ELLIPSOMETRY OF NICKEL-OXIDES AND -HYDROXIDES IN ALKALINE ELECTROLYTE

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ELLIPSOMETRY OF NICKEL-OXIDES AND -HYDROXIDES IN ALKALINE ELECTROLYTE

W. Visscher

To cite this version:

W. Visscher. ELLIPSOMETRY OF NICKEL-OXIDES AND -HYDROXIDES IN ALKA- LINE ELECTROLYTE. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-213-C10-216.

�10.1051/jphyscol:19831044�. �jpa-00223501�

ELLIPSOMETRY OF NICKEL-OXIDES AND "HYDROXIDES IN ALKALINE ELECTROLYTE

W. V i s s c h e r

Laboratory for Electrochemistry, Department of Chemical Technology, Eindhoven University of Technology, P.O. Box SIS, S600 MB Eindhoven, The Netherlands

Résumé

La croissance et l'oxydation de couches minces de Ni (OH)2 déposées sur Ni ont été étudiées par ellipsométrie. Les indices de réfraction de a-Ni (0H>2, g-Ni (0H)2 et y.-NiOOH ont été obtenus à la longueur d'onde 546,1 nm. On a également étudié l'oxydation anodique de Ni dans 0,1 M KOH. La couche de passivation est initiale- ment Ni0.xH2Û. Un cyclage répété des potentiels anodique et cathodique modifie les propriétés optiques de cette couche et conduit à la formation d'une couche d'oxyde de faible densité.

Abstract

The growth and oxidation of thin Ni(OH)2 films deposited on Ni were investigated by ellipsometry. The refractive indices of a-Ni(OH)2, g-Ni(0H)2 and y2-NiOOH were obtained at the wavelength 546.1 nm. Furthermore, the anodic oxidation of Ni in 0.1 M KOH was studied. The passive oxide layer is initially NiO.x H20. Repeated anodic and cathodic potential cycling change the optical properties of this layer and leads to the growth of a low density oxide layer.

I. INVESTIGATION OF THIN Ni(OH)2-ELECTRODES

a-Ni(0H)2 was formed by cathodic deposition [1] from 0.1 M Ni(N03)2 on a smooth Ni substrate (0.5 c m2) . The film growth was monitored ellipsometrically at X = 546.1 nm with a Rudolph Automatic Ellipsometer Model RR 2200. Experimental details are given in [2]. Fig. 1 shows the change of A and ij) with time during deposition at I = 25 ;iA. In a i>-h plot (Fig. 2) these data trace along an egg- shaped curve indicating the formation of a transparent layer. These results are compared with calculated IJJ-A plots (Fig. 2) for the growth of homogeneous trans- parent films with refractive index n = 1.41 and 1.42.

Optical measurements of 6-Ni(OH)2~films were carried out at electrodes at which first an a-film was deposited of known thickness, usually 50-100 nm; the electrode was then converted to g-Ni(0H)2 by hot alkaline treatment [l] outside the optical cell. The A and 1)1 parameters of the g-Ni(OH)2 electrode were measured in the Ni(N03)2-electrolyte. The refractive indices for a- and g-Ni(0H)2 are found to be:

For a-Ni(0H)2 a value of 1.52 - 0 i (X = 632.8 nm) has been reported [3], Density values of 3 Ni(OH)2.2 H20 (= a-Ni(OH)2) and g-Ni(0H)2 differ considerably: 2.6-2.8 g.cm-3 for a-Ni(0H)2; 3.9 g.cm-3 for g-Ni(OH)2 []]; so it is concluded that the thin g-films as deposited here, have a higher H20-content than bulk g-Ni(OH)2. The a-Ni(OH)2 to g-Ni(0H)2 conversion is evident from the difference in the cyclic voltammograms in KOH-electrolyte:

At scan rate 0.5 mV.s- 1 the oxidation peak for a-Ni(0H)2 is at E = 1.36 V vs. RHE and for g-Ni(0H)2 at E = 1.45 V vs. RHE.

Ellipsometrically the oxidation of a- and g-Ni(0H)2 was followed in KOH electrolyte during an applied potential program. The results are shown in Fig. 3 and 4; the changes in A and f are given with respect to their values at E = 1.0 V vs. RHE.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831044

JOURNAL DE PHYSIQUE

Fig. 1

-

-

t ime/min ^min

Fig. 3 Fig. 4

Change in A and $ with respect to the values at E = 1.0 V during oxidation and reduction in KOH-electrolyte.

Fig. 3: a-Ni(0H)z potential range 1.0 to 1.4 V vs. RHE Fig. 4: B-Ni(OH)2 potential range 1.0 to 1.46 V vs. RHE

A characteristic feature of the oxidation of a-Ni(OH)? to y2-NiOOH is that A and $ quickly reach constant values and when the reduction cycle is applied, the original A , $ values of WN~(OH)~ are again obtained. The oxidation of @-Ni(OH)2 is a much slower process; moreover, after reduction A and $ differ from the values before the oxidation started. Evaluation of the refractive index of y2-NiOOH yields n-ki =

assuming no change in thickness).

The oxidation of B-Ni(OH)2 leads to B-NiOOH and then to higher valent oxides. The change in A and $ after cycling indicates change in the thickness of B-Ni(OH)2 or change in the nature of this layer.

11. INVESTIGATION OF ANODICALLY FORMED Ni-OXIDES

When Ni is anodically oxidized in 0.1 M ROH a passive layer is formed. Fig. 5 gives the changes of A and $ during stepwise increase of the potential for a freshly polished, reduced Ni electrode. In the potential range up to ca. I V the oxide-layer grows linearly with potential reaching a few monolayers thickness; its refractive index is n-ki = 2.45

-

A l I

I ^W

With further increase of the potential $ begins to decrease due to conversion in Ni-111-oxide.

Ni0.x H20 is the initially formed Ni-11-oxide layer. It is changed into a struc- turally different oxide by repeated potential cycling from -0.8 to +1.2 V vs. KHE.

At a freshly polished electrode A and $, measured at E = -0.1 V vs. RHE, give the refractive index of bare Ni; the values of A and $ at E = -0.1 V after a complete oxidation-reduction cycle have changed. The difference with respect to the original bare substrate values increases with each cycle and can be interpreted as the growth of an oxide film with n-ki = 1.52 - 0 i; the thickness increase per cycle is ca. 1 nm. This oxide has a much lower density and may be a hydroxide-type layer.

The process can be explained by the following scheme in which the growth proceeds via oxidation and (irreversible) conversion into a non-reducible oxide:

oxidation

Ni j Ni1NiO.x H20 oxidation and 1

reduct ion

&

Ni/Ni(OH)2.y H20 oxidation,Ni/NiO.x ~~0:) Ni(OH)2.y H20 oxidation and

1

reduction

I

Ni/Ni(OH)2.y H20

REFERENCES

JOURNAL

DE

1. Bode H., Dehmelt K. and Witte J., Electrochim. Acta

11

Z. Anorg. Allg. Chemie

366

2. Visscher W. and Barendrecht E., J. Electroanal. Chem., to be published.

3. Hopper M.A. and Ord J.L., J. El$ctrochem. Soc.

120

4. Visscher W. and Barendrecht E.,Surface Sci., to be published.