Boosting photoelectrochemical performance of hematite photoanode with TiO2 underlayer by extremely rapid high temperature annealing

Applied Surface Science 422 (2017) 913–920

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Boosting photoelectrochemical performance of hematite photoanode with TiO2 underlayer by extremely rapid high temperature annealing Dan Wang a,c,1 , Ying Chen a,b,1 , Yang Zhang a,b , Xintong Zhang a,b,∗ , Norihiro Suzuki d , Chiaki Terashima d a

Center for Advanced Optoelectronic Materials Research, School of Physics, Northeast Normal University, 5268 Renmin Street, Changchun 130024, PR China Key Laboratory of UV-Emitting Materials and Technology of Chinese Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, PR China c School of Media and Mathematics & Physics, Jilin Engineering Normal University, Changchun 130052, PR China d Photocatalysis International Research Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, Japan b

a r t i c l e

i n f o

Article history: Received 2 March 2017 Received in revised form 17 May 2017 Accepted 19 May 2017 Available online 9 June 2017 Keywords: Extremely rapid high temperature annealing Hematite photoanode TiO2 underlayer Photoelectrochemical water splitting

a b s t r a c t Extremely rapid high temperature annealing (ER-HTA) was used to boost the photoelectrochemical (PEC) performance of hematite thin film deposited on a TiO2 nanosheet-modified SnO2 :F substrate (FTO-TNHM). The PEC performance of FTO-TN-HM photoanodes were strongly enhanced with increasing ER-HTA temperatures from 700 to 820 ◦ C with a holding time as short as 30 s. The photocurrent density of FTOTN-HM photoanode treated by ER-HTA at 800 ◦ C was 0.49 mA cm-2 and interfacial hole transfer efficiency of 32% was achieved at 1.23 V vs. RHE, which were 18.8 and 16 times as great as FTO-TN-HM photoanodes annealed at 500 ◦ C for 30 min, respectively. The effect of ER-HTA on the PEC performance of FTO-TN-HM photoanodes were studied comparatively, which suggested that the improved crystallinity, decreased recombination through surface states, and enhanced interfacial Ti4+ diffusion all contributed to their advanced PEC performance. Our studies confirm that the ER-HTA treatment is an effective method to improve the PEC properties of hematite photoanodes with TiO2 underlayer and might be applicable for other semiconducting photoelectrodes to get better PEC performance. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Photoelectrochemical (PEC) water splitting is an efficient approach to convert solar energy into non-polluted chemical fuels to resolve energy supply and related environment pollution issues [1–6]. Hematite (␣-Fe2 O3 ) is one of the most desirable photoanode materials due to its narrow band gap (2.0–2.2 eV) [7–9], an appropriate valance band position for oxygen evolution, and excellent chemical stability [10]. However, there are still strong charge recombination in hematite photoanode due to its high structural defects, poor charge transport and transfer properties, which severely limits their water splitting efficiency. To date, many strategies have been developed to improve the PEC properties of hematite. These include fabricating nanostructures to

∗ Corresponding author at: Center for Advanced Optoelectronic Materials Research, School of Physics, Northeast Normal University, 5268 Renmin Street, Changchun, 130024, PR China. E-mail address: [email protected] (X. Zhang). 1 The authors contribute equally to the work. http://dx.doi.org/10.1016/j.apsusc.2017.05.164 0169-4332/© 2017 Elsevier B.V. All rights reserved.

decrease the charge transport length and facilitate the surface collection of photogenerated holes [11–14], donor doping to improve the charge transport and reduce the recombination from charge accumulation [15–17], and surface modification to enhance the surface reaction rate and accelerate the interface charge transfer [2,18–20]. Recent studies have demonstrated that an ultrathin underlayer of wide band-gap deposited between SnO2 :F (FTO) substrate and hematite could suppress charge recombination of the FTO/hematite photoanode [20–23]. In our previous work, an ultrathin TiO2 nanosheets (TiO2 NS) underlayer with high crystallinity prepared by layer-by-layer (LBL) deposition was used to enhance PEC water splitting ability of hematite thin film [24]. The photocurrent of FTO/TiO2 NS/hematite (FTO-TN-HM) photoanode was significantly increased to 373 uA cm−2 at 1.23 V vs. RHE compared to 9.1 uA cm−2 at 1.23 V vs. RHE for FTO/hematite (FTO-HM) photoanode. However, the bulk and surface recombination of hematite still impedes the enhancement of photocurrent due to poor crystallinity. More recently, high temperature annealing (HTA) has been conducted to enhance the PEC performance of hematite photoanode.

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Phuan et al. reported that the HTA could enhance the nanocrystal growth and decrease the grain boundary resistance [25]. Zandi et al. claimed that the high temperature treatment, i.e., sintering at 500 ◦ C for 30 min then annealing at 800 ◦ C for 4 min, could remove the surface states that contribute to recombination, thus, substantially improves the water oxidation performance of hematite photoanodes [26]. Bohn et al. reported that sputtered hematite on FTO-coated glass annealed at 500 ◦ C for 2 h followed by 800 ◦ C for 10 min could result in gradient Sn4+ doping via diffusion and activation that dramatically reduces the resistivity of the film and increases the photocurrents of hematite photoanode [27]. Interestingly, Annamalai et al. reported that HTA on hematite nanorod photoanodes with a TiO2 underlayer on FTO substrates might result in Ti4+ doping effects, which also presented excellent charge transport and collection efficiency [28]. Thus, the HTA may be an efficient technique to promote the photoanode properties of hematite via selective doping to improve the charge transport and increase the crystallinity to simultaneously reduce recombination. However, the severe loss of conductivity for FTO at high temperatures and long processing time decreases the collection efficiency of electrons [29–31]. Therefore, the rapid process for HTA treatment might maximize the electron collection efficiency of FTO substrates while improving the performance of hematite photoanodes. Here, we use extremely rapid HTA (ER-HTA) treatment to improve the PEC performance of FTO-TN-HM photoanode. The ER-HTA treatment is performed at 700–820 ◦ C with a holding time as short as 30 s. The photocurrents of FTO-TN-HM photoanodes are enlarged strongly with increasing ER-HTA temperatures. Comparative studies indicate that the ER-HTA has little effect on the nanocrystal growth but this obviously impacts the structural defects—especially on the surface states. The donor densities gradually increase with increasing ER-HTA temperature due to the interfacial diffusion-induced Ti4+ doping effects. Our research confirms that ER-HTA is an efficient strategy to advance the PEC performance of FTO-TN-HM photoanodes via synergistic effects: reduced recombination, accelerated charge transportation, and increase interface charge transfer. 2. Experimental section 2.1. Materials Ferric nitrate (Fe(NO3 )3 ·9H2 O, 98.5%) was purchased from Aladdin Reagent Company. Ethyl alcohol (C2 H5 OH, 99.7%), hydrogen peroxide (H2 O2 , 30%) and sodium hydroxide (NaOH, 96%) were obtained from Beijing Chemical Reagent Company. Poly(diallyldimethylammonium) chloride (PDDA) solution was bought from Sigma Aldrich Reagent Company. Titanate nanosheets (TiO2 NS) were exfoliated from cesium titanate powers according to our previous work. Deionization water was obtained using reverse osmosis water (Labpartner experimental water system). The SnO2 :F transparent conducting glass (FTO, resistance 17 /eq) substrates were bought from Nippon Sheet Glass. 2.2. Preparation of photoanodes Firstly, the TiO2 NS were deposited on FTO substrate via a layerby-layer (LBL) deposition method, which was used in our previous work. The FTO substrates were dipped into PDDA (20 mg/ml) and then the colloidal suspension of titanate nanosheets for 10 min each; this was repeated twice. Then, hematite thin films were prepared on a TiO2 NS-coated FTO substrate by spray pyrolysis method. 10 mM Fe(NO3 )3 ·9H2 O was dissolved in ethanol as the precursor solution. The hot plate kept at 350 ◦ C during the spray pyrolysis process. The thickness of Fe2 O3 films was controlled with the spray-

ing time. Finally, the as-deposited thin films were further heated at 500 ◦ C in a muffle furnace for 30 min at a heating rate of 10 ◦ C/min or in a rapid thermal processing (RTP) tube furnace at 700, 750, 800, and 820 ◦ C for 30 s. This was named FTO-TN-HM-500, FTO-TNHM-700, FTO-TN-HM-750, FTO-TN-HM-800, and FTO-TN-HM-820, respectively. 2.3. Characterization The hematite thin film were characterized by X-ray diffraction with a Rigaku D/max-2500 using Cu Ka radiation (␭ = 1.5406 Å) and Raman spectra by J-Y UV-lamb micro-Raman spectrometer under an excitation of a 488 nm Ar+ laser. The optical properties were collected by the Perkin Elmer UV Win Lab spectrometer with an integrating sphere. X-ray photoelectron spectroscopy was performed with an Mg K␣ ADES (h = 1253.6 eV) source (VGESCA-LAB MKII). 2.4. Photoelectrochemical experiments The photoelectrochemical (PEC) properties of the samples were tested by Princeton 2273 electrochemical work-station with a standard three-electrode setup. The prepared hematite thin films were used as the working electrode with the active surface area of 2 cm2 . A platinum plate and Ag/AgCl in 3.5 M KCl were the counter and reference electrode, respectively. The 1 M NaOH aqueous solution was used as electrolyte. All measurements under light were illuminated from the backside of FTO using Solar Simulator with a 100 W Xe Lamp (LCS-100 Newport). The light intensity illuminated on the samples were 100 mW/cm2 measured by an optical power meter (PM100D Thorlabs) by adjusting the distance between the samples and the light source. 3. Results and discussion Fig. 1a presents the current-potential curves of FTO-TN-HM photoanodes annealed at different temperatures in the dark and under illumination with 1 M NaOH electrolyte. There are no significant currents at a working potential below 1.6 V vs. RHE for electrodes in the dark. Upon illumination, the photocurrents significantly increase with increasing ER-HTA temperatures. Moreover, the photocurrent densities of FTO-TN-HM photoanodes treated by ER-HTA at 700, 750, 800, and 820 ◦ C are 0.14, 0.34, 0.49, and 0.51 mA cm−2 at 1.23 V vs. RHE corresponding to 5.4, 13.1, 18.8, and 19.6 times as great as the electrodes annealed at 500 ◦ C for 30 min, respectively (Fig. 1b). The onset potential of FTO-TN-HM800 treated by ER-HTA is shifted to 0.9 V vs. RHE, which is negative than the electrodes annealed at 500 ◦ C for 30 min. Clearly, the ER-HTA has a positive effect on the water splitting performance of hematite. The FTO-TN-HM photoanodes treated by ER-HTA at 850 ◦ C are usually broken, and therefore the corresponding PEC performance is not present for comparison. Furthermore, when the annealing temperature increased from 800 to 820 ◦ C, the photocurrent at 1.23 V vs. RHE varies slightly implying that 800 ◦ C is near the optimal condition of ER-HTA. It is known that the morphologies, structures and optical properties of hematite, which might also be affected by HT-RTA, all influence the PEC performance of photoanodes. The SEM images of Fe2 O3 in Fig. S1 annealed at 500 and 800 ◦ C are nearly the same, demonstrating that the morphologies of Fe2 O3 have not been affected by the extremely short annealing treatment. However, the TEM images of Fe2 O3 in Fig. S2 clearly indicate that the extremely rapid high temperature annealing obviously improves the interconnection of Fe2 O3 nanoparticles, which would facilitate the electrons transfer in the PEC measurements. Hematite thin

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Fig 1. (a) Current-potential (J-V) curves of FTO-TN-HM photoanodes annealed at different temperature with a scan rate of 10 mV/s from 0.6 V to 1.7 V vs. RHE in the darkness and under irradiation. (b) The current ratio of FTO-TN-HM photoanodes treated by ER-HTA at 700, 750, 800, and 820 ◦ C to that of FTO-TN-HM-500 at 1.23 V vs. RHE.

Fig. 2. (a) XRD patterns for hematite thin films on silica annealed at 500 ◦ C for 30 min and 800 ◦ C for 30 s; (b) Raman spectra, (c) full-width at half-maximum of Raman peaks, and (d) UV–vis absorption spectra for the FTO-TN-HM-500 and FTO-TN-HM-800.

films were deposited on silica instead of FTO to avoid the diffraction peaks of substrate. Fig. 2a shows the XRD patterns of hematite thin films annealed at 500 ◦ C for 30 min and 800 ◦ C by HT-RTA for 30 s. All of the XRD peaks of Silica-HM-500 and Silica-HM-800 films correspond to the diffractions of ␣-phase hematite according to the standard hematite powder diffraction data (PDF # 89-8104). Therefore, the HT-RTA does not change the phase of hematite, which is further confirmed by Raman data (Fig. 2b) [32,33]. The full width at half maximum (FWHM) of the XRD peaks that related to the

crystalline size of hematite are nearly the same, which implies the extremely short annealing time (∼30 s) does not induce the crystal growth. In addition, the diffraction peaks of ultrathin TiO2 underlayer could not be observed in the XRD spectra because its thickness is only about 1.4 nm. On the other hand, the FWHM intensity of the Raman peaks obviously decrease after HT-RTA as shown in Fig. 2c, which could be attributed to the decrease in local defects of hematite thin films. The elimination of structure defects not only decreases the recom-

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Fig. 3. (a) I-t curves of FTO-TN-HM-800 and FTO-TN-HM-500 collected at 1.1 V vs. RHE with light off/on repeated at an interval of 1.75 s. (b) J-V curves of FTO-TN-HM500 and FTO-TN-HM-800 films with 1 M NaOH electrolyte containing 0.5 M H2 O2 . (c) Interfacial hole transfer efficiencies (␩transfer ) as a function of applied potential for FTO-TN-HM-800 and FTO-TN-HM-500. (d) Cyclic voltametry curves of FTO-TN-HM-800 and FTO-TN-HM-500 collected with a scan rate of 1 V/s in the darkness.

bination, but also enhances the carrier mobilities that facilitate charge transport. Furthermore, the absorbance spectra illustrated in Fig. 2d are very similar to each other, and the absorption edges are around 570 nm that corresponds to the band gap of hematite. It is concluded that the significantly improved PEC performance of FTO-TN-HM-800 is partly due to the repair of structure defects that could reduce the deep energy level in the band gap, thus, decreases the related recombination processes. Moreover, the reduction in carrier scattering from structure defects could also enhance the charge carrier mobility and facilitate charge transport. Fig. 3a presents I-t curves of FTO-TN-HM-800 and FTO-TN-HM500 collected at 1.1 V vs. RHE with light off/on repeated at an interval of 1.75 s. The photocurrent of FTO-TN-HM-500 has a sharp spike with light on of each cycle, which then reaches a steady current of ∼0.023 mA cm−2 . The sharp spikes of photocurrents indicate that strong recombination occurs between the photogenerated electrons and the accumulated holes at the valence band of hematite at a potential of 1.1 V vs. RHE. This indicates poor hole transfer efficiency at the interface [33]. For FTO-TN-HM-800, the spike almost disappears when the light is on. The steady current is increased to 0.17 mA cm−2 , which is about 7 times of that for FTOTN-HM-500. These results suggest that the ER-HTA improves hole transfer efficiency at the interface and decreases the recombination under illumination. To further investigate the interfacial charge transfer properties, J-V curves were measured in electrolyte containing 0.5 M H2 O2

as shown in Fig. 3b. Below 0.91 V vs. RHE, the photocurrent density of FTO-TN-HM-800 is smaller than that of FTO-TN-HM-500. This could be originated from the diminished conductivity of FTO substrate during high temperature annealing process (Fig. S3). Considering that H2 O2 could efficiently capture photogenerated hole, the interfacial hole transfer efficiencies (␩transfer ) are defined as the ratio of JH2O to JH2O2 (JH2O /JH2O2 ) according to the J-V curves with and without H2 O2 , in which the ␩transfer in the presence of H2 O2 is practically ∼100% [34]. Fig. 3c shows that the ␩transfer for FTO-TNHM-800 consistently exceeds FTO-TN-HM-500 when the potential is above 0.91 V vs. RHE. Notably, the ␩transfer for FTO-TN-HM-800 at 1.23 V vs. RHE is more than 32%, which is about 16 times that of FTO-TN-HM-500. Therefore, it could be concluded that the interfacial hole transfer efficiencies are strongly enhanced by ER-HTA, which agrees with Fig. 3a. Cyclic voltametry (CV) measurements were used to further study the surface states acting as carrier trap centers [26]. Before scanning, the photoanodes were illuminated for 100 s at 2 V vs. RHE to photo-oxidize the surface states completely—the hole trap centers were fully filled with photogenerated holes [35]. The CV curves were collected in the darkness, and the scanning starts from 1.7 to 0.3 V vs. RHE with a scanning rate of 1 V/s. Fig. 3d shows a notably reduced peak at 0.6 V vs. RHE for FTO-TN-HM-500. It could be attributed to the electron reduction of photo-oxidized surface states [35]. This process usually corresponds to the indirect recombination via surface holes trap centers in photoelectrochemical

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Fig. 4. Mott-Schottky plots of FTO-TN-HM photoanodes annealed at different temperatures measured at a frequency of 1 KHz in the darkness.

oxidation of water. For the FTO-TN-HM-800 curve, the reduction peak disappears at the same potential confirming that ER-HTA could eliminate the surface states acting as hole trap centers during water oxidation. These holes trap centers at the surface of hematite film could act as hole accumulating sites and result in an obvious spike seen in Fig. 3a. The J-V curves of FTO-HM annealed at different temperatures were also comparatively measured to investigate the effect of the TiO2 underlayer on the PEC performance of photoanode after ERHTA. As depicted in Fig. S4, the current densities of FTO-HM under irradiation are gradually strengthened with increasing annealing temperatures from 700 to 820 ◦ C. Similar to the results of FTOTN-HM photoanode with TiO2 underlayer, the decreased structure defects and surface states reduce the recombination and promote the charge transfers—both could enhance the PEC performance as discussed above. However, for the FTO-HM-800, the photocurrent densities reach 0.06 mA cm−2 at 1.23 V vs. RHE, which is only about 0.12 times as big as FTO-TN-HM-800. Therefore, it can be deduced that the TiO2 underlayer plays an important role in enhancing water splitting of the photoanode under ER-HTA. In our previous work, it had been found that the TiO2 ultrathin underlayer could significantly improve PEC performance of hematite photoanode by suppressing the recombination of electrons back transfer [24]. Recent studies have indicated that the TiO2 underlayer might also act as a Ti4+ doping layer during high temperature annealing [28]. Mott-Schottky spectra were collected at 1 kHz under dark conditions for FTO-TN-HM annealed at different temperatures to investigate the effects of ER-HTA on the donor densities of the electrode. The positive slopes for Mott-Schottky plots in Fig. 4 illustrate that they are all n-type semiconductors. The donor densities (ND ) could be estimated from Mott-Schottky plots according to the equation: 1/C2 =

2 kB T × (V − VFB − ) e␧␧0 ND e

(1)

where C, e, ␧ and ␧0 denote the capacitance, elementary charge, dielectric constant of hematite (␧ = 80) and the permittivity of vacuum, respectively. A, V, VFB , kB and T were the active surface area, the applied potential, the flat band potential, Boltzmann’s constant, and the absolute temperature. For FTO-TN-HM-800 and FTO-TNHM-500, the calculated donor densities are 1.13 × 1020 /cm3 and 3.53 × 1019 /cm3 , respectively, as shown in Table 1. The VFB of FTOTN-HM-800 is about 0.6 V vs. RHE which is about 0.18 V negatively shift compared to that of FTO-TN-HM-500. The calculated donor densities of the samples gradually increase with increasing annealing temperature indicating that the donor densities depend on the

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temperature of ER-HTA. The variation in donor densities might originate from the activation of doped donor or the diffusion-induced doping effects from the substrates (Sn4+ ) or TiO2 underlayers (Ti4+ ) [27,28]. However, the hematite thin film is not prepared with intentional doping, which infers that unactivated donor densities are rare in pure hematite thin films. On the other hand, the slopes of Mott-Schottky plots for FTO-HM (Fig. S5) that correspond to the donor densities do not change obviously after ER-HTA at different temperatures. Thus, the increased donor densities of FTO-TN-HM800 could not originate from the Sn4+ doping from the substrate at high temperatures, but rather come from the effect of TiO2 underlayer during ER-HTA. To study the interface diffusion of the TiO2 underlayer and the Fe2 O3 thin films under ER-HTA, X-ray photoelectron spectroscopy (XPS) was carried out on a sample of titanate nanosheet-covered hematite films (FTO-HM-TN-800) that is prepared with similar conditions to the FTO-TN-HM-800 photoanode. Fig. 5a and b presents XPS spectra of Ti 2p and Fe 2p, respectively. After ER-HTA at 800 ◦ C, the peak intensity of Ti 2p decreases notably while the peak intensity of Fe 2p increases slightly. Meanwhile, the XPS peak of Ti 2p shifts to a higher binding energy after ER-HTA [36,37]. We note that the XPS spectra are very sensitive to the surface chemical state of samples. Therefore, the variation in relative intensity of Ti 2p and Fe 2p implies that the ratio of Ti atoms at the detecting layer is decreased, while the ratio of Fe atoms is increased after ER-HTA. The ratio of Ti 2P3/2 to Fe 2P3/2 for was calculated to obviously decrease versus FTO-HM-TN-500 as depicted in Fig. 5c. Because the TiO2 surface layer could not evaporate during ER-HTA, it is suspected that Ti atoms likely diffuse into Fe2 O3 film along the vertical direction of the thin film. The Ti atoms at the surface layer would then be partly replaced by Fe atoms that diffuse from the hematite film. As a consequence, the local electron cloud densities of Ti atoms would decrease after ER-HTA, and this would weaken the electron screening effect and result in higher binding energies of Ti 2p. With increasing annealing temperatures, more and more Ti atoms diffuse into hematite films and act as donors to increase the carrier density, for further improvement of the photocurrent as previously report [38–40]. The electron densities increase concurrently resulting in an upshift of the Fermi level. As a result, as shown in Mott-Schottky plots, the flat band potential (VFB ) of the electrodes with TiO2 underlayer gradually shift to negative potentials with increasing the annealing temperature. These results further confirm that efficient gradient Ti doping into hematite films under ER-HTA plays an important role in enhancing the photocurrent density of hematite electrodes. Although Jang et al. showed that the Ti underlayer can be doped into the Fe2 O3 as donor doping under high temperature [28], the interfacial diffusion of TiO2 /Fe2 O3 during ER-HTA is still a surprising result considering the extremely short treatment time. Fig. 6a shows the open circuit potential (Voc ) dynamics with light on and off. In the darkness, the Fermi level of the electrode is thermal equilibrium equal to that of the electrolyte which could be considered to be a metal due to its high densities of charge carriers. This process results in a Schottky barrier at the interface of the electrode and electrolyte. When the electrode is illuminated, electrons transition from the valence band (VB) to the conduction band (CB) upshifts the quasi-Fermi level of the electrode, and a photogenerated voltage occurs at the contacts of electrode and electrolyte leading to a step of Voc . A much larger Voc step is obtained for FTOTN-HM-800 suggesting a larger band bending at the interface of the electrode and the electrolyte due to the upshift of Fermi level via Ti4+ doping. The larger band bending could facilitate the interfacial charge transfer efficiencies and decrease the surface accumulation of photogenerated holes in hematite film. This strongly prevents the recombination and enhances the photo-oxidation process. When the light is off, the carrier recombination results in a decay of Voc

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Table 1 Calculated donor densities from Mott-Schottky curves of FTO-TN-HM-500 and FTO-TN-HM treated by ER-HTA at 700, 750, 800, and 820 ◦ C. Annealing temperature(◦ C)

500

700

750

800

820

Donor density (ND )(cm−3 )

3.53 × 1019

4.06 × 1019

6.65 × 1019

1.13 × 1020

1.14 × 1020

Fig. 5. Comparison of XPS spectra of Ti 2p (a) and Fe 2p (b) and the intensity ratio of Ti 2p3/2 to Fe 2p3/2 (c) for FTO-TN-HM-800 and FTO-TN-HM-500.

Fig. 6. (a) Open circuit potential dynamics of FTO-TN-HM-800 and FTO-TN-HM-500 with light off-on-off. (b) Nyquist plots of FTO-TN-HM-800 and FTO-TN-HM-500 measured under illumination with 1 M NaOH electrolyte at a DC potential of 1.23 V vs. RHE and an amplitude of AC potential of 10 mV with frequencies ranging from 100 k to 0.1 Hz. The solid dot represents experimental data, and the line represents fitted curves using Zview software with an equivalent circuit as the inset.

[41]. Usually, the carrier traps could extend the decay time of Voc to tens of seconds or more [42]. Fig. 6a depicts that the Voc of FTO-TNHM-800 decays faster than that of FTO-TN-HM-500 implying that the ER-HTA could eliminate some trap centers in the photoanodes. The effect of ER-HTA on the charge transport and transfer characteristic of hematite photoanodes has also been studied by electrochemical impedance spectroscopy (EIS) measured at a bias of 1.23 V vs. RHE under illumination conditions. The semicircle at high frequency and low frequency regions accounts for charge transport process in the bulk of photoanode and charge transfer process at the Fe2 O3 /electrolyte interface, respectively [17,43]. Fig. 6b shows the Nyquist plots (Z vs. Z ) of FTO-TN-HM-800 and FTO-TN-HM-500 that are fitted using Z-view software with Randles equivalent circuit shown in Fig. 6b inset [19,43]. The equivalent circuit elements include a series resistance of FTO, charge transport resistance (RFe2O3 ) and capacitance (CFe2O3 ) for hematite thin film, and charge transfer resistance (Rct ) and the space-charge layer capacitance (CSC ) at the Fe2 O3 /electrolyte interface. The fitting parameters for the equivalent circuits are summarized in Table 2. The Rs slightly increases after annealing at 800 ◦ C, which may be ascribed to the decreased electrical conductivity of FTO (Fig. S6). The charge transport resistance of Fe2 O3 thin films in FTO-TN-HM800 is reduced, probably due to the increased donor densities and the decreased structure defects. Importantly, a dramatic decrease in Rct is also observed for FTO-TN-HM-800 compared with FTO-

Table 2 Fitted parameters for the Nyquist plots of FTO-TN-HM-500 and FTO-TN-HM-800 samples using Zview software with equivalent circuit model in the inset of Fig. 6b. Sample FTO-TN-HM-500 FTO-TN-HM-800

Rs 9.7 21.6

RFe2O3 283.6 161.6

CFe2O3 −5

9.3 × 10 1.2 × 10−4

Rct

CSC

11845 1334

2.0 × 10−5 3.7 × 10−5

TN-HM-500 electrode, which was attributed to the Ti-doping that induced large band bending and the elimination of surface hole trap centers. To better explain the enhanced PEC performance of FTO-TNHM after ER-HTA, schematic energy bands of FTO-TN-HM-500 and FTO-TN-HM-800 are presented in Scheme 1. For FTO-TN-HM-500, the energy level bends upward (qVD1 ) at the interface ascribed to charge transfer between Fe2 O3 and electrolyte. The introduction of TiO2 underlayer between FTO and Fe2 O3 could block the interface recombination by suppressing the electron back transfer from FTO to Fe2 O3 . However, the deep penetration length of light and the low mobility of charge carriers leads to bulk recombination before photogenerated holes reaching the interface of Fe2 O3 /electrolyte, which denoted as recombination process (I). The defect states provide a pathway for bulk recombination of electrons/holes, i.e., the recombination process (II). The surface recombination via carrier trap centers corresponds to a recombination process (III). For FTO-

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Scheme 1. Illustration of energy bands and the main charge recombination processes for electrodes annealed at (a) 500 ◦ C for 30 min and (b) 800 ◦ C for 30 s. The qVD reflects the band bending between the semiconductor and electrolyte.

TN-HM-800, as discussed above, the ER-HTA obviously reduces the bulk and surface defects. Therefore, the recombination processes of (II) and (III) are prominently prevented. The improved crystallinity also facilitates the holes transport to the electrode surface for oxidizing water. Meanwhile, the interfacial diffusion-induced donor doping in hematite not only reduces the bulk resistance but also enlarges the band bending that enhanced the charge transport and interfacial charge transfer for water oxidation. Therefore, the PEC performances are strongly enhanced by ER-HTA due to the efficient charge separation efficiency, improved charge transport and interface charge transfer efficiency. 4. Conclusion In summary, these comparative studies have shown that ERHTA has a strong effect on the PEC performances of the FTO-TN-HM photoanode. The photocurrent densities of FTO-TN-HM-800 are up to 0.49 mA cm-2 , and the interfacial hole transfer efficiency is up to 32% at 1.23 V vs. RHE. These were 18.8 and 16 times as great as FTO-TN-HM photoanodes annealed at 500 ◦ C for 30 min, respectively. The main reasons for enhanced performance are the increased carrier density induced by doping as well as the increase charge transfer at the interface of the electrode and the electrolyte (enlarged band bending). In addition, both the acceleration of charge transportation from the increased crystallinity and the elimination of surface hole trap centers contribute to the improved PEC performance. These results might be helpful in preparing or designing new functional optoelectronic materials and devices. Acknowledgements The authors sincerely acknowledge Dr. Hancheng Zhu and Haixia Wang for providing technology supporting on characterization of the samples. This work was supported by the Natural Science Foundation of China (Grant Nos. 51502111, 51372036 and 91233204), the Key Project of Chinese Ministry of Education (No. 113020A), the Fund from Jilin Province (Grant No. 20160520113JH). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.05. 164.

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