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Sulfur-doped graphitic carbon nitride incorporated bismuth oxychloride/Cobalt based type-II heterojunction as a highly stable material for photoelectrochemical water splitting

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Highlights

  • Co/S-gC3N4/BiOCl showed high photocurrent density of 393.0 μA cm−2.

  • High stability was achived in PEC water splitting upto 10,800 sec (3 h) at 1.23 V vs. RHE.

  • ABPE efficiency of Co/S-gC3N4/BiOCl was 3.7 and 17-fold greater than S-gC3N4/BiOCl and BiOCl.

  • The PEC activity and stability enhanced by integration of cobalt cocatalyst in the heterojunction.

Abstract

Cobalt incorporated sulfur-doped graphitic carbon nitride with bismuth oxychloride (Co/S-gC3N4/BiOCl) heterojunction is prepared by an ultrasonically assisted hydrothermal treatment. The heterojunction materials have employed in photoelectrochemical (PEC) water splitting. The PEC activity and stability of the materials are promoted by constructing an interface between the visible light active semiconductor photocatalyst and cocatalysts. The photocurrent density of Co-9% S-gC3N4/BiOCl has attained 393.0 μA cm−2 at 1.23 V vs. RHE, which is 7-fold larger than BiOCl and ~3-fold higher than 9% S-gC3N4/BiOCl. The enhanced PEC activity can be attributed to the improved electron-hole charge separation and the boosted charge transfer is confirmed by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) analysis. The fabricated Co/S-gC3N4/BiOCl nanohybrid material has exhibited high stability of up to 10,800 s (3 h) at 1.23 V vs. RHE during PEC water splitting reaction and the obtained photo-conversion efficiency is 3.7-fold greater than S-gC3N4/BiOCl and 17-fold higher than BiOCl. The FESEM and HRTEM images have revealed the formation of heterojunction interface between S-gC3N4 and BiOCl and the elemental mapping has confirmed the presence of cobalt over S-gC3N4/BiOCl. The heterojunction interface has facilitated the photo-excited charge separation and transport across the electrode/electrolyte interface and also the flat-band potential, which is confirmed by Mott-Schottky analysis.

Graphical abstract

Cobalt incorporated sulfur doped graphitic carbon nitride with bismuth oxychloride (Co/S-gC3N4/BiOCl) heterojunction was prepared by ultrasonically aided hydrothermal treatment. The designed catalyst was applied for photoelectrochemical water splitting through type-II heterojunction. High stability was achieved during PEC water splitting up to 10,800 s (3h) at 1.23 V vs. RHE.

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Introduction

Photoelectrochemical (PEC) water splitting has emerged as an eco-friendly and promising green technology approach for the generation of hydrogen energy by means of converting solar energy into chemical energy to resolve the energy demand [1], [2], [3]. The PEC water-splitting reaction is performed as two half-reactions of water oxidation and reduction, however, the oxidation of water is considered as the cornerstone step for determining reaction rate of the PEC water splitting process [4], [5]. Thus, tremendous efforts have been paid for the discovery of appropriate materials with prominent properties [6] such as wide range solar spectrum, low-cost, remarkable photo-stability, suitable band edge, and band alignment for PEC water splitting [7], [8], [9]. A variety of photocatalysts have been explored for PEC water splitting [10] such as metal oxides, nitrides, phosphates, chalcogenides, carbon-based semiconductor materials, etc. [11], [12], [13]. However, the invention of a stable and efficient semiconductor photocatalyst to attain greater hydrogen production during PEC water splitting reaction has always been regarded as a difficult task [14], [15]. In the field of photocatalysis and photoelectrocatalysis, this has enormously struggled with numerous issues [16] such as narrowed light absorption [17], improper band position towards water splitting either water oxidation or reduction activity [18]. To overcome these limitations, synthesis of highly stable and visible-light-active semiconductors is essential. Among the two-dimensional (2D) photoactive semiconductor materials, bismuth-based photocatalyst materials have been implemented for photocatalysis and PEC water splitting [19], [20]. Moreover, bismuth oxyhalides (BiOX) have been comprehensively scrutinized for photocatalysis due to their good optical and electrical properties, and unique layered tetragonal crystal structures [21], [22]. Among the bismuth oxyhalides, the BiOCl semiconductors have been utilized towards the degradation of toxic pollutants, owing to their admirable photocatalytic activity over TiO2 [23], [24], [25]. The BiOCl possesses an underlying inimitable layered configuration that could be distinguished by the layers of [Bi2O2]2+ interleaved with Cl ions [21], [26]. Therefore, it is considered as a promising aspirant due to the internal electric field of BiOCl that rapid charge separation and transfer of photoexcited charge carriers [27], [28]. In this context, the BiOCl semiconductors have been extensively used in photocatalytic applications such as organic pollutants degradation in water [29], dye degradation [30], nitrogen fixation [31], photocatalytic water splitting [32], and CO2 reduction [33]. However, the broad bandgap of BiOCl (3.2 eV) could only utilize the UV region in the solar spectrum that restricts its solar energy utilization to a large extent. Recently, numerous strategies have been adopted to improve these limitations by modification with oxygen vacancies [34], [35], deposition of metals [36], [37], and formation of heterojunction interfaces [38], [39]. Likewise, the graphitic carbon nitride (g-C3N4), a polymeric carbon-based semiconductor is a low-cost, non-toxic, highly stable and ease of fabrication with bandgap of ~2.7 eV and a consequence, it has comprehensively used in photocatalysis [40] and photoelectrocatalysis [41], [42], [43] applications. Recently, M. Shalom and co-workers has reported a benchmark photocurrent density of 353 μA cm−2 at 1.23 V vs. RHE with faradaic and hole-extraction efficiency of 51 and 62%, respectively using carbon nitride films [44]. Besides, the carbon nitride electrode exhibits the photocurrent density of 270 μA cm−2 at 1.23 V vs. RHE in 0.1 M KOH solution with high stability up to 18 h [45]. On the other hand, the sulfur-doped g-C3N4 has improved the photocatalytic activity for hydrogen production under visible light illumination [46]. But, the PEC activity of g-C3N4 is highly affected by their photoinduced charge recombination which obstructs its overall efficiency and exhibits poor stability during PEC reaction. Meanwhile, the stability and PEC activity has been improved by assimilation of cocatalysts into semiconductors photocatalysts [47], [48]. The cocatalysts minimize the activation energies, charge carriers trapping, and charge recombination, leading to improved surface reactions [49]. Moreover, the g-C3N4/BiOCl heterojunctions have been investigated for photocatalytic dye degradation of Rhodamine B [50], Methylene blue [51], and photoelectrochemical detection of ciprofloxacin [52]. The above mentioned facts pertaining to heterojunctions has inspired us to investigate PEC water splitting using cobalt incorporated sulfur doped g-C3N4/BiOCl nanohybrid heterojunction. According to the PEC experimental and analytical characterization results, the reasonable PEC water splitting mechanism is predicted.

Section snippets

Synthesis of S-gC3N4

A 5 g of thiourea was ground using pestle and mortar for 1 h and the fine powder was transferred to alumina crucible. The S-gC3N4 was obtained by pyrolysis of thiourea calcined at 550 °C (2 °C/min) for 4 h.

Synthesis of S-gC3N4/BiOCl nanohybrid material

A desired amount of S-gC3N4 and 0.25 M of Bi(NO)3·5H2O were mixed with water and ethylene glycol (EG), which was subjected to ultrasonication for 1 h to get homogeneous solution A. Simultaneously, 0.25 M of KCl was dissolved in water and EG and it was sonicated for 1 h. The solution was added

Characterization of Co/S-gC3N4/BiOCl nanohybrid material

XRD results was used to scrutinize crystal structure of the synthesized materials that disclosed diffraction peaks corresponding to S-gC3N4 at 2θ values 13.1° (1 0 0) and 27.5° (0 0 2) planes originated from the structural packing of interlayer and aromatic segments of interplanar stacking, respectively (Figure S1A) and it is well-matched with the JCPDS Card No. 00-87-1526 [53], [54]. The XRD pattern of BiOCl is attributed by the well-built correlation with tetragonal phase in p4/nmm space group

Conclusion

The Co/S-gC3N4/BiOCl nanohybrid heterojunction was prepared by a simple ultrasonically aided hydrothermal method and the same was used in photoelectrochemical water splitting. A high current density of 393.0 μA cm−2 at 1.23 V vs. RHE was obtained for Co-9% S-gC3N4/BiOCl, which was 7-fold larger than BiOCl and ~3-fold higher than 9% S-gC3N4/BiOCl. The photo-conversion efficiency reached 0.11% which was 3.7-fold greater than S-gC3N4/BiOCl and 17-fold higher than BiOCl due the formation of

CSIR-CECRI manuscript communication number

CECRI/PESVC/Pubs./2020-020.

CRediT authorship contribution statement

S. Vinoth: Methodology, Investigation, Formal analysis, Data curation, Validation, Writing - original draft. Wee-Jun Ong: Validation, Writing - review & editing. A. Pandikumar: Conceptualization, Project administration, Funding acquisition, Writing - review & editing.

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

Dr. A. Pandikumar thank DST-Technology Mission Division, New Delhi for financial support through Hydrogen and Fuel Cell (HFC) – 2018 Scheme (File No.: DST/TMD/HFC/2K18/101). Mr. S. Vinoth (IF170687) is a recipient of the DST-Inspire Senior Research Fellowship. The authors thank Central Instrumentation Facility, CSIR-Central Electrochemical Research Institute for the instrument facilities.

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