Synthesis of SiO₂@MnCo₂O₄ core–shell nanorattles using layered double hydroxide precursors and studies on their peroxidase-like activity

Abstract

The current work demonstrates a simple soft chemical approach to synthesize SiO₂@MnCo₂O₄ core–shell nanorattles using SiO₂@MnCo-LDH (layered double hydroxide) precursors. XRD analysis indicates the formation of MnCo₂O₄ nanoparticles (NPs) on SiO₂ spheres. FESEM images show hierarchical flower-like MnCo₂O₄ NPs on the surface of the SiO₂ spheres. TEM micrographs show the interstitial space between the core (SiO₂) and the shell (MnCo₂O₄), indicating the core–shell nanorattle morphology of SiO₂@MnCo₂O₄. BET adsorption–desorption isotherms of the SiO₂@MnCo₂O₄ core–shell nanorattles indicate their mesoporous nature with high surface area. Optical studies indicate O²⁻ → Mn²⁺ and O²⁻ → Co³⁺ charge-transfer transitions and d–d transition in pure MnCo₂O₄ NPs and the SiO₂@MnCo₂O₄ core–shell nanorattles. XPS measurements indicate the presence of Si⁴⁺, Mn²⁺, Co²⁺, Mn³⁺, Co³⁺, and O²⁻ in the SiO₂@MnCo₂O₄ core–shell nanorattles. The SiO₂@MnCo₂O₄ core–shell nanorattles show paramagnetic and superparamagnetic behavior at 300 K and 5 K, respectively. After characterization, the SiO₂@MnCo₂O₄ core–shell nanorattles were explored for peroxidase-like activity for the first time. The SiO₂@MnCo₂O₄ core–shell nanorattles act as a peroxidase nanozyme and exhibit better peroxidase-like activity than pure MnCo₂O₄ NPs and horseradish peroxidase.

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Introduction

Recently, core–shell NPs have received considerable attention due to their remarkable properties. Combining two different materials (core and shell) changes the properties of the original core and the shell.¹ Among core–shell NPs, core–shell nanorattles or yolk–shell nanoparticles possess interesting morphological features. Core–shell nanorattles or yolk–shell nanostructures have advantages because of their core@void@shell configuration. Core–shell nanorattles possess enhanced properties due to the void space between the core and the shell.^1,2 Core–shell nanorattles based on metal oxides have several promising applications such as in drug delivery, catalysis, sensing, Li-ion batteries, and adsorption.^2,3 Lee et al. have reported Au@SiO₂ yolk–shell structures as catalyst for the reduction of p-nitrophenol.⁴ Kandula et al. have reported peroxidase-like activity of SiO₂@Co₃O₄ nanorattles.⁵ Chen et al. have reported lithium storage properties of α-Fe₂O₃@SnO₂ nanorattles.⁶ Rokicinska et al. have reported Co₃O₄@SiO₂ core–shell nanorattles for catalytic combustion of toluene.⁷ Hu et al. have reported α-Fe₂O₃@SiO₂ and SnO₂@SiO₂ core–shell nanorattles as anticancer drug carriers.⁸

Silica (SiO₂) is an amorphous insulator, which is stable at high temperatures and in water. The spherical morphology and porous nature of SiO₂ make it a good candidate as a core material for the synthesis of silica-based core–shell NPs/nanorattles.⁹ SiO₂-based nanorattles have been studied for various applications such as Co@SiO₂ core–shell nanorattles as catalysts,¹⁰ α-Fe₂O₃@SiO₂ and SnO₂@SiO₂ core–shell nanorattles as anticancer drug carriers,⁸ SiO₂@Co₃O₄ core–shell nanorattles as nanozymes,⁵ rattle-type gold nanorods/SiO₂ nanocomposites for chemo-photothermal therapy,¹¹ Au@SiO₂ yolk–shell nanostructures as catalyst for reduction of p-nitrophenol,⁴ and Fe⁰@SiO₂ nanoparticles as catalyst for Fenton-like reaction.¹²

MnCo₂O₄ is a spinel-type metal oxide in which Mn²⁺ ions occupy octahedral (O_h) sites, and Co²⁺ and Co³⁺ ions occupy both tetrahedral (T_d) and O_h sites.¹³ Due to the high oxidation potential of cobalt and high electron transport ability of manganese, MnCo₂O₄ exhibits exceptional electrochemical and physicochemical properties.¹⁴ MnCo₂O₄ nanoparticles are used in diverse applications such as asymmetric supercapacitors, anode materials in Li-ion batteries, photocatalysis, adsorption, and oxygen reduction.^14–18 MnCo₂O₄-based core–shell NPs have been used in different applications. For example, Wang et al. have reported MnCo₂O₄@MnCo₂S₄ core–shell nanostructures for asymmetric supercapacitors.¹⁹ Zhao et al. have reported MnCo₂O₄@Ni(OH)₂ core–shell flowers with ultrahigh specific capacitance.²⁰ Li et al. have reported MnCo₂O₄@NiCo₂O₄ core–shell NPs for dye-sensitized solar cells.²¹ Sun et al. have reported MnCo₂O₄@C core–shell nanowires for water splitting.²² Zhao et al. have reported Co₃O₄@MnCo₂O₄ core–shell nanowire arrays for electrochemical energy storage.²³ Mehrez et al. have reported MnCo₂O₄@NiMoO₄ core–shell nanowire arrays for supercapacitor applications.²⁴

Enzymes are biocatalysts that catalyze several biochemical reactions with high efficiency and selectivity.²⁵ Natural enzymes have several limitations. They have low stability, complex storage requirements, high cost, and less adaptability under harsh environmental conditions.²⁶ To overcome the drawbacks of natural enzymes, nanomaterials have been used as an alternative. Metal oxide-based nanozymes have been explored to mimic different natural enzymes such as peroxidase, ferroxidase, catalase, and superoxide dismutase.^26,27 Horseradish peroxidase oxidizes different substrates in the presence of hydrogen peroxide.²⁸ In the literature, different metal oxide-based nanozymes have been used as a peroxidase mimic. For example, Chen et al. have reported CuO nanoparticles for peroxidase-like activity.²⁹ Fu et al. have investigated the peroxidase-like activity of Fe₃O₄ nanoparticles for cancer therapy.³⁰ Gao et al. have reported MnCo₂O₄ nanofibers as a catalyst for peroxidase-like activity.³¹ Ma et al. have reported photoinduced peroxidase-like activity of NiCo₂O₄@MnO₂ nanozymes.³²

Multi-functional core–shell NPs and nanorattles have been synthesized using various soft chemical synthesis routes. For example, Wang et al. have reported the synthesis of MnCo₂O₄@MnCo₂S₄ core–shell nanostructures using the hydrothermal method.¹⁹ Zhao et al. have reported the synthesis of MnCo₂O₄@Ni(OH)₂ core–shell flowers using homogeneous precipitation.²⁰ Mehrez et al. have reported the synthesis of MnCo₂O₄@NiMoO₄ core–shell nanowires using hydrothermal method.²⁴ Zhang et al. have reported the preparation of Co@CoO core–shell nanocomposites using solvothermal method.³³ Gao et al. have reported the synthesis of NiCo₂O₄@SnO₂ hetero-nanostructures using the hydrothermal method.³⁴ Kandula et al. have reported the preparation of SiO₂@NiCo₂O₄ core–shell nanorattles using homogeneous precipitation.³⁵

The current work demonstrates a reliable and cost-effective synthetic strategy to prepare SiO₂@MnCo₂O₄ nanorattles via calcination of SiO₂@MnCo-LDH precursors at 500 °C. The MnCo₂O₄ nanoparticles deposit on the surface of the SiO₂ spheres creating a hollow space (void) between the core and shell. The influence of [Mn²⁺ : Co²⁺] ratio on the thickness of the MnCo₂O₄ shell and void distance between the core (SiO₂) and shell (MnCo₂O₄) has been investigated. The SiO₂@MnCo₂O₄ core–shell nanorattles exhibit better peroxidase-like activity than pure MnCo₂O₄ NPs using 3,3′,5,5′-tetramethylbenzidine (TMB) as the substrate. To the best of the authors’ knowledge, there is no report on the synthesis of SiO₂@MnCo₂O₄ core–shell nanorattles and the SiO₂@MnCo₂O₄ nanorattles have been employed as a peroxidase mimic for the first time.

Experimental

Reagents

Tetraethyl orthosilicate (98%, Sigma-Aldrich), ammonia solution (25%, Rankem), Mn(NO₃)₂.4H₂O (97%, Sigma-Aldrich), Co(NO₃)₂.6H₂O (98%, Merck), urea (99.5%, Rankem), 3,3′,5,5′-tetramethylbenzidine (98%, Spectrochem Chemicals), ethanol (99.9%, Changshu Hongsheng Fine Chemical Co., Ltd.), and H₂O₂ (30%, Rankem, AR). All the chemicals were used as received.

Synthesis of silica microspheres

SiO₂ microspheres were synthesized using a previously reported StÖber's method.³⁶ About 3.7 mL of tetraethyl orthosilicate was added (dropwise) to a mixture of 88 mL of EtOH and 12 mL of NH₄OH solution with constant stirring. The reaction mixture was continuously stirred at RT for 24 h to get a white-colored product. The product was centrifuged, washed with EtOH, and dried in an oven overnight at 70 °C.

Synthesis of SiO₂@MnCo₂O₄ core–shell nanorattles

First, SiO₂@MnCo-LDH precursors were prepared using different molar ratios of [Mn²⁺ : Co²⁺]. The synthetic parameters for the SiO₂MnCo-LDH precursors are given in Table 1. In a typical synthetic experiment, Mn(NO₃)₂·4H₂O, Co(NO₃)₂·6H₂O and urea were dissolved in 100 mL of distilled water in a 250 mL beaker. Then, SiO₂ microspheres (100 mg) were dispersed in the aqueous solution and sonicated for 10 minutes. The contents were heated at 85 °C for six hours with constant stirring. The product was collected by centrifuging and washing with deionized water and ethanol, followed by drying in an oven at 60 °C for about 12 h. The as-prepared SiO₂@MnCo-LDH precursors were calcined at 500 °C (in a muffle furnace) for 3 h (heating rate = 2 °C per minute) in air to get the corresponding SiO₂@MnCo₂O₄ samples. The as-prepared SiO₂@MnCo-LDH precursors, prepared using different molar ratios of [Mn²⁺ : Co²⁺] (0.25 : 0.5, 0.5 : 1, and 1 : 2), will henceforth be referred to as SiO₂@MnCo-LDH-0.25, SiO₂@MnCo-LDH-0.5, and SiO₂@MnCo-LDH-1, respectively. The SiO₂@MnCo₂O₄ samples, obtained on calcination, will henceforth be referred to as SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1, respectively.

Table 1 Synthetic details of SiO₂@MnCo-LDH samples and their nomenclature

Sample code	[Mn^2+] : [Co^2+]	SiO2 (mg)	Urea (g)	Temperature (°C)	Time (h)
SiO2@MnCo-LDH-0.25	0.25 : 0.5	100	1.0	85	6
SiO2@MnCo-LDH-0.5	0.5 : 1	100	1.0	85	6
SiO2@MnCo-LDH-1	1 : 2	100	1.0	85	6
MnCo-LDH	1 : 2	—	1.0	85	6

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Peroxidase-like activity and kinetic analysis

The peroxidase-like activity of the SiO₂@MnCo₂O₄ nanorattles was studied utilizing tetramethylbenzidine (TMB) and H₂O₂ as a substrate, and oxidizing agent, respectively. About 300 μL of 3 mM TMB solution (DMSO) and 31 μL of 100 mM H₂O₂ were mixed with 3 mL of acetate buffer (0.1 M, pH = 5). Then, 30 μL of catalyst dispersion (1 mg of SiO₂@MnCo₂O₄ catalyst dispersed by sonication in 1 mL of deionized water) was added to the reaction mixture. The absorbance values (@λ_max = 652 nm) of the reaction mixture were measured using a UV-Vis spectrophotometer (Shimadzu UV-2600) up to 10 minutes. Kinetic experiments were performed by changing various experimental parameters, such as TMB concentration (0.05 to 0.5 mM), pH (2–12), catalyst dose (5–60 μL), and H₂O₂ concentration (5–200 mM). The kinetic parameters (K_m and V_max) were estimated using the Michaelis–Menten equation and Lineweaver–Burk reciprocal plots.³² The Michaelis–Menten equation is as follows.

1/V = (K_m/V_max) (1/[S]) + 1/V_max

where K_m and V_max denote the Michaelis–Menten constant and maximum reaction velocity, respectively. V and [S] denote reaction velocity and concentration of the substrate (TMB), respectively.

Terephthalic acid was employed as a probe molecule to prove the role of hydroxyl radicals in the peroxidase-like activity of the SiO₂@MnCo₂O₄ nanorattles. In a cuvette, buffer (3 mL, pH = 5) and 100 mM H₂O₂ (31 μL) were mixed, followed by the addition of 30 μL of catalyst dispersion (SiO₂@MnCo₂O₄). The reaction mixture was then treated with 0.5 mL of aqueous terephthalic acid solution (0.5 mM). A spectrofluorometer (Horiba Scientific, Fluoromax-4) was used to measure the PL spectra of the solution (λ_exc = 315 nm) up to 50 minutes.

Characterization

Powder X-ray diffraction patterns of all the samples were recorded using a Bruker AXS D8 Advance powder X-ray diffractometer (Cu-K_α (λ = 1.5406 Å), scan speed = 1° min⁻¹) in the 2θ range of 5° to 90°. FT-IR spectra of the samples (using KBr pellets) were recorded using a Thermo Nicolet Nexus FT-IR spectrophotometer (4000 to 400 cm⁻¹). A PerkinElmer Pyris Diamond instrument was used to perform thermal gravimetric analysis (TGA) of the SiO₂@MnCo-LDH precursors; the precursors were heated at a rate of 10 °C min⁻¹ in air between 30 °C and 1000 °C. A Carl Zeiss Gemini scanning electron microscope (operating voltage = 20 kV) was used to examine the morphology of the samples. For elemental analysis, the SEM attached with an EDXA unit was employed. The core–shell nanorattles were imaged using an FEI Tecnai G2 20S-TWIN transmission electron microscope (operating voltage = 200 kV). Using a Quantum Design MPMS3 superconducting quantum interference device, the magnetic properties of the SiO₂@MnCo₂O₄ core–shell nanorattles were studied. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics make (PHI 5000 Versa Probe III) spectrometer (Al-K_α radiation, energy = 1486.6 eV). Physisorption of N₂ at 77 K was used to determine surface area of the SiO₂@MnCo₂O₄ core–shell nanorattles using a Quantachrome BET surface area analyzer (model = Nova 2200e).

Results and discussion

The results related to SiO₂@MnCo-LDH precursors are given in the ESI.† The results related to SiO₂@MnCo₂O₄ core–shell nanorattles are discussed below.

SiO₂@MnCo₂O₄ core–shell nanorattles

Phase analysis. Pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell NPs were obtained on calcination of MnCo-LDH and SiO₂@MnCo-LDH samples at 500 °C, respectively. The XRD patterns of the MnCo₂O₄ and SiO₂@MnCo₂O₄ samples are shown in Fig. 1. The XRD pattern of MnCo₂O₄ matches with the pattern of cubic MnCo₂O₄ (JCPDS file no. 23-1237). The peaks observed at 2θ = 18.1°, 30.4°, 35.9°, 43.9°, 53.5°, 58.4°, and 64.5° are ascribed to (111), (220), (311), (400), (422), (511), and (440) reflections of cubic MnCo₂O₄. The XRD patterns of the SiO₂@MnCo₂O₄-0.25 and SiO₂@MnCo₂O₄-0.5 samples show peaks at 2θ = 35.9° and 64.5° attributed to the (311) and (440) planes of MnCo₂O₄.

Fig. 1 XRD patterns of MnCo₂O₄ and SiO₂@MnCo₂O₄ samples obtained by calcination of LDH precursors (Table 1) at 500 °C.

The XRD pattern of SiO₂@MnCo₂O₄-1 shows peaks at 2θ = 18.1°, 30.2°, 35.9°, 43.7°, 58.3°, and 64.3° due to (111), (220), (311), (400), (511), and (440) reflections of MnCo₂O₄. The estimated crystallite size (using the Scherrer formula) of MnCo₂O₄ are 9.3 nm, 6.4 nm, 3.5 nm, and 7.5 nm for MnCo₂O₄, SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1, respectively. The SiO₂@MnCo₂O₄ samples possess smaller crystallites of MnCo₂O₄ compared to pristine MnCo₂O₄ NPs.

FT-IR analysis

Fig. S5 (ESI†) displays the FT-IR spectra of MnCo₂O₄ and SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1). The IR spectra of all the samples exhibit bands at about 3438 cm⁻¹, and 1637 cm⁻¹ assigned to stretching and bending vibrational modes of physisorbed H₂O molecules. The IR spectrum of MnCo₂O₄ shows IR bands around 659 cm⁻¹ and 568 cm⁻¹ attributed to stretching vibration of Co–O and Mn–O bonds, respectively.¹⁴ The IR spectra of all the SiO₂@MnCo₂O₄ samples display characteristic bands of SiO₂ at 1105 cm⁻¹ and 486 cm⁻¹ due to υ-(Si–O–Si) and δ-(Si–O–Si), respectively.⁵ In the IR spectra of the SiO₂@MnCo₂O₄ samples, the bands observed at about 660 cm⁻¹ and 565 cm⁻¹ are assigned to the stretching vibrations of Co–O and Mn–O bonds, respectively.¹⁴

Morphological studies and elemental analysis

Fig. 2(a–d) displays FESEM images of pure MnCo₂O₄ and SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1). The FESEM image of pure MnCo₂O₄, (Fig. 2(a)) shows nanosheets (flake-like structures). The FESEM images of the SiO₂@MnCo₂O₄ samples (Fig. 2(b–d)) show that the SiO₂ spheres are uniformly covered with MnCo₂O₄ nanoparticles. Table S4 (ESI†) summarizes the EDX analysis results for all SiO₂@MnCo₂O₄ samples (after calcination at 500 °C). The results indicate that Si, Mn, Co, and O are present uniformly in all the SiO₂@MnCo₂O₄ core–shell NPs. The weight % of Mn and Co in the SiO₂@MnCo₂O₄ samples varies according to the [Mn²⁺] : [Co²⁺] ratio used during the synthesis of their precursors (SiO₂@MnCo-LDH).

Fig. 2 FE-SEM images of (a) MnCo₂O₄, (b) SiO₂@MnCo₂O₄-0.25, (c) SiO₂@MnCo₂O₄-0.5, and (d) SiO₂@MnCo₂O₄-1 core–shell samples calcined at 500 °C.

Fig. 3(a–e) shows the TEM images of SiO₂, pristine MnCo₂O₄, and SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1). A summary of the TEM results is given in Table 2. The TEM image of SiO₂ (Fig. 3(a)) shows spherical particles with mean diameter of 175 ± 14 nm. Fig. 3(b) displays the TEM image of pristine MnCo₂O₄, showing its flake-like morphology (nanosheet). The mean thickness of MnCo₂O₄ nanosheets is 9.3 ± 3.6 nm. The TEM images (Fig. 3(c–e)) of SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1) show interstitial space (void) between the core (SiO₂) and the shell (MnCo₂O₄). Such nanoparticles are known in the literature as core–shell nanorattles or nanoparticles with yolk–shell morphology.^37,38 The TEM images also reveal that the MnCo₂O₄ shell has a porous flower-like structure. From Table 2, it is observed that the mean size of the SiO₂ core in the SiO₂@MnCo₂O₄ core–shell nanorattles varies from 171 ± 13 nm to 190 ± 15 nm. The mean thickness of the MnCo₂O₄ shell lies in the range of 38 ± 8 nm to 60 ± 12 nm. The average void distance between the SiO₂ core and MnCo₂O₄ shell varies from 16 ± 3 nm to 30 ± 6 nm. The mean thickness of the nanosheets of MnCo₂O₄ (shell) in the SiO₂@MnCo₂O₄ core–shell nanorattles varies from 2.6 ± 0.9 nm to 3.3 ± 0.4 nm. The synthetic conditions play an important role in the nanorattle formation. As the concentration (molar ratio) of metal salts used during the synthesis of SiO₂@MnCo-LDH precursors (i.e. Mn(NO₃)₂·4H₂O and Co(NO₃)₂·6H₂O) increases from 0.25 : 0.5 to 1 : 2, more MnCo₂O₄ NPs are formed on the surface of the SiO₂ spheres, which leads to a thick coating of MnCo₂O₄ NPs on the surface of the SiO₂ spheres. When lower concentration of metal salts is used, a thin coating of MnCo₂O₄ NPs occurs and thus in the SiO₂@MnCo₂O₄-0.25 sample, the thin coating of MnCo₂O₄ leads to a good TEM image. EDS mapping and elemental line profiles of SiO₂@MnCo₂O₄ from TEM measurements were done and the results are shown in Fig. S6 and S7 (ESI†). The EDS elemental mapping images and elemental line profile of the SiO₂@MnCo₂O₄ core–shell nanorattles indicate uniform distribution of elements (Si, Mn, Co and O) in the SiO₂@MnCo₂O₄ core–shell nanorattles. The line scan EDS images show that the Mn and Co content is more towards the edge of the spherical particles and the Si content is more in the center of the particles. This suggests coating of MnCo₂O₄ on the SiO₂ spheres.

Fig. 3 TEM images of (a) SiO₂, (b) MnCo₂O₄, (c) SiO₂@MnCo₂O₄-0.25, (d) SiO₂@MnCo₂O₄-0.5 and (e) SiO₂@MnCo₂O₄-1 core–shell samples calcined at 500 °C.

Table 2 Summary of TEM results of SiO₂@MnCo₂O₄ core–shell nanorattles

Sample code	Core size (nm)	Shell thickness (nm)	Void distance (nm)	Flake thickness in the shell (nm)
SiO2@MnCo2O4-0.25	182 ± 13	38 ± 8	30 ± 6	2.6 ± 0.9
SiO2@MnCo2O4-0.5	171 ± 13	41 ± 10	16 ± 3	3.2 ± 0.6
SiO2@MnCo2O4-1	190 ± 15	60 ± 12	22 ± 4	3.3 ± 0.4

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The SAED patterns of pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1) are displayed in Fig. S8(a–d) (ESI†). The SAED pattern of pure MnCo₂O₄ (Fig. S8(a), ESI†) shows rings which indicate the polycrystalline nature of the MnCo₂O₄ NPs. The observed rings are attributed to (220), (311), (400), (422), and (440) reflections of cubic MnCo₂O₄. The SAED patterns of SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1 core–shell nanorattles (Fig. S8(b–d), ESI†) also show rings, which suggests the polycrystalline nature of all the SiO₂@MnCo₂O₄ nanorattles. The observed rings correspond to the (311) and (422) planes of cubic MnCo₂O₄.

BET surface area analysis

BET surface area measurements were carried out to determine the surface area, pore size, and pore volume of the SiO₂@MnCo₂O₄ core–shell nanorattles. SiO₂ and MnCo₂O₄ nanoparticles exhibit a surface area of 88 m² g⁻¹ and 79 m² g⁻¹, respectively. The core–shell nanorattles (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1) exhibit surface area of 278 m² g⁻¹, 302 m² g⁻¹, and 356 m² g⁻¹, respectively. The higher surface area of the SiO₂@MnCo₂O₄ nanorattles is attributed to the void space between the core (SiO₂) and the shell (MnCo₂O₄). Adsorption–desorption isotherms were recorded for pure MnCo₂O₄ nanoparticles and SiO₂@MnCo₂O₄-1. As shown in Fig. S9(a) (ESI†), the adsorption-desorption isotherm of the MnCo₂O₄ nanoparticles shows the characteristics of a porous material with open wedge pores. The adsorption–desorption isotherm of SiO₂@MnCo₂O₄-1 (Fig. S9(b), ESI†) matches with a type IV isotherm.³⁹ The MnCo₂O₄ nanoparticles exhibit an average pore size and pore volume of 6.2 nm and 0.12 cm³ g⁻¹, respectively. The SiO₂@MnCo₂O₄-1 exhibits an average pore size and pore volume of 5.0 nm and 0.44 cm³ g⁻¹, respectively.

Mechanism of formation of SiO₂@MnCo₂O₄ core–shell nanorattles

Fig. S10 (ESI†) shows the proposed mechanism of formation of SiO₂@MnCo₂O₄ core–shell nanorattles. In the first step, Mn²⁺ and Co²⁺ ions from the aqueous solution attach to the OH groups present on the surface of the SiO₂ spheres via electrostatic interaction. At 85 °C, hydrolysis of urea leads to the production of NH₄⁺ and OH⁻ ions. The OH⁻ ions react with Mn²⁺ and Co²⁺ ions present on the surface of SiO₂ forming MnCo-LDH via nucleation and coalescence processes. Finally, flower-like structures consisting of MnCo-LDH nanosheets are formed on the surface of the SiO₂ spheres via Ostwald ripening.^40,41 The flower-like SiO₂@MnCo-LDH precursors are converted into SiO₂@MnCo₂O₄ core–shell nanorattles via calcination at 500 °C with retention of the morphology.

Time-dependent TEM studies were carried out to understand better the formation of SiO₂@MnCo-LDH nanorattles. The TEM images of SiO₂@MnCo-LDH-0.25 recorded after different reaction times (1 h, 2 h, 3 h, 4 h, and 6 h) are shown in Fig. S11 (ESI†). After a reaction time of 1 h, MnCo-LDH crystallites are loosely attached on the outer surface of the SiO₂ spheres. These MnCo-LDH crystallites act as seed for the nucleation and recrystallization. As the reaction progresses (2 h, 4 h), Ostwald ripening (inside-out) dominates and smaller crystallites of MnCo-LDH present in the interior region dissolve, which initiates the hollowing process. Finally, after 6 h, a close-packed MnCo-LDH shell is formed on the surface of the SiO₂ spheres with a void space between the core and shell. SiO₂@MnCo₂O₄ core–shell nanorattles are obtained on calcination of SiO₂@MnCo-LDH at 500 °C with the retention of the morphological features of the LDH precursor.

Optical properties of SiO₂@MnCo₂O₄ core–shell nanorattles

The optical properties of MnCo₂O₄ nanoparticles and SiO₂@MnCo₂O₄ core–shell nanorattles were studied using UV-Vis diffuse reflectance spectroscopy (DRS). The UV-Vis DRS spectra of the MnCo₂O₄ nanoparticles and SiO₂@MnCo₂O₄ core–shell nanorattles are shown in Fig. S12 (ESI†). The DRS spectra of the MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ nanorattles show broad bands at 250 nm and 470 nm, attributed to O²⁻ → Mn²⁺ and O²⁻ → Co³⁺ charge-transfer transitions of MnCo₂O₄, respectively.^43,44 The DRS spectra of all the samples exhibit a broad band at about 738 nm due to d-d transitions of Mn²⁺ and Co³⁺ of MnCo₂O₄ NPs.^42,43

XPS analysis

The oxidation states of Si, Mn, Co, and O in SiO₂, MnCo₂O₄ NPs and SiO₂@MnCo₂O₄-1 nanorattles were determined using XPS analysis.

Fig. S13 (ESI†) displays the XPS spectrum of SiO₂. The Si 2p spectrum of SiO₂ shows a peak at 103.8 eV, indicating the existence of Si⁴⁺. The O 1s spectrum of SiO₂ shows peaks at 532.7 eV and 533.8 eV, indicating the existence of lattice oxygen and surface hydroxyl oxygen in SiO₂, respectively.⁴⁴Fig. 4(a and b) shows the XPS spectra of pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄-1. The Mn 2p spectrum of pure MnCo₂O₄ NPs (Fig. 4(a)) shows peaks due to Mn 2p_3/2 (642.1 eV (Mn²⁺) and 643.5 eV (Mn³⁺) and Mn 2p_1/2 (654.0 eV). The Co 2p spectrum of MnCo₂O₄ NPs (Fig. 4(a)) shows peaks due to Co 2p_3/2 (780.1 eV (Co²⁺) and 782.1 eV (Co³⁺) and Co 2p_1/2 (795.2 eV (Co²⁺) and 797.3 eV (Co³⁺)). The peaks observed at 788.0 eV and 803.8 eV correspond to satellite peaks. The O 1s spectrum of pure MnCo₂O₄ NPs shows peaks at 530.1 eV and 531.9 eV, assigned to lattice oxygen of MnCo₂O₄ and surface hydroxyl oxygen, respectively.^45,46 The Si 2p XPS spectrum of SiO₂@MnCo₂O₄-1 (Fig. 4(b)) shows a peak at 102.7 eV, suggesting the presence of Si⁴⁺. The Mn 2p spectrum of SiO₂@MnCo₂O₄-1 (Fig. 4b) exhibits peaks due to Mn 2p_3/2 (642.2 eV (Mn²⁺) and 644 eV (Mn³⁺)) and Mn 2p_1/2 (654.2 eV (Mn³⁺)). The Co 2p spectrum of SiO₂@MnCo₂O₄-1 (Fig. 4b) shows peaks due to Co 2p_3/2 (781.1 eV (Co²⁺) and 782.7 eV (Co³⁺)) and Co 2p_1/2 (796.8 eV (Co²⁺) and 797.8 eV (Co³⁺)). Two satellite peaks are also observed at 787.5 eV and 803.5 eV. The O 1s spectrum of SiO₂@MnCo₂O₄-1 (Fig. 4b) exhibits peaks at 530.2 eV and 531.4 eV due to lattice oxygen of MnCo₂O₄ and surface hydroxyl oxygen, respectively.^44–46

Fig. 4 XPS spectra of (a) MnCo₂O₄ NPs and (b) SiO₂@MnCo₂O₄-1 core–shell nanoparticles calcined at 500 °C.

Magnetic properties

Magnetic hysteresis (M–H) plots of pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ nanorattles were recorded at RT (300 K) and 5 K up to an applied field of 40 kOe. The M–H curves of pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ samples (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1) are shown in Fig. 5. At 300 K, the absence of hysteresis and negligible coercivity indicate paramagnetic nature of the pure MnCo₂O₄ NPs and all the SiO₂@MnCo₂O₄ samples. Table 3 gives the coercivity and magnetization values of pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ samples at 5 K. At 5 K, the M–H hysteresis loop of the MnCo₂O₄ nanoparticles exhibits coercivity of 5.0 kOe and magnetization of 6.8 emu g⁻¹ (@40 kOe), indicating hard ferromagnetic behavior. At 5 K, the negligible coercivity and finite magnetization of all the SiO₂@MnCo₂O₄ samples indicate superparamagnetic behavior. The coercivity values for SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1 at 5 K are 0.05 kOe, 0.02 kOe, and 0.09 kOe, respectively. For SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1, the magnetization values (@40 kOe) at 5 K are 46.5 emu g⁻¹, 36.3 emu g⁻¹, and 25.5 emu g⁻¹, respectively.

Fig. 5 M–H curves for MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell nanorattles at 300 K and 5 K.

Table 3 Summary of magnetic parameters of MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell NPs at 5 K

Sample code	Coercivity (Hc, kOe)	Magnetization (emu g^−1)	Crystallite size (nm)
MnCo2O4	5.00	6.8	9.3
SiO2@MnCo2O4-0.25	0.05	46.5	6.4
SiO2@MnCo2O4-0.5	0.02	36.3	3.5
SiO2@MnCo2O4-1	0.09	25.5	7.5

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At low temperature, the dominance of magneto-crystalline anisotropy leads to higher coercivity and magnetization of the MnCo₂O₄ nanoparticles and SiO₂@MnCo₂O₄ samples.⁴⁷ The lower coercivity of SiO₂@MnCo₂O₄ nanorattles compared to pure MnCo₂O₄ nanoparticles is attributed to the diamagnetic SiO₂ phase in the nanorattles.⁴⁸ The higher magnetization of SiO₂@MnCo₂O₄ samples compared to pure MnCo₂O₄ NPs is attributed to smaller crystallite size of MnCo₂O₄ NPs in the SiO₂@MnCo₂O₄ samples. In the MnCo₂O₄ nanoparticles, SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1, the crystallite size of MnCo₂O₄ are 9.3 nm, 6.4 nm, 3.5 nm, and 7.5 nm, respectively. The ZFC and FC magnetization curves recorded under 2 kOe for pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ nanorattles (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1) are shown in Fig. 6. The ZFC and FC curves of pure MnCo₂O₄ NPs display a bifurcation at about 100 K and a hump at 25 K, which are assigned as irreversible temperature (T_irr) and blocking temperature (T_B), respectively. All the SiO₂@MnCo₂O₄ core–shell nanorattles show overlapped ZFC and FC curves without any hump, indicating superparamagnetic nature of the samples despite the absence of a maximum in the ZFC curve. The absence of maxima in the ZFC curves of the SiO₂@MnCo₂O₄ samples is due to the absence of ferrimagnetic ordering in these samples.^49–52

Fig. 6 ZFC and FC curves for MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell nanorattles.

Peroxidase-like activity of SiO₂@MnCo₂O₄ nanorattles

The current study demonstrates the peroxidase-like activity of pure MnCo₂O₄ nanoparticles and SiO₂@MnCo₂O₄ core–shell nanorattles (SiO₂@MnCo₂O₄-0.25, SiO₂@MnCo₂O₄-0.5, and SiO₂@MnCo₂O₄-1). The SiO₂@MnCo₂O₄ core–shell nanorattles were tested as catalyst for peroxidase-like activity using TMB and H₂O₂ as substrate and oxidizing agent, respectively. The oxidation of TMB using SiO₂@MnCo₂O₄ core–shell nanorattles in the presence of H₂O₂ leads to formation of TMB˙⁺ (blue-colored charge transfer complex). The UV-Vis spectrum of TMB˙⁺ consists of a characteristic absorption maximum at 652 nm.²⁸ An intense blue-colored solution is obtained using SiO₂@MnCo₂O₄ as the catalyst compared to pure MnCo₂O₄ NPs, which suggests the role of SiO₂@MnCo₂O₄ as the catalyst in peroxidase-like activity.

The UV-Vis spectral results (abs. vs. time plots) and % relative efficiency of peroxidase-like activity for pure MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell NPs are shown in Fig. 7 and Fig. S14 (ESI†), respectively. As shown in Fig. 7, compared to pure MnCo₂O₄ nanoparticles, SiO₂@MnCo₂O₄ nanorattles show better peroxidase-like activity. SiO₂@MnCo₂O₄-1 exhibits the best peroxidase-like activity among the SiO₂@MnCo₂O₄ samples.

Fig. 7 Peroxidase-like activity of MnCo₂O₄ NPs and SiO₂@MnCo₂O₄ core–shell nanorattles as indicated by time dependent absorbance.

Influence of physicochemical conditions on the peroxidase-like activity

In the current study, various experimental parameters were varied to achieve optimal conditions for the peroxidase-like activity of the SiO₂@MnCo₂O₄ core–shell nanorattles. Studies were carried out to investigate the effect of varying pH (2–12) and amount of catalyst (5 μL – 60 μL of dispersion) on the peroxidase-like activity of the SiO₂@MnCo₂O₄ core–shell nanorattles. The influence of varying pH (2–12) on the peroxidase-like activity (Fig. S15(a), ESI†) reveals the maximum peroxidase-like activity of SiO₂@MnCo₂O₄ core–shell nanorattles at pH = 5. At low pH (<5), protonation of amino groups of TMB occurs. The protonation of amino groups leads to the retardation of electron transfer from the substrate (TMB) to the catalyst surface (SiO₂@MnCo₂O₄-1). Hence, in a strongly acidic medium, the peroxidase-like activity of SiO₂@MnCo₂O₄-1 decreases. When pH >5, production of O₂ and H₂O occurs via the break-down of H₂O₂ in the presence of more hydroxyl ions, which leads to suppression of the peroxidase-like activity of SiO₂@MnCo₂O₄-1.^53,54 A linear variation of the peroxidase-like activity of SiO₂@MnCo₂O₄-1 with an increment in catalyst dose from 5 μL to 60 μL of dispersion (1 mg catalyst dispersed by sonication in 1 mL H₂O) is observed (Fig. S15(b), ESI†). A leaching experiment was carried out to understand further the peroxidase-like activity of SiO₂@MnCo₂O₄-1. In a typical experiment, SiO₂@MnCo₂O₄-1 (30 μL dispersion) was incubated in 3 mL buffer (pH = 5) for about 30 minutes. The supernatant solution obtained by centrifugation was further examined for peroxidase-like activity by adding TMB and H₂O₂ solutions. The UV-Vis spectrum of the leached solution was recorded in kinetic mode up to 10 minutes (Fig. S16, ESI†). The negligible absorbance shown by the leached solution confirms the role of MnCo₂O₄ NPs in the peroxidase-like activity and the activity is not due to leached ions.

Kinetic studies

Steady state-kinetic studies on the peroxidase-like activity of SiO₂@MnCo₂O₄-1 nanorattles were performed using different substrates (i.e., TMB and H₂O₂). The kinetic results at different TMB concentrations by fixing H₂O₂ concentration and vice versa are shown in Fig. S17(a and b) (ESI†). The kinetic parameters (K_m and V_max) were determined from the slope and intercept values of Lineweaver–Burk reciprocal plots (Fig. S18(a and b), ESI†). The K_m and V_max values for SiO₂@MnCo₂O₄-1 core–shell nanorattles are summarized in Table 4. Table 4 also gives the kinetic parameters for different metal oxide nanoparticles and core–shell NPs reported in the literature.^5,54–60 The Michaelis–Menten constant (K_m) is inversely proportional to the catalyst's affinity towards a substrate; a smaller K_m value indicates higher affinity of a catalyst towards the substrate.⁵⁴ The K_m and V_max values for SiO₂@MnCo₂O₄-1 core–shell nanorattles towards TMB are 0.032 mM and 1.596 × 10⁻⁸ Ms⁻¹, respectively. The K_m and V_max values for SiO₂@MnCo₂O₄-1 core–shell nanorattles towards H₂O₂ are 2.7 mM and 0.532 × 10⁻⁸ Ms⁻¹, respectively. The reported K_m and V_max values for MnCo₂O₄ nanoparticles towards TMB are 0.063 mM and 2.17 × 10⁻⁵ Ms⁻¹, respectively.⁵¹ For the natural peroxidase enzyme (HRP), the reported K_m and V_max values towards TMB are 0.434 mM and 10 × 10⁻⁸ Ms⁻¹ and towards H₂O₂, the values are 3.70 mM and 8.71 × 10⁻⁸ Ms⁻¹, respectively.⁵⁵ The observed K_m value for SiO₂@MnCo₂O₄-1 nanorattles towards TMB, in the present study, is smaller than that of MnCo₂O₄ nanoparticles and natural peroxidase enzyme (HRP). These results indicate high affinity of SiO₂@MnCo₂O₄-1 core–shell nanorattles towards TMB with better peroxidase-like activity.

Table 4 Comparison of steady-state kinetic parameters (K_m and V_max) for SiO₂@MnCo₂O₄ core–shell nanorattles with those reported in the literature

Catalyst	Substrate	K m (mM)	V max (Ms^−1)	Reference
SiO2@Co3O4 CSNPs	TMB	0.087	0.012 × 10^−8	5
SiO2@Co3O4 CSNPs	H2O2	25.2	0.015 × 10^−8	5
MnCo2O4 nanoparticles	TMB	0.063	2.17 × 10^−5	54
HRP	TMB	0.434	10.0 × 10^−8	54
HRP	H2O2	3.70	8.71 × 10^−8	55
MnO2 nanoparticles	TMB	0.04	5.78 × 10^−6	56
Co3O4 nanoparticles	TMB	0.037	6.27 × 10^−8	57
Co3O4 nanoparticles	H2O2	140.07	12.1 × 10^−8	57
Co3O4@NiO CSNPs	TMB	0.036	—	58
Co3O4@NiO CSNPs	H2O2	8.17	—	58
Fe3O4@CoFe-LDH	TMB	0.395	—	59
Fe3O4@CoFe-LDH	H2O2	47.6	—	59
TiO2@CeOx	TMB	0.28	6.5 × 10^−9	60
TiO2@CeOx	H2O2	6.29	34 × 10^−9	60
SiO2@MnCo2O4-1	TMB	0.032	1.596 × 10^−8	This work
SiO2@MnCo2O4-1	H2O2	2.7	0.532 × 10^−8	This work

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Mechanism of peroxidase-like activity of SiO₂@MnCo₂O₄ nanorattles

Fig. 8 shows the mechanism for the peroxidase-like activity of SiO₂@MnCo₂O₄ nanorattles. In the first step, adsorption of tetramethylbenzidine (TMB) molecules occurs on the surface of SiO₂@MnCo₂O₄ nanorattles. This promotes nitrogen electron pair donation from the NH₂ groups of TMB towards the surface of SiO₂@MnCo₂O₄, resulting in high electron density over SiO₂@MnCo₂O₄.^31,54,61 The SiO₂@MnCo₂O₄ core–shell nanorattles reduce H₂O₂ into OH⁻ and OH˙ species. The OH⁻ and OH˙ species facilitate the oxidation of TMB into TMB˙⁺ (a blue-colored CT complex). The rattle-like hierarchical structure of MnCo₂O₄ NPs present on the surface of SiO₂ spheres eases interaction between tetramethylbenzidine and the catalytic active centers (Mn²⁺ and Co²⁺) leading to improved peroxidase-like activity of the SiO₂@MnCo₂O₄ nanorattles.

Fig. 8 Proposed mechanism of peroxidase-like activity of SiO₂@MnCo₂O₄ core–shell nanorattles using TMB as a substrate.

In the current study, high surface area and rattle-like morphology of SiO₂@MnCo₂O₄ lead to enhanced peroxidase-like activity. The higher peroxidase-like activity of SiO₂@MnCo₂O₄-1 compared to the other samples is attributed to its high surface area (356 m² g⁻¹). The SiO₂@MnCo₂O₄ core–shell nanorattles, synthesized in the current study, exhibit high surface area due to their rattle-type porous morphology. The high surface area of the SiO₂@MnCo₂O₄ core–shell nanorattles is helpful for enhanced peroxidase-like activity. The rattle-type porous nature of hierarchical MnCo₂O₄ NPs on the surface of SiO₂ spheres provides less steric hindrance for the TMB molecules to interact with the catalytic centres (Mn²⁺ and Co²⁺⁾.^62,63 This promotes electron transfer from the SiO₂@MnCo₂O₄ core–shell nanorattles towards H₂O₂ resulting in faster chemical reduction of H₂O₂.

Fig. S19 (ESI†) shows the fluorescence spectra for the detection of OH˙. radicals produced during the peroxidase-like activity. Terephthalic acid (probe molecule, λ_exc = 315 nm) produces fluorescent 2-hydroxyterephthalic acid (λ_em = 425 nm) on reacting with hydroxyl radicals. It can be noted that the fluorescence intensity increases with time due to increasing production of hydroxyl radicals. The PL results suggest the role of hydroxyl radicals in the peroxidase-like activity of SiO₂@MnCo₂O₄ nanorattles.

Detection of H₂O₂ using SiO₂@MnCo₂O₄ nanorattles

SiO₂@MnCo₂O₄ core–shell nanorattles were used for the detection of H₂O₂ using peroxidase-like activity. A calibration plot (Fig. S20, ESI†) was recorded for SiO₂@MnCo₂O₄-1 with varying concentration of H₂O₂ (linear range = 0.05 mM to 1 mM). The equation given below was used for determining the limit of detection (LOD) of H₂O₂.

Limit of detection = 3 × (Standard deviation due to blank/Slope)

The standard deviation was estimated by recording the absorption spectra of a blank solution (i.e. [H₂O₂] = 0) three times. The slope was calculated from the absorbance (@λ_max = 652 nm) versus [H₂O₂] plot (Fig. S20(b), ESI†). The estimated LOD value for H₂O₂ using SiO₂@MnCo₂O₄ core–shell nanorattles is 0.083 mM and it is comparable with that reported in the literature.^58,63

Conclusions

The current work demonstrates a simple and cost-effective synthetic strategy to synthesize SiO₂@MnCo₂O₄ core–shell nanorattles via calcination of SiO₂@MnCo-LDH precursors at 500 °C. MnCo₂O₄ NPs deposit on the surface of SiO₂ spheres creating a hollow space (void) between the core and shell. The [Mn²⁺ : Co²⁺] ratio affects thickness of the MnCo₂O₄ shell and void distance between the core (SiO₂) and the shell (MnCo₂O₄). XRD results confirm the formation of MnCo₂O₄ NPs in the SiO₂@MnCo₂O₄ samples. FESEM and TEM analyses show nanorattle-like morphology of all the SiO₂@MnCo₂O₄ samples. The mean shell thickness and void distance between the SiO₂ core and MnCo₂O₄ shell in the SiO₂@MnCo₂O₄ core–shell nanorattles vary from 38 ± 8 nm to 60 ± 12 nm and 16 ± 3 nm to 30 ± 6 nm, respectively. BET measurements indicate higher surface area of SiO₂@MnCo₂O₄ core–shell nanorattles (278 m² g⁻¹ to 356 m² g⁻¹) compared to the constituents. XPS analysis confirms the presence of Si⁴⁺, Mn²⁺, Co²⁺, Mn³⁺, Co³⁺, and O²⁻ in the SiO₂@MnCo₂O₄ nanorattles. Magnetic measurements indicate paramagnetic and superparamagnetic behavior of SiO₂@MnCo₂O₄ nanorattles at 300 K and 5 K, respectively. The SiO₂@MnCo₂O₄ nanorattles exhibit better peroxidase-like activity than pure MnCo₂O₄ NPs and horseradish peroxidase. The peroxidase-like activity of the core–shell nanorattles can be useful for several sensing applications, such as the detection of glucose, H₂O₂, ascorbic acid, hydroquinone, protein, and dopamine.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Pankaj Rana expresses his gratitude to the UGC and the Ministry of Education, Government of India for awarding him a fellowship (JRF/SRF). The authors express gratitude to Institute Instrumentation Centre (IIC), IIT Roorkee for providing the necessary instrumentation facilities. The authors are also thankful to the Department of Metallurgical and Materials Engineering, IIT Roorkee for providing the TEM facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nj04901e

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