J. Phys. Chem. C 2010, 114, 18853–18859
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Galvanostatic Growth of Nanoporous Anodic Films on Iron in Ammonium Fluoride-Ethylene Glycol Electrolytes with Different Water Contents Hiroki Habazaki,*,† Yoshiki Konno,† Yoshitaka Aoki,† Peter Skeldon,‡ and George E. Thompson‡ DiVision of Materials Chemistry, Faculty of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan, and Corrosion and Protection Centre, School of Materials, The UniVersity of Manchester, M60 1QD, P.O. Box 88, Manchester M60 1QD, United Kingdom ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: October 1, 2010
The growth of porous anodic films on iron has been examined at a constant current density of 50 A m-2 in 0.1 mol L-1 NH4F-ethylene glycol electrolytes containing 0.1-1.5 mol L-1 water. Nanoporous films are formed in all the electrolytes, with the growth rate increasing with the decrease in the water content of the electrolyte. A barrier layer, in which a high electric field is applied during anodizing, thickens in proportion to the formation voltage at a ratio of 1.9 nm V-1, regardless of the water content of the electrolyte. However, there is a transition water content between 0.3 and 0.5 mol L-1, at which growth behavior changes. Above the transition level, the formation voltage is constant after an initial voltage rise, with the constant voltage slightly rising with a decrease in water content. In contrast, the formation voltage increases continuously to more than 150 V when the water contents are below the transition level. The anodic films are poorly crystalline and contain a significant amount of fluoride species. A high enrichment of fluoride species occurs near the metal/film interface when the water content in the electrolyte is below the transition level. Such enrichment is not as significant, or possibly absent, in electrolytes of increased water content. Introduction Relatively thick, porous anodic oxide films formed on aluminum and its alloys have been widely used in industry for corrosion protection, coloring, wear resistance, etc. In the last two decades, porous anodic alumina films have attracted much attention in nanoscience and nanotechnology, which make use of the self-organization process of pore arrays; successful application as templates for the preparation of various nanofibers and nanotubes was demonstrated in the 1990s.1,2 The formation of self-organized porous or nanotubular anodic oxide films has been extended to a range of valve metals, including titanium, zirconium, niobium, tantalum, and tungsten, in the past decade.3-20 Extensive investigations of such nanoporous and nanotubular anodic oxides have been carried out recently due to a wide range of potential applications and also for the scientific interest in the formation mechanisms. Recently, the formation of nanoporous and nanotubular oxide films has been reported on iron in glycerol and ethylene glycol electrolytes containing fluoride and a small amount of water.21-27 The as-formed anodic films are amorphous but are readily converted to R-Fe2O3 by annealing. Because R-Fe2O3 is a promising material for photocatalysts,28-30 gas sensors, lithiumion battery electrodes, and magnetic storage media,31,32 the formation of nanotubular R-Fe2O3 films with a high surface area by simple anodizing is of significant interest. For the fabrication of nanoporous and nanotubular anodic iron oxide films with a controlled morphology at high efficiency, understanding the growth behavior of the porous anodic films is of crucial importance. Albu et al. have developed nanoporous * To whom correspondence should be addressed. Phone/Fax: +81-11706-6575. E-mail:
[email protected]. † Hokkaido University. ‡ The University of Manchester.
anodic oxide films on iron in ethylene glycol electrolytes containing NH4F and water.22 They pointed out the necessity of 0.1-2 mol L-1 water for the growth of nanoporous oxide films in a 0.1 mol L-1 NH4F-ethylene glycol electrolyte. They also revealed the transition from a nanoporous structure to nanotubes by increasing the electrolyte temperature from 293 to 313 K. The importance of the anodizing potential and water content for the transition was also reported by Rangaraju et al.26 Conversely, LaTempa et al. have reported an increased growth rate of the nanotubular iron oxide films at raised electrolyte temperatures, with a reduced influence of anodizing voltage and water content.33 Nanoporous and nanotubular films on iron have usually been formed by potentiostatic anodizing. However, nanoporous anodic films may also be formed under galvanostatic anodizing, which is beneficial for the growth of films at a controlled rate. In this work, a focus is on the correlation between growth, composition, and morphology of nanoporous anodic films formed on iron under galvanostatic conditions in an NH4F-ethylene glycol electrolyte with a range of water contents. Experimental Details Magnetron-sputtered high-purity iron films, with a thickness of ∼300 nm, deposited on flat glass or an aluminum substrate, were used for anodizing. The target was a 99.99% pure iron disk with a diameter of 100 mm and a thickness of 0.5 mm, which was bonded on a copper backing plate. The aluminum substrates were electropolished and then anodized at a constant current density of 50 A m-2 to 200 V in a 0.1 mol L-1 ammonium pentaborate electrolyte to provide a microscopically flat surface. Anodizing was carried out at a constant current density of 50 A m-2 in a 0.1 mol L-1 ammonium fluoride-ethylene glycol
10.1021/jp1078136 2010 American Chemical Society Published on Web 10/15/2010
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Figure 1. Voltage-time responses of the sputter-deposited iron during anodizing at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and various concentrations of water at 293 K.
electrolyte containing 0.1-1.5 mol L-1 water at 293 K. The surfaces and fracture-sections of the resultant anodic films were observed using a JEOL JSM-6500F field emission scanning electron microscope, operated at an accelerating voltage of 15 kV. Further, selected specimens were examined in a JEOL JEM2000FX transmission electron microscope, operated at 200 kV. Electron transparent sections, ∼20 nm thick, were obtained using ultramicrotomy. X-ray photoelectron spectra of the anodized specimens were measured using a JEOL JPS9010MC spectroscope with Mg KR radiation (hν ) 1253.6 eV). Binding energies of the photoelectrons were calibrated using a contaminant carbon peak (285.0 eV). Further, depth profile analyses of the anodic films were undertaken by glow discharge optical emission spectroscopy (GDOES) using a Jobin-Yvon 5000 RF instrument in a neon atmosphere of 900 Pa by applying an rf of 13.56 MHz and a power of 50 W under a pulse mode of 25 Hz and a duty cycle of 0.5. Light emissions of characteristic wavelengths were monitored throughout the analysis with a sampling time of 0.01 s to obtain depth profiles. The wavelengths of the spectral lines used were 385.991, 130.217, 165.701, and 685.602 nm for iron, oxygen, carbon, and fluorine, respectively. The signals were detected from a circular area of approximately 4 mm in diameter.
Figure 2. Cross-sectional scanning electron micrographs of the sputterdeposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and (a) 1.5 mol L-1 water for 500 s, (b) 0.5 mol L-1 water for 500 s, and (c) 0.1 mol L-1 water for 170 s at 293 K.
Results Voltage-Time Responses. The voltage-time responses of the magnetron-sputtered iron during anodizing at a constant current density of 50 A m-2 (Figure 1) reveal a significant influence of water content. Specimens show a voltage surge of ∼4 V, which is mainly associated with the resistivity of the electrolyte. The formation voltage then increases with anodizing time, with the rate being enhanced by reducing the water content in the electrolyte. Following the voltage increase, a steady formation voltage is established when the water content is 0.5 mol L-1 or higher. The steady voltage also increases with a reduction of water content. In contrast, the rise of formation voltage continues to above 150 V in the electrolytes containing 0.1 and 0.3 mol L-1 water, although the rate is reduced at ∼90 and ∼130 V, respectively. During anodizing, gas evolution from the specimen surface was readily evident in the electrolyte containing 1.5 mol L-1 water increased water contents but became less significant by reducing the water content, particularly to less than 0.5 mol L-1. Electron Microscopy Observations. Although the voltagetime responses change significantly with water content of the electrolyte, porous anodic films were formed at all examined water contents. The cross-sectional scanning electron micro-
Figure 3. Cross-sectional scanning electron micrographs of the sputterdeposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and 0.1 mol L-1 water at 293 K for (a) 30 s, (b) 50 s, and (c) 170 s.
graphs of the anodized specimens (Figure 2) disclose the presence of a number of cylindrical pores, which are aligned approximately normal to the film surface. The porous layer and iron substrate are separated by the presence of a thin barrier layer, which thickens with a reduction of water content from 35 ( 2 nm (1.5 mol L-1 water) to 191 ( 3 nm (0.1 mol L-1 water). Thus, the film morphology resembles that of typical porous anodic alumina formed in aqueous acid electrolytes.34 Figure 3 shows a cross-sectional scanning electron micrograph of the anodic films formed in the electrolyte containing 0.1 mol L-1 water for different anodizing times. A barrier-type anodic film is formed predominantly during the initial 30 s of anodizing, where the formation voltage rises almost linearly with time, although a thin porous layer appears to be developed even in this initial stage (Figure 3a). After anodizing for 50 s (Figure
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Figure 4. Cross-sectional scanning electron micrographs of the sputterdeposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and 0.1 mol L-1 water at 293 K for (a) 50 s, (b) 200 s, and (c) 900 s. Figure 6. Transmission electron micrograph of ultramicrotomed sections of the sputter-deposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and (a) 0.1 mol L-1 or (b) 1.5 mol L-1 water at 293 K for 200 s. Selected area electron diffraction patterns taken from the anodic films are also shown.
Figure 5. Scanning electron micrographs of surfaces of the sputterdeposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and 1.5 mol L-1 water at 293 K for (a) 50 s, (b) 200 s, (c) 500 s, and (d) 900 s.
3b), the outer half of the film thickness is porous, with the relative thickness of the outer porous layer increasing to ∼70% after anodizing for 170 s (Figure 3c). The thickness of the barrier layer also increases with increased anodizing time, together with an increase in the formation voltage (Figure 1). Thickening of the porous layer with anodizing time also occurs in the electrolyte containing 1.5 mol L-1 water (Figure 4), but the thickness of the underlying barrier layer is almost constant, in agreement with the steady-state formation voltage. Consequently, it is evident that the water content in the electrolyte is crucial in the growth of the barrier layer. Figure 5 shows the change in the surface morphology of the specimens anodized in the electrolyte containing 1.5 mol L-1 water with anodizing time. The porous nature of the anodic film is revealed even after anodizing for only 50 s, and the pore size enlarges with the anodizing time from 8-15 nm at an anodizing time of 50 s to 10-50 nm at 900 s. It is generally known that porous anodic films grow at the metal/film interface by inward migration of anion species in the barrier layer under the high electric field, not at the film surface. Pore widening at the film surface during anodizing is, therefore, associated with chemical dissolution of film material. Pores became also wider after increased anodizing time in the electrolyte containing 0.1 mol L-1 water. The anodic film formed on iron was further examined by transmission electron microscopy for structural characterization.
Figure 6 shows examples of the specimens anodized in 0.1 and 1.5 mol L-1 water. Because of mechanical damage of the anodic film during sectioning using a diamond knife, only a barrier layer of the anodic film formed in 0.1 mol L-1 water is visible in Figure 6a. The barrier layer is amorphous, as is found from the relatively featureless appearance and a selected area electron diffraction pattern showing a diffuse ring. The anodic film formed in 1.5 mol L-1 water (Figure 6b), with the outer 80% of the thickness being porous, is attached to the iron substrate, which appears at the bottom of the micrograph. In the porous layer, no obvious diffraction contrast is displayed, suggesting that the film comprises mainly amorphous material. Beneath the porous layer, a 38 ( 4 nm thick barrier layer is present. Its thickness is in agreement with the SEM observation (Figure 4b). In contrast to the porous layer, diffraction contrast appears in the barrier layer, indicating the presence of a crystalline phase. The size of the individual crystals is ∼8 nm. The selected area diffraction pattern shown in Figure 6b reveals a diffuse ring with a faint spot, probably corresponding to the crystalline phase. Because of the limited number of diffraction spots, the crystalline phase could not be identified. X-ray Photoelectron Spectroscopy Analysis. To provide compositional information, the anodic film surface was analyzed using X-ray photoelectron spectroscopy. An example of the spectra for the anodic film formed in the electrolyte containing 0.5 mol L-1 water is revealed in Figure 7. Similar spectra were also obtained for the anodic films formed in the other electrolytes used. The peak binding energy of the Fe 2p3/2 electron appears at 711.4 eV, which is assigned to trivalent iron species.35 Significant incorporation of fluoride species into the anodic film from the electrolyte is evident from the intense peak of F 1s. The peak binding energy of 684.8 eV is in agreement with iron fluorides (FeF2 and FeF3).36 The O 1s peak consists of two distinct peaks of 530.2 and 531.8 eV. The former peak is often assigned as oxide, and the latter is hydroxide or water of hydration. The findings indicate that the outer region of the anodic film is composed of a fluoride-containing oxyhydroxide of iron(III). From the semiquantitative analysis using the integrated intensities of individual peaks, the fluoride content at the film surface is approximately one-third that of oxygen. GDOES Depth Profile Analysis. The compositions of the anodic films were further examined by GDOES elemental depth
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Figure 8. GDOES depth profiles of the anodic films formed on the sputter-deposited iron at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and (a) 0.3 mol L-1 water for 170 s and (b) 1.5 mol L-1 water for 500 s at 293 K.
the iron substrate. Such a fluoride-enriched layer is not readily evident in Figure 9c,d. Discussion Figure 7. X-ray photoelectron spectra of (a) Fe 2p, (b) F 1s, and (c) O 1s electrons for the sputter-deposited iron anodized at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and 0.5 mol L-1 water at 293 K for 500 s.
profile analysis (Figure 8). The large increase in the intensity of iron and the decrease in the intensities of oxygen and fluorine at a sputtering time of ∼60 s are associated with the metal/film interface. The presence of fluoride species is clear in the anodic films formed in the electrolytes containing 0.3 and 1.5 mol L-1 water. The carbon content is slightly higher in the anodic film formed in the electrolyte with reduced water content, suggesting enhanced incorporation of ethylene glycol derived species in the anodic film with reducing water content. For clarity in revealing the depth distribution of the three main elements in the anodic films, the relative intensity ratios of iron, oxygen, and fluorine are plotted as a function of sputtering time (Figure 9). The relative ratios of oxygen and fluorine in the anodic films are not significantly dependent upon the water content in the electrolyte, in agreement with the XPS analysis of the film surface. The increased relative oxygen intensity below the sputtering time of ∼5 s may be associated with hydration of the outer film regions. The most significant feature in Figure 9 is the delayed decrease in the fluoride intensity compared with that of oxygen near the metal/film interface for the specimens anodized in the electrolytes with reduced water contents (Figure 9a,b), suggesting the presence of a fluoride-enriched layer near
Growth Rate of the Anodic Films. The findings of the present study reveal that porous anodic films are formed in the NH4F-ethylene glycol electrolytes containing 0.1-1.5 mol L-1 water under galvanostatic conditions. Figure 10 shows the change in the thickness of anodic films with anodizing time in the electrolytes with two different water contents. In both electrolytes, the anodic films thicken linearly with anodizing time up to ∼1 µm in thickness, with the growth rate being higher in the electrolyte with 0.1 mol L-1 water compared with 1.5 mol L-1 water. The enhanced growth rate of anodic films with reducing water content is more evident in Figure 11, in which the growth rate is plotted as a function of water content in the electrolyte. The growth rate increases from 1.1 nm s-1 at 1.5 mol L-1 water to 4.2 nm s-1 at 0.1 mol L-1 water at a constant current density of 50 A m-2. The reduced growth efficiency at increased water contents is mainly associated with a gas generation reaction because gas evolution was readily observed from specimens anodized in electrolytes with increased water contents. The barrier layer in the anodic film formed in 0.1 mol L-1 water is amorphous, whereas nanocrystals are present in that formed in 1.5 mol L-1 water. Suppression of crystalline oxide formation may contribute to the improved growth efficiency by reducing the water content. There is a transition water content between 0.3 and 0.5 mol L-1. At water contents above the transition level, the dependence of the growth rate on water content is relatively small. In addition, the formation voltage during galvanostatic anodizing becomes constant after a certain time of anodizing. In contrast,
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Figure 9. GDOES depth profiles (intensity ratio of Fe, O, and F) of the anodic films formed on the sputter-deposited iron at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and (a) 0.1 mol L-1 water for 170 s, (b) 0.3 mol L-1 water for 170 s, (c) 0.5 mol L-1 for 500 s, and (d) 1.5 mol L-1 water for 500 s at 293 K.
Figure 10. Change in the film thickness with anodizing time for the anodic films formed on the sputter-deposited iron at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and 0.1 or 1.5 mol L-1 water at 293 K.
at water contents below the transition level, the growth rate is increased significantly with reducing water content (Figure 11), and the formation voltage continues to increase up to more than 150 V during anodizing (Figure 1). In separate experiments of the anodizing of a 99.99% iron sheet in the electrolyte containing 0.1 mol L-1 water, dielectric breakdown of the anodic film occurred at ∼160 V. Thus, for the steady growth of porous anodic films, the electrolyte should contain g0.5 mol L-1 water. The formation voltage is largely dependent upon the water content (Figure 1) and is associated with the change in the thickness of the barrier layer located between the porous layer and the iron substrate. Figure 12 shows the change in the thickness of the barrier layer with the formation voltage; the thickness of the barrier layer increases in proportion to the formation voltage at a ratio of 1.9 ( 0.1 nm V-1, with no
Figure 11. Growth rate of anodic films formed on the sputter-deposited iron at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and different concentrations of water at 293 K.
influence of the water content in the electrolyte. This value is nearly twice that for porous anodic alumina (∼1.0 nm V-1)34 and comparable with those for the growth of barrier-type anodic titania and zirconia films.37 Even for anodic films on iron, which is not a valve metal, such a high electric field can be sustained up to a high formation voltage of ∼150 V in the present organic electrolytes. Pore Morphology. In the selected anodizing conditions, nanoporous anodic films are developed on the sputter-deposited iron films. In ethylene glycol electrolytes containing similar concentrations of NH4F and water, nanotubular anodic films have also been grown.22-24,26,33 The key roles of electrolyte temperature and fluoride concentration on the formation of nanotubes have been reported, with the water content and formation voltage playing lesser roles.33 A transition from a nanoporous to a nanotubular morphology is found by increasing
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Figure 12. Correlation between the thickness of the barrier layer and formation voltage for the anodic films formed on the sputter-deposited iron at a constant current density of 50 A m-2 in an ethylene glycol electrolyte containing 0.1 mol L-1 NH4F and different concentrations of water at 293 K.
the electrolyte temperature from 293 to 333 K.22 In the present study, an electrolyte temperature of 293 K was employed such that only nanoporous anodic films are formed. In porous anodic alumina films formed in aqueous acid electrolytes, the cell size (interpore distance) changes with the formation voltage at a proportion ratio of 2.8 nm V-1.34 In contrast, the formation voltage dependence of the interpore distance is relatively small in the present anodic films on iron (Figure 2). The interpore distances at the formation voltages of 14 V (1.5 mol L-1 water) and 150 V (0.1 mol L-1 water) are 54 ( 3 and 120 ( 15 nm, respectively. A doubling of the interpore distance with a 1 order of magnitude increase in the formation voltage suggests that there is no proportionality between the cell size and formation voltage for the present anodic films on iron. One of the possible reasons is that steady film growth has not been achieved for the films formed in the electrolytes containing 0.1 and 0.3 mol L-1 water, for which the formation voltages continuously increased. Distribution of Fluoride Species. The anodic films formed on iron in the present electrolytes contain a relatively high concentration of fluoride species, as confirmed by XPS and GDOES analyses. The fluoride contents in the bulk of anodic films are not strongly dependent upon the water content in the electrolyte, but GDOES depth profiles disclosed enrichment of fluoride species in the anodic films, near the metal/film interface, when the films are formed at voltages of more than 100 V in the electrolytes containing 0.1 and 0.3 mol L-1 water. In contrast, fluoride-enrichment is less significant or not evident at the available resolution of GDOES depth profile analysis for the films formed at low formation voltages (
