Plasmonic MoO2 Nanospheres as a Highly Sensitive and Stable Non

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Plasmonic MoO2 Nanospheres as a Highly Sensitive and Stable Non-Noble Metal Substrate for Multi-Component Surface-Enhanced Raman Analysis Qiqi Zhang, Xinshi Li, Wencai Yi, Wentao Li, Hua Bai, Jingyao Liu, and Guangcheng Xi Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03385 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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Analytical Chemistry

Plasmonic MoO2 Nanospheres as a Highly Sensitive and Stable Non-Noble Metal Substrate for Multi-Component Surface-Enhanced Raman Analysis Qiqi Zhang†, Xinshi Li†, Wencai Yi‡, Wentao Li†, Hua Bai†, Jingyao Liu‡, and Guangcheng Xi†* †

Institute of Industrial and Consumer Product Safety, Chinese Academy of Inspection

and Quarantine, Beijing 100176, P. R. China. ‡

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical

Chemistry, Jilin University, Changchun 130023, P. R. China. Corresponding authors: [email protected] Keywords: metallic molybdenum dioxide, plasma resonance, nanostructures, Raman substrates, surface-enhanced Raman spectroscopy Abstract: Semiconductor-based surface-enhanced Raman spectroscopy are getting more and more attention because of their great price advantage. One of the biggest obstacles to the large-scale application of them is the poor stability. Here, we report that plasmonic MoO2 nanospheres can be used as a highly sensitive and stable semiconducting-substrate material for surface-enhanced Raman scattering (SERS). By using the MoO2 nanospheres as Raman substrates, a series of typical compounds with high attention can be accurately detected. This new non-noble metal substrate material shows a very high detection of limit of 10-8 M, and exhibits great near-field enhancement with one of the highest enhancement factor of 4.8 × 106 reported to date. More importantly, the oxide with intermediate valence displays unexpected ultrahigh stability, which can withstand the corrosion of strong acid and strong alkali as well as 1

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150 °C high temperature oxidation in air. Moreover, the accurate detection of multi-component samples was also successful on this substrate. These results show that some simple metal oxides with intermediate valence may become sensitive and stable SERS substrate materials due to their abundant free electrons and structure that easily causes hot spots. Introduction In 1974, Fleischmann et al. found that the pyridine molecules adsorbed on the roughened silver electrode could produce stronger Raman scattering signals.1 Three years later, VanDuyne and Creighton et al. have independently proved that the intensity of Raman signal of pyridine molecules adsorbed on the roughened surface of silver electrode were about 106 times stronger than that of pyridine molecules in the solution.2-3 The enhancement of Raman scattering phenomenon known as surface enhanced Raman scatting (SERS).4-5 At present, SERS has been developed into an advanced analytical technique, which has outstanding ability in the determination of organic and biological molecules,6-7 the identification of reaction intermediates,8-11 the detection of trace substances and so on.12-16 Under strictly controlled experimental conditions, its lowest detection limit (LOD) even can reach the impressive single-molecule level.17-18 It is generally recognized that one of the key factors determining the performance of SERS is the properties of the substrate materials.19-22 A excellent substrate material requires strong surface plasmon resonance (SPR) effect, high chemical stability, excellent repeatability, low cost and so on.23-25 Up to now nanoscale Au and Ag with strong localized-SPR effects are the most widely used 2


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SERS substrate materials.26-32 Although the price of Au is slightly higher, as a SERS substrate material, Au is almost perfect due to its strong localized-SPR effect and high chemical stability. As for Ag, it is not so lucky. Although the price of Ag is only about one percent of Au, its chemical stability is very poor, which is easily oxidized into Ag2O in air, especially when it is irradiated by laser. Ag is also very easy to react with sulfur compounds in the air to produce a layer of black Ag2S, which will greatly affect its stability. In order to find more SERS candidates, and to further understand the internal mechanism of SERS, in addition to Au and Ag, semiconductors especially metal oxides with nanostructures have been widely studied, including InAs/GaAs quantum dots,33 CdTe nanocrystals,34 TiO2 nanoparticles,35 CuO nanocrystals36 and so on. Although these semiconductor substrates showed a certain degree of SERS activity, their electromagnetic enhancement factors (EFs) are generally low (only 102-103), which are far from the requirements of practical applications. In recent years, with the discovery of a number of new localized-SPR active metal oxides (such as oxygen vacancies-rich WO2.83 nanorods37 and TiO2-x nanosheets)38, to a certain extent this dilemma has been improved. A breakthrough is the discovery of urchin-like W18O49 nanowires with a very high EF (3.4 × 105) and LOD (10-7 M for Rhodamine 6G (R6G)).39 However, the oxidation resistance of these oxygen vacancies-rich metal oxides is usually poor. For example, W18O49 is easily oxidized by air, even at room temperature. Once the oxygen vacancy contained in the W18O49 are occupied by oxygen atoms (namely W18O49 convert into WO3), its localized-SPR effect disappears 3



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rapidly. As another example, Guo et al. recently reported that Cu2O mesoporous spheres display outstanding SERS activity, which has an EF of 8 × 105 and a ultralow LOD of 10-9 for R6G.40 However, like W18O49, Cu2O is also easily oxidized in air, especially the oxidation is almost inevitable when it is exposed to the laser irradiation of the Raman spectrometer. Therefore, for the semiconductor-based SERS substrate materials, how to make it both have high EFs and high stability is one of the most urgent issue at present. As a common and inexpensive metal oxide, molybdenum dioxide (MoO2) is generally used as energy storage materials and catalysts,41-43 while other uses are rarely reported. Very interestingly, MoO2 has many special properties of metal oxides with intermediate valence do not have, such as high chemical stability (insoluble in strong acids and alkalis), high oxidation resistance, and high conductivity.44 High conductivity implies a large number of free electrons in MoO2, which is a necessary condition for the formation of the localized-SPR effect. Earlier theoretical calculations have also shown that MoO2 is a quasi-metal. The characteristics of high stability and high conductivity suggest that MoO2 is most likely to become a potential SERS substrate material with high EFs and high stability. Our recent work initially confirms this conjecture and shows that this material is indeed an excellent enhanced Raman substrate material.45 In order to further improve its detectable limit and identification of complex components, we have conducted a series of systematic studies on this new type of Raman substrate material.

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Here, we synthesized a novel MoO2 nanospheres with very rough surface and proved that it is a outstanding SERS substrate material, which not only has high sensitivity but also has high stability and multi-component detection capability. These MoO2 nanospheres are highly uniform on the morphology and exhibit a strong localized-SPR effect in the visible and near infrared (NIR) range. As a new non-metal SERS substrate, the MoO2 nanospheres achieve a very high EF of 4.8 × 106. A range of high-risk chemical substances, such as clenbuterol hydrochloride (CH, known as lean meat powder in China)46 and methyl parathion (MP, a common pesticide residue), can be accurately detected on this MoO2-based SERS. Compared with the results we recently reported,45 the lowest detectable limit (LOD) reported in the present work increased by an order of magnitude, reaching 10-8 M level, which may be the best among the non-noble metal SERS materials and even reaches or approaches to Au/Ag. More importantly, we have attempted to test a series of multi-component samples by using the MoO2 nanospheres as Raman substrate and obtained very good results. Experimental Section Chemicals All chemicals and materials used in the experimental process are analytic or chromatographic purity and used without further purification. Preparation of Plasmonic MoO2 Nanospheres In a typical synthesis process, 0.326 g (1 mmol) of molybdenyl acetylacetonate (MoO2(acac)2) was added into a mixed solution of distilled water (10 mL), absolute ethanol (30 mL), and isopropanol (IPA, 10 mL), and violently stirred for two hour at 5



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room temperature in air. Then, transfer the mixture to a Teflon-lined stainless steel autoclave and heat it at 180 °C for 8 h. After the reaction is completed, the autoclave is naturally cooled to room temperature, and the obtained black products were separated and collected by high speed centrifugation. Finally, the black powders were washed with ethanol and distilled water for three times and dried at 50 °C in a vacuum drying oven. Characterization The obtained samples were systematicly characterized by a variety detection techniques. 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 (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 Detection

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In order to evaluate the SERS properties of these plasmonic MoO2 nanospheres, a confocal-micro Raman spectrometer (Renishaw-inVia) is used as the characterizing instrument. In all SERS detections, the adopted excitation wavelength is 532 nm and the laser power is 0.5 mW, and the magnification of the objective is × 50 L. A series of standard solutions of highly risk chemicals, such as clenbuterol hydrochloride and methyl parathion with concentrations of 10-4-10-8 M were adopted as the probe molecules. To improve the signal reproducibility and uniformity, 10 mg of MoO2 nanospheres were dipped into a probe molecule aqueous solution to be measured for 15-20 min, and then taken out and dried in air for 2 h (For your attention: keep the drying temperature at room temperature without heating; otherwise, no uniform substrate will be obtained). In all SERS detections, the laser beam is perpendicular to the top of the sample to be tested with a resultant beam spot diameter of 5 µm. Results and Discussion Design of Synthetic Route. Although a lot of MoO3 nanostructure materials have been reported,47-51 there are few reports about MoO2 nanomaterials, which may be due to the fact that it is difficult to control the intermediate valence.52-53 In light of that, we have designed a reductive hydrothermal route to obtain MoO2. A schematic of the synthetic route is shown in Figure 1a. In short, acetylacetone molybdenum (MAA) is firstly dissolved in a mixed solution of water, ethanol (ET), and isopropanol (IPO) with suitable proportion. Then, the obtained mixed solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 8 h. Finally, a black precipitate

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(Figure S-1) was collected from the bottom of the autoclave. The whole experiment was done at ambient pressure, without the protection of inert gas. Crystal Phase and Morphology Characterizations. The phase composition and crystal structure of the obtained black precipitate was detected by powder X-ray diffraction (XRD). Among the common MoO2 of three types, the most stable monoclinic MoO2 has a deformed rutile structure with a space-group of P21/c (a = 5.6068 Å, b = 4.8595 Å, c = 5.5373 Å). In this structure, the O atoms densely packed into MoO6 octahedrons, while the Mo atoms occupy half space of the octahedral voids. The octahedrons are connected along the direction of the a axis to form arrays of octahedrons. The octahedron arrays are connected with each other by top-sharing to form the three-dimensional structure of MoO2 (Figure 1b). The detecting results demonstrate that all the diffraction peaks of our sample (Figure 1c) can be well-indexed as the monoclinic-phase MoO2 (JCPDS. 78-1069). It is should be noted that the diffraction peaks were obviously broadened, which indicates that the grain size of the sample is very small. Based on the maximum half peak width (MHPW) of the 101 diffraction peak, the grain size is calculated to be about 5 nm by the Scherrer equation. Furthermore, the data of laser-confocal Raman spectrum was also clearly demonstrated that the prepared black sample is really monoclinic-phase MoO2 (Figure S-2). In these Raman scattering peaks, the peaks located at 662, 818, and 991 cm−1 can be identified as the O-Mo bond vibration modes of monoclinic-phase MoO2, while the other scattering peaks located at 155, 243, 283, 335, and 378 cm−1 can be 8


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indexed as the phonon vibrations.43 SERS substrate material requires a very clean surface, otherwise it will result in serious interferences. Flourier transformation infrared (FTIR) spectroscopy proved that these synthetic MoO2 samples were very clean (Figure S-3). In addition to the peaks of H2O, CO2, and MoO2, no other infrared absorption was detected. These characterizations are fully demonstrated that the pure monoclinic-phase MoO2 has been synthesized by the facile reductive hydrothermal method. The micrograph of the MoO2 samples was firstly observed by scanning electron microscope (SEM). As shown in Figure 1d, the samples are composed of a large number of uniform spherical particles with a diameter of about 40 nm. High magnification SEM images further revealed that the surface of the nanospheres is very rough and full of sharp projections (Figure 1e and Figure S-4). The images of transmission electron microscope (TEM) also demonstrated that these nanospheres with rough surface are constructed with many small nanoparticles from inside to outside (Figure 1f-g and Figure S-5). These nanoparticles are about 3-8 nm in size, and they interact with each other to form many nanopores. Interestingly, the self-assembled nanospheres are not fragile and loose aggregations of the small nanoparticles, the spherical structure can be completely preserved even they were treated by ultrasonic dispersion for 30 minutes (Figure S-6). The very rough surface and the large number of pores are favorable features of SERS substrate configuration, because the sharp projections and nanoscale gaps will significantly enhance local electromagnetic field intensity and provide a large number of active "hot spots".54-56 9



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High

resolution-TEM

(HRTEM)

characterization

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displays

that

the

MoO2

nanoparticles have clear lattice fringes, suggesting a high crystallinity (Figure 1h). Energy-dispersive X-ray spectroscopy (EDS) measurements shown that that the sample contains only two elements of Mo and O (Figure S-7), and the ratio of them (O : Mo) is 2.14, which is highly consistent with the atom proportion of MoO2. Low-temperature N2 adsorption-desorption measured at -196 °C revealed that the Brunauer-Emmett-Teller (BET) surface area of the MoO2 nanospheres is 36.51 m2/g (Figure S-8). Oxidation State and UV-NIR Absorption. X-ray photoelectron spectroscopy (XPS) was used to detect the composition and valance of surface molybdenum atoms. The survey spectrum shown that the sample composed only of C, O, and Mo (Figure 2a). The weak carbon signal (C1s) located at 283.4 eV can be attributed to the adsorption of CO2 or organic molecules in the air, which is a common interference in the characterization of XPS. The other four strong signals can be referred to as Mo3d (232.08 eV), Mo3p (395.7 and 413.1 eV), and O1s (528.6 eV), respectively. In particular, Figure 2b displays a typical four-peak-shaped of Mo3d spectrum: a pair of strong peaks is accompanied by a pair of weak peaks.43 The pair of strong peaks located at 229.2 and 232.4 eV can be referred to the characteristic peaks of Mo4+ oxidation state, while the pair of weak peaks located at 231.3 and 234.6 eV can be attributed to Mo6+. The sharp contrast between the strong peaks and the weak peaks demonstrates that the amount of Mo6+ in the sample surface is almost negligible.

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Completely different from the only UV-active wide band-gap MoO3 (it only absorbs UV light at wavelengths less than 390 nm), interestingly, these MoO2 nanospheres exhibit strong absorption from near ultraviolet (NUV) to near infrared (NIR) regions, as shown in Figure 2c. All density functional theory (DFT) calculations on the band structure of MoO2 show that its Fermi level enters the conduction band level, and no obvious gap was found between the conduction band and valence band (Figure 2e), which strongly suggests that MoO2 possesses high density of free d-electrons (For details of the calculations, please see support information). Furthermore, the calculation results of electron localization functions (ELF) indicates that the free electron gas density of MoO2 is considerable high, and forms large number of nonpolar Mo-Mo metallic bonds in the MoO2 crystal lattice (Figure 2d). The results show that MoO2 has a certain degree of metallic characteristic. In contrast, the results of the first-principles calculation shown that MoO3 is a typical wide band gap semiconductor, which electrons are highly localized around the oxygen atoms and cannot form metallic bonds between each other (Figure S-9). By combining the calculated results with the UV-vis absorption spectrum, it can be reasonably concluded that this strong absorption from NUV to NIR is likely to come from the collective oscillations of d-orbital free electrons in the electromagnetic field, that is the localized-SPR effect. For comparison, the commercial MoO3 powders were not shown this strong absorption (Figure S-10). Resistance to Corrosion and Oxidation. In general, transition metal oxides with intermediate valence state are often easily oxidized in air, such as Cu2O, W18O49, and 11



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FeO. However, the cough MoO2 nanospheres exhibit unexpected high chemical stability. As shown in Figure 2f, after six months in air, the samples still remain the original color. Furthermore, controlled experiments shown that the MoO2 nanospheres maintains a strong SPR effect even after soaking for 5 hours in concentrated HCl or concentrated NaOH solution with a concentration of 6 mol L-1 (Figure 2g). As a sharp contrast, the recently reported two kinds of semiconductor-based SERS substrate active materials,39-40 urchin-like W18O49 nanowires and Cu2O nanoparticles with high enhancement factors (EFs, 3.4 × 105 for W18O49, 8 × 105 for Cu2O) can't bear the corrosion of strong alkali or acid (Figure S-11). Furthermore, the MoO2 nanospheres have surprisingly high oxidation resistance, which can even withstand 150 °C of heat in air without further oxidation (Figure S-12) and still exhibits a strong localized-SPR phenomenon (Figure 2h). The differential thermal analysis (DTA) curve of the MoO2 nanospheres clearly demonstrated its high oxidation resistance, which clearly shows that oxidation occurs when temperture higher than 280 °C (Figure S-13). By contrast, W18O49 has not been so stable. Its oxidation resistance is very poor, which would undergo an irreversible oxidation process even at room temperature in air (Figure S-14). Acid, alkali, thermal, and photochemical stability test results also show that the no detectable strength change was observed in the localized-SPRs of the MoO2 samples, which also demonstrates that the strong absorption from NUV to NIR regions cannot be attributed to the charge transfer of ligand to MoO2, because the ordinary ligands cannot withstand 150 °C in air or the corrosion of strong acid and alkali. 12


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Detection limit and Near-Field EFs. The combination of strong SPR characteristic and very high stability make the MoO2 nanospheres possible to be an excellent semiconductor-based SERS substrate material. The experimental results have confirmed the conjecture. Figure 3a shows the schematic diagram of the SERS experiment. These MoO2 nanospheres adsorbed with a certain concentration of Rhodamine 6G (R6G) molecules were uniformly dispersed in the glass slide, forming a layer of SERS active substrate. Under the excitation of laser with 532 nm wavelength, the R6G molecules of 10-6 M give a well-defined SERS spectrum (Figure 3b). In this Raman spectrum, the most important 4 characteristic scattering peaks (R1, R2, R3, and R4) can be seen clearly, and almost no background interference was found. In order to find out whether glass slides provide a contribution to the enhanced spectrum, these R6G samples were dispersed on the naked glass slide to detect their Raman signals. The results shown that no Raman signals of R6G were detected under the same conditions, which suggested that the glass slide did not play a role in the enhancement of the Raman signal. The contrast experiment also shown that only the Raman signals of MoO3 itself were detected when the MoO2 nanospheres were instead of MoO3 nanospheres. Considering these MoO3 nanospheres obtained by oxidation of the MoO2 nanospheres have completely lost the activity of localized-SPR (Figure S-15), it is reasonable to believe that the SPR-active MoO2 is the real contributor for the enhancement of Raman signals. The test data of different concentration R6G solution shown that this MoO2-based SERS technology has a very high detection sensitivity, and its LOD (the signal-to-noise ratio﹥3) even is up to 10-8 13



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M level (Figure 3c). Furthermore, it also achieved strong Raman ehanced signals when using the sample treated with acid and base or heated in air as the substrate (Figure S-16). In addition, after repeated three tests, the morphology and microstructure of these MoO2 nanospheres did not change (Figure S-17). The results futher suggest that the new Raman substrate has high stablity. As an important performance parameter of SERS, the EFs of the MoO2 nanospheres were determined by calculating the strength ratio of Raman characteristic peaks of R6G with three distinct concentrations (10-4, 10-5, and 10-6 M) in naked glass slides and MoO2 Substrates (Figure 3d). The Raman scattering peaks of R1 and R2 were selected as the calculation objects. In order to improve the accuracy of the calculated values, the intensity of each characteristic peak at each concentration is obtained by averaging 50 randomly selected measuring points. From the calculation results of the characteristic peak R1, it can be seen that the MoO2 material has a high electromagnetic field enhancement effect, which EF even reached 4.8 × 106 for 10-6 M R6G solution. As for R2, the highest EF also is up to 3.2 ×106. Although the EF of these MoO2 nanospheres is not prominent relative to the noble metal Au and Ag nanostructures, it is a breakthrough among the semiconductor-based SERS substrate materials (Table S1). Reproducibility. For practical SERS applications, the reproducibility of measurement data is another important indicator. In order to achieve high reproducibility, first of all, the uniformity of the substrate should be high. As shown in Figure 4a, the MoO2 nanospheres form a layer of very uniform substrate (50 × 50 µm) on the glass slide. 1 14


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× 10-7 M R6G was used as the probe molecules to detect the repeatability of the MoO2-based substrate. Firstly, 100 measuring point were selected randomly from the substrate, and the results showed that the measured values were highly consistent (Figure 4b). In order to further confirm the high repeatability of it more intuitively, the Raman spectra obtained from 3714 randomly selected measuring points from the area shown in Figure 4a were used to calculate their relative standard deviation (RSD). The SERS mapping over the designated area (contains 3714 measuring points) shows similar intensity distribution at different positions (Figure 4c), reflecting the very homogeneous chemical constitution of the R6G over the MoO2 substrate. The characteristic peak intensities of R1 obtained from the 3714 measuring points were shown in Figure 4d, which RSD is only about 6.9 %. Similarly, we also use the R2 intensities to calculate their RSD, and the results indicates that the RSD is still only about 8.8 % (Figure 4e-f). Furthermore, more data obtained from the different concentration of R6G solutions and different batches of substrates also indicate that the relative low RSDs were achieved on the MoO2 substrates (Figure S-18, 19). These results clearly demonstrate that the measured values obtained from the new type of SERS substrate are highly reproducible. Universality. At the same time, the MoO2 substrate shows a good universality in the detection of compounds. In addition to R6G, other common probe molecules, such as Rhodamine B (RhB), methyl blue (MB), methyl orange (MO), and acid fuchsin (AF) can also be sensitively detected on the MoO2 substrate (Figure S-20). More meaningfully, from the viewpoint of practical applications, the new SERS substrate 15



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also give a high sensitivity for the detection of hazardous compounds that are of particular concern to ordinary consumers. For example, clenbuterol hydrochloride (CH) is a common drug for the treatment of asthma. It also has a use as a feed additive to promote the growth of lean meat pigs, which is known as one of the "lean meat powder" in China. However, the meat containing CH is very harmful to human health due to hormonal effects. Although United States, European Union and China have all banned CH added to animal feeds, there is still a potential risk of contamination because of the huge economic benefits. By using the MoO2-based SERS technology, trace CH can be accurately detected even at the 10-7 M level (Figure S-21a,b). The other two kinds of "lean meat powder", terbutaline sulfate (TS) and albuterol sulfate (AS) also can be detected by this technique with high detection limit (Figure S-21c-f). Furthermore, as shown in Figure S22, methyl parathion (MP, a common pesticide residue) and 2,4-Dichlorophenoxyacetic acid (2,4-D, a common herbicide) also can be calibrated on the MoO2 substrate. Multi-Component Detection. In practical applications, the object to be detected often contains many components, which often cause serious interference to the Raman spectrum. Fortunately, the present MoO2-based SERS technology possesses impressive multi-component detection capability, in other words, the ability to resist interference.

MO,

MB,

and

R6G

of

10-6

M

were

used

as

complex

probe-molecular-system to demonstrate the multi-component detection ability. Over the MoO2 substrate (Figure 5a), the Raman mapping image of the three probe molecules show a uniform component distribution (Figure 5b). By investigating their 16


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Raman spectra with different probe assemblies, it can be seen that the three probe molecules can be well distinguished from the multi-component system (Figure 5c-d). Conclusions In

summary,

in

order

to

solve

the

problem

of

poor

stability

of

semiconductor-based substrates, we report that plasmonic MoO2 nanospheres can be used as a high stable and sensitive SERS substrate material. The MoO2 nanospheres were synthesized by a simple hydrothermal method, which exhibits a very strong localized-SPR effect from NUV to NIR regions. By using the SPR active MoO2 as Raman substrate, a range of highly concerned organic compounds, such as "lean meat powders", herbicides, and insecticides can be accurately detected. The lowest detectable limit (LOD) reaches 10-8 M level, and the maximum EF is up to 4.8 × 106. Compared with previously reported semiconductor-based SERS materials, the MoO2 samples display outstanding stabilities, which can not only resist the corrosion of strong acid and alkali, but also can withstand 150 °C heating in air without further oxidation. The current research results have fortunately broken through an obstacle (poor stability) in the application of non-noble metal-based SERS, and open the prelude for the highly stable MoO2-based semiconductor SERS technology. Supporting Information The calculation of EF, relative standard deviation data, electronic structure calculations, stability test data, SERS of more probe molecules, comparison table of EFs. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements 17



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Figures

Figure 1. Synthetic route, crystal phase, morphology, and microstructure of MoO2 nanospheres. (a) Schematic diagram of the synthetic route. (b) Crystal structure of monoclinic MoO2. (c) XRD pattern of the obtained MoO2 sample. (d-e) SEM images of the MoO2 nanospheres. (f-g) TEM images of the MoO2 nanospheres. (h) HRTEM image of the MoO2 nanoparticles.

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Figure 2. Oxidation states, UV-Vis absorption, band structure, and stability characterizations of the MoO2 nanospheres. (a-b) XPS spectra of the MoO2 samples. (c) UV-vis absorption of the MoO2 nanospheres. (d) Free electron gas density distribution of MoO2 obtained by electron localization functions (ELF). (e) The band structure of MoO2 obtained by all density functional theory (DFT) calculations. (f) After 6 months in air, the samples did not change color. (g-i) Acid, base, thermal, and photochemical stabilities of the MoO2 nanospheres.

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Figure 3. SERS tests of R6G on the MoO2 substrates. a) Schematic diagram of the SERS experiments. b) The SERS spectra of R6G on the different substrates. c) The SERS spectra of R6G with different concentrations. d) EFs at different concentrations.

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Figure 4. Confirmation of the signal repeatability and uniformity of the MoO2 substrate. (a) Optical photograph of the MoO2 nanosphere substrate. (b) SERS spectra of 10-7 M R6G gathered from 100 randomly chosen sites on the substrate. (c-d) The mapping and intensity distribution of R1 characteristic peak (612 cm-1). (e-f) The mapping and intensity distribution of R2 characteristic peak (773 cm-1).

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Figure 5. SERS detection of multi-component probe molecules using the MoO2 nanospheres. (a) Optical photograph of the MoO2-nanosphere substrate. (b) SERS mapping of the multi-components distribution on the substrate. (c-e) SERS spectra of the mixture of methyl orange, methyl blue, and R6G with different combinations, respectively.

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Table of Content

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Plasmonic MoO2 Nanospheres as a Highly Sensitive and Stable Non

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