A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chito by IJSTE

IJSTE - International Journal of Science Technology & Engineering | Volume 4 | Issue 7 | January 2018 ISSN (online): 2349-784X

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes Ravi Divakaran Professor of Chemistry J. C. Bose Centre for Research & Advanced Studies, Toc H Institute of Science & Technology, Arakkunnam P.O., 682313, Kerala, India

Baby Meena K. R. Research Officer J. C. Bose Centre for Research & Advanced Studies, Toc H Institute of Science & Technology, Arakkunnam P.O., 682313, Kerala, India

Abstract A novel method for synthesis of pure metal and metal oxide nanocrystals in the dry state starting from chitosan complexes is discussed, using the preparation of copper (II) oxide as an example. Copper (II) oxide is widely used as catalyst in redox reactions and photochemical splitting of water. It also finds application in heat transfer fluids, semiconductors, gas sensors, batteries and magnetic storage media. It is expected that these properties may be enhanced in nano-sized particles of copper oxide and may lead to novel applications. Chitosan is an eco-friendly, biodegradable organic biopolymer obtained from the waste shells of prawns and crabs from sea food processing industries. Characterization of the synthesized material using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopic (EDS) analyses are also described. The method is amenable to low-cost production of metal, metal oxide and other binary metal compounds on a large scale. The product is pure and does not contain any capping or stabilizers. Keywords: EDS, HRTEM, Copper Oxide, Chitosan, Nanocrystals, Nanomaterials, SEM, XRD ________________________________________________________________________________________________________ I.

INTRODUCTION

Nanomaterials are material particles with at least one dimension ranging in size between 1 to 100 nm. This lies between the size of single atoms and molecules with sizes typically below 1 nm on one side, and conventional materials with particle sizes typically greater than 100 nm consisting of more than about 106 (literally infinite) atoms, molecules or ions on the other side. The properties of atoms and molecules are governed by quantum mechanics and that of macro-scale materials by conventional physics. Nanomaterials lie between these and therefore have peculiar properties attributed to the presence of very large surface area and quantum effects. Study of nano-sized materials is thus a new area in physics and chemistry, with applications in colours and pigments, printing, electronics and semiconductors, sensors, catalysis, surface coatings, optics, drug delivery systems, and medicine. A large number of text books and reviews are available in print as well as on-line on the subject [1][2][3][4][5]. Both physical and chemical methods are available for the synthesis of nanomaterials. Ball milling, laser ablation and vapour deposition are some of the physical methods. Chemical methods involve precipitation from solution, reduction reactions, electrochemical methods, sol-gel methods and polymeric complexing agents. Typically, syntheses of nanoparticles from solutions are carried out in the presence of stabilizers which form a protective layer around the particles which prevent them from coalescing and growing. This is known as capping [1]. Copper (II) oxide: Copper (II) oxide, (cupric oxide, CuO), has been used as a catalyst in several oxidation and reduction reactions, especially in the conversion of CO and CO2 to methanol, and to obtain hydrogen gas by the action of water on methanol [6][7][8]. It has also been used as catalyst to generate hydrogen gas by the photochemical splitting of water [9]. CuO nanoparticles have been used as nanofluids in heat transfer applications [10]. Copper(II) oxide is also a p-type semiconductor with a band gap of 1.2 eV with properties suitable for application in solar cells, gas sensors [11] and electrochemical sensors [12][13]. It is also an antiferromagnetic material and a high temperature superconductor [14]. These properties of copper oxide may become very useful in the development of vehicles using hydrogen as fuel, in solar power generation [15], in the development of new biochemical sensors, for fabricating batteries [16] and new magnetic storage media [17]. The structure and morphology of nano-sized copper (II) oxide depends on the method of preparation. It has been synthesised in the form of nanospheres, nanowires, nanoplates, hollow microspheres, whisker arrays, nanoribbons, nanoleaves and nanoflowers [18][19]. Several methods of preparation and the nature of copper oxide obtained have been reported. Some of these are the hydrothermal method [18], ultrasonic spray pyrolysis [9], spin coating technique [20], sol-gel methods [21], electrochemical methods [22], physical vaporisation to form thin films on substrates [23] and thermal decomposition of precursors [24]. Since many organic synthetic as well as biopolymers containing suitable ligand functional groups can form complexes with metal ions,

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

it was inferred that there is a possibility of these forming nano-sized metal clusters which may be recovered by burning off the organic polymer matrix at low temperatures. No reports of any previous attempts along this line have come to our notice. Chitosan: Chitin is a biopolymer obtained from the shells of crustaceans like crabs and prawns, and is a by-product of seafood processing industries [25]. Chitosan is obtained by the alkali hydrolysis of chitin, and is commercially available. It is biocompatible and biodegradable, with established antimicrobial and fat-absorbing properties. Therefore chitosan is widely used in sunscreen lotions, slimming diets, self-absorbing sutures and drug delivery systems [26]. Its structure is very similar to that of cellulose. It may be chemically described as a co-polymer of glucosamine and N-acetyl glucosamine units linked by 1-4 glucosidic bonds, with the amino group (-NH2) replacing the hydroxy group (-OH) on carbon 2 of many glucose units in cellulose, as shown in figure 1 [27]. This makes it soluble in dilute acid medium, and large numbers of -NH2 and -OH groups in the chain are available for coordination with metal ions. The actual chain length of the polymer and the exact number of amino and hydroxy groups present greatly depend on the source and manufacturing process of the material. Several reports are available on the study of absorption of metal ions by chitosan in waste water treatment [28][29] and in the synthesis of nanomaterials [30].

Fig. 1: Structure of chitosan

In this paper, we report the synthesis of CuO nanocrystals by the incineration of a chitosan-Cu2+ complex at low temperature in the solid state. To the best of our knowledge, this is the first reported synthesis of nanocrystalline copper(II) oxide from a chitosancopper complex. II. MATERIALS AND METHODS About 70% deacetylated chitosan, manufactured from prawn (Penaeus indicus) shells was obtained from India Sea Foods, Kannamaly, Kochi (Ernakulam). Glacial acetic acid, sodium hydroxide and crystalline copper(II) sulphate (CuSO4.5H2O) were of laboratory reagent (LR) grade. Distilled water was used throughout in all experiments. X-ray diffraction (XRD) analysis of powder samples were carried out on a Bruker model AXS D8 Advance spectrometer using Si (Li) PSD detector over a 2 range of 10 to 80. Scanning Electron Microscopy (SEM) and elemental analysis using Energydispersive X-ray Spectroscopy (EDS) were done using JEOL model JSM-6390 instrument. High resolution transmission electron microscopy (HRTEM) was carried out using JEOL model JEM 2100 having 0.24 nm resolution with an acceleration voltage of 200 kV. Acetic acid solution (0.1M) was prepared by dissolving 6 mL of glacial acetic acid in water and making up to 1L. Copper sulphate solution (0.1M) was prepared by dissolving 25g copper(II) sulphate crystals in 1L water. Sodium hydroxide solution (0.2M) was prepared by dissolving 8g NaOH pellets in 1L water. Chitosan solution was prepared by mixing 1g chitosan powder with 100mL acetic acid solution and keeping for two days. Method-1: Chitosan polymer chains are usually cross-linked through H-bonds into tightly coiled configurations in which only a small percentage of ligand groups are exposed on the surface. In order to purify chitosan and open up the strands to make more functional groups available for coordination, chitosan was re-precipitated from solution by adding an equal volume of NaOH solution through a pipette, keeping its tip at the bottom of the solution so that it flowed upwards against gravity in the beaker, thus expanding the precipitated gel and preventing it from settling to the bottom. The gel was washed repeatedly with distilled water flowing in the same manner to remove the excess NaOH till the pH of the mixture came down to about 8. Chitosan was now in the form of silky white strands. Copper sulphate solution was then added in the same manner through a pipette, keeping the tip at the bottom. The metal ions were absorbed by chitosan. The chitosan gel became very dense and bluish-green in colour, settling to the bottom into a granular crystalline mass. Supernatant containing the excess copper sulphate was decanted off. The mass was powdered by pressing with the tip of a glass rod and washed repeatedly with distilled water till the washings were free of any traces of copper sulphate. This was confirmed by the absence of a blue colour when a few drops of the colourless supernatant were mixed with ammonia solution in a test tube. The supernatant was decanted off and the solid transferred to a sheet of filter paper. Water was removed by repeatedly pressing with pieces of filter paper, and the solid chitosan-Cu2+ complex was dried in an air oven at 60C overnight. The lumps were then powdered in a glass mortar.

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

The powder was transferred into a silica crucible and heated slowly over a Bunsen flame. The powder started shimmering like the tip of a lighted cigarette for some time, without any fumes or smell. On stirring with a small glass rod, the material fell into a very fine black powder of copper (II) oxide. Chitosan as well as the powder obtained were subjected to analysis using XRD, SEM and EDS. Method-2: In another experiment, the re-precipitated chitosan gel, after washing to remove excess NaOH, was stirred using a magnetic stirrer while the copper sulphate solution was added rapidly into it. Stirring served to shear the chitosan chains into smaller strands distributed uniformly in the aqueous medium. The chitosan-Cu2+ complex produced was in the form of a very fine crystalline bluish-green precipitate. The crystals settled rapidly when the stirring stopped. Washing and drying this material as before followed by incineration gave a much finer powder of black CuO. The particle size was indeed much smaller than before as indicated by SEM. III. RESULTS AND DISCUSSION Chitosan, the polymeric ligand used in the experiments, was reacted with CuSO4 solution at room temperature (around 30ď&#x201A;°C) to get chitosan-Cu2+ complex. The chitosan used was an amorphous substance, but the copper complex was a highly crystalline material as indicated by SEM and XRD analysis. Figure 2 indicates the two-dimensional structure of chitosan with a texture similar to that of dry leaves. After complexation with copper, the change to 3-dimensional crystalline structure is very clear from figure 3. In figure 4, the presence of only two very broad hills indicate the amorphous nature of chitosan. But the presence of a large number of sharp peaks in figure 5 supports the highly crystalline nature of the chitosan-Cu2+ complex. The exact crystal structure and the planes to which the peaks correlate have not been determined. No prior data is available for comparison as this is the first report containing SEM and XRD patterns of the chitosan-Cu2+ complex. Although an earlier report [31] depicts the SEM of a chitosan-Cu2+ complex, it does not indicate any change in the nature of chitosan as a result of complexation.

Fig. 2: SEM image of chitosan

Fig. 3: SEM image of chitosan-Cu2+ complex

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

Fig. 4: XRD pattern of chitosan

Fig. 5: XRD pattern of chitosan-Cu2+ complex

The black solid copper (II) oxide powder obtained on ignition of the above complex consists of approximately disc-shaped flakes, with thickness less than 100 nm and diameters of about 1ď m or less, as can be seen from SEM data in figure 6. Applying Scherrer equation for calculating particle size from peak broadening to the XRD data in figure 8 gives an average particle size of about 63 nm. The presence of only Cu and O in the sample is clearly seen in the EDS data in figure 7 (a very minute amount of carbon indicated may be the result of incomplete combustion).

Fig. 6: SEM image of CuO nanoflakes prepared using method-1

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

Fig. 7: EDS analysis of CuO nanoflakes

Further proof that the obtained material was CuO can be seen from the XRD pattern of the substance presented in figure 8. All the peaks match well with data for monoclinic CuO given in the archives of the International Centre for Diffraction Data (ICDD, formerly known as the Joint Committee on Powder Diffraction Standards, JCPDS) [18][19][32]. The peaks at 2 = 32.295, 35.323, 38.519, 48.575, 53.229, 58.020, 61.342, 66.028, 67.800, 72.178 and 74.877 correspond to crystal planes (110), (002),(200),(-202),(020),(202),(-113), (-311), (220), (311) and (-222) respectively.

Fig. 8: XRD pattern of CuO nanoflakes

On repeating the preparation with rapid stirring of the chitosan solution while adding copper sulphate solution (method-2), the obtained copper(II) oxide particles were much smaller in size as indicated by its SEM image given in figure 9.

Fig. 9: SEM image of CuO nanoparticles prepared using method-2

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

The actual size of the CuO particles were visualised and measured using HRTEM imaging, the results of which are shown in figure 10 and figure 11.

Fig. 10: HRTEM image of CuO nanocrystals obtained by Method-2

Fig. 11: Individual diameters of CuO particles obtained using Method-2 measured using HRTEM (pink lines) Clockwise from left: 66.45 nm, 71.75 nm, 37.81 nm, 49.47 nm.

IV. CONCLUSIONS Many organic polymers have functional groups that can form complexes with metal ions. Such complexes are insoluble solids and can be easily removed by filtration from the metal ion solution. The polymer part of the complex can then be removed by incineration in presence of air or oxygen to obtain the metal oxide in the form of nano-sized particles in the solid state. The particles are pure and do not have any attached capping. We were able to synthesize nanoflakes of CuO in this manner using chitosan as the polymer matrix. Incineration in an oxygen-free atmosphere may lead to the formation of pure metallic nano particles or compounds other than the oxide. Using other polymers and reaction conditions may drastically change the morphology of nano particles produced. This method can be adapted for the cheap production of a variety of metallic nano particles on a large scale. ACKNOWLEDGEMENT The authors thank the management of Toc H Institute of Science & Technology (TIST) for providing laboratory facilities and partial financial assistance. We are very much obliged to Mr. Everest Edward, Proprietor, India Sea Foods, Kochi for a generous supply of chitosan powder. We also place on record the services of Sophisticated Test and Instrumentation Centre (STIC), Cochin University of Science & Technology (CUSAT) for SEM, HRTEM, XRD and EDS analysis.

A Novel Method for Synthesis of Pure Metal and Metal Oxide Nanocrystals in the Dry State using Chitosan Complexes (IJSTE/ Volume 4 / Issue 7 / 001)

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