Ferroelectric-assisted enhancement of the photocatalytic activity of g-C3N4/TiO2 nanotubes heterojunctions through the addition of strontium

The effect of adding strontium to nanocomposites formed of g-C3N4/TiO2 nanotubes (TiNT) (g-C3N4: 4.0 wt%) on the photocatalytic activity was herein investigated using formic acid as a model target pollutant. g-C3N4 material was prepared by a solvothermal method using a mixture of urea and cyanuric chloride as precursors and acetonitrile as a solvent at 220 °C for 15 h, while TiNT material was obtained by the alkaline hydrothermal method of P25 at 130 °C for 20 h. A series of xSr-gC3N4/TiNT nanocomposites were then synthesized through progressive incorporation of strontium (from 0.2 to 1.0 wt%) at the surface of the composites. Samples were then characterized mainly using N2 adsorption–desorption measurements, X-ray diffraction, UV–vis diffuse reflectance, Raman and photoluminescence (PL) spectroscopies, and scanning and transmission electron microscopy (with energy-dispersive X-ray spectroscopy and Z-mapping). The Sr-containing g-C3N4/TiNT composites were then also fully characterized by electrochemical impedance spectroscopy (EIS) for their dielectric properties, while a deep kinetics analysis was also performed. Such an approach allowed us to establish a correlation between photocatalytic performance and the material's propensity to be polarized. Results emphasize the beneficial role of ferroelectricity induced by adding strontium. Indeed, enhancement of the photocatalytic activity by 2.3 times compared to a Sr-free counterpart was related to the material's ability to be polarized for Sr loadings ≤ 0.6 wt%. At higher Sr loadings, supplementary factors (decrease of the anatase crystallinity, formation of TiO2(B), disruption of the g-C3N4/TiO2 interaction, or lower mobility of photogenerated charges) negatively impact the activity, counterbalancing partly the beneficial ferroelectric effect. In this way, this study emphasizes the role played by ferroelectric effects in enhancing the photocatalytic ability of a given semiconductor.