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The heterojunction of transition-metal dichalcogenides is an important task for their applications in optoelectronics and nanoelectronics owning to substantial enhancing and enticing the material, optical, and electronic properties. A precise and controllable nanostructure preparing method, although highly desired, remains quite challenging, especially for heterojunction growing. In this work, an inspiring strategy is first reported for the synthesis of the MoS2/ZnO composite structure through driving by the nano-photo-thermal energy. The ZnO NCs exhibit a 2D layer growth way on the surface of MoS2 nanosheets. MoS2/ZnO composite structures possess high quality optical properties for the application of heterojunction. The nano-photo-thermal energy drive nanoheterojunction growing technology is a promising strategy towards the facile and in situ controllable method of novel functional materials.
© 2016 Optical Society of America
1. Introduction
The Molybdenum disulfide (MoS2) composite nanostructures has been regarded as one of the most promising strategies for enhancing material properties and thus received much attention during recent decades [1–6]. Combining the electronic properties of the different materials in hybrid heterojunctions offers the possibility to create various new functionalities. MoS2, being layered semiconductors, has a relatively narrow band gap (1.9 eV and 1.2 eV for single- and multilayer MoS2, respectively) and strong near-infrared (NIR) optical absorption [7, 8]. As for the few-layer MoS2, the atomically thin profile results in their limited absorption, and the bandgap of the material determined limited spectral selectivity [9]. By combining MoS2 with other materials to form the composite nanostructures, the ultrathin MoS2 nanosheets can efficiently overcome inherent defects [10], such as the high recombination rate of the photographed electron-hole pairs and the lack of effective emission sites. Wider-range bandgap engineering largely modulated within composite nanostructures is of great significance in constructing high-performance and multifunctional nanomaterials.
Among various solution heterojunction growing nanostructures technologies, the epitaxy layer mainly keep natural growth characteristics depending on crystallographic texture itself [3, 11, 12]. Although the morphology and composition of nanomaterial can be tuning by changing the growth parameters or introducing the surfactants, the effect of the matrix layer on the growth way of the epitaxy layer can be ignored. It can be very necessary to point out that the strain and combination mode between the matrix and the epitaxy layer extremely influence the optical and electrical properties of the heterojunction nanomaterials [13–16]. The combination of MoS2 and ZnO has been demonstrated extremely sensitive optical response due to the increased specific surface area and improved electron-hole pair separation [10, 11, 17–21]. The solution-grown nanomaterials is known to be a simple, rapid, inexpensive, and large-scale preparing technique. However, it can be challenging how to grow ZnO on the surface of the few-layer MoS2 and inhibit the free growth in the solution.
In this communication, we reported MoS2/ZnO composite structures were prepared in the solution via nano-photo-thermal energy technology. The nano-photo-thermal energy has attracted much interest in recent years due to the great promise for nanotechnology in medical applications [7, 8]. Here, we use the heat energy from the MoS2 nanosheets photothermal conversation, not from the laser energy, to realize the growth of ZnO NCs on the surface of MoS2. The ZnO NCs exhibit a two-dimensional (2D) layer growth way and tile on the surface of MoS2 nanosheets. In addition, MoS2/ZnO composite nanostructures possess high quality optical properties for the application of heterojunction. Comparing with the conventional composite structure deposition techniques, the the nano-photo-thermal energy can be adjusted nearly in real time by changing the wavelength, treating time and power of laser, to realize the tuning of nanomertials growth mode.
2. Experimental
Preparation of the few-layer MoS2 nanosheets. MoS2 nanosheets were prepared from commercial layered MoS2 materials (99.0% purity, 50 mg) with a grain size less than 10 μm via a safe yet simple liquid-phase exfoliation method. In brief, MoS2 powder (∼20 mg) were ground with NMP (0.4 mL) for 2h in a mortar. Then the gel-like mixtures were put into a vacuum tube furnace at 60 °C in order to remove the NMP solvents. After being dried for 5h, the powder were dispersed in 45 vol% ethanol/water mixture (20 mL) and sonicated for 3h and then centrifugated at the speed of 6000 rpm for 30 min. Finally, a homogeneous and water-soluble grayish-blue well-dispersed MoS2 aqueous solution was obtained for the further study of their microstructure, optical and photothermal properties.
Photo-thermal energy grow the MoS2/ZnO nanoheterojunctions: The ZnO nanosheets were prepared on the surface of MoS2 via the NIR laser driving. The MoS2/ZnO composite structures were synthesized as follows 0.14 g Zn(CH3COOH)3 H2O (Zn(Ac)2) was dispersed in 50 mL ethanol. After stirring for 30 min at room temperature, MoS2 nanosheets aqueous solution were added and illuminated by NIR laser (808nm, 1.6w) for another 10 min. The precipitates were separated by centrifugation, washed with absolute ethanol, and dried naturally for the further characteristics.
Photothermal conversion of MoS2: For measuring the photothermal conversion performance of MoS2 aqueous solution, 808 nm NIR laser was delivered through a quartz cuvette containing aqueous dispersion (1.0 mL) of MoS2 aqueous solution, and the light source was an external adjustable power 808 nm semiconductor laser device with a 5 mm diameter laser module. The output power was independently calibrated using a handy optical power meter and was found to be 1.6 W for a spot size of ~0.6 cm2. A thermocouple with an accuracy of ± 0.1°C was inserted into the aqueous dispersion of the MoS2 nanosheets perpendicular to the path of the laser and the temperature was recorded one time per 10s.
Determination instruments: UV-Vis absorption spectra were obtained using a Perkin Lambda UV-Vis-near-infrared spectrophotometer. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a JEOL microscope operated at 200 kV, respectively. Surface image of MoS2 sample were recorded using Atomic Force Microscopy (AFM, Micronano Scanning Probe Microscope). All optical measurements were performed at room temperature under ambient condition.
3. Results and discussion
MoS2 is a typical layered material with 2D layers composed of strong S−Mo−S intralayer covalent bonding. We use a simple method for preparing single-layer MoS2 nanosheets via a N-methyl-2-pyrrolidone (NMP) and sonication treatment exfoliation process [22]. The process was briefly described as follows. The commercial MoS2 bulk materials were grinded with NMP for about 2h. NMP acts as a stabilizing surfactant to avoid reassembling of the MoS2 nanosheets during the grinding process. Then, 3h sonication is applied to further exfoliate MoS2 in 45 vol% ethanol/water solution, and then centrifuged for 30 min at 6000 rpm to separate the few-layer MoS2 from the unexfoliated MoS2 sheets.
Figure 1(a) is an AFM image of MoS2 few-layer nanosheets after grinding deposited on a silicon wafer. The nanosheets height observed are in the range of several nanometers, as shown in Fig. 1(b), in agreement with the corresponding TEM data (Fig. 3(a)). It can indicate that the grinding process can thinning MoS2 to only several few layers thickness. Figure 2 shows the schematic illustration of the process of MoS2 and the synthesis of MoS2/ZnO composite structures via a nano-photo-thermal energy driving method. Firstly, MoS2 was roughly exfoliated through grinding with NMP treatment, then the few-layer MoS2 nanosheets were produced through sonication acting. The ZnO nanosheets formed by treating of NIR laser illuminating after Zn ion absorbed on the surface of few-layer MoS2. Figures 3(a)-3(c) show transmission electron microscopy (TEM) images of the as-prepared few-layer, monolayer MoS2, and MoS2/ZnO composite structure, respectively. It can clearly be seen that the MoS2 sheet is the electron beam transparent, compared with the few-layer MoS2 (Fig. 3(a)), which indicated that the MoS2 sheets are obviously thinned. Furthermore, the discontinuous region ZnO nanosheets appear in TEM images of the MoS2/ZnO composite structure (Fig. 3(c)).
Fig. 1 (a) AFM image of MoS2 sample after grinding. (b) AFM height profile across the MoS2 nanosheets in the panel (a).
Fig. 2 (a-d)Schematic illustration of the synthesis of MoS2/ZnO composite structures via a nano-photo-thermal energy driving method. (a)NMP treatment roughly exfoliation MoS2 through grinding. (b)producing fewlayer MoS2 nanosheets through sonication. (c)Zn ion absorbed on the surface of few-layer MoS2. (d)the ZnO nanosheets formed by treating of NIR laser illuminating.
Fig. 3 Low magnfication TEM image for MoS2 after grinding(a), sonication(b), and MoS2/ZnO composite structure (c), respectively.
Detailed the High-Resolution Transmission Electron Microscopy (HRTEM) and the selected area electron diffraction (SAED) analysis were carried out on the MoS2 (Figs. 4(a) and 4(b)) and MoS2/ZnO composite nanomaterials (Figs. 4(c)-4(h)). The image revealed stacking of MoS2(002) layers with an interplanar spacing of 0.62 nm. The fast Fourier transfer (FFT) patterns of the MoS2 in the inset in Fig. 4(a) indicates that the measured lattice spacing of 0.26 nm can be assigned to the {100} planes of the MoS2 nanosheet. HRTEM images of ZnO NCs on MoS2, shows distinguishable lattice fringes for ZnO and MoS2. As shown in the image in Fig. 4(e), ZnO NCs were adsorbed on the surface of the MoS2 nanosheets, which clearly demonstrated the epitaxial growth of ZnO NCs on the MoS2 surface. As shown in Fig. 4(d), the SAED pattern of a MoS2/ZnO nanomaterial, with the basal plane of MoS2 nanosheets normal to the electron beam, gives two sets of diffraction spots, assigned to MoS2 and ZnO, respectively. The diffraction spots of {100} ZnO planes, with a corresponding lattice spacing of 2.8 Å, are aligned with the neighboring spots of {100} MoS2 with a lattice spacing of 2.6 Å. Note that the diffraction spots for {110} ZnO and {110}MoS2 planes are too close to be nearly impossible to distinguish in the SAED pattern. The detailed HRTEM characterization analyzing was carried out (Figs. 4(e)-4(h)) in order to further identify the orientation of the synthesized ZnO NCs,. To precisely measure the crystal-plane spacings, the NC fringes (as shown by red boxes I, II, III, and IV in Fig. 4(c)) in the TEM images were digitally processed using a 2D Fourier transform scheme, contrast enhanced (see inset, Fig. 4(c)), and inverse transformed to obtain the 2D Fourier-transform filtered lattice fringes (Figs. 4(e)-4(h)). The NCs were found to have plane spacings consistent with {100} ZnO planes, indicating that our preparing method was indeed controlling the growth of ZnO NCs along MoS2 {100} planes.
Fig. 4 HRTEM analysis of MoS2 sheets and MoS2/ZnO composite structure (scale bar, 5 nm): (a) high resolution image showing the multilayer nature of the sheets (lower right corner) and high resolution image of the sheets showing the hexagonal structure of MoS2, the inset in (a) is fast Fourier transform of the electron diffraction pattern of a few layers of MoS2. (c) high resolution image showing MoS2/ZnO composite structure, the inset in (c) is fast Fourier transform of the electron diffraction pattern. (b) inverse transforms of contrast-enhanced FFTs of the marked areas in Figure (a). (d) A selected area for electron diffraction patterning of MoS2/ZnO composite structures. (e-f) the nanocrystal fringes (as shown by green boxes I, II, III, and IV in Fig. 1(c)) in the HRTEM images were digitally processed using a 2D Fourier transform scheme, contrast enhanced (see inset, Fig. 2(c)), and inverse transformed to obtain the 2D Fourier-transform filtered lattice fringes.
Figure 5 shows the absorbance spectra of aqueous suspensions of nanosheets samples MoS2 and the MoS2/ZnO sample. The appearance of absorption band of the A exciton, B exciton, and the splitting energy between A and B excitons after exfoliation is remarkable, which might be ascribed to the exfoliation of few-layer MoS2 into the monolayer nanosheets with a few nanometers thickness due to the quantum confinement effect. By compared with the previous theoretical results based on few-layer and bulk MoS2 model [23], the peak of A and B excitons mainly concentrate on the corresponding position of the monolayer MoS2. The background peaks were split into two peaks at 2.71eV and 2.87eV, which originated from the transition in the K-point of the Brillouin zone. The spin-orbit energy is amazedly similar with the difference between A exciton peak and B, which can be helpful to understand the detailed transition process. It can be essential to point out that the photoabsorbance of MoS2/ZnO composite structures changes significantly in the range between 300 to 400nm as the ZnO amount increasing, which proved the ZnO formed in the surface of MoS2 nanosheets. Besides, the excellent photoabsorbance prove the MoS2/ZnO composite structure is with high quality crystal characteristics and good heterjunction properties. The coexist exciton absorbance peaks of both ZnO NCs and MoS2 nanosheets shows that ZnO NCs formed is extremely thin.
Fig. 5 UV-vis absorption spectra of as-synthesized few-layer MoS2 and MoS2/ZnO suspension, all units are eV.
The typical photograph and the Dinydy experiment of Zn(Ar)2 aqueous solution, monolayer MoS2, and MoS2/ZnO from left to right, respectively, is shown in Fig. 6(a). Photographs and the Tyndall experiment of Zn(Ar)2 aqueous solution, monolayer MoS2, and MoS2/ZnO from left to right, respectively. A typical photograph of MoS2 suspension by 2h grinding and 3h sonication after centrifugation is shown in the middle of Fig. 6(a). The aqueous dispersion was stable and transparent over the period of 7 days. When the ZnO NCs were grown on the surface of MoS2 nanosheets, the black-gray precipitation appear in the bottom of the beaker after the solution was treated by 808nm laser because the electro negativity of MoS2 was changed due to the formation of ZnO NCs. The temperature increases of MoS2 and MoS2 + Zn(Ac)2 suspension were illustrated with increasing of laser illuminating time (Fig. 6(b)). The blank experiment demonstrates that the temperature of Zn(Ac)2 is increased by less than 3°C. In order to clearly understand the growth mechanism of the MoS2/ZnO composite structures, a possible mechanism is discussed as follow. Few-layer MoS2 nanosheets structure can efficiently enhance the surface to volume ratio of nanomaterials, which can critically enhance the absorption at the NIR band, which results in the high photothermal conversation. The Zn2+ is extremely easy to be absorbed on the surface of MoS2 nanosheets because the dispersed MoS2 was negatively charged [22]. Besides, it can be necessary to point out that the increasing temperature of MoS2 nanosheets can be more high than the water environment according to the microscopic theory [24]. The ZnO NCs growth temperature can be very easily to be arrived at the low heat energy [25], which should be crucially to avoid to destroy the MoS2 due to high laser energy. Thus, ZnO NCs can be formed on the MoS2 surface assisted by nano-photothermal energy. Moreover, it can be exciting to find that ZnO NCs keep a 2D in situ growth along to the surface of MoS2, which can be ascribed to that the free growth was inhibited through enhancing the in situ growth.
Fig. 6 (a) Photographs and the Tyndall effect of Zn(Ac)2, MoS2 nanosheets, and MoS2/ZnO aqueous solution from left to right, respectively. (b)Photothermal effect of MoS2, MoS2 + Zn(Ac)2, and Zn(Ac)2 aqueous dispersion as a function of irradiation time (10 min) using the NIR laser shining (808 nm, 1.6 W). (c) Photothermal conversion effect of the aqueous dispersion of as a function of irradiation time using the NIR laser shining (808 nm, 1.6 W) for 10 min, and shut off then. (d) Time constant for heat transfer from the system is determined to be τs = 162.6s by applying the linear time data from the cooling period (after 600s) versus negative natural logarithm of driving force temperature, which is obtained from the cooling stage of panel (a).
In order to clearly understand the photothermal conversion properties of MoS2 nanosheets, we further analysis the photothermal conversion of MoS2 solution and determine the system heat transfer time constant based on the macroscopic model. Similar to the ones previously published [26–28], the energy balance can be expressed as:
where m and are the mass and heat capacity of water and T is the solution temperature. The photothermal energy from the nanocrystals QNp can be written aswhere I is the laser power, is the absorbance at the excitation wavelength of laser, and is the photothermal conversation efficiency. The heat lost to the surroundings by the cuvette walls QLoss was given aswherehis heat transfer coefficient, is the surface area of the container, and are the equilibrium temperature and ambient temperature of the surroundings. The temperature profile after the laser is turned on/turn off can be obtained by solution of the Eq. (1). The system heat transfer time constant is determined during the cooling process of solution after the laser was turned off. The heat transfer time constant is importantly reflect the heat energy releasing characteristic of MoS2, which can be given by applying the linear time data from the cooling period vs negative natural logarithm of driving force temperature.The temperature decrease of the MoS2 solution was monitored to determine the heat transfer time constant from the dispersion system to the room temperature, as shown in Fig. 6(c). The thermal equilibrium time constant can effectively evaluate the heat storage capacity, and can be determined by heat transfer equation [27, 28]. The thermal equilibrium time constants of the aqueous dispersion of NFs with different concentrations were obtained for thermal equilibration with the surroundings via conductive and irradiative heat transfer. Figure 6(d) shows a time constant for heat transfer time determined as the negative reciprocal slope of ln(θ) vs. t using temperature versus time data recorded during cooling of the solution. The thermal equilibrium time constant of the samples are calculated to be 162.2s for thermal equilibration with the surroundings via conductive and radiative heat transfer. The characteristic time to establish thermal equilibrium in a single MoS2 nanosheets can be nanosecond-scale values [27, 28], which is larger than the thermal equilibrium time, which tells us that MoS2 nanosheets can keep a stable temperature to ensure the ZnO NCs growing on the surface of MoS2 nanosheets.
4. Conclusion
In conclusion, we have proposed a precise and controllable nano composite structures preparing method, especial heterojunction growing. A typical example first reported for the synthesis of the MoS2/ZnO composite structure by the nano-photo-thermal energy. The heat energy from the MoS2 nanosheets photothermal conversation, not from the laser energy, drive the growth of ZnO NCs on the surface of MoS2. The ZnO NCs exhibit a 2D layer growth way and tile on the surface of MoS2 nanosheets. In addition, MoS2/ZnO composite structures possess high quality optical properties for the application of heterojunction. The nano-photo-thermal energy drive nanoheterojunction growing technology is a promising strategy towards the facile and in situated controllable method of novel functional materials through adjusting nearly in real time by changing the wavelength, treating time and power of laser.
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (11247025).
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