Specific orientation of electrochemically deposited PbO2 crystals

25

J. Elecrroanol. Chem.. 182 (1985) 25-36 Elsevier Sequoia S.A.. Lrkanne - Printed in The Netherlands

SPECIFIC CRYSTALS

JORDANIS

OFUENTATION

C.G. THANOS

OF

and DIETRICH

ELECIROCHER’KICALLY

DEPOSITED

Pb!T)z

W. WABNER

Anorganisch chemisches Ins&~ und Arbeirsgruppe Angewandre Elekrrochemie der Technischen L’niuersiliir Miinchen. Lichrenbergstrasse 4, 8046 Garching bei Miinchen (F. R. G.) (Received

281.h June 1984)

ABSTRACT Lead dioxide anodes were prepared by electrodepcsition of Pb2+ on titanium in acidic Pb(NCl,), solutions at 30, SO, 70. 90 and 130°C. The electrodes piated at higher temperatures revealed a progressive increase in the content of &PbO, relative’to a-PbOa. The individual crystab were larger and a higher 0,/O, overpotential was observed during the electrolysis of water on such anodes in 0.5 M HCIO,. At higher temperatures the 110 orientation parallel to the geometrical plane of the electrode predominates for the crystallographic planes. An opposite trend can be established for the crystaIlographic planes 200 and 101.

(I) INTRODUCTION

The application of PbO, anodes finds an increasing use in industrially important processes [l]. The development of dimensionally stable electrodes [2] on titanium and other substrates [l] enhances the applicability of such anodes. Recently, a process [3] has been developed for an ozone synthesis of long duration on such anodes. During these investigations we noted the importance of manufacturing the PbOz anodec at higher temperatures by the electrodeposition on Ti in a lead nitrate bath. The aim of the present work is to investigate, the similarities and differences of electrodes plated with PbO, at different electrodeposition temperatures. (II) CHEMICALS,

PREPARATION

OF ELECTRODES

AND

INSTRUMENTATION

The chemicals were of analytical grade supplied by Merck, Darmstadt, except for the surfactant FC 98, supplied by 3M. The titanium used has the trade name “Contimet 35” from TEW. The water used was distilled. The “red” varnish from Fa. Blasberg and Teflon has been used for electrica! insulation successfully. Electrodes plated at five different temperatures (30, 58, 70, 90, and 130 “C. f 2 “C) and with i = 40 mA cm-’ have been studied. The bath is described in ref. 2. The electrodes were either plates or wires of 1 mm diameter, according to the demand of the following investigations. The electrical equipment used is described elsewhere [4]. Additionally we used a scanning microscope, Jeol 35C, as well as a mini-computer, 0022-0728/85/%03.30

0 1985 Ekevier

Sequoia SAA.

26

attached to the diffractometer, Siemens D 500, for the digitalisation of the intensities of the X-ray diffractograms. The Cu Ka radiation from a copper target has been used for the X-ray photographs. (111) PRINCIPLE

OF THE

X-RAY

METHOD

(III. I) Crystallile orientation

The technique used for the investigation of the crystallite orientation on a deposited substrate is shown in Fig. 1, where two-dimensional arrays with primitive cell vectors a and b are drawn at a fixed ‘orientation relative to the surface of an electrode plane. The orientation of the two-dimensional crystal considered is fully defined by the vectors a and b. Moreover, the vectors a’ and b’ of a crystallite of the same kind but differently oriented relatively are also shown /al/=/a/ and /b’/ = /b/. For the lattice a, b the reciprocal lattice vectors A and B are also shown. It is assumed that the incident K and reflected K’ wavevectors satisfy thecondition

151 K’-K=G=A+B

(1)

Then, in principle. the reflection (1.1) is observed for the crystallite C, but not for C, for which the condition (I) is not satisfied. If a planar- electrode is positioned pxallel to the speciemen holder of the diffractometer (Fig. 2a) then a diffracticn at the Bragg angle will be observed only

Fig. 1. Two dimensicnA diffraction pauems.

diagrammatic

represenlcitizx

of

favourably

arranged

ctystallites

for

X-ray

27

crystallite

plate

of diffractome.‘er (al

lb)

Fig. 2. (a) Bragg r&ections form

of bent

on planar

electrodes.

(b) Bragg reflections

on PbO?/Ti

eleclrodeposited

in the

wires.

from the crystallites which are favourably

4.j = ( I/IA,

oriented_ We can define

14 Zj/t4,)

(2)

where 1; = intensity of peak of reflection occurring at angle 20; and I,,; = normalised intensity in ASTM tables [6] for the same reflection (Debye-Scherrcr method). In the case of a nonrandom orientation the expected diffraction peaks w~nill have relative intensities deviating from those of a Debye-Scherrer pattern. If cn the other hand plated bent wires (Fig. 2b) are placed on the specimen holder the c~~stallites will be oriented in all directions in space. The magnitude of the factor 6.; below is then a direct measure of the preference of orientations i in comparison to orientationsj. (III. 2) Necessary intensity corrections It may be’seen from Fig. 3 that the number of the crystallites encounterc3 by thz incoming beam, bml, within a distance dx in the PbOz-layer is proportional to dx/sin 8; for this reason the factor G,., below gives a better representation of thz orientation of the crysta!!Ites than cmj: G,_j = c.., (sin fIj/sin

6i)

(3)

For the correction due to the photoelectric correction [7] the following argument (see Fig. 3) is applied: The total path of incident and reflected beams bml and bm2

incoming

Fig.

3. Diagrammatic

representation

of the crystallites

encounteredby

the X-ray

beam.

28

which correspond to a Bragg reflection at depth x is x/sin 8. If p is the absorption coefficient in cm- ‘, I, the intensity of the falling beam, k a”constm_t, p(B) an mguiar factor the influence of which on the intensity of the diffracted beam is already contained in the intensities of Debye-Sche.rrer photographs and I,the total intensi1.y of the diffr:tcted beam we get:

zr = / ‘DZokp (19) exp( 0

- px/sin

B) = -

ykZ_.,(B) exp( -pD/sin

0) - 1

(4)

which shows that I, is proportional to sin 0 which should cancel with the previously discussed geometric factor l/‘sine. However, p for PbO? can be calculated to be of the order of 1000 cm-’ and since the crystal&s are of the order of 10 to 30 pm (see Figs. 5.1 to 5.5), mos? of the X-ray radiation is absorbed within the rough part of the PbO, layer. Hence the sin 8 factor arising from the flat mode! discussed aboLe may be omitted and so Gi.j remains as the fac!or describing at best the orientation features of the crystallites (the main.orientation effects may be also seen, however, from the magn:tude of rhe factor cmj). The applied correction shows that the GiVi factors between parallel planes should be close to unity. This could be verified for the (110) and (220) reflections of /3-PbO,. (IV)

EXPERIMENTAL

RESULTS

In Fig. 4 the diffractograms of PbO,/Ti electrodes electrodeposited at 30 “C and 90°C respectively are shown. The electrodes were similarly pretreated before plating and the plating took place at t40 mA cm- * for 15 n%. The sensitivity of the detector of the diffractometer for Fig. 4 was held constant. It must be noted that the relative intensities of the peaks of a diffraciogram remain unchanged if a planar electrode is oriented in different directions on the specimen holder of the diffractometer (Fig. 2b) as long as the electrode sits flat on it. Also exactly the same X-ray spectra are obtained with a new 90 "C PbOJTi electrode, and one plated at 90 OC on the surface of an older 30 “C PbOJTi electrode, showing that the texture of the PbO, layer is independent of the electrode substrate. The PbO, electrodes which were electrodeposited at higher temperature consist mainly of B-PbO, [2,4,4a]. However, z.t progressively lower temperatures of electrodeporition, increasing amounts of a-PbOz cccur with the &form. Intensities of X-ray diffraction data for same important crystallographic pl&es are shown in Tables 1 and 2 for /3- and cr-PbO, respectively. The width of-the peaks considered amounts to lo. The scz!!c of the intensities is arbitrary, but the relative intensity within either Table 1 or 2 are directly comparable. The relative intensities of the X-ray reflection for two PbO, specimens made at different temperatures tie on avzage indicative of the relative amounts of the specific crystallographic form of

TABLE

1

Some important

X-ray

reflections

of fi-i’bOz

for electrodes

plated at different

temperatures

(40 .rnA cm-‘,

acidic PbfNO,),) Temperature

Brag;:

ASTM

Crystallo-

Index

Experimen-

of electrode

angle

normalized

graphic

i

tal

plating

28

intensity

index of

factor

intensily

planes

G,,

(arbitrary

/“C

in

G ,.5

scale) 30

50

70

90

* The factor G, recorded b With not

F,s =

7605

25.4

loo

110

5

32.1

80

101

1

12577

-

1.0 y6

I.

36.5

90

200

2

7873

-1.7=

52.2

50

220

3

2295

1.2

54.6

50 50

002

4

465

62.0

‘-91

5

6837

67.5

30

202

6

1295

1.5

74.5 83.9

50 30

321

7

1927

312

8

2300

1.4 3.i

25.4

100

110

S

24407

32.i

80

101

3260

0.21

0.26 4.3

1.0

36.5

.90

200

6427

0.42 =

52.2

so

220

5924

0.97

54.6 62.0

002 301 202

350 3896 650

0.060 0.75

67.5

50 50 30

74.5

50

321

5384

83.9

30

312

1940

1.2 0.81

25.4

100

110

S

32.1

80

101

lh

36.5

90

200

2

2433 4120

52.2

50

220

3

8642

54.6

50

002

4

250

62.0

50

301

5

2610

0.027 0.32

67.5 74.5

30 50

202 321

6h 7h

416 7836

0.093 1.1

83.9

30

312

8

1513

0.41

25.4 32.1

100 80

110 101

37800

1.0 0.10 0.17 n 0.91

57195 1060

1.0 0.029 0.078

36.5

90

200

2821

52.2

50

220

11786

54.6

50

002

271

62.0

50

301

1994

67.5 74.5

30

357

50

202 321

9034

83.9

30

312

1397

is in fact slightly smaller because of interference

0.22

=

0.82 0.020 0.16 0.G52 0.87 0.25

with the reflection

200 of n-PbO,

in the

diffract0 yams. specimens

planar 0.82.

made of 70 “C PbO,fi

specimens

the

factor

in the form of wires (Fig.

GiaI is meaningless)

could

be

2.b)

the fotiowing

calculated:

4,

Ft., = 0.60,

factors (for

F,,, = 0.52.

85

80

75

7.1

65

60

55

50

L5

LO

35

30

2; I-

-85 F:g.

4.

80 X-ray

75

diffractograms

70

of PbO,

65

elecwxles

60

55

deposited

50

L5

LO

35

30

25

at 30 and 90 “C.

PbO,

consic.ered during the study of such specimens. A direct comparison of the of a- and p-PbO,, however, is not possible, because we do not know the relative intensities of the reflections oi similar specimens which would contain pure cr- or &PbO,. Other pieces of information in Tables 1 and 2 are: the factor G;, (of a plane assigned the value i also stated in the tables relative to the plane with the strongest P.STM reflection which is assigned the value, s, the Bragg angle 2, the index of the c;ystaUographic plane, the ASTM n~rmahzed intensity and the temperature of electrodeposition of PbO,.

amounts

(.I V.2) Electron micrographs Electron micrographs of 30 “C, 50 “C, 70 “C, and 90 “C PbOJTi electrodes are shown in Fig. 5.1-5.4. A micrograph of an electrode plated at 130 “C in a high pressure cell discussed in ref. 8 is shown in Fig. 5.5.

(IV.3) Electrochemical measurements Semilogarithmic current potential plots for two electrodes (PbO, 90 “C and PbOl 30 “C) during water electrolysis are shown in Fig. 6. The corrections for the

31 TABLE

2

Some important X-ray acidic Pb(NO,)z) Temperature of elecrrode plating

ret&lions

&&PbC&

For electrodes

plated a: different

temperatures

(40 rr4

Bragg angle 28

ASTM normalized intensity

Ct.ySlaliographic index of planes

30

23.2 2S.6 32.6 48.1 50.97 56.0

12 100 70 30 30 15

110 111 002 220 221 113

1 s 2 3 4 5

552 5214 4104 471 1018 1124

0.72 1.0 1.3 0.5 1.1 2.7

50

23.2 28.6 32.6 48.1 50.97 56.0

12 100 70 30 30 15

110 111 002 220 221 113

1 s 2 3 4 5

912 4936 small

1.3 1.0 -

1099 693

1.3 1.8

23.2 28.6 32.6 48.1 50.97 56.0

12 100 70 30 30 15

110 111 002 220 221 113

1 s 2 3 4 5

628 1725

2.5 1.0 -

23.2 28.6 32.6 48.1 50.97 56.0

12 100 70 30 30 15

110 111 002 220 221 113

/“C

70

90

Index

i in factor G1.1

1 S 2

3 4 5

Experimental intensity (arbitrary scale)

small 724 285 446 381 small small ~500 small

cm-‘.

Gi.S

24 2.1 ca. 7.9 1.0 7.6

electrolyte resistance and the TiO, layer between the Ti substrate and PbO, have already been carried out as described elsewhere [8]. The log i/E p!ots have been recorded from high to lower current densities and alter a prepolarisation with + 100 mA cm-’ in a 0.5 it4 HCIO, aqueous solutio:t for 2 rnin at 25 “C. (V) DISCUSSION (V. I) Phase composition, orientation effects :*rom the SEM-photographs it is obvious that under identical conditions the PbO, cry:.tallites of the electrodes deposited ,at higher temperatures are of gradually increasing magnitude. From the X-ray data two observations may be made:

5.1

5.3

Fig. 5.1-5.3.

Electron

micrographs

of ekctrodrposited

(2) 50 “C. (3) 50 “C. The bar corresponds

IO 10 pm.

PbO,

layers at different

~empern~urss:

(1) 79 “C

33

Fig. 5.4 and 5.5. As Fig.

5.1-5.3

but 31 (4) 90 OC mld (5)

130 “C.

The first is, that the amount of a-PbO, is much less at the elwtrodes deposited at higher temperatures (Tables 1. 2). This demonstrates that not only the cur;.ent density is critical for the phase composition [9]. The second observation is the high orientation specificity of the P-PhO, (as \vell as of the minor electrode component. n-PbO?) cq/stallites for the electrodes of higher temperatures. This orientation effect interprets the “anomalous” reflection intensities reported earlier in the literature [lo]. The predominant crystallographic orientations for two different crystallographic planes indicated as i, j are easily recognized from the ratios G,.,/G,., (Tables 1,2). As expected the Lveak peaks are susceptible to large errors in the estimation of the factor G,,,. Despite the possible error reflections (ml,. ml,, ttd3 ) and (II/, . ttl,. tt13) (ttz, tt are small integers e.g. 1.2)

34

lead to very similar Gj_, factors as should be demanded from the ,fact that they originate from parallel planes. It is also seen, that for the specimen studied in the form of wire electrodes (footno!e Table 1) the orientation specificity is largely lost. (For total randomness the factors F;_, should be 1). It is very interesting to note that the strong peak due to reflection 211 (28 = 49’, Fig. 4) is something anticipated from the approximate SEM observations of other workers [ll]. Others authors also comment on preferred orientations of crystallites [3,12] from purely optical observation of PbOz electrodes. From the indifference of the diffractograms obtained by changing the orientation of the electrode on the specimen holder, we may conclude that .the preferred orientations of the crystallites are relative to the electrode/electrolyte only and not

, 1500

1

7700

I

I

I

1900

I

I

,

I

I

I

2100

E/V vs. SCE Fig. 6. log i/E vs. SCE plots during water electrolysis lor two different Pb02 electrodes deposited at (1) 27 “C and (2) 90 “C. Curve (3) recorded with glass PbO, prepared acxrding to ref. 2. Measurements carried out after 5 tin prepolarisation at 100 mA crnm2 (24 “C, 0.5 M I-XIO,).

35

relative to a fixed two dimensional coordinate system with axes parallel to this interphase. Moreover the orientation of the crystals is not due to an epitaxial growth on the substrate, because the nature of the substrate has been seen to be of no importance. If we then suppose that the preferred orientations arjse from the different rates of growth of the crystallographic surfaces at the electrode/electrolyte interphase, there still remains the unanswered question why the di:ference in growth rates is higher at higher temperatures? A very probable explanation of the orientation effect may be given by considering observations made by previous authors on nucleation and growth phenomena: (a) Distinguishable crystallites are observed on 2 Ti substrate at the beginning of PbO, deposition [2]. (b) Substrate inhomogeneities propagaae themselves during plating [13]. (c) Polycrystalline assemblies are observed on the PbOl surface on which /I-PbO, crystallites grow further in all orientations until coalescence occurs [2,12]. Addition of Fin the plating bath increases the deposition over-potential and also the assemblies mentioned above are more pronounced [12]. In our case at higher temperatures the deposition potential is lower and probably these polycrystalline assemblies are less commonly encountered so that disorder effects are less common. Hence although the rates of growth of different crystallographic planes may depend only slightly on temperature, the preferred crystallographic orientations are more pronounced at higher temperatures at which the number of the polycrystalline assemblies on the electrode surface is smaller. (V-2)

Correlation

with oxygen

evolution

potential

Some changes of the log i/E slopes with current density (during water electrolysis in several electrolytes, see Fig. 1 for a HClO, solution) should not necessarily be interpreted by a change of the reaction mechanism as pointed out in our previous work [9]_ Impedance data support this view [4]. The roughness of the electrode is, to a certain extent, responsible for such changes of the slope as well as the diffusion of H+. In general, the “oxygen over-potential” of the anodes made at higher temperatures is higher. This correlates very well with our previous considerations [3] on the evolution of ozone, which is favoured at high temperature electrodes_ Great differences in surface area cannot be established optically from the SEM photographs (since optical observations are very approximate, more information from BET data is needed), so that quantitative conclusions cannot be drawn. The higher potential may be due to the decreased number of crystal edges per geometrical electrode surface, or due to the progressively decreasing amount of cY-PbO,. Ruetschi et al. [14] found a lower O,-overpotential on cy-PbO, than on fi-PbO, for the same BET area of the electrodes. If however the effects of edges of the crystallites are of fundamental importance, the difference in the size of the crystals of the ~1- and &forms could provide an alternative explanation of Ruetschi’s results.

36

A third possibility of the different 02-overpotential on PbO, may lie in the type of crystallographic planes exposed to the electrolyte. For TiO, of the’same crystal structure (t-utile) anisotropy of physical properties like conductivity has ZJready been observed by Blumenthal et ai. [15]. The decrease of the O,-overpotential due to the higher surface density of crystallite edges observed at the low temperature electrodes seems however to be the more acceptable explanation. ACKNOWLEDGEMENTS

der Chemischen Industrie” and Bund der Support of this work by “Fonds Freunde der T.U. Miinchen is gratefully acknowledged 2s well as the grant of a scholarship by the “Institute Alexander Onassis” to J.C.G.Th. REFERENCES 1 2 3 4 4a 5 6

7 8 9 10 11 12 13 I4 15

AT. Kuhn, The Electrochemistry of Lead, Academic Press, London, lS79, pp. 217. D.W. Wabner, Habilitationsschrifb T.U. Miinchen, 1976. H.P. Fritz. J.C.G. Thzmos and D.W. Wabner, Z. Naturforsch. 34b (1979) 1617. J.C.G. Thanos, Disrertation, Mirnchen. 1980. W. TiUmetz, Diplomarbeit. Mimchen. 1982. C. Kittel, lnrroduction to Solid State Physics, Wiley, London, 1971, p. 67. A.S.T.M. Cards of Joint Committee on Powder Diffraction Standards 1972. (a) 25447 Ref.: A.N. Winchell, Elemenrs of Optical Mineralogy. (b) 11-549 Ref.: A.I. Zaslavsky. Yu.D. Kondrashov and S.S. Tolkachcv. DokI. Aksd. Nauk SSSR., 75 (1950) 559. (c) 11-548 Ref.: J. Burbank, J. Electrothem. Sot., 104 (1957) 693. InternaIionaI Tables for X-Ray CrystaIlography, Kynoch Press, Biimingham, Vol. 3. 1968 p. 162. J.C.G. Thanos. H.P. Fritz and D.W. Wabner. J. Appl. Uectrochem.. 14 (1984) 389. A.I. Rusin, Z.I. Zhivilova, N.A. Makhalov and KM. Solov’eva. Translalion from Russian: Zh. Prikl. Khim. Moscow, 49 (1976) 914. A.I. Rusin. Z.I. Zhivilova, Yu.M. Shutova and K.M. Solov’eva. Translation from Russian: Zh. Prikl. Khim. Moskow. 43 (1970) 2614. H. Vaidyanrhan, Ram A. Narasagoudar. T.J. O’Keefe. W-l. James and J.W. Johnson, J. Eleclrochem. Sot., 121 (1974) 876. D. Gihoy and R. Stevens, J. AppI. Electrochem., 10 (1980) 511. Y. Shibasaki. J. EIecIrochem. Sot., 105 (1958) 624. P. Rutxchi, J. Sklarchuk and RT. Angstadt. Elecctrochim. Acta. 8 (1963) 333. RN. Blumenthal. J. Baukus and W.M. Hirthe, J. EIectrochrm. Sot.. 114 (1967)

172.