films which can be structured with the desired properties.
Undoubtedly, the science, technology, and industry of large-area catalytic coatings will continue to gain importance for green applications that will find their place in different fields including architecture and automotive and environmental remediation. There is already an important progress in the last few decades and is already providing answers with growing expectations for the future.
4. Large-Area Applications of Multifunctional Catalytic Films Future Prospects
Up to present, catalytic thin films have been studied from different aspects by numerous researchers as they can be employed on surfaces with a wide range of purposes including optical, optoelectronic, magnetic, electrical, thermal, photoelectrochemical, photovoltaic, mechanical, biomedical, and catalytic applications [178, 213–217]. Figure 7 shows an integrated sol-gel produced material system exhibiting antireflectivity, water repellence, antifogging, and photocatalytic activity where the coating is proposed to be used for photovoltaic applications .
Obviously, high potential of catalytic coatings to combat environmental problems with widespread exploitation is of great significance. Greenhouse gasses are being released constantly and increasingly on global scale [218–220]. Hence, using smart and functional coatings provides an important alternative to this serious green gas emission problem and will make contribution to the solution of environmental pollution [221–224]. Nanotechnology obviously enhances the performance of the coatings considering the final objective of decreasing greenhouse gases by massive environmental remediation. However, it is still not straightforward to commercialize such products because of the industrial difficulties including the levels of catalytic efficiencies in the visible spectral range and economical manufacturing methods for ecological and sustainable coatings with the intended functionalities. On the other hand, as reviewed in this paper, there are numerous possible organic and/or inorganic solutions, which enhance the performance parameters to realize functional and innovative structures .
Considering the incrementally increasing environmental pollution and its impacts on global scale, there is substantial need for development and application of advanced, low-cost, large-scale, green solutions capable of reducing and even eliminating the pollution. In this respect, large-area catalytic thin films are highly promising for green technology applications to save energy and combat pollution and global warming. To date, organic and inorganic catalytic materials have been investigated comprehensively, considering their geometrical and electronic structures, surface reactions, synergistic behavior with other material systems, and stability issues. In general, catalysts lose their effectiveness when they are immobilized in thin films compared to the case when they operate in aqueous environment. However, there is a wide range of methods that have been reported to enhance the efficiency of these catalysts when embedded in coatings.
This review paper covers the developments of photo- and biocatalytic thin films and coatings formed using nanotechnology. Therefore, it is a subset of “catalytics and nano” focusing on formation of thin films containing catalytic nanostructures for large-area applications. The reviewed research work not only includes large-area applications that offer solutions for environmental pollution but also covers the reports that potentially can make a contribution to the low catalytic efficiency and solar spectrum absorption problem. Among these methods, hybridizing catalytic particles with other structures are attention-grabbing considering also enhancement in surface functionalities. Therefore, we gave special attention to the research studies implementing hybrid systems and, specifically, two-dimensional materials are found to be promising because of their superior properties and 2D geometries allowing for mass production capabilities.
To make use of highly appealing functionalities of these immobilized catalytic material systems, it is needed to increase the catalytic efficiency and improve the yield and feasibility. In this review, recent advancements specifically focusing on the catalytic efficiency enhancement and solar spectrum absorption are discussed. Different research reports show that the potential of nanostructuring or hybridizing methods for catalytic thin films is enthusing for the future, also providing novel applications due to enhanced functionalities. As a result, catalytic thin films will provide promising solutions to decontamination as they are embedded into systems to obtain mineralization of a variety of environmental contaminants. However, two critical challenges still remain. To begin with, significant effort continues for understanding the generic mechanisms within the matrix and interactions between the nanocatalysts and the embedded thin film system. For instance, the relationships between the biocatalyst-nanostructured carriers and the polymer matrix are to be yet investigated systematically. Such fundamental information is significantly important in the development of hybrid structures or multifunctional coatings. These fascinating challenges require interdisciplinary collaborations of material scientists, chemists, and engineers. Another impediment is the commercialization. Reducing the cost of production and demonstrating the feasibility and effectiveness of these functional coatings by long-term operation studies are the major hurdle for universalization of large-scale applications.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Nihan Kosku Perkgoz acknowledges the support from Anadolu University (BAP-1404F214 and 1407F335) for this project.
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