Preamble

Funding: This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA0400000).

First Author: HUANG Yucan, male, born in 1997, received his bachelor's degree from Henan Polytechnic University in 2020, and is currently a master's student focusing on power engineering and engineering thermophysics.

Corresponding Author: LONG Dewu, E-mail: longdewu@sinap.ac.cn

Received: 2024-08-30, Revised: 2024-11-14

Title

Nano-hexagonal Boron Nitride Sheet Preparation by Molten Salt Method

Authors: HUANG Yucan¹, SHEN Jiajie¹, GUAN Chengzhi², LONG Dewu², CHEN Bangfu², YANG Aohua¹

¹ Shanghai Maritime University, Shanghai 201306, China

² Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

Abstract

Background: Hexagonal boron nitride (h-BN) possesses a lamellar structure composed of six-membered rings similar to graphene, exhibiting excellent mechanical properties and thermal conductivity, which has attracted widespread attention in numerous applications.

Purpose: This study aims to propose a greener, lower-energy-consumption preparation process for h-BN, addressing the high energy consumption issues inherent in conventional synthesis methods.

Methods: In this work, the molten salt media method was employed as the preparation approach. The process utilized a KCl-NaCl system as the molten salt medium, melamine (C₃N₆H₆) and boric acid (H₃BO₃) as reactants, with nitrogen (N₂) introduced as the reaction atmosphere. The effects of various conditions—including the mass ratio of molten salt to reactants, the nitrogen-to-boron molar ratio in reactants, holding temperature, and holding time—were systematically investigated. Optimal preparation conditions were derived through characterization and analysis of the obtained samples.

Results: Under systematic investigation, the optimal preparation conditions were identified as: a molten salt to reactant mass ratio of 2:1, a nitrogen-to-boron molar ratio of 1:2, and holding at 1000 °C for 10 h. These conditions yielded h-BN nanosheets with an average size of approximately 50 nm, demonstrating controllable particle size, consistent morphology, and good crystallinity.

Conclusions: This work provides a novel green and low-carbon pathway for synthesizing h-BN nanosheets.

Keywords: Hexagonal boron nitride, Graphene, Molten salt, Low-carbon, Nanosheets

Introduction

Hexagonal boron nitride exhibits a hexagonal layered structure similar to graphene, with lattice constants of a = 0.2504 nm and c = 0.6661 nm. It has a theoretical density of 2.27 g·cm⁻³ and a melting point of 3000 °C, and is commonly known as "white graphite" [1]. In the h-BN structure, each layer consists of nitrogen (N) and boron (B) atoms interconnected to form six-membered ring structures. The N and B atoms within each layer are tightly bound by strong sp² covalent bonds, while the layers are connected by weaker intermolecular forces (van der Waals forces) [2-6]. This unique six-membered ring structure endows h-BN with excellent properties including high lubricity, high thermal conductivity, high heat resistance, strong oxidation resistance, strong corrosion resistance, machinability, and chemical stability [7]. Consequently, it finds extensive applications in thermal management, metallurgy, catalysis, lubrication, insulation, hydrogen storage, electronics, high-temperature oxidation-resistant coatings, and aerospace [8-12].

The primary synthesis methods for h-BN nanosheets are high-temperature solid-state sintering and "top-down" approaches [13-14]. Conventional high-temperature solid-state sintering represents an important industrial preparation route, most commonly using borax and urea (or ammonium chloride) as raw materials that react through high-temperature sintering in ammonia or nitrogen atmospheres to obtain h-BN [15-16]. Although this method is technologically mature, it requires prolonged high-temperature sintering, suffers from high energy consumption and low efficiency, and typically yields large crystalline flakes with micrometer-scale diameters and non-uniform size distributions. Li et al. [17] prepared h-BN particles approximately 5 μm in diameter using the boric acid-urea method. Zhang et al. [18] synthesized highly crystalline h-BN through high-temperature sintering of mixtures containing borax, urea, ammonium chloride, and melamine.

The "top-down" method involves breaking down bulk BN into nanosheets through external forces, encompassing mechanical exfoliation, chemical exfoliation, gas exfoliation, ball milling, and ultrasonication. While effective for preparing h-BN nanosheets, these approaches introduce impurities through added chemical reagents or grinding media during the size reduction process, limiting practical applications. Jiang et al. [19] exfoliated BN nanosheets approximately 200 nm in size with thickness less than 5 nm from h-BN using a radiation-induced reduction-exfoliation method in alcohol-water solutions. Goto et al. [20] exfoliated large h-BN nanosheets using cavitation-bubble plasma in water. Ghosh et al. [21] obtained h-BN nanosheets with thicknesses of 50-270 nm and lateral dimensions of 1-3 μm via liquid-phase exfoliation. Yan et al. [22] used starch as a ball-milling aid to exfoliate plate-like h-BN with an average size of 5 μm.

The molten salt method has recently gained attention as a synthesis approach conducted at relatively low temperatures using molten inorganic eutectic salt media. Due to its lower reaction temperature and highly efficient homogeneous reaction environment, this method offers significant advantages. At elevated temperatures, the solubility of reactants in molten salts enables homogeneous reactions, substantially improving contact between reactants, enhancing reaction efficiency, and maintaining process stability. Metin et al. [23] synthesized nanosheets approximately 50 nm in size using H₃BO₃ and NH₄Cl as reactants with a KCl-NaCl molten salt system. Ye et al. [24] prepared highly crystalline h-BN nanosheets via molten salt reduction at 1200 °C under N₂ protection using Na₂B₄O₇ and Mg powder. Tian et al. [25] synthesized h-BN nanosheets through molten salt nitridation using NaCl-KCl as the molten salt medium and borax with melamine as raw materials at 900-1200 °C.

This paper presents the preparation of nano-sized h-BN through nitridation in a molten salt medium. The study systematically investigates the influence of various conditions—including the mass ratio of molten salt to reactants, the nitrogen-to-boron molar ratio in reactants, holding temperature, and holding time—on h-BN synthesis. Under optimal conditions, h-BN nanosheets with controllable particle size and uniform morphology were successfully prepared.

Experimental Methods

Reagents and Materials

Boric acid (AR, ≥99.5%), melamine (CP, ≥99.0%), potassium chloride (CP, ≥99.5%), and sodium chloride (CP, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Corundum crucibles (HERBED, 99% alumina square crucibles) were used as reaction containers.

Experimental Equipment

The experimental setup included a horizontal tube furnace (HWGL-1200, Suzhou Hateng Technology Co., Ltd.), a blast drying oven, an X-ray diffractometer (X'Pert Powder, Malvern Panalytical), a field-emission scanning electron microscope (FIB system, Zeiss Crossbeam 540, Carl Zeiss), and a simultaneous thermal analyzer (STA-449-F3).

Experimental Procedure

Boric acid and melamine were thoroughly mixed at a specific molar ratio to prepare the reactants. The KCl-NaCl molten salt system was prepared at a 1:1 molar ratio. The reactants and molten salt were mixed at a predetermined mass ratio and placed in a corundum crucible. The crucible was then transferred to a horizontal tube furnace, the furnace tube was sealed, and N₂ was introduced. The reaction proceeded through heating at a controlled rate to the target temperature, followed by isothermal holding. Ammonia gas present in the furnace exhaust was absorbed by a dilute sulfuric acid treatment system. After completion of sintering and cooling, the product was retrieved. The product underwent multiple washing cycles with deionized water to remove the molten salt. The filtered cake was then dried in a blast drying oven at 120 °C for 20 hours prior to characterization. This study systematically varied the molar ratio of boron and nitrogen sources in boric acid and melamine, the mass ratio of reactants to molten salt, holding temperature, and holding time to characterize products obtained under different experimental conditions. The reaction process is described by the following equation:

C₃H₆N₆ + 4H₃BO₃ → 4BN + 2NH₃↑ + 3CO₂↑ + 6H₂O↑

Reaction Mechanism

The KCl-NaCl molten salt system forms a low-melting-point eutectic mixture (melting point 657 °C), providing a liquid-phase reaction environment that reduces mass transfer resistance, promotes reactant diffusion and contact, and simultaneously inhibits high-temperature agglomeration. In the reactants, boric acid (H₃BO₃) undergoes thermal dehydration to generate boron oxide (B₂O₃), while melamine (C₃H₆N₆) decomposes at high temperatures to produce nitrogen-containing active gases (such as NH₃, HCN) that supply nitrogen atoms for nitridation. Boric acid and melamine react during the initial heating stage (<300 °C) to form melamine borate (C₃H₆N₆·2H₃BO₃), a stable precursor structure that ensures uniform distribution of boron and nitrogen atoms. In the molten salt medium, B₂O₃ reacts with nitrogen atoms to generate intermediate BN nuclei. The formed BN dissolves and precipitates in the molten salt, growing preferentially along the (002) crystal plane to form hexagonal platelet structures.

The key functional mechanisms of the molten salt include: (1) providing a homogeneous liquid environment for reactions, avoiding incomplete conversion associated with solid-state reactions; (2) molten salt ions adsorbing on BN crystal nuclei surfaces, guiding two-dimensional growth and forming nanosheet structures through a templating effect; and (3) reaction byproducts (such as H₂O, CO) being encapsulated by the molten salt and subsequently removed through water washing after cooling, thereby improving product purity.

Sample Characterization

The X'Pert Powder X-ray diffractometer (XRD) was used to analyze the crystal structure of samples, with results compared against standard h-BD XRD patterns to verify characteristic diffraction peaks. The Zeiss Crossbeam 540 field-emission scanning electron microscope (SEM) was employed to observe sample morphology and measure particle sizes. Size measurements were performed using the SEM scale function to analyze at least 100 particles per sample, enabling statistical determination of average particle size. The STA-449-F3 simultaneous thermal analyzer (TG) was utilized to evaluate chemical stability based on heating curves below 900 °C, analyzing weight loss and gain points to investigate whether oxidation temperatures align with those of h-BN and to determine weight loss percentages.

Results and Discussion

Effect of Molten Salt-to-Reactant Ratio

Reactants were prepared with a melamine-to-boric acid molar ratio of 1:3 (corresponding to a nitrogen-to-boron elemental ratio of 2:1). The KCl-NaCl molten salt was mixed with reactants at mass ratios of 4:1, 3:1, 2:1, 1:1, and without molten salt. The mixed materials were placed in corundum crucibles and reacted in a horizontal tube furnace with a temperature profile of 500 °C for 1 hour followed by 1000 °C for 4 hours. [FIGURE:1] presents X-ray diffraction patterns of BN products prepared under different molten salt ratios, while [FIGURE:2] shows corresponding SEM morphological analysis.

Figure 1 XRD patterns of h-BN powders prepared with different molten salt to reactant mass ratios, fixing the nitrogen-to-boron molar ratio at 2:1 and holding at 1000 °C for 4 h.

Figure 2 SEM images of h-BN powders prepared under different molten salt to reactant mass ratios, fixing the nitrogen-to-boron molar ratio at 2:1 and holding at 1000 °C for 4 h (Molten salt to reactant ratio: a-no molten salt, b-1:1, c-2:1, d-3:1, e-4:1).

The XRD patterns reveal that all five molten salt ratio conditions produced a prominent diffraction peak at 25°-28° and a relatively weaker peak at 41°-44°. The former corresponds to the characteristic (002) crystal plane of h-BN, while the latter represents the incompletely developed (100) and (101) planes. A characteristic (110) plane diffraction peak appears at 75°-77°. These features are consistent with turbostratic boron nitride (t-BN) reported in literature [26], representing an intermediate state between amorphous and hexagonal BN, analogous to the transformation from amorphous carbon to graphite through high-temperature treatment [27]. The XRD patterns indicate that when the molten salt-to-reactant mass ratio is 2:1, the diffraction peaks exhibit the highest intensity, with a relatively prominent (004) crystal plane characteristic peak appearing between 50°-60° and no impurity peaks, indicating the highest crystallinity.

SEM analysis in [FIGURE:2] shows that without molten salt ([FIGURE:2]a), the sample obtained after reaction at 1000 °C for 4 h consists of large aggregated BN particles, consistent with reported t-BN morphologies [26]. When the molten salt-to-reactant mass ratio increased to 1:1 ([FIGURE:2]b), numerous platelet BN structures emerged with diameters of 200-700 nm and thicknesses below 20 nm, demonstrating improved BN crystal growth. At a mass ratio of 2:1 ([FIGURE:2]c), the sample exhibited the optimal morphology with platelet BN diameters of 200-400 nm, thicknesses under 10 nm, and a higher quantity of more uniformly sized nanosheets, indicating the most favorable conditions for BN growth. Further increases in the molten salt-to-reactant mass ratio did not significantly improve sample morphology.

Comprehensive analysis of XRD and SEM results demonstrates that a molten salt-to-reactant mass ratio of 2:1 yields platelet BN with the most uniform size and optimal crystallinity.

Effect of Holding Temperature and Time

Based on the previous results showing that a 2:1 molten salt-to-reactant ratio produced large platelet structures rather than smaller nanoscale h-BN, we investigated the effects of varying holding temperature and duration. For comparative analysis, experiments fixed the nitrogen-to-boron molar ratio at 2:1 and molten salt-to-reactant mass ratio at 2:1, with holding times of 4 h and 10 h at temperatures of 800 °C and 1000 °C. The XRD patterns and SEM images of the resulting samples are presented in [FIGURE:3] and [FIGURE:4].

Figure 3 XRD patterns of h-BN powders prepared under different holding temperatures and times, fixing the nitrogen-to-boron molar ratio at 2:1 and molten salt-to-reactant mass ratio at 2:1 (a-1000 °C, 10 h; b-800 °C, 10 h; c-1000 °C, 4 h; d-800 °C, 4 h).

The XRD patterns in [FIGURE:3] show that increasing the holding temperature from 800 °C to 1000 °C for 4 h slightly enhanced the intensity of the combined (002), (100), and (101) peaks. Comparison between [FIGURE:3]b and [FIGURE:3]d reveals that extending holding time at lower temperature effectively improved BN growth, with significant enhancement in characteristic peak intensity at 800 °C for 10 h, suggesting that prolonged holding can compensate for lower reaction temperatures. The highest intensity of characteristic diffraction peaks, with resolved (100) and (101) peaks, was achieved at 1000 °C for 10 h, yielding results approaching those of standard h-BN.

Figure 4 SEM images of h-BN powders calcined at different holding temperatures and times under conditions of nitrogen-to-boron molar ratio 2:1 and molten salt-to-reactant mass ratio 2:1.

SEM analysis in [FIGURE:4] shows that no platelet structures formed at 800 °C for 4 h, with morphology remaining consistent with t-BN samples. At 1000 °C for 4 h, larger platelet structures appeared with non-uniform sizes of 200-600 nm. Extending the holding time to 10 h at 800 °C produced results similar to those at 1000 °C for 4 h. However, holding at 1000 °C for 10 h yielded finer, more uniform platelet structures with sizes uniformly controlled below 100 nm, consistent with nanoscale h-BN platelet structures. These results demonstrate that extended holding time can compensate for lower reaction temperatures, though the time required is substantially longer. Higher reaction temperatures combined with longer holding times more effectively improve BN growth, producing uniform nanoscale h-BN products.

Effect of Nitrogen-to-Boron Ratio

Based on previous results identifying optimal conditions of 2:1 molten salt-to-reactant mass ratio, 1000 °C, and 10 h holding time for preparing uniform h-BN nanosheets, we fixed these parameters and varied the nitrogen-to-boron molar ratio to 1:2, 1:1, 2:1, 3:1, and 4:1 to investigate its effect on h-BN formation. The XRD and SEM results are presented in [FIGURE:5] and [FIGURE:6].

Figure 5 XRD patterns of h-BN powders prepared with different nitrogen-to-boron molar ratios, fixing the molten salt-to-reactant mass ratio at 2:1 and holding at 1000 °C for 10 h (Molar ratio of nitrogen to boron: a-1:2, b-1:1, c-2:1, d-3:1, e-4:1).

The XRD analysis reveals that a nitrogen-to-boron ratio of 2:1 ([FIGURE:5]c) produced the lowest intensity characteristic diffraction peaks, indicating the poorest crystallinity. As the nitrogen-to-boron ratio decreased, peak intensity progressively increased. At a ratio of 1:2, the characteristic peaks exhibited the highest intensity, sharpest peaks, and smallest half-peak width, with more distinct separation of (100) and (101) peaks, indicating optimal crystallinity. At higher ratios of 3:1 and 4:1, peak intensity increased but remained lower than that of the 1:2 sample, with broader peak shapes. The XRD patterns in [FIGURE:5]d and [FIGURE:5]e show noticeable leftward shifting of the main diffraction peak at 20°-30°, attributed to the t-BN to h-BN transition, where all t-BN diffraction peaks shift leftward by a small proportion. This suggests that increasing nitrogen-to-boron ratios are detrimental to h-BN formation. XRD analysis indicates that a nitrogen-to-boron molar ratio of 1:2 in the reactants is optimal.

Figure 6 SEM images of h-BN powders calcined at different nitrogen-to-boron molar ratios, fixing the molten salt-to-reactant mass ratio at 2:1 and holding at 1000 °C for 10 h (Molar ratio of nitrogen to boron: a-1:2, b-1:1, c-2:1, d-3:1, e-4:1).

SEM results in [FIGURE:6] show that at a nitrogen-to-boron ratio of 1:2, the h-BN platelet structures are approximately 50 nm in size with the most uniform distribution. At a 1:1 ratio, morphology is similar to the 1:2 sample. As the nitrogen-to-boron ratio increases, nanosheet size gradually increases. At a 2:1 ratio, platelet structures are nearly twice the size of those in [FIGURE:6]a. At 3:1, platelet dimensions reach 100-200 nm with noticeably less uniform size distribution. At 4:1, platelet structures increase further in size, with a greater proportion of 200-400 nm structures. Based on XRD and SEM analysis, uniform h-BN nanosheets are obtained under conditions of 2:1 molten salt-to-reactant mass ratio, 1000 °C for 10 h, and a nitrogen-to-boron molar ratio of 1:2.

Particle size distribution analysis was performed on the sample from [FIGURE:6]a by dispersing and measuring 100 particles, with results shown in [FIGURE:7]. The distribution reveals 96 particles under 50 nm and only 4 exceeding 50 nm, demonstrating relatively uniform and controllable size distribution. Comparing all samples in [FIGURE:6], the transformation to h-BN proceeds through initial formation of a t-BN matrix with large 200-400 nm platelet structures. As the reaction progresses, BN nuclei within the t-BN begin to separate, forming smaller 100-200 nm platelets. Continued reaction drives the crystal structure toward h-BN, ultimately yielding uniform, single-nucleus h-BN nanosheets.

Figure 7 Size distribution of 100 particles in the sample shown in Figure 6(a).

Thermogravimetric (TG) Analysis

TG analysis was performed to examine potential volatile impurities in the prepared BN nanosheets ([FIGURE:8]). Curve a corresponds to BN powder prepared with a nitrogen-to-boron ratio of 2:1, molten salt-to-reactant ratio of 2:1, at 1000 °C for 4 h. Curve b corresponds to h-BN powder prepared with a nitrogen-to-boron ratio of 1:2, molten salt-to-reactant ratio of 2:1, at 1000 °C for 10 h. Both BN products exhibit weight loss within ±2%, indicating product purity exceeding 98%. Below 700 °C, curve b shows more pronounced weight loss than curve a, suggesting higher volatile impurity content in product b, likely introduced during the extended 10 h high-temperature treatment. Both curves show an upward trend at 700 °C with an inflection plateau at 800 °C, attributed to surface oxidation of BN to boron oxide. This oxidation temperature is consistent with that of h-BN [28] and higher than the oxidation temperature of graphene [29], confirming the prepared samples as h-BN. The h-BN sample shows a slightly greater weight loss of approximately 2%.

Figure 8 Thermogravimetric analysis (TG) of orthorhombic boron nitride and hexagonal boron nitride (a: nitrogen-to-boron molar ratio 2:1, molten salt-to-reactant mass ratio 2:1, 1000 °C for 4 h; b: nitrogen-to-boron molar ratio 1:2, molten salt-to-reactant mass ratio 2:1, 1000 °C for 10 h).

Conclusion

Using melamine as the nitrogen source, boric acid as the boron source, and KCl-NaCl as the molten salt system under N₂ atmosphere, we successfully prepared uniform, size-controllable nano-sized h-BN. This reaction system was systematically investigated to determine the effects of molten salt-to-reactant mass ratio, holding temperature and time, and nitrogen-to-boron molar ratio on the h-BN product. The results demonstrate that uniform h-BN nanosheets approximately 50 nm in size are obtained under optimal conditions of 2:1 molten salt-to-reactant mass ratio, 1:2 nitrogen-to-boron molar ratio, and 1000 °C for 10 h. By simply adjusting the nitrogen-to-boron ratio, h-BN nanosheets with controllable sizes of 100-200 nm and 200-400 nm can be prepared. Compared with conventional h-BN preparation processes, this work is conducted at temperatures not exceeding 1000 °C, significantly lower than the 1200-1800 °C required for industrial h-BN production. The homogeneous reaction medium provided by the molten salt effectively reduces synthesis temperature [22-23], directly decreasing energy consumption. The prepared h-BN exhibits controllable particle size and uniform morphology, providing a reliable experimental basis for green, low-carbon, large-scale industrial production of BN nanosheets.

Author Contributions

HUANG Yucan conducted the experiments and was responsible for drafting, writing, and revising the manuscript. SHEN Jiajie provided experimental guidance and manuscript review. GUAN Chengzhi provided financial support and experimental guidance. LONG Dewu designed the experimental framework, provided guidance, and reviewed and revised the manuscript. CHEN Bangfu and YANG Aohua provided suggestions and assistance from their respective areas of expertise.

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