One-Dimensional Nanowires of Pseudoboehmite (Aluminum Oxyhydroxide Γ-Alooh)

One-dimensional nanowires of pseudoboehmite (aluminum oxyhydroxide γ-AlOOH)

Sumio Iijimaa,1, Takashi Yumurab, and Zheng Liuc,d,1

aGraduate School of Science and Technology, Meijo University, Nagoya 468-8502, Japan; bDepartment of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan; cInorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan; and dNanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan

Contributed by Sumio Iijima, August 30, 2016 (sent for review June 14, 2016; reviewed by Darren W. Johnson and Alexandra Navrotsky) We report the discovery of a 1D crystalline structure of aluminum clarified even by using the modern crystallography methods such oxyhydroxide. It was found in a commercial product of fibrous as radial distribution function analyses with synchrotron X-ray pseudoboehmite (PB), γ-AlOOH, synthesized easily with low cost. diffraction (16, 17) and NMR (18). To solve such a long-standing The thinnest fiber found was a ribbon-like structure of only two problem with fibrous PB, we made full use of conventional trans- layers of an Al–O octahedral double sheet having a submicrometer mission electron microscopy (TEM) methods, high-resolution TEM length along its c axis and 0.68-nm thickness along its b axis. This (HRTEM) imaging, and electron diffraction (ED), and successfully thickness is only slightly larger than half of the lattice parameter revealed the detailed morphologies and crystal structures, including of the b-axis unit cell of the boehmite crystal (b/2 = 0.61 nm). determination of the fiber axis, of fibrous PB. Moreover, interlayer splittings having an average width of 1 nm inside the fibrous PB are found. These wider interlayer spaces may Results and Discussion have intercalation of water, which is suggested by density func- Structure of Fibrous PB. Fig. 1A shows TEM images of fibrous PB tional theory (DFT) calculation. The fibers appear to grow as al- forming a lacy network of randomly dispersed nanometer-sized most isolated individual filaments in aqueous Al-hydroxide sols fibers prepared by drying a diluted sol of boehmite “F1000” and the growth direction of fibrous PB is always along its c axis. (KAWAKEN Fine Chemical Co., Ltd.). The Debye–Scherrer ED (DS-ED) ring patterns shown in Fig. S2 A and D demon- 1D material | nanowire | TEM | aluminum oxyhydroxide | pseudoboehmite strate that the crystal structure of PB is γ-AlOOH, as reported in the literature (17). However, the width of the DS-ED rings is ow-dimensional materials such as quantum dots (1), nanowires generally broadened with some relatively sharper rings apparent L(2, 3), and nanosheets (4) have attracted interest in academia and as well, and the intensities of these rings differ slightly from those industry. Carbon nanotubes (5), a typical 1D inorganic material, of the well-crystallized PB sample. The differences can be explained have been investigated widely, but new 1D inorganic materials pre- by the crystallite size of the fibrous PB F1000 sample. Such dif- pared by a simple and reliable method have rarely been reported. ferences in crystal size and morphology from the other two samples, The aluminum ore bauxite primarily comprises aluminum hy- provided by the KAWAKEN Fine Chemical Co., namely nanorod droxide possessing different crystalline forms. The thermal de- “10A” and platelet “Powder,” were confirmed as shown in Fig. S2 B hydration of aluminum hydroxide produces alumina Al O one and C, respectively. DS-ED patterns of the three samples (Fig. S2 2 3, D–F of the most important materials in modern industries. γ-Boehmite ) can be easily distinguished from each other in terms of the (γ-AlOOH) (6) is a typical aluminum oxyhydroxide with a crystal ring width and intensity. structure similar to that of lepidocrocite (γ-FeOOH) (7). These A highly magnified image of a region enclosed by rectangle 1 A metal oxyhydroxides, represented by γ-MO(OH), where M = Fe, in Fig. 1 clarifies the ribbon structure appearing as dark, Ni, Mn, Sc, Ti, etc., have attracted interest in fields from energy CHEMISTRY to medicine. In a nickel–hydrogen battery, the formation of Significance poorly crystalline γ-NiOOH on the cathode has been shown to cause the memory effect of the battery (8). Titanate nanosheets Pseudoboehmite (PB) is a poorly crystallized variant of boehm- have recently attracted attention in the electronics community ite, but its crystal structure and morphology have not been because of their semiconducting properties (9). Hydrogen gen- clearly understood. In particular, PB gel crystals are formed in an eration by aluminum–water reactions for onboard vehicular hy- aqueous solution, so the interface between the crystalline sur- drogen storage (10) and the radiolytic generation of hydrogen in face and water is of crucial importance for understanding its conjunction with metal corrosion in water in nuclear energy and properties and growth mechanism. Here we report the detailed waste systems (11) both involve aluminum hydroxides as reaction structures of fibrous PB crystals obtained from high-resolution products. Aluminum hydroxide has also been used as a vaccine transmission electron microscopy and electron diffraction pat- adjuvant (12) and its biochemical mode of action in a recent terns. Some of the fibers consist of only two layers of an Al–O medical technology is being investigated (13). octahedral double-sheet ribbon structure, which is really a 1D Boehmite is synthesized by aging noncrystalline aluminum hy- structure. The findings will broaden the understandings in clas- droxide sol in an aqueous solution at a specific pH, and it has the sical but important fields of colloid science and clay mineralogy. basic crystal structure of double sheet of a metal–oxygen octahe- Author contributions: S.I. designed research; S.I. and Z.L. performed research; Z.L. con- dron layer piled up through hydrogen bonding, as illustrated in tributed new reagents/analytic tools; S.I., T.Y., and Z.L. analyzed data; and S.I. wrote Fig. S1. Poorly crystallized boehmite, often called pseudoboehmite the paper. (PB), forms in various crystal morphologies, such as thin films, Reviewers: D.W.J., University of Oregon; and A.N., University of California, Davis. thin platelets, and nanometer-sized rods or fibers. Importantly, The authors declare no conflict of interest. the characteristics of alumina Al2O3 for industrial use prepared Freely available online through the PNAS open access option. by thermal dehydration are influenced by the morphology of the 1To whom correspondence may be addressed. Email: [email protected] or liu-z@aist. starting boehmite crystals. go.jp. Fibrous PB has a particularly poor crystalline structure (14, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 15), and its crystal structure and morphology have never been 1073/pnas.1614059113/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1614059113 PNAS | October 18, 2016 | vol. 113 | no. 42 | 11759–11764 Downloaded by guest on September 24, 2021 Fig. 1. TEM images of fibrous PB. (A) An electron micrograph shows a thin lacy film of fibrous boehmite samples F1000 from KAWAKEN Fine Chemical Co. (B) An enlarged image of a rectangular region 1 in A shows a ribbon-like crystalline structure. The contrast of the two dark lines suddenly disappeared and changed to a gray ribbon-like contrast, as shown by the arrow, indicating the twisting of the fibrous PB nanoribbon. (C) The dark lines in an enlarged image of the upper rectangle part in B are the (020) lattice image. (D) The corresponding Al–O hydroxides double sheet of crystal model of γ-AlOOH. (E) Another

ribbon-like image of a rectangular region 2 in A. The wider separation indicated by the arrowheads in B and E suggests some species like H2O are intercalated between the adjacent Al–O octahedral double sheet.

parallel straight lines that can be seen in the upper portion of the synthetic aluminum oxyhydroxide. The ribbon consists of a pair image (Fig. 1B), and a further enlarged image in Fig. 1C (for of Al–O octahedral double-sheet layers (imaged as two dark better visualization, this image has been rotated compared with parallel lines), as shown in Fig. 2A. By measuring the interlayer Fig. 1B). A model of γ-AlOOH projected on (100) (Fig. 1D) spacing between two layers of double sheet (i.e., the distances shows that the separation between the two layers of the Al–O between the two dark parallel lines in Fig. 2A), it is noted that the octahedron double sheet is 0.61 nm. We refer to one Al–O oc- average distance is about 0.68 nm, as shown in Fig. 2A,whichis tahedron double sheet as “one layer” in this study for simplicity. 12% wider than the normal (020) lattice spacing (b/2 = 0.607 nm) By comparing the upper four dark lines in Fig. 1C with those of the boehmite crystal. A similar expansion was observed on D from the model shown in Fig. 1 , it can be seen that these dark double layers of graphene (19). One plausible explanation is the lines correspond to the (020) lattice image of the layers that are surface effect in nanoscale crystalline systems as demonstrated by 0.607 nm apart. Another ribbon was collected in an area sur- density functional theory (DFT) calculation. Another possible rounded by rectangle 2 located close to rectangle 1, and the explanation is the intercalation of small molecules, such as H O, E 2 enlarged image is reproduced in Fig. 1 . This particular ribbon between the layers. splits into two pieces with one piece having two layers on the The long double-sheet nanoribbon is bent and twisted, which left side and three additional layers on the right side. The is highlighted by a dotted line in Fig. 2B. The winding ribbon contrast of the two dark lines suddenly disappears and changes B B proves its softness and flexibility. A rectangular area in Fig. 2 to a gray ribbon-like contrast, as shown by the arrow in Fig. 1 , exhibits a clear image of the double-sheet nanoribbon (Fig. 2A). indicating the twisting of the fibrous PB ribbon. Such a narrow Schematic models of the straight, bent, and twisted double-sheet ribbon is expected to be inherently quite soft and flexible, as nanoribbon are illustrated in Fig. 2C.Thethinnestfiberis will be discussed later. a freestanding ribbon structure consisting of only two layers of The anomalous white lines showing the splitting of the ribbons – indicated by the arrowheads in Figs. 1B and 1E can be found Al O octahedral double sheet terminated with (100) and (010) frequently in the present fibrous PB. The width of the white lines surfaces, extending in the [001] direction, as shown in Fig. S1. The atomic structure of the fibrous PB nanoribbon can be ob- (separation between the two black lines) is roughly 1.0 nm, up to D 64% wider than the normal (020) lattice spacing (0.61 nm). The served in HRTEM images at atomic resolution (Fig. 2 ). It re- wider layer spacing is likely associated with initiation of cleavage veals directly the arrangement of Al and O atoms (Al atoms show darker contrast in this image). Differences in the detailed on the (020) planes, suggesting that some species like H2O are intercalated between the adjacent Al–O octahedral double sheet. contrast of the two magnified HRTEM images indicated by two The water intercalation will be discussed further below. rectangles on the right side of Fig. 2D are caused by the bending and twisting along this nanoribbon. The corresponding bending Atomic Structure of Two-Layered Double Sheet of Fibrous PB. One of and twisting models are schematically illustrated in Fig. 2E. The the most important results in the present study is the finding of model in the upper part is rotated by 5° around both the b and c the thinnest fibrous PB ribbon with 1D crystalline structure in a axes relative to the model in the lower part. This HRTEM image

11760 | www.pnas.org/cgi/doi/10.1073/pnas.1614059113 Iijima et al. Downloaded by guest on September 24, 2021 Fig. 2. Two-layered double sheet of fibrous PB. (A) HRTEM images of nanoribbons consisting of two-layered Al–O hydroxide double sheet. The two layers are separated by 0.68 nm. (B) TEM image of a flexible and lengthy double-sheet nanoribbon, highlighted by a dotted line. (C) Schematic models of straight, bent, and twisted double-sheet nanoribbon. (D) HRTEM image with atomic resolution shows local distortion of a two-layered double sheet. The enlarged images of enclosed regions are shown in the right side inserted with the corresponding simulated images (labeled by ”S”) and Al and O atoms overlaid in the lower simulated image. (E) The corresponding models used for simulation in D. The model in the upper part is rotated by 5° along both the b and c axes relative to the model in the lower part.

with atomic resolution further reveals the local distortion of such bound with a monolayer of water molecules was considered. thin PB nanoribbons and proves its flexibility. Different interlayer spacings of PB nanoribbons with and without The mean thickness of the PB nanoribbon was measured from intercalation of a monolayer of water molecules can be found by large numbers of TEM images. A typical TEM image is shown in comparing Fig. 3A and Fig. 3B. The intercalation of only a single Fig. S3A. The arrows indicate occurrence of two-layered double- layer of water molecules strikingly expands the interlayer spacing sheet nanoribbons that happen to be imaged due to their edge on of PB nanoribbons from 0.66 to 0.90 nm. In addition, the in- condition. The histogram of fiber thickness shown in Fig. S3B is terlayer spacing is slightly affected by whether H-atom saturation centered at 1.8 nm, which corresponds to the width of 3–4 layers is involved or not: 0.90 nm (Fig. 3B) and 0.89 nm (Fig. 3C) for of the Al–O double sheet. The frequency of the discernible two- the presence and absence of H-atom saturation, respectively. layered double-sheet nanoribbons is estimated to be more than Within the inner space of the PB nanoribbon containing a

5%. It is noteworthy that, despite frequent appearances of the monolayer of water molecules, the H atoms in water are sepa- CHEMISTRY multilayered double sheets, a single-layered double sheet has never rated by 0.17 nm from the O atoms in the Al–O octahedral sheet been observed in the present fibrous PB. The lack of a single Al–O of the PB nanoribbon, and at the same time the O atoms in water octahedral double-sheet nanoribbon structure may be related to are separated by 0.17 nm from the H atoms terminating the the instability or growth of the double sheet itself in aqueous alu- O atoms in the Al–O octahedral sheet of the PB nanoribbon. minum hydroxides sols, whereas the two-layered nanoribbons are These interatomic separation ranges are suitable for attractive speculated to be nucleated more stably due to the interactions hydrogen-bonding interactions connecting water molecules and between their two layers. Further investigation is necessary. the Al–O octahedral sheet in the PB nanoribbon. The water molecules can act as adhesive agents for connecting Al–O oc- DFT Calculations. To obtain a better understanding of the struc- tahedral double-sheet layers of the PB nanoribbon as shown in tural features of the 1D PB structure observed by HRTEM, spin- Fig. 1 B and E. Similar adhesion effects of water molecules be- polarized DFT calculations were performed under periodic tween a γ-alumina surface and an epoxy resin were also found boundary conditions. Two types of bonding patterns of oxygen (20). The DFT results clearly indicate the influence of interca- atom termination were considered: the presence (Fig. 3 A and B) lated water molecules in enhancing interlayer spacings of PB and absence (Fig. 3C) of H-atom saturation. Fig. 3 displays op- nanoribbons, which provide a hint to understand the splitting timized structures for the PB nanoribbons, whose interlayer observed in the TEM results. The calculated value of the sepa- spacings, corresponding to half the lattice parameter of the b-axis ration, 0.9 nm, seems to support the observation on the large unit cell of the boehmite crystal (blue lines), are given. First, the separation of splitting ribbons (∼1.0 nm, indicated by white ar- interlayer spacing of PB nanoribbons without water molecules rowheads in Fig. 1 B and E) due to water intercalation. Although was calculated to be 0.66 nm (Fig. 3A), which is 8% larger than our calculation model containing three water molecules per unit the normal (020) spacing. Such an expansion fits very well with cell gives rise to an interlayer spacing of 0.9 nm in the b axis, the TEM observations (0.68 nm for two layers of the Al–O decreasing or increasing the number of water molecules within double sheet), demonstrating the interlayer spacing in nanoscale the Al–O octahedral double-sheet layers would diminish or ex- 2D crystalline structure is slightly wider than in the bulk pand interlayer spacing from 0.9 nm. Actually, the splitting width structure. Second, a model including the presence of water in observed in this study has a distribution from 0.83 to 1.5 nm with which the adjacent Al–O octahedral double-sheet layers are an average value of 1.0 nm. However, in this study the occurrence

Iijima et al. PNAS | October 18, 2016 | vol. 113 | no. 42 | 11761 Downloaded by guest on September 24, 2021 well-bundled ones. Two typical DS-ED patterns from these crystals are shown in Fig. 4 A and B. It is noted here that the 020 reflections were not observed in most of the DS-ED patterns. The absence of the 020 reflections was understood as a diffraction phenomenon for an extremely narrow ribbon structure of fibrous PB. Compared with the DS-ED ring pattern from randomly dispersed fibers (Fig. 4A), the pattern for the well-bundled fibers showed characteristic intensity maxima on each DS-ED ring (Fig. 4B). Regardless of which bundled fibers are observed, these maxima always appear at the same positions and can be indexed as labeled in Fig. 4B. The position of the intensity maxima of the 002 reflection always lies on the direction parallel to the bundle axis. This means that the individual fibrous PB crystals grow along the c axis, namely the [001] direction; 200, 002, and 202 reflections appear to belong to h0l reflections from a single crystal of γ-AlOOH, but 120 and 031 reflections do not. In addition, 120 reflections always appear in the same direction as the 200 reflection in the ED pattern, which should not be observed in a single crystal of γ-AlOOH. Such kinds of contradictions can be explained as long as the incident electron beam is perpendicular to the c axis of the fibrous PB crystal. This situation is illustrated in a cross-sectional view of a bundle of fibrous PB in Fig. 4C.This conclusion could explain many other experimental observations regarding the fiber morphology proposed here. An important conclusion is that the appearance of the intensity maxima in the DS-ED pattern (Fig. 4B) is a summation of diffraction intensities from every individual fiber in a bundle. Although a bulk boehmite crystal is an electrical insulator, 1D nanoribbon PB crystals could have different physical properties when intercalated with some ion species, as is known for functionalized carbon nanotubes (21, 22) and 2D materials (23). The electronic structure of intercalated 1D PB nanoribbon is of interest in theoretical and experimental investiga- tions, in terms of the nanoribbon size, layer distance between adjacent Al–O octahedral double sheet, and a sol state where the structure is immersed in an aqueous ionic solution. One of the differences between bulk boehmite and PB is that the latter contains more water. In general, the specific surface area of a crystal is increased with a decrease in crystal size. Therefore, the fibrous PB crystals, which have a larger specific surface area than the bulk boehmite crystals, could adsorb more water on their Fig. 3. Spin-polarized DFT optimized geometries for PB nanoribbons in the surfaces (24). The present study could specify the site for the water absence (A)andpresence(B) of a monolayer of water molecules intercalated. that goes preferentially into the interlayer spaces of the fibrous PB Some terminated oxygen atoms are bound with H atoms. Interspace between nanoribbons having splitting or cleavage on the (010) plane. A Al–O octahedral double sheet with intercalated water molecules, surrounded similar interlayer structure for water intercalation will be taking by purple lines, was magnified (B). (C) Terminated oxygen atoms on PB nano- place in the bundle of the fibrous PB nanoribbons. ribbons are not bound with H atoms. Interspace between Al–O octahedral Synthesis of several different types of morphologies of boehmite double sheet with intercalated water molecules, surrounded by purple lines, crystals has been established in the sol–gel reaction of Al-hydroxide was magnified. Note that the separation of a water H atom to an end O atom on a PB nanoribbon (0.19 nm) is slightly longer than those to a normal O atom solution by controlling the pH, temperature, and reaction time. In on a PB nanoribbon (0.16 and 0.17 nm). The unit of interlayer spacing between contrast with the rod PB and platelet PB, the fibrous PB crystals adjacent Al–O octahedral double sheet, corresponding to the half-lattice pa- extend preferentially in the c-axis direction. Such an extremely rameter of b-axis unit cell of the boehmite crystal (blue lines), and the length of anisotropic linear growth of the fibrous PB-like carbon nanotubes hydrogen bonds (dotted lines in the inserted images), is given in nanometers. (5) may be caused by a reactive site on the tip of the nanoribbon, where Al-hydroxide ionic species could be attracted. The amount of surface negative charge was counted to be slightly higher on the of such kinds of splitting is not so frequent that the intercalated (001) surface than on the (100) and (010) surfaces. Therefore, water content does not obviously affect the total amount of positive Al-hydroxide ion clusters will be continuously attracted absorbed water in the present PB. Moreover, the slight expansion more on the (001) plane that accelerates the growth of c-axis di- of the interlayer spacing from 0.61 to 0.68 nm could be associated rection. The TEM image shown in Fig. 5A andanamplifiedarea with intercalation of small amount of water. The conclusion pre- (white rectangle area in Fig. 5A) showing one of the terminated sented here suggests that the thicker PB nanoribbons showing the fibers (Fig. 5B) will serve to clarify more specifically the growth normal interlayer spacing of boehmite (b/2 = 0.61 nm) contain mechanism of the fibrous PB. The tip shapes of the terminated almost no water in their interlayer spaces. fibers (indicated by an arrow in Fig. 5B) show a geometrical fea- ture and its possible model is illustrated in Fig. 5C. The knife-edge Growth Direction of Fibrous PB. To determine the fiber axial di- shape of the tip appears to fit well with the (101) crystal surface rection of the fibrous PB, we systematically examined the DS-ED and therefore this surface is considered to be a growth front of the patterns of the fibrous PB crystals that tend to agglomerate to fiber. By comparing the (101) surface (Fig. 5C) with the (001) different degrees from completely randomly dispersed fibers to surface (Fig. 5D), it is known that the total amount of charge of

11762 | www.pnas.org/cgi/doi/10.1073/pnas.1614059113 Iijima et al. Downloaded by guest on September 24, 2021 Fig. 4. Bundles of fibrous PB. (A) DS-ED pattern recorded from a randomly dispersed thin film of fibrous PB crystals. (B) DS-ED pattern from bundled fibrous PB crystals, where the bundle axis is shown by the double-headed arrow. Both patterns are indexed as those from a bulk crystal of γ-AlOOH. The (002) reflections, however, are relatively strong in both DS-ED patterns in A and B, suggesting the long ribbon-like structure of PB. Broadening of the (200) reflection is caused by an extremely narrow width of the PB ribbon. The intensity maxima, which look like a single crystal, can be explained by a model in which individual fibers form in a bundle, and align parallel to the [001] direction (i.e., c axis) but randomly rotated around this axis. (C) A cross-sectional view of a bundle is illustrated. Some of the fibers aligned in low-index zone axes are indicated by the index numbers shown in C.

the wedge tip ending with the (101) surface is slightly larger than minimized as much as possible [the beam density during the observations that of the flat (001) surface on the assumption that the alu- ranged from 50 to 150 electrons/(nm2·s)]. A sequence of HRTEM images (up to − − minum ions are fully saturated with O 2 or OH 1 when counting 20 frames) was recorded with an exposure time of 0.5–1sforeachframe,and the amount of the local net charges near the tips. It is reason- after drift compensation, some frames can be superimposed to increase the signal-to-noise ratio for better visualization. able therefore that positively charged Al–O clusters can be DFT calculations were performed by using the projected augmented wave attracted and deposited onto the tip surface, and fibrous PB method, implemented in the Vienna ab initio Simulation Package (VASP 4.6) grows along its c axis. The growth should be influenced by the (30). In the VASP calculations, the Perdew, Burke, and Ernzerhof (PBE) stability of the positively charged multivalent octahedral clusters n functional was used to describe electronic exchange and correlation (31). [Al–O] ,(n = 0 ∼ +3) (25–27). The absence of the monolayered Previous studies confirmed that the PBE functional is suitable for describing Al–O octahedral double sheet is demonstrated by DFT energy hydrogen-bond interactions (32–35), together with metal–oxygen bond in- calculation shown in Fig. S4, in which two-layered Al–O octahedral teractions. All calculations for optimizing PB nanoribbon structures were double sheet is 0.8 eV (per unit cell) and 1.5 eV (per unit cell) more spin polarized. Kinetic energy cutoff of the plane-wave basis is 400 eV. For energetically favorable than monolayered Al–Ooctahedraldouble the calculations of two Al–O octahedral double-sheet layers with a mono- sheet without and with water intercalation, respectively. This may layer of water molecules, we used a square supercell, containing 77 and 68 be a tie into the Al Keggin cluster intermediates and their assembly atoms for the H-atom saturated and unsaturated models, respectively. We allowed full geometry relaxation of PB nanoribbons directed in the (25). It could be interesting to explore further in the future. c direction, where the lattice parameters in the a- and b directions were “ ” CHEMISTRY We conclude that a genuine 1D nanoribbon of fibrous boehm- fixed at 3 nm to avoid significant interactions of the two Al–O octahedral ite has been found in commercial products of PB. Although 1D nanoribbons of two-layered Al–O octahedral double sheet are a minor structure in the present product (more than 5%), it could be synthesized to be a major structure by fully controlling growth processes. It is challenging to form the nanoribbons in other similar metal oxyhydroxides γ-MO(OH). Moreover, the growth mechanism of fibrous PB will broaden deeper understandings in classical but important fields such as nucleation and growth in sol–gel processes, polymorphic transformations, and related aluminum aqueous clusters. Methods Three PB samples examined in the present study are F1000: fibers of 0.7–3-nm thickness, 10A: nanometer-sized rod-like crystallites of 1.2–6-nm thickness and 4–5-nm length, and Powder: platelet crystallites with diameters of 20– 30-nm and 5–10-nm thickness. All are commercial products (KAWAKEN Fine Chemical Co.) prepared by hydrolysis and peptization of aluminum iso- propoxide and their morphologies are controlled by reaction processes. The F1000 is synthesized at the reaction temperature of 150 °C and aged for 6 h with stabilizer of acetic acid (pH 3.4–4.2) (28). The first two samples are aqueous hydroxide sols, and the last one is a dried powder. For the TEM ob- Fig. 5. Tip shapes of the fibrous boehmite – F1000. (A) An electron micro- servation, the samples were diluted in water and collected on a TEM grid scope image showing some terminated fibers enclosed by a rectangle; their covered with a holey carbon film. The TEM specimens were examined by a JEM enlarged image is reproduced in B.(C) A possible structure of the tip of the 2010F electron microscope with a Corrected Electron Optical Systems GmbH fibers terminated with the (101) facet of the boehmite crystal. The tip region (CEOS) spherical aberration corrector operated at 120 keV. Many clay minerals has a slight excess positive charge comparing to a tip terminated with the that contain hydroxyl groups or water are unstable under electron beam ir- (001) facet shown in D. The excess charge would promote an anisotropic radiation (29). In this study, the electron beam damage to the specimen was crystal growth of the fibrous PB.

Iijima et al. PNAS | October 18, 2016 | vol. 113 | no. 42 | 11763 Downloaded by guest on September 24, 2021 double-sheet layers with those located on the neighboring unit cells. A ACKNOWLEDGMENTS. We thank Dr. F. Mizukami (National Institute of Γ-centered 1 × 1 × 2 k-point mesh was used for the geometry optimization. Advanced Industrial Science and Technology) and Dr. N. Nagai (Kawaken The optimization processes were stopped when the forces on all of the Fine Chemicals Co.) for discussions. Z.L. acknowledges Japan Society for the mobile atoms were smaller than 0.01 eV/0.1 nm. Promotion of Science KAKENHI Grant JP 25107003.

1. Brus LB (1984) Electron–electron and electron‐hole interactions in small semi- 20. Semoto T, Tsuji Y, Yoshizawa K (2012) Molecular understanding of the adhesive force conductor crystallites: The size dependence of the lowest excited electronic state. between a metal oxide surface and an epoxy resin: Effect of surface water. Bull Chem J Chem Phys 80(9):4403–4409. Soc Jpn 85(6):672–678.

2. Vrbanic D, et al. (2004) Air-stable monodispersed Mo6S3I6 nanowires. Nanotechnology 21. Zhu WH, et al. (2003) Fluorescent chromophore functionalized single-wall carbon 15(5):635–638. nanotubes with minimal alteration to their characteristic one-dimensional electronic 3. Zhang Z, et al. (2015) Ultrathin inorganic molecular nanowire based on polyoxometalates. states. J Mater Chem 13(9):2196–2201. Nat Commun 6:7731. 22. Liu Z, et al. (2009) Self-assembled double ladder structure formed inside carbon

4. Novoselov KS, et al. (2004) Electric field effect in atomically thin carbon films. Science nanotubes by encapsulation of H8Si8O12. ACS Nano 3(5):1160–1166. 306(5696):666–669. 23. Liu M, et al. (2015) An anisotropic hydrogel with electrostatic repulsion between 5. Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354:56–58. cofacially aligned nanosheets. Nature 517(7532):68–72. 6. Christoph GC, Corbato CE, Hofmann DA, Tettenhorst RT (1979) The crystal structure of 24. Majzlan J, Navrostsky A, Casey WH (2000) Surface enthalpy of boehmite. Clays Clay boehmite. Clays Clay Miner 27(2):81–86. Miner 48(6):699–707. 7. Ewing FJ (1935) The crystal structure of lepidocrocite. J Chem Phys 3(7):420–424. 25. Casey WH (2006) Large aqueous aluminum hydroxide molecules. Chem Rev 106(1):

8. Tkalych AJ, Yu K, Carter EA (2015) Structural and electronic features of β-Ni(OH)2 and 1–16. β-NiOOH from first principles. J Phys Chem C 119(43):24315–24322. 26. Ohlin CA, Villa EM, Rustad JR, Casey WH (2010) Dissolution of insulating oxide ma- 9. Sasaki T, Ebina Y, Kitami Y, Watanabe M (2001) Two-dimensional diffraction of terials at the molecular scale. Nat Mater 9(1):11–19.

molecular nanosheet crystallites of titanium oxide. J Phys Chem B 105(26):6116–6121. 27. Armstrong CR, Casey WH, Navrotsky A (2011) Energetics of Al13 Keggin cluster 10. Petrovic J, Thomas G (2008) Reaction of Aluminum with Water to Produce Hydrogen compounds. Proc Natl Acad Sci USA 108(36):14775–14779.

(US Department of Energy, Washington, DC), Version 1.0, pp 1–26. 28. Nagai N, Mizukami F (2011) Properties of boehmite and Al2O3 thin films prepared 11. Westbrook M, Sindelar R, Fisher D (2015) Radiolytic hydrogen generation from alu- from boehmite nanofibers. J Mater Chem 21(38):14884–14889. minum oxyhydroxide solids: Theory and experiment. J Radioanal Nucl Chem 303(1): 29. Iijima S, Buseck PR (1978) Experimental study of disordered mica structures by high- 81–86. resolution electron microscopy. Acta Crystallogr A 34(5):709–719. 12. Lindblad EB (2004) Aluminium compounds for use in vaccines. Immunol Cell Biol 82(5): 30. Kresse G, Furthmüller J (1996) Efficient iterative schemes for ab initio total-energy 497–505. calculations using a plane-wave basis set. Phys Rev B Condens Matter 54(16): 13. Marichal T, et al. (2011) DNA released from dying host cells mediates aluminum ad- 11169–11186. juvant activity. Nat Med 17(8):996–1002. 31. Perdew JP, Burke K, Ernzerhof M (1996) Generalized gradient approximation made 14. Bugosh J (1961) Colloidal alumina-the chemistry and morphology of colloidal boehmite. simple. Phys Rev Lett 77(18):3865–3868. JPhysChem65(10):1789–1793. 32. Yumura T, Yamasaki A (2014) Roles of water molecules in trapping carbon dioxide 15. Sousa Santos H, Sousa Santos P (1992) Pseudomorphic formation of aluminas from molecules inside the interlayer space of graphene oxides. Phys Chem Chem Phys fibrillar pseudoboehmite. Mater Lett 13(4-5):175–179. 16(20):9656–9666. 16. Brühne S, Gottlieb S, Assmus W, Alig E, Schmidt MU (2008) Atomic structure analysis 33. Ireta J, Neugebauer J, Scheffler M (2004) On the accuracy of DFT for describing hy- of nanocrystalline boehmite AlO(OH). Cryst Growth Des 8(2):489–493. drogen bonds: Dependence on the bond directionality. J Phys Chem A 108(26): 17. Tettenhorst R, Fofmann DA (1980) Crystal chemistry of boehmite. Clays Clay Miner 5692–5698. 28(5):373–380. 34. Zhao Y, Truhlar DG (2005) Benchmark databases for nonbonded interactions and 18. Franceticˇ V, Bukovec P (2008) Peptization and Al-Keggin species in alumina sol. Acta their use to test density functional theory. J Chem Theory Comput 1(3):415–432. Chim Slov 55(4):904–908. 35. Rao L, Ke H, Fu G, Xu X, Yan Y (2009) Performance of several density functional 19. Iijima S (1978-79) Coherent imaging of finitely-sized objects and its application to theory methods on describing hydrogen-bond interactions. J Chem Theory Comput graphitic carbon. Chem Scr 14:117–123. 5(1):86–96.

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