Zahra-Sadat
Pourbakhsh
a,
Kyana
Mohammadi
a,
Ahmad
Moshaii
*a,
Maryam
Azimzadehirani
b and
Amir
Hosseinmardi
a
aDepartment of Physics, Tarbiat Modares University, P.O Box 14115-175, Tehran, Iran. E-mail: moshaii@modares.ac.ir
bDepartment of Chemistry, Tarbiat Modares University, P.O Box 14115-175, Tehran, Iran
First published on 7th October 2019
We have investigated the effect of deposition of a α-Fe2O3 thin layer on a substrate of TiO2 nanoparticles for photoelectrochemical (PEC) water splitting. The TiO2 layer was coated on an FTO substrate using the paste of TiO2 nanoparticles. The α-Fe2O3 layer was deposited on the TiO2 thin film, using the method of Successive Ionic Layer Adsorption and Reaction (SILAR) with different cycles. Various characterizations including XRD, EDX and FE-SEM confirm the formation of α-Fe2O3 and TiO2 nanoparticles on the electrode. The UV-visible absorption spectrum confirms a remarkable enhancement of the absorption of the α-Fe2O3/TiO2/FTO composite relative to the bare TiO2/FTO. In addition, the photocurrents of the composite samples are remarkably higher than the bare TiO2/FTO. This is mainly due to the low band gap of α-Fe2O3, which extends the absorption spectrum of the α-Fe2O3/TiO2 composite toward the visible region. In addition, the impedance spectroscopy analysis shows that the recombination rate of the charge carriers in the α-Fe2O3/TiO2 is lower than that for the bare TiO2. The best PEC performance of the α-Fe2O3/TiO2 sample was achieved by the sample of 70 cycles of α-Fe2O3 deposition with about 7.5 times higher photocurrent relative to the bare TiO2.
In PEC water splitting, hydrogen is generated by the absorption of light in a semiconductor, which must have a suitable bandgap and suitable energies of conduction and valance bands relative to the oxidation and reduction potentials of water. Also, it should be chemically stable in the electrolyte of the PEC process.10–12
In recent years, TiO2 nanoparticles have been extensively studied for PEC water splitting due to their low cost, simple preparation, non-toxicity and chemical stability. However, the high bandgap of TiO2 (3.2 eV for the anatase and 3.0 eV for the rutile phase) only allows absorption of the ultraviolet region of sun light, which covers only 5% of the solar spectrum. This significantly decreases the efficiency of PEC using the TiO2 nanostructures. To improve the PEC properties of TiO2, many methods such as sensitization to the sunlight with doping of nitrogen or heavy metals13–17 and incorporating plasmonic nanoparticles such as Ag, Au, Pt18–20 have been investigated in the literatures. Most of these approaches have the drawback of increasing crystal deformations or introducing defects that act as additional electron–hole recombination centers.21–26 In addition, combining TiO2 with another semiconductor with suitable band alignments has been reported to achieve larger electron/hole separation due to the movement of charge carriers between the different semiconductors. With this regard, Fe2O3 with a bandgap of about 1.9–2.2 eV, which can absorb photons of visible light spectrum,27,28 is a suitable semiconductor to be mixed with TiO2 to extend the adsorption spectrum of the composite efficiency.29 However, two main disadvantages of Fe2O3 are the low conductivity and a short diffusion length of excitons (2–20 nm), which significantly diminish its photocatalytic efficiency.30,31 However, these deficiencies of Fe2O3 can be resolved by combining TiO2 with Fe2O3, due to a strong electric field at the interface of Fe2O3/TiO2.30–32 Accordingly, the combination of these two semiconductors has attracted many attentions recently.33–43
In this work, a composite electrode of α-Fe2O3/TiO2 was introduced as a photoanode for PEC cells. The SILAR method was used for the deposition of α-Fe2O3 on the TiO2 substrate. Fig. 1 shows a schematic of the composite electrode in the water-splitting process. Under visible light irradiation, the composite electrode produces electron–hole pairs and the holes oxidize water at the electrode surface to generate oxygen. The produced electrons migrate to the counter electrode and take part in the reduction of water to produce hydrogen. We studied the optimum thickness of α-Fe2O3 on the TiO2 substrate to obtain the best water-splitting activity. We found that under 70-cycle deposition of α-Fe2O3, the best PEC water-splitting activity occurs, which corresponds to about 7.5 times photocurrent improvement relative to the bare TiO2 electrode. This result is mainly due to extension of the absorption spectrum of the sample into the visible light, due to incorporating α-Fe2O3. In addition, the recombination of charge carriers greatly decreases by using the α-Fe2O3/TiO2 composite.
Fig. 2 A schematic of the SILAR process for deposition of α-Fe2O3 nanoparticles on a sample for one cycle. |
Fig. 3 A photograph of the samples of α-Fe2O3/TiO2/FTO with different SILAR deposition cycles of α-Fe2O3. |
For the electrochemical measurements, the entire substrate was illuminated by a 100 W Xe lamp, so the illumination area of the α-Fe2O3/TiO2 layer was 1 cm × 1 cm. In addition, the illumination was frontside.
In order to investigate the morphology of the samples, the Field Emission Scanning Electron Microscopy (FESEM) of the samples was taken using the MIRA3 TESCAN microscope. The top and the cross-section images of the TiO2 samples are shown in Fig. 5. We see that the thickness of the TiO2 layer on the surface is around 400 nm. While a very thick layer of Fe2O3 (about 18 μm) is produced on the sample by the 70-cycle SILAR deposition.
The EDX analysis was used for elemental determination of the samples. The weight percentages of the elements Ti, O, Fe and Au (which is used for preparation of the layers for EDX/FESEM) for the samples with different SILAR cycles are shown in Fig. 6. It is seen that the iron percentage of the best α-Fe2O3/TiO2 sample for the PEC application (which is the 70-cycle sample) was about 23.15% (wt).
Fig. 6 EDX analysis of various samples of α-Fe2O3/TiO2/FTO with (a) 60, (b) 70, (c) 80 SILAR cycles. |
Fig. 7 shows the UV-vis absorption spectrums of the bare TiO2 and the α-Fe2O3/TiO2 sample with the 70-cycle. The bare TiO2 has an absorption edge around 390 nm (∼3.13 eV), which is related to its wide bandgap. The absorption edge of α-Fe2O3/TiO2 has been transferred to the visible range with a much higher absorption intensity than the bare TiO2 in the range of 400–800 nm.
The bandgaps of the fabricate TiO2 and Fe2O3/TiO2 film are calculated using the Tauc plot. As it showed in Fig. 8, the tangent lines indicates that the Fe2O3 has an indirect band gap of about 2.04 eV and direct band gap of TiO2 is about 3.35 eV for our sample; which is agreed with other observations.42,45
Fig. 8 Tauc plot calculated to find the band gap of the layer from the UV-vis spectrum (a) α-Fe2O3/TiO2 (b) pure TiO2. |
Fig. 9 (a) The J–V measurement curves and (b) chronoamperometry measurements of the α-Fe2O3 loaded TiO2/FTO samples with different deposition cycles. |
Fig. 9(a) shows the photocurrent–voltage (J–V) curves of various samples for both dark and light illuminations. The dark current curves of all samples are close to zero with no photoelectrochemical activity under the dark condition. The bare sample of TiO2 has a small photocurrent density of around 0.04 mA cm−2. The J–V curves of the samples confirm that the current density of the 70 cycles deposition of α-Fe2O3 gives the highest photocurrent (around 0.3 mA cm−2). With increasing the number of deposition cycles to 80 cycles, the current density of the sample decreases. In fact, the 70-cycle deposition of α-Fe2O3 is the optimum thickness for getting the best water-splitting properties from the composite sample.
Fig. 9(b) shows the transient photocurrent curves of various samples for the ON–OFF period of 20 s. We see that the photocurrent rapidly increases when the light is turned on and it immediately decreased to zero when the light is turned off. According to the results of Fig. 9(b), after about 80 seconds, the photocurrent becomes stable and repeatable. In addition, there is a small slope for the current curve under the light ON condition indicating that the electron–hole recombination is small in the complex of α-Fe2O3/TiO2/FTO.
The main reason for increment of the photocurrent density of the α-Fe2O3/TiO2/FTO electrode relative to the bare TiO2 goes back to the higher absorption of the composite material. As shown in Fig. 7, TiO2 mainly absorb short wavelength photons below 400 nm. However, adding α-Fe2O3 to the electrode can extend the optical absorption to the visible spectrum.46 As a result, the low energy photons can be absorbed by the α-Fe2O3 part of the photoelectrode and more photoelectrons can be produced by the composite electrode.47
The next reason is the location of the α-Fe2O3 edge conduction band compare to TiO2 (as shown in Fig. 10). Under UV-vis illumination, electrons can be excited from the valence band (VB) to the conduction band (CB) of the anatase, creating a charge vacancy in the VB. In the absence of the α-Fe2O3, most of electron–holes are combined quickly.39,47
Fig. 10 Schematic of the electron–hole generation and the transferring of the photogenerated charges into the photoanode under the visible light illumination. |
Increasing the coating cycles of hematite (from 50 to 70 cycles), had a significant improvement in current density. According to the results of Fig. 9, the sample of 70-cycle represents the best photocurrent. However, with increasing the number of cycles to 80 cycles, the photocurrent density decreases. Therefore, it is indicated that there was an optimal amount of deposition layer. By increasing the amount of α-Fe2O3 nanoparticles on the surface of the TiO2 nanoparticles, the surface was blocked and light access was greatly reduced, and the production of the electron–hole decreased, so the photocurrent was reduced. So when the amount of α-Fe2O3 was increased from an optimal amount, because photoelectrons accumulated on α-Fe2O3, the recombination probability of the electron–holes increased.48 So α-Fe2O3 being as an electron–hole recombination center, and resulted a reduction in optical activity.49
In order to investigate the charge transport properties of the bare TiO2 and α-Fe2O3/TiO2 samples, the electrical impedance spectroscopy measurements (EIS) were performed for various samples. The applied frequency range was from 100 mHz to 100 kHz (in a 0.1 M Na2SO4 electrolyte at the potential of V = 0.7 V versus the Ag/AgCl reference). The Nyquist plots obtained by the EIS measurements in addition to the equivalent RC circuit for fitting the EIS data are shown in Fig. 11. In the equivalent circuit, Rs indicates a serial resistance between the FTO substrate and the TiO2 layer. Rhf demonstrates the high frequency resistance due to accumulation and trapping of fast charges throughout the α-Fe2O3/TiO2 composite. Rss represents the resistance of charge transfer between the photoanode and the electrolyte. The capacitances Chf and Css are relating to the space charge capacitance of the sample and the capacitance associated with the surface states of the photoanode, respectively.
Fig. 11 (a) Nyquist plots of TiO2/FTO and the sample of α-Fe2O3/TiO2/FTO with 70 cycles. (b) The equivalent circuit used for the fitting of EIS results. |
As shown in Fig. 11, the curve of α-Fe2O3/TiO2 photoanode has a smaller semicircle radius than the bare TiO2, which indicates that both Rhf and Rss are lower for the α-Fe2O3/TiO2 relative to the bare TiO2. This result completely confirms the improvement of charge transfer kinetics of the bare TiO2 sample after the Fe2O3 modification. The faster charge transfer of the 70-cycle sample leads to less surface recombination and improving the photocatalytic activity of this sample relative to all other ones.
It should be mentioned that TiO2 has better charge transport properties relative to Fe2O3, considering the mobility of electrons and the holes in both semiconductors. Therefore, deposition of a thin layer of TiO2 under Fe2O3 in the samples causes that the electrons are better transmitted to the external circuit relative to the case of blank Fe2O3 on FTO. In fact, the recombination between electron and holes at the interface of TiO2/FTO is less than that of Fe2O3/FTO. Consequently, a higher photocurrent is expected from the stack of Fe2O3/TiO2/FTO relative to that of Fe2O3/FTO. This issue has clearly been confirmed by other researchers.42
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