Synthesis, Characterization, and Properties with Potential Applications

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Review

Borocarbonitrides, BxCyNz: Synthesis, characterization and properties with potential applications Chintamani Nagesa Ramachandra Rao, and K Gopalakrishnan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08401 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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ACS Applied Materials & Interfaces

Potential applications of borocarbonitrides 30x30mm (300 x 300 DPI)

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Review

Borocarbonitrides, BxCyNz : Synthesis, characterization and properties with potential applications C. N. R. Rao* and K. Gopalakrishnan Chemistry and Physics of Materials Unit, New Chemistry Unit, International Centre for Materials Science, CSIR Centre of Excellence in Chemistry and Sheik Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, P.O., Bangalore 560064, India.

Abstract

Borocarbonitrides, BxCyNz, constitute a new family of layered two-dimensional materials and can be considered to be derived from graphene. They can be simple composites containing graphene and BN domains or more complex materials possessing B-C and C-N bonds besides BN and C-C bonds. Properties of these materials depend on the composition, and the method of synthesis, wherein one can traverse from the insulating end (BN) to the conducting end (graphene). In this article, we present an up-to-date review of the various aspects of borocarbonitrides including synthesis, characterization and properties. Some of the properties have potential applications, typical of them being in gas adsorption and energy devices such as supercapacitors, fuel cells and batteries. Performance of borocarbonitrides as catalysts in the electrochemical hydrogen evolution reaction is impressive. It is noteworthy that with certain compositions on borocarbonitrides, field effect transistors can be fabricated.

Keywords: Borocarbonitrides, graphene, supercapacitors, field-effect transistors, batteries, hydrogen evolution reaction, oxygen reduction reaction

*E-mail: [email protected]. Phone: +91-80-23653075/22082761. Fax: +91-80-22082760.

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1. Introduction Graphene is known for its fascinating physical properties, such as quantum Hall effect1 and massless Dirac fermions, with potential applications in nanoelectronics.2-3 Single-layer graphene shows very high carrier mobility (∼1.5 × 104 cm2 V−1 s−1 at room temperature).4 In spite of this large carrier mobility, its gapless nature limits its performance as a versatile electronic material.23

Experimental and theoretical studies suggest that substitutional doping of graphene with

nitrogen and/or boron can modify its band structure giving rise to a metal-to-semiconductor transition.5

Borocarbonitrides with the general formula of BxCyNz constitute a new family of 2D materials, whose composition can be varied over a wide range.6 These materials are generally nanosheets containing graphene and BN domains, possibly along with BCN rings (Figure 1). If the ratio of BN to carbon is 1:1 the composition would be BCN. BxCyNz compositions contain hexagonal networks of B–C, B–N, C–N and C–C bonds but no B–B or N–N bonds. On the other hand, lateral combination of graphene and hexagonal boron nitride permits to tune the electronic characteristics as required for a particular application. In this article, we present the synthesis and properties of borocarbonitrides along with potential applications.

Characterization of

borocarbonitrides of have been carried out by electron microscopy, scanning probe microscopy and other techniques. X-ray photoelectron spectroscopy (XPS) is specially useful in determining the composition as well as the nature of bonding. The various applications of borocarbonitrides include their use in supercapacitors, as oxygen reduction reaction (ORR) catalysts in fuel cells, in the hydrogen evolution reaction (HER) and in lithium ion batteries.

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Figure 1. Schematic of borocarbonitride, BxCyNz

2. Synthesis Borocarbonitrides have been generated by several means. These methods include chemical vapor deposition (CVD), pyrolysis of precursors in gas phase and solid state reaction of precursors. Composites of BN and graphene can also be prepared by covalent cross linking. Lateral or vertical heterojunctions of BN and graphene layers can be generated with the use of precursors to deposit them.

2.1 Chemical vapor deposition Chemical vapor deposition is a widely used technique to generate graphene,7 doped graphene8 and boron nitride.9 Levendorf et al.10 obtained graphene-BN lateral heterojunctions on Cu foils using methane, diborane ammonia under CVD conditions. Here, domains of BN and graphene are next to one another to form continuous sheets across the heterojunctions. Figure 2a shows the patterned regrowth of graphene/BN heterojunction while Figure 2b shows optical images. Figure 2c shows a dark-field transmission electron microscope (DF-TEM) image of 3


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graphene1/(graphene2 or h-BN) patterned composite comprising single crystals graphene (see inset Figure 2c). The dark field TEM image (Figure 2d) of the junction areas show crystalline and continuous growth of graphene while Figure 2e shows a plot of grain size as a function of position, the average grain size being ~1.7 µm. Ci et al.11 have made atomic sheets of BCN with a wide range of composition by the decomposition of methane and ammonia-borane precursors on Cu substrates at 1000 oC for 40 mins. BN and graphene layers were deposited one over the other. Atomic layers of graphene/BN are grown on Cu under CVD conditions at 1000 oC for 20 mins.12 Low-pressure chemical vapor deposition yields BCN at 1050 oC using CH4 and ammonia borane vapor in an argon atmosphere.13 Different BN concentrations are obtained by heating ammonia borane from 70-110 oC.

Figure 2. (a) Representation of the formation of graphene/BN (b) Optical image of graphene1. (c) DF-TEM image of a graphene1/(graphene2 or h-BN) patterned area (schematic is shown as an

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inset). (d) Enlarged image of the patterned area. (e) Plot showing average grain size of the heterostructure. Reprinted with permission from ref 10 (Copyright 2012 Nature Publishing Group).

Hexagonal graphene/BN nanosheets have been prepared by atmospheric pressure CVD by using CH4 mixed with argon and ammonia borane on Cu foil at 1057 oC.14 Conversion of graphene to BCN has been made by heating boric acid powder in a flow of NH3 and Ar gas for 2 h at 1000 oC.15 Zhang et al16 have grown graphene/BN vertical heterostructures by annealing Ni(C)/(B,N)/Ni samples in a vacuum furnace in the 900-1050 oC range.

2.2 Solid state urea route

Borocarbonitrides (BCN for simplicity) can be synthesized by reacting urea and boric acid with activated carbon.17 This method yields products with high surface area (1500-2000 m2 g-1). Instead of activated carbon, one can use few-layer graphene has been used as the carbon source. BCN obtained by using graphene contains 5-7 layers. Chu et al18 carried out the synthesis of BCN with graphene oxide instead of activated charcoal and graphene while Lin et al19 prepared BCN by reacting graphene oxide with B2O3 and ammonia at 900 to 1100 °C with different BN contents.

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Figure 3. Experimental set-up of borocarbonitrides by gas phase. Reprinted with permission from ref 20 (Copyright 2011 Elsevier).

2.3 Pyrolysis of precursors and reaction in gas phase Bartlett and coworkers21 carried gas phase synthesis of BxCyNz by the reaction of CH4 with ammonia and BCl3 at 400-700 °C to obtain a composition of B0.35C0.30N0.35 (BCN). A similar gas phase reaction using BBr3, NH3 and ethylene gave BC1.6N containing domains of BN and graphene.20 Figure 3 shows the experimental setup.

Pyrolysis of a pyrrolidine-borane

complex gives BC1.5N0.4 and BC0.9N0.422 while pyrolysis of poly(borosesquicarbodiimide) gives [B2(NCN)3]n.23 High temperature treatment of ethane 1,2-diamineborane gives BCN with gas desorbing properties.24 Thermal decomposition of N,N′-ethylmethylimidazoliumtetracyanoborate gives BC3N.25 Mesoporous BCN obtained from C3N4 and a borane complex exhibits a high surface area.26 Melaminediborate heated at 1000 oC with nitrogen-doped graphene coated nickel foam under Ar/H2 atmosphere yields BCN covered onto nickel foam.27 Yan et al28 have demonstrated growth of bilayer graphene on BN substrate from polymers at 1000 oC using a Ni catalyst.

2.4 Covalently bonded Graphene-BN composites Composites where BN and graphene sheets are covalently bonded have been prepared.29 Covalently cross-linked graphene and boron nitride ((BN)1–xGx) has been obtained by employing

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the carbodiimide reaction. Figure 4 shows the synthetic steps to generate covalently bonded BN and graphene (BN1–xGx, x ≈ 0.25, 0.5, 0.75).

Figure 4. Schematic representation of the preparation of chemically bonded BN and graphene. Reprinted with permission from ref 29 (Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

2.5 Liquid phase generation of BN-Graphene composites BN and graphene sheets can be stacked by mixing dispersions31 or by the use of the liquid-liquid interface.32 Boron nitride powder is sonicated in isopropyl alcohol while graphite powder dissolved in dimethylformamide and supernatants were collected.31 These supernatants are mixed under sonication for 2h to form hybrids of BN with graphene. Graphene/BN hybrid 7


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structures can be obtained by exfoliation in liquid-phase.33 Graphite and BN were added to a mixture (conc. H2SO4 and HNO3) to obtain a hybrid compound. This compound is subjected to thermal treatment to

yield an expanded hybrid structure which is sonicated in

dimethylformamide.

Figure 5. (a-c) Atomic force microscope (AFM) images of graphene-BN heterostructures. (d-g) Topographical image and height profile of graphene and BN films. Reprinted with permission from ref 14 (Copyright 2013 American Chemical Society) (h) AFM image of a BCN film. Reprinted with permission from ref 11 (Copyright 2010 American Chemical Society) (i&j) AFM image of BCN and the height profile. Reprinted with permission from ref 30 (Copyright 2016 Elsevier)

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3. Characterization 3.1 Atomic probe microscopy and transmission electron microscopy

Morphology of BCNs has been examined by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Figure 5 shows AFM images of single crystalline hexagonal BNhexagonal graphene heterostructure flakes.14 The topographical images taken after Cu oxidation are shown in Figures 5(a-c). Figures 5(d-g) shows that the topographical image and height profile of the graphene and BN films (∼0.5 nm). Figure 5h shows an AFM image of a BCN film grown under CVD conditions (2 or 3 layers).11 Figures 5i shows a AFM image of BCN prepared by the urea route, whose height profile in Figure 5j shows that the nanosheets have an average height of 2.4 to 2.6 nm.30

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Figure 6. Formation of misfit dislocations (MD) at graphene-boron nitride (G-BN) interface (>100 nm). (a) STM image of graphene-BN domains (b-e) Interface discontinuities (d) MD relieves interface strain and keeps the continuity of the G-BN boundary. (e) Discontinuities in the BN moiré pattern appear in regions where different BN domains merge. (f) Magnified view of MD in BN Moiré pattern (g) High-resolution image of MD revealing the structure of the dislocation core. (h) Misfit dislocations in BN appear close to graphene’s A edge, while discontinuities in BN close to graphene’s B edge. Reprinted with permission from ref 34 (Copyright 2014 American Chemical Society) (i-l) STM images of monolayer graphene, BN and of graphene-BN heterostructures on Ru(0001). Reprinted with permission from ref 35 (Copyright 2012 American Chemical Society)

Figure 6 shows the in situ scanning tunneling microscope (STM) images of graphene/BN deposited on a Ru(0001) surface.34 A large area image is presented in Figure 6a. Three types of irregularities are observed in BN domains embedded in the matrix of graphene islands (Figures (b-d)). These include random patterns formed at some distances away from the graphene/BN interface, discontinuities and heart-shaped moiré structures. Irregularities away from the interface are shown in Figure 6e. Figures 6d,f,g shows the interface continuities. Two regions A and B are highlighted in Figure 6h. Discontinuities are observed in BN attached to graphene’s 10



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edge A. Heart-shaped dislocations are found in BN attached to edge “B”. Sutter et al35 have used in-situ microscopy to study graphene-BN nanosheets. Figures 6(i-l) show the ultra-high vacuum STM images of graphene-BN heterostructures, which reveal BN domains.

An aberration-corrected high resolution TEM study of BCN sheets made under CVD conditions11 is presented in Figure 7. Fast Fourier transform (FFT) of the image of 2-3 layers BCN in Figure 7a indicates the hexagonal structure with individual atoms resolved from the hexagonal packing (see inset of Figure 7a). The Moiré and FFT patterns are shown in Figure 7b. Figure 7(c & d) shows the hexagonal atomic network of BCN sheets. Figures 7e-g show TEM images of graphene/BN heterostructures.16 Figures 7(e & f) reveal Moire´ patterns, and the wavelength is about 1.28 nm (Figure 7f). Figure 7g shows a HRTEM image and the Moiré pattern is shown in inset. TEM image of graphene edges are shown in Figure 7h. Mostly single and double layers are observed.

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Figure 7. (a & b) Moire patterns of BCN films (inset shows FFT) (c & d) Atomic layers of BCN films (scale bar, 5 nm). Reprinted with permission from ref 11 (Copyright 2010 Nature Publishing Group). (e-f) HRTEM images of graphene/BN heterostructures (inset shows FFT pattern). Scale bars, 1 nm. (g) HRTEM image of graphene. Inset shows zoomed in region (scale bar, 2 nm). (h) TEM images of graphene in different regions (scale bar, 5nm). Reprinted with permission from ref 16 (Copyright 2015 Nature Publishing Group).

3.2 XPS and EELS

X-ray photoelectron spectroscopy (XPS) is effective in unravelling the electronic structure and compositions of borocarbonitrides. The B 1s, C 1s and N 1s core level spectra of BCN prepared from graphene are shown in Figure 8a-c.30 The B 1s feature can deconvoluted into two peaks at 191.3 eV and 192.3 eV corresponding to B-C and B-N bonds. The C 1s signal can be fitted in to peaks centered at 284.4, 285.6 and 286.8 eV due to sp2 carbons of C-C, B-C and C-N bonds respectively. The N 1s signal (Figure 8c) at 398.6 eV is due to pyridinic nitrogen and N-B bonds, and the feature at 400.3 eV is due to graphitic and pyrollic nitrogens. Figures 8d-f show spectra of BCN films grown under CVD conditions.11 The peaks are due to chemically bonded B-C-N atoms.

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Figure 8. (a-c) X-ray photoelectron spectra of BCN prepared from graphene. Reprinted with permission from ref 30 (Copyright 2016 Elsevier). (d-f) XPS of BCN films. Reprinted with permission from ref 11 (Copyright 2010 Nature Publishing Group).

Figure 9 (a) XANES and XES of graphene/BN films (b) Zoomed spectra in the π-π* region. Reprinted with permission from ref 13 (Copyright 2013 American Chemical Society). (c) EEL 13


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spectrum of BCN. Reprinted with permission from ref 6 (Copyright 2013 Royal Society of Chemistry)

Band gap opening in BCN films can be studied by the C K-edge X-ray absorption near edge structure (XANES) and Kα X-ray emission spectroscopy (XES) measurements.13 XAS and XES spectra of BCN samples are shown in Figures 9(a & b). Magnified spectra in the π-π* region is shown in Figure 9b. Graphene and graphene with a BN content 52%, have no band gap. However, the band gap increases up to 600 meV with increase in the concentration of BN. The electron energy loss (EEL) spectrum of BCN6 (Figure 9c) reveals that the splitting of the π* and σ* levels in B 1s, C 1s and N 1s is due to the sp2 hybridization of these atoms.

3.3 Thermogravimetric analysis Thermogravimetric analysis of covalently bonded BN-graphene29 carried out in oxygen atmosphere is shown in Figure 10a. In the 500–700 °C temperature range graphene shows weight loss whereas BN shows no weight loss. The residual weight for BN0.25G0.75, BN0.5G0.5, and BN0.75G0.25 are 24%, 49.5%, and 72% respectively. Figure 10b shows TGA curves of BCN prepared from graphene.30 BCN samples show weight loss ranging from 400-500 °C due to carbon combustion. BCN samples show better thermal stability than graphene.

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Figure 10 (a) Thermogravimetric analysis of graphene and covalently bonded BN-graphene composites. Reprinted with permission from ref 29 (Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim). (b)Thermogravimetric analysis various compositions of BCN in comparison with EG. Reprinted with permission from ref 30 (Copyright 2016 Elsevier)

3.4 Spectroscopy

Figure 11a shows the infrared (IR) spectra of covalently bonded BN-graphene and few-layer BN.29 The composites show IR bands due to the amide bond formation. The IR spectra of BCN of different compositions are shown in comparison with the spectrum of EG (thermally exfoliated graphene) in Figure11b.30 The BCN samples show bands at 1390 and 798 cm-1 due to the B-N and B-N-B bonds respectively. Graphene/BN disks fabricated by lithography show plasmonic absorption as a function of (graphene/BN)n.36 The Fermi level of graphene is characterized in the mid-IR range (Figure 11c). The interband transition is shown in the inset of Figure 11c. Figure 11d displays the extinction spectra of graphene/BN disks, where the plasmonic resonance upshifts (inset in Figure 11d).

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Figure 11. (a) Infrared spectra of covalently bonded BN-graphene composites. Reprinted with permission from ref 29 (Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim). (b) Infrared spectra of graphene and BCNs. Reprinted with permission from ref 30 (Copyright 2016 Elsevier) (c & d) Fermi level characterization of graphene/BN. Reprinted with permission from ref 36 (Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Figure 12. (a) Raman spectra of graphene and BCN. Reprinted with permission from ref 11 (Copyright 2010 Nature Publishing Group).

(b) Raman spectra of graphene, BN and

graphene/BN film. Reprinted with permission from ref 37 (Copyright 2010 Nature Publishing Group) (c & d) Raman spectra of graphene, BCN and BN. Reprinted with permission from ref 15 (Copyright 2014 Nature Publishing Group)

Figure 12a shows the Raman spectrum of a BCN film compared with that of pristine graphene, the BCN curve showing a broader D band at 1360 cm-1 and a shoulder at 1620 cm-1 due to D’ band on the right side of the G band.11 Figure 12b shows Raman spectra collected from graphene/BN heterostructures and also individual films.37 Figures 12(c & d) show the spectra of chemically converted graphene to BCN.15 Graphene is mostly single layered without any defects whereas the BCN film shows the defect band (D band (1350 cm−1)) and the E2g band (1370 cm−1) from h-BN contributing to a band at 1363 cm−1 (Figure 12d).

Forster et al38 have studied

vibrational properties of graphene/BN. BN reduces electron-phonon coupling in graphene.

3.5 Surface area Borocarbonitrides prepared from activated charcoal exhibits high surface areas.17 Brunauer– Emmett–Teller (BET) surface areas for BC1.6N and BC2N are 1509 and 1991 m2 g−1 respectively with type IV behavior (Figure 13). Covalently bonded BN1–xGx composites show surface areas in

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the 180-200 m2 g-1 range.29 Graphene and BN have surface areas of 38 m2 g−1 and 452 m2 g−1 respectively.

Figure 13. Nitrogen adsorption data of (a) BC2N and (b) BC1.6N at 77 K and 1 atm. Reprinted with permission from ref 17 (Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

4. Properties 4.1 Magnetic properties Chemically reduced graphene exhibits weak room-temperature ferromagnetism.39 A similar feature has been observed in graphene prepared by exfoliation of graphite oxide.40 This is not due to impurities, but due to the zig-zag edges and defects. Liu et al.41 report that nitrogen doping increases the magnetization in graphene whereas boron-doped graphene shows weak magnetism.42

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Figure 14. (a) Magnetization curves and (b) Magnetic hysteresis loop of BC2N (c) Magnetization curves and (d) Magnetic hysteresis of BC1.6N. Reprinted with permission from ref 6 (Copyright 2013 Royal Society of Chemistry)

Since BN-doped graphene systems energetically favor spin-polarized ground states, it is interesting to investigate their magnetic properties.43 Borocarbonitrides which contain graphene and BN domains exhibit magnetic hysteresis with weak ferromagnetic behavior just like graphene and boron nitride.6 Magnetization curves of BC2N at 500 Oe in Figure 14a show divergence. Figure 14b shows the hysteresis data curve at 300K. Inset of Figure14b shows the hysteresis loop at 5K. BC1.6N has a lower magnetization than BC2N (Figure 14c). Figure 14d shows the hysteresis loops of BC1.6N at 300 K and 5K. BCN and BC5N reported to show weak ferromagnetic behavior with saturation magnetization values 0.5 and 0.334 at 2 K respectively (0.002 and 0.0084 at 300 K).44

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Figure 15 (a) Magnetoresistance (MR) against external magnetic field (H) of graphene/BN (b) MR vs. H (c) Hall resistivity (ρxy) as a function of H at various VG. (d) Carrier density (n) and mobility (µ) plotted against VG. Reprinted with permission from ref 45 (Copyright 2015 Macmillan Publishers Limited) Gopinadhan et al45 report magnetoresistance (MR) of ~2,000% at 400 K in graphene/BN heterostructures. Figure 15a shows MR of graphene/BN as a function of the external magnetic field at 400K. The MR becomes less on SiO2 substrate due to the degradation in mobility (Figure 15b shows). Hall resistivity (Figure 15c) shows a nonlinear behavior with respect to VG. The carrier density and mobility for VGo and VCNP are shown in Figure 15d.

4.2 Electrical properties

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Graphene is a gapless material3 and its electronic properties can be tuned by chemical doping with boron and nitrogen.5 Figure 16a presents four-probe the electrical resistivity data of BCN samples prepared by urea route.44 BC5N shows lower electrical resistivity than BCN, due to the higher carbon content. The plot of ln R versus T 1/4 is linear as shown in Figure 16b. The transport mechanism is Mott VRH type with domains of BN separated from each other. Reduced graphene oxide/BN composite is reported to show an electrical conductivity of 545 S m−1.46 The electrical conductivity of BCN films can be controlled by alternating the carbon concentration. BN-doped graphene nanosheets prepared from GO, B2O3 and ammonia show an ambipolar behavior.19

Figure 16. (a) Electrical resistivity of BC5N and BCN (b) Fit of the T1/4 law for BC5N. Reprinted with permission from ref 44 (Copyright 2014 The IOP publishing).

4.3 Optical properties 21


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Borocarbonitrides and graphene being quasi-two-dimensional systems with semiconducting and semi-metallic properties respectively, are ideally suited to study the ultrafast dynamics of twodimensional electron–hole plasma generated after optical excitation and nonlinear optical properties.47 Figure 17(a & b) shows time-resolved differential transmission spectra of BCN and RGO suspensions and films at the highest pump fluence of 590 µJ cm-2. The dependence of the time constants and (∆T/T)max on the pump fluence are presented in Figure 17(c & d) for the RGO-suspension (open circles) and the RGO-film (filled circles) along with those for the BCNsuspension (filled stars). It can be seen that the magnitude of the initial transmission (∆T/T)max scales linearly with the pump fluence (Figure 17c) and the time constants decrease with increasing pump fluence for both the RGO-suspension and the RGO-film (Figure 17(d)).

Figure 17. (a) Femto-second differential probe transmission data for (a) BCN and (b) RGO with 3.2 eV pump and 1.57 eV nm probe pulses. Pump fluence was 590 µJ/cm2. The inset in (b)

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shows signals (up to 50 ps) from the RGO-suspension and the RGO-film. (c & d) Summary of the results from non-degenerate pump–probe experiments on the RGO-suspension shown as open circles, RGO-film as filled circles and BCN-suspension as filled stars. Reprinted with permission from ref 47 (Copyright 2010 Elseiver).

Photoluminescence spectra of BCN and BC5N (Figure 18a) show bands at 340 nm and 410 nm.44 It is observed that increase in the carbon content decreases the band gap. Figure 18b show the UV-visible spectra of different BCN films.11 Woessner et al48 have imaged propagating plasmons in graphene/BN using near field microscopy. Figure 18c shows the plasmon fringes of this composite and the laser wavelength to depend on carrier densities. On changing the carrier density there is a change in local optical response as shown in Figure 18d. The plasmon wavelength changes with frequency as shown in Figures 18(e & f). Dai et al49 show nanoinfrared imaging of graphene on h-BN.

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Figure 18 (a) PL spectra of borocarbonitrides of different compositions (...) BCN and (-) BC5N. Reprinted with permission from ref 44 (Copyright 2014 The IOP publishing). (b) Ultraviolet– visible absorption spectra of different BCN films. Reprinted with permission from ref 11 (Copyright 2010 Nature Publishing Group). (c) s-SNOM optical signal (d) Change in complex optical signal (e) Plasmon wavelength dependence on carrier density. (f) Frequency dependence of the plasmon. Reprinted with permission from ref 48 (Copyright 2014 Macmillan Publishers Limited) Gao et al50 have demonstrated electro-optic modulator based on a graphene-BN heterostructure. Graphene/BN sheet have been studied by infrared transmission technique for use in nanoresonators.51 Havener et al52 used a deep ultra violet-visible-near IR hyperspectral microscope for imaging graphene/h-BN heterojunctions. The high fluorescence of BCN can be used for sensing Ag+ ions.53 Density functional theory calculations on core-shell graphene-BN nanoflakes have been carried out to determine the dielectric function and optical quantities.54

4.4 Mechanical properties Mechanical properties are important in nanomaterials for applications.55 Pan et al56 have utilized Raman spectroscopy to study mechanical and thermal properties of graphene/BN heterostructures (Figure 19). Raman spectra of BN-graphene before and after annealing is shown in Figure 19a, where the annealed sample show blue shift of G and 2D bands (inset shows for BN). Figure 19b displays the Raman mapping G or 2D band position. Figure 19c shows the mapping of 2D band position (inset shows an AFM image). The 2D band red-shifts by 13 cm-1 as shown in Figure 19d. Dependence of mechanical properties of BCN films on BN concentration has been studied by density functional theory.55 24



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Figure 19 (a) Raman spectrum of graphene-BN (inset shows for BN) (b) G and 2D band positions of graphene-BN (c) Raman mapping of the 2D band (inset shows an AFM image) (d) Raman spectrum taken at the center of (c). Reprinted with permission from ref 55 (Copyright 2012 Nature Publishing Group).

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Figure 20. (a & b ) permittivity vs. frequency of BCN or RGO. (c) RGO, (d) BCN-AU, (e) BCN-AT frequency dependence. Reprinted with permission from ref 18 (Copyright 2013 Elseiver).

4.5 Microwave absorption

Borocarbonitrides show good microwave absorbing properties due to the adjustable band gap and conductivity.18 BCN was prepared by co-annealing graphene oxide with boric acid and urea at 900 °C and re-heated to 930 °C in an NH3 for 3 h. BCN-AU (ammonia untreated) and BCNAT (ammonia treated) were used to measure the electromagnetic parameters. The complex permittivities are shown in Figure 20a & b. The frequency dependence of the reflection of paraffin composites incorporated with 25 wt.% powders of GO or BCN are shown in Figure 20ce. BCN–AU is shown in Figure 20d and the reflection values are below −20 dB. Frequency dependence of BCN–AT is shown in Figure 20e and the reflection values are below −20 dB. The study reveals BCN to be good for microwave absorbing applications.

5. Results from Theoretical studies Monte Carlo simulations, predict the formation of BN domains in graphene to generate BCNs. Kumar et al6 have examined various configurations of the borocarbonitride, C4B2N2, with a hexagonal supercell containing a random distribution of B, C and N atoms (Figure 21a-d). The 26



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energetic stability of the bonds are in the order, B-N > C-C > C-N > C-B > B-B > N-N. The band gap of graphene can be tuned by varying the BN concentration. (Figure 22a-d).6

Figure 21. Different configurations of C4B2N2. Stability decreases from configuration (a) to (d). Reprinted with permission from ref 6 (Copyright 2013 Royal Society of Chemistry)

Figure 22. (a-c) Change in the band gap (Eg) of graphene with different BN concentrations. Reprinted with permission from ref 6 (Copyright 2013 Royal Society of Chemistry)

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Figure 23. Carrier mobility (µ) of borocarbonitrides (BxCyNz) for different B-C-N concentrations. Reprinted with permission from ref 57 (Copyright 2014 The Royal Society of Chemistry) Banerjee and Pati57 have used density functional theory to study the carrier mobility of BCN. Composition dependence of the intrinsic carrier mobilities is shown in Figure 23. Carbonrich compositions (BCN and BC4N) are metallic whereas the BN predominant compositions (B2.5CN2.5) are semiconducting with high hole mobility (∼106 cm2 V−1 s−1) compare to BCN (∼104 cm2 V−1 s−1) and BC4N (∼105 cm2 V−1 s−1).

A Monte-Carlo simulated annealing process has been employed to investigate electronic and structural properties of BCN over a wide doping range.58 For a given BN doping concentration, the doping-induced band gap can vary from 2.08 up to 10.42%. Kaloni et al59 used density functional theory to compare the electronic properties of BN-doped graphene monolayer, bi-layer, tri-layer, and multi-layer. Doping concentrations are between 12.5% and 75% and

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the obtained band gaps are from 0.02 eV to 2.43 eV. The effect of doping of B and N in graphene has been studied by first-principles electronic structure calculation.60 BN domains seem to be more likely to form in BCN. Effects of geometric shape and size of BCN superlattices have been studied.43 The band gap increases with increase in BN concentration. A study of BNdoping in graphene has shown that the bandgap to be dependent on BN concentration.61 Laterally integrated graphene-BN has been investigated to explore their growth on a Cu substrate, as well the electronic properties, and chemical topology.62 These heterostructures are metallic due to charge transfer effects from the Cu substrate.

6. Potential applications 6.1 Gas Adsorption

Borocarbonitrides synthesized using urea route exhibit high BET surface areas and adsorb remarkable amounts of CO2.17 BC2N (surface area of 1990 m2 g-1) shows a CO2 uptake of 128 wt.% at 195 K and 1 atm (see Figure 24a). At room temperature and 5 MPa, it shows 64 wt.% as shown in the inset of Figure 24a. CH4 adsorption-desorption isotherms for BC2N are shown in Figure 24b. The uptake of CH4 is 15 wt.% at room temperature and 5 MPa and increases by 2 wt.% at 273 K (inset of Figure 24b). Uptake of CH4 increases with the increase in surface area. BCN shows selectivity towards in uptake of CO2 and CH4.6 Adsorption of CO2 is nearly 10 times more selective over N2 at 1 atm and 293 K shown in Figure 24c&d. Selectivity of CO2 over H2, N2 and CH4 are 28:1, 8:1 and 4:1 respectively at 293 K and 5000 kPa.

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Figure 24. (a) CO2 (b) methane adsorption-desorption isotherms of BC2N (insets showing isotherms at room temperature). Reprinted with permission from ref 17 (Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (c) CO2 and N2 uptakes of BCN at 293 K and 1 atm. (d) Adsorption isotherms of CO2, CH4, N2 and H2 at 293 K for BCN. Reprinted with permission from ref 6 (Copyright 2013 The Royal Society of Chemistry)

6.2 Supercapacitors

Graphene shows excellent supercapacitor performance because of its high surface area and high electrical conductivity.63 Nitrogen-doped graphene has shown promising performance in supercapacitors.64-65 Supercapacitors based on B,N-doped graphene show good performance.66 The supercapacitors shows a specific capacitance of 62 F g−1.

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Figure 25. Supercapacitor performance of BCN samples (a) CV curves at 40 mV/s (b) CV curves of BCN-3 (c) Galvanostatic C-D curves (at 1 A/g) (d) Specific capacitance vs. current density. Reprinted with permission from ref 30 (Copyright 2016 Elsevier)

Borocarbonitrides prepared by the urea method show excellent supercapacitor properties.65 BC4.5N shows a specific capacitance value of 178 F g-1 in aqueous electrolyte whereas in an ionic liquid the value is 240 F g-1. BCN made from graphene show excellent supercapacitor performance.30 Cyclic voltammograms (CV) of BCN samples are shown in Figure 25a. BCN-3 (B0.26C0.22N0.52) shows more capacitance than EG, BCN-1 (B0.06C0.73N0.21) and BCN-2 (B0.13C0.49N0.38). Figure 25b shows CV curves of BCN-3 at different scan speeds. The galvanostatic charge-discharge (C-D) curves are shown in Figure 25c. The calculated specific capacitance of BCN-3 is 306 F g-1 at 0.2 A g-1. Figure 25d shows discharge curves measured at increasing current densities. BCN-3 shows specific capacitance of 225 F g-1 at 5 A g-1 which is larger than BCN-1, BCN-2 and EG.

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BN1–xGx composites have been examined for the use as supercapacitor electrodes and the specific capacitance value of BN0.25G0.75 is 238 F g−1.29 Nanostructured graphene-BN composites show enhanced electrical conductivity and supercapacitor performance (824 F g-1).46 BN/graphene synthesized by liquid-phase exfoliation show a specific capacitance of 140 F g−1.67 Shi et al.68 have fabricated the thinnest nanocapacitor electrode consisting of h-BN and graphene with different layers of h-BN ranging from bulk to 2 layers. The electrode with thickness of 0.8 nm (h-BN=2 layers) shows a capacitance of ~ 12 pF.

6.3 Oxygen reduction reaction

Doped graphenes have shown electrocatalytic activity equal to that of platinum with low cost and good durability.5, undoped

69

graphene

Three-dimensional BCN performs as an catalyst for ORR compared to foam

or

B-graphene

foam

and

N-graphene

foam

electrodes.27

Borocarbonitrides prepared with tunable B/N co-doping levels perform as efficient ORR electrocatalysts.70 B12C77N11 found to show superior catalytic activity. Figure 26a presents the CV curves of BCNs, showing that the reduction process occurs at about -0.28 V in the presence of oxygen. Linear-sweep voltammetry (LSV) curves given in Figure 26b show that the ORR activity of B12C77N11 is close to that of the commercial Pt/C catalyst. The electron transfer number n of ORR is close to 4. ORR activity has been studied in high surface area borocarbonitrides.71 The study shows a 4 e− process with better stability compare to Pt/C.

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Figure 26. (a) CV curves of ORR on BCN (b) LSV curves of ORR on BCN. Reprinted with permission from ref 70 (Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Electrocatalytic activity BCN prepared from graphene, boric acid and urea in ORR has been evaluated in alkaline media.30 The samples show ORR peaks at ranging from -0.15 to -0.24 V as shown Figure 27a. Linear-sweep voltammetric (LSV) measurements (Figure 27b) indicate that BCN shows enhanced activity for ORR. Linear-sweep voltammetry curves of BCN-3 by varying the rotations are show in Figure 27c. The electron transfer number (n) is ~3.7-3.9 for the BCN samples from Koutecky−Levich (K-L) plots (Figure 27d). Electrocatalytic activity of carboxylated graphene and covalently bonded BN1–xGx composites for ORR has been studied.29 BN0.25G0.75 shows a better ORR activity than BN0.5G0.5, BN0.75G0.25, and carboxylated graphene.

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Figure 27. (a) CV curves and (b) LSV curves of BCN samples measured in O2-saturated 0.1 M KOH. (c) Linear-sweep voltammetry curves of BCN-3 by varying the rotations (d) Koutecky−Levich (K-L) plots of BCN. Reprinted with permission from ref 30 (Copyright 2016 Elsevier)

6.4 Hydrogen evolution reaction

Platinum (Pt) is the efficient hydrogen evolution reaction (HER) catalyst, its availability and high cost limit its widespread use. Doped-carbon materials are promising catalysts due to their unique physicochemical properties.5 Chhetri et al.72 have investigated HER activity of borocarbonitride of various compositions. The onset potential for HER is -0.28 V for BCN whereas that for Pt/C is -0.23 V (Figure 28a). BCN samples follows the Volmer mechanism giving a Tafel slope of

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100 mV dec−1 (Figure 28b). The enhanced activity is due to the electronic structure of BCN which facilitates the transfer reactions. BCN shows more stability than with Pt/C. From the first principles calculations the, Fermi energy (EF) is found to around -3.2 eV for BC7N2. Calculations also show that BC7N2 and to a smaller extent BC8N are ideally suited for electrochemical hydrogen generation.

Figure 28. Hydrogen evolution reaction activity of BCNs. (a) LSV curves (b) Tafel plots of different BCNs, where the numbers 1-6 are BCNs and 7 Pt/C. Reprinted with permission from ref 72. (Copyright 2016 The Royal Society of Chemistry)

6.5 Lithium ion battery The lithium-ion battery plays a major role in battery technology.5 Borocarbonitrides show high reversible capacity and the charge-discharge curves of BC5N are shown Figure 29A.73 Specific capacities of BC5N, BC1.3N and BC1.5N at a current density of 0.05 A g−1 are shown in Figure 29B (also see Figure 29C). B2C1.5N, BC1.3N, and BC5N show discharge capacities of 1045, 1571

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and 1855 mA h g−1 respectively. BC5N shows a reversible capacity of 150 mA h g−1 (Figure 29C). Increase in B and N content increases the Coulombic efficiency as shown Figure 29D.

Figure 29 (A-D) Battery performance of BxCyNz. Reprinted with permission from ref 73 (Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

6.6 Electrochemical sensors

Borocarbonitrides detect dopamine (DA) and uric acid (UA) selectively in the presence of ascorbic acid.74 Figure 30a shows the differential pulse voltammetry at varying concentrations of dopamine. When the dopamine concentration is increased there is a linear increase in the current as well (Figure 30b), the detection limit being is 2.1 µM for dopamine (Figure 30c). A similar behavior has been observed in the case of uric acid with the detection limit being 4 µM (Figure 30d).

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Figure 30. Differential pulse voltammetry curves of BCN in (a) dopamine and (c) uric acid. Current vs. concentrations of (b) dopamine and (d) uric acid. Reprinted with permission from ref 74 (Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

6.7 Field-effect transistors (FETs) Graphene has a carrier mobility as high as 2 × 105 cm2 V-1 s -1.2-3 Borocarbonitrides allow the possibility of fabricating FETs because of the energy gap present in them. Electrical properties of BCN films is shown in Figure 31a.11 The FET of a BCN sample (40 at.% C) show ambipolar semiconducting behavior. The electron and hole mobilities of BCN devices are in the range of 520 cm2 V-1 s-1. Transfer characteristics of BCN (B0.13C0.49N0.38) is shown in Figure 31b. BCN exhibits carrier mobility values of 10.7 cm2 V-1 s-1.30

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Figure 31. (a) Current-voltage (I–V) curves of BCN. Reprinted with permission from ref 11 (Copyright 2010 Macmillan Publishers Limited) (b) Transfer characteristics of BCN (B0.13C0.49N0.38). Reprinted with permission from ref 30 (Copyright 2016 Elsevier) (c) I-V curves of G-BN devices. Reprinted with permission from ref 75 (Copyright 2013 Nature Publishing Group). (d) I-V curves for a BCN sample at different temperatures. Reprinted with permission from ref 13 (Copyright 2013 American Chemical Society). (e) Characteristics of graphene-BN. Reprinted with permission from ref 10 (Copyright 2012 Macmillan Publishers Limited) (f) I-V measurements of graphene/BN. Reprinted with permission from ref 37 (Copyright 2013 Macmillan Publishers Limited).

Figure 31c shows the transport properties of graphene with photo-induced BN-doping leads to a shift of the charge neutral point.75 Figure 31d shows the FET Ids-Vg transfer 38



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characteristics of the BCN films demonstrating ambipolar behavior (µ=12-60 cm-2 V-1 s-1).13 I-V curves of a hybrid graphene/BN FET device are shown in Figure 31e.10 Graphene shows conducting behavior with the h-BN showing no conductivity (sheet resistance Rsheet >400 TΩ). Graphene/h-BN structure field-effect transistor (FETs) devices are shown in Figure 31f (Insets show I-V curves).37 The mobilities of graphene/h-BN FETs range from ~190 to 2,000 cm2 V-1 s1

2

. Graphene encapsulated by boron nitride is reported to show a carrier mobility up to 80,000 cmV-1 s-1 at room temperature.76

6.8 Photoresponse Graphene exhibits photoconductive behaviour.2-3 Shiue et al76 have demonstrated photodetector based on a hybrid graphene/BN. The detector has been tested using 250 fs pulses at 1800 nm. Figure 32a shows a photocurrent with non-linear photoresponse. Figure 32b plots photocurrent traces as a function of the time delay ∆t between pairs of 250 fs laser pulses for a range of incident powers. The half-width at half-maximum response time τ are plotted in Figure 32c for a sweep of input powers. The minimum response time is approximately ∼3 ps. Figure 32d shows the autocorrelation trace obtained from an auto-correlator (average power of only 1.4 µW (peak power of 67 mW)) which displays a clear dip with a minimum resolution down to 3 ps. Figure 32e shows variations in the current from the BCN (B0.13C0.49N0.38) in response to UV light (365 nm, 7 W).30 The on/off ratios are in the range 1.2-1.3, undoped graphene shows no response.

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Figure 32 (a) Photocurrent of the on-chip graphene detector under pulsed (250 fs) excitation. The black curve shows a fitting of the experimental data (red dots) with IPhoto ∝ Pin0.47±0.003. (b) Autocorrelation traces of the graphene-based autocorrelator. (c) Response time (τ) of the graphene autocorrelator at different input excitation power. (d) Autocorrelation trace measured with low input average power of 1.4 µW, corresponding to a peak power of 67 mW. Reprinted with permission from ref 76 (Copyright 2015 American Chemical Society). (e) UV response (on, blue and off, black) of BCN (B0.13C0.49N0.38). Reprinted with permission from ref 30 (Copyright 2016 Elsevier)

7. Conclusions The discussion in the previous sections should suffice to provide an adequate description of the ways of preparing borocarbonitrides, and also to demonstrate the variety of novel properties exhibited by these materials. Importantly, many of the properties of borocarbonitrides can be exploited for applications. It is noteworthy that borocarbonitrides exhibit properties related to

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energy devices comparable or superior to graphene and other layered materials. Borocarbonitrides are also superior to MoS2 in some of the applications. It is significant that the composition of borocarbonitrides can be varied as also the band gap. Tunability of the band gap is a significant feature that can be exploited for several purposes besides the fabrication of field effect transistors. Use of these materials for the hydrogen evolution reaction, oxygen reduction reaction and in supercapacitors is of practical importance. Borocarbonitrides prepared by some of methods of synthesis are yet to be fully examined for generating materials with desirable features. For example, borocarbonitrides prepared by gas phase reactions may be useful for certain applications. Clearly, there is much to be done in exploiting borocarbonitrides for useful purposes. Furthermore, it may be necessary to determine some of the important properties of borocarbonitrides with stringent control of the composition and structure.

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