General Fabrication of Boride, Carbide, and Nitride Nanocrystals via a

Article pubs.acs.org/IC

General Fabrication of Boride, Carbide, and Nitride Nanocrystals via a Metal-Hydrolysis-Assisted Process Ling Zhou,† Lishan Yang,*,†,‡ Li Shao,† Bo Chen,§ Fanhui Meng,§ Yitai Qian,§ and Liqiang Xu*,§ †

Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research, Ministry of Education, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, China ‡ Changsha Research Institute of Mining and Metallurgy Co. Ltd., Changsha 410012, China § Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China S Supporting Information *

ABSTRACT: Metal boride, carbide, and nitride materials are useful owing to their wide variety of interesting chemical and physical properties. However, the synthesis of these materials with nano or mesoscale sizes is challenging due to the usually required high temperatures and long reaction durations. To our knowledge, the exploration of a number of simultaneous chemical reactions through rapid synthesis still remains a great challenge. In this study, a general route for the reduction and transformation of metal oxides into related metal boride (TiB2, MoB2, DyB4, ErB4, YB4, LaB6, CeB6, SmB6, EuB6), carbide (SiC, TiC, VC, WC, W2C, ZrC, MoC, NbC), and nitride (TiN, VN, BN, AlN, CrN, MgSiN2) nanocrystals were achieved at 150 °C. Here, the exothermic reaction of metal magnesium hydrolysis is utilized to assist the reaction in sealed stainless steel autoclaves. In situ temperature monitoring showed that the inside temperature increased quickly from 139 to 902 °C at the initial stage. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM techniques. The low reaction temperature and cheap raw materials make it possible for large-scale synthesis of those nanomaterials.

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INTRODUCTION Borides, carbides, and nitrides exhibit an unusual combination of physical and chemical properties including high thermal, mechanical, and chemical stabilities, high chemical inertness, wide band gap, high electrical conductivity owing to their features of strong chemical bonds, close-packed lattice, homogeneous and dense structure, etc.1−5 They comprise a class of applications, such as thermionic electron emission sources (LaB6, CeB6), electroanalysis (TiB2, ZrB2, TiN), catalysts (W2C, Mo2C, VC, NbN, MoN),6−10 interconnects in integrated circuits (TiC, NdC), electronic devices (SiC), lubricants and optical coatings (BN),11 high-temperature refractory ceramics (MgSiN2), and Li ion battery materials (SiB3, VN, Ti3C2Tx).12−16 Devising synthesis routes for these borides, carbides, and nitrides and studying their properties and structural polymorphism are active and important areas of current research.17−23 Traditionally, most of these nitrides, carbides, and borides were fabricated by high-temperature conditions, such as solidstate metathesis (SSM, 500−1000 °C),24−26 chemical vapor deposition (CVD, 550−1500 °C),27−29 self-propagating hightemperature synthesis (SHS, 800−1300 °C), etc.30−33 However, those processes usually require high temperatures and long reaction durations. New synthesis routes have been explored to prepare nitrides, carbides, and borides by using © XXXX American Chemical Society

special chemicals via simple and convenient processes at mild temperatures.34,35 Recently, we successfully developed borides, carbides, and nitrides via additive-assisted approaches, such as nitrides (TiN, ZrN, BN, and AlN) have been prepared by using the corresponding elements (Ti, Zr, B, Al) with the assistance of NaN3 and sulfur in a stainless steel autoclave at 250 °C,36,37 and borides (LaB6, CaB6, PrB6, and NdB6) through the coreduction of rare-earth oxides and boric acid by Mg powder with the assistance of I2 at 70−250 °C.38 Compared with those traditional high-temperature methods, the additive-assisted methods can ignite the reaction at a relatively low temperature and are more convenient and efficient. No matter in SSM or additive-assisted synthesis approaches, various chloride- or azide-based precursors (SiCl4, TiCl4, CCl4, and NaN3) have been used to produce metal boride, carbide, and nitride powders.39−41 Compared with halides used in these routes, oxides are more chemically stable, cheaper, and more common in nature. However, owing to their stable chemical properties, the reduction and transformation of oxides always need high temperatures (>1000 °C) and high-activity reducing agents (H2, CO, C, Li, Na, Mg, etc.).42−46 This prompted us to consider the possible use of metal oxide as starting materials in Received: October 14, 2016

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DOI: 10.1021/acs.inorgchem.6b02501 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

temperature to 150 °C at a rate of 10 °C min−1 and kept for 2 h. After that, the autoclave was allowed to cool naturally, and the raw materials were washed with hydrochloric acid (0.2 mol/L), annealed in an air atmosphere at 600 °C for 2 h, and then treated with mixed acid (hot hydrofluoric acid/concentrated nitric acid = 3/1), distilled water, and ethanol and, through centrifugal processing, finally dried in a vacuum oven at 60 °C for 8 h, after which gray-white powders were obtained. Through similar procedures, other carbides were synthesized (only with slightly adjusted proportions of the raw materials). Experimental details and the information about the products are summarized in Table 2.

the synthesis of boride, carbide, and nitride materials by additive-assisted metathesis at a mild temperature. Here, we report a metal-hydrolysis-assisted synthesis (MHAS) strategy for the synthesis of borides (TiB2, MoB2, DyB4, ErB4, YB4, LaB6, CeB6, SmB6, EuB6), carbides (SiC, TiC, VC, WC, W2C, ZrC, MoC, NbC), and nitrides (TiN, VN, BN, AlN, CrN, MgSiN2) from related metal oxides in a lowtemperature range (120−180 °C). The exact mechanism is proposed as follows: The Mg hydrolysis reaction starts at 139 °C, with the released H2 and heat energy, oxides can be deoxidized by reducing agents (Mg, H2, active carbon or amorphous boron) and transformed into related compounds. In this study, a large number of metals and compounds are accessible, including metals and metal hydrides (Si, Mo, W, Zr, ZrH2, TiH2). A self-made temperature-testing instrument is used to observe the reactions inside the autoclave. In situ temperature monitoring showed that the inner system will experience a temperature shock from 139 °C to a temperature above 900 °C and then decline sharply to 200 °C within several seconds.

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Table 2. Experimental Conditions, Structural Information, and Morphologies of the As-Obtained Carbides

EXPERIMENTAL SECTION

Reagents and Synthesis. The raw materials were commercial reagents of TiO2 (amorphous), SiO2, Al2O3, Cr2O3, WO3, ZrO2, Nb2O5, MoO3, Y2O3, La2O3, CeO2, Pr2O3, Sm2O3, Eu2O3, Dy2O3, Er2O3, Mg (200 mesh, >99.5%), activated carbon powder, amorphous boron powder (>99.5%), NH3·H2O (25−28%), anhydrous ethanol, etc. Distilled water was self-made, while amorphous boron powder was purchased from Liaobin Chemical Co. Ltd. (Yingkou, China). All the other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. Preparation of Nitrides. An MHAS strategy was used to fabricate nitrides, carbides, and boride nanocrystals. For the synthesis of TiN: TiO2 (0.799 g, 0.01 mol) and magnesium powder (4.321 g, 0.1 mol) were homogeneously mixed into a 20 mL stainless steel autoclave, and then 2 mL of ammonia was added. Subsequently, the autoclave was sealed and heated from room temperature to 150 °C at a rate of 10 °C min−1 and kept for 2 h. After that, the autoclave was allowed to cool naturally. The raw products were washed in turn with hydrochloric acid (0.2 mol/L), distilled water, and ethanol and, through centrifugal processing, finally dried in a vacuum oven at 60 °C for 8 h. Finally, black-brown powders were obtained. By similar procedures, other nitrides (BN, AlN, CrN, VN, MgSiN2) were synthesized (only with slightly adjusted raw material proportions). Experimental details and the information about the products are listed in Table 1. Preparation of Carbides. For the synthesis of SiC: SiO2 (0.601 g, 0.01 mol), activated carbon powder (0.360 g, 0.03 mol), and Mg powders (5.185 g, 0.12 mol) were homogeneously mixed into a 20 mL stainless steel autoclave, and then 2 mL of distilled water was added. Subsequently, the autoclave was sealed and heated from room

molar ratioa

composition

TiO2

0.01:0.1:2

TiN

V2O5 B2O3

0.005:0.1:2 0.01:0.1:2

VN h-BN

Al2O3

0.003:0.12:2

Cr2O3 SiO2

0.003:0.1:2 0.01:0.1:2

h-AlN, cAlN CrN MgSiN2

molar ratioa

SiO2

1:3:12:10

TiO2 V2O5 ZrO2 W2O3 MoO3 Nb2O5

1:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10

composition 2H-SiC, 4HSiC TiC VC ZrC WC, W2C MoC, Mo2C NbC

morphologies 100−200 nm nanosheet (thickness) ∼100 nm cuboid particles ∼300 nm polygon particles ∼500 nm polygon particles 0.2−1 μm scale polygon particles ∼500 nm polygon particles 0.5−2 μm scale polygon particles

a

Refers to oxides (mol)/activated carbon powder (mol)/Mg (mol)/ distilled water (mol).

Preparation of Borides. For the synthesis of LaB6: La2O3 (1.303 g, 0.004 mol), amorphous boron powder (0.324 g, 0.03 mol) and Mg powders (5.185 g, 0.12 mol) were homogeneously mixed into a 20 mL stainless steel autoclave, and then 10 mL of distilled water was added. Subsequently, the autoclave was sealed and heated from room temperature to 150 °C at a rate of 10 °C min−1 and kept for 2 h. After that, the autoclave was allowed to cool naturally, and the raw materials were washed in turn with hydrochloric acid (0.2 mol/L), distilled water, and ethanol and, through centrifugal processing, finally dried in a vacuum oven at 60 °C for 8 h. Finally, dark purple powders were obtained. By similar procedures, other borides were synthesized (only with slightly adjusted proportions of the raw materials). Experimental details and the information about the products are listed in Table 3.

Table 3. Experimental Conditions, Structural Information, and Morphologies of the As-Obtained Borides

Table 1. Experimental Conditions, Structural Information, and Morphologies of the As-Obtained Nitrides oxides

oxides

oxides

molar ratioa

composition

morphologies

TiO2 MoO3 Dy2O3 Er2O3 Y2O3 La2O3 Ce2O3 Sm2O3 Eu2O3

1:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10 0.4:3:12:10

TiB2 MoB2 DyB4 ErB4 YB4 LaB6 CeB6 SmB6 EuB6

∼200 nm spherical particles ∼200 nm spherical particles ∼300 nm polygon particles ∼500 nm polygon particles ∼500 nm polygon particles 0.5−2 μm polygon particles ∼200 nm polygon particles ∼500 nm polygon particles ∼500 nm polygon particles

a

Refers to oxides (mol)/amorphous boron powder (mol)/Mg (mol)/ distilled water (mol).

morphologies 0.5−1 μm dendrites, ∼500 nm grains ∼200 nm grains 10−30 nm macroporous membranes (thickness) ∼200 nm grains

Caution! Before reaction, all the autoclave must be sealed tightly, and the autoclave can withstand high temperature and high pressure. During the reaction, the inner pressure would experience a highest pressure to 49.15 MPa (900 °C), and then drop to 101.34 KPa af ter the reaction at a room temperature. The autoclaves should be opened caref ully. Characterization. The final products were characterized by X-ray powder diffraction (XRD) measurements, which were determined on a Bruker D8 advanced X-ray diffractometer equipped with graphitemonochromated Cu Kα radiation (λ = 1.5418 Å). The morphology and structure of the products were investigated by scanning electron

∼100 nm grains ∼200 nm grains

a

Refers to oxides (mol)/Mg (mol)/ammonia (mL) [28% ammonia contains ∼0.03 mol NH3 molecule]. B


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Inorganic Chemistry microscopy (FESEM, JEOL SM-6700F), transmission electron microscopy (TEM, Hitachi H-7000), and high-resolution transmission electron microscope (HRTEM, JEOL 2100, 200 kV).

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RESULTS AND DISCUSSION Formation and Characterization of Nitrides. Figure 1 shows the typical XRD patterns of the nitrides (TiN, BN, AlN,

Figure 1. Typical XRD patterns of the synthesized nitrides: (a) TiN (JCPDS No. 38−1420, a1 = 4.245 Å), (b) VN (JCPDS No. 25−1252, c = 4.082 Å), (c) BN (JCPDS No. 34−0421, a2 = 2.510 Å, b2 = 6.659 Å), (d) h-AlN (JCPDS No. 25−1133), c-AlN (JCPDS No. 46−1200), (e) CrN (JCPDS No. 03−1152, c = 4.141 Å), (f) MgSiN2 (JCPDS No. 25−0530).

Figure 2. SEM images of the obtained macroporous BN by MHAS. (inset, upper right) Lattice fringe of HRTEM. (inset, lower left) The SAED pattern.

VN, CrN) prepared via MHAS at 150 °C for 2 h. The calculated cell parameters of these nitrides (listed in the caption of Figure 1) are in agreement with their known values. In the contrast experiments, no significant peak shift was observed when the treatment temperature (150 °C) and time (2 h) were increased, but the diffraction intensity of the peaks increases slightly in part of the nitrides (BN, AlN, CrN). Besides, MgSiN2 is detected in the case of SiO2, which indicated the present route can also be applied to the synthesis of other functional materials. FESEM images (Figure 2) show the product of h-BN (with a micrometer scale in diameter size and thickness of less than 10 nm) in macroporous membranes. As depicted in HRTEM images, the edge for alternating plates were a regular layered crystal structure, and the 0.33 nm distances of neighboring lattice fringes coincide with the (002) lattice spacings of h-BN (JCPDS No. 34−0421). Selection electron diffraction in the lower-left corner of Figure 2 shows that the BN flake with polycrystalline structure47 and diffraction ring, respectively, are indicators for the (002), (101), (102) crystal plane of h-BN (JCPDS No. 34−0421). Details and the morphologies of other carbides are summarized in Table 1. Formation and Characterization of Carbides. This MHAS strategy was also applied to prepare related carbides (SiC, TiC, VC, WC, W2C, ZrC, MoC, NbC). Most of the products could be indexed as the corresponding carbides in terms of XRD patterns (Figure 3). However, unlike TiC, VC, and NbC with high crystallinity and purity, the reaction of ZrO2 and C in our case is not entirely converted to ZrC, but resulted in the mixture of ZrC and ZrO2. It was found that both WC and W2C were observed in the product, which belonged principally to hexagonal crystal system. In the structure of tungsten carbide, it is usually considered that half of the W atoms in octahedral voids are occupied by C atoms. Therefore, WC is usually accompanied by a small amount of W2C under

Figure 3. Typical XRD patterns of the prepared carbides: (a) 2H-SiC (JCPDS No. 29−1130) and 4H-SiC (JCPDS No. 22−1317), (b) TiC (JCPDS No. 65−8807), (c)VC (JCPDS No. 65−8825), (d) ZrC (JCPDS No. 65−8834), (e) WC (JCPDS No. 65−8828) and W2C (JCPDS No. 65−8829), (f) Mo2C (JCPDS No. 15−0457) and MoC (JCPDS No. 65−6664), (g) NbC (JCPDS No. 65−7964), the diffraction peak near 27° can be indexed to be graphite.

low temperature or unstable synthesis conditions. All the carbides in Figure 3 were obtained at 150 °C. Corresponding JCPDS card number of these carbides (listed in the caption of Figure 3) are in agreement with their known values. According to both TEM and SEM images (Figure 4) of 2HSiC, the product presents flakelike shapes with the thickness of 100−200 nm and diameters of up to several micrometers. The HRTEM shows the edge of the flake was a two-dimensional structured lattice, and the 0.25 nm × 0.25 nm lattice spacing corresponds to the (002) planes of 2H-SiC (JCPDS No. 29− 1130) with the angle of 60°. Selection electron diffraction in the C


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Inorganic Chemistry

Figure 4. TEM images of the synthesized macroporous SiC by MHAS. (inset, upper right) Lattice fringe HRTEM and SEM images. (inset, lower left) The SAED pattern.

lower-left corner of Figure 4 shows that the SiC flakes are single crystals.48,49 According to the special structural properties of the hexagonal mesophases material, the index of selected area electron diffraction (SAED; Figure 4) can be standardized. Details and the morphologies of other carbides are summarized in Table 2. Formation and Characterization of Borides. The crystallinity and purity of the obtained borides were investigated by powder XRD. Figure 5 shows the typical XRD patterns of the borides (TiB2, MoB2, DyB4, ErB4, YB4, LaB6, CeB6, SmB6, EuB6) prepared via MHAS. Most of the XRD patterns show sharp and well-defined peaks, which indicated the high crystallinity of the products, except the MoB2 and metallic Mo were obtained at the same time. The LaB6, CeB6, SmB6, and EuB6 can be indexed in the cubic crystal system (space group: Pm3m), and the diffraction peaks can be assigned to the lattice planes of (100), (110), (111), (200), (210), (211), (220), (300), and (310). All the borides in Figure 5 were obtained at 150 °C for 2 h. The calculated cell parameters of these borides (listed in the caption of Figure 5) are in agreement with their known values. In the contrast experiments, no significant peak shift was observed when the treatment temperature and time were increased, but the diffraction intensity of the peaks increases slightly in part of the borides (TiB2, MoB2). The preferred morphology of the as-obtained borides was also confirmed by TEM and SEM characterizations (Figure 6) and reveals the LaB6 consisting of a large number of rectangular cubes with sizes ranging from 500 to 1000 nm. As shown in TEM, the growth directions are determined along the [001] direction, and the terminate face is the (001) plane. Welldefined lattice fringes of the LaB6 indicate the high crystallinity of the hexaborides. The lattice spacing of 0.42 nm × 0.42 nm with the angle of 90° for LaB6 match well with the standard JCPDS (No. 34−0427) values, which correspond to the (100) lattice plane of LaB6. The SAED pattern of LaB6 recorded in

Figure 5. Typical XRD patterns of the synthesized borides: (a) TiB2 (JCPDS No. 65−8698, a = 3.0367, b = 3.239), (b) MoB2 (JCPDS No. 65−8684, a = 3.039, b = 3.064), (c) DyB4 (JCPDS No. 24−1350, a = 7.099, b = 4.013), (d) ErB4 (JCPDS No. 24−1077, a = 7.065, b = 3.990), (e) YB4 (JCPDS No. 07−0057, a = 7.084, c = 4.011), (f) LaB6 (JCPDS No. 34−0427, c = 4.1562), (g) CeB6 (JCPDS No. 38−1455, c = 4.1415), (h) SmB6 (JCPDS No. 65−3293, c = 4.125), (i) EuB6 (JCPDS No. 40−1308, c = 4.182).

Figure 6. TEM images of the synthesized bulk LaB6 by MHAS. (inset) Illustrations for the corresponding images of SEM, lattice fringes, and the results of SAED.

D


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Inorganic Chemistry

Figure 7. (a) Schematic diagram of the autoclave. (b) The temperature evolution inside the autoclave in different experiments: Mg + H2O (black line), Mg + H2O + TiO2 (red line).

experience a gradual phase transformation from anatase (JCPDS No. 21−1272) to rutile (JCPDS No. 21−1276). On the one side, it suggested that too much ammonia causes excess NH3 in reactant, but this does not support the direct reaction of TiO2 and NH3 in mechanism (Path II). On the other side, too much water in the reactant increases the conversion of Mg into Mg(OH)2 or MgO and decreases the reduction effects of Mg and the formation of TiN. The remaining part of anatase gradually transformed into rutile with thermodynamic stability in exothermic reaction process. To test the feasibility and effectiveness of this route, we tried a series of oxides without adding a reducing raw material (B, C, and ammonia) via this MHAS strategy. Results showed that the reduction of oxides (TiO2, SiO2, MoO3, WO3, ZrO2) into related elemental substances or metal hydrides (TiH2, Si, Mo, W, Zr, ZrH2) were also successfully extended. Figure 8 shows

the [001] axis shows that the LaB6 have polycrystalline structure.50,51 According to the characteristic of face-centered cubic structure, the index of SAED (Figure 6) can be standardized. The remaining parameters of other borides can be referred to in Table 3. Chemical Reactions of MHAS. We took the MHAS fabrications of TiN, TiC, and TiB2 from TiO2 as analysis examples. To reveal the underlying reaction mechanism, the thermocouple was inserted into the autoclave to monitor the actual temperature change inside the sample (Figure 7a). The temperature evolution at the beginning of the different reactions is shown in Figure 7b. It was found that the hydrolysis reaction of Mg started at 139 °C and caused the inside temperature increase quickly to 829 °C within minutes, then drop to 200 °C several minutes later. This temperature jump did not happen with the absence of Mg or H2O, suggesting this phenomenon is closely related with the Mg powders hydrolysis. And the inside temperature could reach 902 °C, when TiO2 was mixed with Mg and H2O into the autoclave (red line in Figure 7b). Similar phenomena existed in other temperature evolution testing along with MHAS reactions. On the basis of the above results, two possible reaction paths could happen in the experiment. Rapid hydrolysis oxidation reaction of magnesium powder releases the heat and H2 at ∼139 °C [eq 1, ΔGΘ(1) = −332.90 kJ mol−1], which initiated the transformation reactions of TiO2 with two possible reaction paths according to (Path I), (Path II): Mg + H 2O → MgO + H 2

(1)

Path I: TiO2 will be reduced into metallic Ti when there were enough magnesium sources [eq 2], and then Ti reacted directly with NH3 to fabricate corresponding TiN [eq 3]. The general equation as shown in eq 4 (ΔGΘ(4) = −1009.08 kJ mol−1). When the reaction has no NH3, the new formed Ti will turn to TiH2 [eq 5] instead of TiN [eq 3]. TiO2 + 2Mg/H 2 → Ti + 2MgO/H 2O

(2)

Ti + NH3 → TiN + 3/2H 2

(3)

Figure 8. Typical XRD patterns of the reduced products: (a) the raw product before being washed, (b) TiH2 (JCPDS No. 65−0708), (c) Si (JCPDS No. 27−1402), (d) Mo (JCPDS No. 65−7442), (e) W (JCPDS No. 04−0806), (f) Zr (JCPDS No. 26−1399).

the typical XRD patterns of the substances: the raw product is a mixture of TiH2, MgO, and Mg when the raw products without being washed (Figure 8a); the following are patterns of pure nitrides: (b) TiH2 (JCPDS No. 65−0708), (c) Si (JCPDS No. 27−1402), (d) Mo (JCPDS No. 65−7442), (e) W (JCPDS No. 04−0806), (f) Zr (JCPDS No. 26−1399). The calculated cell parameters of these products are in agreement with their known values. The TiH2, Si, Mo, W, Zr were obtained at 150 °C for 2 h; no significant peak shift was observed when the treatment temperature and time were increased, but the diffraction intensity of the peaks increases slightly in Zr or TiH2. SEM images (Supporting Information, Figure S1a) show the product of TiH2 in spherical particles with rough surface and the average particle size is ∼200 nm. The SAED

TiO2 + 3Mg + H 2O + NH3 → TiN + 3MgO + 5/2H 2 (4)

Ti + H 2 → TiH 2

(5)

Path II: Exothermic reaction of magnesium hydrolysis supports the direct transformation from TiO2 to TiN [eq 6]. 6TiO2 + 8NH3 → 6TiN + 12H 2O + N2

(6)

When the ratio of ammonia (mL) and magnesium powder (g) is increased to 5:0.1, the TiO2 raw materials would E


Article

Inorganic Chemistry (Supporting Information, Figure S1b) and HRTEM (Supporting Information, Figure S1c) illustrations indicated the TiH2 with highly crystallinity. It is important to note that SiO2 system accompany with Si would form a small amount of Mg2Si (Supporting Information, Figure S2). Caution! The reaction between Mg and H2O can occur around 139 °C, and plenty of H2 will be released; slight Mg2Si (byproduct of the reaction between SiO2 and Mg) can react intensively with acid during the purif ication of the raw products. Besides, corresponding productions of boride, carbide, and nitride would be obtained successfully through the MHAS reaction with the elemental powder (Si, Ti, Zr, W) as raw materials at 150 °C, which further confirmed the rationality of the (Path I). Amount them, SiC products with similar size and morphologies were founded by Si powder. XRD result (Supporting Information, Figure S2) shows a higher ratio of 4H-SiC in the product compared to the process of using SiO2 as raw materials. Furthermore, the eq 2 has been reported in the synthesis of nitrides, carbides, and borides through selfpropagating high-temperature synthesis (SHS) process.24,30,52−54 In summary, we believe that the reaction process (Path I) of reduction first and then transformation would be more reasonable in actual case. The general equation (untrimmed) can be described as follows:

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (L. S. Yang) *E-mail: [email protected]. (L. Q. Xu) ORCID

Bo Chen: 0000-0001-6743-9251 Liqiang Xu: 0000-0002-0453-120X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (21471091), Hunan Provincial Natural Science Foundation of China (2015JJ4035), Scientific Research Fund of Hunan Provincial Education Department (14B118) in China, Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization, and the Taishan Scholar Project of Shandong Province (No. ts201511004).

TiO2 + Mg + H 2O + NH3/C/B → TiN/TiC/TiB2 + MgO + H 2 + CO2 /CO/B2O3

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(7)

However, the solid-state reaction is often controlled by the reaction thermodynamics and kinetics,55 so the reaction mentioned in (Path II) could occur when dynamics is feasible. The transformation mechanism of other oxides in this test was similar. As mentioned in previous reaction system, we obtained the relative TiB2, TiC, and TiN in adding reducing raw materials (B, C, and ammonia) via this MHAS strategy selectively. The crystal structure and morphology can be found in Supporting Information Figure S3, with the corresponding XRD results (Figure 5a, Figure 3b, and Figure 1a). Fast Fourier transform illustrations provided further proof of the crystal plane orientation of TiB2, TiC, and TiN.

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CONCLUSIONS In this study, a general route for the reduction and transformation of oxides into related boride (TiB2, MoB2, DyB4, ErB4, YB4, LaB6, CeB6, SmB6, EuB6), carbide (SiC, TiC, VC, WC, W2C, ZrC, MoC, NbC), and nitride (TiN, VN, BN, AlN, CrN, MgSiN2) nanocrystals via a MHAS process has been successfully developed. The sharp increase in reaction temperature (from 139 to 902 °C) is attributed to the exothermic reaction between the additives Mg and H2O. The as-obtained products were characterized by XRD, SEM, TEM, and HRTEM techniques. The specially designed route can provide a guide for the preparation of solid-state compounds with structured on nano or mesoscale and give these samples a good application prospect.

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The SEM, TEM, FFT, and lattice fringe images of TiH2. The XRD patterns of the synthesized SiC from Si powders. The HRTEM images of MHAS preparation of TiB2, TiC, and TiN nanomaterials and their corresponding fast Fourier transform images. Related Gibbs free energy for representative reactions (PDF)

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General Fabrication of Boride, Carbide, and Nitride Nanocrystals via a

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