Induction heating: an efficient methodology for the synthesis of functional core–shell nanoparticles

Abstract

Induction heating has been applied for a variety of purposes over the years, including hyperthermia-induced cell death, industrial manufacturing, and heterogeneous catalysis. However, its potential in materials synthesis has not been extensively studied. Herein, we have demonstrated magnetic induction heating-assisted synthesis of core–shell nanoparticles starting from a magnetic core. The induction heating approach allows an easy synthesis of FeNi₃@Mo and Fe_2.2C@Mo nanoparticles containing a significantly higher amount of molybdenum on the surface than similar materials synthesized using conventional heating. Exhaustive electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy characterization data are presented to establish the core–shell structures. Furthermore, the molybdenum shell was transformed into the Mo₂C phase, and the catalytic activity of the resulting nanoparticles tested for the propane dry reforming reaction under induction heating. Lastly, the beneficial role of induction heating-mediated synthesis was extended toward the preparation of the FeNi₃@WO_x core–shell nanoparticles.

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Bruno Chaudret

After a long-standing relationship with the RSC, in particular as a former member of the Editorial Boards of New J Chem, Dalton Transactions and Catalysis Science & Technology as well as recipient of the Wilkinson Prize, I have been very honored to be nominated as a scientific editor of Materials Horizons. For me the journal was special since each article should bring a new concept and explain it in a separate paragraph. This requirement is largely responsible for the success of Materials Horizons over the past ten years, now a leading journal in the field of materials science.

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New concepts

Using magnetic nanoparticles as heating agents under an alternating magnetic field is a hot research topic due to the high efficiency of heat delivery on a localized spot that these materials enable. For applications, magnetic nanoparticles may need additional functionalization; we show here that magnetic induction allows the preparation of nanoparticles displaying a magnetic core and a shell of molybdenum, a metal difficult to obtain at the zerovalent state, FeNi₃@Mo and Fe_2.2C@Mo. This approach yields significant benefits, depositing approximately twice as much molybdenum on the nanoparticle shell compared to a conventional thermal method in a shorter reaction time and spending lower amounts of Mo(CO)₆. Thus, using magnetic heating instead of thermal heating allows FeNi₃ or Fe_2.2C nanoparticles to serve as both a heat source and a nucleation centre, resulting in an accurate decomposition of Mo(CO)₆ onto the desired spot. The resulting outer layer can be further transformed into Mo₂C under magnetic induction and used for catalytic reactions such as propane dry reforming hence demonstrating the flexibility of the approach. Finally, applying this strategy to an FeNi₃@W system shows that this approach may be extended to functionalize magnetic materials efficiently and accurately. Therefore, the methodology reported herein paves the way for developing innovative complex nanostructures.

1. Introduction

Magnetic nanoparticles (Mag-NPs) can produce high temperatures in short times and in localized spots via hysteresis losses when exposed to an alternating magnetic field (AMF).^1–3 They have found applications in different fields, such as catalysis,^1–3 medicine,⁴ and industrial manufacturing,⁵ where they offer remarkable advantages such as improved energy efficiency, precise localized heating and swift temperature control. However, synthetic implications of the induction heating phenomena (IH) remain almost unexplored at the nanoscale.

Although the literature is teeming with examples of chain-like assemblies, nanosheets and core–shell structures assisted by static magnetic fields,^6,7 there are few instances where an AMF is used, and none that produce core–shell nanoparticles. There have been reports of metal substrates acting as a heating agent upon generating eddy currents under AMF for growing nanostructures. One example is nickel foam anchoring Ni nanoparticles on MoO₂ nanowires and NiFe-layered double hydroxides.⁸ Besides, graphene and carbon nanotubes (CNTs) have been grown on metal substrates by induction heating (IH) in a chemical vapor deposition (CVD) system using Pt monocrystals,⁹ Pt foils,¹⁰ Cu foils,¹¹ and Ni layers¹² as the heating substrates. Cu and Cu₂O nanostructures can also be synthesized on Ni foil using IH and electrodeposition in solution.¹³ Moreover, graphene, a conductive non-metal, serves as a self-heating medium for the preparation of SiC nanorods from SiO₂ NPs.¹⁴ The possibility of using reactors filled with stainless steel balls in a coil for the synthesis of iron oxide nanoparticles¹⁵ or CdSe¹⁶ quantum dots (QDs) has also received attention. To the best of our knowledge, the only systems derived from the use of nanoparticles under an AMF are TiO₂-decorated Fe₃O₄,¹⁷ and a carbon Fe₃O₄ nanocomposite.¹⁸ However, no literature on magnetic nanoparticles acting as the substrate for the growth of metal, metal oxide or metal carbide shells is available.

On the other hand, core–shell nanoparticles are a distinct class of nanomaterials that offer unique advantages. By combining the properties of the core and shell, it is possible to tailor their physical and chemical attributes. In fact, they have found a wide range of uses in materials chemistry,¹⁹ biomedicine,²⁰ and catalysis²¹ due to their broad range of features. Realizing their potential in varieties of applications, numerous synthetic routes were proposed.^21,22 The most popular synthetic method for core–shell nanoparticles is the seed-mediated growth of a shell over a preformed core particle.^21,22 Inspired by this, we aimed to develop a general method for the synthesis of core–shell nanoparticles through the use of ferromagnetic nanoparticles to create metallic or metal carbide shells via precursor decomposition in a reducing atmosphere. Based on this, and to explore the generality of the approach, we have considered two materials for the core, i.e., FeNi₃ and Fe_2.2C nanoparticles, the magnetic properties of which have been studied in our group, and refractory targets which are difficult to prepare at the nanoscale, namely Mo and Mo₂C.

Interestingly, transition metal carbides have recently been proposed as substitutes for noble metals in catalysis due to their similar electronic properties.^23,24 Among them, molybdenum carbide is a promising option due to a catalytic activity similar to platinum.^25,26 In that sense, it has been employed in heterogeneous phases for a variety of processes, including synthesis gas conversion into alcohols,²⁷ water gas shift reaction,²⁸ CO₂ hydrogenation,²⁹ reductive biomass upgradation,^30,31 and the hydrogen evolution reaction (HER).³² In this work, the dry reforming of propane (PDR) was selected to evaluate the catalytic activity of the Mo-based shell. PDR has gathered interest because it leads to the production of synthesis gas while valorising CO₂ (C₃H₈ + 3CO₂ ⇌ 6CO + 4H₂, ΔH° = 620 kJ mol⁻¹), and propane is regarded as a suitable hydrogen carrier due to the already existing infrastructure for its transport and storage.^33–35 Since the PDR reaction occurs at high temperatures (550 to 900 °C) and low pressures, magnetic heating assisted by commercial Fe-wool was used along with the Mag-NPs@Mo nanoparticles.^36,37

In summary, we present herein the synthesis of FeNi₃@Mo and Fe_2.2C@Mo nanoparticles (Mag-NP@Mo) via thermal and induction heating mediated decomposition of a Mo(CO)₆ precursor. The as-prepared samples were thoroughly characterized using transmission electron microscopy (TEM), scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping, powder X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) techniques. The differences between conventional and induction heating (IH) were investigated. Furthermore, the unprecedented functionalization of Mag-NPs with carbidic Mo, the enhancement of the Mo₂C phase by carburation, and its catalytic activity toward the PDR reaction were discussed. The results demonstrate that this approach directs the reaction towards the nanoparticle surface by utilizing the “hot spot” phenomenon. This results in the formation of refractory shells and boosts deposition yield by reducing homogeneous nucleation of molybdenum when compared to the thermal approach. Additionally, the magnetic core is preserved, resulting in high-quality core–shell nanostructures. Finally, the IH-mediated synthesis of FeNi₃@WO_x core–shell nanoparticles was demonstrated for scope extension purposes. The sample abbreviation is shown in Table 1.

Table 1 Abbreviation of the core–shell nanoparticles synthesized in this work

Abbreviation	Magnetic core	Metal shell	Heating mode
FeNi3@Mo-TH	FeNi3	Mo	Thermal
FeNi3@Mo-IH	FeNi3	Mo	Induction
Fe2.2C@Mo-TH	Fe2.2C	Mo	Thermal
Fe2.2C@Mo-IH	Fe2.2C	Mo	Induction
FeNi3@WOx-IH	FeNi3	W	Induction

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2. Results and discussion

At first, FeNi₃@Mo-TH and Fe_2.2C@Mo-TH NPs were prepared by thermal decomposition of Mo(CO)₆ on FeNi₃ NPs or Fe_2.2C NPs, respectively, at 150 °C under a 3 bar H₂ pressure, following a procedure analogous to that previously described in our group for the synthesis of Fe_2.2C@Ru,³⁸ namely a two-step method (see Scheme 1). First, a layer of Mo was deposited, which acted as the nucleation site for the overgrowth of a Mo shell. Each step lasted for 72 hours, and an increasing concentration of Mo was used in each step, (i.e., 0.01 mmol mL⁻¹ and 0.04 mmol mL⁻¹). Elemental analysis data for FeNi₃@Mo and Fe_2.2C@Mo NPs synthesized via thermal and induction heating can be found in entries 1 and 3 in Table 2. The bulk composition of FeNi₃@Mo-TH and Fe_2.2C@Mo-TH NPs determined by ICP-AES analysis proved the presence of Mo in the products in 7 wt% for FeNi₃@Mo-TH and 11 wt% for Fe_2.2C@Mo-TH (Table 2 and Table S1, ESI†).

Scheme 1 Schematic representation of thermal and magnetically induced molybdenum coating on magnetic nanoparticles.

Table 2 Chemical compositions and particle sizes of FeNi₃@Mo and Fe_2.2C@Mo nanoparticles synthesized via thermal and induction heating

Entry	Material	d (nm)	ICP-AES^b (wt%)			XPS^c (at%)
			Fe	Ni	Mo	Fe	Ni	Mo
a Particle size was calculated from the histogram plotted using the BF TEM images (all the histograms are shown in the ESI). b Mass percentage referred to the total mass of material. c Atomic percentage refers to the total amount of metals.
1	FeNi3@Mo-TH	14.9 ± 2.6	18	66	7	39	27	34
2	FeNi3@Mo-IH	15.7 ± 2.6	18	56	14	29	16	55
3	Fe2.2C@Mo-TH	14.8 ± 1.8	71	—	11	76	—	24
4	Fe2.2C@Mo-IH	14.8 ± 1.8	65	—	21	51	—	49

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Taking advantage of the magnetic properties of FeNi₃ and Fe_2.2C precursors, Mo-coated nanoparticles were synthesized using induction heating (300 kHz, 15 mT) for 48 hours. The as-prepared FeNi₃@Mo-IH and Fe_2.2C@Mo-IH contained 14 wt% Mo for FeNi₃@Mo-IH and 21 wt% for Fe_2.2C@Mo-IH as determined by ICP-AES (Table 2 and Table S1, ESI†). Thus, the implementation of induction heating in the synthesis process enabled the achievement of greater quantities of Mo, all within a single cycle of the Mo(CO)₆ decomposition step, while operating under 3 bars of H₂, and using the same concentration of Mag-NPs as for thermal heating. Furthermore, the surface composition of the nanoparticles synthesized via thermal and induction heating was determined using the XPS technique (Table 2). Aligning with the findings observed at the bulk level, the surface analysis reveals a notable enhancement in the deposition of Mo on the bare core (FeNi₃ or Fe_2.2C) nanoparticles when induction heating is employed, as compared to the conventional heating method. The results from XPS data interpretation are discussed in detail in the following sections.

The morphology of the Mo-coated nanoparticles was analyzed via bright-field TEM (BF TEM). The particle size obtained from the histogram is tabulated in Table 2. Fig. S1 and S2 (ESI†) show the BF TEM image of parent FeNi₃ and FeNi₃@Mo-TH NPs, respectively. Both FeNi₃ and FeNi₃@Mo-TH NPs show similar polyhedral shapes. Likewise, the BF TEM image of Fe_2.2C@Mo-TH NPs showed particles that could not be distinguished from the respective parent Fe_2.2C NPs (Fig. S3 and S4, ESI†). The particle size distribution was calculated from the histogram for the as-prepared Mag-NPs and after the Mo coating and was found to be nearly identical (Fig. S1–S4, ESI†).

Additionally, further structural characterization was carried out using the powder XRD technique. The powder XRD pattern of FeNi₃@Mo-TH and Fe_2.2C@Mo-TH NPs was compared with the parent FeNi₃ or Fe_2.2C NPs cores (Fig. S5, ESI†). The diffraction patterns of Mo-coated NPs showed only the peaks corresponding to either FeNi₃ or Fe_2.2C NPs, and no other peak that might be attributed to any Mo species (Fig. S5, ESI†). Furthermore, the presence of the Mo shell was investigated using STEM-EDX elemental mapping. STEM coupled with EDX spectroscopy confirmed the presence of molybdenum on the surface of the nanoparticles (Fig. 1 and 2). The reason why Mo was detected by ICP-AES and STEM-EDX but not by powder XRD results is probably due to the very small amount of surface molybdenum present and to its amorphous nature.

Fig. 1 FeNi₃@Mo-TH NPs, (a) BF TEM image, (b) STEM-HAADF image and STEM-EDX mapping of the sample, (c) Ni (red), (d) Fe (blue), (e) Mo (green), and (f) overlap of Fe, Ni, and Mo maps.

Fig. 2 Fe_2.2C@Mo-TH NPs, (a) BF TEM image, (b) STEM-HAADF image and STEM-EDX mapping showing (c) Fe (blue), (d) Mo (green), and (e) overlap of Fe and Mo maps.

Similarly, the as-prepared FeNi₃@Mo-IH and Fe_2.2C@Mo-IH NPs were also characterized by BF TEM imaging, showing objects indistinguishable from their parent NPs (Fig. 3b and 4b). The FeNi₃@Mo-IH NPs display a mean size of 15.7 ± 2.6 nm and a polyhedral shape (Fig. S6, ESI†), while Fe_2.2C@Mo-IH NPs are spherical with a size of 14.8 ± 1.8 nm (Fig. S7, ESI†). In the same line, the powder XRD patterns matches those of the FeNi₃ or Fe_2.2C cores (Fig. 3a and 4a). Despite a higher concentration of Mo present in FeNi₃@Mo-IH and Fe_2.2C@Mo-IH as compared to the FeNi₃@Mo-TH and Fe_2.2C@Mo-TH, namely a ca. 0.5 nm thick Mo layer, no diffraction peaks corresponding to the Mo (metallic Mo or MoO_x) species were observed. This observation can again be related to the still modest quantity of Mo present at the surface and to its amorphous nature.

Fig. 3 Fe_2.2C@Mo-IH NPs, (a) powder XRD pattern (ICDD: 00-036-1249, red vertical drop lines for standard Fe_2.2C pattern), (b) BF TEM micrograph, (c) HRTEM image, (d) FFT pattern derived from the white square marked zone of image c along 〈0001〉 zone axis, (e) STEM-HAADF image, (f)–(h) STEM-EDX elemental maps showing Fe in blue and Mo in green, and overlap of Fe and Mo maps.

Fig. 4 FeNi₃@Mo-IH NPs: (a) powder XRD pattern (ICDD: 00-038-0419, red vertical drop lines for standard FeNi₃), (b) BF TEM micrograph, (c) HRTEM image of a single nanoparticle, (d) FFT pattern of the marked white square zone from HRTEM figure c along 〈110〉 zone axis, (e) STEM-HAADF image, (f)–(i) individual STEM-EDX maps showing Fe in blue, Ni in red, and Mo in green, and their overlap map, and (j) line scan profile of a single nanoparticle obtained using STEM-EDX technique (Mo L line - green, Fe K line - blue, and Ni K line – red).

Fig. 3c shows the high-resolution transmission electron microscopy (HRTEM) image of the Fe_2.2C@Mo-IH. The fast Fourier transform (FFT) analysis of the core part of the HRTEM image indicates the presence of the hexagonal Fe_2.2C phase (Fig. 3d). In contrast, the FFT pattern of the shell was devoid of any diffraction spots. This finding emphasizes the amorphous nature of the shell. The high-resolution scanning transmission electron microscopy high-angle annular dark field (HR-STEM HAADF) image is shown in Fig. S8 (ESI†). The Fe_2.2C@Mo-IH NPs were observed to have a thin, amorphous shell measuring 1 to 2 nm, which could be attributed to Mo. To corroborate the core–shell structure, STEM-EDX mapping was conducted, and the results are shown in Fig. 3e–h. The combined Fe and Mo EDX maps confirmed that the Fe_2.2C@Mo NPs had a core–shell structure, with Mo located in the outermost layer as seen in Fig. 3h.

The HRTEM image of the FeNi₃@Mo-IH NPs is shown in Fig. 4c. The FFT analysis of the HRTEM image reveals a crystalline cubic FeNi₃ core and an amorphous Mo shell (Fig. 4d), with a thickness of 1 to 2 nm. An amorphous outer layer is also evident in the HR-STEM image (Figure S9, ESI†). The STEM-EDX elemental mapping of the FeNi₃@Mo-IH NPs is shown in Fig. 4e–i. The overlap of the Fe, Ni, and Mo EDX maps establishes the structure: FeNi alloy core and Mo shell. Additionally, EDX line scan analysis confirms the core–shell structure for FeNi₃@Mo-IH, with Mo as the outer layer (Fig. 4j).

In order to investigate the electronic state of the surface Mo layer, X-ray photoelectron spectroscopy (XPS) was performed on samples synthesized via thermal and induction heating. The samples were exposed to air rapidly before introduction into the spectrometer. Fig. 5 shows a tentative fitting for the Mo3d (a) and C1s (b) regions for IH samples. In the Mo3d spectrum, clear signals of oxidized Mo could be identified, with Mo^IV3d_5/2 appearing at 230.0 eV and Mo^VI3d_5/2 at 231.9 eV.^39,40 Additionally, adding components at lower binding energy (BE) makes it possible to achieve a satisfactory fit. Although this fitting (see the ESI† for a detailed procedure) may not precisely quantify Mo species, it helps identify Mo⁰ (≈227.8 eV)^41,42 and carbidic Mo (≈228.4 eV).^43,44 In the case of FeNi₃@Mo, this carbidic peak assignment is also supported by the presence of a peak at ca. 282.8 eV in the C1s region.⁴³ Interestingly, it can be challenging to differentiate whether carbidic and Mo⁰ peaks observed originate from the synthesis or from the decomposition of Mo(CO)₆ on surfaces under the ultra-high vacuum conditions used during analyses.⁴⁵ However, based on the low intensity of the expected CO chemisorbed peak (≈285.6 eV)⁴⁶ and the lack of precursor in the IR analyses (Fig. S12, ESI†), we can assume that the shell composed of carbidic Mo and Mo⁰ originates from the IH-assisted synthesis.

Fig. 5 (a) Mo3d and (b) C1s XPS regions for as-prepared core–shell nanoparticles assisted by induction heating after peak fitting. Note: doublet component colors are merged for a given chemical state for ease of visualization.

To investigate the reasons behind variations in Mo loading during different heating processes, we looked deeper into role of the local reaction temperature. In thermal heating, the temperature of the oil bath was considered the temperature of the solution within the Fisher–Porter bottle. In induction heating, the reaction temperature of the solution during the Mo(CO)₆ decomposition step was measured using an IR camera to be 165 °C (Table S3, ESI†). It is interesting to note that the Mo(CO)₆ boiling point is at 156 °C, and indeed a re-crystallization was observed around the neck of the Fisher–Porter bottle during the reactions in an oil bath at 150 °C. We believe it is likely that, in conventional heating, the decomposition of Mo(CO)₆ and the evaporation of Mo(CO)₆ compete, resulting in the depletion of the overall Mo concentration in the solution. On the other hand, the increased amount of Mo deposited on the magnetic cores is likely due to the generation of localized hot spots in the induction heating. This has previously been demonstrated in the case of Fe_2.2C@Ru where a surface temperature of 251 °C has been deduced from catalytic reactivity, whereas the mesitylene solution remained at 160 °C.⁴⁷ The decomposition of Mo(CO)₆ on the hot nanoparticles minimizes its re-crystallization on the walls of the reactor. Finally, it was observed that, the amount of Mo deposited in FeNi₃@Mo-IH and Fe_2.2C@Mo-IH NPs are associated with the SAR values of the bare cores FeNi₃ (600 W g⁻¹)⁴⁸ and Fe_2.2C (2100 W g⁻¹)⁴⁹ used for the synthesis; the higher SAR values of Fe_2.2C result in higher Mo deposition (Table S3, ESI†).

The hysteresis curves of FeNi₃@Mo-IH and Fe_2.2C@Mo-IH were measured using vibrating sample magnetometry (VSM) technique at 5 and 300 K (Fig. S13, ESI†). The saturation magnetization (M_s, A m² kg⁻¹) and coercive field (H_c, mT) values obtained at 5 and 300 K were tabulated in Table S4 (ESI†). As expected, upon deposition of the non-magnetic element (Mo), both the M_s and H_c decrease for FeNi₃@Mo-IH and Fe_2.2C@Mo-IH as compared to the bare core (Table S4, ESI†). The SAR measured in an AMF at a frequency of 93 kHz and 47 mT, FeNi₃@Mo-IH reached a value of 154 W g⁻¹ and Fe_2.2C@Mo-IH one of 808 W g⁻¹ (Fig. S14, ESI†). These values are substantially lower than those of their bare cores (600 W g⁻¹ for Fe₃₀Ni₇₀ and 2100 W g⁻¹ for Fe_2.2C).^48,49 Upon deposition of non-magnetic materials, the overall heating efficiency decreases for the core–shell NPs, a phenomenon previously reported.³⁸

Finally, the highly loaded Mag-NP@Mo-IH NPs were chosen for induction heating mediated gas-phase propane dry reforming (PDR) reactions. The FeNi₃@Mo-IH and Fe_2.2C@Mo-IH NPs were further diluted in the Siralox (Siralox-5, 5% SiO₂ doped Al₂O₃) using ultrasonication (see the experimental section, ESI†). Due to the dilution of the Mag-NPs with a non-heating Siralox support, the overall heating capacity of the materials decreases. Hence, commercial Fe wool was used as an additional heating agent in order to reach the high temperatures needed for PDR (see the experimental section, ESI†). Such use of Fe wool in gas-phase catalytic reactions under induction heating has been previously demonstrated by our group.^36,37

Before the PDR reaction, the Sirlaox samples were enriched with a Mo₂C phase by being carburized in a propane and hydrogen mixture at 670–680 °C for 90 minutes, as stated in the experimental section (ESI†). This phase is already known to enable CO₂ dissociation in dry reforming.⁵⁰ As a model system, after carburization, the FeNi₃@Mo₂C-IH/Siralox sample was characterized by TEM, XRD, and XPS techniques (see the ESI† for a detailed discussion). The Mo₂C phase was identified using the XRD and XPS techniques (Fig. S15–S17, ESI†). The particle size of the FeNi₃@Mo₂C-IH nanoparticles was slightly higher than the starting FeNi₃@Mo-IH NPs (Fig. S18, ESI†). The propane conversion, CO₂ conversion, carbon monoxide (CO) selectivity, and reaction temperature for different catalysts are shown in Table 3. In a control test, the pure Fe wool did not display any catalytic activity toward the PDR reaction, under the same experimental conditions. When Mo coating is followed by in situ carburization in the case of FeNi₃ and Fe_2.2C catalysts, propane conversion increases. Remarkably, the CO selectivity for all the catalysts was found to be >92%. The TEM images of the as-prepared and the spent catalysts are shown in Fig. S21–S28 (ESI†). In all the spent catalysts, deposition of the carbon flakes and carbon filaments was observed. The β-Mo₂C phase was observed for the spent Mo-containing catalysts (Fig. S29, ESI†).

Table 3 Propane conversion, CO selectivity, and reaction temperature (560–580 °C) are tabulated for different catalysts

Catalyst	Propane Conv. (%)	CO2 Conv. (%)	CO Sel. (%)	T (°C)
FeNi3/Siralox	73	11	97	572
FeNi3@Mo-IH/Siralox	>99	19	93	570
Fe2.2C/Siralox	2	6	94	581
Fe2.2C@Mo-IH/Siralox	29	17	>99	584
FeNi3@WOx-IH/Siralox	60	15	92	560

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Catalytic test conditions under induction heating: 300 kHz coil, catalyst weight 150 mg, iron wool weight 100 mg, total flow of 40 mL min⁻¹ (propane : CO₂ = 1 : 3; propane 10 mL min⁻¹ and CO₂ 30 mL min⁻¹), and WHSV = 16 000 mL g_cat⁻¹ h⁻¹. The catalysts are carburized prior to the catalytic test. Carburization conditions: total flow of 50 mL min⁻¹ (propane : H₂ = 1 : 4; propane 10 mL min⁻¹ and H₂ 40 mL min⁻¹) and at 670–680 °C for 90 min.

The IH technology has been proven to efficiently permit the growth of a molybdenum shell on magnetic NPs by utilizing the magnetic heating properties of precursor cores under mild reaction conditions. In this direction, the advantages of the induction heating mediated synthesis of Mag-NPs@Mo were generalized by demonstrating the synthesis of FeNi₃@WO_x NPs from W(CO)₆. In this synthesis, upon completion of the decomposition of the W(CO)₆ precursor in the presence of FeNi₃ NPs, the nanoparticles are deliberately oxidized via slow air diffusion (experimental section). Remarkably, the chemical stability and semiconducting properties of WO_x have found application in various fields such as electro- and photochromism, pH sensing, photo(electro)catalysis, and acid catalysis.^51–53

The powder XRD pattern of the FeNi₃@WO_x-IH NPs matches the cubic FeNi₃ phase (Fig. 6a). Like previously, the diffraction peaks corresponding to any W (metallic W or WO_x) species were not observed. The particle size and the polyhedral shapes of the FeNi₃@WO_x-IH NPs resemble the bare FeNi₃ NPs (Fig. 6b). Furthermore, through a comprehensive analysis combining HRTEM and FFT analysis, we have substantiated the presence of a cubic FeNi₃ core along with an amorphous WO_x shell (Fig. 6c and d). The incorporation of an amorphous WO_x layer on the FeNi₃ core was also observed in the STEM EDX elemental mapping (Fig. 6e–j) and was evident from the HRSTEM HAADF imaging (Fig. S30, ESI†). The Siralox diluted FeNi₃@WO_x-IH sample was further tested for the PDR reaction (Table 3, entry 5). The catalyst was found active for PDR reaction with ∼92% CO selectivity. Hence, this specific synthesis example evidences the efficiency of the induction-heating approach and its potential to be extended to other complex materials designs.

Fig. 6 FeNi₃@W-IH NPs, (a) powder XRD pattern (ICDD: 00-038-0419, red vertical drop lines for standard FeNi₃), (b) BF TEM micrograph, (c) HRTEM image and the corresponding FFT analysis (d) from the white square marked region, (e)–(j) STEM HAADF image and the respective STEM EDS elemental mapping shows Ni in red, O in yellow, Fe in blue, and W in green, and the overlap maps of W, Fe, and Ni.

Conclusions

This study investigated the benefits of using induction heating to create complex nanomaterials. It has been shown that the combination of magnetic induction heating with Mag-NPs, such as Fe_2.2C and FeNi₃, induces Mo(CO)₆ decomposition and results in the synthesis of well-formed core–shell nanoparticles. These nanoparticles possess a distinctive structure wherein the outer shell contains amorphous carbidic Mo, and the magnetic phases, FeNi₃ or Fe_2.2C, reside in the core. By directing the decomposition towards the nanoparticle surface, this method shows notable advantages. It allows the decomposition of the precursor on the hot spots created by magnetic induction, thus resulting in the deposition of about twice the amount of molybdenum on the nanoparticle shell, in a shorter reaction time and with less Mo(CO)₆ precursor used than in the thermal conditions. These nanoparticles were then supported on Siralox for use in PDR and enriched with the Mo₂C phase in-situ before catalysis under a propane and hydrogen atmosphere. Finally, these new materials were shown to display interesting catalytic properties under magnetic induction.

Overall, we describe a new method for the preparation of complex nanomaterials containing a magnetic core. We have chosen for the shell a metal which is difficult to obtain at the nanoscale in a reduced form, namely molybdenum and have shown that it can be extended to an even more difficult metal, namely tungsten. This methodology can then be used for the synthesis of different complex nanostructures, depending on the desired application.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors gratefully acknowledge the funding from the ERC Advance grant (MONACAT 2015-694159). The authors thank Alain Moreau (LCC, Toulouse) for the ICP analysis.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3mh00908d

‡ Equally contributed.

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