(Special issue- Issue 5 volume 5) Avai - RS Publication

International Journal of Advanced Scientific and Technical Research Available online on http://www.rspublication.com/ijst/index.html

(Special issue- Issue 5 volume 5) ISSN 2249-9954

Synthesis, characterization of combustion derived Zn2TiO4 nanocrystals and its application to adsorption of azo dye R. S. Raveendra1, 2, P. A. Prashanth1#, B.M. Nagabhushana3 1 R & D Center, department of Chemistry, Sai Vidya Institute of Technology, Bengaluru-560 064, India 2 Research & Development Center, Bharathiar University, Coimbatore-641 046, India 3 Department of Chemistry, M.S. Ramaiah Institute of Technology, Bengaluru-560 054, India

ABSTRACT Zn2TiO4 nanocrystals were synthesized by simple self propagating solution combustion method. The synthesized sample was calcined at 900 °C for 4 hours in open air atmosphere. The structural studies are characterized by Powder X-ray Diffraction; from the results, the phase is identified as cubic structured orthozinc titanate (Zn2TiO4 ICDD card number 25-1164) with space group Fd 3 m. Morphology of the sample was analyzed by field emission scanning electron microscopy attached with EDX and high resolution transmission electron microscopy. Further the purity of the sample and M-O bonds formation are confirmed by Fourier transform infrared analysis. Optical energy band gap is calculated by Wood and Tauc method using UV-Vis spectrum. Its adsorption performance was evaluated on malachite green azo dye. The effect of various parameters on adsorption such as initial dye concentration, pH, contact time and dosage were studied. More than 85% of the dye adsorbed onto the Zn2TiO4 nanocrystals in 40 minutes of contact time at an optimum amount of 35 mg/L of dye solution. Keywords: Zn2TiO4 nanocrystals, cubic, malachite green, Wood and Tauc. #Correspondence Author: P. A. Prashanth: [email protected] INTRODUCTION: In the past few years zinc-titanium based oxide materials (Zn-Ti-O) have been used widely because of their outstanding properties and potential scientific and technical applications [1]. Recently, zinc titanates have been investigated for application in many fields such as regenerable sorbents for high-temperature hydrogen sulfide (H2S) removal from coal gasifier gas [2-4], gas sensors [5], humidity sensors [6], paint pigments [7], as dielectric materials [8, 9], and as photocatalysts. It is reported by various authors that there are three compounds existing in the ZnO–TiO2, including cubic inverse-spinel type zincorthotitanate (Zn2TiO4), rhombohedral ilmenite type zinc metatitanate (ZnTiO3) and cubic spinel type structured Zn2Ti3O8 which is considered as low-temperature form of ZnTiO3 [10-12]. Among all these, nano sized crystalline Zn2TiO4 is highly significant material which has been used as a luminescent material and other useful applications, its physical, conducting and optical properties have been studied for various applications [13-15]. Malachite green (MG) is a synthetic azo dye which is commonly used for the dyeing of cotton, silk, paper, leather industries, in manufacturing of paints and printing inks, as a food CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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coloring agent, food additive, medical disinfectant. However, despite its use, MG is a hazardous because it’s adverse effects on the immune and reproductive systems, carcinogenic, genotoxic, mutagenic and teratogenic properties [16, 17]. For these reasons the United States and the European Council have imposed a strict ban on the use of MG in all categories of food. In addition to this, discharge of MG into the hydrosphere can cause severe ecological imbalance as it gives undesirable color to water and reduces sunlight penetration that harms aquatic life. Therefore, it is important to remove MG from aqueous effluents before they are discharged into bodies of water. Therefore in this view, in the current article we present the synthesis and characterization of cubic srtrctured Zn2TiO4 nanocrystals and studied its effectiveness in the adsorptive removal of hazardous MG dye. MATERIALS AND METHODS: Commercially pure zinc nitrate hexa hydrate (Zn(NO3)2 6.H2O, AR 99% Merck), tetra-nbutyl titanate ([Ti(OC2H9)4 AR 99% Aldrich), urea (CO(NH2)2 AR 99% Merck), 1:1 nitric acid (HNO3 Fisher scientific), hydrochloric acid (HCl Fisher scientific), sodium hydroxide (NH4OH Fisher scientific), malachite green (C23H26ON2 Sigma-Aldrich), were used as such without further purification. PREPARATION OF Zn2TiO4 NANOCRYSTALS: Zn2TiO4 nanocrystals were synthesized by facile solution combustion synthesis method [18, 19]. In this method, the reaction mixture was calculated based on the total oxidizing and reducing valances of the oxidizer and fuel required to release the maximum energy for the reaction. Zn2TiO4 nanocrystals were synthesized in two simple steps, PREPARATION OF TITANYL NITRATE: Titanyl nitrate solution was prepared by controlled hydrolysis of tetra n-butyl titanate with distilled water, further reaction of formed titanyl hydroxide with 1:1 HNO3 gives titanyl nitrate. The following reactions take place during the formation. Ti(OC4H9)4 + 3 H2O TiO(OH)2 + 4 C4H9OH ………... (1) TiO(OH)2 + 2 HNO3 TiO(NO3)2 + 2 H2O ……….. (2) COMBUSTION PROCESS: The titanyl nitrate was dissolved in minimum quantity water and the stoichiometric quantities of Zn(NO3)2, and CO(NH2)2 were mixed in the double distilled water and stirred well using a magnetic stirrer for about 30 min. The crystalline dish containing the above solution was introduced into preheated muffle furnace maintained at 500  10 °C. The solution was boiled and resulted in a highly viscous liquid. This viscous liquid catches fire and auto ignited with flames on the surface, which rapidly proceeded throughout the entire volume forming a white powdered product. Then the powder was calcined at 900 °C for 4 hours. The overall reaction can be written as, 2 Zn(NO3)2 + 5 NH2CONH2 + TiO(NO3)2 2 Zn2TiO4 + 5 CO2 + 8 N2 + 10 H2O............ (3)

CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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PREPARATION OF MALACHITE GREEN DYE SOLUTION: An accurately weighed amount of the malachite green dye was dissolved in de-ionized water to prepare stock solution (10 mg/l). Experimental solutions of desired concentration were obtained by successive dilutions. CHARACTERIZATION TECHNIQUES: Zn2TiO4 nanocrystals were characterized by PXRD. Powder X- ray diffraction patterns were collected on a Shimadzu XRD-700 X-ray Diffractometer. Product was morphologically characterized by FE-SEM performed on a ZEISS scanning electron microscope equipped with EDS. TEM and HR-TEM analysis was performed on a Hitachi H-8100. FT-IR studies have been performed on a Perkin Elmer Spectrometer (Spectrum 1000) with KBr pellet technique. To calculate optical energy band gap, UV-Vis spectrum was recorded using Elico SL-159 UV-Vis spectrophotometer. Kemi centrifuge was used to separate dye solution from adsorbent. RESULT AND DISCUSSIONS: PXRD STUDIES: The formation of Zn2TiO4 nanocrystals was confirmed by PXRD measurements. The PXRD of calcined sample confirms the crystallinity with inverse-spinel type cubic Zn2TiO4 (ICDD card number 25-1164). All the diffraction peaks can be indexed to (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (5 3 1), (6 2 0), (5 3 3), (6 2 2), (4 4 4) and (6 4 2) reflections. The broadening of the reflections clearly indicates the inherent nature of nanocrystals. PXRD patterns are shown in the Fig-1 and the crystal structure of the Zn2TiO4 nanocrystals is shown in Fig. 2. The crystallite size is calculated from the full width at half maximum (FWHM ()) of the diffraction peaks using Debye- Scherer’s method [20] using the following equation, … (4)

fig. 1 PXRD patterns of Zn2TiO4 nanocrystals. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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Where‘d’ is the average crystalline dimension perpendicular to the reflecting phases, ‘𝜆’ is the X-ray wavelength, ‘k’ is Scherer’s constant (0.92), ‘β’ is the full width at half maximum (FWHM) intensity of a Bragg reflection excluding instrumental broadening and ‘’ is the Bragg’s angle. The calculated average crystallite size of the sample is found to be 16 nm. The lattice and the structural parameters of the Zn2TiO4 nanocrystals are summarized in Table-1.

Atoms

Oxidation Wyckoff x y state notation Zn 2+ 8a 0.0000 0.0000 Ti 4+ 16d 0.6250 0.6250 O 232e 0.3867 0.3867 Crystal system: cubic; space group: group Fd 3 m (227)

z

Occupancy

0.0000 0.6250 0.3867

1 1 1

Table-1 Lattice and structural parameters of Zn2TiO4 nanocrystals.

Fig. 2 Packing diagram of Zn2TiO4 nanocrystals. FT-IR SPECTROSCOPIC STUDIES: Fig. 3 represents FT-IR spectrum of the Zn2TiO4 nanocrystals recorded to define the vibrational frequency of metal – oxygen and other bonds related to impurities. It can be seen that no major impurity peaks corresponding to the organic impurities was observed. Strong absorption bands at 555 cm-1 and 409 cm-1 can be assigned to the stretching vibration of M–O bonds (M= Zn, Ti) [21].


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Fig. 3 FTIR spectrum of Zn2TiO4 nanocrystals. UV-VIS SPECTROSCOPY STUDIES: In order to determine the optical energy band gap of sample, the UV-Vis absorption spectrum was recorded. The sample shows a strong absorption peak (λmax) at 226 nm at the UV region. Fig. 4 (a) shows the UV-Vis absorption spectrum of calcined sample. This can be attributed to photo excitation of electron from valence band to conduction band. The optical energy band gap (Eg) was calculated (Fig. 4 (b)) by the method proposed by Wood and Tauc [22] according to the following equation,



(h  )  h  Eg

 .......................................................(5) n

where ‘α’ is the absorbance, ‘h’ is the Planck constant, ‘ν’ is the frequency, ‘Eg’ is the optical energy band gap and ‘n’ is a constant associated to the different types of electronic transitions (n= 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively).

Fig. 4 (a).UV-Vis spectrum (b) Optical energy band gap of Zn2TiO4 nanocrystals.


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MORPHOLOGICAL ANALYSIS: Fig. 5 (a, b) show FE-SEM images of calcined Zn2TiO4 nanocrystals. Micrographs revels that the particles are nearly spherical in shape and has uniform size distribution. The particles are highly agglomerated due to sintering of particles during calcination. Energy dispersive spectroscopy was used to analyze the chemical composition of the prepared Zn2TiO4 nanocrystals. No elements other than Zn, Ti and O are seen using energy dispersive spectroscopy (Fig.5c). TEM image of Zn2TiO4 nanocrystals (Fig. 6 a) shows that the particles obtained are in nano regime and has average particle size ~ 20 nm. HR-TEM image (Fig. 6 (b)) shows that the sample is highly crystalline.

Fig. 5 (a, b) FE-SEM (c) EDS micrographs of Zn2TiO4 nanocrystals.

Fig. 6 (a) TEM (b) HR-TEM of Zn2TiO4 nanocrystals. ADSORPTION STUDIES: Adsorption experiments were performed using malachite green dye. Malachite green is a basic triphenylmethane dye with a molecular weight 327. IUPAC name of dye is [4-[(4dimethylaminophenyl)-phenylmethyidene]-1-cyclohexa-2, 5-dienylidene] dimethylazanium with molecular formula C23H25N2+. It has a high solubility in acidic organic solvents but less in water [23]. Batch experiments were carried out at different dose, time and pH. 50 ml of dye solution of concentration (10 ppm) was mixed with different dose (10 to 50 mg) of adsorbent in 100 ml beaker at lab temperature. The dye solution containing adsorbent was stirred magnetically (in CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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absence of light) to increase the contact between the dye solution and the adsorbent. After desired time the adsorbent was separated from the solution by centrifugation at 2000 rpm for 10 min. Residual concentration of dye in supernatant was estimated spectrophotometrically by monitoring the absorbance at 618 nm (λmax) using a UV–Vis spectrophotometer. EFFECT OF CONTACT TIME: Effect of contact time on the adsorption of malachite green onto Zn2TiO4 nanocrystals was studied. It can be observed from the Fig. 7 that the dye adsorption increases with the increasing of stirring time of 60 min. The rate of adsorption is initially quite rapid with most of the compound being adsorbed within the first 40 min. It was found that more than 80% adsorption of dye occurred in the first 40 min; thereafter the rate of adsorption was found to be slow. This shows that equilibrium can be assumed to be achieved after 40 min. It is basically due to saturation of the active site which does not allow further adsorption to take place [24].

Fig. 7 Effect of contact time. EFFECT OF DOSAGE:

Fig. 8 Effect of dose of adsorbent. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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Adsorption of dye is strongly influenced by the quantity of the adsorbent. Adsorption of malachite green onto Zn2TiO4 nanocrystals was studied with changing the amount of adsorbent from 10 mg to 50 mg/L at a constant stirring rate of 40 minutes with optimum dye concentration of 10 ppm. It is observed from the Fig. 8 that with increase in the dose, adsorption of malachite green increases upto optimum quantity of adsorbent. Maximum of 83% dye adsorbed at the dose of 35 mg of adsorbent. Further increase in adsorbent dose, decreases the adsorption percentage. This might be attributed to over-lapping or aggregation of adsorption sites resulting in decrease in total adsorbent surface area available to MG and increase in path length [25]. EFFECT OF PH: The pH of system has a great effect on the adsorption efficiency of organic dyes. Effect pH on malachite green adsorption onto the Zn2TiO4 nanocrystals was carried out at 10 ppm of initial dye concentration with 35 mg mass of adsorbent at 40 min of stirring rate at lab temperature. As given in the Fig. 9, Zn2TiO4 nanocrystals show maximum of 87% dye adsorption at the pH of 9 which decreased to 10% at pH of 2. This confirms that the low pH (2–5) was unfavourable for malachite green adsorption by Zn2TiO4 nanocrystals.

Fig. 9 Effect of pH. CONCLUSION: Zn2TiO4 nanocrystals were successfully prepared by the simple solution combustion method and characterized by PXRD, FT-IR, UV-Vis, FE-SEM with EDS and HR-TEM. Adsorption of hazardous malachite green dye over the Zn2TiO4 nanocrystals was studied. The result showed that the parameters like effect of pH and dosage will play a very important role on the adsorption of azo dye over Zn2TiO4 nanocrystals. ACKNOWLEDGEMENTS: The authors R.S. Raveendra and Dr. P.A. Prashanth thank the Principal and management of Sai Vidya Institute of Technology for their constant encouragement. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS, S J C INSTITUTE OF TECHNOLOGY”

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