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Photocatalytic recovery of Ag ions from wastewater using ZnO nanopowders immobilized on microporous alumina substrates
Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
Photocatalytic recovery of Ag ions from wastewater using ZnO nanopowders immobilized on microporous alumina substrates Sung Park a,∗ , Hye-Jung Park a , Kang Yoo a , Ju-Hyeon Lee b , Jae Chun Lee a a
Department of Materials Science and Engineering, Myongji University, 38-2 San, Nam-dong, Yongin, Kyunggi-do 449-728, Republic of Korea b Department of Electronic Materials Engineering, SunMoon University, Asan, Choongnam 336-708, Republic of Korea Received 14 June 2006; received in revised form 9 October 2006; accepted 9 October 2006 Available online 13 October 2006
Abstract ZnO nanopowders were synthesized by solution combustion method (SCM) and their average particle size was about 30 nm. They were immobilized on microporous alumina substrates by spray coating. The microporous alumina substrates were prepared by anodization and the average pore size was about 150 nm. Ag ions were tried to be recovered from waste photograph developing solution by photocatalytic reaction. Three kinds of samples were prepared to compare their photocatalytic efficiency: commercial ZnO powders (slurry type reaction), SCM ZnO powders immobilized on normal alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates. The commercial ZnO powders without immobilization did not show any photocatalytic reaction even though they have maximum surface area. This is probably ascribed to their poor crystalline quality. However, the Ag ion recovery rate by SCM ZnO powders immobilized on anodized alumina substrates was about 2.5 times higher than that by SCM ZnO powders immobilized on normal alumina substrates. This big difference in photocatalytic efficiency is probably due to the large surface area of the anodized alumina. This result was correspondent to photoluminescence measurement results. © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO nanopowder; Microporous alumina substrate; Photocatalyst; Ag ions; Wastewater
1. Introduction When ZnO is irradiated with UV light of appropriate energy greater than its bandgap, highly mobile electron–hole pairs can be generated. These carriers then migrate to the surface and in turn are trapped by reactants adsorbed on the surface, giving rise to powerful redox chemistry [1]. Therefore, ZnO is of interest as a photocatalyst, especially, in the degradation of environmental pollutants [2]. It has almost the same bandgap energy as titanium dioxide (TiO2 ) and hence its photocatalytic activity is anticipated to be similar to that of TiO2 . But only a limited research has been carried out to realize its full potential as a semiconductor photocatalyst [3,4]. Synthesis of fine ZnO powders was tried by several chemical methods such as spray pyrolysis, sol–gel technique, vapor method, thermal decomposition and precipitation from organic solutions [5–9]. However, the characteristics of ZnO synthesized
∗
Corresponding author. Tel.: +82 31 330 6463; fax: +82 31 330 6457. E-mail address: [email protected] (S. Park).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.10.020
by these methods were not good enough to use for a photocatalyst compared to other materials such as TiO2 . That is the reason why TiO2 is generally used for a photocatalyst. One innovative technique for the synthesis of ZnO powders with high quality is ‘solution combustion method’ which was invented by S. Park et al. [10,11] The solution combustion method begins with the solution which is a mixture of oxidant (such as zinc nitrate) and fuel (such as glycine) dissolved in water. The solution is brought to boil until it ignites and a self-sustaining and rather fast combustion reaction takes place, resulting in high crystalline quality ZnO nanopowders. The combustion reaction can result in the appearance of flames, which can reach temperatures in excess of 1500 ◦ C. In solution combustion method, the energy released from the exothermic reaction between the oxidant and the fuel, which is usually ignited at a temperature much lower than the actual phase transformation temperature, can rapidly heat up the system to a high temperature and sustain it long enough, even in the absence of an external heat source, for the synthesis to occur. Photocatalysis has attracted much attention because of its application potential for water and air purification. Especially,
S. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
for water-treatment, the photocatalyst powder is usually suspended in water, the photoreactor being a slurry reactor. This type of reactor has obvious advantages from the standpoint of the reactor itself. When the photocatalyst powder is perfectly mixed, there is no segregation of phases, and when the photocatalyst powder is small enough, their entire external surface can be irradiated during the reaction time. However, for applications, it cannot be allowed that treated water can still contain solid photocatalyst particles, even though the particles are harmless. Therefore, the remained solid photocatalyst particles should be removed by a liquid–solid separator, the installation and operation of which increase the cost. This is why the immobilization of photocatalyst particles is very important [12–15]. Anodization technique has been known for more than 50 years [16]. This technique provides micropores in aluminum oxide layers, giving rise to large surface areas. These large surface areas of micropores could be utilized to immobilize photocatalyst particles. In this work, ZnO nanopowders synthesized by the solution combustion method were tried to be immobilized on the microporous aluminum oxide layers. The photocatalytic reaction was carried out to evaluate the photocatalytic efficiency of ZnO nanopowders immobilized on the microporous aluminum oxide layers. 2. Experimental procedures 2.1. Synthesis of ZnO nanopowders Zinc nitrate powders [Zn(NO3 )2 ·6H2 O, 3N, High Purity Chemicals Lab., Japan] and glycine powders [H2 NCH2 COOH, Yakuri Pure Chemicals Co. Ltd., Japan] were used for source material(oxidant) and fuel, respectively. The source material was dissolved in distilled water in a beaker and fuel was added to the source material solution. The solution mixture was then heated on a hot plate with stirring. As the distilled water was evaporated, the solution became viscous and generated small bubbles. The viscous solution was then transferred in another stainless steel beaker and heated inside a chamber for ignition. The nitrate group (NO3 − ) reacted with the fuel. At this point, temperature shot up to 1500–1800 ◦ C instantaneously with flame and combustion. This intense heat resulted in instantaneous high pressure, which led to explosion. Therefore, the ZnO powders were formed on high temperature, high pressure and short time reaction conditions. The powders were annealed at 400 ◦ C for 1 h to remove organic residues which might be on powder surface. 2.2. Preparation of microporous alumina substrates by anodization 2.2.1. Pretreatment for anodization Aluminum (1050, 99.5%) and carbon were used for anode and cathode, respectively. The Al plate (1 cm × 2 cm × 6 cm) has been ultrasonically cleaned with DI water and ethanol for 30 min. A solution was prepared by mixing ethanol and perchloric acid with volumetric mixing ratio of 7:1. The solution was again mixed with ethylene glycol monobutyl ether with
31
Fig. 1. Schematic diagram for anodization including (a) aluminum plate, (b) carbon plate, (c) magnetic stirrer and (d) power supplier.
volumetric mixing ratio of 10:1. Using this mixed solution, electropolishing was performed with current density of 250 mA/cm2 at 1–2 ◦ C for 1–2 min. The electropolishing is an electrochemical surface planarization process which requires higher current density than anodization [17]. 2.2.2. Anodization to prepare microporous alumina substrates DC power supplier (HP E3612A model) was used for anodization after electropolishing the Al plate. The electropolished Al and carbon were used for anode and cathode, respectively. The distance between the electrodes was 5 cm. To control the temperature of electrolytic solution, the experiment was carried out in ice bath with magnetic stirring. Fig. 1 shows the schematic diagram for anodization. In general, the pore size of the anodized aluminum oxide depends on kinds of electrolytic solutions. In this work, 0.2 M phosphoric acid was selected to obtain relatively large pores. The Al plate has been anodized at 130V (∼170 mA) for 3 h. The temperature of electrolytic solutions was kept at 5–10 ◦ C. The porous aluminum oxide was cleaned several times with DI water and cut into 1 cm × 1 cm pieces. The synthesized ZnO nanopowders were spray-coated on the anodized alumina substrates. The anodized alumina was characterized by FTIR spectrometer (Perkin-Elmer, System2000), X-ray difractometer (Philips, XRD1825/00) and superprobe (JEOL, JXA-8800SX). The ZnO nanopowders immobilized on the anodized alumina substrates were investigated using photoluminescence (PL) emission spectra (SLM-Aminco spectrofluorometer), SEM (JEOL ABT DX-130S:3 kV) and photocatalytic reaction to remove Ag ions from waste photograph developing solution. 3. Results and discussion To confirm the composition of the anodized aluminum oxide, Al–O vibration characteristics were measured by the FTIR spectrometer. The substrate aluminum has been dissolved by
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S. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
Fig. 2. FTIR spectrum of anodized alumina.
saturated mercury chloride for 30 min in order to separate anodized aluminum oxide layer from the substrate aluminum layer. The separated aluminum oxide layer was cleaned several times by DI water and ethanol. After dry, sample pallets were prepared by mixing the separated aluminum oxide powders and KBr powders. Fig. 2 shows the FTIR measurement result. A weak Al–O bonding absorption peak was observed at near 1150 cm−1 . Strong Al–O stretching mode absorption peaks were also observed at 550–850 cm−1 [17–19]. This result confirms the formation of aluminum oxide layers on aluminum substrates. The microporous aluminum oxide layers were prepared by the voltage reduction sequence method which has been known to provide uniform and pure aluminum oxide layers [20]. XRD measurements were carried out to examine the crystallinity of the anodized aluminum oxide layers. As shown in Fig. 3, a broad band peak was observed at near 15–40◦ . This indicates that the anodized aluminum oxide layers were amorphous rather than crystalline [21]. The surfaces of the anodized aluminum oxide layers were examined by superprobe. As shown in Fig. 4(a), the average pore size was about 150 nm. According to Shawaqfeh and Balus [20], the pore diameter is proportional to applied voltage
Fig. 3. XRD pattern of anodized alumina.
Fig. 4. SEM images of (a) surface and (b) cross-sectional area of anodized alumina.
difference. For example, 1 V difference results in pores of 1.0 nm. Based on this simple calculation, the pores of 130 nm could be obtained by this work in which 130 V was applied for the anodization process. The result of this work (150 nm) is relatively well correspondent to that of the theoretical calculation (130 nm). Fig. 4(b) shows the cross-sectional SEM image of the anodized alumina substrate. Actually, the pores went through the alumina and formed through holes. The photocatalytic reaction was performed to evaluate the photocatalytic efficiency of ZnO nanopowders immobilized on the microporous aluminum oxide layers. Three kinds of samples were prepared for the photocatalytic reaction: commercial ZnO powders (Junsei, Japan), SCM ZnO powders immobilized on alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates. The initial concentration of Ag ions in waste photograph developing solution was about 115 ppm. As shown in Fig. 5(A), the commercial ZnO powders did not show any photocatalytic reaction even though it was a slurry type reactor which has maximum surface areas for the reaction. This means that the surface property of the commercial ZnO powders was not good enough for photocatalytic reaction.
S. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
33
Fig. 5. Photocatalytic removal of Ag ions from wastewater by (A) commercial ZnO powders, (B) SCM ZnO powders immobilized on an alumina substrate and (C) SCM ZnO powders immobilized on an anodized alumina substrate.
The ZnO powders might have a lot of defects on powder surface which act as recombination centers for the generated electrons and holes. However, Fig. 5(B) shows that the SCM ZnO powders immobilized on normal alumina substrates remove Ag ions completely at UV irradiation time of 120 min when the graph is extrapolated, even though they have smaller surface area than that of slurry type. On the other hand, the SCM ZnO powders immobilized on anodized alumina substrates removed Ag ions completely at UV irradiation time of 45 min (Fig. 5(C)). This big difference in photocatalytic efficiency is probably due to the large surface area of the anodized alumina. Fig. 6 represents the SEM photograph showing the SCM ZnO powders immobilized on an anodized alumina substrate. The anodized alumina substrate provided rough surfaces which increased the surface areas of immobilized SCM ZnO powders. Furthermore, the SCM ZnO powders were immobilized even in the inside of pores of the anodized alumina substrate. These two factors allowed us to have large surface areas of the immobilized SCM ZnO powders.
Fig. 6. SEM photograph showing the SCM ZnO powders immobilized on an anodized alumina substrate.
Fig. 7. PL intensity from (a) SCM ZnO powders immobilized on an anodized alumina substrate and (b) SCM ZnO powders immobilized on an alumina substrate.
As shown in Fig. 7, the PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates (Fig. 7(a)) was twice higher than that from the SCM ZnO powders immobilized on alumina substrates (Fig. 7(b)). This is considered to be due to the large surface area of the anodized alumina. Both PL spectra show only band-to-band transition peaks centered at near 385 nm. This indicates that the SCM ZnO powders do not have defect levels inside band gap which could act as recombination centers for the electrons and holes generated by UV irradiation. Electrons showing the band-to-band transition are major contributors to the photocatalytic reaction (reduction of Ag+ ions). The high PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates means that more electrons were generated and resulted in higher photocatalytic efficiency. 4. Conclusions ZnO nanopowders were synthesized by SCM and their average particle size was about 30 nm. To immobilize the SCM ZnO nanopowders, the microporous alumina substrates were prepared by anodization and the average pore size was about 150 nm. In order to evaluate their photocatalytic efficiencies, three kinds of samples such as commercial ZnO powders, SCM ZnO powders immobilized on alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates were prepared. Using these three samples as photocatalysts, Ag ions were recovered from waste photograph developing solution. The Ag ion recovery rate of the SCM ZnO powders immobilized on the anodized alumina substrates was about 2.5 times higher than that of SCM ZnO powders immobilized on normal alumina substrates. This was correspondent to the PL measurement result showing that the PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates was twice higher than that from the SCM ZnO powders immobilized on alumina substrates. This high photocatalytic efficiency seems to be attributed to the large surface area of anodized alumina.
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References [1] A. Mills, S.L. Hunte, J. Photochem. Photobiol. 108 (1997) 1–35. [2] J. Villasenor, P. Reyes, G. Pecchi, J. Chem. Technol. Biotechnol. 72 (1998) 105. [3] H.D. Mansilla, M.C. Yeber, J. Freer, J. Rodriguez, J. Baeza, Water Sci. Technol. 35 (4) (1997) 273. [4] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65. [5] M. Muhammed, Analysis 24 (1996) M12. [6] T. Liu, O. Sakurai, N. Mizutani, M. Kato, J. Mater. Sci. 21 (1986) 3698. [7] G. Westin, M. Nygren, J. Mater. Sci. 27 (1992) 1617. [8] E.A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566. [9] B. Chiou, Y.J. Tsai, J. Duh, J. Mater. Sci. Lett. 7 (1988) 785. [10] S. Park, D.W. Lee, J.C. Lee, J.H. Lee, J. Am. Ceram. Soc. 86 (2003) 1508.
[11] S. Park, K.R. Lee, K.W. Lee, J.H. Lee, J. Mater. Sci. Lett. 22 (2003) 65. [12] N. Serpone, E. Borgarello, R. Harris, P. Cahill, M. Borgarello, E. Pelizzetti, Sol. Energy Mater. 14 (1986) 121. [13] R.W. Matthews, J. Phys. Chem. 91 (1987) 3323. [14] R.W. Matthews, Sol. Energy 38 (1987) 405. [15] H. Al-Ekabi, N. Serpone, J. Phys. Chem. 92 (1988) 5726. [16] F. Keller, M.S. Hunter, D.L. Robinson, J. Electrochem. Soc. 100 (1953) 411. [17] G. Patermarakis, K. Moussoutzanis, Electochem. Acta 6 (1995) 699. [18] S. Ramesh, J. Am. Ceram. Soc. 83 (1) (2000) 89. [19] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Inc., 1996. [20] A.T. Shawaqfeh, R.E. Balus, J. Membr. Sci. 157 (1986) 147. [21] C.H. Lee, Y.M. Hahm, H.S. Kang, Y.H. Chang, J. Korean Ind. Chem. 3 (1998) 1047.
Photocatalytic recovery of Ag ions from wastewater using ZnO nanopowders immobilized on microporous alumina substrates Sung Park a,∗ , Hye-Jung Park a , Kang Yoo a , Ju-Hyeon Lee b , Jae Chun Lee a a
Department of Materials Science and Engineering, Myongji University, 38-2 San, Nam-dong, Yongin, Kyunggi-do 449-728, Republic of Korea b Department of Electronic Materials Engineering, SunMoon University, Asan, Choongnam 336-708, Republic of Korea Received 14 June 2006; received in revised form 9 October 2006; accepted 9 October 2006 Available online 13 October 2006
Abstract ZnO nanopowders were synthesized by solution combustion method (SCM) and their average particle size was about 30 nm. They were immobilized on microporous alumina substrates by spray coating. The microporous alumina substrates were prepared by anodization and the average pore size was about 150 nm. Ag ions were tried to be recovered from waste photograph developing solution by photocatalytic reaction. Three kinds of samples were prepared to compare their photocatalytic efficiency: commercial ZnO powders (slurry type reaction), SCM ZnO powders immobilized on normal alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates. The commercial ZnO powders without immobilization did not show any photocatalytic reaction even though they have maximum surface area. This is probably ascribed to their poor crystalline quality. However, the Ag ion recovery rate by SCM ZnO powders immobilized on anodized alumina substrates was about 2.5 times higher than that by SCM ZnO powders immobilized on normal alumina substrates. This big difference in photocatalytic efficiency is probably due to the large surface area of the anodized alumina. This result was correspondent to photoluminescence measurement results. © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO nanopowder; Microporous alumina substrate; Photocatalyst; Ag ions; Wastewater
1. Introduction When ZnO is irradiated with UV light of appropriate energy greater than its bandgap, highly mobile electron–hole pairs can be generated. These carriers then migrate to the surface and in turn are trapped by reactants adsorbed on the surface, giving rise to powerful redox chemistry [1]. Therefore, ZnO is of interest as a photocatalyst, especially, in the degradation of environmental pollutants [2]. It has almost the same bandgap energy as titanium dioxide (TiO2 ) and hence its photocatalytic activity is anticipated to be similar to that of TiO2 . But only a limited research has been carried out to realize its full potential as a semiconductor photocatalyst [3,4]. Synthesis of fine ZnO powders was tried by several chemical methods such as spray pyrolysis, sol–gel technique, vapor method, thermal decomposition and precipitation from organic solutions [5–9]. However, the characteristics of ZnO synthesized
∗
Corresponding author. Tel.: +82 31 330 6463; fax: +82 31 330 6457. E-mail address: [email protected] (S. Park).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.10.020
by these methods were not good enough to use for a photocatalyst compared to other materials such as TiO2 . That is the reason why TiO2 is generally used for a photocatalyst. One innovative technique for the synthesis of ZnO powders with high quality is ‘solution combustion method’ which was invented by S. Park et al. [10,11] The solution combustion method begins with the solution which is a mixture of oxidant (such as zinc nitrate) and fuel (such as glycine) dissolved in water. The solution is brought to boil until it ignites and a self-sustaining and rather fast combustion reaction takes place, resulting in high crystalline quality ZnO nanopowders. The combustion reaction can result in the appearance of flames, which can reach temperatures in excess of 1500 ◦ C. In solution combustion method, the energy released from the exothermic reaction between the oxidant and the fuel, which is usually ignited at a temperature much lower than the actual phase transformation temperature, can rapidly heat up the system to a high temperature and sustain it long enough, even in the absence of an external heat source, for the synthesis to occur. Photocatalysis has attracted much attention because of its application potential for water and air purification. Especially,
S. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
for water-treatment, the photocatalyst powder is usually suspended in water, the photoreactor being a slurry reactor. This type of reactor has obvious advantages from the standpoint of the reactor itself. When the photocatalyst powder is perfectly mixed, there is no segregation of phases, and when the photocatalyst powder is small enough, their entire external surface can be irradiated during the reaction time. However, for applications, it cannot be allowed that treated water can still contain solid photocatalyst particles, even though the particles are harmless. Therefore, the remained solid photocatalyst particles should be removed by a liquid–solid separator, the installation and operation of which increase the cost. This is why the immobilization of photocatalyst particles is very important [12–15]. Anodization technique has been known for more than 50 years [16]. This technique provides micropores in aluminum oxide layers, giving rise to large surface areas. These large surface areas of micropores could be utilized to immobilize photocatalyst particles. In this work, ZnO nanopowders synthesized by the solution combustion method were tried to be immobilized on the microporous aluminum oxide layers. The photocatalytic reaction was carried out to evaluate the photocatalytic efficiency of ZnO nanopowders immobilized on the microporous aluminum oxide layers. 2. Experimental procedures 2.1. Synthesis of ZnO nanopowders Zinc nitrate powders [Zn(NO3 )2 ·6H2 O, 3N, High Purity Chemicals Lab., Japan] and glycine powders [H2 NCH2 COOH, Yakuri Pure Chemicals Co. Ltd., Japan] were used for source material(oxidant) and fuel, respectively. The source material was dissolved in distilled water in a beaker and fuel was added to the source material solution. The solution mixture was then heated on a hot plate with stirring. As the distilled water was evaporated, the solution became viscous and generated small bubbles. The viscous solution was then transferred in another stainless steel beaker and heated inside a chamber for ignition. The nitrate group (NO3 − ) reacted with the fuel. At this point, temperature shot up to 1500–1800 ◦ C instantaneously with flame and combustion. This intense heat resulted in instantaneous high pressure, which led to explosion. Therefore, the ZnO powders were formed on high temperature, high pressure and short time reaction conditions. The powders were annealed at 400 ◦ C for 1 h to remove organic residues which might be on powder surface. 2.2. Preparation of microporous alumina substrates by anodization 2.2.1. Pretreatment for anodization Aluminum (1050, 99.5%) and carbon were used for anode and cathode, respectively. The Al plate (1 cm × 2 cm × 6 cm) has been ultrasonically cleaned with DI water and ethanol for 30 min. A solution was prepared by mixing ethanol and perchloric acid with volumetric mixing ratio of 7:1. The solution was again mixed with ethylene glycol monobutyl ether with
31
Fig. 1. Schematic diagram for anodization including (a) aluminum plate, (b) carbon plate, (c) magnetic stirrer and (d) power supplier.
volumetric mixing ratio of 10:1. Using this mixed solution, electropolishing was performed with current density of 250 mA/cm2 at 1–2 ◦ C for 1–2 min. The electropolishing is an electrochemical surface planarization process which requires higher current density than anodization [17]. 2.2.2. Anodization to prepare microporous alumina substrates DC power supplier (HP E3612A model) was used for anodization after electropolishing the Al plate. The electropolished Al and carbon were used for anode and cathode, respectively. The distance between the electrodes was 5 cm. To control the temperature of electrolytic solution, the experiment was carried out in ice bath with magnetic stirring. Fig. 1 shows the schematic diagram for anodization. In general, the pore size of the anodized aluminum oxide depends on kinds of electrolytic solutions. In this work, 0.2 M phosphoric acid was selected to obtain relatively large pores. The Al plate has been anodized at 130V (∼170 mA) for 3 h. The temperature of electrolytic solutions was kept at 5–10 ◦ C. The porous aluminum oxide was cleaned several times with DI water and cut into 1 cm × 1 cm pieces. The synthesized ZnO nanopowders were spray-coated on the anodized alumina substrates. The anodized alumina was characterized by FTIR spectrometer (Perkin-Elmer, System2000), X-ray difractometer (Philips, XRD1825/00) and superprobe (JEOL, JXA-8800SX). The ZnO nanopowders immobilized on the anodized alumina substrates were investigated using photoluminescence (PL) emission spectra (SLM-Aminco spectrofluorometer), SEM (JEOL ABT DX-130S:3 kV) and photocatalytic reaction to remove Ag ions from waste photograph developing solution. 3. Results and discussion To confirm the composition of the anodized aluminum oxide, Al–O vibration characteristics were measured by the FTIR spectrometer. The substrate aluminum has been dissolved by
32
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Fig. 2. FTIR spectrum of anodized alumina.
saturated mercury chloride for 30 min in order to separate anodized aluminum oxide layer from the substrate aluminum layer. The separated aluminum oxide layer was cleaned several times by DI water and ethanol. After dry, sample pallets were prepared by mixing the separated aluminum oxide powders and KBr powders. Fig. 2 shows the FTIR measurement result. A weak Al–O bonding absorption peak was observed at near 1150 cm−1 . Strong Al–O stretching mode absorption peaks were also observed at 550–850 cm−1 [17–19]. This result confirms the formation of aluminum oxide layers on aluminum substrates. The microporous aluminum oxide layers were prepared by the voltage reduction sequence method which has been known to provide uniform and pure aluminum oxide layers [20]. XRD measurements were carried out to examine the crystallinity of the anodized aluminum oxide layers. As shown in Fig. 3, a broad band peak was observed at near 15–40◦ . This indicates that the anodized aluminum oxide layers were amorphous rather than crystalline [21]. The surfaces of the anodized aluminum oxide layers were examined by superprobe. As shown in Fig. 4(a), the average pore size was about 150 nm. According to Shawaqfeh and Balus [20], the pore diameter is proportional to applied voltage
Fig. 3. XRD pattern of anodized alumina.
Fig. 4. SEM images of (a) surface and (b) cross-sectional area of anodized alumina.
difference. For example, 1 V difference results in pores of 1.0 nm. Based on this simple calculation, the pores of 130 nm could be obtained by this work in which 130 V was applied for the anodization process. The result of this work (150 nm) is relatively well correspondent to that of the theoretical calculation (130 nm). Fig. 4(b) shows the cross-sectional SEM image of the anodized alumina substrate. Actually, the pores went through the alumina and formed through holes. The photocatalytic reaction was performed to evaluate the photocatalytic efficiency of ZnO nanopowders immobilized on the microporous aluminum oxide layers. Three kinds of samples were prepared for the photocatalytic reaction: commercial ZnO powders (Junsei, Japan), SCM ZnO powders immobilized on alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates. The initial concentration of Ag ions in waste photograph developing solution was about 115 ppm. As shown in Fig. 5(A), the commercial ZnO powders did not show any photocatalytic reaction even though it was a slurry type reactor which has maximum surface areas for the reaction. This means that the surface property of the commercial ZnO powders was not good enough for photocatalytic reaction.
S. Park et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 30–34
33
Fig. 5. Photocatalytic removal of Ag ions from wastewater by (A) commercial ZnO powders, (B) SCM ZnO powders immobilized on an alumina substrate and (C) SCM ZnO powders immobilized on an anodized alumina substrate.
The ZnO powders might have a lot of defects on powder surface which act as recombination centers for the generated electrons and holes. However, Fig. 5(B) shows that the SCM ZnO powders immobilized on normal alumina substrates remove Ag ions completely at UV irradiation time of 120 min when the graph is extrapolated, even though they have smaller surface area than that of slurry type. On the other hand, the SCM ZnO powders immobilized on anodized alumina substrates removed Ag ions completely at UV irradiation time of 45 min (Fig. 5(C)). This big difference in photocatalytic efficiency is probably due to the large surface area of the anodized alumina. Fig. 6 represents the SEM photograph showing the SCM ZnO powders immobilized on an anodized alumina substrate. The anodized alumina substrate provided rough surfaces which increased the surface areas of immobilized SCM ZnO powders. Furthermore, the SCM ZnO powders were immobilized even in the inside of pores of the anodized alumina substrate. These two factors allowed us to have large surface areas of the immobilized SCM ZnO powders.
Fig. 6. SEM photograph showing the SCM ZnO powders immobilized on an anodized alumina substrate.
Fig. 7. PL intensity from (a) SCM ZnO powders immobilized on an anodized alumina substrate and (b) SCM ZnO powders immobilized on an alumina substrate.
As shown in Fig. 7, the PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates (Fig. 7(a)) was twice higher than that from the SCM ZnO powders immobilized on alumina substrates (Fig. 7(b)). This is considered to be due to the large surface area of the anodized alumina. Both PL spectra show only band-to-band transition peaks centered at near 385 nm. This indicates that the SCM ZnO powders do not have defect levels inside band gap which could act as recombination centers for the electrons and holes generated by UV irradiation. Electrons showing the band-to-band transition are major contributors to the photocatalytic reaction (reduction of Ag+ ions). The high PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates means that more electrons were generated and resulted in higher photocatalytic efficiency. 4. Conclusions ZnO nanopowders were synthesized by SCM and their average particle size was about 30 nm. To immobilize the SCM ZnO nanopowders, the microporous alumina substrates were prepared by anodization and the average pore size was about 150 nm. In order to evaluate their photocatalytic efficiencies, three kinds of samples such as commercial ZnO powders, SCM ZnO powders immobilized on alumina substrates and SCM ZnO powders immobilized on anodized alumina substrates were prepared. Using these three samples as photocatalysts, Ag ions were recovered from waste photograph developing solution. The Ag ion recovery rate of the SCM ZnO powders immobilized on the anodized alumina substrates was about 2.5 times higher than that of SCM ZnO powders immobilized on normal alumina substrates. This was correspondent to the PL measurement result showing that the PL intensity from the SCM ZnO powders immobilized on anodized alumina substrates was twice higher than that from the SCM ZnO powders immobilized on alumina substrates. This high photocatalytic efficiency seems to be attributed to the large surface area of anodized alumina.
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