Introduction 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 stable 1 , the light required to activate the catalyst may be long-wavelength 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 recombination 2 . 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 TiO 2 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 centers 3 . There are three main crystal structures for TiO 2 : 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 activities 4 . Formation of Nano-Porous TiO 2 Layer via Anodization in Molten Salts N. Baram , D. Starosvetsky and Y. Ein-Eli E g =3.1 eV The Principle of Photocatalysis Under UV illumination electrons and holes are produced 5,6 : TiO 2 + hv e - + h + The holes are causing to: H 2 O + h + H + + · OH The electrons are causing to: O 2 + e - O 2 - O 2 - + H + · HO 2 · HO 2 + e - H 2 O 2 H 2 O 2 + e - · OH + OH - Hydroxyl radicals have the highest oxidation potential · OH - 2.8V vs. NHE. H 2 O 2 - 1.78V vs. NHE 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 >1m Experimental •Ti Foils 0.5 mm of thickness with known surface area. •The foils were mechanically and chemically etched. • Electrolyte – 50%mol NaNO 2 + 50%mol NaNO 3 • Potential: from 0 V - ~80 V. • Current: different current densities 5-50 mA/cm 2 • Duration: 0.5-4.5 Hours Anode (Ti) Cathode (Ti) V T=300 0 c HRSEM FI B XRD 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 ~2m, which increases the efficiency because the penetration depth of photon is ~1m. •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 TiO 2 in a photocatalysis process. Acknowledgements This work was supported by “NATAF" program at the Israeli Ministry of Industry and Trade, Chief Scientist Office. 0 20 40 60 80 100 120 140 160 180 200 220 240 0 10 20 30 40 50 60 70 80 90 P otential[V ] T im e [m in] w ith 0.1% N aF w ithoutN aF A n o d izatio n in m o lten salt I=5m A/cm 2 Addition of 0.1%wt NaF Addition of NaF: High porousivity but an early breakdown is observed! characteriza tion The thickness of the oxide varies from 2m to a few hundreds of nm •Porous-worm like Structure •Pore size distribution: from 100 nm to 1 micron Cross Section Reference s 1. D.A. Tryk, A. Fugishima, K. Honda, Electrochim. Acta, 45, 2363, (2000). 2. A.G. Agrios, P. Pichat, J. Photochemistry & Photobiology A, (2005). 3. M. Paulose, G.K. Mor, O.K Varghese, K. Shankar, C.A Grimes, J. Photochemistry and Photobiology A, (2005). 4. M. Maeda, T. Watanabe, Thin Solid Films, 489, 320, (2005). 5. A. Makowski, W. Wardas. current topics in biophysics, 25,1, (2005). 6. K.Sunada, Y.Kikuchi, K.Hashimoto, A,Fugishima. Enviromental Science &Tech. 32,5, (1998). 7. D.G.Lovering. Transactions of the Institute of Metal Finishing, 61,3, (1983). 8. C.E.B.Marino, P.A.P.Nascente, S.R.Biaggio, R.C.Rocha-Filho, N.Bocchi, Thin Solid Films 468 (2004). C A B 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 condition 8 . 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 rebuilding 9 . 0 50 100 150 200 250 0 20 40 60 80 100 120 P otential[V ] T im e [m in] 5m A/cm 2 30m A/cm 2 50m A/cm 2 A nodization in M olten S alt- differentcurrents The Mechanism The chatodic reactions 7 : 1 NO 3 - 2O 2- + NO - 2 2NO 2 - 2O 2- + 2NO - 20 30 40 50 60 70 80 A A 2 D - fulla no dization C - 5 0 m A/cm 2 B - 3 0 m a /cm 2 A - 5 m A/cm 2 R R TiR Ti R Ti Ti A Ti F orm ation ofcrysta lline phases d uring a nodization HRSEM photos of TiO 2 grown in 3 different current density (a) 5mA/cm 2 , (b) 30mA/cm 2 , (c) 50mA/cm 2 . Ti Corrosion & Applied Electrochemistry Laboratory (CAEL) Department of Materials Engineering, Technion, Haifa 32000, Israel.
Introduction
Jan 03, 2016
V. Cathode (Ti). Anode (Ti). T=300 0 c. The Principle of Photocatalysis Under UV illumination electrons and holes are produced 5,6 : TiO 2 + hv e - + h + The holes are causing to: H 2 O + h + H + + · OH The electrons are causing to: - PowerPoint PPT Presentation
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IntroductionDifferent 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 long-wavelength 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.
Formation of Nano-Porous TiO2 Layer via Anodization in Molten Salts
N. Baram, D. Starosvetsky and Y. Ein-Eli
Eg=3.1 eV
The Principle of PhotocatalysisUnder UV illumination electrons and holes are
produced5,6: TiO2 + hv e- + h+
The holes are causing to:H2O + h+ H+ +·OH
The electrons are causing to: O2 + e- O2
-
O2- + H+ ·HO2
·HO2 + e- H2O2
H2O2 + e- ·OH + OH-
Hydroxyl radicals have the highest oxidation potential·OH - 2.8V vs. NHE.H2O2 - 1.78V vs. NHE
The efficiency of photocatalytic treatment depends onthe 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 >1m
Experimental•Ti Foils 0.5 mm of thickness with known surface area.
•The foils were mechanically and chemically etched.
•Electrolyte – 50%mol NaNO2 + 50%mol NaNO3
• Potential: from 0 V - ~80 V.
• Current: different current densities 5-50 mA/cm2
• Duration: 0.5-4.5 Hours
Anode (Ti)
Cathode (Ti)
V
T=3000c
HRSEM FIBXRD
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 ~2m, which increases the efficiency because the penetration depth of photon is ~1m.
•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.
AcknowledgementsThis work was supported by “NATAF" program at the Israeli Ministry of Industry and Trade, Chief Scientist Office.
0 20 40 60 80 100 120 140 160 180 200 220 2400
10
20
30
40
50
60
70
80
90
Pot
entia
l [V
]Time [min]
with 0.1% NaF without NaF
Anodization in molten salt
I=5mA/cm2
Addition of 0.1%wt NaF
Addition of NaF: High porousivity but an early breakdown is observed!
characterization
The thickness of the oxide varies from 2m to a few hundreds of nm
•Porous-worm like Structure•Pore size distribution: from 100 nm to 1 micron
Cross Section
References1. D.A. Tryk, A. Fugishima, K. Honda, Electrochim. Acta, 45, 2363, (2000).
2. A.G. Agrios, P. Pichat, J. Photochemistry & Photobiology A, (2005).3. M. Paulose, G.K. Mor, O.K Varghese, K. Shankar, C.A Grimes, J. Photochemistry and
Photobiology A, (2005).4. M. Maeda, T. Watanabe, Thin Solid Films, 489, 320, (2005).5. A. Makowski, W. Wardas. current topics in biophysics, 25,1, (2005).6. K.Sunada, Y.Kikuchi, K.Hashimoto, A,Fugishima. Enviromental Science &Tech. 32,5, (1998).7. D.G.Lovering. Transactions of the Institute of Metal Finishing, 61,3, (1983).8. C.E.B.Marino, P.A.P.Nascente, S.R.Biaggio, R.C.Rocha-Filho, N.Bocchi, Thin Solid Films 468
(2004).9. J.L.Delplancke, R.Winand, Electrochimica Acta, 33 (1988).
C
AB
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.
0 50 100 150 200 2500
20
40
60
80
100
120
Pot
entia
l [V
]
Time [min]
5mA/cm2
30mA/cm2
50mA/cm2
Anodization in Molten Salt - different currents
The Mechanism
The chatodic reactions7:
1 NO3- 2O2- + NO-
2 2NO2- 2O2- + 2NO-
20 30 40 50 60 70 80
A
A
2
D - full anodization
C - 50mA/cm2
B - 30ma/cm2
A - 5mA/cm2
R
R
Ti R
Ti
RTi
Ti
A
Ti
Formation of crystalline phases during anodization
HRSEM photos of TiO2 grown in 3 different current density (a) 5mA/cm2, (b) 30mA/cm2, (c) 50mA/cm2.
Ti
Corrosion & Applied Electrochemistry Laboratory (CAEL)Department of Materials Engineering, Technion, Haifa 32000, Israel.
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