1. Introduction

With increasing use of laser for various fields such as medicine, electronics, scientific research, industry and military, the development of nonlinear optical (NLO) material (saturable absorber and optical limiting (OL) material) which can modulate the light intensity is crucial. Saturable absorber, which allows high intensity light pass, can be employed for Q-switching, mode locking and pulse compression [1–3]. OL material, in contrast, which effectively forbids high intensity light but allows low intensity pass, can be applied to protect human eyes and sensitive instruments from laser induced damage [4]. Despite a variety of optical nanomaterials have been extensively studied for their NLO behavior, for instance, noble metal nanoparticles, quantum dots, organic nanomaterials, carbon-based nanomaterials and graphene [5–9], there is still in great request for highly NLO responsive components.

Layered molybdenum disulfide (MoS₂), an emerging graphene-like two-dimensional (2D) nanomaterial, has aroused the interest of researchers because of its exotic electronic and optical properties induced by the indirect-to-direct transition with decreasing the number of layer [10–21]. Researches demonstrated that layered MoS₂ exhibits strong nonlinear light-matter interaction due to the Van Hove singularities in the density of states [1,22]. Due to the unique performance, layered MoS₂ can be a promising NLO material used in optoelectronics [23–25]. However, the inherent drawbacks including the basic layered building blocks limit its practical applications. Recently, the fabrication of 2D nanocomposites assembled by MoS₂ and other semiconductor materials has been proved as an attractive strategy to obtain novel 2D nanomaterials with enhanced electronic and optoelectronic properties [1,26–30]. The greatest motivation to study the nanocomposites is to determine their unique structures and synergistic coupling effects between the constituents. MoS₂/carbon [29], MoS₂/graphene [1], MoS₂/CdS [28] and MoS₂/ZnO nanocomposites [27] were successfully synthesized and they cannot only boost photocatalytic activity and electrochemical property, but also increase NLO response. Most recently, MoS₂/TiO₂ composite was also prepared, and it displayed excellent lithium storage property with high specific capacity and outstanding rate capability due to the integrated smart architecture [31]. In addition, owing to the more exposed active edge sites compared to the pristine MoS₂ nanosheets, MoS₂/TiO₂ composite exhibited improved hydrogen evolution reaction [32]. However, the NLO response of the MoS₂/TiO₂ nanocomposite remains un-explored so far. Interestingly, Zhang et al reported that the passive Q-switching behavior for the hierarchical MoS₂ thin film was better than that of exfoliated MoS₂ and comparable to that of chemical vapor deposition MoS₂ nanosheets. They inferred that the superior ultrafast pulse emission may be associated with the large NLO absorption which induced by the hierarchical structure. This structure was composed of orthogonally oriented vertical/horizontal layers which had a crowd of exposed edge states [33]. Strong second-order NLO susceptibilities of the 2D MoS₂ crystal were observed by Yin et al most recently, due to the abundant edge states [34]. In addition, TiO₂, as a direct band gap semiconductor, also has been confirmed experimentally that it has excellent NLO property [35]. These factors were believed that investigation NLO property of the MoS₂/TiO₂ nanocomposite is of tremendous significance because the combination of layered MoS₂ and TiO₂ will be likely to enhance the NLO property.

Herein, the MoS₂/TiO₂ composite was successfully synthesized by an electrospinning technique and a hydrothermal process. SEM, TEM, XRD, Raman spectroscopy and XPS were utilized to characterize the MoS₂/TiO₂ nanocomposite. Vertically standing layered MoS₂ nanosheets on TiO₂ nanofibers not only endows structure stability, large surface areas and intimate contact between TiO₂ and MoS₂ for fast electrons transfer, but also possesses maximally exposed edge states [20,21]. The NLO response of the MoS₂/TiO₂/polyvinyl alcohol (PVA) composite was investigated applying an open-aperture (OA) z-scan measurement. The composite film exhibited improved OL property, which demonstrates that the MoS₂/TiO₂/PVA composite is a new promising OL material for laser.

2. Experimental

Typically, 0.5 g polyvinylpyrrolidone (PVP, Mw = 1,300,000, Sigma-Aldrich) power was dissolved in 9.5 mL absolute ethanol which contained 2 mL acetic acid. After stirring for 2 h, 2 mL tetrabutyl titanate (Ti(OBu)₄, Sigma-Aldrich) was added to the homogeneous solution and stirred for another 30 min. The resultant precursor solution was loaded into a 20 mL plastic syringe attached with a blunt metal needle of 0.6 mm inner diameter. The flow rate was 1 mL/h and a grounded stainless steel plate was placed 15 cm below the spinneret to collect the nanofibers. A high voltage of 13 kV was employed. The as-collected electrospun fibers were calcined in a covered corundum crucible in a tube furnace at 500 °C for 2 h under air atmosphere. At the same time, 0.0450 g sodium molybdate (Na₂MoO₄·2H₂O, Sigma-Aldrich) and 0.0090 g thioacetamide (C₂H₅NS, Sigma-Aldrich) were dispersed in 20 mL deionized water under vigorous stirring to achieve a homogeneous solution. Subsequently, 0.0020 g calcined TiO₂ nanofibers were added into the above solution and continuously stirred to get a suspension. The mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 220 °C for 24 h. The reaction system was allowed to cool down to room temperature naturally. Finally, the resulting precipitate was rinsed with ethanol by several times and finally dried at 60 °C for 12 h in vacuum. Then, the products were dispersed in PVA to fabricate the MoS₂/TiO₂/PVA composite film according to the previous work [3]. For comparison, the nanoscale TiO₂/PVA and MoS₂/PVA composite films were also prepared.

Morphologies and sizes of the products were observed by field-emission scanning electron microscope (FESEM, Nova NanoSEM430). Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) characterizations were conducted on a JEOL-2100F field emission microscope operating at 200 kV, the sample was prepared via dispersing in ethanol and dropping into a copper grid. X-ray diffraction (XRD) patterns were recorded with a Bruker diffractometer utilizing Cu Kα radiation (λ = 1.5418 Å). Raman spectra were obtained by a Raman spectrometer (Renishaw inVia, Gloucestershire, UK) with a 785 nm laser as the excitation source. Chemical compositions of the products were characterized by X-ray photoelectron spectra (XPS, K-Alpha, Thermo Scientific, UK) with achromatic 200W Al Kα as the X-ray source and the resolution of 0.05 eV. NLO characteristics of the different samples were measured applying the femtosecond (fs) OA z-scan technique. Laser pulses with a pulse width of 130 fs were generated from a commercial Ti: sapphire laser at the wavelength of 800 nm with a repetition rate of 1 kHz. The normalized transmittance of the product was measured with respect to the incident pump power, which was changed via translating the product along the propagation direction of a focused laser beam with a linear motorized stage controlled by the step motor controller. To more precisely confirm the measured data, CS₂ solution contained in a cuvette (1 mm in thick) was employed to calibrate. All the measurements were performed at room temperature.

3. Results and discussion

Figure 1(a) presents the morphology of the calcined electrospun TiO₂ nanofibers with a diameter of approximately 200 nm. It notes that the surface of the TiO₂ nanofibers which consisting of interconnected TiO₂ particles is highly rough and porous, which is attributed to the removal of PVP porogenic agents by calcination. Figure 1(b) displays that MoS₂ nanosheets with the absence of TiO₂ nanofibers are clustered to generate micron-sized agglomerates. After undergoing an electrospinning technique and a hydrothermal coating reaction, MoS₂/TiO₂ nanocomposite was successfully prepared and exhibited in Fig. 1(c). The TEM image in Fig. 1(d) reveals that the MoS₂ sheets are deeply rooted into the TiO₂ nanofibers. The vertical growth of MoS₂ nanosheets on TiO₂ nanofibers endows a highly structural stability, larger surface areas, intimate contact between TiO₂ and MoS₂, and more exposed MoS₂ edge states. The coated MoS₂ nanosheets are flexible, curly, and transparent, indicative of the ultrathin 2D nanosheets features. The freely suspended MoS₂ sheets show apparent distortion edges and random elastic deformations which facilitate the stability of nanomaterials. Figure 1(e) displays a HRTEM image taken from the edge of the MoS₂/TiO₂ nanocomposite. The uniform d interlayer spacing of about 0.642 nm is observed, corresponding to the (002) plane of hexagonal-phase MoS₂. As directly evidenced from the folded edge in Fig. 1(e), the MoS₂ nanosheets mainly contain 3 sandwiched S-Mo-S layers, that is, the thickness is below 3 nm. In addition, Fig. 1(e) also apparently presents the lattice fringes with a spacing of 0.374 nm, which is in good agreement with the (101) plane of anatase TiO₂. Selected area electron diffraction (SAED) pattern in Fig. 1(f) further manifests the existence of TiO₂ and MoS₂ crystal phases in the MoS₂/TiO₂ nanocomposite.

 

Fig. 1 (a) SEM images of TiO₂ nanofibers, (b) TEM images of pure MoS₂ nanosheets, (c) SEM, (d) TEM, (e) HRTEM images and SAED pattern of MoS₂/TiO₂ nanocomposite.

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Optical absorption spectra of the TiO₂ nanofibers, pure MoS₂ nanosheets and MoS₂/TiO₂ nanocomposite were depicted in Fig. 2(a). Two absorption bands at 380-450 and 600-700 nm regions are also observed in the MoS₂/TiO₂ nanocomposite, indicative of the features of 2D layered MoS₂ nanosheets with hexagonal symmetry. The dual peaks located at 619 and 667 nm are assigned to the inter-band excitonic transitions at the K point of the 2D Brillouin zone of MoS₂, identified as the B and A transitions [23]. In addition, according to a distribution for 2D MoS₂ nanosheets with different thickness reported by Wang et al [16], we can also deduce that the average thickness of the MoS₂ nanosheets in MoS₂/TiO₂ nanocomposite is less than 3 nm from the A exciton position (667 nm).

 

Fig. 2 (a) Optical absorption spectra, (b) XRD patterns and (c) Raman spectra of the TiO₂ nanofibers, pure MoS₂ nanosheets and MoS₂/TiO₂ nanocomposite, respectively.

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XRD technique is applied to investigate the crystal structure of the resultant material. As shown in Fig. 2(b), all of the diffraction peaks can be well-indexed to anatase-phase TiO₂ (JCPDS card no. 21-1272) and hexagonal-phase MoS₂ (JCPDS card no. 24-0513), respectively. No obvious peaks from the impurity phase are observed in the XRD patterns, indicating the MoS₂/TiO₂ nanocomposite containing only TiO₂ and MoS₂ components. It is worth mentioning that the (002) peak of MoS₂, corresponding to the periodicity in c-axis of MoS₂ plane, is very weak in the MoS₂/TiO₂ nanocomposite compared to the pristine MoS₂, which declared that the c-axis stacking of MoS₂ happening in pure MoS₂ is effectively restrained in MoS₂/TiO₂ nanocomposite.

Raman spectrum is conducted to identify the atomic structure arrangement of sample. As presented in Fig. 2(c), the phonon vibrational mode of the pure TiO₂ nanofibers at 143.9, 398.2, 516.2 and 638.9 cm⁻¹ coincide with the $E_g^1$, $B_{1g}^1$, $A_{1g}+B_{1g}^2$ and $E_g^2$ modes of anatase TiO₂, respectively [36]. While two characteristic peaks of the pristine MoS₂ at 383.2 and 406.6 cm⁻¹ can be assigned to the in-plane $E_{2g}^1$ and out-of-plane $A_{1g}$ vibrational modes of hexagonal MoS₂, respectively [23]. In the Raman spectrum of the MoS₂/TiO₂ nanocomposite, there is a blue shift of ~3.5 cm⁻¹ for $E_g^1$ mode compared to the individual TiO₂ nanofibers, which is mostly owing to the surface strain, it is possibly caused by the TiO₂ surface. Also, a surface strain mostly formed in the coated MoS₂ nanosheets, which interprets the red shift of ~1.3 cm⁻¹ for $E_{2g}^1$ mode and ~1.2 cm⁻¹ for$A_{1g}$ mode compared with pristine MoS₂ [36].

The XPS was studied to further understand the chemical composition and bonding configuration of the as-prepared samples and the XPS measurements were presented in Fig. 3(a)-3(c). For pristine MoS₂ nanosheets, the binding energies of Mo 3d_3/2, Mo 3d_5/2, S 2p_1/2 and S 2p_3/2 peaks [Fig. 3(a)-3(b)] were located at 232.3, 229.1, 163.1 and 161.8 eV, respectively [37]. After MoS₂ nanosheets vertically rooting into the TiO₂ nanofibers, these peaks shift to 232.0, 228.7, 162.7 and 161.5 eV, respectively, which are lower than the homologous values of pristine MoS₂ nanosheets. Such shifts imply electronic interaction between TiO₂ nanofibers and MoS₂ nanosheets [38]. For O 1s peaks of the MoS₂/TiO₂ nanocomposite in Fig. 3(c), apart from the peak at 529.2 eV ascribed to the Ti-O-Ti bond, 532.3 eV corresponding to O-H bonds of the surface-adsorbed water, the peak at 530.9 eV emerged which cannot be observed in pristine TiO₂ nanofibers, it might be associated with the formation of the Ti-O-Mo bonds between MoS₂ nanosheets and TiO₂ nanofibers [32,39].

 

Fig. 3 XPS spectra pure MoS₂ and MoS₂/TiO₂ nanocomposites: (a) Mo 3d peaks and (b) S 2p peaks of MoS₂ and MoS₂/TiO₂, respectively. (c) O1s peaks of TiO₂ and MoS₂/TiO₂, respectively.

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Figure 4(a) illustrates the typical OA z-scan curves for the MoS₂/TiO₂ nanocomposite, obvious reverse sarturable absorption (RSA) is observed. The MoS₂/TiO₂ nanocomposite exhibits increased RSA, characterized by the significantly reducing valley value of transmittance with increasing the incident pump power. In other words, the MoS₂/TiO₂ nanocomposite can effectively inhibit high intensity light but allow low intensity light to pass, which clearly suggest the fs OL effects. Furthermore, the MoS₂/TiO₂ nanocomposite shows much stronger nonlinear absorption than that of the pristine TiO₂ nanofibers and pure MoS₂ nanosheets in the same input pump power [Fig. 4(b)]. Owing to the lack of the any pronounced nonlinear behavior for the pure PVA film, we can suggest that the significant RSA mainly originate from the MoS₂/TiO₂ nanocomposite. Fitting the OA z-scan curves [Fig. 4(b)] using formula given in the previous reports [3], the nonlinear absorption coefficient β ~23.68 cm/GW of MoS₂/TiO₂ nanocomposite was calculated, which is larger than the sum of that of TiO₂ nanofibers (β ~5.90 cm/GW) and MoS₂ nanosheets (β ~11.03 cm/GW). The imaginary part of the third-order NLO susceptibility Im χ⁽³⁾ is directly associated with β, so that Im χ⁽³⁾ ~3.32 x 10⁻¹² esu of the MoS₂/TiO₂ nanocomposite can be deduced. To eliminate the influence of the linear absorption α₀, the nonlinear figure of merit (FOM) is used to evaluate the NLO response of the materials. The larger FOM signifies the better NLO property of the sample. FOM for the third-order optical nonlinearity is defined as FOM = |Im χ⁽³⁾/α₀| [23]. Hence, FOM of the MoS₂/TiO₂ nanocomposite is determined to be 3.10 x 10⁻¹⁴ esu cm⁻¹, which is much larger than the sum of that of TiO₂ nanofibers (FOM ~0.77 x 10⁻¹⁴ esu cm⁻¹) and MoS₂ nanosheets (FOM ~1.44 x 10⁻¹⁴ esu cm⁻¹). In addition, The OL threshold (F_OL, deifind as the input fluence at which the normalized transmittance falls to 50%) is a crucial parameter to evaluate the OL effects of a given material [24]. The fs OL responses of the samples in different input fluences were presented in Fig. 4(c), it is clear that the F_OL value of the MoS₂/TiO₂ nanocomposite is 22.3 mJ/cm², significantly lower than that of the TiO₂ nanofibers and MoS₂ nanosheets. All these results indicate that the MoS₂/TiO₂ nanocomposite has great potential as a type of excellent NLO material for OL utilization.

 

Fig. 4 (a) OA z-scan curves of the MoS₂/TiO₂ nanocomposite with incident pump powers from 10.7 to 68.0 μW, (b) OA z-scan curves of the TiO₂ nanofibers, pure MoS₂ nanosheets and MoS₂/TiO₂ nanocomposite with an incident pump power of 21.0 μW, (c) OL responses of the TiO₂ nanofibers, pure MoS₂ nanosheets and MoS₂/TiO₂ nanocomposite under different input fluences and (d) schematic diagram of the nonlinear absorption in MoS₂/TiO₂ nanocomposite.

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The improved nonlinear light-matter interaction between the MoS₂/TiO₂ nanocomposite and fs laser, other than the contribution of the strong nonlinear absorption of MoS₂ nanosheets and TiO₂ nanofibers, can be further understood as follows: the hierarchical structure with vertical layered MoS₂ nanosheets rooting into TiO₂ nanofibers can afford a crowd of exposed edge states and larger surface areas to absorb light, leading to the significant enhancement of NLO property [3,33]. In addition, such unique structure also can enable the efficiently intimate contact between TiO₂ and MoS₂. When excited by the fs laser, the highly efficient interface electron transfer between TiO₂ and MoS₂, and excited state absorption (ESA) may occur, as shown in Fig. 4(d). The electrons are excited to either the excited state (ES) of MoS₂ or TiO₂ via multiphoton absorption (MA). The upward band bending and the interfacial band energy build up a strong internal electric filed. Under this driving force, the photoexcited electrons flow into TiO₂. Hence, owing to interface charge transfer, charge accumulates in the ES of TiO₂, which would improved the ESA process of TiO₂, and developed a strong potential between the ES and the ground state (GS), which also promotes MA for TiO₂ and MoS₂ [40].

4. Conclusions

We have successfully fabricated the MoS₂/TiO₂ nanocomposite with vertical layered MoS₂ nanosheets rooting into highly porous electrospum TiO₂ nanofibers through a hydrothermal approach. The MoS₂/TiO₂ nanocomposite exhibited enhanced OL effects with very low threshold F_OL ~22.3 mJ/cm², identified by an OA z-scan measurement. Apart from the contribution of the strong OL responses of MoS₂ nanosheets and TiO₂ nanofibers, the hierarchical structure with a crowd of exposed edge states, large surface areas and fast electron transfer from TiO₂ and MoS₂ also contribute to the enhancement of OL property of MoS₂/TiO₂ nanocomposite. This work advances the development of the novel 2D nanocomposite for efficiently improved NLO performance.