The effect of acidic and basic catalysts and of thermal treatment on the preparation of TiO 2 by the sol-gel meth- od has been studied. The resultant solids were ...
React. Kinet. Catal. Lett., Vol. 48, No. i, 295-300 (1992)
RKCL2092
E F F E C T OF HYDROLYSIS C A T A L Y S T TITANIA
Universidad
T. L o p e z
a n d E.
Autonoma
Metropolitana-
55-534 Mexico
A.P.
AND THERMAL
TREATMENT
ON
SYNTHESIS VIA SOL-GEL Sanchez
D.F.
Iztapalapa
09340 Mexico
Received June 30, 1992 Accepted August 5, 1992
The e f f e c t of a c i d i c treatment
and basic
on the p r e p a r a t i o n
od has b e e n
studied.
i z e d by U V - V i s
The r e s u l t a n t
spectroscopy.
due to a f u n d a m e n t a l
TaK~e
HOMO~D~ ~ble
B~HHHMe
CHeKTpOCKOHHH.
CMe~eHHe
Ha6~a~H
o6~aCTb
~pH
5onee
KaTa~HSaTOpOB,
B HpHPOTOB~eHHH
MeTo~a. c
was ob-
used.
O~pH6OTKH
xapaKTepH3oBanH
character-
of the b a n d
to l o w e r e n e r g i e s
were
TBep~o-Fe~eo0paBHOFO
o6pasH~
solids were
KHC~MX H OCHOBHMX
HX T e p M H q e c K o H
a n d of t h e r m a l
A displacement
transition
served when acid catalysts
Hcc~e~oBa~H
catalysts
of T i O 2 by the s o l - g e l m e t h -
a
TiO 2 C
~o~yqeHH~e
TBep-
HOMO~BD Y~ H BH~HMOH
HCHO~bBOBaHHH
KHC~BIX K a T a ~ H B a T O p O B
HO~OCbl ~NH~aMeHTH~BHbIX n e p e x o ~ o s
S
HH3KHX 9 H e p P H ~ .
INTRODUCTION Sol-gel
chemistry
sizing polymeric tanium dioxide
materials,
exhibits
lytic properties. methods
has p r o v e d
for e x a m p l e
superficial,
For t h i s
for o b t a i n i n g
to be a u n i q u e
reason,
The p r o p e r t i e s
structure
[1-2].
in s y n t h e -
supports.
semiconductive
it is i m p o r t a n t
this material.
s o l i d d e p e n d on the c r y s t a l l i n e
catalyst
tool
Ti-
and c a t a to k n o w the of this
I s o b e et al.
Akad6miai Kiad6, Budapest
LOPEZ, SANCHEZ: HYDROLYSIS CATALYST
[3-4] studied p r e v i o u s l y mechanical Lopez
activation
et al.
[5] also c o n c l u d e d
of the chemical ment
bond between
significantly
titania
altered.
that changes
in this case,
(diffuse reflectance)
tronic
structure
analyses
used to study t r a n s i t i o n s tion bands.
band energy
spectroscopy of titania between
method
is o b t a i n e d
but it
in the crystal
structure
band energy
proper-
(Eg). UV-
are suitable for elec-
gels.
This technique
the valence
In this work the r e l a t i o n s h i p
ture and p r e p a r a t i o n
treat-
ratio Ti/O=i/2 is
a wide range of s e m i c o n d u c t i v e
ties and it has then a 3.9 eV forbidden Vis
in the property
material,
if the elemental
The density of point defects
provides,
and
hydrogels.
the dehydration.
is a dielectric
into a s e m i c o n d u c t o r
treatment
of titania
Ti 4+ and OH- after thermal
influenced
Stoichiometric turns
the effect of the thermal
on the structure
is presented,
from the UV-Vis
is
band and conduc-
between
crystal
and the
forbidden
struc-
spectra.
EXPERIMENTAL The synthesis the dissolution,
of metal
oxides
in organic
by sol-gel m e t h o d
solvents, of a metal
is then h y d r o l y z e d with a c o n t r o l l e d lyzed w i t h a c o n t r o l l e d tion is c a t a l y z e d
amount of water.
amount of water.
by acids
and bases.
is basically
a!koxide,
The hydro-
The hydrolysis
In this work,
which
reac-
five sam-
ples of t i t a n i u m dioxide were p r e p a r e d by the sol-gel method, using t i t a n i u m tilled water
tetraethoxide
(3.2 mol)
(ii) H2C204,
(iii)
were t h e r m a l l y
integration
296
and
by UV-Vis
using a Varian, sphere.
ethanol
and the hydrolysis
CH3COOH
(80 ml),
dis-
catalysts: (i) HCI,
~iv) NH4OH.
treated at 70,~300,
were c h a r a c t e r i z e d photometry
(0.2 mol),
The gels o b t a i n e d
600, and 900~
The solids
(diffuse reflectance)
Cary 17 D i n s t r u m e n t
spectro-
with a coupled
LOPEZ, SANCHEZ : HYDROLYSIS CATALYST
RESULTS AND DISCUSSION Hydrolysis
catalyst
In the U V - V i s lysis c a t a l y s t , them the
(Band A)
two w e l l
is v e r y
transition
tanium
and of l o w e r
by S a d e g h i
surface,
characteristic
Previous
peak around value
studies
report
peak
around
if the
impurity
creasing
This
~ of the
with
(Table
to l o w e r e n e r g y
toward but
increases. also
a second
higher energies
it g r o w s w i t h
1
different h y d r o l y s i s
catalysts
t g (h)
E g (eV)
Band A (eV)
Band B (eV)
HCI
40
2.96
3.70
4.96
CH3COOH
26
3.12
3.75
5.00
H2C204
24
3.35
3.80
5.00
H20
20
3.31
3.80
4.96
NH4OH
6
3.36
3.84
5.06
T
g g
- Forbidden - Gelation
When titania,
in-
impurities.
Hydrolysis catalyst
E
i).
reflectance
impurities
shifts
of h y d r o g e n
Table prepared
function
i m p u r i t i e s causes
is high,
(Band B),
[7] and on h y d r o g e n doped
of h y d r o g e n
concentration
to ti-
at 5.0 eV a n d
of a c h a r a c t e r i s t i c
last peak
One of
b e i n g due to
The o t h e r
is l o c a t e d
TiO 2
hydro-
2p o r b i t a l
transition).
This p e a k grows and s h i f t s
concentration
Titanium
at 3.8 eV,
[6] as the w o r k
of t h e s e o r i g i n a l 5 eV.
were observed.
f r o m the o x y g e n
on i n t r i n s i c
if the c o n c e n t r a t i o n
The p r e s e n c e
without
of the t y p e n 3 s e m i c o n d u c t o r s
the p r e s e n c e
4 eV.
bands
intensity,
et al.
prepared
located
(fundamental
assigned
titania
defined
intense,
of e l e c t r o n s
3d o r b i t a l s
being wide
effect
s p e c t r u m of t i t a n i a
band energy time
acid hydrolysis B a n d A was
catalysts
shifted
were
used
in the g e l a t i o n
to l o w e r e n e r g i e s
with
of
an i n c r e a s e 297
LOPEZ, SANCHEZ: HYDROLYSIS CATALYST
in i n t e n s i t y , hydrolysis culated
which
corresponds
catalysts
used.
f r o m the s p e c t r a
the c a t a l y s t s when NH40H
increase
According
band energy
diminishes as the a c i d i c
(Table
to the v a l u e s
the use of HCI for t i t a n i a ,
acid catalysts the X - r a y
forbidden
character
i). The o p p o s i t e
of the
(Eg)
cal-
character
of
effect occurs
is used.
of B a n d A, structure
The
to the a c i d i c
favors
calculated
favors
while
the use of b a s i c
the a n a t a s e
diffractograms
for E a n d the p o s i t i o n g f o r m a t i o n of a r u t i l e
the
structure.
of t h e s e m a t e r i a l s
catalyst This
or w e a k
agrees
reported
with
formerly
[5]. Temperature effect When t h e peratures,
samples were thermally
the c o r r e s p o n d i n g
described
bands.
In this
of B a n d A t o w a r d upon
thermal
is a c o n s e q u e n c e of the
case,
rises
In the
(Table
synthesized
with
600~
t h a t the v a l e n c e
ions
in the
the p e a k s
b a n d and the
sample.
Temperature (oC)
298
synthesized
E
band
in the s t r u c t u r e
[8]. catalyst,
4.96 eV p r o b a b l y
due
is c a l -
This e f f e c t r e v e a l s b a n d are closer.
2
w i t h HCI as c a t a l y s t
Band A g
a shift
of this
If the m a t e r i a l
forbidden
Table Titania
present
HCI and h y d r o l y s i s
are d i s p l a c e d .
tem-
formerly
3). T h i s w i d e n i n g
formation
effect
are observed at 3.9 3.70 and
to the H + and CIc i n e d at
samples
2 and
of o x y g e n v a c a n c i e s
solid
different
and the w i d e n i n g
s o l i d due to a t e m p e r a t u r e
three peaks
at
s h o w e d the two
all the
lower energies
treatment
treated
spectra
(ev)
(ev)
Band
B
(ev)
70
2.96
3.70
4.96
300
2.91
3.67
5.00
600
2.94
3.64
5.10
900
2.94
3.56
5.20
LOPEZ, SANCHEZ: HYDROLYSIS CATALYST
It has b e e n o b s e r v e d cases
on s a m p l e s
material X-ray
treated
undergoing
diffraction:
rutile.
In the HCI
anatase
decreases
treated
at 3 0 0 ~
broad,
showing
in all
thermally.
of the
structural titania
of E diminish g This ~ a c o n s e q u e n c e
reanrangements,
is t r a n s f o r m e d
(pH 3) c a t a l y z e d
has not c r y s t a l l i z e d .
Table Titania
synthesized
Temperature (~ 70
is small.
treated
the c o n v e r s i o n
In the
solids
to r u t i l e
of titania
only
E g (eV)
Band A
Band B
(eV)
(eV)
3.36
3.84
5.06
3.11
3.74
5.06
2.95
3.73
4.90
900
2.96
3.60
4.96
If the t r e a t i n g
3.84 eV p e a k b r o a d e n s
tile.
In this case,
the E
value
defect
density.
g The
2p band.
in the
is i n c r e a s e d ,
the c r y s t a l l i n e
phase,
At
900~
now,
to l o w e r e n e r g y
Two peaks the a shoul-
is o n l y ruregions
and
diminishes. features
in the
3d b a n d
depend
in t i t a n i a ,
in the
but
in the bond,
in the o x y g e n
it w i l l
in s t o i c h i o m e t r y
b a n d or e l e c t r o n s
on the p o i n t
of TiO 2 are u s e d
3d b a n d any more,
is full
but v a r i a t i o n s
2p o x y g e n
of t i t a n i a
If the d - e l e c t r o n s
located
As the
a dielectric,
are p r e s e n t e d .
temperature
Band A shifts
semiconductive
t h e y are not
bands
to l o w e r e n e r g y v a l u e s .
der at 3.5 eV a p p e a r s ;
total.
as c a t a l y s t
300
are o b s e r v e d .
around
is a l m o s t
3
with NH4OH
3, the t i t a n i a / N H 4 O H
are
The h i g h
fraction
600
In T a b l e
solids
The X - r a y p e a k s
that a significant
But in 9 0 0 ~
remains,
size
t h e n to
the a m o u n t of
is i n c r e a s e d .
is o b s e r v e d .
t h a t the c r y s t a l l i t e suggests
as s h o w n by
to a n a t a s e
preparahions
if the t e m p e r a t u r e only anatase
background level
5% of a n a t a s e
t h a t the v a l u e s
behave
can p r o d u c e
3d b a n d of TiO2,
as
holes the
299
LOPEZ, SANCHEZ: HYDROLYSIS CATALYST
oxide will,
then, behave
In the c o n d u c t i o n
as a type
and valence
sub-bandtDaconduction
semiconductor.
transitions
sub-band could exist,
of this kind are located than the forbidden
"n" extrinsic
bands,
from a valence
and t r a n s i t i o n s
in the UV region at higher energies
energy band.
CONCLUSION A relationship
is p r e s e n t e d
between
used in the first step of the sol-gel Eg,
the hydrolysis
titania
catalyst
preparation
the energy position of the band due to the f u n d a m e n t a l
sition.
It was observed
comes more basic, Another
important E
g with w i d e n i n g
effect
versity,
USA)
Conacyt
tranbe-
increases.
when the gel is
and Band A shifts
of the band,
financial
catalyst
band energy value
is also present:
increases
in the number of v a c a n c i e s Acknowledgement.
as the hydrolysis
the forbidden
treated thermally, energies
that,
and
toward lower
which indicates
an increase
in the solid.
and N.S.F. support
(UAM-I, M e x i c o - T u l a n e
Uni-
is acknowledged.
REFERENCES i. L.C. Klein Preforms, Jersey, 2. C.J.
(ed.) : Sol-gel Electronics
Technology
and S p e c i a l i t y
for Thin Films, Shapes.
Fibers,
Noyes,
New
1987.
Brinker:
J. Non-Cryst.
Solids,
i00,
31
(1988).
3. T. Isobe,
M. Senna:
J. Solid State Chem.,
93,
368
(1991).
4. T. Isobe,
M. Senna:
J. Solid State Chem.,
93,
358
(1991).
5. T. Lopez,
E. Sanchez,
Chem. 6. H.R.
and Phys. Sadeghi,
7. R.K.J.
300
P. Bosch,
Y. Meas,
R. Gomez:
Mat.
(in press).
V.E.
Henrich:
Dwivedi: Mater
Sci.
J. Catal., Lett.,
~,
109,
373
1 (1988).
(1986).
