Design of p-n homojunctions in metal-free carbon nitride photocatalyst for overall water splitting

doi:10.1016/S1872-2067(20)63670-1

Chinese Journal of Catalysis

Volume 42, Issue 3, March 2021, Pages 501-509

https://doi.org/10.1016/S1872-2067(20)63670-1 Get rights and content

Abstract

Two-dimensional (2D) carbon nitride (CN) photocatalysts are attracting extensive attention owing to their excellent photocatalytic properties. In this study, we successfully prepared CN materials with heterogeneous structures via hydrothermal treatment, high-temperature roasting, ball milling, sintering, and other processes. Benefitting from interface interactions in hybrid architectures, the CN photocatalysts exhibited high photocatalytic activity. The rate of hydrogen production using these CN photocatalysts reached 17028.82 μmol h⁻¹ g⁻¹, and the apparent quantum efficiency was 11.2% at 420 nm. The ns-level time-resolved photoluminescence (PL) spectra provided information about the time-averaged lifetime of fluorescence charge carriers; the lifetime of the charge carriers causing the fluorescence of CN reached 9.99 ns. Significantly, the CN photocatalysts displayed satisfactory results in overall water splitting without the addition of sacrificial agents. The average hydrogen and oxygen production rates were 270.95 μmol h⁻¹ g⁻¹ and 115.21 μmol h⁻¹ g⁻¹ in 7 h, respectively, which were promising results for the applications of the catalysts in overall water splitting processes. We investigated the high efficiency of the prepared CN photocatalysts via a series of tests (UV-vis diffuse reflectance spectroscopy, photocurrent response measurements, PL emission spectroscopy, time-resolved PL spectroscopy, and Brunauer-Emmett-Teller analysis). Furthermore, the Mott-Schottky plot and current-voltage curve were acquired via electrochemical tests. The fabricated CN photocatalyst had a small p-n junction in its heterogeneous structure, which further enhanced its photocatalytic efficiency. Therefore, this work can promote the development of CN photocatalysts.

Graphical Abstract

In this work, we successfully prepared a CN material with a heterogeneous structure via a clever design for the first time. Benefitting from the interface interactions in hybrid architectures, the CN photocatalysts exhibited high photocatalytic activity.

Introduction

Two-dimensional (2D) graphene-like carbon nitride (CN) photocatalysts have been recently receiving significant attention from researchers owing to their particularly good response to visible light, relatively large specific surface area, non-toxicity, low cost, and other advantages [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. In 2009, Wang et. al [11] successfully used block g-C₃N₄ as a photocatalyst in water splitting for the production of hydrogen. Subsequently, high-quality g-C₃N₄ or composite structures (metallic oxide/g-C₃N₄) were prepared via various methods [12, 13, 14, 15, 16, 17]. For example, She et al. designed α-Fe₂O₃ nanosheets that could actively promote the exfoliation of g-C₃N₄, producing 2D hybrid structures that exhibited good photocatalytic efficiency in the production of hydrogen [18]. All these reports showed that g-C₃N₄ had numerous application prospects in the field of water splitting.

However, there are two difficulties encountered during the production of hydrogen. The first is that, like metal-free CN photocatalysts, metal-based composite CN materials are often prepared [19, 20, 21, 22, 23, 24, 25, 26, 27]. The second is the need to add a sacrificial agent in water splitting; otherwise, the effect of the catalyst is extremely low [28, 29, 30, 31, 32, 33]. Therefore, these two issues limit the practical applications of CN to metal-free photocatalysts. In 2015, Kang’s research group used carbon quantum dots with CN to conduct a full splitting water experiment via an all-metal-free photocatalyst without the use of precious metals and sacrificial agents for the first time; they proposed the two-electron channel phenomenon in Science [34]. Since then, there have been few major breakthroughs in the use of metal-free CN photocatalysts for the full splitting of water.

In our previous work, we investigated a lot of literature on CN. We found that almost all CN photocatalysts assumed n-type structures owing to specific ratios of carbon and nitrogen [35, 36, 37, 38, 39, 40, 41]. Generally, most researchers use p-type metal-based materials and n-type CN photocatalysts for the development of composites, which was inspired by the fundamental p-n junction principle applied in silicon solar cells. This structure can greatly improve the light-conversion efficiency of hybrid photocatalysts. For example, Sun et al. reported that a ZnO/g-C₃N₄ composite photocatalyst exhibited better photocatalytic activity than pure g-C₃N₄ [42]. However, the synthesis of p-type metal-free CN photocatalysts has rarely been reported. Therefore, we aimed to design a metal-free CN photocatalyst with a small p-n junction structure.

Recently, we prepared a new 2D CN-precursor organic material with a thickness of approximately 1 nm [43]. The carbon content of the prepared CN was higher than the nitrogen content, and the prepared material was a p-type CN semiconductor [43]. Thus, the prepared material could be compounded with conventional n-type CN semiconductor materials. A pure metal-free CN photocatalyst had a heterogeneous structure with a small p-n junction and was prepared after grinding and calcining. This type of non-metallic CN photocatalyst displayed superior photocatalytic effect (hydrogen production rate of 17028.82 μmol h⁻¹ g⁻¹) than other pure CN photocatalysts under the same experimental conditions. The apparent quantum efficiency (AQE) was 11.2% at 420 nm. The ns-level time-resolved photoluminescence (PL) spectra provided information about the time-averaged lifetime of fluorescence charge carriers; the lifetime of the charge carriers causing the fluorescence of CN reached 9.99 ns. Significantly, the CN photocatalyst demonstrated satisfactory results in overall water splitting without the addition of sacrificial agents (the average hydrogen and oxygen production rates were 270.95 μmol h⁻¹ g⁻¹ and 115.21 μmol h⁻¹ g⁻¹, respectively, within the first 7 h).

We investigated the high efficiency of this CN photocatalyst via a series of tests (UV-vis diffuse reflectance spectroscopy, photocurrent response measurements, PL emission spectroscopy, time-resolved PL spectroscopy, and Brunauer-Emmett-Teller (BET) analysis). Furthermore, the Mott-Schottky plot and current-voltage curve were acquired via electrochemical tests. The fabricated CN photocatalyst had a small p-n junction in its heterogeneous structure, which further enhanced its photocatalytic efficiency. Therefore, this work can promote the development of CN photocatalysts.

Section snippets

Preparation

Analytical-grade ammonium fluoride, melamine, triethanolamine, and alcohol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai). All reagents were used as received.

First, 5 g melamine as the raw material was heat-treated via hydrothermal synthesis at 180 °C for 24 h, filtered, and dried. Then, the material was transferred into a tubular furnace under the protection of high-purity argon at 550 °C for 1 h. Second, 5 g melamine was treated with 5 g ammonium fluoride at 180 °C for 24

Results and discussion

The flow diagram of the preparation of the CN photocatalyst with a heterogeneous structure is shown in Fig. 1. First, melamine was used as the raw material, and it was heat-treated under hydrothermal synthesis at 180 °C for 24 h, filtered, and dried. Then, the material was transferred into a tubular furnace under the protection of high-purity argon at 550 °C for 1 h. Second, melamine was treated with ammonium fluoride at 180 °C for 24 h, filtered, dried, and calcined (at 550 °C for 1 h) leading

Conclusions

CN photocatalysts with heterogeneous structures were successfully synthesized via a clever design for the first time. Benefitting from the interface interactions in hybrid architectures, the CN photocatalysts exhibited high photocatalytic activity. The hydrogen production rate of these CN photocatalysts reached 17028.82 μmolh⁻¹g⁻¹, the AQE was 11.2% at 420 nm, and the lifetime of the charge carriers causing the fluorescence of CN reached 9.99 ns. Significantly, the CN photocatalysts displayed

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version.

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: This work was supported by the National Natural Science Foundation of China (51802177, 51672109, 11504134), the Major Basic Program of the Natural Science Foundation of Shandong Province (ZR2018ZC0842), and Natural Science Foundation of Shandong Province (ZR2018BEM019).

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ArticleDesign of p-n homojunctions in metal-free carbon nitride photocatalyst for overall water splitting

Abstract

Graphical Abstract

Introduction

Section snippets

Preparation

Results and discussion

Conclusions

Appendix A. Supplementary material

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Article
Design of p-n homojunctions in metal-free carbon nitride photocatalyst for overall water splitting