Exploring Diverse Synthesis Pathways for Copper- and Silica-Based Janus Nanoparticles

Abstract

Janus nanoparticles (JNPs) synthesis has caught the scientific community’s attention due to their amphiphilic properties and extensive areas of application. In this work, different new copper–silica-based and silica-based JNPs were synthesized using a novel masking methodology and a self-assembly method based on sol–gel procedures, respectively. Moreover, various techniques were used to characterize the developed nanomaterials, including scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Two types of copper–silica-based Janus nanoparticles were synthesized with a 40 to 70 nm size, while SiO₂-based JNPs of around 135 nm were obtained. The duality of different JNPs was confirmed by SEM and by a simple and economical route based on an emulsion stabilization path: analyzing dispersion/aggregation and associated behavior at the immiscible solvent interface. JNPs exhibited an extended residence time over 20 days at an immiscible solvent interface, thereby enhancing the resulting emulsion interface stability. This behavior highlighted their amphiphilic characteristics in comparison to conventional nanoparticles. Consequently, a procedure to determine nanoparticle amphiphilicity could be further standardized.

1. Introduction

Janus nanoparticles (JNPs) are a unique class of nanomaterials characterized by their asymmetrical structure due to two distinct chemical compositions or functional groups within a single nanoparticle (NP). This dual composition confers amphiphilic properties, which generate fascinating performances, e.g., distinctive surface properties between both sides and a capacity to develop clusters with an engineered size and shape [1,2]. Therefore, different applications have been proposed for energy storage [3], biomedicine [4], environmental remediation [5], and oriented catalysts [6], among many others [7,8,9].

Nanomaterial synthesis is categorized into top-down and bottom-up methods. In the top-down approach, NPs are obtained by modifying existing bulk particles through general physical techniques such as ball milling, lithography, and laser ablation. In the bottom-up approach, nanomaterials are synthesized from atomic or molecular species through chemical reactions for nucleus formation in a controlled growth process, allowing nanoparticle size and morphology control. The most used methods are chemical vapor deposition, hydrothermal, and sol–gel techniques [10]. Amphiphilic JNPs are usually synthesized by masking, self-assembling, and phase separation. One of the most common pathways is a self-assembly process, where building blocks associate spontaneously [11]. Silica NPs are popular for their ease of modification, high porosity, and excellent adsorption capacity. They offer superior colloidal stability, mechanical strength, and thermal stability compared to polymer-based NPs. Furthermore, they are chemically inert, biocompatible, and non-toxic [12].

The amphiphilic SiO₂ JNPs leverage their dual interfacial properties in a range of applications, functioning as solid surfactants and interface stabilizers. In oil–water separation, JNPs enable efficient demulsification and pollutant removal. Magnetic Janus-like SiO₂/polymer nanoparticles adsorb at oil–water interfaces and, under a magnetic field, coalesce and remove emulsified droplets, achieving >99% separation efficiency and >98% recyclability [13]. Likewise, Janus-based membranes exhibit high performance: a lignin–nanoparticle-bridged Janus paper achieved >99% separation efficiency for emulsified oils, delivered ultra-high fluxes (~5500–6300 L·m⁻²·h⁻¹·bar⁻¹), and maintained antifouling properties over 10 cycles [14]. In enhanced oil recovery (EOR), Janus SiO₂ materials significantly improve displacement efficiency. Temperature-responsive Janus SiO₂ particles formed stable emulsions and reduced interfacial tension, boosting oil recovery by ~16.5% with just 0.03 wt. % addition [15]. Similarly, Janus-reinforced polymer gel microspheres (conformance agents + interfacial activity) achieved ~19.7% extra recovery in core flooding, outperforming conventional polymer systems [16]. Janus polymer nanosheets at 1–50 mg∙L⁻¹ acted as ultra-efficient emulsifiers, lowering IFT and altering core wettability, resulting in enhanced flooding and reduced injection pressures [17]. Amphiphilic Janus carbon nanotubes formed stable O/W nano-emulsions, reduced oil viscosity by ~99%, and converted oil-wet rock toward neutral-wet, enabling >20% EOR efficiency [18]. In foam-assisted gas injection EOR, SiO₂ JNPs enhanced foam stability across CH₄, CO₂, and air media. Oleic acid-modified JNPs reduced surface tension by ~25 mN∙m⁻¹ compared to bare SiO₂ and maintained foam height (~80%) for over 2 h [19]. Beyond oil applications, Janus SiO₂ has been integrated into coatings and colloidal systems: snowman-shaped polymer JNPs encapsulated silicone oil droplets in a thin shell, which, when added to paint, improved stain resistance without typical cratering effects [20]. More broadly, JNPs act as hybrid surfactant–particle emulsifiers in nanofluids, stabilizing Pickering emulsions and dispersions that require both steric and amphiphilic effects [13,21].

Metallic hybrid JNPs are also being exploited for several applications, such as Ag-TiO₂-L-arginine for pathogenic bacteria suppression [22] and TiO₂-Au JNPs for antitumor therapy [23]. However, the use of abundant, non-toxic metals such as copper remains merely promising. Across the compiled Cu-based Janus nanoparticle literature, the reported systems cluster into three material families defined by the bimetallic Janus domain, “soft–hard” hybrid architectures, and a template-mediated interfacial growth family; each is typically associated with a characteristic synthesis route. Cu–Fe “Janus”-like bimetallic nanoparticles were produced by the simultaneous electrical explosion of twisted Cu and Fe wires in argon [24]. CuO–Ag JNPs were generated by the simultaneous electrical explosion of twisted Cu–Ag wires in an argon/oxygen buffer gas mixture [25]. Cu–Ag JNPs were prepared by a wet-chemical route mixing AgNO₃(aq), CuCl₂, glucose, and hexadecylamine and heating at 100 °C for 2 h [26]. Au–Cu Janus nanostructures appear in two works via seeded growth [27,28]. In Cu₂−xSe–Au Janus nanoparticles, Au cores were first formed, followed by surface functionalization [29]. Cu₃P–Cu₂O JNPs embedded in porous carbon were enabled at scale via the carbonization of Cu²⁺-containing ion-exchange resins with KOH [30]. CuS–ZnS JNPs were synthesized by thermolysis in octadecene/dodecanethiol [31]. In the second category, soft–hard hybrid JNPs include Prussian Blue/CuS [32], and polydopamine–Cu (PDA/Cu) JNPs [33]. Beyond these, the template-mediated interfacial growth group can be illustrated by Cu-sSiO₂&rPMO mesoporous JNPs (sSiO₂ means hydrophilic SiO₂ nanospheres and rPMO refers to periodic mesoporous organosilica) [34]; and a biomedical “decorated Janus” variant couples copper peroxide (CP) nanodots onto a gold nanostructure/silica nanorod Janus scaffold, assembled via silica nanorod sol–gel growth [35]. The current development of mesoporous JNPs is primarily constrained to functionally inert SiO₂ and polymer materials. Furthermore, the synthesis of metal compound Janus architectures, including the selective assembly of Cu on the hydrophilic SiO₂ compartment, is deemed “urgently needed” yet poses considerable challenges. Further research into new and versatile synthesis methods for Cu–SiO₂ Janus nanoparticles is necessary, as the functionalization of the silica side holds significant potential for applications across various fields.

Moreover, JNP applications face two main challenges: high synthesis costs and large-scale production complexity. To address these issues, research on silica JNPs can be focused on optimizing reactants, characterization techniques, and scalability [2,36]. Various advanced techniques are employed to characterize Janus nanomaterials, exhibiting distinct physical and chemical properties on opposite sides. These techniques include scanning electron microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray diffraction, Fourier Transform Infrared Spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). In addition, wettability tests frequently allow for the demonstration of their two-faced amphiphilicity through water contact angle measurements [37]. These results could also be supported by dynamic light scattering (DLS) and zeta potential measurements (ζ), which allow for assessing size distributions and colloidal stability [14,19], providing light upon the size and charge differences. Other methods are based on how Janus SiO₂ particles adsorb strongly at oil–water interfaces; tensiometry confirms that they dramatically reduce interfacial tension. Despite the efficacy of these methods, their inefficiencies in accurately determining the duality of some specific JNPs have been documented in the literature [2]. Notably, their ability to disperse in both aqueous and oily phases—demonstrated via colloidal dispersion tests—is directly linked to their oppositely functionalized hemispheres [20].

This limitation underscores the pressing need to develop innovative, cost-effective methodologies to reliably assess the unique behaviors and functionalities of Janus nanomaterials. Different strategies have been proposed, e.g., employing a measurable hydrophilic–lipophilic balance (HLB) (estimated with surfactants) [38], and assessing the impact on interfacial tension between two immiscible liquids [39]. Two other cost-effective approaches, solubility tests and interface stability tests, have recently been discussed to meet the previously mentioned requirements [2].

In this work, two types of JNPs were synthesized and systematically characterized. The first type, novel copper–silica JNPs, was produced using a novel masking method and a self-assembly procedure based on the sol–gel technique, where the Cu nanoparticle seeds were modified with functionalized SiO₂ nanoparticles in a single step. The masking technique employed hexadecyltrimethylammonium bromide (CTAB) to stabilize the emulsion and enable the deposition of modified silica on the metallic seeds, yielding JNPs named GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu. The second type, denoted as SiO₂ JNPs, comprised fully silica-based JNPs obtained through a sol–gel self-assembly strategy employing (3-glycidyloxypropyl) trimethoxysilane (GPTMOS) and trimethoxy (2-phenylethyl) silane (PhTMOS) to introduce chemically distinct hydrophilic and hydrophobic functionalities, respectively, in several steps. The asymmetric character of the resulting SiO₂@Cu JNPs was evidenced by SEM, while two different cost-effective interfacial stability assays in immiscible solvent systems were employed to determine the Janus character of the second type of SiO₂ JNPs, thereby demonstrating their potential for EOR. These methodologies enabled accurate assessment of the structural asymmetry and interfacial behavior of the synthesized nanoparticles.

2. Results and Discussions

2.1. SEM and EDS Characterization

Nanomaterials were characterized using SEM to assess size distribution and morphology. Copper-based nanoparticles displayed spherical shapes, suggesting the presence of a rough surface covered by smaller spherical particles (Figure 1). Similarly, spherical SiO₂-based JNPs were achieved (Figure S1a). Smaller spherical particles were attributed to modified silicon oxide nanoparticles, as revealed by the EDS and XPS analysis discussed in the next sections. For some particles, it was observed that a rough surface is concentrated on one side of the nanoparticle, as expected in nanoparticles with Janus character (Figure 1b,d); during the preparation of samples for SEM, the random positioning of nanoparticles resulted in a heterogeneous JNP orientation. This variability provides distinct perspectives on their Janus characteristics, as illustrated in Figure 1e. In addition, fine, filamentous wires were observed connecting some nanoparticles (e.g., in Figure 1c). These intriguing structures were hypothesized to originate from the formation of silica-based linkages between different copper-based JNPs. It was advisable to mitigate this network through ultrasonication prior to any application to ensure uniform dispersion. Size distributions of SiO₂-JNPs and SiO₂@Cu-JNPs for a population of 100 nanoparticles (each) were as displayed in Figure 2.

Figure 1. SEM images of GPTMOS-SiO₂@Cu (a,b), and PhTMOS-SiO₂@Cu JNPs (c,d). Representation of different Janus orientations (i–v) of SiO₂@Cu JNPs (e); adapted with permission from [2].

Figure 2. Size distribution of the synthesized nanoparticles: (a) SiO₂@Cu-based Janus nanoparticles functionalized with PhTMOS and GPTMOS, and (b) SiO₂ Janus nanoparticles.

The results revealed that GPTMOS-SiO₂@Cu JNPs had an average size of 44.0 ± 7.8 nm. PhTMOS-SiO₂@Cu JNPs presented an average size of 53.0 ± 9.8 nm. This finding suggested that the silane monomer choice did not significantly impact either the morphology or size distribution for both types of SiO₂@Cu JNPs.

The polydispersity of the SiO₂ Janus nanoparticles observed in Figure 3 arises from the intrinsic kinetics of the sol–gel synthesis, in which variations in precursor concentration, hydrolysis and condensation rates, and mixing efficiency lead to overlapping nucleation and growth processes [40]. These effects, together with minor fluctuations in temperature and pH, result in particles forming at different growth stages, giving rise to a broad size distribution.

Figure 3. XRD patterns for Cu NPs without modification (light blue line, up pattern), PhTMOS-SiO₂@Cu JNPs (purple line), and GPTMOS-SiO₂@Cu JNPs (dark blue line).

In addition, post-synthesis processing plays a significant role. During drying, capillary forces and interparticle interactions can promote agglomeration, particularly in the presence of residual water, further broadening the apparent size distribution [41]. This behavior was quantified by analyzing SEM micrographs of over 200 particles, from which the mean diameter, standard deviation, and coefficient of variation were determined.

All JNPs were subjected to analysis via SEM and EDS to ascertain their Janus character and evaluate sample composition (Figure S1). SiO₂-JNPs presented notable complexities for analysis using this technique. These complexities primarily stemmed from the nanoparticles’ small size and non-conductive nature, which hindered the acquisition of clear images and precise measurements. Furthermore, during SEM examination, the nanoparticles tended to become charged due to interaction with the electron beam. This charging effect not only distorted the imaging results but also compromised analysis stability. Despite the complexity of the analysis, spherical nanoparticles of approximately 134.2 ± 30.6 nm in size were imaged.

Nanomaterial agglomeration impeded an accurate Janus character assessment, and the EDS results did not show conclusive evidence in determining the Janus character. Therefore, it was necessary to employ an alternative technique to provide more robust evidence regarding this characteristic, such as solubility and interface tests [2]. Further EDS analysis of the copper-based Janus nanoparticles confirmed the presence of copper and silicon (Si) dispersed across the sample (Figure S1c,d), as opposed to commercial copper nanoparticles (Figure S1b).

Synthesized amphiphilic Janus nanoparticles are schematically represented in Scheme 1, where SiO₂ with epoxy and phenyl group functionalization derived from the sol–gel method provided SiO₂ nanoparticles with their Janus characteristic (Scheme 1a). Copper oxide nanoparticles functionalized with SiO₂-based epoxy or phenyl group modification obtained by a modified solvothermal method are represented in Scheme 1b and Scheme 1c, respectively.

Scheme 1. Hypothetical schematic representation of Janus nanoparticles. (a) GPTMOS-PhTMOS-SiO₂ JNP, (b) GPTMOS-SiO₂@Cu JNP, and (c) PhTMOS-SiO₂@Cu JNP.

2.2. XRD Analysis

XRD diffraction patterns of copper nanoparticles before and after modification were obtained, as seen in Figure 3, while the SiO₂ JNPs’ XRD pattern showed an amorphous phase (Figure S2). In the copper-based JNPs’ XRD spectra, it is possible to observe the presence of metallic copper, copper (I) oxide, and copper (II) oxide. In the case of bare copper, intense peaks at 43.34° 2θ, 50.48° 2θ and 74.17° 2θ are assigned to the (111), (002), and (022) planes of metallic copper (ICSD card N° 98-005-3247), whereas peaks located at 36.42° 2θ and 61.37° 2θ correspond to the copper planes (111) and (022) of (I) oxide (ICSD card N° 98-005-2043). The presence of oxide is related to Cu NPs’ ambient oxidation.

The XRD patterns also show that modifying Cu NPs with either PhTMOS or GPTMOS leads to copper oxide appearance. Specifically, Cu modified with PhTMOS (PhTMOS-SiO₂@Cu JNPs) presents intense peaks located at 36.49° 2θ, 42.39° 2θ, and 61.50° 2θ, associated with the (111), (002), (022) planes, respectively, of copper (I) oxide (ICSD card N° 98-002-6963). In addition, other peaks corresponding to copper (II) oxide at 35.55° 2θ (11-1), 38.67° 2θ (111) and 48.84° 2θ (20-2) (Tenorite, ICSD card N° 98-004-3179) are visible. In contrast, in copper nanoparticle modification with GPTMOS silane (GPTMOS-SiO₂@Cu JNPs), the XRD pattern exhibits a mostly amorphous character due to a high proportion of amorphous silica, similar to the results obtained in refs. [42,43], higher than in PhTMOS-SiO₂@Cu JNPs. However, this revealed the presence of two crystalline peaks corresponding to (11-1) and (111) planes, at 35.54 and 38.64° 2θ, which are attributed to the presence of copper (II) oxide (ICSD card N° 98-004-3179), and the peak located at 43.19° 2θ corresponding to metallic copper (ICSD card N° 98-005-3247). The phenyl group exhibits a passivating effect on nanomaterial surfaces by forming a protective layer, which limits copper Cu NPs’ oxidation. In contrast, GPTMOS groups possess hydrophilic properties, which results in an increased rate of Cu nanoparticle oxidation.

2.3. XPS Characterization

Both functional groups in GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs were attached to the silicon dioxide surface via silanization to obtain JNPs. XPS analyses of C 1s, O 1s, Si 2p, and Cu 2p were conducted on both samples to obtain comprehensive and high-resolution spectra (Figure 4 and Figure 5, respectively). The C 1s spectra of the GPTMOS-SiO₂@Cu JNP sample was deconvoluted into three distinctive peaks (Figure 4a). The first peak, located at 284.68 eV, corresponds to C-C/C-H bonds. The peak at 286.07 eV is assigned to the C–O bond, attributed to the epoxy group presence [44,45]. The third at 288.39 eV is ascribed to the O–C=O bond [46]. Similarly, the C 1s spectra of the PhTMOS-SiO₂@Cu JNP sample was deconvoluted into four curves (Figure 5a). The peak located at 284.62 eV is assigned to the C-C/C-H bonds and the one at 284.46 eV is related to the C-OH bond, which is in agreement with the aromatic ring corresponding to the phenyl group [47,48]. These findings are in agreement with the representation proposed in Scheme 1b, which displays a short hydrocarbon chain with a terminal epoxide moiety for GPTMOS-SiO₂@Cu JNPs, and a phenyl group termination for PhTMOS-SiO₂@Cu JNPs.

Figure 4. (a) C 1s, (b) Si 2p, (c) O 1s, and (d) Cu 2p core levels’ XPS spectra of the GPTMOS-SiO₂@Cu JNPs. Purple hollow dots correspond to raw data, the black solid line corresponds to envelope/fitting curve, and colored areas correspond to deconvoluted components.

Figure 5. (a) C 1s, (b) Si 2p, (c) O 1s, and (d) Cu 2p core levels’ XPS spectra of PhTMOS-SiO₂@Cu JNPs. Purple hollow dots correspond to raw data, black solid line corresponds to envelope/fitting curve, and colored areas correspond to deconvoluted components.

Silicon recorded in the XPS spectra was attributed to silicon in silanes. The Si 2p spectrum is displayed in Figure 4b and Figure 5b. After Gaussian–Lorentzian fitting, two main components were observed in both samples.

For GPTMOS-SiO₂@Cu JNPs, the peak located at 101.73 eV is assigned to Si-O bonding, and the one at 102.30 eV is attributed to the combined contribution of Si-C and Si-O bonding [49]. The peak contribution to the Si 2p XPS spectra is 77.97% and 22.03%, respectively. Peak deconvolutions for the PhTMOS-SiO₂@Cu JNP sample were found at 102.15 eV and 102.59 eV. These peaks are attributed to Si-O and Si-C/Si-O bonding, respectively.

Figure 4c and Figure 5c show the O 1s spectra, for GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs, respectively. For both spectra, each curve was deconvoluted into five distinctive curves, corresponding to Si-O-C, C-O, C=O, Cu-O, and O-OH bond groups (for detailed information, see Table S1). Although the same bonds are suggested for both samples, the Cu-O bond peak for the GPTMOS-SiO₂@Cu JNP sample has a higher peak area (33.55%) compared to that of PhTMOS-SiO₂@Cu JNPs (9.78%). This could be related to different copper oxidation states coexisting in both samples, also indicating a slight Cu²⁺ increase in the GPTMOS-SiO₂@Cu JNP sample, in line with the XRD results (Figure 3).

In the Cu 2p_3/2 spectra for GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs (Figure 4d and Figure 5d, respectively), it was observed that the characteristic peak and its corresponding satellite were separated by a binding energy of ~8 eV, as reported before [50]. Previous studies indicated that these satellites were related to the partially filled Cu²⁺ oxidation state d-orbital (d⁹), suggesting that their presence and intensity were directly correlated with the Cu²⁺ surface concentration [51,52,53]. This corroborated the XRD results, which showed that JNPs are composed of copper and its oxides.

The relative concentrations are calculated by the following equations:

$$\%\left({Cu}^0{+Cu}^{+}\right)={\displaystyle \frac{A2}{A+B}}·100={\displaystyle \frac{A-A1}{A+B}}·100,$$

(1)

$$\%{Cu}^{2+}={\displaystyle \frac{A1+B}{A+B}}·100,$$

(2)

where B is the total area of the satellite peak, and A is the total area of the main peak, which are contributions of the Cu²⁺ (A1)-related area and area related to (Cu⁰ + Cu⁺) (A2). (A1S/BS) is the ratio of the main peak/shake-up peak areas for the 100% pure Cu²⁺ sample equal to 1.89 for the CuO reference at 20 eV pass energy [51]. Area peaks were calculated using Casa XPS Version 2.3.25PR1.0 software (peak area/relative sensitivity factor; 16.73 for Cu 2p3/2). The (Cu⁰ + Cu⁺) and Cu²⁺ calculated content was 23.53% and 76.46%, respectively, for PhTMOS-SiO₂@Cu JNPs. Similarly, for Cu in GPTMOS-SiO₂@Cu JNPs, the percentage of (Cu⁰ + Cu⁺) was 18.65% and 81.34% for Cu²⁺.

From these results, it was concluded that the oxidation of copper nanoparticles to copper oxides occurred in both cases. Still, higher oxidation with GPTMOS-SiO₂@Cu modification was observed (Cu²⁺). This enhanced oxidation may be related to the hydrophilic character of the GPTMOS [54], which contains epoxy functional groups known for their ability to interact with environmental water, thereby promoting the oxidation of the copper nanoparticles. The formation of these oxide layers can enhance chemical stability by passivating the surface, while also modifying surface chemistry and interactions with silane groups. Therefore, the resulting particles can be more accurately described as copper-based nanoparticles with oxide-rich cores.

2.4. FTIR Characterization (Silica-Based JNPs, Cu-Based JNPs)

The FTIR spectra and band summaries for different vibration modes and corresponding chemical functional groups for silica (Scheme 2) and SiO₂@Cu-based JNPs are shown in Figure 6 and Table 1, respectively. Analysis of these results, starting from Zone IV, where the asymmetric and symmetric stretching vibration of the Si–O–Si bond, denoted as ν(SiOSi), at approximately 1080.0 and 795.0 cm⁻¹, respectively, confirms the formation of silica-based nanoparticles through the siloxane group of Si–O–Si mode. Furthermore, the band observed at 985.9 cm⁻¹, attributed to ν(SiOH), is characteristic of free silanol molecules, proving hydrolysis occurrence [55]. Zone III provides information regarding the organic functional groups of monomers used in the JNPs’ synthesis and their transformation.

Scheme 2. Representation of functional groups (in red) for silane-based monomer structures employed in JNPs synthesis: (a) tetra-ethyl orthosilicate (TEOS), (b) (3-glycidyloxypropyl) trimethoxysilane (GPTMOS), (c) trimethoxy [3-(methylamine) propyl] silane (APTMOS), and (d) trimethoxy (2-phenylethyl) silane (PhTMOS).

Figure 6. FTIR spectra of (a) GPTMOS-SiO₂@Cu JNPs, (b) PhTMOS-SiO₂@Cu JNPs, (c) SiO₂ JNPs (SiO₂ JNPs), and (d) SiO₂ JNPs treated by annealing process at 600 °C (SiO₂ NPs H). The colored zones I–IV correspond to the classifications outlined in Table 1.

Table 1. Assignment of infrared spectrum bands to different vibration modes and corresponding chemical functional groups. The color codes associated with Zones I through IV correspond to the details presented in Figure 6.

Zone I		Zone II		Zone III		Zone IV
ν (cm−1)	Vibrations	ν (cm−1)	Vibrations	ν (cm−1)	Vibrations	ν (cm−1)	Vibrations
3475.3	ν OH (water) and ν Si-OH	2899.8	νs Csp3-H2	1290.7	δ Csp3-O-H	733.0	γ Csp3-H
3741.3	ν N-H	2939.6	νas Csp3-H2	1395.4	γ Csp3-O-H	794.8	νs Si-O-Si
		2980.0	ν Csp2-H	1447.8	δs Csp3-H	853.4	νas Si-O-Si
				1484.9	δas Csp3-H	985.9	ν Si-O-H
				1626.9	δs N-H	1080.0	ν Si-O-Si
				1840.0–1985.0	Aromatic overtones

The epoxy group presence belonging to GPTMOS (Scheme 2b) can be confirmed by in-plane bending at 1290.7 cm⁻¹ and out-of-plane bending vibrations of the γ C-O-H band at 1395.4 cm⁻¹ due to epoxy cycled structure rupture. In addition, the single-bond CH₂ scissoring bending vibration (δ C_SP3-H₂) presented in the alkyl groups shows a doublet at 1447.8 and 1484.9 cm⁻¹ since the sample is analyzed in solid form [56].

PhTMOS monomer aromatic groups (Scheme 2d) are identified by overtones (harmonics) of benzene ring vibrational modes, which are also sensitive to the substitution pattern. These appear as stretching frequencies in the fingerprint region of the infrared spectrum between 2100 and 1750 cm⁻¹, which are visible in Figure 6c,d. Both show the presence of PhTMOS during synthesis, but not in Figure 6b due to silica’s low presence in the Cu JNPs’ structure. Furthermore, a spectral band observed at 1626.9 cm⁻¹ corresponds to the bending vibration of the N-H bond of the APTMOS monomer utilized in the synthesis process (Scheme 2c). In Zone II, two signals at 2899.8 and 2939.6 cm⁻¹ are visible due to alkyl and/or aromatic groups’ -CH₂ vibration bands presented in the different silane monomers. Also, a peak at 2980.0 cm⁻¹ can be attributed to the Csp₂-H vibration of the phenyl group present in the PhTMOS monomer. Finally, Zone I displays a broad peak around 3400.0 cm⁻¹, corresponding to the -OH stretching vibration band associated with adsorbed water, and the stretching modes originated from Si-OH. This peak is reduced after SiO₂ nanoparticles’ annealing at 600 °C, highlighting a prominent double band around 3700 cm⁻¹. This double band is attributed to the N-H stretching vibration mode of secondary amine in APTMOS (see Scheme 2c).

A slight silica presence within the structure of the copper nanoparticle (Cu JNP), combined with copper’s significant impact on the vibrational modes, has attenuated bands associated with silica face formation. Nonetheless, Si-O-Si bond formation is unequivocally confirmed by the band around 1080 cm⁻¹ (Figure 6a,b), thereby validating sol–gel synthesis process attainment. Furthermore, low-intensity peaks observed at 2939.6 cm⁻¹ (Figure 6b) are attributed to the phenolic compound (PhTMOS) (see Scheme 2d), which is not present during GPTMOS-SiO₂@Cu JNPs’ synthesis.

A comparative analysis of SiO₂ JNPs (Figure 6c) and SiO₂ NPs (Figure 6d) shows that the bands in Zone IV diminished following annealing treatment. This is attributed to the thermal process, which alters material structural composition. Epoxy group degradation is induced by heat, as confirmed by the absence of spectral bands in the range of 1390.0 to 1490.0 cm⁻¹ (Zone III, Figure 6d). Intensity aromatic overtones increase due to the band intensity reduction between 1840.0 and 1985.0 cm⁻¹, corresponding to region III (Figure 6c,d). Furthermore, the ν Csp₂-H observed at 2980 cm⁻¹ exhibits a significant reduction after the heating process; however, it does not entirely disappear. This phenomenon can be attributed to benzene group proximity to the silica structure (see Scheme 2d).

2.5. Zeta Potential Measurement

Zeta potential (ζ) measurements as a function of pH were performed to probe the acid–base behavior and surface charging properties of the nanoparticles. The ζ–pH behavior arises from the collective protonation–deprotonation of surface sites. In contrast, Janus-type structures—characterized by the spatial separation of hydrophilic and hydrophobic chemical domains—may exhibit nonlinear charging trends, slope discontinuities, or suppressed magnitude of ζ-values, reflecting the coexistence of chemically distinct hemispheres with different protonation–deprotonation equilibria. Although ζ-potential analysis alone cannot conclusively demonstrate Janus morphology, the presence of deviations from the behavior expected for fully homogeneous SiO₂ NPs provides supporting evidence when interpreted in conjunction with the functionalization strategy, interfacial behavior, and emulsion tests. Therefore, the ζ–pH curves serve as a complementary approach to assess the asymmetric surface chemistry that underlies amphiphilicity in JNPs [57].

Figure 7 shows the ζ-potential vs. pH curves of the SiO₂ JNPs, and SiO₂ NPs without modification in deionized water. A clear variation in ζ-potential with pH was observed for both samples, with SiO₂ JNPs demonstrating a more pronounced effect. The variation in SiO₂ JNPs ζ-potential at elevated pH was steep, a phenomenon attributable to their larger surface area [58]. This increased surface area results in a greater number of functional groups available for deprotonation when compared to SiO₂ NPs, due to further steps during the synthesis process associated with GPTMOS and PhTMOS monomers.

Figure 7. Dependence of ζ-potential on pH for suspension of SiO₂-based JNPs, and homogeneous SiO₂ NPs.

On the one hand, the amorphous SiO₂ NPs exhibited a negative surface charge over the pH range 3–11. It is also appreciable that the negative charge on the nanoparticles’ surface has a slight increase with an increasing pH, which was associated with an increasing silanol group formed on the silica surface. The silica surface attains a net negative charge primarily due to the deprotonation (H⁺) of silanol groups, resulting in a surface that exhibits an increasing negative charge (Figure 7) [59]. On the other hand, the SiO₂-based JNPs showed a positive ζ-potential for pH < 4.9, and negative otherwise. At this pH, the JNPs exhibit the isoelectric point, confirming the absence of this phenomenon for SiO₂ NPs, which are negatively charged in the studied pH range. The analysis of the curve indicated that the behavior of the SiO₂ JNPs exhibited amphoteric properties [60]. This observation was further strengthened by additional visual evidence.

2.6. Janus Character Tests in Immiscible Liquid Systems for SiO₂ JNPs

Determining the SiO₂ JNPs’ Janus properties was not achievable through the SEM technique, in contrast to copper-based nanomaterials. Consequently, further analyses were conducted exclusively on SiO₂ JNPs and their precursors (see Table 2) by interface tests (Figure 8 and Figure 9), as reported in ref. [2]. Janus character assessment involved examining the emulsion stabilization formed at the interface. Figure 8 presents a comparative analysis of various precursors and the resultant SiO₂ JNPs. It is noteworthy that the liquid systems in a 1:1 mixture facilitate the assessment of interfacial separation between the two immiscible phases (water and cyclohexane, H₂O:C₆H₁₂), a process that exhibits delays when considering the final SiO₂ JNPs. This behavior provides insight into the potential for emulsion stabilization, suggesting applications of JNPs in the recovery of heavy oil from diverse media [15,16,17,18]. SiO₂ JNPs exhibited distinct Janus behavior in a 1:1 H₂O:C₆H₁₂ mixture, as illustrated in Figure 8(IVc,Vc).

Table 2. Nomenclature and pertinent information concerning JNPs developed in this research.

Samples	Precursors	Treatments	Step
SiO2 seeds	TEOS	Stored 7 days	0
PhTMOS-SiO2 NP	SiO2 seeds + PhTMOS	Stored 3 days	1
GPTMOS-SiO2 NP	SiO2 seeds + GPTMOS	Stored 3 days	2
SiO2 JNPs	SiO2–Ph + APTMOS + SiO2–G	Centrifugated and dried at 30 °C	3
Cu seeds	Cu NPs + CTAB	Ultrasonicated 30 min	1
GPTMOS-SiO2@Cu JNPs	Cu NPs + TEOS + GPTMOS	Ultrasonicated for 40 min, centrifugated, washed, and dried at 30 °C	2
PhTMOS-SiO2@Cu JNPs	Cu NPs + TEOS + PhTMOS		2

Figure 8. Interface test focusing on separation of two immiscible phases (1:1 H₂O:C₆H₁₂ mixture) over time [(I): immediately after vigorous stirring and ultrasonication for 10 min; (II): after 30 min of settling; (III): after 24 h of settling; (IV): 20 days later; (V): schematic representation of case IV)]. Conditions studied included (a) without nanoparticles, (b) SiO₂ NPs, (c) SiO₂ JNPs, (d) GPTMOS-SiO₂ NP, and (e) PhTMOS-SiO₂ NP. In this depiction, the blue sphere illustrates the core of the SiO₂ nanoparticles (NPs). The chemical structures with two red circles (oxygen atoms) depict GPTMOS units, while those with a benzene group represent PhTMOS units.

Figure 9. Emulsion stabilization tests in the 31.6:15.8:52.6 C₁₂H₂₆:PEG 400:H₂O (v/v) ternary system using different amounts of SiO₂ JNPs. Nanoparticle concentrations evaluated: (i) 0 g (control), (ii) 0.001 g, (iii) 0.005 g, (iv) 0.015 g, (v) 0.020 g, and (vi) 0.025 g. Images were recorded at the following times: (A) immediately after addition of dodecane to the PEG 400 solution; (B) after vigorous stirring and 10 min ultrasonication; (C) after 30 min; (D) after 4 h; (E) after 24 h; and (F) after 20 days. Long-term interface stabilization was observed for systems iv–vi.

Initially, a rapid separation of immiscible phases was observed in the absence of nanoparticles (Figure 8(Ia–Va)). A comparable phase separation behavior was also noted when using homogeneous SiO₂ NPs, which lack amphiphilic properties (Figure 8(Ib–Vb)). These results suggest that the Janus character plays a critical role in stabilizing the emulsification process, likely due to preferential localization at the interface between the two immiscible phases (Figure 8(Ic–Vc)).

The schematic in Figure 8V depicts the behavior of nanoparticles in the immiscible 1:1 C₆H₁₂:H₂O system, where the different nature of the nanoparticles is evident, and the resulting effects on the system are shown. Furthermore, PhTMOS-SiO₂ NPs demonstrated a low reversion degree to the initial state, attributable to their hydrophobic characteristics due to the phenyl group, with a high affinity for the C₆H₁₂ phase (Figure 8(Ve)); meanwhile, GPTMOS-SiO₂ NPs (Figure 8(Id–Vd)) demonstrated properties comparable to those of homogeneous SiO₂ NPs (Figure 8(Ib–Vb)), attributable to their hydrophilic nature.

2.7. Discussion on Emulsion Stabilization in a Multicomponent System: Potential Application in EOR

The potential of SiO₂ JNPs to stabilize emulsions was further evaluated using a ternary system composed of 31.6% dodecane (C₁₂H₂₆), 15.8% polyethylene glycol 400 (PEG 400), and 52.6% water (v/v). The influence of the nanoparticle concentration on interfacial stability was assessed, as shown in Figure 9. The amphiphilic nature of the JNPs significantly enhanced the stability of the oil–water interface, with long-term stabilization observed at higher nanoparticle loadings. After more than 20 days, systems iv, v, and vi exhibited a persistent and well-defined interface, indicating effective stabilization. Notably, a concentration of 0.015 g of JNPs was sufficient to achieve robust interfacial stability (system iv, panels D–F), suggesting that only small amounts of nanoparticles are required to impart long-term structural integrity to the emulsion.

While these results demonstrate a clear proof-of-concept application, it is important to note that the C₁₂H₂₆–PEG 400–H₂O formulation has not been optimized in terms of phase composition. Further studies are being planned to determine the optimal ratios and to evaluate performance under conditions more closely aligned with industrial practice. The findings reveal the emerging potential of the developed SiO₂ JNPs for EOR. However, achieving and maintaining colloidal stability under conditions that mimic reservoir environments—such as pH fluctuations and varying JNP concentrations—presents intricate challenges. These complexities highlight the need for a more comprehensive understanding of JNP interactions and behaviors under these specific conditions, which is essential for optimizing their application in EOR processes. This management is essential for the effective application of SiO₂ JNPs in injection-based oil recovery processes.

3. Materials and Methods

3.1. Materials and Reagents

Trimethoxy [3-(methylamine) propyl] silane (APTMOS), (3-glycidyloxypropyl) trimethoxysilane (GPTMOS), trimethoxy (2-phenylethyl) silane (PhTMOS), and tetraethyl orthosilicate (TEOS) (all reactant grade, Sigma-Aldrich, GmbH, Albuch, Germany) were employed as silane monomers in the synthesis process. Ethanol, acetonitrile, and cyclohexane (all absolute grade for analysis, Merck, KGaA, Darmstadt, Germany) were employed as solvents for the sol–gel method. Synthesis was performed via basic hydrolysis by adding 25% NH₄OH solution procured from Merck KGaA, Darmstadt, Germany. Copper nanoparticles with an average particle size of 60–80 nm, as reported by the supplier (Merck, KGaA, Darmstadt, Germany), were purchased as nano-powder. Scanning electron microscopy (SEM) of the powder further confirmed the particle size. CTAB (99% hexadecyltrimethylammonium bromide) from Aldrich-Sigma Chemicals Company (GmbH, Albuch, Germany) was used as a stabilizer agent in the masking method.

3.2. Characterization and Equipment

Morphological characterization was conducted using scanning electron microscopy (SEM, FEI Verios 460 L, Hillsboro, OR, USA), and size distributions were determined by “ImageJ” Version 1.54k software. Elemental mapping was achieved using a Tescan FE Mira II LMU (Brno, Czech Republic) instrument equipped with an energy-dispersive X-ray spectroscope (EDS) detector (Bruker AXS, Inc., AZtec Version 4.2 software package released by Oxford Instruments, Abingdon, UK). Surface chemical composition analysis was performed utilizing X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Supra spectrometer (Manchester, UK) equipped with monochromatic Al Kα X-ray radiation, operating at an emission current of 15 mA and employing the hybrid lens mode. For XPS analysis, the passing energy was set at 80 eV for detailed spectra and 20 eV for broad spectra. The resulting XPS spectra were processed and analyzed using CasaXPS Version 2.3.25PR1.0 software, with a calibration reference based on the C 1s peak fixed at 284.7 eV. Spectra fitting was achieved by employing the Shirley algorithm for background subtraction and Gaussian–Lorentzian (30–70%) line shape modeling.

Furthermore, crystalline phase composition was investigated using an X-ray diffractometer (XRD) with a high-brightness source (Rigaku SmartLab 9 kW CF2). XRD scans were conducted in Bragg–Brentano geometry, employing Cu Κα radiation within a scan range spanning from 20° to 80°, with a scan speed of 4°·min⁻¹. ζ-potential was measured with a LITESIZER 500 instrument from Anton Paar (Hampton Falls, NH, USA) by preparing a dispersion of 0.01 mg/mL of SiO₂ JNPs in water and homogenizing with an ultrasonic probe; dependence on pH from 2 to 12 values was tested by employing NaOH/HCl buffer solutions with an automatic dosing system.

3.3. Experimental Procedures

This work examines the efficacy of sol–gel and masking techniques in developing doubly functionalized silica (SiO₂-JNPs) and copper–silica (GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs) structures, respectively (Scheme 3). A brief description of processes and a summary of developed nanomaterials are outlined in the next sections and Table 2.

Scheme 3. Representation of employed self-assembled synthesis for obtaining (a) SiO₂ JNPs and annealing-based post-treatment to remove amphiphilic character (SiO₂ NPs), and (b) SiO₂@Cu-based JNPs (GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs).

3.3.1. Preparation of Janus Silica Nanoparticles SiO₂-JNPs

A silica–ethanol sol produced by a sol–gel method, as reported earlier [61], was applied to achieve SiO₂-based JNPs (Scheme 3a). The specific procedure comprised the following steps:

Step 0 or initial: TEOS (silica precursor) was mixed with absolute ethanol (solvent), then 25% NH₄OH solution and water were added together and magnetically stirred at 30 °C for two hours. After reaction, sol was allowed to age in a water bath at 25 °C for 7 days. Afterward, samples were refluxed in ethanol to remove ammonia in the sample, obtaining silica nano-seeds (around 70 nm) to be modified [61] (step 0, see Table 1).

Subsequently, a nano-silica dispersion with amphiphilic properties and a double-particle structure was produced by following these steps:

Step 1: Silica–ethanol sol (obtained in step 0) with a concentration of 3 wt. % was mixed with PhTMOS, continuously stirred at 30 °C for 2 h, and aged at 30 °C for 3 days. A lipophilically modified silica–ethanol sol was then acquired.

Step 2: Silica–ethanol sol with a concentration of 3 wt. % was mixed with GPTMOS at 30 °C for 2 h and aged at 30 °C for 3 days to attain a hydrophilic modified silica–ethanol sol.

Step 3: After 3 days, modified sols obtained in step 1 and step 2 were mixed and combined by adding APTMOS and continuously stirred at 30 °C for 3 h to develop the nano-silica dispersion with amphiphilic properties and a double-particle structure (SiO₂-JNPs). Later, the suspension was repeatedly centrifugated at high velocity and washed with water and ethanol.

3.3.2. Preparation of SiO₂@Cu-JNPs

SiO₂@Cu-based JNPs were synthesized using a modified solvothermal method adapted from [62], where Fe₃O₄ nanoparticles were originally employed as seeds. In the present work, these seeds were replaced by Cu nano-seeds, followed by functionalization of the silica face to achieve GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs with epoxy and phenyl groups, respectively, a key novelty of this work.

Step 1: Initially, a copper nanoparticle dispersion with a concentration of 1 g∙L⁻¹ was ultrasonicated. Subsequently, CTAB was incorporated into the dispersion and subjected to continuous ultrasonication for 30 min.

Step 2: SiO₂ nanoparticles were prepared in situ with the addition of TEOS, and other silanes were used for functionalization (TEOS combined with GPTMOS or TEOS combined with PhTMOS). They were added along with a solution containing 25% NH₄OH. The reaction was maintained under stirring for an additional 40 min at 40 °C.

Step 3: The reaction was concluded by ethanol addition. The resultant GPTMOS-SiO₂@Cu and PhTMOS-SiO₂@Cu JNPs were then subjected to repeated centrifugation and washing processes with both ethanol and water.

3.4. Determination of Amphiphilic Character of SiO₂-Based JNPs and EOR Assessment

The Janus character of the SiO₂@Cu-based nanoparticles was assessed using SEM images, in which functionalization was clearly observed in EDS elemental maps, and was corroborated by FTIR, XRD, and XPS measurements.

SiO₂-based JNPs amphiphilic characteristics (Janus nature) were assessed for the samples listed in Table 2, utilizing a straightforward and cost-effective interface stability test [2]. This approach revealed the dual hydrophobic and hydrophilic JNPs’ properties by showing emulsion stabilization at the interface between two immiscible solvents. A 1:1 mixture of water (H₂O) and hexane (C₆H₁₂) was used to compare the precursors of SiO₂ JNPs with the final material. Subsequently, the resulting dispersions were subjected to vigorous stirring and ultrasonic treatment for 5 min for each mixture.

EOR assessment was performed by employing a heterogeneous mixture of dodecane (C₁₂H₂₆, 31.6% v/v), polyethylene glycol PEG 400 (15.8% v/v), and water (52.6% v/v). Briefly, PEG 400 was dissolved in a specified volume of water, to which varying quantities of SiO₂ JNPs were systematically added (0.001, 0.005, 0.015, 0.020, and 0.025 g). Subsequently, dodecane was incorporated into the mixtures, and samples were subjected to ultrasonication at 6000 rpm for 5 min. Given the hydrophobicity of dodecane (a typical oil-like solvent), this test was also employed to assess the potential of JNPs for EOR, a widely proposed application.

After performing both approaches, emulsions were allowed to equilibrate, and phase separation phenomena for all samples were documented using a COOLPIX B500 Nikon camera at specified time intervals, assessing the extent to which they revert to the original state with both phases fully separated (reversion degree). Additionally, to illustrate the amphiphilic properties of developed SiO₂-based JNPs, a specific portion of synthesized SiO₂ JNPs underwent a post-treatment process involving annealing at 600 °C for 4 h in a conventional tube furnace. This procedure resulted in surface group degradation that conferred amphiphilic characteristics on these materials. Consequently, new SiO₂ nanoparticles designated as SiO₂ NPs, lacking the dual nature exhibited by original JNPs, were produced.

4. Conclusions

Amphiphilic silica@copper and silica-based nanomaterials with a median diameter of approximately 135 nm, as determined by SEM, were obtained using two sol–gel-based methods. Comprehensive chemical characterization using XRD, FTIR, and XPS confirmed the incorporation of functional epoxy and phenyl groups. Novel SiO₂@Cu JNPs, PhTMOS-SiO₂@Cu and GPTMOS-SiO₂@Cu JNPs, were synthesized using an innovative masking technique. These nanoparticles measured 40–70 nm in diameter, as verified by SEM, which also provided direct visual evidence of their Janus morphology. The presence of functional epoxy and phenyl groups on the silica surface was corroborated through the same set of chemical characterization techniques. Nevertheless, determining the Janus character of nanomaterials is complex and requires multifaceted techniques such as dynamic light scattering (DLS) for ζ-potential measurements, as was the case for SiO₂ JNPs. Consequently, the amphiphilic characteristics of these silica-based JNPs were further corroborated through interfacial and emulsion stabilization tests, which assessed the interfacial behavior of JNPs within biphasic and triphasic liquid systems. These methodologies are straightforward and cost-effective for evaluating Janus nature.

SiO₂ JNPs demonstrated excellent emulsion-stabilizing capabilities, maintaining stability over 20 days in 1:1 H₂O:C₆H₁₂ systems and in ternary mixtures containing 31.6%, 15.8%, and 52.6% of C₁₂H₆/PEG400/H₂O, respectively. Furthermore, the methodologies established in this work not only increased the understanding of the synthesized JNPs but also enabled the reliable and cost-effective identification of their Janus character. The functionalization of the silica face not only enhances amphiphilicity but also highlights the potential for flexible design and architectures, holding the potential to significantly enhance the scalability and applicability of Janus nanomaterials across various disciplines, including EOR.