Exploiting the dynamic Sn diffusion from deformation of FTO to boost the photocurrent performance of hematite photoanodes
Graphical abstract
Introduction
Hematite (α-Fe2O3) is widely studied for photoelectrochemical (PEC) water splitting due to its favorable band gap (Eg~2.1 eV), maximum theoretical efficiency (12.9%), good electrochemical stability, abundance, non-toxicity, and cost-effectiveness [1], [2]. However, its PEC activity is limited by ultra-fast charge recombination (τ~10 ps), small hole diffusion length (2–4 nm), and poor carrier mobility (0.2 cm2 V−1 s−1) [3], [4]. The PEC activity can be improved by increasing the charge carrier density via intentional or unintentional doping [1], [4], [5], by increasing the hematite–electrolyte interface area via engineering nanostructure morphology [2], [6], [7], and by reducing the over-potential via surface treatments [7], [8], [9]. Annealing is one of the influential parameters that decide the physico-chemical properties of hematite. The photoactivity of hematite fabricated on FTO (F:SnO2) substrate at low temperature is very poor and requires a thermal activation at high temperatures (650–800 °C) to induce Sn doping from FTO for improvement [3], [10]. Conventionally, hematite is activated for efficient PEC performance via two-step heat treatment that involves initial low-temperature heat treatment at 500–550 °C for 2–10 h and natural cooling until room temperature, followed by high-temperature annealing at 800 °C for 10–20 min [3], [7], [11], [12]. The purpose of low-temperature heat treatment for synthesis of hematite electrodes by different methods such as colloidal, doctor-blade, hydrothermal and sputtering was reported to be for removing the organics, bringing about the nanostructured growth and particle connectivity. The improvement in photoactivity due to high-temperature annealing was mainly attributed to the n-type doping of Sn cations diffused from the deformed FTO substrate. Such cationic incorporation dramatically decreases the resistivity of the film as a result of enhanced donor density, thus positively affecting the photoactivity of the photoanodes. However there are two decisive factors that may limit the photoactivity of hematite. One is behavior of FTO conductivity during two-step annealing and another is formation of oxide layer during the first low-temperature annealing. As FTO's role is to efficiently drive the photo-generated electrons to the back contact, maintaining its conductivity to optimum level is crucial during high-temperature annealing. We noticed that the loss of conductivity in FTO is severe for the hematite photoanodes fabricated by conventional annealing approach as a result of more FTO deformation due to longer hours of processing time. The pre-formed oxide layer at the interface of FTO may also pose interference for the dynamic diffusion of Sn ions during re-crystallization of hematite lattice at high-temperature annealing. Importantly, the Sn ions leached during FTO deformation have to diffuse into the hematite lattice in a dynamic way so that the structural ordering of hematite is unperturbed. Therefore, there is a need to maintain a trade-off between the amount of Sn ions leached from FTO and minimum loss in conductivity of FTO. To overcome this difficulty, we used a direct one-step activation of hematite using as-grown iron-coated FTO electrodes at high temperature for optimum duration followed by a mild quenching in air. Such heat treatment not only enables the cost-effective synthesis of hematite but also yields improved PEC performance.
In this work, it is demonstrated that different post-growth annealing conditions can significantly influence the photocurrent response of hematite photoanodes. We report on efficient utilization of Sn diffusion from the underlying FTO (F:SnO2) substrate during hematite activation from FTO deformation at high temperature (HT) to boost the photocurrent of α-Fe2O3 photoanodes. We used two different states of electrodes for hematite activation: As-grown Fe/FTO (one-step activation at HT) and Fe2O3/FTO (two-step activation at 550 °C+HT). The annealing temperature and the time for HT activation of hematite were optimized systematically by means of photocurrent performance. The state of material to be annealed is found to affect the dynamics of Sn diffusion, electron transport and the PEC performance of photoanodes. The effect of Sn diffusion into hematite matrix on the changes in donor density of hematite, shift in onset potential of water oxidation, deterioration in FTO conductivity, and structural ordering are discussed. The possible mechanism during hematite activation is discussed to understand annealing phenomena.
Section snippets
Synthesis of α-Fe2O3 photoanodes
The iron films were grown on fluorine doped tin oxide (FTO, F:SnO2, 10–15 sq. cm−1)-coated glass substrates by a facile pulse reverse electrodeposition (PRED) method according to our previous study [13]. The process parameters of PRED such as amplitude of square wave pulse, duty cycle, pulse period, and deposition time were [10 V (−6/+4 V)] 20%, 10 ms, and 45 s, respectively. The as-grown samples were rinsed in deionized water and dried immediately using nitrogen stream. For one-step annealing, the
Photoelectrochemical study of α-Fe2O3 photoanodes
Fig. S1a and b shows the current density–voltage (J–V) curves and photocurrent density (Jph) variation measured at 1.23 V vs. RHE (VRHE) for hematite photoanodes prepared with different one-step annealing temperatures. The one-step-annealed hematite prepared at 800 °C for 15 min showed Jph of 561 µA cm−2 at 1.23 VRHE, which is 11% higher than that of the two-step-annealed hematite prepared at the same high temperature condition. The onset potential (Vonset) of the water oxidation photocurrent is in
Conclusions
Hematite activation from FTO deformation at high temperature (viz. 800 °C) has been the choice of common practice to induce Sn doping in order to improve electronic conductivity and hence the PEC performance of hematite. This work introduces an effective way to exploit the dynamic diffusion of Sn ions leached during hematite activation from FTO deformation at 800 °C for 13.5 min by avoiding an intermediate low-temperature annealing step commonly employed for conventional approach. Such activation
Acknowledgments
This research was supported by BK Plus and the Basic Science Research Programs of the National Research Foundation of Korea (NRF, 2012R1A6A3A04038530). This subject is supported by Korea Ministry of Environment (2014000160001) as Public Technology Program based on Environmental Policy.
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