**3. Impact of surface defects and optical band-gap energy on photocatalytic activities of titanate-based perovskite nanomaterial**

In recent times, substantial work has been done to enhance the physiochemical properties of TBP nanomaterial, more especially surface defects and band-gap energy. This is because of their role in the photocatalytic activities of this group of perovskites. Attempts have been made to modify these two parameters via reaction conditions and synthesis techniques. In this section, the role of reaction conditions on these physiochemical properties was highlighted. The impact of surface defects and bandgap energy on photocatalytic activities of TBP nanomaterial was also discussed.

Zhuang and co [43] reported three different morphologies of CaTiO3 formulated from three different Ti precursors through hydrothermal techniques for removal of As (III). After 40 mins of exposure to UV source, CaTiO3 sample with (Ti(OC3H7)4) (TIP) as a precursor showed the highest activities (98.4%) in removal of As(III). **Figure 3** indicates the induced changes in the concentration of the As(III) under UV source against different morphologies of CaTiO3. The higher photocatalytic activity of CaTiO3 (TIP) was associated with its fern-like morphology and the higher specific area (108.14<sup>2</sup> /g) when compared to other two.

Cai et al. [44] fabricated surface disordered CaTiO3 using modified hydrogenation method. The as-prepared samples were utilized as a model to study roles of surface oxygen vacancies (SOVs) on photocatalytic H2 evolution under a visible light source. They demonstrated that CaTiO3 hydrogenated at 700°C induced more SOVs with a very high photocatalytic H2 evolution rate (2.96 mmol g<sup>1</sup> h<sup>1</sup> ). A value that is almost 49 times greater than the value of the pristine CaTiO3 without SOVs. They suggested that the significant increase in H2 production in the hydrogenated samples was because of the induced SOVs, which lower surface recombination of photogenerated e-h pair and increase charge separation.

Portia et al. [46] studied the influence of annealing temperature on photocatalytic activity of CaTiO3 NPs under the visible light source. The samples were prepared via sol–gel method and were annealed at various temperatures of 500°C, 700°C, and

#### **Figure 3.**

*The induced changes in the concentration of the As(III) under UV-254 nm for different morphologies of CaTiO (A) CaTiO3 TTC (B) CaTiO3 TNB (C) CaTiO3 TIP and (D) No photocatalyst. Reprinted with permission from [43].*

900°C. The photocatalytic activities of the samples were assessed against MO and Rhb dyes. The sample annealed at 900°C exhibited the highest degradation efficiency (DE) 88% and 78% for Mo and Rhb, respectively, while sample annealed at 500°C showed DE of 53% and 38% for Mo and Rhb, respectively. The photocatalytic activity results also correlate with the result of UV–visible diffuse reflectance spectroscopy, which indicates the absorption of the sample annealed at 500°C to be 340 nm while the sample annealed at 900°C exhibited outstanding and improved visible—absorption with its peaks at 450 nm. The kinetic fit of the samples against MO and RhB dyes were shown in **Figure 4**. They associated the enhancement in photocatalytic activities of the sample annealed at 900°C to a decrease in the optical band-gap energy of the CaTiO3 sample, specific surface area with porous features as well as a low recombination rate of the e—h<sup>+</sup> pairs.

Lazono-Sánchez et al. [47] reported influence of Eu3+ doping and heat treatment on photocatalytic activities of CaTiO3 under visible and near-infrared light sources. The results showed that samples doped with 1% Er3+ exhibited the highest degradation reaction rate against MB (4.54 <sup>10</sup><sup>5</sup> <sup>s</sup> 1 ), a value, which is about 2.5 times higher than that of undoped sample 1.86 <sup>10</sup><sup>5</sup> <sup>s</sup> 1 . Furthermore, the as-prepared samples were calcined at 850°C and their photocatalytic activity against MB was also evaluated under UV–vis–NIR irradiation. They reported a significant increase in the photodegradation reaction rate of the calcined sample against MB. The author attributed improvement in the photocatalytic reaction rate of the samples to introducing a new energy state in the optical band-gap of the CaTiO3 and increased crystallinity. In another work, Yang et al. [48] reported photodegradation of MB by Fe-doped CaTiO3 under UV–visible light irradiation. They found that doping CaTiO3 with Fe enhanced its photocatalytic activity against MB. Nevertheless, the photocatalytic activities of Fe doped CaTiO3 sample calcined at a high temperature of 500°C exhibited higher activity of about 100%. Yang et al. associated enhancement in the light absorption of Fe doped CaTiO3 in the visible region with the high calcination temperature.

Recently, Chen et al. [45] reported Na + codoped CaTiO3:Eu3+ fabricated via solution combustion method using stoichiometric ratio of Ca1xTiO3:Eu3+x (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025). The photocatalytic activity of the samples was

#### **Figure 4.**

*The kinetic fit of the samples against MO and RhB dyes exposed to a visible light source. Reprinted with permission [46].*

*Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities… DOI: http://dx.doi.org/10.5772/intechopen.106253*

#### **Figure 5.**

*Ca1xTiO3:Eu3+x samples degradation of MB under UV light. (b) Ca1xTiO3:Eu3+x kinetics plots of photocatalytic degradation of MB (c) Ca1xTiO3:Eu3+x first-order apparent rate constants (k) in photodegradation of MB Reprinted with permission [45].*

evaluated against MB under UV light source. **Figure 5**(a, b, and c) indicates the Ca1-xTiO3:Eu3+x degradation of MB, kinetics plots of photocatalytic degradation, and first-order apparent rate constants (k) in photodegradation of MB. They recorded significant improvement in the photocatalytic activity of CaTiO3 after being doped with Eu3+. The CaTiO3 sample doped with 5% Eu3+ exhibited the highest DE relative to other samples, which the author attributed to the smaller optical band-gap energy of the sample relative to other samples. They also noted that Eu3+ doping led to a decrease in particle size of CaTiO3 hence enhancing its light absorption potential.

Bhat et al. [50] reported the influence of Rh occupying sites in BaTiO3 electronic structure. The samples were prepared by one-pot hydrothermal method and photocatalytic activities of the samples were evaluated against MB under visible light source. In the experiment, the authors found that samples doped with Rh exhibited higher photocatalytic activities than the pristine sample; with 0.5 Rh doped BaTiO3 having the highest photodegradation against MB. They attributed enhancement noted in Rh doped sample to a decrease in band-gap energy. **Figure 6**(a and b) describes the photocatalytic of the sample against MB and the rate constants (K) of pristine BaTiO3 and Rh-doped BaTiO3 against MB. The DFT studies also predicted that an increase in Rh concentration causes a decrease in the optical band-gap energy of the samples, which further increases the absorption light of the sample within the visible region.

Similarly, Khan and co [52] reported effects of 1, 3, and 5% Ag doping on the optical properties and photocatalytic activities of BaTiO3 against Rhb dye under visible light illumination. The decomposition efficiency of BaTiO3, 1, 3, and 5% Ag doped BaTiO3 NPs are 41, 46, 58, and 79%, respectively, as indicated in **Figure 7**. The linear increase in photocatalytic activities of BaTiO3 was credited to the observed decrease in band-gap energy of BaTiO3 NPs as Ag concentration is increased as well as the decrease in the e—h<sup>+</sup> recombination rate.

Nishioka and Maeda [51] investigated H2 evolution of Rh doped BaTiO3 nanocrystal synthesized via the hydrothermal method with different precursors. The H2 evolution was evaluated using aqueous ethanol at wavelength of >420 nm. They claimed

that H2 evolution via photocatalytic reaction of the samples depends strongly on the following; preferred TiO2 and Rh precursor, precursor ratio, and postheating process. A sample that consists of anatase NPs as its main phase indicated higher photocatalytic

**Figure 6.**

*(a) The photocatalytic degradation of MB and (b) the rate constants (K) of the pristine BaTiO3 and Rh-doped BaTiO3 against MB. Reprinted with permission [50].*

**Figure 7.** *Decomposition efficiency of the as-prepared samples against Rh B dye under visible light Illumination. Reprinted with permission [52].*

*Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities… DOI: http://dx.doi.org/10.5772/intechopen.106253*

activity than sample with rutile precursor. They attributed improved photocatalytic activities of post-treated Rh doped BaTiO3 to higher temperature treatment.

Lu et al. [40] explored the effect of optical and structural properties of SrTiO3- BiFeO3 solid solution on its photocatalytic H2 production. The as-prepared sample was prepared via solid-state method using stoichiometric ratio as Sr1xBixTi1xFexO3 (0 ≤ x ≤ 0.4). The photocatalytic activities of the samples were studied by investigating hydrogen production from Na2SO3 (aq) solution under visible light illumination. At wavelength (≥250 nm), an obvious increase in H2 production was noted in the solid solution relative to pristine SiTiO3. The highest activity was noted in Sr0.9Bi0.1Ti0.9Fe0.1O3 sample with mean H2 production rate of 180 mol/h. This value corresponds to apparent quantum efficiency of 2.28% while pristine SrTiO3 H2 production activity rate was 17 mol/h. The pristine SrTiO3 value is one order of magnitude lower than that of the solid solution as shown in **Figure 8**. The authors attributed the higher activity rate of a solid solution to the observed larger surface area, which in turn creates more surface reaction sites at the surface. In addition, SrTiO3 indicated a distinct absorption edge in the UV region while the solid solution indicated a large absorption shoulder in the visible light region, which they link to metal-to-metal charge transfer.

He et al. [41] reported photocatalytic activities of SrTiO3/reduced graphene oxide (SrTiO3-RGO) composites, prepared through a one-pot hydrothermal process. The asprepared sample SrTiO3–0.8%RGO composites exhibited a considerable increase in photocatalytic production of H2 (363.79 mmol (g h)<sup>1</sup> ) than SrTiO3 under UV light exposure. The higher H2 production rate observed in the composite sample was associated with generation of more reactive sites and low recombination rate of the e—h<sup>+</sup> caused by a suitable amount of RGO composition in SrTiO3. Moreover, the photoelectrochemical and electrochemical impedance spectroscopy analyses as shown in **Figure 9**(a and b) revealed that SrTiO3–0.8%RGO had higher photocurrent and lower impedance value than SrTiO3. This is an indication of enhanced charge transport, separation of the photo-generated electrons, and holes in the composite sample.

Pei et al. [49] formulated network-like hierarchical nanosized CaTiO3 via one-pot hydrothermal method and polyvinyl alcohol (PVA) as a structure guiding agent. The

#### **Figure 8.**

*Photocatalytic H2 production of the samples when exposed to UV and visible light sources. Reprinted with permission [40] Copyright Elsevier 2017.*

**Figure 9.** *Photocatalytic H2 production and (b) photoelectrochemical spectroscopy results of the samples. Reprinted with permission [41].*

synthesized sample photocatalytic H2 evolution was evaluated under visible light illumination (>400 nm). It was noted that sample synthesized with PVA had a smaller band-gap value of 2.57 V compared to band-gap of sample synthesized without PVA at 3.55 eV, a value which is typical optical band-gap energy of perovskite CaTiO3. Under visible irradiation PVA-CaTiO3 H2 generation rate was 14.19 mol h<sup>1</sup> g<sup>1</sup> while CaTiO3 alone indicated a negligible H2 generation rate. The H2 generation rate of the samples is shown in **Figure 10**(i and ii). The authors ascribed the improvement in H2 generation by PVA-CaTiO3 to presence of defects such as oxygen vacancies (Vo). They found out that Vo creates an energy level at 0.95 eV below the CB, acted as e donor, and aided charge transport and separation.

Abirami et al. [53] synthesized undoped PbTiO3 and 0.01, 0.02, and 0.03% Ag–Fe codoped PbTiO3 NPs via hydrothermal method. The photocatalytic activities of the asprepared samples were evaluated using MB dyes under UV light source. The results showed that 0.02% Ag-Fe codoped PbTiO3 exhibited the highest photocatalytic decoloration of MB (80.4%) at pH 0f 6 within 2 h. From **Figure 11**, it appears that pH

**Figure 10.** *(i) Depicts the H2 generation rate of PVA-CaTiO3 and pristine CaTiO3 Reprinted with permission [49].*

*Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities… DOI: http://dx.doi.org/10.5772/intechopen.106253*

#### **Figure 11.**

*The decolorization efficiency of the sample at the different pH a) at pH of 2. B) at pH of 4, and c) at pH of 6 Reprinted with permission [53].*

also played a critical role in photodegradation of MB; the higher the pH value, the better the discolorization efficiency. According to Devi et al. [56], the adsorption of dyes on SC photocatalysts relies on pH as a result of the changes in the point zero charge of a SC photocatalyst at different pH. The author's associated improvement in the decolorization to reduction in e—h<sup>+</sup> recombination rate caused by trapping photoexcited electrons by Ag and Fe.

Pirgholi-Givi et al. [54] studied the influence of the synthesis methods on photocatalytic activities of mixed BiTi3O12 and Bi12TiO20 NPs. The samples were formulated via microwave-assisted (S1) and hydrothermal synthesis (S2) methods.

**Figure 12.** *Photocatalytic activities of S1- sample and S2-sample against MB. Reprinted with permission [54].*

Their findings revealed that photocatalytic activity of the S1-sample was about 1.4 times higher than S2-sample at a higher pH value (12.5); S1-sample degraded 98% of MB in 28 min while S2-sample degraded 98% of MB in 40 min. The presence of Bi12Ti12O20 phase in Bi4Ti3O12 substrates elevated charge carrier span, enhanced charge separation, and photodegradation of the MB. **Figure 12** shows a decline in percentage of degradation of the test dye.

### **4. Conclusions**

Use of titanante-based perovskite nanomaterial in photocatalytic application seems to be a promising and effective approach to mitigate the challenges that are associated with water pollution. However, considerable numbers of TBP have large optical bandgap energy, which allows photocatalytic reaction only with a UV source. In addition, the majority of TBP have excellent photocatalytic ability but with a high rate of recombination of the photogenerated e-h<sup>+</sup> pair which significantly affects its efficiency. Some of these challenges made scientists explore ways to optimize and improve photocatalytic activities of TBP material.

In this chapter, we made efforts to avail the overview of basic principles and mechanisms of an SC photocatalyst and recent synthesis techniques that have been deployed in preparing TBP nanomaterial. Special effort was made to highlight the influence of reaction conditions and approaches such as doping, codoping, composites, temperature, and pH on the surface defects and optical properties of TBP nanomaterial. Particularly, how surface effects and optical properties of these materials impact their photocatalytic activities were also discussed. Finally, the future perspective of TBP was proposed in the chapter.

Deploying appropriate synthesis methods and precursors is quite essential as it creates room to control the particle size, crystal structure, shapes, and morphology of a TBP photocatalyst. These physiochemical parameters play a vital role in modification of the electronic bandgap and surface defects of TBP photocatalysts. Much effort should be made to adopt appropriate methods and control over physiochemical

*Role of Surface Defects and Optical Band-gap Energy on Photocatalytic Activities… DOI: http://dx.doi.org/10.5772/intechopen.106253*

properties of these group SC photocatalysts. Attention should also be given to reaction conditions such as temperature and pH of the system during synthesis. Regulating pH and annealing temperature have shown improved crystallinity of TBP, which in turn aids its photocatalytic activities under visible light irradiation. Other strategies that have been efficient in improving photocatalytic activities of TBP material include doping, codoping, and composites with nonmetal, metals, and perovskites material. These strategies have shown to increase charge carrier span, reactive sites, surface oxygen vacancies, decrease in bandgaps, and recombination rate of photogenerated e- h+ pair, improve charge separation, and photocatalytic absorption in the visible light region.

In spite of the progress made in recent times in understanding reaction processes and path involved in degradation of organic wastes and the role of doping TBP with other elements or TBP composite plays in altering its optical band-gap. Nevertheless, the sturdiness of these materials over time and their catalytic active cores require more detailed explanation. Hence there is a need to advance more accurate densityfunctional models to help optimize the photocatalytic configurations of these materials. In addition, more work should be done to optimize the reaction conditions of TBP nanomaterial, especially pH and temperature selection. Appropriate control to both parameters can enhance its photocatalytic activities as demonstrated by few authors. More work should be done to study photocorrosion of titanate-based perovskite nanomaterial.

### **Acknowledgements**

We thank Engr. Emeka Okwuosa for the generous sponsorship of April 2014, July 2016, July 2018, and July 2021 conferences/workshops on applications of nanotechnology to Energy, Health and Environment, and for providing some research facilities.

### **Funding information**

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

### **Conflict of interest**

The authors declare that they have no conflict of interest.
