Preparation of Coatings Containing At Least One In-Plane Oriented Layer of Anisotropic Shaped Objects

Abstract

The subject matter of the present invention concerns the preparation of a coated solid surface wherein the coating contains at least one in-plane oriented layer of anisotropic shaped objects through a specific spraying method, and the device enabling this method.

Claims

1. A process of preparation of a coated solid surface wherein the coating contains at least one in-plane oriented layer of anisotropic shaped objects, comprising: a) the preparation of at least one solution, suspension or dispersion of interacting objects, b) the spraying of at least one solution, suspension or dispersion of step a) on a solid surface said spraying is done with: an angle inferior to 80, preferably inferior to 30, with respect to the plane formed by the solid surface to be coated, a droplet speed superior to 0.1 meters per second, in particular wherein the combination of the droplet and gas in movement generates a movement of the newly formed thin layer on the surface of the substrate to be coated, c) previously, simultaneously or subsequently to step b), an optional spraying or application of at least one complementary interacting chemical species, such as an interface and/or cohesion agent, onto the coated and/or uncoated surface, steps a), b) and/or c) being reproduced as many times as necessary to obtain the desired coating, and d) retrieving the coated solid surface.

2. The process according to claim 1, characterized in that the solution, suspension or dispersion of step a) comprises at least one non-volatile solvent or non-volatile liquid vehicle.

3. The process according to claim 1, characterized in that the objects are nano-objects preferably chosen in the list consisting of nanowires, nanorods, nanobelts, nanoribbons, nanorice, nanotubes, nanofibers, microfibrils, and/or microfibers.

4. The process according to claim 1, characterized in that at least one complementary interacting chemical species, such as an interface and/or cohesion agent, preferably a polymer, is sprayed on the coated or uncoated solid surface with an angle ranging between 70 to 90 with respect to the solid surface to be coated.

5. The process according to claim 1, characterized in that the complementary interacting chemical species is negatively charged or positively charged.

6. The process according to claim 1, characterized in that the complementary interacting chemical species is an organic polymer or a mineral polymer.

7. The process according to claim 1, characterized in that a step f) is added wherein the coating is removed from the solid surface, preferably via the delamination of the coating or dissolution, melting or vaporization of said solid surface.

8. The process according to claim 1, characterized in that a supplementary gas nozzle is added to push the liquid film across the solid surface.

9. The process according to claim 1, characterized in that a supplementary step e) of activation of the surface of the solid to be coated is added between step a) and b).

10. A coating comprising at least one in-plane oriented layer of anisotropic shaped objects characterized in that the nematic order parameter of said coating is comprised between 0.5 and 1, preferably the coating comprises at least one complementary interacting chemical species.

11. The coating according to claim 10, characterized in that it is multi-layered with a controlled sequence of layers of differently oriented or of different chemical species.

12. The coating according to claim 10, characterized in that the in-plane orientation angle of each oriented layer of objects shifts from one layer to the next with respect to the parallel plane defined by the solid surface.

13. The coating according to claim 12, characterized in that the shifting angle is constant from one layer to the next.

14. (canceled)

15. A device for a spraying process in order to obtain a coating comprising at least one in-plane oriented layer of anisotropic shaped objects characterized in that said device comprises: at least one means of fixing a substrate to be coated on a support, said support being optionally arranged to allow the possibility of an angular orientation adjustment; at least one spray holder, comprising at least one spray nozzle mounted in a fixed or movable way through at least a translation movement back and forth along the orthogonal direction of the spraying.

16. (canceled)

Description

FIGURES

[0171] FIG. 1: SEM pictures of silver nanowires sprayed for 10 s (a), 20 s (b), 40 s (c) and 200 s (d), with the measured coverage as function of spraying time (respectively 15, 28, 41 and 61%).

[0172] FIG. 2: SEM pictures of silver nanowires sprayed on various substrates, with the distribution of nanowire angle with respect to the spraying direction and the corresponding nematic order parameter S. The substrates are a) glass coated with PEI, b) glass activated with a plasma treatment, c) silicon coated with PEI, d) PMMA coated with PEI, e) gold coated with PEI, f) aluminum coated with PEI and g) stainless steel coated with PEI.

[0173] FIG. 3: SEM (a, b and c) and AFM pictures (d and e) of a) silver nanowires, b) zinc oxide nanowires, c) gold nanorods, d) carbon nanotubes, and e) cellulose microfibrils sprayed on PEI coated glass substrates, with the corresponding angle distribution with respect to the spraying direction and the nematic order parameter.

[0174] FIG. 4: sketch of the nozzle and substrate, showing the spraying angle cp, and the nematic order parameter as function of the spraying angle for silver nanowires deposited on PEI-coated glass.

[0175] FIG. 5: sketch and picture of the moveable linear guide device, which allows translational movement in the parallel and perpendicular directions with respect to the spraying direction. The substrate is placed on a holder which allows rotational movement in order to adjust the orientation between different layers.

[0176] FIG. 6: map of the large area samples for both scanning velocities, with the nematic order parameter calculated from SEM pictures of aligned silver nanowires taken at various locations on the substrate.

[0177] FIG. 7: SEM pictures (a, c: top view and b, d: cross-section) of multilayer samples (a, b: 5 nanowire layers oriented in the same direction and c, d: 5 nanowire layers each one oriented at 45 from the underlying layer orientation).

[0178] FIG. 8: SEM pictures of perpendicularly oriented silver nanowires separated by a) 5 polyelectrolyte bilayers, b) 10 polyelectrolyte bilayers and c) 15 polyelectrolyte bilayers.

[0179] FIG. 9: Comparison of monolayer films obtained according to the invention of the present application and a film obtained through the process of U.S. 2014/0044865. The conditions of spraying are identical (distance between the nozzle and the substrate of 1 cm, liquid flow rate of 1 ml/min, gas flow rate 30 L/min, angle of spraying 20, coating of 1 minute, AgNW in water on a substrate of SiO.sub.2 covered with PEI). Only the temperature of the substrate is different: ambient temperature for the process according to the present application and 100 C. for the prior art method (i.e. at a temperature wherein at least one solvent is partially volatile which thus does not enable the effect reported in the present application). It can be clearly seen on the two pictures (optical microscopy and electronic microscopy) that the method of the present application enables the obtaining of a much more homogeneous coated layer.

[0180] FIG. 10 represents scanning electron microscopy pictures taken from the side of multilayer anisotropic objects coatings according to the present invention. The coatings comprise respectively 2, 4, 6 and 8 layers, showing the gradual increase in thickness and the homogeneity of the coatings.

EXAMPLES

[0181] 1. Spraying conditions

[0182] The grazing incidence spraying of nanowires and nanorods has been performed with a Spraying Systems stainless steel nozzle (model B1/4J-SS). The nozzle is held at a distance of 1 cm from the substrate. Unless otherwise specified, the angle between the angle and the substrate plane is 20. The nozzle is fed by compressed air (air flow 30 L/min) and the nanowire suspension is delivered by a HPLC pump (1 mL/min). The spraying time is 200 seconds, unless otherwise specified. Most experiments (unless otherwise specified) have been performed with a suspension of silver nanowires in Milli-Q water. The nanowire length and diameter are 42 m and 405 nm respectively, and the nanowires are functionalized in situ during the synthesis by PVP (poly(vinyl pyrrolidone)). The suspension has been deposited on silicon oxide glass substrates unless otherwise specified. The substrates are coated with a PEI (poly(ethylene immine)) layer deposited by orthogonal spraying followed by a rinsing step with water. After deposition of the silver nanowires, the substrate is rinsed with water and dried using a gentle air flow. [0183] 2. Characterization of the alignment

[0184] The thin layers formed by grazing incidence spraying are imaged by Scanning Electron Microscopy (SEM). The nanowire angle distribution relative to the spraying direction is extracted from the SEM picture using an automated procedure based on the evaluation of the structure tensor in a local neighborhood (Biomech Model Mechanobiol 2012, 11, 461-473). The nematic order parameter S has been used to characterize the quality of alignment, where:

[00001]$S=\frac{3.Math.{\cos .Math.}^2.Math.-1}{2}$

with being the angle between the nanowire long axis and the spraying direction. The nematic order parameter S is equal to 0 for a fully random nanowire orientation, whereas it is equal to 1 for a perfectly parallel set of nanowires. [0185] 3. Working embodiments [0186] 3.1. monolayers of varying densities

[0187] The density of nanowires deposited on the surface can be tuned easily by varying the spraying time. [0188] Different samples have been prepared by varying the deposition between 10 and 200 seconds (FIG. 1). The coverage increases from 15% to 61% with the spraying time. A longer deposition time doesn't increase significantly the coverage. [0189] 3.2. Deposition on various substrates

[0190] Silver nanowires have been deposited on various substrates, including silicon oxide glass, silicon, PMMA, gold, aluminum and steel.

[0191] The oriented deposition of silver nanowires is effective on a broad range of substrates, provided that there is a sufficient interaction between the nanowire and the substrate to be coated. Coating the substrate with a layer of PEI can provide this attractive interaction, but this is not mandatory, as exemplified by the successful deposition on bare glass activated by a plasma treatment. The nematic order parameter is almost substrate-independent for glass, silicon, gold and PMMA. The ordering is slightly reduced for the steel and aluminum substrates, probably due to the higher surface roughness which may modify the liquid flow at the liquid/substrate interface.

[0192] The detailed substrate preparation was as follows: [0193] FIGS. 2a, c and d): the substrate (glass, silicon or PMMA) is successively soaked in ethanol in an ultrasound bath for 15 minutes, in MilliQ water in an ultrasound bath for 15 minutes, in a PEI solution in MilliQ water for 15 minutes, then rinsed in MilliQ water for 15 minutes, and finally dried with a gentle air stream. [0194] FIG. 2b): the glass substrate is successively soaked in ethanol in an ultrasound bath for 15 minutes and in MilliQ water in an ultrasound bath for 15 minutes, followed by an exposure to plasma cleaner. [0195] FIG. 2e): the gold surface (gold sputtered on a glass slide) is exposed to plasma cleaner, the successively soaked in a PEI solution in MilliQ water for 15 minutes and in MilliQ water for 15 minutes, and finally dried with a gentle air stream. [0196] FIGS. 2f and g): the aluminum or steel substrates are mechanically polished, then successively soaked in acetone in an ultrasound bath for 30 minutes, in ethanol in an ultrasound bath for 30 minutes, in MilliQ water in an ultrasound bath for 15 minutes, in a PEI solution in MilliQ water for 15 minutes, then rinsed in MilliQ water for 15 minutes, and finally dried with a gentle air stream. [0197] 3.3. Deposition of various anisotropic nanoparticles

[0198] In order to demonstrate the flexibility of the oriented deposition towards nanoparticle type, various anisotropic nanoparticles have been deposited on PEI-coated glass substrates. These particles show very different chemical nature and surface chemistries: metals, oxide, carbon-based material. It is not mandatory for the nanoparticle to be coated by a stabilizing molecule, provided that the particle itself has sufficient interaction with the PEI layer deposited on the substrate to ensure its adsorption.

[0199] Silver nanowires (coated with PVP) are 2-6 m long and have a diameter of 40 nm (aspect ratio 50-150). Zinc oxide nanowires (uncoated) are 4-5 m long and have a diameter of 300 nm (aspect ratio15). Gold nanorods (coated with CTAB) are 100-400 nm long and have a diameter of 15 nm (aspect ratio10-25). Carbon nanotubes (coated with PSS) are 1-5 m long and have a diameter of 1.5 nm (aspect ratio600). Cellulose microfibrils (uncoated) are 2-20 m long and have a diameter of 1-50 nm (aspect ratio30-500). [0200] All samples show significant orientation. The variation in nematic order parameter could be ascribed to the aspect ratio variation, as low aspect ratio nano-objects (such as gold nanorods) are harder to orient compared to very elongated particles. The lower nematic order parameter for carbon nanotubes is due to artifacts in the image treatment induced the by the circular bright spots, which are due to impurities in the suspension, and which enlarges the angle distribution. The actual nematic order parameter is probably in the range 0.7-0.9. The density variation is due to the different affinities of the considered nanoparticle (or its coating) with the PEI coating applied on the glass substrate. [0201] 3.4. Influence of the spraying angle [0202] The spray angle and droplet velocity depend essentially on the liquid and gas flow rate as well as the distance and spray angle between the nozzle and the substrate. The nematic order parameter of aligned silver nanowires on PEI-coated glass substrates has been measured as function of the spraying angle between 0 and 80. The nematic order parameter is higher than 0.5 for all angles below 70, with a maximum around =20. [0203] 3.5. Deposition on large areas

[0204] In order to coat large areas, a linear guide device which allows a translational movement of the nozzle above the substrate while keeping the spraying angle and nozzle to substrate distance constant.

[0205] Large PEI-coated silicon substrate (108 cm.sup.2) have been used. The flow of liquid on the substrate is only present in front of the spraying nozzle, and thus a static liquid film stays on the surface after moving the spray nozzle to a different position. In order to reduce the adsorption of non-oriented nanowires from liquid which is not moving on the surface, an additional air flow has been added just next to the spraying nozzle in order to remove the remaining liquid once the nozzle is moved. Two different translational scanning velocities have been used perpendicularly to the spraying direction: 2 mm/s and 20 mm/s. The nozzle has not been moved in the parallel direction with respect to the spraying direction.

[0206] The use of the described device allows depositing nanowires on large areas with a significant orientation, although the nematic order parameter is below the one obtained without moving the nozzle with respect to the substrate. The quality of ordering is pretty homogeneous on the surface, being slightly reduced on the opposite side of the spray nozzle, possibly because the liquid flows with a lower velocity when the distance between nozzle and substrate increases. [0207] 3.6. Multilayers

[0208] Multilayer oriented nanoparticle assemblies can be made by combining the grazing incidence spraying with the well known Layer-by-Layer assembly technique. A complementary species has to be deposited on the nano-object layer, on top of which a subsequent layer of nano-objects can be deposited. It is also possible to adsorb polyelectrolyte layer pairs (alternating layers of positively and negatively charged polymers, sometimes also called bilayers) in order to tune the interlayer spacing. One advantage of the assembly technique is that the direction of alignment can be chosen independently in each oriented layer.

[0209] FIG. 7 show SEM pictures of samples comprising 5 layers of silver nanowires separated by 6 layers of polyelectrolyte bilayers. The full structure of the thin film can be written as Si/PEI/AgNW/[PEI(PSS/PAH).sub.5PSS/PEI/AgNW].sub.4, where PEI is deposited in the layer beneath and above silver nanowires due to its strong affinity with the PVP-coated silver nanowires. 5 layer pairs of PSS/PAH are inserted between the silver nanowires to act as a spacer. Two examples are shown, in which the orientation is either the same in each silver nanowire layer (FIG. 7a, b), either shifted by 45 between consecutive silver nanowire layers (FIG. 7c, d).

[0210] FIG. 8 shows 3 examples of 2 perpendicularly oriented layers of silver nanowires separated by different number of polyelectrolyte bilayers which act as spacers. The increased spacing between the nanowire layers can be distinguished by the apparent disappearance of the underlying layer when the total number of polyelectrolyte bilayers is increased. The full structure of the film can be described as Si/PEI/AgNW/PEI(PSS/PAH).sub.nPSS/PEI/AgNW, where n=5, 10 and 15 for FIGS. 8a, b and c respectively.