Article pubs.acs.org/JPCC
Obviously Angular, Cuboid-Shaped TiO2 Nanowire Arrays Decorated with Ag Nanoparticle as Ultrasensitive 3D Surface-Enhanced Raman Scattering Substrates Zhigao Dai,†,⊥ Gongming Wang,‡,⊥ Xiangheng Xiao,*,†,‡ Wei Wu,† Wenqing Li,† Jianjian Ying,† Junfeng Zheng,† Fei Mei,†,§ Lei Fu,Ψ Jiao Wang,Ψ and Changzhong Jiang† †
Department of Physics, Hubei Nuclear Solid Physics Key Laboratory and Center for Ion Beam Application, Wuhan University, Wuhan 430072, People’s Republic of China § School of Electrical & Electronic Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China Ψ College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, People’s Republic of China ‡ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: In recent years, surface-enhanced Raman scattering (SERS) has received renewed attention, because of its nondestructive, ultrasensitive, and rapid analysis, detection, and imaging. Development of SERS substrates with high sensitivity and excellent stability is of great importance to realize its practical applications in trace analysis, bio diagnosis, and in vivo studies. In this work, we demonstrate wafer-scale Ag-nanoparticle-decorated, obviously angular, quasi-vertically aligned cuboid-shaped TiO2 nanowire arrays (TiO2-NWs), as ultrasensitive and uniform 3D SERS substrates. A detection limit of 10−15 M rhodamine 6G molecules and an analytical enhancement factor of 1012 were achieved on the cuboid-shaped TiO2-NWs arrays with 9 min Ag-sputtering. This is the best result obtained among the literature values on Ag-modified semiconductor SERS substrates. More importantly, the optimized TiO2-Ag also exhibits excellent stability and uniformity. The excellent SERS performance is attributed to the “cusp” and “gaps” formed on the Ag-NPs coated TiO2 nanowire arrays, which create a huge number of SERS “hot spots”. The experimental results were further confirmed by theoretical calculations of the spatial distributions of electromagnetic field intensity. The prepared TiO2-Ag SERS substrates with such low detection limit and high sensitivity will provide a promising candidate for practical chemical and biological detection.
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INTRODUCTION
noble metal nanostructures for SERS application, and significant process has been achieved. However, the substrates prepared by these techniques are too technologically dependent, and it is also difficult and too expensive for large-scale synthesis. Recent studies on semiconductor−noble metal nanocomposites have shown great potentials to achieve the above goals. Most SERS substrates were composited with noble metal structures (Au, Ag, and Cu). The noble metal structures were multiple shapes, for example, nanowires, nanotriangles, nanocubes, and core−shell nanoparticles.33,34 Interestingly, Rijana and co-workers found the strong SERS enhancement on semiconducting nanoparticles without noble metals, as a result of a hybrid charge transfer system.35 Because of the contributions from both the electromagnetic enhancement and the semiconductor-induced chemical enhancement, larger
Surface-enhanced Raman scattering (SERS) is an influential method to acquire chemical and biological molecules’ information because of its extremely high surface sensitivity.1,2 The mechanisms of SERS enhancement are related to the electromagnetic and chemical enhancement induced by noble metal structure.3−7 For the electromagnetic enhancement, the electromagnetic field is enhanced by the localized surface plasmons of noble metal structure under laser irradiation, and therefore results in more intense Raman scattering of the molecules on the metal structure surface.8−13 Chemical enhancement is attributed to the charge interactions between the absorbed molecules and noble metal structures.14−16 To meet the practical applications, the SERS substrate is required to be highly stable, repeatable, cheap, and facile to be prepared.17,18 Currently, various techniques such as vacuum evaporation of Ag film on nanospheres (AgFON),19−28 nanosphere lithography (NLS) to fabricate periodic nanoparticle arrays,29,30 electron beam lithography (EBL),31 and focused ion beam milling (FIB)32 have been used to fabricate © 2014 American Chemical Society
Received: July 28, 2014 Revised: September 9, 2014 Published: September 10, 2014 22711
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Preparation of Polystyrene Masks for Ag Film on Nanospheres. The polystyrene (PS) masks were prepared by self-assembly of monodisperse PS particles with a diameter of 1100 nm as in our previously reported NLS method.29,30 The obtained PS nanosphere films were deposited with Ag nanoparticles by ultra high vacuum magnetron sputtering. Assembly of Ag-NPs on TiO2-NWs and PS Masks. AgNPs were deposited onto the as-prepared cuboid-shaped TiO2NWs and PS masks at room temperature in an ultra high vacuum magnetron sputtering system (ULVAC, ACS-4000C4). The vacuum chamber was evacuated to a base pressure of less than 5 × 10−6 Pa, and then the deposition proceeded with a working pressure of 5 × 10−1 Pa. The working gas was Ar, and the distance between the targets and the substrate was kept at 130 mm. Characterization. The surface morphology was characterized by scanning electron microscopy (SEM, FEI Sirion FEG). Transmission electron microscopy (TEM) images were performed by a JEOL JEM-2010 (HT) transmission electron microscope operated at 200 kV. X-ray diffraction (XRD) experiment was performed using Rigaku Cu Kα radiation. Raman spectroscopy was performed with laser confocal microRaman spectrometers (RenishawinVia, Renishaw, 532 and 633 excitation wavelength; Horiba LabRAM HR800, excitation wavelength, 488 and 785 nm). The laser beam was focused onto the sample through a 100× objective lens. For dark-field (DF) scattering spectra, unpolarized white light from a 100 W halogen lamp was sent to the sample through a dark-field condenser (NA = 1.20−1.44). Scattered light was collected by the 100× objective.
Raman enhancement can be achieved on the composites of semiconductors and noble metals.36−44 TiO2 has received extraordinary attention as semiconductor/nobel metal hybrid SERS substrates, due to its well chemical in SERS substrates, strong chemical stability, low cost, and facile synthesis.36,39,45 For example, Ag nanoparticles (Ag-NP) were deposited on the electrospun anatase-phase TiO2 nanofibers, via an electroless plating method.36 Yet SERS “hot spots” work inefficiently, because of the large gaps between the Ag nanoparticles on the TiO2 nanofibers. Moreover, the TiO2 nanotube (NTs) was not obviously angular, and the Au nanoparticles were randomly deposited on the tubes, which could not produce enough efficient “hot spots” for ultrasensitive and uniform SERS substrates.39 Herein, a wafer-scale obviously angular, quasi-vertically aligned cuboid-shaped TiO2 nanowire array was synthesized, and the side and top of the TiO2 nanowire arrays were deposited with Ag-NPs with different sizes as ultrasensitive and uniform 3D SERS substrates. The synthetic process is shown schematically in Figure 1. First, obviously angular, quasi-
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Figure 1. Schematic diagram of the preparation of arrayed obviously angular, cuboid-shaped TiO2-NWs decorated with Ag-NPs on FTO glass. There is a kind of “cusp” on the cuboid-shaped TiO2-NWs coated with Ag-NPs and three kinds of “gaps” between the Ag-NPs to form 3D “hot spots” as indicated as “1st”, “2nd”, “3rd”, and “4th”, respectively. “1st” stands for the edges of the cuboid-shaped TiO2NWs coated with Ag-NPs; “2nd” stands for the gaps between the AgNPs located on the side surface of the same cuboid-shaped TiO2-NW; “3rd” stands for the gaps between the two Ag-NPs located on the side surface of two neighboring cuboid-shaped TiO2-NWs; and “4th” stands for the gaps between the two larger, dense Ag-NPs located on the tops of two neighboring NRs.
RESULTS AND DISCUSSION Rutile TiO2-NWs were prepared on a FTO glass substrate by our previously reported hydrothermal method.46 Large-area, quasi-vertically aligned, and cuboid-shaped TiO2-NWs on a FTO glass are presented in Figure 2a. TiO2-NWs are homogeneous with a rectangular cross section (Figure 2a), sharp side edges, and smooth side surface (Figure 2a, inset). The diameters of nanowires are 80−120 nm, and the TiO2-
vertically aligned cuboid-shaped TiO2 nanowire arrays (TiO2NWs) were synthesized on a fluorine-doped tin oxide (FTO) glass substrate by hydrothermal method. Second, Ag-NPs were deposited on the side of the TiO2-NWs by high vacuum magnetron sputtering for 1 min. To further increase the density of “hot spots”, huge Ag-NPs were further deposited on the top of each cuboid-shaped TiO2-NW by increasing the magnetron sputtering time. Finally, the SERS performance will be studied detailedly in terms of enhancement capability, light stability, and reproducibility, to optimize the experimental parameters of Ag-coated cuboid-shaped TiO2-NWs as ultrasensitive SERS substrates.
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Figure 2. (a) SEM image of the wafer-scale arrays of cuboid-shaped TiO2-NWs. Inset: Enlarged image of side view of the cuboid-shaped TiO2-NWs arrays. SEM images of the TiO2-NWs after Ag-sputtering for 1 min (b), 9 min (c), and 17 min (d); the scale bars in the SEM images represent 2 μm. Insets: Enlarged images of side view of the cuboid-shaped TiO2-NWs arrays after Ag-sputtering for 1 min (b), 9 min (c), and 17 min (d); the scale bars in the insets represent 500 nm.
EXPERIMENTAL METHODS Preparation of TiO2 Nanowire Arrays. Rutile cuboidshaped TiO2-NWs were prepared on a FTO glass substrate using the previously reported hydrothermal method.46,47 The obtained TiO2 nanowire arrayed films were annealed in air at 550 °C for 3 h before being used for silver deposition. 22712
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Figure 3. SERS spectra of R6G collected on cuboid-shaped TiO2-NW arrays with different Ag-sputtering times (sputtering time is indicated on the upper-right of each spectrum) before (a) and after (b) ultraviolet and visible light irradiation. Exposed to a 10−7 M R6G solution for 1, 3, 5, and 7 min; excitation, 532 nm; power, 0.025 mW; data collection, 5 s; and for 9, 11, 13, and 17 min; excitation, 532 nm; power, 0.025 mW; data collection, 1 s. (c) The relationship between the Raman intensity at 1360 cm−1 of R6G and the Ag-sputtering times before (red) and after (blue) exposure under ultraviolet and visible light. Each datum represents the average value for 5−15 samples, and the error bar indicates the standard deviation.
NWs lengths are 2−3 μm. The Ag-NPs were deposited onto the cuboid-shaped TiO2-NWs by magnetron sputtering deposition. After 1 min sputtering (TiO2-Ag-1), not only do the tiny Ag-NPs in diameter of 10 nm grow on the side of the TiO2-NWs (Figure 2b, inset), but larger Ag-NPs are also produced on the top of the TiO2-NWs, as shown in Figure 2b and the TEM image (Supporting Information Figure 1a). With the increase of sputtering times, the Ag-NPs on the side and on the top of the cuboid-shaped TiO2-NWs continue to grow. For the sputtering time of 9 min, the mean distance between the Ag-NPs on the top of two neighboring TiO2-NWs is about 10 nm (see Figure 2c). The minimum gaps can be several nanometers. To more clearly observe the side of the cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min (TiO2Ag-9), the side view of TiO2-Ag-9 with high magnification, as shown in the inset of Figure 2c and Supporting Information Figure S1b, shows that the sides of the TiO2-NWs are covered with the diameter of 65 nm Ag-NPs. Also, the edges of TiO2Ag-9 are still sharp. The size histograms of the diameter of cuboid-shaped TiO2-NW arrays with different Ag-sputtering times were shown in Supporting Information Figure S2. After 17 min sputtering, the Ag-NPs on the top of the TiO2-NWs aggregate to form larger spherical particles (Figure 2d and inset). To acquire the structural information, the silver-coated cuboid-shaped TiO2-NWs are further characterized XRD. The XRD pattern is presented in Supporting Information Figure S3. The diffraction lines of the cubic metal silver located at around 2θ ≈ 38°, 45°, and 65° are the (111), (200), and (220) reflections, respectively. The diffraction lines located at 2θ ≈ 36.5° and 63.5° are assigned to the (101) and (002) reflections for the rutile TiO2, respectively. For SERS studies, rhodamine 6G (R6G) was used as Raman molecules to characterize the sensitivity of the as-prepared 3D SERS substrates. The series of cuboid-shaped TiO2-NWs with different Ag-sputtering times were immersed in a 1 × 10−7 M solution of R6G at room temperature, and then dried with N2 gas after being rinsed in each for 3 min with acetone, ethanol, and water to ensure only a monolayer of R6G molecules adsorbed on substrates. The SERS activities of the as-prepared substrates are gradually improved with the sputtering time ranging from 1 to 9 min but decreased with further increasing the sputtering time to 13, 15, and 17 min (see Figure 3a). The SEM images of 3, 5, 7, 13, and 15 min Ag-coated TiO2-NWs are shown in Supporting Information Figure S4. The strongest Raman signals are achieved on the TiO2-Ag-9 (red spectrum in
Figure 3a). To test the light stability property, the R6G molecules adsorbed series of TiO2-Ag for different times placed in water under ultraviolet and visible light irradiation for 15 min. After that, the R6G molecules adsorbed TiO2-Ag were dried in air followed by another SERS test as shown in Figure 3b. The intensities at 1360 cm−1 of R6G on these different sputtering time SERS substrates before and after ultraviolet and visible light irradiation are plotted in Figure 3c. The strongest Raman signals are still achieved on the TiO2-Ag-9, and the SERS activity decreases substantially (1 order of magnitude) after sputtering 1−3 min. However, the SERS activity decreases mildly for the substrates with sputtering time of more than 7 min. The Ag sputtering time-dependent SERS sensitivity and light exposure stability can be ascribed to their particular structures. After short 1 min sputtering, a small number of AgNPs with wide distances between each Ag NP is present on the side of the same cuboid-shaped TiO2-NWs. Therefore, the SERS activity is not high enough with short sputtering time. After increasing the sputtering time from 3 to 9 min, the diameter of the Ag-NPs on the side of the cuboid-shaped TiO2NWs also increases (Figure 2c). Not only do the “hot spots” of the Ag-NPs covered the same cuboid-shaped TiO2-NW induce the larger SERS intensity, but also two neighboring Ag-NPs covered on different TiO2-NWs become more efficient. The Raman activity reduces speedily after the sputtering time for more than 9 min, because of the formation of larger Ag NPs with fewer gaps when increasing sputtering time. The large AgNPs smooth the sharp edges of TiO2-NWs, and thus the efficiency of “hot spots” is minimized. In comparison with the previously reported fabrication strategies with only few and small Ag-NPs or Au-NPs formed on the surface or the top of the TiO2 nanostructures,36,39 the SERS activity decreases rapidly under light irradiation, probably due to photocatalytic degradation of the probe molecules. In our strategy, we can manipulate the exposed surface area of TiO2 NWs to the probe molecules by controlling the sputter times, and thus modify the light stability of the TiO2-Ag substrates. This is can help to understand why the SERS intensity decays so quickly on the substrates with short sputtering time, but mildly on the substrates with longer sputtering time. Thus, experimental results exhibited that TiO2-Ag-9 has maximum SERS sensitivity and better light stability. To investigate the excitation wavelength-dependent SERS sensitivity of the optimized TiO-Ag-9 substrates, we conducted SERS characterization using four different excitation lasers, ranging from visible to near-infrared (NIR). All of the SERS 22713
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Figure 4. SERS spectra of R6G normalized to the Raman band of Si wafer at 520 cm−1 under excitation of 488, 532, 633, and 785 nm lasers on the same arrays of cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min, respectively. (b) Comparison of Raman intensities of the band at 1360 cm−1 (average intensity and standard deviation for five measurements). (c) Dark-field scattering spectrum of cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min.
spectra exhibit well-defined Raman peaks with reasonable intensity (Figure 4a), and the intensity of SERS on TiO2-Ag-9 is excitation wavelength dependent with a maximum value at 532 nm. Quantitatively, the peak intensity at 1360 cm−1 with 532 nm laser wavelength is about 45, 15, and 350 times larger than the 488, 633, or 785 nm laser wavelength, respectively (Figure 4b). The plasmonic properties of TiO2-Ag-9 were further studied by DF scattering spectrum (Figure 4c). Interestingly, two different plasmon modes (peaks at 510 and 580 nm) are shown in the DF scattering spectrum of TiO2-Ag9. The two peaks originate from a plasmon hybridization model. The plasmon responses from the metal nanostructures with elementary geometries created new modes by that plasmon’s interaction.48 It is well-known that the SERS activity can be efficiently increased when the surface plasmon resonance of the substrate matched with the excitation wavelength.49,50 Furthermore, the surface plasmon resonance of the TiO2-Ag-9 is near the absorption peak of R6G molecules (about 532 nm).51 The 532 nm excitation wavelength was used for the SERS studied below. To further prove the excellent SERS activity and low detection limit of the 3D TiO2-Ag-9 hybrid substrate, SERS spectra were collected on TiO2-Ag-9 with different concentrations of R6G as shown in Figure 5a. The 1 × 10−5 L of 10−8−10−16 M solutions of R6G was dripped on the TiO2-Ag-9 arrays. Excitingly, the R6G characteristic peaks are still distinguishable even at 10−15 M, as illustrated in Figure 5b. In comparison, the signals collected from commercial SERS substrate Klarite were almost invisible with 10−9 M R6G.52 The extremely high SERS detection limit of the TiO2-Ag-9 substrate is attributed to both the unique structures with high “hot-spot” density and the excellent adsorptive action of the 3D TiO2-Ag-9 arrays with high surface area. We also compared our results to literature reported values, in terms of detection limit and analytic enhancement factor (AEF), as shown in Table 1. The analytical enhancement factor was estimated using the following equation:53,54
Figure 5. Collected on the arrays of cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min exposed to different R6G concentrations (indicated in the upper middle position of each spectrum), for 10−8 M; excitation, 532 nm; power, 0.05 mW; data collection, 1 s; for 10−9 to 10−16 M; excitation, 532 nm; power, 0.05 mW; data collection, 30 s. (b) The enlarged SERS spectra of R6G collected on the substrate exposed to 10−15 and 10−16 M R6G solution, and the Raman spectra of R6G collected on the arrays of cuboid-shaped TiO2-NWs without Agsputtering exposed to 10−1 M R6G solution (excitation, 532 nm; power, 0.025 mW; data collection, 1 s).
AEF = (ISERS/CSERS)/(IRS/C RS)
where IRS is the Raman intensity of an analyte with a concentration CRS on a non-SERS substrate. ISERS can be obtained from 3D SERS substrate TiO2-Ag-9 with an analyte concentration CSERS. The laser wavelength, microscopic magnification, and spectrometer were identical in the study. For the non-SERS substrate, the Raman spectrum of 10−1 M R6G was collected on cuboid-shaped TiO2-NWs arrays without Ag-sputtering as shown in Figure 5b. Using IRS and ISERS of the peaks at 1510 cm−1, the AEF of the TiO2-Ag-9 is about 1012. By
comparison, it clearly shows that our Ag-TiO2 NWs substrate achieves the best detection limit and analytic enhancement factor, among the reported Ag-coated semiconductor nano22714
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light wavelengths of 532 nm, where |E| = |Elocal/Ein| and Elocal and Ein are the local and incident electric fields, respectively. As for SERS, the |E|4 is the Raman enhancement factor of the EM effect. The maximum SERS enhancements of EM effect on the single TiO2-Ag-9 NW and the single TiO 2-Ag-1 NW correspond to 1.2 × 107 and 6.7 × 105 in theories, respectively. The enhancement electric field distribution of the TiO2-Ag-9 array is shown in Figure 6c. The maximum electromagnetic enhancement |E|2 was similar to that of single TiO2-Ag-9 NW. Yet the deepness of electromagnetic enhancement reduces to 250 nm, when a lot of single TiO2-Ag-9 NWs form TiO2-Ag-9 NWs arrays vertically with several nanometer gaps as shown in Figure 6c. The maximum electromagnetic enhancement |E|2 and deepness of |E|2 of the single TiO2-Ag-1 NW, single TiO2Ag-9 NW, and TiO2-Ag-9 NW array were shown in Table 2. As
Table 1. Reported Detection Limits and Analytical Enhancement Factors on Different Heterostructures between Semiconductors (Si, ZnO, and TiO2) and Noble Metals (Au and Ag) substratea Ag NPs on TiO2 nano-felt Ag nanocluster−ZnO nanowire array Au semishells on TiO2 spheres Au NPs on TiO2 nanotube arrays Au-coated ZnO nanorods Ag NPs on ZnO nanorod arrays Ag NPs on flowerlike ZnO Ag NPs-coated Si NWs Ag nanodesert roses on Si Ag-coated cuboid-shaped TiO2-NWs a
detection limit (M) −7
analytical enhancement factor
ref
b
b 3.2 × 108
36 37
10−9
1.4 × 105
38
10
10−9
b
39
10−12 10−12
1 × 108 3.58 × 107
10−13 10−14 10−15 10−15
1.17 2.3 2 1
× × × ×
107 108 1010 1012
40 41
Table 2. Maximum Electromagnetic Enhancement |E|2 and Deepness of |E|2 of the Single TiO2-Ag-1 NW, Single TiO2Ag-9 NW, and TiO2-Ag-9 NW Array
42 43 44 this study
substrate single TiO2-Ag-1 NW single TiO2-Ag-9 NW TiO2-Ag-9 NW array
NP: nanoparticle. NW: nanowire. bNot reported.
structure. Therefore, the prepared highly sensitive SERS substrate with such low detection limit will provide new opportunities in trace detection applications. The SERS enhancement is widely considered to be the combined contribution of the electromagnetic (EM) effect8,55−57 and the chemical (CHEM) effect.14,58 The electromagnetic field intensity distributions of the single TiO2-Ag-1, single TiO2-Ag-9 NW, and TiO2-Ag-9 NW arrays were compared by the finite difference time domain (FDTD) method with periodic boundary conditions. The FDTD simulation parameters were coincident with the experimental conditions. The lengths of the side of the TiO2 NWs were set as 100 nm. The diameters of the Ag-NPs were set as 10 nm (65 nm) in TiO2-Ag-1 (TiO2-Ag-9). The k- and E-vectors are the incident direction of laser light and the polarization direction, respectively. The electric field enhancement distribution of the single TiO2-Ag-1 NW and single TiO2-Ag-9 NW are shown in Figure 6a and b, respectively. The electromagnetic enhancement |E|2 of the single TiO2-Ag-9 NW is stronger than that of the single TiO2-Ag-1 NW (3.4 × 103 vs 8.2 × 102) at incident
figure
maximum |E|2
deepness of |E|2 (nm)
6a 6b 6c
8.2 × 10 3.4 × 103 3.4 × 103
300 650 250
2
compared to the SEM of TiO2 in Figure 2a, a part of TiO2 is quasi-vertically aligned with a smaller inclination angle. So the deepness of electromagnetic enhancement will increase to form more hot spot 3D SERS substrate. Furthermore, the contribution of CHEM has been reported to be 10−102, which is much less than EM. The enhancement of surfaceenhanced resonant Raman scattering (SERRS) of R6G is 103. Therefore, the AEF of the TiO2-Ag-9 is estimated to be about 1012. Besides sheer SERS enhancement, the homogeneity of the SERS signal across a large area is another important parameter in evaluating the usefulness of an SERS substrate. In addition, SERS signal in the uniformity of large area is another important parameter to evaluate the SERS substrate.52 To demonstrate the homogeneity of TiO2-Ag-9 as uniform SERS sensor, stepby-step Raman mapping was collected on two random selected 24 × 24 μm2 areas with a 3 μm step length on TiO2-Ag-9 as illustrated in Figure 7. Figure 7 shows the Raman intensity
Figure 6. FDTD calculated local electric field enhancement (log|E|2) of single TiO2-Ag-1 (a), single TiO2-Ag-9 NW (b), and TiO2-Ag-9 NW arrays (c), respectively. 22715
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onto the side and top of TiO2-NWs via magnetron sputtering. The excellent SERS performance is attributed to a kind of “cusp” and three kinds of “gaps” formed on the Ag-NPs coated TiO2 nanowire arrays that create a huge number of 3D SERS “hot spots”. The detection limit of rhodamine 6G on the cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min substrate can be as low as 10−15 M, and the analytical enhancement factor is about 1012. The experimental results were also confirmed by theoretical calculations. Combined with the uniformity and stability, our obviously angular, quasivertically aligned cuboid-shaped TiO2-NWs decorated with AgNPs system can be applied as highly sensitive SERS substrate for chemical, biological, medicinal, and environmental pollutant-sensor applications.
Figure 7. Two Raman mappings of randomly selected areas (21 × 21 μm2) of the 10−8 M R6G signals at 1360 cm−1 on the cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min.
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mapping at 1360 cm−1. The distribution of the Raman intensity of 1360 cm−1 peak strongly proves that the TiO2-Ag-9 is homogeneous in a large area with good SERS reproducibility. The phenomenally realistic screenshots of Raman intensity mapping processes of two random selected area and random selected SERS spectra are shown in Supporting Information Figure S5. Finally, to further demonstrate the benefits of TiO2-Ag-9, we compared the SERS spectra of the R6G molecule on the AgFON, a well-studied SERS substrate,19−28 as shown in Figure 8 (Experimental Methods). In the studies, all of the other parameters, including the Ag coated time (9 min), microscopic magnification, R6G concentration, laser parameters, and spectrometer, were the same. Quantitatively, when the SERS spectra of R6G were collected on the TiO2-Ag-9, the average Raman intensity at 1360 cm−1 is about 7 times higher than that when the SERS spectra of R6G were collected on the AgFON (Figure 8c).
ASSOCIATED CONTENT
* Supporting Information S
(1) TEM image of the cuboid-shaped TiO2-NWs with Agsputtering for 1 and 9 min. (2) Size histograms of the diameter of cuboid-shaped TiO2-NW after Ag-sputtering for 0, 1, 9, and 17 min. (3) XRD pattern of the cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min. (4) SEM image of the cuboid-shaped TiO2-NWs with Ag-sputtering for 3, 5, 7, 13, and 15 min. (5) Phenomenally realistic screenshots of Raman intensity mapping processes of two random selected area and random selected SERS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-27-68752481. E-mail:
[email protected]. Author Contributions
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⊥
CONCLUSIONS We have developed an ultrasensitive 3D SERS substrate based on TiO2-NWs coated with Ag-NPs array. The wafer-scale arrays of obviously angular, quasi-vertically aligned cuboid-shaped TiO2-NWs were fabricated on FTO glass by hydrothermal method. Ag-NPs of different sizes were concurrently deposited
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Shunping Zhang and Wen Chen for Raman testing. This work was partially supported by the NSFC
Figure 8. (a) SERS spectra of 10−7 M R6G collected on AgFON and cuboid-shaped TiO2-NWs with Ag-sputtering for 9 min. (b) SEM image of AgFON. (c) Comparison of intensities of the band at 1360 cm−1 (average intensity and standard deviation for five measurements). 22716
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(51171132, U1260102, 51201115, 51371079, 51371131, and 11375134), NCET (12-0418), China Postdoctoral Science Foundation (2014M550406), Hubei Provincial Natural Science Foundation (2011CDB270, 2012FFA042), the Fundamental Research Funds for the Central Universities (T201220203), Jiangsu Provincial Natural Science Foundation (BK20141217), Wuhan Planning Project of Science and Technology (2014010101010019), and the experimental technology project of Wuhan University.
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