A eletrodepositação galvanostática de filmes de dióxido de manganês com espessuras en tre 0 e 1000 nm foi investigada por elipsometria in situ. Os resultados obtidos podem ser ajustados em termos de uma anisotropia uni ax ial do filme para o intervalo completo de espessuras. Os índices ópticos e as espessuras foram calculados. As propriedades anisotrópicas podem ser relacionadas a uma orientação preferencial do depósito.The galvanostatic electrodeposition of man ga nese di ox ide films in the thick ness range from 0 to 1000 nm was in ves ti gated by in situ ellipsometry. The re sults ob tained can be fit into the whole thick ness range in terms of the uni ax ial ani so tropy of the film. The op ti cal in di ces and thick nesses were cal cu lated. The anisotropic prop er ties may be re lated to a pref er en tial orien ta tion of the de pos its. Key words: ellipsometry, man ga nese ox ide, ani so tropy In tro duc tionThe man ga nese ox ides are nu mer ous and many of their struc tures are poorly known 1 . Mn0 2 has been widely studied and many phys i cal and chem i cal prop er ties such as elec tri cal con duc tiv ity, po ros ity, man ga nese con tent, surface area, elec trode po ten tial, pore size, and par ti cle shape and size, have been mea sured and dis cussed in terms of the dry cell per for mance 2 . The ac tiv ity of the man ga nese ox ide lay ers de pends strongly on the de po si tion con di tions 3,4 . The op ti cal char ac ter iza tion of Mn0 2 is very valu able for the cor re la tion of in situ charge stor age ca pac ity and structural changes as a func tion of the ap plied po ten tial. However, ellipsometric stud ies are lim ited to a few ar ti cles 5,6 . Re cently, an od i cally de pos ited Mn0 2 films were in ves tigated by ellipsometry 7 . Op ti cal re sults could not be explained as the growth of iso tro pic or anisotropic lay ers in the case of thick films and the re ported data were in terpreted as sum ing a de pend ence of the ex tinc tion co ef fi cient on the thick ness 6,7 .In the pres ent pa per, we re port an in situ ellipsometric in ves ti ga tion of an odic man ga nese ox ide films grown galvanostatically. The re sults ob tained give ev i dence of the uni ax ial ani so tropy of the films, which may be re lated to a pref er en tial ori en ta tion of the de pos its.Ar ti cle
The electrodeposition of manganese oxide films onto a platinum substrate was investigated by means of in situ ellipsometry. In the thickness range from 0 to 150 nm, the anodic oxide behaves as an isotropic single layer with optical constants that are independent of thickness. Deviations at higher thickness are explained in terms of anisotropic properties of the film. The electroreduction of thin films ͑up to ca. 150 nm͒ in an alkaline electrolyte leads to a decrease in both the refractive index and the extinction coefficient and is accompanied by a thickness increase of ca. 10%. The Mn͑IV͒ to Mn͑III͒ conversion takes place from the oxide/electrolyte interface inwards. © 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1825951͔ All rights reserved. The wide spread use of MnO 2 for battery electrodes has continued over the years and has been extended today to nonaqueous lithium cells. [1][2][3][4] Manganese dioxide can exist as various polytypes denoted ␣-, -, ␥-, ramsdellite, ␦-and -forms, with the ␥-form, referred to as nsutite, being the most active electrochemically. Among the samples referred to as ␥-MnO 2 and depending on their origin, natural MnO 2 , chemical MnO 2 ͑CMD͒, and electrolytic MnO 2 ͑EMD͒ can be distinguished. The differences are associated to subtle structural changes. Electrolytic MnO 2 films have been prepared using galvanostatic and potentiostatic methods. 5,6 Amorphous hydrous manganese oxides anodically deposited from MnSO 4 solutions of different pH show an acceptable capacity, high reversibility, and high pulse-power properties for electrochemical supercapacitors. 7,8 For battery-active manganese dioxides used in primary cells ͑␥-and -MnO 2 ), 9 the electroreduction process during the first cycle in alkaline electrolytes has been extensively studied. Kozawa and coworkers observed that the reduction process depends on KOH concentration. 9,10 Amarilla et al. 11 have studied the influence of KOH concentration on the MnO 2 redox mechanism using step potential electrochemical spectroscopy ͑SPECS͒ and X-ray diffraction ͑XRDS͒. In 1 M KOH the voltamperometric and XRD data show that the redox mechanism can be described as a H ϩ /e Ϫ insertion/ desinsertion process with good reversibility. For strongly alkaline KOH solutions, a different redox mechanism is observed after the first cycle and a loss of electrochemical activity is also noticed.In general, the reduction of ␥-MnO 2 occurs in two potential regions. The first step corresponds to a homogeneous reduction while the second step is a heterogeneous reduction process. investigated the reduction of electrodeposited manganese dioxide ͑EMD͒, birnessite, and Bi-birnessite electrodes and found that each type of manganese dioxide underwent a homogeneous reduction followed by a heterogeneous reduction stage.The electrochemical process of insertion/desinsertion of H ϩ was investigated by studying its influence on the evolution of the crystallographic structure of ␥-MnO 2 in EMD samples during the discharge and recharge processes of an alkaline batt...
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