Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies

Abstract

Surface-enhanced Raman spectroscopy (SERS) represents a very powerful tool for the identification of molecular species, but unfortunately it has been essentially restricted to noble metal supports (Au, Ag and Cu). While the application of semiconductor materials as SERS substrate would enormously widen the range of uses for this technique, the detection sensitivity has been much inferior and the achievable SERS enhancement was rather limited, thereby greatly limiting the practical applications. Here we report the employment of non-stoichiometric tungsten oxide nanostructure, sea urchin-like W₁₈ O₄₉ nanowire, as the substrate material, to magnify the substrate-analyte molecule interaction, leading to significant magnifications in Raman spectroscopic signature. The enrichment of surface oxygen vacancy could bring additional enhancements. The detection limit concentration was as low as 10^-7 M and the maximum enhancement factor was 3.4 × 10⁵ , in the rank of the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without 'hot spots'.

ARTICLE

Received 31 Oct 2014 | Accepted 11 Jun 2015 | Published 17 Jul 2015

Shan Cong1,*, Yinyin Yuan1,2,*, Zhigang Chen1, Junyu Hou3, Mei Yang1, Yanli Su3, Yongyi Zhang1, Liang Li2, Qingwen Li1, Fengxia Geng3 & Zhigang Zhao1

Surface-enhanced Raman spectroscopy (SERS) represents a very powerful tool for the identication of molecular species, but unfortunately it has been essentially restricted to noble metal supports (Au, Ag and Cu). While the application of semiconductor materials as SERS substrate would enormously widen the range of uses for this technique, the detection sensitivity has been much inferior and the achievable SERS enhancement was rather limited, thereby greatly limiting the practical applications. Here we report the employment of non-stoichiometric tungsten oxide nanostructure, sea urchin-like W18O49 nanowire, as the substrate material, to magnify the substrateanalyte molecule interaction, leading to signicant magnications in Raman spectroscopic signature. The enrichment of surface oxygen vacancy could bring additional enhancements. The detection limit concentration was as low as 10 7 M and the maximum enhancement factor was 3.4 105, in the rank of

the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without hot spots.

DOI: 10.1038/ncomms8800 OPEN

Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies

1 Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China.

2 Key Laboratory for Ultrane Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China. 3 College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China.* These authors contributed equally to this work. Correspondence and requests for materials should be addressed to Z.Z. (email: mailto:[email protected]

Web End [email protected] ).

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The surface-enhanced Raman scattering (SERS) effect is a surface-sensitive technique characterized by an increase in the Raman intensity by orders of magnitude for molecules

adsorbed on particular surfaces such as rough metals with respect to that expected from the same number of non-adsorbed molecules in solution or the gas phase1,2. The enhanced sensitivity makes the technique serve as one of the most powerful analytical tools for the unequivocal identication of chemical and biological analytes and additionally in numerous other elds, such as electrochemistry, surface science, catalysis, chemical and biomolecular sensing and so on1,3,4. While the exact mechanism for the enhancement effect is still a matter of debate, two widely accepted theories have been adopted in most cases to explain the phenomenon5. The primary one is electromagnetic mechanism predicting that the electric eld is magnied when excitation takes place within localized surface plasmon resonances of substrate materials, leading to enhancement factors (EFs) up to the order of 106. The induced enhancement of local eld by plasmonic coupling is called electromagnetic hot spots. The other is a chemical mechanism proposing the formation of charge-transfer complexes between the chemisorbed species and the substrate materials and obtaining enhancement when the excitation frequency is in resonance with the charge-transfer transition with EFs around 10100. SERS experiments have been essentially dominated by adsorbates on rough metallic surfaces, especially noble and alkali metals such as Au, Ag and Cu, because their plasmon resonance frequencies locate within excitation wavelength ranges commonly used in Raman spectroscopy. While precise tuneable position of adjacent metallic nanoparticles is a key factor to bring about strong plasmonic coupling, fabricating perfect plasmonic nanostructures with high density hot spots to achieve high enhancements usually requires delicate procedures and high cost. In addition, noble metals typically show poor stability and biocompatibility, and therefore the search of other alternative materials for working as SERS substrate becomes an urgent task. A few semiconducting materials have been proven to show Raman enhancement, for example, InAs/GaAs quantum dots6, CuTe nanocrystals7, Cu2O nanospheres8 and TiO2 nanostructures9,10, in which charge transfer at the semiconductoranalyte interface plays a major role in Raman scattering enhancement. The TiO2 SERS substrate offers not only a biocompatibility, but also a greater chemical and mechanical stability against variation in pH or temperature of the environment9. The effective utilization of semiconductor SERS-active substrates would greatly expand the applications of SERS in many elds, such as direct monitoring of interfacial chemical reactions on individual nanoparticles11. However, one signicant problem of metal-free SERS-active substrates is that the EFs were typically quite low, usually in the range of 10102, far from sufcient for applications in biological and biomedical analysis and diagnosis. It is consequently one formidable challenge but greatly desirable to nd efcient semiconductor SERS substrate materials and obtain EFs comparable to noble metals.

The surface states of substrate material and its interaction with analyte molecule is a key to the SERS effect. Among the numerous methods in tuning the surface states of semiconducting oxide materials, adjusting oxygen deciency represents one main strategy and may work as an efcient and simple way to achieve the goal of high sensitivity comparable to that of noble metals. Tungsten oxide materials, a traditional semiconductor, have attracted considerable attention because they possess distinctive physical and chemical properties, which endow them to be effective candidates in a wide range of applications, covering photocatalysts, gas sensors and electrochromic devices1214. Importantly, rich phases of substoichiometric composition (WO3 x) can be obtained by the reduction and formation of

various types of defect structure, such as WO2.72 (W18O49),

WO2.8 (W5O14), WO2.92 (W25O73) and so on1517,which

makes the system work as an ideal platform for investigating the effect of vacancies or defects on SERS phenomenon. WO2.72

(W18O49) with the largest vacancy has been reported as the only oxide that can be isolated in a pure form, which contains tungsten ions of mixed valency16,18.

Herein, employing vacancy-containing W18O49 as the substrate

material, we achieve greatly enhanced SERS effect for the rst time on function-rich tungsten oxide material, and the enhancement is further improved by creating surface deciencies, which gives SERS EF as high as 3.4 105 and 10010,000 times higher

than the previously reported values for most other semiconductor SERS-active substrates, and even comparable to noble metals without hot spots.

ResultsSample characterizations. W18O49 was synthesized using a previously reported approach, WCl6 dissolved in ethanol followed by hydrothermal reaction at 180 C for 24 h (ref. 13). Phase and purity of the sample was veried by X-ray diffraction characterizations as shown in Fig. 1a. The hydrothermally synthesized sample was crystallized in a monoclinic phase of W18O49 (P2/m, JCPDS no. 84-1516) with lattice constants rened to be a 18.31(2), b 3.839(8), c 14.00(1) and

b 115.19(9). The relatively high intensities and narrowing

exhibited by (010) and (020) peaks suggest that the crystals preferably grew along the b axis as a result of retarded growth along the close-packed (010) planes. For comparison study, tungsten trioxide, WO3, was acquired by annealing the W18O49 sample in air. The transformed WO3 was also of high purity with all reections perfectly tted in stoichiometric monoclinic (no. 14, P21/n) tungsten trioxide with cell dimensions a 7.305(5),

b 7.523(4), c 7.689(4) and b 90.90(9). To obtain more

information about the intrinsic nature of the samples, ultraviolet visible absorption spectra of the two samples were collected (Fig. 1b). Distinct from the absorption to 500 nm normally observed in tungsten oxide materials, W18O49 sample exhibited an obvious blue shift of the absorption edge. In addition, an absorption tail beyond the absorption edge in the visible and near-infrared region was observed, up to the measurement limit, 800 nm, which can be considered to be closely related with the free electrons and/or oxygen deciency-induced small polarons15, providing clear evidence that the as-synthesized W18O49 sample

contains a large number of oxygen vacancies. Morphology of the samples were characterized by scanning electron microscope observations, which suggested that the as-grown W18O49 sample

comprised of thin nanowires with average length estimated to be about several micrometres while width in the range of 1020 nm (Fig. 1c). The nanowires were entangled featuring a prickly, spherical and sea urchin-like morphology. High-resolution transmission electron microscopy on one single nanowire discerned clear lattice fringes belonging to (010) planes of monoclinic W18O49 with an interplanar spacing of B0.38 nm (inset in Fig. 1c), which undoubtedly indicates that the nanowires preferably grow along the [010] direction, in good agreement with X-ray diffraction observations. After annealing in air and oxidizing into WO3, the morphology was featureless, showing irregular particles of several micrometres (Fig. 1d).

The defect structure of W18O49 is illustrated in Fig. 2, in comparison with the perfect structure of WO3, projected along the [010] direction. The crystal structure of WO3 consists of slabs of corner-sharing WO6 octahedra (ReO3-type), which have an innite extension in two dimensions (ac plane) and a nite, characteristic width in the [010] direction. The slabs are mutually

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a b

010

503

-

W18O49

204

020

124

204

Absorbance

W18O49

-

Intensity

002

002

002

WO3

WO3

112

120

122220

022

222

004

040

400

420

20 30 40 50 60 70

Wavelength (nm)

300 400 500 600 700 800

2[afii9835] ()

c d

0.38 nm

1 m 500 nm

Figure 1 | Characterization of W18O49 and annealed WO3 samples. (a) X-ray diffraction patterns of as-obtained W18O49 sample and annealed WO3 with all reections perfectly indexed; (b) ultravioletvisible prole comparison between W18O49 sample and annealed WO3, showing a blue shift and an obvious absorption tail beyond the edge that may arise from the presence of oxygen vacancies; (c,d) scanning electron microscopy images for W18O49 and WO3

samples, respectively. Inset in (c): high-resolution transmission electron microscopy image on one single nanowire illustrating clear lattice fringe of0.38 nm, which suggested that the nanowire growth was along the [010] direction.

a b

c

c

a

a

Hexagonal channel

Figure 2 | Structure illustration for WO3 and W18O49 in [010] projection. (a) The crystalline WO3 structure consists of stacking of innite corner-sharing WO6 octahedra layers, while (b) the defects in W18O49 structure lead to the formation of hexagonal channels.

linked, forming channels between the octahedra. In contrast, the reduction of WO3 to W18O49 leads to structural changes of the

ReO3-type structure and the formation of tungsten pentagonal columns causes the arising of additional hexagonal channels19. The presence of oxygen defects may enrich the surface states of semiconductor and enhance the interaction afnity between the adsorbent and adsorbate, providing promises for enhancements in Raman signals.

Raman enhancement for W18O49 sample. The Raman enhancement behaviour of the materials was examined using laser dye Rhodamine 6G (R6G) as a Raman probe, the molecular structure of which is provided in inset of Fig. 3a. R6G is a strongly uorescent xanthene derivative that shows a molecular resonance effect when excited into its visible absorption band. Figure 3a shows the Raman spectra of R6G (10 6 M) with the excitation wavelength being 532.8 nm on substrates deposited with W18O49

nanowires, annealed WO3, along with bare SiO2/Si. While no clear signals related with R6G molecule were discerned on bare

SiO2/Si substrate and WO3 except the uorescence background, a substantial Raman enhancement was observed on W18O49, and

four characteristic bands of R6G centred at 612, 773, 1,360 and 1,650 cm 1, named as P1, P2, P3 and P4, were clearly detected, which suggests that W18O49 could work as SERS-active substrate. Raman characterizations on bare W18O49 in the absence of probe molecules were also performed, in which only the signals related to OWO bending and WO stretching modes were found (Supplementary Fig. 1), further conrming that the P1, P2, P3 and P4 bands originated from R6G molecules. P1 and P2 can be assigned to in-plane and out-of-plane bending motions of carbon and hydrogen atoms of the xanthenes skeleton, respectively; P3 and P4 correspond to aromatic CC stretching vibration modes20. Raman spectra at four different R6G concentrations (the range was selected according to adsorption isotherms in Supplementary Fig. 2), decreasing from 10 4, 10 5 and 10 6 to 10 7 M, were collected, showing obvious intensity enhancement until an extreme dilute solution was used, 10 7 M, in which just detectable signal was obtained (Fig. 3b). Therefore, the detection limit for W18O49 material can be determined to be 10 7 M. Such a low detection limit enables a high sensitivity towards trace amounts of analyte molecules, and makes it possible for a systematic investigation of different vibration modes in the SERS proles.

Two signature bands, P1 and P3, were selected for calculating EFs based on the magnication of Raman intensity compared with that on bare substrate (details in Supplementary Methods)2123, the variation of which was depicted in Fig. 3c as a function of R6G analyte concentrations (CR6G). It is clear that

the greater enhancement occurs for P1 at each concentration, which was found to be surprisingly high as 1.9 105 with the

R6G concentration of 10 6 M, about 10010,000 times higher than that previously reported for most other semiconductor

SERS-active substrates (Supplementary Table 1). Furthermore, the intensity variation was not uniform and instead strongly dependent on the band identity. The selective enhancement of the

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a b

Intensity (a. u.)1,000 cps

R6G 104M

P3

1,000 cps

P4

R6G 106M

P1

P2

W18O49

Si/SiO2

WO3

Raman shift (cm1)

Intensity (a. u.)

600

800

1,000

1,200

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106M

107M

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EF 105

50 cps

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P3

600 900 1,200 1,500 1,800

600

1,000

1,400

1,800

106 104

105

105M

Raman shift (cm1)

C0 (M)

Figure 3 | SERS properties. (a) Raman prole of R6G (10 6 M) on substrates deposited with W18O49 sample compared with that for WO3 and bare SiO2/ Si substrate. Inset: molecule structure of R6G. (b) Raman spectra collected for W18O49 at four different concentrations, 10 4, 10 5, 10 6 and 10 7 M, suggesting the detection limit was as low as 10 7 M (Inset: with narrowed y scale for 10 7 M). (c) The statistical evolution of EF as a function of R6G concentration plotted in logarithmic scale, with the analysis carried out over 30 different regions per sample. The Raman enhancement typically increased with decreasing concentrations and selective enhancement occurred with different bands.

34 36 38 40

different vibration modes indicates that the SERS on W18O49

sample largely depends on the distinct binding and geometry of R6G molecules on the W18O49 surface, which is a character of the chemical enhancement mechanism. Specically, the change of P1 was more obvious than that for P3. The relative lower intensity exhibited by P3, especially at low concentrations, indicates that the long axis plane of R6G molecule was not parallel to the sample surface or the aromatic rings were separated by chemical groups including CH bond24. With dilution of the R6G solutions, the signal intensity of P3 gradually increases, suggesting the gradual decrease of tilt angles of R6G molecules on the substrate. The emergence of greatly enhanced SERS sensitivity for W18O49 compared with WO3 can probably be attributed to the presence of oxygen vacancies in W18O49

structure (vide ante, Fig. 2).

Surface vacancy related Raman enhancement. To conrm the role of oxygen vacancy played in SERS, additional surface vacancy was deliberately created by annealing the W18O49 sample

in Ar/H2 atmosphere25. After the treatment at 300 C for 1 h, the crystalline phase of W18O49 and the sea urchin-like morphology were still well kept (Fig. 4a). X-ray photoelectron spectroscopy was used to check the surface states, and Fig. 4b displays the spectra of W4f core levels for pristine W18O49 and the

Ar/H2-treated samples. The W4f core-level spectra could be deconvoluted into three doublets (W6 , W5 and W4 ) for all samples. On the tting analysis, it was found that the atomic percentage of W6 , W5 and W4 for pristine W18O49 was54.1, 30.4 and 15.5%, roughly agreeing the documented values26. By contrast, the percentage for W5 increased to 35.4 and 47.5%,

W4 to 21.4 and 13.9% for Ar and H2 thermal-treated samples, respectively. The increased percentage for reduced tungsten,

W4 and W5 , is likely to be related with the creation of

positively charged oxygen vacancies with accompanying charge-compensating electrons27. With increasing oxygen vacancy, the intensity of the absorption tail in the visible and near-infrared region increased (Supplementary Fig. 3), consistent with the recorded results28. The sample colour changed from pale blue for pristine W18O49 to cyan and

deep blue for Ar- and H2-treated samples, respectively, conrming the increment of oxygen deciencies present in the examined samples.

The Raman spectra of R6G with concentration of 1 10 6 M

for the two treated samples are given in Fig. 5a along with the

a

b

W6+ W5+

Intensity (a. u.)

H2-treated

Ar-treated

Intensity (a. u.)

Pristine W18O49

2[afii9835] ()

250 nm

20 30 40 50 60 70

Binding energy (eV)

Figure 4 | Characterization of W18O49 samples with modulated surface oxygen vacancies. (a) X-ray diffraction patterns of Ar- and H2-treated samples along with that for as-prepared W18O49 sample for comparison purpose, indicating that the crystalline phase remained during the annealing. Inset: the corresponding SEM images showing the morphology was also unchanged. (b) XPS spectra of W4f core levels for W18O49

samples after treatment in Ar and H2 atmosphere at 300 C and pristine W18O49 sea urchin-like aggregates. Right: corresponding optical images of the three samples, showing colour change from pale blue for pristine W18O49 to cyan and deep blue for Ar- and H2-treated samples.

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spectrum of pristine W18O49 sample for comparison. Both treated samples clearly manifested the four characteristic peaks of R6G molecule. Neither a new peak nor an obvious shift of the characteristic peak was detected, indicating that no formation or vanish of chemical bonds between R6G and the substrates were induced on oxygen vacancy alternation. Notably, the Ar- and H2-treated samples were found to display further Raman intensity increase on some degree compared with pristine W18O49

sample, corroborating the fact that oxygen vacancy may help increase Raman detect sensitivity. The Raman intensity of the four feature bands for all the three samples were summarized in Fig. 5b, and it is obvious that the Raman enhancement was in the order of H2-treated W18O494Ar-treated W18O494pristine

W18O49. The P1 band of H2-treated W18O49 displayed the greatest enhancement, and the corresponding EF was evaluated to be 3.4 105. Such high EF has been rarely observed in

semiconducting materials, even comparable to that for noble metals without hot spots20, which may come from the combined function of intrinsic and deliberately created oxygen deciencies.

It seems the presence of oxygen vacancies in semiconducting oxide materials, either intrinsic ones or deliberately created on the surface, could bring signicant Raman enhancement for R6G molecule. To examine the interaction between tungsten oxide materials with adsorbate molecule, ultravioletvisible spectrum of R6G molecules deposited on W18O49 was collected in comparison

with those for neat R6G and W18O49, which is presented in Fig. 6a. The spectrum for the hybrid features occurrence of a new band with an optical absorption onset at B580 nm. The band locates between the bandband transition absorption edge for W18O49, 420 nm, and the photoabsorption threshold for R6G dye, 670 nm, providing clear evidence for the efcient charge transfer between R6G and W18O49 material. This observation of high

Raman enhancement on W18O49 may be based on the charge-transfer mechanism in an adsorbatesemiconductor system.

Moreover, oxygen vacancy may play an irreplaceable role in enriching the surface states of semiconductor to provide magnied afnity for the adsorbentadsorbate interaction, which results in a further increase to the Raman signals for the probe molecules adsorbed on the semiconductor substrate surface through charge transfer involved in a CM mechanism. Providing with efcient charge transfer processes between the matching energy levels of adsorbed probe molecules and the semiconductor, both the polarizability tensor and the electron density distribution of the molecule would be modied, leading to the observation of non-totally symmetric SERS modes. As shown in Fig. 6b, R6G as a typical SERS probe molecule has the highest-occupied molecular orbital and lowest-unoccupied molecular orbital levels at 5.70 and 3.40 eV, respectively29. The valence

band and conduction band of semiconductor W18O49 locate at

7.71 and 5.11 eV (ref. 30),respectively, with oxygen vacancy-

associated electronic state (VO) well separated from the bottom of the CB, located at about 0.51.0 eV below the conduction band minimum31. It can be expected that contributions from several types of thermodynamically feasible charge transfer resonance may be related to the overall Raman enhancement in our W18O49R6G system at an excitation of 532 nm, including molecule resonance (mmol) of R6G, exciton resonance (mex) of W18O49 defect states, and the photon induced charge transfer resonance (mPICT) together with the ground-state charge transfer resonance (mGSCT) from matched energy level between W18O49 and R6G molecules. These resonances will lead to a magnication of Raman scattering cross-section.

In fact, defect state |Vi stemming from oxygen vacancies in the

lattice of semiconductor has a crucial contribution to the molecular polarizability tensor via the vibronic coupling with molecular ground state |Ii and molecular excited states |Ki,

besides the conduction band state |Si and valence band state |S0i

of semiconductor (details are illustrated in Supplementary Methods). Under the incident laser frequency higher than the molecular resonance (o04oIK), the polarizability tensor asr can be expressed by a simple formula asr A B C32, where A

represents the contribution of the molecular resonance. B and C represent the contributions from the photoinduced charge

a b

Intensity (a.u.)

H2 300 C

Ar 300 C

EF 105

4

3

2

1

P1

P3

P1

600

Pristine

Raman shift (cm1)

1,000 1,400 1,800

P3

0 Pristine Ar-300 H2-300Samples

Figure 5 | Improved SERS properties of W18O49 samples after Ar/H2 annealing treatment. (a) Raman signals of R6G molecule on pristine

W18O49 and the samples after annealing treatment (in Ar/H2 at 300 C for 1 h). The tested concentration of R6G was 1 10 6 M. (b) A comparison of

Raman EF for the two respective vibration modes P1 and P3. Data reported in this histogram resulted from Raman spectra acquired over 30 different regions per sample and provide an indication of the EF for each Raman mode. The H2-treated sample shows the greatest enhancement at band P1, the EF for which was evaluated to be 3.4 105.

Absorbance (a.u.)

300 400 500 600 700 800Wavelength (nm)

a b

E

LUMO

3.40 eV

5.70 eV

W18O49-R6G

3.5 4.0 4.5

5.0 5.5 6.0

6.5 7.0 7.5

8.0

W18O49

PICT

PICT

GSCT

mol

CB

Vo

5.11 eV

7.71 eV VB

W18O49

R6G

HOMO

ex

R6G

Figure 6 | Charge transfer between R6G and W18O49. (a) Absorption spectra for R6G on W18O49 compared with neat W18O49 and R6G dye. (b) Energy-level diagram of R6G on oxygen-decit W18O49 measured in a vacuum.

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transfer of the molecule-to-semiconductor and semiconductor-to-molecule, respectively, which are both related to the defect states in the semiconductor. It can be evidenced that additional possible resonant contribute to the total enhancement in defect-rich semiconductormolecule system, compared with its non-defect counterpart. When these predicted multiplicatively coincide, large EFs are expected (Supplementary Fig. 4). Thus, the observed high SERS enhancement indicates that modulation of the surface vacancy of semiconductor substrate is a simple but effective means in design of a molecule-semiconductor system with high SERS enhancement.

DiscussionIn summary, metal-free SERS substrate with highly sensitive detection has been successfully fabricated employing nonstoichiometric tungsten oxide, W18O49 as an example, which achieved a limit detection as low as 10 7 M and EF up to3.4 105, an outstanding observation for semiconducting mate

rials and even comparable to noble metal without hot spots. The oxygen vacancy played a critical role in amplifying the spectroscopic signatures of probe molecules, since tungsten trioxide, WO3, only gave extremely weak signals. Moreover, the articial creation of oxygen deciencies by annealing the material substrate in inert or reducing atmosphere (Ar/H2) brought about further Raman enhancements, providing unambiguous evidence that the presence of oxygen vacancy, either intrinsic or post-created ones, could help magnify the Raman signals. The extremely high SERS sensitivity on semiconducting materials can probably be attributed to the presence of oxygen deciencies, either intrinsic or on the surface, and the resultant strengthened interaction with probe molecules via vibronic coupling. These observations provide important clues in the future strategy design of efcient semiconducting SERS substrate, and may bring important fundamental advance in material science and chemistry.

Methods

Synthesis of sea urchin-like W18O49 nanowires and WO3 nanoparticleson Si/SiO2. W18O49 nanowires in sea urchin-like morphology on Si/SiO2 substrate were prepared according to a recently published hydrothermal approach13. In brief, WCl6 (0.099 g) was dissolved in 30 ml absolute ethanol, which was transferred to a

Teon-lined stainless steel autoclave holding several horizontally oriented Si/SiO2 substrates. The autoclave was then sealed and heated at 180 C for 12 h. After nishing the reaction, the substrates were rinsed thoroughly with absolute ethanol and dried naturally at room temperature before use. The W18O49 nanowires on substrate were transformed to the corresponding trioxide form, WO3, by annealing in air. The temperature was set at 500 C and the sample was kept at the temperature for 1 h.

Modulating surface vacancy states. Modulation of the surface oxygen vacancy states was achieved through annealing the as-prepared W18O49 nanowires by heating in Ar/H2 atmosphere at 300 C for 1 h. The velocity of Ar/H2 was xed at 400 and 200 ml min 1, respectively.

Raman measurement. To study the Raman enhancement effect by the tungsten oxide materials, R6G dissolved in deionized water was employed to be the probe molecule because R6G has a large Raman scattering cross-section at the laser excitation wavelength applied for the SERS experiments, 532.8 nm (ref. 33). The tested concentration ranged from 10 4 to 10 7 M. Stock solution of 10 3 M was initially made, and solutions of other concentrations were obtained by successive dilution by factors of 101 or 102. After dropping an aliquot of the respective solutions on the substrate and drying for at least 5 h, Raman spectra were subsequently collected on a high-resolution confocal Raman spectrometer (LabRAM HR-800) using the same instrumental settings for ready comparisons. The excitation wavelength was 532.8 nm and a 50 L objective was used to focus

the laser beam. The spectra were acquired for 30 s with three accumulations and the laser power was maintained at 0.3 mW with an average spot size of 1 mm in diameter in all acquisitions. For each sample, Raman spectra from different areas were collected, and the signal intensity was averaged for nal analysis, from which relative s.d. values for EFs were estimated (Supplementary Methods and Supplementary Figs 58).

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (51372266), the Natural Science Foundation of Jiangsu Province (BK20130348) and Suzhou Industrial Science and Technology Programm (ZXG201426). F.G. acknowledges support from the National Natural Science Foundation of China (51402204), Thousand Young Talents Program and Jiangsu Specially-Appointed Professor Program. Y.Z. and Q.L. acknowledge the support of the National Natural Science Foundation of China (51202282) and National Basic Research Program by Ministry of Science and Technology (2011CB932600).

Author contributions

Z.Z. and F.G. conceived the project and designed the experiments. S.C., Y.Y., Z.C., J.H., M.Y. and Y.S. performed material synthesis, structural characterization and Raman

measurements. S.C., Y.Z., L.L., Q.L., F.G. and Z.Z. analysed the data. S.C., F.G. and Z.Z. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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How to cite this article: Cong, S. et al. Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies. Nat. Commun. 6:7800 doi: 10.1038/ncomms8800 (2015).

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