Sonochemical synthesis of a new nano-sized cerium(III) coordination polymer and its conversion to nanoceria

Accepted Manuscript Sonochemical synthesis of a new nano-sized cerium(III) coordination polymer and its conversion to nanoceria Parviz Gohari Derakhshandeh, Janet Soleimannejad, Jan Janczak PII: DOI: Reference:

S1350-4177(15)00035-8 http://dx.doi.org/10.1016/j.ultsonch.2015.02.003 ULTSON 2791

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

14 December 2014 1 February 2015 2 February 2015

Please cite this article as: P. Gohari Derakhshandeh, J. Soleimannejad, J. Janczak, Sonochemical synthesis of a new nano-sized cerium(III) coordination polymer and its conversion to nanoceria, Ultrasonics Sonochemistry (2015), doi: http://dx.doi.org/10.1016/j.ultsonch.2015.02.003

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Sonochemical synthesis of a new nano-sized cerium(III) coordination polymer and its conversion to nanoceria Parviz Gohari Derakhshandeha, Janet Soleimannejad*a, Jan Janczakb a

School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6455,

Tehran, Iran b

Institute of Low Temperture and Structure Research, Polish Academy of Sciences, P.O.

Box 1410, 50-950 Wrocław, Poland Abstract Nanoparticles of a new two-dimensional cerium(III) coordination polymer compound, [Ce(pzdc)(pzdcH)(H2O)3]n (1), (H2pzdc = 2,3-pyrazinedicarboxylic acid), have been synthesized by a sonochemical process and characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), FT-IR spectroscopy and elemental analyses. Compound 1 was structurally characterized by single crystal X-ray diffraction and was shown that it consists of 2D sheets that construct a three-dimensional supramolecular architecture via non-covalent interactions i.e. hydrogen bonding. The thermal stability of compound 1 both for its crystals and nanostructures has been studied by the thermal gravimetric (TG) method and compared with each other. The role of ultrasound irradiation power and the concentration of initial reactants on the size and morphology of the nano-structured compound 1, has been investigated. Calcination of compound 1 at 800 ºC under air atmosphere yields ceria nanoparticles. Furthermore, the fluorescent properties of compound 1 at room temperature were studied. Keywords: Sonochemical synthesis; Coordination polymer; Nanoceria; Crystal structure; Cerium(III) 1. Introduction Coordination polymers (CPs) have attracted remarkable attention due to their fascinating architectures and topologies, and also because of their vast number of applications [1-6].

*) J. Soleimannejad (
) e-mail: [email protected] ; Tel/Fax: +98 2166495291

Considering the fact that the intrinsic advantages of CPs are associated with the used metal ions, an upsurge in the use of lanthanide elements for constructing CPs has been seen in the recent decade [7-10]. However, they have been studied far less in comparison with other metals. Cerium-based CPs have a broad range of applications including luminescence, molecular recognitions, optoelectronic devices, sensors, gas absorption, catalysis, magnetism, ion exchange and material sciences [11-14]. Nano-CPs benefit from certain attributes associated with nanomaterials as an asset to the abovementioned major characteristics of CPs, which lead to the introduction of a unique set of advantages that furthermore expand the breadth of their applications e.g. in medicine [15]. Furthermore nano-CPs have presented a new class of applications as precursors to nanomaterials such as nano-metal oxides [16-19]. Nano-size CeO2, nanoceria, has demonstrated utility in a variety of applications and its properties change dramatically from that of the bulk material. It has been synthesized using different methods such as co-precipitation, solvothermal, sol-gel, micro emulsion, etc [20]. There are also a few reports on the synthesis of nanoceria from coordination polymers [2123]. However none have used the nano-CPs of cerium(III) as the precursor, and the ultrasonic technique as the method for the synthesis of nano-CPs. Herein, we report a facile route to nanoceria from a new cerium-based nano-coordination polymer, [Ce(pzdc)(pzdcH)(H2O)3]n (1), (where H2pzdc is 2,3-pyrazinedicarboxylic acid) which has been synthesized using the ultrasonic method. Ultrasonic synthesis is a simple, efficient, low cost and environmentally friendly approach to the preparation of nano-CPs in comparison with traditional synthetic techniques [24,25]. In this method the powerful ultrasound radiation (20 kHz-1 MHz) induces local hot spots with very high temperatures (approx. 5000 °C) and pressures (approx. 500 atm), and very short lifetimes through the process of acoustic cavitation [26]. These unique conditions not only drive chemical reactions, but they can also help the formation of nano-sized coordination polymers through high-energy chemical reactions. We also report on the crystal structure, thermal behaviors and fluorescent properties of the abovementioned nano-coordination polymer (1). 2. Experimental 2.1. Materials and Physical Techniques All reagents for the synthesis and analysis were commercially available from Merck and Aldrich Company and used as received. Ultrasonic syntheses were carried out on a SONIC 2

3MX, (maximum 160 W at 37 kHz). Melting points were measured on an Electrothermal 9100 apparatus. Elemental analyses (C, H and N) were performed using ECS 4010 CHNSO analyzer. FT-IR spectra were recorded on Bruker Enquinox 55 spectrometer equipped with a single reflection Diamond ATR system over the range of 600-4000 cm-1. Thermogravimetric analyses (TGA) were carried out under argon atmosphere on a TGA Q50 V6.3 Build 189 instrument at the heating rate of 20 °C min-1. Scanning Electron Microscopy (SEM) was performed on a KYKY EM-3200. The simulated XRD powder pattern based on single crystal data were prepared using Mercury software [27]. X-ray powder diffraction (XRPD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu-Kα radiation (λ=1.54056 Å). X-ray single crystal intensity data for 1 were collected using graphite monochromatic MoKα radiation on a four-circle κ geometry KUMA KM-4 diffractometer with a twodimensional area CCD detector. The ω-scan technique with ∆ω = 1.0 o for each image was used for data collection. The 900 images for six different runs covering over 99% of the Ewald sphere were performed. One image was used as a standard after every 50 images for monitoring of the crystals stability and the data collection. No correction on the relative intensity variations was necessary. Data collections were made using the CrysAlis CCD program [28]. Integration, scaling of the reflections, correction for Lorentz and polarization effects and absorption corrections were performed using the CrysAlis Red program [28]. The structures were solved by the direct methods using SHELXS-97 and refined using SHELXL97 program [29]. The hydrogen atoms joined to aromatic carbon atoms were introduced in their geometrical positions and refined as riding. All non-hydrogen atoms were refined anisotropically. The data were deposited in Cambridge crystallographic data center with deposition number CCDC 1002354. Visualizations of the structures were made with the Diamond 3.0 program [30]. Fluorescence analyses were performed on Perkin-Elmer LS50 model luminescence spectrometer. 2.2. Synthesis of [Ce(pzdc)(pzdcH)(H2O)3]n (1) 0.09 g (0.2 mmol) of Ce(NO3)3.6H2O and pzdcH2 (0.05 g, 0.3 mmol) were refluxed for 1 h in 5 ml of water. The resulting solution was filtered and left to stand at room temperature. Paleyellow crystals suitable for X-ray crystallography were obtained by slow evaporation after two weeks (yield 68%), m.p. >350°C. Anal. Calcd for C12H11CeN4O11: C 27.3, H 2.1, N 3

10.6%. Found: C 27.8, H 1.9, N 11.1%. IR (cm-1): 780 (s), 868 (s), 1117 (s), 1167 (s), 1361 (s), 1326 (s), 1452 (m), 1627 (vs), and 3330 (br). 2.3. Synthesis of [Ce(pzdc)(pzdcH)(H2O)3]n (1) nanoparticles To synthesis the nanoparticles of 1, 20 ml solution of cerium(III) acetate (0.015 M) in water was positioned in an ultrasonic bath, operating at 37 kHz with a maximum power output of 160 W. Into this aqueous solution a 20 ml solution of the ligand 2,3-pyrazinedicarboxylic acid (0.0225 M) was added dropwise. The obtained precipitates were filtered off, washed with water and then dried in air (yield 59%), m.p. >350°C. (Found C 27.6, H 2.0, N 10.9%). IR (cm-1) selected bands: 841(m), 1117(s), 1355(s), 1389(s), 1443(m), 1565(vs), and 3330(br). To probe the effect of concentration of the initial compounds and the role of ultrasound irradiation power on size and morphology of the nano-structured coordination polymer 1, the above experiment was repeated with a concentration of 0.075 M and electrical powers of 70% and 100% from the ultrasonic generators. 2.4. Preparation of nanoceria For the preparation of nano-sized cerium(IV) oxide, nanoparticles of compound 1 were placed at 800 °C in static atmosphere of air for 3 h. 3. Results and discussion Reaction of 2,3-pyrazinedicarboxylic acid with cerium(III) nitrate hexahydrate leads to formation of a 2D coordination polymer [Ce(pzdc)(pzdcH)(H2O)3]n (1). The structure of coordination polymer 1 was characterized by single-crystal X-ray diffraction method. The detailed crystallographic data and structure refinement parameters for 1 are summarized in Table 1. Single-crystal X-ray analysis revealed that compound 1 crystallizes in the centrosymmetic space group Pbca of the orthorhombic system. The asymmetric unit of 1 contains one Ce(III) cation coordinated by pzdc2- and Hpzdc- anionic ligands and three water molecules. The molecular structure of the fundamental building unit for coordinating polymer 1 is shown in Fig. 1. The central Ce(III) cation is nine coordinated to seven oxygen atoms from three water molecules and four carboxylate groups, and two nitrogen atoms from two different pzdc2- and Hpzdc- anionic ligands. The corresponding bond lengths and angles around the Ce(III) center are listed in Table 2. Each cerium atom is bound to two pzdc units 4

through a single carboxylate oxygen atom, and the remaining two pzdc units are bound to the metal centers in a bidentate fashion through the pyrazine ring and a carboxylate oxygen atom forming 2D coordinating polymer (Fig. 2). Within the 2D coordinating polymer each pzdc units connect the two cerium sites. The most remarkable features on the construction of its 3D supramolecular architecture in 1 is that the hydrogen bonding serves to connect these 2D coordination polymer sheets to 3D supramolecular framework, as shown in Fig. 3. In the title compound, the coordinated water molecules contribute to the formation of intermolecular hydrogen bonds involving carboxylate O atoms. These 2D coordinating polymers extended to a 3D supramolecular network, via relatively strong O6H...O4iv, O6H...O11 iii and O7—H···N4iv (symmetry code as in Table 3) hydrogen bonds between the water molecules and uncoordinated (O4 and O11) oxygen atoms of carboxylate groups and N4 nitrogen atom of pyrazine ring of pzdc2anionic ligand. Corresponding hydrogen bonding distances and angles are listed in Table 3. Nano-structures of coordination polymer 1 were synthesized utilizing the ultrasonic technique at an ambient condition. The size and morphology of nano-structures of compound 1 were examined by scanning electron microscopy (SEM), Fig. 4-6. It is evident from these images that particles of compound 1 prepared using ultrasonic irradiation are nano-sized and they mostly inherit rod like morphology. The role of concentration of initial reactants on the nature of products was investigated with two different concentrations of 0.015 and 0.075 M of the initial reactants with a sonication power of 160 W. Comparison between samples with different concentrations shows that particle sizes produced using lower concentrations of initial reactants (0.015 M, Fig. 4) are smaller than particle sizes obtained using higher concentrations (0.075 M, Fig. 5). The IR spectra and XRPD patterns of typical samples of compound 1 prepared by the sonochemical process at concentrations of 0.015 and 0.075 M are also similar to the crystalline sample. Thus the concentration of 0.015 M of initial reactants has been considered as the optimized value. To study the effect of ultrasound power on the size and morphology of the nanostructures, a similar reaction with a sonication power of 112 W at the optimized concentration of initial reactants was carried out. It was found that lowering the ultrasonic generator’s power, lessens the quality of nanostructures (Fig. 6). The elemental analysis and IR spectra of the nanostructures produced by the sonochemical method and of the crystalline material produced by the reflux method are identical (Fig. 7). The symmetric and asymmetric vibrations of the carboxylate group are observed as two

5

strong bands at 1454 and 1627 cm–1, respectively. The ∆(υas-υsym) values of 173 cm-1 indicates that the carboxylate groups coordinate to the cerium(III) center in a monodentate fashion. Fig. 8 illustrates the XRPD pattern of the typical sample of coordination polymer 1 prepared by the sonochemical method (Fig. 8a) in comparison with the simulated XRPD pattern from single crystal X-ray data of coordination polymer 1 (Fig. 8b). Acceptable matches are observed for the patterns indicating the presence of only one crystalline phase in the samples prepared using the sonochemical method. In other words, the nano-sized coordination polymer obtained by the sonochemical method has an identical structure to that of the crystalline CP determined by single crystal diffraction. The thermal gravimetric analyses of the nanostructures and the single crystals of [Ce(pzdc)(pzdcH)(H2O)3]n (1) were carried out in a range of temperatures between 25 and 1000 °C at a heating rate of 20 °C min-1 under argon flow. As depicted in Fig. 9, the crystalline compound 1 is stable up to the temperature of 200 oC and its decomposition occurs between 200 and 800 ºC with a mass loss of 65.86% (calc. 67.9%). Nano-sized compound 1 is less stable and starts to decompose at 75 °C. Detectable decomposition of the nanoparticles of 1 thus starts about 125 degrees earlier than that of its bulk counterparts which is probably due to the much higher surface to volume ratio of the nano-particles and therefore more heat is needed to collapse the lattices of the bulk materials. At higher temperatures, the decomposition of compound 1 and nano-sized materials occur to ultimately give solid that appear to be CeO2 (observed: 34.14, calculated: 32.1%). The fluorescent property of the title compound [Ce(pzdc)(pzdcH)(H2O)3]n (1) was also investigated at room temperature, with the results shown in Fig. 10. Coordination polymer exhibits emission peak at about 360 nm upon excitation at 250 nm. Compared to the free pzdc ligands, compound 1 shows a slightly red shift. This indicates that the emission of 1 may be attributed to the π-π* transition of the ligand. Nano-sized CeO2 particles have been prepared by thermal decomposition of nanostructured compound 1 at 800 oC. The XRPD pattern of the residue confirms the formation of CeO2, as matches well with the standard pattern of CeO2 (Fig. 11 and JCPDS card No. 44-1001). The reflection peaks are strong and sharp and no impurity peaks are observed. These suggest that the as-prepared products are well crystallized and indicate the high purity of the final ceria product. A SEM image of the residue obtained from the direct calcination of compound 1 illustrates the formation of nano-sized CeO2 particles (Fig.12). It can be seen that the nanoparticles have a uniform morphology with diameters ranging from 30 to 50 nm. 6

4. Conclusions A new cerium(III) 2D nano-coordination polymer compound, [Ce(pzdc)(pzdcH)(H2O)3]n (1), has been synthesized using the ultrasound technique. Morphologies and sizes of the nanoparticles depend on the power of ultrasound irradiation as well as the concentration of initial reactants. Appropriate nano-sized materials of compound 1 were obtained at a concentration of 0.015 M. Compound 1 proved to be an appropriate precursor for nanoceria, to which it converts upon calcination at 800 ºC under air atmosphere. Furthermore, the fluorescent properties of compound 1 at room temperature were studied, the results of which indicate that the emission of compound 1 may be originated from the π- π* transition of the pzdc ligand. Acknowledgements The authors thank University of Tehran for all the supports. References

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Table 1. Crystal data and refinement details of [Ce(pzdc)(pzdcH)(H2O)3]n (1) Empirical formula

C12H11CeN4O11

Formula weight

527.370

Temperature

298(2)K

Wavelength

0.71073Å

Crystal system

Orthorhombic

Space group

Pbca

Unit cell dimensions

a = 7.9321(2)Å b = 14.7305(3)Å c = 27.7895(6)Å

Volume

3247.0(1)Å3

Z

8

Density (calculated)

2.150Mg/m3

Absorption coefficient

2.878 mm-1

F(000)

2056

Crystal size

0.22 x 0.15 x 0.12 mm3

Theta range for data collection

2.93 to 29.73°.

Index ranges

-10≤ h ≤10 -16≤ k ≤20 -37≤ l ≤37

Reflections collected

36926

Independent reflections

4369 [R(int) = 0.0366]

Absorption correction

Numerical

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters 2

4369 / 0 / 274

Goodness-of-fit on F

1.003

Final R. [I>2sigma(I)]

R1 = 0.0223, wR2 = 0.0402

R indices (all data)

R1 = 0.0388, wR2 = 0.0425

Largest diff. peak, hole

0.565 and -0.630 e.Å-3

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Table 2. Selected bond lengths [Å] and angles [°] for [Ce(pzdc)(pzdcH)(H2O)3]n (1)

Bond (Å) Ce−O3i

2.4283(16)

Ce−O5

2.4970(17)

Ce−O12 Ce−O2

2.4382(15) 2.4493(15)

Ce−O6 Ce−O7

2.5050(16) 2.5954(17)

Ce−O14ii Ce−N11

2.4600(15) 2.8502(18)

Ce−N1

2.7982(18)

O2−Ce−O6 O14ii−Ce−O6

83.86(6) 87.22(6)

O14ii−Ce−O5 O6−Ce−O7

133.22(6) 68.21(6)

O3ii−Ce−O6 O5−Ce−O6

139.28(6) 74.45(7

O12−Ce−O6 O12−Ce−N1

134.38(5) 68.73(5)

O3i−Ce−O7 O12−Ce−O7

71.25(5) 144.89(6)

O2−Ce−N1 O14ii−Ce−N1

60.37(5) 135.62(5)

O5−Ce−O7 O2−Ce−O7

133.92(6) 71.56(6)

O14ii−Ce−O7 O3i−Ce−O12

71.38(5) 80.54(5)

O3i−Ce−O2 O3i−Ce−O14ii

87.06(6) 76.43(6)

O12−Ce−O2 O12−Ce−O14ii

128.33(5) 82.20(5)

O2−Ce−O14ii O12−Ce−O5

142.59(6) 81.13(6)

O3i−Ce−O5

141.86(6)

Angle (˚)

Symmetry code: (i) x-1/2,y,-z+1/2; (ii) -x,-y+2,-z; (iii) x+1/2,y,-z+1/2 .

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Table 3. Hydrogen bonds geometry (Å, º). D−H···A

D−H

H···A

D···A

D−H···A

O5−H51···O1 i

0.82(2)

1.93(2)

2.734(2)

167(3)

O5−H52···O13 ii

0.82(2)

1.96(2)

2.777(3)

173(3)

O6−H61···O11 iii

0.82(2)

1.92(2)

2.713(2)

162(3)

O6−H62···O4iv

0.82(2)

1.96(2)

2.737(2)

158(3)

O7−H71···O1v

0.82(2)

2.06(2)

2.862(2)

168(3)

O7−H72···N4iv

0.82(2)

2.05(2)

2.865(3)

179(3)

N14−H14···O4 vi

0.87(2)

1.74(2)

2.611(2)

176(2)

Symmetry code: (i) x+1/2, y, −z+1/2; (ii) −x+1, −y+2, −z; (iii) −x+1/2, y−1/2, z; (iv) −x, y−1/2, −z+1/2; (v) x−1/2, y, −z+1/2; (vi) −x+1/2, −y+2, z−1/2.

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Captions for Figures Fig. 1. The coordination environment of Ce(III) cation in the [Ce(pzdc)(pzdcH)(H2O)3]n (1). Symmetry codes: (A) -x,-y+2,-z; (B) x-1/2,y,-z+1/2. Fig. 2. A fragment of the 2D coordination polymer sheet in compound 1, viewed along b direction. Fig. 3. A fragment of the 3D supramolecular framework in compound 1, viewed along a direction. Dashed lines represent the O−H…O and N−H…O hydrogen bonds. Fig. 4. SEM photographs of compound 1 nanostructures prepared by ultrasonic generator 160 W at concentration of initial reactants of 0.015 M. Fig. 5. SEM image of agglomerated compound 1 particles prepared by ultrasonic generator 160 W at concentration of initial reactants of 0.075 M. Fig. 6. SEM photograph of compound 1 nanostructures prepared by ultrasonic generator 112 W at concentration of initial reactants of 0.015 M. Fig. 7. IR spectra of (a) bulk materials as synthesized of 1, (b) nano-particles of compound 1 produced by sonochemical method. Fig. 8. XRD patterns of (a) nano-sized particle of coordination polymer 1 prepared by sonochemical method, (b) simulated pattern based on single crystal X-ray data of coordination polymer 1. Fig. 9. Thermal behavior of compound 1 as single crystals (bulk), and as nanostructures. Fig. 10. Emission spectrum of [Ce(pzdc)(pzdcH)(H2O)3]n (1). Fig. 11. XRD pattern of CeO2 nanoparticles, prepared by calcination of compound 1 at 800 ºC. Fig. 12. SEM image of CeO2 nanoparticles, prepared by calcination of compound 1 at 800 ºC.

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Figure 1

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Figure 4(1)

Figure 4(2)

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Figure 10

Figure 11

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Highlights •

A new cerium(III) coordination polymer, [Ce(pzdc)(pzdcH)(H2O)3]n (1), was synthesized.

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Nanostructures of [Ce(pzdc)(pzdcH)(H2O)3 ]n were prepared by a sonochemical process.

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Nanoceria was obtained from the nano-compound of [Ce(pzdc)(pzdcH)(H2O)3]n.

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Crystal structure of new cerium(III) CP were determined by X-ray crystallography.

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The fluorescent properties of cerium(III) CP at room temperature were studied.

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