Recent Achievements in Development of TiO2-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting

Abstract

Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H₂ production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e^-/h⁺) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO₂ and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO₂ based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO₂-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

Keywords: H2 production; TiO2 heterojunction; photocatalytic degradation; photocatalytic water splitting; semiconductor coupling; water treatment.

Figure 1

Photocatalytic reaction mechanism over semiconductor material.

Figure 2

Photocatalytic degradation mechanism over TiO₂/WO₃ composite.

Figure 3

Photocatalytic degradation mechanism over TiO₂/Fe₂O₃ composite.

Figure 4

Proposed mechanism for the tetracycline (TC) photodegradation process using N-doped TiO₂/CaFe₂O₄/ diatomite [44].

Figure 5

Photocatalytic degradation mechanism over TiO₂/Cu₂O composite.

Figure 6

Photocatalytic degradation mechanism over TiO₂/Bi₂O₃ composite.

Figure 7

General photocatalytic degradation mechanism over TiO₂/MS (M = Cd or Cu) or MS₂ (M = Mo and Sn) composite.

Figure 8

Photocatalytic mechanisms Ti³⁺-TNTs/Ag₃PO₄ (a) conventional heterojunction, and (b) Z-scheme heterojunction.

Figure 9

Photocatalytic reaction mechanism of TiO_2−X/Ag₃PO₄ under visible light irradiation.

Figure 10

Photocatalytic degradation mechanism over TiO₂/Ag₂O composite.

Figure 11

The principle photocatalytic water splitting mechanism over illuminated TiO₂ nanoparticle.

Figure 12

Valence band (VB) and conduction band (CB) band positions of various (a) n-type semiconductors; (b) p-type semiconductors [111].

Figure 13

Separation mechanisms of charge carriers in hybrid materials: (A) Schottky junction; (B) Type II Heterojunction; (C) p-n Heterojunction; and, (D) Direct Z-scheme Heterojunction [112].

Figure 14

Schematic diagram of proposed photocatalytic mechanism in the CO-NG/TiO₂ system.

Figure 15

The proposed photocatalytic mechanism in MoS₂-CdS-TiO₂ photocatalyst.

Figure 16

The type Z-heterojunction with the use of different solid electron mediators (multiwall carbon nanotubes (MWCNTs), carbon quantum dots (CQDs), rGO).