Formation of Nano-Porous TiO2 Layer via
Anodization in Molten Salts
N. Baram, D. Starosvetsky and Y. Ein-Eli
Corrosion & Applied Electrochemistry Laboratory (CAEL)
Department of Materials Engineering, Technion, Haifa 32000, Israel.
Introduction
The Principle of Photocatalysis
Different aspects of water treatment are considered the most urgent
topics at the present and will influence our future life. Photocatalytic
oxidation of organic compounds is comparatively new method for
removal of impurities from water. Titanium dioxide is close to being the
ideal photocatalyst in several ways: relatively inexpensive, chemically
stable1, the light required to activate the catalyst may be longwavelength UV such as the natural UV component of the sunlight and
the produced oxidant is powerful with elimination potential of most
types of microorganisms. The main problem of this process is the low
efficiency due to electron/hole recombination2.
In this work we present anodization of Ti in molten salt solution based
on sodium nitrite and sodium nitrate mixture in order to create nano
TiO2 catalyst. Basic photocatalytic events occur at the surface or close
to it so increasing the surface area will improve the efficiency of the
photocatalysis process. In order to obtain high surface area, the
surface of the catalyst should have a nano-porous structure.
In addition, the catalyst is needed to be crystalline because amorphous
structure provides carrier recombination centers3. There are three
main crystal structures for TiO2: brookite, anatase (metastable
phases), and rutile (thermodynamically stable phase). The difference
in structure can exert influences on the energy of the conduction band
and therefore on catalytic properties and hence, anatase is more
suitable for photocatalytic activities4.
2O2-
2 2NO2-
TiO2 + hv
•Ti Foils 0.5 mm of thickness with known
surface area.
•The foils were
chemically etched.
e - + h+
The holes are causing to:
H2O + h+
H+ +·OH
Eg=3.1 eV
+
120
NO-
Formation of crystalline phases
during anodization
•Electrolyte – 50%mol NaNO2 + 50%mol
NaNO3
• Potential: from 0 V - ~80 V.
The electrons are causing to:
O2 + eO2·HO
O2- + H+
2
·HO + eH2O2
2
·OH + OHH2O2 + e-
• Current: different
5-50 mA/cm2
Hydroxyl radicals have the highest oxidation potential
·OH
- 2.8V vs. NHE.
H2O2 - 1.78V vs. NHE
V
The efficiency of photocatalytic treatment depends on
the amount of generated holes, which is typically low,
due to the high electron-hole recombination rate.
Holes concentration may be enhanced by:
1. Retarding the electron-hole recombination process –
crystalline phase,
2. Increasing the effective surface area of the catalyst,
3. Sufficient thickness of oxide >1mm
Anode (Ti)
Cathode (Ti)
T=3000c
XRD
5mA/cm
2
30mA/cm
2
50mA/cm
C
60
Anodization in molten salt
B
40
2
A
with 0.1% NaF
without NaF
80
20
70
0
50
100
150
200
Potential [V]
0
250
Ti
R
Ti
D - full anodization
60
50
40
30
20
10
A
C - 50mA/cm
2
0
0
A
B - 30ma/cm
A
40
I=5mA/cm
90
R
30
FIB
HRSEM
Addition of 0.1%wt NaF
Time [min]
20
densities
80
Ti
Ti R
current
• Duration: 0.5-4.5 Hours
Ti
Ti
and
2
100
2O2- + 2NO-
R
mechanically
Anodization in Molten Salt - different currents
Potential [V]
1 NO3
-
Under UV illumination electrons and holes are
produced5,6:
characterization
The Mechanism
The chatodic reactions7:
Experimental
A - 5mA/cm
50
60
70
2
2
20 40 60 80 100 120 140 160 180 200 220 240
Time [min]
•Porous-worm like Structure
•Pore size distribution: from 100 nm to 1 micron
Addition of NaF: High porousivity but an early
breakdown is observed!
80
2
The anodization curves have 2 main areas:
1. Rapid increase in the voltage - In this area there is a rapid
growth of the oxide under high field condition8.
2. Slow rise of voltage - In this area, around 65V, sparks are
observed and breakdown in the oxide is caused by: a). phase
change from anatase to rutile; (b). destruction of the oxide layer
and rebuilding9.
Cross Section
Summery and Future work
•Anodization in molten salts of sodium nitrite
and sodium nitrate mixture allow us to grow a
thick nano-porous oxide layer on Ti substrate
in a relatively short period (0.5Hr – 4Hr)
without the need of special equipment.
• The oxide layer thickness is ~2mm, which
increases the efficiency because the
penetration depth of photon is ~1mm.
The thickness of the
oxide varies from
2mm
to
a
few
hundreds of nm
•Addition of 0.1%wt NaF forms more porous
oxide layer.
•Optimization of the anodization process is
planned.
•The next step is to evaluate the efficiency of
TiO2 in a photocatalysis process.
HRSEM photos of TiO2
grown in 3 different current
density (a) 5mA/cm2, (b)
30mA/cm2, (c) 50mA/cm2.
References
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(2004).
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Acknowledgements
This work was supported by “NATAF"
program at the Israeli Ministry of Industry
and Trade, Chief Scientist Office.