strategies in addressing the low-efficiency issue will be discussed.Photocatalysis mainly deals with electron and energy transfer processes. Prior to the discussion, it is necessary to learn the basic behaviors of the excited state of a molecule, which can enhance the understanding on electron transfer and energy dissipation in semiconductors. Generally, the first process is the absorption of a photon by a molecule (in the time scale of femtoseconds (fs)), where the ground state is lifted energetically to the first excited singlet state. Subsequently, a simplified Jablonski diagram is reconsidered, [2] as shown in Figure 1. The charge carriers generated in one molecule experience several possible processes with varying possibilities: [3] a) Vibrational relaxation (VR): it is relaxation of excited state electrons to the lowest energy level which generally occurs in picoseconds (ps). It can happen from each excited state to each non-excited state including the ground state. This VR results in loss of energy because excessive vibrational energy is converted into heat. b) Fluorescence: After the relaxation to the lowest vibrational level, the excited molecule can finally get back to the ground state by emitting a photon. This is named as fluorescence, which occurs in a relatively long time, ranging from ps to nanoseconds (ns). c) Internal conversion (IC): it is a crossover process in which an electronically excited molecule moves from one electronic state to a lower one of the same multiplicity (singlet-to-singlet or triplet-to-triplet states) and can be measured from ps to fs. [4] d) Intersystem crossing (ISC): A transition from one electronic state to another one with a different spin multiplicity is called ISC. e) Phosphorescence: After the molecule transitions through ISC to the triplet state, further deactivation occurs through phosphorescence. And its lifetime ranges from one millisecond (ms) to hundreds of seconds. Apart from these, other processes such as vibrational cooling are also possible, which are not illustrated here.The above-mentioned processes in a single molecule can be used as a reference when discussing a semiconductor photocatalyst. Analogously, electrons are excited to the conduction band (CB) after absorption. Afterward, they will undergo several decay processes or they will finally migrate to the surface and participate in a specific redox reaction. These decay processes are briefly introduced here by comparing with those in a molecule (Figure 1): a) Relaxation of electrons to the lowest CB energy states. b) Radiative recombination of electrons and holes via emitted as fluorescence; c) Non-radiative decay, also referred to as Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of...
