Japanese Journal of Applied Physics
Vol. 44, No. 7B, 2005, pp. 5397–5399#2005 The Japan Society of Applied Physics

Atomic Force Microscopy on Imogolite, Aluminosilicate Nanotube,Adsorbed on Au(111) Surface
Yoshihisa O HRAI, Tomoaki GOZU, Shoji Y OSHIDA, Osamu T AKEUCHI,
Sumio I IJIMA1 and Hidemi S HIGEKAWA
Institute of Applied Physics, University of Tsukuba, 21st COE, CREST-JST, Tsukuba, Ibaraki 305-8573, Japan
1Faculty of Science and Technology, Meijo University 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan(Received February 16, 2005; accepted April 15, 2005; published July 26, 2005)

Natural imogolite and its adsorption structure were studied on a Au (111) substrate by tapping- and non contact (NC)-mode
atomic force microscopy (AFM). When the length of the imogolite tubes was over 2–3 micrometers, a tangled structure wasobserved. In contrast, when the length was shortened to 1 mm, a layered structure consisting of well-ordered imogolite tubes
was formed. When the length was shortened further, imogolite tubes dispersed on the substrate. These results indicate that theadsorption structure is inﬂuenced by the interaction between imogolite tubes depending on their length, suggesting thecontrollability of the structures of this material. The structure of a single imogolite tube itself is also discussed.
[DOI: 10.1143/JJAP.44.5397]
KEYWORDS: Nanotube, AFM, Imogolite, Nanomaterial, SAM, Noncontact AFM

1.

Introduction

Imogolite is an inorganic aluminosilicate nanotube discovered in volcanic ash soil as a natural material.1) The innerand outer walls of the imogolite tube are formed by
gibbsitelike sheets with SiOH and AlOH groups, respectively (Fig. 1). This material has attracted considerable
attention because of the functions derived from its small
tubular structure, and methods for its chemical synthesis
have been developed. According to previous studies,2) theexperimentally obtained diameters of natural and synthesized imogolite tubes are 2.1–2.3 nm and 2.5–2.8 nm,
respectively. The diameter obtained by simulations is2.62–2.89 nm, which is close to the value of the synthetic
tubes. The reason for the diﬀerence in natural and synthetic
diameters has not yet been clariﬁed.
Since imogolite has a hollow tublar structure that has ahigh surface-to-volume ratio, it exerts signiﬁcant eﬀects on
ion exchange and water adsorption in soil.3) For theapplication of these functions, imogolite tubes have beenstudied as a heat exchange material, a fuel storage medium
and an eﬀective desiccant. The synthesis of new nanomaterials by changing the hydroxyl groups of imogolite toother functional groups has been proposed.4) For suchapplications, analysis of the adsorption structure of imogolite tubes on a substrate is of great importance as is theclariﬁcation of the structures themselves.

Imogolite tubes have been studied thus far by atomic force
microscopy (AFM), transmission electron microscopy(TEM) and secondary electron microscopy (SEM).1,3,5–7) In
the electron microscopy studies,5,6) imogolite tubes werehung on a grid mesh, and the analysis of the structure of
imogolite itself was the ﬁnal goal. In the AFM studies,
imogolite tubes were observed on a glass substrate studies.
However, only random adsorption structures were observed,
and any well-ordered structures of imogolite tubes were notfound on the substrate. The diﬀerence in the molecularlength did not aﬀect the adsorption structure. The diameterof the observed single imogolite tube, 20–30 nm, could not
be discussed either, because the apparent size of small
structures is inﬂuenced by the curvature radius of the AFM
cantilever, and it is diﬃcult to determine the precise size byAFM.5–7)
In this study, imogolite tubes adsorbed on a Au(111)substrate were observed by tapping-AFM in air and bynoncontact-AFM (NC-AFM) in ultrahigh vacuum. It wasrevealed that the adsorption structure depends on the tubelength. When the molecular length was over 2–3 micrometers, tangled imogolite tubes were observed, while alayered structure consisting of well-ordered imogolite tubes
was formed when the length was shortened to 1 mm. Whenthe length was shortened further, imogolite tubes dispersedon the substrate. These results suggest that the adsorption
structure of the imogolite tubes can be controlled bychanging the interaction among tubes or between tubes
and the substrate. The structure of a single imoglite tube wasstudied by NC-AFM. Adsorption of particulate solids onto
the imogolite outer surface was also conﬁrmed.
2.

Fig. 1. Schematic image of imogolite. Larger circles of atoms represent
front side ones.

Experimental

Natural imogolite tubes were dissolved in distilled waterand dispersed by ultrasoniﬁcation. The aqueous solution wasthen dropped on a Au(111) substrate deposited on a clean
mica surface and dried for one day in air at room
temperature. In this study, tapping AFM and NC-AFM
measurements were carried out at room temperature in airand under ultra high vacuum conditions, respectively.

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http://dora.ims.tsukuba.ac.jp
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Jpn. J. Appl. Phys., Vol. 44, No. 7B (2005)

(a)

(c)

(b)

(d)

Fig. 2. Tapping-AFM images of imogolite tubes on Au(111) (1000 
1000 nm2 ) substrate: (a) outside, (b) at edge of, and (d) around center areaof thin layer formed by droplet; (c) image obtained between (b) and (d).

3.

Y. OHRAI et al.

(a) 1500×1500(nm 2 )(d) 500×500(nm 2 )(b) 1300×1300(nm2 )(e) 400×400(nm 2 )(c) 2000×2000(nm2 )(f) 600×600(nm2 )Results and Discussion

3.1 Adsorption structure of imogolite tubes
On a sample surface prepared by the procedure described,
a half-transparent thin layer corresponding to the size of theoriginal droplet of the imogolite solution remained. Figure 2
shows a tapping-AFM image of this surface. A clean
Au(111) surface structure was observed outside the thinlayer as shown in Fig. 2(a). In contrast, the surface was very
rough at the central area of the thin layer, and threedimensional islands of dense deposit were observed[Fig. 2(d)]. From the center to the edge of the thin layer,
the roughness of the surface decreased and thread-likestructures were observed [Fig. 2(c)]. Around the edge of thethin layer, the density of the deposit decreased and eachthread on the surface was clearly observed [Fig. 2(b)]. Thesethreads were considered to be the imogolite tubes, thestructure of which is discussed later.
In order to investigate the adsorption structure ofimogolite tubes, we observed the sample surface aroundthe edge of the thin layer by tapping AFM. The length of theimogolite tubes was changed by controlling the duration ofultrasoniﬁcation carried out before adsorption.7) For theimogolite tubes of 1 mm length, the formation of a wellordered layer structure was observed for the ﬁrst time.
With a short ultrasoniﬁcation time, tangled imogolite
tubes were observed as shown in Fig. 3(a). Thick bundles of
threadlike structures branch into thinner bundles as shown inthe magniﬁed image [Fig. 3(d)]. In the area of the thickbundles, threads intertwine with each other. It was also
observed that some bundled threads follow the grain
structures on the substrate. These results indicate that
imogolite is a soft material in contrast to the carbon
nanotube, suggesting the possibility of ﬂexible structuralcontrol of this material on the nanoscale. Since threads werebundled, the length of each molecule could not bedetermined; however, the length of the observed bundles
was over 2–3 micrometers.

Fig. 3. Tapping-AFM images of characteristic adsorption structures
around edge of thin layer formed by droplet: (a) tangled molecular, (b)well-ordered layer, and (c) dispersed structures. (d), (e) and (f) aremagniﬁed images of (a), (b) and (c), respectively.

When the imogolite tubes were shortened to 1 mm withfurther ultrasoniﬁcation, a ﬂat layered structure consisting of
domains was observed [Fig. 3(b)]. As shown in themagniﬁed image [Fig. 3(e)], well-ordered imogolite tubes
were observed in each domain. These threads are equallyspaced and have the same width as that of the thin branches
observed in Fig. 3(a), and no further branching wasobserved. Thus, we concluded that the threads in Fig. 3(e)are single imogolite tubes. The placement of these domains
was independent of the steps, grain boundaries, and crystal
orientation of the Au(111) substrate surface. Even when thelayers overlapped, threads were well-ordered in each layer.
These results suggest the existence of a strong interaction
among threads in a layer.
For the molecules of shorter length [Fig. 3(c)], a ﬂatstructure similar to that in Fig. 3(b) was observed. However,contrary to the well-ordered structure in Fig. 3(b), short
imogolite tubes 0:1 mm long dispersed, as shown in themagniﬁed image [Fig. 3(f)]. In some places, longer threads
exhibited the local formation of an ordered layer structure.
These results indicate that the adsorption structure isstrongly inﬂuenced by the length of imogolite tubes, namely,
the change in the tube-tube interaction. Since the substrate
structure has no eﬀect on the adsorption site and orientationof the short imogolite tubes, the interaction between

Jpn. J. Appl. Phys., Vol. 44, No. 7B (2005)Y. OHRAI et al.

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(a)

(b)

Height (nm)Height (nm)

(c)

1.5
1.0
0.5

6.0
5.0
4.0050100150200250

Length (nm)0.001020304050

Length (nm)Fig. 5. NC-AFM images of single imogolite tube and its line proﬁle:(a) height-image, (b) diﬀerential images, and (c) cross section along linein (a).

Fig. 4. Magniﬁed tapping-AFM image of well-ordered layer structure andits line proﬁle.

imogolite tubes and the substrate is weak in this case. It isnoteworthy that the structural change shown in this study
was not observed in the case of the glass substrate.7)
3.2 Structure of single imogolite tube
The diameter of a single thread observed in Figs. 2 and 3
was 20–30 nm, which is signiﬁcantly larger than the value
predicted by the theory, 2.1–2.3 nm. Although the observedheight by tapping AFM was 2–3 nm, the actual diametercannot be determined from this value alone. In this section,
we consider this point in detail.
In the AFM experiment, objects of apparently larger width
are generally observed, caused by the inﬂuence of thecurvature radius of the AFM cantilever. As the size of theobject becomes 2–3 nm, as in this study, this eﬀect becomes
particularly prominent. This is one reason why the molecular
structure was not discussed in previous AFM studies.
In order to determine the diameter, we investigated thelayered structure shown in Fig. 3(b). In this ﬁgure, thethreads in the domain are equally spaced and well-ordered.
Since an attractive interaction between the tubes causes thisstructure, these threads are supposed to contact each other.
Therefore, the diameter of the thread can be obtained bymeasuring the interval between the ordered threads. Figure 4
shows a magniﬁed image of the ordered threads and its crosssection along the arrow in the image. The averaged interval
between the threads was 10–15 nm, which is 5–8 times
larger than the theoretical value of the imogolite diameter. In
order to see the details, we carried out NC-AFM on thismaterial.
An NC-AFM image of a single thread and its cross section
are shown in Figs. 5(a) and 5(c), respectively. The observed
surface is very rough (2–3 nm) as shown in the cross section,
and a spiral pattern can be seen. In order to emphasize thelocal corrugation, the diﬀerential image of Fig. 5(a) isshown in Fig. 5(b). The pitch of the spiral in Fig. 5(b) isapproximately 5 nm, which is much larger than the periodicity expected from the atomic structure of the imogolite
tube. It is also notable that granular adsorbates with adiameter of 5 nm exist near the thread as shown inFig. 5(b). The adsorbates were also observed in other areas
on the substrate.
From these results, the imogolite tube may be covered by
the granular adsorbate, resulting in the observed large
diameter. The material may be a fragment of ﬂattened
imogolite tubes on the substrate due to their softness, whichis under examination.
Since the intermolecular interaction between imogolite
tubes should be inﬂuenced by the adsorbates on themolecular surface, variation in the adsorbate may be anotherpossibility to control the ordering of the imogolite tubes.
4.

Conclusions

Natural imoglite, one of the smallest inorganic tubes, wasadsorbed on a Au(111) substrate and observed by AFM. It
was revealed that the adsorption structure depends on thetube length. For tubes with a length of 1 mm, a well-orderedstructure was observed for the ﬁrst time. The diameter of theobserved imogolite was 10–15 nm, which is 3–6 times larger
than the theoretically predicted value. Adsorption of somegranular substance on the outer surface of imogolite,
resulting in an observed larger diemeter, was also conﬁrmed.
Details of the adsorbate are under investigation.
Acknowledgements
This work was supported by a Grant-in Aid for ScientiﬁcResearch from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Ms. Rie Yamashita, in our group at University of Tsukuba, for her help inpreparing this paper.
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