Proton conductivity of fluorite based rare earth titanates (LnxTi1-x)4O8-2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765).

Solid solutions of rare earth titanates with high contents of rare earth oxides of up to 50-62% have been synthesized by the co-precipitation method and their structure, microstructure and conductivity in dry and wet air have been studied. Proton conductors have been found for the first time in solid solutions of rare earth titanates with a high content of Ln2O3 (>50%) with a nominal formula composition of (LnxTi1-x)4O8-2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765). Among (LnxTi1-x)4O8-2x (Ln = Yb, Er, Ho, x = 0.684), (HoxTi1-x)4O8-2x (x = 0.684) showed the maximum conductivity in wet air. In this context, four additional compositions (HoxTi1-x)4O8-2x (x = 0.718, 0.734, 0.75, and 0.765) were synthesized in the holmium series. An increase in the holmium content leads to an increase in the proton transfer coefficients; at the same time, a more complex nature of the dependence of the conductivity under dry and wet atmospheres is observed. For the fluorite-like solid solution (HoxTi1-x)4O8-2x (0.701 ≤ x ≤ 0.765), the proton transfer coefficients were found to be ∼0.9 in the range of 200-450 °C. As the temperature continues to rise, the proton conductivity decreases quite sharply and the transfer coefficient becomes as low as 0.3 at 700 °C. The increase in proton conductivity in the Yb-Er-Ho series is associated with an increase in the hydrophilic properties of rare earth cations. In the (HoxTi1-x)4O8-2x (x = 0.667 ≤ x ≤ 0.765) series, the conductivity in wet air was ∼1 × 10-6 S cm-1 at 450 °C for most compositions. The conductivity of ceramics with x = 0.701 and 0.75 is about 2 times higher, which may be due to the optimal size of pyrochlore nanodomains in the fluorite matrix for x = 0.701 and the formation of pure fluorite for x = 0.75, respectively.

Nitrogen-Doped Carbon Nanodots Produced by Femtosecond Laser Synthesis for Effective Fluorophores.

Understanding the effect of heteroatom doping is crucial for the design of carbon nanodots (CNDs) with enhanced luminescent properties for fluorescence imaging and light-emitting devices. Here, we study the effect and mechanisms of luminescence enhancement through nitrogen doping in nanodots synthesized by the bottom-up route in an intense femtosecond laser field using the comparative analysis of CNDs obtained from benzene and pyridine. We demonstrate that laser irradiation of aromatic compounds produces hybrid nanoparticles consisting of a nanocrystalline core with a shell of surface-bonded aromatic rings. These nanoparticles exhibit excitation-dependent visible photoluminescence typical for CNDs. Incorporation of nitrogen into pyridine-derived CNDs enhances their luminescence characteristics through the formation of small pyridine-based fluorophores peripherally bonded to the nanoparticles. We identify oxidation of surface pyridine rings as a mechanism of formation of several distinct blue- and green-emitting fluorophores in nanodots, containing pyridine moieties. These findings shed additional light on the nature and formation mechanism of effective fluorophores in nitrogen-doped carbon nanodots produced by the bottom-up route.