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Synthesis and Functional Properties of BiFeO₃ and Bi₂FeCrO₆ based Nanostructures and Thin Films.

Li, Shun (2014). Synthesis and Functional Properties of BiFeO₃ and Bi₂FeCrO₆ based Nanostructures and Thin Films. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 219 p.

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There is an increasing interest in developing and characterizing multiferroic materials, in which both ferromagnetic and ferroelectric orders coexist, as they exhibit rich physical properties and offer exciting opportunities for data storage, spintronics, sensors, electromagnets and photovoltaic (PV) applications. Among all multiferroic materials studied so far, BiFeO3 (BFO) has attracted considerable attention because it shows intrinsic ferroelectric (TC ~ 1103 K) and Gtype antiferromagnetic (TN ~ 643 K) orders simultaneously well above room temperature. In addition, multiferroic BiFeO3, with band gap energy of 2.2-2.8 eV, has been recently identified as a promising candidate for PV devices and photocatalysts in the visible range. Moreover, the coupling between ferroic orders in BiFeO3 materials offers new modes for investigating and controlling the PV effect, which may endow next generation solar and photoelectrochemical (PEC) cells with multiple functionalities. On the other hand, considerable interest has been attributed to multiferroic BiFeO3 nanostructures in the quest of miniaturizing devices and discovering interesting fundamental physics at nanoscale. BiFeO3 nanomaterials with various sizes and shapes such as nanoparticles, nanotubes, nanowires, and nano-/micro cubes have been reported so far and exhibit quite different physical and chemical properties compared to the bulk form of BiFeO3 crystals due to the nanosize effect. Therefore, the synthesis of multiferroic BiFeO3 nanostructures and investigation of their functional properties are considered important for both fundamental research as well as designing new multifunctional materials combining magnetic, ferroelectric and optoelectronic properties. Meanwhile, recent emergence of a novel double perovskites multiferroic material Bi2FeCrO6 (BFCO), with functional properties well above room temperature, opens new opportunities for practical applications of multiferroics. Bi2FeCrO6 has a similar crystal structure as BiFeO3 and exhibits a particularly Fe/Cr cationic ordering along the [111] pseudocubic direction. Recent works demonstrated that an ordered Bi2FeCrO6 phase can be obtained in both thin film and nanostructured form using pulsed laser deposition (PLD) technique. The reported Bi2FeCrO6 thin films possess a remnant polarization of about 55 C/cm2 along the [001] pseudocubic direction, and are ferrimagnetic with a magnetic moment depending on Fe/Cr cationic ordering, about 1.8 B per formula unit, far exceeding the properties of parent BiFeO3. In addition, theoretical studies showed that Fe and Cr mixed d orbital transition allow a small band gap around 2.3 eV. Therefore, Bi2FeCrO6 is expected to be a promising candidate for efficient PV devices and PEC cells using sun light. The work performed in this thesis was therefore driven by two main objectives: (1) synthesis and understanding the fundamental physical properties (i.e. ferroelectric and magnetic) of various low-dimensional BiFeO3 nanostructures; (2) design and investigate BiFeO3 and Bi2FeCrO6 based nanomaterials and thin film devices for high efficiency solar energy conversion (solar to chemical/electrical energy) applications. The results obtained in this work are resumed in two sections as follows: In the first section, we have synthesized and investigated the ferroelectric, magnetic and photocatalytic properties of BiFeO3 nanomaterials (1D nanowires and 2D nanoplates). We studied the ferroelectric properties of 1D single-crystalline BiFeO3 nanowires using piezoresponse force microscopy (PFM). PFM measurements demonstrated that the assynthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both in plane and out of plane directions, thereby confirming the ferroelectric nature of the wires. We explained our results by estimating the shape of the piezoelectric tensor for the rhombohedral symmetry. We have also studied the photocatalytic solar water splitting properties of the BiFeO3 nanowires and discovered that the nanowires exhibit better visible-lightdriven photocatalytic activity for generation of O2 from water than other BiFeO3 materials (e.g. nanocubes) reported previously, which could be attributed to the unique morphology of the nanowires. To further enhance the photocatalytic activity, we designed and synthesized a hybrid Au/BiFeO3 nanocomposite photocatalyst consisting of single crystalline BiFeO3 nanowires and laser ablated Au nanoparticles by a functionalization-step-free solution process. We found that 1.0 wt% Au nanoparticle decorated BiFeO3 nanowires exhibit significantly higher photocatalytic activity (~30 times) of water oxidation for O2 than that of the parent wires during the first 4 h of the reaction. Their superior catalytic activity can be attributed to the role of the Au as electron trapping centers as well as the unique surface-chemistry features of the laser ablated Au nanoparticles that can strengthen the interaction and promote charge transfer. Meanwhile, we observed that the localized surface plasmonic resonance (LSPR) effect of Au nanoparticles could also contribute to the enhancement of the photoactivity. In addition, we developed a novel approach to synthesize (100) pseudocubic facets exposed 2D single crystalline BiFeO3 nanoplates, with thickness ranging from 20 to 120 nm and lateral size of sub-micrometers, via a rapid (1-2 min) microwave-assisted hydrothermal method. The BiFeO3 nanoplates exhibited weak ferromagnetic properties at room temperature, which we attribute to the size-confinement effect on magnetic ordering. The second section is focused on solar energy conversion (i.e. PV and PEC) applications of Bi2FeCrO6 thin film based cells. First, we presented the optical and PV properties of Bi2FeCrO6 epitaxial thin films grown on (100)-oriented SrTiO3 substrate buffered with SrRuO3 electrode deposited via PLD. In this part of work, we have achieved a wide band gap tunability from 1.4 to 2.5 eV in the epitaxial Bi2FeCrO6 films with significant polarization by tuning the ordering of transition-metal element Fe and Cr cations, which is remarkably large as compared with reported values from other doped ferroelectrics, opening up the possibility of discovering new narrow band gap multiferroic materials and designing high efficient oxide solar cells. With optimized PLD deposition conditions, we got a record power conversion efficiency of 3.3% under AM1.5G illumination (100 mW/cm2) in the Bi2FeCrO6 thin film based solar cells. Additionally, we demonstrated the use Bi2FeCrO6 epitaxial thin film as a new photocathode material for the visible-light-driven reduction of water to hydrogen. PEC measurements showed that the highest photocurrent up to −1.0 mA/cm2 at a potential of −1.0 V versus reversible hydrogen electrode (RHE) was obtained in p-type Bi2FeCrO6 thin film grown on CaRuO3/SrTiO3 substrate. For the positively poled Bi2FeCrO6 thin film, the photocurrent density was further enhanced by a factor of ~2, as a result of the modulation of the internal electric field gradient resulting from the ferroelectric polarization.

Type de document: Thèse
Directeur de mémoire/thèse: Rosei, Federico
Informations complémentaires: Résumé avec symboles
Mots-clés libres: nanostructures à base de BiFeO₃; propriétés ferroélectrique; propriétés magnétique; propriétés photocatalytique; énergie solaire; photovoltaïque; photo-électrochimique; Bi₂FeCrO₆
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 17 mars 2015 20:30
Dernière modification: 24 nov. 2015 16:16
URI: http://espace.inrs.ca/id/eprint/2625

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