Solar-to-Chemical Conversion

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 E g, as shown in Figure 2.4.

Figure 2.4A 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 E g, 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 2.5Proposed 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 to high crystallinity that diminishes the formation of charge recombination defect sites. Construction of various kinds of nanostructures such as nanowires (belts) and nanosheets may also facilitate charge transportation and promote charge separation efficiency. As compared to nanoparticle, one‐dimensional nanostructures exhibit better photocatalytic activity because they have better charge mobility and can reduce the charge recombination. Furthermore, creation of “junctions” with built‐in electric fields or chemical potential differences is also an effective strategy for improving charge separation efficiency [18]. The surface catalytic reaction is a successive step of charge separation. In principle, a photocatalytic reaction consists of two half‐reactions, reduction reaction and oxidation reaction. The electrons in CB may initiate the reduction reaction, and the reduction capability is determined by the position of CB; the holes in the VB involve the oxidation reaction, and the oxidation capability is determined by the position of VB. For the water splitting reaction, the position of CB of a semiconductor photocatalyst should be more negative than the redox potential of H +/H 2(0 V vs. normal hydrogen electrode [NHE], pH = 7), while the energy level of VB should be more positive than the redox potential of O 2/H 2O (1.23 V vs. NHE, pH = 7). Sometimes, some particular surface sites of a semiconductor can act as the active centers themselves, especially for oxidation reaction on the surface of metal oxide semiconductors. However, in most cases, efficient photocatalytic reactions proceed only after loading noble metal and oxide cocatalysts on semiconductors.

Hydrocarbons as important products of photosynthesis provide almost all energy for living things that take advantage of chemical energy to maintain activities by oxidizing organics. In photosynthesis system, carbon dioxide is the mainly carbon source and transformed into hydrocarbons. Inspired by the nature, scientists attempt to hydrogenate CO 2to produce hydrocarbons as substitutes to traditional fossil fuels. In general, CO 2is one of the most thermodynamically stable carbon compounds. The photocatalytic reduction of carbon dioxide to hydrocarbons requires to consume large amounts of energy for dissociation of the C=O bond and formation of C—H bonds. Since the carbon atoms in CO 2are in the highest oxidation state, this process may occur only with the participation of reducing agents providing a certain amount of electrons. On the other hand, the corresponding protons participate oxidative reactions, such as water oxidation. Meanwhile, hydrogen in water can further give rise to CO 2reduction for formation of C–H. However, the reduction of CO 2by water to give organic compounds such as methanol or methane entails a reaction with a high positive Gibbs free energy change: CO 2+ 2H 2O → CH 3OH + 1.5O 2(Δ G 0= 702.2 kJ mol −1); CO 2+ 2H 2O → CH 4+ 2O 2(Δ G 0= 818.3 kJ mol −1). Therefore, to overcome these energy obstacles requires external energy source such as thermal energy, plasma, electrics, and solar irradiation. For example, in 1973, Inoue et al. reported that HCHO and CH 3OH were produced by the reduction of CO 2with H 2O in aqueous suspension systems under Xe lamp irradiation and electric field involving a variety of semiconductor photocatalysts such as TiO 2, ZnO, and WO 3[19]. Meanwhile, trace formic acid and methane were detected over specific photocatalysts (SiC, GaP) by using special quantitative analysis methods. In contrast, in the absence of light source, no reduced products were detected in these systems, which strongly demonstrate that light excitation triggers the reduction reaction between CO 2with H 2O in room temperature and normal pressure over the semiconductor photocatalysts. As shown in Figure 2.6, it is demonstrated that there are many reaction paths in reduction of CO 2under external field excitation, leading to the diversity of products so that selectivity is another important parameter to determine the performance of CO 2reduction photocatalysts. The possible reactions that can occur in the reduction of CO 2in the aqueous medium, under band‐gap illumination, ranging from one‐ to eight‐electron transfer, with thermodynamic potentials vs. NHE, are mentioned below:

(2.2)