# Facile and Green Synthesis of Monodisperse Sub-10 nm Copper and Tin Nanoparticles Using L-ascorbic Acid as a Reducing Agent

Abdennour Benabbas¹*, Grégoire Breyton², Catherine Especel¹, Anthony Le Valant¹, Christian Ricolleau², Guillaume Wang², Tzonka Mineva³, Jaysen Nelayah², Hazar Guesmi³, Florence Epron¹*

1. CNRS, Université de Poitiers, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Poitiers, France

2. Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Cité - CNRS, Paris, France

3. Institut Charles Gerhardt de Montpellier (ICGM), Université de Montpellier, CNRS, ENSCM, Montpellier, France

*florence.epron@univ-poitiers.fr, abdennour.benabbas@univ-poitiers.fr

## Abstract

We first optimized a simple and low-cost polyol-based synthesis route for the preparation of stable and monodisperse sub-10 nm copper nanoparticles. Building on this robust approach, we extended the method to tin and succeeded in producing tin nanoparticles stabilized in the unconventional α-Sn phase, which is remarkable given the metastable character of this phase under ambient conditions. The resulting α-Sn nanoparticles exhibit excellent resistance to oxidation together with long-term colloidal stability in air, enabling further processing for potential applications. In both cases, inexpensive commercial precursors and mild conditions (80 °C, aqueous or polyol solvents, ascorbic acid as sole reducing agent, no inert atmosphere or additional stabilizers) were employed. The nanoparticles were characterized using TEM, UV-visible spectroscopy, ATR-FTIR, ICP-OES, and XPS.

(key words: copper nanoparticles, tin nanoparticles, green chemistry, soft chemistry, colloidal synthesis, non-noble metal nanomaterials, stable nano-colloids)

## 1. Introduction

Non-noble metals attract a growing attention as potential candidates for replacing the scarce and expensive precious platinum group metals (PGMs) that are already in excessive use for various industrial applications. Naturally abundant and relatively nontoxic and inexpensive metals, particularly copper and tin, enable accessing to some enhanced physicochemical properties at the nanoscale compared to bulk metals. Thus, they may be considered as promising alternatives with the potential for high impact on future sustainable processes in catalysis, energy storage, optics, electronics, photovoltaics, and magnetism.¹, ², ³, ⁴, ⁵, ⁶

Unfortunately, this objective remains difficult to achieve due to the challenges in accessing to oxidation-/corrosion-resistant zerovalent copper or tin nanoparticles in a small size range, uniform size distribution, and sufficient yields.⁷ So far, robust synthetic procedures for efficient nano-engineering of copper and tin are very few compared to the extensive and already established studies found on gold or silver for instance.⁸,⁹
The "wet chemical reduction" route, classified among bottom-up strategies, offers several advantages such as tunable nanoparticle size and morphology, moderate cost, and suitability for up-scaling, which are crucial for real large-scale practical use.¹⁰ The size control depends on nucleation and growth rates, initiated by the reduction of metal ions in liquid phase.¹¹ Initially, the synthesis efforts were developed on the basis of the studies conducted on noble metals. For instance, the well-known classic polyol process proposed by Fievet in the 1980s,¹² was successfully extended to the synthesis of various transition metals nanoparticles with controlled sizes.¹³ The reduction by polyols, such as ethylene glycol (EG) or diethylene glycol (DEG), which also act as capping agents, depends on the nature of the polyol, pH, and particularly the reaction temperature, which is typically high (>160 °C). For example, copper nanocrystals of 2 nm average diameter were obtained by Kawasaki *et al*.¹⁴ following a surfactant-free polyol method using ethylene glycol as the reducing agent activated by microwave radiation at high temperature, 185 °C, in the presence of NaOH. Further examples include the use of strong reducing agents, efficient at low temperatures such as hydrazine, sodium borohydride or sodium hypophosphite.¹⁵,¹⁶ Nevertheless, concerns quickly raised regarding the presence of impurities, chemical instability or potential hazard to human health and environment.¹⁷

In addition, special attention must be given to ensuring the chemical stability of the produced nanoparticles (NPs) against oxidation by air or water, whether during synthesis, storage or processing steps. For instance, Cu NPs synthesized following a thermal decomposition of copper acetylacetonate in oleylamine at 230 °C under N₂ atmosphere quickly become coated with a cuprous oxide (Cu₂O) layer upon exposure to air.¹⁸ Sn NPs are also naturally coated with an amorphous layer of SnO₂.¹⁹ To remedy this issue, typical, heavy surface protective agents such as polyvinylpyrrolidone (PVP) or long chain amines are employed, that are then difficult to remove.²⁰,²¹,²²,²³ This highlights the urgent need to find optimal synthetic conditions for producing purely metallic nanoparticles that are inherently resistant to oxidation and corrosion, without relying on heavy surface capping agents.

In alignment with the principles of green chemistry, biocompatible, biodegradable, and non-toxic reducing agents, such as plant, vegetable, and fruit extracts, are increasingly emerging as a promising approach in green formulations carried out under mild conditions.²⁴ Ascorbic acid (AA, C₆H₈O₆), an anti-oxidant extracted from fruits and vegetables, is an efficient reducing agent for the synthesis of metal nanoparticles.²⁵ It is a mild reducing agent, and its reactivity depends strongly on the pH (E (C₆H₈O₆) = 0.46 – 0.07 pH (V/SHE) for 1 ≤ pH≤ 13).¹⁷,²⁶ The oxidation of AA leads to the formation of dehydroascorbic acid (DHA) according to a 2 e⁻ pathway. Since AA is a diprotic acid (H₂A), it can undergo, depending on the pH, two deprotonation steps to form first ascorbate ion HA⁻ (pKa=4.2) then ascorbate dianion A²⁻ (pKa=11.8). Consequently the redox potential of the DHA/AA couple strongly decreases with the increase in pH and AA is thus considered as a weak reducing agent in non-alkaline solution.²⁷ Thus, the pH of the reaction medium has certainly an effect on the reduction rate, and consequently, the subsequent nucleation and growth process is faster in basic medium than in acidic one.¹⁷ Obviously, reactions conducted in a non-alkaline medium are carried out at high temperatures to accelerate the kinetics of the redox reaction and decrease the redox potential of ascorbic acid. Finally, the redox reaction depends also on the concentrations of O₂ and of the nature of metal species in the medium.²⁸ AA is, in general, used as a reducing agent in the presence of stabilizers such as citrate, sodium dodecyl sulphate or polyvinylpyrrolidone in aqueous solution.²⁹,³⁰ However, it can act as both a reducing agent and a capping agent. Xiong *et al*.³¹ proposed that polyhydroxyl structure of DHA, resulting from the oxidation of AA is hydrolyzed in water, and facilitates the complexation of copper nanoparticles and their stabilization by formation of hydrogen bonding. They have demonstrated that it is possible to obtain highly stable dispersion of copper nanoparticles with narrow size distribution (d < 2 nm) by reduction of copper chloride in water using L-ascorbic acid at 80 °C.³¹ The obtained nanoparticles were characterized by UV-visible spectroscopy, Fourier transform infrared (FTIR)
spectroscopy, and transmission electron microscopy (TEM). To the best of our knowledge, while the colloidal stability was proven, the oxidation state of the nanoparticles was not characterized.

In this work, we report the synthesis strategy not described before for the preparation of sub-10 nm tin nanoparticles in ethylene glycol using ascorbic acid as the sole reducing agent, starting from a classical precursor salt and without any surfactant. By optimizing the solvent conditions initially developed for stable, zero-valent Cu nanoparticles, we successfully extend the method to tin, obtaining highly stable suspensions of sub-10 nm particles with remarkable oxidation resistance. Notably, the unconventional α-Sn phase is formed instead of the commonly observed β-Sn, highlighting both mechanistic and material novelty. The obtained nanoparticles may be used in various fields including printed electronics, conductive inks, battery materials, catalysis or in optoelectronics since α-Sn is a semiconductor-like material as SnO₂.

## 2. Experimental

### 2.1. Materials

Copper(II) chloride dihydrate ($CuCl_2 \cdot 2H_2O$, 99%, Alfa Aesar), tin chloride dihydrate ($SnCl_2 \cdot 2H_2O$, 99%, Sigma), L-ascorbic acid ($C_6H_8O_6$, ≥ 99%, Sigma-Aldrich), ethylene glycol ($C_2H_6O_2$, 99%, Sigma), diethylene glycol ($C_4H_{10}O_3$, ≥ 99%, Roth) and hydrochloric acid (HCl, 35%, VWR) were purchased and used as received without further purification. The water used in the experiments was ultrapure (Resistivity ≥ 18.2 MΩ.cm).

### 2.2. Synthesis protocol

#### 2.2.1. Synthesis of copper nanoparticles

The general procedure involves the preparation of a solution containing 4 mmol of copper precursor vigorously dissolved in 20 mL of a selected solvent (water, ethylene glycol (EG) or diethylene glycol (DEG)). The solution was loaded into a round-bottom flask equipped with a bubbler and was heated-up to 80 °C in an oil bath under continuous stirring. When the temperature stabilized at 80 °C, a solution containing 8 mmol of L-ascorbic acid (AA) already well-dissolved in 20 mL of the same solvent was quickly added into the pre-heated copper precursor solution. The concentration of $Cu^{2+}$ in solution is 0.1 mol L⁻¹ and the molar ratio between AA/Cu²⁺ is equal to 2. The reaction is then allowed to proceed for 16 h. The schematic diagram describing the aforementioned procedure is presented in Figure S1.

Following this protocol, several reactive mixtures were tested, with the solvent being the only experimental parameter varied. Each reaction mixture was subjected to a 16 h reaction at 80 °C. It is worth noting that, in some experiments, a specified volume of 35% HCl was added.

#### 2.2.2. Synthesis of tin nanoparticles

The same procedure as for the Cu NPs synthesis described above was applied to the synthesis of Sn NPs. Tin chloride dihydrate ($SnCl_2 \cdot 2H_2O$), L-ascorbic acid and ethylene glycol were used as the precursor salt of tin, the reducing agent and the solvent, respectively. The molar ratio between L-ascorbic acid and tin(II) ions was kept to 2 as above.
## 2.3. Characterization

UV-visible analyses were recorded in the spectral range between 200-900 nm on a Varian-Agilent Carry 5000 UV-NIR spectrophotometer.

Standard transmission electron microscopy (TEM) analysis was performed on a JEOL 2100 operated at 200 kV, this served for general observation to check the NPs formation in each synthesis and to establish the particle diameter histograms. The aqueous suspensions were directly loaded onto a holey carbon film of Au grids (300 mesh), and then dried while the suspensions prepared in polyalcohols were first diluted in ethanol at a 1:1 volume ratio. A minimum of 200 particles were counted to establish representative size histograms and determine the mean particle diameter.

High resolution TEM/STEM imaging for the characterization of single NPs with atomic resolution was performed with a JEOL ARM 200F double aberration corrected microscope operated at 200 kV. For high resolution imaging, the colloidal suspensions of Cu and Sn NPs in ethylene glycol were first diluted in ethanol in a 1:4 ethylene glycol-to-ethanol ratio. For TEM/STEM observations, 2 µl of the diluted suspensions were taken and drop-casted on nickel (for Cu NPs) or copper (for Sn NPs) holey-carbon TEM grids.

X-ray photoelectron spectroscopy (XPS) measurements were recorded using a Kratos Axis Ultra DLD equipment with a monochromatic Al X-ray source (1486.6 eV, 10 mA, 15 kV). To collect enough signal, colloidal suspensions droplets were deposited either directly on a steel XPS sample holder (Cu NPs in EG) or on a TEM Au grid, which was then attached on the sample holder using an Ag paste (Sn NPs in EG). The samples were dried before being introduced in the apparatus. The spectra were obtained with a pass energy of 20 eV on an analysis area of 300 µm×700 µm. The spectral region corresponding to Cu 2p or Sn 3d core levels was recorded for each sample. Data were acquired with 0.1 eV steps. The data treatment was performed with the CasaXPS software.

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was conducted using a Thermo Nicolet 5700 spectrometer. The samples were analysed directly without further preparation; a droplet of the suspension was simply poured on a diamond disk, and pressed with a steel probe with the droplet for analysis. The analysis was performed in the spectral range of 650–4000 cm⁻¹ with a spectral resolution of 2 cm⁻¹. A background spectrum was first recorded, followed by the sample spectrum, which was averaged over 64 successive scans.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used for the determination of metal concentration in aqueous and organic metal suspensions. The liquid samples did not require acid digestion and were only diluted in ultrapure water. The analysis was performed on a 5110 Agilent VDV spectrometer.

# 3. Results and discussion

## 3.1. Synthesis of copper nanoparticles (Cu NPs)

### 3.1.2. Reduction of Cu(II) precursor in aqueous medium

Amid the growing enthusiasm for developing green and eco-friendly methods for synthesizing monodisperse metal nano-colloids, based on the work of Xiong et al.<sup>31</sup>, we have initially selected water as the reaction solvent as it is naturally abundant, and it dissolves a wide range of common metal precursors. Water also dissolves O₂ from the air in important proportions, which represents a challenge
for yielding oxidation-resistant metal particles. One can note that experiments were carried out in a closed flask (Figure S1).

When the solution of AA is added to the solution of CuCl₂ precursor, the light turquoise aqueous solution (pH = 2) instantly turns transparent, and a white solid simultaneously forms. This result suggests that a very fast partial reduction of Cu(II), initially present as hexaaqua copper complex [Cu(H₂O)₆]²⁺, to Cu(I) takes place following the introduction of L-ascorbic acid. The white solid represents around 74 % of Cu molar content in the synthesis, deduced from the quantification of the residual Cu in the liquid supernatant by ICP-OES, and is attributed to cuprous chloride (CuCl) powder.³² One can note that the appearance of this white solid was not mentioned by Xiong *et al*.³¹ A higher AA/[Cu(II)] ratio of 6, instead of 2, was tested, but the formation of CuCl was not avoided. Normally, Cu(I) ions are unstable in water, they easily disproportionate into Cu(II) and Cu(0) or are easily reduced to Cu(0) in the presence of a reducing agent. However, the reducibility of the CuCl intermediate is difficult due to its very low solubility constant in water. During the reaction, the CuCl solid do not disappear, while the starting transparent supernatant changes to yellow, orange and then finally to brown after 16 h of reaction (Figure S2). TEM images of the brown supernatant are presented in Figure 1, showing Cu NPs with sizes ranging from 1 to 15 nm.

<figure><img src="image_1.png" /><figcaption>**Figure 1.** TEM images of Cu NPs obtained from the reduction of CuCl₂ by AA in water at 80 °C for 16 h.</figcaption></figure>

In order to suppress the formation of the white solid of CuCl, a complementary experiment was performed where 5 mL HCl (35%) were added to the CuCl₂ aqueous solution in order to have an excess of Cl⁻ in the medium. The pH of the reaction medium shifted then from 2 to 1. Thus, even if it was demonstrated that in the presence of chloride ions in the medium the reduction of Cu²⁺ favours the one-electron redox process stabilizing the Cu(I) oxidation state,³³ the formation of the white CuCl solid is not observed upon the introduction of AA due to the formation of soluble Cu(I) chloro-complexes namely CuCl₂⁻, CuCl₃²⁻ and CuCl₄³⁻ in the presence of excess Cl⁻.³⁴,³⁵ However, only large Cu NPs with a mean diameter of 26.8 ± 7.9 nm are observed in TEM images (Figure S3). In addition, the colloidal suspension is unstable, with the nanoparticles gradually coalescing into a surface coating in the bottom of the reaction flask within a few days.
In conclusion, water is not the most suitable solvent for the synthesis of sub-10 nm Cu NPs especially when ascorbic acid is used as a reducing agent.

### 3.1.3. Reduction of Cu(II) precursor in organic medium

Ethylene glycol (EG) was chosen as organic solvent since CuCl₂ precursor salt dissolves well in it. EG is able to reduce metal ions at high temperature, but below 120 °C, it presents a limited reducing power. Overall, in contrast with water, EG is a versatile solvent that can play the role of a bidentate ligand forming complexes *in situ* during reaction in addition to the role of a surface agent using its non-bonding doublets of its alcoholic (-OH) functional group.³⁶ In this solvent, the predominant Cu(II) species is the [CuCl₂(EG)₂] complex, which is characterized by its intense green color.³⁷ This complex is very stable and non-reducible even at a temperature as high as 210 °C, higher than the boiling point of EG (197.6 °C) usually used to enhance the reducing effect of EG.³⁸ When AA is added to the solution of CuCl₂ in EG at 80 °C, the uniform green solution undergoes changes analogous to those observed in an aqueous medium, with CuCl solid as the insoluble intermediate (Figure S4), indicating that the [CuCl₂(EG)₂] complex is likely to be destabilized since the reduction reaction occurs. In contrast to water, the brown supernatant is darker and is composed of monodisperse Cu NPs of an average size of 2.4 ± 0.7 nm as seen in the TEM image and size histogram presented in Figures 2 (a) and (b). The darker brown shade in EG is a qualitative indication of a higher Cu concentration in the supernatant. According to ICP-OES analysis, the sedimenting CuCl represents only 19 % of the total Cu concentration in the reactive mixture. When 0.4 mL of HCl (35%) is added to the precursor solution before reaction, the outcome of the synthesis is improved by the excess of Cl⁻ ions, as observed in water (Figure S4). Note that in ethylene glycol, a smaller amount of Cl⁻, 0.4 mL instead of the previously used 5 mL HCl in water, is required to prevent the formation of insoluble CuCl. The resulting Cu NPs keep their perfect homogeneity with a slight increase of the average diameter to 3.3 ± 1.4 nm, compared to those obtained in the absence of HCl, as shown in Figures 2 (c) and (d). This is due to the greater amount of Cu²⁺ reduced, which promotes nanoparticle growth. One can note that the suspensions of copper NPs thus obtained are very stable over time, with no destabilisation after 12 months of storage.

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** TEM images and particle diameter histograms of Cu NPs obtained from the reduction of CuCl₂ by AA in various solvents at 80 °C for 16 h: (a-b) in ethylene glycol, (c-d) in ethylene glycol with the addition of 0.4 mL of HCl (35%) and (e-f) in diethylene glycol.</figcaption></figure>

To monitor the progress of the reaction over time, aliquots of the Cu NPs suspensions prepared in EG with added HCl were taken at different times of the reaction and analyzed by UV-visible spectroscopy after dilution (Figure 3). The spectra were compared to those of witness solutions of fresh AA dissolved in EG and CuCl₂ dissolved in EG (Figure S5). The solution of the fresh AA in EG gives an absorption peak at ca. 245 nm (indexed (1) on Figure 3) that we can follow. The recording of absorption spectra of the reactive mixture starts at 2 min of reaction, slightly after the addition of AA to Cu(II) solution. The difference in the absorbance observed between the 2 min spectrum and the fresh AA can be associated to a first step reduction of Cu²⁺ to Cu⁰ occurring instantly. Then, in the reactive mixture of Cu NPs suspension, the peak representative of AA in EG continuously decreases during the reaction (downward pink arrow for peak (1)), indicating a slow gradual degradation of ascorbic acid due to a slow reduction of Cu²⁺ or Cu⁺. After 13 hours of reaction, the peak (1) has completely disappeared, suggesting that the reaction is likely completed due to the total consumption of AA from the medium. In addition, the spectrum stabilization between 13 h and 14 h suggests that the Cu NPs nucleation/growth has already reached its equilibrium upon 13 h of reaction. It is noteworthy that the resulting spectrum attributed to Cu NPs was characterized by the gradual appearing of a band at ca.
284 nm (upward pink arrow for peak labelled (2)). The absorption of the presumed forming Cu NPs do not carry similarities with the one recorded on the starting Cu²⁺ ions.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** UV-visible spectra representing the evolution in time of the CuCl₂ reduction by AA at 80 °C in ethylene glycol in the presence of 0.4 mL added HCl (35%).</figcaption></figure>

In the experimental conditions chosen, especially keeping the reaction temperature below 100 °C, the oxidation of EG is unlikely. This point was checked by infrared spectroscopy analysis of the suspension after 16 h of reaction in the presence of HCl. The ATR-FTIR spectrum (Figure S6) of Cu NPs suspension displays the same absorption bands with the same intensities as the spectrum of fresh EG (grey line) and the well-known EG oxidation products (acetaldehyde, glycolaldehyde, glyoxal, etc) were not detected.¹³ Consequently it can be inferred from the UV-Visible and ATR-FTIR spectroscopies that the reducing agent of Cu²⁺ is likely to be AA, as it occurred in water, and not EG.

The direct access to crystallographic information about the Cu NPs dispersed in organic solvent using HR TEM or HR STEM-HAADF was limited due to organic contamination under the electron beam, which acts as a veil, as shown in Figure S7. Consequently, to remove the organic solvent, the TEM grids for high-resolution TEM/STEM imaging were heated under H₂ at 300 °C for 1h (temperature ramp 10°C min⁻¹) prior to observation. Despite this cleaning procedure, organic contamination during TEM observation could not be entirely eliminated and thus, electron microscopy imaging of the Cu NPs was performed in TEM mode to minimize contamination of the sample under the electron beam (Figure 4 (a)). Figures 4 (b) and (c) show high resolution TEM images of two representative NPs. The Cu NPs have an average size of 4 nm possibly due to interparticle sintering at 300 °C. The corresponding fast Fourier transform (FFT) is shown in inset. The interplanar distances and interplanar angles from the TEM
images were consistent with the face-centred cubic (FCC) structure of copper viewed along the [001] zone axis. For instance, the interplanar distances obtained from the analysis of the FFT image of the NP in Figure 4 (b) (1.28 Å for (220) planes (in red) and 1.84 Å for (200) planes (in green)), as well as the interplanar angles between these planes (45° between the (220) and (200) planes), match well with those of the FCC phase of bulk Cu (1.27 Å for (220) planes and 1.80 Å for (200) planes). This structural analysis also rules out the presence of a crystallized oxide copper phase in the sample.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** (a) HR TEM image of Cu NPs synthesized in EG and 0.4 mL HCl (35%) after H₂ treatment at 300 °C for 1 hour. (b) and (c) HR TEM image of two representative Cu NPs in [001] zone axis with its indexed FFT.</figcaption></figure>

In order to confirm that the synthesis of Cu NPs in EG using AA as reducing agent yields zerovalent copper metal (Cu⁰), the suspension was further analysed by XPS (Figure S8(a)). The peaks of Cu 2p₃/₂ and Cu2p₁/₂ are located at 932.2 eV and 952.1, respectively, showing that Cu(II) (at ca. 933.8 and 953.8 eV, respectively) is not present. This confirms that Cu is in a reduced state, likely as Cu(0), in agreement with the TEM results. However, the presence of Cu(I) cannot be entirely ruled out, as the binding energies are only approximately 0.1 eV away from those of Cu(0) species.³⁹

Diethylene glycol (DEG), a four-carbon dimer of EG, dissolves as well CuCl₂ salt and presents a higher chelating property than EG due to the ether functional group (-O-). Consequently, it was also tested as a solvent and chelating agent for the synthesis of Cu NPs. The reduction of CuCl₂ by AA in diethylene glycol at 80 °C results in a uniform brown solution (Figure S4). Interestingly, the common intermediate CuCl solid observed so far is visibly not formed and excess Cl⁻ ions by addition of HCl is not required anymore. The resulting uniform brown solution in DEG corresponds to highly stable small size Cu nanocrystals of around 2.2 ± 0.8 nm as highlighted in Figures 3 (e) and (f). As one can see from Figure 5, these nanoparticles are flattened; their thickness is lower than what observed for Cu NPs synthesized in EG medium. The plate-like shape is due to the modified adsorption character of DEG that has 2 types of "O" moieties: alcohol and ether.³⁷ Chemically, the NPs correspond to a FCC structure of Cu as highlighted by the diffraction spots of the (200) and (220) family plan on HR TEM.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** a) HR TEM picture of Cu NPs synthesized in DEG. b) Reduced Fourier transform of the whole area showing the three family plans of lower indices for FCC structures: (200) (red), (111) (blue) and (220) (green). c) HRTEM image of a specific area of a) containing an NP. d) Reduced Fourier transform of b) matching with Cu FCC in [100] direction.</figcaption></figure>

Overall, the Cu NPs synthesized in either of the diols (EG and DEG) display a fascinating colloidal stability during more than 12 months and cannot be destabilized using centrifugation even at higher rotation speeds (11 000 rpm) and duration (1 h). The measurement of ζ-potential helps in understanding the mechanism of colloidal stability, the dilution in water was necessary for the experiment to reach a satisfactory current conduction for the analysis in solutions containing non-polar organic solvents in classic cuvettes. The water-diluted suspension of Cu NPs synthesized in EG remained stable and kept its uniform brown colour, the ζ-potential value was 0 V. This result demonstrates the steric stabilization of the Cu NPs due to the effect of EG molecules, potentially combined with that of dehydroascorbic acid, while indicating that the negatively charged Cl⁻ ions present in the medium do not play a role. In contrast, the suspension of Cu NPs synthesized in DEG immediately turns green upon water addition, and a white solid form, suggesting the reversibility of the NP formation process, in a fashion comparable of what occurs in a purely aqueous medium as discussed earlier. Thus, these NPs are highly sensitive to oxidation by water, leading to further corrosion/dissolution. DEG at the surface of the NPs is permeable to O₂ diffusion and this explains the harsh corrosion phenomenon.³⁴ Consequently, among the solvents tested, only EG is able to prevent Cu NPs from oxidation.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** UV-visible absorption spectra of as prepared Cu NPs in varied solvent conditions.</figcaption></figure>

Finally, to verify the formation of sub-10 nm Cu NPs in the polyol media (EG, EG with HCl, and DEG), UV-visible absorption spectra were recorded at the end of the experiment, i.e., after 16 h of reaction (Figure 6). First, the absence of localized surface plasmon resonance (LSPR) peak in the 550-650 nm spectral range in the three media is a proof of the absence of large copper nanoparticles.⁴⁰ In addition, the absorption bands highlighted with the dashed pink line at 218, 256, 284 and 395 nm for Cu NPs synthesized in EG and 203, 256 and 284 nm in DEG are characteristic of small NPs.¹⁴,⁴⁰,⁴¹ Besides, they do not correspond to the absorption bands of CuCl₂ dissolved in the same solvent at the same concentration, in the absence of AA. The position and shape of these bands are influenced by the molecular environment at the metal surface and closely aligned with those measured by Kawasaki *et al.*, for Cu NPs with diameters less than 3 nm.¹⁴ Comparatively, when HCl is added in the synthesis medium to suppress CuCl precipitation and enhance the efficiency of Cu²⁺ reduction, the absorbance increases (from grey to red spectrum) because the concentration of Cu nanocrystals in the solution rises.

## 3.2. Synthesis of tin nanoparticles (Sn NPs)

Sn NPs obtained through various synthetic routes are subject to systematic oxidation upon exposure to air. An oxidation shell, often of stannic oxide (SnO₂), spontaneously forms.⁴²,⁴³ As EG has demonstrated to be an efficient solvent for obtaining small nanoparticles of metallic Cu, synthesis of Sn NPs was performed in the same solvent. SnCl₂ is easily dissolved in EG resulting in a colourless solution. When AA is added to the colourless SnCl₂ precursor solution, the reactive mixture gradually takes on a yellow shade, eventually reaching a uniform light golden-yellow appearance after 16 h of reaction (Figure S4). Sn NPs are stable in the solvent and cannot be precipitated by centrifugation procedures. As shown in Figures 7 (a) and (c), small Sn NPs are obtained (average size ≈ 3.5 ± 1.2 nm). Figures 7 (a) and (b) show high resolution STEM-HAADF images of Sn NPs. In Figure 7 (b), FFT analyses
of the atomically-resolved images of two NPs are consistent with the α-Sn diamond-like structure. One is in the [110] zone axis (violet box) and the other one is oriented along the [111] zone axis (yellow box). Indeed, the interplanar distances measured (3.11 Å for (200) planes, 2.18 Å for (220) planes and 3.46 Å for (111) planes) are very close to those of α-Sn with lattice parameter a= 6.49 Å (interplanar spacings of 3.25 Å for (200) planes, 2.30 Å for (220) planes and 3.75 Å for (111) planes). Moreover, these FFT are not consistent with the tetragonal structure of β-Sn with a=5.82 Å and c=3.17 Å. The contours of the particles are poorly defined, probably due to the strong presence of adsorbed organic ligands. A few more particles alongside their structures are presented in the supplementary material (Figure S9). XPS analysis also confirms the presence of zerovalent metallic Sn (Sn⁰) as well as tin oxide (SnO₂), likely due to the formation of a passivation shell around the Sn⁰ NPs (Figure S8(b)).

Interestingly, the Sn NPs resulting from colloidal synthesis reported in literature crystallized into the white tin phase (β-Sn), which is a tetragonal system.<sup>44,45</sup> The stabilization of Sn NPs in the grey tin phase (α-Sn), which displays a diamond (FCC) cubic lattice, is rarely observed under conventional conditions of wet chemistry syntheses.<sup>46,47</sup> The change of the crystalline system modifies the metal's properties, β-Sn has metal-like properties while α-Sn is likely a semi-conductor. This unusual result may be synthesis-mediated, demonstrating the strength and versatility of ethylene glycol (EG) as both a synthesis medium and more specifically as a surface agent.

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** STEM-HAADF images of Sn NPs dispersed in ethylene glycol after SnCl₂ reaction with AA at 80 °C for 16 h. (a) Low resolution and (b) HR TEM image of two Sn NPs in [110] and [111] zone axis with its indexed FFT. (c) Particle diameter histogram.</figcaption></figure>

**4. Conclusion**

In summary, a straightforward colloidal approach was developed for the synthesis of stable copper nanoparticles under ambient conditions by employing a modified polyol process at 80 °C. This approach was successfully extended to the synthesis of tin nanoparticles, making it a generalizable method for the synthesis of other non-noble metals that are highly prone to oxidation. Ethylene glycol served as the solvent, while L-ascorbic acid functioned as the reducing agent. This method yielded sub-10 nm nanoparticles, with Cu and Sn predominantly in their zerovalent states, and with an unconventional phase for Sn. The resulting nanoparticles exhibited remarkable long-term stability against both aggregation and oxidation.

**Acknowledgement**

This research work was funded by the French Agence Nationale de la Recherche (ANR-21-CE07-0029). The authors gratefully acknowledge the financial support from the European Union (ERDF) and the French Région Nouvelle Aquitaine. This work pertains to the French government program "Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010).

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**Supplementary information**

# Facile and Green Synthesis of Monodisperse Sub-10 nm Copper and Tin Nanoparticles Using L-ascorbic Acid as a Reducing Agent

Abdennour Benabbas¹*, Grégoire Breyton², Catherine Especel¹, Anthony Le Valant¹, Christian Ricolleau², Guillaume Wang², Tzonka Mineva³, Jaysen Nelayah², Hazar Guesmi³, Florence Epron¹*

1. CNRS, Université de Poitiers, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Poitiers, France

2. Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Cité - CNRS, Paris, France

3. Institut Charles Gerhardt de Montpellier (ICGM), Université de Montpellier, CNRS, ENSCM, Montpellier, France

*florence.epron@univ-poitiers.fr, abdennour.benabbas@univ-poitiers.fr
<figure><img src="image_10.png" /><figcaption>**Figure S1.** Schematic representation of the Cu NPs synthesis procedure (a) and Experimental set-up for the synthesis of Cu and Sn nanoparticles. The bubbler is filled with pristine EG. The heating bath is filled with Si-Oil. Example in the figure: Cu NPs synthesised in EG with HCl addition (35%) (b).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure S2.** Representative images of the synthesis of Cu NPs after 16 h of reaction conducted in water.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure S3.** TEM image (a) and particle diameter histogram (b) of Cu NPs obtained from the reduction of CuCl₂ by AA in water with 5 mL of HCl (35%) at 80 °C for 16 h.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure S4.** Representative images of the synthesis of Cu NPs (left) and Sn NPs (right) conducted in polyol media. Cu NPs were synthesised in: EG (with CuCl precipitate), EG + HCl, and DEG. Sn NPs were synthesised in EG.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure S5.** UV-visible spectra of CuCl₂ and ascorbic acid (AA) in ethylene glycol solution.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S6.** ATR-FTIR spectra of fresh ethylene glycol EG solvent (grey line), suspension of Cu NPs synthesized in ethylene glycol in the presence of HCl (red line) and suspension of Sn NPs synthesized in ethylene glycol (blue line).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure S7.** STEM-HAADF images of Cu NPs synthesized in ethylene glycol with the addition of HCl (0.4 mL) at low (a) and high (b) magnitudes.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure S8.** XPS spectra of: (a) Cu NPs (lines Cu 2p<sub>3/2</sub> and Cu 2p<sub>1/2</sub>) synthesized in EG in the presence of AA and HCl and (b) Sn NPs (lines Sn 3d<sub>5/2</sub> and Sn 3d<sub>3/2</sub>) synthesized in EG.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure S9.** Supplementary STEM-HAADF images of Sn NPs dispersed in ethylene glycol after reaction with AA at 80 °C for 16 h. Sn NPs in figures (a), (b) and (c) are in [001], [110] and [111] zone axes respectively. The indexed FFT is given for each image according to a colour code.</figcaption></figure>