MXene: Synthesis, Characterization, and Potential Application as

Aug 28, 2017 - Two-Dimensional Titanium Nitride (Ti2N) MXene: Synthesis, Characterization, and Potential Application as Surface-Enhanced Raman Scatter...
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Two-Dimensional Titanium Nitride (Ti2N) MXene: Synthesis, Characterization, and Potential Application as Surface-Enhanced Raman Scattering Substrate Bhuvaneswari Soundiraraju and Benny Kattikkanal George* Analytical, Spectroscopy and Ceramics Group, Vikram Sarabhai Space Centre, Thiruvananthapuram 695022, Kerala, India S Supporting Information *

ABSTRACT: We report on the synthesis, characterization, and application of Ti2N (MXene), a two-dimensional transition metal nitride of M2X type. Synthesis of nitride-based MXenes (Mn+1Nn) is difficult due to their higher formation energy from Mn+1ANn and poor stability of Mn+1Nn layers in the etchant employed, typically HF. Herein, the selective etching of Al from ternary layered transition metal nitride Ti2AlN (MAX) and intercalation were achieved by immersing the powder in a mixture of potassium fluoride and hydrochloric acid. The multilayered Ti2NTx (T is the surface termination) obtained was sonicated in DMSO and centrifuged to obtain few-layered Ti2NTx. MXene formation was verified, and the material was completely characterized by Raman spectroscopy, XRD, XPS, FESEM-EDS, TEM, STM, and AFM techniques. Surfaceenhanced Raman scattering (SERS) activity of the synthesized Ti2NTx was investigated by fabricating paper, silicon, and glass-based SERS substrates. A Raman enhancement factor of 1012 was demonstrated using rhodamine 6G as the model compound with 532 nm excitation wavelength. Detection of trace level explosives with a simple paper-based SERS substrate with Ti2N (MXene) as active material was also illustrated. KEYWORDS: MXene, Ti2NTx, 2D materials, SERS, trace detection

O

element), and X is carbon and/or nitrogen. MAX phases possess hexagonal crystal structure with P63/mmc symmetry, where M layers are nearly close packed with X atoms at octahedral sites and adjacent M−X layers interleaved with A layers. This intrinsic nanolayered structure bestows a unique combination of metal-like and ceramic-like properties, which unwraps its potential for diverse applications.7 More than 70 MAX phases and a still larger number of solid solutions of MAX phases are known to exist.8,9 However, MXenes synthesized to date include only Ti2C, Nb2C, V2C, Mo2C,

wing to the unprecedented wealth of properties, twodimensional (2D) materials have acquired tremendous interest among the scientific community. The quest for exploring 2D materials and its properties commenced when graphene was discovered by Novoselov, Geim, et al.1 Hexagonal boron nitride (BN),2 transition metal dichalcogenides (TMDs),3 metal oxides, and hydroxides4 are a few 2D materials investigated other than graphene.2−5 In 2011, Naguib et al.6 discovered a new family of 2D materials named “MXenes”, which are produced from their corresponding three-dimensional (3D) MAX phases upon selective etching of the A element with a suitable etchant. MAX phases are layered ternary metal carbides, nitrides, or carbonitrides with the general formula Mn+1AXn where M is an early transition metal, A is an A group element (mainly 13 and 14 group © 2017 American Chemical Society

Received: May 5, 2017 Accepted: August 28, 2017 Published: August 28, 2017 8892

DOI: 10.1021/acsnano.7b03129 ACS Nano 2017, 11, 8892−8900

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Figure 1. (a) Raman spectrum of Ti2AlN (MAX) and Ti2N (MXene), (b) XRD pattern of Ti2AlN (MAX) and Ti2N (MXene), (c and d) FESEM image and EDS spectrum of Ti2AlN (MAX), (e and f) FESEM image and EDS spectrum of Ti2N (MXene).

Ti 3 C 2 , Zr 3 C 2 , Hf 3 C 2 , Ta 4 C 3 , Ti 4 N 3 , Nb 4 C 3 , Ti 3 CN, (Ti0.5V0.5)2C, (Mo2Ti2)C3, (Ti0.5Nb0.5)2C, (V0.5Cr0.5)3C2, (Nb0.8Ti0.2)4C3, (Nb0.8Zr0.2)4C3, Mo2TiC2, Mo2Ti2C3, and Cr2TiC2.5,8−13 In MAX phases, the Mn+1Xn layers are characterized by strong covalent M−X bonds, interleaved with A atoms through weaker M−A bonds.7 Exploiting the difference in the nature of the bonds, it is possible to selectively etch the A layers to generate Mn+1XnTx, resulting in weakly bound stacks of 2D sheets. The resulting M−X layers are terminated depending upon the etchant and solvent used for exfoliation and delamination. The termination generally is denoted as Tx in the general formula Mn+1XnTx.5,6 Selective etching of A layers is generally carried out by treating the precursor with an aqueous solution containing fluoride ions, such as aqueous hydrofluoric acid (HF), ammonium bifluoride, a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl), or molten salts of fluoride to yield MXenes.5,9,12,14 Delamination of layers after etching is usually done by immersing it in dimethyl sulfoxide (DMSO), tetrabutyl ammonium hydroxide, alcohols, choline hydroxide, or n-butylamine.5,12,15 MXenes are considered potential candidates for a myriad of applications ranging from electrochemical energy storage materials (lithium-ion and sodium-ion energy storage systems), conductive additives to polymers, water purification, sensors, electronic devices, antibacterial materials, and catalysts.12 Among all the applications listed, most explored is their use in energy storage applications, and Ti3C2Tx is a widely studied MXene.5,10 In contrast to carbides and carbonitrides MXenes, nitride-based MXenes possess several advantages due to their higher electronic conductivity. Transition metal nitrides are considered to be promising candidates for plasmonics, electrochemical capacitors, optics, and metamaterial devices.9 However, nitride-based MXenes remain unexplored due to the intricacy in synthesis and due to their low stability in etchants such as HF. Ti4N3 is the only reported MXene to date, which was synthesized at higher temperatures using a mixture of fluoride salts.9,12

Herein we report the synthesis of Ti2N (MXene) from its 3D counterpart, Ti2AlN. Selective etching of Al and intercalation of layers were achieved by immersing Ti2AlN in a potassium fluoride (KF)−hydrochloric acid (HCl) mixture. The multilayered MXene (ML-Ti2NTx) obtained thereafter was delaminated in DMSO. We explored the sensitivity of Ti2NTx for fabrication of a surface-enhanced Raman scattering (SERS) substrate. Although SERS application of Ti3C2Tx MXene hybrids with silver, gold, and platinum nanoparticles was reported by Elumalai et al.,16 the activity of MXene alone as a substrate material for SERS is yet to be explored. The synthesis of Ti2N (MXene) and its characterization and application for fabrication of a SERS substrate to detect trace levels of explosives are discussed in this paper.

RESULTS AND DISCUSSION Synthesis of Two-Dimensional Ti2N (MXene). The 3D MAX phase precursor Ti2AlN was synthesized by heating titanium and aluminum nitride in an argon atmosphere up to 1500 °C. The density of the as-synthesized sample was 4.26 g cm−3, which is close to the theoretical density of Ti2AlN (4.31g cm−3).8,19 The average particle size of the powder analyzed by light-scattering technique was 11.8 μm (Supporting Information, Figure S1). Formation of Ti2AlN was confirmed from Raman spectral and X-ray diffraction (XRD) analyses (Figure 1a and b). The four Raman-active modes (2E2g + E1g + A1g) of Ti2AlN were observed at ∼145(ω1), 225 (ω2), 230 (ω3), and 360 cm−1 (ω4).17 In general, scattering in the optic range (400−650 cm−1) is governed by the vibrations of lighter ions, whereas the scattering in the acoustic range (150−300 cm−1) is determined by vibrations of heavy ions. Since, the X atom’s vibrations are not involved in Raman-active modes of 211-type MAX phases,17,18 no peaks were observed above 400 cm−1. E2g modes of Ti2AlN emanate from the shear vibrations of Ti and Al atomic planes, whereas E1g and A1g modes arise from the parallel and perpendicular vibrations of Ti atomic planes. The XRD pattern (Figure 1b) confirms the presence of Ti2AlN along with traces of unreacted titanium and TiAl as a phase 8893

DOI: 10.1021/acsnano.7b03129 ACS Nano 2017, 11, 8892−8900

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Figure 2. Characterization of Ti2NTx (MXene) after delamination with DMSO: (a) low-magnification TEM image; inset: SAED pattern; (b) HRTEM image; (c) FESEM image; (d) EDS spectrum of FL-Ti2NTx; (e) STM image; (f) AFM topography image; (g and h) line profiles across the solid and dashed line in image (f).

impurity.19 The c-lattice parameter (c-LP) of synthesized Ti2AlN was 13.59 Å, which is in close agreement to the c-LP of Ti2AlN reported.8,19 A field emission scanning electron microscopic (FESEM) image (Figure 1c) shows the typical layered platelet morphology of MAX phases, and the energy dispersive X-ray spectrum (EDS) (Figure 1d) confirms the presence of Ti, Al, and N. The peak at ∼2.5 keV arises from gold sputter-coated on the powder. The exact stoichiometry of Ti2AlN could not be verified using EDS analysis due to the strong peak overlap of nitrogen−K (0.392 keV) and titanium− Lα (0.395 keV) lines.20,21 Since Naguib22 reported futile attempts to synthesize Ti2N using 10% and 20% HF, we chose to employ 5% HF in this study to exfoliate the layers. Immersion in 5% HF for 24 h resulted in the formation of oxides on the surface along with a fused morphology (Supporting Information, Figure S2a and b). Thus, in order to prevent the formation of oxides and deflocculation of Ti2N layers after removal of Al, an alternate short-duration approach with heating and sonication was adopted. Ti2AlN was immersed in different etchants, viz., HF, a HF−HCl mixture, KF, and a KF−HCl mixture for 3 h followed by heating for an hour at 40 °C and sonication. Treatment with other etchants except the KF−HCl mixture formed a white precipitate during the process. The precipitate was separated and analyzed by Raman spectroscopy, and the powder after washing was examined by FESEM−EDS. FESEM images and EDS spectra of the powder obtained after etching with HF, the HF−HCl mixture, and KF are included in the Supporting Information (Figure S2). The FESEM image reveals that there was no evident exfoliation with HF (Supporting Information, Figure S2c), which could be due to the shorter duration of the treatment. Immersion in the HF− HCl mixture and KF resulted in the loss of the layered morphology typical of that of MXenes along with formation of nanostructures on the surface (Supporting Information, Figure S2e and g). The Raman spectrum of the white precipitate confirms the formation of titanium dioxide (anatase) (Supporting Information, Figure S3), as reported by Naguib.22 From these investigations, it could be concluded that the KF− HCl mixture might serve as a suitable etchant to remove Al. Unlike HF, not only will the KF−HCl mixture act as an

etchant, but K+ ions with radii 138 pm may also intercalate and weaken the M−X layers. The exfoliation and intercalation were confirmed from the Raman spectrum and XRD pattern. The powder obtained after treatment with the KF−HCl mixture is determined to be multilayered Ti2N (ML-Ti2NTx). Only two broad peaks at 205 and 335 cm−1 corresponding to E1g and A1g modes were observed in the Raman spectrum of Ti2AlN after treating with the KF−HCl mixture. The disappearance of E2g modes along with a red-shift in the E1g and A1g modes in Figure 1a (top spectrum) indicates the exfoliation in the Ti2AlN phase. During the process of etching, Al atoms are interchanged with lighter atoms whose vibrations are often suppressed by strong E1g oscillations and are not observed in the spectrum. To get insight into the nature of surface terminations, the IR spectrum of the powder was recorded (Supporting Information, Figure S4), which clearly confirms the presence of −OH, −F, and −O− groups in MLTi2NTx. The absence of an intense peak from the (103) plane along with a downshift in the peak position of the (002) plane from 2θ = 13.07° to 10.2° is clear evidence of exfoliation of Al from Ti2AlN. The sharp peak at 2θ = ∼25° due to the (004) reflection in Ti2AlN was not observed after treatment; instead a broad peak between 2θ = 15° and 30° with the peak maxima at ∼20° was observed, which is assigned to the reflection from the (004) plane. Removal of Al was further confirmed by an increase in c-LP from 13.59 Å to 17.33 Å. Unlike HF-etched MXenes, the FESEM image (Figure 1e) showed the stacked layers typical of that of MXene’s morphology observed when a mixture of fluoride salt and acid (i.e., LiF and HCl)12,23,24 was employed as etchant. It is reported23,24 that the morphology of MXenes etched with LiF and HCl appears compact without any visible delamination due to Li+ intercalation. In the present investigation, when the KF−HCl mixture is used to remove Al, water and/or K+ ion intercalates between the layers and results in stacked layers, and the EDS spectrum (Figure 1f) confirms the presence of O and F along with Ti and N moieties. The possibilities of unreacted precursor and oxides of Al and Ti were eliminated, as there were no characteristic peaks or reflection in the Raman spectrum and XRD pattern. However, Al was present in a few spots in the EDS spectrum recorded from other particles after treatment with the KF−HCl mixture 8894

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Figure 3 shows the X-ray photoelectron spectrum (XPS) and deconvoluted high-resolution Ti 2p, F 1s, N 1s, and O 1s

(Supporting Information, Figure S5), which could be attributed to traces of impurity or byproducts that could not be removed completely at this stage. The supernatant, after treating with a KF−HCl mixture containing the byproduct, was characterized using Raman spectroscopy. The Raman spectrum confirms the presence of AlCl3 (Supporting Information, Figure S6a), along with a complex spectrum with characteristic vibrations of metal fluoride (Al−F), titanate anion, and −OH (Supporting Information, Figure S6b). It was observed that etching with a KF−HCl mixture produced a mixture of fluoride and chloride salts. However, on the basis of the above results possible reactions of Ti2AlN with the KF−HCl mixture could be summarized as follows: 2Ti 2AlN + 6KF + 6HCl = 2Ti 2NTx + K3AlF6 + AlCl3 + 3KCl + 3H 2↑

2Ti 2AlN + 6KF + 6HCl = 2Ti 2NTx + 2AlF3 + 6KCl + 3H 2↑

where Tx could be −OH/−F/−O− as indicated from IR and EDS spectra. ML-Ti2NTx was then delaminated in DMSO to obtain few-layered Ti2N (FL-Ti2NTx). The delamination in DMSO could be attributed to the co-intercalation of moisture along with solvent during sonication as reported by Naguib et al.5 A detailed investigation on the kinetics and mechanism of exfoliation is in progress. The transmission electron microscopic (TEM) image indicates the transparency of sheets to electrons (Figure 2a), which substantiates the formation of FL-Ti2NTx. A considerable fraction of the sheets were found to be wrinkled and curved at their edges in TEM, FESEM, and atomic force microscopic (AFM) images (Figure 2a, c, and f), proving their flexibility similar to graphene.6,25 The selected area electron diffraction (SAED) pattern shown as an inset in Figure 2a proves that the flakes retain the hexagonal symmetry of the parent MAX phase after delamination. The high-resolution TEM (HRTEM) image in Figure 2b shows the cross section of stacked layers in FL-Ti2NTx, and the gap between the layers compliments the removal of Al. The delaminated Ti2N shows a stacked lamellar morphology (Figure 2c) with lateral dimensions in the micrometer range. Interestingly, the small particles seen in the FESEM image after treating Ti2AlN with a KF−HCl mixture (Figure 1e) were not observed in FL-Ti2NTx (Figure 2c), and the EDS spectrum showed only the presence of Ti, N, O, and F moieties. To ensure the absence of Al in FLTi2NTx, it was subjected to inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis, and the results showed the absence of Al. The scanning tunneling microscopic (STM) image in Figure 2e shows the hexagonal close packing (top view) of titanium in FL-Ti2NTx. The height profiles in Figure 2g and h along the solid and dashed lines in the AFM topography image (Figure 2f) indicate the existence of 1 to 3 layers of MXene. An increase in the specific surface area from 1.03 m2 g−1 (Ti2AlN) to 12.6 m2 g−1 (ML-Ti2NTx) upon etching and to 32.4 m2 g−1 (FL-Ti2NTx) upon intercalation compliments the delamination of the layers. The four-probe conductivity measurements of discs of 13 mm diameter and 2 mm thick Ti2AlN and FL-Ti2NTx were carried out and were found to be 28570 and 4950 S cm−1. The drastic decrease in conductivity is attributed to the removal of metallically bonded Al layers in Ti2AlN.

Figure 3. XPS spectra of delaminated MXene: (a) wide survey spectrum; deconvoluted high-resolution spectra of (b) Ti 2p, (c) F 1s, (d) N 1s, and (e) O 1s.

spectra of FL-Ti2NTx. The peaks at 684.05, 530.70, 458.60, and 396.50 eV in the wide survey spectrum (Figure 3a) confirm the presence of Ti, F, N, and O moieties,26 and their deconvoluted high-resolution spectra are shown in Figure 3b to e, respectively. Atomic concentrations (%) of the elements were quantified from a wide spectrum with respect to their peak intensity. The binding energies (BEs) and their corresponding peak assignments and fraction of individual components calculated from high-resolution XPS spectra are tabulated in Table 1. The BE of Ti 2p3/2 is higher than its MAX phase precursor27 (454.40 eV), indicating the replacement of Al layers by electronegative groups such as −F, −O−, and −OH. The presence of the peak in F 1s at 684.70 eV corresponding to Ti− F compliments the removal of Al and formation of Fterminated Ti2NTx. The percentage of Al is negligible in the XPS analysis and EDS spectral analysis (Figure 2d), and ICPAES analysis of delaminated MXenes showed no traces of Al. The possibility of the existence of TiO2 and TiC in FL-Ti2NTx is eliminated, as there are no peaks at 458.60 and 460.40 eV in 8895

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ACS Nano Table 1. XPS Peak Fitting Results of Deconvoluted HighResolution Spectra and Their Binding Energies, Atomic Concentration, and Assignments region

atomic conc (%)

Ti 2p

43.5

F 1s

7.6

N 1s

20.8

O 1s

28.0

a

BE (eV) 459.70 (2p1/2) 455.60 (2p3/2) 464.25 (2p1/2) 458.25 (2p3/2) 463.25 (2p1/2) 460.65(2p3/2) 456.85 684.70 686.00 396.31 399.88 533.30 532.20 530.70

assigned to

fraction of individual components

Ti−N27

0.41

Ti−F26,28

0.31

Ti−O26,29

0.17

Ti−OH29 Ti−F26,28 Al−F30a Ti−N27 satellite peak28 Ti−H2O(adsorbed)29 Ti−OH26,29 Ti−O26,29

0.11 0.79 0.21 0.73 0.27 0.10 0.20 0.70

Atomic concentration (%) of Al 0.09.

the Ti 2p region.26 The absence of a peak in O 1s at 530.30 eV further confirms the absence of TiO2.26,31 XPS analysis confirms the removal of Al from Ti2AlN and formation of Ti2N with−OH, −F, and −O− surface terminations along with adsorbed water. SERS Activity of Two-Dimensional Ti2NTx (MXene). The SERS activity of the synthesized MXene was explored by fabricating a simple paper (P)-based SERS substrate. Filter paper was chosen as the substrate to utilize its potential to collect a sample from any sort of surface. The paper substrate might serve as a potential SERS candidate due to its high specific area and ability to accumulate the analyte. In addition to that, the fibrous morphology of the paper along with wrinkles on the surface imparts a natural roughness to it compared to other planar substrates. Planar silicon (Si) and glass (G) substrates were also fabricated for comparison. Figure 4a shows the dispersion of FL-Ti2NTx in water, and Figure 4b is a photograph of bare filter paper and fabricated (P, Si, and G) and commercial (C) SERS substrates. The zeta potential of the FL-Ti2NTx dispersion (Figure 4c) was −38 mV, which suggests the stability of MXenes in water. The MXene dispersion possesses absorption in the UV region (Figure 4d), as evidenced by a broad peak in the range 250−350 nm with a peak maximum at ∼290 nm, which could be attributed to the presence of surface terminations.16 The absorption edge was observed at ∼700 nm (i.e., ∼2.8 eV), which covers the resonance range of the excitation laser used for SERS studies (i.e., 532 nm ≡ 2.3 eV). However, direct correlation for the existence of plasmons was not observed, which could be attributed to the nature of the nanoparticles.32,33 Le Ru et al.32 reported that a direct correlation between UV absorption and plasmons holds true only for metallic nanoparticles such as silver and gold. Doherty et al.33 also observed that there was little or no correlation between absorption spectra and excitation wavelength employed for SERS. Hence, in cases where the particle size and shape varies, the relationship between absorption maxima and SERS enhancement is not direct and could even be misleading. It is also reported by Rupali Das et al.34 that SERS enhancement not only stems from surface plasmons but also depends on factors such as surface

Figure 4. (a) Photograph of FL-Ti2NTx dispersed in water, (b) photographs of bare filter paper and SERS substrates, (c) zeta potential distribution curve of an FL-Ti2NTx dispersion, (d) UV− visible spectrum of an FL-Ti2NTx dispersion, (e) AFM topography image of bare filter paper, (f) AFM topography image of the paper substrate (P), (g) FESEM image of the paper substrate (P), and (h) EDS spectrum recorded on the paper substrate (P).

roughness, size, shape,35 and local curvatures of nanoparticles. The absence of absorption at 210 nm36 confirms the absence of alumina in the MXene dispersion used for fabricating SERS substrates. Optical images (Supporting Information, Figure S7) of fabricated SERS substrates showed that the particles were not adhered properly onto planar substrates, whereas uniform surface coverage was observed in the case of the paper substrate. The surface roughness of the filter paper calculated from the AFM image was 0.332 μm, which after loading the MXene was reduced to 0.114 μm, indicating the homogeneous adsorption of MXene on the paper substrate. AFM topography images in Figure 4e and f clearly show the fibrous structure of bare filter paper and the platelet structure of MXene on the filter paper. The FESEM image (Figure 4g) of the substrate further compliments the observations by AFM analysis, and the EDS spectrum (Figure 4h) recorded showed no traces of Al. SERS sensitivity of fabricated substrates was investigated using rhodamine-6G (R6G) as the model compound. Figure 5a−c show the stack plots of Raman spectra with actual intensity counts for various concentrations of R6G recorded on paper-, silicon-, and glass-based SERS substrates. It should be noted that there was no signal from the substrates or from MXene in the region of interest when the laser power was 8896

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Figure 5. (a−c) Stack plots of Raman spectra (with actual intensities on the y-axis) for various concentrations of R6G on a SERS substrate: (a) paper (P), (b) silicon (Si), and (c) glass (G). The bottom spectra (black color) in (a), (b), and (c) correspond to 1 millimolar R6G (1 mM*) on bare P, Si, and G substrates.

of 1012, which clearly dictates its usefulness in “single-molecule detection”. Due to the higher surface area, the paper substrate could efficiently concentrate the analyte and thereby results in intense Raman signal relative to planar substrates. The absence of a characteristic peak shift in R6G points toward the domination of an electromagnetic enhancement mechanism.37 The high efficiency of a paper-based SERS substrate compared to the planar substrates arises from its hierarchical fiber-like morphology, which facilitates uniform adsorption of MXene on the paper substrate. The bent and curved surfaces of MXenes, as evidenced from FESEM and AFM images, serve as hot spots, thereby enhancing the Raman signal. In addition to that, the wrinkles on the surface of the paper offer a natural roughness to the paper, thereby resulting in signal enhancement. The relative standard deviations of femtomolar R6G on a paper-based substrate with respect to 615 and 1575 cm−1 peaks from 10 measurements were calculated to be 6.33% and 4.15%, respectively (Supporting Information, Figure S8). The homogeneous adsorption of analyte (R6G) on the filter paper was proved by mapping the area (Supporting Information, Figure S9). The SERS sensitivity of Ti2N (MXene) was further validated by analyzing micromolar concentrations of military-grade explosives such as PETN, HMX, and RDX. Figure 6d shows the structure of the explosives recorded, and Figures 6a−c are the stack plots of Raman spectra with actual intensities obtained for PETN, HMX, and RDX on different SERS substrates. The intensity comparison of characteristic peaks of explosives on different substrates is shown in Figure 6e, and Table 340−42 tabulates the peak assignments of the explosives analyzed. It is evident from the Raman spectra that the paper substrate could detect traces of explosives with a better sensitivity compared to planar and commercial substrates. The same concentration of explosives when analyzed using bare filter paper did not give rise to any signal. Our work demonstrates the potential for developing flexible Ti2N (MXene)-based SERS substrates for trace detection of explosives, which can be further extended to the investigation of other contaminants.

maintained at 0.54 mW. The enhancement factor (EF) was calculated using the following equation:37 EF = (C RS/IRS)(ISERS/CSERS)

where CRS is the concentration of analyte solution and IRS is the intensity of the Raman signal under non-SERS conditions. ISERS is the intensity of the Raman signal under identical experimental conditions (i.e., with the same power and laser wavelength, microscope objective, volume of analyte, etc.) for the same analyte on a SERS substrate, with different concentrations (CSERS). The enhancement factors calculated for the most intense peaks of femtomolar concentration R6G in the case of the paper substrate and micromolar concentration R6G on planar substrates and their peak assignments38 are tabulated in Table 2. IRS and ISERS of P, Si, and G are included in Table S1. Table 2. Peak Assignment of R6G and Calculated Enhancement Factors calculated enhancement factors on SERS substrates (CRS = 10−3 M) Raman shift (cm−1) 615 1364 1510 1574 1654

peak assignment C−C−C ring bend aromatic C−C stretching aromatic C−C stretching aromatic C−C stretching aromatic C−C stretching

P

Si

G

(CSERS = 10−15 M)

(CSERS = 10−6 M)

(CSERS = 10−6 M)

2.70 × 1012

6.33 × 103

2.57 × 103

2.82 × 1012

4.19 × 103

5.70 × 103

2.97 × 1012

3.44 × 103

6.10 × 103

2.74 × 1012

3.66 × 103

1.07 × 103

1.93 × 1012

3.27 × 103

7.32 × 103

The enhancement of signal in the presence of Ti2NTx MXene could be attributed to the high electron density distribution on the N atom, as a result of transfer of electrons from the Ti atom.39 Planar substrates could detect only up to micromolar concentrations of R6G with an enhancement factor of 103. Conversely, a paper-based SERS substrate could detect a femtomolar concentration of R6G with an enhancement factor 8897

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Figure 6. (a−c) Stack plots of Raman spectra (with actual intensities on the y-axis) of 1 μM explosives recorded using different SERS substrates (P, Si, G, and C denote paper, silicon, glass, and commercial SERS substrates): (a) PETN, (b) HMX, (c) RDX, and (d) structure of explosives analyzed and (e) intensity comparison of characteristic peaks of PETN, HMX, and RDX on various substrates employed.

and further endorsed by the detection of explosives at the micromolar level.

Table 3. Peak Position and Peak Assignments for the Explosives Analyzed explosive

Raman shift (cm−1)

PETN

622 870 1289

HMX

844 940 1224 1318

RDX

878 1263 1316

MATERIALS AND METHODS

peak assignment C−C−C deformation and NO2 rocking O−N stretching along with C−C stretching NO2 symmetric stretching along with −CH2 wagging symmetric ring stretching along with O−N−O bending asymmetric ring stretching with −CH2 rocking −CH2 scissoring NO2 symmetric stretching along with −CH2 wagging symmetric ring breathing mode N−N stretching along with O−N−O stretching N−N stretching and −CH2 twist

Synthesis of Ti2AlN (MAX). Ti2AlN was synthesized by ball milling titanium (assay 99.8%; M/s, Surabhi Industries, India) and aluminum nitride (assay >99.99%; Acros Organics, India) powder in the molar ratio 2:1 for 6 h using zirconia balls at 400 rpm. The mixture was heated in a tubular furnace under an argon atmosphere up to 1500 °C at a heating rate of 10 °C/min with a soak time of 2 h.22 The resulting powder was further milled at a speed of 400 rpm for 1 h to reduce the size and subsequently characterized. Synthesis of Ti2N (MXene). Ti2AlN (∼2 g) was immersed in 20 mL of a potassium fluoride (assay >99%; Merck Chemicals) and hydrochloric acid (Merck Chemicals) mixture at RT for 3 h. The KF− HCl mixture was prepared by dissolving 6 g of KF in 100 mL of 6 M HCl. The suspension was further heated at 40 °C for 1 h along with bath sonication to facilitate the exfoliation and intercalation. The supernatant was removed, and deionized water was added to the powder mass and centrifuged at 3500 rpm for 5 min to remove the soluble fluorides. The procedure was repeated until the pH of the supernatant was close to 6. After decanting, isopropyl alcohol was added to the powder and the mixture was centrifuged at 3500 rpm for 0.5 h and filtered to get ML-Ti2NTx. Further, delamination of the layers was carried out by immersing 1 g of the powder in 20 mL of DMSO and sonicating for an hour. The mixture was left undisturbed overnight, the supernatant was removed, deionized water was added, and the mixture was centrifuged at 3500 rpm for 1 h. The supernatant suspension was vacuum filtered and dried to collect few-layered Ti2N (FL-Ti2NTx). The yield of FL-Ti2NTx was 87%. Etchants such as 5% HF, a HF−HCl mixture (5% HF + 100 mL of 6 M HCl), and 6% KF

CONCLUSIONS In summary, we report the synthesis of a two-dimensional transition metal nitride of M2X type, Ti2N (MXene), by selective etching of Al from a Ti2AlN precursor using a mixture of potassium fluoride and hydrochloric acid. Multilayered Ti2NTx (T = −OH, −F, −O−) was delaminated to few-layered Ti2NTx using DMSO. Fabrication of a simple and efficient paper-based SERS substrate using Ti2N (MXene) was established, and an enhancement factor of 10 12 was demonstrated for R6G. The SERS efficiency of the paper substrate was compared with planar and commercial substrates 8898

DOI: 10.1021/acsnano.7b03129 ACS Nano 2017, 11, 8892−8900

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ACS Nano were also employed to selectively remove aluminum from the precursor. Fabrication and Characterization of the SERS Substrate. SERS substrates based on paper, silicon, and glass were fabricated. To fabricate the paper-based SERS substrate, FL-Ti2NTx was dispersed in water by sonicating it for 0.5 h and loaded onto a laboratory filter paper (Whatman 44 grade) by immersing it in the suspension for 12 h. Upon removal, the paper with adsorbed Ti2NTx was dried in a vacuum oven at 60 °C and used as the SERS substrate. The surface of the glass and silicon wafer was cleaned prior to immersing in the suspension of FL-Ti2NTx, and the procedure followed to fabricate the paper-based SERS substrate was adopted to prepare silicon- and glass-based SERS substrates. Raman enhancement factors were calculated using varying concentrations (10−3 to 10−15 M) of rhodamine-6G (S.D.’s Lab Chem. Industry, India) as a model compound with 532 nm as excitation wavelength. The fabricated SERS substrates were used to detect micromolar concentrations of military-grade explosives, and the results were compared with a commercially available nanosilver-based SERS substrate from YashNanotech, India. Micromolar (1 μM) concentrations of explosives such as pentaerythritol tetranitrate (PETN), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), and cyclotrimethylenetrinitramine (RDX) were prepared using acetonitrile as solvent. Characterization. Raman spectral analyses and investigation on SERS activity of synthesized MXenes were carried out using a WITec alpha 300R (Germany) confocal Raman microscope with a 532 nm excitation laser. A 2 μL of analyte was dropped onto the SERS substrates, and 10 Raman spectra were accumulated for 5 s using a 50× air objective with a numerical aperture of 0.55. The laser beam was focused on a selected area through the confocal microscope, and the laser power was kept constant (0.54 mW) throughout the investigation. XRD analysis was carried out on a Rigaku Miniflex 120 II-C (Japan) diffractometer using Cu Kα radiation at a step size of 0.02° at 2θ = 5−80°. The IR spectrum of the powder was recorded in a ThermoScientific Nicolet iS50 FTIR spectrometer in the wavelength range 4000−400 cm−1 with a resolution of 2 cm−1. The particle size distribution of Ti2AlN was determined using a Malvern Mastersizer 2000 (UK) particle size analyzer. The BET surface area of the powders was analyzed using a Quantachrome Nova 1200e surface area analyzer. FESEM images and EDS spectra of gold-coated powder samples were recorded using a Carl Zeiss Sigma HD (Germany) field emission scanning electron microscope equipped with an Oxford Inca (UK) energy dispersive X-ray spectrometer. Inductively coupled plasmaatomic emission spectrometry (ICP-AES) analysis of the powder after treating with a KF−HCl mixture was carried out in a PerkinElmer Optima 4300 V (USA) inductively coupled plasma-atomic emission spectrometer. The powder was digested in a mixture of HF and concentrated HNO3 and was then injected into the plasma (detection limit of Al in ICP-AES is 0.1 ppm). TEM imaging and SAED studies of the delaminated MXene sheets were done using a FEI Tecnai G2 30 STWIN (USA) at an accelerating voltage of 200 kV. AFM analysis of MXene and the SERS substrate was conducted in contact mode with an Agilent 5500 (USA) scanning probe microscope. FL-Ti2NTx was dispersed in acetone by sonication and drop casted onto a copper grid and silicon wafer, respectively, for TEM and AFM studies. Conductivity measurements of the discs were carried out in a Keithley 6221 current source 2182A nanovoltmeter with a four-probe head. Xray photoelectron spectral measurements were done in an Axis Ultra from Kratos Analytical (USA). Zeta potential measurement of the MXene dispersion was carried out in a Malvern Nano Zen 3601 (UK) nanoparticle size analyzer. The UV−visible spectrum of the MXene dispersion was recorded in a PerkinElmer Lambda 950 (USA) UV− visible spectrometer.

mixture, KF, and KF−HCl mixture; Raman spectral characterization of white precipitate obtained during etching with HF, HF−HCl mixture, and KF; IR spectrum of ML-Ti2NTx and Raman spectral characterization of the supernatant after etching with a KF−HCl mixture; optical images of fabricated SERS substrates and intensities of characteristic peaks on non-SERS and SERS substrates; Raman spectra and mapping recorded on a paper substrate (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Bhuvaneswari Soundiraraju: 0000-0001-8531-8904 Author Contributions

B.S. conceived the idea, carried out the experiments, and analyzed the data. B.K.G. participated in the discussion and guided the work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the Director of the Vikram Sarabhai Space Centre for granting permission to publish this work. We thank Nellori Dileep Kumar, Liquid Propulsion Systems Centre, Thiruvananthapuram, India, for FESEM and EDS characterization and Sarath and Arya Raju, Amrita Centre for Nanosciences, Kochi, India, for XPS and HRTEM characterization. REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−435. (3) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. (4) Ma, R.; Sasaki, T. Nanosheets of Oxides and Hydroxides: Ultimate 2D Charge-Bearing Functional Crystallites. Adv. Mater. 2010, 22, 5082−5104. (5) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Adv. Mater. 2014, 26, 992−1005. (6) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253. (7) Radovic, M.; Barsoum, M. W. MAX phases: Bridging the Gap between Metals and Ceramics. Am. Ceram. Soc. Bull. 2013, 92, 20−27. (8) Barsoum, M. W. MAX Phases. In Properties of Machinable Ternary Carbides and Nitrides; Wiley-VCH Verlag GmbH & Co. KGaA, 2013; pp 1−12, 16. (9) Urbankowski, P.; Anasori, B.; Makaryan, T.; Er, D.; Kota, S.; Walsh, L. P.; Zhao, M.; Shenoy, V. B.; Barsoum, M. W.; Gogotsi, Y. Synthesis of Two-Dimensional Titanium Nitride Ti4N3 (MXene). Nanoscale 2016, 8, 11385−11391. (10) Lei, J. C.; Zhang, X.; Zhou, Z. Recent advances in MXene: Preparation, Properties, and Applications. Front. Phys. 2015, 10, 107303−1−11.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03129. Particle size analysis of synthesized Ti2AlN; FESEM images and EDS spectra of Ti2AlN in HF, HF−HCl 8899

DOI: 10.1021/acsnano.7b03129 ACS Nano 2017, 11, 8892−8900

Article

ACS Nano

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DOI: 10.1021/acsnano.7b03129 ACS Nano 2017, 11, 8892−8900