Zinc Oxide Coating Effect for the Dye Removal and Photocatalytic Mechanisms of Flower-Like MoS2 Nanoparticles

Flower-like MoS2 nanoparticles (NPs) consist of ultra-thin MoS2 nanosheets are synthesized via a facile one-pot hydrothermal method. The MoS2/ZnO p-n heterostructure is formed by coating n-type ZnO on the surface of flower-like MoS2 NPs through the seed-mediate route and post-annealing treatment. The effects for the dye removal and photocatalytic performances after ZnO coating are systematically investigated. The results demonstrated that the coating of ZnO nanoparticles has a positive promotion to the photodegrading properties while negative effect on the adsorption capacity of the MoS2/ZnO heterostructures. The related mechanisms on the relationship of adsorption capacity and photocatalysis are discussed in detail. Electronic supplementary material The online version of this article (doi:10.1186/s11671-017-2005-0) contains supplementary material, which is available to authorized users.


Background
Synthetic dyes are massively produced and widely used in many areas nowadays, while most of them are the hazardous materials in the wastewater. Because they are environmentally harmful and not quite prone to degradation, the daily life and health of human beings are imperceptibly influenced by these synthetic dyes [1]. Therefore, the development of efficient methodology or technology to decompose these synthetic dyes becomes emergency issues for environment remediation. Various reports demonstrated that adsorption and photocatalytic degradation are the conventional, effective, and economical methodologies in wastewater purification and environmental protection over the past decades [2][3][4].
In fact, the MoS 2 -based composite photocatalysts that are coupled with TiO 2 , ZnO, CdS, or reduced graphene oxide (RGO) have been reported recently [11,[21][22][23][24]. For example, the P-doped 2D-2D ZnO/MoS 2 nanocomposites show excellent photocatalytic activity in methyl blue degradation [22]. The P-doped ZnO nanosheets were prepared through conventional chemical vapor transportation and condensation (CVTC) method, and the 2D MoS 2 was prepared via liquid exfoliation method, then the ZnO nanosheets/MoS 2 hybrid photocatalysts were formed by ultrasonically handling and magnetically stirring. This was a complicated preparation process, and the ZnO and MoS 2 will be in loose contact with each other only by physically mixing, which is not beneficial for the efficient transfer of energy and carriers. In addition, most of the previous studies on MoS 2 -based composite photocatalysts reveal that the enhanced photocatalytic performance could be attributed to the synergetic effect of effective light-response, p-n heterojunctions, and large surface areas. However, there are scarcely researches that focused on the influence of the coating effect to the relationship between adsorption capacity and photocatalysts. Indeed, the adsorption and desorption equilibrium between the dyes and the photocatalysts before light illumination show positive promotion effect on the photodegradation process. It has been reported that strong surface adsorption performance is beneficial for the effective degradation of organic dyes during the photocatalytic reaction [25,26]. For example, the BiOI microspheres have more effective surface adsorption than random BiOI nanoplatelets, which lead to more excellent performance for the degradation of tetracycline hydrochloride (TC) [27].
Therefore, the relationship of adsorption and photocatalysts need to be understood before the application. Good adsorption ability means the organic dyes tend to be in closer contact with the photocatalysts, which is beneficial for the transmission of photogenerated carriers from photocatalysts to organic dye molecules. Strong surface adsorption ability portends more active sites for the photodegrading reaction as well [3]. Moreover, different components will lead to dual removal mechanisms between surface adsorption and photocatalytic degradation in the process of organic dye removal. For instance, the composite photocatalysts combining TiO 2 with highly adsorptive multi-walled carbon nanotubes (MWCNT) demonstrated high removal rate of methyl orange (MO) dyes in aqueous solutions [28].
Herein, we prepared the MoS 2 /ZnO heterostructures by coating the tunable ZnO layers onto the surface of flower-like MoS 2 nanoparticles with facile methods. The adsorption and photocatalytic abilities of MoS 2 NPs have been investigated before and after ZnO coating. The adsorption results illustrate MoS 2 is the main contributor to the adsorption capacity, which is non-selective adsorption to the organic dyes. After ZnO coating, the adsorption capacity is decreasing and the photocatalytic ability is increasing. And hence the relationship between the surface adsorption and photocatalytic degradation was discussed in detail.

Synthesis of MoS 2 Nanoparticles
In a typical process, 93.75 mg of Na 2 MoO 4 · 2H 2 O and 185 mg of thiourea was dispersed into 30 ml of H 2 O by rigorous stirring for 5 min. Then, 1 ml of concentrated hydrochloric acid (35% wt.) was added into the solution with stirring to form a homogenous mixture. The mixture was transferred into a 50-ml Teflon-lined autoclave after another 5 min continuous stirring. Finally, the sealed autoclave was placed into the oven and kept for 24 h at 200°C. The precipitate was centrifuged and sequentially washed by H 2 O for several times. The assynthesized products were dried (60°C, 12 h) for further measurement.

Adsorption and Photocatalytic Degradation Test
For the adsorption tests, 10 mL of organic dyes (RhB, MB, MO, and Rh6G) in aqueous solution with different concentrations were added into the quartz tube with 5 mg of the as-prepared samples. The suspension was homogenized by sonication and placed in the dark environment with continuous vigorous stirring. The adsorption/desorption equilibrium curves were measured at 5-min intervals by the UV-vis spectra on the Shimadzu UV-2550 spectrophotometer. The photocatalytic performances of MoS 2 /ZnO heterostructures were characterized by investigating the degradation of RhB with 10 mg/L. The preliminary homogenized solutions were illuminated under the simulated sunlight environment after reaching the adsorption/desorption equilibrium. The light source is mercury and tungsten mixed light lamp (300 W). And the corresponding UV-vis spectra were measured at 15 min intervals.

Characterization
Scanning electron microscopy (SEM) was carried out on a FEI Nova 400 NanoSEM at accelerating voltage of 20 kV. Transmission electron microscopy (TEM), highresolution TEM (HRTEM) images, and energy-dispersive X-ray spectroscopy (EDX) were performed on a JEOL JEM-2100F transmission electron microscope at 200 kV. The X-ray diffraction (XRD) patterns were recorded in a PANalytical X'Pert PRO X-ray diffractometer with Co Ka radiation. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Fisher ESCALAB 250Xi system equipped with a monochromatic Al Ka (1486.6 eV) as the radiation source.

Results and Discussion
The morphology of the as-obtained naked MoS 2 nanoparticles are characterized by SEM and TEM. The representative SEM image is shown in Fig. 1a, in which the surface of flower-like MoS 2 nanoparticles consisted of several or even single layers of ultra-thin MoS 2 nanosheets. The average size of individual flower-like MoS 2 nanoparticle is about 200 nm. As shown in Fig. 1c, highresolution TEM image demonstrates that the thickness of MoS 2 monolayer is ca. 0.7 nm, and the nanosheets consisted of about two to five layers, which could be indexed well with the theoretical thickness of MoS 2 monolayer (S-Mo-S unit cell) [29,30]. The EDX spectra of as-obtained MoS 2 nanoparticles are shown in Fig. 1b, the elements of Mo, S, C, and Cu could be found. The structure of the naked MoS 2 nanoparticles is further characterized by XRD (Fig. 1d), all the characteristic peaks can be assigned to the molybdenite-2H phase of MoS 2 (JCPDS card no. 37-1492). The above results reveal the successful synthesis of MoS 2 NPs.
Subsequently, the MoS 2 /ZnO heterostructures with different concentrations of zinc precursor are prepared. The detailed microstructural characterizations are presented in Fig. 2a-e, which show that the ZnO nanoparticles were successfully deposited on the surface of MoS 2 . Along with the increase of the concentration of Zn 2+ , both of the sizes and coverage area of ZnO nanoparticles are increased gradually. Compared with the uncoated MoS 2 nanoparticles (Additional file 1: Figure S1), the morphology of MoS 2 /ZnO (3:2) shows no significant change because only a few ZnO nanoparticles are coated on the surface of MoS 2 (Fig. 2a). When the Zn 2+ is increased to 0.02 M, some small ZnO particles appeared (as marked by the red circle in Fig. 2b). When the Zn 2+ concentration further increases to 0.03 M, most of MoS 2 are coated by ZnO nanoparticles, the uncovered area of MoS 2 was marked with red cycle in Fig. 2c. In addition, the size of ZnO nanoparticles grew much larger than 0.02 M. In Fig. 2d, the SEM image of MoS 2 /ZnO (3:10) shows that all the surface of MoS 2 was covered with ZnO nanoparticles and the ZnO nanoparticles presented as triangle-shape. When the Zn 2+ concentration elevated to 0.1 M, the ZnO particles with spindle-like structure that appeared on the surface of MoS 2 nanosheets are observed. The TEM images of MoS 2 /ZnO are shown in Fig. 2a′-   could be attributed to high mass percentage of ZnO (with poor photodegradation performance), and finally decreased the total degradation performance of MoS 2 / ZnO heterostructures. Generally, the reaction kinetics of photocatalytic degradation is followed by the pseudo first-order rate model: [6].
where k is the photodegradation rate. In    [32,33] and ZnO (4.7 eV) [29,34]. Then, the photogenerated electrons at the CB of MoS 2 can overcome the barrier and transfer to the CB of ZnO; meanwhile, the photogenerated holes will be in a position to transfer from the VB of ZnO to the VB of MoS 2 . Consequently, the photogenerated electron-hole pairs are separated efficiently at the contact interfaces of MoS 2 and ZnO, thus, improving the photocatalytic abilities of the heterostructural catalysts finally.
Meanwhile, when we measured the photocatalytic performance of MoS 2 /ZnO, it can be found that the surface adsorption plays an important influence in photocatalytic degradation process. It can be seen that 99% of RhB dyes can be absorbed by naked MoS 2 nanoparticles before the light illumination (Fig. 6a), while 27.7% of RhB dyes are absorbed when using the MoS 2 /ZnO (3:10) heterostructures as the photocatalysts (Fig. 6b). Thus, we measured the absorption capacities of different samples in dark environment for 30 min. The comparative absorbance of all the samples was carried out in Fig. 6c. It can be seen that the MoS 2 /ZnO photocatalysts exhibit decreased adsorption capacity along with the increasing of the concentration of Zn precursor. Obviously, the ZnO coating can prevent the MoS 2 to adsorb the dyes. The adsorption abilities of MoS 2 /ZnO photocatalysts are better than pure ZnO nanoparticles (12%, red dotted line in Fig. 6c). Furthermore, the naked MoS 2 after annealing treatment show decreased adsorption abilities, only 89% of the RhB dyes were adsorbed. The reason may be resulted from the surface morphologic change of the flowerlike MoS 2 nanoparticles (Additional file 1: Figure S1) that decreases the adsorption sites. The corresponding Brunauer-Emmett-Teller specific surface areas (S BET ) of MoS 2 nanoparticles before and after annealing were investigated by nitrogen adsorption (Beishide, 3H-2000PS2). In contrast, the S BET for flower-like MoS 2 nanoparticles is about 54.7 m 2 /g, which is much larger than that of MoS 2 nanoparticles after annealing treatment with a surface area of 13.2 m 2 /g (Additional file 1: Figure S2). The reason was largely attributed to the morphologic change of MoS 2 nanoparticles, resulting in the decrease of the adsorption capacities to MG dyes finally.
The selective adsorption to organic dyes is also a critical factor for the photocatalytic performance of catalyst. As the main contributor to the adsorption capacity of the MoS 2 /ZnO heterostructures, the adsorptive capacity of the naked MoS 2 was further measured with different organic dyes, including RhB, Rh6G, MB, and MO. The asselective dyes are very stable in routine environment. In addition, different concentrations of organic dyes (5, 10, and 20 mg/L) are tested. Additional file 1: Figure S3 displays the relationship between the adsorption time and concentrations. It is found that the adsorption process can be finished in 5 min, and there is no continued adsorption with prolonging adsorption time. The experiment results are in agreement with the report that the adsorption equilibrium can be reached in a relatively quick process of about 5 min [30]. The adsorption capacity is calculated based on the experimental dates and according to the mass balance relationship: [35].
where Q eq is the amount of dye adsorbed onto the naked MoS 2 , and C 0 and C eq are the initial and equilibrated dyes concentrations, respectively. V is the volume of solution, and m is the mass of the adsorbent. Figure 7 shows the isotherms for dyes adsorption on the asobtained naked MoS 2 nanoparticles. All the adsorption isotherms that fitted to the experimental dates are modeled using the Langmuir and Freundlich isotherm  (Table 1) [2]. The theoretical Langmuir and Freundlich isothermal parameters can be represented by the following Eqs. 3 and 4 [36,37].
Q e is the amount of dyes adsorbed onto the adsorbent at equilibrium, Q max is the maximum amount of adsorption, K L is the Langmuir adsorption equilibrium constant, K F is the Freundlich constant representing the adsorption capacity, and n is the constant depicting the adsorption intensity. The maximum adsorption capacities calculated from the experimental date for RhB, MB, MO, and Rh6G are 55.2, 28.4, 18.6, and 18.8 mg/g, respectively. The results reveal that the naked MoS 2 has a non-selective adsorption to the organic dyes with perfect adsorption capacities. From the nonlinear-curve-fitting results (R 2 ), we can see that the Langmuir isotherm present well with the experimental dates than that of Freundlich isotherm for RhB (0.9851 > 0.8628) and Rh6G (0.9921 > 0.9794), while the adsorption value is concurred with agreement with the Freundlich model than Langmuir model from the regression coefficient R 2 of MB (0.9506 < 0.9741) and MO (0.9921 < 0.9999). For the comparison, the absorption capacity of pure ZnO particles is tested. The corresponding effect of contact time and concentration is shown in Additional file 1: Figure S4a. The maximum adsorption amount is only 2.4 mg/g (R 2 = 0.9985) from the Langmuir model fitting result in the Additional file 1: Figure S4b.
From the adsorption isotherm studies, we can draw the conclusion that the pure ZnO nanoparticles has a poor adsorption to RhB dyes, which decreased the adsorption capacity of MoS 2 /ZnO heterostructures. MoS 2 nanoparticles act as the main adsorption substrate in the heterostructural system and show perfect and non-selective adsorption capacity to the organic dyes. The Langmuir and Freundlich modeled results can be confirmed that the organic dyes inhibited monolayer adsorption on the surface of MoS 2 nanoparticles and further indicated that the homogeneous nature of MoS 2 surface has a same adsorption capacity of all the surfaces [36]. Moreover, the adsorption intensity (1/n) gives an indication of the favorability of adsorption. When the n > 1 (or 10 > 1/n > 1), it represents favorable adsorption condition [38,39]. The calculated values of n are listed in Table 1, confirming the favorable adsorption condition of dyes on the surface of MoS 2 nanoparticles.

Conclusions
In summary, the naked flower-like MoS 2 nanoparticles are synthesized successfully via a hydrothermal method.
The MoS 2 /ZnO heterogeneous photocatalysts are synthesized and formed as p-n heterojunction. The coating of ZnO nanoparticles shows positive promotion to the photodegrading properties while negative effect on the adsorption capacity of the MoS 2 /ZnO heterostructures. The MoS 2 /ZnO (3:10) heterostructures showed 3.7 times than pure ZnO for the decomposition of RhB dyes. The adsorption property of naked MoS 2 was further investigated with RhB, MB, MO, and Rh6G. The adsorption relationship was described with Langmuir and Freundlich isotherm, showing that the adsorption of dye molecules on the surface of MoS 2 is monolayer adsorption and it is favorable for the adsorption of organic dyes in our study. Q e (mg/g) is the amount of dyes adsorbed onto the adsorbent at equilibrium, Q max (mg/g) is the maximum amount of adsorption, K L (L/mg) is the Langmuir adsorption equilibrium constant, K F (mg 1-1/n L 1/n g −1 ) is the Freundlich constant representing the adsorption capacity, n (dimensionless) is the constant depicting the adsorption intensity We envision that the MoS 2 /ZnO photocatalysts will become a potential candidate in the field of environmental remediation.

Additional file
Additional file 1: Figure S1. Supporting information is available online from the Wiley Online Library or from the author. (DOCX 1281 kb)