Chemisorption-Driven Roughening of Hydrothermally Grown KTa_1–xNb_xO₃ Nanoparticles

Abstract

The chemical and physical properties of nanoparticle surfaces have significant effects on their growth processes and the resulting morphology. Hydrothermally grown KNbO3, KTaO3, and KTa1–x Nb x O3 were studied to examine the complex relationships between surface composition, phase, chemistry, and energetics and how these may be used to model and thereby control nanoparticle growth mechanisms. Two different composition-dependent growth modes were identified, where one type formed smooth surface facets, while the other resulted in roughened nanoparticle morphologies. Electron microscopy characterization, density functional theory calculations, and mathematical growth models were used to illuminate the role of surface properties and chemisorption on the nanoparticle growth morphology. Surface energy reduction by chemisorption can increase the rate of terrace nucleation, driving the roughening of these lower energy nanoparticle surfaces.

Complex Fluorine Chemical Potential Effects on the Shape and Compositional Heterogeneity of KTa1–xNbxO3 Nanoparticles

“…Significant morphology changes corresponding to increasing KF/KOH concentrations were also observed. The nanocuboids shown in Figure were typical of particles grown at lower KF/KOH concentrations, and their morphologies matched the nucleation and growth via surface terraces behavior studied previously. , In contrast to the nanocuboids, anisotropically shaped particles also formed at higher concentrations of KF (KF/KOH > 0.20). These particles were flat rectangular flakes on the order of several hundred nanometers wide and ∼100 nm thick (Figure ), and EDS maps of these particles shown in Figure S1 did not demonstrate any significant composition segregation as the cuboidal KTN particles did.…”

“…It was shown that the composition gradient could be inverted from having a tantalum-rich interior and a niobium-rich surface to a niobium-rich interior and a tantalum-rich surface by predissolving niobium oxide in a basic KOH solution before hydrothermal synthesis. 16 However, the same niobium oxide dissolution procedure could not be replicated with tantalum oxide. The inverted Ta−Nb composition gradient effect was mirrored here by the addition of high concentrations of KF (Figure 2), which implied that the KF, or more specifically the fluoride, similarly affected the dissolution of the oxides in solution.…”

“…The difference in reaction rates between KOH and the metal oxides, Nb 2 O 5 and Ta 2 O 5 , to form KTN perovskite nanoparticles were compared in a previous study. It was shown that the composition gradient could be inverted from having a tantalum-rich interior and a niobium-rich surface to a niobium-rich interior and a tantalum-rich surface by pre-dissolving niobium oxide in a basic KOH solution before hydrothermal synthesis . However, the same niobium oxide dissolution procedure could not be replicated with tantalum oxide.…”

“…15 Regions I, II, and III may also all be interlinked; when the growth of KTN nanoparticles with different surface chemistries (Ta or Nb), which could be controlled by their relative stabilities in region I, were compared, it was shown that the chemisorption behavior in region II could impact the growth morphology in region III. 16 The relationships between regions I and IV and their effects on KTN and KTO particles are the focus of this study. Potassium fluoride was added to the aqueous hydrothermal solution to affect the relative solution stabilities of the reagents during reaction and to act as a driving force behind the formation of bulk planar defects in the produced materials.…”

“…For instance, it was demonstrated in the hydrothermal synthesis of KTaO 3 (KTO) that the relative kinetics of the step-flow growth mechanisms in Region III were controlled by the changing thermodynamic properties of Region I over the course of the synthesis reaction . Regions I, II, and III may also all be interlinked; when the growth of KTN nanoparticles with different surface chemistries (Ta or Nb), which could be controlled by their relative stabilities in region I, were compared, it was shown that the chemisorption behavior in region II could impact the growth morphology in region III …”