Sn self-doped α-Fe2O3 nanobranch arrays supported on a transparent, conductive SnO2 trunk to improve photoelectrochemical water oxidation
Introduction
Semiconductor-based artificial photosynthesis, especially photoelectrochemical (PEC) water splitting, has been considered to be an ideal and environmentally friendly method for solar hydrogen generation [1]. The main challenge of PEC water splitting is in the development of a suitable photoanode with a narrow bandgap energy, appropriate bandedge position, high chemical stability, and low fabrication cost [2]. A hematite (α-Fe2O3), one of the most prevalent materials on Earth, with a band gap of 1.9–2.2 eV can satisfy the key criteria [3]. Nevertheless, intrinsic natures like the short hole diffusion length (2–20 nm) and poor carrier mobility limits its charge separation and collection efficiency as a PEC anode [4].
On-going efforts have been pursued to overcome this problem by altering its morphology to promote charge collection [5], incorporating dopant into α-Fe2O3 to improve charge-diffusion distance [6], [7], [8], [9], or both [10], [11]. Wang et al. fabricated the nanonet-based α-Fe2O3 nanostructure with short diffusion distance and large surface area through an atomic layer deposition technique, achieving 1.6 mA/cm2 at 1.23 V vs. a reversible hydrogen electrode (RHE) [12]. Zou et al. have obtained a photocurrent of 0.7 mA/cm2 at 1.23 V/RHE by Ti4+ doped α-Fe2O3 films at one order higher carrier concentrations than with an undoped sample [13]. Remarkably, the best photocurrent of 2.3 mA/cm2 at 1.23 V/RHE was reported on the Si doped dendritic α-Fe2O3 nanostructure that was synthesized by a chemical vapor deposition (CVD) protocol [14]. Therefore, the combination of the doping and nanoarchitecturing methods has been regarded as a promising approach to improve the PEC performance of the α-Fe2O3 photoanode [10], [14], [15], [16], [17], [18].
However, it is still difficult to control both the morphology and doping-level simultaneously in a wet chemical route that is simple, inexpensive, and scalable for industrial production [19], [20]. It is due to critical morphology changes, such as an agglomeration by a dopant source [15]. A solution to the problem could be found in the pioneer work of Sivula et al., in which Sn atoms could be diffused from the FTO glass substrate and incorporated into α-Fe2O3 thinfilm by high temperature (800 °C) sintering. Consequently, Sn doped α-Fe2O3 thinfilm was obtained [21]. Thus, post-annealing with a dopant source can be an alternative approach to solution-based doping. We developed a Sn self-doped, three dimensional (3D) α-Fe2O3 nanorod (NRs) nanostructure by using a conductive SnO2 nanobelt (NBs) array as a dopant source and a backbone. Sn atoms were successfully diffused from SnO2 NBs to Fe2O3 NRs by post annealing at relatively low temperatures (650 °C). The Sn doped and hierarchical α-Fe2O3 nanostructure yields a superior PEC performance than the doped or branched α-Fe2O3 electrodes.
Section snippets
Materials and synthesis
The thermal evaporation method was used to obtain the one-dimensional SnO2 conductive backbone. Sn (99.5%, Samchun, Korea) powders and FTO glass (TEC-8, Pilkington, USA) were utilized as the source material and substrate, respectively. To enhance the electrical conductivity of the SnO2 backbone, small amounts of Sb (99.9%, High Purity Chemicals, Japan) powders were mixed with Sn powders (about 10%). Au nanoparticles were deposited on the FTO substrate by an ion coater (IB-3, Eiko Engineering,
Material synthesis and characterization
XRD graphs in Fig. 1 reveal the crystal structure and phase purity of the three samples, which is α-Fe2O3 NRs fabricated on SnO2 NB arrays by a CBD method for 2 h, 4 h, and 12 h. All samples were further heat treated for phase-transition from FeOOH to α-Fe2O3. As the CBD reaction time increased, the diffraction peaks corresponding to α-Fe2O3 increased while that of SnO2 decreased. Moreover, as shown in normalized XRD patterns on the basis of SnO2 (110) peaks (Fig. S1), it is clearly revealed
Conclusion
A 3D hierarchical nanostructure composed of a conductive SnO2 backbone and Sn-doped α-Fe2O3 branch has been successfully developed. Surprisingly, Sn4+ was considerably diffused and self-doped into the α-Fe2O3 lattice at low annealing temperatures (650 °C) in that each α-Fe2O3 nanorod was directly junctioned with SnO2 nanobelts. An increase in the flat band potential and carrier density was achieved by incorporating Sn4+ into α-Fe2O3 nanorods. In optimum conditions, the Sn doped α-Fe2O3/SnO2
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2012R1A2A2A01045382). This work was also supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2011-0031574).
References (35)
- et al.
Microemulsion-mediated solvothermal synthesis of ZnS nanowires
Mater Lett
(2007) - et al.
Comparative study of gas sensor performance of SnO2 nanowires and their hierarchical nanostructures
Sens Actuators B Chem
(2010) - et al.
Solar water splitting cells
Chem Rev
(2010) - et al.
Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook
Energ Environ Sci
(2013) - et al.
Charge carrier trapping, recombination and transfer in hematite (α-Fe2O3) water splitting photoanodes
Chem Sci
(2013) Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting
Chem Soc Rev
(2013)- et al.
Nanostructured hematite: synthesis, characterization, charge carrier dynamics, and photoelectrochemical properties
Energ Environ Sci
(2012) - et al.
Enhancement of photoelectrochemical hydrogen production from hematite thin films by the introduction of Ti and Si
J Phys Chem C
(2007) - et al.
Improved photoelectrochemical performance of Ti-doped alpha-Fe2O3 thin films by surface modification with fluoride
Chem Commun
(2009) - et al.
Pt-doped α-Fe2O3 thin films active for photoelectrochemical water splitting
Chem Mater
(2008)