Effects of Processing and Composition on the Electrical, Optical and Photocatalytic Properties of Ba, Ni Modified KNbO3 Ceramics

Abstract

Ferroelectric materials are widely used in capacitors, actuators, non-volatile memory, electro-optic materials for data storage applications and thermistors. In recent years ferroelectrics with narrow bandgap offer tremendous scope for applications in the field of photocatalysis and photovoltaics because of their efficient ferroelectric polarization-driven carrier separation which in principle can lead to better photocatalytic activity compared to conventional semiconducting oxides. However, the efficiency reported so far for conventional ferroelectrics is still too low to be considered for practical application due to wide band gap (>3 eV) of these materials. The invention of visible light absorbing semiconducting ferroelectric oxide (1-x)KNbO3-x(BaNi12Nb12O3−δ) (KBNNO) by Grinberg et al. augmented global research activity in the area of semiconducting ferroelectric. However, the synthesis of KBNNO by solid state route, require high calcination temperature (850oC) and sintering at further higher temperature (1100oC). Due to this prolonged heat treatment at high temperature, potassium volatizes which results in non-stoichiometry may have an adverse effect on densification, dielectric and optical properties. Very few reports are available on the effect of dopants on structural, electrical and optical properties. Hence, a systematic study of the effect of suitable dopant atoms in such oxide perovskite would throw light on the structure-band gap microstructure-electronic property relationship in these materials which is one of the objectives of the present work. In addition to band gap of a photocatalyst, particle size and particle morphology also have an important role on the photocatalytic activity of oxide semiconductor. Furthermore, it will be interesting to study the synthesis of KBNNO at significant lower temperature compared to solid state method. There are no reports available on the detailed study of photocatalytic mechanism in KBNNO. Coupling of narrowband gap oxide semiconductors with conventional ferroelectrics, photocatalytic efficiency can be enhanced further by preventing the charge carriers from rapid recombination and increase in visible light activity. However, there are no reports available on the formation of heterojunction composite with KBNNO. The purpose of this proposed work is focused on the preparation of KNbO3, KNb1- x/2Ni x/2O3-δ (KNNO), [KNbO3]1-x[BaNb1/2Ni1/2]xO3-δ (KBNNO) (x = 0.05, 0.1, 0.15 and 0.2) visible light absorbing ferroelectric perovskite through solid state method and characterization of their structural, dielectric, ferroelectric, electrical, optical and photocatalytic properties. XRD and Raman spectroscopy confirm the orthorhombic structure in synthesized ceramics. Ni substitution in KNbO3 showed appearance of absorption peaks in visible region. Furthermore, a shifting in absorption edge to higher wavelength was observed for Ba, Ni co-doped KNbO3. Raman spectroscopy shows weakening of long range polar order with increase in Ba-Ni doping (x>0.1). FESEM micrographs show the drastic reduction in grain size in KBNNO ceramics. Ni doping shifted the Curie temperature slightly towards the room temperature and in Ba, Ni co-doped samples, a broad hump can be observed around phase transition. From Impedance spectroscopy, the decrease in conductivity of Ba, Ni modified KNbO3 compared to Ni-doped KNbO3 may be due to the reduction of oxygen vacancies created by acceptor doping (Ni+2) in KNbO3 which in turn led to better photocatalytic activity for KBNNO ceramics. Raman, P-E hysteresis, PL, Impedance spectroscopy indicate optimum dopant concentration that is necessary to get visible absorption with retention of ferroelectricity and better photocatalytic activity in KBNNO. These results are useful in the understanding of phase evolution, bandgap tunability, electrical conductivity and photocatalysis in KNbO3 and show the potential of such materials in photocatalysis and photovoltaic application.
In order to reduce the phase formation temperature and to prepare nano structured KBNNO ceramics, solution combustion method was explored. (1-x)KNbO3-x(BaNi12Nb12O3−δ) (x = 0, 0.05, 0.1, 0.15 and 0.2) ceramics were successfully synthesized at low temperature using citrate-nitrate solution combustion method at 600oC. The fuel-to-oxidizer ratio (Φe) (0.6–1.0) has significant effect on the combustion process and phase evolution. TEM micrograph of KBNNO 0.1 shows that the particles are in nano meter range and the average particle size is around 17 nm. Raman and P-E loop indicate polar structure and photoluminescence spectroscopy displays the lowest electron-hole recombination, in KBNNO 0.1 and this may be the reason for the best photocatalytic activity. The rate constant of KBNNO 0.1 for RhB degradation is 3.17 times and 1.4 times higher than KNbO3 and P25 (commercial TiO2 based photocatalyst), respectively under similar visible light illumination. The mechanism behind photocatalytic activity and photo-stability have also been studied for the best composition. In order to further reduce the phase formation temperature and to make oriented nanostructure (nanorods/nano wires), hydrothermal method was explored. Phase pure KBNNO 0.1 were successfully synthesized via hydrothermal method at 200oC. In hydrothermal synthesis of KBNNO 0.1, the effect of KOH concentration (1M-18M) and the effect of reaction time (30min-12h) on phase evolution and powder morphology were investigated. We found that to get complete phase purity atleast 10M KOH concentration and 12h soaking time were required. The Powder morphology varies from spherical to irregular shape, to rod like nature, to cube shape when KOH concentration of the starting solution in the range of 4-6M, 8-12M and 14-18M, respectively. However, its photocatalytic properties are poorer compared to combustion derived samples and comparable to that of solid state samples. To further enhance the photocatalytic property of KBNNO 0.1, nanostructured ferroelectric/conventional semiconductor heterostructure (KBNNO-Ag2O and KBNNO-Bi2O3) has been explored. In context, a series of KBNNO-Ag2O/Bi2O3 composites with varying weight ratios (75:25, 50:50 and 25:75) by a simple precipitation technique/solid state method. Preparation method and processing temperature have significant effect on phase stability and interface formation for KBNNO-Ag2O/Bi2O3 composites. UV-Vis spectroscopy shows that the synthesized composites exhibited higher visible light absorption and PL spectroscopy indicates reduced recombination time of charge carriers than the parent materials. Photocatalytic studies shows that KBNNO:Ag2O (50:50) composite sample can completely mineralize the dye within 25 min. However, the best composition for KBNNO:Bi2O3 (25:75) can completely mineralize RhB in 45 min. Radical trapping experiment shows that O2− , h+ and .OH are the major reactive species helping in mineralization of RhB in KBNNO:Bi2O3 system and h+ and O2− are the major reactive species in KBNNO:Ag2O (50:50) system. The significant absorption in visible region and reduced recombination time of charge carriers in the composite than the parent materials were responsible for excellent photocatalytic properties. The mechanism for degradation was also studied in detail. Moreover, a reasonable degradation of 95% (on an average) was observed after 5 cycles, suggesting a good photocatalytic stability of the composites. Ag nanoparticle dispersed orthorhombic KBNNO are synthesized by photoreduction method. The photoreactivity of Ag/KBNNO nanocomposites as a function of Ag content (0.5-4 wt%) is studied toward aqueous rhodamine B degradation under visible light. The photocatalytic degradation of RhB under visible light irradiation indicates that 3wt% Ag-KBNNO has the highest rate of degradation (0.056 min-1) and can completely mineralize RhB in 60 min. However, the rate constant and degradation time is much lower than the best composition of KBNNO/Ag2O (50:50) which are 0.113 min-1 and 25 min respectively. In Ag-KBNNO, radical trapping experiments show that O2− and .OH are the major reactive species involved in photodegradation of RhB.