Elsevier

Nano Energy

Volume 83, May 2021, 105783
Nano Energy

Full paper
Introducing spin polarization into atomically thin 2D carbon nitride sheets for greatly extended visible-light photocatalytic water splitting

https://doi.org/10.1016/j.nanoen.2021.105783Get rights and content

Highlights

  • An approach of fluorination-defluorination in Se vapor is used to narrow the bandgap of atomically thin 2D CNs.

  • A downward electron spin polarization is introduced into the structure of CNs.

  • The visible-light photocatalytic water splitting of Se-CNs photocatalyst significantly exceeds CNs photocatalyst.

Abstract

Atomically thin 2D carbon nitride sheets (CNs) became one of the most promising solar energy conversion materials. However, the application of CNs is still limited due to two reasons: (i) the bandgap of CNs is wider than its counterpart due to the quantum size effect, which reduces its effective utilization of the entire solar spectrum, and (ii) the visible-light photocatalytic activity of CNs is still low due to its faster recombination of photogenerated carriers than photocatalytic reaction. Here, we achieve a strong visible-light absorption band in CNs through fluorination followed by thermal defluorination in Se vapor (Se-CNs). Experimental results and theoretical calculations confirm that the formation of cyano groups accompanied with in-situ Se doping expands the absorption edge of CNs from 416 to 584 nm. More importantly, a downward electron spin polarization in the CNs structure improves dramatically the efficiency of charge separation and surface catalysis reaction. The hydrogen generation rate of Se-CNs with 3 wt% Pt under visible-light irradiation (> 420 nm) reaches up to 5411.2 μmol h−1 g−1 that is 176.5 times of the hydrogen generation of CNs. Additionally, the visible-light photocatalytic oxygen evolution of Se-CNs acquires tremendous improvements. This work provides a new approach for improving electron structure of atomically thin 2D non-metal semiconductor materials.

Graphical Abstract

An approach is developed to narrow the bandgap of atomically thin 2D CNs by introducing spin polarization. The CNs-based photocatalyst shows a superior photocatalytic capability for water splitting under visible-light irradiation.

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Introduction

Among all renewable energy sources, solar energy has been recognized as one of the most potential sources because of its widespread existence and environmental friendliness. Photocatalytic water splitting to generate hydrogen and oxygen is considered as a promising way to address the environmental and global energy problems [1], [2], [3], [4]. In the past few decades, various wide bandgap semiconductor oxides, such as titanium dioxide, zinc oxide, and tin dioxide were developed as photocatalysts that were driven by ultraviolet light to generate hydrogen or oxygen through water splitting [4]. However, ultraviolet light only accounts for 5%, and visible light accounts for 43% of the solar spectrum [5]. Therefore, it is urgent and essential to develop an efficient and stable visible-light-response semiconductor photocatalyst material. As a visible-light photocatalyst, layered carbon nitride (CN) can be used to split water to produce hydrogen and oxygen. However, bulk CN is limited by high exciton binding energy, small specific surface area, and rapid recombination of photogenerated electron-hole pairs. All these deficiencies lead to extremely low photocatalytic efficiency of bulk CN and undoubtedly restrict its’ large-scale promotion and application in the field of energy and environmental photocatalysis.

Atomically thin 2D materials are potential photocatalysts to split water for hydrogen and oxy generation, due to their short diffusion path of photogenerated carriers and high specific surface area for enhancing the separation efficiency of photogenerated carriers [6], [7]. Among many 2D semiconductor materials, 2D metal-free polymer CN sheets (CNs) has been widely used in solar energy conversion [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. However, the bandgap of atomically thin CNs is about 3.06 eV, which makes it can only absorb the solar spectrum below a wavelength of 420 nm, [17] and miss the visible light that occupies 43% of the solar spectrum. Therefore, it is important to expand the visible-light absorption range of 2D CNs to enhance solar energy utilization. In addition, constructing the atomically thin structure can shorten the diffusion distance of photoexcited carriers from the bulk to the surface, while the catalytic reaction is much slower than the migration and recombination of the photoexcited carriers. Furthermore, if photogenerated carriers that migrate to the surface do not undergo a catalytic reaction in time, they still face serious recombination risk [18]. Fortunately, it was recently reported that the introduction of electron spin polarization with parallel arrangement (the electron spin at the Fermi surface has a single orientation) in titanium oxide materials improved the efficiency of charge separation and surface reaction and further significantly improved the photocatalytic activity [19]. Therefore, the introduction of electron spin polarization on the surface of 2D CNs can largely avoid the recombinations of photogenerated carriers. Nevertheless, atomically thin CNs has two obvious shortcomings: (i) CNs does not have unpaired electrons exist, thereby only exhibits intrinsic diamagnetism [20], and (ii) the thermal stability of the long-range ordered 3-s-triazine unit maintained by interlayer hydrogen bonds and van der waals force is poor [17]. Therefore, using conventional methods to both introduce electron spin polarization and expand the visible-light absorption of atomic 2D CNs is still a challenge.

Here, the 2D CNs was fluorinated and then annealed in Se vapor to realize strong visible-light absorption band. Experimental results combined with DFT calculations, show that spin polarization with parallel arrangement can be introduced into the CNs structure through the formation of cyan groups and in-situ Se doping, and the bandgap can be narrowed from 3.06 to 2.73 eV. Furthermore, cyano groups and Se atoms are introduced into the CNs framework as the capture sites of electrons and holes, which can effectively facilitate the separation and transfer of electrons. Thus, the ability of atomic 2D CNs in visible-light photocatalytic water splitting has been drastically improved.

Section snippets

Preparation of atomically thin CNs

10 g of urea was calcinated in muffle furnace with static atmosphere at 550 ℃ for 3 h (the heating rate was 0.5 ℃ min−1) to synthesize bulk CN. During the heating process, the covered crucible was wrapped by aluminum foil to avoid complete decomposition and the following rapid evaporation of urea. The obtained light yellow polymer was then ground into powder in mortar. The procedure of preparing atomically thin CNs from the above bulk CN by means of thermal oxidation etching is as follows:

Results and discussion

Atomically thin 2D CNs was synthesized through thermally etching the bulk CN that was formed from thermal polymerization of urea in the air. The formation mechanism of atomically thin CNs by thermal oxidation etching is described as follows. The interlayer of the bulk CN is bonded by a weak van der Waals force, and the inner layer is composed of melon units bonded by hydrogen bonds. Therefore, nitrogen atoms are oxidized and form nitrogen vacancies during thermal oxidation, especially for

Conclusions

In summary, we have reported a method of fluorination followed by thermal defluorination in Se vapor to realize strong visible-light absorption band in atomically thin CNs. This method can effectively reduce the bandgap of CNs from 3.06 to 2.73 eV and realize wide spectrum visible-light absorption. Experimental results and DFT calculations show that the bandgap is narrowed through in-situ Se doping, cyano groups introduced into CNs, and the opened heptazine structural units. Especially, the

CRediT authorship contribution statement

Author contributions: Rusen Yang conceived the project and designed the experiments with Yong Wang. Yong Wang and Yu Zhang contributed to sample fabrication. Yong Wang and Boye Zhou performed the preliminary TEM characterization of the samples. Yong Wang, Yizhang Wu, Zhaokun Wang, and Lin Fu carried out photocatalytic activity measurements. Wei Xu performed the DFT calculations. Rusen Yang and Yong Wang analyzed the data. Yong Wang, Wei Xu, Tingting Wang, Liang Cheng, Jianzhang Shi, and Hong

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Xidian University for its support. This work was financially supported by the Fundamental Research Funds for the Central Universities (No. JC2003), Fundamental Research Funds for the Central Universities (Grant No. JB191402), the National Natural Science Foundation of Shaanxi Province under Grant No. 2019JCW-17 and 2020JCW-15.

Yong Wang is currently a lecturer at the Academy of Advanced Interdisciplinary Research, Xidian University. He received his Ph.D. degree in Condensed Matter Physics from Nanjing University in 2020. His current research interest is focused on low-dimensional semiconductor materials.

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    Yong Wang is currently a lecturer at the Academy of Advanced Interdisciplinary Research, Xidian University. He received his Ph.D. degree in Condensed Matter Physics from Nanjing University in 2020. His current research interest is focused on low-dimensional semiconductor materials.

    Wei Xu is currently pursuing his Ph.D. degree under the supervision of Prof. Wei Zhong at School of Physics, Nanjing University, China. His research is focused on computational studies of the structural and electronic properties of materials.

    Yu Zhang is currently a lecturer in the Department of Physics at Shaanxi University of Science and Technology. She received her Ph.D. degree in Condensed Matter Physics from Nanjing University in 2020. Her research mainly focuses on the microstructure, mechanism of hydrogen evolution reaction (HER) and HER performance of the photocatalytic or electrocatalytic materials, as well as the simple application of DFT theory to calculate the density of electronic states of the materials.

    Yizhang Wu is currently a doctoral candidate in department of Physics, Nanjing University. His recent research interests focus on g-C3N4-based and MXene-based photocatalysis and their extended applications on biosensors, biotherapy (PTT and PDT) and electromagnetic wave absorption.

    Zhaokun Wang is currently a candidate for a master’s degree in the School of Physics at Nanjing University. His main research interests concern the synthesis and utilization of CNS and graphene.

    Lin Fu is currently a Postdoc in Department of Physics, Fudan University. He received Ph.D. degree in School of Physics from Nanjing University in 2020. His research interests are focused on the magnetic and transport properties of low-dimensional materials (especially the carbon-based materials and manganite oxides).

    Fulan Bai is currently a master of physics in Nanjing University. She is mainly engaged in the preparation and performance research of nano semiconductor materials. She's recently working on two-dimensional molecular crystals, which has shown great potential in the field of photocatalytic antibacterial.

    Boye Zhou is currently a Ph.D. student in the School of Physics at Nanjing University. Now, she is working in the field of energy conversion of novel low-dimensional nanomaterials.

    Tingting Wang is currently a lecturer in the School of Physics and Electronic-Electrical Engineering at Ningxia University. She received her Ph.D. degree in Condensed Matter Physics from Nanjing University in 2019. Her research interest is focused on microstructure, performance and physical mechanism of highly active electrocatalytic materials.

    Liang Cheng is currently a lecturer in the School of Physics and Electronic-Electrical Engineering at Ningxia University. He received his Ph.D. degree in Physics from Xiamen University in 2019. Now his research interest is focused on microstructure and gas sensing properties of semiconductor functional nanomaterials, as well as the structural instability of low-dimensional nanomaterials as induced by in-situ electron beam irradiated in Transmission Electron Microscope.

    Jianzhang Shi is currently an associate professor in the School of Advanced Materials and Nanotechnology at Xidian University. He received his Ph.D. degree in Electronic Science and Technology from Xi’an Jiaotong University in 2009. His most interests focus on first-principle calculations on low dimensional materials for photocatalyst and hydron energy production.

    Prof. Dr. Hong Liu is a professor in State Key Laboratory of Crystal Materials, Shandong University. He received his Ph.D. degree in 2001 from Shandong University (China). His current research is mainly focused on chemical processing of nanomaterials for energy related applications including photocatalysis, tissue engineering, especially the interaction between stem cell and nanostructure of biomaterials, as well as the nonlinear optical crystals.

    Rusen Yang is currently a professor in the School of Advanced Materials and Nanotechnology at Xidian University. He received his Ph.D. degree in Materials Science and Engineering from Georgia Institute of Technology in 2007, where he continued as Post-Doctoral Associate. He has discovered novel nanostructures, such as ZnO, SnO2, Zn3P2, and investigated their application potentials. His most recent work on energy harvester based on piezoelectric nanomaterials made significant contribution in the field of renewable energy.

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    These authors contributed equally to this work.

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