Ionic liquid based dopant-free band edge shift in BiVO4 particles for photocatalysis under simulated sunlight irradiation

Foreign elemental doping is a widely utilized strategy to modify the electronic structure of semiconductors. Herein, we present a dopant-free novel synthesis approach to control the electronic structure of a semiconductor. Utilizing butyl methyl imidazolium ([BMIM]Cl) and methoxyethyl methyl imidazolium ([M(MOE)Im][Cl]) chloride ILs, we prepared four different Bi and V based ILs: 3-butyl-1-methyl-1H-imidazol 3-ium vanadate [BMIm][VO3], 3-(2-methoxyethyl)-1-methyl-1H-imidazol-3-ium vanadate [M(MOE)Im][VO3], 3-butyl-1-methyl-1H-imidazol-3-ium tetrachlorobismate [BMIm][BiCl4] and 3-(2-methoxyethyl)-1-methyl 1H-imidazol-3-ium tetrachlorobismate [M(MOE)Im][BiCl4]. Owing to the bimetallic oxide nature of BiVO4, these gels were mixed either with each other or with Bi/V commercial salts and simply heat-treated to obtain monoclinic BiVO4. Depending on the IL, the bandgap energy of pure BiVO4 will be redshifted (2.44 to 2.25 eV). The IL based synthesis induced oxygen vacancies and uplifted the BiVO4 valence band edge as observed in the X-ray photoelectron spectroscopy (XPS). These effects were profound for IL anchored Bi; however, the side effects of this synthesis were chemisorption of a higher oxygen content and low reactivity of Bi with V to form an additional V2O5 phase. ILs acted as templates to form smooth spherical particles with improved crystallinity. [M(MOE)Im] based synthesis resulted in lower-order crystallinity and a large V–O bonding length of BiVO4 compared to [BMIm] which may be ascribed to its lower-order cationic–anionic electrostatic attraction associated with the presence of oxygen in the ether-group for [M(MOE)Im]. [BMIm] cation-based synthesis suppressed photogenerated charge-recombination and resulted in a five-fold O2 evolution of B30 mmol for 3 h (AM 1.5G illumination) compared to pure BiVO4 which was better compared to the sample prepared by the conventional hydrothermal process. It also improved the photocurrent, and the MS plots have shown that the conduction band was not much affected; however, the defect density was larger for IL based synthesi

Enhanced magnetoelectric response of cofired ceramic layered composites by adjusting the grain boundary conductivity of the magnetostrictive component

[EN] Magnetoelectric composites provide large enough room-temperature functional responses to enable a range of novel technologies, like uncooled high-sensitivity magnetic sensors. Magnetoelectricity in these composites appears as a product property of the piezoresponses of two elastically coupled ferroic phases. Coefficients then depend not only on the magnetostriction and piezoelectricity of the components, but also on interface quality. Different composite approaches are under development, among which cofired ceramic layered structures of ferrimagnetic spinel and ferroelectric perovskite oxides stand out because direct bonding between phases is attained, which facilitates miniaturization. An aspect rarely investigated is the role of the electrical characteristics of the magnetostrictive material beyond their effect on ferroelectric poling. We report here a three-fold enhancement of magnetoelectric coefficients by the only adjustment of the grain boundary conductivity of the spinel oxide. Significant differences in poling level are ruled out, and the effect is related with changes in the composite effective permittivity.This work was supported by the Coordenaçao de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES) - Finance Code 001 (Process No. 88881.030500/2013–01), Sao Paulo Research Foundation FAPESP (Grants No.2013/00134–7 and 2017/17872–1), Brazilian National Council for Scientific and Technological Development CNPq (Grant No. 150979/2020-7) and Spanish MICINN (Project MAT2017-88788-R

Processing issues and their influence in the magnetoelectric performance of (K,Na)NbO3/CoFe2O4-based layered composites

Low temperature processing of environmentally-friendly (K,Na)NbO- and CoFeO-based layered composites has been accomplished by hot pressing, and their electrical, piezoelectric and magnetoelectric properties have been characterized. High quality composite three-layer CoFeO/KNaNbO/CoFeO structures with defect-free interfaces were obtained by hot pressing at only 1000 °C, when highly reactive CoFeO powders obtained by Pechini were used. Negligible chemical reactions at interfaces or interdiffusion across them took place during processing, while alkali volatilization was minimized. Dense, crack-free CoFeGaO/(KNa)LiNbTaO/CoFeGaO structures incorporating phases with enhanced piezoresponses have been also obtained, though in this case magnetoelectric coefficients were lower than expected. This is shown to be related with the presence of a core-shell type microstructure within the piezoelectric layer characteristic of (KNa)LiNbTaO, which is stabilized by the low temperature processing. Procedures for further enhancement of the magnetoelectric response are proposed.Research funded by Brazilian PVE/CAPES (process88881.030500/2013-01) and grant #2013/00134-7 Sao Paulo Research Foundation (FAPESP) and by Spanish MINECO through projects MAT2014-58816-R and MAT2017-88788-R.Peer Reviewe

Magnetoelectric coupling in lead-free piezoelectric Lix(K0.5Na0.5)1 − xNb1 − yTayO3 and magnetostrictive CoFe2O4 laminated composites

To replace lead zirconium titanate in magnetoelectric (ME) composites owing to concerns regarding its toxicity, we investigate the ME coupling in bilayer composites comprising lead-free Lix(K0.5Na0.5)1 − xNb1 − yTayO3 (LKNNT) (piezoelectric) and CoFe2O4 (magnetostrictive) phases. We prepare the LKNNT ceramics and measure its piezoelectric coefficient d31, a crucial ingredient determining ME couplings, for several Li (x = 0.03, 0.035, 0.04) and Ta (y = 0.15, 0.2, 0.25) concentrations, and find that the highest d31 occurs at y = 0.2 for all the values of x studied here. We then evaluate both the transverse (αE,31) and the longitudinal (αE,33) low-frequency ME coupling coefficients of our composites, for each the above composition of (x, y). At x = 0.03, we find the usual scenario of αE,31 and αE,33, i.e., the strongest ME coupling occurs when d31 is maximal, namely at y = 0.2. On the other hand, interestingly, we also obtain the strongest ME coupling when the LKNNT layer has a relatively weaker , e.g., at y = 0.25 for x = 0.035 and y = 0.15 for x = 0.04, following from the interplay of d31 and other ingredients (e.g., dielectric constant). Our calculated ME couplings, with αE,31 in magnitude around twice of αE,33, are comparable to those in lead-based composites. The effect of the volume fraction and interface parameter on the ME coupling is also discussed.This work was supported by CAPES, FAPESP (Grant #2013/00134-7), PRP/USP (Q-NANO), and Natural Science Foundation of China (Grant No. 11004120)

Environmentally-friendly magnetoelectric ceramic multilayer composites by water-based tape casting

Supplementary data associated with this article can be found, in theonline version, at: https://doi.org/10.1016/j.jeurceramsoc.2018.10.009Magnetoelectric composites are an enabling material technology for a range of novel devices like electrically-tunable magnetic microwave components or room-temperature-operation high-sensitivity magnetic sensors. Among the different approaches under development, cofired ceramic layered composites provide large effective magnetoelectric coefficients and improved reliability. However, miniaturization and processing up-scaling remain an issue. This can be addressed by using tape casting technology to prepare multilayer structures, as it is industrially done for multilayer ceramic capacitors. We report here the processing of ceramic multilayer composites of environmentally-friendly piezoelectric (KNa)LiNbTaO and magnetostrictive CoFeMnO by water-based tape casting. Dense ceramic multilayers with high quality interfaces were obtained, and their functional response characterized. Effects of the multilayer geometry, characterized by ceramic layer thickness of tens of microns and large number of layers, have been defined. A distinctive enhancement of functionality as compared with conventionally-prepared layered structures is demonstrated and related to strain/stress relaxation characteristics across layers.Research co-funded by the Coordenação de Aperfeiçoamento dePessoal de Nível Superior–Brasil (CAPES)–Finance Code 001 (process88881.030500/2013-01), São Paulo Research Foundation FAPESP(grants #2013/00134-7 and #2017/17872-1), and Spanish MINECO (projects MAT2014-58816-R and MAT2017-88788-R).Peer Reviewe

Controlling colloidal processing of (K,Na)NbO3-based materials in aqueous medium

[EN] K0:5Na0:5NbO3-based materials are serious candidates to replace leadbased piezoceramics since they show excellent electrical and piezoelectric properties. The tape casting technique can be used to obtain highly textured KNN-based ceramics; however, despite industrial and environmental advantages of water-based processing, there are not reports about the control of colloidal processing conditions to obtain optimized K0:5Na0:5NbO3-based slurries in aqueous medium. This paper reports a procedure for controlling colloidal stability and rheological behavior of aqueous (K0:5Na0:5)0:97Li0:03Nb0:8Ta0:2O3 suspensions. Zeta potential and cationic solubility measurements as a function of pH showed that pH 8.5 is adequate for concentrated suspensions, while ow curves analysis allowed optimizing processing parameters, such as, powder content, amount of de occulant and binder, and sonication time. Optimized colloidal suspensions were prepared and used to obtain high quality tapes. Processed ceramics from these stacked tapes show equivalent properties to those processed directly from powders, which demonstrates the effectiveness of the colloidal route reported here.Research co-funded by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - Brasil (CAPES) - Finance Code 001 (process 88881.030500/2013-01), Sao Paulo Research Foundation FAPESP (grants#2013/00134-7 and #2017/17872-1), and Spanish MINECO (projects MAT2014-58816-R, MAT2015-67586-CR-2-R, and MAT2017-88788-R)

Enhanced piezomagnetic coefficient of cobalt ferrite ceramics by Ga and Mn doping for magnetoelectric applications

See the supplementary material for XRD patterns and SEM micrographs of CFG and CFM compositions. [ftp://ftp.aip.org/epaps/journ_appl_phys/E-JAPIAU-125-083908][EN] The magnetic, magnetostrictive, and electrical properties of Ga- and Mn-doped cobalt ferrite are reported as a function of composition. Materials with improved functionality for magnetoelectric composites are obtained. Magnetic characterizations reveal the effectiveness of the dopants to reduce the typically high magneto-crystalline anisotropy of cobalt ferrite and significantly enhance piezomagnetic coefficients. CoGa0.15Fe1.85O4 ceramic shows large effective piezomagnetic coefficient q11, 3.9 × 10−6 kA−1 m, which is among the highest values reported for cobalt ferrite-based ceramics. Additionally, a two order of magnitude increase of resistivity is found after doping, which makes this material specially suitable for particulate composites. On the contrary, CoMn0.25Fe1.75O4 ceramic has the highest value of q11+q21 (∼1.9 × 10−6 kA−1 m), which is the relevant parameter for laminated composites. Analytical calculations of the transverse magnetoelectric coefficient αE31 for bilayers containing these optimized magnetostrictive phases are also reported, and they demonstrate their high potential for developing new magnetoelectric composites.The research was co-funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES) —Finance Code 001 (Process No. 8881.030500/2013-01), São Paulo Research Foundation FAPESP (Grant Nos. 2013/00134-7and 2017/17872-1), and Spanish MINECO (Project Nos. MAT2014-58816-R and MAT2017-88788-R).Peer reviewe

Selection and optimization of a K0.5Na0.5NbO3-based material for environmentally-friendly magnetoelectric composites

[EN] Li-and Ta-modified KNaNbO compounds are among the most promising lead-free ferroelectrics for high-sensitivity piezoelectric ceramic materials, and are potentially capable of replacing Pb(Zr, Ti)O. They are also being investigated as piezoelectric components in environmentally friendly magnetoelectric composites. However, most suitable modifications for this application have not been identified. We report here a simulation study of how the magnetoelectric voltage responses of layered composite structures based on Lix(KNa)NbTayO varies with the chemical composition of the piezoelectric. Instead of relying on material coefficients from the literature, which would have required using different sources, an ad hoc set of materials was prepared. This demanded tailoring preparation by conventional means to obtain dense ceramics while controlling alkali volatilization, perovskite phase and microstructure, as well as characterizing their dielectric, elastic and electromechanical properties. This provided the set of relevant material coefficients as a function of composition, which was used to obtain the magnetoelectric responses of model layered structures including a reference magnetostrictive spinel oxide by simulation. The piezoelectric material leading to the highest magnetoelectric coefficient was identified, and shown to be different to that showing the highest piezoelectric coefficient. This reflects the dependence of the magnetoelectric response on all material coefficients, along with the complex interplay between composition, processing and properties in KNaNbO-based ceramics.The research was co-funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES)—Finance Code 001 (process number 88881.030500/2013-01), the São Paulo Research Foundation FAPESP (grant numbers 2013/00134-7 and 2017/17872-1), and Spanish MINECO (project MAT2017-88788-R)