In the present study, the application of the lattice strain engineering to a newly-designed lead-free ferroelectric mesocrystalline nanocomposite has been demonstrated. Some specific properties of the ferroelectric materials, including the Curie temperature (Tc), piezoelectric and dielectric responses can be improved by an enormous strain in the heteroepitaxial interface constructed by two kinds of crystals with different lattice parameters. Generally, the lattice strain engineering is mainly applied to the thin film materials because the heteroepitaxial interfaces are relatively easy to be fabricated by heteroepitaxial crystal growth of the film materials, but it is a high cost for the fabrication of thin film materials.
The mesocrystals not only have some potential properties based on the individual nanocrystals, but also exhibit unique collective properties of nanocrystal ensembles. The mesocrystalline nanocomposite constructed by two kinds of nanocrystals is a promising material for the lattice strain engineering to improve the ferroelectricity because it has high density of the heteroepitaxial interface and is low-cost. It is noteworthy that the mesocrystalline nanocomposites exhibit both improved Tc and piezoelectric response, which cannot be achieved simultaneously in the thin film materials or bulk materials without mesocrystalline nanostructure, as far as I know. Therefore, this approach provides a new concept to design the high-performance lead-free piezoelectric materials.
In addition, there has been a recent surge of interest in perovskite solar cells (PSCs) due to soaring power conversion efficiencies (PCEs). However, the fundamental understanding of organic-inorganic halide perovskites employed as the absorber in the
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PSCs is still limited. Consequently, systematic studies on further improvements of the materials and device structures for the commercialization have been severely hampered.
In the present study, the relationship between structure and ferroelectricity of MAPbI3-xClx perovskite has been investigated by the structural analysis and the measurements of the piezoelectric, ferroelectric, dielectric and ferroelastic responses.
The transformation from antiferroelectric MAPbI3-xClx phase to its ferroelectric phase by the poling treatment has been uncovered for the first time.
The swapping behavior between antiferroelectric and ferroelectric phases of the MAPbI3-based perovskites suggest that the ferroelectricity would affect charge separation performance, and the ferroelectric phase can possess a higher charge separation effect than that of the non-ferroelectric phase in the PSCs; hence, current-voltage (J-V) hysteresis behaviors for the PSCs can be well explained based on this behavior. The results conclude that the J-V hysteresis is one of the solid evidence for the exhibition of higher charge separation effect of the ferroelectric perovskites than that of non-ferroelectric perovskites and the reverse scan J-V curve should be used to evaluate PSCs of the antiferroelectric or ferroelectric perovskites because it corresponds to the ferroelectric semiconductor charge separation effect.
The main results and points of the present study are summarized as follow:
In Chapter I, some reviews on the synthesis, the formation mechanisms, characterizations, and the applications of conventional mesocrystals were described.
The general introduction for the topochemical synthesis, and the layered protonated titanate as a precursor for the topochemical synthesis of the mesocrystals were mentioned. In addition, the perovskite and perovskite-related halides were described also as they possess several interesting properties, such as electron-acceptor behavior, a
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large optical transmission domain and piezoelectric etc. Furthermore, the purposes of the present study were clarified.
In Chapter II, the ferroelectric mesocrystalline BT/BNT nanocomposite synthesized from a layered titanate H1.07Ti1.73O4 (HTO) by a facile two-step topochemical process, namely first-step solvothermal process and second-step solid-state process, was introduced. The BT/BNT nanocomposite is constructed from well-aligned BT and BNT nanocrystals with the same crystal-axis orientation. The BT/BNT heteroepitaxial interface in the nanocomposite is promising for the enhanced piezoelectric performance by using the lattice strain engineering, which gives a giant piezoelectric response with a d*33 value of 408 pm/V. The introduced lattice strain at the BT/BNT heteroepitaxial interface causes transitions of pseudo-paraelectric BT and BNT nanocrystals to the ferroelectric nanocrystals in the mesocrystalline nanocomposite, which enlarges ferroelectric, piezoelectric and dielectric responses. The lattice strain also results in the elevated Curie temperatures (Tc) of BT and BNT and a new intermediate phase transition.
In Chapter III, the ferroelectric mesocrystalline BT/BBT nanocomposite synthesized from the layered titanate HTO by a facile two-step topochemical process, namely first-step solvothermal process and second-step solid state process, was exhibited. The BT/BBT nanocomposite is constructed from well-aligned BT and BBT nanocrystals oriented along the [110] and [11-1] crystal-axis directions respectively. The lattice strain is introduced into the nanocomposite by the formation of the BT/BBT heteroepitaxial interface, which causes a greatly elevated Curie temperatures from 400 to 700 °C and an improved piezoelectric response with d*33=130 pm/V. In addition, the BT/BBT nanocomposite is stable up to a high temperature of 1100 oC, hence, the mesocrystalline
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ceramic can be fabricated as a high-performance ferroelectric material.
In Chapter IV, the ferroelectric and semiconducting properties of the CH3NH3PbI3-xClx perovskites were studied by structural analysis, measurements of the ferroelastic behavior, the ferroelectric hysteresis loops, the piezoelectric response and conductivity. The results reveal that the CH3NH3PbI3-xClx perovskite exhibits the antiferroelectric and semiconducting natures, and the antiferroelectricity can be switched to ferroelectricity by poling treatment, which gives a solid evidence to put an end to the heated argument between the non-ferroelectric and ferroelectric nature for the MAPbI3-based perovskites and paves the way for the fabrication of high-performance perovskite solar cells by using ferroelectric and antiferroelectric phases.
The results described above conclude that the in situ topochemical mesocrystal conversion reaction process is an attractive approach. This approach can be employed to the development of the platelike functional titanate ferroelectric mesocrystalline nanocomposite. The nanocrystal size, morphology, structure, and composition of the mesocrystalline nanocomposite can be controlled by adjusting the reaction conditions in the in situ topochemical mesocrystal conversion reaction process. These mechanisms will serve also as a guide to develop the topochemical syntheses of other materials in the solvothermal processes and solid-state processes. Therefore, both the solvothermal chemical processes and solid-state processes accompanying with the in situ topochemical conversion reaction are of notable significance for the fundamental research, and can provide important knowledge for controlling the chemical reaction process to achieve the materials with advanced functions.
The study of mesocrystalline nanomaterials constructed from well-aligned oriented nanocrystals has increasingly become an intense and major interdisciplinary research
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area in the recent decade owing to their potential applications to catalysis, sensing, ferroelectric, and energy storage and conversion. In addition, strain engineering has been used to alter the electronic structure of materials, which can greatly change a series of physical properties of the materials, since its impact is delivered directly on the lattice. Up to now, the strain engineering has been widely applied to 2D materials with simple 2D heteroepitaxial interface, while its application to 3D bulk materials have been rarely reported. In the 3D systems, strain can be effectively introduced to a bulk material by either pulling or squeezing the lattice. However, the 3D heteroepitaxial interface is very difficult to be constructed in the 3D bulk materials. Therefore, the application of the strain engineering to a newly-designed mesocrystalline nanocomposite with a 3D heteroepitaxial interface is a big breakthrough for making 3D bulk materials with newly excellent properties. The success in developing these mesocrystalline nanocomposites not only expand mesocrystalline nanomaterials chemistry and offers a good opportunity to understand the formation process of this unique mesocrystalline nanocomposite structure, but also paves a way for the application of the mesocrystalline materials to improve ferroelectric, piezoelectric, and dieletric nanomaterials via the strain engineering.
On the other hand, our findings for the MAPbI3-xClx perovskite not only put an end to the heated argument between its non-ferroelectric and ferroelectric natures, but also pave a new avenue toward the fabrication of high-performance PSCs using the antiferroelectric and ferroelectric semiconductor perovskites with optimizing the cell performances in future developments for the commercialization.
In our next challenges, firstly, figuring out the connection between the atomic arrangement structure and the anomalous ferroelectric behavior of the mesocrystalline
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nanocomposite is significant. Of course, this requires the help of some state-of-art technologies, such as the scanning transmission electron microscopy (STEM), the energy-dispersive X-ray spectroscopy (EDS) and the electron energy loss spectroscopy (EELS). Given that ferroelectric mesocrystalline nanocomposites constructed from the different nanocrystals with the same perovskite structure tend to transform into the solid solution phase under high heating temperature, therefore, the application of the mesocrystalline nanocomposite to its ceramic counterpart is scarcely possible. Therefore, the formation of its film materials or/and polymer-based composite applied to nanoelectronic devices is promising.
As mentioned above, the lattice strain engineering has been mainly applied to the ferroelectric super-structured film materials with 2D heteroepitaxial interfaces, which has been widely studied. Whereas, the ferroelectric mesocrystalline nanocomposite film materials should exhibit much more complicated 3D heteroepitaxial interface, which needs much more refined STEM image analysis and other assistant methods to find out the atomic arrangements near the interfaces and its connection to the anomalous ferroelectric behavior. Although, some preliminary works have been done, the further study is still needed.
Besides, the further research on the ferroelectricity of other types of halide perovskites used for PSCs and discussing its connection to the power conversion efficiency are meaningful. The ferroelectric semiconductor charge separation mechanism would not be limited in the halide perovskites, and can be applied also to other ferroelectric semiconductors, such as metal oxides and metal sulfides. It could offer a better avenue for the development of the high-performance PSCs with the help of piezoresponse force microscopy (PFM) technique based on atomic force microscopy
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(AFM) or the strain-electric-field (S-E) texting system.
Publications
Publications in Journals
1. Wenxiong Zhang; Hao Ma; Sen Li; Dengwei Hu; Xinggang Kong; Shinobu Uemura; Takafumi Kusunose; Qi Feng. Anomalous piezoelectric response of ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 nanocomposites designed by strain engineering. Nanoscale 2018, 10, (17), 8196-8206.
2. Wenxiong Zhang; Sen Li; Hao Ma; Dengwei Hu; Xinggang Kong; Shinobu Uemura; Takafumi Kusunose; Qi Feng. Ferroelectric Mesocrystalline BaTiO3/BaBi4Ti4O15 Nanocomposite: Formation Mechanism, Nanostructure, and Anomalous Ferroelectric Response. Nanoscale 2019, DOI: 10.1039/C8NR07587E 3. Dengwei Hu; Wenxiong Zhang; Yasuhiro Tanaka; Naoshi Kusunose,; Yage Peng;
Qi Feng. Mesocrystalline Nanocomposites of TiO2 Polymorphs: Topochemical Mesocrystal Conversion, Characterization, and Photocatalytic Response. Crystal Growth & Design 2015, 15, (3), 1214-1225.
4. Dengwei Hu; Xiaomei Niu; Hao Ma; Wenxiong Zhang; Galhenage A. Sewvandi;
DesuoYang; Xiaoling Wang; Hongshei Wang; Xinggang Kong; Qi Feng.
Topological relations and piezoelectric responses of crystal-axis-oriented BaTiO3/CaTiO3 nanocomposites. RSC Adv. 2017, 7, (49), 30807-30814.
5. Dengwei Hu; Wenxiong Zhang; Fangyi Yao; Fang Kang; Hualei Cheng; Yan Wang;
Xinggang Kong; Puhong Wen; Qi Feng. Structural and morphological evolution of
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an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process.
CrystEngComm 2018, 20, (6), 728-737.
6. Ma, H.; Wenxiong Zhang; Xinggang Kong; Shinobu Uemura.; Takafumi Kusunose.; Qi Feng. BaTi4O9 mesocrystal: Topochemical synthesis, fabrication of ceramics, and relaxor ferroelectric behavior. Journal of Alloys and Compounds 2019, 777, 335-343.
7. Wenxiong Zhang; Galhenage A. Sewvandi; Sen Li; Xinggang Kong; Dengwei Hu;
Shinobu Uemura.; Takafumi Kusunose.; Qi Feng. Compelling Evidences for Antiferroelectric to Ferroelectric Transition of MAPbI3-xClx Perovskite in Perovskite Solar Cells. (In the submission)
Publications in Conferences
1. Wenxiong Zhang, Qi Feng. Topochemical Synthesis of BaTiO3 Platelike Mesocrystals from Layered Titanate by Flux Method. 第21回ヤングセラミスト ミーティングin中四国, p103-104, Shimane, 2014/11/15.
2. Wenxiong Zhang, Hao Ma, Qi Feng. Synthesis and Characterization of Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. The 54th Symposium on Basic Science of Ceramics, p82, Saga, 2016/01/07-08.
3. Wenxiong Zhang, Hao Ma, Qi Feng. Fabrication and Characterization of Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. 日本化学会 中国四国支部大会, ポスター, 2016/11/05-06.
4. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3
Nanocomposites and Their Ferroelectric Behavior. The 55th Symposium on Basic
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Science of Ceramics, p109, Okayama, 2017/01/12-13.
5. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3
Nanocomposites and Their Ferroelectric Behavior. International Symposium on Advanced Materials: Golden Era in Hydrothermal Research, p36-37, Kochi, 2017/03/27-30.
6. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain Engineering. The 56th Symposium on Basic Science of Ceramics, p9, Tsukuba, 2018/01/11-12.
7. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain Engineering. 2018 ISAF-FMA-AMF-AMEC-PFM Joint Conference, p49, Hiroshima, 2018/05/25-06/01.
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Acknowledgment
I could not have completed this dissertation without the help of many people that have influenced and supported me scientifically, financially and socially over the past 5 years at Kagawa University, Japan.
I would like to firstly thank my supervisor, Prof. Qi Feng, for his kind guidance, cultivation, and continuous supervision, excellent advice and continuous encouragement towards the completion of this present research successfully in time. His rigorous and pragmatic academic attitude is worth studying for my life. Besides the fundamental knowledge he had taught me, more importantly, he realized early that I have a strong desire to research, anything, and you gave me the freedom to develop. And I’m also appreciated for Associate Prof. Lin Yu (Okayama Shoka University), for her much supports and patience on our research supervisor team, for her precious guidance and perspectives on my daily life.
I would also like to express my thanks to my vice supervisors Prof. Takafumi Kusunose and Associate Prof. Shinobu Uemura for their kind advice, valuable suggestion, necessary support, and enthusiastic assistances to my Ph. D study. In addition, I would like to express my thanks to Prof. Chengling Pan (Anhui University of Science and Technology) for recommending me to study in Japan. Grateful acknowledgements are to Senior Dengwei Hu (Baoji University of Arts and Science), Changdong Chen (Liaoning Shihua University), Yien Du (Jinzhong University) and Sewvandi Asha Galhenage (University of Moratuwa) for their valuable suggestion, enthusiastic assistances and life experience.
I would like to acknowledge the former and current administrative staff at Kagawa University. Thank you very much for making my life easier in Japan: Especially, I