Nanocubes or Nanorhombohedra? Unusual Crystal Shapes of

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J. Phys. Chem. C 2008, 112, 9209–9213

9209

Nanocubes or Nanorhombohedra? Unusual Crystal Shapes of Corundum-Type Indium Oxide Aleksander Gurlo,* Stefan Lauterbach, Gerhard Miehe, Hans-Joachim Kleebe, and Ralf Riedel Fachbereich Material - und Geowissenschaften, Technische UniVersita¨t Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany ReceiVed: January 28, 2008; ReVised Manuscript ReceiVed: March 29, 2008

Sol-gel processing via the modified nonalkoxide route resulted in the formation of nanocrystals of the corundum-type In2O3 polymorph, which revealed characteristic rhombohedra with a morphology very close to cubes. Because the cubic bixbyite-type In2O3 polymorph is more abundant under ambient conditions, the seemingly cube-shaped morphology might confuse the experimentalist. As shown by transmission electron microscopy, this morphology does not reflect any relation to a cubic structure but appears accidentally with form {012} at a specific axis ratio of corundum-type structures. The pseudocubic particles have also been reported for R-Fe2O3, also in this case the pseudocubic nanorhombohedra are terminated by {012} facets. 1. Introduction The corundum (R-Al2O3) structure (space group R3jc, No. 167) is important for its common occurrence; i.e., the majority of the trivalent transition metal sesquioxides, among them R-Fe2O3 (hematite), crystallize in the corundum structure. Several recent papers reported on sol-gel and solvothermal synthesis of cube-shaped crystals of R-Fe2O31–18 and of corundumtype In2O3 polymorphs19,20 (rh-In2O3). The seemingly cubic morphology is a rather surprising finding, because one would expect the cube-shaped crystals in cubic space groups and not in the trigonal crystal system, wherein the corundum-type structure belongs to. Moreover, because under normal conditions In2O3 crystallizes in the cubic bixbyite-type structure (C-type structure of rare-earth oxides, space group Ia3j, No. 204), the seemingly cubic morphology might confuse the experimentalist, leading to the false phase identification. To clarify this point, we have performed structural characterization of the rh-In2O3 rhombohedra with seemingly cubic morphology. The corundum-type In2O3 is usually described as a highpressure-high-temperature In2O3 polymorph,21–24 which is metastable and transforms irreversibly into cubic bixbyite-type In2O3 upon heating.25 The rh-In2O3 can also be synthesized under ambient pressure in hydrothermal and sol-gel based routes.19,20,26–34 The stabilization of the rh-In2O3 structure under ambient pressure was also observed (i) in solid solutions of In2O3-SnO232,35–40 and In2O3-Fe2O3,41–43 (ii) in films grown via MOCVD on sapphire substrates,44 and (iii) in nitrogen-doped In2O3 thin films.45

Figure 1. X-ray diffraction pattern of rh-In2O3 showing both the observed (open circles) and calculated (solid line) intensities (Rietveld refinement). The difference curve is given at the bottom of the graph. The calculated peak positions are denoted by solid bars.

2. Experimental Section Synthesis was performed by the modified nonalkoxide sol-gel method19 based on the ammonia-induced hydrolysis of indium nitrate in methanol. The processed powder (calcined in air at 500 °C for 1 h) was used for the structural characterization. X-ray powder data were collected at a STOE STADI P diffractometer operating in Debye-Scherrer geometry. With Mo * To whom correspondence should be addressed. Phone: +49 6151 166342. Fax: +49 6151 166346. E-mail: [email protected].

Figure 2. SEM micrograph of the rh-In2O3 powder sample revealing nanocrystals ranging between 50 and 100 nm in size. Note that these crystallites appear to be of cubic symmetry.

KR1 radiation and a position-sensitive detector with 6° aperture, intensities were measured from 2θ ) 8 to 38° in steps of 0.02°. A full-profile Rietveld structure refinement was done using the program FullProf.46

10.1021/jp800823g CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

9210 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Gurlo et al.

Figure 3. (a) Low-magnification TEM micrograph of the rh-In2O3 powder sample revealing nanocrystals which also appear to accommodate cubic symmetry (similar to the SEM image given in Figure 2). The inset shows the corresponding diffraction pattern (powder rings). However, some particles are seen which reveal a hexagonal crosssection. An enlarged image of such a “hexagonal” grain is given in b. Figure 5. (a) TEM micrograph of a pseudocube. (b) Enlarged image of the boxed area. (c) Fourier-filtered image of the boxed area in b. The deviation of the nearly vertical rows of lattice points (dashed line) from the vertical direction contradicts the 4-fold symmetry of a cube.

Figure 6. (a) TEM micrograph of one pseudocubic rh-In2O3 powder particle. The lines clearly show the deviation of the angle from 90°. (b) Corresponding diffraction pattern along the 〈241〉 zone axis.

Figure 4. (a) TEM micrograph of a rh-In2O3 powder particle with rather rough surface facets. (b) Corresponding SAD pattern strongly resembling zone [110] of a fcc structure. (c) Fourier-filtered image of the boxed area in a.

The particle morphology was characterized by scanning electron microscopy (SEM) utilizing a FEI Quanta600 instrument (FEI, Eindhoven, The Netherlands), with an acceleration voltage of 25 kV. Transmission electron microscopy (TEM) was performed at a FEI CM20 microscope, equipped with a Philips double-tilt holder at a nominal acceleration voltage of 200 kV. TEM samples were prepared from a small amount of powder which was dispersed in ethanol by ultrasonic treatment for 5 min. A drop was applied to a copper grid covered with a holey carbon film. Because of the small particle size, all diffraction patterns have been taken with a strongly converging beam. 3. Results and Discussion: Corundum-type In2O3 The X-ray powder diffractogram can undoubtedly be attributed to rh-In2O3. Figure 1 shows the quality of the Rietveld fit. No internal standard was used; therefore, the cell parameters might suffer from a systematic error. Despite this shortcoming

the results of refinement [a ) 5.4911(2) Å; c ) 14.5255(7) Å; c/a ) 2.6453(2); V ) 379.30(5) Å3; In (12c), 0, 0, z (z ) 0.35725(5)); O (18e), x, 0, 1/4 (x ) 0.299(1))] are very close to the values found by other authors, e.g., Prewitt et al.23 SEM imaging of the rh-In2O3 powder sample revealed nanosized crystals with a size ranging between 50 and 100 nm. Most of the particles appeared as perfect cubes, as can be seen in the inset of Figure 2. A TEM image of the same powder sample is given in Figure 3a. The inset shows the electron diffraction pattern of an area with a 4 µm diameter. The diameter and intensity of the powder rings appear as expected for rh-In2O3. It should be noted that the limited size of the selected area diffraction aperture limits the number of crystallites contributing to the observed spotted ring pattern. These crystals are not in a crystallographic orientation with each other except that they reveal different inplane rotations resulting in individual spots constructing the ring pattern. Outside the agglomerated regions, individual particles can be seen, which are predominantly cube-shaped. Some grains were oriented in a way that a hexagonal shape appeared. An enlarged image of such a hexagonal morphology is given in Figure 3b. Note that this nanocrystal shows some faint contrast change between the center and the rim, which is due to the thickness difference of the outer facet (thinner) and inner core

Corundum-Type Indium Oxide Unusual Crystal Shapes

Figure 7. Orientation of the {012} pseudocube with respect to the unit cell. The slim rhombohedron represents the primitive setting of the R-centered cell.

(thicker), indicating that this is not a hexagonal basal plane projected onto the screen. High-resolution TEM imaging (HRTEM) of the sample also showed, at first sight, a cubic symmetry of these nanocrystallites (Figure 4 and Figure 5). The SAD pattern (Figure 4b) corresponding to the grain imaged in Figure 4a strongly resembles zone [110] of a fcc structure. Similarly, the nanocrystal shown in Figure 5 does not display a projection of a cubic basal plane onto the projection screen. By a closer inspection of the crystal lattice shown in Figure 5c, which reveals an enlarged Fourierfiltered image of Figure 5a, it becomes apparent that the lattice in fact is not precisely cubic, since the atomic arrangement slightly deviates from 90° angles, as indicated by arrows in the upper right corner of the image.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9211 Detailed examination of one selected nanocrystal with pseudocubic morphology shows a slight deviation from 90° (Figure 6 a); the corresponding electron diffraction patterns can be indexed as shown in Figure 6b. It turned out that the pseudocubes have to be described as rhombohedra, terminated by (012), (1j02), (102j), (01j2j), (11j2), and (1j12j) facets (i.e., {012} form hereafter, Figure 7). The calculated angles between the (012) and the (1j02) planes is 92.86°. Accordingly, the identical edges as well as the angles, which only slightly deviate from 90°, generate the impression of a perfectly shaped cubic crystal. The crystal shapes observed by HRTEM can be understood as follows: (i) “square” (Figure 5; the rhombohedron {012} viewed along one of its edges, e.g., along [241]); (ii) “rectangle” (Figure 4; the rhombohedron {012} viewed along one of its face diagonals, i.e., along either [100] or [1j11]); and (iii) “hexagon” (Figure 3; the rhombohedron {012} viewed along its body diagonal, i.e., along [001] (perfect hexagon) or along [841] (distorted hexagon)). To the best of our knowledge, among many corundum-type oxides only R-Fe2O3 and rh-In2O3 are reported to form the pseudocubic particles (Table 1). There have been only two reports19,20 on cube-shaped rh-In2O3 nanocrystals, but no explanation was given for such “pseudocubes”. R-Fe2O3 is described to form the pseudocubic particles terminated by {012} facets.47 The similarity between the shape of form {012} and that of a cube is accidental and does not reveal a relation to any cubic structure. The structure of corundum is described by two independent atoms, each of them having one free positional parameter: O in position 18e (x, 0, 1/4) and the cation in position 12c (0, 0, z). With x(O) ≈ 1/3 and c ≈ 26d(O-O)sas in corundumsthe oxygen atoms form a hexagonal closest packing arrangement with six layers (ABABAB) per cell in the c direction. The cations occupy two-thirds of the octahedral interstices in such a way that pairs of face-sharing octahedra are formed and that the length of the a-axis becomes 3d(O-O). Hence for an “ideal” corundum structure the ratio c/a ) 8 ) 2.828. The angle between faces (1j02) and (012) is 95.22°. For rh-In2O3, the value c/a ) 2.645 deviates from the ideal value by -6.5% and the angle between faces (1j02) and (012) is 92.85°; for R-Fe2O3, the value c/a ) 2.729 deviates by -4.5% and the angle between faces (1j02) and (012) is 94.00°. Since the hcp-arrangement of anions possesses only one 3-fold axis, there is no relation to any cubic structure. The geometric arrangement of the face-sharing octahedra might be the reason for the prominence of the form {012}.

TABLE 1: “Cube-Shaped” Nanocrystals of Corundum-type r-Fe2O3 (Hematite) and of Corundum-type In2O3 Polymorphs oxide

shapea

edge size

200-1200 nm R-Fe2O3 cubic pseudocubic (subparticles in pseudocubic polycrystals) 12-16 nm subparticles in pseudocubic 1500 nm polycrystals cubic 1100-1200 nm cube 40-50 nm pseudocubic (subparticles in pseudocubic polycrystals) 60-650 nm (polycrystal), 43-53 nm subparticles pseudocubic (subparticles in pseudocubic polycrystals) 350-1100 nm (polycrystal), 16-20 nm subparticles cubic particles 400 nm cube-shaped 67-86 nm nanocubes 220-250 nm nanocubes 12 nm nanocubes 15 nm cube 250 nm rh-In2O3 cubic 50-80 nm nanocubes 10 nm a

facetsb {012} (012) (110) (102j)

Original shape description, taken from the corresponding reference. b Planes identified in the HRTEM and /or SAED experiments.

ref 1–3 4–7,47 8,9 10 11 12 13 14 15 16 17 18 19 20

9212 J. Phys. Chem. C, Vol. 112, No. 25, 2008 On the other hand, a sequence of six close-packed oxygen layers per unit cell may also be realized by the stacking sequence of a cubic close packing, ABCABC. The spinel structure is known to be based on this arrangement. The form {012} of rh-In2O3 and R-Fe2O3 corresponds to the form {100} of the cubic structure. The ratio c/a for the rhombohedral setting of any cubic face centered structure is c/a ) 6 ) 2.4495. This ”ideal” value deviates by 8.0 and 11.5% from the value for rhIn2O3 and R-Fe2O3, respectively. Hence the geometric reason for the cubelike shape of form {012} is the small difference of about 15% between the values of 6 and 8. The form {012} of corundum-type structures corresponds to form {100} of the cubic cell. The conditions for the formation of pseudocubic particles of corundum-type In2O3 and R-Fe2O3 are not fully understood. As known, the preferred equilibrium shapes of nanoparticles depend on the growth conditions, especially on ligand type and degree of ligand coverage.48,49 In the sol-gel synthesis the pseudocubic shapes can also result from kinetically driven growth paths.50 The pseudocubic rh-In2O3 and R-Fe2O3 particles appeared after dehydroxylation of the corresponding oxyhydroxides, i.e., InOOH19,20 and β-FeOOH (akageneite),2,5,10,15,51 respectively. For R-Fe2O3 the {014} rhombohedron is described as an usual morphology;52 the growth of the {012} pseudocubic rhombohedra was ascribed to the specific adsorption of chloride ions and/or chloroferric complexes to the {012} faces restraining the growth in the directions normal to the {012} faces.5,51 However, it is well-established that the growth morphology strongly depends on the growth environment. Since ammonia-induced hydrolysis of indium nitrate in methanol was used to process the initial In2O3 powder, calcined at 500 °C, it is thought that the variation in chemical environment such as the presence of ammonia and/or organics strongly affect the surface energy of the crystal facets, consequently favoring the growth of {012} surface planes. As a result, the pseudocubic morphology of the rh-In2O3 nanocrystals was observed here. Lee et al.53 reported on important guiding parameters for the construction of MnS and CdS nanocrystalline architectures and concluded that, besides kinetic control and temperature, activation energy modulations of surfaces are crucial for growth anisotropy. 4. Conclusion The characteristic cube-shaped nanocrystals of corundumtype In2O3 represent in fact trigonal rhombohedra with pseudocubic morphology. These rather isometric rhombohedra are terminated by {012} planes with a deviation of (2.8° from the orthogonal angle. Because In2O3 crystallizes under normal conditions in the cubic bixbyite-type structure, the seemingly cube-shaped morphology might confuse the experimentalist, leading to the false phase identification. This finding points out how careful one should be in the interpretation of the experimental data, especially dealing with the correlation of the crystal shape and structure. It should be noted though that it is not clear yet why this unusual morphology of the crystallites formed under the given processing conditions. Acknowledgment. We are indebted to Dmytro Dzivenko, Technische Universita¨t Darmstadt, Institute of Material Science and Geoscience, for helping in obtaining the SEM pictures. The work was financially supported by the Fonds der Chemischen Industrie, Frankfurt, Germany. References and Notes (1) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509.

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