A critical mini-review on doping and heterojunction formation in ZnO-based catalysts

Abstract

This mini-review on doping and heterojunctions for catalysis applications provides a comprehensive overview of key aspects. Doping, when carried out adequately with a uniform distribution, creates a new energy level that significantly enhances charge transfer and light absorption. This new level alters the material's morphology and enhances intrinsic defects. For instance, ZnO, despite its exceptional band edge concerning oxygen reduction and water oxidation redox potentials, faces the issue of electron-hole recombination. However, forming a heterojunction can effectively aid charge transfer and prolong electron-hole relaxation without recombination. This is where the role of doping and heterojunctions becomes crucial. Additionally, incorporating noble metals with S- and Z-scheme heterojunctions offers a promising mechanism for charge transfer and visible light harvesting, further amplifying the catalytic properties.

Fig. 1. Band edge potentials and band gap of some selected semiconductors. The band edge potentials are shown relative to water and oxygen's oxidation and reduction potentials, respectively.

Fig. 2. (a) Acceptor; (b) donor-type defects. The shallow donor and acceptor cavities are located near the CB and VB, respectively. The deep electron and deep hole trap defects were created next to the shallow-level defects of CB and VB, respectively.

Fig. 3. Scheme showing probable charge transfer within cobalt-doped ZnO: valence to conduction band, cobalt(iii) level to host conduction band, cobalt(iii) to host valence band, and dopant d–d transition. The charge transfer enhances the electron–hole relaxation time and degrades dye pollutants such as methylene blue by oxidising agents.

Fig. 4. The possible type II (staggered type) charge transfer mechanism within semiconductor I and semiconductor II: (a and b) before and after contact, respectively; (c) during light irradiation and electron–hole generation; and (d) electron–hole migration.

Fig. 5. The possible Z-scheme charge transfer mechanisms within semiconductor I and semiconductor II: (a and b) before-and-after contact; (c) during light irradiation and electron–hole generation; and (d) allowed and forbidden electron–hole migrations.

Fig. 6. The possible Z-scheme charge transfer mechanism in the presence of noble metals within semiconductors I and II: (a) before contact; (b) during light irradiation and electron–hole generation in the existence of noble metal as an electron sink; and (c) allowed and forbidden electron–hole migration.

Fig. 7. The possible S-scheme charge transfer mechanisms within semiconductor I and semiconductor II: (a and b) before-and-after contact; (c) during light irradiation and electron–hole generation; and (d) allowed and forbidden electron–hole migration.

Buzuayehu Abebe

Neeraj K. Gupta

Dereje Tsegaye