Title: ETH_Sem6 / ACIV / Section2

Q: Solid-state Synthesis via Ceramic Method
A: Heat a mixture of metal oxides -> \(500 - 3000 \, \mathrm{^{\circ}C}\) energy intensive!
\(\Rightarrow\)Solid to solid diffusion already at \(\frac{mp(\mathrm{K})}{2}\) (Tamman temperature)
\(\Rightarrow\)Usually gives thermodynamic product

Q: Solid-state Synthesis via Molten Salt Approach advantages (2)
A: - Decrease mp. by eutectic mixture, eg. NaOH, KOH in 1:1 mixture \(\Rightarrow 165 \, \mathrm{^{\circ}C}\)
- Access to different morphologies, as kinetic control

Q: Molten Salt Approach: Adams Method
A: 1. Mixing of metal chlorides in water
2. Addition of large excess \(\ce{NaNO3}\)
3. Homogenizing (stirring, sonication) and drying of the mixture
4. Heating in open crucible (typically \(350-500 \, \mathrm{^{\circ}C}\) for 1-3 h
5. Washing with water and drying

Q: Thermolytic Molecular Precursors
A: Tune synthesis with molecules (anions) that decompose at low temperature (e.g. \(\ce{CO3^{2-}}, \ce{C2O4^{2-}}\)), which give the respective oxides upon heating or \(\ce{SiX}\) to get the silicates. Low temperatures enable access to kinetic products.

Q: Atomic layer deposition
A: A film is grown on a substrate by exposing the metal oxide surface to alternate gaseous species.
Step 1 is the reaction of TMA (trimethylamine); Step 2 is a purge or evacuation step, Step 3 is the reaction of water, and Step 4 is a purge or evacuation step.
In contrast to chemical vapor deposition, the precursors are never present simultaneously in the reactor, but they are inserted as a series of sequential, non-overlapping pulses. In each of these pulses the precursor molecules react with the surface in a self-limiting way, so that the reaction terminates once all the available sites on the surface are consumed.

Q: Both, "classical" chemical vapor deposition (CVD) and atomic layer deposition (ALD) are gas-phase deposition techniques. What are the similarities and differences between these two methods? What is the main advantage of ALD?
A: Similarity:
- Both used for the synthesis of thin films using volatile precursors.
Differences:
- CVD: the surface is usually exposed to simultaneous flows of precursor vapor and reactant gas, the precursors are decomposed in the gas phase, and the products deposit at the surface. The growth rate of the film usually depends on the local gas flux. The final film thickness is a function of both, reaction rate and time.
- ALD: two self-limiting and complementary reactions are used sequentially in such a way that a single layer is established in each cycle.
- Advantage ALD: much better control of the thickness of the film, which can be simply modified via the number of cycles and as ALD is based on chemisorption it requires reactive surface sites such as OH-groups -> area-selective ALD is possible.

Q: Why is CVD less convenient for coating of 3D shape surface structures such as nanoporous materials compared to ALD?
A: In CVD the growth rate depends on the local gas flux. Due to limited diffusion of the gaseous precursors into the pores, the local gas flux is higher at the entrance of the pores, leading to thicker films in these regions, which is not an issue in ALD.

Q: Write down the two sequential, self-limiting surface half-reactions for \(\ce{SiO2}\) ALD from \(\ce{SiCl4}\) and explain the role of pyridine that was added first. Propose an experimental approach to study the role of pyridine.
A: The half-reactions are (* indicates surface species):
After these two reactions, we have introduced one new layer.
\[\ce{Si-OH^* + SiCl4 -> SiO-SiCl3^* + HCl}\]\[\ce{SiCl^* + H2O -> Si-OH^* + HCl}\]Pyridine acts as a Lewis Base cat. Strong hydrogen bonding to isolated hydroxyl groups increases the nucleophilicity of the oxygen and therefore enhances the nucleophilic attack on \(\ce{SiCl4}\).
To confirm the role of pyridine, one could do pyridine adsorption experiments: IR analysis at different pyridine pressures and comparing the OH frequencies, which should be red-shifted due to the interaction of the nitrogen lone pair with the hydrogen on the OH.

Q: Disadvantage of the molten salt method given that a molten salt matrix is used in addition to the pure metal oxide precursors?
A: It can be much harder to purify the material, as the salt matrix cannot always be rinsed off with water.

Q: Phases of Sol-Gel synthesis (4)
A: 1. Hydrolysis of the metal precursor: the monomers for the inorganic polymerization are generated.
2. Nucleation via condensation: the formation and precipitation of particles is associated with a kinetic barrier (which is why it is possible to generate a supersaturated solution). The height of this barrier depends on the concentration of monomer in solution. Phase 2 is triggered when the monomer concentration (generated by hydrolysis) reaches a certain threshold.
3. Gelation and growth via condensation: once nuclei have been formed, they can grow by condensation to form particles that precipitate from the solution.
4. Aging via Ostwald Ripening: as the concentration of the monomeric species approaches the value of equilibrium, (slow) dissolution and reprecipitation processes enable Ostwald ripening: due to size-dependent solubility, the small particles dissolve whereas large particles continue to grow.

Sol-gel: Monomer concentration in solution as a function of time.

Q: Sol-gel: Number of particles and mean particle size as function of time.
A: Ideally, all new particles are formed during phase II, during phase III the particle number should stay approximately constant, and only growth should occur. During phase IV, Ostwald ripening leads to dissolution of small particles (decrease of particle number) and large particles continue to grow (increase in mean size).

Q: Explain why the duration of the period during which nucleation occurs affects the width of the particle size distribution. Should this duration be long or short if you were interested in obtaining a narrow size distribution? How should the rate of hydrolysis and the rate of nucleation compare for a narrow size distribution?
A: The longer the period of nucleation the larger the polydispersity. Some nuclei will have more time and more monomers available, while those that are formed later have less time and lower concentration of monomers. The goal for narrow polydispersity is that all nuclei are formed at the same instant in time: they all compete for the same number of monomers in solution and the growth duration is constant. The rate of hydrolysis needs to be slower than the rate of nucleation and roughly of the same order of growth. Nucleation and growth should be separated in time.

Q: Assuming spherical particles, how should the rate of growth \((\dd{r}{t})\) ideally depend on particle size to narrow the size distribution during the period of growth?
A: The growth rate should also have an inverse dependence on the radius: in this way small particles grow faster and have a chance of ”catching up” with the larger particles.
This is the case for diffusion-limited growth conditions.

Q: Describe the phenomenon of Ostwald ripening.
A: Ostwald ripening is associated with size-dependent solubility of particles. Due to increased curvature, small particles have a higher surface energy and are therefore more likely to dissolve than large particles. In a suspension of colloids, small particles will dissolve whereas large particles will grow. This may be undesirable in the synthesis of monodisperse nanocrystals as it leads to size-defocusing.

Q: Exact absolute electronegativity of an isolated atom \(\chi_a\)
A: \[\chi_a = \frac{1}{2}(I_v + A_v)\]half the sum of the ionization energy and electron affinity.

Q: Electronegativity Equalisation Principle
A: Since electronegativity is a chemical potential and chemical potentials of individual species are equalized in equilibrium, electron transfers and therefore partial charge accumulation is completed as soon as all electronegativities are equalized and every atom in the molecule has the same electronegativity.

Q: How can we describe the pH dependence of the standard chemical potential of protons in solution?
A: Using the Nernst equation (at \(T = 298 \, \mathrm{K}\)) we get\[\mu_H = \mu_H^0 + RT \ln(\ce{a_H}) = \mu_H^0 - 0.059 \, \mathrm{V} \, pH\]

Q: What are the required (empirical) conditions for condensation of \(\ce{[M(OH)_h(OH2)_{n-h}]^{(z-h)+}}\)?
A: The partial charges have to fulfill\[\delta(\ce{OH}) < 0 \qquad \delta(\ce{M}) > +0.3\]Nucleophilic OH group, electrophilic metal site

Q: Names of the two distinct condensation mechanisms
A: Olation and oxolation

Q: Mechanism for condensation via olation and requirement.
A: Nucleophilic metal hydroxo and electrophilic metal aquo species react in an S_N2-like, S_N1-like or associative displacement under the release of water forming a metal hydroxide.
As the aquo ligand acts as a leaving group in this mechanism, olation requires \(\delta(\ce{OH2}) < 0\) (otherwise the aquo ligand is not stable and is deprotonated forming hydroxo or oxo).

Q: Mechanism for condensation via oxolation and requirement.
A: Oxolation is either base- or acid-catalyzed condensation between two metal hydroxo-species resulting in the formation of a metal oxide in both cases under release of water and the catalyst (\(\ce{OH−}\) for base-catalysis, \(\ce{H3O+}\) for acid catalysis). As this process occurs when no aquo-ligand is present in the initial coordination sphere, it is present for cases where \(\delta(\ce{OH2}) > 0\).
Should be -H3O+

Olation is typically favored with elements having small charge and large size (soft cations) such as \( \ce{Al^3+}, \ce{Cr^3+}, \ce{Ni^2+}\) and \(\ce{Co^2+}\)  while oxolation is favored with elements having large charge (≥ 4 ) and small size (hard cations) such as \(\ce{Si^4+}\), \(\ce{Ti^4+}\).

Q: Consider Alumina, Silica and Titania. Which follows olation and which oxolation condensation mechanism?
A: As olation is favoured for soft cations and oxolation for hard cations, the soft cation \(\ce{Al^3+}\) will condense via olation, forming a metal hydroxide, while the harder cations \(\ce{Ti^4+}, \ce{Si^4+}\) will condense via the oxolation mechanism, forming metal oxides.

Q: Why is it important to carry out sol-gel synthesis of materials at specific pH values which vary for each material? What pH suits \(\ce{Al, Ti, Si}\)?
A: To ensure a predominance of neutral charged monomers of the hydrolysed metal precursor. The condensation of charged species will always be self-limited, because the charge on the inorganic polymer increases with every condensation of two charged entities, which gradually decreases the stability of the polymer (charge accumulation).
Alumina condenses via olation, the condensation rate is thus pH-independent, the synthesis can be performed over a broad range of pHs as long as the neutral aquo species are predominant (pH = 5-10).
For silica and titania, a pH that is distinct from the PZC has to be chosen to enable the acid/base catalyzed oxolation. The points of zero charge of Silica and Titania are ≈3 and ≈6 respectively. Depending on whether the pH is higher or lower than the PZC, different types of aggregates are obtained.

Q: Unique advantages of the sol-gel method (6)
A: - Comparatively low temperatures and pressures
- Access meta-stable materials with non-equilibrium crystalline or amorphous structure
- Porous, high surface area materials
- Modify and fine-tune the synthesis by varying the precursors, solvent composition, temperatures, and concentrations
- Structure-directing agents can be used to fine-tune porosity or colloid size.
- No additives required, therefore high purity (e.g. for medicinal technology, like implants)

Q: Basis Allred Rochow electronegativity and reason often used
A: Electrostatic interaction between \(e^-\) and nuc. \(\frac{Z_{eff}}{r^2}\)
\(\chi_{AR} \propto \frac{Z_eff}{r^2}\quad \eta_a\propto\frac{1}{r} \) where \(r\) is the covalent radius and \(\eta\) the hardness
It is often used as it can be easily calculated for every element.
Allred Rochov accounts for size and shape effects.

Q: Example molecule that shows weakness of partial charges model:
A: Both have the same sum formula and thus same partial charges (which is clearly not physical) -> Unable to distinguish constitutional isomers.

Q: Upon treating \(\ce{ VO(OCH(CH3)2)3}\) with water, the acidity of the solution increases. Rationalize this observation and describe the possible underlying reaction as a function of the hydrolysis ratio \(h\).
A: The precursor dissociates, allowing the access of \(\ce{V^5+}\) ions in solution, which at first are hydrolysed by polar \(\ce{H2O}\). However, because of the strong polarizing power of hard \(\ce{V^5+},\) the coordinated \(\ce{H2O}\) will be partially deprotonated, leading to a pH decrease according to
\[\ce{[V(OH2)6]^5+ + h H2O <-> [V(OH)_h(OH2)_{6-h}]^{(5-h)+} + h H3O+}\]

Q: Hardness
A: The hardness \(\eta = \frac{1}{\sigma} = \frac{1}{2} \frac{\delta}{\delta N} \frac{1}{2} (I - A)\) is the resistance to a change of chemical potential as a function of the number of electrons \(N\) and defines the extent to which partial charges are created in the molecule. It depends on the charge and size: \(\eta \propto \frac{1}{r}\) as \(E \propto \frac{q^2}{r}\).

Q: Sealed tube/Flow reactor approach
A: Similar to ceramic method, but better for sensitive materials such (e.g. sulfides) as carried out in sealed tube:\[\ce{4Si(powder) + S8 -> 4SiS2}\]in flow reactor: sample in boat-like crucible and gas flows through:\[\ce{M + [oxidant gas O2, Cl2)] -> MO_x}\]

Q: Rapid metathesis reaction with example
A: Highly exothermic salt exchange reaction between two solids giving thermodynamic product (often used for metal nitrides). Example:\[\ce{3ZrCl4 + 4Li3N <=> Zr3N4 + 12LiCl}\]Disadvantage: Only works on mg scale!

Q: Rate of hydrolysis and nucleation dependence on precursor
A: - Metal: More lewis acidic \(\Rightarrow\) faster hydrolysis: TM faster for \(\ce{Si}\), \(\ce{Al, Ti} > \ce{Si}\)
- Alkoxide: The bulkier, the slower the condensation
- Chelating ligands: Can slow down condensation/hydrolysis (but not necessarily to the same extent). Problem with mixed metal oxides: inhomogeneous product if hydrolysis and condensation rates of precursors differ \(\Rightarrow\) fine-tuning of alkoxide precursors to have similar hydrolysis and condensation rates

Q: Limitations of sol-gel (3)
A: - Fast evaporation of solvent might cause improper gel formation and cracking of gel
- Particle surface may be covered with residues of \(\ce{Cl}\) or \(\ce{OR}\)
- Cannot really obtain hydrophobic solid, as we are in aq. solution

Q: Adsorption isotherm type I
A: Microporous: fast inset at beginning as micropores are filled at low \(p\), then plateau at which \(n_{ads}\) is constant, then inset due to condensation at \(p_0\). For microporous materials the majority of SA is inside the pores, which are already filled at low \(p\).

Q: Adsorption type II
A: Macroporous or non-porous: lower inset at beginning than I (occupation of high \(E\) sites first, leads to non-linearity of curve), then linear increase, no cap. cond. at intermediate \(p\) as pores are very large

Q: Adsorption isotherm type IV
A: Mesoporous: small inset at beginning, then \(2^{nd}\) inset with (optional) hysteresis at \(0.5 < p/p_0 < 0.8\) (delayed cap- cond. in mesopores). The larger the pore diameter the larger \(p\) at which cap. cond. occurs. Hysteresis is more likely for larger than for smaller pores.

Q: Sol Gel
Xerogels
A: - made by drying the gel
- usually mesoporous materials
- material retains some porosity of the polymer network (< 50%) and shrinks considerably

Q: Sol Gel
Aerogels
A: - produced by adjusting the temperature and pressure such that the solvent becomes supercritical -> solvent can be removed with very little loss of volume and porosity
- materials have very low densities, are exceptional thermal insulators

Q: Sol Gel
Homogeneous nucleation
A: - occurs without any additives -> generally a significant barrier of activation
Two thermodynamic counteracting factors:
1. surface with a certain surface energy is created -> disfavours nucleation -> positive term to the Gibbs free energy -> scales with the area of the created particle -> \(4\pi r^2\gamma \) (r: particle radius, \(\gamma\): surface free energy per unit area)
2. generation of M-O-M bonds -> negative free energy of formation -> favours nucleation -> negative term to the Gibbs free energy -> scales with volume of the created particle: \(\frac{4\pi}{3}r^3\Delta G_V\) with \(\Delta G_V = \frac{RT \ln S}{V_m}\) being the free energy per unit volume of a crystal
\(S = \frac{C}{C_S}> 1\): supersaturation of the solution -> actual concentration of monomers divided by the eq. monomer concentration of the bulk solid
\(V_m\): molar volume of the material

Q: Sol Gel
Speed of nucleation
A: - depend on S (supersaturation) and T
- @high T and S -> critical radius is smaller, activation barrier for nucleation is lowered -> nucleation is faster
- if T or S too low -> barrier of activation is higher than the thermally available energy -> no homogeneous nucleation will occur