Polystyrene

Apr 23, 2009 - In order to confirm that nanocomposites retained conductive properties of TiO2 nanoparticles, we subsequently removed the SEO matrix us...
1 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2009, 113, 8601–8605

8601

Conductive Properties of Inorganic and Organic TiO2/Polystyrene-block-Poly(ethylene oxide) Nanocomposites Junkal Gutierrez, Agnieszka Tercjak, Laura Peponi, and In˜aki Mondragon* Materials + Technologies Group, Dpto, Ingenierı´a Quı´mica y M. Ambiente, Polytechnic School, UniVersidad del Pais Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastian, Spain ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: April 2, 2009

In this work, atomic and electrostatic force microscopy measurements on titanium dioxide nanoparticles embedded in an amphiphilic polystyrene-block-poly(ethylene oxide) (SEO) block copolymer have been performed. TiO2 nanoparticles were generated via sol-gel synthesis and simultaneously confined into poly(ethylene oxide) domains of a SEO block copolymer. Silicon wafer substrates were used to prepare nanocomposite thin films with different amounts of TiO2 nanoparticles. In order to confirm that nanocomposites retained conductive properties of TiO2 nanoparticles, we subsequently removed the SEO matrix using ultraviolet light treatment, which finally led to a film surface completely covered with semiconductor titanium dioxide nanoparticles, with average diameters of 40 nm. Introduction Development of nanoscience and nanotechnology relies on research related to metal oxide nanoparticle synthesis, characterization of their structural, chemical, and physical properties, and their assembly into larger structures extending over several scales of lengths. The unique characteristics of metal oxides make them the most diverse class of materials in the field of nanotechnology because of their optical, electronic, electrical, photoelectronic, catalytic, and magnetic properties. These unique properties are also led by nanocomposites with small-sized nanoparticles and large specific surface areas. Moreover, the great variety of structures and properties of designed nanocomposites allow for their application in diverse fields such as nanophotonics,1 electrochemistry,2 energy storage and conversion,3 catalysis,4 biomedical applications,5 sensors and actuators,6-8 etc. Block copolymers (BC) are an attractive alternative for semiconductor patterning applications because they can act as templates for defining integrated circuit elements because of their ability to self-assemble into different nanostructures such as spheres, hexagonally packed cylinders, and lamellae, with dimensions on the nanometer scale.9 The quality of obtained self-assembled scaffolds depends on preparation conditions (annealing time and temperature, film thickness, solvent, vapor exposure time, etc.).10,11 Here, it should be pointed out that in this area block copolymers have been used as templates for synthesis of regular arrays of inorganic oxide semiconductors.12-14 On the other hand, sol-gel synthesis offers an appropriate route for generation of inorganic oxide semiconductors because of several advantages, including good homogeneity, easy composition control, low processing temperature, and low equipment cost. In recent years, amphiphilic polystyrene-bpoly(ethylene oxide) (SEO) block copolymers have been intensively studied as templating agents, coupled with sol-gel chemistry to control the selective location and morphology of hybrid nanocomposites, specifically systems containing TiO2 nanoparticles as the inorganic part.15-23 These kind of amphiphilic block copolymers consist of a hydrophilic poly* To whom correspondence should be addressed. E-mail: inaki. [email protected].

Figure 1. Schematic illustration of EFM measurements performed for a TiO2/SEO nanocomposite (Vt-tip voltage).

(ethylene oxide) (PEO) block, which strongly absorbs onto the substrate and interacts with the inorganic nanoparticles as well as a hydrophobic polystyrene (PS) block that builds the matrix.24,25 Morphology and nanoparticle size distribution in TiO2/SEO nanocomposites containing different amounts of inorganic nanoparticles have been reported by Sun and co-workers by means of surface force microscopy, scanning electron microscopy, and small-angle X-ray scattering analysis.15-17 Additionally, Kim et al.18,19 used atomic force microscopy and ultraviolet (UV) treatment in order to study the surface morphology generated after thermal or UV degradation of the organic part. Moreover, many research groups20-23 have studied photoluminescence properties of the films as well as morphological evolution of the nanostructures as a function of preparation conditions. However, to the best of our knowledge, conductive properties of the TiO2 nanoparticles in a SEO-based matrix have not been yet reported. In this study, conductive properties of TiO2 nanoparticles synthesized via the sol-gel method and simultaneously embed-

10.1021/jp900858f CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

8602

J. Phys. Chem. C, Vol. 113, No. 20, 2009

Figure 2. TM-AFM and EFM phase images (2 µm × 2 µm) for SEO block copolymer.

ded in amphiphilic polystyrene-b-poly(ethylene oxide) block copolymer were investigated by electrostatic force microscopy (EFM). Results confirm that nanoparticles retain their conductive properties in the obtained inorganic and organic hybrids. Experimental Section Materials and Sample Preparation. Polystyrene-b-poly(ethylene oxide) block copolymer contents (MnPS ) 125000 g/mol, MnPEO ) 16100 g/mol, and Mw/Mn ) 1.04) was purchased

Gutierrez et al. from Polymer Source, Inc. and was used as received. The sol-gel solution used to obtain the TiO2 nanoparticles consisted of titanium isopropoxide {Ti[OCH(CH3)2]4, TTIP} as a precursor (Aldrich), isopropanol (IPA), an aqueous hydrochloric acid (HCl) solution (37%), and toluene. All chemicals used in this experiment were analytical grade, which were used as received. TiO2/SEO nanocomposites were synthesized according to the procedure published by Sun et al.14-16 and Cheng.21The sol-gel solution was prepared mixing IPA (5 mL), TTIP (0.125 mmol), HCl (0.125 mmol), and toluene (5 mL), under vigorous stirring for 1 h. Afterward, the desired amount of sol-gel (5, 10, and 20 vol %) was added to SEO solutions (10 wt %) and stirred for 30 min. Finally, the mixed solutions were filtered, using a 0.2 µm PTFE filter. Nanocomposites and neat block copolymer thin films were spin coated onto precleaned Si (100) wafers at 2000 rpm for 30 s, using a Telstar Instrument P-6708D spin coater. In order to obtain inorganic arrays of TiO2 nanoparticles, we irradiated nanocomposites films with UV light of 254 nm (XX15S, UVP Inc.) at room temperature for 2 h to remove the organic matrix. Techniques. Atomic force microscopy (AFM) and electrostatic force microscopy were performed in tapping mode (TM)

Figure 3. TM-AFM and EFM phase images (2 µm × 2 µm) for different TiO2/SEO nanocomposites based on varying vol % sol-gel additions.

Conductive Properties of TiO2/PS-b-PEO

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8603

using a Dimension 3100 NanoScope IV (Veeco). Etched singlebeam cantilever (110-140 µm length) silicon nitride probes were used, having a tip nominal radius of 10 nm. Scan rates ranged from 0.7 to 1.2 Hz s-1. Measurements were performed with 512 scan lines and a target amplitude around 0.9 V. In order to obtain repeatable results of the nanocomposites morphology, we scanned different regions of the specimens. Electrostatic force microscopy was used to study the conductivity capability of TiO2 nanoparticles in the SEO block copolymer matrix. These measurements were performed, using the same scanning probe microscopy operating in the lift mode (lift height was 180 nm), in ambient conditions and equipped with an integrated Co/Cr-coated MESP tip having a resonance frequency around 75 kHz. The secondary imaging mode, derived from the tapping mode that measures the electric field gradient distribution above the sample surface, was detected by applying voltage to the cantilever tip. Quantitative voltage measurements were made of the relative voltages within a single image. Electrical conductivity of the nanocomposites was measured using a semiconductor characterization system (Keithley model 4200-SCS). Results and Discussion Electrostatic force microscopy is a versatile method for studying the electric field gradient distribution above the sample surface, which consequently allows distinguishing different conductive parts of the sample.26-28 Figure 1 shows the scheme of EFM measurements in TiO2/SEO nanocomposites. Here, it should be pointed out that, when a voltage is applied to the tip, only charged domains of the sample surface are detected in the EFM phase image.29 The unique component that responds to the applied voltage in the investigated TiO2/SEO nanocomposite samples is the TiO2 nanoparticles. In order to confirm the conductive properties of the TiO2 nanoparticles embedded in the PEO block of the block copolymer, we performed EFM measurements. Figure 2 shows TM-AFM and EFM phase images of the neat SEO block copolymer. In the case of the TM-AFM image, the PEO block domains appear as dark regions dispersed in the PS block matrix, which are light regions.23 EFM measurements of a neat block copolymer demonstrate that, when a bias of 0 V was applied to the tip, no charge domains were detected on the surface. The same phenomenon occurred when a bias of 6 V was applied to the tip; no charge domains were detected on the surface of the neat SEO block copolymers. Taking into account these results, we conclude that, under the measurement conditions, the SEO polymeric matrix does not respond to the applied voltage because the block copolymer is an uncharged material, even though PS and PEO blocks show different surface potentials. Figure 3 shows a comparison of the TM-AFM phase images with the EFM phase images for TiO2/SEO nanocomposites with a different volume % of sol-gel. In the case of the TM-AFM images of synthesized TiO2/SEO nanocomposites, the obtained morphology was not significantly altered with respect to the morphology of the neat SEO. It is well-known that during sol-gel synthesis, a hydrophilic TiO2 nanoparticle network is formed,16 which interacts with the hydrophilic PEO block, leading to the formation of hydrogen bonds. All nanocomposites show small bright dots selectively located in the PEO block domains of the block copolymer. These bright spherical domains correspond to TiO2 nanoparticles. As shown in Figure 3, with an increasing volume % of sol-gel, more PEO block domains appear filled with titanium dioxide nanoparticles. Thus, more PEO block domains turned from dark to bright because TiO2

Figure 4. EFM phase images (2 µm × 2 µm) applying (a) a positive bias, (b) a negative and positive bias, and (c) current-voltage (I-V) curves to a 20 vol % sol-gel/SEO nanocomposite.

nanoparticles located in the PEO block became harder than those in the PS block domain. All compositions from 5 to 20 vol % sol-gel showed individual nanoparticles, with a diameter between 25 and 30 nm. In order to confirm the noninfluence of the topographic effect, 0 bias voltage was applied to the EFM tip (Figure 3). The EFM phase image indicated no charge on the sample surface; thus, any charged domains were distinguished.30 On the contrary, when a bias of 6 V was applied to the EFM tip, spherical bright domains appeared well-dispersed on the sample surface. Morphology detected for the 6 V EFM phase image of nanocom-

8604

J. Phys. Chem. C, Vol. 113, No. 20, 2009

Gutierrez et al.

Figure 5. TM-AFM and EFM phase images (2 µm × 2 µm) for different TiO2/SEO nanocomposites based on varying vol % sol-gel addition and exposed 2 h to UV radiation.

posites was not significantly altered with respect to that of the TM-AFM phase images. On the other hand, one can easily note that a higher quantity of bright nanoparticles appeared with an increasing vol % of sol-gel. Consequently, taking into account that all of the samples were imaged with the same EFM scale bar (from 0° to 2°), in the case of 20 vol % sol-gel, we detected the TiO2/SEO nanocomposite sample with the highest intensity of the EFM phase image. The higher amount of charged domains, related to higher TiO2 nanoparticle content, led to high contrast EFM phase images, when 6 V was applied to the samples. As a consequence, the EFM phase image of the 20 vol % sol-gel TiO2/SEO showed a large difference between the charged and uncharged domains, which allowed us to clearly distinguish the charged domains on the investigated systems in EFM phase images. Figure 4a shows a 20 vol % sol-gel TiO2/SEO nanocomposite sample with different applied bias (0, 3, 6, 9, and 12 V). The performed measurements clearly indicate a higher EFM phase image contrast, with increasing applied bias. Additionally, in Figure 4b, negative and positive biases were applied to the sample. These measurements allow us to conclude that applying the same level of positive or negative bias to the EFM tip results in almost the same contrast in the EFM phase image. Moreover, no differences were observed when the same bias value switched

from negative to positive, indicating that investigated nanocomposites were able to respond, regardless of the sign of the applied voltage. In order to measure the quantitative conductive responses of the TiO2 nanoparticles in the 20 vol % sol-gel TiO2/SEO nanocomposite, current-voltage (I-V) curves were performed by means of a Keithley model 4200-SCS semiconductor analyzer. Two-point probe experiments were carried out, applying voltage from -4 to 4 V and from 4 to -4 V to record the response of the synthesized nanocomposite (Figure 4c). Results demonstrate that TiO2 nanoparticles maintained conductive properties and simultaneously responded on the increasing or decreasing voltage cycles almost without hysteresis. Arrangements of the conductive TiO2 nanoparticles on the surfaces of the silica wafers were obtained by degradation of the organic component of nanocomposites by means of UV light irradiation. Figure 5 shows AFM phase images of TiO2/SEO nanocomposites with different inorganic percentages after exposure to UV irradiation for 2 h. As shown in the TM-AFM images (Figure 5), the arrays of titanium domains were unaffected by the removal of the polymeric matrix via degradation by UV light exposure. The interparticle center-to-center distance decreased with an increasing sol-gel amount in the nanocomposites. All compositions from 5 to 20 vol % sol-gel showed individual nanoparticles, with a diameter between 40

Conductive Properties of TiO2/PS-b-PEO

J. Phys. Chem. C, Vol. 113, No. 20, 2009 8605 dots were generated on the completely covered surfaces of the silica wafers. These conductive nanomaterials have possible applications in the field of dye-sensitized solar cells, along with other applications determined by the electrical conductivity of the nanocomposites. Acknowledgment. Financial support is gratefully acknowledged from the Basque Country Government in the frame of Grupos Consolidados (IT-365-07), the ETORTEK/inanoGUNE project, and the Spanish Ministry of Education and Science (MAT2006-06331). Additionally, J.G. thanks Eusko Jaurlaritza/ Gobierno Vasco (Programas de becas para formacio´n y perfeccionamiento de personal investigador) for the grant supplied for this work. References and Notes

Figure 6. Three-dimensional view of the height of an AFM image (2 µm × 2 µm) of a 20 vol % sol-gel/SEO nanocomposite exposed 2 h to UV radiation.

and 50 nm. In the case of the 20 vol % sol-gel TiO2/SEO nanocomposite, the sample surface was completely covered by TiO2 nanoparticles. Moreover, Figure 5 also shows EFM images, when the voltage between the tip and substrate was not applied (0 V). All nanocomposites presented the same behavior; no charged dots were distinguished on the samples surfaces. Contrarily, applying 6 V to the EFM tip allowed us to clearly distinguish the charged and uncharged domains in the TiO2/ SEO nanocomposite surfaces after UV exposure. Here, it should be pointed out that only TiO2 nanoparticle domains respond to the voltage. As has been observed in 6 V EFM images, TiO2 nanoparticles are easily detected as a result of degradation of the organic part after UV exposure. Consequently, a higher amount of nanoparticles, visualized as bright dots, completely covered the surface of the silica wafer. Thus, one can obtain higher conductive surfaces, compared to those of the same samples before UV treatment. Figure 6 shows a three-dimensional view of the height contrast of an AFM image of a 20 vol % sol-gel TiO2/SEO nanocomposite exposed 2 h to UV radiation. This topographic image confirms the highly dense arrays of uniformly sized titanium dots. Therefore, one can conclude that the generated thin film surface was completely covered by semiconductive nanoparticles that were able to respond to the applied voltage. Conclusion Electrostatic force microscopy was used to study the conductive properties of titanium dioxide nanoparticles in TiO2/PS-bPEO nanocomposites with different inorganic contents, prepared via sol-gel synthesis. Results confirmed that TiO2 nanoparticles retained conductive properties in the generated nanocomposites and responded to the voltage applied to the EFM tip. Welldispersed conductive TiO2 nanoparticles were located in the PEO block of the block copolymer template, allowing us to obtain materials in which conducting and nonconducting domains were clearly distinguished. Nanocomposite thin films were exposed to UV light in order to remove the organic matrix. Well-ordered conductive TiO2

(1) Yuwono, A. H.; Zhang, Y.; Wang, J.; Zhang, X. H.; Fan, H. M.; Ji, W. Chem. Mater. 2006, 18, 5876. (2) Wang, J.; Liu, G. D.; Engelhard, M. H.; Lin, Y. H. Anal. Chem. 2006, 78, 6974. (3) Kim, J. H.; Kang, M.-S.; Kim, Y. J.; Won, J.; Park, N.-G.; Kang, Y. S. Chem. Commun. 2004, 1662. (4) Wang, J. C.; Neogi, P.; Forciniti, D. J. Chem. Phys. 2006, 125, 194717. (5) Jones, J. R. J. Eur. Cer. Soc. 2009, 29, 1275. (6) Bonini, N.; Carotta, M. C.; Chiorini, A.; Guidi, V.; Malagu, C.; Martinelli, G.; Paglialonga, L.; Sacerdoti, M. Sens. Actuators, B 2000, 68, 274. (7) Garzella, C.; Comini, E.; Bontempi, E.; Depero, L. E.; Frigeri, C.; Sberveglieri, G. Sens. Actuators, B 2002, 83, 230. (8) Zhao, Y.; Zhai, J.; Wei, T.; Jiang, L.; Zhu, D. J. Mater. Chem. 2007, 17, 5084. (9) Forster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (10) Hamley, I. W. The Physics of Block Copolymers; University Press: New York, 1998. (11) Li, M.; Ober, C. K. Mater. Today 2006, 9, 9. (12) Kim, H. C.; Jia, X.; Stafford, C. M.; Kim, D. H.; McCarthy, T. J.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2001, 13, 795. (13) Yeh, S. W.; Wei, K. H.; Sun, Y. S.; Jeng, U. S.; Liang, K. S. Macromolecules 2003, 36, 7903. (14) Fahmi, A. W.; Braun, H. G.; Stamm, M. AdV. Mater. 2003, 15, 1201. (15) Sun, Z.; Gutmann, J. S. Physica A 2004, 339, 80. (16) Sun, Z.; Wolkenhauer, M.; Bumbu, G. G.; Kim, D. H.; Gutmann, J. S. Physica B 2005, 357, 141. (17) Sun, Z.; Kim, D. H.; Wolkenhauer, M.; Bumbu, G. G.; Knoll, W.; Gutmann, J. S. ChemPhysChem 2006, 7, 370. (18) Kim, D. H.; Kim, S. H.; Lavery, K.; Russell, T. P. Nano Lett. 2004, 4, 1841. (19) Kim, D. H.; Sun, Z.; Russell, T. P.; Knoll, W.; Gutmann, J. S. AdV. Funct. Mater. 2005, 15, 1160. (20) Cheng, Y. J.; Gutman, J. S. J. Am. Chem. Soc. 2006, 128, 4658. (21) Li, X.; Yang, H.; Li, C.; Xu, L.; Zhang, Z.; Kim, D. H. Polymer 2008, 49, 1376. (22) Cheng, Y. J.; Zhou, S.; Gutmann, J. S. Macromol. Rapid Commun. 2007, 28, 1392. (23) J. Gutierrez, J.; Tercjak, A.; Garcı´a, I.; Peponi, L.; Mondragon, I. Nanotechnology 2008, 19, 155607 (8 pp). (24) Vairac, P.; Cretin, B. Appl. Phys. Lett. 1996, 68, 461. (25) Levy, L.; Feltin, N.; Ingert, D.; Pileni, M. P. J. Phys. Chem. Bull. 1997, 101, 9153. (26) Chiang, D. M.; Liu, W. L.; Chen, J. L.; Susuki, R. Chem. Phys. Lett. 2005, 412, 50. (27) Qiu, X. H.; Qi, G. C.; Yang, Y. L.; Wang, C. J. Solid State Chem. 2008, 181, 1670. (28) Li, J. K.; Zou, S.; Rider, D. A.; Manners, I.; Walker, G. C. AdV. Mater. 2008, 20, 1989. (29) Jiao, Z.; Wan, X.; Guo, H.; Wang, J.; Zhao, B.; Wu, M. Ultramicroscopy 2008, 108, 1371. (30) Tercjak, A.; Garcia, I.; Mondragon, I. Nanotechnology2008, 19, 275701 (5 pp).

JP900858F