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Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies.
Cong S
,
Yuan Y
,
Chen Z
,
Hou J
,
Yang M
,
Su Y
,
Zhang Y
,
Li L
,
Li Q
,
Geng F
,
Zhao Z
.
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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 W18O49 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 × 10(5), in the rank of the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without ''hot spots''.
Figure 1. Characterization of W18O49 and annealed WO3 samples.(a) X-ray diffraction patterns of as-obtained W18O49 sample and annealed WO3 with all reflections perfectly indexed; (b) ultraviolet–visible profile 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 of 0.38 nm, which suggested that the nanowire growth was along the [010] direction.
Figure 2. Structure illustration for WO3 and W18O49 in [010] projection.(a) The crystalline WO3 structure consists of stacking of infinite corner-sharing WO6 octahedra layers, while (b) the defects in W18O49 structure lead to the formation of hexagonal channels.
Figure 3. SERS properties.(a) Raman profile 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.
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.
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.
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-deficit W18O49 measured in a vacuum.
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