Materials Letters 65 (2011) 783–785

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Materials Letters

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Na2Cd2I6L2(H2O)6 [L=Urotropine]: An interesting precursor for synthesizingCdO particles

Sandip Mondal a, Tanmay Chattopadhyay b, Swarup Kumar Neogi c, Totan Ghosh a,Aritra Banerjee c, Debasis Das a,⁎a Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata-700 009, Indiab Department of Chemistry, Panchakot Mahavidyalya, Neturia, Purulia, 723121, Indiac Department of Physics, University of Calcutta, 92, A. P. C. Road, Kolkata-700 009, India

⁎ Corresponding author. Tel.: +91 33 2350 8386x516E-mail address: dasdebasis2001@yahoo.com (D. Das

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.11.055

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2010Accepted 23 November 2010Available online 27 November 2010

Keywords:Coordination polymerPyrolytic techniqueCdO particleOptical propertySemiconductorsNanoparticles

A coordination polymer, Na2Cd2I6L2(H2O)6 [L=Urotropine] has been employed as sole precursor tosynthesize CdO particles. Two different preparation methods viz (i) pyrolysis of the title compound at 700 °Cfor 2 h and (ii) forming cadmium hydroxide from the title compound followed by pyrolysis at 700 °C for 2 hhave been used for the synthesis of nano sized CdO-I and CdO-II, respectively. From powder XRD data thelattice parameters (0.4701 and 0.4696 nm respectively for the two samples) and particle size (~77 and 30 nmfor CdO-I and CdO-II) have been evaluated. Morphology of the two varieties of CdO is widely different as isevident from their SEM images. The estimated values of the band gap of 2.53 eV and 2.59 eV for CdO-I andCdO-II respectively are obtained from the optical spectral analysis.

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1. Introduction

Cadmium oxide (CdO) is an important n-type semiconductorwhich has a direct band gap of 2.3 eV and an indirect band gap of1.98 eV [1,2]. It is a promising material having a wide application inthe field of solar cells, photo transistors, transparent electrodes,catalysts and gas sensors etc. [3–6]. Particle size, porosity, and specificsurface area are of major significance for those applications. Up tonow, a number of CdO particles having different sizes and morphol-ogies have been synthesized by adopting various methodologies [7–16]. In most cases although the main precursors are simple cadmium(II) salts of common inorganic or organic acids, the synthesismethodologies demand co-precursors (template) or catalysts for thepreparation of nano-sized CdO. Coordination polymers are interestingfor several reasons but their use as precursors for the preparation ofinorganic micro- and nano-scale-materials has not yet been investi-gated in detail. In this paper we have described the utilization of acoordination polymer, Na2Cd2I6L2(H2O)6 (L=Urotropine) as the onlyprecursor to synthesize CdO particles having different sizes andmorphologies depending on the mode of the preparation of the latterfrom that coordination polymer either by direct pyrolysis or via the

synthesis of Cd(OH)2 followed by pyrolysis without using any catalystor template.

2. Experimental

2.1. Materials

Analytically highly pure CdI2, urotropine and sodium dicyanamidewere obtained from Sigma-Aldrich chemical company. Twice distilledwater was prepared in our own laboratory.

2.2. Physical measurements

The phase purity of the synthesized samples has been checked usingpowder X-Ray Diffractometer with Cu–Kα radiation. Two differentdiffractometers [both of make Philips (PANalytical), Model: PW1830and X'pert PRO MRD X-ray diffractometer] has been employed for thispurpose. All the X-Ray Diffraction (XRD) measurements were carriedout in the range of 20°≤2θ≤80°. Further, the lattice parameter andparticle size of the synthesized samples has been estimated using theobtained XRD data. Themorphology and size of CdOparticles have beencharacterized by Hitachi S-3400N scanning electronmicroscopy (SEM).Infrared spectra were recorded on KBr disks (400–4000 cm−1) with aPerkin-Elmer RXI FTIR spectrophotometer. UV–VIS spectra weremeasured on Hitachi U-3501 spectrophotometer in the wavelengthrange of 200 to 800 nm.

Fig. 1. SEM image of (a) CdO-I particles and (b) CdO-II particles.

784 S. Mondal et al. / Materials Letters 65 (2011) 783–785

2.3. Syntheses

Na2Cd2I6L2(H2O)6 has been synthesized and characterized by theprocedure reported before [17]. The specific-sized CdO particle wasprepared from that precursor by following two different methods:

(a) Firstly, the pure Na2Cd2I6L2(H2O)6 was taken in a quartzcrucible and calcined in air at 700 °C for 2 h. The complex led toproduce CdO particles (hereafter CdO-I) inside quartz-crucibleafter oxidation.

(b) Secondly, Na2Cd2I6L2(H2O)6, was dissolved in double distilledwater. Ammonium hydroxide solution was then added to thatsolution drop-wise under stirring until the final solutionachieved the pH 8. During this time precipitation was formed.The resulting precipitate was filtered and washed several timeswith double distilled water to remove the impurities. Thehydroxide, thus formed was dried at 100 °C and grinded into apowder. Then it was calcined in air at 700 °C for 2 h whichproduced CdO particles (hereafter CdO-II).

3. Results and discussion

The FT-IR spectrum of synthesized Cd(OH)2 exhibits a sharp andvery intensive band at 3571 cm−1 (SI file). This IR band is attributedto the stretching vibrations of structural OH groups of γ-Cd(OH)2[13]. The IR bands in the region of 1500 to 1200 cm−1 are due to aresidual organic component. Ristic´ et al. has observed IR bands at 452and 347 cm−1 and assigned to Cd(OH)2 [13]. However, in our case wehave got IR band at 607 cm−1 is probably due to Cd(OH)2. The IR bandof Cd(OH)2 at 3571 cm−1 has completely been disappeared in the IRspectra of CdO-I and CdO-II. The formed CdO has been characterizedby an intense and very broad IR band at 1024 cm−1 in both cases (SIfile) as is reported recently by Morsali et al. [14].

Wemeasured theX-Raypowderdiffractionpatterns of bothCdO-I andCdO-II (SI file). All the visible peaks can be well indexed to the cubic CdOphase [18]. We have calculated the lattice parameter of the synthesizedsamples from the indexed XRD peaks and using the Nelson-Relay (NR)function [18,19]. The estimated values of the lattice parameter for the twosamples as shown in Table 1, are in close accordance with those reportedrecently [18]. Further, the crystallite size was calculated by using theScherrer formula:

d = kλ= βs cosθ

where d is the average size of the crystallite grains, k is 0.9, λ isthe wavelength of Cu–Kα radiation, βs is the full width at halfmaximum (FWHM) of the diffracted peak of sample and is given byffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiβ Ex

2−β Ins

q2 [18]. θ is the Bragg angle of diffraction. It is noteworthy to

mention that, instrumental broadening (βIns) plays a dominant role inthe broadening of the XRD peaks [19,20]. Hence, while estimating thecrystallize size from the XRD pattern instrumental broadening ofindividual XRD instruments has been used accordingly. The estimatedparticle sizes of the CdO-I and CdO-II presented in Table 1.

Cube-like micro-particles with irregular ends, sphere and fiberlike morphologies for Cd(OH)2 (SI File), CdO-I [Fig. 1(a)] and CdO-II[Fig. 1(b)], respectively have been visualized in their respective SEM

Table 1Comparison of the values of particle sizes and lattice parameters of CdO particles of ourwith ref.[18].

CdO-I CdO-II Ref.[18]as-grown

Ref.[18]400 °C

Ref.[18]500 °C

Particle size(nm) 75.11 30.51 16.80 44.84 31.54Lattice parameter (nm) 0.4701 0.4696 0.4708 0.4696 0.4695

micrographs. The particle sizes obtained from the SEM images are inwell agreement with those calculated from the XRD data. The opticalproperties of CdO-I and CdO-II nano-particles have been evaluatedfrom their UV–Vis absorption spectral studies (SI File). It is wellknown that the relationship between absorption coefficient (α) nearthe absorption edge and the optical band gap (Eg) obeys the followingequation for a semiconductor [21,22]:

αhνð Þn = B hν−Egð Þ

where B is the parameter that relates to the effectivemasses associatedwith the valence and conduction bands, hν is the photon energy, and nis either two for a direct transition or half for an indirect transition. Theabovementioned equation, also known as Tauc relation, can be used inevaluating the band gap (Eg) of the synthesizedCdO samples. The Eg ofthe samples is obtained using this equation, when the linear region ofthe (αhν)n against hν plot is extrapolated to intersect the energy axis,since Eg=hνwhen (αhν)n=0. Reported result shows both direct andindirect Eg in the same CdO sample [16]. Following this we attemptedto obtain both direct and indirect Eg from the (αhν)2 vs. hν plot and(αhν)1/2 vs. hν plot, respectively. The results, in accordance with thevery recent report of S. Aksoy et al., indicate that synthesized CdOnanomaterials posses direct band gap [18] and a comparison of the ourobserved parameters with their work are presented in Table 1.

(a)

(b)

1.751.50 2.00 2.50 2.75 3.00 3.25 3.502.25

8.0x1011

6.0x1011

4.0x1011

2.0x1011

0.0

(αhν

)2 (cm

-1eV

)2

1.0x1012

8.0x1011

6.0x1011

4.0x1011

2.0x1011

0.0

(αhν

)2 (cm

-1eV

)2

Energy (eV)

1.751.50 2.00 2.50 2.75 3.00 3.25 3.502.25Energy (eV)

Fig. 2. The plots of (αhν)2 versus hν for (a) CdO-I particles and (b) CdO-II particles.

785S. Mondal et al. / Materials Letters 65 (2011) 783–785

The plots of (αhν)2 versus hν are shown below (Fig. 2(a) and (b)).The estimated values of the band gap are 2.53 and 2.59 eV for CdO-Iand CdO-II respectively. Both energies of the CdO nano materials are

slightly higher than reported values for the direct band gap of bulkCdO, which indicates the quantum confinement effect of thesynthesized CdO nano materials [12].

4. Conclusion

In summary, we have described here a simple pyrolytic techniqueto synthesize CdO nano-particles exploiting coordination polymerwithout using any catalyst or template. The choice of method canstrongly affects the morphologies of CdO nanostructure as well as thedirect band gap.

Acknowledgments

The financial support from the Centre for Research on NanoScience and Nano Technology, University of Calcutta is gratefullyacknowledged.

References

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