# Graphite decoration with copper nanodots: Influence of graphite pre-treatment and copper particle synthesis route

Charlotte Metral, Amélie Veillère, Catherine Debiemme-Chouvy, Angélique Wilson, Christine Labrugère-Sarroste, Jean-François Silvain

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Charlotte Metral, Amélie Veillère, Catherine Debiemme-Chouvy, Angélique Wilson, Christine Labrugère-Sarroste, et al. Graphite decoration with copper nanodots: Influence of graphite pre-treatment and copper particle synthesis route. *Surfaces and Interfaces*, 2026, 86, pp.108702. 10.1016/j.surfin.2026.108702. hal-05506554

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https://hal.sorbonne-universite.fr/hal-05506554v1

Submitted on 12 Feb 2026

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# Graphite decoration with copper nanodots: Influence of graphite pre-treatment and copper particle synthesis route

Charlotte Metral<sup>a,b,*</sup>, Amélie Veillère<sup>a</sup>, Catherine Debiemme-Chouvy<sup>c,*</sup>, Angélique Wilson<sup>b,D</sup>, Christine Labrugère-Sarroste<sup>d</sup>, Jean-François Silvain<sup>a,D</sup>

## ARTICLE INFO

### Keywords:

- Graphite

- Cu nanodots

- Interface

- Nucleation and growth

- Surface treatment

## ABSTRACT

Metal matrix composites have proven to be promising materials in which carbonaceous materials, such as graphite, carbon fiber, diamond, graphene, or carbon nanotube, have been introduced as reinforcements due to their intrinsic properties. The dispersion of the carbon materials, more specifically for nanoscale ones, still remains a challenge to this day. The functionalization of carbon reinforcements is a way to help their dispersion in a metal matrix. In this work, graphite decorated with copper (Cu) nanodots is fabricated through a liquid or solid route. The surface of graphite was pre-treated with nitric acid, phosphoric acid or an ester phosphate (CP213, dispersing agent) prior to nanodot deposition. Two fabrication routes were investigated: the solid route based on thermal treatments of dendritic copper particles and graphite under controlled atmosphere (air then hydrogen), and the liquid route based on Cu(II) precipitation in solution in the presence of graphite. These synthesis techniques enable the control of the interface through the growth of Cu nanoparticles that are chemically bonded to the graphite. Depending on the graphite surface pre-treatment and on the technique used, the size and the surface density of the Cu nanodots vary. Via the solid route, the size of the nanodots is between 20 and 300 nm whereas the liquid route leads to Cu nanodots with a size distribution between 50 and 150 nm.

## 1. Introduction

Metal matrix composites are used in various fields due to their thermal, electrical and mechanical properties. Constituted of a matrix (metal or alloy) (M) and reinforcements (R) [1], they are chosen according to target properties. In the power electronic field, thermal and electrical properties are important for heat dissipation and electricity conduction [2–4]. Copper (Cu) matrix with diamond (D) reinforcements have been reported for such properties [5]. The incorporation of D particles inside a metal matrix allows a high thermal conductivity, making it a composite material of choice for thermal applications [6]. In the case of power distribution, electric properties are most regarded. In breakers and contactors, silver (Ag)-based composites are used; the reinforcements being graphite (Gr) flakes or tungsten carbide (WC) particles [7]. In composite materials, the microstructures and the properties of the matrix-reinforcement (M-R) interface are critical to allow proper transfer of properties between both materials. They can be separated in

two categories: reactive system and non-reactive system [8]. Most of the time, metal matrix and reinforcements are non-reactive, i.e., there is no chemical reactivity between M and R. However, when the matrix and the reinforcements have no chemical affinities, it leads to a pure mechanical interfacial property transfer, which can degrade during operation. The absence of chemical reactivity exists with systems such as Ag/C or Cu/C. In this case, several routes can be used to create a chemical bond between M and R to allow the properties transfer. These routes include the addition of a carburigen element [9,10] or the creation of metallic nanoparticles on functionalized reinforcement surface to optimize the interface properties [5,11–13].

For non-reactive systems, reinforcements decorated with Cu nanodots are used to increase the physical and mechanical properties of metal matrix composite (MMC) materials. In this study, the objective being the fabrication of Ag/Gr MMC, the decoration of Gr with Cu metallic nanoparticles has been chosen due to the fact that Cu and Ag form a solid solution [14]. In the case of carbon reinforcements such as graphite (Gr),

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026, Pessac, France</footnote>

<footnote><sup>b</sup> Metalor Technologies Electrotechnics (France) SAS, Courville-sur-Eure, France</footnote>

<footnote><sup>c</sup> Sorbonne Université, CNRS, Laboratoire Interfaces et Systèmes Electrochimiques, UMR 8235, Paris, France</footnote>

<footnote><sup>d</sup> CNRS, Univ. Bordeaux, Placamat, UAR 3626, Pessac, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: charlotte.metral@icmbc.cnrs.fr (C. Metral), catherine.debiemme-chouvy@sorbonne-universite.fr (C. Debiemme-Chouvy).</footnote>
carbon nanotubes (CNT) or graphene, surface property plays a crucial role in the chemical reaction between the C substrate and the Cu metallic particles. The use of nanometric reinforcements such as CNT or gra- phene leads to aggregation issues or inhomogeneous repartition of the reinforcements using powder metallurgy processes, due to size differ- ence between the matrix powder (micrometric) and the reinforcement powder (nanometric). The positive impact on the dispersion of the re- inforcements in a solvent leads to more homogeneous composites, limiting aggregation [15]. Then, the functionalization of R can allow the chemical bond between the matrix and the reinforcements. Two types of functionalization exist for those carbon materials: non-covalent and covalent functionalization [16]. Non-covalent functionalization through π-interactions can offer the possibility of attaching functional groups to graphene or CNT without disturbing the electronic network [17,18]. Adsorption of organics such as surfactant molecules or polymers on carbon reinforcements is used to modify their surface [19–21]. In this case, covalent functionalization is a better option to create a chemical bond between the metal matrix and the carbon reinforcements. Covalent functionalization is based on the addition of chemical groups which induces a hybridization change of the C atoms from sp² to sp³. Oxygen, nitrogen or fluorochemical groups can be used for this purpose [16,22]. Oxygen functions through carbon oxidation by an acid such as nitric acid (HNO₃) or phosphoric acid (H₃PO₄) is a common route for graphene or CNT reinforcements [23–27].

We report in this article the influence of graphite (Gr) surface treatment with HNO₃ or H₃PO₄ or a dispersant (CP213, phosphate ester) on the growth of Cu nanoparticles on graphite flakes. The nucleation and growth of Cu nanoparticles on Gr substrate is obtained via two different routes: liquid and solid. The liquid route consists in Cu(II) precipitation onto the Gr flakes using a Cu(II) salt followed by a reduction step [11]. The solid route can be summarized as Cu deposition using metallic Cu particles and heat treatments under controlled atmosphere (air then hydrogen) [28].

## 2. Experimental

In the first step, graphite flakes undergo a pre-treatment with acids or dispersing agent in order to functionalize their surface. Then, two different routes are used to decorate the graphite with Cu nanoparticles: a liquid and a solid route. A schematic diagram of the process is pre- sented in Fig. 1. Finally, the chemistry and the microstructure of treated and non-treated decorated graphite flakes are studied.

### 2.1. Materials

Gr was purchased from AMG Graphite. Copper nitrate (Cu(NO₃)₂) and perchloric acid (HClO₄) were obtained from Acros organics. Sodium hydroxide (NaOH) and orthophosphoric acid (13.8 M) were purchased from Fischer Chemicals. Cu oxide (CuO) was purchased from Sigma Aldrich. Dendritic Cu powder (Cu₄) was purchased from Eckart Granules Poudmet. CP213, a phosphate ester (dispersant agent) was purchased from Cerampilot. Nitric acid (14.5 M) was obtained from Scharlau.

### 2.2. Functionalization method

The functionalization of Gr was studied using HNO₃, H₃PO₄ and CP213. For the treatment with acids [25], 1 g of Gr sheets was added to a round-bottom flask with 100 mL of acid (14.5 M HNO₃ or 13.8 M H₃PO₄) and heated in a reflux set-up at 100 °C for 30 min. Then, the Gr flakes were collected by centrifugation of the suspension and rinsed three times with pure water, followed by three additional times with ethanol (CH₃CH₂OH). Finally, they were dried in an oven at 70 °C for 4 hours.

For the Gr functionalization with CP213, the method differs depending on the chosen route. For the solid route, dendritic Cu pow- ders and Gr flakes (40 vol. %) were first mixed with a planetary mixer (1200 rpm, 5 min). Then, a mixture of ethanol and 2-butanone (CH₃COCH₂CH₃) (40/60 vol. %) was prepared [13] and the dispersant CP213 was added to this solution (0.5 wt. % on the dry powder basis). Then, Gr + Cu powders were added and mixed for 4 h through a 3D mixer (200 rpm) before to be dried at 70 °C for 4 h. Finally, the powder was submitted to a 2 h heat treatment at 400 °C under air. For the liquid route, the process was similar: instead of using Cu powder + Gr flakes, only Gr flakes were used.

### 2.3. Decoration of gr with Cu nanodots

For the solid and the liquid routes, the Gr flakes have undergone a chemical surface pre-treatment (HNO₃ or H₃PO₄ or phosphate ester (CP213)).

#### 2.3.1. Solid route

This section is inspired by the work of Guillemet et al. [13] who used the process with Cu and D particles, and the work of Silvain et al. [29] who worked with Cu, carbon fibers and silicon wafers. First, Gr flakes and Cu dendritic particles (50 µm) were mixed for 4 h at 200 rpm in a 3D mixer, under air. Then, this mixture was heated at 400 °C for 2 h under air, leading to the oxidation of the Cu and the growth of nano-sized wire-shaped CuO on their surface. Finally, a second thermal treatment under Ar-H₂ (5 vol. %) for 1 h leads to the sublimation of the CuO nanowires and the nucleation and growth of Cu nanoparticles on the surface of the Gr flakes. This route is called Cu₄.

#### 2.3.2. Liquid route

This section is inspired by the work of Hwang et al. [11] and Bident et al. [30] who used the process with copper nitrate salt (Cu(NO₃)₂) to decorate graphene. Herein two different Cu(II) precursors were used leading to two denominations: CuNO₃ route for Cu(NO₃)₂ precursor and CuO route for CuO precursor. For the CuNO₃ route, 1 g of Gr was added to 100 mL of ethanol. Then, a 0.04 M alcoholic solution of Cu nitrate was added and finally a 0.5 M solution of sodium hydroxide was added dropwise to increase the solution pH and obtain Cu(II) precipitation. For the CuO route, Gr was added to 100 mL of a 0.04 M solution of Cu²⁺ obtained by the dissolution of CuO in water by adding a 11.4 M HClO₄ solution. Then, a 0.5 M solution of sodium hydroxide was added drop- wise. The obtained suspensions were centrifuged, rinsed 3 times with

<figure><img src="image_2.png" /><figcaption>Fig. 1. Schematic diagram of the process methods for Gr decoration with Cu nanodots.</figcaption></figure>
ethanol and finally dried at 70 °C for 4 h. A heat treatment under reducing atmosphere was performed in order to convert the Cu(II) precipitate into Cu nanodots. This is achieved by heating the powder at 400 °C under Ar-H₂ (5 vol. %) for 1 h.

## 2.4. Characterization

X-Ray Diffraction (XRD) patterns were obtained using an X-Ray Diffraction spectrometer (BRUKER D8 Discover) with CuKα radiation to identify the phase structures of the samples. The morphological aspect of the materials was analyzed using a Field Emission Gun Scanning Electron Microscope (FEG-SEM, Ultra55, Zeiss) and a Scanning Electron Microscope (SEM, Tescan Vega 2 SBH). An analysis of the number of Cu nanodots on the surface of the different Gr (untreated, HNO₃, H₃PO₄ and CP213) was performed using the software LAS Application Suite, on three different micrographic images per sample on a 4000 px² area (16 µm² area). The impact of the Gr pre-treatment was analyzed by X-ray photoelectron spectroscopy (XPS). XPS analyses were performed using a K-Alpha ThermoFisher Scientific instrument, with a monochromatic Al radiation source. The size of the X-ray spot was approximately 400 µm. The survey spectra were obtained in an energy range of 0–1100 eV with a pass energy of 200 eV, while the high-resolution spectra were measured using a pass energy of 40 eV, without charge compensation. Ultrasonic treatment, using with a Bandelin Sonoplus Titanteller TT13 apparatus, was applied to decorated Gr flakes in order to analyze the bounding strength between Gr surface and Cu nanodots.

# 3. Results and discussion

## 3.1. Surface analysis of the untreated and treated Gr flakes

XPS analyses have been performed to characterize the surface chemistry of the untreated Gr and the Gr treated with HNO₃, H₃PO₄ and CP213. The survey spectra are presented in Fig. 2.

The quantification of C and O (at. %) is reported in Table 1. HNO₃ treatment has no specific effect on the variation of the oxygen content in opposition with the H₃PO₄ treatment for which remains oxygen coupled to the phosphorus presence. This behavior is unexpected due to the chemical reactivity of the two acids: HNO₃ is a stronger acid than H₃PO₄. Similar results have already been found for graphene [25].

The deconvolution of C 1 s spectra of the untreated Gr, HNO₃ Gr, H₃PO₄ Gr and CP213 Gr are shown in Fig. 3. These deconvolutions are inspired by the work of Biesinger [31]. For the untreated Gr six

<figure><img src="image_3.png" /><figcaption>Fig. 2. XPS survey spectra of untreated Gr, HNO₃ Gr, H₃PO₄ Gr and CP213 Gr.</figcaption></figure>

<table><caption>**Table 1**
XPS surface concentration of elements (in at. %).</caption><thead><tr><th></th><th>C</th><th>O</th><th>N</th><th>P</th></tr></thead><tbody><tr><td>Untreated Gr</td><td>97.8</td><td>2.2</td><td>N/A</td><td>N/A</td></tr><tr><td>Gr HNO<sub>3</sub></td><td>96.8</td><td>2.9*</td><td>0.3</td><td>N/A</td></tr><tr><td></td><td></td><td>CO: 2.0</td><td></td><td></td></tr><tr><td>Gr H<sub>3</sub>PO<sub>4</sub></td><td>51.9</td><td>39.3**</td><td>N/A</td><td>8.8</td></tr><tr><td></td><td></td><td>CO: 4.1</td><td></td><td></td></tr><tr><td>Gr CP213</td><td>98.1</td><td>1.9</td><td>N/A</td><td>0.02</td></tr></tbody></table>

N/A = not available. * and **: oxygen contributions are related not only to Gr oxidation but also to the presence of nitrate (0.3 × 3 = 0.9 at. %) and phosphate (8.8 × 4 = 35.2 at. %), respectively.

components are present in the spectrum, at 284.5 eV, 285.1 eV, 286.3 eV, 287.8 eV, 289.1 eV and 291.0 eV. They can be attributed to Csp², Csp³, (C—OH, C—O-C), C = O, O—C = O bonds and π-π* due to Csp² environments, respectively.

Comparing the four Gr samples, there is no clear discrepancy between the different pre-treatments of Gr on the C 1s spectra (Fig. 3). Comparing the full width at half maximum (FWHM) of the Csp² component, H₃PO₄ leads to higher FWHM, which indicates a more oxidized surface than for the other treatments. For the π-π* bonding, there is a decrease in the area ratio between H₃PO₄ Gr and the other Gr (0.03 to 0.14 for the untreated Gr, 0.13 for HNO₃ Gr and 0.16 for CP213 Gr) due to a degradation of the graphite surface (Table 2).

For the P 2p region of H₃PO₄ and CP213 treated Gr, one component is present at 134.3 eV which is attributed to P(V) (Fig. 4). For CP213 Gr, the P 2p spectrum is similar, it was measured with a pass energy of 100 eV due to the low concentration of P (Table 1).

These observations allow us to conclude the following:

* To compare the amount of oxygen bound to carbon (graphite oxidation) after the different treatments, elements which do not correspond to CO bonds must be removed from the total oxygen contribution (Table 1). The oxygen contribution for Gr with H₃PO₄ pre-treatment (39.3 at. %) is related to the presence of phosphate (8.8 × 4 = 35.2 at. %) and CO bonding (4.1 at. %). In the case of HNO₃ pre-treatment, the oxygen contribution (2.9 at. %) is related to the presence of NO₃ (0.3 × 3 = 0.9 at. %) and CO bonding (2 at. %). Thus, the higher oxygen contribution due to the CO bonding is related to the H₃PO₄ treatment, in good agreement with the shape of the C 1s spectra (Fig. 3). This finding is unexpected since HNO₃ is a stronger acid than H₃PO₄. This can be explained with the reaction between HNO₃ and carbon [32]:

$$C + 4 \text{HNO}_3 \rightarrow \text{CO}_2\uparrow + 4 \text{NO}_2\uparrow + 2 \text{H}_2\text{O} \quad (1)$$

In the present case, the HNO₃ concentration used is very high (14 M). It induces extreme oxidation of Gr leading to CO₂ evolution (Eq. (1)). This Gr surface oxidation explains the decrease of the atomic concentration of oxygen linked to C for the sample treated with nitric acid compared to phosphoric acid (Table 1).

## 3.2. Cu decoration of Gr

Surface pre-treatment of Gr leads to the creation of functional groups on its surface (C—O, O—C = O, and P-O bonds). These groups can serve as sites for the nucleation and the growth of metal particles via oxygen bridges (C—O-Cu and/or C—O-P-Cu or C-P-O—C). After acid or CP213 pre-treatments, Gr flakes were decorated with Cu nanodots by two different routes: solid and liquid routes.

### 3.2.1. Solid route

For the Cu₉ method, the XRD patterns of Cu + Gr mixtures, obtained
<figure><img src="image_4.png" /><figcaption>Fig. 3. Fitted C 1s spectra focused on the oxidized carbon components for a) untreated Gr, b) HNO₃ Gr, c) H₃PO₄ Gr and d) CP213 Gr.</figcaption></figure>

<table><caption>**Table 2**
Proportions of carbon components versus the major Csp2 one for the different Gr pre-treatments.</caption><thead><tr><th>Energy (eV)</th><th>Untreated Gr</th><th>Gr HNO<sub>3</sub></th><th>Gr H<sub>3</sub>PO<sub>4</sub></th><th>Gr CP213</th></tr></thead><tbody><tr><td>284.5 (Csp<sup>2</sup>)</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>285.1 (Csp<sup>3</sup>)</td><td>0.09</td><td>0.07</td><td>0.06</td><td>0.11</td></tr><tr><td>286.3 (C—OH, C—O-C)</td><td>0.13</td><td>0.13</td><td>0.11</td><td>0.13</td></tr><tr><td>287.8 (C = O)</td><td>0.05</td><td>0.04</td><td>0.02</td><td>0.04</td></tr><tr><td>289.1 (O—C = O)</td><td>0.01</td><td>0.02</td><td>0.02</td><td>0.01</td></tr><tr><td>291.0 (π-π*)</td><td>0.14</td><td>0.13</td><td>0.03</td><td>0.16</td></tr></tbody></table>

before and after air treatment and Ar-H₂ (5 vol. %) treatment are shown in Fig. 5. Before thermal treatment, diffractogram (a) shows the presence of C and Cu. After thermal treatment under air, CuO, Cu₂O and C

are detected (diffractogram (b)). Finally, the Ar-H₂ (5 vol. %) treatment of the thermal air treated material shows the presence of Cu and C (diffractogram (c)). CuO has a monoclinic structure and a space group C2/c, and Cu₂O a cubic structure with a space group Pn-3m. The conversion of CuO into Cu can be described by Eqs. (2) and (3):

$$2 \text{CuO} + \text{H}_2 \rightarrow \text{H}_2\text{O} + \text{Cu}_2\text{O} \quad (2)$$

$$\text{Cu}_2\text{O} + \text{H}_2 \rightarrow 2 \text{Cu} + \text{H}_2\text{O} \quad (3)$$

Fig. 6a) and 6b) show the micrographs of dendritic Cu particles with Gr flakes before and after thermal treatment under air atmosphere, respectively. This treatment triggers the oxidation of Cu into cupric (CuO) and cuprous (Cu₂O) oxides, leading to the formation of Cu oxide nanowires on the surface of Cu dendrites (Fig. 6b). This phenomenon was already observed and discussed in the literature [13,33]. After

<figure><img src="image_5.png" /><figcaption>Fig. 4. Deconvolution of P 2p spectrum of H₃PO₄ Gr.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** XRD patterns of the Gr powders from the solid route, Gr + Cu<sub>d</sub> (a) before and (b) after thermal treatment under air, and (c) after air and Ar-H<sub>2</sub> (5 vol. %) treatment.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** SEM micrographs of Gr flakes and Cu powder (a) after the mixing step, (b) after air treatment and (c,d,e,f) after Ar-H<sub>2</sub> (5 vol. %) treatment; (g) non-functionalized Gr, (h) HNO<sub>3</sub> Gr, (i) H<sub>3</sub>PO<sub>4</sub> Gr and (j) CP213 Gr.</figcaption></figure>

Ar-H₂ (5 vol. %) treatment, Fig. 6c, 6d, 6e and 6f show the presence of Cu nanodots on Gr flakes. During the reduction step, CuO nanowires sublimated settled onto nucleation sites of the Gr surface. Those nucleation sites can be chemical sites through C—O-P-O-Cu and/or C-P-O—Cu [13,28], or structural defects such as steps or vacancies [34–36]. The size of Cu nanodots, measured from SEM micrographs, is between 20 and 300 nm. In our experimental conditions, Cu deposition is not uniform: some flakes are not covered by Cu nanodots. The Gr functionalization seems to have an impact on the number of Cu nanodots on the Gr surface: the number of nanodots is lower with HNO₃ treatment (Fig. 6d) than other treatments (Fig. 6c, 6e and 6f). The Gr surface coated with Cu is more important with CP213 treatment (Fig. 6f). Some nanodots are bigger with the untreated Gr and the CP213 treatment (Fig. 6c and 6f). Based on the morphology of the Cu nanodots, one can conclude that the Cu nanodots originate from a 3D growth process (Volmer-Welber growth model, strong adatoms-adatoms interactions [37]). Direct heating under H₂ atmosphere of a mixture of graphene oxide (GO) and copper formate has allowed Shi *et al.* to obtain reduced GO sheets decorated with Cu dots [38].

Fig. 7 presents the Cu nanodots size distribution with the different Gr pre-treatments.

The average particle size of Cu nanodots is between 20 - 60 nm, 20 - 40 nm, and 200 - 300 nm, for untreated Gr, HNO₃ Gr and H₃PO₄ Gr, respectively.

### 3.2.2. Liquid route

SEM micrographs shown in Fig. 8 confirm that Cu(II) precipitation occurs at the surface of the Gr flakes since some Cu particles are detected on the untreated Gr flakes. The shape of these particles depends on the route followed (CuNO₃ or CuO). The same results were obtained for graphite after acid or CP213 pre-treatments. To specify the nature of these particles, XRD analyses were performed.

Fig. 9 presents the XRD patterns of Gr powders before and after Ar-H₂ (5 vol. %) treatment for the CuNO₃ and the CuO methods. For CuNO₃ route, diffractogram 9a shows the presence of C and Cu₂(OH)₃NO₃ which has a monoclinic structure with a space group P21. For the CuO route, diffractogram 9b shows the presence of C and CuO. After Ar-H₂ thermal treatment of the samples obtained following both routes (only
<figure><img src="image_8.png" /><figcaption>Fig. 7. Histogram of Cu particle size distribution on Gr for the solid route (on a 16 µm² area).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. SEM micrographs of the untreated Gr flakes after treatment following (a) CuNO₃ route and (b) CuO route.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9. XRD patterns of the Gr powder from the liquid route, before thermal treatment for (a) CuNO₃ route, (b) CuO route and c) after Ar-H₂ (5 vol. %) thermal treatment of both routes.</figcaption></figure>

one is presented since both are identical), diffractogram 9c shows the presence C, Cu and Cu₂O.

Before thermal treatment, the presence of Cu₂(OH)₃NO₃ on the Gr flakes is due to the reaction described by Eq. (4):

$$2 \mathrm{Cu}^{2+} + \mathrm{NO}_{3}^{-} + 3 \mathrm{OH}^{-} \rightarrow \mathrm{Cu}_{2}(\mathrm{OH})_{3}\mathrm{NO}_{3} \quad (4)$$

The conversion of Cu₂(OH)₃NO₃ into Cu after Ar-H₂ (5 vol. %)

treatment can be described by Eq. (5), (6) and (7). For the CuO route, the reduction of CuO to Cu can also be described by Eqs. (6) and (7). Eq. (6) can explain the presence of Cu₂O at 2θ = 36.4° in diffractogram (c), the reduction Cu(II) is not totally complete.

$$\mathrm{Cu}_{2}(\mathrm{OH})_{3}\mathrm{NO}_{3} \rightarrow 2 \mathrm{CuO} + \mathrm{H}_{2}\mathrm{O} + \mathrm{HNO}_{3} \quad (5)$$
$$2 \mathrm{CuO} + \mathrm{H}_2 \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{Cu}_2\mathrm{O} \quad (6)$$

$$\mathrm{Cu}_2\mathrm{O} + \mathrm{H}_2 \rightarrow 2 \mathrm{Cu} + \mathrm{H}_2\mathrm{O} \quad (7)$$

Fig. 10 shows the micrographs of Gr flakes decorated with Cu nanodots after Ar-H₂ (5 vol. %) treatment. EDS analyses performed on some of these samples confirm the presence of Cu (Figure S1, S2 and S3) and for one of the samples pre-treated with H₃PO₄ (CuO route) the presence of P (Figure S3). The size of the Cu nanodots, measured from SEM micrographs, varies from 50 to 150 nm. For the untreated graphite, both routes are equivalent (Fig. 10a and 10b). The Gr pre-treatment has an impact on the number of Cu nanodots and the particle size present on the Gr flakes. Smaller nanodots are present in the micrographs of Fig. 10c, 10e, 10f and 10g The covered surface is lower in Fig. 10d and 9h, and similar for the other micrographs. The Cu particles size distribution histograms are presented in Fig. 11 for the CuNO₃ route and in Fig. 12 for the CuO route.

For the CuNO₃ route, untreated Gr and H₃PO₄ Gr lead to a smaller average size of Cu particles (between 20 and 40 nm) compared to HNO₃ Gr and CP213 Gr treatments (100 and 150 nm).

For the CuO route, the average particles size is between 100 and 150 nm for all Gr treatments.

An adhesion test was performed in order to evaluate the adhesion of the Cu nanodots onto the Gr flakes, which should be related with chemical bond between the Cu nanodots and the Gr surface. For this adhesion test, Gr powders covered with Cu nanodots obtained from the different routes (untreated and treated Gr) were introduced in an ethanol solution and placed under an ultrasonic probe for 1 min. Bident et al [30] have shown that when Cu nanodots are grown on untreated graphene (therefore without chemical bonding), they detach from the

graphene during the adhesion test, whereas when chemical bonds link the Cu nanodots on graphene, the nanodots remain attached during the adhesion test. In our system, SEM observations after the adhesion test show that the Cu nanoparticles are still present on the Gr surface after this ultrasonic treatment. The results are similar for all the routes and Gr pre-treatments. Therefore, we can conclude that there is a chemical bound between Cu and Gr induced by the formation of C—O, O—C = O and P-O groups. These functional groups serve as nucleation sites through oxygen bridges (C—O-Cu and/or C—O-P-Cu or C-P-O—C). This behavior differs from graphene or CNT. Actually, a functionalization treatment is not necessary for graphite to create a chemical bound between Gr and Cu, most likely due to the fact that Gr is already partially oxidized, unlike graphene. This assumption is in good agreement with Ruiz et al [39] who have shown that chemical decoration of reduced graphene oxide (rGO) with copper dot, using CuSO₄ and NaBH₄ as reducing agent, is possible due to the remaining oxygenated groups on the surface of the rGO.

### 3.3. Impact of functionalization

LAS Suite Application was used to compare the different routes and Cu deposition methods. The average percentage covered surface on different Gr was investigated (Fig. 13) (on a 16 µm² surface). The average size of Cu nanodot is between 50 and 150 nm. The Cu nanodot deposition for the solid route is not uniform: some flakes are not recovered by Cu nanodots. Thus, the chosen micrographs are representative of a high surface decoration.

For untreated Gr flakes, both liquid routes lead to a similar Cu coverage (14 % and 13 %) and the solid route is superior (36 %). For HNO₃ pre-treatment, the Gr covered surface following the CuNO₃ liquid

<figure><img src="image_11.png" /><figcaption>Fig. 10. SEM micrographs of the Gr flakes from CuNO₃ and CuO liquid routes after Ar-H₂ (5 vol. %) treatment, (a,b) untreated Gr, (c,d) HNO₃ Gr, (e,f) H₃PO₄ Gr and (g,h) CP213 Gr, following (a,c,e,g) CuNO₃ route and (b,d,f,h) CuO route.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 11. Histogram of Cu particles size distribution on Gr for the CuNO₃ route (on a 16 µm² area).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. Histogram of Cu particles size distribution on Gr for the CuO route (on a 16 µm² area).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 13. Average surface covered with Cu on Gr flakes obtained by LAS Suite Application.</figcaption></figure>

route is higher than for the CuO route (from 14 % to 8 %) and the solid route (1 %). For H₃PO₄ pre-treatment, the CuNO₃ liquid route is better than the CuO one (11 % compared to 4 %), and the solid route is better than both liquid routes (34 %). Finally, the covered surface with CP213 treatment is higher for the solid route (37 %) compared to the liquid routes (14 % and 12 %). Concerning the pre-treatments performed on Gr, the highest Cu coverage is achieved in solid route with CP213 one. This result is contradictory to the conclusions drawn from the XPS analysis: H₃PO₄ treatment seems to have more effect on the Gr surface (with the C—OH contribution). Oxygen element can be associated to defects, due to the modification of carbon hybridization from sp² to sp³ [30]. R. Burgess *et al.* [26] report that -OH bounding is a more effective surface functional group for nucleation growth of gold particles on Gr than C = O. Silvain *et al.* [40] report that chemical reactions at the interface between polyethylene terephthalate and aluminum lead to C—O-Al groups and the formation of Al-O bounds at the interface. In our case, H₃PO₄ treated Gr leads to the largest atomic concentration of -OH bounds. The increase of surface concentration of oxygen on Gr H₃PO₄ can explain the increase of Cu particle quantity with Cu<sub>d</sub> method. The creation of phosphorous nucleation sites on the surface of Gr with H₃PO₄ or CP213 increases the number of Cu particles on the Gr surface for Cu<sub>d</sub>
method.

For the liquid route, all the Gr flakes are decorated, whereas inhomogeneous decoration can be observed for the solid route. This difference is linked to specific behavior of both methods. For the liquid route, the Gr and Cu(II) salts are homogeneously distributed in the stirred solution and during NaOH addition, Cu(II) precipitation occurs homogeneously. For the solid route, the distribution of Cu dendritic particles and Gr flakes is relatively inhomogeneous. Considering the mean free path of sublimated Cu atoms, if the Gr particles are not close to the Cu particles, the sublimated Cu atoms will not deposit on Gr flakes.

In the case of graphite, as opposed to graphene, the surface treatment does not contribute to increase the deposition and adhesion of copper nanodots. This observation suggests that the surface morphology of graphite plays a significant role in the observed phenomenon. Graphene and CNT do not exhibit sufficient surface defects. Consequently, their surface modification through functionalization is necessary.

## 4. Conclusion

In this work, two methods for copper nanodots formation on Gr substrate were studied, with an investigation on the impact of acid functionalization treatment (HNO₃ or H₃PO₄) or dispersing agent (ester phosphate, CP213) on Gr substrate. Two liquid route methods with Cu salts (Cu(NO₃)₂ and CuO) and a suspension of Gr were used, and one solid route with a mixture of copper dendritic powder and Gr followed by a thermal treatment.

This study allows us to conclude the following:

* The liquid route allows homogeneous copper deposition on Gr compared to the solid one,

* For untreated Gr, both liquid routes are equivalent, while solid route results in a higher deposition of Cu dots, less homogeneous.

* HNO₃ and H₃PO₄ treatments lead to lower decoration for liquid route, while CP213 gives nearly the same coverage as untreated Gr.

* For the solid route, HNO₃ leads to a low surface cover rate while H₃PO₄ and CP213 lead to the highest surface decoration.

* If low cover surface is the objective, priority should be given to liquid route. Alternatively, if high cover surface is the aim, solid route should be selected.

* In this study, Gr functionalization is not necessary to have Cu nanodots on its surface, contrary to graphene or CNT.

## Funding

This study was supported by Metalor and by the ANRT (Association Nationale de la Recherche et de la Technologie) with a CIFRE (Convention Industrielle de Formation par la REcherche) fellowship granted to C. Metral.

## CRediT authorship contribution statement

**Charlotte Metral:** Writing – original draft, Investigation. **Amélie Veillere:** Validation, Supervision, Conceptualization. **Catherine Debienne-Chouvy:** Writing – review & editing, Validation, Supervision, Methodology. **Angélique Wilson:** Writing – review & editing, Validation, Supervision. **Christine Labrugère-Sarroste:** Writing – review & editing, Investigation. **Jean-François Silvain:** Writing – review & editing, Validation, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Catherine Debienne-Chouvy reports financial support was provided by Metalor Technologies SA. If there are other authors, they declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors would like to acknowledge Eric Lebraud from ICMCB for his technical and scientific support for the X-ray diffraction experiments and Florence Billon from LISE for the SEM observations.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.surfin.2026.108702.

## Data availability

Data will be made available on request.

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