Accepted Manuscript Artocarpus gomezianus aided green synthesis of ZnO nanoparticles: Luminescence, Photocatalytic and Antioxidant Properties D. Suresh, R.M. Shobharani, P.C. Nethravathi, M.A. Pavan Kumar, H. Nagabhushana, S.C. Sharma PII: DOI: Reference:

S1386-1425(15)00069-4 http://dx.doi.org/10.1016/j.saa.2015.01.048 SAA 13216

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

1 November 2014 31 December 2014 14 January 2015

Please cite this article as: D. Suresh, R.M. Shobharani, P.C. Nethravathi, M.A. Pavan Kumar, H. Nagabhushana, S.C. Sharma, Artocarpus gomezianus aided green synthesis of ZnO nanoparticles: Luminescence, Photocatalytic and Antioxidant Properties, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.01.048

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Artocarpus gomezianus aided green synthesis of ZnO nanoparticles: Luminescence, Photocatalytic and Antioxidant Properties D. Suresh1*, R.M.Shobharani1, P.C. Nethravathi1, M.A. Pavan Kumar2, H. Nagabhushana3, S.C.Sharma4, 5 1 Department of Studies and Research in Chemistry, Tumkur University, Tumkur 572 103, Karnataka, India, Department of Studies and Research in Biochemistry, Tumkur University, Tumkur 572 103, Karnataka, India, 3 Prof. C. N. R. Rao Centre for Advanced Materials, Tumkur University, Tumkur 572 103, Karnataka, India, 4 Vice Chancellor, Chattisgarh Swami Vivekanand Technical University, Bhilai, Chattisgarh, India. 5 Academic Mentor and Honorary Professor of Eminence, Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur, 572 103, Karnataka, India 2

Abstract We report green synthesis of multifunctional ZnO nanoparticles (NPs) using Artocarpus gomezianus (AG) extract as fuel by solution combustion synthesis. The formation of NPs was confirmed by Powder XRD, SEM, TEM and UV – Visible studies. The NPs were subjected for photoluminescence, photodegradative and antioxidant studies. XRD data reveals that the ZnO NPs possess wurtzite structure. UV - Visible spectrum shows absorbance maximum at 370 nm which corresponds to the energy band gap of 3.3 eV. Morphology studies indicate the highly porous nature of the NPs. PL spectra of NPs found to display very interesting blue, green and red emissions upon excitation at 325 nm. The NPs exhibit potential photocatalytic activity towards the degradation of Methylene blue (MB) dye upon exposure to Sun light and UV light. ZnO NPs found to have considerable antioxidant activity against DPPH (2, 2-diphenyl-1-picrylhydrazyl) free radicals. The study successfully demonstrates a simple and eco-friendly method for the synthesis of efficient multifunctional ZnO nanoparticles using green synthetic approach. Key-words: X-ray techniques, Luminescence, ZnO, Nano particles, Artocarpus gomezianus, Green synthesis. ____________________________________________________________________________ *Corresponding Author: E-mail: [email protected]; Phone No: +91 9886465964

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1. Introduction Nanoparticles are endowed with a variety of properties due to the presence of large surface area. ZnO nanoparticles are one of the widely studied classes of nanoparticles for various applications. ZnO is one of the best likely materials for performing photocatalytic task, as an alternative to the extensively used, comparatively expensive titanium oxide (TiO2). Although researchers recognized comparable photocatalytic mechanisms with both TiO2 and ZnO, they showed that ZnO was a better photocatalyst in degrading the herbicide triclopyr, pesticide carbetamide, pulp mill bleaching wastewater, phenol, 2-phenylphenol, blue 19, and acid red 14 [1]. This advantage of ZnO photocatalytic activity is because it contains large number of active sites, highly effective in generating hydrogen peroxide and higher reaction rates [1, 2]. Owing to its straight, wide band gap of 3.37 eV, ZnO has a wide variety of applications in optoelectronic devices such as photodetectors, light-emitting diodes and p-n homojunctions [3]. The large exciton binding energy of 60 meV [3] compared to that of Gallium Nitride (GaN) epilayers (approximately 25 meV) [4], improves the luminescence effectiveness of the emitted light even at room temperature in case of ZnO. The visible photoluminescence (PL) emission at around 2.5 eV (around 495 nm), initiated from intrinsic defects [5], creates ZnO fit for applications in vacuum fluorescent displays and field emission detectors. Several routes were used for fabrication of ZnO nanoparticles namely, pulsed laser deposition [6], chemical vapor deposition [7], molecular beam epitaxy [8], hydrothermal synthesis [9], sputtering [10] and oxidation of metallic zinc powder [11-12] for numerous applications. Nano particulate form improves the photocatalytic activity due to its vast surface area and the occurrence of vacancies and un-coordinated particles at the edges and corners. The photocatalytic activity can also be improved by band gap engineering due to quantum

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confinement effect [13-15]. A well controlled synthesis is desired for the economical usage of ZnO in catalytic applications such as aquatic treatment and other environmental claims. Herein, we are reporting, for the first time, a direct and simple method for the green synthesis of nanocrystalline ZnO via combustible method using fruit extract of Artocarpus gomezianus (AG) as fuel which belongs to the family of Artocarpus. It is well known traditionally for treatment against inflammation, malaria, diarrhoea, diabetes and tapeworm infection. It is widely distributed in southern India in the Western Ghats-South, Central and Maharashtra, Sahyadris and Sri Lanka [16]. Artocarpus species are found to be rich in phenolic compounds including flavonoids, stibenoids, aryl benzofurons and jacalin, a lectin [17]. These phytochemical constituents with highly reducing properties may act as good fuel during solution combustion synthesis that reduce metal nitrates to form useful metal oxide nanoparticles.

Moreover, there are no reports till date

exemplifying the use of Artocarpus gomezeanus towards the synthesis of nanoparticles. Therefore, an effort has been made for the first time to exploit naturally occurring fruit extract of AG as a combustible fuel by green synthetic approach for the synthesis of multifunctional ZnO NPs. This process involves a self sustained reaction in homogeneous solution of Zinc nitrate as an oxidizer and fruit extract of AG as fuel.

2. Materials and Methods 1, 1 - Diphenyl-2-picrylhydrazyl was procured from Sigma-Aldrich India Company. Ascrobic acid, Gallic acid, Vanillin, Phloroglucinol, Methylene blue, Zn(NO3)3.6H2O and Methanol were from S. D. Fine Chemicals Company. All other solvents were of AR grade and distilled before

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use. Distilled water was employed for all the experiments. The plant material was shade dried and powdered into 100 mesh size and stored at room temperature in an airtight container. 2.1 Preparation of the extract About 1:10 proportion of AG fruits powder to solvent was taken in a round bottomed flask. Extraction was carried out at boiling temperature of water with a reflux arrangement for about 3 hours with constant stirring. The extract was filtered and centrifuged to remove any suspended particles and then dried using roto evaporator. The prepared extract was stored in air tight bottles. 2.2 Polyphenols Assay Folin Ciocalteu reagent (FC reagent) (0.1 N) was prepared by diluting commercially available FC reagent (1:20) with distilled water to get the required concentration. Sodium carbonate (7.5%) was prepared by dissolving 7.5 gm of sodium carbonate in 100 ml of de-ionized water. Gallic acid (standard) stock I (conc. 0.1 mg/ml) was prepared by dissolving 1 mg of Gallic acid in 10 ml with 50% Methanol. For making standard graph of Gallic acid, concentration range of 2 - 20 µg/ml was used. The assay was carried out by using Singleton and Rossi method [18]. In a typical process, 1000 µl of FC reagent was added to 200 µl of 50% methanol/standard/test sample with various concentrations, mixed and incubated at RT for 5 min. followed by addition of 800 µl of 7.5% sodium carbonate solution. The resulting solution was mixed and incubated at 37oC for 30 min. The absorbance was recorded at 750 nm against blank using spectrophotometer. Color correction was given with the same concentration of the test sample in 50% methanol without FC reagent.

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2.3 Flavonoids Assay Vanillin, an aromatic aldehyde condenses with the flavon-3-ols and oligomers to form soluble pigments in acidic medium with an absorbance maximum at 500 nm, which can be detected by UV-Visible spectrophotometer.

Vanillin Reagent (1%) was prepared by dissolving 1 g of

crystallized vanillin in 100 ml of 70% Conc. H2SO4.Conc. H2SO4 (70%) was prepared by diluting 70 ml on Conc. H2SO4 in 100 ml de-ionized water. 10 mg of Phloroglucinol was dissolved and made up to a volume of 10 ml with 50% Methanol followed by centrifugation at 12,000 rpm for 10 min and labeled as Stock I. Stock II was prepared by diluting stock – I to a conc. of 0.1mg/ml with 50 % methanol. For making standard graph of Phloroglucinol, 1 – 10 µg ml-1 concentration range was used. The flavonoid assay was carried out using Swain and Hillis method [19]. In a typical experiment, to a 400 µl of distilled water / positive control / test sample with various concentrations, 800 µl of 1% vanillin reagent was added, mixed and incubated at RT for 15 min. The absorbance was recorded at 500 nm against blank using spectrophotometer. Color correction was given with the same concentration of the test sample in distilled water without vanillin reagent. The flavonoid content in the plant extract was measured with reference to the standard Gallic acid values. 2.4 Synthesis ZnO nanoparticles were prepared by solution combustion synthesis of AG fruit extract as fuel. Stoichiometric amount of Zn(NO3)3.6H2O was dissolved in 10 ml of plant extract, and kept in a preheated muffle furnace at 400oC [20]. A milky white powder was obtained with a vigorous reaction in about 4 minutes. The experiment was carried with 15, 20, 25, 30 ml of plant extract.

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2.5 Characterization The phase purity and the crystallanity of the powders were examined by Shimadzu-7000 powder X-ray diffractometer using Cu Kα (1.541 Å) radiation with a nickel filter. The optical properties such as UV-Visible, Photoluminescence studies were carried out by employing Thermo Scientific Spectrophotometer and Horiba Spectroflourimeter respectively. Surface morphology of the product was studied by employing Hitachi table top (7000) Scanning Electron Microscope and TEM studies were performed using TECNAIF-30. 2.6 Photocatalytic degradation of dye Photocatalytic experiments were carried out in a 150 X 75 mm batch reactor. A catalytic load of 50 mg in 100 ml of 5 ppm MB dye was prepared. The slurry containing dye solution & catalyst was placed in the reactor and stirred magnetically for agitation with instantaneous exposure to Sun light or UV light. Known volume or the slurry was withdrawn at various intervals of time such as 0, 30, 60, 90, 120 minutes. Then they were centrifuged to get rid off any interference due to the presence of catalyst and absorbance was measured using spectrophotometer at 663 nm to assess the rate of degradation. The % degradation was calculated using the relation %     =

 −   100 

Where Ci & Cf are the initial and final concentrations of the dye. The same procedure was repeated for different catalytic load of samples (100, 150 and 200 mg) and at different concentration of dye (10, 15 and 20 ppm) keeping the concentration of catalyst constant and also at different pH (viz., 2, 4, 6, 8, 10, 12) and the % degradation was determined [20].

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2.7 Antioxidant activity Antioxidant activity was carried out by DPPH assay using modified method of Brand-Williams [21]. DPPH (oxidized form) is a stable free radical with purple color. In the presence of an antioxidant which can donate an electron to DPPH radical decays, and the change in absorbance at 520 nm is followed which can be measured spectrophotometrically. Exactly 39.4 mg of DPPH was dissolved in 100 ml of methanol to get 0.14 mM concentration of DPPH in the assay. Ascorbic acid standard stock I (conc. 200 µg/ml) was prepared by dissolving 2 mg of Ascorbic acid and made up to a volume of 10 ml with de-ionised water. For making standard graph of ascorbic acid 2, 4, 6, 8, 10 µg/ml concentration range was used. In brief, to an 860 µl of 50% methanol/ascorbic acid/test sample with various concentrations, 140 µl of 1 mM DPPH was added, mixed and incubated at 37 oC for 30 min. The absorbance was measured at 520 nm against 50 % methanol blank using spectrophotometer.

A control sample was maintained

without addition of the test sample. The antioxidant activity was measured with reference to the standard ascorbic acid absorbance values. The actual absorbance was taken as the absorbance difference of the control and the test sample and IC50 value was determined.

3. Results and Discussion Plants are usually found to contain significant amounts of bioactive secondary metabolites such as polyphenols. These classes of compounds act as good reducing agents during the solution combustion synthesis of nanoparticles, consequently, they act as fuels. The water extract of Artocarpus gomezianus was subjected for the determination of polyphenol content. It was found that the extract contains 17.16 % of polyphenols. Also, the flavonoids, which form another important class of secondary metabolites, may also act as good source of reducing 7

agents. Our results indicate that the extract was found to possess 22% of flavonoids. These results suggest that the extract is a very good source of bioactive secondary metabolites which could efficiently perform the role of reducing agents during the synthesis of nanoparticles.

30ml

Intensity (au)

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(103) (200) (112)

(102)

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2θ Fig. 1. PXRD Pattern of ZnO NPs prepared with different concentrations of AG extract

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β cosθ

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4sinθ Fig. 2. W-H plots of ZnO NPs prepared with different concentrations of AG extract

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Micro strain (x10-4)

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47

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1.1631

51

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5

42

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64

Sample

By W-H plots (nm)

ε

Table 1. Particle sizes, strain, dislocation density, lattice pacing and micro strain of ZnO NPs prepared with different concentrations of AG extract

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Figure 1 shows XRD patterns of ZnO nanoparticles synthesized. The experiments were carried out employing Shimadzu-7000 Powder X-Ray diffractometer with Cu-Kα (λ= 1.54 Å) radiation. The characteristic patterns are corresponding to the diffraction patterns of hexagonal wurtzite phase of ZnO. No other peaks appeared in the patterns. The cell constants were observed to be a = 3.25 A˚ and c = 5.21 A˚ and all the peaks were well matched with the JCPDS card No. 361451. The sharp diffraction peaks reveal the crystallinity of the as prepared material. The area under the diffraction peaks increase with increase in the ratio of fuel and Zinc nitrate. This increase in area under the peak results in crystallite sizes decrease below 20 nm. The particle size was calculated using the Debye Sherrer formula and strain produced in the as synthesized material is calculated by plotting W-H plots.

=

. 

-------------------------- (1)

Where D is the crystallite size, λ is the wavelength of X-rays, β is the full width half maximum, θ is the Braggs diffraction angle, the average crystallite sizes were calculated are around 11.53 nm. A graph is plotted (Fig. 2) fuel concentration v/s particle sizes by Debye Sherrer formula and W-H plots. It is clear from the graphs that particle size decreases and then increases followed by steep decrease with increase in fuel concentration. Dislocation is a crystallographic defect or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. Where Dislocation densities are calculated using the formula δ = 1/D2 ……………………..(2).

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Where D is the crystallite size δ is the dislocation density, and the number of unit cells was calculated using the formula (table 1). n=π(4/3)x(D/2)3x(1/V) ………………… (3) Figure 3 shows the room temperature UV- Visible spectra of the ZnO nanoparticles prepared with various concentrations of the plant extract. The maximum absorbance was observed at 370 nm, corresponding to a band gap of 3.3 eV. Quantum size effects on electronic energy bands of semiconductors become more prominent when the size of the nano crystallites is less than the bulk excitation Bohr radius. Columbic interactions between holes and electrons play a crucial role in nano sized solids. The quantum confinement of charge carriers modifies valence and conduction bands of semiconductors.

Absorbance (a.u)

370 nm

10 15 20 25 30

mL mL mL mL mL

1

400

500

600

Wavelength (nm)

Fig. 3. UV - Visible Spectrum of ZnO NPs prepared with different concentrations of AG Extract

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Fig. 4. SEM image of ZnO NPs synthesized using water extract of Artocarpus gomezianus fruit Figure 4 shows the SEM micro graphs of as prepared ZnO Nano particles it is observed that almost spherical in nature further, the particles are agglomerated to form foam like bunch of particles. The agglomeration could be induced by densification resulting in the narrow space between particles. When gas is escaping with high pressure, pores are formed with the

Fig. 5. TEM image of ZnO NPs synthesized using water extract of Artocarpus gomezianus fruit

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simultaneous formation of small particles. The morphology of the powders reflects the inherent nature of the combustion process.

The observations of the SEM studies crystallite size

PL Intensity (au)

determination calculations were supported by TEM analysis (Fig. 5).

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Fig. 6. PL spectra for ZnO NPs prepared with different concentrations of AG Extract Fig. 6 shows room temperature Photoluminescence (PL) spectra of as prepared ZnO NPs. The spectra have four emission peaks centered at 378, 481, 546 and 650 nm when the material was excited of 325 nm. The spectra shows characteristic blue band edge emission at 378 nm and the peaks was found to be less intense and broad. The deep-level emission was observed due to oxygen deficiencies and zinc interstices and structural defects in the ZnO Nps. The major edge was at 546 nm corresponds to the green emission and second major edge was at 481 nm which 13

corresponds to the blue emission which is due to the quantum confinement effect. The third major emission edge was observed at 650 nm which is pure red emission. Thus, the emission resulted from the recombination of a photo-generated hole with a singly ionized charge state of the specific defects (oxygen vacancies and zinc interstices). Various concentrations of methylene blue such as 5, 10, 15, 20 ppm/100 ml of reaction mixture were tested against fixed catalytic load of 50 mg. Then reaction mixture was exposed to UV and Sun light. In both the cases of exposure, the photocatalytic activity remains almost same. It was observed that 5 ppm/100 ml solution was degraded very effectively by ZnO nanoparticles having a catalytic load of 50 mg/100 ml. With the increase in the concentration of dye (say 10, 15, 20 ppm) then the degradative activity decreases gradually. However it is not very appreciable. Therefore 5 ppm/100 ml of dye would be optimum for further studies.

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Fig. 7. Percentage degradation of methylene blue with different concentration of dye under a) UV light b) Sun light The amount of catalyst used is directly related to number of active sites in the reaction mixture. The catalytic load also affects the penetration of light in to the reaction mixture. By keeping the

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concentration of dye constant (5 ppm) & varying the catalytic load with increasing order such as 50, 100, 150 and 200 mg/100 ml were tested against 5 ppm /100 ml concentration of dye. From the Fig. 7 and 8, it can be concluded that with the increase in catalytic load, the degradation increases. But this increase in degradation was not very significant with increase in catalytic load. Therefore the lowest catalytic load of 50 mg/100 ml of reaction mixture was considered for further studies.

30

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50 mg 100 mg 150 mg 200 mg

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Fig. 8. Percentage degradation of methylene blue with different concentration of catalytic load under a) UV light b) Sun light The effect of pH on the degradation of dye was assessed by keeping the catalytic load (50 mg) and dye concentration (5 ppm/ 100 ml) constant and varying pH in the range of 2 – 12 (Fig. 9). It was observed that the degradation was quite low at lower pH. However the degradation was found to be very high in alkaline pH. This degradation may also be attributed to instability of the dye in alkaline pH range [22].

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Fig. 9. Percentage degradation of methylene blue at different pH under a) UV light b) Sun light DPPH, a stable free radical with a characteristic absorption at 517 – 520 nm, was used to study the radical scavenging effects. The decrease in absorption is taken as a measure of the extent of radical scavenging.

% Inhibition

50

IC50of ZnO = 10.8 mg/mL

0 0.0

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Fig. 10. DPPH Free radical Scavenging activity of ZnO Nanoparticles. The radical-scavenging activity (RSA) values were expressed as the ratio percentage of sample absorbance decrease and the absorbance of DPPH˙ solution in the absence of extract at 520 nm 16

(Fig. 10). The ZnO nanoparticles were proved to be inhibiting the DPPH free radical scavenging activity with IC50 value of 10.8 mg/ml. Conclusion We have developed greener approach for Zinc oxide Nanoparticles synthesis. The synthesis method is faster, economical & greener since this avoids multiple reaction steps conventional energy sources & harmful chemicals. This preparation of Zinc oxide nanoparticles using Artocarpus gomezianus is eco-friendly & can be an effective substitute for the large scale synthesis of ZnO nanoparticles. XRD studies shows that the ZnO nanoparticles synthesized has a wurtzite structure. Electron microscope studies indicate highly porous nature of the nanoparticles. UV- Visible studies show that the absorbance value is 370 nm which corresponds to the energy band gap of 3.3 eV. PL spectra show the emission of prepared ZnO nanoparticles has good photocatalytic activity towards the photodegradation of methylene blue. ZnO Nanoparticles found to exhibit antioxidant property. The study successfully demonstrates facile, economical and ecofriendly method of synthesis of multifunctional ZnO nanoparticles.

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Research Highlights •

ZnO NPs synthesized using extract of Artocarpus gomezianus fruits

•

PXRD, SEM and UV – Visible studies confirm the formation of Nps

•

PL spectra display blue, green and red emissions upon excitation at 325 nm

•

Nps exhibit excellent photocatalytic activity

•

NPs show good antioxidant activity against DPPH radicals

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GRAPHICAL ABSTRACT

PL Intensity (au)

ZnO-Ag-2(1)

http://www.biotik .org/india/species/a/artogoze/artogoze_12.jpg

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