ARTICLE pubs.acs.org/JPCC
Complete Structural Characterization of Ni3Si2O5(OH)4 Nanotubes: Theoretical and Experimental Comparison F. Alvarez-Ramírez,* J. A. Toledo-Antonio, C. Angeles-Chavez, J. H. Guerrero-Abreo, and E. Lopez-Salinas Programa de Ingeniería Molecular, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, 07730, Mexico, Distrito Federal, Mexico ABSTRACT: This work reports a complete structural characterization of hydrothermal synthesized tubular Ni3Si2O5(OH)4 correlating experimental X-ray diffraction (XRD) and transmission electron microscopy (TEM) results with theoretical nanotubular model characteristics. Nanotubes of single and multiple layers were evidenced by TEM. The d-spacing of the planes displayed in the bidimensional projection of the nanotube was measured in the high-resolution TEM image. An atomic model of the nanotube was constructed to reproduce experimental XRD and TEM data. Parameter lattice and reflection planes were obtained with theoretical calculus. Finally, it was calculated a high-resolution TEM image from atomic model. Both, experimental and calculated highresolution images show an excellent match.
1. INTRODUCTION The discovery of carbon nanotubes1,2 has been the driving force of the scientific community to synthesize and to characterize inorganic materials with similar morphologies due to their unique properties.3 Considerable research effort has subsequently been directed toward the creation of inorganic nanotubes of various chemical composition and structure. Typically, the best candidates for the synthesis of tubular or fibrillar morphology are layered inorganic compounds with structures comparable to the structure of graphite such as metal dichalcogenides (sulfides, selenides, and tellurides), halides (chlorides, bromides, and iodides), oxides, and hydroxides. However, it has been rarely recognized that nanotubes were known to geoscientists for more than 40 years prior to the discovery of carbon nanotubes. Inorganic nanotubes such as cylindrite,4 chrysotile,5 halloysite,6 imogolite,7 tochilinite,8 and asbestos-like serpentines occur in nature displaying heterogeneity in composition and morphology. In particular, the toxicity associated with the natural asbestos impedes the study of their properties and applications.912 However recently, synthetic chrysotile Mg3Si2O5 (OH)4 has been proposed as a nontoxic reference to investigate optical, electronic, and surface properties these materials. The synthesis of Mg3Si2O5(OH)4 fiber13 and its isomorphic analogues, where the magnesium in the octahedral layer is replaced by other d metal cations, such as iron, cobalt, and nickel, has been reported in the literature,14 opening the possibilities and renewing the interest in asbestos-like materials for nanotechnology applications. Although some of these nanomaterials have been studied for much longer than 1 century, the elucidation of their structural characteristics has proved unusually difficult and their identification has been correspondingly uncertain. In spite of the dominion of the synthesis of these materials r 2011 American Chemical Society
Table 1. Structural Parameters of the Optimized Ni3Si2O5(OH)4 Basic Cell cell parameters
symmetry
R = 90.0°
a = 5.3621 Å
space group name
P31M
β =90.0° γ =120.0°
b = 5.3621 Å c = 7.3784 Å
int table no.
157
element
x
y
z
occ
Si
0.333 33
0.666 67
0.105 54
1.00
O
0.333 33
0.666 67
0.112 09
1.00
Ni
0.000 00
0.329 83
0.271 94
1.00
O
0.000 00
0.659 35
0.401 08
1.00
H
0.000 00
1.648 88
0.534 21
1.00
O
0.514 48
1.000 00
0.187 23
1.00
H
0.000 00
0.000 00
0.001 55
1.00
O
0.000 00
1.000 00
0.131 08
1.00
a complete correlation between structural tubular morphology and experimental data such as X-ray diffraction (XRD) and transmission electron microscopy (TEM) has not yet been carried out. Electron microscopy has shown that nickel-substituted Mg3Si2O5(OH)4 minerals can be present as rod- or tube-shaped particles or as platy or fluffy particles,15 the characterization of the rod shape being the most difficult to elucidate. Therefore, the aim Received: February 28, 2011 Revised: April 21, 2011 Published: May 19, 2011 11442
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Figure 1. (a) View of the Ni3Si2O5(OH)4 basic cell. (b) c axis layer distribution of the Ni3Si2O5(OH)4 cell.The interlayer distance was DFT calculated, giving a value of 7.4 Å, whereas the cell length along the b axis gives 5.36 Å. (c) Expansion of the Ni3Si2O5(OH)4 basic cell along b and c axes. The green notes indicate the axial direction in the fiber with a periodicity of 9.3 Å. (d) Three-dimensional view of the Ni3Si2O5(OH)4 fiber. (e) Fiber’s transversal section. The green square corresponds to the green square in b. (f) Axial view of the fiber.
Figure 3. Energy dispersive X-ray spectrum of Ni3Si2O5(OH)4 nanotubes. The spectrum displays the presence of Ni, Si, and O in the tubular structure, whereas the Na peak is due to the presence of residual sodium, kept in the sample after the washing procedure. Finally, the peaks of C and Cu are associated with the copper grid used in the TEM sample preparation.
of this work is to generate rod shape atomistic models of the nickel analogue of chrysotile, Ni3Si2O5(OH)4, naturally known as pecoraite,16 and correlate their morphology aspects with experimental results of XRD and TEM.
Figure 2. (a) Global view of Ni3Si2O5(OH)4 nanotubes showing the dispersion in the distribution size of the nanotubes, observing mainly the presence of two- to three-walled and multiwalled nanotubes. (b) View of a multiwall nanotube. (c) View of two- to three-walled nanotube.
2. EXPERIMENTAL SECTION Pecoraite was first synthesized in the early 1950s by Noll et al.17 using a hydrothermal methodology, this procedure being the most commonly used in the synthesis of these materials. However, Kloprogge et al.18 report the preparation of pecoraite under nonhydrothermal conditions of 90 °C and 1 atm. In our case, we used the hydrothermal procedure. The synthesis was done as follows: 9 mL of 0.5 M Ni(NO3)2 solution and 9 mL of 0.5 M Na2SiO3 were added to a mixture of ethanol (37.5 mL) and ethylene glycol (15 mL). Then, 4.5 g of NaOH was added to form a slurry precipitate which was sealed into a Teflon-lined 11443
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Figure 4. (a) TEM image of Ni3Si2O5(OH)4 nanotube where two main crystallographic orientations, labeled as 1 and 2, are observed. (b) Zoom of region 1 in (a). (c) Zoom of region 2 in (a). (d) Simulated HREM based in the model of Figure 1. (e) Zoom of the simulated images corresponding to region 1. (f) Zoom of the simulated images corresponding to region 2.
autoclave and heated at 220 °C during 18 h. The as-obtained nanotubes were filtered and washed first with ethanol and then with deionized water in order to eliminate the organic material occluded inside the nanotubes. The resulting green powder was characterized by XRD, TEM, and energy dispersive X-ray spectroscopy (EDXS). The XRD pattern was measured in air at room temperature with a Bruker D-8 Advance diffractometer with the BraggBrentano θθ geometry, Cu KR radiation, a Ni 0.5% Cu Kβ filter in the secondary beam, and a one-dimensional position-sensitive silicon strip detector (Bruker, Lynxeye). The diffraction intensity as a function of 2θ angles was measured between 5 and 80°, with a 2θ step of 0.019 447. TEM images were obtained using a transmission electron microscope JEM-2200FS which operates at 200 kV, and it is equipped with a Schottky field-emission electron gun, an ultrahigh-resolution (UHR) configuration (Cs, 0.5 mm; Cc, 1.1 mm; point to point resolution, 0.19 nm), and in-column omega energy filter.
3. THEORETICAL SECTION To simulate the structural properties of the pecoraite fibers, a layer structure of Ni3Si2O5(OH)4 was considered . The construction of this layer should be carried out, in principle, using experimental results. Unfortunately, there is no atomic position refinement of the pecoraite cell, the cell parameters of the basic unit by Song et al.19 and Faust et al.,20 being the only ones reported. However, Song et al. and Faust et al. argue analogies of the X-ray diffraction pattern of tubular pecoraite,
Ni3Si2O5(OH)4, and the tubular clinochrysotile, Mg3Si2O5(OH)4. Therefore, if we assume an isomorphic transformation of the atomic coordinates from clinochrysotile to pecoraite, then it is possible to construct a basic unit cell for pecoraite, considering the atomic coordinates of clinochrysotile. The clinochrysotile atomic coordinates are reported by Whittaker21 and Gualtieri and Artioli.22 The transference of the clinochrysotile atomic coordinates the pecoraite cell parameters generates a two-layer structure set up by a distorted Ni octahedral layer and Si tetrahedral with symmetry group Cm. The cell formed in this way was density functional theory (DFT) geometry optimized in order to establish the stability of this structure. The geometry optimization was carried out using the code Castep23 implemented in the interface Materials Studio 5.5.24 The minimum energy cell model was determined using the generalized gradient approximation (GGA) of Perdew and Wang25 (PW91). Additionally, Ultrasoft pseudopotentials26 were used in the calculation with a plane wave cutoff energy of 380 eV. The default Castep MonkhorstPack27 3 3 2 scheme was used for choosing k-points. The positions of all atoms and cell angles and lengths were allowed to relax, keeping the Cm symmetry. The DFT geometry optimization of the Ni3Si2O5(OH)4 cell, keeping the symmetry group Cm, generates an unstable structure where the Ni octahedral arrangement disappears. On the other hand, the relaxation of the Ni3Si2O5(OH)4 geometry without symmetry restrictions produces a stable cell whose analysis gives a structure with symmetry (P31m; IT no., 157), that is, isomorphic to nepouite.28 The parameters of the relaxed cell and the 11444
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Figure 5. (a) TEM images of the first crystallographic orientation, labeled as region 1 in Figure 4. (b) Planar distance assignment based on the fast Fourier transformation pattern of region 1. (c) TEM images of the second crystallographic orientation, labeled as region 2 in Figure 4. (d) Planar distance assignment based on the FFT pattern of region 2.
characteristics of the DFT optimization are given in Table 1. This nepouite layer structure was used as the basis for the construction of the fibrilar pecoraite. The creation of the fibrilar pecoraite was based on the isomorphism between the chrysotile and pecoraite fibers. In the case of the chrysotile crystal, it is made up of layered structures29 where a sheet of SiO tetrahedra joins with a sheet of MgO octahedra through sharing apical O atoms to form a layer.30 Accordingly, when the layers are rolled-up cylindrically, the tubules will be formed.31 The nanotube model was built assuming a Ni3Si2O5(OH)4 layer rolled-up into a scroll configuration using most of the geometric parameters of the crystallite. Figure 1ac show the crystal characteristics of the Ni3Si2O5(OH)4 cell, whereas Figure 1df display the fiber analogous to the crystal.
4. RESULTS AND DISCUSSION A global TEM analysis revealed that the sample was formed by one-dimensional nanostructures, which are mainly composed by two types of nanotubes: (i) two- or three-layer and (ii) multilayer (higher than three) nanotubes, (see Figure 2a). In the multilayer nanotubes the external diameter was around 30 nm, while their internal diameter was variable (see Figure 2b). On the other hand, the two- to three-layer nanotubes displayed 21 and 26 nm inner and outer diameters, respectively, with a d-spacing of 0.85 nm (see Figure 2c). A lower number of staking layers of the nanotubes increases the interlayer space, for instance, from multilayer to two- or three-layer nanotube resulting in 0.75 to
0.85 nm of interlayer space, respectively. The nanotube length, in both cases, is similar, with values around 200 nm. Chemical analysis performed in this morphology displays characteristic peaks of O, Na, Si, and Ni in the EDX spectrum (see Figure 3). The Na peak signal on the EDX spectrum is due to the presence of residual sodium kept in the sample after the washing procedure. A higher magnification of a multilayer nanotube is shown in Figure 4a. If we assume that a single nanotube is a prototype of the rest of the nanotubes on the sample, then the nanotubes are mainly formed in two crystallographic orientations. The first crystallographic orientation is located at the external of the nanotube, being characterized by several layers orientated along the nanotube axis (see Figure 4b). The d-spacing between layers was around 0.77 nm. Additionally, a plane perpendicular to the axial direction of the nanotube with a d-spacing of 0.261 nm was detected. The second crystallographic orientation was located in the nanotube center, where a hexagonal arrangement formed by a family of planes with a d-spacing of 0.456 nm was observed (see Figure 4c). All of these distances were corroborated in the fast Fourier transformation (FFT) pattern of both regions (see Figure 5). With a view to reveal the origin of the TEM patterns and validate our theoretical model, we simulated the optical transforms of a perfect Mg3Si2O5(OH)4 fiber model, by the multislice simulation technique implemented in the cerius2 interface,32 using the experimental parameters: defocus, ΔF = þ70 nm, and spherical aberration, Cs = 0.5 mm. The simulated TEM image (see Figure 4d) displays most of the geometric aspects observed in the experimental image. Additionally, there is a close similarity 11445
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank “Fondos Sectoriales SENER-CONACyT” for financial support of this work (Grant 116233). ’ REFERENCES
Figure 6. (a) XRD of our nanotubes where peaks are identified for pecoraite (JCPDS 49-1859) and nickel (JCPDS 04-0850). (b) Comparison between theoretical and experimental X-ray diffraction pattern. Theoretical curve fits all the peak positions with the exception of the peak in 2θ = 44° that corresponds to the presence of metallic nickel. (c) View of the atomistic model where the X-ray diffraction pattern was calculated.
between the zoom regions labeled as 1 and 2 in Figure 4a, in comparison with the simulated counterparts (see Figure 4e,f). The X-ray diffraction pattern reported in Figure 6a shows the reflection lines that match well with the tubular nickel serpentine mineral pecoraite (pecoraite, JCPDS 49-1859).33 A diffraction line at 2θ = 44.5° arises from metallic nickel (Ni°), suggesting that decomposition of the organic components (ethylene glycol and/or ethanol) upon hydrothermal reaction induced the reduction of a fraction of the Ni precursor into Ni° (Ni, JCPDS 04-0850). Additionally, a comparison between the experimental and theoretical X-ray diffraction patterns is displayed in Figure 6b, observing the agreement in the 2θ position of all reflection peaks, except the above mentioned peak at 44.5° that corresponds to metallic nickel. The theoretical X-ray diffraction pattern was obtained using the powder diffraction code implemented in the interface Materials Studio 5.5,34 taking the periodic boundary model described in Figure 6c. As in the case of TEM, the agreement between the experimental and theoretical data validates the theoretical model.
5. CONCLUSION In summary we found an excellent correlation between the proposed rod-shape model XRD pattern and TEM images with those obtained from our experimental results. In particular the origin of most of the main peaks observed experimentally by XRD patterns of Ni3Si2O5(OH)4 rods were elucidated. Additionally, our simulated TEM images reproduce most of the geometric aspects observed in the experimental image, validating our theoretical model.
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