Visible-Light-Enhanced Electroless Deposition of Nanostructured Iron

Feb 5, 2010 - The crystal structure of the as-deposited thin films was a metastable phase of iron oxyhydroxide ... Materials Science Forum 2017 893, 1...
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J. Phys. Chem. C 2010, 114, 3707–3711

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Visible-Light-Enhanced Electroless Deposition of Nanostructured Iron Oxyhydroxide Thin Films Kai Kamada,* Takeo Hyodo, and Yasuhiro Shimizu Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki UniVersity, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan ReceiVed: NoVember 19, 2009; ReVised Manuscript ReceiVed: January 23, 2010

The present study demonstrated that the visible light irradiation promoted the electroless deposition of iron oxyhydroxide thin films on a bare Pt substrate in FeSO4 aqueous solution. Fe2+ is oxidized to higher valence under the presence of dissolved oxygen and then is precipitated as iron(III) oxyhydroxide on the Pt substrate. The light irradiation on the Pt substrate induced the formation of photocarriers (i.e., electrons and holes) in the preformed iron oxyhydroxide with a narrow band gap. The resultant holes in the valence band photocatalytically oxidized Fe2+ at the oxyhydroxide/solution interface as well as the Pt substrate. That is, the oxyhydroxide nuclei themselves acted as a sensitive layer for photoelectroless deposition. The photoactivation effect significantly depended on photon energy. Although the deposition rate was accelerated only under visible light irradiation, as stated above, the irradiation of UV light caused the reductive photocorrosion of the preformed deposits rather than the photocatalytic oxidation of Fe2+. The crystal structure of the as-deposited thin films was a metastable phase of iron oxyhydroxide containing sulfate ions, which is known as “schwertmannite”. The films were composed of nanosized whiskers, and such nanostructures with a large surface area remained unchanged even after the phase transition to hematite by postannealing at 873 K for 1 h. In this paper, the photoelectroless deposition mechanism of iron oxyhydroxide was discussed in detail, and the positive effects of visible light irradiation on film quality were demonstrated. Introduction Effective utilization of visible light occupying the greater part of sunlight has attracted much interest of researchers in the past decades. With respect to photoelectrochemistry of semiconductors, much research efforts have been devoted to the transformation of sunlight to electrical or chemical energy conversion systems.1,2 Recently, the photon energy has been also employed for development of functional inorganic materials or improvement of their properties. Among them, the photodeposition of noble metal nanoparticles on oxide semiconductors reported took the lead in the progress of related research fields.3 Recently, Torimoto and co-workers described a novel synthesis method of monodispersed nanoparticles of sulfide semiconductors through a wavelength-controlled photocorrosion process.4 A number of reports have dealt with the photoelectrochemcial etching of single-crystalline or polycrystalline oxide semiconductors in order to fabricate unique microstructures at their surfaces.5,6 On the other hand, the photoeffects on electrodeposition have already been studied for compound semiconductor films (CdTe, etc.).7 Our group also investigated the effect of light irradiation during the anodic electrodeposition of CeO2 thin films in the aqueous solution containing Ce3+.8 As a result of light illumination equivalent to or larger than the band gap energy, the deposition amount and the crystal size of CeO2 clearly increased as compared with the deposition in the absence of illumination, that is, the dark process. Among various oxide semiconductors, most iron oxides and oxyhydroxides show semiconducting properties with relatively narrow band gaps that can be excited by visible light.9,10 Hence, the thin film formation of these materials will be also assisted * To whom correspondence should be addressed. E-mail: kkamada@ nagasaki-u.ac.jp.

by combining the electrochemical process with the photoirradiation, as mentioned above. Because of prominent chemical durability and rich natural resources, such films are promising as photoanodes for hydrogen gas production and electrodes for chemical sensors or lithium-ion batteries.11,12 Therefore, the present study discussed the influence of visible light illumination on the formation of iron(III) oxyhydroxide thin films in Fe2+ solution via an oxidative electroless deposition process. Though the deposition of CeO2 is required to apply anodic bias for the oxidation of Ce3+, as stated above,8 Fe2+ is easily oxidized by dissolved oxygen molecules and insoluble iron oxyhydroxide is formed on a conductive substrate without external bias (electroless deposition). Under visible light irradiation, the electrochemical reactions are promoted by generated photocarriers at the deposits|solution interface. This enhances the deposition amount and presumably the crystallinity as compared with the dark process. In addition, it is revealed that the film obtained has a unique nanostructure based on the crystal morphology of iron oxyhydroxide minerals. The development of photoelectroless processing of semiconductor films may explore a novel aspect with respect to utilization of visible light. Experimental Methods Electroless deposition of iron oxyhydroxide thin films was carried out in a quartz glass cell including a mixed aqueous solution of 0.03 M FeSO4 + 0.03 M CH3COOH (pH ) 3.1). The water used in all processes was doubly deionized water. The coexistence of acetate ions brings about a weak acidity to stabilize Fe2+ in the solution. In fact, the colorless and transparent solution immediately became turbid in the absence of acetic acid (pH ) 4.4). For electroless deposition, the Pt substrate (apparent surface area ) 1 cm2) was immersed in a

10.1021/jp911014f  2010 American Chemical Society Published on Web 02/05/2010

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Figure 1. Reaction time dependence of electroless deposition amounts of Fe on the Pt substrate under dark or visible light irradiation (>400 nm). The Fe amounts on the Ti substrate under dark and illumination (solid and open squares, respectively) were plotted as well, but only for time ) 18 h. The inset shows the effect of irradiation wavelength on the deposition amount at 24 h reaction time (µg cm-2).

naturally aerated solution at 308 K for various periods, where the Ti foil was also employed in a few cases. During the reaction, the substrate was exposed to the light from a 300 W Xe lamp where UV radiation (400 nm). The Fe amount increased monotonically with reaction time. The linear relationship between the amount deposited and the time predicts the easy controllability of the deposition amount in a microgram level by selecting the time. In the dark state, the film formation on the Pt substrate is caused due to a local cell mechanism. The dissolved oxygen molecule oxidizes Fe2+ by an electron transfer via the Pt substrate. Considering a catalytic activity of Pt for oxygen reduction, the redox reaction prefers

Kamada et al. to occur on the Pt rather than in the bulk solution. The opencircuit potential of Pt was gradually shifted to the negative side during the deposition (Figure 1S, Supporting Information). Because the Fe2+ to be oxidized was present in abundance, its concentration was not changed over the whole reaction period. Taking into account a small content of dissolved oxygen being responsible for the cathodic reaction, the deposition rate would be limited by diffusion of oxygen molecules toward the Pt surface. As a result, it is considered that the negative shift of the open-circuit potential was originated from the reduction of partial current density for the cathodic reaction. These facts support the assumption of a local cell mechanism for the electroless deposition. Because of a lower solubility of Fe3+ than the reduced species, the Fe3+ would be deposited as a solid phase of oxyhydroxide (FeO(OH)) on the Pt substrate. The consumption of protons by the oxygen reduction might raise the local pH near the Pt surface and incite the formation of the iron oxyhydroxide.13 In general, not only iron oxides (Fe2O3) but also polymorphous oxyhydroxides (FeO(OH)) behave as n-type semiconductors having narrow band gaps that can be excited by visible light (Eg ) 2.0-2.5 eV).14,15 Indeed, it was established that the as-deposited layer acts as an n-type semiconductor by the photoelectrochemical evaluation (Figure 2S, Supporting Information). During the reaction, the exposure of the substrate to visible light brings about the production of photocarriers, that is, electrons in the conduction band and holes in the valence band. Hence, the Fe2+ ions in solution are oxidized by the holes to form new nuclei on the original ones because the redox potential of Fe2+/FeO(OH) is more negative than the valence band edge.15 This may proceed via the reaction between Fe2+ and OH radicals, which are generated by the reaction of H2O with the holes. That is, the preformed oxyhydroxide nuclei act as a sensitive layer for photoelectroless deposition. As shown in Figure 1, the deposition rate under visible light illumination was more than 3 times faster than that of the dark reaction even at the same bath temperature (2.6 µg/h > 0.8 µg/h). In the case of the dark reaction, the film growth through the local cell mechanism will be suppressed once the substrate is covered with deposit, which is not very conducting. Several groups have studied the photoelectrochemical fabrication of thin films on semiconductor electrodes, such as doped silicon, SrTiO3, and TiO2.16 They have reported that holes (n-type) or electrons (ptype) produced photochemically at the valence or conduction band can promote the electrochemical reaction of chemical species in solution. However, if the deposits scatter or reflect the incident photon energy, the light intensity reaching the semiconductor electrode would be attenuated and the number of photogenerated charge carriers would decrease. In contrast, because photoactive FeO(OH) is always exposed at the surface in the proposed system, continuous growth of FeO(OH) is achieved independent of substrate type. In fact, we have confirmed that the photoirradiation was effective not only for the Pt but also the Ti substrate (Figure 1), even though the deposition amount on the Ti was considerably reduced as compared with the Pt substrate having excellent oxygen reduction ability. In summary, it was demonstrated that the visible light irradiation significantly improves the deposition rate of iron oxyhydroxide thin films on the basis of the photoassisted oxidation of Fe2+ at the solid-solution interface. The inset in Figure 1 indicates the influence of irradiation wavelength on the deposition amount of Fe. Interestingly, it was revealed that the growth rate of iron oxyhydroxide films was dramatically affected by the wavelength for the excitation.

Electroless Deposition of Iron Oxyhydroxide Thin Films

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Figure 3. UV-vis transmission spectra of as-deposited (a) and postannealed (b) films on the FTO substrate after photoelectroless deposition for 24 h. Figure 2. Raman spectra of as-deposited (a) and postannealed (b) films prepared by photoelectroless deposition under irradiation of visible light (>400 nm) for 24 h. S, H, and M after the peak wavenumber indicate schwertmannite, hematite, and magnetite, respectively.

The visible light irradiations above 400 or 420 nm resulted in increments of Fe amounts depending on the photon energies. However, no formation was confirmed under the white light, including the UV region. The result may be due to photoreductive dissolution of Fe3+-OH surface groups in the lattice. Under the irradiation of photon energy significantly exceeding the band gap, the photogenerated electrons trapped at the oxyhydroxide|solution interface give rise to reduction of surface Fe3+ and subsequent dissolution of reduced species. In the case of γ-Fe2O3 (maghemite), it was reported that the quantum efficiency for the photoreductive dissolution in the presence of EDTA increases with decreasing irradiation wavelength and pH.17 In the present study, hence, the iron oxyhydroxide exposed to the UV light would be dissolved in the acidic solution readily after deposition. Thus, with the photoirradiation, two contradictory effects for the electroless deposition of iron oxyhydroxide take place, that is, activation of Fe2+ oxidation and dissolution of solid products. The former remarkably appeared under the visible light irradiation, and the latter is more prominent for the UV light. Judging from these facts, it can be concluded that the photoactivation of electroless deposition appears only under visible light illumination. Because the XRD patterns of as-deposited samples (Figure 3S, pattern a, Supporting Information) showed no diffraction peaks, Raman spectroscopy was utilized for assessment of the crystal structure of the prepared films. No Raman-active mode could be detected in the spectrum of the film prepared in the dark, implying that the film was amorphous (not shown). In contrast, the film deposited under illumination exhibited several broad bands, as shown in Figure 2, spectrum a. These results support that the visible light irradiation would induce the crystallization and growth of iron oxyhydroxide particles. All peaks in Figure 2, spectrum a, were assigned to schwertmannite (Fe8O8(OH)8-2x(SO4)x, x ) 1-1.75), which is a metastable iron oxyhydroxide mineral containing sulfate ions.18 Indeed, the Raman band attributed to SO42- also existed at 981 cm-1 (not shown). The crystal structure of schwertmannite is akin to akagane´ite (β-FeO(OH)), and the sulfate ions occupy the tunnel space surrounded by FeO6 octahedrons.19 It is known that the mineral is spontaneously formed in Fe-rich acidic solution, including sulfate ions at pH ) 2.5-4.8 (e.g., in acid mine drainage). Though the growth of schwertmannite slowly proceeds in acidic solution, the presence of iron oxidizing bacteria increases the formation rate.20 Because the present reaction bath containing

sulfate ions is close to the production environment of natural schwertmannite, the analogous formation process would occur during the electroless deposition. That is, Fe3+ ions produced at the Pt or the deposit surface were transformed into Fe2(OH)2(SO4)x(2-x)+ dimers by hydrolysis and then immediately precipitated as schwertmannite phase.21 On the other hand, the postannealing brought about the phase transition to thermodynamically stable hematite (R-Fe2O3) by dehydroxylation and crystallization (Figure 2, spectrum b, and Figure 3S, pattern b, Supporting Information), where the additional Raman mode attributed to magnetite (Fe3O4) was observed as similar to the report that dealt with the electrochemical synthesis of hematite films in ferric solution.11 Figure 3 shows UV-vis absorption spectra of the fabricated thin films on the transparent FTO substrate. The as-deposited film absorbed the light having a shorter wavelength than 470 nm (2.6 eV). The absorption edge was located at the higher energy side than the band gap energy of β-FeO(OH) having an equivalent crystal structure with schwertmannite and other iron oxyhydroxides (2.0-2.5 eV).14,15 There are few studies on the optical absorption properties of schwertmannite at present. The observed blue shift of the absorption shoulder may be related to low crystallinity of schwertmannite owing to incorporation of the large sulfate ion into the narrow tunnel space in the β-FeO(OH) structure. As shown in the inset in Figure 1, the slight photoeffect in the case of visible light irradiation above 420 nm may be attributed to less overlapping between the absorption wavelength of schwertmannite and the light source. On the other hand, the appearance of the film was changed from pale yellow to redbrown after the postannealing at 873 K for 1 h. The absorption edge of the postannealed films was shifted to the longer wavelength (590 nm ∼ 2.1 eV) according to the phase transition to hematite, as shown in Figure 2. Figure 4 shows the surface SEM images of as-deposited (a) and postannealed (b) films prepared by the photoelectroless deposition for 24 h. The as-deposited film retains the nanostructure composed of numerous whiskers with diameters of 10-30 nm. Though this nanostructure is similar to the “hedgehog” morphology of schwertmannite frequently reported, the growth mechanism of the anisotropic particles is now under discussion.21,22 The light irradiation might lead to preferential photooxidation of Fe2+ and stimulate anisotropic crystal growth at the apex of the whiskers. As shown in Figure 4c, the anodic photoelectrodeposition at +0.8 V versus Ag|AgCl for 24 h caused the dense coating of the Pt substrate in the equivalent solution.23 Accordingly, it should be noted that the nanostructure observed in Figure 4a is one of the features of the photoelectroless deposition technique. Figure 4b displays the surface microstructure of the films after postannealing at 873 K for 1 h.

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Kamada et al. above ca. + 0.7 V. The occurrence of anodic photocurrent was ascertained under the illumination, suggesting that the film shows n-type semiconducting properties. The crystalline R-Fe2O3 can absorb photon energy in the visible region, and holes are created in the valence band as stated already. The current spikes and their decays observed readily after the irradiation are attributed to recombination of photogenerated electron-hole pairs at the solid|solution interface. Another possible process contributing to the photocurrent decay is that electrons excited in the conduction band reduces the oxidized species in the electrolyte solution.24 As shown in Figure 5, the steady-state photocurrent after converging the decay was small (ca. 5 µA cm-2) even under the presence of methanol as a hole scavenger, implying a low electron mobility and short diffusion length of holes in pure hematite.10 In addition, the thin films postannealed at higher temperatures (973 and 1073 K) showed comparable photoelectrochemical performances with Figure 5. Therefore, the present photoelectroless deposition technique will be extended to fabricate nanostructured and doped hematite films25 to enhance the photoanode performances in the near future. Conclusions

Figure 4. SEM images of as-deposited (a) and postannealed (b) films fabricated by photoelectroless deposition, where the surface picture of film photoelectrochemically deposited at +0.8 V vs Ag|AgCl for 24 h is also depicted in (c) for comparison.

Figure 5. Anodic polarization curve of R-Fe2O3/Pt in 1 M CH3OH + 0.1 M K2SO4 at room temperature with a scan rate of 10 mV s-1, where the electrode was illuminated with visible light (>420 nm) intermittently.

It was revealed that the unique nanostructure of as-deposited films was unchanged even after the postannealing. The above-mentioned nanostructure is believed to be useful for various applications because of large surface area and low grain boundary density. For instance, the applications as the anode of photoelectrochemical cells for water splitting, detection electrode for gas sensors, and the positive electrode for Li-ion batteries may be promising. We have evaluated the photoelectrochemical performance of the film on the Pt substrate after the postannealing at 873 K (R-Fe2O3/Pt). Figure 5 indicates that the anodic polarization curve of the R-Fe2O3/Pt was recorded in 1 M CH3OH + 0.1 M K2SO4 (pH 6.2), where the R-Fe2O3/ Pt electrode was exposed to visible light (>420 nm) intermittently (“on” and “off”). Under the dark (lower line), the broad peak centered at ca. + 0.5 V was assigned to oxidation of Pt, and oxygen gas evolution from H2O molecule was confirmed

The present study investigated the influence of visible light irradiation on the electroless deposition of iron oxyhydroxide thin films on the Pt substrate in the FeSO4 aqueous solution. According to the local cell mechanism, Fe2+ is oxidized to Fe3+ by dissolved oxygen via the Pt substrate as an electron mediator, and then the produced Fe3+ was deposited as iron oxyhydroxide on the substrate. The visible light irradiation during the electroless deposition induced the occurrence of photocarriers at the preformed oxyhydroxide surface with a narrow band gap. The holes in the valence band having strong oxidizing ability induce further oxidation of Fe2+ in the solution. As a result, the deposition rate of films was considerably increased. The degree of photoactivation depended on the wavelength of incident light. No film formation was observed under the illumination of UV light, which preferentially caused photoreductive dissolution of the deposits. The crystal structure of asdeposited thin films corresponded to the metastable phase of iron oxyhydroxide, including sulfate ions, which is known as schwertmannite. The film was composed of numerous nanosized whiskers that had a roughly perpendicular orientation to the substrate. This nanostructure remained unchanged even after phase transition to hematite by postannealing. The huge surface area of the nanostructured iron oxide films may be variable as an active electrode for several electrochemical devices employing interface reactions. In addition to iron oxide, there are many semiconducting metal oxides that are formed by oxidation of metal ions with lower valence in solution (TiO2, CeO2, etc.). The photoassisted effect is also predicted for electroless deposition of these oxides on various conducting substrates. Moreover, the photochemical technique enables on-site fabrication because the reaction occurs only on the irradiated area. It is expected that the present technique will be used to fabricate many different kinds of metal oxide and complex oxide thin films with excellent functions. Acknowledgment. This study was partly supported by the Special Coordination Funds for Promoting Science and Technology, MEXT, Japan: “The Nagasaki University Strategy for Fostering Young Scientists”. Supporting Information Available: Figures showing time evolution of the open-circuit potential during photoelectroless

Electroless Deposition of Iron Oxyhydroxide Thin Films deposition, electrochemical photoresponse of iron hydroxide thin films, and X-ray diffraction patterns of as-deposited and postannealed films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gra¨tzel, M. Nature 2001, 414, 338. (2) (a) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (b) Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (3) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. (b) Sato, S.; Sobczynski, A.; White, J. M.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E. J. Photochem. Photobiol., A 1989, 50, 283. (4) (a) Torimoto, T.; Paz Reyes, J.; Iwasaki, K.; Pal, B.; Shibayama, T.; Sugawara, K.; Takahashi, H.; Ohtani, B. J. Am. Chem. Soc. 2003, 125, 316. (b) Torimoto, T.; Kontani, H.; Shibutani, Y.; Kuwabata, S.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. B 2001, 105, 6838. (5) (a) Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. J. Phys. Chem. B 2000, 104, 4873. (b) Nakanishi, S.; Tanaka, T.; Saji, Y.; Tsuji, E.; Fukushima, S.; Fukami, K.; Nagai, T.; Nakamura, R.; Imanishi, A.; Nakato, Y. J. Phys. Chem. C 2007, 111, 3934. (6) (a) Masuda, H.; Kanezawa, K.; Nakao, M.; Yokoo, A.; Tamamura, T.; Sugiura, T.; Minoura, H.; Nishio, K. AdV. Mater. 2003, 15, 159. (b) Oekermann, T.; Yoshida, T.; Nakazawa, J.; Yasuno, S.; Sugiura, T.; Minoura, H. Electrochim. Acta 2007, 52, 4325. (7) (a) Sugimoto, Y.; Peter, L. M. J. Electroanal. Chem. 1995, 386, 183. (b) Murase, K.; Matsui, M.; Miyake, M.; Hirato, T.; Awakura, Y. J. Electrochem. Soc. 2003, 150, C44. (8) Kamada, K.; Higashikawa, K.; Inada, M.; Enomoto, N.; Hojo, J. J. Phys. Chem. C 2007, 111, 14508. (9) (a) Kay, A.; Cesar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 15714. (b) Duret, A.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 17184. (c) Bjo¨rkste´n, U.; Moser, J.; Gra¨tzel, M. Chem. Mater. 1994, 6, 858. (d) Khader, M. M.; Vurens, G. H.; Kim, I.-K.; Salmeron, M.; Somorjai, G. A. J. Am. Chem. Soc. 1987, 109, 3581. (10) (a) Lindgren, T.; Wang, H.; Beermann, N.; Vayssieres, L.; Hagfeldt, A.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 2002, 71, 231. (b) Beermann, N.; Vayssieres, L.; Lindquist, S.-E.; Hagfeldt, A. J. Electrochem. Soc. 2000, 147, 2456.

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