Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass

IJSTE - International Journal of Science Technology & Engineering | Volume 3 | Issue 10 | April 2017 ISSN (online): 2349-784X

Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method Mr. Chirag Mevada Research Scholar Department of Chemical Engineering SVNIT, Surat, 395007, India

Dr. Bina Sengupta Associate Professor Department of Chemical Engineering The M S University of Baroda, Baroda 390001, India

Abstract Copper oxide (CuO) was synthesized on glass substrates using hydrothermal method. Heating time and precursor concentration were found to largely affect the morphology and size of CuO microstructures formed. Material characterization was performed using X-ray Diffraction and Fourier Transform Infrared Spectroscopy. FEGSEM was employed to determine the morphology of CuO structures grown on the substrates. 3D flower-like microstructures were formed from nanorods and nameplates. The asprepared CuO microstructures all showed good catalytic activity for the degradation of methylene blue (MB) in presence of hydrogen peroxide (H2O2).

Keywords: Copper oxide, Hydrothermal, Parametric variations, Methylene blue ________________________________________________________________________________________________________ I.

INTRODUCTION

Metal oxide nanostructures have attracted attention because of their extraordinary properties in different fields of optics, optoelectronics, electronics, catalysts and sensors. As a p-type transition metal oxide with a narrow band gap (1.23 eV), CuO has received considerable attention in recent years due to its exotic properties and wide application, as heterogeneous catalysts [1], gas sensors [2], solar cells [3], electrode materials [4], super hydrophilic materials [5] and high temperature superconductors.

Chemical and physical properties of CuO are dependent on its size and shape. CuO structures, such as nanorods [3], nanowires [6], nanoribbons [6], nanoparticles [7], nanoplatelets [8], nanoneedles [9], nanosheets [4], and nanorings [10] have extensively been studied. Apart from these some complex structures of CuO have also been reported in the literature, such as flower-like [11], Dandelions [12], sheaf-like [13], Honeycombs [14], dendrites [15], and doughnut-like [16]. Various methods for the preparation of CuO structures have been investigated, including thermal decomposition [16], hydrothermal methods [8], hydrolysis route [11], and electrochemical methods [17]. The hydrothermal process is an excellent method as a wet chemical process with advantages of low temperature, use of simple equipment, environmentally safe process and ease of scale up.

In the present investigation CuO was synthesized on glass substrates via the hydrothermal method using copper sulfate as the precursor and ammonia as a complexing agent. The effect of temperature and precursors concentration on the morphology of the CuO was investigated. X-ray diffraction, Fourier Transform Infrared Spectroscopy was used for characterization and FEGSEM was employed to determine the morphology. The catalytic activity of CuO for the oxidation of methylene blue was studied in the presence of hydrogen peroxide.

II. EXPERIMENTAL SECTION Materials and Synthesis Copper sulfate, acetate, chloride and nitrate were supplied by S.D. Fine-Chem (Mumbai). Aqueous ammonia (NH3•H2O, 25 wt%) from LOBA chemicals was used as the complexing agent. In order to investigate catalytic activity of CuO, methylene blue from Qualigens and hydrogen peroxide 30 wt% form MERCK Specialties Pvt Ltd. were used. All chemicals were of analytic grade, and were used without further purification Substrate Preparation: Copper oxide was synthesized on glass substrates via the hydrothermal method. The glass substrates were cleaned with soap solution followed by cleaning with acetone and sonicating for 10 min in an ultrasonic bath.

Synthesis of copper oxide: 50 ml of 40mM precursor solution was transferred to 100 ml pyrex glass bottles with polypropylene autoclave able screw caps.

Liquid ammonia was added so as to attain the required mole ratio and the pH of the solution was recorded. Previously cleaned

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Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method (IJSTE/ Volume 3 / Issue 10 / 002)

glass substrates were immersed into the solution. The bottles were then transferred to a PID temperature controlled oven (Model PNX-14A) and kept in the oven in order to convert copper ammonical complex to copper oxide. The deposition temperature varied from 70 0C to 105 0C at a constant pH and time. The bottles were then taken out from the oven and the coated substrates were rinsed with water in order to remove loosely bonded CuO particles and dried. The particles deposited at the bottom of the bottles were separated from the reagents by centrifugation, washed several times with water and dried at 60 0 C for 3 hours.

Catalytic activity of CuO Catalytic activity of CuO was demonstrated by degrading MB (Methylene Blue). A typical catalytic reaction was carried out in a 250 mL round bottom glass flask, which contained 50 mL of aqueous MB (10 mg/L) and 10 mg of as-prepared CuO. After adding 10 mL of aqueous H2O2 (30 wt.%), the mixture was allowed to react at room temperature under stirring condition.

Samples were withdrawn at regular time intervals and absorption spectra were measured using an UV–visible spectrophotometer.

A calibration curve for MB concentration was generated by measuring the peak intensity at λ max = 665 nm for a series of standard solutions.

Characterization X-ray diffraction (XRD) patterns of CuO were recorded on a Bruker D2 phaser using Cu Kα radiation (λ = 1.5418 nm) operated at 40 kV and 40 mA. FTIR spectra were recorded on a spectrophotometer (Thermo Scientific Nicolet iS5 with iD1 Transmission) with 32 scans. The morphology of the synthesized particles was determined using field emission gun scanning electron microscopy (FEGSEM) SUPRA 40 Carl Zeiss SMT AG (Germany) with emitter thermal field emission type acceleration voltage 1.5-30 kV and probe current 4 pA-10nA. The catalytic activity of as prepared CuO was characterized using UV–visible recording spectrophotometer UV-260 (Shimadzu) in the range of 500-700 nm and water was used as reflectance standard.

III. RESULTS AND DISCUSSION Growth mechanism For copper ions in aqueous solutions, when the ion product (IP) of the solution is higher than the solubility product (SP) the precipitation of Cu(OH)2 occurs. The degree of super saturation (S), defined as the ratio of IP to SP, is an important parameter to evaluate the precipitation process in aqueous solutions. If S is lower than 1, no precipitation occurs. If S is higher than 1 but lower than a critical value Sc, a heterogeneous precipitation occurs on the walls of container and substrate, since the super saturation is not sufficient to induce nuclei formation in the bulk solution. If S is higher than S c, a homogenous precipitation occurs in the bulk solution [18]. Based on this, S value for copper ions must be fixed in between 1 and S c in order to get heterogeneous nucleation on the substrate.

Reaction Mechanism The insitu generation and deposition of CuO nanoparticles on glass substrates proceeds via a simple precipitation–dehydration process. The copper oxide nanoparticles are formed during the process according to the following reactions: [19] Cu2+(aq) + 4NH3•H2O(aq) [Cu(NH3)4] 2+(aq) + 4H2O ……..………………..(1) [Cu (NH3)4] 2+(aq) + 2OH− (aq) + 4H2O Cu(OH)2 (s)

Cu(OH)2 (s) + 4NH3•H2O(aq) ……..(2)

CuO (s) + H2O…………………………………………………. (3)

The formation of copper oxide takes place through the formation of a copper ammonical complex, [Cu(NH 3)4]2+. Copper ions react with ammonia to form a deep blue solution containing [Cu(NH 3)4]2+ complex ions. Ammonia used in the precipitation reaction, plays a vital role during the synthesis and determines the degree of super saturation S. Due to the presence of excess OH− in the system, orthorhombic Cu(OH)2 first precipitates in solution with strong H bonds. The alkaline media, in addition to temperature, accelerates the decomposition of Cu(OH) 2 to CuO so that the intermediate compound Cu(OH)2 is not observed. The CuO crystalline nuclei formed by the dehydration of Cu(OH) 2 precipitate and gradually grow on the substrate and wall of the reactor The hydrothermal treatment, transforms the orthorhombic Cu(OH) 2 into monoclinic CuO nanostructures as a result of a dehydration reaction in which the inter planer H-bonds are broken [20].

Synthesis of CuO on glass substrates Parameters that can affect the size and morphology of the CuO particles grown on substrates are temperature, precursor concentration, additives, substrates, alkaline media and heating manner. In view of this, the effect of parametric variations on the growth of CuO particles was studied.

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Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method (IJSTE/ Volume 3 / Issue 10 / 002)

Effect of Temperature The morphology of products is highly dependent upon the temperature of the hydrothermal treatment in the synthesis. The effect of the reaction temperature on the morphology of the resulting CuO was examined for temperature a range from 70 0C to 1050C for a heating time of 3 and 5 hours at two pH values 10 and 11. Figure 1 shows the FEGSEM images of the CuO structures obtained at varying temperatures.

When the reaction was carried out at 700C for 3 hours, the solution remain in completely ammonical complex form, no precipitates were observed at the bottom of bottle as well as no visible growth was observed on the glass substrate. However, the FEGSEM image of the substrate (Figure 1(1)), showed that the ammonical complex was not converted at 11 pH and 70 0C, the intermediate product Cu(OH)2 of size 200 nm was observed this was identical to the morphology observed at 105 0C, 13pH and heating time of 3 hours.

Further increase in temperature from 70 0C to 850C at 11pH, the solution was still ammonical complex, but growth on the substrate was observed. Rosettes of size ~1 μm composed of sheets were observed (Figure 1(2)). Low temperature results in growth of nanoplates and irregular shaped structures. Low temperature and low pH cannot provide enough energy for the reaction. High temperature results in fast nucleation leading to numerous nuclei formation which coalesces in order to decrease the surface energy of the system with time. The coalesced nuclei then grow larger to many CuO nanosheets which assemble to flower like structures.

At 1050C, the solution in bottle after 3 hours still remained as the blue complex but the glass substrate was uniformly coated with CuO. XRD patterns of the glass substrate and the FTIR analysis of the collected precipitates (Figure 2 and 3) confirming the formation of monoclinic CuO. The density of the nano sheets in the microflower assemblage increased, and the size of the microflowers obtained were ~10 μm. Further, some of the microflowers combined together and agglomerated with each other (Figure 1(3)).

When the hydrothermal treatment temperature was 850C and the heating time was 5 hour, the solution of the bottle still remained in ammonical complex and the growth of CuO on the glass substrate was not uniform. Figure 1 (4) shows the high density and closely packed nanosheet assemblages of CuO. Further increasing the temperature to 105 0C at 3 and 5 hours, the complex was not a fully converted but glass substrate was uniformly coated by CuO. FTIR and XRD analysis (Figure 2 and 3) of the glass substrate and collected precipitate showed the crystalline nature and monoclinic phase of CuO. Figure 1(5) shows the well-defined 3D microflowers grown on the substrate with size ~10 μm.

Thus, increasing the temperature from 70 0C to 1050C resulted in an increase in size of structure since the number of nuclei increased which further coalesce to decrease the surface energy of the system. Jia et al. (2009) observed that increasing the temperature from (900C to 1050C) of the hydrothermal treatment the size of the obtained structure was increased from nanosphere to microsphere. [21]

Fig. 1: FEG-SEM images of CuO particles grown on glass substrates at varying temperature. (1)70 0C, 3h,11pH, (2) 850C, 3h, 11pH, (3)1050C, 3h,11pH, (4) 850C, 5h,11pH, (5)105 0C, 5h,11pH.

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Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method (IJSTE/ Volume 3 / Issue 10 / 002)

Fig. 2: FTIR spectra of CuO precipitates at varying heating times (pH 11, temperature1050C).

Fig.3: XRD patterns of the CuO coated on the glass substrates at varying heating times times. (pH=11, temperature 1050C). (Vertical lines represent the standard diffraction data of CuO from JPCDS file (no. 44-0607)).

Effect of Precursor Concentration The concentration of precursor has an important effect on the morphology of final products. In the present work, it was also interestingly noticed that not only the size of the synthesized CuO, but also the quantity of intermediate products at certain temperature greatly depended on the concentration of reactants.

Figure 4 shows the SEM images of the synthesized CuO particles at varying initial concentration of CuSO4•5H2O (40mM and 12mM) as precursor at pH10 and pH11, hydrothermal temperature 105 oC grown for 3 hours.

Fig. 4: FEGSEM images of CuO structures obtained at varying precursors concentration (1) 12mM 10pH, (2) 40mM 10pH,(3) 12mM 11pH, (4) magnified image of 3, (5) 40mM 11pH, (6) magnified image of 5 (time 3 hours and temperature 105 0C)

When the initial concentration of precursor was 12mM, Figure 4 (1) shows the 3D sphere morphology of CuO obtained of size ~1.5 Âľm. Further, increasing the concentration of precursor from 12mM to 40mM, at the same pH and temperature, only some of the assemblages were fully grown to spheres while some were in the process of attaining the spherical shape. There was a substantial increase in the packing density of sheets due to the higher concentration of the precursor.

Hence, it can be concluded that the morphology of the structures obtained for varying precursor concentration are the same but the packing density of attached leaves was different. Xu et al. (2007) tuned the size of the microflowers from 2 mm to several

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Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method (IJSTE/ Volume 3 / Issue 10 / 002)

tens of micrometers by adjusting the precursor concentration in the range 0.0225–0.086 M. Jia et al. (2009) reported that when precursors concentration was raised from0.025 M to 0.1 M, than the size of the nanoflower turned into microflower. [21, 22] Catalytic Activity of grown CuO Microstructures The catalytic activity of CuO synthesized under different conditions (5h 11pH 105 0C, and 3h 11pH 105 0C 3h 9pH 1050C) was carried out. Degradations tests were also performed without addition of CuO. Figure 5 shows standard curve generated for MB of concentration vs absorbance. Figure 6 shows the decrease in concentration of MB with time for CuO microstructures grown at varying conditions.

Fig. 5: Standard curve of MB (Concentration vs Absorbance)

Fig. 7: Second-order kinetic plot of (1/[MB]-1/[MB]0) vs time of MB (10 mg/L, 50 mL) degradation in presence of flower like CuO (10 mg) and H2O2 (30 wt.%, 10 mL) at room temperature.

Fig. 6: Time profile of MB degradation using different concentration

Fig.8: UV–visible spectra obtained during the MB oxidative degradation over CuO.

When the reaction was carried out without addition of CuO (H 2O2+MB) 35% degradation was observed after 4 hours, with the addition of the CuO the degradation reached up to ~ 92% in just 2 hours (Figure 6). The MB absorbance dropped from 1.04 to 0.098 in 2 hours revealing that the fully grown microstructure exhibited maximum degradation. The degradation reaction followed the second-order kinetics with rate constant of k = 0.831 h-1 (Figure 7). In contrast, the plots of zero-order and firstorder kinetics give poor linear correlation values. It can be concluded, that the fully grown microstructure CuO exhibited good catalytic activity.

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Effect of Temperature and Precursor Concentration on Morphology of Copper Oxide Synthesized on Glass Substrates Via Hydrothermal Method (IJSTE/ Volume 3 / Issue 10 / 002)

IV. CONCLUSIONS Increasing the hydrothermal heating time resulted in an increase in the size of microstructure grown. The electrostatic attraction between the nanosheets reduced, resulting in the opening up of the individual sheets. Finally, each nanosheet in the microstructure became identifiable. The size of the CuO structure can be facilely and selectively controlled from several hundred nanometers to a few micrometers simply by variation in the precursor concentration. The catalytic performance of as-prepared CuO products in the oxidation of MB was studied in presence of H 2O. All the obtained CuO structures showed good catalytic activity in the degradation of aqueous MB. In addition, results showed that a full grown microstructure was more active.

ACKNOWLEDGMENT Authors would like to acknowledge Prof J. Datta Majumdar (Professor - in charge of FEGSEM Lab.) and Mr. Mithun Das for FEGEM analysis, Prof. Rahul Mitra (In charge of SEM Lab.) and senior scientific officer Dr. R N Maiti from IIT Kharagpur for SEM analysis and Prof. T K Chaudhari for all XRD analysis.

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