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Au nanowire arrays as highly sensitive and recyclable SERS sensor
Sensors & Actuators: B. Chemical 279 (2019) 313–319
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Visible-light-driven charge transfer to significantly improve surfaceenhanced Raman scattering (SERS) activity of self-cleaning TiO2/Au nanowire arrays as highly sensitive and recyclable SERS sensor
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Xin Zhao, Wenzhong Wang , Yujie Liang, Junli Fu, Min Zhu, Honglong Shi, Shijing Lei, Chunjiang Tao School of Science, Minzu University of China, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface-enhanced Raman scattering TiO2 nanowire array Au nanoparticle Charge transfer Recyclable detection
We apply a facile photoreduction deposition strategy to grow Au nanoparticles (NPs) on the surfaces of TiO2 nanowire arrays (TiO2/Au NWAs) as a Raman sensitive substrate. The fabricated TiO2/Au NWAs can be used as a highly sensitive and recyclable SERS sensor for detecting the rhodamine 6 G (R6 G) molecules with a concentration as low as 10−9 M. Moreover, when irradiated with UV light, the SERS sensor with high sensitivity can be fully reproduced due to the superior self-cleaning capability of the substrate. By a comparative study on the SERS activities of R6 G molecules over the substrates of pure TiO2 NWAs, pure Au NPs and TiO2/Au NWAs, as well as the photoresponse performance of pure TiO2 and TiO2/Au NWAs under visible light illumination, we provide an illuminating insight into enhanced mechanism of SERS activity for TiO2/Au NWAs. The photoresponse performances and SERS activities provide experimental evidence that enhanced SERS activity of TiO2/ Au NWAs is ascribed to the efficient charge transfer among Au, TiO2 and R6 G. This work throws a new light on enhanced mechanism for SERS activity of semiconductor/noble-metal substrates, which are expected to find potential applications to fabricate highly sensitive SERS sensors for the detection of analytes.
1. Introduction As a powerful technique to detect spectroscopic signals of molecules in the single molecule level [1] surface-enhanced Raman scattering (SRES) has attracted extensive attention and has found increasingly wide applications in many fields [2,3]. Thus various Raman substrates have been developed recently for detecting the SERS signals of analytes. However, the choice of an appropriate substrate is a key to achieve highly sensitive, recyclable and stable SERS signals [4]. Although noble-metal micro/nanostructures are still considered as the optimal SRES substrates due to their capability to present satisfactory SERS activity [4,5], photothermal and local heating of the noble-metal micro/nanostructures possibly result in low reproducibility for SERS data [6–9]. Therefore, it is highly desirable to design and fabricate other substrates to achieve highly sensitive, reproducible and stable SERS signals. Recently, it has demonstrated that the semiconductor/noble-metal heterostructure substrates fabricated by combining suitable semiconductor nanostructures with noble-metal nanoparticles (NPs) show highly sensitive, reproducible, and stable SERS activity [10–12]. In ⁎
contrast with noble-metal substrates, the superior SERS activity of these substrates is attributed to their following unique properties. (1) The capability to preconcentrate and absorb the probe molecules on the active sites of the substrate due to the unique surface wettability of semiconductor nanostructure. (2) The capability to produce an evanescent field by semiconductor nanostructures to enhance the SERS sensitivity and reproducibility with the negligible perturbation for probe molecules. (3) The capability to provide highly sensitive, reproducible and stable SERS activity due to its superior self-cleaning ability under UV or visible light irradiation. These multiple functionalities make semiconductor/noble-metal nanostructures to be as suitable substrates for fabricating SERS sensors, which have been widely applied in detecting and analyzing structural information of chemical and/or biological analytes [13–17]. However, for the practical applications, the SERS sensor needs to have good sensitivity, recyclability and stability, thus a key issue to achieve the semiconductor/noblemetal substrate-based SERS sensor with high sensitivity, recyclability and stability is to understand the enhanced mechanism of SERS activity. In few published works, a charge transfer mechanism has been applied to interpret the SERS enhancement of semiconductor/noble-metal
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.snb.2018.10.010 Received 24 May 2018; Received in revised form 28 September 2018; Accepted 4 October 2018 Available online 06 October 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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samples were studied by a Lambda 950 spectrometer.
nanostructure substrates [18–20]. For instance, the enhanced SERS activity of Ag/TiO2 nanoparticle substrate was attributed to the charge transfer from TiO2 to the probe molecule of 4-Mercaptobenzoic acid (4MBA) and from Ag to 4-MBA molecule through TiO2 conduction band (CB) [21]. Another important example is the Ag/N719/TiO2 nanocomposite substrate (where N719 is dye (Bu4N)2[Ru(dcbpyH)2(NCS)2]). In this substrate, the enhanced SERS activity was ascribed to the charge transfer from Ag to N719 molecule and from N719 molecule to TiO2, besides the contributions from the electromagnetic enhancement of Ag NPs and resonance effect of N719 molecule [20]. These studies have demonstrated that the selective SERS enhancement for specific Raman signals of molecules is usually considered as the evidence that enhanced SERS activity is attributed to the charge transfer between constituents of the substrate. However, it has been proven that the selective SERS enhancement for specific Raman signals of molecules is greatly depended on the light wavelength and the testing molecules [19,20]. In the present study, we apply a facile photoreduction deposition strategy to grow Au NPs on the surfaces of TiO2 nanowire arrays (TiO2/ Au NWAs) as a Raman sensitive substrate. The fabricated TiO2/Au NWAs can be used as a highly sensitive and recyclable SERS sensor for detecting the rhodamine 6 G (R6 G) molecules with a concentration as low as 10−9 M. Moreover, when irradiated with UV light, the SERS sensor with high sensitivity can be fully reproduced due to the superior self-cleaning capability of the substrate. By a comparative study on the SERS activities of R6 G molecules over the substrates of pure TiO2 NWAs, pure Au NPs and TiO2/Au NWAs, as well as the photoresponse performance of pure TiO2 and TiO2/Au NWAs under visible light illumination, we provide an illuminating insight into the enhanced mechanism of SERS activity for TiO2/Au NWAs. This work throws a new light on enhanced mechanism for SERS activity of semiconductor/ noble-metal substrates, which is expected to find potential applications to fabricate other semiconductor/noble-metal SERS sensors with high sensitivity for the detection of analytes.
2.4. Photoresponse evaluation The photoresponse activities of the pure TiO2 and TiO2/Au NWAs were studied by a photoelectrochemical (PEC) cell containing working electrode (TiO2 or TiO2/Au NWAs), reference electrode (Ag/AgCl) and counter electrode (a Pt wire). All experiments were conducted in Na2SO4 electrolyte solution (0.1 M) under illumination with λ > 420 nm visible light (AM 1.5 G, 100 mW/cm2). 2.5. SERS measurements The SERS properties of achieved TiO2/Au NWAs were investigated through detecting the Raman spectra of R6 G molecules at different concentrations (10−4 to 10−9 M). For SERS testing, the R6 G solution with concentrations of 10−4 to 10−9 M was dropped on the TiO2/Au NWAs. A confocal microprobe Raman spectrometer (Horiba LABRAMHR-800) was applied to collect Raman spectra of samples excited with 633 nm light at room temperature. To study enhanced mechanism for SERS performance of TiO2/Au NWAs, the SERS activities of pure Au NPs and bare TiO2 NWAs were also investigated by testing the Raman spectra of R6 G molecules at the same conditions. 2.6. Reproducible characterization
2. Experimental section
For reproducible SERS activity characterization, after the SERS performance was measured, the used TiO2/Au NWAs were placed into ultrapure water and then irradiated for 2 h by a Xe lamp. The irradiated TiO2/Au NWAs were rinsed with ultrapure water to remove residues such as ions and molecules, followed by drying under room temperature. Then, the substrate of the cleaned TiO2/Au NWAs was applied again to detect the Raman signals of R6 G molecules. The reproducible tests were repeated five times to evaluate the reproducibility and stability of the fabricated TiO2/Au NWAs.
2.1. Preparation of TiO2 NWAs
3. Results and discussion
The TiO2 NWAs were grown on glass (fluorine-doped tin oxide, FTO) substrate via an easy hydrothermal process described as the following. 15 mL HCl (38%) was firstly added into 15 mL ultrapure water, and then 0.5 mL titanium butoxide was added drop by drop under stirring. Finally, the solution was put into an autoclave, followed by placing a cleaned FTO piece against the autoclave wall. Afterwards, the autoclave was sealed and heated for 3.5 h at 150 °C. Then, the sample was washed with ultrapure water and ethanol, dried and annealed for 30 min at 450 °C.
3.1. XRD and SEM characterizations Fig. 1a shows the XRD patterns of the samples. Before growing Au NPs, the diffraction peaks of the sample can be assigned to those of rutile phase TiO2 (JCPDS No. 78-2485), besides the diffraction peaks from FTO substrate. After growing Au NPs, the sample also shows the dominate diffraction peaks of rutile phase TiO2. However, a weak shoulder peak of one diffraction peak of FTO at around 38.0° and an obvious diffraction peak at around 44.6° are observed. The enlarged patterns (insets of Fig. 1a) show that these two peaks can be easily assigned to (111) and (200) planes of Au (JCPDS No. 4-784), demonstrating the formation of Au NPs in sample. Fig. 1b shows the SEM images of the sample before growing the Au NPs. It can be found that the sample is composed of aligned nanowires (NWs) with end diameter of about 150 nm. Thus the rutile phase TiO2 NWAs were prepared on the FTO substrate. Another feature is that the surface of the NWs is smooth. After growing the Au NPs, the SEM images evidently show that a lot of Au NPs are grown on the surfaces of NWs as shown in Fig. 1 c,d. The nominal particle size determined by SEM images is about 3–63 nm. The particle sizes were obtained by statistically analyzing over 300 particles through an image processing software of ImageJ, in which the particle sizes were determined by measuring the greatest pixel length between the boundaries of the particles and converting to nm as used in previously published article [22]. SEM studies demonstrate that TiO2/Au NWAs have been prepared by using a facile photoreduction deposition method to grow Au NPs on the surface of TiO2 NWAs.
2.2. Fabrication of TiO2/Au NWAs The TiO2/Au NWAs were fabricated by using a facile photoreduction deposition method to grow Au NPs on the nanowire surface of TiO2 NWAs. Briefly, the FTO covered with TiO2 NWAs was placed into 60 mL aqueous solution containing 10 mL of 1.5 mg mL−1 gold chloride (HAuCl4⋅3H2O), and then irradiated for 3 min by a Xe lamp (300 W) with λ < 420 nm light. After that, the FTO sample was thoroughly washed with ultrapure water, dried and heated for 1 h at 450 °C. 2.3. Characterization X-ray powder diffraction (XRD) was applied to study the composition and crystalline phase of the samples. The experiments were carried out on a XD-D1 diffractometer employing Cu Kα (λ = 1.5406 Å) irradiation. Shape and size of the samples were observed by a scanning electron microscope (SEM). Optical absorption properties of the 314
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Fig. 1. (a) XRD patterns of TiO2 and TiO2/Au samples. (b) SEM images of pure TiO2 NWAs. (c) low- and (d) high-magnification SEM images of TiO2/Au NWAs.
3.2. SERS activity The SERS properties of bare TiO2 and TiO2/Au NWAs were investigated through detecting the Raman vibrational signals of R6 G molecules. To well understand the enhanced mechanism for SERS performance of TiO2/Au NWAs, the SERS performance of pure Au NPs was also studied at the same conditions. The pure Au NPs were grown on the FTO substrate by photoreduction deposition method applied to grow Au NPs on the surface of TiO2 NWAs at the same conditions. Fig. 2a shows the SERS spectra of R6 G molecules with the concentration of 10−4 M over substrates of TiO2/Au NWAs, bare TiO2 NWAs and pure Au NPs. When the bare TiO2 NWAs are used as SERS substrate, no Raman signals of R6 G molecules are detected. For the SERS substrate of pure Au NPs, the SERS spectrum only shows weak Raman signals of R6 G molecules at 1601, 1360, 1320 and 612 cm-1. In addition, some characteristic Raman signals are not showed. Because the amount of Au NPs grown FTO substrate is small (Fig. S1), the Raman signals of R6 G molecules over the pure Au nanoparticle substrate are weak, and some characteristic Raman signals are not detected. However, when applied as SERS substrate, the TiO2/Au NWAs not only show Raman signals of R6 G molecules at 1648, 1601, 1570, 1506, 1360, 1320, 1186, 774 and 612 cm−1 [23,24], but also significantly enhance the intensities of these Raman signals. The SERS properties undoubtedly show that the TiO2/Au NWAs achieve remarkably improved SERS activity for detecting Raman signals of R6 G molecules compared to pure Au NPs. In addition, The SERS properties also demonstrate that improved SERS activity of TiO2/Au NWAs is attributed to the synergistic effect of Au, TiO2 and R6 G. A highly sensitive performance is essential to a SERS sensor for practical application. The R6 G molecules with different concentrations were selected to investigate the SERS sensitivity of the TiO2/Au NWAs. Fig. 2b shows the SERS spectra of R6 G molecules with different
Fig. 2. (a) SERS spectra of R6 G molecules with 10−4 M concentration over the substrates of bare TiO2 NWAs, pure Au NPs and TiO2/Au NWAs. (b) SERS spectra of R6 G molecules with concentrations of 10−4 to 10−9 M over the substrate of TiO2/Au NWAs.
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Fig. 3. SERS spectra of R6 G molecules after the decomposition by TiO2/Au NWAs under the different irradiation time with UV light.
concentrations over the TiO2/Au NWAs. It can be found that the obvious Raman signals of R6 G molecules can be detected at the concentration as low as 10−9 M, demonstrating the highly detective ability of the TiO2/Au NWAs for R6 G molecules. A comparison of SERS performance with the reported Au/semiconductor substrates for the detection of R6 G molecules also demonstrates that the TiO2/Au NWAs show highly sensitive SERS activity (Table S1). 3.3. Self-cleaning ability The recyclability is another key issue to a SERS sensor for practical application. It has been reported that the TiO2/Au nanostructures show ability to decompose absorbed dye molecules under UV or visible light irradiation [25,26]. In this work, the self-cleaning ability of the TiO2/ Au NWAs to decompose the R6 G molecules was evaluated under the UV light irradiation. Fig. 3 gives the SERS spectra of R6 G molecules with concentration of 10−4 M after the decomposition by TiO2/Au NWAs under the different irradiation time. It can be found that the Raman signals of R6 G molecules gradually disappear with prolonging the time of light irradiation. After irradiation with UV light for 2 h, the Raman signals of the R6 G molecules almost disappear completely, demonstrating that the TiO2/Au NWAs exhibit a superior capability to decompose R6 G molecules by self-cleaning process under UV light irradiation.
Fig. 4. (a) SERS spectra of the R6 G molecules with 10−4 M on the TiO2/Au NWAs before and after self-cleaning over five cycles. (b) Raman intensities for 1360 cm-1 of R6 G molecules with 10-4 concentration during repeated measurement and self-cleaning with UV light irradiation.
3.5. Optical absorption and visible light photoresponse activity To elucidate the enhanced mechanism for SERS activity of TiO2/Au NWAs, the light absorption and visible light photoresponse activity were studied. The light absorption spectrum of TiO2 NWAs shows a strong absorption peak at about 370 nm. And no light absorption is observed over 400 nm region as shown in Fig. 5a. However, the spectrum of TiO2/Au NWAs shows obvious light absorption from 500 to 600 nm with a main peak at about 570 nm, besides a strong absorption at about 370 nm. The light absorption from 500 to 600 nm in TiO2/Au NWAs is induced by the surface plasmon resonance (SPR) absorption of plasmonic Au NPs. This wide light absorption from 500 to 600 nm is attributed to the wide size distribution of Au NPs, which can be verified by SEM studies. The SEM images and size distribution histogram show that the Au NPs display a size distribution from 3 to 63 nm with an average size of 24 nm (Fig. S2). Thus this wide optical absorption arises from the wide size distribution of Au NPs. The bandgap values for TiO2 and TiO2/Au NWAs are ca. 3.0 eV, which are obtained by combining Kubelka-Munk function with the Taus plot [27] and the tangent line intercept of (F(R)hν)1/2 vs. hν plot as shown in Fig. 5b. The SERS activity shows that the TiO2/Au NWAs exhibit highly sensitive SERS ability to detect the Raman signals of R6 G molecules under 633 nm light illumination. Thus the investigation for visiblelight-driven photoresponse activity of pure TiO2 and TiO2/Au NWAs is favorable for elucidating enhanced SERS performance of TiO2/Au NWAs. In this work, we use photocurrent density, which was achieved under visible light illumination (λ > 420 nm, AM 1.5 G, 100 mW/cm2) in a PEC cell using 0.1 M Na2SO4 as electrolyte, to evaluate the photoresponse ability of pure TiO2 and TiO2/Au NWAs. At a potential of 0 V versus Ag/AgCl, the TiO2/Au NWAs produce an obvious photocurrent density (around 9 μA cm−2) (Fig. 5c). However, the pure TiO2 NWAs almost don’t display any photoresponse activity. At a potential of 0.5 V
3.4. SERS recyclable performance As shown in Fig. 3, the superior self-cleaning ability allows the TiO2/Au NWAs to be easily regenerated for detecting the SERS signals of the R6 G molecules. To evaluate the recyclable detection ability for R6 G molecules, the TiO2/Au NWAs were recycled five cycles with UV light irradiation for 2 h. Fig. 4a shows the SERS spectra of the R6 G molecules with concentration of 10−4 M over the TiO2/Au NWAs before and after self-cleaning. It can be evidently found that the characteristic Raman signals of R6 G molecules almost disappear completely after the UV light irradiation. However, when the R6 G molecules are absorbed again on the cleaned TiO2/Au NWAs, the strong SERS signals of R6 G molecules are observed, which are similar to those of the new TiO2/Au NWAs. Fig. 4b displays the Raman intensities for 1360 cm−1 of R6 G molecules with 10−4 concentration during repeated measurement and self-cleaning with UV light irradiation, showing that the TiO2/Au NWAs exhibit superior ability to retain SERS activity during the five recycles. The studies on the Raman properties experimentally demonstrate that the fabricated TiO2/Au NWAs show highly sensitive and recyclable SERS activity for the detection of R6 G molecules. However, in order to find further applications for the detection of other probe analytes, a key issue is to rationally interpret the mechanism for enhanced SERS activity of TiO2/Au NWAs. 316
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Fig. 5. (a) UV–vis absorption spectra of TiO2 and TiO2/Au NWAs. (b) The bandgap values of TiO2 and TiO2/Au NWAs, calculated by fitting absorption spectra. The curves of photocurrent densities for the pure TiO2 and TiO2/Au NWAs at (c) 0 V and (d) 0.5 V vs. Ag/AgCl under illumination with λ > 420 nm visible light (AM 1.5 G, 100 mW/cm2).
(LUMO) level of R6 G. (2) The contribution resulted from the charge excitation and transfer between the LUMO and the highest occupied molecular orbital (HOMO) energy levels of the R6 G. (3) The contribution offered by the charge transfer from the TiO2 to the LUMO level of R6 G. (4) The contribution provided by the transfer of “hot electrons” from the Ef of Au to the LUMO level of R6 G via the CB of TiO2. Thus the positions of Ef for Au, the CB and VB for TiO2, and the LUMO and HOMO for R6 G are crucial parameters to determine whether the charge excitation and transfer can be achieved in TiO2/R6 G, Au/R6 G and TiO2/Au/R6 G structures under the irradiation of 633 nm light. As reported previously, the Ef of Au is 5.1 eV [30], the positions of CB and VB for rutile phase TiO2 are 4.04 and 7.07 eV, respectively [31,32], the level positions of the LUMO and HOMO for R6 G are 3.4 and 5.7 eV versus vacuum level, respectively [33]. Based on the energy band structure of Au, rutile phase TiO2 and R6 G, we can give a rational elucidation for the SERS activity of pure TiO2, pure Au and TiO2/Au substrate, respectively. For the pure TiO2 NWAs, the 633 nm light is incapable of exciting TiO2 to generate electron-hole pairs, because the bandgap of TiO2 NWAs is round 3.0 eV, larger than the light energy of 633 nm (1.96 eV). This result has yet been confirmed by visible-light-driven photoresponse activity as shown in Fig. 5 a,b. In addition, the light of 633 nm is also unable to excite R6 G to produce charge carriers, because the energy of 1.96 eV is less than the bandgap (2.3 eV) of R6 G molecule. These results show that the SERS activities induced by the charge transfer from the TiO2 to the LUMO level of R6 G, and between the LUMO and HOMO energy levels of R6 G can’t be achieved in TiO2/R6 G structure. Therefore, no Raman signals of R6 G molecules are detected over substrate of pure TiO2 NWAs as shown in Fig. 2a. For the pure Au substrate, the energy difference between Ef of Au and the LUMO level of
versus Ag/AgCl, the TiO2 NWAs also don’t show any photoresponse, whereas the photocurrent density of TiO2/Au NWAs increases to 13 μA cm−2 as shown in Fig. 5d. Thus we experimentally show that the TiO2/ Au NWAs not only exhibit a rapid photoresponse activity, but also generate an obvious photocurrent density under illumination with λ > 420 nm visible light. The light absorption shows that the TiO2/Au NWAs exhibit an obvious SPR absorption with a main peak at about 570 nm. Thus, when irradiated by λ > 420 nm visible light, an enlarged enhanced electromagnetic (EM) field can be generated due to the strong SPR effect of the Au NPs, and then the enlarged EM field can excite “hot electrons” of the Au nanoparticle to the CB of TiO2 nanowire [28]. In addition, a Schottky barrier established at the interface of Au and TiO2 will further facilitate the transfer of “hot electrons” to the CB of TiO2 and prevent the “hot electrons” from coming back to Au NPs [29]. Moreover, because the λ > 420 nm light is not able to excite the electrons in the valence band (VB) of TiO2 to its CB, no holes are produced in the VB of TiO2. Thus, the “hot electrons” accumulated on the CB of TiO2 will transfer to the counter electrode to split water, resulting in the produce of photocurrent in a PEC cell. No photoresponse activity and photocurrent are achieved in bare TiO2 NWAs because they can’t be excited to produce photoinduced electrons and holes under λ > 420 nm visible light irradiation. 3.6. Mechanism for enhanced SERS activity of TiO2/Au NWAs There are four possible contributions to enhanced SERS performance of the TiO2/Au NWAs for detecting the Raman signals of R6 G molecules. (1) The contribution offered by the charge transfer from the Fermi level (Ef) of Au to the lowest unoccupied molecular orbital 317
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based SERS sensors with high sensitivity for the detection of analytes. However, the detection concentration for R6 G molecules of the fabricated TiO2/Au NWAs is still too high to practical applications as a SERS sensor for the detection of analytes. By controlling size and loading content of Au NPs, we will further improve sensitivity of TiO2/Au NWAs to detect analytes with concentration of 10−11 to 10−13 M. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant Nos. 61575225, 11374377, 11074312 and 11404414, and the Undergraduate Research Training Program of Minzu University of China under Grant Nos. GCCX2017110006, GCCX2017110007 and URTP20170007. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2018.10.010. References [1] R. Zhang, Y. Zhang, Z.C. Dong, S. Jiang, C. Zhang, L.G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J.L. Yang, J.G. Hou, Chemical mapping of a single molecule by plasmon-enhanced Raman scattering, Nature 498 (7452) (2013) 82–86. [2] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, SERS: materials, applications and the future, Mater. Today 15 (1–2) (2012) 16–25. [3] I. Alessandri, J.R. Lombardi, Enhanced Raman scattering with dielectrics, Chem. Rev. 116 (24) (2016) 14921–14981. [4] M.J. Banholzer, J.E. Millstone, L. Qin, C.A. Mirkin, Rationally designed nanostructures for surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 37 (5) (2008) 885–897. [5] H. Wang, N.J. Halas, Mesoscopic Au “meatball” particles, Adv. Mater. 20 (4) (2008) 820–825. [6] Z. Ioffe, T. Shamai, A. Ophir, G. Noy, I. Yutsis, K. Kfir, O. Cheshnovsky, Y. Selzer, Detection of heating in current-carrying molecular junctions by Raman scattering, Nat. Nanotechnol. 3 (12) (2008) 727–732. [7] D.R. Ward, D.A. Corley, J.M. Tour, D. Natelson, Vibrational and electronic heating in nanoscale junctions, Nat. Nanotechnol. 6 (1) (2011) 33–38. [8] N.J. Hogan, A.S. Urban, C. Ayala-Orozco, A. Pimpinelli, P. Nordlander, N.J. Halas, Nanoparticles heat through light localization, Nano Lett. 14 (8) (2014) 4640–4645. [9] M. Moskovits, Persistent misconceptions regarding SERS, Phys. Chem. Chem. Phys. 15 (15) (2013) 5301–5311. [10] C.Y. Huang, C.X. Xu, J.F. Lu, Z.H. Li, Z.S. Tian, 3D Ag/ZnO hybrids for sensitive surface-enhanced Raman scattering detection, Appl. Surf. Sci. 365 (2016) 291–295. [11] Y. Zhao, L. Sun, M. Xi, Q. Feng, C.Y. Jiang, H. Fong, Electrospun TiO2 nanofelt surface-decorated with Ag NPs as sensitive and UV-cleanable substrate for surface enhanced Raman scattering, ACS Appl. Mater. Interfaces 6 (8) (2014) 5759–5767. [12] F.A. Harraz, A.A. Ismail, H. Bouzid, S.A. Al-Sayari, A. Al-Hajry, M.S. Al-Assiri, Surface-enhanced Raman scattering (SERS)-active substrates from silver platedporous silicon for detection of crystal violet, Appl. Surf. Sci. 331 (2015) 241–247. [13] X. Han, H. Wang, X. Ou, X. Zhang, Highly sensitive, reproducible, and stable SERS sensors based on well-controlled silver nanoparticle-decorated silicon nanowire building blocks, J. Mater. Chem. 22 (28) (2012) 14127–14132. [14] E.Z. Tan, P.G. Yin, T.T. You, H. Wang, L. Guo, Three dimensional design of largescale TiO2 nanorods scaffold decorated by silver NPs as SERS sensor for ultrasensitive malachite green detection, ACS Appl. Mater. Interfaces 4 (7) (2012) 3432–3437. [15] K. Zhang, J. Ji, Y. Li, B. Liu, Interfacial self-assembled functional nanoparticle array: a facile surface-enhanced Raman scattering sensor for specific detection of trace analytes, Anal. Chem. 86 (13) (2014) 6660–6665. [16] H.W. Qiu, M.Q. Wang, L. Li, J.J. Li, Z. Yang, M.H. Cao, Hierarchical MoS2-microspheres decorated with 3D AuNPs arrays for high-efficiency SERS sensing, Sens. Actuators B 255 (2018) 1407–1411. [17] M.S. Yang, J. Yu, F.C. Lei, H. Zhou, Y.S. Wei, B.Y. Man, C. Zhang, C.H. Li, J.F. Ren, X.B. Yuan, Synthesis of low-cost 3D-porous ZnO/Ag SERS-active substrate with ultrasensitive and repeatable detectability, Sens. Actuators B 256 (2018) 268–275. [18] H. Fang, C.X. Zhang, L. Liu, Y.M. Zhao, H.J. Xu, Recyclable three-dimensional Ag nanoparticle-decorated TiO2 nanorod arrays for surface-enhanced Raman scattering, Biosens. Bioelectron. 64 (2015) 434–441. [19] X. Jiang, X. Li, X. Jia, G. Li, X. Wang, G. Wang, Z. Li, L. Yang, B. Zhao, Surfaceenhanced Raman scattering from synergistic contribution of metal and semiconductor in TiO2/MBA/Ag(Au) and Ag(Au)/MBA/TiO2 assemblies, J. Phys. Chem. C 116 (27) (2012) 14650–14655. [20] X.L. Wang, Y. Wang, H.M. Sui, X.L. Zhang, H.Y. Su, W.N. Cheng, X.X. Han, B. Zhao, Investigation of charge transfer in Ag/N719/TiO2 interface by surface-enhanced Raman spectroscopy, J. Phys. Chem. C 120 (24) (2016) 13078–13086. [21] L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, J.R. Lombardi, Charge-transferinduced surface-enhanced Raman scattering on Ag-TiO2 nanocomposites, J. Phys.
Fig. 6. Illustration for charge transfer process of (a) Au/R6 G structure and (b) TiO2/Au/R6 G structure.
R6 G is 1.7 eV (Fig. 6a), which is less than the photo energy of 633 nm (1.96 eV). So the “hot electrons” of plasmonic Au NPs can be motivated to the LUMO level of R6 G, resulting in the obvious SERS signals of R6 G as presented in Fig. 2a. For the substrate of TiO2/Au NWAs, the energy difference between the Ef of Au and the CB of TiO2 is 1.06 eV (Fig. 6b), less than the energy (1.96 eV) of 633 nm light. Thus the “hot electrons” of the plasmonic Au NPs can be excited to the CB of TiO2. This conclusion has yet been experimentally verified by the visible-light-driven photoresponse activity as shown in Fig. 5 c,d. Moreover, as shown in Fig. 6b, the energy difference between the CB of TiO2 and the LUMO level of R6 G is 0.64 eV, thus the “hot electrons” motivated to the CB of TiO2 can be further excited to the LUMO level of R6 G. Therefore, besides the contribution provided by the charge transfer from the Ef of Au to the LUMO level of R6 G, the charge transfer from the Ef of Au to the LUMO of R6 G via the CB of TiO2 also contributes an additional enhancement for SERS signals, leading to significant enhancement for SERS signals as demonstrated in Fig. 2. The experiments and discussion show that the significantly improved SERS activity for TiO2/Au NWAs is ascribed to the synergistic effect and effective charge transfer among the Au, TiO2 and R6 G, which is illustrated in Fig. 6b. 4. Conclusions In conclusion, the heterogeneous TiO2/Au NWAs were fabricated by an easy photoreduction deposition method. Used as a SERS substrate, the TiO2/Au NWAs show highly sensitive and recyclable SERS activity for detecting R6 G molecules with a concentration as low as 10−9 M. Furthermore, the substrate of TiO2/Au NWAs exhibits superior selfcleaning capability, making it to fully reproduce high sensitivity after irradiating with UV light. The SERS activities and photoresponse performances achieved under visible light irradiation provide experimental evidence that improved SERS activity of TiO2/Au NWAs is attributed to the efficient charge transfer among the Au, TiO2 and R6 G. This work throws a new light on enhanced mechanism for high SERS activity of semiconductor/noble-metal substrates, which are expected to find potential applications to fabricate semiconductor/noble-metal substrate318
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Chem. C 113 (36) (2009) 16226–16231. [22] N.A. Mashayekhi, Y.Y. Wu, M.C. Kung, H.H. Kung, Metal nanoparticle catalysts decorated with metal oxide clusters, Chem. Commun. 48 (2012) 10096–10098. [23] C.W. Cheng, B. Yan, S.M. Wong, X.L. Li, W.W. Zhou, T. Yu, Z.X. Shen, H.Y. Yu, H.J. Fan, Fabrication and SERS performance of silver-nanoparticle-decorated Si/ ZnO nanotrees in ordered arrays, ACS Appl. Mater. Interface 2 (7) (2010) 1824–1828. [24] Y.X. Du, Y. Zhao, Y. Qu, C.H. Chen, C.M. Chen, C.H. Chuange, Y.W. Zhu, Enhanced light-matter interaction of grapheme-gold nanoparticle hybrid films for high performance SERS detection, J. Mater. Chem. C 2 (23) (2014) 4683–4691. [25] C. Feng, Z.C. Yu, H.J. Liu, K.K. Yuan, X.Q. Wang, L.Y. Zhu, G.H. Zhang, D. Xu, Enhanced photocatalytic performance of Au/TiO2 nanofibers by precisely manipulating the dosage of uniform-sized Au nanoparticles, Appl. Phys. A 123 (8) (2017) 519. [26] S.T. Kochuveedu, D.P. Kim, D.H. Kim, Surface-plasmon-induced visible light photocatalytic activity of TiO2 nanospheres decorated by Au nanoparticles with controlled configuration, J. Phys. Chem. C 116 (3) (2012) 2500–2506. [27] R. López, R. Gómez, Band-gap energy estimation from diff ;use reflectance measurements on sol–gel and commercial TiO2: a comparative study, J. Sol–Gel Sci. Technol. 61 (1) (2012) 1–7. [28] H. Lee, Y.K. Lee, E. Hwang, J.Y. Park, Enhanced surface plasmon effect of Ag/TiO2 nanodiodes on internal photoemission, J. Phys. Chem. C 118 (2014) 5650–5656. [29] Y. Shiraishi, N. Yasumoto, J. Imai, H. Sakamoto, S. Tanaka, S. Ichikawa, B. Ohtanie, T. Hirai, Quantum tunneling injection of hot electrons in Au/TiO2 plasmonic photocatalysts, Nanoscale 9 (24) (2017) 8349–8361. [30] N.D. Orf, I.D. Baikie, O. Shapira, Y. Fink, Work function engineering in low-temperature metals, Appl. Phys. Lett. 94 (11) (2009) 113504. [31] Q.C. Zhang, X.G. Liu, Dye-sensitized solar cell goes solid, Small 8 (24) (2012) 3711–3713. [32] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R.A. Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W. Watson, T.W. Kea, P. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798–801. [33] X.T. Wang, W.S. Shi, G.W. She, L.X. Mu, Using Si and Ge nanostructures as substrates for surface-enhanced Raman scattering based on photoinduced charge transfer mechanism, J. Am. Chem. Soc. 133 (41) (2011) 16518–16523.
Professor Wenzhong Wang is Professor of Physics at the Minzu University of China since 2006. He received his Ph. D. degree in condensed matter physics from the Nanjing University (China) in 2002. In 2003 he worked as a post-doctoral fellow in the Institute of Materials Research at the Pennsylvania State University (University Park, USA). From 2004 to 2006 he worked as a research fellow in the Department of Physics, Boston College (USA). His recent research focuses on the design and fabrication of low-dimensional nanostructures and their applications involving solar energy conversion, energy storage and SERS-based sensors. Associate Professor Yujie Liang is associate professor in Physics at the Minzu University of China since 2017. She received her Ph. D. degree in physics from the Beijing Normal University (China) in 2009. From 2009 to 2011 she worked as a post-doctoral fellow in physical electronics at the University of Electronic Science and Technology of China. Her current research interest focuses on the design and fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Associate Professor Junli Fu is associate professor in Physics at the Minzu University of China since 2013. She received her Ph. D. degree in physics from the Lanzhou University (China) in 2008. Her current research interest focuses on the design and synthesis of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Associate Professor Min Zhu is associate professor in Physics at the Minzu University of China since 2010. She received his Ph. D. degree in physics from the Beijing Normal University (China) in 2008. His current research interest involves the fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Dr. Honglong Shi received his Ph. D. degree in physics from the Institute of Physics, Chinese Academy of Sciences in 2011. He currently is a Lecture at the Minzu University of China. His research interest involves the synthesis of nanostructures and their microstructure analysis using transmission electron microscopy. Ms Shijing Lei is a junior. She is pursuing her bachelor degree in nanomaterials and technology at the Minzu University of China.
Ms Xin Zhao received her bachelor degree in 2015 from the Beijing Institute of Technology (China) and her mater degree in 2018 from Minzu University of China. Her research interest lies in the fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes.
Mr Chunjiang Tao is a junior. He is pursuing his bachelor degree in nanomaterials and technology at the Minzu University of China.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Visible-light-driven charge transfer to significantly improve surfaceenhanced Raman scattering (SERS) activity of self-cleaning TiO2/Au nanowire arrays as highly sensitive and recyclable SERS sensor
T
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Xin Zhao, Wenzhong Wang , Yujie Liang, Junli Fu, Min Zhu, Honglong Shi, Shijing Lei, Chunjiang Tao School of Science, Minzu University of China, Beijing 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface-enhanced Raman scattering TiO2 nanowire array Au nanoparticle Charge transfer Recyclable detection
We apply a facile photoreduction deposition strategy to grow Au nanoparticles (NPs) on the surfaces of TiO2 nanowire arrays (TiO2/Au NWAs) as a Raman sensitive substrate. The fabricated TiO2/Au NWAs can be used as a highly sensitive and recyclable SERS sensor for detecting the rhodamine 6 G (R6 G) molecules with a concentration as low as 10−9 M. Moreover, when irradiated with UV light, the SERS sensor with high sensitivity can be fully reproduced due to the superior self-cleaning capability of the substrate. By a comparative study on the SERS activities of R6 G molecules over the substrates of pure TiO2 NWAs, pure Au NPs and TiO2/Au NWAs, as well as the photoresponse performance of pure TiO2 and TiO2/Au NWAs under visible light illumination, we provide an illuminating insight into enhanced mechanism of SERS activity for TiO2/Au NWAs. The photoresponse performances and SERS activities provide experimental evidence that enhanced SERS activity of TiO2/ Au NWAs is ascribed to the efficient charge transfer among Au, TiO2 and R6 G. This work throws a new light on enhanced mechanism for SERS activity of semiconductor/noble-metal substrates, which are expected to find potential applications to fabricate highly sensitive SERS sensors for the detection of analytes.
1. Introduction As a powerful technique to detect spectroscopic signals of molecules in the single molecule level [1] surface-enhanced Raman scattering (SRES) has attracted extensive attention and has found increasingly wide applications in many fields [2,3]. Thus various Raman substrates have been developed recently for detecting the SERS signals of analytes. However, the choice of an appropriate substrate is a key to achieve highly sensitive, recyclable and stable SERS signals [4]. Although noble-metal micro/nanostructures are still considered as the optimal SRES substrates due to their capability to present satisfactory SERS activity [4,5], photothermal and local heating of the noble-metal micro/nanostructures possibly result in low reproducibility for SERS data [6–9]. Therefore, it is highly desirable to design and fabricate other substrates to achieve highly sensitive, reproducible and stable SERS signals. Recently, it has demonstrated that the semiconductor/noble-metal heterostructure substrates fabricated by combining suitable semiconductor nanostructures with noble-metal nanoparticles (NPs) show highly sensitive, reproducible, and stable SERS activity [10–12]. In ⁎
contrast with noble-metal substrates, the superior SERS activity of these substrates is attributed to their following unique properties. (1) The capability to preconcentrate and absorb the probe molecules on the active sites of the substrate due to the unique surface wettability of semiconductor nanostructure. (2) The capability to produce an evanescent field by semiconductor nanostructures to enhance the SERS sensitivity and reproducibility with the negligible perturbation for probe molecules. (3) The capability to provide highly sensitive, reproducible and stable SERS activity due to its superior self-cleaning ability under UV or visible light irradiation. These multiple functionalities make semiconductor/noble-metal nanostructures to be as suitable substrates for fabricating SERS sensors, which have been widely applied in detecting and analyzing structural information of chemical and/or biological analytes [13–17]. However, for the practical applications, the SERS sensor needs to have good sensitivity, recyclability and stability, thus a key issue to achieve the semiconductor/noblemetal substrate-based SERS sensor with high sensitivity, recyclability and stability is to understand the enhanced mechanism of SERS activity. In few published works, a charge transfer mechanism has been applied to interpret the SERS enhancement of semiconductor/noble-metal
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.snb.2018.10.010 Received 24 May 2018; Received in revised form 28 September 2018; Accepted 4 October 2018 Available online 06 October 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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samples were studied by a Lambda 950 spectrometer.
nanostructure substrates [18–20]. For instance, the enhanced SERS activity of Ag/TiO2 nanoparticle substrate was attributed to the charge transfer from TiO2 to the probe molecule of 4-Mercaptobenzoic acid (4MBA) and from Ag to 4-MBA molecule through TiO2 conduction band (CB) [21]. Another important example is the Ag/N719/TiO2 nanocomposite substrate (where N719 is dye (Bu4N)2[Ru(dcbpyH)2(NCS)2]). In this substrate, the enhanced SERS activity was ascribed to the charge transfer from Ag to N719 molecule and from N719 molecule to TiO2, besides the contributions from the electromagnetic enhancement of Ag NPs and resonance effect of N719 molecule [20]. These studies have demonstrated that the selective SERS enhancement for specific Raman signals of molecules is usually considered as the evidence that enhanced SERS activity is attributed to the charge transfer between constituents of the substrate. However, it has been proven that the selective SERS enhancement for specific Raman signals of molecules is greatly depended on the light wavelength and the testing molecules [19,20]. In the present study, we apply a facile photoreduction deposition strategy to grow Au NPs on the surfaces of TiO2 nanowire arrays (TiO2/ Au NWAs) as a Raman sensitive substrate. The fabricated TiO2/Au NWAs can be used as a highly sensitive and recyclable SERS sensor for detecting the rhodamine 6 G (R6 G) molecules with a concentration as low as 10−9 M. Moreover, when irradiated with UV light, the SERS sensor with high sensitivity can be fully reproduced due to the superior self-cleaning capability of the substrate. By a comparative study on the SERS activities of R6 G molecules over the substrates of pure TiO2 NWAs, pure Au NPs and TiO2/Au NWAs, as well as the photoresponse performance of pure TiO2 and TiO2/Au NWAs under visible light illumination, we provide an illuminating insight into the enhanced mechanism of SERS activity for TiO2/Au NWAs. This work throws a new light on enhanced mechanism for SERS activity of semiconductor/ noble-metal substrates, which is expected to find potential applications to fabricate other semiconductor/noble-metal SERS sensors with high sensitivity for the detection of analytes.
2.4. Photoresponse evaluation The photoresponse activities of the pure TiO2 and TiO2/Au NWAs were studied by a photoelectrochemical (PEC) cell containing working electrode (TiO2 or TiO2/Au NWAs), reference electrode (Ag/AgCl) and counter electrode (a Pt wire). All experiments were conducted in Na2SO4 electrolyte solution (0.1 M) under illumination with λ > 420 nm visible light (AM 1.5 G, 100 mW/cm2). 2.5. SERS measurements The SERS properties of achieved TiO2/Au NWAs were investigated through detecting the Raman spectra of R6 G molecules at different concentrations (10−4 to 10−9 M). For SERS testing, the R6 G solution with concentrations of 10−4 to 10−9 M was dropped on the TiO2/Au NWAs. A confocal microprobe Raman spectrometer (Horiba LABRAMHR-800) was applied to collect Raman spectra of samples excited with 633 nm light at room temperature. To study enhanced mechanism for SERS performance of TiO2/Au NWAs, the SERS activities of pure Au NPs and bare TiO2 NWAs were also investigated by testing the Raman spectra of R6 G molecules at the same conditions. 2.6. Reproducible characterization
2. Experimental section
For reproducible SERS activity characterization, after the SERS performance was measured, the used TiO2/Au NWAs were placed into ultrapure water and then irradiated for 2 h by a Xe lamp. The irradiated TiO2/Au NWAs were rinsed with ultrapure water to remove residues such as ions and molecules, followed by drying under room temperature. Then, the substrate of the cleaned TiO2/Au NWAs was applied again to detect the Raman signals of R6 G molecules. The reproducible tests were repeated five times to evaluate the reproducibility and stability of the fabricated TiO2/Au NWAs.
2.1. Preparation of TiO2 NWAs
3. Results and discussion
The TiO2 NWAs were grown on glass (fluorine-doped tin oxide, FTO) substrate via an easy hydrothermal process described as the following. 15 mL HCl (38%) was firstly added into 15 mL ultrapure water, and then 0.5 mL titanium butoxide was added drop by drop under stirring. Finally, the solution was put into an autoclave, followed by placing a cleaned FTO piece against the autoclave wall. Afterwards, the autoclave was sealed and heated for 3.5 h at 150 °C. Then, the sample was washed with ultrapure water and ethanol, dried and annealed for 30 min at 450 °C.
3.1. XRD and SEM characterizations Fig. 1a shows the XRD patterns of the samples. Before growing Au NPs, the diffraction peaks of the sample can be assigned to those of rutile phase TiO2 (JCPDS No. 78-2485), besides the diffraction peaks from FTO substrate. After growing Au NPs, the sample also shows the dominate diffraction peaks of rutile phase TiO2. However, a weak shoulder peak of one diffraction peak of FTO at around 38.0° and an obvious diffraction peak at around 44.6° are observed. The enlarged patterns (insets of Fig. 1a) show that these two peaks can be easily assigned to (111) and (200) planes of Au (JCPDS No. 4-784), demonstrating the formation of Au NPs in sample. Fig. 1b shows the SEM images of the sample before growing the Au NPs. It can be found that the sample is composed of aligned nanowires (NWs) with end diameter of about 150 nm. Thus the rutile phase TiO2 NWAs were prepared on the FTO substrate. Another feature is that the surface of the NWs is smooth. After growing the Au NPs, the SEM images evidently show that a lot of Au NPs are grown on the surfaces of NWs as shown in Fig. 1 c,d. The nominal particle size determined by SEM images is about 3–63 nm. The particle sizes were obtained by statistically analyzing over 300 particles through an image processing software of ImageJ, in which the particle sizes were determined by measuring the greatest pixel length between the boundaries of the particles and converting to nm as used in previously published article [22]. SEM studies demonstrate that TiO2/Au NWAs have been prepared by using a facile photoreduction deposition method to grow Au NPs on the surface of TiO2 NWAs.
2.2. Fabrication of TiO2/Au NWAs The TiO2/Au NWAs were fabricated by using a facile photoreduction deposition method to grow Au NPs on the nanowire surface of TiO2 NWAs. Briefly, the FTO covered with TiO2 NWAs was placed into 60 mL aqueous solution containing 10 mL of 1.5 mg mL−1 gold chloride (HAuCl4⋅3H2O), and then irradiated for 3 min by a Xe lamp (300 W) with λ < 420 nm light. After that, the FTO sample was thoroughly washed with ultrapure water, dried and heated for 1 h at 450 °C. 2.3. Characterization X-ray powder diffraction (XRD) was applied to study the composition and crystalline phase of the samples. The experiments were carried out on a XD-D1 diffractometer employing Cu Kα (λ = 1.5406 Å) irradiation. Shape and size of the samples were observed by a scanning electron microscope (SEM). Optical absorption properties of the 314
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Fig. 1. (a) XRD patterns of TiO2 and TiO2/Au samples. (b) SEM images of pure TiO2 NWAs. (c) low- and (d) high-magnification SEM images of TiO2/Au NWAs.
3.2. SERS activity The SERS properties of bare TiO2 and TiO2/Au NWAs were investigated through detecting the Raman vibrational signals of R6 G molecules. To well understand the enhanced mechanism for SERS performance of TiO2/Au NWAs, the SERS performance of pure Au NPs was also studied at the same conditions. The pure Au NPs were grown on the FTO substrate by photoreduction deposition method applied to grow Au NPs on the surface of TiO2 NWAs at the same conditions. Fig. 2a shows the SERS spectra of R6 G molecules with the concentration of 10−4 M over substrates of TiO2/Au NWAs, bare TiO2 NWAs and pure Au NPs. When the bare TiO2 NWAs are used as SERS substrate, no Raman signals of R6 G molecules are detected. For the SERS substrate of pure Au NPs, the SERS spectrum only shows weak Raman signals of R6 G molecules at 1601, 1360, 1320 and 612 cm-1. In addition, some characteristic Raman signals are not showed. Because the amount of Au NPs grown FTO substrate is small (Fig. S1), the Raman signals of R6 G molecules over the pure Au nanoparticle substrate are weak, and some characteristic Raman signals are not detected. However, when applied as SERS substrate, the TiO2/Au NWAs not only show Raman signals of R6 G molecules at 1648, 1601, 1570, 1506, 1360, 1320, 1186, 774 and 612 cm−1 [23,24], but also significantly enhance the intensities of these Raman signals. The SERS properties undoubtedly show that the TiO2/Au NWAs achieve remarkably improved SERS activity for detecting Raman signals of R6 G molecules compared to pure Au NPs. In addition, The SERS properties also demonstrate that improved SERS activity of TiO2/Au NWAs is attributed to the synergistic effect of Au, TiO2 and R6 G. A highly sensitive performance is essential to a SERS sensor for practical application. The R6 G molecules with different concentrations were selected to investigate the SERS sensitivity of the TiO2/Au NWAs. Fig. 2b shows the SERS spectra of R6 G molecules with different
Fig. 2. (a) SERS spectra of R6 G molecules with 10−4 M concentration over the substrates of bare TiO2 NWAs, pure Au NPs and TiO2/Au NWAs. (b) SERS spectra of R6 G molecules with concentrations of 10−4 to 10−9 M over the substrate of TiO2/Au NWAs.
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Fig. 3. SERS spectra of R6 G molecules after the decomposition by TiO2/Au NWAs under the different irradiation time with UV light.
concentrations over the TiO2/Au NWAs. It can be found that the obvious Raman signals of R6 G molecules can be detected at the concentration as low as 10−9 M, demonstrating the highly detective ability of the TiO2/Au NWAs for R6 G molecules. A comparison of SERS performance with the reported Au/semiconductor substrates for the detection of R6 G molecules also demonstrates that the TiO2/Au NWAs show highly sensitive SERS activity (Table S1). 3.3. Self-cleaning ability The recyclability is another key issue to a SERS sensor for practical application. It has been reported that the TiO2/Au nanostructures show ability to decompose absorbed dye molecules under UV or visible light irradiation [25,26]. In this work, the self-cleaning ability of the TiO2/ Au NWAs to decompose the R6 G molecules was evaluated under the UV light irradiation. Fig. 3 gives the SERS spectra of R6 G molecules with concentration of 10−4 M after the decomposition by TiO2/Au NWAs under the different irradiation time. It can be found that the Raman signals of R6 G molecules gradually disappear with prolonging the time of light irradiation. After irradiation with UV light for 2 h, the Raman signals of the R6 G molecules almost disappear completely, demonstrating that the TiO2/Au NWAs exhibit a superior capability to decompose R6 G molecules by self-cleaning process under UV light irradiation.
Fig. 4. (a) SERS spectra of the R6 G molecules with 10−4 M on the TiO2/Au NWAs before and after self-cleaning over five cycles. (b) Raman intensities for 1360 cm-1 of R6 G molecules with 10-4 concentration during repeated measurement and self-cleaning with UV light irradiation.
3.5. Optical absorption and visible light photoresponse activity To elucidate the enhanced mechanism for SERS activity of TiO2/Au NWAs, the light absorption and visible light photoresponse activity were studied. The light absorption spectrum of TiO2 NWAs shows a strong absorption peak at about 370 nm. And no light absorption is observed over 400 nm region as shown in Fig. 5a. However, the spectrum of TiO2/Au NWAs shows obvious light absorption from 500 to 600 nm with a main peak at about 570 nm, besides a strong absorption at about 370 nm. The light absorption from 500 to 600 nm in TiO2/Au NWAs is induced by the surface plasmon resonance (SPR) absorption of plasmonic Au NPs. This wide light absorption from 500 to 600 nm is attributed to the wide size distribution of Au NPs, which can be verified by SEM studies. The SEM images and size distribution histogram show that the Au NPs display a size distribution from 3 to 63 nm with an average size of 24 nm (Fig. S2). Thus this wide optical absorption arises from the wide size distribution of Au NPs. The bandgap values for TiO2 and TiO2/Au NWAs are ca. 3.0 eV, which are obtained by combining Kubelka-Munk function with the Taus plot [27] and the tangent line intercept of (F(R)hν)1/2 vs. hν plot as shown in Fig. 5b. The SERS activity shows that the TiO2/Au NWAs exhibit highly sensitive SERS ability to detect the Raman signals of R6 G molecules under 633 nm light illumination. Thus the investigation for visiblelight-driven photoresponse activity of pure TiO2 and TiO2/Au NWAs is favorable for elucidating enhanced SERS performance of TiO2/Au NWAs. In this work, we use photocurrent density, which was achieved under visible light illumination (λ > 420 nm, AM 1.5 G, 100 mW/cm2) in a PEC cell using 0.1 M Na2SO4 as electrolyte, to evaluate the photoresponse ability of pure TiO2 and TiO2/Au NWAs. At a potential of 0 V versus Ag/AgCl, the TiO2/Au NWAs produce an obvious photocurrent density (around 9 μA cm−2) (Fig. 5c). However, the pure TiO2 NWAs almost don’t display any photoresponse activity. At a potential of 0.5 V
3.4. SERS recyclable performance As shown in Fig. 3, the superior self-cleaning ability allows the TiO2/Au NWAs to be easily regenerated for detecting the SERS signals of the R6 G molecules. To evaluate the recyclable detection ability for R6 G molecules, the TiO2/Au NWAs were recycled five cycles with UV light irradiation for 2 h. Fig. 4a shows the SERS spectra of the R6 G molecules with concentration of 10−4 M over the TiO2/Au NWAs before and after self-cleaning. It can be evidently found that the characteristic Raman signals of R6 G molecules almost disappear completely after the UV light irradiation. However, when the R6 G molecules are absorbed again on the cleaned TiO2/Au NWAs, the strong SERS signals of R6 G molecules are observed, which are similar to those of the new TiO2/Au NWAs. Fig. 4b displays the Raman intensities for 1360 cm−1 of R6 G molecules with 10−4 concentration during repeated measurement and self-cleaning with UV light irradiation, showing that the TiO2/Au NWAs exhibit superior ability to retain SERS activity during the five recycles. The studies on the Raman properties experimentally demonstrate that the fabricated TiO2/Au NWAs show highly sensitive and recyclable SERS activity for the detection of R6 G molecules. However, in order to find further applications for the detection of other probe analytes, a key issue is to rationally interpret the mechanism for enhanced SERS activity of TiO2/Au NWAs. 316
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Fig. 5. (a) UV–vis absorption spectra of TiO2 and TiO2/Au NWAs. (b) The bandgap values of TiO2 and TiO2/Au NWAs, calculated by fitting absorption spectra. The curves of photocurrent densities for the pure TiO2 and TiO2/Au NWAs at (c) 0 V and (d) 0.5 V vs. Ag/AgCl under illumination with λ > 420 nm visible light (AM 1.5 G, 100 mW/cm2).
(LUMO) level of R6 G. (2) The contribution resulted from the charge excitation and transfer between the LUMO and the highest occupied molecular orbital (HOMO) energy levels of the R6 G. (3) The contribution offered by the charge transfer from the TiO2 to the LUMO level of R6 G. (4) The contribution provided by the transfer of “hot electrons” from the Ef of Au to the LUMO level of R6 G via the CB of TiO2. Thus the positions of Ef for Au, the CB and VB for TiO2, and the LUMO and HOMO for R6 G are crucial parameters to determine whether the charge excitation and transfer can be achieved in TiO2/R6 G, Au/R6 G and TiO2/Au/R6 G structures under the irradiation of 633 nm light. As reported previously, the Ef of Au is 5.1 eV [30], the positions of CB and VB for rutile phase TiO2 are 4.04 and 7.07 eV, respectively [31,32], the level positions of the LUMO and HOMO for R6 G are 3.4 and 5.7 eV versus vacuum level, respectively [33]. Based on the energy band structure of Au, rutile phase TiO2 and R6 G, we can give a rational elucidation for the SERS activity of pure TiO2, pure Au and TiO2/Au substrate, respectively. For the pure TiO2 NWAs, the 633 nm light is incapable of exciting TiO2 to generate electron-hole pairs, because the bandgap of TiO2 NWAs is round 3.0 eV, larger than the light energy of 633 nm (1.96 eV). This result has yet been confirmed by visible-light-driven photoresponse activity as shown in Fig. 5 a,b. In addition, the light of 633 nm is also unable to excite R6 G to produce charge carriers, because the energy of 1.96 eV is less than the bandgap (2.3 eV) of R6 G molecule. These results show that the SERS activities induced by the charge transfer from the TiO2 to the LUMO level of R6 G, and between the LUMO and HOMO energy levels of R6 G can’t be achieved in TiO2/R6 G structure. Therefore, no Raman signals of R6 G molecules are detected over substrate of pure TiO2 NWAs as shown in Fig. 2a. For the pure Au substrate, the energy difference between Ef of Au and the LUMO level of
versus Ag/AgCl, the TiO2 NWAs also don’t show any photoresponse, whereas the photocurrent density of TiO2/Au NWAs increases to 13 μA cm−2 as shown in Fig. 5d. Thus we experimentally show that the TiO2/ Au NWAs not only exhibit a rapid photoresponse activity, but also generate an obvious photocurrent density under illumination with λ > 420 nm visible light. The light absorption shows that the TiO2/Au NWAs exhibit an obvious SPR absorption with a main peak at about 570 nm. Thus, when irradiated by λ > 420 nm visible light, an enlarged enhanced electromagnetic (EM) field can be generated due to the strong SPR effect of the Au NPs, and then the enlarged EM field can excite “hot electrons” of the Au nanoparticle to the CB of TiO2 nanowire [28]. In addition, a Schottky barrier established at the interface of Au and TiO2 will further facilitate the transfer of “hot electrons” to the CB of TiO2 and prevent the “hot electrons” from coming back to Au NPs [29]. Moreover, because the λ > 420 nm light is not able to excite the electrons in the valence band (VB) of TiO2 to its CB, no holes are produced in the VB of TiO2. Thus, the “hot electrons” accumulated on the CB of TiO2 will transfer to the counter electrode to split water, resulting in the produce of photocurrent in a PEC cell. No photoresponse activity and photocurrent are achieved in bare TiO2 NWAs because they can’t be excited to produce photoinduced electrons and holes under λ > 420 nm visible light irradiation. 3.6. Mechanism for enhanced SERS activity of TiO2/Au NWAs There are four possible contributions to enhanced SERS performance of the TiO2/Au NWAs for detecting the Raman signals of R6 G molecules. (1) The contribution offered by the charge transfer from the Fermi level (Ef) of Au to the lowest unoccupied molecular orbital 317
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based SERS sensors with high sensitivity for the detection of analytes. However, the detection concentration for R6 G molecules of the fabricated TiO2/Au NWAs is still too high to practical applications as a SERS sensor for the detection of analytes. By controlling size and loading content of Au NPs, we will further improve sensitivity of TiO2/Au NWAs to detect analytes with concentration of 10−11 to 10−13 M. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant Nos. 61575225, 11374377, 11074312 and 11404414, and the Undergraduate Research Training Program of Minzu University of China under Grant Nos. GCCX2017110006, GCCX2017110007 and URTP20170007. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2018.10.010. References [1] R. Zhang, Y. Zhang, Z.C. Dong, S. Jiang, C. Zhang, L.G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J.L. Yang, J.G. Hou, Chemical mapping of a single molecule by plasmon-enhanced Raman scattering, Nature 498 (7452) (2013) 82–86. [2] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, SERS: materials, applications and the future, Mater. Today 15 (1–2) (2012) 16–25. [3] I. Alessandri, J.R. Lombardi, Enhanced Raman scattering with dielectrics, Chem. Rev. 116 (24) (2016) 14921–14981. [4] M.J. Banholzer, J.E. Millstone, L. Qin, C.A. Mirkin, Rationally designed nanostructures for surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 37 (5) (2008) 885–897. [5] H. Wang, N.J. Halas, Mesoscopic Au “meatball” particles, Adv. Mater. 20 (4) (2008) 820–825. [6] Z. Ioffe, T. Shamai, A. Ophir, G. Noy, I. Yutsis, K. Kfir, O. Cheshnovsky, Y. Selzer, Detection of heating in current-carrying molecular junctions by Raman scattering, Nat. Nanotechnol. 3 (12) (2008) 727–732. [7] D.R. Ward, D.A. Corley, J.M. Tour, D. Natelson, Vibrational and electronic heating in nanoscale junctions, Nat. Nanotechnol. 6 (1) (2011) 33–38. [8] N.J. Hogan, A.S. Urban, C. Ayala-Orozco, A. Pimpinelli, P. Nordlander, N.J. Halas, Nanoparticles heat through light localization, Nano Lett. 14 (8) (2014) 4640–4645. [9] M. Moskovits, Persistent misconceptions regarding SERS, Phys. Chem. Chem. Phys. 15 (15) (2013) 5301–5311. [10] C.Y. Huang, C.X. Xu, J.F. Lu, Z.H. Li, Z.S. Tian, 3D Ag/ZnO hybrids for sensitive surface-enhanced Raman scattering detection, Appl. Surf. Sci. 365 (2016) 291–295. [11] Y. Zhao, L. Sun, M. Xi, Q. Feng, C.Y. Jiang, H. Fong, Electrospun TiO2 nanofelt surface-decorated with Ag NPs as sensitive and UV-cleanable substrate for surface enhanced Raman scattering, ACS Appl. Mater. Interfaces 6 (8) (2014) 5759–5767. [12] F.A. Harraz, A.A. Ismail, H. Bouzid, S.A. Al-Sayari, A. Al-Hajry, M.S. Al-Assiri, Surface-enhanced Raman scattering (SERS)-active substrates from silver platedporous silicon for detection of crystal violet, Appl. Surf. Sci. 331 (2015) 241–247. [13] X. Han, H. Wang, X. Ou, X. Zhang, Highly sensitive, reproducible, and stable SERS sensors based on well-controlled silver nanoparticle-decorated silicon nanowire building blocks, J. Mater. Chem. 22 (28) (2012) 14127–14132. [14] E.Z. Tan, P.G. Yin, T.T. You, H. Wang, L. Guo, Three dimensional design of largescale TiO2 nanorods scaffold decorated by silver NPs as SERS sensor for ultrasensitive malachite green detection, ACS Appl. Mater. Interfaces 4 (7) (2012) 3432–3437. [15] K. Zhang, J. Ji, Y. Li, B. Liu, Interfacial self-assembled functional nanoparticle array: a facile surface-enhanced Raman scattering sensor for specific detection of trace analytes, Anal. Chem. 86 (13) (2014) 6660–6665. [16] H.W. Qiu, M.Q. Wang, L. Li, J.J. Li, Z. Yang, M.H. Cao, Hierarchical MoS2-microspheres decorated with 3D AuNPs arrays for high-efficiency SERS sensing, Sens. Actuators B 255 (2018) 1407–1411. [17] M.S. Yang, J. Yu, F.C. Lei, H. Zhou, Y.S. Wei, B.Y. Man, C. Zhang, C.H. Li, J.F. Ren, X.B. Yuan, Synthesis of low-cost 3D-porous ZnO/Ag SERS-active substrate with ultrasensitive and repeatable detectability, Sens. Actuators B 256 (2018) 268–275. [18] H. Fang, C.X. Zhang, L. Liu, Y.M. Zhao, H.J. Xu, Recyclable three-dimensional Ag nanoparticle-decorated TiO2 nanorod arrays for surface-enhanced Raman scattering, Biosens. Bioelectron. 64 (2015) 434–441. [19] X. Jiang, X. Li, X. Jia, G. Li, X. Wang, G. Wang, Z. Li, L. Yang, B. Zhao, Surfaceenhanced Raman scattering from synergistic contribution of metal and semiconductor in TiO2/MBA/Ag(Au) and Ag(Au)/MBA/TiO2 assemblies, J. Phys. Chem. C 116 (27) (2012) 14650–14655. [20] X.L. Wang, Y. Wang, H.M. Sui, X.L. Zhang, H.Y. Su, W.N. Cheng, X.X. Han, B. Zhao, Investigation of charge transfer in Ag/N719/TiO2 interface by surface-enhanced Raman spectroscopy, J. Phys. Chem. C 120 (24) (2016) 13078–13086. [21] L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, J.R. Lombardi, Charge-transferinduced surface-enhanced Raman scattering on Ag-TiO2 nanocomposites, J. Phys.
Fig. 6. Illustration for charge transfer process of (a) Au/R6 G structure and (b) TiO2/Au/R6 G structure.
R6 G is 1.7 eV (Fig. 6a), which is less than the photo energy of 633 nm (1.96 eV). So the “hot electrons” of plasmonic Au NPs can be motivated to the LUMO level of R6 G, resulting in the obvious SERS signals of R6 G as presented in Fig. 2a. For the substrate of TiO2/Au NWAs, the energy difference between the Ef of Au and the CB of TiO2 is 1.06 eV (Fig. 6b), less than the energy (1.96 eV) of 633 nm light. Thus the “hot electrons” of the plasmonic Au NPs can be excited to the CB of TiO2. This conclusion has yet been experimentally verified by the visible-light-driven photoresponse activity as shown in Fig. 5 c,d. Moreover, as shown in Fig. 6b, the energy difference between the CB of TiO2 and the LUMO level of R6 G is 0.64 eV, thus the “hot electrons” motivated to the CB of TiO2 can be further excited to the LUMO level of R6 G. Therefore, besides the contribution provided by the charge transfer from the Ef of Au to the LUMO level of R6 G, the charge transfer from the Ef of Au to the LUMO of R6 G via the CB of TiO2 also contributes an additional enhancement for SERS signals, leading to significant enhancement for SERS signals as demonstrated in Fig. 2. The experiments and discussion show that the significantly improved SERS activity for TiO2/Au NWAs is ascribed to the synergistic effect and effective charge transfer among the Au, TiO2 and R6 G, which is illustrated in Fig. 6b. 4. Conclusions In conclusion, the heterogeneous TiO2/Au NWAs were fabricated by an easy photoreduction deposition method. Used as a SERS substrate, the TiO2/Au NWAs show highly sensitive and recyclable SERS activity for detecting R6 G molecules with a concentration as low as 10−9 M. Furthermore, the substrate of TiO2/Au NWAs exhibits superior selfcleaning capability, making it to fully reproduce high sensitivity after irradiating with UV light. The SERS activities and photoresponse performances achieved under visible light irradiation provide experimental evidence that improved SERS activity of TiO2/Au NWAs is attributed to the efficient charge transfer among the Au, TiO2 and R6 G. This work throws a new light on enhanced mechanism for high SERS activity of semiconductor/noble-metal substrates, which are expected to find potential applications to fabricate semiconductor/noble-metal substrate318
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Professor Wenzhong Wang is Professor of Physics at the Minzu University of China since 2006. He received his Ph. D. degree in condensed matter physics from the Nanjing University (China) in 2002. In 2003 he worked as a post-doctoral fellow in the Institute of Materials Research at the Pennsylvania State University (University Park, USA). From 2004 to 2006 he worked as a research fellow in the Department of Physics, Boston College (USA). His recent research focuses on the design and fabrication of low-dimensional nanostructures and their applications involving solar energy conversion, energy storage and SERS-based sensors. Associate Professor Yujie Liang is associate professor in Physics at the Minzu University of China since 2017. She received her Ph. D. degree in physics from the Beijing Normal University (China) in 2009. From 2009 to 2011 she worked as a post-doctoral fellow in physical electronics at the University of Electronic Science and Technology of China. Her current research interest focuses on the design and fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Associate Professor Junli Fu is associate professor in Physics at the Minzu University of China since 2013. She received her Ph. D. degree in physics from the Lanzhou University (China) in 2008. Her current research interest focuses on the design and synthesis of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Associate Professor Min Zhu is associate professor in Physics at the Minzu University of China since 2010. She received his Ph. D. degree in physics from the Beijing Normal University (China) in 2008. His current research interest involves the fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes. Dr. Honglong Shi received his Ph. D. degree in physics from the Institute of Physics, Chinese Academy of Sciences in 2011. He currently is a Lecture at the Minzu University of China. His research interest involves the synthesis of nanostructures and their microstructure analysis using transmission electron microscopy. Ms Shijing Lei is a junior. She is pursuing her bachelor degree in nanomaterials and technology at the Minzu University of China.
Ms Xin Zhao received her bachelor degree in 2015 from the Beijing Institute of Technology (China) and her mater degree in 2018 from Minzu University of China. Her research interest lies in the fabrication of surface-enhanced Raman scattering (SERS) substrates and their applications as sensors for the detection of analytes.
Mr Chunjiang Tao is a junior. He is pursuing his bachelor degree in nanomaterials and technology at the Minzu University of China.
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