WO2019195916A1 - Self-cleaning and anti-fouling graphene oxide/tio2 photoactive coating and large scale bonding-to- surface process thereof - Google Patents

Abstract

Embodiments described herein related to photocatalytically active coatings having improved photocatalytic activity. Embodiments described herein also relate generally to methods for producing graphene-oxide/TiO₂ materials and new applications based on the improved photoactivity of the materials. For example, the graphene-oxide/TiO₂ can be used in enhanced self-cleaning coatings for surfaces including cement, metal, glass, and wood. Embodiments described herein also relate generally to methods of producing functionalized graphene oxide-TiO₂ materials and methods of coatings by surface modification of a substrate and functionalization of the functionalized graphene oxide-TiO₂, the functionalized composite being coated onto the target surface after proper surface treatment.

Description

SELF-CLEANING AND ANTI-FOULING GRAPHENE OXIDE/TiO₂ PHOTOACTIVE COATING AND LARGE SCALE BONDING-TO- SURFACE PROCESS THEREOF

Cross-Reference To Related Applications

{0001] This application claims priority to and the benefit of U.S. Application No.

62/657,220, filed April 13, 2018, entitled“Self-Cleaning and Anti-Fouling Graphene Oxide/ TiO₂ Photoactive Coating and Large Scale Bonding-To-Surface Process Thereof,” the disclosure of which is incorporated herein by reference in its entirety.

Background

{0002) Embodiments described herein related to photocatalytically active coatings having improved photocatalytic activity and methods of making the same. Surfaces exposed to the weather and airborne organic matter typically get dirty over time because of the accumulation of debris including organic matter. Conventional approaches for cleaning outdoor surfaces such as pressure washing or scrubbing are labor-intensive and require the use of cleaning materials such as water, solvents, chemicals, and/or soap. Existing approaches for self-cleaning surfaces are generally not durable and experience a decay of self-cleaning effectiveness over time. Thus, the surface must often be scraped and re-coated to continue to achieve self-cleaning properties.

Summary

10003] Embodiments described herein related to photocatalytically active coatings having improved photocatalytic activity. Embodiments described herein also relate generally to methods for producing grapheie-oxide/TiO₂ materials and new applications based on the improved photoactivity of the materials. For example, the graphene- oxide/TiO₂ can be used in enhanced self-cleaning coatings for surfaces including cement, metal, glass, and wood. Embodiments described herein also relate generally to methods of producing functionalized graphene oxide-TiO₂ materials and methods of coating the functionalized graphene oxide-TiO₂ onto the surface after proper surface treatment. Brief Description of the Drawings

[0004] FIG. 1 is a schematic illustration of a photocatalytically active surface, according to an embodiment.

[0005] FIG. 2 is a schematic illustration of a photocatalytic oxidation phenomena, according to an embodiment.

[0006] FIG. 3 is a schematic illustration of a photocataly tied ly active surface, according to an embodiment

[0007] FIG. 4 is a flow diagram illustrating a method of forming a photocatalytically active composition, according to an embodiment.

[0008] FIG. 5 is a schematic illustration of a method of forming a photocatalytically active coated substrate, according to an embodiment.

Detailed Description

[0009] Embodiments described herein related to photocatalytically active coatings having improved photocatalytic activity. Embodiments described herein also relate generally to methods for producing graphene-oxide/TiO₂ materials and new applications based on the improved photoactivity of the materials. For example, the graphene- oxide/TiO₂ can be used in enhanced self-cleaning coatings for surfaces including cement, metal, glass, and wood. Embodiments described herein also relate generally to methods of producing functionalized graphene oxide-TiO₂ materials and methods of coating the functionalized graphene oxide-TiO₂ onto the surface after proper surface treatment.

[0010] As described herein, photocatalytic coatings using nano-TiO₂ can be an attractive way of degrading environmental pollutants and harmful microorganisms. Many of the current methods for applying nanoscale coating products require a sol-gel process, which can be energy-intensive, expensive^ and often requires toxic chemicals. These drawbacks have hindered large-scale applications of TiO₂ photocatalysis. On the other hand, current commercially available nano-TiO₂ coatings are typically bound physically to the substrate, meaning the stability of attachment of the coating to the substrate can be quite low. Another disadvantage of TiO₂ by itself as a material for photocatalytic phenomena is the small quantum efficiency and low visible light activity of TiO₂. In feet, thick, white coatings of TiO₂ are often required to fulfill the requirements of antimicrobial action and organic compound degradation over practical time scales. Therefore^ there is a need for additional photocatalytic efficiency in addition to mechanical stability of the coating on the substrate and for coatings that are effectively bound to and mechanically stable on the surface, and that provide sufficient visible light activity at the same time.

[0011] Graphene can include a single, one atomic thick layer of the common mineral graphite and can be a truly two-dimensional material. Graphene can also be transparent, extremely flexible yet still rigid, and an excellent electrical and thermal conductor. However, industrial production of a dispersible graphene with pristine crystalline quality can be quite difficult, On one hand, high quality graphene with minimal defects and crystalline imperfection is often important for many applications of graphene. On the other hand, pristine graphene suffers from limited dispersibility in solvents and polymers, which impedes significant addition into such matrices. Hence, increasing the dispersibility of graphene is important to transfer the superior property of graphene into the host matrix.

[0012] Embodiments described herein related to photocatalytically active coatings having improved photocatalytic activity. In some embodiments, a method for forming the photocatalytically active coating includes the deposition of elements such as Pt, Au, Ni or Ag within the crystal lattice, also referred to herein as doping. Without wishing to be bound by any particular theory, these doped atoms may lead to an increase in the absorption edge wavelength and a decrease in the band gap energy of TiO₂ nanoparticles. The doped TiO₂ nanoparticles in general may show higher photocatalytic activities than pure TiO₂ nanoparticles. In some embodiments, doping the TiO₂ nanoparticles shifts the absorption edge to the visible region.

[0013] In some embodiments, the method for forming the photocatalytically active coating includes adding a functionalized graphitic compound to help prevent the recombination of the excited electron (going back to unexcited state). Without wishing to be bound by any particular theory, adding a functionalized graphitic compound (e.g., graphene or graphene oxide) may help the electrons and electron vacancies to reach the surface of the particle by allowing them to travel at a faster speed through the particle.

[0014] Without wishing to be bound by any particular theoty, by applying these two strategies, the absorption spectrum of the photocatalytic semiconductor may be extended to the visible region and the overall capture of incident photons can be greatly improved, increasing the photocatalytic efficiency. In particular, without adding graphene and Ni to the photocatalytic semiconductor, ultraviolet radiation with a wavelength from 10 nm to 400 nm is necessary to accomplish photocatalytic degradation of organic compounds. By doping Ni to the range, the photocatalytic phenomenon can be accomplished using ultraviolet radiation as well as visible light, which is corresponds to a wavelength range of > 380 nm nanometers (nm). As a result, the coating can also be used for indoor applications because the ultraviolet radiation from direct sunlight is not required to activate the photocatalytic capabilities of the surface coating. In addition, the resulting photocatalytic coating has effective antimicrobial activity because of the production of aggressive radicals that attack the bacteria/viruses wall and prevent DNA clonal processing. The attacking mechanism of TiO₂ photocatalysis is related to cell membrane damage. Subsequently, further oxidation can destroy the internal cellular components, eventually causing cell death.

[0015] FIG. 1 illustrates one possible phenomena by which organic compounds may be degraded using a photocatalytically active surface. Without wishing to be bound by any particular theory, an oxidation agent (e.g., TiO₂) may absorb ultraviolet radiation from sunlight or an illuminated light source and produce pairs of electrons and vacancies. For example, the electron in the valence band of the oxidation agent may become excited when illuminated by light. The excess energy of this excited electron may promote the electron to the conduction band of the oxidation agent, therefore creating the negative electron (e-) and positive vacancy (h+) pair. The vacancies may have a potential that is sufficiently positive to generate hydroxyl (OH) radicals from water molecules adsorbed onto the phoiocatalytically active surface, which may oxidize organic compounds. The electrons may react with oxygen molecules to form the superoxide anion, O₂-. These active oxygen species can oxidatively degrade organic compounds, forming carbon dioxide and water. In other words, the positive-vacancy of the oxidation agent may break apart the water molecule to form hydrogen gas and the hydroxyl radical, and the negative-electron may react with the oxygen molecule to form the super oxide anion. This cycle may continue as long as ultraviolet radiation is available to the photocatalytically active surface.

[0016] Without wishing to be bound by any particular theory, photocatalytic efficiency may depend on the competition between the process in which the electron reacts with a chemical species and the electron- vacancy recombination process. This reaction may result in heat or radiation release. One aspect of particular interest in this type of catalytic process is the usage of the sun as the energy source. In other words, the sunlight that provides the ultraviolet radiation that causes electron promotion and vacancy production competes with the recombination process, in which the electron falls from the conduction band back into the valence band (i.e., into a vacancy). Therefore, an electron vacancy pair disappears and the energy of recombination may be emitted as a photon of light and/or heat.

[0017] Nevertheless, the possibility of electron-vacancy recombination occurring greatly limits the photocatalytic activity and thus several efforts are envisaged to allow a more efficient charge carrier separation, Among the available photocatalytic semiconductors, TiO₂ is generally considered an excellent material for such an application. In fact, TiO₂ exhibits many of the properties that are desirable for an efficient photocatalytic process, except that it does not absorb visible light. Furthermore, TiO₂ is nontoxic, thermally stable, chemically inert, photostable, readily available, and relatively cheap. TiO₂ also shows band edges that are well positioned, exhibits strong oxidizing power at ambient temperature and pressure, and the photogenerated electrons are able to reduce oxygen to the superoxide. This phenomena may be useful for increasing the photocatalytic activity of a coating, especially when combined with excitement promoters that prevent excited electron and vacancy recombination.

[0018] FIG. 2 is a schematic illustration of a photocatalytically active surface 200 that includes a substrate 210 and a photocatalytic coating 220 applied to the substrate 210. The photocatalytic coating 220 includes an oxidation agent 230 configured to cause the oxidative degradation of organic matter photocatalytically and an excitation promoter 240 configured to increase the oxidative potential of the oxidation agent 230, perhaps due to improved electronic properties. The substrate 210 includes a surface that has a first organic matter accumulation rate, and the photocatalytic coating 220 defines a second surface that has a second organic matter accumulation rate less than the first organic matter accumulation rate. Without wishing to be bound by any particular theory, the photocatalytically active surface 200 may degrade organic compounds according to the phenomena shown in FIG. 1.

[0019] In some embodiments, the substrate 210 can include any surface that is prone to dirt or debris accumulation and that is regularly exposed to sunlight In some embodiments, the substrate 210 can include a wall, support structure, window, roof, bridge, door, gate, edifice, fence, tower, column, or other feature. In some embodiments, the substrate 210 can be formed from concrete, cement, bricks, tile, glass, metal, ceramic, wood, polymeric materials such as polyvinyl chloride, polytetrafluoroethylene, polyethylene, polyethylene terephthalate, polycarbonate, and high-density polyethylene, rubber, silicone, stone, minerals, tile, any other suitably durable building material, or combinations thereof.

[0020] in some embodiments, the substrate 210 can be inherently textured such that a surface coating will adhere easily to the substrate 210. In some embodiments, a texture can be imparted to the substrate 210 by mechanically or chemically etching, scratching, burning, scraping, grinding, scoring, or otherwise degrading the substrate 210. In some embodiments, texture can be imparted to the substrate 210 by applying material to the substrate 210. In some embodiments, the substrate 210 can be configured to cross-link with a polymer or other moiety naturally, upon application of a chemical such as an initiator, upon application of heat, or in any other way.

[0021j In some embodiments, the photocatalytic coating 220 can be applied to a portion of the substrate 210 in order to reduce the accumulation of organic material on the portion of the substrate 210. In some embodiments, the photocatalytic coating 220 can be applied to the substrate 210 after the substrate is in place at the final building location or before installation. In some embodiments, the photocatalytic coating 220 can be applied to the substrate 210 by dip-coating, spraying, spin-coating, rolling, brushing, aerosolized application, or in any other manner.

[0022] In some embodiments, the photocatalytic coating 220 can be made by dissolving the oxidation agent 230 and the excitation promoter 240 in a solvent and then heating the solvent to reduce the oxidation agent 230 and achieve the deposition of the excitation promoter 240 onto the oxidation agent 230. In some embodiments, the oxidation agent 230 can be substantially or completely dissolved in the solvent before the excitation promoter 240 is added. In some embodiments, the solution of the oxidation agent 230 in the solvent can be heated while the excitation promoter 240 is added to facilitate dissolution.

[0023] In some embodiments, the solvent can include at least one of acetone, ethanol, water, methyl acetate, ethyl acetate, hexane, petrol ether, terpenes, toluene, turpentine, acetic acid, toluene, butyl acetate, butanol, amyl acetate, ethyl cellosolve, pyrobenzene, xylene, white spirit, cyclohexanone, pentane, cyclopentane, benzene, cyclobenzene, 1,4- dioxane, chloroform, dichloromethane, tetrahydrofuran, dimethylformamide, acetonitrile, nitromethane, propylene carbonate, formic acid, n-butanol, isopropyl alcohol, «-propanol, methanol, and combinations thereof.

[0024] In some embodiments, the solvent can be heated during dissolution of the oxidation agent 230 and/or excitation promoter 240 to a temperature of at least about 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65°C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, or greater than 100 °C. In some embodiments, the solution including the solvent and the oxidation agent 230 and/or excitation promoter 240 can be stirred at a speed of greater than about 1 ipm, 2 rpm, 5 rpm, 10 rpm, 20 rpm, 30 rpm, 60 rpm, 100 rpm, 200 rpm, 500 rpm, or greater than about 1 ,000 rpm. In some embodiments, the solution can be heated and stirred during dissolution of the oxidation agent 230 and/or excitation promoter 240 to optimize homogenization ofthe solution.

(0025] In some embodiments, the oxidation agent 230 and excitation promoter 240 can be recovered from the homogenous solution by filtration, rinsing, and/or drying. In some embodiments, the finished mixture of oxidation agent 230 and excitation promoter 240 can be surface-modified using a silane coupling agent such as 3-triethoxysiiylpropylamine (APTES). In some embodiments, the mixture of oxidation agent 230 and excitation promoter 240 can be dispersed in a solvent such as deionized water using stirring and/or ultrasonication. The silane coupling agent can then be added to a concentration of less than about 10 wt%, 9 wt%, 8 wt%, 7 wt%, 6 wt%, 5 wt%, 4 wt%, 3 wt%, 2 wt%, or 1 wt%.

[0026] In some embodiments, the mixture of the solvent, the oxidation agent 230, the excitation promoter 240, and the silane coupling agent can then be refluxed. In some embodiments, refluxing can be accomplished at greater than about 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65°C, 70 °C, 75 °C. 80 °C, 85 °C, 90 ^»C, 95 °C, 100 °C, or greater than 100 °C. In some embodiments, the refluxing can be conducted for more than about one minute, two minutes, five minutes, ten minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, or greater than about one hour. During at least one ofthe above-mentioned steps, the oxidation agent 230 and excitation promoter 240 can become coupled together. Without wishing to be bound by any particular theory, the composite of oxidation agent 230 and excitation promoter 240 can be formed by intercalation. [0027] In some embodiments, the dispersed composite particles can be separated from the solvent by centrifbging for greater than about one minute, five minutes, ten minutes, 30 minutes, one hour, or more at a speed of greater than about 1,000 rpm, 2,000 rpm, 3,000 rpm, 4,000 rpm, 5,000 rpm, 6,000 rpm, 7,000 rpm. 8,000 rpm, 9,000 rpm, 10,000 rpm, 15,000 rpm, 20,000 rpm, or faster. In some embodiments, the separated composite particles can include very little of the solvent originally used, for example less than about 10 wt%, 9 wt%, 8 wt%, 7 wt%, 6 wt%, 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1 wt% or less. In some embodiments, the separated composite particles can be rinsed, for example using distilled water, and again centrifuged for at least one, two, three, or more cycles. The resulting photocataJytic coating 220 composition is highly photocatalytically active and can be applied to the substrate 210 to achieve the“self-cleaning" phenomena.

[0028] In some embodiments, the substrate 210 can be treated using, for example, a plasma treatment and then amino-salinized before the photocatalytic coating 220 is applied.

Without wishing to be bound by any particular theory, the plasma treatment may provide texture to the surface while the amino-salinization may aid in covalent bonding between the substrate 210 and the photocatalytic coating 220.

[0029] In some embodiments, the photocatalytic coating 220 composition can be applied to the treated surface via spray coating or any other suitable method. In some embodiments, the coated substrate 210 can be dried, for example in an oven at greater than about 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65°C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, or greater than 100 °C. In some embodiments, the coated substrate 210 can be dried for greater than about one hour, two hours, five hours, ten hours 15 hours, 24 hours, 48 hours, 72 hours, or greater than about 96 hours. In some embodiments, the oven dried, coated substrate 210 can be cooled for between about ten minutes and about one week to form the photocatalytically active surface 200. Without wishing to be bound by any particular theory, the photocatalytic coating 220 may be highly stably bonded to the substrate 210 because of covalent bonding. In some embodiments, the covalent bonding may be characterized using at least one of Fourier-transform infrared radiation spectrophotometry (FT-IR) and x-ray photoelectron spectroscopy (XPS).

[0030] The resulting coating also exhibited self-cleaning characteristics, as measured by a reduction in the accumulation of organic compounds on the surface over time, as compared to uncoated substrate. Further discussion of methods of forming photocatalyticaily active surfaces 200 are described below in reference to FIG. 4.

[0031] In some embodiments, the oxidation agent 230 is configured to photocatalyticaily create strong oxidation agents that break down organic matter into gaseous products. The oxidation agent 230 can include a metal oxide such as titanium dioxide (TiO₂), zinc oxide (ZnO), iron (III) oxide (Fe₂O₃), tungsten trioxide (WO₃), tin dioxide (SnO₂), and zirconium dioxide (ZrO₂), cadmium sulfide (CdS), zinc sulfide (ZnS), and other suitable photocatalytic semiconductors that acts to clean a surface using photocatalysis and, optionally, hydrophilicity. In other words, the oxidation agent 230 can cause the breakdown of organic material, which can effectively reduce organic compounds to volatile or gaseous molecules such as carbon dioxide, oxygen, and/or water. In some embodiments, the oxidation agent 230 can be ball milled, for example in a liquid environment. In some embodiments, the ball milling can include a one-step medium speed wet ball milling or attritor milling system with jars or balls, for example made of non- electrically conductive ceramic materials such as tungsten carbide and/or agate. In some embodiments, the oxidation agent 230 can have a planar structure and include few or multiple layers. In some embodiments, the oxidation agent 230 can be between 2 and 100 layers. In some embodiments, the oxidation agent 230 can have a lateral sheet size of between about 10 nm and about 500 microns. In some embodiments, the oxidation agent 230 can have a particle size of between about 10 nm and about 900 nm, about 50 nm to about 500 nm, and about 100 nm and about 200 nm.

[0032] In some embodiments, the oxidation agent230 can be furtherfiinctionalized with the excitation promoter 240 (e.g., reduced graphene oxide (rGO)) to form a composite. Without wishing to be bound by any particular theory, adding the excitation promoter 240 to the oxidation agent 230 may prevent the recombination of the excited electron (going back to the unexcited state) and may also help the electrons and the vacancies to reach the surface of the particle by allowing them to travel at a faster speed through the particle. In other words, more electron pairs may reach the surface of the particle and subsequently the photocatalytic effect increases.

[0033] In some embodiments, the oxidation agent 230 can be added to and dispersed within a resin or binder that is configured to adhere to the substrate 210. In some embodiments, a silane agent can be used to modify the surface of the substrate 210 and/or the functionalized composite (e.g., rGO-TiO₂ composite) and durably adhere the composite on the substrate 210.

[0034] The excitation promoter 240 can also be configured to improve the durability of the photocatalytic coating 220 in addition to increasing the efficiency of oxidation of organic material by the oxidation agent 230. In some embodiments, the excitation promoter 240 can include a carbonaceous material, a highly organized carbonaceous material, a crystallized carbonaceous material, carbonaceous material having a lattice structure such as a hexagonal lattice structure, and combinations thereof. In some embodiments, the excitation promoter 240 can include a graphitic material such as graphite, graphene, GO, rGO, functionalized GO, few-layer graphene, carbon nanotubes, and combinations thereof.

[0035] The addition of the excitation promoter 240 may increase the durability of the photocatalytic coating 220 on the substrate210 and increase the life of the photocatalytically active surface 200, In some embodiments, the excitation promoter 240 may also reduce electron and vacancy recombination, allowing for greater oxidation of organic compounds.

[0636] The photocatalytic coating 220 can further include a noble metal such as Pt, Au, or Ag within the crystal lattice structure. Without wishing to be bound by any particular theory, the doped noble metal atoms may act as electron scavengers, decreasing the rate of recombination of excited electrons and electron vacancies. This may increase an interfacial transfer rate of the charge-carriers (particles free to move, carrying an electric charge, e.g. electrons, ions, and vacancies).

[0037] FIG. 3 depicts a photocatalytically active surface 300 that includes a substrate 310 being a first surface having a first organic matter accumulation rate and a photocatalytic coating 320 applied to the substrate 310 to form a second surface having a second organic matter accumulation rate less than the first organic matter accumulation rate. In some embodiments, the photocatalytic coating 320 can include an oxidation agent 330 and an excitation promoter 340. In some embodiments, the oxidation agent 330 and the excitation promoter 340 can be substantially similar to the oxidation agent 230 and the excitation promoter 240. In some embodiments, the photocatalytic coating 320 can include a binder 350 configured to bind the photocatalytic coating 320 to the substrate 310, [0038] In some embodiments, the binder can include glues, polymers, silicon-based binders, rubber materials, cross-linking adhesives, acrylate-based polymers, polychloroprene, hot adhesives, multi-component adhesives, polyester resins, polyols, acrylic polymers, and ultraviolet light curing adhesives. In some embodiments, the binder can be incorporated into the photocatalytic coating 320 or it can form an interlace between the photocatalytic coating 320 and the substrate 310.

[0039] In some embodiments, the oxidation agent 330 is TiO₂ and the excitation promoter 340 is rGO. In some embodiments, the rGO functions as an enhancer of the photocatalytic effect of the TiO₂. In some embodiments, the photocatalytic coating 320 may lose some photocatalytic efficiency over time. When the photocatalytic efficiency decreases, the photocatalytic coating 320 can be removed from the substrate 310 and a fresh photocatalytic coating 320 can be applied to the substrate 310. In some embodiments, the photocatalytic coating 320 should be reapplied to the substrate 310 every about 10 years, about 8 years, about 6 years, about 5 years, about 4 years, about 3 years, about 2 years, and about 1 year, inclusive of all values and ranges therebetween.

[0040] FIG. 4 illustrates a method 400 for making a photocatalytic coating (e.g., photocatalytic coating 210) for a photocatalytically active surface (e.g., photocatalytica!ly active surface 200) including synthesizing a uniform rGO-TiO₂ composite 410, adding functional groups to graphene-TiO₂ composite 420, treating the substrate 430, functionalizing the substrate with self-assembled monolayers 440, and depositing the functionalized graphene-TίOz composite onto the substrate to produce a stable, homogenous, and transparent film 450. The method 400 is provided for a composite including graphene oxide and titanium dioxide, however any combination of oxidation agent (e.g., 230) and excitation promoter (e.g., 240) can be used as disclosed herein.

[0041] In some embodiments, synthesizing a uniform rGO-TiO₂ composite 410 includes a process of co-precipitation. In some embodiments, the rGO-TiO₂ composite can be a graphene-Ni/TiO₂ composite. In some embodiments, Ni-doped TiO₂ can be coprecipitated using NaOH as a precipitating agent. In some embodiments, Ni(NO₃)2·6H₂0 can be dissolved in deionized water followed by the addition of glycerol which favors the formation of small Ni particles, prohibiting aggregation of Ni particles. The nickel-glycerol complex can be precipitated on TiO₂ nanoparticles by adding NaOH into the suspension until a final pH of -8.5 is achieved. The solution can then be continuously stirred for between about 0.5 hours and about 4 hours prior to filtering and the precipitate can be dried at between about 40 °C and about 130 °C. The dried photocatalyst can then be calcined in air at between about 350 °C and about 1 ,000 °C for between about 1 hour and about 10 hours. The loading of Ni into the TiO₂ nanoparticles can be between about 1 wt% and about 40 wt%. The rGO-Ni/TiO₂ composite can be obtained by dissolving graphene oxide (GO) in a solution of distilled H2O and ethanol by ultrasonic treatment for between about 30 minutes and about 5 hours. In some embodiments, TiO₂ particles can be added to the solution and stirred for between about 30 minutes and about 5 hours to get a homogeneous suspension. In some embodiments, the suspension can then be placed in an autoclave and maintained at between about 90 °C and about 150 °C for between about 30 minutes and about 5 hours to simultaneously achieve the reduction of GO and the deposition of Ni/TiO₂ on the carbon substrate. In some embodiments, the resulting composite can be recovered by filtration, and the composite can be rinsed several times using deionized water, and dried at room temperature.

[0042] In some embodiments, adding functional groups to the graphene- TiO₂ nanoparticles 420 can include a first carboxylation process. In some embodiments, about

50 wt% to about 80 wt% of the graphene-TiO₂ nanoparticles can be mixed with H₂SO₄ while about 20 wt% to about 50 wt% can be mixed with HNO3, and then the two solutions mixed together. In some embodiments, the mixture can be centrifuged for between about ten minutes and about 10 hours at between about 5,000 rpm and about 30,000 rpm and the supernatant decanted and distilled water added. In some embodiments, the rinsing process can be repeated until a pH of about 7 is achieved and then a solvent is added and the process repeated until the water is removed. In some embodiments, the solution can then be allowed to dry at room temperature, dried using an oven or through accelerated evaporation of the solvent, or by any other suitable method.

[0043] In some embodiments, adding functional groups to the graphene-TiO₂ nanoparticles 420 can include a second carboxylation process. In some embodiments, the graphene-TiO₂ nanoparticles can be sonicated for about 3 hours with an excess of trichloroacetic acid. The solution can be rinsed with deionized water and the solution filtered through hydrophilic filter paper and dried under vacuum at about 50 °C, [0044] In some embodiments, adding functional groups to the graphene-TiO₂ nanoparticles 420 can include immersing the nanoparticles in an electrolyte consisting of carboxylic acids in order to functionalize the TiO₂ with carboxyl groups. In some embodiments, the carboxylic acid is carboxylic acid or acetic acid. In some embodiments, the ball milling is accomplished with an electrolyte such as carboxylic acid, acetic acid, deprotonic acid, a weak oxidizing agent, sulfuric acid, NaNO₃, a surfactant, sodium dodecyl sulfate, thionin acetate salt, or combinations thereof. In some embodiments, the mass ratio of ball milling balls and oxidation agent can be between about 1:5 and about 100:1, between about 1 :3 and about 50:1, between about 1 :2 and about 25:1 , between about 1 :1 and about 10:1, inclusive of all values and ranges therebetween. In some embodiments, the ball mill is rotated with speeds from about 10 rpm to about 500 rpm, about 25 rpm to about 250 rpm, about 50 rpm to about 100 rpm, inclusive of all values and ranges therebetween. In some embodiments, the pH of the electrolyte solution in the ball milling step is between about 0 and about 5, between about 0 and about 3, between about 1 and about 4, between about 1 and about 3, inclusive of all values and ranges therebetween.

|0045] In some embodiments when the TiO₂ particles are milled, GO is then added into the functionalized TiO₂ and the GO-TiO₂ compound is produced. In some embodiments, the GO-TiO₂ compound can be incorporated into a mixture of mineral and silicon resin and the mixture sprayed over the substrate to form an enhanced self-cleaning coating to protect against organic and environmental pollutants.

[0046] In some embodiments, treating the substrate 430 can be accomplished by plasma treating. In some embodiments, a substrate can be placed in the discharge path of a plasma jet such that the electrons generated in the discharge impact the surface with energies 2 to 3 times the energy necessary to break the molecular bonds on the surface of the substrate. This creates very reactive free radicals. These free radicals in the presence of oxygen can react rapidly to form various chemical functional groups on the substrate surface. Functional groups resulting from this oxidation reaction can be effective at increasing surface energy and enhancing chemical bonding to a resin matrix. In some embodiments, these can include carbonyl (-C=O-), carboxyl (HOOC-), hydroperoxide (HOO-), and/or hydroxyl (HO-) groups. 10047] In some embodiments, functionalizing the substrate with self-assembled monolayers 440 can include functionalizing with monolayers of aminosilanes. In some embodiments, 3-Triethoxysilylpropylamine (APTES) can be used as a silane coupling agent to modify the surface of the GO-TiO₂ nanoparticles. In some embodiments, GO-Ni/TiO₂ can be dispersed in deionized water by ultra-sonication for between about l min and about 1 hour. In some embodiments, the silane coupling agent can be have a concentration of between about 0.01 wt% and about 10 wt%. In some embodiments, the mixture can be refluxed at about 60 °C for between about 1 min and about 1 hour. In some embodiments, the method can further include separating dispersed particles from the solvent (e.g., using a centrifuge for about 10 min at about 10,000 rpm) followed by washing with distilled water ibr between 2 and 4 cycles.

10048] In some embodiments, depositing the functionalized TiO₂ nanoparticles to the substrate 450 can be accomplished by any of the application methods described above or a combination thereof. In some embodiments, a binder or resin material can be applied to the substrate and the fiinctionalized TiO₂ nanoparticles can be applied to the binder or resin material. In some embodiments, the functionalized TiO₂ nanoparticles include functional groups that facilitate their attachment onto the substrate. In some embodiments, the functional groups can include carbonyl (-C=O-), carboxyl (HOOC-), hydroperoxide (HOO- ), and hydroxyl (HO-) groups. In some embodiments, the functional groups can cause increased binding strength for the fiinctionalized TiO₂ nanoparticles to the substrate.

[0049] In some embodiments, the method 400 for forming a photocatalytically active surface includes functionalizing metal oxide particles with carboxylic groups using a first milling process step and increasing the affinity of graphene and water using surfactants such that electrostatic forces chemically bond graphene with the functionalized GO particles through a second milling process step. In some embodiments, the two steps can be carried out as two discrete steps or as a continuous process in one milling run. In some embodiments, the temperature during ball milling is maintained such that electrolytes (e.g., water) do not appreciably evaporate during ball milling. In some embodiment, the temperature during ball milling is about room temperature. In some embodiments, the ball milling is stopped every 15, 30, 45 or 60 minutes for gas evacuation. |0050] FIG. 5 illustrates a method for making a photocatalytie coating (e.g., photocatalytie coating 210) for a photocatalytically active surface (e.g., photocatalytically active surface 200) including co-precipitation of synthesized TiO₂ nanopartides with a nickel-containing material, preparation of a Ni-doped TiO₂/graphene oxide composite, functionalization of the surface of the Ni-doped TiO₂/graphene oxide nanopartides, preparation of a receiving substrate, depositing the functionalized graphene-TiO₂ composite onto the substrate to produce a stable, homogenous, and transparent film, and optionally heat treating the coated substrate.

[0051] In some embodiments, pre-synthesized ΊΊO2 nanopartides (NPs) can be doped with nickel. In some embodiments, NaOH can be used as a precipitating agent and a nickel- containing material can be used as the precursor. In some embodiments, the precursor can include nickel (II) nitrate hexahydrate. In some embodiments, the precursor can be dissolved in distilled water along with glycerol in order to prevent Nik aggregation. In some embodiments, nanopartides of TiO₂ can be mixed with the nickel-glycerol complex until substantially homogeneous. In some embodiments, the precipitation can be carried out by dropwise addition of NaOH into the suspension until a pH of about 8.5 is achieved. In some embodiments, dropwise addition of NaOH can be carried out while mixing the solution. In some embodiments, the solution can be mixed for about two hours. In some embodiments, the stirring and pH increase may contribute to the formation of a precipitant that includes nickel-doped TiO₂ nanopartides. In some embodiments, the precipitant can be filtered to remove excess water and the filtered precipitant can be dried at about 105 °C. In some embodiments, the dried precipitant can be calcined for about 5 hours at about 550 °C. In some embodiments, the amount of nickel doping applied onto the TiO₂ nanopartides can be optimized based upon the photocatalytie behavior of the resulting nickel-doped TiO₂ nanopartides under visible light.

10052] In some embodiments, the method can include a Ni-doped TiO₂/graphene oxide composite formation step in which the Ni-doped TiO₂ nanopartides are deposited onto graphene oxide (GO) substrates. In some embodiments, GO powder can be dispersed into distilled water and ethanol. In some embodiments, ultrasonication can be carried out on the mixture for about one hour to achieve a homogenized mixture. In some embodiments, Ni- doped TiO₂ nanoparticles can be added and the mixture can be sonicated for approximately an additional two hours. In some embodiments, the suspension is then heated in an autoclave, e.g., at about 120 °C for about three hours, to reduce the GO and cause the deposition of the Ni-doped TiO₂ nanoparticles onto the reduced GO substrates. In some embodiments, the resulting Ni-doped TiO₂/GO composite can be recovered from the solution by filtration, rinsed using distilled water, and dried at room temperature.

[0053] In some embodiments, the method can include a surface modification step in which the Nί-doped TiO₂/GO nanoparticles are surface functionalized. In some embodiments, (3-Aminopropyl)triethoxysilane (APTES) can be used to functionalize the Ni-doped TiO₂/GO nanoparticles. In some embodiments, to cause functionalization, the Ni- doped TiO₂/GO nanoparticles can be mixed with trimethylamine and ethanol at about 60 °C until the mixture is substantially homogenized. In some embodiments, ammonium hydroxide, deionized water, and APTES (e.g., about 1 wt%) can be added to the solution. In some embodiments, the mixture can be stirred continuously for greater than about four hours, about five hours, about six hours, about seven hours, about eight hours, about nine hours, about ten hours, about eleven hours, about twelve hours, about thirteen hours, about fourteen hours, or greater than about fifteen hours.

[0054] In some embodiments, the resulting solution can be centrifuged at about 4.500 rpm for about 15 minutes. In some embodiments, the supernatant after centrifugation can be discarded and the pellet can be rinsed multiple times with ethanol. In some embodiments, the resulting functionalized Nί-doped TiO₂/reduced GO composite material (e.g., in powder form) can be stored temporarily in toluene or another suitable solvent while additional preparation steps are carried out. In some embodiments, the functionalized Ni-doped TiO₂/reduced GO composite material can be created just in time for disposition to the prepared substrate and therefore no storage in toluene or another suitable solvent is used.

[0055] In some embodiments, the method can include a substrate preparation step in which the receiving substrate is cleaned and/or modified. In some embodiments, the substrate can first be cleaned according to any suitable method whereby no residue remains on the substrate after cleaning that would interfere with deposition of the surface coating to the substrate. In some embodiments, the cleaned substrate can be treated using a plasma process, e.g., using a CO₂ plasma. Without wishing to be bound by any particular theory, treatment of the cleaned substrate using a plasma process such as COz plasma treatment can help create carboxylic functional groups on the surface of the substrate. In some embodiments, an approximately 1:1 mixture of l-Ethyl-3-(3- dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS), on a molar basis, can be applied to the plasma treated substrate. Without wishing to be bound by any particular theory, application of the mixture of EDC-NHS may activate the carboxylic groups on the surface of the substrate. In some embodiments, the resulting solution can be applied to the substrate using any suitable method and left for a period of time to activate the carboxylic functional groups on the surface of the substrate. In some embodiments, the solution can be rinsed off the substrate using deionized water. In some embodiments, the functionalized Ni-doped TiO₂/reduced GO nanoparticles can be applied to the prepared substrate using any suitable method, including but not limited to spray coating, dip-coating, or spin coating. In some embodiments, the applied coating can be left to chemically bind to the prepared substrate for a period of time, e.g. greater than about two hours. In some embodiments, the coated substrate can be rinsed with deionized water and dried at room temperature.

[0056] In some embodiments, an alternative method for coating the surface can include heat treatment. In some embodiments, the prepared functionaiized Ni-doped TiO₂/reduced GO nanoparticle solution can be applied to the prepared substrate and left at room temperature. In some embodiments, the coated substrate can then be heat treated at about 200 °C for about five hours, In some embodiments, after heat treatment, the coated substrate can be rinsed with deionized water and/or wiped to remove any unattached functionalized Ni-doped TiO₂/reduced GO nanoparticles.

Working Example

[0057] A TiO₂-GO composite was prepared by dissolving 2 mg of GO in a solution of 20 mL of distilled water and 10 mL of ethanol using ultrasonic treatment for one hour. 0.2 g of TiO₂ was added to the GO solution and stirred for two hours to get a homogenous suspension. The suspension was then placed in an autoclave and maintained at 120 °C for three hours to simultaneously achieve the reduction of GO and the deposition of P25 on the carbon substrate. The resulting composite was recovered by filtration, rinsed by deionized water several times, and dried at room temperature.

[0058] Surface modification of the TiO₂ nanoparticles was accomplished by using 3- triethoxysilylpropylamine (APTES) as a silane coupling agent to modify the composite surface. 0.5 mg of TiO₂-GO was dispersed in 50 mL of deionized water by ultrasonication for 10 minutes. The silane coupling agents were added to a concentration of 2 wt%. The mixture was kept refluxing at 60 °C for 10 minutes. Dispersed particles were separated from solvent by centrifuging for ten minutes at 10,000 rpm, followed by washing with distilled water for two cycles.

[0059] The polyvinyl chloride (PVC) substrate was treated by plasma treatment and then amino-salinized. The photoactive coating was then applied to the treated surface via spray coating and the coating was dried in an oven at 100 °C for 24 hours and cooled for one hour at room temperature. The coating Is also more stably bonded to the substrate, perhaps due to the covalent bonding between the coating and the substrate. The bonding was characterized using Fourier-transform infrared radiation spectrophotometry (FT-IR) and x-ray photoelectron spectroscopy (XPS). The resulting coating is capable of increased photoactivity due to an approximately 50% increase in the quantity of electron pairs that can reach the surface of the coating. The resulting coating also exhibited self-cleaning characteristics, as measured by a reduction in the accumulation of organic compounds on the surface over time, as compared to uncoated substrate.

[0060] All literature and similar material cited in this application, including, but not limited to, patents, patent applications, articles, books, treatises, and web pages, regardless of the format of such literature and similar materials, are expressly incorporated by reference in their entirety. In the event that one or more of the incorporated literature and similar materials differs from or contradicts this application, including but not limited to defined terms, term usage, described techniques, or the like, this application controls.

[0061] While the present teachings have been described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments or examples. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art.

[0062] While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure arc directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0063] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

[0064] The indefinite articles“a” and“an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean“at least one." Any ranges cited herein are inclusive.

[0065] The terms“substantially” and“about” used throughout this Specification are used to describe and account for small fluctuations. For example, they may refer to less than or equal to +5%, such as less than or equal to +2%, such as less than or equal to +1%, such as less than or equal to +0.5%, such as less than or equal to +0.2%, such as less than or equal to +0.1 %, such as less than or equal to +0.05%.

[0066] The phrase“and/or," as used herein in the specification and in the claims, should be understood to mean“either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e.,“one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the“and/or" clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to“A and/or B”, when used in conjunction with open-ended language such as“comprising" may refer, in some embodiments, to A only (optionally including elements other than B); in some embodiments, to B only (optionally including elements other than A); in yet some embodiments, to both A and B (optionally including other elements); etc.

[0067] As used herein in the specification and in the claims,“or” should be understood to have the same meaning as“and/or” as defined above. For example, when separating items in a list,“or” or“and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of" or“exactly one of,” or, when used in the claims,“consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e.“one or the other but not both") when preceded by terms of exclusivity, such as“either, "“one of,”“only one of,” or“exactly one of.”“Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[00681 As used herein in the specification and in the claims, the phrase“at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase“at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a nonlimiting example,“at least one of A and B” (or. equivalently, "at least one of A or B," or, equivalently“at least one of A and/or B") may refer, in some embodiments, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in some embodiments, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet some embodiments, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. [0069] As used herein“at%” refers to atomic percent and“wt%” refers to weight percent. However, in certain embodiments when“at%” is utilized the values described may also describe“wt%.” For example, if“20 at%” is described in some embodiments, in other embodiments the same description may refer to“20 wt%.” As a result, all“at%” values should be understood to also refer to“wt%” in some instances, and all“wt%” values should be understood to refer to“at%” in some instances.

[0070] In the claims, as well as in the specification above, all transitional phrases such as '“comprising,”“including,"“carrying,”“having,”“containing,”“involving,”“holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases“consisting of" and“consisting essentially of" shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 21 1 1.03.

[0071] The claims should not be read as limited to the described order or elements unless stated to that effect, lt should be understood that various changes in form and detail may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. All embodiments that come within the spirit and scope of the following claims and equivalents thereto are claimed.

Claims

Claims

1. A method of preparing a Ni/TiO₂-graphene oxide composite, the method comprising;

preparing a Ni-doped TiO₂ catalyst via co-precipitation using NaOH as a precipitating agent; and

preparing the Ni/TiO₂-graphene oxide composite.

2. The method of claim 1 , further comprising:

modifying a surface of the Ni/TiO2-graphene oxide composite.

3. The method of claim 2, wherein modifying the surface is modifying the surface using

3-triethoxysilylpropylamine.

4. A method of applying a Ni/TiO₂-graphene oxide coating to a substrate, the method comprising:

treating the substrate using a plasma jet;

amino-salinizing the substrate; and

depositing the Ni-TiO₂-graphenc oxide coating to the substrate.

5. A photocatalytically active surface, comprising:

a substrate having a first organic compound accumulation rate;

a ball milled material including graphene oxide and titanium dioxide, the ball milled material disposed to the substrate and configured to form the photocatalytically active surface having a second organic compound accumulation rate less than the first organic compound accumulation rate.