- Email: [email protected]
Application of electroless deposition for surface modification of the multiwall carbon nanotubes
Accepted Manuscript Research paper Application of electroless deposition for surface modification of the multiwall carbon nanotubes M. Kurkowska, S. Awietjan, R. Kozera, E. Jezierska, A. Boczkowska PII: DOI: Reference:
S0009-2614(18)30349-X https://doi.org/10.1016/j.cplett.2018.04.056 CPLETT 35612
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
1 February 2018 29 April 2018
Please cite this article as: M. Kurkowska, S. Awietjan, R. Kozera, E. Jezierska, A. Boczkowska, Application of electroless deposition for surface modification of the multiwall carbon nanotubes, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.04.056
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Application of electroless deposition for surface modification of the multiwall carbon nanotubes M.
KURKOWSKA1,
S.
AWIETJAN2,
R.
KOZERA3,E.
JEZIERSKA4,
A.
BOCZKOWSKA5 1,2,3,4,5
Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, 02-507
Warsaw, Poland 1
Corresponding author, e-mail: [email protected] tel. +48 22 2345713
2
e-mail: [email protected]
3
e-mail: [email protected]
4
e-mail: [email protected]
5
e-mail: [email protected]
Abstract. The paper describes modification of carbon nanotubes surface by attaching the grains of Ni-P, Ni-B, Co-B and Fe-B. The modification was obtained by electroless metallization using sodium hypophosphite (NaH2PO2). We have investigated the parameters of electroless metallization process of CNTs. The uniformity of the coating on the carbon nanotubes was related to proper surface activation. While optimizing the electroless deposition, a range of catalyst concentrations from 0.1 to 1.0 gPd/l were tested. Deposition was used to improve the electrical properties of the later composite materials CNT-NiP/epoxy. The best results of electroless deposition were obtained for Ni-P and Ni-B coatings. Keywords: carbon nanotubes, nanocomposite, electroless deposition, coatings, Pd activation
1.Introduction In the 80s of the twentieth (XX) century, the industrial production of the material that we know as multiwall carbon nanotubes was began. However, a huge interest in this material started after 1990, when the paper of Iijma with the description of multiwall carbon nanotubes appeared in Nature. Two years later, the paper written by the same author about single wall carbon nanotubes was published. Next papers describe the unique properties of carbon nanotubes, such as the Young’s module approaching 1 TPa [1], [2] a tensile strength of about 60GPa [1], [2] for SWCNT and even 1,5TPa for MWCNT [2]. In addition, the electrical conductivity of carbon nanotubes is up to 109 A/cm2[2], [3] while the thermal conductivity is 2000-6000 W/m K at room temperature [4], [5] but for MWCNT we can observe values around 3000 W/mK. Furthermore, commercially available multiwall carbon nanotubes are characterized by reduced electrical and thermal properties compared to singlewall carbon 1
nanotubes, which is compensated by various surface modifications. In addition, the modification is intended to improve the wettability of the nanofiller by polymer [6], [7], ceramic [8] and metallic matrix [9]. The most popular method to produce industrial carbon nanotubes is chemical vapor deposition (CVD) [10]. The most commonly used variant of this method is catalytic chemical vapor deposition. This method as opposite to other methods, provides a high degree of process control and the ability to produce reproducible material [5], [11]. Among the most common catalysts there are Fe, Ni or Co or combination of them [5], [11-13]. This method allows to scale up in production, to use cheaper raw materials, to increase efficiency and to reduce the energy demand of the process. All these aspects have led to the final product’s price reduction, what made it possible to use in everyday products. Large-scale carbon nanotubes are contaminated from the substrate, catalyst and amorphous carbon. Therefore, a purification process was introduced to remove mentioned impurities [14]. This process can be carried out by the action of high temperature in the presence of N2/O2 or O2, H2O, Cl2 itself [15]. Although thermal purification removes amorphous carbon, catalyst and residue from substrate cannot be removed by this method. Chemical purification in nitric acid or in mixture of nitric and sulfuric acids is also often used [15], [16]. This method allows for full cleaning of the carbon nanotubes. Moreover, the surface of carbon nanotubes is functionalized in carboxyl, carbonyl and hydroxyl groups [15-17]. In many papers, it has been proven that these groups improve the wetting of the carbon nanotubes by the matrix (i.e. metal, polymer) and this is necessary to deposition metal ions onto carbon nanotubes [17], [18]. The disadvantage of chemical purification is the introduction of defects into the structure of carbon nanotube and the shortening of CNT length. Therefore, chemical purification requires control of parameters to determine the balance between CNT purification, functionalization and degradation. The next step is modification of carbon nanotubes surface. Carbon nanotubes due to their large surface area, nanometer size and tendency to tangle are not a simple substrate for depositing by metallic coating. There are a few studies about electroless deposition of metallic coating on surface of carbon nanotubes. Modification may be covalent, then the modified material forms a covalent bond to the carbon in the structure of carbon nanotubes. Described case of carbon nanotubes modification is related to the change from sp2 to sp3 hybridization. Modification may also have a non-covalent character where the modifying component is adsorbed on the surface without changing the hybridization of carbon in the structure of tubes. 2
Among non-covalent methods, we are focused on electroless deposition. It has many advantages that make it applicable to a wide variety of substrates such as metallic, ceramic and polymeric. This method can be also applied to substrates with highly complex geometry and no external power source or complex apparatus are needed [19]. Electroless deposition is a well-known method of surface modification [20]. Nickel coatings (mostly Ni-P and Ni-B alloys) are usually with this method. Deposition of a Ni-B coating when we used a dimethylamine boran as a reducing agent is characterized by a higher deposition efficiency. However, attention should be paid to the toxicity of the mentioned reducing agent. Therefore, the Ni-P coating is much more common in the literature. In addition, it is associated with a low producing cost and an easy controlling of parameters during electroless deposition [17]. Both coatings are characterized by good electrical conductivity, excellent bonding to the substrate, low porosity and exceptional electromagnetic performance [20], [21].
2. Materials and methods In experiments described in this paper CN3100 multiwall carbon nanotubes supplied by Nanocyl® were used with the following characteristics: 9.5nm in diameter, 1.5µm in length. The purity of the powder was over 95%. Raman spectroscopy was applied to characterize the carbon nanotubes. The study was performed using Renishaw’s InVia Raman spectrometer. All samples were exposed to a wavelength of 514 nm. Power of laser was 10% of output power. During the measurement, five different places were irradiated and analyzed. The X-ray fluorescence (XRF) method was used to investigate composition of the residue of impurities and their content. The XRF measurements were performed using PANalytical’s Epsilon 3XLE spectrometer. Measurements were made using the Omnian software. Coatings on the surface of carbon nanotubes after various types of deposition were analyzed based on STEM and TEM in the bright and dark field and electron diffraction. Microstructure observations were performed by using high resolution scanning electron microscopy Hitachi S5500 and transmission JEOL JEM 3010 (300kV).
3.Experimental The initial step of CNT’s surface preparation was purification from amorphous carbon and catalyst residues. The carbon nanotubes were purified in acid mixture in a ratio of 3:2 (68% nitric acid: 98% sulfuric acid) for 1.5 hour at 70°C. The mixture of acids and nanotubes 3
was filtered and rinsed to neutral pH (pH=7). The carbon nanotubes were dried in an oven at 110°C. The purified carbon nanotubes were pulverized in a mortar. Then nanotubes were sensitized in a mixture of tin chloride (SnCl2) in the presence of hydrochloric acid (HCl) and the surface was activated by treatment with a solution of palladium chloride (PdCl2) in hydrochloric acid (HCl). Optimal activation of the surface of the CNTs is crucial for the deposition of Ni-P coating on their surface. Therefore, a part of experiments was done with different concentrations of palladium (cat.0.1-1.0 g/l) to determine its appropriate value. The effect of activation was observed with the use of electron microscopy. Between each step rinsing in distilled water was carried out. To improve the CNT’s surface accessibility, the stages of initial preparation to the deposition were carried out in an ultrasonic cleaner. The basic electroless deposition bath included: nickel sulfate as a source of nickel, sodium hypophosphite (phosphate (I) sodium) as a reducing agent and glycine as the complexing and buffering agent. The scale of the CNTs surface area had a significant impact on the deterioration of the stability of solution for electroless deposition. Therefore, thiourea was added to the solution as a stabilizer. The hydrophobic and strongly entangled structure of carbon nanotubes required the addition of the surfactant- CTAB. Process was carried out under the 70˚C and in pH=8,5. To enhance the accessibility of the carbon nanotubes surface the metallization was conducted in the presence of ultrasounds (ultrasonic probe and ultrasonic cleaner). After deposition, the modified carbon nanotubes were rinsed in distilled water in ultrasound bath and dried in an oven for 2h at 110°C. Final tests were carried out to compare Ni-P coating with coatings obtained by a Graphene Laboratory in Warsaw (i.e. Ni-B, Co-B and Fe-B coatings). These coatings were deposited a result of electroless method.
4. Results and Discussion. 4.1Characterization of substrates. Characterization of NC3100 carbon nanotubes was started with Raman spectroscopy (Fig.1). The ID/IG ratio was calculated from spectra collected from five different places and was 1,52±0,13.
4
D
Normalized intensity
G
2D
100
600
1100
1600
2100
Raman Shift,
2600
3100
cm-1
Fig.1.Raman spectra of NC3100 carbon nanotubes surface, in the region of 100-3200 cm-1.
This value indicates the surface degradation of CNT. For spectra, peak below 500 cm-1was not observed. This result confirmed that the NC3100 carbon nanotubes are multiwalled. The appearance of additional peaks in region 1200 cm-1 – 1500 cm-1 may indicate the presence of amorphous carbon in the sample. The XRF method was used to determine the production contamination of MWCNT. The results of the XRF study were summarized in Table.1. Table1 XRF test for NC3100, elemental and quantitative (wt.%) material composition.
NC3100
NC 3100 Element Concentration [wt.%]
Element
Concentration [wt.%]
Mg
46,7 ppm
Ca
218,5 ppm
Al
48,2 ppm
Cr
21,5 ppm
Si
25,6 ppm
Fe
509,7 ppm
P
19,9 ppm
Co
189,5 ppm
S
109,9 ppm
C,O,H*
99,866 %
Cl
146,3 ppm
*the XRF spectrometer is not able to accurately analyze light elements, hence the total values of carbon, oxygen and hydrogen in the samples tested.
The NC3100 carbon nanotubes contain small amount of impurities (<0,15wt%) which were introduced during the CNTs manufacturing process. Elevated content of Fe and Co are associated with their use as catalysts in CNT production procedure. Part of the impurities is related to the substrate (Mg, Al. Si) used. Other contaminations may be related to pollution of the carbon source (e.g. Ca, Cl).
5
4.2 Purification of CNTs and Electroless Deposition The use of the mixture of acids at elevated temperature caused partial break up of tightly entangled agglomerates of carbon nanotubes (which increased the surface availability for electroless metallization) and the nanotubes were shortened (Fig. 2ab). The use of an ultrasonic cleaner and an ultrasonic probe in successive stages of mixing was associated with an additional shattering of the agglomerates and shortening of nanotubes. Electron diffraction of purified carbon nanotubes was also performed. a)
b)
Fig. 2. STEM and TEM images of the surface of NC3100 carbon nanotubes: a) STEM of carbon nanotubes after acid treatment, b) TEM of carbon nanotubes after acid treatment.
Another important step after purification is activation. At the lowest concentrations of 0.1, 0.4gPd/l, the amount of catalyst on the surface of the carbon nanotube was insufficient. A small quantity of grains did not activate the carbon nanotube surface, so it was not possible to deposit the Ni-P coating. The two highest concentrations (0.8, 1.0gPd/l) led to the formation of large catalyst agglomerates on the surface. Removing agglomerates to electroless bath causes destabilization of the solution and consequently leads to decomposition. Concentration of 0.6gPd/l (Fig. 3) allowed for even surface activation of carbon nanotubes. Electron diffraction and dark field of activated CNT was performed. White points on the surface of carbon nanotubes (Fig. 3c, dark field image) are catalyst grains with a crystalline structure satisfying the Bragg low due to the same structure and orientation of nanocrystallites.
6
a)
b)
c)
d)
Fig. 3. Images of the activated surface of carbon nanotubes: palladium concentration of 0.6 gPd/l, (a,b) STEM images of NC3100, c) bright field TEM and d) dark field TEM showing the nanocrystalline particles.
The resulting coating contains about 3wt.% of phosphorus. This content is related to process conditions (T=70°C, pH=8,5) and to the concentration of the electroless deposition bath components. In addition, the Ni-P coating on the surface of carbon nanotubes was in the form of adsorbed grains (Fig.4d, e) not continuous coating, as confirmed in another publication [22]. The developed coating process allowed for uniformity.
7
a)
b)
c)
d)
e)
f) 8
Fig. 4. STEM images of carbon nanotubes with the electroless deposited layers of Co-B (a), Fe-B (b), Ni-B (c), Ni-P (d, e) and f) BF-TEM image of Ni-P coating consisting of nanocrystallites forming the chain.
Comparative experiments were conducted. The coating deposition was carried out by the same method (electroless deposition) using a different reducing agent. In all cases, for Fe, Co and Ni coating deposition, sodium tetrahydroborate was used. The use of a different reducing agent forced the application of different components of the solution and operation in a strongly alkaline environment. Sodium tetrahydroborate in addition to the reduction of individual metal sources, added small amounts of boron, which created the alloy type of coating. The results of the deposition of Co-B, Fe-B and Ni-B were compared using the STEM (Fig.4, a-e). Deposition of the Co-B leads to a large agglomerate of coating with embedded carbon nanotubes (Fig. 4a). The coating grew independently on substrate geometry. Images taken using SEM microscope showed that there are areas with the total absence of the coating. Preservation of properties in the entire volume of obtained powder is associated with the need for uniformity of coating on the surface of the CNTs. In the case of cobalt this was not obtained and requires further research. Another coating applied to carbon nanotubes was Fe-B. The experiment results are much better than for Co-B, showing layer growth along the carbon nanotubes. Nevertheless, the areas with a total lack of coating were still present. It should also be noted that the alloy coating is not continuous but in the form of adsorbed Fe-B grains (Fig.4b). The best results were obtained for Ni-B coatings (Fig. 4c). It grew while maintaining the geometry of the CNTs. The coating still grew in the form of adsorbed grains. I order to examine the quality and crystallinity of the obtained coatings electron diffraction method was carried out. The following samples were tested: carbon nanotubes
9
after chemical purification, carbon nanotubes after activation step and additionally carbon nanotubes with Ni-P coating. The first selected area electron diffraction pattern was performed on NC3100 carbon nanotubes after chemical purification. The diffractogram (Fig. 5a) clearly shows the halo from amorphous carbon film and diffused diffraction spots on several rings due to carbon nanotubes, which means that those nanotubes are nanocrystalline. In addition, this result is associated with a large degradation of the NC3100. The thickness of the carbon nanotubes walls is relatively small, which indicates a small number of single walls in MWCNT. The number of single walls was estimated to be between 8-10. a)
b)
c)
Fig.5. Electron diffraction images from a) multiwalled carbon nanotubes (NC3100) after chemical purification, b) carbon nanotubes (NC3100) after activation in PdCl2, c) carbon nanotubes(NC3100) with Ni-P coating.
The next diffraction pattern was performed on carbon nanotubes after activation step. Diffused spots related to nanocrystallites with different orientations were achieved in this case on diffraction rings. The catalyst grain size ranges from a few nm to about 35 nm. 10
The electron diffraction studies showed that the Ni-P coating structure was crystalline (Fig. 5c). The Nickel-Phosphorus phase diagram describes the system of Ni crystallites and nickel phosphorus crystals (mainly Ni3P) [23]. This crystal structure was confirmed using several electron diffraction patterns from various areas and identification was done according to the PDF method. As a result, Ni3P tetragonal phase was defined with the space group I4ത (82). The highest intensity was obtained for the interplanar distance of 2.16 for the (3 2 1) planes. The lower intensity of reflexes on the electron diffraction was associated with very small volume of nanocrystals and incomplete stoichiometry in grains.
5. Conclusions Commercially available multiwall carbon nanotubes are usually characterized by a large surface degradation. These defects have negative impact on mechanical and electrical properties of CNTs. Therefore, it is necessary to modify surface of multiwalled carbon nanotubes to improve their properties. A method that allows the deposition of a uniform coating, even on geometrically complex substrates, which does not require complicated apparatus and can be used in mass production is electroless deposition. This method is commonly used for metal, polymer and ceramic surfaces but rarely used for powders. A small number of articles concerns the direct metallization of CNTs by electroless deposition which was the scope of this work. The reason of this situation is a high instability of the electroless deposition bath in the presence of the powder. Mostly multiwalled carbon nanotubes are used as an additive to the Ni-P coating. In addition, carbon nanotubes have a residue of CCVD’s catalysts, residues of the substrate on which they have grown and an amorphous carbon. These impurities inhibit electroless deposition process and deteriorate the properties of the CNTs. Therefore, purification in a mixture of nitric and sulfuric acids was used. This method has been effective in removing of all types of these impurities. The first step in obtaining a uniform coating is to uniformly activate the surface of carbon nanotubes, which takes place during the initial sensitization and activation. In this work, activation was performed using Pd in the concentration range of 0.1-1.0 gPd/l. This allowed to determine the optimum distribution of the Pd(0.6gPd/l) catalyst on the surface of the carbon nanotubes before electroless deposition. The catalyst grains adsorbed on the surface of the carbon nanotube have a nanocrystalline structure, which was confirmed with electron diffraction study. In this paper, a methodology for the deposition of a uniform Ni-P 11
coating with a low phosphorus content throughout the CNT surface has been developed. The coating’s structure is nanocrystalline and consists of a mixture of Ni crystallites and nickel phosphide crystals (the most stable structure is Ni3P). The lower intensity of signals from the crystallites on the diffraction pattern compared to the catalyst grain is associated with the smaller Ni-P coating grains and still-formed structure with incomplete stoichiometry. The effect of intensive mixing (ultrasonic cleaner, ultrasonic probe) was observed during the preparation of the substrate (sensitization and activation) and electroless deposition. It breaks up the agglomerates, entangled carbon nanotubes and increases the availability of CNTs surface. In the bath for electroless deposition, a CTAB surfactant and thiourea as a stabilizer were used. CTAB improved the availability for components from electroless bath to CNT’s surface, while thiourea provided stability during electroless deposition. Surface modification using electroless deposition was used to improve the electrical conductivity of multiwall carbon nanotubes. This material in further research will be added to the polymer matrix (epoxy) and tested in terms of electrical conductivity.
Acknowledgments The project was funded by the National Science Centre based on decision no. DEC2011/03/D/ST8/04921.The authors would like to thank the Graphene Laboratory for performing Ni-B, Co-B and Fe-B coatings on carbon nanotubes.
References [1] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: A review of the mechanical properties of carbon nanotube-polymer composites, Carbon 44 (2006) 1624-1652. [2] A.S. Khurshed, A.T. Bilal, Synthesis of carbon nanotubes by catalytic chemical vapour deposition: A review on carbon sources, catalysts and substrates, Mater. Sci. in Semicond. Process. 41 (2016) 67-82. [3] R. Khare, S. Bose, Carbon Nanotube Based Composites – A Review, J. Miner Mater. Charact. Eng. 4(1) (2005) 31-46. [4] Z. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review, Prog. Polym. Sci. 36 (2011) 914-944. [5] K. Donaldson, R. Aitken, L. Tran, V. Stone, R. Duffin, G.Forrest, A. Alexander, Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety, Toxicol. Sci. 92 (1) (2006) 5-22. [6] F.H. Gojny, J. Nastalczyk, Z. Rosłaniec, K. Schulte, Surface modified multi-walled carbon nanotubes in CNT/epoxy-composites, Chem. Phys. Lett. 370 (2003) 820-824.
12
[7] J.A. Kim, D.G. Seong, T.J. Kang, J.R. Youn, Effects of surface modification on rheological and mechanical properties of CNT/epoxy composites, Carbon 44 (10) (2006) 1898-1905. [8] J. Cho, A.R. Boccaccini, M.S.P. Shaffer, Ceramic matrix composites containing carbon nanotubes, J. Mater. Sci. 44 (2009) 1934-1951. [9] A.D. Moghadam, E. Omrani, P.L. Menezes, P.K. Rohatgi, Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanptubes (CNTs) and graphene – A review, Composities Part B77 (2015) 402-420. [10] M. Kumar, Carbon nanotube synthesis and growth mechanism. In Carbon nanotubes - synthesis, characterization, applications.Yellampalli S., Ed., InTech, 2011, DOI 10.5772/19331. [11] A.C. Dupuis, The catalyst in the CCVD of carbon nanotubes – a review, Prog.Mater. Sci. 50(8) (2005) 929-961. [12] J. Zhang, Z.-H. Xie, H. Chen, C. Hu, L. Li, B. Hu, Z. Song, D. Yan, G. Yu, Electroless deposition and characterization of a double-layered Ni-B/Ni-P coating on AZ91D Mg alloy from eco-frendly fluoride-free baths, Surf. Coat. Technol. 342 (2018) 178-189. [13] J.F. Colomera, C. Stephanb, S. Lefrantb, G. Van Tendelooc,Willemsa IS, Kónyaa, A. Fonsecaa, laurentd Ch., J.B. Nagya, Large-scale synthesis of single-wall carbon nanotubes by catalytic chamical vapor deposition (CCVD) method, Chem. Phys. Lett. 317(1-2) (2000) 83-89. [14] P-X. Hou; Ch. Liu; H.-M. Cheng, Purification of carbon nanotubes, Carbon 46(2008)2003-2025. [15] H. Peng-Xiang, L. Chang, Ch. Hui-Ming, Purification of carbon nanotubes, Carbon 46(15) (2008) 2003-2025. [16] M. Peng-Cheng, A. Naveed Siddiqui, M. Gad, K. Jang-Kyo, Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review, Composites Part A 41(10) (2010) 1345-1367. [17] Q. Wang, M. Callisti, A. Miranda, B. McKay, I. Deligkiozi, T. Kosanovic, A. Zoikis-Karathanasis, K. Hrissagis, Evolution of structure, mechanical and tribological properties of Ni-P/ MWCNT coatings as a function of annealing temperature, Surf. Coat. Technol. 302 (2016) 195-201. [18] M. Peng-Cheng, M. Shan-Yin, T. Ben-Zhong, K. Jang-Kyo, Dispersion, interfacial interaction amd reagglomeration of functionalized carbon nanotubes in epoxy composites, Carbon 48(6) (2010) 1824-1834. [19] F. Wang, S. Arai, M. Endo, The preparation of multi-walled carbon nanotubes with a Ni-P coating by an electroless deposition process, Carbon 43 (2005)1716-1721. [20] J.-R. Choi, Y. Sil Lee, S.-J. Park, A study on thermal conductivity of electroless Ni-B plated multiwalled carbon nanotubes-reinforced composites, J. Int. Eng. Chem. 20 (2014) 3421-3424. [21] J. Liang, H. Li, L. Qi, W. Tian, X. Li, J. Zhou, D. Wang, J. Wei, Influence of Ni-CNTs additions on the microstructure and mechanical properties of extruded Mg-9Al alloy, Mater. Sci. Eng. A 678(2016) 101-109.
13
[22] L.M. Ang, T.S. Hor, G.Q. Xu, C. Tung, V Zhao, L.S. Wang, Electroless plating of metals onto carbon nanotubes activated by a single -step activation method, Chem. Mater. 11 (1999) 2115-2118. [23] C. Schmetterer, J. Vizdal, H. Ipser, A new investigation of the system Ni-P, Intermetallics 17(10) (2009) 826-834.
Graphical abstract
Highlights:
•
A method of electroless metallization for surface modification of MWCNT is
proposed. •
A methodology of obtaining uniform coatings on surface of CNT is developed.
•
Ni-P coating is intended to improve the electrical conductivity of carbon carbon nanotubes.
•
Ni-P coating grains on the CNT surface have a nanocrystalline structure.
14
S0009-2614(18)30349-X https://doi.org/10.1016/j.cplett.2018.04.056 CPLETT 35612
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
1 February 2018 29 April 2018
Please cite this article as: M. Kurkowska, S. Awietjan, R. Kozera, E. Jezierska, A. Boczkowska, Application of electroless deposition for surface modification of the multiwall carbon nanotubes, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.04.056
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Application of electroless deposition for surface modification of the multiwall carbon nanotubes M.
KURKOWSKA1,
S.
AWIETJAN2,
R.
KOZERA3,E.
JEZIERSKA4,
A.
BOCZKOWSKA5 1,2,3,4,5
Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, 02-507
Warsaw, Poland 1
Corresponding author, e-mail: [email protected] tel. +48 22 2345713
2
e-mail: [email protected]
3
e-mail: [email protected]
4
e-mail: [email protected]
5
e-mail: [email protected]
Abstract. The paper describes modification of carbon nanotubes surface by attaching the grains of Ni-P, Ni-B, Co-B and Fe-B. The modification was obtained by electroless metallization using sodium hypophosphite (NaH2PO2). We have investigated the parameters of electroless metallization process of CNTs. The uniformity of the coating on the carbon nanotubes was related to proper surface activation. While optimizing the electroless deposition, a range of catalyst concentrations from 0.1 to 1.0 gPd/l were tested. Deposition was used to improve the electrical properties of the later composite materials CNT-NiP/epoxy. The best results of electroless deposition were obtained for Ni-P and Ni-B coatings. Keywords: carbon nanotubes, nanocomposite, electroless deposition, coatings, Pd activation
1.Introduction In the 80s of the twentieth (XX) century, the industrial production of the material that we know as multiwall carbon nanotubes was began. However, a huge interest in this material started after 1990, when the paper of Iijma with the description of multiwall carbon nanotubes appeared in Nature. Two years later, the paper written by the same author about single wall carbon nanotubes was published. Next papers describe the unique properties of carbon nanotubes, such as the Young’s module approaching 1 TPa [1], [2] a tensile strength of about 60GPa [1], [2] for SWCNT and even 1,5TPa for MWCNT [2]. In addition, the electrical conductivity of carbon nanotubes is up to 109 A/cm2[2], [3] while the thermal conductivity is 2000-6000 W/m K at room temperature [4], [5] but for MWCNT we can observe values around 3000 W/mK. Furthermore, commercially available multiwall carbon nanotubes are characterized by reduced electrical and thermal properties compared to singlewall carbon 1
nanotubes, which is compensated by various surface modifications. In addition, the modification is intended to improve the wettability of the nanofiller by polymer [6], [7], ceramic [8] and metallic matrix [9]. The most popular method to produce industrial carbon nanotubes is chemical vapor deposition (CVD) [10]. The most commonly used variant of this method is catalytic chemical vapor deposition. This method as opposite to other methods, provides a high degree of process control and the ability to produce reproducible material [5], [11]. Among the most common catalysts there are Fe, Ni or Co or combination of them [5], [11-13]. This method allows to scale up in production, to use cheaper raw materials, to increase efficiency and to reduce the energy demand of the process. All these aspects have led to the final product’s price reduction, what made it possible to use in everyday products. Large-scale carbon nanotubes are contaminated from the substrate, catalyst and amorphous carbon. Therefore, a purification process was introduced to remove mentioned impurities [14]. This process can be carried out by the action of high temperature in the presence of N2/O2 or O2, H2O, Cl2 itself [15]. Although thermal purification removes amorphous carbon, catalyst and residue from substrate cannot be removed by this method. Chemical purification in nitric acid or in mixture of nitric and sulfuric acids is also often used [15], [16]. This method allows for full cleaning of the carbon nanotubes. Moreover, the surface of carbon nanotubes is functionalized in carboxyl, carbonyl and hydroxyl groups [15-17]. In many papers, it has been proven that these groups improve the wetting of the carbon nanotubes by the matrix (i.e. metal, polymer) and this is necessary to deposition metal ions onto carbon nanotubes [17], [18]. The disadvantage of chemical purification is the introduction of defects into the structure of carbon nanotube and the shortening of CNT length. Therefore, chemical purification requires control of parameters to determine the balance between CNT purification, functionalization and degradation. The next step is modification of carbon nanotubes surface. Carbon nanotubes due to their large surface area, nanometer size and tendency to tangle are not a simple substrate for depositing by metallic coating. There are a few studies about electroless deposition of metallic coating on surface of carbon nanotubes. Modification may be covalent, then the modified material forms a covalent bond to the carbon in the structure of carbon nanotubes. Described case of carbon nanotubes modification is related to the change from sp2 to sp3 hybridization. Modification may also have a non-covalent character where the modifying component is adsorbed on the surface without changing the hybridization of carbon in the structure of tubes. 2
Among non-covalent methods, we are focused on electroless deposition. It has many advantages that make it applicable to a wide variety of substrates such as metallic, ceramic and polymeric. This method can be also applied to substrates with highly complex geometry and no external power source or complex apparatus are needed [19]. Electroless deposition is a well-known method of surface modification [20]. Nickel coatings (mostly Ni-P and Ni-B alloys) are usually with this method. Deposition of a Ni-B coating when we used a dimethylamine boran as a reducing agent is characterized by a higher deposition efficiency. However, attention should be paid to the toxicity of the mentioned reducing agent. Therefore, the Ni-P coating is much more common in the literature. In addition, it is associated with a low producing cost and an easy controlling of parameters during electroless deposition [17]. Both coatings are characterized by good electrical conductivity, excellent bonding to the substrate, low porosity and exceptional electromagnetic performance [20], [21].
2. Materials and methods In experiments described in this paper CN3100 multiwall carbon nanotubes supplied by Nanocyl® were used with the following characteristics: 9.5nm in diameter, 1.5µm in length. The purity of the powder was over 95%. Raman spectroscopy was applied to characterize the carbon nanotubes. The study was performed using Renishaw’s InVia Raman spectrometer. All samples were exposed to a wavelength of 514 nm. Power of laser was 10% of output power. During the measurement, five different places were irradiated and analyzed. The X-ray fluorescence (XRF) method was used to investigate composition of the residue of impurities and their content. The XRF measurements were performed using PANalytical’s Epsilon 3XLE spectrometer. Measurements were made using the Omnian software. Coatings on the surface of carbon nanotubes after various types of deposition were analyzed based on STEM and TEM in the bright and dark field and electron diffraction. Microstructure observations were performed by using high resolution scanning electron microscopy Hitachi S5500 and transmission JEOL JEM 3010 (300kV).
3.Experimental The initial step of CNT’s surface preparation was purification from amorphous carbon and catalyst residues. The carbon nanotubes were purified in acid mixture in a ratio of 3:2 (68% nitric acid: 98% sulfuric acid) for 1.5 hour at 70°C. The mixture of acids and nanotubes 3
was filtered and rinsed to neutral pH (pH=7). The carbon nanotubes were dried in an oven at 110°C. The purified carbon nanotubes were pulverized in a mortar. Then nanotubes were sensitized in a mixture of tin chloride (SnCl2) in the presence of hydrochloric acid (HCl) and the surface was activated by treatment with a solution of palladium chloride (PdCl2) in hydrochloric acid (HCl). Optimal activation of the surface of the CNTs is crucial for the deposition of Ni-P coating on their surface. Therefore, a part of experiments was done with different concentrations of palladium (cat.0.1-1.0 g/l) to determine its appropriate value. The effect of activation was observed with the use of electron microscopy. Between each step rinsing in distilled water was carried out. To improve the CNT’s surface accessibility, the stages of initial preparation to the deposition were carried out in an ultrasonic cleaner. The basic electroless deposition bath included: nickel sulfate as a source of nickel, sodium hypophosphite (phosphate (I) sodium) as a reducing agent and glycine as the complexing and buffering agent. The scale of the CNTs surface area had a significant impact on the deterioration of the stability of solution for electroless deposition. Therefore, thiourea was added to the solution as a stabilizer. The hydrophobic and strongly entangled structure of carbon nanotubes required the addition of the surfactant- CTAB. Process was carried out under the 70˚C and in pH=8,5. To enhance the accessibility of the carbon nanotubes surface the metallization was conducted in the presence of ultrasounds (ultrasonic probe and ultrasonic cleaner). After deposition, the modified carbon nanotubes were rinsed in distilled water in ultrasound bath and dried in an oven for 2h at 110°C. Final tests were carried out to compare Ni-P coating with coatings obtained by a Graphene Laboratory in Warsaw (i.e. Ni-B, Co-B and Fe-B coatings). These coatings were deposited a result of electroless method.
4. Results and Discussion. 4.1Characterization of substrates. Characterization of NC3100 carbon nanotubes was started with Raman spectroscopy (Fig.1). The ID/IG ratio was calculated from spectra collected from five different places and was 1,52±0,13.
4
D
Normalized intensity
G
2D
100
600
1100
1600
2100
Raman Shift,
2600
3100
cm-1
Fig.1.Raman spectra of NC3100 carbon nanotubes surface, in the region of 100-3200 cm-1.
This value indicates the surface degradation of CNT. For spectra, peak below 500 cm-1was not observed. This result confirmed that the NC3100 carbon nanotubes are multiwalled. The appearance of additional peaks in region 1200 cm-1 – 1500 cm-1 may indicate the presence of amorphous carbon in the sample. The XRF method was used to determine the production contamination of MWCNT. The results of the XRF study were summarized in Table.1. Table1 XRF test for NC3100, elemental and quantitative (wt.%) material composition.
NC3100
NC 3100 Element Concentration [wt.%]
Element
Concentration [wt.%]
Mg
46,7 ppm
Ca
218,5 ppm
Al
48,2 ppm
Cr
21,5 ppm
Si
25,6 ppm
Fe
509,7 ppm
P
19,9 ppm
Co
189,5 ppm
S
109,9 ppm
C,O,H*
99,866 %
Cl
146,3 ppm
*the XRF spectrometer is not able to accurately analyze light elements, hence the total values of carbon, oxygen and hydrogen in the samples tested.
The NC3100 carbon nanotubes contain small amount of impurities (<0,15wt%) which were introduced during the CNTs manufacturing process. Elevated content of Fe and Co are associated with their use as catalysts in CNT production procedure. Part of the impurities is related to the substrate (Mg, Al. Si) used. Other contaminations may be related to pollution of the carbon source (e.g. Ca, Cl).
5
4.2 Purification of CNTs and Electroless Deposition The use of the mixture of acids at elevated temperature caused partial break up of tightly entangled agglomerates of carbon nanotubes (which increased the surface availability for electroless metallization) and the nanotubes were shortened (Fig. 2ab). The use of an ultrasonic cleaner and an ultrasonic probe in successive stages of mixing was associated with an additional shattering of the agglomerates and shortening of nanotubes. Electron diffraction of purified carbon nanotubes was also performed. a)
b)
Fig. 2. STEM and TEM images of the surface of NC3100 carbon nanotubes: a) STEM of carbon nanotubes after acid treatment, b) TEM of carbon nanotubes after acid treatment.
Another important step after purification is activation. At the lowest concentrations of 0.1, 0.4gPd/l, the amount of catalyst on the surface of the carbon nanotube was insufficient. A small quantity of grains did not activate the carbon nanotube surface, so it was not possible to deposit the Ni-P coating. The two highest concentrations (0.8, 1.0gPd/l) led to the formation of large catalyst agglomerates on the surface. Removing agglomerates to electroless bath causes destabilization of the solution and consequently leads to decomposition. Concentration of 0.6gPd/l (Fig. 3) allowed for even surface activation of carbon nanotubes. Electron diffraction and dark field of activated CNT was performed. White points on the surface of carbon nanotubes (Fig. 3c, dark field image) are catalyst grains with a crystalline structure satisfying the Bragg low due to the same structure and orientation of nanocrystallites.
6
a)
b)
c)
d)
Fig. 3. Images of the activated surface of carbon nanotubes: palladium concentration of 0.6 gPd/l, (a,b) STEM images of NC3100, c) bright field TEM and d) dark field TEM showing the nanocrystalline particles.
The resulting coating contains about 3wt.% of phosphorus. This content is related to process conditions (T=70°C, pH=8,5) and to the concentration of the electroless deposition bath components. In addition, the Ni-P coating on the surface of carbon nanotubes was in the form of adsorbed grains (Fig.4d, e) not continuous coating, as confirmed in another publication [22]. The developed coating process allowed for uniformity.
7
a)
b)
c)
d)
e)
f) 8
Fig. 4. STEM images of carbon nanotubes with the electroless deposited layers of Co-B (a), Fe-B (b), Ni-B (c), Ni-P (d, e) and f) BF-TEM image of Ni-P coating consisting of nanocrystallites forming the chain.
Comparative experiments were conducted. The coating deposition was carried out by the same method (electroless deposition) using a different reducing agent. In all cases, for Fe, Co and Ni coating deposition, sodium tetrahydroborate was used. The use of a different reducing agent forced the application of different components of the solution and operation in a strongly alkaline environment. Sodium tetrahydroborate in addition to the reduction of individual metal sources, added small amounts of boron, which created the alloy type of coating. The results of the deposition of Co-B, Fe-B and Ni-B were compared using the STEM (Fig.4, a-e). Deposition of the Co-B leads to a large agglomerate of coating with embedded carbon nanotubes (Fig. 4a). The coating grew independently on substrate geometry. Images taken using SEM microscope showed that there are areas with the total absence of the coating. Preservation of properties in the entire volume of obtained powder is associated with the need for uniformity of coating on the surface of the CNTs. In the case of cobalt this was not obtained and requires further research. Another coating applied to carbon nanotubes was Fe-B. The experiment results are much better than for Co-B, showing layer growth along the carbon nanotubes. Nevertheless, the areas with a total lack of coating were still present. It should also be noted that the alloy coating is not continuous but in the form of adsorbed Fe-B grains (Fig.4b). The best results were obtained for Ni-B coatings (Fig. 4c). It grew while maintaining the geometry of the CNTs. The coating still grew in the form of adsorbed grains. I order to examine the quality and crystallinity of the obtained coatings electron diffraction method was carried out. The following samples were tested: carbon nanotubes
9
after chemical purification, carbon nanotubes after activation step and additionally carbon nanotubes with Ni-P coating. The first selected area electron diffraction pattern was performed on NC3100 carbon nanotubes after chemical purification. The diffractogram (Fig. 5a) clearly shows the halo from amorphous carbon film and diffused diffraction spots on several rings due to carbon nanotubes, which means that those nanotubes are nanocrystalline. In addition, this result is associated with a large degradation of the NC3100. The thickness of the carbon nanotubes walls is relatively small, which indicates a small number of single walls in MWCNT. The number of single walls was estimated to be between 8-10. a)
b)
c)
Fig.5. Electron diffraction images from a) multiwalled carbon nanotubes (NC3100) after chemical purification, b) carbon nanotubes (NC3100) after activation in PdCl2, c) carbon nanotubes(NC3100) with Ni-P coating.
The next diffraction pattern was performed on carbon nanotubes after activation step. Diffused spots related to nanocrystallites with different orientations were achieved in this case on diffraction rings. The catalyst grain size ranges from a few nm to about 35 nm. 10
The electron diffraction studies showed that the Ni-P coating structure was crystalline (Fig. 5c). The Nickel-Phosphorus phase diagram describes the system of Ni crystallites and nickel phosphorus crystals (mainly Ni3P) [23]. This crystal structure was confirmed using several electron diffraction patterns from various areas and identification was done according to the PDF method. As a result, Ni3P tetragonal phase was defined with the space group I4ത (82). The highest intensity was obtained for the interplanar distance of 2.16 for the (3 2 1) planes. The lower intensity of reflexes on the electron diffraction was associated with very small volume of nanocrystals and incomplete stoichiometry in grains.
5. Conclusions Commercially available multiwall carbon nanotubes are usually characterized by a large surface degradation. These defects have negative impact on mechanical and electrical properties of CNTs. Therefore, it is necessary to modify surface of multiwalled carbon nanotubes to improve their properties. A method that allows the deposition of a uniform coating, even on geometrically complex substrates, which does not require complicated apparatus and can be used in mass production is electroless deposition. This method is commonly used for metal, polymer and ceramic surfaces but rarely used for powders. A small number of articles concerns the direct metallization of CNTs by electroless deposition which was the scope of this work. The reason of this situation is a high instability of the electroless deposition bath in the presence of the powder. Mostly multiwalled carbon nanotubes are used as an additive to the Ni-P coating. In addition, carbon nanotubes have a residue of CCVD’s catalysts, residues of the substrate on which they have grown and an amorphous carbon. These impurities inhibit electroless deposition process and deteriorate the properties of the CNTs. Therefore, purification in a mixture of nitric and sulfuric acids was used. This method has been effective in removing of all types of these impurities. The first step in obtaining a uniform coating is to uniformly activate the surface of carbon nanotubes, which takes place during the initial sensitization and activation. In this work, activation was performed using Pd in the concentration range of 0.1-1.0 gPd/l. This allowed to determine the optimum distribution of the Pd(0.6gPd/l) catalyst on the surface of the carbon nanotubes before electroless deposition. The catalyst grains adsorbed on the surface of the carbon nanotube have a nanocrystalline structure, which was confirmed with electron diffraction study. In this paper, a methodology for the deposition of a uniform Ni-P 11
coating with a low phosphorus content throughout the CNT surface has been developed. The coating’s structure is nanocrystalline and consists of a mixture of Ni crystallites and nickel phosphide crystals (the most stable structure is Ni3P). The lower intensity of signals from the crystallites on the diffraction pattern compared to the catalyst grain is associated with the smaller Ni-P coating grains and still-formed structure with incomplete stoichiometry. The effect of intensive mixing (ultrasonic cleaner, ultrasonic probe) was observed during the preparation of the substrate (sensitization and activation) and electroless deposition. It breaks up the agglomerates, entangled carbon nanotubes and increases the availability of CNTs surface. In the bath for electroless deposition, a CTAB surfactant and thiourea as a stabilizer were used. CTAB improved the availability for components from electroless bath to CNT’s surface, while thiourea provided stability during electroless deposition. Surface modification using electroless deposition was used to improve the electrical conductivity of multiwall carbon nanotubes. This material in further research will be added to the polymer matrix (epoxy) and tested in terms of electrical conductivity.
Acknowledgments The project was funded by the National Science Centre based on decision no. DEC2011/03/D/ST8/04921.The authors would like to thank the Graphene Laboratory for performing Ni-B, Co-B and Fe-B coatings on carbon nanotubes.
References [1] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: A review of the mechanical properties of carbon nanotube-polymer composites, Carbon 44 (2006) 1624-1652. [2] A.S. Khurshed, A.T. Bilal, Synthesis of carbon nanotubes by catalytic chemical vapour deposition: A review on carbon sources, catalysts and substrates, Mater. Sci. in Semicond. Process. 41 (2016) 67-82. [3] R. Khare, S. Bose, Carbon Nanotube Based Composites – A Review, J. Miner Mater. Charact. Eng. 4(1) (2005) 31-46. [4] Z. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review, Prog. Polym. Sci. 36 (2011) 914-944. [5] K. Donaldson, R. Aitken, L. Tran, V. Stone, R. Duffin, G.Forrest, A. Alexander, Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety, Toxicol. Sci. 92 (1) (2006) 5-22. [6] F.H. Gojny, J. Nastalczyk, Z. Rosłaniec, K. Schulte, Surface modified multi-walled carbon nanotubes in CNT/epoxy-composites, Chem. Phys. Lett. 370 (2003) 820-824.
12
[7] J.A. Kim, D.G. Seong, T.J. Kang, J.R. Youn, Effects of surface modification on rheological and mechanical properties of CNT/epoxy composites, Carbon 44 (10) (2006) 1898-1905. [8] J. Cho, A.R. Boccaccini, M.S.P. Shaffer, Ceramic matrix composites containing carbon nanotubes, J. Mater. Sci. 44 (2009) 1934-1951. [9] A.D. Moghadam, E. Omrani, P.L. Menezes, P.K. Rohatgi, Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanptubes (CNTs) and graphene – A review, Composities Part B77 (2015) 402-420. [10] M. Kumar, Carbon nanotube synthesis and growth mechanism. In Carbon nanotubes - synthesis, characterization, applications.Yellampalli S., Ed., InTech, 2011, DOI 10.5772/19331. [11] A.C. Dupuis, The catalyst in the CCVD of carbon nanotubes – a review, Prog.Mater. Sci. 50(8) (2005) 929-961. [12] J. Zhang, Z.-H. Xie, H. Chen, C. Hu, L. Li, B. Hu, Z. Song, D. Yan, G. Yu, Electroless deposition and characterization of a double-layered Ni-B/Ni-P coating on AZ91D Mg alloy from eco-frendly fluoride-free baths, Surf. Coat. Technol. 342 (2018) 178-189. [13] J.F. Colomera, C. Stephanb, S. Lefrantb, G. Van Tendelooc,Willemsa IS, Kónyaa, A. Fonsecaa, laurentd Ch., J.B. Nagya, Large-scale synthesis of single-wall carbon nanotubes by catalytic chamical vapor deposition (CCVD) method, Chem. Phys. Lett. 317(1-2) (2000) 83-89. [14] P-X. Hou; Ch. Liu; H.-M. Cheng, Purification of carbon nanotubes, Carbon 46(2008)2003-2025. [15] H. Peng-Xiang, L. Chang, Ch. Hui-Ming, Purification of carbon nanotubes, Carbon 46(15) (2008) 2003-2025. [16] M. Peng-Cheng, A. Naveed Siddiqui, M. Gad, K. Jang-Kyo, Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review, Composites Part A 41(10) (2010) 1345-1367. [17] Q. Wang, M. Callisti, A. Miranda, B. McKay, I. Deligkiozi, T. Kosanovic, A. Zoikis-Karathanasis, K. Hrissagis, Evolution of structure, mechanical and tribological properties of Ni-P/ MWCNT coatings as a function of annealing temperature, Surf. Coat. Technol. 302 (2016) 195-201. [18] M. Peng-Cheng, M. Shan-Yin, T. Ben-Zhong, K. Jang-Kyo, Dispersion, interfacial interaction amd reagglomeration of functionalized carbon nanotubes in epoxy composites, Carbon 48(6) (2010) 1824-1834. [19] F. Wang, S. Arai, M. Endo, The preparation of multi-walled carbon nanotubes with a Ni-P coating by an electroless deposition process, Carbon 43 (2005)1716-1721. [20] J.-R. Choi, Y. Sil Lee, S.-J. Park, A study on thermal conductivity of electroless Ni-B plated multiwalled carbon nanotubes-reinforced composites, J. Int. Eng. Chem. 20 (2014) 3421-3424. [21] J. Liang, H. Li, L. Qi, W. Tian, X. Li, J. Zhou, D. Wang, J. Wei, Influence of Ni-CNTs additions on the microstructure and mechanical properties of extruded Mg-9Al alloy, Mater. Sci. Eng. A 678(2016) 101-109.
13
[22] L.M. Ang, T.S. Hor, G.Q. Xu, C. Tung, V Zhao, L.S. Wang, Electroless plating of metals onto carbon nanotubes activated by a single -step activation method, Chem. Mater. 11 (1999) 2115-2118. [23] C. Schmetterer, J. Vizdal, H. Ipser, A new investigation of the system Ni-P, Intermetallics 17(10) (2009) 826-834.
Graphical abstract
Highlights:
•
A method of electroless metallization for surface modification of MWCNT is
proposed. •
A methodology of obtaining uniform coatings on surface of CNT is developed.
•
Ni-P coating is intended to improve the electrical conductivity of carbon carbon nanotubes.
•
Ni-P coating grains on the CNT surface have a nanocrystalline structure.
14