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Strategies to Facilitate the Charge Separation and Transportation

Facilitating the charge separation and transportation of a photoelectrode has been regarded as an effective and powerful method for significantly enhancing its PEC performance. Indeed, continuous efforts have been devoted to develop efficient strategies to facilitate the charge separation and transportation in photoelectrodes in the past years. Therein, element doping, heterostructure construction, and morphological control are the extensively used approaches.

Element Doping

Besides the contribution to the bandgap narrowing and increased light absorption as discussed above, element doping has also been a useful method to improve the conductivity of photoelectrodes and consequently facilitate the charge separation and transportation processes [9,128]. In fact, poor electrical conductivity has long been a fatal issue for some photoanodes such as hematite, which strictly limits their practical applications [88]. Taking hematite photoanodes as examples, Ti- [91], Sn- [92], and P-doping [63,136], as well as the introduction of oxygen vacancies [94] have been widely used to increase the carrier concentration and the electrical conductivity, which consequently can suppress the electron-hole recombination and enhance the PEC activity.

Among various doping methods, gradient doping emerged as a special and useful one because the concentration of dopants varies from the surface to the bulk [128]. Consequently, the band bending of a gradient photoelectrode not only occurs on the surface, but also continues in the bulk, which thus provides an intrinsic electric field throughout the bulk material. In this case, the charge separation and transportation to the surface is easier in a gradient photoelectrode. Recently, the P-gradiently doped hematite photoanode [136] and W-gradiently doped BiV04 photoanode [137] have also showed great potential in terms of PEC water splitting performance.

Heterostructure Construction

The construction of heterojunction photoanodes by coupling a semiconducting light absorber with a second semiconductor (not limited to two semiconductors) is another frequently used effective strategy for facilitating the charge separation and transportation [9,128]. Generally, the band structure of two photoelectrode materials should meet several requirements for forming an optimum heterojunction. Taking a photoanode as an example (Figure 6.9), the VB of material A should be more positive than material B, whereas the CB of material В should be more negative than material A [128]. This type-II band alignment would facilitate the electron and hole transfer from one to the other. In recent years, a great deal of heterojunction photoelectrodes with greatly enhanced PEC performance have been reported, such as W0,/BiV04 [106], Fe,0,/Fe2Ti05 [98,99], p-Cu,0/n-TaON [138], and Ti02/Fe,Ti05 [139].

The band structure for the different heterojunctions

FIGURE 6.9 The band structure for the different heterojunctions. CB stands for conduct band and VB stands for valance band. (Reprinted with permission from Hu, J. et al., Coatings, 9, 309, 2019. Copyright (2019) MDPI.)

Morphology Control

The diffusion length of a minority carrier is another key factor for the charge separation efficiency of photoelectrodes. An effective approach is to control the morphology of nanostructured photoelectrodes, as it can shorten their diffusion distance for the minority carrier and offer a large interfacial area between the photoelectrode and the electrolyte [128,140]. For example, hematite photoanodes with nanoparticles, dendrites, and mesoporous nanostructures usually suffer from severe charge recombination and poor transport across grain boundaries between particles [141]. Hematite films of ID nanostructures such as nanowires, nanotubes, and nanorods with high aspect ratios and large surface areas can improve charge carrier collection by minimizing hopping transport, and thus reduce recombination losses at grain boundaries [142]. Moreover, ID hematite nanostructures with smaller diameters can minimize the diffusion distance of photogenerated holes from inside to the hematite/electrolyte interface, thereby avoiding the poor charge transport limitation [140].

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