Introduction

The non-linear optical (NLO) properties of large organic molecules and polymers have been the subject of extensive theoretical and experimental investigations during the past two decades, and they have been investigated widely due to their high-NLO properties, rapid response in electro-optic effect and large second- or third-order hyperpolarizibilities compared to inorganic NLO materials [1]. Anthracene is one of the organic molecular crystals, which exhibits peculiar optical and electronic properties. It forms colourless monoclinic prismatic crystals with melting point 216 °C. The gel-aided solution method [2] was used to grow bigger and better quality crystals of anthracene at ambient temperatures.

The scintillation property of anthracene grown from Double Run Selective Self-seeding Vertical Bridgman technique (DRSSVBT) was studied [3]. Anthracene crystals were also grown using sublimation growth and Czochralski growth by Karl [4]. Anthracene crystals were grown from vertical Bridgmann technique by Chakkaravarthi [5]. Anthracene crystals were also grown from the solutions of benzene and n-pentane and from the vapour phase. Recently, we reported the growth of high-quality anthracene crystals by a simple solution technique [6]. Metastable zone width is an essential parameter for the growth of good crystals from solution, since it is the direct measure of the stability of the solution in its super saturated region [7]. Thermal, spectral and X-ray analyses are very useful techniques for material characterization [8–42]. In the present study, the growth and the metastable zone width of anthracene in CS2, CHCl3 and CCl4 as solvents were determined. Various thermal, spectral and X-ray analyses were also used to characterize solution grown anthracene. The nucleation parameters of solution grown anthracene were determined using the interfacial tension and reported for the first time.

Experimental

Crystal growth

The anthracene used to grow crystals from solution was blue fluorescent grade. This anthracene was further purified by re-crystallization from carbon tetra chloride for several times. The final crystal was taken for growth.

Solubility of anthracene in CS2, CCl4 and CHCl3 at various temperatures was determined. The solvents used were analar grade. Solubility determination shows that the solubility increases with temperature. This is shown by the solubility curves of anthracene in CS2, CCl4 and CHCl3 in Figs. 1, 2 and 3, respectively.

Fig. 1
figure 1

Solubility and metastability of anthracene in CS2

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Fig. 2
figure 2

Solubility and metastability of anthracene in CCl4

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Fig. 3
figure 3

Solubility and metastability of anthracene in CHCl3

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Measurements

Powder X-ray diffractometry (XRD) analysis was performed with a graphite monochromated Cu Kα radiation.

FT-IR spectra were recorded using an AVATAR 330 FT-IR by KBr pellet technique in the range 400–4000 cm−1.

UV–Visible absorption spectra were recorded using a Hitachi UV–VIS spectrophotometer in the spectral range 250–1200 nm.

The thermogravimetric (TG) and differential thermal analysis (DTA) were carried out using a NETZSCH STA 409C thermal analyzer in nitrogen atmosphere. The sample was heated between 30 and 550 °C at a heating rate of 10 °C/min.

The NMR spectrum of solution grown anthracene was recorded using JEOL instrument, model GSX400, using CDCl3 as solvent.

Determination of metastable zone width

Metastable zone width is an essential parameter for the growth of good crystals from solution, since it is the direct measure of the stability of the solution in its supersaturated region. After solubility determination, the metastable zone width of anthracene in CS2, CCl4 and CHCl3 was determined. Saturated solutions of anthracene in these solvents at different temperatures were allowed for systematic slow cooling. The temperature at which the first nucleation was observed corresponds to their width of metastable zones. The metastable zone widths of anthracene in CS2, CCl4 and CHCl3 are shown in Figs. 1, 2 and 3, respectively. The differences in metastable zone width of anthracene in CS2, CCl4 and CHCl3 at various temperatures (30, 35 and 40 °C) are shown in Table 1. An important behaviour noted that the metastable zone width depends on solvent nature. At a particular temperature, the enhancement of metastable zone width is from CS2 to CHCl3 and from CHCl3 to CCl4. That is, larger metastable zone width is observed in CCl4 than in CHCl3 and CS2. After solubility and metastable zone width determination, the saturated solutions of anthracene were allowed for slow evaporation in room temperature. The crystals of different morphology were obtained in 1 or 2 days. The crystals were carefully harvested and subjected to characterization studies, viz., UV, NMR, FTIR, XRD and TG–DTA.

Table 1 Comparison of metastable zone widths in various solvents
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Induction periods and interfacial energies

There are several methods of measuring the induction period depending upon the solubility of the materials. Here the visual observation method was followed. Solutions of anthracene in CS2 at different super saturation values were prepared and subjected to systematic slow evaporation. The time period that elapses between the achievement of super saturation and appearance of visible nuclei is taken as the induction period (t). Several trial runs were performed to minimize the error. Experiments were repeated for super saturation(s) like 1.10, 1.15 and 1.20 at two different temperatures. From the results obtained, a plot of ln t against 1/(ln s)2 is drawn and is shown in Fig. 4. The interfacial tension was calculated from the slope of the curves using the equation

$$ \ln \,t = \ln \,A + 16\pi \gamma^{3} V^{2} N/3RT(\ln \,s)^{2} $$

where A is a constant related to the pre-exponential factor of the nucleation rate expression, V is the molar volume, N is the Avagadro number and R is the gas constant. The factor 16π/3 in the above equation refers to the spherical nuclei. The interfacial tension between the anthracene and CS2 is calculated by measuring the slope value of the curve obtained at the two temperatures.

Fig. 4
figure 4

A plot of ln t vs. (1/ln s)2 for anthracene grown from CS2 at 303 and 308 K

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Similar experiments were made in the case of CCl4 and CHCl3 solvents also. From the results obtained, plots of ln t against 1/(ln s)2 are drawn and shown in Figs. 5 and 6. The effect of solvent and temperature on interfacial tension is presented in Table 2.

Fig. 5
figure 5

A plot of ln t vs. 1/(ln s)2 for anthracene grown from CCl4 at 303 and 308 K

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Fig. 6
figure 6

A plot of ln t vs. 1/(ln s)2 for anthracene grown from CHCl3 at 303 and 308 K

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Table 2 Effect of temperature and solvent on interfacial tension
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According to the classical homogenous nucleation theory, the Gibbs free energy required to form anthracene nucleus is given by

$$ \Updelta G = (4/3)\pi r^{3} \Updelta G_{\text{v}} + 4\pi r^{2} \gamma $$
(2)

where ∆G v is the Gibbs energy change per unit volume and r is radius of the nucleus. At the critical state, the free energy of formation obeys the condition that d(∆G)/dr = 0. Hence, the radius of the critical nucleus is expressed as

$$ r^{*} = - 2\gamma /\Updelta G_{\text{v}} $$

where

$$ \Updelta G_{\text{v}} = - KT\,\ln \,s/V $$
(3)

where V is the molar volume and s = C/C*, where C is the actual concentration and C* is the equilibrium concentration.

Hence

$$ r^{*} = 2v\gamma /KT\,\ln \,s $$
(4)

The critical Gibbs free energy is given by

$$ \Updelta G^{*} = 16\pi \gamma^{3} v^{2} /\Updelta G_{\text{v}}^{2} $$
(5)

The number of molecules in the critical nucleus is expressed as

$$ i^{*} = 4\pi (\gamma^{*} )^{3 } /3V $$
(6)

Therefore, using the interfacial tension value, the radius of the critical nuclei (r*), the Gibbs free energy change for the formation of a critical nucleus (∆G*) and the number of molecules in the critical nucleus (i*) were calculated at two different temperatures for anthracene in CS2 and presented in Table 3.

Table 3 Nucleation parameters of anthracene crystal in CS2
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It was noted that with the increase in super saturation, the Gibbs free energy change for the formation of a critical nucleus (∆G*) decreases with radius (r*). This favours the easy formation of nucleation in CS2 solutions at higher super saturations.

Similar type of calculations used to calculate the nucleation parameters of anthracene in CHCl3 and CCl4, and the values are given in Tables 4 and 5, respectively.

Table 4 Nucleation parameters of anthracene crystal in CHCl3
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Table 5 Nucleation parameters of anthracene crystal in CCl4
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The surface tension of the solvents CS2, CHCl3 and CCl4 is in decreasing order. The effect of surface tension on interfacial tension is given in Table 6. The interfacial tension between the anthracene and CS2, CHCl3 and CCl4 is also in decreasing order. The effect of viscosity of solvents on interfacial tension is given in Table 7. As viscosity increases, the interfacial tension decreases. The effect of density of solvents on interfacial tension is presented in Table 8. As density increases, the interfacial tension decreases.

Table 6 Effect of surface tension of solvents on interfacial tension
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Table 7 Effect of viscosity of solvents on interfacial tension
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Table 8 Effect of density of solvents on interfacial tension
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UV spectral analysis

To analyse the transmission range and the suitability of solution grown anthracene crystals for optical applications, the UV–Visible spectrum was recorded and shown in Fig. 7. The spectrum shows the characteristic absorption of anthracene between 300 and 400 nm which is assigned to aromatic ring. The UV spectrum proves the highly transparent nature of the material between 400 and 800 nm, which is one of the important requirements to a material for NLO applications.

Fig. 7
figure 7

UV spectrum of anthracene

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NMR spectral studies

There are three sets of different kinds of protons present in anthracene molecule (C14H10). Therefore, we expect three types of signals [43, 44]. The recorded 1H NMR spectrum shown in Fig. 8 is well in accordance with theoretical spectrum. A singlet at 8.4 ppm is assigned to two protons. A double doublet at 8.0 ppm is assigned to four protons of one kind and another double doublet at 7.48 ppm is assigned to another kind of four protons.

Fig. 8
figure 8

Expanded 1HNMR spectrum of solution grown anthracene

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FTIR spectral studies

The FTIR spectrum is an important record which gives sufficient information about the structure of a compound. The FTIR spectral studies were performed using the KBr pellet technique. The FTIR spectrum of anthracene grown form CS2, CHCl3 and CCl4 is shown in Fig. 9. The middle of the IR spectrum of anthracene provides a characteristic peak due to aromatic C–H stretch at 3047 cm−1 which is very sharp. The sharpness of the peak shows that the hydrogen atoms in the anthracene ring are not exerting any bonding interaction with molecules. The skeletal vibrations (C=C bond) of the ring could be assigned to peaks 1619 and 1447 cm−1. The presence of two sets of four adjacent hydrogen atoms is evidenced by the peak at 734.4 cm−1 due to C–H out of plane bending vibration.

Fig. 9
figure 9

FTIR spectrum of solution grown anthracene

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The peak at 883 cm−1 is due to C–H out of plane bending vibration which corresponds to two isolated hydrogen atoms. Overtones or combination bands are found at 1926.7–1719.4 cm−1. Out of plane (C=C bond) could be assigned to 469.3–438 cm−1 [43, 44].

X-ray diffraction studies

The powder X-ray diffraction studies were done, and lattice parameters were calculated. Most of the lattice parameters evaluated by powder X-ray diffraction studies of anthracene in CS2 and CCl4 as shown in Figs. 10, 11 and 12 are in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) values. The prominent faces identified in the powder XRD pattern of anthracene grown from CS2 are (1,0,0) \( (\bar{1},1,0) \) (2,0,0) \( (\bar{1},0,2) \) (2,0,2) which is shown in Fig. 10. The prominent faces identified in the XRD pattern of anthracene grown from CCl4 and CHCl3 are (1,0,0) (1,1,0) (0,1,1) (2,0,0) (1,0,2) (3,0,0) which is shown in Figs. 11 and 12. This reveals that the morphology of anthracene grown from different solvents is different.

Fig. 10
figure 10

XRD pattern of anthracene grown from CS2

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Fig. 11
figure 11

XRD pattern of anthracene grown from CCl4

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Fig. 12
figure 12

XRD pattern of anthracene grown from CHCl3

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Thermal studies

TG and DTA were carried out on the crystal samples for qualitative analysis. The weight change in the sample with temperature was studied by TG, and the energy change in the sample with temperature was studied by DTA. The analysis was performed in nitrogen atmosphere.

The TG and DTA curves of anthracene grown from CS2, CCl4 and CHCl3 given in Fig. 13 show the purity of the harvested anthracene crystals. The TG curve indicates that it is thermally stable up to 150 °C. Thermal studies show that the sharp exothermic peak (on DTA curve) corresponding to 216.40 °C indicate phase transition due to melting point, which exhibits the high purity of the grown crystals best and suitable for fabrication of best optical scintillator device.

Fig. 13
figure 13

TG and DTA curves of solution grown anthracene crystal

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Conclusions

The metastable zone width of anthracene in CS2, CCl4 and CHCl3 solvents was determined for the first time. It was found that the metastable zone width depends on solvent nature. The effect of temperature and solvent on interfacial tension was determined. Using the interfacial tension value, the nucleation parameters such as radius of the critical nuclei (r*), the free energy change for the formation of a critical nucleus (∆G*) and the number of molecules in the critical nucleus (i*) were also calculated for all these solvents at two different temperatures. The effect of surface tension, viscosity and density of these solvents are correlated with interfacial tension. The UV spectrum proves the highly transparent nature of the material between 400 and 800 nm, which is one of the important requirements to a material for NLO applications. Thermal studies also support the purity of solution grown anthracene. All the characterization studies of solution grown anthracene are well in agreement with standard and theoretical value.