Carbon-coated Fe₃O₄ core–shell super-paramagnetic nanoparticle-based ferrofluid for heat transfer applications

Abstract

Herein, we report the investigation of the electrical and thermal conductivity of Fe₃O₄ and Fe₃O₄@carbon (Fe₃O₄@C) core–shell nanoparticle (NP)-based ferrofluids. Different sized Fe₃O₄ NPs were synthesized via a chemical co-precipitation method followed by carbon coating as a shell over the Fe₃O₄ NPs via the hydrothermal technique. The average particle size of Fe₃O₄ NPs and Fe₃O₄@C core–shell NPs was found to be in the range of ∼5–25 nm and ∼7–28 nm, respectively. The thickness of the carbon shell over the Fe₃O₄ NPs was found to be in the range of ∼1–3 nm. The magnetic characterization revealed that the as-synthesized small average-sized Fe₃O₄ NPs (ca. 5 nm) and Fe₃O₄@C core–shell NPs (ca. 7 nm) were superparamagnetic in nature. The electrical and thermal conductivities of Fe₃O₄ NPs and Fe₃O₄@C core–shell NP-based ferrofluids were measured using different concentrations of NPs and with different sized NPs. Exceptional results were obtained, where the electrical conductivity was enhanced up to ∼3222% and ∼2015% for Fe₃O₄ (ca. 5 nm) and Fe₃O₄@C core–shell (ca. 7 nm) NP-based ferrofluids compared to the base fluid, respectively. Similarly, an enhancement in the thermal conductivity of ∼153% and ∼116% was recorded for Fe₃O₄ (ca. 5 nm) and Fe₃O₄@C core–shell (ca. 7 nm) NPs, respectively. The exceptional enhancement in the thermal conductivity of the bare Fe₃O₄ NP-based ferrofluid compared to that of the Fe₃O₄@C core–shell NP-based ferrofluid was due to the more pronounced effect of the chain-like network formation/clustering of bare Fe₃O₄ NPs in the base fluid. Finally, the experimental thermal conductivity results were compared and validated against the Maxwell effective model. These results were found to be better than results reported till date using either the same or different material systems.

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1. Introduction

The dispersion of nanoparticles (NPs) in various fluids enhances their thermal conductivity, which have potential application in heat transfer as coolants in various systems such as automobile radiators, refrigerators, process engineering systems, electronic devices, solar energy heaters, pulsating pipes, and thermosyphons.^1–5 There are several typical base fluids used for heat transfer applications such as water, ethylene glycol, ethanol, methanol, dimethyl formamide, polyalfaolefin, and oils.^6–13 Ferrofluids or magnetic fluids are stable colloidal homogeneous suspensions of magnetic NPs (∼10 nm in diameter) in an aqueous or a non-aqueous carrier liquid.¹⁴ The dispersion of NPs in these base fluids results in the formation of a nanofluid, and if NPs are magnetic in nature, the resulting fluid is termed a ferrofluid. Generally, NPs show enhanced electrical, thermal, optical and mechanical properties because of their large surface to volume ratio.^15–17 Magnetic NPs such as Fe₃O₄ have attracted great interest from the scientific community because of their unique properties such as biocompatibility, high electrochemical response and thermal stability.¹⁸ Consequently, Fe₃O₄ NP-based ferrofluids exhibit enhanced thermal and electrical conductivities.^19–21 However, NPs have high surface energy, which results in their agglomeration or settling at the bottom of the dispersion liquid when used for a long period. Therefore, the coating of inorganic or organic materials over Fe₃O₄ NPs is necessary for their good dispersion and long-term stability.¹⁸ The oxidation of magnetite (Fe₃O₄) NPs leads to the formation of a stable phase of iron oxide, which is γ-Fe₂O₃ NPs (maghemite), and a more stable form, i.e., α-Fe₂O₃ (hematite) under particular conditions.²² Therefore, it is necessary to protect superparamagnetic Fe₃O₄ NPs from physical and chemical changes by encapsulating them with other materials. Accordingly, capping Fe₃O₄ NPs with another sub-material (shell) can result in their good dispersion and stability, as reported by Sharma et al.¹⁸ Hence, various core–shell NPs have been reported thus far, especially Fe₃O₄@C core–shell NPs, for e.g., Xuan et al. synthesized carbon-encapsulated Fe₃O₄ core–shell particles via the reduction of glucose.²³ Conversely, Wang et al. synthesized single carbon layer-coated ultra-small Fe₃O₄ NPs using a one-step hydrothermal technique for surface-enhanced Raman spectroscopy studies,²⁴ and He et al. used in situ-synthesized carbon-encapsulated Fe₃O₄ NPs as an anode material for lithium ion batteries.²⁵ Moreover, Zhao et al. employed the hydrothermal method for the synthesis of interconnected carbon nanospheres covering Fe₃O₄ NPs.²⁶ Similarly, Liang et al. synthesized Fe₃O₄@Au core–shell NPs for the ultrasensitive detection of carbohydrate–protein interactions.²⁷ A magnetic metal nanocomposite was coated by carbon to prepare FeNi@C core–shell NPs.²⁸ Li et al. synthesized Cu@C core–shell NPs via a simple state reduction method and investigated their optical properties.²⁹ Similarly, Ag@C core–shell NPs were synthesized via the wet chemical route and catalysed under hydrothermal conditions, which exhibited hydrophilic and unique optical properties.³⁰ Moreover, the reverse micelle method was employed to prepare Fe@Fe-oxide and Fe@Au core–shell nanostructures.^31,32 These core–shell NPs possessed an added advantage over other NPs as their shell can protect their core from physical and chemical changes. Thus, bio-incompatible, highly reactive and toxic NPs can be covered with biologically compatible, non-reactive and environmentally friendly materials in the form of shells.

To further explore the unique properties of core–shell NPs, they were introduced in a dispersion liquid to make stable ferrofluids for various applications. Recently, ferrofluids were prepared by dispersing Fe₃O₄ NPs in mineral oil and γ-Fe₂O₃ NPs in various base fluids.^19–21 Their thermal conductivities were enhanced multiple folds compared to that of the base liquids. Similarly, Cui et al. synthesized superparamagnetic Fe₃O₄ NPs, which were dispersed in perfluoropolyether base fluid, and the resulting ferrofluid exhibited enhanced viscosity and thermal conductivity.³³ Zupan et al. prepared a ferrofluid by dispersing iron(II,III) oxide NPs in water for heat transfer application.³⁴ Furthermore, Fe₃O₄ NPs were dispersed in water and a maximum 200% enhancement in thermal conductivity was observed.³⁵

Core–shell NP-based ferrofluids also exhibit good biocompatibility and improved properties. Accordingly, the other applications of core–shell NP-based ferrofluids include multimodal imaging, hyperthermia, drug delivery, cytocompatibility tumour targeting, cancer chemotherapy and thermotherapy and other possible biological applications.^36–38 Similarly, magnetic NP systems are widely used in the MRI-guided delivery of magneto-electric drug nano-carriers to the brain.³⁹ Magneto-electric core–shell NPs exhibit enhanced cell uptake and control drug release under the influence of an applied and magnetic field.⁴⁰ To manage central nervous system (CNS) diseases, surface-engineered magnetic NPs have been employed as a tool via image-guided therapy and theranostics.⁴¹ Magneto-electric NPs (MENPs) are stimulus-responsive nanosystems for controlled drug release and cell uptake.^42,43 The futuristic application of these magnetic core–shell nanostructures may be projected in the biomedical field, where previous studies showed the use of magnetic core–shell nanoparticles as an antimicrobial agent.⁴⁴ Targeted drug delivery and drug delivery to the brain are limited due to their complicated methods and structural behavior. Thus, to achieve this goal, magnetic core–shells have been suggested as important nanocarriers.^45,46

Although nanofluids based on non-magnetic NPs are widely studied, to date, few studies investigating the thermal conductivity of magnetic core–shell NPs for heat transfer applications have been performed.^47–49 To the best of our knowledge, the electrical and thermal conductivities of superparamagnetic Fe₃O₄@C core–shell NPs have rarely been reported. Therefore, herein, we synthesized Fe₃O₄ NPs using a chemical co-precipitation method followed by the hydrothermal synthesis technique, which led to the formation of Fe₃O₄@C core–shell NPs. Furthermore, these NPs were characterized using various techniques and their electrical and thermal conductivities were measured. The super-paramagnetic Fe₃O₄@C core–shell NPs exhibited good thermal and electrical properties, and thus can be exploited for heat transfer applications, such as cooling of electronic devices, fuel cells, and solar cells.⁵⁰

2. Experimental

2.1. Materials and methods

Ferrous sulphate heptahydrate (FeSO₄·7H₂O), ferric chloride hexahydrate (FeCl₃·6H₂O) and sodium hydroxide (NaOH) were purchased from Sigma Aldrich. Absolute alcohol and hydrochloric (HCl) acid were procured from Chem-Lab (Belgium) and Scharlau (Spain), respectively. All chemicals were of research grade and used without further purification. Similarly, fructose of analytical grade was purchased from Gem-Chem (India) and used as received. Deionized (DI) water obtained from an ultrapure water unit (Puris-Expe water system) was used during the synthesis of the Fe₃O₄ NPs.

2.2. Synthesis of Fe₃O₄ NPs

Fe₃O₄ NPs were synthesized via a chemical co-precipitation method. In a typical co-precipitation method, 0.32 mol of ferrous sulphate heptahydrate (FeSO₄·7H₂O) and 0.64 mol of ferric chloride were dissolved in 100 mL DI water separately. The Fe²⁺/Fe³⁺ ions were mixed properly before dispersing them in an alkali solution. The ion mixture was then transferred to a 100 mL burette. In a 250 mL round-bottom flask, 1.5 M NaOH solution was prepared and the volume was made up to 100 mL. The iron salt ions were then added dropwise to the above alkali solution under vigorous stirring. The pH of the solution was maintained at around 11–12 during the reaction. An additional amount of NaOH solution could be added to the reaction vessel if needed. As soon as the iron salts were added to the alkali solution, precipitation of the salts started, resulting in the formation of a black-colored co-precipitate. The precipitate was then stirred for 30 min at room temperature. Subsequently, the solution was allowed to settle for 30 min and with the help of a strong magnet, the precipitate was separated from the unreacted solution. Finally, the obtained precipitate was washed several times with distilled water. Similarly, by tuning the pouring of aqueous solution of iron salt (drops) into the aqueous solution of alkali and stirring rate, Fe₃O₄ NPs with three different sizes were obtained including 5 nm, 12 nm and 25 nm.

2.3. Synthesis of Fe₃O₄@C core–shell NPs

The as-synthesized Fe₃O₄ NPs of different sizes were dispersed separately in a 5 M aqueous solution of glucose and kept in a tight capped 500 mL bottle, which was half-filled with the solution. To coat a carbon shell over the Fe₃O₄ NPs, the sample solution was shaken properly and then left in an autoclave for 4–5 h at high pressure and 180 °C. The carbonization of glucose occurred at 180 °C during the hydrothermal treatment. After this reaction, the system was allowed to cool to room temperature. A dark black-colored solution smelling like sugarcane vinegar was obtained, which was decanted with the help of a strong magnet. Subsequently, the sample was washed with distilled water several times. Finally, the black-colored precipitate was collected and dried at 80 °C for 4–5 h in an oven.^18,23 Consequently, Fe₃O₄@C core–shell NPs with different sizes including 7 nm, 14 nm and 28 nm were obtained using this technique.

2.4. Preparation of ferrofluids

We introduced fine powders of Fe₃O₄ and Fe₃O₄@C core–shell NPs into DI water in separate beakers. Given that the NPs were highly magnetic in nature, the mixtures were sonicated for 3–4 hours for better dispersion of the NPs. Eventually, we obtained a highly monodispersed NP solution. The prepared ferrofluids were found to be stable without any external applied magnetic field for a long time. Before their application in a heat exchanger, the ferrofluids were sonicated again for better heat transfer results. Different concentrations of ferrofluids were prepared in a similar way for heat transfer and electrical conductivity measurements.

2.5. Characterization

The phase and crystallinity of the as-synthesized Fe₃O₄ and Fe₃O₄@C powder samples were identified by XRD (D8 AαS Advance X-ray diffractometer using Cu Kα radiation, λ = 1.54156 Å). Raman analyses were conducted using a Raman spectrometer (Aramis LabRam spectrometer). The microstructure analyses of the as-synthesized powders were carried out using a transmission electron microscope (TEM) (JEOL, JEM2100F) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed for elemental information using an X-ray photoelectron spectrometer (Thermo Fisher, USA). Moreover, the functional groups were identified using FT-IR (ATR-FT-IR Nicolet iS 10). The magnetic properties of the as-synthesized powders were analyzed using an ADE 3473-70 Technologies vibrating sample magnetometer (VSM) in the range of −10 kOe to 10 kOe with a magnetization error of ±1%. The zeta potential values of the prepared ferrofluids were obtained using a Zetasizer (Nano-ZS; Malvern, UK) operated at room temperature. The electrical conductivities of the ferrofluids were measured using an HI 2300 NaCl/TDS/EC meter (HANNA Instruments). For the measurement of the thermal conductivity of the ferrofluids with different concentrations, a WL-373 (GUNT Hamburg, Germany) instrument was used. This unit is particularly suited for the determination of the coefficients of thermal conduction of liquids and gaseous materials. The unit was comprised of a double-walled cylinder with an integrated heater acting as the heat source, and the surrounding cylinder as the heat sink. There was a narrow slot in the unit, which was sufficient to prevent heat by convection, and therefore the heat of transfer in the slot is due to the thermal conduction. Due to the constant width of the slot, thermal conduction occurs in a plane wall. The heat transferred, Q, can be calculated using Fourier's law as follows:where Q_a = heat transfer, λ = thermal conductivity coefficient, ΔT = temperature gradient, A = surface area and δ = thickness of the slot.

3. Results and discussion

Fig. 1a and b show the XRD patterns of the as-synthesized Fe₃O₄ powder and Fe₃O₄@C powder samples, respectively. Fig. 1a shows diffraction peaks at 2θ values of around 30.09°, 35.45°, 43.02°, 53.59°, 57.20° and 62.72°, which correspond to the (220), (311), (400), (422), (511) and (440) crystal planes, respectively. The peak positions and relative intensities of the Fe₃O₄ powder sample matched with the JCPDS card no. 19-629, which confirmed the crystalline nature and cubic spinel structure of the powder sample. Similarly, for the Fe₃O₄@C powder sample, the diffraction peaks at around 30.2°, 35.5°, 43.17°, 53.59°, 57.37° and 62.72° correspond to the (220), (311), (400), (422), (511) and (440) crystal planes, respectively, as shown in Fig. 1b.^18,23 There is a negligible shift in the diffraction peaks, which suggest that the Fe₃O₄ powder sample did not transform into another iron oxide phase during the synthesis of the Fe₃O₄@C powder sample.^18,23Fig. 1c and d show the FT-IR spectra of the Fe₃O₄ and Fe₃O₄@C powder samples, respectively. The peaks observed at 571 and 631 cm⁻¹ correspond to the vibrations of Fe–O (Fig. 1c).^18,23,51 However, for the Fe₃O₄@C powder sample, the Fe–O vibration bands were observed at 575 and 635 cm⁻¹. The small shift in the Fe–O vibration band is due to the carbon coating over Fe₃O₄. Moreover, the decrease in the vibration frequency of the bond indicates the addition of carbon. The peaks at 2924 cm⁻¹ and 2862 cm⁻¹ refer to the C–H stretching vibrations, and the peaks at 1416 cm⁻¹ and 1066 cm⁻¹ refer to the C–H bending vibrations and C–O stretching vibrations, respectively, for the Fe₃O₄@C NPs. The broad band observed at 3300–3450 cm⁻¹ and the band at 1611 cm⁻¹ present in both spectra correspond to the O–H stretching mode and the H–O–H bending mode, respectively, indicating the presence of interstitial water molecules.^18,23,52 The Raman spectrum of the Fe₃O₄@C powder sample is shown in Fig. 1e, where the intense D band and G band were observed at around 1352 and 1598 cm⁻¹, respectively. The D band can be attributed to sp² carbon, which indicates a disordered graphitic structure and the G band can be attributed to sp³ carbon. The other significant peaks at around 396 cm⁻¹ and 487 cm⁻¹ in the Raman spectrum suggest the T_2g phonon vibration mode in Fe₃O₄.⁵³ The peaks at 574 cm⁻¹ and 667 cm⁻¹ suggest the Fe=O vibration, which is known as the A_1g phonon vibration mode and generally observed due to the symmetric stretching of the O atom along the Fe–O bonds in Fe₃O₄.⁵³

Fig. 1 XRD patterns of (a) Fe₃O₄ and (b) Fe₃O₄@C core–shell NPs. FTIR spectra of (c) Fe₃O₄ and (d) Fe₃O₄@C core–shell NPs and (e) Raman spectrum of Fe₃O₄@C core–shell NPs.

Fig. 2a and b show the low-magnification TEM images of the as-synthesized Fe₃O₄ and Fe₃O₄@C powder samples, and their corresponding size distribution histograms are shown in Fig. 2c and d, respectively. It was observed that the as-synthesized powder samples were comprised of NPs, which were monodispersed on the TEM Cu grid. The average particle size of the Fe₃O₄ NPs and Fe₃O₄@C NPs were found to be around 5 nm and 7 nm, respectively. Similarly, the TEM images of two different sized Fe₃O₄ NPs (12 nm and 25 nm) and Fe₃O₄@C core–shell NPs (14 nm and 28 nm) together with their corresponding size distribution histograms are shown in Fig. S1 (ESI†). Fig. 3a and b show the high-magnification TEM and HRTEM images of the Fe₃O₄ NPs, respectively. As shown in Fig. 3b, three different types of planes were identified with the d spacings of 0.29, 0.26 and 0.21 nm, which correspond to the (220), (311) and (400) planes of cubic Fe₃O₄.⁵³ The HRTEM results also confirmed the crystalline nature of the as-synthesized Fe₃O₄ NPs and suggest that they did not transform into another stable form of iron oxide. Similarly, Fig. 3c and d show the high-magnification TEM and HRTEM images of the carbon-encapsulated Fe₃O₄ (Fe₃O₄@C) NPs, respectively. The carbon-shell coating over the Fe₃O₄ NPs was found to be amorphous in nature, and the thickness of the carbon shell over a random Fe₃O₄ core NP was found to be approximately in the range of 0.8–1.2 nm.

Fig. 2 Low magnification TEM images of (a) Fe₃O₄ NPs (5 nm) and (b) Fe₃O₄@C core–shell NPs (7 nm) and (c) and (d) their corresponding size distribution histograms, respectively.

Fig. 3 High-magnification TEM images of (a) Fe₃O₄ NPs and (c) Fe₃O₄@C core–shell NPs. (b) and (d) HRTEM images of Fe₃O₄ NPs and a random Fe₃O₄@C core–shell NP.

Fig. 4a shows the XPS survey scans of the bare Fe₃O₄ NPs (red color) and Fe₃O₄@C core–shell NPs (black color). Three significant peaks corresponding to the Fe 2p, O 1s and C 1s core levels were obtained for the carbon-coated Fe₃O₄ NPs, which confirmed the composition of the Fe₃O₄@C core–shell NPs, whereas the C 1s core level peak was missing for the bare Fe₃O₄ NPs. Fig. 4b shows the high-resolution spectrum of Fe 2p core level, i.e., Fe 2p_3/2 and Fe 2p_1/2, which correspond to the binding energies 710.5 eV and 724.1 eV, respectively. No peak was observed at 719 eV, which corresponds to α-Fe₂O₃, indicating that no phase transformation occurred.⁵³ Fe exists in two states, i.e., Fe²⁺ and Fe³⁺, in Fe₃O₄. The peak at 710.3 eV (pink) is attributed to the Fe²⁺ state in Fe 2p_3/2. The peak observed at 712.7 eV (purple) corresponds to the Fe³⁺ state in Fe 2p_3/2. Similarly, for Fe 2p_1/2, a peak appeared at 725.7 eV (blue), which correlates to Fe³⁺, and another peak appeared at 723.7 eV (cyan), corresponding to Fe²⁺. The HR O 1s core level spectrum is shown Fig. 4c. The peaks at 531.7 eV (green) and 529.9 eV (wine) are attributed to the Fe–O bond in the Fe₃O₄ NPs. The other peak observed at 533.1 eV (blue) corresponds to the O–H bond. The HR C 1s core level spectrum is shown in Fig. 4d. The peaks at 28.63 eV (blue) and 285.2 eV (green) correspond to the C–C sp³ and C=C sp² carbon, respectively. The small peaks observed at 287.5 eV (purple) and 289.1 eV (pink) are attributed to C–O and C=O, respectively, which may originate from airborne organic contaminants. Moreover, it was also confirmed that no reaction between the Fe₃O₄ NPs and carbon coating occurred.

Fig. 4 (a) XPS survey scan of bare Fe₃O₄ NPs and carbon-coated Fe₃O₄ (Fe₃O₄@C) core–shell NPs. HR scans of (b) Fe 2p_3/2 and Fe 2p_1/2 and (c) O 1s and (d) C 1s core level spectra.

Fig. 5 shows the magnetic characterization of the small average-sized Fe₃O₄ NPs (ca. 5 nm) and Fe₃O₄@C core–shell NPs (ca. 7 nm) with respect to different magnetic fields at 300 K. An unusual behavior in magnetic properties was observed in NPs compared to their bulk counterpart. The magnetization was observed to be 60 emu g⁻¹ for Fe₃O₄ NPs (ca. 5 nm), and it was obvious that there was no coercivity and remanence. Hence, the as-synthesized Fe₃O₄ NPs (ca. 5 nm) were found to be superparamagnetic in nature.²⁴ Similarly, the as-synthesized Fe₃O₄@C core–shell NPs (ca. 7 nm) also exhibited super-paramagnetic behavior, and the magnetization was found to be 30 emu g⁻¹ with no coercivity and remanence. The saturation magnetization was found to be much lower in the case of the core–shell NPs. The low value of the magnetic saturation may be attributed to the carbon coating over the Fe₃O₄ NPs as carbon materials are diamagnetic in nature.⁵⁴ A low coercive field was also observed in the case of both NPs which indicate their spherical shape.⁵⁵ The stabilities of the prepared ferrofluids were evaluated in terms of zeta potential and the results are shown in Table 1 and Fig. 6. The zeta potentials were measured for the different sized Fe₃O₄ and Fe₃O₄@C NP-based ferrofluids at 0.7 vol% of NPs. The results suggest coagulation/agglomeration behavior for the Fe₃O₄ NP-based ferrofluids (zeta potential <±5), leading to the formation of networks, whereas the Fe₃O₄@C NP-based ferrofluids showed moderate stability (±10 < zeta potential < ±30).^56,57 An increase in the stability of NPs or their good dispersibility in the base fluid is generally observed for core–shell NPs.^58,59

Fig. 5 Magnetic hysteresis loop of Fe₃O₄ NPs (ca. 5 nm) and Fe₃O₄@C core–shell NPs (ca. 7 nm).

Table 1 Zeta potential of the different sized Fe₃O₄ and Fe₃O₄@C NP-based ferrofluids at 0.7 vol% of NPs

Ferrofluid sample	Nanoparticles	Size (nm)	Zeta potential (mV)
1	Fe3O4	5	6.49
2	Fe3O4	12	−1.12
3	Fe3O4	25	−7.18
4	Fe3O4@C	7	−18.7
5	Fe3O4@C	14	−19.4
6	Fe3O4@C	28	−20.2

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Fig. 6 Zeta potential plots of (a) 5 nm Fe₃O₄, (b) 12 nm Fe₃O₄, (c) 25 nm Fe₃O₄, (d) 7 nm Fe₃O₄@C, (e) 14 nm Fe₃O₄@C and (f) 28 nm Fe₃O₄@C NP-based ferrofluids at 0.7 vol% NP concentration.

The electrical conductivities (ECs) of the as-synthesized Fe₃O₄ NPs (ca. 5 nm) and Fe₃O₄@C core–shell NP (ca. 7 nm)-based ferrofluids with different concentrations and temperature were measured. The EC of the ferrofluid was found to increase with an increase in the concentration of both types of NPs, as shown in Fig. 7a and b. Fig. 7a shows the ECs of distilled water (base fluid) and the ferrofluids with different concentrations of Fe₃O₄@C core–shell NPs at various temperatures. The EC of the ferrofluids was found to be many times higher than that of the base fluid. Similarly, Fig. 7b shows the ECs of Fe₃O₄ NP-based ferrofluids, which followed the same trend as observed in Fig. 7a for the Fe₃O₄@C core–shell NP-based ferrofluids. However, the ECs of the bare Fe₃O₄ NP-based ferrofluids were found to be higher than the core–shell NP-based ferrofluids. This may be attributed to the amorphous nature of the carbon coating, which resulted in a decrease in the EC. Fig. 7c shows a comparison of the ECs of the Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids at 50 °C. It was quite obvious that the Fe₃O₄ NP-based ferrofluid at a particular concentration of Fe₃O₄ NPs exhibited a higher EC compared to the core–shell NP-based ferrofluid. Fig. 7d demonstrates the percentage enhancement in the EC compared to that of the base fluid. It was found that the Fe₃O₄ NP-based ferrofluid exhibited the highest value of EC, i.e., 205 μS cm⁻¹ for 0.7 vol% of NPs at 50 °C. Conversely, the EC value of 130 μS cm⁻¹ was recorded for the Fe₃O₄@C core–shell NP-based ferrofluid at the same concentration and temperature. The percentage enhancement was calculated using the following equation:

where σ₀ = EC of DIW and σ = EC of the ferrofluid.

Fig. 7 Electrical conductivities of (a) Fe₃O₄@C NP (7 nm)-based ferrofluids and (b) Fe₃O₄ NP (5 nm)-based ferrofluids at different temperatures. (c) Comparison of the electrical conductivities of the Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids at 50 °C and (d) Comparison of the percentage enhancement in electrical conductivity for the Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids at 50 °C with respect to DI water.

The maximum enhancement in the EC of the Fe₃O₄ NP- and Fe₃O₄@C core–shell NP-based ferrofluids was obtained as ∼3222% and ∼2015%, respectively, for 0.7 vol% of NPs at 50 °C. There are many factors that affect the EC of a nanofluid or a ferrofluid such as volume fraction, temperature, structure of NPs, types of NPs, and types of fluids.⁶⁰ The EC of a nanofluid/ferrofluid generally increases with an increase in the volume fraction of the nanofluid/ferrofluid and with temperature. The ions on the surface of the colloidal NPs are attracted to the oppositely charged ions, which results in the formation of a layer. Another layer of ions is also formed, where the ions from the suspension become attached to the first layer by Coulomb force, which electrically screens the first layer. The ions in the second layer are loosely associated and move due to the electric force of attraction and thermal motion. Hence, as the temperature increases, the free ions move rapidly into the suspension of the ferrofluid, which results in a high EC. An increase in the concentration of NPs also enhanced the EC as more solid NPs suspended into the fluid were available to expose their surfaces toward the ions and formed an electrical double layer. However, a relatively low EC in the case of the Fe₃O₄@C core–shell NPs was observed as the carbon-coated suspended Fe₃O₄ NPs had a lower number of electrons on their surfaces to get attracted toward the free ions compared to the bare Fe₃O₄ NPs. The electrons were more populated near the Fermi level in the semiconductor (Fe₃O₄) NPs compared to the non-metallic amorphous carbon. Hence, relatively low ECs were observed for the Fe₃O₄@C core–shell NP-based ferrofluids compared to the Fe₃O₄ NP-based ferrofluids.

Similarly, the thermal conductivities of the Fe₃O₄ NP- and Fe₃O₄@C core–shell NP-based ferrofluids were measured. Fig. 8a and b show the graphs of the coefficient of thermal conductivity versus temperature gradient for the Fe₃O₄@C core–shell NP (7 nm)- and Fe₃O₄ NP (5 nm)-based ferrofluids, respectively. For both types of NPs, with concentrations in the range of 0.1 vol% to 0.7 vol%, it was observed that the coefficient of thermal conductivity (λ) increased consistently. The average thermal conductivity coefficients for water was found to be 0.71 W m⁻¹ K⁻¹, and for the ferrofluids at different concentrations of Fe₃O₄@C core–shell NPs, i.e., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 vol% they were found to be 0.82, 0.90, 0.99, 1.19, 1.22, 1.46 and 1.54 W m⁻¹ K⁻¹, respectively, as shown in Fig. 8a. A similar trend was observed for the Fe₃O₄ NP-based ferrofluids, and the values of the coefficient of thermal conductivity for various concentrations, i.e., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 vol% were obtained as 0.84, 0.92, 1.02, 1.16, 1.21, 1.48 and 1.80 W m⁻¹ K⁻¹, respectively, as shown in Fig. 8b. The comparison of thermal conductivities and percentage enhancement of Fe₃O₄ NP- and Fe₃O₄@C core–shell NP-based ferrofluids are shown in Fig. 8c and d, respectively. Fig. 8d depicts the increase in percentage enhancement with respect to DI water from 16% to 116% as the concentration of Fe₃O₄@C core–shell NPs varied from 0.1 vol% to 0.7 vol%. Similarly, a 17% to 153% enhancement with respect to DI water was observed for the Fe₃O₄ NPs as their concentration varied from 0.1 vol% to 0.7 vol%.

Fig. 8 Thermal conductivity with ΔT (a) Fe₃O₄@C core–shell NP (7 nm)-based ferrofluids and (b) Fe₃O₄ NP (5 nm)-based ferrofluids. (c) Comparative thermal conductivity coefficient and (d) comparative percent enhancement in thermal conductivity of Fe₃O₄ NP- and Fe₃O₄@C core–shell NP-based ferrofluids with respect to DI water.

The maximum enhancement was observed for 0.7 vol% of Fe₃O₄ NPs. This may be attributed to the clustering effect of the NPs as their surface energies were quite high owing to their high surface to volume ratio and the formation of a chain-like network of small-sized NPs, which increased the effective volume fraction of heat conductive phases in the ferrofluid. Moreover, the dispersed NPs in the water-based ferrofluid executed Brownian motion, which led to collisions between the NPs and with the molecules of the water.^61,62 Conversely, in the case of the Fe₃O₄@C core–shell NPs, the thermal capacity of the carbon coating and percolation pathways for heat conduction were responsible for the thermal conductivity enhancement.^61,62 It should be noted that at only 0.7 vol%, the thermal conductivity of the Fe₃O₄@C core–shell NP-based ferrofluid was significantly less than that of the Fe₃O₄ NP-based ferrofluid. Moreover, the effects of different NPs sizes on the enhancement of the electrical and thermal conductivities were also studied, as shown in Fig. 9a and b (Fig. S2 and S3, ESI†), respectively. Fig. 9a shows the variation in the electrical conductivity enhancement of the Fe₃O₄ NP- and Fe₃O₄@C NP-based ferrofluids with NP size. The EC was found to be enhanced with a decrease in the size of the NPs for both types of ferrofluids. The increase in EC with a decrease in the NP size is attributed to the increase in surface area and electrophoretic mobility of the NPs.^63,64 A similar trend was observed for the thermal conductivity enhancement, which increased with a decrease in NP size for both types of ferrofluids, as shown in Fig. 9b. The exceptional enhancement in the thermal conductivity especially in the case of the bare Fe₃O₄ NP (5 nm)-based ferrofluid compared to the larger-size Fe₃O₄ NP (12 nm and 25 nm)-based ferrofluids is due to the more pronounced effect of chain-like network formation/clustering of the NPs, as inferred from the zeta potential values (Table 1 and Fig. 6). However, owing to the carbon coating over the bare Fe₃O₄ NPs, relatively better dispersion and less agglomeration of the different sized Fe₃O₄@C NPs (as inferred from their more negative zeta potential values, Table 1 and Fig. 6) in the base fluid (water) resulted in a lower enhancement in thermal conductivity for the Fe₃O₄@C NP-based ferrofluids compared to the Fe₃O₄ NP-based ferrofluids.

Fig. 9 Effect of NP size on (a) electrical conductivity enhancement and (b) thermal conductivity enhancement of ferrofluids.

To theoretically validate the obtained results, the experimental values of the thermal conductivities of the Fe₃O₄ NP (ca. 5 nm)- and Fe₃O₄@C NP (ca. 7 nm)-based ferrofluids were fitted with the existing Maxwell model,⁶⁵ as shown in Fig. 10. Based on the effective medium theory, the effective thermal conductivity of the Fe₃O₄ NPs coated with carbon was calculated using the following equation:⁶⁶

where k_ep is the equivalent thermal conductivity of the Fe₃O₄@C NPs, k_p is the thermal conductivity of the Fe₃O₄ NPs, γ is the ratio of coated nanolayer thermal conductivity to the particle thermal conductivity, and β is the ratio of coated layer thickness to that of the particle radius. The thermal conductivity of the ferrofluids based on Fe₃O₄ NPs and Fe₃O₄@C NPs was calculated using the Maxwell equation as follows:where k_ff, k_bf and k_np are thermal conductivities of the ferrofluid, base fluid and NP (k_ep was used instead of k_np in the case of the carbon-coated Fe₃O₄ NPs), respectively, and ⌀_np is the concentration of NPs (vol%) in the base fluid. It can be seen in Fig. 10 that the values predicted by the Maxwell model are higher at low concentrations for the Fe₃O₄ NP (ca. 5 nm)-based ferrofluids. In contrast, for the Fe₃O₄@C (ca. 7 nm) NP-based ferrofluid, the predicted values of thermal conductivity closely fit the experimental data. The slight deviation in the thermal conductivity values of the Fe₃O₄ NP-based ferrofluids is attributed to the clustering or aggregation of the NPs, as inferred from the zeta potential values (Table 1 and Fig. 6).⁶⁷ Therefore, the Maxwell model validated the experimental results for the Fe₃O₄@C NP-based ferrofluid. These obtained results were compared with the results reported in the literature, as presented in Table 2.

Fig. 10 Comparison of experimental and predicted values of thermal conductivities of Fe₃O₄ (ca. 5 nm) and Fe₃O₄@C (ca. 7 nm) NP-based ferrofluids.

Table 2 Comparison of the electrical and thermal conductivities of our Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids with that in the literature

Material	Size (nm)	Conc./volume fraction	Base fluid	Electrical conductivity enhancement (%)	Thermal conductivity enhancement (%)	Ref.
Fe3O4	14.2	0.5	Water	360	—	68
SiO2	12	1.1–2.4	Water	—	1.1	69
TiO2	27	3.1–4.3			10.8
Al2O3	13	1.3–4.3			32.4
Graphene	—	0.5/0.1	Ethylene glycol/water	1664	10.47	70
Fe@C	—	1.5 wt%	Polyethylene glycol	—	30	39
Al@C					40
Cu@C					49
Iron oxide(II,III)	50	1 g L^−1	Water	—	34	34
Fe3O4	5–10	2.5 g L^−1	Mineral oil	—	51	19
γ-Fe2O3	12	1.5 g L^−1	50 : 50 oil mixture	—	51	21
CuO	29	0–6	Ethylene glycol/water	—	60	71
Al2O3	53	0–10			69
γ-Fe2O3	13	1.5 g L^−1	Liquid paraffin/sunflower/mineral oil	—	20.6	20
					45.2
					77
Fe3O4@C	7	0.7 vol%	Water	2015	116	This work
Fe3O4 NPs	5			3222	153

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4. Conclusion

In conclusion, Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids were prepared to investigate their thermal and electrical conductivities. Firstly, we synthesized different sized Fe₃O₄ NPs via a chemical co-precipitation method using iron salts and alkali solution as precursors. The successfully synthesized Fe₃O₄ NPs were introduced in an autoclave with a solution of glucose at 180 °C and high pressure. The hydrothermal process enabled the formation of a carbon shell over the Fe₃O₄ NPs. The average particle sizes of the Fe₃O₄ and Fe₃O₄@C core shell NPs were found to be in the range of 5–25 nm and 7–28 nm, respectively. The structural and elemental analyses revealed that no phase transformation of Fe₃O₄ occurred during the formation of core–shell NPs. The magnetic characterization revealed that the as-synthesized small average-sized Fe₃O₄ NPs (ca. 5 nm) and Fe₃O₄@C core–shell NPs (ca. 7 nm) were superparamagnetic in nature. Finally, the electrical and thermal conductivities of the Fe₃O₄ and Fe₃O₄@C core–shell NP-based ferrofluids were measured at different concentrations of NPs and with different sized NPs. Exceptional results were obtained, where the electrical conductivity was enhanced up to ∼3222% and ∼2015% for the Fe₃O₄ (ca. 5 nm) and Fe₃O₄@C (ca. 7 nm) core–shell NP-based ferrofluids, respectively, compared to distilled water (base fluid). Similarly, the enhancement in the thermal conductivity was determined to be ∼153% and ∼116% for the Fe₃O₄ (ca. 5 nm) and Fe₃O₄@C (ca. 7 nm) core–shell NPs, respectively.

Author contributions

Mohd Imran: Conceptualization, methodology, investigation, writing-original draft preparation, supervision; Nasser Zouli: visualization, investigation, supervision; Tansir Ahamad: characterization, investigation, funding; Saad M. Alshehri: characterization, investigation, funding; Mohammed Rehaan Chandan: visualization, validation, review & editing; Shahir Hussain: conceptualization, methodology, investigation, electrical conductivity experiment; Abdul Aziz: resources, thermal conductivity experiment; Mushtaq Ahmad Dar: data curation, characterization; Afzal Khan: visualization investigation, supervision, review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors gratefully acknowledge the Deanship of Scientific Research at King Saud University for funding this project (RG-1438-026). The authors are also grateful to King Saud University, Riyadh, and Jazan University, Jazan, Kingdom of Saudi Arabia for providing experimental facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1na00061f

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