*4.3. Z-Scheme Heterojunction*

Normally, whether the band structures of heterojunctions changed after contact with each other, these systems cannot have high-efficient charge separation and strong redox ability at the same time [94]. Many researchers tried to find a different way to produce a new form of photocatalyst, which possessed high-efficient charge separation without sacrificing the redox ability of pristine catalyst since the construction of heterojunction is a perfect way to improve the photoactivity. Z-scheme photocatalyst is such an ideal system. By combining two or more kinds of materials, the new composite could respond to visible light and the charge separation can be improved, but the redox abilities of these catalysts were unharmed, like the system of g-C3N4/Au/BiOCl [49] and BiOCl/RGO/protonated g-C3N4 [50]. Z-scheme photocatalysts could be synthesized by not only chemical methods but also mechanical force, just like the system of WO3/NaNbO3 [95]. According to Yang Bai and co-workers, directly combining g-C3N4 and BiOCl could construct a Z-scheme catalyst through a facile method [66]. Typically, Z-scheme catalysts can be divided into three types, which could be labeled as PS-C-PS, PS-PS, and PS-A/D-PS [96–98]. In the PS-A/D-PS system, there exists an acceptor/donor pair as a common electron mediator. The schematic diagram of Z-scheme electron transfer in the PS-A/D-PS system is shown in Figure 10(a).

**Figure 10.** Schematic diagram of Z-scheme electron transfer in (**a**) PS-A/D-PS, (**b**) PS-C-PS, and (**c**) PS-PS system. Reproduced with permission from Zhou P et al, Advanced Materials; published by John Wiley and Sons, 2014.

Because the system of PS-C-PS is more stable than PS-A/D-PS, the PS-C-PS catalyst can be used in many different circumstances. Some researchers found the system of PS-A/D-PS was eroded after being used several times [99,100]. The PS-C-PS and PS-PS systems have a wider application range. These two systems also called all-solid-state Z-scheme photocatalysts. The schematic diagrams of Z-scheme electron transfer in the system of PS-C-PS and PS-PS are shown in Figure 10b,c.

The g-C3N4/Au/BiOCl heterojunction could be classified as a typical PS-C-PS catalyst, Au was employed as the mediator. Z-scheme catalyst of g-C3N4/BiOCl can be classified as a PS-PS type catalyst. It is believed that the conductive mediator of the PS-C-PS system could block the visible light to lower the efficiency of light energy absorption due to the surface plasmon resonance effect [101]. In this point of view, the system of PS-PS Z-scheme photocatalyst turns out to exhibit better photocatalysis performance. Because of the better photoactivity, Z-scheme catalysts drew a lot of attention, recently. Like the systems discussed in the former two sections, the enhancement of the Z-scheme catalysts based on g-C3N4 and BiOCl could be ascribed to better separation of charge carriers and wavelength expansion of light absorption.

To date, only one kind of Z-scheme g-C3N4/BiOCl photocatalyst was reported [66]. The proposed catalytic mechanism of the BiOCl/g-C3N4 system is shown in Figure 11. The variation of the band structure during the synthesis of the heterojunction was not taken into account. Results of the trapping experiments indicated that the main reactive species were hydroxyl and superoxide radicals. This was considered as evidence of the formation of the Z-scheme heterojunction, according to the study.

**Figure 11.** Direct Z-scheme photocatalytic mechanism of BiOCl/g-C3N4. Reproduced with permission from Bai Y et al, RSC Advances; published by Royal Society of Chemistry, 2014.

When the heterojunction was exposed under the illumination of visible light, the generation of •OH decreased. At the same time, the generation of superoxide radicals was not affected. The theoretical values of the band's redox potentials were directly adopted without being verified further.

The g-C3N4/Au/BiOCl Z-scheme system also adopted a similar theory that considered that the band structures of the three materials stayed the same after combined. The main reactive species during the degradation of RhB was the photo-induced holes. The aforementioned study of BiOCl/RGO/protonated g-C3N4 also expected the band structures stayed the same during the formation of the heterojunction. The main reactive species of the antibiotic TC degradation were holes and •O2 −, and the presence of them was taken as the evidence of the Z-scheme mechanism.

Therefore, all the Z-scheme photocatalysts mentioned above did not take the alignment of Fermi energy levels during the synthesis of the heterojunctions into account. Then,

theoretical values of the redox potentials were adopted to explain the proposed mechanisms. The presence of specific reactive species was used as proof of the proposed mechanism.

It is common sense that the enhancement caused by the construction of the heterojunction is due to better charge separation and expended wavelength of light absorption. However, when it comes to the explanation of the mechanism, there are three different scenarios. As far as we know, the CB of BiOCl and the VB of g-C3N4 is not negative and positive enough to generate superoxide and hydroxyl radicals, respectively. However, just like the study mentioned above, though the VB of C60/g-C3N4 was not supposed to be positive enough to produce hydroxyl radicals, the presence of hydroxyl radicals was still detected in an anoxic environment, which excluded the possibility that hydroxyl radicals might be produced from superoxide radicals. According to the CNB heterojunctions mentioned above, a similar phenomenon was observed. The ionic-liquid-assisted solvent-thermal route synthesized BiOCl/g-C3N4 generated hydroxyl radicals under the illumination of visible light [44], but the radicals might be produced in the VB of g-C3N4. The authors carried out no further tests to figure out the source of the radicals.

Additionally, some studies mentioned above might be explained by more than one mechanism. The improvement of the heterojunctions could be translated by all the three mechanisms. For example, the Z-scheme mechanism was used to describe photocatalytic reaction over the g-C3N4/BiOCl heterojunction [66], but it is still plausible to explain the process by the mechanism of CNB heterojunction. Photo-induced electrons and holes accumulated on the CB of BiOCl and the VB of g-C3N4, respectively. So, it is reasonable to think that the main species were superoxide radicals and holes according to the mechanism showed in Figure 5. The generation of •OH could be ascribed to •O2 −. The presence of 420 nm cutoff filter meant that BiOCl cannot be photo-excited, so the concentration of •O2 <sup>−</sup> decreased, then the chain of the reaction generated •OH was cut off. The results of the experiments could be evidence of the relation between those two reactive species.

The photocatalytic reaction could be also explained by the mechanism of PCNB heterojunction. The band structures changed after being combined. The electrons and holes accumulated on the CB of g-C3N4 and the VB of BiOCl just as shown in Figure 6. The results of trapping experiments could be evidence of the PCNB mechanism. The generation of •OH could be ascribed to •O2 −, either.

Therefore, it is difficult to testify the charge transfer mechanism over the g-C3N4/BiOCl photocatalysts. However, there is still no direct evidence of the mechanisms mentioned above. All the theories used to explain them seem very reasonable. Although trapping experiments, ESR tests, and other methods could detect the presence of the main reactive species, we still cannot figure out exactly where they come from. So, there are still many steps to take in order to precisely describe the mechanism.
