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Improved Surface-Enhanced Raman Spectroscopy Sensitivity on Metallic Tungsten Oxide by the Synergistic Effect of Surface Plasmon Resonance Coupling and Charge Transfer Wei Liu, Hua Bai, Xinshi Li, Wentao Li, Junfeng Zhai, Junfang Li, and Guangcheng Xi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01624 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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The Journal of Physical Chemistry Letters

Improved Surface-Enhanced Raman Spectroscopy Sensitivity on Metallic Tungsten Oxide by the Synergistic Effect of Surface Plasmon Resonance Coupling and Charge Transfer Wei Liu, Hua Bai, Xinshi Li, Wentao Li, Junfeng Zhai, Junfang Li, and Guangcheng Xi* Institute of Industrial and Consumer Product Safety, Institution Chinese Academy of Inspection and Quarantine, No. 11,Ronghua South Road, Beijing 100176, P. R. China. AUTHOR INFORMATION Corresponding Author [email protected]

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ABSTRACT: Increasing the sensitivity of non-noble metal surface-enhanced Raman spectroscopy (SERS) is an urgent issue that needs to be solved at present. Herein, metallic W18O49 nanowires with strong localized surface plasmon resonance (LSPR) effect are prepared. Interestingly, the LSPR peaks of these nanowires would undergo a strong blue shift from near infrared (NIR) to visible light regions as the aggregation degree of the nanowires increases. By narrowing the gap between the LSPR absorption peak and the Raman excitation wavelength (532 nm), the oriented W18O49 bundles with a LSPR peak centered at 561 nm have greatly improved SERS sensitivity compared with that of the dispersed nanowires with a LSPR peak centered at 1025 nm. Enhancement mechanism investigation shows the high sensitivity can be attributed to the synergistic effect of LSPR coupling among the oriented ultrathin nanowires and oxygen vacancy (Vo)-assisted charge Transfer.

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Benefit from its rich molecular structure information, SERS has become a powerful analytical tool in chemistry, physics, material fields and so on. 1-5 Of the various factors that affect SERS performance, substrate material is the most critical.6-8 At present, Au and Ag are the most common SERS substrate materials.9-14 Although Au and Ag do not have Raman scattering properties themselves, they can produce strong LSPR behavior under light irradiation. In this enhanced electromagnetic field caused by the strong LSPR, the Raman signal intensity of the analyte molecules adsorbed on the surface of the substrate material will be strongly enhanced, which is the so-called electromagnetic enhancement (EM). In addition to the EM-based Au and Ag substrates, the non-noble metal SERS substrates, such as metal oxides, 15-18 graphen,19 Si,20 metal tellurides21 and other semiconducting materials22,23 proved to be a new type of SERS substrate materials. Of particular interest is that semiconducting organic films also exhibit a 10 3level Raman scattering enhancement factor (EF).24 For these non-noble metal SERS substrates, the charge transfer (CT) mechanism is widely used to explain their Raman signal enhancement.25,26 Generally, the EFs of non-noble metal SERS substrates are 2-3 orders of magnitude lower than that of noble metals represented by Au and Ag. If a SERS substrate material can have both EM and CT ability, its SERS activity may be greatly improved by the synergistic effect of EM and CT. Generally, semiconductors do not have enough free electrons due to their large band gap, which determines that typical semiconductors such as TiO2 and WO3 have no LSPR effect or "hot spots" under illumination. However, the conductivity of semiconductors will be greatly improved due to the increase in the number of free electrons by introducing high concentration Vo-defects.27,28For example, Vo-rich TiO2-x nanosheets29 and WO3-x nanorods30 have been shown theoretically and experimentally to have dorbital free electrons and the corresponding LSPRs. However, SERS activity induced by LSPR


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effect cannot be generated on these Vo-rich semiconductors since their LSPR peaks in NIR region are far away from the commonly used SERS excitation wavelength (532 or 632 nm). However, on the other hand, the defect state energy levels caused by V o can significantly improve the charge transfer between the adsorbed molecules and the substrates, greatly improving the SERS activity induced by CT way.17Therefore, if the LSPRs of the Vo-rich semiconductors can be regulated from the NIR region to the visible region (close to the SERS excitation wavelength), their SERS activity would further increase. Recent studies have shown that strong LSPR coupling between closely adjacent nanoparticles will lead to the shift of LSPR peaks,31 which inspired us to combine the strong LSPR coupling between particles and the charge transport channels induced by Vo-levels to create a highperformance, non-noble metal SERS substrate. Herein, we synthesized a highly dense and highly oriented W18O49 ultrafine nanowire bundles with high concentration of V o and found that it has a strong LSPR effect in visible light region (561 nm). Thanks to the strong EM and CT enhancements at the same time, this ultrafine nanowire bunches exhibit two orders of magnitude of enhancement factor (EF) compared with that of dispersed W 18O49 nanowires, reaching 2.8 × 107. We chose W18O49 as the candidate because of a large number of oxygen vacancies in this compound compared with WO3.32As shown in Figure 1, the band structure of W 18O49 was predicted by the first-principles calculation, which reveals that W 18O49 has an intrinsic metallic feature with the Fermi level crossing some bands (red frame), and most states of W 18O49 around the Fermi level were composed from W5d orbitals. In stark contrast, there is no free electron distribution near the Fermi level of WO3 (Figure S1). Moreover, these Vo defects also produce a defect level near the valence band (blue frame). This metallicity (means free electrons) may


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cause the LSPR effect and large number of "hot spots" (sites with particularly high electromagnetic field intensity under light irradiation), while these Vo-levels will facilitate the CT between the molecules and substrate. In order to obtain a blue-shifted LSPR in the visible region by the strong coupling between the nanowires, we designed and synthesized highly dense and highly oriented ultrafine W18O49 nanowire bundles (synthetic details see Experimental Section in Supporting Information).

Figure 1. Upper: the band structure of W18O49 and the two possible SERS enhancement ways in W18O49: (1) EM and (2) CT; Lower: LSPR coupling between nanowires. By the first-principles calculations, it was found that monoclinic phase W 18O49 has a strong tendency to grow along the b axis (Figure S2), which provides a theoretical basis for us to prepare highly oriented W18O49 ultrafine nanowire bundles. By adjusting the concentration of the precursor (WCl6) and the reaction temperature, three different morphology products were obtained respectively. When using high precursor concentrations and high reaction temperatures, a large number of highly dense and highly oriented W18O49 ultrafine nanowire bundles were synthesized, as shown in Figure 2a. With the decrease of precursor concentration and reaction temperature, the degree of aggregation of nanowire bundles is significantly reduced (Figure 2b),


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eventually becoming dispersed nanowires (Figure 2c). High-resolution transmission electron microscopy (HRTEM) image shows that these highly oriented nanowires shown in Figure 2a have high crystallinity and display clear lattice fringes (Figure 2d). Based on the resulting fringe spacing of 0.38 nm, these nanowires do indeed grow along the [010] direction, which is exactly the same as the theoretical predictions. The diameter of these nanowires is high uniform and only about 1 nm. In the [010] direction, such fine nanowires have not even fully developed into a complete unit cell (Figure 2e), which means that more Vo will be generated on the surface of nanowires. It is very interesting that as the aggregation degree of these nanowires decreases, their absorption spectra undergo a great deal of change as shown in Figure 2f. As the ultrathin nanowire bundles gradually became thicker (that is the aggregation degree is increased), their absorption peaks gradually shifted from the NIR(1025 nm) to the visible (561 nm) regions, transferring almost 500 nm. According to the mentioned-above theoretical predictions and the earlier reports,[37,39] these strong light absorption can be referred to as the LSPRs of the V o-rich W18O49 nanowires. What needs to be mentioned is that although the LSPR maximum absorption peaks of these three samples have changed dramatically, their X-ray powder diffraction (XRD) patterns (Figure S3) and X-ray photoelectron spectroscopy (XPS) characterizations (Figure S4S5) are almost the same, which indicates that this apparently blue-shifted LSPR phenomenon is most likely caused by the strong electromagnetic oscillation coupling between these metallic ultrafine W18O49 nanowires.31,33 In order to distinguish these samples, we named them W 18O49561, W18O49-743, and W18O49-1025 according to their LSPR peaks. Combining this strong LSPR in visible light region and the Vo-defect state level, there is reason to believe that this W 18O49561 ultrathin nanowire bundles is very likely to have excellent SERS properties.


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Figure 2. Structure and optical properties of the W18O49 samples.(a-c) TEM images of the W18O49-561,W18O49-743,and W18O49-1025. (d) HRTEM image of the W18O49-561. (e) the cross section of a 1.0 nm nanowire inside one W 18O49 unit cell oriented along the [010] direction. (f) UV-Visible-NIR absorption spectra of the W 18O49-561, W18O49-743, and W18O49-1025. In order to confirm this conjecture, the SERS properties of these samples were systematically evaluated. In the SERS experiments, Rh6G was used as the probe molecule and a 532 nm laser was used as the excitation light. As shown in Figure 3a, for 10 -7 M Rh6G, these W18O49-561 ultrathin nanowire bundles show the highest sensitivity. While for W 18O49-743 and W18O491025, their sensitivities to the analyte molecules greatly decreases. It needs to be pointed out that if the Rh6G is directly dropped on the slide, no SERS signal is generated, which makes it clear


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that the SERS signals should be attributed to the metallic W 18O49 nanowires with high concentration of Vo. A series of controlled experiments were carried out to investigated the SERS enhancement mechanism over the W18O49 samples. First, when the excitation wavelength changed from 532 nm to 785 nm, none of the three samples could give the distinguishable signal to the 10 -7 M Rh6G. When the concentration of the Rh6G was increased to 10-5 M, both W18O49-561 and W18O49-743 showed obvious SERS signals, while W 18O49-1025 still had no signal (Figure 3b). These phenomena are obvious features of the EM enhancement mechanism: with the decrease of excitation energy, the SERS signal intensity of substrate material would corresponding decrease. Furthermore, a notable fact is that at the excitation of 785 nm laser, the intensity of SERS signal obtained on W18O49-743 is higher than that obtained on W 18O49-561, which is just the opposite from the result obtained when use 532 nm laser as the excitation light. The reason for this phenomenon is that the 785 nm excitation light is closer to the position of the LSPR peak of the W18O49-743, which further confirms that the EM enhancement mechanism plays an important role in the W18O49-561 and W18O49-743 substrates. To clarify whether the CT mechanism is present in these W 18O49 ultrathin nanowire bunches, the surface of W18O49-561 was coated with a thin layer of amorphous SiO 2 (named SiO2/W18O49561, Figure S6-S8) and then examined their sensitivity. As shown in Figure 3c, the signal intensity obtained on the SiO2/W18O49-561 is obviously lower than that achieved on the W 18O49561. This decrease can be attributed to the fact that this SiO 2 coating maybe block the charge transport channel between the substrate and the adsorbed molecules, which in turn proves that the CT enhancement mechanism is also present in this V o-rich tungsten oxide. This CT enhancement is likely to be attributed to a large number of Vo defects contained in the samples.


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To demonstrate this point, samples with different Vo-defect concentrations were prepared to test their sensitivity. As shown in Figure 3d, when the W 18O49-561 was heated in the air for 1 hour at 100 °C (the sample is named W18O49-561-1h), its SERS sensitivity was significantly reduced. Once the W18O49-561 has been completely oxidized to WO3, their SERS activities are completely disappears, which only displays its own Raman vibration modes. Interestingly, although the CT enhancement pathway were isolated by the SiO 2 coating, these samples still showed distinguishable signals, which also demonstrated that EM enhancement mechanisms exist in this substrate. The above experimental results reveal that the ultrahigh SERS sensitivity of these W 18O49-561 is derived from the combination of EM and CT enhancement mechanism (Figure 3e). As for the EM mode, d-orbit free electrons generate a strong LSPR under the excitation of the 532 nm laser. Because these metallic W18O49 nanowires are highly densely arranged in parallel, intense LSPR coupling occur inevitably between ultrathin nanowires, causing blue-shift from NIR to the visible region,38 while forming a large number of “hot spots” in the gaps between the nanowires. As for the CT enhancement, the large number of defect states formed by V o plays a key role. Specifically, as for the probe molecule of Rh6G, its highest occupied molecule orbital and lowest unoccupied molecule orbital are at -5.7 and -3.4 eV. While for the SERS substrate, the conduction band (CB) and valence band (VB) of W18O49 lie at -5.1 and -7.7 eV, respectively. The defect state level induced by oxygen vacancies is located at -6.6 eV. The defect state effectly promotes the charge transfer between W18O49 and Rh6G according to the thermodynamically reasonable processes as shown in Figure 3e (right).


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Figure 3. The SERS activity and enhancement mechanism investigation of the W 18O49substrates.(a) SERS signals of 10-7 M Rh6G molecules on the different substrates under the excitation light of 532 nm. (b) SERS signals of 10 -5 M Rh6G molecules on the different substrates under the excitation light of 785 nm. (c) SERS signals of 10 -7 M Rh6G molecules on the SiO2/W18O49-561 and W18O49-561. (d) As the Vo concentration decreases, the corresponding Raman signal intensity of Rh6G also decreases accordingly. (e) The proposed enhancement mechanism contains: (1) EM and (2) CT. The SERS substrate assembled from these W 18O49 ultrathin nanowire bundles has good morphology uniformity (Figure S9). Furthermore, the linear scanning spectra obtained from 20 points in 20 m distance clealy show that the intensities of the SERS signals have high


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uniformity (Figure 4a-b). At the same time, the SERS mapping at 612 cm -1over 35 × 35 m area (contains 3600 randomly selected points) also demonstrates that the excellent signal uniformity (Figure 4c). In order to further clearly show the signal repeatability of this substrate, the relative standard deviation (RSD) of the signal intensity at 612 cm-1 of 3600 points was counted. The calculation result displays that the RSD is only about 4.8% (Figure 4d).

Figure 4. Uniformity characterization of the W18O49-561 SERS substrate.(a) Linear Scanning SERS Spectra recorded from 20 points in 20 m distance with a step size of 1 m. The concentration of the Rh6G is 10-7 M and the excitation wavelength is 532 nm. (b) Variation of SERS signal intensity at three different Raman vibration modes of 612, 773, and 1361 cm -1, respectively. (c) The SERS intensity mapping recorded at 612 cm-1 vibration mode in a area of 35 × 35 m. (d) The intensity distribution and calculated RSD from 3600 spectra at 612 cm -1.


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For these Vo-rich W18O49 ultrathin nanowires, their photocatalytic performance is further improved due to their strong LSPR compared with that of WO 3. Under the visible light irradiation (with a 420 nm filter), Rh6G aqueous solution with a concentration of 10 -5 M would be fully degradated in 20 min by using the W18O49-561 as a photocatalyst (Figure S10a). With the help of its excellent performance of the photocatalytic degradation of organics, this W 18O49561 can be used repeatedly when used as a SERS substrate. As shown in Figure S10b, after five cycles, the SERS activity of these W18O49-561 substrate to 10-7 M Rh6G remains high, no significant performance degradation. In order to accurately evaluate the EF of this kind of substrate, the concentration and integration time of the reference experiments have been greatly improved, as shown in Figure S11a. The calculated EF is 2.8 × 107, which value is an improvement of 2 orders of magnitude compared to using dispersed W18O49 nanowires without LSPR coupling as the SERS substrate. Furthermore, compared with other semiconductor-based SERS active materials, the EF of the W18O49-561 SERS substrate material, which contains both EM enhancement and CT enhancement, is also greatly improved (Table S2). In addition, parallel experiments have also shown that common contaminants such as methyl orange (MO) and 2,4-dichlorophenol (2,4DCP) can also be sensitively detected, which proves that this W 18O49-561SERS substrate has a good universality (Figure S11b). In summary, by introducing Vo-defects into ultrafine nanowire structures and self-assembling them into highly dense and highly oriented bundle structures, a W 18O49 SERS substrate with strong and wavelength-tunable LSPRs is obtained. High concentration of Vo plays an important role in the acquisition of the high EF. On the one hand, the introduction of Vo allows W18O49present metallicity and the resulting LSPR coupling effect. On the other hand, the defect


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energy level derived from the Vo improves the transfer efficiency of the charge between the adsorbed molecules and the substrate, causing an increase in the polarity of the molecules. Combining these two aspects, the W18O49 therefore has an ultrahigh EF. Furthermore, W18O49substrate also shows an excellent recyclability under visible light irradiation. ASSOCIATED CONTENT Supporting Information is free of charge at the ACS publication website. AUTHOR INFORMATION Corresponding Author G.C.X.: Email: [email protected] Notes The authors declare no completing financial interest. ACKNOWLEDGMENT This work received financial support from the Natural Science Foundation of China (51472226). Supporting Information Available: experimental section, theoretical calculation, enhancement factor measurement, XRD patterns, XPS spectra, UV-Vis absorption spectra of the W 18O49 samples. REFERENCES (1)

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