Synthesis and dielectric properties of nanocrystalline oxide perovskites, [KNbO3]1−x[BaNi0.5Nb0.5O3−δ]x, derived from potassium niobate KNbO3 by gel collection†
Inorganic materials synthesis techniques that can approach low temperature routes akin to chemical solution processing are attractive for their ability to prepare nanocrystalline oxides. These methods can offer thin film integration options more compatible with modern device platforms such as spray coated or printed devices (2D, 3D) and flexible electronics. A chemical solution processing method based on sol–gel chemistry was used to obtain a set of perovskite compounds of the formula [KNbO3]1−x[BaNi0.5Nb0.5O3−δ]x, called KBNNO, a class of visible light absorbing ferroelectric photovoltaic materials, with a tunable bandgap as a function of x. The materials produced were fully crystallized with average nanoparticle sizes of 15–20 nm (KNO) and 20 nm (KBNNO). Control over the composition of KBNNO was based on the synthesis of nanocrystalline potassium niobate KNbO3 (KNO) via potassium and niobium ethoxides, with subsequent chemical reaction of complimentary barium and nickel alkoxides and methoxyethoxides. Characterization by Raman, TEM, SEM, XRD, and EDS confirms structure and composition. Following the introduction of Ba and Ni, a transition from the original orthorhombic Amm2 unit cell (x = 0) to a more complex atomic arrangement in cubic Pm3m (x > 0.1) is observed. This synthetic route to KBNNO, previously only synthesized by solid state processing at 1050–1200 °C, provides a lower temperature (<525 °C) approach to doping ferroelectric KNbO3 with Ba and Ni, which inserts Ba2+ onto the A-site, and Ni2+ onto the B-site with the addition of oxygen vacancies for charge compensation. Frequency dependent dielectric measurements, performed on KNO–PFA (poly furfuryl alcohol) and KBNNO–PFA nanocomposites, show stable effective dielectric constants of 41.2, 70.8, 94.0, and 108.3 for KNO, KBNNO x = 0.1, 0.2, and 0.3 respectively at 1 MHz. Using full error analysis and the modified interphase model, a Maxwell-Garnett based micromechanics approach, the dielectric constant of the individual nanoparticles of KNO, KBNNO x = 0.1, x = 0.2, and x = 0.3 were calculated to be 154, 180, 225, and 255 respectively. The decrease in observed values relative to bulk films is attributed to a potential particle size suppression of the ferroelectric behavior.