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Materials Letters 62 (2

Novel growth morphologies of nano- and micro-structured cadmium oxide

A.K. Srivastava ⁎, S. Pandey 1, K.N. Sood, S.K. Halder, R. Kishore

National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi-11012, India

Received 9 April 2007; accepted 18 June 2007Available online 26 June 2007

Abstract

Powder particles of various morphologies and dimensions of CdO via metal – catalyst free – vapor phase were grown using a simple thermalevaporation technique under atmospheric pressure at the temperature of 1273 K. CdO deposited at quartz (slide)-substrate exhibited the formationof different morphologies including tubular, cylindrical, horse-shoe and semi-spherical. In contrast the CdO grown inside quartz-crucible resultedthe more precise geometrical shapes of faceted cuboids interspersed with spheroids and highly dense long rods of even surfaces. These novelmicrostructures have elucidated crystallographically preferred growth along 002 plane in a cubic lattice, instead of 111 plane observed in a bulkCdO. Evolution of different microstructures on quartz-substrate (slide) and inside crucible is compared and discussed to understand themechanism of formation of these fascinating microscopic objects.© 2007 Elsevier B.V. All rights reserved.

Keywords: CdO; Electron microscopy; Microstructure; Growth morphology

1. Introduction

Fine scale objects of variety of materials are of great interestdue to their various tunable microstructure, phase transforma-tions and quantum confinements, required for potential usage[1–3]. The nano-particles, wires, rings, combs and tetrapods ofsemiconducting oxides such as ZnO [4–6], WO3 [7–9], TiO2

[10,11] and CdO [12–15] are most desired for technologicalapplications. Their novel properties can be tailored significantlyby producing them at micro- and nano-scale in differentmorphologies. Among these CdO is a unique wide band gapsemiconductor (2.4 eV) possessing excellent electrical, me-chanical and optical properties. It has a cubic (NaCl type, fcc;a=0.469 nm) crystal structure with alternating Cd and O atomslocated at lattice points, with high density (8150 kg m−3) andmelting point (1500 °C). It is a promising candidate for opto-electronics, solar cells, phototransistors, photodiodes, transpar-ent electrodes and gas sensors. Dimensional reduction of suchmaterials results to improved non-linearity, controlled by thequantum size effects and other mesoscopic influences. So farpertaining to fine microstructural features, only the random-

⁎ Corresponding author. Tel.: +91 11 25742610.E-mail address: aks@nplindia.ernet.in (A.K. Srivastava).

1 Permanent address: Indian Institute of Technology Kanpur-208016, India.

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.06.044

shaped particles and a limited work on oriented-film growth ofCdO are attempted [13,15]. Unfortunately not much work hasbeen done on CdO due to its toxic nature. Investigations ondifferent scale microstructures, respective morphologies andcrystallographic interpretations of ultrafine CdO are stillwarranted. The present work envisages a detailed microscopyand spectroscopy of thermally evaporated microscopic objectsof CdO.

2. Experimental details

CdO microstructures were synthesized by solid–vapour de-position technique, using high purity Cd granules in a quartz-crucible covered with a quartz-slide and placed in a horizontalmuffle furnace. Since the boiling point of Cd is 1040 K, thefurnace was maintained at a temperature of 1273 K to make themolten Cd little volatile for its easy reaction with surroundingO. Oxidation of Cd vapours led to the nucleation and growth ofCdO particles on the quartz-slide (referred as sample 1) andinside quartz-crucible (referred as sample 2). A scanningelectron microscope (SEM model LEO 440) equipped with anenergy dispersive spectrometer (EDS model Oxford Link ISIS300) was used to study the topological features and thecomposition of Cd and O. Microstructural characterization athigh magnifications and crystallographic analysis was carried

Fig. 1. Sample 1 showing morphologies of tubular, cylindrical, horse-shoe andsemi-spherical. Inset shows prominent tubular growth.

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out using a transmission electron microscope (TEM, modelJEOL JEM 200CX, accelerating voltage: 200 kV). Furthercrystallographic interpretations were performed by X-raydiffraction (XRD, model D8 Advanced Bruker Diffractometer).

3. Results and discussion

CdO powder collected from the quartz-slide (sample 1) revealed theformation of particles of variable size (∼200 nm to 5 μm) with irregularmorphologies, viz., tubular, cylindrical, horse-shoe, semi-spherical andcuboidal (Figs. 1 and 2). EDS analysis of the respective materialdelineated that the overall (area) composition is close to stoichiometricCdO, whereas the composition of tubular growth (spot analysis, inset inFig. 1) is deficit of O (Table 1).

The powder collected from the quartz-crucible (sample 2) depicteda more regular morphology of cuboids with well defined facetsinterspersed with a few spheroidal particles in between (Fig. 3a and b).The edges of these facets ranged between 200 nm to 4 μm in length.These cuboids are either grown adjacent to each other (Fig. 3a) or insome cases smaller cuboids grown on facets of larger cuboids (Fig. 3b).In some regions a large fraction of long smooth rods (along theircircumference) of diameters between 50 to 100 nm are seen (Fig. 3c).Inset (Fig. 3c) shows a rod of diameter about 50 nmwith a conical tip ofcurvature about 3 nm. Similar to the sample 1, in this case (sample 2),the composition of cube and rods are found deficit in O fromstoichiometric CdO (Table 1). TEM investigations on cuboids and long

Fig. 2. Histogram showing variation of different particle sizes with number ofparticles.

rods revealed that these fine objects are single crystalline. A mi-crograph and a corresponding selected area electron diffraction pattern(SADP) of ultrafine cuboids are depicted in Fig. 3d. SADP reveals asingle crystal diffraction pattern of CdO recorded along slightlydeviated from [011] zone axis of fcc-crystal structure. Two importantcrystallographic planes, viz. 200 and 1ī1, are indicated as 1 and 2,respectively, on SADP (inset in Fig. 3d). In general majority of thesefascinating objects are less than 500 nm at least in one of theirdimensions (Fig. 2).

The present investigations indicate that the choice of substratematerial can strongly affect the morphologies of CdO microstructure.The CdO powder obtained from the quartz-slide (sample 1) exhibitedirregular shapes with prominent tubular growth while the CdO powderobtained from the quartz-crucible (sample 2) depicted more precisecuboidal and rod geometries. These microstructures can be explainedon the basis of nucleation and growth conditions that persist duringthermal evaporation. It is important to note that crystallographicallyquartz-slide is entirely different from freshly nucleated CdO crystalsand may result in random growth of the resultant product. On contrary,CdO crystals growing in quartz-crucible find a similar crystallographicenvironment due to pre-oxidized molten Cd on the crucible-innersurface. Moreover, CdO grown in the crucible will experience longerperiod of annealing and slower cooling. Under these conditions CdOfrom crucible will grow in a more systematic way preserving itscrystallographic cubic-symmetry in general. Our XRD measurementsfurther supported the present microscopy interpretations. It has beenobserved that the important planes reflected in X-ray diffractogram are:111, 200, 311 and 400 with corresponding interplanar spacings (d) andrelative peak intensities (I) of 0.27108 nm (I=10), 0.23472 nm(I=100), 0.14158 nm (I=3) and 0.11738 nm (I=10), respectively.However the X-ray powder JCPDS file of CdO states that standardvalues correspond to the cubic CdO planes: 111, 200, 311, 400 showsthe reflections corresponding to d and I values as: 0.2712 nm (I=100),0.2349 nm (I=88), 0.1416 nm (I=28) and 0.11742 nm (I=5). TheseXRD observations states that instead of preferred 111 plane of cubic(fcc) crystal, the 200 has become more dense plane at lattice scale andhas taken the crystallographically preferred growth direction duringthermal evaporation of CdO. It is an interesting phenomenon which hasbeen affirmed by the CdO nano-objects especially cuboidal morphol-ogy. In this clear facets with equal edge length forming perfect squareshas been delineated on cuboid particles.

It has been discussed [13] that 2 lattice points in the 200 plane and1.875 lattice points in the 111 plane within the single CdO cell, makes200 plane (bigger planar density of lattice points) of lower surfaceenergy. As a result the grains easily grow along the 200 direction withlower surface energy instead of 111 direction. However the experi-ments were carried out on oriented thin films [13], otherwise in generalthe CdO has been grown along 111-textured orientation [13,15]. It isworth mentioning that in present work the preferred orientation of 200has been delineated in free particles which has been affirmed by asystematic microstructure–crystal structure correlation. For a betterunderstanding, a schematic in Fig. 4 further elucidates the atomic sites

Table 1Composition analysis of sample 1 and 2

Different samples Cd (at.%) O (at.%)

Sample 1 (area analysis, Fig. 1) 42.99 57.01Sample 1 (spot analysis, inset in Fig. 1) 60.93 39.07Sample 2 (area analysis, Fig. 3a) 51.79 48.21Sample 2 (spot analysis, Fig. 3b) 58.67 41.33

Fig. 3. SEM micrographs (sample 2) showing different size cuboids (a, b), nano rods (c), inset(c) shows a nanotip. TEM micrograph (d) shows two adjacent cuboids,inset shows a SADP.

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on the 111 and 200 planes of CdO unit cell. Our spectroscopicmeasurements (Table 1) illustrates that the tubular, cuboid and rod-shaped objects are deficient in O and therefore leading vacant sites instoichiometric CdO with a composition of CdO0.64 and CdO0.71. In ourprevious work [6], it has been postulated that the O vacant sites havebeen created in stoichiometric ZnO by controlling the partial pressureduring thermal evaporation of Zn to prepare ZnO. The microstructuraltransformation from nano-particles to wires takes place when the Znand O in zinc oxide alter from ZnO0.87 to ZnO0.57 [6]. In the presentwork, although the actual transformation from particles to tubular orcuboids and/or rods has not been studied, but it is worth mentioningthat the preferred novel growth morphologies (tubular, cuboids androds) are evolved only when there is deficiency of O in stoichiometricCdO. These O vacant sites at lattice scale along 200 normally actas nucleant sites for preferred growth of sharp-edged cuboids. Howeverpresumably the O vacant sites on other planes (preferably 111, nextmost atomically dense) of fcc unit cell of CdO (Fig. 4) lead to theformation of tubular and rod-shaped morphologies. A detailed study

Fig. 4. Schematic of fcc-CdO unit cell marked with 111, 200 planes and latticepoints.

in this direction is under way to correlate the effect of processconditions like evaporation temperature, substrate for deposition andlimited O pressure while Cd evaporation with the nucleation andgrowth of novel nano-scaled objects of CdO. These objects are furtherbeing characterized for their electrochromic and photoluminescenceperformance.

4. Conclusions

A detailed analysis of different single crystalline growthmorphologies of CdO is investigated. Among these, the tubular,cuboid and rod-shaped are quite fascinating. These morphol-ogies are mainly temperature sensitive during growth, becausethe process is entirely catalytic free followed with solid–vapourmechanism. The different shapes with smooth surfaces are welldefined when the time of cooling with the reaction of Cd and Ois uniform.

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