CHARACTERIZATION OF ZNO NANOPARTICLES PREPARED IN CURCUMA AROMATICA SALISB. ROOT EXTRACT

The root extract of the locally available Curcuma Aromatica Salisb. was used with the ZnO precursors to study the effect of the Curcuma Aromatica Salisb. extract on the growth and optical absorption behaviour of the ZnO nanoparticles. ZnO nanoparticles were synthesized in various concentrations of Curcuma Aromatica Salisb. root extract. X-ray diffraction studies show that the prepared samples consist of nanocrystallites of having sizes in the range between 17 nm to 26 nm. The optical absorption studies reveal that the eco-dying on ZnO nanoparticles using Curcuma Aromatica Salisb. root extract has enhanced the optical absorption behaviour of ZnO to the visible region of the spectrum.


INTRODUCTION
Curcuma aromatica Salisb., the wild turmeric, 'Vanaharidra' in Ayurveda, belongs to the 'ginger family' Zingiberaceae. This wild turmeric is an aromatic medicinal plant and is commonly known as "kasturi manjal" (musk turmeric) in south India. The rhizome of the plant is loaded with alkaloids, flavonoids, curcuminoids, tannins and terpenoids. The medicinal properties of this rhizome are being used in many traditional systems of medicines. The rhizomes have characteristic fragrance. The paste of rhizome is used for facial application to reduce acne and excessive hair growth and also to improve skin tone and complexion by village women in South India [1][2][3] . The colouring matter in the rhizomes of Curcuma aromatica Salisb. is predominantly curcumin and demethoxycurcumin [4] . Revathi et al., studied the antibacterial activity of the rhizome of Curcuma aromatica Salisb. and reported that the hexane extract of the rhizome of Curcuma aromatica Salisb [5] . contains the compounds such as Aromadendrene, a-Vatrenene, Epiglobulol,Germacron, 20 ml of 0.75 M zinc acetate dihydrate was prepared in de-ionized water then 10ml of the prepared Curcuma aromatica Salisb. solution of different concentrations were added slowly to the 0.75 M zinc acetate dihydrate solution under magnetic stirring. The mixture was stirred for 10 minutes by keeping the hotplate at 70 o C. while stirring, 20 ml of 1M NaOH solution added dropwise to the mixture of zinc acetate dihydrate and tannin using a burette. A precipitate of ZnO was obtained and the sodium acetate formed during the reaction process is soluble in water, that can be removed by repeated washing. The supernatant is removed using a syringe and the precipitate was washed many times by continuing the syringe method. After, repeated washing, the obtained precipitate was dried and the resultant powder was collected for analysis. X-ray diffraction (XRD) analysis of the powder samples were done at wavelength 0.1546 nm, running at 40 kV and 30 mA in X-ray diffractometer (Bruker D8 Advance). X-ray diffractograms of zinc oxide nanoparticles were recorded in the region from 10 o to 80 o at a scan speed of 2 o per minute. For UV spectroscopic analysis, prepared powder of ZnO nanoparticles were dispersed in distilled water (1 mg/ mL) and scanned in Agilent Cary 5000 UV-vis spectrometer at 25 o C in the range of 250-900nm. Fourier Transform Infrared (FTIR) analysis of the ZnO were done by mixing ZnO nanoparticles and potassium bromide to form a salt plate. Spectra between 4000 and 400 cm -1 were recorded using a Thermo Nicolet Avtar 370 spectrometer.

RESULTS AND DISCUSSION
Curcuma aromatica Salisb. solution of various concentrations (0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 mg per 10 ml of deionized water) were used to enhance the absorption characteristics of the ZnO nanoparticles. Curcuma aromatica Salisb. solution consists of many number of organic compounds [5] . The extracts confirmed the presence of phenolic (-OH) groups [7] and the FTIR spectrum of the solution is shown in Figure 1. The analysis was performed in a frequency range of 4000-400 cm -1 at room temperature. The observed peaks and corresponding vibrations are tabulatd in the Table 1. The O-H stretch band around 3400 cm -1 is associated with the moisture. From the intensity of the absorption, it is inferred that the 0.8T sample has little higher moisture than other samples. The spectrum presented a band at around 890 cm −1 and around 424 cm -1 signals the characteristic bond of Zn -O, which confirms the presence of zinc oxide in the sample.    The UV-visible absorption spectra of the prepared liquid sample of the concentration 0.2T was obtained as given in the Figure 2. It is observed that the Curcuma aromatica Salisb. has absorption peak around 270 nm and absorption edge around 340 nm. Neerja et al. reported UV absorption peaks at around 240,420 nm for curcumin 251 and 423 for demethoxycurcumin [4] . The observed spectrum of our Curcuma aromatica Salisb. solution shows that the samples have absorption in the visible region and the solution were seen as yellowish in color. This absorption in the visible region helps us to sensitize ZnO nanoparticles to expand ZnO spectrum to visible region. Also, the broadening of the absorption spectra is desirable for harvesting the solar spctra in visible region that leads to a higher photocurrent from the dye sensitized solar cell. Then the absorption spectra of the prepared ZnO nanoparticles powder were studied and the spectra is shown in the Figure 3. The absorption edge observed at 400 nm (3.1 eV) for the pristine ZnO white powder. But, the absorption spectra of the ZnO prepared in the various concentrations of the tamarind tannin extended to visible region (650 nm) with the same absorption peak seen in the spectrum of the Curcuma aromatica Salisb. solution alone. The absorption edge due to the ZnO in the eco-dyed sample has not moved remarkably from the position of the pristine sample, it means that the size of the ZnO has not much changed when they are prepared in the presence of Curcuma aromatica Salisb.
The band gap of the prepared powder material has been determined by the modified tauc plot method proposed by Ragib Ahsan et al [9] .
For preparing Tauc plot for nanoscale powders, Ragib Ahsan et al [9] assumed that From this equation they derived as Where A is the absorbance, α is the absorption coefficients, Io and IT are the incident photon intensity and transmitted photon intensity respectively.
Using the equation (4), the absorption coefficients (α) calculated from absorbance spectrum using the modified equation [9] . Here the importance is given only to the linear part of the Tauc plot and approximated the value of absorption coefficients by considering absorption coefficient is proportional to the absorbance [9]. This value of alpha is substituted in the tauc plot equation (5) [10] .
Where h is plank's constant, is the photon frequency, is the absorption coefficient, is the band gap and A is proportionality constant. The value of the exponent denotes the nature of the electronic transition, such as n=1/2 for direct allowed transition, n=3/2 for direct forbidden transition, n=2 for indirect allowed transition, n=3 for indirect forbidden transition. Generally the allowed transitions dominate the basic absorption process, either n=1/2 or n=2, for direct and indirect transitions respectively. By plotting and by finding the best fit of the ( ℎ ) 1 vs ℎ using n=1/2 and n=2 gives the correct transition type. Band gap is found from the intersecting point of the curve in the energy axis. Here it is assumed that the optical absorption strength depends on the difference between the photon energy and the band gap.
ZnO has a direct allowed transition [6] and drawn the Tauc plot by substituting n=1/2. At low energies the photon energy absorption approached to zero and at higher energies the absorption process saturate and the curve again deviate from linear, which is the characteristic of the Tauc Plot. At the near of the band gap the the absorption gets stronger and shows a region of linearity in the plot. This linear regions used to extrapolate to the xaxis intercept to find the band gap value.
The Tauc plots of the prepared powder samples are given in the Figure 4 and the band gaps found from the plots are given in the Table 2. It is observed that the band gap has not shown much changes with the concentration of the Curcuma aromatica Salisb. solution. This implies that the Curcuma aromatica Salisb. has not any remarkable effect on controlling the particle size of the ZnO nanoparticlses when ZnO nanoparticles prepared by this method.  The UV-visible absorption spectra also gives an information about the Urbach tails related to the width of the localized states available in the optical band gap of the ZnO nanoparticles. The band structure in the semiconductor may be damaged due to the disorder in the crystals or may be due to the addition of other extra atoms. Therefore the Urbach energy found below the absorption band edge gives us an idea about the amount of the damage happened to the crystal. For highly imperfect crystal, Urbach energy is large. The generation of absorption edge at the band gap energy is due to the exciton phonon interaction or may be due to the electron phonon interaction. The defects in crystals also absorb light but does not contribute to free electrons instead it recombined or trapped there. This is the curve in the tauc plot after the absorption edge. Urbach energy is the energy that gives the spectral dependence of the absorption coefficients that are examined at photon energies, which are less than the band gap of the material. That is, the formation of localized states with energies at the boundaries of the energy gap which is one of the effects of the structural disorder on the electronic structure of the material.These defect states traps the excited electrons and prevent the movement of electron to the conduction band when the samples are irradiated with light of a particular wavelength. This absorption tail is called the Urbach tail and is associated with the Urbach energy. The equation for calculating the Urbach energy is Where is the absorption coefficient, Eu is the Urbach Energy. The Urbach energy is calculated by plotting ln( ) vs ℎ . Figure 5 shows the Urbach energy plots for all the samples. The slope and intercept of the linear region found by using linear quick fit gadget tool in the Origin Software. The reciprocal of the slope of the linear fit, gives the Urbach energy values and the Eu values are tabulated in Table 3 with band tgap energies. There is not seen any trend of changing the disorders as varying the tamarind seed coat tannin concentrations. The XRD pattern shows that the prepared ZnO has hexagonal wurtzite structure (with a = b =3.25 Ǻ, c = 5.20 Ǻ) belonging to the C46v space group (P63mc). The broadening of peak in the XRD pattern clearly implies that small nanocrystals are present in the samples. There is no evidence of bulk form of the materials or any impurity. In the XRD pattern, the (101) diffraction peak is much stronger than other peaks. This indicates that the formed ZnO nanocrystals have a preferential crystallographic (101) orientation. The average crystallite size of prepared sample was calculated by the Debye-Scherrer's formula [11] i.e., = 0.9 where, D is the particle size, λ the wavelength of x-rays (λ = 1.5406 Å for Cu Kα), θ the Bragg angle and β is the full width at half maxima (FWHM). Average particle size (D) of synthesized ZnO nanoparticles were found to be 17 nm using this equation (7).
The induced micro strain in the crystal due to the crystal imperfection and distortion was calculated using the equation [12] = 4 (8) The strain and particle size can also be found from the Williamson-Hall Plot. Williamson-Hall Plot were drawn using the equation  The plots were made by taking 4 along x-axis and along y-axis for all the ZnO samples as shown in Figures 8,9 and 10. From the linear fit, the crystallite size was estimated from the y intercept and the strain from the slope of the fit [12] . Geometric parameters of the prepared ZnO nanoparticles are tabulated for the samples S1, Tm0.2 and Tm1.0 are given in the Table 4,5 and 6. Table 7 shows the values obtained from Scherrers formula and from the Williamson-Hall plot. When comparing the tabulated values, it is seen that the size of the crystallites have increased little from 17 nm to 26 nm in the case of Tm0.2 sample and to 21nm in the case of Tm1.0 sample. But, this increase is not evident from the optical absorption analysis. TEM and HRTEM images of the smaple S1 and 1.0T are shown in Figure 10 and 11 respectively. The figures shows that the sizes of the prepared particles are below 50 nm. The interplar spacing measured as 0.22 nm corresponds to the (101) plane.

CONCLUSIONS
The extracts of Curcuma aromatica Salisb. was obtained in deionized water for eco-dying ZnO nanoparticles and their FTIR and absorption were studied. The particle size of the prepared ZnO particles were calculated as approximately 17 nm and the particle sizes were seen increased when they prepared in Curcuma aromatica Salisb. extract by the propsed method. The UV-visible absorption spectroscopic results shows that the optical behaviour is enhanced and the eco-dyed ZnO can absorb a wide spectrum of light from 400 nm to 650 nm. Curcuma aromatica Salisb. extract can be successfully employed as a natural dye to sensitize ZnO nanoparticles without changing their size much.