**2. Mechanism of photocatalysis**

When photocatalyst such as titanium dioxide (TiO<sup>2</sup> ) absorbs Ultraviolet (UV)\* radiation comes from sun or any other illuminated light source (e.g., fluorescent lamps), pairs of electrons and holes are produced, see **Figure 2**. As a result of the light illumination, the electron of the valence band of titanium dioxide becomes excited. Excited electron transits to the conduction band of titanium dioxide with excess energy to create pair of charges; the negative-electron (e-) and positive-hole (h+). This behaviour is well known as the semiconductor's photoexcitation' state. The 'Band Gap' is defined as a result of the difference in energy between the valence band and the conduction band. The necessary wavelength of the light required for the photo-excitation is given according to 1240 (Planck's constant, h)/3.2 eV (band gap energy) and equal to 388 nm [3]. The hole with positive charge in titanium dioxide may split the water molecule into both of the hydrogen gas and hydroxyl radical. On the other side, the electron with negative charge reacts with oxygen molecule forming the super oxide anion. The continuity of this cycle depends on the availability of the light [3].

Perovskite Strontium Doped Rare Earth Manganites Nanocomposites and Their Photocatalytic Performances http://dx.doi.org/10.5772/intechopen.79479 131

**Figure 2.** Schematic diagram showing the photocatalysis mechanism by producing both holes and electrons as a result of illumination [3].

Solar energy is clean and till now its utilization is limited. A strong need to develop a sustainable and cost-effective manner for harvesting solar energy to satisfy the growing energy demand of the world with a minimal environmental impact [4]. Photo-catalysis plays an important role for the conversion of solar energy into chemical fuel, electricity, the decomposition of organic pollutants etc.

The degradation behaviors were studied by Sher Bahadar Khan et al. [5] and the degradation pattern of AO by Langmuir–Hinshelwood (L–H) model was defined and given from the relationship between the rate of degradation and the initial concentration of AO in photo-catalytic reaction [6].

The rate of photo-degradation was calculated according to the following equation; Eq. (1)

$$\mathbf{r} = -\mathbf{d} \mathbf{C}/\mathbf{dt} = \mathbf{K} \mathbf{r} \mathbf{K} \mathbf{C} = \mathbf{K} \mathbf{a} \mathbf{p} \mathbf{p} \mathbf{C} \tag{1}$$

where r in this equation is defined as the degradation rate of organic pollutant, *K*r is describing the reaction rate constant, *K* is constant equal to the equilibrium constant, C is the concentration of the reactant. From Eq. 1, we can neglect KC when C becomes very small so this equation could describe the first order kinetic. Applying the following initial conditions, (t = 0, C = C0 ) in Eq. (1), that may lead to a new equation; Eq. (2).

$$-\ln \mathbf{C} / \mathbf{C}\_0 = k\mathbf{t} \tag{2}$$

Half-life, t1/2 (in min) is

which the activation energy is reduced that may lead to acceleration of the reaction. In general, light is used to activate a substance, which modifies the rate of a chemical reaction without being involved itself, and the photocatalyst is the substance, which can modify the rate of chemical reaction using light irradiation [1]. Chlorophyll of plants is good example for the natural pho-

photocatalyst and chlorophyll of plants is a typical natural photocatalyst [2].

**Figure 1**) [2] is, usually chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, while photocatalyst creates strong oxidation agent and electronic holes to breakdown the organic matter to carbon dioxide and water in the presence of photocatalyst, light and water [2]. So many materials are developed daily to be applied as photocatalysis and nanocompsites that have perovskites-like structure are promising materials for these applications.

from sun or any other illuminated light source (e.g., fluorescent lamps), pairs of electrons and holes are produced, see **Figure 2**. As a result of the light illumination, the electron of the valence band of titanium dioxide becomes excited. Excited electron transits to the conduction band of titanium dioxide with excess energy to create pair of charges; the negative-electron (e-) and positive-hole (h+). This behaviour is well known as the semiconductor's photoexcitation' state. The 'Band Gap' is defined as a result of the difference in energy between the valence band and the conduction band. The necessary wavelength of the light required for the photo-excitation is given according to 1240 (Planck's constant, h)/3.2 eV (band gap energy) and equal to 388 nm [3]. The hole with positive charge in titanium dioxide may split the water molecule into both of the hydrogen gas and hydroxyl radical. On the other side, the electron with negative charge reacts with oxygen molecule forming the super oxide anion.

photocatalyst (see

) absorbs Ultraviolet (UV)\* radiation comes

tocatalyst. The difference between chlorophyll photocatalyst and nano TiO<sup>2</sup>

The continuity of this cycle depends on the availability of the light [3].

**2. Mechanism of photocatalysis**

**Figure 1.** Nano TiO2

130 Nanocomposites - Recent Evolutions

When photocatalyst such as titanium dioxide (TiO<sup>2</sup>

$$\mathbf{t}\_{1\sharp} = \,^0\mathbf{t}\,\mathbf{693}/\mathrm{k}\tag{3}$$

The photo degradation of AO in the presence of CeO<sup>2</sup> **1** nano-particles is shown in **Figure 3**.

Different materials are used as photocatalysis and research is going on to apply a new material for this applications. The rare earth manganite is one of the promising materials for photocatalysis and so in the present proposal we develop the strontium doped neodymium manganites nanocomposites within perovskite like structure as photocatalysis and studying its performance and so the main goals are; −synthesis new perovskite materials enhanced the photocatalysis performance applying the obtained results for solar energy utilizations.

applications in the environmental remediation and self -cleaning surfaces. The technique is widely known but still hampered by one significant limitation. The materials generally absorb ultra violet UV light but we need to develop active materials for visible light, see **Figure 4**.

Perovskite Strontium Doped Rare Earth Manganites Nanocomposites and Their Photocatalytic Performances

 perovskites are very essential family of oxide materials because they possess very interesting physical and chemical properties. These unusual properties may lead to use these mate-

evolution [8–10], catalysts [11, 12], photo/electro- catalysts for hydrogen production

pervoskite [61, 62]. Therefore, we can say that the perovskite compounds

was studied as superior photocatalyst under visible light, different

to get high harvesting of photons and enhancing

perovskite which prepared by sol–gel

play very important role in transfer of oxygen and electrons easily and may lead to nonstoichiometry of oxygen [8–23]. Moreover, the mixed valence states of the transition metal at B-site are also important term in such perovskite-type oxides, which affect their activity. Nevertheless a

and pollutants degradation [13–19] and electrode material used in fuel cells [13]. The synthesis of perovskite materials could be done using different methods such as solid state reaction [24–28], chemical co-precipitation [29–33], sol–gel [34–38]. In each method there are parameters to play with in order to improve the properties of the required materials. A lot of perovskite oxides have been synthesized such as tantalate [39–43], titanate [14, 44–50], ferrite, [51, 52] vanadium-and niobium-based perovskites [53–56], and manganites [57, 58] and they have shown visible light photocatalytic activity as a result of their unique electronic properties and crystal structures [59]. The reduced band-gap energy values in the doped alkaline rare-earth transition metal perovskite-like structure oxides focus more attention because this property enhances the separation of charge carriers (photogenerated electrons and holes) [60]. Intensive studies have been done on these materials because of the capability of tuning their electrical and optical properties, indicating a control of their rational design structure by substitutions

are one of the promising structure that are adapting the bandgap values to harvest visible-light absorption and the potentials of band edge to tailor the needs of particular photocatalysis.

Furthermore, the lattice distortion existed in the rare earth transition metal perovskite compounds strongly affects the separation of photogenerated charge carriers [59, 63, 64]. The distortion in the bond angles resulted from both; metal-ligand or the metal-ligand-metal into perovskite framework are significantly related to their charge carriers and band gap values [65–67]. The crystallinity, phase structure, size, and surface area affect the efficiency of photocatalysts. Consequently, control of the shape of perovskites and the size and crystal phase is essential and significant parameter for assessing their phase-dependent photoactivity and promoting perovskites-based driven visible light photocatalysts. According to Abdel-Latif

the migration and separation of the photogenerated charge carriers through the photocatalytic reaction [61–65]. For the first time, the impact on phase structures and photocatalytic efficien-

lattice site in these materials

http://dx.doi.org/10.5772/intechopen.79479

133

perovskites, such as electrocata-

**3. Perovskites as photocatalytic**

rials in potential applications. The corner-shared octahedral BO<sup>6</sup>

lot of applications depend on the A and B cations in the ABO3

ABO3

lysts for O2

of cationic in ABO3

et al. [66], Nd0.6Sr0.4MnO<sup>3</sup>

modifications of perovskite Nd0.6Sr0.4MnO<sup>3</sup>

cies under visible light of the annealed Nd0.6Sr0.4MnO<sup>3</sup>

Metal oxide photocatalysis is based on metal oxide like titanium dioxide as light-activated catalysts [7]. Three types of materials are used in the degradation of organic matter which has

**Figure 3.** Photo-degradation of AO in the presence of CeO<sup>2</sup> **1** nanoparticles [5].

**Figure 4.** Schematic representation; top light with energy higher than band gap leads to charge separation, with electron reducing a donor (usually oxygen) and hole oxidizing a donor (usually water); summary of processes occurring. Image based on Bahnemann (2004) [7].

applications in the environmental remediation and self -cleaning surfaces. The technique is widely known but still hampered by one significant limitation. The materials generally absorb ultra violet UV light but we need to develop active materials for visible light, see **Figure 4**.
