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J. Phys. Chem. B 1999, 103, 7190-7194
Deposition of Metallic Oxides on TiO2 Electrode Using the Photoelectrochemical Epitaxial Growth Technique (PEEG), and Electrochemical Behavior of the PbO2/TiO2 Electrode Yasumichi Matsumoto,* Masayuki Noguchi, and Tetsuhiro Matsunaga Department of Applied Chemistry, Faculty of Engineering, Kumamoto UniVersity, Kurokami 2-39-1, Kumamoto 860-8555, Japan ReceiVed: January 6, 1999; In Final Form: June 14, 1999
The deposition of metallic oxides onto both single crystal and polycrystalline TiO2 electrodes using the photoelectrochemical technique was carried out, and the electrochemical behavior of the prepared PbO2/TiO2 electrode was studied. The photoelectrodeposition of PbO2, Fe2O3 and MnO2 occurred under illumination but not in the dark. The crystal orientation of the deposited oxides strongly depended on the crystal misfit between the oxides and the TiO2 substrate. Even pulse illumination brought about the deposition, because the OH radical produced on the surface by a hole significantly contributes to the oxide deposition during the initial step. The PEEG (photoelectrochemical epitaxial growth) model was proposed, and compared with the AFM images. From the measurements of the current-potential curve of the PbO2/TiO2 electrode, it was found that the PbO2 deposited on the polycrystalline TiO2 produces a surface state leading to a large anode tunneling current and that Fermi level pinning occurs at the PbO2/TiO2 interface. The present PbO2/polycrystalline TiO2 electrode was very stable in acidic solution, and therefore, will be very useful as an inert anode.
Introduction The development of photoelectrochemistry of semiconductor electrode1-5 has brought about two important areas of photoelectrochemical cell6-8 and photocatalysis9-13 in the application, where TiO2 is one of the most important material for these applications. Photoelectrochemical epitaxial growth (PEEG)14-16 in electrodeposition is the other important application of the TiO2 electrode in photoelectrochemistry, because epitaxial films of high performance oxides, which are very important for the preparation of superlattice, quantum dot, and other high performance materials,17-19 can be easily prepared under mild condition and at low cost. The PEEG was observed during the photoelectrochemical deposition of some metallic oxides onto the surface of singlecrystal oxide semiconductor electrodes such as TiO2 and SrTiO3.14-16 In PEEG, the photoproduced OH radical on the illuminated electrode surface significantly contributes to the deposition of the metallic oxides because the deposition is very difficult under a dark condition even if an anodic current flows. A small tunneling current bringing about the subsequent deposition under anodic bias was also observed at the metallic oxide/TiO2 electrode after the initial photoelectrochemical deposition during the PEEG process.15,16 However, the details of the interface condition of the prepared metal oxide/TiO2 and the crystal growth in the PEEG of the deposited oxides have not yet been clarified. It is well-known that PbO2 is very useful as an inert anode in acidic solution.20-22 PbO2/Ti electrodes where R- and β-PbO2 are anodically deposited on a Ti plate in the solution containing Pb2+ play very important roles in practical applications.22 In this case, the TiO2 film formed on the Ti plate must be released20 or some conducting mixed oxides must be coated on the Ti substrate21,22 before the PbO2 electrodeposition, because the TiOx oxide produced on the Ti substrate under the anodic bias behaves as an insulator and disturbs the electrodeposition of PbO2.
The photoelectrodeposition of PbO2 onto the TiO2 semiconductor is possible if we use the PEEG technique as stated above.15 Moreover, the prepared PbO2 coated TiO2 (PbO2/TiO2) electrode may be useful as an inert anode material in acidic solution, because a tunneling current flows via the PbO2 crystal formed at the surface under anodic bias. In this paper, the PEEG model was proposed and was compared with the AFM images for the deposited PbO2, MnO2, and Fe2O3, and the electrochemical behavior of the prepared PbO2/TiO2 electrode was measured. PEEG Model. Metallic oxides have been sometimes electrodeposited at metal electrode in acidic solution under anodic bias by the following steps.23-25
H2O f OH + H+ + e- (at metal electrode)
(1)
Mn+ + nOH f MOn + nH+
(2)
The first step consists of the production of OH radical (eq 1) in this electrodeposition process, although some complex steps will be contained in the subsequent step (eq 2) that produces the oxidation reaction of the metallic cation (from Mn+ to M2n+(MOn)). In the case of the PEEG process, the OH radical is also produced at n-type oxide semiconductor such as TiO2 by the following reaction between a hole and water under illumination.
H2O + h+ f OH + H+ (at TiO2 electrode)
(3)
Therefore, the OH radical produced on the surface of the n-type TiO2 semiconductor electrode under illumination must necessarily catch the metallic cation and then bring about its oxide deposition during the subsequent steps ( eq 2). Figure 1 shows the postulated model of the PEEG for the metallic oxide deposition on the TiO2 surface. Reactions similar to those in Figure 1 will occur for other titanate oxides producing the OH radical on their surface under illumination (for example
10.1021/jp9900685 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/06/1999
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Figure 1. Model of the PEEG for the metallic oxide deposition on the TiO2 surface.
SrTiO3, etc.). The OH radical produced by a hole on the TiO2 surface (step A, eq 3) will react with the metallic cation (Mn+) to form some crystal nuclei of the metallic oxide MOn (step B, eq 2), where a monolayer of the oxide will form only at the surface part covered with the OH radical. Diffusion of some nuclei with a unit cell size and/or OH radical26 may simultaneously occur on the TiO2 surface in step B. There will be some differences in the production rate of the OH radical depending on the surface point. For example, the production rate of the OH radical will be relatively fast at the defect, step, and kink sites27 compared with the perfect surface. In the following steps, C and D, the oxide crystal growth due to the tunneling current via surface states of metallic oxide nuclei as well as the production of the MOn nuclei by the OH radical on the bare TiO2 (eqs 1 and 2) will occur. The same reactions as eqs 1 and 2 at metal electrode will occur at the surface of the MOn nuclei under the tunneling current. Consequently, many metallic oxide nuclei epitaxially grow on the entire surface of the TiO2 because of the single crystal surface, although some oxide hills are produced where fast production of the OH radical occurs are present. The degree of the roughness, of course, will strongly depend on the electrolysis current density and temperature affecting the surface diffusion of the formed oxide nuclei. Therefore, a relatively smooth surface can be obtained at a low current density and under high-temperature conditions as already reported,15,16 because the surface diffusion is fast at high temperature and the production of excess new crystal nuclei is suppressed in low current density. On the other hand, crystal growth is different in morphology under the pulse illumination technique where the illumination is applied from few seconds to a few minutes during the initial electrolysis and then the deposition proceeds due to a tunneling current in the dark (eqs 1 and 2). In this case, the crystal growth will occur only at the oxide nuclei initially formed under the pulse illumination, but the deposition will not occur at the bare surface of the TiO2, because only the deposited oxide nuclei act as the surface state bringing about the tunneling current and the active sites for the subsequent oxide deposition (eq 2) as already reported. This model is illustrated in Figure 1E and F where some large island type crystals will sparsely grow on the TiO2 surface (Supporting Information).
Experimental Section The polycrystalline TiO2 electrodes were prepared by heating a Ti plate at 400 °C. All of the thermally formed TiO2 had a rutile structure according to an X-ray diffraction (XRD) analysis. In the case of the single-crystal electrode, the rutile type TiO2 with a (100) plane parallel with the electrode surface was reduced in H2 at 800 °C so as to increase conduction. Indium was deposited on the backside of the TiO2 for the purpose of ohmic contact. The electrodeposition of PbO2, MnO2 and Fe2O3 onto the TiO2 electrodes was carried out under anodic bias at a constant current density under illumination at 80 °C or at room temperature. A 500 W ultrahigh-pressure Hg lamp (3 × 105 lux) was used as the light source. The solutions for the PbO2, MnO2 and Fe2O3 depositions were Pb(NO3)2, Mn(NO3)2, and FeCl2, respectively. A 0.1 M Pb(NO3)2 solution was sometimes adjusted to pH ) 1 by HNO3 titration, since the solution of 0.1 M Pb(NO3)2 was initially pH ) 4. The structures of their deposited oxides and their morphology were analyzed by the X-ray diffraction (XRD, Cu KR radiation, 1.54 Å wavelength) and atomic force microscopy (AFM), respectively. The morphological state of the deposited oxide on the electrode surface was observed by an electron probe microanalysis (EPMA). The deposited PbO2 film was dissolved by HCl and then analyzed by inductively coupled plasma (ICP) spectroscopy to determine the amount of Pb. The electrochemical behavior of the PbO2/TiO2 electrodes was obtained in H2SO4. Ag/AgCl and Hg/Hg2SO4 electrodes were used as the references in Pb(NO3)2 and H2SO4 solutions, respectively. Results and Discussion Morphology of the Deposited Oxides. Some OH radicals produced on the TiO2 surface (eq 3) will react with each other as represented by the following steps to produce H2O2 and O2, except for the deposition reaction (eq 2).28
OH + OH f H2O2
(4)
H2O2 + 2h+ f O2 + 2H+
(5)
The current efficiency for the PbO2 production was about 100% according to the analysis of the amount of the deposited PbO2
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Figure 2. Potential-time curve of the TiO2 (100) single-crystal electrode at 50 µA/cm2 in 0.1 M Pb(NO3)2 (pH ) 1) at room temperature.
Figure 3. XRD patterns (Cu KR, 1.54 Å) of the β-PbO2 (A) and Fe2O3 (B) deposited at 50 µA/cm2 for 3 h on the single-crystal TiO2(100) surface at 80 °C in 0.1 M Pb(NO3)2 (pH ) 1) and 0.1 M FeCl2 (pH ) 4), respectively.
using ICP spectroscopy and the calculation using eqs 1-3 where n ) 2. This means that eqs 4 and 5 reactions bringing about the O2 evolution reaction (OER) scarcely proceed in the PbO2 deposition process. H2O2 as an intermediate in eq 4 never reacts with Pb2+ to produce PbO2, because this is an uphill reaction. Consequently, only the PEEG of PbO2 proceeds by the reaction steps represented by eqs 1-3 in the present electrolysis conditions. Figure 2 shows the time dependence of the potential in the electrolysis at 50 µA/cm2 under illumination for the PEEG of PbO2. The deposition occurred even at 50 µA/cm2 in the present test, although the deposition was difficult at room temperature in our previous work.15 This difference may be based on that in pH. The solution was pH ) 4 at ref 15, but was pH ) 1 at the present test. A large photoresponce of the potential at the initial electrolysis indicates that the photoelectrodeposition to form the PbO2 nuclei is more preferential than the crystal growth due to the tunneling current. The photoresponce of the potential scarcely appears after about 40 min electrolysis, where the PbO2 completely covers the TiO2 surface and the current 50 µA/cm2 is only due to the tunneling bringing about the crystal growth (eqs 1 and 2). The constant potential was about 0.95 V (vs SCE) which is more positive than the equilibrium potential of PbO2/ Pb2+ (0.76 V). Figure 3 shows XRD patterns of the β-PbO2 (A) and Fe2O3 (B) deposited at 50 µA/cm2 for 3 h on the single-crystal TiO2 (100) surface. The same crystal orientation (100) was observed for the β-PbO2 because of the same rutile structure (The lattice mismatch in the surface oxygen anion arrangement is from 7.2% ([010] direction) to 14.4% ([001] direction)). On the other hand, a (110) orientation was observed for the Fe2O3. The lattice mismatch at the (110) plane of the Fe2O3 between the (100)
Figure 4. AFM images of the β-PbO2 depoited at 50 µA/cm2 in 0.1 M Pb(NO3)2 (pH ) 1) for 30 min on the single-crystal TiO2 (100) surface. The illumination time was 30 min (A) and 10 s (B) at room temperature, and the illumination time was 30 min at 80 °C (C).
plane of the TiO2 is the smallest (from 0.07% ([010] direction of the substrate TiO2) to 3.58% ([001] direction of the TiO2)). Similar phenomenon in the crystal orientation depending on the degree of the lattice mismatch has also been observed for the PEEG of Co3O4.16 Figure 4 shows the AFM images of the β-PbO2 deposited on the (100) surface of the TiO2 single crystal in 0.1 M Pb(NO3)2 at 50 µA/cm2 for 30 min deposited under various illumination and temperature conditions. The epitaxial growth of β-PbO2 was observed under various conditions. The PbO2 almost covered the entire TiO2 surface as shown in (A, room temperature) and (C, 80 °C) in Figure 4, where the illumination was made for the total period during the electrolysis and corresponds to the case of the step D in Figure 1. On the other hand, a large islandlike crystal growth was observed for the PbO2 (B, room temperature), where short period illumination for 10 s was done during the initial electrolysis. This condition corresponds to the steps E and F in Figure 1. The crystal size increased with an increase in the electrolysis temperature C, but scarcely depended on the current density in the range from 50 to 500 µA/cm2. These results mean that the diffusion of the crystal nuclei and/ or OH radical produced on the surface (B in Figure 1) strongly depends on the temperature and that the production rate of the OH radical does not bring about the increase in the number of the PbO2 crystal nuclei. The latter phenomenon is quite difference in the usual electrodeposition process of the oxide crystal,29 where many nuclei of the deposited oxide are produced at high current density on account of the supersaturation of the intermediate. Probably the surface area bringing about the production of the OH radical will be limited on some parts of the surface of the TiO2 at the initial stage, and the area will not spread out with the increase in the rate of the produced OH radical. Morphologies similar to A and B in Figure 4 were also
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Figure 5. Current-potential curves of the single-crystal TiO2 (A) and PbO2/single-crystal TiO2 (B) in 0.5 M H2SO4.
Figure 6. Current-potential curves of the polycrystalline TiO2 (A) and PbO2/polycrystalline TiO2 (B) in 0.5 M H2SO4.
observed for the Fe2O3 as shown in Figure 1S (Supporting Information). The Fe2O3 covered the entire TiO2 surface with crystal hills in the case of the illumination during the total period in the electrolysis (A), while relatively large islandlike crystal growth was observed in the case of the short period illumination for one minute. Relatively small crystal size of the Fe2O3 (B) in Figure 1S (in Supporting Information) compared with that of PbO2 (B) in Figure 4 may be due to the difference in the conductivity. The conductivity of Fe2O3 is small, leading to the suppression of the electrodeposition itself. In conclusion, the experimental observations based on the AFM images are in fair agreement with the models of the PEEG shown in Figure 1. Therefore, we can control the crystal shape of the deposited metallic oxides by the illumination time as well as the temperature. The photoelectrochemical deposition of MnO2 was also observed on the TiO2 electrode, but it was amorphous. Its crystallization will need a higher temperature. Figure 2S (Supporting Information) shows the XRD patterns of the PbO2 photoelectrochemically deposited onto the polycrystalline TiO2 electrode in 0.1 M Pb(NO3)2 with pH ) 4 (A) and pH ) 1 (B) at 500 µA/cm2 under illumination. No deposition occurred again in dark. It was found from Figure 2S (Supporting Information) that the product mainly consisted of a polycrystalline β-PbO2 in low pH but that a large amount of polycrystalline R-PbO2 was also deposited at a relatively high pH. This result is quite similar to the case for the deposition of the PbO2 onto the metal electrode.20,30 The presence of R-PbO2 did not affect the electrochemical behavior of the TiO2/PbO2 electrode but affected the long-term stability as an anode in acidic solution as stated later. Electrochemical Behavior of the PbO2/TiO2 Electrode. Figure 5 shows the current-potential curves of the single-crystal TiO2 (A) and PbO2/single-crystal TiO2 (B) where the PbO2 was photoelectrochemically deposited at 500 µA/cm2 for 30 min. The deposited PbO2 completely covered the surface of the TiO2 according to an EPMA and SEM measurements. A well-known photocurrent-potential curve was observed for the single-crystal TiO2 (A), and its flatband potential (Efb) was about -800 mV (vs Hg/Hg2SO4) according to a Mott-Schottky plot measurement. On the other hand, no photocurrent was observed for the PbO2/TiO2 electrode (B) because the PbO2 completely interrupts the illumination of the TiO2 surface. Dark current was scarcely observed under the anodic bias, while the cathodic current due
to the PbO2 reduction was observed in the potential region more negative than about the Efb as shown in Figure 5 (B). Figure 6 shows the current-potential curves of the polycrystalline TiO2 (A) and PbO2/polycrystalline TiO2 electrode (B), where the PbO2 was deposited at 500 µA/cm2 for 30 min in order to completely cover the TiO2 surface. The PbO2 film thickness was about 10 µm. The photocurrent-potential curve (A in Figure 6) and its Efb were similar to the case of the singlecrystal TiO2 shown in Figure 5A, while the current-potential curve of the PbO2/polycrystalline TiO2 (B in Figure 6) is quite different from Figure 5B. That is, a large dark tunneling current due to the OER was observed under the anodic bias, and a cathodic current due to the reduction from PbO2 to PbSO4 was observed in the potential region more negative than the equilibrium potential of PbO2/PbSO4 (EPbO2/PbSO4) as shown in Figure 6B. The electrochemical behavior of the present PbO2/ polycrystalline TiO2 electrode is quite similar to the case of the general metal electrode. However, no dark tunneling current was observed at the MnO2 or Fe2O3 deposited polycrystalline TiO2 electrodes under anodic bias. Therefore, it is concluded that the electrochemical behavior like a metal electrode is a characteristic for the PbO2/polycrystalline TiO2 electrode. Figure 7 shows the band models in the PbO2/TiO2 interface of the above PbO2/TiO2 electrodes. In the case of the singlecrystal TiO2, surface state bringing about a large dark tunneling current due to the OER under anodic bias in the present potential region will scarcely exist at the interface A. Under a cathodic bias, a large cathodic current due to the reduction of PbO2 flows, because the Schottky barrier scarcely forms at the PbO2/TiO2 interface in the potential region more negative than about the Efb (B). On the other hand, the Fermi level pinning will occur at the PbO2/polycrystalline TiO2 interface where a surface state with a large density of states will be formed as shown in Figure 7A and B. Fermi level pinning is confirmed by the Mott-Schottky plot shown in Figure 3S (Supporting Information). The value of 1/c2 was almost independent of the potential in the region more positive than about -0.5 V for the PbO2/polycrystalline TiO2 electrode, indicating that the Schottky barrier is independent of the potential (On the other hand, the 1/c2 value was proportional to the potential for the PbO2/single-crystal TiO2 electrode, indicating that the Schottky barrier increases with the potential). This surface state will consist of Ti-Pb-O which will be formed at the crystal edges or grain boundaries or some
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Matsumoto et al. tion are necessary for the industrial evaluation of the present PbO2/polycrystalline TiO2 electrode. Acknowledgment. The present work was supported by the RFTF Program in the Japan Society for the Promotion of Science (Grant JSPS-RFTF96R06901) and a Grant-in Aid for Scientific Research (Grant 09650909) from the Ministry of Education, Science, Sports and Culture. Supporting Information Available: Supplementary explanation for model of the PEEG section. Figures showing AFM image of the deposited Fe2O3, XRD for the deposited PbO2 on polycrystalline TiO2, Mott-Schottky plot for the PbO2/TiO2 electrodes, and potential-time curve for the PbO2/polycrystalline TiO2 electrode. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 7. Band models in the PbO2/TiO2 interface under anodic bias (A) and cathodic bias (B).
defects at the polycrystalline TiO2 in the interface, because dark tunneling current was scarcely observed for the PbO2/singlecrystal TiO2 and Fe2O3, MnO2/polycrystalline TiO2 electrodes as stated already. In conclusion, a large tunneling current due to the OER flows (A), and the cathodic current flows in the potential region more negative than EPbO2/PbSO4 (B). The metallic electrode-like behavior for the PbO2/polycrystalline TiO2 electrode shown in Figure 6B means that this electrode has the possibility to act as an inert and insoluble anode in acidic solution. Therefore, the long-term stability as an anode was tested at 30 mA/cm2 in 0.5 M H2SO4 solution as shown in Figure 4S (Supporting Information). The R- and β-PbO2 mixture/TiO2 electrode which was prepared by the deposition at 500 µA/cm2 for 30 min in 0.1 M Pb(NO3)3 solution with pH ) 4 was very stable compared with the β-PbO2/TiO2 electrode which was prepared in 0.1 M Pb(NO3)2 solution with pH ) 1. The anodic stability of the PbO2/TiO2 electrode may significantly depend on the degree of the residual stress between the PbO2/TiO2 interface produced by the deposition, where the presence of R-PbO2 with a relatively small mismatch to the TiO2 rutile will decrease the stress, leading to maintaining the adhesion between the PbO2 and TiO2.21 Thus, the presence of R-PbO2 may also be important for the stability of the PbO2/ TiO2 electrode prepared by the present photoelectrochemical deposition technique. In conclusion, the present photoelectrochemical method is very useful for the preparation of the PbO2/ TiO2 anode, although some further tests under hard electrolysis conditions such as high temperature and high H2SO4 concentra-
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