Formation of In2O3 Microrods in Thermal Treated InSe Single Crystal

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Formation of In2O3 Microrods in Thermal Treated InSe Single Crystal Tiziana Siciliano,* Antonio Tepore, Gioacchino Micocci, Alessandra Genga, Maria Siciliano, and Emanuela Filippo Dipartimento di Scienza dei Materiali, Universita del Salento, I-73100 Lecce, Italy

bS Supporting Information ABSTRACT: In2O3 microrods were grown by thermal oxidation of InSe single crystal under a mixture of argon
oxygen flow without the presence of a catalyst. Microrods were obtained at the temperature of about 640 °C after a thermal treatment of 180 min. The morphology, structure, and composition of the prepared materials were studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction spectroscopy (XRD). The microrods had a hexagonal cross-section with a diameter range of 1
3 μm and a length of between 20 and 30 μm. Structural analysis showed that the microstructures were cubic In2O3 crystal with a lattice constant of a = 10.115 Å. The possible mechanism of the formation of In2O3 microstructures is also discussed in this work.

1. INTRODUCTION Low-dimensional elongated semiconductor structures such as wires, tubes, or rods have attracted a large amount of interest in the past decade due to their potential applications in future optical and electronic micro- and nanodevices. Various inorganic elongate materials such as oxides,1
4 sulfides,5
7 and nitrides8 as well as elemental materials9,10 have been fabricated. Indium oxide (In2O3) is a material of particular interest for fundamental research and various device applications. This material is used in liquid crystal displays,11 solar cells,12,13 solid electrolyte cells,14 optoelectronic devices,15 information storage,16 biosensors,17 and gas sensors.18 Recently, the principal interest to In2O3 nanomaterials lies in the possibility of using In2O3 nanocrystals as nonlinear optical materials.19,20 In particular, Kityk et al. using photoinduced secondorder nonlinear optical methods determined that monodisperse In2O3 nanocrystals incorporated into polymer matrices possessed second-order optical susceptibilities almost one order higher than that of traditional nanolayers of In2O3. According to the literature, indium oxide elongated micro- and nanostructures have been usually synthesized using various growth methods, including chemical vapor deposition,21,22 laser ablation,23,24 sol
gel,25 and physical evaporation.26 In the present work, a fairly uniform ensemble of well-formed vertically aligned In2O3 microrods with defined facets and hexagonal cross-section were grown by thermal treatment of InSe single crystal under a mixture of argon
oxygen flow without the presence of catalyst. To the best of our knowledge, the morphological modification r 2011 American Chemical Society

of the surface of layered III
VI compounds by air oxidation has been only reported by Balitskii27 and Bakhtinov et al.28 The advantage of this method lies in the fact that it leads to the growth of micro- and nanostructures directly on the sample surface which acts as source as well as substrate.

2. EXPERIMENTAL SECTION InSe single crystal was grown in our laboratory by means of the Bridgman
Stockbarger method from an indium-rich polycrystalline melt with 52 atom % indium and 48 atom % selenium. The details of the experimental procedure of crystal growth and the composition analysis are reported elsewhere.29 Samples were obtained by cleaving the ingot parallel to the layers with a razor blade. The samples were then placed in a sapphire crucible in the center of a horizontal furnace and thermally treated with argon
oxygen flow of 100 sccm at different temperatures and for different times. It was found that treatment in the temperature range of 620
650 °C led to the growth of well-developed microrods structures on the surface of the samples. Therefore, in this work the results of samples thermally treated at the temperature of 640 °C for times of 60, 90, and 180 min were reported. General morphology of synthesized samples was determined using scanning electron microscopy (ESEM, FEI XL30). For the elemental analysis, the microscope was equipped with an energy dispersive X-ray Received: January 21, 2011 Revised: March 9, 2011 Published: March 23, 2011 1924

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Crystal Growth & Design (EDS) microanalysis detector. In our experiment, scanning electron microscopy (SEM) observations and microanalysis were carried out at a chamber pressure of 5 Pa (0.05 Torr); in such conditions the eventual contribution of oxygen coming from the environmental gas to the X-ray spectra could be negligible.30 The structure and crystal phases were determined by X-ray diffraction (XRD, Rigaku MiniFlex diffractometer) measurements with CuKR radiation (λ = 1.54056 Å), employing a scanning rate of 0.02° s
1 in the 2θ range from 15° to 75°.

3. RESULTS AND DISCUSSION The synthesized materials obtained at different thermal annealing times were first analyzed by SEM to follow the surface morphology evolution process. SEM images reported in Figure 1 clearly showed the surface morphology was heavily affected by thermal treatment. Figure 1a shows that the surface of no thermally treated starting InSe sample was smooth and uniform. During the thermal treatment, formation of microcrystalline structures with different morphologies was observed. In the initial stage of the growth process (t = 60 min), the SEM image

Figure 1. Typical SEM image of (a) as-prepared InSe single crystal and of the same crystal after a thermal treatment of (b) 60 min, (c) 90 min, and (d) 180 min. Inset: Cross-section of a single microrod.

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(Figure 1b) clearly indicates the formation of grains with welldefined facets over the whole sample surface. In the subsequent development of the structures (t = 90 min), the growth of embryonic microrods nucleated on these grains was observed (Figure 1c). Finally, as annealing proceeded (t = 180 min), these structures continued to grow into vertical microrods with a length in the range of 20
30 μm and diameter in the range of 1
3 μm (Figure 1d). The inset of Figure 1d shows a higher magnification of a single microrod and reveals that the rods had hexagonal cross sections with well-defined facets. A careful SEM observation at higher magnification demonstrated that some microrods exhibited rough microparticles on the top (Figure 2a). It is evident that those microparticles were just settled on the rod and they were not fused with them. More complex structures as flower-like (Figure 2b) or branched structures (Figure 2c) were also occasionally observed. Cross-sectional SEM analysis of the sample treated for 180 min (Figure 3) shows the detail between the substrate, the grains, and the microrods. These observations suggest that the structures grown after an annealing of 60 and 90 min were a preliminary stage of growth compared to those observed after a treatment of 180 min; such a continuous growth mechanism first led to the formation of the grains on which, in a second stage, the microrods directly nucleated and grew. Moreover, we speculate that the microparticles on the top of the rods were formed after the heating was switched off through the condensation of the vapor present in the furnace. Figure 3 also allowed us to estimate that the thickness of the grains on which microrods grew was in the range of 9
19 μm. The elemental chemical composition of the observed structures was performed by energy dispersive X-ray (EDS) microanalysis in SEM. Typical EDS spectra of the grains, microrod stems, and microparticles are shown in Figure 4a
c, respectively. It is evident that the detected elements were oxygen, selenium, and indium in all observed microstructure and that the EDS spectra acquired on the grains were very similar to the spectra acquired on the stems; moreover, it was found that the selenium content was much higher in the microparticle than in the grains and in the stem of the rods. In detail, while in grains and microrods selenium was detected in amounts ranging from about 1.7 atom % to a maximum of about 4.3 atom %, in the microparticles on the top of the rods the typical concentrations ranged from 24.7 to 39.7 atom %. Further comparison of the oxygen and indium atomic content revealed that in grains and microrods oxygen was in the range of 62.1
65.1 atom % and indium was rather constant with values of 33.2
33.6 atom %. A careful analysis of the previous data allowed us to deduce the

Figure 2. (a) Higher magnification SEM image of the microrods exhibiting rough microparticles on the top; (b) flower-like; and (c) branched microstructures observed on the sample. 1925

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Crystal Growth & Design selenium content anticorrelated to that of oxygen, indicating that probably during the thermal annealing oxygen replaced selenium in its site. The microparticles on the top of the rods exhibited oxygen and indium atomic content in the range of 48.5
31.7 atom % and 26.8
28.6 atom %, respectively. In order to further analyze the formation of the protruding elongated microstructures created on the InSe crystal surface, the compositional distribution of In, Se, and O was examined by plotting the intensity of the In LR1, Se LR1, and O KR peaks

Figure 3. Cross-section image of the microrods showing their growth mechanism.

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across and along their axis. Elemental line scan profiles were recorded along and across the longitudinal axis of the single microrod. The isolated microrod with the corresponding chemical concentration profiles are reported in Figure 5a,b. They revealed the uniform and continuous spatial distribution of the constituent elements throughout the microrod across and along its longitudinal axis (including the substrate). This suggested that after thermal annealing proceeded for 180 min, the oxidation process involved all the InSe crystal and not only its surface. Moreover, Figure 5a showed that selenium content strongly increased in the microparticle of the top of the rod as expected from previous microanalysis data. The crystallinity of the microstructures formed at different growth stages was then analyzed using XRD. Figure 6 shows the XRD pattern of the starting material. θ-2θ scan data exhibit strong 2θ peaks at 21.38°, 32.28°, 43.44°, and 67.34°, respectively, corresponding to the (006), (009), (0012), and (0018) diffraction planes of hexagonal crystal structure of InSe with lattice parameters of a = 4.003 Å and c = 24.947 Å. These values are comparable with the standard values (a = 4.002 Å and c = 24.946 Å) reported in the literature (JCPDS-Card No. 42-0919). No discernible peaks due to other indium
selenium phases or impurities appear in this XRD spectrum. The samples annealed in an Ar
O2 flow exhibited a completely different structure, indicating that after the thermal treatment InSe was transformed to a new crystal structure. Figure 7 shows the XRD spectra of the oxidized samples after annealing at a temperature of 640 °C for 60, 90, and 180 min.

Figure 4. Typical EDS spectra recorded from the (a) grain, (b) stem of the microrod, and (c) microparticles located on the top of the microrod.

Figure 5. Elemental concentration profile (a) along the longitudinal axis of the microrod and (b) across the microrod. 1926

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Figure 6. XRD pattern of InSe used as source material.

Figure 8. SEM images of the microstructures observed on the samples annealed for (a
c) 60 min, (d) 90 min, and (e) 180 min: (a
c) different degrees of truncated microcubes, (d) truncated octahedron with microrod nucleating on its facet, and (e) truncated octahedron with microrod grown on its facet and capped by a rough microparticle. Figure 7. XRD spectra of In2O3 microrods obtained at 640 °C for different growth times.

The most intensive peak presented is shown at 2θ = 30.68°. The peaks of these patterns could be indexed to a body-centered cubic structure of In2O3 with a lattice constant of a = 10.115 Å, which is consistent with the standard value (JCPDS 06-0416). From Figure 7, it is clearly evident that the prolonged annealing tended to improve the crystallinity of In2O3 microstructures. In fact, as annealing time increased, the intensity of the (222) diffracted peak became more intense and sharper, suggesting a higher degree of crystallization. No other peaks related to Se, Se
O, or In
Se compounds were detected in the spectra within the detection limit of the X-ray diffraction. The XRD pattern of the sample treated for 180 min has been compared with the standard diffraction pattern of bcc-In2O3 (Figure S1, Supporting Information). Comparing the ratio of the intensity of two diffraction peaks (222) and (400) (I222/I400) of the standard bulk with that of the microrods, it was found that this ratio increased from 3.3 to 8.3. This observation indicated that [111] orientation has become dominant and suggested that the microrods might have the preferential [111] growth direction. This observation was further confirmed by the growth mechanism proposed for the hexagonal rods. The evolution of the morphological features of the InSe surface under heating was carefully investigated through AFM images of sample thermally treated for various oxidation times by Bakhtinov et al,.28 and it was associated with the formation of oxide crystallites. In particular, the oxidation of the InSe surface has been found to be an evolutionary process consisting of several stages: the replacement of selenium by oxygen in the

crystal lattice; oxidation of the chalcogen and the subsequent evaporation of the volatile compounds; the formation of crystalline In2O3 and recrystallization of In2O3 during prolonged annealing. Since atomic radius of oxygen is 0.66 Å and that of selenium is 1.3 Å, the diffusion of oxygen through selenium vacancies caused the plastic deformation of the InSe layer. Moreover, the layered semiconductors are characterized by highly anisotropic elastic properties due to the presence of two types of bonding in the crystal (strong bonding in a layer and weak interlayer bonding). As a result of this feature, a significant expansion along the c axis is accompanied by lateral compression. Such a deformation involved not only the uppermost surface layer but also the deeper crystal layers. In the successive step of the oxidation process of the layered anisotropic InSe crystal, nucleation of oxide nanocrystals and their growth and coalescence took place on plastically deformed surface. It is clear that the peculiar feature of the oxidation process of InSe single crystal was that the surface oxide film was formed via successive phase transformations, which were accompanied by drastic variations in the crystal structure. In order to explain the successive anisotropic hexagonal growth of the microrods, it is necessary to investigate the morphological evolution of the grains on which the microrods nucleated and grew. Figure 8 shows the general morphologies of the products obtained at different annealing times. Careful observation of the grains formed on the samples annealed for 60 and 90 min revealed the presence of two main distinct morphologies, truncated microcubes and truncated octahedrons, which are both consistent with the cubic crystal structure of In2 O3 . Figure 8a displayed the SEM image of a truncated microcube 1927

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Crystal Growth & Design

Figure 9. Schematic illustration of (a) and (b) the morphological evolution of grains and (c) growth process of microrods.

observed on the surface of the sample treated for 60 min with the corners truncated and replaced by triangular facets. Figure 8b,c revealed that they were truncated to different degrees. Similar observations on In2O3 microparticles have been already reported by Shi et al.31 Truncated octahedron represented the main morphology of the grains observed on the sample thermally treated for 90 and 180 min (Figure 8, panels d and e, respectively); it was clear that the microrods nucleated and grew on a hexagonal facet of such octahedrons. The dominant factors for the formation of anisotropic morphologies of the observed microstructures were found to be the vapor-phase concentration and the surface energy of the growing surface planes; namely, the gas-phase supersaturation of the atoms or molecules of the growing material determines the growth rate of the crystal, and the surface energy of the growing surface planes determines the significance of which in the final crystal.31 Wang32 claimed that the growth of crystals is related to the relative growth rate of different crystal facets, and the difference in the growth rates of various crystal facets results in a different outlook of the crystallites. For In2O3 with a bcc structure, the surface energy relationship of three low-index crystallographic planes is γ {111} < γ {100} < γ {110}. The highest surface energy facets of the crystal have the fastest growth rate, and the fastest growing planes should disappear to leave behind the lower surface energy facets as the planes of the product. Moreover, the shape of a cubic crystal phase is mainly determined by the ratio (r) between the growth rates along the [100] and [111] directions.32 Octahedrons and tetrahedrons bound by the most-stable {111} planes will be formed when r = 1.73 and cubic-like particles bound by the less-stable {100} planes will result if r = 0.58. However, if 0.58 < r < 1.73, both the {100} and {111} facets will appear, resulting in the formation of truncated cubes. The sample thermally treated for 60 min exhibited also some not-faceted crystallites. The formation of such oxide crystallites was due to the plastic deformation of InSe surface during oxidation process, as explained by Bakhtinov et al.28 Therefore, In2O3 vapors generated during thermal treatment nucleated on these grains. This mechanism led to the formation of grains with a faceted morphology, and, as heating continued, the different planes grew accordingly to their surface energy. In fact, in the duration of the reaction time, the concentration of In2O3 vapors increases33 and thus progressively reduced the growth rate along the [111] direction and/or enhanced the growth rate along the [100]

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direction, and enhanced r to a value higher than 0.58.31 This process favored the formation of grains shaped as truncated microcubes and octahedrons. The {111} facets started to be formed on the surface of these grains and even became larger, until reaching a hexagonal shape in the truncated octahedron shape. The hexagonal microrods were believed to originate from such micrograins which exhibited a [111] facet with a hexagonal shape (Figure 8d,e). The microrods started to grow along the downward [111] direction because the {111} facets were adequately exposed and had much chance to absorb the In2O3 vapor.31 A schematic illustration of the proposed gradual morphological evolution of the grains and growth process of the microrods is given in Figure 9. Moreover, the excess vapor concentration and saturation leads to the formation of rough microparticles that are attached on the top of some In2O3 microrods, similar to the microstructures observed by Yin et al.33 The results mentioned above demonstrated that our experimental conditions favored high growth rate and appearance of low surface energy {111} planes. Thus, the low surface energy planes guided the formation of anisotropic truncated microcubes and truncated octahedrons from which hexagonal microrods started their growth. Because no catalyst was used, the above-described growth process of In2O3 microstructures can be interpreted by a vaporphase (VS) mechanism.34 In our experiment, the substrate was located at the center of the furnace, and the gas flew from the right to the left. The majority of the microstructures were formed on the left part of the substrate. Such observation suggests that In2O3 vapors generated from a part of the substrate were transported by the gas flow and deposited on the adjacent part of substrate itself.

4. CONCLUSIONS In summary, thermal annealing of InSe single crystals at a temperature of about 640 °C under a mixture of Ar
O2 gas flow caused the decomposition of the compound and formation of body-centered cubic In2O3 microstructures with shapes depending on the duration of thermal treatment. In the final stage of growth, well-formed microrods with hexagonal cross-section were observed. The rods had a diameter in the range of 1
3 μm and a length of between 20
30 μm. This method led to the growth of the single crystal microstructures directly on the InSe sample surface, which acted as source as well as substrate without the presence of any catalysts. We proposed a multistep process to explain the growth mechanism of the In2O3 microrods starting from InSe single crystal. In the first stage, oxidation and plastic deformation of InSe surface occurred, as discussed above, leading to the formation of oxide crystallites. As heating proceeded, the increase of the concentration could favor the formation of In2O3 grains shaped as truncated microcubes and truncated octahedrons exhibiting {111} facets with hexagonal shape. The hexagonal microrods were believed to originate from such truncated octahedrons starting from {111} facet. It is evident that the initial deformation of the InSe crystal surface and the formation of In2O3 crystallites were essential for the subsequent formation of faceted grains. Moreover, EDX microanalysis demonstrated that the In2O3 microrods formed from InSe single crystal still contained a modest amount of selenium. This feature could influence for example the sensing properties of In2O3 microstructures. Among the sensing materials, semiconductors oxides such as In2O3, 1928

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Crystal Growth & Design ZnO, SnO2, etc. are widely used in the electrical detection of the pollutant gases due to their ability to change the conductivity when they come in contact with the gas molecules. In order to attain a higher response and selectivity, different approaches such structure control and doping have been adopted to modify the sensing properties of semiconductor metal oxide gas sensors. It is well-known that the sensing mechanism is based on the surface reaction of the structure with the exposed gas (adsorption and desorption of the test gas molecules). As adsorption is a surface effect, one can increase the adsorption of gas molecules by decorating with nanoparticles or doping with an element which has a stronger chemical affinity for that particular gas molecule.35 In particular, selenium has a good affinity for Hg at the low levels,36 and recently Se nanostructures have been investigated as promising gas sensors of volatile organic compounds.37 In this context, Se-containing In2O3 microrods can be explored as an innovative pollutant sensor.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 (XRD spectra). This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]. Phone: þ39 0832 297073. Fax: þ39 0832 297062.

’ ACKNOWLEDGMENT The authors thank A. R. De Bartolomeo and G. D’Elia for their technical assistance during the measurements. ’ REFERENCES (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829–R858. (2) Xu, N.; Liu, Z.-H.; Ma, X.; Qiao, S.; Yuan, J. J. Nanopart. Res. 2009, 11, 1107–1115. (3) Zhang, K.; Rossi, C.; Tenailleau, C.; Alphonse, P.; Chane-Ching, J.-Y. Nanotechnology 2007, 18 (275607), 1–8. (4) Park, J.; Ryu, Y.; Kim, H.; Yu, C. Nanotechnology 2009, 20 (105608), 1–8. (5) Kim, J. H.; Park, H.; Hsu, C.-H.; Xu, J. J. Phys. Chem. C 2010, 114, 9634–9639. (6) Peng, H.; Jiang, L.; Huang, J.; Li, G. J. Nanopart. Res. 2007, 9, 1163–1166. (7) Zhang, Y.; Du, Y.; Xu, H.; Wang, Q. CrystEngComm 2010, 12, 3658–3663. (8) Gupta, S.; Kang, H.; Strassburg, M.; Asghar, A.; Kane, M.; Fenwick, W. E.; Dietz, N.; Ferguson, I. T. J. Cryst. Growth 2006, 287, 596–600. (9) Yang, X.; Wang, L.; Yang, S. Mater. Lett. 2007, 61, 2904–2907. (10) Fan, S.; Li, G.; Zhang, X.; Mu, H.; Zhou, B.; Gong, L.; Liang, H.; Guo, L.; Guo, J. Cryst. Growth Des. 2009, 9, 95–99. (11) Smith, J. F.; Aronson, A. J.; Chen, D.; Class, W. H. Thin Solid Films 1980, 72, 469–474. (12) Hara, K.; Sayama, K.; Arakawa, H. Sol. Energ. Mater. Sol. C 2000, 62, 441–447. (13) Zhou, Z. B.; Cui, R. Q.; Pang, Q. J.; Wang, Y. D.; Meng, F. Y.; Sun, T. T.; Ding, Z. M.; Yu, X. B. Appl. Surf. Sci. 2001, 172, 245–252. (14) Jeong, J. I.; Moon, J. H.; Hong, J. H.; Kang, J. S. J. Vac. Sci. Technol. A 1996, 14, 293–298.

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