Structural and Compositional Characteristics of Ball-Milled Lepidocrocite Alkali Titanate and the Correlation to Its Surface Acidic–Basic Properties

Abstract

The studies on mechanical treatments of layered alkali metal oxides are limited despite their diverse compositions/ structures and potential for property tuning. In this work, we vibratory mill Cs 0.7 Zn 0.35 Ti 1.65 O 4 , K 0.8 Zn 0.4 Ti 1.6 O 4 , and Cs 2 Ti 6 O 13 for up to 4 h, during which the lepidocrocite-type structure and the plate-like morphology are well preserved. X-ray diffraction (XRD) indicates a tiny (≤0.6 Å) interlayer expansion accompanied by the enhancement of the preferred orientation along the stacking direction. Chemical analyses across multiple length scales suggest Cs deintercalation, elemental redistributions, and bulk-to-surface (or crystal edge) Cs migration. This ball-milling-induced Cs-rich moiety partially blocks the surface acid sites, although the solids still show a dominating acidic character. The ball-milled samples Cs 0.7−p Zn 0.35−q Ti 1.65 O 4−δ contain vacancies between the sheets (p) and at the sheets (q and δ). It is deduced from Sanderson's electronegativity equalization principle and experimentally verified by Xray photoelectron spectroscopy (XPS) that ball milling increases (decreases) the partial charge at the surface acidic Ti 4+ /Zn 2+ (basic O 2− ) sites. These nonporous solids (≤20 m 2 •g −1 ) contain water sorbed on the external surface as high as 1.1 mol•mol −1 , which is comparable to that in a water-intercalated sample. Our work expands the current understanding of the reactivity vs robustness in layered alkali titanates under physically demanding conditions, complementing knowledge gathered via the soft chemistry approach.

Electrochemical and electrical characteristics of ball milled Cs2Ti6O13 modified by the surface-to-bulk migration of hydroxyl groups

“…The method follows our recent works, 11,12 where a stoichiometric mixture of Cs 2 CO 3 and anatase-type TiO 2 was calcined under air (800 °C for 20 h, 10 °C min −1 ), giving the "as made" product. Next, ∼5 g of the powder was added with a few drops of 1/1 ethanol/water and vibratory milled for 2 h with a ≤3 min stop every 30 min.…”

“…38,39 Herein, we characterized the bulk and surface of cesium titanate Cs 2 Ti 6 O 13 , either as made or after a 2 h vibratory ball milling. 11,12 This composition is the less-understood, peculiar member 40 of the lepidocrocite titanate family with an unusually low alkali-to-Ti ratio (0.33) (compared to the typical one, i.e., Cs/Ti = 0.42 in Cs 0.7 Zn 0.35 Ti 1.65 O 4 ). 4,21,22,41,42 To determine the effect of generated hydroxyl groups, we evaluated the specific capacitance in 1 M Na 2 SO 4 (as in typical electrochemical supercapacitors) by galvanostatic charge-discharge (GCD), supplemented by electrochemical impedance spectroscopy (EIS).…”

“…Very recently, we have reported 11,12 the soft vibratory ball milling of lepidocrocite-type Cs 0.7 Zn 0.35 Ti 1.65 O 4 where the morphology and crystal structure were preserved, although the interlayer distance slightly expanded. X-ray photoelectron spectroscopy (XPS) suggested the bulk-to-surface migration of the interlayer Cs + ions.…”

“…10 In addition, the hydroxyl groups can be created by 'soft' mechanochemical treatments such as vibratory ball milling where the surfaces are modified but the crystal structure and morphology of the starting solid are well preserved. 11,12 Among several families of layered metal oxides, layered alkali/protonic titanate A 2 O•nTiO 2 and its metal-doped counterpart A 2 O•mM x O y •nTiO 2 13,14 (A = alkali/proton, n = integer) have received increasing attention as ion battery electrodes, [15][16][17] electrochemical supercapacitors, 18,19 ion/ proton conductors and dielectrics, [20][21][22][23] and precursors of molecularly thin nanosheets. 24,25 The mechanical treatments of these solids, either high-power or 'soft', have received increasing attention for selective synthesis, size reduction, and surface functionalization.…”

“…4,43,44 A simple model is then proposed. While the ball milling of layered alkali/protonic titanate has been reported including the (micro)structural characteristics and acidic/basic properties, 11,12,17,[29][30][31][32] the surface-to-bulk migration of hydroxyl groups (or protons) is a new phenomenon, affecting the electrochemical and electrical properties alike, which have not been investigated from this point of view. The understanding of this phenomenon would benefit the rational design and wider application of layered titanates in, e.g., electrochemical capacitors, ion conductors, and dielectrics.…”