Full Length ArticleCriss-crossed α-Fe2O3 nanorods/Bi2S3 heterojunction for enhanced photoelectrochemical water splitting
Graphical abstract
Introduction
To fight against climate change and exhausting energy sources, researchers have looked for numerous ways to replace fossil fuels with carbon-free substitutes such as hydrogen (H2). Photoelectrochemical cells (PECs) have the potential to provide hydrogen fuel through artificial photosynthesis, a rising renewable innovation that utilizes solar light to drive a chemical reaction such a way to split water into hydrogen and oxygen [1], [2], [3]. The success of the PEC cell lies not only in how its photoelectrode responds to light to produce hydrogen, but also oxygen. Extensive work on discovering a potential material that can fulfill all the criteria for efficient PECs water splitting is still a long-standing challenge [4]. Hence there has been growing attention to the development and modification of semiconductors possessing high stability and high PECs water splitting capability [5]. Moreover, creating low-cost photoanodes that can work under ambient conditions with high catalytic activity is of great prominence and a challenge for environmentally friendly practical application [6].
Literature surveys show that hematite (α-Fe2O3) is one of the most promising materials to be used as a photoanode for PEC cells. α-Fe2O3 is an n-type semiconductor, possessing numerous properties useful for water oxidation like amazing chemical stability, cost-effectiveness, natural abundance of its constituent elements, non-toxicity, and suitable valence band position for water oxidation [7], [8], [9], [10]. Additionally, α-Fe2O3 possesses a suitable bandgap of 2.1 eV that theoretically provides a huge current density of 12.6 mA cm−2 at Air Mass (AM) 1.5G [4], [11]. Besides all of these advantages, the PEC performance of α-Fe2O3 is still low due to certain intrinsic drawbacks including poor reaction kinetics for oxygen evolution reaction (OER), short hole diffusion length (2–4 nm), high carrier recombination rate, and positive conduction band edge relative to the hydrogen redox potential. Therefore, it needs an external bias potential to complete the photoelectrochemical water splitting reaction [12], [13], [14], [15], [16], [17]. Many strategies have been applied to overcome the impact of these drawbacks such as changing the electrical conductivity of α-Fe2O3 by varying the crystallite size (nanostructuring engineering), elemental doping, surface’s modification of α-Fe2O3 by using various oxygen evolution catalysts e.g. Ni, Co, Ir, and Ru based one to improve the photocurrent and reduce the overpotential [18], [19], [20], [21], [22], [23], [24], [25]. Among various material modification techniques formation of heterojunction of α-Fe2O3 with various semiconductors is an efficient way to upgrade the material efficiency by creating a potential gradient inside the photoelectrode that helps in the effective charge separation and enhances the charge transfer rate at the semiconductor electrolyte heterojunction [26], [27]. There have been several studies revealing the coupling of α-Fe2O3 with certain metal chalcogenides like CdS, Sb2S3, MoS2 and Bi2S3[28], [29], [30], [31], [32].
Among binary metal chalcogenides Bi2S3 has attracted much attention because of its small direct bandgap (1.3–1.9 eV), high absorption coefficient (104-105 cm−1), sensible photon to current conversion efficiency, and also less toxic as compared to other transition metals like Cd, Hg, Pb based chalcogenides [33], [34], [35], [36]. Thus, fabrication of Bi2S3 with α-Fe2O3 will result in the formation of appropriate heterojunction that provides a better band alignment leading to better separation of photogenerated electron and hole and also enhancing the visible light utilization of the solar spectrum. Recently, Mahadeo et al. developed Bi2S3 nanosheet (NS)/Zr doped Fe2O3 nanorod (NR) heterojunction photoelectrode and indicated that deposition of Bi2S3(NS) over Zr doped Fe2O3 NR enhances the photocurrent density [37]. Yunji et al. synthesized 3D leaf-like Bi2O3-Bi2S3 leaf NS on Fe2O3 nanoparticles (NPs), which enhances light absorption efficiency, favoring light refraction and improving charge separation[38]. Recently, Xiang et al. investigated Bi2S3 SILAR cycle impact on Fe2O3–Bi2S3 heterostructure on photoelectrochemical hydrogen production in concentrated Na2SO3-Na2S solution, indicating the number of SILAR cycles of Bi2S3 deposition deposition alters on Fe2O3–Bi2S3 photoresponse [39]. Although α-Fe2O3–Bi2S3 heterojunctions have been studied to improve the photoelectrochemical performance of bare α-Fe2O3, however enhancement in the photocurrent density is still far away from commercialization. Thus, it has been anticipated to develop new structures and combine various techniques to achieve more robust PEC performance by fabricating α-Fe2O3/Bi2S3 heterojunction.
In the present work, we have synthesized α-Fe2O3/Bi2S3 heterojunction on FTO substrate by using three different synthetic techniques.α-Fe2O3 nanorods synthesis was carried out by the hydrothermal approach while Bi2S3 deposition was performed by combining successive ionic layer adsorption reaction (SILAR) and solvothermal method. The main idea behind using the SILAR process is to provide a seed layer before the solvothermal method for uniform distribution of Bi2S3 over entire α-Fe2O3 nanorods. This α-Fe2O3/Bi2S3 heterojunction was found to effectively utilize the visible region of the solar spectrum and enhance the effectiveness of charge separation at the interface leading to an improved PEC performance as compared to pristine α-Fe2O3.
Section snippets
Chemicals and materials
Iron (III) chloride anhydrous (FeCl3, min 98 %), bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 98%), Urea (CO(NH2)2 99.0–101.5%), Thiourea (CH4N2S, ≥99.0%), sodium sulfide nonahydrate (Na2S·9H2O, ≥98.0%), Sodium sulfite (Na2SO3 95%), ethylene glycol (C2H6O2 ≥ 99.0%), hydrochloric acid (HCl 37%), acetone (C3H6O 99.5%), ethyl alcohol (C2H5OH 99.9%) and fluorine-doped tin oxide (FTO, ∼8 Ω/Sq.) were purchased from Sigma–Aldrich and Merck. All chemicals were of analytical grade.
Synthesis of α-Fe2O3 nanorods
α-Fe2O3 nanorods
Results and discussions
Fig. 1(a) shows the top view FESEM images of pristine α-Fe2O3 grown over FTO substrate. FESEM images show that α-Fe2O3 nanorods grow in a crisscross manner randomly crossing each other and densely covering the FTO substrate. Fig. 1(b) shows the FESEM image of α-Fe2O3/Bi2S3.6 h photoanode synthesized without seed layer by solvothermal approach with a reaction time of 6 h. It has been observed that urchin-shaped Bi2S3 are randomly distributed over α-Fe2O3, however, most parts of the α-Fe2O3 are
Conclusion
In summary, α-Fe2O3/Bi2S3 heterojunction photoanodes based on FTO substrate have been synthesized to improve the photoelectrochemical performance of α-Fe2O3 for efficient water splitting. The seed layer approach plays an important role in the uniform distribution of Bi2S3 over α-Fe2O3 to make a perfect heterojunction connection. PEC studies show that among all the photoanode, α-Fe2O3/Bi2S3 6 h shows a maximum photocurrent density of 2.542 mA cm2 at 1.23 V RHE which is almost 20 times higher
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
The author greatly acknowledges The Scientific and Technological Research Council of Turkey (TUBITAK) 2216 Research fellowship program for foreign citizens and Scientific Research Projects Unit (FBA-2019-12171) of Çukurova University for supporting this work.
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