Effect of zinc acetate addition on crystal growth, structural, optical, thermal properties of glycine single crystals

2.1 Chemicals used

Glycine (C₂H₅No₂ aminoaceticacid) SD-fine AR (99.5%) M.wt: 75.07 g/mol, zinc acetate dihydrate (C₄H₆O₄Zn·2H₂O) SD-fine AR (99.5%) M.wt: 219.50 g/mol and double distilled water were used for the crystal growth experiment.

2.2 Crystal growth

In series the analytical reagent chemicals of glycine and zinc acetate dihydrate were mixed in different molar ratios such as 1:0.2,1: 0.4, 1:0.6, 1:0.7, 1:0.8, and 1:0.9, respectively in 100 ml double distilled water to identify and confirm the presence of γ-polymorph in the aqueous solution. The prepared solutions were placed in a separate 100 ml beaker and the mixtures were stirred well continuously up to 6 h to get the homogenous saturated solutions. The saturated solutions were filtered with whatman 41 filter paper. The filtered saturated solutions were closed with a perforated cover and kept in a dust free environment and allowed to crystallize by slow evaporation at ambient temperature. After a nucleation period of 5–7 days of solvent evaporation, the solution becomes supersaturated and tiny crystals were found in the beakers containing molar ratios such as 1:0.9, 1:0.8, 1:0.7, and 1:0.6, respectively. The nucleation period τ for the grown crystals is presented in Table 1. From the growth experiment, it is clear that higher molar concentration of zinc acetate dihydrate (0.6–0.9) induces quick nucleation sites than that of lower molar concentration of zinc acetate (0.2, 0.4).

2.3 Crystal morphology

The morphology of the grown crystal changes from α to γ-polymorph when the concentration of zinc acetate exceeds 0.4 M. The α- and γ-polymorphs of glycine were confirmed by Single crystal XRD, Powder XRD and FTIR studies. The grown crystals were harvested after a period of 18 days. The harvested seed crystal of α-glycine and its bulk size are shown in Figs. 1 and 2. The morphology of the crystal resulted from lower molar concentrations of zinc acetate is shown in Figs. 3 and 4. Higher molar concentrations of zinc acetate lead to γ-polymorphs which are presented in Figs. 5–8.

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Figure 1

Harvested seed crystal of α-Glycine.

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Figure 2

Bulk size of α-Glycine crystal.

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Figure 3

α-Glycine crystal (1:0.2).

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Figure 4

α-Glycine crystal (1:0.4).

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Figure 5

γ-Glycine crystal (1:0.6).

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Figure 6

γ-Glycine crystal (1:0.7).

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Figure 7

γ-Glycine crystal (1:0.8).

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Figure 8

γ-Glycine crystal (1:0.9).

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3.1 Single crystal XRD and ICP-OES studies

Single crystal X-ray diffraction data were recorded using ENRAF NONIUS CAD4-MV31 X-ray diffractometer with MoK_α radiation (λ = 0.71073 Ǻ) to reveal the crystal structure and unit cell dimensions of the grown crystal. The unit cell dimensions were obtained for the grown single crystals from single crystal XRD analysis and are presented in Table 2. This study confirms the α- and γ-polymorphs of glycine. The cell values closely agreed with the reported values (Sekar and Parimaladevi, 2009). To determine the concentration of zinc element present in the grown crystals, the inductively coupled plasma optical emission spectrometry (ICP-OES) analysis using Perkin Elmer Optima 5300 DV ICP-OES spectrometry was carried out. The presence of zinc element in the grown crystals is listed in Table 3. Emission spectrometry is based on the principle that atoms or ions in an excited state tend, to revert back to the ground state and in doing so emit characteristic wavelength and the intensity of that light is proportional to the concentration of that particular element in the sample solution. This technique is used for qualitative and quantitative determination of zinc present in the γ-glycine. For this analysis, the grown crystals were crushed into pieces and finely grounded in an agate mortar. These powdered samples were transferred into a 25 ml volumetric flask with the help of a funnel and were diluted to volume with deionized water. This diluted sample was analysed by ICP-OES system. The amount of zinc element present in the γ-glycine sample prepared from 0.9 M concentration of zinc acetate was found to be 0.73 ppm and its characteristic wavelength is 213.89 nm. It is determined that the concentration of zinc is so low as to form a complex and hence it can be concluded that the zinc can be incorporated in the void space within the crystal lattice of γ-glycine (Dillip et al., 2012; Anbuchezhiyan et al., 2010).

3.2 Powder X-ray diffraction analysis

Powder X-ray diffraction studies were carried out using a Bruker D8 advance diffractometer with CuK_α (λ = 1.5406 Ǻ) radiation. The powdered α- and γ-glycine sample from 0.4 to 0.9 M concentration of zinc acetate respectively was scanned over a range 5–70° at the scan rate of 1° min. The reflection peaks corresponding to different crystal h k l planes in the recorded XRD profile were indexed. The indexed powder X-ray diffraction pattern for α- and γ-glycine crystal is shown in Figs. 9 and 10 for comparison. The observed powder X-ray diffraction pattern and position of the peaks of α- and γ-glycine crystal were found to be in good agreement with the reported pattern (Index to the Powder Diffraction File, 2002). The data obtained from the XRD spectrum such as Angle 2θ, d-value, h k l, peak intensity, intensity (%) and full width half maximum value (FWHM) of every prominent peak in the spectrum of α- and γ-glycine are tabulated in Tables 4 and 5. The sharp and strong peaks in the XRD pattern confirm good crystallinity of the grown crystal. From the XRD data, the lattice parameters of the grown α- and γ-glycine crystal were calculated using UNIT CELL software and are tabulated in Table 6.

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Figure 9

Indexed powder XRD profile of α-Glycine.

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Figure 10

Indexed powder XRD profile of γ-Glycine.

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3.3 FTIR spectral analysis

The infrared spectroscopy is effectively used to identify the functional groups of the samples. FTIR spectra of α- and γ-glycine sample from 0.4 to 0.9 M concentration of zinc acetate respectively are shown in Figs. 11 and 12 for comparison. In the FTIR spectrum of α-glycine, the peaks observed at 502, 606 and 695 cm⁻¹ are attributed to the carboxylic groups while the peak observed at 1513 cm⁻¹ corresponds to the ${\text{NH}}_3^{+}$ group. The peaks at 1030 and 898 cm⁻¹ are attributed to C–C–N asymmetric and C–C–N symmetric stretching modes of vibration. The absorption peak observed at 1328 cm⁻¹ is attributed to CH₂ twisting mode. The peak observed at 1109 cm⁻¹ corresponds to ${\text{NH}}_3^{+}$ rocking. The strong asymmetric CO₂ stretching vibration occurs at 1609 cm⁻¹. The combination bond occurs at 2121 cm⁻¹. The peaks at 2614 and 3169 cm⁻¹ are due to ${\text{NH}}_3^{+}$ stretching vibration.

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Figure 11

FTIR spectrum of α-Glycine.

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Figure 12

FTIR spectrum of γ-Glycine.

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The spectrum of γ-glycine displays the peaks at 501, 606 and 683 cm⁻¹ that are attributed to the carboxylic groups while the peak observed at 1491 cm⁻¹ corresponds to the ${\text{NH}}_3^{+}$ group. Frequencies observed at 1040 and 887 cm⁻¹ are attributed to C–C–N asymmetric and C–C–N symmetric stretching vibration. The absorption peaks observed at 927 and 1327 cm⁻¹ are attributed to CH₂ rocking and CH₂ twisting mode. The peak observed at 1124 cm⁻¹ corresponds to ${\text{NH}}_3^{+}$ rocking. The prominent band near 2171 cm⁻¹ may be assigned to combination bond. The other peaks around 2601 and 3105 cm⁻¹ have been attributed to the ${\text{NH}}_3^{+}$ stretching group. The FTIR data comparisons of α- and γ-glycine single crystals are presented in Table 7. The observed functional groups of the grown α- and γ-glycine single crystals are in good agreement with the literature values (Sekar and Parimaladevi, 2009; Dhanraj and Rajesh, 2009; Dillip et al., 2012).

3.4 Differential scanning calorimetry analysis

DSC studies of powdered sample of α- and γ-glycine sample from 0.4 to 0.9 M concentration of zinc acetate respectively were recorded by employing NETZSCH DSC 200F3 instrument between 30 and 500 °C. The recorded DSC thermograms of α- and γ-glycine sample are shown in Figs. 13 and 14. The α-glycine DSC thermogram shows a peak at 253.9 °C which corresponds to the decomposition of glycine. Whereas in γ-glycine DSC thermogram a minor peak occurs at 187.3 °C, a phase transformation peak from γ to α-polymorph. The sharpness of the exothermic peak shows a good degree of crystallinity. From the DSC curve of γ-glycine, it is observed that an exothermic event begins at 243.5 °C and then a sharp peak appears at 256.9 °C which corresponds to decomposition point of the sample. The phase transition temperatures are reported by several researchers such as 172, 189, and 182 °C (Narayan Bhat and Dharmaprakash, 2002; Balakrishnan et al., 2008; Dhanraj and Rajesh, 2009). It is concluded from their studies that the transition temperature between α and γ-polymorphs can range between 165 and 201 °C (Perlovich et al., 2001). Thus, in the present work, the phase transition temperature appears at 187.3 °C.

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Figure 13

DSC thermogram of α-Glycine.

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Figure 14

DSC thermogram of γ-Glycine.

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3.5 Optical transmittance studies

Good optical transmittance and lower cut-off wavelength are essential properties for NLO crystals. The UV transmission spectra of α-glycine and γ-glycine grown in the presence of 0.4 and 0.9 M concentrations of zinc acetate respectively are shown in Figs. 15 and 16 for comparison. The lower cut-off wavelength of γ-glycine was at 212 nm and it was determined by chromophores such as the aminoacid and carboxyl group, which show no absorption and are transparent in the UV region. The bandgap of γ-glycine is calculated using the formula E_g = 1240/λ (nm). It is found to be 5.84 eV. The percentage of transmittance of γ-glycine is 94%. The observed spectrum and band gap value are in good agreement with the literature values (Narayan Bhat and Dharmaprakash, 2002; Sekar and Parimaladevi, 2009; Dillip et al., 2011; Esthaku Peter and Ramasamy, 2010; Balakrishnan et al., 2008; Dhanraj and Rajesh, 2009).

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Figure 15

UV spectrum of α-Glycine.

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Figure 16

UV spectrum of γ-Glycine.

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3.6 SHG efficiency measurement

The second harmonic generation efficiency of the grown γ-glycine crystal was checked using the technique developed by Kurtz and Perry technique (Kurtz and Perry, 1968). The crystal was ground into powder and densely packed in a micro capillary tube. A Q-switched Nd: YAG laser beam of wavelength 1064 nm was made to fall normally on the sample cell. The emission of green light confirmed the SHG of γ-glycine crystal. Thus, the efficiency of γ-glycine crystal is 3.66 times higher than that of potassium dihydrogen phosphate (KDP). The SHG signal of γ-glycine crystal is 165 mV, while the standard inorganic material potassium dihydrogen phosphate (KDP) gave a signal of 45 mV for the same input energy.