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Motasim Billah, Yukana Terasawa, Mostafa Kamal Masud, Toru Asahi, Mohamed Barakat Zakaria Hegazy, [Takahiro Nagata](https://orcid.org/0000-0002-8591-2943), [Toyohiro Chikyow](https://orcid.org/0000-0003-3860-4806), [Fumihiko Uesugi](https://orcid.org/0000-0003-3346-4218), Md. Shahriar A. Hossain, Yusuke Yamauchi

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[Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O<sub>3</sub> thin film](https://mdr.nims.go.jp/datasets/ca60e096-57d4-4a9f-9e72-65bc2ff73cb0)

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Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmChemicalScienceEDGE ARTICLEOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueGiant piezorespoaAustralian Institute for Bioengineering andQueensland, Brisbane, QLD 4072, Austrayamauchi@uq.edu.aubSchool of Mechanical and Mining Engineeand Information Technology (EAIT), The U4072, AustraliacKagami Memorial Research Institute for MUniversity, 2-8-26 Nishiwaseda, Shinjuku-kudFaculty of Advanced Science and TechnologKurokami, Kumamoto-shi, Kumamoto 86kumamoto-u.ac.jpeDepartment of Life Science & Medical BioEngineering, Waseda University, 2-2 WakJapanfDepartment of Chemistry, Faculty of SciencgResearch Center for Electronic and Optical MScience (NIMS), 1-1 Namiki, Tsukuba, IbarahCenter for Basic Research on Materials,(NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-iResearch Network and Facility Services DivScience (NIMS), 1-2-1 Sengen, Tsukuba, IbajDepartment of Plant and Environmental NeDeogyeong-daero, Giheung-gu, Yongi-si, GyekDepartment of Materials Process EngineNagoya University, Nagoya 464-8603, Japan† Electronic supplementary informationSEM, XRD, EDS, STEM, XPS, dielectriccurrent density. See DOI: https://doi.org/1Cite this: Chem. Sci., 2024, 15, 9147All publication charges for this articlehave been paid for by the Royal Societyof ChemistryReceived 14th December 2023Accepted 22nd April 2024DOI: 10.1039/d3sc06712brsc.li/chemical-science© 2024 The Author(s). Published bynse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin film†Motasim Billah,ab Yukana Terasawa, *cd Mostafa Kamal Masud, a Toru Asahi,eMohamed Barakat Zakaria Hegazy, f Takahiro Nagata,g Toyohiro Chikyow,hFumihiko Uesugi,i Md. Shahriar A. Hossain *ab and Yusuke Yamauchi *ajkLattice strain effects on the piezoelectric properties of crystalline ferroelectrics have been extensivelystudied for decades; however, the strain dependence of the piezoelectric properties at nano-level hasyet to be investigated. Herein, a new overview of the super-strain of nanoporous polycrystallineferroelectrics is reported for the first time using a nanoengineered barium calcium zirconium titanatecomposition (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT). Atomic-level investigations show that the controlled porewall thickness contributes to highly strained lattice structures that also retain the crystal size at theoptimal value (<30 nm), which is the primary contributor to high piezoelectricity. The strain field derivedfrom geometric phase analysis at the atomic level and aberration-corrected high-resolution scanningtransmission electron microscopy (STEM) yields of over 30% clearly show theoretical agreement withhigh piezoelectric properties. The uniqueness of this work is the simplicity of the synthesis; moreover thepiezoresponse d33 becomes giant, at around 7500 pm V−1. This response is an order of magnitudegreater than that of lead zirconate titanate (PZT), which is known to be the most successful ferroelectricover the past 50 years. This concept utilizing nanoporous BCZT will be highly useful for a promisinghigh-density electrolyte-free dielectric capacitor and generator for energy harvesting in the future.Nanotechnology (AIBN), The University oflia. E-mail: md.hossain@uq.edu.au; y.ring, Faculty of Engineering, Architectureniversity of Queensland, Brisbane, QLDaterials Science and Technology, Waseda, Tokyo 162-0051, Japany, Kumamoto University, 2-39-1 Chuo-ku,0-8555, Japan. E-mail: terasawa@cs.science, School of Advanced Science andamatsu-cho, Shinjuku, Tokyo 162-8480,e, Tanta University, Tanta 31527, Egyptaterials, National Institute for Materialski 305-0044, JapanNational Institute for Materials Science0044, Japanisioin, National UInstitute for Materialsraki 305-0047, Japanw Resources, Kyung Hee University, 1732onggi-do 446-01, South Koreaering, Graduate School of Engineering,(ESI) available: Supplementary gures;constant, dielectric loss, and leakage0.1039/d3sc06712bthe Royal Society of ChemistryIntroductionPiezoelectric materials have been widely used in sensors,transducers, undersea sonar, and small devices for medicaldiagnostic applications. Researchers have not previously deeplyexamined the possibility of energy harvesting using piezoelec-tric devices as a rival to large-scale traditional energy sources,such as coal, hydrocarbons, and other known sources.1 The keycomponent in the design of piezoelectric generators is the useof highly efficient, stable, environmentally friendly, and low-cost piezoelectric materials that can respond effectively toapplied mechanical stress that distorts the crystal structure andelectric voltage generation. The most rigorous work has beendone on lead zirconate and lead titanate, Pb(Zr,Ti)O3 (PZT), withvarious doping of a variety of composites, with a high piezo-electric charge constant (d33) (650 pC N−1) and electrome-chanical coupling coefficient (k33) (0.78), in a directionlongitudinal to the applied stress.2 For half a century, the PZTceramic family has been representative of a large class oftechnologically important materials—that is, piezoelectrics—that convert mechanical stress (strain) and electrical voltage(charge), representing the piezoelectric effect and the inversepiezoelectric effect.3 The highest d33 (∼2500 pC N−1) and k33(>0.9)4 have been reported in prototypes of Pb-based relaxors,Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT), which are only in the form of a single crystal.5Chem. Sci., 2024, 15, 9147–9154 | 9147http://crossmark.crossref.org/dialog/?doi=10.1039/d3sc06712b&domain=pdf&date_stamp=2024-06-15http://orcid.org/0000-0002-3882-1213http://orcid.org/0000-0002-5346-3135http://orcid.org/0000-0003-2525-0092http://orcid.org/0000-0002-7291-9281http://orcid.org/0000-0001-7854-927Xhttps://doi.org/10.1039/d3sc06712bhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bhttps://pubs.rsc.org/en/journals/journal/SChttps://pubs.rsc.org/en/journals/journal/SC?issueid=SC015024Chemical Science Edge ArticleOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThese highly toxic lead-based materials are commerciallydominant piezoelectric materials. However, researchers world-wide have been searching for lead-free piezoelectric materialsfor more than a decade, aiming to replace these toxic materials.6This goal—that is, the invention of a Pb-free piezoelectricmaterial with a performance equivalent to or even superior tothat of PZT-based piezoelectric materials—has not yet beenfullled. In recent years, barium titanate (BaTiO3, BTO),7bismuth sodium titanate (Bi0.5Na0.5TiO3, NBT),8 potassiumsodium niobate (K0.5Na0.5NbO),9 and bismuth ferrite (BiFeO3)10have been widely studied as Pb-free sustainable materials. Inparticular, BTO has shown potential for use in piezoelectricapplications, such as multilayer ceramic capacitors, as it hasa high dielectric constant and low dielectric loss.11 However, thevalues of d33 (191 pC N−1) and k33 (0.494) in BTO12 are still farbehind those of its commercially available Pb-basedcounterparts.Consequently, researchers worldwide have attempted toimprove the performance of BTO. Cao et al. reported thata reduction in the grain size to 1.6 mm is responsible forenhancing d33 to a value as high as 460 pC N−1.13 More recently,Wada et al. synthesized grain-oriented ceramics of BTO by thetemplated grain growth (TGG) method, obtaining a remarkabled33 value of 788 pC N−1.14 However, the TGG method requirescomplex remixing and recalcination of spherical and ake-structured presynthesized BTO in different ratios. This givesthe desired planar orientation (111)—where the dipole rotationhappens to be in its least energy-invasive state to transfersurface charge, resulting in the best piezoelectric response. Inanother report, a seed-passivated texturing process was utilizedto fabricate textured PZT ceramics through microplatelet tem-plating.15 Though this strategy achieved a piezoelectric coeffi-cient d33 of 760 pC N−1, it involves multi-step preparation. Byconsidering all these complex and extended synthesis methods,a simple synthesis approach with a very high yield is required toprepare a new BTO candidate that is highly competitive withcommercially available Pb-based materials.Many efforts to dope BTO with different elements havedemonstrated its potential to compete with commerciallyavailable PZT. A calcium and zirconium co-doped barium tita-nate system (Ba(Ti0.8Zr0.2)O3-(Ba0.7Ca0.3)TiO3, BZT-BCT) witha noticeably enhanced d33 (∼620 pC N−1) was also reported byLiu et al.16 The orthorhombic Ba0.85Ca0.15(Ti0.9 Zr0.1)O3 (BCZT)in its morphotropic phase boundary (MPB) is thermodynami-cally unstable in a ferroelectric phase, with polarization rotationoccurring within the minimum energy state in the crystalsystem, and it can be easily manipulated by external stress or anelectric eld with minimum effort, yielding the best ferroelec-tric properties. In addition, according to some earlier publica-tions on BCZT polycrystalline ceramics, the coexistence ofpolymorphic phase transitions can play a signicant role inenhancing the dielectric and piezoelectric properties.16–18 Thevariations in the composition of Ca (14–18 mol%) trigger thepolymorphic phase (the coexistence of orthorhombic-tetragonalphases), tending toward ferroelectric relaxor behavior. Conse-quently, a wide range of piezoelectric and ferroelectric proper-ties can be tailored in such novel single BCZT structural models9148 | Chem. Sci., 2024, 15, 9147–9154by simply varying their stoichiometry. The highest dielectricconstant 3r (5000) has been reported in bulk BCZT by varying theCa content between 12 and 16 mol%.19 As another strategy, ourprevious study on nanoporous lms hinted that the existence ofelongated nanopores certainly leads to a successful strainengineering process for enhancing piezoelectricity. Meanwhile,the improved surface area enhances the dipole moment at thesurface, thereby enhancing the piezoelectric response.20Hence, we prepared highly strained nanoporous Ba0.85-Ca0.15(Ti0.9Zr0.1)O3 (BCZT) by a so-templating method and weinvestigated the strain pattern developed in the BCZT lm. Wealso studied the atomic-scale strain mapping using high-resolution electron microscopy and analyzed it by using thepresence of strong Bragg reections in its Fourier transform. Inprinciple, strain and local deformation can be determineddirectly by measuring the displacement of the lattice fringes onthe atomic scale. The technique of quantitative measurement ofsuch displacement and strain eld analysis in this study isbased upon a theoretical method developed by M. J. Hytch,2,9regarded as geometric phase analysis. Piezoresponse forcemicroscopy (PFM) study is also popular for conrming piezo-electricity in the nano-regime and to calculate the piezoelectriccoefficient (d33).11,21 The PFM study in this work gathers anaccurate vertical piezoresponse (d33) mapping, a hysteresis-buttery loop for quantitative analysis. It is found that the ob-tained d33 and strain values in the synthesized nanoporousBCZT are in good agreement and indicate ten-fold higher d33compared to its non-porous bulk counterpart. Such a nano-porous lm shows great promise for high-density energyharvesting.Results and discussionSynthesis of nanoporous BCZT lm and observation of porousstructureNanoporous BCZT lms were synthesized by a sol–gel-basedmethod using a diblock copolymer (PS-b-PEO) as a pore-directing agent. The role of PS-b-PEO as a pore-directing agentwas demonstrated in our previous report.22 In the precursorsolution, the PS-b-PEO polymers form spherical micelles (i.e.,a template) where the PS blocks are the core and the PEO blocksare the shell. The resulting nanoporous BCZT presents bothhighly crystallized and amorphous/poorly crystallized phases,as evidenced by the different morphologies (light and dark) inthe SEM image (Fig. 1a). The growth architecture of the pores onthe top of the lm surface calcined at 650 °C is elongated due tothe framework crystallization, which is further veried by theSTEM image (Fig. 1b). The coexistence of two phases is alsofurther conrmed by the difference in stiffness in the phaseimages from atomic force microscopy (AFM) (Fig. 1c and d).19The boundary between the two phases is clearly separated bythe dotted line marked in the topographical image in Fig. 1c.The difference in contrast is evident in the region with highcontrast as a highly crystalline region, whereas low contrastindicates the amorphous/poorly crystalline region. Without theuse of a pore-directing agent, the bulk BCZT (calcined at 700 °C)© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bFig. 1 Electron micrograph images of nanoporous BCZT. (a) Scanning electron microscope (SEM) image showing surface morphology. Theboundary between highly crystallized and amorphous/poorly crystallized phases is indicated by a square. (b) Scanning transmission electronmicroscope (STEM) image showing pore walls. (c and d) Atomic force microscopy (AFM) mapping: (c) topographical image showing highlycrystallized and amorphous/poorly crystallized phases, and (d) phase image for distinguishing between these two phases.Edge Article Chemical ScienceOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinepossesses random grains of several tens of nanometers in size(Fig. S1a†).On the other hand, with the use of PS-b-PEO, nanopores startto form aer calcination at 400 °C due to the removal of thetemplate (Fig. S2a†). The nanopores become large by joiningwith their neighbouring pores and are elongated as the calci-nation temperature increases (Fig. S2b†). Crystallinity canhardly be observed in the lms calcined at 400 °C and 500 °C(Fig. S2c†). Abundant robust nanopores with highly crystallizedframeworks can obviously be observed at 650 °C (Fig. S1b, c andS2c†). The expansion and partial collapse of the nanoporesoccurs in a temperature range higher than 650 °C. Therefore,650 °C is xed as the optimized temperature. From the aboveresults, it is found that the resulting nanoporous architectureshows a very different morphology from the BCZT bulk lm (itshould be noted that we attempted to synthesize a bulk lmwithout PS-b-PEO at a calcination temperature of 650 °C, but nosignicant XRD peaks were found, indicating amorphous BCZT;therefore, the calcination temperature was increased to 700 °C(Fig. S1c†).As shown in Fig. S1c,† both bulk and nanoporous BCZT lmsshow a perovskite phase. In the case of the BCZT bulk lm,some peaks other than those of BCZT (i.e., impurities) are© 2024 The Author(s). Published by the Royal Society of Chemistryobserved in the low-angle region. However, the addition of PS-b-PEO can facilitate a homogeneous mixing of inorganic sourceswithout precipitation, thereby forming an almost pure BCZTphase. The average crystallite sizes of the bulk and nanoporousBCZT roughly calculated from the Scherrer equation are ca.10 nm and 23 nm, respectively. It is interesting that nanoporousBCZT becomes more crystallized with the addition of a sotemplate, and the calculated crystallite size is smaller than thethickness of the pore walls (Fig. 1b).Atomic distribution in nanoporous BCZT lmThe ADF-STEM image and EDS mappings for nanoporousBCZT are shown in Fig. 2. The low-magnication ADF-STEMimage shows that this sample is a collection of crystal grainsof less than 100 nm. A high-resolution ADF-STEM image of thegrains shows low-index crystal band axes ([100] or [001]). It canbe deduced that the strong-intensity columns are Ba and theweak ones are Ti, since this is an ADF-STEM image, in whichthe intensity is approximately proportional to the square ofthe elemental number. The EDS mappings are obtained fromthe same area as the ADF-STEM images. Since the Ba and Tipeaks appear at almost the same energy position in the EDSspectra (Fig. S3†), they are also almost identical in the map.Chem. Sci., 2024, 15, 9147–9154 | 9149http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bFig. 2 ADF-STEM image and EDS mappings for nanoporous BCZT. A high-resolution ADF-STEM image with low-index crystal band axes ([100]or [001]) magnified from the low-magnification ADF-STEM image. The raw EDS intensity mappings for Ba, Ti, Ca and Zr, which are indicated byBa-L, Ti-K, Ca-K, and Zr-K, respectively. The processed EDS intensity mappings for Ca and Zr, which are indicated by Ca-K (proc.) and Zr-K(proc.), respectively. EDS maps (green) of Ca or Zr overlaid with the ADF-STEM image (red), which are indicated by ADF and Ca, and ADF and Zr,respectively.Chemical Science Edge ArticleOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineTherefore, the columns of Ba and Ti cannot be distinguishedfrom the EDS mapping. However, it can be said that theprevious assumption of Ba in the intense column and Ti in theless intense column is correct by combining the ADF-STEMand EDS results. Additionally, the substitution positions forCa and Zr can be estimated by comparing the high-resolutionADF-STEM images and the EDS mappings. It is difficult toidentify the positions of Ca and Zr due to their weak intensity.Therefore, image processing is performed to enhance thevisibility of each position. These images are superimposed onthe high-resolution ADF-STEM images. The results show thatCa and Zr are substituted for Ba and Ti. These mappingimages are direct evidence of the successful doping of Ca andZr atoms. The Ba, Ca, Ti, and Zr compositional ratio coincideswith the ICP result, supporting the formation of Ba0.85-Ca0.15(Ti0.9Zr0.1)O3. The ADF-STEM images for nanoporousBTO are shown in Fig. S4.† The details of the doping levels areevaluated from XPS (Fig. S5†).9150 | Chem. Sci., 2024, 15, 9147–9154Measurement in PFM analysisQuantitative studies of piezoelectric behavior and polarization-related properties at the nanoscale can be challenging andextremely sensitive to the experimental conditions of piezores-ponse force microscopy (PFM). However, near-accurate resultscan be achieved by carefully optimizing several factors. Theseinclude (i) electrostatic contributions due to cantilever-surfacenonlocal and tip-surface local capacitive forces, (ii) tip bias tocalibrate and compensate for linear dielectric contributions,and (iii) choice of driving voltage (VAC) and bias window ofmeasurement.23 In the measurement process, the electrostaticcontribution can be represented by the equation: Ael = G(u)(VDC− Vsurf), where Ael represents the electrostatic contribution, G(u)denotes the frequency-dependent electromechanical responsefunction, VDC is the direct current voltage, and Vsurf representsthe surface potential. Unlike the piezoelectric contribution tothe signal, which remains independent of the tip bias for linearpiezoelectric or dielectric components, the electrostatic© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bEdge Article Chemical ScienceOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinecontribution varies linearly with bias and becomes zero whenthe null condition VDC = Vsurf is met. This null condition can beidentied by carefully assessing the uniformity of the grayscalecontribution in the amplitude image in PFM. In an unoptimizedimage, surface charge (non-piezoresponse components) mayappear as very bright contrasts (white highlights). The pro-portionality factor G(u) is strongly dependent on frequency andcan signicantly amplify the contribution to the PFM signal,particularly at frequencies near 300 kHz.24A heavily doped (conductive)-silicon cantilever was used witha spring constant of 2.7 N m−1 and medium stiffness with highcontact resonance, and fairly tight adhesion to the samplesurface (essential for compensating for local capacitive forces)by the cantilever was found to be best suited for all studiedmaterials including the reference sample. A contact resonanceof 200–300 kHz was present in the studied sample duringscanning. A test reference sample of PZT lm (the same lmthickness as the studied sample) with known d33 was taken forwhich the contact resonance lay within the same frequencyrange and the VAC modulation window of the cantilever wastuned to resemble the known piezoresponse (d33) of PZT tocompensate for the proportionality factor G(u) due to anyfrequency artefact that may have occurred. 1 V was found to bestable and was kept constant in all studied samples since all thesamples were fairly responsive below this threshold. Therefore,sample deformation becomes Aout-plane = d33 at 1 V (ref. 11) andthe amplitude image of PFM becomes a direct representation ofFig. 3 Amplitude distribution of d33 mapping for (a) bulk and (b)nanoporous BCZT. The mapping areas for the bulk and nanoporoussamples are 1 × 1 mm2 and 5 × 5 mm2, respectively. Approximately 10times the amplitude is observed for nanoporous BCZT compared tobulk BCZT.© 2024 The Author(s). Published by the Royal Society of Chemistrythe vertical piezoresponse (d33), as shown by the plotted data inFig. 3. Generally, the unit for d33 is C N−1 or m V−1.Piezoresponse and strain in nanoporous BCZT lmThe mapping of piezoresponse, representing vertical ampli-tudes with respect to the lm substrate, is illustrated in Fig. 3.The amplitudes for the bulk (Fig. 3a) and nanoporous lms(Fig. 3b) go from 0 to 600 pm V−1 and from 4.5 to 7.5 nm V−1,respectively. Therefore, approximately 10 times the amplitude isobserved for nanoporous BCZT compared to that of the bulk.The maximum amplitude for the bulk lm is 600 pm and thatfor the nanoporous lm is 7.5 nm. These results are reected inthe d33 values at 1 V in the amplitude–voltage loops, which willbe discussed later. This is because the amplitude in Fig. 3indirectly means d33, as a voltage of 1 VAC is applied in the PFMexperiment. Surprisingly, the result is that this large remnantd33 value of nanoporous BCZT far exceeds those of other Pb-based materials.2,4,7–10,12–16 The strain is calculated with high-resolution scanning transmission electron microscopy (HR-STEM) (Hitachi HF5000 aberration-corrected STEM). A Bragg-ltered image of the lattice strain is calculated using the FFTpatterns (see details in the ESI;† the validity of strain values andFig. S6†). Fig. 4 shows the strain eld images for bulk andnanoporous BCZT lms.Surprisingly, there is a large difference in the calculatedstrain distribution between bulk and nanoporous BCZT lms.The values are$0.3% and over ∼30% for bulk and nanoporousBCZT lms, respectively. The strain for the nanoporous BCZTlm is approximately 100 times larger than that of bulk BCZTlm. The mathematical relationship between strain (obtainedquantitatively from geometric phase analysis; Fig. 4) and pie-zoresponse (obtained from PFM; Fig. 5) shows they are directlyrelated where our quantitative ndings satisfy the boundedrelation equation, as follows:3 = d33EG(u)−1,where 3 is strain, d33 is the vertical piezoresponse, E is thecoercive eld, G(u)−1 is a proportionality factor calibrated for anoptical beam deection sensor (OBD) equipped with a DARTPFM, provided by the PFM manufacturer Oxford Instruments,USA and given in the ESI† (Fig. S6).Piezoresponse in nanoporous BCZT lmThe nano-indented piezoelectric properties were evaluated withthe amplitude–voltage, phase–voltage, and piezoelectrichysteresis loops from piezoresponse force microscopy (PFM)(Oxford Instruments Cypher High Voltage Dual AC ResonanceTracking; DART) under off-eld conditions. Fig. 5 shows theaveraged normalized plot (red dotted line), compiled from fouriterations, which correspond to four sets of data obtainedduring nano-indentation probing at the same location. Thisapproach allows for an extended duration of “polling” under theapplied electric eld compared to a single probing or iteration.Also, it ensures the repeatability and quality of the collecteddata. The relationship between deection and voltage for bulkChem. Sci., 2024, 15, 9147–9154 | 9151http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bFig. 4 Bragg-filtered image (left) and strain field image (right) for (a) bulk and (b) nanoporous BCZT films. The bar on the right-hand side of thestrain field images represents the degree of “strain”.Fig. 5 Remanent piezoresponse (off-field). (a) Deflection–voltage hysteresis loop, (b) phase–voltage hysteresis loop, (c) d33–voltage hysteresisloop of bulk (non-porous) BCZT; and (d) deflection–voltage hysteresis loop, (e) phase–voltage hysteresis loop, and (f) d33–voltage hysteresisloop for nanoporous BCZT. The averaged normalized plot (red dotted line) was compiled from four iterations (black lines), which correspond tofour sets of data obtained during nano-indentation probing at the same location.9152 | Chem. Sci., 2024, 15, 9147–9154 © 2024 The Author(s). Published by the Royal Society of ChemistryChemical Science Edge ArticleOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bEdge Article Chemical ScienceOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online(non-porous) and nanoporous BCZT is depicted by deection–voltage hysteresis loops (Fig. 5a and d). The amplitude valuesfor the nanoporous BCZT lm are approximately ten timeslarger (7500 pm V−1 vs. 550 pm V−1) than those for the bulkBCZT lm (Fig. 5c and f). In the case of the nanoporous BCZTlm, the coercive voltage Vc at the cross-point of the loop isapproximately ±50 V, which is indicative of a larger internalstrain (since the strain is directly proportional to d33) comparedto the bulk BCZT lm (Fig. 5f). Meanwhile, the relationshipbetween phase and voltage for nanoporous BCZT is depicted byrectangular loops with a squareness ratio near unity, which areabsent in non-porous BCZT (Fig. 5b and e) due to the highenergy content possessed by nanoporous BCZT. 180° phase ipsare observed for both nanoporous and bulk lms (Fig. 5b and e).The d33–voltage hysteresis loops, which are ferroelectric polar-ization–electric eld-like hysteresis loops, can be observed(Fig. 5c and f). The remnant d33 value is 7500 pm V−1 (Fig. 5f).This value of remnant d33 is more than 10 times larger than thatfor the bulk BCZT lm and for the previously reported BZT-BCT.15 Compared to those of non-doped BTO and PZT, i.e.,representative Pb-based materials, the value of d33 for nano-porous BCZT is much larger.2,12 Furthermore, the obtainedvalue in this study is demonstrated to be reasonable accordingto 3r (Fig. S7†). The large dielectric constant (3r = 1500 and 1000for nanoporous and BCZT, respectively) and adequately lowleakage current density (J = 10−6 A cm−2 for nanoporous BCZTin comparison to 10−7 A cm−2 for bulk BCZT) for practical useare conrmed in both nanoporous and bulk lms (Fig. S8†).Moreover, the dielectric loss tan d values for both nanoporousand bulk lms are almost the same at approximately 0.03 to0.04 in the frequency range from 0.001 to 0.01 MHz, whereasthey show an increase above 0.01 MHz (Fig. S7†). In addition,the tan d value for the nanoporous BCZT lm shows a moregradual increase than that for the bulk BCZT lm. It isapproximately 0.12 at 1 MHz, which is approximately half thatfor the bulk lm. Therefore, higher 3r and lower tan d for thenanoporous lm than for the bulk lm are observed over thisfrequency range. In these measurements, the piezoresponse isnot symmetrical along the vertical axes, indicating a strainmemory effect. This effect is microscopically attributed todomain switching in non-180° domains which was previouslyreported in PZT.25 Additionally, the values of the coercive volt-ages of Vc− and Vc+ are not the same in the d33–voltage hysteresisloops, which indicates the existence of an internal bias voltage.The calculated values of Vc− and Vc+ in the d33–voltage hysteresisloops for bulk BCZT are y−3.5 V and y8.2 V, respectively(Fig. 5c), while for nanoporous BCZT they are y−62.8 V andy57.5 V, respectively (Fig. 5f). Such asymmetry is remarkable inthe deection–voltage hysteresis loop (Fig. 5a and d). Twodistinct values are observed at 0 V, indicating a signicantinternal bias voltage. This is due to the offset voltage beingapplied by the doping and porous structure. The coercive eld,Vc (Fig. 5f), is notably ve times higher than that of its bulkcounterpart (Fig. 5c). Additionally, the energy generationequation, E ¼ 12CVc2, highlights the signicance of the“square” term of Vc, serving as another breakthrough indicator© 2024 The Author(s). Published by the Royal Society of Chemistryfor mainstream energy harvesters, alongside the discovery ofthe massive d33. Such a phenomenon is not observed in bulkBCZT materials.ConclusionsWe have investigated the piezoelectric properties of a nano-porous BCZT lm with articially engineered strains at thenanoscale using a so-templating approach. Deformation ari-ses from the pore wall structure, leading to strain veriedthrough geometric phase analysis. As is evident from the PFMmeasurements, the d33 of nanoporous BCZT is 10 times greaterthan that of bulk BCZT. The introduction of the porous struc-ture causes strong anisotropic stress on the oxygen octahedronin the BCZT crystal, which signicantly distorts the crystallattice. As a result, spontaneous polarization occurs in a partic-ular orientation, which is thought to have resulted in largepiezoelectricity. Our nanoporous BCZT lm demonstratessignicant piezoelectricity solely through the introduction ofa nanoporous structure, eliminating the necessity for creatingintricate architectures, such as nanowires, nanobers, ornanocore/shell structures.26–28 This study introduces a newapproach for the cost-effective production of Pb-free piezoelec-tric materials using much lower annealing temperatures whileachieving an ultra-high piezoresponse, d33. Furthermore, itopens up a new avenue for high-density energy harvestingapplications, serving as a renewable alternative to fossil fuels—an aspect that has not previously been explored.Data availabilityData are available from the authors upon reasonable request.Author contributionsThe concept for the experiment was initially developed by M. B.,M. S. A. H. and Y. Y. Nanoporous BCZT was synthesized by M.B., M. K. M. and Y. T. XPS, dielectric constant, dielectric loss,leakage current density, etc. were conducted by M. B. under thesupervision of M. S. A. H. and Y. Y. ADF-STEM was performedand analyzed by T. A., Y. T., M. B. Z. H., T. N., T. C. and F. U. M.B. Z. H., M. K. M. and Y. T. wrote the manuscript. All authorsdiscussed the results at all stages and participated in thedevelopment of the manuscript.Conflicts of interestThe authors declare no conict of interest.AcknowledgementsThis work was funded by the JST-ERATO project (grant number:JPMJER2003) and the ES program (via Nagoya University) and ispartially supported by the start-up company Schnell Energy,Melbourne, Australia, the Australian Research Council(LP200200689), an Australian Government Research TrainingProgram (RTP) Scholarship at the University of QueenslandChem. Sci., 2024, 15, 9147–9154 | 9153http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712bChemical Science Edge ArticleOpen Access Article. Published on 20 May 2024. Downloaded on 10/9/2024 6:27:47 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineto M. B., and Joint Research Support Project at KumamotoUniversity. Additionally, this work was partially conducted atthe Queensland node of the Australian National FabricationFacility, a company established under the National Collabora-tive Research Infrastructure Strategy to provide nano- andmicro-fabrication facilities for Australian researchers (Australia)and NIMS Electron Microscopy Analysis Station, NanostructuralCharacterizaion Group (Japan).References1 J.-H. Lee, J. Kim, T. Y. Kim, M. S. A. Hossain, S.-W. Kim andJ. H. Kim, J. Mater. Chem. A, 2016, 4, 7983–7999.2 L. Li, Z. Xu, S. Xia, Z. Li, X. Ji and S. Long, J. 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Published by the Royal Society of Chemistryhttps://doi.org/10.1016/0022-460X(72)90684-0http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3sc06712b Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,... Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin filmElectronic supplementary information (ESI) available: Supplementary figures; SEM, XRD,...