2.1. Materials and Test Equipment

Silver nitrate (AgNO₃) and stearic acid (C₁₈H₃₆O₂) were purchased from Sinopharm Chemical Reagent Co. Formaldehyde (HCHO) was purchased from Shanghai McLean Biochemical Technology Co., Ltd. Anhydrous ethanol (C₂H₅OH) was purchased from Tianjin Fuyu Fine Chemical Co. Ammonia (NH₃·H₂O) was purchased from Shanghai McLean Biochemical Technology Co., Ltd. All chemical reagents were used as received without further purification. Sodium hydroxide (NaOH) was synthesized in the laboratory. Deionized water (H₂O) was used for all experiments.

The instruments used in this study include electronic balance (FA2004, Shanghai Sunyu Hengping Scientific Instrument Co., Ltd.), constant speed electric stirrer (JJ-1B 200W, Changzhou Surui Instrument Co., Ltd.), circulating water vacuum pump (SHZ-D, Shanghai Kesheng Instrument Co., Ltd.), ultrasonic cleaner (KQ-3200E, Kunshan Meimei Ultrasonic Instrument Co., Ltd.), and vacuum drying oven (DZE, Beijing Yongmingming Medical Instrument Co.).

2.2. Preparation of Silver Nanoparticles

First, 1.08 g of silver nitrate was weighed and added to 360 mL of deionized water to form a solution. Thereafter, the solution was stirred using an electronic stirrer under the application of a magnetic field. Afterward, 4.5 mL of ammonia and 2 mL of 20 g/L sodium hydroxide were injected into the silver nitrate solution to adjust the pH value to ∼11. Next, 0.06 g of stearic acid was dissolved in alcohol and added to the solution under continuous stirring. We then rapidly added 4.8 mL of formaldehyde, which caused the solution color to transition from brownish-yellow to black. The mixture was stirred for 2 h to form a black suspension. After filtration through a Büchner funnel and two ethanol washes, the product was vacuum-dried to yield micro/nanoscale silver powder. This powder was used for the initial experiments (Figures 1(a) and 1(b)).

In this study, we systematically varied the magnetic field configuration by using different numbers of magnets (1–4). During chemical reduction, we monitored the field strength in the reaction beaker by using a Gauss Meter. The measurements revealed field strength variations that depended on both the number of magnets and their spatial orientation (horizontal vs. vertical), consistent with the magnets’ anisotropic properties.

2.3. Preparation of Silver Nanoparticles on Silicon Single-Crystal Wafers

Silicon wafer (100) was cut into 1 cm × 1 cm × 1 cm pieces, and a measured 4.8 mL formaldehyde was added dropwise using a pipette. Thereafter, the silver nitrate solution with a concentration of 3 g/L was configured on the single-crystal wafer such that the silver nanoparticles were directly deposited on the silicon single-crystal wafer. After 2 h, the silicon single-crystal wafer was collected and three or four sets of experiments were performed in this manner.

2.4. Characterization of the Sample Performance

For each test method, we used three samples prepared under the same batch conditions. The micromorphology of the nanomaterials was observed using a scanning electron microscope (Nova NanoSEM 230, FEI Czech Republic s.r.o., Czech Republic), and the particles were structurally analyzed and characterized using Fourier transform infrared (IR) spectroscopy (model Nicolet iS50, Thermo Scientific, United States). The sample surface was qualitatively, quantitatively, and structurally analyzed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific, United States). Thermogravimetric IR gas chromatography (PerkinElmer TG8000-FTIR-GCMS-ATD, PerkinElmer, United States) was used to determine the thermophysical properties of the sample with respect to temperature or time to characterize the molecular structure of the substance, structure of the polymer space, changes in molecular motion to determine the solid–liquid interface, and other relevant features of the substance.

3.1. Microstructure Analysis

In the macroscopic analysis, all samples obtained via the reaction under the application of different magnetic field strengths were earthy yellow suspensions with negligible variability. After extraction and filtration, the samples were obtained as silvery mixtures, which were washed to yield gray–black powders. At the microscopic level, scanning electron microscopy (SEM) images reveal different particle sizes and morphologies under different magnetic field conditions. When a magnetic field was not applied (Figure 2(a)), the sample surface was rough with small particles at the edges. After the application of a horizontally oriented magnetic field of H = 18.07 mT (Figure 2(b)), the particle surfaces are smooth without unevenness. The particle surfaces are smoother and uniformly distributed under the condition of H = 28.65 mT (Figure 2(c)). Under the conditions of H = 38.71 and 49.27 mT, the surfaces are smooth but possess a few small particles (Figures 2(d) and 2(e)). Overall, SEM observations demonstrated that the samples were formed via the mesoscale assembly of primary nanoparticles, which possessed a spherical or spherical-like morphology, with diameters of 10–100 nm.

The SEM analysis results of the samples are presented in Figure 2(f). Particle size analysis demonstrated that increasing magnetic field strength during synthesis reduces the average silver nanoparticle size from 490 to 340 nm while narrowing the size distribution. The range decreases from 240 to 140 nm. The related data are presented in the form of a line graph in Figure 2(f), indicating that the magnetic field affects the nucleation and growth of the powder particles during the preparation of silver nanoparticles via stirring and injection.

Based on the sample preparation method described in Section 2.2, a magnetic field was applied underneath the beaker and the reaction was performed via the stirring and injection method. Afterward, the silver nanoparticle samples were extracted, filtered, and cleaned. The SEM images of the samples shown in Figure 3 reveal that the particle size and morphology of the silver nanoparticles obtained under different conditions vary. In Figure 3(a), the sample surface is rough under the condition without a magnetic field. In Figure 3(b), after applying a magnetic field of H = 63.06 mT in the vertical direction, the particle surface is smooth and the particle size distribution is uniform. In Figure 3(c), after applying a magnetic field of H = 114.21 mT in the vertical direction, the silver nanoparticles are agglomerated, with smooth surfaces and a uniform particle size distribution. After applying a magnetic field of H = 157.04 mT in the vertical direction (Figure 3(d)), a small number of particles with small particle sizes and smooth surfaces appear. Figure 3(e) presents a smooth surface of the powder after the application of a vertically oriented magnetic field of H = 180.78 mT.

The experimental results reveal that with an increase in the strength of the magnetic field during the reaction process, the average particle size of the prepared silver nanoparticles exhibited an overall decreasing trend and the particle size distribution of the powder particles became narrower. The specific data are presented in the form of a line graph in Figure 3(f). These findings are consistent with the experimental results obtained with the application of the magnetic field in the horizontal direction, i.e., when the strength of the magnetic field increases, the size of the as-prepared sample particles gradually decreases. The width of the distribution of the powder particle size distribution decreases.

3.2. Analysis of IR Spectral Results

Figure 4 presents the IR spectra of the first and second groups of samples. The sample spectra demonstrated flatter peaks at 3450.03 and 1654.62 cm⁻¹. The spectra of the nine groups of experimental samples revealed small peaks at approximately 3450 and 1650 cm⁻¹, which were attributed to O–H stretching vibration and H–O–H bending vibration of water present in the free state in the tested sample powders [72, 73]. Interestingly, in Samples 3 and 4, prepared under magnetic fields of 28.65 and 38.71 mT, respectively, an additional absorption peak near 1100 cm⁻¹ was detected. This peak is commonly assigned to C–O or C–N stretching vibrations, often originating from residual organic species such as citrate ions or other capping/reducing agents used during synthesis. The selective appearance of this peak suggests that the applied magnetic field in this range may have altered the nanoparticle surface structure or growth dynamics. Specifically, moderate-intensity magnetic fields could influence the diffusion of reactants and the alignment of growing crystal facets, possibly resulting in a more developed or porous surface morphology. Such changes may increase the surface area or active sites available for organic species to adsorb, making their IR signatures more detectable. In contrast, samples without a field (sample 1) or with very low (18.07 mT) or high (49.27 mT) magnetic field strengths may undergo different growth regimes. In these conditions, the particle surfaces may be either too compact (leading to minimal surface adsorption) or too kinetically controlled (limiting organic residue binding), hence the absence of the 1100 cm⁻¹ peak.

In addition, a weak but consistently observed absorption band around 2900 cm⁻¹ was detected in all IR spectra, regardless of the applied magnetic field. This region corresponds to the C–H stretching vibrations of methylene (–CH₂–) and methyl (–CH₃) groups. The ubiquitous presence of this signal suggests trace amounts of stearic acid from the synthesis process, which remain weakly adsorbed on the silver nanoparticle surfaces.

3.3. Analysis of XPS Results

To determine the surface valence of the prepared silver nanoparticles, the experimental samples were analyzed via XPS. Figure 5 depicts the XPS full spectra of the silver nanoparticles obtained under the conditions of no magnetic field and horizontally oriented magnetic fields with different strengths. Ag 3d peaks are detected at 373.76 and 367.80 eV, which are attributed to 3d_3/2 and Ag 3d_5/2, respectively. The XPS full spectra reveal the presence of Ag 3p_1/2, Ag 3p_3/2, C 1s, O 1s, and N 1s signals.

In the XPS full spectra of all aforementioned five sets of images, strong Ag  3d_3/2, Ag  3d_5/2, Ag  3p_1/2, Ag  3p_3/2, and C 1s peaks were observed. Furthermore, peaks corresponding to N and O elements were observed in the XPS full spectra. The peak of the O element was noted at ∼531.60 eV. An analysis of the XPS database revealed that the prepared silver nanoparticles were partially oxidized to form AgO. In addition, all peaks of the N element appeared at ∼399.40 eV. Compared with those in the standard Ag 3d_3/2 spectrum, the Ag 3d_3/2 binding energy in all aforementioned Ag spectra increased and all the peaks shifted toward high binding energies. These findings indicate that the generation environment of the silver nanoparticles in the various aforementioned reaction systems differently changed such that the density of the outer electron cloud decreased, resulting in a considerable reduction in its shielding effect on the inner electrons. Furthermore, the binding energy of the inner electrons considerably increased. The peak in the 1s energy spectrum of element N shifts toward low binding energies, indicating that N atoms have gained electrons during the preparation process of the silver nanoparticles.

XPS and IR analyses together provide a comprehensive understanding of the silver nanoparticles’ surface chemistry. XPS revealed partial oxidation to AgO, with the O1s peak at ∼531.60 eV, indicating oxidation and changes in the electronic environment of the silver. IR spectra, showing peaks at 3450 and 1650 cm⁻¹, identified water molecules adsorbed on the nanoparticle surface.

In the chemical reduction method, the role of strong mechanical stirring in diminishing the impact of magnetic fields on silver nanoparticle nucleation is significant [74, 75]. The agitation caused by stirring disrupts the alignment of nanoparticles in the magnetic field, which weakens the magnetic influence during the initial nucleation phase. As a result, we hypothesize that the magnetic field plays a limited role in the early stages of nanoparticle formation, where the nucleation process is primarily driven by chemical reduction without substantial interference from the magnetic forces.

However, the situation changes during the growth phase of silver nanoparticles, where aggregation becomes a key issue. In the absence of a magnetic field, nanoparticles tend to aggregate due to Brownian motion, van der Waals forces, and mechanical stirring [76, 77]. These interactions lead to the formation of larger particle clusters, which can negatively impact the material’s uniformity and performance. This is particularly important when considering applications that require precise control over particle size and dispersion, such as in electronics and catalysis.

When a magnetic field is introduced, the situation changes dramatically. Silver atoms and nanoparticles experience Lorentz forces, which act perpendicular to both their direction of motion and the magnetic field. The exact direction of these forces depends on the orientation of the magnetic field relative to the stirring motion. When the magnetic field aligns parallel to the stirring direction, the silver atoms are pushed toward the top of the container, whereas, if the magnetic field is perpendicular to the stirring direction, the atoms are directed toward the container’s exterior [78–80]. These deviations from the original stirring motion result in a more controlled trajectory for the nanoparticles, reducing their interaction frequency with other particles. As a consequence, the reduced frequency of atomic and particle collisions diminishes the likelihood of nanoparticle aggregation [81]. This mechanism is particularly advantageous in preventing the formation of larger clusters, which often leads to broad-size distributions and compromised material properties. Interestingly, the extent of this effect is directly correlated with the strength of the applied magnetic field: the stronger the magnetic field, the more pronounced the deviation in particle trajectories, which enhances dispersion and results in smaller, more uniform particle sizes.

The application of magnetic fields in chemical reduction for the synthesis of silver nanoparticles offers substantial advantages in terms of particle purity, uniformity, and size control. By improving dispersion and reducing aggregation, this technique enhances the overall quality of the synthesized nanoparticles, making them more suitable for advanced applications in electronics, catalysis, and antimicrobial treatments. Precise control of particle growth and size distribution optimizes material performance while enabling tailored property adjustments for specific applications.

Looking to the future, further optimizations in magnetic field control could lead to even more significant improvements in the synthesis process. Potential breakthroughs in this area may unlock new possibilities in the development of high-performance materials for use in advanced electronics, next-generation catalysts, and cutting-edge medical treatments. These advancements could pave the way for more efficient, sustainable, and customizable nanoparticle-based technologies, underscoring the critical role of magnetic fields in controlling nanoparticle synthesis.