Application of BaTiO₃-based catalysts for piezocatalytic, photocatalytic and piezo-photocatalytic degradation of organic pollutants and bacterial disinfection in wastewater: A comprehensive review

2.1 Piezo-photocatalytic experiment based on suspended powder catalyst

Piezo-photocatalytic experiments using powder catalyst involves dispersing a certain amount of the catalyst into a contaminated solution. Since powder catalyst offers high surface to volume ratio, the solution mixture consisting of the catalyst is stirred for a certain time, normally for 30 min to reach an adsorption–desorption equilibrium in the dark. Thereafter, the solution mixture gets exposed to a light source. Some of the important parameters which need to be considered during conducting piezo-photocatalytic experiments in suspension systems includes the type of the light source (solar or UV light), UV light power, Ultrasonic power, UV intensity, the amount of the material used (dosage), reaction time and pH of the solution. Recently, a majority of the piezoelectric semiconductors such as BaTiO₃ have been modified to convert their absorption from UV region to visible region (reduce their band gap) to utilize the visible light as a source of light, which constitutes of 43 % of the solar energy. For instance, the activity of piezoelectric semiconductors like ZnO and BaTiO₃ were tested under different light sources such as sunlight and artificial visible light (Xenon lamp 1000 W, which emits visible light in the wavelength between 400 and 800 nm). Under solar light irradiation, the total organic carbon (TOC) results showed complete mineralization of phenol at lower concentrations as compared to artificial visible light irradiation (Pardeshi and Patil, 2008).

The type of the vibration normally employed in piezo-photocatalysis process is ultrasonic vibration. Ultrasonic excitation, can be used to induce piezoelectric materials to produce piezoelectric potential, which can effectively encourage the deterioration of organic dyes. However, under stress the generated free carriers will move in a specific direction to their end two poles and shield the piezo-potential, reducing the driving force. As a result, to maintain the electric field during the piezocatalysis process, continual oscillation is necessary. The ultrasound has the capacity to deliver continuous stress as a physical expression of mechanical energy (Lu et al., 2022). It is important to note that prolonged ultrasonic vibration will have both sonochemical and piezoelectric effects on materials that are made of piezoelectric components (Torres et al., 2008). The sonochemical effect can also help in the degradation of organic or inorganic wastewater pollutants.

2.2 Piezo-photocatalytic experiment based on thin film electrode catalyst.

The issues associated with powder piezo-photocatalyst such as low separation efficiency, poor recovery and regeneration ability, could be resolved by fabricating piezo-photocatalysts supported on the substrate to produce thin film electrodes. Typically, powder catalysts are separated from aqueous solution via filtration and centrifugation process, thus time consuming and some of the catalyst residue might remain in the solution and lead to secondary pollution. Piezo-photocatalyst thin film electrodes offer a good recoverability and recyclability, unlike powder catalysts. However, thin film electrodes during degradation process do not offer the full contact with the solution as compared to powder catalyst. Due their limited contact with the solution or low surface area, thin films exhibit slow degradation rate and low degradation efficiency. Besides that, growing interest is being shown in thin films with nanostructures that are directly formed on the surface of the substrate and are particularly susceptible to exposure to the dye solution. The degradation of organic pollutants via piezo-photocatalytic processes can be illustrated in Fig. 3. As shown in the Fig. 3 experiment, the prepared piezo-photocatalyst thin film is dispersed into a solution containing organic pollutants, thereafter exposed to light and ultrasonic irradiation. Just like piezo-photocatalytic experiment in suspension system, the parameters such as; the distance between the thin film electrode and light source, distance between thin film electrode and ultrasonic probe, ultrasonic power and light source need to be considered. Recently, floatable thin films are designed, which freely moves atop the water offering better utilization of sunlight. Unlike, steady thin film which requires photoreactor and external light source. Furthermore steady thin film requires a specific platform to control the distance between the light source and thin film electrode, which obviously raises the cost of scalable water purification (Yaozhong Zhang et al., 2019).

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Fig. 3

Piezo-photocatalytic degradation experiment based on thin film (Masekela et al., 2022a).

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Another form of using thin film electrode is via sono (piezo)-photoelectrocatalytic degradation processes. Sono(piezo)-photoelectrocatalytic processes is a combination of sonocatalysis/piezocatalysis, photocatalysis and electrocatalysis. These processes have not yet been extensively investigated. In this experiment, light irradiation, ultrasonic vibration and bias voltage is applied on the surface of the thin film electrode. The degradation experiment is conducted using potentiostat/galvanostat, the prepared piezo-photocatalyst thin film is employed as a working electrode in the presence of a reference (Ag/AgCl) and counter electrode (platinum wire). Generally, the fabricated thin film electrode is positioned vertically opposite to the ultrasonic probe and light source (Fig. 4).

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Fig. 4

Photo assisted sonoelectrochemical degradation experiment (Ojo et al., 2022).

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4.1 Methods to prepare powdered BaTiO₃

4.1.1

4.1.1 Hydrothermal method

Hydrothermal synthesis is one of the popular methods for the fabrication of powdered BaTiO₃ since it is not expensive and can form stable and pure materials. This method involves a reaction between Barium (Ba) and Titanium (Ti). During their synthesis the most widely used precursors include barium chloride (BaCl₂), barium hydroxide (Ba(OH)₂), TiCl₄ and TiO₂ materials. The hydrothermal reactions take place in an autoclave at temperatures above 100 °C. Several parameters including reaction time, temperature, solvents and solution pH can influence the morphology, particle size and crystal structure of BaTiO₃. Xia et al. prepared BaTiO₃ nano/microcrystals using commercial titanium dioxide (TiO₂) and Ti(OH)₄ as a titanium (Ti) precursor mixed with Ba(OH)₄ as barium (Ba) precursor (Xia et al., 1996). A very well crystalline and dispersed BaTiO₃ with a crystallite size < 100 nm was formed when Ti(OH)₄ gel and Ba(OH)₂ solution were used as precursors. The results showed that the starting precursors also had a strong impact on the morphology and crystallite size of the prepared BaTiO₃. Furthermore, the hydrothermal reaction temperature had a strong influence on the crystal structure. As shown in Table 1, the lattice constant “a” slightly decreased with an increase in reaction temperature. According of the study conducted by Wen et al., it was found that lattice parameter a can affect photocatalytic activity of the semiconductor (Wen et al., n.d.). The photocatalyst (TiO₂) anatase material with same composition, morphology, phase, and surface states but different lattice parameter ‘’a’’ were employed for photocatalytic degradation and photo-reduction of toluene and Cr(VI), respectively. However, greater catalytic activity was achieved by TiO₂ with the extended lattice parameter than standard TiO₂. Increasing in the length of the lattice parameter ‘’a’’ caused the bottom of the TiO₂ conduction band to move higher, thus improving its photocatalytic activity.

Table 1 Influence of hydrothermal reaction temperature on lattice constant (Xia et al., 1996).

Temperature (°C)	Lattice constant “a” (Å)	Unit cell volume Vc (Å3)
75	4.031	65.50
100	4.025	65.21
150	4.016	64.77
250	4.012	64.58
300	4.008	64.38
400	4.003	64.14

Moreover, Habib et al. showed that the structural morphology of the powdered BaTiO₃ was temperature dependent (Habib et al., 2008). According to their result, the hydrothermal BaTiO₃ obtained at low temperature (90 °C) had less pores compared to those attained at 120 and 150 °C. The study involving the relationship between photocatalytic activity of BaTiO₃ thin film with porosity and surface area was conducted by Augurio et al.(Augurio et al., 2022). The porous BaTiO₃ thin films exhibited higher photocurrent response than non-porous BaTiO₃ thin film, indicating that porosity is beneficial in photocatalysis. This suggested that porous BaTiO₃ can enhance interfacial charge transfer whilst lowering the charge carrier recombination rates, thus improving the photocatalytic activity. In another study, Zhan et al. controlled the hydrothermal reaction time (from 15 min to 480 h) to obtain BaTiO₃ nanoparticles (Zhan et al., 2012). The XRD results showed no diffraction peaks after 15 min of hydrothermal reaction, demonstrating that the material lacked crystalline phases. However, longer hydrothermal reaction times (20 min to 48 h) led to the appearance of diffraction peaks in the XRD patterns that were attributed to the cubic BaTiO₃. A continuous increase in the intensity of the diffraction peaks was observed with an increase in reaction time, demonstrating a persistent rise in the crystallinity and size of the crystals. Surmenev et al. produced BaTiO₃ nano and micro rods via the hydrothermal method. The BaTiO₃ nano- and micro rods were obtained at a temperature of 160–210 °C, using 0.02 and 0.15 M (NaOH) concentration and within 45–90 min (Surmenev et al., 2021). The XRD results showed that BaTiO₃ purity drastically increased as NaOH concentration increased from 0.025 to 0.15 M. Furthermore, BaTiO₃ tetragonal phase was clearly visible after 6 hrs of hydrothermal synthesis at 210 °C and varied alkalinities (from 0.025 to 0.15 M), whereas 45 and 90 min produced a combination of cubic or tetragonal phases. The results showed that the hydrothermal reaction conditions such as temperature, alkalinity and time, have a great impact on the formation of BaTiO₃ structures with different morphologies. Wei et al. controlled the size of BaTiO₃ nanoparticles via hydrothermal approach with Fe doping and ethylenediamine (en) addition (Wei et al., 2008). The crystal size of the synthesized BaTiO₃ were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HR-TEM). The results showed that BaTiO₃ crystal size decreased as it was doped with Fe, indicating that Fe-doping suppress the crystal growth. It was further noticed that as Fe doping concentration increases, the average particle size also decreases (Fig. 6). Additionally, the addition of en, which served as both a solvent and a capping agent, may inhibit particle growth and cause a contained effect that changed the shape of the particles from spherical to cubic. It has been reported that semiconductors with smaller particle sizes have excellent photocatalytic activity as compared to those with large particles.

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Fig. 6

SEM images of BaTiO₃ doped with different concentrations (a)Fe0%, (b) Fe2%, (c) Fe4%, and (d) Fe8% (Wei et al., 2008).

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BaTiO₃ heterostructures are easily fabricated using the hydrothermal method. Li et al. prepared BaTiO₃/TiO₂ heterostructure nanotube arrays using a straight forward hydrothermal process, the hydrothermal reaction was carried out at different reaction times, temperature and concentration of Ba(NO₃)₂ (R. Li et al., 2013)_. Zhao et al. and Kappadan et al. demonstrated the preparation of Ag₂O/BaTiO₃ and BaTiO₃/ZnO heterostructures, respectively, using hydrothermal method (Zhao et al., 2020)(Kappadan et al., 2020a). Based on their experimental results, BaTiO₃ nanoparticles were anchored on hexagonal rod-shaped ZnO (Kappadan et al., 2020a). The combination of hydrothermal and microwave method could be used to fabricate BaTiO₃ (Sun et al., 2006). For example, Amaechi et al. prepared Fe-doped BaTiO₃ via ultrafast microwave-assisted hydrothermal method (Amaechi et al., 2021). Furthermore, the hydrothermal approach could be used with the electrospinning method. By combining an electrospinning and a hydrothermal technique, Ren et al. developed ZnO/BaTiO₃ nanofiber heterostructures (Ren et al., 2012). As seen in Fig. 7(a), BaTiO₃ nanofibers had a rather smooth surface and formed a network topology. BaTiO₃ nanofibers ranged from 300 to 400 nm in diameter and up to several micrometers in length. The ZnO nanoparticles were uniformly dispersed on the rough surface of BaTiO₃ nanofibers (Fig. 7(b)). The elemental composition of the pure BaTiO₃ nanofibers and ZnO/BaTiO₃ nanofiber heterostructures showed the presence of Ba, Ti, O and Zn. The detected Al element was from aluminium foil.

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Fig. 7

SEM images of (a) pure BaTiO₃ nanofibers, (b) ZnO/BaTiO₃ heterostructures, (c) EDS of BaTiO₃ nanofibers and (d) EDS of ZnO/BaTiO₃ heterostructures (Ren et al., 2012).

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4.1.2

4.1.2 Sol-gel method

The sol–gel method is another simple method that is used to prepare barium titanates such as BaTiO₃, BaTi₄O₉, Ba₂TiO₄ and BaTi₂O₅. Normally, barium acetate (Ba(OAc)₂ and titanium (VI) isopropoxide (C₁₂H₂₈O₄Ti) are used as barium and titanium precursors, respectively. The mixture of titanium (VI) isopropoxide and barium acetate solution tends to form BaTiO₃-gel which is further dried and calcined at high temperatures (400–1200 °C) (Kavian and Saidi, 2009). The synergic sol gel and template method was used to fabricate BaTiO₃ nanotubes (Cao et al., 2006). The formation of BaTiO₃-gel was attained via mixing (Ba(OAc)₂ and titanium isopropoxide, thereafter the nanostructured BaTiO₃-gel grew on the porous alumina membrane (pore size 200 nm). The resultant alumina template covered with BaTiO₃-gel was calcined at 700 °C to form BaTiO₃ nanotubes with 50 µm length. The calcination temperature had a significant impact on the BET surface area of the BaTiO₃. Pffaf (Pfaff, 1992) indicated that the BET surface area of BaTiO₃ prepared via sol–gel method decreased as the calcination temperature increased (Table 2). The high specific surface was obtained when BaTiO₃-gel was calcined at low temperature (200 °C). It has been reported that at elevated temperatures, nanoparticles tend to agglomerate extensively thus resulting in a significant reduction in BET surface area and pore diameter (Zhang et al., 2015).

Table 2 Effect of calcination temperature on the BET surface area of BaTiO₃ (Pfaff, 1992)_.

Calcination temperature (°C)	Specific surface area (m2/g)	Average particle diameter (µm)
200	125	0.01
400	105	0.01
500	80	0.01
600	55	0.02
700	37	0.03
800	23	0.04
900	14	0.07

However, XRD diffraction showed highly pure crystalline BaTiO₃ obtained at higher temperature. At high calcination temperatures above 800 °C, the crystal structure of BaTiO₃ transformed from cubic to tetragonal structure. This crystal structure transformation (cubic to tetragonal) was depicted by XRD peak splitting at 2θ value approximately 45° (Fig. 8) [70]. From this study, it can be noted that crystallinity did have an influence on the optical properties of the semiconductors. According to literature, the optical band energy gap of the semiconductors decreases with an increase in crystallinity. Nishioka and Maeda (Nishioka and Maeda, 2015) studied the influence of the post heating of the hydrothermally synthesized Rhodium-doped barium titanate (BaTiO₃:Rh) which could increase crystallinity and further improve photocatalytic activity. The XRD patterns were stronger and narrower after post heating 900 °C, thus confirming crystallization. However, the specific surface area was reduced from 8 to 4 m² g⁻¹. UV–vis diffuse reflectance spectroscopy (DRS) was employed to investigate the optical properties of BaTiO₃:Rh. Upon post heating, DRS exhibited a series of changes as the temperature increased (with exception of the sample at 1150 °C). At higher temperatures, Rh⁴⁺ species induced greater absorption at longer wavelengths. This would make sense because at a high-temperature heat treatment increased the oxidation of Rh³⁺ to Rh⁴⁺ in BaTiO₃:Rh. In terms of photocatalytic activity, unheated samples tend to show low activity, while on the other hand the activity increased with an increase in post heating temperature until 1000 °C. At elevated temperature above 1000 °C, the photocatalytic activity of BaTiO₃:Rh decreased drastically.

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Fig. 8

(a) and (b) XRD patterns for BaTiO₃ (El-Sayed et al., 2020).

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Sol gel was combined with low temperature hydrothermal reaction procedure to prepare BaTiO₃ nanopowder (Wang et al., 2013). In the study reported by Wang et al., it was found that experimental conditions such as potassium hydroxide concentration (KOH), reaction temperature and time had a significant role on the crystallinity and morphology of BaTiO₃ powder (Wang et al., 2013). A highly crystalline pure BaTiO₃ with a cubic structure was obtained at 120 °C (after 2 h of reaction time) with KOH concentration over 1.0 M. The hydrothermal and reaction time showed less effect on the crystallinity and morphology, whereas KOH concentration showed a significant impact on the crystallinity and morphology. It was observed that, when the KOH concentration rised from 1.0 M to 8.0 M, the average size of the BaTiO₃ particles decreased from 370 nm to 100 nm.

4.1.3

4.1.3 Solid state-method

Solid state synthesis is a common method which is usually employed to produce polycrystalline materials such as barium titanate (BaTiO₃). This method requires a very high temperature, however its benefits include simplicity and high yield production. The main factors which affect solid state reaction include reaction temperature, pressure, chemical and morphological properties of the starting reagents/materials. The solid state synthesis of BaTiO₃ nanoparticles was reported by Qi et al. (Qi et al., 2020). In their studies, different molar ratios of Barium nitrate (Ba(NO₃)₂) and Ti powder (Ba/Ti) as starting materials were varied and calcined at different temperatures (500, 550 and 600 °C). The calcination temperature played a crucial role in the formation of BaTiO₃. This was confirmed by the XRD pattern which showed that there was no formation of BaTiO₃ at temperatures below 500 °C, since only XRD peaks for starting materials (Ba(NO₃)₂ and Ti) were revealed (Fig. 9). At high temperature (600 °C), the peaks were almost indexed to BaTiO₃ material, thus now confirming the effect of calcination temperature on these materials. Other studies reported the thermal decomposition reaction of barium carbonate (BaCO₃) and titanium dioxide (TiO₂) for the formation of BaTiO₃ (Pithan et al., 2005). Since the rate of reaction is controlled by the diffusion rate of Ba ions into Titanium dioxide (TiO₂) lattice, the shape and size of the BaTiO₃ produced was more influenced by the TiO₂ morphology. The formation of titanates were explained in detail in the literature (Beauger et al., 1983). Trzebiatowski et al. reported that the formation of barium titanate (BaTiO₃) and barium orthotitanate (Ba₂TiO₄) occurs simultaneously via the below chemical reaction (Brdi et al., 1950);

(1)

BaCO₃ + TiO₂ → BaTiO₃ + CO₂

(2)

2BaCO₃ + TiO₂ → Ba₂TiO₄ + 2CO₂

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Fig. 9

(a) and (b) XRD patterns for BaTiO₃ (Qi et al., 2020).

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In other studies they have reported that Ba₂TiO₄ forms when BaTiO₃ reacts with TiO₂ as shown in equation (4), thereafter the formed Ba₂TiO₄ reacts with the remaining TiO₂ to produce meta titanate as shown in equation (5) (Beauger et al., 1983).

(4)

BaTiO₃ + BaCO₃ → Ba₂TiO₄

O3

Ba₂TiO₄ + TiO₂ → 2BaTi

Solid state reaction method could be combined with sol–gel method. Mi et al. prepared nano BaTiO₃ ceramics using TiO₂ precursor gel and BaCO₃ as starting raw materials (Mi et al., 2020). The XRD results showed the initial formation of BaTiO₃ at calcination temperatures of 600 °C. A cubic BaTiO₃ structure was formed when the calcination temperature reached 800 °C. At 900 °C calcination temperature, the diffraction peak of (2 0 0) separated into peaks of (0 0 2) and (0 0 3), thus suggesting phase transition from cubic to tetragonal phase. In another experiment, Ren et al. used a solid state method to fabricate Bi₂O₃/BaTiO₃ heterostructure (Ren et al., 2013). Firstly, BaTiO₃ was prepared from Ba(CH₃COO)₂ and TiCl₄via the hydrothermal treatment process. Thereafter, Bi₂O₃/BaTiO₃ heterostructure were prepared through ball milling and calcination process using the prepared BaTiO₃ and commercial Bi₂O₃ (mass ratio BaTiO_3: Bi₂O₃ = 4:1). After the calcination procedure, it was discovered that Bi³⁺ had dissolved in the BaTiO₃ lattice and that a chemical connection had been created at the interface between Bi₂O₃ and BaTiO₃.

4.1.5

4.1.5 Co-precipitation

Co-precipitation method is the most frequently utilized synthesis approach for metal oxides (Rao et al., 2017). This method involves dissolving of metals salts in an appropriate solvent, followed by the addition of a precipitating agent. The most widely used precipitating agents include sodium hydroxide (NaOH), ammonium hydroxide (NH₄OH) and potassium hydroxide (KOH). In case of preparing BaTiO₃ using oxalate co-precipitation, it is challenging to obtain optimal conditions where both Barium (Ba) and Titanium (Ti) precipitates at the same time. Since Titanium (Ti) precipitates as titanly oxalate in the presence of alcohol at pH $\le$ 2 whereas Barium (Ba) precipitates as BaC₂O₄ at pH $\ge$ 4. Titanium generates soluble anionic species such as TiO(C₂O₄)₂^2– in the pH between 2 and 4, thus influencing the stoichiometry ratio of Ba/Ti simultaneously (Geetha et al., 2016). It has been reported that through manipulation of several chemical conditions such as pH, reactants, and reaction medium, it is possible to make Ba and Ti to precipitate at the same time. Prasadarao et al. investigated the influence of pH (range 2–10) on the synthesis of BaTiO₃ from barium chloride (BaCl₂) and potassium titanyl oxalate (KTO) (Prasadarao et al., 2001). The formation of barium titanyl oxalate was obtained at pH 2.5 and an increase in pH to 5 led to the formation of barium titanyl hydroxy oxalate. At higher pH values (7–9), precipitation reactions yielded a mixture of titanium dioxide (TiO₂) and barium oxalate (BaC₂O₄). He et al. also prepared BaTiO₃ powder via the co-precipiation of BaCl₂ and TiOCl₂ in an highly-alkaline environment (He et al., 2014). The pH solution and concentration of the starting precursors (BaCl₂ and TiOCl₂) had an effect on the particle grain size and homogeneity of the BaTiO₃ powder. An average particle size of approximately 80 nm was obtained at pH 14 and reaction temperature of 80 °C. In another study, Zhang et al. used BaCl₂, TiCl₄ as starting raw materials and tartaric acid as a precipitant agent for the preparation of tetragonal BaTiO₃ nano-powder (X. Zhang et al., 2021). The white precipitated were formed by adding slowly a solution of ammonium hydroxide solution. Followed by thermal treatment at different calcination temperatures (750–1050 °C) for 4 hrs. The microwave assisted co-precipitation was reported to produce BaTiO₃@rGO nanocomposite (Khan et al., 2021a). BaTiO₃ and GO as starting materials were prepared separately via the sol–gel method and modified Hummers method, respectively. Thereafter, a certain amount of BaTiO₃ and rGO were added to 50 ml of deionised water and stirred for 1 hr at room temperature. Then, NaOH solution was slowly added to the above mixture solution, and heated for 1 hr in a microwave oven. The reduction of GO into rGO was confirmed by color change of the solution from brown to black. The co-precipitated nanocomposite was washed with mixture of ethanol/water and dried at 60 °C in an oven for 12 hr. The TEM images of pure BaTiO₃ and BaTiO₃@rGO are shown in Fig. 10 (a-b). As shown in Fig. 10(a), pure BaTiO₃ exhibits spherical nanoparticles with a size distribution of 10–30 nm, whereas Fig. 10(b) shows spherical BaTiO₃ nanoparticles with an average particle size range of 15–34 nm, which are uniformly distributed on the surface of rGO sheet. Table 3 highlights the summary of some of the synthetic methods for BaTiO₃ powder.

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Fig. 10

TEM images for (a) pure BaTiO₃ and (b) BaTiO₃@rGO (Khan et al., 2021a).

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Table 3 Various synthetic procedures and morphologies.

Materials	Synthesis method	Starting materials	Morphology	Ref
BaTiO3	Hydrothermal	Ba(OH)2·8H2O, TiO2	Cubic phase	(Zhan et al., 2012)
BaTiO3	Hydrothermal	Ba(OH)2·8H2O, Ti(OBu)4	Cubic, Tetragonal	(Ji et al., 2022)
BaTiO3	Sol-gel	C₄H₆BaO₄, C12H28O4Ti	Tetragonal phase	(Kavian and Saidi, 2009)
BaTiO3	Sol-gel template	Ba(OAc)2, Ti(OPri)4	Nanotubes	(Cao et al., 2006)
BaTiO3	Hydrothermal	(Ba(NO3)2, TiO2	Spherical like, Tetragonal	(Li et al., 2020)
BaTiO3	Sol-gel and solid state	BaCO3, TiO2	Whisker like, Cubic, Tetragonal	(Mi et al., 2020)
BaTiO3/TiO2	Hydrothermal	(Ba(NO3)2, TiO2	Nanotube arrays	(R. Li et al., 2013)
BaTiO3	Hydrothermal	Ba(OH)2·8H2O, Na2Ti3O7, TiO2	Nanoparticles, Nanowires, Nanosheets	(Yu et al., 2021)
BaTiO3@rGO	Microwave assisted- co-precipitation	C₄H₆BaO₄, C12H28O4Ti	Spherical	(Khan et al., 2021a)
BaTiO3	Sonochemical	Ba(OH)2, C12H28O6Ti	Irregular Bowl-like structure	(Utara and Hunpratub, 2018b)
BaTiO3	Microwave-hydrothermal	Ba(OH)2·8H2O, (Ba(NO3)2 BaCl2, Ti(OBu)₄	Nanocuboid	(Chen et al., 2016)

These techniques are mostly applied to prepare powdered BaTiO₃, however powdered piezo-photocatalyst are difficult to be recycled in practical applications. For instance, after the degradation process, some parts of the powdered catalyst may persist in the aqueous solution, thus leading to secondary pollution. Therefore, recently piezo-photocatalyst based thin films are being developed for better recoverability, thus in the next section some common techniques that are used to produce BaTiO₃ based thin films will be highlighted.

4.2 Preparation BaTiO₃ based thin films

There are various methods implemented for the preparation of BaTiO₃ thin films, these include physical and chemical techniques. The physical methods include sputtering deposition, pulsed laser deposition (PLD), spin coating, dip coating and the Dr Blade method (Asadzadeh et al., 2021; Cernea, 2004). Chemical methods include chemical vapour deposition (CVD), sol–gel method and hydrothermal method. All of these have their own advantages and disadvantages. Cernea et al. explained most of these methods basic principle and their own benefits (Cernea, 2004). In this review, a few common physical and chemical methods are discussed below.

4.2.3

4.2.3 Chemical vapour deposition (CVD)

Chemical vapour deposition is a widely used method to produce 2D nanomaterials and thin films. In this process, a solid material is deposited from the vapour by a chemical reaction occurring on or in the vicinity of a typically heated substrate (Mittal et al., 2021). The thin film nanostructures and thickness can be tuned by controlling the deposition conditions and the CVD system key factors. These include the substrate material and precursors, composition of reaction gas mixture, total pressure gas flows and temperature. Suzuki and Kijima (Suzuki and Kijima, 2006) prepared nanostructured BaTiO₃ thin film from bis-dipivaloylmethanate barium (Ba(DPM)₂₎ and titanium (IV) isopropoxide (Ti(OiPr)₄ deposited on platinum/alumina/silica/silicon (Pt/Al₂O₃/SiO₂/Si) substrate using the CVD technique assisted with Inductively Coupled Plasma (ICP). The size of the resultant nanostructured BaTiO₃ thin film was greatly influenced by substrate temperature. The single phase BaTiO₃ structure and particles sizes of approximately 30 nm were successfully obtained at temperature of 500 °C (Fig. 11(a)). The deposited spherical BaTiO₃ nanoparticles on the surface of the substrate were more agglomerated with less pores. Fig. 11(b) shows a cross section image of the deposited dense nanoparticles on the substrate surface, however the thickness of the thin film was not determined. At substrate temperatures above 600 °C, the deposited BaTiO₃ nanoparticles fused into a columnar form as shown in Fig. 11(c)-(d). The bottom of the thin films formed, exhibited a columnar structure, whereas the structure surrounding the surface was made of nanoparticles.

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Fig. 11

Surface SEM images and cross section images of the films deposited at different substrate temperatures; (a) surface image at 500 °C, (b) cross section at 500 °C, (c) surface at 600 °C and (d) cross section at 600 °C (Suzuki and Kijima, 2006).

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4.2.4

4.2.4 Dr Blade /Tape casting method

Tape casting known as the Dr.Blade method has been widely used for the production of ceramic thin films. This technique is usually used to obtain thin films with a thickness ranging from 10 to 1000 μm. In this method, the powdered starting materials are mixed with appropriate solvents and binders to form a homogeneous mixture which is tape casted on the solid substrate (Asadzadeh et al., 2021). Thereafter, the tape casted substrate is dried at certain temperatures. The drying rate and temperature tend to be the most important factors which control the crack free of the thin film. Other factors which can affect the thin film properties and thickness include relative content of ceramic powder (starting materials), solvent and binder. Lilge et al. hydrothermally synthesized BaTiO₃ powder from BaCl₂.8H₂O and Ti [OCH(CH₃)₂]₄ (Lilge et al., 2020). The hydrothermal reaction place was performed in a microwave for 120 min at 140 °C. The resultant BaTiO₃ powder was further used to prepare a photoanode electrode using the Dr Blade method. For the preparation of the photoanode electrode, the powdered BaTiO₃ was mixed with ethylene glycol Triton X-100 and ethanol. The slurry mixture was then taped casted on the FTO substrate (area of 1 cm²) to form a thin film. The BaTiO₃ pasted on the surface of FTO appeared to be spherical in shape and agglomerated (Fig. 12).

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Fig. 12

(a) and (b) Surface SEM images of FTO/BaTiO₃ (Lilge et al., 2020).

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5.1 Tailoring BaTiO₃ morphology and particle size

The surface morphology and particles size of the semiconductor photocatalyst/piezocatalyst plays an important role in the catalytic degradation of organic waste pollutants. It has been reported that BaTiO₃ with different morphological structures including nanowires, nanofibers, nanorods, nanotubes, nanocubes and nanoparticles exhibits different piezoelectricity. For example, 1-D fiber/wire piezoelectric materials show a superior piezocatalytic response as compared to spherical particles. Whereas, thin sheet-like 2-D structures also generate more piezoelectricity under mechanical vibration (Mondal et al., 2022). As piezocatalyst, Liu et al. explored different nanostructures (nanocubes (NCs), nanoparticles (NPs) and nanofibers (NFs)) of BaTiO₃ for piezocatalytic degradation of Rhodamine B (Rh B) (D. Liu et al., 2020). BaTiO₃ nanofibres showed greater piezocatalytic performance as compared to nanocubes (NCs) and nanoparticles (NPs) due to a higher surface area, easy deformation structure and fine crystal size. Moreover, Jiao et al. prepared different BaTiO₃ nanostructures via the hydrothermal route at different reaction times (starting from 4 to 16 hr) (Jiao et al., 2017). Spherical BaTiO₃ nanoparticles formed at 4–8 hrs were more effective for photocatalytic degradation of Rh B dye than other morphological nanostructures such as bowl like and agglomerated spherical particles. To further understand the enhancement in photocatalytic activity due to morphological-tuning of the semiconductor photocatalyst, different analyses including BET surface area, photon energy, electrochemical Impedance Spectroscopy (EIS), photoluminescence (PL) and photocurrent also need to be conducted. Since the method of preparation has an impact on the final morphological product, various authors have also synthesized these piezomaterials using varying methods Xiong et al. fabricated BaTiO₃ nanocubes, since cubic structures are known to have the best ability to reduce crystal defects and increase the surface-to-volume ratio (Xiong et al., 2015). The effect of reaction time (24, 48, 74 hr) using the hydrothermal method was employed to produce the cubic like BaTiO₃ structure (Fig. 13(a)-(f)). The particle size increased with an increase in hydrothermal synthesis duration. Furthermore, the edges of the cube got sharper as the reaction time increased, showing an increase in the cubic phase's crystallinity. The BaTiO₃ nanocubes formed over period of 48 hrs exhibited impressive photocatalytic performance under light irradiation. This better performance was due to more uniform morphological distribution, higher crystallinity, small particle size and higher surface area which lead to more active sites, reduction in migration path of charge carriers, narrowing the energy band gap and reducing the rate of charge carrier’s recombination.

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Fig. 13

(a)-(f) TEM images of cubic like structure of BaTiO₃ synthesized at various temperatures [90].

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The optical properties of the BaTiO₃ nanocubes calcined at different temperature were investigated by photoluminescence (PL) and UV vis spectrophotometer. The calcination temperature had an effect on the optical properties of the hydrothermally synthesized BaTiO₃ reported by Hasbullah et al.(Hasbullah et al., 2019). The energy band gap calculated from tauc’s plot (Fig. 14) for BaTiO₃ calcined at 500, 600, 700 and 1000 °C were 3.18, 2.87, 2.83 and 2.74 eV, respectively. Upon increasing the calcination temperature, the energy band gaps of BaTiO₃ were expected to increase due to their high crystallinity. However, BaTiO₃ resulted in lower energy band gap than expected. This could be due to inadequate oxygen delivery during the calcination process in ambient air resulted in oxygen deficiency in BaTiO₃ structures (Orhan et al., 2005). As a result, the calcined BaTiO₃ had a greater density of oxygen vacancy or non-bridging oxygen. The oxygen vacancy has the potential to change the BaTiO₃ structure and cause localized electronic states. Therefore, resulting in reduction of energy band gaps for extremely crystalline BaTiO₃ structures.

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Fig. 14

Tauc’s plot for BaTiO₃ calcined at different temperatures (Hasbullah et al., 2019).

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Moreover, photoluminescence (PL) was employed to study the rate of photogenerated electrons and holes recombination. As seen in Fig. 15(a)-(f), the PL intensity was expected to decrease with increase in crystallinity. However, in this study the PL intensity reached its highest peak as the calcination temperature was elevated to 1000 °C. It was hypothesized that the increase in photoluminescence intensities is due to the presence of a localized state within BaTiO₃ structures. With sufficient stimulation, the localized state effectively lowers the band gap of BaTiO₃ structures, hence resulting in strong photoluminescence intensity.

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Fig. 15

(a)-(f) Photoluminescence emission and Gaussian deconvolution plots for BaTiO₃ calcined at different temperatures (Hasbullah et al., 2019).

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5.2 Doping

Metal or non-metal doping is one of the most popular methods used to modify semiconductor photocatalysts to improve their optical properties such as a reduction of band gap and photogenerated charge carriers (electron-holes), increase in photocurrent response and interfacial charge carries. The improvement of these properties tends to enhance the photocatalytic and piezo-photocatalytic degradation of organic wastewater pollutants. It has been reported that the modification of photocatalyst semiconductors such as TiO₂ (Khairy and Zakaria, 2014), ZnO (Kaur and Singhal, 2014), WO₃ (Peleyeju and Viljoen, 2021), BiVO₄ (Orimolade and Arotiba, 2020)and BaTiO₃ (Ray et al., 2021) by metal ion doping can successfully shift their optical absorption to the visible light region, thus narrowing their band gaps. Recently, a lot of research has shifted towards metal doping rather than no-metal doping since metal doping synthesis is easily achievable. To date, many transition metals including copper (Cu), Iron (Fe), Manganese (Mn), Tungsten (W) and Cerium (Ce) to mention a few have been explored as BaTiO₃ photocatalyst dopants for their improved break down of several organic contaminates like methylene blue (MB), tetraclycline (TC), methyl orange (MO), and atrazine. Among these transition metal dopants, Cu has been shown to be the most efficient BaTiO₃ dopants due to the fact it has shown greater improvement in degradation of organic pollutants as compared to Mn-, Fe-, Ce-, W- and Cr doped BaTiO₃ (I. C. Amaechi et al., 2019; Ifeanyichukwu C. Amaechi et al., 2019; Basaleh and Mohamed, 2020; Nageri and Kumar, 2018; Senthilkumar et al., 2019). Basaleh and Mohamed (Basaleh and Mohamed, 2020) investigated the degradation activity of undoped and cu-doped BaTiO₃ for the removal atrazine from wastewater. According to their outcomes, 5 wt% Cu/BaTiO₃ showed the highest degradation efficiency of 100 % after 60 min, which was 33 times better compared to the undoped BaTiO₃. The addition of Cu to the BaTiO₃ surface reduced the band gap of undopoed BaTiO₃ sample from 3.28 to 2.77 eV, thus improving the photocatalytic activity of the Cu-doped BaTiO₃ sample.

Noble metals including gold (Au), silver (Ag), platinum (Pt) and palladium (Pd) have been shown to improve the BaTiO₃ piezo-photocatalyst sensitivity either under visible light or ultrasonic vibration. These noble metals are receiving more attention from researchers because of their superb utilisation of the solar spectrum, from visible to infrared, through the SPR effect (Chao et al., 2020; Cui et al., 2013). The BaTiO₃ plasmonic photocatalyst have been fabricated from doping these noble metals with pure BaTiO₃ sample. Under solar irradiation, the plasmonic photocatalyst generates an internal electric field which causes the photogenerated charge carriers to move in opposite directions. According to the charge transfer process in plasmonic photocatalysts, electrons from noble metal NPs can travel to the photocatalyst's CB and vice versa. Therefore, resulting in improved separation of charge carriers of plasmonic photocatalyst for better photocatalytic performance. In a study conducted by Xu et al., plasmonic piezo-photocatalyst (Ag/BaTiO₃) compared to pure BaTiO₃ showed an improved absorption under simulated solar irradiation (Xu et al., 2019). Due to this improvement, they discovered that Ag/BaTiO₃ had a greater photocatalytic effectiveness than pure BaTiO₃. The SPR of silver (Ag) nanoparticles resulting from internal band transitions from the 5d band to and within the 6sp band of the noble metal resulted in an increase in the piezo-photocatalytic activity of the modified BaTiO₃.

Another way of adjusting the band gap and improving the photocatalytic performance of the semiconductor is via non-metal doping. Unlike noble metals which are very expensive, non-metal materials are less expensive and can be applied as dopants for several photocatalyst to be used in wastewater treatment. Carbon based materials have been widely used as non-metal dopants to improve piezo-photocatalytic performance of BaTiO₃ since they can improve the rate of electron transfer and also reduce the electron-hole recombination rate. Some of the widely used carbon-based materials include carbon nanotubes (CNTs), activated carbon nanofibres (ACFs), graphene oxide (GO), Biochar, and Carbon nanodots (CNDs) to mention a few (Orimolade et al., 2021a). These distinct carbonaceous materials have different morphologies (surface area and pore size) and surface chemical characteristics (functional groups, hydrophobicity, and hydrophilicity) which all have a significant role in photocatalytic degradation of waste pollutants. Over past years, several few types of these carbonaceous materials have been employed to modify BaTiO₃ structure. However, the utilization of graphene oxide (GO) has been shown to be the most effective strategy. Unlike other carbons, GO offers a variety of benefits including high UV–visible light transmittance, quick electrical and thermal conductivity, superior mechanical and tribological characteristics, and corrosion resistance. In addition, the delocalization of pi (π) network of the layers effectively suppresses electron-hole recombination thus resulting in improved photocatalytic performance (Zou et al., 2019). For instance, Zhao et al. showed an improved photocatalytic performance of BaTiO₃ after loading it with different mass ratios of graphene oxide (Zhao et al., 2018). Firstly, the graphene oxide was prepared from the oxidation of graphite powder using the Hummer’s method and, later the freeze drying method was employed for the preparation of graphene oxide-BaTiO₃ hybrid photocatalyst (Fig. 16(a)). Under light exposure, the hybrid material had superior photocatalytic performance than the unmodified BaTiO₃, as shown in Fig. 16(b-c). Similar observations were reported by Rastogi et al. and Wang et al., whereby the introduction of graphene oxide (GO) into BaTiO₃ lattice structure resulted in a higher photoresponse under the ultraviolet region or in the visible region than BaTiO₃ pristine (Rastogi et al., 2016b) (Wang et al., 2015).

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Fig. 16

(a) schematic illustrations of BaTiO₃/GO synthesis, (b) UV–vis absorption spectra and (c) Tauc’s plot (Photon energy curves) for composites (Zhao et al., 2018).

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5.3 Formation of semiconductor heterojunction photocatalyst

The combination of BaTiO₃ with other several semiconductors to form BaTiO₃ based heterojunction photocatalyst is another approach of improving piezo-photocatalytic efficiency of the BaTiO₃ pristine. Mostly metal oxide semiconductors such as TiO₂, ZnO, SnO₂, Bi₂O₃, Bi₂WO₆ and Cu₂O which have an unequal band gap as BaTiO₃ are used to form heterojunctions (Mengying et al., 2017; Ray et al., 2021; Sharma et al., 2016; Wang et al., 2021). The formation of a heterojunction results in a band alignment which promotes the extension lifetime of the photoexcited holes and electrons within the heterostructured catalyst, thus reducing the rate of electron and holes recombination. Heterojunctions such as p-n (between a p-type semiconductor and an n-type semiconductor), n-n (between two n-type semiconductors), and p-p (between two p-type semiconductors) can be created depending on the kind of semiconductors that are combined. Furthermore, the band alignment of the heterostructured catalyst can be classified as Type I (straddling), Type II (staggered), and Type III (broken). In semiconductors, type II (including Z scheme) have been reported to efficiently improve electrons and holes separation (Orimolade and Arotiba, 2020). The charge transfer mechanism of type II was explained more in detail by Orimolade et al., Zhang et al. and Peleyeju et al. (Orimolade and Arotiba, 2020)(Zhang and Jaroniec, 2018)(Peleyeju and Arotiba, 2018). As shown in Fig. 17, electron transfer within a heterojunction interface commonly follows a two-step pathway depending on the Femi energy level of the coupled semiconductors. On the first pathway mechanisms (Fig. 17(a)), when the Fermi energy level of SC-1 (p-type semiconductor) is smaller than that of SC-2 (n-type semiconductor), electrons (e^-) migrate from SC-1 conduction band (CB) to SC-2 conduction band (CB), while holes (h⁺) migrate from SC-2′s valence band (VB) to SC-1′s valence band (VB). However, when the Fermi energy level of SC-1 is greater than that of SC-2, electrons (e^-) from SC-2 merge with the holes (h⁺) from SC-1 following band alignment in the heterojunction, thus resulting in electrons and holes separation from SC-1 and SC-2 in Fig. 17(b). The accessible separated holes (h⁺) in SC-2 and electrons (e^-) in SC-1 are responsible for piezo-photocatalytic breakdown of organic waste pollutants. This type mechanism pathway of electrons and holes separation is known also as Z-scheme (Fig. 17(b)).

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Fig. 17

(a) Electrons and holes separation pathway in type II heterojunction and (b) Z-scheme heterojunction charge separation pathway (Zhang and Jaroniec, 2018).

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Several BaTiO₃ based heterojunctions have been fabricated using different synthetic methods such as hydrothermal, sol–gel, solid state method and co-precipitation method for various applications, including wastewater treatment. In water and wastewater treatment, BaTiO₃ have been coupled with several metal oxide semiconductors including ZnO, SnO₂, TiO₂, Bi₂O₃, Fe₂O₃ and MnO₂ for better piezocatalytic/photocatalytic removal performance. Other non-metal oxides including g-C₃N₄, Ag₃PO₄ and AgBr have also been coupled with BaTiO₃ for improved photocatalytic/piezocatalytic activity (Mengying et al., 2017; Ray et al., 2021). Feng et al. synthesized BaTiO₃/SnO₂ hybrid heterostructured catalyst using the hydrothermal method for piezocatalytic degradation of organic contaminates (Feng et al., 2020). The effect of SnO₂ loading on BaTiO₃ had a huge impact of piezo-current response, as shown in Fig. 18(a), BaTiO₃ loaded with SnO₂ and SnO₂-Sb generated greater piezoelectrochemical current response than pure BaTiO₃. Furthermore, electrochemical impedance measurements showed the evidence of improved electron mobility via reduction in charge transfer resistance (Rct) of the composites (BaTiO₃/SnO₂ and BaTiO₃/SnO-Sb) (Fig. 18(b)).

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Fig. 18

(a) Piezoelectrochemical current response and (b) electrochemical impedance (EIS) for BaTiO₃, BaTiO₃/SnO₂ and BaTiO₃/SnO-Sb (Feng et al., 2020).

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6.1 Decomposition organic dyes

In the past, BaTiO₃ based catalysts have been extensively investigated for their piezocatalytic and piezo-photocatalytic removal ability of several wastewater pollutants including organic dyes. For example, Wu et al. synthesized BaTiO₃ nanoparticles and nanowires using a two-step hydrothermal method for piezocatalytic removal of methyl orange (MO) from wastewater (Wu et al., 2018b). Under ultrasonic vibration, BaTiO₃ nanowires (NWs) were easily deformed therefore showed better piezocatalytic performance as compared to BaTiO₃ nanoparticles (NPs) with poor deformability. The highest piezocatalytic efficiency obtained by BaTiO₃ NWs under ultrasonic vibration (power 80 W) was about 92 % within 160 min, with reaction processes following pseudo-first order kinetics model (Fig. 20(a)). Scavenger studies were conducted to investigate the reactive oxygen species that were more effective in breaking down of MO into CO₂ and H₂O. Various trapping agents such as tert-butyl alcohol (TBA), benzoquinone (BQ) and disodium ethylene diamine tetra-acetate dehydrates (EDTA-2Na) were used to supress hydroxyl radicals (•OH), superoxide (•O^2–) and holes (h⁺), respectively. As shown in Fig. 20(b), upon the addition of TBA, the degradation efficiency reduced dramatically thus confirming that hydroxyl radicals (•OH) were the most effective ROS species for MO break down, followed by superoxide radicals ((•O^2–). Another study was conducted by Hong et al., were BaTiO₃ as a piezocatalyst generated more hydroxyl radicals (•OH) and superoxide radicals ((•O₂^–) to break down Acid orange 7 (AO7) dye into CO₂ and H₂O (Hong et al., 2012). Under piezoelectric effect, the strained BaTiO₃ dendrites decomposed about 80 % of the AO7 dye after 90 min. Several factors including influence of pH, catalyst dose and initial concentration which can affect the piezocatalytic process were investigated. In case of catalyst loading, the piezocatalytic efficiency increased with an increase in catalyst dosage until reaching a plateau region with 0.025 g of BaTiO₃ catalyst. This was due to more available strained induced charges on the surface of BaTiO₃ as its total surface-active sites increased with an increase in the amount of catalyst dosage. Therefore, resulting in enhancement of piezocatalytic degradation efficiency. The pH solution and initial AO7 concentration significantly influenced piezocatalytic processes, the piezocatalytic efficiency decreased with an increase in intial AO7 concentration. It was speculated that as initial concentration increases, more AO7 molecules increases which cover less active surface sites of the catalyst thus leading to a decrease in piezocatalytic efficiency. The highest piezocatalytic efficiency was slightly reduced in alkaline media and enhanced in acidic media. In acidic conditions, the surface of the BaTiO₃ dendrites is protonated (positively charged) thus enlarges electrostatic interaction between anionic AO7 dye (negatively charged) and positively charged BaTiO₃ surface, and resulting in higher piezocatalytic removal of AO7.

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Fig. 20

(a) Piezocatalytic degradation of MO by BaTiO₃ NWs and (b) Scavenger studies (Wu et al., 2018b).

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To improve the photocatalytic activity of BaTiO₃, Li et al. fabricated new hybrid composites (Ag₂O-BaTiO₃) by combining BaTiO₃ ferroelectric with Ag₂O semiconductor. Under ultrasonic vibration, an internal electric field was generated by ferroelectric BaTiO₃ nanocrystal to reduce the rate of electrons and holes recombination thus enhancing the photocatalytic performance of the hybrid composite (Ag₂O-BaTiO₃) (Li et al., 2015). Fig. 21 shows the effect of ultrasonic vibration on the photocatalytic degradation of Rh B dye using hybrid Ag₂O-BaTiO₃ photocatalyst. Four photocatalyst materials such as commercial P25 nanoparticles, Ag₂O, BaTiO_3, mixture of BaTiO₃ and Ag₂O were used for photocatalytic degradation comparison study. As depicted in Fig. 21(a), P25, BaTiO₃ nanocubes, or Ag₂O nanoparticles were not effective in the degradation of Rh B under ultrasonic irradiation only. However, the physical mixture of Ag₂O and BaTiO₃ as well as the Ag₂O-BaTiO₃ hybrid composite showed a slight deterioration of Rh B. These results shows that the combination of ferrolectric BaTiO₃ nanoctrystal and Ag₂O semiconductor can improve piezocatalysis/sonocatalysis performance of the Ag₂O-BaTiO₃ hybrid piezocatalyst. Fig. 21(b) illustrates the photocatalytic degradation of Rh B with all four samples in the absence of an ultrasonic irradiation. The synthesized BaTiO₃ showed no photocatalytic degradation towards Rh B, whereas Ag₂O, Ag₂O-BaTiO₃ and their physical mixtures showed higher photocatalytic degradation performance towards the removal of Rh B. Under both UV light and ultrasonic irradiation, Ag₂O-BaTiO₃ hybrid photocatalyst completely degraded all Rh B within a short space of time (1.5 h) (Fig. 21(c-d)). Their piezo-photocatalytic or sono-photocatalytic mechanisms were explained as follows: under UV light irradiation, the Ag₂O surface generates electrons (e^-) and holes (h⁺), these charge carriers (e^-, h⁺) are required to be separated in order to produce reactive oxygen species (ROS) such as hydroxyl and superoxide radicals for deterioration of Rh B. Under ultrasonic vibration, the ferroelectric BaTiO₃ nanocrystal in the Ag₂O-BaTiO₃ hybrid composite generated an internal piezo-electric field which acted as a driving force for the separation of charge carrier’s (electrons and holes) (Fig. 21(e-f)). The suppression of rapid electrons and holes recombination separation led to an improved photocatalytic activity of the hybrid composite (Ag₂O-BaTiO₃).

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Fig. 21

(a) sonocatalytic, (b) photocatalytic, (c) sono-photocatalytic degradation of Rh B (d) sono-photocatalytic kinetics and (e-f) sono-photocatalytic mechanism (Li et al., 2015).

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In addition, BaTiO₃ photocatalytic performance was further improved by modifying it with several co-catalysts such as ZnO, SnO_2, Fe₂O₃, TiO₂ and Bi₂O₃. Heterostructured BaTiO₃/ZnO composites were prepared using the sol–gel and hydrothermal methods for the degradation of MB, MO, and RhB (Kappadan et al., 2020b; Karunakaran et al., 2014; L. Wang et al., 2019). In the study conducted by Kappadan et al., hydrothermal method was employed to prepare n-n heterojunction photocatalyst of BaTiO₃/ZnO using Barium acetate, titanium tetra isopropoxide and Zinc nitrate hexahydrate as precursors (Kappadan et al., 2020b). The scanning electron microscopic images of the composite (BaTiO₃/ZnO) clearly showed spherical BaTiO₃ nanoparticles anchored on hexagonal rod-shaped ZnO. The formation of n-n heterojunction resulted in improved photocatalytic performance, as evidenced by the reduction of band gap energy from 3.1 to 2.97 eV and charge transfer resistance from 871 to 745 Ω. The mechanisms, as shown in Fig. 22(a), revealed that an improved charge carriers separation was achieved by forming a typical type II band alignment within the BaTiO₃/ZnO heterojunction interface, which allowed BaTiO₃ photogenerated electrons to migrate into the conduction band (CB) of ZnO, whereas ZnO photogenerated holes were transported into the valence band (VB) of BaTiO_3. The prepared heterostructured BaTiO₃/ZnO (BTZ) photocatalyst showed good stability since even after the 3rd cycle the degradation efficiency was above 91 % for methylene blue (MB) (Fig. 22(b)). However, after the 4th cycle the photocatalytic degradation efficiency slightly reduced from 91 to 86 %. The total mineralisation of MB dye was calculated using TOC, under UV light irradiation, the BTZ heterostructure recorded a TOC removal of 87.1 % after 60 min. A recent report by Liu et al. modified BaTiO₃ with TiO₂ via hydrothermal process to form BaTiO₃-TiO₂ core–shell heterostructures (Liu et al., 2019). The heterostructured photocatalyst was applied for photocatalytic deterioration of Rh B dye from wastewater. The ratio of BaTiO₃:TiO₂ played a significant role in the photocatalytic degradation of Rh B. All BaTiO₃-TiO₂ core–shell heterostructures with different molar ratios exhibited better photocatalytic removal towards Rh B as compared to pure BaTiO₃. The BaTiO₃-TiO₂ core–shell heterostructures (1.2:1) showed the greatest photocatalytic performance compared to other samples, its performance was 1.8 times greater than pure TiO₂. The improved photodegradation activity was due to the fact that BaTiO₃-TiO₂ core–shell heterostructures (1.2:1) exhibited the lowest photoluminiscent (PL) intensity thus lowering the rate of electrons and holes recombination. Wu et al. boosted the photocatalytic activity of heterostructured BaTiO₃/TiO₂ nanocomposites with piezotronic effect under ultrasonic vibration (J. Wu et al., 2020). Under ultrasonic activation, the built-in electric field exhibited by ferroelectric BaTiO₃ facilitated the charge transfer and separation within BaTiO₃/TiO_2, thus improving its photocatalytic activity. Comparing with other BaTiO₃ based metal oxides (Alex et al., 2019; Cui et al., 2017; Fan et al., 2012; Karunakaran et al., 2014; Lin et al., 2007; Liu et al., 2019; Selvarajan et al., 2017; Zhou et al., 2019), BaTiO₃/TiO₂ composite outperformed them in terms of photocatalytic performance due to excellent suppression of electrons and holes recombination.

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Fig. 22

(a) Photocatalytic degradation mechanism of MB using BaTiO₃/ZnO and (b) re-usability of BaTiO₃/ZnO (Kappadan et al., 2020b).

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Plasmonic photocatalysts have also attracted a lot of attention in the photocatalytic removal of organic pollutants in wastewater. Several noble metals including platinum (Pt), gold (Au), rhodium (Rh) and silver (Ag) have been doped with BaTiO₃ to promote chemical redox reaction under UV light and ultrasonic irradiation. For example, chao et al. fabricated a heterostructured Au@BaTiO₃ photocatalyst using the hydrothermal method for the breakdown of Rh B under UV light exposure (Chao et al., 2020). The plasmonic heterostructured photocatalyst showed an enhanced photocatalytic activity towards the removal of Rh B. The heterostructured composite (Au@BaTiO₃) nearly degraded about 100 % of Rh B after 36 min and the reaction process followed Langmuir-Hinshelwood model. The degradation rate constant (k) obtained from Langmuir-Hinshelwood model were 0.05446 and 0.01118 min⁻¹ for Au@BaTiO₃ and pure BaTiO_3, respectively. The apparent rate constant (k) for Au@BaTiO₃ was 4.9 times than that of pristine BaTiO₃. These results confirmed an enhancement in the photocatalytic properties of heterostructured Au@BaTiO₃ photocatalyst. Several studies investigated the photocatalytic performance of silver (Ag) doped BaTiO₃ photocatalyst for the catalytic degradation of hazardous organic dyes (Rh B and MO) (Cui et al., 2013; Lin et al., 2021; Nithya and Devi, 2019; Xu et al., 2019). For instance, Khan et al. reported Ag-doped BaTiO₃ prepared via sol–gel method for photodegradation of Rh B (Khan et al., 2021b). The XRD and HR-TEM results confirmed their tetragonal phase and crystallite size range of 46–54 nm, respectively. The incorporation of silver (Ag) dopants into BaTiO₃ reduced its band gap from 3.87 to 3.47 eV. Furthermore, they observed a reduction in photoluminescent peak intensity upon addition of silver ions into tetragonal BaTiO_3, confirming restrain in electrons and holes recombination. The BET surface of Ag (5 %)-doped BaTiO₃ had a higher surface area of 20.1 m².g⁻¹ as compared to pure BaTiO₃ (15.4 m².g⁻¹). Therefore, due to Ag-doped BaTiO₃ having a higher surface area and lower band gap, their photocatalytic performance was improved. Under light irradiation (400 W sodium lamp), 1 %Ag@ BaTiO₃, 3 %Ag@BaTiO₃ and 5 %Ag@BaTiO₃ showed better photocatalytic performance than pure BaTiO_3. The improved photocatalytic activity of Ag-doped BaTiO₃ could be due to the reduced rate of electrons and holes recombination, surface area increase and reduction in band gap energy after Ag-deposition. The highest photocatalytic removal for Rh B was 79, 58, 46 and 51 % when 5 %Ag@BaTiO_3, 3 %Ag@ BaTiO_3, 1 %Ag@ BaTiO₃ and BaTiO₃ were applied as photocatalysts, respectively. Niu and Xu observed the same photocatalytic enhancement when Ag-BaTiO₃ was further co-doped with other metal dopants such as Ni, Pd and Pd–Sn–Ni through a one-step ball milling process (Niu and Xu, 2019). The co-doped Ag@BaTiO₃ rate constant was 4.5 times greater than undoped BaTiO_3. Table 4 summarizes an overview application of BaTiO₃-based piezo/photocatalyst for catalytic degradation of organic dyes in water and wastewater.

6.2 Pharmaceuticals

In recent decades, antibiotics have been identified as emerging contaminants owing to their endurance in aquatic ecosystems. They are widely used for treating numerous bacterial infections. However, they find their way into several waterbodies through incorrect disposal of unused or expired medications and human excretion. In ground and surface water, antibiotics are found in lower concentrations ranging from μg/L to mg/L (Cabeza et al., 2012; Orimolade et al., 2021b; Wang and Wang, 2016). Unfortunately, the detection of these pharmaceuticals in aquatic environment can lead to several harmful biological and economic impacts. For example, continuous ingestion of drinking water contaminated with pharmaceuticals can result in the formation of drug-resistant bacterium strains to humans and animals. The elimination of several emerging pharmaceuticals by BaTiO₃ based ferroelectric/piezo-photocatalyst system have attracted so much attention recently. Kurniawan et al. investigated the photocatalytic performance of BaTiO₃ and BaTiO₃/TiO₂ composites for the removal of acetaminophen (Ace) from distilled water under UV–vis irradiation (Kurniawan et al., 2018). The XRD patterns confirmed the cubic phase structure of BaTiO₃ nanoparticles. The BaTiO₃/TiO₂ composites as compared to individual pristine (BaTiO₃ and TiO₂) performed better, thus confirming that the synergetic effect of the two semiconductors can improve visible light absorption and efficient charge separation. Furthermore, it was found that the photocatalytic performance of the composites (BaTiO₃/TiO₂) can be enhanced by varying the molar weight ratios (w/w) of BaTiO₃ and TiO_2. Under optimal conditions, dosage of 1 g/L, pH 7, 4 hrs reaction time and initial Ace concentration of 5 mg/L, BaTiO₃, TiO₂ and BaTiO₃/TiO₂ photocatalytically degraded about 18, 33 and 95 % of Ace respectively. As shown in Fig. 23, initial degradation pathways for Ace were through hydroxylation and photolysis. The intermediates formed during this route included hydroquinone and 1,4-benzoquinone, which were consistent with the results obtained by Zhang et al. and Aguilar et al. (Zhang et al., 2008) (Aguilar et al., 2011). The hydroxyl radicals (•OH) further attacked these intermediates to form hydroxylation products. The first detected intermediate was quinoneimine which was effortlessly hydrolized to 1,4-benzoquinone. The intermediates were then oxidized further into carboxylate acid and carbon dioxide by destroying their aromatic structures. The total mineralization via photocatalysis process generally does not occur quickly, however after some few hours these organics can be totally mineralized. Demircivi and Simsek reported tungsten-doped BaTiO₃ (W-BaTiO₃) for enchanted photocatalytic removal of tetracycline under and visible light-driven and UV-A light irradiation (Demircivi and Simsek, 2019). The composite was prepared through a simple hydrothermal method using a Teflon-lined stainless-steel autoclave at 200 °C, followed by washing, drying and calcination for 2 hrs at 700 °C. The effect of tungsten (W) loading on BaTiO₃ was investigated for tetracycline degradation. It was found that BaTiO₃ doped with low amounts of tungsten (W) exhibited higher photocatalytic activity than pure BaTiO₃ and BaTiO₃ doped with higher amount of tungsten (W). According to this study, pH played a significant role in the photocatalytic degradation of tetracycline. The photocatalytic degradation removal percentage increased with an increase in pH solution. The degradation efficiency was recorded to be 3, 80 and 90 % at pH 3, 5.60 and 10, respectively. In addition, tungsten-doped BaTiO₃ (W-BaTiO₃) was used to treat spiked real water samples.

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Fig. 23

Photocatalytic degradation pathway for acetaminophen (Ace) (Kurniawan et al., 2018).

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(tap and drinking water). Under light irradiation (after 3 hrs), the composite achieved a degradation efficiency of 74 and 76 % in drinking and tap water, respectively. The decrease in the degradation efficiency from tap and drinking water was due to other various pollutants or ions detected in the water matrices, which has some negative effect on the photocatalytic process (Bilgin Simsek, 2017). In a study conducted by Demircivi et al., decorated BaTiO₃ with carbon fibers (CFs) were synthesized for enhanced photocatalytic degradation of tetracycline (Demircivi et al., 2020). The incorporation of CFs to BaTiO₃ reduced the band gap of the composite and showed enhancement in photocatalytic activity of BaTiO₃/CF. Under UV and visible light irradiation, BaTiO₃/CF showed the highest degradation efficiency of 96 % as compared to BaTiO₃ (W-BaTiO₃). According to re-usability studies, the composite showed the highest stability within 5 cycles. However, after 6 cycles, a sharp decline in degradation efficiency was observed from 96 to 77 %. This could be due to the loss of tiny photocatalyst powder with an increase in recyclability. Almost a 100 % degradation efficiency was reported when Ti₃₂-oxo-cluster/BaTiO₃/CuS p-n heterojunction was employed for wastewater treatment under both visible light irradiation and mechanical vibration (Piezo-photocatalysis) (Zhou et al., 2021).

The piezoelectric/ferroelectric materials including ZnSnO₃, MoS₂, NaNbO₃, BiFeO₃, KNbO₃, BiOCl, Bi₄ Ti₃O₁₂ and BaTiO₃ nanoparticles in powder forms have been utilized as promising piezocatalysts for several applications. However their applications are limited in cleaning water due to the inability of being recovered from aqueous solution (Lin et al., 2020). Sharma et al. investigated the piezocatalytic activity of cement-based BaTiO₃ composites for the removal of several pollutants in water such as pharmaceutical (paracetamol) and dyes ((Rh B), (MO) and (MB)) (Sharma et al., 2020). Sharma and co-workers, combined powdered BaTiO₃ nanoparticles with cement to form cement-ferroelectric composites which can be easily recovered from aqueous solution after wastewater treatment. Under ultrasonic vibration, the poled BaTiO₃ cement composites showed significant piezocatalytic removal of all organic pollutants. The poled composite exhibited the highest piezocatalytic degradation of approximately > 90, 86, 85, and 79 % for Rh B, MB, MO and paracetamol, respectively. The composites could be reused up to the 5th cycle of piezocatalysis under ultrasonic vibration. Another antibiotic drug which is in the class of fluoroquinolone antibiotics is Norfloxacin (NFX). This antibiotic drug is not easily degradable and can further contribute to antibiotic resistance when used. Therefore, the double Z-scheme of BiFeO₃/CuBi₂O₄/BaTiO₃ was fabricated by Zhang et al., and employed for photocatalytic degradation of Norfloxacin (NFX) under solar-light irradiation (Zhang et al., 2020). The combination of BiFeO₃, CuBi₂O₄ and BaTiO₃ to form the composites, extended the light absorption to UV–visible and near-infrared (NIR) light to allow efficient use of the whole solar light spectrum. As shown in Fig. 24(a), the nanocomposites Z-scheme of BiFeO₃/CuBi₂O₄/BaTiO₃ strongly absorbed in the 200–800 nm range. The calculated optical band energy gap from the tauc’s plot were approximately 3.30, 1.76, 2.29 and 2.20 eV for BaTiO₃, CuBi₂O_4, BiFeO₃ and BiFeO₃/CuBi₂O₄/BaTiO₃, respectively (Fig. 24(b)). The effect of irradiation time, catalyst amount and initial NFX concentration were investigated on the photocatalytic performance of the Z-scheme composites. It was noticed that as irradiation time and catalyst dosage increased, the degradation efficiency also increased (Fig. 24(c-d)). The degradation rate increased with an increase in initial NFX concentration within a particular concentration range (Fig. 24(e)), this was due to the fact that more NFX molecules remained in aqueous solution at higher concentrations. Under the same optimal parameters (dosage of 1.0 g/L, within 60 min, and NFX concentration of 2.5 mg/L), BiFeO₃/CuBi₂O₄/BaTiO₃ nanocomposites had a better photocatalytic activity than individual samples of BaTiO₃, CuBi₂O_4, BiFeO₃. The composite reached its highest degradation of 93.5 % which could be attributed to a better separation of electrons (e^-) and holes (h⁺) to improve the redox ability thus enhancing the photocatalytic activity of the catalyst. The electrons and holes separation were confirmed by photoluminescence (PL) spectrum (Fig. 24(f)). Generally, a low PL intensity reveals greater electrons (e^-) and holes (h⁺) separation efficiency. As shown in Fig. 24(f), BiFeO₃/CuBi₂O₄/BaTiO₃ had the lowest PL intensity than BaTiO₃, CuBi₂O₄ and BiFeO₃, thus indicating better enhancement in photocatalytic activity of the composite. The extent of mineralization of NFX (10.0 mg/L) obtained from TOC was 3.9, 6.8, 40.9 and 60.3 % for BaTiO₃, CuBi₂O₄, BiFeO₃ and BiFeO₃/CuBi₂O₄/BaTiO₃, respectively. However, the extent of mineralization for NFX (2.5 mg/L) reached up to 93.5 % within 1 hr. According to scavenger experiments conducted, it was found that hydroxyl radicals (•OH) and holes (h⁺) played an important role in the deterioration of NFX than superoxide radicals (•O₂^–).

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Fig. 24

(a) UV–vis DRS spectra and (b) Tauc’s plot curves for BiFeO₃, CuBi2O₄, BaTiO₃ and BiFeO₃/CuBi₂O₄/BaTiO₃, (c) effect of irradiation time (d) effect of catalyst dosage (e) effect of initial concentration o photocatalytic degradation and (f) (PL) spectra’s of materials (Zhang et al., 2020).

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Pharmaceuticals used in aquaculture are also a source of pollution that is delivered directly into surface water. For example, atrazine is a well-known herbicide that is used to control broadleaf and grassy weeds in water. It was banned in most countries because of its negative impact on aquaculture and humans (Cavas, 2011). Due to that, Basaleh and Mohamed (Basaleh and Mohamed, n.d.) developed Copper (Cu)-doped BaTiO₃ photocatalyst for the removal of this toxic herbicide (atrazine) from wastewater so as to provide clean water to the environment. The photocatalyst (Cu-BaTiO₃) was prepared through the hydrothermal and photo-assisted deposition method. The prepared photocatalyst was added into 300 ml atrazine solution (50 ppm) and irradiated with Xenon lamp. The photocatalytic removal for atrazine using 0.5 Cu/BaTiO₃, 1.0 Cu/BaTiO₃, 3.0 Cu/BaTiO₃ and 5.0 Cu/BaTiO₃ were recorded to be 45, 65, 100 and 100 %, respectively. As for pure BaTiO₃, the removal efficiency was low as 3 % due to the high electron and holes recombination. These results confirmed that doping BaTiO₃ with Cu can suppress the rate of electrons and holes recombination, thus increasing photocatalytic performance of Cu-BaTiO₃. The suppression of electrons and holes recombination was confirmed by photocurrent response and photoluminescence (PL) spectrum. The Cu/BaTiO₃ had a greater photocurrent response of 12.8 mA cm⁻² than pure BaTiO₃ (2.8 mA cm⁻²). In another study, BaTiO₃ was co-loaded with two electrocatalyst (Pt and RuO₂) to promote sufficient redox ability for piezocatalytic degradation of tricyclazole under mechanical vibration (Feng et al., 2019). The platinum (Pt) was selected since it has been considered as the best catalyst for oxygen reduction, whereas RuO₂ can produce large amounts of hydroxyl radicals and facilitate protons transport during electrocatalytic reaction. The loading of co-catalysts to BaTiO₃ resulted in surface area increment from 25.5 to 28.8 m²g⁻¹. Specific surface area of the materials is another factor which can influence the photocatalytic performance of the photocatalyst. Under ultrasonic vibration (40 kHz, 110 W), the composite achieved the highest removal percentage of 86 % which was higher than the values obtained using RuO₂/t-BaTiO₃ (51.0 %) and Pt/t-BaTiO₃ (75.9 %). According to apparent rate constant values (k) obtained from pseudo-first order kinetics, the piezocatalytic reaction rate for the composites (k = 0.0320 min⁻¹) was 3.11 times greater than pure BaTiO₃ (k = 0.0103 min⁻¹) and the sum of k values of Pt/t-BaTiO₃ (0.0125 min⁻¹) and RuO₂/t-BaTiO₃ (0.0124 min⁻¹). The outstanding performance of the composite confirmed that coupling ferroelectric materials with a good catalyst can yield a synergistic enhancement effect on the piezocatalytic degradation. Another BaTiO₃ based heterostructured catalyst which has been employed for piezocatalytic/photocatalytic and piezo-photocatalytic removal of pharmaceutical pollutants is BaTiO₃/La₂Ti₂O₇ heterojunction. Li et al. prepared BaTiO₃/La₂Ti₂O₇ composites via a two-step hydrothermal and microwave hydrothermal synthesis for piezo-photocatalytic degradation of ciprofloxacin (Y. Li et al., 2021). After 90 min of photocatalytic degradation, BaTiO₃/La₂Ti₂O₇ recorded a degradation efficiency of 16.5 and 22.7 % greater than that of BaTiO₃ and La₂Ti₂O_7, respectively, thus indicating that the formation of a heterojunction improved the photocatalytic degradation process. Under ultrasonic vibration (piezocatalysis), the degrading efficiency of CIP over BaTiO₃/La₂Ti₂O₇ (37.7 %) was roughly 19 % greater than that of BaTiO₃ and La₂Ti₂O_7. The degradation rate constant (k value) obtained from pseudo first-order kinetics model was 0.00593 min⁻¹ for BaTiO₃/La₂Ti₂O₇, which is roughly 2.8 and 2.3 times greater than BaTiO₃ and La₂Ti₂O₇, respectively. The higher degradation efficiency of 50.2 % was achieved when piezocatalysis and photocatalysis were merged. For all samples, the performance of piezo-photocatalysis was superior to that of photocatalysis or piezocatalysis. This might be due to photocatalysis's low visible-light absorption and low carrier separation efficiency. The catalytic performance was likewise low during the piezocatalytic procedure due to the restricted amount of free charge carriers created. Photogenerated charge carriers were efficiently separated under the combined action of visible light and ultrasound, and CIP degradation efficiency was greatly increased when compared to sole-ultrasound and sole-visible-light irradiation. The composites showed higher catalytic activity than individual samples, thus indicating that the BaTiO₃/La₂Ti₂O₇ heterojunctions boost the catalytic process. The photocatalytic degradation of tetracycline and Rh B was reported by Zheng et al., using a Z-type BaTiO₃/γ-Bi₂O₃ heterojunction which was prepared via hydrothermal, co-precipitation, and calcination (Zheng et al., 2022). The morphological structure of the prepared samples appeared to be of a tetrahedron shape, irregular nano-particles and a mixture of both shapes (tetrahedron and nanoparticles) for γ-Bi₂O₃, BaTiO₃ and BaTiO₃/γ-Bi₂O₃ heterojunction, respectively (Fig. 25(a-c)). It was found that the calcination temperature had no effect on the morphology, the obtained average particles for γ-Bi₂O₃, BaTiO₃ and BaTiO₃/γ-Bi₂O₃ (HS3) were roughly around 5.9 μm, 425.9 nm, and 1.9 μm, respectively. The effect of catalyst dose, pH of the solution and different water bodies on photodegradation of tetracycline was explored, however these parameters had a little impact on the photocatalytic degradation process (Fig. 25(d-g)). The degradation efficiency for both pristine (γ-Bi₂O₃ and BaTiO₃) were below 67 %, for γ-Bi₂O₃ and BaTiO₃ were found to be 59.65 and 66.28 %, respectively. However, for all Z-type BaTiO₃/γ-Bi₂O₃ (HS) heterojunctions with different molar ratio, reaction time and calcination temperature, the degradation removal percentages were above 93 %, with HS3 exhibiting the highest degradation efficiency of 97.95 % for tetracycline. Even for Rh B dye, the degradation efficiency for single γ-Bi₂O₃ and BaTiO₃ were below 67 %. It was found that γ-Bi₂O₃ and BaTiO₃ degraded about 63.34 and 45.35 %, respectively. The photocatalytic degradation efficiencies for all HS heterojunction samples were above 73 %, with HS3, HS4, and HS5 breaking down the Rh B molecule entirely. The enhanced photocatalytic activity was due to electrons transferred via Z-type from Bi₂O₃ conductor band (CB) to BaTiO₃ valence band (VB) by work function and charge density difference which resulted in charge separation.

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Fig. 25

(a) Effect of catalyst dosage, (b) initial concentration on degradation of tetracycline, as well as effect of pH and water bodies on the degradation of (f) tetracycline and (g) Rh B (Zheng et al., 2022).

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Another type of hydrothermally synthesized Z scheme heterojunction of La(OH)₃@BaTiO₃ (LB) composite was investigated for deterioration of an A-ring of tetracycline (Zheng et al., 2022). The reactive oxygen species (ROS) such as hydroxyl radicals (OH), superoxide (O₂^–), holes and electrons completely degraded 100 % of tetracycline. The scavenger studies confirmed that four active species played a role in their photocatalytic degradation, as follows: h⁺ = •O₂⁻ >•OH > e^- (before 20 min reaction) and h⁺ > •OH >•O₂^– > e^- (after 20 min reaction). According to these results, it means that holes (h⁺) played a major role in the photocatalytic degradation whereas electrons (e^-) showed the least contribution during the photocatalytic degradation of tetracycline. The application of BaTiO₃-based catalyst for removal of pharmaceuticals is summarized in Table 5.

Despite using BaTiO₃-based catalyst for piezo-photocatalytic degradation of organic dyes and pharmaceuticals, there are some factors which affects the piezo-photocatalytic degradation process negatively. Some of these factors include nature of the semiconductor, amount of the catalyst, solution pH, reaction time, light intensity, dissolved reactive oxygen species and temperature. For examples, some organic pollutants showed maximum adsorption removal and piezo-photodegradation at lower (acidic media) or higher pH (basic media) due their complex structure. Therefore, limiting their applications in real wastewater treatment because it means prior to degradation processes, the pH of the real wastewater samples needs to be adjusted. Another critical factor is the surface area of the piezo-photocatalyst, since the piezo-photodegradation efficiency increases with an increase in surface area of the catalyst. It is very important to select piezo-photocatalyst with very high surface area because of more active sites, which assist in the enhancement of piezo-photodegradation. Furthermore, this processes requires reactive high amount of reactive oxygen species (ROS) such as hydroxyl radicals and superoxide to completely oxidize organic dyes and pharmaceutical into less harmful by-products such as carbon dioxide (CO₂) and water (H₂O). Therefore, lower levels of reactive oxygen species would result in incomplete oxidation of organic pollutants.

6.3 Pathogenic bacteria

Piezocatalytic, photocatalytic and piezo-photocatalytic disinfection has attracted more attention in elimination of pathogenic bacteria. These processes usually use reactive oxygen species (ROS) such as hydroxyl radicals, superoxide, hydrogen peroxide and h⁺, e^- generated when piezo-photocatalyst is exposed under light irradiation or mechanical vibration to kill bacteria (J. He et al., 2021). Generally, the idea of photocatalytic disinfection is to first remove each bacterium's cell wall, removing its protection, and then to damage its cytoplasmic membrane, causing the cellular material inside the newly torn cell envelope to degrade (Fig. 26). Ferroelectric based catalysts such as barium titanate (BaTiO₃) have been used as piezocatalyst/photocatalysts for bacterial disinfection due to its exceptional optical and piezoelectric/ferroelectric properties. For example, Zhao et al. fabricated a novel p-n type Cu₂MgSnS₄/BaTiO₃ (CMTS@BaTiO₃) heterojunction for the degradation of organic dyes and bacterial disinfection (Ali et al., 2021). In terms of bacterial disinfection, CMTS@BaTiO₃ obtained 72–76 % and 84–90 % inhabitation for E.coli and S.aureus, respectively, which was three times greater than pure CMTS and BaTiO₃. Furthermore, CMTS@BaTiO₃ heterojunction was tested for inactivation of E.coli and S.aureus in the presence of wastewater containing dyes (MG and MB). When compared to as-grown bacteria, inactivated levels of bacterial percentages employing CMTS@BaTiO₃ composites were around 94.22–101.24 % with MB and MG, respectively, against E.coli and 97.59 to 87.96 % for S.aureus. In comparison to CMTS@BaTiO₃, Kumar et al. demonstrated piezocatalytic, photocatalytic and piezo-photocatalytic disinfection of E.coli using unpoled and poled BaTiO₃ ceramic under UV light and ultrasonic vibration (Kumar et al., 2019a). During the piezocatalytic process (under ultrasonic vibration), unpoled BaTiO₃ showed a 56 % of bacterial disinfection within 30 min. Under light exposure and ultrasonic vibration, piezo-photocatalysis improved the bacterial degradation rate and the piezo-photocatalytic degradation of E.coli increased from 56 to 70 % within the same reaction time. When the poled BaTiO₃ ceramic were employed for piezocatalytic degradation (under ultrasonic vibration), it was found that about 97 % of bacteria were killed. Under the piezo-photocatalysis process (ultrasonic vibration and light exposure), poled BaTiO₃ ceramic showed some catalytic enhancement towards catalytic degradation of bacteria (99.99 % recorded within 20 min). From these results, it can be concluded that poled BaTiO₃ ceramic have better photocatalytic activity than unpoled BaTiO₃ ceramic and coupling piezocatalysis with photocatalysis further enhanced the catalytic degradation activity of BaTiO₃.

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Fig. 26

Possible photocatalytic bacterial disinfection mechanism (Laxma Reddy et al., 2017).

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Kumar et al. investigated the photocatalytic and antibacterial activity of poled BaTiO₃ prepared via a solid state method (Kumar et al., 2019b). From the FESEM analysis, it was found that BaTiO₃ consisted of large dense grains with clear grain boundaries (Fig. 27(a)). The calculated average grain size of BaTiO₃ sample ranged from 40 to 60 μm (Fig. 27(b)). Both unpoled and poled BaTiO₃ were shown to respond under UV irradiation. The poled BaTiO₃ exhibited the highest photocurrent response of about 0.006 μA, which was>100 times greater than that of unpoled BaTiO₃. Thus, indicating that poled BaTiO₃ had a higher photocatalytic activity (Fig. 27(c-d)). This was due to its remnant polarization which inhibited recombination of photogenerated charges. However, under dark conditions both samples (poled and unpoled BaTiO₃) showed no response to UV irradiation.

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Fig. 27

(a-b) SEM images and particle size of BaTiO₃, (c-d) photocurrent response under dark and UV light, catalytic degradation of bacteria under (e) dark and (f) UV light (Kumar et al., 2019b).

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The colony forming unit (CFU) method was employed to study the effect of poled BaTiO₃ samples on absolute bacterial mortality. As shown in Fig. 27(e), the results display that there was no substantial antibacterial activity with unpoled BaTiO₃ and negative side of poled samples in the dark. However, the positive side of poled BaTiO₃ exhibited strong antibacterial activity with about 90 % bacterial destruction after 60 min when UV light was irradiated (Fig. 27(f)). In the absence of a catalyst (BaTiO₃ poled or unpoled), the UV light alone showed antibacterial activity since it can prevent bacterial growth by destroying their structural DNA. Kushwana and co-workers reported (Kushwaha et al., 2015) Li-Doped Bi_0.5Na_0.45 K_0.5TiO₃–BaTiO₃ (BNKLBT) for antibacterial activity against E.coli and A.flavus using standard disc diffusion method. This method involves bacterial growth using Luria–Bertani (LB) broth–agar in a petridish disc. Three different concentrations (10, 50 and 100 μg) of BNKLBT were tested against bacteria and compared with commercial antibiotics (kanamycin (k₃₀)). Based on their results, the zone of inhibition increased as BNKLBT concentration increased on the disc. The reason for this trend was not explained in this work. In another report, BaTiO₃ nanoparticles were tested for antibacterial efficacy against S.aureus and P. aeruginosa by Shah et al.(Shah et al., 2018). The diffusion method was employed to investigate the antimicrobial activity of BaTiO₃ nanoparticles. The optimal concentration of 100 μg/ml (BaTiO₃) achieved bacterial inhibition of 85 ± 3.5 % and 80 ± 3 % against S.aureus and P.aeruginosa biofilms, respectively.

Shuai et al. prepared PVDF/xAg-pBT composites via laser sintering method for bacterial disinfection (Shuai et al., 2020a). According to their studis, PVDF/xAg-pBT composites generated piezoelectric potential/voltage. Furthmore, it was found that with an increase in Ag concentration in the PVDF/pBT composites, the piezoelectrical current and voltage firstly rose then dropped. Babu et al. and Parl et al. observed comparable enhancement after adding conductive fillers to polymer-ceramic composites (Babu and de With, 2014) (Park et al., 2012). Their assumptions on the output voltage and current improvement were based on the conductivity enhancement. The antibacterial activity of the prepared scaffolds (PVDF/4Ag-pBT and PVDF/pBT) were evaluated using zone of inhibition method. Fig. 28(a-b) shows the bacterial inhibition zones of the scaffolds. PVDF/4Ag-pBT was shown to be more successful in inhibiting the growth of E.coli as compared to PVDF/pBT scaffold. Further test including Turbidimetric test were conducted to verify antibacterial activity of the scaffolds. The transparent vials including and excluding E.coli suspensions were labelled as control and blank group, respectively. The turbidity was the same for vials containing PVDF/4Ag-pBT and control. Surprisingly, the vial containing E.coli solution incubated with PVDF/4Ag-pBT was clear as a blank group, thus indicating that the scaffold inhibited bacterial growth of E.coli (Fig. 28(c)). The SEM images also confirmed that PVDF/4Ag-pBT scaffold raptured and destroyed the whole rod-shape structure of E.coli, whereas PVDF/pBT scaffold had minimal impact on the smooth rod-shape of E.coli (Fig. 28(d-e)). Fig. 28(f-g) displays bacterial inhibition rate of scaffolds and cumulative or non-cumulative of silver ion concentration released by PVDF/4Ag-pBT. The PVDF/4Ag-pBT scaffold inhibited bacteria at a rate of over 81 %, while the PVDF/pBT scaffold had no antibacterial activity.

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Fig. 28

(a-b) Photographs of zone of inhibition, (c) turbidity test, (d-e) SEM images of bacteria, (f) bacterial zone of inhibition rate and (g) cumulative or non-cumulative of silver ion concentration released by PVDF/4Ag-pBT (Shuai et al., 2020a).

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Overall, BaTiO₃-based catalyst have been shown to be appropriate for applications involving the combination of photocatalysis, piezocatalysis and other catalysis processes. An overview of the previous studies addressing the use of BaTiO₃-based catalyst for water disinfection is summarized in Table 6. The use of piezo-photocatalyst based materials for bacterial disinfection is still in its infancy. The process of piezo-photocatalysis has been widely applied including hydrogen production (H₂), degradation of organic pollutants in wastewater, carbon dioxide (CO₂) conversion and hydrogen peroxide (H₂O₂) production. In piezo-photocatalytic disinfection, it is challenging to develop a general mechanism for bacteria inactivation because of a wide range of pathogens and their cell complexity. Therefore, deeper understanding of the mechanisms the piezo-photocatalytic process is still required since suggested mechanisms highly rely on the generated reactive oxygen species during redox reactions. It is still debatable whether polarized charge carriers participate in the redox catalytic process. According to certain studies, photogenerated electrons and holes play a crucial part in redox processes, and the polarization potential of piezoelectric materials merely helps to separate photogenerated charge carriers (Fu et al., 2022).