Z-Scheme Photocatalytic Systems for Solar Water Splitting

Abstract

As the world decides on the next giant step for the renewable energy revolution, scientists have begun to reinforce their headlong dives into the exploitation of solar energy. Hitherto, numerous attempts are made to imitate the natural photosynthesis of plants by converting solar energy into chemical fuels which resembles the "Z-scheme" process. A recreation of this system is witnessed in artificial Z-scheme photocatalytic water splitting to generate hydrogen (H₂). This work outlines the recent significant implication of the Z-scheme system in photocatalytic water splitting, particularly in the role of electron mediator and the key factors that improve the photocatalytic performance. The Review begins with the fundamental rationales in Z-scheme water splitting, followed by a survey on the development roadmap of three different generations of Z-scheme system: 1) PS-A/D-PS (first generation), 2) PS-C-PS (second generation), and 3) PS-PS (third generation). Focus is also placed on the scaling up of the "leaf-to-tree" challenge of Z-scheme water splitting system, which is also known as Z-scheme photocatalyst sheet. A detailed investigation of the Z-scheme system for achieving H₂ evolution from past to present accompanied with in-depth discussion on the key challenges in the area of Z-scheme photocatalytic water splitting are provided.

Keywords: Z‐scheme; artificial photosynthesis; electron mediators; hydrogen; water splitting.

Figure 1

Schematic illustration of solar water splitting technologies: PV‐EC, PEC, and photocatalysis.

Figure 2

Fundamental photocatalytic water splitting mechanism in particulate system. The potential difference between CB and H⁺/H₂ is abbreviated as ɳ _HER while the potential difference between VB and O₂/H₂O is abbreviated as ɳ _OER.

Figure 3

Schematic illustration of a) single component photocatalysts, b) type‐II heterojunction photocatalysts, and c) Z‐scheme photocatalytic system.

Figure 4

Timeline for the evolution in different generation of Z‐scheme.

Figure 5

Graphical representation illustrating two‐step photoexcitation system in natural photosynthesis of green plant.

Figure 6

Schematic band energy diagram of PS‐A/D‐PS Z‐scheme system.

Figure 7

Effect of midgap states as electron donor level. Proposed band structure of Rh‐doped SrTiO₃. Adapted with permission.43 Copyright 2004, American Chemical Society.

Figure 8

Degree of r_A+B/r_A ratio with cutoff wavelength of light. UV–vis DRS of a) WO₃, b) CaTaO₂N, and c) BaTaO₂N. Adapted with permission.35 Copyright 2009, American Chemical Society.

Figure 9

Properties of Fe³⁺/Fe²⁺ redox system. a) Adsorption isotherms of Fe³⁺ and Fe²⁺ on TiO₂ powder. b) Photocatalytic generation of Fe²⁺ and O₂ using Fe³⁺ as the electron acceptor. The reaction was carried out in 0.05 dm³ of aqueous FeCl₃ containing TiO₂ powder. Adapted with permission.45 Copyright 1997, American Chemical Society.

Figure 10

Optical properties of Sillén‐Aurivillius class of bismuth oxyhalides. a) UV–vis of Bi₄NbO₈Cl, Bi₄NbO₈Br, BiOCl and BiOBr. b) Schematic of band structure for Bi₄NbO₈Cl and BiOCl. Adapted with permission.49 Copyright 2016, American Chemical Society.

Figure 11

Optical properties of doped TiO₂. a) UV–vis DRS of TiO₂ and doped TiO₂. b) Schematic illustration of band structure for TiO₂ doped with different atoms. Adapted with permission.52 Copyright 2017, Royal Society of Chemistry.

Figure 12

Electronic configuration of various doped metal oxide. A) Band structure for Ir and La co‐doped BaTa₂O₆. B) UV–vis DRS of ATa₂O₆:Ir,La where A is equal to a) Ca, b) Sr, and c–e) Ba. Samples of (a–c) were prepared via borate‐flux method whereas (d,e) were prepared using NaCl‐flux and SSR methods. Adapted with permission.[qv: 37d] Copyright 2017, Royal Society of Chemistry.

Figure 13

Z‐scheme water splitting of doped metal oxide in [Co(bpy)₃]^3+/2+ redox system. Mechanism of water splitting reaction containing Ru‐doped BaTa₂O₆:Ir,La as HEP, BiVO₄ as OEP in solution with [Co(bpy)₃]SO₄ complex under visible light. Adapted with permission.[qv: 37d] Copyright 2017, Royal Society of Chemistry.

Figure 14

Separated gas evolution of H₂ and O₂ from photocatalytic overall water splitting. a,b) Schematic illustration of on‐site separation of H₂ and O₂ gases from two‐compartment PS‐A/D‐PS system. Adapted with permission.54 Copyright 2013, American Chemical Society.

Figure 15

Schematic band energy diagram of PS‐C‐PS Z‐scheme system.

Figure 16

All‐solid‐state Z‐scheme with Au as electron mediator. a) HRTEM image and b) schematic illustration of charge transfer in CdS/Au/TiO₂. Adapted with permission.23 Copyright 2006, Nature Publishing Group.

Figure 17

a) Charge transfer mechanism of CdS/Au/TiO_1.96C_0.04. b) UV–vis of TiO_1.96C_0.04 and CdS/Au/TiO_1.96C_0.04. Adapted with permission.68 Copyright 2011, American Chemical Society.

Figure 18

Synthetic protocol of preparation of CdS/Au/ZnO Z‐scheme structure via two‐step assembly method. Adapted with permission.[qv: 67a] Copyright 2013, Royal Society of Chemistry.

Figure 19

Geometry of PS‐C‐PS system that resembles butterfly wings. a,b) FESEM images of WA‐TiO₂. c) TEM image of WA‐TiO₂. d,e) TEM images of CdS/Au/WA‐TiO₂. Adapted with permission.69 Copyright 2013, Elsevier.

Figure 20

PS‐C‐PS system with in situ generation of conductor. a) Synthetic protocol of Ag₂S/Ag/TiO₂. b,c) FESEM and HRTEM images of Ag₂S/Ag/TiO₂ and d) the corresponding EDX mapping. Adapted with permission.70 Copyright 2015, Springer.

Figure 21

a) Computed band structure of bulk TiO₂ and Ag₂S. b) Schematic of electron flow in Ag/Ag₂S system. c) Computed differential charge distribution at interface of Ag₂S and Ag. d) Schematic illustration of Z‐scheme formation for Ag₂S/Ag/TiO₂. Adapted with permission.70 Copyright 2015, Springer.

Figure 22

a) Synthetic protocol and b) charge transfer profile of CdS/Cd/ZnO Z‐scheme system. Adapted with permission.66 Copyright 2012, Wiley.

Figure 23

Schematic illustration of charge transfer profile for Ru/SrTiO₃:La,Rh‐Ir‐CoO_x/Ta₃N₅ Z‐scheme system. Adapted with permission.71 Copyright 2014, American Chemical Society.

Figure 24

A) C 1s XPS spectra of a) GO, b) hydrazine‐reduced GO, c) PRGO/BiVO₄, d) PRGO/SrTiO₃:Rh, and e) Ru/SrTiO₃:Rh‐(PRGO/BiVO₄). B) Schematic illustration of charge transfer mechanism in Ru/SrTiO₃:Rh‐(PRGO/BiVO₄). Adapted with permission.25 Copyright 2011, American Chemical Society.

Figure 25

Charge transfer mechanism of Pt/metal sulfides‐CoO_x/BiVO₄ in a) RGO‐mediated Z‐scheme and b) PEC. c) Current‐potential curves of BiVO₄, CoO_x/BiVO₄, and Pt/CuGaS₂. d) Overall water splitting of Pt/CuGaS₂‐RGO‐BiVO₄ Z‐scheme system with and without CoO_x under visible light. Adapted with permission.78 Copyright 2016, American Chemical Society.

Figure 26

Heteroatoms doped GO in Z‐scheme. a) Schematic illustration of charge transfer mechanism of NGO‐QDs. b) Representative HRTEM image of NGO‐QDs. c) Energy band energy diagram of NGO‐QDs. Adapted with permission.80 Copyright 2014, Wiley.

Figure 27

Schematic illustration of a) Zn_0.5Cd_0.5S‐CNTs‐TiO₂ and b) its charge transfer mechanism. c) HRTEM image of Zn_0.5Cd_0.5S‐CNTs‐TiO₂. d) Band energy diagram of Zn_0.5Cd_0.5S‐CNTs‐TiO₂. Adapted with permission.[qv: 17a] Copyright 2017, Elsevier.

Figure 28

Schematic illustration of a) charge transfer mechanism and b) its corresponding band position of BiVO₄‐CQDs‐CdS Z‐scheme system. Adapted with permission.85 Copyright 2017, Elsevier.

Figure 29

Schematic band energy diagram of PS‐PS or direct Z‐scheme system.

Figure 30

Physical formation of direct Z‐scheme. Optical microscope images of aqueous suspension containing Ru/SrTiO₃:Rh and BiVO₄ under different pH. a) 7, b) 4, c) 3.4, and d) 2.5. e) Schematic illustration of contact behaviors of Ru/SrTiO₃:Rh‐BiVO₄ suspension under neutral and acidic conditions. Adapted with permission.27 Copyright 2009, American Chemical Society.

Figure 31

Chemical formation of direct Z‐scheme. a) Schematic illustration of band structure of ZnO/CdS direct Z‐scheme. b) Fluorescence emission decay spectra of ZnO, CdS and (ZnO)₁/(CdS)_0.2. Adapted with permission.[qv: 26a] Copyright 2009, Royal Society of Chemistry.

Figure 32

Metal loading method for direct Z‐scheme verification. a,b) TEM and HRTEM images of anatase/rutile TiO₂ nanocomposites loaded with Pt nanoparticles. c) Plausible electronic profile of anatase/rutile TiO₂ nanocomposites during photodeposition of Pt. Adapted and reproduced with permission.93 Copyright 2014, Elsevier.

Figure 33

Charge transfer mechanism in direct Z‐scheme and heterojunction. a) Direct Z‐scheme and b) type‐II heterojunction of BP‐BiVO₄ coupling during sacrificial testing. Reproduced with permission.94 Copyright 2018, Wiley.

Figure 34

Radical testing for direct Z‐scheme verification. a) PL spectra of g‐C₃N₄/TiO₂ nanocomposites in solution containing 5 × 10⁻⁴ M TA under irradiation (excitation at 315 nm). b) Effect of scavengers on degradation of ISN using g‐C₃N₄/TiO₂ nanocomposites. c) Charge transfer mechanism of g‐C₃N₄/TiO₂ in both direct Z‐scheme and type‐II heterojunction systems. Adapted and reproduced with permission.97 Copyright 2015, Elsevier.

Figure 35

XPS method for direct Z‐scheme verification. High resolution XPS spectra of a) C 1s and b) N 1s of g‐C₃N₄ and g‐C₃N₄/ZnO. High resolution XPS spectra of c) Zn 2p and d) O 1s of ZnO and g‐C₃N₄/ZnO. Adapted with permission.99 Copyright 2015, Royal Society of Chemistry.

Figure 36

Formation of direct Z‐scheme. Schematic illustration of electronic band structure diagram of g‐C₃N₄/ZnO before and after contact. Reproduced with permission.22 Copyright 2017, Wiley.

Figure 37

Heterojunction and direct Z‐scheme in g‐C₃N₄/W₁₈O₄₉. a,b) Electronic band structure diagram and c,d) energy diagram during Fermi level alignment in type‐II heterojunction and Z‐scheme system. Adapted and reproduced with permission.100 Copyright 2017, Elsevier.

Figure 38

Fermi level difference between g‐C₃N₄ and W₁₈O₄₉ with and without addition of sacrificial reagent measured by a) OCP and b) DFT. Calculated potential measured by DFT of c) g‐C₃N₄ and d) W₁₈O₄₉. Adapted with permission.100 Copyright 2017, Elsevier.

Figure 39

High resolution XPS of TEOA‐adsorbed g‐C₃N₄/W₁₈O₄₉, pure g‐C₃N₄/W₁₈O₄₉ and their respective individual components of a) O 1s and b) N 1s. Adapted with permission.100 Copyright 2017, Elsevier.

Figure 40

Electronic configuration of direct Z‐scheme in g‐C₃N₄/Fe₂O₃. Calculated WF of a) g‐C₃N₄ and b) Fe₂O₃. c) Estimated band configuration of g‐C₃N₄/Fe₂O₃. d) HRTEM image of Pt‐loaded g‐C₃N₄/Fe₂O₃. Adapted with permission.101 Copyright 2018, Wiley.

Figure 41

Electronic configuration of p–n junction in g‐C₃N₄/Bi₄Ti₃O₁₂. a) Band structure diagram of g‐C₃N₄ and Bi₄Ti₃O₁₂. b) Charge transfer mechanism of g‐C₃N₄/Bi₄Ti₃O₁₂ nanocomposites. Adapted and reproduced with permission.102 Copyright 2016, Elsevier.

Figure 42

Technological map of three different solar H₂ production approaches for practical solar energy conversion. Adapted and reproduced with permission.3 Copyright 2019, Royal Society of Chemistry.

Figure 43

a) Large‐scale photocatalytic H₂ production using reactor containing immobilized Pt/g‐C₃N₄. Adapted with permission.113 Copyright 2015, Wiley. b) Photocatalytic water splitting panel with immobilized RhCrO_x/Al:SrTiO₃. Adapted with permission.9 Copyright 2018, Elsevier.

Figure 44

Particulate Z‐scheme photocatalyst sheets. Band configuration and plausible charge transfer mechanism in particulate Z‐scheme photocatalyst sheets. Adapted with permission.114 Copyright 2018, American Chemical Society.

Figure 45

a) Preparation of Z‐scheme photocatalyst sheets using particle transfer method. b) Schematic illustration of charge transfer mechanism and overall water splitting reaction of SrTiO₃:La,Rh/Au/BiVO₄:Mo Z‐scheme sheets. Adapted with permission.28 Copyright 2016, Nature Publishing Group.

Figure 46

SEM–EDX mapping of a–e) top view and f–j) cross‐sectional view of SrTiO₃:La,Rh‐C‐BiVO₄:Mo photocatalyst sheets. Adapted with permission.115 Copyright 2017, American Chemical Society.

Figure 47

a) PESA results for Au, graphite, sputtered carbon and glassy carbon. b) Effect of background pressure on overall water splitting performance of Ru‐loaded SrTiO₃:La,Rh‐C‐BiVO₄:Mo photocatalyst sheets. Adapted with permission.115 Copyright 2017, American Chemical Society.

Figure 48

Photocatalyst sheets with direct Z‐scheme configuration. a) False‐colored SEM image of Si/TiO₂ nanotree arrays. b) Photographs of TiO₂, Si and Si/TiO₂ films compared to a US quarter (right). c,d) Enlarged SEM images of Si/TiO₂ nanotree arrays. Adapted with permission.116 Copyright 2013, American Chemical Society.

Figure 49

Mechanism of electron flows in Si/TiO₂ nanotree arrays. a) Schematic illustration of charge transfer in Si/TiO₂ nanotree arrays. b) Electronic configuration of Si/TiO₂ nanotree arrays in overall water splitting. Adapted with permission.116 Copyright 2013, American Chemical Society.

Figure 50

Relationship between STH conversion efficiency and maximum wavelength of photons available for water splitting at different values of AQY for photocatalytic water splitting. Adapted and reproduced with permission.119 Copyright 2014, Springer.

Figure 51

A) Electronic band structure of photocatalysts after interstitial doping of heteroatoms. Adapted with permission.122 Copyright 2012, American Chemical Society. B) UV–vis DRS of a) SrTiO₃, b) SrTiO₃:Rh, c,d) SrTiO₃:La/Rh prepared by one‐step SSR and two‐step SSR. Adapted with permission.71 Copyright 2014, American Chemical Society.

Figure 52

Photocatalytic overall water splitting of WO₃‐based Z‐scheme redox system. Rate of gas evolution over PtO_x/H‐Cs‐WO₃ as OEP and Pt/SrTiO₃:Cr,Ta as HEP in I₃ ⁻/I⁻ and IO₃ ⁻/I⁻ system under different pH. Adapted with permission.[qv: 36c] Copyright 2013, Royal Society of Chemistry.

Figure 53

A) Consumptions of H₂ and O₂ due to backward reactions on Pt/SrTiO₃:Rh‐BiVO₄ (Triangle) and Ru/SrTiO₃:Rh‐BiVO₄ (Circle). B) Overall water splitting of a) Ru/SrTiO₃:Rh‐BiVO₄ and b) Pt/SrTiO₃:Rh‐BiVO₄. Adapted with permission.[qv: 37c] Copyright 2008, Elsevier.

Figure 54

a) HRTEM image of Rh/Cr₂O₃‐loaded (Ga_{1− x}Zn_x)(N_{1− x}O_x). b) Schematic of overall water splitting mechanism on nanostructured core/shell co‐catalyst loaded HEP. Adapted and reproduced with permission.125 Copyright 2006, Wiley. c) ORR activity of C/Ti and Au/Ti electrodes. Adapted with permission.115 Copyright 2017, American Chemical Society.