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Atomic force microscopy of synthetic imogolite
Geoderma 118 (2004) 209 – 220 www.elsevier.com/locate/geoderma
Atomic force microscopy of synthetic imogolite M. Tani 1, C. Liu 2, P.M. Huang * Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A8 Received 4 June 2002; accepted 2 May 2003
Abstract The morphology of imogolite has been substantially studied using transmission electron microscopy (TEM) under ultra-high vacuum. However, the surface feature of imogolite at a nanometer scale under ambient conditions remains to be uncovered. The surface feature and the particle size of synthetic imogolite were investigated using contact- and tapping-mode atomic force microscopy (AFM) in the present study. The imogolite was synthesized at an initial Si concentration of 1.6 mmol l 1, a Si/Al molar ratio of 0.5, and an OH/Al molar ratio of 2.0. The X-ray diffractogram and infrared (IR) spectrum of the precipitates indicate that imogolite was the dominant reaction product. The in situ surface features of imogolite were revealed for the first time by using AFM under ambient conditions. The morphological features of synthetic imogolite were more clearly observed by using the tapping-mode AFM rather than the contact-mode AFM. Compared with the contact-mode AFM, the tapping-mode AFM required a shorter sample-drying period. Use of the contact-mode AFM tips, which have low aspect ratios, resulted in enlarging the observed diameter values of imogolite threads, which were subjected to the optimal ultrasonification (at 150 W for 5 s), by 21% apparently due to tip-sample convolution. The imogolite, which was subjected to the optimal ultrasonification (at 150 W for 5 s) that resulted in the least alteration of imogolite particles, appears in the tapping-mode AFM three-dimensional images as curved threads, varying in diameter from 40.2 to 95.5 nm (standard error < F 0.1 nm for each thread). The tappingmode AFM is a powerful and reliable technique to investigate the morphological features of imogolite and its related surface chemistry in soil environments. D 2003 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Contact-mode and tapping-mode; Imogolite; Surface features; Thread diameter
1. Introduction Imogolite is a naturally occurring hydrous aluminosilicate polymer with a unique tubular structure * Corresponding author. Tel.: +1-306-966-6838; fax: +1-306966-6881. E-mail address: [email protected] (P.M. Huang). 1 Current address: Department of Agro-Environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Inada, Obihiro, Hokkaido 080-8555, Japan. 2 Current address: Kuo Testing Labs, Inc., 337 S. 1st Avenue, Othello, WA 99344, USA. 0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0016-7061(03)00204-0
(Cradwick et al., 1972; Farmer et al., 1983; Wada, 1989). It exists in soils as a para-crystalline mineral and has been extensively isolated from the B horizons of Andisols (Parfitt and Henmi, 1980; Wada, 1989) and Spodosols (Brydon and Shimoda, 1972; Ross and Kodama, 1979; Farmer et al., 1980; Anderson et al., 1982; Gustafsson et al., 1995). Imogolite and protoimogolite allophanes can make up at least 6% of the B2 horizon soil (Farmer et al., 1980). The walls of an imogolite tube unit consist of curved gibbsite-like sheets with SiOH groups on the inside and AlOH groups on the outside. Because of its reactive surface,
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imogolite exerts significant effects on ion exchange properties, surface acidity, water adsorption, and other physicochemical properties of soils (Farmer et al., 1983; Wada, 1989). Many nutrients and pollutants such as heavy metal cations, phosphate, and arsenate can form chemical bonds with the outer surface of imogolite (Cradwick et al., 1972; Parfitt et al., 1974). Furthermore, imogolite has been proposed as a shaped-selective catalyst due to its tubular structure (Pohl et al., 1996). The structure, morphology, and surface properties of imogolite, therefore, deserve special attention. Transmission electron microscopy (TEM) investigations have shown that natural and synthetic imogolite particles mostly have thread-like structures (Yoshinaga and Aomine, 1962; Russell et al., 1969; Tait et al., 1978; Inoue and Huang, 1984; Koenderink et al., 1999). Under transmission electron microscope, imogolite appears as threads, varying in diameter from 10 to 30 nm and extending up to several micrometers in length (Yoshinaga and Aomine, 1962), and consists of assemblies of a tube unit with inner and outer diameters of 1.0 and 2.0 – 2.7 nm, respectively (Yoshinaga and Aomine, 1962; Yoshinaga et al., 1968; Wada, 1989; Koenderink et al., 1999). Tazaki (1979) also reported that imogolite threads formed from plagioclase grains in volcanic ash are less than 50 nm in diameter and about 0.3 Am in length under TEM. Although TEM and scanning electron microscopy (SEM) are useful tools to observe the surface morphology of mineral colloids, the high-vacuum condition under which the TEM and SEM are operated could result in the alteration of the surface morphology. Scanning probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), represents an ever-increasing class of microscopic techniques that provide three-dimensional images of solid surfaces at high resolution (Rugar and Hansma, 1990; Nagy and Blum, 1994; Bertsch and Hunter, 1998). The distinctive advantage of STM and AFM compared with SEM and TEM is that the samples do not have to be coated and subjected to ultra-high vacuum condition (Bertsch and Hunter, 1998). Therefore, SPM work under conditions in air and at room temperature is appropriate for environmental studies (Maurice, 1998). However, the surface
feature of imogolite has not been revealed by using AFM. In this study, we investigated the morphological features of synthesized imogolite under ambient conditions by using AFM and compared the surface features of imogolite observed by contact- and tapping-mode AFM.
2. Materials and methods 2.1. Preparation of imogolite The imogolite was prepared from a solution containing monomeric orthosilicic acid and AlCl3 using the method of Inoue and Huang (1984). Monomeric orthosilicic acid was obtained by passing Na-metasilicate solution through a H+-saturated Dowex 50WX8 cation exchange resin column (Alexander, 1953). To avoid polymerization of Si, the initial concentration of silicic acid was maintained below 2 mmol l 1 (Iler, 1979). The silicic acid was mixed with AlCl3 solution to give a Si/Al molar ratio of 0.5. The solution was then titrated with 0.1 M NaOH with continuous stirring at the rate of 0.5 ml min 1 to an OH/Al molar ratio of 2.0. The final concentration of Si was 1.6 mM and the pH of the solution was 4.36. The solution was heated at 98 jC for 110 h. After cooling to room temperature, the pH of the suspension was determined. The precipitate was collected by ultra-filtration using a cellulose nitrate membrane filter of 0.1-Am pore size. The precipitate was dialyzed (molecular weight cut off = 3500) against deionized distilled water until Cl free, freeze dried, and then ground gently to pass through a 0.5-mm sieve. 2.2. Elemental, XRD, and FTIR analyses The contents of Si and Al in the precipitate were determined according to the methods of Weaver et al. (1968) and Hsu (1963), respectively, following the dissolution of the precipitate with 0.5 M NaOH. The X-ray diffraction analysis was carried out on a Rigaku D/Max-RBX X-ray diffractometer (Rigaku, Tokyo) with FeKa radiation using a parallel-oriented sample dried on a glass slide. Infrared (IR) spectrum was recorded on a Perkin-Elmer 983 infrared spectrometer
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(Perkin-Elmer, Buckinghamshire, England) with a KBr disc incorporating 1 mg of the freeze-dried material with 200 mg KBr. 2.3. Investigation by atomic force microscopy The imogolite was dispersed by ultrasonification at 150 W for 5, 15, 30, 60, and 120 s in an ice bath. Two drops of the extremely diluted imogolite suspension (20 mg l 1) were deposited on a watch glass and then dried for 1 day at room temperature (23.5 F 0.5 jC). Another replicate sample was dried for 1 month under the same condition. The watch glass was then fastened to a magnetized stainless steel disk with double-sided tape. Finally, the AFM three-dimensional images of the imogolite were taken by using a NanoScope III atomic force microscope in contact- and tappingmode (Digital Instrument, Santa Barbara, CA) in air and at room temperature. The scanner was type 1881E and the scanner size was 15 Am. The image was recorded after scanning from 1 to 100 times. The image obtained after scanning 1 time was referred to as the image obtained after the tip moved from one edge of the frame to another edge of the frame. The images were not filtered and not modified. In contact-mode AFM, a silicon nitride cantilever with spring constant of 0.12 N m 1 was used. The scanning rate was 2.2 Hz. Since deflection image is more suitable for emphasizing topographic features and clearer than height image (Fischer et al., 1996), the defection image was selected for the contact-mode AFM analysis. The deflection images with the scanning size of 10 10 or 15 15 Am were displayed to describe the morphological features of imogolite in this study. The intensity of gray coloration in deflection images represented the difference between neighboring height values. The methodology described by Liu and Huang (1999) was followed in the AFM investigation in this study. In tapping-mode AFM, an etched silicon cantilever with resonant frequency of 281 –506 kHz and length of 125 Am was used. The scanning rate was 1.4 Hz. Since there is no deflection image and the height image is clear in the tappingmode AFM analysis, the height image with the scanning size of 10 10 or 15 15 Am was recorded in the present study. The AFM used in this study was calibrated by using mica standard (Digital Instrument, 1993). After
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the image was enlarged, the diameter of imogolite threads was measured by using Section analysis of AFM (Digital Instrument, 1993) through drawing a line across the imogolite thread and the length of imogolite threads was determined directly by drawing a line along the thread. The measurements were carried on 50 different particles to represent typical imogolite particles. In selecting the imogolite particles, aggregated imogolite particles and the particles that were located in the lower layers were avoided.
3. Results and discussion The precipitate formed from the solution containing monomeric orthosilicic acid and AlCl3 at a Si/Al molar ratio of 0.5 and an OH/Al molar ratio of 2.0 was predominantly imogolite as indicated by X-ray diffraction peaks at 2.20, 0.89, and 0.62 nm (Fig. 1) and IR absorption bands at 987, 935, 695, 564, 488, 423, and 341 cm 1 (Fig. 2). The X-ray diffractogram shows the presence of a small amount of bayerite as indicated by the d-value at 0.47 nm (Fig. 1). The IR spectrum indicates the presence of a small amount of pseudoboemite (1069 cm 1, Fig. 2) in the precipitate. The contents of Si and Al in the precipitate were 221
Fig. 1. X-ray diffractogram of the synthetic imogolite.
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were not observed; these AFM images show the presence of spheroidal and ill-defined shaped particles (Fig. 3b). After 100 times of scanning with the
Fig. 2. Infrared (IR) spectrum of the synthetic imogolite.
and 546 g kg 1 as SiO2 and Al2O3, respectively. The SiO2/Al2O3 molar ratio was 0.69 and thus was lower than the theoretical ratio of 1 for imogolite. This may be due to the presence of bayerite and pseudoboemite in the precipitate. Since imogolite forms hollow tubes and its structural stability is relatively lower than many other soil minerals, we have to pay attention to sample preparations such as ultrasonic dispersion and evaporation of the suspension, and effects of physical forces between the tip and the samples on the resolution and quality of AFM images. The effects of ultrasonification and air-drying period of samples on the surface features of imogolite were investigated in the present study. After deposition of the imogolite suspension, which was dispersed by ultrasonification at 150 W for 5 s on a watch glass, the solution was allowed to evaporate for 1 day at 23.5 jC. At the initial scanning, the distinctive thread-like structure of imogolite was observed in the contact-mode AFM image (Fig. 3a). However, after repeating the scanning in the same area for 5 times and 10 times, the surface features were altered and the thread-like morphology of smaller particles gradually disappeared (not shown). After scanning for 100 times, thread-like structures Fig. 3. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite air-dried for 1 day on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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scanning size of 15 15 Am, the buildup of eroded materials along the edges of the scanning spot was observed in the deflection image (Fig. 3c). Maurice (1998) reported that frictional forces, adhesive forces, and chemical interactions between tip and sample could result in etching of a sample surface along microtopographic features such as a step edge during the scanning process. Delawski and Parkinson (1992) showed that the edges of pits of metal chalcogenides were eroded parallel to the scan direction as found for imogolite in the present study (Fig. 3c). They reported that the etching process was related to the relative humidity. When the relative humidity was >30%, the etching increased with the increase of the relative humidity. However, after the imogolite suspension, which was dispersed by ultrasonification at 150 W for 5 s, was deposited on a watch glass and air-dried for 1 month at 23.5 jC, the scanning times (repeating 1 to 100 times) had no effect on the morphological features of imogolite (Fig. 4a and b). Even when the scanning was repeated for 100 times, the typical thread-like structure of imogolite was still observed in the contact-mode AFM deflection image (Fig. 4b). Although small accumulation of eroded materials along the edges of the scanning site was observed in the 15 15-Am2 image (Fig. 4c), the effect of etching processes was almost negligible. These results suggest that it will take a long time to evaporate moisture from the suspension and leave the particles firmly attached to the surface of the support in the case of imogolite, which has a high affinity to water. The relative humidity of the ambient air was observed to affect the etching rate of the substrate (Delawski and Parkinson, 1992). Thus, in applying the contact-mode AFM to investigate the surface features of the fragile imogolite, the moisture conditions of the sample and the ambient air are critical to avoid the erosion of the sample surface. Tapping-mode AFM is a relative new technique that circumvents the problem of large lateral frictional
Fig. 4. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite air-dried for 1 month on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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forces, where the tip contacts the sample surfaces at known numerous frequencies (Bertsch and Hunter, 1998; Maurice, 1998). Tapping-mode AFM inherently prevents the tip from sticking to the surface and causing damage during scanning. This innovation is excellent for imaging of soft and fragile surfaces, such as humic substances and soil bacteria (Maurice, 1998). The height images of tapping-mode AFM of imogolite, which was dispersed by ultrasonification at 150 W for 5 s and then subjected to 1-day air-drying, are given in Fig. 5. Similar thread structures of imogolite were observed after scanning 1 or 100 times (Fig. 5a and b). The larger-size images (15 15 Am2) (Fig. 5c) further reveal that unlike contact-mode AFM, tapping-mode AFM did not cause damage of imogolite thread structure during scanning after the sample was air-dried only for 1 day. Although the easily deformable thread-like structures of imogolite can be observed distinctly by using the contact-mode AFM in combination with adequate desiccation of imogolite deposit on the holder before AFM imaging (Fig. 4), the image of imogolite observed by tappingmode AFM is much clearer than that observed by contact-mode AFM under the same condition (Fig. 5). The three-dimensional deflection images of contact-mode AFM of the reaction product after airdrying for 30 days as influenced by different ultrasonification periods are given in Fig. 6. Without ultrasonification, no distinctive thread structure was observed in the AFM image (Fig. 6a). This is evidently due to aggregation of imogolite particles, as found in the AFM studies on illite, smectite, and goethite (Blum, 1994; Fischer et al., 1996). After ultrasonification for 5 s, imogolite appeared in the AFM image as curved threads (Fig. 6b). Many electron microscopic studies have already shown the thread-like tubular structures of imogolite (Yoshinaga and Aomine, 1962; Russell et al., 1969; Henmi and Wada, 1976; Inoue and Huang, 1984,1985; Lou and Huang, 1989). After the ultrasonification for 15 s to 2 min, the thread-like structures which are characteristic
Fig. 5. The tapping-mode AFM three-dimensional height images of the synthetic imogolite air-dried for 1 day on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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Fig. 6. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite after different ultrasonification times: (a) 0 s, (b) 5 s, (c) 15 s, (d) 30 s, (e) 1 min, and (f) 2 min. The suspension after ultrasonification was air-dried on the sample holder for 1 month. The image was captured after scanning 2 times. The same scale is used from (a) to (f).
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of imogolite were still observed (Fig. 6c – f). The average values of the diameter and length of imogolite threads after different ultrasonification times were obtained from 50 different imogolite particles and statistical analysis was carried out by using the Fisher – LSD test (Table 1). With the increase in the ultrasonification time, the diameters of thread-like structures of imogolite decreased slightly, but there was no significant difference. In contrast, the length of imogolite threads decreased significantly with the increase in the ultrasonification time (Table 1 and Fig. 6). The imogolite threads after the ultrasonification for 5 s were 3 times longer compared with those after 2 min of treatment. This must have resulted from cutting of the thread-like structures by physical force of the ultrasonic dispersion. Henmi et al. (1999) reported that the imogolite structure is extremely weak. The present study shows that ultrasonification of imogolite suspension is necessary to disperse imogolite particles, but ultrasonification time is critical to avoid alteration of imogolite. The data indicate that 5 s of ultrasonification at 150 W is the optimal dispersion for this synthetic imogolite after freezedrying. The three-dimensional height images of tappingmode AFM of the product after air-drying for 30 days as influenced by different ultrasonification periods are Table 1 The diameter and length of imogolite threads (air-dried for 30 days) after different ultrasonification times obtained by contact-mode AFM Time
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Range
Average
Range
43.8 – 101.2 42.5 – 105.9 55.4 – 97.9 44.5 – 97.9 43.8 – 85.0
N.A. 1.58 e 1.20 d 0.89 c 0.67 b 0.53 a
0.75 – 2.28 0.73 – 1.67 0.55 – 1.42 0.36 – 0.98 0.28 – 0.89
c
N.A. 78.8 ad 77.5 a 77.3 a 73.3 a 70.1 a
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter for each particle was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.06 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
given in Fig. 7. Similar effect of ultrasonification on the imogolite thread structure was observed in the tapping-mode AFM images (Fig. 7). However, the diameter of imogolite threads observed by tappingmode AFM (Table 2) was consistently and significantly ( p = 8.64 10 4) smaller in comparison with that observed by contact-mode AFM (Table 1). For example, the diameter of imogolite threads, which were subjected to 5-s ultrasonification, air-dried for 30 days, and observed by tapping-mode AFM, was 21% smaller compared with that observed from the contactmode AFM image. The Si3N4 contact-mode AFM tips have low aspect ratios, which apparently resulted in enlarging the observed width of small-size features. The aspect ratio is calculated as the ratio of the vertical and horizontal size of the tip. When the aspect ratio of a tip is very high, the sidewall angles of the tip to the surface approach 90j; when the aspect ratio of a tip is low, the sidewall angles of the tip to the surface are much less than 90j (Fig. 8). When a tip moves along the surface of the sample which is not very flat, the tip with a low aspect ratio cannot be as close to the surface as the tip with a high aspect ratio. This is due to steric hindrance caused by the limitation of the sidewall of the tip to contact the surface of the sample as illustrated in Fig. 8. However, the lengths of imogolite threads observed by contact-mode and tapping-mode AFM were not significantly different (Tables 1 and 2). Apparently, the aspect ratio of the AFM tip did not significantly affect the measurement of the length of imogolite threads. This is because the length of imogolite threads is much greater than the errors caused by the aspect ratio effect. The diameter of synthetic imogolite threads (after 5-s ultrasonification) observed by tapping-mode AFM ranged from 40.2 to 95.5 nm with an average of 62.2 nm (Table 2), which was larger than that of naturally occurring imogolite threads (10 –50 nm in diameter) reported in the literature (Yoshinaga and Aomine, 1962; Tazaki, 1979). Imogolite threads are referred to as the imogolite that contains little, if any, mesoporosity associated with the random twisting of tube bundles (Hoshino et al., 1996; Pohl et al., 1996). Therefore, the diameter of imogolite threads would greatly depend on the number of imogolite tubes which twist together. This may explain the great variation of the diameter of imogolite threads.
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Fig. 7. The tapping-mode AFM three-dimensional height images of the synthetic imogolite after different ultrasonification times: (a) 0 s, (b) 5 s, (c) 15 s, (d) 30 s, (e) 1 min, and (f) 2 min. The suspension after ultrasonification was air-dried on the sample holder for 1 month. The image was captured after scanning 2 times. The same scale is used from (a) to (f).
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Table 2 The diameter and length of imogolite threads (air-dried for 30 days) after different ultrasonification times obtained by tapping-mode AFM Time
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Average
Range
c
N.A. 62.2 ad 61.8 a 60.2 a 60.6 a 61.5 a
40.2 – 95.5 39.6 – 97.4 40.1 – 96.8 39.0 – 94.1 39.0 – 90.0
N.A. 1.50 e 1.22 d 0.95 c 0.67 b 0.57 a
Table 3 The diameter and length of imogolite threads (air-dried for 1 day) after different ultrasonification times obtained by tapping-mode AFM Time
Range 0.65 – 2.15 0.63 – 2.00 0.51 – 1.67 0.40 – 1.05 0.28 – 1.00
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Range
Average
Range
N.A. 48.7 – 98.0 40.0 – 98.5 41.6 – 96.4 45.4 – 94.1 38.7 – 85.2
1.47 1.25 0.92 0.68 0.55
0.67 – 2.45 0.63 – 2.32 0.49 – 1.62 0.39 – 1.05 0.27 – 0.96
c
N.A. 67.6 ad 65.1 a 68.8 a 66.2 a 63.5 a
e d c b a
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.06 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.07 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
The imogolite structure is weak and fragile; its structure is easily damaged by dry grinding and its thermal stability is low (Yoshinaga, 1968; Henmi and Yoshinaga, 1981). The average diameter of imogolite threads, which were subjected to 1-day air-dried period (Table 3), was consistently and significantly ( p = 1.15 10 2) larger than that of imogolite threads which were subjected to 1-month air-dried period (Table 2). Further, the diameter of imogolite, which was subjected to 5-s ultrasonification and then ovendried at 105 jC for 24 h and observed by tappingmode AFM, ranged from 38.4 to 92.6 nm with an
average of 59.2 nm. The 24-h oven drying resulted in a significantly ( p = 3.67 10 2) smaller diameter of imogolite compared with 1-month air drying. Ovendrying and length of air-drying affected the degree of dehydration of a sample and modified the structure of the sample. Therefore, ultra-high vacuum conditions of the TEM should result in reduced diameter of imogolite threads. Sorption and catalysis occur on the surface of soil minerals in natural environments. The reactivity of soil minerals is governed by the surface reactive sites which in turn are influenced by the specific surface, surface
Fig. 8. Schematic diagram depicting the difference in the observed diameters of imogolite threads using (a) a tapping-mode AFM tip with a high aspect ratio and (b) a contact-mode AFM tip with a low aspect ratio. The diameter of the circle represents the intrinsic diameter of imogolite threads. Steric hindrance caused by the limitation of the sidewall of the tip with a low aspect ratio to contact the surface of the sample is illustrated.
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characteristics, and site density. These surface properties of thread-like imogolite are influenced by its diameter and length and related other surface features. Atomic force microscopy is in situ analysis. Research using AFM is conducted in air and at ambient temperature, and experimental conditions are, thus, appropriate for environmental studies. Therefore, tapping-mode AFM is a powerful and reliable technique to investigate the surface features of imogolite and its related surface chemistry pertaining to environmental quality.
4. Conclusions The in situ surface features of imogolite were revealed for the first time by using the AFM under ambient conditions. The morphological features of synthetic imogolite can be more clearly observed by using the tapping-mode AFM rather than the contactmode AFM. The tapping-mode AFM also required a shorter sample drying period. The imogolite appeared in the AFM three-dimensional images as threads, which were curved, as found in the TEM studies. The data obtained in the present study show that adequate ultrasonification is essential to disperse imogolite particles. The length of imogolite threads observed greatly depended on the ultrasonification. Over-ultrasonification can cause the breakdown of imogolite threads. The diameter values of imogolite threads subjected to 5 s and 150 W of ultrasonification, air-dried for 30 days, and then determined by tapping-mode AFM in the present study are 40.2– 95.5 nm (standard error < F0.1 nm for each thread). Sufficient air drying of imogolite on the sample holder prior to contact-AFM scanning is critical to avoid the etching of imogolite surface by the frictional forces and adhesive forces between the tip and the sample. Use of the contact-mode AFM tip resulted in enlarging the observed diameter of imogolite threads due to tip-sample convolution.
Acknowledgements This study was supported by the Ministry of Education of Japan and Discovery Grant GP2383Huang of the Natural Sciences and Engineering Research Council of Canada.
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the determination of aluminum with aluminon. Soil Sci. 96, 230 – 238. Iler, R.K., 1979. The Chemistry of Silica. Wiley, New York. Inoue, K., Huang, P.M., 1984. Influence of citric acid on the natural formation of imogolite. Nature 308, 58 – 60. Inoue, K., Huang, P.M., 1985. Influence of citric acid on the formation of short-range ordered aluminosilicate. Clays Clay Miner. 33, 312 – 322. Koenderink, G.H., Kluijtmans, S.G.J.M., Philipse, A.P., 1999. Note on the synthesis of colloidal imogolite fibers. J. Colloid Interface Sci. 216, 429 – 431. Liu, C., Huang, P.M., 1999. Atomic force microscopy and surface characteristics of iron oxides formed in citrate solutions. Soil Sci. Soc. Am. J. 63, 65 – 72. Lou, G., Huang, P.M., 1989. Nature and charge properties of X-ray noncrystalline aluminosilicates as affected by citric acid. Soil Sci. Soc. Am. J. 53, 1287 – 1293. Maurice, P.A., 1998. Scanning probe microscopy of environmental surfaces. In: Huang, P.M., Senesi, N., Buffle, J. (Eds.), Structure and Surface Reactions of Soil Particles. Wiley, Chichester, UK, pp. 109 – 153. Nagy, K.L., Blum, A.E., 1994. Scanning probe microscopy of clay minerals. Clay Mineral Society Workshop Lectures, vol. 7. Clay Mineral Society. Boulder, CO. Parfitt, R.L., Henmi, T., 1980. Structure of some allophanes from New Zealand. Clays Clay Miner. 28, 285 – 294. Parfitt, R.L., Thomas, A.D., Atkinson, R.J., Smart, R.S.C., 1974. Adsorption of phosphate on imogolite. Clays Clay Miner. 22, 455 – 456.
Pohl, P.I., Faulon, J.-L., Smith, D.M., 1996. Pore structure of imogolite computer models. Langmuir 12, 4463 – 4468. Ross, G.J., Kodama, H., 1979. Evidence for imogolite in Canadian soils. Clays Clay Miner. 27, 297 – 300. Rugar, D., Hansma, P., 1990. Atomic force microscopy. Phys. Today 43, 23 – 30. Russell, J.D., McHardy, W.J., Fraser, A.R., 1969. Imogolite: a unique aluminosilicate. Clay Miner. 8, 87 – 99. Tait, J.M., Yoshinaga, N., Mitchell, B.D., 1978. The occurrence of imogolite in some Scottish soils. Soil Sci. Plant Nutr. 24, 145 – 151. Tazaki, K., 1979. Scanning electron microscopic study of imogolite formation from plagioclase. Clays Clay Miner. 27, 209 – 212. Wada, K., 1989. Allophane and imogolite. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environment, 2nd ed. Soil Science Society of America, Madison, WI, pp. 1051 – 1087. Weaver, R.M., Syers, J.K., Jackson, M.L., 1968. Determination of silica in citrate – bicarbonate – dithionite extracts of soils. Soil Sci. Soc. Am. J. 32, 497 – 501. Yoshinaga, N., 1968. Identification of imogolite in the filmy gel materials in the Imaichi and Shichihonzakura pumice beds. Soil Sci. Plant Nutr. 14, 238 – 246. Yoshinaga, N., Aomine, S., 1962. Imogolite in some Ando soils. Soil Sci. Plant Nutr. 8, 22 – 29. Yoshinaga, N., Yotsumoto, H., Ibe, K., 1968. An electron microscopic study of soil allophane with an ordered structure. Am. Mineral. 53, 319 – 323.
Atomic force microscopy of synthetic imogolite M. Tani 1, C. Liu 2, P.M. Huang * Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A8 Received 4 June 2002; accepted 2 May 2003
Abstract The morphology of imogolite has been substantially studied using transmission electron microscopy (TEM) under ultra-high vacuum. However, the surface feature of imogolite at a nanometer scale under ambient conditions remains to be uncovered. The surface feature and the particle size of synthetic imogolite were investigated using contact- and tapping-mode atomic force microscopy (AFM) in the present study. The imogolite was synthesized at an initial Si concentration of 1.6 mmol l 1, a Si/Al molar ratio of 0.5, and an OH/Al molar ratio of 2.0. The X-ray diffractogram and infrared (IR) spectrum of the precipitates indicate that imogolite was the dominant reaction product. The in situ surface features of imogolite were revealed for the first time by using AFM under ambient conditions. The morphological features of synthetic imogolite were more clearly observed by using the tapping-mode AFM rather than the contact-mode AFM. Compared with the contact-mode AFM, the tapping-mode AFM required a shorter sample-drying period. Use of the contact-mode AFM tips, which have low aspect ratios, resulted in enlarging the observed diameter values of imogolite threads, which were subjected to the optimal ultrasonification (at 150 W for 5 s), by 21% apparently due to tip-sample convolution. The imogolite, which was subjected to the optimal ultrasonification (at 150 W for 5 s) that resulted in the least alteration of imogolite particles, appears in the tapping-mode AFM three-dimensional images as curved threads, varying in diameter from 40.2 to 95.5 nm (standard error < F 0.1 nm for each thread). The tappingmode AFM is a powerful and reliable technique to investigate the morphological features of imogolite and its related surface chemistry in soil environments. D 2003 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Contact-mode and tapping-mode; Imogolite; Surface features; Thread diameter
1. Introduction Imogolite is a naturally occurring hydrous aluminosilicate polymer with a unique tubular structure * Corresponding author. Tel.: +1-306-966-6838; fax: +1-306966-6881. E-mail address: [email protected] (P.M. Huang). 1 Current address: Department of Agro-Environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Inada, Obihiro, Hokkaido 080-8555, Japan. 2 Current address: Kuo Testing Labs, Inc., 337 S. 1st Avenue, Othello, WA 99344, USA. 0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0016-7061(03)00204-0
(Cradwick et al., 1972; Farmer et al., 1983; Wada, 1989). It exists in soils as a para-crystalline mineral and has been extensively isolated from the B horizons of Andisols (Parfitt and Henmi, 1980; Wada, 1989) and Spodosols (Brydon and Shimoda, 1972; Ross and Kodama, 1979; Farmer et al., 1980; Anderson et al., 1982; Gustafsson et al., 1995). Imogolite and protoimogolite allophanes can make up at least 6% of the B2 horizon soil (Farmer et al., 1980). The walls of an imogolite tube unit consist of curved gibbsite-like sheets with SiOH groups on the inside and AlOH groups on the outside. Because of its reactive surface,
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imogolite exerts significant effects on ion exchange properties, surface acidity, water adsorption, and other physicochemical properties of soils (Farmer et al., 1983; Wada, 1989). Many nutrients and pollutants such as heavy metal cations, phosphate, and arsenate can form chemical bonds with the outer surface of imogolite (Cradwick et al., 1972; Parfitt et al., 1974). Furthermore, imogolite has been proposed as a shaped-selective catalyst due to its tubular structure (Pohl et al., 1996). The structure, morphology, and surface properties of imogolite, therefore, deserve special attention. Transmission electron microscopy (TEM) investigations have shown that natural and synthetic imogolite particles mostly have thread-like structures (Yoshinaga and Aomine, 1962; Russell et al., 1969; Tait et al., 1978; Inoue and Huang, 1984; Koenderink et al., 1999). Under transmission electron microscope, imogolite appears as threads, varying in diameter from 10 to 30 nm and extending up to several micrometers in length (Yoshinaga and Aomine, 1962), and consists of assemblies of a tube unit with inner and outer diameters of 1.0 and 2.0 – 2.7 nm, respectively (Yoshinaga and Aomine, 1962; Yoshinaga et al., 1968; Wada, 1989; Koenderink et al., 1999). Tazaki (1979) also reported that imogolite threads formed from plagioclase grains in volcanic ash are less than 50 nm in diameter and about 0.3 Am in length under TEM. Although TEM and scanning electron microscopy (SEM) are useful tools to observe the surface morphology of mineral colloids, the high-vacuum condition under which the TEM and SEM are operated could result in the alteration of the surface morphology. Scanning probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), represents an ever-increasing class of microscopic techniques that provide three-dimensional images of solid surfaces at high resolution (Rugar and Hansma, 1990; Nagy and Blum, 1994; Bertsch and Hunter, 1998). The distinctive advantage of STM and AFM compared with SEM and TEM is that the samples do not have to be coated and subjected to ultra-high vacuum condition (Bertsch and Hunter, 1998). Therefore, SPM work under conditions in air and at room temperature is appropriate for environmental studies (Maurice, 1998). However, the surface
feature of imogolite has not been revealed by using AFM. In this study, we investigated the morphological features of synthesized imogolite under ambient conditions by using AFM and compared the surface features of imogolite observed by contact- and tapping-mode AFM.
2. Materials and methods 2.1. Preparation of imogolite The imogolite was prepared from a solution containing monomeric orthosilicic acid and AlCl3 using the method of Inoue and Huang (1984). Monomeric orthosilicic acid was obtained by passing Na-metasilicate solution through a H+-saturated Dowex 50WX8 cation exchange resin column (Alexander, 1953). To avoid polymerization of Si, the initial concentration of silicic acid was maintained below 2 mmol l 1 (Iler, 1979). The silicic acid was mixed with AlCl3 solution to give a Si/Al molar ratio of 0.5. The solution was then titrated with 0.1 M NaOH with continuous stirring at the rate of 0.5 ml min 1 to an OH/Al molar ratio of 2.0. The final concentration of Si was 1.6 mM and the pH of the solution was 4.36. The solution was heated at 98 jC for 110 h. After cooling to room temperature, the pH of the suspension was determined. The precipitate was collected by ultra-filtration using a cellulose nitrate membrane filter of 0.1-Am pore size. The precipitate was dialyzed (molecular weight cut off = 3500) against deionized distilled water until Cl free, freeze dried, and then ground gently to pass through a 0.5-mm sieve. 2.2. Elemental, XRD, and FTIR analyses The contents of Si and Al in the precipitate were determined according to the methods of Weaver et al. (1968) and Hsu (1963), respectively, following the dissolution of the precipitate with 0.5 M NaOH. The X-ray diffraction analysis was carried out on a Rigaku D/Max-RBX X-ray diffractometer (Rigaku, Tokyo) with FeKa radiation using a parallel-oriented sample dried on a glass slide. Infrared (IR) spectrum was recorded on a Perkin-Elmer 983 infrared spectrometer
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(Perkin-Elmer, Buckinghamshire, England) with a KBr disc incorporating 1 mg of the freeze-dried material with 200 mg KBr. 2.3. Investigation by atomic force microscopy The imogolite was dispersed by ultrasonification at 150 W for 5, 15, 30, 60, and 120 s in an ice bath. Two drops of the extremely diluted imogolite suspension (20 mg l 1) were deposited on a watch glass and then dried for 1 day at room temperature (23.5 F 0.5 jC). Another replicate sample was dried for 1 month under the same condition. The watch glass was then fastened to a magnetized stainless steel disk with double-sided tape. Finally, the AFM three-dimensional images of the imogolite were taken by using a NanoScope III atomic force microscope in contact- and tappingmode (Digital Instrument, Santa Barbara, CA) in air and at room temperature. The scanner was type 1881E and the scanner size was 15 Am. The image was recorded after scanning from 1 to 100 times. The image obtained after scanning 1 time was referred to as the image obtained after the tip moved from one edge of the frame to another edge of the frame. The images were not filtered and not modified. In contact-mode AFM, a silicon nitride cantilever with spring constant of 0.12 N m 1 was used. The scanning rate was 2.2 Hz. Since deflection image is more suitable for emphasizing topographic features and clearer than height image (Fischer et al., 1996), the defection image was selected for the contact-mode AFM analysis. The deflection images with the scanning size of 10 10 or 15 15 Am were displayed to describe the morphological features of imogolite in this study. The intensity of gray coloration in deflection images represented the difference between neighboring height values. The methodology described by Liu and Huang (1999) was followed in the AFM investigation in this study. In tapping-mode AFM, an etched silicon cantilever with resonant frequency of 281 –506 kHz and length of 125 Am was used. The scanning rate was 1.4 Hz. Since there is no deflection image and the height image is clear in the tappingmode AFM analysis, the height image with the scanning size of 10 10 or 15 15 Am was recorded in the present study. The AFM used in this study was calibrated by using mica standard (Digital Instrument, 1993). After
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the image was enlarged, the diameter of imogolite threads was measured by using Section analysis of AFM (Digital Instrument, 1993) through drawing a line across the imogolite thread and the length of imogolite threads was determined directly by drawing a line along the thread. The measurements were carried on 50 different particles to represent typical imogolite particles. In selecting the imogolite particles, aggregated imogolite particles and the particles that were located in the lower layers were avoided.
3. Results and discussion The precipitate formed from the solution containing monomeric orthosilicic acid and AlCl3 at a Si/Al molar ratio of 0.5 and an OH/Al molar ratio of 2.0 was predominantly imogolite as indicated by X-ray diffraction peaks at 2.20, 0.89, and 0.62 nm (Fig. 1) and IR absorption bands at 987, 935, 695, 564, 488, 423, and 341 cm 1 (Fig. 2). The X-ray diffractogram shows the presence of a small amount of bayerite as indicated by the d-value at 0.47 nm (Fig. 1). The IR spectrum indicates the presence of a small amount of pseudoboemite (1069 cm 1, Fig. 2) in the precipitate. The contents of Si and Al in the precipitate were 221
Fig. 1. X-ray diffractogram of the synthetic imogolite.
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were not observed; these AFM images show the presence of spheroidal and ill-defined shaped particles (Fig. 3b). After 100 times of scanning with the
Fig. 2. Infrared (IR) spectrum of the synthetic imogolite.
and 546 g kg 1 as SiO2 and Al2O3, respectively. The SiO2/Al2O3 molar ratio was 0.69 and thus was lower than the theoretical ratio of 1 for imogolite. This may be due to the presence of bayerite and pseudoboemite in the precipitate. Since imogolite forms hollow tubes and its structural stability is relatively lower than many other soil minerals, we have to pay attention to sample preparations such as ultrasonic dispersion and evaporation of the suspension, and effects of physical forces between the tip and the samples on the resolution and quality of AFM images. The effects of ultrasonification and air-drying period of samples on the surface features of imogolite were investigated in the present study. After deposition of the imogolite suspension, which was dispersed by ultrasonification at 150 W for 5 s on a watch glass, the solution was allowed to evaporate for 1 day at 23.5 jC. At the initial scanning, the distinctive thread-like structure of imogolite was observed in the contact-mode AFM image (Fig. 3a). However, after repeating the scanning in the same area for 5 times and 10 times, the surface features were altered and the thread-like morphology of smaller particles gradually disappeared (not shown). After scanning for 100 times, thread-like structures Fig. 3. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite air-dried for 1 day on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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scanning size of 15 15 Am, the buildup of eroded materials along the edges of the scanning spot was observed in the deflection image (Fig. 3c). Maurice (1998) reported that frictional forces, adhesive forces, and chemical interactions between tip and sample could result in etching of a sample surface along microtopographic features such as a step edge during the scanning process. Delawski and Parkinson (1992) showed that the edges of pits of metal chalcogenides were eroded parallel to the scan direction as found for imogolite in the present study (Fig. 3c). They reported that the etching process was related to the relative humidity. When the relative humidity was >30%, the etching increased with the increase of the relative humidity. However, after the imogolite suspension, which was dispersed by ultrasonification at 150 W for 5 s, was deposited on a watch glass and air-dried for 1 month at 23.5 jC, the scanning times (repeating 1 to 100 times) had no effect on the morphological features of imogolite (Fig. 4a and b). Even when the scanning was repeated for 100 times, the typical thread-like structure of imogolite was still observed in the contact-mode AFM deflection image (Fig. 4b). Although small accumulation of eroded materials along the edges of the scanning site was observed in the 15 15-Am2 image (Fig. 4c), the effect of etching processes was almost negligible. These results suggest that it will take a long time to evaporate moisture from the suspension and leave the particles firmly attached to the surface of the support in the case of imogolite, which has a high affinity to water. The relative humidity of the ambient air was observed to affect the etching rate of the substrate (Delawski and Parkinson, 1992). Thus, in applying the contact-mode AFM to investigate the surface features of the fragile imogolite, the moisture conditions of the sample and the ambient air are critical to avoid the erosion of the sample surface. Tapping-mode AFM is a relative new technique that circumvents the problem of large lateral frictional
Fig. 4. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite air-dried for 1 month on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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forces, where the tip contacts the sample surfaces at known numerous frequencies (Bertsch and Hunter, 1998; Maurice, 1998). Tapping-mode AFM inherently prevents the tip from sticking to the surface and causing damage during scanning. This innovation is excellent for imaging of soft and fragile surfaces, such as humic substances and soil bacteria (Maurice, 1998). The height images of tapping-mode AFM of imogolite, which was dispersed by ultrasonification at 150 W for 5 s and then subjected to 1-day air-drying, are given in Fig. 5. Similar thread structures of imogolite were observed after scanning 1 or 100 times (Fig. 5a and b). The larger-size images (15 15 Am2) (Fig. 5c) further reveal that unlike contact-mode AFM, tapping-mode AFM did not cause damage of imogolite thread structure during scanning after the sample was air-dried only for 1 day. Although the easily deformable thread-like structures of imogolite can be observed distinctly by using the contact-mode AFM in combination with adequate desiccation of imogolite deposit on the holder before AFM imaging (Fig. 4), the image of imogolite observed by tappingmode AFM is much clearer than that observed by contact-mode AFM under the same condition (Fig. 5). The three-dimensional deflection images of contact-mode AFM of the reaction product after airdrying for 30 days as influenced by different ultrasonification periods are given in Fig. 6. Without ultrasonification, no distinctive thread structure was observed in the AFM image (Fig. 6a). This is evidently due to aggregation of imogolite particles, as found in the AFM studies on illite, smectite, and goethite (Blum, 1994; Fischer et al., 1996). After ultrasonification for 5 s, imogolite appeared in the AFM image as curved threads (Fig. 6b). Many electron microscopic studies have already shown the thread-like tubular structures of imogolite (Yoshinaga and Aomine, 1962; Russell et al., 1969; Henmi and Wada, 1976; Inoue and Huang, 1984,1985; Lou and Huang, 1989). After the ultrasonification for 15 s to 2 min, the thread-like structures which are characteristic
Fig. 5. The tapping-mode AFM three-dimensional height images of the synthetic imogolite air-dried for 1 day on the sample holder and then scanned (a) 1 time (full scale of 10 Am), (b) 100 times (full scale of 10 Am), and (c) 100 times (full scale of 15 Am). The imogolite suspension was ultrasonified for 5 s before air-drying and AFM imaging.
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Fig. 6. The contact-mode AFM three-dimensional deflection images of the synthetic imogolite after different ultrasonification times: (a) 0 s, (b) 5 s, (c) 15 s, (d) 30 s, (e) 1 min, and (f) 2 min. The suspension after ultrasonification was air-dried on the sample holder for 1 month. The image was captured after scanning 2 times. The same scale is used from (a) to (f).
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of imogolite were still observed (Fig. 6c – f). The average values of the diameter and length of imogolite threads after different ultrasonification times were obtained from 50 different imogolite particles and statistical analysis was carried out by using the Fisher – LSD test (Table 1). With the increase in the ultrasonification time, the diameters of thread-like structures of imogolite decreased slightly, but there was no significant difference. In contrast, the length of imogolite threads decreased significantly with the increase in the ultrasonification time (Table 1 and Fig. 6). The imogolite threads after the ultrasonification for 5 s were 3 times longer compared with those after 2 min of treatment. This must have resulted from cutting of the thread-like structures by physical force of the ultrasonic dispersion. Henmi et al. (1999) reported that the imogolite structure is extremely weak. The present study shows that ultrasonification of imogolite suspension is necessary to disperse imogolite particles, but ultrasonification time is critical to avoid alteration of imogolite. The data indicate that 5 s of ultrasonification at 150 W is the optimal dispersion for this synthetic imogolite after freezedrying. The three-dimensional height images of tappingmode AFM of the product after air-drying for 30 days as influenced by different ultrasonification periods are Table 1 The diameter and length of imogolite threads (air-dried for 30 days) after different ultrasonification times obtained by contact-mode AFM Time
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Range
Average
Range
43.8 – 101.2 42.5 – 105.9 55.4 – 97.9 44.5 – 97.9 43.8 – 85.0
N.A. 1.58 e 1.20 d 0.89 c 0.67 b 0.53 a
0.75 – 2.28 0.73 – 1.67 0.55 – 1.42 0.36 – 0.98 0.28 – 0.89
c
N.A. 78.8 ad 77.5 a 77.3 a 73.3 a 70.1 a
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter for each particle was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.06 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
given in Fig. 7. Similar effect of ultrasonification on the imogolite thread structure was observed in the tapping-mode AFM images (Fig. 7). However, the diameter of imogolite threads observed by tappingmode AFM (Table 2) was consistently and significantly ( p = 8.64 10 4) smaller in comparison with that observed by contact-mode AFM (Table 1). For example, the diameter of imogolite threads, which were subjected to 5-s ultrasonification, air-dried for 30 days, and observed by tapping-mode AFM, was 21% smaller compared with that observed from the contactmode AFM image. The Si3N4 contact-mode AFM tips have low aspect ratios, which apparently resulted in enlarging the observed width of small-size features. The aspect ratio is calculated as the ratio of the vertical and horizontal size of the tip. When the aspect ratio of a tip is very high, the sidewall angles of the tip to the surface approach 90j; when the aspect ratio of a tip is low, the sidewall angles of the tip to the surface are much less than 90j (Fig. 8). When a tip moves along the surface of the sample which is not very flat, the tip with a low aspect ratio cannot be as close to the surface as the tip with a high aspect ratio. This is due to steric hindrance caused by the limitation of the sidewall of the tip to contact the surface of the sample as illustrated in Fig. 8. However, the lengths of imogolite threads observed by contact-mode and tapping-mode AFM were not significantly different (Tables 1 and 2). Apparently, the aspect ratio of the AFM tip did not significantly affect the measurement of the length of imogolite threads. This is because the length of imogolite threads is much greater than the errors caused by the aspect ratio effect. The diameter of synthetic imogolite threads (after 5-s ultrasonification) observed by tapping-mode AFM ranged from 40.2 to 95.5 nm with an average of 62.2 nm (Table 2), which was larger than that of naturally occurring imogolite threads (10 –50 nm in diameter) reported in the literature (Yoshinaga and Aomine, 1962; Tazaki, 1979). Imogolite threads are referred to as the imogolite that contains little, if any, mesoporosity associated with the random twisting of tube bundles (Hoshino et al., 1996; Pohl et al., 1996). Therefore, the diameter of imogolite threads would greatly depend on the number of imogolite tubes which twist together. This may explain the great variation of the diameter of imogolite threads.
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Fig. 7. The tapping-mode AFM three-dimensional height images of the synthetic imogolite after different ultrasonification times: (a) 0 s, (b) 5 s, (c) 15 s, (d) 30 s, (e) 1 min, and (f) 2 min. The suspension after ultrasonification was air-dried on the sample holder for 1 month. The image was captured after scanning 2 times. The same scale is used from (a) to (f).
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Table 2 The diameter and length of imogolite threads (air-dried for 30 days) after different ultrasonification times obtained by tapping-mode AFM Time
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Average
Range
c
N.A. 62.2 ad 61.8 a 60.2 a 60.6 a 61.5 a
40.2 – 95.5 39.6 – 97.4 40.1 – 96.8 39.0 – 94.1 39.0 – 90.0
N.A. 1.50 e 1.22 d 0.95 c 0.67 b 0.57 a
Table 3 The diameter and length of imogolite threads (air-dried for 1 day) after different ultrasonification times obtained by tapping-mode AFM Time
Range 0.65 – 2.15 0.63 – 2.00 0.51 – 1.67 0.40 – 1.05 0.28 – 1.00
0s 5s 15 s 30 s 1 min 2 min
Diameter (nm)a
Length (Am)b
Average
Range
Average
Range
N.A. 48.7 – 98.0 40.0 – 98.5 41.6 – 96.4 45.4 – 94.1 38.7 – 85.2
1.47 1.25 0.92 0.68 0.55
0.67 – 2.45 0.63 – 2.32 0.49 – 1.62 0.39 – 1.05 0.27 – 0.96
c
N.A. 67.6 ad 65.1 a 68.8 a 66.2 a 63.5 a
e d c b a
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.06 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
a The diameter of imogolite threads was measured using Section analysis of AFM (Digital Instrument, 1993). The standard error of the measurements of the diameter was less than F 0.1 nm. b The length of imogolite threads was determined directly by drawing a line along the thread and the standard error of the measurements was less than F 0.07 Am. c Not applicable. d Values within each column followed by the same letter are not significantly different at the 0.05 level by using Fisher – LSD test.
The imogolite structure is weak and fragile; its structure is easily damaged by dry grinding and its thermal stability is low (Yoshinaga, 1968; Henmi and Yoshinaga, 1981). The average diameter of imogolite threads, which were subjected to 1-day air-dried period (Table 3), was consistently and significantly ( p = 1.15 10 2) larger than that of imogolite threads which were subjected to 1-month air-dried period (Table 2). Further, the diameter of imogolite, which was subjected to 5-s ultrasonification and then ovendried at 105 jC for 24 h and observed by tappingmode AFM, ranged from 38.4 to 92.6 nm with an
average of 59.2 nm. The 24-h oven drying resulted in a significantly ( p = 3.67 10 2) smaller diameter of imogolite compared with 1-month air drying. Ovendrying and length of air-drying affected the degree of dehydration of a sample and modified the structure of the sample. Therefore, ultra-high vacuum conditions of the TEM should result in reduced diameter of imogolite threads. Sorption and catalysis occur on the surface of soil minerals in natural environments. The reactivity of soil minerals is governed by the surface reactive sites which in turn are influenced by the specific surface, surface
Fig. 8. Schematic diagram depicting the difference in the observed diameters of imogolite threads using (a) a tapping-mode AFM tip with a high aspect ratio and (b) a contact-mode AFM tip with a low aspect ratio. The diameter of the circle represents the intrinsic diameter of imogolite threads. Steric hindrance caused by the limitation of the sidewall of the tip with a low aspect ratio to contact the surface of the sample is illustrated.
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characteristics, and site density. These surface properties of thread-like imogolite are influenced by its diameter and length and related other surface features. Atomic force microscopy is in situ analysis. Research using AFM is conducted in air and at ambient temperature, and experimental conditions are, thus, appropriate for environmental studies. Therefore, tapping-mode AFM is a powerful and reliable technique to investigate the surface features of imogolite and its related surface chemistry pertaining to environmental quality.
4. Conclusions The in situ surface features of imogolite were revealed for the first time by using the AFM under ambient conditions. The morphological features of synthetic imogolite can be more clearly observed by using the tapping-mode AFM rather than the contactmode AFM. The tapping-mode AFM also required a shorter sample drying period. The imogolite appeared in the AFM three-dimensional images as threads, which were curved, as found in the TEM studies. The data obtained in the present study show that adequate ultrasonification is essential to disperse imogolite particles. The length of imogolite threads observed greatly depended on the ultrasonification. Over-ultrasonification can cause the breakdown of imogolite threads. The diameter values of imogolite threads subjected to 5 s and 150 W of ultrasonification, air-dried for 30 days, and then determined by tapping-mode AFM in the present study are 40.2– 95.5 nm (standard error < F0.1 nm for each thread). Sufficient air drying of imogolite on the sample holder prior to contact-AFM scanning is critical to avoid the etching of imogolite surface by the frictional forces and adhesive forces between the tip and the sample. Use of the contact-mode AFM tip resulted in enlarging the observed diameter of imogolite threads due to tip-sample convolution.
Acknowledgements This study was supported by the Ministry of Education of Japan and Discovery Grant GP2383Huang of the Natural Sciences and Engineering Research Council of Canada.
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