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step Solution Processing of Ag

AbstractWe report on one step hybridization of silver, gold and palladium nanoparticles from solution onto exfoliated two dimensional (2D) Ti3C2 titanium carbide (MXene) nanosheets. The produced hybrid materials can be used as substrates for surface enhanced Raman spectroscopy (SERS). An approximate analytical approach is also developed for the calculation of the surface plasmon resonance (SPR) frequency of nanoparticles immersed in a medium, near the interface of two dielectric media with different dielectric constants. We obtained a good match with the experimental data for SPR wavelengths, 440nm and 558nm, respectively for silver and gold nanoparticles. In the case of palladium, our calculated SPR wavelength for the planar geometry was 160nm, demonstrating that non spherical palladium nanoparticles coupled with 2D MXene yield a broad, significanlty red shifted SPR band with a peak at 230nm. We propose a possible mechanism of the plasmonic hybridization of nanoparticles with MXene. The as prepared noble metal nanoparticles on MXene show a highly sensitive SERS detection of methylene blue (MB) with calculated enhancement factors on the order of 105. These findings open a pathway for extending visible range SERS applications of novel 2D hybrid materials in sensors, catalysis, and biomedical applications.IntroductionOwing to their physicochemical properties, 2D materials such as graphene, layered metal oxides, sulfides, and nitrides have attracted much attention in SERS1. Graphene has recently been studied the most due to its unique 2D structure, superior mechanical strength, and attractive electronic and optical properties2. The surface plasmon resonances (SPRs) of graphene, however, are found to be in the infrared and THz range, limiting its electromagnetically modified (EM) enhancement in the visible range SERS3. Nevertheless, attempting to get compensation from chemically modified (CM) enhancement, graphene has been hybridized by ex situ and in situ methods with various nanomaterials including polymers, metal oxides and metal nanoparticles (NPs). For this, numerous hydrothermal, wet chemical, electrochemical deposition, and photoreduction methods have been applied4,5,6,7,8,9. This can be useful not only in SERS, but also in biomedical, imaging, and sensing (even at a single molecule level) applications.In order to benefit from both EM and CM enhancements in the visible SERS applications, new 2D material hybrids are necessary. Some members of the MXene family, recently discovered early transition metal 2D carbides/nitrides10,11, are predicted to possess SPRs from infrared to ultraviolet (UV) range. MXenes are composed of layered P63/mmc symmetry hexagonal Mn+1XnTx structures, where M is a transition metal or a combination of such, X is either C and/or N, Tx stands for the functional terminations (such as O, OH and F), and n=1, 2 or 312. The plasmonic properties of MXenes are therefore dependent on the layers' size, number and stacking order, as well as functional terminations13,14,15. To date, layered MXene nanosheets have been proposed for many applications including super capacitors, batteries, hydrogen storage and hybrid devices16,17. Particularly, exfoliated 2D hybrids with nanoparticles and polymers can be used as building blocks for fabricating novel functional hybrid materials18,19. Among them, the most notable is a MXene Cu2O composite for catalytic applications, as well as a MXene poly vinyl alcohol (PVA) composite film, and a sandwich like structure of MXene/carbon nanotube (CNT) paper for application as highly capacitive battery anodes for energy storage20,21,22. The tunability of their chemical and physical properties renders MXenes very promising for hybridization with nanomaterials. Nevertheless, no MXene based plasmonic nanostructures of any kind have been reported so far. On the other hand, hybridizing NPs has exhibited advantageous properties for SERS in the past decade due to their high surface to volume ratio, surface control, and excellent plasmonic properties23,24,25. The single delocalized d electrons as well as surface roughness of NPs can selectively enhance electromagnetic radiation at specific excitation wavelengths. The enhancement factor can be further tuned by NP surface functionalization and grouping9,26. There have been numerous studies on colloidal solutions of NPs hybridized by graphene, 2D metal sulfides, and others27. Yoshimura et al. developed a soft processing (or soft solution processing) method with the aim to fabricate materials of desired shape, size, composition, and structure in solutions within a shorter time frame28,29,30,31. The method has already succeeded in producing large scale functionalized 2D carbon based materials for electrochemical and catalytic applications5,32. In this work, we extend this approach to achieve a one step environmentally benign hybridization of silver, gold and palladium NPs (Ag, van arpels and cleef necklace knock off Au and Pd) with Ti3C2Tx MXene in an aqueous solution, without an external reducing agent or surfactant, as illustrated in Fig. 1. We suggest a NP growth model and explain the plasmonic hybridization of NPs with MXene. We demonstrate the SERS applicability of the as synthesized materials on methylene blue (MB), a well known dye used as a probe molecule.ResultsFigure 2 shows ultra violet and visible (UV Vis) spectroscopy results on delaminated Ti3C2Tx MXene (0.04mg/mL concentration) and colloidal MXene NP hybrid suspensions derived from it. It can be seen from the photographs (inset of Fig. The broad UV absorption spectrum of the delaminated MXene in dilute aqueous medium exhibits peaks at 225 and 275nm in Fig. 2. This is expected because the surface of MXene was functionalized with different groups after etching out the A element from its precursor ternary transition metal carbide (so called MAX phase) during synthesis, and the flakes were delaminated by dimethyl sulfoxide (DMSO). The MXene colloid also exhibits high absorption in the UV region within the range from 225 to 325nm. This absorption may correspond to the band gap energy of the oxidized MXene, which was also predicted by theoretical calculations16,17. This indicates the presence of NPs (Ag and Au, respectively) in the colloidal solution. The SPR band for Pd NPs is usually observed in ultraviolet region33.To visualize the morphology of the as synthesized NPs on MXene, we performed transmission electron microscopy (TEM) analysis. Moreover, to highlight the constitution of these NPs, we simultaneously performed energy dispersive X ray (EDX) mapping and spectroscopy (EDS) on the same areas as analyzed by TEM. The cross section of layered MXene structure (Fig. 3a) and the distributed NPs (Ag, Au and Pd) on MXene flakes are shown in the TEM micrographs in Fig. 3b d, respectively. The bare MXene sample consists of single and few layer MXene flakes (Fig. 3a) with lateral sizes around 2 3m (Supplementary Fig. S1a). The corresponding high resolution TEM images of MXene indicate thickness from one to few nanometers (Supplementary Fig. S1b). The EDS spectrum and EDX replica van arpels and cleef necklace mapping reveal that the delaminated MXene consists of only Ti and C with no Al (Supplementary Fig. S1), suggesting successful etching. From the high resolution TEM micrographs (Supplementary Figs S2 S4), it is clear that the NPs were directly attached to the MXene. The size and shape of the NPs vary for different metals synthesized in this study. The Ag and Au NPs are rounded with sizes varying from 10 70 and 40 50nm, respectively. The size of Ag NPs on MXene is harder to control because the highly reactive Ag+ ions are likely to undergo rapid reduction under the ultrasonication even when using a low precursor concentration (0.1mM AgNO3 solution). Nevertheless, under the same experimental conditions, a more homogenous size distribution of Au NPs (40 50nm in average) was obtained, suggesting a slower reduction of Au3+ ions. S4), suggesting that the reduction step was different from that of the Ag and Au. The distribution and elemental EDX mapping analysis for each MXene NP hybrid confirmed the presence of the synthesized metal nanoparticles on MXene (Supplementary Figs S2 S4).To assess the structure of the metal NPs on the MXene, we performed X ray diffraction (XRD) analysis before (Fig. 4a) and after hybridization of MXene (Fig. 4b d) with Ag, Au, and Pd, as shown in Fig. 4. The XRD patterns of the delaminated MXene have very weak peaks comparable to those previously reported12. The disappearance of the non basal peak at 60 (2) suggests the full delamination of MXene and no crystallographic stacking of MXene sheets12. We also found intense peaks corresponding to the (111), (200), (220) and (311) planes of the Au and Ag NPs, which further confirm the successful hybridization. The nm range thickness of the planar NPs hinders the detection of reflected light from planes other than the (111). These observations are consistent with the shape analysis performed by TEM, in Fig. 3.Figure 4The powder X ray diffraction patterns of (a) delaminated MXene nanosheets, and after hybridization with (b) Ag, (c) Au and (d) Pd nanoparticles.The surface chemical bonding of the MXene and MXene hybrids were examined by X ray photoelectron spectroscopy (XPS). The individual core level high resolution XPS deconvolution spectra have been background corrected using the Shirley algorithm prior to curve resolution, presented in Fig. 5.As seen from the high resolution XPS spectra of Ti 2p, the O and C contribution mainly comes from the TiOx and carbon of TiC, respectively. Moreover, the peaks at 454.9eV (sp3) and 461.2eV (sp1) represent a contribution from Ti C bonds, and the peaks at 456.6eV and 462.7eV correspond to Ti O bonds. Figure 5b shows the 3d Ag core level spectrum with doublet peaks of Ag 3d5/2 and 3d3/2 of the two chemically distinguished spin orbit pairs observed at 368.4eV and 374.4eV, respectively. The low binding energy component (at 368.4eV) is a characteristic peak for electron emission from the Ag nanocore. The difference in binding energy (6eV) of this doublet clearly shows evidence of the Ag atoms present in the drop coated film. 5c) shows peaks at 84.7eV and 88.4eV in which the difference in binding energy (3.7eV) indicates a reduced form of Au0, further confirming the presence of Au nanoparticles in the sample. 5d). The back scattered light from planar Pd nanoparticles is, however, screened by the MXene flakes, causing the low signal to noise ratio. We also inspected the XPS deconvolution core level spectra of Ti 2p, C 1s and O 1s of MXene and MXene hybrids. A survey spectrum of MXene revealed the presence of Ti, C and O atoms (Supplementary Fig. S5). The C 1s and O 1s spectra of MXene suggest that after the delamination process, the MXene surface was predominantly functionalized with OH groups, the contributions from graphitized carbon, and carbides (Supplementary Figs S7 and S8, respectively). The functional groups of the delaminated MXene and MXene hybrids were further analyzed by Fourier transmission infrared (FTIR) spectroscopy in the attenuated total reflection (ATR) mode, as displayed in Fig. 6. The IR spectrum of the delaminated MXene has a vibrational mode at 3742cm1, which, due to the absence of atmospheric moisture (the samples were blow dried in N2 flow prior to the measurements), is assigned to the OH functional group out of plane vibrations of MXene. This is verified by first principles calculations of Ti3C2(OH)2 in the Ti3C2 block34. These are assigned to the OH/H2O adsorbed on the NPs surface, since these bands were not observed in the case of delaminated MXene flakes.DiscussionThe hybridization of exfoliated 2D nanosheets is potent not only in tailoring the physicochemical properties of hybridized species, but also in showing catalytic activity in concert with coupling between the 2D materials and NPs. Au NPs were previously deposited on delaminated MXene flakes with sodium borohydrate (NaBH4) as a reducing agent35. Moreover, it was observed that a homogeneous size distribution was achieved without a reducing agent by direct addition of Au salt in dark conditions or by photocatalytic assistance of UV irradiation35. Metal NPs (Pt) were also deposited directly on MXene flakes36. Herein, we propose a general in situ reduction method of noble metal NPs on MXene flakes by direct addition of metal salts in ambient air. Firstly, the effect of sonication on the hybridization of MXene flakes was systematically tested by performing treatments both with and without ultrasonication. The ex situ TEM analysis revealed that a homogeneous deposition of NPs (on individual MXene flakes) and subsequently, highly dispersed MXene hybrids (case of Au) were accomplished by ultrasonication. In the case when ultrasonication was not used, non uniformly deposited NPs on stacked MXene flakes (>4 nanosheets) were observed (Supplementary Fig. S9). Thus, we conclude that the ultrasonic mixing helped to obtain a more uniform NPs distribution on finer dispersed MXene nanosheets. 7, involving Ag DMSO complexes. We note that based on preliminary results of35 another salt reduction process without the DMSO complexes could possibly occur on the MXene flakes, either by or without photocatalytic activation. Nevertheless, we hypothesize hereby a mechanism involving DMSO intercalants attached to the few nm thick delaminated MXene flakes12 serving as reduction/nucleation sites for the NPs37. As metal ions (0.1mM Ag+) are added into the MXene colloidal solution under sonication, the formation of [Ag+ DMSO] complex monomers occurs in the MXene solution (stage 1). The [Ag+ DMSO] complexes cause an immediate color change by forming Ag+ DMSO (monomer) strong surface bonds with the OH functional groups of MXene. Thereafter, a fast electron transfer causes the formation of Ag+ [DMSO] from oxygen's lone pair electron ([S (CH3)2]) (stage 2). In the next step (stage 3), the charge transfer between Ag1 MXene complexes initializes the MXene Ag DMSO dimerization. Since the bond with the OH functional group is stronger, this MXene Ag dimeric complexes then undergo a further reduction and form stable (Ag0)n nano clusters (stage 4). This cluster formation leads to further nucleation and subsequent growth to form stable Ag NPs on the MXene surface (stage 5). Eventually, this MXene localized surface reduction mechanism yields plasmonic 2D substrates decorated by NPs that are highly dispersed in aqueous solution. The coverage of the NP can be further tuned by chemical manipulation of the OH functional groups on the MXene substrate.Next, to understand the nature of the noble metal NPs plasmon peaks observed by the UV vis spectroscopy, as shown in Fig. 2, we calculated in the limits of quasistatic approximation the SPR frequency of an Ag or Au sphere placed near the interface of two dielectric media with dielectric constants denoted as a and b. The case of Pd is discussed separately. We apply the imaging method38 to reduce the problem of the NP immersed in a medium near the MXene (b) sheet to a problem of two NPs in a homogeneous medium. For solving this reduced problem, we involve the approximate analytical approach developed for two spheres39. We consider a NP of a radius (R) in the range of 5 to 35nm where the developed quasistatic approximation van cleef and arpel clover necklace knock off is valid. Considering the charge distribution coefficient of the image NP, , (Supplementary information), we obtain the following relation:where y is the minimal distance between NP and MXene, () is the dielectric function of the NP depending on frequency . Eventually, inserting experimental values of dielectric constants15,40 in equation (1), we obtain a good match with the experimental data for SPR wavelengths, 440nm and 558nm, respectively for Ag and Au, in the case of a reasonable value of y/R0.015. Our calculations (Supplementary information) also show that the SPR linewidths of Fig.As seen from Fig. 3, the case of Pd structures deposited on MXene flakes drastically differs from that of the Ag and Au NPs. According to the TEM analysis results, we model these Pd nanostructures by highly oblate spheroids (disc shaped particles). By using Pd optical constants41 and our calculated value of the effective refractive index (Supplementary information), we obtain a SPR peak in the UV range at 160nm. The more sophisticated planar NP geometry and coupling can indeed yield a more red shifted SPR value, evident in Fig. 2. Nevertheless, when we model Pd NPs by spheres of the same size as those of the Ag and Au NPs, we obtain an SPR of 375nm, which is in accordance with the literature42. The absence of such a peak in Fig. 8a) and after NP hybridization (Fig. 8b d) by soaking the glass deposited samples in a 106M ethanol solution of MB and subsequently drying it. In order to improve the signal to noise ratio of the detected Raman signal, we use a hybridization of NPs with increased particle size and amount by adding a 5 fold raised (0.5mM) concentration of each precursor metal salt during the hybridization process. This could result in a fast reduction of the precursor, causing non uniform and larger size NP coating (possibly aggregated) on the MXene surface. Characteristic Raman peaks of partially oxidized Ti3C2Tx10,16 can be seen in Fig. 8a. The oxidation could have occurred due to reaction with ethanol or MB, or from the high laser power (35mW). Nevertheless, no Raman spectral features of MB can be seen in Fig. 8a even though MB molecules were adsorbed on MXene. This clearly comes from the absence of the NPs on the MXene surface, which are necessary for enhancing the Raman signal of MB. The characteristic peaks of MB around 443 and 1615cm1 which have been assigned to C N C skeletal bending and C C stretching, respectively, are evident in the SERS spectra for the three NP hybrids (with varying intensities), indicating that the molecules were adsorbed onto the substrates43. The SERS characteristic bands for (C S C) and (C S) at 559cm1 and 1181cm1, respectively, are observed suggesting that the chemisorbed MB molecules were attached to the MXene hybrids surface via sulphur metal bond, enriching C S bond intensities. 8c,d. The produced 2D MXene hybrid substrates are hence expected to exhibit high SERS enhancement. As a reference, 1% of liquid MB (in ethanol) was used to acquire a Raman spectrum for the calculations of the enhancement factors of 2D MXene hybrids samples (Supplementary Fig. S10). This suggests possible SERS applications of the proposed material.Figure 8(a) Raman spectrum of Ti3C2Tx after soaking in MB dispersed in ethanol and subsequent drying.In conclusion, herein we report a strategy for synthesis of noble metal nanoparticle MXene hybrids by one step soft solution processing. Through TEM and simultaneous EDX mapping analysis, we show that a uniform coating of Ag, Au and Pd NPs on MXene nanosheets was accomplished. The plasmonic nature of these novel 2D hybrid nanostructured substrates was demonstrated by surface enhanced Raman scattering (SERS). The advantage of this method is the in situ controlled synthesis approach, as well as the EM and CM combined nature, favorable for the SERS enhancement. Moreover, the simplicity of the solution based approach and the absence of reducing agents and expensive instrumentation allow exploration of other MXene NP hybrids and for applications including optical sensors (colorimetric detection), electrochemistry, catalysis and interface studies.MethodsSynthesis of aqueous MXene nanosheets colloidal solutionThe MXene powder of Ti3C2Tx was prepared by etching with a fluoride salt (LiF in HCl) at 35C for 24h from Ti3AlC2 (MAX phase). The obtained MXene powder was used for the production of colloidal suspension of MXene nanosheets by delamination of powder and subsequent bath sonication (Bransonic M3800 Ultrasonic bath, 110W and Frequency 40kHz). For delamination, 0.3g of MXene powder was mixed

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