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The surface fractal investigation on carbon nanotubes modified by the adsorption of poly(acrylic acid)
Surface & Coatings Technology 190 (2005) 394 – 399 www.elsevier.com/locate/surfcoat
The surface fractal investigation on carbon nanotubes modified by the adsorption of poly(acrylic acid) Hou Qing-Feng a, Lu Xian-Cai b, Liu Xian-Dong b, Hu Bai-Xing a, Cui Ju-Qing a, Shen Jian a,c,* a
Research Center of Surface and Interface Chemical Engineering Technology, 22 Hankou Road, Nanjing University, Nanjing 210093, PR China b The State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China c Department of Applied Chemistry, Nanjing Normal University, Nanjing 210024, PR China Received 9 September 2003; accepted 18 March 2004 Available online 10 May 2004
Abstract In this paper, surface fractal analysis is carried out to study the surface of carbon nanotubes after the adsorption of poly(acrylic acid) (PAA) using the thermodynamic method. The fractal dimension (dSF ) of a fractal surface, BET surface area (SBET) and pore size distribution (PSD) are calculated from low-temperature nitrogen adsorption isotherms. The value of dSF declines as the adsorption amount of PAA increases, which means the adsorption may lower the surface roughness of the carbon nanotubes. The PSD pattern was modified obviously after the adsorption of PAA because of the pore-blocking effect. Additionally, an excellent linear decrease in the BET surface area of carbon nanotubes is found as the adsorption amount of PAA increases, which can be attributed to both the pore-blocking effect and the surfacescreening effect. The results of the present work may facilitate our understanding of the interaction between polymer and the carbon nanotubes at the microscale occurring on irregular interfaces. D 2004 Elsevier B.V. All rights reserved. Keywords: Fractal dimension of surface; Adsorption; Carbon nanotubes; Surface area; Pore size distribution
1. Introduction Since the early work of Mandelbrot [1] explored the replication of structure on an increasingly finer scale, i.e., the quality of self-similarity, much attention has been paid to the application of fractal analysis to surface science in recent years. Some valuable information of fractal geometry can be deduced from nitrogen adsorption isotherms of the examined solid surface, the fractal geometry of which is a key parameter affecting the interactions between adsorbed molecules and surface. A scale-independent parameter named value of fractal dimension (dSF) is proposed to quantify the degree of surface irregularity. Usually, the value of dSF lies between 2 and 3. A regular and smooth surface possesses dSF = 2, and a higher dSF value suggests a more irregular and space-filling surface. Therefore, dSF can be considered as an operative measurement of the surface roughness. The easiest * Corresponding author. Research Center of Surface and Interface Chemical Engineering Technology, 22 Hankou Road, Nanjing University, Nanjing 210093, PR China. Tel.: +86-2583-594933; fax: +86-2583594404. E-mail address: [email protected] (J. Shen). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.016
method to calculate dSF is to fit a single nitrogen adsorption isotherm to a fractal isotherm equation having dSF as a parameter. Presently, there are two popular methods for calculating dSF from a single nitrogen adsorption isotherm. One is the FHH method which evaluates dSF through the slope of a log –log plot of N/Nm vs. ln[( P/P0)] 1 from a single nitrogen adsorption isotherm in the range of capillary condensation [2 –6]. Here Nm can be computed from the BET equation. The other one is the so-called thermodynamic method proposed by Neimark et al. in 1992 [7 –9]. The value obtained using this method is independent to the structure model of the porous network or the type of adsorption process. Instead, it is based on an integral thermodynamic relationship between the area of the liquid –vapor interface and the amount adsorbed at a given relative pressure. However, this method is valid only for describing the capillary condensation mechanism because it completely ignores surface forces. The thermodynamic method concludes a very simple relationship between the surface area of the adsorbed liquid film, S, and the mean radius of the curvature of this adsorbed film, r, lnS ¼ const ðdSF 2Þlnr
ð1Þ
Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399 Table 1 Parameters of all the experimental samples Sample
Adsorption amount of PAA (mg/g)
SBET (m2/g)
dSF
rmin (nm)
rmax (nm)
CNT-0 CNT-1 CNT-2 CNT-3 CNT-4 CNT-5 CNT-6
0 13.09 43.35 89.60 118.28 146.53 268.98
200.33 F 6 198.43 F 3 189.97 F 4 171.41 F 1 156.88 F 2 150.04 F 1 112.50 F 1
2.56 2.56 2.56 2.57 2.58 2.53 2.51
0.35 0.35 0.35 0.38 0.38 0.38 0.38
40.56 16.90 17.28 17.64 17.36 17.36 17.50
395
nomena occurring at surface and interface of carbon nanotubes such as adsorption, surface modification and coating by some polymer. In this paper, thermodynamic method is used to study the changes in surface fractal dimension of carbon nanotubes due to the adsorption of poly(acrylic acid) (PAA). Six treated carbon nanotube samples with different adsorption amount of PAA and one pure sample (CNT-0) as a reference are analyzed. Low-temperature ( 196 jC) nitrogen adsorption measurements is used to calculate the surface parameters of carbon nanotubes including dSF, BET surface area (SBET) and pore size distribution (PSD).
In this equation, the surface area of the film is calculated from the Kiselev equation S ¼ ðRT =cÞ
Z
2. Experimental
Nmax
lnðP0 =PÞdN
ð2Þ
NðP=P0 Þ
where Nmax denotes the amount adsorbed when P = P0 tends toward unity, and c is the surface tension of the liquid adsorbate. Meanwhile, the Kelvin equation (Eq. (3)) is used to convert the equilibrium pressure P to the mean radius of the curvature r. ln
P 2cvL ¼ P0 rRT
ð3Þ
where vL denotes molar volume of the adsorbate in the condensed state. Presently, surface fractal analysis is often employed to survey the detail surface information of porous particles. For example, Venkatraman et al. [2] have studied the influence of the temperature of calcination on the surface fractal dimension of two types of mesoporous absorbents. Meng et al. [3] has carried a comparative study of surface fractality between polymeric and particulate titania aerogels. Lee et al. [4] practised surface fractal analysis to investigate the surface-screening effect and pore-blocking effect of Camontmorillonites after the exchange with both metal cations and organic cations. Those studies prove that surface fractal analysis can be helpful to explain some interface – surface properties of the porous particles at the microscale. Carbon nanotube has become another very important topic in material science and technology since Iijima reported it first in 1991 [10]. The crystalline porous structure and large surface area of the carbon nanotube make it an ideal adsorbent for polymer. Usually, the adsorption of polymer on the carbon nanotubes can be surveyed by some traditional techniques such as SEM, STM and Raman spectra. However, more valuable information on the modification of surface properties such as surface irregularities and defects can be monitored by the fractal analysis. Plenty of investigations have been carried out on the synthesis, purification, structure characterization and application of the carbon nanoparticles [11 – 23], few of which are related to the surface fractal analysis. Hence, the surface fractal analysis maybe a feasible approach to explain some phe-
2.1. Materials The solid adsorbent is the lab-synthesized carbon nanotubes by catalytic decomposition of benzene vapor [24]. About 20 mg Fe –Co/g-Al2O3 as catalyst was placed in a quartz tube with 10 mm inner diameter inside a large quartz tube with 35 mm inner diameter and 550 mm length, mounted in a tube furnace with 300 mm length. The catalyst was preheated to a certain reaction temperature between 560 and 810 jC under nitrogen atmosphere at a flow rate of 70 ml/min. Subsequently, benzene vapor was led into the system by means of the nitrogen flow passing through a triplet benzene saturator at 25 jC. The nitrogen and benzene vapor flow lasted for 60 min for growing carbon nanotubes. The furnace was then cooled down to ambient temperature in nitrogen flow. The carbon nanotubes are washed in hydrochloric acid at 140 jC for 5 h then washed by distilled water. The diameters of carbon nanotubes range from 10 to 30 nm. Poly(acrylic acid) is obtained by polymerization of
Fig. 1. The diagram of BET surface area of the carbon nanotubes vs. the adsorption amount of PAA.
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monomer in aqueous solution at 70 jC using about 30% (NH4)2S2O8 as initiator. Number-average molar mass (Mn) and polydispersity index (Mw/Mn) for the PAA samples are 7.073 104 and 1.283, respectively, which are measured on the miniDAWN multi-angle laser scatter analyzer (Wyatt, CA) using the ASTRA software 4.90.07.
thence. The cake precipitate is dried at 40 jC in vacuum for 24 h to remove moisture and then used as the sample for further analysis. The adsorption amount of PAA is calculated from the concentration difference of the solution before and after the adsorption measured by the titration. The related parameters of the treated carbon nanotubes are listed in Table 1.
2.2. PAA adsorption experiment 2.3. Techniques of measurements Six shares of 2.500 g carbon nanotubes are dispersed respectively in six 100-ml PAA solution with different concentration in small vessels which could be firmly closed with suitable caps. The suspension is stirred in water bath at 25 jC for 48 h to the adsorption equilibrium and filtered
Low-temperature ( 196 jC) nitrogen adsorption – desorption isotherms are measured on an ASAP2010 Accelerated Surface Area and Porisimetry Adsorption Analyzer (Micromeritics). Prior to nitrogen adsorption, the samples
Fig. 2. The nitrogen adsorption – desorption isotherms of the experimental samples.
Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399
are degassed to the pressure about 0.68 Pa at 40 jC for about 24 h. Adsorption – desorption of nitrogen is carried out in the relative pressure range of 0.001– 0.995 using about 1.000 g of experimental samples. In this paper, SBET is determined from nitrogen adsorption isotherms in the relative pressure range of 0.05 –0.20 based on the traditional method. Harkins and Jura (HJ) cylinder pore model is employed to calculate PSD according to the classical Kelvin equation from the nitrogen adsorption isotherm.
3. Results and discussion 3.1. BET surface area The original carbon nanotubes have the largest SBET as 200.33 m2/g which decreases gradually to 112.50 m2/g,
397
while the adsorption amount of PAA increases from 0 to 268.98 mg/g. The decline is proportional to the adsorption amount of PAA as plotted in Fig. 1. Similar decrease in SBET of porous particles after the adsorption of some organic substance has also been reported in other literatures [4,25,26]. 3.2. Pore size distribution The nitrogen adsorption – desorption isotherms of the examined samples are shown in Fig. 2 in the linear scales. All of the isotherms have similar shape which belongs to type IV according to the classification proposed by Brunauer [27]. These isotherms exhibit remarkable hysteresis loops which reflect a rather broad distribution of mesopores in these samples. As demonstrated in Fig. 3, the adsorption of PAA influences the PSD of the samples distinctly. Two
Fig. 3. The diagram of PSD of the experimental samples.
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Fig. 4. The plots drawn with the standard coordinates of ln S vs. ln r. The value of dSF can be calculated from the slope in the linear range.
types of mesopores should be identified clearly in the pure carbon nanotubes (CNT-0). One narrow peak located at 3 nm can be identified as the intraparticle pores, a typical cylinder-shaped mesopores of the carbon nanotubes [28]. The other broad multi-peaks between 7 –100 nm can be identified as the interparticle pores due to the strong aggregation among the carbon nanotubes [28]. The intraparticle pores at 3 nm reduce gradually and disappear eventually as the adsorption amount of PAA increases which indicates those original intraparticle pores are blocked up by PAA. Remarkable change is also found in the interparticle pores. The interparticle pores reduces as a whole, and those pores wider than 100 nm in the pure carbon nanotubes disappear entirely after the adsorption of PAA, which can be attributed to two possible reasons. PAA could fill these interparticle pores. Moreover, PAA has made the aggregation among the carbon nanotubes more closely and rigidly. It should be pointed out that a new dominant pores at about 1.4 nm is found in CNT-4, CNT-5 and CNT6, which indicates a new type of micropore is formed when the adsorbed PAA solidified at the surface of carbon nanotubes. As the assembly of PAA on the surface of carbon nanotubes is difficult to observe directly, the mechanism of origination of these micropores is not clear yet. It is suspected that the width of these micropores is related to the size of molecular PAA. The reduction in the surface area of both intraparticle and interparticle pores of the carbon nanotubes proves that the pore-blocking effect of PAA is one predominant reason for the total decrease of SBET of the examined samples.
face area vs. the radius of curvature plots of all nitrogen isotherms is given in Fig. 4 in double-logarithmic scales according to Eq. (1). The value of dSF can be calculated from the slope of those fitting lines. The range of r where linearity present is also listed in Table 1. The lower limit for r of all the examined samples is approximately 0.35 nm, corresponding to the limit of applicability of the Kelvin equation. Nevertheless, The upper limit r of the six treated sample is about 17.36 nm which is much smaller than that of the original carbon nanotubes (40.56 nm). The value of dSF of the original carbon nanotubes is 2.56. In contrast to the linear decrease of SBET, dSF does not decrease proportional to the adsorption amount of PAA. As shown in Fig. 5, the change of dSF after the adsorption of PAA can be divided into two stages, and one transition at CNT-4 can be observed clearly. The value of dSF increases firstly (CNT-0 to CNT-4) then decreases sharply (CNT-4 to CNT-6) as the adsorption amount of PAA increases. No change is found in dSF among CNT-0, CNT-1 and CNT-2, which means the surface roughness of these samples is almost at the same level. Accordingly, the process of the adsorption of PAA could be described as below. In the beginning, the adsorption amount of PAA is too low to change the original surface of the carbon nanotubes obviously, which results in the invariability of dSF in CNT-0, CNT-1 and CNT-2. As the adsorption amount of PAA increases continuously such as CNT-3 and CNT-4, more and more conglomeration of PAA become present at the original surface of the carbon nanotubes which may enhance the roughness of the surface as proved by the increase of dSF. When the adsorption amount of PAA is high enough to screen the whole original surface, a new relatively slippery surface is formed as proved by the sharp decrease of dSF. The surface-screening effect of PAA results in a change in dSF of the samples discussed above. Meanwhile, the pores of the carbon nanotubes are also blocked up by PAA
3.3. Surface fractal analysis The value of dSF is calculated from the nitrogen isotherms according to the thermodynamic method. The inter-
Fig. 5. The diagram of dSF vs. the adsorption amount of PAA of the experimental samples.
Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399
gradually, which can be proved by the changes in the PSD pattern of the treated samples. Since the surface area of a smooth surface is lower than that of a rough surface, the surface-screening effect of PAA should partially result in the decrease of SBET in some examined samples such as CNT-5 and CNT-6, which have smoother surfaces than the untreated sample CNT-0. On the other hand, based on the surface fractal analysis, CNT-3 and CNT-4 have more irregular and space-filling surfaces, which should have a bigger SBET than that of other samples. In fact, the SBET decreases proportionally to the adsorption amount of PAA as shown in Fig. 1, which implies the reduction of the pores surface, i.e., the pore-blocking effect has the dominating contribution to change in SBET of the carbon nanotubes. Hence, both pore-blocking effect and surface-screening effect can result in a linear decrease in SBET of the carbon nanotubes. However, the pore-blocking effect should be the main cause for the decrease in SBET of the examined samples.
4. Conclusions Surface fractal analysis is a valid method to monitor the surface properties of carbon nanotubes after the adsorption of PAA. The adsorption of PAA greatly influences the surface properties of original carbon nanotubes. SBET of the carbon nanotubes decreases after the adsorption of PAA, which can be attributed to two possible reasons: the poreblocking effect and the surface-screening effect. Integrating the analysis on dSF and PSD, the pore-blocking effect, i.e., the reduction of the pore surface should be the main cause for the decrease in SBET. The surface roughness is well characterized by the dSF and consists of the change in both SBET and PSD of the examined samples well. Additionally, the adsorption of PAA on the carbon nanotubes can be divided into two stages which is monitored by the change in the dSF. The value of dSF increases a little then decreases sharply as the adsorption amount of PAA increases. When the adsorption amount is high enough, a new type of micropore at about 1.4 nm is found associated with the formation of the new surface of PAA.
Acknowledgements This work was supported by the National Science Foundation of China under grant no. 40003002. The authors
399
especially wish to thank Prof. Allan Matthews and anonymous reviewers for valuable comments to make improvements in this paper.
References [1] B.B. Mandelbrot, Les Objets Fractals: Forme, Hasard et Dimension, Flammarion, Paris, 1975. [2] A. Venkatraman, L.T. Fan, W.P. Walawender, J. Colloid Interface Sci. 182 (1996) 578. [3] F. Meng, J.R. Schlup, L.T. Fan, J. Colloid Interface Sci. 197 (1998) 88. [4] J.F. Lee, C.K. Lee, L.C. Juang, J. Colloid Interface Sci. 217 (1999) 172. [5] D. Avnir, M. Jaroniec, Langmuir 14 (1990) 221. [6] D. Avnir, M. Jaroniec, Langmuir 10 (1994) 1532. [7] A.V. Neimark, Physica A 191 (1992) 258. [8] A.V. Neimark, M. Hanson, K.K. Unger, J. Phys. Chem. 97 (1993) 601. [9] A.V. Neimark, K.K. Unger, J. Colloid Interface Sci. 150 (1993) 412. [10] S. Iijima, Nature 354 (1991) 56. [11] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [12] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chappelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Nature 888 (1997) 756. [13] E.G. Gamaly, T.W. Ebbesen, Phys. Rev. B 52 (1995) 2083. [14] J.C. Charlier, X. Gonze, J.P. Michenaud, Europhys. Lett. 29 (1995) 43. [15] Y. Chen, L.P. Guo, S. Patel, Y. Ye, D.T. Shaw, Appl. Phys. Lett. 73 (1998) 2119. [16] K. Kaneko, C. Ishii, Colloid Surf. 67 (1992) 203. [17] M. Jaroniec, K. Kaneko, Langmuir 13 (1997) 6589. [18] Z.M. Wang, K. Kaneko, J. Phys. Chem. 99 (1995) 16714. [19] T. Kyoutani, L. Tsai, A. Tomita, Chem. Mater. 7 (1995) 1427. [20] K. Morishige, H. Fujii, M. Uga, D. Kinukawa, Langmuir 13 (1997) 3494. [21] N.A. Seaton, J.P.R.B. Walton, N. Quike, Carbon 27 (1991) 853. [22] R.C. Haddon, G.E. Scuseria, R.E. Smalley, Chem. Phys. Lett. 38 (1997) 272. [23] E.G. Gamaly, T.W. Ebbesen, Phys. Rev. B 52 (1995) 2083. [24] X.Z. Wang, Z. Hu, Q. Wu, Y. Chen, Chin. Phys. 10 (2001) 76. [25] M. Kruk, M. Jaroniec, Microporous Mesoporous Mater. 44 (2001) 725. [26] A. Bose, R.K. Gilpin, M. Jaroniec, J. Colloid Interface Sci. 240 (2001) 224. [27] S. Brunauer, The Adsorption of Gases and Vapors, Physical Adsorption, vol. 1, Princeton Univ. Press, Princeton, NJ, 1943. [28] S. Inoue, N. Ichikuni, T. Suzuki, T. Uematsu, K. Kaneko, J. Phys. Chem. B 102 (1998) 4690.
The surface fractal investigation on carbon nanotubes modified by the adsorption of poly(acrylic acid) Hou Qing-Feng a, Lu Xian-Cai b, Liu Xian-Dong b, Hu Bai-Xing a, Cui Ju-Qing a, Shen Jian a,c,* a
Research Center of Surface and Interface Chemical Engineering Technology, 22 Hankou Road, Nanjing University, Nanjing 210093, PR China b The State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China c Department of Applied Chemistry, Nanjing Normal University, Nanjing 210024, PR China Received 9 September 2003; accepted 18 March 2004 Available online 10 May 2004
Abstract In this paper, surface fractal analysis is carried out to study the surface of carbon nanotubes after the adsorption of poly(acrylic acid) (PAA) using the thermodynamic method. The fractal dimension (dSF ) of a fractal surface, BET surface area (SBET) and pore size distribution (PSD) are calculated from low-temperature nitrogen adsorption isotherms. The value of dSF declines as the adsorption amount of PAA increases, which means the adsorption may lower the surface roughness of the carbon nanotubes. The PSD pattern was modified obviously after the adsorption of PAA because of the pore-blocking effect. Additionally, an excellent linear decrease in the BET surface area of carbon nanotubes is found as the adsorption amount of PAA increases, which can be attributed to both the pore-blocking effect and the surfacescreening effect. The results of the present work may facilitate our understanding of the interaction between polymer and the carbon nanotubes at the microscale occurring on irregular interfaces. D 2004 Elsevier B.V. All rights reserved. Keywords: Fractal dimension of surface; Adsorption; Carbon nanotubes; Surface area; Pore size distribution
1. Introduction Since the early work of Mandelbrot [1] explored the replication of structure on an increasingly finer scale, i.e., the quality of self-similarity, much attention has been paid to the application of fractal analysis to surface science in recent years. Some valuable information of fractal geometry can be deduced from nitrogen adsorption isotherms of the examined solid surface, the fractal geometry of which is a key parameter affecting the interactions between adsorbed molecules and surface. A scale-independent parameter named value of fractal dimension (dSF) is proposed to quantify the degree of surface irregularity. Usually, the value of dSF lies between 2 and 3. A regular and smooth surface possesses dSF = 2, and a higher dSF value suggests a more irregular and space-filling surface. Therefore, dSF can be considered as an operative measurement of the surface roughness. The easiest * Corresponding author. Research Center of Surface and Interface Chemical Engineering Technology, 22 Hankou Road, Nanjing University, Nanjing 210093, PR China. Tel.: +86-2583-594933; fax: +86-2583594404. E-mail address: [email protected] (J. Shen). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.016
method to calculate dSF is to fit a single nitrogen adsorption isotherm to a fractal isotherm equation having dSF as a parameter. Presently, there are two popular methods for calculating dSF from a single nitrogen adsorption isotherm. One is the FHH method which evaluates dSF through the slope of a log –log plot of N/Nm vs. ln[( P/P0)] 1 from a single nitrogen adsorption isotherm in the range of capillary condensation [2 –6]. Here Nm can be computed from the BET equation. The other one is the so-called thermodynamic method proposed by Neimark et al. in 1992 [7 –9]. The value obtained using this method is independent to the structure model of the porous network or the type of adsorption process. Instead, it is based on an integral thermodynamic relationship between the area of the liquid –vapor interface and the amount adsorbed at a given relative pressure. However, this method is valid only for describing the capillary condensation mechanism because it completely ignores surface forces. The thermodynamic method concludes a very simple relationship between the surface area of the adsorbed liquid film, S, and the mean radius of the curvature of this adsorbed film, r, lnS ¼ const ðdSF 2Þlnr
ð1Þ
Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399 Table 1 Parameters of all the experimental samples Sample
Adsorption amount of PAA (mg/g)
SBET (m2/g)
dSF
rmin (nm)
rmax (nm)
CNT-0 CNT-1 CNT-2 CNT-3 CNT-4 CNT-5 CNT-6
0 13.09 43.35 89.60 118.28 146.53 268.98
200.33 F 6 198.43 F 3 189.97 F 4 171.41 F 1 156.88 F 2 150.04 F 1 112.50 F 1
2.56 2.56 2.56 2.57 2.58 2.53 2.51
0.35 0.35 0.35 0.38 0.38 0.38 0.38
40.56 16.90 17.28 17.64 17.36 17.36 17.50
395
nomena occurring at surface and interface of carbon nanotubes such as adsorption, surface modification and coating by some polymer. In this paper, thermodynamic method is used to study the changes in surface fractal dimension of carbon nanotubes due to the adsorption of poly(acrylic acid) (PAA). Six treated carbon nanotube samples with different adsorption amount of PAA and one pure sample (CNT-0) as a reference are analyzed. Low-temperature ( 196 jC) nitrogen adsorption measurements is used to calculate the surface parameters of carbon nanotubes including dSF, BET surface area (SBET) and pore size distribution (PSD).
In this equation, the surface area of the film is calculated from the Kiselev equation S ¼ ðRT =cÞ
Z
2. Experimental
Nmax
lnðP0 =PÞdN
ð2Þ
NðP=P0 Þ
where Nmax denotes the amount adsorbed when P = P0 tends toward unity, and c is the surface tension of the liquid adsorbate. Meanwhile, the Kelvin equation (Eq. (3)) is used to convert the equilibrium pressure P to the mean radius of the curvature r. ln
P 2cvL ¼ P0 rRT
ð3Þ
where vL denotes molar volume of the adsorbate in the condensed state. Presently, surface fractal analysis is often employed to survey the detail surface information of porous particles. For example, Venkatraman et al. [2] have studied the influence of the temperature of calcination on the surface fractal dimension of two types of mesoporous absorbents. Meng et al. [3] has carried a comparative study of surface fractality between polymeric and particulate titania aerogels. Lee et al. [4] practised surface fractal analysis to investigate the surface-screening effect and pore-blocking effect of Camontmorillonites after the exchange with both metal cations and organic cations. Those studies prove that surface fractal analysis can be helpful to explain some interface – surface properties of the porous particles at the microscale. Carbon nanotube has become another very important topic in material science and technology since Iijima reported it first in 1991 [10]. The crystalline porous structure and large surface area of the carbon nanotube make it an ideal adsorbent for polymer. Usually, the adsorption of polymer on the carbon nanotubes can be surveyed by some traditional techniques such as SEM, STM and Raman spectra. However, more valuable information on the modification of surface properties such as surface irregularities and defects can be monitored by the fractal analysis. Plenty of investigations have been carried out on the synthesis, purification, structure characterization and application of the carbon nanoparticles [11 – 23], few of which are related to the surface fractal analysis. Hence, the surface fractal analysis maybe a feasible approach to explain some phe-
2.1. Materials The solid adsorbent is the lab-synthesized carbon nanotubes by catalytic decomposition of benzene vapor [24]. About 20 mg Fe –Co/g-Al2O3 as catalyst was placed in a quartz tube with 10 mm inner diameter inside a large quartz tube with 35 mm inner diameter and 550 mm length, mounted in a tube furnace with 300 mm length. The catalyst was preheated to a certain reaction temperature between 560 and 810 jC under nitrogen atmosphere at a flow rate of 70 ml/min. Subsequently, benzene vapor was led into the system by means of the nitrogen flow passing through a triplet benzene saturator at 25 jC. The nitrogen and benzene vapor flow lasted for 60 min for growing carbon nanotubes. The furnace was then cooled down to ambient temperature in nitrogen flow. The carbon nanotubes are washed in hydrochloric acid at 140 jC for 5 h then washed by distilled water. The diameters of carbon nanotubes range from 10 to 30 nm. Poly(acrylic acid) is obtained by polymerization of
Fig. 1. The diagram of BET surface area of the carbon nanotubes vs. the adsorption amount of PAA.
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Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399
monomer in aqueous solution at 70 jC using about 30% (NH4)2S2O8 as initiator. Number-average molar mass (Mn) and polydispersity index (Mw/Mn) for the PAA samples are 7.073 104 and 1.283, respectively, which are measured on the miniDAWN multi-angle laser scatter analyzer (Wyatt, CA) using the ASTRA software 4.90.07.
thence. The cake precipitate is dried at 40 jC in vacuum for 24 h to remove moisture and then used as the sample for further analysis. The adsorption amount of PAA is calculated from the concentration difference of the solution before and after the adsorption measured by the titration. The related parameters of the treated carbon nanotubes are listed in Table 1.
2.2. PAA adsorption experiment 2.3. Techniques of measurements Six shares of 2.500 g carbon nanotubes are dispersed respectively in six 100-ml PAA solution with different concentration in small vessels which could be firmly closed with suitable caps. The suspension is stirred in water bath at 25 jC for 48 h to the adsorption equilibrium and filtered
Low-temperature ( 196 jC) nitrogen adsorption – desorption isotherms are measured on an ASAP2010 Accelerated Surface Area and Porisimetry Adsorption Analyzer (Micromeritics). Prior to nitrogen adsorption, the samples
Fig. 2. The nitrogen adsorption – desorption isotherms of the experimental samples.
Q.-F. Hou et al. / Surface & Coatings Technology 190 (2005) 394–399
are degassed to the pressure about 0.68 Pa at 40 jC for about 24 h. Adsorption – desorption of nitrogen is carried out in the relative pressure range of 0.001– 0.995 using about 1.000 g of experimental samples. In this paper, SBET is determined from nitrogen adsorption isotherms in the relative pressure range of 0.05 –0.20 based on the traditional method. Harkins and Jura (HJ) cylinder pore model is employed to calculate PSD according to the classical Kelvin equation from the nitrogen adsorption isotherm.
3. Results and discussion 3.1. BET surface area The original carbon nanotubes have the largest SBET as 200.33 m2/g which decreases gradually to 112.50 m2/g,
397
while the adsorption amount of PAA increases from 0 to 268.98 mg/g. The decline is proportional to the adsorption amount of PAA as plotted in Fig. 1. Similar decrease in SBET of porous particles after the adsorption of some organic substance has also been reported in other literatures [4,25,26]. 3.2. Pore size distribution The nitrogen adsorption – desorption isotherms of the examined samples are shown in Fig. 2 in the linear scales. All of the isotherms have similar shape which belongs to type IV according to the classification proposed by Brunauer [27]. These isotherms exhibit remarkable hysteresis loops which reflect a rather broad distribution of mesopores in these samples. As demonstrated in Fig. 3, the adsorption of PAA influences the PSD of the samples distinctly. Two
Fig. 3. The diagram of PSD of the experimental samples.
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Fig. 4. The plots drawn with the standard coordinates of ln S vs. ln r. The value of dSF can be calculated from the slope in the linear range.
types of mesopores should be identified clearly in the pure carbon nanotubes (CNT-0). One narrow peak located at 3 nm can be identified as the intraparticle pores, a typical cylinder-shaped mesopores of the carbon nanotubes [28]. The other broad multi-peaks between 7 –100 nm can be identified as the interparticle pores due to the strong aggregation among the carbon nanotubes [28]. The intraparticle pores at 3 nm reduce gradually and disappear eventually as the adsorption amount of PAA increases which indicates those original intraparticle pores are blocked up by PAA. Remarkable change is also found in the interparticle pores. The interparticle pores reduces as a whole, and those pores wider than 100 nm in the pure carbon nanotubes disappear entirely after the adsorption of PAA, which can be attributed to two possible reasons. PAA could fill these interparticle pores. Moreover, PAA has made the aggregation among the carbon nanotubes more closely and rigidly. It should be pointed out that a new dominant pores at about 1.4 nm is found in CNT-4, CNT-5 and CNT6, which indicates a new type of micropore is formed when the adsorbed PAA solidified at the surface of carbon nanotubes. As the assembly of PAA on the surface of carbon nanotubes is difficult to observe directly, the mechanism of origination of these micropores is not clear yet. It is suspected that the width of these micropores is related to the size of molecular PAA. The reduction in the surface area of both intraparticle and interparticle pores of the carbon nanotubes proves that the pore-blocking effect of PAA is one predominant reason for the total decrease of SBET of the examined samples.
face area vs. the radius of curvature plots of all nitrogen isotherms is given in Fig. 4 in double-logarithmic scales according to Eq. (1). The value of dSF can be calculated from the slope of those fitting lines. The range of r where linearity present is also listed in Table 1. The lower limit for r of all the examined samples is approximately 0.35 nm, corresponding to the limit of applicability of the Kelvin equation. Nevertheless, The upper limit r of the six treated sample is about 17.36 nm which is much smaller than that of the original carbon nanotubes (40.56 nm). The value of dSF of the original carbon nanotubes is 2.56. In contrast to the linear decrease of SBET, dSF does not decrease proportional to the adsorption amount of PAA. As shown in Fig. 5, the change of dSF after the adsorption of PAA can be divided into two stages, and one transition at CNT-4 can be observed clearly. The value of dSF increases firstly (CNT-0 to CNT-4) then decreases sharply (CNT-4 to CNT-6) as the adsorption amount of PAA increases. No change is found in dSF among CNT-0, CNT-1 and CNT-2, which means the surface roughness of these samples is almost at the same level. Accordingly, the process of the adsorption of PAA could be described as below. In the beginning, the adsorption amount of PAA is too low to change the original surface of the carbon nanotubes obviously, which results in the invariability of dSF in CNT-0, CNT-1 and CNT-2. As the adsorption amount of PAA increases continuously such as CNT-3 and CNT-4, more and more conglomeration of PAA become present at the original surface of the carbon nanotubes which may enhance the roughness of the surface as proved by the increase of dSF. When the adsorption amount of PAA is high enough to screen the whole original surface, a new relatively slippery surface is formed as proved by the sharp decrease of dSF. The surface-screening effect of PAA results in a change in dSF of the samples discussed above. Meanwhile, the pores of the carbon nanotubes are also blocked up by PAA
3.3. Surface fractal analysis The value of dSF is calculated from the nitrogen isotherms according to the thermodynamic method. The inter-
Fig. 5. The diagram of dSF vs. the adsorption amount of PAA of the experimental samples.
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gradually, which can be proved by the changes in the PSD pattern of the treated samples. Since the surface area of a smooth surface is lower than that of a rough surface, the surface-screening effect of PAA should partially result in the decrease of SBET in some examined samples such as CNT-5 and CNT-6, which have smoother surfaces than the untreated sample CNT-0. On the other hand, based on the surface fractal analysis, CNT-3 and CNT-4 have more irregular and space-filling surfaces, which should have a bigger SBET than that of other samples. In fact, the SBET decreases proportionally to the adsorption amount of PAA as shown in Fig. 1, which implies the reduction of the pores surface, i.e., the pore-blocking effect has the dominating contribution to change in SBET of the carbon nanotubes. Hence, both pore-blocking effect and surface-screening effect can result in a linear decrease in SBET of the carbon nanotubes. However, the pore-blocking effect should be the main cause for the decrease in SBET of the examined samples.
4. Conclusions Surface fractal analysis is a valid method to monitor the surface properties of carbon nanotubes after the adsorption of PAA. The adsorption of PAA greatly influences the surface properties of original carbon nanotubes. SBET of the carbon nanotubes decreases after the adsorption of PAA, which can be attributed to two possible reasons: the poreblocking effect and the surface-screening effect. Integrating the analysis on dSF and PSD, the pore-blocking effect, i.e., the reduction of the pore surface should be the main cause for the decrease in SBET. The surface roughness is well characterized by the dSF and consists of the change in both SBET and PSD of the examined samples well. Additionally, the adsorption of PAA on the carbon nanotubes can be divided into two stages which is monitored by the change in the dSF. The value of dSF increases a little then decreases sharply as the adsorption amount of PAA increases. When the adsorption amount is high enough, a new type of micropore at about 1.4 nm is found associated with the formation of the new surface of PAA.
Acknowledgements This work was supported by the National Science Foundation of China under grant no. 40003002. The authors
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especially wish to thank Prof. Allan Matthews and anonymous reviewers for valuable comments to make improvements in this paper.
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