- Email: [email protected]
New Application of Stainless Steel
Available online at www.sciencedirect.com SCIENCE
DIRECTe
JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2006, 1 3 ( 1 ) : 62-66
New Application of Stainless Steel YANG Jia-long’“
,
LI Ying’ ,
WANG Fu’
,
ZANG Zheng-gui’
,
LI Si-jun’
( 1 . College of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning , China; 2. Ironmaking Plant, Wuhan Iron and Steel Group Co, Wuhan 430083, Hubei, China)
Abstract: Several rigid substrates such as stainless steel, titanium alloy, aluminum alloy, nickel foil, silicon, and sodium lime glass have been employed for manufacturing high quality TiO, films by metal organic chemical vapor deposition (MOCVD). T h e as-deposited TiO, films have been characterized with SEM/EDX and XRD. T h e photocatalytic properties were investigated by decomposition of aqueous orange II . UV-VIS photospectrometer was employed to check the absorption characteristics and photocatalytic degradation activity. T h e results show that films synthesized on metal substrates display higher photoactivities than that on absolute substrates such as silicon and glass. It is found that solar light is an alternative to UV-light used for illumination during photodegradation of orange II. Ti02 film on stainless steel substrate was regarded as the best one for photocatalysis. Key words: MOCVD; photodegradation; orange JJ ; metal substrate: inert substrates
Titanium dioxide ( TiO, ) has been extensively investigated in many applications, such as photocatalytic purification, photovoltaic cells, anti-reflective coatings, dielectric layer, optical filters, sensors, fog-proof ( frost-proof ) , anti-cancer, anti-bacterial T h e advantages of using and self-cleaning etc“’ TiO, in the photocatalytic decomposition of organic pollutants are based on its remarkable activity, low cost, chemical and radiation stability and is not prone to or photocorrosion; strong oxidizing agents, such as HzOz, aren’t in needCZ1 ; and it is non-toxic. is a textile azo-dye which is resistant Orange t o light degradation, and not easy t o react with 0, , common acids and bases. It does not undergo biological degradation in waste water treatment plantsi3’. In anaerobic conditions, it can be reduced t o potential carcinogenic aromatic amines. Traditional methods like ultra-filtration, extraction, air stripping, carbon adsorption, incineration and oxidation via ozonatiod4’ or hydrogen peroxide are non-destructive; the pollutant is transferred from one phase to another, and it can’t be eliminated. TiO, photocatalysis is the best choice for degradation of azo-dye. The use of aqueous suspensions limits practical applications due to the problems of separation of fine parti-
.
o3
cles of T i 0 2 and recycling photocatalysts. Therefore, many techniques have been proposed for the immobilization of TiOz on solid supports to solve this problem. Most of the general methods were reported in a recent reviewC51. There are few studies on TiO, photoactivities among different rigid substrates at the same condition. U p t o now, there are no related reports about TiOz deposition on different substrates with MOCVD system t o check their photocatalystic properties. In present study, TiOz film was deposited on various substrates such as austenite stainless steel, glass, titanium alloy, nickel foil, aluminum alloy, silicon ( 100 1 and silicon ( 111) , by MOCVD technique. T h e as-deposited film were used for checking microstructural properties and the photoactivities for degradation dye solution (orange fl 1. Some important conclusions have been reached about the influence of types of substrates and the source of illumination light on the photoactivity of TiO, films. And some unusual performances were discussed.
1
Experimental
Deposition of TiO, thin films was carried out in a homemade low-pressure vertical metal organic
Foundation Item: Item Sponsored by National High-Tech Research and Development Plan (2003AA331080) E-mail: jialongyangB163. com; Biography:YANG Jia-long(1964-) , Male, Doctor, Professoriate Engineer;
Revised Date: August 17, 2004
New Application of Stainless Steel
'go. 1
chemical vapor deposition ( LP-MOCVD ) system. ritanium isopropoxide, TIP(Ti[OCH(CH, ) 2 ] 4 was ised as single molecular precursor. T h e general deptisition conditions are as follows : temperature ranges from 300 "C to 800 C ; Pressure ranges from 13. 33 t o 666. 6 Pa; and the flow rate of diluting gas, T I P carrier gas and O2 are ( 1 - 4 ) X l o p 6 m'/min, (12 ) X l o p 6 m3/min and 2 X m3/min respectively. The deposition time is 240 min. All of the metal substrates such as stainless steel, whose composition (mass percent, % 1 is : Cr 29.5,Ni4.l,Mo1.3,CO.l, Si0.9, Mn1.8and Fe 62. 3 , titanium alloy, whose composition (mass percent, %) is: C 0. 03, N 0.01, Fe 0. 2 , Al 6.34, V 3. 99, 0 0.1 7 , Y
+
63
transparent polymeric container with a diameter of 30 mm and height of 5 mm. T h e solution container was located in a homemade chamber equipped with 200 W mercury lamp that can produce UV and visible light irradiated on TiO, films through a Pyrex filter cover. The light intensity was measured with a power meter (Newport 1815-c). In this study, the intensity of light was maintained to 5. 5 mW cm-'. Each time four samples and a controlled solution were used for irradiation under UV light. All of the experiments were carried out at ambient temperature (25 - 40 'C ) and ordinary pressure. The irradiation time changed from 0. 5 h to 6 h.
2
Results and Discussion
Stainless steel ( PFR41) , titanium alloy ( PT41) , aluminum alloy (PA411, nickel foil (PN411, silicon ( 1 0 0 1 (PS41 and untreated high clean glass (PG41) were employed as substrates of TiO, films deposition. Each sample was deposited by MOCVD within 4 h at an operational temperature of 400 "C, and a thickness of about 2 000 nm was obtained for each sample. 6. 5 mL and 1X 10 ' mol/L orange fl solution was used for each film sample. U V light intensity was maintained at 5. 5 m W / c m z , illuminating time changed from 0. 5 to 6 h. Fig. 1 shows the UV-VIS absorption spectra after 1. 5 h of irradiation. T h e other UV-VIS absorption spectra of specimens with different illuminating time are not shown here. T h e photodegradation proportions or photoactivities were illustrated in Fig. 2. For more quantitative comparison, reaction kinetic analysis is necessary. T h e apparent reaction rate constants of TiO, films 0.40
I
0.30
.
29 0.20
.
a
c
3
B
0.10 .
0. 200
Fig. 1
400 600 Wavelengthhm
800
UV-VIS absorption spectra of photocatalytic degradation of dye solution by TiO, film synthesized on different substrates
64
Vol. 13
Journal of Iron and Steel Research, International
*
100
5.5 4.5
80
3.5
3 2.5
60
9-
= ;
$ c,
40
1.5
0.5 0
20
I
2 4 Irradiation timeh
0
6
0
Fig. 3
Relationship between In(C/Co) and irradiation time
100
80
-
0"
3d C
60
n
40
s 20
0 0
Fig. 2
duced (in Fig. 4). From Fig. 1- 4 , i t can be concluded that TiO, film deposited on metal substrates (high conductivity materials) possesses related higher photocatalytic activity in degradation orange solution compared with that on inert substrates. T h e degrading reaction rate of TiOz films on stainless substrate is four times as high as that on glass substrate. For the same type of conductive materials, there are some differences, TiO, film on stainless steel expressed highest reaction rate. Films on aluminum alloy, however, displayed lowest degrading reaction rate. According to the values of degradation reaction rates, the following consequence can be obtained corresponding to decreasing order of photoactivity : TiO, /stainless steel TiO, /Ti alloy > TiO, /Ni foil TiO, /A1 alloy>TiOZ /Si(100) >TiO, /glass. Fig. 5 shows the FE-SEM top-graph of TiO, film on various substrates deposited at 400 %. It is clear that there are obvious differences in film microstructure for different substrates. For metal substrates, almost all of the TiO, films display loosely
2 4 Irradiation timeih
6
Influence of Ti02 thin film's substrates on activity of photocatalytic degradation of dye solution
on different substrates can be calculated. Generally the photocatlytic degradation follows a Langmuir-hinshelwood mechanismLg1 , indicating that the reaction rate is proportional to the coverage 8. kB=kKc/(l + K c ) (1) where k is true rate constant that includes various parameters such as the mass of catalyst, the flux of efficient photons, the coverage of oxygen dissolved in solution, etc, and K is adsorption constant. Bewas only cause the initial concentration of orange 1X l o p 5 mol/L, and the value is very small, Kc in denominator can be neglected. Therefore the rate becomes the first order: r= -ddC/dt=kK,=k.C (2) where k, is apparent rate constant of pseudo-first order. The solution of the first order equation is as follows: (3) In(C,/C> =k,t Curves in Fig. 2 could he changed into lines in Fig. 3 , and by calculating slops of lines, k, corresponding to films on various substrates could be de-
>
In
n
>
o.902
0.8
0.677
&'
0.4
0.337 0.253
0
Fe
Fig. 4
Ti
Ni Al Substratematerial
Si
Be
Degrading reaction apparent constants of Ti02 film on various substrates
'Qo.1
New Application of Stainless Steel
Fig. 6
*
65
*
FE-SEM images of T i 9 films deposited at 400 "c on titanium alloy (a) and silicon (100) substrate (b)
relative bigger particles are just scattered on the film surface. As shown in Fig. 6 ( b ) , there was even no porosity under the film surface. So it is difficult for this kind of microstructure t o gain higher activity of degradation, hence with a lower activity than that of
Fig. 5
FE-SEM top morphology of T i 9 film ( 2 000 nm) deposited on various substrates at 400 "C
scattered particulates. As particles are separately distributed on the surface, and for TiO, films grown by MOCVD system, the as-produced film usually has a columnar structure because of its eptixial growth characteristics, it is believed that loosely scattered columns should be the main microstructure under the surface of film deposited on metal supports [in Fig. 6 (a)]. This kind of structure of TiO, film can improve the solid-liquid contact condition, thus enhance the activity of photodegradation. Image (Be) shows the top view morphology of TiO, film on substrate of glass. This image illustrates that the film surface is composed of larger and compact heaped particulates. There are very few porosities existing between particles. Photodegradation reactions could take place only on external surface. So 1 he rates of dye degradation reactions would be limited. The kinetic condition of degradation reaction would not be as good as that of loosely scattered columnar structure. Therefore the overall degradation activities of TiO, film on glass substrate in this group of experiments are maintained at a relative
conductive substrates. Fig. 7 shows the XRD patterns of Ti02 film on different substrates. Film on substrates of stainless steel, nickel foil and titanium alloy, there is only one dominant anatase orientation. So TiO, grows mainly on one direction of (220). The relative intensity on ( 2 2 0 ) is the strongest one for film on stainless steel among all substrates. So it can be deduced that TiO, film on this support will have well-distributed columnar microstructure. Indeed images of ( F e ) , ( N i ) , ( T i ) show that TiO, particles almost grow in one direction. So it is reasonable that well scattered columnar structure would be obtained while depositing Ti02 film on these three substrates especially on stainless steel. For aluminum alloy substrate, there are three main peaks with almost the same intensity, indicating that TiO, will grow at different directions. Image ( Al) displays an uneven distribution of TiO, particulates; both the crystallite size and the growth direction are different. Therefore well-scattered columnar microstructure is unavailable. T h u s the activity of TiOz film in this situaA(101) I
r . . JL /
1
I
A(220) I
liu /
Silicon r
, L
iower level. Comparing the morphologies of these two inert substrates, it is found that TiO, particles on silicon substrate are larger than that on glass substrate. The film on Si support therefore possesses relative higher activity compared with that on glass. But the
A(200) A(211)
/
h
Al alloy
L U
20
30
40
MI
60
70
80
Z@/(Oj
Fig. 7
XRD patterns of T i 9 film on various substrates
66
Journal of Iron and Steel R e s e a r c h , International
tion shows the lowest value among the conductive substrates. Other possible reasons that cause the enhancement of photodegradation for films on conductive substrates can be elucidated as follows: as U V light irradiated the surface of T i 0 2 film with hu>3. 2 e V , the electron-hole pairs will produce on the surface of film. Generally holes follows located on the outside and electron inside. If these electron-hole pairs are not trapped by acceptor, the electron and hole will recombine and produce heat. No redox reactions will occur. So trapping holes or electrons immediately after their generation will enhance the photodegradation reactions. Using metals as TiO, film substrates can make the photogenerated electrons move to the substrates where they will react with absorbed oxygen and form superoxide anion radical and then lead to the formation of several active oxygen species through the following reactions: 0 2 + e - + O2
O2 - + H + + H O O HOO +HOO +HzOz+Oz HZO,+Oz - + O H +OH-+02 +OHHzOn+eb,-+OH T h u s , hydrogen peroxide, perhydroxyle and hydroxyl radicals can be formed. T h e generation of hydroxyl radicals can enhance the reaction of degradation. Because the side and back surface of conductive material can absorb oxygen, electrons generated by UV light irradiation which is transferred to the conductive substrate can move freely in metal material. T h u s the kinetic condition of degradation reaction is created. Besides the holes will maintain for a long time to catch O H - or H 2 0 and produce oxidant
Vol. 13
of hydroxyl radicals. Therefore employing conductive substrates would be beneficial for improving the efficiency of degradation reaction.
3
Conclusions
When deposition is conducted at lower tempera, TiO, films on metal substrates demture (400 "C) onstrate higher photodegradation efficiency compared with those on inert substrates, no matter what kinds of light source are employed. Generally the decreasing order of TiO, film photoactivity is as follows : TiO, /stainless steel >TiO, / T i alloy >TiOz/ Ni foil>Ti02 /A1 alloy>TiOZ /Si>Ti02 /glass. Stainless steel is one of the best materials for the growth of T i 0 2 films in photodegradation of harmful organic substances. T h e authors would like to express thankfulness to those experts: Dong-hyuk Kim, W. I. Park and Hyun Woong, Park Professor Wong Yong Choi, for their collaboration of doing experiments. References:
c11 c21 c31 c41
[sl
Roberto L Pozzo, Maguel A. Supported Titanium Oxide as Photocatalyst in Water Decontamination [J]. State of Art Catalysis Today, 1997, 93(7): 219-231. Braun A , Pelizzetti E. Photochemical Conversion and Storage of Solar Energy [MI. Dordrecht: Kluwer, 1991. Halmann M. Photodegradation of Water Pollutants [MI. Boca Raton: CRC Press, 1996. Cooper P. Color in Dyehouse Effluent [MI. Landon: Society of Dyers and Colourists Press, 1995. Rachel A , Subrahmanyam M, Boule P. Comparison of Hot* talcaytic Efficiencies of Ti02 in Suspended and Immohilised Form for the Photocatalytic Degradation of Nitrobenzenesulfonic Acids [J]. Applied Catalysis B: Environmental, 2002, 37 ( 3 ) : 301-308.
DIRECTe
JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2006, 1 3 ( 1 ) : 62-66
New Application of Stainless Steel YANG Jia-long’“
,
LI Ying’ ,
WANG Fu’
,
ZANG Zheng-gui’
,
LI Si-jun’
( 1 . College of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning , China; 2. Ironmaking Plant, Wuhan Iron and Steel Group Co, Wuhan 430083, Hubei, China)
Abstract: Several rigid substrates such as stainless steel, titanium alloy, aluminum alloy, nickel foil, silicon, and sodium lime glass have been employed for manufacturing high quality TiO, films by metal organic chemical vapor deposition (MOCVD). T h e as-deposited TiO, films have been characterized with SEM/EDX and XRD. T h e photocatalytic properties were investigated by decomposition of aqueous orange II . UV-VIS photospectrometer was employed to check the absorption characteristics and photocatalytic degradation activity. T h e results show that films synthesized on metal substrates display higher photoactivities than that on absolute substrates such as silicon and glass. It is found that solar light is an alternative to UV-light used for illumination during photodegradation of orange II. Ti02 film on stainless steel substrate was regarded as the best one for photocatalysis. Key words: MOCVD; photodegradation; orange JJ ; metal substrate: inert substrates
Titanium dioxide ( TiO, ) has been extensively investigated in many applications, such as photocatalytic purification, photovoltaic cells, anti-reflective coatings, dielectric layer, optical filters, sensors, fog-proof ( frost-proof ) , anti-cancer, anti-bacterial T h e advantages of using and self-cleaning etc“’ TiO, in the photocatalytic decomposition of organic pollutants are based on its remarkable activity, low cost, chemical and radiation stability and is not prone to or photocorrosion; strong oxidizing agents, such as HzOz, aren’t in needCZ1 ; and it is non-toxic. is a textile azo-dye which is resistant Orange t o light degradation, and not easy t o react with 0, , common acids and bases. It does not undergo biological degradation in waste water treatment plantsi3’. In anaerobic conditions, it can be reduced t o potential carcinogenic aromatic amines. Traditional methods like ultra-filtration, extraction, air stripping, carbon adsorption, incineration and oxidation via ozonatiod4’ or hydrogen peroxide are non-destructive; the pollutant is transferred from one phase to another, and it can’t be eliminated. TiO, photocatalysis is the best choice for degradation of azo-dye. The use of aqueous suspensions limits practical applications due to the problems of separation of fine parti-
.
o3
cles of T i 0 2 and recycling photocatalysts. Therefore, many techniques have been proposed for the immobilization of TiOz on solid supports to solve this problem. Most of the general methods were reported in a recent reviewC51. There are few studies on TiO, photoactivities among different rigid substrates at the same condition. U p t o now, there are no related reports about TiOz deposition on different substrates with MOCVD system t o check their photocatalystic properties. In present study, TiOz film was deposited on various substrates such as austenite stainless steel, glass, titanium alloy, nickel foil, aluminum alloy, silicon ( 100 1 and silicon ( 111) , by MOCVD technique. T h e as-deposited film were used for checking microstructural properties and the photoactivities for degradation dye solution (orange fl 1. Some important conclusions have been reached about the influence of types of substrates and the source of illumination light on the photoactivity of TiO, films. And some unusual performances were discussed.
1
Experimental
Deposition of TiO, thin films was carried out in a homemade low-pressure vertical metal organic
Foundation Item: Item Sponsored by National High-Tech Research and Development Plan (2003AA331080) E-mail: jialongyangB163. com; Biography:YANG Jia-long(1964-) , Male, Doctor, Professoriate Engineer;
Revised Date: August 17, 2004
New Application of Stainless Steel
'go. 1
chemical vapor deposition ( LP-MOCVD ) system. ritanium isopropoxide, TIP(Ti[OCH(CH, ) 2 ] 4 was ised as single molecular precursor. T h e general deptisition conditions are as follows : temperature ranges from 300 "C to 800 C ; Pressure ranges from 13. 33 t o 666. 6 Pa; and the flow rate of diluting gas, T I P carrier gas and O2 are ( 1 - 4 ) X l o p 6 m'/min, (12 ) X l o p 6 m3/min and 2 X m3/min respectively. The deposition time is 240 min. All of the metal substrates such as stainless steel, whose composition (mass percent, % 1 is : Cr 29.5,Ni4.l,Mo1.3,CO.l, Si0.9, Mn1.8and Fe 62. 3 , titanium alloy, whose composition (mass percent, %) is: C 0. 03, N 0.01, Fe 0. 2 , Al 6.34, V 3. 99, 0 0.1 7 , Y
+
63
transparent polymeric container with a diameter of 30 mm and height of 5 mm. T h e solution container was located in a homemade chamber equipped with 200 W mercury lamp that can produce UV and visible light irradiated on TiO, films through a Pyrex filter cover. The light intensity was measured with a power meter (Newport 1815-c). In this study, the intensity of light was maintained to 5. 5 mW cm-'. Each time four samples and a controlled solution were used for irradiation under UV light. All of the experiments were carried out at ambient temperature (25 - 40 'C ) and ordinary pressure. The irradiation time changed from 0. 5 h to 6 h.
2
Results and Discussion
Stainless steel ( PFR41) , titanium alloy ( PT41) , aluminum alloy (PA411, nickel foil (PN411, silicon ( 1 0 0 1 (PS41 and untreated high clean glass (PG41) were employed as substrates of TiO, films deposition. Each sample was deposited by MOCVD within 4 h at an operational temperature of 400 "C, and a thickness of about 2 000 nm was obtained for each sample. 6. 5 mL and 1X 10 ' mol/L orange fl solution was used for each film sample. U V light intensity was maintained at 5. 5 m W / c m z , illuminating time changed from 0. 5 to 6 h. Fig. 1 shows the UV-VIS absorption spectra after 1. 5 h of irradiation. T h e other UV-VIS absorption spectra of specimens with different illuminating time are not shown here. T h e photodegradation proportions or photoactivities were illustrated in Fig. 2. For more quantitative comparison, reaction kinetic analysis is necessary. T h e apparent reaction rate constants of TiO, films 0.40
I
0.30
.
29 0.20
.
a
c
3
B
0.10 .
0. 200
Fig. 1
400 600 Wavelengthhm
800
UV-VIS absorption spectra of photocatalytic degradation of dye solution by TiO, film synthesized on different substrates
64
Vol. 13
Journal of Iron and Steel Research, International
*
100
5.5 4.5
80
3.5
3 2.5
60
9-
= ;
$ c,
40
1.5
0.5 0
20
I
2 4 Irradiation timeh
0
6
0
Fig. 3
Relationship between In(C/Co) and irradiation time
100
80
-
0"
3d C
60
n
40
s 20
0 0
Fig. 2
duced (in Fig. 4). From Fig. 1- 4 , i t can be concluded that TiO, film deposited on metal substrates (high conductivity materials) possesses related higher photocatalytic activity in degradation orange solution compared with that on inert substrates. T h e degrading reaction rate of TiOz films on stainless substrate is four times as high as that on glass substrate. For the same type of conductive materials, there are some differences, TiO, film on stainless steel expressed highest reaction rate. Films on aluminum alloy, however, displayed lowest degrading reaction rate. According to the values of degradation reaction rates, the following consequence can be obtained corresponding to decreasing order of photoactivity : TiO, /stainless steel TiO, /Ti alloy > TiO, /Ni foil TiO, /A1 alloy>TiOZ /Si(100) >TiO, /glass. Fig. 5 shows the FE-SEM top-graph of TiO, film on various substrates deposited at 400 %. It is clear that there are obvious differences in film microstructure for different substrates. For metal substrates, almost all of the TiO, films display loosely
2 4 Irradiation timeih
6
Influence of Ti02 thin film's substrates on activity of photocatalytic degradation of dye solution
on different substrates can be calculated. Generally the photocatlytic degradation follows a Langmuir-hinshelwood mechanismLg1 , indicating that the reaction rate is proportional to the coverage 8. kB=kKc/(l + K c ) (1) where k is true rate constant that includes various parameters such as the mass of catalyst, the flux of efficient photons, the coverage of oxygen dissolved in solution, etc, and K is adsorption constant. Bewas only cause the initial concentration of orange 1X l o p 5 mol/L, and the value is very small, Kc in denominator can be neglected. Therefore the rate becomes the first order: r= -ddC/dt=kK,=k.C (2) where k, is apparent rate constant of pseudo-first order. The solution of the first order equation is as follows: (3) In(C,/C> =k,t Curves in Fig. 2 could he changed into lines in Fig. 3 , and by calculating slops of lines, k, corresponding to films on various substrates could be de-
>
In
n
>
o.902
0.8
0.677
&'
0.4
0.337 0.253
0
Fe
Fig. 4
Ti
Ni Al Substratematerial
Si
Be
Degrading reaction apparent constants of Ti02 film on various substrates
'Qo.1
New Application of Stainless Steel
Fig. 6
*
65
*
FE-SEM images of T i 9 films deposited at 400 "c on titanium alloy (a) and silicon (100) substrate (b)
relative bigger particles are just scattered on the film surface. As shown in Fig. 6 ( b ) , there was even no porosity under the film surface. So it is difficult for this kind of microstructure t o gain higher activity of degradation, hence with a lower activity than that of
Fig. 5
FE-SEM top morphology of T i 9 film ( 2 000 nm) deposited on various substrates at 400 "C
scattered particulates. As particles are separately distributed on the surface, and for TiO, films grown by MOCVD system, the as-produced film usually has a columnar structure because of its eptixial growth characteristics, it is believed that loosely scattered columns should be the main microstructure under the surface of film deposited on metal supports [in Fig. 6 (a)]. This kind of structure of TiO, film can improve the solid-liquid contact condition, thus enhance the activity of photodegradation. Image (Be) shows the top view morphology of TiO, film on substrate of glass. This image illustrates that the film surface is composed of larger and compact heaped particulates. There are very few porosities existing between particles. Photodegradation reactions could take place only on external surface. So 1 he rates of dye degradation reactions would be limited. The kinetic condition of degradation reaction would not be as good as that of loosely scattered columnar structure. Therefore the overall degradation activities of TiO, film on glass substrate in this group of experiments are maintained at a relative
conductive substrates. Fig. 7 shows the XRD patterns of Ti02 film on different substrates. Film on substrates of stainless steel, nickel foil and titanium alloy, there is only one dominant anatase orientation. So TiO, grows mainly on one direction of (220). The relative intensity on ( 2 2 0 ) is the strongest one for film on stainless steel among all substrates. So it can be deduced that TiO, film on this support will have well-distributed columnar microstructure. Indeed images of ( F e ) , ( N i ) , ( T i ) show that TiO, particles almost grow in one direction. So it is reasonable that well scattered columnar structure would be obtained while depositing Ti02 film on these three substrates especially on stainless steel. For aluminum alloy substrate, there are three main peaks with almost the same intensity, indicating that TiO, will grow at different directions. Image ( Al) displays an uneven distribution of TiO, particulates; both the crystallite size and the growth direction are different. Therefore well-scattered columnar microstructure is unavailable. T h u s the activity of TiOz film in this situaA(101) I
r . . JL /
1
I
A(220) I
liu /
Silicon r
, L
iower level. Comparing the morphologies of these two inert substrates, it is found that TiO, particles on silicon substrate are larger than that on glass substrate. The film on Si support therefore possesses relative higher activity compared with that on glass. But the
A(200) A(211)
/
h
Al alloy
L U
20
30
40
MI
60
70
80
Z@/(Oj
Fig. 7
XRD patterns of T i 9 film on various substrates
66
Journal of Iron and Steel R e s e a r c h , International
tion shows the lowest value among the conductive substrates. Other possible reasons that cause the enhancement of photodegradation for films on conductive substrates can be elucidated as follows: as U V light irradiated the surface of T i 0 2 film with hu>3. 2 e V , the electron-hole pairs will produce on the surface of film. Generally holes follows located on the outside and electron inside. If these electron-hole pairs are not trapped by acceptor, the electron and hole will recombine and produce heat. No redox reactions will occur. So trapping holes or electrons immediately after their generation will enhance the photodegradation reactions. Using metals as TiO, film substrates can make the photogenerated electrons move to the substrates where they will react with absorbed oxygen and form superoxide anion radical and then lead to the formation of several active oxygen species through the following reactions: 0 2 + e - + O2
O2 - + H + + H O O HOO +HOO +HzOz+Oz HZO,+Oz - + O H +OH-+02 +OHHzOn+eb,-+OH T h u s , hydrogen peroxide, perhydroxyle and hydroxyl radicals can be formed. T h e generation of hydroxyl radicals can enhance the reaction of degradation. Because the side and back surface of conductive material can absorb oxygen, electrons generated by UV light irradiation which is transferred to the conductive substrate can move freely in metal material. T h u s the kinetic condition of degradation reaction is created. Besides the holes will maintain for a long time to catch O H - or H 2 0 and produce oxidant
Vol. 13
of hydroxyl radicals. Therefore employing conductive substrates would be beneficial for improving the efficiency of degradation reaction.
3
Conclusions
When deposition is conducted at lower tempera, TiO, films on metal substrates demture (400 "C) onstrate higher photodegradation efficiency compared with those on inert substrates, no matter what kinds of light source are employed. Generally the decreasing order of TiO, film photoactivity is as follows : TiO, /stainless steel >TiO, / T i alloy >TiOz/ Ni foil>Ti02 /A1 alloy>TiOZ /Si>Ti02 /glass. Stainless steel is one of the best materials for the growth of T i 0 2 films in photodegradation of harmful organic substances. T h e authors would like to express thankfulness to those experts: Dong-hyuk Kim, W. I. Park and Hyun Woong, Park Professor Wong Yong Choi, for their collaboration of doing experiments. References:
c11 c21 c31 c41
[sl
Roberto L Pozzo, Maguel A. Supported Titanium Oxide as Photocatalyst in Water Decontamination [J]. State of Art Catalysis Today, 1997, 93(7): 219-231. Braun A , Pelizzetti E. Photochemical Conversion and Storage of Solar Energy [MI. Dordrecht: Kluwer, 1991. Halmann M. Photodegradation of Water Pollutants [MI. Boca Raton: CRC Press, 1996. Cooper P. Color in Dyehouse Effluent [MI. Landon: Society of Dyers and Colourists Press, 1995. Rachel A , Subrahmanyam M, Boule P. Comparison of Hot* talcaytic Efficiencies of Ti02 in Suspended and Immohilised Form for the Photocatalytic Degradation of Nitrobenzenesulfonic Acids [J]. Applied Catalysis B: Environmental, 2002, 37 ( 3 ) : 301-308.