electrolyte solution

Solid State Ionics 16 (1985) 141-146 North-HollandPublishingCompany

141

SURFACE CHARGES AT THE INTERFACE OXYDIC SEMICONDUCTOR / ELECTROLYTE SOLUTION

H.N.

STEIN

Laboratory of Collo’id Chemistry, Eindhoven Box 513, 5600 MB Eindhoven, The Netherlands

Different in donor chemical solutions.

pretreatments concentrations, properties of

University

of

Technology,

of

ZnO or TiO2 leading to significant do no cause distinct differences in suspensions of these solids in aqueous

However, 1.

INTRODUCTION The present

ing

the

lid

state

tors

parameters

of

aqueous

aims

whether

question,

influence

ties

differences the colloid electrolyte

the

use

semiconducting

investigation

at

answer-

for

the

in

the

the

question

changes

of

oxidic

the

colloid

dispersions

of

so-

semiconduc-

chemical these

propersolids

results for

oxides

when

whether

obtained

data

isolating,

changes

influence

with

interpreting

in

colloid

raises

solid

state

chemical

pheno-

mena.

in

In TiO,

solutions.

of

oxides

same

character

P.O.

the

present

were

used

investigation,

as

solids.

ZnO

Their

and

solid

sta-

L

This a.

question

Dispersions

a.0.

in

is

currence

e.g.

be

found

In

to

lids

on such

With

the

pretreatment

dispersions as

oxides

rence

of

that

over

interface.

in the This

semiconducting

flatband

dope

behaviour

these

oxides low

/

been

model

so-

oxides,

latices.

one

meets

the

conductivity the

can

of

measurement

potential

be

as a shift

were

changed

or reducing

diffe-

EXPERIMENTAL

2 m

ZnO: -1 g

ex Merck It

*

a continuous OC

for

stream

4 are

p.a.,

was

BET surface

pretreated

stream hrs;

of

oxygen

samples

indicated

(1

in

atm)

at

in

an

cooled

by

3.66

450

O2 Treatment

ZnO/02.

are

indicated

by

as

determined

by

Interstitial

Norman’s 23 * 10 75

ppm

in

ZnO/H

2’ upon

g = 1.965

interstitial TiO

2

4

per *

ESR indicates cooling and

in

at

Zn a

to in

in

a

pronounced

6 ppm (= ZnO/02,

atoms

per

significant

H of 2’ g = 2.003,

Zn and O-,

:

m3)

1o24

cooling

avoid

, amounted

atoms (=

to

Zn, 1)

method Zn

to

at

0 167-2738/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

area heating

ZnO/H2.

measured the

by

H was restricted 2 H stream in order to 2 sintering; such samples

crease

in

heating

materials

solution

potential.

by

atmospheres.

with has

electrolyte

parameter

of

solids.

research

2.

the

concen-

characteristics

in oxidizing

could

might

flow

overall

oxide

oc-

t-educing

prevents the

such

If

polystyrene

the

frequently

changes

or

of the

oxides

containing

AgI

used

under

the of

te

a possibility

chemistry,

dispersions

difficulty,

for

changing

or pastes

by

coagulation. of

affect

colloid

focused

of

conditions,

suspensions b.

of

by

oxidizing

behaviour

properties by

are

profoundly

absence

influenced

significance:

solids

The flow

chemical

tration,

oxidic

influenced

or

colloid be

of

ceramics.

systems

or

has a two-fold

the

3

and m3i in-

signals

ascribed 2) respectively .

to

H.N. Stein / Surface

142

a.

ex Merck

b.

ex

The

(referred

with

pretreated

water

(72

ting.

Pretreatment

tions

leading

heating

for

stream

of

to

at

by

30

passing

oxidizing

at

600

minutes

in

of

heating

finally

by a broad TiO /H 2 2’ absent in TiO /O 2 2’

signal

3.

in

the

f i rmed

The

= 486

of

time.

tion

experiment,

pre-

02;

leading for

0

15

a.

of

the

suspension.

700 rpm,

nm) was

light

followed

Directly the

extinc-

as

before

the

ZnO was

a funccoagula--

dispersed

by

treatment;

19%

12 ( E )_O EO with E = extinction, for

at

and

600

Ti3+

“C

con-

ions

at

g

in

=

the

1.96

TiO

suspension

zz

440

at

measure

a

was

light

extinction

measured

The

(E2).

a measure

giving com4) 7). employed . in

treatment

ultrasonic later

as

method

rest,

was

nm)

taken

rate. simpler

2’ a data only

parative a

was

coagulation

For

as

CEO)

ratio

of

directly and

E /E 2 0 stability

the

th after

two

hours

was

taken of

the

suspension.

consisted

is

adsorbed

fast

(H+

or

following for

expressed In

as

those

cess

employed.

b.

Electrophoresis.

As

radius

particle of

from

the the

poten-

vs.

at

va-

the

pH

of

Hf

amount surface

a slow to

shows

for

Zeta the

time

potentials

0,4-

we took

BET surface

area

as for

kinetics

for ZnO in a stirred 3, 5) (diameter 15

were

cylindrical mm) containing

charge in

( ao) 10 -2

n

X 0

pro=

X 0

X& V

O-

x9

-0 Z-

0

-PH 10

xi@”

xx

method the

particles

ZnO/H2

T~H--H+I (PM&)

0. .2-

a

mobili-

Wiersema-Loeb-Overbeek radius

2

surface and

KC1 solutions.

were

electrophoretic

primary

the

ZnO/O

charge where

by

1

pH

of

average calcula-

-0 .4-

@ X

spherical

particles. Coagulation

RESULTS

8

from the

4.

amount

cases

were

by

OH-1 the

net

adsorption was followed 4) values extrapolated ,

calculated

of

maintaining

The

value.

C.mw2).

too,

adsorption

ions by

present

in short,

Fig. the

KOH necessary

a

are described - 7) In

: of

pH values,

HCl or at

of

determining

rious

3)

elsewhere

Measurement

tial

c.

speed

ultrasonic

followed

ESR data

of

employed

methods detail

they

ted

(h

tion

of

in 10 ml of

METHODS

more

ties 8) .

of

H2 during stream.

cooling

tion

hea-

2’ a N2 stream,

a H 2 presence

and

in

heating

in

extracby

condi-

conditions,

a stream

stirrer, a stirrer

solutiotz

a continuous

cooling

reducing

tic

consisted

“C in

semiconductor/electrol_~te

At

Soxhlet

under

and

‘C

as

followed

consisted

600

by

by

Ti02/H2,

20 hrs

Ti02/H2,

hrs.

to

oxydic

DP ‘25).

as M808);

hrs),

to

O2’ under

treatment

to

(referred

p.a.

was

TiO2

tion

p.a.

Degussa

charges at the interface

investigated

- 0.6 I

vessel a magne-

Surface ZnO/H2

Fig. 1. charge vs. pH for ( 0 ) in 10e2 M KCl.

Zn0/02

(

X )

and

143

H.N. Stein / Surface charges at the interface oxydic semiconductor/electrolyte solution

Differences

in

noticeable

differences

neither

the

point

slope

the

pretreatment

do

in

of

lead

surface

zero

(pzc)

signifi-

changed

2

Fig.

shows

to

foreign ding the

impurities cations

to

lues

are

of

ditions.

The

towards

lower

pzc

in

sorption shift

is

cing

the in

is

pH = 8.6

is

re-

notably

to

ZnO,

ZnO lea-

solution

towards

addition change

(when

the

peak

va-

the of is

the

coagulation

zeta

potential

ment

(fig.

chloride

not

affected

were made about the -2 H KC1 solutions): 10

(in rate

is

determined

irrespective

of

by

the

the

pretreat-

3).

tl

the

cations

rate

the

!

dE/dt E2 0

0.24

first

effected

by

ZnO under reducing con-3 M KC1 is shifted 10

pH values when compared with -2 indicating chemiM KCl, 10

of

not

10

portions,

on no

pxc

the

-3

in

from

these

two

conditions;

reducing tion

of in

Again,

pretreatment

the

of

observed

portion).

the

in

saturation

added

near

desorbed

hydroxides

ZnO is

results

The peak

KC1 solution. lated

similar

observations

coagulation

nor

cantly.

Similar

to

charge:

charge

is

doO/dpH

not

ions. by

this

pretreatment

thus,

noticeably

But

Cl-

influenced

under

chemisorpby

OL

pzc

redu-

pretreatment.

the

Hamaker

constant

between

particles

attraction YoH-H+) Q.LM~-* I

-30

-40

Fig. 3. The initial coagulation rate as a function of the zeta potential in 10e2 H KCl, for ZnOI02 ( 0 1 and for ZnO/Hg ( X ).

Thus, 0.6 I

-20

-10

‘)

is

gree

not by

influenced

changes

in

to

the

describing in a

the

suspension

noticeable

pretreatment

de-

procedu-

re. For

an

found:

for

of

at

ao

tive

sorption Fig. 2. pH for charge vs. ( 0 ) in 10M3 H KCl.

ZnO/O2

(

X )

and

is

found

for

or

shift

more

(fig.

posicomzeta

5);

thus

a reducing

behind

plane,

counterions

is

TiO /H as 2 2 4). The

by

compensated

slipping

circum-

systematic

towards

effected

are

of

a

pH,

some

pretreatment

TiO /O (fig. 2 2 not influenced

differences

trokinetic

under of

H808,

given

is

treatment

Surface ZnO/H2

Ti02 a

with

potential

ions.

influence

values

pared

all

however,

Ti02,

stances

by

the

preelec-

additional

desorption

adof

co-

144

H.N.Stein

/ Surface

charges at the interface

oxydic

semiconductor~ele~trol~te

sol~ltio~

~PH 0

1 -

-_--.il

1

5

6

7

8

10

9

I

I -20 1

I -40

t I

-6O}

I

-80t I Zeta and (TiO2

-

inspection,

reduction

towards

pears

not

to

self:

for

From during

more

of

analysis

ions

Soxhlet

while TiO 2

ions

DP

are

results wards

is

of

)

u

values the

25.

and

reduction,

and

if

these

they

the

more positive

was

on

will

promote OH-

observed values.

/

it-

H+

strongly adsorp-

adsorption. shift

Wc.nr;?

t

-04t

o i-

4

sulfide

are

of

*la% .

that 2SO 4 found

by

contamination

0 1 KN03

on

0

obtained

appeared

If

counteract in

wash-water it

( N

ap-

TiO2

contaminated

such

formed

and

the

extraction,

no

chemisorbed, tion

of

TiO2/02 10-2

O HI --20

shift

positive

a property

0

TiO

I4808

TiO2

in

be

the

(

DP 25, practically identi2 relationsships (5 are found (pH) 0 Ti02/02 and TiO2/H2 (fig. 6).

with

(

Fig. 5. pH for X ) in

’ 02

Fig. 4. charge vs. pH for TiO/Op ( X ) in 1O-2 E4 KN03

closer

cal

vs.

o.zt

Surface TiO2/H2

On

potential Ti02/H2 : N 808).

Surf ace charge and TiO2/H2 (TiO2: DP 25).

Fig. pH

vs.

(

o

)

6 for in

Ti;2/02 3

(

)

l

solutions

This u

0

toAlternatively contamination 2+ Fe or Fe will

one in

could TiO

2 be formed.

think

of

M808;

on

Fe

3+

as

reduction

a

H.N. Stein / Surface charges at the interface oxydic semiconductor/electrolyte

desorption

stronger 2+ Fe could A

explain

no iron

ever, water

be

Atomic

by

(detection The

the

could

limit

2 * IO-’

of

the

summarized

fig.

no difference Ti02/H2

the

DP

25

the

how-

should

wash

sion

and

the

dispersions

are

staare

by local

to

of

groups

E2&

0.3

IEP_,

I

6

10

8

is

Stability Ti02 : Ti02/H2.

of

DP

the

vs. (

pH 0 1

tained

in

that ned

the

their

the

present

colloid

(surface

donor The

value,

charge,

net

slipping are

rather

the

investigation

chemical

electrokinetic constant)

face

data

indicate

parameters charge plane

insensitive

ob-

3)

in

result

elusion

holds

during

cooling

surface

layer

after is

reoxidized

be the

of

have,

at

coagulation

to

cause

rates.

given rates

that

an

a factor the a

10

Hamaker distinct

A similar

(Fig.

2

a

conclusion,

of

the

result,

con-

7).

Hamaker

towards

the

6.

CONCLUSIONS Pretreatment

cannot

from

the

concentration. possibility

between

parts

Zn by

enough

no layer

additively

a change

coagulation

OH

constant, surface

experimental

the

TiO

0.4

attraction up

ZnO/H 2

for

+

OH nmm2).

various

the

in

difference

(8.3

any

interstitial

large in

+

similar

increase

change

9.3

the

to

does

not

:

Hamaker

the

and

leads

excluded,

that

reduction, by a contaminant

a

res,

although

trations does

in not

lead

.

Ti02/H2

of

concer-

behind and

by

2 potential,

de10)

+

+ 0.2

made

Thus

constant

DISCUSSION

at

is

ZnO/O

was

5 .l

distinct

the

because

particles.

This analysis

:

no

of

particles

that

the

: Ti02/02

ZnO/O 2

influence

expected,

(fig.

Taken

case

area.

nm2;

however,

contributions

Pig. 7. dispersions Ti02 25. ( X ) TiO2702;

is,

between

nm2.

and ZnO/H2 (8.5

zeta

5.

per

between

significant

two

reoxi-

cooling

hydrogen were

per

ZnO.

In

4

surface

found

found

nme2)

pH

interface.

superficial during

active

OH groups

For

the

parameters

predominantly

difference

unit

OH groups

was

0.

near

elec-

TiO 10 and TiO /H was 2 2 2 2 to the number of hydroxyl

by

values

0.8

these

the

regard

per

termined

surface

plane:

against

of

conclu-

the

behind

Ti02/H2

with

the

to

charge

significant

surfaces found

draw

determined

conditions

a

The

be

indication

that

to

This

H20).

regard

slipping

expected

dation

/ O2 and

found.

with net

145

(e.g.,

cautious

the

trokinetic to

employed

make us

An Ti02

H2

mentioned

charge

Spectroscopy

regard

in

7.:

between

is

in

of

mol/l) .

with

Ti02

in

than

shift;

UC

detected

Adsorption

observations

bility

Pe3+

of

solution

with

H 2

at

increasing ZnO to

and

high

the TiO

changes

2

temperatu-

donor

concen-

significantly, in

the

colloid

H.N. Stein 1 Surface

146 chemical charge,

plane,

investigated:

parameters chemisorption

charge

behind

charges at the interface

the

of

surface

4)

net

counterions,

electrokinetic

o.xydic serllicondlr~torlelectrol~.te

Electrolyte

5)

“The

Janssen,

Thesis,

slipping

and Hamaker constant.

M.J.G.

6)

Logtenberg

1)

V.J.

Norman,

Analyst

2)

R.D.

Iyengar

and N. Codell,

Adv.Coll.Int.Sci. E.H.P. the loid Ph.D.

3,

Logtenberg,

Solid

State

Chemical Thesis,

89,

261

365

Relation

Properties

Eindhoven

and of

1983.

Zinc

Ph.D.

and H.N.

Stein,

print)

and H.N.

Stein,

J.Coll.Int.Sci.

Janssen

P.H.

and

H.N.

9)

Col-

Oxide”, 10)

P.C.

A.L.

Wiersema,

Overbeek, Between the

/

Stein,

to

be

published. 8)

(1972).

“The

Behaviour

(1964).

M.J.G.

to

Interface”,

(in

Logtenberg

submitted 7)

3)

E.H.P.

Dioxide

1984.

J.Coll.Int.Sci.

REFERENCES

Titanium

Solution Eindhoven

E.H.P.

solutim

J.Coll.Int.Sci.

Hiemenz,

Chemistry,

1977,

412.

T.

Morimoto

Soc.Japan

46,

22,

Principles

Surf ace p.

Loeb

of

Marcel

and

H.

2000

(1973).

Naono,

and

J.Th.G.

78 (1966). Colloid Dekker

and Inc.

Bull.Chem.