A review on g-C3N4 for photocatalytic water splitting and CO2 reduction

doi:10.1016/j.apsusc.2015.08.173

Applied Surface Science

Volume 358, Part A, 15 December 2015, Pages 15-27

https://doi.org/10.1016/j.apsusc.2015.08.173 Get rights and content

Highlights

•
New synthesis strategy for g-C₃N₄ production was summarized.
•
Novel ideas on extending the absorption edge of g-C₃N₄ were reviewed.
•
The overall water splitting and CO₂ photoreduction were highlighted.

Abstract

Solar fuel generation through water splitting and CO₂ photoreduction is an ideal route to provide the renewable energy sources and mitigate global warming. The main challenge in photocatalysis is finding a low-cost photocatalyst that can work efficiently to split water into hydrogen and reduce CO₂ to hydrocarbon fuels. Metal-free g-C₃N₄ photocatalyst shows great potentials for solar fuel production. In this mini review, we summarize the most current advances on novel design idea and new synthesis strategy for g-C₃N₄ preparation, insightful ideas on extending optical absorption of pristine g-C₃N₄, overall water splitting and CO₂ photoreduction over g-C₃N₄ based systems. The research challenges and perspectives on g-C₃N₄ based photocatalysts were also suggested.

Graphical abstract

Introduction

Solar fuel generation through water splitting and CO₂ photoreduction over photocatalysts has great potential to supply the renewable energy for the future and alleviate the environmental issues [1], [2], [3], [4], [5], [6], [7], [8]. However, the positive Gibbs free energy change, 237 kJ mol⁻¹ for water splitting to H₂ and 1135 kJ mol⁻¹ for CO₂ photoreduction to CH₄, makes the solar hydrogen production and CO₂ photoreduction be thermodynamically unfavorable reactions [9], [10], [11]. Attractively, a photocatalyst with sufficiently high conduction band (CB) bottom and deeply low valence band (VB) top can be applied to facilitate these photocatalytic redox reactions. Up to now, more than one hundred of materials have been demonstrated as potential photocatalysts for photocatalytic H₂ generation and CO₂ reduction [12], [13]. However, most of developed photocatalysts are wide bandgap semiconductors, and thereby lead to the weak light harvesting in visible light region that accounts for ∼43% of the solar energy. Meanwhile, these photocatalysts are almost all metal-containing materials, which is not only costly for large-scale applications but also but also raising environmental contamination concerns because of the metal ion leaching in usages. As a result, researchers have been stimulated to exploit metal-free photocatalysts with reasonable activities for solar-to-fuel conversion.

Recently, polymeric g-C₃N₄ has become a focus of attention for photocatalytic reactions [14], [15], [16], [17], [18], [19]. g-C₃N₄ is a visible-light-response material (2.7 eV bandgap), and the energy position of CB and VB is at −1.1 and 1.6 eV vs normal hydrogen electrode (NHE), respectively [14]. In addition, g-C₃N₄ has very high resistance to attack from heat, strong acid, and strong alkaline solution [20]. Unlike the metal-containing photocatalysts that need expensive metal salts for preparation, g-C₃N₄ photocatalyst can be facilely prepared by thermally polycondensing the cheap N-rich precursors, such as dicyanamide, cyanamide, melamine, and urea [14], [21], [22]. These excellent properties enable g-C₃N₄ to be used in water splitting, CO₂ photoreduction, organic contaminants purification, catalytic organic synthesis, and fuel cells [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. There have been several excellent reviews on g-C₃N₄ preparation and applications in the last 5 years, and readers can refer to these review articles [15], [16], [17], [18], [19].

In this mini review, we mainly focused on the most current advances on the design, preparation, and photocatalytic activities for H₂ generation and CO₂ reduction over g-C₃N₄ in the past 2 years. We started to introduce the novel design idea and new synthesis strategy for g-C₃N₄, and then highlighted the enlightened ideas on extending the optical absorption of g-C₃N₄. Next, the overall water splitting and CO₂ photoreduction over g-C₃N₄ based systems were summarized. Finally, the research status of g-C₃N₄ was briefly summarized. Also, the research challenges on g-C₃N₄ were concluded and the perspectives in future researches were also suggested.

Section snippets

New fabrication strategies of g-C₃N₄

Traditionally, g-C₃N₄ is generally synthesized by thermal condensation of N-rich precursors, such as urea, dicyandiamide, melamine, and so on [14], [21], [22]. However, the photocatalytic activity of as-obtained g-C₃N₄ is usually restricted by low efficiency due to fast recombination of photogenerated electron–hole pairs. Therefore, new strategy must be designed and fabricated to solve this problem.

g-C₃N₄/g-C₃N₄ heterojunction

The low quantum efficiency of solar fuel generation over photocatalysts has restricted the practical applications of photocatalysis. The low efficiency in photocatalytic reactions mainly caused by the fast recombination of photoinduced electron–hole pairs. To suppress this recombination, a heterojunction photocatalyst coupling g-C₃N₄ with other semiconductors can not only significantly facilitate the separation of photogenerated charge carriers but also broaden the solar light absorption, and

Important breakthrough oxygen production on g-C₃N₄

Comparing to the water reduction for H₂ generation, water oxidation involves the four electrons to yield O₂ and has been long regarded as the bottleneck of water splitting. Therefore, designing highly active and stable water oxidation photocatalysts is urgent for developing light-driven water splitting.

Presently, the photocatalytic O₂ evolution activity of g-C₃N₄ is an order of magnitude lower than that of H₂ evolution because of the relatively negative valence band position [113]. In order to

Conclusions and perspectives

Significant progresses have been achieved in the design and fabrication of g-C₃N₄ materials for efficient photocatalytic hydrogen and CO₂ photoreduction in the past two years. A very recent work by Kang and co-workers show that g-C₃N₄, coupling with carbon nanodots, can stably photocatalyze the pure water splitting into H₂ and O₂ to 600 nm light irradiation without the assistance of any co-catalysts for 200 days reaction. It is noteworthy that the extraordinary high solar energy conversion

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51572003, 11374013, 51002001), Anhui Provincial Natural Science Foundation (1508085ME105, 1408085MB22), the State “211 Project” of Anhui University, China, the Open Fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (KHK1403) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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¹: These authors contributed equally to this work.

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ReviewA review on g-C3N4 for photocatalytic water splitting and CO2 reduction

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

New fabrication strategies of g-C3N4

g-C3N4/g-C3N4 heterojunction

Important breakthrough oxygen production on g-C3N4

Conclusions and perspectives

Acknowledgements

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Review
A review on g-C₃N₄ for photocatalytic water splitting and CO₂ reduction

New fabrication strategies of g-C₃N₄

g-C₃N₄/g-C₃N₄ heterojunction

Important breakthrough oxygen production on g-C₃N₄