NANOSTRUCTURES OBTAINED THROUGH THE GROWTH OF

ZINC-OXIDE NANORODS OR MICRORODS ON

UNSUPPORTED GRAPHENE NANOPLATELETS IN

SUSPENSION

The present invention relates to the nanotechnology sector and more precisely to an innovative method for the production of graphene nanoplatelets decorated with nanorods or microrods of zinc oxide (possibly doped with metal), with improved electrical, electronic, and mechanical properties. The graphene nanoplatelets (GNPs) are uniformly coated over their entire surface (on both sides of the flake) with nanorods (NR) or microrods (MR) of zinc oxide (ZnO), possibly doped with metals. The morphological properties of the ZnO nanostructures and the coating density of the surface of the GNPs can be controlled during the growth process.

The process takes place in aqueous or hydroalcoholic suspension and leads to the production of nanomaterials that can be used as fillers in polymer matrices to obtain nanocomposites that present specific electrical, electromagnetic, and electromechanical properties.

The method used in the present invention is simple, economically advantageous, scalable for mass production, does not require the use of a catalyst, and the end product is free from impurities.

Field of the invention

The present invention has been developed within a research framework aimed at providing new nanostructured and graphene-based materials with controlled electrical, electromagnetic, and electromechanical properties. GNPs decorated with ZnO nano/microrods are of considerable applicational interest in multiple sectors, either as mechanical reinforcement in composite materials, or for possible exploitation of their piezoelectric or electroactive properties, for the production of composites with desired electrical and/or electromagnetic properties, such as, for example, radar- absorbent materials, as well as for catalytic or energy-harvesting applications.

Innovative aspects of the invention

According to the present invention, ZnO-GNP hybrid nano/microstructures are produced in the presence of a seed layer that favours the nucleation of ZnO structures and leads to a uniform and high-density coating of the GNPs. Deposition of the seed layer is carried out while maintaining the suspension of GNPs under stirring by various techniques, as described hereinafter, for the control of the density and uniformity of the coating .

The main innovative characteristics of the invention rely on the simplicity and cheapness of the process proposed, which, by means of appropriate definition of the operating conditions during the steps of deposition of said seed layer and of growth of the ZnO micro/nanostructures, enables production of ZnO- decorated GNPs, having controlled morphological characteristics.

Some studies present in the literature show that the growth of ZnO nanorods on GNPs from aqueous solution is possible in the absence of prior deposition of the seed layer on the GNPs. However, from these studies it emerges that in this way there is no satisfactory control on the quality and morphology of the ZnO micro/nanostructures produced, and there is found a poor coating density of the surface of the GNPs, accompanied by a limited uniformity of the distribution of the micro/nanostructures thereon.

In other studies it is shown how the presence of a seed layer makes it possible to obtain a high coating density of flakes of GO (graphene oxide) or rGO (reduced graphene oxide) or supported graphene.

Hydrothermal growth of ZnO nanostructures is usually carried out, according to the literature, under static conditions.

Instead, according to the present invention, for the growth of ZnO micro/nanorods (possibly doped with metals) with the desired morphology, three different growth techniques are proposed (namely, static hydrothermal - HT - growth, dynamic HT growth, and growth by probe sonication - PS) in order to be able to control the morphological characteristics of the structures produced (i.e., diameter and length).

The production technique developed according to the present invention is economically advantageous and suitable for mass production.

Summary of the prior art

Composite materials based on graphene and zinc oxide (ZnO) nanostructures form the subject of numerous studies owing to their potential applications in the production of new multifunctional materials with improved electrical and mechanical properties, and of new devices for the electronic and photonic sectors [1, 2]. It has been shown in the literature that, when inorganic materials, such as zinc oxide, are integrated with graphene, their electronic properties are considerably improved [3]. ZnO nanostructures generally behave as n-type semiconductors, and hence have the capability of enabling electron doping on graphene. Consequently, decoration of graphene by means of ZnO nanostructures enables production of new graphene nanomaterials with additional features, such as a higher electrical conductivity, an improved capacity of absorption of the energy associated to electromagnetic fields, when used as fillers in polymer matrices, higher electron emission and detection capabilities [4, 5, 6], and better electrochemical properties [7, 8] . In a prior study [9] there have been produced composite materials made up of graphene nanoplatelets (GNPs) and ZnO nanowires by using suspension mixing and vacuum filtering. The porous composite material thus produced has revealed interesting electrical properties. In particular, it has been highlighted that there exists an optimal concentration in weight of ZnO nanowires with respect to the GNPs, corresponding to a maximum improvement of the electrical conductivity of the composite, confirming the hypothesis that ZnO nanostructures can induce an electronic doping effect on graphene.

The present invention consequently falls within this research framework and proposes an innovative technique for the production of graphenes decorated with nano/microstructures (in the case in point nanorods or microrods with controlled morphological properties) of ZnO or else of ZnO doped with metals. The graphene nanoplatelets according to the present invention constitute the growth substrate for the ZnO nano/microstructures, consequently enabling maximization of the effect of interaction between nanostructured ZnO and graphene. Growth of the ZnO nanostructures takes place in aqueous solution, in which the graphene nanoplatelets are dispersed in suspension. Appropriate definition of the process conditions, and in particular thedeposition of a seed layer on the surface of the GNPs and use of growth techniques, involving continuous mixing of the suspension, enable control of the morphology of the nanostructures and a high and uniform coating density of the surface of the GNPs to be obtained.

Various studies, both in the scientific literature and in the patent literature, have dealt with the production of hybrid composites generally constituted by graphene oxide (GO) and ZnO nano/microstructures [10, 12] . The majority of these studies, however, use graphene oxide as the starting material [7, 10], which is well known for not being is a good electrical conductor. Consequently, in these studies there is always involved a processing step dedicated to reduction of the GO-ZnO hybrid material obtained, above all in applications in which - as described previously - the aim is to exploit the effect of enhancement of the electrical/electronic properties of the final material resulting from the effect of interaction between ZnO nanostructures and graphene, as, for example, in capacitors, light detectors, and sensors.

Specifically, Junwei Ding et al. [10] have grown mixed zinc-oxide structures, constituted by nanoparticles and microspindles, via hydrothermal synthesis in aqueous solution on reduced graphene oxide and without the use of a seed layer. The GO was obtained via a modified Hummer's method and subsequently reduced via glucose and ammonia. ZnO microspindles present, however, a coating density of the rGO flakes lower than the one that can be obtained according to the present patent.

In another study, Chien-Te Hsieh et al. [11] report the growth of ZnO nanocrystals via microwave heating . Under high shear stress mixing, the GO obtained via modified Hummer's method is coated with the ZnO nanocrystals produced . Also in this case no seed layer was used.

The few studies that use, instead, non-oxidized graphene as the growth substrate for ZnO nanostructures, in any case start from GO and implement, before growth of the nanostructures, a step of reduction of the GO to rGO, which frequently involves the use of toxic and dangerous reagents.

In any case, in the aforementioned studies, the quality of the grown nanostructures is poor. The typical morphology that is noted is that of nanoparticles or nanorods with very low aspect ratio, with rather uneven characteristics, and a rather low density of coating of the surface of the graphene. Moreover, the possibility of growing ZnO nanostructures doped with metals is not present.

Other studies show the growth of ZnO nanostructures on supported graphene sheets, grown by means of either chemical- vapour deposition (CVD) techniques[l l], or obtained by mechanical exfoliation of graphite [12] . Ren-Jel Chung et al. [11] synthesized ZnO nanorods using the hydrothermal technique on supported graphene, grown via CVD and using a 100-nm seed layer obtained via evaporation activated by an electron beam. The process used, in addition to being costly, is not easily scalable. Yong-Jin Kim et al. [12] report the growth of ZnO microrods on multilayer graphene obtained by mechanical exfoliation and without the use of a seed layer. The structures grown have diameters of 230 to 800 nm. The process used is laborious, costly, and not easily scalable.

It is to be noted, however, how none of the aforementioned techniques are suitable for mass production of ZnO-decorated non-supported graphene flakes, to be used for the production of new materials and multifunctional devices.

Also the patent literature, which will be briefly analysed hereinafter, does not provide innovative procedures that are such as to enable a high-density growth of ZnO nano/microstructures on non-supported GNPs that will be uniform over the entire surface of the GNPs and will be economically viable and suited for mass production.

The patent KR20140037518 (A) "ZnO nanostructure including graphene" regards deposition of a ZnO nanostructured layer on a graphene sheet grown by CVD and supported by a substrate, for applications in devices used for photodetection. This patent, KR20140037518, involves the use of a support for the growth of graphene, presents high costs, is not easily scalable, and enables growth of the ZnO nanostructures on just one of the faces of the graphene plane.

The patent CN 102157315 (A) "Emitting cathode based on composite material of graphene/zinc oxide nanowire and preparation of same" regards the production of an electron field- emission cathode for applications in FEDs (Field-Emission Displays), constituted by a conductive electrode coated with graphene and overlaid by an array of ZnO nanowires. The conductive electrode (usually constituted by a glass coated via PVD with a metal layer), is coated with a graphene layer, starting from a previously sonicated aqueous suspension, using various techniques, such as spraying or electrophoresis. Then, the ZnO nanowires can be grown via hydrothermal technique or CVD or PVD, preferably CVD. As in the previous case, also the patent CN102157315 involves the use of a growth substrate and the use of costly apparatuses. The nanostructures are moreover grown on just one face of the graphene plane.

The patent US 20130099196 Al "Semiconductor-Graphene Hybrids Formed Using Solution Growth" regards the production of a composite constituted by graphene and ZnO nano-microrods by carrying out growth in aqueous solution (without the use of a seed layer) of ZnO on graphene grown by CVD and supported on a PMMA layer, to obtain a UV detector. An innovative aspect of this patent is the absence of the seed layer and the possibility of carrying out the growth with a graphene substrate placed face down, on the free surface of the reaction beaker, or face up on the bottom of the beaker, and supported in this case by a Si or glass substrate.

This U.S. patent, however, does not enable growth of ZnO micro/nanostructures on both faces of the graphene sheets and involves the use of a substrate and of costly apparatuses.

The patent CN 103734188 (A) "Preparation method and applications of zinc oxide-graphene oxide composite nanomaterial" regards the production of a graphene oxide-zinc oxide composite, starting from a suspension of GO in an alcohol, and its use as antibacterial agent, in particular as antiseptic agent for biomedical instrumentation and equipment. This patent regards the production of a composite of GO and ZnO nanoparticles, for the production of which it is necessary to obtain first GO, applying a modified version of the well-known Hummer's method, and hence with the use of dangerous reagents.

The patent CN 104007236 (A) "Preparation method and application of graphene/zinc oxide nanocomposite material" regards the production of a hybrid nanocomposite based on graphene/ZnO, obtained in aqueous suspension using ultrasounds. The patent involves ultrasound dissolution of Zn nitrate in aqueous solution, followed by the addition of GO and reduction in situ, once again by means of ultrasounds, via use of hydrazine. Also the patent CN104007236 involves the use of aggressive reagents both for the step of production of GO and for its subsequent reduction. Moreover, the various steps of production comprise a large number of processes, which render implementation thereof long and technically demanding . The patent CN 103435033 (A) "Simple and convenient method for preparing graphene-zinc oxide nanorod composite material in ultrasonic wave" regards the production of a hybrid graphene-ZnO nanorod composite starting from sonication of finely ground metallic-Zn powder in the presence of expanded graphite in suspension in an aqueous ethanol solution.

Production of the hybrid graphene-ZnO nanorod composite is obtained via a long ultrasound process lasting 6-10 h.

The patent CN 102580716 (A) "Method for synthesizing zinc oxide/graphene composite by solvothermal method" reports a method for producing hybrid composites constituted by ZnO- decorated GO with photocatalytic properties. The method involves a solvothermal synthesis at temperatures below 200°C (and simultaneous partial reduction of GO), carried out on a mixture of GO and zinc acetylacetonate in appropriate solvent (generally ethylene glycol or ethanol). This patent, CN102580716, involves the use of strong acids and oxidizing agents for the production of GO through a modified version of the known Hummer's method.

The patent CN 101857222 (A) "Preparation method of large-area and continuous graphene/zinc oxide composite structure" regards the preparation of a composite formed by G/GO and ZnO nanostructures starting from an aqueous suspension or a suspension in organic solvents of graphene or GO. After suspending the G/GO into the solvent via sonication, the ZnO nanostructures (containing various geometries that include nanodots and nanowires/nanorods) are produced by means of hydrothermal growth starting from different reagents according to the structures to be obtained (zinc nitrate and HMTA or zinc acetate). Reduction is then carried out with hydrazine. Also this patent involves the use of aggressive reagents both for the step of production of GO and for its subsequent reduction. The different steps of production further comprise a large number of processes, which render implementation thereof long and problematical.

The patent CN 10342614 (A) "Preparation method of graphene-ZnO nanoparticle composite material" regards the preparation of a GO-ZnO nanoparticle composite starting from simple Zn salts (nitrate, acetate, sulphate, chloride, etc.). The product is obtained by simple mixing of the salts in aqueous GO solution and subsequent thermal treatment between 150°C and 1000°C (particle size of 10 nm). The patent CN103482614 uses GO as precursor for the preparation of the GO-ZnO nanoparticle composite. The process employed moreover involves the use of high temperatures and long reaction times that may even reach 10 h.

There is consequently markedly felt the need for a process for the growth of nano/microstructures of ZnO (possibly doped with metals) on non-supported GNPs, dispersed in aqueous suspension, that will coat the entire surface of the GNPs (on both sides of the flake) and that will enable mass production, where the morphological properties of the ZnO nanostructures and the coating density of the surface of the GNPs can be controlled throughout the growth process.

The experiments conducted by the present applicants have revealed the possibility of growing micro/nanostructures of ZnO (possibly doped with metals) on both sides of non-supported GNPs and in aqueous suspension, with a homogeneous and high- density coating, without resorting to any step of reduction that involves the use of dangerous reagents. There has moreover been highlighted the correlation existing between the process conditions used for growth and the morphology, density, and homogeneity of the ZNO nanorods and microrods grown on nonsupported GNPs.

An innovative procedure has thus been developed, suitable for mass production, in which the ZnO-GNP hybrid nano/microstructures are produced in the presence of a seed layer that favours nucleation of the ZnO structures and leads to a uniform and high-density coating of the GNPs on both faces. Moreover, the deposition of the seed layer is carried out by keeping the GNP suspension under stirring using various techniques as described in detail hereinafter, for control of the density and uniformity of the coating of the graphene flakes, thus enabling control of the final properties of the material. Another innovative characteristic of the present invention regards the possibility of doping the ZnO micro/nanostructures, grown on GNPs, with metals. This characteristic is not described in the literature. The method developed comprises the following steps:

• Step 1 : production of aqueous suspension of graphene nanoplatelets (GNPs);

· Step 2: deposition of a seed layer on non-supported

GNPs in aqueous suspension;

• Step 3 : growth of ZnO nanorods/microrods on nonsupported GNPs in aqueous suspension.

The innovative aspects that characterize the present invention regard Steps 2 and 3 of the process of growth of the ZnO nano/microstructures, as outlined below.

In Step 2, during initial deposition of a seed layer (SL) on GNPs in suspension, the uniformity and size of the nanoparticles that constitute the SL are controlled via :

the use of a mixing technique (of a magnetic or mechanical type) or else of probe sonication; and the temperature of the subsequent step of heating in an oven.

In Step 3, for the growth of micro/nanorods of ZnO (possibly doped with metals) with desired morphology, three different growth techniques (static HT growth, dynamic HT growth, and growth by PS) are, instead, proposed in order to be able to control the morphological characteristics of the structures produced (i.e., diameter and length). In particular, the following techniques are proposed :

i) hydrothermal growth under static conditions for the growth of ZnO microrods (usually characterized by a diameter of the rods in the 100 nm÷300 nm range, and a length of up to 1-2 pm);

ii) hydrothermal growth under dynamic conditions (i.e. continuous stirring of the suspension) to obtain ZnO nanorods with high aspect ratio (usually characterized by a diameter of the rods of approximately 20-40 nm and a length of up to 500 -800 nm);

Hi) growth via probe sonication for the production of ZnO nanorods with reduced aspect ratio (generally characterized by a diameter of between 20 and 40 nm and a length generally not greater than 300 nm). Further characteristics and advantages of the present invention will emerge clearly from the ensuing detailed description, which illustrates in succession the three steps of the production process.

Step 1 - Production of aqueous suspension of GNPs

The GNPs are produced using the thermochemical exfoliation technique starting from intercalated graphite compounds according to the prior art [13, 14, 15] . However, the methods of deposition of the seed layer and of the ZnO nano/microstructures work in a similar way on graphenes obtained from epitaxial growth, chemical vapour deposition, mechanical exfoliation, and reduced graphene oxide (rGO). The advantage of using GNPs as starting graphene material lies in the better properties of electrical conductivity of GNPs as compared to rGO [14] and in the process of production of GNPs, which is economically advantageous, suitable for mass production, and does not make use of toxic substances.

Step 2 - Deposition of seed layer on non-supported GNPs in aqueous solution

The seed layer for the growth of ZnO nano/microrods is deposited on GNPs in aqueous suspension, which is constituted by a mixture of water and isopropanol, dissolved in which is an appropriate amount of zinc acetate dihydrate (Zn(CH3COO)2-2H2O) in a concentration lying in the 1 mM

÷10 mM range [16].

The process of deposition of the seed layer consists in the substeps described below.

i) An aqueous solution of zinc acetate dihydrate (between 1 mM and 10 mM) is mixed with isopropanol via magnetic stirring for a time of between 20-60 min at a rate of between 400-600 rpm.

// The GNPs obtained in Step 1 (or GO or rGO or graphene nanoplatelets of a commercial type) are rinsed with isopropanol, and the solvent is removed by centrifugation. The GNPs are then dispersed in the aqueous solution containing zinc acetate dihydrate via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker and subjected to one of the two different treatments described hereinafter:

a) mechanical stirring or stirring with magnetic bar at a rate of between 100 rpm and 500 rpm, for a time of between 10 min and 60 min;

b) probe sonication for a time of between 5 min and 30 min, fixing the amplitude of oscillation of the probe between 20% and 80%;

Hi) The suspension thus obtained is further centrifuged for removal of the growth solution of the seed layer;

iv) The sediment obtained is thermally treated in an oven at a temperature of between 200°C and 400°C, for a time of between 10 min and 60 min, to obtain GNPs coated with nanoparticles that constitute the seed layer, of which it is possible to control the dimensions and coating density (over GNPs) by choosing the technique of mechanical/magnetic stirring or else the sonication technique and by appropriately fixing the process temperature and times.

Step 3 - Growth of ZnO nanorods/microrods on nonsupported GNPs in aqueous suspension

Growth of the ZnO nanorods on non-supported GNPs dispersed in aqueous suspension is carried out using the hydrothermal method [17] at low temperature in static or dynamic conditions or else using the probe-sonication technique [18], following the substeps described below.

i) The aqueous solution for the growth of ZnO nano/microrods is prepared by dissolving equimolar amounts of zinc nitrate hexahydrate and hexamethylenetetramine (HMTA) in double-distilled (DI) water, in a concentration ranging from 2 mM to 0.5 M. For the production of metal-doped ZnO nano/microrods s, a metal nitrate hexahydrate or anhydrous metal nitrate is further added to the growth solution . The resulting solution is then magnetically stirred for a time between 20 min and 60 min at a rate in the 300 rpm÷700 rpm range at room temperature, thus obtaining a turbidity-free solution. ii) The GNPs coated with the seed layer are rinsed again with DI H ₂ 0, and the water is removed by centrifugation.

Hi) The GNPs recovered from the sediment are then homogeneously dispersed in the growth aqueous solution via vigorous stirring in a centrifuge tube.

iv) This is followed by the growth of ZnO nano/microrods by means of one of the techniques described hereinafter:

a. Hydrothermal growth in static conditions: the suspension of GNPs in the aqueous growth solution (transferred into a glass beaker) is put into an oven preheated at a temperature between 70°C and 150°C for a time ranging between 1 h and 10 h.

b. Hydrothermal growth in dynamic conditions: the suspension of GNPs in the aqueous growth solution (transferred into a glass beaker) is put on a hot plate heated at a temperature between 40°C and 120°C and mixed by a mechanical (or magnetic) stirrer for a time ranging between 1 h and 10 h. During the reaction the temperature of the solution is kept constant between 30° and 80°C.

c. Probe sonication: the suspension of GNPs in the aqueous growth solution (transferred into a glass beaker) is subjected to probe sonication for a timef between 5 min and 60 min, fixing the amplitude of oscillation of the probe between 20% and 100% (of its maximum value).

v) Once the growth step is completed, the suspension is centrifuged to remove the growth solution and washed twice with double-distilled water.

vi) The precipitate obtained is dried in an oven at a temperature between 70°C and 180°C for a time between 10 and 30 min. The end product is constituted by GNPs coated with ZnO nano/microrods.

Further features of the present invention are illustrated via the examples provided hereinafter, which illustrate, in a purely explanatory and non-limiting way, various steps of the process, with reference to the attached artworks, in which :

Figures la and lb show SEM images at two different magnifications of GNPs coated by seed layer obtained through magnetic stirring;

Figures 2a and 2b show SEM images at two different magnifications of GNPs coated by seed layer obtained through probe sonication;

Figures 3a and 3b show SEM images at two different magnifications of GNPs coated with ZnO micro/nanorods obtained by producing the seed layer via mechanical stirring, followed by hydrothermal growth under static condition;

Figures 4a and 4b show SEM images at two different magnifications of GNPs coated with ZnO nanorods obtained by producing the seed layer by means of mechanical stirring, followed by hydrothermal growth under dynamic conditions;

Figures 5a and 5b show SEM images at two different magnifications of GNPs coated with ZnO nanorods obtained by producing the seed layer by means of probe sonication, followed by hydrothermal growth under dynamic conditions;

Figures 6a and 6b show SEM images at two different magnifications of GNPs coated with ZnO nanorods obtained, by producing the seed layer by means of probe sonication, followed by growth at room temperature via probe sonication; and

Figures 7a and 7b show SEM images of ZnO microrods doped with magnesium at two different magnifications.

Examples

Example 1 - Preparation of the GNPs The GNPs are produced using a graphite intercalated compound (GIC) as precursor, according to a method similar to the one reported in [13-15]. In brief, the GICs are thermally expanded in a muffle furnace at 1150°C for 5 s. An amount of 20 mg of expanded graphite is then immersed in ethanol (instead of acetone, or an acetone-DMF mixture or an acetone-NMP mixture as described in [13-15]) and exfoliated in liquid phase via probe sonication in pulsed regime for a total time between 15 min and 30 min, fixing the amplitude of oscillation of the probe at 70% and controlling the temperature of the suspension at 15°C, through the use of a recirculation bath connected to a thermocryostat. The sonication process produces a colloidal suspension of GNPs with lateral size of 1 pm to 5 pm and a thickness of 1 nm to 20 nm. The solvent is then removed by centrifugation.

Example 2 - Deposition of the seed layer on GNPs via magnetic stirring

The solution for the deposition is prepared by dissolving zinc acetate dihydrate (with a concentration between 0.001 M and 0.010 M) in isopropanol via magnetic stirring for 20÷60 min at a rate of 400-600 rpm. Prior to deposition of the seed layer, the GNPs obtained in the previous step are rinsed with isopropanol, and the solvent is removed by centrifugation. The GNPs are then dispersed in the solution for formation of the seed layer via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker and magnetically stirred at 250 rpm for 30 min to obtain a uniform coating of GNPs by the seed layer. The suspension is then further centrifugated (at 3095 g for 30 min) to remove the seed-layer growth solution. The sediment obtained is then thermally treated in a muffle furnace at a temperature between 200°C and 400°C for a time between 10 min and 60 min, to obtain GNPs coated with ZnO nanoparticles, which constitute the seed layer.

Figure 1 shows the SEM images at different magnifications of GNPs with seed layer obtained by means of magnetic stirring. The nanoparticles have a size usually between 10 nm and 30 nm.

Example 3 - Deposition of the seed layer on GNPs via probe sonication

The solution for the deposition of the seed layer is obtained as described in Example 2. The suspension of GNPs is prepared as described in Example 2. The suspension is then transferred into a glass beaker and sonicated by means of a sonication probe for a time between 5 min and 30 min, fixing the amplitude of oscillation of the probe between 20% and 80% of the maximum value. The suspension is then further centrifugated (at 3095 g for 30 min) to remove the seed-layer growth solution. The sediment obtained is then thermally treated in a muffle furnace at a temperature between 200°C and 400°C for a time between 10 min and 60 min to obtain GNPs coated with ZnO nanoparticles, which constitute the seed layer.

Figure 2 shows images obtained under the scanning electron microscope of GNPs coated with seed layer obtained by means of probe sonication. As compared to Example 2, it may be noted that the nanoparticles that provide the seed layer are of a size generally smaller than 10÷20 nm and uniformly coat the surface of the GNPs. The coating density of the surface of the GNPs is higher in the case of sonication as compared to the case of magnetic stirring.

Example 4 - Growth of ZnO nanorods on GNPs, from a seed layer produced via mechanical stirring, using a hydrothermal method under static conditions ZnO micro/nanorods are grown on GNPs previously coated with seed layer produced by mechanical stirring, as described in Example 2. The aqueous growth solution is prepared as described on page 15, Phase 3, step / ^' ). The resulting solution is then mechanically mixed via magnetic stiring at room temperature to obtain a turbidity-free solution. Prior to the growth, the GNPs coated with the seed layer are rinsed again with double-distilled water, and the water is removed by centrifugation. The GNPs recovered from the sediment are then homogeneously dispersed in the growth solution via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker and put in an appropriately preheated oven between 70°C and 150°C for a time between 1 h and 10 h (hydrothermal technique under static conditions). Once the growth step is completed, the suspension is centrifugated (at 3095 g for 30 min) for removal of the growth solution and washed twice with double-distilled water. The precipitate obtained is dried in an oven at a temperature between 70°C and 180°C for a time between 10 min and 60 min. The end product is constituted by GNPs coated by ZnO nano/microrods. As may be noted from SEM micrographs shown in Figure 3, they have a diameter between approximately 40 nm and 150 nm and a length in the 500 nm ÷2 pm range.

Example 5 - Growth of ZnO nanorods on GNPs from a seed layer produced via mechanical stirring, using the hydrothermal methodunder dynamic conditions

ZnO nanorods are grown on GNPs previously coated with a seed layer produced by means of mechanical stirring, as described in Example 2. The growth aqueous solution is prepared as described on page 15, Phase 3, Step / ^' ). The resulting solution is then mixed mechanically using a mechanical stirrer at room temperature to obtain a turbidity-free solution. Prior to the growth, GNPs coated by the seed layer are rinsed again with double-distilled water, and the water is removed by centrifugation. The GNPs recovered from the sediment are then homogeneously dispersed in the growth solution via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker, positioned on a hot plate heated between 40°C and 100°C and mixed continuously by means of a magnetic stirrer for a time between 1 h and 10 h. During the reaction, the solution is kept at constant temperature. Once the growth step is completed, the suspension is centrifugated (at 3095 g for 30 min) to remove the growth solution and washed twice with double-distilled water. The precipitate obtained is dried in oven at a temperature between 70°C and 180°C for a time between 10 min and 60 min. The end product is constituted by GNPs coated with ZnO nanorods. As may be noted from the SEM micrographs shown in Figure 4, they have a diameter ranging between approximately 20 nm and 30 nm and a length of between approximately 400 nm and 600 nm.

Example 6 - Growth of ZnO nanorods on GNPs from a seed layer produced by probe sonication, using the hydrothermal method under dynamic conditions

ZnO nanorods are grown on GNPs previously coated with a seed layer produced by probe sonication, as described in Example 3. The aqueous growth solution is prepared as described on page 15, Phase 3, Step / ^' . The resulting solution is then stirred via magnetic stirring at room temperature to obtain a turbidity-free solution. Prior to the growth, GNPs coated with the seed layer are rinsed again with double-distilled water, and the water is removed by centrifugation. The GNPs recovered from the sediment are then homogeneously dispersed in the growth solution via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker, positioned on a hot plate heated between 40°C and 100°C and mixed continuously by means of a magnetic stirrer for a time between 1 h and 10 h. During the reaction the solution is kept at constant temperature. Once the growth step is completed, the suspension is centrifugated (at 3095 g for 30 min) to remove the growth solution, and washed twice with double-distilled water. The precipitate obtained is dried in an oven at a temperature between 70°C and 180°C for a time between 10 min and 60 min. The end product is constituted by GNPs coated with ZnO nanorods. As may be noted from the micrographs represented in Figure 5, they have a diameter ranging between approximately 40 nm and 70 nm and a length between approximately 300 nm and 400 nm. The coating density of the surface of the GNPs is very high, and the distribution of the nanostructures very uniform.

Example 7 - Growth of ZnO nanorods on GNPs, from a seed layer produced by probe sonication, using probe sonication

ZnO nanorods are grown on GNPs previously coated with a seed layer produced by probe sonication, as described in Example 3. The growth aqueous solution is prepared as described on page 15, Phase 3, Step / ^' ). The resulting solution is then magnetically stirred to obtain a turbidity-free solution. Prior to the growth, GNPs coated with the seed layer are rinsed again with double-distilled water, and the water is removed by centrifugation. The GNPs recovered from the sediment are then homogeneously dispersed in the growth solution via vigorous stirring in a centrifuge tube. The suspension is then transferred into a glass beaker and subjected to probe sonication for a time between 5 min and 60 min at room temperature, fixing the amplitude of oscillation of the probe between 20% and 100% (of its maximum value). Once the growth step is completed, the suspension is centrifugated (at 3095 g for 30 min) to remove the growth solution and washed twice with double-distilled water. The precipitate obtained is dried in an oven between 70°C and 180°C for a time between 10 min and 60 min. The end product is constituted by GNPs coated with ZnO nanorods. As may be noted from the micrographs represented in Figure 6, they present a state of flower-like aggregation shape. The diameter of the nanostructures ranges between approximately 20 nm and 40 nm, and the length is between approximately 150 nm and 300 nm. The coating density is lower than in the case reported in Example 6 as a result of probe sonication during the growth step of the nanostructures.

Example 8 - Growth of ZnO nanorods doped with magnesium on GNP

Microrods of ZnO doped with magnesium are grown according to the procedure described on page 15 (Phase 3), adding magnesium nitrate hexahydrate, in the preparation step of the micro/nanostructure-growth solution,.

As shown in Figure 7, the structures thus obtained present a perfectly hexagonal cross section, with a diameter up to 500 nm and a length up to 2÷3 pm.

Conclusions

From the foregoing description and the examples provided above the innovative aspects and the advantages of the present invention emerge clearly.

• The growth of the ZnO nanostructures takes place on graphene nanoplatelets (GNPs) and not only on reduced- graphene-oxide (rGO) nanoplatelets as reported in the literature. This represents a considerable advantage in so far as the production of GNPs does not involve the use of toxic and dangerous reagents, which are, instead, necessary for the production of rGO.

· Decoration of the GNPs with ZnO nanorods takes place using a hydrothermal technique, and not the CVD technique, as reported in the literature. This renders the process economically advantageous and easily scalable for mass production.

· The growth of the ZnO nanostructures takes place on both faces of the GNPs, which, for this reason, are suspended in aqueous solution. Instead, in the studies present in the literature, the decoration of the graphene with ZnO nanostructures generally takes place on graphene sheets or nanoplatelets placed on substrates and hence only on the free surface of the graphene. Moreover, the growth in aqueous suspension makes it possible to obtain GNP powders decorated with ZnO nanorods by means of a simple drying process. These powders can then be used as fillers in matrices of different nature to obtain multifunctional materials.

· Control of the size of the ZnO nanostructures and of their coating density on the GNPs is obtained by appropriate definition of the process of deposition of the seed layer and the creation of a system for the hydrothermal growth under dynamic conditions. The modality of mixing of the suspension of growth enables control of the morphologies of the structures. This aspect is not found in the existing literature, including the patent literature, and represents a substantial improvement introduced by the present invention as compared to existing techniques.

Main areas of application

The sector of interest of the present invention is that of nanostructured and nanocomposite materials with enhanced electrical, electronic, electromagnetic, mechanical, and catalytic properties. Possible subjects interested in the invention are firms that operate in the sector of advanced materials and composite materials and piezoresistive and piezoelectric materials, in the sensors field, and in the production of water-based paints for providing radar-absorbent thin coatings or coatings with sensing properties.

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