Both morphologies of ML-Ti3C2Tx and FL-Ti3C2Tx have been shown in Fig. 1a, 1b and 1c, 1d, 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 [20–22]. 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 . 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 might are mainly terminated with -O and -OH groups, respectively [19, 24].
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 [25, 26]. 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 . 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 Fig. S1, which further confirm that FL-Ti3C2Tx are terminated by more groups with negative charges.
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 , implying that there are more functional groups have been terminated on FL-Ti3C2Tx . 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 is obviously lower than that of ML-Ti3C2Tx. Such weakened absorbance in the visible and near-infrared range implies that FL-Ti3C2Tx and ML-Ti3C2Tx have been terminated by -OH and -O, respectively . FL-Ti3C2Tx with more -OH terminal groups shows hydrophilicity and electrostatic repulsion between sheets [25, 30]. As a result, FL-Ti3C2Tx can be well dispersed in the aqueous solution. As for ML-Ti3C2Tx with more -O terminals, it can only form a suspension in the beginning and will deposit subsequently as shown in 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 have been shown in Fig. 3. All corresponding detailed information regarding the surface states have been summarized in 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 Fig. S1. Apart from the quantity of the terminal groups, the analysis of XPS results also reveal 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 there are 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 . As a result, ML-Ti3C2Tx presents a better oxidation resistance with a lower TiO2 ratio (13.98%) than FL-Ti3C2Tx (19.60%).
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 , while for FL-Ti3C2Tx, it is mainly terminated by Ti3C2(OH)2, which helps FL-Ti3C2Tx to be well-dispersed in aqueous solutions .
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, 33, 34]. 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 Fig. S2b. However, Fig. 4a reveals that ML-Ti3C2Tx substrate obviously performs better SERS activities than FL-Ti3C2Tx substrate. 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, 14, 33, 35], it therefore can be concluded that ML-Ti3C2Tx with stronger SERS activity may result from the stronger near-field effect induced by the relatively stronger surface plasmon resonance as shown in Fig. 2b.
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 have been shown in 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 [36, 37]. 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 . 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 , 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.