**3. Inorganic compounds**

The H-MW method was used for preparation of free or supported *elemental metals* (Cu, Ni, Co, Ag) long ago (Komarneni et al., 1995). Thus, microwave-hydrothermal processing in combination with polyol process was used to prepare Ago-, Pto- or Pdo-intercalated montmorillonite (Komarneni, Hussein, et al., 1995). Sub-nanometer metal clusters were introduced into the interlayers while some larger metal particles of 5-100 nm were crystallized on the external surfaces. The small metal clusters in the interlayers and on the external surfaces may be useful in certain catalytic applications. The reduction of chlorocomplexes of gold(III) from muriatic solutions by nanocrystal powders of palladium and platinum at 110 and 130oC under H-MW conditions was studied, revealing Au-Pd and Au-Pt bimetallic particles with a core-shell structure according to the scheme shown in Fig. 2 (Belousov et al, 2011). The obtained particles had a core of the metal reductant covered with a substitutional solid (Au, Pd) solution in case of palladium, and isolated by a gold layer in the case of platinum. It was shown by the example of the Au-Pd system that the use of microwave irradiation allowed one not only to accelerate the synthesis of particles but also to obtain more homogeneous materials in comparison with conventional heating. In addition, magnetic FeNi3 nanochains were synthesized by reducing iron(III) acetylacetonate and nickel(II) acetylacetonate with hydrazine in ethylene glycol solution without any template according to the mechanism shown in Fig. 3 (Jia et al., 2010). The size of the aligned nanospheres in the magnetic FeNi3 chains could be adjusted from 150 to 550 nm by increasing the amounts of the precursors. Magnetic measurement revealed that the FeNi3 nanochains showed enhanced coercivity and saturation magnetization. As an example of core-shell-type gold nanoparticles, the Au/SnO2 core-shell structure was synthesized using the H-MW method (Yu & Dutta, 2011). In MW preparation, the peak position of the UVvisible plasmon absorption band of Au nanoparticles was red shifted from 520 to 543 nm, due to the formation of an SnO2 shell. An SnO2 shell (thickness 10-12 nm) formation was complete within 5 min.

**Figure 2.** Core-shell Au/Pd particle formation scheme. With permission.

108 The Development and Application of Microwave Heating

2011).

this study.

**2. Typical equipment** 

**3. Inorganic compounds** 

requires a long time (typically half to several days) and high electric power (over a thousand Watts), microwave-assisted heating is a greener approach to synthesize materials in a shorter time (several minutes to hours) and with lower power consumption (hundreds of Watts) as a consequence of directly and uniformly heating the contents. Particular aspects of these techniques were examined in several reviews (Shangzhao Shi & Jiann-Yang Hwang, 2003; Komarneni, 2003; Komarneni & Katsuki, 2002) and a book chapter (Guiotoku et al.,

In this Chapter, we try to describe briefly main aspects of hydro/solvothermal processes under simultaneous microwave heating (H-MW or S-MW). Reactions, carried out by consecutive application of hydro/solvothermal and microwave treatment, are out of scope of

Typical commercial equipment, used for MW hydro/solvothermal processing, is shown in Fig. 1. Its cost is usually about 30,000 USD. Several reports describe also home-made

combinations of MW-heating and hydro/solvothermal reactions.

**Figure 1.** Typical equipment (MARS), used for MW hydro/solvothermal processing.

The H-MW method was used for preparation of free or supported *elemental metals* (Cu, Ni, Co, Ag) long ago (Komarneni et al., 1995). Thus, microwave-hydrothermal processing in combination with polyol process was used to prepare Ago-, Pto- or Pdo-intercalated montmorillonite (Komarneni, Hussein, et al., 1995). Sub-nanometer metal clusters were introduced into the interlayers while some larger metal particles of 5-100 nm were crystallized on the external surfaces. The small metal clusters in the interlayers and on the external surfaces may be useful in certain catalytic applications. The reduction of chlorocomplexes of gold(III) from muriatic solutions by nanocrystal powders of palladium and platinum at 110 and 130oC under H-MW conditions was studied, revealing Au-Pd and

**Figure 3.** Illustration of a proposed mechanism for the formation of FeNi3 nanochains. With permission.

Among *metal oxides*, TiO2 is the compound, received obviously main attention of researchers due to its numerous applications, in particular in nanostructurized forms. Similar applied methods led to a variety of its distinct crystalline phases obtained in different reports. Thus,

nanoparticles of *brookite*-type TiO2 were prepared at 200oC (H-MW-heating time 5 min) for 0- 60 min starting from the titanium peroxo glycolate complex in basic solution (Morishima et al., 2007). The activity in photodecomposition of oxalic acid by the samples prepared using H-MW technique was higher than activities of brookite nanoparticles prepared by the conventional hydrothermal method. On the contrary, mesoporous titanium dioxide with highly crystalline *anatase* phase and high surface area, a promising material for energy and environmental application, was obtained (Huang et al., 2011) *via H-*MW route using stable and water-soluble titanium citrate complexes as the precursors. It was shown that the synthesized TiO2 contained mainly anatase phase with crystallite size of 5.0-8.6 nm at various hydrothermal temperatures and durations ranging from 150 to 180oC and from 30 to 120 min, respectively. The mesoporous nanocrystals synthesized at 180oC were then used to prepare the TiO2 photoelectrode using screen-printing deposition method. The MW180-120 based TiO2 photoanode exhibited a good efficiency on photocurrent conversion and the conversion efficiency was in the range 4.8-7.1%, depending on active area and film thickness. In addition, TiO2 of the shuttle-like *rutile* phase (10 nm) was prepared using TiCl4 and HCl by H-MW method (Chen et al., 2008). Interestingly, the use of H-MW method resulted in the formation of TiO2 *nanotubes* comprising anatase and rutile phases (Sikhwivhilu et al., 2010). Conventional hydrothermal heating resulted in the formation of tubes with a titanate structure. The two methods yielded tubular structures with similar size dimensions, surface areas and morphologies. The two methods gave 100 % yields of tubes with different degrees of crystallinity. At last, the combination of sonication and H-MW (three methods at once) led to preparing fluorinated mesoporous TiO2 *microspheres* (500 nm size) (Zhu et al., 2010). The authors achieved the fabrication of mesoporous TiO2, doping of fluorine by sonication and then hydrothermal treatment of a solution containing TiO2 precursor sol and sodium fluoride.

Zinc oxide, the principal nanotechnological object, was also intensively studied and reported in distinct forms, in particular as nanobar-structured ZnO thin film (Li et al., 2011) (obtained from Zn salt solution (nitrate, chloride, acetate, and/or sulfate) and hexamethylenetetramine solution as raw materials). The wide interest in ZnO has resulted from the following fundamental characteristic features with potential applications in electronic, structural and bio-materials: direct band gap semiconductor (3.37 eV), large excitation binding energy (60 meV), near UV emission and transparent conductivity. ZnO nanorods were synthesized using zinc nitrate and methenamine aqueous solutions in a H-MW process (Shojaee et al., 2010). It was revealed that concentration of precursors and irradiation power displayed significant influences on the compaction and dimensions of the grown nanorods. The 1D ZnO nanostructures and microstructures with a hexagonal crosssection growing in the (0002) direction were obtained under H-MW method (MW 2.45 GHz) at 130oC for 30 min (de Moura et al., 2010). In addition, the intriguing results were reported (Huang, J. et al., 2008) for a facile H-MW route employing the reaction of Zn(NO3)2· 6H2O and NaOH to synthesize a single-crystal zinc oxide 1D nanostructure with a 3D morphology (Fig. 4). A substantial reduction in the reaction time as well as the reaction temperature is observed compared with the hydrothermal process. Fig. 5 shows a condensed illustration of the authors' strategies in the morphology control of ZnO nanostructures. First, ZnO nuclei generally evolve into nanorods by preferential c-axis ([002] direction) oriented 1D growth. Second, nanorods can be converted into nanowires by a multiple nanorods growth along the [002] direction and simultaneous local attachment of the polar (0001) surfaces or nanospindles by an increase in diameter and local dissolution. Third, multiple nanorods grow from center results in nanodandelions. Fourth, when the crystal growth along the [002] direction is suppressed, nanoslices can be obtained due to quasi 1D growth. Finally, when multiple nanoslices grow further, nanothruster vanes can be formed by selfassembled growth. The MW-hydrothermal mechanism of ZnO nanostructures can be considered as follows (reactions 1-2):

110 The Development and Application of Microwave Heating

precursor sol and sodium fluoride.

nanoparticles of *brookite*-type TiO2 were prepared at 200oC (H-MW-heating time 5 min) for 0- 60 min starting from the titanium peroxo glycolate complex in basic solution (Morishima et al., 2007). The activity in photodecomposition of oxalic acid by the samples prepared using H-MW technique was higher than activities of brookite nanoparticles prepared by the conventional hydrothermal method. On the contrary, mesoporous titanium dioxide with highly crystalline *anatase* phase and high surface area, a promising material for energy and environmental application, was obtained (Huang et al., 2011) *via H-*MW route using stable and water-soluble titanium citrate complexes as the precursors. It was shown that the synthesized TiO2 contained mainly anatase phase with crystallite size of 5.0-8.6 nm at various hydrothermal temperatures and durations ranging from 150 to 180oC and from 30 to 120 min, respectively. The mesoporous nanocrystals synthesized at 180oC were then used to prepare the TiO2 photoelectrode using screen-printing deposition method. The MW180-120 based TiO2 photoanode exhibited a good efficiency on photocurrent conversion and the conversion efficiency was in the range 4.8-7.1%, depending on active area and film thickness. In addition, TiO2 of the shuttle-like *rutile* phase (10 nm) was prepared using TiCl4 and HCl by H-MW method (Chen et al., 2008). Interestingly, the use of H-MW method resulted in the formation of TiO2 *nanotubes* comprising anatase and rutile phases (Sikhwivhilu et al., 2010). Conventional hydrothermal heating resulted in the formation of tubes with a titanate structure. The two methods yielded tubular structures with similar size dimensions, surface areas and morphologies. The two methods gave 100 % yields of tubes with different degrees of crystallinity. At last, the combination of sonication and H-MW (three methods at once) led to preparing fluorinated mesoporous TiO2 *microspheres* (500 nm size) (Zhu et al., 2010). The authors achieved the fabrication of mesoporous TiO2, doping of fluorine by sonication and then hydrothermal treatment of a solution containing TiO2

Zinc oxide, the principal nanotechnological object, was also intensively studied and reported in distinct forms, in particular as nanobar-structured ZnO thin film (Li et al., 2011) (obtained from Zn salt solution (nitrate, chloride, acetate, and/or sulfate) and hexamethylenetetramine solution as raw materials). The wide interest in ZnO has resulted from the following fundamental characteristic features with potential applications in electronic, structural and bio-materials: direct band gap semiconductor (3.37 eV), large excitation binding energy (60 meV), near UV emission and transparent conductivity. ZnO nanorods were synthesized using zinc nitrate and methenamine aqueous solutions in a H-MW process (Shojaee et al., 2010). It was revealed that concentration of precursors and irradiation power displayed significant influences on the compaction and dimensions of the grown nanorods. The 1D ZnO nanostructures and microstructures with a hexagonal crosssection growing in the (0002) direction were obtained under H-MW method (MW 2.45 GHz) at 130oC for 30 min (de Moura et al., 2010). In addition, the intriguing results were reported (Huang, J. et al., 2008) for a facile H-MW route employing the reaction of Zn(NO3)2·

and NaOH to synthesize a single-crystal zinc oxide 1D nanostructure with a 3D morphology (Fig. 4). A substantial reduction in the reaction time as well as the reaction temperature is observed compared with the hydrothermal process. Fig. 5 shows a condensed illustration of the authors' strategies in the morphology control of ZnO nanostructures. First, ZnO nuclei

6H2O

$$\text{Zn(NO}\_3\text{)}\_2 + 2\text{NaOH} \rightarrow \text{Zn(OH)}\_2\downarrow + 2\text{NaNO}\_3 \tag{1}$$

$$\text{Zn(OH)}\_{2} + 2\text{H}\_{2}\text{O} \rightarrow \text{Zn(OH)}\_{4}^{2-} + 2\text{H}^{+} \rightarrow \text{ZnO} + 3\text{H}\_{2}\text{O} \tag{2}$$

**Figure 4.** FE-SEM images of the ZnO nanocrystals with different morphologies: nanorods (a, temperature = 413 K, [Zn2+] = 1.6 mol L−1, filling ratio = 70%, time = 20 min, middle: HRTEM image and right: selected area electron diffraction (SEAD) pattern), nanowires (b, temperature = 453 K, [Zn2+] = 1.6 mol L−1, filling ratio = 70%, time = 20 min), nanothruster vanes (c, temperature = 393 K, [Zn2+] = 1.6 mol L−1, filling ratio = 70%, time = 20 min), nanodandelions (d, temperature = 373 K, [Zn2+] = 1.6 mol L−1, filling ratio = 70%, time = 20 min) and radial nanospindles (e, pressure = 3.0 MPa, [Zn2+] = 0.8 mol L−1, filling ratio = 70%, time = 20 min). With permission.

Other simple and mixed/complex oxides are extensively reported. Thus, yttria stabilized zirconia (YSZ) is the main material for preparing functional device such as oxygen sensor, solid state oxide fuel cell and high temperature humidity transducer (Zhao et al., 2007). Its nanopowders were prepared by H-MW method with programmable MARS-5 microwave digester in strong basic media at temperature from 100-120oC and time from 1 h to 5 h, while the temperature is 190-250oC by conventional hydrothermal heating (CH). The result

showed that compared with CH, H-MW can reduce the reaction time, and influence the content of product. The CH and H-MW techniques were also used for production of nanocrystalline zirconium and hafnium dioxides (8-20 nm) at 180 and 250oC and highly dispersive powders of barium zirconate and hafnate at 150oC (Maksimov et al., 2008). HfO2 was also H-MW-obtained in a rice form (Eliziari et al., 2009) according to the following reactions 3-5:

$$\text{HfCl}\_4 + 2\text{H}\_2\text{O} \rightarrow \text{Hf}\text{(OH)}\_2\text{Cl}\_2 + 2\text{HCl} \tag{3}$$

$$2\text{ Hf(OH)}\_{2}\text{ Cl}\_{2} + 2\text{KOH} \rightarrow \text{Hf(OH)}\_{4} + 2\text{KCl} \tag{4}$$

$$\text{Hf(OH)}\_{4} \rightarrow \text{HfO}\_{2} + 2\text{H}\_{2}\text{O} \tag{5}$$

**Figure 5.** Schematic of the shape-controlled synthesis of ZnO nanorods, nanowires, nanothruster vanes, nanodandelions and radial nanospindles *via* a microwave hydrothermal route. With permission.

The effect of microwave radiation on the formation of smaller and uniform -Fe2O3 powders from FeCl3 solution at 100-140oC was investigated (Katsuki, 2009). As a practical application, a new red pigment on their basis for porcelain was developed. Structures of tin oxides in different oxidation states are known, for instance SnO2 nanoparticles (Krishna & Komarneni, 2009) or SnO powders (Pires et al., 2008); the last ones were obtained by the H-MW technique using SnCl2. 2H2O as a precursor. By changing the hydrothermal processing time, temperature, the type of mineralizing agent (NaOH, KOH, or NH4OH) and its concentration, SnO crystals having different sizes and morphologies could be achieved. Plate-like form was found to be the characteristic morphology of growth. CuO with lesscommon sea urchin-like morphology is also known (Volanti et al., 2010). Single crystalline Co3O4 nanorods (Li, W.-h., 2008) or Co3O4 mesoporous nanowires (Zeng et al., 2011) with average single crystalline grain sizes of 8 nm, 12 nm, 25 nm, and 45 nm were synthesized by sintering the last nanostructure of H-MW-processed belt-Co(OH)2 precursors at 300-500oC for 2 h. The interesting finding was made that room temperature ferromagnetism appeared at 350oC in the high orientation samples. A mixture of crystalline Co3O4/CoO nanorods (length of around 80 nm and an average diameter of 42 nm) with non-uniform dense distribution was synthesized by H-MW technique (Al-Tuwirqi et al., 2011). The band energy gap of the product was 1.79 eV which lies between the energy gap of CoO and that for Co3O4. As synthesized mixed Co3O4/CoO nanorods can be very useful for supercapacitor devices application. Magnetic hysteresis loops at room temperature of the as synthesized mixed oxides (Co3O4/CoO) nanorods exhibit typical soft magnetic behavior.

112 The Development and Application of Microwave Heating

reactions 3-5:

showed that compared with CH, H-MW can reduce the reaction time, and influence the content of product. The CH and H-MW techniques were also used for production of nanocrystalline zirconium and hafnium dioxides (8-20 nm) at 180 and 250oC and highly dispersive powders of barium zirconate and hafnate at 150oC (Maksimov et al., 2008). HfO2 was also H-MW-obtained in a rice form (Eliziari et al., 2009) according to the following

4 2 <sup>2</sup> <sup>2</sup> HfCl 2H O Hf OH Cl 2HCl (3)

2 2 <sup>4</sup> Hf OH HfO 2H O (5)

**Figure 5.** Schematic of the shape-controlled synthesis of ZnO nanorods, nanowires, nanothruster vanes, nanodandelions and radial nanospindles *via* a microwave hydrothermal route. With permission.

<sup>2</sup> 2 4 Hf OH Cl 2KOH Hf OH 2KCl (4)

MoO3 nanoflowers were synthesized on a Si substrate by a facility H-MW method (Wei et al., 2009). The nanoflowers consisted of tens of nanobelts and the nanobelts were about several micrometers in length, several tens to several hundreds of nanometers in width, and tens of nanometers in thickness. As-grown MoO3 nanobelts exhibited a good field-emission property and have great potential for applications in field-emission devices. In case of tungsten oxide, its compounds with different composition were reported, for example monodisperse crystalline WO3. 2H2O (H2WO4. H2O) nanospheres, which were prepared by (+)-tartaric acid-assisted H-MW process (Sun et al., 2008), meanwhile the synthesis of crystalline W18O49 with nanosheet like morphology was carried out by low cost MW irradiation method without employing hydrothermal process (Hariharan et al., 2011). The W18O49 nanosheets had the average dimensions of the order of 250 nm in length and around 150 nm in width. The band gap energies to be 3.28 and 3.47 eV for WO3. H2O and W18O49 samples, respectively. An hierarchically structured WO3. 0.33H2O "snowflakes" were synthesized by a template-free and H-MW method (Li, J. et al., 2011). Their particles had an oriented growth along six equivalent <100> directions in (001) plane to form the snowflakelike microstructure, which is significantly different from the sample prepared at conventional hydrothermal conditions. Moreover, microwave heating was considered by authors to accelerate the oriented crystal growth along <100> directions. Mixed Mo-W nanostructures are also known. Thus, W0.4Mo0.6O3 and carbon-decorated WO*x*-MoO2 (*x* = 2 and 3) nanorods were synthesized (Yoon & Manthiram, 2011). The carbon-decorated WO*x*-MoO2 nanorods exhibited excellent capacity retention as the carbon provides an elastic matrix for absorbing the volume expansion-contraction smoothly and prevents aggregation

of the nanorods during cycling. In addition, solid solutions of titanium-tin oxide Ti*x*Sn1-*x*O2 were prepared by H-MW method from a solution containing TiCl3 and SnCl4 (Yang et al., 2011). The advantage and contribution of this technique was revealed to be effective reduction of the difference in the formation rate of TiO2 and SnO2, resulting in the precise control of the solid solution composition. Enlarging the crystal size of single-phase, rutiletype Ti*x*Sn1-*x*O2 solid solutions can be achieved by annealing in air and the crystal phase is stable at 800oC.

A series of reports is devoted to lanthanide oxides, in particular to distinct CeO2 nanostructures, for instance virtually nonaggregated, primarily hexagonal CeO2 nanoparticles (Ivanov et al., 2009). In another research (Dos Santos et al., 2008), crystalline CeO2 nanoparticles were prepared by a simple and fast H-MW method at 130oC for 20 min and then were calcinated at 500oC for 1, 2 and 4 h. Ceria powders were found to have, in this case, a spherical shape with particle size below 10 nm, a narrow distribution, and exhibit weak agglomeration. In addition, ceria hollow nanospheres composed of CeO2 nanocrystals were synthesized *via* a template-free H-MW method (Cao et al., 2010). An Ostwald ripening mechanism (Fig. 6) coupled with a self-templated, self-assembly process, in which amorphous solid spheres are converted to crystalline nanocrystals and the latter selfassemble into hollow structures, was proposed for the formation of these ceria hollow structures, which showed an excellent adsorption capacity for heavy metal ions, for example, 22.4 mg. g-1 for As(V) and 15.4 mg. g-1 for Cr(VI). The authors noted that these ceria hollow nanospheres are also excellent supports for gold nanoparticles, forming a Au/CeO2 composite catalyst. Nanocrystalline Nd2O3 precursor particles were prepared by a H-MW route from a solution containing Nd(CH3COO)3. H2O (Zawadzki, 2008). Further thermal treatment of the as-prepared precursors resulted in the formation of the well-crystallized Nd2O3 (cubic or trigonal) nanoparticles with fibrous or rod-like morphology (specific surface area 130 m2/g).

**Figure 6.** Illustration of the Ostwald ripening coupled self-templated, self-assembly process of the ceria precursor. With permission.

A small number of *hydroxides*, obtained by H-MW technique, are known. Thus, the CH and H-MW methods were used to synthesize layered double hydroxides (LDHs) (Wang et al., 2011). The microwave treatment LDHs (of MgAl and NiMgAl) were found to have higher crystallinity and smaller crystal sizes than the conventional hydrothermal treatment LDHs. It was indicated that the interactions of both OH- -CO32- and CO32--CO32- in NiMgAl-LDH, obtained by H-MW technique, are weaker. Also, the thermal decomposition of OH and CO32- in the NiMgAl-LDH sample, obtained by H-MW technique, occurred earlier and faster than that of other LDHs. Nanostructural β-Ni hydroxide β-Ni(OH)2 plates were prepared using the H-MW method at a low temperature and short reaction times (de Moura et al., 2011). An NH3 solution was employed as the coordinating agent, which reacts with [Ni(H2O)6]2+ to control the growth of β-Ni(OH)2 nuclei. It was revealed that the samples consisted of hexagonal-shaped nanoplates with a different particle size distribution. Hierarchically nanostructured -AlOOH microspheres self-assembled by nanosheets were prepared *via* H-MW technique at 160oC for 30 min, by using AlCl3. 6H2O and NaOH as raw materials and cetyltrimethyl ammonium bromide (CTAB) as surfactant, respectively (Liu et al., 2011). The morphology-contained -Al2O3 can be obtained through the thermal decomposition of -AlOOH precursors at 500oC for 2 h. Both of -AlOOH and -Al2O3 microspheres were used to adsorb Congo red from water solution. Cubic-shaped In(OH)3 particles with average size of 0.348 m were precipitated from a mixed aqueous solution of InCl3 and urea by a H-MW method (Koga & Kimizu, 2008). No intermediate compound was found during the course of thermal decomposition from cubic-In(OH)3 to cubic-In2O3. In addition, GaOOH nanorods were synthesized from Ga(NO3)3 *via* a facile H-MW method (Sun, M. et al., 2010). It was revealed that the as-synthesized sample was consisted of rodlike particles. The results for degradation of aromatic compounds (such as benzene and toluene) in an O2 gas stream under UV light illumination demonstrated that GaOOH nanorods exhibited superior photocatalytic activity and stability as compared to commercial TiO2 in both benzene and toluene degradation.

114 The Development and Application of Microwave Heating

stable at 800oC.

example, 22.4 mg.

area 130 m2/g).

precursor. With permission.

g-1 for As(V) and 15.4 mg.

route from a solution containing Nd(CH3COO)3.

It was indicated that the interactions of both OH-

of the nanorods during cycling. In addition, solid solutions of titanium-tin oxide Ti*x*Sn1-*x*O2 were prepared by H-MW method from a solution containing TiCl3 and SnCl4 (Yang et al., 2011). The advantage and contribution of this technique was revealed to be effective reduction of the difference in the formation rate of TiO2 and SnO2, resulting in the precise control of the solid solution composition. Enlarging the crystal size of single-phase, rutiletype Ti*x*Sn1-*x*O2 solid solutions can be achieved by annealing in air and the crystal phase is

A series of reports is devoted to lanthanide oxides, in particular to distinct CeO2 nanostructures, for instance virtually nonaggregated, primarily hexagonal CeO2 nanoparticles (Ivanov et al., 2009). In another research (Dos Santos et al., 2008), crystalline CeO2 nanoparticles were prepared by a simple and fast H-MW method at 130oC for 20 min and then were calcinated at 500oC for 1, 2 and 4 h. Ceria powders were found to have, in this case, a spherical shape with particle size below 10 nm, a narrow distribution, and exhibit weak agglomeration. In addition, ceria hollow nanospheres composed of CeO2 nanocrystals were synthesized *via* a template-free H-MW method (Cao et al., 2010). An Ostwald ripening mechanism (Fig. 6) coupled with a self-templated, self-assembly process, in which amorphous solid spheres are converted to crystalline nanocrystals and the latter selfassemble into hollow structures, was proposed for the formation of these ceria hollow structures, which showed an excellent adsorption capacity for heavy metal ions, for

hollow nanospheres are also excellent supports for gold nanoparticles, forming a Au/CeO2 composite catalyst. Nanocrystalline Nd2O3 precursor particles were prepared by a H-MW

treatment of the as-prepared precursors resulted in the formation of the well-crystallized Nd2O3 (cubic or trigonal) nanoparticles with fibrous or rod-like morphology (specific surface

**Figure 6.** Illustration of the Ostwald ripening coupled self-templated, self-assembly process of the ceria

A small number of *hydroxides*, obtained by H-MW technique, are known. Thus, the CH and H-MW methods were used to synthesize layered double hydroxides (LDHs) (Wang et al., 2011). The microwave treatment LDHs (of MgAl and NiMgAl) were found to have higher crystallinity and smaller crystal sizes than the conventional hydrothermal treatment LDHs.

obtained by H-MW technique, are weaker. Also, the thermal decomposition of OH-

CO32- in the NiMgAl-LDH sample, obtained by H-MW technique, occurred earlier and faster than that of other LDHs. Nanostructural β-Ni hydroxide β-Ni(OH)2 plates were prepared

g-1 for Cr(VI). The authors noted that these ceria

H2O (Zawadzki, 2008). Further thermal


and

A considerable number of publications are devoted to H-MW fabrication of *oxygen-containing salts,* in particular to metal *titanates*, both simple as BaTiO3 (Sun, W. et al., 2007; Nyutu et al., 2008) and more complex non-stoichiometric as Ba0.75Sr0.25Zr0.1Ti0.9O3 (Chen & Luo, 2008) or Bi0.5Na0.5TiO3. (Lv et al., 2009). Influence of reaction conditions on their synthesis and structures has been studied. Thus, the role of *in situ* stirring under H-MW conditions on the preparation of barium titanate was investigated (Komarneni & Katsuki et al., 2010). It was established that stirring under H-MW conditions in the temperature range of 150-200oC led to enhanced crystallization of Ba titanate as revealed by yields compared to the static condition. In addition, stirring led to smaller and more uniform crystals under H-MW conditions compared to those crystallized without stirring. Nanoparticles (20-40 nm) of barium titanate doped with different amounts of Sn2+ consisting of single phase perovskite structure were synthesized by using a S-MW reaction (Xie et al., 2009). Ceramic bodies (Fig. 7) were obtained using a spark plasma sintering method under argon atmosphere avoiding the disproportionation and oxidation of Sn2+ in the air. It was considered that Sn2+ entered into A site of the perovskite formula, ABO3, because the lattice parameters decreased with increasing the amount of doped Sn2+. The particle sizes were about 20–40 nm and increased with increasing the amount of doped Sn2+. The H-MW method was used also to synthesize crystalline barium strontium titanate (Ba0.8Sr0.2TiO3) nanoparticles (BST, 40-80 nm) in the temperature range of 100-130oC (Simoes et al., 2010). Comparing this and conventional techniques, it was shown that the H-MW synthesis route is rapid, cost effective, and could serve as an alternative to obtain BST nanoparticles. In addition, as a combination of three techniques, MW-heating was applied in the sonocatalyzed hydrothermal preparation of tetragonal phase-pure lead titanate nanopowders with stoichiometric chemical composition (Tapala et al., 2008).

**Figure 7.** SEM images of a) BaTiO3, b) Ba0.90Sn0.10TiO3, and c) Ba0.80Sn0.20TiO3 ceramic body. With permission.

Related *niobates, molybdates* (for example SrMoO4 (Sczancoski et al., 2008)), *zirconates,* and *tungstates* were also widely reported. Thus, crystallization of 1D KNbO3 nanostructures (generally unstable due to the natural tendency to form non-stoichiometric potassium niobates) was carried out through the reaction between Nb2O5 and KOH under H-MW preparation (Paula et al., 2008). The use of this method made possible a very fast preparation of single crystalline powders. Crystalline, single-phase, needle-like Ba(Mn1/3Nb2/3)O3 ceramics possessing high anisotropy were also described (Dias et al., 2009). Large crystals could be prepared from a direct combination of nanosized crystals under H-MW processing, which use leads to fast nucleation and production of nanosized particles, which could undergo a multiplying growth *via* a "cementing mechanism". It was established that the Mn ions exhibit a particular role in the lattice dynamics in complex perovskites. BaMoO4 powders were prepared by the coprecipitation method and processed in a domestic H-MW equipment (Cavalcante et al., 2008). It was shown that the BaMoO4 powders present a polydisperse particle size distribution, are free of secondary phases and crystallize in a tetragonal structure. In another closely related research of the same authors, octahedron-like BaMoO4 microcrystals were synthesized by the same method at room temperature and processed in H-MW equipment at 413 K for different times (from 30 min to 5 h) (Cavalcante et al., 2009). The researchers revealed that as-prepared BaMoO4 microcrystals present an octahedron-like morphology with agglomerate nature and polydisperse particle size distribution. It was also indicated that the microcrystals grow along the [001] direction. A fast and economical route based on H-MW reaction was developed to synthesize pancake-like Fe2(MoO4)3 microstructures (Zhang et al., 2010). It was established that several factors, including the amt. of nitric acid, reaction time, temperature and iron source, play crucial roles in the formation of the Fe2(MoO4)3 multilayer stacked structures. The oriented attachment and layer-by-layer self-assembly of nanosheets is responsible for the formation of these structures. Additionally, micro-sized decaoctahedron BaZrO3 powders (Fig. 8) were synthesized by means of a H-MW method at 140oC for 40 min (Moreira et al., 2009). A theoretical model derived from previous first principle calculations allowed authors to discuss the origin of the photoluminescence emission in BaZrO3 powders which can be related to the local disorder in the network of both ZrO6 octahedral and dodecahedral (BaO12) hence forming the constituent polyhedron of BaZrO3 system. The H-

MW processing was also employed to synthesize nanostructured alkaline-earth-metal (Ca, Ba, Sr) tungstate compounds in environmentally friendly conditions (110oC for times ranging from 5 to 20 min) (Siqueira et al., 2010).

116 The Development and Application of Microwave Heating

permission.

**Figure 7.** SEM images of a) BaTiO3, b) Ba0.90Sn0.10TiO3, and c) Ba0.80Sn0.20TiO3 ceramic body. With

Related *niobates, molybdates* (for example SrMoO4 (Sczancoski et al., 2008)), *zirconates,* and *tungstates* were also widely reported. Thus, crystallization of 1D KNbO3 nanostructures (generally unstable due to the natural tendency to form non-stoichiometric potassium niobates) was carried out through the reaction between Nb2O5 and KOH under H-MW preparation (Paula et al., 2008). The use of this method made possible a very fast preparation of single crystalline powders. Crystalline, single-phase, needle-like Ba(Mn1/3Nb2/3)O3 ceramics possessing high anisotropy were also described (Dias et al., 2009). Large crystals could be prepared from a direct combination of nanosized crystals under H-MW processing, which use leads to fast nucleation and production of nanosized particles, which could undergo a multiplying growth *via* a "cementing mechanism". It was established that the Mn ions exhibit a particular role in the lattice dynamics in complex perovskites. BaMoO4 powders were prepared by the coprecipitation method and processed in a domestic H-MW equipment (Cavalcante et al., 2008). It was shown that the BaMoO4 powders present a polydisperse particle size distribution, are free of secondary phases and crystallize in a tetragonal structure. In another closely related research of the same authors, octahedron-like BaMoO4 microcrystals were synthesized by the same method at room temperature and processed in H-MW equipment at 413 K for different times (from 30 min to 5 h) (Cavalcante et al., 2009). The researchers revealed that as-prepared BaMoO4 microcrystals present an octahedron-like morphology with agglomerate nature and polydisperse particle size distribution. It was also indicated that the microcrystals grow along the [001] direction. A fast and economical route based on H-MW reaction was developed to synthesize pancake-like Fe2(MoO4)3 microstructures (Zhang et al., 2010). It was established that several factors, including the amt. of nitric acid, reaction time, temperature and iron source, play crucial roles in the formation of the Fe2(MoO4)3 multilayer stacked structures. The oriented attachment and layer-by-layer self-assembly of nanosheets is responsible for the formation of these structures. Additionally, micro-sized decaoctahedron BaZrO3 powders (Fig. 8) were synthesized by means of a H-MW method at 140oC for 40 min (Moreira et al., 2009). A theoretical model derived from previous first principle calculations allowed authors to discuss the origin of the photoluminescence emission in BaZrO3 powders which can be related to the local disorder in the network of both ZrO6 octahedral and dodecahedral (BaO12) hence forming the constituent polyhedron of BaZrO3 system. The H-

**Figure 8.** FE-SEM image of nearly spherical BaZrO3 powders obtained for (a) 10 and (b) 20 min, and of decaoctahedral BaZrO3 powders obtained for (c) 40, (d) 80, and (e) 160 min. With permission.

*Ferrite* salts, such as BiFeO3 (Prado-Gonjal et al., 2009), and related compounds are common for H-MW synthesis. Thus, the pure phase of BiFeO3 powder was synthesized by the H-MW method with FeCl3. 6H2O and Bi(NO3)3. 5H2O powder as raw materials, KOH as mineralizer and Tween 80 as a surfactant (Zheng et al., 2011). It was indicated that the single phase of BiFeO3 powder could be prepared *via* the H-MW method at 200oC for 1 h with KOH of 1.5 mol/L, the volume fraction of Tween 80 of 0-1.7% and a mole ratio of Fe to Bi of 1.0. When the hair ball-like BiFeO3 powder was used as a catalyst, degradation rate of Rhodamine B increased from 48.92% to 79.71% after 4 h UV light irradiation. The formation of CoFe2O4 nanocrystals (particle size 6-11 nm) under hydrothermal conditions at 130oC was investigated (Kuznetsova et al., 2009). The hydrothermal medium was heated by 2 different methods, *i.e.,* the microwave (H-MW, with a reaction time from 1 min to 2 h) and conventional (with a time from 30 min to 45 h) methods. The use of microwave heating considerably accelerates the formation of CoFe2O4 particles. Preliminary ultrasonic treatment for 3 min increases the phase formation rate in the case of microwave heating and hardly affects the occurrence of the process upon conventional heating. In addition, (Ni0.30Cu0.20Zn0.50)Fe2O4-*x*MnO (*x* = 0,0.01, 0.02, 0.03, 0.04) nanopowders were synthesized by H-MW method, and sintered into dense-ceramics under the conditions of 900o/4 h (Xu et al., 2007). It was established that Mn content can influence lattice parameters of samples; Mndoping increases density of sintered Ni-Cu-Zn ferrites. Among other iron-containing compounds, we note nanocrystalline Yttrium Iron garnet (YIG) Y3Fe2(FeO4)3 with improved magnetic properties (Sadhana et al., 2009) and tantalum oxide-added MgCuZn ferrite powders (Krishnaveni et al., 2006).

Other oxygen-containing salts are represented by a relatively small number of examples. Thus, by using H-MW crystallization approach, LiFePO4 nanoparticles were synthesized in several minutes without the use of any organic reducing agent and argon protection (Yang, G. et al., 2011). A preferential orientation of crystal growth occurs upon MW hydrothermal field. The LiFePO4 crystals present 1) a change from nanoparticle to nanosheet with the increasing reaction time from 5 to 20 min, 2) a couple of redox peaks in their CV profiles, whose pairs correspond to the charge/discharge reaction of the Fe3+/Fe2+ redox couple. The authors stated that, because of the LiFePO4 samples prepared without any carbon, the initial charge/discharge capacities and cycleability of absolutely are affected by the crystal structure which is controlled by the MW irradiation condition. Flower-like PbGeO3 microstructures were prepared at low temperature *via* H-MW solution-phase approach (Li, Z.-Q. et al., 2012). It was revealed that the entire structure of the architecture is composed of a large quantity of individual nanorods with 100-300 nm in width and several micrometers in length. Optical test showed that the absorption edge of the PbGeO3 sample was 315 nm, corresponding to a bandgap of 3.94 eV. The CoAl2O4 pigment commonly used for coloring ceramic products was synthesized by the H-MW processing, and ink-jet printing with this aqueous pigment ink was performed to decorate porcelain (Obata et al., 2011). The synthesized CoAl2O4 particles were regular octahedrons measuring approximately 70 nm and were used to prepare an aqueous suspension, which was then used for printing on tiles by an ink-jet printing system. A convenient H-MW synthesis of nanostructured Cu2+-substituted ZnGa2O4 spinels was reported (Conrad et al., 2010). A difference was observed in the coordination environments with Zn mostly situated on the tetrahedral sites of the spinel lattice whereas Cu is located on the octahedral sites of the nanostructured ZnGa2O4:Cu2+ materials. Eu3+, Dy3+ and Sm3+ doped nano-sized YP0.8V0.2O4 phosphors were synthesized by a simple and facile microwave assisted hydrothermal process (Jin et al., 2011). Under UV excitation, the YP0.8V0.2O4:Ln3+ showed the VO43- self-emission band at approximately 452 nm and the characteristic emission of doped lanthanide ions (Ln3+). In addition, a facile H-MW route was developed to prepare oxynitride-based (Sr1-*x-y*Ce*x*Tb*y*)Si2O2-N2+ phosphors (Hsu & Lu, 2011). The emitting colors of the microwave-hydrothermally derived phosphors can be tuned over a wide range under UV excitation. Hectorite (white clay mineral) Na0.4Mg2.7Li0.3Si4O10(OH)2 was prepared by aging the gel precursor by H-MW treatment at 393 K for 16 h (Vicente et al., 2009). Thus fabricated hectorite had higher purity (60%) than hectorite prepared by conventional heating (45%).

*Sulfides, nitrides, and phosphides*. To reduce the reaction time, electrical energy consumption, and cost, binary -NiS--NiS was synthesized by a H-MW within 15 min at temperatures of 160-180oC (Idris et al., 2011). At 140oC, pure hexagonal NiAs-type -NiS phase was identified; with increasing reaction temperature (160-180oC), an increasing fraction of rhombohedral millerite-like -NiS is formed as a secondary phase. TEM imaging confirmed that needle-like protrusions connect the clusters of -NiS particles.

powders (Krishnaveni et al., 2006).

conventional heating (45%).

magnetic properties (Sadhana et al., 2009) and tantalum oxide-added MgCuZn ferrite

Other oxygen-containing salts are represented by a relatively small number of examples. Thus, by using H-MW crystallization approach, LiFePO4 nanoparticles were synthesized in several minutes without the use of any organic reducing agent and argon protection (Yang, G. et al., 2011). A preferential orientation of crystal growth occurs upon MW hydrothermal field. The LiFePO4 crystals present 1) a change from nanoparticle to nanosheet with the increasing reaction time from 5 to 20 min, 2) a couple of redox peaks in their CV profiles, whose pairs correspond to the charge/discharge reaction of the Fe3+/Fe2+ redox couple. The authors stated that, because of the LiFePO4 samples prepared without any carbon, the initial charge/discharge capacities and cycleability of absolutely are affected by the crystal structure which is controlled by the MW irradiation condition. Flower-like PbGeO3 microstructures were prepared at low temperature *via* H-MW solution-phase approach (Li, Z.-Q. et al., 2012). It was revealed that the entire structure of the architecture is composed of a large quantity of individual nanorods with 100-300 nm in width and several micrometers in length. Optical test showed that the absorption edge of the PbGeO3 sample was 315 nm, corresponding to a bandgap of 3.94 eV. The CoAl2O4 pigment commonly used for coloring ceramic products was synthesized by the H-MW processing, and ink-jet printing with this aqueous pigment ink was performed to decorate porcelain (Obata et al., 2011). The synthesized CoAl2O4 particles were regular octahedrons measuring approximately 70 nm and were used to prepare an aqueous suspension, which was then used for printing on tiles by an ink-jet printing system. A convenient H-MW synthesis of nanostructured Cu2+-substituted ZnGa2O4 spinels was reported (Conrad et al., 2010). A difference was observed in the coordination environments with Zn mostly situated on the tetrahedral sites of the spinel lattice whereas Cu is located on the octahedral sites of the nanostructured ZnGa2O4:Cu2+ materials. Eu3+, Dy3+ and Sm3+ doped nano-sized YP0.8V0.2O4 phosphors were synthesized by a simple and facile microwave assisted hydrothermal process (Jin et al., 2011). Under UV excitation, the YP0.8V0.2O4:Ln3+ showed the VO43- self-emission band at approximately 452 nm and the characteristic emission of doped lanthanide ions (Ln3+). In addition, a facile H-MW route was developed to prepare oxynitride-based (Sr1-*x-y*Ce*x*Tb*y*)Si2O2-N2+ phosphors (Hsu & Lu, 2011). The emitting colors of the microwave-hydrothermally derived phosphors can be tuned over a wide range under UV excitation. Hectorite (white clay mineral) Na0.4Mg2.7Li0.3Si4O10(OH)2 was prepared by aging the gel precursor by H-MW treatment at 393 K for 16 h (Vicente et al., 2009). Thus fabricated hectorite had higher purity (60%) than hectorite prepared by

*Sulfides, nitrides, and phosphides*. To reduce the reaction time, electrical energy consumption, and cost, binary -NiS--NiS was synthesized by a H-MW within 15 min at temperatures of 160-180oC (Idris et al., 2011). At 140oC, pure hexagonal NiAs-type -NiS phase was identified; with increasing reaction temperature (160-180oC), an increasing fraction of rhombohedral millerite-like -NiS is formed as a secondary phase. TEM imaging confirmed that needle-like protrusions connect the clusters of -NiS particles. Uniform nanorod and nanoplate structured Bi2S3 thin films were prepared on ITO substrates using a H-MW-assisted electrodeposition method (Wang et al., 2010). The obtained thin films are composed of orthorhombic phase Bi2S3 with good crystallinity. With the increase in the hydrothermal temperature (the best is 130oC), the crystallinity of the obtained films gradually improves and then decreases. In a related report (Thongtem et al., 2010), Bi2S3 nanorods in flower-shaped bundles were synthesized by decomposition of Bi-thiourea complexes under the microwave-assisted hydrothermal process. It was shown that Bi2S3 has the orthorhombic phase and appears as nanorods in flower-shaped bundles. Their UV-visible spectrum showed the absorbance at 596 nm, with its direct energy band gap of 1.82 eV. In addition, silver sulfide nanoworms (diameter of 50 nm and hundreds of nanometers in length) were prepared *via* a rapid H-MW by reacting silver nitrate and thioacetamide in the aqueous solution of the Bovine Serum Albumin (BSA) protein (Xing et al., 2011). It was shown that the nanoworms were assembled by multiple adjacent Ag2S nanoparticles and stabilized by a layer of BSA attached to their surface. *In vitro* assays on the human cervical cancer cell line HeLa showed that the nanoworms exhibited good biocompatibility due to the presence of BSA coating. The authors stated that this combination of features makes the nanoworms attractive and promising building blocks for advanced materials and devices. As an example of practical application of metal calcogenides, obtained by H-MW technique, a red pigment, consisting of zirconium silicate-encapsulated particles of cadmium selenide sulfide, was prepared (Wang, F. et al, 2010). The GaN nanorods were synthesized by means of a combination of H-MW process and ammoniation at high temperature using Ga2O3 as raw material (Li, D. et al., 2011). It was found that GaN nanorods with aspect ratio of 5:1 are composed of highly oriented nanoparticles. The nanorods belong to hexagonal structure, whose crystal orientation is (002). The efficiencies of two methods of synthesizing InP micro-scale hollow spheres (Fig. 9) were compared (Xiuwen Zheng et al., 2009) *via* the analogous solution–liquid–solid (ASLS) growth mechanism, either through a traditional solvothermal procedure, or *via* a microwave-assisted method (S(H)-MW). InCl3. 4H2O, HAuCl4 ethanol solution, P4 and KBH4 were used as precursors with ethylenediamine as solvent. MW synthesis was carried out for ca. 30 min under 180–220oC at the microwave power 600 W. For traditional solvothermal route, long time (10 h) is necessary to obtain the micrometer hollow spheres, however, for the microwave-assisted route, 30 min is enough for hollow spherical products. Fig. 10 describes the formation mechanism of InP micro-scale hollow spheres, which, according to the authors, is as follows: under the thermal MW irradiation conditions, the reaction between P4 molecules and In is on the surface of the In/Au droplets. Thereafter, the formed InP nanoparticles undergo the solidification to form compact InP layer on the surface of the Au/In core/shell droplets, which block the further reaction of P4 with In molecules in the beads. As a result, when removing the unreacted In by diluted HCl solution, the inner Au cores separate from the outermost InP shells, and finally produce InP hollow spheres. Due to the loss of the support, some collapsed hollow spheres are formed.

*Complex fluorides* (Fig. 11) with well-defined cubic morphologies KMF3 (M = Zn, Mn, Co, Fe, materials of technological importance, were synthesized (Kramer et al., 2008; Parhi et al.,

2008) from KF and MCl2 (M = Zn, Mn, Co, Fe) in a Parr hydrothermal vessel, subjected to MW-heating in a domestic microwave operating for 4 min at 2.45 GHz with a maximum power of 1100 W, according to the reaction (6):

2 3 3KF MCl KMF 2KCl (6)

**Figure 9.** The SEM images (a and b) for **T**-InP (obtained by a **traditional** solvothermal procedure) samples and (c and d) for **M**-InP (obtained by a **microwave** solvothermal procedure) samples after HCl treatment. Inserted in (c) is the close-up of hollow nature for **M**-InP samples. With permission.

**Figure 10.** The proposed formation mechanism for InP hollow spheres. With permission.

Microwave Hydrothermal and Solvothermal Processing of Materials and Compounds 121

**Figure 11.** SEM images of (a) KZnF3, (b) KMnF3, (c) KCoF3, and (d) KFeF3 synthesized by H-MW method. With permission.
