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Immobilization of TiO2 nanoparticles on carbon nanocapsules for photovoltaic applications
Thin Solid Films 511 – 512 (2006) 203 – 207 www.elsevier.com/locate/tsf
Immobilization of TiO2 nanoparticles on carbon nanocapsules for photovoltaic applications Hui-Chi Huang a,b, Gan-Lin Huang b, Hsin-Lung Chen a, Yu-Der Lee a,* a
Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC b Polymer Technology Department, Union Chemical Laboratories, Hsinchu, Taiwan 300, ROC Available online 19 January 2006
Abstract In this work, TiO2 nanoparticles were immobilized on the carbon nanocapsules (CNC) treated by H2SO4/KMnO4 using a sol-gel process. The TEM images of the TiO2-coated CNC suggested that introducing crystallographic defects by acid functional groups tended to facilitate TiO2 immobilization onto the nanocapsules. The TiO2-coated CNC exhibited effective quenching on the emission of a light-emitting conjugated polymer, poly(2-phenyl-3-phenyl-4-(3V,7V-dimethyloctyloxy)-1,4-phenylene vinylene) (DPO-PPV). Consequently, the composites of the TiO2coated CNC and conjugated semiconducting polymers have potentials for photovoltaic applications. D 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Carbon nanocapsules; Nanoparticle immobilization; Photovoltaic
1. Introduction Composites of inorganic nanoparticles and conjugated polymers have been one of the major materials for enhancing solar energy conversion due to the possibility of combining the optoelectronic properties of organic polymers with the superior conductivity of inorganic nanoparticles [1–4]. Among the wide variety of inorganic nanoparticles, titanium dioxide (TiO2) has received much attention for application in the fields of photocatalytic degradation and photocells. Therefore, a number of efforts have been devoted to developing modern functional materials by coupling TiO2 with other inorganic or organic materials. Liu et al. [5] had labeled the defects of single-wall carbon nanotubes (SWNTs) using TiO2 nanoparticles as markers. Boccaccini et al. [6] had investigated TiO2 coatings as oxidation protective barrier on silicon carbide and carbon fibers. Hanprasopwattana et al. [7] immobilized TiO2 on silica gel as catalyst support. Several studies had also revealed the morphology of anatase TiO2 on multiwall carbon nanotubes [8–10]. A new nano-material called ‘‘carbon nanocapsule (CNC)’’ has been disclosed recently [11]. This type of material consists of concentric layers of closed graphitic sheets, leaving a nanoscale cavity at its center. Numerous metals such as iron, cobalt and * Corresponding author. Tel.: +86 886 3 5713204; fax: +86 886 3 5715408. E-mail address: [email protected] (Y.-D. Lee). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.127
nickel, may be filled into the nanocapsules by the arc method, thereby leading to various potential nanotechnological applications [11– 13]. To our knowledge, there is no prior report on the growth of TiO2 nanoparticles onto the hollow CNC. The aim of the present study is to immobilize TiO2 onto CNC surface by a sol-gel process. The TiO2-coated CNC is then combined with a conjugated polymer, poly(2-phenyl-3-phenyl-4-(3V,7V-dimethyloctyloxy)-1,4-phenylene vinylene) (DPO-PPV), to produce a composite having potential for photovoltaic application due to effective quenching of photoluminescence (PL) efficiency. 2. Experimental 2.1. Materials Titanium(IV) isopropoxide (Ti(OPri)4 (Acros organics), bromomalonic acid diethyl ester (TCI), KMnO4 (Showa), HCl (Scharlan) were of the highest purity available and were used as received without further purification. CNC was produced by the arc method in which a voltage was applied across the cathode and the anode as a pulse current [14]. The synthetic route of DPO-PPV has been described elsewhere [15]. The number average molecular weight (M n ) and the polydispersity index of the polymer were 415,196 g/ mol and 1.107, respectively, as measured using gel permeation chromatography (GPC).
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2.2. Functionalization of CNC
nanocapsules because of the different chemical reactivity between pentagons and hexagons [18].
The nanocapsules were functionalized by acids according to a procedure similar to the functionalization of C60 by malonic acid and described by Lamparth and Hirsch [16]. Briefly, diethyl bromomalonate and 1,8-diazabicyclo[5,4,0]undec-7ene (DBU) were added to a suspension of CNC in dry toluene at 140 -C, which resulted in a color change from violet to dark red. The solids were collected followed by refluxing in 1 M HCl aqueous solution for 12 h, to incorporate malonic acid derivatives in CNC. The resultant nanocapsules were hence called ‘‘diethyl bromomalonate/DBU-treated CNC’’. In order to understand the role of carboxylic groups in immobilizing TiO2 nanoparticles on carbon nanocapsules, the other pathway of functionalization was done according to Hiura et al. [17]. A low concentration of KMnO4 solution was gradually added to the 1 N H2SO4 solution in which CNC was dispersed. The whole solution was refluxed at 140 -C for 1.5 h followed by filtration to get the functionalized nanocapsules, called ‘‘H2SO4/KMnO4-treated CNC’’. The chemical oxidation generally resulted in the formation of carboxylic groups at the crystallographic defects, especially in the corner of the
a
2.3. Immobilization of TiO2 by sol-gel processes Titanium dioxide was immobilized on H2SO4/KMnO4treated CNC by a sol-gel process. Titanium(IV) isopropoxide (Ti(OPri)4) solutions were slowly added into the suspension of functionalized CNC in aqueous HCl (pH = 2) followed by stirring for 8 h. All impregnated CNC were filtered with isopropanol, and then thermally treated at 300 -C for 2 h to yield the TiO2-coated CNC. Neat titanium dioxide nanoparticles were prepared under the same condition but without adding functionalized CNC. 2.4. Characterization The nanostructure and elemental composition of the nanocapsules were revealed using transmission electron microscope (TEM) (JEOL, JEM-2010) operated at 200 keV in conjunction with energy dispersive spectrometer (EDS). The crystalline structure of TiO2-coated CNC was elucidated from
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Fig. 1. C1s XPS spectra of (a) raw CNC, (b) diethyl bromomalonate/DBU-treated CNC, (c) H2SO4/KMnO4-treated CNC, and (d) TiO2-coated CNC.
H.-C. Huang et al. / Thin Solid Films 511 – 512 (2006) 203 – 207
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X-ray diffraction measurement (Scintag, model XDS 008) (CuKa = 0.15418 nm). The thermal behavior and the amount of TiO2 immobilized on the CNC were analyzed by thermogravimetric analysis (Seiko instruments, TG/DT 6200) under airflow (100 ml/min). The chemical bonding was analyzed by PHI 1600 X-ray photoelectron spectroscopy (XPS) with spherical capacitor analyzer. TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC were blended with DPO-PPV, respectively, to form composites by spin coating. The concentrations of three doping species, TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC, were carefully controlled at 21.8 wt.%, 22.0 wt.% and 22.3 wt.%, respectively, by accurate measurement of the weight of these substances using an electronic balance (Mettler-Toledo GmbH, model AX26DR) with the standard deviation of the repeatability tested out of 10 measurements of 0.0017 mg and the tolerance of 0.0040 mg. Consequently, the concentration errors of the doping species in these composites were minimized. The thicknesses of the composite films were measured by means of a TENCOR P-10 surface profiler. Ultraviolet (UV) absorption spectra were recorded using a Perkin-Elmer Lambda 19 UV/Visible spectrophotometer. PL spectra were measured using a Jobin Yvon Fluoro Max-3 spectrophotometer. The substrate was arranged with an incident angle of 30- by excitation beam. 3. Results and discussion Fig. 1 shows the C1s XPS results of raw CNC, diethyl bromomalonate/DBU-treated CNC, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC. For all samples, a main peak due to the C –C bonds is observed at 284.6 eV. Additional peaks located at 285.2, 287.4, and 288.4 eV are identified except for CNC; these peaks are attributed to C –O, CfO, and COO groups, respectively. A further oxidation on the surface of the TiO2-coated CNC occurred during the sol-gel process: therefore, the three peaks shift slightly to the higher binding energy in comparison with those associated with diethyl bromomalonate/DBU-treated CNC and H2SO4/KMnO4-treated CNC. The concentration of carboxyl groups determined quantitatively by back titration was 2.96 10 4 mol/g in the diethyl bromomalonate/DBU-treated CNC but was negligible in the H2SO4/KMnO4-treated CNC. However, XPS results revealed the existence of carboxyl groups in both functionalized CNC: this implies that the carboxyl groups were dangling at the surface of the diethyl bromomalonate/DBU-treated CNC but were hidden in the defects in the H2SO4/KMnO4-treated CNC. The high resolution TEM micrographs of diethyl bromomalonate/DBU-treated and H2SO4/KMnO4-treated CNC are shown in Fig. 2a and b, respectively. These images indicate that the oxidation introduced crystallographic defects, where the defect concentration is high in the H2SO4/KMnO4-treated sample but low in the diethyl bromomalonate/DBU-treated one. The TEM and HRTEM micrographs of the TiO2-coated CNC are presented in Fig. 3a and b, respectively. It can be seen from Fig. 3a that TiO2 immobilizes and disperses well on the H2SO4/KMnO4-treated CNC. Furthermore, the interfaces
Fig. 2. TEM micrographs of the CNC samples: (a) HRTEM of diethyl bromomalonate/DBU-treated CNC; (b) HRTEM of H2SO4/KMnO4-treated CNC.
between the CNC and TiO2 nanoparticles with size less than 5 nm is clearly seen in HRTEM (Fig. 3b). The result indicates that TiO2 nanoparticles are well attached on the outermost shell of the H2SO4/KMnO4-treated CNC. It is noted that TiO2 nanoparticles did not deposit on the surface of the diethyl bromomalonate/DBU-treated CNC; but they rather formed large aggregates. On the other hand, TiO2 nanoparticles were able to coat onto the H2SO4/KMnO4-treated CNC. In this sense, the introduction of relatively high concentration of crystallographic defects at the surface of CNC favored the TiO2 immobilization. The corresponding EDS spectrum is shown in Fig. 3c. The presence of Ti element verified the attachment of TiO2 on CNC. TiO2 synthesized in all samples after thermal treatment at 300 -C exhibits anatase phase, as revealed by the XRD pattern. The amount of TiO2 immobilized on the CNC was assessed by TGA. In this case, the sample was treated at 900 -C for 45 min in air to burn the CNC completely. The amount of TiO2 was then calculated from the residual weight of the sample and was found to be ca. 7.5 wt.%. TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC were mixed with DPO-PPV respectively to form composite
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a Luminescence (a.u.)
DPO-PPV H2SO4/KMnO4-treated CNC/DPO-PPV TiO2/DPO-PPV TiO2-coated CNC/DPO-PPV
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Wavelength (nm) Fig. 5. Photoluminescence spectra of the composite films with DPO-PPV.
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Fig. 3. (a) TEM and (b) HRTEM micrographs of TiO2-coated CNC. TiO2 nanoparticles are marked by the circles. The corresponding EDS spectrum in (c) shows the presence of Ti element. The Cu element peak originates from the copper grid.
Absorbance (a.u.)
DPO-PPV H2SO4/KMnO4-treated CNC/DPO-PPV TiO2/DPO-PPV TiO2-coated CNC/DPO-PPV
˚ . Figs. 4 films by spin coating with thicknesses of 1200 T 100 A and 5 display the UV –Vis (normalized) and PL spectra of the films, respectively. Neat DPO-PPV exhibits two absorption peaks at 281 nm and 438 nm, attributable to the CYC* transition in the conjugated polymer. The UV – Vis spectra of DPO-PPV and H2SO4/KMnO4-treated CNC/DPO-PPV composite display an absorption edge between 500 and 550 nm. This tail, however, is absent in the other two samples, implying that the additions of TiO2 and TiO2-coated CNC suppress the aggregation of DPO-PPV chains [19 – 21]. The PL spectra of pure DPO-PPV and that of its composites with TiO2, H2SO4/ KMnO4-treated CNC, and TiO2-coated CNC are shown in Fig. 5. The amounts of TiO2 in the TiO2/DPO-PPV and TiO2-coated CNC/DPO-PPV composites are 21.8 wt.% and 1.67 wt.%, respectively. The PL intensity drops significantly in the TiO2 containing composites as compared with that of pure DPOPPV. It is noted that the large intensity reduction cannot be simply attributed to the lower amount of DPO-PPV in the composites. Instead, the PL quenching by TiO2 due to its electron-accepting properties is the main cause of the intensity drop [21,22]. Only little PL quenching is observed in the composites of DPO-PPV with 22.0 wt.% H2SO4/KMnO4treated CNC. However, loading a trace amount of TiO2 (1.67 wt.%) onto the CNC induced a larger PL quenching than the CNC-free TiO2/DPO-PPV which contains a much larger amount of TiO2 (21.8 wt.%). Therefore, the combination of TiO2 with H2SO4/KMnO4-treated CNC shows a higher PL quenching than that exhibited by the individual components. The PL quenching may be attributed to the energy or charge transfer from the polymer to the inorganic semiconductors. The quenching of emission intensity is the most significant in the presence of TiO2-coated CNC; consequently, the CNC tends to promote the charge transfer from the polymer to TiO2. 4. Conclusions
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Wavelength (nm) Fig. 4. Normalized ultraviolet – vis spectra of the composite films with DPO-PPV.
We have demonstrated the immobilization of TiO2 on the CNC. Our results suggested that the presence of acid functional groups but without introducing significant concentration of defects on CNC could not facilitate the TiO2 immobilization.
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XPS spectra of the TiO2-coated CNC showed the further oxidation during the sol-gel process. Coating of a slight amount of TiO2 onto the CNC surface significantly promoted the quenching efficiency of the light emission of DPO-PPV as compared with the CNC-free TiO2/DPO-PPV composite that contained a much higher TiO2 content. This implied that CNC promoted the charge transfer from the conjugated polymer to TiO2. Our results suggested that the present organic– inorganic heterojuction may be exploited as a potential active layer in photovoltaic and photoelectrochemical devices. Acknowledgments The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. 93RFA0400421, NSC 93-2216-E-007-009. References [1] J.S. Salafsky, Phys. Rev., B 59 (1999) 10885. [2] H. Ago, K. Petritsch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Adv. Mater. 11 (1999) 1281. [3] A.C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.H. Horhold, Adv. Mater. 12 (2000) 1689. [4] B.Q. Sun, E. Marx, N.C. Greenham, Nano Lett. 3 (2003) 961. [5] X.H. Li, J.L. Niu, J. Zhang, H.L. Li, Z.F. Liu, J. Phys. Chem., B 107 (2003) 2453.
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[6] A.R. Boccaccini, P. Karapappas, J.M. Marijuan, J. Mater. Sci. 39 (2004) 851. [7] A. Hanprasopwattana, T. Ricker, A.G. Sault, A.K. Datye, Catal. Lett. 45 (1997) 165. [8] S.W. Lee, W.M. Sigmund, Chem. Commun. 6 (2003) 780. [9] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Beguin, S. Bonnamy, Carbon 42 (2004) 1147. [10] S.H. Lee, S. Pumprueg, B. Moudgil, W. Sigmund, Colloid Surf., B 40 (2005) 93. [11] T.W. Ebbesen, Carbon Nanotubes Preparation and Properties, 1st edR, CRC Press, 1997, p. 249. [12] P.J.F. Harris, Carbon Nanotubes and Related Structures, 1st edR, Cambridge University Press, 1999, p. 156. [13] P.Z. Si, Z.D. Zhang, D.Y. Geng, C.Y. You, X.G. Zhao, W.S. Zhang, Carbon 41 (2003) 247. [14] G.L. Hwang, US Patent Application, 20030159917 (2003). [15] C.C. Chiu, K.F. Lin, H.L. Chou, J. Polym. Sci., Part A, Polym. Chem. 41 (2003) 2180. [16] L.L. Dugan, D.M. Turetsky, C. Du, D. Lobner, M. Wheeler, C.R. Almli, C.K.F. Shen, T.Y. Luh, D.W. Choi, T.S. Lin, Proc. Natl. Acad. Sci. 94 (1997) 9434. [17] H. Hiura, T.W. Ebbesen, K. Tanigaki, Adv. Mater. 7 (1995) 275. [18] T.W. Ebbesen, Carbon Nanotubes Preparation and Properties, 1st edR, CRC Press, 1997, p. 132. [19] B.D. Yang, K.H. Yoon, Synth. Met. 142 (2004) 21. [20] B.D. Yang, K.H. Yoon, K.W. Chung, Synth. Met. 143 (2004) 25. [21] A. Petrella, M. Tamborra, M.L. Curri, P. Cosma, M. Striccoli, P.D. Cozzoli, A. Agostiano, J. Phys. Chem., B 109 (2005) 1554. [22] A.C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.H. Ho¨rhold, Adv. Mater. 12 (2000) 1689.
Immobilization of TiO2 nanoparticles on carbon nanocapsules for photovoltaic applications Hui-Chi Huang a,b, Gan-Lin Huang b, Hsin-Lung Chen a, Yu-Der Lee a,* a
Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC b Polymer Technology Department, Union Chemical Laboratories, Hsinchu, Taiwan 300, ROC Available online 19 January 2006
Abstract In this work, TiO2 nanoparticles were immobilized on the carbon nanocapsules (CNC) treated by H2SO4/KMnO4 using a sol-gel process. The TEM images of the TiO2-coated CNC suggested that introducing crystallographic defects by acid functional groups tended to facilitate TiO2 immobilization onto the nanocapsules. The TiO2-coated CNC exhibited effective quenching on the emission of a light-emitting conjugated polymer, poly(2-phenyl-3-phenyl-4-(3V,7V-dimethyloctyloxy)-1,4-phenylene vinylene) (DPO-PPV). Consequently, the composites of the TiO2coated CNC and conjugated semiconducting polymers have potentials for photovoltaic applications. D 2005 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Carbon nanocapsules; Nanoparticle immobilization; Photovoltaic
1. Introduction Composites of inorganic nanoparticles and conjugated polymers have been one of the major materials for enhancing solar energy conversion due to the possibility of combining the optoelectronic properties of organic polymers with the superior conductivity of inorganic nanoparticles [1–4]. Among the wide variety of inorganic nanoparticles, titanium dioxide (TiO2) has received much attention for application in the fields of photocatalytic degradation and photocells. Therefore, a number of efforts have been devoted to developing modern functional materials by coupling TiO2 with other inorganic or organic materials. Liu et al. [5] had labeled the defects of single-wall carbon nanotubes (SWNTs) using TiO2 nanoparticles as markers. Boccaccini et al. [6] had investigated TiO2 coatings as oxidation protective barrier on silicon carbide and carbon fibers. Hanprasopwattana et al. [7] immobilized TiO2 on silica gel as catalyst support. Several studies had also revealed the morphology of anatase TiO2 on multiwall carbon nanotubes [8–10]. A new nano-material called ‘‘carbon nanocapsule (CNC)’’ has been disclosed recently [11]. This type of material consists of concentric layers of closed graphitic sheets, leaving a nanoscale cavity at its center. Numerous metals such as iron, cobalt and * Corresponding author. Tel.: +86 886 3 5713204; fax: +86 886 3 5715408. E-mail address: [email protected] (Y.-D. Lee). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.127
nickel, may be filled into the nanocapsules by the arc method, thereby leading to various potential nanotechnological applications [11– 13]. To our knowledge, there is no prior report on the growth of TiO2 nanoparticles onto the hollow CNC. The aim of the present study is to immobilize TiO2 onto CNC surface by a sol-gel process. The TiO2-coated CNC is then combined with a conjugated polymer, poly(2-phenyl-3-phenyl-4-(3V,7V-dimethyloctyloxy)-1,4-phenylene vinylene) (DPO-PPV), to produce a composite having potential for photovoltaic application due to effective quenching of photoluminescence (PL) efficiency. 2. Experimental 2.1. Materials Titanium(IV) isopropoxide (Ti(OPri)4 (Acros organics), bromomalonic acid diethyl ester (TCI), KMnO4 (Showa), HCl (Scharlan) were of the highest purity available and were used as received without further purification. CNC was produced by the arc method in which a voltage was applied across the cathode and the anode as a pulse current [14]. The synthetic route of DPO-PPV has been described elsewhere [15]. The number average molecular weight (M n ) and the polydispersity index of the polymer were 415,196 g/ mol and 1.107, respectively, as measured using gel permeation chromatography (GPC).
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2.2. Functionalization of CNC
nanocapsules because of the different chemical reactivity between pentagons and hexagons [18].
The nanocapsules were functionalized by acids according to a procedure similar to the functionalization of C60 by malonic acid and described by Lamparth and Hirsch [16]. Briefly, diethyl bromomalonate and 1,8-diazabicyclo[5,4,0]undec-7ene (DBU) were added to a suspension of CNC in dry toluene at 140 -C, which resulted in a color change from violet to dark red. The solids were collected followed by refluxing in 1 M HCl aqueous solution for 12 h, to incorporate malonic acid derivatives in CNC. The resultant nanocapsules were hence called ‘‘diethyl bromomalonate/DBU-treated CNC’’. In order to understand the role of carboxylic groups in immobilizing TiO2 nanoparticles on carbon nanocapsules, the other pathway of functionalization was done according to Hiura et al. [17]. A low concentration of KMnO4 solution was gradually added to the 1 N H2SO4 solution in which CNC was dispersed. The whole solution was refluxed at 140 -C for 1.5 h followed by filtration to get the functionalized nanocapsules, called ‘‘H2SO4/KMnO4-treated CNC’’. The chemical oxidation generally resulted in the formation of carboxylic groups at the crystallographic defects, especially in the corner of the
a
2.3. Immobilization of TiO2 by sol-gel processes Titanium dioxide was immobilized on H2SO4/KMnO4treated CNC by a sol-gel process. Titanium(IV) isopropoxide (Ti(OPri)4) solutions were slowly added into the suspension of functionalized CNC in aqueous HCl (pH = 2) followed by stirring for 8 h. All impregnated CNC were filtered with isopropanol, and then thermally treated at 300 -C for 2 h to yield the TiO2-coated CNC. Neat titanium dioxide nanoparticles were prepared under the same condition but without adding functionalized CNC. 2.4. Characterization The nanostructure and elemental composition of the nanocapsules were revealed using transmission electron microscope (TEM) (JEOL, JEM-2010) operated at 200 keV in conjunction with energy dispersive spectrometer (EDS). The crystalline structure of TiO2-coated CNC was elucidated from
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Fig. 1. C1s XPS spectra of (a) raw CNC, (b) diethyl bromomalonate/DBU-treated CNC, (c) H2SO4/KMnO4-treated CNC, and (d) TiO2-coated CNC.
H.-C. Huang et al. / Thin Solid Films 511 – 512 (2006) 203 – 207
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X-ray diffraction measurement (Scintag, model XDS 008) (CuKa = 0.15418 nm). The thermal behavior and the amount of TiO2 immobilized on the CNC were analyzed by thermogravimetric analysis (Seiko instruments, TG/DT 6200) under airflow (100 ml/min). The chemical bonding was analyzed by PHI 1600 X-ray photoelectron spectroscopy (XPS) with spherical capacitor analyzer. TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC were blended with DPO-PPV, respectively, to form composites by spin coating. The concentrations of three doping species, TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC, were carefully controlled at 21.8 wt.%, 22.0 wt.% and 22.3 wt.%, respectively, by accurate measurement of the weight of these substances using an electronic balance (Mettler-Toledo GmbH, model AX26DR) with the standard deviation of the repeatability tested out of 10 measurements of 0.0017 mg and the tolerance of 0.0040 mg. Consequently, the concentration errors of the doping species in these composites were minimized. The thicknesses of the composite films were measured by means of a TENCOR P-10 surface profiler. Ultraviolet (UV) absorption spectra were recorded using a Perkin-Elmer Lambda 19 UV/Visible spectrophotometer. PL spectra were measured using a Jobin Yvon Fluoro Max-3 spectrophotometer. The substrate was arranged with an incident angle of 30- by excitation beam. 3. Results and discussion Fig. 1 shows the C1s XPS results of raw CNC, diethyl bromomalonate/DBU-treated CNC, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC. For all samples, a main peak due to the C –C bonds is observed at 284.6 eV. Additional peaks located at 285.2, 287.4, and 288.4 eV are identified except for CNC; these peaks are attributed to C –O, CfO, and COO groups, respectively. A further oxidation on the surface of the TiO2-coated CNC occurred during the sol-gel process: therefore, the three peaks shift slightly to the higher binding energy in comparison with those associated with diethyl bromomalonate/DBU-treated CNC and H2SO4/KMnO4-treated CNC. The concentration of carboxyl groups determined quantitatively by back titration was 2.96 10 4 mol/g in the diethyl bromomalonate/DBU-treated CNC but was negligible in the H2SO4/KMnO4-treated CNC. However, XPS results revealed the existence of carboxyl groups in both functionalized CNC: this implies that the carboxyl groups were dangling at the surface of the diethyl bromomalonate/DBU-treated CNC but were hidden in the defects in the H2SO4/KMnO4-treated CNC. The high resolution TEM micrographs of diethyl bromomalonate/DBU-treated and H2SO4/KMnO4-treated CNC are shown in Fig. 2a and b, respectively. These images indicate that the oxidation introduced crystallographic defects, where the defect concentration is high in the H2SO4/KMnO4-treated sample but low in the diethyl bromomalonate/DBU-treated one. The TEM and HRTEM micrographs of the TiO2-coated CNC are presented in Fig. 3a and b, respectively. It can be seen from Fig. 3a that TiO2 immobilizes and disperses well on the H2SO4/KMnO4-treated CNC. Furthermore, the interfaces
Fig. 2. TEM micrographs of the CNC samples: (a) HRTEM of diethyl bromomalonate/DBU-treated CNC; (b) HRTEM of H2SO4/KMnO4-treated CNC.
between the CNC and TiO2 nanoparticles with size less than 5 nm is clearly seen in HRTEM (Fig. 3b). The result indicates that TiO2 nanoparticles are well attached on the outermost shell of the H2SO4/KMnO4-treated CNC. It is noted that TiO2 nanoparticles did not deposit on the surface of the diethyl bromomalonate/DBU-treated CNC; but they rather formed large aggregates. On the other hand, TiO2 nanoparticles were able to coat onto the H2SO4/KMnO4-treated CNC. In this sense, the introduction of relatively high concentration of crystallographic defects at the surface of CNC favored the TiO2 immobilization. The corresponding EDS spectrum is shown in Fig. 3c. The presence of Ti element verified the attachment of TiO2 on CNC. TiO2 synthesized in all samples after thermal treatment at 300 -C exhibits anatase phase, as revealed by the XRD pattern. The amount of TiO2 immobilized on the CNC was assessed by TGA. In this case, the sample was treated at 900 -C for 45 min in air to burn the CNC completely. The amount of TiO2 was then calculated from the residual weight of the sample and was found to be ca. 7.5 wt.%. TiO2, H2SO4/KMnO4-treated CNC, and TiO2-coated CNC were mixed with DPO-PPV respectively to form composite
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a Luminescence (a.u.)
DPO-PPV H2SO4/KMnO4-treated CNC/DPO-PPV TiO2/DPO-PPV TiO2-coated CNC/DPO-PPV
50 nm
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Wavelength (nm) Fig. 5. Photoluminescence spectra of the composite films with DPO-PPV.
c cps
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Fig. 3. (a) TEM and (b) HRTEM micrographs of TiO2-coated CNC. TiO2 nanoparticles are marked by the circles. The corresponding EDS spectrum in (c) shows the presence of Ti element. The Cu element peak originates from the copper grid.
Absorbance (a.u.)
DPO-PPV H2SO4/KMnO4-treated CNC/DPO-PPV TiO2/DPO-PPV TiO2-coated CNC/DPO-PPV
˚ . Figs. 4 films by spin coating with thicknesses of 1200 T 100 A and 5 display the UV –Vis (normalized) and PL spectra of the films, respectively. Neat DPO-PPV exhibits two absorption peaks at 281 nm and 438 nm, attributable to the CYC* transition in the conjugated polymer. The UV – Vis spectra of DPO-PPV and H2SO4/KMnO4-treated CNC/DPO-PPV composite display an absorption edge between 500 and 550 nm. This tail, however, is absent in the other two samples, implying that the additions of TiO2 and TiO2-coated CNC suppress the aggregation of DPO-PPV chains [19 – 21]. The PL spectra of pure DPO-PPV and that of its composites with TiO2, H2SO4/ KMnO4-treated CNC, and TiO2-coated CNC are shown in Fig. 5. The amounts of TiO2 in the TiO2/DPO-PPV and TiO2-coated CNC/DPO-PPV composites are 21.8 wt.% and 1.67 wt.%, respectively. The PL intensity drops significantly in the TiO2 containing composites as compared with that of pure DPOPPV. It is noted that the large intensity reduction cannot be simply attributed to the lower amount of DPO-PPV in the composites. Instead, the PL quenching by TiO2 due to its electron-accepting properties is the main cause of the intensity drop [21,22]. Only little PL quenching is observed in the composites of DPO-PPV with 22.0 wt.% H2SO4/KMnO4treated CNC. However, loading a trace amount of TiO2 (1.67 wt.%) onto the CNC induced a larger PL quenching than the CNC-free TiO2/DPO-PPV which contains a much larger amount of TiO2 (21.8 wt.%). Therefore, the combination of TiO2 with H2SO4/KMnO4-treated CNC shows a higher PL quenching than that exhibited by the individual components. The PL quenching may be attributed to the energy or charge transfer from the polymer to the inorganic semiconductors. The quenching of emission intensity is the most significant in the presence of TiO2-coated CNC; consequently, the CNC tends to promote the charge transfer from the polymer to TiO2. 4. Conclusions
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Wavelength (nm) Fig. 4. Normalized ultraviolet – vis spectra of the composite films with DPO-PPV.
We have demonstrated the immobilization of TiO2 on the CNC. Our results suggested that the presence of acid functional groups but without introducing significant concentration of defects on CNC could not facilitate the TiO2 immobilization.
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XPS spectra of the TiO2-coated CNC showed the further oxidation during the sol-gel process. Coating of a slight amount of TiO2 onto the CNC surface significantly promoted the quenching efficiency of the light emission of DPO-PPV as compared with the CNC-free TiO2/DPO-PPV composite that contained a much higher TiO2 content. This implied that CNC promoted the charge transfer from the conjugated polymer to TiO2. Our results suggested that the present organic– inorganic heterojuction may be exploited as a potential active layer in photovoltaic and photoelectrochemical devices. Acknowledgments The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. 93RFA0400421, NSC 93-2216-E-007-009. References [1] J.S. Salafsky, Phys. Rev., B 59 (1999) 10885. [2] H. Ago, K. Petritsch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Adv. Mater. 11 (1999) 1281. [3] A.C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.H. Horhold, Adv. Mater. 12 (2000) 1689. [4] B.Q. Sun, E. Marx, N.C. Greenham, Nano Lett. 3 (2003) 961. [5] X.H. Li, J.L. Niu, J. Zhang, H.L. Li, Z.F. Liu, J. Phys. Chem., B 107 (2003) 2453.
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