Steering charge transfer for boosting photocatalytic H2 evolution: Integration of two-dimensional semiconductor superiorities and noble-metal-free Schottky junction effect

https://doi.org/10.1016/j.apcatb.2018.12.011Get rights and content

Highlights

  • Synergistically utilize 2D semiconductor superiorities and noble-metal-free Schottky junction to boost the photocatalytic performance.

  • The stronger redox capability and the fast charge kinetics synergistically boosted the photocatalytic performance.

  • By ESR analysis and theoretical calculation, the photocatalytic mechanism was researched in detail.

Abstract

Sunlight-driven photocatalysis holds great promise for alleviating the energy and environmental crises. For the visible-light-driven bare semiconductor, there is an irreconcilable contradiction between the light absorption and strong redox capabilities. Here, we reported a predictable design for improving the photocatalytic performance via regulating the bandgap and accelerating the charge kinetics of the semiconductor. Taken together, utilize two-dimensional (2D) structure to essentially increase the bandgap of the semiconductor for gaining the higher transfer and separation of the photogenerated electron-hole pairs and the stronger redox capabilities; and accelerate charge kinetics via the driving force from the Schottky junction. Meanwhile, the Schottky barrier prevents the photogenerated electrons trapped by a noble-metal-free electron acceptor from dually recombining. Additionally, the energy transfer process of the photocatalytic reaction was also researched in detail, aligning well with the photocatalytic mechanism.

Graphical abstract

Utilize the two-dimensional semiconductor superiorities and noble-metal-free Schottky junction effect to boost the photocatalytic performance.

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Introduction

Photocatalysis as the most promising technology for addressing energy and environmental crises is widely used in water spitting, CO2 reduction as well as wastewater treatment [[1], [2], [3], [4], [5], [6], [7], [8]]. The photocatalytic reaction is a redox reaction participated in by the photogenerated electrons and holes from the semiconducting photocatalyst. The conduction band (CB) and valence band (VB) of the semiconductor determine the redox capability and the rate of the photocatalytic redox reaction depends on the charge kinetics of the semiconductor [[8], [9], [10]]. Within certain limits, the CB and VB of the semiconductor should be higher (more negative) and lower (more positive), respectively, meaning a wide bandgap (Eg) semiconductor possesses the strong redox capability [[10], [11], [12]]. As the wide band-gap semiconductor can only absorb the short-wavelength UV light, however, there is an irreconcilable contradiction between the visible light absorption and strong redox capabilities because UV light with short-wavelength only takes ∼4% and the visible light with long wavelength takes ∼46% of the incoming solar energy. For another, the photogenerated charge carriers from the bare semiconductor can only diffuse irregularly due to the lack of the charge driving force, resulting in the slow charge kinetics [9,10]. To rationally design the photocatalyst, the above factors need to be considered.

For the band structure of the semiconductor, it should be increased as high as possible without limiting the absorption of the visible light, that is the absorption edge should be larger than 400 nm (λ > 400 nm, Eg < 3.1 eV). Generally, decreasing the size of the bare semiconductor is an effective method for increasing the bandgap. Especially, when the size is reduced to the nano level, the semiconductor will possess the quantum confinement effect, which can effectively increase the bandgap of the semiconductor [[13], [14], [15], [16]]. As there are so many indeterminacies for simultaneously decreasing three-dimensional (3D) sizes of the semiconductor for the photocatalytic reaction, however, 2D semiconductors with a nano-size in the Z direction have attracted lots of attentions [17,18]. 2D semiconducting photocatalysts with an ultrathin structure possess a larger bandgap than that of the bulk one and the higher transfer and separation efficiencies of the charge carriers from the bulk to the surface [[19], [20], [21]]. Thus, controllably adjusting the thicknesses of some semiconductors is a high-efficiency method for increasing the bandgap.

To accelerate charge kinetics of the photocatalytic reaction, integrating the conductors (mainly noble metal such as Pt, Au and Ag) to the semiconductors is a commonly effective method for improving the photocatalytic performance [9,[22], [23], [24], [25], [26]]. There are mainly two combination modes for conductor-semiconductor heterojunction (the Schottky junction and the ohmic contact), as shown in Fig. 1. Take n-Type semiconductor as an example, when the work function of the conductor (Wc) is larger than that of the n-Type semiconductor (Ws) and them come into contact, to build up an equilibrium state between the Fermi levels of the conductor (Efc) and n-Type semiconductor (Efs), the electrons from the n-Type semiconductor will diffuse to the conductor at a lower energy level, resulting in the upward band bend from semiconductor to conductor and the Schottky barrier (ϕSB) (Schottky junction) will form (the left side of Fig. 1a) [9,27,28]. Certainly, Wc may be smaller than Ws, the downward band bending will form and no barrier forms, meaning an ohmic contact (the left side of Fig. 1b) [9,29]. For the p-Type semiconductor, the Schottky junction and ohmic contact are shown in the right side of Fig. 1. It is obvious that the charge carriers transfer in ohmic contact should be smoother compared with that of the Schottky junction due to no barrier in ohmic contact [29]. However, the charge carriers can also return to the semiconductor by the ohmic contact due to no barrier, resulting in the recombination of the charge carriers. Generally, the efficiency of the Schottky junction is higher than that of the ohmic contact for the separation or suppression of the charge carriers [30,31]. Although the noble metals, especially platinum (Pt), possess the absolute advantages in the conductor-semiconductor system due to the large work function, this is not acceptable for the commerce [[32], [33], [34], [35]]. Thus, the noble-metal-free conductors are naturally considered.

In this work, we synergistically utilized 2D semiconductor superiorities and noble-metal-free Schottky junction effect to boost the photocatalytic H2 evolution performance. As a proof of concept, we chose CuS with a large work function as an electron acceptor to assemble on the surface of the ultrathin 2D g-C3N4 with a larger bandgap and the higher transfer and separation efficiencies compared with the bulk g-C3N4 via the in-situ growth method. Compared with the bulk g-C3N4, the photocatalytic H2 evolution performance of 2D g-C3N4 was improved from zero to “one” without any co-catalyst. And the introduction of CuS in 2D g-C3N4 further boosted the photocatalytic activity. The process of the charge carriers transfer and the photocatalytic mechanism were researched in detail.

Section snippets

Chemicals

Melamine (C3H6N6, >99.0%), Copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, >99.0%), Thiourea (CH4N2S, >99.0%), Ethylene glycol (EG, C2H6O2, ≥99.0%), Cetyl trimethyl ammonium bromide (CTAB, C19H42BrN, ≥99.0%), Ethanol absolute (C2H6O, ≥99.7%) and Triethanolamine (TEOA, C6H15NO3, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were used as received without further purification. Deionized water was used through all experiments.

Synthesis of the samples

2D g-C3N4 was prepared by a modified

Results and discussion

As only when there is an intimate contact between an electron acceptor and a semiconductor can the Schottky junction be formed [9]. In this case, the in-situ growth method was used to synthesize composite photocatalysts with the Schottky junction. In the proof-of-concept demonstration, 2D g-C3N4 and CuS were employed as a model 2D photocatalyst and an electron acceptor to boost the photocatalytic H2 production from H2O under the visible light integrating 2D semiconductor superiorities and

Conclusions

In this work, we have taken advantages of 2D semiconductor superiorities and Noble-Metal-free Schottky junction effect for boosting the photocatalytic performance. 2D g-C3N4 can provide a wide-band structure (a stronger redox capability) and a high efficiency of transfer and separation of the photogenerated electron-hole pairs. When CuS comes into 2D g-C3N4, the photogenerated electrons can be trapped by CuS via the formed Schottky junction which also can prevent these electrons trapped by CuS

Acknowledgments

The authors genuinely appreciate the financial support of this work by the National Natural Science Foundation of China (21776118, 21476097). A Project Funded by Natural Science Foundation of Jiangsu Province (BK20180870). A Project Funded by China Postdoctoral Science Foundation (2017M620193) and the Priority Academic Program Development of Jiangsu Higher Education Institutions, High-tech Research Key laboratory of Zhenjiang (SS2018002). The calculation in this work was supported by the high

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