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II. Meanwhile, CO2 from the air is captured by the leaf through diffusion process and brought to the chloroplast. Finally, the chlorophyll II excited by incident light catalyzes water split and releases hydrogen and oxygen. After that, the oxygen atoms are formed dioxygen and escape from chloroplast to air. Meantime, the hydrogen atoms react with CO2 molecules into hydrocarbons, such as glucose. Until now, the plants fulfill the great photosynthesis process, which creates the suitable ecosystem for advanced living things, in which chemical fuels and O2 can be further consumed to provide energy for aerobic organisms.
By systematically analyzing the process of natural photosynthesis, it can be found that CO2 and water are the initial reactants and chlorophyll II as a catalyst is necessary to accelerate the reaction under sunlight irradiation. Generally speaking, CO2 and water are abundant and cheap in the Earth, which can be obtained easily. Nevertheless, the reaction between CO2 and water cannot proceed in thermodynamic aspect [6]. Therefore, for chemists, it is necessary to develop an efficient catalyst for triggering the photosynthesis reaction, which helps that the O2 and chemical fuels can be produced in a cheap and sustainable way in a factory. Based on this inspiration, simulating natural photosynthesis process is a relentless pursuit for scientists, which is called as artificial photosynthesis [7].
2.2.2 Artificial Photosynthesis
Scientists around the world have been trying to replicate the natural reactions that occur during photosynthesis and have come across the science of artificial photosynthesis. The term artificial photosynthesis is used to refer to any mechanism made to capture light and store energy from the sun in chemical bonds of a solar fuel [8]. In general, as shown in Figure 2.2, the artificial photosynthesis includes three main steps: (i) The first step in artificial photosynthesis is for the reactants coming together. The reactants include sunlight along with water and carbon dioxide that is available in the atmosphere. (ii) These reactants then go through the whole process of photosynthesis artificially. Scientists have been known to use artificial leaves to split water, producing both oxygen and hydrogen. They are now creating artificial leaves using ruthenium and manganese complexes to mimic the natural process of photosynthesis (redox reaction of nicotinamide adenine dinucleotide phosphate (NADPH/NADP+)). (iii) The products of the reactions are created as soon as water is split, producing both oxygen and hydrogen. The hydrogen is then either used directly as a fuel or a reductant for carbon dioxide to produce organic fuels [10].
Figure 2.2 Schematic diagrams of (a) natural photosynthesis and (b) artificial photosynthesis based on molecular systems.
Source: Liu et al. [9].
2.3 Principles of Photocatalysis
The term “photocatalysis” is often used in papers on catalytic reaction excited by simulated or natural light irradiation, where a catalyst has been used as a reaction center [11]. To simulate the efficient natural photosynthesis reaction, the use of a catalyst is inevitable in the view of physiochemical field. Because biological scientists find that in leave cells the enzyme is the key role in prompting the non‐thermodynamic reaction, i.e. carbon dioxides reacting with water to produce dioxygen and hydrocarbons. Therefore, to accelerate this reaction under certain conditions, a photocatalyst often is used as similar in many industrial fields, such as sulfuric acid products, synthetic ammonia, and syngas. As shown in Figure 2.3, in general, there are many active sites existing on the surface of catalyst, which can act as reaction site and accelerate the transformation of substrates to products under certain reaction conditions [12]. Similarly, the catalytic reaction process is triggered at the active sites on the surface of photocatalyst under light irradiation. It is reported that the electrons and holes can be produced in the photocatalysts, which further participate the redox reaction [13].
Figure 2.3 (a) Traditional catalytic processeson the surface of catalysts. (b) Classic photocatalytic paths over photocatalysts.
In photocatalysis, many photocatalysts have been developed and investigated in the past decades [14]. Here, taking excessively studied semiconductor photocatalyst as an example, the whole photocatalytic process is described. Semiconductor is a kind of material with electrical conductivity between conductor (such as metals) and insulator (such as ceramic) [15]. The conductivity of a semiconductor usually increases with the increase of the temperature, which is opposite to that of a metal. The unique electronic property of a semiconductor is characterized by its valence band (VB) and conduction band (CB). The VB of a semiconductor is formed by the interaction of the highest occupied molecular orbital (HOMO), while the CB is formed by the interaction of the lowest unoccupied molecular orbital (LUMO). There is no electron state between the top of the VB and the bottom of CB. The energy range between CB and VB is called forbidden band gap (also called energy gap or bandgap), which is usually denoted as Eg, as shown in Figure 2.4.
Figure 2.4 A band‐gap diagram showing the different sizes of band gaps for conductors, semiconductors, and insulators.
The band structure, including the band gap and the positions of VB and CB, is one of the important properties for a semiconductor photocatalyst, because it determines the light absorption property as well as the redox capability of a semiconductor [16]. As shown in Figure 2.5, the photocatalytic reaction initiates from the generation of electron–hole pairs upon light irradiation. When a semiconductor photocatalyst absorbs photons with energy equal to or greater than its Eg, the electrons in VB will be excited to CB, leaving the holes in VB. These photogenerated electron–hole pairs may further be involved in the following three possible processes: (i) successfully migrate to the surface of semiconductor, (ii) be captured by the defect sites in bulk and/or on the surface region of semiconductor, and (iii) recombine and release the energy in the form of heat or photon. The last two processes are generally viewed as deactivation processes because the photogenerated electrons and holes do not contribute to the photocatalytic reaction. Only the photogenerated charges that reached to the surface of semiconductor could be available for photocatalytic reactions. The defect sites in the bulk and on the surface of semiconductor may serve as the recombination centers for the photogenerated electrons and holes, which will decrease the efficiency of the photocatalytic reaction.
Figure 2.5 Proposed mechanism of photocatalytic reactions on semiconductor photocatalysts.
Source: Ma et al. [16].
Efficient charge separation is the most important factor that determines the photocatalytic activities. Various strategies could be applied for improving charge separation efficiency [17]. For example, preparation of semiconductor photocatalysts at high temperatures may lead