New Insights into the Fundamental Principle of Semiconductor Photocatalysis

Abstract

Although photocatalysis has been studied for many years as an attractive way to resolve energy and environmental problems, its principle still remains unclear. Some confusions and misunderstandings exist in photocatalytic studies. This research aims to elaborate some new thoughts on the fundamental principle of semiconductor photocatalysis. Starting from the basic laws of thermodynamics, we first defined the thermodynamic potential of photocatalysis. A concept, the Gibbs potential landscape, was thus then proposed to describe the kinetics of photocatalysis. Photocatalysis is therefore defined as a light-driven chemical reaction that still needs heat activation, in that light and heat play their different roles and interact with each other. Photocatalysis should feature an activation energy functioning with both light and heat. The roles of light and heat are correlative and mutually inhibit at both levels of thermodynamics and kinetics, so it is impossible for an intrinsic light-heat synergism to happen. Two criteria were further proposed to determine an intrinsic light-heat synergism in photocatalysis. Experiments were also carried out to calculate the thermodynamic potential and can agree well with the theory. Experimental results proved that there is no intrinsic light-heat synergism, in accordance with our theoretical prediction. This research clarified some misunderstandings and gained some new insights into the nature of photocatalysis; this is important for the discipline of semiconductor photocatalysis.

Figure 1

(A) Schematic of the kinetic processes of the charge carriers (holes and electrons) generated from the electronic transition from the valence band to the conduction band under light illumination. (1. Surface recombination; 2. Bulk recombination; 3. Electron transfer; 4. Hole transfer). The electron transfer and hole transfer are in direct connection with the photocatalytic effect. (B) Diagrammatic comparison between the band potentials (the conduction band potential and the valence band potential) and the reduction potential of an electron acceptor (A/A^–) and the oxidative potential of a hole acceptor (D/D⁺). The electric potentials of hydrogen evolution and oxygen evolution are also shown. The electrons and holes can be generated by light-induced electronic excitation; they can also be generated by heat via the lattice–electron interaction in a statistic way.^,,

Figure 2

Relation between the interfacial transfer of electrons from the CB to the VB of a semiconductor and the induced photocatalytic effect.

Figure 3

(A) Gibbs free energy landscape (G-potential landscape) of downhill photocatalytic reactions via the IT, such as PCOs; (B) G-potential landscape (G-potential landscape) of uphill photocatalytic reactions via the IT, such as the water splitting and the CO₂ reduction.

Figure 4

Kinetic diagram for determining the synergistic effect in semiconductor photocatalysis. (A) To determine the synergistic effect of heat on light; (B) to determine the synergistic effect of light on heat.

Figure 5

Schematic diagram of the electron IT pathway for organic PCO.

Figure 6

(A) Linear dependence of the ΔG_IT and semi-log dependence of the acetone conversions on the light intensities (A) and temperatures (B).

Figure 7

Correlation between the experimental-derived acetone photocatalytic conversions in the case of different light intensities (A) and temperatures (B).

Figure 8

(A) Arrhenius plots of the PCOs and thermocatalytic oxidations of acetone by TiO₂ at different temperatures; (B) Arrhenius plot of the photocatalysis of the PCOs of formaldehyde over TiO₂ at different temperatures; (C) log–log dependence of the acetone PCO rates on light intensities (experiments were done at 65 °C); (D) log–log dependence of the formaldehyde PCO rates on light intensities (the experiments were done at 125 °C).