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Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate
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APSUSC-32489; No. of Pages 5
Applied Surface Science xxx (2016) xxx–xxx
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Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate Yunhan Ling a,∗ , Yuqing Zhuo a , Liang Huang a , Duolu Mao b a b
Lab of Advanced Materials, School of Materials Science & Engineering, Tsinghua University, Beijing 100084, PR China School of Physical and Electronic Information Engineering, Qinghai Nationalities University, Xining, Qinghai 810007, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Available online xxx Keywords: TiO2 nanotubes array Surface-enhanced Raman scattering (SERS) Silver nanoparticle Photocatalysis Self-cleaning
a b s t r a c t A simple strategy for synthesizing Ag-loaded TiO2 nanotube film for use as multifunctional photocatalyst and recyclable surface-enhanced Raman scattering (SERS) substrate is introduced. Highly aligned TiO2 nanotube arrays (TNTA) prepared via electrochemical anodization were used as a 3D rough host for silver nanoparticles. Ag deposits were sputtered in a vacuum, and it was found that their morphologies were mainly influenced by the diameters of nanotubes and the UV irradiation induced aging process, especially the self-migration of silver along the tubular wall. SERS and the self-cleaning effect were observed using Rhodamine 6G (R6G) as the probe molecule. The results showed that narrow nanotube and silver nanoparticles embedment contributed significantly to both the phenomenal SERS and recyclability. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since it was first observed for adsorbed pyridine on both silver and copper by Fleischmann’s group [1], Surface-enhanced Raman scattering (SERS) has attracted great interest due to its remarkable enhancement, excellent sensitivity, the “fingerprinting” ability and was determined to play an important role in molecular detection, including applications in analytical chemistry and biological science [2,3]. Metal nanomaterials have usually been employed as SERSactive substrates because of their strong SERS enhancement originated from their unique surface plasmon resonance properties [4,5]. Noble metals (such as Ag, Au, Pd, and Pt), transitional metals (such as Cu, Ni, Ti, and Co) and even single crystals of semiconductor (NiO and TiO2 ) were reported to exhibit SERS for pyridine and other organic molecules [6,7]. The key to sensitive detection using SERS lies in the optimization of the enhancing surface. Apart from the active metallic nanostructure, SERS was always found on the combination of nanostructure substrate supporters; and it is no doubt that the morphology of the substrate has a synergistic effect on the SERS as well as the metallic particle itself. Mondal et al. [2] used nanoporous anodic alumina as a template decorated with silver nanoparticles to detect R6G; Lin et al. [8] reported SERS on porous
∗ Corresponding author. E-mail address: [email protected] (Y. Ling).
Si substrates with silver micro- and nanocrystallites; Sun et al. [9] used the silver-coated silicon nanowire arrays for SERS with R6G as the probe molecule; Although much progress has been achieved using different metals and rough nanostructures as the SERS substrate, the major drawback of such SERS substrates is that they cannot be reused, due to the contamination of absorbed target molecules. In recent years, semiconductor and noble metal complex nanomaterials, as the representative renewable SERS, have attracted ever-increasing attention because of the realization of multiple functionalities in a single entity [10–15]. Among these TiO2 has aroused much interest due to its self-cleaning function. Interestingly, the electron transfer at the noble metal-TiO2 interface exhibited much higher photocatalytic activity than bare TiO2 film individual. In this manner, the substrate is able to self-clean and be reused for a new SERS detection cycle. As a popular photocatalyst, nanostructured titanium dioxide has received much attention in environmental purification [16], and titanium dioxide nanotube arrays were nearly determined to be the ideal substrate for SERS and recyclability. Besides the high sensitivity and reproducibility, low-cost and reusability of the substrates are also very important to the acceptance of SERS as a general analysis tool from the application viewpoint. Au-coated TiO2 nanotubes array prepared using hydrothermal method [17] was reported to exhibit the self-cleaning effect; and Silver-coated TNTA for SERS has been used to detect pyridine by Maria Janik-Czachor and coworkers [18–21].
http://dx.doi.org/10.1016/j.apsusc.2016.01.257 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
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ARTICLE IN PRESS Y. Ling et al. / Applied Surface Science xxx (2016) xxx–xxx
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Fig. 1. Morphologies of silver particles on TNTA with migration time of (a) 2 h (b) 1 d (c) 5 d (d) 10 d.
Silver was widely and economically used as active noble metal for SERS, the interaction between silver clusters and the geometry of TNTA, however, is still a moot question. This including the oxidation of silver and the fast migration of Ag ions, and the latter one induced microstructure rearrangement may undoubtedly play an important role in catalysis and SERS. Therefore, more efforts need to be invested to improve the SERS property of silver-capped TNTA. In this study, highly aligned TiO2 nanotubes array was proposed as the rough 3D matrix for SERS, while the nanoparticles of silver were coated onto the microstructure through plasma sputtering. The effect of silver migrated and embedded TNTA on the SERS and the self-cleaning properties for R6G were investigated and analyzed. 2. Experimental Ti foil (purity >99.9%, with size of 2.0 cm × 2.0 cm and 0.05 mm in thickness) was initially cleaned in acetone and ethanol solution for 15 min through ultrasonication, and then anodization was performed in an ethylene glycol electrolyte with the addition of 0.5 vol.% HF. The treated Ti foil was used as an anode, and graphite
paper was chosen as a cathode. The foil was anodized for 15 min and at constant voltages of 20 V, 30 V, 40 V, 50 V, and 60 V, respectively. The as-prepared nanotube film is amorphous and has poor adhesion to the substrate, and a final thermal annealing was conducted at 500 ◦ C in air for 2 h to convert the amorphous oxide into the anatase phase and to improve its mechanical stability. The silver coating on the as-prepared nanotube arrays was performed by vacuum plasma sputtering technique under a pressure of 6 Pa for 10 min, and the silver deposition was about 0.03 mg/cm2 . To control the morphology of silver-coated TNTA, the sample was either kept in humid air (70%R.H.) for aging and exposed to UV light (365 nm) to promote the migration of silver. For concision, the terminology used to describe the specimen of TiO2 nanotube arrays anodized at voltage of xV were indicated as NTx (for example, NT30 represents the specimen of NTNA anodized at voltage of 30 V). The aged silver coated TiO2 nanotube arrays under UV exposure in aqueous solution for 30 min would be called Ag-NTx. The morphologies of the samples were examined by field emission scanning electron microscopy (HITACHI S-4800), and transmission electron microscopy (JEOL 2011), respectively. R6G is
Fig. 2. TEM morphologies of silver embedded TNTA, inset in (a) is SEAD for Ag while HRTEM of TiO2 in (b).
Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
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prepared in an aqueous solution with a concentration of 10−6 M. The sample was immersed in it for 30 min and blow-dried. The Raman spectra of the sample were obtained by a LabRAM HR800 Raman spectrometer using a 633 nm HeNe laser for the excitation. The spectra were captured in double scans with an accumulation time of 10 s at a laser power of 0.47 mW. 3. Results and discussion The anodization process resulted in the formation of TiO2 nanotubes with perpendicular to the Ti substrate and separated from each other. Fig. 1 shows the morphologies of silver on the TNTA surface at different stages of the aging process. SEM examinations revealed that the average diameter of the nanotubes was approximately 50 nm at anodization voltage of 30 V. Careful inspection of Fig. 1(a) found that the sputter-deposited Ag formed nanoparticles. The diameter of these particles varies from 20 nm to 50 nm. They are located at the top of the nanotubes. The substrates were aged in humid air for 10 days while silver on the TNTA surface migrated as time went by. Fig. 1d depicts the surface microstructure after 10 days of aging, and shows that the silver particle ring likely comprised of silver and silver oxide was epitaxial to the TiO2 nanotube, and that some of the silver was filled even in the tube to form nanorods. It has been reported that the plasmon resonance would lead to selective oxidation of silver particle to silver-oxide under visible light [22]. Meanwhile, with the photocatalytic effect of the TiO2 and UV light radiation, the silver oxide can be reduce to metallic silver again and adhering to the TNTA surface. From a TEM micrograph, one can see that the diameter of the silver rod (Fig. 2a) in the tube is about 50 nm, the same diameter as the TiO2 nanotube. The electron diffraction pattern obtained from the selected area corresponds to (111) (311), clearly indicating the orientation of the silver nanorods, the inset in Fig. 2a was indexed to silver crystal; while inset in Fig. 2b corresponded to the plane (101) of anatase TiO2 . Some smaller nanoparticles were adhered on the sidewalls of nanotubes (Fig. 2b). By comparison, it is obvious that aging in a humid environment and exposure to UV radiation promote the migration of Ag, and the as-sputtered silver agglomerates, rings, nanorods, and nanoparticles reconstruct the microstructure and create a new
Fig. 3. The inner diameter of TNTA as a function of anodic voltage.
artificial roughness with sharp edges on the surface of TNTA. The migration of Ag to the inner of TiO2 nanotubes during the aged process might be related to the galvanic effect due to the different electrode potentials between Ag (0.7996 V vs. NHE) and dissolved O2 in water film under humidity (1.229 V vs. NHE), accompanying with re-deposition by reduction. Based on these results, it can be inferred that both the humidity and UV induced ion migration and photo-reduction re-deposition process promoted the reconstruction of Ag nanostructures. As reported, the anodization voltage has a significant influence on the diameter of the TiO2 nanotube [23–25]. Figs. 3 and 4 display the size of TNTA and morphologies of silver on TiO2 nanotubes array anodized at different voltages after aging process. It is clear that nanotubes can be formed at a certain threshold voltage (>10 V), and that the average inner diameter of the nanotubes increased with increasing voltage, as it changed from 40 nm, 60 nm, 80 nm, 100 nm for 20 V, 30 V, 40 V and 50 V, respectively. Due to the difference in nanotube diameter, the behavior of sputtered silver particles should vary greatly during the aging process. It is obviously noted
Fig. 4. Morphologies of silver on TNTA with anodization voltages of (a) 20 V, (b) 30 V, (c) 40 V and (d) 50 V; The oxidation time was fixed to 15 min.
Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
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Fig. 5. SERS spectra of (a) Ag-NT20; (b) Ag-NT30; (c) Ag-NT40; (d) Ag-NT50; (e) Ag-NT60; and (f) NT immersed into 10−6 M R6G solution for 30 min, respectively.
that the silver particles (about 50 nm) are all located at the top of the nanotubes, for the diameter of the tube anodized at 20 V is so small (Fig. 3a). As the anodic voltage increases, the diameter of the tube gets bigger, and silver starts to migrate into the nanotubes anodized at 30 V (Fig. 3b). Nanorings epitaxial to the nanotubes likely consist of silver and silver oxide and nanorods inside the nanotubes are also observed, and most silver particles migrate into the tube at a higher voltage of 40 V and 50 V (Fig. 3cd). Since R6G is always used as the model molecule for the SERS spectra [2,6,9,26], we choose R6G as the molecule for examining the performance of the silver coated TiO2 nanotubes array as the SERS substrate. Fig. 5 exhibits the SERS spectra of R6G on the silver coated substrate anodized at different voltages after the aging process and immersion into the 10−6 M R6G solution for 30 min. The Raman peak at 636 cm−1 can be assigned to anatase, while the Raman peaks at 616, 727, 1129, 1180, 1315, 1361, 1514, 1576 and 1656 cm−1 can be attributed to R6G. The spectra agree well with data in the literature [2,6,9,26]. SERS spectra without silver coated on (Fig. 5A inset), which has no signals except 636 cm−1 peak (anatase), were compared and analyzed (the signal seemed no difference under different anodization voltage). It can be seen that the overall Raman signals significant increase as the Ag decorated. The differences in the signals intensity observed in nanotubes with different anodization voltages in Fig. 5B could be explained by the geometric configuration of silver coated TNTA. Contrasted with Ag-NT20, the SERS effect is remarkable for Ag-NT30, wherein a larger number of silver nanoparticles are formed, increasing the roughness on the surface (Fig. 2b). The SERS enhancement intensity from Ag-NT30 at 1180 cm−1 is about 7 times higher as compared to that of Ag-NT20 (Fig. 2a, only large silver particles on the surface). The SERS activity is not proportional to the diameters and it is noticed that the greater enhancement were observed on the AgNT30 and Ag-NT60 specimens. The enhancement factors on various roughened metallic surfaces strongly depend on the size, shape, distribution and interaction with TNTA. The higher SERS activity of the Ag-NT60 can be explained by the dominant contribution of R6G molecules adsorbed in the narrow slits between the silver particles. The silver particles present in the Ag-NT40 and Ag-NT50 are
20
30
40 50 Potential/V
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Fig. 6. SERS spectra detected by (a) Ag-NT20, (b) Ag-NT30, (c) Ag-NT40, (d) A g-NT50 and (e) Ag-NT60 irradiated by UV light, respectively.
relatively isolated, with the diameter a little smaller than the nanotube, whereas those in the Ag-NT60 provide a substantial amount of relatively narrow slits between silver particles. It is interesting to note that there is nearly no Raman scattering signal from Ag-NT30 after being irradiated by UV light, while the intensity of the SERS signal at 1180 cm−1 is about 1/20 of initial (Fig. 6). This might be attributed to the photocatalytic activity of silver coated TNTA for the decomposition of various environmental pollutants [27], and the main step in this reaction process is the generation of the electron-hole pairs in TNTA corresponding to the proper light irradiation. In this condition, it can be inferred that the R6G molecular was decomposed and the substrate was refreshed under UV irradiation. However, since Ag ions will be leached by solution gradually, more investigation should be done in the future to evaluate the durability of such material and device. 4. Summary and conclusion A facile technique was developed to fabricate silver decorated TiO2 nanotubes array, surface-enhanced Raman scattering (SERS) and the self-cleaning effect of the composite nanostructures have been observed in this study. We have examined the morphologies of silver particle on TNTA during the migration. It might be concluded that the reconstructed Ag embedment via ion migration and photo reduction was configuration dependent, an enhanced SERS signal could be obtained using nanotube array oxidized at voltage of 30 V. Moreover, the R6G can be degraded by the substrate under UV irradiation while the substrate was refreshed. We believe that this method will be very useful for obtaining the SERS substrates in a controllable process where the electric field enhancement can be manipulated by tailoring the particle size distribution and the substrates can be recycled. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No. 91023037, U1430118 and the National Basic Research Program of China (973 Program) under grant No. 2011CB61050.
Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
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Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
ARTICLE IN PRESS
APSUSC-32489; No. of Pages 5
Applied Surface Science xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate Yunhan Ling a,∗ , Yuqing Zhuo a , Liang Huang a , Duolu Mao b a b
Lab of Advanced Materials, School of Materials Science & Engineering, Tsinghua University, Beijing 100084, PR China School of Physical and Electronic Information Engineering, Qinghai Nationalities University, Xining, Qinghai 810007, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Available online xxx Keywords: TiO2 nanotubes array Surface-enhanced Raman scattering (SERS) Silver nanoparticle Photocatalysis Self-cleaning
a b s t r a c t A simple strategy for synthesizing Ag-loaded TiO2 nanotube film for use as multifunctional photocatalyst and recyclable surface-enhanced Raman scattering (SERS) substrate is introduced. Highly aligned TiO2 nanotube arrays (TNTA) prepared via electrochemical anodization were used as a 3D rough host for silver nanoparticles. Ag deposits were sputtered in a vacuum, and it was found that their morphologies were mainly influenced by the diameters of nanotubes and the UV irradiation induced aging process, especially the self-migration of silver along the tubular wall. SERS and the self-cleaning effect were observed using Rhodamine 6G (R6G) as the probe molecule. The results showed that narrow nanotube and silver nanoparticles embedment contributed significantly to both the phenomenal SERS and recyclability. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since it was first observed for adsorbed pyridine on both silver and copper by Fleischmann’s group [1], Surface-enhanced Raman scattering (SERS) has attracted great interest due to its remarkable enhancement, excellent sensitivity, the “fingerprinting” ability and was determined to play an important role in molecular detection, including applications in analytical chemistry and biological science [2,3]. Metal nanomaterials have usually been employed as SERSactive substrates because of their strong SERS enhancement originated from their unique surface plasmon resonance properties [4,5]. Noble metals (such as Ag, Au, Pd, and Pt), transitional metals (such as Cu, Ni, Ti, and Co) and even single crystals of semiconductor (NiO and TiO2 ) were reported to exhibit SERS for pyridine and other organic molecules [6,7]. The key to sensitive detection using SERS lies in the optimization of the enhancing surface. Apart from the active metallic nanostructure, SERS was always found on the combination of nanostructure substrate supporters; and it is no doubt that the morphology of the substrate has a synergistic effect on the SERS as well as the metallic particle itself. Mondal et al. [2] used nanoporous anodic alumina as a template decorated with silver nanoparticles to detect R6G; Lin et al. [8] reported SERS on porous
∗ Corresponding author. E-mail address: [email protected] (Y. Ling).
Si substrates with silver micro- and nanocrystallites; Sun et al. [9] used the silver-coated silicon nanowire arrays for SERS with R6G as the probe molecule; Although much progress has been achieved using different metals and rough nanostructures as the SERS substrate, the major drawback of such SERS substrates is that they cannot be reused, due to the contamination of absorbed target molecules. In recent years, semiconductor and noble metal complex nanomaterials, as the representative renewable SERS, have attracted ever-increasing attention because of the realization of multiple functionalities in a single entity [10–15]. Among these TiO2 has aroused much interest due to its self-cleaning function. Interestingly, the electron transfer at the noble metal-TiO2 interface exhibited much higher photocatalytic activity than bare TiO2 film individual. In this manner, the substrate is able to self-clean and be reused for a new SERS detection cycle. As a popular photocatalyst, nanostructured titanium dioxide has received much attention in environmental purification [16], and titanium dioxide nanotube arrays were nearly determined to be the ideal substrate for SERS and recyclability. Besides the high sensitivity and reproducibility, low-cost and reusability of the substrates are also very important to the acceptance of SERS as a general analysis tool from the application viewpoint. Au-coated TiO2 nanotubes array prepared using hydrothermal method [17] was reported to exhibit the self-cleaning effect; and Silver-coated TNTA for SERS has been used to detect pyridine by Maria Janik-Czachor and coworkers [18–21].
http://dx.doi.org/10.1016/j.apsusc.2016.01.257 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257
G Model APSUSC-32489; No. of Pages 5
ARTICLE IN PRESS Y. Ling et al. / Applied Surface Science xxx (2016) xxx–xxx
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Fig. 1. Morphologies of silver particles on TNTA with migration time of (a) 2 h (b) 1 d (c) 5 d (d) 10 d.
Silver was widely and economically used as active noble metal for SERS, the interaction between silver clusters and the geometry of TNTA, however, is still a moot question. This including the oxidation of silver and the fast migration of Ag ions, and the latter one induced microstructure rearrangement may undoubtedly play an important role in catalysis and SERS. Therefore, more efforts need to be invested to improve the SERS property of silver-capped TNTA. In this study, highly aligned TiO2 nanotubes array was proposed as the rough 3D matrix for SERS, while the nanoparticles of silver were coated onto the microstructure through plasma sputtering. The effect of silver migrated and embedded TNTA on the SERS and the self-cleaning properties for R6G were investigated and analyzed. 2. Experimental Ti foil (purity >99.9%, with size of 2.0 cm × 2.0 cm and 0.05 mm in thickness) was initially cleaned in acetone and ethanol solution for 15 min through ultrasonication, and then anodization was performed in an ethylene glycol electrolyte with the addition of 0.5 vol.% HF. The treated Ti foil was used as an anode, and graphite
paper was chosen as a cathode. The foil was anodized for 15 min and at constant voltages of 20 V, 30 V, 40 V, 50 V, and 60 V, respectively. The as-prepared nanotube film is amorphous and has poor adhesion to the substrate, and a final thermal annealing was conducted at 500 ◦ C in air for 2 h to convert the amorphous oxide into the anatase phase and to improve its mechanical stability. The silver coating on the as-prepared nanotube arrays was performed by vacuum plasma sputtering technique under a pressure of 6 Pa for 10 min, and the silver deposition was about 0.03 mg/cm2 . To control the morphology of silver-coated TNTA, the sample was either kept in humid air (70%R.H.) for aging and exposed to UV light (365 nm) to promote the migration of silver. For concision, the terminology used to describe the specimen of TiO2 nanotube arrays anodized at voltage of xV were indicated as NTx (for example, NT30 represents the specimen of NTNA anodized at voltage of 30 V). The aged silver coated TiO2 nanotube arrays under UV exposure in aqueous solution for 30 min would be called Ag-NTx. The morphologies of the samples were examined by field emission scanning electron microscopy (HITACHI S-4800), and transmission electron microscopy (JEOL 2011), respectively. R6G is
Fig. 2. TEM morphologies of silver embedded TNTA, inset in (a) is SEAD for Ag while HRTEM of TiO2 in (b).
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prepared in an aqueous solution with a concentration of 10−6 M. The sample was immersed in it for 30 min and blow-dried. The Raman spectra of the sample were obtained by a LabRAM HR800 Raman spectrometer using a 633 nm HeNe laser for the excitation. The spectra were captured in double scans with an accumulation time of 10 s at a laser power of 0.47 mW. 3. Results and discussion The anodization process resulted in the formation of TiO2 nanotubes with perpendicular to the Ti substrate and separated from each other. Fig. 1 shows the morphologies of silver on the TNTA surface at different stages of the aging process. SEM examinations revealed that the average diameter of the nanotubes was approximately 50 nm at anodization voltage of 30 V. Careful inspection of Fig. 1(a) found that the sputter-deposited Ag formed nanoparticles. The diameter of these particles varies from 20 nm to 50 nm. They are located at the top of the nanotubes. The substrates were aged in humid air for 10 days while silver on the TNTA surface migrated as time went by. Fig. 1d depicts the surface microstructure after 10 days of aging, and shows that the silver particle ring likely comprised of silver and silver oxide was epitaxial to the TiO2 nanotube, and that some of the silver was filled even in the tube to form nanorods. It has been reported that the plasmon resonance would lead to selective oxidation of silver particle to silver-oxide under visible light [22]. Meanwhile, with the photocatalytic effect of the TiO2 and UV light radiation, the silver oxide can be reduce to metallic silver again and adhering to the TNTA surface. From a TEM micrograph, one can see that the diameter of the silver rod (Fig. 2a) in the tube is about 50 nm, the same diameter as the TiO2 nanotube. The electron diffraction pattern obtained from the selected area corresponds to (111) (311), clearly indicating the orientation of the silver nanorods, the inset in Fig. 2a was indexed to silver crystal; while inset in Fig. 2b corresponded to the plane (101) of anatase TiO2 . Some smaller nanoparticles were adhered on the sidewalls of nanotubes (Fig. 2b). By comparison, it is obvious that aging in a humid environment and exposure to UV radiation promote the migration of Ag, and the as-sputtered silver agglomerates, rings, nanorods, and nanoparticles reconstruct the microstructure and create a new
Fig. 3. The inner diameter of TNTA as a function of anodic voltage.
artificial roughness with sharp edges on the surface of TNTA. The migration of Ag to the inner of TiO2 nanotubes during the aged process might be related to the galvanic effect due to the different electrode potentials between Ag (0.7996 V vs. NHE) and dissolved O2 in water film under humidity (1.229 V vs. NHE), accompanying with re-deposition by reduction. Based on these results, it can be inferred that both the humidity and UV induced ion migration and photo-reduction re-deposition process promoted the reconstruction of Ag nanostructures. As reported, the anodization voltage has a significant influence on the diameter of the TiO2 nanotube [23–25]. Figs. 3 and 4 display the size of TNTA and morphologies of silver on TiO2 nanotubes array anodized at different voltages after aging process. It is clear that nanotubes can be formed at a certain threshold voltage (>10 V), and that the average inner diameter of the nanotubes increased with increasing voltage, as it changed from 40 nm, 60 nm, 80 nm, 100 nm for 20 V, 30 V, 40 V and 50 V, respectively. Due to the difference in nanotube diameter, the behavior of sputtered silver particles should vary greatly during the aging process. It is obviously noted
Fig. 4. Morphologies of silver on TNTA with anodization voltages of (a) 20 V, (b) 30 V, (c) 40 V and (d) 50 V; The oxidation time was fixed to 15 min.
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Fig. 5. SERS spectra of (a) Ag-NT20; (b) Ag-NT30; (c) Ag-NT40; (d) Ag-NT50; (e) Ag-NT60; and (f) NT immersed into 10−6 M R6G solution for 30 min, respectively.
that the silver particles (about 50 nm) are all located at the top of the nanotubes, for the diameter of the tube anodized at 20 V is so small (Fig. 3a). As the anodic voltage increases, the diameter of the tube gets bigger, and silver starts to migrate into the nanotubes anodized at 30 V (Fig. 3b). Nanorings epitaxial to the nanotubes likely consist of silver and silver oxide and nanorods inside the nanotubes are also observed, and most silver particles migrate into the tube at a higher voltage of 40 V and 50 V (Fig. 3cd). Since R6G is always used as the model molecule for the SERS spectra [2,6,9,26], we choose R6G as the molecule for examining the performance of the silver coated TiO2 nanotubes array as the SERS substrate. Fig. 5 exhibits the SERS spectra of R6G on the silver coated substrate anodized at different voltages after the aging process and immersion into the 10−6 M R6G solution for 30 min. The Raman peak at 636 cm−1 can be assigned to anatase, while the Raman peaks at 616, 727, 1129, 1180, 1315, 1361, 1514, 1576 and 1656 cm−1 can be attributed to R6G. The spectra agree well with data in the literature [2,6,9,26]. SERS spectra without silver coated on (Fig. 5A inset), which has no signals except 636 cm−1 peak (anatase), were compared and analyzed (the signal seemed no difference under different anodization voltage). It can be seen that the overall Raman signals significant increase as the Ag decorated. The differences in the signals intensity observed in nanotubes with different anodization voltages in Fig. 5B could be explained by the geometric configuration of silver coated TNTA. Contrasted with Ag-NT20, the SERS effect is remarkable for Ag-NT30, wherein a larger number of silver nanoparticles are formed, increasing the roughness on the surface (Fig. 2b). The SERS enhancement intensity from Ag-NT30 at 1180 cm−1 is about 7 times higher as compared to that of Ag-NT20 (Fig. 2a, only large silver particles on the surface). The SERS activity is not proportional to the diameters and it is noticed that the greater enhancement were observed on the AgNT30 and Ag-NT60 specimens. The enhancement factors on various roughened metallic surfaces strongly depend on the size, shape, distribution and interaction with TNTA. The higher SERS activity of the Ag-NT60 can be explained by the dominant contribution of R6G molecules adsorbed in the narrow slits between the silver particles. The silver particles present in the Ag-NT40 and Ag-NT50 are
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Fig. 6. SERS spectra detected by (a) Ag-NT20, (b) Ag-NT30, (c) Ag-NT40, (d) A g-NT50 and (e) Ag-NT60 irradiated by UV light, respectively.
relatively isolated, with the diameter a little smaller than the nanotube, whereas those in the Ag-NT60 provide a substantial amount of relatively narrow slits between silver particles. It is interesting to note that there is nearly no Raman scattering signal from Ag-NT30 after being irradiated by UV light, while the intensity of the SERS signal at 1180 cm−1 is about 1/20 of initial (Fig. 6). This might be attributed to the photocatalytic activity of silver coated TNTA for the decomposition of various environmental pollutants [27], and the main step in this reaction process is the generation of the electron-hole pairs in TNTA corresponding to the proper light irradiation. In this condition, it can be inferred that the R6G molecular was decomposed and the substrate was refreshed under UV irradiation. However, since Ag ions will be leached by solution gradually, more investigation should be done in the future to evaluate the durability of such material and device. 4. Summary and conclusion A facile technique was developed to fabricate silver decorated TiO2 nanotubes array, surface-enhanced Raman scattering (SERS) and the self-cleaning effect of the composite nanostructures have been observed in this study. We have examined the morphologies of silver particle on TNTA during the migration. It might be concluded that the reconstructed Ag embedment via ion migration and photo reduction was configuration dependent, an enhanced SERS signal could be obtained using nanotube array oxidized at voltage of 30 V. Moreover, the R6G can be degraded by the substrate under UV irradiation while the substrate was refreshed. We believe that this method will be very useful for obtaining the SERS substrates in a controllable process where the electric field enhancement can be manipulated by tailoring the particle size distribution and the substrates can be recycled. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No. 91023037, U1430118 and the National Basic Research Program of China (973 Program) under grant No. 2011CB61050.
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Please cite this article in press as: Y. Ling, et al., Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.257