Synthesis of niobium(IV) carbide nanoparticles via an alkali-molten-method at a spatially-limited surface of mesoporous carbon

Abstract

One-pot synthesis of niobium carbabide (NbC) nanoparticles with ca. 30–50 nm was achieved via a rationally designed novel alkali-molten salt method using niobium oxide (Nb₂O₅), potassium carbonate (K₂CO₃), and mesoporous carbon (MPC). In this reaction, potassium niobate (KNbO₃) was produced as an intermediate and carbonization of KNbO₃ proceeds at a spatially limited external surface encompassed by the mesopores of MPC due to the repulsive characteristics of ionic KNbO₃ toward hydrophobic MPC, which affords the size-controlled NbC nanoparticles with a narrow particle distribution. The particle sizes tended to become smaller as the pore sizes of MPCs or the temperature on the calcination under the nitrogen stream decreased. Elemental reactions along the one-pot synthesis of NbC nanoparticles were clarified by X-ray spectroscopic, thermogravimetric, and mass spectrometric measurements.

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Introduction

Catalytic reactions such as hydrogen production, methanol synthesis, Fischer–Tropsch synthesis, and reverse water gas shift reaction are essential to realizing carbon neutrality using sustainable energy production aiming to emit essentially no carbon dioxide.^1–5 In these catalytic processes, noble metals such as platinum have been widely used as active sites in catalysis. However, due to their scarcity, their use in sustainable energy production is currently not realistic. From the background, alternative materials of the noble metals are strongly desired.

Transition metal carbides (TMCs) are expected as alternatives to noble metals since TMCs possess high melting point, high corrosion resistance, and significant electronic and catalytic characteristics.^6–18 Among the many TMCs, niobium carbide (NbC) has attracted chemists because of its mechanical and chemical stability, and excellent electrochemical potential.¹⁹ NbC was conventionally synthesized by calcination at high temperature after mixing niobium(V) oxide (Nb₂O₅) and carbon sources, affording adamant NbC with large particle sizes.²⁰ However, the growth of the particle size causes a decrease in the amount of active sites per weight.

Regarding this, the preparation of nano-sized NbC has been addressed by a lot of researchers to increase active sites per weight on the catalysis.^6,21–32 Recently, Tour and Zhao et al. synthesized NbC nanoparticles with ca. 20 nm of size via novel flash Joule heating in the reactor composed of capacitor banks, electrodes, quartz and a carbon source.²¹ Takanabe and his co-workers succeeded in the synthesis of various TMCs including NbC nanoparticles with the size of <10 nm by a skillful strategy using ethanol solution of metal precursors and mesoporous graphitic carbon nitride (mpg-C₃N₄), in which the TMC nanoparticles were produced in the mesopores of mpg-C₃N₄.²² As we can see in the pioneered study such as aforementioned literature, the key point on the preparation of size-controlled TMC nanoparticles is “reaction field”. In other words, the size-controlled TMC nanoparticles are afforded resulting from precise design of the reaction field of a carbonization of metal species precursor.

Here, we reported a rationally designed new one-pot synthetic method of NbC nanoparticles with 30–50 nm by using Nb₂O₅, potassium carbonate (K₂CO₃), and hydrophobic mesoporous carbon (MPC). The key strategy of this method is utilizing a prohibition of capillary action (CA) of intermediated potassium niobate (KNbO₃) into the nano-sized pores of MPC due to significantly low affinity between them, resulting in the formation and growth of NbC particles at limited external surface encompassed by the pores as a reaction field (Fig. 1). Pore size of MPC had a direct relationship to area of the external surface unit, hence the size of resulting NbC nanoparticles was also changed by varying the pore size of MPC. Moreover, the nanoparticle sizes of the synthesized NbC gradually became smaller as the calcination temperature decreased, indicating a possibility of precise adjustment of particle sizes. Furthermore, we propose elemental reactions during the synthesis of NbC nanoparticles from Nb₂O₅.

Fig. 1 Schematic representation of the alkali-molten method proposed in this study.

Results and discussion

The NbC nanoparticles were synthesized from Nb₂O₅ and MPC possessing 150 nm of mean pore size (MPC-150) with K₂CO₃ under N₂ atmosphere. Fig. 2 displays X-ray diffraction (XRD) patterns of niobium species after calcination of the physical mixture of Nb₂O₅ and MPC with/without K₂CO₃ at 1150 °C under N₂ stream. In the case of the absence of K₂CO₃, NbC was formed accompanied by residual reduced niobium oxide species (NbO₂, Nb₁₂O₂₉) with the NbC yield of 31.8% estimated by reference intensity ratio (RIR) method, while Nb₂O₅ was completely converted to NbC under the presence of K₂CO₃. This finding indicates that K₂CO₃ promoted the formation of NbC. To explore the impact of carbon sources on the synthesis of NbC nanoparticles, graphite was used instead of MPC. Although XRD measurements revealed that NbC could be produced regardless of carbon source, crystalline size (D) of NbC synthesized with graphite (D = 55 nm, Fig. S1†) was larger than that with MPC-150 (D = 32 nm). Moreover, transmission electron microscopic (TEM) images showed that NbC prepared with graphite tended to grow compared to NbC obtained by MPC-150 because the average particle sizes calculated by manual estimation of diameter of 200 of NbC particles were 77, and 51 nm on using graphite and MPC-150, respectively (Fig. 3a–f, S2 and S3†). Moreover, the significantly wider distribution of particle size of NbC on graphite than MPC-150 implies the presence of spatially limited area on MPC-150.

Fig. 2 XRD patterns of products after calcination of the mixture of Nb₂O₅ (0.1 g) and MPC-150 (0.1 g) with or without K₂CO₃ at 1150 °C for 10 h under N₂ stream atmosphere. The yield of NbC was estimated to be 31.8% on the absence of K₂CO₃.

Fig. 3 (a–d and g–j) TEM images of synthesized NbC particles with (a and b) graphite, (c and d) MPC-150, (g and h) MPC-30, and (i and j) MPC-10 as carbon sources. (e, f, k and l) size distributions of the NbC nanoparticles synthesized with (e) graphite, (f) MPC-150, (k) MPC-30, and (l) MPC-10. The distribution was estimated by the measuring particle sizes of NbC in TEM images. The terms RSD and RSE indicate “relative standard deviation” and “relative standard error”, respectively.

In this context, NbC was synthesized with MPCs which possess pore sizes of 30 nm (MPC-30) and 10 nm (MPC-10) to investigate the effect of the pore. The XRD measurement revealed the D values of NbC synthesized with MPC-30 (D = 30 nm) or MPC-10 (D = 27 nm), indicating that crystalline sizes were not dependent on the pore sizes of MPCs (Fig. S4†). On the other hand, TEM observation clearly revealed that the size of NbC nanoparticles synthesized with MPC-30, in which average particle size was 30 nm, decreased compared to that prepared with MPC-150 (Fig. 3g, h, k and S5†). Notably, the average particle size of NbC obtained from MPC-10 (average size: 29 nm) was almost identical to the condition under the usage of MPC-30, and the size was larger than their pore size (Fig. 3i, j, l and S6†). This finding indicates that the reaction does not proceed in the mesopore but at the external surface. To visualize the reaction field, scanning electron microscopy (SEM) was performed (Fig. 4). Intriguingly, few NbC particle exists in the pore of MPC, demonstrating the formation of NbC occurs at the external surface of MPC. The origin of differences in particle size on the used MPC was considered by the calculation of the external surfaces area (S^ext) for each MPC from Brunauer–Emmett–Teller surface area (S^BET) and the internal surface area of the pore (S^int) estimated by N₂ adsorption/desorption experiments: S^ext = S^BET − S^int (Fig. S7,† and Table 1). The S^ext for the MPC-150, MPC-30, and MPC-10 were 26.0, 49.9, and 32.3 m² g⁻¹, respectively. The slight difference in S^ext compared to S^BET indicates that the MPC with smaller pores possesses a larger number of pores per weight. In such a situation, the area of a limited external surface encompassed by mesopores as a reaction field became smaller. Hence, the smaller particle sizes were afforded in the case of MPC with small pores.

Fig. 4 SEM images of synthesized NbC by the calcination of the mixture of Nb₂O₅ (0.1 g) MPC-150 (0.1 g), and K₂CO₃ (0.035 g) at 1150 °C for 10 under N₂ stream atmosphere.

Table 1 Brunauer–Emmett–Teller surface areas (S^BET), internal surface areas arising from mesopores (S^int) estimated by Barrett–Joyner–Halenda (BJH) method, and external surface area (S^ext) of MPC-150, MPC-30, and MPC-10. S^ext was calculated by the equation as follow: S^ext = S^BET − S^int

Type of MPC	S^BET/m^2 g^−1	S^int/m^2 g^−1	S^ext/m^2 g^−1
MPC-150	203.2	177.2	26.0
MPC-30	695.0	645.1	49.9
MPC-10	1055.9	1023.6	32.3

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The impact of calcination temperatures on the formation of NbC was also investigated. Fig. 5 shows XRD patterns of the products after calcination at different temperatures (800–1000 °C). NbC could be successfully synthesized alone above 1000 °C (see Fig. 2 for the calcination at 1150 °C), while potassium niobate (KNbO₃) was observable at 900 °C, and other potassium niobate species (K₄Nb₆O₁₇) was also detected in addition to KNbO₃ in the case of calcination at 800 °C. However, KNbO₃ completely deteriorated and only diffraction peaks arising from NbC appeared on the calcination at 900 °C for 20 h (Fig. S8†). Weakening peaks of K₄Nb₆O₁₇ and KNbO₃ accompanied with strengthen of the NbC peak was also observed by calcination at 800 °C for 40 h, too (Fig. S8†). Notably, no NbC was generated on the calcination at 900 °C at 20 h in the absence of K₂CO₃ (Fig. S9†), therefore the niobates were reaction-active species. Since K₄Nb₆O₁₇ is an intermediate in the formation of KNbO₃ from Nb₂O₅ and K₂CO₃,^33,34 these findings indicate that NbC is formed by the reaction of carbon and KNbO₃. Here, the intermediated KNbO₃ is expected to have an extraordinary contribution to selective reaction at the external surface of MPC. Capillary action (CA) is generally the dominant driving force on incursion of liquid into the nano-scale pores and its magnitude is strongly dependent on the affinity between the liquid and substrate possessing pores.³⁵ Since ionic KNbO₃ molten, composed of K⁺ and [NbO₆]⁷⁻ octahedra,^36,37 disfavors hydrophobic MPC, the CA is not preferred, resulting in the prohibition of the incursion of KNbO₃ molten into the pore of MPC. The D values of NbC became smaller as the calcination temperature decreased, i.e. D = 36, 31, and 17 nm for 1150 °C (10 h), 1000 °C (10 h), and 900 °C (20 h), respectively. The average particle sizes of NbC estimated from TEM observation also coincided with this tendency (Fig. S10 and S11†). The decrease in the particle sizes was due to preventing sintering, and particle sizes of NbC could be changed in our novel alkali-molten synthetic method with MPC by the variation calcination temperature.

Fig. 5 XRD patterns of products after calcination of the mixture of Nb₂O₅ (0.1 g) MPC-150 (0.1 g), and K₂CO₃ (0.035 g) at different temperatures for 10 h under N₂ stream atmosphere. The yields of NbC were estimated to be 60.8, 85.5, and 85.5% on the calcination at 800, 900, 1000 °C, respectively.

According to the result mentioned above, Nb₂O₅ stepwise changed to NbC via KNbO₃ intermediate. To clarify the reaction mechanism on the first step, from Nb₂O₅ to KNbO₃, thermogravimetry (TG) was performed for Nb₂O₅, K₂CO₃, and the mixture of Nb₂O₅ and K₂CO₃ (Fig. 6). Weight was identical on the elevation of temperature in the case of Nb₂O₅, while the clear weight loss from 50 °C to 175 °C of temperature range was observable on the K₂CO₃ and their mixture. The decrease in the weight is attributed to the dehydration of K₂CO₃.³⁸ In addition to the dehydration, second 1.13 mg of weight loss could be seen in only the mixture. A gaseous by-product was responsible for the weight loss since KNbO₃ was never evaporated at 900 °C, hence mass spectrometry of possible gaseous products during temperature elevation was performed for the mixture of Nb₂O₅ and K₂CO₃ (Fig. S12†). The mass signals strongly indicate that only CO₂ (m/z = 44) was produced as gaseous products, which is reasonable because the theoretical stoichiometric amount of emitted CO₂ in the reaction was 1.02 mg, coinciding with the experimental value (1.13 mg). Thus, the first step in the synthesis of NbC can be described as eqn (1), which is well-matched with the result mentioned in the effect of calcination temperature.

Nb₂O₅ + K₂CO₃ → 2KNbO₃ + CO₂ (1)

Here, K₂CO₃ used in this experiment (2.53 × 10⁻⁴ mol) was stoichiometrically lacking to Nb₂O₅ (3.76 × 10⁻⁴ mol), but a complete transformation of Nb₂O₅ to KNbO₃ could be explained by the discussion mentioned below.

Fig. 6 TG profiles of Nb₂O₅, K₂CO₃ and their mixture during temperature elevation at the rate of 10 °C min⁻¹ under N₂ atmosphere.

The reaction mechanism in the second step was hence tracked by using commercial KNbO₃ and MPC-150. The XRD measurement strongly suggests that KNbO₃ is the reactant for the formation of NbC (Fig. S13†). The weight of the mixture of KNbO₃ and MPC-150 decreased by 92.0 mg after calcination. Here, K₂O is known to be degraded to K₂O₂ and K above 360 °C.³⁹ The K₂O₂ again afford K₂CO₃ by reacting with CO,³⁹ while K evaporates above 757 °C.⁴⁰ Hence, not only the gaseous materials but also K₂O can be the product responsible for weight loss. If the CO₂ was formed in this process under estimation of evaporation of K₂O, the theoretical weight loss was 76.9 mg, while the weight loss was 88.1 mg in the case of the formation of CO. Hence, CO is preferred as the product because of well matching the experimental data. Therefore, a possible reaction can be depicted as follow.

2KNbO₃ + 7C → 2NbC + 5CO + K₂O (2)

It should be noted that K₂O likely to reacts with Nb₂O₅ in the case of the existence of Nb₂O₅.⁴¹ Indeed, the reaction of Nb₂O₅ and KOH at 700 °C afforded KNbO₃ (Fig. S14†), where reaction temperature was set to 700 °C to laten carbonization of KNbO₃ and KOH transformed into K₂O by dehydration at this temperature.⁴² The reaction of K₂O with Nb₂O₅ well explains the fact that no Nb₂O₅ remains despite the condition of the stoichiometric lacking of K₂CO₃ to Nb₂O₅ for all experiments. Hence the following equation can be described.

Nb₂O₅ + K₂O → 2KNbO₃ (3)

The entire reaction mechanism was proposed from the findings of experiments. On the direct reaction at the first step, Nb₂O₅ was transformed into KNbO₃ by the reaction with K₂CO₃ (eqn (1)) and K₂O was generated as a by-product. The K₂O further makes Nb₂O₅ change KNbO₃ (eqn (3)) or reacts with CO₂ to regenerate K₂CO₃ (ref. 43) (eqn (4)). On the other hand, since NbO₂ was also formed in the absence of K₂CO₃ (Fig. 2), indirect paths from NbO₂ should be also considered (eqn (5)–(7)). The second step is the carbonization of intermediated KNbO₃ on the external surface of MPC (eqn (2)). The selective proceeding at the external surface is due to the inhibition of CA as mentioned above. All considerable elemental reactions in the synthesis of NbC nanoparticles using MPC are shown as follows.

First step

(Direct reaction from Nb₂O₅)

Nb₂O₅ + K₂CO₃ → 2KNbO₃ + CO₂ (1)

Nb₂O₅ + K₂O → 2KNbO₃ (3)

K₂O + CO₂ → K₂CO₃ (4)

(Indirect reaction via NbO₂)

Nb₂O₅ + C → 2NbO₂ + CO (5)

Nb₂O₅ + CO → 2NbO₂ + CO₂ (6)

2NbO₂ + K₂CO₃ → 2KNbO₃ + CO (7)

Second step

2KNbO₃ + 7C → 2NbC + 5CO + K₂O (2)

Experimental

Synthesis of niobium(IV) carbide (NbC)

0.1 g of graphite (8.33 × 10⁻³ mol, KANTO CHEMICAL Co., Inc.) or MPC (8.33 × 10⁻³ mol, TOYO TANSO Co., Ltd), 0.1 g of Nb₂O₅ (3.76 × 10⁻⁴ mol, FUJIFILM Wako Pure Chemical Co., Ltd), and 0.035 g of K₂CO₃ (2.53 × 10⁻⁴ mol, FUJIFILM Wako Pure Chemical Co., Ltd) were mixed by mortar and obtained mixture was added on the platinum boat. The reactants on the platinum boat were placed in the oven and calcined at different temperature (800–1150 °C) for 10 h under N₂ atmosphere. After cooling to room temperature, NbC was obtained. On the investigation of reaction mechanism, KNbO₃ (KOJUNDO CHEMICAL LABORATORY Co., Ltd) was also used as reactants.

X-ray diffraction (XRD) measurement

The XRD analysis was performed using Ultima-IV apparatus (Rigaku Co., Ltd). All samples were irradiated with Cu Kα (λ = 0.154 nm) at a voltage of 40 kV and current of 40 mA. The scanning range and scanning speed were set to 5°–80° and 2.0° min⁻¹ for all the measurements. The “D/teX Ultra” was utilized as a detector. A crystalline size (D) was calculated by Scherrer equation as shown below:where, K, β, and θ represent shape factor, dull widths at half maximum, and diffraction angle, respectively. The composition of the resulting materials was simply quantified by using reference intensity ratio (RIR) method.

Nitrogen (N₂) adsorption/desorption test

N₂ adsorption/desorption isotherms were recorded by a BELSORP-mini II apparatus (MicrotracBEL Co., Ltd). 10 mg of sample was placed in a glass cell and was heated at 150 °C for 2 h in vacuo as pre-treatment. N₂ adsorption experiments were performed at −196 °C.

Thermogravimetry (TG)

Thermogravimetry-differential thermal analyzer (Thermo plus EVO, Rigaku Co., Ltd) was used for TG-DTA, and the sample was added into a platinum pan (SHIMADZU). The α-alumina was used as a reference. On the investigation of mechanism, the sample was heated from 25 to 900 °C at the rate of 10 °C min⁻¹ under N₂ stream condition.

Temperature-programed mass spectrometry

The mixture of Nb₂O₅ (0.1 g) and K₂CO₃ (0.035 g) in the glass cell was heated to 1000 °C at the rate of 5.0 °C min⁻¹ by using BELCAT II apparatus (MicrotracBEL Co., Ltd). The ejected gaseous component was analyzed by mass spectrometry using BELMASS (MicrotracBEL Co., Ltd).

Transmission electron microscopy (TEM)

All samples were added in ethanol and the suspensions were sonicated. The suspensions were dip on the Cu microgrid (Okenshoji Co., Ltd) and dried. TEM images were observed at 200 kV using a JEM-2100F (JEOL) system.

Scanning electron microscopy (SEM)

The resulting NbC was placed onto carbon tape equipped with alumina substrate (Okenshoji Co., Ltd) and SEM images of it were observed by field-emission scanning electron microscopy (FE-SEM, JSM-7400F, JEOL) with a measurement condition as follow: 5 kV of voltage, 1.0 × 10⁻¹⁰ A of current, and 8 mm of working distance.

Conclusions

We succeeded in the fabrication of NbC nanoparticles with a size of 30–50 nm by a novel alkali-molten salt method using MPCs. The NbC nanoparticles became smaller when MPC possessing small pore size was used due to the decreasing in area of the external surface encompassed by pores as reaction field. The particle sizes of NbC were also changeable by calcination temperature. Furthermore, all considerable elemental reactions were demonstrated by the X-ray spectroscopic, thermogravimetric, and mass spectrometric analyses. We believe that the strategy in the present study has an extraordinary contribution to the field of material chemistry and accelerates development of nano-sized functional materials.

Author contributions

Keigo Tashiro: preparation of manuscript, TEM observation, analyses. Shogo Kobayashi: preparation of manuscript, experiments, analyses. Hinako Inoue: experiments. Shuhei Shimoda: preparation of manuscript, TEM observation. Akihide Yanagita: TEM observation. Shigeo Satokawa: preparation of manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the Joint Usage/Research Center for Catalysis, Hokkaido University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra03254j

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