Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5

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An Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5 Hierarchical Microspheres for Surface Enhanced Raman Spectroscopy Yahui Li, Hua Bai, Junfeng Zhai, Wencai Yi, Junfang Li, Haifeng Yang, and Guangcheng Xi Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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Analytical Chemistry

An Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5 Hierarchical Microspheres for Surface Enhanced Raman Spectroscopy Yahui Li,† Hua Bai,† Junfeng Zhai,† Wencai Yi,‡ Junfang Li,† Haifeng Yang,† 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. Laboratory of High Pressure Physics and Material Science，School of Physics and

‡

Physical Engineering, Qufu Normal University, Qufu, 273165, China E-mail: [email protected]

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ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Compared with noble metals, improving the sensitivity of semiconducting surface-enhanced Raman scattering (SERS) substrates is of great significance to their fundamental research and practical application of Raman spectroscopy. In this communication, it is found that the SERS sensitivity is increased by 10000 times by reducing the semiconducting TiO2 microspheres to quasi-metallic Ti3O5 microspheres. Its lowest detectable limit is up to 10−10 M, which may be the best among the non-noble metal substrates and even reaches or exceeds certain Au/Ag nanostructures to the best of our knowledge. This new type of non-noble metal SERS substrate breaks through the bottleneck of poor stability of conventional semiconductor substrate and can withstand high temperature oxidation at 200 °C and strong acid-base corrosion without performance degradation. Benefiting from its excellent ability of visible-light photocatalytic degradation of organic molecules, the substrate can be reused. Moreover, the new material also exhibits excellent photothermal conversion properties.

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Introduction As a powerful analytical tool, surface-enhanced Raman spectroscopy (SERS) has been widely used in the detection of trace substances, catalytic intermediate screening, biological tissue imaging and other fields.1-5 The main advantages of this technique are as follows: 1. high sensitivity, which detection limit can even reach the level of single molecule; 2. detection time is very short, often a detection cycle is less than 10 s. Current studies have shown that the most important factor determining the performance of SERS is the properties of its substrate, except for the influence of excitation sources.6-10 The most widely used SERS-substrate material are Au nanomaterials.11-13 However, from the perspective of large-scale utilization, the high cost of Au substrate limits its promotion to a certain extent, such as entry-exit inspection and quarantine. Because of its very high Raman signal enhancement factor (EF), Ag is another widely studied noble-metal-SERS substrate material.14-17 Although the price of silver is only about one percent of the gold price, the main defect of Ag is its poor stability, which is especially easy to oxidize in air. In addition to these traditional noble-metal materials, more and more semiconductor SERS substrate materials have been reported recently,18-20 such as W18O49,21,22 Cu2O,23 ZnO,24 BN,25 TiO226-28 and so on. Recently our group also reported a series of non-noble metal SERS substrate materials, such as MoO229,30 and WO2.31 For semiconductor substrates, the main obstacles to their large-scale application are low sensitivity, which is mainly due to the different enhancement mechanisms between semiconductor and noble metal substrates.32,33 For Au, Ag and 3



other noble metal substrates, their enhancement mechanism is traditionally attributed to the localized surface plasma resonance (LSPR) effect caused by free electrons, that is, the so-called electromagnetic enhancement (EM).34,35 As for semiconductors, their enhanced scattering signal are believed to originate from charge transfer (CT) behavior between the substrate and the adsorbed probe molecules.36,37 Is it possible to possess EM and CT enhancement behavior on a single substrate? If feasible, this may be an effective way to improve the sensitivity of the substrate. In fact, this problem has perplexed researchers for a long time. Obviously, for noble metals whose electronic structure is almost unchanged, it is difficult for them to have the CT mechanism. For semiconductors, this is possible because their band gaps and free electron densities can be regulated by a variety of ways. In this communication, to answer this question and to find an effective way to improve the sensitivity of semiconducting SERS-substrates, an interesting hypothesis is proposed in the present work. As shown in Scheme 1, if a large number of oxygen vacancy (Vo) defects are introduced into a typical wide-band gap semiconductor (such as TiO2) in some way, the density of free electrons of the material will greatly increase due to the unsaturated oxygen coordination environment. Under specific exciting light irradiation, the free electrons may show a LSPR effect, and then lead to the EM enhancement behavior. At the same time, due to the narrowing of the band gap, the transfer of photon-generated carriers between the substrate and probe molecules will be promoted, thus improving the CT enhancement mechanism. That is to say, by introducing oxygen vacancy defects, typical semiconductors would be 4


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converted into quasi-metals, which may have both CT and EM enhancement capabilities. Experimental Section Large Scale Synthesis of Anatase-TiO2 Hierarchical Microspheres. 0.4 L of titanium isopropoxide was dissolved in a mixed solution of 2 L ethanol and 1.0 L of isobutanol. The resulting mixture was stirred evenly and transferred to a 5-L Teflon lined autoclave (Figure S1). The autoclave was heated to 180 °C and maintained for 10 hours at this temperature. When the reaction system naturally cooled to room temperature, the generated white flocculent products were collected. After washing and drying with distilled water and ethanol, the quality of these white products is about 105 g (Figure S2). Large Scale Synthesis of γ-Ti3O5 Hierarchical Microspheres. The obtained anatase-TiO2 powder (100 g) is evenly spread in two flat bottom alumina crucibles, and the two crucibles are placed in the chamber of a vacuum atmosphere furnace. The Ar gas was introduced into the chamber until heated to 500 °C at a heating rate of 5 °C min-1, then the hydrogen gas was injected into the chamber and held at 500 °C for one hour; the powder cooled naturally in atmospheric argon flow, and finally, the white TiO2 transformed to black Ti3O5 powder. Characterization. The obtained samples were systematically characterized by a variety detection technique. X–ray powder diffraction (XRD) patterns of the products were measured on a Bruker D–8 focus X–ray diffractometer by using CuKα radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) and high–resolution TEM 5



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(HRTEM) observations were completed with a Tecnai G F30 operated at 300 kV accelerating

voltage.

Scanning

electron

microscopy

(SEM)

images

and

energy-disperse X-ray spectrum (EDS) spectrums were obtained on a Hitachi S-4800 with a accelerating voltage of 15 KV. The X-ray Photoelectron Spectroscopy (XPS) measurments were performed in a Theta probe (ESCALab-250Xi Thermo Fisher) using monochromated Al Kα X-rays at hυ = 1486.6 eV. Peak positions were internally referenced to the C1s peak at 284.6 eV. UV–Vis absorption spectra were detected with a Shimadzu UV-3600. The Fourier transform infrared (FTIR) spectra were measured from THERMO Iz-10. The specific surface area was detected in a Micro Tristar II 3020. Raman Property Test. A laser-confocal-micro-Raman spectrometer (Renishaw-inVia) was used as the measuring equipment to detect the SERS properties of the as-prepared metallic γ-Ti3O5 hierarchical microspheres. In all the experiments mentioned in this work, if there is no definite indication, the used excitation wavelengths are all 532 nm, the magnification of the objective is × 50 L, and the excitation power is 1 mW. A series of standard solutions, such as Rh6G aqueous solutions with concentrations of 10-5-10-10 M were used as the probe molecules. To improve the signal reproducibility and uniformity of the SERS substrates, 0.05 g of the γ-Ti3O5 microspheres were dipped into a probe molecule aqueous solution (10 mL) to be measured and maintained for 10 min. After 5 minutes of sonication, the obtained uniform suspension was uniformly coated on a glass slide by spin coating (300 rm-1), and then dried in air for 5 min under the irradiation of an infrared light. In all SERS detections, the laser 6


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beam is perpendicular to the top of the sample to be tested with a resultant beam spot diameter of 5 m. Note: in order to obtain the SERS spectra of R6G molecules on a single γ-Ti3O5 microsphere, the suspension concentration of the γ-Ti3O5 microspheres should be as low as possible, no more than 1 mg mL-1. Results and Discussion Density of States and Free Electron Gas Distributions of γ-Ti3O5 and TiO2. In order to confirm whether the hypothesis illustrated in Scheme 1 is correct or not, TiO2 was used as the research object. TiO2 is chosen because it is one of the most common wide-band gap semiconductor, and several of its nanostructures have been proved to be relatively sensitive semiconducting SERS substrate.27,28 Therefore, if this hypothesis can be successfully demonstrated on TiO2 nanostructures, it will be highly representative. By reduction, TiO2 can be reduced to a variety of low valence titanium oxides, such as TiO and Ti3O5 and so on. Although the valence of Ti in TiO is lower, because of its poor oxidation resistance (would be oxidized in air at room temperature), we chose to reduce TiO2 to a stable non-stoichiometric compound, γ-Ti3O5.38 Figure 1a-c shows the projected density of states (PDOS) of anatase TiO2 and γ-Ti3O5. The PDOS of anatase TiO2 distinctly presents the semiconductor character with a wide band gap, and the highest occupied states mainly composed from O 2p orbitals. Compared with TiO2, the PDOS of γ-Ti3O5 shows an intrinsic metal character with the fermi level crossing some bands, which benefits from the raise of fermi level induced by oxygen vacancy defects, and most states of γ-Ti3O5 around the 7



fermi level were composed from Ti 3d orbitals. Furthermore, the density of the free electron gas distribution (Figure 1b-d), which was simulated by calculating the electron localization functions (ELF), indicates that the free electron gas density of γ-Ti3O5 is far higher than that of TiO2. The results of the first-principles calculation clearly show that γ-Ti3O5 contains a narrowed band gap and a large number of free electrons. This means that although γ-Ti3O5 is a semiconductor, it has some properties of metals, so we call it a quasi-metal. This dual property are the structural basis for the enhancement of EM and CT. Synthesis and Structure Characterization of γ-Ti3O5 Microspheres. Direct synthesis of γ-Ti3O5 nanostructure is very difficult, almost no records on γ-Ti3O5 nanostructure synthesis have been reported in the literatures. In order to get well-defined γ-Ti3O5 nanostructure, we designed a reasonable synthetic route. As shown in Figure 2a, firstly, we fabricated a hierarchical TiO2 microsphere assembled from ultrathin nanosheets in large scale by a simple solvothermal method. After washing and drying with distilled water and ethanol, the quality of these white products is up to 101.7 g (Figure S1). XRD pattern shows that these white product is composed of anatase-structured TiO2, and no other crystalline products have been detected (Figure S2). Raman spectrum also demonstrated that the white product is anatase TiO2 (Figure S3). Then, SEM observations show that these anatase-structured TiO2 are actually made up of microspheres with diameters of 5-10 m (Figure S4a). Furthermore, the enlarged SEM images revealed that these microspheres were actually assembled from a large number of ultrathin nanosheets with a thickness of 8


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only 2-3 nm (Figure S4b-c). By the way, the fragments ruptured by ultrasound indicate that the inside of these microspheres is also composed of ultra-thin nanosheets (Figure S5). Then, the obtained TiO2 hierarchical microspheres were reduced into γ-Ti3O5 by a temperature programmed H2 reduction method. After this reduction process, a black product appeared (Figure S6). The as-synthesized black product shows a positive temperature coefficient of resistance (PTC), which measured resistivity value is only about 7.6 × 10-3 Ω cm at 300 K measured by a pressing plate method (Figure S7), suggesting it possesses a feature of electrical conductivity of metal as expected. XRD pattern proves that this black product is γ-Ti3O5 (Figure S8). No other crystalline products were found. As shown in Figure 2b-e, SEM images show that these black γ-Ti3O5 retain the same morphological characteristics as the original TiO2 microspheres. HRTEM image (Figure 2f) demonstrated that the prepared γ-Ti3O5 nanosheets possesses excellent crystallinity, and the lattice stripes with a distance of 0.335 nm can be represented as (002) crystal faces of γ-Ti3O5. Figure S9 give the Raman spectrum of the as-synthesized γ-Ti3O5 hierarchical microspheres, compared with TiO2, it has more Raman scattering peaks, which indicates that its band structure is more abundant. The elementary composition of the black γ-Ti3O5 was further confirmed by energy-dispersive X-ray spectroscopy (EDS) (Figure 5g), which only contained Ti and O signals. The O/Ti atomic ratio is 1.71, which is highly consistent with the theoretical atomic ratio of Ti3O5. Fourier transform infrared spectroscopy (FTIR) characterization shows no absorption peaks were found except for the 9



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absorption peaks of water and titanium oxide, which suggests that the surface of the γ-Ti3O5 microspheres is very clean (Figure S10).

The nitrogen adsorption method

measurement revealed that the specific surface area of the γ-Ti3O5 hierarchical microspheres is up to 405.8 m2g-1 (Figure S11), which is an importance parameter in the performance measurement for the SERS-active nanoparticles since high surface area will greatly increase the adsorption capacity of the analysts. Furthermore, it should be noted that these sharp edges and gaps between the nanosheets are very conducive to generating strong Raman signals, because these areas tend to generate a large number of “hot spots”.39 Localized Surface Plasma Resonance Effect γ-Ti3O5 Microspheres. The valence state of Ti ions was investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3a, Ti2p spectrum shows that the sample contains both Ti4+ and Ti3+ (Ti4+/Ti3+ atomic ratio: 1/2). At the same time, O1s spectrum demonstrates that high concentration of oxygen vacancy defects exists in the lattice of these samples (Figure 3b). These oxygen vacancy defects are obviously caused by these unsaturated coordinated Ti3+ ions. At the same time, these unsaturated coordinated Ti3+ ions are also the source of a large number of free electrons. The effect of this high concentration oxygen vacancies is directly reflected in its absorption spectrum. As shown in Figure 3c, the black γ-Ti3O5 hierarchical microspheres present strong absorption in the range of 300-600 nm, and the strongest absorption occurs near 550 nm. In contrast, the white TiO2 microspheres did not absorb in the range of visible light. Obviously, this strong absorption is caused by the LSPR effect induced by the 10


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excitation of free electrons under light irradiation. This strong LSPR effect of the γ-Ti3O5 hierarchical microspheres will provide a powerful EM enhancement path for the following SERS tests. The presence of these high oxygen vacancy defects is also reflected in their photoluminescence (PL) behavior. As shown in Figure 3d, under the light excitation of 295 nm, these γ-Ti3O5 microspheres almost showed no fluorescence, which can be attributed to fluorescence quenching caused by oxygen defects. While for the TiO2 microspheres, their fluorescence emission is obvious. In addition, under the light excitation of 532 nm (the excitation wavelength is the same as that of Raman spectrum), these γ-Ti3O5 microspheres also showed no fluorescence (Figure S12), which can be attributed to the low energy of excited light. These XPS, UV-Vis, and PL characterizations undoubtedly confirm that these γ-Ti3O5 microspheres contain a large number of oxygen vacancy defects. On the other hand, the high concentration free electrons make the γ-Ti3O5 microspheres have excellent photothermal conversion properties. Photothermal Effect of the γ-Ti3O5 Microspheres. These γ-Ti3O5 hierarchical microspheres also exhibit excellent photothermal conversion properties. As shown in Figure 3e, the surface temperature of the film formed by microspheres rapidly rises from room temperature to 103 °C in 10 seconds under the simulated solar radiation (100 mW m-2). Under the irradiation of the 300W xenon lamp, the temperature of the aqueous solution containing these microspheres increased to 74 °C after 2 minutes (Figure 3f). This excellent photothermal conversion property can be reasonably

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attributed to the strong plasma resonance effect and optical absorption efficiency of these microspheres. Surface Enhanced Raman Spectroscopy Properties on One Single γ-Ti3O5 Microsphere. The SERS performance of one single γ-Ti3O5 microsphere was investigated by evaluating its sensitivity of Rhodamine 6G (R6G). As shown in Figure 4a, in order to obtain the SERS spectrum on a single γ-Ti3O5 microsphere, 10-7 M R6G aqueous solution contained very small amount of γ-Ti3O5 microspheres was dispersed on a piece of glass with clean surface. A laser beam with a wavelength of 532 nm is irradiated perpendicularly to the sample surface as excitation light (the experimental details please see Experimental Section). Figure 4b give a Raman mapping image of one single γ-Ti3O5 microsphere adsorbed R6G molecules, which shows a uniform distribution of SERS signals. The SERS spectra recorded in the three white box regions show that the obtained Raman scattering signals on this microsphere are very strong and can be completely indexed as the Raman spectra of R6G molecules (Figure 4c). In these Raman spectra, the most important 4 characteristic scattering peaks (R1, R2, R3, and R4) of R6G can be seen clearly. Furthermore, by analyzing the SERS peak intensity of R1 of 20 points randomly selected on the surface of the microsphere, the calculated RSD is only 4.1% (Figure 4d), which is very important for practical testing. When the concentration of R6G is reduced to 10-8 M, the γ-Ti3O5 microsphere substrate also shows strong Raman scattering signals (Figure 4e). Even when the 10-10 M R6G was used as the probe molecules, the microsphere still shows the effective response (S/N ≧ 3) to the 12


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detected object, which detection limit is probably the highest for non-noble metal SERS substrates. Moreover, in the broad range of 10-6 to 10-10 M, the Raman peak intensity (R1) of R6G has a good linear relationship with the concentration, which is very necessary for quantitative chemical detection. In addition, in order to identify the contribution of enrichment effect of the nanosheet microspheres to Raman signal intensity, we directly drop the R6G solution to the γ-Ti3O5 microsphere substrate. The results show that the LOD has indeed been reduced, but it can still reach 10-9 M (Figure S13), which indicates that the main enhancement effect is contributed by SERS of the γ-Ti3O5 microsphere. It should be noted that this γ-Ti3O5 microsphere SERS substrate not only has good sensitivity to R6G, but also has responsiveness to a variety of common harmful chemicals. For example, bisphenol A (BPA), a substance strictly prohibited in consumer goods for children, can be effective detected on this new kind of SERS substrate at 10-6 M level (Figure S14a). In addition, as a common environmental hormone, 2, 4-dichlorophenol (2, 4-DCP) can also be sensitively detected (Figure S14b). These results prove that the γ-Ti3O5 microsphere SERS substrate has good universality, which is very important for its application in practical detection. For pure TiO2 nanostructured materials without LSPR effect, such as TiO2 photonic microarray and TiO2 shell-based resonators,27,28] Alessandri group and Zhang group reported that they exhibited SERS activity based on nanostructure effect and chemical transfer mechanism. However, the reported lowest detection limit (LOD) is only 10-6 M for R6G molecules, and the sensitivity of these TiO2 nanostructure 13



substrates is needed to improve. By reducing the wideband gap TiO2 to quasi metallic γ-Ti3O5, the detection limit of R6G is improved to 10-10 M, which suggests that increasing the free electron density (that is LSPR effect) of non-noble metals is very helpful to improve their SERS sensitivity. For comparison, the intensity of Raman scattering signal decreased significantly when TiO2 hierarchical microspheres without LSPR effect were used as SERS substrate under 532 nm excitation light (black spectrum in Figure 4f). Furthermore, when the excitation wavelength is changed from 532 nm to 633 nm for the γ-Ti3O5 microspheres, the intensity of the Raman scattering peaks is also obviously weakened (blue spectrum in Figure 4f). However, this reduction in peak strength is a unified reduction, and there is no obviously change in the relative strength between peaks (compare the red spectrum and the blue spectrum), which clearly demonstrated that EM mechanism plays a dominant role in the γ-Ti3O5 microsphere substrate.40 On the other hand, the SERS spectrum of TiO2 microspheres dominated by the CT mechanism and the SERS spectrum dominated by the EM mechanism vary greatly (compare the red spectrum and the black spectrum in Figure 4f).41 Another key factor for the high SERS sensitivity of these quasi-metallic γ-Ti3O5 microspheres is that their LSPR wavelength (551 nm) is very close to the wavelength of the incident excitation light (532 nm), and strong resonance is easy to occur. In addition to sensitivity, considering that the SERS substrate materials are exposed to laser irradiation during the Raman testing process, their stability is also an important index to evaluate its performance. Recently, several non-noble metal substrates with SERS activity, such as W18O49 and Cu2O nanostructures,21-23 have 14


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been reported. However, because they contain a large number of intermediate valence ions (W4+, W5+, and Cu+), their chemical stability is poor, such as lower oxidation resistance and corrosion resistance, which limits their application in practical testing. Interestingly, although containing a large number of intermediate valence ions (Ti3+), the γ-Ti3O5 powders possess a high oxidation resistance and corrosion resistance. For example, after immersed in 8M sodium hydroxide or hydrochloric acid for 10 h, these γ-Ti3O5 microspheres still present almost invariable LSPR effect (Figure 5a). Even if heated to 200 °C for 5 h in air, the γ-Ti3O5 sample is still stable. XRD patterns and SEM images also demonstrated that the structure and morphology of the samples have not changed. Moreover, when these γ-Ti3O5 microspheres treated by corrosive compounds or high temperature were used as SERS substrates, the Raman scattering signal intensity was almost unchanged (Figure 5b). In terms of cost reduction and environmental protection, it would be significant if the SERS substrate could be reused. TiO2 is an excellent UV photocatalyst, which has excellent ability to decompose organic molecules. Interestingly, these γ-Ti3O5 microspheres exhibit better photocatalytic activity because of their strong light harvesting ability in the visible light region. As shown in Figure 5c, with these γ-Ti3O5 microspheres as catalysts, the R6G molecules contained in 10-5 M R6G aqueous solution (100 mL) will be completely degraded within 30 minutes under the visible light irradiation (300W Xe lamp with a 420 nm filter, 100 mW cm-2). Benefited from this highly efficient photocatalytic degradation of organic molecules, the γ-Ti3O5 microspheres can be used as a reusable SERS substrate (Figure 5d), which 15



would greatly reduce their cost of use. It should be pointed out that the 532 nm laser used for Raman spectroscopy excitation has too long wavelength and low power to cause photocatalytic decomposition of the molecule to be measured. As shown in Figure S15, controlled photocatalytic experiments showed that the concentration of R6G (10-5 M) does not change significantly even after 40 minutes of exposure to light irradiation (300W Xe lamp with a 525 nm filter, 45 mW cm-2). Considering that the power of xenon lamp is much larger than that of Raman spectrum, it is reasonable to think that as SERS substrate, the photocatalytic properties of these γ-Ti3O5 microspheres can not play a role. Conclusion In summary, a highly sensitive and stable SERS substrate was obtained by converting semiconducting TiO2 microspheres without LSPR effect into quasi metallic γ-Ti3O5 microspheres with strong LSPR effect. The new SERS substrate has an ultralow LOD of 10-10 M and is resistant to oxidation at 200 °C and corrosion by strong acids and bases. Moreover, the new SERS substrate can be reused benefited from it highly efficient photodegradation ability. This method to improve the sensitivity of SERS substrate is simple, reliable and universal, and is expected to be extended to other metal oxide systems, such as tungsten oxide (WO3 to WO3-x) and molybdenum oxide (MoO3 to MoO3-x). Acknowledgements This work received financial support from the Natural Science Foundation of China (51472226). 16


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Supporting Information Electronic

structure

calculations;

linear

relationship

measuring;

resistivity

measurement; photocatalytic property test; SEM images, XRD pattern and Raman spectrum of the TiO2 hierarchical microspheres; Raman spectrum, FTIR spectrum, specific surface area, I-V behaviors and XRD pattern of the γ-Ti3O5 hierarchical microspheres; SERS spectra of BPA and 2,4-DCP; PL spectra of the γ-Ti3O5 hierarchical microspheres obtained under the excition of 532 nm laser. References (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at A Silver Electrode. Chem. Phys. Lett. 1974, 26, 163-166. (2) Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry: Part I. Heterocyclic, Aromatic, and Aliphatic Amines Adsorbed on the Anodized Silver Electrode. J. Electroanal. Chem. 1977, 84, 1-20. (3) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science. 1997, 275, 1102-1106. (4) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature. 2010, 464, 392-395. (5) Lim, D. K.; Jeon, K. S.; Hwang, J. H.; Kim, H.; Kwon,S.; Suh,Y. D.; Nam, J. M. Highly Uniform and Reproducible Surface-Enhanced Raman Scattering from DNA-Tailorable Nanoparticles with 1-nm Interior Gap. Nat. Nanotech. 2011, 6, 452-460. 17



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(6) Qian, X. M.; Peng, X. H.; Ansari, D. O.; Yin-Goen, Q. Q.; Chen,G. Z.; Shin, D. M.; Yang, L. L.; Young, A. N.; Wang, M. D.; Nie, S. M. In Vivo Tumor Targeting and Spectroscopic Detection with Surface-Enhanced Raman Nanoparticle Tags. Nat. Biotechnol. 2008, 26, 83-90. (7) Zhu, C. H.; Meng, G. W.; Zheng, P.; Huang, Q.; Li, Z. B.; Hu, X. Y.; Wang, X. J.; Huang, Z. L.; Li, F. D.; Wu, N. Q. A Hierarchically Ordered Array of Silver ‐ Nanorod Bundles for Surface-Enhanced Raman Scattering Detection of Phenolic Pollutants. Adv. Mater. 2016, 28, 4871-4876. (8) Shen, W.; Lin, X.; Jiang, C. Y.; Li, C. Y.; Lin, H. X.; Huang, J. T.; Wang, S.; Liu, G. K.; Yan, X. M.; Zhong, Q. L.; Ren, B. Reliable Quantitative SERS Analysis Facilitated by Core-Shell Nanoparticles with Embedded Internal Standards. Angew. Chem. Int. Ed. 2015, 54, 7308-7312. (9) Schlücker, S. Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications. Angew. Chem. Int. Ed. 2014, 53, 4756-4795. (10) Kubackova, J.; Fabriciova, G.; Miskovsky, P.; Jancura, D.; Sanchez-Cortes, S. Sensitive

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Figures

Scheme 1. By introducing oxygen vacancies, wide-band gap semiconductors are transformed into quasi-metals with rich free electrons. As SERS substrates, they would have both EM enhancement caused by LSPR effect and chemical enhancement caused by CT route.

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Figure 1. Electronic structures of the anatase TiO2 and γ-Ti3O5. (a,c) The projected density of states of anatase TiO2 (a) and γ-Ti3O5 (c), the magenta dash line is fermi level. (b,d) The calculated ELF of anatase TiO2 (b) and γ-Ti3O5 (d), respectively. Green to red indicates the gradually increased charge localization.

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Figure 2. (a) The synthetic route of the γ-Ti3O5 hierarchical microspheres. (b-e) SEM images with different magnification of the as-synthesized γ-Ti3O5 hierarchical microspheres. (f) HRTEM image of the highly crystalline γ-Ti3O5 nanosheets. (g) EDS spectrum of the sample.

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Figure 3. (a,b) XPS spectra of the γ-Ti3O5 hierarchical microspheres. (c) UV-Vis absorption spectrum of the Infrared imaging based on temperature, which shows strong LSPR absorption near 550 nm. Inset: UV-Vis absorption of the anatase TiO2 hierarchical microspheres. (d) PL spectra of the γ-Ti3O5 hierarchical microspheres and anatase TiO2 hierarchical microspheres, respectively. (e) Infrared imaging based on temperature of the γ-Ti3O5 microsphere film. (f) Infrared imaging based on temperature of the γ-Ti3O5 microsphere aqueous solution (1 mg mL-1).

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Figure 4. SERS properties of the γ-Ti3O5 hierarchical microspheres. (a) Schematic diagram of Raman testing over a single γ-Ti3O5 hierarchical microsphere. (b) The Raman single mapping of the 10-8 M R6G adsorbed on one γ-Ti3O5 hierarchical microsphere. (c) Three Raman spectra obtained from the areas shown in the Figure 4b, which shows the uniformity of the Raman signals. (d) The calculated RSD obtained from randomly selected 20 points from a single microsphere. (e) There is a linear relationship between concentration and signal intensity in the range of 10-6-10-10 M. (f) The obtained SERS signals of R6G from different substrates and excitation wavelengths. 28


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Figure 5. (a) These γ-Ti3O5 microspheres with intermediate valence state show unexpected high oxidation resistance and corrosion resistance. (b) After high temperature and acid alkali treatment, these γ-Ti3O5 microspheres showed almost unchanged SERS activity. (c) The γ-Ti3O5 microspheres show excellent visible-light photocatalytic propeties. (d) These γ-Ti3O5 microspheres can be reused as SERS substrates by utilizing their outstanding capability of photocatalytic degradation organic molecules.

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For the TOC

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Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5

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