In2O3 Nanotowers: Controlled Synthesis and Mechanism Analysis You-Guo Yan,* Ye Zhang, Hai-Bo Zeng, and Li-De Zhang* Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, P. R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 940-943
ReceiVed October 16, 2006; ReVised Manuscript ReceiVed January 24, 2007
ABSTRACT: Two kinds of novel In2O3 nanotowers were synthesized in an improved chemical vapor deposition (CVD) system through controlling the kinetics factors (saturation ratio). The morphologies and crystalline structures were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). A periodical one-dimensional (1-D) and persistent 0-D growth was proposed to explain the formation of nanotower I, and the formation of nanotower II was ascribed to the alternate 1-D and 0-D growth. The growth mechanism analysis was useful to realize the relation between the kinetics factors and the complex nanostructure. The synthesis of complex tower-shaped structures not only enriched the synthesis science but also provided new blocks in future architecture of functional nanodevices. 1. Introduction In2O3, an important wide band gap (3.6 eV) and transparent semiconductor, has been widely used in the opto-electronic field as window heaters, solar cells, flat panel display materials, and gas sensors.1-3 There is increasing interest in the synthesis and properties of In2O3 nanostructures due to their novel characteristics and important potential applications in sensing,4,5 lightemitting diodes, and nanoscale transistors,6,7 Up to now, many methods have been developed to synthesize various In2O3 nanostructures,suchasnanowires,8-11 nanorods,12-14 nanobelts,15-18 nanotubes,19,20 nanotubes filled with In,21 nanoparticles (cubes, octahedra, pyramids),22-28 nanochains,29,30 etc. It is well-known that the properties of nanostructures strongly depend on their morphologies. Different morphologies have special applications, such as nanowires for optoelectronics,11,31,32 efficient field emission,33,34 gas sensors,35,36 chemical sensors,37 and biosensors,38 nanoparticles for efficient field emission and toxic gas sensing,27,39 etc. In previous reports, most of the efforts were focused on the synthesis and properties of single morphology nanostructures. Research on the complex nanostructure was limited, while investigation of the synthesis and properties of complex nanostructures represented developing directions of nanoscience and nanotechnology, which have important potential applications in realizing the multiple functions of nanodevices. In our article, two kinds of novel tower-shaped structures were synthesized through controlling the kinetics factor (saturation ratio), and the growth mechanisms are discussed at length. The growth mechanism analysis was helpful to understand the relationship between the kinetics factors and the complex structures, and it was valuable to realize the controlled synthesis of complex nanostructures. The synthesis of complex, towershaped nanostructures not only enriched the synthesis science but also provided new building blocks in the future architecture of functional nanodevices. 2. Experimental Section The synthesis of these two kinds of In2O3 nanotowers was performed in an improved chemical vapor deposition (CVD) system (as shown in Figure 1). The system had two primary superiorities in comparison with the conventional CVD system. The first was that the tube was * Corresponding authors. E-mail:
[email protected] (Y.-G.Y.); fax: +86551-5591434;
[email protected] (L.-D.Z.).
Figure 1. Configuration of the improved CVD system. one-ended, which made the airflow reverse and increased the saturation ratio of the reagent species around the sealed end, and the second was the location of the substrate, which was put at the upside of the source, not downstream like conventional CVD systems. This design ensured that the reagent vapor could be directly translated to the substrate and avoided the consumption in carrying material downstream. In a typical experimental procedure (experiment I), 0.5 g of high-purity indium particles was put into an alumina boat. A silicon wafer coated with 3-5 nm Au film was placed at the upside of the indium particles. Before heating, the tube chamber was purged for 30 min with 100 mL‚min-1 high purity Ar, and then the furnace was heated to 700 °C at a rate of 25 °C/min and kept at that temperature for 60 min. After the reaction, a layer of yellow product was found deposited onto the silicon wafer. In experiment II, a mixture of oxide indium and active carbon power (molar ratio ∼1:6) was adopted as the source, the reaction temperature was 900 °C, and a cleaned catalyst-free silicon wafer was used as the substrate. Keeping other experimental conditions unchanged, after the reaction, a layer of yellow products was found deposited onto the substrate. The crystalline phase and morphological and structural features of the products were investigated by X-ray diffraction (XRD) (Philips X’pert-PRO, Cu KR (0.15419 nm) radiation), field emission scanning electronic microscope (FESEM, Sirion 200 FEG), and high-resolution transmission electron microscopy (HRTEM, JEOL 2010, at 200 kV).
3. Results and Discussion Figure 2 shows the XRD spectra of the obtained products in experiments I and II. All the peaks in the two samples could be indexed to cubic In2O3 crystalline with a lattice constant of a ) 10.11 Å (JCPDS card no. 06-0416), Moreover, no other impurity phases, such as In and Au, were detected. As to the product in experiment I, a low-magnification FESEM image (Figure 3a) showed that some tapered nanowhiskers and a layer of particles were deposited onto the silicon wafer. A magnified FESEM image (Figure 3b) showed that the tapered nanowhisker presented a multilayered structure. We
10.1021/cg0607194 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
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Figure 2. XRD spectra of the obtained products: (a) nanotower I and (b) nanotower II.
defined this structure as nanotower I. Careful observation revealed that there was a particle on the tip of each nanotower. The inset on down-left corner in Figure 3b shows a typical nanotower. It is clearly seen that the tower is a layered structure with a decreasing size from the bottom to the tip. Each layer of the tower had an octahedral configuration with a truncated bottom and tip. Detailed observations of the particle film (in Figure 3b) indicated that the particle had an imperfect octahedral configuration; no particles were found on their surface. A low-magnification FESEM image (Figure 3c) of the products in experiment II indicated that bulk rodlike structures dispersed on the silicon wafer. A magnified image (Figure 3d) revealed that the nanorod consisted of linked octahedra with a truncated bottom and tip and ended with an imperfect octahedral cap. We defined this structure as nanotower II. Unlike the nanotower I, the nanotower II had a relatively uniform size of about 400 nm. The insets on the upper-right corner in Figure 3d gave closeup images of the body and the tip of the tower, which further confirmed the tower-shaped structure and revealed
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that the tower was composed of connected truncated octahedra of relative uniform size. The nanotower I was further characterized by transmission electron microscopy (TEM). Figure 4a was a low magnification TEM image of the nanotower I, which revealed the layered structures of nanotower I. Figure 4b, the HRTEM image of the black pane in Figure 4a and the corresponding selected area electron diffraction (SAED) pattern, indicated that the growth direction of the nanotower was along the [001] and the lattice plane spacing of (002) was 0.506 nm. As to nanotower I, its growth was not controlled by the single vapor-liquid-solid (VLS) mechanism. Alhough a catalyst particle also existed at the tip of the nanotower, the gradually increasing size from the tip to the bottom and the periodical appearance of a truncated octahedron along the growth direction could not be explained well by the VLS mechanism. We proposed following growth mechanism to explain the growth of this tower-shaped structure: periodical one dimensional (1D) growth controlled by a VLS mechanism and persistent 0-D growth controlled by a vapor-solid (VS) mechanism. It is well-known that, in the growth process of VLS, four consecutive steps are involved as shown in Figure 5a: (1) the transport of the reagent species in the vapor phase and on the surface of the growing nanostructures; (2) the dissolution of the species at the droplet surface; (3) the diffusion of the species inside the droplets; and (4) precipitation, incorporation, and crystal growth at the liquid-solid interface between the catalyst droplet and the solid nanowire. In experiment I, the low temperature (700 °C) restricted the abundant evaporation of indium atoms, and so, we presumed that the supply of reagent atoms in the droplet was insufficient to maintain continuous 1-D growth. Furthermore, the transfer rate of the reagent species was not high under low temperature (700 °C). As a result, some of these absorbed reagent species on the surface of the growing nanostructure would incorporate into the lateral surface of the growing nanostructure, while not all the absorbed species transported to the top catalyst particle maintaining the 1D
Figure 3. (a, b) Low- and high-magnification FESEM images of nanotower I; the inset in (b) shows a typical spectra of the nanotower; (c, d) lowand high-magnification FESEM images of nanotower II; the insets in (d) give closeup images of the tip and body of nanotower II.
942 Crystal Growth & Design, Vol. 7, No. 5, 2007
Figure 4. (a, b) TEM and HRTEM images showing nanotower I; the inset in (b) shows the corresponding electron diffraction (ED) pattern.
Figure 5. (a) Schematic illustration VLS growth steps for 1-D nanostructures; the growth map of the nanorods; (b) schematic illustration of 0-D growth with octahedral configuration under the VS mechanism.
Figure 6. (a, b) Growth process maps of nanorods, nanotower I, and nanotower II, respectively.
growth. On the basis of the above two points, we proposed following growth process of nanotower I (as shown in Figure 6a). In the stage of heating, the Au film split into small Au particles and acted as the preferential adsorbing sites to the
Yan et al.
reagent species, and the adsorbed species then dissolved into the liquid droplet. The In comes from the evaporation of metal In particles, and the oxygen maybe comes from (1) the residue oxygen in the alumina tubes and the adsorbed oxygen on the surface of alumina boats because we did not vacuumize the alumina tubes in advance; (2) the leakage of the reaction system.25 When the vapor pressure of the reagent species in the catalyst droplet reached supersaturation, the 1-D growth controlled by VLS initiated, and the solid In2O3 nanorod grew out of the liquid droplet with a size confined by the catalyst particle. As mentioned above, the supply of reagent atoms in the droplet was insufficient to maintain continuous 1-D growth. In the process of 1-D growth, abundant reagent species were consumed and the saturation ratio of reagent species in the droplet decreased, and the 1-D growth stopped. Subsequently, some of the absorbed reagent species on the surface of the growing nanorods incorporated into the crystal lattice maintaining the lateral growth under the VS mechanism. It is well-known that the growth rate perpendicular to different planes is proportional to their surface energies. For In2O3 with a bcc structure, the surface energy relationships among three low-index crystallographic planes should correspond to γ{111} < γ{100} < γ{110}, so the growth rates of three growth directions have such relationships: r< r< rr. As shown in Figure 5b, after the 0-D growth, the (110) plane would disappear earlier, and the (111) plane would be preserved. The lateral growth of the nanorods accorded with the above 0-D growth, and the nanorod would possess an octahedral configuration. During the 0-D growth, the catalyst droplet continually dissolved the reagent species from the vapor phase and the adsorbed reagent species, which transported from the side surface of growing nanorod. When the vapor pressure reached supersaturation again, the 1-D growth restarted, and a new segment of nanorod formed. After the 1-D growth, based on the above analysis, the saturation ratio in the catalyst droplet decreased and a new turn of 0-D growth restarted. It is worth pointing out that the 0-D growth was continuous without intermission during the whole growth process. The periodical 1-D and continuous 0-D growth finally resulted in the formation of nanotower I. The increasing size of the nanotower from tip to bottom could be ascribed to the different growing time. These octahedra at the bottom formed earlier and had a longer lateral growth time than these ones at the tip and certainly possessed a larger size. As to the octahedral particles deposited on the silicon wafer, it may be the result of continuous 0-D growth without the inducement of Au catalyst droplets. In experiment II, the In source came from the carbothermal reduction reaction,40 and the oxygen came from the residue and leakage of the system. As to the nanotower II, we assumed that the supply of indium species was also not enough to maintain the continuous 1-D growth. We proposed a periodical 1-D and 0-D growth mechanism to nanotower II. The periodical growth was controlled by the fluctuation of the saturation ratio. As shown in Figure 6b, at the stage of heating, the indium vapor gradually evaporated out of the In oxide through a carbothermal reduction reaction, so the saturation ratio was low at this stage. Under a low saturation ratio, the 0-D nucleation and growth began, and a truncated octahedron formed under a VS mechanism. During the 0-D growth, the indium vapor uninterruptedly evaporated, and the saturation ratio gradually increased and finally reached the critical value of 1-D growth. Then the 1-D growth at the top plane of the truncated octahedron along the [200] direction began. The 1-D growth consumed abundant reagent species and resulted in a decrease in the
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saturation ratio, so the 1-D growth stopped and new 0-D growth began. The 0-D growth would make the newly formed nanorod have an octahedral configuration. It is worth mentioning that the earlier formed octahedron did not thicken during this turn of 0-D growth, which could be ascribed to the high transfer rate of reagent atoms at high temperature (900 °C). These species absorbed on the side surface of the earlier formed octahedron had a high transfer rate and would transfer to the surface of the nanorod, which had high surface energy, while not incorporating into the low energy plane ((111)) of earlier formed octahedron. During the 0-D growth, the saturation ratio increased, and a new round of 1-D growth began. After the alternating 0-D and 1-D growth, the body of tower-shaped structure formed. As to the octahedral shaped cap, it was considered formed at the end of the reaction under a low saturation ratio. 4. Conclusions In summary, an improved CVD system has been adopted to synthesize two kinds of In2O3 tower-shaped nanostructures, and the growth mechanism was discussed at length. As to nanotower I with a tapered shape, the periodical fluctuation of the saturation ratio in the catalyst droplet induced periodical 1-D growth under a VLS mechanism. At low temperature (700 °C), the absorbed atoms on the side surface of the forming nanostructures had a low transfer rate. Some of these atoms would incorporate into the side surface maintaining the lateral growth under a VS mechanism. The 0-D growth on the side surface of newly formed nanorods made the nanorod an octahedral configuration. The 0-D growth on the surface of the earlier formed octahedron would make them thicken. The periodical 1-D and continuous 0-D growth finally resulted in the formation of nanotower I. As to nanotower II, the fluctuation of the saturation ratio around the substrate induced alternate 1-D and 0-D growth, which explained the formation of nanotower II. At 900 °C, these absorbed atoms on the side surface had a high transfer rate and would transfer to the top growing surface, which had high surface energy and was the preferential absorbing sites, so the lateral growth rate was very low even stopped. As a result, the formed nanotower II would have a relatively uniform size. Through modulating the saturation ratio, two kinds of novel tower-shaped nanostructures were obtained. Our designed experimental route could be extended to the synthesis of other complex nanostructures. Furthermore, these two kinds of novel nanostructures may have potential applications as functional building blocks in future nanodevices. Acknowledgment. This work was financially supported by the National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No. 2005CB23603), the Special Fund for President Scholarship, Chinese Academy of Sciences, and the National Natural Science Foundation of China (Grant No. 90406008). References (1) Granqvist, C. G. Appl. Phys. A: Solids Surf. 1993, 57, 19. (2) Hamburg; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123.
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