What Role Does the Incident Light Intensity Play in Photocatalytic Conversion of CO₂: Attenuation or Intensification?

Abstract

Artificial photoreduction of CO2 is vital for the sustainable development of human beings via solar energy storage in stable chemicals. This process involves intricate light‐matter interactions, but the role of incident light intensity in photocatalysis remains obscure. Herein, the influence of excitation intensity on charge kinetics and photocatalytic activity is investigated. Model photocatalysts include the pure graphitic carbon nitride (g‐C3N4) and g‐C3N4 loaded with noble/non‐noble‐metal cocatalysts (Ag, TiN, and CuO). It is found that the increase of light intensity does not always improve the electron utilization. Overly high excitation intensities cause charge carrier congestion and changes the recombination mechanism, which is called the light congestion effect. The electron transport channels can be established to mitigate the light‐induced effect via the addition of cocatalyst, leading to a nonlinear growth in the reaction rate with increasing light intensity. From experiments and simulations, it is found that the light intensity and active site density should be collectively optimized for increasing the energy conversion efficiency. This work elucidates the effect of light intensity on photocatalytic CO2 reduction and emphasizes the synergistic relationship of matching the light intensity and the photocatalyst category. The study provides guidance for the design of efficient photocatalysts and the operation of photocatalytic systems.

Revealing the stochastic kinetics evolution of photocatalytic CO2reduction

“…A high light intensity enhances the total electron concentration of photocatalysts, but it also affects the probability density distribution of surface electrons at various active sites in our previous work. 32…”

“…Each active site is assumed to be non-overlapping. According to the approximate size of photocatalysts (1.5 μm) and cocatalyst (10 nm) obtained by SEM results and our previous work, 32 the maximum number is assumed as 900. Besides, too few reaction sites will result in a very low reaction rate in the simulation process which is far from the experimental activity.…”

“…According to the result of previous works, the recombination probability is nonlinear and positively correlated with light intensities. 32 The recombination rate is enhanced due to a high concentration of photogenerated electrons. And the subsequent photoluminescence experiments in this work also show that with the increase in temperature, trap-assisted recombination begins to strengthen.…”

“…30,31 For instance, the density and recombination rate of charge carriers were reported to vary nonlinearly with light intensity. [32][33][34] The collision of surface molecules and the phonon-assisted transition of charge carriers may be changed by a rise in system temperature. 35,36 So far, many photocatalytic models have been established based on semiconductor equations for analyzing the mechanism of photocatalytic processes.…”

“…54,55 Electrons and holes move freely in photocatalysts, and a multiple-trapping method is utilized for calculating the migration of photogenerated charge carriers, programming details of this method can be found in previous works. 32,42,56 While charge carriers are moving within photocatalysts, they may be captured by defects or surface active sites. The defect filled by an electron may become the recombination center.…”