With the rapid development of industry, the discharge of dye wastewater from the textile and printing industries poses a huge threat to aquatic ecosystems and human health. Exploring efficient catalytic technologies to reduce water pollution is highly anticipated. Photocatalysis, as an efficient and environmentally friendly advanced oxidation technology for decomposing organic dyes, has attracted considerable attention. However, the low utilization rate of solar energy and severe photogenerated carrier recombination restrict the practical application of photocatalysis. In addition to solar energy, the thermal energy generated by cold-hot alternation can be also used as a clean and renewable energy source to induce positive and negative charges and generate pyroelectric potential, thereby achieving pyrocatalysis. Combining pyrocatalysis with photocatalysis can simultaneously collect solar energy and alternating heat energy, and the pyroelectric potential can serve as an internal driving force for dissociating photo generated exciton. This work constructed g-C₃N₄/NaNbO₃ composite materials for the absorption of g-C₃N₄ to visible light to enhance the utilization efficiency of sunlight. The Mott-Schottky curve was obtained by electrochemical workstation to analyze the conduction type and flat band potential of g-C₃N₄ and NaNbO.

A hydrated NaNbO₃ sample was obtained via hydrothermal reaction at 160 ℃ for 4 h with Nb₂O₅ and NaOH as raw materials. The hydrated NaNbO₃ prepared was then mixed, ground with a certain mass of melamine powder (in a mass ratio of 1∶2), and sintered at 500 ℃ for 2 h to obtain g-C₃N₄/NaNbO₃ composite material. The phase, morphology, and optical properties of the product was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible spectroscopy (UV-Vis). The pyrophotocatalytic performance of g-C₃N₄/NaNbO₃ composite materials was evaluated via degrading mixed dyes (i.e., methylene blue MB, methyl orange MO, rhodamine RhB, and acid orange AO7) under light irradiation and cold-hot cycling. The environmental adaptability of g-C₃N₄/NaNbO₃ composite materials was analyzed by changing the concentration, pH value, and water source of the mixed dye solution. The stability of g-C₃N₄/NaNbO₃ composite material was estimated through five repeated uses and the XRD patterns before and after use. The influence of g-C₃N₄/NaNbO₃ on the degradation performance of the mixed dyes was analyzed via adding different active substance trapping agents, and the main active species in the catalytic reaction was determined. The work function was calculated based on density functional theory (DFT) to analyze the electron transfer after contacting g-C₃N₄ and NaNbO₃.

The g-C₃N₄/NaNbO₃ composite material prepared by a two-step method of hydrothermal and mixed sintering is sheet-like g-C₃N₄ covering the surface of NaNbO₃ nanorods with the diameters of 100-300 nm. The photocatalytic degradation efficiencies of g-C₃N₄, NaNbO₃, and g-C₃N₄/NaNbO₃ composite materials for RhB are 19%, 71%, and 84%, respectively. Combined with the pyroelectric catalysis, the photocatalytic efficiency of g-C₃N₄/NaNbO₃ increases to 97.3%. The results show that the formation of g-C₃N₄ and NaNbO₃ composites can enhance the photocatalytic performance, and the pyroelectric potential generated by temperature fluctuations can further separate the photo-generated electron hole pairs, thus improving the photocatalytic degradation efficiency. Under illumination and cold-hot cycling, g-C₃N₄/NaNbO₃ exhibits a pyrophotocatalytic degradation efficiency of 96.9% for mixed dyes, with a degradation reaction rate of 0.038 6 min^-1. The degradation efficiency of g-C₃N₄/NaNbO₃ decreases with the increase of initial concentrations of mixed dye solution (i.e., 2×10^-5 mol/L, 4×10^-5 mol/L, 6×10^-5 mol/L and 8×10^-5 mol/L). The pyro-photocatalytic degradation efficiency of the mixed dye is 96.9%, 70.8%, and 54.3% when the pH value of the solution is 2.6, 7.1 and 11, respectively, indicating that g-C₃N₄/NaNbO₃ composite catalyst is suitable for working in acidic and neutral environments. Also, g-C₃N₄/NaNbO₃ composite material has a better catalytic activity in deionized water and tap water, while its pyrophotocatalytic performance reduces in river water and lake water. The experimental results of adding sacrificial agents show that ·O₂^-, ·OH, and h⁺ are all active substances in the thermal electric photocatalytic degradation, among which ·O₂⁻ and ·OH play a particularly significant role in the catalytic degradation. The results of UV-Vis spectra, the Mott-Schottky curve and DFT calculations indicate that g-C₃N₄/NaNbO₃ composite material broadens the absorption range of the solar spectrum, forms an S-type heterojunction, and the pyroelectric potential generated by the pyroelectric effect promotes the dissociation of photo generated carriers, thus improving the pyrophotocatalytic performance.

The constructed g-C₃N₄/NaNbO₃ composite material utilized a wide solar spectral absorption range and a photothermal effect of g-C₃N₄. The formation of S-type heterojunction between g-C₃N₄ and NaNbO₃ was conducive to the separation and migration of photo generated carriers. Also, the pyroelectric potential induced by temperature fluctuation could result in band bending, and further promoted the separation of photo generated carriers in NaNbO₃, demonstrating an enhanced pyrophotocatalytic performance. This work could provide an efficient catalyst that simultaneously utilized solar energy and environmental thermal energy to achieve sewage treatment and environmental remediation.