Method for preparing a hydroalcoholic suspension and hydroalcoholic suspension thus obtained

Abstract

Method for preparing a hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiO2 nanocrystals, comprising the steps of preparing a solution (S1) of a titanium alcoholate Ti(OR)4 in alcohol ROH; hydrolysing said solution (S1) in an aqueous solution (S2) containing zinc nitrate hexahydrate Zn(NO3)2 • 6H2O, thus obtaining a suspension (S3) of zinc-doped titanium dioxide TiO2 nanocrystals; adjusting the pH of the suspension (S3) until a pH between 9 and 11 is obtained so as to obtain a first hydroalcoholic suspension (S') of zinc-doped titanium dioxide TiO2 nanocrystals; and maintaining the first hydroalcoholic suspension (S') under stirring at the process temperature (TP), for a time interval (ΔT1), so as to obtain the hydroalcoholic suspension (S).

Description

[0001] Method for preparing a hydroalcoholic suspension and hydroalcoholic suspension thus obtained

[0002] The present invention relates to a method for preparing a hydroalcoholic suspension applicable to a glassy surface to make it self-cleaning and / or anti-fogging, a hydroalcoholic suspension obtained by said method, a method for making a glassy surface self-cleaning and / or anti-fogging, as well as a self-cleaning and / or anti-fogging glassy surface thus obtained. In particular, the hydroalcoholic suspension according to the invention can be used to make the glassy surface of a solar panel self-cleaning.

[0003] Solar panels are designed and structured to use solar energy and transform it into energy useful for human activities. They are usually classified into thermal solar panels, aimed at heating a fluid, photovoltaic solar panels, aimed at the production of electricity, and hybrid solar panels, intended for the production of thermal energy and electricity at the same time. The term photovoltaic system is a commonly used term used to indicate a system capable of generating electrical energy by direct conversion of sunlight. Each photovoltaic system comprises a plurality of photovoltaic panels, each of which is an optoelectronic device comprising a photosensitive face covered by a transparent plate, normally made of glass, and consisting of multiple photovoltaic cells, in which the conversion of solar radiation into electric current takes place by photoelectric effect.

[0004] Precisely because they are designed to collect solar radiation, these panels must be installed outdoors, for example in an elevated position on rooftops or on the ground, but must necessarily be tilted and facing the sun. The orientation and inclination of the photovoltaic panels with respect to the direction of the incident solar rays are very important: in fact, the efficiency of a single photovoltaic panel is at maximum when the solar rays strike the surface of the panel perpendicularly.

[0005] Due to their location outdoors, the panels are highly exposed to atmospheric agents and are subject to deposits of dust and substances that tend to opacify them. Specifically, the dust naturally present in the air, the micro-particles resulting from pollution, as well as the excrement of birds, the resins of the trees that are near the plant and all kinds of dirt that can be carried by the wind create an opaque patina over the photosensitive face of the photovoltaic panels. Opacification leads to a reduction in overall plant efficiency of about 10-15%. In fact, a single dirty photovoltaic panel not only does not produce energy, but, by limiting the overall pass-through current, reduces the overall efficiency of the entire system. Natural rains are not enough to eliminate these traces of dirt, and for good energy efficiency it is therefore necessary to methodically clean the panels. The photovoltaic solar panelscurrently in use are very resistant and their useful life is on average between 25 and 30 years. Good cleaning is extremely necessary and should not be limited to the warranty period, but extended for the entire life of the system, to optimise energy productivity and economic yield.

[0006] To overcome this drawback, photovoltaic systems are normally washed periodically, on average every six months and sometimes even more frequently if their location is particularly critical in speeding up soiling. Currently, the cleaning of solar photovoltaic panels is mainly done by hand, by personnel and specialized companies. Manual cleaning is expensive and can often be a hazard to the safety of personnel.

[0007] Alternatively, washing systems using pressurized water jets directly on the surface of solar panels have been proposed and adopted. However, their effectiveness appears poor and in any case limited, since the water that remains on the surface of the panels, then drying out due to evaporation, can leave limescale deposits and / or impurities that still tend to opacify the surface itself.

[0008] More articulated washing methods are also known. For example, it is known to follow pressurized water washing with forced drying by means of a pressurized air jet, so that no deposits or substances that may limit the absorption of solar radiation remain.

[0009] Mechanical systems capable of brushing the surface of the panel during a targeted spraying of pressurised water are also known. For example, an electric cleaning machine for solar panels is known, equipped with a platform that can be moved, both horizontally and vertically, over the entire surface of the panel. The cleaning systems mentioned so far use deionized water, to avoid calcareous deposits upon drying.

[0010] Although detergents and surfactants can remove dirt from the panel surface more effectively than water alone, their use in cleaning solar panels is strongly discouraged, not only to limit environmental pollution, but also because detergents and surfactants can leave traces of opacifying deposits on the surface.

[0011] Various types of organic-polymeric and inorganic coatings are also known that can be deposited on the surface of the panel with the aim of protecting it from environmental soiling. However, these coatings, although thin and transparent, have an appreciable absorption of solar radiation: they therefore have the disadvantage of lowering the efficiency of the underlying solar panel.

[0012] Some coatings contain photocatalytic substances, such as semi-metal compounds, which catalyse the oxidation of organic residues when activated by sunlight. The band gap of the photocatalytic substance limits the activation spectrum of photocatalysis, which generallydoes not include visible light and mainly includes ultraviolet (UV) radiation. As a result, the efficiency of photocatalysis, which is activated by only a small portion of solar radiation (UV radiation accounts for about 4-5% of solar radiation), is also limited.

[0013] An object of the present invention is to provide a method of preparing a hydroalcoholic suspension applicable to a glassy surface to make it self-cleaning and / or anti-fogging that is economical and effective.

[0014] Another object is to provide a hydroalcoholic suspension that allows to give a glassy surface more efficient self-cleaning characteristics compared to coatings and similar known substances.

[0015] A further object is to provide a method of making the glassy surface of a solar panel selfcleaning in an optimal manner, i.e. maximizing the self-cleaning properties of the surface without decreasing the efficiency of the underlying panel.

[0016] A first aspect of the invention provides a method for preparing a hydroalcoholic suspension according to claim 1.

[0017] A second aspect of the invention provides a hydroalcoholic suspension according to claim 5.

[0018] A third aspect of the invention provides a method for making a glassy surface self-cleaning and / or anti-fogging according to claim 9.

[0019] A fourth aspect of the invention provides a self-cleaning solar panel according to claim 13. A fifth aspect of the invention provides a self-cleaning and / or anti-fogging glass according to claim 14.

[0020] The invention can be better understood and implemented with reference to the attached drawings which show exemplary and non-limiting embodiments, wherein:

[0021] Figure 1 schematically illustrates the main steps of a method for preparing a hydroalcoholic suspension according to the invention;

[0022] Figure 2 schematically illustrates an optional step of the method of Figure 1;

[0023] Figure 3 shows a size distribution of nanocrystals suspended in a hydroalcoholic suspension according to the invention, obtained by DLS (Dynamic Light Scattering) measurements.

[0024] Referring to Figures 1, 2 there is illustrated a method of preparing a hydroalcoholic suspension S of Zn-doped titanium dioxide TiOi nanocrystals (hereinafter also Zn-TiOi for brevity) according to the invention. The hydroalcoholic suspension S is in particular applicable to a glassy surface, such as the exposed surface of a solar panel, the surface of a mirror, a window or other object comprising a glassy surface, to make it self-cleaning and / oranti-fogging.

[0025] Titanium dioxide TiCh is a known photocatalytic compound having a wide band gap and consequently an activation spectrum of its photocatalytic activity, or activation band, limited to UV radiation. Titanium dioxide TiCh crystals are found in nature in different allotropic forms, of which the main ones are called anatase, brookite and rutile. Among them, the most efficient in photocatalytic processes is anatase, while rutile is the least efficient form in photocatalytic processes, but more thermodynamically stable and therefore more easily obtainable. The brookite form has promising photocatalytic activity, but is less studied and applied than the other two forms as it is less thermodynamically stable and therefore more difficult to obtain.

[0026] The photocatalytic activity of titanium dioxide TiCh can be improved by doping with metals and non-metals; the transition metals are particularly effective, in particular zinc. In fact, the zinc ion Zn2+and the titanium ion Ti4+have very similar atomic radii, 0.74 A and 0.75 A, respectively. This facilitates the substitution, within the crystalline lattice of titanium dioxide TiCh crystals, of Ti4+titanium ions with Zn2+zinc ions without disturbances to the crystalline structure and thus allowing a stable anatase form to be obtained.

[0027] Advantageously, doping titanium dioxide TiCh crystals with zinc is able to increase the photocatalytic power of the undoped titanium dioxide TiCh and to extend its activation band in the UV / visible radiation spectrum. In addition, when irradiated with visible light, Zn-TiOi (in anatase form) exhibits greater photocatalytic activity than pure titanium dioxide TiOi or titanium dioxide TiCh doped with other metals, since doping with zinc causes the formation of superoxide anion radicals and increases the effectiveness of electron-hole separation. Specifically, the photocatalytic activity of Zn-TiCh increases as the zinc content increases until it reaches a peak activity at a doping of 4-5%.

[0028] A first step of the inventive method for the preparation of the hydroalcoholic suspension S of Zn-TiOi nanocrystals provides for preparing a 6%-10% molar percentage solution SI of a titanium alcoholate Ti(OR)4 in ROH alcohol. Unless otherwise indicated, the mole percentage of a solution means the percentage of moles of solute (in this case, moles of titanium alcoholate Ti(OR)4) with respect to the total moles of solution (in this case, the sum of the moles of titanium alcoholate Ti(OR)4 and alcohol ROH). R denotes a straight or branched alkyl group having 1 to 6 carbon atoms. Preferably, the alkyl group R is selected among a methyl group, an ethyl group, a propyl group, a 1 -methylethyl group, a butyl group, a 2-methylpropyl group, a 1,1 -dimethylethyl group, a pentyl group, a 3-methylbutyl group, a hexyl group, a 4 -methylpentyl group.The method then provides for hydrolysing the solution SI of titanium alcoholate Ti(0R)4in alcohol ROH by slowly dropping it into an aqueous solution S2. The aqueous solution S2 contains zinc nitrate hexahydrate Zn(NOs)2 • 6H2O. A mole percentage of the zinc nitrate hexahydrate Zn(NOs)2 • 6H2O of the aqueous solution S2 is comprised between 1% and 5% of the number of moles of titanium alcoholate Ti(0R)4 of the solution SI of titanium alcoholate.

[0029] During the hydrolysis, the aqueous solution S2 is kept under stirring at a process temperature TP, in particular comprised between 50°C and 100°C, in particular between 80°C and 95°C. The hydrolysis allows obtaining a suspension S3 of titanium dioxide TiC nanocrystals doped with zinc at a mole percentage comprised between 1% and 5% of the number of total moles of titanium Ti4+and zinc Zn2+cations contained in the aforementioned nanocrystals. At the end of the hydrolysis, i.e. when all the titanium alcoholate Ti(0R)4 of the solution SI has been hydrolysed in titanium dioxide TiCh and alcohol ROH, the method provides for adjusting the pH of the suspension S3 by adding dropwise a solution B of a strong base, such as sodium hydroxide NaOH, potassium hydroxide KOH, lithium hydroxide LiOH, barium hydroxide Ba(0H)2 or the like, until a pH between 9 and 11 is obtained so as to obtain a first hydroalcoholic suspension S’ of zinc-doped titanium dioxide TiO2 nanocrystals.

[0030] The method then provides for maintaining the first hydroalcoholic suspension S’ of zinc-doped titanium dioxide TiO2 nanocrystals under stirring at the process temperature TP, for a time interval ATI comprised between 0.5 hours and 5 hours, in particular comprised between 1 hour and 3 hours, so as to obtain the hydroalcoholic suspension S of zinc-doped titanium dioxide TiCh nanocrystals.

[0031] In the hydroalcoholic suspension S thus obtained, the Zn-TiCh nanocrystals have 90%-99.5% anatase form and 0.5%-10% brookite form.

[0032] As known in the art, anatase-brookite mixtures with a prevalence of anatase form have greater photocatalytic activity than single -phase particles, i.e. present in a single crystalline form, for example 100% in anatase form or 100% in brookite form. The increased photocatalytic activity of anatase-brookite mixtures is generally attributed to synergistic effects between the two types of nanocrystals.

[0033] On the other hand, due to the instability of the brookite form, pure brookite and anatasebrookite mixtures are notoriously difficult to obtain and generally require high temperatures and pressures.

[0034] The method described above allows instead to obtain an anatase-brookite mixture, therefore with high photocatalytic activity, under less extreme temperature and pressure conditionsand therefore more easily and economically feasible.

[0035] The nanocrystals of the hydroalcoholic suspension S have positive zeta potential (surface charge ,) in a pH range of 4 to 7, corresponding to the ambient pH.

[0036] The method may comprise the further step of aggregating the zinc-doped titanium dioxide TiCh nanocrystals of the hydroalcoholic suspension S onto hydroxyapatite HAp nanocrystals doped with zinc (hereinafter also Zn-HAp for brevity) at a percentage comprised between 1% and 4%, in particular between 1% and 2%.

[0037] In such a case, a weight percentage of the zinc-doped HAp hydroxyapatite nanocrystals to the weight of the zinc-doped TiCh titanium dioxide nanocrystals is 1% to 10%.

[0038] The photocatalytic activity of Zn-TiCh nanocrystals can be modified by aggregating the nanocrystals onto different substrates. In particular, by aggregating Zn-TiCh nanocrystals onto different inorganic substrates (carbon, graphite, carbonates and nano structured apatites), Zn-HAp zinc-doped hydroxyapatite appears to be the substrate associated with the best photocatalytic activity, thanks to the strong electrostatic interaction between the Zn-TiCh nanocrystals and the Zn-HAp crystals.

[0039] To have a positive effect on the photocatalytic activity of the aggregates, the zinc doping of the hydroxyapatite HAp nanocrystals must not exceed approximately 4%: higher percentages hinder the crystallization of the hydroxyapatite HAp, compromising the correct formation and functionality of the aggregates.

[0040] The aggregation of the Zn-TiCh nanocrystals of the hydroalcoholic suspension S onto Zn-HAp nanocrystals may comprise a slow mixing of the hydroalcoholic suspension S of zinc-doped titanium dioxide TiC nanocrystals with a suspension of zinc-doped hydroxyapatite HAp nanocrystals S4, under slow stirring at a constant mixing temperature MT, in particular comprised between 40°C and 90°C. The Zn-TiOi nanocrystals bind spontaneously to the surface of the Zn-HAp nanocrystals thanks to the electrostatic interaction between the positive zeta potential of the former and the negative zeta potential of the latter.

[0041] The invention also relates to a hydroalcoholic suspension S of zinc-doped titanium dioxide TiCh nanocrystals for making a glassy surface self-cleaning and / or anti-fogging, comprising from 5% to 20% by weight titanium dioxide TiCh nanocrystals doped with zinc at a percentage comprised between 1% and 5%, wherein the nanocrystals have 90%-99.5% anatase form and 0.5%-10% brookite form.

[0042] The zinc-doped TiCh titanium dioxide nanocrystals have , positive zeta potential in a pH range of 4 to 7.

[0043] The hydroalcoholic suspension S may comprise an alcohol having 1 to 6 carbon atoms,particularly at least one of methanol, ethanol, 1 -propanol, 2-propanol, 1 -butanol, 2-methyl-1 -propanol, 2-methyl-2-propanol, 1 -pentanol, 3 -methyl- 1 -butanol, 1 -hexanol, 4-methyl-l-pentanol.

[0044] The zinc-doped titanium dioxide TiCh nanocrystals of the hydroalcoholic suspension S may be aggregated onto hydroxyapatite HAp nanocrystals doped with zinc at a percentage comprised between 1% and 4%, with a percentage by weight of the zinc-doped hydroxyapatite HAp nanocrystals relative to the weight of the zinc-doped titanium dioxide TiCh nanocrystals comprised between 1% and 10%.

[0045] The invention also relates to a method for making a glassy surface self-cleaning and / or antifogging, comprising:

[0046] providing a glassy surface;

[0047] removing any soiling and / or contaminating elements and / or substances from the glassy surface;

[0048] dispensing onto the glassy surface the hydroalcoholic suspension S of zinc-doped titanium dioxide TiCh nanocrystals described above or obtained with the method described above.

[0049] Subsequent to the dispensing of the hydroalcoholic suspension S, the liquid phase of the hydroalcoholic suspension S evaporates and the nanocrystals of the hydroalcoholic suspension S, thanks to their positive zeta potential and therefore opposite to the negative zeta potential of the glassy surface, bind to the latter by heterocoagulation (bonding between particles having opposite surface charges).

[0050] The nanocrystals are distributed over the entire glassy surface uniformly, but strongly discontinuously, therefore, advantageously, without completely and continuously covering any extended area of the glassy surface: this preserves the transparency of the glassy surface, therefore the light (for example, solar radiation) that may be incident is transmitted through the glassy surface substantially without losses. The morphology of the individual nanocrystals is lamellar, with a tendency to form nano structured aggregations having an average size of 100-200 nm (Fig. 3).

[0051] Therefore, the method described above to make the glassy surface self-cleaning and / or antifogging allows the latter to be nanostructured. Nano structuring with Zn-TiCh nanocrystals (possibly aggregated onto Zn-HAp nanocrystals) makes the glassy surface superhydrophobic (in the absence of radiation capable of activating photocatalysis).

[0052] This result is an example of biomimicry. Biomimetic is defined as the scientific approach that aims to reproduce natural biological processes and materials in the laboratory. In thespecific case, the effect reproduced is the so-called lotus effect: the leaves of the lotus plant are self-cleaning because they are coated with nanometric crystals of a hydrophobic wax (nano-roughness), which makes them hydrophobic. Thanks to such crystals, the actual contact area between a water droplet and the respective bearing surface on the leaf is about 3% of the apparent bearing surface, so the weight of the droplet makes it roll away. The nano-roughness of the leaf, which determines its hydrophobicity, is essential for the selfcleaning effect or lotus effect, because the drops, rolling and not slipping as happens on a normal surface, take away the dirt with them.

[0053] The self-cleaning properties of the glassy surface treated according to the above method are not limited to superhydrophobicity. Taking for example the case of a glassy surface installed outdoors, such as the covering of a solar panel or the glass of a window, during the day it is hit by the radiation of sunlight, in particular by UV / visible radiation belonging to the activation band of Zn-TiCh nanocrystals. The photocatalysis by the Zn-TiCh nanocrystals is therefore activated and the nanocrystals catalyze the oxidation of organic residues and pollutants such as nitrogen oxides NOx, sulfur oxides SOx, carbon oxides COx and volatile organic compounds (VOC). The activated Zn-TiOi nanocrystals are also superhydrophilic, thus facilitating the accumulation of water (e.g. atmospheric moisture or precipitation) on the glassy surface. In the absence of activation radiation (for example, at night), the glassy surface returns to being superhydrophobic: the water accumulated during the day tends to fragment into droplets and slip away from the surface, taking the dirt with it and creating the lotus effect particularly effectively. Therefore, advantageously, a glassy surface installed outdoors, such as the covering of a solar panel, made self-cleaning with the method according to the invention is subjected to a cleaning cycle every day thanks to the night-day alternation.

[0054] The anti-fogging properties conferred by the method according to the invention are in turn due to the superhydrophilicity of the Zn-TiCh nanocrystals activated by appropriate radiation. In fact, the fogging of a glassy surface is due to the drops of water that condense on the surface and, due to their spherical or otherwise curved shape, diffuse light in multiple directions. When the treated glassy surface becomes superhydrophilic, any water droplets in contact with the glassy surface spread completely on the glassy surface itself with a contact angle of approximately zero, forming a uniform film that substantially does not disturb the trajectory of the light that crosses it: in these conditions of complete wetting, the glassy surface does not fog up.

[0055] Advantageously, the nanostructuring of the glassy surface obtained as described above is notsubstantially altered by atmospheric agents. If necessary, the removal of the treatment is still possible by washing with a hydroalcoholic solution and mechanical removal means such as brushes or the like.

[0056] The aforementioned dispensing of the hydroalcoholic suspension S may comprise dispensing between 3 mL / m2and 10 mL / m2of the hydroalcoholic suspension S of zinc-doped titanium dioxide TiCh nanocrystals onto the glassy surface, in particular between 5 mL / m2and 6 mL / m2.

[0057] In fact, the greater or lesser discontinuity of the spatial distribution of the nanocrystals on the glassy surface is closely linked to the applied dispensing conditions. The Applicant has found that the best distribution is achieved by dispensing the hydroalcoholic suspension S with a ratio between volume of dispensed hydroalcoholic suspension S and dispensing surface area of 3-10 mL / m2, preferably 5-6 mL / m2.

[0058] On the other hand, by dispensing more than 10 mL / m2of hydroalcoholic suspension S, a less discontinuous distribution of Zn-TiOi nanocrystals (possibly aggregated onto Zn-HAp nanocrystals) is obtained, which increases the absorption of solar radiation by the nanocrystals and reduces both the lotus effect and the amount of solar radiation transmitted by the glassy surface.

[0059] Dispensing the hydroalcoholic suspension S may comprise nebulizing the hydroalcoholic suspension S, in particular by means of an airbrush. The Applicant has ascertained that a nebulization of the hydroalcoholic suspension S into small droplets, for example of average diameter less than 500 pm, improves the uniformity of the distribution of the nanocrystals on the glassy surface.

[0060] The glassy surface to be made self-cleaning can be the surface of a solar panel.

[0061] The method may comprise, before distributing the hydroalcoholic suspension S on the glassy surface,

[0062] - cleaning the latter with water under pressure and with means of mechanical removal of contaminating and soiling elements;

[0063] - drying the glassy surface completely.

[0064] By way of non-limiting example, a method for preparing a hydroalcoholic suspension S of Zn-TiCL nanocrystals according to the invention may comprise the following steps:

[0065] - preparing 156 kg of a solution SI of titanium isopropoxide, or titanium isopropoxide Ti(OR)4in isopropanol ROH, where R indicates an isopropyl group CH3-CH+-CH3, or 1- methylethyl group, by mixing 111 kg of isopropanol with 45 kg of titanium isopropoxide. - hydrolysing the titanium isopropoxide solution S 1 by slowly dripping it into an aqueoussolution S2 comprising 444 kg of water and containing zinc nitrate hexahydrate ZniNOsJi • 6H2O. A mole percentage of such zinc nitrate hexahydrate is 1% to 5% of the number of moles of titanium isopropoxide present in the solution SI. The aqueous solution S2 is kept under stirring at a process temperature TP comprised between 80°C and 95°C. A suspension S3 of titanium dioxide TiC nanocrystals doped with zinc at a mole percentage comprised between 1% and 5% of the number of total moles of titanium Ti4+and zinc Zn2+cations contained in the nanocrystals themselves is thus obtained.

[0066] - at the end of the hydrolysis, after approximately 4-5 hours from its start, adjusting the pH of the suspension S3 by adding a solution of sodium hydroxide NaOH dropwise until a pH between 9 and 11 is obtained so as to obtain a first hydroalcoholic suspension S’ of Zn-TiOi nanocrystals.

[0067] - maintaining the first hydroalcoholic suspension S’ of Zn-TiCh nanocrystals under stirring at the process temperature TP, for a time interval ATI comprised between 1 hour and 3 hours, so as to obtain the hydroalcoholic suspension S of Zn-TiCh nanocrystals according to the invention.

[0068] A structural characterization carried out by the Applicant of the Zn-TiCh nanocrystals of the hydroalcoholic suspension S made by the method described above showed that they have an anatase form at about 98% and brookite form at about 2%. The morphology of Zn-TiCh nanocrystals is lamellar, with a tendency to form aggregations of sizes of the order of 100-200 nm.

[0069] The hydroalcoholic suspension S according to the invention is simple and inexpensive to prepare, since the preparation method according to the invention provides for few steps, easily available reagents and easily achievable temperature and pressure conditions (atmospheric pressure, temperatures below 100°C). In addition, the Zn-TiCh nanocrystals of the hydroalcoholic suspension S are mainly in the anatase form (90%-99.5%) and for the remaining part in the brookite form, therefore they have a particularly high photocatalytic activity.

[0070] The method according to the invention for making a glassy surface self-cleaning and / or antifogging is extremely simple and can be carried out substantially by anyone, not only on glassy surfaces of new objects (solar panels, window glass, mirrors and so on), but also of objects already in use. The treated glassy surface does not lose its transparency, thanks to the strongly discontinuous (but uniform) distribution of the nanocrystals deposited according to the provided method.

[0071] It should be noted that the known methods provide for a calcination step at temperaturesabove 100°C (and generally in the order of 400°C-800°C) to obtain powdered nanocrystals from a hydroalcoholic suspension and / or to remove the liquid component of the hydroalcoholic suspension from a glassy surface treated therewith. On the contrary, the method according to the invention simply requires the delivery of the hydroalcoholic suspension S on the glassy surface to be treated and the evaporation of the liquid phase takes place at room temperature. Therefore, no calcination is necessary, which simplifies and shortens the preparation method compared to those known, also avoiding having to heat to temperatures above 100°C and therefore saving energy.

[0072] The hydroalcoholic suspension S confers better self-cleaning characteristics than other known approaches, thanks to the expansion of the photocatalysis activation band obtained by doping the titanium dioxide TiOi nanocrystals with zinc. Zn-TiOi nanocrystals are even more efficient in embodiments where they are aggregated onto Zn-HAp nanocrystals. The Applicant has found that treatment of a solar panel already in use with the hydroalcoholic suspension S according to the invention leads to an increase of the panel yield of about 3%.

[0073] The nano structuring of the glassy surface obtained as described above is advantageously durable, as atmospheric agents alone are not sufficient to significantly alter it. At the same time, it is reversible with a suitable washing with a hydroalcoholic solution accompanied by a mechanical removal of the Zn-TiOi nanocrystals.

Claims

CLAIMS1. Method for preparing a hydroalcoholic suspension (S) of zinc-doped TiOi titanium dioxide nanocrystals, in particular applicable to a glassy surface to make it self-cleaning and / or anti-fogging, comprising the steps:- preparing a 6%-10% mole percentage solution (SI) of a titanium alcoholate Ti(0R)4 in ROH alcohol, R being a straight or branched alkyl group having 1 to 6 carbon atoms,- hydrolysing said solution (SI) of titanium alcoholate Ti(0R)4 in alcohol ROH by slow dripping thereof into an aqueous solution (S2) containing zinc nitrate hexahydrate Zn(NOs)2 • 6H2O, a mole percentage of said zinc nitrate hexahydrate being comprised between 1% and 5% of the number of moles of said titanium alcoholate Ti(0R)4 present in said solution (SI) of titanium alcoholate, said aqueous solution (S2) being maintained under stirring at a process temperature (TP), in particular comprised between 50°C and 100°C, in particular between 80°C and 95°C, thus obtaining a suspension (S3) of titanium dioxide TiCh nanocrystals doped with zinc at a mole percentage comprised between 1% and 5% of the number of total moles of titanium Ti4+and zinc Zn2+cations contained in said nanocrystals;- at the end of said hydrolysis, adjusting the pH of said suspension (S3) by adding a solution (B) of a strong base dropwise until a pH between 9 and 11 is obtained so as to obtain a first hydroalcoholic suspension (S’) of zinc-doped titanium dioxide TiCh nanocrystals;- maintaining said first hydroalcoholic suspension (S’) of zinc-doped titanium dioxide nanocrystals under stirring at the process temperature (TP), for a time interval (ATI) comprised between 0.5 hours and 5 hours, in particular comprised between 1 hour and 3 hours, so as to obtain said hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiCL nanocrystals, wherein said nanocrystals have 90-99.5% anatase form and 0.5-10% brookite form.

2. Method according to claim 1, wherein said alkyl group R is chosen among a methyl group, an ethyl group, a propyl group, a 1 -methylethyl group, a butyl group, a 2- methylpropyl group, a 1,1 -dimethylethyl group, a pentyl group, a 3-methylbutyl group, a hexyl group, a 4 -methylpentyl group.

3. Method according to claim 1 or 2, comprising the step of aggregating said zinc-doped titanium dioxide TiCL nanocrystals of said hydroalcoholic suspension (S) onto hydroxyapatite HAp nanocrystals doped with zinc at a percentage comprised between1% and 4%, in particular between 1% and 2%, wherein a percentage by weight of said zinc-doped hydroxyapatite HAp nanocrystals relative to the weight of said zinc -doped titanium dioxide TiC nanocrystals is comprised between 1% and 10%.

4. Method according to claim 3, wherein said aggregating comprises a slow mixing of said hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiOi nanocrystals with a suspension of zinc-doped hydroxyapatite HAp nanocrystals (S4) under slow stirring at a constant mixing temperature (TM), in particular comprised between 40°C and 90°C.

5. Hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiOi nanocrystals for making a glassy surface self-cleaning and / or anti-fogging, comprising from 5% to 20% by weight titanium dioxide TiOi nanocrystals doped with zinc at a percentage comprised between 1% and 5%, wherein said nanocrystals have 90%-99.5% anatase form and 0.5%-10% brookite form.

6. Hydroalcoholic suspension (S) according to claim 5, comprising an alcohol having 1 to 6 carbon atoms, particularly at least one of methanol, ethanol, 1 -propanol, 2-propanol, 1 -butanol, 2-methyl-l -propanol, 2-methyl-2-propanol, 1 -pentanol, 3 -methyl- 1 -butanol, 1 -hexanol, 4-methyl-l -pentanol.

7. Hydroalcoholic suspension (S) according to claim 5 or 6, wherein said zinc-doped titanium dioxide TiOi nanocrystals are aggregated onto hydroxyapatite HAp nanocrystals doped with zinc at a percentage comprised between 1% and 4%, a percentage by weight of said zinc-doped hydroxyapatite HAp nanocrystals relative to the weight of said zinc-doped titanium dioxide TiOi nanocrystals being comprised between 1% and 10%.

8. Method for making a glassy surface self-cleaning and / or anti-fogging, comprising:- providing a glassy surface;- removing any soiling and / or contaminating elements and / or substances from said glassy surface;- dispensing onto said glassy surface a hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiOi nanocrystals according to one of claims 5 to 7 or obtained with the method according to one of claims 1 to 4.

9. Method according to claim 8, comprising dispensing between 3 mL / m2and 10 mL / m2of said hydroalcoholic suspension (S) of zinc-doped titanium dioxide TiOi nanocrystals onto said glassy surface, in particular between 5 mL / m2and 6 mL / m2.

10. Method according to claim 8 or 9, wherein said dispensing comprises nebulizing said hydroalcoholic suspension (S), in particular by means of an airbrush.

11. Method according to any one of claims 8 to 10, wherein said glassy surface is a surface of a solar panel, said method comprising, prior to said dispensing said hydroalcoholic suspension (S) onto said glassy surface,- cleaning the latter with water under pressure and with means of mechanical removal of contaminating and soiling elements;- drying said glassy surface completely.

12. Self-cleaning solar panel obtained with the method according to any one of claims 8 to 11.

13. Self-cleaning and / or anti-fogging glass obtained by the method according to any one of claims 8 to 10.

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