Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

Progress in Organic Coatings 76 (2013) 1869–1873

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Effect of TiO2 supporting layer on Fe2 O3 photoanode for efficient water splitting Bo-Ra Kim a , Hyo-Jin Oh a , Kang-Seop Yun a , Sang-Chul Jung b , Wooseung Kang c,∗ , Sun-Jae Kim a,∗ a

Nano Materials Lab., Institute/Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, South Korea Department of Environmental Engineering, Sunchon National University, Jeonnam 540-742, South Korea c Department of Metallurgical & Materials Engineering, Inha Technical College, Incheon 402-752, South Korea b

a r t i c l e

i n f o

Article history: Available online 2 July 2013 Keywords: Titanate nanotube TiO2 Fe2 O3 Water splitting Photoelectrochemical

a b s t r a c t For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 ◦ C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO2 /FTO coated film; this red shift was achieved by the formation of thin hematite-Fe2 O3 and anatase-TiO2 phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe2 O3 /TiO2 /FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe2 O3 /TiO2 /FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe2 O3 /FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe2 O3 /FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO2 base layer beneath the Fe2 O3 layer. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen production through photoelectrochemical (PEC) water splitting process has been paid considerable attention as a promising solution to the global energy problem [1–4]. Transition metal oxide semiconductors have been considered candidate materials for this process. Among them, Fe2 O3 -based materials are one of the most extensively explored [5–7]. However, some major issues have been raised in the fabrication of Fe2 O3 photoanodes, such as low electrical conductivity of the material and necessity for less than 5 nm particle synthesis to avoid electron–hole recombination related to the extremely short diffusion distance of holes, among others [8,9]. Along with transition metal oxide semiconductors, TiO2 has also been extensively investigated as an electrode material because of its favorable characteristics, such as photoelectrochemical stability, non-toxicity, and low fabrication costs [10–12]. These two approaches of obtaining high photocurrents have been independently studied in the ranges of ultraviolet (UV) or visible light. In the present study, we focused on the fabrication and optimization

of a nano-sized Fe2 O3 thin layer supported on TiO2 -based nanotube materials to combine Fe2 O3 and TiO2 materials under UV/visible (UV–vis) light illumination for enhanced PEC cell performance for the water splitting process. The materials are expected to exhibit the advantages of the porous TiO2 nanotube thin film support given that they are fully utilized to enhance the performance of the supported Fe2 O3 nanoparticles. There is ample interest in the combination of TiO2 and Fe2 O3 thin films in overcoming the aforementioned limitations in PEC application. Therefore, we fabricated a nanotubular TiO2 film by repetitive self-assembly method. Fluorine-doped tin oxide (FTO) substrates were used to support the Fe2 O3 on TiO2 nanoparticles. The Fe2 O3 /TiO2 /FTO multilayer film system was characterized by Xray diffraction (XRD), Raman spectroscopy, and optical absorbance spectroscopy. Its I–V PEC performance under UV–vis light illumination for the water splitting process was carefully investigated to optimize the Fe2 O3 particles on the support compared with the Fe2 O3 /FTO film. The effective TiO2 base layer beneath the Fe2 O3 layer was also monitored. 2. Experimental

∗ Corresponding authors. Tel.: +82 234083780; fax: +82 234084342. E-mail addresses: [email protected] (W. Kang), [email protected] (S.-J. Kim). 0300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2013.05.031

Initially, the repetitive self-assembly of oppositely charged ions in an aqueous solution was conducted to directly coat the

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hydrogen-titanate (H-TiNT) powder obtained from a hydrothermal technique in a previous report by our group, on a transparent conducting substrate [13,14]. In the current study, the FTO (Asahi Glass Co.), which was surface treated for 20 min in 0.2 M polyethyleneimine (PEI, Aldrich) solution containing positively charged ions, was used as a transparent conductive substrate. All aqueous solutions were prepared using distilled water with 1.8 M. The surface-treated FTO glass was immersed for 20 min in an aqueous 10 g/L H-TiNT solution dispersed with 0.2 M tetrabutylammonium hydroxide (TBAOH, Aldrich) to produce negatively charged ions. After this, using the same method, an H-TiNT-treated film was immersed in 0.2 M aqueous polydiallyldimethylammonium chloride (PDDA, Aldrich) solution, which contained positively charged ions. The repetition of this process yielded an H-TiNT/FTO glass coated with an ∼700 nm thick H-TiNT film. The glass was dried under UV–vis light irradiation (200W, Super-Cure, SAN-EI Electric) to remove the remaining water before coating of the Fe2 O3 nanoparticles on the substrates without any sintering of the H-TiNT particles. Based on our previous report [15], the H-TiNT particles were found to have photocatalytic properties with optical energy band gap of ∼3.5 eV, which could photocatalytically remove the surfactants used during the coating process, resulting in the porous structure of H-TiNT film favorable for the following Fe2 O3 precursor coating. Then, for the supporting or coating process of the Fe2 O3 nanoparticles, the substrates were dipped in an aqueous Fe(NO3 )3 solution with controlled Fe concentrations and dipping times, and then subjected to heat treatments at 500 ◦ C for 10 min in air. This method was performed in air inside a box furnace to produce the final photoanode thin film with hematite-Fe2 O3 phase for the water splitting process. The microstructures, crystallinities, and I–V electrochemical properties of the as-prepared heat-treated thin films were analyzed by scanning electron microscopy (SEM; S-4700, Hitachi), Raman spectroscopy (Renishaw, inVia Raman microscope), and cyclic voltammetry (CV; ␮Autolab type III, Micro Autolab), respectively. To measure the I–V electrochemical property, a calomel electrode and Pt wire were used as the reference and counter electrodes, respectively, when the as-prepared, heat-treated coated Fe-oxide/H-TiNT/FTO composite films were used as the working electrodes in an aqueous 1.0 M NaOH deaerated solution under 100 mW/cm2 UV–vis irradiation (Hg–Xe 200W lamp, Super-Cure, SAN-EI Electric). The measured potentials versus calomel were converted to the reversible hydrogen electrode (RHE) scale.

3. Results and discussions Fig. 1 shows the UV–vis absorbance results of the FTO glass, HTiNT/FTO film, Fe-oxide/H-TiNT/FTO (corresponding to 65.48 wt% Fe2 O3 ) film, and Fe-oxide/FTO (corresponding to Fe2 O3 27.16 wt%) film after heat treatment at 500 ◦ C for 10 min in air. Using the Kubelka–Munk function [16,17], the values from the slopes of ((F(R∞ )h)(1/) ), obtained by converting the absorbance data corresponding to the optical energy band gaps were 3.85, 3.24, 3.11, and 3.53 eV. The optical energy band gaps obtained were almost the same as the values obtained from where the wavelengths met with the slopes of the absorbance data. The amount of Fe2 O3 was calculated using the measured concentration of combined Ti and Fe elements by energy dispersive spectroscopy, assuming a complete conversion into TiO2 and Fe2 O3 from H-TiNT and Fe(NO3 )3 , respectively. As reported previously [18,19], the original FTO glass and H-TiNT film without heat treatment exhibited 3.85 and 3.55 eV of optical energy band gaps, respectively. However, the optical energy band gap of the heat-treated H-TiNT/FTO film almost coincided with that of the typical anatase-TiO2 phase. The heat-treated Feoxide-coated FTO and TiO2 /FTO films became narrower because

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0 300

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3.53 eV 3.24 eV 3.11 eV

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400

450

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Wavelength (nm) Fig. 1. UV–vis absorbance results and band-gap energy values. Band-gap energy values (a) FTO film = 3.85 eV, (b) Fe2 O3 (=27.16 wt%)/FTO film = 3.53 eV, (c) TiO2 /FTO film = 3.24 eV, (d) Fe2 O3 (=65.48 wt%)/TiO2 /FTO film = 3.11 eV.

of red-shifting by the very thin Fe-oxide film. Here, larger energy band gap of Fig. 1(b) and (d) compared to bulky-Fe2 O3 material (Eg = 2.2 eV) was very thin layer supported on the FTO glass. Conversely, the phase or crystallinity of the specimens in Fig. 1 was confirmed by Raman spectroscopy; the XRD patterns (not shown) showed that the crystallographic characteristics of the heat-treated FTO, Fe-oxide, and H-TiNT films were pure FTO film, hematiteFe2 O3 phase, and anatase-TiO2 phase with a minor phase similar to that of as-prepared titanate, respectively. Fig. 2 shows the SEM images of the surface microstructures of the Fe2 O3 and TiO2 films coated on the FTO glass, which had been heat treated at 500 ◦ C for 10 min in air. Fig. 2a shows the surface of the heat-treated FTO glass with 300–500 nm grains and a surface roughness of ±200 nm. Fig. 2b displays the image of 27.16 wt% of Fe2 O3 coated on the FTO glass. Fig. 2c and d illustrates the surfaces of the TiO2 film converted from HTiNT on the FTO substrate obtained by repetitive self-assembly method as described in experimental procedure and the Fe2 O3 film with 65.48 wt% on the TiO2 /FTO film obtained after similar processing. Fig. 2b and d depicts the surfaces of the FTO and TiO2 /FTO films, respectively, partially covered and homogeneously distributed with less than 10 nm Fe2 O3 particles in diameter, and where the Fe distributions were confirmed with EDS elemental mapping. In Fig. 2c, the TiO2 nanoparticles consisting of spherical and longitudinal particles, which transformed from ∼10 nm wide and ∼100 nm long original nanotubular shapes, can be observed. The specimens (Fig. 1) that functioned as photoanodes were applied in water splitting, in which their I–V electrochemical characteristics were evaluated using linear sweep voltammetry in the range of +0.7 to +1.5 V versus the RHE under 100 mW/cm2 of UV–vis light illumination (Fig. 3). First, the dark current signals of all the specimens in Fig. 3A were detected with an onset voltage of about 1.1 V versus RHE. Compared with that of the FTO glass, their dark current density increases to a greater extent with Fe2 O3 than with TiO2 coatings, showing the maximum value with Fe2 O3 /TiO2 composite coating on the FTO glass as a substrate. We confirmed the various Fe2 O3 loading amounts on the FTO and TiO2 /FTO glasses, and verified that 27.16 wt% Fe2 O3 on the FTO glass and 65.48 wt% Fe2 O3 on the TiO2 /FTO glass had maximum dark current densities, as reported in our previous study [19]. Under UV–vis illumination (Fig. 3B), however, both the heattreated FTO and TiO2 /FTO films yielded very low photocurrents of down to 1.3 V versus the RHE of the applied potential, indicating that water splitting is electrochemically difficult at 1.23 V versus RHE. When both the Fe2 O3 films partially covered

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Fig. 2. SEM surface images of the Fe oxide films and H-TiNT film coated on the FTO film, which were heat treated at 500 ◦ C for 10 min in air. (a) FTO film, (b) Fe2 O3 (=27.16 wt%)/FTO film, (c) TiO2 /FTO film, and (d) Fe2 O3 (=65.48 wt%)/TiO2 /FTO film.

(Fe2 O3 = 27.16 wt%) and the TiO2 /FTO glass (65.48 wt%) well covered the FTO glass, their photocurrents significantly increased at 1.23 V versus RHE, whereas the Fe2 O3 film, which fully covered the surface of the FTO and TiO2 /FTO glasses, yielded relatively low values, although they were higher than those of the heattreated FTO and TiO2 /FTO films. This variation indicates that all the

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photo-electrons generated by the full Fe2 O3 film coating cannot be completely collected through the FTO conducting layer compared with those generated by the thin or partial Fe2 O3 film coating. In these results, the Fe2 O3 film was photo-activated under UV–vis illumination, which decreased its onset-voltage dramatically by the existence of the TiO2 base layer converted from the nanotubular titanate particles layer. As shown in Fig. 3B, the photocurrent at 1.23 V versus RHE peaked at 4.81 mA/cm2 with 65.48 wt% of the supported Fe2 O3 nanoparticles on the TiO2 /FTO film. This value reflects 3.5 times more photocurrent than that on the Fe2 O3 /FTO film. Fig. 4 shows the onset voltage data collected from their I–V electrochemical characteristics curves, with varied Fe2 O3 loading amounts supported on the FTO and TiO2 /FTO films. The loading amounts were varied by controlling the dipping time and Fe ion concentrations in an aqueous Fe(NO3 )3 solution, respectively. This procedure was followed by heat treatment at 500 ◦ C for 10 min in air to obtain the hematite-Fe2 O3 phase. As the supported loading amount of Fe2 O3 nanoparticles increased, the onset voltage of the Fe2 O3 /FTO glass decreased abruptly from 1.1 to 0.88 V, even with a small loading of Fe2 O3 (=14.23 wt%). On the other hand, that of the Fe2 O3 /TiO2 /FTO glass decreased continuously to

(c) TiO2/FTO (d) FTO 0.8

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Potential (V vs RHE) Fig. 3. (A) I–V characteristics curves of Fe2 O3 supported on TiO2 /FTO and FTO films for non-illuminating UV light source conditions in a dark room and (B) which were illuminating UV light source condition. (a) Fe2 O3 (=65.48 wt%)/TiO2 /FTO film, (b) Fe2 O3 (=27.16 wt%)/FTO film, (c) TiO2 /FTO film, and (d) FTO film.

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Current Density (mA/cm2)

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Consequently, the maximum photocurrent can be obtained when supported by the appropriate amount of hematite-Fe2 O3 nanoparticles on the TiO2 /FTO film. The comparison of the Fe2 O3 /FTO film and the TiO2 /FTO film shows that the TiO2 coating alone cannot yield a sufficient photocurrent. However, this coating can contribute substantially to the generation of an enhanced photocurrent as an easy path layer by reducing the recombination of the photo-generated electrons and holes. Although Fe2 O3 nanoparticles produce photocurrents, larger amounts or thick Fe2 O3 nanoparticles result in the loss of photo-generated currents by increasing the recombination of electrons and holes. Thus, it can be said that the insertion of an easy electron path layer beneath the Fe2 O3 top layer is necessary to enhance the photocurrent for an efficient water splitting process.

Potential (V vs RHE) 4. Conclusions

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Potential (V vs RHE) Fig. 5. (A) Cycle voltammograms of Fe2 O3 supported on TiO2 /FTO and FTO films with illuminating UV light condition. (B) Fe2 O3 (=65.48 wt%) supported on the TiO2 /FTO film for illuminating and non-illuminating UV light source conditions in a dark room. Each number represents the potential voltage value of each peak.

65.48 wt% loading of Fe2 O3 (0.7 V versus RHE), and then increased abruptly when it began approaching the value of the Fe2 O3 /FTO glass. This finding shows that the TiO2 layer plays an important role in the easy collection of photo-generated electrons into the current collector by the Fe2 O3 layer with optimized loading. The oxidation–reduction characteristics according to the existence of the TiO2 base layer are shown in Fig. 5 using CV. Fig. 5A shows the CV data of the specimens in Fig. 1 under UV–vis light illumination, in which the curve of the FTO glass itself was omitted because it did not show oxidation–reduction peaks at all. Fig. 5B shows the characteristics of Fe2 O3 /TiO2 /FTO with/without UV–vis light illumination. The TiO2 /FTO film has almost no or very small detectable oxidation–reduction peaks, but the Fe2 O3 /FTO film has a large reduction value at −0.9 V. The Fe2 O3 /TiO2 /FTO film has distinct, large values of oxidation–reduction peaks (oxidation: −1.07 and −0.79 V; reduction: −1.11 and 0.87 V). Compared with the Fe2 O3 /FTO film, the Fe2 O3 /TiO2 /FTO film has stronger oxidation–reduction characteristics at the interface between Fe2 O3 and water with the help of the TiO2 base layer. The dark characteristic shape in Fig. 5B is almost the same as that of the illuminated Fe2 O3 /FTO film in Fig. 5A. However, the UV–vis light characteristic shape in Fig. 5B changed with additional oxidation–reduction peaks. Even the illuminated TiO2 /FTO film has no peaks; thus, the Fe2 O3 and TiO2 layers play a combined role under illumination in the Fe2 O3 /TiO2 /FTO system. These results indicate that the Fe2 O3 thin layer acts photocatalycally as a photoanode, and the photocurrents increase considerably because of the existence of the TiO2 base layer.

To overcome the limitations of the Fe2 O3 material for PEC application, the nanotubular TiO2 film prepared by repetitive selfassembly of titanate particles on FTO substrates and successive heat treatment at 500 ◦ C for 10 min in air was utilized to support the Fe2 O3 nanoparticles. The material was characterized, and its I–V electrochemical performance under UV–vis light illumination for the water splitting process was carefully investigated to determine the optimized Fe2 O3 particle content and confirm the effect of TiO2 base layer beneath the Fe2 O3 layer. The heat-treated hematite-Fe2 O3 film showed superior photocurrent generation and collection, with appropriate amounts of Fe2 O3 on the TiO2 /FTO to the FTO films. The onset voltage at 65.48 wt% of the supported Fe2 O3 decreased considerably to approximately 0.7 V versus RHE, and its photocurrent peaked at 4.81 mA/cm2 at 1.23 V versus RHE. Although the Fe2 O3 nanoparticles produced photocurrents, larger amounts or thick Fe2 O3 nanoparticles resulted in the loss of photo-generated current through the increase in the recombination of electrons and holes. In conclusion, the insertion of an easy electron path layer beneath the Fe2 O3 top layer is necessary to enhance the photocurrent for an efficient water splitting process.

Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (No. 20110016699).

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