Effects of reaction parameters on the electrochemical formation of multilayer films composed of manganese oxides and tet
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Multilayered manganese oxide films were prepared on a platinum electrode by potentiostatic oxidation of aqueous Mn2+ ions in the presence of n-tetra-alkylammonium compounds. Alkylammonium cations were intercalated between manganese oxide layers to balance the negative layer charge. Effects of several preparative parameters such as the size of alkylammonium molecules, counteranions, and bath composition on the structure of products were investigated. The interlayer distance of the products increased with increasing alkyl chain length up to C4, and the change became obviously small among C4–C6 compounds. The multilayer formation was achieved only when the manganese concentration was lower than 10 mM, and the highest crystallinity was obtained from a bath composed of 2 mM manganese sulfate and 50 mM alkylammonium chloride. At low concentrations of alkylammonium ( PTS− > ClO4− > BF4−, is not consistent with the order of ionic seizes, Cl− (0.172 nm in radius) < BF4− (0.232 nm) < ClO4− (0.240 nm) < PTS−,21,22 it is certain that the smallest anion yields the largest pattern. An increase in the diffraction intensity can be ascribed to an increase in the layer ordering. Anions in solution form ion-pairs with alkylammonium cations until just before the formation of Mn oxide, and it is reasonable to assume that self-assembling of the alkylammonium cations to the negatively charged Mn oxide is interrupted by bulkier anions. PTS−, which exhibits the second largest pattern, might facilitate the assembly of the alkyl cations with a specific affinity due to its organic moiety.
J. Mater. Res., Vol. 21, No. 12, Dec 2006 Downloaded: 11 Mar 2015
IP address: 138.251.14.35
M. Nakayama et al.: Effects of reaction parameters on electrochemical formation of films of Mn oxides and tetra-alkylammonium ions
FIG. 3. XRD patterns of the electrodeposited films prepared from 2 mM MnSO4 solutions containing 50 mM Et4N+ salts of various anions by applying a constant potential of +1.0 V. The electric charge passed during electrolysis was always 810 mC cm−2.
C. Effect of bath composition
Effects of bath composition on the structure of manganese oxide films were investigated with the use of Et4N+ chloride. Figure 4 illustrates XRD patterns of the films obtained with different MnSO4 concentrations in the range from 2 to 10 mM while maintaining the Et4NCl concentration at 50 mM. The pattern (䡲) due to the Et4N+-intercalated phase shows a maximum intensity at [MnSO4] ⳱ 2 mM, which decreases gradually with an increase in manganese concentration and then disappears completely at 10 mM. On the other hand, the product deposited at [MnSO4] 艋 1 mM was in powder form and adhered poorly to the substrate. Increasing the Mn2+ concentration causes an increase in the formation rate of manganese oxide, meaning that the reaction conditions become comparable to those of the usual electrodeposition of manganese dioxide, as formulated by Mn2+ + 2H2O → MnO2 + 4H+ + 2e−
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(1)
On the other hand, at manganese concentrations of 3–5 mM the pattern (●) with d001 ⳱ 0.7
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