Chloride Flux Growth of La₂TiO₅ Crystals and Nontopotactic Solid-State Transformation to LaTiO₂N Crystals by Nitridation Using NH₃

Abstract

Oxynitride perovskites and related phases have received considerable attention due to their potential application for visible-light-responsive photocatalyst and nontoxic inorganic pigments. The changes in bonding and structure by a partial replacement of O2– by N3– give rise to interesting dielectric behavior. Here, we report on the fabrication of highly crystalline La2TiO5 crystals by chloride flux growth method and their subsequent nitridation to form the LaTiO2N crystals using NH3 gas. The flux-grown La2TiO5 crystals had a columnar structure grown in the ⟨001⟩ direction. Using the NaCl flux, larger columnar La2TiO5 crystals were grown compared to those grown using the KCl flux. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased significantly. The columnar La2TiO5 crystals with smooth surface were readily converted by nitridation at 950 °C for 45 h followed by acid treatment into the LaTiO2N crystals with a highly porous structure that formed from the strong segregation of nanocrystals, leading to the largest specific surface area (16.5–18.4 m2·g–1). For the La2TiO5 crystals grown using the chloride fluxes, the wavelength of the absorption edges was approximately 320 nm (E g = 3.87 eV), whereas the absorption edges exhibited by the LaTiO2N crystals obtained by nitridation were approximately 600 nm (E g = 2.06 eV). Particularly, the LaTiO2N crystals prepared in this study by nitriding the precursor La2TiO5 crystals did not show a noticeable absorption in the near-infrared region above 600 nm, which is generally attributable to some reduced Ti3+ species and nitrogen deficiency, even after a long nitridation process. The fabricated LaTiO2N crystals with low defect density will be advantageous for various applications that specially require higher specific surface area.

KCl flux-induced growth of isometric crystals of cadmium-containing early transition-metal (Ti 4+ , Nb 5+ , and Ta 5+ ) oxides and nitridability to form their (oxy)nitride derivatives under an NH 3 atmosphere for water splitting application

“…In this study, we have attempted to experimentally validate the results of previous theoretical calculations predicting the possible formation of the CdTiO 3-x N y , CdNbO 2 N, and CdTaO 2 N phases by applying conventional one- [11] and two-step [12,13] [16] have already been reported, this study will seek to understand the flux-induced growth of their isometric crystals and conversion to their corresponding (oxy)nitride derivatives under high-temperature NH 3 atmosphere. The effects of solute concentration and cooling rate on the flux growth of the CdTiO 3 , Cd 2 Nb 2 O 7 , and Cd 2 Ta 2 O 7 crystals and the impacts of nitridation temperature and time on the formation of their corresponding (oxy)nitride derivatives under high-temperature NH 3 atmosphere were investigated.…”

“…highly porous structures were formed possibly through the lattice condensation process caused by the replacement of O 2− with N 3− in the anionic network and the volatilization of cadmium[13,[23][24][25]. The surface textures of the samples obtained by nitridation of the Cd 2 Nb 2 O 7 and Cd 2 Ta 2 O 7 crystals are nearly identical compared to that of the sample obtained by nitriding the CdTiO 3 crystals.…”

Preparation of LaTiO2N Using Hydrothermally Synthesized La2Ti2O7 as a Precursor and Urea as a Nitriding Agent

“…In contrast, a product with a greenbrown color was obtained after calcination of the unwashed product (the mixture of La 2 Ti 2 O 7 and La(OH) 3 ) and urea (Figure 2d). [54] These results indicated that while pure La 2 Ti 2 O 7 cannot be directly nitrided to LaTiO 2 N by urea, this nitridation can be achieved when the precursor contains La(OH) 3 . Though some diffraction peaks assignable to the La 2 O 3 phase also co-existed, these impurities could be removed by washing with acidic solution (details are discussed later).…”

UVA- and visible-light-driven photocatalytic activity of three-layer perovskite Dion-Jacobson phase CsBa2M3O10 (M=Ta, Nb) and oxynitride crystals in the removal of caffeine from model wastewater

“…The highest photocatalytic activity under UVA light was related to the crystal structure. Moreover, the presence of defects in LaTiO2N related to the reduced titanium species and anion vacancies was observed to be responsible for its higher photocatalytic activity under UVA light[47][48][49].The lowest photocatalytic activity under UVA light was obtained with CsBa2Ta3O10 andCaTaO2N crystals (40-45% CAF removal) due to their larger crystal sizes and band gap energies. Although the highest removal of CAF from aqueous solution by the flux-grown crystals was within the first 20 min of photodegradation reaction, the complete removal of CAF was not observed within 60 min of photodegradation reaction.…”

“…Interestingly, among all samples, only LaTiO2Ncrystals showed a strong background absorption beyond the absorption edge wavelength, which is generally characteristic of the presence of defects related to reduced titanium species and anion vacancies[47][48][49] because of higher electronegativity of Ti 4+ (χ = 1.54) compared to Ta 5+ (χ = 1.50). The light absorption at longer wavelength of CsBa2Nb3O10 with respect to CsBa2Ta3O10 can be related to the fact that Nb 5+ (χ = 1.60) is more electronegative than Ta 5+ (χ = 1.50).…”