Preparation of In/Sn Nanoparticles (In₃Sn and InSn₄) by Wet Chemical One-Step Reduction and Performance Study

Abstract

The preparation of binary alloys by surfactant-assisted chemical reduction in aqueous solution at room temperature has become a hot topic. In this article low melting point tin/indium (Sn/In) nanoparticles are synthesized. The formation process of the alloy was studied. Scanning electron microscopy, energy spectrometry, and X-ray diffraction are used to determine the morphology, composition, and crystal structure of the nanoparticles. Study found that fully alloyed indium-tin nanoparticles can be obtained by wet chemical method and the main phases of indium-tin alloy are β-phase (In₃Sn) and γ-phase (InSn₄). However, the Sn phase appears at a low content of indium (40 wt%). When the content of indium increases to 45 (wt%), the tin phase disappears. In addition, the most important finding is that the composition of the indium-tin alloy can be changed by ratio control, and the content of In₃Sn increases with the increase of indium content. The relative content of In₃Sn attains a maximum when the content of indium increases to 60 (wt%). In contrast, the content of InSn₄ decreases. Finally, differential scanning calorimetry measurements is performed to understand the melting behavior of the nanoparticles and low melting temperatures are achieved for a wide range of indium compositions (from 40% to 60%). The melting temperature is found to be in the range of 125–132 °C and it increased with increasing In₃Sn (also the increase of indium content). This gives us a new understanding into the binary alloy nano-system and gives important information for the application of low temperature alloy solders. The choice of composition can be based on the corresponding melting point.

1. Introduction

Nanotechnology has received extensive attention in recent years, especially bimetallic nanoparticles [1,2,3,4,5]. Because the electronic and crystal structure of bimetallic nanoparticles surface changes significantly, these nanoparticles show physical and chemical properties that traditional materials do not have, affecting the special properties of their materials [6,7,8,9,10]. Especially in the application of antimicrobial materials and catalysts, bimetallic nanoparticles are widely supported. One of the silver-based bimetallic nanoparticles is mainly used for antibacterial materials [11,12]. Other bimetallic nanoparticles have also made great progress in catalysts [13,14,15,16]. At present, several lead-free nano-templates have been proposed and used. The methods of manufacturing are diverse, thus creating various shapes (spherical, tubular, bar, slatted, oval) [17]. Due to simple conditions, short-time chemical synthesis routes are noted [18]. In the research field, bimetallic nanoparticles are starting to be noticed. In the beginning, most people studied silver and gold nano-alloys. Then other systems appeared (Ag-Cu, Ag-Pt, Ag-Pd, Ag-Co, Ag-Sn, Ag-Al, Sn-In Sn-Ag-Cu) [19,20,21,22,23,24]. In addition, complexes of other systems have been studied and developed in nanostructures. By changing the coordination spheres of ions by ultrasonics and acoustochemistry, the structure of the crystal was changed and the material properties were improved [25,26,27,28,29].

Photovoltaic tile stacking technology is a technology in which cells are sliced and then welded into strings with special conductive materials [30]. The cells are connected in front and back stacking, making full use of the surface area of the cells and therefore significantly increasing the solar energy conversion efficiency [31]. Conductive adhesive is currently used. However, the conductive adhesive is expensive and also subject to aging, so a material with the same performance as the conductive adhesive is needed instead of the conductive adhesive [32]. Therefore, a low-melting-point solder, indium-tin alloy, is chosen as the connection material for the cells. However, the size of indium tin nanoparticles prepared by other methods is large and not applicable with microelectronic packaging technology. Therefore, wet chemical preparation of indium-tin nanoparticles is a potentially promising method.

Indium-tin nanomaterials have also been studied, especially their superconducting properties [33,34,35,36,37,38]. Ashish Chhaganlal Gandhi et al. [39]. successfully prepared low-temperature nano-superconductors of In₃Sn and InSn₄ by reducing metal slats. Takashi Mochiku et al. [40]. prepared In₃Sn and InSn₄ low-temperature superconductors by alloying with gallium at room temperature. Shu Yang et al. [41] used InCl₃ and SnSO₄ as metal precursors and NaBH₄ as a reducing agent to study In/Sn solders with melting points of 115–118 °C. InSn₄ with particle sizes in the tens of nanoparticles was successfully formulated, and In/Sn alloy nanowires were also studied at 0 °C [42]. The main components are InSn₄ phase, In phase, and Sn phase. Finally, the In/Sn nanoparticles were subjected to heat treatment at different temperatures, and the equilibrium transition temperature of the In/Sn nanoalloys was successfully found, and low melting point In/Sn nano-solders (In₃Sn and InSn₄) were prepared, providing important information for the study of electronic packaging [43].

In our work, our In/Sn nano-alloys (In₃Sn and InSn₄) are synthesized based on tin (bulk melting point of 231.96 °C) and indium (bulk melting point of 156.63 °C). We successfully prepared nanoparticles with the composition of In₃Sn phases and InSn₄ phases by adding anhydrous ethanol during the reaction based on Shu Yang et al. We have successfully prepared nanoscale In₃Sn and InSn₄ particles without heat treatment. The relative contents of In₃Sn and InSn₄ can be controlled by controlling the In/Sn ratio. The melting point of the In/Sn eutectic alloy was measured at 130 °C, and the corresponding melting point increased as the indium content increased. This is slightly higher than the conventional In/Sn eutectic alloy (melting point of 118 °C), which is still being further explored. Finally, we explain the possible formation processes of In₃Sn and InSn₄ based on their enthalpies of formation and chemical reduction potentials.

2. Material and Methods

2.1. Materials

Sodium dodecyl sulfate (SDS) (99.9%), tin sulfate (SnSO₄, 99.99%), indium chloride (InCl₃, 99.99%) were purchased from Aladdin. Sodium borohydride (NaBH₄, 99.9%) was purchased from Koyi Chemical Glass. Pure water was obtained from a Lidl purifier. Anhydrous ethanol and all other chemicals were used directly.

2.2. Synthesis of In/Sn Nanoparticles

In/Sn nanoparticles were synthesized by surfactant-assisted chemical reduction. In this work, ambient temperature is controlled at 25 °C. The mass ratios of In/Sn 40/60, 45/55, 50/50, 52/48, 60/40 were studied (these are the mass ratios that can form the beginning and end of In₃Sn phase and InSn₄ phase). First, SDS was added into pure water from an 8 mM solution. Meanwhile, a certain amount of the SnSO₄ and InCl₃ was dissolved in deionized water as metal precursor. Then, the pre-made SnSO₄ and InCl₃ metal precursors were added to the SDS solution and mixed for 4–5 min. This situation was kept at the ambient temperature with constant stirring (700 r/min). A concentration of 1 mol/L of reducing agent sodium borohydride was injected into the solution. Then the solution was kept for 40 min to allow for the formation of In/Sn nanoparticles. Adding anhydrous ethanol during the reduction process eliminates the foam generated from the reaction. After the reaction was completed, the nanoparticles were separated from the solution by centrifugation using an Anake TGL-16C (Shanghai anting, Shanghai, China) at 13,000 r/min and washed 5 times in pure water and 5 times with ethanol through dispersion and centrifuge cycles.

2.3. Characterization

The morphology and elemental composition of the In/Sn nanoparticles were characterized with the emission-scanning electron microscope (JEP-7800, Jeol Ltd., Tokyo, Japan), and an energy dispersive X-ray spectrometer (EDS, Thermo Fisher, Bremen, Germany) were used to examine the morphology of the nanoparticles as well as to identify the chemical composition of nanoparticles. XRD was performed on the samples to analyze the physical phase and relative content of the samples by X-Ray diffraction. DSC was used to determine the phase transition temperature of the samples, with the temperature increase rate was 5 °C/min.

3. Result and Discussion

3.1. Morphological Analysis

The morphology of the In/Sn nanoparticles was observed by scanning electron microscopy. Figure 1a–f shows the morphology of In/Sn ratios of 40/60, 45/55, 50/50, 52/48, 55/45, 60/40, respectively. The spherical shape is the main morphology of nanoparticles, accompanied by some ellipsoidal, rod-shaped and block-shaped particles. The size distribution of spherical particles is uneven in all sample tubes, with the larger ones having 300 nm and the smaller ones only 20 nm. It is interesting to find some small holes on the top of the blocks, spheres, or rods, which we guess are caused by the incomplete maturation due to Ostwald ripening. Since Ostwald ripening is a way to sacrifice small particles for the growth of large particles, we speculate that the small holes may be caused by attaching to the surface of large particles and shedding them. However, in this sample with an In/Sn ratio of 55/45 we found a high degree of sphericity of the particles, as well as a uniform distribution of their particles, with a poor dispersion of the smaller size (around 30 nm), and overall, a spherical particle with an average size of 50 nm.

Energy spectrum analysis was done on eutectic composition samples from Figure 2. Oxygen was found on the surface of some points (as shown in

Table 1. Indium-tin raising quality ratio (%).

Serial Number	O (wt%)	In (wt%)	Sn (wt%)
1	0	60.68	39.32
2	5.13	51.85	43.00
3	0	65.57	34.43
4	5.05	65.37	29.57
5	0	61.46	38.54
6	0	62.06	37.94

). But the strange thing is that oxygen was not found in 1, 3, 5, and 6. Points 2 and 4 reveal that the samples contain at least 5 wt% oxygen. So, during the synthesis of indium-tin nanoparticles, hydrolysis plays a significant but negative role in nanoparticle formation. The hydrolysis reactions of InCl₃ and SnSO₄ are shown below:

$${\mathrm{InCl}}_3+3{\mathrm{H}}_2\mathrm{O} =\mathrm{In}{\left(\mathrm{OH}\right)}_3 +3\mathrm{HCl}$$

(1)

$${\mathrm{SnSO}}_4+ 2{\mathrm{H}}_2\mathrm{O} = \mathrm{Sn}{\left(\mathrm{OH}\right)}_2+{ \mathrm{H}}_2{\mathrm{SO}}_4$$

(2)

According to the reaction equation, after hydrolysis of part of the indium-tin alloy, the generation of acid inhibits the hydrolysis from proceeding. Here the oxygen content of the analyzed sample is obtained by hydrolysis. In addition, the InSn mass ratio on the surface of each nanoparticle is different, which does not match the ratio of In₃Sn and InSn₄. This shows that each nanoparticle is not a separate phase, but consists of the In₃Sn phase and InSn₄ phase. This conclusion is consistent with the previous XRD findings.

3.2. Structure and Research

The structure of the indium-tin alloy was investigated by XRD measurements, and the Figure 3a shows the phase structure for a mass fraction of indium of 40/60 45/55 50/50 55/45 60/40. We reference the standard patterns of InSn₄(PDF#04-003-2161), In₃Sn (PDF#04-004-7736), tin (PDF#04-009-6172). Most of the prominent peaks matched the peaks of In₃Sn and InSn₄. This indicates that the structure of the nanoalloy is mainly composed of In₃Sn and InSn₄ phases. But a small amount of tin phases can be seen in an indium a mass fraction of 30 (refer to a very small peak of tin (200), between InSn₄ (001) and In₃Sn (101)). With the increase of indium content, the tin phase in the nanoalloy becomes less and less and finally disappears. In addition, the peak of InSn₄ will relatively come to be obvious between the peak InSn₄ and the peak In₃Sn. However, no possible oxide materials were observed (including SnO, SnO₂, Sn₃O₄, In₂O₃, and In₅O₆) from the XRD patterns. This may be due to the relatively small amount of oxides located mainly on the nanoparticle surface. The phases of In₃Sn and InSn₄ are basically the same when the composition of indium rises to about 45. Meanwhile, no tin was found after 45 for indium. We have refined the XRD to initially calculate the relative masses of In₃Sn and InSn₄. We found that the relative mass of In₃Sn is increasing while the relative mass of InSn₄ is decreasing. This result is consistent with the ratios we expect. The XRD results showed that the generated nanoparticles have the same crystal structure by different ratios, the only difference being the content of In₃Sn and InSn₄. As the indium content increases, the peaks of In₃Sn are higher, indicating a higher tin content. Therefore, we are able to control the content of In₃Sn and InSn₄ by controlling the mass ratio of In/Sn. The results are shown in the Figure 3b. This result differs significantly from the results reported in the literature [43]. The literature shows that the crystal structure of In₃Sn does not appear in the preparation of indium-tin alloys with different compositions, but instead the crystal structure of monometallic indium appears. However, in this paper no monometallic crystal structure appears and the result is a pure alloy phase (In₃Sn and InSn₄).

3.3. Melting Point Test Analysis

The melting temperature of the tin/indium nanoalloys was measured by differential scanning calorimetry (DSC). The results of differential scanning calorimetry analysis of tin/indium nanoalloys with different compositions are shown in Figure 4 for nanoparticles with compositions ranging from 40–60% of indium. We found the presence of the main peak between 125 °C (40% indium) and 132 °C (60% indium) and did not find the appearance of the eutectic peak of In/Sn, as well as the appearance of the peaks of singlet indium and tin. However, in the literature [43], the presence of monometallic indium in the prepared alloy particles led to the appearance of multiple melting peaks during the treatment. Only the final formation of a single alloy phase agrees with our results, with only one melting point peak in the indium-tin alloy. This indicates that our experimental results yielded a melting point peak for a mixture of the simple In₃Sn phase and the InSn₄ phase. According to the XRD obtained from the experiments, we can see that the content of In₃Sn increases with the increase of indium content, and conversely, the content of InSn₄ decreases, while the change of DSC melting point increases with the increase of In₃Sn. So, we can control the melting point of our desired In/Sn alloy nanoparticles by controlling the indium content. This is very significant for us to want specific melting point studies.

3.4. The Formation Process of Liquid Phase Process

In/Sn nanoparticles are mainly composed of two phases, In₃Sn and InSn₄, which are in competition for Sn²⁺ during the eutectic reduction process. The redox potential Sn²⁺/Sn (E = −0.136 V) is higher than In³⁺/In (−0.34 V), so the Sn²⁺ ion is preferentially reduced in the reduced system. This process could be described as Equations (5) and (6). Since this gap is not particularly large, we can reduce the gap caused by this potential by adjusting the molar ratio of these two metals. Yuetian Ji et al. [24] also mentioned in the preparation of Au-Ag alloys that adjusting the concentration of metal precursors can make Au ions and Ag ions have the same nucleation growth rate. In our work, we studied the reaction at different ratios of In/Sn, and as the indium content increases, the effect due to the potential difference and the resulting effect gradually diminish to reach a consistent reduction rate. In this case, Sn2⁺ is reduced to tin atoms and In³⁺ is reduced to indium atoms. This process could be described as follows:

$${\mathrm{Sn}}^{2+} + 2{\mathrm{BH}}_4^{-} \to \mathrm{Sn} +{ \mathrm{H}}_2 +{ \mathrm{B}}_2{\mathrm{H}}_6$$

(3)

$${\mathrm{In}}^{3+} + 6{\mathrm{BH}}_4^{-} \to 2\mathrm{In} + 3{\mathrm{H}}_{2 } + 3{\mathrm{B}}_2{\mathrm{H}}_6$$

(4)

$${\mathrm{Sn}}^{2+}+2\mathrm{e}={\mathrm{Sn} \mathrm{E}}^0= -0.136 \mathrm{eV}$$

(5)

$${\mathrm{In}}^{3+}+3\mathrm{e}={\mathrm{In} \mathrm{E}}^0= -0.34 \mathrm{eV}$$

(6)

The newly reduced tin atom and indium atom (Active atoms) combine to form In₃Sn and InSn₄ phases. The formation enthalpies of In/Sn alloys have been systematically calculated by Jain, Anubhav et al. [44,45]. From their calculations we know the formation enthalpies of In₃Sn (0.034 eV) and InSn₄ (0.134 eV). From their calculations, we know that In₃Sn is first formed in our reaction system due to the low enthalpy of formation of In₃Sn. Therefore, In₃Sn is mainly formed at the beginning of the reaction, and InSn₄ is mainly formed at this time because the concentration of indium atoms decreases at a later stage. No new nuclei appear when the indium atoms in the reactants are depleted. We found a small fraction of tin in the reaction products, as the indium content was low. When In₃Sn and InSn₄ reach stability, the final reaction system no longer interconverts. The reaction pattern of In/Sn nanoparticles prepared by the reduction method is shown in Figure 5.

$${\mathrm{In}}_{\left(\mathrm{reacted}\right)}+{ \mathrm{Sn}}_{\left(\mathrm{reacted}\right)} \to { \mathrm{In}}_3\mathrm{Sn}+{\mathrm{InSn}}_4$$

(7)

In summary, we described the formation of In/Sn nanoparticles as follows: first black particles are precipitated, followed by white particles when the reducing agent is added to the metal precursor. Most of the indium and tin cations are reduced to the metallic form and nucleated separately. The active indium and tin atoms preferentially form the intermetallic phase In₃Sn under mechanical stirring due to the low formation energy of In₃Sn, accompanied by the formation of InSn₄ by mutual adsorption of the atoms. As the indium content increases and the reaction proceeds more and more favorably to the formation of In₃Sn.

4. Conclusions

Low melting point In/Sn nanoparticles were successfully synthesized by surfactant-assisted one-step chemical reduction. This is an environmentally friendly method and all used chemicals are non-toxic. The experiments concluded that the wet chemical method is capable of synthesizing spherical indium-tin nanoalloys, while the content of two intermetallic compounds (In₃Sn and InSn₄) in indium-tin alloys can be controlled by adjusting the ratio of indium-tin metal precursors. In₃Sn content increases and InSn₄ content decreases with increasing indium content. No other alloy phase is observed. Moreover, the melting point of indium-tin alloys increases with increasing In₃Sn content (relative content based on XRD calculations). At indium content of 40%, the melting point peak appears at 125 °C, and the maximum peak reaches 132 °C for indium content of 60%. This result indicates that the properties of In/Sn alloys can be modified by adjusting the composition of the In/Sn alloy. This is important for the development of low temperature solders; In/Sn solders will be more utilized in the field of packaging electronics.