Formation of semiconductor composite

As mentioned in the beginning of this section, the principal of photocatalysis could be considered in three steps: 1) photoexcitation of charge carrier (electron and holes); 2) charge carrier separation and diffusion to the photocatalyst surface; 3) oxidation and reduction reaction on the photocatalyst surface. For the first step, it could be

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controlled by modifying the band structure of photocatalyst. However, in the second step, the recombination of photogenerated electron and hole occurs in photocatalysts, which means that the photoexcited charge carriers could recombine each other before the formation of oxidative radicals. The recombination of charge carriers reduces the quantum efficiency and thus, the efficiency of photocatalytic reaction could be improved via restraining or postponing the recombination electron-hole pairs.

Several methods could be used to inhibit recombination, such as increasing the crystallinity of photocatalyst for decreasing the density of crystal defect; and reducing the particle size of photocatalyst for shortening the diffusion pathway. One favored strategy is using different co-catalyst to reduce charge carrier recombination probability. Three types of co-catalyst are usually used, metal nanoparticle, conductive material and other photocatalysts.

Composite photocatalyst

In contrast with other strategy, combing two or three different photocatalysts to form heterojunction composites in order to reduce charge carrier recombination is widely used and reported. The basic principle for a photocatalyst composite in direct contact will be explained in the following part and corresponding examples will also be provided to support.

A system containing two different components will be adopted to clearly explicate the fundamental principle behind photocatalyst composite. In that case, the formed heterojunction composite could be classified into three different types, which are depicted in Figure 1.8.

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Figure 2.1 Different types of semiconductor heterojunctions

For type I heterojunction composite, both the VB and CB of semiconductor B are higher than that of semiconductor A. Hence, the photogenerated electron can transfer from CB (semiconductor B) to CB (semiconductor A); and the holes are simultaneously transferred from VB (semiconductor A) to VB (semiconductor B). As a result, electron and holes are spatially separated from each other and consequently, the reduction of charge carrier recombination are accomplished. CdS/TiO2 composite is a typical example of type I heterojunction composite system and have been widely investigated. The band gaps of CdS and TiO2 are 2.4 and 3.2 eV, respectively [268,269]. However, because the VB of CdS (+1.5 V vs SHE) is smaller than the VB (+2.87 V vs SHE) of TiO2 and the CB of CdS is higher than the VB of TiO2, thus the composite of these two photocatalysts belongs to the type I heterojunction composite. Under visible light irradiation, the photon can only excite electron-hole pairs in CdS but not in TiO2. Therefore, photogenerated holes accumulate in the CdS and the electrons transfer to TiO2 resulting in the separation of charge carrier. The as-prepared CdS/TiO2 composite have been widely used for the degradation of organic compounds or H2 production [270–273]. The photocatalytic activity of CdS/TiO2 has been significantly improved under visible light, comparing with that of each component. Afterwards, CdS have been combined with various photocatalyst,

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such as ZnO, K4Nb6O17, Ta2O5 and ZnFe2O4 for enhancing photocatalytic activity [274–

277]. Nowadays, several of type I photocatalytic composite systems have been developed to enhance photocatalytic activity [19,278,279].

For type II heterojunction composite, the transfer of charge carriers is as same as in type I. The only difference is that the band positions are even further so that the VB of semiconductor B is higher than CB of semiconductor A. This arrangement of band position is also known as broken-gap situation. However, this model have hardly been used to improve the separation of charge carriers for photocatalysis.

For type III heterojunction composite, the VB of semiconductor B is lower than that of semiconductor A, and the CB of semiconductor B is higher than that of semiconductor A. When the charge carriers are generated by photon with enough energy, the electrons can transfer from CB (semiconductor B) to CB (semiconductor A); and the holes cans be transferred from VB (semiconductor B) to VB (semiconductor A). As a result, all charge carriers are accumulated on semiconductor A and thus charge carrier separation could not be achieved. BiOI/TiO2 composite is a typical heterojuction composite with type III [280,281]. The photocatalytic experiment proved that BiOI/TiO2

(1:1) exhibit significantly improvement on degradation of MO under visible light, comparing with that of each individual component. A rational explanation is that, under visible light illumination, electrons in the valence band of BiOI could be excited up to a higher position, which is more active than that of TiO2, and hence, photogenerated electrons on the BiOI would easily transfer to TiO2. In this way, the photogenerated electrons and holes are effectively separated and therefore, the BiOI/TiO2 composite could exhibit better photocatalytic properties than that of BiOI or TiO2 on the degradation of MO under visible-light irradiation.

Metal nanoparticle or conductive material loading

Another important strategy for the separation of photogenerated electrons from semiconductor to reduce charge carrier recombination is to produce composite with metal nanoparticle. When metal nanoparticle is in contact with semiconductor, electrons prefer

Literature review 51 to transfer from the semiconductor to nanoparticle due to fact that the Fermi energy of metal nanoparticle is usually lower than that of semiconductor [282–284]. The most commonly used metal for this purpose is noble metals, such as platinum, palladium, gold, cobalt, nickel and rhodium [285–289]. In addition to being an electron scavenger, these metal nanoparticle also provide a more efficient active sites for the photocatalytic degradation reaction [285].

Beside metal nanoparticles, the composite consists of photocatalyst and conductive materials is also an option to prohibit charge carriers recombination. The most commonly used material is carbon compounds; graphene and C3N4 are widely studied [290–293]. All these material are outstanding conductors with high electron mobility, so the photogenerated electrons prone to transfer from the CB of semiconductor to carbon conductor, and hence, are readily separated from photocatalyst.