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Even though traditional wet chemistry routines using hot plate / stirrer accessories are still popular despite their typical long durations, more advanced heating techniques, such as solvothermal methods and microwave-assisted reactions, are absolutely required in order to successfully, efficiently, and rapidly carry out some specific soft chemistry approaches. Further details of these two common heating techniques will be provided in the following sections.

1.2.2.1 Solvothermal Methods

In simple words, a solvothermal reaction is performed under modest temperature and high pressure, and in an appropriate solvent.46 Typically a mixture of reactants and a solvent is enclosed inside a PTFE-lined cylinder (bomb), and heated in an oven up to 100-500 ºC and under high pressures.32 Pictures of a PTFE-lined cylinder and a typical sealable autoclave (stainless steel container), are presented in Figure 1-9. In case of using water as the solvent, the

solvothermal reaction is specifically called hydrothermal.1,32,136 Even though higher pressures can be easily reached via connection to an external pressure control, the amount of pressure in the vessel can also be estimated based on the filling percentage of the mixture in the bomb and the reaction temperature.32,137 In other words, the second function of the solvent in such reactions is to transmit pressure via forming convection streams in the confined reaction vessel (for

example due to the existence of water / steam in a hydrothermal method).32 Solvothermal

Figure 1-8: A typical exfoliation reaction of HLaNb2O7 perovskite due to the

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reactions have been vastly used for the production or modification of various inorganic materials.88,100,136,138–147

1.2.2.2 Microwave-Assisted Reactions

Microwave irradiations have been used as an important method of heating since

commercializing the first generation of microwave ovens in 1950s.148,149 Use of microwaves in performing chemical transformation was first reported as a published work in 1986,150,151 and have been extensively expanding since then in facilitating processes in different fields such as analytical chemistry, biochemistry, photochemistry, catalysis, as well as the synthesis of inorganic materials, organometallics, polymers, and most importantly microwave-assisted organic synthesis (MAOS).152–155 _{Microwave-assisted research was initially based on the use of}

kitchen microwave ovens, which tremendously increased the confusion among the chemists in 1990’s due to non-reliable temperature / pressure monitoring and higher chances of explosion. Even though employing domestic household microwave ovens is not yet eliminated in the published works (about 30% in 2009), the majority of the researchers today take advantage of dedicated microwave stations and their interesting features (monitoring both temperature and pressure on-line and accurately, higher safety controls, robust cavities that withstand possible explosions, and possibility of stirring the reaction vessels which increases the homogeneity).152 Despite availability of dedicated microwave reactors in lower prices these days, they are still harder to afford than conventional heating equipment.152

Figure 1-9: PTFE-lined cylinder and parts of a typical autoclave (left) and a sealed autoclave (right).

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Heating a reaction vessel using an external heat source (conventional thermal heating) typically causes a temperature gradient within the sample with the temperature increasing in layers closer to the reaction vessel (Figure 1-10a). The key feature of microwave-assisted reaction, is the simultaneous exposure of the whole reaction mixture to microwaves (internal or in core volumetric heating), causing a uniform temperature increase throughout the entire sample (Figure 1-10b).32,152,156

The frequency of microwaves is in 0.3 – 300 GHz range of the electromagnetic irradiation, corresponding to wavelengths of 1 mm – 1 m.32,152,156 Microwaves are used for transmission of either energy or information. All domestic microwave ovens and dedicated microwave stations operate at a specific frequency (2.45 GHz) to avoid interference with telecommunication frequencies.152 This small frequency does not cleave any molecular bonds and is also lower than Brownian motion, proving that microwaves will not induce any chemical reactions via direct absorption (as opposed to ultraviolet or visible radiation).152 Like any electromagnetic irradiation, microwaves also transmits as a transverse oscillating wave of electric and magnetic fields.152,156 The electric component of microwaves is mainly responsible for heating of the materials via two major mechanisms: dipolar polarization and ionic

conduction. Molecules with dipoles constantly try to align themselves with the oscillating

electric field and realign as the field quickly changes. In this process, heat is generated due to molecular friction and dielectric loss. In ionic conduction, heating is based on the collisions of

Figure 1-10: Comparison of heat distribution in: (a) conventional methods with an external heat source, and (b) microwave-assisted reactions

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charged particles and ions as they oscillate back and forth under the impression of the alternating electric field.152 _{In both of these mechanisms, the microwaves are absorbed by dipolar molecules,}

ions, or charged particles, and generate heat throughout the sample. However, there is a third mechanism which applies to semiconducting or conducting materials (such as metals) where microwaves are mainly reflected rather than absorbed: resistive (ohmic) heating mechanism. In ohmic heating, the electric field will direct the free flow of electrons on the surface of the

material, which causes heating due to the intrinsic resistance of the system.152,155 Even though the electric field is the component that is responsible for heating most of the microwave-assisted processes, the interactions with the magnetic field could also be of interest in some cases (for instance for transition metal oxides).157–159

The dielectric properties of a certain material will directly impact their ability to efficiently convert this electromagnetic irradiation into heat (dielectric heating). Microwave- absorbing feature of the materials is typically evaluated by the so-called loss factor or loss

tangent (tan δ). This factor is obtained by dividing the dielectric loss of the material (ε") by its

dielectric constant (ε'), respectively defined as the efficiency of the material to convert the electromagnetic radiation into heat, and polarizability of the molecules in the electric field. Solvents used in microwave chemistry can be classified based on their loss factor: high (tan δ > 0.5, such as ethanol), medium (0.1 < tan δ < 0.5, such as water), and low microwave-absorbing (tan δ < 0.1, such as toluene). It should be noted that the loss factor is strongly frequency and temperature dependent. For instance, the loss tangent of pure water and most of the organic solvents drops with increasing the temperature; as in water heating via microwaves gets very difficult past 100 ºC, to a point that water becomes transparent to microwaves at its supercritical temperature. In the opposite scenario (in case of the materials that become more microwave- absorbing at elevated temperatures), the chances of overheating and explosion highly increases.152

Microwave-assisted reactions significantly decrease the reaction times due to the minimization of wall effects and efficient internal heating of the reaction mixture. This allows the chemists screen for new target compounds in a few hours, and move on to more decision points without wasting days waiting for the result.152 Due to the enormous instantaneous energy

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provided in microwave heating, thermodynamically controlled reactions can also take place, as opposed to the conventional heating techniques were mainly kinetic products are obtained (the easiest path with the lowest activation energy due to the mild conditions in conventional methods).156 _{This so-called “microwave flash heating” is believed to be the main reason of the}

rate-enhancement in the majority of microwave-assisted reactions. However, there are more perplexing aspects to microwave-matter interactions (microwave effects),152 which are also known to enhance the rate of microwave-assisted reactions. Full ramifications of the microwave

effects have not been realized yet.

1.3– Nanomaterials

Nanotechnology is based on the manipulation of materials in nanoscale – from

subnanometer to several hundred nanometers. However, the decisive aspect of nanotechnology is

the appearance of a novel property, so-called “nano-effect”, which is achieved by going down in

the crystallite size of a particular material .160,161 Even though materials in the microscale have properties very similar to that of bulk, nanoscale materials offer distinctively different features than that of their bulk.160 As an example, gold has no catalytic properties in bulk, however, gold nanocrystals are known to be great low-temperature catalysts.160 Gold also changes its typical yellow color when a critical size is reached: becoming blue at about 50 nm and purple at about 20 nm (the nano-effect in this specific example is the plasmon resonance revealed in the smaller size range).161 Since the 1980s, nanotechnology has been growing tremendously in different areas such as therapeutic drugs, information storage, refrigeration, chemical/optical computers, improved ceramics and insulators, harder metals, thin film precursors, environmental chemistry (solar cells, remediation, water purification, and destructive adsorbents), catalysts, sensors, smart magnetic fluids, and batteries.4,161–163 Some of the aforementioned fields should be considered more as nanoscience, as they are still a step away from well-realized technologies.161 The cornerstone of nanoscience is the ability to fabricate and process nanostructures and

nanomaterials: materials with at least one dimension in the nano-scale. Properties of a material can strictly change by changing its dimensionality; designing desirable zero-, one-, two-, and three-dimensional nanostructures (like nanoparticles, nanorods, nanosheets, and nanoflowers, respectively).4,164 _{Two-dimensional (2D) materials, having two dimensions outside of}

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nanometric size range by definition, are the specific class of nanomaterials that was the focus of this work.4