Preparation of PEG8K-Pd Nanoparticles and Their Characterization by DLS Technique Project Work Submitted to the Department of Applied Sciences for the partial fulfillment of the degree ofMasters of Science inApplied Chemistry By Miss Ritu Kumari Under the supervision of Dr Rupesh Kumar Kirori Mal College, University of Delhi

AcknowledgementsFirst and foremost, I would like to extend my sincere gratitude to my supervisor, Dr. Rupesh Kumar for allowing me to work on this project. His enthusiasm and integral view on research and mission for providing high quality work has made a deep inspiration on me and at the same time enriched my growth as a researcher. I greatly acknowledge his, guidance, effort and for being a backbone of my training and this thesis.I am highly thankful to the Principle, Kirori Mal College, for allowing me to do my training. Apart from my efforts, the success of any project largely depends largely on the encouragement and guidelines of many others. I am also thankful to Miss Umisha Singh and Miss Priyanka Dutta for their continuous support and knowledge throughout my internship at KMC. I also convey my thanks to staff members of the laboratories of the college for allowing me to avail the facilities available there.I would also like to thank my internal coordinator Dr. Joydeep Dutta, for being supportive and encouraging my research work. His valuable inputs, wisdom, knowledge and commitments always inspired and motivated me.

Ritu Kumari M.Sc Applied Chemistry

Bonafide CertificateI approve the project of Miss Ritu Kumari, entitled To Study PEG8K- Pd nanoparticles by DLS Technique is worthy of consideration for the award of degree of Master of Science and is the record of original and bonafide research work carried out by her under my supervision. The results contain in it have not been submitted in part or full to any other university or institute for award of any degree/diploma.

Ritu Kumari Dr. Joydeep Dutta (Candidate) Internal Co-ordinator

AbstractThis work presents the result of preparation of Pd nanoparticles by chemical reduction method. In this work a systematic study of preparation of PEG8K-Pd nanoparticles and characterization of Pd nanoparticles using DLS technique is reported. The samples have been synthesized using PEG8K and palladium acetate solution using 1,4-dioxane with methanol as reducing agent. Scheme 1: Synthesis of PEG8K-Pd nanoparticles

Table of Contents Introduction 9

Chapter 1 Instrumentation

1.1 Dynamic Light Scattering studies10

1.2 Transmission Electron Microscope13

1.3 Scanning Electron Microscope13

1.4 X-Ray Diffraction14

1.5 Thermogravimetric Analysis15

Chapter 2 Literature survey on the role of PEG, Palladium Nanoparticles

2.1 PEG Chemistry16

2.2 Palladium Chemistry19

2.3 Applications of Pd nanoparticles20

2.4 Transition Metal Nanoparticles Palladium 21

Chapter 3 Preparation of PEG8K-Pd Nanoparticles and Their Characterization

3.1 Material 23

3.2 Solvents23

3.3 Instrumentations23

3.4 Experimental Discussion24

3.5 Experimental Procedure26

3.6 Results27

3.7 Conclusion 30

Abbreviations

DLSDynamic Light ScatteringTEMTransmission Electron MicroscopyHPLC High Performance Liquid ChromatographyK 1000Mw Average Molecular WeightMN Number Average Molecular WeightNp NanoparticlesPd PalladiumPd(OAc)2 Palladium DiacetatePEG Poly Ethylene GlycolPEG8K Polyethylene Glycol 8,000PEG8K-Pd Polyethylene Glycol 8,000-PalladiumPEO Polyethylene OxidePOE PolyoxyethyleneQELS Quasi-Electric Light ScatteringSEM Scanning Electron MicroscopeTGA Thermo Gravimetric AnalysisXRD X-Ray Diffraction

Introduction

Nanotechnology represents on the major breakthroughs modern science, enabling materials of distinctive size and composition to be formed. Such nanodimensional materials (in the 1-100nm size domain) are seen as a bridge between atomic and bulk material and have been shown to exhibit a variety of unique chemical, physical and electronic properties. The studies of these properties has become an increasingly important area in chemistry, physics, biology and medicine and material sciences. However reliable preparation of nanomaterials are required for their exploitation and this remains an area of active research. Whilst much research has focused on nanomaterials of coinage metal (especially gold), interest in the properties of transition metal nanoparticles is also considerable and growing. The high surface area-to-volume ratio makes nanomaterials highly desirable for use as potential catalysts. Given that palladium is one of the most efficient metal in catalysis, the study of palladium based nanomaterials is hugely important and valuable. As a consequence of, nanoparticles of palladium have been extensively studied in the wide range of catalytic applications including hydrogenation, oxidation, carbon-carbon bond formation and electrochemical reactions in fuel cells. However it should be noted that the applications of palladium go beyond catalysis. In the present report the synthesis of the palladium nanoparticles with well controlled particle sizes and shapes of high monodispersity is a key technology in producing materials that are more effective and efficient than the current state of art. For example, particle size can play a critical role in catalytic process and monodispersed particle with an optimal size enables the most efficient use of the valuable metal and the highest selectivity in the subsequent reaction.

Chapter 1: Instrumentation1. Characterization of Nanoparticles1.1. Dynamic light scattering (DLS)DLS , sometimes referred to as Quasi Electric Light Scattering (QELS), is a non invasive, well-establishing technique for measuring the shape and size distribution of molecules and particles typically in the submicron region , and with latest technology lower than 1nm.

Figure 1 Dynamic Light Scattering InstrumentHere a solution containing particles is placed in the path of monochromatic beam of light and the temporal fluctuations of the scattering light due to the Brownian motion of the particles are determined. Let us assume that a detector of light is fixed at some angle with respect to the direction of the incident beam and at some fixed distance from the scattering volume, which contains a large number of particles. Here the small particles will be undergoing a diffusive Brownian motion. Hence the distance that scattered wave travels to the detector varies a function of time. The scattered waves then can interfere constructively or destructively depending on the differences in the distance travelled to the detector and the result is an average intensity with superimposed fluctuations. Smaller particles will move rapidly and can cause high frequency of fluctuations, whereas larger particles will move slowly and can cause low frequency of fluctuations. With the help of a spectrum analyzer, all these frequencies can be measured. But the most efficient concept, which is used to analyze these fluctuating signals, is correlation. When two variable or two signals are highly correlated then a change in one can be used to predict the change in the other. Thus correlation can be defined as the average of the product of the two quantities, mathematically. Auto-correlation is just the average of the product of a variable with a delayed version of itself. In DLS, one of the important terms is sample time. When the total time over which a measurement is made, is divided into small intervals of time then this small interval of time is called sample time. In each of these intervals the scattered light intensity, as represented by the number of electrical pulses registered during sample time, fluctuates about a mean value. The intensity of auto-correlation function is formed by averaging the product of intensity is of these small time intervals as the function of time between the intervals. Auto correlation function, C(t), is defined as a function of the time between intervals. As t increases, correlation is lost and the function approaches a constant background term B. for shorter time, correlation is high. In between these two limits, the function decays exponentially for a monodisperse suspension of rigid, globular particles and is given by C(t)=Ae-2t + B Where, A is an optical constant which is determined. is related to the relaxation of fluctuations and is expressed as = D q2 The value of q is calculated from the scattering angle , the wavelength of laser 0, and the refractive index, n, of the suspending liquid: q = (4n/0) sin/2the translational diffusion coefficient, D, is the main quantity measured by DLS. Now the translational diffusion coefficient, D, is related to the particle size of different shapes like sphere, ellipsoid cylinder size etc. The most useful assumption of the particle size is the spherical assumption. For a sphere, D is given by the Strokes-Einstein equation: D = kT / 3dWhere k is the Boltzmanns constant (1.38054x10-23joules/kelvin), T is the temperature in kelvin, is the viscosity of the liquid in which the particle is suspended, d is the particle diameter. Thus by substituting the value of T, D and , the size of the particle can be determined. This equation assumes that particles are moving independently, so the measurements must be made in dilute suspensions. The molecular weight of the polymers in the solution can be calculated from the translational diffusion coefficient:Typical applications of dynamic light scattering are the characterization of particles emulsions or molecules, which have been dispersed or dissolved in a liquid. The Brownian motion of particles or molecules in suspension causes laser light to be scattered at different intensities. Analysis of these intensity fluctuations yields velocity of the Brownian motion and hence the particle size using the Strokes-Einstein relationship. In our experiments, 2ml sonicated aqueous/methalonic dispersion of nanoparticles was used for laser light scattering experiment. QELS measurements for determining the size of nanoparticles were performed using photoCor-FC instrument with open modular architecture goniometer. Air cooled He/Ne laser was operated at 633nm and 200mw was light source. The time dependence of the intensity auto correlation function of scattering was derived by using a 288 channel digital co-relator. All measurements were done at 25C. The particle was automatically calculated from the autocorrelation function using Strokes-Einstein equation. 1.2 Transmission Electron Microscope (TEM)The Transmission electron Microscope (TEM) has evolved over many years into a highly sophisticated instrument that has found widespread application across the scientific disciplines. Because the TEM has has an unparalleled ability to provide structural and chemical information over a range of length scales down to the level of atomic dimensions, it has developed into an indispensable tool for scientists who are interested in understanding the properties of nano structured and in manipulating their behavior. Image formation in TEM is more complicated in practice than in the case for the optical microscope. Strong magnetic fields are needed for focusing the electron beam, and these cause electrons to take a spiral trajectory through the lens field. Modern day TEMs operating at 200 or 300KeV have resolution limits well below 2.0A0, which is comparable to the spacing between atoms. Individual columns of atoms thus can be resolved in crystalline materials, which must first, however be oriented so that the incident electron beam is aligned along some major crystallographic zone axis of the sample. In the standard TEM operating mode, which is commonly referred to as amplitude or diffraction contrast imaging, only a fraction of those electrons that have passed through the sample are used to form the highly magnified final image. 1.3 Scanning Electron Microscope (SEM)The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a light microscope. The SEM has a large depth, which allows a large amount of the sample to be focus in one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of samples is relatively easy since most SEM only require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makes the SEM one of the most heavily used instruments in research area today. In light microscopy, a specimen is viewed through the series of lenses that magnify the visible-light image. However, the scanning electron microscope (SEM) does not actually view a true image of the specimen, but rather produces an electronic map of the specimen that is displayed on the cathode ray tube (CRT). The SEM has a secondary electron detector. The signals produced by the secondary electrons is detected and sent to a CRT image. The SEM has compensating advantages, though including the ability to image a comparatively large area of the specimen; the ability to image bulk materials; and the variety of analytical models available for measuring the composition and nature of the specimen.1.4 X-Ray Diffraction (XRD)X-ray diffraction is a versatile, non-destructive analytical technique for identification and quantitative determination of the various crystalline forms, known as phases of the compounds present in the powdered and solid samples. When a monochromatic X-ray beam with wavelength is incident on the lattice crystals on lattice planes in a crystal at an angle , diffraction peaks occur when the distance travelled by the ray reflected from successive planes differs by a complete number n of wavelengths. This is described by Braggs Equation: n = 2d SinWhere d is the spacing between the planes. By varying the angle , Braggs equation is satisfied by different d spacing in polycrystalline material.

1.5 Thermogravimetric Analysis (TGA)Thermal gravimetric analysis (TGA) is a method of thermal analysis in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate) or as a function of time (with constant temperature and/or constant mass loss). TGA can provide information about physical phenomena, such as second phase transition, including vaporization, sublimation, absorption and adsorption and desorption. Likewise, TGA, can provide information about chemical phenomena including chemisorptions, desolvation (especially dehydration), desorption and solid-gas reactions (e.g. oxidation and reduction). TGA is commonly used to determine selected characteristics of material that exhibit mass loss or gain due to decomposition, oxidation, or loss of volatiles (such aas moisture). Common applications of TGA are:1. Materials characterization through analysis of characteristic decomposition patterns,2. Studies of degradation mechanism and reaction kinetics,3. Determination of organic content in a sample, and4. Determination of inorganic (e.g. ash) content in a sample, which may be useful for corroborating predicted material structures or simply used a chemical analysis. It is an especially used technique for the study of polymeric materials, including thermoplastics, thermosets, elastomer, composites, plastic films, fibers, coating and paints. Discussion of the TGA apparatus, methods and trace analysis will be elaborated upon below. Thermal stability, oxidation and combustion, all of which are possible interpretations of TGA traces, can be predicted.

Chapter 2: Literature Survey

2.1 PEG ChemistryPEG is also known as polyethylene oxide (PEO) or polyoxy ethylene(POE) depending upon their molecular weight. Polyethylene glycol is the polymer of non ionic ether with a molecular formula H-(-O-CH2-CH2)n-OH. PEG, PEO, or POE refers to an oligomer of ethylene oxide. PEG has tended to refer to oligomers and polymers with a molecular mass below 20,000g/ml, PEO to polymers with a molecular mass above 20,000g/ml, and POE to a polymer of any molecular weights. PEG and PEO are liquids or low melting solids depending upon their molecular weights. PEGs are prepared by polymerization of ethylene oxide and are commercially available over a wide range of molecular weights from 300g/ml to 10,000,000g/ml. While PEG and PEO with different molecular weights find use in different applications, and have different physical properties (e.g.viscosity) due to chain length effects, their chemical properties are nearly identical. Different forms of PEG are also available, depending on the initiator used for the polymerization process-the most common initiator is a monofunctional methyl ether PEG or methoxypoly(ethylene glycol), abbreviated mPEG. Lower-molecular-weight PEGs are also available as purer oligomers, reffered to as monodisperse, uniform, or discrete. Very high purity PEG has recently been shown to be crystalline, allowing determination of a crystal structure by x-ray diffraction. Since purification and separation of pure oligomers is difficult, the price for this type of quality is often 10-1000 fold that of polydisperse PEG. Figure 2 . Polyethylene Glycol

PEGs are also available with different geometries:1. Branched PEGs have three to ten PEG chains emanating from a central core group.2. Star PEGs have 10 to 100 PEG chains emanating from a central core group.3. Comb PEGs have multiple peg chains normally grafted onto a polymer backbone.Polyethylene glycol is produced by the interaction or ethylene oxide with water, ethylene glycol or ethylene glycol oligomers. The reaction is catalyzed by acidic basic catalysts. Ethylene glycol and its oligomers are referable as a starting material instead of water, because they allow the creation of polymers with a low polydispersity (narrow molecular weight distribution). Polymer chain length depends on the ratio of reactants.HOCH2CH2OH+ n(CH2CH2O) HO(CH2CH2O)N+1HDepending on the catalyst type, the mechanism of polymerization can be cationic or anionic. The anionic mechanism is preferable because it allows one to obtain PEG with a low polydispersity, polymerization of ethylene oxide is an exothermic process. Overheating or contaminating ethylene oxide with catalysts such as alkalis or metal oxides can lead to runaway polymerization, which can end in an explosion after a few hours.Polyethylene oxide, or high-molecular weight polyethylene glycol, is synthesized by suspension polymerization. It is necessary to hold the growing polymer chain in solution in the course of the polycondensation process. The reaction is catalyzed by magnesium, aluminium or calcium-organoelement compounds. To prevent coagulation of polymer chains from solution chelating additives such as dimethyl glyoxime are used. High molecular weight PEG(e.g.8000) has been shown to be a dietary preventive agent against colorectal cancer in animal models. The injection of PEG 2000 into the bloodstream of guinea pigs after spinal cord injury leads rapid recovery through molecular repair of nerve membranes. The effectiveness of this treatment to prevent paraplegia in humans after an accident is not known yet.PEG is being used in the repair of motor neurons damaged in crush or laceration incidents in vivo and in vitro. Since PEG is a flexible, water soluble polymer, it can be used to create very high osmotic pressures (on the order of tens of atmospheres). It also is unlikely to have specific interactions with biological chemicals. These properties make PEG one of the most useful molecules for applying osmotic pressure in biochemistry and biomembranes experiments, in particular when using the osmotic stress technique. PEG is used in a number of toothpastes as a dispersant. In this application, it binds water and helps keep xanthun gum uniformly distributed throughout the toothpaste. The generation and stabilization catalytically active nanoparticles in water have several important advantages over both traditional homogeneous and supported transition-metal catalysts in terms of low cost, absence of expensive ligands and organic solvents. 1. Here, PEG act as a stabilizer, which absorb to the particle surface, control particle size, and prevent agglomeration. The resulting material bearing longer polyether chains has been successfully used as a stabilizer for the preparation of water soluble palladium nanoparticles. Water is an excellent solvent for this PEG capped nanoparticles in catalytic system. 2. Possibly the polar capping agent forms a more open structure in water, so that reactants could easily get in contact with the metal surface. 3. Using highly biocompatible PEG molecules as reducing and stabilizing agent, two types of water-soluble stable monometallic NPs (PEG8K-Pd) were successfully synthesized. 4. The size of the resulting NPs was controllable by the concentration of metal/polymer ratio precursors. 5. The electron transfer between the metal ions and hydroxyl groups of solvent methanol and PEG polymer results in the reduction of metal ions to zero valent metal. 6. The prepared polymeric nanoparticles can be preserved for months without any change of physical and chemical properties. Thus, the polymer stabilized metal NPs can be more effective catalysts for varied catalytic applications. 2.2 Palladium ChemistryElectronic configuration of palladium metal is 1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s0, 4d10. Atomic radius of palladium metal is 137 pm. Standard reduction potential palladium metal is 0.938V. Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas which itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas. Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as platinum group metals (PGMS). These have similar chemical properties, but palladium has the lowest melting point and is the least dense of them. Palladium plays a key role in the technology used for fuel cells which combine hydrogen and oxygen to produce water, heat and electricity. Palladium belongs to group 10 in the periodic table. Palladium is a soft silver white metal that resembles platinum. It is soft and ductile when annealed and greatly increases its strength and hardness when cold-worked. Palladium dissolves slowly in hydrochloric sulfuric and nitric acid. This metal also does not react with oxygen at normal temperature (and does not tarnish in air). Palladium heated to 8000C will produce a layer of palladium(II) oxide(PdO). It lightly tarnishes in moist atmosphere containing sulfur. Common oxidation states of palladium are 0, +1, +2, +3 and +4. Although originally +3 was thought of as one of the fundamental oxidation states of palladium, there is no evidence for palladium occurring in the +3 oxidation states, this has been investigated via X-ray diffraction for a number of compounds, indicator a dimer of palladium (II) and palladium(IV).

2.3 Applications of PalladiumThe largest use of palladium today is in catalytic converters. Palladium is also used in jewelry in dentistry, watch making, in blood sugar test strip in aircraft spark plugs and in the production of surgical instrument and electrical contacts. Palladium is also used to make professional transverse flute. A large number of carbon-carbon bond forming reactions in organic chemistry (such as the Heck and Suzuki coupling) are facilitated by catalysis with palladium compounds. In addition, palladium, when dispersed on conductive materials, proves to be excellent electro catalyst for oxidation of primary alcohols in alkaline media. Palladium is also a versatile for homogenous catalysis. Palladium itself has been used as a precious metal in jewelry since 1939, as an alternative to platinum for making white gold. 2.4 Transition Metal NanoparticlesTransition metal nanoparticles have attracted a great deal of attention in the last 10 years, their preparation, structure determination, and applications are capable of current interest. Nanoparticles are defined as having 1-50 nm diameter a size range where metals can show size dependent properties. The smaller the cluster of atoms, the higher the percentage of atoms are on the surface, rendering nanoparticles very interesting in catalysis. Thus a nanoparticle of 10nm diameter has about 10% of its atom in the surface, but one of 1nm has 100%. The metal atoms constituting nanoparticles can be generated by (i) chemical reduction of a metal salt, (ii) thermal, photochemical, or sonochemical decomposition of a metal (0) complex, (iii) hydrogenation of a coordinating oelifinic moiety, and (iv) the vapor phase deposition. To this list proposed by Bradley should be added (v) electrochemical reduction of valent species of the metal. During the generation of nanoparticles, the following steps have been identified (i) generation of atoms as above; (ii) nucleation to form an initial cluster of atoms; (iii) growing of cluster until a certain volume is reached; and (iv)surrounding the cluster by a protecting shell that prevents agglomeration. Therefore, nanoparticles should be formed in the presence of a protecting agent. These protectors can be divided into two categories: those providing electrostatic and those providing steric stabilization. The electrostatic stabilization is based upon the double electric layer formed when the ions of the same sign are adsorbed at the nanoparticle surface. The counter ions forms a second layer that repels the neighboring nanoparticle; for example sodium citrate acts by this mechanism. In other cases, protecting molecules of considerable length interact attractively with the surface of the particles. The volume of these surrounding molecules prevents mutual approach of metal surfaces at bonding distance. Popular protecting agents are polymers (poly(vinylpyrrolidone)), cyclodextrins, dendrimers, and so forth. Particularly well known is the mechanism of stabilization by large molecules featuring functional groups with high affinity for the metals, i.e. thiols, sulfides, amines or phosphines. Other common stabilizers are ionic surfactants i.e. sodium dodecyl sulfate and lauryl trimethyl ammonium chloride. These Compounds protect nanoparticles by both electrostatic and steric mechanism.Size and dispersity are important properties of nanoparticles. The development of uniform nanometer sized particles have been intensively pursued because of the many technological and fundamental scientific interests associates with these nanoparticles. These nanoparticle materials often exhibit very interesting electronic, optical, magnetic and chemical properties, which are unachievable for their bulk counterparts. The formation of metal nanoparticles is carried out by reduction of metal ions in the presence of stabilizers like polymers, dendrimers, microgel, surfactants, and colloids which prevent the nanoparticles from aggregation and serve as carriers. In general, the catalytic properties of metal nanoparticles are a function of their size, crystal lattice parameters and the properties of carrier systems.

Chapter 3Preparation of PEG8K-Pd Nanoparticles and Their Characterization 3.1 MaterialsPurity of the chemicals have a great influence on the properties and stability of the nanoparticles. Therefore it is equally important to discuss about the purity of the chemicals and materials used in the experiments. In the following experiments, a chemical reduction method was used which consisted of polyethylene glycol having molecular weight 8000, palladium acetate, HPLC grade methanol, HPLC grade deionized water and HPLC grade 1,4-dioxane.

3.2 Solvents(a) HPLC grade methanol and 1,4-dioxane was purchased from Sigma-Aldrich of HPLC grade.(b) HPLC grade deionized water was used to prepare all the solutions.

3.3 InstrumentationThe PEG8K-Pd nanoparticle catalysts were characterized by QELS, TEM, EDX, SEM, XRD, IR, TGA, and UV-Visible techniques. QELS measurements for determining the size of nanoparticles were performed using Photo Core-FC instrument with open modular architecture goniometer. Air cooled He/Ne was operated at 633nm and 20mw as light source. The time dependence of the intensity autocorrelation function of the scattered intensity was derived by using a 288-channel digital correlator.

3.4 Experimental The goal is to obtain nanoparticle narrow sized distribution and well stabilized palladium PEG capped nanoparticles.Controlling size and polydispersity of nanomaterials is a key requirement for most of the applications. In this work we described the synthesis and characterization of polymer PEG8K capped palladium nanoparticles of smallest size with low polydispersity. The use of water as a solvent and design of recyclable catalysts are some of the promising directions in this field. Among water soluble catalysts, a great deal of attention has been focused on the metal nanoparticles. Water-soluble Pd (0) nanoparticles are a promising class of catalysts in some organic processes, mainly in hydrogenation, oxidation and cross coupling reaction for the formation of C-C bonds. A wide of stabilizers for the preparation of nanoparticles is known, in order to prevent their aggregation. Polymer protected noble metal colloids are usually prepared from suitable metal precursors by various in situ reactions, such as chemical reduction, photo reduction and thermal decomposition.These protective polymers employed should fulfill the following requirements:1. They should be soluble in different solvents and be thermally stable at the temperature used for the preparation and technical applications of the colloids. 2. There should be good protective function of the polymer for the stabilization of metal colloids, therefore good interaction with the metal surface is essential.3. The polymer should ideally interact with the metal precursor, for instance for the complex and the ion-pair formation. The number of nuclei formed at the very beginning of the determines the number and size of the resultant particles. The atoms formed at the later period are used mainly to the collision with the nuclei already formed instead of the formation of new nuclei and therefore lead to the formation of larger particles. Among them polymers provide stabilization by entrapment, both through their steric bulk and through the weak dative bonds between the nanoparticle surface and the hetero atoms present in the structure of the protecting agent. In this context our work is focused on the use of polyethylene glycol (PEG) as the stabilizing agents for metal nanoparticles. The PEG chains are commercially available in various molecular weights which are soluble in water and are insoluble in diethyl ether. In this work, it has been synthesized a new PEG8K-Pd nanoparticles. The resulting material bearing longer polyether chains has been successfully used as stabilizer for the preparation of water soluble palladium nanoparticles. A common method for the preparation of metal nanoparticles involves the reduction of metal ions in the presence of stabilizers as surfactant and polymers. In the present study we used PEG8K as a stabilizer for palladium nanoparticles. We report a facile and novel route for the preparation of Pd nanoparticles by exploiting PEG, molecular weight 8000 (MW8K), which was found to act as both reducing agent and stabilizer.Preparation of polymeric PEG8K-Pd nanoparticles were achieved successfully (Scheme 1) by the reaction of a solution of Pd(OAc)2 in 1,4-dioxane with aqueous solution of PEG8K in methanol at room temperature in a 30ml closed vial for 5 hours.Pd(II)ions reduced to Pd(0) because of the presence of the terminal OH functional present in methanol as reducing agent as well as solvent in the system.

Scheme 1: Synthesis of PEG8K-Pd nanoparticlesThe experimental condition such as amount of protecting polymer, the concentration of metal ions are systematically changed to achieve the smallest size PEG8K-Pd nanoparticles. The size and morphology of the resulting PEG8K-Pd nanoparticles are sensitive to a number of different reaction conditions. These include the polymer PEG8K used, the reducing agent employed (methanol), the reaction time, the nature of the stabilizing ligand (polyethers), and the ratio of the palladium (palladium acetate) precursor to the other reagents (PEG8K). In general, to prepare small nanoparticles it is beneficial to use a large excess of long chain polymer with an excess of reducing agent.

3.5 Experimental Procedure Procedure for preparation of PEG8K-Pd nanoparticles1. In the typical experiment a mixture of palladium acetate Pd(OAc)2 (5.09 x 10-3 M in 1,4-dioxane solution) and molecular weight 8000 aqueous solution (3.050%) in methanol (15ml).2. The mixture is then allowed to for stirring on the magnetic stirrer for 5 hours.3. With the course of time the color of the solution turned orange to brown and finally turned black, indicating the formation of PEG8K capped Pd(0) metal nanoparticles.4. Size of the prepared nanoparticles were characterized by DLS technique. The reduction of Pd2+ ions followed an analogous polyol process in the current study. When Pd(II) ions were added into the methanol is solution, electropositive palladium ions are rapidly trapped by electronegative oxygen forming weak metal ion complex followed by analogous polyol process. In this system electron transfer between metal ions and hydroxyl group leads to the reduction of Pd2+ to Pd(0). 5. For TEM analysis the samples with best DLS results are re-prepared in 3sets, again DLS of those samples is done and then sample is diluted and finally the sample is analyzed through TEM.

3.6 Results Characterization of prepared PEG8K-Pd nanoparticles by DLS

a. b. c.

d. e. f. g. h. i.

Table 3.1: DLS data of prepared PEG8K-Pd nanoparticles

Sample NamePEG8K (ml)Pd(OAc)2 (ml)Mean Size (nm)Polydispersity (s.d./mean2)

RI-10.90.1NA36.79

RI-20.80.254.860.341

RI-30.70.359.700.042

RI-40.60.467.430.600

RI-50.50.545.480.052

RI-60.40.636.350.427

RI-70.30.730.830.205

RI-80.20.832.880.342

RI-90.10.9NA480.8

We selected samples Fig.(b) (sample name: RI-2) and (h) (sample name: RI-8) as the best results from above table . The samples that were collected are reprepared in a set of 3 to check the reproducibility of the procedure so that the procedure can be standardized.

DLS data of re-prepared Samples a b c d e f

Table 3.2 : DLS data of reprepared samples

DLS evaluation of the nanoparticles indicates that the size distributions of the particles are very narrow. We begin the studies with some preliminary investigations of the particle core size by using QELS. The palladium nanoparticles are capped by PEG8K molecules that provided sufficient hydrophobicity to the nanoparticles. These particles are stable upto several months at room temperature. Using DLS data we found that sample RI-2b with size 51.50nm and polydispersity 0.643 and RI-8a with mean size 43.48nm and polydispersity 0.082 are the best prepared nanoparticles among the reported samples.

3.7 ConclusionThe utilization of nanodimensional materials offer significant benefits in a range of different applications. In order to maximize their usefulness, reliable synthesis are required that can generate well-defined nanoparticles with a high degree of polydispersity. This aim is being achieved in the synthesis of PEG8K-Palladium nanoparticles by using polyethylene glycol 8,000 sterically bulky molecules to control the synthesis. This enables the properties such as the size, shape, solubility and surface functionality of the resulting nanoparticles to be carefully tuned. Such materials are being explored for many different applications, especially in catalysis, where palladium can effectively catalyze a range of different transformations. From the DLS data we found that sample RI-2b with size 51.50nm and polydispersity 0.643 and RI-8a with mean size 43.48nm and polydispersity 0.082 are the best prepared nanoparticles among the reported samples. Thus we achieved our goal of preparing smallest size nanoparticles with highest polydispersity using chemical reduction method.

References:1. Berne, B. Pecora, R.: Dynamic Light Scattering with applications to chemistry, biology and physics, Wiley-Interscience, New York 2. A. Moores and F. Goettmann, New J. Chem., 2006, 30 (8), 1121 3. M. C. Daniel, D. Astruc., Chem., Rev., 2004, 104, (1), 293 4. N. Semagina, N. Renken and L. Kiwi Minsker, J. Phy., Chem., C, 2007, 111, (37) 139335.O. M. Wilson, M. R. Knecht, J. C. Garcia-Martinez and R. M. Crooks, J. Am. Chem. Soc., 2006, 128, (14), 4510 6. N. Dimitratos, F. Porta and L. Prati, Appl. Catal. A: Gen 2005, 291, (1-2), 210 7. Z. Hou, N. Theyssen, A. Brinkmann and W. Leitner, Angew Chem. Int. Ed. 2005, 44, (9), 1346

10