J. Phys. Chem. C 2008, 112, 12943–12950
12943
Aggregation of Synthetic Chrysotile Nanotubes in the Bulk and in Solution Probed by Nitrogen Adsorption and Viscosity Measurements B. G. Olson,† J. J. Decker,† S. Nazarenko,*,† V. E. Yudin,‡ J. U. Otaigbe,† E. N. Korytkova,§ and V. V. Gusarov§ School of Polymers and High Performance Materials, the UniVersity of Southern Mississippi, Hattiesburg, Mississippi 39406-0001 and Institutes of Macromolecular Compounds and of Silicate Chemistry, Russian Academy of Science, St. Petersburg, Russia ReceiVed: February 20, 2008; ReVised Manuscript ReceiVed: April 28, 2008
Chrysotile nanotubes (ChNTs) were synthesized under hydrothermal conditions. The shape and size of individual ChNTs were examined by transmission electron microscopy (TEM). Specific surface area (SSA) of nanotubes surface treated with a silane coupling agent and of pristine nanotubes in the bulk was determined by BET analysis of N2 adsorption at 77 K. The theoretical SSA of a single nanotube and of nanotubes organized in a bundle was calculated as a function of nanotube geometrical parameters and the bundle size, that is, number of nanotubes in the bundle. A comparison of experimental and theoretically calculated SSA values indicated that nanotubes form bundles in the bulk. The characteristic bundle size in the bulk was estimated. The tendency of ChNTs to form bundles was also investigated in polar (ethanol) and nonpolar (xylene) solvents by measuring the complex viscosity behavior of the corresponding colloidal solutions. Viscosity measurements showed that nanotubes form bundles and that they are larger, consisting of more nanotubes, in nonpolar xylene than in polar ethanol. The tendency of ChNTs to aggregate in the bulk and in the solution was reduced by surface treatment of nanotubes with a silane coupling agent. I. Introduction One of the most commonly identified and broadly used structures in the area of nanotechnology is the nanotube (NT). Carbon nanotubes (CNTs) are frequently reported due to their unique mechanical, thermal, and electronic properties.1–3 Consisting of a nanoscale outer diameter, and in many instances a hollow channel like interior, CNTs are characterized by a fairly large specific surface area (SSA). This opens an opportunity of using CNTs also for gas storage applications, in particular to store hydrogen, which is important for fuel-cell applications.4,5 Specific surface area (SSA), defined as the ratio of surface area to mass, of powders or porous materials is routinely determined by measuring the amount of gas adsorption, typically N2 at 77 K, on their surface. By applying the Brunauer-EmmettTeller (BET) method6 to the resulting isotherms, the specific surface area can then be determined. Predictably, there has been a fairly large number of N2 adsorption studies reported for various CNTs, with the reported SSA varying between 300-1300 m2/g.7–9 To a certain extent, this wide range of SSA can be attributed to a variation of external and internal diameters and most importantly to the multiwalled nature of some of these CNTs. The number of walls does not lead to a strong increase of the surface area accessible for N2 adsorption but does considerably increase the weight of the CNT, thus leading to a smaller SSA. Furthermore, the nanotubes can aggregate due to secondary forces forming tight bundles in which the interstitial regions are not always accessible for gas adsorption. In this case, the SSA of a bundle is smaller than a sum of individual * Corresponding author. E-mail:
[email protected]; phone: (601)266-5967. † University of Southern Mississippi, Hattiesburg. ‡ Institute of Macromolecular Compounds. § Institute of Silicate Chemistry.
contributions. Recently, Peigney et al. developed a model that allowed calculating SSA for bundles consisting of a different number of individual nanotubes.10 Therefore, the information on experimentally determined SSA can potentially be used to infer the morphology of bulk nanotube samples. The analysis presented by Peigney et al. is limited to very long nanotubes, which is the common case for most commonly studied CNTs. Subsequently, the nanotube end surfaces were not considered as their contribution is small as compared to the overall lateral surface. The end surfaces, however, would be a contributing factor in the case of a bundle consisting of short nanotubes. Compared to carbon nanotubes, synthetic inorganic nanotubes exhibiting a hollow channel-like interior, similar to CNTs, are less well-known and little investigated. Recently, some interest has been expressed toward synthesis, structural characterization, and application of chrysotile nanotubes (ChNTs), which crystallographically and morphologically mimic natural chrysotile, also known as white asbestos.11–13 Asbestos is a typical hydrated magnesium silicate (chemical formula Mg3Si2O5(OH)4), a compound which belongs to the serpentine group of minerals. Morphologically, asbestos exists in the form of long hollow fibers with typical outer and inner diameters of 20-40nm and 2-8nm respectively, whereas an individual fiber length can be 1 cm or longer. The fibers are arranged in close-packed hexagonal arrays (bundles). Asbestos is important for its thermal and chemical resistance, flameretardant and thermal insulation properties.14,15 Interestingly, BET analysis using nitrogen adsorption measurements revealed that SSA for naturally occurring asbestos generally range from 10 to 20 m2/g, which is considerably smaller than that of CNTs.15 In contrast to natural asbestos, ChNTs are considerably shorter, and still retain a tendency to aggregate by forming bundles.13 In contrast to natural asbestos, ChNTs can be more
10.1021/jp801522q CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
12944 J. Phys. Chem. C, Vol. 112, No. 33, 2008
Olson et al.
readily dispersed in polymeric media. In our recent study we used ChNTs in combination with certain polyimides to prepare nanocomposite structures with enhanced mechanical and gas barrier properties, as well as reduced dielectric constant.16 These nanocomposites potentially represent a new material platform for development of advanced protective coatings that may be used for electronic applications. The main intent of this work was to investigate nitrogen adsorption characteristics of synthetic chrysotile nanotubes. Gas adsorption data for this nanotube system have never been previously reported. In this article we aimed to (a) resolve a question about the availability for gas adsorption of the nanotube inner channels; (b) determine the SSA of ChNTs, using BET analysis, and understand the extent of nanotube aggregation and bundle formation in the bulk; (c) explore the tendency of ChNTs to aggregate and form bundles in different solvents by conducting the viscosity measurements of the corresponding colloidal solutions; and (d) study if silane surface treatment reduces the aggregation of ChNTs in the bulk and in the solution. II. Experiments and Methodology Sample Preparation. Chrysotile nanotubes were synthesized in a high pressure autoclave from a mixture of magnesium and silicon oxides under hydrothermal conditions.12 The processing conditions were as follows: molar ratio between MgO and SiO2 in initial mixture was equal to 1.5, which corresponds to the stoichiometric ratio of these compounds; temperature and pressure were 350 °C and 70 MPa; NaOH content in hydrothermal solution was 1 wt %; overall reaction duration was 24 h. Pristine ChNTs were also functionalized with m-aminophenyltrimethoxysilane (Gelest. Inc.) coupling agent according to procedure described elsewhere.17,25 To obtain uniform monolayer coverage the amount of silane was calculated based on the specific wetting surface of silane, ∼350 m2/g, and on the SSA of a single ChNT, ∼100 m2/g. One gram of ChNTs was dispersed in 50 mL of ethanol and sonicated with a Fisher Ultrasonic Bath for 1 h. Then, 0.3 mL of silane was added to the suspension followed by sonication for an additional 10 min. Finally, the suspension was centrifuged, which led to precipitation of ChNTs on the bottom of the tube. The functionalized ChNTs were briefly rinsed twice with ethanol. Drying from ethanol and curing of the silane layer was conducted for 5 h at 60 °C under vacuum. ChNTs were dispersed in ethanol and xylene. Dispersions containing from 1 to 10% (vol/vol) of nanotubes in solution were prepared and sonicated for 1 h using a Fisher Ultrasonic Bath (frequency 40 kHz, average sonic power 45 W). Note that all the solutions (dispersions) remained fairly stable for a long period of time (g 1 week). The Shape, Size and Organization of ChNTs Examined by Transmission Electron Microscopy. Prior to the measurements, ChNTs were dispersed in ethanol (USP grade) at 0.05% (wt/wt) concentration and sonicated for 2 h in an ultrasonic bath. A small droplet was placed on each transmission electron microscopy (TEM) grid and dried in air. After being dried, the grids were inspected with a JEM-2100 LaB6 TEM. Bright field images were analyzed using the Digimizer V5 image analysis software. Determination of Specific Surface Area Using ASAP 2010. Nitrogen adsorption-desorption measurements at 77 K were carried out using a standard static volumetric technique (ASAP 2010, Micromeritics Instrument). ChNT bulk samples weighing approximately 0.3 g were initially pretreated overnight in vacuum at 200 °C to remove water. The ChNT modified with
Figure 1. TEM micrograph of nanotubes at (a) lower magnification and (b) higher magnification.
silane was pretreated in vacuum at 100 °C for 3 h. Pretreatment at higher temperatures or for longer times risked removal of the silane from the ChNT surface. The mass of the outgassed sample was then measured and recorded. The adsorption and desorption isotherms were measured over a range of relative pressures P/P°, where P° is the saturated vapor pressure, from 10-5 to 1.0, for all samples. Samples were measured multiple times to ensure reproducibility, which results in at most a 2% uncertainty in determining the specific surface area. A strain-controlled dynamic rheometer ARES from TA Instruments was used to measure the complex shear viscosity, |η*| ) (η′2 + η′′2), of colloidal suspensions of ChNTs in different solvents at 25 °C using a cone and plate configuration. The diameter of the plate was 25 mm and the cone angle was 0.1 rad. Measurements were conducted using an angular frequency (ω) of 1 rad/s and a strain amplitude (γo) of 1%. Special precautionary measures were undertaken to ensure that solvent evaporation did not occur during the experiment. III. Results and Discussion TEM Analysis of Nanotubes. Figure 1 shows the characteristic TEM micrograph, at low and high magnification, of ChNTs prepared via dispersion of nanotubes in ethanol at low concentration followed by solvent evaporation in air. The images revealed the presence of single nanotubes as well as nanotube bundles. Cylinder-in-cylinder morphology, the defect form of chrysotile tubular structure, can also be occasionally observed
Specific Surface Area of Chrysotile Nanotubes
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12945
Figure 3. Nitrogen isotherms for nanotube bulk (powder) samples measured at 77 K. (a) Pristine nanotubes; (b) nanotubes modified with silane coupling agent; open symbols denote adsorption and closed symbols are desorption.
Figure 2. Statistical analysis of nanotube dimensions: (a) length (L); (b) outer diameter (D); (c) aspect ratio (L/D).
in the micrographs. As expected, all the nanotubes appeared lighter (lower electron density) in the nanotube central region, indicative of the inner channel. The length (L), outer diameter (D), inner diameter (d), and aspect ratio (L/D) of at least 500 nanotubes were determined using image analysis and multiple TEM micrographs. The statistical analysis allowed the construction of histograms for each of these characteristics. The histograms of length, outer diameter, and aspect ratio are shown in Figure 2. The nanotube length varied from about 10 to 600 nm, the outer diameter from about 9 to 22 nm, and the aspect ratio varied from about 1 to 45. The average nanotube length was found to be 85.2 nm, average outer diameter was 14.3 nm, and average aspect ratio was 6. The average inner diameter of the channel was 3 nm. The average outer and inner diameter of ChNTs were found to be about twice as small, and the length several times shorter, than the values reported in the literature.12,13 The possibility that the ultrasonic treatment in the solution led to brittle failure of nanotubes, chopping them up into shorter sections, was ruled out by comparing the nanotube average length with and without ultrasonic treatment. No measurable difference in the nanotube average length was found. Therefore,
the shorter than expected nanotube average length as well as somewhat smaller outer and inner diameter as compared with those reported in the literature,12 for which the same source of ChNTs was used, perhaps was due to some variability of nanotube dimensions from batch to batch. Nitrogen Adsorption Analysis of Nanotubes in the Bulk. Figure 3 shows typical nitrogen adsorption-desorption isotherms conducted at 77 K for pristine and surface modified with silane ChNT bulk (powder) samples. According to the IUPAC classification,18,19 all measured isotherms were of type II, commonly obtained from a nonporous or macroporous adsorbent with no hysteresis observed in adsorption/desorption cycle. This implies that at least the inner channel of the ChNT was inaccessible for nitrogen adsorption. The nature of the inner channels clogging agent is not exactly clear, however one possibility is water, because chrysotile is highly hydrophilic. The type II isotherm, representing unrestricted monolayermultilayer adsorption, allowed for the application of the Brunauer-Emmett-Teller (BET) method6 to calculate the monolayer capacity, and hence the SSA, of the adsorbent. The BET equation, derived for multilayer adsorption of an adsorbent,6,20 is given by eq 1,
p 1 (C + 1) p ) a + a Va(p° - p) Vm C VmC p°
(1)
where Va is the quantity of molecules adsorbed, in cm3 [STP]/ g, and Vam is the monolayer capacity of the adsorbent. The parameter C, according to the BET theory, is exponentially related to the enthalpy of adsorption in the first layer. In practice, however, the relationship between the enthalpy of adsorption and the parameter C is not as straightforward as the BET theory
12946 J. Phys. Chem. C, Vol. 112, No. 33, 2008
Olson et al. TABLE 2: Specific Surface Area As Calculated from the TEM Analysis (SSATEM), and BET analysis of the nitrogen adsorption isotherm (SSAN2) SSATEM (m2/g)
Figure 4. The BET plots for nitrogen adsorbed at 77 K on pristine ChNTs (open symbols) and ChNTs modified with silane coupling agent (closed symbols).
TABLE 1: BET Parameters for Nitrogen Adsorption on Pristine and Surface Modified Synthetic Chrysotyle Nanotube sample
a Vm (cm3 [STP]/g)
C
pm/p°
ChNT ChNT/silane
13.92 17.86
192 67
0.07 0.11
1 s+i s C) +1 i
(2) (3)
where s is the slope and i is the intercept of the best fit line. The BET plots for ChNTs with and without silane surface modification are shown in Figure 4. The linear range was found to include the relative pressures from 0.01 to 0.35 for all samples. The resulting BET parameters are shown in Table 1, along with the value of the relative pressure when one monolayer is complete, pm/p°. The SSA of a substance covered by a monolayer of adsorbed gas can be determined from eq 4,
SSA ) NAamnm
(4)
where am is the area occupied by an adsorbate molecule in the monolayer, nm is the number of moles of adsorbate in the monolayer per gram of adsorbent, and NA is the Avogadro constant. For measurements of gas adsorption employing the static volumetric technique, the amount of gas adsorbed is more conveniently expressed as volume of gas, reduced to STP, per gram of adsorbent, Vam. Therefore, eq 4, after using the ideal gas law, becomes eq 5.
SSA ) NAam
a Vm P RT
sample
SSAN2 (m /g)
open
blocked
ChNT ChNT/silane
61 ( 1 78 ( 2
147
124
from eq 5 using BET analysis for pristine ChNTs and those surface modified with a silane coupling agent are listed in Table 2. The specific surface area of ChNTs was also estimated from average geometric dimensions of NT determined by TEM, SSATEM. Taking into account the inner and outer cylindrical surfaces, along with the ring shaped ends, the surface area of a hollow cylinder of inner diameter d, outer diameter D, and length L, can be calculated in the general case for a single nanotube (SNT) by eq 6.
[
ASNT ) πLD 1 +
D d d2 + 2L L 2LD
]
(6)
The mass of a ChNT with a density F can be determined from eq 7.
assumes.18 Therefore, the parameter C is used as an indication of the magnitude of the adsorbent-adsorbate interaction energy and is not converted to enthalpies of adsorption. A plot of [Va(p°/ p - 1)]-1 against p/p° should result in a straight line, where the monolayer capacity and parameter C can be found from eqs 2 and 3, a Vm )
2
(5)
The area occupied by a nitrogen molecule at 77 K is taken as am(N2) ) 0.162 × 10-18 m2, as calculated from the density of liquid nitrogen and assuming the same close packing in the monolayer as in the bulk liquid.6 The SSAN2, as determined
π MSNT ) (D2 - d2)LF 4
(7)
This simple geometrical analysis allows calculating ASNT assuming both a hollow d * 0 (open channel) and closed d ) 0 (blocked channel) cylinder. Naturally, the corresponding open and blocked cylinder specific surface areas SSASNT can be calculated as SSASNT ) ASNT/MSNT, where MSNT is constant and assumes open channel geometry. The specific surface areas with open and blocked channels are shown in Table 2, using the average nanotube geometric dimensions obtained from TEM, along with the density of chrysotile, 2.56 g/cm3, as determined from X-ray. The internal channel surface constitutes about 19% of the external surface of an average-sized ChNT. The calculated values of SSAN2 for both pristine (61 m2/g) and surface-modified (78 m2/g) ChNTs were somewhat larger than the values of SSAN2 reported for naturally occurring asbestos, which generally range from 10 to 20 m2/g,15 but were considerably smaller than the values of SSAN2 reported for CNTs (350-1300 m2/g).10 Using eqs 6 and 7 and the geometry of individual asbestos hollow fibers (see Introduction), SSA was estimated at 50-90 m2/g. Therefore, there is a significant discrepancy between calculated SSA for a fiber and SSA in the bulk as determined from nitrogen adsorption data, as was also previously noticed in the literature.15 The smaller than expected SSA of asbestos in the bulk, as measured from nitrogen adsorption data, is consistent with the morphology of this material as described above, that is, closely packed large arrays (bundles) of long fibers with inner channels and interstitial regions not accessible for N2 adsorption because these free spaces are completely or partially filled with solid material or small molecules.15 In fact, Pundsack studied the structure of blocks of asbestos using electron microscopy. He concluded that the aperture of interstitial spacing between the fibers in the close-packed bundles for asbestos was comparable with that of the inner-fiber channel.21 Similarly, the shape of nitrogen adsorption-desorption isotherms for ChNTs in the bulk implied that the inner-fiber channels as well as interstitial regions between nanotubes in the bundles were inaccessible for N2 adsorption. The SSA of
Specific Surface Area of Chrysotile Nanotubes
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12947 A relatively straightforward algorithm can be generated for the surface area as a function of complete layers in a bundle. The first layer of the bundle would be complete upon the introduction of a 7th nanotube, whereas the 19th nanotube would complete the second layer (see Figure 5b). The number of nanotubes in layer l is NSNT ) 6l, and the number of interstitial l SNT regions created by the addition of the lth layer is NIS l ) 2Nl - 6 ) 12l - 6. The total number of nanotubes in a bundle of l complete layers is then l
NSNT(l) ) 1 +
∑
l
NiSNT ) 1 + 6
i)1
∑i
(11)
i)1
and the total number of interstitial regions is Figure 5. Schematics indicating (a) interstitial region and (b) a nanotube bundle of three complete layers, consisting of 37 individual nanotubes. Solid circles indicate “corner” nanotubes and dashed circles indicate “internal” (see text).
ChNT bulk samples, as determined from BET analysis, were found to be smaller than the value obtained from geometric considerations, by about half, assuming either open or blocked channels. Naturally, this behavior was attributed to aggregation of nanotubes into bundles. In the next section we expanded the model proposed by Peigney et al. so the analysis can be applied for bundles consisting of short nanotubes. As Peigney et al. was concerned with carbon nanotubes of aspect ratios greater than 1000, the surface area of the ends of the CNTs could be ignored. For ChNTs studied here, with aspect ratios on the order of 10, the surface area of the tube ends can not be neglected. It was assumed that each bundle consisted of the same-sized individual nanotubes in a hexagonal packing arrangement, Figure 5, where inner channels and the interstitial regions were assumed inaccessible to N2 adsorbate. An algorithm was derived for the SSA as a function of the number of nanotubes in the bundle. Assuming the inner channel of the chrysotile nanotubes are blocked, the surface area of one end of the tube is ASNT End ) SNT π/4D2, and the area of the cylindrical surface is ACyl ) πDL. The surface area of a bundle of two nanotubes, including both ends, is simply SNT SNT AB2 ) 2(ACyl + 2AEnd )
1 SNT 1 D2 ) √3 - π AIS ) ATri - 3 AEnd 6 2 4
) (
)
(9)
For a bundle of 3-6 cylinders, the surface area of the end of IS the bundle will increase by ASNT End + A as each cylinder is added, and the outer cylindrical surface will increase by (1/2)ASNT Cyl . The surface area of N nanotubes will then be, including both ends,
1 SNT SNT ANB ) AB2 + (N - 2) ACyl + 2(AEnd + AIS) 2
[ ] π 1 2√3 ) [2 + (N - 2)]πDL + [(N - 2) + (N + 2)] D 2 π 4
2
where 3 e N e 6.
(10)
l
l
i)1
i)1
∑ NiIS ) -6 + 12∑ i
(12)
The surface area of the ends of a bundle of l complete layers, including both ends, is then B SNT SNT AEnd (l) ) 2(AEnd N (l) + AISNIS(l))
(13)
The surface area of the cylindrical surface of a bundle of l complete layers is determined by noting that the nanotubes on the “corner” of each layer, located at the vertices of the hexagons inscribed in Figure 5b, and the “internal” nanotubes along the edges of the inscribed hexagons, contribute different surface areas to the cylindrical surface of the bundle. The corner nanotubes would be, for example, nanotube 9, 11, 13, and 22 of Figure 5b, and the central nanotubes would be numbers 8, 10, 20, and 21, for example. The corner nanotubes contribute two thirds of their cylindrical surface area, and central nanotubes of a layer contribute one half of their cylindrical surface of the bundle surface area. There are always six corners per layer, however the number of central nanotubes increase with each layer. There are no central nanotubes in the first layer, but there are six in the second layer, and the number increases by six for each successive layer. The cylindrical surface area is therefore given by eq 14.
2 SNT 1 SNT B ACyl (l) ) 6 ACyl + 6(l - 1) ACyl 3 2
(
(8)
As one can see in Figure 5a, for a bundle of three nanotubes there is an interstitial region for which the surface area needs to be determined. An equilateral triangle with vertices at the center of each cylinder has sides of length D and an area of ATri ) 3(D2/4). The area of the triangle includes that of the interstitial region, and one sixth of the surface area of each cylinder end. The surface area of the interstitial region is therefore given by eq 9
(
NIS(l) )
)
(
)
(14)
The total specific surface area of a bundle containing l complete layers is then given by eq 15.
SSAB(l) )
B B ACyl + AEnd
NSNT(l)MSNT
(15)
Substituting eqs 11–14 into eq 15 gives eq 16,
SSAB(l) )
4D(3l + 1) l
+
(1 + 6 ∑ i)(D - d )F i)1
2
[
2
2π - 4√3 l
1+6∑i i)1
]
+ √3
4D2 (16) π(D2 - d2)LF
where the first and seconds terms are the cylindrical and end surface contributions to the total specific surface area of the bundle, respectively. In the limit of a bundle containing an infinite number of layers, the term in eq 16 associated with the cylindrical surface goes to zero, whereas the term related to the ends of the bundle reduces to eq 17.
12948 J. Phys. Chem. C, Vol. 112, No. 33, 2008
SSAB(l f ∞) )
4√3D2 π(D2 - d2)LF
Olson et al.
(17)
The specific surface area of ChNTs as a function of the number of complete layers in a bundle l is shown in Figure 6, using the average nanotube diameter and length as determined from TEM analysis. The predicted SSAB(l) dependence exhibited a somewhat hyperbolic shape with the maximum, 77 m2/g at l ) 1, decreasing rapidly at smaller l and then slowly approaching 10.6 m2/g, according to eq 17, as l goes to infinity. The experimental bulk value of SSAN2 (Table 2) for pristine ChNTs 61 ( 2 m2/g was found somewhat in between the calculated SSAB(1) ) 77 m2/g (7 nanotubes in the bundle) and SSAB(2) ) 53 m2/g (19 nanotubes in the bundle). The typical bundle size in the bulk is perhaps closer to 19 than to 7 nanotubes per bundle. After the silane surface treatment of ChNTs, experimental SSAN2 was reduced and approached a value very close to that corresponding to one complete layer (7 nanotubes) per bundle. Therefore, the degree of aggregation was reduced in the bulk after treating nanotubes with a silane coupling agent. Seeing that real chrysotile nanotubes displayed considerable variability of their dimensions (see Figure 2), the sensitivity of the model to the outer diameter D and the length L has been explored. Figure 7 shows the dependencies of SSAB as a function of D (L ) 85.2 nm and d ) 3nm) and L (D ) 14.3 nm and d ) 3 nm) for a single nanotube, and bundles consisting of 7 (l ) 1), 19 (l ) 2), 91 (l ) 5), and 1261 (l ) 20) nanotubes generated using equation 16. The range of lengths and diameters both lied well within the TEM measured distributions. The corresponding SSAB value decreased hyperbolically with D and L. Interestingly, the difference in SSAB(l) for bundles having a progressively larger number of complete layers decreases with the increase of D, whereas as a function of L it practically remains the same within the entire range of lengths. Because of the hyperbolic nature of SSA(D) and SSA(L), the contribution of thinner and shorter nanotubes to the specific surface area is more predominant than their thicker and longer counterparts, assuming the same size of bundles. Also, considering the practical range of observed ChNT dimensions (D ) 7-20 nm, L ) 20-200 nm) the effect of D on SSAB is somewhat stronger than L. In the conclusion of this analysis, the variability of nanotube dimensions may lead to a somewhat larger value of the specific surface area as compared to that calculated from the model and based on mean values of D and L. Another factor
Figure 6. The theoretical specific surface area of a bundle of ChNTs, as a function of the number of complete layers in the bundle. The solid line represents the SSA of a single nanotube. The dotted-dashed line is the measured bulk SSA for pristine ChNTs, and the dashed line is the bulk SSA measured for silanated ChNTs.
Figure 7. Theoretical specific surface area as a function of (a) outer diameter of a nanotube D (L ) 8.52 nm, d ) 3 nm) and (b) the length of a nanotube L (D ) 14.3 nm, d ) 3 nm) for (b) a single nanotube and bundles of (O) 1, (0) 2, (∆) 5, and ()) 20 complete layers.
that should be mentioned here with regards to nanotube size distribution is that, in comparison to the model, real bundles consist of nanotubes exhibiting different D and L values. This mismatch of diameters and lengths may lead to the existence of open surface patches and gaps between the nanotubes. All this may also contribute to the increase of experimentally observed specific surface area. The final question of this part, which the previous analysis can answer, is what factor is primarily responsible for the considerably lower SSA of natural asbestos in the bulk (10–20 m2/g) relative to bulk (61 m2/g) ChNTs? One of the major differences between asbestos and ChNTs is the length of nanofibers. Asbestos nanofibers are very long (∼1 cm) as compared to the fairly short synthetic nanotubes (∼100 nm). However, as Figure 7b indicates, the length is a relatively conservative parameter with regards to defining the SSA. The dependence SSA(L) practically levels out after 50-70 nm. The specific surface area, however, is strongly affected by the size of a bundle, that is, the number of complete layers l. Assuming nanofiber outer diameter D ) 14.3 nm and length 1 cm, with 5 complete layers (91 nanotubes) SSA equals 20.1 m2/g, and for 10 complete layers (331 nanotubes) SSA equals 10.7 m2/g. Therefore, the primary factor that explains the considerably lower SSA for natural asbestos in the bulk as compared to ChNTs is the size of the bundle. Rheological Behavior of ChNTs in Solutions. In addition to the surface analysis in the bulk, it was interesting to explore the tendency of chrysotile synthetic nanotubes to aggregate (form bundles) in solution. The existence of multiple hydroxyl groups on the surface of ChNTs help chrysotile nanotubes to aggregate due to hydrogen bonding interactions and/or Van der Waals forces. Therefore, it was interesting to investigate the
Specific Surface Area of Chrysotile Nanotubes
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12949 IV. Conclusions
Figure 8. Complex viscosity as a function of the chrysotile nanotube volume fraction in (O) ethanol, (∇) xylene, and (0) silane modified ChNTs in xylene.
dispersion of SNT in a polar solvent such as ethanol and a nonpolar solvent such as xylene. When the concentration of nanoparticles in a solution approach a critical value, the viscosity of the colloidal dispersion rises steeply, due to the formation of a percolation network that is characteristic of a fractal gel.25–27 Rheological measurements can be used to trace this behavior.22,23 Figure 8 shows complex viscosity |η*| as a function of the chrysotile nanotube volume fraction in ethanol and xylene. Qualitatively, all the dispersions exhibited a similar trend associated with gelation upon adding the particulates. When the volume fraction of particulates approach the onset of long-range connectivity (φ f pc), |η*| starts increasing exponentially and then slows down rapidly after the percolation threshold is reached (φ ) pc), and the solution undergoes gelation. Mean field theories are able to predict the threshold pc in many cases. In particular, when particulates exhibit a cylindrical shape, the prediction is as follows,24
pc ) 0.6 ⁄ r
(18)
where r ) L/D is the cylinder aspect ratio. Equation 18 is only a rough estimation, but it helps to foresee the relationship between the effective aspect ratio of the cylindrical particulates and the percolation threshold. Using the mean values of geometric parameters found from TEM analysis already discussed, the anticipated threshold pc ) 10% was estimated. Experimentally, |η*| for ChNT colloidal dispersions in both solvents leveled out at φ e 10%, that is, about 3% for the ethanol and ChNT dispersion and well above 7% for the xylene and ChNT dispersion. The question can be posed why the percolation threshold for the xylene and ChNT dispersion was noticeably larger than for the ethanol and ChNT dispersion. One plausible explanation of this behavior is that hydrophilic chrysotile nanotubes are much less thermodynamically compatible to nonpolar xylene than to polar ethanol; therefore, at equilibrium the bundles in the former case contain less nanotubes than in the later case. Subsequently, the more fibers in a bundle the smaller the aspect ratio (r) of the bundle and hence, according to eq 18, must result in a larger percolation threshold (pc). The surface treatment of ChNTs with a silane coupling agent disrupts attraction of the nanofibers so less fibers form a bundle, and the percolation threshold is shifted toward smaller ChNTs loadings, as shown in Figure 8. This behavior is in qualitative agreement with that observed for specific surface area of chrysotile nanotubes probed by N2 adsorption, suggesting that the surface treatment with a silane coupling agent can effectively reduce the tendency of ChNTs to form larger bundles.
Chrysotile nanotubes were synthesized under hydrothermal conditions. These nanotubes crystallographically and morphologically mimic white asbestos. However, ChNTs are significantly shorter than asbestos nanofibers. Nitrogen adsorption-desorption isotherms were conducted at 77 K on bulk (powder) samples of pristine ChNTs and ChNTs surface-modified with silane. All measured isotherms were of type II, with no hysterisis observed in the adsorption/desorption cycle. This implied that the inner channel of the ChNTs was inaccessible for nitrogen adsorption. BET analysis was applied to adsorption isotherms, and specific surface areas for pristine and surface modified ChNTs in the bulk have been determined. The SSA measured for both pristine (61m2/g) and surfacemodified (78 m2/g) ChNTs were smaller than calculated for a single nanotube (124 m2/g). Smaller SSA values for ChNTs in the bulk were attributed to the fact that nanotubes form bundles with interstitial regions inaccessible for nitrogen adsorption. A model was developed that allowed for calculating SSA as a function of nanotube geometrical parameters and the bundle size, that is, number of nanotubes in the bundle. The model took into account both the cylindrical surfaces and the ends of nanotubes. Experimentally determined and theoretically calculated SSA values were compared. A comparison indicated that nanotubes in the bulk form bundles consisting of about 19 nanotubes (2 complete layers). The size of characteristic bundles was reduced to about 7 layers per bundle (1 complete layer) after nanotubes were surface treated with a silane coupling agent. The tendency of ChNTs to form bundles was also investigated in polar (ethanol) and nonpolar (xylene) solvents by measuring the complex viscosity behavior of the corresponding colloidal solutions. Viscosity measurements revealed that nanotubes form bundles, especially in nonpolar xylene. As in the case of ChNTs in the bulk, the tendency of ChNTs to aggregate in the solution was weakened after surface treatment of nanotubes with silane. Acknowledgment. We acknowledge the U.S. National Science Foundation for financial support of this research under award Nos. CBET-0317646 and MRSEC Award DMR0213883. Financial support of this work by the Russian Foundation of Basic Research under contract grant No. 07-0300846-a is also gratefully acknowledged. References and Notes (1) Iijima, S. Nature 1991, 56, 354. (2) Subramoney, S. AdV. Mater. 1998, 10, 1157–1173. (3) Iqbal Z.; Goyal A., In Functional Fillers for Plastics: Carbon nanotubes/Nanofibers and Carbon Fibers; Xanthos, M., Ed.; Willey-VCH Verlag: Weinheim, 2005; p 176. (4) Dilon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (5) Ding, R. G.; Lu, G. Q.; Yan, Z. F.; Wilson, M. A. J. Nanosci. Nanotechnol. 2001, 1, 1–23. (6) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (7) Inoue, S.; Ichikuni, N.; Suzuki, T.; Uematsu, T.; Kaneko, K. J. Phys. Chem. 1998, 102, 4689. (8) Fujiwara, A.; Ishii, K.; Suematsu, H.; Kataura, H.; Maniwa, Y.; Suzuki, S.; Achiba, Y. Chem. Phys. Lett. 2001, 336, 205. (9) Estwaramoorthy, M.; Sen, R.; Rao, C. N. R. Chem. Phys. Lett. 1999, 304, 207–210. (10) Peigney, A.; Laurent, Ch.; Flahaut, E.; Bacsa, R. R.; Rousset, A. Carbon. 2001, 39, 507. (11) Yada, K.; Gishi, K. Am. Mineral. 1977, 62, 958–965. (12) Korytkova, E. N.; Maslov, A. V.; Pivovarova, L. N.; Drozdova, I. A.; Gusarov, V. V. Glass Phys. Chem. 2004, 30, 51–55. (13) Falini, G.; Foresti, E.; Lesci, G.; Roveri, N. Chem. Commun. 2002, 1512. (14) Wittaker, E. J. W. Acta Crystallogr. 1957, 10, 149–155.
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