Surface engineering of graphitic carbon nitride polymers with cocatalysts for photocatalytic overall water splitting

Abstract

Graphitic carbon nitride based polymers, being metal-free, accessible, environmentally benign and sustainable, have been widely investigated for artificial photosynthesis in recent years for the photocatalytic splitting of water to produce hydrogen fuel. However, the photocatalytic stoichiometric splitting of pure water into H₂ and O₂ with a molecular ratio of 2 : 1 is far from easy, and is usually hindered by the huge activation energy barrier and sluggish surface redox reaction kinetics. Herein, we provide a concise overview of cocatalyst modified graphitic carbon nitride based photocatalysts, with our main focus on the modulation of the water splitting redox reaction kinetics. We believe that a timely and concise review on this promising but challenging research topic will certainly be beneficial for general readers and researchers in order to better understand the property-activity relationship towards overall water splitting, which could also trigger the development of new organic architectures for photocatalytic overall water splitting through the rational control of surface chemistry.

Fig. 1. Schematic illustration of photocatalytic overall water splitting over a semiconductor photocatalyst modified with both H₂- and O₂-evolution cocatalysts.

Fig. 2. Schematic description of the energy diagram of a semiconductor modified with both H₂ and O₂ evolution cocatalysts under light irradiation for non-sacrificial photocatalytic water splitting. E _a1: activation energy without cocatalysts; E _a2: activation energy with cocatalysts; ΔG: Gibbs free energy change; ΔG ⁰: Gibbs free energy change under standard conditions; r: redox reaction process.

Fig. 3. Schematic illustration of photocatalytic water reduction for H₂ evolution in the presence of an electron donor driven by a semiconductor modified with H₂ evolution cocatalysts. TEOA: triethanolamine.

Fig. 4. Dependence of the H₂ evolution rate, when using Pt-loaded g-C₃N₄ under visible light, on the loading amount of Pt. Reprinted with permission from ref. 56. Copyright 2009, American Chemical Society.

Fig. 5. Size distribution of Pt particles prepared by (a) photoreduction (298 K) and (b) H₂ reduction (673 K); (c) H₂ evolution activities in the presence of an electron donor. Reprinted with permission from ref. 86. Copyright 2014, Royal Society of Chemistry.

Fig. 6. (a) The rate of H₂ production over mpg-C₃N₄ loaded with different amounts of MoS₂ or Pt, (b) TEM image of MoS₂/g-C₃N₄ and (c) illustration of the deposition of layered MoS₂ on the surface of g-C₃N₄ for photocatalytic H₂ evolution. Reprinted with permission from ref. 57. Copyright 2013, Wiley-CVH.

Fig. 7. (a) Comparison of the photocatalytic activity of samples with different amounts of Ni(OH)₂ loaded on g-C₃N₄ polymers, (b) cyclic H₂-evolution curve for the Ni0.5 sample. Reprinted with permission from ref. 80. Copyright 2013, Royal Society of Chemistry.

Fig. 8. (a) Structures of Ni-, Co-, and Fe-based cocatalysts and g-C₃N₄, (b) photocatalytic H₂-evolution for different cocatalyst modified g-C₃N₄ samples. Reprinted with permission from ref. 94. Copyright 2012, Wiley-CVH.

Fig. 9. (A) Comparison of the photocatalytic activities of samples with different loading amounts of graphene on g-C₃N₄ polymers, (B) proposed mechanism for the enhanced electron transfer process of the graphene/g-C₃N₄ polymers. Reprinted with permission from ref. 48. Copyright 2011, American Chemical Society.

Fig. 10. Schematic illustration of photocatalytic water oxidation for O₂ evolution in the presence of an electron acceptor driven by a semiconductor modified with O₂ evolution cocatalysts.

Fig. 11. (a) Schematic illustration of surface modification and bulk doping modification of CoO_x with g-C₃N₄. Photocatalytic water oxidation activities of the g-C₃N₄, B–Co–g-C₃N₄ and S–Co–g-C₃N₄ samples under (b) UV (λ > 300 nm) and (c) visible light (λ > 420 nm) irradiation. Reprinted with permission from ref. 107. Copyright 2016, American Chemical Society.

Fig. 12. (a) TEM image of Pt deposited g-C₃N₄ polymers prepared by in situ photo-reduction; (b) overall water splitting activities of g-C₃N₄ polymers deposited with different amounts of Pt; (c) long reaction time overall water splitting by a Pt–Co–g-C₃N₄ polymer; (d) overall water splitting activities with the light on and off. Reprinted with permission from ref. 110. Copyright 2016, Royal Society of Chemistry.

Fig. 13. Time course of the photocatalytic evolution of H₂ and O₂ using (a) Co₃O₄/HCNS/Pt and (b) (Co₃O₄ + Pt)/HCNS under UV irradiation (λ > 300 nm). Reprinted with permission from ref. 40. Copyright 2016, Wiley-CVH.