Kinetic study of boron doped carbon nanotubes synthesized using chemical vapour deposition

Accepted Manuscript Kinetic Study of Boron Doped Carbon Nanotubes Synthesized Using Chemical Vapour Deposition Anita Sharma, Ashwin Patwardhan, Kinshuk Dasgupta, Jyeshtharaj B. Joshi PII: DOI: Reference:

S0009-2509(19)30533-0 https://doi.org/10.1016/j.ces.2019.06.030 CES 15051

To appear in:

Chemical Engineering Science

Received Date: Revised Date: Accepted Date:

5 February 2019 11 June 2019 20 June 2019

Please cite this article as: A. Sharma, A. Patwardhan, K. Dasgupta, J.B. Joshi, Kinetic Study of Boron Doped Carbon Nanotubes Synthesized Using Chemical Vapour Deposition, Chemical Engineering Science (2019), doi: https:// doi.org/10.1016/j.ces.2019.06.030

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Kinetic Study of Boron Doped Carbon Nanotubes Synthesized Using Chemical Vapour Deposition Anita Sharma1, Ashwin Patwardhan1, Kinshuk Dasgupta2, Jyeshtharaj B. Joshi1,3 1 Department

of Chemical Engineering, Institute of Chemical Technology, Mumbai 400019, India

2Materials 3Homi

Group, Bhabha Atomic Research Centre, Mumbai 400085, India

Bhabha National Institute, Anushaktinagar, Mumbai 400094, India



Corresponding author. Present address :Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-19, Phone: +91 22 3361 2106 Email address : [email protected]

1

Abstract Boron doped carbon nanotubes were synthesized using chemical vapour deposition using acetylene as carbon source, boric acid as boron source and Ferrocene/MgO as catalyst/support. Boric acid was directly used as a solid precursor for doping boron in the CNT lattice. A kinetic model was established by varying the temperature of the reactor, partial pressure of reactants, flow rates and catalyst concentration. The rate controlling steps were studied and the optimal reaction conditions were established so as to synthesize the desired quantity and quality of boron doped carbon nanotubes. Boron doping of 6.31-6.71 at. % was obtained. Two different mechanisms were found to control the rate of reaction at different range of temperatures. The activation energy for the two mechanisms was found to be 18.21 kJ/mol and 6.73 kJ/mol respectively. A mechanism was proposed and validated using the experimental data so as to understand the growth of B-CNTs. The synthesized B-CNTs were purified using concentrated hydrochloric acid and characterized by using SEM, TEM to understand the surface characteristics; FTIR and XPS to detect and quantify the boron present in the sample, Raman spectroscopy and TGA analysis to determine the purity of the product. Keywords: Boron, carbon nanotubes, doping, kinetics, chemical vapour deposition, mechanism

2

1. Introduction The hexagonally packed carbon nanotubes (CNTs) have been widely studied since the time of its discovery (Mitri and Sotirchos, 2005). Their unique electronic, mechanical and chemical properties have been explored so as to utilise it in an efficient manner for high end applications (Ebbesen, 1996; Ruoff and Lorents, 1995; Saito et al., 1998). These properties, especially the electronic properties of the CNTs can be specifically tuned by deliberately introducing the defects in the CNTs. Besides, the stability of CNTs with respect to its chemical reactivity has always been a hindrance for its use in different reaction environments. For this reason, heteroatom doping has been extensively studied (Ayala et al., 2008; Ewels et al., 2007; Trasobares et al., 2002). Nitrogen and Boron are two most popularly used dopants as they can be directly intercalated within the carbon or substitutionally doped by replacing one or more carbon atoms(Ewels et al., 2010). This helps in changing the electronic configuration thereby making the stable CNTs more reactive. Doping of CNTs can be successfully carried out using different methods (Terrones, 2003), out of which chemical vapour deposition (CVD) is one of the simplest and the most efficient techniques (Kumar and Ando, 2010). Although there have been extensive studies on nitrogen doped carbon nanotubes (N-CNTs), with regards to doping mechanism (MacKenzie et al., 2010; Mohammad, 2014; Susi et al., 2011) , kinetics of doping (Sharma et al., 2017) and large scale production of N-CNTs (Huang et al., 2012), boron doped carbon nanotubes (B-CNTs) are a way behind. There is a greater need to explore the doping methodology of B-CNTs for better quality and quantity of production. The addition of boron atoms into the CNT lattice depends on choice of precursor material, reaction temperature, catalyst type and concentration, reactant concentration, gas flow rate, partial pressure of the gas as well as the reactor total pressure. The combined effect of all these parameters is a base to produce desired quantity and quality of B-CNTs. The literature studies suggest that B-CNTs have been synthesized using different forms of CVD (Lyu et al., 3

2011; Panchakarla et al., 2010; Watanabe et al., 2010) as well as other methods like chemical doping of CNTs (Redlich et al., 1996). Even though a huge literature is available, there is a lack of systematic evaluation of the data which can help predict the exact nature of assynthesized B-CNTs. For instance, there is no proper growth mechanism described for B-CNTs comparable to that of N-CNTs, wherein many theories (Mohammad, 2014; Susi et al., 2011) have been postulated for N-CNTs. The morphology of B-CNTs is also not well defined which makes it difficult to predict whether presence of boron has caused the morphology to differ or whether the morphological changes can be attributed to the synthesis parameters. The outcomes are so scattered and contrasting in manner, that no absolute conclusion can be drawn, with regards to, influence of a particular parameter on the morphology or chemical characteristics of the B-CNTs. For instance, (Handuja et al., 2009) suggests that when the concentration of boron is increased in the CNTs, the alignment of the CNTs increase while the crystallinity reduces. Whereas, (Wang et al., 2007) suggest that the undoped CNTs have a proper alignment compared to B-CNTs, wherein the high concentration of boron reduces the alignment and increases the surface roughness. Such contrasting studies make it difficult to bring about proper conclusions. Most of the boron precursors available are hazardous for the environment or highly toxic. The cheapest source of boron which is boric acid has been rarely utilised directly for BCNT synthesis (Tetana et al., 2017). Although literature mentions the use of boric acid by dissolving it in liquid precursors (Handuja et al., 2009; Preston et al., 2016), the direct decomposition of boric acid to produced B-CNTs has not yet been attempted. Besides all this, there is no kinetic data available which can help understand the feasibility of the B-CNTs formed and the optimum parameters that can be employed to bulk synthesize the B-CNTs. The Table 1 below lists the different precursors and different CVD methods used till date for synthesis of B-CNTs. As observed from the Table 1 mostly liquid precursors are used for the

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B-CNT synthesis with an additional carbon source along with it. There are limited solid boron precursors available that can be directly decomposed. Boric acid is one such precursor which is stable even at higher temperatures and can be used for synthesis of B-CNTs. The direct use of boric acid as a precursor for B-CNT synthesis using CVD has not yet been made as per the knowledge of authors. The present study, is concerned with the systematic understanding of growth of BCNTs with respect to its kinetics. As per the knowledge of the authors, there is no such study performed/reported, for understanding the kinetics of B-CNTs formation. The different parameters responsible for the growth of B-CNTs are considered and varied consequently to study the effect of each parameter with precision. The study is carried out in a fixed bed reactor, on similar lines to that of N-CNTs that were synthesized in our previous work (Sharma et al., 2017). The objectives of the present research are; 1. To understand the feasibility of boric acid as a direct precursor for CVD 2. To understand the role of different parameters on the growth rate of B-CNTs 3. Obtaining the optimum parameters for growth rate of B-CNT. 4. Determining the rate controlling step and plausible mechanism for growth of B-CNTs. 5. To establish the kinetics of the extrinsic chemical reactions.

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Table 1: List of different precursors and CVD doping techniques for B-CNT formation Reference

C-source

B-Source

Inert

Catalyst

Temperature

Pressure

Power

Synthesis

Type of

(oC)

(Pa)

(Watt)

Method

CNT

Boron Doping (%)

(Chen et al., 2003)

Methane

Trimethyl Borate

Hydrogen

Fe/Silica

700

2000

700

MPCVD

CNT arrays

NM

(Wang et al., 2007)

Methane

Diborane

Hydrogen

Iron Oxide/silica

650

30

700

ECR-CVD

MWNTs

NM

(Daothong et al., 2009)

Triethyliso propylborat e

Triethylisopropylbo rate

Hydrogen

Fe/MgO

800-900

0.0001

NA

HVCVD

SWCNTs

0.6

(Handuja et al., 2009)

Xylene

Boric acid

Argon

Ferrocene

900

NA

NA

Solution Injected CVD

MWCNTs

NM

(Koós et al., 2010)

Toluene

Triethyl Borane

Argon

Ferrocene

800-1100

NA

NA

Aerosol CVD

MWCNTs

0-0.5

Triisopropyl borate

Hydrogen/ Argon

Fe-Mo/MgO

900

NA

NA

CCVD

DWCNTS

0.8-3.1

Triethyl borate/Tri isopropyl borate

Hydrogen

Fe/MgO

700-900

9.33*E-3

NA

HVCVD

SWCNTs

1

(Lyu et al., 2011) (Monteiro et al., 2012)

Tetrahydro furan Triethyl Borate/Trii sopropyl borate

(Keru et al., 2015)

Toluene

Triethyl Borane

Hydrogen/ Argon

Ferrocene

900

NA

NA

FCCVD

MWCNTs

0.4-2.5

(Preston et al., 2016)

Ethanol

Boric Acid

Argon/Hyd rogen

Co-Mo/MgO

850

NA

NA

Solution Injected CVD

FWCNTs

2.787.22

6

Nature of BCNTs formed Short Length, no bamboo shape and less density The higher content of boron leads to high surface roughness Addition of sulphur increase the yield of B-CNT Alignment of BCNTs is random at low concentration cotton wool like bundles with knees and undulations well defined and high quality tubes The B-CNTs formed were of 0.81.3nm diameter kinks, thick walls, elbow joints and Yjunctions The nature of BCNTs were attributed to the nature of catalyst synthesized

2. Experimental procedure The experimental setup used for the synthesis is similar to the previous studies (Sharma et al., 2017). The two zone furnace used comprised of a reaction zone (Z2) in which the support material was kept and the precursor zone (Z1), which inhabits the ferrocene catalyst and boron precursor boric acid. The contents were kept in quartz boats in the respective zones. The reaction takes place in Z2 where the support magnesium oxide was kept on the quartz boat. Argon gas (I) was used as the carrier gas and acetylene gas (A) was used as a source of carbon. When the reaction temperature was reached (Z2), the temperature of the Z1 was increased to vapourize ferrocene and this vapour was carried inside Z2 by argon. Ferrocene decomposed in zone Z2 and Fe nanoparticles were deposited mostly on the quartz boat (S) containing MgO and some amount around the quartz tube (Q) near the reaction zone Z2. After depositing the catalyst the temperature was further increase to 550oC to vapourise the boric acid. After the vapourising temperature was reached, acetylene gas was charged into the reactor wherein both the boric acid and the acetylene gas decomposed to give B-CNTs. B-CNTs were collected from the quartz tube (Q) as well as quartz boat (S). The reaction was carried out for a time span of 25 minutes after which the reaction was stopped and reactor was cooled to room temperature, with constant passage of argon gas to avoid oxidation of the product and maintaining the inert conditions. The sample was then collected, washed with conc. hydrochloric acid to remove the catalyst and finally washed with deionized water, filtered, dried and stored. Table 2 gives the details about the experiments carried out, by varying different parameters like temperature, partial pressure, and total flow rate and catalyst concentration. The activation energy was calculated by varying temperature of synthesis in the range of 650 to 950oC. Also the effect of temperature on the morphology of the B-CNTs was studied in detail. Magnesium oxide was used as a support material. As per the kinetic study of N-CNTs which was previously carried out using the same support and the same reactor dimensions, it 7

was found that the particle size of less than 75 µm was essential to eliminate the effect of internal mass transfer or pore diffusion control (Sharma et al., 2017). So, the particle size in the range of 25-75 µm was used in this study. The total flow rate was varied to understand the effect of external mass transfer. The influence of chemical reaction was studied by varying the partial pressure of reactant gas phases and the amount of catalyst. Table 2 Experimental parameters for B-CNT synthesis Study of variation in parameters

Partial pressure (atm.)

Temperature (oC)

Total Flow Rate (sccm)

Catalyst weight (g)

Particle size (µm)

Effect of temperature

0.25

650-950

800

0.8

25-75

Effect of partial pressure

0-1

800

800

0.8

25-75

Effect of total flow rate and catalyst concentration

0.25

800

0-3200

0-3.2

25-75

Effect of parameters

The purified B-CNTs were then characterized using different analytical techniques like SEM (Scanning Electron Microscopy), Transmission Electron Microscopy (TEM) for confirming the presence of different structural features. High Resolution Transmission Electron Microscopy (HRTEM) were used for determining the number of walls of N-CNTs formed and its nature. Thermo Gravimetric Analysis (TGA) was used for understanding the stability of the formed B-CNTs. X-ray Photoelectron Spectroscopy (XPS) analysis was used for calculating the boron content of the sample and Raman for determining the purity of the sample.

8

3. Results and discussions 3.1 Effect of temperature One of the important parameters for growth of B-CNTs is its temperature of synthesis. The synthesis temperature decides the nature of the B-CNTs formed depending upon the amount of B incorporated in the sample. As seen from Table 2, for the present study the temperature of synthesis was varied from 650oC-950oC. Figure 1 shows the effect of temperature on the rate of B-CNT formation. As indicated from the Figure 1, two different mechanisms were observed in two different range of temperatures. For the temperature range from 650oC-750oC, appreciable increase in the rate of formation of B-CNTs is indicated with activation energy of 18.21 kJ/mol. The rate decreases for temperature of 800oC and again starts increasing upto the temperature of 950oC with an activation energy of 6.73 kJ/mol. This indicates that two different mechanisms are being followed within a given temperature range. The low activation energies in case of both the mechanisms suggests slower diffusion rates. The further clarification can be made by understanding the overall effect of other parameters. To clarify what exactly is the nature of the products formed, the TEM and HRTEM images were studied. Figures 2 (a) (b) and (c) shows TEM image of B-CNTs synthesised at different temperatures. The B-CNT synthesized at 650oC shows that it has kinks and joints at different points and morphology is not well defined. The diameter of the tube is non uniform and varies along the length. This can be due to the low synthesis temperature. B-CNTs formed at 700oC, show a mixture of nanotubes and nanochains formed. The walls are thick with interstitial bonding at wide spaces. In comparison to the morphology of N-CNTs which had a typical bamboo shaped morphology, the bonds in case of B-CNTs formed in the present case are not well defined.

9

1/T (K-1) 7.5E-04 -10.3

8.5E-04

9.5E-04

1.1E-03

E1= 18.21 kJ/mol

ln(rA)

-10.4

-10.5

E2 = 6.73 kJ/mol -10.6

Figure 1: Plot of rate of reaction versus T-1 (Temperature variation from 650oC to 950oC) The morphology at 750oC is a chainlike structure wherein catalyst particles are entrapped within each module. There is no tube structure formed in this case, but formation of chain structure with some amount of amorphous carbon is visible. At 800oC chain structures were found to merge and very few nanofibres were formed including large amount of nanotubes. There was no visible internal demarcation within the tubes and the structure had similar kinks and joints as observed for B-CNTs at 650oC.

Figure 2(a): TEM and HRTEM images for B-CNTs synthesised at 650oC and 700oC 10

The structures formed at 850oC were clear and free from kinks. Carbon nanochains were also found to be present, but they did not predominate as the ones which were formed at 750oC. The tube walls were thick, which was the characteristic of all the B-CNT tubes observed till now.

Figure 2 (b): TEM and HRTEM images for B-CNTs synthesised at 750oC and 800oC At 900oC the B-CNTs were found to fuse due to higher temperatures and large amount of amorphous structures were observed. Nanofibers were predominantly observed at this temperature. A very few tubes were seen which had very thick walls and internal demarcation in some tubes were found to be closely spaced. As observed for N-CNTs, the B-CNTs didn’t give a distinct identity of presence of bamboo shaped morphology or uniform tube diameter throughout the length. Kinks and corrugation of the walls were a dominant feature that was observed.

11

Figure 2 (c): TEM and HRTEM images for B-CNTs synthesised at 850oC and 900oC The temperature studies showed two different reaction rates at different range of temperatures. Most of the samples produced were a mixture of carbon nanochains, carbon nanotubes and carbon onions. There was no demarcating difference that could suggest the formation of entirely different products. The possibility that the presence water vapor and HBO2 (metaboric acid) in the reaction zone due to the decomposition of boric acid might have given rise to changed interactions with the acetylene or with the catalysts at different temperatures, which in-turn might have resulted in different reaction rates. As per (Koós et al., 2010) , when the percentage of boron doping is high there are chances of formation of B2O3 film at the surface of the carbonaceous materials. Boron does not directly affect the formation of B-CNTs but alters the catalyst and substrate interactions. These interactions might possibly have slowed down the penetration of carbon atoms through the catalyst surface thereby leading to slower reaction rates and a mass transfer controlled reaction As will be discussed later, the XPS analysis shows that there is no boron in the samples synthesized at 750oC which had formed carbon nanochains and no nanotubes. This shows that 12

there was a strong resistance to the doping of boron at 750oC. In other words, it can be said that the possible combination of carbon molecules at temperature of 750oC favoured the formation of carbon nanochains compared to carbon nanotubes. This might be the reason for the highest reaction rate at 750oC. As per (Zhang et al., 2014),the carbon nanochain formation mechanism is a bit different from the conventional CNT formation mechanism. A carbon nanochain is formed by combining multiple units of Carbon Nano Onions (CNOs). The mechanism suggests that, the catalyst, during the formation of carbon nanochains, is not stationary and can separate out with quasi Carbon Nano Onions (that help building a carbon nanochain) and is continuously utilized for the formation of multiple such units. As the catalyst is continuously consumed by carbon, there are limited chances for the interaction of boron with the catalyst, which forms an important part of the doping process. The understanding of carbon nanochains can form a separate set of studies which might require detailed knowledge of reaction kinetics. As for the present case, it is the best to focus on mechanism of boron doping in the CNT lattices at temperatures at which requisite amount of CNTs are formed. The further studies were carried out taking into account the following points from the literature; (i) As per the earlier kinetic studies on CNTs, the best temperature window for synthesizing CNTs using acetylene was between 700 to 807 oC (Dasgupta et al., 2014). (ii)As per (Keru et al., 2015) the best temperature window for production of B-CNTs was from 760 to 840oC. Based on the present studies, it was found that, at a temperature of 750oC, nanochains were formed which was not the objective of the study. Besides, the literature studies strongly suggested the best temperature range to be between 750 and 850oC. It was thus decided, that the range of temperature for further optimization studies should be 800oC. 3.2 Partial Pressure studies The studies were undertaken to understand the partial pressure at which optimal amount of B-CNTs were detected. The temperature of 800oC was taken to be the most appropriate 13

temperature as per the kinetics of CNTs obtained from earlier studies (Dasgupta et al., 2014) and the present temperature studies. The rate of formation of B-CNTs were based on a total flow rate of 800 sccm and partial pressure of acetylene and argon were varied by keeping partial pressure of boric acid constant. The results suggested that partial pressure of acetylene had negligible effects for the reaction rates at low partial pressures but showed increasing trend for higher acetylene partial pressures.

Rate of formation of B-CNTs (gmB-CNT gcat-1 sec-1)

5.0E-4

4.0E-4

3.0E-4

2.0E-4

1.0E-4

0.0E+0 0

0.2

0.4

0.6

0.8

1

1.2

Partial Pressure (atm.) Figure 3: Plot of rate of reaction versus partial pressure of Acetylene (Partial Pressure variation from 0-1 atm. at temperature of 800oC) The B-CNTs obtained for the given partial pressures were characterized to determine the formation of a CNT by using SEM. A typical SEM image is as shown in the Figure 4. Typically four partial pressures were chosen (0.125, 0.25, 0.38 and 0.5 atm. respectively) for depicting the nature of tubes formed from SEM images. At partial pressure of 0.125 atm., lesser number of tubes, having lengths of about 1 µm were detected. The image showed more agglomerates compared to the tubes. The partial pressure of 0.25 atm. shows presence of large number of tubes entangled with each other with more or less similar dimensions. For the partial pressure of 0.38 atm. again fewer number of tubes were observed with increase in agglomerates. The partial pressure at 0.5 atm. showed no proper tube like structures. The partial pressures beyond 1 atm. had larger proportions of amorphous carbon. Thus,

14

an optimal partial pressure was required for getting the desired nanotubes. Therefore, for further studies the partial pressure of acetylene was kept to be 0.25 atm.

;

Figure 4: SEM images for B-CNTs formed at different partial pressures 3.3 Effect of Flow rate and Catalyst concentration The flow rate studies were carried out at a temperature of 800oC.The experiments were carried out by changing the concentration of the catalyst and total flow rate simultaneously, keeping the space velocity constant. The total flow rate and the amount of catalyst were changed in such a manner that the rate of B-CNT formation becomes independent of the flow rate, that is, any external mass transfer limitation is eliminated. Internal mass transfer diffusion limitations were already made negligible by using the particle size in the range of 25-75 µm which were found to be free of pore diffusional resistances. If F is the flow rate and C is the amount of catalyst, then the changes were made in the manner (F, C); (2F, 2C); (3F, 3C) and (4F, 4C) respectively. As seen in Figure 5, the rate of reaction linearly increased with increase in flow rate and catalyst concentration. Beyond the flow rate of 2400 sccm and catalyst quantity of 2.4 g. After this, the reaction rate remained more or less a constant so that it can be said that 15

diffusion resistances were eliminated. Thus, the total flow rate of > 2400 sccm and catalyst weight of > 2.4 g could be responsible for totally eliminating the bulk diffusion resistances and temperature gradient effect when all the intraparticle resistances are eliminated. 3.0E-05

rA (gm B-CNT.gcat-1. s-1)

2.9E-05

2.8E-05

2.7E-05

800

1600

2400

3200

2.6E-05 600

1100

1600

2100

2600

3100

3600

Total Flow Rate (sccm)

Figure 5: Plot of reaction rate versus total flow rate at different catalyst concentration (total flow rate variation from F = 800sccm, 2F = 1600 sccm, 3F =2400 sccm, 4F = 3200 sccm and catalyst variation from C = 0.8 g, 2C = 1.6 g, 4C = 2.4 g, 4C = 3.2 g). 3.4 Surface characteristics of B-CNTs 3.4.1

Fourier Transforms Infrared Spectroscopy (FTIR)

FTIR was essential as initial studies, to understand whether boron is present or not in the CNT lattice. The boron bonding with C and O in the structure verified the presence of boron in the lattice. The Figure 6 shows the FTIR spectra for the B-CNTs synthesized at different temperatures. The multiple bonds between 500-1000 cm-1 indicate that BO2 or BO4 groups may be present. B-O stretching is indicated around 1330cm-1. The bond is not prominent but is an indication of presence of boron in the CNT lattice. The peak found around 3450 cm-1 indicates the O-H stretching vibrations in the CNT lattice. The medium peaks between 800-1100 cm-1 indicate C-H stretching and bending frequencies. For quantification of boron present in the sample XPS was used.

16

3661.97 3607.89

3946.45 3870.17 3824.37 3763.44

3500

3000

2500 2000 Wavenumber cm-1

3500

3000

2500 2000 Wavenumber cm-1

Figure 6: FTIR spectra for B-CNTs synthesized at different temperatures

D:\AWP\ANITA\9FEB2017\SAMPLE6.0 D:\AWP\ANITA\9FEB2017\SAMPLE6.0

SAMPLE6

SAMPLE6

Page 1/1

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SOLID

SOLID

1500

1500 1000

C-H, C=C

C-H, C=C

1000

SOLID

1000

1000

472.07 418.36 363.73

800

574.06

SOLID

470.60 418.62

1000

574.99

558.63

348.89

460.28

1000

470.60 418.33

470.55 418.33

573.18

809.45

700

694.52

873.00 800.46

Single channel 0.90 0.85

C-H, C=C

573.62

1500

808.79 750.99 696.36

Page 1/1

500

470.60 418.33

SOLID

1074.34

1380.03 1309.22 1257.43

1462.61

1629.05

1739.04

0.80

1000

573.62

1500

804.88

1500

914.36

750

1115.63 1072.16

1243.21

1330.63

1428.66

Page 1/1

1072.65

1370.87 1312.11 1255.19

1534.46 1466.61

1589.91 1533.22

1776.28 1729.49

SOLID

804.77

1064.28

1710.14 1617.63

1500

1066.06

2500 2000 Wavenumber cm-1

1259.15

2500 2000 Wavenumber cm-1

1381.90

1627.31

1921.27 1875.19

2131.09 2077.05 2015.06

2363.59

1.0

SOLID

804.77

Page 1/1

1500

1066.06

900 1814.08

2500 2000 Wavenumber cm-1

1259.15

SAMPLE5

2500 2000 Wavenumber cm-1

1463.69 1411.90 1379.56

D:\AWP\ANITA\9FEB2017\SAMPLE5.0

3000

2500 2000 Wavenumber cm-1

1463.69 1411.90 1379.56

3500 SAMPLE4

1623.63

D:\AWP\ANITA\9FEB2017\SAMPLE4.0

3000

1623.63

3500 SAMPLE

2214.12

2921.27 2852.77 2771.43 2713.07 2657.70

3454.90

3664.74 3608.10

3890.03 3834.81 3772.52

0.9

3000

1925.81

3000

2367.34

D:\AWP\ANITA\9FEB2017\SAMPLE3.0

SAMPLE2

2367.62

3000

2367.62

3500 SAMPLE1

2763.51 2689.46 2643.28 2594.69 2538.47 2493.35 2433.42 2388.83 2296.09

3041.97 2989.63 2921.77

D:\AWP\ANITA\9FEB2017\SAMPLE2.0

2921.83 2855.73

3381.51 3327.58 3272.19 3213.79 3159.28 3103.62 3047.42

Single channel 0.7 0.8

3500

2919.71 2852.07

3454.49

3662.51 3604.30 3552.98 3474.15

3890.31 3832.52

0.6

D:\AWP\ANITA\9FEB2017\SAMPLE1.0

2919.45 2851.92

3449.18

3661.13

3884.85

%channel Transmittance Single 0.95 0.86 0.88 1.00 0.90 0.92 0.94 0.96 0.98 1.00

3500

2919.45 2851.92

3449.18

3661.97 3607.89

3946.45 3870.17 3824.37 3763.44

Single Singlechannel channel Single channel 0.60 0.80 0.5 0.6 0.7 0.85 0.8 0.90.90 1.0 0.60 0.65 0.70 0.75 0.80 0.85 0.65 0.90 0.95 0.70 1.00 0.4 0.75

471.43 418.09

575.32

807.91

1069.10

1257.29

1382.46

1465.94

1630.25

2921.57 2853.89

3452.30

0.95

1.00 0.4

0.5

Single channel 0.6 0.7 0.8

B-O

C-H, C=C

650

500

09/02/2017

B-O

500

09/02/2017

C-H, C=C B-O

500

C-H, C=C

Page 1/1 09/02/2017

B-O

B-O 500

Page 1/1 09/02/2017

850

500

B-O

09/02/2017

500

09/02/2017

09/02/2017

0.9

1.0

3.4.2

X-ray Photoelectron Spectroscopy (XPS)

The XPS analysis was carried out for B-CNTs synthesized at 650oC, 750oC and 850oC. The XPS (Figure 7) shows the wide range scan which indicates the prominent peaks of oxygen, carbon and weak peaks of boron. The 1s spectra of oxygen was prominently detected at binding energy of 532eV. This oxygen was present in bonding with a chain of carbon species. 1s carbon was found to be present at 285eV Figure 8 shows the high ultrascan for boron The boron was detected in B-CNTs synthesized at 650oC, 800oC and 850oC which was found to be 6.31 at.%, 6.48 at. % and 6.71 at.%, respectively. Negligible amount of boron was detected for B-CNTs synthesized at 750oC. As seen from the TEM images, the products synthesized at 750oC were carbon nanochains and no CNTs were found to be present. Also, negligible boron doping is detected that suggests there might be a hindrance to doping whilst the formation of carbon nanochains. There is even a possibility that the amount of boron doped was in undetectable range. This also may be the reason of the different formation mechanisms that were observed for different range of temperatures. Thus, it can be said that appreciable amount of doping was achieved but there was no proper control over the amount of boron doped. It can be attributed to the low partial pressures of boric acid which was directly decomposed in its solid state. The boron element is detected in the range of 175 eV to 200 eV. Elemental boron are usually oxidised when present at the surface. As it is clearly seen there are no prominent peaks visible for the XPS spectra taken at 750oC (represented by yellow line), which depicts absence of boron in it. The normal boron species expected includes, B2O3 around 192 eV and H3BO3 species between 193-194eV.

18

3.50E+05

4.00E+05

O 1s

3.00E+05

3.00E+05 Counts per second

XPS at 650oC

2.50E+05 Counts per second

O 1s

3.50E+05

2.00E+05 650

C 1s

1.50E+05

XPS at 750oC

2.50E+05 2.00E+05

750

C 1s

1.50E+05

1.00E+05 1.00E+05

B 1s

5.00E+04

0.00E+00 1195

995

795

395

195

0.00E+00

-5

1195

3.00E+05

3.00E+05

2.50E+05

2.50E+05

XPS at 850oC

2.00E+05

995

795

3.50E+05

O 1s

Counts per second

Counts per second

3.50E+05

595 Binding energy (eV)

B 1s

5.00E+04

850

C 1s

1.50E+05

595 Binding energy (eV)

395

195

-5

O 1s

XPS at 800oC

2.00E+05

800

C 1s

1.50E+05

1.00E+05

1.00E+05

B 1s

5.00E+04

B 1s

5.00E+04

0.00E+00

0.00E+00 1195

995

795

595 Binding energy (eV)

395

195

1195

-5

995

Figure 7: Wide XPS spectra for B-CNTs

19

795

595 Binding energy (eV)

395

195

-5

1700

Counts per second

1600 750 850

1500

650 800

1400

1300

1200 175 177 179 181 183 185 187 189 191 193 195 197 199 Binding Energy (eV)

Figure 8: XPS scan for boron 3.4.3

Raman spectra

The Raman analysis for B-CNTs was carried out for the temperature of 800oC. Figure 9 shows the Raman spectra wherein the defect band is comparatively lower than the graphitic band. The ID/IG = 0.49 which shows that the crystallinity is very well maintained. The sharp peak of 2D or G' band also indicates that good graphitic structure. The number of layers in the multiwalled tubes leads to a more prominent and a proportionately increased G´band (Hodkiewicz, 2010). This is also evident from the HRTEM images which depict the presence of large number of tube walls of B-CNTs. This is well in contrast with the literature studies which concludes that higher boron content in the B-CNTs leads to reduced crystallinity (Keru et al., 2015).

20

2000 G-Band

G'-Band

Relative Intensity

1600 1200

D-Band

800 400 0 600

1100

1600

Raman Shift

2100

2600

(cm-1)

Figure 9: Raman spectra for B-CNTs synthesized at 800oC 3.4.4

Thermogravimetric (TGA) analysis

The TGA studies were carried out for B-CNTs synthesized at 800oC in the presence of air to study the decomposition. As seen from Figure 10, the initial weight loss upto about 60oC can be attributed to the loss of volatiles. This is also evident from the sharp dent in the dTG curve. The percentage weight loss indicate increase in the weight between the temperature 400480oC which is also evident from the dTG peak which shows a sharp rise around that temperature range. This increase in the weight can be attributed to different oxidation reactions taking place. An increase in weight at 450oC is due to the oxidation of Fe metal trapped inside the B-CNTs. A sharp weight loss is observed at about 500 oC and the weight loss continues till 690oC. The initial degradation starting at 500oC is due to the disorders that are present in the B-CNT lattice and the stable bonds finally decompose at higher temperatures. It can be said that the B-CNTs can be stable upto 690oC. The TGA data also depicts that the weight loss is around 30% and rest of the material that remains behind is the catalyst. The reason behind large amount of catalyst remaining as a residue is that, a comparatively large amount of catalyst is

21

trapped in the B-CNTs which cannot be removed due to their thick walls. (Supplementary Material)

0.01 100

% Weight

90 80

0

dTG

% Weight dTG

70 60 50 0

200

400 600 Temperature (oC)

800

-0.01 1000

Figure 10: Percentage weight loss curve for B-CNTs synthesized at 800oC The TGA data also helped in determining the different types of carbon present in the B-CNTs synthesized at particular temperature. The quantification of these carbon types are given in Table 4. Table 4: Different types of carbon structures in B-CNTs

*B-CNT/ Total Carbon Amorphous Defect BContent Carbon CNTs Nanochains (%) (%) (%) (%) 40.4 27.6 65.04 7.36 49.5 6.5 47.74 45.76 29.28 7.1 20.99 71.91 51.22 50.46 19.9 29.64 the ratios and yields of amorphous carbon, defect carbon and B-

Temperature 700 †750

800 850 Table 4 shows

CNT/nanochains. It is very difficult to differentiate whether the structure is carbon nanochain or nanotube as they have almost the same decomposition temperature. These ratios are given

The values suggest presence of graphitic B-CNTs or Carbon Nanachains or combination of both combined Complete carbon nanochains were synthesized at 750oC

** †

22

in the last column. The defect B-CNTs (4th column) are the ones which have a larger defect and decompose faster compared to well-structured and graphitic B-CNTs. As the best temperature window for synthesizing CNTs is between (700-850oC), the variation in the types of carbon is discussed between these temperatures. The values were estimated based on the TGA data obtained at different temperatures. The amorphous carbon was considered to decompose at around 350oC. The defect B-CNTs and graphitic B-CNTs/nanochains were considered to decompose beyond 450oC. 3.5 Mechanism of B-CNT formation To suggest a proper reaction mechanism for the formation of B-CNTs using acetylene and boric acid, a series of rate laws were proposed. The reaction mechanism was formulated in accordance with the different steps involved in catalytic reactions (Fogler, 1999). The temperature studies showed the existence of two different reaction rates for lower and higher temperature ranges. However, the TEM and the HRTEM images did not provide a great demarcation between the products formed. The activation energies for both the range of temperatures were quite low (< 20kJ/mol), which suggested the presence of diffusion limitations. The temperature of 800oC was considered while formulating the reaction steps as the product formed constituted solely of nanotubes and not nanochains. While proposing the rate laws following assumptions were made; (a) Reactants are present in a single fluid phase at the reaction temperature (b) All the surface sites have same energies for adsorption (c) Adsorbed molecules do not interact with each other. (d) The supersaturated complex completely precipitates to give B-CNT without any saturated remnant. (e) The catalyst remains unaltered.

23

Acetylene was the only carbon precursor that was used. Ferrocene was considered to be totally contributing to the catalyst formation and the contribution of carbon from ferrocene was neglected. The boric acid decomposition takes place at 170oC where it forms HBO2 (metaboric acid) vapours (Sevim et al., 2006). When boric acid is heated slowly, it loses water and converts to metaboric acid. Dehydration of boric acid takes place in two steps; 2H3BO3 →2HBO2 + 2H2O

1

2HBO2 →B2O3 + 2H2O

2

On further heating metaboric acid combines to form B2O3 (Boron Oxide) which is a hygroscopic solid. Therefore, understanding the decomposition of boric acid the following steps can be proposed with respect to interaction of boric acid (metaboric acid vapours) with acetylene and its formation to B-CNTs. The different steps involved in the B-CNT formation in accordance with literature (Dasgupta et al., 2014) were considered as follows; 1. Mass transfer of acetylene on the surface of the catalyst pellet. 2. Diffusion of the reactants from pore mouth of the catalyst through the catalyst pores. 3. Adsorption of acetylene on the catalyst surface. 4. Reaction on the catalyst surface. 5. Formation of saturated and supersaturated complexes 6. Desorption of B-CNTs from the catalyst surface. 7. Mass transfer of the products from external surface to the bulk fluid. Step 1 includes external mass transfer control regime which was eliminated by maintaining a suitable flow rate for the reactants to diffuse to the catalyst surface (Flow rate studies)

24

Step 2 can be ignored as the catalyst pore diffusion is already eliminated by using the support size between 25-75 µm Step 7 can be eliminated as the products include solid B-CNTs and these are precipitated on the surface by the supersaturated complex. The remaining steps are enlisted as follows; Step 3: Acetylene (C2H2) adsorbs on a vacant side (S) to become (C2H2.S) Step 4: The adsorbed as well as gaseous phase acetylene reacts with the metaboric acid vapours to form intermediate, saturated complex boron carbide structure with the release of water vapour. Step 5: The intermediate further reacts with the acetylene in the gas phase to form the supersaturated complex. Step 6: The supersaturated complex desorbs into B-CNTs. Thus it can be proposed that; Adsorption k1 C2H2 + S ↔ C2H2.S k-1

Step 3

Surface Reaction k2

C2H2.S + 2HBO2 + 2C2H2→(C6B2)s.S + 4H2O↑ k3 (C6B2)s.S + 𝐶2H2 ↔ (C6B2)ss.S + 4H2↑ k-3

Step 4 Step 5

B-CNT formation k4

Step 6

(C6B2)ss.S → B - CNT + S The rate equations for the above proposed mechanism can be written as follows,

25

rad = k1PC2H Cv - k - 1f(C2H2).s 2

rs1 = k2fC2H .s p2HBO2p2C2H2 2.

rs2 = k2f(C6B2)s.spC2H2 - k - 3f(C6B2)ss.s pH2

rB - CNT = k4(C6B2)ss.s

Assuming pseudo steady state hypothesis (Fogler, 1999) the values of intermediates were determined. The rates for each step were expressed in terms of fractional coverage which is the ratio of concentration of intermediates to total catalyst sites given as follows; CC2H2.s

f(C2H2).s =

f(C6B2)s.s =

f(C6B2)ss.s =

Ct C(C6B2)s.s Ct C(C6B2)ss.s Ct

The concentration of vacant sites is given by, Cv =

X D

Where X = k - 1 + k2p2HBO2p2C2H2 D = k - 1 + k2p2HBO2p2C2H2 + k1 pC2H2 + (k1k2 k3k4)((k - 3pH2 + k4)(p2HBO2p2C2H2) + (k1k2 k4) p2HBO2p3C2H2 The final rate expression after evaluating all the fractional coverage is given as follows; rad =

X1 D

;rs1 =

X2 X3 X4 ;rs2 = ;rs4 = D D D

Where, 26

X1 = X2 = X4 = k1k2p2HBO2p3C2H2 The equations were solved to evaluate the rate constants for all the plausible mechanisms and the best fit was estimated. Figure 11 shows the experimental and the model values for the proposed mechanism;

rate of B-CNT formation (gm* gcat -1*sec-1)

5.0E-04

Experimental

4.0E-04

adsorption surface reaction

3.0E-04

2.0E-04

1.0E-04

0.0E+00 0

0.2

0.4

0.6

0.8

1

Partial Pressure (atm.)

Figure 11: Model validation for proposed mechanism As seen from the Figure 11 the rate expression for adsorption of the species fits reasonably well with the experimental data. Also, as seen from the temperature studies the diffusion limitation is present as the activation energy is about 20 kJ/mol. Here, the data suggests that there is some sort of a difficulty to the adsorption of species on the active catalyst surface. The boron source interacts with the catalyst surface thereby avoiding further deposition of the carbon species thus influencing the adsorption (Handuja et al., 2009). The reaction rate constant values as calculated from the rate expressions are given as follows; k1 = 0.08

lit

- 5lit atm.g.sec k - 1 = 1.5 * 10 (atm).g.sec

27

k2 = 3.8 * 105lit/((atm)^4.g.sec) k-3 = 1

mol

mol k3 = 1.5 * 10 - 2 g.sec

g.sec; k4 = 20

lit

g.sec

As observed from the rate constants it is clearly understood that the slowest reaction is the adsorption reaction wherein the molecules attached find it difficult to undergo a reversible reaction. Also the surface reaction, which is conversion of the adsorbed species into the saturated complex, is a very fast process which is evident by the large reaction rate constant, contrary to that, the formation of supersaturated complex is a bit slower than the formation of saturated species. The reason for that may be formation of CO2/CO by the reaction of oxygen and/or water species with the hydrocarbon gas in the reaction chamber. Finally the desorption step or the formation of B-CNT is a relatively fast process as compared to the supersaturation. The predicted model indicates that adsorption of acetylene on the site is a rate limiting step. This may be due to the fact that the interaction of the catalyst with the acetylene is affected due to the presence of boron and /or boron oxides, in the vicinity of the catalyst, which somehow poisons the catalyst site thereby affecting the catalyst and substrate interaction. As for the mechanism that is proposed it can only be said that the direct decomposition of boric acid affects the adsorption capacity of acetylene by hindering the interaction between acetylene and the catalyst site. B-CNT formation does not entirely depend on acetylene adsorption but it can be said that adsorption of acetylene is affected by the interaction of various components present in the gas mixture like decomposed products from boric acid which includes water. The present model can be validated for species that has separate precursor for boron and a separate carbon source. The mechanism cannot be assuredly said to be fully understood as there are still many gaps in the literature pertaining to the study of doping boric acid and its interaction with the carbon precursor as well as the catalyst. Thus, more studies are required for ensuring the validation of the proposed model with a few more different experimental 28

studies, as no in-depth studies have yet been performed in a manner in which the present synthesis of B-CNTs is carried out. 4. Conclusions 1. The objectives set forth for the present research were systematically studied. It was found that boric acid when directly decomposed can successfully form B-CNTs. The amount of doping however cannot be systematically controlled by varying different parameters, as was reported in literature. 2. As reported by Wang et al., 2007 high concentration of boron in the CNT lattice increases the surface roughness and reduces the alignment. The B-CNTs that were formed for present study were random without any particular orientation. The TEM and HRTEM images showed that the boron doping resulted into non-uniform and etched surfaces which eventually increased the surface roughness. However, only high Bdoping cannot be solely responsible for the surface roughness but can be one of the factors that are responsible. 3. The B-CNTs formed were full of kinks, elbow joints and Y-junctions which was similar to the one as observed by Keru et al., 2015. However, the high crystalline nature as depicted by them was in contrast with the observations made in the present study. As per the present study, it was found that even though the amount of boron doped was high, the crystal structure was found to be stable and highly graphitic (ID/IG <1). The B-CNTs were stable and did not disintegrate with high boron loading. 4. The effect of different parameters were studied which brought to light the fact that adsorption of the carbon species and rate of supersaturation were amongst the slowest steps which controlled the overall rate of reaction.

29

5. There was no boron doping observed for the CNT synthesis at 750oC, which showed the formation of carbon nanochains from the TEM images. The mechanism is not yet clearly understood, as to what exactly caused the formation of carbon nanochains and what caused the boron from not doping into the CNT lattices. 6. The temperature studies showed the presence of two different reaction rates in two different temperature ranges. The TEM images however did not indicate the formation of any new species other than CNTs and carbon nanochains. The interaction of boric acid with the catalyst might have given rise to different activation energies. 7. The flow rate and catalyst concentration studies showed that the total flow rate of >2400 sccm and catalyst weight of >2.4g was essential for the removal of external mass transfer limitations 8. The boron doping in the CNT lattice was found to be between 6.31 to 6.71 at.% and boron was present in the CNT lattice which was concluded from the XPS, FTIR spectra and TEM, HRTEM images. 9. The TEM and HRTEM images suggested that the number of walls of B-CNTs was between 30-40. Even though a reaction rate mechanism was predicted and the model was successfully validated for the obtained experimental data, there is yet a need to understand the basic growth mechanism of B-CNTs which has not yet been emphasized in greater details.

30

Acknowledgement The authors acknowledge DAE-ICT centre for supporting the project. The authors would also like to acknowledge the Sophisticated Analytical Instrument Facility (SAIF) department of IIT Bombay for providing the TEM and HRTEM images for B-CNTs. The authors would also like to acknowledge Central Surface Analytical Facility (ESCA), Department of Physics, IIT Bombay for providing the XPS results.

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35

Highlights 1. Boron doped carbon nanotubes synthesized using solid boron source, boric acid, with boron doping between 6-7 atom percent. 2. Existence of two different reaction mechanisms in different range of temperatures. 3. Reaction kinetics were studied and a kinetic model was proposed for validating the experimental results. 4. The temperature at 750oC helps in synthesizing carbon nanochains with absence of carbon nanotubes.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

NA

37