Glucose-Mediated Microstructure Refinement of Electroless Silver Coatings on Atomized Fe Particles

Abstract

Electroless silver (Ag) plating has emerged as a simple yet effective surface modification technique, garnering significant attention in consumer electronics and composite materials. This study systematically investigates the influence of glucose dosage on the microstructural refinement of Ag coatings deposited from silver–ammonia solutions onto iron (Fe) particles while also evaluating the oxidation resistance of Ag-plated particles through thermogravimetric analysis. Optimal results were achieved at a silver nitrate concentration of 0.02 mol/L and a glucose concentration of 0.05 mol/L, producing Fe particles with a uniform and dense silver coating featuring an average Ag grain size of 76 nm. The moderate excess glucose played a dual role: facilitating Ag⁺ ion reduction while simultaneously inhibiting the growth of Ag atomic clusters, thereby ensuring microstructural refinement of the silver layer. Notably, the Ag-plated particles demonstrated superior oxidation resistance compared to their uncoated counterparts. These findings highlight the significance of fine-grained electroless Ag plating in developing high-temperature conductive metal particles and optimizing interfacial structures in composite materials.

1. Introduction

Electroless plating, as a non-electrodeposition technique, is a widely used surface modification method for depositing specific metals onto substrates through redox reactions [1,2,3]. This process has gained extensive applications due to its advantages, such as operational simplicity, good feasibility, and adaptability to complex metallic structures. Currently, a variety of metals can be deposited via electroless plating, including Cu [4], Ni [5], Au [6], and Ag [7,8].

Silver, as a soft metal, exhibits excellent oxidation resistance along with outstanding electrical and thermal conductivity [9,10,11]. These properties make it a preferred choice for electroless plating coatings in composite materials to enhance performance. On the other hand, Fe particles, commonly employed as reinforcing phases in composites [12,13,14,15], demonstrate favorable magnetic properties and electrical conductivity. However, they suffer from poor wettability with matrix materials and weak inherent oxidation resistance. Research has shown that these limitations can be mitigated by applying various coating techniques to improve the oxidation resistance and surface properties of Fe particles.

The electroless silver plating technique for powder surfaces has garnered widespread attention due to its simple processing and cost-effectiveness. With its exceptional uniform coating capability, this method effectively overcomes the issue of uneven coating thickness—commonly encountered in traditional encapsulation techniques due to variations in particle morphology—making it a highly efficient approach for preparing composite powder materials [16,17]. By constructing a continuous silver coating on conventional powder surfaces, the resulting composites can exhibit superior comprehensive performance compared to pure silver materials.

With advancements in surface treatment technologies, the electroless silver plating process has been continuously optimized, enabling the large-scale application of silver-coated composite powders in various fields, including electronics, oxidation resistance, and antibacterial applications. Güler et al. [18] fabricated composites by hot-pressing Al₂O₃ particles with either silver-coated Cu powder or uncoated Cu powder. Their findings revealed that electroless silver plating improved the uniform distribution of Al₂O₃ particles, thereby enhancing both the hardness and electrical conductivity of the composites. In a subsequent study, Güler et al. [19] developed a bilayer functionally graded material using silver-coated Cu powder and pure Cu powder, demonstrating significantly improved oxidation resistance compared to the original pure Cu powder. Varol et al. [20] synthesized Cu-Ag alloys using silver-coated Cu particles and observed that increasing the content of silver-coated Cu particles enhanced the mechanical properties, electrical conductivity, and oxidation resistance of the alloy. Dong et al. [21] formulated a silver-coated Cu paste by blending silver-coated Cu powder with Ag powder. Their results indicated that a paste containing 70 wt.% Ag exhibited comparable electrical conductivity to pure Ag paste while maintaining excellent stability. Hsueh et al. [22] deposited an Ag/collagen coating on porous TiO₂ surfaces. The presence of collagen was found to influence the aggregation size of Ag, and the resulting composite exhibited enhanced hydrophilicity, antibacterial properties, and reduced cytotoxicity.

Although silver plating is considered a well-developed electroless deposition process, the existing literature primarily focuses on increasing coating thickness and deposition rates, with limited systematic investigation into the microstructural refinement of silver coatings. Moreover, reports on electroless silver plating using Fe substrates remain scarce. To address these research gaps, this study aims to regulate the grain size of electroless silver coatings on atomized iron particles by controlling the glucose concentration in the plating solution. The influence of silver coatings on the high-temperature oxidation resistance of iron particles is investigated.

2. Materials and Methods

2.1. Electroless Silver Plating

Spherical Fe particles with 99 wt.% purity, prepared by the spray-atomization method (average particle size: ~20 μm), were selected as substrates for silver deposition. The electroless plating procedure, as schematically illustrated in Figure 1, comprised the following sequential steps: (1) surface coarsening, (2) sensitization–activation, (3) cleaning, (4) electroless plating, (5) cleaning, and (6) desiccating. Prior to plating, the micron-sized Fe particles underwent surface pretreatment. First, oxide removal and surface roughening were performed by immersing the particles in 1% dilute hydrochloric acid solution under mechanical stirring for 5 min. Subsequently, sensitization–activation treatment was conducted using the solution with the composition specified in Figure 1. During this 30 min mechanical stirring process, Pd²⁺ ions in the activation solution were reduced to metallic Pd nanoparticles, which were then adsorbed onto the Fe particle surfaces to serve as catalytic sites for subsequent silver deposition. The activated Fe particles were thoroughly rinsed with deionized water for 5 min before electroless plating. The loading of particles in the solution was fixed at a concentration of 20 g/L. The total electroless plating time was 40 min. The plating solution formulation (Figure 1) consisted of glucose as a reducing agent, potassium sodium tartrate as a complexing agent, and silver nitrate as a metal ion source. The initial pH of the plating bath was adjusted to 12 using 2 g/L NaOH solution. To maintain optimal deposition conditions, additional NaOH was introduced at 15 min and 25 min intervals during the plating process to compensate for pH variation. The concentration of glucose, a critical parameter affecting reduction kinetics, was systematically varied at 0.02, 0.05, and 0.1 mol/L to investigate its influence on the plating characteristics.

2.2. Microstructure and Performance Characterization

The surface morphology of silver-plated Fe particles prepared with different glucose concentrations was examined using field emission scanning electron microscopy (SEM, FEI Quanta 450 FEG, Hillsboro, OG, USA). Quantitative analysis of Ag particle size distribution was performed using Image J v1.6.5 software based on SEM images. Phase composition was characterized by X-ray diffraction (XRD, Tongda TDM-20, Dandong, China) with Cu Kα radiation, employing a scanning range of 20–90° at a rate of 2°/min. The oxidation resistance of silver-coated Fe particles was evaluated by a simultaneous thermogravimetric differential scanning calorimetry analyzer (TGA-DSC, Setaram Themys ONE, Caluire-et-Cuire, France) from room temperature to 650 °C with a heating rate of 20 K/min.

3. Results

3.1. Microstructure

Figure 2 presents the surface microstructures of Fe particles after electroless silver plating with three different glucose concentrations. At a 0.02 mol/L glucose concentration, the silver coatings covered most of the Fe particle surfaces, though some exposed areas are clearly visible in Figure 2c. This incomplete coverage results from an insufficient concentration of the reducing agent during the later reaction stages, failing to reduce adequate metallic Ag. When the glucose concentration increased to 0.05 mol/L, uniform and dense silver coatings formed on the Fe particle surfaces. These coatings exhibited complete coverage without exposed areas, silver particle agglomeration, or noticeable pores. However, at the highest glucose concentration (0.1 mol/L), the coating uniformity deteriorated with localized protrusions of large silver particle clusters. This non-uniform growth occurs because the excessive reducing agent concentration accelerates the plating rate dramatically, promoting inhomogeneous silver cluster formation. In addition, Figure 2a–c show some white loose clusters absorbed on the Fe particles, which should indeed correspond to Ag clusters formed during the growth process. During electroless silver plating, while some Ag⁺ was reduced at Pd particle sites to form tightly bonded coatings (heterogeneous nucleation), another portion was freely precipitated in the plating solution through the silver mirror reaction, forming larger clusters (homogeneous nucleation). These loose Ag clusters float randomly in the plating solution and may occasionally adhere to the Fe particle surfaces without forming strong bonds. Consequently, their poor conductivity results in brighter SEM images due to charging effects. The presence of these loose Ag particle clusters appears independent of glucose concentration and warrants further investigation. In future studies, the use of reducing agents and buffering additives designed to suppress homogeneous nucleation may help reduce the formation of such loose Ag particle aggregates.

The particle size distribution serves as a crucial indicator for evaluating the uniformity and stability of nanocrystalline grains. Based on the SEM images in Figure 2, grain boundaries were reconstructed and a statistical analysis of Ag grain diameters was performed using Image J software. The results demonstrate that the silver coatings obtained at all three glucose concentrations exhibited normal distribution of grain sizes. Specifically, the 0.02 mol/L glucose concentration produces Ag grains with an average size of 62.4 ± 2.0 nm, with most grains (approximately 68%) distributed in the 20–100 nm range. Increasing the concentration to 0.05 mol/L, the Ag coating shows a larger average grain size of 76.2 ± 2.6 nm, representing a 22% increase compared to that at 0.02 mol/L. At the highest concentration (0.1 mol/L), the average grain size further increases to 87.5 ± 6.2 nm (40% larger than at 0.02 mol/L), with the appearance of large Ag clusters (~3 μm) in the coating, as shown in Figure 2h. Overall, as clearly shown in Figure 3, the Ag grain size exhibited a glucose concentration-dependent growth tendency. This phenomenon can be attributed to the enhanced reaction kinetics at higher glucose concentrations—the increased reducing agent concentration accelerates the initial deposition rate, thereby promoting the formation of larger Ag grains through accelerated nucleation and growth processes.

To characterize the phase composition of silver coatings on Fe particles, XRD analysis was conducted on samples prepared with three different glucose concentrations, as shown in Figure 4a. All silver-plated spherical Fe particles exhibited three sharp diffraction peaks corresponding to the α-Fe phase, while the Ag phase showed weaker diffraction intensities. This indicates significantly lower Ag content compared to Fe, suggesting relatively thin silver coatings in all three cases. Notably, the sample with a 0.1 mol/L glucose concentration displayed a higher Ag peak intensity than the other concentrations, consistent with the thicker coating observed in Figure 2i. Comparatively, the 0.05 mol/L sample showed intermediate Ag diffraction intensity—stronger than 0.02 mol/L but weaker than 0.1 mol/L—indicating better surface coverage while maintaining a relatively thin coating thickness. In addition, the crystallite size can be calculated based on the XRD patterns using Scherrer’s equation:

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }}}$$

(1)

where D represents the Ag crystallite size, K is the shape factor (0.89 for spherical crystals), λ is the X-ray wavelength (0.15406 nm for Cu Kα radiation), β is the full width at half maximum (FWHM), and θ is the diffraction angle. As shown in Figure 4b, the calculated crystallite sizes were 56 nm, 70 nm, and 79 nm for glucose concentrations of 0.02 mol/L, 0.05 mol/L, and 0.1 mol/L, respectively. These values show good agreement with the SEM statistical results (Figure 3) and further confirm the increasing trend of Ag grain size with higher glucose concentrations, as shown in Figure 4b.

3.2. Oxidation Resistance

Figure 5 presents the thermogravimetric analysis (TGA) results of the original spherical Fe particles and the electroless silver-coated Fe particles prepared with three different glucose concentrations (0.02, 0.05, and 0.1 mol/L). All samples exhibited significant oxidative weight gain during heating, revealing distinct oxidation resistance behaviors. In the low-temperature regime (0–400 °C), the sample prepared with 0.05 mol/L glucose demonstrated the most gradual weight gain (slope: 0.12%/°C), indicating enhanced oxidation resistance. A comparative analysis showed a 25% reduction in oxidation rate for the 0.05 mol/L sample versus its 0.02 mol/L counterpart in this temperature range. At 650 °C, the total weight gains were quantified as 112.1 ± 1.0% (original Fe particles), 110.1 ± 0.8% (0.02 mol/L), 107.1 ± 0.6% (0.05 mol/L), and 107.9 ± 0.7% (0.1 mol/L), demonstrating the optimal protection efficiency of the 0.05 mol/L coating. These results demonstrate that the oxidation resistance of Fe particles is improved after electroless silver coating, and a medium glucose concentration (0.05 mol/L) achieves optimal balance between coating continuity and microstructural stability, providing superior oxidation protection.

The oxidation resistance of silver-plated Fe particles is related to the thickness of the silver plating layer and the grain size on the surface. When the glucose concentration is too low, the silver plating layer does not play a good role in oxidation protection because the surfaces of many Fe particles are not completely encapsulated by the Ag layer. When the glucose concentration is 0.1 mol/L, there are more large grains in the silver plating layer, and the size distribution is uneven, which means that cracks can easily form and the invasion of oxygen atoms cannot be effectively protected against, resulting in poor antioxidant capacity. When the glucose concentration is moderate (0.05 mol/L), the silver plating layer has high coverage and a dense coating structure, which can effectively delay the progression of high-temperature oxidation.

4. Discussion

In this experiment, electroless silver plating was performed using glucose as the reducing agent, potassium sodium tartrate as the complexing agent, and silver nitrate as the oxidizing agent. After activation, the Ag⁺ ions in the silver–ammonia solution were reduced to Ag and initially deposited onto the palladium (Pd) metal to form nucleation sites. Subsequently, the reduction of Ag⁺ continued on the newly formed silver nuclei, ultimately achieving electroless silver plating on the particle surfaces. The reaction process between glucose (as the reducing agent) and Ag⁺ is as follows:

$$\mathrm{R}-\mathrm{C}\mathrm{H}\mathrm{O}+2{\mathrm{A}\mathrm{g}}^{+}+3{\mathrm{O}\mathrm{H}}^{-}=\mathrm{R}-{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+2\mathrm{A}\mathrm{g}\downarrow +2{\mathrm{H}}_2\mathrm{O}$$

(2)

Equation (2) shows that electroless silver plating is a classic redox reaction that consumes free hydroxyl ions (OH⁻) in the plating solution. As we know, the reducing ability of glucose mainly comes from the aldehyde group in its molecular structure [23] and strengthens with increasing pH of the solution. Therefore, to facilitate the reaction, this study controlled the initial pH of the solution at 12. Additionally, sodium hydroxide (NaOH) solution (2 g/L) was added after plating times of 15 min and 25 min to replenish the free OH⁻ required for the reaction.

Given that the silver plating reaction continuously consumes OH⁻, the change in OH⁻ concentration (i.e., pH) reflects the progression of the electroless plating process. During the plating process, the pH of the solution was monitored every 5 min, and the results are shown in Figure 6. The slope of the pH change correlates with the rate of electroless silver plating, allowing the plating speed to be roughly estimated based on the rate of pH decline. Clearly, within the 0–10 min period, the plating speed was relatively fast. After adding NaOH solution at 15 min, the subsequent significant drop in pH indicates that the silver plating reaction resumed upon OH⁻ replenishment. However, after adding NaOH solution at 25 min, the lack of significant pH variation suggests that the Ag⁺ in the plating solution had been nearly depleted, making it insufficient to sustain further plating reactions. Additionally, Figure 6 also demonstrates that the glucose concentration significantly affects the plating rate. When a glucose concentration of 0.02 mol/L was used, the relatively low concentration of the reducing agent (glucose) resulted in a slower plating rate. In contrast, when glucose concentrations of 0.05 mol/L and 0.1 mol/L were employed, the electroless silver plating proceeded at a faster rate. Moreover, in the later stages of plating, the pH of the solution remained almost unchanged, suggesting that the Ag⁺ in the solution was completely depleted after 25 min of plating.

The open-chain structure of glucose is CH₂OH-(CHOH)₄-CHO, which contains one aldehyde group (-CHO) and five hydroxyl groups (-OH). In the electroless silver plating reaction, the aldehyde group reduces Ag⁺ to metallic Ag⁰ under alkaline conditions, forming face-centered cubic (FCC)-structured Ag atomic clusters in the plating solution, as illustrated in Figure 7. Meanwhile, according to Equation (2), glucose is oxidized to gluconic acid (CH₂OH-(CHOH)₄-COOH) after reducing Ag⁺. The abundant -OH groups in gluconic acid readily adsorb onto specific crystal planes of Ag clusters, such as {100} [24], thereby inhibiting the rapid growth of the Ag clusters. These fine Ag clusters nucleate on the Pd atoms present on the Fe particle surfaces, ultimately forming a silver coating composed of fine grains, as shown in Figure 2. In electroless plating, Pd atoms (or small atomic clusters) derived from the SnCl₂-PdCl₂ sensitization–activation treatment adsorb onto the substrate surface [25,26], which serve as nucleation sites for coating growth due to their similar crystal structure to the deposited metal.

Herein, glucose serves a dual function: as a reducing agent and as a grain refiner. When the glucose concentration increases, its reducing capability becomes more pronounced. However, the resistance to Ag cluster growth also intensifies, still resulting in a fine-grained Ag coating. Nevertheless, the adsorption kinetics of -OH groups require sufficient time for effective surface coverage. When the glucose concentration is excessively high, the newly reduced Ag clusters may grow rapidly before complete -OH adsorption occurs, leading to the formation of coarse Ag particle aggregates. Therefore, an optimal glucose concentration—such as 0.05 mol/L used in this study—achieves a balance between the reduction rate and adsorption kinetics, facilitating the formation of a fine-grained Ag coating on the Fe substrate.

5. Conclusions

This study presents an optimized electroless silver plating process for spherical Fe particles in an ethanol-based system, using silver nitrate as the metal source and glucose as the reducing agent at room temperature. The key findings are summarized as follows:

• The grain size of silver coatings exhibits a strong dependence on glucose concentration, with higher concentrations leading to coarser Ag grains. The optimal glucose concentration of 0.05 mol/L enables the formation of a dense and well-covered silver layer on Fe particles, featuring an average Ag grain size of approximately 76 nm.
• Thermogravimetric analysis reveals that silver-plated Fe particles prepared with 0.05 mol/L of glucose demonstrate superior oxidation resistance, showing only 107.1% weight gain at 650 °C. In contrast, coatings with coarse Ag microstructures exhibit significantly inferior oxidation resistance compared to their completely coated fine-grained counterparts.
• Glucose serves dual functions in the plating process: as a reducing agent through its aldehyde group for Ag⁺ reduction and as a grain growth barrier via the selective adsorption of hydroxyl groups on Ag {100} facets to inhibit grain growth.

The fundamental insights gained from this work provide new theoretical foundations for developing advanced conductive metal powders and controlling interface structures in composite materials.