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Interfacial Functionalization of TiO2 with Smart Polymers: pH-Controlled Switching of Photocurrent Direction Da Chen†,‡ and Jinghong Li*,‡ College of Materials Science & Engineering, China Jiliang UniVersity, Hangzhou 310018, China, and Department of Chemistry, Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, China ReceiVed: February 01, 2010; ReVised Manuscript ReceiVed: May 10, 2010
TiO2/P(NIPAM-AA) nanocomposites were successfully obtained by the incorporation of TiO2 nanoparticles (P25, commercial TiO2 nanoparticles from Degussa) with pH-stimuli responsive smart polymer [poly(Nisopropylacrylamide-co-acrylic acid), P(NIPAM-AA)]. Compared with the pure TiO2 nanoparticles, the incorporation of TiO2 nanoparticles with smart polymer P(NIPAM-AA) produced a significant effect on its photocurrent activities. The photocurrent behavior of TiO2/P(NIPAM) nanocomposites photoelectrode was tunable in response to pH stimuli. It was found that the nanocomposites photoelectrode produced an anodic photocurrent at pH 3.0, whereas it generated a relatively small cathodic photocurrent at pH 10.0. The pHdependent swelling behavior of smart polymer played an important role in the pH-induced photocurrent switching, which actually was an intrinsic feature resulting from a specific electronic structure of the surfacemodified semiconductor with smart polymer. The combination of the photoelectrochemical properties of inorganic semiconductor nanoparticles with external stimuli-responsive properties of organic smart polymers provides a new opportunity for controllable photocurrent switching. 1. Introduction Stimuli-responsive smart polymer interfaces, which switch their physical and chemical properties in response to external stimuli such as temperature, pH, humidity, ionic strength, and solvent composition, have great potential in many technologically important areas such as drug delivery, controlled permeation membrane, actuate microdevices constitution, separation, and so on.1-5 On the other hand, semiconductor nanocrystals, such as titania (TiO2) nanostructures, as another important material, have attracted extensive research interests due to their excellent physicochemical properties and photoinduced activities6,7 and play an important role in applied fields such as photovoltaics, photocatalysis, and photoinduced superhydrophilicity.8,9 Thus, it is expected that the combination of semiconductor nanostructures with these smart polymers may yield functional nanocomposites with synergetic properties and functions. The recent surge of research interest in the materials field is focused on the preparation of these stimuli-responsive polymer-inorganic nanocomposites as well as their fascinating properties and potential applications. For example, Zhu et al.10 synthesized thermosensitive Au/poly(N-isopropylacrylamide) (PNIPAM) hybrid microgels via a covalent bond, and these hybrid microgels exhibited a sharp, reversible, clear-opaque transition in solution between 25 and 30 °C. Kuang and co-workers11 have fabricated multicolor-encoded microspheres by confining CdTe nanocrystals into PNIPVP and studied their controlled release by pH stimuli. Our group also reported the synthesis of a highly photoluminescent CdTe/PNIPAM hydrogel and revealed that its photoluminescence (PL) was reversible and sensitive to external temperature stimuli.12 * To whom correspondence should be addressed: Tel +86 10 6279 5290; Fax +86 10 6279 5290; e-mail
[email protected]. † China Jiliang University. ‡ Tsinghua University.
In addition, molecular and nanoscale assemblies capable of performing logic gate functions have recently attracted significant interest, motivated by the desire to find new ways of information storage and processing.13 In this context, semiconductor-based photoelectrodes capable of stimuli-responsive switching of the photocurrent direction are of tremendous practical significance because of the possibility to control the information processing. During the past years, a substantial amount of research work has been carried out on the design of functional nanostructured semiconductor interfaces that can respond to the external environment and, hence, reversibly regulate the switching of photocurrent direction.14,15 Considering the pioneering work in this area, modification of the semiconductor interfaces with smart polymer has been shown to constitute a useful methodology for controlling their physical and chemical properties. To the best of our knowledge, no attention has been paid to the photocurrent switching behavior of the semiconductor/smart polymer nanocomposites photoelectrode to date. Herein, we described the interfacial functionalization of TiO2 nanoparticles with pH-stimuli responsive smart polymer (poly(N-isopropylacrylamide-co-acrylic acid), P(NIPAMAA)), and further investigated their corresponding pH-dependent photocurrent behaviors. 2. Experimental Section 2.1. Materials and Reagents. TiO2 particles are commercial products (Degussa P25, which consists of about 30% rutile and 70% anatase and a particle size of about 20 nm). N-Isopropylacrylamide (NIPAM, Aldrich) and N,N′-methylenebis(acrylamide) (BA, Aldrich) were recrystallized from hexane and methanol before use, respectively. Acrylic acid (AA, Alfa Aesar) and potassium persulfate (KPS, Alfa Aesar) were used as supplied, without further purification. Unless otherwise specified, all other reagents and materials involved were obtained commercially from the Beijing Chemical Reagent Plant (Beijing,
10.1021/jp100969a 2010 American Chemical Society Published on Web 05/25/2010
Functionalization of TiO2 with Smart Polymers China) and used as received without further purification. Milli-Q purified water (the resistivity g18 MΩ cm) was used during the experimental process. The experiments were carried out at room temperature and humidity. 2.2. Synthesis of Smart Polymer (Poly(N-isopropylacrylamide-co-acrylic acid)). Poly(N-isopropylacrylamide-coacrylic acid) (P(NIPAM-AA)) was synthesized by a similar method reported by Bradley et al.16 The polymerization reaction was carried out in a 500 mL reaction flask, fitted with a reflux condenser, a mechanical stirrer, and a glass nitrogen inlet tube. Typically, a 1.2500 g sample of NIPAM, 0.1289 g of AA, and 0.1251 g of BA were dissolved in 200 mL of deionized water. The reaction mixture was stirred for 30 min, with a nitrogen purge to remove the dissolved oxygen. The oil bath temperature was raised to 70 °C, and 0.1235 g of KPS was added to initiate polymerization. The polymerization reaction system was then kept for 24 h in a 70 °C oil bath under an atmosphere of nitrogen. The obtained polymer products were allowed to cool and then exhaustively dialyzed against Milli-Q purified water to remove any unreacted monomer and other impurities (changing the dialysate twice daily for a week). 2.3. Incorporation of TiO2 with Poly(NIPAM-AA). Before preparing the composites, nanoparticles of TiO2 powders (P25) were suspended in deionized water with the pH adjusted to 1.5 using 37% (v/v) HCl to maintain a positive charge on the particle surface. Large aggregates in the suspension were removed by centrifugation so as to obtain a more homogeneous dispersion of TiO2. A poly(NIPAM-AA) aqueous solution was then mixed with the P25 suspension in a desired loading ratio of 1:5 (w/w) under stirring overnight, and the final pH was adjusted to ∼6, which is close to the known isoelectric point for Degussa P25 of ∼6.2. After aging for about 24 h, the P25/P(NIPAM-AA) composite particles settled to the bottom and the supernatant was removed. The reactant could be further purified by centrifugation at 7000 rpm and redispersed in deionized water. 2.4. Material Characterizations. Transmission electron microscopy (TEM) images were taken with a Hitachi model H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectra were carried out using Perkin-Elmer spectrometer in the frequency range of 4000-600 cm-1 with a resolution of 4 cm-1. The thermal gravimetry (TG) analysis was performed with a STA 409C thermal analyzer (NETZSCH, Germany). Measurements were conducted by heating the samples from 50 to 800 °C at a heating rate of 10 °C/min under flowing argon atmosphere. Mott-Schottky (MS) spectra were carried out on a PARSTAT 2273 potentiostat/galvanostat (Advanced Measurement Technology Inc.) at a frequency of 1 kHz by using one three-electrode cell with the P25 or P25/P(NIPAM-AA) photoanode as the working electrode, a platinum wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. The electrolyte was 0.1 M KCl aqueous solution at different pH values. 2.5. Photocurrent Measurements. The P25 or P25/P(NIPAMAA) paste for the fabrication of a photoanode was obtained by mixing 2 mL of ethanol and 50 mg of P25 or P25/P(NIPAMAA) powder homogeneously. The paste was then spread on the FTO (F-doping SnO2, 15 Ω/sq) conducting glass with a glass rod, using adhesive tapes as spacers. The paste films were further treated at 60 °C in a vacuum overnight for drying. The film thickness measured with a profilometer was about 4 µm. Photocurrent action spectra were measured in a two-electrode configuration home-built experimental system, where the ob-
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Figure 1. Transmission electron microscopy (TEM) images of (A) TiO2 P25 nanoparticles and (B) P25/P(NIPAM-AA) nanocomposites.
tained photoanode served as the working electrode and a platinum wire as the counter electrode.17 A 500 W Xe lamp with a monochromator was used as the light source. The photoelectrochemical cell was illuminated from the FTO side of the photoanode electrode by incident light. The generated photocurrent signal was collected by using a lock-in amplifier (Stanford instrument SR830 DSP) synchronized with a light chopper (Stanford instrument SR540). The monochromatic illuminating light intensity was about 15 µW/cm2 estimated with a radiometer (Photoelectronic Instrument Co. IPAS). The illumination area of the photoanode was about 0.12 cm2. 3. Results and Discussion 3.1. Interfacial Functionalization of TiO2 with Smart Polymers. To make the composites, the pH of the P25 suspension was adjusted to 1.5 by using 37% (v/v) HCl, introducing a positive charge to the surface of P25 nanoparticles.18 Then a poly(NIPAM-AA) aqueous solution was mixed with the P25 suspension under stirring overnight, and the final pH was adjusted to ∼6. During this course, the incorporation of the positively charged P25 nanoparticles with the negatively charged poly(NIPAM-AA) was obtained by the attractive electrostatic forces.19 In addition, it is known that deprotonated sulfonic and carboxylic acid groups can well functionalize inorganic oxide (such as TiO2) surfaces.20-22 Thus, the existence of acrylic acid groups within P(NIPAM-AA) could further facilitate the incorporation of P25 nanoparticles with P(NIPAMAA). Parts A and B of Figure 1 show the TEM images for the TiO2 P25 nanoparticles and the P25/P(NIPAM-AA) nanocomposites, respectively. As shown, particles of P25, having a crystal size of about 20 nm, were discretely distributed, and there were no significant changes in the size and shape of P25 particles before and after the incorporation of P(NIPAM-AA). Compared to the P25 particles, however, the particles of P25/P(NIPAMAA) were more clear, and more agglomerates were observed for the P25/P(NIPAM-AA) nanocomposites, which should be attributed to the accumulation and adhesion of polymer P(NIPAM-AA) coverage on the surface of P25 particles. Attenuated total internal reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was performed to characterize the composition and structure of the P25/P(NIPAM-AA) nanocomposites. Figure 2 shows the spectra taken from P25 particles, pure polymer P(NIPAM-AA), and P25/P(NIPAM-AA) nano-
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Figure 2. ATR-FTIR spectra of (a) P25 nanoparticles, (b) pure P(NIPAM-AA), and (c) P25/P(NIPAM-AA) nanocomposites.
composites. Figure 2a corresponds to the bare TiO2 P25 particles. The bands at lower wavenumber range (500-800 cm-1) were mostly assigned to the vibration of Ti-O-Ti groups. In the spectrum of the P(NIPAM-AA) reference sample (Figure 2b), the N-H stretch at ∼1535 cm-1 and the CdO stretch at ∼1640 cm-1 confirmed the presence of amide groups in the polymer, and the characteristic doublet at 1385 and 1362 cm-1 indicated the presence of the isopropyl group. Additionally, the bands at 1724, 1458, and 1260 cm-1 arose from the carbonyl (CdO) stretching, CH2 stretching, and the C-O stretching of the acrylic acid carboxyl groups in PAA, respectively. The band in the region 2800-3000 cm-1 corresponded to the CH2 and CH3 groups of polymer backbone. These characteristic peaks were consistent with the reported literatures,23,24 indicating the successful synthesis of P(NIPAM-AA). As shown in Figure 2c, however, there appeared a broad band region of 1300-1700 cm-1, which could be associated with various vibrational modes of P(NIPAM-AA). The bands at 1227 and 1148 cm-1 could be assigned to the stretching of C-O-C and C-O-Ti, characteristic of the acrylic acid carboxyl groups linked to titanium.25 In addition, the region of bands below 800 cm-1 was assigned to Ti-O-Ti groups. The above results of FT-IR spectra permit to prove the formation of the P25/P(NIPAM-AA) composite particles. In addition, the composition and structure of the P25/ P(NIPAM-AA) nanocomposites were also characterized using the thermogravimetric analysis. The TG curves for the P25 nanoparticles, the pure polymer P(NIPAM-AA), and P25/ P(NIPAM-AA) nanocomposites are shown in Figure 3. In the case of P25 nanoparticles, it showed the initial weight loss of surface water below 100 °C and the following loss of structural water around 200 °C. And the total weight loss of P25 nanoparticles at the whole temperature range of 50-800 °C was less than 5%, which indicated that P25 had the excellent thermal stability even at high temperature (300-800 °C). For the polymer P(NIPAM-AA), the initial mass loss of ∼8.4% at low temperatures between 50 and 250 °C corresponded to a consequence of anhydride formation, with loss of physically absorbed water and structural water molecules. The major weight loss of ∼63.8% spanned from ∼250 to 450 °C, involving the removal and decomposition of the polymer P(NIPAM-AA) molecules. Careful examination of the TG curve within this range indicates that there might be two separated processes. The first one occurred between ∼250 and 350 °C, corresponding to the anhydride decomposition from the PAA part of P(NIPAMAA), with loss of CO2.26 The second transition from ∼350 to 450 °C was believed to be associated with the polymer backbone
Figure 3. TGA curves of (a) TiO2 P25 nanoparticles, (b) P(NIPAMAA), and (c) P25/P(NIPAM-AA) nanocomposites.
decomposition of P(NIPAM-AA). Subsequently, the decomposition of all organic species between 450 and 600 °C resulted in a nearly complete loss of mass, suggesting the completely thermal decomposition of P(NIPAM-AA) molecules at about 600 °C. As the P25 was incorporated with the smart polymer P(NIPAM-AA), the TG behavior of P25/P(NIPAM-AA) nanocomposites presented synergetic characteristics of P25 nanoparticles and P(NIPAM-AA), as shown in Figure 3c. The TG curve of P25/P(NIPAM-AA) nanocomposites showed a weight loss below 200 °C, which was caused by the evaporation of physically adsorbed free water and the release of structural water in the nanocomposites, while further loss at the higher temperature between 200 and 400 °C was assigned to the depolymerization and anhydride decomposition of P(NIPAM-AA). The major weight loss, between 400 and 500 °C, was attributed to the decomposition of polymer P(NIPAM-AA). The resultant residue of P25/P(NIPAM-AA) nanocomposites, therefore, mainly comprised P25 nanoparticles and possessed a good thermal stability up to 500 °C. In addition, the final weight loss at 600 °C was about 17.8%, indicating that the nanocomposites contained ∼15 wt % P(NIPAM-AA) and ∼85 wt % P25 nanoparticles, which was in agreement with the initial feeding ratio (1:5, w/w) of P(NIPAM-AA) and P25. 3.2. Photocurrent Actions. In order to examine the influence of the environmental pH changes on the photoelectrochemical properties of P25/P(NIPAM-AA) nanocomposites, measurements of the photocurrent action spectra were performed in a home-built photoelectrochemical experimental system. Figure 4 shows the photocurrent action spectra of the P25 nanoparticles and P25/P(NIPAM-AA) nanocomposites in 0.1 M KCl aqueous solution under different pH values. As shown, both samples showed a photocurrent spectrum with the maximum wavelength at about 340 nm corresponding to the band gap of nanocrystalline TiO2, which was blue-shifted from the band gap of bulk TiO2 (387 nm, 3.2 eV), mainly due to the quantum confinement effect.27,28 For the P25 nanoparticles, the generated anodic photocurrent was much higher in an acidic solution at pH 3 than that in an alkaline solution at pH 10. Interestingly, in the case of the P25/P(NIPAM-AA) nanocomposites, the nanocomposites photoelectrode exhibited quite different photocurrent behaviors in the acidic solution (pH 3) and alkaline solution (pH 10). At pH 3.0, the nanocomposites photoelectrode kept a similar photocurrent behavior to that of the P25 nanoparticles photoelectrode with a slightly reduced photocurrent intensity. At pH 10, however, the nanocomposites photoelectrode gener-
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Figure 5. pH-induced switching of photocurrent direction at a P25/ P(NIPAM-AA) nanocomposites electrode upon illumination at λ ) 340 nm in a 0.1 M KCl aqueous solution (at pH ) 3.0 and pH ) 10.0).
Figure 4. (A) Photocurrent action spectra of TiO2 P25 nanoparticles in 0.1 M KCl aqueous solution (at pH ) 3 and pH ) 10). (B) Photocurrent action spectra of P25/P(NIPAM-AA) nanocomposites in 0.1 M KCl aqueous solution (at pH ) 3 and pH ) 10).
ated a relatively small reverse (cathodic) photocurrent; that is, the photocurrent direction was switched by changing the pH of the solution. In addition, the reversible variations in photocurrent for the P25/P(NIPAMA-AA) nanocomposites photoelectrode were recorded as the pH was repeatedly cycled between values of 3.0 and 10.0, as shown in Figure 5. With the pH switching between 3.0 and 10.0, the generated photocurrent of the nanocomposites photoelectrode could be repeatedly cycled between anodic and cathodic photocurrent. Besides, the peak photocurrent gradually descended with an increase of cycle numbers. A possible explanation for the peak photocurrent gradual decrease could be ascribed to the change of the nanocomposites structure due to the exposure of the material to successive pH changes. Desorption and deformation of the polymer from the P25/P(NIPAMA-AA) nanocomposites was possible when successive pH changes were produced on the composite structure. Furthermore, to advance in the understanding of the electrontransfer properties of the synthesized P25/P(NIPAM-AA) nanocomposites, Mott-Schottky (MS) measurements were performed by using the impedance technique.29,30 Figure 6 shows the MS plots of the electrodes based on the P25 TiO2 nanoparticles and P25/P(NIPAM-AA) nanocomposites. Reversed sigmoidal plots were observed with an overall shape consistent with that typical for n-type semiconductors, and the reproducible flat-band potentials, that is, the potentials corresponding to the situation in which there was no charge
Figure 6. Mott-Schottky (MS) plots of (A) P25 nanoparticles and (B) P25/P(NIPAM-AA) nanocomposites thin film electrodes. The MS plots were obtained at a frequency of 1 kHz in 0.1 M KCl aqueous solution with different pH values.
accumulation in the semiconductor so that the energy bands showed no bending, could be obtained from the x-intercepts of the linear region. Compared to the P25 photoelectrode at pH 3.0, the P25 nanoparticles photoelectrode at pH 10.0 showed a large negative shift of the conduction band, which was consistent with the reported literature.31-33 In addition, the P25/P(NIPAMAA) nanocomposites electrode at pH 10.0 also showed a significantly negative shift in the flatband potential as compared
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with the P25/P(NIPAM-AA) nanocomposites electrode at pH 3.0. In most cases, the negative shift in flatband potential with pH change for pure TiO2 could be attributed to the protonation/ deprotonation equilibrium of the electrode surface.34 Perhaps for the P25/P(NIPAM-AA), the negative shift in flatband potential was also due to a similar principle. Notably, the presence of two regions in the MS plot was another matter of discussion and was a characteristic feature observed in P25/ P(NIPAM-AA) nanocomposites photoelectrode at pH 10.0. It was suggested that this effect could be attributed to the generated new surface states in the P25/P(NIPAM-AA) interface at pH 10, which could contribute to the cathodic photocurrent response in the photocurrent action spectrum. The above results indicate that the photocurrent generation of P25 nanoparticles and P25/P(NIPAM-AA) nanocomposites was to a large extent affected by the pH of the solution. Compared to the generated photocurrent at pH 10, the photocurrent enhancement of the P25 nanoparticles photoelectrode at pH 3 was not only due to the positive shift of flatband potential of P25 TiO2 but also due to the more generated photoelectrons and higher electron diffusion in acidic solutions35 as well as the decreased surface resistance of P25 nanoparticles photoelectrode at lower pH.36 Particularly, when the P25 nanoparticles were functionalized with smart polymer P(NIPAMAA), the generated photocurrent of the P25/P(NIPAM-AA) nanocomposites photoelectrode showed a distinct pH dependence. An anodic photocurrent was obtained in acidic solution (pH 3.0), whereas in alkaline solution (pH 10.0) a relatively small cathodic photocurrent was observed. The origin of this effect could be multifold. On the one hand, the combination of the physicochemical properties of inorganic TiO2 nanocrystals with those of an organic smart polymer P(NIPAM-AA) affected the photocurrent actions. It is well-known that the smart polymer P(NIPAM-AA) has pH-dependent swelling behavior. At low pH, the carboxylate groups of P(NIPAM-AA) are hardly ionized, and the polymer is in a relaxed conformation, whereas at high pH, the carboxylate groups are fully ionized, and ionic repulsion between the COO- groups expands the polymer chain to form a stretched structure with more negative charge on its surface.37,38 Thus, the photoelectrochemical properties of TiO2 were the intrinsic driving force for the photocurrent generation on a large scale, and the photocurrent generation also could be tunable by the pH-dependent swelling behavior of the smart polymer P(NIPAM-AA). The dissociation state of the terminal carboxyl group in smart polymer P(NIPAM-AA) could be varied by changing the pH of the solution, which could to some extent lead to the switching of photocurrent direction. In alkaline solutions (at pH ) 10), the carboxylate groups of P(NIPAMAA) were fully ionized, and the surface of P25/P(NIPAM-AA) should be covered with anionic groups such as Ti-O- and -COO-, formed by deprotonation of surface Ti-OH and -COOH. It could be thus assumed that chemical adsorption of oxygen molecules occurred on the surface of P25/P(NIPAMAA) in alkaline solutions through a charge transfer interaction between surface Ti-O- and -COO- group as an electron donor and O2 molecules as an electron acceptor. This led to an increase in the density of chemically adsorbed O2 molecules as well as the rate of electron transfer from the conduction band of TiO2 to the adsorbed O2 molecules, in contrast to the case of only weak physical adsorption of O2 in acidic and neutral solutions. Accordingly, the generation of cathodic photocurrents for P25/ P(NIPAM-AA) in alkaline solutions could be attributed to the effective capture of photogenerated electrons in TiO2 particles by chemisorbed oxidative species (such as O2 molecules) present
Chen and Li in a high density in the solutions, while the observation of the anodic photocurrent for P25/P(NIAPM-AA) in acidic solutions (at pH ) 3) under this condition could thus be explained by assuming that the reaction of holes with water was faster than the capture of electrons by dissolved oxidative species (such as O2 molecules), owing to its weak physical adsorption. On the other hand, many n-type semiconductor photoelectrodes have been reported to generate cathodic photocurrents due to the formation of an inversion layer39 or preferred reaction of the surface with electron acceptors.40-42 The appearance of the cathodic photocurrent in P25/P(NIPAM-AA) at pH 10 could be attributed to efficient electron transfer from the conduction band of P25 nanoparticles to surface states of chemically adsorbed species, the density of which was largely increased in alkaline solutions through a interfacial charge-transfer interaction between surface anionic groups such as Ti-O- as an electron donor and some electron acceptors such as oxygen molecules. The pH-induced photocurrent switching was an intrinsic feature resulting from a specific electronic structure of the surface-modified semiconductor with smart polymer. Nonetheless, the specific explanation for the pH-induced photocurrent switching of P25/P(NIPAM-AA) described here has not been well-known for the moment, and further investigation is still in progress. 4. Conclusions Poly(N-isopropylacrylamide) (PNIPAM), poly(acrylic acid) (PAA), and their copolymers, which are easily synthesized, as well as chemically and mechanically robust, attract extensive research interest for application in the engineering of smart surfaces because of their excellent responding ability to environmental changes in temperature, ionic strength, pH, and solvent. In this work, P25 TiO2 nanoparticles were functionalized by means of the incorporation of smart polymer P(NIPAMAA), and their photocurrent activities of P25/P(NIPAM-AA) nanocomposites were subsequently investigated. The TEM, TGA, and FT-IR characterization results indicated that P25/P(NIPAM-AA) nanocomposites were successfully prepared, and the majority of smart polymer P(NIPAM-AA) were located around the surface of P25 nanoparticles. Compared with the mere P25 nanoparticles, the incorporation of P25 nanoparticles with smart polymer P(NIPAM-AA) produced a significant effect on its photocurrent activities. It was found that P25/P(NIPAM-AA) nanocomposites photoelectrode exhibited pH-dependent switchable photocurrent behaviors. At pH 3.0, an anodic photocurrent was obtained at the nanocomposites electrode, whereas it produced a relatively small cathodic photocurrent at pH 10.0. This phenomenon of pH-induced photocurrent switching must be ascribed to the incorporation of P25 with smart polymer P(NIPAM-AA), leading to the changes of interfacial structures of P25 nanoparticles as well as their photoelectrochemical properties. The change of the dissociation state of the terminal carboxyl group in smart polymer P(NIPAM-AA), caused by changing the pH of the solution, could to some extent lead to the switching of photocurrent direction. At the same time, the P25/P(NIPAMAA) nanocomposites, different from those of similar characteristics, were not composites comprising of two types of semiconductors but rather uniform surface-complex materials. The pH-induced photocurrent switching was an intrinsic feature resulting from a specific electronic structure of the surfacemodified semiconductor with smart polymer. Thus, the present work provides a simple alternative method to effectively modulate the switching of photocurrent direction through the
Functionalization of TiO2 with Smart Polymers coupling of semiconductor nanoparticles and smart polymers, although the mechanism of the pH-induced photocurrent switching still remains an intriguing issue and needs to be explored further. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20975060 and No. 60808016), National Basic Research Program of China (No. 2007CB310500), and Scientific Research Foundation for Talented Scholars in China Jiliang University (01101000278). References and Notes (1) Kiser, P. F.; Wilson, G.; Needham, D. Nature 1998, 394, 459. (2) Chu, L.; Li, Y.; Zhu, J.; Chen, W. Angew. Chem., Int. Ed. 2005, 44, 2124. (3) Park, T. G. Biomaterials 1999, 20, 517. (4) Xi, J. Z.; Schmidt, J. J.; Montemagno, C. D. Nat. Mater. 2005, 4, 180. (5) Buchholz, B. A.; Doherty, E. A. S.; Albarghouthi, M. N.; Bogdan, F. M.; Zahn, J. M.; Barron, A. E. Anal. Chem. 2001, 73, 157. (6) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (7) Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. J. Phys. Chem. B 2002, 106, 2967. (8) Goto, H.; Hanada, Y.; Ohno, T.; Matshmura, M. J. Catal. 2004, 225, 223. (9) Liu, S. H.; Zhang, Z. H.; Han, M. Y. AdV. Mater. 2005, 17, 1862. (10) Zhu, M. Q.; Wang, L. Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656. (11) Kuang, M.; Wang, D.; Bao, H.; Gao, M.; Mo¨hwald, H.; Jiang, M. AdV. Mater. 2005, 17, 267. (12) Li, J.; Hong, X.; Liu, Y.; Li, D.; Wang, Y. W.; Li, J. H.; Bai, Y. B.; Li, T. J. AdV. Mater. 2005, 17, 163. (13) Long, M. C.; Beranek, R.; Cai, W. M.; Kisch, H. Electrochim. Acta 2008, 53, 4621. (14) Beranek, R.; Kisch, H. Angew. Chem., Int. Ed. 2008, 47, 1320. (15) Szacilowski, K.; Macyk, W.; Stochel, G. J. Am. Chem. Soc. 2006, 128, 4550. (16) Bradley, M.; Ramos, J.; Vincent, B. Langmuir 2005, 21, 1209. (17) Chen, D.; Zhang, H.; Hu, S.; Li, J. H. J. Phys. Chem. C 2008, 112, 117. (18) Coutinho, C. A.; Harrinauth, R. K.; Gupta, V. K. Colloids Surf., A 2008, 318, 111.
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