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Photoelectrochemical switch based on cis-Azobenzene chromophore modified TiO2 nanowires
Optics Communications 284 (2011) 4991–4995
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Photoelectrochemical switch based on cis-Azobenzene chromophore modified TiO2 nanowires Xiaojun Lv a,⁎, Haixin Chang c, Hao Zhang b a b c
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Department of Chemistry, Key Lab. of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, China WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8578, Japan
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 21 June 2011 Accepted 23 June 2011 Available online 7 July 2011 Keywords: TiO2 nanowires Azobenzene modified Photoelctrochemical switch Higher photocurrent
a b s t r a c t Carboxylated-azobenzene chromophore modified TiO2 nanowire composites were prepared and characterized. Photocurrent measured with monochromatic incident light irradiation results showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response covering range of 350–650 nm, and the wavelength position corresponding to the maximum photocurrent was red shift to about 470 nm. After alternate irradiation with UV and visible light, the azobenzene modified TiO2 nanowire electrode exhibited obvious photoelectrochemical switching properties. Furthermore, the photocurrent under visible light irradiation was much higher than that under UV irradiation due to the cis-totrans isomerization transformation of azobenzene chromophore. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The photochromism of azobenzene and its derivatives, originating from the uniquely reversible trans–cis photoisomerization, has been studied with immense interest, because of its potential applications in areas such as optical storage [1], photoactive biomaterials [2], diffractive optical elements [3], and molecular switching devices [4]. And azobenzene modified material surface has attracted a great deal of interests in the past decade because of their numerous potential applications [5–8]. Azobenzene modified Si surface has good switchable photoisomerizability in response to alternating UV and visible light exposure, which can be expected to be applied in microelectronics and molecular electronics [5]. Paolo Samori [8] et al. found that azobenzene SAMs incorporated in a junction between an Au (111) surface and a mercury drop were able to lift the “heavy” Hg drop, and reversibly photoswitch the current flowing through the junction. All these investigations illustrated that a new class of devices based on the optically switchable nanoelectronic circuitry by azobenzene molecules organized in films can be fabricated. TiO2 as a wide bandgap n-type metal oxide semiconductor has been extensively investigated in some fields due to their good stability, nontoxicity, low cost, and high catalytic activity. And the photoelectrochemical switching of hybrid semiconductor electrode based TiO2 nanoparticle researches have also been widely ⁎ Corresponding author. Tel.: + 86 10 82543520. E-mail address: [email protected] (X. Lv).
studied [9–11]. Some bilayers or core–shell composites of organic polymers and TiO2 [12], or TiO2–N and CuI (p-type) hybrid deposited on ITO [9], as well as TiO2 modified with FeII complexes [13] or with a ruthenium cluster dye [14] showed good photoelectrochemical switching. Compared to TiO2 nanoparticle, TiO2 nanowires have better charge transport properties and larger specific surface area than those of bulk or two dimensional (2D) nanostructures. Thus, TiO2 nanowires are considered as highly desired structures for both photovoltaic and photoelectrochemical applications. However, azobenzene modified TiO2 nanowires for the photoelectrochemical switchable photoisomerizability are few. The azobenzene molecules contained strong donor and acceptor groups, which can also enhance the photocurrent of TiO2 electrode in visible light region as dyestuffs. And lots of investigates had proven that the carboxyl can link to TiO2 surface through Ti\O binding bonds [15]. In this paper, carboxylated-azobenzene modified TiO2 nanowires were prepared and their photoelectrochemical switching was investigated by photocurrent versus time test under alternate irradiation with UV and visible light. Photocurrent measured with monochromatic incident light results showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response. After alternate irradiation with UV and visible light, the azobenzene modified TiO2 nanowire electrode exhibits obvious photoelectrochemical switching properties. These results indicated that one new type photocontrolled switching process on a functional molecule-modified TiO2 nanowire surface was possibility to realize.
0030-4018/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.06.047
4992
X. Lv et al. / Optics Communications 284 (2011) 4991–4995
2. Experimental sections 2.1. Preparation of TiO2 nanowire arrays TiO2 nanowires were prepared by a modified method developed by Choi et al. [16]. Typically, the polished Ti foil (0.25 mm thick, 99.38% purity, from Beijing General Research Institute for Nonferrous Metals) was pretreated by rinsing in an ultrasonic bath of acetone, alcohol and deionized water for 5 min in turn, then chemically etched by immersing in a mixture solution of HF, HNO3 and H2O acids in the ratio of 1:4:5 (v/v) for 30 s, rinsed in an ultrasonic bath of acetone, isopropanol and methyl alcohol for 5 min in turn, and finally rinsed in deionized water. At the end, the obtained substrate was dried with N2 at room temperature. The anodization process was carried out at room temperature using a direct current power supply (Dahua Wireless Instrument Co., Beijing) in a two-electrode electrolytic cell, with the pretreated Ti foil serving as the anode and Pt foil serving as the cathode. The anodizing voltage increased gradually from 0 to 50 V with an increasing rate of 100 mV/s and was then kept at 50 V for 10 h. The electrolyte was 15 mL of ethylene glycol (EG) solution containing NH4F (0.2 wt.%) and 500 μL deionized water. Generally, a small amount of water in the EG solution is essential for the formation of anodic TiO2 nanowires at a high potential. After the anodization, the obtained substrate was immediately rinsed with deionized water and dried with a N2 stream. Then, the anodized substrate was annealed at 450 °C in oxygen for 3 h with the cooling rate of 2 °C min − 1 to convert the amorphous phase to the anatase crystalline phase. 2.2. Synthesis of carboxyl azobenzene and linked to TiO2 nanowires surface Synthesis of carboxyl azobenzene (shown in Fig. 1) followed a method that is developed by Takahiro Seki et al. with some modifications [17]. The process of introducing the carboxyl azobenzene was carried out by solution immersion. TiO2 nanowires were annealed at 450 °C for 3 h and then cooled in air to 80 °C, then immediately immersed in 0.1 mmol/L carboxyl azobenzene ethanol solution for two days, and finally rinsed for three times with ethanol to wipe off the superfluous azobenzene on the surface of TiO2 nanowires. 2.3. Material characterization SEM images were acquired on a field emission (FE) scanning electron microscope (JEOL JSM-7401 F) operated at 3.0 kV. Fourier transform infrared (FTIR) spectra were carried out using Perkin Elmer spectrometer in the frequency range of 4000–500 cm − 1 with a resolution of 4 cm − 1. Light absorption properties were measured using a UV–vis spectrophotometer (UV-160A) with a wavelength range of 250–600 nm. 2.4. Photoelectrochemical measurements Photocurrent action spectra were measured in a two-electrode configuration, home-built experimental system, where the sintered TiO2 photoanode served as the working electrode with an active area of about 1 cm 2 defined using Teflon tape and a platinum wire was
Fig. 1. Molecule strucuture of carboxyl azobenzene.
used as the counter electrode. The generated photocurrent signal was collected by using a lock-in amplifier (Stanford instrument SR830 DSP) synchronized with a light chopper (Stanford instrument SR540). Photocurrent density vs bias potential characteristic was conducted with a three-electrode cell, using the sintered photoanode as the working electrode, a platinum wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. Photocurrent density vs bias potential characteristic spectra was obtained with a PARSTAT-2273 Advanced Electrochemical System (Princeton Applied Research) controlled by a computer. A 500-W Xe lamp with a monochromator and equipped with AM 1.5 filter (filter off lights with wavelength shorter than 400 nm for 100 mW/cm 2 visible light measurements) respectively, was used as the light source. The electrolyte was 0.1 M KNO3 aqueous solution. All measurements were carried out after bubbling N2 for 20 min and controlled automatically by a computer. 3. Results and discussion 3.1. Structure characterization of cis-azobenzene chromophore linked TiO2 nanowire arrays Fig. 2 A shows that the azobenzene molecules displayed photochromic behavior in ethanol solution under different irradiation. The spectra are typical of azobenzene chromophore. The maximum absorption peak centered at 350 nm is attributed to the electronic transition along the long axis of the trans isomer (π–π* transition, see curve (a) of Fig. 2 A). Irradiation with UV light for 30 s caused the absorption peak at 350 nm obviously decreased, but the absorption peak at near 450 nm increased (n–π*, curve (b) of Fig. 2 A), indicating the occurrence of trans to cis photoisomerization. And from Fig. 2 A (c) we can see that the cis isomer thus produced could be obviously reconverted to the trans isomer again under visible illumination for 30 s, which resulted in a recovery of the spectral features of trans isomer. It is seen that the recovered absorbance of trans-azobenzene is slightly lower than that before irradiation. However, the change in absorbance became completely reversible upon subsequent cycles of UV and visible light irradiations [18]. Fig. 2 B compares the FTIR spectra of azobenzene and azobenzene modified TiO2 nanowire surface with the transmittance infrared spectrum of azobenzene dispersed in KBr. From Fig. 2 B we observed that the IR spectra of the azobenzene and azobenzene modified TiO2 nanowires are similar. Both Fig. 2 B (a) and 2 B (b) spectra contained absorptions for typical antisymmetric and symmetric C–H stretching vibration band at 2840–3000 cm − 1, the peak at 3434 cm − 1 was OH stretching, and the N\N deformation band was at 1393 cm − 1, the C\O and C–O stretching vibration peaks were at 1691 cm − 1 and 1237 cm − 1, respectively. And the peak at 1601 cm − 1 and 1502 cm − 1 was attributed to aryl. From Fig. 2 B (b) we can see that the band at 1691 cm − 1 assigned to the C\O stretching of the carboxylic acid almost disappeared upon adsorption on TiO2 nanowires. Meanwhile, new bands at 1380 and 1630 cm − 1 assigned to the carboxylate symmetric and asymmetric –COO-stretching appeared, indicating a strong chemical bond interaction between the carboxylate group of the adsorbed azobenzene molecules and the TiO2 nanowire substrate. Fig. 2 C and D shows typical FESEM images of the morphologies of TiO2 nanowires and azobenzene modified TiO2 nanowires. As Fig. 2 C shows, the TiO2 nanowires were more than 10 μm in length and 60 nm in width, and the bottom TiO2 nanotubes with a diameter of about 100 nm remained underneath the nanowires. TiO2 nanowires were fabricated by vertically splitting off into several parts by the electric-field-directed etching of the highly organized nanotube arrays which were formed at the beginning of the anodization. The splitting led to the formation of TiO2 nanowires on the entire TiO2 nanotube arrays as shown in Fig. 2 C. And after linked azobenzene, the surface of most nanowires absorbed thickly dotted azobenzene (seen
X. Lv et al. / Optics Communications 284 (2011) 4991–4995
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Fig. 2. (A) UV photochromic spectra of azobenzene. (B) FTIR spectra of azobenzene and azobenzene modified TiO2 nanowires. (C) The FESEM images of TiO2 nanowires. (D) The FESEM images of azobenzene modified TiO2 nanowires.
in Fig. 2 D), which also showed that the TiO2 can directly link with the carboxyl in the azobenzene [16].
3.2. Photoelectrochemical characterization Fig. 3 A shows that the photoelectrochemical properties for both TiO2 nanowire and azobenzene modified TiO2 nanowire electrodes. As shown in Fig. 3 A, TiO2 nanowire electrodes showed the maximum photocurrent with wavelength at 375.8 nm corresponding to the band gap of nanocrystalline TiO2, and exhibited little photoresponse in the visible light region. However, azobenzene modified TiO2 nanowire electrode showed the maximum photocurrent at about 470 nm, and higher and broader visible light response covering the range of 350– 650 nm, which demonstrated that the azobenzene chromophore had attached and covered the surface of TiO2 nanowire electrode, and the azobenzene chromophore had absorbed the visible light and generated electric as good dyestuff. The results indicated that azobenzene molecule contained strong donor and acceptor groups and π-conjugated orbitals, and azobenzene modified TiO2 nanowire electrodes facilitated a strong and sustainable visible photoresponse. To further investigate the photoinduced behavior of the generated photocurrent, the photocurrent responses of TiO2 nanowires with and without attached azobenzene thin films upon the on–off illumination with a monochromatic incident light (at 450 nm) were also investigated (Fig. 3 B). When the light was subsequently switched on and off, a series of almost identical electric signals were obtained. For the azobenzene modified TiO2 nanowire electrodes, there was
obvious photocurrent with the illumination at 450 nm (curve a), and little or no photocurrent was detected for TiO2 nanowire film upon light irradiation at 450 nm, which indicated that the azobenzene modified TiO2 nanowire electrodes had better photocurrent response in visible area.
3.3. Photoelectrochemical switching The photochemical switching properties of azobenzene modified TiO2 nanowire electrodes were assessed by the photocurrent I–t curve, as seen in Fig. 4 A. The I–t curves displayed a drastically different photocurrent on different azobenzene molecular conformation. Fig. 4A shows that there was no obvious photocurrent under dark conditions. However, under UV irradiation for 30 s, the typical I–t curve immediately showed that there was obvious photocurrent generated, and the illumination was turned off, photocurrent immediately disappeared. However, under 30 s visible light irradiation, the transient generated photocurrent became to a higher current compared to under UV irradiation, which was because the cis-to-trans isomerization of azobenzene under visible irradiation. Thus, the azobenzene modified TiO2 nanowire electrodes exhibit clearly photochemical switching properties under alternate irradiation with UV and visible light. The scheme of photoresponsive TiO2 surface modified azobenzene was demonstrated in Fig. 4 B. And after much cycles, the switching characteristic of two kinds of isomers shows well stability and reproducibility, which indicates that the azobenzene species maintained good photochromic properties in the TiO2
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X. Lv et al. / Optics Communications 284 (2011) 4991–4995
Fig. 4. (A) I–t response of azobenzene modified TiO2 nanowires electrodes upon UV and Vis irradiation with the same irradiation intensity. (B) Scheme of photoresponsive TiO2 surface modified azobenzene-containing molecules Atom labels: gray (C), red (O), blue (N); H atoms are omitted.
Fig. 3. (A) Normalized of photocurrents of (a) TiO2 nanowires and (b)azobenzene modified TiO2 nanowires as photoanodes in 0.1 M KNO3 solution under Xe lamp irradiation. (B) Transient photocurrent generated under pulse visible light illumination at 450 nm (I–t) without bias in a two-electrode configuration for the (a) TiO2 nanowires and (b) azobenzene modified TiO2 nanowire as photoanodes in 0.1 M KNO3 solution.
nanowire electrodes. Trans and cis structural azobenzene molecules modified TiO2 nanowire surfaces show a difference in photocurrent characteristic, which is likely to be the result of the fact that a trans isomer has a more favorable electronic conjugate along the long axis than a cis isomer [5].
electrodes. Photocurrent measured showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response. These results show that azobenzene modified TiO2 nanowire is expected to be promising candidates in potential applications such as optical storage and molecular switching devices. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 20975060), National Basic Research Program of China (No. 2007CB310500). Thanks professor Jinghong Li for the advice and suggestion of experiments and discussion. H.X.C. acknowledges the WPI Initiative funding for young researchers from JSPS and MEXT, Japan. References
4. Conclusion In summary, azobenzene modified TiO 2 nanowires were successfully prepared and their photochemical switching properties were researched through I–t measured under alternate irradiation with UV and visible light. And the I–t curve measured results showed that the azobenzene modified TiO2 nanowire electrodes exhibited obvious photochemical switching properties under alternating irradiation with UV and visible light. The trans/cis conformational transformation of azobenzene on the TiO2 nanowire electrodes can be achieved by alternating UV and visible light irradiation. After many cycles, the switching characteristic of two kinds of isomers shows well stability and reproducibility, which indicated that the azobenzene species maintained good photochromic properties in the TiO2 nanowire
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Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Photoelectrochemical switch based on cis-Azobenzene chromophore modified TiO2 nanowires Xiaojun Lv a,⁎, Haixin Chang c, Hao Zhang b a b c
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Department of Chemistry, Key Lab. of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, China WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8578, Japan
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 21 June 2011 Accepted 23 June 2011 Available online 7 July 2011 Keywords: TiO2 nanowires Azobenzene modified Photoelctrochemical switch Higher photocurrent
a b s t r a c t Carboxylated-azobenzene chromophore modified TiO2 nanowire composites were prepared and characterized. Photocurrent measured with monochromatic incident light irradiation results showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response covering range of 350–650 nm, and the wavelength position corresponding to the maximum photocurrent was red shift to about 470 nm. After alternate irradiation with UV and visible light, the azobenzene modified TiO2 nanowire electrode exhibited obvious photoelectrochemical switching properties. Furthermore, the photocurrent under visible light irradiation was much higher than that under UV irradiation due to the cis-totrans isomerization transformation of azobenzene chromophore. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The photochromism of azobenzene and its derivatives, originating from the uniquely reversible trans–cis photoisomerization, has been studied with immense interest, because of its potential applications in areas such as optical storage [1], photoactive biomaterials [2], diffractive optical elements [3], and molecular switching devices [4]. And azobenzene modified material surface has attracted a great deal of interests in the past decade because of their numerous potential applications [5–8]. Azobenzene modified Si surface has good switchable photoisomerizability in response to alternating UV and visible light exposure, which can be expected to be applied in microelectronics and molecular electronics [5]. Paolo Samori [8] et al. found that azobenzene SAMs incorporated in a junction between an Au (111) surface and a mercury drop were able to lift the “heavy” Hg drop, and reversibly photoswitch the current flowing through the junction. All these investigations illustrated that a new class of devices based on the optically switchable nanoelectronic circuitry by azobenzene molecules organized in films can be fabricated. TiO2 as a wide bandgap n-type metal oxide semiconductor has been extensively investigated in some fields due to their good stability, nontoxicity, low cost, and high catalytic activity. And the photoelectrochemical switching of hybrid semiconductor electrode based TiO2 nanoparticle researches have also been widely ⁎ Corresponding author. Tel.: + 86 10 82543520. E-mail address: [email protected] (X. Lv).
studied [9–11]. Some bilayers or core–shell composites of organic polymers and TiO2 [12], or TiO2–N and CuI (p-type) hybrid deposited on ITO [9], as well as TiO2 modified with FeII complexes [13] or with a ruthenium cluster dye [14] showed good photoelectrochemical switching. Compared to TiO2 nanoparticle, TiO2 nanowires have better charge transport properties and larger specific surface area than those of bulk or two dimensional (2D) nanostructures. Thus, TiO2 nanowires are considered as highly desired structures for both photovoltaic and photoelectrochemical applications. However, azobenzene modified TiO2 nanowires for the photoelectrochemical switchable photoisomerizability are few. The azobenzene molecules contained strong donor and acceptor groups, which can also enhance the photocurrent of TiO2 electrode in visible light region as dyestuffs. And lots of investigates had proven that the carboxyl can link to TiO2 surface through Ti\O binding bonds [15]. In this paper, carboxylated-azobenzene modified TiO2 nanowires were prepared and their photoelectrochemical switching was investigated by photocurrent versus time test under alternate irradiation with UV and visible light. Photocurrent measured with monochromatic incident light results showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response. After alternate irradiation with UV and visible light, the azobenzene modified TiO2 nanowire electrode exhibits obvious photoelectrochemical switching properties. These results indicated that one new type photocontrolled switching process on a functional molecule-modified TiO2 nanowire surface was possibility to realize.
0030-4018/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.06.047
4992
X. Lv et al. / Optics Communications 284 (2011) 4991–4995
2. Experimental sections 2.1. Preparation of TiO2 nanowire arrays TiO2 nanowires were prepared by a modified method developed by Choi et al. [16]. Typically, the polished Ti foil (0.25 mm thick, 99.38% purity, from Beijing General Research Institute for Nonferrous Metals) was pretreated by rinsing in an ultrasonic bath of acetone, alcohol and deionized water for 5 min in turn, then chemically etched by immersing in a mixture solution of HF, HNO3 and H2O acids in the ratio of 1:4:5 (v/v) for 30 s, rinsed in an ultrasonic bath of acetone, isopropanol and methyl alcohol for 5 min in turn, and finally rinsed in deionized water. At the end, the obtained substrate was dried with N2 at room temperature. The anodization process was carried out at room temperature using a direct current power supply (Dahua Wireless Instrument Co., Beijing) in a two-electrode electrolytic cell, with the pretreated Ti foil serving as the anode and Pt foil serving as the cathode. The anodizing voltage increased gradually from 0 to 50 V with an increasing rate of 100 mV/s and was then kept at 50 V for 10 h. The electrolyte was 15 mL of ethylene glycol (EG) solution containing NH4F (0.2 wt.%) and 500 μL deionized water. Generally, a small amount of water in the EG solution is essential for the formation of anodic TiO2 nanowires at a high potential. After the anodization, the obtained substrate was immediately rinsed with deionized water and dried with a N2 stream. Then, the anodized substrate was annealed at 450 °C in oxygen for 3 h with the cooling rate of 2 °C min − 1 to convert the amorphous phase to the anatase crystalline phase. 2.2. Synthesis of carboxyl azobenzene and linked to TiO2 nanowires surface Synthesis of carboxyl azobenzene (shown in Fig. 1) followed a method that is developed by Takahiro Seki et al. with some modifications [17]. The process of introducing the carboxyl azobenzene was carried out by solution immersion. TiO2 nanowires were annealed at 450 °C for 3 h and then cooled in air to 80 °C, then immediately immersed in 0.1 mmol/L carboxyl azobenzene ethanol solution for two days, and finally rinsed for three times with ethanol to wipe off the superfluous azobenzene on the surface of TiO2 nanowires. 2.3. Material characterization SEM images were acquired on a field emission (FE) scanning electron microscope (JEOL JSM-7401 F) operated at 3.0 kV. Fourier transform infrared (FTIR) spectra were carried out using Perkin Elmer spectrometer in the frequency range of 4000–500 cm − 1 with a resolution of 4 cm − 1. Light absorption properties were measured using a UV–vis spectrophotometer (UV-160A) with a wavelength range of 250–600 nm. 2.4. Photoelectrochemical measurements Photocurrent action spectra were measured in a two-electrode configuration, home-built experimental system, where the sintered TiO2 photoanode served as the working electrode with an active area of about 1 cm 2 defined using Teflon tape and a platinum wire was
Fig. 1. Molecule strucuture of carboxyl azobenzene.
used as the counter electrode. The generated photocurrent signal was collected by using a lock-in amplifier (Stanford instrument SR830 DSP) synchronized with a light chopper (Stanford instrument SR540). Photocurrent density vs bias potential characteristic was conducted with a three-electrode cell, using the sintered photoanode as the working electrode, a platinum wire as the counter electrode, and a standard Ag/AgCl in saturated KCl as the reference electrode. Photocurrent density vs bias potential characteristic spectra was obtained with a PARSTAT-2273 Advanced Electrochemical System (Princeton Applied Research) controlled by a computer. A 500-W Xe lamp with a monochromator and equipped with AM 1.5 filter (filter off lights with wavelength shorter than 400 nm for 100 mW/cm 2 visible light measurements) respectively, was used as the light source. The electrolyte was 0.1 M KNO3 aqueous solution. All measurements were carried out after bubbling N2 for 20 min and controlled automatically by a computer. 3. Results and discussion 3.1. Structure characterization of cis-azobenzene chromophore linked TiO2 nanowire arrays Fig. 2 A shows that the azobenzene molecules displayed photochromic behavior in ethanol solution under different irradiation. The spectra are typical of azobenzene chromophore. The maximum absorption peak centered at 350 nm is attributed to the electronic transition along the long axis of the trans isomer (π–π* transition, see curve (a) of Fig. 2 A). Irradiation with UV light for 30 s caused the absorption peak at 350 nm obviously decreased, but the absorption peak at near 450 nm increased (n–π*, curve (b) of Fig. 2 A), indicating the occurrence of trans to cis photoisomerization. And from Fig. 2 A (c) we can see that the cis isomer thus produced could be obviously reconverted to the trans isomer again under visible illumination for 30 s, which resulted in a recovery of the spectral features of trans isomer. It is seen that the recovered absorbance of trans-azobenzene is slightly lower than that before irradiation. However, the change in absorbance became completely reversible upon subsequent cycles of UV and visible light irradiations [18]. Fig. 2 B compares the FTIR spectra of azobenzene and azobenzene modified TiO2 nanowire surface with the transmittance infrared spectrum of azobenzene dispersed in KBr. From Fig. 2 B we observed that the IR spectra of the azobenzene and azobenzene modified TiO2 nanowires are similar. Both Fig. 2 B (a) and 2 B (b) spectra contained absorptions for typical antisymmetric and symmetric C–H stretching vibration band at 2840–3000 cm − 1, the peak at 3434 cm − 1 was OH stretching, and the N\N deformation band was at 1393 cm − 1, the C\O and C–O stretching vibration peaks were at 1691 cm − 1 and 1237 cm − 1, respectively. And the peak at 1601 cm − 1 and 1502 cm − 1 was attributed to aryl. From Fig. 2 B (b) we can see that the band at 1691 cm − 1 assigned to the C\O stretching of the carboxylic acid almost disappeared upon adsorption on TiO2 nanowires. Meanwhile, new bands at 1380 and 1630 cm − 1 assigned to the carboxylate symmetric and asymmetric –COO-stretching appeared, indicating a strong chemical bond interaction between the carboxylate group of the adsorbed azobenzene molecules and the TiO2 nanowire substrate. Fig. 2 C and D shows typical FESEM images of the morphologies of TiO2 nanowires and azobenzene modified TiO2 nanowires. As Fig. 2 C shows, the TiO2 nanowires were more than 10 μm in length and 60 nm in width, and the bottom TiO2 nanotubes with a diameter of about 100 nm remained underneath the nanowires. TiO2 nanowires were fabricated by vertically splitting off into several parts by the electric-field-directed etching of the highly organized nanotube arrays which were formed at the beginning of the anodization. The splitting led to the formation of TiO2 nanowires on the entire TiO2 nanotube arrays as shown in Fig. 2 C. And after linked azobenzene, the surface of most nanowires absorbed thickly dotted azobenzene (seen
X. Lv et al. / Optics Communications 284 (2011) 4991–4995
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Fig. 2. (A) UV photochromic spectra of azobenzene. (B) FTIR spectra of azobenzene and azobenzene modified TiO2 nanowires. (C) The FESEM images of TiO2 nanowires. (D) The FESEM images of azobenzene modified TiO2 nanowires.
in Fig. 2 D), which also showed that the TiO2 can directly link with the carboxyl in the azobenzene [16].
3.2. Photoelectrochemical characterization Fig. 3 A shows that the photoelectrochemical properties for both TiO2 nanowire and azobenzene modified TiO2 nanowire electrodes. As shown in Fig. 3 A, TiO2 nanowire electrodes showed the maximum photocurrent with wavelength at 375.8 nm corresponding to the band gap of nanocrystalline TiO2, and exhibited little photoresponse in the visible light region. However, azobenzene modified TiO2 nanowire electrode showed the maximum photocurrent at about 470 nm, and higher and broader visible light response covering the range of 350– 650 nm, which demonstrated that the azobenzene chromophore had attached and covered the surface of TiO2 nanowire electrode, and the azobenzene chromophore had absorbed the visible light and generated electric as good dyestuff. The results indicated that azobenzene molecule contained strong donor and acceptor groups and π-conjugated orbitals, and azobenzene modified TiO2 nanowire electrodes facilitated a strong and sustainable visible photoresponse. To further investigate the photoinduced behavior of the generated photocurrent, the photocurrent responses of TiO2 nanowires with and without attached azobenzene thin films upon the on–off illumination with a monochromatic incident light (at 450 nm) were also investigated (Fig. 3 B). When the light was subsequently switched on and off, a series of almost identical electric signals were obtained. For the azobenzene modified TiO2 nanowire electrodes, there was
obvious photocurrent with the illumination at 450 nm (curve a), and little or no photocurrent was detected for TiO2 nanowire film upon light irradiation at 450 nm, which indicated that the azobenzene modified TiO2 nanowire electrodes had better photocurrent response in visible area.
3.3. Photoelectrochemical switching The photochemical switching properties of azobenzene modified TiO2 nanowire electrodes were assessed by the photocurrent I–t curve, as seen in Fig. 4 A. The I–t curves displayed a drastically different photocurrent on different azobenzene molecular conformation. Fig. 4A shows that there was no obvious photocurrent under dark conditions. However, under UV irradiation for 30 s, the typical I–t curve immediately showed that there was obvious photocurrent generated, and the illumination was turned off, photocurrent immediately disappeared. However, under 30 s visible light irradiation, the transient generated photocurrent became to a higher current compared to under UV irradiation, which was because the cis-to-trans isomerization of azobenzene under visible irradiation. Thus, the azobenzene modified TiO2 nanowire electrodes exhibit clearly photochemical switching properties under alternate irradiation with UV and visible light. The scheme of photoresponsive TiO2 surface modified azobenzene was demonstrated in Fig. 4 B. And after much cycles, the switching characteristic of two kinds of isomers shows well stability and reproducibility, which indicates that the azobenzene species maintained good photochromic properties in the TiO2
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Fig. 4. (A) I–t response of azobenzene modified TiO2 nanowires electrodes upon UV and Vis irradiation with the same irradiation intensity. (B) Scheme of photoresponsive TiO2 surface modified azobenzene-containing molecules Atom labels: gray (C), red (O), blue (N); H atoms are omitted.
Fig. 3. (A) Normalized of photocurrents of (a) TiO2 nanowires and (b)azobenzene modified TiO2 nanowires as photoanodes in 0.1 M KNO3 solution under Xe lamp irradiation. (B) Transient photocurrent generated under pulse visible light illumination at 450 nm (I–t) without bias in a two-electrode configuration for the (a) TiO2 nanowires and (b) azobenzene modified TiO2 nanowire as photoanodes in 0.1 M KNO3 solution.
nanowire electrodes. Trans and cis structural azobenzene molecules modified TiO2 nanowire surfaces show a difference in photocurrent characteristic, which is likely to be the result of the fact that a trans isomer has a more favorable electronic conjugate along the long axis than a cis isomer [5].
electrodes. Photocurrent measured showed that azobenzene modified TiO2 nanowire electrode had obviously higher photocurrent and broader visible light response. These results show that azobenzene modified TiO2 nanowire is expected to be promising candidates in potential applications such as optical storage and molecular switching devices. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 20975060), National Basic Research Program of China (No. 2007CB310500). Thanks professor Jinghong Li for the advice and suggestion of experiments and discussion. H.X.C. acknowledges the WPI Initiative funding for young researchers from JSPS and MEXT, Japan. References
4. Conclusion In summary, azobenzene modified TiO 2 nanowires were successfully prepared and their photochemical switching properties were researched through I–t measured under alternate irradiation with UV and visible light. And the I–t curve measured results showed that the azobenzene modified TiO2 nanowire electrodes exhibited obvious photochemical switching properties under alternating irradiation with UV and visible light. The trans/cis conformational transformation of azobenzene on the TiO2 nanowire electrodes can be achieved by alternating UV and visible light irradiation. After many cycles, the switching characteristic of two kinds of isomers shows well stability and reproducibility, which indicated that the azobenzene species maintained good photochromic properties in the TiO2 nanowire
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