Process for covering articles with graphene oxide films

Abstract

The present invention relates to a process for covering articles with a graphene oxide film, to obtain a coating capable of protecting said articles from corrosion due to atmospheric and chemical agents. In particular, the invention finds application in the field of the protection and preservation of artistic artefacts.

Description

[0001] DESCRIPTION of the industrial invention entitled:

[0002] “PROCESS FOR COVERING ARTICLES WITH GRAPHENE OXIDE FILMS”

[0003] ********************

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to a process for covering articles with thin graphene oxide films, in order to obtain a coating capable of protecting said articles from corrosion due to atmospheric and chemical agents while not altering the appearance thereof. In particular, the invention finds application in the field of the protection and preservation of artistic and historical items made of metal alloy.

[0006] BACKGROUND ART

[0007] Graphene and its derivatives have been studied in view of possible applications in the field of anticorrosion; these compounds are promising candidates for the development of nanostructured coatings to protect metals from corrosion and oxidation. High-quality, highly crystalline single-layer graphene of monoatomic thickness is known for its impermeability to various gases, which, in principle, makes it an ideal barrier since it is thin and transparent. However, the early approaches to the protection of metal articles with graphene have not yielded satisfactory results. The article “Graphene as a long-term metal oxidation barrier: worse than nothing”, M. Schriver et al., ACS Nano 2013, 7, 7, 5763-5768, reports that a film of pure graphene offers protection against the corrosion of metals (exemplified by Cu) in the short term, but over the long term the corrosion is more pronounced than on a bare surface of the same metals; a possible explanation is that the high electrical conductivity of graphene can facilitate galvanic reactions in defects and exposed grain boundaries, affecting both the surface and the mass of the metal substrates. Furthermore, the transfer of graphene directly on the substrate by means of chemical vapour deposition (CVD) requires temperatures of 1000 °C or higher, which can alter the substrate itself; see, for example, the article “Chemical vapour deposition”, L. Sun et al., Nature Reviews, Methods Primers, January 2021. Alternative methods include the use of a polymeric medium in which graphene is transferred and then deposited by etching the polymer or by means of roll-to-roll transfer (see the article to L. Sun et al. cited). However, neither of the two methods is suitable for metal items, due to the use of acidic solutions and the difficultiesassociated with the surface roughness of the articles to be protected, in the case of roll-to-roll processes.

[0008] The article “Present status and perspectives of graphene and graphene-related materials in cultural heritage”, E. Galvagno et al., Adv. Funct. Mater. 2024, 34, 2313043, is a review which describes the use of graphene-related materials for the protection of artistic and historical items; in particular, graphene oxide (GO) and reduced graphene oxide (rGO) are mentioned, again referring to CVD and roll-to-roll methods for covering the substrates.

[0009] US patent application 2019 / 0345345 A1 describes the use of graphene films or derivatives thereof in combination with anti-corrosive pigments or sacrificial metals (e.g., Zn or Al) for anticorrosion applications; the films are formed on the substrate to be protected from corrosion by deposition from a liquid phase and subsequent drying.

[0010] Other methods suggested in the field involve the use of polymer films containing graphene or derivatives thereof; however, these polymer films are relatively thick, from tens to hundreds of microns, and can alter the visible features (for example, the colour) of the articles to be protected, an undesirable effect especially when the latter are works of art.

[0011] Patent application CN 106930095 A describes a method for covering carbon fibres with graphene oxide films obtained with the Langmuir-Blodgett (LB) technique. The method is described in general terms, without specifying particular features of the graphene oxide. It is the aim of the method described in this document to increase the chemical compatibility of carbon fibres with polymer materials for producing fibre-reinforced composites, and no information is given regarding the degree of covering of the fibres.

[0012] The article “General self-assembly method for deposition of graphene oxide into uniform close-packed monolayer films”, A. Holm et aL, Langmuir 2019, 35, 4460-4470, describes the covering of silicon or silicon oxide surfaces with singlelayers of graphene oxide obtained with the LB technique. The conclusions of this article clearly highlight that, to obtain an efficient coverage of the substrate, the LB single-layer must be obtained by adding a cationic surfactant to the graphene oxide (1 ,2-di-(9Z-octadecenoyl)-3-trimethylammonium-propane chloride salt isexemplified); however, cationic surfactants too could promote undesired reactions at the metal surface of the article to be protected and generally, as organic molecules, are subject to rapid degradation under environmental conditions.

[0013] The literature indicates that it is impossible to obtain GO films with a high coverage percentage, which condition is absolutely necessary to ensure effective protection against corrosion. Furthermore, the transfer of such films on rough surfaces represents an unresolved and particularly complex problem. In the article “General Self-Assembly Method for Deposition of Graphene Oxide into Uniform Close-Packed Monolayer Films”, Holm et al., Langmuir 2019, 35, 13, 4460-4470, it is reported “It is apparent that the conventional LB (Langmuir-Blodgett) method can be used only to deposit overlapping films while it does not allow obtaining compact and non-overlapping films ... Furthermore, it is possible to obtain only multilayer films”.

[0014] The article “How exfoliated graphene oxide nanosheets organize at the water interface: evidence for a spontaneous bilayer self-assembly”, N. Bonatout et aL, Nanoscale, vol. 9, n. 34 (2017), reports a study on the organisation of exfoliated graphene oxide layers at the water-air interface.

[0015] The article “Highly transparent and conducting ultralarge graphene oxide / single-walled carbon nanotube hybrid films produced by Langmuir-Blodgett assembly”, Q. Zheng et aL, Journal of Materials Chemistry, vol. 22, no. 48 (2012), describes the production of uniform transparent hybrid films with a wide surface, consisting of graphene oxide and single-walled carbon nanotubes by means of an assembly process with the LB technique.

[0016] The article “Excellent optoelectrical properties of graphene oxide thin films deposited on a flexible substrate by Langmuir-Blodgett assembly”, X. Lin et aL, Journal of Materials Chemistry C, vol. 1, no. 41 (2013), describes the production of transparent conductive graphene oxide films on a polymer substrate (PET) by means of LB technique and subsequent reduction of the films with hydroiodic acid and chemical doping.

[0017] Therefore, there is still a need in the art to have a method available for covering articles, in particular historical and artistic articles sensitive to oxidation, which overcomes the drawbacks of the known methods. In particular, it is fundamental toobtain GO films with a high coverage (above 90%), capable of adhering effectively to highly heterogeneous and rough surfaces. In fact, films with partial coverage are not suitable for protecting metal surfaces and can even aggravate the corrosive process at the boundaries of the covered areas.

[0018] SUMMARY OF THE INVENTION

[0019] This object is achieved by the present invention which, in a first embodiment thereof, relates to a process for covering articles sensitive to oxidation and corrosion with a graphene oxide film obtained with the Langmuir-Blodgett technique. Said process comprises the following steps:

[0020] a) spreading, on the surface of a liquid phase consisting of water, sheets of graphene oxide dispersed in a mixture of water and a water-miscible organic solvent, in a watersolvent volume ratio between 1 :3 and 1 :5;

[0021] b) compressing the graphene oxide sheets on the surface of the liquid phase up to a surface pressure between 20 and 40 mN / m by moving one or more movable barriers that move parallel to the surface of the liquid phase and are partially immersed therein;

[0022] c) immersing or extracting the article to be covered in or from the liquid phase, with a direction of movement perpendicular to the surface of said liquid phase, maintaining the surface pressure applied to the GO layer within the range between 20 and 40 mN / m by means of the movement of said one or more barriers;

[0023] d) optionally, further immersing and / or extracting the article to be covered in or from the liquid phase for a number n of times, with n = 1-4, with a direction of movement perpendicular to the surface of said liquid phase;

[0024] characterized in that the graphene oxide sheets used have lateral dimensions between 20 and 50 pm with a monomodal size distribution and an oxidation degree between 35% and 50%, where said oxidation degree is understood as the percentage by weight of oxygen in the film measured by CHNS(O) elemental analysis.

[0025] In a second embodiment of the process, a composite consisting of graphene oxide sheets and cerium oxide nanoparticles is used, in which the cerium oxide is present in an amount between 0.5% and 10% by weight with respect to the sum ofthe weights of graphene oxide and cerium oxide.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 shows surface pressure graphs of graphene oxide films (surface pressure as a function of surface area per unit mass of material) during the compression and decompression steps, with one film of the invention and with three comparative films.

[0028] Figure 2 shows SEM micrographs of a silicon substrate covered with a singlelayer oxide film, and the results of a thickness measurement of the single-layer film obtained by means of an atomic force microscopy analysis.

[0029] Figure 3 shows SEM micrographs of an oxidized copper substrate covered with a single-layer graphene oxide film, together with atomic force microscopy analyses.

[0030] Figure 4 shows photographs of a bare oxidized copper plate and one protected with a graphene oxide film after exposure to hydrochloric acid vapours, together with SEM micrographs of the respective aged samples.

[0031] Figure 5 depicts polarization curves obtained using the Tafel method on a bare oxidized copper plate and on one protected with a graphene oxide film after exposure to an aqueous solution containing sodium chloride.

[0032] Figure 6 shows the surface pressure graph of films of a graphene oxide / cerium oxide nanoparticle composite as a function of specific surface area, during compression and decompression steps.

[0033] Figure 7 depicts photographs of an unprotected oxidized copper plate, an oxidized copper plate protected with a graphene oxide film, and an oxidized copper plate protected with a film of a graphene oxide / cerium oxide nanoparticle composite, after exposure to vapours of hydrochloric acid and subsequent exposure to humidity for 11 days.

[0034] DETAILED DESCRIPTION OF THE INVENTION

[0035] In the description below, the following abbreviations common in the field and the following definitions are adopted:

[0036] - GO: graphene oxide;

[0037] - LB: Langmuir-Blodgett (can refer to the technique or the film obtained); - the definition “graphene oxide layer” means a layer formed by a single planeof carbon atoms; however, the definition admits the presence of oxygenated species above or below said plane of carbon atoms (as well as at the edges thereof). The “graphene oxide sheets” have the same feature of comprising a single plane of carbon atoms, but, unlike the layers, they have micrometric lateral dimensions;

[0038] - a “graphene oxide film” in the present invention can consist of a single layer of GO, or of a multilayer containing up to five of said layers.

[0039] The invention relates to a process for obtaining GO films on corrosion-sensitive articles. The films of the invention consist of one or more layers, each of which is obtained from GO sheets arranged edge-to-edge, are homogeneous, continuous, and with total coverage by virtue of the absence of defects.

[0040] The process essentially consists of two steps: a first step consists in forming GO layers, and the second step consists in depositing one or more of the layers thus obtained, to form the film of the invention, on the article to be protected from corrosion.

[0041] The thickness of the GO films of the invention is between 1 and 6 nm, depending on the number of layers; this thickness ensures no or very low alteration of the colour of the protected article, as demonstrated below by means of experimental examples, while maintaining protective properties suitable for the purpose. Therefore, the films of the invention form a physical barrier that prevents contact between the surface to be protected and the chemical species present in the environment, which are typically in the form of gases or vapours, but can also be liquid or solid.

[0042] The inventors have verified that, surprisingly, by selecting sheets of GO having lateral dimensions between 20 and 50 pm, with a monomodal distribution in said range, essentially continuous layers of the material are obtained which show the behaviour required for an optimal transfer of an anticorrosive protective layer, i.e., curves practically devoid of hysteresis in the compression and relaxation cycles of the layer on the free surface of the liquid; more in detail, this means that the surfacepressure curves of the layer (mN / m) as a function of the surface area (cm2per mg of GO) measured in compression and relaxation are almost exactly superimposed. The inventors have observed that GO layers with this behaviour are stable and, therefore, can be transferred onto an article, forming an essentially continuousbarrier already with a single deposited layer and a fully continuous barrier with the addition of a second layer superimposed on the first. A surprising effect observed by the inventors is that an essentially continuous layer, almost free of defects and holes which allow contact between the article and the environmental chemical species, is obtained by using sheets of relatively large size. In principle, it would have been expected that more continuous layers would form with sheets of very small lateral size, minimizing the interstices between adjacent GO sheets. To the contrary, the experimental observation is that better results in terms of layer continuity are obtained with sheets of relatively large lateral dimensions, despite the edges thereof with irregular geometry. Furthermore, the layer is capable of adapting to complex surfaces with a high degree of roughness. This property is a consequence of the perfect meeting between the 20-50 pm size of the GO with the roughness of the substrate in the range of hundreds of nanometres and tens of micrometres, in particular substrates with a surface characterized by a roughness between 150 nm and 200 pm. Roughness of these orders of magnitude can be measured by means of atomic force microscopy (AFM).

[0043] For carrying out the process of the invention, the selection of GO sheets having the features indicated above is important.

[0044] The GO sheets can be obtained by methods known to those skilled in the art, such as, for example, by electrolysis using graphite electrodes in an aqueous solution of ammonia or oxidizing electrolytes (NO2", SO42", ...) or by means of oxidation with a plasma torch. However, the most used methods are those of chemical oxidation; the conventional method is described in the article “Preparation of Graphitic Oxide”, W. S. Hummers Jr. etal., J. Am. Chem. Soc. 1958, 80, 6, 1339, consisting in the oxidative exfoliation of graphite in the presence of KMnO4 and NaNOa in concentrated sulfuric acid, carried out at temperatures below 45 °C; a recent modification of the method consists in using KMnO4 in a 1:9 mixture of H2SO4 / H3PO4, without potassium nitrate. With these methods, hydroxyl and epoxide groups mainly form on the two main surfaces of the sheets, and carbonyl and carboxyl groups at the edges thereof. The useful oxidation degree of GO for the objects of the invention is between 35 and 50%, understood as the percentage by weight of oxygen on the material sample. Compliance with the oxidation degreerequired for the invention can be verified with CHNS(O) elemental analysis, also known as “organic elemental analysis” or “elemental microanalysis”, performed in an automated manner with special commercial instruments. If the elemental analysis indicates an oxidation degree less than 35%, the oxidation procedure can be repeated until obtaining a value in the 35-50% range. Graphene oxide is also commercially available from various suppliers; also in this case, measuring the oxidation degree with elemental analysis is possible for verifying the suitability of the product prior to the use thereof in the process of the invention.

[0045] GO sheets with lateral size in the useful range for the objects of the invention can be selected by various methods. A first possible method is filtration with physical filters, selecting the particles that pass through a 300 mesh filter and are retained by a 635 mesh filter; filters with openings of these sizes, made from steel, titanium, or polymer (e.g., nylon) wires, are commercially available. Alternatively, it is possible to select GO sheets by centrifugation of an aqueous suspension thereof; in this case, two steps are typically carried out, where the first step can be carried out at an acceleration between 6250 G and 6660 G for a duration of 40-60 minutes, while the second step can occur at an acceleration between 4800 G and 5100 G for a duration of 30-50 minutes. With the first step the sheets of size greater than 20 pm sediment, whereby, by removing the supernatant, the sheets of size smaller than this value are eliminated. In the second step, instead, sheets of size greater than 50 pm sediment, whereby, by recovering the supernatant (and eliminating the sediment), a suspension of GO sheets having size in the desired range is obtained.

[0046] Finally, the third feature that the GO sheets must exhibit to be useful in the process of the invention is a monomodal size distribution; in other words, the distribution function must exhibit a single population and a low polydispersity index. This requirement can also be verified before the use of a sample of GO sheets in the process of the invention, by means of automated instruments that are widely available on the market, the operation of which is based on different physical principles, for example on the sedimentation velocity of particles in a liquid, electron microscopy or methods of laser light diffraction of particles in a liquid suspension.

[0047] In the first step of the process of the invention (step a), the GO sheets thus selected are spread on the surface of a liquid phase consisting of water, with amixture of water and a water-miscible organic solvent, in a watensolvent volume ratio in a range between 1 :3 and 1 :5. The initial liquid phase is contained in a tank of appropriate shape and size and adapted to the subsequent steps of immersing the article in the liquid phase. Apparatuses of various forms for implementing the LB technique exist: for example, the article “A miniaturized radial Langmuir trough for simultaneous dilatational deformation and interfacial microscopy”, S. K. Kale etal., Journal of Colloid and Interface Science, vol. 582(B), 1085-1098, describes an LB system of radial type, in which the compression on the surface of the liquid phase of the species which will form the layer occurs from the exterior towards the interior of a polygonal shape defined by elastic movable barriers. Generally, however, the tank of an LB system is quadrilateral, typically square or, more commonly, rectangular.

[0048] The organic solvent can be selected from acetone, acetonitrile, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide and, preferably, C1-C3 alcohols. Preferably, the volume ratio between water and the organic solvent is between 1 :3 and 1 :5.

[0049] The spreading of GO sheets on the surface of the liquid is preferably carried out by preparing a concentrated suspension of the sheets in water, or in a mixture of one of the above-mentioned solvents with water, and then depositing this suspension, for example by means of a pipette or a syringe, onto the free surface of the liquid present in the tank in which the LB procedure is carried out. It is necessary to spread the dispersion in a uniform manner on said free surface of the liquid, with the precaution of not adding material always at the same points. After depositing the initial suspension on the surface of the liquid phase in the tank, the system is left at rest for at least one hour, or preferably for a longer time, so that it can reach equilibrium by virtue of a homogeneous distribution of the sheets of GO on the surface of the liquid phase.

[0050] In the second step of the process of the invention (step b), the GO sheets are compressed on the surface of the liquid phase according to the well-known LB technique, moving one or more barriers partially immersed in the liquid phase; in the common case of a single barrier, this is moved along one of the two main axes of the tank. The movement of the barrier is controlled by a microprocessor thatcontinuously records the change in surface pressure, which increases as the compression applied to the GO sheets increases. In this step the GO sheets, the main surfaces of which are parallel to the surface of the liquid phase, mutually approach along the edges thereof, forming the desired layer. The inventors have observed that, for obtaining of a continuous layer from the sheets, it is necessary to reach a surface pressure of at least 20 mN / m, while at values of surface pressure higher than 40 mN / m, the layer collapses with partial superimposition of the parts thereof and loss of material in the liquid solution. The preferred surface pressure values for carrying out the process of the invention are between 25 and 35 mN / m, even more preferably of about 30 mN / m.

[0051] In the next step of the process of the invention (step c), the article to be covered with the film is immersed in the liquid phase or extracted therefrom. The GO layer is thus transferred from the surface of the liquid phase to that of the article, and adheres uniformly to the latter. During this step, the barrier (or barriers) of the system is moved (or are moved) so as to keep the surface pressure applied to the GO layer essentially constant, and in any case within the indicated limits (with fluctuations of ± 0,5 mN / m with respect to the chosen pressure).

[0052] As for the transfer of the GO layer onto the surface of the article, the extraction from the liquid phase obviously occurs with a movement in the upwards direction, which movement is defined in the field as “upstroke”, while immersion occurs with a movement directed downwards, referred to as “downstroke”.

[0053] In the case of “upstroke" transfer, the article can have been completely immersed in the liquid phase before the preceding step of forming the GO layer by compression of the sheets on the surface of the liquid phase itself; alternatively, it can have been completely immersed in the liquid phase before spreading the GO sheets on said surface. In these cases, this step of the process simply consists in extracting the article from the liquid phase, once the pressure ranges indicated above have been reached.

[0054] In the case of upstroke transfer, if the article had not been immersed in the liquid phase previously, the immersion occurs as the first step of this operation, with a pressure of the barrier (or barriers) on the GO sheets less than that necessary to form a cohesive layer, while the second step of the operation is the extraction fromthe liquid phase.

[0055] The extraction, and possibly the immersion, of the article from or in the liquid phase are carried out with a movement direction perpendicular to the liquid surface and therefore to the surface of the GO layer, at a constant speed between 1 mm / min and 20 mm / min.

[0056] In the case of immersion in and subsequent extraction of the article from the liquid phase, with the GO layer already formed and maintained at constant surface pressure, a double covering is obtained, with a first film layer deposited on the article in the downstroke phase and a second film layer deposited on the article in the upstroke phase.

[0057] In general, if the article is passed through the surface of the liquid phase while maintaining a surface pressure between 20 and 40 mN / m, the transfer of a layer onto the article is obtained; if the passage of the article through the surface of the liquid phase occurs with zero surface pressure, or in any case less than 20 mN / m, a modest transfer is obtained and in any case less than that necessary to obtain a coverage of at least 90%. Therefore, by adjusting the surface pressure applied by the one or more movable barriers in the various steps, it is possible to achieve the desired sequence of steps of upstroke / downstroke transfer of GO layers onto the article.

[0058] The inventors have observed, with electron microscope analyses, that a film consisting of one layer of GO already exhibits a very high coverage degree, above 90%, and generally above 97%, of the surface of the article.

[0059] Optionally, the above operation can be repeated from one to four times in step d); in this case, the inventors observed, already with only a single second step of transferring the GO layer and therefore with a double-layer film, 100% coverage of the article surface.

[0060] In an alternative embodiment of the method, cerium oxide (CeO2) nanoparticles are added to the GO sheets. The cerium oxide nanoparticles have catalytic properties such that the covering film of the article performs not only a physical barrier action (obtained with GO) but also a chemical action against oxidizing agents and free radicals.

[0061] Useful CeO2 nanoparticles for the purposes of the invention can be producedaccording to the procedure reported in the article “Repeated topical administration of 3 nm cerium oxide nanoparticles reverts disease atrophic phenotype and arrests neovascular degeneration in AMD mouse models”, A. Badia et al., ACS Nano, Vol.

[0062] 17 / 2.

[0063] The invention will be further shown by the following examples.

[0064] EXPERIMENTAL TOOLS, METHODS AND CONDITIONS

[0065] Selection of GO sheets

[0066] The size selection operations of the GO lamellae were carried out with a Sartorius centrifuge mod. G-16C.

[0067] Particle size analysis

[0068] The size analyses of the GO sheets were performed with a Mastersizer 3000 Laser Granulometer, dispersing, for each measurement, 0,13 mg of GO sheets, possibly in combination with CeO2 nanoparticles, in 2,2 ml of a mixture of water and an organic solvent in a 1 :4 volume ratio.

[0069] Elemental analysis

[0070] CHNS(O) elemental analyses were performed with a UNICUBE® instrument.

[0071] Langmuir-Blodgett apparatus

[0072] The apparatus used for producing the GO layers was the KSV 3000 instrument (KSV Instrument Ltd, Helsinki, Finland).

[0073] Spin Coater Tool

[0074] APT GmbH - Waffer Spinner - Spinl 50

[0075] Scanning electron microscope (SEM)

[0076] The micrographs for evaluating the features of the films were obtained with a FE-SEM JSM 7500 FA SEM.

[0077] Atomic Force Microscope (AFM)

[0078] The micrographs to evaluate the morphology and roughness of the films were obtained with a Bruker Dimension Icon® AFM instrument.

[0079] In detail, the surface morphology of the samples was analysed by means of atomic force microscopy operating in dynamic contact operating conditions in the tapping mode. The acquisitions were performed on areas with sizes varying from 1 x 1 pm to 30 x 30 pm. The surface roughness was quantified by means of the arithmetic mean roughness parameter (Ra), calculated as the mean of the absolutevalues of the deviations in height with respect to the mean plane of the surface. Colorimetric analysis

[0080] The colorimetric analyses were performed with a Konika Minolta CR-400 instrument.

[0081] Materials

[0082] GO sheets: supplier Avanzare company of Navarrete (Spain), catalogue number av-GOX-70, in the form of a suspension in water.

[0083] The Ce02 nanoparticles were synthesized following the protocol reported in the article to A. Badia et al. cited.

[0084] The other chemicals were purchased from the Sigma-Aldrich catalogue.

[0085] EXAMPLE 1

[0086] This example relates to the verification of the ideality of a GO layer obtained in accordance with the invention and, for comparison, of three GO layers obtained under conditions not according to the invention.

[0087] A commercial GO suspension in water (Avanzare) was diluted 1:10 in water and then centrifuged at 6654 G for 40 minutes. The supernatant was discarded. The sediment was redispersed in water and centrifuged at 5095 G for 30 minutes. The sediment was discarded and the supernatant was the starting suspension fortesting in accordance with the invention.

[0088] The selected suspension was sonicated for 30 minutes.

[0089] The tank of the LB apparatus was filled with ultra pure MilliQ water (resistivity = 18 MQ cm, pH = 5.6 at 20 °C) obtained with the Simplicity apparatus (Millipore, Italy) coupled with Milli-Q setup (Millipore, Italy). The water temperature was set to 20 °C, and the room temperature was set to 23 °C.

[0090] 600 pL of GO suspension in water were collected and diluted with 2200 pL of methanol (obtaining a GO concentration of 0.062 g / L in water / methanol 1 :4); the suspension was sonicated for 5 minutes and then distributed dropwise onto the surface of the water in the tank of the LB apparatus.

[0091] After distributing the solution, the system was allowed to equilibrate for 1 hour, to obtain a uniform spreading of the GO sheets on the surface of the liquid phase.

[0092] The spreading isotherms, surface pressure versus area, were collected at a compression speed of 10 mm / min. The GO layer was compressed up to a surfacepressure of 30 mN / m to study the properties thereof by analysing the features of the isotherm, and then expanded to verify the stability thereof.

[0093] The obtained graph is shown in Fig. 1.a and shows an optimal behaviour, with a negligible hysteresis. For comparison, the test was then repeated with the suspension of not size-selected GOs as obtained from the supplier, with a suspension of GOs having a size less than 20 pm (the supernatant eliminated after centrifugation at 6654 G), and with a suspension of GOs having a size greater than 50 pm (obtained by redispersing in water the sediment obtained from centrifugation at 5095 G). The three curves obtained with these three suspensions are depicted in Figures 1.b, 1.c, and 1.d, respectively. Furthermore, in the case of the two suspensions of GO < 20 pm and GO > 50 pm, during the expansion, the curve does not return to the initial value, indicating a loss of material in the underlying aqueous phase; this high hysteresis also has the consequence of making any type of transfer of the films onto a substrate to be covered difficult.

[0094] EXAMPLE 2

[0095] This example relates to the production of a GO film on a silicon substrate, to verify the covering efficiency of a substrate with a smooth surface.

[0096] A fragment of silicon wafer of rectangular shape, with width of 1 cm, length 5 cm and thickness 0,5 mm, was inserted into the tank of the LB apparatus of Example 1. The wafer had previously been treated with Piranha solution (a mixture of H2SO4 and H2O2) to remove organic contaminants from the surface.

[0097] In the tank, a layer of GO was then produced, operating in the modes described in Example 1 for producing the film in accordance with the invention (the film in Fig. 1.a).

[0098] The GO single-layer film was transferred onto the surface of the Si wafer at a constant surface pressure of 20 mN / m and with a substrate extraction speed from the tank (upstroke mode) of 10 mm / min.

[0099] The transfer efficiency was checked by measuring the parameter TR, defined as follows:

[0100] TR = AA / As,

[0101] where:

[0102] AA is the reduction of the surface area of the single layer of GO on the surfaceof the liquid phase in the tank of the LB apparatus during the deposition process; As is the area of the substrate which is coated (i.e., the area of the surface that physically comes into contact with the single layer of GO).

[0103] A TR value equal to about 1 indicates an efficient transfer (the amount of GO layer removed from the tank corresponds to the amount deposited on the substrate with the same orientation and density).

[0104] The measured TR value was only slightly above 1 , indicating good film transfer efficiency.

[0105] Fig. 2.a shows an SEM image of the film on the surface of the substrate. As noted in the figure, the film appears to adhere well to the surface, with a minimal amount of openings, quantified with image analysis methods at about 3% of the surface.

[0106] Fig. 2.b shows an image acquired with AFM on the same sample of the image in Fig. 2.a, in a zone between two sheets of GO deposited on the surface of the substrate. Fig. 2.d is a graph obtained from an AFM measurement along the white line indicated in Fig. 2.b, confirming an optimal coverage with a thickness between 1 and 2 nm.

[0107] The sample thus obtained was allowed to dry for 24 hours and the procedure was repeated under identical conditions.

[0108] Fig. 2.c shows the SEM image of the surface of the sample thus obtained; as can be seen in the figure, after the transfer of a second layer of GO the covering of the substrate is complete, without openings.

[0109] EXAMPLE 3

[0110] This example relates to the production of GO films on a copper substrate oxidized at the surface, representative of metal archaeological articles.

[0111] The test of Example 2 was repeated using as substrates copper plates of thickness 2 mm and lateral dimensions 2 x2 cm, with the surface oxidized.

[0112] In order to have surfaces with a controlled and reproducible oxidation degree, the surfaces of the copper plates were lapped with abrasive papers and diamond pastes to obtain a 1 pm finish, then degreased with ethanol. The samples were then immersed for 30 minutes in a 1 M HNO3 solution to remove the oxidized species formed in the atmosphere. Thereafter, the plates were thoroughly rinsed with Milli-Q water to remove the copper nitrates which had formed.

[0113] Oxidation was then induced by immersing the samples for 3 hours in a 1 :5 solution of H2O2 and H2O. Finally, to stabilize the oxidation, the samples were placed in an oven at 35 °C overnight.

[0114] Unlike the silicon wafer of Example 2, the substrates of this example are heterogeneous and exhibit high surface roughness, which requires a tailored approach to obtain satisfactory results. Fig. 3.a shows an SEM micrograph of a copper plate oxidized as described above, and Fig. 3.b shows a micrograph obtained by atomic force microscopy of the surface morphology of the plate in Fig.

[0115] 3.a. With some preliminary orientation tests, it was verified that the best conditions for transferring the GO layer onto the substrate were in the upstroke mode with extraction speed of 1 mm / min. Under these conditions, a TR value of about 1.5 was measured, which reflects the fact that the deposited GO layer must conform to a surface with a greater development than the geometric area of the plate.

[0116] The procedure of transferring the GO layer onto the substrate was repeated a second time, also in this case in the upstroke mode; the sample obtained in the first deposition step was quickly immersed in the aqueous phase, to avoid transfer of material in this step (TR in the immersion = 0).

[0117] Fig. 3.c shows an SEM image of the GO film obtained after a film transfer (one layer) on the substrate. Also in this case, despite the irregularity of the surface of the substrate, the film appears continuous.

[0118] Fig. 3.d shows an AFM image which demonstrates the perfect adhesion between the GO sheets and the underlying rough substrate. This arises from the choice of the lateral width which perfectly follows the asperities of the surface.

[0119] EXAMPLE 4

[0120] This example relates to the assessment of the corrosion protection properties by a GO film of the invention.

[0121] The test of Example 3 was repeated with a second oxidized copper plate covered with a film consisting of two layers of GO. For comparison, the test was carried out on an identical oxidized copper plate, not covered with a GO film.

[0122] The two plates were masked on half of the surface thereof with a layer of Parafilm and exposed to HCI vapours at 40 °C for 5 hours.After this treatment, the protection was removed from the plates, which were initially visually evaluated. As shown in Fig. 4.c, the plate covered with GO film in accordance with the invention appears uniform, no differences in colour are observed between the part exposed to HCI and the non-exposed part; vice versa, on the plate not protected with GO film, shown in Fig. 4.a, an apparent darkening effect is observed in the right part, the one not protected during exposure to HCI vapours.

[0123] Fig. 4.b shows an SEM micrograph of the unprotected area of the plate in Fig.

[0124] 4.a: the arrows show the formation of crystals of copper chlorides due to the corrosive phenomenon.

[0125] Fig. 4.d shows an SEM micrograph of the unprotected area with a GO singlelayer of the plate in Fig.4.c; the arrows show the formation of a highly limited number of crystals of copper chlorides.

[0126] The visual inspection was confirmed with an instrumental colorimetric measurement. To this end, the parameter AE was evaluated, defined as follows:

[0127] AE = [(AL*)2+ (Aa*)2+ (Ab*)2]1 / 2

[0128] where AL*, Aa* and Ab* are the variation of the colorimetric parameter of the CIELAB 1976 system before and after exposure to HCI vapours. A lower AE value implies a smaller colour variation, and therefore a smaller alteration of the sample, after exposure to the corrosive environment.

[0129] The values obtained were AE = 4.84 ± 0.08 for the sample protected with the GO film, and AE = 32.22 ± 0.50 for the unprotected plate, in accordance with the naked-eye observation.

[0130] The protective effect of the GO film obtained in this example was further confirmed by electrochemical measurement, using an oxidized copperplate covered with a GO film in accordance with the invention and, for comparison, an identical unprotected plate. The tests consist of Tafel polarization measurements, conducted using the copper oxide surface as the working electrode, a platinum wire as the counter electrode, and an Ag / AgCI electrode as the reference electrode. The corrosive environment was a 0.1 M NaCI solution, with 50 pL of this solution applied to a confined area of the working electrode (0.12 cm2). The obtained curves are shown in Fig. 5; the dashed curve in the figure relates to the measurement carriedout on the plate not covered with a GO film, while the solid curve relates to the plate covered in accordance with the invention. The analysis of the anodic and cathodic Tafel regions from the polarization curves has confirmed a reduction of the corrosion current density (icon-) in the case of the plate protected with GO film with respect to the uncovered substrate.

[0131] These tests confirm the ability of the GO films of the invention to protect the substrates from corrosion.

[0132] EXAMPLE 5

[0133] This example relates to a comparison between the protection features of substrates obtained with the GO films of the invention and with polymer film coverings known in the art.

[0134] Another oxidized copper plate covered with a GO film prepared as described in Example 3 was used as a representative sample of the invention.

[0135] For comparison, oxidized copper plates identical to that used in Example 3 were covered with thin layers of polymer materials widely used in the conservation and restoration field.

[0136] A first plate was coated by the spin coating technique with 100 pL of a 10% by weight solution of Paraloid™ B72 dissolved in ethyl acetate. The process was carried out by setting a maximum speed of 5000 rpm for 60 seconds, achieved with an acceleration of 100 rpm / sec.

[0137] A second plate was covered by means of the spin coating technique with 100 pL of 10% Zapon lacquer (Roth®) dissolved in ethyl acetate. The process was carried out by setting a maximum speed of 5000 rpm for 60 seconds, achieved with an acceleration of 100 rpm / sec.

[0138] On the three samples thus obtained, the percentage protection factor, PF (%), was measured, defined as:

[0139] PF (%) = |(AEcP- AEsp) I AEsp | x 100

[0140] where:

[0141] AECp = AE measured on a sample with a protective layer;

[0142] AEsp = AE measured on a sample without protective layer (bare plate); and the notation with two vertical bars indicates the modulus of the value of the ratio (AEcp- AEsp) / AEsp.On the three samples thus obtained, the HCI vapour exposure procedure of Example 4 was repeated, obtaining the following results:

[0143]

[0144] As shown by the results in the table, the film of the invention offers a protection factor comparable to that of Paraloid™ B72, but with a halved impact on colour change (AE). As compared to Zapon, it is about 4% less protective, but nonetheless three times less impactful on colour. Therefore, the film of the invention exhibits a significantly lower alteration of the colour (AE), a particularly relevant aspect in the field of the conservation of works of art.

[0145] EXAMPLE 6

[0146] This example concerns the preparation of a composite film of GO and nanoparticles (NP) of CeO2 and the verification of the behaviour thereof in compression / relaxation cycles.

[0147] To an aqueous dispersion of GO sheets at a concentration of 0.7 mg / ml, 300 ppm of CeO2 NP were added to achieve a final concentration of 0.6 mg / ml of GO and 0.033 mg / ml of cerium oxide. The solution was then mixed until complete mixing of the two components was achieved. Sodium hydroxide was then added to the mixture of GO and cerium oxide to reach a concentration of 0.25 M. The alkaline environment promotes the removal of the citrate capping agents, which is necessary to promote the precipitation of the NPs onto the GO sheets and the consequent formation of the composite. The sample was further centrifuged at 7500 g for one hour, to ensure the complete precipitation of the GO-CeO2 composite. The supernatant was discarded and the sediment was redispersed. The centrifugation was repeated three times at 7500 g for 30 minutes to completely purify the composite material and, each time, the supernatant was discarded while the sediment was redispersed in the solvent.

[0148] Then, 600 pL of the aqueous suspension thus obtained were collected anddiluted with 2200 pL of methanol. The isotherm, collected in the same manner as in Example 1 and depicted in Fig. 6 as a dashed curve, exhibits a small hysteresis with respect to GO alone (solid curve added in figure for comparison).

[0149] The corrosion protection properties of a copper plate protected with a film consisting of a double layer of GO / CeO2 composite were then evaluated.

[0150] Three oxidized copper plates were initially exposed to HCI 1M vapours at 40 °C for 5 hours. The first of these plates was not protected with a film, the second was protected with a film of only GO, and the third with a film of GO / CeO2. The samples were then exposed for 11 days to a highly humid environment created with a closed receptacle filled with water containing 3.5% p / V of NaCI at 80 °C.

[0151] The images in Fig. 7 depict the micrographs obtained under an optical microscope on the samples thus obtained.

[0152] Fig. 7.a shows an optical microscope image of an oxidized copper plate after aging with HCI vapours. Fig. 7.b shows an optical microscope image of the same copper plate oxidized with HCI vapours following exposure to moisture; the arrows indicate the copper hydroxychloride phases formed as a result of the corrosive phenomenon. Fig. 7.c shows an optical microscopy image of a copper plate oxidized with HCI vapours protected by a GO single-layer after exposure to moisture; the arrow indicates the presence of some corrosion phases. Fig. 7.d shows an optical microscope image of a copper plate oxidized with HCI vapours, protected with a GO / CeO2 single-layer after exposure to moisture; the arrow indicates the presence of some phases of corrosion, in a highly limited number with respect to the control and also with respect to that protected with GO.

Claims

CLAIMS1. A process for covering articles sensitive to oxidation and corrosion with a graphene oxide film obtained with the Langmuir-Blodgett technique, said process comprising the following steps:a) spreading, on the surface of a liquid phase consisting of water, sheets of graphene oxide dispersed in a mixture of water and a water-miscible organic solvent, in a watensolvent volume ratio between 1 :3 and 1 :5; b) compressing the graphene oxide sheets on the surface of the liquid phase up to a surface pressure between 20 and 40 mN / m by moving one or more movable barriers that move parallel to the surface of the liquid phase and are partially immersed therein;c) immersing or extracting the article to be covered in or from the liquid phase, with a direction of movement perpendicular to the surface of said liquid phase, maintaining the surface pressure applied to the GO layer within the range between 20 and 40 mN / m by means of the movement of said one or more barriers;d) optionally, further immersing and / or extracting the article to be covered in or from the liquid phase for a number n of times, with n = 1-4, with a direction of movement perpendicular to the surface of said liquid phase; characterized in that the graphene oxide sheets used have lateral dimensions between 20 and 50 pm with a monomodal size distribution and an oxidation degree between 35% and 50%, where said oxidation degree is understood as the percentage by weight of oxygen in the film measured by CHNS(O) elemental analysis.

2. The process according to claim 1, wherein cerium oxide nanoparticles are added to the graphene oxide sheets used in step a), in an amount between 0.5% and 10% by weight with respect to the sum of graphene oxide and cerium oxide.

3. The process according to any one of claims 1 or 2, wherein the graphene oxidesheets are produced by a technique selected from electrolysis using graphite electrodes in an aqueous solution of ammonia or an oxidizing electrolyte, oxidation with a plasma torch, or by oxidative exfoliation of graphite in the presence of KMnO4 and NaNOs in concentrated sulfuric acid at a temperature below 45 °C or in the presence of KMnO4 in a 1 :9 mixture of H2SO4 / H3PO4.

4. The process according to any one of the preceding claims, wherein the oxidation degree of the graphene oxide is verified by CHNS(O) elemental analysis .

5. The process according to any one of the preceding claims, wherein graphene oxide sheets with lateral dimensions between 20 and 50 pm are selected with a technique chosen from:- filtration with physical filters, selecting the particles that pass through a filter with openings of 50 pm (300 mesh) and are retained by a filter with openings of 20 pm (500 mesh);or- by centrifugation of an aqueous suspension of graphene oxide sheets, with a sequence of steps comprising: a first centrifugation step carried out with an acceleration between 6250 G and 6660 G for a duration of 40-60 minutes; elimination of the liquid phase with recovery of the sediment; redispersion of the sediment in water; a second centrifugation step with an acceleration between 4800 G and 5100 G for a duration of 30-50 minutes; elimination of the sediment and recovery of the liquid phase, which forms a suspension of graphene oxide sheets having dimensions in the indicated range.

6. The process according to any one of the preceding claims, wherein said water- miscible organic solvent is selected from acetone, acetonitrile, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and C1-C3 alcohols.

7. The process according to any one of the preceding claims, wherein thegraphene oxide sheets are compressed on the surface of the liquid phase at a surface pressure between 25 and 35 mN / m.

8. The process according to any one one of the preceding claims, wherein during step c), the one or more movable barriers of the system are moved so as to maintain the surface pressure applied to the film within the limits indicated in step b).

9. The process according to any one of the preceding claims, wherein step c) is carried out at a constant speed between 1 mm / min and 20 mm / min.

10. An article covered with a film comprising from one to five layers of graphene oxide, optionally with the addition of cerium oxide nanoparticles, obtained according to any one of the preceding claims, wherein, prior to covering with graphene oxide film, said article has a surface characterized by a roughness (Ra) between 150 nm and 200 pm, calculated as the average of the absolute values of the deviations in height with respect to the mean plane of the surface obtained in an atomic force microscopy measurement in the tapping mode, and wherein a single layer of graphene oxide, or graphene oxide with the addition of cerium oxide nanoparticles, forms a coverage above 90% of said surface.

11. The article according to claim 10, which has a colour alteration value AE compared to the uncovered article of less than 5, calculated with the formula AE = [(AL*)2+ (Aa*)2+ (Ab*)2]1 / 2where AL*, Aa* and Ab* are the variation of the colorimetric parameter of the CIELAB 1976 system.

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