Semiconductor Nanomaterial Photocatalysts for Water-Splitting Hydrogen Production: The Holy Grail of Converting Solar Energy to Fuel

Abstract

Nanomaterials have attracted attention for application in photocatalytic hydrogen production because of their beneficial properties such as high specific surface area, attractive morphology, and high light absorption. Furthermore, hydrogen is a clean and green source of energy that may help to resolve the existing energy crisis and increasing environmental pollution caused by the consumption of fossil fuels. Among various hydrogen production methods, photocatalytic water splitting is most significant because it utilizes solar light, a freely available energy source throughout the world, activated via semiconductor nanomaterial catalysts. Various types of photocatalysts are developed for this purpose, including carbon-based and transition-metal-based photocatalysts, and each has its advantages and disadvantages. The present review highlights the basic principle of water splitting and various techniques such as the thermochemical process, electrocatalytic process, and direct solar water splitting to enhance hydrogen production. Moreover, modification strategies such as band gap engineering, semiconductor alloys, and multiphoton photocatalysts have been reviewed. Furthermore, the Z- and S-schemes of heterojunction photocatalysts for water splitting were also reviewed. Ultimately, the strategies for developing efficient, practical, highly efficient, and novel visible-light-harvesting photocatalysts will be discussed, in addition to the challenges that are involved. This review can provide researchers with a reference for the current state of affairs, and may motivate them to develop new materials for hydrogen generation.

Keywords: green source; hydrogen production; nanomaterial; photocatalysis; semiconductor materials; water splitting.

Figure 3

Shows a schematic diagram of hydrogen production from a particulate photocatalytic system. Printed with permission from the Royal Chemical Society [43].

Figure 6

Representing the band gap of material obtained via a solid solution of wide and narrow-band-gap materials. Printed with permission from Wiley Online Library [62].

Figure 1

The basic principle of water splitting on the photocatalyst surface. Printed with permission from the Royal Chemical Society [21].

Figure 2

Hydrogen production using (a) semiconductor photocatalysts (b) photo electrolytic process and (c) particular photocatalytic system. Printed with permission from the Royal Chemical Society [42].

Figure 4

Band gap structure of a cation-doped wide band gap semiconductor. Printed with permission from Wiley Online Library [57].

Figure 5

Anion-doped wide band gap semiconductor with visible-light activity. Printed with permission from Wiley Online Library [46].

Figure 7

Semiconductor catalyst with co-catalyst. Printed with permission from Frontiers [63].

Figure 8

Dual photocatalysts system employing a redox shuttle. Printed with permission from Wiley Online Library [46].

Figure 9

Z-scheme mechanism for photocatalytic water splitting. Printed with permission from Elsevier [80].

Figure 10

S-scheme pathway for photocatalytic water splitting. Printed with permission from Elsevier [81].

Figure 11

Dual-functionalized mixed Keggin- and Lindqvist-type [Cu^I₂₄(µ₃-Cl)₈ µ₄-Cl]₆] based POM@MOFs as H₂ and O₂ evolution photocatalysts. Printed with permission from the American Chemical Society [86].

Figure 12

Diagram of an instance of type-II MIL-167 and MIL-125- NH₂ heterojunctions. Printed with permission from American Chemical Society [87].

Figure 13

UV/visible diffuse reflectance spectra of Sm₂Ti₂O₇ and Sm₂Ti₂S₂O₅. Printed with permission from American Chemical Society [120].

Figure 14

UV–visible diffuse reflectance spectra of Ta₂O₅, TaON, Ta₂N₃, and MtaO₂N (M: Ca, Sr, Ba). Printed with permission from Elsevier [125].

Figure 15

(A) Powder X-ray diffraction patterns and (B) UV/visible diffuse reflectance spectra of (a) GaN, (b) GaN:ZnO, (c) GaN:ZnO, (d) GaN:ZnO, and (e) ZnO. Printed with permission from American Chemical Society [132].

Figure 16

Hydrogen production rate from an aqueous solution of Na₂S and Na₂SO₃ in visible light over solid solutions of Cd_1-xZn_xS with different zinc concentrations (0.2, 0.25, 0.30, and 0.35) in 150 mL of reactant solution with a 0.1 M concentration of Na₂S and 0.04 M concentration of Na₂SO₃. Printed with permission from Elsevier [145].

Figure 17

Schematic diagram of g-C₃N₄TiO₂ band gap reduction and working efficiency. Printed with permission from Elsevier [148].