preface
For example, previous researchers used supercritical CO2 to induce a phase transition, introduced CO2 into 2H-MoS2 to convert part of the MoS2 of the 2H phase into the 1T phase, and then introduced Au nanoparticles into the 1T-2HMoS2/Au heterostructure by sol-gel method in the 1T-2HMoS2 structure.
The excellent SERS performance of this heterostructure substrate has a detection limit of up to 10-10M, which is mainly attributed to the plasma resonance effect generated by gold nanoparticles and the efficient charge transfer between the transverse/longitudinally distributed 1T-2HMoS2/Au heterostructure and R6G, but its complex preparation process is not conducive to the large-scale application of transverse/longitudinally distributed 1T-2HMoS2/Au heterostructures.
Previous researchers used seed growth to obtain Ag nanowires from Ag nanospheres, and then prepared AgNWs@MoS2 core-shell composites by sol-gel method, and detected R6G and N719 dye molecules, and found that their concentration limits can reach 10-5M.
Although the SERS signal based on this core-shell structure is only enhanced by a few orders of magnitude, the ultra-thin 3nm MoS2 shell can protect the silver nanowires from oxidation, so the preparation of composite materials with high sensitivity can not only improve the SERS performance of the substrate, but also broaden the application range of the substrate.
In this paper, rose-like MoS2 nanoparticles were synthesized by hydrothermal method, and Au nanoparticles were grown on the surface of MoS2 by redox method to form MoS2@Au composite materials, and we used MoS2 and MoS2@Au as SERS substrates and R6G as probe molecules for trace detection, and by comparing the detection concentration limits of these two substrates, we elucidated the synergistic effect of EM and CM mechanisms to effectively improve the performance of SERS.
Experimental materials and methods
2.1 Experimental materials and equipment
Experimental materials: sodium molybdate, analytical pure, Tianjin Xiensi Biochemical Technology Co., Ltd.; Thioacetamide, Rhodamine 6G, Analytical Purity, Shanghai Aladdin Biochemical Technology Co., Ltd.; Tetrachloroauric acid tetrahydrate, analytical pure, Beijing Huawei Ruike Chemical Co., Ltd.; Absolute ethanol, analytical pure, Sinopharm Chemical Reagent Co., Ltd.; Deionized water, laboratory homemade, all chemicals are unpurified, direct use.
Characterization equipment: In this paper, scanning electron microscopy was used to characterize the morphology and elemental distribution of samples. In order to further explore the crystal structure of the sample, the above prepared sample was characterized by X-ray diffraction instrument, in which the voltage was 36kV, the scanning speed was set to 2 degrees/min, and the structure and molecular vibration information of the sample were characterized by confocal Raman spectrometer.
2.2 Experimental process and method
The first step, the preparation of rosette-like MoS2 nanoparticles, Dissolve 0.25g of Na2MoO4 powder in 60mL of deionized water, stir with a magnetic stirrer for 10min, then add 0.2gCH3CSNH2 powder, continue stirring for 30min, pour the evenly stirred solution into a stainless steel reactor with a lining capacity of 100mL, set the temperature of the drying box to 220°C, heat for 16h, and finally wash it ultrasonically with deionized water and ethanol, and after multiple centrifugations, put it into a vacuum drying oven and dry at 60°C for 12h to obtain black MoS2 powder.
In the second step, MoS2@Au the preparation of composite materials, weigh 0.152g of MoS2 powder with a balance, dissolve it in 70mL of deionized water, stir with a magnetic stirrer for 20min to form a uniform MoS2 aqueous solution, take 10mLMoS2 aqueous solution and add 20mL of HAuCl4 solution with a concentration of 1×10-3mol/L, stir well for 1h, and finally centrifuge dry to obtain MoS2@Au composites.
2.3 SERS performance test
First, R6G is used as the probe molecule, and different concentrations (10-4~10-10mol/L) of R6G solution are prepared with absolute ethanol as the solvent, and then, the same amount of ethanol is added to MoS2 and MoS2@Au composites to form a uniformly dispersed solution, before the SERS performance test, we drop MoS2 and MoS2@Au ethanol solution on a glass slide, and after ethanol volatilization at room temperature, drop different concentrations of R6G solution, After sufficient adsorption, MoS2/R6G and MoS2@Au/R6G systems containing different concentrations of adsorption can be obtained.
Finally, the confocal Raman spectrometer equipped with a 532nm laser was used to test the SERS performance of MoS2/R6G and MoS2@Au/R6G systems, in order to prevent the sample from being burned out, the irradiation power of the laser was set to 1%, and the acquisition time was 120s.
Results & Analysis
3.1 Topography analysis
Figure 1 (a) is the SEM diagram of MoS2, which clearly shows that MoS2 is rosette-shaped, with a diameter of about 200 nm, and Figure 1(b) is the SEM diagram of the MoS2@Au composite, which can clearly see a large number of Au nanoparticles on the surface modification of the rosette-like MoS2, and from the element distribution map of Figure 1(c), it can be seen that the Au element is evenly distributed on the surface of the rosette-like MoS2, at the same time, from Figure 1 (d) and (e) The elements Mo and S are also observed to be evenly distributed throughout the rosette-like MoS2.
As can be seen from Figure 2(a), Rosette-like MoS2 has a distinct layered structure with thin and sharp edges, and from Figure 2(b), it can be observed that Au nanoparticles grow on the edge of MoS2 rose petals, confirming the formation of MoS2@Au composites.
The illustration in Figure 2(b) is a selective electron diffraction (SAED) plot showing the diffraction rings of each crystal plane of the MoS2@Au composite, where the crystal plane spacing of 0.160 and 0.260 nm (red circle) corresponds to the (100) and (110) planes of MoS2, respectively, confirming that the prepared MoS2 is a hexagonal structure.
The crystal plane spacing of 0.240nm (yellow circle) corresponds to the (111) surface of Au, which is consistent with Au of the face-centered cubic structure, indicating that the structure of Au has not changed when synthesizing MoS2@Au composites.
Figure 1
Figure 2
The phase purity and crystal structure of the rosette-like MoS2 and MoS2@Au substrates were analyzed by XRD spectra, as shown in Figure 3, the characteristic diffraction peaks of MoS2 at 14.36°, 33.38°, 39.74°, 48.45°, and 59.15° correspond to the (002), (100), (103), (105) and (108) crystal faces of hexagonal phase MoS2 (whose JCPDS card code is #73-1508), respectively.
While the characteristic diffraction peaks of 38.23°, 44.42°, 64.61° and 77.53° came from the (111), (200), (220) and (311) crystal planes of the cubic phase Au (whose JCPDS card code is #12-1095), the XRD spectra of the MoS2@Au composite showed the characteristic diffraction peaks of MoS2 and Au, further indicating that the Au nanoparticles were successfully modified on the rosette-like MoS2 surface.
Figure 3
Considering the influence of Au nanoparticle modification on the structure of MoS2, Raman spectroscopy of MoS2 and MoS2@Au were performed, respectively.
Figure 4 is the Raman spectrum of rose-like MoS2 and MoS2@Au composites, both of which have two vibration modes, in-plane vibration (E12g) and out-of-plane vibration (A1g), and both have significantly unequal redshift, from the figure, it can be seen that the difference between the vibration frequency of E12g and A1g of MoS2 is 26.25cm-1, while MoS2 The vibration frequency difference between E12g and A1g in @Au decreased to 24.38cm-1, indicating that the interaction between Au and MoS2 enhanced the vibration between layers.
Figure 4
The Raman spectroscopy test of R6G powder was carried out, a small amount of R6G powder was taken on a glass slide, and the results obtained were shown in Figure 5, and we can observe that part of the Raman characteristic peaks of R6G molecules appeared at 609, 770, 1187, 1306, 1360, 1502, 1537, 1570 and 1646cm-1, and their characteristic peak attribution is shown in Table 1.
Among them, the characteristic peaks at 1360, 1502, 1570 and 1646cm-1 were all caused by the C-C bond telescopic vibration in the oxanthene ring, the characteristic peaks at 609cm-1 belonged to the C-C-C bond deformation in the torus, while the characteristic peaks at 770cm-1 were caused by the deformation of the C-H bond outside the torus, the peaks of 1187cm-1 were caused by the C-H bond in the xanthene torus and the N-H bond bending outside the plane, respectively, and the peak of 1306cm-1 corresponded to C= C double bond telescopic vibration, and the peak of 1537cm-1 corresponds to the expansion vibration of C = O double bond.
Table 1
SERS performance of 3.2 substrate
In order to explore the effect of gold nanoparticle modification on the sensitivity of MoS2 substrate SERS detection, we added different concentrations and equal amounts of R6G solution to MoS2 and MoS2@Au composite substrates, and the measured SERS spectra were shown in Figure 7(a) and (b).
Compared with MoS2 for R6G molecules detection limit of 5×10-6M, MoS2@Au composite can reduce the detection limit to 5×10-9M, significantly increased by 3 orders of magnitude, in addition, we selected 1360cm-1 peak as the characteristic peak to quantitatively analyze the change of SERS intensity, Figure 7(c) and (d) represent the natural logarithm of 1360cm-1 peak intensity in MoS2/R6G and MoS2@Au/R6G systems, respectively (In[ intensity] vs. logarithmic R6G concentration (-lg[CR6G]).
From Figure 7 (c), it can be seen that the intensity of I1360 has a good linear correlation with the concentration of R6G (10-4~10-6M), which meets the standard equation: y=10.775-0.896x, y represents the logarithm of I1360's intensity, x represents the logarithm concentration of the R6G molecule, and its correlation coefficient R2=0.7958, after the modification of Au nanoparticles, it can be seen from Figure 7(d) that the MoS2@Au/R6G system satisfies the standard equation: y= 12.303-0.600x, correlation coefficient R2=0.9817, closer to 1, the results show that the modification of Au nanoparticles can make the intensity of I1360 in the system and the concentration of R6G (10-4~10-9M) meet a good linear relationship.
Figure 7
SERS signal uniformity of 3.3MoS2@Au substrate
In order to detect the uniformity of SERS signal on the MoS2@Au substrate, an R6G solution with a concentration of 5×10-5M was used as the probe, as shown in Figure 6(a), the Raman spectrum obtained by randomly selecting 10 regions on the MoS2@Au substrate showed that the SERS enhancement effect of R6G on the MoS2@Au substrate had good consistency and high SERS intensity.
In addition, the Raman intensity of the characteristic peak at 1502cm-1 was statistically analyzed, as shown in Figure 6(b), and the relative standard deviation (RSD) was calculated to be 12.2%, indicating that the SERS signal of the MoS2@Au substrate had good uniformity, which confirmed that the prepared MoS2@Au composite was uniform and ordered in structural characteristics.
3.4 SERS enhancement mechanism diagram of the substrate
In order to reveal the Raman enhancement effect, we propose SERS enhancement mechanisms for MoS2/R6G and MoS2@Au/R6G systems, respectively, as shown in Figure 8, according to the literature, the lowest unoccupied molecular orbital energy (LUMO) and the highest occupied molecular orbital energy (HOMO) of R6G molecules are -3.4 and -5.7 eV, respectively, and the valence band (VB) and conduction band (CB) of MoS2 are located at -5.9 and -4.22 eV, respectively, and the Fermi level (Ef) of Au is -5.1eV.
As shown in Figure 8(a), when the 532nm (~2.33eV) laser irradiates the probe molecule R6G, the electrons in the HOMO energy level of the R6G molecule absorb light energy more easily to jump to the CB of the substrate MoS2, at this time effective charge transfer occurs between R6G and MoS2, because of its chemical enhancement mechanism to produce SERS enhancement effect, considering the modification of Au nanoparticles, the adsorption of R6G molecules is discussed in two situations, as shown in Figure 8(b,c).
Figure 8
The first case: when the R6G molecule is adsorbed on the surface of the Ag nanoparticle, as shown in Figure 8(b), using 532nm laser irradiation, the electron of the HOMO level of the R6G molecule is excited to the LUMO level, and the electron at the high energy level is unstable back to the Ef level of Au, and the thermal electron jumps from Au's Ef to the CB of MoS2 and finally back to MoS2's VB due to the LSPR effect generated on the surface of Au nanoparticles, thereby achieving charge transfer.
The second case: when the R6G molecule is adsorbed on the surface of MoS2, as shown in Figure 8(c), the electrons in the HOMO level of the R6G molecule are excited into the CB of MoS2 by laser irradiation at 532nm.
The LSPR effect generated on the surface of Au nanoparticles makes the thermal electrons jump from Au's Ef to MoS2's CB, and finally back to MoS2's VB, thereby realizing charge transfer electrons, according to the analysis of the above two conditions, it can be concluded that the LSPR effect of Au nanoparticles and charge transfer between substrate and molecules synergistic promote the improvement of SERS signal of R6G molecules.
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