Ultrasonics Sonochemistry 19 (2012) 1227–1233

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Ultrasonics Sonochemistry

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Sonochemical syntheses of a nanoparticles cadmium(II) supramoleculeas a precursor for the synthesis of cadmium(II) oxide nanoparticles

Vahid Safarifard, Ali Morsali ⇑Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838 Tehran, Islamic Republic of Iran

a r t i c l e i n f o

Article history:Received 30 April 2011Received in revised form 5 February 2012Accepted 24 February 2012Available online 24 March 2012

Keywords:Coordination polymerNano-structureSonochemicalThermal decomposition

1350-4177/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.ultsonch.2012.02.013

⇑ Corresponding author. Tel.: +98 21 82884416; faxE-mail address: morsali_a@yahoo.com (A. Morsali

a b s t r a c t

Nanoparticles of a three-dimensional supramolecular Cd(II) compound, [Cd(L)2(H2O)2] (1), (L� = 1H-1,2,4-triazole-3-carboxylate), have been synthesized by a sonochemical process and characterized byscanning electron microscopy, X-ray powder diffraction, IR spectroscopy and elemental analyses. Thethermal stability of compound 1 in both its bulk and nano-size has been studied by thermal gravimetric(TG) and differential thermal analyses (DTA) and compared with each other. Concentration of initialreagents effects on size and morphology of nano-structured compound 1, have been studied. Calcinationof the single crystals and nano-sized compound 1 at 650 �C under air atmosphere yields CdOnanoparticles.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Chemical and physical properties of solid materials strongly de-pend on both the size and the shape of the microscopic particlesthey are made up from. This is especially true for materials withmorphological features smaller than a micron in at least onedimension, which are commonly called nano-scale materials, orsimply nano-materials. In these materials the ratio of surface areato volume is vastly increased when compared to compounds withlarger grain sizes and quantum mechanical effects such as the‘‘quantum size effect’’ begin to play a significant role. These effectsonly play a minor role when going from macro to micro dimen-sions, but become increasingly important when reaching the nano-meter size range [1–5]. Supramolecular architectures, on the otherhand, exhibit potential applications as molecular wires [6], electri-cal conductors [7], molecular magnets [8], in host–guest chemistry[9] and in catalysis [10]. In contrast to inorganic materials, the spe-cific syntheses of nano-structured supramolecular compoundsseem to be surprisingly sparse. Equally, the use of organometallicsupramolecular compounds as precursors for the preparation ofinorganic nanomaterials has not yet been investigated thoroughly.Cd2+, as a d10 metal ion, is particularly suited for the construction ofsupramolecular compounds and networks. The spherical d10 con-figuration is associated with a flexible coordination environment,so that geometries of these complexes can vary from tetrahedral(CN = 4) to dodecahedral (CN = 8) and severe distortions in theideal polyhedron easily occur. Furthermore, due to the general

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lability of Cd(II) complexes, the formation of coordination bondsis reversible, which enables metal ions and ligands to rearrangeduring the process of polymerization to give highly ordered net-work structures. Consequently, Cd can readily accommodate allkind of architectures and a selection of topological types of 1D,2D and 3D polymers is given [11–16]. Thus, their preparation ischallenging owing to their ability to tailor their physical andchemical properties [17]. On the other hand, cadmium oxide(Eg � 2.3 eV) is an n-type degenerate semiconductor with highelectrical conductivity. Due to its large linear refractive index(n0 = 2.49), it is a promising candidate for optoelectronic applica-tions and other applications including solar cells, photo transistors,photodiodes, transparent electrodes and gas sensors [18,19]. Be-cause of these interesting applications, efforts to prepare nanopar-ticles of CdO using a variety of methods have been reported in theliterature. Among others, solvothermal synthesis [20] and a micro-emulsion method [21] have been reported for preparing CdO nano-particles [22–26]. 1H-1,2,4-triazole-3-carboxylic acid (HL) contains1,2,4-triazole and one carboxylic acid functional group and to datesome interesting supramolecular compounds with the ligand havebeen reported [27–31]. In this paper we describe a simplesynthetic sonochemical preparation of nano-structures of a cad-mium(II) supramolecular with ligand 1H-1,2,4-triazole-3-carbox-ylate. Sonochemistry is the research area in which moleculesundergo a reaction due to the application of powerful ultrasoundradiation (20 kHz–10 MHz) [32]. Ultrasound induces chemical orphysical changes during cavitation, a phenomenon involving theformation, growth, and instantaneously implosive collapse ofbubbles in a liquid, which can generate local hot spots havingtemperatures of roughly 5000 �C, pressures of about 500 atm,

1228 V. Safarifard, A. Morsali / Ultrasonics Sonochemistry 19 (2012) 1227–1233

and a lifetime of a few microseconds [33]. These extreme condi-tions can drive chemical reactions, but they can also promote theformation of nano-sized structures, mostly by the instantaneousformation of a plethora of crystallization nuclei [34]. This has beenwidely used to fabricate nano-sized structures of a variety of com-pounds [35], and in recent years many kinds of nano-sized materi-als have been prepared by this method [36–42]. So far littleattention has been given on synthesis of nano-sized supramolecu-lar compounds.

Scheme 1. Materials produce

Fig. 1. IR spectra of (a) nano-particles of compound 1 produced by s

2. Experimental

2.1. Materials and physical techniques

All reagents for the synthesis and analysis were commerciallyavailable from Merck Company and used as received. Doubly-dis-tilled water was used to prepare aqueous solutions. Ultrasonic gen-erators were carried out on a SONICA-2200 EP, input: 50–60 Hz/305 W. Melting points were measured on an Electrothermal 9100

d and synthetic methods.

onochemical method and (b) bulk materials as synthesized of 1.

Fig. 2. XRD patterns; (a) simulated pattern based on single crystal data of compound 1 and (b) nanoparticles of compound 1 prepared by sonochemical process.

Fig. 3. SEM photograph and the corresponding particle size distribution histogram of compound 1 nanoparticles prepared in concentration of initial reagents[Cd2+] = [L�] = 0.01 M.

V. Safarifard, A. Morsali / Ultrasonics Sonochemistry 19 (2012) 1227–1233 1229

Fig. 4. SEM photograph and the corresponding particle size distribution histogram of compound 1 nanoparticles prepared in concentration of initial reagents[Cd2+] = [L�] = 0.05 M.

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apparatus. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. IR spectra were recorded using Perkin–Elmer 597and Nicolet 510P spectrophotometers. The thermal behavior wasmeasured with a PL-STA 1500 apparatus between 25 and 700 �Cin a static atmosphere of argon. The simulated XRD powder patternbased on single crystal data were prepared using Mercury software[43]. X-ray powder diffraction (XRD) measurements were per-formed using a Philips X’pert diffractometer with monochromatedCo-ka radiation (k = 1.78897 Å). The samples were characterized bya scanning electron microscope (SEM) (Philips XL 30 and S-4160)with gold coating.

2.2. Synthesis of [Cd(L)2(H2O)2] (1)

Compound 1 was prepared using the branched tube method[44]: 1H-1,2,4-triazole-3-carboxylic acid (0.117 g, 1 mmol) andcadmium(II) chloride (0.114 g, 0.5 mmol) were placed in the mainarm to be heated. Water was carefully added to fill both arms, andthen the arm to be heated was placed in a bath at 60 �C. After10 days, colorless crystals were deposited in the cooler arm whichwere filtered off, washed with water and air dried. (0.104 g, yield55.8%), m.p. > 300 �C. (Anal. calc. for C6H8CdN6O6: C, 19.34%;H, 2.16%; N, 22.56% found; C, 19.52%; H, 2.07%; N, 21.94%). IR(cm�1) selected bands: 829(m), 1306(m), 1369(m), 1480(s),1621(vs), 3095(br) and 3329(br).

2.3. Synthesis of [Cd(L)2(H2O)2] (1) nanostructure by a sonochemicalprocess

To prepare nano-sized [Cd(L)2(H2O)2] (1), 50 ml solution of cad-mium(II) chloride (0.01 M) in water was positioned in a high-den-sity ultrasonic probe, operating at 50 Hz with a maximum poweroutput of 305 W. Into this aqueous solution a 50 ml solution ofthe ligand 1H-1,2,4-triazole-3-carboxylic acid (0.01 M) and sodiumhydroxide (0.01 M) were added dropwise. The obtained precipi-tates were filtered off, washed with water and then dried in air.m.p. > 300 �C. (Found C, 19.60; H, 2.24; N, 22.03%). IR bands:646(m), 1309(m), 1379(m), 1482(s), 1619(vs) and 3337(br).

For the study of the effect of concentration the initial reagentson size and morphology of nano-structured compound 1, the aboveprocesses were done with concentration of 0.05 M.

In order to investigate the size effects of precursor compound 1on the size and morphology of the produced CdO, compound 1 ob-tained under the mentioned conditions was calcinated at 650 �C ina furnace and static atmosphere of air for 4 h.

3. Results and discussion

Reaction of 1H-1,2,4-triazole-3-carboxylate (L�) with cad-mium(II) chloride leads to formation of a 3D supramolecular[Cd(L)2(H2O)2] (1). Nanoparticles of compound 1 were obtained

Fig. 5. A fragment of the 3D framework in compound 1, viewed along c direction.

Fig. 6. Thermal behavior of compound 1 as bulk and nanoparticle.

V. Safarifard, A. Morsali / Ultrasonics Sonochemistry 19 (2012) 1227–1233 1231

in aqueous solution by ultrasonic irradiation, while single crystalsof compound 1 were obtained using a heat gradient applied to an

Fig. 7. XRD pattern of CdO nanostructure prepar

aqueous solution of the reagents (the ‘‘branched tube method’’)[44]. Scheme 1 gives an overview of the methods used for the syn-thesis of [Cd(L)2(H2O)2] (1) using the two different routes.

The elemental analysis and IR spectra of the nano-structure pro-duced by the sonochemical method and of the bulk material pro-duced by the branched tube method are indistinguishable(Fig. 1). The symmetric and asymmetric vibrations of the carboxyl-ate group are observed as two strong bands at 1480 and 1621 cm–1,respectively. The D(tas-tsym) values indicate that the carboxylateanions coordinate to the Cd(II) center in bridging mode [45]. Thebroad band near 3350 shows the existence of the water molecule[27,45].

Fig. 2 shows the simulated XRD pattern from single crystal X-ray data (see below) of compound 1 (Fig. 2a) in comparison withthe XRD pattern of the typical sample of compound 1 preparedby the sonochemical process (Fig. 2b). Acceptable matches, withslight differences in 2h, were observed between the simulatedand experimental powder X-ray diffraction patterns. This indicatesthat the compound obtained by the sonochemical process as nano-particles is identical to that obtained by single crystal diffraction.

ed by calcination of compound 1 at 650 �C.

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The significant broadening of the peaks indicates that the particlesare of nanometer dimensions.

The reaction between 1H-1,2,4-triazole-3-carboxylate (L�) withcadmium(II) chloride provided a crystalline material of the generalformula [Cd(L)2(H2O)2] (1). The morphology and size of compound1 prepared by the sonochemical method was characterized byscanning electron microscopy (SEM) and shows that it is composedof particles with sizes of about 62 nm. Fig. 3 shows the scanningelectron microscopy (SEM) and the corresponding particle sizedistribution histogram of the compound 1. Also, different concen-tration of cadmium(II) chloride and ligand 1H-1,2,4-triazole-3-carboxylate (L�) solution (0.05 M) were tested (Fig. 4).

Appropriate nano-sized particles of compound 1 were obtainedat both concentrations of 0.01 and 0.05 M (Figs. 3 and 4). Particlesizes of the nanoparticles depend on the concentration of initial re-agents. In order to investigate the role of concentration of initial re-agents on the nature of products, reactions were performed withtwo different concentrations. Comparison between the sampleswith different concentrations shows that high concentration of ini-tial reagents increases particles size. Thus, particles sizes producedusing lower concentrations of initial reagents (0.01 M, Fig. 3) aresmaller than particles size produced using higher concentration(0.05 M, Fig. 4). The IR spectrum and XRD pattern of typical sam-ples of compound 1 prepared by the sonochemical process at con-centrations of 0.01 and 0.05 M are also same with the crystallinesample. These experiments indicate that the reaction at two differ-ent concentrations produces the same product but that the sizes ofthe particles are different.

Fig. 8. SEM image of agglomerated CdO nanoparticles prepared by thermolyses ofcompound 1 at 650 �C.

The structure of compound 1 has been reported by J. Zhu et al.[29]. The complex in the solid state is a 3D supramolecular frame-work that consists of one Cd(II) ion, two de-protonated L� and twoterminal coordinated water molecule. A view of the coordinationenvironment around the cadmium(II) ion and packing of com-pound 1 is shown in Fig. 5. The title complex crystallizes in themonoclinic space group P21/c.

To examine the thermal stability of the nano-sized particles andthe single crystals of compound [Cd(L)2(H2O)2] (1), thermal gravi-metric (TG) and differential thermal analyses (DTA) were carriedout between 25 and 700 �C under argon flow (Fig. 6). The com-pound 1 is stable up to 150 �C. Decomposition of compound 1 oc-curs between 150 and 630 �C with a mass loss of 60.09% (calc.65.4%). Nano-sized compound 1 is less stable and starts to decom-pose at 142 �C. Detectable decomposition of the nano-particles of 1thus starts about 8� earlier than that of its bulk counterparts, prob-ably due to the much higher surface to volume ratio of the nano-sized particles, as more heat is needed to annihilate the latticesof the single crystals. Mass loss calculations of the end residueand the XRD pattern of the final decomposition product (Fig. 7)show the formation of CdO. The DTA curves display two distinctexothermic effects at 182 and 382 �C for the single crystals of com-pound 1 (Fig. 6). The DTA curve of the nano-structured material hasthe same appearance as those of their single crystalline counter-parts and the exothermic effects are retained for nano-structuredcompound 1. Some differences between the maximum intensitiesindicate, in agreement with TGA results, a somehow lowerstability of the nanostructures when compared with their singlecrystals.

Fig. 7 provides the XRD pattern of the residue obtained fromcalcination of compound 1. The obtained pattern matches withthe standard pattern of CdO with the lattice parameters (a =4.6953 Å, S.G. = Fm3m(225) and z = 4), which is the same as the re-ported values (JCPDS card number 05-0640). An SEM image of theresidue which is obtained from the direct calcination single crys-tals of compound 1 at 650 �C shows the formation of agglomeratedCdO nanoparticles (Fig. 8). As the calcination process was success-ful for the preparation of CdO nanoparticles, we used the nano-sized compound 1 prepared by the sonochemical process at a con-centration of 0.01 M for the preparation of CdO nanoparticles. TheXRD pattern of the residue shows that the resulting residue wasagain CdO with the lattice parameters mentioned above. TheSEM image of the resulting residue shows the formation of CdOnanoparticles (Fig. 9).

Fig. 9. SEM image of agglomerated CdO nanoparticles prepared by thermolyses ofcompound 1 nanoparticles at 650 �C.

V. Safarifard, A. Morsali / Ultrasonics Sonochemistry 19 (2012) 1227–1233 1233

4. Conclusions

A nano-sized Cd(II) supramolecular, [Cd(L)2(H2O)2] (1), (L� =1H-1,2,4-triazole-3-carboxylate) was synthesized by sonochemicalirradiation and compared with its crystalline structure reported byJ. Zhu et al. [29]. Compound 1 was characterized by X-ray powderdiffraction (XRD), IR spectroscopy, thermal gravimetric (TG) anddifferential thermal analysis (DTA). To prepare the nanostructureof compound 1, two different concentrations of initial reagents,0.01 and 0.05 M, were tested. Appropriate nano-sized particles ofcompound 1 were obtained at both concentrations. Particle sizesof the nanoparticles depend on the concentrations of initial re-agents. Results show a decrease in the particles size as the concen-trations of initial reagents is decreased. Calcination of compound 1at different sizes produced nanoparticles of CdO.

Acknowledgements

Support of this investigation by Tarbiat Modares University isgratefully acknowledged.

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