Understanding the Crystallization Mechanism of Delafossite CuGaO₂ for Controlled Hydrothermal Synthesis of Nanoparticles and Nanoplates

The delafossite CuGaO₂ is an important p-type transparent conducting oxide for both fundamental science and industrial applications. An emerging application is for p-type dye-sensitized solar cells. Obtaining delafossite CuGaO₂ nanoparticles is challenging but desirable for efficient dye loading. In this work, the phase formation and crystal growth mechanism of delafossite CuGaO₂ under low-temperature (<250 °C) hydrothermal conditions are systematically studied. The stabilization of Cu^I cations in aqueous solution and the controlling of the hydrolysis of Ga^III species are two crucial factors that determine the phase formation. The oriented attachment (OA) growth is proposed as the crystal growth mechanism to explain the formation of large CuGaO₂ nanoplates. Importantly, by suppressing this OA process, delafossite CuGaO₂ nanoparticles that are 20 nm in size were successfully synthesized for the first time. Moreover, considering the structural and chemical similarities between the Cu-based delafossite series compounds, the understanding of the hydrothermal chemistry and crystallization mechanism of CuGaO₂ should also benefit syntheses of other similar delafossites such as CuAlO₂ and CuScO₂

Synthesis, Exfoliation, and Electronic/Protonic Conductivity of the Dion–Jacobson Phase Layer Perovskite HLa₂TiTa₂O₁₀

Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion–Jacobson phase HLa₂Ti₂TaO₁₀ in the temperature range of interest (250–475 °C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa₂Ti₂TaO₁₀, which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa₂Ti₂TaO₁₀·<i>n</i>H₂O (<i>n</i> = 1–2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa₂Ti₂TaO₁₀·<i>n</i>H₂O lost intercalated water at temperatures between 100 and 200 °C, but X-ray diffraction patterns up to 500 °C did not show evidence of collapse of the interlayer galleries that has been observed with the structurally similar compound HCa₂Nb₃O₁₀. Under humid hydrogen atmosphere, the conductivity of HLa₂Ti₂TaO₁₀ followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10^–9 to 10^–5 S cm^–1 depending on the preparation conditions and temperature. Modification of the stoichiometry to produce A-site or B-site (vacancy or substitution) defects decreased the conductivity slightly. The conductivity was approximately 1 order of magnitude higher in humid hydrogen than in humid air atmospheres, suggesting that the dominant mechanism in the intermediate temperature range is electronic. A-site substitution (Sr²⁺ for La³⁺) beyond the Ruddlesden–Popper phase limit converted the layered pervoskite to a cubic perovskite Sr_2.5□_0.5Ti₂TaO₉ with 2 orders of magnitude higher conductivity than HLa₂Ti₂TaO₁₀ at 475 °C