Effect of poling and porosity on BaTiO 3 for piezocatalytic dye degradation

The presence of both organic and inorganic pollutants in water can represent a threat to our ecosystems and pose a challenge to long-term sustainability. As a result, there is a need to investigate novel methods for addressing environmental remediation. Among a variety of techniques available, piezocatalysis has attracted attention due to its abililty to harness the piezoelectric effect for efficient degradation of pollutants. Herein, porous ceramic barium titnate (BaTiO 3 ) pellets for piezocatalytic dye degradation were synthesized using polymethyl methacrylate (PMMA) as a pore former in 0–30 wt% proportion through solid state reaction method. The synthesized porous BaTiO 3 pellets were characterized in detail by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and field emission scanning electron microscopy. An increase in the degradation of a methylene blue (MB) dye with an increase in porosity within the BaTiO 3 materials, where a maximum degradation was observed for 30 wt% PMMA pellet which has a degradation rate that was ~ 1.75 greater than the dense (0 wt% PMMA) BaTiO 3 ceramic pellet. Furthermore, the synthesized porous BaTiO 3 ceramic pellets were pulse poled, where the piezoelectric coefficient ( d 33 ) decreased with an increase in porosity. The poled 30 wt% PMMA porous BaTiO 3 pellet showed approximately ~ 57% MB dye degradation in 180 min, which was comparable with 30 wt% PMMA unpoled BaTiO 3 and dense 0 wt% PMMA poled BaTiO 3 ceramic pellet. The study provides insights on the influence of poling of a low density porous ceramic pellets, which are utilised as the piezocatalyst for water remediation.


Introduction
The sudden expansion in the industrialization and urbanisation of society has exacerbated environmental challenges for our planet.One of the most important concerns is the continual release of organic effluents into water bodies, whereby human and animal life are being affected [1], [2].These contaminants, such as dyes, contain harmful toxic compounds that lead to harmful effects on the ecosystem and human health [3].
To address this challenge, piezoelectric materials are being considered for the removal of organic contaminants in water.These materials are able to convert mechanical energy into electrical energy via the direct piezoelectric effect [4,5].Since these materials exhibit a high efficiency for energy conversion, the process of piezocatalysis has emerged that uses the direct piezoelectric effect as an advanced oxidation process [6][7][8][9][10][11][12].During piezocatalysis, or the piezoelectric-chemical coupling process, the charges generated under mechanical stimulation are separated and react with O 2 /OH − to give rise to strong oxidizing active species, which are accountable for weakening organic contaminants [13][14][15].The degradation efficiency of organic dyes using this intriguing process is enhanced by tailoring the micro and nano-structure, the use of doping elements, building heterojunctions, varying the amount of catalyst and careful design of the experimental conditions [16][17][18][19][20][21][22].Further, synergistic effect of piezocatalysis with photocatalysis process is currently gaining exceptional traction and garnering significant interest for its potential in applied research areas such as environmental remediation and energy conversion [23][24][25].
In this regard, ferroelectric-based piezoelectric materials have gained popularity for water cleaning applications since they have the potential to generate an electric potential on their surface [26][27][28].While a piezocatalyst is generally used in powder form, there are subsequent treatment challenges since its recovery becomes difficult, even after applying several separation methods [29].
Porous ceramics are therefore an alternative approach to powder-based materials, where their functionalities can be expanded significantly.The specific characteristics of porous ceramics include good thermal insulation, low dielectric constant, low density, enhanced surface area, high toughness and high resistivity to thermal shock; these beneficial properties open up new doors for a number of applications [30].Porous ferroelectric ceramics processed by a polymer burn-out approach could therefore be a possible route to develop high surface area piezocatalysts for degrading organic pollutants.However, after the sintering process, porous pellets exhibit no piezoelectric properties [31] since dipoles are randomly organised with no net remnant polarization.However, a porous ceramic can be poled via a pulse poling technique since the traditional approach of poling these materials generally produces a low increase in piezoelectric properties of porous ceramic pellets [32].Generally, conventional dc poling is applied to piezoelectric materials, which takes several hours for their dipole orientation [33], [34], and such a procedure of dipole alignment is not efficient in the case of porous ceramics as the applied electric field concentrates in low permittivity porous phase [35].In addition, aligning the dipoles via conventional poling can affect productivity in terms of mass production.While poling via a pulse voltage is not a common phenomenon, its potential has been reported in the literature [36,37].Thus, the present study examines the effect of pulse poling of porous BaTiO 3 ceramic pellets and the resulting piezoelectric and piezocatalytic properties.The influence of pulse poling on piezocatalytic activity of porous ferroelectric material in water remediation is examined and the piezocatalytic activity of the poled porous ceramic is compared with unpoled porous ceramic pellet, as undertaken in our previous work [38].This study represents the first report for assessing piezoactivity of pulse poled porous ceramic by degrading MB dye.

BaTiO 3 ceramicpowder
Barium carbonate (BaCO 3 ) and titanium dioxide (TiO 2 ) with a high purity (~ 99%) were used as precursors for synthesis of tetragonal barium titanate oxide (BaTiO 3 ).Polyvinyl alcohol (PVA) was used as a binder in fabricating the BaTiO 3 pellets, where polymethyl methacrylate (PMMA) was used as a pore former agent.The precursors of BaCO 3 and TiO 2 were mixed manually in accordance to their stoichiometric ratio in a acetone medium to attain a high homegeneity.Subsequently, the mixed precursors were calcined at 1200 °C for 4 h; resulting in the synthesis of BaTiO 3 ceramic powder via solid state reaction method.

Porous BaTiO 3 ceramic
The BaTiO 3 powder was then combined with PMMA microspheres at a ratio of 0 to 30 weight% (step of 10% weight).The BaTiO 3 with PMMA microspheres was subsequently combined with PVA (~ 4 wt%), where it was further moulded into 20 mm diameter pellet/disk by applying an 8 tonne pressure through a hydraulic press.The green pellets were then sintered at a rate of 2 °C/min from ambient temperature to 240 °C, 1 °C/

Pulsepolingofporous BaTiO 3 ceramic
The synthesized porous ceramic pellets/disks were poled via a pulse poling technique by applying silver paste as electrodes on both sides of the pellets.Pulse poling was achieved by using a high voltage pulse generator capable of generating an output power source of ~ 400 kV.The pellets were initially heated (~ 100 °C) and were then subjected to six pulse shocks.The piezoelectric coefficient (d 33 ) of poled porous sample was determined using an APC International YE2730A piezoelectric d 33 meter.

Characterizationofporous BaTiO 3 ceramic
X-Ray diffraction patterns analysed in 20-75° range at a scanning rate of 2 °/min, which was used to determine phase formation of the synthesised BaTiO 3 ceramic pellets via X-Ray diffractometer (Rigaku Corporation) (9 kW rotating anode and Cu-K source (λ = 1.54 Å).Raman spectroscopy (HORIBA (Model-Lab RAM HR Evolution, Japan) was used to investigate vibrational modes of the BaTiO 3 ceramics powder using a 532 nm wavelength green laser.The microstructure of the pellets was determined using a field emission scanning electron microscope (FE-SEM) (FEI SEM NOVA Nanosem 450, Hillsboro, OR).The element information was revealed from photoelectron spectrophotometer facilitating XPS technique.Further, Brunauer-Emmett-Teller (BET) technique was used to investigate the volume and size of pores.

Piezocatalyticevaluationofporous BaTiO 3 (unpoledandpoled)
The piezocatalytic dye degradation performance of the poled and unpoled porous BaTiO 3 pellets was compared via experiments using MB dye (10 ml of ~ 5 mg/L concentrated).To achieve adsorption-desorption equilibrium, the poled/unpoled porous BaTiO 3 pellets were soaked in the dye solution for 24 h before beginning piezocatalytic tests in a dark environment.Piezocatalysis experiments were undertaken through an ultrasonicator (120 W, 40 kHz).The water used as an ultrasonic medium was replaced after every 30 min and care was taken to avoid temperature rises ( maintained at < 15 °C) during testing that causes thermocatalytic dye degradation.At 30 min intervals, a 1000 µl aliquot solution was analysed by UV-visible spectrophotometer (ShimadzuUV-2600) and reintroduced into the main solution.The quantification of MB dye degradation was achieved using Eq. 1 [39].
where, D%denotes the percentage of dye degradation,

Scavengertestforreactionspecies detection
Using an indirect scavenger method, the active species of piezocatalysis process were determined.A dye solution containing 10 mM each of ethylenediamine tetraacetic acid (EDTA), p-benzoquinone (BQ), and isopropyl alcohol (IPA) were used to arrest reactive oxygen species such as holes (h + ), superoxide (•O 2 ), and hydroxyl (•OH).The identification of the major active radical is enabled by capture of reactive species causing the maximum decline in piezocatalytic performance.

Resultsanddiscussions
Figure 1 shows the thermogravimetric (TG) analysis of powdered polymethyl methacrylate (PMMA) which was used as a pore former for BaTiO 3 ceramic pellet.The TG profiles revealed that PMMA started to lose weight as early as ~ 320 °C and continued to lose weight until 400 °C, when it was completely removed.In addition, from Fig. 1 the endothermic breakdown of PMMA at 375 °C is visible.Therefore, the role of the PMMA powder as a pore forming agent in the synthesis of porous BaTiO 3 ceramic pellets is supported by TG analysis.
Figure 3a-d depicts the surface SEM micrographs of the synthesized BaTiO 3 ceramic pellets.Figure 3a illustrates that limited porosity is present, clearly showing grain boundaries with nearly hexagonal platelet grains.However, due to the addition of PMMA in the BaTiO 3 ceramic, small pores with an irregular shape that change from spherical to elliptical in BT-10, BT-20, and BT-30, respectively, are observed.According to the findings of the SEM analysis, as the quantity of PMMA increases, small scale porosity begins to emerge on the surface of the ferroelectric materials.These small pores then begin to agglomerate at higher contents, which leads to the formation of larger pores with an elliptical shape.The micrographs shown in Fig. 3a-d demonstrated that fine and evenly distributed open cells were obtained in synthesized BaTiO 3 ceramic.In addition, as physical picture of BT-30 is shown in Fig. 3e where porosity is clearly discernible.
The XPS survey of BaTiO 3 in a range of 0-1250 eV confirms the presence of barium (Ba), titanium (Ti) and oxygen (O) elements as illustrated in Fig. 4a.Furthermore, Fig. 4b shows the XPS spectrum of Ba3d where binding energy (eV) ~ 778 eV and ~ 793 eV is accounted for Ba 3d5/2 and Ba 3d3/2, respectively.The difference between of Ba 3d5/2 and Ba 3d3/2 ~ 5.2 eV indicates + 2 vacancy of Ba element [46].In addition, peaks at ~ 529 and ~ 531 eV are attributed to lattice oxygen and surface adsorbed oxygen as shown in Fig. 4c [47].
The pore size dustribution is calculted via Barrett-Joyner-Halenda (BJH) through adsorption-desorption mechanism [48].Therefore, using the BJH technique, the adsorption-desorption branch of N 2 (77.35K and 0.35 P/P o relative pressure), pore size distribution and volume was calculated.Figure 5a shows a BJH analysis revealing the pore size distribution of the BT-20 ceramic pellet.The pore radius calculated from the adsorption and desorption distributions were estimated as 22 Å and 25 Å, respectively.The porous ceramics can be divided into three pore size categories: macro-porous (d > 50 nm), meso-porous (2 nm < d < 50 nm), and micro-porous (d < 2 nm) [49].Thus, from the SEM and BJH analysis, the pores in the synthesized BT-20 ceramic resulted in meso-porous ceramic having open structure [30], [50].The density of the synthesized BaTiO 3 ceramic pellets was determined using the Archimedes' principle represented in Eq. 4.
where W 2 and W 1 are the weight (in grams) of the BaTiO 3 pellets in air and water, respectively.
The densities ( ) of BT-0, BT-10, BT-20 and BT-30 were found to be 5.836, 5.104, 4.529 and 4.213 g/cm 3 , as shown in Fig. 5b.When compared with density of BaTiO 3 (theoretical density taken as ~ 6.02 g/cm 3 ) O1s spectrum [51], the densities of BT-0, BT-10, BT-20 and BT-30 were 96.94%, 84.78%, 75.23% and 69.98% of BaTiO 3 (theoretical density).An increase in the amount of PMMA in the BaTiO 3 ceramic powder increased the volume of the green pellets prior to sintering, when subject to the sintering process that leads to burnout of the polymeric pore former agent; PMMA.The complete burnout of PMMA created the pores within the BaTiO 3 ceramic.During the sintering process, the BaTiO 3 powder diffused under the effect of temperature, leading to a decrease in density from 5.8 to 4.2 g/ cm 3 .In addition, the calculated relative density of the porous pellets BT-10, BT-20, and BT-30 in relation to the non-porous pellet BT-0 was 12%, 22%, and 29%, respectively.
For a porous piezoelectric ceramic, the relative permittivty (ɛ 33 ), or dielectric constant, will decrease due to the significant dielectric difference between the high permittivity piezoelectric ceramic (ferroelectric matrix) and the low permittivity air (pores) [52].The relative permittivity (ɛ 33 ), piezoelectric charge coefficient (d 33 ) and the piezoelectric voltage coefficient (g 33 ) are related as shown in Eq. 5.
where s 33 denotes elastic compliance and k represents a contant factor.By introducing pores, the compliance,s 33 , generally increases [53], and there is also a decrease in the dielectric constant (ɛ 33 ) with a further decrease in piezoelectric component that leads to a reduction in the piezoelectric charge coefficient (d 33 ) [54].
The piezoelectric coefficient (d 33 ) of the nonporous and porous pellets BaTiO 3 exhibited a zero value when measured in the d 33 piezo-meter.When Since the poling field preferentially concentrates in the low permittivity pores, there is a decrease in the piezoelectric charge coefficient (d 33 ) [55] with an increase in porosity, where the difficulty of poling a porous ceramic is well reported in the literature [55].Nevertheless, the achievement of a 30 pC/N value for PBT-30 i.e., the BaTiO 3 ceramic with the highest porosity and poled using pulse poling technique is of interest.This accomplishment highlights the significance of pulse poling in rapidly orienting dipoles, thereby creating an opportunity for poling porous ferroelectric bulk ceramic.Thus, pulse poling, which orients the dipoles within a fraction of seconds, opens up a space for poling porous ferroelectric bulk ceramic.The alignment of the dipoles along the applied field direction during poling is attributed to the increase in the d 33 of both the non-porous and porous materials.The mobile ferroelastic domain dependent on the applied poling field leads to a change in the remanent polarization and piezoelectric coefficient, where the influence of pulse poling on the piezoelectric coefficient is previously reported in the literature [36,37].Furthermore, the poled porous BaTiO 3 pellets were subjected to ultrasonic waves to determine their piezocatalytic activity when loaded at similar conditions to ultrasound activated piezocatalysis.The poled pellets were electroded with silver paste before being submerged in deionized water, where it can be seen from Fig. 6a-d on applying ultrasonic waves, significant open circuit signals (AC voltage) were recorded via an oscilloscope.The peak to peak (piezovoltage in Volts (V)) were recorded (~+/-) 1.5 V, 0.6 V, 0.2 V and 0.1 V for PBT-0, PBT-10, PBT-20, and PBT-30, respectively.Figure 6a-d shows the repeated signal recordings in ultrasound on and off circumstances.The measured piezoelectric voltage results validate the piezoactive nature of poled porous pellets with a decreasing trend of output piezovoltage due to the lower d 33 coefficients of porous ceramic pellets.A schematic repesentation of the process of poling a porous pellet via pulse poling and piezovolatge generation is illustrated in Fig. 7a and b.
During the piezocatalysis process, the mechanical stress induces a local polarization which can be used for controlling generation of carriers and their seperation, transportation and recombination of charges depends on the electronic state of piezocatalyst [56][57][58][59].The piezocatalysis process is similar to conventional electrocatalysis process, however, an external power source separates the charge (electron) during piezocatalysis, whereas during the electrocatalysis process the electrical potential separates the charge (electron).The applied mechanical stimulation seperates out the charges or induces a piezoelectric polarization and produces charges on the surface of the ferroelectric material, where the seperated charges react with O 2 and OH − to form highly reactive species responsible for weakening of organic dye.It is important to note that even with no external force, the electron in the conduction band (CB) can reduce O 2 molecules to •O 2 radicals as the energy level of conduction band in BaTiO 3 is − 0.83 V (vs.normal hydrogen electrode (NHE)), which is more negative than standard redox potential of O 2 molecule to •O 2 radical conversion i.e., − 0.33 V vs. NHE [60].In addition, the valance band (VB) of BaTiO 3 has an energy level of + 2.31 V vs. normal hydrogen electrode.which can oxidise OH − to •OH radicals (+ 1.9 V vs. normal hydrogen electrode) [61]; this process happens as a result of thermally activated charges above 0 K.However, the free charge concentration in BaTiO 3 is negligible so that no oxidation and reduction process occurs.When a mechanical stimulation is applied to the BaTiO 3 material, a piezoelectric potential is created due to charge seperation and reaction with the dissolved oxygen molecule and hydroxyl to form .O 2 and .OH radicals [62,63].
The piezoelectric potential causes the BaTiO 3 sample's CB and VB to tilt, where a tilted VB would efficiently attract h + to would oxidise OH − and produce •OH, while the tilted CB can collect e − in order to diminish O 2 and produce •O 2 [64].However, the piezoelectric potential has a tendency to decrease and become zero when there is an accumulation of external screening charge on the surface, when it is sufficiently high to balance the polarisation charges.This suppresses the driving force for charge transfer, whereby a new potential equilibrium is attained, which in turn slows down the redox reactions.Furthermore, the decreased pressure from a pulsating wave of mechanical vibrations will aid in building new potential equilibrium in BaTiO 3 and again redox reaction functions [4].A schematic of the piezocatalysis process for weakening of organic dye is outlined in Fig. 7c.The MB dye degradation using the synthesized ceramic can therefore be understood by following Eqs.( 6)-( 16) [17].
At the anode (negatively charges side of BaTiO 3 ) OH .+ dye ⇒ degradation product ofMB dye (15) e − + dye ⇒ degradation product of MB dye (16) h + + dye ⇒ degradation product of MB dye dye. Figure 8(b) shows the concentration/original concentration (C/C o ) vs. time plot for the MB dye degradation after every 30 min using BT-0, BT-10, BT-20, and BT-30 samples, respectively.The pseudo kinetic rate k was calculated as 0.0021, 0.0031, 0.0039, and 0.0045 min −1 for BT-0, BT-10, BT-20, and BT-30, respectively, as shown in Fig. 8c.The MB dye degradation was found to be ~ 31, ~42, ~ 49, and ~ 54% after 180 min of ultrasonication using BT-0, BT-10, BT-20, and BT-30 samples, respectively, as seen in Fig. 8d.Clearly, there is an increase in the degradation of the MB dye with an increase in the level of porosity.It is known that after the sintering process the dipoles in the ferroelectric material are randomly oriented at a macroscopic level, leading to cancellation of the dipoles and a zero net polarization.However, at the local or single domain structure there is suffucient alignment to show piezoelectric activity [3,65].As a result, a porous ceramic material can be categorised into two main categories based upon its porous structure; open and closed structure [30].An open pore structure allows the dye to be permeable which increases the surface area of porous BaTiO 3 ceramic pellet and the local orientation of dipoles and domains exhibits piezoactivity, resulting in enhanced degradation of MB dye.As a result, an increase in MB dye degradation is observed with an increase in porosity porosity level, where a ~ 1.7 times MB dye degradtion efficiency was achieved using the most porous BT-30 sample compared to the dense BT-0 sample.
Figure 9a represents the absorbance spectrum of the MB dye using the pulse poled PBT-0 sample after 180 min of ultrasonication.The percentage degradation of MB dye after every 30 min can be seen in C/C o vs. time plot as shown in Fig. 9b.The first order kinetic rate k was calculated as 0.0064,0.0049,0.0046 and 0.0048 min −1 for PBT-0, PBT-10, PBT-20, and PBT-30, respectively depicted in Fig. 9c.The pulse poled porous materials of PBT-10, PBT-20 and PBT-30 therefore showed nearly the same kinetic rate, wheras the maximum kinetic rate was found for the dense poled material, PBT-0.A degradation of ~ 68, ~57, ~ 55, and ~ 57% was measured with PBT-0, PBT-10, PBT-20, and PBT-30, respectively shown in Fig. 9d.There was no considerable difference in degradation of MB dye using poled porous BaTiO 3 pellets.To assess the cyclic stability of the poled porous samples, a series of 4 degradation cycles of MB dye were conducted, as illustrated in Fig. 9e.The results revealed that the degradation range of MB dye varied across the samples: 14.7% (with a maximum of ~ 68% and a minimum ~ 58%) for PBT-0, 26.31% (with a maximum ~ 57% and a Figure 10a illustrates a comparison of the unpoled and poled BaTiO 3 ceramic with porosity, showing their MB dye degradation efficiency.A clear increase in degradation efficiency can be observed with an increase in porosity for the unpoled materials, however, when the porous poled BaTiO 3 ceramic pellets are considered, there is a gradual decrease in the degradation efficiency when porosity was intially introduced at 10wt.% PMMA, which remained nearly constant up to 30wt.%PMMA.It is difficult to pole and orient the dipoles in particular direction in a porous ceramic with conventional DC poling setup.However, by application of a high electric field for short peroid of time ( < < 1 s) through pulse poling, orientation of dipoles towards the applied field became conceivable.This is evidenced by the piezoelectric d 33 coefficient (~ 120, 95, 80 and 30 pC/N for PBT-0, PBT-10, The increase in MB dye degradation through porous unpoled BaTiO 3 pellets is due to enhanced surface area possible due to open porous structure in BaTiO 3 , whereas in case of poled porous BaTiO 3 pellets the orientation of dipoles and the high d 33 piezoelectric coefficient had more influence.This can be evident as the dense PBT-0 was polarised more compared to PBT-30 sample through pulse poling in same condition (six pulse shock each) i.e., where the d 33 of PBT-0 (120 pC/N) was more than the d 33 of PBT-30 (30 pC/N), leading to a decrease in degradation for the pulse poled porous ceramic.In pulse poled porous samples, both poling and porosity factors come into play.On the other hand, in porous samples, only the porosity factor (effective surface area) contributes.Moreover, in pulse poled samples, the piezovoltage experiences a decline as porosity increases, leading to a decrease in the impact of poling, while the effective surface area increases.Collectively, with an escalation in porosity within pulse poled porous samples, the influence of the surface area becomes more prominent, as the impact of poling diminishes.In the case of the PBT-30 sample, the pronounced effect of surface area due to its high porosity counteracts the impact of poling, resulting in an overall higher piezocatalytic activity compared to the PBT-20 sample.
During the piezocatalysis process, there is a generation of reactive oxygen species (ROS), which are ultimately accountable for the degradation of organic dye.The primary reactive oxygen species (ROS) which are responsible are holes (h + ), hydroxyl radicals (•OH), electrons (e − ) and superoxide radicals (•O 2 ).An indirect scavenger test using ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ) and isopropyl alcohol (IPA) scavengers was performed for determining the contribution of each ROS [66][67][68].Figure 10b shows the scaveneger test performed for assessing the MB dye degradation using the BT-30 sample for 3 h.The degradation achieved were ~ 54, ~39, ~ 35 and ~ 19% using no scavenger, IPA, EDTA, and BQ, respectively.Each scavenger traps/arrests particular reactive oxygen species, such as ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ), and isopropyl alcohol (IPA) scavengers trap/arrest holes (h + ), hydroxyl (•OH), and superoxide (•O 2 ) reactive species.As BQ exhibits the minimum degradation compared to other scavenger, the participation of hydroxyl (.OH) was least in piezocatalysis process.Thus, hydroxyl (•OH), was the primary reactive species responsible for MB dye degradation followed by holes (h + ) and superoxide (•O 2 ).

Conclusions
Porous ceramics, formed by a fabrication process of using burned out polymer spheres, were produced with isometric pores and have been extensively studied for various applications.The degradation of an organic MB dye exhibited an increase in degradation with an increase in porosity in BaTiO 3 pellet and this is due to enhanced surface area of the porous material.While porous ceramics are not easily poled, the present study highlights initial progress in poling porous ceramic via pulse poling technique which was further utilized for degrading MB dye.The studies showed that it was feasible to pole porous BaTiO 3 ceramic where a considerable amount of MB dye was demineralized through these poled porous ceramic pellets.The enhanced degradation with increasing porosity was thought to be due to the enhanced surface area and the piezoelectric coefficient in case of poled porous ceramic, where the non-porous poled ceramic countered the porous poled ceramic (30wt.%PMMA) due to its higher piezoelectric d 33 coefficient developed via pulse poling.The study enabled the poling of a low density porous ceramic pellets, which are utilised as the piezocatalyst for water remediation.

Fig. 5 a
Fig. 5 a Barrett-Joyner-Halenda (BJH) analysis showing adsorption and desorption curves b Effect of density, realtive density (with respect to dense/non-porous ceramic i.e., BT-0) and piezoelectric d 33 coefficient with porosity

Fig. 6
Fig. 6 Piezoelectric open circuit voltage of pulse poled materials measured through oscilloscope for a PBT-0 b PBT-10 c PBT-20 and d PBT-30 Figure8arepresents the absorbance spectra of MB dye using unpoled BT-20 sample for 180 min.There is a decreased absorbance spectrum of the MB dye at ~ 664 nm wavelength, indicating degradation of MB

Fig. 7 a
Fig. 7 a Pulse poling effect on dipole orientation b Piezoelectric open circuit voltage of porous pellets measured through osciloscope c Plausible mechanism of piezocatalysis process

Fig. 8 a
Fig. 8 a Absorbance spectra of MB dye using BT-30 for 3 h of ultrasonication b C/C o vs. time plot showing percentage degradation of MB dye after every 30 min using unpoled BT-0, BT-10, BT-20 and BT-30 samples c Kinetic rate of each sample d % degradation of MB dye using unpoled BT-0, BT-10, BT-20 and BT-30 samples in 3 h of ultrasonication

Fig. 9 a
Fig. 9 a Absorbance spectra of MB dye using PBT-0 in 3 h of ultrasonication b C/C o vs. time plot showing percentage degradation of MB dye after every 30 min using pulse poled PBT-0, PBT-10, PBT-20 and PBT-30 samples c Kinetic rate of each sample d percentage degradation of MB dye using pulse poled PBT-0, PBT-10, PBT-20 and PBT-30 samples in 3 h of ultrasonification and e Piezocatalytic performance of pulse poled PBT-0, PBT-10, PBT-20 and PBT-30 samples for 4 cycles

Fig. 10 a
Fig. 10 a Comparison of percentage MB dye degradation using unpoled/poled samples in 3 h of ultrasonication with wt% PMMA addition b Scavenger test revealing the reactive species accountable for degradation of MB dye o and C o represent the initial absorbance and concentration of the MB dye solution, respectively, where [40]d C represent the absorbance and concentration of MB dye solution at particular time.The pseudofirst-order rate constant is denoted by k , and the time reaction is by t as shown in Eqs. 2 and 3[40].