Zn-Cr LDH/g-C3N4 heterostructure for estrone photodegradation: what is the effect of synthesis methods on materials’ properties and degradation efficiency?

Photocatalysis is a promising water purification technology, that harnesses the power of light-induced reactions to degrade contaminants. Utilizing visible light in the reactions is imperative, as it comprises a significant portion of the solar spectrum, which aligns with sustainable practices. Graphitic carbon nitride (g-C₃N₄) is a photocatalyst active in visible light, characterized by high chemical and thermal stability, ease of synthesis, and relatively low cost. However, its performance is limited by charge carrier mobility and charge recombination. These limitations might be addressed by synthesizing heterostructures, i.e. composites containing two or more semiconductors. Heterojunctions between g-C₃N₄ and layered double hydroxides (LDH) have shown increased photocatalytic efficiency. LDH are crystalline, hydrotalcite-like materials that enhance the charge separation and light absorption by the heterostructure. These materials can be obtained through various synthesis methods, including coprecipitation and hydrothermal treatment, influencing their properties. Thus, this study aimed to compare heterostructures obtained by different synthesis routes and asses their photocatalytic efficiency.

Three synthesis methods were used to obtain Zn-Cr LDH/g-C₃N₄ heterostructures: coprecipitation, adsorption/coprecipitation, and hydrothermal treatment. For g-C₃N₄ preparation, melamine was heated at 550°C for 5 h. The coprecipitation method involved dissolving zinc and chromium nitrates in DI water, then adding it to a suspension of g-C₃N₄, while simultaneously adding a solution of NaOH and Na₂CO₃. The adsorption/coprecipitation method was based on adding zinc and chromium nitrates to a g-C₃N₄ suspension, then after 30 minutes adding a solution of NaOH and Na₂CO₃. The hydrothermal method included dissolving zinc and chromium nitrates in the g-C₃N₄ suspension, adding NaOH and Na₂CO_3, then placing the suspension in an oven for 24 h at 100°C. The obtained materials were characterized by XRD, SEM/TEM, XPS, TRFL, N₂ adsorption/desorption, and photoelectrochemical measurements. The photocatalytic activity of heterostructures was assessed in batch experiments in aqueous solutions containing 1 ppm of estrone, with a 150 W LED lamp as a source of visible light.

The XRD patterns of obtained materials confirmed the formation of layered phases. The hydrothermal heterostructure was characterized by sharper reflections, which suggested higher structural order and/or larger crystallites. The average specific surface values (S_BET) showed that the hydrothermal composite was characterized by the highest S_BET - 124 m²/g. This indicated its higher porosity, compared to the materials obtained by other methods. The TRFL studies proved that the heterojunctions have lower recombination rates and longer lifetimes of charge carriers. The results of photochemical measurements suggested superior electronic conductivity and charge transfer efficiency for the hydrothermal heterostructure. The determined photoelectrochemical properties agreed well with the photocatalytic activity of heterostructures, which was the highest for the hydrothermal composite. This material led to a 99.5% estrone concentration loss after 60 minutes of reaction, in comparison to 49.9% and 75.8% loss assessed for the materials obtained by coprecipitation and adsorption/coprecipitation, respectively. The results of this study demonstrated the advantage of using the hydrothermal method to obtain LDH/g-C₃N₄ heterostructures with a high efficiency of pollutant photodegradation from aqueous solutions.

This research was funded by the AGH University of Science (Krakow, Poland), grant number 16.16.140.315.