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Hierarchical TiO2–Ag composite with three-dimensional hot spots for trace detection
Journal of Alloys and Compounds 811 (2019) 151994
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hierarchical TiO2eAg composite with three-dimensional hot spots for trace detection Xiaoyun Xu, Yanhong Feng* Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, Key Laboratory of Polymer Processing Engineering of the Ministry of Education, National Engineering Research Centre of Novel Equipment for Polymer Processing, South China University of Technology, Guangzhou, 510641, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 22 July 2019 Accepted 23 August 2019 Available online 27 August 2019
The construction of sensitive materials for trace detection is significant for various applications, including structure analysis, environment monitoring, and biological analysis. A critical challenge is the construction of materials with a combination of high sensitivity, excellent reliability, and good reusability. Here, we describe a hierarchical AgeTiO2 composite that can provide three-dimensional hot spots for local electric field enhancement. This substrate can offer stable surface-enhanced Raman scattering (SERS) signal with an absolute intensity that is ~51 times higher than that of conventional Ag substrate. The SERS signal from the substrate is highly homogenous over a wide region, without any obvious peak fluctuations. Moreover, the composite can be regenerated through an in situ photocatalytic reaction of TiO2. This innovative, recyclable SERS substrate can provide an efficient platform for in situ structure analysis and trace detection. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ag-TiO2 Composite Surface enhanced Raman scattering Trace detection
1. Introduction Due to the enhanced attention focused on the environmental monitoring, there is a rapidly growing need for sensitive materials that can perform complex functions, such as trace organic detection with high sensitivity and in situ structure analysis, and are also simultaneously recyclable [1e5]. Metallic nanostructures (e.g., Au and Ag) have attracted significant attention, because the plasmon resonance induced by photon excitation may generate extremely intense and enhanced local electric field (104) [6e8]. This unique feature may give rise to surface-enhanced Raman scattering (SERS)da phenomenon that can be used for detecting trace species at single molecule level [9e12]. The existing methods for the fabrication of metallic nanostructures include electron-beam/ optical lithography, focused ion-beam milling, chemical deposition, assembly, solution reduction, thermal evaporation, and photoconversion [13e23]. Particularly, the design and construction of hierarchical AgeTiO2 composite by using the TiO2 as the host have been attracted much attention in the past several years, because it can be potentially elaborated into the recyclable SERS substrate for trace organic detection [24e29].
* Corresponding author. E-mail address: [email protected] (Y. Feng). https://doi.org/10.1016/j.jallcom.2019.151994 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Here, we focus our attention on another hierarchical AgeTiO2 composite with unique taper configuration because the taper structure is favorable for efficient light coupling. The composite was fabricated via the photo-reduction of Ag on taper TiO2 nanostructure. The composite, with Ag nanoparticles homogenously and densely distributed along the taper TiO2 nanostructure, can provide three-dimensional hot spots for giant local electric field enhancement. It helps to generate high intense fingerprint signals of trace organics and allows their in situ structure analysis. Furthermore, the composite can be utilized as a recyclable SERS substrate. 2. Experimental section 2.1. Methods and synthesis Taper TiO2 nanostructures were grown on a glass substrate. In a typical experiment, several pieces of silica glass substrates were ultrasonically cleaned and placed on a 25 ml Teflon-lined stainless steel autoclave. The reaction solution was composed of 9 ml n-octane and 0.9 ml mixture of titanium tetrachloride and concentrated hydrochloride acid (HCl, 37 wt%), with the volume ratio varying from 1:1 to 1:16. The reaction was conducted at 160 C, and the reaction time was varied from 45 min to 5 h. When the reaction reached completion, the container was cooled to room
2
X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
temperature, and the substrate was washed carefully with ethanol for several times. The substrate covered with dense taper TiO2 was heat-treated at 300 C for 2 h in air to remove the reaction coproducts. Hierarchical TiO2eAg composite was fabricated by the photoreduction method. In a typical experiment, 0.021 g AgNO3 was dissolved in a mixed solution containing 2.5 ml water and 2.5 ml alcohol. The taper TiO2 substrate was immersed into the mixed solution, and the photochemical reaction was triggered by irradiating it with ultra-violet light for various time durations. The wavelength of light could be tuned from 172, 222e308 nm. The obtained composite was stored for further experiments.
scattering spectrum and SERS were recorded using the Renishaw InVia spectrometer with a 633 nm laser as the excitation source. For mapping measurement, the laser beam was focused using an objective lens ( 40, numerical aperture ¼ 0.6). The laser power was controlled at 0.2 mW, and the acquisition time for spectrum collection was 0.5 s. The mapping region was 100 mm 50 mm, with a step size of ~2 mm2. X-ray photoelectron spectroscopy (XPS) was performed on Amicus apparatus (150 W Al/Ka radiation at an energy of 1468.6 eV). 3. Results and discussion 3.1. Synthesis and characterizations of hierarchical TiO2
2.2. Characterization The crystalline phases of taper TiO2 and TiO2eAg composite were identified by X-ray diffraction (XRD) with Cu/Ka radiation (X'pert Powder, PANalytical, Netherlands). The morphologies of the products were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The Raman
The harsh reaction conditions were considered to be favorable for the growth of hierarchical TiO2 [30,31]. Considering this, we created a strong acid reaction environment by employing concentrated HCl (37 wt%) as the reaction media. The influence of the volume ratio of Ti precursor (TiCl4) and HCl on the product was studied. Fig. 1 shows the SEM images of the products with TiCl4:HCl
Fig. 1. Influence of TiCl4:HCl ratio on the growth of hierarchical TiO2. (ael) SEM images of the hierarchical TiO2 grown on the reaction solution with various TiCl4:HCl ratios: 1:1 (a and d), 1:2 (b and e), 1:4 (c and f), 1:8 (g and j), 1:12 (h and k), and 1:16 (i and l).
X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
ratios of 1:1, 1:2; 1:4; 1:8; 1:12, and 1:16. The low resolution SEM images suggest that decreasing the TiCl4:HCl ratio reduces the yield of the hierarchical TiO2. For TiCl4:HCl ratios of 1:1, 1:2, and 1:4, dense TiO2 covers the surface of the glass substrate. A different scenario is observed at the other ratios. At TiCl4:HCl ratios of 1:8; 1:12, and 1:16, relatively separated TiO2 islands are precipitated, and the density gradually decreases with increasing TiCl4:HCl ratio. High-resolution SEM provides valuable information on the microstructure of TiO2. It can be observed that all the TiO2 structures exhibit taper configuration. Interestingly, the size of the taper increases with increasing TiCl4:HCl ratio. At TiCl4:HCl ratio of 1:1, the size of the TiO2 tip is estimated to be 24 nm (Fig. 1a and d). It grows to 47 nm as the TiCl4:HCl ratio changes to 1:12 (Fig. 1h and k). An extreme case is the sample with TiCl4:HCl ratio of 1:16; a single TiO2 tree with a major trunk and several branches could be clearly observed (Fig. 1i and l). Some nanocrystals with square shape found on the surface of substrate can be ascribed to the TiO2 nanocrystals (Fig. 1l). The growth process was studied by SEM characterization of the sample with TiCl4:HCl ratio of 1:4, at various time points from
3
45 min to 5 h (Fig. 2). At a reaction time of less than 2 h, sparsely distributed TiO2 with various hierarchical configurations (Fig. 2d and e) could be observed. With the progress of reaction time, TiO2 kept growing, forming dense layers. At the same time, the characteristic size of the sample gradually decreases, indicating that the hierarchical TiO2 further grows along the major trunk and branches. The final thickness of the TiO2 layer reached ~500 mm. The crystalline phase of the covered hierarchical TiO2 was identified by XRD (Fig. 3a). Several characteristic peaks at 27.4 , 36.1, 41.1, 54.2 , 56.3 , and 62.8 could be observed, which could be ascribed to the (110), (101), (111), (211), (002), and (301) crystalline planes, respectively, of rutile TiO2 (JCPDS, No. 77e0441). Raman spectroscopy, which can provide information on the chemical bonding, was employed to characterize the hierarchical TiO2 (Fig. 3b). The clear characteristic peaks at 250, 447, and 615 cm1 could be ascribed to the second-order line, Eg mode, and A1g mode of rutile TiO2, respectively [32]. The results confirm the pure single phase of the hierarchical TiO2. The morphology and microstructure of the hierarchical TiO2 were further characterized by TEM (Fig. 3c). The TEM image shows that hierarchical TiO2 is
Fig. 2. Influence of reaction time on the growth of hierarchical TiO2. (ael) SEM images of the hierarchical TiO2 grown on the solution with TiCl4:HCl ratio of 1:4 for 0.75 h (a and d), 1 h (b and e), 2 h (c and f), 3 h (g and j), 4 h (h and k) to 5 h (i and l).
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X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
Fig. 3. Structural characterizations of the hierarchical TiO2: (aed) XRD pattern (a), Raman spectra (b), TEM image (c), high resolution TEM image (d), electron diffraction pattern (e), and Fourier-transform diffractogram (f).
Fig. 4. Morphology of hierarchical TiO2eAg composite. (a, b) SEM image of hierarchical TiO2eAg composite obtained using hierarchical TiO2 synthesized after growth duration of 2 h and Ag growth duration of 3 min. (c, d) SEM image of hierarchical TiO2eAg composite obtained using hierarchical TiO2 synthesized after growth duration of 5 h and Ag growth duration of 5 min.
taper-shaped, with the taper angle estimated to be ~30 . Highresolution TEM image shows that the taper TiO2 is composed of extremely tiny rutile TiO2 particles with a typical size of ~5 nm, indicating that the hierarchical TiO2 taper is formed through order
attachment of secondary nanostructures (Fig. 3d). The electron diffraction pattern confirms the polycrystalline feature of the taper TiO2 (Fig. 3e). Fig. 3f highlights the Fourier-transform diffractogram of a typical particle, confirming the excellent crystallinity of TiO2.
X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
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Fig. 5. Structural characterizations of hierarchical TiO2eAg composite. (a) XRD pattern. (b) High resolution TEM image. (c) The full scale XPS spectrum. (d) The characteristic XPS band of Ag.
3.2. Synthesis and characterizations of hierarchical TiO2eAg composite TiO2 is a typical photocatalyst, and it can be used to reduce metallic nanostructures. This inspired us to consider the possibility for construction of TiO2eAg composite through in situ photoredox reaction. To test this, a mixture of AgNO3, water, and alcohol was employed as the reaction solution, and the hierarchical TiO2 substrate with different taper configuration was used. The substrate was immersed into the solution, and the photoreduction was triggered by ultraviolet light irradiation. The SEM images of the fabricated TiO2eAg composites show that the Ag nanoparticles can homogeneously precipitate on the entire surface of taper TiO2 (Fig. 4). Interestingly, the size of Ag nanoparticles can be finely tuned from 65 to 150 nm by precisely controlling the irradiation time from 3 to 5 min. The photon-energy dependent growth of Ag nanoparticles on taper TiO2 was also investigated by employing three different irradiation sources of wavelengths 172, 222, and 308 nm. No obvious difference was observed, which could be ascribed to the absorption of taper TiO2 over a broad band. The structure of TiO2eAg composite was characterized by XRD (Fig. 5a). The intense XRD peaks at 38.2 , 44.3 , 64.4 , and 77.4 could be ascribed to the (111), (200), (220), and (311) crystalline plane, respectively, of Ag (JCPDS, No. 04e0783). This confirms the successful precipitation of dense Ag nanostructures on taper TiO2. A typical TEM image shows two tiny Ag nanoparticles attached on the surface of taper TiO2 (Fig. 5b). The chemical state of the Ag species was further studied by XPS. The full scale XPS spectrum clearly shows the signal from Ti and Ag species (Fig. 5c). The high resolution spectrum of the Ag3d states indicates that the binding
energies of the Ag3d5/2 and Ag3d3/2 states are around 367.8 and 373.8 eV, which match well with the binding energies of the Ag0 species (368.0 and 374.0 eV) (Fig. 5d). The results demonstrate that the Ag species predominantly exist in the metallic form.
3.3. Application of hierarchical TiO2eAg composite for trace detection The in situ precipitation of Ag nanostructures on hierarchical TiO2 may give rise to intriguing plasmonic properties. On one hand, the diameter of TiO2 taper gradually changes along the taper, imparting a gradual change in the refractive index and facilitating light-matter interaction [33e35]. Our theoretical simulation indicates that the higher intensity of electric field is located at the middle and the bottom of the tapered TiO2, indicating that the tapered TiO2 is favorable for efficient optical coupling. On the other hand, the dense distribution of Ag on TiO2 taper may generate an enhanced strong three-dimensional electric field through nearfield coupling between particles on a single TiO2 taper or different tapers (Fig. 6a). The above two factors are believed to be favorable for efficient photon utilization and field amplification, which are critical for highly sensitive trace detection. To check this, low concentration of benzenethiol (107 M) was employed as a prototype pollutant, and two materials, hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructures, were studied as the sensing substrates. Fig. 6b shows the collected SERS signals; a significantly intense signal can be observed for hierarchical TiO2eAg composite. The sharp fingerprint bands at 1011, 1031, 1082 and 1570 cm1 could be ascribed to the vibrations of n12, n18a, n1, and C]C/8a/8b, respectively. The absolute signal intensity
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X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
Fig. 6. Trace detection of hierarchical TiO2eAg composite. (a) Schematic showing the fabrication of hierarchical TiO2eAg composite via photoreduction. (b) SERS spectra of low concentration benzenethiol (107 M) on hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructure. (c) Comparison of SERS intensity of low concentration benzenethiol (107 M) on hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructure. The peak used for comparison is the characteristic band at ~1570 cm1. The absolution intensity is estimated and used for comparison.
Fig. 7. Recycling performance of the fabricated hierarchical TiO2eAg composite. (a) SERS signal intensity and (b) peak shift mapping on the fingerprint band of benzenethiol at 1570 cm1 for the sample after five detection cycles. (c) A summary of the change in SERS signal intensity in five cycles.
of hierarchical TiO2eAg composite is estimated to be more than 51 times higher than that of the conventional two-dimensional Ag nanostructures (Fig. 6c). More importantly, the characteristic bands at 1117 and 1491 cm1 from benzenethiol, which are not visible in the two-dimensional Ag substrate, can be clearly identified on the hierarchical TiO2eAg composite. The above results firmly demonstrate that the constructed hierarchical TiO2eAg composite can not
only be extended as an ultra-sensitive detection substrate but may also provide unprecedented opportunity for performing in situ structure analysis. The additional advantage of the hierarchical TiO2eAg composite is that it can be a potential reusable sensing substrate, because TiO2 can decompose organics under extended ultra-violet light irradiation through photocatalytic reactions. As a demonstration, the
X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
substrate was repeatedly used for detection of benzenethiol (107 M) with a 633-nm laser as the excitation source for SERS measurement and 325-nm incoherent light as an external field for photon-decomposition. Five-cycle tests were performed, and the results are summarized in Fig. 7. Raman mapping on the fingerprint band of benzenethiol at 1570 cm1 for the sample after five cycles was performed, and the results are presented in Fig. 7a and b. Encouragingly, the hierarchical TiO2eAg composite substrate could provide highly homogenous SERS signal in a 100 mm 50 mm region (Fig. 7a). Moreover, Raman mapping on the peak position indicate that no obvious peak shift occurred during SERS measurement (Fig. 7b). Importantly, 50% sensitivity was retained after five cycles, indicating its potential for reuse (Fig. 7c). The performance degradation mainly occurred in the first cycle, which is probably associated with the violent structure evolution in the fresh sample. The above results collaboratively demonstrate that the constructed SERS substrate can act as a reliable sensor for trace organic detection. 4. Conclusions In summary, our study suggests that hierarchical TiO2eAg composite can be a novel plasmonic material for SERS detection. This novel substrate had high sensitivity, excellent reliability, and good reusability. The studies on the giant local electric field enhancement through creation of three dimensional hot spots enable an improved understanding of the light-matter interaction in nanostructured materials. The findings also open the possibility for constructing plasmonic platforms for trace detection, structure analysis, and biological analysis. Acknowledgments The authors acknowledge financial support from The National Natural Science Foundation of China (No. 51873073, 51435005), the Science and Technology Planning Project of Guangdong Province. China (No. 2017B090901062), and the Special Support Program of Guangdong Province, P. R. China (No. 2015TX01X151). References [1] E.M. Larsson, C. Langhammer, I. Zori c, B. Kasemo, Nanoplasmonic probes of catalytic reactions, Science 326 (2009) 1091e1094. [2] R.A. Alvarez-Puebla, L.M. Liz-Marz an, Traps and cages for universal SERS detection, Chem. Soc. Rev. 41 (2012) 43e51. [3] S. Schlücker, Surface-enhanced Raman spectroscopy: concepts and chemical applications, Angew. Chem. Int. Ed. 53 (2014) 4756e4795. [4] S. Laing, L.E. Jamieson, K. Faulds, D. Graham, Surface-enhanced Raman spectroscopy for in vivo biosensing, Nat. Rev. Chem. 1 (2017). UNSP 0060. [5] C. Zong, M. Xu, L. Xu, T. Wei, X. Ma, X. Zheng, R. Hu, B. Ren, Surface-enhanced Raman spectroscopy for bioanalysis: reliability and challenges, Chem. Rev. 118 (2018) 4946e4980. [6] K. Ueno, S. Juodkazis, V. Mizeikis, K. Sasaki, H. Misawa, Clusters of closely spaced gold nanoparticles as a source of two-photon photoluminescence at visible wavelengths, Adv. Mater. 20 (2008) 26. ~ ez, A. Rivera, A. Prada, G. Tardajos, J. Gonza lez[7] G. Gonz alez-Rubio, P. Díaz-Nún ~ ares, P. Llombart, L.G. Macdowell, M. Alcolea-Palafox, L.M. LizIzquierdo, L. Ban ~ a-Rodríguez, A. Guerrero-Martínez, Femtosecond laser Marz an, O. Pen reshaping yields gold nanorods with ultranarrow surface plasmon resonances, Science 358 (2017) 640e644. nez de Aberasturi, L.M. Liz-Marz [8] J. Reguera, J. Langer, D. Jime an, Anisotropic metal nanoparticles for surface enhanced Raman scattering, Chem. Soc. Rev. 46 (2017) 3866e3885. [9] D. Graham, D.G. Thompson, W.E. Smith, K. Faulds, Control of enhanced Raman scattering using a DNA-based assembly process of dye-coded nanoparticles, Nat. Nanotechnol. 3 (2008) 548e551. [10] D.K. Lim, K.S. Jeon, H.M. Kim, J.M. Nam, Y.D. Suh, Nanogap-engineerable Raman-active nanodumbbells for single-molecule detection, Nat. Mater. 9 (2010) 60e67.
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[11] R. Chikkaraddy, B. de Nijs, F. Benz, S.J. Barrow, O.A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess, J.J. Baumberg, Single-molecule strong coupling at room temperature in plasmonic nanocavities, Nature 535 (2016) 127e130. [12] J. Lee, K.T. Crampton, N. Tallarida, V.A. Apkarian, Visualizing vibrational normal modes of a single molecule with atomically confined light, Nature 568 (2019) 78. [13] R.C. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Schatz, J.G. Zheng, Photoinduced conversion of silver nanospheres to nanoprisms, Science 294 (2001) 1901e1903. [14] K. Ueno, S. Juodkazis, V. Mizeikis, K. Sasaki, H. Misawa, Spectrally-resolved atomic-scale length variations of gold nanorods, J. Am. Chem. Soc. 128 (2006) 14226e14227. [15] A.K. Pal, D.B. Mohan, SERS enhancement, sensitivity and homogeneity studies on bi-metallic Ag-Cu films through tuning of broad band SPR towards red region, J. Alloy. Comp. 698 (2017) 460e468. [16] L. Lin, M. Chen, H. Qin, X. 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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hierarchical TiO2eAg composite with three-dimensional hot spots for trace detection Xiaoyun Xu, Yanhong Feng* Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, Key Laboratory of Polymer Processing Engineering of the Ministry of Education, National Engineering Research Centre of Novel Equipment for Polymer Processing, South China University of Technology, Guangzhou, 510641, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 May 2019 Received in revised form 22 July 2019 Accepted 23 August 2019 Available online 27 August 2019
The construction of sensitive materials for trace detection is significant for various applications, including structure analysis, environment monitoring, and biological analysis. A critical challenge is the construction of materials with a combination of high sensitivity, excellent reliability, and good reusability. Here, we describe a hierarchical AgeTiO2 composite that can provide three-dimensional hot spots for local electric field enhancement. This substrate can offer stable surface-enhanced Raman scattering (SERS) signal with an absolute intensity that is ~51 times higher than that of conventional Ag substrate. The SERS signal from the substrate is highly homogenous over a wide region, without any obvious peak fluctuations. Moreover, the composite can be regenerated through an in situ photocatalytic reaction of TiO2. This innovative, recyclable SERS substrate can provide an efficient platform for in situ structure analysis and trace detection. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ag-TiO2 Composite Surface enhanced Raman scattering Trace detection
1. Introduction Due to the enhanced attention focused on the environmental monitoring, there is a rapidly growing need for sensitive materials that can perform complex functions, such as trace organic detection with high sensitivity and in situ structure analysis, and are also simultaneously recyclable [1e5]. Metallic nanostructures (e.g., Au and Ag) have attracted significant attention, because the plasmon resonance induced by photon excitation may generate extremely intense and enhanced local electric field (104) [6e8]. This unique feature may give rise to surface-enhanced Raman scattering (SERS)da phenomenon that can be used for detecting trace species at single molecule level [9e12]. The existing methods for the fabrication of metallic nanostructures include electron-beam/ optical lithography, focused ion-beam milling, chemical deposition, assembly, solution reduction, thermal evaporation, and photoconversion [13e23]. Particularly, the design and construction of hierarchical AgeTiO2 composite by using the TiO2 as the host have been attracted much attention in the past several years, because it can be potentially elaborated into the recyclable SERS substrate for trace organic detection [24e29].
* Corresponding author. E-mail address: [email protected] (Y. Feng). https://doi.org/10.1016/j.jallcom.2019.151994 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Here, we focus our attention on another hierarchical AgeTiO2 composite with unique taper configuration because the taper structure is favorable for efficient light coupling. The composite was fabricated via the photo-reduction of Ag on taper TiO2 nanostructure. The composite, with Ag nanoparticles homogenously and densely distributed along the taper TiO2 nanostructure, can provide three-dimensional hot spots for giant local electric field enhancement. It helps to generate high intense fingerprint signals of trace organics and allows their in situ structure analysis. Furthermore, the composite can be utilized as a recyclable SERS substrate. 2. Experimental section 2.1. Methods and synthesis Taper TiO2 nanostructures were grown on a glass substrate. In a typical experiment, several pieces of silica glass substrates were ultrasonically cleaned and placed on a 25 ml Teflon-lined stainless steel autoclave. The reaction solution was composed of 9 ml n-octane and 0.9 ml mixture of titanium tetrachloride and concentrated hydrochloride acid (HCl, 37 wt%), with the volume ratio varying from 1:1 to 1:16. The reaction was conducted at 160 C, and the reaction time was varied from 45 min to 5 h. When the reaction reached completion, the container was cooled to room
2
X. Xu, Y. Feng / Journal of Alloys and Compounds 811 (2019) 151994
temperature, and the substrate was washed carefully with ethanol for several times. The substrate covered with dense taper TiO2 was heat-treated at 300 C for 2 h in air to remove the reaction coproducts. Hierarchical TiO2eAg composite was fabricated by the photoreduction method. In a typical experiment, 0.021 g AgNO3 was dissolved in a mixed solution containing 2.5 ml water and 2.5 ml alcohol. The taper TiO2 substrate was immersed into the mixed solution, and the photochemical reaction was triggered by irradiating it with ultra-violet light for various time durations. The wavelength of light could be tuned from 172, 222e308 nm. The obtained composite was stored for further experiments.
scattering spectrum and SERS were recorded using the Renishaw InVia spectrometer with a 633 nm laser as the excitation source. For mapping measurement, the laser beam was focused using an objective lens ( 40, numerical aperture ¼ 0.6). The laser power was controlled at 0.2 mW, and the acquisition time for spectrum collection was 0.5 s. The mapping region was 100 mm 50 mm, with a step size of ~2 mm2. X-ray photoelectron spectroscopy (XPS) was performed on Amicus apparatus (150 W Al/Ka radiation at an energy of 1468.6 eV). 3. Results and discussion 3.1. Synthesis and characterizations of hierarchical TiO2
2.2. Characterization The crystalline phases of taper TiO2 and TiO2eAg composite were identified by X-ray diffraction (XRD) with Cu/Ka radiation (X'pert Powder, PANalytical, Netherlands). The morphologies of the products were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The Raman
The harsh reaction conditions were considered to be favorable for the growth of hierarchical TiO2 [30,31]. Considering this, we created a strong acid reaction environment by employing concentrated HCl (37 wt%) as the reaction media. The influence of the volume ratio of Ti precursor (TiCl4) and HCl on the product was studied. Fig. 1 shows the SEM images of the products with TiCl4:HCl
Fig. 1. Influence of TiCl4:HCl ratio on the growth of hierarchical TiO2. (ael) SEM images of the hierarchical TiO2 grown on the reaction solution with various TiCl4:HCl ratios: 1:1 (a and d), 1:2 (b and e), 1:4 (c and f), 1:8 (g and j), 1:12 (h and k), and 1:16 (i and l).
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ratios of 1:1, 1:2; 1:4; 1:8; 1:12, and 1:16. The low resolution SEM images suggest that decreasing the TiCl4:HCl ratio reduces the yield of the hierarchical TiO2. For TiCl4:HCl ratios of 1:1, 1:2, and 1:4, dense TiO2 covers the surface of the glass substrate. A different scenario is observed at the other ratios. At TiCl4:HCl ratios of 1:8; 1:12, and 1:16, relatively separated TiO2 islands are precipitated, and the density gradually decreases with increasing TiCl4:HCl ratio. High-resolution SEM provides valuable information on the microstructure of TiO2. It can be observed that all the TiO2 structures exhibit taper configuration. Interestingly, the size of the taper increases with increasing TiCl4:HCl ratio. At TiCl4:HCl ratio of 1:1, the size of the TiO2 tip is estimated to be 24 nm (Fig. 1a and d). It grows to 47 nm as the TiCl4:HCl ratio changes to 1:12 (Fig. 1h and k). An extreme case is the sample with TiCl4:HCl ratio of 1:16; a single TiO2 tree with a major trunk and several branches could be clearly observed (Fig. 1i and l). Some nanocrystals with square shape found on the surface of substrate can be ascribed to the TiO2 nanocrystals (Fig. 1l). The growth process was studied by SEM characterization of the sample with TiCl4:HCl ratio of 1:4, at various time points from
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45 min to 5 h (Fig. 2). At a reaction time of less than 2 h, sparsely distributed TiO2 with various hierarchical configurations (Fig. 2d and e) could be observed. With the progress of reaction time, TiO2 kept growing, forming dense layers. At the same time, the characteristic size of the sample gradually decreases, indicating that the hierarchical TiO2 further grows along the major trunk and branches. The final thickness of the TiO2 layer reached ~500 mm. The crystalline phase of the covered hierarchical TiO2 was identified by XRD (Fig. 3a). Several characteristic peaks at 27.4 , 36.1, 41.1, 54.2 , 56.3 , and 62.8 could be observed, which could be ascribed to the (110), (101), (111), (211), (002), and (301) crystalline planes, respectively, of rutile TiO2 (JCPDS, No. 77e0441). Raman spectroscopy, which can provide information on the chemical bonding, was employed to characterize the hierarchical TiO2 (Fig. 3b). The clear characteristic peaks at 250, 447, and 615 cm1 could be ascribed to the second-order line, Eg mode, and A1g mode of rutile TiO2, respectively [32]. The results confirm the pure single phase of the hierarchical TiO2. The morphology and microstructure of the hierarchical TiO2 were further characterized by TEM (Fig. 3c). The TEM image shows that hierarchical TiO2 is
Fig. 2. Influence of reaction time on the growth of hierarchical TiO2. (ael) SEM images of the hierarchical TiO2 grown on the solution with TiCl4:HCl ratio of 1:4 for 0.75 h (a and d), 1 h (b and e), 2 h (c and f), 3 h (g and j), 4 h (h and k) to 5 h (i and l).
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Fig. 3. Structural characterizations of the hierarchical TiO2: (aed) XRD pattern (a), Raman spectra (b), TEM image (c), high resolution TEM image (d), electron diffraction pattern (e), and Fourier-transform diffractogram (f).
Fig. 4. Morphology of hierarchical TiO2eAg composite. (a, b) SEM image of hierarchical TiO2eAg composite obtained using hierarchical TiO2 synthesized after growth duration of 2 h and Ag growth duration of 3 min. (c, d) SEM image of hierarchical TiO2eAg composite obtained using hierarchical TiO2 synthesized after growth duration of 5 h and Ag growth duration of 5 min.
taper-shaped, with the taper angle estimated to be ~30 . Highresolution TEM image shows that the taper TiO2 is composed of extremely tiny rutile TiO2 particles with a typical size of ~5 nm, indicating that the hierarchical TiO2 taper is formed through order
attachment of secondary nanostructures (Fig. 3d). The electron diffraction pattern confirms the polycrystalline feature of the taper TiO2 (Fig. 3e). Fig. 3f highlights the Fourier-transform diffractogram of a typical particle, confirming the excellent crystallinity of TiO2.
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Fig. 5. Structural characterizations of hierarchical TiO2eAg composite. (a) XRD pattern. (b) High resolution TEM image. (c) The full scale XPS spectrum. (d) The characteristic XPS band of Ag.
3.2. Synthesis and characterizations of hierarchical TiO2eAg composite TiO2 is a typical photocatalyst, and it can be used to reduce metallic nanostructures. This inspired us to consider the possibility for construction of TiO2eAg composite through in situ photoredox reaction. To test this, a mixture of AgNO3, water, and alcohol was employed as the reaction solution, and the hierarchical TiO2 substrate with different taper configuration was used. The substrate was immersed into the solution, and the photoreduction was triggered by ultraviolet light irradiation. The SEM images of the fabricated TiO2eAg composites show that the Ag nanoparticles can homogeneously precipitate on the entire surface of taper TiO2 (Fig. 4). Interestingly, the size of Ag nanoparticles can be finely tuned from 65 to 150 nm by precisely controlling the irradiation time from 3 to 5 min. The photon-energy dependent growth of Ag nanoparticles on taper TiO2 was also investigated by employing three different irradiation sources of wavelengths 172, 222, and 308 nm. No obvious difference was observed, which could be ascribed to the absorption of taper TiO2 over a broad band. The structure of TiO2eAg composite was characterized by XRD (Fig. 5a). The intense XRD peaks at 38.2 , 44.3 , 64.4 , and 77.4 could be ascribed to the (111), (200), (220), and (311) crystalline plane, respectively, of Ag (JCPDS, No. 04e0783). This confirms the successful precipitation of dense Ag nanostructures on taper TiO2. A typical TEM image shows two tiny Ag nanoparticles attached on the surface of taper TiO2 (Fig. 5b). The chemical state of the Ag species was further studied by XPS. The full scale XPS spectrum clearly shows the signal from Ti and Ag species (Fig. 5c). The high resolution spectrum of the Ag3d states indicates that the binding
energies of the Ag3d5/2 and Ag3d3/2 states are around 367.8 and 373.8 eV, which match well with the binding energies of the Ag0 species (368.0 and 374.0 eV) (Fig. 5d). The results demonstrate that the Ag species predominantly exist in the metallic form.
3.3. Application of hierarchical TiO2eAg composite for trace detection The in situ precipitation of Ag nanostructures on hierarchical TiO2 may give rise to intriguing plasmonic properties. On one hand, the diameter of TiO2 taper gradually changes along the taper, imparting a gradual change in the refractive index and facilitating light-matter interaction [33e35]. Our theoretical simulation indicates that the higher intensity of electric field is located at the middle and the bottom of the tapered TiO2, indicating that the tapered TiO2 is favorable for efficient optical coupling. On the other hand, the dense distribution of Ag on TiO2 taper may generate an enhanced strong three-dimensional electric field through nearfield coupling between particles on a single TiO2 taper or different tapers (Fig. 6a). The above two factors are believed to be favorable for efficient photon utilization and field amplification, which are critical for highly sensitive trace detection. To check this, low concentration of benzenethiol (107 M) was employed as a prototype pollutant, and two materials, hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructures, were studied as the sensing substrates. Fig. 6b shows the collected SERS signals; a significantly intense signal can be observed for hierarchical TiO2eAg composite. The sharp fingerprint bands at 1011, 1031, 1082 and 1570 cm1 could be ascribed to the vibrations of n12, n18a, n1, and C]C/8a/8b, respectively. The absolute signal intensity
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Fig. 6. Trace detection of hierarchical TiO2eAg composite. (a) Schematic showing the fabrication of hierarchical TiO2eAg composite via photoreduction. (b) SERS spectra of low concentration benzenethiol (107 M) on hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructure. (c) Comparison of SERS intensity of low concentration benzenethiol (107 M) on hierarchical TiO2eAg composite and conventional two-dimensional Ag nanostructure. The peak used for comparison is the characteristic band at ~1570 cm1. The absolution intensity is estimated and used for comparison.
Fig. 7. Recycling performance of the fabricated hierarchical TiO2eAg composite. (a) SERS signal intensity and (b) peak shift mapping on the fingerprint band of benzenethiol at 1570 cm1 for the sample after five detection cycles. (c) A summary of the change in SERS signal intensity in five cycles.
of hierarchical TiO2eAg composite is estimated to be more than 51 times higher than that of the conventional two-dimensional Ag nanostructures (Fig. 6c). More importantly, the characteristic bands at 1117 and 1491 cm1 from benzenethiol, which are not visible in the two-dimensional Ag substrate, can be clearly identified on the hierarchical TiO2eAg composite. The above results firmly demonstrate that the constructed hierarchical TiO2eAg composite can not
only be extended as an ultra-sensitive detection substrate but may also provide unprecedented opportunity for performing in situ structure analysis. The additional advantage of the hierarchical TiO2eAg composite is that it can be a potential reusable sensing substrate, because TiO2 can decompose organics under extended ultra-violet light irradiation through photocatalytic reactions. As a demonstration, the
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substrate was repeatedly used for detection of benzenethiol (107 M) with a 633-nm laser as the excitation source for SERS measurement and 325-nm incoherent light as an external field for photon-decomposition. Five-cycle tests were performed, and the results are summarized in Fig. 7. Raman mapping on the fingerprint band of benzenethiol at 1570 cm1 for the sample after five cycles was performed, and the results are presented in Fig. 7a and b. Encouragingly, the hierarchical TiO2eAg composite substrate could provide highly homogenous SERS signal in a 100 mm 50 mm region (Fig. 7a). Moreover, Raman mapping on the peak position indicate that no obvious peak shift occurred during SERS measurement (Fig. 7b). Importantly, 50% sensitivity was retained after five cycles, indicating its potential for reuse (Fig. 7c). The performance degradation mainly occurred in the first cycle, which is probably associated with the violent structure evolution in the fresh sample. The above results collaboratively demonstrate that the constructed SERS substrate can act as a reliable sensor for trace organic detection. 4. Conclusions In summary, our study suggests that hierarchical TiO2eAg composite can be a novel plasmonic material for SERS detection. This novel substrate had high sensitivity, excellent reliability, and good reusability. The studies on the giant local electric field enhancement through creation of three dimensional hot spots enable an improved understanding of the light-matter interaction in nanostructured materials. The findings also open the possibility for constructing plasmonic platforms for trace detection, structure analysis, and biological analysis. Acknowledgments The authors acknowledge financial support from The National Natural Science Foundation of China (No. 51873073, 51435005), the Science and Technology Planning Project of Guangdong Province. China (No. 2017B090901062), and the Special Support Program of Guangdong Province, P. R. China (No. 2015TX01X151). References [1] E.M. Larsson, C. Langhammer, I. Zori c, B. Kasemo, Nanoplasmonic probes of catalytic reactions, Science 326 (2009) 1091e1094. [2] R.A. Alvarez-Puebla, L.M. Liz-Marz an, Traps and cages for universal SERS detection, Chem. Soc. Rev. 41 (2012) 43e51. [3] S. Schlücker, Surface-enhanced Raman spectroscopy: concepts and chemical applications, Angew. Chem. Int. Ed. 53 (2014) 4756e4795. [4] S. Laing, L.E. Jamieson, K. Faulds, D. Graham, Surface-enhanced Raman spectroscopy for in vivo biosensing, Nat. Rev. Chem. 1 (2017). UNSP 0060. [5] C. Zong, M. Xu, L. Xu, T. Wei, X. Ma, X. Zheng, R. Hu, B. Ren, Surface-enhanced Raman spectroscopy for bioanalysis: reliability and challenges, Chem. Rev. 118 (2018) 4946e4980. [6] K. Ueno, S. Juodkazis, V. Mizeikis, K. Sasaki, H. Misawa, Clusters of closely spaced gold nanoparticles as a source of two-photon photoluminescence at visible wavelengths, Adv. Mater. 20 (2008) 26. ~ ez, A. Rivera, A. Prada, G. Tardajos, J. Gonza lez[7] G. Gonz alez-Rubio, P. Díaz-Nún ~ ares, P. Llombart, L.G. Macdowell, M. Alcolea-Palafox, L.M. LizIzquierdo, L. Ban ~ a-Rodríguez, A. Guerrero-Martínez, Femtosecond laser Marz an, O. Pen reshaping yields gold nanorods with ultranarrow surface plasmon resonances, Science 358 (2017) 640e644. nez de Aberasturi, L.M. Liz-Marz [8] J. Reguera, J. Langer, D. Jime an, Anisotropic metal nanoparticles for surface enhanced Raman scattering, Chem. Soc. Rev. 46 (2017) 3866e3885. [9] D. Graham, D.G. Thompson, W.E. Smith, K. Faulds, Control of enhanced Raman scattering using a DNA-based assembly process of dye-coded nanoparticles, Nat. Nanotechnol. 3 (2008) 548e551. [10] D.K. Lim, K.S. Jeon, H.M. Kim, J.M. Nam, Y.D. Suh, Nanogap-engineerable Raman-active nanodumbbells for single-molecule detection, Nat. Mater. 9 (2010) 60e67.
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