Incorporate boron and nitrogen into graphene to make BCN hybrid nanosheets with enhanced microwave absorbing properties

Accepted Manuscript Incorporate Boron and Nitrogen into Graphene to Make BCN Hybrid Nano‐ sheets with Enhanced Microwave Absorbing Properties Yue Kang, Zengyong Chu, Dongjiu Zhang, Gongyi Li, Zhenhua Jiang, Haifeng Cheng, Xiaodong Li PII: DOI: Reference:

S0008-6223(13)00394-1 http://dx.doi.org/10.1016/j.carbon.2013.04.085 CARBON 8016

To appear in:

Carbon

Received Date: Accepted Date:

5 March 2013 28 April 2013

Please cite this article as: Kang, Y., Chu, Z., Zhang, D., Li, G., Jiang, Z., Cheng, H., Li, X., Incorporate Boron and Nitrogen into Graphene to Make BCN Hybrid Nanosheets with Enhanced Microwave Absorbing Properties, Carbon (2013), doi: http://dx.doi.org/10.1016/j.carbon.2013.04.085

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Incorporate Boron and Nitrogen into Graphene to Make BCN Hybrid Nanosheets with Enhanced Microwave Absorbing Properties Yue Kang a,b , Zengyong Chua,*, Dongjiu Zhangb, Gongyi Lia, Zhenhua Jianga, Haifeng Chengb, Xiaodong Lia a

Department of Chemistry and Biology, College of Science, National University of Defense

Technology, Changsha 410073, P. R. China. b

Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of

Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, P. R. China.

ABSTRACT This work demonstrates the conversion of graphene oxide into BCN hybrid nanosheets by reaction with boric acid and urea at 900℃, during which the boron and nitrogen atoms are incorporated into graphene atomic sheets. X-ray diffraction pattern and X-ray photoelectron spectroscopy reveal the existence of h-BN. High-resolution electron microscopy and Raman spectrum indicate the presence of graphene-like layers with h-BN nanodomains. The content of h-BN in the BCN nanosheets can also be tuned by further heat-treatment in ammonia environment, which in turn affects the band gap of these nanosheets. The electromagnetic parameters suggest that these samples can be used as good microwave absorbing materials at G band (5.6-8.2GHz) and X band (8.2-12.4GHz). This study provides a simple route to BCN hybrid nanosheets with tunable band gap and adjustable conductivity for microwave absorbing applications.

Corresponding Author: Zengyong Chu (Z.Y.Chu) Fax: 0086-731-84574250 E-mail address: [email protected]

1. Introduction Graphene, which consists of atom-thick sheets of carbon, organized in a honeycomb structure that resembles chicken wire, has attracted tremendous attention from both academic and commercial communities owing to its long-range π-conjugation yielding extraordinary thermal, mechanical and electrical properties [1-5]. Its two-dimensional (2D) network is the fundamental building block of other carbon-based materials, such as 3D graphite and diamond, 1D carbon nanotubes, 0D fullerene. The chemistry of graphene reported in the literature mainly concerns the chemistry of graphene oxide (GO) with chemically reactive oxygen functionalities, including carboxylic acid groups at the edges of GO, and epoxy and hydroxyl groups on the basal planes

[6]

. GO is a wide band gap material but a controlled

oxidation/reduction process provides tenability of the electronic, optical, and mechanical properties including the possibility of accessibly zero-band-gap graphene via complete removal of the C-O bonds. After annealed, graphene oxide becomes graphene. As a zero-band-gap material, graphene has a very poor on/off current ration. To realize graphene-based logic circuits and electrical systems, researchers have been trying to find ways to open the zero-band-gap of graphene, for example by making nanostructures including graphene quantum dots

[7]

. Several other groups have

reported methods to open the band gap of graphene by using suitable underlying substrates [8], electrical gating [9] or putting molecules on graphene [10-14]. Alternatively, theoretical and experimental studies have suggested that substitutional doping can

modulate the band structure of graphene, resulting in a metal-to-semiconductor transition in graphene

[15-18]

. When doping atoms such as B and N replace carbon in

the graphene lattice to form covalent bonds with neighboring carbon atoms, the lattice structure of graphene is changed, thus leading to the modification of the electronic structure of graphene and opening the band gap [15-16]. In this aspect, the application of graphene will be greatly widened, used such as supercapacitors [20-21]

[19]

and memoristors

, based on the different doping elements and their contents. Their microwave

absorbing properties should also be different from those of the graphene. Recently, Bartlettand

[22]

and co-workers prepared a composition close to B1C1N1 by the

reaction of BCl3, NH3 and acetylene. Ci et al

[23]

used the chemical vapor deposition

(CVD) method to synthesize BCN. Kalyan and co-workers

[24]

obtained BCN from

charcoal, boric acid and urea, which is simple but not easy to obtain separable sheets. Wu et al [25] used GO, B2O3 vapor and NH3 to synthesize BCN through co-annealing, which could lead to the doped graphene layers but not in large-scale. Herein, we demonstrate an improved annealing route to make BCN hybrid nanosheets in a relatively large scale. GO was used as the starting template. Boron and nitrogen atoms were incorporated by co-annealing with boric acid and urea. It is noteworthy that the reaction occurs at the temperature as low as 900 . The content of BN in the BCN hybrid nanosheets can also be tuned by further heat-treatment in an ammonia environment, which in turn affects the band gap of these nanosheets. The band gaps of the doped graphene were opened and adjusted, making their microwave absorbing properties greatly improved.

2. Methods 2.1 Materials Boric acid (99.5%) was purchased from Taishan Shiji Co. Ltd. (Taishan, China). Urea (99.0%) was obtained from Xiangke Chemical Work. (Changsha, China). Graphite particles (10~15 m) were purchased from Xinghe Graphite Co. Ltd. (Qingdao, China). High purity ammonia (99.999%) was obtained from Date Gas Co. Ltd. (Dalian, China). All these chemicals were used without further purification. Deionized water used in all the experiments was produced from a Millipore-ELIX water purification system. 2.2 Preparation Large and single-layer GO nanosheets were prepared by a modified Hummers method as reported

[26-27]

. The obtained GO (0.2g) was thoroughly mixed with a

solution of boric acid (0.03g) and urea (0.36g) in demonized water (50ml). The mixture was warmed to 80

to form a thick slippery liquid, which was then dried at

the same temperature for 36h in vacuum. The dried mixture was then heated at 900 for 10h in N2 atmosphere. The sample was cooled to the room temperature, and re-heated to 930

in an NH3 environment with duration of 3h. The black products

obtained before and after the ammonia treatment are both BCN samples, named BCN-AU (ammonia untreated) and BCN-AT (ammonia treated) respectively. As a comparison, reduced GO (rGO) was obtained from GO using the same heating procedure as BCN-AU, namely, heating reduced at 900 ℃ for 10h in N2 atmosphere.

2.3 Characterization The surface morphologies of BCN nanosheets were characterized by scanning electron microscope (SEM) using JSM-6700F microscope. The crystalline structure was investigated by X-ray diffraction (XRD) on a D8ADVANCE type, using CuKa radiation with 2θ from 10o to 90o. Raman spectroscopy (514 nm, Ar+ ion laser) was used to study the doping defect structure. The Raman spectra were obtained using a laser confocal Raman spectrometer (LABRAM-010) in the range from 400 to 2000cm-1. UV–Visible (UV/Vis) spectra were recorded with a UV-1800 spectrophotometer and quartzcells with 1 cm path length. Transmission electron microscopy (TEM) was conducted using a JEM-2100F electron microscope at an acceleration voltage of 200 kV with a CCD camera. The sheet resistance and conductivity was measured by a four-point probe method (Model ST-21 multimeter) at room temperature, during which the sample was pressed into thin membrane. X-ray photoelectron spectroscopy (XPS) was investigated using K-Alpha 1063 type with focused monochromatized Al Kα radiation (1486.6 eV), to determine changes in the atomic ratios and the existence of functional groups. The relative complex permittivity ε = ε′ - jε′′ (ε′ and ε′′  are the real and imaginary parts of the complex permittivity, respectively) of the BCN–paraffin composites were measured by Agilent 8720ET vector network analyzer, over the microwave frequency range of 2–18 GHz. The reflection coefficients, R(dB), of the rGO, BCN-AU and BCN-AT were simulated and obtained using our self-programmed RAMCAD software.

3. Results and Discussion It is noteworthy that the lowest temperature for the successful incorporation of BN into GO is 1500℃ if N2 is used in the doping experiment [25]. However, the use of gaseous ammonia enables the doping process at much lower temperatures (900oC-930oC)

[25]

. A previous study of the synthesis of BCN nanotubes via

substitution reaction also shows that the reaction temperature can be lowered by using ammonia to replace nitrogen gas

[28]

. The relatively low reaction temperature is

advantageous because most furnaces can be fit for the reaction. Typical SEM images of BCN samples are shown in Fig.1. As displayed in Fig.1 (a-c), BCN-AU has a honeycomb-like structure at the micrometer scale. It seems that the ultrathin graphene oxide nanosheets are interconnected with B2O3 (or BN) flakes. The size of the flakes is ranging from 0.5 um to 3 um. After ammonia treatment, as shown in Fig.1 (d-e), the flakes look disappeared on the surface of BCN-AT. This is mainly because B2O3 reacts with NH3, producing uniform BN, which is in agreement with the following XPS analysis.

Fig.1- Typical SEM images of (a-c) BCN-AU and (d-e) BCN-AT.

Fig.2 shows the XRD patterns of the dried samples from the raw material mixture to the BCN nanosheets. The indication of GO is found at 5o (2θ) in Fig.2 (a) [26-27]

. While for the BCN samples shown in Fig.2 (b), only hexagonal phase of BN

could be observed. It shows characteristic peaks centered at about 26o (2θ), belonging to the (002) interlayer reflection of h-BN. The broadening of the peaks indicates the formation of nanosized domains and low correlation lengths. The Scherrer formula applied to the (002) reflection yields a crystalline spacing of 0.33 nm, in agreement with the following high-resolution TEM (HRTEM) analysis.

 

Fig.2- XRD patterns of (a) mixed GO, urea and boric acid and (b) BCN samples.

Typical Raman spectra of GO, rGO and BCN samples are shown in Fig.3. For GO and rGO, the peaks centered at around 1360cm-1 and 1590cm-1 correspond to D and G bands, respectively. G band is generally observed in single crystalline graphite and attributed to in-plane bond stretching of pairs of sp2-C atoms; D mode is associated with defects or lattice distortion

[29-31]

. 2D band centered at around 2703

cm-1 is typically used to indicate the quality of graphene films [29-31]. For BCN-AT, the three dominant peaks of D, G and 2D bands are located at 1399cm-1 1582cm-1 and 2696cm-1 respectively. Both D and G bands are broadened in

BCN samples compared to GO and rGO. It indicates the increase of defects and lattice distortion when B and N are incorporated. Especially after the ammonia treatment, the two bands look combined together. In addition, the up shift of 2D peak in BCN samples is observed, possibly due to the presence of BN nanodomain incorporated in graphene leading to the increase of disorders. So all the above Raman features suggest that disorder is increasing from rGO, to ammonia-untreated BCN-AU and to ammonia-treated BCN-AT.

Fig.3- Raman spectra of GO, rGO and BCN samples.

As expected, a homogeneous heteroatom doping in graphene was realized because X-ray photoelectron spectroscopy (XPS) reveals that significant contents of N and B are incorporated in graphene. Fig.4 shows the XPS spectra of BCN-AU. Full spectrum in Fig.4 (a) indicates the existence of B, C, N and O elements. The B1s peak in Fig.4 (b) can be deconvolved into three peaks at 188.5eV, 190.8eV and 192.9eV, respectively. The

main peak at 190.8eV is very close to the reported value for B1s (190.75eV) in BN nanosheets synthesized by the CVD method [32]. Furthermore, the peak at a relatively lower binding energy (188.5eV) corresponds to B-C bonding, and at the higher binding energy (190.8eV) to B-O bonding. This is in good agreement with the reported values for BCN

[23-25]

. The N1s peak in Fig.4 (d) can also be deconvolved

into three peaks corresponding to N-B (397.9eV), B-N-C (398.8eV) and graphitic N (401.3eV), respectively

[19]

. The middle peak corresponds to N bonded to both C and

B, and the last is N bonding in a graphitic-like configuration. According to the intensity and energy of the major peak in B1s and N1s spectra, it can be concluded that B and N are mainly bonding as B-N, which strongly implies the existence of BN domains in the BCN nanosheets.

Fig.4-XPS spectra of BCN-AU. The curves are deconvolved by Gaussian fitting, thereby indicating possible bonding structures.

The C1s peak in Fig.4 (c) can be deconvolved into five peaks centered at 283.1eV, 284.6eV, 285.9eV, 287eV and 288.38eV, respectively. The C1s peak was composed mostly of C-C (284.6eV) and C-N (285.9eV) components with additional broadening due to disorder and a possible C-O (287eV) contribution

[33]

. Moreover, a

shoulder at the lower binding energy was apparent, which indicates possible boron carbide (283.1eV) formation. The peak at the higher binding energy was possible C-R (288.38eV), which is the carboxylic acid groups at the edges of GO. Fig.5 shows the XPS spectra of BCN-AT. The B1s peak in Fig.5 (b) can be deconvolved into three peaks at B-C (189.97eV), B-N (190.76eV) and B-O (192.97eV), respectively. The N1s peak in Fig.5 (d) can be deconvolved into three peaks of N-B (397.9eV), B-N-C (398.78eV) and graphitic-like N (400.7eV), respectively. The C1s signal in Fig.5 (c) can be deconvolved into bands at 283.11eV, 284.6eV, 285.82eV, 287.08eV and 288.38eV, respectively, assigned to five different kinds of carbon atoms bonded to B, C, N, O and R respectively. Compared to Fig.4 for BCN-AU nanosheets, the intensities of B-O and C-O bonding in the ammonia-treated BCN-AT nanosheets are much weakened, suggesting the removal of oxygen functionalities in the ammonia environment as well as the transformation of B2O3 into BN. So during the ammonia treatment, possible reactions include, B-O(s) + NH3 (g) → B-N(s) + B-O(s)

H2O(g)

→ BO (g)

C-O(s) + NH3 (g) → C-N(s) +

(1) (2)

H2O(g)

(3)

→ CO (g)

(4)

+ H2O(g) → CO (g) + H2 (g)

(5)

C-O(s) C(s)

Among them, Eq.1-4 are leading to O decrease, Eq.4-5 C decrease, Eq.2 B decrease and Eq.1,3 N increase. The elemental changes were obtained from XPS analysis and are listed in Table 1. We can see that the chemical formulas for BCN-AU and BCN-AT are B1.0C1.5N0.7O0.6 and B1.0C1.0N0.9O0.2 respectively. Because Eq.2 is suppressed by Eq.1 in NH3, we can make an assumption that B remains stable during the ammonia treatment. Thus, it can be concluded that O and C decreased and N increased during the ammonia treatment. The reduction of C is probably due to the contaminated water in the ammonia gas according to reaction of Eq.5, in addition to the reaction of Eq.4.

Fig.5-XPS spectra of BCN-AT. The curves are deconvolved by Gaussian fitting, thereby indicating possible bonding structures.

Table 1.The atomic ratios of BCN samples Elements B C N O

BCN-AU At. % Ratio 26.56 1.0 39.33 1.5 17.64 0.7 16.47 0.6

BCN-AT At.% Ratio 31.36 1.0 32.64 1.0 28.32 0.9 7.68 0.2

Typical TEM images of BCN-AT are shown in Fig.6. The images indicate the presence of BCN layers similar to few-layered graphene. HRTEM in Fig.6 (c) indicates a flake-like nanodomain assigned to h-BN, incorporated into the GO lattice. The flakes are transparent and stable under the electron beam. To further evaluate the nanodomain, SEAD was carried out on the layer and is shown as an insert image. It clearly reveals the distinctive hexagonal structure of h-BN films [34-35].

Fig.6-Typical TEM images of BCN-AT. (c) HRTEM image indicates h-BN nanodomain combined with the GO lattice. The insert in (c) is the corresponding SEAD of the h-BN region. Previous investigations have shown that bulk h-BN is a wide gap material, which is a very good insulator [36]. It is also confirmed that the electronic properties of BCN

nanosheets are located between those of the graphene and the monolayer BN

[35-36]

.

Furthermore, the band gap of BCN compounds can be tuned by varying the atomic configuration and their composition. In this study, the UV/Vis absorption spectrum was carried out to investigate the band gap of the BCN samples based on their optically induced transition. The following Tauc’ equation was used to determine the optical band gap of BCN nanosheets, Eg[37].

ω 2ε = (hω − Eg ) 2

(6)

Where ε is the optical absorbance and ω = 2π / λ is the angular frequency of the

incident radiation. On the basis of Tauc’ formulation, it is speculated that the plot of

ε 1 / 2 / λ versus 1 / λ should be a straight line at the absorption range. Therefore, the intersection point with X axis is 1 / λg . The optical band gap can be calculated based

on Eg = hc / λg . As shown in Fig.7, B2O3 (or BN) and graphene domains coexist in the BCN nanosheets. For BCN-AU in Fig.7 (a), the first absorption edge corresponds to a band gap of 5.12 eV, which belongs to B2O3 (or BN) domains in the nanosheets. The second absorption edge suggests a band gap of 2.87 eV, related to B, C and N combined domains. As shown Fig. 7 (b), the absorption spectrum displays only one sharp adsorption peak at 206nm, and the calculated gap wavelength is about 389 nm, corresponding to a band gap of 3.18 eV. It indicates the success of incorporating B and N atoms and h-BN is relative uniformly distributed on the GO nanosheets.

Fig.7-Plot of ε 1 / 2 / λ versus 1 / λ for (a) BCN-AU and (b) BCN-AT. Parameters are derived from the UV/Vis absorption spectra in the insets. GO itself is a kind of insulator with infinite resistance but graphene is a very good conductor [1-5]. As shown in Table 2, the rGO nanosheets have a sheet resistance and conductivity of 0.19 Ω/sq and 3.37×106 S/m, respectively. h-BN is also a good insulator, so its incorporation in graphene will influence the electrical conductivities. When B and N incorporated, for BCN-AU, the sheet resistance increased to 25.94 Ω/sq and the conductivity decreased to 1.54×104 S/m, respectively. After the ammonia treatment, for BCN-AT, the sheet resistance was nearly doubled companied with a slight decrease in conductivity. Table 2. Electrical properties of rGO and BCN membranes Samples Sheet Resistance Conductivity Thickness (Ω/sq) (S/m) (μm) 6 rGO 0.19 3.37×10 1.56 4 BCN-AU 25.94 1.54×10 2.50 2.67 BCN-AT 46.95 0.85×104

There has been increasing applications of microwaves, for example, using G-band (5.6-8.2 GHz) and X-band (8.2-12.4 GHz) for radar systems. Microwave

absorbing materials are thus in great demand. BCN is a good candidate for microwave absorbing due to its adjustable band gaps and conductivities. In order to evaluate the microwave absorbing properties, we prepared paraffin composites incorporated with 25 wt% BCN samples and carried out measurement of the electromagnetic parameters. The real part (ε′) and the imaginary part (ε′′) of the complex permittivity are shown in Fig.8 (a) and (b) respectively. Complex permeability ( = ′+ ′′) is not shown because the sample has no magnetic properties, namely, ′≈1, ′′≈0. We can see that ε′ and ε′′ of the permittivity of BCN-AT is lower than those of BCN-AU, while those of rGO are much higher than both of them. This indicates that the incorporation B and N atoms lowers the complex permittivity and the ammonia treatment could further reduce the value by increasing more content of h-BN.

Fig.8-(a) The real part (ε′) and (b) the imaginary part (ε′′) of the complex permittivity plotted against frequency for the paraffin composites with 25 wt% rGO or BCN samples. (c) rGO, (d)BCN-AU, (e) BCN-AT, Frequency dependences of the reflection coefficient for the paraffin composites at different thicknesses from 0.2 mm to 3 mm.

According

to

transmission

line

theory,

the reflection

coefficient

of

electromagnetic wave, R(dB), under perpendicular wave incidence at the surface of a single-layer material backed by a perfect conductor can be defined by [38] 2πjfd εμ μ tanh( ) −1 c ε R (dB) = 20 lg 2πjfd εμ μ tanh( ) +1 c ε

(7)

Where, R is the reflection value, f is the frequency, d is the thickness of the material, c the light velocity in vacuum, j is the imaginary unit. In addition, the complex permittivity is ε=ε′+ ε′′; the complex permeability is = ′+ ′′. If the reflection value of a material is below -10dB, 90% of microwave energy will be absorbed. In this case, the material is good or acceptable for microwave absorbing applications. Furthermore, if the reflection value is below -20dB, the absorption value will be as high as 99%. It will be a very good microwave absorbing material. The frequency dependences of the reflection for the paraffin composites incorporated with 25 wt% powders were simulated using our self-programmed RAMCAD software. Fig.8 (c), (d) and (e) give the detailed reflection coefficients for rGO, BCN-AU and BCN-AT respectively. The thickness is ranging from 0.2 mm to 3 mm. For BCN-AU shown in Fig.8 (d), the reflection values below -20 dB were obtained in the 8.24-14.48 GHz range with a thickness of 1.4-2.4 mm, and a minimum value of -28 dB was observed at 11.28 GHz with a thickness of 1.8 mm; For BCN-AT shown in Fig.8 (e), the reflection values below -20 dB were obtained in the 7.76-17.84

GHz range with a thickness of 1.6-3mm, and a minimum value of -33.6 dB was observed at 15.28 GHz with a thickness of 1.6 mm. While the reflection values of rGO are much larger than those of BCN samples, no chance to reach -20 dB. It can be concluded that BCN samples, both BCN-AU (B1.0C1.5N0.7O0.6) and BCN-AT (B1.0C1.0N0.9O0.2), have a much widened absorption band below -20dB. The B and N incorporation and ammonia treatment can adjust the location of the reflection peaks, and influence the reflectivity intensities. Due to the 2D planar structure of the graphene oxide, we plan to blend the BCN hybrid nanosheets with some other polymers to form film-like or coating-purpose microwave absorbing materials.

4. Conclusion

It has been possible to synthesize a BCN composition by reacting graphene oxide with urea and boric acid. It has a two-dimensional layered structure. XRD and XPS results suggest that the treatment produces BCN that consists predominantly of domains of hexagonal BN and graphene. After the ammonia treatment, this behavior correlates to the increased surface roughness, hexagonal lattice intercalation, surface defects and disorder at boundaries by TEM, SEM, XRD, Raman and XPS analysis. UV/Vis spectra of the BCN samples show that the band gap of graphene is widened by the incorporation of h-BN in the nanosheets. The electromagnetic parameter suggests that the BCN composites can be used as potential microwave absorption materials at G band (5.6-8.2GHz) and X band (8.2-12.4GHz). The method of synthesizing nanosheets proposed in this study may be feasible for future large-scale

production of BCN hybrid nanosheets. The results indicate that B and N incorporation can open and modulate the band gap of graphene, and widen their applications in the microwave absorbing field.

5. Acknowledgements

The project was financially supported by National Natural Science Foundation of China (No. 51073172) and Basic Science Foundation of NUDT (No. JC11-01-01).

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Table 1. The atomic ratios of BCN samples Table 2. Electrical properties of rGO and BCN membranes

Fig.1-Typical SEM images of (a-c) BCN-AU and (d-e) BCN-AT. Fig.2-XRD patterns of (a) mixed GO, urea and boric acid and (b) BCN samples. Fig.3-Raman spectra of GO, rGO and BCN samples. Fig.4-XPS spectra of BCN-AU. The curves are  deconvolved by Gaussian fitting, thereby indicating possible bonding structures. Fig.5-XPS spectra of BCN-AT. The curves are deconvolved by Gaussian fitting, thereby indicating possible bonding structures. Fig.6-Typical TEM images of BCN-AT. (c) HRTEM image indicates h-BN nanodomain combined with the GO lattice. The insert in (c) is the corresponding SEAD of the h-BN region. Fig.7- Plot of ε 1 / 2 / λ versus 1 / λ for (a) BCN-AU and (b) BCN-AT. Parameters are derived from the UV/Vis absorption spectra in the insets. Fig.8-(a) The real part (ε′) and (b) the imaginary part (ε′′) of the complex permittivity plotted against frequency for the paraffin composites with 25 wt% rGO or BCN powders. (c) rGO, (d)BCN-AU, (e) BCN-AT, Frequency dependences of the reflection coefficient for the paraffin composites at different thicknesses from 0.2 mm to 3 mm.  

Table 1.The atomic ratios of BCN samples Elements B C N O

BCN-AU At. % Ratio 26.56 1.0 39.33 1.5 17.64 0.7 16.47 0.6

BCN-AT At.% Ratio 31.36 1.0 32.64 1.0 28.32 0.9 7.68 0.2

       

Table 2. Electrical properties of rGO and BCN membranes Samples Sheet Resistance Conductivity Thickness (Ω/sq) (S/m) (μm) 6 rGO 0.19 3.37×10 1.56 BCN-AU 25.94 1.54×104 2.50 4 BCN-AT 46.95 0.85×10 2.67