One-dimensional Anisotropic Electronic States in Needle-shaped La₅Ti₂CuS₅O₇ Single Crystals Grown in Molten Salt in Bridgman Furnace

Abstract

High-quality needle-like crystals of La5Ti2CuS5O7 (a ternary transition-metallic oxysulfide) as long as 1 mm and a few μm in diameter were grown in CsCl molten-salt flux. This synthesis process was performed in a Bridgman furnace with slow motion of charge across the heating zone. Transmission electron microscopy/diffraction analysis indicated that they are product crystallites without crystalline intergrowth, and that the longitudinal direction of needle crystals is parallel to crystallographic b axis of La5Ti2CuS5O7. Optical polarized ultraviolet–visible reflectance spectroscopy provided the first experimental evidence of the one-dimensional electronic excitation along the b axis of this material. The spectroscopic peaks are assigned to the transitions between the electronic bands aided by numerical calculations on the basis of density functional theory (DFT). The advantages of this synthetic process will be applicable in general for oxysulfides that are useful as optical and photocatalytic materials.

Unveiling charge dynamics of visible light absorbing oxysulfide for efficient overall water splitting

“…In addition, Supplementary Fig. 7 displays that the α abs of Y 2 Ti 2 O 5 S 2 is similar to that of other well-studied visible-light absorbing oxysulfide (La 5 Ti 2 CuS 5 O 7 ) 35 , however, one order less than the metal oxides (BiVO 4 ; α-Fe 2 O 3 ) 36,37 and nitride (Ta 3 N 5 ) 27,28 photocatalysts. Consequently, the oxysulfides absorb the ultraviolet solar spectrum from 3.1 to 4.2 eV within the depth of 50 nm to 600 nm, whereas metal oxides and nitride absorb within 10 to 50 nm from the electrolyte interface.…”

“…Considering 100% EQE, the photocatalytic materials should be tailored for narrow band-gap energy <2. 35 eV or light absorption edge >527 nm to achieve STH efficiency beyond 10% for practical applications.…”

Understanding the Photocatalytic Activity of La5Ti2AgS5O7 and La5Ti2CuS5O7 for Green Hydrogen Production: Computational Insights

“…32 All zero-dipole symmetric slabs up to a maximum Miller index of two were considered in this study. The (001) surface was added, and the (1−12) and (−211) surfaces replaced the (112) and (211) surfaces as they were observed on selected area electron diffraction (SAED) patterns by Iwase et al 19,20 The pairs of positive and negative index surfaces were identical, but the negative index slabs were included for consistency with experimentally reported nomenclature. Slab calculations used the same energy cut off as bulk, with k-point meshes derived from the bulk, keeping the same reciprocal space spacing.…”

“…Neither system displays the extent of anisotropy described by Iwase et al, likely due to the experimental conditions favoring elongation of crystals. 19,20 The surface energy anisotropy, based on the coefficient of variation from weighted surface energy, is 0.098 for LTA and 0.111 for LTC, making LTC ever so slightly more anisotropic. Both materials are predicted to be more anisotropic than 88% of the elemental crystals.…”

“…18 Recently, analysis of needle-shaped LTC single crystals has proven the existence of one-dimensional anisotropic electronic states along the b-axis. 19 LTA and LTC have been researched as potential watersplitting materials with the focus on various dopant−cocatalyst combinations as well as the materials' unique structure. 15,17,19−23 Solid solutions of LTA and LTC have been computationally studied, but no comprehensive study of the surface energetics has been performed so far.…”