Oxygen defects-mediated Z-scheme charge separation in g-C3N4/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution

https://doi.org/10.1016/j.apcatb.2017.01.067Get rights and content

Highlights

  • g-C3N4/oxygen-defective ZnO heterojunction photocatalysts were fabricated.

  • Oxygen vacancies improved the light absorption and mediated the Z-scheme mechanism.

  • Z-scheme charge transfer enhanced the charge separation efficiency.

  • Nanocomposite enhanced visible-light degradation of 4-chlorophenol and H2 evolution.

Abstract

g-C3N4 nanosheets were coupled with oxygen-defective ZnO nanorods (OD-ZnO) to form a heterojunction photocatalyst with a core-shell structure. Multiple optical and electrochemical analysis including electrochemical impedance spectroscopy, photocurrent response and steady/transient photoluminescence spectroscopy revealed that the g-C3N4/OD-ZnO heterojunction exhibited increased visible-light absorption, improved charge generation/separation efficiency as well as prolonged lifetime, leading to the enhanced photocatalytic activities for the degradation of 4-chlorophenol under visible-light illumination (λ > 420 nm). An oxygen defects-mediated Z-scheme mechanism was proposed for the charge separation in the heterojunction, which involved the recombining of photoinduced electrons that were trapped in the oxygen defects-level of OD-ZnO directly with the holes in the valence band of g-C3N4 at the heterojunction interface. The detection of surface generated reactive species including radical dotO2 and radical dotOH clearly supported the Z-scheme mechanism. Moreover, the g-C3N4/OD-ZnO photocatalysts also exhibited enhanced visible-light Z-scheme H2 evolution activity, with an optimal H2 evolution rate of about 5 times than that of pure g-C3N4. The present work not only provided an alternative strategy for construction of novel visible-light-driven Z-scheme photocatalysts, but also gained some new insights into the role of oxygen-defects of semiconductors in mediating the Z-scheme charge separation.

Introduction

Recently, a metal-free semiconductor, graphitic carbon nitride (g-C3N4) has emerged as an attractive material in the fields of environment and sustainability. The facile synthesis, high physicochemical stability and suitable band-gap (∼2.7 eV) make it a promising visible-light photocatalyst [1], [2]. However, the low efficiency of this polymeric photocatalyst caused by the high recombination rate of photoinduced charges limits its practical application [3], [4]. Strategies have been developed to suppress the charge recombination in g-C3N4, such as textural design [5], doping [6] and noble metal loading [7], etc. In particular, coupling g-C3N4 with other semiconductors to form a heterojunction has been widely investigated owing to the efficient spatial separation of charges [2].

Basically, two types of heterojunctions with similar staggered band structure but different pathways for interfacial charge transfer, namely type-II heterojunction and solid-state Z-scheme system, have been well established [8], [9]. In a type-II heterojunction, the photoinduced electrons transfer to a less negative conduction band (CB) while the holes to a less positive valence band (VB), leading to their lower redox ability. However, in a Z-scheme system, the photoinduced electrons from less negative CB and holes from less positive VB will recombine at the interface, leaving behind the electrons and holes with stronger redox ability separately on two semiconductors. Therefore, the Z-scheme system exhibits dual advantages of efficient charge separation and high redox ability, which are more suitable for the photocatalytic process requiring high redox potentials (especially for H2 evolution) [8]. Based on this, various g-C3N4-based Z-scheme photocatalysts such as g-C3N4/TiO2, g-C3N4/BiOI and g-C3N4/WO3, etc. have been fabricated and they showed enhanced performances in multiple photocatalytic processes including decomposition of organics, H2 evolution and CO2 conversion [10], [11], [12], [13], [14]. Nevertheless, to date, g-C3N4-based heterojunction photocatalysts are still dominated by the type-II heterojunction. In addition, although an increasing number of g-C3N4-based Z-scheme photocatalysts have been recently reported, only a handful of them provided convincible experimental evidences to support the proposed mechanisms [1]. Given these conditions, it is still desirable to develop novel strategies for construction of g-C3N4-based Z-scheme photocatalysts as well as explore their interfacial charge separation mechanism.

Another main drawback of g-C3N4 is its limited adsorption edge of 460 nm. When constructing a g-C3N4-based Z-scheme photocatalyst, the visible-light utilization ability of the other component should be considered. Many g-C3N4-based Z-scheme systems containing wide-band semiconductors such as TiO2, ZnO and BiOCl, etc. usually need to work efficiently under UV illumination [10], [13], [15], [16]. On the other hand, some narrow band-gap semiconductors used in the Z-scheme system, such as CdS and Ag3PO4 [17], [18], etc. are unfortunately unstable and suffer from serious photodecomposition. Therefore, many efforts have been devoted to the coupling of g-C3N4 with band-engineered semiconductors for improved visible-light absorption. For example, g-C3N4 was hybridized with S-doped TiO2 [19] or N-doped ZnO [20] to form Z-scheme photocatalysts with enhanced visible-light absorption, which were attributed to the formation of dopants-isolated energy levels in TiO2 and ZnO. In addition to foreign elemental doping, introducing defects as “self-doping” to create defects-isolated levels in semiconductors is an alternative approach to extend light absorption. Specially, the oxygen defects have been demonstrated to be effective in adjusting the band levels of many metal oxides-bearing Z-scheme systems [21], [22], [23], [24], [25]. For example, various semiconductors with rich oxygen vacancies such as Bi20TiO32 [24], ZnO [22], SnO2-x [23], [25] etc., were coupled with g-C3N4 and the obtained Z-scheme heterojunction showed enhanced visible-light photocatalytic degradation of organics and CO2 reduction. It was generally suggested that oxygen-defective metal oxides extended the visible-light absorption owing to their defects-induced band gap narrowing. However, in these reported g-C3N4-based heterojunction systems, the critical roles of oxygen defects in mediating the Z-scheme charge separation were not fully identified. Further insight into the interfacial charge transfer mechanisms will facilitate the construction of visible-light-driven Z-scheme photocatalysts by employing oxygen defects.

Herein, g-C3N4 nanosheets were coupled with oxygen-defective ZnO nanorods (OD-ZnO) to form a core-shell heterojunction. The rich surface oxygen vacancies in OD-ZnO played dual-function roles of improving visible-light absorption and mediating the Z-scheme charge separation. Multiple optical and electrochemical analyses were employed to evaluate the charge separation efficiency of the heterojunction, and the Z-scheme mechanism was convinced by detecting the surface generated reactive species. The enhanced charge generation and separation efficiency of the g-C3N4/OD-ZnO photocatalysts led to their superior-visible-light activities for degradation of 4-chlorophenol. Moreover, the oxygen defects-mediated Z-scheme H2 evolution from a g-C3N4-based photocatalyst was demonstrated for the first time.

Section snippets

Synthesis of photocatalysts

Commercial chemicals of analytical grade and deionized water with a resistivity >18  cm−1 were used in all the experiments. Commercial ZnO powder (denoted as C-ZnO, >99.9%) with few defects was used as a reference in this work and was purchased from Sigma-Aldrich.

Oxygen-defective ZnO nanorods (OD-ZnO) were synthesized from ε-Zn(OH)2 precursor by a solution conversion method based on our previous work with modifications [26]. In brief, ε-Zn(OH)2 was prepared by drop-wise addition of 50 mL of 2.0 

Results and discussion

Oxygen-defective ZnO nanorods (OD-ZnO) were simply synthesized by a solution-conversion approach using ε-Zn(OH)2 as precursor. Fig. 1 shows that the OD-ZnO displays a needle-like morphology with diameters of 50–200 nm and lengths of 0.5–4 μm. The XRD pattern (Fig. S2) reveals the hexagonal würtzite structure. A few damaged domains (marked by dashed yellow circles) could be observed in the HRTEM image of the nanorods (Fig. 1b). Such a lattice deformation should be mainly caused by the surface

Conclusion

In summary, heterojunction photocatalysts were fabricated by coupling g-C3N4 nanosheets with oxygen-defective ZnO nanorods. The rich oxygen vacancies in OD-ZnO play dual-function roles of improving visible-light absorption and mediating the efficient Z-scheme charge separation of the g-C3N4/OD-ZnO heterojunction, leading to their enhanced visible-light photocatalytic performances for degradation of 4-chlorophenol as well as H2 evolution. The present work demonstrated an effective strategy for

Acknowledgements

This work was financially supported by the National Science Foundation of China (Nos. 51234003 and 51374138), National Science Foundation of Jiangsu Province (BK20160328) and National Key Technology Research and Development Program of China (2013BAC14B02). Jing Wang is grateful for the scholarship from China Scholarship Council (No. 201506210232).

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