**2.5 W+6 oxides**

The structure of tungsten oxide concerning its inorganic chemistry carefully reflects molybdenum oxide. Numerous ortho-tungstate compounds such as Cs2WO4, Li2WO4, Rb2WO4, Na2WO4, and Na2WO4 holds isolated sites for WO4

**Figure 3.**

*Structures of dehydrated surface monoxo MoOx species (a) isolated monoxo MoO4/MoO5 and (b) polymeric monoxo MoO6 [15].*

**9**

[Se2Mo (=O)2]

**Figure 4.**

**3. Synthetic approaches**

2− and [Se2W(=O)2]

*Rational Design and Advance Applications of Transition Metal Oxides*

that are identified. Infrequently tungstate compounds procedure polymeric WO4 compounds as illustrated in **Figure 4**. One exception related to it is MgW2O7 that involves couples of distributing WO4 units. Interchanging polymeric sites of WO4 and WO6 are existing in the poly-tungstate chains of Na2W2O7 as well as (NH4)2W2O7. Ca3(WO5)Cl2 is the compound in which presence of an isolated WO5 coordinated site has been governing. Coordinated units of Isolated WO6 are found in the Wolframite structure such as ZnWO4, FeWO4, NiWO4, MnWO4, and CoWO4. Poly-tungstate chains are consistent with coordinated units of WO6 which are existing in Li2W2O7 and Ag2W2O7. Clusters of Tungsten oxide comprises polymeric units of WO6 which have been recognized with fluctuating the number of tungstate units: 12-membered includes para-tungstate (NH4)10(H2W12O42.10H2O and metatungstate (NH4)6(H2W12O40), 10-membered involves NH4BuW10O32, 6-membered comprises (NBu4)2W6O19, and 4-membered consist of Ag8W4O16. Furthermore, bulk WO3 is assembled up of 3D structure of somewhat misleading WO6 units. Numerous gas-phase mono-oxo tungsten oxyhalides (X4W=O) are identified such as X = F, Cl, and Br [36, 37]. The gas-phase complex of F4W=O displays its W=O vibrations at 1055 cm−1 unfortunately the vibrations of the gas phase monoxo complexes Cl4W=O and Br4W=O have not been experimentally determined. However, it is probable to approximate the vibrational frequency through the likeness with the corresponding oxy-halides such as X4Mo = O and X3V=O that are correspondingly guided via electronegativity order of the halide ligands. This kind of assessment proposes the mono-oxo W=O vibrations for oxy-halides such as Cl4W=O and Br4W=O must arise respectively around 1024 and 1010 cm−1. Furthermore, vibrational spectra analysis of X2W(=O)2 oxy-halides (di-oxo) have not been regulated, but IR fingerprints for Br2Mo(=O)2 have been reported and display their *v*s/*v*as vibrations in the range 995/970 cm−1. Same values for the ions such as

*Structures of dehydrated surface monoxo WOx species. (a) Isolated surface monoxo (WO4 and WO5) and (b)* 

888/845 cm−1 [38, 39]. It is worth mentioning, that the selenium covering di-oxo ions display alike vibrations. Beyond this range of W-containing ion vibrates lies in the order 10–24 cm−1 which is greater related to agreeing to Mo-containing ion. From this discussion, it can be suggested that oxy-halide of gas-phase Br2W(=O)2 would vibrate in the range 1020/980 cm−1 by similarity with Br2Mo(=O)2 [40, 41].

To prepare transition metal oxides, a variety of routes can be employed such as high temperatures and pressures, hydrothermal conditions, controlled reducing and oxidizing atmospheres, and so on. A ceramic method is commonly utilized to prepare these oxides, involving continuous grinding and heat treatment of reactant materials (e.g. carbonates, oxides, etc). These oxides have got the attention to prepare under the suitable conditions of milder and minor energy-consumption. For homogeneous mixing of reactants on an atomic scale, precursor method has been utilized [42]. Compared to ceramic technique, diffusion distance is effectively diminishes by this approach from 10,000 Å to 100 Å. Furthermore, solid solutions

2− are respectively observed around 864/834, and

*DOI: http://dx.doi.org/10.5772/intechopen.96568*

*polymeric surface monoxo surface [15].*

*Rational Design and Advance Applications of Transition Metal Oxides DOI: http://dx.doi.org/10.5772/intechopen.96568*

**Figure 4.**

*Transition Metal Compounds - Synthesis, Properties, and Application*

polymolybdates for MoO6 latter [32].

bonds existing in polymolybdates [33–35].

**2.5 W+6 oxides**

interchanging MoO4 and MoO6 units which are NaMo2O7. Coordination of isolated MoO4 is still somewhat mutual for ortho-molybdates as an example MgMoO4, CuMoO4, Na2MoO4, MnMoO4, K2MoO4, and CaMoO4. Extremely misleading coordination of isolated MoO4 is discovered in Gd2(MoO4)3, Fe2(MoO4)3, Cr2(MoO4)3, and Al2(MoO4)3. Whereas, extremely misleading units of MoO5 are existing in Bi2(MoO4)3 [20, 30]. Further, clusters of polymolybdate are constituted with 6 to 8 MoO6; whereas coordinated units are also recognized for example (NH4)4Mo8O26, (NH4)6Mo7O24, and [NH3P3(NMe2)6]2Mo6O19. Bulk MoO3 (alpha) is comprised of a 3D structure prepared up of extremely misleading units of MoO6. The great misleading existing in bulk MoO3 (alpha) origins the sixth O atom to be positioned extremely distant respected to Mo and, therefore, the structure of the relevant bulk MoO3 (alpha) is well pronounced as comprising of MoO5 units. The bulk MoO3 (beta) crystalline period is one more MoO3 3D structure fabricated up of minute misleading of MoO6 units [31]. Numerous gas-phase mon-oxo molybdenum oxyhalides (X4Mo = O) are also recognized, structural analysis illustrated in **Figure 3**. The Mo = O vibrations fluctuate in the range 1008–1039 cm−1 with the increment in electronegativity of halide in the order Cl *<* F. The gas-phase di-oxo Br2Mo(=O)2 growths to the bands at 995 (*v*s) as well as 970 (*v*as) cm−1 owing to an order of electronegativity as Br *<* Cl *<* F. Therefore, the structure of molybdenum oxides with respect to its inorganic chemistry involves the coordinated units as MoO4, MoO5, and MoO6, with a first choice in

For non-SiO2 supported MoOx catalysts, MoOx coordination for the dehydrated surface is subjected to the exposure of surface molybdena and particular oxide support. At faint surface coverage of molybdena that ranges from 5–15% of a monolayer, mainly surface coordinated groups of MoO4 are exist on Al2O3 and TiO2. The parallel Raman spectrum of the above-discussed catalysts also agrees to the extent of minute surface coverage. In addition, MoO4 surface species are correspondingly isolated on both oxide supports. This phenomenon is also authenticated through UV–vis spectra that display huge bandgap energy related to isolated classes. Species with monolayer owns the surface exposure of molybdena, sustained MoO3/TiO2 was establishing to hold MoO6 coordinated groups, and sustained MoO3/Al2O3 was set up to retain a combination of MoO4 as well as MoO6 coordinated species. For monolayer MoO3/Al2O3, the supplementary occurrence of surface MoO6 was also revealed in minor bandgap value of this catalyst. Therefore, UV–vis analysis and Raman measurements for samples discussed above (dehydrated MoO3/ZrO2 and MoO/Al2O3) were quite alike and recommend the similar surface species such as MoOx occur on the supports together by a certain surface exposure. Measurements are taken from Raman approach also discloses characteristics of linking Mo–O–Mo

The structure of tungsten oxide concerning its inorganic chemistry carefully reflects molybdenum oxide. Numerous ortho-tungstate compounds such as Cs2WO4, Li2WO4, Rb2WO4, Na2WO4, and Na2WO4 holds isolated sites for WO4

*Structures of dehydrated surface monoxo MoOx species (a) isolated monoxo MoO4/MoO5 and (b) polymeric* 

**8**

**Figure 3.**

*monoxo MoO6 [15].*

*Structures of dehydrated surface monoxo WOx species. (a) Isolated surface monoxo (WO4 and WO5) and (b) polymeric surface monoxo surface [15].*

that are identified. Infrequently tungstate compounds procedure polymeric WO4 compounds as illustrated in **Figure 4**. One exception related to it is MgW2O7 that involves couples of distributing WO4 units. Interchanging polymeric sites of WO4 and WO6 are existing in the poly-tungstate chains of Na2W2O7 as well as (NH4)2W2O7. Ca3(WO5)Cl2 is the compound in which presence of an isolated WO5 coordinated site has been governing. Coordinated units of Isolated WO6 are found in the Wolframite structure such as ZnWO4, FeWO4, NiWO4, MnWO4, and CoWO4. Poly-tungstate chains are consistent with coordinated units of WO6 which are existing in Li2W2O7 and Ag2W2O7. Clusters of Tungsten oxide comprises polymeric units of WO6 which have been recognized with fluctuating the number of tungstate units: 12-membered includes para-tungstate (NH4)10(H2W12O42.10H2O and metatungstate (NH4)6(H2W12O40), 10-membered involves NH4BuW10O32, 6-membered comprises (NBu4)2W6O19, and 4-membered consist of Ag8W4O16. Furthermore, bulk WO3 is assembled up of 3D structure of somewhat misleading WO6 units. Numerous gas-phase mono-oxo tungsten oxyhalides (X4W=O) are identified such as X = F, Cl, and Br [36, 37]. The gas-phase complex of F4W=O displays its W=O vibrations at 1055 cm−1 unfortunately the vibrations of the gas phase monoxo complexes Cl4W=O and Br4W=O have not been experimentally determined. However, it is probable to approximate the vibrational frequency through the likeness with the corresponding oxy-halides such as X4Mo = O and X3V=O that are correspondingly guided via electronegativity order of the halide ligands. This kind of assessment proposes the mono-oxo W=O vibrations for oxy-halides such as Cl4W=O and Br4W=O must arise respectively around 1024 and 1010 cm−1. Furthermore, vibrational spectra analysis of X2W(=O)2 oxy-halides (di-oxo) have not been regulated, but IR fingerprints for Br2Mo(=O)2 have been reported and display their *v*s/*v*as vibrations in the range 995/970 cm−1. Same values for the ions such as [Se2Mo (=O)2] 2− and [Se2W(=O)2] 2− are respectively observed around 864/834, and 888/845 cm−1 [38, 39]. It is worth mentioning, that the selenium covering di-oxo ions display alike vibrations. Beyond this range of W-containing ion vibrates lies in the order 10–24 cm−1 which is greater related to agreeing to Mo-containing ion. From this discussion, it can be suggested that oxy-halide of gas-phase Br2W(=O)2 would vibrate in the range 1020/980 cm−1 by similarity with Br2Mo(=O)2 [40, 41].

#### **3. Synthetic approaches**

To prepare transition metal oxides, a variety of routes can be employed such as high temperatures and pressures, hydrothermal conditions, controlled reducing and oxidizing atmospheres, and so on. A ceramic method is commonly utilized to prepare these oxides, involving continuous grinding and heat treatment of reactant materials (e.g. carbonates, oxides, etc). These oxides have got the attention to prepare under the suitable conditions of milder and minor energy-consumption. For homogeneous mixing of reactants on an atomic scale, precursor method has been utilized [42]. Compared to ceramic technique, diffusion distance is effectively diminishes by this approach from 10,000 Å to 100 Å. Furthermore, solid solutions

of hydroxides nitrates, and carbonates, have been frequently utilized to aim for this purpose besides the precursor compounds. Novel oxides that acquire challenging scheme to prepare can also be synthesized by this method. Similarly, topochemical reactions produce rare oxides such as synthesis of MoO3 and ReO3 structure by topochemical dehydration. By this dehydration reaction, Mo1-xWxO3 has also been synthesized. Further examples of synthesizes of rare oxides by topochemical reaction are reported in the literature [43]. A worth mentioning topochemical reaction is the addition of atomic species in oxides hosts. Thus, alkali metals and lithium have been injected into the different types of oxides such as MnO2, Fe3O4, TiO2, VO2, and ReO3. In the literature, intercalation phenomenon has been reviewed sufficiently. By employing slight oxidizing conditions, deintercalation of lithium and some alkali metals can be carried out steadily. Several innovative examples of deintercalation and intercalations phenomenons are being continuously conveyed. Recently, lithium injection to W19O55 and topochemical reactions of LixNbO2 has been reported. Ion exchange can be executed in the close-packed arranges of oxides, tunnel and layered structures. These reactions are also associated as topochemical and can be executed in molten media e.g., conversion to HNbO3 from LiNbO3 with hot aqueous acid [44]. The procedure of this reaction is contrary to transformation of ReO3 to LiReO3 (rhombohedral). Hydrogen can also be injected into holes of oxide with the company of Pt catalyst. Diversities of exchange reactions are huge for synthetic purposes. In the literature, many exchange reactions have been mentioned; two recent examples are given as the exchange properties of Na4Ti9O20 (X) H2O and intercalated effect of alkylammonium ion on cation (+) exchange properties of H2Ti3O7. Preparation of layered K2Ti4O9 and metastable TiO2 using a topotactic dihydroxylation is also an interesting example [42, 45].

The vapor deposition method is a well-known technique among other synthesis methods. Complex oxides (Mo and Mo bronzes) have been synthesized by employing fused salt electrolysis. Under oxidizing conditions, the pyrochlores Bi[Ru2-x <sup>5</sup> *Bi*<sup>x</sup> + ] O7-Y and Pb2[Ru2-x <sup>4</sup> *pb*<sup>x</sup> <sup>+</sup> ]O7-Y has been synthesized from an alkaline medium [45, 46]. The sol–gel approach is more efficient in preparing multiple oxides and superconducting cuprates. Although arc melting process can prepare many oxides a novel technique is a crucible-free method. Synthesis by high-pressure methods has been reviewed. This greater pressure reasons to stabilize the states of rare oxidation (e.g. GdNiO3, La2Pd2O7, etc.). Recently, under high oxygen pressure YBa2CU4O8 has been synthesized [45, 47, 48].

## **3.1 Transition**−**metal oxides nanostructures**

Ended to the previous few decades, transition metal oxides nanostructures (TMON) have been extensively considered owing to attain excessive potential in optical, electronic, and magnetic applications. To accomplish extraordinary and exceptional performances, TMONs have been assimilated into the assortment of devices that consists of efficient photocatalysis, and enhanced gas sensing [49, 50]. In TMOs, although the electrons are permanently occupied in the s − shells of +ve metallic ions, the d − shells of TMOs may not be entirely occupied. This distinctive carries numerous exceptional properties in them, that comprises decent electrical characteristics [51–53] high dielectric constants [54, 55], reactive electronic transitions [56, 57], wide band gaps [58, 59], and so on. Meanwhile, TMOs owns several states including, ferrimagnetic, ferromagnetic, and semi-conductive state. Hence, TMOs are reflected in the absolute interesting functional materials. Catalysts are liquefied into liquid alloy droplets, which also comprise corresponding source metal. When alloy droplets attain supersaturated condition then the respective source metal initiates to precipitate which turns into metal oxide followed by the flow of

**11**

room temperature is γ-WO3 [2].

*Rational Design and Advance Applications of Transition Metal Oxides*

structure, properties, and morphology of the product [2, 67–69].

To prepare one dimensional (1-D) metal-oxide nanostructures such as wires/ fibers [70, 71], nanorods [72–74], nanotubes [75, 76], hemitubes [77], nanobelts [78, 79], and needles/tips [70, 80], enormous attempts have been made. To enhance the morphological parameters, VS and VLS are the two main growth mechanisms used in vapor phase method. By changing variables such as assisting electric field, substrate, catalyst, pre-treatment, deposition temperature, etc., morphologies of required products can be controlled. Vapor phase method in the presence of oxygen obtained WO3 1-D nanostructures which have high aspect ratios (**Figure 5a**) showed exceptional results in field emission display (**Figure 5b**) and also in some other applications such as gas sensors, photodetectors, and so on. It's convenient to comprehend monoclinic formation (three unequal axes) of γ-WO3 phase which is stable at 17–320 °C by assuming the growth temperature under 1000 °C, transition of phase in WO3 is not completely reversible while the most stable phase reported at

Heterogeneous substrates are used to grow 1-D nanostructures [70, 83], affected

by the substrate surface, mostly, they exhibited {001} growth direction beside length (**Figure 5c**), while W + Si supported Au film or nanowires on Si wafer showed {010} or {100}/{010} growth direction (**Figure 5d, e**). Due to lack of oxygen gas WO2 nanowires were synthesized caused by oxidation of Ni, by restoring the substrate with Si + W succeeded by Ni film (**Figure 5f**) [83]. By using vapor phase method, WO3-τ (0 < τ < 1) 1-D nanostructures (e.g. W18O29) can be manufactured with poor oxygen atmosphere (react with slighter oxygen source or gas like carbon dioxde) [84, 85]. Because of closely packed planes such as {010}, one-dimensional W18O29 nanostructures (e.g. nanoneedles, nanowire, nanotip, *etc.*, substrates

oxygen. Generally, as−synthesized metal oxides especially rise along specific alignment, which resulted in the establishment of 1D nanostructure. Up to now, preparation approach for the metal oxide nanowires including In2O3, [60] CdO [61], TiO2 [62], ZnO [63], and SnO2 [64] have been accomplished using VLS mechanism. The VLS procedure corresponds to catalyst−aided growth whereas; VS route is attributed to the catalyst−free growth [65, 66]. The progression of VS method includes the reactants which are first heated to produce vapors followed by high temperature and then unswervingly condensed on the substrate. In this substrate, the seed crystals will be assisted to nucleation sites located and acquire shape. Facilitate directional growth followed will minimize the surface energy of product.

In 1970s, the hydrothermal route was primarily hired to synthesize the various types of crystalline structures. Using this strategy, reactants are positioned in the sealed vessel that followed water as the solvent (reaction medium). A reaction in hydrothermal approach proceeds in the presence of high temperature that causes to produce high pressure. This procedure can speed up the reactions among ions and finally endorse the hydrolysis. Eventually, self−assembly, as well as the growth and of crystals, will be succeeded as the consequence of reaction mechanism in solution. Merits of this process contain mild reaction conditions, easy monitoring, and importantly low cost. Morphology, crystallographic structure, and the properties of final product acquired through hydrothermal route can be accomplished by altering the experimental limitations that involve the variance in time, reaction medium, temperature, and pressure, etc. Surfactants are familiarized with the arrangement to advance hydrothermal route. The surfactant-promoted method has been verified to results in an efficacious manner in order to fabricate metal oxide owing to an assortment of morphologies. Three phases are always involved in the system firstly, oil phase secondly, surfactant phase, and lastly, aqueous phase. In the progression of route, surfactants can restrain the growth of final product. Meanwhile, pH value, concentration of reactants, and temperature also has necessary guidance on the

*DOI: http://dx.doi.org/10.5772/intechopen.96568*

#### *Rational Design and Advance Applications of Transition Metal Oxides DOI: http://dx.doi.org/10.5772/intechopen.96568*

*Transition Metal Compounds - Synthesis, Properties, and Application*

topotactic dihydroxylation is also an interesting example [42, 45].

O7-Y and Pb2[Ru2-x <sup>4</sup> *pb*<sup>x</sup>

been synthesized [45, 47, 48].

**3.1 Transition**−**metal oxides nanostructures**

The vapor deposition method is a well-known technique among other synthesis methods. Complex oxides (Mo and Mo bronzes) have been synthesized by employing fused salt electrolysis. Under oxidizing conditions, the pyrochlores Bi[Ru2-x <sup>5</sup> *Bi*<sup>x</sup>

The sol–gel approach is more efficient in preparing multiple oxides and superconducting cuprates. Although arc melting process can prepare many oxides a novel technique is a crucible-free method. Synthesis by high-pressure methods has been reviewed. This greater pressure reasons to stabilize the states of rare oxidation (e.g. GdNiO3, La2Pd2O7, etc.). Recently, under high oxygen pressure YBa2CU4O8 has

Ended to the previous few decades, transition metal oxides nanostructures (TMON) have been extensively considered owing to attain excessive potential in optical, electronic, and magnetic applications. To accomplish extraordinary and exceptional performances, TMONs have been assimilated into the assortment of devices that consists of efficient photocatalysis, and enhanced gas sensing [49, 50]. In TMOs, although the electrons are permanently occupied in the s − shells of +ve metallic ions, the d − shells of TMOs may not be entirely occupied. This distinctive carries numerous exceptional properties in them, that comprises decent electrical characteristics [51–53] high dielectric constants [54, 55], reactive electronic transitions [56, 57], wide band gaps [58, 59], and so on. Meanwhile, TMOs owns several states including, ferrimagnetic, ferromagnetic, and semi-conductive state. Hence, TMOs are reflected in the absolute interesting functional materials. Catalysts are liquefied into liquid alloy droplets, which also comprise corresponding source metal. When alloy droplets attain supersaturated condition then the respective source metal initiates to precipitate which turns into metal oxide followed by the flow of

<sup>+</sup> ]O7-Y has been synthesized from an alkaline medium [45, 46].

+ ]

of hydroxides nitrates, and carbonates, have been frequently utilized to aim for this purpose besides the precursor compounds. Novel oxides that acquire challenging scheme to prepare can also be synthesized by this method. Similarly, topochemical reactions produce rare oxides such as synthesis of MoO3 and ReO3 structure by topochemical dehydration. By this dehydration reaction, Mo1-xWxO3 has also been synthesized. Further examples of synthesizes of rare oxides by topochemical reaction are reported in the literature [43]. A worth mentioning topochemical reaction is the addition of atomic species in oxides hosts. Thus, alkali metals and lithium have been injected into the different types of oxides such as MnO2, Fe3O4, TiO2, VO2, and ReO3. In the literature, intercalation phenomenon has been reviewed sufficiently. By employing slight oxidizing conditions, deintercalation of lithium and some alkali metals can be carried out steadily. Several innovative examples of deintercalation and intercalations phenomenons are being continuously conveyed. Recently, lithium injection to W19O55 and topochemical reactions of LixNbO2 has been reported. Ion exchange can be executed in the close-packed arranges of oxides, tunnel and layered structures. These reactions are also associated as topochemical and can be executed in molten media e.g., conversion to HNbO3 from LiNbO3 with hot aqueous acid [44]. The procedure of this reaction is contrary to transformation of ReO3 to LiReO3 (rhombohedral). Hydrogen can also be injected into holes of oxide with the company of Pt catalyst. Diversities of exchange reactions are huge for synthetic purposes. In the literature, many exchange reactions have been mentioned; two recent examples are given as the exchange properties of Na4Ti9O20 (X) H2O and intercalated effect of alkylammonium ion on cation (+) exchange properties of H2Ti3O7. Preparation of layered K2Ti4O9 and metastable TiO2 using a

**10**

oxygen. Generally, as−synthesized metal oxides especially rise along specific alignment, which resulted in the establishment of 1D nanostructure. Up to now, preparation approach for the metal oxide nanowires including In2O3, [60] CdO [61], TiO2 [62], ZnO [63], and SnO2 [64] have been accomplished using VLS mechanism. The VLS procedure corresponds to catalyst−aided growth whereas; VS route is attributed to the catalyst−free growth [65, 66]. The progression of VS method includes the reactants which are first heated to produce vapors followed by high temperature and then unswervingly condensed on the substrate. In this substrate, the seed crystals will be assisted to nucleation sites located and acquire shape. Facilitate directional growth followed will minimize the surface energy of product.

In 1970s, the hydrothermal route was primarily hired to synthesize the various types of crystalline structures. Using this strategy, reactants are positioned in the sealed vessel that followed water as the solvent (reaction medium). A reaction in hydrothermal approach proceeds in the presence of high temperature that causes to produce high pressure. This procedure can speed up the reactions among ions and finally endorse the hydrolysis. Eventually, self−assembly, as well as the growth and of crystals, will be succeeded as the consequence of reaction mechanism in solution. Merits of this process contain mild reaction conditions, easy monitoring, and importantly low cost. Morphology, crystallographic structure, and the properties of final product acquired through hydrothermal route can be accomplished by altering the experimental limitations that involve the variance in time, reaction medium, temperature, and pressure, etc. Surfactants are familiarized with the arrangement to advance hydrothermal route. The surfactant-promoted method has been verified to results in an efficacious manner in order to fabricate metal oxide owing to an assortment of morphologies. Three phases are always involved in the system firstly, oil phase secondly, surfactant phase, and lastly, aqueous phase. In the progression of route, surfactants can restrain the growth of final product. Meanwhile, pH value, concentration of reactants, and temperature also has necessary guidance on the structure, properties, and morphology of the product [2, 67–69].

To prepare one dimensional (1-D) metal-oxide nanostructures such as wires/ fibers [70, 71], nanorods [72–74], nanotubes [75, 76], hemitubes [77], nanobelts [78, 79], and needles/tips [70, 80], enormous attempts have been made. To enhance the morphological parameters, VS and VLS are the two main growth mechanisms used in vapor phase method. By changing variables such as assisting electric field, substrate, catalyst, pre-treatment, deposition temperature, etc., morphologies of required products can be controlled. Vapor phase method in the presence of oxygen obtained WO3 1-D nanostructures which have high aspect ratios (**Figure 5a**) showed exceptional results in field emission display (**Figure 5b**) and also in some other applications such as gas sensors, photodetectors, and so on. It's convenient to comprehend monoclinic formation (three unequal axes) of γ-WO3 phase which is stable at 17–320 °C by assuming the growth temperature under 1000 °C, transition of phase in WO3 is not completely reversible while the most stable phase reported at room temperature is γ-WO3 [2].

Heterogeneous substrates are used to grow 1-D nanostructures [70, 83], affected by the substrate surface, mostly, they exhibited {001} growth direction beside length (**Figure 5c**), while W + Si supported Au film or nanowires on Si wafer showed {010} or {100}/{010} growth direction (**Figure 5d, e**). Due to lack of oxygen gas WO2 nanowires were synthesized caused by oxidation of Ni, by restoring the substrate with Si + W succeeded by Ni film (**Figure 5f**) [83]. By using vapor phase method, WO3-τ (0 < τ < 1) 1-D nanostructures (e.g. W18O29) can be manufactured with poor oxygen atmosphere (react with slighter oxygen source or gas like carbon dioxde) [84, 85]. Because of closely packed planes such as {010}, one-dimensional W18O29 nanostructures (e.g. nanoneedles, nanowire, nanotip, *etc.*, substrates

dependent) commonly revealed monoclinic (unequal axes) phase with the selective growth along {010} direction (**Figure 6**).

Substrates are conventionally utilized for growth of hierarchical structures in vapor phase method. On the Si substrate surface along with polystyrene spheres monolayer, 0-D and 2-D structures of α-Fe2O3 can be attained by PLD-CVD at an oxygen pressure 60 and 6 Pascal, respectively [87], as shown in **Figure 7a**-**d**. Since there is deficiency of Fe atoms and O atoms are in excess in {110} plane, so for preferential growth along {110} direction, it can be assumed to be driving force. Single dimensional-based 3-D Fe3O4 successfully synthesized in an autoclave on its wall (**Figure 7e** and **f**) through the pyrolysis of ferrocene (supercritical carbon dioxide at 450 °C) Cao *et al.* [88] increasing Fe sources resulted in the formation of 2-D nanosheets while decreasing the amount CO2 sources led to the reduction of

#### **Figure 5.**

*(a) The cross-sectional SEM image of as-prepared WO3 nanowires, and (b) Arabic numerals and Chinese characters displayed by the double-gated FED [81] (c) TEM micrographs showing the lattice fringes and the diffraction pattern (insets) of individual tungsten oxide nanowires [82] (d) the SEM image of* γ*-WO3 nanowires, (e) typical HRTEM images of* γ*-WO3 nanowire and (f) WO2 nanowire [83].*

#### **Figure 6.**

*(a) SEM images of three-dimensionally aligned W18O49 nanowires on carbon microfibers, (b) a typical TEM image of a single W18O49 nanowire (c) selected area electron diffraction (SAED) pattern of the nanowire [86].*

**13**

**Figure 8.**

*(f, h, j) TEM images of hollow Fe2O3 microboxes [95].*

*Rational Design and Advance Applications of Transition Metal Oxides*

nanorods length. On the substrate of FTO (**Figure 7g**–**i**), nanoplatelet of α-Fe2O3 can be obtained at room temperature in a PECVD system, whose thickness can be

*(a) SEM images of as-deposited samples at an oxygen pressure of 6 Pa (0D based, a and b) and 60 Pa; (a), (c) top surface; (b), (d) cross-section [87]. (e and f) typical FESEM images of 3D Fe3O4 networks [88] (g) HRTEM image and (h) HAADF-STEM micrograph representing the hierarchical morphology of the hematite* 

Multiple FeOx arranged nanostructures can be prepared through simple solution method, precursor based method, template-directed, and solvo/hydrothermal reaction in liquid phase method. By precursor based method [90, 91] and solvo/hydrothermal reaction [92], 0-D based FeOx arranged nanostructures (mesoporous particles such as spheres, cubes, super-structures, hollow spheres/ bowls, etc. (**Figure 8a**–**c**) are commonly prepared. Metal–organic frameworks (MOFs) have received great attention as an advanced type of precursors with controllable properties such as shape, composition, size, and internal structure for MOX arranged nanostructures. For example, Fe2O3 microboxes synthesized by Lou *et al.* [95] with different shell structures (**Figure 8e**–**j**) based on appropriate annealing of pre-formed PB (Prussian blue) microcubes (**Figure 8d**) [2]. As-synthesized Fe2O3 micro boxes having unique shell structures and distinguish cycling performance unveiled high lithium storage capacities when evaluated for

*(a) A TEM image of a single Fe3O4 microsphere, with a corresponding SAED pattern (inset) [91] (b) a SEM image of Fe3O4 hollow microspheres (the inset is the corresponding TEM image) [93] (c) SEM images of the bowl-like hollow Fe3O4/r-GO composites [94] (d) a FESEM images of PB microcubes; (e, g, i) FESEM and* 

increase by increasing the amount of Fe sources [89].

*platelets; (i) a cross-sectional SEM image of hematite nanoplatelet arrays [89].*

*DOI: http://dx.doi.org/10.5772/intechopen.96568*

**Figure 7.**

*Rational Design and Advance Applications of Transition Metal Oxides DOI: http://dx.doi.org/10.5772/intechopen.96568*

#### **Figure 7.**

*Transition Metal Compounds - Synthesis, Properties, and Application*

growth along {010} direction (**Figure 6**).

dependent) commonly revealed monoclinic (unequal axes) phase with the selective

Substrates are conventionally utilized for growth of hierarchical structures in vapor phase method. On the Si substrate surface along with polystyrene spheres monolayer, 0-D and 2-D structures of α-Fe2O3 can be attained by PLD-CVD at an oxygen pressure 60 and 6 Pascal, respectively [87], as shown in **Figure 7a**-**d**. Since there is deficiency of Fe atoms and O atoms are in excess in {110} plane, so for preferential growth along {110} direction, it can be assumed to be driving force. Single dimensional-based 3-D Fe3O4 successfully synthesized in an autoclave on its wall (**Figure 7e** and **f**) through the pyrolysis of ferrocene (supercritical carbon dioxide at 450 °C) Cao *et al.* [88] increasing Fe sources resulted in the formation of 2-D nanosheets while decreasing the amount CO2 sources led to the reduction of

*(a) The cross-sectional SEM image of as-prepared WO3 nanowires, and (b) Arabic numerals and Chinese characters displayed by the double-gated FED [81] (c) TEM micrographs showing the lattice fringes and the diffraction pattern (insets) of individual tungsten oxide nanowires [82] (d) the SEM image of* γ*-WO3*

*(a) SEM images of three-dimensionally aligned W18O49 nanowires on carbon microfibers, (b) a typical TEM image of a single W18O49 nanowire (c) selected area electron diffraction (SAED) pattern of the nanowire [86].*

*nanowires, (e) typical HRTEM images of* γ*-WO3 nanowire and (f) WO2 nanowire [83].*

**12**

**Figure 6.**

**Figure 5.**

*(a) SEM images of as-deposited samples at an oxygen pressure of 6 Pa (0D based, a and b) and 60 Pa; (a), (c) top surface; (b), (d) cross-section [87]. (e and f) typical FESEM images of 3D Fe3O4 networks [88] (g) HRTEM image and (h) HAADF-STEM micrograph representing the hierarchical morphology of the hematite platelets; (i) a cross-sectional SEM image of hematite nanoplatelet arrays [89].*

nanorods length. On the substrate of FTO (**Figure 7g**–**i**), nanoplatelet of α-Fe2O3 can be obtained at room temperature in a PECVD system, whose thickness can be increase by increasing the amount of Fe sources [89].

Multiple FeOx arranged nanostructures can be prepared through simple solution method, precursor based method, template-directed, and solvo/hydrothermal reaction in liquid phase method. By precursor based method [90, 91] and solvo/hydrothermal reaction [92], 0-D based FeOx arranged nanostructures (mesoporous particles such as spheres, cubes, super-structures, hollow spheres/ bowls, etc. (**Figure 8a**–**c**) are commonly prepared. Metal–organic frameworks (MOFs) have received great attention as an advanced type of precursors with controllable properties such as shape, composition, size, and internal structure for MOX arranged nanostructures. For example, Fe2O3 microboxes synthesized by Lou *et al.* [95] with different shell structures (**Figure 8e**–**j**) based on appropriate annealing of pre-formed PB (Prussian blue) microcubes (**Figure 8d**) [2].

As-synthesized Fe2O3 micro boxes having unique shell structures and distinguish cycling performance unveiled high lithium storage capacities when evaluated for

#### **Figure 8.**

*(a) A TEM image of a single Fe3O4 microsphere, with a corresponding SAED pattern (inset) [91] (b) a SEM image of Fe3O4 hollow microspheres (the inset is the corresponding TEM image) [93] (c) SEM images of the bowl-like hollow Fe3O4/r-GO composites [94] (d) a FESEM images of PB microcubes; (e, g, i) FESEM and (f, h, j) TEM images of hollow Fe2O3 microboxes [95].*

#### **Figure 9.**

*Morphology of the hollow spheres composed of ZnO nanorods. (a) TEM image of the samples (b, c) typical magnified TEM images of hollow spheres (d, e) SEM image of the samples (f) typical magnified SEM image of a hollow sphere (g) the EDS spectrum of hollow spheres [98].*

lithium-ion batteries as potential anode material. Furthermore, using controlled chemical etching, hollow interiors could be generated inside the PB nanoparticles in poly (vinylpyrrolidone) presence, [96] porous nanostructures of iron oxide having hollow interiors, various phases of these PB nanoparticles (preliminary precursors) can be synthesized by controlled calcination.

Due to the potential uses in various fields like waste removal, biologically active agent protection, chemical, biological sensors, catalysis, and bimolecular-release systems, well-defined 0-D ZnO hollow structures have attracted much attention. So in past few years, many successful attempts were made to prepare hollow structures of ZnO. The template-assisted technique is now the main focus of researchers which conventionally employed spherobacteria, carbon spheres, polystyrene spheres, and so on as template for hollow structures growth of ZnO. Under hydrothermal conditions, conversion of Zn(NH3 <sup>2</sup> 4) <sup>+</sup> reported by Gao *et al.* [97] resulted in hollow spheres of ZnO formation which have an inner and outer diameter as 100 nm and 600 nm, respectively. These hollow spheres were made up of ZnO nanorods (**Figure 9**). Ethanol volume ratio with respect to solution and initial mixture pH value both have a significant role in hollow spheres formation. Meanwhile, results obtained from characterization, ZnO hollow spheres showed remarkable photoluminescence properties (at room temperature) with UV emission peak at 390 nm.

#### **4. Advanced applications**

Over the past decade, due to unique electronic, magnetic, and optical applications metal oxide materials arising as potential candidates with fruitful functionalities have been extensively studied. These applications will be discussed briefly in this section.

#### **4.1 Photovoltaics**

In photovoltaics stable and environment-friendly metal oxide semiconductors are used in dye−sensitized solar cells (DSSCs) as photoelectrode or to design p-n

**15**

**Figure 10.**

*Rational Design and Advance Applications of Transition Metal Oxides*

junctions of metal oxide. Materials have been examined for photoelectrodes purpose in DSSCs (**Figure 10**) such as binary metal oxides (ZrO2, Fe2O3, TiO2, Al2O3, ZnO, Nb2O5) and ternary compounds (SrTiO3, Zn2SnO4). Due to high thermal and chemical stability, a hole blocking property, and suitable electron selectivity Nb2O5,

In technology, lithium-ion batteries made up of metal oxide nanoparticles (SnO2, Co3O4, Fe2O3, TiO2, and complex metal oxides) enable superior rate capability; better cycling performance and high specific capacity are arising as the best choice for portable electronics. Its applications include electronics, electric vehicles, etc. Transition metal oxides hold boundless potential towards high-energy-density anode due to their better capacities than those which are commercially utilized as

In most highlighted photocatalytic areas TiO2 has been the most promising material as a photocatalyst. In last 3 decades, TiO2 attracted notable scientific and technological consequences (**Figure 11**). Similarly, to study other photocatalytic oxidation properties metal oxides (ZnO, SnO2, Fe2O3, WO3, Cu2O, SrTiO3) have been studied in detail. High crystallinity and large surface area with more active sites reduce recombination rate of photo−generated electron–holes pairs are the properties of the best photocatalyst. For oxygen (O2) evolution by photocatalysis from H2O under irradiation of visible light, highly−arranged tungsten oxide (m − WO3) hybridized with reduced graphene-oxide has been synthesized. Tremendous photocatalytic properties have been shown by CdS nanorods/reduced graphene-oxide composites had excellent photocatalytic properties with a rate constant was around three times

Electrical conductance sensitive to ambient gas composition, rising from interac-

tions of charges with volatile organic compounds, reactive gases (O2, CO, NOx), hydrocarbons, and semiconducting metal oxides (WO3, TiO2, SnO2, ZnO) are utilized for gas sensing applications. The effort was made to acquire better results towards low pollutant gas concentrations under low operating temperatures for gas sensing materials. For the detection of harmful gases and large scale, thermal stability under operating conditions of sensors SnO2 nanostructures has attracted

*Schematic diagram of the nanowire dye-sensitized solar cell based on a ZnO wire array [99].*

ZnO, and TiO2 are excellent expectant as a photoelectrode [2, 99, 100].

*DOI: http://dx.doi.org/10.5772/intechopen.96568*

anode material such as graphite [2, 101, 102].

greater than CdS nanorods for the degradation of MO [2, 103].

**4.2 Lithium-ion batteries**

**4.3 Photocatalysis**

**4.4 Gas-sensing**

the most attention [2, 104].

junctions of metal oxide. Materials have been examined for photoelectrodes purpose in DSSCs (**Figure 10**) such as binary metal oxides (ZrO2, Fe2O3, TiO2, Al2O3, ZnO, Nb2O5) and ternary compounds (SrTiO3, Zn2SnO4). Due to high thermal and chemical stability, a hole blocking property, and suitable electron selectivity Nb2O5, ZnO, and TiO2 are excellent expectant as a photoelectrode [2, 99, 100].
