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Effect of PVP molecular weight on the formation of Ag nanoparticles on echinoid-like TiO2
Materials Letters 96 (2013) 214–217
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of PVP molecular weight on the formation of Ag nanoparticles on echinoid-like TiO2 Inseok Jang, Kyungho Song, Jun-Hwan Park, Minkyung Kim, Dae-Wook Kim, Seong-Geun Oh n Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2012 Accepted 12 January 2013 Available online 23 January 2013
Ag nanoparticles were immobilized on echinoid-like TiO2 particles via an alcohol reduction process with poly(vinyl pyrrolidone) (PVP) of different molecular weights (Mw.). The sizes of Ag nanoparticles prepared by using PVP K15 (Mw.: 10,000) and K30 (Mw.: 40,000) were 12–15 nm and 19–31 nm, respectively. On the other hand, Ag nanoparticles were not observed when PVP K12 (Mw.: 3500) was used. The crystal structure of Ag nanoparticles formed on TiO2 was face-centered cubic silver phase and the chemical states of Ag nanoparticles were Ag þ and Ag0. The photocatalytic performance was enhanced by Ag deposition. Among the samples, the prepared Ag/TiO2 composite by using PVP K15 exhibited better photocatalytic activity than other composites due to its higher Ag coverage on the TiO2 surface. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ag nanoparticles Echinoid-like TiO2 Poly(vinyl pyrrolidone) Alcohol reduction Photocatalytic activity
1. Introduction In recent years, the metal/inorganic composite particles have attracted much attention in the fields of antibacterial material, photonic crystals, and photocatalyst [1–3] due to their outstanding features such as electric conductivity, photovoltaic performance, and photocatalytic activity [4–6]. Various processes have been proposed to prepare metal/inorganic composites: selfassembly, electroless plating, ultrasound irradiation methods, and so on [7–9]. However, these synthesis methods usually need chemical reducing agents and it is difficult to control the particle size since the reaction rate is too fast. These problems can be overcome by using the alcohol reduction method known as a simple and chemical reducing agent free method. In this method, poly(vinyl pyrrolidone) (PVP) is generally employed as a reduction potential accelerator and a stabilizer because the electron lone pairs on imide groups in PVP easily promote the nucleation of metal ions and PVP prevents Ag nanoparticles from agglomerating through the steric effect [10]. Shin et al. reported how the growth of the Ag nanoparticles is affected by variation in the molecular weight of PVP [11]. Also, Chou et al. investigated the effect of PVP molecular weights on the formation Ag nanoparticles. They found that the size of Ag nanoparticles could be controlled by using different molecular weights of PVP [12]. In this study, Ag nanoparticles were deposited on echinoid-like TiO2 particles having a large surface area via the alcohol reduction method. In order to improve photocatalytic activity, the size and
n
Corresponding author. Tel.: þ82 2 2220 0485; fax: þ82 2 2294 4568. E-mail address: [email protected] (S.-G. Oh).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.045
coverage of Ag nanoparticles were controlled by using PVP (K12, K15, and K30) with different molecular weights.
2. Experimental Synthesis of echinoid-like TiO2 particles: Echinoid-like TiO2 particles were synthesized by the same process reported in our previous research [13]. Poly(vinyl alcohol) (PVA, 0.6 g) was dissolved in dimethyl sulfoxide (DMSO, 18 mL) under stirring at 100 1C for 1 h. Titanium tetraisopropoxide (TTIP, 9 mL) was added into 2-propanol (30 mL) under stirring "at room temperature for 1 h. PVA/ DMSO solution and TTIP/2-propanol solution were added dropwise into acetic acid (600 mL). The resultant solution was refluxed at 100 1C for 4 days. After the reaction, the synthesized particles were washed with acetone and dried. Finally, the particles were calcined at 450 1C for 2 h. Preparation of Ag/TiO2 composites: For the modification of TiO2 surface with amine groups as a chemical binder between Ag and TiO2 particles [14], the synthesized TiO2 powder (0.1 wt%) was dispersed in 3-aminopropyltrimethoxysilane aqueous solution (1.0 wt%). The obtained particles were washed with distilled water and then re-dispersed in ethanol. PVP (4.0 wt%) and silver nitrate (0.01 wt%) were added into the solution and the mixture was heated at 78 1C for 4 h. After the reaction, the prepared Ag/ TiO2 composites were washed with ethanol and dried. The echinoid-like TiO2 and Ag/TiO2 composites synthesized by using PVP K12, K15, and K30 were identified as ET, ST12, ST15, and ST30, respectively. Investigation into the photocatalytic activities of Ag/TiO2 composites: Ag/TiO2 composites were calcined at 450 1C for 2 h to remove the residual organic reagents completely and all samples
I. Jang et al. / Materials Letters 96 (2013) 214–217
were treated through the hydroxylation process for the functionalization of TiO2 surface with hydroxy groups [13]. Methylene blue (MB) was used to evaluate the photocatalytic property of samples and commercial P25 was employed as a reference. The 0.05 g of the powder was added into 100 mL of MB aqueous solution (10 ppm) and then the photocatalytic reaction was performed under UV irradiation. The experiments were repeated three times and the standard deviation is shown with error bars.
3. Results and discussion As shown in Fig. 1 (a), no Ag nanoparticles were observed in ST12. On the contrary, the Ag nanoparticles of ST15 and ST30, respectively were well deposited on the surface of TiO2 particles in Fig. 1(b) and (c). The sizes of Ag nanoparticles of ST15 and ST30 were 12–15 nm and 19–31 nm, respectively. The average size and the size distribution of Ag nanoparticles in ST15 were smaller and narrower than those of ST30. The SAED pattern of ST15 exhibits the (101) of anatase and (200) of the face-centered cubic (FCC) silver phases as illustrated in Fig. 1(d). In Fig. 1(e), the XRD pattern of ST12 shows
215
only the anatase crystal structure of TiO2 particles (JCPDS No. 211272). On the contrary, the diffraction patterns of ST15 and ST30 display not only the anatase phase but also the peak at 2y ¼44.2771 (200) of the FCC silver phase (JCPDS No. 04-0783), indicating that the Ag nanoparticles were successfully immobilized on the TiO2 surface. The Ag peak intensity of ST15 was higher than that of ST30 and it means that more grafted Ag nanoparticles exist in ST15. The XPS survey spectra of ET and ST15 are shown in Fig. 1(f). ET contains Ti, O, and C elements, which are assigned to Ti 2p, O 1s, and C 1s, respectively, and Ag 3d photoelectron peak was additionally observed in the ST15 [15]. The carbon peak is attributed to the residual carbon from the sample and/or adventitious hydrocarbon from the XPS instrument itself. Fig. 1 (g) shows the Ti 2p peaks of ET. The splitting width between Ti 2p1/2 and Ti 2p3/2 was 5.7 eV, indicating a normal state of Ti4 þ [15]. The shift of Ti 2p peaks and their smaller intensities after Ag deposition demonstrate that Ag nanoparticles were chemically adsorbed on TiO2 surface [16,17]. In Fig. 1(h), two O 1s peaks in ET were found and the earlier and latter peaks are attributed to oxygen on the surface and in the bulk of TiO2, respectively [15]. After Ag deposition, the O 1s surface peak shifted to lower binding energy, resulting from the change of
Fig. 1. HR-TEM images of ST12 (a), ST15 (b), ST30 (c), SAED pattern of ST15 (d), XRD patterns (e), XPS survey spectra (f), high-resolution XPS spectra of Ti 2p (g), O 1s (h), and Ag 3d (i).
216
I. Jang et al. / Materials Letters 96 (2013) 214–217
chemical environment of oxygen atoms by Ag immobilization [18]. Fig. 1(i) shows the Ag 3d3/2 and Ag 3d5/2 peaks of Ag/TiO2 composites. The Ag 3d5/2 peaks consist of Ag2O (Ag þ ) and Ag (Ag0) [15,18]. The Ag 3d5/2 peaks of ST12 show that Ag2O (Ag þ ) peak is predominant over Ag (Ag0) peak although a weak and broad Ag (Ag0) peak was observed on the ST12 from XPS data. It indicates that the reduction of Ag þ did not sufficiently occur since the polyvinyl chain of PVP K12 is too short to accelerate the reduction potential [14]. Therefore, Ag particles in ST12 might be too small to be detected from HR-TEM and XRD pattern. On the other hand, the Ag (Ag0) peaks in the Ag 3d5/2 peaks of ST15 and ST30 are superior to the Ag2O (Ag þ ). The Ag (Ag0) peak of ST15 has a higher intensity and narrower FWHM compared with those of ST30. These results indicate that more Ag particles were formed on TiO2 surface in ST15 than those of ST30 [17,18]. From the above results, PVP K15 showed the most excellent reduction potential in the alcohol reduction process and ST15 had the highest Ag coverage on TiO2 among the samples. The estimations for properties of Ag/TiO2 composites are summarized in Table 1. Fig. 2 describes the mechanism for formation of Ag/TiO2 composites which is influenced by the molecular weight of PVP in the system. Molecular weight of PVP K30 is higher and the kinetic behavior of chains is more complex than those of PVP K15 in the solution [11]. Thus, the closer capped Ag ions within chains of PVP K30 can be aggregated more easily as the Ag ions are reduced and Table 1 Estimations for the properties of Ag/TiO2 composites. Sample PVP molecular weight
ST12 ST15 ST30 n
3500 10,000 40,000
HR-TEM
XPS
Size of Ag (nm)
FWHM of Ag0 (eV)
n
– 12–15 19–31
1.62 1.29 1.91
31.45 83.43 72.14
Ag0 atom (%)
Ag0 atom (%) ¼Ag0/(Ag0 þ Ag þ ) 100.
Fig. 3. Photocatalytic activities of samples on the degradation of methylene blue.
grown. Therefore, ST30 has a larger average size and a wider size distribution of Ag particles. As shown in Fig. 3, the photocatalytic performance of echinoidlike TiO2 was higher than that of P25 and the photocatalytic activity was enhanced after Ag deposition. This is because the recombination was prevented by Ag immobilization with the formation of Schottky barrier [19]. ST15 showed the higher decomposition efficiency of MB than that of ST30 due to its higher Ag coverage and this result can be explained by the following reasons. First of all, the reflection of the incident light increases with the growth of the Ag nanoparticles. This reduces the number of photons absorbed by TiO2 and thus lowers the photo-quantum efficiency of photocatalytic reaction [20]. Secondly, the electrons are more accumulated in Ag nanoparticles as the size of Ag nanoparticles increases. As a result, positively charged holes will be attracted to the negatively charged Ag nanoparticles by electrostatic interaction. Thus, the prepared Ag/TiO2 composite by using PVP K15 showed the highest efficiency for MB degradation. 4. Conclusions In summary, we reported that Ag nanoparticles were immobilized on echinoid-like TiO2 particles via an alcohol reduction process using PVP with different molecular weights (PVP K12, K15, and K30). Ag nanoparticles were successfully formed on the TiO2 surface when PVP K15 and K30 were used. However, Ag nanoparticles were not observed when PVP K12 was used. The crystal structure and main chemical state, respectively of the Ag nanoparticles of ST15 and ST30 were FCC phase and Ag (Ag0). The degradation of MB was improved by the grafted Ag nanoparticles. The prepared Ag/TiO2 composite by using PVP K15 exhibited better photocatalytic activity than those of other samples due to their higher Ag coverage on TiO2 surface. Acknowledgment This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded from the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2012-0000591). References
Fig. 2. Schematic illustration for the effect of PVP molecular weight on the formation of ST15 (a) and ST30 (b).
[1] Amin SA, Pazouki M, Hosseinnia A. Powder Technol 2009;196:241–5. [2] Wang Z, Chan CT, Zhang W, Ming N, Sheng P. Phys Rev B 2001;64:113108.
I. Jang et al. / Materials Letters 96 (2013) 214–217
[3] [4] [5] [6] [7] [8] [9] [10] [11]
Chen CW, Serizawa T, Akashi M. Chem Mater 1999;11:1381–9. Croce F, Appetecchi GB, Persi L, Scrosati B. Nature 1998;394:456–8. Pinnavaia TJ. Science 1983;220:365–71. Winiarz JG, Zhang L, Lal M, Friend CS, Prasad PN. J Am Chem Soc 1999;121:5287–95. Dokoutchaev A, James JT, Koene SC, Pathak S, Prakash GKS, Thompson ME. Chem Mater 1999;11:2389–99. Kobayashi Y, Tadaki Y, Nagao D, Konno M. J Colloid Interf Sci 2005;283:601–4. Pol VG, Grisaru H, Gedanken A. Langmuir 2005;21:3635–40. Zhang Z, Zhao B, Hu L. J Solid State Chem 1996;121:105–10. Shin HS, Yang HJ, Kim SB, Lee MS. J Colloid Interf Sci 2004;274:89–94.
[12] [13] [14] [15] [16] [17] [18] [19] [20]
217
Chou KS, Lai YS. Mater Chem Phys 2004;83:82–8. Jang I, Song K, Oh SG. Chem Lett 2012;41:173–4. Lee JM, Jun YD, Kim DW, Lee YH, Oh SG. Mater Chem Phys 2009;114:549–55. Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Minnesota: Perkin-Elmer Corporation; 1992. Amarjargal A, Tijing LD, Kim CS. Dig J Nanomater Biostruct 2011;6:1957–65. Luo St. K, Clair TP, Lai X, Goodman DW. J Phys Chem B 2000;104:3050–7. Han Z, Zhang J, Yu Y, Cao W. Mater Lett 2012;70:193–6. Linsebigler AL, Lu G, Yates JT. Chem Rev 1995;95:735–58. Vamathevan V, Amal R, Beydoun D, Low G, McEvoy S. J Photochem Photobiol A Chem 2002;148:233–45.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of PVP molecular weight on the formation of Ag nanoparticles on echinoid-like TiO2 Inseok Jang, Kyungho Song, Jun-Hwan Park, Minkyung Kim, Dae-Wook Kim, Seong-Geun Oh n Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2012 Accepted 12 January 2013 Available online 23 January 2013
Ag nanoparticles were immobilized on echinoid-like TiO2 particles via an alcohol reduction process with poly(vinyl pyrrolidone) (PVP) of different molecular weights (Mw.). The sizes of Ag nanoparticles prepared by using PVP K15 (Mw.: 10,000) and K30 (Mw.: 40,000) were 12–15 nm and 19–31 nm, respectively. On the other hand, Ag nanoparticles were not observed when PVP K12 (Mw.: 3500) was used. The crystal structure of Ag nanoparticles formed on TiO2 was face-centered cubic silver phase and the chemical states of Ag nanoparticles were Ag þ and Ag0. The photocatalytic performance was enhanced by Ag deposition. Among the samples, the prepared Ag/TiO2 composite by using PVP K15 exhibited better photocatalytic activity than other composites due to its higher Ag coverage on the TiO2 surface. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ag nanoparticles Echinoid-like TiO2 Poly(vinyl pyrrolidone) Alcohol reduction Photocatalytic activity
1. Introduction In recent years, the metal/inorganic composite particles have attracted much attention in the fields of antibacterial material, photonic crystals, and photocatalyst [1–3] due to their outstanding features such as electric conductivity, photovoltaic performance, and photocatalytic activity [4–6]. Various processes have been proposed to prepare metal/inorganic composites: selfassembly, electroless plating, ultrasound irradiation methods, and so on [7–9]. However, these synthesis methods usually need chemical reducing agents and it is difficult to control the particle size since the reaction rate is too fast. These problems can be overcome by using the alcohol reduction method known as a simple and chemical reducing agent free method. In this method, poly(vinyl pyrrolidone) (PVP) is generally employed as a reduction potential accelerator and a stabilizer because the electron lone pairs on imide groups in PVP easily promote the nucleation of metal ions and PVP prevents Ag nanoparticles from agglomerating through the steric effect [10]. Shin et al. reported how the growth of the Ag nanoparticles is affected by variation in the molecular weight of PVP [11]. Also, Chou et al. investigated the effect of PVP molecular weights on the formation Ag nanoparticles. They found that the size of Ag nanoparticles could be controlled by using different molecular weights of PVP [12]. In this study, Ag nanoparticles were deposited on echinoid-like TiO2 particles having a large surface area via the alcohol reduction method. In order to improve photocatalytic activity, the size and
n
Corresponding author. Tel.: þ82 2 2220 0485; fax: þ82 2 2294 4568. E-mail address: [email protected] (S.-G. Oh).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.045
coverage of Ag nanoparticles were controlled by using PVP (K12, K15, and K30) with different molecular weights.
2. Experimental Synthesis of echinoid-like TiO2 particles: Echinoid-like TiO2 particles were synthesized by the same process reported in our previous research [13]. Poly(vinyl alcohol) (PVA, 0.6 g) was dissolved in dimethyl sulfoxide (DMSO, 18 mL) under stirring at 100 1C for 1 h. Titanium tetraisopropoxide (TTIP, 9 mL) was added into 2-propanol (30 mL) under stirring "at room temperature for 1 h. PVA/ DMSO solution and TTIP/2-propanol solution were added dropwise into acetic acid (600 mL). The resultant solution was refluxed at 100 1C for 4 days. After the reaction, the synthesized particles were washed with acetone and dried. Finally, the particles were calcined at 450 1C for 2 h. Preparation of Ag/TiO2 composites: For the modification of TiO2 surface with amine groups as a chemical binder between Ag and TiO2 particles [14], the synthesized TiO2 powder (0.1 wt%) was dispersed in 3-aminopropyltrimethoxysilane aqueous solution (1.0 wt%). The obtained particles were washed with distilled water and then re-dispersed in ethanol. PVP (4.0 wt%) and silver nitrate (0.01 wt%) were added into the solution and the mixture was heated at 78 1C for 4 h. After the reaction, the prepared Ag/ TiO2 composites were washed with ethanol and dried. The echinoid-like TiO2 and Ag/TiO2 composites synthesized by using PVP K12, K15, and K30 were identified as ET, ST12, ST15, and ST30, respectively. Investigation into the photocatalytic activities of Ag/TiO2 composites: Ag/TiO2 composites were calcined at 450 1C for 2 h to remove the residual organic reagents completely and all samples
I. Jang et al. / Materials Letters 96 (2013) 214–217
were treated through the hydroxylation process for the functionalization of TiO2 surface with hydroxy groups [13]. Methylene blue (MB) was used to evaluate the photocatalytic property of samples and commercial P25 was employed as a reference. The 0.05 g of the powder was added into 100 mL of MB aqueous solution (10 ppm) and then the photocatalytic reaction was performed under UV irradiation. The experiments were repeated three times and the standard deviation is shown with error bars.
3. Results and discussion As shown in Fig. 1 (a), no Ag nanoparticles were observed in ST12. On the contrary, the Ag nanoparticles of ST15 and ST30, respectively were well deposited on the surface of TiO2 particles in Fig. 1(b) and (c). The sizes of Ag nanoparticles of ST15 and ST30 were 12–15 nm and 19–31 nm, respectively. The average size and the size distribution of Ag nanoparticles in ST15 were smaller and narrower than those of ST30. The SAED pattern of ST15 exhibits the (101) of anatase and (200) of the face-centered cubic (FCC) silver phases as illustrated in Fig. 1(d). In Fig. 1(e), the XRD pattern of ST12 shows
215
only the anatase crystal structure of TiO2 particles (JCPDS No. 211272). On the contrary, the diffraction patterns of ST15 and ST30 display not only the anatase phase but also the peak at 2y ¼44.2771 (200) of the FCC silver phase (JCPDS No. 04-0783), indicating that the Ag nanoparticles were successfully immobilized on the TiO2 surface. The Ag peak intensity of ST15 was higher than that of ST30 and it means that more grafted Ag nanoparticles exist in ST15. The XPS survey spectra of ET and ST15 are shown in Fig. 1(f). ET contains Ti, O, and C elements, which are assigned to Ti 2p, O 1s, and C 1s, respectively, and Ag 3d photoelectron peak was additionally observed in the ST15 [15]. The carbon peak is attributed to the residual carbon from the sample and/or adventitious hydrocarbon from the XPS instrument itself. Fig. 1 (g) shows the Ti 2p peaks of ET. The splitting width between Ti 2p1/2 and Ti 2p3/2 was 5.7 eV, indicating a normal state of Ti4 þ [15]. The shift of Ti 2p peaks and their smaller intensities after Ag deposition demonstrate that Ag nanoparticles were chemically adsorbed on TiO2 surface [16,17]. In Fig. 1(h), two O 1s peaks in ET were found and the earlier and latter peaks are attributed to oxygen on the surface and in the bulk of TiO2, respectively [15]. After Ag deposition, the O 1s surface peak shifted to lower binding energy, resulting from the change of
Fig. 1. HR-TEM images of ST12 (a), ST15 (b), ST30 (c), SAED pattern of ST15 (d), XRD patterns (e), XPS survey spectra (f), high-resolution XPS spectra of Ti 2p (g), O 1s (h), and Ag 3d (i).
216
I. Jang et al. / Materials Letters 96 (2013) 214–217
chemical environment of oxygen atoms by Ag immobilization [18]. Fig. 1(i) shows the Ag 3d3/2 and Ag 3d5/2 peaks of Ag/TiO2 composites. The Ag 3d5/2 peaks consist of Ag2O (Ag þ ) and Ag (Ag0) [15,18]. The Ag 3d5/2 peaks of ST12 show that Ag2O (Ag þ ) peak is predominant over Ag (Ag0) peak although a weak and broad Ag (Ag0) peak was observed on the ST12 from XPS data. It indicates that the reduction of Ag þ did not sufficiently occur since the polyvinyl chain of PVP K12 is too short to accelerate the reduction potential [14]. Therefore, Ag particles in ST12 might be too small to be detected from HR-TEM and XRD pattern. On the other hand, the Ag (Ag0) peaks in the Ag 3d5/2 peaks of ST15 and ST30 are superior to the Ag2O (Ag þ ). The Ag (Ag0) peak of ST15 has a higher intensity and narrower FWHM compared with those of ST30. These results indicate that more Ag particles were formed on TiO2 surface in ST15 than those of ST30 [17,18]. From the above results, PVP K15 showed the most excellent reduction potential in the alcohol reduction process and ST15 had the highest Ag coverage on TiO2 among the samples. The estimations for properties of Ag/TiO2 composites are summarized in Table 1. Fig. 2 describes the mechanism for formation of Ag/TiO2 composites which is influenced by the molecular weight of PVP in the system. Molecular weight of PVP K30 is higher and the kinetic behavior of chains is more complex than those of PVP K15 in the solution [11]. Thus, the closer capped Ag ions within chains of PVP K30 can be aggregated more easily as the Ag ions are reduced and Table 1 Estimations for the properties of Ag/TiO2 composites. Sample PVP molecular weight
ST12 ST15 ST30 n
3500 10,000 40,000
HR-TEM
XPS
Size of Ag (nm)
FWHM of Ag0 (eV)
n
– 12–15 19–31
1.62 1.29 1.91
31.45 83.43 72.14
Ag0 atom (%)
Ag0 atom (%) ¼Ag0/(Ag0 þ Ag þ ) 100.
Fig. 3. Photocatalytic activities of samples on the degradation of methylene blue.
grown. Therefore, ST30 has a larger average size and a wider size distribution of Ag particles. As shown in Fig. 3, the photocatalytic performance of echinoidlike TiO2 was higher than that of P25 and the photocatalytic activity was enhanced after Ag deposition. This is because the recombination was prevented by Ag immobilization with the formation of Schottky barrier [19]. ST15 showed the higher decomposition efficiency of MB than that of ST30 due to its higher Ag coverage and this result can be explained by the following reasons. First of all, the reflection of the incident light increases with the growth of the Ag nanoparticles. This reduces the number of photons absorbed by TiO2 and thus lowers the photo-quantum efficiency of photocatalytic reaction [20]. Secondly, the electrons are more accumulated in Ag nanoparticles as the size of Ag nanoparticles increases. As a result, positively charged holes will be attracted to the negatively charged Ag nanoparticles by electrostatic interaction. Thus, the prepared Ag/TiO2 composite by using PVP K15 showed the highest efficiency for MB degradation. 4. Conclusions In summary, we reported that Ag nanoparticles were immobilized on echinoid-like TiO2 particles via an alcohol reduction process using PVP with different molecular weights (PVP K12, K15, and K30). Ag nanoparticles were successfully formed on the TiO2 surface when PVP K15 and K30 were used. However, Ag nanoparticles were not observed when PVP K12 was used. The crystal structure and main chemical state, respectively of the Ag nanoparticles of ST15 and ST30 were FCC phase and Ag (Ag0). The degradation of MB was improved by the grafted Ag nanoparticles. The prepared Ag/TiO2 composite by using PVP K15 exhibited better photocatalytic activity than those of other samples due to their higher Ag coverage on TiO2 surface. Acknowledgment This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded from the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2012-0000591). References
Fig. 2. Schematic illustration for the effect of PVP molecular weight on the formation of ST15 (a) and ST30 (b).
[1] Amin SA, Pazouki M, Hosseinnia A. Powder Technol 2009;196:241–5. [2] Wang Z, Chan CT, Zhang W, Ming N, Sheng P. Phys Rev B 2001;64:113108.
I. Jang et al. / Materials Letters 96 (2013) 214–217
[3] [4] [5] [6] [7] [8] [9] [10] [11]
Chen CW, Serizawa T, Akashi M. Chem Mater 1999;11:1381–9. Croce F, Appetecchi GB, Persi L, Scrosati B. Nature 1998;394:456–8. Pinnavaia TJ. Science 1983;220:365–71. Winiarz JG, Zhang L, Lal M, Friend CS, Prasad PN. J Am Chem Soc 1999;121:5287–95. Dokoutchaev A, James JT, Koene SC, Pathak S, Prakash GKS, Thompson ME. Chem Mater 1999;11:2389–99. Kobayashi Y, Tadaki Y, Nagao D, Konno M. J Colloid Interf Sci 2005;283:601–4. Pol VG, Grisaru H, Gedanken A. Langmuir 2005;21:3635–40. Zhang Z, Zhao B, Hu L. J Solid State Chem 1996;121:105–10. Shin HS, Yang HJ, Kim SB, Lee MS. J Colloid Interf Sci 2004;274:89–94.
[12] [13] [14] [15] [16] [17] [18] [19] [20]
217
Chou KS, Lai YS. Mater Chem Phys 2004;83:82–8. Jang I, Song K, Oh SG. Chem Lett 2012;41:173–4. Lee JM, Jun YD, Kim DW, Lee YH, Oh SG. Mater Chem Phys 2009;114:549–55. Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Minnesota: Perkin-Elmer Corporation; 1992. Amarjargal A, Tijing LD, Kim CS. Dig J Nanomater Biostruct 2011;6:1957–65. Luo St. K, Clair TP, Lai X, Goodman DW. J Phys Chem B 2000;104:3050–7. Han Z, Zhang J, Yu Y, Cao W. Mater Lett 2012;70:193–6. Linsebigler AL, Lu G, Yates JT. Chem Rev 1995;95:735–58. Vamathevan V, Amal R, Beydoun D, Low G, McEvoy S. J Photochem Photobiol A Chem 2002;148:233–45.