Osm Physics by Ashish

Reactive Methods for Synthesis of Nanostructures Reactive Methods for the Synthesis of Nanostructures 1. Introduction Reactive methods involve the formation of nanostructures through controlled chemical reactions such as reduction, oxidation, hydrolysis, condensation, and decomposition. These methods allow precise control over particle size, morphology, and composition. 2. Fundamental Mechanism (i) Generation of Reactive Species M n+ → M 0 (reduction) M(OR) n + H 2 O → M(OH) n (ii) Nucleation Formation of stable nuclei under supersaturation conditions. (iii) Growth Controlled by diffusion or surface reaction mechanisms. (iv) Stabilization Surfactants prevent agglomeration and control particle size. 3. Types of Reactive Methods 3.1 Sol–Gel Method Principle Transformation of a sol into a gel through hydrolysis and condensation. Reactions Hydrolysis: M(OR) n + H 2 O → M(OH) n + ROH Condensation: M–OH + M–OH → M–O–M + H 2 O S...

Precipitation Method (Co-precipitation) Precipitation Method (Co-precipitation Method) 1. Introduction The precipitation (co-precipitation) method is a widely used wet chemical technique for synthesizing nanostructured materials such as metal oxides, hydroxides, and composites. It involves the formation of an insoluble solid (precipitate) when soluble precursors react in solution. 2. Basic Principle The method is based on supersaturation, leading to nucleation and growth of nanoparticles. Chemical Reaction: M n+ + X m- → MX ↓ Example: Fe 3+ + 3OH - → Fe(OH) 3 ↓ 3. Mechanism of Formation (i) Supersaturation Occurs when ion concentration exceeds solubility Drives nucleation (ii) Nucleation Formation of small stable nuclei Can be homogeneous or heterogeneous (iii) Growth Controlled by diffusion and surface reactions (iv) Ostwald Ripening Smaller particles dissolve and redeposit on larger ones 4. Co-pr...

General Methods for the Synthesis of Nanostructures Nanostructures are synthesized mainly through  bottom-up approaches , where atoms, ions, or molecules assemble into nanoscale materials via  nucleation and growth mechanisms . Control over  thermodynamics and kinetics  plays a crucial role in determining  size, morphology, crystallinity, and functionality . 1. Precipitation Method (Co-precipitation Method) 2. Reactive Methods (Chemical Synthesis Methods) 3. Hydrothermal / Solvothermal Method

4. Effect on the Phonon Density of States Another effect of size reduction can be seen in the lattice dynamics of nanometric particles. When the surface-to-volume ratio reaches a certain value, the phonon spectrum broadens. Phonons are quasi-particles representing the vibrational modes of atoms in the lattice. On the low frequency side, the broadening is due to the contribution of surface atoms which have softer modes. The broadening toward higher frequencies is due to the lattice contraction which corresponds to increased rigidity in the system, itself the consequence of increased interatomic forces. These changes in the distribution of the phonon spectrum also affect the thermodynamic properties of the system. Note in particular that there is an increase in the vibrational entropy and that the specific heat deviates from a T^3 dependence at low temperatures. The increase in the surface-to-volume ratio when the size decreases also has a significant effect on the melting temperature of...

Effect on the Lattice Parameter 3. Effect on the Lattice Parameter Let us now consider the effects of the increase in the surface-to-volume ratio as the object size decreases. We first analyze the simple case of a liquid sphere of diameter R . Due to curvature, a pressure difference ΔP is generated between the inside and outside of the sphere. In hydrostatic equilibrium, this is given by the Laplace equation: ΔP dV = γ dA where dV is the volume change corresponding to a change dA in surface area. For a sphere, this becomes: ΔP = 2γ / R For a spherical solid, the specific surface energy must be replaced by the surface stress tensor g ij . Considering a simple cubic structure (isotropic case), we have: g = γ + dγ/dA Surface stress accounts for deformation effects, unlike surface energy which only accounts for surface creation. The compressibility is defined as: χ = -ΔV / VΔP where V is the atomic volume of the solid, whi...

Specific Surface Energy and Surface Stress 2. Specific Surface Energy and Surface Stress The specific surface energy γ (J/m²) can be defined as the energy required to create a new surface by cleaving a crystal. More generally, it represents the work needed to increase the surface area of a material. If the surface area increases by an amount dA , the work done is given by: dW = γ dA Here, γ is the specific surface energy. The increase in surface area can occur by moving atoms from the bulk to the surface. Alternatively, the area can be increased by stretching the surface while keeping the number of surface atoms constant. In the case of stretching, the work done is: dW = g ij dA where g ij represents the surface stress (J/m²) . It is a tensor quantity because it depends on crystallographic directions. Surface stress is related to elastic stresses generated due to deformation (strain) at the surface. The relation betwee...

  Fraction of Surface Atoms Consider a homogeneous solid material of compact shape (let us say spherical) and macroscopic dimensions (let us say millimetric). Most of its properties will be related to its chemical composition and crystal structure. This is what is traditionally studied in the physics and chemistry of solids. For an object of this size, the surface atoms comprise a negligible proportion of the total number of atoms and will therefore play a negligible role in the bulk properties of the material. Note, however, that surface atoms will nevertheless play a predominant role in properties involving exchanges at the interface between the object and the surrounding medium. This is the case, for example, when we consider chemical reactivity (and catalysis) and crystal growth.  It can be seen from Fig. that, when the size of the object is reduced to the nanometric range, i.e., < 10 nm, the proportion of surface atoms is is about 20%, whilst at 2nm (around 500 atoms),...

 Size Effects There are two ways of approaching the properties of nanoscale objects: the bottom-up approach and the top-down approach. In the first, one assembles atoms and molecules into objects whose properties vary discretely with the number of constituent entities, and then increases the size of the object until this discretisation gives way in the limit to continuous variation. The relevant parameter becomes the size rather than the exact number of atoms contained in the object. In the second case, one considers the evolution of the properties of a sample as its size is whittled down from macroscopic toward nanometric lengths. It is this approach that we shall examine here, whilst mentioning zones of overlap and exclusion between the two approaches. Fraction of Surface Atoms Specific Surface Energy and Surface Stress Effect on the Lattice Parameter Effect on the Phonon Density of States