Abstract:

The thermodynamics and kinetics of photocatalytic processes provide the scientific foundation for the optimization of reaction conditions and establishment of reaction mechanisms. Because of the limited availability of techniques that can provide in situ thermodynamics coupled with spectral information during photo-driven processes, research regarding the thermodynamics of the photo-driven processes is rare and in-depth studies on their kinetics remain inadequate. Herein, a novel photocalorimetry-fluorescence spectroscopy system composed of a photocalorimeter and laser-induced fluorescence spectrometer based on a 405 nm laser was developed. This system could simultaneously monitor the thermal and spectral information during photocatalytic processes, providing a correlation between nonspecific thermodynamic and specific molecular fluorescence spectral results. A highly efficient, bionic Z-type g-C₃N₄@Ag@Ag₃PO₄ nano-composite photocatalyst was developed and characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In situ thermodynamic and spectral kinetic information for Rhodamine B (RhB) degradation on g-C₃N₄@Ag@Ag₃PO₄ was obtained at five temperatures by synchronously monitoring the calorimetric and spectrometric results using the newly developed photocalorimetry-fluorescence spectroscopy system and the effects of temperature on various parameters were investigated. The catalytic decomposition comprised three stages at different temperatures: (ⅰ) photoresponse of RhB and photocatalyst, (ⅱ) competition between the endothermic photoresponse and exothermic RhB photodegradation, and (ⅲ) stable exothermic period of RhB photodegradation. The in situ heat flux and fluorescence spectra could be combined to estimate the concentration characteristics of the different photocatalytic reactions: (1) the spectral information suggested that the competitive endothermic and exothermic reactions followed first order kinetics, and the reaction rate constants (k) at five temperatures were calculated. The results also indicated that the degradation rate increased with increasing temperature. The activation energy at each temperature interval was determined, and yielded an average value of 23.82 kJ·mol⁻¹. (2) The calorimetric results revealed that the subsequent stable exothermic period was a pseudo-zero-order process. The exothermic rates at 283.15, 288.15, 293.15, 298.15, and 303.15 K were determined to be 0.4668 ± 0.3875, 0.5314 ± 0.3379, 0.5064 ± 0.3234, 0.5328 ± 0.3377, and 0.5762 ± 0.3452 μJ·s⁻¹, respectively. The novel photocalorimetry-fluorescence spectroscopy technique could concurrently obtain thermodynamic, thermo-kinetic, and molecular spectral information, allowing for the direct correlation of the thermodynamics, thermo-kinetics, and spectrokinetics with the underlying mechanisms of the reaction. This in situ technique integrated the thermal information with spectral information for improved understanding of the microscopic mechanisms of photo-driven processes, providing scientific support for the establishment of photothermal spectroscopy.

Key words: g-C₃N₄@Ag@Ag₃PO₄, Photocatalysis, Photocalorimetry-fluorescence spectroscopy, Thermodynamics, Temperature effect