Cadmium Oxide Octahedrons and Nanowires on the Micro

Jan 23, 2009 - Kolkata 700 032, India, Nuclear and Atomic Physics DiVision, Saha Institute of Nuclear Physics, 1/AF. Bidhannagar, Kolkata 700 064, Ind...
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CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1287–1292

Articles Cadmium Oxide Octahedrons and Nanowires on the Micro-Octahedrons: A Simple Solvothermal Synthesis Tandra Ghoshal,† Subhajit Biswas,† P. M. G. Nambissan,‡ Gautam Majumdar,§ and S. K. De*,† Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India, Nuclear and Atomic Physics DiVision, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India, and Department of Mechanical Engineering, JadaVpur UniVersity, Kolkata 700 032, India ReceiVed February 23, 2008; ReVised Manuscript ReceiVed December 15, 2008

ABSTRACT: Cadmium oxide (CdO) micro-octahedrons and nanowires were synthesized by a simple solvothermal process using ethanol as a solvent. The amount of NaOH and the synthesis temperature were the key parameters to control the phase and morphology of the as-synthesized products. The phase purity of the samples was determined through X-ray diffraction (XRD). Lower concentration of NaOH and higher-synthesis temperature favored the formation of CdO micro/nanostructures. Morphologies of the products were identified through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). At higher temperature and low NaOH concentrations, nanowires were protruded from the octahedron facets. HRTEM images and SAED patterns reveal the single-crystalline nature of the octahedrons and polycrystalline nature of the nanowires. Cd(OH)2 samples, obtained at low synthesis temperature, were annealed at different temperatures to test the feasibility of the products to transform to CdO. Positron annihilation measurements were carried out to study the defects of the nano/microstructures. Electrical resistivity measurement showed semiconducting behavior of CdO samples. Introduction Over the past decade, one-dimensional (1D) nanostructures of semiconductors have received considerable attention from the scientific and engineering communities because they exhibit distinct properties that are different from those of bulk materials and they are promising candidates for realizing nanoscale electronic, optical, and mechanical devices. 1-7 In addition, full control of the architecture, size, morphology, and pattern in inorganic crystals along three dimensions has become a dominant feature in materials science. Such control gives additional variables in tailoring crystal properties, and significantly widens the possibility of fabricating new devices in various fields. Among these interesting complex structures, pyramid and octahedrons of different inorganic materials (GaN, In2O3, etc.) with sharp tip have been widely studied because of their efficient field-emission properties.8-11 The IIB-VIA binary oxide semiconductors are an important group of technological materials. Most of them crystallize in either cubic zinc-blende or hexagonal wurtzite structure (or both) * Corresponding author. E-mail: [email protected]. Tel.: 91-33-24734971. Fax: 91-33-24732805. † Indian Association for the Cultivation of Science. ‡ Saha Institute of Nuclear Physics. § Jadavpur University.

where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. They have been extensively used as transparent conducting oxide (TCO) materials. But despite having enormous applications, there is a lack of report on the synthesis and characterization of 1D and hierarchal nanostructures of CdO, which is one of the most promising member of TCO family. CdO-based transparent conductive oxides have been of interest because of their relatively simple crystal structures, high carrier mobility, and sometimes nearly metallic conductivities.12-14 Bulk cadmium oxide (CdO) is an n-type semiconductor, with a wide direct band gap of 2.27 eV and a narrow indirect band gap of 0.55 eV.15 The difference in band gap originates from cadmium and oxygen vacancies and strongly depends on the synthesis procedures.16 Because of its large linear refractive index (n0 )2.49), it is a promising candidate for optoelectronics applications and other applications, including solar cells,17 phototransistors,18 photodiodes,19 transparent electrodes,20 and gas sensors.21 Several techniques have been used to prepare CdO films,22-24 microwhiskers,25 nanoneedles,26 belts,27 cubes,28 nanoclusters,29 nanoparticles,30 wires/rods,31,32 etc. But a simple chemical synthesis of CdO crystals directly from the solution and proper understanding of the effect of different synthesis conditions on the shape, size, and crystal structure of them are very important from not only the crystal growth point of view but also for better understanding

10.1021/cg800203y CCC: $40.75  2009 American Chemical Society Published on Web 01/23/2009

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of the TCO materials. Because defects, especially vacancies and their clusters, play an important role in modifying the properties of materials with nanoscale structural composition, one can use positron annihilation spectroscopy to identify them and understand their properties in materials.33,34 In this report, a simple solvothermal method was utilized to prepare single-crystalline cadmium oxide (CdO) micro-octahedron, polycrystalline CdO nanowires and cadmium hydroxide (Cd(OH)2) nanorods by only varying the amount of NaOH and synthesis temperature. To the best of our knowledge, this is the first report on the fabrication of CdO octahedrons. Positron annihilation spectroscopy (PAS) study was employed to identify the defects present in the samples. Semiconducting nature of the samples was revealed by electrical resistivity measurement. Experimental Section For the synthesis of the CdO and Cd(OH)2 crystals, a Teflon lined cylindrical stainless steel chamber of capacity 50 mL was used. All the reagents were of analytical grade and were used without any further purification. Different amount of sodium hydroxide (NaOH) pellets (1, 2, 3, 4 g) was mixed with 35 mL of ethanol (C2H5OH). The solution was stirred until NaOH dissolved completely. A white precipitate appeared when 3.084 g of Cadmium nitrate (Cd(NO3)2,4H2O) was mixed with the above solution under constant stirring. After 5 min the solution was transferred into Teflon-lined stainless steel cylindrical chamber. The closed chamber was then placed inside a preheated box furnace at a desired temperature for 12 h. After 12 h, the system was allowed to cool naturally. The precipitate was collected, washed with distilled water and ethanol several times and dried in air. The synthesis was performed at different temperatures of the box furnace varied as 453, 473, and 493 K. Synthesized Cd(OH)2 samples were annealed in ambient air atmosphere at 473 and 573 K for 3 h. The products were characterized by X-ray powder diffraction (XRD, Bruker aXS, D8 ADVANCE) with Cu KR radiation. Microstructures of the products were obtained by scanning electron microscopy (SEM, Hitachi S-2300), field-emission scanning electron microscopy (FESEM, JEOL, JSM 6700F) and transmission electron microscopy (TEM, JEOL JEM 2010). For the positron annihilation studies, the radioactive source, 22 NaCl in dilute HCl, was deposited and dried on a thin (∼2 µm) Ni foil and folded to form the experimental source. It was kept immersed in the volume of the powdered sample taken in a glass tube and maintained under dry vacuum conditions during the experiments. The sample surrounded the source from all sides in sufficient thickness to ensure annihilation of positrons within it. The positron lifetime measurements were carried out using a slow-fast coincidence spectrometer, having a time resolution (fwhm) of 200 ps for the γ rays from 60Co source. For Doppler broadening measurements, the positron annihilation gamma ray spectra were recorded using a HPGe detector with resolution 1.14 keV.

Results and Discussion Structural Characterization. In the solvothermal process, the decomposition of the precursors in a particular solvent depends on the temperature and pressure within the reaction vessel. Here, pressure was related to the filling fraction of the solvent and was kept constant for all the experiments. The synthesis was performed with different amounts of NaOH at different temperatures (453, 473, and 493 K) keeping other experimental parameters unchanged. Figure 1a shows typical XRD pattern of the as synthesized products prepared at 473 K. The diffraction peaks of the samples synthesized with lower amounts of NaOH (1 and 2 g) were indexed to crystalline cubic structured cadmium oxide (JCPDS card No. 05-0640). An increase in the amount of NaOH resulted in the formation of either cubic cadmium oxide along with hexagonal (JCPDS card No. 31-0228) and monoclinic phase (JCPDS card No. 40-0760) of Cd(OH)2 or the mixed phases of Cd(OH)2. At lower temperature (453 K) with a lower amount of NaOH (1 and 2 g),

Figure 1. XRD pattern of the samples synthesized with different amount of NaOH at temperatures of (a) 473, (b) 453, and (c) 493 K.

primarily Cd(OH)2 along with traces of CdO were formed (Figure 1b); however, a higher amount of NaOH (3 and 4 g) resulted in pure Cd(OH)2. When the synthesis was performed at higher temperature (493 K), XRD patterns reveal the formation of crystalline CdO regardless of the amount of NaOH (Figure 1c). Lower concentrations of NaOH and higher synthesis temperatures are the preeminent conditions for the synthesis of phase pure CdO. Morphological Study. The morphologies, sizes, and crystal structures of the as-synthesized products were determined from the SEM, FESEM and TEM studies. Figure 2 shows the SEM

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Figure 3. SEM images of the samples synthesized with (a) 1 and (b) 3 g of NaOH at temperature of 493 K.

Figure 2. SEM images of the samples synthesized at 473K with (a) 1 and (b) 4 g of NaOH for 12 h. (c) SEM image of the sample with 2 g of NaOH for 6 h. Inset of (a) shows the interpenetrating nature of the octahedrons.

images of the samples synthesized at 473 K. Figure 2a depicts the formation of octahedrons well in abundance for the sample synthesized with 1 g of NaOH. These octahedrons consist of two inverted pyramids attached at their square base and bounded by eight triangular facets. The edges between the facets are sharp and surface of all the facets were very smooth. The side length of square base of octahedrons varied in the range 1.5-3.5 µm. Inset of Figure 2a exhibits the growing status of few octahedrons with interpenetrative growth without any distortion of their shape. Similar octahedrons were obtained with the increase of the amount of NaOH to 2 g. The samples synthesized with higher amount of NaOH shows different morphological features of bundle of nanorods along with large number of nanoparticles attached to the surfaces of these nanorods. Representative SEM image of the sample synthesized with 4 g of NaOH is shown in Figure 2b. The synthesis was also performed for smaller reaction time with lower amount of NaOH (1 and 2 g) to clarify

the growth stages of the formation of octahedrons. Along with the octahedrons, large number of nanosheets with uniform size and well-defined hexagonal shape were observed for 6 h of synthesis (Figure 2c). Similar sheetlike morphology was obtained when the synthesis was carried out at 453 K. The samples synthesized at 493 K reveal the formation of octahedrons (Figure 3). Few nanowires were growing out from the surfaces of the octahedrons for the samples synthesized with lower concentrations of NaOH (1 and 2 g) (Figure 3a). Increased amount of NaOH initiates the nanowire growth from the surfaces of the octahedrons (Figure 3b). One sample with mixed phase of CdO and Cd(OH)2 (synthesized with 3 g of NaOH at temperature 473 K) and another pure Cd(OH)2 sample (synthesized with 4 g of NaOH at temperature 473 K) were annealed in ambient air atmosphere at temperature 473 K and 573K for 3 h to transform them to phase pure CdO. The XRD pattern of the samples confirmed the complete conversion of Cd(OH)2 to CdO (Figure 4a). Figure 4b depicts the presence of a large number of nanowires with high aspect ratio growing from the surfaces of few crystals when the mixed phase sample was annealed at 473 K for 3 h. When annealing temperature was increased to 573 K, crystals were the dominant product, along with a few nanowires emerging from them (Figure 4c). A similar feature was obtained through annealing of the pure Cd(OH)2 sample. The morphology and crystal structure of the nanowires were further investigated by TEM and HRTEM studies as shown in Figure 5. TEM image (Figure 5a) confirms the growth of large number of nanowires from the surface of the octahedrons at higher synthesis temperature. Closer view of a single nanowire shown in the inset of Figure 5a reveals that the nanowire was composed of large number of nanostructured grains. HRTEM

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Figure 4. (a) XRD pattern of Cd(OH)2 sample annealed at different temperature. SEM and FESEM images of the sample prepared with 3 g of NaOH at 473 K when annealed at (b) 473 and (c) 573 K for 3 h.

image reveals the polycrystalline nature of the nanowires as well (Figure 5b). Figure 5c shows the representative TEM image of the annealed sample. High-resolution TEM image (Figure 5d) shows clear lattice fringes from the nanostructured grains revealing the polycrystalline nature of the nanowires. SAED pattern shown in the inset of Figure 5d also confirms the polycrystalline nature of the nanowires. Growth Mechanism. Formation mechanism of cadmium hydroxide in the presence of NaOH can be explained on the basis of buffer action of cadmium ions. Cadmium ions in the solution becomes hydrated and transformed to solid cadmium hydroxide through stepwise coordination of hydroxyl ions.35 But,

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depending upon the concentration of base and the synthesis temperature, cadmium hydroxide transformed into cadmium oxide through dehydration. It is believed that self-assembly of colloidal particles into crystallographic orientation of the aggregated particles resulted in the formation of different micro/nanostructures depending upon the concentration of NaOH in the solution and on the synthesis temperature. To understand the formation mechanism of octahedrons, we carried out temperature- and time-dependent experiments, which revealed that octahedrons were formed by the attachments of nanosheets (Figure 2c). The anisotropic growth of the nanocrystals in a template free method is related to the different surface energy of different crystal planes of the nanocrystals. Those planes with high surface energy have a strong tendency to capture monomers from the reaction solution in order to reduce their surface energy. At 453 and 473 K and at lower concentrations of NaOH, nanoparticles can coalesce through oriented attachment under solvothermal treatment to form irregular hexagonal sheets. The impetus for the aggregation of nanoparticles in this case should still the reduction of total surface energy through elimination of the higher surface energy of the lattice faces by the aggregation. With sufficient thermal energy provided by the solvothermal system at 473 K, the resulting sheet were experienced an attachment process. This behavior is reasonable because, on the one hand, the surface energy of an individual sheet was quite high with two exposed flat planes and thus they tended to be attached through different angles to decrease the surface energy by greatly reducing exposed areas. On the other hand, with the well-matched crystal lattice and active surface, the adjacent plates were prone to fuse to each other driven by the gaining of free energy and latticefree energy.36 This process occurring for the sheets eventually led to the formation of octahedrons. This is the thermodynamic equilibrium shape determined by considering the surface energies of all facets and strongly associated with the intrinsic structure of the material. When two or more nano/microstructures were formed from close assembly of nucleation centers, penetration between them occurred. But lower temperature does not provide sufficient thermal energy to overcome intense Brownian motion inside the reaction vessel. Although few nanosheets experienced the attachment process to form octahedrons, nanosheets remained as the principal products. At a higher temperature of 493 K, the octahedrons were formed through the similar attachment process described above. But the concentration of NaOH plays a significant role in the formation of nanowires. Theoretically, when the concentration of NaOH increases, the viscosity (η) of the liquid phase increases and thus the diffusion rate (D) of nanoparticles should decrease, according to the Stoke-Einstein equation, D ) constant/η.37 Thus, a lower concentration of NaOH favors a higher diffusion rate between the nanoparticles. The nanoparticles that are attached to the surface of the octahedron aggregated into onedimensional assembly, which resulted in the formation of nanowires via diffusion of the nanoparticles. Positron Annihilation Studies. The results of positron lifetime and Doppler broadening measurements on the two samples, one pure octahedron sample (synthesized with 1 g of NaOH at a temperature of 473 K), and another sample with a nanowire growing out of the octahedron surface (synthesized with 1 g of NaOH at a temperature of 493 K) can be summarized as follows. The positron lifetime spectrum of each sample was found to be composed of three exponential terms, each representing the lifetime of positrons getting annihilated through different mechanisms and environments. The longest component

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Figure 5. (a) TEM and (b) HRTEM image of pure CdO nanowire and octahedron sample prepared with 1 g of NaOH at 493 K. Inset of part (a) reveals polycrystalline nature of a nanowire. (c, d) TEM and HRTEM image of the annealed sample at 473K for 3 h. Inset of (d) shows the SAED pattern of the nanowire.

τ3 (∼1.3-1.4 ns) is ignored, as its intensity I3 (∼0.6%) in both the cases was negligibly small. This is expected since the fraction of positrons diffusing to the octahedron surfaces and forming positronium is extremely small. The other two lifetimes τ1 and τ2 were of values 134 and 323 ps in the case of sample containing only octahedrons and 140 and 342 ps in another sample. In both the cases, their intensities were the same as 79.1 and 20.3%, respectively. In nanocrystalline materials, positrons can thermally diffuse out to the grain surfaces prior to annihilation, since the average thermal diffusion length of positrons in typical materials varies around 50-100 nm. But if the grain sizes are larger than these lengths, as is the case in these particular samples, positrons can hardly reach the surfaces. As the edge lengths of CdO microoctahedrons varying in the range 1.5-3.5 µm, positron annihilations will be taking place within the octahedrons and not on the octahedron surfaces. Similarly, for the sample synthesized at 493 K, the octahedrons are also of sizes larger than the diffusion lengths for positrons. The only differing environment in the two samples is the nanowires in the second sample. Here in this case, a fraction of the positrons can annihilate on the nanowire surfaces. Because the surfaces of nanowires are generally rich in defects because of the highly disordered atomic arrangement, positrons can get trapped into the defects over there and get an enhanced lifetime. Perhaps this could account for the slightly higher lifetimes (τ1 and τ2) in this sample. Although the magnitude of this change is not significantly high, it is certainly outside the limits of statistical errors (2 and 8 ps, respectively) and can hence hint toward such a distinct possibility. Surprisingly, the additional trapping of positrons on the nanowire surfaces did not yield any change in the intensity I2 or in the Doppler broadening of the spectrum. The octahedron dimensions in both the cases are too large to have any differences between them. If we assume that there are defects present inside the octahedron, then the values of τ2 can be attributed to positron trapping in very large vacancy clusters. Note that the positron lifetimes in the second sample are larger,

Figure 6. Electrical resistivity (F) vs temperature (T) curve for the sample prepared (a) with 1 g of NaOH at 493 K (black) and (b) by annealing the Cd(OH)2 sample at 473 K for 3 h (yellowish brown). Insets shows the variation in ln F with (1000/T) of the corresponding samples.

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which is consistent with the enhancement in positron lifetime expected for annihilation at grain boundaries in microcrystalline samples. Electrical Resistivity Study. Electrical resistivity measurements of the samples were carried out using dc two-probe method in the temperature range 303-573 K under a vacuum. It was observed that the CdO samples prepared directly by solvothermal synthesis is black and that of prepared by annealing is yellowish brown. Panels a and b in Figure 6 depict the electrical resistivity (F) vs temperature (T) curve for two of the representative CdO samples with similar morphology but different color, one prepared at 493 K with 1 g of NaOH (black) and another sample prepared by annealing the Cd(OH)2 sample at 473 K for 3 h (yellowish brown), respectively. For both the samples, electrical resistivity decreases with the increase in temperature, indicating semiconductor behavior of the samples. Black colored CdO shows a smaller resistivity compared to yellowish brown sample. Black CdO shows very low resistivity of 4.7 × 10-4 ohm cm at higher temperature (573 K), which is comparable to the reported values for CdO thin films.38,39 The high electrical conductivity of the black colored sample might be due to the presence of oxygen vacancies or excess metal interstitial atoms. EDAX analysis also confirmed the presence of excess cadmium (Cd:O ) 62:38 at %) for the black sample. Insets of panels a and b in Figure 6 shows the variation of dark electrical resistivity (ln F) with temperature (1000/T) (K-1) of the respective samples. The straight line curves indicate same type of conduction mechanism is involved for both the samples. The activation energy, which represents the location of trap levels below the conduction band, was calculated from the straight lines. In the case of the black colored sample, calculated activation energy Ea ) 0.096 eV, and for the annealed sample, Ea ) 0.088 eV. Conclusions Single-crystalline cadmium oxide micro-octahedrons and polycrystalline nanowires were successfully synthesized by a simple solvothermal process using ethanol as a solvent. The different morphologies could be synthesized by this easily controllable technique just varying the NaOH concentration and the synthesis temperature. The nanowires could be grown on the octahedron facets at higher temperature with lower NaOH concentration. The possible growth mechanisms for the formation of octahedrons and nanowires have been proposed. Positron annihilation measurements show an enhancement of lifetime for the sample containing polycrystalline nanowires, which could be attributed to the larger defects due to the highly disordered atomic arrangement. Black CdO powder samples prepared directly by solvothermal synthesis show higher electrical conductivity compared to the yellowish brown sample prepared by annealing the Cd(OH)2 sample because of the presence of oxygen vacancies or excess metal interstitial atoms. Acknowledgment. T.G. gratefully acknowledges the financial support received from Council of Scientific and Industrial Research, New Delhi (India). The authors thank Prof. Subhadra Chaudhuri (Deceased) for useful discussions.

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