Preparation of Boron Nitride Nanoparticles with Oxygen Doping and a

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Preparation of Boron Nitride Nanoparticles with Oxygen Doping and a Study of Their Room-Temperature Ferromagnetism Qing Lu, Qi Zhao, Tianye Yang, Chengbo Zhai, Dongxue Wang, and Mingzhe Zhang* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, oxygen-doped boron nitride nanoparticles with roomtemperature ferromagnetism have been synthesized by a new, facile, and efficient method. There are no metal magnetic impurities in the nanoparticles analyzed by X-ray photoelectron spectroscopy. The boron nitride nanoparticles exhibit a parabolic shape with increase in the reaction time. The saturation magnetization value reaches a maximum of 0.2975 emu g−1 at 300 K when the reaction time is 12 h, indicating that the Curie temperature (TC) is higher than 300 K. Combined with first-principles calculation, the coupling between B 2p orbital, N 2p orbital, and O 2p orbital in the conduction bands is the main origin of room-temperature ferromagnetism and also proves that the magnetic moment changes according the oxygen-doping content change. Compared with other room temperature ferromagnetic semiconductors, boron nitride nanoparticles have widely potential applications in spintronic devices because of high temperature oxidation resistance and excellent chemical stability. KEYWORDS: boron nitride nanoparticles, oxygen-doping, room temperature ferromagnetic, first-principles calculations, spin-polarized

1. INTRODUCTION Diluted semiconductors can combine information processing with information storage functions by using the charge characteristics and spin characteristics of electrons at the same time.1−3 Especially, dilute magnetic semiconductors with room-temperature ferromagnetism and high enough magnetic saturation strength can potentially be widely used in spin quantum devices, magnetic storage devices, and microelectronics industries.4−9 In recent years, graphene material has received increasing attention by the scientific and engineering communities.10,11 Theoretical calculations and experiments showed that graphene and related materials could also produce the ferromagnetism signal;12,13 traditionally, magnetism in condensed phases is attributed to 3d or 4f electrons in transition metals,14−17 rare earth-doped sulfides,18,19 and the like. Graphene and related metal-free materials attracted more attention. Most important, some graphene materials even display relatively high Curie temperature.20,21 Hexagonal boron nitride (h-BN), which is a structural analogue graphite, has more important advantageous properties than graphene, such as high thermal conductivity,22 biological applicability,23−25 anti-oxidation ability, chemical inertness, and electrical insulation.26,27 Theoretical calculations have already demonstrated that there are a number of factors that can change the magnetic state of boron nitride (BN), such as defects,28 fluorination,29,30 and doping.31,32 For example, An et al.33 investigated the possible magnetic properties of three types of BN atomic chains, BnNn, BnNn−1, and BnNn+1 types; it was shown that all the three types of isolated BN atomic chains are magnetic. Wu et al.34 found that replacing carbon with one boron or one nitrogen atom in © XXXX American Chemical Society

the BN nanotubes can lead to spontaneous magnetization. Recently, there are article reports of magnetism in BN nanosheets. For example, Si et al.35 reported the observed room-temperature ferromagnetism of the fluorinated h-BN nanocages. Weng et al.36 have found that the BN nanosheets with oxygen doping have paramagnetism. However, to our best knowledge, the origin of room-temperature ferromagnetism of oxygen-doped BN nanoparticles is not very clear. Therefore, it is of great significance to develop a feasible method to synthesize oxygen-doped BN nanoparticles and analyze the corresponding origin of ferromagnetism. Herein, we present a new, facile, and efficient method to synthesize oxygen-doped BN nanoparticles. This synthesis method has several advantages: it has two steps, the raw materials are easy to obtain, and it is inexpensive. The structural, optical, and magnetic properties of the product are analyzed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and superconducting quantum interference device (SQUID). Using the Vienna Ab initio Simulation Package (VASP) based on the density functional theory (DFT), the spin polarization density of states (DOS) theoretical calculations were performed, which can explain the origin of magnetism of the synthesized BN nanoparticles. Received: November 24, 2017 Accepted: March 29, 2018

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DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All the reagents (guaranteed reagent) were used as purchased without further purification. The deionized water was used in the whole experiment. In a typical process, the whole experiment can be divided into two steps: the preparation of ammonium borate hydrate (NH4B5O8·4H2O) particles as a precursor and sintering of NH4B5O8·4H2O particles at 900 °C in the ammonia atmosphere to obtain BN nanoparticles. In the first step, 0.05 mol of B(OH)3 was dissolved into 100 mL of deionized water to form a uniform transparent solution with stirring. Subsequently, the solution was moved to the reaction chamber and then reacted with the flowing NH3 gas at a rate of 60 mL/min, which was placed in an ultrasonic bath to avoid particle agglomeration and promote the formation of a new liquid surface. At the beginning, a lot of nuclei were formed and began to grow at the gas−liquid interface. Under the influence of ultrasound, the particles were dispersed in the solution and the new liquid surface appeared. This process was repeated until the reaction was complete. Last, the reaction solution was dried in vacuum at 60 °C and white NH4B5O8·4H2O particles were obtained. It could be seen that the diffraction peaks were consistent with JCPDS card files no. 3143, as shown in Figure S1. In the second step, NH4B5O8·4H2O particles were placed in an alumina boat. The boat was then loaded into a horizontal quartz tube and heated at 900 °C for different reaction times at a heating rate of 15 °C min−1 in ammonia atmosphere. After the reaction, the quartz tube was cooled to room temperature naturally in ammonia atmosphere. The prepared white products were gathered and were centrifuged at 10 000 rpm for 3 min with aqueous solution of hydrochloric acids, deionized water, and anhydrous ethanol twice, and impurities were successfully removed. White BN nanoparticles were obtained after being dried in air. 2.2. Characterization. The purity and the structure of the phase of the prepared powder were characterized by XRD with Cu Kα1 radiation (λ = 1.54056 Å) in the range of 10−90° (2θ) at a scanning rate of 4°/min. To further analyze the microstructures and size of the sample, field-emission SEM images were obtained by using a Magellan 400 FEI microscope operating at 5 kV. The TEM, SAED, and HRTEM images were performed on a JEOL JEM-2200FS microscope. XPS (ESCALAB MK II spectrometer) can examined the chemical state of the sample. Fourier transform infrared (FT-IR) spectroscopy spectra were collected on a Nicolet 6700 FT-IR spectrophotometer. UV−vis spectra were recorded using a UV-3150 spectrophotometer at room temperature. The excitation and emission spectra have been analyzed by a FluoroMax-4 fluorescence spectrophotometer (HORIBA Scientific) equipped with a 450 W xenon arc lamp. The initial magnetization curve in the range of 0−6000 Oe and magnetizations versus applied magnetic field (M−H) in the range of −6000 to 6000 Oe were both tested on a vibrating sample magnetometer (VSM) (Lakeshore model 7410) at 300 K. The zero-field-cooled/field-cooled (ZFC/FC) curves were carried out by using SQUID (Quantum Design). The specific procedure was as follows: (a) temperature was first set to 2 K at zero magnetic fields, and then warming process was investigated as temperature gradually increased to 300 K at a 500 Oe magnetic field to obtain the ZFC curve; (b) temperature was set to 2 K at the 500 Oe magnetic field initially, and the warming process was done with temperature increasing to 300 K to obtain the FC curve. 2.3. First-Principles Calculations. Using the VASP based on DFT, the spin polarization DOS theoretical calculations were performed. The exchange and correlation functions are realized by the generalized gradient approximation (GGA) and the Perdew− Burke−Ernzerhof (PBE) gradient correction function. In the calculations, a supercell containing 50 atoms and a cutoff energy of 420 eV was used. The Brillouin zone was modeled with a k-point mesh of 2 × 2 × 1 by the scheme of Monkhorst−Pack.

Figure 1. Structural characteristics and elemental analysis of synthesized BN nanoparticles. (a) SEM image of BN nanoparticles. The inset is the high magnification. (b) TEM image of BN nanoparticles. (c) XRD pattern of BN nanoparticles. The inset is the SAED pattern. (d) HRTEM of BN nanoparticles.

nanoparticles are shown in Figure 1c. They are wellcorresponding to JCPDS no. 45-0893, and the BN nanoparticles have the hexagonal structure with the lattice constant a = 2.502 Å and c = 6.66 Å, space group no. 164. Furthermore, there are no other peaks except the three peaks at the (002), (100), and (004) reflection positions, which illustrates that the product doesn’t have other compounds in addition to boron nitride. SAED and HRTEM can further provide more information on the structures of BN nanoparticles. The SAED image (in the inset of Figure 1c) shows three obvious diffraction rings consistent with the (002), (100), and (004) crystal planes of the h-BN structure. The HRTEM image (Figure 1d) provides more information on the BN nanoparticle structures and shows clear lattice fringes that are about 0.333 nm which correspond to the (002) plane of the h-BN structure (JCPDS card no. 45-893). The bonding nature of the synthesized BN nanoparticles is also further illustrated by IR spectroscopy, as shown in Figure 2a. The BN nanoparticles display two sharp absorption peaks at 780 and 1400 cm−1, which is consistent with out-of-plane B− N−B bending vibration and in-plane B−N stretching vibration, respectively.37 There are extra three absorption peaks at 930, 1105, and 3385 cm−1, which are ascribed to B−N−O bending, B−O bending, and O−H bending, respectively.38 The other absorption peaks at 1020 and 2362 cm−1 are ascribed to C−O bending. The B−O bending confirmed oxygen doping in the BN nanoparticles. The broad absorbed absorption peak at 3385 cm−1 is attributed to water absorbed on the sample. The presence of B−O bonds in the BN nanoparticles is also confirmed by XPS. Figure 2b shows the XPS full survey spectrum of the BN nanoparticles where the peaks of B, N, O, and C are clearly observed. Figure 2c shows two peaks at 191.2 and 192 eV in the high-resolution B 1s spectrum of BN nanoparticles, which are consistent with B−N and B−O bonds,

3. RESULTS AND DISCUSSION The BN nanoparticles were synthesized under 900 °C for 12 h. SEM and TEM images of the nanoparticles (Figure 1a,b) clearly illustrate that the sample has a flake-like morphology with a diameter from 50 to 70 nm. The XRD patterns of BN B

DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) FT-IR spectra of BN nanoparticles. (b) XPS spectra of the BN nanoparticles. (c) B 1s XPS spectra of the BN nanoparticles. (d) N 1s XPS spectra of the BN nanoparticles.

Figure 3. (a) UV−vis spectra of oxygen-doped BN nanoparticles. (b) PL spectra of oxygen-doped BN nanoparticles.

At 900 °C, a small amount of ammonia decomposes, and the undecomposed ammonia is in a metastable and highly reactive condition.41 The ammonia adsorbed on the surface of ammonium borate diffuses into the interior of ammonium borate because of the content difference. According to Fick’s law, with the increase of time, the number of ammonia gas molecules diffused into the interior of ammonium borate increased, and the oxygen atoms in ammonium borate are gradually nitrided by ammonia, inducing formation of B−N bonds,38 as shown in Figure 4. However, parts of the boron− oxygen covalent bond are not completely broken, inducing the incomplete nitride process. That is, there are a few incomplete nitride oxygen atoms remaining at the boron−oxygen covalent bond, resulting in oxygen doping. With the increase of the reaction time, the oxygen atom content decreases, resulting in the products of different reaction times containing different contents of oxygen doping (ON). The reaction is carried out in an atmosphere of excess ammonia to avoid N vacancy defect (VN) formation. Combined with IR and XPS analysis, the final product contained the B−O bond, which also demonstrated that the as-synthesized product contained O impurities. Before studying the intrinsic ferromagnetism properties of prepared BN nanoparticles, any other magnetic impurities should be excluded in the first place. The XPS spectroscopy

respectively. In addition, Figure 2d illustrates one peak centered at 398.4 eV corresponding to the N 1s spectrum of BN nanoparticles which can be ascribed to B−N bonds. A small peak shown in Figure 2c indicates that O is successfully doped into the BN nanoparticles. The O peak except B−O binds can be attributed to CO2 or H2O which presents in the face of the specimen. The peaks for C can be attributed to the C or CO2 adsorbed on the face of the specimen. Figure 3a shows the results of investigation of the UV−vis absorption spectra of the BN nanoparticles. The standard h-BN only at the deep UV range (≤230 nm) has optical absorption in accordance with the derived band gap value of 5.4 ev.36 The prepared oxygen-doped BN nanoparticles display absorptions within the total UV range and show a strong absorption peak at 262 nm, as shown in Figure 3a. The estimated optical band gap of this compound is 2.7 eV. PL spectroscopy can further illustrate the optical band gap properties. Because the standard h-BN has a wide band gap, it will not display visible PL. It is known that a low level of lattice defects in BN materials can generate PL.39,40 Figure 3b shows the PL spectra of the samples at room temperature. Three emission peaks located at 404, 425, and 450 nm are all observed in the sample, which can be expected to present the O atom doping in BN nanoparticles. C

DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

emu g−1 as the reaction time increases from 3 to 12 h and then decreases with a further increase of the reaction time. The Ms value is much larger than that reported for fluoridation of BN nanosheets and nanocages.29,35 The oxygen-doping content exhibits gradually a decline trend with the increase of the reaction time. According to the XRD patterns of samples 1−6 (as shown in Figure S3), it can be found that the (002) plane presents a slight shift toward the small-angle direction with the increasing reaction time, which indicates the oxygen-doping content of the samples are gradually decreased. This is because the ionic radius of O2− is smaller than N3−. Furthermore, according to the first-principle calculation results, the magnetic moment value of the sample increases initially and then decreases with the sustained reduction of the oxygen-doping content. Accordingly, the magnetic moment of the BN nanoparticles exhibits a parabolic shape with the increase in the reaction time. In this experiment, when the reaction time reached 12 h, the oxygen content decreased to the corresponding maximum magnetic moment. Consequently, controlling the reaction time could efficiently modulate the magnetic moment of BN nanoparticles. The M−H curve showing a typical FM order proved that the long-range magnetic order still controls the BN even at room temperature. To further study the magnetic properties of sample 4, the SQUID test is carried out. Figure 5b shows the ZFC and FC curves of the sample in the field of 500 Oe, with the temperature ranging from 2 to 300 K. For ferromagnetic samples, the magnetic domains of the sample have been frozen in their preferred direction when the temperature dropped to 2 K of the ZFC test, and then the warming process was investigated as temperature gradually increased to 300 K at a 500 Oe magnetic field. Magnetic domains are gradually along the direction of the magnetic field arrangement, and the thermal energy barely weakens magnetizations, thereby inducing increase of magnetization. The ZFC curve reaches the maximum value at 105 K and is defined as the blocking temperature (TB) caused by the interaction between magnetic aeolotropism energy and thermal energy. As measured in the FC curve, the magnetic domains always keep the same direction with the magnetic field. From the inset of Figure 5b, it can be clearly found that ZFC and FC curves are still not coincident until 300 K, which indicates that the thermal energy cannot disturb the magnetic ordered state. The furcation between FC and ZFC magnetization curves indicates that the ferromagnetic transition temperature of the sample is higher

Figure 4. Schematic representation of the intermediate phase formation at different reaction stages, finally generating h-BN with O doping.

confirms no magnetic metal elements in the sample. Therefore, the magnetic impurities could be excluded from the magnetic source. There must be other mechanisms for the magnetic source of the metal-free sample. According to the growth mechanism, oxygen doping will be generated in the synthetic process, and the reaction time is an efficient way to adjust the defect content, which may be the origin of ferromagnetism. Herein, six BN nanoparticle samples obtained under 900 °C at 3 h (sample 1), 6 h (sample 2), 9 h (sample 3), 12 h (sample 4), 15 h (sample 5), 18 h (sample 6) have been selected to study the effect of the reaction time on the ferromagnetism performance. The initial magnetization curve of sample 4 is shown in Figure S2a. The hysteresis loops of samples 1−6 are tested by a VSM at 300 K. As shown in Figure 5a, all the samples exhibit obvious room-temperature ferromagnetic hysteresis phenomena with clear coercivities, suggesting the existence of room-temperature ferromagnetism. As exhibited in the inset I of Figure 5a, the saturation magnetization (Ms) value reaches a maximum value of 0.2975

Figure 5. Ferromagnetism of prepared BN nanoparticles. (a) Hysteresis loops for BN nanoparticles with different reaction times in the field between −6000 and 6000 Oe at 300 K. The Ms with different reaction times is shown in the inset (I). The enlarged view of hysteresis loops with different reaction times in the magnetic field between −200 and 200 Oe at 300 K is shown in the inset (II). (b) ZFC and FC temperature-dependent magnetization curves under an applied magnetic field of 500 Oe at a temperature range of 2−300 K. The inset shows the high magnification of the corresponding intersection. D

DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 6. Spin-polarized-projected DOS of four BN systems. (a) Ideal model with B25N25. (b) ON defects of five O atom substitution for five N atoms (B25N20O5). (c) ON defects of three O atom substitution for three N atoms (B25N22O3). (d) ON defects of one O atom substitution for one N atom (B25N24O1).

moment presents a downward trend which is ascribed to excessive oxygen-doping content. The percolation of the bound magnetic polaron theory can illustrate this phenomenon.42,43 The increased antiferromagnetic interaction between adjacent oxygen ions inhibits the ferromagnetic behavior at high oxygendoping content, resulting in a decrease of the Ms value. Hence, the magnetic moment values correspond to the oxygen-doping content. In addition, as shown in Figure S5, the magnetic moments of six models are 0.32 μB of B25N20O5, 0.948 μB of B25N21O4, 1.101 μB of B25N22O3, 0.737 μB of B25N23O2, 0.341 μB of B25N24O1, and 0 μB of B25N25.

than 300 K. The aforementioned investigations confirm that the as-synthesized samples have room-temperature ferromagnetism. 3.1. First-Principles Calculations and the Magnetic Mechanism. VASP software is used for the theoretical calculation based on the DFT and the projector-augmented wave pseudopotential to reveal the origins of the stable roomtemperature ferromagnetism. The exchange and correlated functions have been achieved by the GGA with the PBE gradient-corrected functions. The Brillouin zone was modeled with a k-point mesh of 2 × 2 × 1 by the scheme of Monkhorst−Pack. A single layer h-BN model is applied in our first-principle calculation. The curvature effects and the interlayer interaction of the BN nanoparticles are not involved in the calculation. A vacuum space is set to be 20 Å to avoid the interaction between adjacent layers. To perform a detailed study on magnetism properties of BN nanostructures with various oxygen contents, six model configurations of the BN nanostructure have been built (Figure S4). Figure 6 displays the spin-polarized DOS spectra of four models. (a) Ideal model (B25N25); (b) five O atom substitution for five N atoms (B25N20O5); (c) three O atom substitution for three N atoms (B25N22O3); (d) one O atom substitution for one N atom (B25N24O1). It can be observed that the ideal model (Figure 6a) presents symmetric total spin-polarized DOS, indicating no resultant spin polarization and magnetic moments. The total DOS and partial DOS of the ON defect are shown in Figure 6b−d. The TDOS confirms that because of the O substitution at the N site, the ON defect induces a significant spin polarization of the valence band near the Fermi level. The origin of magnetic properties of the sample is attributed to mutual coupling of B 2p orbital, N 2p orbital, and O 2p orbital in the conduction band, in which the B 2p orbital dominates and O 2p and N 2p orbitals contribute less. As oxygen-doping content increases, the total magnetic moment exhibits a parabolic shape and reaches a maximum of 1.101 μB of the B25N22O3 model. When the oxygen-doping content continues to increase, the magnetic

4. CONCLUSIONS In summary, the oxygen-doped BN nanoparticles with roomtemperature ferromagnetism have been synthesized via the reaction of ammonia gas and boric acid. The raw materials only contain four elements of B, N, O, and H, and this synthetic approach is greatly conducive to obtain BN nanoparticle products with less impurities. The Ms value reaches a maximum value of 0.2975 emu g−1 as the reaction time is 12 h. Hence, the reaction time is the key factor to modulate the oxygen-doping content of BN nanoparticles and the change in the Ms value is closely related to the oxygen-doping content. The firstprinciples calculations indicates that B25N22O3 has the largest magnetic moment of the six built models and also shows that the room-temperature ferromagnetic properties mainly originate from the mutual coupling between B 2p orbital, N 2p orbital, and O 2p orbital in the conduction bands. This work can provide experimental and theoretical methods for designing the high spin-polarized semiconductor nanocrystals.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17932. Initial magnetization curve, structural characteristics, and calculation models (PDF) E

DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mingzhe Zhang: 0000-0002-6987-5572 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China, no. 11474124. We also acknowledge the High Performance Computing Center of Jilin University for Calculation Resources.



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DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b17932 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX