CHAPTER 4 GROWTH AND CHARACTERIZATION OF 4-NITROPHENOL UREA SINGLE CRYSTALS

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1 89 CHAPTER 4 GROWTH AND CHARACTERIZATION OF 4-NITROPHENOL UREA SINGLE CRYSTALS 4.1 INTRODUCTION Organic crystals find applications in frequency doubling, frequency mixing, electro optic modulation, optical storage and optical communications. The organic compounds with electron rich (donor) and deficient (acceptor) substituents, provide the asymmetric charge distribution in the electron system and show large nonlinear optical responses. NLO crystals should meet several requirements, such as large phase - matchable nonlinear optical co-efficient, a wide optical and chemical stability and a high damage threshold [13]. A majority of organic crystals have their absorption in the blue region and some of them have a cut-off wavelength lower than 450 nm. This indicates the possibility of reduced conversion efficiency of SHG due to self-absorption of materials when using a semiconductor laser with 800 nm band [142, 143]. Recently, there has been a search for newer organic NLO materials with blue light transmittance [144]. Organic crystals have been extensively studied due to their nonlinear optical coefficients being often larger than that of inorganic materials. In addition to large NLO coefficient, an organic NLO crystal should be transparent in the UV region [13, 145]. NLO applications require materials with very large macroscopic second order susceptibilities which are usually constituted from molecules with large molecular first hyperpolarizability and oriented in a noncentrosymmetric arrangement [54]. Most of the commercial materials for second order applications are inorganics, especially for high power use. Organic materials are perceived as being

2 structurally more diverse and therefore are believed to have more long term promise than inorganics. 90 Jonie Varjula et al., [146] developed a new semi-organic nonlinear optical sodium paranitrophenolate paranitrophenol dihydrate single crystal using methanol as solvent by slow evaporation technique. The second harmonic generation (SHG) efficiency of the crystal measured by Kurtz's powder technique infers that the crystal has NLO coefficient 5 times greater than that of KDP crystal. Srinivasan et al., [147] have grown single crystals of dimethyl amino pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP) using acetone as solvent with ph 3.26 at constant temperature (30 C). The relative second harmonic efficiency of the compound was found to be 15 times greater than that of KDP. The laser induced surface damage threshold for the grown crystal was measured as 2.24 GW/cm 2 with Nd:YAG laser assembly. A good optical quality bulk single crystal of a semiorganic non-linear optical material, lithium p-nitrophenolate trihydrate was grown by Dinakaran et al [148]. The solubility of the material was measured in water before attempting the growth by cooling. The grown crystal was subjected to X-ray diffraction for phase identification and high resolution X-ray diffraction study for assessing its crystalline quality. A rocking curve with full width half maximum of 18 arcs was observed, which exhibits the good crystalline quality of the crystal. Jose et al., [149] have developed potassium p-nitrophenolate dihydrate single crystals by slightly adjusting the ph and growth temperature. Between 510 nm and 2000 nm, the material was observed to be nearly transparent allowing it to be explored for potential use in device fabrication. In addition, the photoluminescence spectrum of the grown crystal at room temperature shows a stable broad violet - blue emission around the nm wavelengths with the maximum centered at 436 nm. Urea is one among the organic crystals, which has been used in practical applications [43, ]. It has large NLO coefficients, a high degree of birefringence and relatively high laser damage threshold. Since, urea has transparency range extends up to 200 nm in the short wavelength limit, it is one of the most promising materials for nonlinear applications in UV region. The powder second harmonic generation (SHG) of urea is 2.5 times greater than that of ADP. The damage

3 91 threshold of urea is 3 GW cm -2 at 532 nm and 5 GW cm -2 at µm for 10 ns pulse. However, the growth of large high quality crystals of urea is difficult due to unfavorable growth properties. The molecular structure of urea is shown in Figure 4.1. Figure 4.1 Molecular Structure of Urea 4-Nitrophenol (also called p-nitrophenol or 4-hydroxynitrobenzene) is a phenolic compound that has a nitro group at the opposite position of hydroxy group on the benzene ring. Ionic crystals belonging to the paranitrophenol family are found to the most materials for nonlinear optical applications, in the view of their high second order nonlinear optical coefficient, wide optical transparency, large band gap and the stronger ionic bond between the nitrophenoxy ion and the organic ligand [153]. The molecular structure of 4-Nitrophenol is shown in Figure 4.2. On the basis of molecular engineering procedure, some enhanced properties can be expected of the derivatives and mixed systems of urea [43, 44]. 4-Nitrophenol urea is one such system and the structural analysis of 4-Nitrophenol urea was carried out by Zhao and Li [154] using dimethyl formamide as solvent. Figure 4.2 Molecular Structure of 4-Nitrophenol The structure of 4-Nitrophenol urea is shown in Figure Nitrophenol molecules are linked to urea molecules by O H O and N H O hydrogen bonds, forming a network structure [154].

4 92 Figure 4.3 Structure of 4-Nitrophenol Urea with atomic numbering showing displacement ellipsoids at 50% probability level In the present work, 4-Nitrophenol urea crystals were grown by slow evaporation technique. The grown crystals were subjected to various characterization methods such as XRD studies, FTIR studies, UV-Visible measurements, TGA, DSC, second harmonic generation studies and etching studies. 4.2 GROWTH OF 4-NITROPHENOL UREA SINGLE CRYSTALS Analytical reagents of urea, 4-Nitrophenol were procured from Loba Chemicals and were used as such. Generally, for the growth of good quality crystals, the choice of the solvent is very important. Urea is soluble in water, alcohol and other organic solvents over a wide spectrum of solubility with good temperature coefficients. Generally, if the solubility is large then there is a huge difficulty in growing high quality crystals. But for 4-Nitrophenol, the solubility is moderate in water compared to that of other solvents. Hence, triple distilled water was taken as the solvent for the growth of 4-Nitrophenol urea crystals. The adduct of 4-Nitrophenol urea was prepared by taking urea and 4-Nitrophenol in an equimolar ratio (1:1) as shown in Figure g of 4-Nitrophenol was first dissolved in 100 ml of water. The mixture was stirred continuously for 3 hours. In order to increase the solubility, the solution was heated to about 50 o C. 6 g of urea was then dissolved separately in 100 ml of triple distilled water. The two solutions were mixed together and stirred continuously for three hours. The solution was filtered to remove the solid impurities in the parent solution.

5 The crystal obtained is basically non - hygroscopic in nature. The photograph of the as grown crystals is shown in Figure Figure 4.4 Mechanism of formation of 4-Nitrophenol urea crystal Figure 4.5 Photograph of the as grown crystals of 4-Nitrophenol urea 4.3 RESULTS AND DISCUSSION Single crystal X-ray diffraction analysis Single crystal X-ray diffraction studies for the grown crystals were carried with MoK radiation ( = Å). The accurate cell parameters of the grown crystals at room temperature were obtained from the least-squares refinement of the setting angles of 25 reflections. The lattice parameters were calculated using triclinic crystallographic equation and compared with the literature values. The lattice parameters of the grown crystal from the present work are: a = Å, b = Å, c = Å, = o, = o, = o and V = Å 3. The lattice parameters are in good agreement with the literature [154].

6 Powder X-ray diffraction studies The grown crystals were also subjected to powder XRD studies. Fine powders of the crystal were packed tightly between glass plates. The Powder XRD pattern was recorded using powder SEIFERT X-ray diffractometer with CuK 1 radiation ( = Å). The powdered samples were scanned over the range at a rate of 1 o per minute. The XRD patterns were indexed with INDX and UNIT CELL software. The lattice parameters of the grown crystal from the powder XRD are : a = Å, b = Å, c = Å, = o, = o, = o and V = Å 3. The indexed powder XRD pattern is shown in the Figure (0 2 0) Intensity(cps) (1-2 1) (1 0 1) 2000 (0 1 1) (0 0 2) (0 2 1) (1 0 0) (1 0 2) (0 0 4) (0 4 0) (0 4 1) (0 1 5) (0 4 3) Two Theta Thetha(degrees) Figure 4.6 Indexed Powder XRD pattern of 4-Nitrophenol urea

7 95 Table 4.1 Indexed Powder XRD data of 4-Nitrophenol urea S.No h k l d(obs) (Å) d(calc) (Å) Diff (d) (Å) (obs) (deg) 2 (calc) (deg) Diff (2 ) (deg) FTIR spectral analyses The FTIR spectral analysis was carried out to identify the functional groups of the material. The FTIR spectrum was recorded using Bruker IFS-66V spectrophotometer in the region cm -1 using KBr pellet technique. The grown crystals were powdered and mixed with KBr. The mixture was then pelletized. The FTIR spectrum is presented in Figure 4.7. Aromatic nitro compounds have strong absorptions due to asymmetric and symmetric stretching vibrations of the NO 2 group at cm -1 and cm -1 respectively [155]. The two strong peaks at 1487 and 1337 cm -1 are attributed to NO 2 asymmetric and symmetric stretching respectively. The scissoring mode of NO 2 vibrations often give rise to only IR bands in the region cm -1 whereas the wagging mode shows a strong absorption in the region cm -1. These are observed in the spectrum with broad peaks at 863 cm -1 and 752 cm -1. The CH stretching vibrations of benzene derivatives generally appear above 3000 cm -1 [151]. The band at 3145 cm -1 in the

8 96 FTIR spectrum is attributed to the aromatic ring CH asymmetric stretching vibration. The corresponding symmetric stretching vibration appears at 3111 cm -1. The peak at 3390 cm -1 corresponds to NH 2 stretching. The peak at 3579 cm -1 corresponds to OH stretching due hydrogen bonding. The peak at 1584 cm -1 is due to CN stretching vibration. In 4-Nitrophenol, the OH vibrations generally occurs at 3325 cm -1 whereas in the spectrum of 4-Nitrophenol urea, this peak is missing indicating that the free OH is linked to C=O of urea. The other vibrations are similar to those of urea and 4-Nitrophenol [ , 139, 156]. In the FTIR spectrum, there are two strong peaks at 633 cm -1 and 696 cm -1 which are assigned to C-NO 2 stretching and C=C bending respectively. The detailed vibrational assignments are given in Table Transmittance (%) Wavenumber (cm -1 ) W avenumber [ cm-1] Figure 4.7 FTIR spectrum of 4-Nitrophenol urea crystal

9 97 Table 4.2 Vibrational assignments for 4-Nitrophenol urea crystal Wave number (cm -1 ) Vibrational Assignments 3579 (vw) OH stretching 3492 (vs ) OH stretching 3452 (vs ) NH 2 asymmetric stretching 3390 (vs ) NH 2 symmetric stretching 3240 (vs) CH stretching 3145 (s) CH asymmetric stretching 3111 (vs) CH asymmetric stretching 2948 (vs) CH asymmetric stretching 2738 (vs) CH symmetric stretching 2565 (vs) CH symmetric stretching 1916 (vw) C = C stretching 1682 (vs) C = O stretching 1584 (vs) CN stretching 1487 (vs) NO 2 asymmetric stretching 1395 (s) OH bending 1337 (vs) NO 2 symmetric stretching 1286 (vs) CH in plane bending 999 (vs) Ring stretching vibration 946 (s) CN in plane bending 886 (vs) OH out of plane bending 863 (vs) NO 2 scissoring 848 (vs) CH out of plane bending 752 (vs) NO 2 wagging 696 (s) C = C bending 633 (s) C - NO 2 stretching 535 (s) NO 2 wagging Optical studies The UV-Vis spectrum gives information about the structure of the molecule because the absorption of UV and visible light involves promotion of the electron in and orbital from the ground state to the higher energy states. An NLO

10 98 material can be widely used if it has a wide transparency range. The optical absorption spectrum was recorded in the range up to 800 nm using CARY 5E UV-VIS-NIR spectrophotometer and the spectrum is shown in Figure Absorbance(AU) Wavelength (nm) Figure 4.8 UV-Visible absorption spectrum of 4-Nitrophenol urea From the spectrum, the cut off wavelength is found to be 370 nm and the crystal possesses a good transparency in the region nm. Vanishri et al., [157] have determined the cut off wavelength for sodium p-nitrophenolate crystal as 480 nm. It is also found that higher percentage of transmission was observed, when water was used as solvent. Vijayan et al., [158] reported that the organic NLO material 8-hydroxyquinoline had a minimal absorption in the wavelength regime 300 to 1200 nm. The cut off wavelength of this crystal was found to be 300 nm. Absence of absorption in the region between 300 nm and 1200 nm is an advantage, as it is the key requirement for materials having NLO properties [51, 130]. This transparent nature in the visible region makes the 4-Nitrophenol urea crystal, a potential candidate for NLO applications.

11 Thermal analyses The thermal properties of the 4- Nitrophenol urea crystals were studied by using TGA / DSC studies. A sample mass of 6 mg was taken in the crucible for the thermal studies. Alumina was taken as the reference material. The TGA / DSC was carried out in nitrogen atmosphere at a heating rate of 20 C/min in the temperature between 50 C to 800 C. The TGA/DSC curve is shown in Figure 4.9. There are two stages of decomposition of the crystal. The first stage of decomposition is dominant, whereas the second stage of decomposition is less significant. The first stage of decomposition starts at a temperature of 170 C. The loss of mass in the first stage is about 95% which is much significant portion of the mass of the specimen. In the second stage of decomposition, which is less significant, only 4.5% of the sample is decomposed. The residual mass which is left in the crucible is only 0.8 % at a temperature of 796 C. Since the sample completely decomposes in the first stage, the volatile gases such as carbon, nitrogen and oxygen would have decomposed leaving behind a small residue. The DSC trace shows two sharp peaks. The first peak is an endotherm, which starts at 117 C and ends at 135 C with peak at 120 C. This peak corresponds to the melting point of the crystal. The area under the first peak is 205 J/g. An endotherm is centered at a temperature of 260 C and the area under the second peak is J/g. Generally, the organic materials have moderate thermal stability as compared to inorganic materials. It was found by Vijayan et al., [158] that, the 8-hydroxyquinoline crystal is stable up to a temperature of 113 C. Chen et al., [159] have reported that another NLO crystal, 2,6-diaminopyridinium 4-nitrophenolate 4-nitrophenol starts decomposition at a temperature of 155 C. Thus, 4-Nitrophenol urea crystal has sufficient thermal stability required for a NLO crystal.

12 100 Weight (%) Heat flow (mw/mg) Temperature ( o C) Figure 4.9 TGA/DSC curve of 4-Nitrophenol urea crystal Second harmonic generation studies The crystal was powdered to a crystallite size of µm. This powder was sandwiched between glass plates and exposed under 1064 nm laser beam from a Q switched Nd:YAG laser. Inorder to test the NLO property, the output from the laser was used as the source and was incident on the powdered sample. The output from the crystal was determined using a power meter in the energy range µj. The second harmonic generation (SHG) was seen as bright green flash emission from the sample. An input power of 1.39 V was given, which yielded corresponding output of 8.27 mv. For standard KDP, the input power of V was given and the corresponding output was mv. Thus, 4-Nitrophenol urea crystal is a material with very high SHG efficiency, 3.5 times greater than that of KDP and can be used for device applications Etching studies The etching studies were carried out on the grown crystals of 4-Nitrophenol urea using Carl Zeiss High resolution optical microscope. Ethanol was used as etchant. The photographs were taken with an etching time of 30 seconds and

13 seconds and the etch patterns are shown in the Figures 4.10 and 4.11 respectively. It is observed that initially when t = 30 s, less etch pits are formed as compared to t = 60 s. It is observed that the deformation of the crystal is maximum in the process of etching with increased time. The grain pattern is observed when t = 30 s. The rectangular pattern is observed when t = 60 s. There is a change in the shape and nature of the etch pits under different conditions. Figure 4.10 Etch Patterns for 4-Nitrophenol urea crystals with ethanol (t = 30 s) Figure 4.11 Etch Patterns for 4-Nitrophenol urea crystals with ethanol (t = 60 s)

14 CONCLUSION Single crystals of 4-Nitrophenol urea were grown by slow solvent evaporation technique. XRD studies confirmed the lattice parameters. UV-Visible studies and FTIR studies reveal the absorption range and functional groups for the given material. TGA and DSC studies reveal the thermal stability of the crystals. The SHG studies indicate that the 4-Nitrophenol urea crystals have NLO efficiency 3.5 times greater than that of standard KDP crystal. The etch patterns indicates the growth of the crystal. Thus, the good NLO properties, excellent optical quality makes 4-Nitrophenol urea crystals, a strong candidate for NLO applications.