Synthesis of Copper, Silver, and Copper–Silver Powders by Hydrogen-Assisted Ultrasonic Spray Pyrolysis

Abstract

Copper (Cu), silver (Ag), and copper–silver (Cu–Ag) powders were synthesized using ultrasonic spray pyrolysis (USP) combined with hydrogen-assisted reduction in order to examine how key processing parameters influence particle characteristics. The effects of reduction temperature, gas atmosphere, and precursor molar ratio on particle morphology, size distribution, and elemental composition were systematically investigated. Aqueous precursor solutions of copper nitrate trihydrate and silver nitrate were atomized in a USP reactor and thermally treated under hydrogen-containing or argon atmospheres at temperatures between 500 and 700 °C. The resulting powders were characterized by scanning electron microscopy (SEM), particle size analysis using ImageJ, and energy-dispersive X-ray spectroscopy (EDS). The results showed that both temperature and gas atmosphere strongly affected particle formation. Hydrogen-assisted synthesis promoted efficient reduction and high metal purity but was associated with increased particle coalescence, whereas argon atmospheres yielded finer and more uniform particles through thermally driven decomposition. In the case of Cu–Ag powders, the precursor molar ratio played a decisive role in particle stability. A 1:1 Cu:Ag ratio produced uniform particles with reduced susceptibility to surface oxidation, while Ag-rich compositions (1:3 Cu:Ag) showed increased agglomeration and partial oxidation after synthesis. Overall, this study demonstrates that careful adjustment of gas atmosphere, synthesis temperature, and precursor composition enables control over the morphology and compositional stability of Cu, Ag, and Cu–Ag powders produced by USP. These findings provide practical guidance for the scalable preparation of mono- and bimetallic metal powders for applications in electronics, catalysis, and energy-related technologies.

1. Introduction

Copper (Cu) and silver (Ag) are technologically important metals owing to their high electrical and thermal conductivity, chemical stability, and broad applicability in electronics, catalysis, and functional coatings [1]. In recent years, Cu–Ag bimetallic systems have attracted increasing attention because the combination of these two metals can result in synergistic properties that differ from those of the individual components, such as enhanced catalytic selectivity, improved oxidation resistance, and tunable electrical behavior. These characteristics make Cu–Ag materials promising candidates for applications in conductive inks, electrocatalysis, plasmonic devices, and energy-related technologies [2,3,4,5].

Despite strong interest in Cu, Ag, and Cu–Ag powders for conductive inks and catalytic systems, practical implementation is often limited by (i) oxidation and conductivity loss of Cu during storage and processing, (ii) particle sintering/agglomeration at elevated temperatures, (iii) surface contamination from organic reagents used in wet-chemical routes, and (iv) scalability and batch-to-batch reproducibility challenges. These issues motivate scalable gas-phase synthesis routes capable of producing clean surfaces and controllable morphologies under continuous conditions.

A wide range of synthesis methods has been reported for Cu, Ag, and Cu–Ag powders, including wet chemical reduction, electrodeposition, solvothermal routes, and physical vapor-based techniques. While these approaches allow fine control over particle size and composition, they often rely on organic reducing agents, surfactants, or stabilizers, which can introduce impurities, complicate post-treatment, and limit scalability. In addition, batch-type synthesis routes may suffer from poor reproducibility and limited throughput, restricting their suitability for large-scale production [3,6,7,8,9,10,11,12,13].

Ultrasonic spray pyrolysis (USP) represents an alternative gas-phase synthesis technique that enables continuous production of fine powders from liquid precursor solutions. In USP, precursor droplets generated by ultrasonic atomization undergo solvent evaporation, thermal decomposition, and particle formation within a high-temperature reactor. Compared to wet chemical methods, USP offers several advantages, including a continuous and scalable process, relatively short residence times, precise control over precursor composition, and reduced risk of contamination from organic additives. When combined with a reducing atmosphere, USP can be adapted for the synthesis of metallic and bimetallic particles with controlled morphology and composition [14,15,16].

Prior USP and spray-based studies have demonstrated the feasibility of producing metallic Cu and Ag powders and, in some cases, Ag–Cu composites; however, most reports focus on single-metal systems or do not systematically compare the coupled effects of temperature, gas atmosphere, and Cu:Ag precursor ratio under hydrogen-assisted conditions. As a result, parameter–morphology–stability relationships for USP–HR-derived Cu–Ag powders remain insufficiently established.

Despite these advantages, systematic studies on the synthesis of Cu, Ag, and Cu–Ag powders via USP—particularly under hydrogen-assisted reduction conditions—remain limited. Existing reports often focus on single-metal systems or rely on inert atmospheres, while the combined influence of synthesis temperature, gas atmosphere, and precursor ratio on particle morphology and compositional stability has not been thoroughly examined for Cu–Ag systems produced by USP. In particular, understanding how process parameters affect particle size distribution, agglomeration behavior, and susceptibility to post-synthesis oxidation is essential for tailoring materials for practical applications.

In this work, Cu, Ag, and Cu–Ag powders are synthesized using ultrasonic spray pyrolysis combined with hydrogen reduction (USP–HR). The effects of reduction temperature, gas atmosphere, and precursor molar ratio on particle morphology, size distribution, and elemental composition are systematically investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Rather than proposing new formation mechanisms, this study aims to provide process-level insight into how USP–HR parameters influence the characteristics of mono- and bimetallic Cu–Ag powders. The results contribute to the development of scalable synthesis strategies for Cu-based and Ag-based materials with tunable properties, relevant for applications in electronics, catalysis, and energy-related technologies.

It should be emphasized that the objective of the present work is not to propose new fundamental nucleation or growth mechanisms for Cu–Ag bimetallic systems, which have been extensively investigated in wet chemical and electrochemical synthesis routes. Instead, this study focuses on evaluating how processing parameters in a gas-phase ultrasonic spray pyrolysis–hydrogen reduction (USP–HR) system influence particle morphology, size distribution, and compositional stability under continuous synthesis conditions.

2. Results

2.1. Copper Powders Produced via USP–HR 105

Particle size analysis in this study was performed using SEM images evaluated by ImageJ 1.54 (Windows, Java 8) software, with a minimum of 100 particles measured per sample. Average particle diameters are therefore reported together with standard deviation to reflect the width of the size distributions rather than as absolute values. Elemental compositions obtained by EDS are presented as semi-quantitative results, as the technique is inherently limited by interaction volume effects, surface sensitivity, and the absence of absolute error estimation. Consequently, EDS data are interpreted in terms of compositional trends and relative changes between samples rather than precise quantitative phase analysis.

2.1.1. Temperature-Dependent Morphology and Particle Size Evolution of Cu Particles

Temperature had a pronounced influence on the morphology, particle size, and compositional trends of copper particles synthesized via ultrasonic spray pyrolysis (USP). Figure 1 shows SEM images (1 μm magnification) of particles obtained from a 1 M copper nitrate precursor solution at synthesis temperatures of 550, 600, 650, and 700 °C.

At 550 °C, the particles exhibited cauliflower-like agglomerates characterized by fused clusters and a broad particle size distribution. With increasing temperature under USP/H₂ conditions, the particles became finer and more discrete; however, particle agglomeration also became more pronounced at the highest temperatures.

Figure 2. Average size of Cu particles produced at 1 mol/L. (a) 550 °C, (b) 600 °C, (c) 650 °C and (d) 700 °C.

Figure 2. Average size of Cu particles produced at 1 mol/L. (a) 550 °C, (b) 600 °C, (c) 650 °C and (d) 700 °C.

The average particle size decreased from approximately 580 nm (at 550 °C) to about 100 nm (at 700 °C), as shown at Figure 2. Between 550 °C and 650 °C, this size reduction was accompanied by a narrower particle size distribution.

2.1.2. Elemental Composition of Cu Particles

As shown in

Table 1. EDS results of Cu produced at different temperatures at 1 mol/L.

Temperature (°C)	Element (wt. %)
	Cu	O	Cl	Si	Al	C	Total
550	90.5	8.9	-	0.6	-	-	100
600	94.6	5.4	-	-	-	-	100
650	94.7	4.6	0.3	-	0.4	-	100
700	98.1	1.2	-	-	-	0.7	100

, EDS analysis revealed that the copper content remained nearly constant across all temperatures, while the oxygen content decreased with increasing temperature, indicating improved reduction efficiency and higher metal purity. The trace amount of silicon detected in the 550 °C sample (0.6 wt%) is attributed to minor contamination originating from the quartz reactor tube or sample handling, which is commonly observed in high-temperature spray pyrolysis systems. A similar explanation applies to the other trace impurities detected.

2.2. Silver Powders Produced via USP–HR

SEM analysis revealed that silver nanoparticles synthesized at 600 °C were mostly spherical, smooth, and well-defined, with limited agglomeration indicating minimal sintering.

Figure 3. SEM and Particle size distribution of Ag particles. (a) 600 °C and (b) 700 °C.

Figure 3. SEM and Particle size distribution of Ag particles. (a) 600 °C and (b) 700 °C.

The average particle diameter was approximately 296 nm, exhibiting a narrow size distribution, as shown at Figure 3. At 700 °C, the particles maintained a spherical morphology but displayed a more compact and uniform size distribution compared to those obtained at 600 °C. ImageJ analysis indicated an average particle diameter of about 178 nm, confirming a significant reduction in size with increasing temperature.

Figure 4. EDS spectrums for Ag particles produced at (a) 600 °C and (b) 700 °C.

Figure 4. EDS spectrums for Ag particles produced at (a) 600 °C and (b) 700 °C.

EDS analysis verified that samples synthesized at both 600 °C and 700 °C consisted entirely of metallic silver, with no detectable impurities, confirming complete reduction of silver nitrate, as shown at Figure 4. This indicates that the USP–HR process efficiently converted the precursor to metallic Ag under both conditions. For comparison, silver nanoparticles were additionally synthesized under an argon atmosphere, in the absence of hydrogen, to assess the influence of the reducing environment.

2.3. Silver Powders Produced Without Hydrogen (Only Argon)

For this synthesis, the same silver nitrate precursor solution (S2) was used, with argon as the carrier gas at a fixed flow rate of 1 L/min, producing smooth silver particles at both 600 °C and 700 °C. Under these conditions, ultrasonic spray pyrolysis (USP) produced predominantly spherical. At 600 °C, the morphology appeared heterogeneous, consisting of both large and small spheres with slight agglomeration. Increasing the temperature to 700 °C led to more uniform, well-packed spheres with improved homogeneity and fewer irregularities compared to the 600 °C sample. Quantitative ImageJ analysis confirmed these morphological observations. The average particle diameter at 600 °C was approximately 225 nm, while at 700 °C, it decreased to around 148 nm, as shown at Figure 5, indicating that higher temperatures promoted the formation of finer and more uniform particles.

Figure 5. Particle size distribution of silver (Ag) Produced without hydrogen at (a) 600 °C, and (b) 700 °C.

Figure 5. Particle size distribution of silver (Ag) Produced without hydrogen at (a) 600 °C, and (b) 700 °C.

Figure 6. EDS analysis of silver (Ag) (only Ar) at (a) 600 °C and (b) 700 °C.

Figure 6. EDS analysis of silver (Ag) (only Ar) at (a) 600 °C and (b) 700 °C.

EDS analysis at Figure 6 revealed the presence of only silver peaks, confirming that the particles were composed of pure metallic silver. This demonstrates that complete precursor reduction occurred even under argon, despite the absence of an external reducing agent such as hydrogen.

2.4. Cu-Ag Produced via USP–HR

2.4.1. 1:1 Cu-Ag Ratio

Cu–Ag composite particles were synthesized at 650 °C using 0.5 M copper nitrate trihydrate and 0.5 M silver nitrate precursor solutions, with hydrogen and argon flow rates each fixed at 1 L/min.

Figure 7. SEM (a) and EDS (b) results of 1:1 Cu:Ag NPs at 650 °C.

Figure 7. SEM (a) and EDS (b) results of 1:1 Cu:Ag NPs at 650 °C.

SEM observations at Figure 7 revealed that the powders consisted of uniformly spherical particles with minimal whisker formation, indicating efficient particle growth and a stable morphology at this synthesis temperature. SEM-based ImageJ analysis indicates an average particle diameter of 244 nm, reflecting a moderately narrow size distribution. EDS surface enrichment suggests preferential silver localization at the particle surface; however, definitive confirmation of a core–shell structure would require transmission electron microscopy, which is beyond the scope of the present study.

2.4.2. 1:3 Cu-Ag Ratio

The 1:3 Cu:Ag sample indicates larger particles (≈280 nm) and more pronounced clustering compared to the 1:1 Cu:Ag sample, which showed smaller, more uniform spheres averaging ≈244 nm. The observed differences indicate a strong dependence of particle morphology on the Cu:Ag ratio.

Figure 8. SEM (a) and EDS (b) results of 1:3 Cu:Ag NPs at 650 °C.

Figure 8. SEM (a) and EDS (b) results of 1:3 Cu:Ag NPs at 650 °C.

EDS spectra at Figure 8 revealed the presence of both Cu and Ag, with surface-enriched Ag signals relative to Cu, suggesting preferential silver localization at the particle surface. The 1:3 Cu:Ag powder contained approximately 10 wt% oxygen, likely resulting from post-synthesis oxidation during storage, while the 1:1 Cu:Ag sample remained oxygen-free. The latter also displayed smoother and more spherical particles, suggesting enhanced reduction efficiency and morphological uniformity at this stoichiometric ratio.

3. Discussion

3.1. Influence of Temperature on Particle Morphology and Purity

Higher synthesis temperatures accelerate solvent evaporation and reduction kinetics, promoting rapid nucleation and limiting particle growth during the early stages of particle formation. However, excessive heating, especially at temperatures around 700 °C, enhances interparticle interactions and sintering, which explains the increased agglomeration observed at the highest temperature. The formation of smaller and more uniform particles at elevated temperatures under a hydrogen-containing atmosphere suggests that hydrogen facilitates reduction kinetics and promotes burst nucleation, leading to the rapid formation of numerous fine nuclei. In contrast, at lower temperatures such as 600 °C, reduced nucleation rates allow fewer nuclei to grow for longer periods before precursor depletion, resulting in larger particle sizes.

Comparable temperature-dependent trends in particle morphology have been reported in previous spray-based and thermal synthesis studies, although the dominant mechanisms strongly depend on the synthesis route and operating conditions [10]. Additionally, literature reports on copper oxide systems indicate that elevated temperatures can influence the oxidation state of copper, as evidenced by residual oxygen contents after thermal treatment, which is consistent with the compositional trends observed in the present work [11].

3.2. Influence of Gas Atmosphere on Particle Formation

The influence of gas atmosphere on particle size and morphology in the present study is interpreted on a phenomenological basis, considering differences in reduction kinetics, nucleation rates, and particle growth behavior under hydrogen-containing and inert atmospheres.

From a thermodynamic perspective, hydrogen provides a strongly reducing environment that favors conversion of transient oxide or nitrate-derived intermediates to the metallic state at elevated temperatures, thereby shifting reactions toward metal formation. Under argon, metal formation relies on thermal decomposition pathways and transient oxide intermediates, which alters the nucleation–growth balance and can lead to different coalescence behavior. In the present work, these thermodynamic tendencies are used only to support a qualitative, process-level interpretation consistent with SEM/EDS trends.

The SEM and EDS results for silver powders synthesized under different gas atmospheres highlight the critical role of the reaction environment in determining particle morphology and compositional trends. EDS spectra showed only Ag-related signals within the detection limits of the technique, indicating effective reduction of the silver nitrate precursor under both hydrogen-containing and argon atmospheres. Such compositional purity is particularly important because the electrical, catalytic, and plasmonic properties of silver particles are highly sensitive to oxide formation and surface contamination [4].

In hydrogen-assisted synthesis, the reducing atmosphere is expected to enhance reduction kinetics, leading to rapid conversion of the precursor and promoting metallic silver formation. The accelerated reduction in hydrogen leads to rapid formation of metallic nuclei at an early stage, which favors subsequent particle growth and coalescence within the high-temperature zone, resulting in larger average particle sizes. In contrast, under argon, silver nitrate reduction proceeds primarily through thermally driven decomposition, with Ag₂O acting as a transient intermediate before conversion to metallic Ag. As argon does not provide reactive reducing species, metal formation relies on thermally driven decomposition, which delays nucleation and increases supersaturation within droplets, promoting the formation of finer and more uniform particles. At lower temperatures, slower nucleation rates favor the formation of fewer nuclei and consequently larger particles with broader size distributions. Increasing the synthesis temperature enhances evaporation rates and supersaturation within the droplets, generating more nucleation sites and resulting in finer and more uniform particles.

The reduction of silver under argon conditions can also be described by an autocatalytic deposition mechanism, in which initially formed silver nuclei catalyze further reduction of remaining silver species [12]. In the present study, silver powders produced under hydrogen-containing and argon atmospheres exhibited comparable compositional trends within the semi-quantitative limits of EDS analysis, suggesting that sufficiently high synthesis temperatures enable effective silver reduction even in the absence of hydrogen. Nevertheless, the observed differences in particle size and morphology indicate that the gas atmosphere remains an important parameter for tailoring particle characteristics in USP-based synthesis.

3.3. Influence of Precursor Ratio on Morphology and Stability

Cu–Ag composite powders synthesized at 650 °C exhibited distinct morphological and compositional differences depending on the Cu:Ag precursor ratio. These observations are consistent with the well-documented immiscibility of copper and silver, which promotes phase segregation tendencies commonly associated with core–shell-type configurations rather than homogeneous solid solutions [13]. During hydrogen-assisted reduction, silver atoms—owing to their lower surface energy—tend to migrate toward the particle surface, resulting in Ag-enriched outer regions relative to the copper-rich interior.

Such Ag-enriched surface configurations have been reported in the literature to enhance oxidation resistance by shielding the copper component and may impart favorable surface characteristics for catalytic or plasmonic applications [17]. The effective reduction achieved at 650 °C demonstrates that the USP–HR process is capable of producing metallic Cu–Ag composites under continuous synthesis conditions.

The Ag-rich (1:3 Cu:Ag) sample exhibited larger particle sizes and a higher degree of clustering compared to the equimolar composition. This behavior can be attributed to the higher silver content, which influences particle growth dynamics and promotes aggregation during synthesis. Excess silver enhances surface diffusion and coalescence, leading to localized particle growth and denser packing. At the same time, Ag-rich particles showed a greater tendency toward post-synthesis oxidation, as indicated by the oxygen detected by EDS. In contrast, the 1:1 Cu:Ag sample displayed improved morphological uniformity and compositional stability, suggesting a more favorable balance between homogeneity, oxidation resistance, and structural integrity.

Overall, these results indicate that increasing silver content can enhance surface mobility but may compromise long-term stability, whereas an equimolar Cu:Ag ratio provides more uniform and stable particles under the investigated conditions. While Cu–Ag bimetallic systems have shown promising performance in applications such as electrochemical CO₂ reduction, further optimization and comprehensive characterization are required to address remaining challenges related to stability and performance.

The Ag-enriched surface features observed in this study are consistent with prior reports on Cu–Ag systems synthesized via wet chemical and electrochemical methods, where differences in surface energy and reduction kinetics favor silver segregation. No new nucleation or shell-formation mechanisms are proposed here. Rather, the present results demonstrate that similar compositional tendencies can be achieved under gas-phase USP–HR conditions without the use of surfactants or post-synthesis treatments, underscoring the capability of this approach to reproduce known structural features in a scalable and continuous manner.

It should be emphasized that, in the absence of X-ray diffraction and transmission electron microscopy, the present discussion is limited to compositional and morphological trends inferred from SEM and EDS analysis. Consequently, definitive determination of crystallographic structure, phase composition, and internal particle architecture is beyond the scope of this study.

4. Materials and Methods

4.1. Materials

Copper nitrate trihydrate and silver nitrate were selected due to their high solubility in water and their well-documented thermal decomposition behavior reported in the literature, which makes them suitable precursors for spray-based thermal synthesis routes [12]. Four sets of precursor solutions were prepared and are designated as S1–S4, as shown in Figure 9.

Figure 9. Precursor solutions used in Ultrasonic spray pyrolysis experiments: (a): 1 M Cu(NO₃)₂·3H₂O solution. (b): 1 M AgNO₃ solution. (c): Cu–Ag precursor solution with a 1:1 molar ratio (0.5 M Cu²⁺: 0.5 M Ag⁺). (d): Cu–Ag precursor solution with a 1:3 molar ratio (0.25 M Cu²⁺: 0.75 M Ag⁺).

Figure 9. Precursor solutions used in Ultrasonic spray pyrolysis experiments: (a): 1 M Cu(NO₃)₂·3H₂O solution. (b): 1 M AgNO₃ solution. (c): Cu–Ag precursor solution with a 1:1 molar ratio (0.5 M Cu²⁺: 0.5 M Ag⁺). (d): Cu–Ag precursor solution with a 1:3 molar ratio (0.25 M Cu²⁺: 0.75 M Ag⁺).

Under constant stirring, 1 mol/L copper nitrate trihydrate and 1 mol/L of silver nitrate precursor solutions were prepared separately; the silver solution was kept protected from light to avoid photodecomposition. Two Cu–Ag precursor mixtures were then formulated: one with a 1:1 molar ratio (0.5 M each) and another with a 1:3 molar ratio (0.25 M Cu²⁺: 0.75 M Ag⁺). These compositions were designed to study how precursor ratio affects the structural, catalytic, and oxidation-resistance properties of the synthesized Cu–Ag powders.

4.2. Methods

Copper (Cu), silver (Ag), and copper–silver (Cu–Ag) powders were synthesized via Ultrasonic Spray Pyrolysis combined with Hydrogen Reduction (USP–HR). A schematic representation of the USP–HR experimental setup is shown in Figure 10, while the synthesis parameters used for Cu, Ag, and Cu–Ag powders are summarized in Table 2 In this process, a liquid precursor solution is ultrasonically atomized into fine droplets that are transported through a high-temperature reactor, where solvent evaporation, salt decomposition, and reduction yield solid metallic particles. The precursor solutions were ultrasonically atomized using a laboratory-scale ultrasonic nebulizer operating at a frequency of 1.7 MHz. Thermal decomposition and reduction were carried out in a horizontal tubular electric furnace equipped with a quartz reaction tube.

Figure 10. Schematic view of the USP–HR setup used for the synthesis of Cu, Ag, and Cu–Ag powders, showing the introduction of the aqueous precursor (stock) solutions into the ultrasonic atomizer.

Figure 10. Schematic view of the USP–HR setup used for the synthesis of Cu, Ag, and Cu–Ag powders, showing the introduction of the aqueous precursor (stock) solutions into the ultrasonic atomizer.

After the furnace temperature stabilized, hydrogen gas (1 L/min) was introduced as a reducing agent, resulting in residence times of approximately 15.8 s under argon and 7.9 s with hydrogen. The atomizer temperature was maintained at 23 °C using a thermostatically controlled water-cooling system, and the setup was purged with argon before and after each experiment to ensure safe operation. Copper powders were synthesized from copper nitrate trihydrate at temperatures ranging from 550 to 700 °C to investigate the effect of temperature on particle properties. Silver powders were produced from silver nitrate under two atmospheres: H₂/Ar and pure Ar at 600 °C and 700 °C, to study the role of hydrogen reduction. Cu–Ag bimetallic powders were prepared using two precursor ratios.

Table 2. Parameters used for the synthesis of Copper, Silver, and Copper–Silver Powders.

Solutions	Sample Code	Temp. °C	Concentration (mol/L)	H2 Flow Rate (L/min)	Ar Flow Rate (L/min)
S1	Cu-1	550	1	1	1
	Cu-2	600	1	1	1
	Cu-3	650	1	1	1
	Cu-4	700	1	1	1
S2	Ag-6	600	1	1	1
	Ag-7	700	1	1	1
	Ag-8	600	1	0	1
	Ag-9	700	1	0	1
S3	Cu-Ag	650	0.5 M each	1	1
S4	Cu-Ag	650	0.25 Cu and 0.75 M Ag	1	1

Table 2. Parameters used for the synthesis of Copper, Silver, and Copper–Silver Powders.

Solutions	Sample Code	Temp. °C	Concentration (mol/L)	H2 Flow Rate (L/min)	Ar Flow Rate (L/min)
S1	Cu-1	550	1	1	1
	Cu-2	600	1	1	1
	Cu-3	650	1	1	1
	Cu-4	700	1	1	1
S2	Ag-6	600	1	1	1
	Ag-7	700	1	1	1
	Ag-8	600	1	0	1
	Ag-9	700	1	0	1
S3	Cu-Ag	650	0.5 M each	1	1
S4	Cu-Ag	650	0.25 Cu and 0.75 M Ag	1	1

The selection of the synthesis temperature range (500–700 °C) was guided by literature-reported thermal decomposition behavior of copper nitrate trihydrate and silver nitrate, which decompose at substantially lower temperatures than those applied in the present USP–HR experiments. Thermogravimetric and calorimetric analyses were not performed in this study; therefore, precursor decomposition temperatures were inferred from established references. The experimental design was structured to address different process-related questions for each material system while ensuring stable operation of the continuous USP–HR reactor. For copper powders, multiple synthesis temperatures (550–700 °C) were investigated to systematically evaluate the influence of temperature on particle size evolution and reduction efficiency. For silver powders, a narrower temperature range was selected and the gas atmosphere was varied to isolate the effect of hydrogen-assisted reduction relative to thermally driven decomposition under argon. In the case of Cu–Ag bimetallic powders, synthesis was conducted at a fixed intermediate temperature of 650 °C, identified based on the monometallic Cu and Ag experiments, in order to examine the influence of precursor molar ratio on particle morphology and oxidation behavior without introducing additional temperature-related variables.

5. Conclusions

This study demonstrates that gas atmosphere, synthesis temperature, and precursor molar ratio have a pronounced influence on the morphology and elemental composition of Ag, Cu, and Cu–Ag powders produced by ultrasonic spray pyrolysis combined with hydrogen reduction. Variations in processing conditions resulted in clear differences in particle size distribution, agglomeration behavior, and susceptibility to surface oxidation. For copper powders, synthesis at intermediate temperatures (600–650 °C) led to more uniform particle morphologies, whereas higher temperatures promoted finer particles but with increased agglomeration. In Cu–Ag systems, the precursor ratio played a critical role, with Ag-rich compositions exhibiting enhanced particle coalescence and a higher tendency toward post-synthesis oxidation compared to equimolar Cu–Ag powders.

Overall, the results highlight the strong sensitivity of USP-based synthesis to processing parameters and demonstrate that controlled adjustment of temperature, gas atmosphere, and precursor composition provides an effective means of tailoring particle characteristics. The conclusions drawn in this work are based on morphological and compositional trends observed by SEM and EDS and are intended to provide process-oriented insight rather than mechanistic interpretation at the atomic scale.

The present study does not constitute a full parametric optimization of the USP–HR process, and temperature-dependent effects in Cu–Ag bimetallic systems were not systematically explored. These aspects will be addressed in future work through expanded experimental matrices.

Future work will focus on incorporating complementary characterization techniques to overcome the limitations of the present analysis. X-ray diffraction (XRD) will be employed to assess crystallinity and phase composition, while TEM/HRTEM will be used to resolve internal particle structures; additionally, X-ray photoelectron spectroscopy (XPS) will be considered to evaluate surface oxidation states where relevant. Additional techniques, such as UV–Vis spectroscopy, will be considered where optical properties are relevant. Furthermore, systematic investigations of oxidation stability, particle growth behavior, and application-oriented performance—such as catalytic or plasmonic activity—are required to establish robust structure–property relationships. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses will also be incorporated in future studies to enable a more precise correlation between precursor decomposition behavior and particle formation during the USP–HR process.