Anodically Prepared Films

Films of oxides may also be prepared by sputtering. The most intensively studied material is irO because of its possibie application as an electrochromic materiai (81-83). Fiims prepared by this method seem to be more similar to films prepared thermally, than to those prepared by anodic oxidation (see next section). Aurian-Biajeni et al. (81) have interpreted the two time constants in the impedance spectrum as caused by a two-iayer structure in the oxide. 

Many transition and nobie metai oxides can be anodicaiiy oxidized in aqueous eiectroiyte soiutions to form oxide coatings, in some cases. 

In some cases, the behavior of the anodic films is significantly different from the thermally prepared materials. Hepel et al. (88) found that the energy of interaction of protons with IrOj was about the same for sputtered films and for single crystals, but differed significantly from that associated with anodic films. 

Confirmation of cationic (H3O+) involvement in the charging process for anodic ir02 was obtained by McIntyre et al. (90) who examined the composition of oxidized and reduced films using RBS and nuclear reaction analysis of deuterated specimens. The average D lr ratio was found to be 1.51 in oxidized and 2.28 in bleached films. The composition of the films was also found to be nonuniform. The interior of the films becomes 

Using chemical analysis, Pickup and Birss (91) found evidence of non proton cation involvement in the redox processes in aqueous solutions. 

---

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

Fig. 10.1.5 SEM photographs of the carbon tubes prepared by carbon deposition of propylene (a, b) carbon deposition period of I h on the anodic oxide film with 30-nm channels (c, d) a period of 12 h on the film with 230-nm channels. (From Ref. 12.)...

By the template technique using anodic oxide films and pyrolytic carbon deposition, one can prepare monodisperse carbon tubes. Since the length and the inner diameter of the channels in an anodic oxide film can easily be controlled by changing the anodic oxidation period and the current density during the oxidation, respectively, it is possible to control the length and the diameter of the carbon tubes. Furthermore, by changing the carbon deposition period, the wall thickness of the carbon tubes is controllable. This template method makes it possible to produce only carbon tubes that are not capped at both ends. Various features of the template method are summarized in Table 10.1.1 in comparison with the conventional arc-discharge method. 

G. Mende, F. Fenske, H. Flietner, M. Jeske, and J. W. Schultze, Localized anodic oxide films on Si Preparation and properties, Electrochim. Acta 39(8/9), 1259, 1994. 

N. Nourbakhsh, Anodic Alumina Films Preparation, Characterization and Investigation of Reaction and Transport Properties. Ph.D. Thesis, USC, May 1990. 

Fig. 3. SEM micrographs of anodic alumina films prepared in (a) basic electrol3fte mixture containing KH2PO4, NH4(H2P04) and Na2C03 (b) electrolyte (a) + Ni(CH3COO)2 (c) electrolyte (a) + (NH4)2Cr04 (d) electrolyte (a) + (NH4)sMo7024.

Porous anodic alumina films prepared in oxalic and tartaric acids were demonstrated to reveal properties of birefringent or photonic band gap media depending on the pore size and distance. 

Amorphous silicon demonstrates different features than those of crystalline silicon when subjected to lithiation at room temperature. 8tudies on an amorphous silicon thin-film anode prepared by magnetron sputtering and an amorphous silicon anode obtained through chemical delithiation of Lii2Si7 phase show two distinctive. 

These examples show that EXAFS in reflection mode may provide good data on the in situ structure of even highly disordered or even amorphous anodic oxide films. It yields data for the whole film and not only of the surface and is applicable also for rough films. Therefore, it is a good alternative and complimentary to scanning probe methods like STM and AFM. However, the experimental effort with respect to equipment (synchrotron radiation source) and specimen preparation is relatively large. 

H. Xia, S. Tang and L. Lu, Properties of amorphous Si thin film anodes prepared by pulsed laser deposition. Mater. Res. Bull. 42,2007,1301-1309. 

Xian Y, Liu M, Cai Q, li H, Lu J, Jin L (2001) Preparation of microporons aluminium anodic oxide film modified Pt nano array electrode and application in direct measurement of nitric oxide release fiom myocardial cells. Analyst 126 871—876... 

---