Green synthesis and growth mechanism of new nanomaterial: Zn(salen) nano-complex

Journal of Crystal Growth 431 (2015) 39–48

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Green synthesis and growth mechanism of new nanomaterial: Zn (salen) nano-complex Maryam Mohammadikish n Faculty of Chemistry, Kharazmi University, Tehran, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 14 July 2015 Received in revised form 28 August 2015 Accepted 31 August 2015 Communicated by: M. Roth Available online 10 September 2015

Zn(salen) nano-complex was synthesized by facile hydro/solvothermal route in water as green solvent at various times and temperatures without using any surfactant or capping agent. The morphology of the products varied from irregular microcrystals to nanosheets by adjusting the temperature and time of the reaction. Based on the growth process with respect to the morphological structure, a novel growth mechanism is revealed which involves a unique multistep pathway, including reaction–nucleation, aggregation, crystallization, dissolution–recrystallization, and Ostwald ripening. The photoluminescence properties of the compounds were also examined and exhibited strong fluorescence emissions. & 2015 Elsevier B.V. All rights reserved.

Keywords: A1. Crystal morphology A1. Nanostructures A1. X-ray diffraction A2. Hydrothermal crystal growth B1. Nanomaterials B1. Zinc compounds

1. Introduction Nanotechnology and nanoscience are promising to the extent that they have significant potential to have a remarkable impact on society. The preparation of nanomaterials from defined precursors includes an important research area due to the substantial interest in materials that have practical applications in physics, chemistry, biology, and related fields. Well-structured architectures of nanomaterials provide memorable features in this regard. It is essentially a ‘bottom-up’ approach which allows the simple and rapid formation of designed compounds [1,2]. Up to now, most reported nanostructures are based on inorganic materials. In recent years, transition metal complexes have attracted more and more interests, because they exhibit many advantages and applications in catalysis, biological modeling, material chemistry and molecular magnets [3–8]. Thus, the synthesis and characterization of these nanomaterials with different particle sizes and morphologies are very important in view of basic science as well as technological applications. The transition metal complexes with Schiff bases as ligands are of paramount scientific interest, due to their multiple implications [9]. These complexes play important role in the development of coordination chemistry related to catalysis and enzymatic reactions, magnetism and molecular architectures as well as for liquid-crystal technology [10,11]. n

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http://dx.doi.org/10.1016/j.jcrysgro.2015.08.029 0022-0248/& 2015 Elsevier B.V. All rights reserved.

In recent years, the synthesis of nanometer sized transition metal complexes with novel controlled shape and structure has been attracted an increasing interest owing to their excellent physical and chemical properties. Saghatforoush et al. synthesized Ni-Schiff base [12] and Cu-Schiff base [13] complexes by sonochemical and solvothermal methods. Liu et al. used N-pnitrophenylsalicylaldimine with zinc acetate and cadmium chloride, respectively, as precursors to synthesize Zn-Schiff base [14] and Cd-Schiff base [15] nanoribbons via solvothermal routes. Bis (8-hydroxyquinoline) magnesium [16], bis(8-hydroxyquinoline) mercury [17], bis(8-hydroxyquinoline) zinc [18] and bis(8-hydroxyquinoline) cadmium [19] nanoribbons have been synthesized via the solvothermal method by Wang et al. Recently, with development of synthetic technology of nanomaterials, some wet-chemical methods have been explored and various nano-complexes were synthesized. However, there are few works published on the synthesis of transition metal nanocomplexes in aqueous solution [20]. In this report, [N,N0 -Bis(salicylidene)ethylenediamine] zinc nano-complex, Zn(Salen), with different morphologies was synthesized via a facile hydro/solvothermal route without the use of template, catalyst or surfactant. Characterization of the prepared nanomaterials indicated that they consisted of nanorods, nanosheets, nanoparticles and some agglomerated micro-shapes. The effect of reaction conditions on the morphology of the products was investigated and a possible formation mechanism was proposed.

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2. Experimental 2.1. Materials and methods All reagents and solvents were of analytical grade and were used as received without further purification. The elemental analysis was carried out on a Perkin–Elmer 2400. The electronic spectra were recorded on Perkin–Elmer lambda 25 spectrometer. Fourier-transform infrared spectra were recorded using Perkin– Elmer Spectrum RXI FT-IR spectrometer, using pellets of the materials diluted with KBr. 1H NMR spectra were recorded on a Bruker Avance 300 spectrometer. The 1H NMR chemical shifts in ppm are reported from tetramethylsilane (TMS) as internal reference. X-ray diffraction patterns were recorded by a Rigaku D-max CIII, X-ray diffractometer using Ni-filtered Cu Kα radiation. Scanning electron microscopy (SEM) images were obtained on KYKYEM3200. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Diamond thermogravimeter in the temperature range from room temperature to 800 °C at a heating rate of 10 °C min  1 in air. Photoluminescence measurements were performed on an Ava spec-3684 fluorescence spectrophotometer. 2.2. General procedure for the synthesis of Zn(salen) complex 2.2.1. Hydro/solvothermal synthesis of Zn(salen) complex In a typical synthesis, 0.75 mmol of Zn(NO3)2  4H2O dissolved in 30 mL H2O was added to 0.75 mmol of N,N0 -Bis(salicylidene) ethylenediamine dispersed in 70 mL H2O. The resulting mixture stirred for 15 min and then transferred into a teflon lined stainless steel autoclave with 150 mL capacity, sealed and maintained at various times and temperatures. After hydrothermal synthesis, the autoclave cooled down to room temperature. The pale yellow precipitate was collected and washed several times with distilled water and absolute ethanol. In another experiments, in the solvothermal process, 30 mL of C2H5OH or CH3CN was used as solvent instead of water. 2.2.2. Reflux synthesis of Zn(salen) complex For comparison, Zn(salen) complex was prepared in reflux conditions. 0.75 mmol of N,N0 -Bis(salicylidene)ethylenediamine and 0.75 mmol of Zn(NO3)2  6H2O were dissolved in 30 mL methanol and refluxed for 2 h. After evaporation of solvent, fine crystals of pale yellow Zn(salen) were appeared, which was filtered and washed with water and ethanol. The solid Zn(salen) complexes (Scheme 1) were synthesized with hydro/solvothermal treatment of salen ligand with zinc nitrate in 1:1 molar ratio at different temperatures (120 °C, 140 °C, 160 °C and 180 °C), times (6 h, 13 h, 18 h and 24 h) and solvents (H2O, C2H5OH and CH3CN). Structural and morphological characterization of the complexes were carried out by elemental analysis, 1H NMR, FT-IR, UV–vis, PL spectroscopy and XRD, SEM, TGA techniques. 1- Zn(salen). 0.4 H2O: in reflux conditions: pale yellow crystal (38%); Elemental anal. Calc. for C16H14N2O2Zn. 0.4 H2O: C, 56.71; H, 4.40; N, 8.27. Found: C, 57.00; H, 4.16; N, 8.62. 1H NMR (DMSO-d6): 0 0 δ ¼8.42 (s, 2H, N ¼CH), 7.13 (d, 2H, H4,4 ), 7.10 (d, 2H, H2,2 ), 6.60 (d, 1,10 3,30 6,60 2H, H ), 6.41 (t, 2H, H ), 3.71 (s, 4H, H ) ppm. FT-IR (KBr, cm  1): 3427 (υO–H, H2O), 3043, 3012 (υC–H–aromatic), 2929, 2900 (υC–H–aliphatic), 1639 (υC ¼ N), 1598, 1539, 1472 (υC ¼ C), 1150 (υC–O). UV–vis (λmax, nm, DMSO): 316, 384. 2- Zn(salen). 1.0 H2O: in 140 °C–6 h hydrothermal conditions: pale yellow (38%); Elemental anal. Calc. for C16H14N2O2Zn. 1.0 H2O: C, 54.96; H, 4.61; N, 8.01. Found: C, 55.14; H, 4.32; N, 8.03. 1H NMR 0 (DMSO-d6): δ ¼ 8.42 (s, 2H, N ¼CH), 7.13 (d, 2H, H4,4 ), 7.10 (dd, 2H,

Scheme 1. Schematic representation of the Zn(salen) complex.

0

0

0

0

H2,2 ), 6.60 (d, 2H, H1,1 ), 6.41 (dt, 2H, H3,3 ), 3.71 (s, 4H, H6,6 ) ppm. FT-IR (KBr, cm  1): 3426 (υO–H, H2O), 3042, 3010 (υC–H–aromatic), 2920, 2898 (υC–H–aliphatic), 1642 (υC ¼ N), 1600, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 318, 384. 3- Zn(salen). 1.0 H2O: in 140 °C–13 h hydrothermal conditions: pale yellow (42%); Elemental anal. Calc. for C16H14N2O2Zn. 1.0 H2O: C, 54.96; H, 4.61; N, 8.01. Found: C, 54.89; H, 4.33; N, 8.11. 1H NMR 0 (DMSO-d6): δ ¼8.42 (s, 2H, N ¼CH), 7.13 (d, 2H, H4,4 ), 7.10 (d, 2H, 0 2,20 1,10 3,30 H ), 6.60 (d, 2H, H ), 6.41 (t, 2H, H ), 3.71 (s, 4H, H6,6 ) ppm. 1 FT-IR (KBr, cm ): 3427 (υO–H, H2O), 3041, 3010 (υC–H–aromatic), 2926, 2898 (υC–H–aliphatic), 1641 (υC ¼ N), 1599, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 318, 385. 4- Zn(salen). 1.0 H2O: in 140 °C–18 h hydrothermal conditions: pale yellow (38%); Elemental anal. Calc. for C16H14N2O2Zn. 1.0 H2O: C, 54.96; H, 4.61; N, 8.01. Found: C, 54.92; H, 4.42; N, 8.00. 1H NMR 0 (DMSO-d6): δ ¼8.42 (s, 2H, N¼ CH), 7.13 (d, 2H, H4,4 ), 7.10 (dd, 2H, 0 0 0 0 H2,2 ), 6.60 (d, 2H, H1,1 ), 6.41 (dt, 2H, H3,3 ), 3.70 (s, 4H, H6,6 ) ppm. 1 FT-IR (KBr, cm ): 3428 (υO–H, H2O), 3041, 3011 (υC–H–aromatic), 2920, 2898 (υC–H–aliphatic), 1641 (υC ¼ N), 1599, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 318, 382. 5- Zn(salen). 1.0 H2O: in 140 °C–24 h hydrothermal conditions: pale yellow (42%); Elemental anal. Calc. for C16H14N2O2Zn. 1.0 H2O: C, 54.96; H, 4.61; N, 8.01. Found: C, 55.03; H, 4.39; N, 8.08. 1H NMR 0 (DMSO-d6): δ ¼8.42 (s, 2H, N¼ CH), 7.13 (d, 2H, H4,4 ), 7.10 (dd, 2H, 0 0 0 0 H2,2 ), 6.60 (d, 2H, H1,1 ), 6.41 (t, 2H, H3,3 ), 3.71 (s, 4H, H6,6 ) ppm. 1 FT-IR (KBr, cm ): 3430 (υO–H, H2O), 3041, 3011 (υC–H–aromatic), 2926, 2901 (υC–H–aliphatic), 1642 (υC ¼ N), 1600, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 317, 384. 6- Zn(salen). 1.0 H2O: in 120 °C–18 h hydrothermal conditions: pale yellow (38%); Elemental anal. Calc. for C16H14N2O2Zn. 1.0 H2O: C, 54.96; H, 4.61; N, 8.01. Found: C, 55.03; H, 4.56; N, 8.03. 1H NMR 0 (DMSO-d6): δ ¼8.42 (s, 2H, N¼ CH), 7.13 (dd, 2H, H4,4 ), 7.10 (d, 2H, 0 0 0 0 H2,2 ), 6.60 (d, 2H, H1,1 ), 6.41 (t, 2H, H3,3 ), 3.71 (s, 4H, H6,6 ) ppm. FT-IR (KBr, cm  1): 3424 (υO–H, H2O), 3042, 3010 (υC–H–aromatic), 2926, 2901 (υC–H–aliphatic), 1641 (υC ¼ N), 1599, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 317, 384. 7- Zn(salen). 0.9 H2O: in 160 °C–18 h hydrothermal conditions: pale yellow (42%); Elemental anal. Calc. for C16H14N2O2Zn. 0.9 H2O: C, 55.24; H, 4.58; N, 8.05. Found: C, 55.15; H, 4.15; N, 8.02. 0 1 H NMR (DMSO-d6): δ ¼8.41 (s, 2H, N¼ CH), 7.13 (d, 2H, H4,4 ), 7.10 2,20 1,10 3,30 (dd, 2H, H ), 6.60 (d, 2H, H ), 6.41 (t, 2H, H ), 3.70 (s, 4H, H6, 60 ) ppm. FT-IR (KBr, cm  1): 3434 (υO–H, H2O), 3042, 3011 (υC–H– aromatic), 2926, 2898 (υC–H–aliphatic), 1642 (υC ¼ N), 1600, 1541, 1472 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 317, 383. 8- Zn(salen). 0.9 H2O: in 180 °C–18 h hydrothermal conditions: pale yellow (42%); Elemental anal. Calc. for C16H14N2O2Zn. 0.9 H2O: C, 55.24; H, 4.58; N, 8.05. Found: C, 55.22; H, 4.27; N, 8.01. 0 1 H NMR (DMSO-d6): δ ¼8.41 (s, 2H, N¼ CH), 7.13 (d, 2H, H4,4 ), 7.10 2,20 1,10 3,30 (dd, 2H, H ), 6.61 (d, 2H, H ), 6.41 (dt, 2H, H ), 3.71 (s, 4H, 0 H6,6 ) ppm. FT-IR (KBr, cm  1): 3426 (υO–H, H2O), 3041, 3009 (υC–H– aromatic), 2925, 2898 (υC–H–aliphatic), 1641 (υC ¼ N), 1600, 1541, 1471 (υC ¼ C), 1152 (υC–O). UV–vis (λmax, nm, DMSO): 315, 385.

M. Mohammadikish / Journal of Crystal Growth 431 (2015) 39–48

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Fig. 1. FT-IR spectra of Zn(salen) prepared at (a) reflux, and (b) hydrothermal conditions 180 °C–18 h.

3. Results and discussion 3.1. Synthesis of the complexes The elemental analysis results of the hydrothermal products are in very good agreement with the Zn(salen) complex that was synthesized in reflux conditions; although the CHN results of the solvothermal reactions e.g. 140 °C–6 h in ethanol and 140 °C–6 h in acetonitrile are somewhat different from calculated data. All the complexes have coordinated or adsorbed water molecules to saturate the metal coordination spheres. 3.2. Spectroscopic characterization The FT-IR spectra of two samples (reflux, and hydrothermal prepared at 180 °C–18 h) can be seen in Fig. 1. The presence of water molecule is confirmed by a broad absorption band between

2500 and 3500 cm  1 in FT-IR spectra of the complexes, indicating intermolecular H-bonding [21]. The spectra exhibited some weak bands at 2800–3100 cm  1 which are related to aromatic and aliphatic C–H stretching vibrations. The imine band, υ(C ¼N), was found at lower frequencies in the complexes, suggesting coordination of the metal through nitrogen atoms [22]. The peaks at wavenumbers around 1600–1470 cm  1 are assigned to the C ¼C stretching vibrations. The peak at 1152 cm  1 indicates C–O stretching vibration that undergoes a shift toward higher wavenumbers with respect to the ligand [23]. The complexes were further characterized by 1H NMR spectroscopy. The 1H NMR spectra of all the complexes prepared in different temperatures and times are quite similar with the sample synthesized at reflux condition. The coordination of two phenolic oxygens to zinc center was confirmed by the absence of 10– 12 ppm signal, typical of phenolic OH groups in Schiff base ligand.

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Fig. 2. 1H NMR spectra of Zn(salen) complex prepared at (a) reflux, and (b) hydrothermal conditions, 180 °C–18 h.

In all of the spectra, one intense singlet at δ ¼8.42 ppm is assigned to azomethine protons clearly suggesting their equivalent environment. The aromatic proton signals were appeared in the 7.13,

0

0

0

0

7.10, 6.60 and 6.41 ppm for H4,4 , H2,2 , H1,1 and H3,3 , respectively. Furthermore, there is a sharp singlet at δ ¼ 3.71 ppm assigned to 0 the four aliphatic hydrogens (H6,6 ). For comparison, the 1H NMR

M. Mohammadikish / Journal of Crystal Growth 431 (2015) 39–48

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Fig. 3. XRD patterns of the as-prepared Zn(salen) complex at (a) 140 °C–6 h (b) 140 °C–18 h, (c) 180 °C–18 h in H2O and (d) simulated from single crystal X-ray data of Zn (salen) [25].

spectra of two samples (reflux and hydrothermal 180 °C–18 h) was shown in Fig. 2. In the solvothermally prepared samples (140 °C– 6 h in ethanol and 140 °C–6 h in acetonitrile) all of these signals exist as well as some impurities. The electronic spectra of the zinc complexes were similar and revealed two broad peaks around 317 and 384 nm. The higher energy band can be assigned to the π–π* transition in the ligand and the lower energy absorption at 380–385 nm is assigned as a charge transfer transition [24]. 3.3. Morphological analysis The crystallinity of the as-synthesized Zn(salen) complexes was examined by XRD analysis. Because products with different shapes have the same XRD patterns, just three samples were selected as examples. Fig. 3 shows the diffraction patterns of the as-synthesized Zn(salen) hydrothermal products at 140 °C–6 h, 140 °C–18 h, 180 °C– 18 h as well as simulated pattern of Zn(salen) obtained from its single crystal X-ray data [25]. The strong and sharp diffraction peaks confirm the high crystallinity of the products. The crystallinity of the resultant products was changed a little by varying the reaction time and temperature. Acceptable matches were observed between the simulated and experimental powder X-ray diffraction patterns. This indicates that the compounds obtained by a hydrothermal method have crystalline phase with Pmnb space group. All of the diffraction peaks were indexed by comparison with the simulated XRD powder

patterns [25]. The higher intensity of (1 1 1) peak revealed the preferred growth at the {1 1 1} orientation at different reaction times and temperatures. The SEM images of Zn(salen) complexes are shown in Figs. 4–7. Fig. 4 shows the SEM images of the complex synthesized in reflux condition and indicates nanorods of complex with diameter about 100 nm. Also, some nanoparticles can be seen in these images. The effect of temperature on the size and morphologies of the obtained products was investigated at 120, 140, 160 and 180 °C for 18 h (Fig. 5). At 120 °C, some microcrystals with undefined shapes and different sizes formed. With increasing the reaction temperature to 140 °C, some nanoplates (red circles) as well as microcrystals were observed. At the same time but increased temperature to 160 °C nanosheets (red circles in Fig. 5c) with thickness of about 57 nm were obtained. Moreover, isolated nanorods with 60 nm in diameter and stacked nanorods (yellow circle) can be seen in Fig. 5c. When the temperature was raised to 180 °C, nanosheets and in less extent microcrystals of Zn(salen) were produced (Fig. 5d). In general, increasing the reaction temperature causes the change of morphology from microcrystals to nanosheets. In addition to the reaction temperature, the effect of reaction time on the morphology of the products was also studied at 6, 13, 18 and 24 h at 140 °C (Fig. 6). The product obtained after 6 h is consisted of microcrystals with cubic, rhombus, and other structures (Fig. 6a). Extending the reaction time to 13 h led to the formation of

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Fig. 4. SEM images of Zn(salen) nanorods prepared in reflux conditions.

Fig. 5. SEM images of Zn(salen) nano-complexes prepared in H2O at 18 h and various temperatures; (a) 120 °C, (b) 140 °C, (c) 160 °C, (d) 180 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

some nanoparticles in addition of microcrystals (Fig. 6b). A mixture of nano and microplate were formed after 18 h (Fig. 6c). When the reaction time was increased to 24 h, the stacked nanosheets with thickness of about 45 nm were seen (Fig. 6d). In addition to the reaction time and temperature effects on the morphology of the products, some solvothermal reactions were also carried out at 140 °C and 6 h in H2O, C2H5OH and CH3CN to investigate the effect of solvent. In hydrothermal conditions, in the presence of H2O, some microcrystals with different morphologies like cubic, rhombus etc. were obtained (Figs. 6a and 7a). Another experiment was carried out by using ethanol as solvent; however, the mixture of nano/microspheres was obtained (Fig. 7b). Many small nanoparticles with diameters about 50 nm and some bigger

nanoparticles with 600 nm in diameter can be seen. In acetonitrile, many nanorods with thickness about 55 nm as well as nanoparticles with 80 nm in diameter were obtained (Fig. 7c and d). 3.4. Growth mechanism Based on the above experimental observations, a growth evolution from initial irregular aggregates to final nanosheet structures could be seen (Scheme 2). The formation of Zn(salen) complex is based on a multistep pathway, including reaction–nucleation, aggregation, crystallization, dissolution–recrystallization, and Ostwald ripening growth mechanism [26]. Before the hydrothermal reaction, there are insoluble N, N0 -Bis(salicylidene)ethylenediamine

M. Mohammadikish / Journal of Crystal Growth 431 (2015) 39–48

Fig. 6. SEM images of Zn(salen) nano-complexes prepared in H2O at 140 °C and various times; (a) 6 h, (b) 13 h, (c) 18 h, (d) 24 h.

Fig. 7. SEM images of Zn(salen) nano-complexes prepared at 140 °C–6 h and various solvents; (a) H2O, (b) EtOH, (c, d) CH3CN.

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Scheme 2. Schematic illustration of a proposed mechanism for the formation of the Zn(salen) nanosheets.

Fig. 8. TGA, DTG and DTA curves of Zn(salen) nano-complex prepared at 180 °C–18 h in H2O. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

and soluble Zn2 þ cation in the water solvent. At the beginning of the hydrothermal reaction, with increasing temperature, the precursors begin to dissolve. When the heterogeneous critical nucleation concentration is reached, the Zn2 þ and salen are transformed into a few Zn(salen) nuclei at the surface or edges of the precursors, which are not completely dissolved in solution. While the nucleation phase is not terminated, some nuclei involve in crystal growth phase. The formed Zn(salen) complex precipitated on the surface of initial nuclei results in faster crystal growth rate in a short reaction time and formation of irregular microcrystals. This complex procedure could contain three main stages. First, at the early stage of growth, only irregular aggregates of microcrystals, which could be composed of small nanoparticles and/or nanocrystals, were obtained (Figs. 5a and 6a). Second, nanosheets as well as microcrystal products appeared at the following growth stage (Fig. 5b, c and Fig. 6b, c). Third, with further increasing the reaction time or temperature, there is a transition from microcrystal to nanosheets. This transition could be thought as a typical Ostwald ripening process [27,28], during which microcrystals dissolve and recrystallize to form 2D nanosheets. As we can see from Figs. 5d and 6d, sheets rather than irregular aggregates were the main products. In this process, reaction time, temperature (discussed above), and solvent play the major factors influencing the morphology of the products. Under solvothermal conditions, soluble Salen serves as a chelating ligand and coordinates to Zn2 þ to form Zn(salen) complex from which aggregates into nanorods in acetonitrile or nanoparticles in ethanol. Probably, the type of solvent influences the growth on certain directions. The crystal growth mechanism in

solution is so complicated that the actual crystallization mechanism remains a challenging question. 3.5. Thermogravimetric analysis Thermal behavior of the Zn(salen) sample prepared at 180 °C– 18 h was monitored by thermogravimetric analysis. As can be seen in Fig. 8 the TGA curve shows a two-step process. The TGA curve of the prepared Zn(salen) complex (Fig. 8, red curve) gave a weight loss of about 4.9% on heating in the range 100–170 °C. The corresponding DTA curve in this region (Fig. 8, blue curve) indicated an endothermic effect, which could be attributed to the loss of trace amount of adsorbed water. The next TGA peak is a weight loss of about 70.3% in the range 350–440 °C. The DTA peak in this region is exothermic which is assigned to combustion of the organic ligand in the Zn(salen). There is no weight loss above 450 °C and finally the zinc oxide was left. 3.6. Photoluminescence analysis The study of photoluminescence (PL) properties of Zn(II) complexes is not only of fundamental interest, but also significant for different applications (optical amplifiers, optical waveguides, OLED, etc.) [29]. Since salen ligand and its complexes are reported to be luminescent [29,30], the photoluminescence spectra of two complexes (180 °C–18 h and reflux samples) were recorded in DMSO and depicted in Fig. 9. The emission spectra were obtained by excitation at the longest wavelength of the absorption peaks. It has been reported that the ligand shows a photoluminescence with emission around 512 nm in DMSO [31]. Upon complexation of the salen

M. Mohammadikish / Journal of Crystal Growth 431 (2015) 39–48 50000 reflux 180C-18h

Intensity

40000

30000

20000

10000

0 400

450

500

550

wavelenght (nm)

600

650

700

Fig. 9. The emission spectra of the Zn(salen) complexes in DMSO.

ligand with Zn(II) ion, intense emissions are observed at 481 nm for reflux and 488 nm for sample prepared at 180 °C–18 h conditions. The photoluminescence spectra of the Zn(II) complexes were assigned to energy transfer between the HOMO and LUMO of the deprotonated ligands with the HOMO mainly being π-bonding in character and the LUMO mainly being π*-antibonding in character [32]. No emission originating from metal-centered excited states is expected for the Zn(II) complexes, since they are difficult to oxidize or reduce due to their d10 configuration. Thus, the emission observed in the complexes is assigned to the (π–π*) intraligand fluorescence and these are blue shifted by o30 nm. The enhancement of the fluorescence of the complexes compared to their respective ligands is due to CHEF (chelation enhancement of fluorescence emission) [33]. Factors like a simple binding of the ligand to the metal ions [34], an increase in rigidity of structure [35], a restriction in the photo-induced electron transfer (PET) [33,36] etc. are assigned to the appearance and enhancement of the PL emission. Also, the Zn(salen) with various morphologies have the same luminescent emission with nearly similar λmax.

4. Conclusion In summary, a simple hydro/solvothermal synthetic route is devised to produce a Zn(salen) nano-complex from microcrystal to ultrathin sheets. The morphology of the product is dependent on the experimental conditions such as the solvent, heating temperature and heating time. From the morphology evaluations, the formation process of microcrystal to nanosheets contains multistep pathway, including reaction–nucleation, aggregation, crystallization, dissolution–recrystallization, and Ostwald ripening growth mechanism which started from irregular microcrystals aggregates to ultrathin nanosheets. The blue emission of these compounds suggests that they may be potential candidates for blue-light emitting materials.

Acknowledgments The author is grateful to Kharazmi University for financial support.

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