Template synthesis of novel monolayer B4C ultrathin film

Author’s Accepted Manuscript Template Synthesis of Novel Monolayer B4C Ultrathin Film Lei Zhou, Jian Gao, Yang Liu, Jingshuang Liang, Muhammad Javid, Asif Shah, Xinglong Dong, Hongtao Yu, Xie Quan www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)33033-5 https://doi.org/10.1016/j.ceramint.2018.10.216 CERI19928

To appear in: Ceramics International Received date: 21 June 2018 Revised date: 11 October 2018 Accepted date: 26 October 2018 Cite this article as: Lei Zhou, Jian Gao, Yang Liu, Jingshuang Liang, Muhammad Javid, Asif Shah, Xinglong Dong, Hongtao Yu and Xie Quan, Template Synthesis of Novel Monolayer B4C Ultrathin Film, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.216 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Template Synthesis of Novel Monolayer B4C Ultrathin Film Lei Zhoua, Jian Gaoa, Yang Liua, Jingshuang Lianga, Muhammad Javida, Asif Shahb, Xinglong Dong*a, Hongtao Yuc and Xie Quan*c a Key Laboratory of Materials Modification by Laser, Ion and Electron Beams(Ministry of Education), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China b Department of Metallurgy and Materials Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan c Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Abstract The excellent physical and chemical properties of two-dimensional (2D) ultrathin film of boron carbide (B4C) make it suitable for various applications. Nonetheless, the related fabrication strategies have not been well established yet. In this paper, the branches of dendritic magnesium oxide (MgO) nanofibers have been firstly used to serve as the template for synthesis of high-quality monolayer B4C ultrathin film. As a thermal source, DC arc-discharge plasma was adopted to co-evaporate the raw mixture target comprising of boron, graphite and magnesium oxide species. In the evaporation and subsequent condensation processes, the scaffolds of crossed MgO nanofibers were built at higher temperature prior to heterogeneous nucleation of B4C nanosheets on them. It is indicated that the kneaded B4C films can wrap MgO nanofibers into a round-shaped sphere and were entirely stretched out by removal of MgO scaffolds through a simple water-washing purification. The monolayer B4C ultrathin film is about 1.5 nm in thickness and several micrometers in length. UV-vis diffuse reflectance spectra reveal that the band gap of B4C films is ~1.37 eV, which is around 50% higher than that of normal B4C films. This work describes the formation mechanism of such monolayer B4C ultrathin film in aspect of nucleation/growth processes under the high-temperature plasma conditions. Address correspondence to Xinglong Dong, [email protected]; Xie Quan, [email protected] 

Keywords: MgO scaffolds, B4C ultrathin films, DC arc-discharge plasma, band gap

1 Introduction Boron carbide mostly known as black diamond is the third hardest material after diamond and cubic boron nitride [1]. It was firstly discovered in 1858 [2], and the stoichiometric formula of B4C was later assigned in 1934 [3]. It has rhombohedral crystal structure, which belongs to D3d5-R3m space lattice with the lattice constants a = 0.519 nm, c = 1.12 nm and angle α = 66°18'. Crystallographic unit cell of B4C consists of 12 B11C icosahedral atomic clusters, those are interconnected by covalent bound C-B-C three-atom chains on the diagonal of orthorhombic hexahedron. Boron atoms and carbon atoms can be substituted for each other between icosahedral and three-atom chains (BBB, BCB, CBC, CCC, et al.), which is the reason for so many isomers of boron carbide [4,5]. B4C has been widely used for high-tech applications e.g.

high-temperature

wear-resistant

material,

boron

carbide

thermocouple,

lightweight armor, nuclear reactor control rod, nuclear reaction shielding material, solid-state neutron detector etc. Due to its unique properties such as high Young’s modulus, high melting point, excellent chemical resistance, low stiffness, low density, excellent thermoelectric properties and good cross section absorption of thermal neutrons. Among a large variety of nanostructures, 2D materials can supply a broader platform to venture into largely unexplored regions of the materials science. In comparison with zero or one-dimensional materials, the ultimate thinness makes 2D films extremely promising candidate for the electronic materials [6,7,8]. Meanwhile, 2D materials are relatively easy to be configured into complex structures. Graphene, the typical two-dimensional carbon film [9,10], has been widely studied concerning its rich physics [11,12,13] and its high mobility [14]. Pristine graphene exhibits zero bandgap, nevertheless a proper bandgap is essential for many applications [9]. So far, in the case of p-type semiconductor, low-dimensional boron carbide has been observed as significant among the most promising inorganic materials [15,16]. The

band gap depends on the boron to carbon stoichiometry which makes them potential candidates for electronic devices. In comparison to conventional semiconductors, 2D B4C thin films could be a platform to produce hybrid structures, which can provide high energy density and long lasting durability to betavoltaic devices with improved structural and mechanical properties of the metal matrix composites [17]. So far, researchers have been focused on synthesis techniques for low-dimensional boron carbide materials and much techniques have been developed such as carbon thermal reduction process [18,19], high-energy ball milling [20], self-propagating high temperature synthesis [21], elements of direct synthesis [22], mechanical alloying [23], chemical vapor deposition [24] and low temperature pyrolysis (sol gel method) [25,26]. Out of these all technique, most of the preparation techniques associated with the zero or one-dimensional B4C structures e.g. nanoparticles, nanowires and nanobelts, few methods are being effectively used for the synthesis of 2D B4C thin films. Nersisyan [9] et al. reported the synthesis of high-quality 2D B4C nano sheets by a facile combustion reaction of the mixture comprising boron oxide (B2O3), Mg and polyvinylchioride (C2H3Cl)n. Du [27] et al. synthesized B4C nanoflakes by a thermal reduction on raw bamboo wood i.e. the dried bamboo stems were beforehand ball-milled into powder and dispersed in an ethanol emulsion containing KF, B2O3 and Ni(NO3)26H2O. Later, dried mixture was placed in a sealed graphite boat and heated at 1600℃ to produce B4C nanoflakes. Su [17] et al. used a carbothermal reduction method to prepare B4C nanoflakes, emphasizing the morphological modifications. Although, all mentioned routes had been succeeded in synthesis of 2D boron carbides with drawback of thicknesses that were relatively higher as 10-150 nm, due to the methodological or technological limitations. To best of our knowledge, the reports on ultrathin or even single-layered B4C films in several nanometers thickness had not been developed yet. Mounet [6] et al. ever reported their theoretical estimation on formation of two-dimensional materials, based on 1,825 compounds those were easily or possibly peelable into two-dimensional films, unfortunately B4C was not involved and discussed. It is theoretically infeasible to anticipate the thinner or even single-layered B4C films

probably stripped off from bulks. Synthesis of 2D B4C ultrathin films hence remains a considerable challenge to researchers. In this work, the monolayer B4C ultrathin film has been successfully synthesized with the assistance of a dendritic MgO template which consists of nanoscale fibers. Using arc-discharge plasma as the thermal source, raw materials were co-evaporated into gaseous state, and subsequently undergo nucleation, growth and passivation processes to harvest B4C/MgO precursor powders. A water-washing treatment was carried out on the precursor to remove MgO nanofibers contained, thus the well-stretched monolayer B4C ultrathin films were finally obtained. The structural characterization, optical behavior and formation mechanism of these B4C ultrathin films were studied and discussed. It is anticipated that this simple preparation technique presented here may open a novel way for synthesis of ultrathin film of B4C.

2 Experimental 2.1. Preparation of as-prepared precursor powders The micron-sized amorphous boron, graphite and MgO powders were used as raw materials. These coarse powders were mixed in a molar ratio of 4.0 : 1.0 : 0.2, and then pressed into a bulk target. Later, this bulk target was served as anode in arc-discharge plasma machine and be consumed (evaporated) into the gaseous atoms. A carbon rod with 6 mm in diameter was used as cathode of arc-discharge, and the mixture of Ar/H2 with ratio of 2 : 1 (vol.%) was introduced into reaction chamber during operation. The reaction current and voltage of power supply were set as 90 A and 45 V, respectively. Following the processes of evaporation, nucleation/growth and finally cooling to room temperature, a passivation process was carried out on the fresh powder product for 10 hours in an inert atmosphere containing ~5% of oxygen and finally collected them from the chamber. 2.2. Removal of MgO The as-prepared nanopowders were comprised of B4C/MgO particles, deionized water was used to remove MgO phase. Two samples were prepared, each sample containing 0.5 g of the precursor powders. Later, samples were placed into a beaker

containing 200 ml deionized water, and then ultrasonically stirred at 80℃. After 0.5 hour, Sample-1 taken out and Sample-2 further treated for one hour. After centrifugation process, the upper clarification liquid was poured out and remaining settled down precipitation powder was further dried for 12 hours in a furnace at 60℃. 2.3. Structural characterization The detailed morphology, microstructure, thickness and size of as-prepared B4C/MgO precursor particles and 2D B4C ultrathin films were characterized using X-ray diffractometer with Cu Kα radiation (XRD-6000, Japan), High resolution transmission electron microscopy (Tecnai G20 S-Twin, USA) operated at 200 kV, Field emission scanning electron microscope (SUPARR 55, Germany), and Atomic Force Microscopy (DI-Multimode NS3A-02). An in-Via Raman spectrometer (Renishaw England) was used to analyze the carbon-related structures, using the light source of a laser with wavelength of 632.8 nm and the scanning range of 100~1800 cm-1. X-ray photoelectron spectrometer (ESCALAB 250Xi, Englamd) and UV spectrophotometer (UV-550, Japan) were used to identify the chemical bonds and band gap, respectively.

3 Results and discussion 3.1. Morphology and Crystal structure of MgO scaffold

Figure 1. TEM, HRTEM and XRD results of pure MgO nanofibers synthesized by DC arc-discharge plasma. (a) TEM image of MgO nanofibers. (b) HRTEM image of single MgO nanofiber. (c) Magnified HRTEM image of the crystal lattice of MgO. (d) XRD pattern of MgO nanofibers.

In this work, it is thought that the 2D B4C ultrathin films grew on dendritic surface of the MgO nanofibers. Such nanostructure MgO scaffold was instantaneously generated prior to the nucleation of B4C seeds in the same co-evaporation process under the circumstances of arc-discharge plasma. In order to study the characteristic of MgO nanofibers without extra species such as boron and carbon atoms. Separate powder was prepared under the same preparation condition with that of co-evaporation case, i.e. same atmosphere and power supply of arc-discharge plasma. As shown in Fig. 1(a), the MgO scaffolds consist of crossed nanofibers, with ~15 nm in diameter and 0.5~1.0 m in length range. Surface of MgO nanofibers looks uneven constituted by crystallographic facets (see magnified image of Fig. 1b), which can provide favorable condition for nucleation points for B4C seeds and further growth into films. Details of MgO grain are well displayed in the lattice image as shown in Fig. 1(c) and as mentioned in the selected square region in Fig. 1(b). Fig. 1(d) shows XRD pattern of pure MgO nanofibers without any impurities involved. During fabrication of B4C films by co-evaporation of B, C, Mg and O containing species, it is observed that the combination between Mg and O will be preferentially accomplished due to its higher melting point, resulting in nucleation and growth of MgO nanofibers, which will act as the scaffold template for subsequent growth of B4C films. 3.2. Morphology and Crystal structure of B4C ultrathin film

Figure 2. SEM (a, b, c) and TEM (a, b, c) images of the initial B4C/MgO precursor and its water-washed products. (a, a) the as-prepared B4C/MgO precursor powder. (b, b) the washed powder, operated at 80℃ for 0.5h. (c, c) the washed powders, operated at 80℃ for 1h. (d) AFM image of ultrathin B4C film.

The SEM and TEM images of B4C/MgO precursor powder and its water-washed products show the morphological changes after washing treatment (Fig. 2). As shown in Fig. 2(a, a), the as-prepared B4C/MgO nanoparticles are mostly regular spheres with 50~100 nm in diameters, its XRD analysis result (see Fig. 3a) indicates that B4C phase is dominant phase in the precursor. Film-like B4C cannot be observed at this stage because of the B4C films grew on branches of MgO scaffold that would tightly

wrap up fibers into a cloth-like kneaded sphere. Removal of the MgO scaffolds was carried out by a simple water-washing process, i.e. putting the precursor powder in deionized water in a beaker and subsequently ultrasonically washed at 80℃. After washing for 0.5 hour, the upper portion of the MgO scaffolds were taken out (Fig. 2b, b). Further washing of settled down powder was carried out for one more hour, thus the well-stretched B4C films were obtained as shown in Fig. 2(c) and (c). It is observed that sufficient washing time is necessary to entirely dissolve MgO nanofibers and remove them off. Finally, the kneaded B4C films have been fully spread out to be petal-like shapes. The thickness of these monolayer 2D B4C ultrathin films are measured using AFM technique as shown in Fig. 2(d), the load substrate is a single-crystal silicon wafer. The contact pattern indicates that the thickness of single-layered B4C film is about 1.5 nm.

Figure 3. XRD patterns and Raman spectra of the initial B4C/MgO precursor and its water-washed product of B4C films. (a) XRD profiles. (b) Raman spectra.

Fig. 3(a) shows XRD patterns of the as-prepared B4C/MgO precursor (upper portion) and the B4C films (settled down portion). It is indicated that the crystal B4C phase can be indexed to a rhombohedral structure [28] (JCPDS No. 35-0798; a = 5.600 Å，c = 12.086 Å). Both samples show strong diffraction peaks at 34.9° and 37.8° which is associated to (021) and (104) planes of B4C crystals, respectively. The crystallographic plane of (021) is significant for this B4C film, it is further proved by HRTEM analysis (Fig. 4) that the B4C film consists of small grains, all of them are

in-plane grown sheets along the direction of [021]. Residual graphite phase is detected in the washed powder from the diffraction peak at 26.4°, similar results have been also observed by many researchers [29,30]. Two diffraction peaks at 42.8° and 62.2° confirm the existence of MgO scaffolds in the as-prepared B4C/MgO precursor, later these were removed by the water-washing purification. Raman spectra of the as-prepared B4C/MgO precursor (upper portion) and the B4C films (settle down portion) were measured and presented in Fig. 3(b). Both spectra exhibit the same peaks in their positions and intensities, implying water-washing process doesn’t have any influence on the B4C film product. In low-frequency region, the Raman signal doublet at 264 cm-1 and 319 cm-1 are attributed to the bending modes of linear 3-atoms (C-B-B or C-B-C) chain along the diagonal of orthorhombic B4C unit cell [31,32]. The intensity of these both peaks are directly related to the carbon content of B4C. Appearance of these two peaks in Raman spectra indicates that the B4C film consists of boron-rich boron carbide crystal sheets [33]. In range of 400-600 cm-1, the peaks at 476 cm-1 and 529 cm-1 are corresponding to the Eg modes of B4C [34] i.e. 476 cm-1 is related to a rotational mode of the 3-atoms linear chain and 529 cm-1 is associated with the vibrational mode of the icosahedral unit within one B4C cell. In high-frequency region, the peaks at 715, 822, 996 and 1077 cm-1 are typically assigned to intra- and inter-icosahedral bonds, the vibrational modes of icosahedral units based on projected density of the states and experimental studies [35]. Peak at 715 cm-1 is the characteristic response from B6.5C cluster, which is further support evidence for the boron-rich unit composed of B4C film [35]. According to theoretical analyses, the linear 3-atoms chains yield to minor contribution in high-frequency region. Nevertheless, sp2 hybridized carbon bonds to create strong Raman peaks at 1350 cm-1 (D band) and 1585 cm-1 (G band). It distinctly indicates the presence of graphite scraps in B4C films that have also been clearly indicated in XRD results. The Raman peak of MgO at 268cm-1 and 1340cm-1 were were covered by the same peak of B4C, and the weak band at 440 cm-1 of MgO can not found, that all due to relatively lower content of MgO phase.

Figure 4. HRTEM images of B4C ultrathin films. (a) The as-prepared B4C/MgO precursor powder. (b) The water-washed B4C films. Square regions (I ,II, III, IV) in (a) and (b) are further analyzed in below magnified images. (c) Schematic illustrations on B4C unit cell and the crystallographic relationship between in-plane B4C film and the crystal growth direction of [021].

The as-prepared B4C/MgO precursor is anticipated the kneaded B4C films grow on the dendritic MgO nanofibers to wrap these scaffolds inside and to form the granular morphology. This presumption is confirmed by HRTEM analysis on the selected regions of (I, II) in Fig. 4(a), their detailed lattice images (a1, a2, a3, a4)

reveal the existence of B4C sheets those are totally arranged in (021) planes. Accordingly, image analysis on B4C films under high magnification [see Fig. 4(b), (b1), (b2), (b3) and (b4)] further supports that all B4C films are composed of consistently orientated gains in (021) planes with the corresponding lattice spacing of 0.231 nm. The SAED analysis on the B4C films as shown in the inset of Fig. 4(b), the diffraction rings show that the monolayer B4C films are made of polycrystalline grains with diffractions from (021), (104) and (125) planes. The cross size of single B4C grain is approximately larger than 4 nm.

Figure 5. X-ray photoelectron spectra of 2D B4C ultrathin films. (a) Survey spectrum. (b) Binding energies of B1s electrons. (c) Binding energies of O1s electrons. (d) Binding energies of C1s electrons.

XPS was used to investigate the chemical surface states of monolayer 2D B4C ultrathin films. The survey spectrum of B4C films is shown in Fig. 5(a), it shows the emissions from B, C and O elements, detailed photoelectron core-level spectra reappear in Figs. 5(b)-5(d). In Fig. 5(b), the binding energies of B1s electrons and the

main peak at 187.5 eV is attributed to the B-C bond of B4C. Meanwhile, peak at 192.2 eV associated to the B-O bond of BCO2 [36]. Peak at 188.8 eV corresponding to the B-O bond of oxidized boron (B2O3 or BO) [37]. These all findings exhibit that B4C species is dominant on the film surface, and has been slightly oxidized into oxide layer during the passivation process. Presence of BCO2 and boron oxides can be further confirmed by the binding energies of O 1s electrons in Fig. 5(c) i.e. the strong C=O bond of BCO2 at 531.9 eV and C-O bond of oxidized boron (B2O3 or BO) at 531.1 eV. B4C (282.2 eV) and BCO2 (287.5 eV) appear in the binding energies of C1s electrons (see Fig. 5d), meanwhile a mass of carbonous species (C-C bonds) emerge at 284.5 eV and 284.8 eV, both species are ascribed to the graphite and carbon contamination. 3.3. Formation mechanism of B4C ultrathin films

Figure 6. Schematic diagram on the formation of B4C ultrathin film. First stage: the raw materials co-evaporated into a gaseous state. Second stage: with the decrease of temperature, MgO first nucleation and grow into scaffolds, then B4C nucleation on the scaffold and grow into film, and unable to stretch due to the shackles of the MgO scaffold. Third stage: removal of MgO scaffold by hot water washing, and get the stretched films.

Based on above structural characterization results, the formation of monolayer

2D B4C ultrathin film is well comprehended in schematic of Fig. 6. Formation of B4C films has been roughly divided into three stages i.e. the evaporation, nucleation/growth and water-washing processes. At first stage, the raw materials of bulk MgO, amorphous boron and graphite are completely co-evaporated by high-energy arc-plasma into a gaseous state. At second stage, the evaporated atoms of raw species will undergo nucleation and growth processes in sequence, depending on the melting points of MgO (2852℃) and B4C (2350℃) phases. Prior to B4C, the MgO seeds will be nucleated due to its higher melting point, and subsequently will grow on nanofibers which will serve as a template (scaffold) for the heterogenous nucleation of B4C seeds on it. On MgO nanofibers scaffolds, the nucleated B4C seeds will grow along favorable directions into nanosheet-like films. The MgO nanofibers can restrict the extension of grown B4C films within nanometer-scale. Thus the as-prepared B4C/MgO precursor powders exhibit sphere morphologies, and a passivation process make them protected by ultra-thin oxide layers. At third stage of the water-washing process, the MgO scaffolds will be dissolved in water and washed out, meanwhile the kneaded B4C will be eventually stretched out into the free films.

3.4. UV–vis absorption spectroscopy of B4C ultrathin films

Figure 7. UV-vis light absorption and the band gas of 2D B4C ultrathin films. (a) UV-vis diffuse reflectance spectrum. (b) Plot of the transformed Kubelka-Munk function vs. the photon energy absorbed by B4C films.

Fig. 7(a) shows the UV-vis diffuse reflectance spectrum of monolayer 2D B4C ultrathin films. As the wavelength increases from 200 to 400 nm, rise in optical

absorption up to the maximum value of 400 nm is found, it slowly decreases within visible light range. Consequently, the optical absorption of 2D B4C ultrathin films is mainly concentrated in the range of visible light. As semiconductor, the band gap of B4C can be estimated by the Tauc drawing, according to the equation below: 𝛼ℎ ∝ (ℎ − 𝐸𝑔 )𝑛

(3.1)

where, 𝛼 is the absorbance of the solution sample, ℎ is the Planck constant,  is the photon frequency,

is 1/2 for direct transition and 2 for indirect transition. When

the absorption value is measured directly by solid powder, the formula (3.1) is transformed into formula (3.2) by using Kubelka-Munk equation, the expression 3.3 and 3.4 give the relationship between A, F(R∞) and R. 𝐹(𝑅∞)ℎ ∝ (ℎ − 𝐸𝑔 )𝑛

(3.2)

𝐴 = −𝑙𝑜𝑔(𝑅)

(3.3)

𝐹(𝑅∞) = (1 − 𝑅)2 /2𝑅

(3.4)

where 𝐴 is the absorbance of a solid powder and 𝑅 is the reflectance of a solid powder. As B4C is an indirect band gap semiconductor [38], so

is considered as 2.

In the following Tauc drawing, the photon energy (ℎ) keeps as the abscissa, while 𝐹(𝑅∞)ℎ

1/2

as the ordinate (Fig.7b). In this plot, the abscissa value at the intersection

of the tangent line drawn at the maximum point of curve is considered as the indirect band gap of B4C films, it is measured as 1.37 eV. Lee [39] et al. had measured the optical band gap of boron carbides varying its composition from 2.4 to 50 (the atom ratio of boron to carbon), and found that the lowest bad gap (0.77 eV) was obtained at the highest carbon concentration, the band gaps varied from 0.77 to 1.80 eV, and for B4C band gap of 0.90 eV was measured. This experimentally measured bad gap of B4C ultrathin films (1.37 eV) is larger than the reported value (0.90 eV). The reasons are thought as: one is from the small size effect, possibly happened due to several nanometers in the thickness of B4C film; the other is that the boron-rich composition of B4C films may contribute an increase in the band gap [28]. The oxygen-containing functional group C=O can also enlarge the band gap, but its influence is so limited [40]. The band gap of monolayer 2D B4C ultrathin film is approximately 50% higher than that of normal B4C film that makes it potential candidate in wide range of

electronic applications.

Conclusions As an efficient thermal source, the DC arc-discharge plasma has been applied for synthesis of monolayer 2D B4C ultrathin films, using compacted coarse powders of graphite, amorphous boron and MgO as the raw target, under the preparation atmosphere of H2 and Ar gases. The MgO nanofibers formed in as-prepared B4C/MgO precursor nanoparticles are able to serve as the template (scaffold) to favor heterogeneous nucleation and further growth of kneaded B4C nanosheets. B4C ultrathin films can be completely stretched out by removal of the MgO nanofibers through a water-washing purification process. The B4C ultrathin films are typically 1.5 nm in thickness and 0.5~2.0 micrometers in length. It is indicated that the band gap of such monolayer 2D B4C ultrathin film is 1.37 eV, approximately 50% higher than that of thicker B4C films. The preparation technique reported in this work may become a novel process for synthesis of ultrathin films consisted of small atoms such as B, C, N and O. The unique properties of these monolayer B4C ultrathin films make them promising candidate for applications in electronic devices

Acknowledgements Financial support from National Natural Science Foundations of China (Nos. 51331006 and 51331006).

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method

for

zero-

and

two-dimensional

B13C2

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