WO2014064427A1

WO2014064427A1 - Low refractive index particles

Info

Publication number: WO2014064427A1
Application number: PCT/GB2013/052703
Authority: WO
Inventors: Martin Gardener; James Best
Original assignee: Oxford Energy Technologies Limited
Priority date: 2012-10-24
Filing date: 2013-10-16
Publication date: 2014-05-01
Also published as: GB201219126D0

Abstract

This invention relates to porous organosilica nanoparticles having a BET surface area of at least 400m²/g and/or a pore volume of at least 0.7cm³/g, wherein the nanoparticles have a mean pore diameter of l-50nm, a mean diameter in the range 1-100nm and wherein the pores of the nanoparticles are randomly oriented. This invention also relates to solutions, dispersions, powders, compressed pellets and coatings comprising a plurality of the porous organosilica nanoparticles, as well as to methods for preparing the porous organosilica nanoparticles.

Description

LOW REFRACTIVE INDEX PARTICLES

[001] This invention relates to porous organosilica nanoparticles having a high pore volume and surface area, and a low refractive index. This invention also relates to coatings comprising the nanoparticles, optionally in a suitable binder, which are transmissive preferably to visible light, and preferably provide anti-reflective properties, and optionally provides other additional functionality. The coatings are particularly, but not exclusively, suitable for application to ophthalmics and eyewear, photovoltaic cells, displays, windows, light emitting diodes and solar concentrators. The coatings also find application in low dielectric constant (low k) materials for integrating circuits.

[002] Eyewear, solar cells and displays generally consist of an outer substrate exposed to the environment consisting of a sheet of glass or polymer. These typically have a refractive index of 1.5 - 1.7 and reflect about 4 - 5% of incident sunlight on each surface - energy which reduces visibility through the substrate or which is lost to a solar cell. These substrates may be coated with an anti-reflective coating layer that reduces this reflection to less than 2%. Fig. 1 illustrates schematically a conventional single-layer anti- reflective (AR) coating 1 on a substrate 2. The thickness of the AR coating 1 is h. The reflectance is reduced if the light reflected off the front and back surfaces of the AR coating 1 is arranged to destructively interfere. This is achieved (for normal incidence) if the thickness of the coating 1 is equal to a quarter of the wavelength of the incident light in the medium of the coating, i.e.:

where λ is the wavelength of the light in vacuum, and ni is the refractive index of the coating. This assumes that the refractive index ni of the coating 1 is less than the refractive index n_m of the substrate 2, such that there is a π phase change of the light reflected at the interface between the coating 1 and the substrate 2. The thickness h may, of course, be any odd integer multiple of one quarter of the wavelength of the light in the coating. For complete destructive interference, the amplitude of the two reflected waves must be equal to each other. This can be achieved if the refractive indices are matched such that:

rearranging this gives: n_x = ^n₀n_m

For air no = 1, and for glass n_m = 1.5, which gives the ideal refractive index of the coating as ni = 1.22. [003] The degree of reflection from a given surface is related to its refractive index, a higher refractive index resulting in greater reflection. At normal light incidence this may be simply calculated using the equation below - for a typical polycarbonate surface of refractive index n_m = 1.586, the reflection R is 5.1% per surface, giving a total of 10.2% reflected light.

[004] Reflections are significantly enhanced at higher incident angles and even poorly reflecting surfaces can appear mirror-like at glancing angles. At an incident angle of 65° a typical surface will reflect over 25% of the light striking each surface. The equations describing this behaviour are known as the Fresnel Equations and further information can be found in any optics text, for example Hecht E, Optics, 2002 ppll3-122.

[005] As mentioned above, the anti-reflective coating layer thickness governs the phase difference between the two waves and the refractive index of the layer governs the amplitude of the reflected waves. The behaviour of the coating system is described by the equation below, in which a coating of refractive index ni is applied to a surface of refractive index n_m.

R ⁿl (l ^{~ n}m f ^cos2 k₀h + (n_m -

)sin k₀h

ni (l + ⁿm f ^cos2 k₀h + {n_m + n )sin ² k₀h

[006] The terms ko and h refer to the phase angle of the incident light and the optical thickness of the film respectively. For an incident light wavelength of λο and a film thickness of d= λο/4 ni equation 2 simplifies to:

[007] Therefore, reflectance R=0% when the refractive index of the coating is the square root of the refractive index of the surface. So, for a polycarbonate surfaces, a llOnm thick coating of refractive index 1.26 on a surface would have zero reflection at 550nm, the centre of the visible spectrum. [008] This is the simplest solution to the reflection problem, but options for such a coating have been limited. Materials with the lowest known refractive indices tend to be fluorides such as MgF₂ (refractive index=1.38) or CaF₂ (refractive index=1.43) which only reduce the reflection from a typical lens surface to 1.3-1.5% and in any case are water soluble, necessitating encapsulation layers which further degrade the anti-reflective properties.

[009] Low refractive index mesoporous silica nanoparticles such as those described in international patent application numbers PCT/GB2010/000490 and PCT/GB2011/051565 are suitable for use in anti-reflective coatings due to their high pore volume (60 to 70%) and consequently low refractive index (1.16).

[0010] Binder material may be included with the particles in a coating layer in order to try and impart mechanical durability while retaining a layer refractive index of 1.23 to 1.27. However, a trade-off always exists between optical properties and mechanical strength. A higher binder content in the coating provides greater mechanical strength, but results in a corresponding increase in refractive index. Conversely, a lower binder content in the coating provides a lower refractive index, but results in reduced mechanical strength. Thus, providing a particle of lower refractive index will allow the use of more binder material in a coating of a particular refractive index, and hence a stronger film results. This invention seeks to provide such lower refractive index particles.

[0011] A relatively small reduction in the refractive index of the particles in the coating can result in an increase in the amount of binder that can be used to achieve a particular refractive index of the coating, giving a significant increase in the mechanical strength of the coating. Thus, a reduction of particle refractive index of as little as 0.01 or more would prove to be highly advantageous over the current particle technology.

[0012] The dielectric constant of a material is related to the refractive index and, as such, low refractive index silica nanoparticles may also be used as low dielectric constant (low-κ) materials. In integrated circuits signal delays have traditionally been as a result in delays within the transistors themselves. The transistors are interconnected by wires which are surrounded and insulated by a dielectric layer. However, as transistor sizes become smaller the interconnects become increasingly tightly packed and crosstalk between them induces capacitance. The resultant interconnect delay effect goes as the inverse square of the interconnect line spacing and there is a rapid increase in capacitance as feature size decreases, to the extent that it becomes dominant over the transistor gate delay. The interconnect delay may be reduced by either reducing the resistivity of the interconnect wires or by reducing the dielectric constant of the material between the wires, conventionally silica in which k=3.9.

[0013] Using the Clausius-Mossitti relationship and the known void fractions of mesoporous silica nanoparticles we can estimate that mesoporous silica nanoparticle based anti-reflection coatings in a silicate binder will have a dielectric constant of 2.15. This would further be reduced by reducing the density and increasing the porosity of the particles and hence improving these properties would make a coating of the particles attractive for use as a low-K dielectric in an integrated circuit (see, for example, BD Hatton et al, Materials Today, Volume 9, number 3, pp 22-31 (2006)).

[0014] Known methods for producing mesoporous silica nanoparticles generally involve forming the particles from a mixture of tetraethoxysilane and phenyltriethoxysilane with triethanolamine. The triethanolamine acts as both a base catalyst and a pore templating agent. The templating agent is removed by centrifugation and washing with acidified alcohol before final dispersion in alcohol. [0015] It has surprisingly been found by the inventors that incorporation of bridged silanes (such as bis silanes or tris silanes) in silica nanoparticles retains the advantageous structure of the particles, but reduces the density by incorporation of carbon bridges thereby leading to a lower refractive index than previously achieved. In particular, higher BET surface areas and higher pore volumes can be attained.

[0016] Bis silanes have previously been used in the production of mesoporous organic materials (see M. A. Wahab, I. Kim, C. Ha, J. Solid State Chemistry, 177 (2004), 3439-3447). However, these materials were not in the form of nanoparticles, and were intended for use as catalyst supports.

[0017] Accordingly, this invention relates to porous organosilica nanoparticles having a BET surface area of at least 400m²/g and/or a pore volume of at least 0.7cm³/g. BET is a standard technique for the measurement of surface area by the absorption of gas molecules (for example by nitrogen adsorption-desorption isotherms). Pore volumes were determined using BJH analysis (again by nitrogen adsorption-desorption isotherms). All measurements were in accordance with IS09277. [0018] In relation to this invention, the term "organosilica" is used in relation to the nanoparticles to refer to particles comprising carbon-silicon bonds. Such particles are also referred to as carbon-doped silica nanoparticles.

[0019] Preferably, the nanoparticles have a pore volume of at least 0.8cm³/g, more preferably at least 0.9cm³/g, even more preferably at least 0.95cm³/g. It is preferred that the nanoparticles have a BET surface area of at least 500m²/g, preferably at least 550m²/g, more preferably at least 600m²/g.

[0020] It is preferred that the nanoparticles have a refractive index of less than 1.16, preferably 1.14 or less, more preferably 1.13 or less. In some embodiments, the refractive index of the nanoparticles is less than 1.12, and can be as low as 1.10 or less. The refractive index of the nanoparticles is calculated using ellipsometry. More specifically, the refractive index of particular nanoparticles are calculated by measuring by ellipsometry the refractive index of coatings comprising the particles and a binder at a specific ratio, plotting these results on a graph of refractive index against the particle loading (ie wt%) of a particular coating and drawing a linear fit of the data points, and extrapolating the linear fit to determine the refractive index of a coating having 100wt% particle loading. As will be appreciated, at least two different particle loadings need to be measured in order to do this, although more loadings provide a more accurate result. In a particularly preferred embodiments, the refractive index of the nanoparticles is calculated by preparing, and measuring the refractive indices of, coatings having 0wt%, 10wt%, 20wt%, 30wt%, 40wt%, 50wt% and 60wt% particle loadings.

[0021] In a preferred embodiment, the nanoparticles have a refractive index such that, when they are formulated into a coating comprising about 30wt% nanoparticles and about 70%wt silica, the coating has a refractive index as measured by ellipsometry of 1.40 or less, more preferably 1.38 or less, in some embodiments 1.36 or less.

[0022] It is preferred that the particles have a pore volume greater than 50% of the volume of the particle, preferably greater than 55%, more preferably greater than 60%. It is preferred that the particles have a density of less than 3g/cm³, preferably less than 2.5g/cm³, more preferably less than 2.2g/cm³. [0023] The organosilica nanoparticles of the present invention are porous, preferably having a mean pore diameter in the range l-50nm. It is preferred that the nanoparticles are mesoporous, ie they have a mean pore diameter in the range 2-50nm. Preferably, substantially all of the pores (more preferably all of the pores) have a mean pore diameter in the range 1-lOnm, preferably in the range l-5nm, more preferably in the range l-3nm.

[0024] The pores of the nanoparticles preferably have an internal surface at least partially comprising a hydrophobic layer. The hydrophobic layer is preferably an organic layer, more preferably a polymer. The polymer can comprise polystyrene and/or polyvinyl butadiene. Alternatively, the organic layer comprises phenyl and/or alkyl groups. These groups can be substituted with halogen and/or amine groups. In some embodiments, the organic layer comprises one or more trialkylamines or triethanolamine. The hydrophobic layer is preferably less than 50wt% of the weight of each particle, more preferably less than 40wt%, even more preferably less than 30wt%. In some embodiments, the hydrophobic layer is a monolayer. [0025] It is preferred that the nanoparticles have a continuous porous structure, ie they are not hollow. Preferably, none of the pores has a pore diameter greater than 50% of the diameter of the nanoparticle. More preferably, none of the pores has a pore diameter greater than 25%, even more preferably 15%, of the diameter of the nanoparticle.

[0026] The term "nanoparticles" is used in relation to this invention to refer to particles having a mean diameter in the range 1-lOOnm. Preferably, the nanoparticles have a mean diameter in the range l-50nm, more preferably in the range 10-40nm, even more preferably in the range 20-30nm. The porous particles are preferably in the size range 20-30nm in order to reduce any surface roughness of the film to less than 30nm.

[0027] It is preferred that the pores of the porous silica nanoparticles are randomly oriented. The term "randomly oriented" is used in relation to this invention to refer to pores which do not form a repeating (and/or does not form a partly or entirely symmetrical) structure. Examples of this are pores which have a tortuous path, and/or are disordered, and/or are non-uniform, and/or are non-periodic, and/or are irregular and/or are asymmetric and/or having no axis of symmetry. Thus, the preferred pore structure is not in the form of an array of pores (ie the pores are not arranged in rows and/or columns). In some embodiments, the pore structure is not in the form of an array of pores (ie the pores are not arranged in rows and/or columns) at any scale.

[0028] Without wishing to be bound to any theory, it is thought that by providing porous silica nanoparticles having randomly oriented pores, this tortuous pore path has the effect of reducing liquid ingress (ie ingress of the binder in the coating). The nanoparticles used in the invention can consist of a random collection of pores which are arranged in a complex tortuous path. This type of structure, optionally in conjunction with the hydrophobic layer on the internal surface of the pores described above, can help to block binder ingress into the particle core, maintaining the low refractive index of the particles when they are immobilised in the binder. Thus, the refractive index of the particles in the coating can become an average of that of air and the material of the particles.

[0029] The random orientation of the pores of the organosilica nanoparticles means that when the nanoparticles are formulated into a coating with a binder, the pores are primarily air filled (ie preferably at least 50% of the volume of the pore is air) except for the optional thin hydrophobic internal surface. Due to the random nature of the internal pore structure there is preferably substantially no binder ingress into the pores. In addition to the low refractive index of the particles, this assists in allowing the refractive index of the coating to be maintained at less than 1.20. This effect may be enhanced if the pores have an internal surface at least partially comprising a hydrophobic layer and the binder is hydrophilic. Preferably, the external surface of the nanoparticles (ie excluding the pores) is hydrophilic. This can be achieved in some embodiments of the invention by ensuring that there is substantially no surface functionalisation of the surface of the nanoparticles. It is preferred that the nanoparticles are distributed within the binder.

[0030] In some embodiments, the organosilica nanoparticles comprise functional groups on their surface in order to improve compatibility with a binder and/or a solvent. Preferably, the organosilica nanoparticles comprise acrylate-containing groups, thiol-containing groups, carboxylic acid- containing groups, glycidyl ether-containing groups and/or amine-containing groups on their surface, more preferably acrylate-containing groups. It is preferred that the organosilica nanoparticles comprise methacrylate- containing groups on their surface. Preferably, the methacrylate-containing groups comprise 3-(methacryloxy) propyl groups.

[0031] The term "hydrophilic" is used in relation to this invention to refer to a substance whose surface has a water contact angle of less than 90°. The term "hydrophobic" is used in relation to this invention to refer to a substance whose surface has a water contact angle of greater than 90°.

[0032] This invention also relates to a method for preparing porous organosilica nanoparticles comprising the step of: (a) mixing a bis and/or tris silane with an organic templating agent, a surfactant and a base in order to form porous silica nanoparticles.

Preferably the method is for preparing the particles as defined above.

[0033] The term "organic templating agent" is used in relation to this invention to refer to a compound whose presence in the method described above directs the pore structure of the resulting nanoparticles. In some embodiments, the organic templating agent and the base are the same component, ie both functionalities are provided by one chemical component. A preferred organic templating agent is triethanolamine (TEA). TEA can act as both an organic templating agent and a base. A preferred surfactant is cetyltrimethylammonium chloride (CTAC). [0034] The terms "bis silane" and "tris silane" are used in relation to this invention to refer to bridged silanes. The term "bis silane" is used to refer to a silane compound having two silane groups. The term "tris silane" is used to refer to a silane compound having three silane groups. [0035] It is preferred that the silicon atoms in the silane groups are linked by at least one intervening atom. This at least one intervening atom is known as the bridge or linking group. Preferred bridges comprise alkyl groups, alkylether groups, ketones, esters, sulphides (for example, tetrasulphides), azines and/or amides. Preferably, the bridge comprises a methyl, ethyl, propyl, butyl or pentyl group, more preferably an ethyl group.

[0036] It is preferred that step (a) comprises mixing a bis silane, preferably a bis(alkyloxy) silane, more preferably a bis(trialkyloxy) silane. Preferred bis silanes are 1,2 -bis (triethoxysilyl) ethane and 1,2 -bis (triethoxysilyl) methane. Without wishing to be bound to any theory, it is thought that bis and/or tris silanes provide a more open, porous particle structure with a reduced density. An example of a tris silane is 1,1,2 -tris (triethoxysilyl) ethane.

[0037] The inclusion of an organic templating agent results in structural modification of the particle and assists in the development of a randomly oriented pore structure. For example, in the method of the invention, the space occupied by the organic templating agent cannot be occupied by the silica, and hence the organosilica grows around the organic templating agent, resulting in an intimately mixed organic/inorganic particle.

[0038] In a preferred embodiment, the method comprises providing a solution comprising a bis and/or tris silane and mixing this with a solution comprising the organic templating agent, the surfactant and the base. [0039] It is preferred that mixing step (a) additionally comprises another silane, preferably a mono silane. The term "mono silane" is used to refer to a silane compound having one silane group. Preferred mono silanes are tri or tetra alkoxy silane. Particularly preferred mono silanes include tetralkoxysilanes, such as tetramethoxysilane and/or tetraethoxysilane, and/or organotriethoxysilanes. A particularly preferred organotriethoxysilane is phenyltriethoxysilane. Preferably, the mono silane comprises a combination of a tetralkoxysilane and an organotriethoxysilane, more preferably tetraethoxysilane and phenyltriethoxysilane, preferably with the bis silane 1,2 -bis (triethoxysilyl) ethane. Preferably, this silane or silanes is/are provided in the solution comprising the bis and/or tris silane.

[0040] The reaction typically proceeds via hydrolysis of an alkoxysilane, followed by co-condensation of the hydrolysed precursor to produce an inorganic silica polymer. [0041] The reaction is catalysed by the presence of a base, which accelerates the condensation reaction. Preferred bases include ammonia, sodium hydroxide, potassium hydroxide, amines and triethanolamine. The most preferred base is triethanolamine. Thus, the reaction is normally performed in an alkaline solution, which is typically an aqueous solution of the base. This reaction can then result in spherical silica particles.

[0042] It is preferred that the mixing of the silane(s) with the organic templating agent, the surfactant and the base is to form a colloidal suspension of organosilica nanoparticles, preferably in the form of micelles. The overall particle size is normally controlled by forming an oil in water emulsion. The emulsion droplets can act to halt growth of the particle beyond the domains of the droplet. The droplet size can be controlled by the ratio of oil, water and emulsifying agent type and concentration. Under appropriate conditions, particle diameter and distribution of diameters can therefore be kept within the most preferred range of 20-30 nm.

[0043] It is preferred that the method additionally comprises, after step (a), the step of (b) removing the organic templating agent. This is preferably done by adding a solution comprising an acid and an alcohol. A preferred acid is hydrochloric acid. A preferred alcohol is ethanol.

[0044] It is preferred that the method additionally comprises, after step (c), one or more steps of washing the precipitate with a solution of ammonium nitrate in alcohol, and/or with water. A preferred alcohol is ethanol.

[0045] Removal of the organic templating agent, by a solvent that dissolves the agent and not silica, results in a silica particle with pores resulting from the organic templating agent removal. Organic templating agent removal is never complete because the surface energy increase of completely removing the agent from the silica surface is too large. Hence a degree of organic templating agent is retained within the silica nanoparticles on the internal surfaces of the pores. [0046] In addition, this invention relates to porous organosilica nanoparticles obtainable by the methods described above.

[0047] This invention also relates to a solution for forming a coating, preferably an optical coating, comprising a solvent and a plurality of porous organosilica nanoparticles as defined above. In some embodiments, the solution also comprises a binder. In other embodiments, a further solution for forming a coating comprising a binder and a solvent is provided.

[0048] This invention also relates to a dispersion for forming a coating comprising a solvent and a plurality of porous organosilica nanoparticles as defined above, and optionally a suitable binder.

[0049] This invention also relates to a coating comprising a plurality of porous organosilica nanoparticles as defined above and a binder. It is preferred that the nanoparticles are distributed within the binder. This invention also relates to powders and/or pellets, preferably compressed pellets, comprising a plurality of porous organosilica nanoparticles as defined above. [0050] In relation to this invention, the binder may be either inorganic or organic. In the coating, the binder surrounds the particles and acts to provide mechanical strength to the film. The binder may comprise at least one of a silicate, silica, silicone based polymer, siloxane based polymer, acrylate based polymer, cellulose, cellulose derivatives, isocyanate, or vinyl alcohol. The binder is preferably a hydrophilic binder. Particularly preferred binders include tetraethoxysilane (TEOS), pentaerythritol triacrylate (PETA) or a siloxane-based hardcoat comprising 3-glycidoxypropyltrimethoxysilane (GPTMS) (known as a hardcoat in optical applications). It can be advantageous to select a binder which is compatible with the optional surface groups on the nanoparticles. A particularly preferred combination of acrylate polymer and surface groups is a polymer formed from pentaerythritol triacetate monomer with 3-(methacryloxy) propyl surface groups. [0051] Selecting a binder which has similar properties to the underlying substrate (ie chemical compatibility such that the binder will adhere to the substrate) can reduce or substantially eliminate brittle and interface film failure even under loads that induce significant distortion to the coating. Examples of suitable combinations include (i) a TEOS or siloxane-based binder and a glass substrate, and (ii) an acrylate binder and a TAC substrate.

[0052] In some embodiments, the coating additionally comprises a fluorine- containing compound (also known as a fluorocompound). Fluorocompounds may be included in the coating at a level of between 0.1 and lwt%. Fluorocompounds may be included in the coating and/or applied to the coating in order to impart a low surface energy to the coating surface. The effect of a low surface energy is improved abrasion resistance, hydrophobicity and anti-fingerprint properties. [0053] A variety of fluorocompounds exist that may be used including fluorine-substituted alkoxysilanes and fluoroacrylates such as fluoroalkylacrylates and fluoromethacrylates. Examples of suitable compounds are perfluoroalkylethyl acrylate esters (available through DuPont as Zonyl® TA-N) and fluoroalkylmethacrylate esters (available through DuPont as Zonyl® TM). Examples of fluorine-substituted alkoxysilanes include hydrolysed/oligomerised perfluorooctyltrimethoxysilane and fluorosilicones. The inclusion of fluorocompounds in the coating can be advantageous because they migrate to the coating surface, thereby lowering the surface energy of the coating.

[0054] This invention also relates to the combination of a coating as described above and a substrate.

[0055] Preferred substrates include polycarbonate, glass, quartz, silica, triacetate cellulose (TAC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or polymethylmethacrylate (PMMA).

[0056] The substrates, particularly if the substrate is polycarbonate, may comprise a hardcoat (for example MP-1154D, a siloxane-based hardcoat comprising 3-glycidoxypropyltrimethoxysilane, GPTMS), which is preferably thermal and/or UV curable, onto which the coating is applied, either with or without a surface treatment such as application of a primer. The term "hardcoat" is used in relation to this invention to mean a compound that is harder than the substrate onto which it is coated and which is chemically bonded to the substrate. Suitable hardcoats include silicone and acrylate polymers.

[0057] In some embodiments, the surface of the substrate to which the coating is to be applied is treated before application of the coating solution. In some embodiments, this surface treatment can be in the form of the application of a primer to the substrate or hardcoat in order to enhance adhesion between the coating and the substrate. Suitable primers include polyurethane based primers such as PR1165, which is polyurethane in water. PR1165 is particularly suitable for use between a polycarbonate substrate and a layer comprising the siloxane MP-1154D. [0058] In other embodiments, the surface treatment can involve altering the chemical or physical properties of the surface of the substrate. This can be done to increase the surface energy of the substrate. Such treatments can include treatment with an acid (eg hydrochloric or sulphuric acid) or a base (eg sodium hydroxide), or plasma or corona treatment. Acid or base treatment can hydrolyse the surface of a substrate, and all of these treatments can be used to oxidise and/or etch the surface of a substrate. Hydrolysing the bonds on the surface of the substrate can provide a more polar surface, thereby increasing polar interactions. Oxidising and etching can increase the surface roughness and contact area. Hydrolysis, oxidising and etching can all be used improve compatibility (and therefore adhesion) between the substrate and the binder. [0059] Preferred surface treatments for polycarbonate substrates (with or without a hardcoat as described above) include plasma treatment, preferably in oxygen (preferably 1 bar for 1 minute).

[0060] Preferred surface treatments for TAC substrates include treatment with a base. It is preferred that the base is sodium hydroxide, preferably in solution with water, normally at about 10%w/w concentration. It is preferred that treatment with a base is followed by washing with water.

[0061] Preferred surface treatments for PMMA substrates include treatment with an acid or treatment with a base. A preferred acid is sulphuric acid, preferably a 3M aqueous solution. A preferred base is ethylamine diamine, preferably a 1M solution in isopropanol. Preferably, treatment with an acid or base is followed by washing with water and/or IPA. [0062] Preferred surface treatments for silica or glass substrates include treatment with an acid, a base, or plasma or corona treatment; or a polymer such as polycarbonate, polymethylmethacrylate (PMMA), polyethylene naphthalate (PEN, polyfethylene 2,6-naphthalate)), PET (polyethylene terephthalate), or CR-39 (also known as allyl diglycol carbonate or ADC).

[0063] An adhesion promoter may also be used to enhance bonding between the substrate and the binder, for example, organo functionalised silanes may be used to bond acrylate based coatings to silica based substrates. Typical examples of systems are XIAMETER® OFS-6032 silane or methacryloxypropyltrimethoxysilane (XIAMETER® OFS-6030), both manufactured by Dow Corning.

[0064] All substrates are preferably washed prior to use, either before or after surface treatment. Washing can be with a non-ionic surfactant solution and/or isopropanol and/or acetone and/or water, optionally with sonication. Preferably, the non-ionic surfactant has a hydrophilic polyethylene group and a hydrophobic group, such as Triton X-100 (preferably a lwt% solution in water). It is preferred that ultrasonication is followed by washing with water and/or isopropanol.

[0065] A further coating may be applied to the coating of the invention to improve its resistance to abrasion. A preferred coating comprises a soluble fluoropolymer. Preferred soluble fluoropolymers include fluorinated silicones, fluorinated polyethers (such as Fluorolink™-S10, ie a perfluoropolyether with ethoxysilane terminal groups) or hyperbranched fluoropolymers. These can be applied to the coating in a solution, for example by dissolving them in an appropriate solvent and coating as a very thin layer (<10nm) on the surface to give the desired low surface energy. Suitable solvents include isopropanol, preferably with water and/or acetic acid. [0066] In addition, the present invention relates to the use of porous organosilica nanoparticles as defined above in the manufacture of a coating. [0067] Another aspect of the invention provides a method of fabricating a coating, the method comprising:

preparing either (i) a solution for forming a coating comprising a binder, a solvent, and a plurality of porous organosilica nanoparticles as defined above, or (ii) two solutions for forming a coating, one comprising a binder and a solvent and one comprising a plurality of porous organosilica nanoparticles as defined above;

applying the solution or solutions to a substrate; and

removing solvent from the solution or solutions to form the coating. [0068] If two coating solutions are prepared in the method described above, they are preferably applied to the substrate separately and sequentially.

[0069] Preferably, the solvent used in the coating solution(s) comprises an alcohol and/or a ketone. However, the solvent may also comprise glycols and/or ethers such as ethylene glycol and/or a glycol ether, as well as optionally including aliphatic hydrocarbons, aromatic hydrocarbons, esters, and/or aldehydes. The alcohol is preferably methanol, ethanol, propanol or butanol, more preferably isopropanol. It is preferred that the ketone is methyl ethyl ketone and/or methyl isobutyl ketone.

[0070] The coating solution(s) may additionally comprise other components such as water, acid (preferably hydrochloric acid), and/or silicone. These additional components are useful in controlling the viscosity of the coating solution and the dispersion of the particles. [0071] The coating solution(s) described above can be applied to a substrate by standard wet chemical coating techniques, including but not limited to spin coating, dip coating, roll to roll coating, spray coating and webcoating on a substrate.

[0072] The coating solution may be dried and optionally cured on the substrate to form the coating. The drying is a process to remove the solvent, optionally involving heating. The drying can be performed simultaneously with the curing or can constitute a separate process. In some embodiments, the curing is performed by maintaining the temperature in the range of from 50 to 250°C, more preferably from 80 to 140°C; alternatively UV curing is performed at ambient or elevated (ie above 25°C) temperature. The elevated temperature used can be chosen by the skilled person depending upon the substrate and on the binder.

[0073] The combination of the coating and the underlying substrate can be matched by manipulation of the ratio of particles to binder and by the choice of binder. This matching allows the coating to flex under continuous pressure or during an impact, for example from a sand particle hitting the surface whilst maintaining the hardness of the coating.

[0074] It is preferred that the coating comprises 20-65wt% nanoparticles, preferably 25-50wt%, preferably when the substrate has a refractive index of 1.5. Preferably, the remainder of the coating is the binder and optionally any additives which have been used.

[0075] Preferably, when the binder is TEOS, the coating solution includes an acid, preferably hydrochloric acid. The hydrochloric acid catalysis the hydrolysis of TEOS, the hydrolysis releasing an alcohol and producing reactive silanol (Si-OH) groups. These silanol groups then undergo a condensation reaction which forms -Si-O-Si- bonds, resulting in a continuous silica network. The inclusion of an acid is also advantageous because it slows the condensation reaction, resulting in polymeric silica chains that are not large enough to scatter light (ie keeping the material optically transparent).

[0076] Preferably the coating is an optical coating, in particular an anti- reflective (AR) coating, and/or a low-κ dielectric coating (ie having a lower dielectric constant than silicon dioxide). The term "anti-reflective coating" is used in relation to the present invention to refer to a coating which, when applied to a substrate, reduces the amount of incident light (or other electromagnetic radiation) which is reflected by the substrate. The "term low-K dielectric coating" refers to a coating which has a dielectric constant <3.9, preferably <3.0, more preferably <2.2, at a frequency in the 1-lOGHz range.

[0077] Preferably the coating can exhibit a hardness of typically greater than 0.7 GPa, or more preferably greater than 1.0 GPa, as measured by nanoindentation. Also preferably, the coating has an elastic modulus greater than half and less than twice the elastic modulus of the underlying substrate. More preferably the coating has an elastic modulus within ±25%, even more preferably ±10%, in some embodiments substantially identical to, the elastic modulus of the substrate. In this way the elastic modulus can match the underlying substrate, which indicates that the film is capable of significant flex. It is preferred that the coating embodying the invention will flex without brittle failure (ie without plastic deformation, for example cracking and/or delamination) to ten times (preferably greater than 10 times) the coating thickness on flexible substrates, for example polymer substrates. This flex is even seen when a coating comprising an inorganic binder is used on a polymer substrate. This surprising result is another aspect to the composite porous silica-organic structure of the nanoparticles. [0078] The coating typically has a refractive index in the range 1.0 to 1.4. It is preferred that the coating has a refractive index of ±20% of the square root of refractive index of the substrate, more preferably ±15%, even more preferably ±10%. A glass substrate typically has a refractive index of 1.5, a polycarbonate substrate normally has a refractive index of 1.58. The binder will typically have a refractive index of about 1.5 and the nanoparticles have a refractive index of less than 1.16. The refractive index of the mixture of the particles and the binder can therefore be tailored to a specific substrate by varying the ratio of binder to nanoparticles. This allows the system to optimise the refractive index of the coating to minimise the reflectivity of the optical coating in the case of an anti-reflective coating film.

[0079] The coating preferably has a mean thickness in the range from 75 to 500 nm, more preferably 75 to 300 nm, even more preferably 100 to 200 nm. It is preferred that the coating has a average surface roughness in the range from 2 to 50 nm, more preferably 5 to 30nm, even more preferably 10 to 30nm, as measured by atomic force microscopy (AFM) or interferometry. [0080] Preferably, the reflectance for incident light on a substrate having one surface coated with the coating of the invention at at least one wavelength in the range from 300nm to 1900nm is less than 2%, more preferably less than 1.5%. [0081] In the present specification, the term "optical" is used, for example in "optical coating"; however, this term is not intended to imply any limitation to visible light only. The invention may, if required, be applied to other parts of the electromagnetic spectrum, for example including at least ultraviolet (UV) and infrared (IR). The coating of the invention is also referred to as a film in some contexts. [0082] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a conventional uniform-thickness, single-layer AR coating provided on a substrate;

Fig. 2 is a Transmission Electron Microscopy (TEM) image of the nanoparticles of Example 1;

Fig. 3 is a reflectance curve in the visible wavelength range showing the anti-reflective performance of the coating of Example 2;

Fig. 4 is a graph showing the refractive index of coatings comprising various loadings of the nanoparticles of Example 1;

Fig. 5 is a TEM image of the nanoparticles of Example 4;

Fig. 6 is a reflectance curve in the visible wavelength range showing the anti-reflective performance of the coating of Example 5;

Fig. 7 is a graph showing the refractive index of coatings comprising various loadings of nanoparticles of Example 4;

Fig. 8 is a TEM image of the nanoparticles of Example 7; and

Fig. 9 is a reflectance curve in the visible wavelength range showing the anti-reflective performance of the coating of Example 8;

[0083] EXAMPLES

[0084] Example 1 - Synthesis of 30% loading l,2-Bis(triethoxysilyl)ethane nanoparticles

[0085] A solution A was prepared comprising lg triethanolamine and 9g de- ionised water. A solution B was also prepared by adding 450g of de-ionised water to 50g of a 25% aqueous CTAC solution. 9.6g of solution A and 480g of solution B were then placed in a flask, stirred and heated to 70°C. [0086] A silane solution was prepared by mixing 32.68g tetraethoxysilane, 5.38g of phenyltriethoxysilane and 11.92g of l,2-Bis(triethoxysilyl)ethane. After stirring for 30 minutes, 38.4ml of the silane solution was added to the reaction flask and left stirring at 70°C for 1 hour.

[0087] An acidified ethanol solution was prepared by diluting 200g of concentrated HC1 to 2L with ethanol. The mixture was removed from the heat and 460mL of the acidified ethanol was added to the reaction mixture and stirred. This was then decanted into 24 centrifuge tubes and then spun at 15000rpm for lOmins. The supernatant was poured off and 18mL of ammonium nitrate solution (lOg ammonium nitrate in 500mL ethanol) was added to each tube. The solid was then re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was again poured off and 18mL of the acidified ethanol was added to each tube. The solid was again re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was poured off again and lOmL of ethanol was added to each tube. The solid was then re-dispersed using vortex mixing and ultra- sonication. The resultant dispersions were recombined into a single container. A final dispersion of 240mL ethanol comprising 2.40wt% nanoparticles was obtained. A transmission electron microscopy image of the nanoparticles produced by this method is shown in Figure 2.

[0088] Example 2 - Optical properties of an anti-reflection coating using particles obtained in Example 1 [0089] A 2.1wt% Si0₂ binder solution was prepared by mixing 17.5g TEOS, 200g ethanol and 17.5g 0.1M HC1. This mixture was stirred for 24 hrs to pre- hydrolyse the TEOS prior to use. [0090] After this period, a coating solution comprising a 30% ratio of the weight of particles as obtained in Example 1 to the weight of the binder was prepared. This was done by mixing l.Og of the binder solution with 0.375g of particle dispersion from Example 1 and 2.63g of ethanol. This resulted in a coating solution with 0.75wt% solids.

[0091] This coating solution was then spin coated onto a glass slide substrate with a blackened back surface. The substrate was spun at 4000 rpm and 8 coats of ΙΟΟμί, of the coating solution were applied 10 seconds apart. [0092] Figure 3 shows a reflection curve for the anti-reflection coating produced in this example.

[0093] Example 3 - Refractive index measurement of particles obtained in Example 1

[0094] A 2.1wt% S1O2 binder solution was prepared by mixing 17.5g TEOS, 200g ethanol and 17.5g 0.1M HC1. This mixture was stirred for 24 hrs to pre- hydrolyse prior to use. [0095] Coating solutions comprising various weight ratios of nanoparticles of Example 1 to binder were prepared. These ratios were 0% (ie no nanoparticles), 10%, 20%, 30%, 40%, 50% and 60%. In each case, this was done by mixing the particle dispersion obtained in Example 1 with the binder solution and ethanol to achieve a coating solution with 2.1wt% solids. For example, a 50% coating solution was prepared by mixing 1.749g of the Example 1 particle dispersion at 2.401wt% with 2.0g of the binder solution and 0.25g ethanol.

[0096] Each coating solution was coated on to a silicon wafer substrate. The substrate was spun at lOOOrpm and 4 coats of 500μί, of the coating solution were applied 30 seconds apart. The coated substrates obtained were then measured and the refractive index of the layer obtained by ellipsometry. The refractive index of each coating is plotted in Figure 4 as a function of particle loading. The linear relationship between loading and refractive index (to discount the effect of any air gaps) was plotted and the refractive index of the particles is taken where the plot intersects with the y-axis at a coating comprising 100% particles. Figure 4 shows that the particles produced in Example 1 have a refractive index of 1.13. [0097] Example 4 - Synthesis of 50% loading l,2-Bis(triethoxysilyl)ethane nanoparticles

[0098] A solution A was prepared comprising lg triethanolamine and 9g de- ionised water. A solution B was also prepared by adding 450g of de-ionised water to 50g of a 25% aqueous CTAC solution. 9.6g of solution A and 480g of solution B were then placed in a flask, stirred and heated to 70°C.

[0099] A silane solution was prepared by mixing 24.02g tetraethoxysilane, 5.55g of phenyltriethoxysilane and 20.44g of l,2-Bis(triethoxysilyl)ethane. After stirring for 30 minutes, 38.4mL of the silane solution was then added to the reaction flask and left stirring at 70°C for 1 hour.

[00100] An acidified ethanol solution was prepared by diluting 200g of concentrated HC1 to 2L with ethanol. The mixture was removed from the heat and 460mL of the acidified ethanol was added to the reaction mixture and stirred. This was then decanted into 24 centrifuge tubes and then spun at 15000rpm for lOmins. The supernatant was poured off and 18mL of ammonium nitrate solution (lOg ammonium nitrate in 500mL ethanol) was added to each tube. The solid was then re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was again poured off and 18mL of the acidified ethanol was added to each tube. The solid was again re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was poured off again and lOmL of ethanol was added to each tube. The solid was then re-dispersed using vortex mixing and ultra- sonication. The resultant dispersions were recombined into a single container. A final dispersion of 240mL ethanol comprising 1.51wt% nanoparticles was obtained.

[00101] Figure 5 shows a transmission electron micrograph of the particles produced by the method of this example.

[00102] Example 5 - Optical properties of an anti-reflection coating using particles obtained in Example 4

[00103] A 2.1wt% Si0₂ binder solution was prepared by mixing 17.5g TEOS, 200g ethanol and 17.5g 0.1M HC1. This mixture was stirred for 24 hrs to pre-hydrolyse prior to use.

[00104] After this period, a coating solution comprising a 30% ratio of the weight of the particles as obtained in Example 4 to the weight of the binder was prepared. This was done by mixing l.Og of the binder solution with 0.375g of particle dispersion from Example 4 and 2.63g of ethanol. This resulted in a coating solution with 0.75wt% solids. [00105] This coating solution was then spin coated onto a glass slide substrate with a blackened back surface. The substrate was spun at 4000 rpm and 8 coats of ΙΟΟμί, of the coating solution were applied 10 seconds apart.

[00106] Figure 6 shows a reflection curve for the anti-reflection coating produced in this example. [00107] Example 6 - Refractive index measurement of particles obtained in Example 4

[00108] A 2.1wt% Si0₂ binder solution was prepared by mixing 17.5g TEOS, 200g ethanol and 17.5g 0.1M HC1. This mixture was stirred for 24 hrs to pre-hydrolyse prior to use.

[00109] Coating solutions comprising various weight ratios of nanoparticles of Example 4 to binder were prepared. These ratios were 0% (ie no nanoparticles), 10%, 20%, 30%, 40%, 50% and 60%. In each case, this was done by mixing the particle dispersion obtained in Example 4 with the binder solution and ethanol to achieve a coating solution with 2.1wt% solids. For example, a 50% coating solution was prepared by mixing 1.749g of the Example 4 particle dispersion at 2.401wt% with 2.0g of the binder solution and 0.25g ethanol.

[00110] Each coating solution was coated on to a silicon wafer substrate. The substrate was spun at lOOOrpm and 4 coats of 500μί, of the coating solution were applied 30 seconds apart. The coated substrates obtained were then measured and the refractive index of the layer obtained by ellipsometry. The refractive index of each coating is plotted in Figure 7 as a function of particle loading. The linear relationship between loading and refractive index (to discount the effect of any air gaps) was plotted and the refractive index of the particles is taken where the plot intersects with the y- axis at a coating comprising 100% particles. Figure 7 shows that the particles produced in Example 4 have a refractive index of 1.10.

[00111] Example 7 - Synthesis of 30% loading 1.2- Bis(triethoxysilyl) methane nanoparticles [00112] A solution A was prepared comprising lg triethanolamine and 9g de-ionised water. A solution B was also prepared by adding 450g of de- ionised water to 50g of a 25% aqueous CTAC solution. 2.4g of solution A and 120g of solution B were then placed in a flask, stirred and heated to 70°C. [00113] A silane solution was prepared by mixing 7.92g tetraethoxysilane, 1.3 lg of phenyltriethoxysilane and 2.77g of Bis(triethoxysilyl)methane. After stirring for 30 minutes, 9.6mL of the silane solution was then added to the reaction flask and left stirring at 70°C for 1 hour.

[00114] An acidified ethanol solution was prepared by diluting 200g of concentrated HC1 to 2L with ethanol. The mixture was removed from the heat and 120mL of the acidified ethanol was added to the reaction mixture and stirred. This was then decanted into 6 centrifuge tubes and then spun at 15000rpm for lOmins. The supernatant was poured off and 18mL of ammonium nitrate solution (lOg ammonium nitrate in 500mL ethanol) was added to each tube. The solid was then re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was again poured off and 18mL of the acidified ethanol was added to each tube. The solid was again re-dispersed using vortex mixing and ultra-sonication. Each tube was then filled with de-ionised water, shaken and spun at 21000rpm for lOmins. The supernatant was poured off again and lOmL of ethanol was added to each tube. The solid was then re-dispersed using vortex mixing and ultra- sonication. The resultant dispersions were recombined into a single container. A final dispersion of 60mL ethanol comprising 2.98wt% nanoparticles was obtained.

[00115] Figure 8 shows a transmission electron micrograph of the particles produced by the method of this example.

[00116] Example 8 - Optical properties of an anti-reflection coating using particles obtained in Example 7 [00117] A 2. lwt% Si0₂ binder solution was prepared by mixing 17.5g TEOS, 200g ethanol and 17.5g 0.1M HC1. This mixture was stirred for 24 hrs to pre-hydrolyse prior to use.

[00118] After this period, a coating solution comprising a 30% ratio of the weight of the particles as obtained in Example 7 to the weight of the binder was prepared. This was done by mixing l.Og of the binder solution with 0.375g of particle dispersion from Example 7 and 2.63g of ethanol. This resulted in a coating solution with 0.75wt% solids. [00119] This coating solution was then spin coated onto a glass slide substrate with a blackened back surface. The substrate was spun at 4000 rpm and 8 coats of ΙΟΟμί, of the coating solution were applied 10 seconds apart. [00120] Figure 9 shows a reflection curve for the anti-reflection coating produced in this example.

[00121] Example 9 - Surface area and pore volume analysis of particles obtained by Examples 1, 4, and 7

[00122] BET surface area analysis is a technique used to determine the specific surface area of particles and porous materials, the values are expressed in m²g ^-1. Clean surfaces adsorb surrounding gas molecules and Brunauer, Emmett and Teller (BET) theory provides a mathematical model for this process of gas sorption. This physical adsorption of a gas over the surface of a material and the filling of pores is called physisorption and is used to measure the total surface area and conduct pore size analysis of nanopores, micropores and mesopores within the particles.

[00123] Table 1 below sets out the surface areas and pore volumes of the nanoparticles as measured by a Micromeritics Gemini VI BET surface analyser.

Table 1

[00124] APPLICATIONS

The coating of the invention can be used numerous fields such as optics (including fibre optics), ophthalmics (eg ophthalmic elements such as lenses), displays (including both emissive and reflective displays, for example LCD backlit, LED and/or E Ink display such as that used in the Amazon Kindle), solar collection (including solar cells and components thereof, for example as an anti-reflective coating on an S13N4 coating in a silicon solar cell), lighting components, windows (eg windows for buildings, vehicle windows (e.g. automotive), laser windows, self-cleaning windows, as well as anti-static windows such as ZnO:Al, indium tin oxide (ITO) or other transparent coated windows), glass for protecting pictures/paintings, display cases, fish tanks/aquaria, and display panels such as instrument display panels. One exemplary application of the coating is on a glass or polymer window on top of a photovoltaic solar cell. The solar cell may be of any suitable kind, such as monocrystalline silicon, polycrystalline silicon, thin-film silicon and hybrid technologies. The coating may be used on other optical components, known as solar concentrators, used for collecting and directing sun light to a photovoltaic cell. Suitable polymer materials for such components include, but are not limited to, polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), and polyolefins such as biaxially oriented polypropylene (BOPP). However, the coating embodying the invention may also be used in general displays, and general window applications - for example for thermal management of buildings. A coating embodying the invention can also be employed in ophthalmic elements, whether made of glass or plastics materials, for example spectacle lenses. Since the dielectric constant of a material is related to the refractive index the particles of the present invention may also be used, in conjunction with a suitable binder, in low dielectric constant coatings in integrated circuits.

Claims

1. Porous organosilica nanoparticles having a BET surface area of at least 400m²/g and/or a pore volume of at least 0.7cm³/g, wherein the nanoparticles have a mean pore diameter of l-50nm, a mean diameter in the range 1-lOOnm and wherein the pores of the nanoparticles are randomly oriented.

2. Porous organosilica nanoparticles as claimed in claim 1 having a refractive index of less than 1.16.

3. Porous organosilica nanoparticles as claimed in either claim 1 or claim 2, wherein the nanoparticles have a refractive index such that, when they are formulated into a coating comprising about 30wt% nanoparticles and about 70%wt silica, the coating has a refractive index as measured by ellipsometry of 1.38 or less.

4. Porous organosilica nanoparticles as claimed in any one of claims 1-3 having a pore volume of greater than 50% of the volume of the particle.

5. Porous organosilica nanoparticles as claimed in any one of claims 1-4 having a density of less than 3g/cm³.

6. Porous organosilica nanoparticles as claimed in any one of claims 1-5, wherein the particles are mesoporous.

7. Porous organosilica nanoparticles as claimed in any one of claims 1-6, wherein the pores of the nanoparticles have an internal surface at least partially comprising a hydrophobic layer.

8. A solution or a dispersion for forming a coating, the solution or dispersion comprising a solvent and a plurality of porous organosilica nanoparticles as claimed in any one of claimed 1-7.

9. A coating comprising a plurality of porous organosilica nanoparticles as claimed in any one of claimed 1-7 and a binder.

10. The combination of a coating as claimed in claim 9 and a substrate.

11. An optical element, ophthalmic element, solar cell, window, glass panel or low-κ dielectric comprising a coating as claimed in claim 9.

12. A display panel comprising a coating as claimed in claim 9.

13. A method for preparing porous organosilica nanoparticles comprising the step of:

(a) mixing a bis and/or tris silane with an organic templating agent, a surfactant and a base in order to form porous silica nanoparticles.

14. A method as claimed in claim 13, wherein the mixing step forms a colloidal suspension of porous organosilica nanoparticles in the form of micelles.

15. A method as claimed in either claim 13 or claim 14, wherein the bis silane is a bis(alkyloxy) silane, preferably a bis(trialkyloxy) silane.

16. A method as claimed in claim 15, wherein the bis(trialkyloxy) silane is 1,2 -bis (triethoxysilyl) methane or 1,2 -bis (triethoxysilyl) ethane.

17. A method as claimed in any one of claims 13-16, wherein the mixing step (a) additional comprises a mono silane, preferably 1,2- bis(triethoxysilyl) ethane.

18. A method as claimed in any one of claims 13-17, wherein the organic templating agent is triethanolamine.

19. A method as claimed in any one of claims 13-18, wherein the base is triethanolamine.

20. A method as claimed in any one of claimed 13-19, wherein the surfactant is cetyltrimethylammonium chloride.