Preparation through the Vapor Transport and Growth Mechanism of

Preparation through the Vapor Transport and Growth Mechanism of the First-Order Hierarchical Structures of MoO3 Belts on Sillimanite Fibers Petre Badica

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 794-801

Institute for Materials Research, Tohoku UniVersity, 2-1-1 Katahira, Sendai 980-8577, Japan, and National Institute of Materials Physics, Bucharest-Magurele, POB MG-7, 077125, Romania ReceiVed December 7, 2006; ReVised Manuscript ReceiVed January 23, 2007

ABSTRACT: First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of R-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47° and 94°. The growth mechanism is of vaporsolid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo-O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite fibers), on which the MoO3 belts are growing, are important and can be used for the directional growth control of the belts and formation of hierarchical structures. The vapor transport method allows the formation of β-MoO3 belts that grow in the b-axis direction. 1. Introduction Several oxides are known in the Mo-O system with R-MoO3 being the most stable and investigated. This oxide has an orthorhombic crystal structure (a ) 0.3962 nm, b ) 1.3858 nm, and c ) 0.3697 nm), and it is composed of bilayers of MoO6 quasi-octahedrons stacked together by weak van der Waals forces.1-4 Low-temperature meta-stable phases, β-MoO32-6 and h-MoO37 crystallizing as monoclinic (a ) 0.71228 nm, b ) 0.5366 nm, c ) 0.5566 nm, and β ) 92.01°; or viewed as pseudo-cubic, similar to ReO32,4) and hexagonal (a ) 1.0531 nm, c ) 1.4876 nm, JCPDS 21-0569) polymorphs, respectively, were also often reported. Solid, vapor, and liquid states of MoO3 are all of practical interest. For example, solid R-MoO3 alone or in combination with other materials can work as a sensing material of different gases,8-13 as a cathode material in batteries,7,14 as a conductive glass-fiber,15 as a charge-generation layer in electrophosphorescent organic light emitting devices,16 or as a component of composite materials for supercapacitors to be used in lightweight electronic fuses, memory back-up power sources, and in protection devices to pulse power sources for smart weapons.17 Also, as a nanocomposite core-shell particle, in combination with TiO2, it is reported to decrease the photoabsorbtion energy of TiO2,18 while stacked thin films with Au or Pt are showing enhanced photochromism and electrochromism.19,20 Electronic applications of MoO3 are also very promising, and, for example, the field-emission effect demonstrated by different groups21,22 recommends this phase as a candidate for vacuum microelectronic devices such as large-area flat panel displays. It is also well known that supported MoO3 is widely used as a catalyst in the petroleum and chemical industry; catalysts of MoO3 mixed with various oxides are intensively investigated, and in many cases they exhibit improved efficiency.23-28 Intercalationdeintercalation,6,29,30 spreading,31 flux,32,33 clustering properties,31,34 and gas-phase reactivity35 are all extremely useful and important in designing new processing methods, materials and applications, or for improving the existing ones. * Corresponding author. Phone: 81-22-215-2148. Fax: 81-22-215-2149. E-mail: [email protected].

Recent developments and experiments on the effects of dimensionality and geometry in the nanolimit are further significantly enhancing this range of possibilities [e.g., see review article36] toward miniaturization, energy saving and green technologies, better performance, and new devices and applications. To realize and take advantage of this huge potential, synthesis understanding and control at nano- and microscales are priority and challenging tasks. R-MoO3 phase is known to develop into 1D objects, the most frequent being ribbon-shaped belts of micrometer or nanometer diameters. Methods by which they are obtained are vapordeposition,12,21,22,37-41 thermal conversion from β-MoO3 at temperatures higher than 380 °C6 in the air, calcinations in the air of a mixture of a complex Mo-based compound and carbon nanofibers42 or carbon nanotubes43 used as template, step-edge decoration through electrodeposition,44 electrospinning applied to mixtures of oxide-sol-gel and polymer solutions,45 and the hydrothermal method.5,46-52 Hierarchical structures of MoO3 can also form, and the following articles report them: Spherical and flower-like hierarchical structures of R-MoO3 are presented in ref 5. Four- or two-armed forklike R-MoO3 nanostructures as well as assembled objects into more complex morphologies such as centrally holed nanorods, tridents, and paintbrushes are reported in ref 50. Forklike units are shown to act as template for TiO2 shaped into square or horseshoe geometry.46 Finally, prism-like rods, nanowires (cylinder-like), bundles of fibers, and nanotubes of R-MoO3 are introduced in refs 48, 51-53, 44, and 52-54, respectively. β-MoO3 phase was obtained in thin films by rf sputtering,2 powders,3,4 as nanobelts by the hydrothermal route,5 and as complex fibrous aggregates by deintercalation from MoO3(4,4′bipyridyl)0.5 crystals.6 Micro-rods of h-MoO3 were produced by chemical precipitation followed by hydrothermal treatment.7 As one can observe, there is a limited number of articles on complex and shape- or size-controlled objects of MoO3. Furthermore, we note that the presented hierarchical structures or organized structures are all generated by soft-chemistry

10.1021/cg060893s CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007

Structures of MoO3 Belts on Sillimanite Fibers

Figure 1. X-ray diffraction pattern of the sillimanite fibers in the powder state. Diffraction lines were identified according to JCPDS file no. 38-0471.

methods, for example, the hydrothermal technique. Vapor deposition seems to allow less phase, directional, shape, and organization-assembly control than do soft chemistry methods, although island clusters and networks of clusters were reported in ref 31 and an array of parallel R-MoO3 nanobelts was synthesized in ref 22. On the other hand, comparative analysis of the soft chemistry methods versus vapor deposition routes indicates some advantages of the latter ones: vapor deposition methods are very simple, result in large-scale products, treatment temperatures are relatively low, and large amounts of contaminated waste materials are avoided. Therefore, exploration and improvement of vapor deposition methods of MoO3 is of interest, and our work will focus on preparation by vapor deposition of MoO3 belts on sillimanite microfibers, resulting in the first-order hierarchical branched structures. Belts are of R-MoO3, but few β-MoO3 were also observed. The growth mechanism will be discussed and compared to literature data. 2. Experimental Section For the synthesis of MoO3 belts, a conventional electrical muffle furnace (Yamato FO100) was used. A plate of Mo-metal was placed in an Al2O3 crucible and heated in the air. The heating time was from 30 min to 24 h. Airflow, ensuring vapor transportation and growth conditions, is produced by the fan of the furnace. The gap between the door and the furnace was enlarged to have a 3-4 mm free space for easy airflow. Belts of MoO3 grow in a certain region of the furnace directly on the fibers of refractor wool that is the thermal insulation of the furnace. Temperature in the mentioned region was 360-380 °C for heating temperatures of 1000-1080 °C. X-ray diffraction (XRD) from Figure 1 shows that powders obtained by grinding the fibers are mainly composed of sillimanite, Al2SiO5 (denoted SAO). Some peaks were not identified (e.g., 13.79°, 20.98°, 21.78°, 48.03°, and 49.36°), suggesting that some other phases are present in the fibers. These data are likely supported by energydispersive spectroscopy (EDS) analysis that indicates a composition, Al0.95-1.19SiNa0.02-0.06Ox (normalized to Si), different from the stoichiometric sillimanite composition. Most frequent, Al/Si ratio values were 0.16 for the analyzed fibers (about 25), and, in some cases, Na was not detected. Crystal quality of the fibers is relatively poor as deduced from the broad lines and their low intensity. No preferential orientation was detected by XRD on fibers. XRD spectra were measured using a PANalytical/Phillips diffractometer (Cu KR radiation). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed with JEOL JSM-6400F/EDAX and JEOL JEM-3010 systems, respectively.

3. Results and Discussion Individual MoO3 belts, complex structures, different stages of growth, XRD patterns, SEM, TEM, and electron diffraction

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(ED) images, and energy-dispersive spectroscopy (EDS) results are presented in Figures 2-5. Schematic representation of the growth processes is shown in Figure 6. 3.1. Individual r-MoO3 Belts. Individual MoO3 belts extracted from MoO3-sillimanite complex structures (see section 3.3) show the following features: (i) Belts are of white color and transparent. They have a ribbon-like shape with width from 0.1 to 40 µm. Thickness also varies in a wide range from 0.02 to 7 µm. Maximum length is of about 1.5 mm (Figure 2a-g). In some cases, belts have large width (300-600 µm), and they can be considered as being plates rather than belts (Figure 2h and Figure 5a,d). Layered features of the belts can be easily observed, and along them belts can easily cleave (Figure 2k). (ii) XRD patterns from Figure 3 indicate that belts consist of phase R-MoO3. Observation of only (0l0) lines in the diffraction pattern of the as-grown belts shows that the thickness of the belts is along the b-axis. Lattice parameters are a ) 0.3958 nm, b ) 1.38493 nm, and c ) 0.3689 nm. These values are approximately similar to or slightly smaller than those presented in the literature and standard powder diffraction files JCPDS 76-1003/35-0609/5-0508. A slightly smaller unit cell might correlate with the deficiency of oxygen observed from EDS; the average O/Mo ratio is 2.8 below the stoichiometric ratio 3. This observation requires further analysis. Additional details on EDS compositions of the belts are presented in part (vi). (iii) The surface of the belts is clean, flat, and the only defects that are sometimes detected are stripes running parallel to the length of the belt (Figure 4a) and incomplete ac-plane layers (Figure 2e). TEM images on the stripes’ border showed that there are no defects associated with it. Incomplete ac-plane layers are more frequently found at the tip region (Figure 2e). The two defects are generating steps, and more details are addressed in section 3.3. A relatively rare defect is the one resembling ac-plane twins. To establish whether this is a classic twin defect, presented data are not sufficient. The twinning angle is 54° in Figure 5a, and this value is similar to one of the values measured at the triangular-like shape tip of the belts (see part (v)). (iv) TEM and ED (Figure 4a,b) show that belts are of R-MoO3 phase, and this result is consistent with XRD data from Figure 3. TEM and ED images also indicate that the length is along the c-axis direction, and this is the main growth direction. This result is in agreement with many reports.1,5,21,47,49 Literature gives the explanation for this behavior based on crystal chemistry. Briefly, the idea is that, in R-MoO3, distorted MoO6 octahedrons share both edges (along [001] direction) and corners (along [100] direction). In the b-axis direction, layers are held together by van der Waals weak forces, while on the c-axis there are strong covalent bonds. Hence, more energy will be released if growth occurs along the c-axis, making this direction energetically favorable. Growth in the c-axis direction will proceed by stacking MoO6 octahedrons in a zigzag growth sequence1,47,49 when one observes the projection of the tetrahedrons in the ac-plane (Figure 6). Such stacking sequence is expected to be reflected in the morphology of the belts. (v) Most of the belts show a triangular-like shape of the tip. The angles in the ac-plane are experimentally found to be of certain values: 15°, 20°, 30°, 47°, 54°, 94°, 108°, 150°, and 180° (Figure 2a-j). These values show some scattering, and one can observe that some of them are combinations or are double the smaller ones. The highest occurrence is for the angles of 47° and the double value 94°. The belts with the lower angle values are usually without symmetric tips, while those with

796 Crystal Growth & Design, Vol. 7, No. 4, 2007

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Figure 2. SEM images of MoO3-sillimanite structures: (a) general image and direction of air flow; (b) treelike structure in which SAO fiber stalk can be visualized; (c) image showing MoO3 preferential orientation within a treelike structure; (d and e) general view and detail of flower-like structure (note also incomplete layers for the marked belt); (f) MoO3 belts with triangular tips at both ends; (g) belts showing ribbon-like morphology with clean, flat surfaces and sharp edges and triangular tips; (h) plate with zigzag edge; (j) belts with low angle tips; (k) belt with triangular tip showing cleavage between ac-plane layers; (l) sillimanite fibers after short time growth of belts; (m) detail on fibers from (l) showing surface Mo-O spreading and island formation; (n) belt detaching at the tip in the direction perpendicular to the sillimanite fiber; (o) belt detaching at the tip in the direction parallel to the sillimanite fiber; and (p) plate on sillimanite fiber with stripes and apparently with main growth direction (arrow) parallel to the length of the fiber. Note large width of the plate as compared to the diameter of the fiber.

higher values show some degree of symmetry. Projection of the octahedrons on the ac-plane can be approximated with an in-plane rhomb having the angle in the c-axis direction of about 94° (φ ) 2 arctan[(a/2)/(c/2)] ) 94.03°). One-half of this angle is 47°. For 94°, the tip is in many cases symmetric, and this result suggests that the smallest building units to obtain this kind of tip are octahedrons. For the half-angle case, preserving

the same zigzag growth sequence, to obtain a 47° in-plane nonsymmetric tip angle, the smallest building units should be 1/2, 1/ , or 1/ of the MoO octahedron. These units are obtained 4 8 6 through cross sectional cutting of one octahedron along bc-plane, bc-plane + ab-plane, and bc-plane + ab-plane + ac-plane, respectively. Kihlborg1 noted that MoO3 represents a transitional stage between octahedral and tetrahedral coordination, providing


Figure 3. XRD patterns on as-grown (W) and ground (P) R-MoO3belts extracted from the MoO3-sillimanite structures.

Figure 4. TEM and ED images of R-MoO3 belt with stripes (top images (a), (b), and inset) and of β-MoO3 belt (bottom images (c), (d), and inset).

arguments that 1/2 octahedral building units would be possible. In his work, a tetrahedron, that is, one-half of an octahedron, is obtained by cutting along ac-plane, and this is very different from our case. Kihlborg also suggested that there is a direct relationship between solid and gaseous MoO3 based on the tetrahedral representation. In the vapor state, depending on temperature, various MoxOy, x ) 1-5 and y ) 1-15, oxide ion clusters were experimentally detected.55 These ion clusters


Figure 5. SEM images showing the location where EDS local compositions were determined. Composition ratio r ) O/Mo of the EDS-spots labeled with numbers are: (1) 2.9001, (2) 3.8192, (3) 2.7878, (4) 3.2017, (5) 2.8823, (6) 3.2808, (7) 2.7119, (8) 2.7847, (9) 2.075, (10) 3.5955, (11) 2.248, (12) 4.216, (13) 3.3206, (14) 3.5780, (15) 3.5106, (16) 2.1979, (17) 3.098, (18) 1.6252, (19) 2.7693.

form different structures from simple (linear) to complex (ring).35,55 In the presence of different environmental gases, clusters show different behavior of fragmentation and reaction with them. If the relationship mentioned by Kihlborg between the structure of the ion clusters in the vapor state and the solid phase is extrapolated and it is valid for geometries other than the tetrahedral one, that means controlling environmental gas in the deposition atmosphere, that is, controlling the abundance of a certain Mo-O cluster with a given structure, it would be possible to control the tip morphology of the belt or perhaps the morphology of the belt itself. Although this highly speculative hypothesis may be a hint to explain the tip angles and its symmetry, data are not enough to make any conclusion. Just for the record, growth of belts from this work was done in natural laboratory air relatively humid as it is during Japanese summer days (70-90% humidity). Another explanation for the occurrence of different tip angles and its symmetry is related to the stacking of the MoO6 octahedrons in the ac-plane. An example showing the formation of a belt with the nonsymmetric small angle tip at one end and of a symmetric high angle tip at the other is schematically shown in Figure 6a. Different stacking rates along the a-axis and c-axis might easily generate tips with different angles keeping MoO6 octahedrons as the smallest building units. The reason for different stacking of the octahedrons, resulting in different tip angles, might be related to gradients versus flow of gas and Mo-O vapors in the growth atmosphere and not necessarily to the ion clusters type and their structure in the vapor state. If this is the case, development of the tip’s morphology would be controlled by relative orientation and location of the belt against SAO fiber and spatial thermal, partial pressure, and compositional gradients triggered to some extent by the flow direction. Because belts are growing with a certain orientation on SAO substrates forming hierarchical


Figure 6. Schematic representation of growth: (a) triangular tips with different angles and symmetry by MoO6-octahedrons in-plane stacking; (b) initial growth stages, vapor transport, spreading, island and rhombic MoO3 crystals formation; and (c) advanced stages of growth, tip detachment resulting in formation of the belts and hierarchical structures and 2D layer-by-layer thin-film-like growth enhancing thickness.

structures and these structures show preferential orientation against the direction of the gas flow (see section 3.3), this may suggest that this idea is plausible and it is likely more probable than the previous one. However, this second hypothesis also needs in-depth investigations. Here, it is worthy to note that belts have well-defined sharp edges including the tips. Droplets at the tip as in ref 38 are not observed. This result and predefined various tip angles make MoO3 belts from this work of interest for field-emission application studies. This is because it was observed that electrons are emitted more easily from sharp corners, edges, or tips.21 Sometimes, plates with a zigzag edge composed of triangles having tip angles similar to those for belts (Figure 2h, Figure 5a,d) are found. The growth of plates is discussed in section 3.3. (vi) The ratio r ) O/Mo of the as-grown fresh MoO3 belts from energy-dispersive spectroscopy (EDS) is on average around 2.8, while for the belts stored for 9 months in the open air of our laboratory it was higher, around 3.7. Scattering is observed, and it is relatively large with values from 2 to 4.2 for fresh belts (Figure 5) and 2.4 to 5.9 for stored ones. Stored belts seem to slightly change their color to yellow. It is possible that belts absorb oxygen and/or water. Seguin et al.3 reviewed MoO3‚ nH2O phases and noted that MoO3‚H2O is of yellow color. Storage in Ar or sealed vessels preserved initial r-values. Values of r higher than the stoichiometric (r ) 3) in the as-grown belts were also found by other authors.37 EDS compositions determined at different locations on one belt or plate are approximately constant (e.g., compare r for EDS-spots 1, 3, 5, 7, and 8, Figure 5), but, in some cases, very different compositions were measured (compare r for EDS-spots 1, 3, 5, 7, and 8 with EDS-spot 4 in Figure 5a, and for EDS-spot 16 with EDS-spot

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17 in Figure 5c). Attempts to find a relationship between the morphology of the belts and composition were not successful. (vii) Many belts show triangular tips at both ends (Figure 2f). This can be explained considering the growth mechanism (see section 3.3). 3.2. Individual β-MoO3 Belts. Few belts grown at 360 °C (furnace temperature 1000 °C) with width of 50-150 nm apparently did not show the layered structure along the thickness, and the tip angles were 180°. TEM and ED images (Figure 4c,d) revealed that they are β-MoO3 belts. In the literature, temperatures of 250-380 °C depending on atmosphere2,4,6 were found to be suitable to obtain β-MoO3 phase. Belts of β-MoO3 grow along their b-axis. ED pattern and growth direction are similar to those reported in ref 5. The authors of ref 5 noted that β-MoO3 involves a corner-connected distorted octahedron in a ReO3-type structure, where preferential growth is not favored. They explained 1D growth of the belt considering adsorption of the cationic surfactant cetyltrimethylammonium bromide (CTAB) to the (010) plane during the growth process of the MoO3 belt under hydrothermal conditions. This will change the degree of saturation of the side surfaces, accelerating growth in the indicated direction. The reader of ref 5 might get the impression that the growth of β-MoO3 belts is possible only through the presented method, or at least through similar soft chemistry methods. It is interesting that the vapor transport method may also result in the formation of β-MoO3 belts, and to my knowledge this is the first report in this regard. Explanation based on differential saturation may also work in this case, and principles are rather universal, but their application is different depending on the specifics of each growth method. The difference is that, for the hydrothermal growth, the environment is the liquid media, while for the vapor transport method it is the growth atmosphere. Parameters such as temperature, pressure, and composition gradients in the growth atmosphere are important for growth, and in particular for directional growth (see also section 3.3). 3.3. Hierarchical Structures and Growth Mechanism of MoO3 Belts. Belts of MoO3 form a first-order branched tree structure (T-structure), and a general image is given in Figure 2a. A more attentive look shows that the stalk of the tree is made of SAO fiber with a diameter of 1-5 µm (Figure 2b,lp). It is interesting that most tree structures are growing with their SAO-stalk parallel to each other and parallel to the airflow (indicated by the arrow in Figure 2a). This might be related to local thermal/pressure/compositional gradients distribution and dynamics. A second important observation related to T-structures is that most MoO3 belts grow parallel to each other and perpendicular to the SAO-stalk fiber (Figure 2c). More precisely, belts (see sections 3.1 and 3.2) from the T-structures are with the acplane of the R-MoO3 parallel to the surface of the sillimanitestalk fiber, and their c-axis is usually perpendicular to the length of the SAO fiber (Figure 2c,n). In few cases, belts or plates with c-axis approximately parallel to SAO-stalk fiber were observed (Figure 2o,p). In the literature, growth mechanisms of the MoO3 belts through the vapor transport method are vapor-solid (VS)12,21,22 and vapor-solid-liquid (VLS).38 In the VS mechanism, belts are growing from the bottom-end similar to the grass-like growth, while in the classic VLS mechanism they grow from a liquid droplet located at the tip. Without droplet, VLS-growth is not possible. Li et al.38 found droplets at the tip of MoO3 belts grown by a vapor transport method and appreciated that this is the signature of the classic tip-like VLS. As we have



Table 1. Analysis of the Thickness Enhancement Mechanism mechanism of thickness enhancement thin-film-like growth

spreading/migration from growth site to the tip

supporting arguments occurrence of steps thin film growth of MoO3 is reported in ref 2 it is not in contradiction with the proposed VS- mechanism occurrence of nonparallel steps

discussed for the BiSrCaCuO whiskers,56 a bottom-end mechanism, which for the BiSrCaCuO whiskers is mainly a solidliquid (SL) one, can also produce whiskers with droplets at the tip. Furthermore, the substrate temperatures in the experiments from ref 38 are high so that occurrence of a liquid, in a basically VS-like mechanism leading to formation of droplets at the tip, would not be a surprise. For the MoO3 belts from this work, droplets were not found, but the presence of a liquid phase or a similar state is possible. It was noted in the literature that MoO3 has a high spreading ability, for example, over several hundred micrometers on alumina surface.31 Several models were proposed for this behavior, and, depending on conditions, this phenomenon resembles wetting of the materials by a liquid. Hence, a modified VS mechanism involving liquid phases and denoted V(L)S is considered for the growth of MoO3 belts. Spreading can be accompanied or followed by formation of island clusters.31,34 Formation of islands on SAO fibers is presented in Figure 2 (l and m, top fiber). In more advanced stages (Figure 2m, bottom fiber) one can see that islands are larger with a planar and approximately rhombic shape. At this stage, growth into larger islands might be favored by Oswald ripening, that is, growth of larger crystals at the expense of the smaller ones from the surface energy minimization considerations. Oswald ripening hints once more at the presence of a liquid. When a crystal becomes large enough relative to the SAO fiber diameter, its tip will detach from the surface of the fiber (Figure 2n) and a belt with triangular tips can develop in one or two opposite directions (of the c-axis for a R-MoO3 belt). Schematically, the process of growth is presented in Figure 6. In a similar manner, β-MoO3 may also form in the b-axis direction. The curved surface of the SAO fiber plays an important role in promoting directional growth: the most convenient situation for tip detachment is when the MoO3 belt is perpendicular to the SAO fiber, and once the tip is free it may be in a different condition, for example, at lower temperature due to gas flow, than the part attached to the fiber. Combined effects of curved surface and gradients promote directional growth and explain formation of hierarchical structures and their preferential orientation versus gas flow (SAO is parallel to flow direction). Extra arguments supporting the proposed growth sequence and V(L)S-like mechanism are: (a) The initial MoO3 rhomb-crystals formed on SAO are randomly oriented. Therefore, directional growth is not controlled by the MoO3-SAO epitaxial relationship. (b) Higher order of branching does not occur, and this means that the SAO fiber is the support in a bottom-end VS-like growth mechanism. Criteria for the substrate to allow formation of belts are not clear. Materials used as substrate in the growth of MoO3 belts by the vapor transport method are Al2O3,12,38 Si,21,22 and SiO2.37,39,41 Carbon nanotubes and stepped graphite plates were used as substrate/template within other growth methods.43,44 The material of the substrate seems less important than its geometry (e.g.,

nonsupporting arguments

surface diffusion (spreading) length L of MoO3 is up to hundreds of micrometers,13 but the maximum length of the belts is higher (i.e., 1.5 mm); hence, L should be at least 750 µm when the belt grows into opposite directions and the growth site is located at the half-length growth site is located between MoO3 and SAO; for enhancement of thickness, transport or shift of material along the b-axis (across ac-plane) has to be taken into consideration

size, roughness, shape, surface defects, other). The same idea also results from our observations: MoO3 are growing on SAO fibers (of certain geometry, size, and surface quality), and no belts formed on the other refractor (planar, porous, and rough) ceramic insulation of the furnace. Tips of the MoO3 belts with c-axis parallel to the SAO fibers may also detach, although this case is rare. Such a situation can be visualized in Figure 2o,p. The image from Figure 2p may also suggest that growth may proceed in the a-axis direction, enhancing the width of the belt. Gradients in the growth atmosphere may favor such growth, possibly by influencing stacking rates of MoO6 octahedrons along the a-axis and c-axis. Data are not sufficient to conclude. Through a similar differential-stacking-rate mechanism, plates with or without zigzag edges may grow. Plates may also form through epitaxial sintering along the c-axis between 2 or more belts. If the initial thickness of the merging belts is different, apparently stripes are observed. Different tips of the initial belts may result in zigzag edges. As mentioned (see section 3.1), stripes are more like surface defects. This makes difficult a clear delimitation between stripes and incomplete ac-plane layers. Both defects, stripes and incomplete layers, generate steps on the surface of the belt/plate, and this is actually what one can observe. Steps are of two types: running parallel (Figure 4a) and non-parallel (Figure 2e) to the length of the MoO3 belt. Classification of the growth defects, their growth mechanism, and their location and orientation versus belt geometry may give valuable information in understanding growth mechanism. The reason is that growth defects are “witness” elements of the belts growth history. This idea was applied to study and to give an answer to the question on how the thickness of the belts enhanced from less than 20 nm on the initial stages of growth to values of 1 or 2 orders of magnitude higher. Unfortunately, incomplete understanding of the defects’ origin in MoO3 allows only a partial answer: steps are evidence for a 2D layer-by-layer growth resulting in enhancement of thickness. The next step was to consider two possibilities for this 2D growth: (1) a thinfilm-like growth with direct supply of atomic species to the already formed belt, or (2) a growth through spreading/migration to the tip from the growth site located at the MoO3-SAO joint. Arguments or weak points in supporting each mechanism based on available data are presented in Table 1. Analysis of Table 1 suggests that the thin-film-like mechanism is more probable, but extra experiments are necessary. At long growth times (over 6 h), new structures are observed. These new structures are flower-like (F-structure) hierarchical structures. SAO stalk cannot be observed (Figure 2d,e). It is very possible that these structures form at the tip of the SAO fibers that often have a droplet shape and belts grow in the same manner as those from the T-structures. Imperfect orientation of MoO3 belts in the F-structure versus SAO stalk is probably due to more available orientations of the initial crystals from a half-


sphere surface than a cylinder, and due to interaction between the belts when their density is high. For a structure with a high density of belts and imperfect orientation, one has also to consider the possibility that between edges or faces of the neighbor MoO3 belts, near the SAO fiber, angles may become favorable and promote nucleation of new belts. 4. Conclusion MoO3 belts with diameters in the nanometer and micrometer range were obtained by the vapor transport method on sillimanite fibers. MoO3 and SAO fibers form first-order hierarchical structures of two types: branched treelike with the stalk being SAO fiber and flower-like. Higher order branching does not occur. The growth mechanism of MoO3 belts on SAO fibers is mainly of VS-like type with some specific features, and it is summarized as follows: (a) Vapors transport to SAO fibers; (b) arrival of the vapor species on the SAO fiber, their spreading and formation of islands; (c) formation of small platelike MoO3crystals randomly oriented on SAO fiber; (d) growth of MoO3 crystals due to Oswald ripening; (e) favorably oriented MoO3 crystals versus SAO fiber and growth environment conditions develop into MoO3 belts through detachment of the tip from the surface of SAO fiber; curved surface of the substrate (sillimanite fibers) is important for the directional growth control resulting in the formation of MoO3-SAO hierarchical structures; (b)-(e) are likely accompanied by the presence of a liquid-like phase; (f) on the already formed belts, a 2D layer-by-layer growth seems possible; consequently, thickness of the belts enhances; available data suggest that this process probably takes place through direct deposition of vapors as in the thin films growth; and (g) in the advanced stages of growth, flower-like structures occur; it is thought that this is due to the high density of belts from the T-structures and the SAO fibers’ sphere geometry at the ends. Some growth mechanism aspects require more research. Most belts are of R-MoO3 phase, and they show at both endings triangular-shape tips with certain angles. Such morphology is expected to be useful in, for example, field-emission applications. The mechanism by which triangular tips are generated is not clear, although some possible scenarios are discussed. It is shown that the vapor transport method allows the growth of β-MoO3 belts contrary to the current impression that they can be obtained only by solution growth methods. Growth defects, composition, and aging of the MoO3 belts are reported. Acknowledgment. The author thanks Y. Hayasaka and E. Aoyagi (Tohoku University, Japan) for technical assistance with SEM and TEM measurements and Prof. K. Togano (NIMS, Japan) for continuous encouragement and support of this work. References (1) Kihlborg, L. Ark. Kemi 1963, 24, 357. (2) Carcia, P. F.; McCarron, E. M. Thin Solid Films 1987, 155, 53. (3) Seguin, L.; Figlarz, M.; Cavagnat, R.; Lassegues, J.-C. Spectrochim. Acta, Part A 1995, 51, 1323. (4) Parise, J. B.; McCarron, E. M., III; Von Dreele, R.; Goldstone, J. A. J. Solid State Chem. 1991, 93, 193. (5) Wang, S.; Zhang, Y.; Ma, X.; Wang, W.; Li, X.; Zhang, Z.; Qian, Y. Solid State Commun. 2005, 136, 283. (6) Wei, M. X.; Zeng, C. H. J. Phys. Chem. 2003, 107, 2619. (7) Song, J.; Wang, X.; Ni, X.; Zheng, H.; Zhang, Z.; Ji, M.; Shen, T.; Wang, X. Mater. Res. Bull. 2005, 40, 1751.

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CG060893S

Preparation through the Vapor Transport and Growth Mechanism of

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