In-situ construction of photoanode with Fe2O3/Fe3O4 heterojunction nanotube array to facilitate charge separation for efficient water splitting
Graphical Abstract
Introduction
At present, the use of semiconductor electrode materials to drive photoelectrochemical (PEC) water splitting for hydrogen production has been widely concerned. In particular, effective photoanodes with good performance and long-term stability play a vital role in the system, thus gradually becoming pursuing target [1], [2], [3]. Water oxidation over anode materials involving multi-electron transfer and multi-proton transfer is often considered as the rate-limiting step throughout the PEC reaction process. Therefore, it is particularly important to select an appropriate photoanode material.
Currently, Bi2WO6, TiO2, Fe2O3, WO3, BiVO4, and Bi2O3 are semiconductors that have been widely investigated as photoanodes [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. As a resource-rich candidate, hematite has been paid much attention owing to its suitable band gap (~2.1 eV), outstanding chemical stability in alkaline aqueous solutions, and excellent theoretical photocurrent density of 12.6 mA cm−2 [14], [15], [16]. However, the application of pure Fe2O3 is limited by its slow water oxidation kinetics, short hole diffusion length, and easy electron-hole recombination [6], [17]. To overcome these limitations, it is necessary to modify Fe2O3 with morphology regulation, element doping, and creating heterojunction or passivation layer [18], [19], [20].
Nanostructures have been considered effective in reducing carrier transfer length for enhanced PEC performance. By constructing nanoscale photoanodes, especially one-dimensional nanostructures such as nanowires (NWs) [21], [22], [23], [24], nanorods (NRs)[25], [26], and nanotube arrays (NTAs)[27], [28], the separation and transfer of photogenerated charges can be facilitated effectively. Fe2O3 nanotubes with unique hollow structures can shorten the transfer path of photogenerated carriers and hence improve photoelectrical efficiency. Moreover, the highly ordered nanotubes prepared by anodization have a large specific surface area with respect to the compact planar film [29], [30]. Nevertheless, the performance improvement through introducing nanostructure is not sufficient enough to meet the demands. Chang et al. [31] prepared dense α-Fe2O3 NTAs as the photoanode, which achieved an improved photocurrent density of 0.72 mA cm−2 at 0.5 V (νs. Ag/AgCl). To accelerate water oxidation kinetics for further enhancing PEC water oxidation, much effort has been devoted to reducing the recombination of photogenerated electrons and holes by doping various metal ion species (including Ti4+, Mg2+, Rh3+, Y3+, and Pt4+) or constructing surface co-catalysts such as Pt, Au, Pd, Ag, CdS, and SnO2 [32], [33]. Furthermore, constructing heterojunctions induces an internal electric field, which separates photogenerated electron-hole pairs, thus improving carrier transportation efficiency and carrier lifetime. For example, Kyesmen et al. loaded CuO onto α-Fe2O3 as a supporting catalyst, and the resultant electrode realized a photocurrent density of 0.53 mA·cm−2 at 1 V νs. RHE (reversible hydrogen electrode) [34]. Wang et al. constructed Fe2TiO5/Fe2O3/Pt type-II heterojunctions to enhance the electron-hole pair separation effectively, promoting the photocurrent density to 2.4 mA·cm−2 at 1.6 V νs. RHE [35]. In addition, there are some researches currently using in-situ methods to construct heterojunctions. Ruan et al. [36] and Wei et al. [37] fabricated TiO2 and CuO heterojunctions in situ by a series of methods, separately. In-situ heterojunctions accelerate the rapid transport of interfacial charges and improve the transfer efficiency of carriers.
Magnetite (Fe3O4) is widely used as catalyst because of its good electrical conductivity and high chemical stability [38], [39]. Some studies indicate that Fe3O4/Fe2O3 heterojunction can effectively promote charge transfer [40], [41], so as to demonstrate predictable application prospects in photoelectrocatalytic water splitting. However, it is worth noting that the relative position and relative content ratio of Fe3O4 to Fe2O3 have a great influence on photoelectrochemical performance. Hiralal et al. [42] found that the Fe3O4 layer located between Fe substrate and Fe2O3 hinders photo-induced electron transportation, yielding only a photocurrent density of 0.18 mA·cm−2 at 1.23 V νs. RHE. Leduc et al. [40] synthesized α-Fe2O3/Fe3O4 heterostructural films on FTO. The Fe3O4 located on heterojunction surface enhanced photo-induced electron transportation and promoted photocurrent density to 0.48 mA cm−2 at 1.23 V νs. RHE.
Therefore, it is necessary to design reasonable photo-anode structures to facilitate electron transfer and reduce the recombination of electrons and holes for obtaining high photocurrent density and PEC reaction efficiency. In present work, Fe2O3/Fe3O4 heterojunction NTAs were constructed in situ by sequential annealing in different atmospheres, reaching a photocurrent density of 2.5 mA cm−2 at 1.6 V νs. RHE in 1 M NaOH and a hydrogen production of 23.84 μmol cm−2 h−1. The in-situ prepared Fe2O3/Fe3O4 heterojunction facilitates the electron transport at interface and effectively reduces electron-hole pair recombination, thus solving the problem of short hole diffusion length and slow water oxidation kinetics.
Section snippets
Synthesis of samples
A commercial iron foil with a thickness of 0.2 mm (purity of 99.99 %) was cut into 10 mm × 30 mm pieces, and then mechanically polished to remove surface oxide layer. A piece of iron foil was ultrasonically cleaned with acetone for 30 min, and then washed with alcohol and deionized water. Fe2O3 nanotube arrays were prepared by anodic oxidation on a stabilized DC voltage supply (model YK-AD1003D). The iron foil was anodized in 100 mL of NH4F ethylene glycol (EG) solution consisting of 0.55 g of
Results and discussion
The morphology of vertically aligned iron oxide nanotubes prepared by anodic oxidation is shown in Fig. S1a. Fe2O3-NTAs with uniform distribution and identical size on Fe substrate are obtained. From cross-sectional SEM image, it can be seen that the diameter of Fe2O3 nanotubes is 30–50 nm, and the length of the nanotubes is about 3 µm. Generally, one-dimensional structure provides more active sites for PEC reactions and shorter hole transport distance from the bulk to the electrode/electrolyte
Conclusion
In this work, Fe2O3/Fe3O4 heterojunction is constructed in situ by two-step annealing. When Fe2O3-NTAs are annealed at high temperature in argon atmosphere, lattice oxygen readily spills out of Fe2O3 crystal as oxygen atoms and leaves behind electrons of equivalent charge, thus reducing surrounding Fe3+ to Fe2+. The oxidation state and symmetry of Fe atoms change from Fe3+ octahedral coordination to Fe2+ tetrahedral coordination environment, which transfers the crystal lattice from hexagonal
CRediT authorship contribution statement
Jinbo Xue: Project administration, Writing – review & editing, Supervision. Narui Zhang: Data curation, Investigation, Writing – original draft. Qianqian Shen: Funding acquisition, Carried out laboratory research, Provision of reagents, materials, and computing resources. Qi Li: Carried out laboratory research. Xuguang Liu: Formal analysis, Writing – review & editing. Husheng Jia: Carried out laboratory research, Funding acquisition. Rongfeng Guan: Verification of the experimental design,
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.
Acknowledgment
The financial support is gratefully acknowledged from the National Natural Science Foundation of China (NSFC) (Grant No. 62004137, 21878257 and 21978196), Natural Science Foundation (NSF) of Shanxi Province (Grant No.20210302123102), Key Research and Development Program of Shanxi Province (Grant No.201803D31042), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (Grant No.2019L0156), Research Project Supported by Shanxi Scholarship Council of China (
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