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J. Phys. Chem. C 2007, 111, 14508-14513
Photoassisted Anodic Electrodeposition of Ceria Thin Films Kai Kamada,* Keigo Higashikawa, Miki Inada, Naoya Enomoto, and Junichi Hojo Department of Applied Chemistry, Faculty of Engineering, Kyushu UniVersity, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ReceiVed: June 27, 2007; In Final Form: July 31, 2007
This study investigated the effect of UV light irradiation on a Pt working electrode during the anodic electrodeposition of ceria (CeO2) thin films in an aqueous solution containing Ce3+. UV light irradiation on the Pt anode induced the formation of holes in the valence band of anodically preformed CeO2 nuclei, which serve as photoabsorbers. The resultant holes oxidized Ce3+ to CeO2 at the CeO2/solution interface as well as at the bare Pt surface. That is, CeO2 itself acted as a sensitive layer for photoelectrochemical deposition. Although the current efficiency was reduced by the UV irradiation, the deposition amount and crystal size of CeO2 clearly increased as compared to the dark process. It was verified that the oxygen saturation of the electrolyte solution was also effective in enhancing the deposition. In this paper, the photoelectrochemical deposition mechanism of CeO2 is discussed in detail, and the positive effects of photoirradiation on film quality are demonstrated.
Introduction Thin films of ceria (CeO2) and its solid solutions have many important characteristics, such as electrochromic,1,2 UV light shielding,3 and ion conducting4,5 properties. Recently, CeO2 films have attracted attention as anti-corrosion coatings for galvanized steel6,7 and aluminum alloys8 because they show selfhealing properties,9 similar to conventional chromate coatings containing toxic hexavalent chromium ions. In the past few decades, a number of reports has dealt with the electrochemical preparation of CeO2thin films.10-17 Conventional electrolysis was performed in aqueous solutions of Ce3+ salts, and CeO2 films were deposited via electrochemical reactions on conductive substrates. It is well-known that CeO2 films can be electrodeposited by both cathodic10-16 and anodic17 processes. In the former case, applying a more negative bias than the hydrogen evolution potential of H2O raises the pH near the electrode surface because of the simultaneous formation of hydroxide ions. It appears that, as a result, Ce3+ is precipitated as a hydroxide on the electrode surface. Finally, Ce(OH)3 is transformed into CeO2 by dehydration and oxidation. Anodic polarization allows electrochemical oxidation of Ce3+ to soluble Ce(OH)22+ (eq 1). The supersaturated Ce(OH)22+ at the anode surface is deposited as solid CeO2 (eq 2).
Ce3+ + 2H2O f Ce(OH)22+ + 2H+ + e-
(1)
Ce(OH)22+ f CeO2 + 2H+
(2)
Electrochemical preparation of metal oxide thin films is recognized as a soft solution method because the process can be performed under environmentally friendly conditions.18-20 Furthermore, the film composition, thickness, and morphology can be controlled by adjusting electrochemical parameters such as potential, current, and temperature. However, there are some disadvantages associated with oxide film deposition: (1) poor * Corresponding author. Tel.: +81-92-802-2861; fax: +81-92-802-2861; e-mail:
[email protected].
Figure 1. Schematic model for anodic electrodeposition of CeO2 in the Ce3+ solution without (a) or with (b) UV light irradiation on the metal electrode.
crystallinity of films consisting of hydro- or hydrated oxide, (2) difficulty in fabricating a thick film because of the potential drop through the deposited layer on the metal electrode, and (3) crack formation in films due to gas evolution caused by the decomposition of electrolytes or solvents (sub-reaction). These problems are often circumvented through the addition of organic reagents to the bath13 or postannealing of the as-prepared film.14 It is believed that for the electrochemical preparation of dense and crystalline CeO2 films, anodic polarization is more appropriate than cathodic polarization because the former method allows the oxide form to be directly obtained from soluble Ce(OH)22+ without the formation of hydroxides (eqs 1 and 2). CeO2 can harvest UV light (∼3.1 eV) by the indirect transition of electrons from the valence band (O2p) to the empty Ce4f states.21,22 Figure 1a schematizes CeO2 deposition under an applied anodic potential. CeO2 can be formed on the metal electrode in Ce3+ solution by eqs 1 and 2. However, film growth will be suppressed once the electrode is covered with CeO2. In other words, Ce3+ cannot be oxidized on the preformed CeO2 layer, which is not very conducting. If a large overpotential is applied to increase the film thickness, structural defects (cracks, pinholes, and peeling) will be induced by oxygen evolution from H2O. The present study proposes UV light-assisted anodic electrochemical synthesis for the fabrication of CeO2 films of excellent quality (dense, crystalline, and strong adhesion with
10.1021/jp074999p CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
Photoassisted Anodic Electrodeposition of CeO2 Thin Films
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14509
substrate). Figure 1b schematizes the proposed photoelectrochemical technique. The electrode surface is irradiated with UV light to activate the electrochemical reaction on CeO2. Holes (h+) are produced in the valence band of the CeO2 nuclei, formed anodically in advance. The Ce3+ ions in solution are oxidized by the h+ to form new CeO2 nuclei on the original nuclei (eqs 3 and 2) because the redox potential of Ce3+/4+ is more negative than the valence band edge. Equation 3 may proceed via the reaction between Ce3+ and OH radicals, which are generated by the reaction of H2O with h+.23 As a result, the continuous growth of CeO2, which is difficult by conventional anodic electrolysis, can be achieved under illumination
Ce3+ + H2O + h+ f Ce(OH)22+ + 2H+
(3)
Several groups have studied the photoelectrochemical fabrication of thin films on semiconductor electrodes such as doped silicon,24-27 SrTiO3,28 and TiO2.28,29 They have reported that holes (n-type) or electrons (p-type) 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.24 In contrast, since UV active CeO2 is always exposed at the surface of the electrode in the proposed system, continuous growth of CeO2 is achieved independently of electrode type. It is believed that this would enhance the density, thickness, and presumably also the crystallinity as compared to the dark process. The photoeffects on electrodeposition already have been studied for compound semiconductor films (CdTe, etc.).30 The present study investigates in detail the feasibility of the proposed photoelectrochemical deposition technique, and UV irradiation was demonstrated to be effective in improving the quality of CeO2 thin films without any posttreatment. Experimental Procedures Electrochemical preparation of CeO2 thin films was carried out in a single-compartment quartz glass cell. A Pt or SUS304 foil was adopted as the working electrode (apparent electrode area: 1 cm2). A Pt plate (6 cm2) and saturated Ag/AgCl were used as counter and reference electrodes, respectively. In this paper, the working electrode potentials refer to the reference electrode unless otherwise stated. The water used in all processes was doubly deionized water. The electrolyte solution was prepared by mixing identical volumes of a 0.1 M Ce3+ salt (nitrate, acetate, and chloride) with 0.1 M CH3COOH (i.e., [Ce3+] ) 0.05 M). The presence of acetate ions in the electrolyte solution allows Ce3+ to be stabilized as a complex ion.17 The solution pH was adjusted to 5 by the addition of a minute amount of 3 M NaOH. Before electrolysis, the electrolyte solution was naturally aerated or saturated with O2 gas over 30 min in a few cases. Electrolysis was performed by applying a constant potential to the working electrode at 313 K, where the working electrode was exposed to UV light from a 500 W ultrahigh pressure Hg lamp. The films obtained were washed with water and dried at room temperature. The morphology, composition, and crystal structure were observed and analyzed by using SEM, EDS, XRD, UV-vis, and Raman spectroscopy. The amount of Ce deposited was measured by ICP-MS after dissolving a film in HNO3 solution. The electrocatalytic performance of Pt/CeO2 for methanol oxidation was assessed by anodic polarization in an aqueous solution of 5 M CH3OH + 0.1 M K2SO4 at room temperature. The anti-corrosion
Figure 2. Anodic polarization curves (2 mV/s) of (a) bare Pt electrode in 0.05 M Ce(CH3COO)3 and (b) Pt/CeO2 electrode in 0.1 M K2SO4 under UV illumination. Pt/CeO2 was fabricated by anodic polarization at 0.8 V for 1 h under UV irradiation.
properties of a CeO2 coated SUS substrate were determined by measuring the anodic polarization curve in a 0.1 M NaCl aqueous solution at room temperature. Results and Discussion Among the three cerium salts (nitrate, acetate, and chloride), Ce(CH3COO)3 and CeCl3 were adequate for the present technique from the viewpoint of the stability of Ce3+ at pH 5 in the absence of an electric field (i.e., no precipitation occurred in the bath under UV irradiation). However, it was difficult to obtain a uniform film morphology using the CeCl3 solution. In this case, a translucent film consisting of a large amount of deposit with poor adhesion was observed on the electrode. The reversible redox potential of Ce3+ was reported to depend on the counteranion. The presence of chloride ions shifts it to a significantly more negative potential.31 Such a potential shift might increase the current density during an anodic electrolysis, which would hinder homogeneous deposition in the CeCl3 solution. Moreover, Cl- contamination may have a negative effect on the anti-corrosion properties of the CeO2 film. Consequently, a Ce(CH3COO)3 solution, which can be used to fabricate transparent thin layers, was employed as an electrolyte solution in further investigations. Figure 2a shows the anodic polarization curve of the Pt working electrode in a Ce(CH3COO)3 solution from an open circuit potential (OCP: +0.4 V) without UV illumination. The broad peak centered at 0.75 V was assigned to the oxidation of Pt, and oxygen gas evolution from a H2O molecule (OER) was confirmed above ca. 1.0 V. On the basis of a comparison with the polarization curve for a Ce3+-free solution, no electrochemical response related to Ce3+ was detected in Figure 2a. This was because of the more positive oxidation potential of Ce3+ (1.14 V vs SHE) as compared to that of H2O (0.92 V vs SHE) at pH 5, where the estimations were carried out by assuming [Ce3+] ) [Ce(OH)22+] and pO2 ) 0.2 at 298 K.32 The stabilization of Ce3+ by weak hybridization with CH3COOligands (log Kf ) 1.68)17 might also increase the overpotential. Potentiostatic dark electrolysis at +0.8 V (and more positive potential) induced the formation of a film on the Pt electrode. The fact that the characteristic X-rays of Ce and O were detected for the as-deposited film by EDS analysis indicates that CeO2 was formed during electrolysis via eqs 1 and 2. To investigate the photoelectrochemical properties of asdeposited CeO2 thin films (Pt/CeO2), anodic polarization curves
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Kamada et al.
Figure 3. Effect of UV irradiation on current density during potentiostatic electrolysis at 0.8 V and 313 K. Typical current change in the O2-saturated solution under UV illumination is also shown.
(Figure 2b) were measured in 0.1 M K2SO4 (pH ) 5), where the Pt/CeO2 electrode was exposed to UV light intermittently (on and off). The polarization behavior in the dark state (off) was similar to that shown in Figure 2a. As already stated, crystalline CeO2 can absorb photon energy in the UV region, and holes are created in the valence band, as depicted in Figure 1b. The absorption in the UV-vis region was confirmed in the reflectance spectra of Pt/CeO2 (Figure S1 in the Supporting Information). In general, the turning point of the photocurrent (from a cathodic to an anodic photocurrent) in the potential approaches the flat band potential (Vfb). Although Vfb depended slightly on the electrolyte solution and the amount of CeO2, the occurrence of anodic photocurrent was verified above ca. Vfb ) 0.2 V. The photocurrent increased with increasing anodic potential (about 20 µA at 0.8 V). It must be noted that the surface oxide on Pt under an anodic bias also exhibits a photoresponse.33 The anodic photocurrent of the Pt/PtOx electrode without a CeO2 layer (Figure S2 in the Supporting Information), however, was sufficiently small as compared to the Pt/CeO2 electrode (
