Yolk-shell ZnO@C-CeO2 ternary heterostructures with conductive N-doped carbon mediated electron transfer for highly efficient water splitting

https://doi.org/10.1016/j.jcis.2021.07.052Get rights and content

Highlights

  • A facile synthesis route is utilized to produce Z-scheme yolk-shell ZnO@C-CeO2.

  • After calcination, a conductive N-doped graphitic carbon layer is generated.

  • The photocatalysts showed enhancement toward visible light absorption.

  • A superior PEC water splitting performance is achieved: 7.43 mA/cm2 at 1.18 V RHE.

  • N-doped carbon electron-transfer layer prolonged life-time of light in yolk-shell.

Abstract

Herein, carbon-incorporated yolk-shell ZnO@C-CeO2 ternary heterostructures are employed as visible light responsive photocatalyst for highly efficient photoelectrochemical (PEC) water splitting. Compared to conventional ZnO/CeO2 semiconductors, introduction of a thin PDA shell layer assures the generation of a conductive N-doped graphitic carbon layer after a calcination post-treatment with mesoporous hollow morphologies. The evaluation of PEC water splitting performance of ZnO@C-CeO2 photoanodes reveals the maximum photocurrent density as 7.43 mA/cm2 at 1.18 V RHE under light whereas almost no response is recorded at dark. These superior PEC H2 evolution performance strongly implies efficient charge separation, facilitated charge transfer between photoanode and electrolyte interface as well as within the semiconductor bulk by means of rapid electron transfer ability of N-doped graphitic carbon layer and prolong life time of light inside yolk-shell structure. Furthermore, considerable depression in PL intensity of ZnO@C-CeO2 photoanodes compared to ZnO clearly reveals a higher photon absorption due to the reflection of light in hollow region and increase in electron hole separation efficiency. Moreover, plausible Z-scheme charge transfer mechanism using ZnO@C-CeO2 photoanodes under visible light illumination is verified using radical trapping experiments and X-ray photoelectron spectroscopy (XPS) methods, suggesting new generation of heterostructures for sufficient conversion of sunlight to H2 fuels.

Introduction

Photoelectrochemical (PEC) systems with good potential of solar energy-to-hydrogen (STH) efficiency have been expected to be an alternative approach for traditional fossil fuels since 1970 [1], [2], [3]. In a typical PEC water splitting system, the design and fabrication of feasible photoelectrodes with efficient light harvesting, accelerated surface redox reactions, and promoted charge separation are key parameters to achieve a high solar-to-hydrogen efficiency [4], [5], [6]. Due to their unique features, inorganic semiconductors (i.e., TiO2, WO3, BiVO4, CdS, metal halide perovskite, and etc.) with hierarchical shapes contributed to the PEC applications individually or together with their interfaces (i.e., solid–liquid or solid–solid between photoelectrode-electrolyte and inside photoelectrode, respectively) [7], [8], [9], [10], [11], [12], [13], [14].

Recently, development of low-cost and affordable noble metal-free photoelectrodes have attracted the great attention of research community toward typical TiO2 based photocatalysis to fulfill the above mentioned highly efficient PEC system requirements [15], [16]. Among various photoelectrode materials, ZnO has been preferred due to its high electron mobility (210 cm2V-1s−1) to achieve an efficient solar-to-hydrogen conversion [17]. However, the wide band gap (∼3.37 eV) limits its photocatalytic activities for only UV region of solar light which consists only 5% of incident solar light spectrum, providing low utilization of visible light [18]. Therefore, most research efforts have focused on the development of effective strategies such as interface engineering to extend the photoresponsivity of ZnO based photoelectrodes in visible region during PEC water splitting processes [17], [19]. Hence, ceria (CeO2) is a promising n-type candidate (with band gap of ∼3.2 eV) that attracted the considerable attention thanks to the abundant oxygen vacancies and good oxidation reduction properties for photocatalytic applications [20]. Early studies showed that ZnO/CeO2 structures demonstrated enhanced visible light photocatalytic activity thanks to the suppression of the recombination of photogenerated electron-hole pairs [21], [22], [23], [24]. Although early studies have provided remarkable findings, the poor conductivity of ZnO/CeO2 semiconductors leaving room for the fabrication of electrically conductive star shape yolk-shell nanostructures that have not been employed for PEC water splitting until now. Generally, yolk-shell structures with single-core@single-shell morphologies were considerably utilized as an alternative photocatalytic platform with remarkable enhancement in light scattering and solar photon conversion efficiency [25].

Herein, carbon-incorporated yolk-shell ZnO@C-CeO2 heterostructures were employed for highly efficient photoelectrochemical water splitting. The carbonization of multifunctional polydopamine (PDA) shell layer significantly provided the electrical conductivity of hybrid yolk-shell ZnO(core)@C-CeO2(shell) nanostructures. Compared to conventional ZnO/CeO2 semiconductors, the introduction of PDA not only facilitated the auto-redox reaction for reduction of CeO2 by catechol moieties, but also more importantly created a N-doped graphitic carbon layer with mesoporous hollow morphologies (after calcination). The physico-chemical properties and photoelectrochemical analysis of newly synthesized heterostructures were performed. Additionally, the Z-scheme charge transfer mechanism of ZnO@C-CeO2 photoanodes was verified by radical trapping experiments and XPS as well. To the best of our knowledge, this one-step fabrication procedure is a first attempt for synthesis of yolk-shell ZnO@C-CeO2 ternary heterostructures with highly enhanced photocatalytic water splitting under visible light illumination.

Section snippets

Chemicals

Dopamine hydrochloride, N,N-dimethylformamide (DMF, anhydrous 99.8%), Zinc acetylacetonate (Zn(acac)2), sodium hydroxide (NaOH), cerium nitrate hexahydrate (Ce(NO 3)3·6H2O), polyvinylpyrrolidone (PVP), acetone (99.8%), benzoquinone (BQ), terephthalic acid (TPA), and sodium sulfate (Na2SO4) were obtained from Sigma-Aldrich, USA. Absolute ethanol was purchased from Merck, Germany.

Synthesis of carbon-incorporated yolk-shell ZnO@C-CeO2 heterostructures

Briefly, ZnO nanoparticles (NPs) with star shape morphologies were synthesized according to the literature [26]. Then,

Results and discussion

Scheme 1 represented the synthetic pathway for yolk-shell ZnO@C-CeO2 heterostructures. As seen here, star-shaped ZnO NPs were selected as starting template, followed by PDA coating and CeO2 deposition. The obtained ZnO@PDA-CeO2 composites were calcinated @450 °C, achieving the successful synthesis of yolk-shell ZnO@C-CeO2 ternary heterostructures.

Fig. 1A shows the SEM images of ZnO NPs, realizing a narrow and monodisperse size distributions. After PDA coating onto ZnO NPs, the as-synthesized

Conclusion

In summary, mesoporous yolk-shell ZnO@C-CeO2 ternary heterostructures are synthesized by introduction of PDA layer between ZnO (as starting template) and CeO2 (as shell layer) and subsequently a calcination post treatment is conducted in order to carbonize PDA. The incorporation of a PDA shell layer assures dual purposes such as generation of CeO2 by catechol functional groups and fabrication of a conductive N-doped graphitic carbon layer between ZnO and CeO2 as well. The good engineering of

CRediT authorship contribution statement

Nuray Celebi: Investigation, Writing - review & editing, Resources. Kouroush Salimi: Methodology, Investigation, Writing - review & editing, Visualization, Resources, Supervision.

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.

Acknowledgement

This study was partially supported by Scientific and Technical Research Council of Turkey (TÜBİTAK, Project No. 119 M076) and Projects Office of Ankara Yildirim Beyazit University (Project No. 5706 and FMG-2020-2019).

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