Spectral and Thermal Studies of Gel Grown Benzophenone Crystal

Spectral and Thermal Studies of Gel Grown Benzophenone Crystal

Divya R, Lekshmi. P. Nair, B.R.Bijini, K.Rajendra Babu Department of Physics

MG College Thiruvananthapuram, India-695004

K. Rajendra Babu

Department of Science

Heera College of Engineering & Technology Thiruvananthapuram, India-695568

AbstractBenzophenone has significant relevance in non linear optical applications. Benzophenone crystals had already been grown by several methods. In this work, single crystals of benzophenone were successfully grown by gel method. The single crystal X ray diffraction technique was carried out to obtain the crystallographic data from which valence electron plasma energy and Fermi energy was also calculated. The grown crystal was subjected to FTIR spectral analysis to confirm the presence of functional groups. The optical band gap of the crystal was determined from the UV Visible NIR absorbance spectrum and found to be 3.18 eV. The electronic polarisability of the grown crystal was calculated from the band gap. The nature of the spectrum shows its potential as an NLO material. Thermal properties and thermal stability were studied by conducting Thermo-gravimetric and Differential Thermal Analysis. Here the melting point was observed as 49.5 degree Celsius which is well matching with the theoretical value. It is also thermally stable up to 150 degree Celsius.

KeywordsBenzophenone, gel growth, Single crystal Xray diffraction, FTIR, UV-Vis-NIR spectral analysis, Thermal analysis

1. INTRODUCTION

   Crystals play an inevitable role in modern technology. So crystal growth is a prominent area in the scientific and technological research. Since the crystal growth has immense applications, it is an interdisciplinary subject covering physics, chemistry, materials science, chemical engineering, metallurgy, crystallography, mineralogy etc. There is growing interest on crystal growth to meet the demand of materials for technological applications [1].

   The growth of NLO materials has become trend in recent years. They are having technological importance in the field of optoelectronics, lasers, data storage systems and optical communication [2]. These materials should possess large second order optical non linearities, short transparency, cut off wavelength and thermal stability [3]. NLO response is larger in organic materials when compared to inorganic materials due to the presence of active bonds [4]. So we focus our studies on organic materials.

   Benzophenone is one of the most important organic materials showing NLO property [5]. It is an important compound in organic photochemistry and perfumery as well as in organic

   synthesis. It is also used as a photo-initiator of UV curing applications in inks, adhesive and coatings, optical fiber as well as in printed circuit boards[5]. Rapid crystal growth of benzophenone by low temperature solution growth and unidirectional seeded single crystal growth from solution of benzophenone [5, 6] has already been reported.

   Here we have grown benzophenone crystals by gel method to improve the quality of the crystals. The gel growth is simple in technique, effective in growing single crystals of compounds that cannot be easily grown by other methods [7]. In this paper, we are presenting the XRD characterization and spectral and thermal studies of gel grown benzophenone crystals.

2. EXPERIMENTAL – GROWTH OF SINGLE

   CRYSTALS

   Single crystals of benzophenone were formed by gel diffusion technique. The technique involves the setting of gel and addition of required top solution over the set gel. The crystallization apparatus for the growth consists of borosilicate glass test tube of length 20 cm and diameter 2.5 cm placed vertically on a stand. The solution for gel having specific gravity 1.03-1.05 was prepared by dissolving sodium meta silicate (SMS) in double distilled water. The solution was then acidified with 1M glacial acetic acid to get the pH in the range 4 – 7(in steps of 0.5) and taken about 30 ml of each in different test tubes. They were kept undisturbed for gel setting. Over the set gel, the top solution prepared by dissolving AR grade benzophenone in ethanol was added drop wise through the side of the test tube to prevent the gel breakage. The test tubes were covered with transparent plastic sheets to avoid evaporation and contamination of solution. The crystals were found growing over the gel surface within 1 day and the growth lasted for about 50 days. It is also found that pH = 6 and gel density 1.04 g/cc was the optimum condition for the growth of best quality benzophenone crystals.

3. RESULTS AND DISCUSSIONS

   A. Single crystal X-ray diffraction

   The crystallographic data of gel grown benzophenone crystal were obtained by using Bruker Kappa Apex II X ray diffractometer and is shown in table I.

   TABLE I

   	a	=	7.968A0
   	b	=	10.304 A0
   Unit cell parameters	c	= =	12.098 A0
   		=	900
   		=
   Volume	993.2 (A0)3
   Lattice type	Orthorhombic P

   Absorbance

   Absorbance

   The valence electron plasma energy, p is given by

   In the higher wavelength region, the peak at 3054.80 cm-1 is associated with aromatic C-H stretching. The peak at 1650 cm-1 represents C=O stretching. The skeletal vibrations are represented by the peaks 1591.44 cm-1 and 1443.87cm-1. The region between 1274.65 and 1024.87 cm-1 represents the in plane bending modes of C-H while the peaks below 1000 cm-1 represents out of plane bending modes.

   C. UV-Vis NIR spectral analysis

   The UV-visible absorption spectrum of benzophenone crystal shown in Fig. 2 was recorded in the range 214 nm to 1200 nm using the instrument Varian Cary 5000 UV Vis NIR spectrometer.

   1.4

   1.2

   1.0

   1

   3285.91

   3054.80

   3285.91

   3054.80

   28.8 Z 2

   (1)

   0.8

   p M

   0.6

   where Z is the total number of valence electrons, is the density of the crystal, M is the molecular mass of the crystal[8].

   1982.20

   1982.20

   1650.04

   1650.04

   The Fermi Energy EF can be calculated using the formula [8]

   1591.44

   1443.87

   1591.44

   1443.87

   3

   3

   4

   0.4

   0.2

   0.0

   1274.65

   1274.65

   EF 0.2948(

   p )

   (2)

   200 400 600 800 1000

   Wavelength(nm)

   1024.87

   937.77

   1024.87

   937.77

   699.06

   629.84

   699.06

   629.84

   For the grown crystal, we have Z = 68, = 1.2185 g/cc and M = 182.2179 g/mol. The calculated values of valence electron plasma energy and Fermi energy of benzophenone crystal are 19.42 eV and 15.39 eV respectively.

   B. Fourier Transform Infrared Spectroscopic Studies (FTIR)

   The FTIR spectrum of gel grown benzophenone crystal is shown in the Fig. 1.

   100

   %Transmittance

   %Transmittance

   80

   60

   40

   20

   0

   4000 3500 3000 2500 2000 1500 1000 500

   Wave numbers(cm-1)

   Fig. 1. FTIR spectrum of Benzophenone crystal

   Fig. 2. UV visible spectrum of Benzophenone crystal

   The transmission range and cut off wavelength of the crystals are very crucil in many important practical applications. Here a strong absorption is taking place at 377 nm, which is the lower cut off wavelength. There is no significant absorption in the visible range. This transparent region makes the benzophenone crystal suitable for optoelectronic applications [9, 10].

   The optical band gap Eg can be calculated from the absorption spectrum using the equation

   (h)n = A(Eg – h) (3)

   where A is a constant, Eg is the optical band gap, h Plancks constant and frequency of the incident photons, absorption coefficient[11].

   Fig. 3 shows a graph drawn between (h)3 versus h . Eg is calculated as 3.18 eV by extrapolating the linear part of the graph.

   300

   250

   200

   (h)3

   (h)3

   150

   100

   50

   0

   1 2 3 4 5 6

   Energy h(eV)

   Fig. 3. Plot of (h)3 versus photon energy

4. CONCLUSION

Good quality benzophenone crystals were grown by gel diffusion technique with optimum conditions of gel density

1.04 g/cc and pH value 6. The valence electron plasma energy and Fermi energy was calculated as 19.42 eV and

1. eV respectively from the crystallographic data obtained by single crystal XRD technique. FTIR spectrum has revealed the vibrational modes which identifies the grown crystal. The optical band gap of the crystal was determined as 3.18 eV from the UV visible spectrum. The electronic polarisability of the crystal was also calculated from the band gap .The crystal is having a transparent region which makes it suitable for NLO applications. The crystal is thermally stable up to 1500C and the melting point was determined as 49.550C.

   ACKNOWLEDGMENT

   The authors thank STIC, Cochin University of science and technology for analytical facilities.

   The electronic polarisability of the crystal can be calculated

   from the band gap using the equation [12]

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   1

   Eg M

   0.396 1024 cm3

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      4.06

      (4)

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      and its value is found to be 3.321023 cm3 .

      D. Thermal Analysis

      Thermo-gravimetric analysis (TGA) and Differential thermal analysis (DTA) were carried out for thermal studies. TGA and DTA curves are shown in Fig. 4.

      TGA

      DTA

      TGA

      DTA

      100 15

      90 20

      Heat flow endo down(mW)

      Heat flow endo down(mW)

      80 25

      70 30

      Weight (%)

      Weight (%)

      60 35

      50 40

      40 45

      30 50

      20 55

      10 60

      0 65

      50 100 150 200

      Temperature (0C)

      Fig. 4. TGA/DTA curves of Benzophenone crystal

      In the DTA graph, the endothermic peak at 49.550C corresponds to the melting point of the crystal. Here the melting point is found to be more than the melting point of benzophenone crystals grown by other method (41.360C) [6].The sharpness of the endothermic curve shows good degree of crystallinity. The TGA graph shows that the crystal is thermally stable up to 1500C.

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