Formation of a thin MnIII oxide film on noble metal electrodes from amanganese solution

M. Nakayama *, C. Matsushima, K. Ogura

Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan

Received 25 March 2002; received in revised form 12 September 2002; accepted 14 September 2002

Abstract

MnII�/glycine complex in slightly alkaline solution is oxidized at a less positive potential (�/0.25 to �/0.6 V vs. Ag j AgCl) where

free Mn2� ion is not oxidizable, leading to the formation of a thin uniform film containing MnIII oxide. The film prepared by

continuous cyclic scans in the potential region between �/0.2 and �/0.6 V exhibited an electron spin resonance (ESR) peak attributed

to MnII(H2O)6, but this resonance peak disappeared when the lowest potential was shifted to 0 V. From the electrochemical quartz

crystal microbalance (EQCM) measurements, the film deposited anodically can be represented as MnIII2 O6[MnII(H2O)6]3 which

corresponds to a structure where MnII in Mn5O6 (MnIII2 MnII

3 O6) is not bound to the oxygen atom in the MnIII�/O network but is

coordinated with water molecules. The MnII(H2O)6 in the film was suggested to be released during the reduction of the oxide matrix

and oxidized to form the MnIII�/O bond at more positive potentials than �/0.6 V.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: ESR; EQCM; Manganese oxide; Glycine

1. Introduction

Metal oxide films are prepared by chemical vapor

deposition, radiofrequency sputtering, sol�/gel and spin

coating, and recently by electrochemical deposition.

Among these methods, the electrodeposition technique

has been paid much attention since very thin and

uniform oxide films can be obtained with a high

reproducibility even on complicated substrates. Films

grown electrochemically in aqueous media show a high

degree of hydration, offering sufficient charge transfer

kinetics owing to ease of ion transport. Manganese

oxides are good candidates as materials for positive or

negative electrodes of primary and secondary batteries.

Although the redox process of manganese oxides,

especially MnO2, has been extensively studied by means

of various voltammetric and spectroscopic techniques

[1�/3], only a few studies were made for elucidating the

formation process. Recently, Messaoudi et al. have

investigated manganese oxide films formed by the

anodic polarization of the Mn electrode in NaOH [4].

They revealed from in situ Raman spectroscopy that the

electrode surface is covered by Mn3O4, Mn2O3 and

MnO2 as the potential is shifted towards more positive

values. Chigane and Ishikawa have prepared thin MnOx

films by electrolysis of the manganese�/ammine complex

at various potentials, and the structure of the films was

examined using X-ray diffraction (XRD) and X-ray

photoelectron spectroscopy (XPS) [5]. Their XRD

results indicated that the electrolysis at potentials lower

than �/0.3 V versus Ag j AgCl gives films mainly

composed of g-Mn2O3 and/or Mn3O4 (hausmannite)

and that at higher potentials produces Mn7O13 �/5H2O.

We have previously reported that metal�/amino acid

complexes can be oxidized at less positive potentials

compared to uncomplexed ions, resulting in the deposi-

tion of CuO [6] and Co3O4 [7] films. In this case, the

adsorption of complexing ligands onto the electrode

surface was suggested to lower the overpotential for the

oxidation of metal ions. In the present study, anodic

oxidation of Mn2� was carried out similarly in the

presence of glycine, and thin films containing MnIII

oxide generated at positive potentials less than �/0.6 V

* Corresponding author. Tel.: �/81-836-85-9223; fax: �/81-836-85-

9201

E-mail address: nkymm@po.cc.yamaguchi-u.ac.jp (M. Nakayama).

Journal of Electroanalytical Chemistry 536 (2002) 47�/53

www.elsevier.com/locate/jelechem

0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 1 9 1 - 9

were characterized by means of electrochemical quartz

crystal microbalance (EQCM), electron spin resonance

(ESR) and Fourier-transform infrared (FTIR) techni-

ques. ESR is able to detect MnII and MnIV, whereasMnIII is not detected due to the large splitting of energy

levels [8]. Also, ESR is effective only to paramagneti-

cally isolated species. For instance, MnII oxide (MnO)

provides no distinct ESR signal because of a strong

magnetic interaction of the MnII sites [9], but MnII

dispersed in a matrix shows well-resolved peaks at

ordinary temperature [10]. Hence, the use of ESR

spectroscopy will be of great help in studying the natureof manganese species in the deposited films.

2. Experimental

The electrolytic solutions were prepared by dissolving

an appropriate amount of manganese(II) sulfate and

glycine in distilled water. The pH of the solutions was

adjusted by adding concd. NaOH. All chemicals usedwere of reagent grade, and purchased from the Wako

Chemical Company.

All electrochemical experiments were carried out in a

three-electrode system. The counter and reference elec-

trodes were a Pt plate and an Ag j AgCl j sat. KCl

electrode, respectively. The working electrode (WE) in

voltammetric and FTIR measurements was a Pt plate (1

cm2). EQCM experiments were carried out with a 6MHz AT-cut quartz crystal, which was supplied with a

thin film of gold deposited on both sides. A one-sided

Au-plated crystal was exposed to the electrolyte and

served as the WE. The active electrochemical area of this

electrode was 1.55 cm2. The oscillation circuit was

controlled with an automatic polarization system (Ho-

kuto Denko, HZ-3000) and an EQCM controller

(Hokuto Denko, HQ-101B). The current passingthrough the EQCM WE and the frequency of oscillation

of the quartz disk were measured simultaneously. The

frequency shift observed is linearly related to the added

mass per unit surface area according to the Sauerbrey

equation [11].

Sample films for ESR measurements were deposited

on a Pt rod (1 mm f�/30 mm). The electrode obtained

was ultrasonicated in a water bath for at least 10 minand rinsed with copious amounts of water to remove

physically adsorbed species. Following this treatment,

the electrode was placed in a quartz tube (3 mm inner

diameter) connected with a vacuum line, evacuated to be

dried, and then exposed to water vapor at 259/1 8Covernight without contact to air, unless otherwise noted.

ESR spectra were recorded at room temperature with a

JEOL JES-FE1X spectrometer working at X-bandfrequency region.

Samples for FTIR spectroscopy were prepared by the

following procedure. A thick film deposited on a Pt

plate was peeled off and dried under vacuum. Two

milligram of the sample was mixed with 40 mg of KBr,

and this mixture was used to prepare a compressed

pellet. The data were recorded on a Shimadzu DR-8000FTIR spectrometer.

3. Results and discussion

3.1. Anodic behavior of Mn2� in aqueous glycine solution

Fig. 1a shows the voltammogram of a Pt electrode ina 2 mM MnSO4�/20 mM glycine solution of pH 8. An

anodic current starts to appear at �/0.25 V and increases

again from �/0.6 V. The former current cannot be

observed in a similar solution of pH 5.7 (Fig. 1b) and in

a MnSO4 solution of pH 8 (Fig. 1c). On the other hand,

the latter current is seen in all the solutions containing

Mn2�, and is attributed to the oxidation of free Mn2�

ion.The voltammetric measurements were made similarly

in MnSO4 solutions of various pHs in the presence of 20

mM glycine. In Fig. 2a, the maximum current density at

the former wave (denoted as ja1, see Fig. 1a) was plotted

as a function of the solution pH. The pH dependence of

ja1 up to pH 9 seems similar to that of the amount of

glycine in the anionic form (pKa2�/9.60) (Fig. 2b). Since

the anionic glycine forms a complex with manganeseion, the amount of MnII�/glycine complex should

increase with increasing solution pH. An abrupt de-

crease appearing above pH 9 is probably due to the loss

of soluble Mn2� as a result of the precipitation of

manganese hydroxide. On the other hand, Fig. 2c

displays the variation of ja1 with respect to the glycine

concentration in the 2 mM MnSO4 solution of pH 8. It

is found that the current density increases roughly

Fig. 1. Voltammograms of a Pt electrode obtained in 2 mM MnSO4

solutions with (a, b) and without (c) 20 mM glycine. pHs of the

solutions were 8.0 (a, c) and 5.7 (b). Scan rate, 10 mV s�1.

M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47�/5348

linearly with the glycine concentration. This also in-dicates that the increase of MnII�/glycine complex causes

an increase in the anodic current because a certain

portion of the added glycine always dissociates at pH 8.

Thus, the anodic current from �/0.25 V can be

attributed to the oxidation of MnII�/glycine complex.

3.2. Characterization of deposited films

3.2.1. EQCM studies

EQCM measurements were conducted in a 2 mM

MnSO4�/20 mM glycine solution of pH 8 duringcontinuous potential cycling between 0 and �/0.6 V.

The potential scan rate was 10 mV s�1. Fig. 3 shows

current�/potential (a) and mass change�/potential (b)

curves recorded for initial five cycles. At the first scan,

the anodic current from �/0.25 V is found to accompany

an increase in the electrode mass. This suggests that

MnII�/glycine complex is oxidized to form a deposited

layer containing MnIII on the electrode. The mass

increase continues at the reverse scan to attain a

constant value around �/0.3 V. Toward more negative

potential, a reduction current starts to appear at �/0.23

V together with a decrease in the electrode mass, which

corresponds to a transport of matter from the anodically

deposited species. The mass change during each poten-

tial cycle does not return to the original value, resulting

in an increase in the total mass of the electrode with each

scan. After cycling the potential 90 times, a brown film

characteristic of manganese oxide appeared, and this

film gave a FTIR spectrum demonstrating the forma-

tion of the Mn�/O bond and the absence of glycine or

SO2�4 ; as indicated later. It is therefore suggested from

the above observations that the deposited film is

composed of MnIII oxide. Furthermore, the current

response is noted to become larger as the number of

cycles is increased while the mass change associated with

each cycle gradually decreases both for the positive and

negative going scans. This tendency continued until a

slight mass change was observed constantly, meaning

that the mass transport related to the redox process of

the grown film is much smaller than that taking place

during the early stage of the film deposition.

To determine the species involved in the first anodic

and subsequent cathodic processes, the apparent molar

mass, i.e. the mass change per mole of electrons, was

Fig. 2. (a, c) ja1 in the voltammograms of a Pt electrode plotted vs. the

solution pH (a) and the concentration of glycine (c). The voltammo-

grams were obtained in 2 mM MnSO4 solutions of various pHs with 20

mM glycine (a) and of pH 8 with various concentrations of glycine (c).

Scan rate, 10 mV s�1. (b) Distribution of glycine species as a function

of pH in the glycine�/water system.

Fig. 3. (a) CV and (b) mass change�/potential curves obtained with an

Au electrode in a 2 mM MnSO4�/20 mM glycine solution of pH 8.

Scan rate, 10 mV s�1.

M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47�/53 49

estimated. The mass change (Dm ) and the electric charge

passed (DQ ) at an interval of 20 mV (2 s) calculated

from the data shown in Fig. 3 were used to estimate the

molar mass according to the following equation.

molar mass�FDm=DQ (1)

where F stands for the Faraday constant (96 500 C

mol�1). In Fig. 4, the molar mass data thus obtained for

the anodic (a) and cathodic (b) processes are shown as a

function of the electrode potential. The anodic process(Fig. 4a) yields an almost constant value of about 315 g

per mol of electrons in the region between 0.33 and 0.55

V. This value is considerably larger compared to those

expected for the formation of pure MnIII-containing

oxides such as Mn2O3, Mn3O4 and Mn5O6 from MnII

(79, 114 and 185 g per mol of electrons, respectively).

For the cathodic process (Fig. 4b), two constant values

of about �/280 (0.08 VB/E ) and �/170 g per mol ofelectrons (E B/0.08 V) can be seen, suggesting that

different reactions take place depending on the poten-

tial. Such large decreases in the electrode mass cannot be

ascribed to the ion transport for charge compensation

during the reduction of the grown film, as already

described, and it is probably due to the dissolution of a

certain portion of the film deposited anodically, because

the mass change detected by EQCM involves electricallynon-charged species. The assignments of the species

involved in both anodic and cathodic processes will be

dicussed later.

3.2.2. ESR studies

Fig. 5 shows the ESR spectra of the manganese oxide

films which were obtained by cycling the potential 90

times between �/0.2 (a) or 0 (b) and �/0.6 V in a 2 mM

MnSO4�/20 mM glycine solution of pH 8. The spectrum

of the film obtained in the former potential region

presents the resonance peak with a hyperfine structure,whereas this feature is not seen in the latter film. The

observed sextet signal with a g factor of 2.00 and a

hyperfine coupling parameter, A , of 95 Gauss can be

identified as the Ms�/�/1/2l/�/1/2 transition of MnII,

and these values are almost the same as those reported

for MnII(H2O)6 (g�/2.009/0.002, A�/ 96.5 Gauss) [12].

It is evident that this signal is not associated with the

MnII sites in the reduced manganese oxide because itclearly appears when the potential scan was stopped

before the reduction of the film. Also, it is known that

manganese oxides show no hyperfine structure owing to

a strong magnetic interaction of the Mn sites [9]. Hence,

the observed signal can be attributed to MnII(H2O)6

incorporated in the MnIII oxide matrix. The MnII(H2O)6

species is considered to interact electrically with the

negative charge of unshared oxygen in the MnIII oxide.On the other hand, the disappearance of the signal in the

negative potential region suggests that MnII(H2O)6 is

released from the film during the reduction of MnIII

oxide. This view coincides with the large decrease in the

electrode mass during the cathodic process (Fig. 3b and

Fig. 4b). As also described later, these observations can

be recognized by considering that the reduction of MnIII

Fig. 4. Molar mass plotted vs. the electrode potential for the first

anodic (a) and subsequent cathodic processes (b) from the data shown

in Fig. 3.

Fig. 5. ESR spectra of the manganese oxide films deposited on a Pt

electrode by cycling the potential 90 times between �/0.2 (a) or 0 (b)

and �/0.6 V in a 2 mM MnSO4�/20 mM glycine solution of pH 8. Scan

rate, 10 mV s�1.

M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47�/5350

oxide leads to the formation of MnII(OH)2 by incorpor-

ating protons, and the MnII(H2O)6 held by anionic

oxygen of the oxide matrix is expelled from the film.

Fig. 6 shows the ESR spectra of the films prepared by

potentiostatic electrolysis at �/0.4 (a), �/0.8 (b) and �/

1.2 (c) V in the same solution, where the electric charge

for the film preparation was fixed. The film prepared at

�/0.4 V provides signal due to MnII(H2O)6 similar to

that described above. The signal intensity decreases

markedly in intensity at �/0.8 V and diminishes almost

completely at �/1.2 V, suggesting that the MnII(H2O)6

species in the film is oxidized at such positive potentials.

In Fig. 7, ESR spectra of the films in various

environments are shown, where the sample films depos-

ited at a constant potential of �/0.4 V were evacuated

once to dry them, (a) and then exposed to methanol (b)

or iso -propanol (c) vapor at 259/1 8C overnight. In the

case of the film treated with methanol vapor, the signal

due to MnII(H2O)6 is almost the same as that of the

hydrated sample (Fig. 6a), confirming that the coordi-

nated water molecules remain in the film after evacua-

tion. However, no absorption is seen for the films dried

(a) and treated with iso -propanol (c). A similar phe-

nomenon has been reported by Brouet et al. for MnII-

impregnated aluminophosphate molecular sieves

(MnIIAlPO) [13], in which the ESR signal due to MnII

in the hydrated MnIIAlPO became smaller when the

sample was evacuated. The authors explained this

change by an increase of the spin�/spin interaction of

the MnII sites as a consequence of which MnII ions were

closer to each other in the dry state. Furthermore, as

small molecules such as methanol, which can enter the

AlPO channels, were adsorbed, the spectrum showed the

same signal as the hydrated sample, while molecules

larger than the channel entrance, such as o-xylene,

yielded a spectrum similar to that of an evacuated

sample. Hence, the spectral change observed here may

reflect the effect of the adsorbed molecules on the

magnetic interaction between the incorporated

MnII(H2O)6 and the MnIII oxide matrix. That is, only

water and methanol can permeate through the film and

reduce their magnetic interaction, which implies that the

MnIII oxide matrix and the incorporated MnII(H2O)6

form a quite dense film.

Fig. 8 shows the FTIR spectra of the electrodeposited

films in the region between 2000 and 450 cm�1, where

the film deposition was carried out under potentiostatic

conditions in a 2 mM MnSO4�/20 mM glycine solution

of pH 8. No absorption related to glycine is noticed for

all the compounds examined. In the spectrum of the film

prepared at �/0.4 V (Fig. 8a), a broad band appears in

the region below 750 cm�1. This absorption can be

assigned to the metal�/oxygen (MO) stretching mode

[14,15], which is for the formation of the Mn�/O bond.

The small absorption around 1080 cm�1 is attributable

to the bending vibration of H�/O� � �MO [14,16], suggest-

ing the presence of surface hydroxide in the film. The

peak observed at 1630 cm�1 is due to structural water.

The film prepared at �/1.2 V (Fig. 8b) provides a larger

Mn�/O absorption than that of the film at �/0.4 V. Fig.

8c shows the spectrum of the film which was first

deposited at �/0.4 V and cycled between �/0.2 and �/0.8

V in a Na-borate solution. In this spectrum, the intensity

of the Mn�/O band relative to the absorptions at 1630

and 1080 cm�1 is larger than that of the as-deposited

film (Fig. 8a). This is probably because the MnII(H2O)6

Fig. 6. ESR spectra of the manganese oxide films on a Pt electrode

prepared potentiostatically at �/0.4 (a), �/0.8 (b) and �/1.2 (c) V by

applying a constant electric charge of 75.6 mC cm�2 in a 2 mM

MnSO4�/20 mM glycine solution of pH 8.

Fig. 7. ESR spectra of the manganese oxide films which were

evacuated (a) and then exposed to methanol (b) and iso -propanol (c)

at vapor pressure at 259/1 8C overnight. The oxide film was prepared

at �/0.4 V on a Pt electrode by applying an electric charge of 75.6 mC

cm�2 in a 2 mM MnSO4�/20 mM glycine solution of pH 8.

M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47�/53 51

incorporated in the film prepared at �/0.4 V was

oxidized to form the MnIII�/O bond at the positive

potential.

3.3. Mechanism of the film deposition

From the above results, it is revealed that the

oxidation of MnII�/glycine complex at a positive poten-

tial less than �/0.6 V yields a thin film consisting ofMnIII oxide and MnII(H2O)6. Taking into account the

film being electrically neutral, the deposited substance

can be represented in the simplified formula of

MnIII2 O2x[MnII(H2O)6]2x�3 and is considered to be

formed according to the following reaction from

MnII�/glycine (Gly�) complex.

(2x�1)MnII�(Gly�)n�(14x�18)H2O

0 MnIII2 O2x[MnII(H2O)6]2x�3�4xH�

�(2x�1)nGly��2e� (2)

when x is 3 (/MnIII2 O6[MnII(H2O)6]3; MW, 694), the

molar mass expected for this reaction is estimated to

be 347 g per mol of electrons, and becomes close to the

value observed in Fig. 4a (315 g per mol of electrons).

This formula corresponds to a structure where MnII inMn5O6 (MnIII

2 MnII3 O6) is not covalently bound to

oxygen atoms in the MnIII�/O network but coordinated

with water molecules.

MnII(H2O)6 is released from the film at a negative

potential where the MnIII oxide is reduced. As described

above, the release of such a cationic species during the

reduction of the film cannot be explained by iontransport relating to the redox reaction of the film.

Hence, it is reasonable to consider that MnIII oxide is

reduced to MnII hydroxide by the injection of proton

and electron in a way similar to usual manganese oxides

[1], and this causes a release of MnII(H2O)6 and H2O.

MnIII2 O6[MnII(H2O)6]3�8H��2e�

0 2MnII(OH)2�2H2O�3MnII(H2O)6 (3)

Based on this equation, the mass decrease per mole of

electrons is calculated to be 262 g, being similar to the

observed molar mass (�/280 g per mol of electrons) at

less negative potentials than 0.08 V in the cathodic

process (Fig. 4b). The decrease in apparent molar mass

at potentials more negative than 0.08 V is probablyrelated to the involvement of a small amount of

MnII(H2O)6 in the film. That is, it is suggested that

reaction (3) takes place in the outer part of the deposited

film, while the inner part is not completely transformed

to MnII(OH)2, because the diffusion of H�, H2O or

MnII(H2O)6 should be more difficult. The apparent

molar mass (�/170 g per mol of electrons) at potentials

more negative than 0.08 V is close to MnII(H2O)6

(M.W., 162) per mole of electrons, and thus the

following reduction scheme can be given.

MnIII2 O6[MnII(H2O)6]3�4H��2e�

0 2MnIIO(OH) �MnII(H2O)6�2MnII(H2O)6 (4)

4. Conclusions

MnII�/glycine complex was oxidized at a less positive

potential than �/0.6 V to form a thin film of a mixture of

MnIII oxide and MnII(H2O)6, which can be represented

as MnIII2 O6[MnII(H2O)6]3: This formula corresponds to a

structure where MnII sites in Mn5O6 (MnIII2 MnII

3 O6) are

coordinated with water molecules, and not bound tooxygen atoms in the MnIII�/O network. MnII(H2O)6

within the initially deposited film is expelled from the

film during the reduction of the MnIII oxide to MnII

hydroxide. On the other hand, at more positive poten-

tials than �/0.6 V, the incorporated MnII(H2O)6 is

suggested to be oxidized to form the MnIII�/O bond.

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