Effect of Formic Acid as a Dopant in Tuning the Physical Properties of L-Arginine Acetate

P.   V.    Radhika; K.    Jayakumari; C.   K.    Mahadevan

doi:10.17577/IJERTV3IS050263

Volume 03, Issue 05 (May 2014)

Effect of Formic Acid as a Dopant in Tuning the Physical Properties of L-Arginine Acetate

DOI : 10.17577/IJERTV3IS050263

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Authors : P. V. Radhika, K. Jayakumari, C. K. Mahadevan
Paper ID : IJERTV3IS050263
Volume & Issue : Volume 03, Issue 05 (May 2014)
Published (First Online): 14-05-2014
ISSN (Online) : 2278-0181
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Effect of Formic Acid as a Dopant in Tuning the Physical Properties of L-Arginine Acetate

*P. V. Radhika,** K. Jayakumari, ***C. K. Mahadevan

*Department of Physics, Lekshmipuram College of Arts & Science, Neyyoor 629 802, Tamil Nadu, India.

**Department of Physics, Sree Ayyappa College for Women, Chunkankadai 629 003, Tamil Nadu, India.

***Physics Research Centre, S.T. Hindu College, Nagercoil 629 002, Tamil Nadu, India.

Abstract – L- arginine acetate is one of the most common amino acid derivative. Single crystals of L-arginine acetate (LAA), a nonlinear optical (NLO) material, have been grown by the free evaporation method and characterized chemically, structurally, thermally, optically, mechanically and electrically. Effect of formic acid as an impurity (added at different concentrations) on the properties of LAA has been investigated. X-ray diffraction analysis indicates the crystal system as monoclinic. The functional groups have been identified using Fourier transform infrared spectral analysis. The crystals are found to be thermally stable up to 191 C. The UV-Vis spectral analysis shows that these crystals have wide transparency range in the entire visible region. Second harmonic generation measurement shows the NLO property. Microhardness measurement indicates that the grown crystals come under soft material category. Dielectric measurements were carried out at various temperatures ranging from 40- 140 C and with various frequencies ranging from 1 kHz to 100 kHz by the parallel plate capacitor method. The AC activation energies were also estimated. The low dielectric constant values observed for pure and doped LAA indicate that the crystals grown are not only promising NLO materials but also low dielectric constant value dielectric materials.

Key words: L-arginine acetate, slow evaporation technique, doping effects.

INTRODUCTION

crystals were reported to have promising NLO properties comparable to that of the well-known inorganic crystal KDP. L-arginine acetate is a nonlinear optical material having a powder second harmonic generation (SHG) efficiency nearly three times that of KDP [8].

A considerable interest has been shown recently in studying the effect of impurities (both organic and inorganic) on the growth and properties of some hydrogen bonded crystals. Presence of impurity molecules, even at lower concentrations in the parent solute, may have considerable effect on the growth kinetics and other properties of LAA crystals as already reported by several research workers on their systems [8 – 13].

In the present work, we have made an attempt to grow hybrid material single crystals by doping LAA with formic acid taken at different concentrations and to investigate the effect of doping on the various properties of LAA crystals. Results obtained are reported herein.

EXPERIMENTAL
Densities of the grown crystals were measured by the floatation method with a mixture of carbon tetra chloride (density-1.592 g/cc) and hexane (0.652 g/cc) taken as high and low density liquids respectively.

CHNS analysis was carried out using an Elementar Vario EL III to estimate the carbon, hydrogen and nitrogen atom contents in the grown crystals.

The grown crystals were subjected to single crystal X- ray diffraction (SXRD) study using an ENRAF NONIUS CAD4 single crystal XRay diffractometer. Powder X-Ray diffraction (PXRD) analysis was also carried out to understand the crystallinity of the grown crystals using an XPERT-PRO diffractometer with monochromated CuK radiation ( =1.54056 AU). The reflections were indexed following the procedure of Lipson and Steeple [14].

FTIR spectra were recorded in the mid-infrared region (wavenumber range 400-4000 cm-1) using a MAGNA 550 model spectrometer. In order to understand the optically transparent nature of the grown crystals, UV- Vis absorption spectra were recorded in the wavelength range 200-800 nm using a Vario Cary 500 scan spectrophotometer.

SHG measurements were made on the grown crystals using a 1064 nm Quanta ray laser and a Molectron powermeter.

Thermal study(TGA, DTA and DSC analyses) was carried out for LAA and LAAF4 using a SDT Q600 V8.3 Build 101 analyser in a temperature range of room temperature to 800 C in air atmosphere to understand the thermal stability of the grown crystals.

Mehanical strength of the grown crystals were studied by measuring the microhardness on the large area face with the help of a Leitz microhardness tester.

The capacitance (Ccrys) and dielectric loss factor (tan) measurements were carried out to an accuracy of Â±1

% on all the five grown crystals having the large area faces touching the electrodes by the parallel plate capacitor method [2] using an LCR meter (Agilant 4284A) at various temperatures ranging from 40-140 C with three different frequencies, viz.1 kHz, 10 kHz, and 100 kHz. The temperature was controlled to an accuracy of Â±1 C. The observations were made while cooling the sample. The dimensions of the crystals were measured using a traveling microscope. Air capacitance (Cair) was also measured. Since the variation of air capacitance with temperature was found to be negligible, air capacitance was measured only at the lower temperature considered. The crystals were shaped and polished and the opposite faces were coated with graphite to form a good conductive surface layer (ohmic contact). The sample was mounted between th silver electrodes and annealed at 140 C for about 30 min to homogenize the sample before taking the readings.

= r = (Aair/Acrys) ((Ccrys Cair) (1- Acrys/Aair))/Cair)

where Ccrys is the capacitance with crystal (including air), Cair is the capacitance of air, Acrys is the area of the crystal touching the electrode and Aair is the area of the electrode.

The AC electrical conductivity, AC, was calculated using the relation

AC = 0r tan

where 0 is the permittivity of free space (8.85×10-12 C2N- 1m-2), is the angular frequency ( =2f; f=1, 10 and 100 kHz in the present study) and tan is the dielectric loss factor.
RESULTS AND DISCUSSION

Densities, lattice parameters and chemical composition

The crystals grown in the present study, both pure and doped LAA, are found to be colourless and transparent

.The photographs of the sample crystals grown are shown in Figure 1. It is found that, as dopant concentration increases the transparency of the crystal increases.

(a)

(b)

Figure 1: Photographs of the sample crystals grown – (a) LAA and (b) From right LAAF1, LAAF2, LAAF3 and LAAF4

The observed weight percentages of carbon, hydrogen and nitrogen present in both pure and doped LAA are provided in Table.1. Formic acid also contains C and H atoms. Variations in C, N and H concentrations observed are found to be insignificant. However, the results observed indicate that the basic material of the grown crystals is L-arginine acetate. This analysis is not sensitive enough to estimate the impurity concentration as the impurity concentration considered in the present study is very small.

Table 1: Chemical analysis of pure and formic acid doped LAA

Crystal	Composition (%)
Crystal	C	H	N
LAA	40.98	12.82	24.52
LAAF1	40.85	12.89	24.30
LAAF2	40.86	12.52	24.40
LAAF3	40.82	12.32	24.16
LAAF4	40.84	12.48	24.40

Results obtained from density and SXRD measurements are provided in Table 2. The measured density is found to increase with the increase in impurity concentration indicating that the impurity molecules have entered into the LAA crystal matrix. The observed variation in lattice volume also endorses this. Lattice parameters observed in the present study for the pure LAA crystal compare well with those reported in the literature [12].

The PXRD patterns observed in the present study are shown in Figure 2. The sharp peaks observed indicate the higher crystalline nature of the grown crystals. The observed patterns confirm the material of the grown crystals as basically LAA [12].

Figure 2: The observed PXRD patterns

Table 2: Single crystal XRD data for pure and formic acid doped LAA crystals

Crystal	Lattice parameters							Crystal system	Space group	Density (g/cc)
Crystal	a ()	b ()	c ()	()	()	()	Volume (3)
LAA	9.189 (9.174)	5.166 (5.172)	13.050 (13.478)	90	109.6 (110.1)	90	584.0 (600.4)	Monoclinic	P21	1.323
LAAF1	9.220	5.178	13.110	90	109.7	90	590.0		P21	1.366
LAAF2	9.226	5.168	13.077	90	109.6	90	587.0	Monoclinic	P21	1.367
LAAF3	9.207	5.173	13.080	90	109.0	90	587.2	Monoclinic	P21	1.449
LAAF4	9.205	5.173	13.079	90	109.5	90	587.3	Monoclinic	P21	1.469

Reported values are given in parenthesis [12]

The FTIR spectra recorded in the present study for pure and doped LAA crystals are shown in Figure 3. The spectra are found to be complex as a result of various functional groups present in the pure and doped LAA crystals. The vibrational band assignments are given in Table 3.

Figure 3: The observed FTIR spectra

Both the pure and doped LAA crystals show a broad envelope between 2100-3500 cm-1. It is due to overlapping of peaks of NH (NH3+), OH (COOH) and CH (CH2 and CH3) vibrations. The absorptions at around 3750- 2300 cm-1 show the presence of NH and CH stretching vibrations. The peak observed at around 2025 cm-1 is for symmetrical NH3+ bending. The spectrum of LAA shows a strong stretching absorption near 1550 cm-1 which is the characteristic of acyclic NH stretching and OH stretching. All other peaks below 1440 cm-1 are due to COO- and other bending modes. The peak, due to torsional NH oscillations of NH3+ is observed at 543 cm-1[13].

FTIR spectra of doped crystals are almost similar to that of pure LAA crystal. However, a significant change in absorption level is seen due to stretching modes, in between the band ranging from 1600 to 1200 cm-1, when the doping concentration is increased. From these variations, we can understand the dopant inclusion into the LAA crystal matrix.

Table 3: The vibrational band assignments

LAA (Reported [8])( cm-1)	LAA (present work ) (cm-1)	LAAF1 (cm-1)	LAAF2 (cm-1)	LAAF3 (cm-1)	LAAF4 (cm-1)	Assignment
3750-2300	3900-2300	3900-2300	3900-2300	3900-2300	3900-2300	3750-2300 NH and CH stretching vibrations
2025	2025.35	2025.75	2025.74	2025.93	2025.65	+ bending AsymmetricNH3
1532	1557.39	1560.11	1555.43	1556.04	1557.93	Asymmetric stretching modes of COO-
			1395.79	1414.63	1399.87	CH3 symmetric deformation
		1322.85	1322.85	1323.04	1356.10	Stretching vibration of CO
					1340.38	CH3 wagging
					1323.18	CH2 twisting
1228	1229.10	1228.94	1228.82	1229.06	1229.15	C-C-COO vibrations
1197	1197.02	1196.87	1196.22	1197.05	1197.13	-COO vibrations

1089.62

1093	1089.74	1089.56	1089.75	1089.75	C-CN stretching vibrations
928	928.93	928.94	928.88	928.98	928.89	C-CH bending
		914.91		915.09		Overtone of torsional oscillation of NH + 3
670	675.87	671.32	653.46	671.39	653.16	NH out of plane bending

Thermal, optical and mechanical properties

The TGA, DTA and DSC patterns observed for LAA and LAAF4 are shown in Figure 4. It is observed that the decomposition of LAA starts at 204 C. This is supported by the results of DTA and DSC analyses where the endothermic reaction is observed at 202 C. It can be seen from the thermal analyses that LAA subjected to continuous heating does not remain stable for a long temperature change. However, it is decomposed at a temperature 582 C and forms a residue. Above 632 C, the product is stable without any prominent weight loss. LAAF4 starts decomposing at 191 C. Other decompositions observed for LAAF4 are at temperatures 286, 457 and 587 C, and above 741 C there is no prominent weight loss observed.

Since there is no endothermic or exothermic transition below 200 C, the materials are proved to be stable in this region. The resistivity of the materials against thermal crack is evident as the DSC trace is smooth up to nearly 200 C. It is the property observed for compounds where the lattice force is more predominating than the covalent bonding forces in molecules. It is also observed that the electrostatic force that originated as a result of perfect proton transfer between acetic acid and L-arginine becomes dominating to provide resistance to melting before decomposition [16].

(a)

(b)

(c)

(d)

(e)

Figure 4: The observed TG, DTA and DSC

patterns [(a), (b) and (c) are respectively the TG, DTA and DSC patterns of LAA crystals; and (d) and (e) are respectively the TG/DTA and DSC patterns of LAAF4 crystal]

The range and percentage of optical transmission of pure and doped LAA crystals were studied by using the UV Vis spectra of the grown crystals recorded in the wavelength range 200 800 nm. The spectra of pure and formic acid doped LAA observed in the present study are shown in Figure 5.

1.2

1.1

1.0

wavelength is around 236 nm, and the transparency conveniently extends to 800 nm.

SHG efficiency of the grown crystals was confirmed by adopting the Kurtz and Perry method. Urea crystal was used as the reference material in the SHG measurement. The SHG efficiencies observed for the grown crystals are found to be comparable to that of urea.

Hardness is defined as the resistance offered by a material to external mechanical action endeavouring to scratch, indent, or any other way affecting its structure. In the present study, microhardness measurements were carried out on all the five grown crystals using a Vickers Hardness Indenter. Vickers Hardness Number (HV) was calculated using the relation

HV = 1.8554P/d2 kgmm-2

0.9

Abs (arb. unit)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

for pure LAA LAAF1 LAAF2 LAAF3 LAAF4

200 300 400 500 600 700 800

Wavelength (nm)

Figure 5: The observed UV-Vis absorption spectra

where P is the load applied and d is the diagonal length of the indented impression.

pure LAA LAAF1

LAAF2

LAAF3 LAAF4

110

100

Figure 6 shows the plots between load P and hardness number HV observed in the present study. Results obtained indicate that the hardness number increases with the increasing load for all the five crystals grown and characterized in the present study. Also, it is understood that doping decreases the Hardness but nonlinearly with the dopant concentration. The nonlinear variation due to dopant concentration may be understood as due to the randomness in formate of dopant replacing the acetate in LAA or in occupying the interstitials in LAA crystal matrix.

90

80

70

60

50

40

30

20

H (Kg/mm2 )

v

Crystal sample	Absorption edge(nm)	SHG efficiency(in urea unit)	Work hardening coefficient(n)
LAA	236.4	1.05	3.361
LAAF1	232.9	1.19	3.212
LAAF2	229.4	0.89	3.276
LAAF3	233.8	0.81	3.190
LAAF4	237.2	1.06	3.168

Table 4: The optical absorption edges, SHG efficiencies and work hardening coefficients observed in the present study

20 30 40 50 60 70 80 90 100 110

Load P (g)

The optical absorption edges, SHG efficiencies and work hardening coefficients observed are provided in Table 4. It is observed that the lower cut off

Figure 6: Variation of Vickers hardness number with load for pure and doped LAA crystals

From Figure 6, it can be understood that the hardness number of the crystal increases with increasing load for both pure and doped samples. The work hardening coefficients n for the pure and doped LAA crystals were estimated from the plots (not shown here) between logP and logd. If n>2, the microhardness number Hv increases with increasing load and if n<2, Hv decreases with increasing load [17]. Also, if n>1.6 the crystal is considered to be of soft materials category. Results obtained in the present study indicate that the crystals grown in the present study are of soft materials category. Also, the crystals exhibit the normal size indentation effect.

Electrical properties

4.4

4.2

4.0

3.8

3.6

3.4

3.2

100kHz

4.6

LAAF2 LAAF3 LAAF4

LAAF1

pure LAA

dielectric constant ( )

r

The study of dielectric constant of a material gives an outline about the nature of atoms, ions and their bonding in the material. The dielectric parameters, viz. dielectric constants, dielectric loss factors and AC electrical conductivities observed in the present study for all the five crystals grown are shown respectively in Figures 7, 8 and

9. The AC activation energies (EAC) were estimated by fitting the AC electrical conductivity data to the Arrhenius relation

AC = AC0exp(-EAC/(kT))

where AC0 is the material dependent constant, k is the Boltzmann constant and T is the absolute temperature. The estimated AC activation energies are given in Table 5.

40

60

80

100

120

140

temperature (oC)

(b)

(c)

0.05

d e m o

1kHz

d e m o

0.06

4.5

4.4

4.3

4.2

4.1

4.0

3.9

3.8

3.7

4.7

4.6

for pure LAA

LAAF1

LAAF2 LAAF3 LAAF4

dielectric constant ( )

r

Figure 7: The observed dielectric constants

1kHz

d e m o

140

120

100

80

60

40

0.01

0.02

d e m o

<>d e m o

0.03

d e m o

0.04

d e m o

d e m o for pduermeo LAA LAAF1 LAAF2

d e m o d e m o

LAAF3

LAAF4

temperature (0C)

140

120

100

80

60

40

dielectric loss factor (tan )

(a)

temperature (0C)

0.040

d e m o

0.045

d e m o

0.050

d e m o

10kd eHmzo

d e m o

0.055

0.060

d eLmAoAF2

LAAF3 LAAF3

pure LAA LAAF1

d e m o

4.0

4.5

10kHz

for pure LAA LAAF1 LAAF2 LAAF3 LAAF4

dielectric constant ( )

r

(a)

0.015

d e m o

0.010

40

60

80

100

120

140

temperature (0C)

0.020

d e m o

0.025

d e m o

0.030

d e m o

0.035

temperature (0C)

140

120

100

80

60

40

dielectric loss factors (tan)

(b)

40

60

80

100

120

140

temperature (0C)

140

120

100

80

60

40

10

d e m o

20

30

d e m o

40

50

d e m o

60

d e m o

70

80

d e m o

100

90

d e m o

110

10kHz

d e m o

120

130

LAAF3

LAAF4

d e m o for pud reem oLAA

LAAF1

LAAF3

d e m o d e m o

d e m o

d e m o LdAe mAoF3

LAAF4

d e m o

d e m o pdue rmeo LAA d e m o 100 kdHe zm o

LAAF1 LAAF2

d e m o

0.042

0.040

0.038

0.036

0.034

0.032

0.030

0.028

0.026

0.024

0.022

0.020

0.018

0.016

0.014

0.012

0.010

0.008

0.006

0.004

dielectric loss factors (tan )

AC conductivity ( x10-9mho/m)

AC

(b)

(c)

d e m o

d e m o d e m o

LAAF1 LAAF2

d e m o

1kHz

d e m o

for pure LAA

d e m o d e m o

12

d e m o

d e m o d e m o

14

d e m o

400

d e m o

500

600

d e m o

700

d e m o

100kHz

d e m o

800

d e m o

900

LAAF3

LAAF4

LAAdFe 2m o

d e m o

d e m o for pd ue mreo LAA

LAAF1

AC conductivity ( X10-9mho/m)

AC

Figure 8: The observed dielectric loss factors

d e m o

temperature (0C)

140

120

100

80

60

40

2

4

d e m o

6

d e m o

8

d e m o

LAAF4

d e m o

10

LAAF3

temperature (0C)

140

120

100

80

60

40

100

d e m o

200

d e m o

300

AC conductivity ( x10-9mho/m)

AC

(c)

Figure 9: The observed AC electrical conductivities

(a)

All the three dielectric parameters considered are found to increase with the increase in temperature. Moreover, the r and tan values are found to decrease whereas the AC values are found to increase with the increase in frequency. This is a normal dielectric behaviour [2,12]. The dielectric loss factors observed are found to be low which indicates that the grown crystals are of high quality. The activation energy values observed are found to be less which indicates the presence of oxygen vacancies in the temperature range considered.

Table 5: The estimated AC activation energies

Crystal sample

AC activation energy, EAC(eV) for frequencies

1 kHz

10 kHz

100 kHz

LAA LAAF1 LAAF2 LAAF3 LAAF4

0.057

0.134

0.124

0.117

0.129

0.153

0.109

0.123

0.115

0.127

0.157

0.188

0.192

0.149

0.148

The low r values observed for the pure LAA (at low temperature) and doped LAA crystals (at all temperatures considered) indicate that the crystals grown in the present study are not only potential NLO materials but also promising low r value dielectric materials, expected to be useful in both the photonics and microelectronics industries [2]. Moreover, the doped LAA crystals can be considered to be more useful as they exhibit significantly less temperature dependent variation of dielectric constant. The present study, thus, indicates that doping with formic acid has tuned significantly the dielectric properties of LAA crystal.

Doping leads to decrease of dielectric constant significantly. However, the variation is found to be nonlinear with the dopant concentration. The other dielectric parameters also vary nonlinearly with the dopant concentration. This can be explained as due to the randomness in formate of dopant replacing the acetate in LAA and occupying the interstitials in the LAA crystal matrix.

CONCLUSIONS

Pure and formic acid doped LAA single crystals have been grown successfully by the slow evaporation method. SXRD measurements confirm that all the five (pure and four doped) grown crystals belong to the monoclinic crystal system. ThePXRD patterns observed confirm the crystallinity. PXRD, FTIR spectral and CHNS elemental analyses indicate that LAA is the basic material of all the five crystals grown. The observed variations of density and lattice volume with doping concentration used for the growth of crystals indicate that the dopant molecules have entered into the LAA crystal matrix. Thermal analyses indicate that the grown crystals are thermally stable at least up to 191oC. Results of optical, mechanical and electrical measurements indicate that doping with formic acid tunes these physical properties of LAA crystal significantly. Nonlinear variation of physical parameters observed with dopant concentration could be understood as due to the randomness in formate of dopant replacing the acetate in LAA and occupying the interstitials in the LAA crystal matrix. The present study indicates that the formic acid doped LAA crystals are not only potential NLO materials but also low r value dielectric materials expected to be highly useful in photonics and microelectronics industries.

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Effect of Formic Acid as a Dopant in Tuning the Physical Properties of L-Arginine Acetate

Key words: L-arginine acetate, slow evaporation technique, doping effects.

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