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
90
structurally more diverse and therefore are believed to have more long term promise
than inorganics.
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/cm2 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 383 - 550 nm
wavelengths with the maximum centered at 436 nm.
Urea is one among the organic crystals, which has been used in practical
applications [43, 150 - 152]. 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
91
threshold of urea is 3 GW cm-2 at 532 nm and 5 GW cm-2 at 1.064 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 4.3. 4-Nitrophenol
molecules are linked to urea molecules by O–H …O and N–H … O hydrogen bonds,
forming a network structure [154].
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 4.4. 13 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 oC. 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.
93
The crystal obtained is basically non - hygroscopic in nature. The photograph of the
as grown crystals is shown in Figure 4.5.
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
4.3.1
Single crystal X-ray diffraction analysis
Single crystal X-ray diffraction studies for the grown crystals were carried
with MoK radiation ( = 0.71073 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 = 3.765 Å, b = 10.248 Å, c = 11.823 Å,
= 98.61o,
= 92.44o,
= 99.39o and
V = 443.9 Å3. The lattice parameters are in good agreement with the literature [154].
94
4.3.2
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 ( = 1.5406 Å). The powdered samples were scanned over the range
10° - 70° at a rate of 1o 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 = 3.765 Å, b = 10.247 Å, c = 11.822 Å,
= 98.61o,
= 92.42o,
= 99.30o and V = 444.10 Å3. The indexed powder XRD pattern is shown in the
Figure 4.6.
(0 2 0)
14000
10000
(0 4 3)
(0 2 1)
(0 1 1)
2000
(0 0 2)
4000
(0 1 5)
6000
(1 0 2)
(0 0 4)
(0 4 0)
(0 4 1)
(1 0 1)
(1 -2 1)
8000
(1 0 0)
Intensity(cps)
Intensity (cps)
12000
0
10
20
30
40
50
60
70
Two
(degrees)
Two Theta
Thetha(degrees)
Figure 4.6 Indexed Powder XRD pattern of 4-Nitrophenol urea
95
Table 4.1 Indexed Powder XRD data of 4-Nitrophenol urea
d(obs)
(Å)
S.No h k l
1 0 1 1 7.05268
2
(calc)
(deg)
Diff (2 )
(deg)
d(calc)
(Å)
Diff (d)
(Å)
(obs)
(deg)
7.0513
0.00138
12.54
12.542
-0.002
2
0 0 2 5.83154
5.83083
0.00072
15.18
15.182
-0.002
3
0 2 0 4.99257
4.99218
0.00039
17.75
17.751
-0.001
4
0 2 1
4.3475
4.34643
0.00108
20.41
20.415
-0.005
5
1 0 0 3.70774
3.70774
0
23.98
23.98
0
6
1 0 1 3.46602
3.46602
0
25.68
25.68
0
7
3.16946
0
28.13
28.13
0
8
1 2 1 3.16946
1 0 2 3.14686
3.14686
0
29.39
29.39
0
8
0 0 4
2.9153
2.91541
-0.00012
30.64
30.639
0.001
9
0 4 0 2.49593
2.49609
-0.00016
35.95
35.948
0.002
10
0 4 1 2.36526
2.36514
0.00012
38.01
38.012
-0.002
11
0 1 5 2.19531
2.19528
0.00003
41.08
41.081
-0.001
12
0 4 3 1.96324
1.96338
-0.00014
46.2
46.196
0.004
4.3.3
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 450 - 4000 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
NO2 group at 1570 - 1485 cm-1 and 1370 - 1320 cm-1 respectively [155]. The two
strong peaks at 1487 and 1337 cm-1 are attributed to NO2 asymmetric and symmetric
stretching respectively. The scissoring mode of NO2 vibrations often give rise to only
IR bands in the region 800 - 890 cm-1 whereas the wagging mode shows a strong
absorption in the region 700 - 760 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
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 NH2 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 [132 - 134, 139, 156]. In the FTIR spectrum, there are two strong peaks
at 633 cm-1 and 696 cm-1 which are assigned to C-NO2 stretching and C=C bending
respectively. The detailed vibrational assignments are given in Table 4.2.
100
Transmittance
(%)
Transmittance (%)
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
-1
W a v e n u m b e r(cm
[ c m )- 1 ]
Wavenumber
Figure 4.7 FTIR spectrum of 4-Nitrophenol urea crystal
500
97
Table 4.2 Vibrational assignments for 4-Nitrophenol urea crystal
Wave number (cm-1)
4.3.4
Vibrational Assignments
3579 (vw)
OH stretching
3492 (vs )
OH stretching
3452 (vs )
NH2 asymmetric stretching
3390 (vs )
NH2 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)
NO2 asymmetric stretching
1395 (s)
OH bending
1337 (vs)
NO2 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)
NO2 scissoring
848 (vs)
CH out of plane bending
752 (vs)
NO2 wagging
696 (s)
C = C bending
633 (s)
C - NO2 stretching
535 (s)
NO2 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
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 4.8.
6
Absorbance (AU)
Absorbance(AU)
5
4
3
2
1
0
200
300
400
500
600
700
800
Wavelength
Wavelength(nm)
(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 370-800 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.
99
4.3.5
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 -58.3 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
113
crystal
is
stable
up
to
a
temperature
of
°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.
Weight (%)
Heat flow (mW/mg)
100
Temperature (oC)
Figure 4.9 TGA/DSC curve of 4-Nitrophenol urea crystal
4.3.6
Second harmonic generation studies
The crystal was powdered to a crystallite size of 10 - 20 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 300 - 600 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 0.381 V was given and the
corresponding output was 0.657 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.
4.3.7
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
101
60 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)
102
4.4
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.