Nanoscale Materials Chemistry (21NTC201J) Course Notes PDF

NANOSCALE MATERIALS CHEMISTRY (21NTC201J) Course Instructor: Dr. Venkata Ravindra A Assistant Professor Department of Physics and Nanotechnology SRM Institute of Science and Technology Em...

NANOSCALE MATERIALS CHEMISTRY (21NTC201J) Course Instructor: Dr. Venkata Ravindra A Assistant Professor Department of Physics and Nanotechnology SRM Institute of Science and Technology Email: [email protected] Phone: 8019448666 3 August 2023 1  Time Table 3 August 2023 2 3 August 2023 3 3 August 2023 4 3 August 2023 5 Unit I: Chemistry of Nanoparticles' Synthesis Chemical bonding and surface properties Introduction to chemical bonding Atomic bonding Types of bond: metallic, ionic, covalent, and Vander Waals bonds Surface energy Chemical potential as a function of curvature Electrostatic stabilization Surface charge density Electric potential at the proximity at solid surface Vander Waals attraction potential DLVO theory and steric stabilization Influence of kinetic energy on the surface of nanomaterials 3 August 2023 6  Chemical Bonding Introduction: 118 elements on the periodic table combine to make millions and millions of chemical compounds This is because chemical bonds between atoms result in new substances that are very different from the elements they are made of. Chemical Bonding: Process of holding atoms together in solids is called Bonding. Intermolecular forces are responsible for holding the atoms to stay together in materials. When a force holds atoms together long enough to create a stable, independent entity, that force can be described as a chemical bond. The work done by these forces represents potential energy. The potential energy versus interatomic distance relation is important for understanding properties of solids. 3 August 2023 7  Bonding Force and Potential Energy vs. Interatomic Separation 3 August 2023 8 Potential Energy vs. Interatomic Separation 3 August 2023 9  Atomic Bonding Introduction: All materials are made up of atoms. These atoms are held together by forces called interatomic bonds which are incredibly important in determining materials properties. The bonds act like springs, linking each atom to its neighbour. There are several different types of bonds that form between atoms. The type of bonding between atoms can give rise to very different properties. For example, graphite and diamond are both carbon, however, due to the nature of their atomic bonding, they exhibit exceptionally different material characteristics. Atoms are arranged in different ways in different materials. Two important aspects of atomic packing are the number of bonds per unit area and the bond angle. 3 August 2023 10  Primary Bonds Primary bonds involve sharing or donating electrons between atoms to form a more stable electron configuration. Primary bonding occurs when electrons are lost or gained so that the outer shell is filled. All elements except inert gases have an unfilled valence shell. For example, sodium has a nucleus containing 11 protons and orbiting shells containing 11 electrons. The outer shell has one valence electron. If a sodium atom loses its valence electron, it is left with a full outer shell of electrons and if a chlorine atom, which has only seven electrons in the outer shell, gains an electron, its outer shell is then full. Ex.: Metallic, Ionic, Covalent 11 3 August 2023  Metallic Bonds Metallic bonds are the chemical bonds that hold atoms together in metals. They differ from covalent and ionic bonds because the electrons in metallic bonding are delocalized, that is, they are not shared between only two atoms. Instead, the electrons in metallic bonds float freely through the lattice of metal nuclei. This type of bonding gives metals many unique material properties, including excellent thermal and electrical conductivity, high melting points, and malleability. Metallic bonds are formed when the valence electrons of metal atoms are shared by more than one neighboring atom. The metal atoms are held together by a “sea” of electrons floating around. 12 3 August 2023  Metallic Bonds Metals consist of a lattice of positive ions through which a cloud of electrons moves. The positive ions will tend to repel one another, but are held together by the negatively charged electron cloud. The mobile electrons, known as conduction electrons, can transfer thermal vibration from one part of the structure to another i.e., metals can conduct heat. Ex.: The metal atoms Na, Cu, Ag, Fe etc., are bound to each other in their crystals by metallic bond. 13 3 August 2023  Ionic Bond Ionic bond is also called electrovalent bond. It is formed from the electrostatic attraction between oppositely charged ions in a chemical compound. It forms when the valence electrons of one atom are transferred permanently to another atom. The atom that loses the electrons becomes a positively charged ion (cation), while the one that gains them becomes a negatively charged ion (anion). Ionic bonding results in compounds known as ionic, or electrovalent, compounds, which are best exemplified by the compounds formed between nonmetals and the alkali and alkaline- earth metals. 14 3 August 2023  Ionic Bonds In ionic crystalline solids of this kind, the electrostatic forces of attraction between opposite charges and repulsion between similar charges orient the ions in such a manner that every positive ion becomes surrounded by negative ions and vice versa. In short, the ions are so arranged that the positive and negative charges alternate and balance one another, the overall charge of the entire substance being zero. The magnitude of the electrostatic forces in ionic crystals is considerable. Accordingly, these substances tend to be hard and nonvolatile. 15 3 August 2023  Ionic Bonds A covalent bond is formed when electrons from both participating atoms are shared equally. The pair of electrons involved in this type of bonding is known as a shared pair or bonding pair. Molecular bonds are another name for covalent bonds. The sharing of bonding pairs will ensure that the atoms achieve stability in their outer shell, similar to noble gas atoms. Elements with extremely high ionization energies are incapable of transferring electrons, and elements with extremely low electron affinity are incapable of absorbing electrons. The atoms of such elements tend to share electrons with atoms of other elements or atoms of the same element in such a way that both atoms achieve octet configuration in their respective valence shells and thus achieve stability. 16 3 August 2023  Covalent Bonds A covalent Bond refers to such an association formed by the sharing of electron pairs among different or similar kinds. If sharing a single electron pair between atoms does not satisfy an atom’s normal valence, the atoms may share more than one electron pair between them. 17 3 August 2023  Types of Covalent Bonds 18 3 August 2023  Secondary Bonds Secondary bonds are weak in comparison to primary bonds. They are found in most materials, but their effects are often overshadowed by the strength of the primary bonding. Secondary bonds are not bonds with a valence electron being shared or donated. They are usually formed when an uneven charge distribution occurs, creating a dipole Dipole: The total charge is zero, but there is slightly more positive or negative charge on one end of the atom than on the other. These dipoles can be produced by a random fluctuation of the electrons around what is normally an electrically symmetric field in the atom. 19 3 August 2023  Secondary Bonds Once a random dipole is formed in one atom, an induced dipole is formed in the adjacent atom. This is the type of bonding is known as Van Der Waals Bonding. Secondary bonding may also exist when there is a permanent dipole in a molecule due to an asymmetrical arrangement of positive and negative regions. Molecules with a permanent dipole can either induce a dipole in adjacent electrically symmetric molecules, and thus form a weak bond, or they can form bonds with other permanent dipole molecules. 20 3 August 2023  Van Der Waals (VDW) Bonding 21 3 August 2023  Hydrogen Bonding 22 3 August 2023 3 August 2023 23  Surface Energy Atoms or molecules on a solid surface possess fewer nearest neighbors or coordination numbers, and thus have dangling or unsatisfied bonds exposed to the surface. Because of the dangling bonds on the surface, surface atoms or molecules are under an inwardly directed force and the bond distance between the surface atoms or molecules and the sub-surface atoms or molecules, is smaller than that between interior atoms or molecules. The extra energy possessed by the surface atoms is described as surface energy, surface free energy or surface tension. Surface energy (γ) is the energy required to create a unit area of “new” surface. where A is the surface area 24 3 August 2023  Surface Energy (γ) ‘Nb’ is the number of broken bonds ‘ε’ is bond strength. ‘ρa’ is the surface atomic density: the number of atoms per unit area on the new surface Surface energy of a face-centered cubic (fcc) crystal structure: Nb = 4 Nb = 5 Nb = 3 25 3 August 2023  Mechanisms to Reduce Surface Energy 1. Surface relaxation: 2. Surface restructuring: 3. Surface adsorption: 4. composition segregation or impurity enrichment on the surface through solid-state diffusion 26 3 August 2023  Chemical Potential as a Function of Surface Curvature Chemical potential is dependent on the radius of curvature of a surface Let us consider transferring material from an infinite flat surface to a spherical solid particle As a result of transferring of dn atoms from a flat solid surface to a particle with a radius of R, the volume change of spherical particle Ω is atomic volume μc is the chemical potential on the particle surface μꝏ is the chemical potential on the flat surface Young-Laplace equation: An atom in a spherical surface An atom in any type of curved surface27 3 August 2023  Chemical Potential as a Function of Surface Curvature For a convex surface, the curvature is positive, and thus the chemical potential of an atom on such a surface is higher than that on a flat surface. Mass transfer from a flat surface to a convex surface results in an increase in surface chemical potential. When mass is transferred from a flat surface to a concave surface, the chemical potential decreases. Thermodynamically, an atom on a convex surface possesses the highest chemical potential an atom on a concave surface has the lowest chemical potential 28 3 August 2023  Vapor Pressure as a Function of Surface Curvature: μv is the chemical potential of a vapor atom k is the Boltzmann constant Pꝏ and Pc are the equilibrium vapor pressure of flat and curved solid surfaces, respectively T is temperature. An atom in any type of curved surface An atom in a spherical surface This equation is known as the Gibbs-Thompson relation Solubility as a Function of Surface Curvature: Sꝏ and Sc are the solubility of flat and curved solid surfaces, respectively This equation is known as the Gibbs-Thompson relation 29 3 August 2023  Variation in solubility of silica with radius Vapor pressure of a number of liquids as a of curvature of surface function of droplet radius 30 3 August 2023  Stabilization Mechanisms In nanostructure fabrication and processing, it is very important to overcome the huge total surface energy to create the desired nanostructures. It is equally important to prevent the nanostructures from agglomeration As the dimension of nanostructured materials reduces, van der Waals attraction force between nanostructured materials becomes increasingly important Stabilization mechanisms are required for the prevention of agglomeration of individual nanostructures There are two major stabilization mechanisms widely used: Electrostatic stabilization Steric stabilization Electrostatic stabilization kinetically stable Steric stabilization thermodynamically stable 31 3 August 2023  Electrostatic Stabilization Surface charge density: When a solid emerges in a polar solvent or an electrolyte solution, a surface charge will be developed through one or more of the following mechanisms: Preferential adsorption of ions Dissociation of surface charged species Isomorphic substitution of ions Accumulation or depletion of electrons at the surface Physical adsorption of charged species onto the surface Surface electrical charge density or electrode potential The surface potential of a solid varies with the concentration of the ions in the surrounding solution Can be either positive or negative The surface charge in oxides is mainly derived from preferential dissolution or deposition of ions 32 3 August 2023  Surface charge density: In the oxide systems, typical charge determining ions are protons and hydroxyl groups The concentration is described by pH The concentration of charge determining ions corresponding to a neutral or zero-charged surface is defined as a point of zero charge (p.z.c.) or zero-point charge (z.p.c.). At pH > p.z.c., the oxide surface is negatively charged, since the surface is covered with hydroxyl groups, OH- At pH < p.z.c., H+ is the charge determining ions and the surface is positively charged 3 August 2023 33  Electrostatic Stabilization Electric potential at the proximity of solid surface: When a surface charge density of a solid surface is established, there will be an electrostatic force between the solid surface and the charged species in the proximity to segregate positive and negatively charged species However, there also exist Brownian motion and entropic force, which homogenize the distribution of various species in the solution Although charge neutrality is maintained in a system, distributions of charge determining ions and counter ions in the proximity of the solid surface are inhomogeneous and very different The distributions of both ions are mainly controlled by a combination of the following forces: Coulombic force or electrostatic force Entropic force or dispersion Brownian motion 34 3 August 2023  Electric potential at the proximity of solid surface: The inhomogeneous distributions of ions in the proximity of the solid surface lead to the formation of so-called double layer structure The double layer consists of two layers Stern layer Gouy layer These two layers are separated by the Helmholtz plane When two particles are far apart, there will be no overlap of two double layers and electrostatic repulsion between two particles is zero. However, when two particles approach one another, double layer overlaps and a repulsive Electrical double layer structure and the force develops electric potential near the solid surface 35 3 August 2023  Van der Waals attraction potential: When particles are small, typically in micrometers or less, and are dispersed in a solvent, van der Waals attraction force and Brownian motion play important roles, whereas the influence of gravity becomes negligible Van der Waals force is a weak force and becomes significant only at a very short distance. Brownian motion ensures that the nanoparticles are colliding with each other all the time. The combination of van der Waals attraction force and Brownian motion would result in the formation of agglomeration of the nanoparticles. The total interaction energy or attraction potential A is a positive constant termed the Hamaker constant 36 3 August 2023  Van der Waals attraction potential: The van der Waals attraction potential between two particles are different from that between two flat surfaces. The interaction between two molecules is significantly different from that between two particles. The attraction force between two particles decay much slowly and extends over distances of nanometers. Van der Waals interaction energy between two molecules can be simply represented by As a result, a barrier potential must be developed to prevent agglomeration 37 3 August 2023  Interactions between two particles DLVO Theory The total interaction between two particles, which are electrostatic stabilized, is the combination of van der Waals attraction and electrostatic repulsion: The electrostatic stabilization of particles in a suspension is successfully described by the DLVO theory, named after Derjaguin, Landau, Venvey and Overbeek. Assumptions of DLVO Theory: Infinite flat solid surface Uniform surface charge density No redistribution of surface charge the surface electric potential remains constant No change of concentration profiles of both counter ions and surface charge determining ions the electric potential remains unchanged Solvent exerts influences via dielectric constant only no chemical reactions between the particles and solvent 38 3 August 2023  DLVO Theory: At a distance far from the solid surface, both van der Waals attraction potential and electrostatic repulsion potential reduce to zero Near the surface is a deep minimum in the potential energy produced by the van der Waals attraction. A maximum is located a little farther away from the surface, as the electric repulsion potential dominates the van der Waals attraction potential. The maximum is also known as repulsive barrier. If the barrier is greater than ~ 10 kBT, Schematic of DLVO potential: where kB is Boltzmann constant, the VA = attractive van der Waals potential collisions of two particles produced by VR = repulsive electrostatic potential Brownian motion will not overcome the barrier and agglomeration will not occur. 39 3 August 2023  The electric potential is dependent on the concentration and valence state of counter ions The van der Waals attraction potential is almost independent of the concentration and valence state of counter ions The overall potential is strongly influenced by the concentration and valence state of counter ions An increase in concentration and valence state of counter ions results in a faster decay of the electric potential As a result, the repulsive barrier is reduced and its position is pushed Variation of the total interaction energy towards the particle surface between two spherical particles, as a function If secondary minimum is established, of the closest separation distance between their surfaces, for different double layer particles are likely to be associated thickness (K-1) obtained with different with each other, which is known as monovalent electrolyte concentrations. flocculation. The electrolyte concentration is C (mo1.L-1) = 1015 K2 (cm-1). 40 3 August 2023  When two particles are far apart: ❖ The distance between the surfaces of two particles is larger than the combined thickness of two electric double layers of two particles ❖ There would be no overlap of diffusion double layers ❖ There would be no interaction between two particles When two particles move closer: ❖ The two electric double layers overlap ❖ A repulsion force is developed As the distance reduces, the repulsion increases It reaches the maximum when the distance between two particle surfaces equals the distance between the repulsive barrier and the surface. Schematic illustrating the conditions for the occurrence of electrostatic repulsion between two particles. 41 3 August 2023  Understanding the repulsion force: The repulsion derives from the overlap of electric potentials of two particles. Osmotic flow: When two particles approach one another, the concentrations of ions between two particles where two double layers overlap, increase significantly, since each double layer would retain its original concentration profile. As a result, the original equilibrium concentration profiles of counter ions and surface charge determining ions are destroyed. To restore the original equilibrium concentration profiles, more solvent needs to flow into the region where the two double layers overlap. Such an osmotic flow of solvent effectively repels two particles apart. The osmotic force disappears only when the Schematic illustrating the distance between the two particles equals to or conditions for the occurrence of becomes larger than the sum of the thickness of the electrostatic repulsion between two particles. two double layers. 42 3 August 2023  Electrostatic Stabilization is limited by: 1) Electrostatic stabilization is a kinetic stabilization method. 2) It is only applicable to dilute systems. 3) It is not applicable to electrolyte sensitive systems. 4) It is almost not possible to redisperse the agglomerated particles. 5) It is difficult to apply to multiple phase systems, since in a given condition, different solids develop different surface charge and electric potential. 43 3 August 2023  Steric Stabilization Steric stabilization is also called polymeric stabilization It is widely used in stabilization of colloidal dispersions and in the synthesis of nanoparticles However, it is less well understood as compared with electrostatic stabilization method Polymeric stabilization does offer several advantages over electrostatic stabilization 1. It is a thermodynamic stabilization method, so that the particles are always redispersible. 2. A very high concentration can be accommodated, and the dispersion medium can be completely depleted. 3. It is not electrolyte sensitive. 4. It is suitable to multiple phase systems. 5. Can lead to a narrow size distribution in the synthesis of nanoparticles Non-solvable polymers cannot be used for the steric stabilization Polymer layer adsorbed on the surface of nanoparticles serves as a diffusion barrier to the growth species, resulting in a diffusion-limited growth in the subsequent growth of nuclei 44 3 August 2023  Solvents: Solvents can be grouped into aqueous solvent, which is water, H2O, and non- aqueous solvents or organic solvents. Solvents can also been categorized Protic solvent: exchanges protons. Ex. methanol, ethanol Aprotic solvent: Cannot exchange protons. Ex.: benzene When a solvable polymer dissolves into a solvent, the polymer interacts with the solvent. Such interaction varies with the system as well as temperature. Good solvent: When polymer in a solvent tends to expand to reduce the overall Gibbs free energy of the system Poor solvent: When polymer in a solvent tends to coil up or collapse to reduce the Gibbs free energy For a given combination of polymer and solvent, whether the solvent is a “good” or “poor” solvent is dependent on the temperature. At high temperatures: polymer expands; At low temperatures: polymer collapses The temperature at which a poor solvent transfers to a good solvent is called “Flory-Huggins theta temperature” or “theta (θ) temperature” At T= θ, the solvent is considered to be at the theta state, at which the Gibbs free energy does not change whether the polymer expands or collapses. 45 3 August 2023  46 3 August 2023  Polymers: Depending on the interaction between polymer and solid surface, a polymer can be grouped into. Anchored polymer: which irreversibly binds to solid surface by one end only, and typically are di-block polymer Adsorbing polymer: which adsorbs weakly at random points along the polymer backbone Non-adsorbing polymer: Schematic of different polymers according which does not attach to solid to the interaction between polymers and surface and thus does not contribute solid surface: to polymer stabilization (a) anchored polymer (b) absorbing polymer 47 3 August 2023  Interaction between solvent and polymer: The interaction between a polymer and solid surface is limited to adsorption of polymer molecules onto the surface of solid. The adsorption can be either by forming chemical bonds between surface ions or atoms on the solid and polymer molecules or by weak physical adsorption. Furthermore, there is no restriction whether one or multiple bonds are formed between solid and polymer. No other interactions such as chemical reactions or further polymerization between polymer and solvent or between polymers are considered 48 3 August 2023  Interactions between polymer layers: First let us consider two solid particles covered with terminally anchored polymers as schematically illustrated below When two particles approach one another, the attached polymers interact only when the separation distance (H) between the surfaces of two particles is less than twice the thickness of polymer layers (L) Beyond this distance, there is no interaction between two particles and their polymer layers on surfaces When the distance reduces to less than 2L, but is still larger than L, there will be interactions between solvent and polymer and between two polymer layers. But there is no direct interaction between the polymer layer of one particle and the solid surface of the opposite particle Schematic of interactions between polymer layers: (a) the schematic of two approaching polymer layers (b) the Gibbs free energy as a function of the distance between two particles. 49 3 August 2023  In a good solvent: Case 1: If the coverage of polymer on the solid surface is not complete, particularly less than 50% coverage When the concentration of polymer in the solvent is insufficient Two polymer layers tend to interpenetrate so as to reduce the available space between polymers. Such an interpenetration of two polymer layers of two approaching particles would result in a reduction of the freedom of polymers, which leads to a reduction of entropy, i.e. ΔS < 0. As a result, the Gibbs free energy of the system would increase, assuming the change of enthalpy due to the interpenetration of two polymer layers negligible, i.e. ΔH = 0 So two particles repel one another and the distance between two particles must be equal to or larger than twice the thickness of polymer layers Case 2: When the coverage of polymer is high, particularly approaching 100% There would be no interpenetration. As a result, the two polymer layers will be compressed, leading to the coil up of polymers in both layers. The overall Gibbs free energy increases, and repels two particles apart. When the distance between the surfaces of two particles is less than the thickness of polymer layers, a further reduction of the distance would force polymers to coil up and result in an increase in the Gibbs free energy. 50 3 August 2023  In a poor solvent: Case 1: If the coverage of polymer on the solid surface is not complete, particularly less than 50% coverage When the distance between two particles is less than twice the thickness of polymer layers but larger than the thickness of single polymer layer, i.e. L < H < 2L, polymers adsorbed onto the surface of one particle surface tend to penetrate into the polymer layer of the approach particle. Such interpenetration of two polymer layers will promote further coil up of polymers Result in a reduction of the overall Gibbs free energy. Two particles tend to associate with one another Case 2: When the coverage of polymer is high, particularly approaching 100% Similar to polymer in a good solvent, there would be no penetration and the reduction in distance results in a compressive force Leads to an increase in the overall free energy When the distance between two particles is less than the thickness of the polymer layer, a reduction in distance always produces a repulsive force and an increase in the overall Gibbs free energy Regardless of the difference in coverage and solvent, two particles covered with polymer layers are prevented from agglomeration by the space exclusion or steric stabilization. 51 3 August 2023  In a poor solvent: Schematic of interactions between polymer layers: (a) the schematic of two approaching polymer layers and (b) the Gibbs free energy as a function of the distance between two particles. The physical basis for the steric stabilization is: (i) a volume restriction effect arising from the decrease in possible configurations in the region between the two surfaces when two particles approach one another (ii) an osmotic effect due to the relatively high concentration of adsorbed polymeric molecules in the region between the two particles. 52 3 August 2023