Introduction

Ti3C2Tx, a typical two-dimensional layered transition metal carbide with a graphene-like structure, has attracted great attention due to its wide potential applications in fields of catalysis, energy, and medicine thanks to its unique properties, especially large specific surface area and so on [1,2,3,4,5,6]. It has been demonstrated that the physical and chemical performance of Ti3C2Tx could be determined by its terminal groups, referred as Tx in the formula (usually are –F, –O and/or –OH), which can be adjusted by choosing different preparation procedures [7, 8]. For example, some experimental results indicate that the hydrophilic hydrophobic equilibrium of Ti3C2Tx can be modulated by interacting some agent groups with –O terminal groups on Ti3C2Tx [9], and the Pb adsorption capacity can be improved by connecting with hydroxyl groups on Ti3C2Tx [10]. In the meantime, some theoretical works have determined that the attached methoxy groups could improve the stability of Ti2C and Ti3C2 [11], and O-related terminal groups could enhance the lithium ion storage capacity of various nanosheets [12]. Apart from the multifarious applications by taking advantage of the unique layered structure with certain terminal groups, it is found that Ti3C2Tx can present plasmonic performance as well, and the resonance wavelength can be tuned by the terminals and/or thickness [13], indicating that Ti3C2Tx could confine electromagnetic field under excitation and eventually can be employed as broadband perfect absorbers [14, 15], Terahertz shielding devices [16], and photonic and/or molecular detectors or sensors [17,18,19]. However, most of previous works either concerned the etching condition dependent terminal groups [20] or focused on the overall plasmonic performance [21]. Therefore, it is interesting to systematically study the relationship between the terminal groups of Ti3C2Tx with different layers and their near-field enhancement effect, since such effect has been widely employed in many optical related fields, such as surface-enhanced Raman scattering detection, due to the strong confined electromagnetic field [22,23,24].

In this work, in order to simplify the terminal options and avoid using hazardous HF, the mixed etching agent of LiF and HCl has been used to minimize the fluorine terminals (–F) in the etching process [25]. Furthermore, the procedure of sonication in water has been carried out to delaminate the multilayered Ti3C2Tx (ML-Ti3C2Tx) into few-layered Ti3C2Tx (FL-Ti3C2Tx) without introducing any other reagents. As a result, the obtained Ti3C2Tx with different layers in this work will be mainly terminated by either O- or OH-related groups, which make ML-Ti3C2Tx or FL-Ti3C2Tx nanosheets reveal different physical and chemical properties and eventually present different near-filed enhancement performance. In addition, the hybrid structures composed of Ti3C2Tx and Ag nanoparticles have been prepared and the corresponding coupling effects have been explored as well. Such exploration regarding terminal dependent plasmonic performance of these Ti3C2Tx with different layers and configurations could help people to select suitable Ti3C2Tx-based materials in some specific optical fields.

Methods

Preparation of Ti3C2Tx Nanosheets

ML-Ti3C2Tx was prepared by following a modified previously reported method [26]. The typical etching process started with the preparation of LiF solution by dissolving 1 g of LiF in 20 mL of dilute HCl solution (6 M) with stirring. Subsequently, 1 g of Ti3AlC2 powder was slowly added into the above solution, and the etching process was kept at 70 °C for 45 h under stirring. The wet sediment was then washed several times with deionized water until the pH of the suspension liquid was bigger than 6. Afterward, the suspension was collected and named as ML-Ti3C2Tx. To obtain FL-Ti3C2Tx, ML-Ti3C2Tx was further delaminated by sonication for 2 h in Ar atmosphere and followed by centrifugation at 3500 rpm for 1 h.

Preparation of Ag/Ti3C2Tx Nanocomposites

The synthesis of the hybrid materials was started with the preparation of the mixed solution of AgNO3 (12.5 mL, 2 mmol/L) and NaC6H5O7 (12.5 mL, 4 mmol/L) at room temperature. After rapidly adding PVP solution (25 mL, 0.1 g/mL), Ti3C2Tx solution (5 mL, 0.05 mg/mL) was then slowly added into the mixed solution with stirring for 10 min at room temperature. Subsequently, the above-mixed solution was heated up to 70 °C to react for 45 h. After centrifuging, the products were kept in water and named as Ag/ML-Ti3C2Tx and Ag/FL-Ti3C2Tx, respectively, according to the type of Ti3C2Tx used in the procedure.

Characterization

A field emission scanning electron microscope (Carl ZEISS Sigma) and two transmission electron microscopes (JEM-2100F and JEM-1400Flash) have been employed to determine the morphologies of the samples. The X-ray diffraction (XRD) patterns in the range of 2θ = 5°–80° with a step of 0.02° were recorded on a powder diffractometer (X'Pert PRO MPD). Zeta potentials and surface states of ML-Ti3C2Tx and FL-Ti3C2Tx were measured by a Malvern Zetasizer (Nano-ZS90) and an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi), respectively. The absorption and Raman performance of samples were recorded by a UV–Vis spectrophotometer (CARY 5000) and a Raman spectroscopy (LabRAM HR Evolution), respectively. The excitation wavelength of Raman detection was 532 nm, and the laser powers for usual Raman measurements and surface enhanced Raman scattering (SERS) characterizations were 12.5 mW and 0.05 mW, respectively.

Results and Discussion

Both morphologies of ML-Ti3C2Tx and FL-Ti3C2Tx are shown in Fig. 1a, b and c, d, respectively. It can be seen that FL-Ti3C2Tx looks more transparent, indicating that its layer number is much less than ML-Ti3C2Tx. Figure 1e shows the XRD patterns of all samples. Ti3AlC2 and ML-Ti3C2Tx show their typical phase features, which agree well with some previous reports [26,27,28]. It can be readily observed that the intense (002) peak of ML-Ti3C2Tx shifts to the lower angle comparing with that of Ti3AlC2, implying the removal of Al atoms from the MAX phase and the expanding along the c axis. Compared with the diffraction peaks of ML-Ti3C2Tx, both broadened (002) peak and disappeared (004) and (008) peaks of FL-Ti3C2Tx determined the successful preparation of the few-layered sample [29]. Moreover, the (002) peak of FL-Ti3C2Tx locates at a little higher angle than that of ML-Ti3C2Tx, indicating that ML-Ti3C2Tx and FL-Ti3C2Tx should be terminated with different groups, which can be attributed to -O and -OH, respectively, since the as-prepared Ti3C2Tx (ML-Ti3C2Tx) will not be mainly terminated with -F without HF as etching agent and the corresponding c parameters attracted from the XRD patterns agree well with what previous works reported [25, 30].

Fig. 1
figure 1

Morphology and phase determinations. a, b SEM and TEM images of ML-Ti3C2Tx. c, d SEM and TEM images of FL-Ti3C2Tx. e XRD patterns of Ti3AlC2, ML-Ti3C2Tx and FL-Ti3C2Tx

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Figure 2a shows Raman spectra of ML-Ti3C2Tx and FL-Ti3C2Tx. As it can be seen that the Raman signals in the range of 200–800 cm−1 for both samples are quite similar. Among them, the peak at 717 cm−1 is due to the A1g symmetrical out-of-plane vibration of Ti and C atoms, while the peaks at 244, 366 and 570 cm−1 are arising from the in-plane (shear) modes of Ti, C and surface terminal groups, respectively [31, 32]. As for the Raman signals ranging from 800 to 1800 cm−1, comparing with ML-Ti3C2Tx, FL-Ti3C2Tx not only shows stronger Raman signal at 1580 cm−1 (G band), but also presents two emerging Raman bands at 1000–1200 cm−1 and 1300 cm−1 (D band). Herein, the appearance of D band indicates that some Ti atoms have been peeled away and more C atoms are exposed to the surroundings [33]. Therefore, the integrated Raman intensity of FL-Ti3C2Tx in this range is slightly larger than that of ML-Ti3C2Tx, implying that FL-Ti3C2Tx adsorbs more terminal groups. Zeta potentials of ML-Ti3C2Tx and FL-Ti3C2Tx are −4.38 and −26.9 mV, respectively, as shown in Additional file 1: Fig. S1, which further confirm that FL-Ti3C2Tx are terminated by more groups with negative charges.

Fig. 2
figure 2

a Raman spectra and b Normalized absorption spectra of FL-Ti3C2Tx and ML-Ti3C2Tx. The inset in b presents the absorption bands of FL-Ti3C2Tx and ML-Ti3C2Tx in the UV region

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The UV–Vis spectra shown in Fig. 2b reveal that both FL-Ti3C2Tx and ML-Ti3C2Tx present two dominant absorption bands. In the UV region (225–325 nm), FL-Ti3C2Tx displays relatively stronger absorption band which corresponds to the band gap transition [34], implying that there are more -OH groups have been terminated on FL-Ti3C2Tx [35]. On the other hand, the comparison between the long wavelength absorption bands (600-1000 nm) of both samples shows that the relative intensity of FL-Ti3C2Tx in this range is obviously lower than that of ML-Ti3C2Tx, indicating that ML-Ti3C2Tx are mainly terminated by –O [35]. FL-Ti3C2Tx can be well dispersed in the aqueous solution since the terminated –OH groups shows hydrophilicity and electrostatic repulsion between sheets [31, 36]. As for ML-Ti3C2Tx with more –O terminals, it can only form a suspension in the beginning and will deposit subsequently as shown in Additional file 1: Fig. S2a.

In order to shed more light on the surface groups terminated on ML-Ti3C2Tx and FL-Ti3C2Tx, XPS spectra of both samples were collected and are shown in Fig. 3. All corresponding detailed information regarding the surface states are summarized in Additional file 1: Table S1. The fraction of Ti-C in FL-Ti3C2Tx (9.80%) is lower than that in ML-Ti3C2Tx (17.31%), while the ratio of C–C in FL-Ti3C2Tx (44.62%) is higher. Such surface states changing evidences the loss of Ti atoms and the more exposed C atoms on the surface of FL-Ti3C2Tx, which agrees with the emerging D band in its Raman spectrum shown in Fig. 2a. The increased C-Ti-Tx ratio in FL-Ti3C2Tx (21.27%) indicates that there should be more active terminal groups adsorbed on its surface than ML-Ti3C2Tx, which agrees with the Zeta potential results shown in Additional file 1: Fig. S1. Apart from the quantity of the terminal groups, the analysis of XPS results also reveals that FL-Ti3C2Tx and ML-Ti3C2Tx have been terminated by different dominant functional groups, which also has been suggested by the (002) diffraction peaks shown in Fig. 1e. Regarding O 1 s spectra of these two samples, it can be clearly seen that more O-related states have been found on the surface of ML-Ti3C2Tx, and some of them are adsorbed oxygen molecules, which can dissociate to form Ti3C2Ox and therefore will repel O2 in air to prevent further oxidation of ML-Ti3C2Tx [37]. As a result, ML-Ti3C2Tx seems present a better oxidation resistance with a lower TiO2 ratio (13.98%) than FL-Ti3C2Tx (19.60%).

Fig. 3
figure 3

XPS spectra of ML-Ti3C2Tx and FL-Ti3C2Tx a Ti2p, b C1s, c O1s

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Based on the observations and analyses of Figs. 1, 2 and 3, it can be concluded that although both ML-Ti3C2Tx and FL-Ti3C2Tx are terminated by some functional groups with negative charge, the amount and dominant type of the groups are quite different. On one hand, the quantity of terminal groups on FL-Ti3C2Tx is larger than that of ML-Ti3C2Tx. On the other hand, the dominant terminal structure on ML-Ti3C2Tx is Ti3C2O2, which makes ML-Ti3C2Tx to be more stable in the air [38], while for FL-Ti3C2Tx, it is mainly terminated by Ti3C2(OH)2, which helps FL-Ti3C2Tx to be well-dispersed in aqueous solutions [36].

Ti3C2Tx with functional terminal groups could reveal good adsorption performance and therefore could act as a surface-enhanced Raman scattering (SERS) substrate to improve the Raman activity of positively charged probe molecules [3, 39, 40]. Comparing with ML-Ti3C2Tx, FL-Ti3C2Tx should present better adsorption ability since it has been determined that it is terminated with more negative charges. Such better adsorption performance has been demonstrated by the optical photographs of the mixed solution with R6G and FL-Ti3C2Tx as shown in Additional file 1: Fig. S2b. However, Fig. 4a reveals that the ML-Ti3C2Tx substrate obviously performs better SERS activity than FL-Ti3C2Tx one. Considering ML-Ti3C2Tx with –O terminal presents stronger absorption band centered at around 800 nm, which can be assigned to the surface plasmon resonant absorption [3, 15, 39, 41], it therefore can be concluded that ML-Ti3C2Tx with stronger SERS activity should result from the stronger near-field effect induced by the relatively stronger surface plasmon resonance as shown in Fig. 2b.

Fig. 4
figure 4

a SERS spectra of R6G (10–3 M) with ML-Ti3C2Tx and FL-Ti3C2Tx. b SERS spectra of R6G (10–6 M) with Ag/ML-Ti3C2Tx and Ag/FL-Ti3C2Tx. c Schematic diagram of electron transfer from FL-Ti3C2Tx to Ag NP due to their work function difference. Wm and Ws represent the work functions of Ag NP and FL-Ti3C2Tx, respectively

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In order to further explore the relationship between the terminal groups and the near-filed effect of Ti3C2Tx nanosheets, the hybrid structures composed of Ti3C2Tx nanosheets, including few layered and multilayered, and Ag nanoparticles (NPs) have been synthesized, which are accordingly labeled as Ag/FL-Ti3C2Tx and Ag/ML-Ti3C2Tx, respectively. The morphologies of both hybrid samples are shown in Additional file 1: Fig. S3. The insets indicate the corresponding size distributions of Ag NPs loading on ML-Ti3C2Tx (5–40 nm) is larger than that on FL-Ti3C2Tx (2–20 nm). Intuitively, it might be concluded that Ag/ML-Ti3C2Tx could perform better SERS activity than Ag/FL-Ti3C2Tx since both larger Ag NPs and relative stronger surface plasmon resonance of ML-Ti3C2Tx are beneficial to confine stronger near-field. However, the SERS spectra shown in Fig. 4b reveal a counterintuitive result. It is clear that the enhancement effect offered by Ag/FL-Ti3C2Tx is nearly 3 times of that by Ag/ML-Ti3C2Tx, implying that the coupling between Ag NPs and FL-Ti3C2Tx should play an important role during the detection process. As confirmed above that FL-Ti3C2Tx has been mainly terminated by -OH groups with lots of surface electrons, which will result in the formation of Ti3C2(OH)2 structure with a work function of 1.6–2.8 eV [42, 43]. As shown in Fig. 4c, the abundant surface electrons will therefore transfer from FL-Ti3C2Tx to Ag NPs with a work function of 4.7 eV [44]. With the extra injection of hot electrons from FL-Ti3C2Tx, Ag NPs with smaller size could present stronger resonance under the excitation and eventually perform better SERS activity due to the coupling induced stronger electromagnetic effect. It is worth noting that the work function of Ti3C2O2 structure formed on the surface of ML-Ti3C2Tx is around 6.0 eV [43], which will result in electron transfer from Ag NPs surface to ML-Ti3C2Tx nanosheets and therefore will weaken the near-field enhanced effect supported by the Ag NPs. On the other hand, not like FL-Ti3C2Tx with -OH terminals, ML-Ti3C2Tx with -O terminals cannot offer sufficient electrons under excitation [42]. It is therefore reasonable that the SERS activity of Ag/ML-Ti3C2Tx is worse than that of Ag/ FL-Ti3C2Tx.

Conclusions

In summary, ML-Ti3C2Tx and FL-Ti3C2Tx terminated with different dominant functional groups have been successfully prepared. It has been demonstrated that ML-Ti3C2Tx is more stable in the air due to the surface structure of Ti3C2O2 and show stronger SERS activity than FL-Ti3C2Tx because it can reveal stronger near-field effect. However, FL-Ti3C2Tx terminated by Ti3C2(OH)2 can be well dispersed in aqueous solution and will show better SERS performance after coupling to the Ag NPs due to the sufficient electron injection. Such research regarding the terminal groups-dependent near-field enhancement performance will help people to expand the potential applications of Ti3C2Tx in the optical related fields.