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Effect of boron and nitrogen additives on structure and transport properties of arc-produced carbon

We have studied the effect of introduction of boron, nitrogen or both elements into an electric arc on the morphology and the conductivity of the resultant carbon products. Scanning and transmission electron microscopies showed that the use of a boron-filled graphite electrode and a nitrogen gas during the arc discharge synthesis strongly affects the growth kinetics of carbon nanoparticles. The addition of boron promotes the formation of short, defective carbon nanotubes. In contrast, involvement of nitrogen in the synthesis process produces more perfect carbon nanostructures, including graphitic plates. Evaporation of a boron-filled electrode in a nitrogen atmosphere leads to BN co-doping of the carbon product. The concentration of each dopant is ca. 1 at.% and this value is twice greater than that for the cases of individual dopants. Among the studied materials, the BN-doped one possessed the highest conductivity, and this was attributed to the synergetic effect of co-doping. A substitution of carbon atoms by boron or nitrogen resulted in the por n-type doping of the samples, respectively. The evolution of conductivity with temperature and magnetic field showed that transport properties of the arc discharge synthesis products are strongly dependent on the charge carrier concentration, morphology and crystallinity of carbon nanoparticles. © 2018 Elsevier Ltd. All rights reserved.


Introduction
Arc-discharge is a capable process for mass production of metalfree carbon nanotubes (CNTs) [1,2]. The extremely high temperature generated in the electric arc leads to atomization of the graphitic electrode. Subsequent cooling of the gas jet gives carbon nanostructures with very little disorders that is a great advantage of the arc-discharge synthesis. However, due to the complexity of process, CNTs are formed simultaneously with other forms of carbon, such as fullerenes [3], polyhedral particles [4], graphene and nano-graphites [5]. The content of co-products depends on the synthesis parameters, such as gas type and pressure, current density, discharge power, etc. [6e8].
The doping of carbon nanostructures with heteroatoms, such as boron or nitrogen, is one of the effective ways to tune their properties in accordance with the particular application's requirements. Heteroatom doping can be performed either during the synthesis or by post-synthesis treatments, including ion implantation [9], plasma treatment [10], and annealing in nitrogen-containing atmosphere [11,12]. Several studies have been devoted to the synthesis of the B-and N-doped carbon nanostructures using arc discharge [6,8,13e25]. These nanostructures are formed in a nitrogen atmosphere [18,19] and using boron-filled graphite electrodes [17,21,24,25]. In the co-doped deposit, boron and nitrogen are found as individual defects [6,13,20,22] or BN domains [13,14,16,21]. Some reports showed that the introduction of dopants in an electric arc results in enhancement of the crystallinity of carbon nanomaterials [13,15,17], while others reported formation of more defective products [6,17,18,23]. Electronic, optical and conducting properties of the B-and N-doped multi-walled CNTs (MWCNTs) have been reported in Refs. [24e30]. Particularly, it was shown that even a small amount of dopant (<1 at.%) can significantly enhance the conductivity of nanotubes [24,25,28,29]. On the other hand, dopants could act as additional defects, deteriorating the conductivity [30].
The present communication aimed to explore the modification of morphology and charge transport properties of carbon materials arising from addition of nitrogen and/or boron species in the electric arc. The morphology of nanoparticles was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the defect density and chemical composition were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The transport properties were studied by measuring the temperature dependence of conductivity and magnetoconductivity (MC).

Synthesis
The setup used for the syntheses is described in details elsewhere [31]. Briefly, an upper movable cathode was made from a graphite rod of 60-mm diameter. A graphite anode had a crosssection of 14 Â 14 mm and a length of 200 mm. The anode was placed on a water-cooled holder in bottom of the chamber opposite the cathode. All syntheses were carried out at arc current of 500 A, arc voltage of 50 V and gas pressure of 10 5 Pa, which have been proved to be the optimal parameters for growth of MWCNTs [7,25]. The synthesis duration was 25 min.
Undoped and N-doped carbon samples were produced by evaporation of solid graphite rod in helium and nitrogen atmosphere, respectively. To produce B-doped and BN co-doped samples, a cylindrical cavity in central part of the rod was filled by powder of amorphous boron mixed with graphite powder in a ratio of 1:1. The concentration of boron in the rod was ca. 5 wt.%. The electrode was evaporated in helium or in nitrogen atmosphere to produce either B-doped or BN-doped samples.
The arc discharge evaporation of anodes produces dense deposits on the cathode surface and soot on the water-cooled walls of the chamber. The initial weight of the anodes was ca. 70 g, the mass of the deposits was ca. 10 g. The samples for the investigations were taken from central part of the deposits. Although the storage of a carbon sample in laboratory conditions usually results in p-type doping by atmospheric oxygen [32], further, we will refer the pure carbon material as "undoped" sample meaning that it does not contain nitrogen or boron impurities.

Methods
The morphology of samples was examined by SEM on a JEOL JSM 6700F microscope and TEM on a JEOL 2010 microscope. Raman spectra were collected with a Spex 1877 triple spectrometer using the 488-nm line from an argon laser. XPS and NEXAFS spectra were recorded at the Berliner Elektronenspeicher ring für Synchrotronstrahlung (BESSY II) using radiation from the Russian-German beamline. A powder sample was attached to a scratched copper substrate and placed into a chamber maintained at a pressure of 10 À7 Pa. The XPS spectra were measured at a monochromatic radiation of 800 eV. After subtraction of a Shirley background, the C, N, and B 1s XPS spectra were fitted using Gaussian/Lorentzian functions within Casa XPS 2.3.15 software. The NEXAFS spectra near C-, N-, and B K-edge were recorded in the total-electron yield mode with an experimental resolution better than 0.1 eV.
The direct current (DC) conductivity s measurements of parallelogram-shaped samples (1 Â 1 Â 5 mm) were carried out using afour-probe technique in a temperature range of 4.2e292 K. The Hall effect and MC were measured by an eight-contact method at the temperature of liquid helium (4.2 K). The MC was defined as sðHÞ sð0Þ À 1, where sð0Þ and sðHÞ are the DC conductivity without and with the magnetic field H applied, respectively.

Characterization
Typical SEM and TEM images of the samples are presented in Figs. 1 and 2, respectively. All samples are highly nonhomogeneous. They consist of straight MWCNTs, polyhedral nanoparticles and graphitic plates of different shapes and sizes. The simultaneous growth of nanotubes together with other co-products is typical for the arc discharge process [1,7]. Involvement of nitrogen or boron in the synthesis influences the characteristics of arc-discharge, which might differ from the optimal conditions for the nanotubes growth. The evaporation of the solid graphite rode in a nitrogen atmosphere promotes the formation of graphitic plates and strongly limits the growth of MWCNTs, whereas the B-doped and BN-doped samples are enriched with short MWCNTs and polyhedral nanoparticles. This fact suggests that the addition of amorphous boron into graphite electrode results in disordering of graphitic structures that terminates the growth of long nanotubes.
Raman spectra of the samples exhibited two peaks in the region between 1000 and 2000 cm À1 (Fig. 3), which are typical for graphite [33]. The G band at 1577 cm À1 is assigned to the in-plane vibrations of CeC bonds and the D band at 1358 cm À1 is activated in the presence of disorder in graphite lattice. Since the intensity of D band increases with the number of defects and functionalities, the intensity ratio of D and G bands (I D /I G ) is commonly used to estimate structural disorder in graphitic materials. The I D /I G ratio is equal to 0.2 for the undoped and N-doped samples and this value is far smaller than the ratio of 0.7 for the B-doped and BN-doped samples. Additionally, the full width at half maximum (FWHM) of the G band for the B-and BN-doped samples (32 and 29 cm À1 ) is larger than that for the undoped and N-doped samples (25 and 17 cm À1 ). Therefore, arc-discharge evaporation of graphite in helium or nitrogen atmospheres results in the formation of carbon nanoparticles with graphite-like structure. At that, the N-doped sample has the most perfect sp 2 -hybridized network among all the studied samples. The addition of amorphous boron in the synthesis strongly increases disorder in graphitic shells. Furthermore, for the B-doped and BN-doped samples we observe upshifts of D band by ca. 7 cm À1 and G band by ca. 11 cm À1 , which are more probably attributed to the hole doping [34].
Overall XPS spectra of the samples revealed the signals from carbon, oxygen, nitrogen and boron. The intensity ratios of these lines were used to evaluate the surface concertation of elements ( Table 1). The sample produced in nitrogen atmosphere contains ca. 0.5 at% of nitrogen. The temperature of the electric arc near the anode reaches 4000e5000 C [35], that is enough for the nitrogen gas dissociation, which occurred at about 3500 C [36]. The content of boron in the B-doped sample was also ca. 0.5 at%. The fraction of both boron and nitrogen increases to ca. 1 at% in the BN-doped sample. Since the XPS signal from oxygen can appear not only from the samples but also from copper holder, the oxygen content is difficult to compare.
The XPS lines and NEXAFS spectra were used to determine the chemical states of the elements. The C 1s XPS spectra of the samples are presented by asymmetric peaks located at 284.5 eV (Fig. 4a), corresponding to the sp 2 -hybridized carbon [37]. The main difference between the spectra is the width of the peak, responsible for atomic ordering in graphitic layers. The FWHM value for the undoped, Ndoped, B-doped, and BN-doped samples is equal to 0.8, 1.0, 1.1, and 1.5 eV, respectively. The increase of FWHM values with doping indicates disorder in the graphitic network. A low-energy shift of the C 1s spectrum of the B-doped sample by 0.3 eV as compared to other spectra could be due to the p-type doping effect [38].
The C K-edge NEXAFS spectra of all samples show two main peaks at 285.4 and 293.0 eV (Fig. 4b) assigned to 1s/p* and 1s/s* transitions, respectively [37,39]. The sharpness of these resonances is a sign of good atomic ordering within the graphitic layers in carbon particles. Since the probing depth of NEXAFS is ca. 10 nm, the higher intensity of the p* resonance for the N-doped sample evidences the higher crystallinity of inner layers of the nanoparticles. While surface-sensitive XPS technique indicates that nanoparticles in the undoped sample have the better quality of external layers. A weak peak at 288.6 eV labeled c is likely contributed by carbon bonded with oxygen. The spectra of the Bdoped and BN-doped samples have a low-energy peak a at 284.5 eV associated with downshift of the Fermi level due to transfer of electron density from carbon cagesto incorporated boron. This agrees well with the upshifts of the D and G Raman bands. The spectrum of the BN-doped sample exhibits a lowering of the p*resonance and increase of relative intensities of the peaks b and c located at 287.8 and 288.6 eV, respectively (Fig. 4b). These peaks could be attributed to the formation of covalent bonds between carbon and boron, as well as carbon and nitrogen (B-C-N species).  The N 1s XPS spectrum of the N-doped sample was fitted by two components positioned at 401.5 and 398.5 eV (Fig. 5a). The former component denoted N gr corresponds to the nitrogen atoms replacing the carbon atoms in a hexagonal lattice (i.e., graphitic N) [40,41]. The latter wide component denoted N def could be assigned to the nitrogen atoms, being different from the graphitic one. According to the previous works, the N 1s line at 398.5 eV is usually attributed to pyridine-like nitrogen [39,40]. However, we do not expect that the dangling bonds could persist under the extremely high temperature of the electric arc. Most likely, that the N def peak corresponds to the two-fold coordinated nitrogen atoms located on the edges of graphitic particles or three-fold coordinated N atoms in topological defects.
The N K-edge NEXAFS spectrum of the N-doped sample shows two states before the s*-edge [42]. Taking into account the XPS data, a peak at 402 eV is assigned to the p* states of graphitic N (N gr ), while a sharp peak at 399.7 eV is attributed to N def . According to quantum-chemical studies, the insertion of nitrogen in carbon pentagons near the nanotube caps is energetically more preferable than in the nanotube body [43,44]. The pentagons are needed to close the nanotube ends and to produce spherical or polyhedral nanoparticles, which are abundant in the arc-produced samples (Fig. 2). The N 1s spectrum of the BN-doped sample also has two peaks denoted as N gr and N def. (Fig. 5a). However, nitrogen bonded to boron also contributes to the latter peak [45]. The occurrence of BN species in this sample is supported by an intense peak at 401.4 eV in the N K-edge spectrum [42]. The component arisen from BN species is located at 190.8 eV in the B 1s XPS spectrum (Fig. 6a) and 192.0 eV in the NEXAFS B K-edge spectrum (Fig. 6b) [45,46]. Further, lowenergy weak peaks in the XPS B 1s spectrum (ca. 186.7 eV, Fig. 6a) and NEXAFS B K-edge spectrum (ca. 189.0 eV, Fig. 6b) of the BN-doped sample indicate that boron atoms interact also with carbon atoms. Additionally, the spectra show formation of B-O species in the BN-doped and B-dopes samples.
Segregation of sp 2 -hybridized carbon and BN phases has been observed for arc-produced samples previously [13,14,16,21]. This was associated with a weaker coupling between C and B or C and N in comparison with C-C and B-N bonding [16,47] and stabilization of C-BN heterojunctions [47]. We suppose that obtained BN-doped nanoparticles contain BN-domains embedded into sp 2 -hybridized carbon network. This is different from the structure of N-and Bdoped nanoparticles, which contain point substitutional defects. The regions of carbon and BN in nanoparticles doped with BN are probably linked through C e B bonds. Actually, analysis of the XPS data shows that the fraction of these species is about two times greater than the content of C-N species (n C-B ¼ 0.30 at% and n C- It should be noted that we did not observe the peak at 405 eV in the XPS N 1s spectra and the split resonance at 401 eV in the NEXAFS N K-edge spectra from molecular nitrogen [48e50]. Thus, in contrast to N-doped MWCNTs produced by chemical vapor  Table 1 Concentration of carbon, nitrogen, boron, and oxygen in the samples estimated by XPS.

Sample
Element concentration, at% deposition technique using N-containing precursors, the arcproduced N-doped and BN-doped carbon samples do not contain encapsulated N 2 molecules.  [24,25]. The conductivity of the BN-doped sample is significantly higher (493 S/cm) than that for the samples doped with nitrogen or boron solely. This enhancement can be assigned to the formation of B-terminated BNdomains in carbon network. The extended B-rich interfaces donate holes in the electronic system of carbon nanoparticle more effectively than isolated boron atoms. This indicates the synergetic effects of simultaneous nitrogen and boron incorporation into the carbon network of obtained nanoparticles.

Transport properties
The conductivity curves of all samples have two regions with different behavior. At temperatures above 100 K, the conductivity can be expressed by any exponential behavior. In particular, approximation of the conductivity by the exp(-E/T) reveals that doping decreases the tunneling barrier E. For the undoped and Ndoped samples, E is quite high (about 153 and 93 K, respectively), while it drops to about 4.5 and 7 K for the B-and BN-doped samples. Besides, both two-and three-dimensional variable-range hopping transports (as well as their combinations) are suitable to approximate the conductivity dependencies. It is likely, that highly nonhomogeneous structure of the samples causes a simultaneous movement of electrons (or holes) along different paths. The s(T) dependences of the undoped and N-doped samples can also be fitted by a power law T 1.5 . Such temperature behavior is typical for doped semiconductors, whose conductivity is determined by impurity scattering [51]. Therefore, both the tunneling between neighboring nanoparticles and the intrinsic conductivity of nanotubes and graphitic plates contribute to the transport properties of the samples containing well-ordered graphitic layers (undoped and N-doped). The addition of boron into electric arc enhances disorder in carbon network that, in particular, is exhibited as the formation of many small polyhedral nanoparticles. On the other hand, strong hole doping of the samples enhances a coupling between neighboring nanoparticles. As a result, conductivities of the B-doped and BN-doped samples weakly depend on the temperature and have a metallic behavior above 100 K.
Below 100 K, the conductivity of all samples follows a logarithmic law (lnT, Fig. 7c) due to the contributions of quantum effects inherent to the disordered systems [52]. For such systems, the transport mechanism can be described by a diffuse conductance within the concept of weak localization (WL). When two paths of an electron wave function form a closed loop, they become phase coherent and the constructive interference tends to localize electrons.
The contribution of WL to low-temperature transport is confirmed by the MC measurements, which are sensitive to the crystallinity of nanoparticles, their shapes and sizes (Fig. 8). The undoped, B-doped and BN-doped samples have a positive dependence of MC (Fig. 8a). When electrons are localized, magnetic field breaks the constructive quantum interference. The higher the magnetic field, the weaker the localization effects, the higher the conductivity. MC follows quadratic law H 2 at low fields and increases as lnH at higher field (Fig. 8 bed). These asymptotes are specific for materials, whose charge transport is governed by twodimensional WL effects, which is widely used to explain positive MC in disordered thin metal films [53], amorphous carbon materials [54,55], disordered graphite [56], carbon nanotubes [57], graphene [58,59]. Although the electron-electron interaction effect causes the same dependence of MC(H) [52], we should rule it out due to rather small values of the magnetic field. On the other hand, the arc-discharge products are nonhomogeneous materials, and electrons are diffusively scattered at boundaries of crystallites that restricts their free paths [60]. The diffusion scattering and WL effects will make comparable contributions to the macroscopic conductivity of nanostructure with a linear dimension of about or less than 10 nm [61]. The magnetic field induces extension of the mean free path, thus increasing the conductivity of the nanoparticles as H 2 [62].
The field dependence of MC has qualitatively different behavior for the N-doped sample (Fig. 8a). The conductivity increases from zero to 0.3 T and then a negative contribution to MC is observed (~-H 2 , Fig. 8e). Such behavior could be related to the ordinary Lorentz cyclic motion of electrons in magnetic field [63]. Similar crossover from positive to negative MC was found for bulk graphite materials with different particle sizes [61]. In particular, the conductivity of graphite composed of 30-nm particles increases with the magnetic field due to WL effects and diffuse scattering at the boundaries of crystallites. When the size of the particles reaches several micrometers, the conductivity decreases as~-H 2 . Therefore, we suppose that the crossover in the s(H) dependence of the Ndoped sample is due to the compensation of quantum transport

Conclusion
Multicomponent carbon samples doped with nitrogen and/or boron with a concertation of foreign elements less than 1 at% have been synthesized by the arc-discharge method using amorphous boron and nitrogen gas as dopant sources. SEM and TEM methods have found MWCNTs, polyhedral nanoparticles and graphitic plates in the samples. Among the synthesized samples, the N-doped material is distinguished by the fact that it comprised a large fraction of the plates. As shown by Raman scattering, XPS and NEXAFS, this material also had a high graphitization degree comparable to the other samples. The B-and BN-doped samples were enriched by small nanoparticles and short MWCNTs. It was shown that arc-discharge evaporation of a boron-stuffed graphite electrode enhanced disorder in a honeycomb carbon lattice. A comprehensive characterization revealed the presence of C-N and C-B bonds confirming the incorporation of dopants in the lattice. That induced either electron or hole doping depending on whether nitrogen or boron has participated in arc-discharge process. We suggest that the co-doped sample also showed the hole doping due to the preferable involvement of nitrogen in the formation of B-N bonds, while residual boron atoms and boron atoms located at the C-BN heterojunctions acted as electron acceptors. Our data suggested that the charge carrier concentration is a dominant factor in the manifestation of semiconducting or metallic conductivity behavior of arc-discharge carbon samples. From the MC measurements, we found that both quantum correction and diffusive scattering at the crystallite boundaries govern the lowtemperature charge transport properties of the undoped, Bdoped, and BN-doped samples. Above some critical value of magnetic field, the opposite trend in the conductivity was found for the N-doped samples. It is supposed that the dominant mechanism responsible for compensation of the conductivity increase is a cyclotron motion of charges on surfaces of graphitic plates. Probably, either size of graphitic plates or their crystallinity are not enough to countervail the positive MC effects in other samples. The found relationships between synthesis conditions, morphology and conductivity of arc produced materials opens a new route toward controllable and easy to use technology of either p-or n-type doped carbon components fabrication. Their transport properties and carrier concentrations could vary within the same process, just adding either the amorphous boron or nitrogen gas, or both, to the arc discharge.

Acknowledgment
This work was done with the support of RFBR (grant 17-52-04077), BRFFR (grant F17RM-068) and H2020 RISE 734164 Graphene 3D. The work was partially supported by the bilateral Program "Russian-German Laboratory at BESSY II" in the part of XPS and NEXAFS measurements.