Green, Nonchemical Route for the Synthesis of ZnO Superstructures

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Green, non chemical route for the synthesis of ZnO superstructures, Evaluation of its applications towards Photocatalysis, Photoluminescence and Bio-sensing - Udayabhanu, G. Nagaraju, H. Nagabhushana, R. B. Basavaraj, G. K. Raghu, D. Suresh, H. Rajanaika, and S. C. Sharma Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00936 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Green, non-chemical route for the synthesis of ZnO superstructures, Evaluation of its applications towards Photocatalysis, Photoluminescence and Bio-sensing Udayabhanua, G. Nagarajua*, H. Nagabhushanab R. B. Basavarajb, G. K. Raghua, D. Sureshc, H. Rajanaikad and S. C. Sharmae. a b

Dept. of Chemistry, Siddaganga Institute of Technology, Tumakuru, India

Prof. CNR Rao Centre for Advanced Materials, Tumkur University, Tumakuru, India c

Dept. of Studies and Research in Chemistry, Tumkur University, Tumakuru, India d

e

Dept. of Studies and Research in EVS, Tumkur University, Tumakuru, India

Dept. of Mechanical Engineering, Dayananda Sagar University, Tumakuru, India

Abstract: In the present work, we have developed a novel, eco-friendly method for the synthesis of ZnO superstructures for the first time through the thermal decomposition of zinc nitrate precursor without using any fuel. The synthesised materials were thoroughly characterized using of various analytical tools such as XRD, FTIR, UV-Vis, SEM & TEM. Further it has been used as a Photocatalyst for the degradation of one of the environmental pollutants such as Methylene Blue and also as bio-sensor towards the detection of Dopamine (DA) at trace level. The as synthesised ZnO Nanoparticles (NPs) showed superior catalytic activity towards the degradation of methylene blue dye with high degree of recyclability with yellow light emitting Photoluminescence. The developed sensor showed a linear range for dopamine upto 300 µM with a detection limit of 1 µM with reproduce results over periods of several months without any deviation in its electrochemical performance. Key words: Super structures, Photocatalyst, Methylene blue, Dopamine and Zinc Oxide Corresponding Email address: [email protected] 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 40

# Dedicated to 109th Birthday (born April 1, 1907) of Dr. Sree Sree Sree Shivakumara Mahaswamiji, Siddaganga Mutt, Tumakuru, Karnataka, India. 1. Introduction: Metal oxide nanoparticles have gained a significant attention in a scientific community due to its extensive range applications from chemistry/physics to biology/medicine to electronics worldwide. The possibility to establish these applications are due to its unique catalytic, electronic properties and noble performance over their bulk counterparts [1]. The electrochemical as well as catalytic properties of these nanoparticles are associated with their reduced size, large surface to volume ratio and availability of specific surface binding sites [2]. In this direction, numerous nobel metal nanoparticles including Gold, Silver, Platinum, palladium etc. have been extensively used to serve the catalytic and electrocatalytic applications. These metals showed significant achievements in the respective fields but offered high synthetic cost [3]. Hence towards to minimize the economy efforts have been made towards to replace these high cost nobel metal with low cost metal oxides like ZnO, CuO, MgO, NiO, TiO2 etc. which inturn meet all necessary requirements as that of cost Nobel metals in regard to electrochemical and catalytic properties [4]. Among these aforesaid oxide based materials, Zinc oxide nanostructures are considered as a robust candidate for constructing platforms for various multidimensional applications in the areas of photo catalytic degradation of environmental pollutants in particular organic dyes, developing optical devices, LED as well as in the sensor technology etc,. These applications of ZnO are owing to its exceptional features including low cost, availability, easy preparation, high quantum efficiency, large bandgap, massive charge carrier recombination, optical transparency, high surface area, electrochemical activity, facile electron shuttling ability during photocatalytic reactions. 2 ACS Paragon Plus Environment

Page 3 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Zinc oxide nanostructures have been widely synthesized through various approaches including

electrodeposition,

hydrothermal,

solvothermal,

ionothermal,

co-precipitation,

microwave, sonochemical physical deposition methods etc,. [5-12]. However these methods results ZnO in nano dimensions but associated with high temperature treatments, long preparation time, use of higher end equipments, use of surfactants, oxidants, high toxic solvents etc,. Hence there is a pressing need for the development of new methodology for the synthesis of ZnO nanostructures which consumes less time in terms of minutes without using any other additives. As a result of this, in the present work we aimed to synthesizing ZnO nanostructures through the thermal decomposition of zinc nitrate hexahydrate solution in an open atmosphere for 10 min. and then in a muffle furnace at 500 °C for 10 minutes which inturn result in the formation of ultra pure light yellow colored ZnO NPs. Usage of synthetic dyes in our daily life is continuously increasing and they are extensively coming out in the research area of carcinogenic dye removal in waste water treatments. Many conventional methods like adsorption foam floatation, membranes and coagulation have been employed for the elimination of dye from waste waters [13-17]. Among these methods, semiconductor assisted photocatalytic degradation is one of the best method for the decomposition of organic wastes in water treatment [18-19]. ZnO is the most stable photo catalyst used for the photo assisted catalytic degradation of organic contaminants mainly due to its wide band gap of 3.37 eV [20]. Dopamine (DA) is an important neurotransmitter which plays a vital role in the central nervous, hormonal and cardiovascular systems [21]. Neurotransmitters (NTs) are chemical messengers that transmit a message from one neuron to the next. As a result of this, the dysfunction of the dopaminergic system in the central nervous system (CNS) has been related to neurological disorders such as schizophrenia and Parkinson’s disease [22]. 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 40

Therefore it is adequate to quantify dopamine at trace concentration levels. Till today, the development of electrochemical sensing interface by metal oxides with tunable properties towards particular target analyte is a challenging task. In view of this, in the present work we have used ZnO NPs as a electrocatalyst and subsequently applied in the quantification of biomolecule by taking dopamine as a standard probe. Further it has also been focused as a catalyst towards to degrade organic environmental pollutant like Methylene blue (MB) and LED applications too. 2. Experimental details 2.1. Materials used All chemicals used were of analytical grade. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and Methylene blue dye were purchased from sd fine chemicals limited Company, Mumbai, India and used without further purification. 2.2. Synthesis of self-assembled ZnO super structures: Self-assembled hexagonal pyramids of ZnO nanostructured particles were synthesized by a simple and cost effective thermal decomposition of zinc nitrate hexahydrate solution (Scheme1). Briefly, 2 g of zinc nitrate hexahydrate crystals were taken in a conical flask and allowed at ambient conditions to an extent of 10 minutes. Under these conditions, the zinc nitrate hexahydrate crystals undergo endothermic reaction with the absorption of 0.2805 g of moisture (H2O) from the environment and subsequently solution is formed. Then the resulting zinc nitrate solution was kept in a muffle furnace at 180 °C, 250 °C for 2 Hour and 500 °C for 10 minutes to yield ultra pure light yellow colored of ZnO NPs. The main factors which influence the formation of ZnO nanostructures have been listed in table 1.

4 ACS Paragon Plus Environment

Page 5 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Characterization techniques: The samples were characterized using Shimadzu-7000 X-ray diffractometer with monochromatized Cu Kα radiation. Morphology of the products examined using Hitachi-7000 and JEOL JSM-6490LV Scanning Electron Microscope (SEM) and JEOL 3010 Transmission Electron Microscope (TEM). UV–Vis diffused reflectance spectra were analyzed by perkinelmer Lambda-35 spectrophotometer. Photoluminescence (PL) spectra were recorded using Horiba Fluorolog -3 Spectroflourimeter at room temperature.

2.4. Photocatalytic activity: The photocatalytic activity of the as synthesized ZnO NPs were evaluated by considering the degradation of Methylene blue (MB) in an aqueous solution at room temperature using a 120 W mercury lamp as radiance source. In a typical procedure, 50-200 mg of photocatalyst was added to 100 mL of methylene blue solutions of different concentrations (5-20 ppm) in a 150 x 75 mm sized batch reactor, maintained the gap between the radiation source and the reactor was 18 cm. The solution was continuously stirred in the dark chamber for about 30 minutes to ensure the complete organization of an adsorption as well as the desorption equilibrium between the MB dye and photocatalyst. Then, 2 mL volume of the suspension was withdrawn from the above solution at a regular sequence of 30 minutes intervals. After removal of the photo catalyst from the solution by centrifugation, the concentration of left over aqueous solution was monitored using UV-Vis spectrophotometer at a fixed wavelength of 664 nm and the % degradation of the MB has been calculated based on the Beer Lambert law using the formula. % of degradation = Ci - Cf X 100...................... (i) Ci



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 40

Where Ci and Cf are the initial and final concentrations of dye solution in ppm respectively. The photocatalytic experiment was repeated by changing various parameters such as catalytic load, concentration of dye, catalyst recycling and pH of the solution. 2.5. Procedure for the modification of electrode and electrochemical study: Prior to modification, the surface of rigid glassy carbon (GC) electrode was cleaned by the uniform and unidirectional hand rubbing of its surface against micro pads having alumina particles of different grain sizes say 1.0, 0.3 and 0.05 micron respectively. Then the surface is washed thoroughly with deionized water and sonicate for 5 minutes in order to remove any adhered alumina particles. The cleaned glassy carbon electrode surface was modified with aqueous colloidal solution of ZnO NPs (1 mg/ml) through drop casting method and air dried for overnight. Cyclic voltammetry has been used to identify the electrocatalytic property of the ZnO nanoparticles. It has been carried out using CH Instrument with a single electrochemical cell setup of 10 mL volume consisting of ZnO NPs modified glassy carbon electrode as a working electrode, silver - silver chloride electrode as a reference electrode and a platinum wire as a counter electrode. The voltammograms were recorded at bare as well as ZnO NPs modified glassy carbon electrode in presence of quiescent solution of known concentrations of dopamine analyte in the potential window from - 1.0 to 1.0 V in a phosphate buffer solution of pH 7 with a scan rate of 50 mV/s. Amperometry has been carried out by the successive addition of 50 µM DA into a continuously stirred phosphate buffer solution of pH 7 at an applied potential of 0.3 V with regular intervals of time.


Page 7 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and Discussion: The crystal structure, size, surface and applicability of the ZnO NPs has been studied and discussed in detail through various physico-chemical techniques like XRD, IR, UV-Vis DRS, SEM, TEM, PL and Cyclic Volatametry (CV). 3.1 Powder X - ray diffraction (PXRD): The PXRD pattern of as synthesized ZnO nanocrystals is shown in figure 1. This XRD pattern has been used in calculating crystallite size and strain of the sample (Table 2) The pattern showed significant characteristic diffraction peaks for ZnO and is in good agreement and with hexagonal zincite (wurtzite) structure [23]. All the peaks of ZnO are well matches with the standard card (JCPDS no. 5-664) and Zinc nitrate hydroxide peaks matches with the (JCPDS no. 18-1486). This demonstrates that the ZnO has been well crystallized and there are no extra peaks were observed in the samples, which inturn indicates that the synthesized material is quite free from any kind of impurities. The crystallite size of the synthesized material was calculated using Debye - Scherer equation (ii) with respect to highly intensified peak.

D=

0.89 λ .......... .......... .......... (.ii ) β cos θ

Where, D is crystalline size, λ is wavelength of X-ray, β is full-width at half-maximum and θ is the angle of diffraction. From equation (ii), the average crystallite size of the synthesized ZnO is tabulated in table 2. In addition to this, Williamson and Hall (W-H) plot have also been used in order to estimate the grain size and strain using the equation (iii).

β cos θ =

kλ + 4ε sin θ .......... .......... .......... .(.iii ) D



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Where

Page 8 of 40

kλ represent the intercept and ε represent the micro strain in the sample. The D

above equation represents a straight line between ‘4sinθ’ (x-axis) and ‘βcosθ’ (y-axis). The slope of the line gives the strain and intercept of this line on y-axis gives grain size (D). Figure 2 represents the typical W - H plot of ZnO sample from which the grain size and strain can be calculated and were found to be 75 nm and 37 nm respectively. The XRD pattern has been analysed employing Rietveld technique with the help of the Fullprof Suite program using the

P63mc space group. The X-ray diffraction patterns along with Rietveld refined data has been shown in the Figure 3 (a & b). It could be seen that the profiles for observed and calculated ones are matching to each other and all the experimental peaks are allowed Bragg 2θ positions for

P63mc space group. The oxygen positions have been taken as free parameters during the refinement. The atomic fractional positions for zinc have been fixed. Isothermal parameters and occupancies are fixed for both zinc and oxygen. Other parameters such as lattice constants, scale factors and shape parameters have been taken as free parameters during the fitting. The global parameters such as background and scale factors were refined in the first step of refinement. In the next step, the structural parameters such as lattice parameters, profile shape, preferred orientation, and atomic coordinates were refined in sequence. Background has been fitted with sixth order polynomial while the peak shapes have been described by pseudo-voigt profiles. The Reliability factors (R factors) Rp, Rwp, RBragg, RF and χ2 obtained from refinement are listed in the Table 3. The value of goodness of fit or χ2 comes out to be equal to 0.654 (for ZnO at 250 ºC) and 0.793 (for ZnO at 500 ºC), which may be considered to be very good for estimations. The profile fitting procedure adopted was minimizing the χ2 function. The packing diagrams for the ZnO samples are drawn using DIAMOND software and are shown in Fig 3 (c &d).


Page 9 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Diffuse Reflectance spectra (DRS) The diffuse reflectance spectra (DRS) of ZnO NPs were shown in Fig.4 a. The spectra displays a strong band in the wavelength region ~ 450 nm was ascribed due to absorption of the host lattice. The Kubelka–Munk (K-M) theory was utilized to determine the energy band gap (Eg) synthesized ZnO NPs from DR spectra. The intercept of the tangents to the plots of

[F(R∞ ) hν ]1/ 2 versus photon energy hν was shown in Fig.5b. The Kubelka–Munk function F (R∞) and photon energy ( hν ) was calculated by following equations [24]. 2 ( 1 − R∞ ) -------------------- (iv) F (R∞ ) =

2R∞

hν =

1240

λ

-------------------- (v)

Where R∞; reflection coefficient of the sample, λ; the absorption wavelength. The enrgy gap was found to be around ~ 3.09 eV which is in good agreement with the earlier reported values. The variation in Eg values were mainly attributed to degree of structural order and disorder in the matrix as well as change the distribution of energy levels within in the band gap [25].

3.3 Fourier Transform Infrared Spectra (FTIR) FTIR spectra of the products prepared at 180, 250 and 500°C recorded from 350–4000 cm1

as shown in figure 5. In case of ZnO synthesized at 250°C as well as 500 °C (Fig. 6a, 6b), a

strong absorption peak was found at 445 cm-1and is due to Zn - O stretching vibrations inturn confirms that the product obtained during the synthetic protocol is zinc oxide [26]. FTIR spectrum of Zinc nitrate hydroxide (Fig. 6c) there is no appearance of strong peak at 445 cm-1, 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 40

instead of that it shown an another two broad peaks at 3524 and 1427 cm-1 which are due to the stretching of hydroxyl group of H2O molecules and N-O functionalities [27]. The absence of peak at 445 cm-1 case of ZnO and Zinc nitrate hydroxide respectively, clearly confirms the formation of zinc oxide NPs [28]. From FTIR spectra clearly shows that 180 C is not sufficient energy to undergo thermal decomposition to produce ZnO.

3.4 Scanning Electron Microscopy (SEM) The scanning electron microscopic images of zinc nitrate hydroxide and self-aggregated ZnO hexagonal pyramids are depicted in figure 6. Figure 6(a1, a2 & a3) shows the flaky like morphologies of zinc nitrate hydroxide which were synthesized at 180°C. From Fig. 6(b1, b2 & b3), it clearly shows the several aggregates of irregularly arranged ZnO hexagonal pyramids synthesized by mixing water to the zinc nitrate hexahydrate at 500 °C. Even though water evopartes at 100 ºC but it affects the surface morphology, for that we got flower shaped morphology (Fig 6 (b3)). The typical size of the aggregates of ZnO pyramids is in µm. The large ZnO hexagonal pyramids are in the range of 5-20 µm and built from a large number of hexagonally arranged pyramids. It is well known that the thermal decomposition of zinc nitrate hexahydrate leads ultimately to ZnO. Thermal decomposition of Zn (NO3)2 .6H2O can be divided into two phases: dehydration and pyrolysis [29-31]. The dehydration of the compound takes place when temperature is in the range of 100 to 150 ºC. At this temperature the chemical composition may be represented by Equation (vi). Zn (NO3)2.6H2O → Zn (NO3)2 + 6H2O.............. (vi) Above the temperature of 150 ºC, there is a conversion of zinc nitrate to zinc hydroxide, at 180°C we found that there is a mixture of zinc nitrate hydroxide (Fig. 1c). At 240±10 ºC, Zinc


Page 11 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nitrate hydroxide decomposed into ZnO with the evolution of oxides of nitrogen gas. Decomposition of compound in this stage can be represented by Equation (vii). 2 Zn (NO3)2→ 2ZnO + 4 NO2 + O2................. (vii) As outcome of the thermal decomposition of zinc nitrate, breaking down the bulk solid into micrometer sized ZnO pyramids. In order to study the detailed morphology of selfaggregated hexagonal pyramids, image (c2) in Figure 6 indicates the self-aggregation of the small ZnO hexagonal pyramids. The well clear facets of the small hexagonal pyramids are visible. The typical height of the small ZnO pyramids is in the range of 5 to 10 µm and also there are number of small pyramids grown on the large pyramids. The process of formation of ZnO hexagonal pyramids has shown in the scheme 2. In addition to this, it is very clear that all the structures are fully filled, solid, hard ZnO super structures formed by the aggregation of ZnO nanoparticles, which was clearly confirmed by the TEM images (Fig. 8). Further the texture of the super structure are brittle, clear and polished like surfaces, as a result of this the dye molecules may easily adsorb and desorbed. So this crystal structure acts as a stable photocatalyst for the degradation of organic dyes. Some of the crystals are also joined together by their bases to forms a hexagonal bipyramidal structure (Fig. 6 (c3)). A large number of tests show that it is hard to grow nanoscale ZnO hexagonal pyramids when using top-down synthesis scheme. Such restrictions on the size of ZnO rely on the top-down synthesis schemes itself. The bottom-up approach begins with ions, atom or molecules with building blocks and assembles nanoscale clusters or bulk material from them. So it is quite easy to form nanoscale ZnO with the bottomup synthesis strategy [32, 33]. Sujinnapram et.al. Reported that synthesis of nanocrystalline ZnO powder by thermal decomposition of zinc nitrate hexahydrate for 400, 600 and 800 ºC, they have not tried for lower temperatures and has not provided TEM information [34]. Auffredic and 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 40

Louer given that the physico-chemical parameters such as decomposition temperature, heating rate, residual pressure and sample mass were important in determining the microstructure of the synthesized ZnO material [35]. The primary differences between the top-down and bottom-up synthesis strategies decide the differences in the morphology and size of the resultant ZnO materials. Figure 7 shows the SEM images of ZnO flowers synthesized at 250 °C. Due to the low temperature, zinc nitrate hexahydrate requires 2 h for decomposition and formation of ZnO Reason for getting different morphologies at 250 °C is because, at the initial stages, the zinc nitrate hexahydrate solution boils and forms a paste at the top layer (bottom of crucible) on the wall of the crucible, then the paste undergo thermal decomposition and forms ZnO. In this way periodically the top layers forms a ZnO flowers. Depending on the surrounding environment each layers form with different, good and attractive morphologies. If we compare the SEM images synthesized at 500°C and 250°C, all images at higher temperature (500 °C) are similar because, at this temperature all zinc nitrate solution undergo thermal decomposition at higher rate at a time and forms a similar hexagonal pyramids with a sharp edges. But in case of 250 °C, periodically it undergo slower rate thermal decomposition and forms a different shaped ZnO flowers, buds and grains without sharp edges. D. Kavyashree and co-workers have synthesized ZnO super structures using plant extracts and they explained the evolution of structures is due to the variation of organic content concentration in the plant extracts [36, 37]. But we are the first to reporting the synthesis of flower shaped ZnO at low temperature without the addition of organic content in this work. We have compare the morphology obtained with the naturally available structures. Fig. 7(a2) resembles the tulip flower, (b2) resembles the Lotus bud, (c2) represents the custard apple, (d2) resembles the dahlia flower like structure, (e1) compared with the sunflower 12 ACS Paragon Plus Environment

Page 13 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and (f1) compared with the wheat grains respectively. All these super structures obtained during thermal decomposition of zinc nitrate hexahydrate at 250 °C only. TEM and HRTEM images as shown in Figure 8 correspond to ZnO compounds obtained for different temperatures. The TEM images shows the agglomerated small particles of ZnO prepared at 250 °C and 500 °C temperatures. The high resolution TEM image shows the well defined crystal planes with an average spacing of ∼0.227 nm and ∼0.25 nm, respectively for the above mentioned two compositions. There is an increase in the interplanar spacing with more ordered superstructures of ZnO. Further, it explains that the crystallinity of the compounds is increased with an increase in the temperature.

4. Photocatalytic activity of hexagonal pyramidal ZnO NPs: Under the irradiation of light, the semiconductor absorbs the photon of energy which is higher than the band gap of semiconductors, and then it create an electrons and holes in the valence band and conduction band. If the charge carriers do not recombine, then they can migrates on the surface where the free electrons forms the reduction of oxygen and forms the peroxides and superoxide’s and created holes oxidises the water and forms the OH˙, these generated species are highly reactive and unstable, ultimately leads to the degradation of organic dyes. Photocatalytic action on dyes are enhanced by many factors ie., phase composition, crystallinity, particle size, size distribution, morphology, band gap, surface area and surface hydroxyl density of the photocatalyst [38]. Synthesized ZnO NPs were used as photocatalyst to examine the degradation of methylene blue dye under UV-light. 2 mL of aliquots sample was withdrawn at regular interval of time (30 min) end. Centrifuged the solution and absorption of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 40

the samples were recorded. By measuring change in intensity, we can calculate the rate of degradation of dye at 664 nm (λmax).

4.1. Effect of dye concentration Photocatalytic activity of ZnO NPs mainly depends on the concentration of the dye under investigation. Therefore, in order to evaluate the optimal concentration of dye towards the efficient oxidation, the experiments were conducted with constant catalytic loading (100 mg) and by altering the concentration of dye from 5 to 20 ppm under UV-light is as shown in Fig. 9. It clearly shows that with increase the concentration of dye from 5 ppm to 20 ppm there was decreases the photocatalytic degradation from 80 to 20 % and 5 ppm was the best effectual concentration with respect to degradation of dye. In general as the concentration of dye increases, more no. of dye molecules adsorbed on the surface of the ZnO NPs and therefore decreasing the rate of degradation. On the other hand the adsorbed dye molecules were not degraded

faster, because the

light intensity

and

catalyst load

is

constant, When the

concentration of dye was more, penetrating power of light is less, because of this reason generation of hydroxyl and superoxide radicals were less which indicates that the photocatalytic degradation is less at higher concentration [39].

4.2 Effect of catalytic load: Figure 10 shows the degradation spectra of methylene blue dye with varying the catalytic load (50 - 200 mg), by keeping dye concentration as constant (100 mL of 5 ppm). Figure 10 clearly showed that with increase in the catalytic load the rate of degradation of dye increases from 80 to 100% at the end of 180 min. This is because the availability of more active sites with the increase of the catalytic load [40].


Page 15 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.3 Effect of pH: To determine the optimum pH for the photodegradation of methylene blue dye, experiments were performed at different pH from 3 to 11 by keeping catalyst (100 mg) and dye concentration (5 ppm) as constant and the results are represented in Fig. 11. It clearly shows that the degradation of the MB was effective in basic medium [41-42] and highest rate of degradation is observed at pH 11. Above this pH the degradation decreases and which can be explained on the basis of zero potential charge. For ZnO the zero potential charge was found to 9.0±0.3 and above this value the surface is negatively charged due to adsorbed OH¯¯ ions. Due to the presence of many OH¯¯ ions on the surface of catalyst causes the production of OH˙ radicals and acts as primary oxidizing agents, which are accountable for the degradation of methylene blue dye [43-46].

4.4 Catalyst recycling: To estimate the stability of the photocatalyst, recycled experiments for the degradation of the methylene blue dye was performed (Fig. 12). The experiments were carried out with 100 mg catalyst and 100 mL of 5ppm dye. The degradation efficiency of methylene blue was almost same for 6 cycles. This figure clearly represents that it degrades nearly 80 % efficiency for all 6 cycles. This efficiency of degradation is mainly due to the stable hexagonal ZnO pyramids.

4.5 Mechanism: Scheme 3: Shows the schematic pictorial representation for the degradation of methylene blue using ZnO super structures.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

4.6 Detection of OH˙ radicals: During photocatalytic degradation reaction, Hydroxyl radicals are the major reactive species. The rate of formation and detection of OH˙ during the degradation reaction can be measured by a

simple,

sensitive and rapid photoluminescence

(PL)

technique

using

Coumarin as a probe molecule. OH˙ reacts with Coumarin to generate highly fluorescent compound 7-hydroxyl Coumarin at 456 nm. In this process, 130 mg of ZnO was dispersed in 50 mL of 1 mM Coumarin aqueous solution in a borosil jar. The solution mixture was allowed to 10 min for adsorption-desorption equilibrium between the ZnO, water and Coumarin previous to irradiation. This reaction was irradiated under UV-light of intensity 60 W/m2 as a light source. 2 mL aliquots were withdrawn for every 10 minute time interval and measured the photoluminescence spectra using Agilent Technologies Cary Eclipse spectrophotometer. Figure 13 clearly indicates that PL intensity at 456 nm increase linearly with increase in time and it clearly indicated that formation of OH˙ at ZnO surface was directly proportional to the irradiation time [47]. It shows that formation of OH˙ increases with increase in time. This OH˙ is responsible for the degradation of organic dye.

5. Photoluminescence (PL) studies: In semiconductors the recombination of the photo generated free charge carriers leads to photoluminescence emission spectrum. Figure 14 shows the emission excitation and spectrum of ZnO NPs recorded at room temperature. The near-band-edge (NBE) excitation peak at 397 nm 16 ACS Paragon Plus Environment

Page 17 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was recorded at an emission wavelength of 588 nm. The emission spectrum was monitored at 397 nm wavelength in pure ZnO NPs showed a broad yellow emission at 588 nm along with weak peaks at 394 and 488 nm, which indicates the existence of large number of surface defects. The broad 588 nm peak may be due to the transition between single charged oxygen vacancy and the photo excited holes in the valence band of the ZnO NPs [48]. The color clarity of any luminescent material was expressed in terms of chromaticity coordinates, called Commission International De I’Eclairage (CIE). The CIE chromaticity diagram of ZnO with water under 397 nm excitation was shown in Fig. 15. The corresponding CIE coordinates (x, y) obtained were given in inset of Fig. 16. It is cleared from the CIE diagram that pure ZnO emits broad yellow light under 397 nm excitation [49, 50].

6. Electrochemical behavior of ZnO Nanoparticles: Sensing of Dopamine In order to establish the electrochemical applicability of the as synthesized ZnO NPs, cyclic voltammetric technique has been used by taking dopamine as a model system. Figure 17 Depicts the cyclic voltammetric voltamogram of ZnO modified glassy carbon electrode in the absence of DA (a) and bare glassy carbon electrodes (b) and of ZnO modified glassy carbon electrode (c) in presence of 10 mM of dopamine in the potential window from - 1.0 to 1.0 V in a phosphate buffer solution of pH 7 with a scan rate of 50 mV/s. The voltammograms recorded at ZnO modified glassy carbon electrode in the absence of DA will not show any voltammetric signature in the selected potential window. This indicates that the ZnO has not shown any peaks for the oxidation of zinc into zinc ions. From this we can conclude that the ZnO modified glassy carbon electrode can be used as a sensor interface in order to study the electrocatalytic behavior of ZnO with respect to DA analyte molecule. Further, the cyclic voltammogram of 10 mM of DA has been recorded at bare GC in order to distinguish the electrochemistry of DA at bare as well 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

as modified electrode. The cyclic voltammogram recorded at bare GC has showed an oxidation peak at peak potential of 0.14 V with a peak current of 7.4 µA. Whereas the ZnO Modified electrode has shown an oxidation peak at a peak potential of 0.26 V with a peak current of 61 µA which inturn 6 fold more than that of obtained at bare glassy carbon electrode. These results are in good agreement with reported literatures [20]. From this it can be concluded that the use of ZnO nanoparticles synthesized through thermal decomposition method can be used as a suitable candidate in the designing of ZnO based dopamine sensors. In order to evaluate the analytical applicability of the developed sensing platform towards the determination of dopamine from various environmental samples it is adequate to establish a linear dependence of oxidation current with respect to the concentration of analyte at very low concentration ranges. As a result of this, we have carried out chronoamperometric experiments. It has been carried out by the successive addition of 50 µM of dopamine into a continuously stirred solution of phosphate buffer solution of pH 7 at an applied potential of 0.3 V. The amperogram exhibited a rapid and sensitive response to the variation of concentration of dopamine and increase of current for the oxidation of dopamine increases linearly with the increase of its concentration upto 300 µM (Fig. 18) with a detection limit of 1 µM (3 σ). Fluorescence spectroscopic technique was also used for the detection of dopamine [51]. Table 4 shows the comparison of detection limit of dopamine using ZnO nanoparticles [52-55].

7. Conclusion: In the present work we are the first reporting the synthesis of ZnO superstructures at lower temperatures through a simple thermal decomposition of zinc nitrate hexahydrate precursor without adding any fuel for combustion. Further, this method offers several advantages like low cost, ecofriendly, nontoxic, low temperature, simple and quick etc,. XRD pattern shows 18 ACS Paragon Plus Environment

Page 19 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that the ZnO crystals are hexagonal in structure, the average crystallite size as 38 nm. DRS spectrum show the band gap of 3.04 and 3.24 eV for synthesized ZnO superstructures. It is evident from SEM studies that there is uniform distribution of shape and size of ZnO at higher temperature (500ºC) and different flowers, buds and grains were obtained at low temperature (250 ºC). The photoluminescence band in the visible luminescence range resulting from the higher surface interstitial defects reduces the electrons or holes recombination and consequently increases the photocatalytic activity. The prepared ZnO NPs shows more stability and good photocatalytic activity for the photodegradation of methylene blue. Finally, the electroanalytical applicability of the as synthesized ZnO nanostructures have been evaluated by quantifying dopamine as a model system at trace (1 µM) concentration levels.

8. Acknowledgement: Authors thank DST Nanomission (Project No. SR/NM/NS-1262/2013 (G), New Delhi, Govt. of India for financial support to carry out the research work. Mr. Udayabhanu and Dr. G. Nagaraju thanks to Siddaganga Institute of Technology, Tumakuru for encouraging and providing all the research facilities.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

9. References: (1) Daowen Xiong.; Tao Fang.; Linpeng Yu.; Xiaofeng Sima.; Wentao Zhu. Science of The

Total Environment 2011, 409, 1444. (2) Agnieszka Kołodziejczak Radzimska.; Teofil Jesionowski. Materials 2014, 7, 2833. (3) Jingyi Chen.; Byungkwon Lim.; Eric P. Lee.; Younan Xia. Nanotoday 2009, 4, 81. (4) Yassine Ghandour.; Arbia ben Khelifa.; Mohamed Salah Belkhiria.; Carole Daiguebonne.; Stephane Freslon; Olivier Guillou.; Thierry Roisnel. Polyhedron 2016, 115, 1. (5) Abdel Hamid, Z.; Abdel Aal A.; Ali Shaaban.; Hassan, H. B. Surface and Coatings

Technology 2009, 203, 3692. (6) Yan Bao.; Cheng Wang.; Jian zhong Ma. Ceramics International 2016, 42, 10289. (7) Hatem Moussa.; Emilien Girot.; Kevin Mozet.; Halima Alem.; Ghouti Medjahdi.; Raphaël Schneider. Applied Catalysis B: Environmental 2016, 185, 11. (8) Tangxiang Cun.; Chengjun Dong.; Qiang Huang. Applied Surface Science 2016, 384, 73. (9) Vijayaprasath, G.; R. Murugan,; Asaithambi, S.; Sakthivel, P.; Mahalingam, T.; Hayakawa, Y.; Ravi, G. Ceramics International 2016, 42, 2836. (10) Hossein Ajamein.; Mohammad Haghighi. Energy Conversion and Management 2016, 118,

231. (11) Huawa Yu.; Huiqing Fan.; Xin Wang.; Jing Wang.; Pengfei Cheng. Optik International

Journal for Light and Electron Optics 2015, 126, 4397. (12) Pradip Sekhar Das.; Dipak Kr.; Chanda.; Arjun Dey.; Ashok, K.; Mandal.; Kajari Das Gupta.; Nitai Dey.; Anoop, K.; Mukhopadhyay. Ceramics International 2014, 40, 6365. (13) Arab, M.; Chamjangali.; Bagherian, G.; Javid, A.; Boroumand, S.; Farzaneh,

N.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2015, 150, 230. 20 ACS Paragon Plus Environment

Page 21 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Gomez, V.; Larrechi, M. S.; Callao, M. P. Chemosphere 2007, 69, 1151. (15) Lin, S. H.; Lo, C. C. Environ. Technol. 1996, 17, 841. (16) Arami, M.; Yousefi Limaee, N.; Mahmoodi, N. M. Chemosphere 2006, 65, 1999. (17) Papic, S.; Koprivanac, N.; Bozic, A. L. Color. Technol. 2000, 116, 352. (18) Suresh, D.; Nethravathi, P. C.; Udayabhanu.; Pavan Kumar, M. A.; Raja Naika, H.; Nagabhushana, H.; Sharma, S. C. Materials Science in Semiconductor Processing 2015, 40, 759. (19) Alamelu, K.; Ramasami, H.; Raja Naika, H.; Nagabhushana, H.; Ramakrishnappa, T.; Geetha, R. Balakrishna.; Nagaraju, G. Materials Characterization 2015, 99, 266. (20) Wang, N.; Sun, C.; Zhao, Y.; Zhou, S.; Chen, P.; Jiang, L. J. Mater. Chem. 2008, 18,

3909. (21) Andrea, E., Steuer.; Martina, I.; Boxler.; Lorena Stock.; Thomas Kraemer. Toxicology

Letters 2016, 243, 48. (22) Bankiewicz, K. S.; Plunkett, R. J.; Mefford, I.; Kopin, I. J.; Oldfield, E. H. Progress in

Brain Research 1990, 82, 561. (23) Su, Y. L.; Zhang, Q. Y.; Pu, C. Y.; Tang, X.; Zhao, J. J. Solid State Communi.

2015, 223, 19. (24) Morales, A. E.; Mora, E. S.; Pal, U. Revista Mexicana de Fisica S 2007, 53, 85. (25) De Mello Donega, C.; Meijerink, A.; Blasse, G. J. Phys Chem Solid. 1995, 56, 673. (26) Mitra, P.; Mondal, S.; Prog. Theor. Appl. Phys. 2013,1,17. (27) Madan, H. R.; Sharma, S. C.; Udayabhanu.; Suresh, D.; Vidya, Y. S.; Nagabhushana, H.; Rajanaik, H.; Anantharaju, K. S.; Prashanthah, S. C.; Sadananda Maiya, P. Spectrochimica Acta

Part A: Molecular and Biomolecular Spectroscopy 2016, 152, 404. (28) Umar, A.; Rahman, M. M.; Al Hajry, A.; Hahn, Y. B.; Talanta 2009, 78, 284. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

(29) Nikolic, R.; Zec, S.; Maksimovic, V.; Mentus, S. J. Therm. Anal. Cal. 2006, 86, 423. (30) Campbell, I. D. Faraday Trans. 1977, 73, 487. (31) Bao gai Zhai.; Qing lan Ma.; Yuan Ming Huang. Optoelectron. Mater. 2015, 1, 1. (32) Huang, Y. M.; Ma, Q. L.; Zhai, B. G. Mater. Chem. Phys. 2014, 147, 788. (33) Ma, Q. L.; Huang, Y. M. Mater. Lett. 2015, 148, 171. (34) Sujinnapram, S.; Termsuk, U.; Charoentam, A.; Sutthana, S. Adv. Mater. Res. 2013, 770,

68. (35) Auffredic, J. P.; Louer, D. React. 1987, 4, 105. (36) Lakshmeesha, T. R.; Sateesh, M. K.; Daruka Prasad, B.; Sharma, S. C.; Kavyashree, D.; Chandrasekhar, M.; Nagabhushana, H. Cryst. Growth Des. 2014, 14, 4068. (37) Kavyashree, D.; Shilpa, C. J.; Nagabhushana, H.; Daruka Prasad, B.; Sreelatha, G. L.; Sharma, S. C.; Ashoka, S.; Anandakumari, R.; Premkumar, H. B. ACS Sustainable Chem.

Eng. 2015, 3, 1066. (38) Talebian, N.; Nilforoushan M. R. Thin Solid Films 2010, 518, 2210. (39) Manjunath, K.; Ravishankar, T. N.; Dhanith Kumar.; Priyanka K. P.; Thomas Varghese.; Raja Naika, H.; Nagabhushana, H.; Sharma, S. C.; Dupont, J.; Ramakrishnappa, T.; Nagaraju, G. Materials Research Bulletin 2014, 57, 325. (40) Suresh, D.; Udayabhanu.; Nethravathi, P. C.; Lingaraju, K.; Rajanaika, H.; Sharma, S. C.; Nagabhushana, H. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

2015, 136, 1467. (41) Khodja, A. A.; Sehili, T.; Pilichowski, J. F.; Boule, P. J. Photochem. Photobiol. Part A:

Chem. 2001, 141, 231.


Page 23 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Anandan, S.; Vinu, A.; Lovely, K.; Gokulakrishnan, N.; Srinivasu, P.; Mori, T. J. Mol.

Cat. A Chem. 2007, 226, 149. (43) Poulios, I.; Tsachpinis, I. J. Chem. Technol. Biotech. 1999, 74, 349. (44) Liqiang, J.; Xiaojun, S.; Baifu, X.; Baiqi, W.; Weimin, C.; Honggang, F. J. Solid

State Chem. 2004, 177, 3375. (45) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. Rev. 1993, 22, 417. (46) Rengaraj, S.; Li, Z. X. Int. Environ. Pollut. 2006, 27, 20. (47) Xiang, Q.; Yu, J.; Wong, P. K. J. Colloid Interface Sci. 2011, 357, 163. (48) Li, H.; Cui, Y.; Hong, W. Appl. Surf. Sci. 2013, 264, 581. (49) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J.

Appl. Phys. 1996, 79, 7983. (50) Devaraja, P. B.; Avadhani, D. N.; Prashantha, S. C.; Nagabhushana, H.; Sharma, S. C.; Nagabhushana, B. M.; Nagaswarupa H. P. Spectrochimica Acta Part A: Molecular and

Biomolecular Spectroscopy 2014, 118, 847. (51) Xiaodong Zhang.; Xiaokai Chen.; Siqi Kai.; Hong-Yin Wang.; Jingjing Yang.; Fu-Gen Wu.; Zhan Chen. Anal. Chem. 2015, 87, 3360. (52) Reddy Yadav, L. S.; Archana, B.; Lingaraju, K.; Kavitha, C.; Suresh, D.; Nagabhushana, H.; Nagaraju, G. International Journal of Nanoscience 2016, 15, 2. (53) Manjunath, K.; Lingaraju, K.; Kumar, D.; Nagabhushan, H.; Samrat, D.; Reddy, V.; Dupont, J.; Ramakrishnappa, T.; Nagaraju, G.; Int. Journal of Nanoscience 2015. 14, 3. (54) Cao Xia.; Wang Ning.; Wang Long.; Guo Lin. Sensors and Actuators B 2010. 147, 629.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

Figure Captions: Figure 1. PXRD pattern of (a) ZnO NPs synthesized at 500°C, (b) ZnO NPs synthesized at 250°C, (c) Zinc nitrate hydroxide synthesized at 180 ºC

Figure 2. W-H plots of ZnO NPs Figure 3. (a) Rietveld refinement spectra and (b) Packing diagram of ZnO synthesized at 250 ºC. (c) Rietveld refinement spectra and (d) Packing diagram of ZnO synthesized at 500 ºC.

Figure 4. Diffuse reflectance spectra (a) and Direct band gap energy of ZnO (b). Figure 5. FTIR spectrum of (a) ZnO NPs synthesized at 500°C, (b) ZnO NPs synthesized at 250°C, (c) Zinc nitrate hydroxide synthesized at 180 ºC.

Figure 6. SEM images of (a1, a2, a3) Zinc nitrate hydroxide synthesized at 180 °C. (b1, b2, b3) ZnO crystals synthesized at 500 °C with addition of water. (c1, c2, c3) ZnO hexagonal pyramids synthesized at 500 °C with absorption of moisture.

Figure 7. SEM images of ZnO super structures synthesized at 250 °C Figure 8. TEM images of synthesized ZnO nanoparticles at 250°C (a1, a2, a3 (HRTEM)) and 500°C (b1, b2, b3(HRTEM))

Figure 9. Degradation spectra of methylene blue dye with 100 mg of ZnO NPs and varying the dye concentration

Figure 10. Degradation spectra of methylene blue dye with 5 ppm of dye and varying the catalytic load

Figure 11. Degradation spectra of methylene blue dye with varied pH Figure 12. Recycling of 100 mg catalyst for the degradation of 100 mL of 5 ppm dye Figure 13. Concentration of Hydroxyl radicals against irradiation time Figure 14. Excitation spectrum of ZnO NPs: emission centered at 588 nm 24 ACS Paragon Plus Environment

Page 25 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 15. Emission spectrum of ZnO NPs: excited at 397 nm Figure 16. CIE diagram for ZnO NPs Figure 17. Cyclic voltammetric profile of ZnO modified GC in the absence of DA (a), GC electrode in presence of 10 mM of DA (b) and ZnO modified GCE (c) in presence of 10 mM of dopamine in a phosphate buffer solution of pH 7.0 with a scan rate of 50 mV/s

Figure 18. Amperogram of ZnO modified glassy carbon electrode in presence of 50, 100, 150, 200, 250 and 300 µM of dopamine with a constant applied potential of 0.3 V. Inset Calibration plot

Scheme Captions: Scheme 1: Schematic representation for the synthesis of ZnO NPs Scheme 2: Schematic mechanism for the growth of self aggregated ZnO hexagonal pyramids Scheme 3: Schematic mechanism for the degradation of methylene blue dye

Table captions: Table 1. Detailed Reaction Conditions and the Obtained Products Table 2. Estimated crystallite size and strain of ZnO NPs synthesized at different temperature Table 3: Refinement parameters of ZnO synthesized at 250 and 500 oC Table 4. Comparison of detection limit of dopamine.



Figures:

Intensity (a.u)

5 0 0 oC

( c )

Intensity (a.u)

2 5 0 oC

32

34

36

38

2 θ (d e g re e )

1 0

2 0

3 0

4 0

2 θ

5 0

6 0

7 0

(202)

( a )

(004)

(112)

(103)

(110)

(102)

(002)

(110)

(101)

( b )

8 0

( d e g r e e )

Figure 1. PXRD pattern of (a) ZnO NPs synthesized at 500°C, (b) ZnO NPs Synthesized at 250°C, (c) Zinc nitrate hydroxide synthesized at 180 ºC. 5 0 0 0C

βcosθ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

2 5 0 0C

1 .0

1.5

4 sin θ

2 .0

2.5

Figure 2. W-H plots of ZnO NPs


Page 27 of 40

(c)

(a) Intensity (a.u)

Y-obs Ycalc Yobs-Ycalc Bragg position

10

20

30

40

50

60

70

80

2θ (degrees)

(b)

(d) (d)

Y-obs Ycalc Yobs-Ycalc Bragg position

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

50

60

70

80

2θ (degrees)

Figure 3. (a) Rietveld refinement spectra and (b) Packing diagram of ZnO synthesized at 250 ºC. (c) Rietveld refinement spectra and (d) Packing diagram of ZnO synthesized at 500 ºC.



70

(a) 60 50

(b)

250 0C 500 0C

250 oC 500 oC

F(R)2

40 30 20

(3.25 eV)

10

(3.09 eV)

0 300

400

500

600

700

800

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Energy (eV)

Wavelength (nm)

Figure 4. Diffuse reflectance spectra (a) and Direct band gap energy of ZnO (b).

(c ) 1427

3524

(b ) 445

200

% Transmittance

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

(a ) Z n -O 3500

3000

2500

2000

1500

W a v e n u m b e r (c m

1000 -1

500

)

Figure 5. FTIR spectrum of (a) ZnO NPs synthesized at 500°C, (b) ZnO NPs synthesized at 250°C, (c) Paragon Zinc nitrate synthesized at 180 ºC. ACS Plushydroxide Environment

28

Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a1)

(a2)

(a3)

(b1)

(b2)

(b3)

(c1)

(c2)

(c3)

Figure 6. SEM images of (a1, a2, a3) Zinc nitrate hydroxide synthesized at 180 °C. (b1, b2, b3) ZnO crystals synthesized at 500 °C with addition of water. (c1, c2, c3) ZnO hexagonal pyramids synthesized at 500 °C with 29 absorption of moisture. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a1

a2

Page 30 of 40

e1

a3

Tulip flower

b1

b2

e2

b3

Sunflower

Lotus bud

c1

c2

f1

c3

Custard apple

d1

d2

f2

d3

Dahlia Flower

Wheat grains

Figure 7. SEM images of ZnO super structures synthesized at 250 °C

(a) 30 ACS Paragon Plus Environment

Page 31 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a1)


(a2)

(a3)

d= 0.22 nm

(b1)

(b2)

(b3)

d= 0.27 nm

Figure 8. TEM images of synthesized ZnO nanoparticles at 250°C (a1, a2, a3 (HRTEM)) and 500°C (b1, b2, b3(HRTEM)).



100

80

5 ppm 10 ppm 15 ppm 20 ppm

60

50 mg 100 mg 150 mg 200 mg

80

% degradation

% degradation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

40

20

60

40

20

0

0 0

30

60

90

120

150

180

0

30

Time (min)

60

90

120

150

180

Time(min)

Figure 9. Degradation spectra of methylene blue dye with 100 mg of ZnO NPs and varying the dye concentration

Figure 10. Degradation spectra of methylene blue dye with 5 ppm of dye and varying the catalytic load


Page 33 of 40

100 75

60

3 pH 5 pH 7 pH 9 pH 11 pH

40

20

% degradation

80

% degradation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

25

0 0

30

60

90

120

150

Time (min)

Figure 11. Degradation spectra of methylene blue dye with varied pH

1

2

3

4

5

6

Recycling (no.)

Figure 12. Recycling of 100 mg catalyst for the degradation of 100 mL of 5 ppm dye

33 ACS Paragon Plus Environment Figure 13. Concentration of Hydroxyl radicals against irradiation time.


7

1.2x10

250 oC 500 oC

λemi = 588 nm

397 nm

535 nm

1.0x10

6

λexi = 397 nm

8.0x10

588 nm

6

6.0x10

488 nm

468 nm

PL Intensity (a.u)

7

PL Intensity (a.u)

6

4.0x10

6

2.0x10

0.0

300

350

400

450

500

400

550

450

Wavelength (nm) Figure 14. Excitation spectrum of ZnO NPs: emission centered at 588 nm

500

550

600

650

Wavelength (nm)

Figure 15. Emission spectrum of ZnO NPs: excited at 397 nm

0.9

ZnO

λexi = 397 nm 0.6

Y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

0.3

ZnO 250 0 c 500 0C

0.0 0.0

0.2

X 0.44204 0.46335

0.4

0.6

Y 0.45151 0.47285

0.8

X Figure 16. CIE diagram for ZnO NPs ACS Paragon Plus Environment

34

-6.0x 10

-5

-4.0x 10

-5

-2.0x 10

-5

c

b 0 .0

2.0x 10

a

-5

-1 .0

-0 .5

0.0

0 .5

1 .0

P o ten tial (V ) Figure 17. Cyclic voltammetric profile of ZnO modified GC in the absence of DA (a), GC electrode in presence of 10 mM of DA (b) and ZnO modified GCE (c) in presence of 10 mM of dopamine in a phosphate buffer solution of pH 7.0 with a scan rate of 50 mV/s.

-0.10

10

Current µ ( A)

8

-0.08

6

4

Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current (A)

Page 35 of 40

-0.06

2

50

100

150

20 0

250

300

C o n cen tration o f D o p am in e ( µ M )

-0.04

-0.02

0.00 0

50

100

150

200

250

300

Potential (V) Figure 18. Amperogram of ZnO modified glassy carbon electrode in presence of 50, 100, 150, 200, 250 and 300 µM of dopamine with a constant applied potential of 0.335 V. Inset - Calibration plot. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

Scheme 1. Schematic representation for the synthesis of ZnO NPs

Scheme 2. Schematic mechanism for the growth of self aggregated ZnO hexagonal pyramids


Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O2

*

M. Blue

e-

M. Blue

CB e- ee-

-

O2

+

M. Blue

h+ h+

h+

h+

OH

˙

VB

H2O Scheme 3. Schematic mechanism for the degradation of methylene blue dye



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

Table 1. Detailed Reaction Conditions and the Obtained Products

Sl. No

Starting material

Moisture

(°C) 180

Time for completion (min) 180

Zn (NO3)2. 2Zn(OH)2

Moisture

250

120

ZnO

3

Moisture

500

10

ZnO

4

Water

500

10

ZnO

1 2

Zinc nitrate

Source of addition

Temperature

Product Obtained

Table 2. Estimated crystallite size and strain of ZnO NPs synthesized at different temperature ZnO

250 o C

500 °C

(hkl)

2ϴ (degree)

FWHM β x(10−3)

Crystallite size (nm)

(110)

31.833

3.573

40.33

(002)

34.505

3.575

40.61

(101)

36.471

3.577

40.84

(110)

56.65

5.219

30.16

(110)

32.06

3.763

38.31

(002)

34.72

3.762

38.60

(101)

36.55

3.690

39.55

(110)

56.86

5.717

27.57

Average Crystallite size (nm)

Average strain є x(10−3)

38

10

36

63


Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Refinement parameters of ZnO synthesized at 250 and 500 oC

Crystal System : Hexagonal

Space Group : P 63 m c

Hall Symbol

: P 6c –2c

Parameters

a=b (Å)

c (Å)

Rp

Rwp

Rexp

RBrag

RF

χ2

GoF

Density (g/cm3)

Unit cell volume (Å3)

ZnO 250 oC

3.2520

5.2094

3.95

5.33

6.60

2.71

1.95

0.654

0.80

34.432

47.712

ZnO 500 oC

3.2492

5.2048

3.10

4.22

4.74

1.40

0.981

0.793

0.87

34.063

47.587

Table 4. Comparison of detection limit of dopamine.

Material and

Fuel/Reducing/ Surfactant

methods

used

Detection Limit (µM)

Reference

ZnO- Combustion

Cajanas cajan

60

39

ZnO- Combustion

Sugar cane juice

60

52

ZnO- Combustion

Pigeon pea powder

60

53

PVP, glycol, L-ascorbic acid

0.06

54

Nothing

1.0

Our work

ZnO- Co precipitation ZnO-Thermal decomposition



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

“For Table of Contents Use Only”

Green, non-chemical route for the synthesis of ZnO superstructures, Evaluation of its applications towards Photocatalysis, Photoluminescence and Bio-sensing Udayabhanu, G. Nagaraju, H. Nagabhushana R. B. Basavaraj, G. K. Raghu, D. Suresh, H. Rajanaika and S. C. Sharma

Table of Contents Graphic

ZnO

250˚C Moisture

2h

Zinc nitrate hexahydrate

Zn(NO3)2. 2Zn(OH)2

500˚C Moisture

10 min

ZnO

ZnO Photocatalytic activity

Electrochemical Sensing

Photoluminescence studies

Brief Synopsis

ZnO superstructures were synthesized by Green, non-chemical route via thermal decomposition of zinc nitrate at different temperatures without using any fuels/surfactants/capping agents/reducing agents etc. Samples were characterized by XRD, SEM/TEM and UV-Visible spectroscopy. The obtained ZnO superstructures were tested for Photocatalytic, Photoluminescence and Bio-sensing applications. The results show that it is a potential candidate for the above applications.


Green, Nonchemical Route for the Synthesis of ZnO Superstructures

Recommend Documents