Lowering the Onset Potential of Fe - ACS Publications

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Lowering the Onset Potential of Fe2TiO5/Fe2O3 Photoanodes by Interface Structures: F and Rh Based Treatments Jiujun Deng, Xiaoxin Lv, Kaiqi Nie, Xiaolin Lv, Xuhui Sun, and Jun Zhong ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Lowering the Onset Potential of Fe2TiO5/Fe2O3 Photoanodes by Interface Structures: F and Rh Based Treatments Jiujun Deng, Xiaoxin Lv, Kaiqi Nie, Xiaolin Lv, Xuhui Sun, and Jun Zhong* Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China

ABSTRACT: Hematite is recognized as a promising photocatalyst for solar water oxidation. However, its practical performance at a low bias is still very low due to large onset potential. Here we report a combination of F-treatment and Rh-treatment on Fe2TiO5/Fe2O3 to lower the onset potential with a large value of 230 mV. The final onset potential is 0.63 V vs. RHE comparable to the lowest value ever reported for hematite. The F and Rh co-treated photoanode yields a high photocurrent of 1.47 mA/cm2 at 1.0 V vs. RHE, which is more than 3 times of that for the pristine sample. X-ray photoelectron spectroscopy reveals the existence of surface Ti-F bonds after F-treatment. Moreover, the surface groups can be further modified after immersion in the working NaOH solution, which can form an interfacial hydrogen-bond network to accelerate the hole transfer. The surface Fe atoms are also partly reduced to accelerate the hole transport. Rh-treatment can further lower the onset potential with a good stability. The enhanced performance can be attributed to a synergetic effect of F and Rh based treatments, in which F

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based interface structure accelerates the hole transfer while Rh based material improves the catalytic performance.

KEYWORDS: Hematite; interface structure; lowering the onset potential; F-treatment; Rhtreatment; solar water splitting.

INTRODUCTION Hematite is widely recognized as a good photocatalyst for solar water oxidation due to its favorable band gap (2.1-2.2 eV), good stability in oxidative solution, low cost and wide abundance.1-6 However, its performance remains insufficient due to various factors such as low conductivity, improper conduction band position, poor oxygen evolution reaction (OER) kinetics and short hole diffusion length.7-10 Especially, the performance of hematite at a low bias is not high enough due to large onset potential.11-16 It originates from the improper conduction band position of hematite which needs an external bias about 0.4 V vs. RHE.4 Moreover, various factors such as poor OER kinetics and surface recombination also lead to additional bias.11-16 To address these problems, many effective methods have been developed. Surface cocatalysts such as IrO2, Co-Pi, and FeNiOx were widely used to catalyze the OER process and then lower the onset potential.4-6 The way to reduce the electron-hole recombination at the surface was also widely studied.12-17 By constructing a passivation layer, the surface recombination can be directly blocked.12-14,17 However, the passivation layer should be well coupled with hematite and allow the hole transfer to the electrolyte with good light transparency.13 Some other methods to reduce surface recombination were also developed. For example, high temperature annealing was used to decrease the surface defects to suppress the recombination.18 Jang et al. reported a re-growth


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method of hematite to reduce surface disorder for less recombination.10 Acid treatments were also reported as an effective way for the suppression of electron-hole recombination.15,16 Although various methods were used, the convenient way to modify hematite with surface groups and then construct an interface structure has only very limited study for lowering the onset potential. Here we show a facile F-treatment of hematite to build up an interfacial hydrogen-bond network for faster hole transfer, which can effectively suppress the surface recombination and then lower the onset potential. Although F-treated hematite photoanodes have been previously reported, the enhanced performance was simply attributed to F-doping or the band position shift.7,8 The detailed interface structure of F-treated hematite to enhance the performance, especially that in the working solution, has not been investigated.7,8 Recently, F-treated TiO2 was reported to form surface Ti-F bonds and then create an interfacial hydrogen-bond network in solution to accelerate the hole transfer.19 Since Ti based treatments in hematite have been widely used with great success,20-28 similar interface structure can also be expected for Ti-treated hematite. The formation of Fe2TiO5 in hematite (Fe2TiO5/Fe2O3) was widely reported with high performance and here we use Fe2TiO5/Fe2O3 as a starting material for further enhancement.23-27 The F-treated Fe2TiO5/Fe2O3 exhibits a large cathodic shift of the onset potential (160 mV) when compared to the sample before F-treatment. X-ray photoelectron spectroscopy (XPS) reveals the existence of various Ti-F bonds similar to that in the literature.19 Interestingly, after immersion in the working NaOH solution, the F-treated Fe2TiO5/Fe2O3 photoanode shows a modified interface structure to form the hydrogen-bond network. The surface Fe atoms are also partly reduced to accelerate the hole transfer. Moreover, the F-treated sample can be well coupled with Rh based cocatalyst to further lower the onset potential with a total value of 230 mV. The final onset


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potential is 0.63 V vs. RHE comparable to the lowest value ever reported for hematite.4-6,11-16,29 The F and Rh co-treated photoanode is capable of yielding a high photocurrent of 1.47 mA/cm2 at 1.0 V vs. RHE, which is more than 3 times of that for the pristine sample (0.39 mA/cm2). The F and Rh co-treated photoanode can also overcome the stability problem of Rh reported in the literature.30 The excellent performance is attributed to a synergetic effect of F and Rh, in which F based interface structure accelerates the hole transfer while Rh based material improves the catalytic performance.

EXPERIMENTAL SECTION Preparation of photoanodes: A modified hydrothermal method was used to prepare the pristine hematite photoanodes grown on a fluorine-doped SnO2 (FTO, Nippon Sheet Glass, Japan, 14 ohm/sq) glass substrate.3,24,31 Briefly, a Teflon-lined stainless steel autoclave was filled with 80 mL aqueous solution containing 1.62 g ferric chloride (FeCl3.6H2O, Sinopharm Chemical Reagent Co., Ltd.) and 0.48 g glucose (C6H12O6, Sinopharm Chemical Reagent Co., Ltd). Then the autoclave with FTO glass slide (50 mm×30 mm×2 mm) was heated at 95 °C for 4 h to produce FeOOH on FTO. The FeOOH-coated substrate was then sintered in air at 550 °C for 2 h and at 750 °C for additional 15 min. The product was labeled as the pristine hematite (Fe2O3). To prepare Ti-modified hematite, the FTO substrate was firstly immersed in a TiCl4 aqueous solution at 75 °C for 30 min, then annealed in air at 180 °C for 15 min. The Ti-modified FTO was then used to prepare hematite by using the same steps as that for the pristine hematite. The final product was labeled as Ti-modified hematite (Fe2TiO5/Fe2O3). To prepare F-treated sample, the Ti-modified hematite was immersed in a solution containing 0.37 g NH4F (Sinopharm Chemical Reagent Co., Ltd), 5 mL H2O2 (30%, Sinopharm Chemical


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Reagent Co., Ltd) and 5 ml H2O at 60 oC for 5 min. Then the sample was heated in air at 200 oC for 20 min. The sample was labeled as F-treated hematite (with Ti-modification). For Rhtreatment, photo-assisted electrodeposition was applied by using a three-electrode PEC cell. The F-treated sample as the working electrode was firstly cycled for 5 times in 1 M NaOH solution from -0.4 to 0.8 V vs. Ag/AgCl under AM 1.5G illumination (scan rate, 50mV/s). The sample was washed. Then 0.25 mM RhCl3 solution was added in the PEC cell and the F-treated sample was cycled for 5 times again. The final produce was washed and labeled as Rh-F-treated sample. The Rh-treated sample without F was prepared by the same method but the deposition sample was not treated by NH4F and H2O2 solution. Structural characterization: Scanning Electron Microscope (SEM, FEI Quanta 200F), X-ray Diffraction (XRD, PANalytical, Zmpyrean), X-ray photoelectron Spectrometer (XPS, Kratos AXIS UltraDLD) and Transmission Electron Microscopy (TEM, FEI Tecnai G2 F20 S-TIWN) were used for the morphology and structural characterization. X-ray absorption spectra were collected at the National Synchrotron Radiation Laboratory (NSRL, XMCD beamline) and the Beijing Synchrotron Radiation Facility (BSRF, Soft X-ray beamline). PEC Measurements: The working area of hematite photoanode was about 0.1 cm2 with the left part covered by non-conductive Hysol epoxy. All PEC measurements were performed by using an electrochemical workstation (CHI 660D). It was also used for the electrochemical impedance spectra (EIS) measurement. The pH value of the electrolyte (1 M NaOH) was controlled at 13.6. The measured voltage was converted into the potential vs. reversible hydrogen electrode (RHE). The potential was swept from 0.6 V to 1.8 V vs. RHE at a scan rate of 50 mV/s. Xenon High Brightness Cold Light Sources (XD-300) equipped with AM 1.5 ﬁlter were used as the light


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source with a power density of 100 mW/cm2. IPCE were measured using a Xenon lamp (CELHXF300/CEL-HXBF300, 300W) coupled with a monochromator (Omni-λ3005).

RESULTS AND DISCUSSION Here we use Ti-modified hematite (Fe2TiO5/Fe2O3) as the starting material. Ti based treatments were widely used to improve the performance of hematite.20-28 Especially, it is easy to form a Fe2TiO5-Fe2O3 heterostructure to enhance the hole transport due to a favorable valence band position of Fe2TiO5.24-27 Recently, the Fe2TiO5-incorporation in bulk hematite by a pretreatment of the FTO substrate was also reported to accelerate the charge separation and hole transfer and then improve the performance.23 Although mainly Fe2TiO5 located in the surface or near surface regions serves as a heterojunction to improve the surface hole transfer, a bulk Fe2TiO5-incorporation can always keep a Ti-based surface for the further F-treatment, which can avoid the removal of surface Fe2TiO5 in the treatment. In this work we also treat the fluorinedoped SnO2 (FTO) with Ti precursor and then grow hematite on the Ti-modified FTO substrate to prepare Fe2TiO5-incorporated hematite.23 The experimental procedure has been shown in the experimental section and Supporting Information Figure S1. The TEM mapping in the Supporting Information Figure S2 clearly shows the existence of Ti in hematite. XRD data in the Supporting Information Figure S3 also confirms the hematite crystal structure (JCPDS 33-0664). The synchrotron radiation X-ray absorption spectroscopy (XAS) data at the Ti L-edge are shown in the Supporting Information Figure S4, which clearly reveal the formation of Fe2TiO5 in the Timodified sample with a single peak between the separated peaks B1 and B2 for TiO2.23,24 We thus label the Ti-modified hematite sample as Fe2TiO5/Fe2O3 for further treatments.


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Figure 1. (a-c): SEM images of Fe2O3, Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3, respectively. (d-f): High-resolution TEM images of Fe2O3, Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3, respectively. Fe2TiO5/Fe2O3 was then treated by the NH4F and H2O2 water solution to produce F-treated sample (labeled as F-Fe2TiO5/Fe2O3). The detailed treatment can be found in the experimental section. Figure 1 shows the SEM and TEM images of the pristine (Fe2O3), Ti-modified (Fe2TiO5/Fe2O3) and F-treated (F-Fe2TiO5/Fe2O3) hematite nanostructures. From the SEM images, all three samples show a nanorod shape similar to the literatures.3,24,31 All the samples show a thickness around 250 nm from SEM cross section images (data not shown). TEM images clearly reveal the hematite crystal structure. No obvious surface coating can be observed after Ti or F based treatments. The TEM mapping in the Supporting Information Figure S5 confirms the decoration of F in F-Fe2TiO5/Fe2O3. XRD data in the Supporting Information Figure S6a also reveals that the F-treated sample keeps hematite crystal structure and no additional XRD peak


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can be observed. Absorption spectra of the samples before and after F-treatment are shown in the Supporting Information Figure S6b and no obvious difference can be observed.

Figure 2. (a) J-V curves of the pristine (Fe2O3) and Ti-treated (Fe2TiO5/Fe2O3, by various concentrations of TiCl4) hematite photoanodes. (b) J-V curves of Fe2O3, Fe2TiO5/Fe2O3 and FFe2TiO5/Fe2O3. Figure 2a compares the photocurrent density versus applied potential (J-V) scans of the hematite photoanodes before and after Ti-modification, while Figure 2b shows the J-V scans of Fe2O3, Fe2TiO5/Fe2O3 and the F-treated sample (F-Fe2TiO5/Fe2O3). The pristine sample shows a photocurrent of 0.87 mA/cm2 at 1.23 V vs. RHE, while the optimized Ti-modified hematite (4 mM) shows a significantly enhanced photocurrent of 1.61 mA/cm2, strongly confirming the presence of Fe2TiO5 structure in hematite to improve the performance.23 Moreover, after Ftreatment the F-Fe2TiO5/Fe2O3 sample in Figure 2b exhibits a significant cathodic shift of the onset potential with a value of 160 mV (we use the potential at the intersection point of dark current and the tangent at maximum slop of photocurrent to calculate the onset potential shift).15 Surface Ti-F bonds in TiO2 were recently reported to accelerate the hole transfer to the electrolyte by forming an interfacial hydrogen-bond network, which was observed by in-situ IR


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spectroscopy.19 Importantly, different fluorination configurations such as the adsorbed F onto the terminal Ti (Ft) and the substituted F at a bridge position between Ti atoms (Fbridge) were revealed to form the hydrogen bonds.19 Here the F-treated Fe2TiO5/Fe2O3 sample may also form surface bonds to improve the hole transfer due to the existence of Ti in the hematite sample.

Figure 3. (a) XPS survey spectra of Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3. (b)-(e): Highresolution XPS spectra at the F 1s, Fe 2p, O 1s and Ti 2p edges, respectively. The FFe2TiO5/Fe2O3 sample immersed in NaOH is labeled as F-Fe2TiO5/Fe2O3-NaOH.


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To confirm the hypothesis, XPS results were shown in Figure 3 to explore the F-treated interface structure. The XPS survey spectra of Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3 are shown in Figure 3a with strong Fe and O signals. Ti signal can be observed in both samples confirming the existence of Ti. Sn and C signals are also observed which come from the Sn diffusion from FTO substrate and the adsorbed carbon contaminations, respectively.3,31 F signal can be observed in FFe2TiO5/Fe2O3 and Figure 3b shows the high-resolution F 1s XPS spectra. Obviously, before Ftreatment no F signal can be observed, while after F-treatment the F-Fe2TiO5/Fe2O3 sample shows two prominent features for Ti-F bonds. According to the literature,19 the main peak at around 684.8 eV can be assigned to the fluorination configuration of Ft with F- adsorbed onto the terminal Ti, while another feature at around 687.1 eV can be attributed to the substituted F at a bridge position between Ti or other atoms (Fbridge) (see Figure 4b). A clear energy shift at the Ti 2p edge in Figure 3e can also be observed for F-Fe2TiO5/Fe2O3 when compared to Fe2TiO5/Fe2O3, indicating the effect of Ti-F bonds. The different chemical state of Ti has been confirmed by an energy shift of the main feature in the Ti L-edge XAS spectrum (Supporting Information Figure S7). The XPS results strongly confirm the existence of various fluorination configurations in F-Fe2TiO5/Fe2O3, which can form a hydrogen-bond network in solution.19 The surface structures before and after F-treatment have been illustrated in Figure 4a and 4b, respectively.


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Figure 4. Illustration of the surface structures for Fe2TiO5/Fe2O3 (a), F-Fe2TiO5/Fe2O3 (b), and F-Fe2TiO5/Fe2O3 immersed in the NaOH solution (c). The electronic structure of F-Fe2TiO5/Fe2O3 was further studied by immersing the sample in a NaOH solution. It was reported that the surface Ti-F bonds could be replaced by OH- groups in a NaOH solution.19 Since the hematite photoanode is working in a NaOH solution for the J-V scans, we further study the electronic structure of F-Fe2TiO5/Fe2O3 after an immersion in the NaOH solution for 10 min (washed for XPS experiments). The XPS results are also shown in Figure 3. Interestingly, after NaOH immersion the surface Ti-F bonds (684.8 eV) in Figure 3b


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have almost no change, while the bridge F at 687.1 eV has almost disappeared. It can be attributed to the unstable bridge site of F between Ti and Fe (Figure 4b). Figure 3d also shows the O 1s XPS spectra and after NaOH immersion an increased feature at around 533.0 eV can be observed, indicating the formation of OH- groups (the difference between F-Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3 can be attributed to the existence of Fe2TiO5). The OH- groups can replace the bridge F and connect to the neighboring Ti and Fe sites. However, the additional electron in OHgroup may reduce the neighboring Fe or Ti atoms. Actually, in Figure 3c the Fe 2p spectrum for the sample after immersion clearly shows a reduced feature for Fe2+ confirming the results. It is very interestingly to observe that F-treatment may lead to the reduction of surface Fe atoms. Ti 2p spectra before and after NaOH immersion show almost no difference in Figure 3e, suggesting the Ti sites have not been strongly affected. Thus the surface Ti-F bonds, OH- groups at the bridge sites and free OH- groups in the NaOH solution can form a hydrogen-bond network for faster hole transfer, as illustrated in Figure 4c. The Fe2+ can also accelerate the hole transfer from the bulk to the surface.23 Then the performance of F-Fe2TiO5/Fe2O3 can be significantly improved as shown in Figure 2. It should be noted that the Ti-treatment in hematite is important for the further F-treatment. Almost no F signal can be observed for the F-treated pristine hematite from the XPS (data not shown). J-V scans of the pristine hematite (without Ti-treatment) before and after F-treatment are shown in the Supporting Information Figure S8a. Although a cathodic shift of the onset potential after F-treatment can be observed, it is much smaller (60 mV) than that of the Ti-modified sample (160 mV). Since the F-treatment is performed in the H2O2 and NH4F solution, the slightly increased performance of F-Fe2O3 might be attributed to the acid effect.16 The results strongly


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confirm the important role of surface bonds to form a hydrogen-bond network for better performance.

Figure 5. J-V curves of Fe2TiO5/Fe2O3 (a) and F-Fe2TiO5/Fe2O3 (b) with (red) and without (black) H2O2 (0.5 M) as a hole scavenger. (c) Charge separation efficiencies (ηsurf) of Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3. The accelerated hole transfer has been confirmed by the enhanced charge separation efficiency of F-Fe2TiO5/Fe2O3 in Figure 5. The charge separation efficiencies of Fe2TiO5/Fe2O3 and FFe2TiO5/Fe2O3 at the solid-liquid interface have been calculated by adding H2O2 (0.5 M) as a hole scavenger. H2O2 is a well-known hole scavenger with high reactivity and the surface charge separation of H2O2 oxidation is essentially complete (100%).14 Thus the surface charge separation efficiency (ηsurf) in water can be calculated by the comparison of the photocurrent in H2O2 (JH2O2) with that in water (JH2O).14 It can be described as the equation: ηsurf = JH2O / JH2O2. The J-V curves of Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3 with (red) and without (black) H2O2 (0.5 M) have been shown in Figure 5a and 5b, respectively. Figure 5c shows the calculated surface charge separation efficiencies (ηsurf) of Fe2TiO5/Fe2O3 and F-Fe2TiO5/Fe2O3. It is clear that the surface charge separation efficiency has been significantly enhanced by F-treatment, confirming


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the faster hole transfer.19 The electron-hole recombination can thus be suppressed and the onset potential can be lowered as shown in Figure 2b. The experimental conditions of F-Fe2TiO5/Fe2O3 were optimized. Since the F-treatment was performed in the H2O2 and NH4F solution, the samples treated by H2O2 or NH4F alone are also tested and the performances are shown in the Supporting Information Figure S8b, which are obviously lower than the sample treated in the H2O2 and NH4F solution. The concentration of H2O2 and NH4F solution is also optimized and the results are shown in the Supporting Information Figure S9.

Figure 6. High-resolution TEM image and elemental mappings of the Rh-treated sample (Rh-FFe2TiO5/Fe2O3). The onset potential of F-treated sample can be further lowered by improving the OER using a surface cocatalyst. For example, Co-Pi and FeNiOx were widely used in the literatures.5,6 Noble metal elements are well-known to have good catalytic properties. However, Pt mainly enhanced the photocurrent instead of lowering the onset potential.32 IrO2 was used as an efficient


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cocatalyst but it was not stable in a long time.4 Rh based cocatalyst was also reported but they have the stability problem.30 Here we use a photo-assisted electrodeposition method with Rh precursor in a NaOH solution to prepare Rh based cocatalyst on hematite. The detailed Rhtreatment can be found in the experimental section and the samples after Rh-treatment are labeled as Rh-Fe2TiO5/Fe2O3 or Rh-F-Fe2TiO5/Fe2O3. Interestingly, we found the hematite sample incorporated with Fe2TiO5 can enhance the stability of Rh based cocatalyst. The performance of Rh-Fe2TiO5/Fe2O3 is shown in the Supporting Information Figure S10, which exhibits a large cathodic shift (120 mV) of the onset potential when compared to Fe2TiO5/Fe2O3. The photocurrent of Rh-Fe2TiO5/Fe2O3 is also stable in a long time (data not shown), suggesting that the incorporation of Fe2TiO5 in hematite can improve the stability of Rh on hematite.30 Rh cocatalyst was further coupled to the F-Fe2TiO5/Fe2O3 sample for better performance. Figure 6 shows the TEM image and elemental mappings of Rh-F-Fe2TiO5/Fe2O3, which clearly show the decoration of Rh based material on hematite. Some amorphous structure (labeled by red curve in the left high-resolution image) can be clearly observed on the surface, indicating the deposition of Rh based cocatalyst. SEM images of F-Fe2TiO5/Fe2O3 and Rh-F-Fe2TiO5/Fe2O3 are also shown in the Supporting Information Figure S11 and no obvious difference can be observed. XPS data of Rh-F-Fe2TiO5/Fe2O3 are shown in the Supporting Information Figure S12. Rh 3d features in Figure S12a can be clearly observed, with an energy position indicating the existence of Rh3+.30 The O 1s XPS spectrum of Rh-F-Fe2TiO5/Fe2O3 in Figure S12b also shows an enhanced feature around 532.5 eV (labeled by arrow), indicating the increased OH groups in the cocatalyst.33 Thus the Rh based material can be assigned to Rh(OH)3. Rh(OH)3 was previously reported as efficient oxygen evolution catalyst.30 Rh3+-OH can catch holes and thus be oxidized to Rh4+-OXO, which releases O2 and transfers to Rh2+.30 Since the oxygen evolution by Rh4+-


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OXO is much faster than by hematite, the catalytic performance can be greatly enhanced by Rh cocatalyst.30

Figure 7. J-V curves of Fe2TiO5/Fe2O3, F-Fe2TiO5/Fe2O3 and Rh-F-Fe2TiO5/Fe2O3. Figure 7 shows the performance of Rh-F-Fe2TiO5/Fe2O3. A further cathodic shift (70 mV) of the onset potential in Rh-F-Fe2TiO5/Fe2O3 can be observed when compared to the sample before Rh-treatment. The totally shifted onset potential after F and Rh based treatments can be a large value of 230 mV, which is a remarkable value compared to the literatures.4-6,11,23 The onset potential can thus be improved to be around 0.63 V vs. RHE, which is comparable to the lowest onset potential ever reported for hematite.4-6,11-16,29 Notably, the photocurrent at a low onset potential of 1.0 V vs. RHE can be significantly improved from 0.39 mA/cm2 to achieve a high value of 1.47 mA/cm2, which is more than 3.5 times enhancement.6 The photocurrent of Rh-FFe2TiO5/Fe2O3 also increases to 2.12 mA/cm2 at 1.23 V vs. RHE. The decorated Rh(OH)3 cocatalyst also exhibits a good stability (Supporting Information Figure S13, at 1.0 V vs. RHE) when compared to the unstable Rh based catalyst in the literature.30 The results suggest that the


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combination of F-treatment and Rh-treatment can be a good strategy to lower the onset potential of Ti-treated hematite. The incident photon-to-current conversion efficiencies (IPCE) of Rh-FFe2TiO5/Fe2O3, F-Fe2TiO5/Fe2O3 and Fe2TiO5/Fe2O3 (at 1.0 V vs. RHE) are shown in the Supporting Information Figure S14. Significant improvement in the IPCE is observed for Rh-FFe2TiO5/Fe2O3, confirming the higher performance. A maximum IPCE of 37% is observed for Rh-F-Fe2TiO5/Fe2O3 at 370 nm, versus 10% for the Fe2TiO5/Fe2O3 sample.

Figure 8. (a) Electrochemical impedance spectra (EIS) of Fe2TiO5/Fe2O3, F-Fe2TiO5/Fe2O3 and Rh-F-Fe2TiO5/Fe2O3 measured at 1.0 V vs. RHE in 1 M NaOH electrolyte under illumination. The inset shows the equivalent circuit. (b) Parameters of equivalent circuit elements.


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Electrochemical impedance spectroscopy (EIS) under the light illumination has also been used to confirm the working mechanism of hematite photoanodes after various treatments. The EIS data of Fe2TiO5/Fe2O3, F-Fe2TiO5/Fe2O3 and Rh-F-Fe2TiO5/Fe2O3 have been shown as the form of Nyquist plots in Figure 8a. The equivalent circuit (EC) to simulate the Nyquist plot is shown in the inset.34 The EIS spectra in the dark (at 1.0 V vs. RHE) are shown for comparison in the Supporting Information Figure S15. The impedance values in the dark are much higher than those obtained under illumination. The EIS spectra under illumination show two arcs while the spectra in the dark show only one arc. According to the references,26,35 the additional arc in the right at lower frequencies (under illumination) can be attributed to the charge transfer of surface states. The curve for F-Fe2TiO5/Fe2O3 exhibits a smaller diameter compared to that for the sample before F-treatment, indicating faster charge transfer kinetics at the electrode interface.34 The fitting parameters of EC elements in EIS are also shown in Figure 8b. Interestingly, the Rct (resistance for the interfacial charge transfer) for F-Fe2TiO5/Fe2O3 is much lower than that for Fe2TiO5/Fe2O3, suggesting that the F-treatment is very effective to improve hole transfer.34 The Rct value for Rh-F-Fe2TiO5/Fe2O3 is only slightly lower than that for F-Fe2TiO5/Fe2O3, suggesting that Rh based cocatalyst mainly enhances the catalytic performance instead of accelerating charge transfer. The EIS spectra at a low potential of 0.6 V vs. RHE (at which the reaction does not take place, under illumination) are also shown in the Supporting Information Figure S16. However, the obtained impedance values are very large, indicating that the charge transfer resistance is too high to develop any photocurrent.26 The results confirm the synergetic effect of F and Rh in hematite to lower the onset potential, in which F based interface structure accelerates the hole transfer while Rh based material improves the catalytic performance.


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CONCLUSIONS F and Rh based treatments were used to lower the onset potential of Fe2TiO5/Fe2O3. The sample after F-treatment shows a large cathodic shift of the onset potential (160 mV) when compared to the sample before F-treatment. XPS experiments clearly reveal the existence of various Ti-F bonds. Moreover, when the sample was immersed in the working NaOH solution, a modified interface structure had been observed which can form a hydrogen-bond network in solution for faster hole transfer. The faster hole transfer has been confirmed by the increased charge separation efficiency with H2O2 as a hole scavenger. Moreover, the F-treated sample can be well coupled with Rh based cocatalyst to further lower the onset potential with a total value of 230 mV. The final onset potential is 0.63 V vs. RHE comparable to the lowest value ever reported for hematite. The F and Rh co-treated photoanode also show a high photocurrent of 1.47 mA/cm2 at 1.0 V vs. RHE. The enhanced performance can be attributed to a synergetic effect of F and Rh based treatments, which can be used as a favorable strategy to lower the onset potential of Ti-modified hematite for solar water oxidation.

ASSOCIATED CONTENT Experimental setup, TEM elemental mappings, XRD spectra, Ti L-edge XAS spectra, absorption spectra, J-V scans, SEM images, XPS spectra, photochemical stability curves, IPCE and EIS spectra of Fe2TiO5/Fe2O3, F-Fe2TiO5/Fe2O3 and Rh-F-Fe2TiO5/Fe2O3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Author information: *Corresponding Author: E-mail: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the support from NSRL and BSRF for the XAS experiments. This work is supported by the National Natural Science Foundation of China (U1432249, 11275137). This is also a project supported by Collaborative Innovation Center of Suzhou Nano Science & Technology, Soochow University-Western University Centre for Synchrotron Radiation Research, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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