Ya
Chen
a,
Penghe
Su
a,
Xiaotong
Liu
a,
Hongchi
Liu
a,
Baolin
Zhu
a,
Shoumin
Zhang
a and
Weiping
Huang
*abc
aCollege of Chemistry, Nankai University, Tianjin 300071, China
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China
cThe Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
First published on 15th November 2018
Novel 3D Cu2S–MoS2(x:y) nanocomposites with different proportions of Cu2S (x) and MoS2 (y) are synthesized successfully by a one-step hydrothermal method with the assistance of the ionic liquid [BMIM]SCN. The characterization results show that the nanocomposites are self-assembled from nanosheets of Cu2S and MoS2, they display nanoflower morphology and a typical mesoporous structure. The fabrication mechanism of the nanocomposites is investigated using time-dependent experiments, which indicate the key role of the ionic liquid (IL) in the synthesis process. Furthermore, TAA is used as a sulfur source instead of the IL to form a Cu2S–MoS2 nanocomposite, with the aim of further investigating the effects of the IL on the morphology of the composite. Photodegradation of MB under visible light irradiation experiments were used as probe reactions to evaluate the photocatalytic performance of the as-prepared samples. All the nanocomposites show better catalytic activity than Cu2S and MoS2 monomers. Among the different Cu2S–MoS2(x:y) nanocomposites, the Cu2S–MoS2(1:1) composite exhibits the most excellent photocatalytic performance and cycling stability.
Molybdenum disulfide (MoS2) has a layered structure consisting of Mo–S–Mo sandwiched in a graphite-like manner. MoS2 nanostructures have been found to have a superior response to visible-light illumination due to their layered structures. Hence, MoS2 nanostructures are usually used as co-catalysts to improve photocatalysts in terms of catalytic activity and stability against photo corrosion.35–39 Nano-MoS2 has a high conduction band energy level, which makes it an electronic trapper and it can easily transfer electrons to Cu2S. The extended electronic channels allow longer separation-times of photo-generated electron–holes. However, the reported Cu2S–MoS2 composites were usually synthesized by complex preparation procedures and have small specific surface areas.40,41
In this paper, Cu2S–MoS2 composites are facilely synthesized by a one-pot strategy with the assistance of an IL and hydrothermal conditions. Comparing with the other synthesis methods, a one-pot strategy is relatively simple and convenient. The fabrication mechanism of the nanocomposites with the assistance of the IL is investigated using time-dependent experiments. Furthermore, the photocatalytic performance of the Cu2S–MoS2 nanocomposites, Cu2S and MoS2 are investigated using the photodegradation of MB solution under visible-light irradiation. The influence of the proportions of Cu2S and MoS2 on the morphology, structure and photocatalytic performance are discussed. Moreover, a Cu2S–MoS2 composite is synthesized using TAA as a sulfur source instead of the IL, and the key role of the ionic liquid in the synthesis of Cu2S–MoS2 composites is also discussed.
Cu2S was synthesized under the same conditions with 0.066 g Cu(CH3COO)2·H2O, 0.366 g [BMIM]SCN and 16 mL deionized water without the molybdenum precursor.
MoS2 was also synthesized under the same conditions with 0.066 g (NH4)6Mo7O24·4H2O, 0.366 g [BMIM]SCN and 16 mL deionized water.
Cu2S–MoS2(1:1)-T was synthesized using thioacetamide (TAA) as the sulfur source instead of [BMIM]SCN: 0.066 g Cu(CH3COO)2·H2O, 0.066 g (NH4)6Mo7O24·4H2O and 0.134 g TAA were dispersed in 16 mL deionized water. After stirring for 30 min, the solution was transferred into a 20 mL autoclave for hydrothermal synthesis at 200 °C for 24 h. Cu2S–MoS2(1:1)-T was obtained after washing and drying procedures.
Fig. 1 (a–e) SEM images of Cu2S–MoS2(x:y): (a) 1:3, (b) 1:2, (c) 1:1, (d) 2:1 and (e) 3:1; (f) XRD patterns of samples. |
N2 adsorption–desorption experiments were carried out to investigate the textural and structural characteristics of the resultant samples, and the results are shown in Fig. 2 and Table 1. All the samples exhibit the typical type IV adsorption–desorption isotherm with a H3 hysteresis. This means that the samples are characterized by a mesoporous structure and slit-like pores. In addition, all of the nanocomposites show a similar pore size distribution with the maximum values located at 3.8 nm. As the proportion of Cu2S increases, the SBET of the samples increases continuously (Table 1) and all of the composites with different proportions exhibit a much higher specific surface area than that of pure Cu2S and MoS2. The differences in specific surface area may affect the catalytic performance of the composites.
x:y | 1:3 | 1:2 | 1:1 | 2:1 |
S BET (m2 g−1) | 21.94 | 35.87 | 49.31 | 67.52 |
x:y | 3:1 | Cu2S | MoS2 | (1:1)-T |
S BET (m2 g−1) | 81.93 | 5.55 | 16.67 | 28.94 |
TEM images of the Cu2S–MoS2(1:1) nanocomposite are shown in Fig. 3a and b, and two different lattice stripes can be found in the HRTEM image of the sample. In Fig. 3b, two orthogonal lattice fringes in the top inset show crystal spacing of 0.39 and 0.56 nm, which can be assigned to the {110} and {100} planes of Cu2S (53-0522), respectively. According to the literature,43 the layered spacing of 0.89 nm in the bottom inset should be the classical {002} plane of MoS2 (37-1492), which maybe resulted from hydrothermal synthesis of MoS2 without annealing treatment. The existence of Cu, Mo and S is confirmed by the EDS spectrum and elemental mapping shown in Fig. 3c. As the X-ray can penetrate 1 μm thickness, we could deduce that Mo, Cu and S are all distributed in the sample with high abundance. Meanwhile, the interlayer distance of the peak at 9.0° in the XRD pattern is about 0.9 nm according to the Bragg equation, which agrees with the result of the TEM. From the electron microscopy images, EDS-mapping and XRD analysis, it is demonstrated that nanoflower like composite materials have been successfully prepared in one-pot, and these are made up of Cu2S and MoS2.
Fig. 3 (a and b) HRTEM images and (c) EDS spectrum and elemental mapping of S, Cu and Mo of the Cu2S–MoS2(1:1) nanocomposite. |
In order to further confirm the composition of the composite materials, XPS and AES were performed to investigate the valence state of the elements in Cu2S–MoS2(1:1), the results are displayed in Fig. 4. In Fig. 4a, the high-resolution spectrum of Cu 2p shows two peaks with binding energy (BE) of 932.2 eV and 952.2 eV, which correspond to Cu 2p1/2 and Cu 2p3/2, respectively, indicating the existence of Cu1+. Because no satellite peaks are detected, the existence of Cu2+ is excluded.44 The AES (Fig. 4b) further confirms that the valence state of Cu is +1, since the characteristic peak associated with LMM at 569.6 eV is in accordance with the literature.45 The two peaks of Mo with BE of 229.3 eV and 232.5 eV in Fig. 4c are assigned to Mo 3d3/2 and Mo 3d5/2, respectively, which indicate that the Mo in the composite is tetravalent.46 According to a previous report,47 the peaks of the S 2p spectra at 162.3 eV (2p1/2) and 161.1 eV (2p3/2) in Fig. 4d are characteristic of S2−. The results demonstrate that the nanoflower like composite materials consist of Cu2S and MoS2. The BE values of the Cu and Mo spectra shift slightly compared with the literature as mentioned. This can be attributed to the interaction of electrons between the Cu2S and MoS2 components.
Fig. 4 (a, c and d) High resolution X-ray photoelectron spectra of Cu 2p, Mo 3d and S 2p in the Cu2S–MoS2(1:1) nanocomposite, respectively; (b) LMM spectrum of Cu in the Cu2S–MoS2(1:1) nanocomposite. |
In order to explore the formation mechanism of Cu2S–MoS2 nanocomposites, a series of time dependent experiments during the preparation of Cu2S–MoS2(1:1) were performed by intercepting the intermediates during continuous reaction time. Fig. 5a is the SEM image of the white precipitate collected after 2 h hydrothermal reaction. The image shows the morphology and the micro-bulks are about 10 μm and micro-rods about 40 μm in size. When the reaction time is prolonged to 4 h, the micro-rods become shorter around 20 μm and form hollow structures, while the micro-bulks decompose to nanoparticles (Fig. 5b). When the reaction time reached 8–12 h, the nanoparticles gather to form nanoflowers with size about 200 nm, which are assembled with nanosheets (Fig. 5c and d), while the micro-rods dissolve to less than 10 μm. After 16 h, bulges begin to form on the surfaces of nanoflowers (Fig. 5e) and the micro-rods dissolve to less than 5 μm. After 20 h, the micro-rods are fully dissolved to form nanosheets and grow together with the nanosheets in the nanoflowers. When the time is extended to 24 h, the two kinds of nanosheet keep growing together, showing the flower like appearance of the final products (Fig. 1c). As confirmed by the XRD results (Fig. 5g), the XRD curves at 2 h should be mixed peaks of micro-bulks and micro-rods, which mainly show the signal of [BMIM]2Mo4O13 according to the literature.43 With the reaction time prolonged from 4 h to 8 h, the [BMIM]2Mo4O13 peaks (8.6°–10.5° and 21°–49°) become weaker. After 12 h, the peaks of Cu2S and MoS2 appear. After 16 h, only Cu2S and MoS2 peaks remain, which is in agreement with the final XRD curves. UV-Vis DRS was carried out to investigate the optical properties of the as-prepared samples and the results are shown in Fig. 5h. When the synthetic reaction time was extended from 2 h to 24 h, the light absorption in the visible region becomes more and more broad and the absorbance capability of the composite becomes higher and higher. Meanwhile, the light absorption of the composite in the UV region is maintained. This result could be due to the more regular morphology and increased crystallinity obtained from the prolonged reaction time. So the well-developed Cu2S–MoS2(1:1) composite at 24 h should have the best response capability and light trapping properties in the whole visible region. In addition, the UV-Vis DRS results are in accordance with the SEM analysis (Fig. 5a–f).
Based on the above results, the growth mechanism of the Cu2S–MoS2 composite materials can be described as a four-step process (Fig. 6). In the first step (within 2 h), two kinds of metal soluble salt react with the ionic liquid. The Mo4O132− transformed from (NH4)6Mo7O24·4H2O could react with [BMIM]+, forming [BMIM]2Mo4O13 micro-rods, while Cu2+ reacts with SCN−, forming Cu2S micro-bulks. The morphologies of the sample in this step are micro-rods and micro-bulks (Fig. 5a) and the XRD curves show the strong characteristic peaks of [BMIM]2Mo4O13 (Fig. 5g). The second step is the reaction within 8 h. In this step, the [BMIM]2Mo4O13 micro-rods begin to react with H2S on their surfaces and form MoS2 crystal nuclei, while the inner part of the micro-rods would diffuse to the surface forming hollow structures by the Kirkendall effect (Fig. 5b). At the same time, the Cu2S micro-bulks dissolve into nanoparticles and gather to form nanoflowers by Ostwald ripening (Fig. 5c). The XRD curves of this step show that the characteristic peaks of the Mo precursor become weaker (Fig. 5g). When the reaction time was prolonged to 12–16 h, the sample is in the third reaction step. In this step, [BMIM]2Mo4O13 decomposes rapidly. The micro-hollow-rods are hard to see, while the Mo precursor just remains as nanoplates (Fig. 5d and e). The main morphology of the sample is nanoflowers with ridges on their surfaces. These should be the MoS2 crystal nuclei adsorbed on the Cu2S nanosheets. The XRD peaks of [BMIM]2Mo4O13 disappeared and Cu2S–MoS2 peaks appeared, which can support the above view (Fig. 5g). The last step is the reaction from 16 h to 24 h. The MoS2 nanosheets grow together with the Cu2S nanosheets (Fig. 1c), forming the final composite material. As a result, the composite material is made up of Cu2S and MoS2, and should exhibit good photocatalytic activity.
To further explore the effects of the IL on the morphology of Cu2S–MoS2 composites, TAA was used as a substitute for the IL to synthesize Cu2S–MoS2, this was labeled Cu2S–MoS2(1:1)-T. Its XRD pattern and SEM image are shown in Fig. 7a and b, respectively. As seen in Fig. 7a, Cu2S–MoS2(1:1)-T exhibits similar diffraction peaks to Cu2S–MoS2(1:1), but the intensity of the diffraction peaks is obviously weaker than those of Cu2S–MoS2(1:1). This phenomenon demonstrates that the addition of the ionic liquid could improve the long-term order of the compound structure. In addition, compared with the irregular morphology and smaller specific surface area of Cu2S–MoS2(1:1)-T (Fig. 7b and Table 1), Cu2S–MoS2(1:1) shows a regular 3D nanoflower morphology and much larger specific surface area (Fig. 1c and Table 1). This indicates that the ionic liquid plays an important role in controlling the morphology of the composites during the preparation process, which is consistent with the literature.48 The different morphologies and specific surface areas may have impacts on the photocatalytic performance.
Fig. 7 XRD patterns of Cu2S–MoS2 (1:1) and Cu2S–MoS2 (1:1)-T (a); SEM image of Cu2S–MoS2 (1:1)-T (b). |
The photocatalytic results for all the catalysts are shown in Fig. 8 as are their corresponding curves of ln(C0/C) vs. irradiation time. The rate constant values (k) of the different samples are listed in Tables 2 and 3. As seen in Fig. 8a, all of the Cu2S–MoS2(x:y) samples show excellent adsorption capacity to methylene blue in darkness while there is great difference in photocatalytic performance. All the Cu2S–MoS2(x:y) composites present better photocatalytic performance than Cu2S or MoS2. This result indicates that the existence of MoS2 could obviously enhance the photoactive performance of the composites under visible irradiation. The more regular 3D nanoflower structure and larger specific surface area could be the reason for the enhanced photocatalytic performance compared to that of pure Cu2S and MoS2. In addition, the efficient electron transport between the Cu2S and MoS2 interface of the composites may have a positive influence on photocatalysis. Among the Cu2S–MoS2(x:y) samples, the Cu2S–MoS2(1:1) nanocomposite exhibits the most outstanding performance with k = 0.0471 min−1. As the ratio of Cu2S to MoS2 increases from 1:3 to 1:1, the photocatalytic efficiency increased obviously. This result may be due to the increased proportion of Cu2S to MoS2 which results in larger specific surface area and more regular morphology. The regular 3D structure and larger specific surface area could supply more active sites on the surface for transfer of charge-carriers and a larger contact area for substrates, thus promoting the photocatalytic performance. In addition, the number of transferred electrons on the interface of Cu2S–MoS2 composites could increase with the increased proportion of Cu2S. In this case, the interface between Cu2S and MoS2 could improve the electron transfer efficiency and effectively inhibit charge recombination. However, as the ratio of Cu2S to MoS2 increases further from 1:1 to 3:1, the degradation rate of MB decreased dramatically despite of the ever-increasing specific surface area. This phenomenon should be caused by excessive MoS2 covering the Cu2S component, and the decreased exposed amount of Cu2S could hinder electron transfer on the interface between Cu2S and MoS2, thus in turn leading to the decrease in photoactivity. Furthermore, as the ratio of Cu2S to MoS2 increases from 1:1 to 3:1, the morphology of the composites becomes irregular, which can also negatively affect the photoactivity. MoS2 has excellent absorption ability for visible light and transfer ability for electrons, which could result in shortened lifetimes of photo-induced charge-carriers. Therefore, a low MoS2 proportion could depress the composite's photocatalytic performance. On the other hand, the gradual irregular morphology (shown in Fig. 1) may have a negative impact on the photocatalytic activity of the nanocomposites.
Sample | k (min−1) | Sample | k (min−1) |
---|---|---|---|
1:3 | 0.0135 | 2 h | 0.0027 |
1:2 | 0.0247 | 4 h | 0.0049 |
1:1 | 0.0471 | 8 h | 0.0085 |
2:1 | 0.0077 | 12 h | 0.0162 |
3:1 | 0.0058 | 16 h | 0.0223 |
P-25 | 0.0006 | 20 h | 0.0360 |
Blank | 0.0002 | 24 h | 0.0471 |
Cu2S | 0.0037 | MoS2 | 0.0032 |
Cu2S–MoS2(1:1)-T | 0.0049 |
Cycle | 1st | 2nd | 3rd | 4th |
---|---|---|---|---|
k (min−1) | 0.0471 | 0.0381 | 0.0288 | 0.0245 |
In Fig. 8a, the Cu2S–MoS2(1:1) nanocomposite shows much higher catalytic activity than Cu2S–MoS2(1:1)-T, which corresponds with the XRD, SEM and BET results. This indicates that ionic liquid-assisted Cu2S–MoS2 nanocomposites have excellent photocatalytic performance for the degradation of MB under visible light.
Fig. 8b shows the photocatalytic results of the Cu2S–MoS2(1:1) nanocomposite with different reaction times at 200 °C. With prolonged reaction time, the photocatalytic performance of the samples is significantly enhanced and the sample within 24 h exhibits the best catalytic performance. With the extension of the reaction, the samples exhibit more regular morphology and their response ability to visible light enhance obviously. The catalytic results are also in accordance with the SEM and UV-Vis DRS analysis.
Cycling experiments were carried out to investigate the lifetime and cycle stability of the Cu2S–MoS2(1:1) nanocomposite and the results are shown in Fig. 8c. It can be seen that the sample still exhibits good degradation of MB after four cycles. The decreased photocatalytic performance may result from the adsorption of the substrate and products on the catalyst in reaction, which may decrease the number of active sites. In addition, this result is also attributed to the loss of catalyst in the recovery process.
After the cycling experiment, the Cu2S–MoS2(1:1) nanocomposite was characterized by XRD and XPS, and the results are shown in Fig. S5 and S6 (ESI†), respectively. As can be seen from the XRD pattern of the composite in Fig. S5 (ESI†), the composite still retains its structural characteristics after the experiment although peak intensity was slightly different from that before the reaction. From Fig. S6 (ESI†), it can be seen that the valence states of the elements in the Cu2S–MoS2(1:1) composite did not change during the experiment. This means that the nanocomposite still retains its internal structure after cycling and that it is an excellent and stable catalyst for the degradation of MB under visible irradiation.
Due to the structural advantages and excellent absorption ability for light in the whole region, these Cu2S–MoS2 nanocomposites may be expected to be promising catalysts in other fields of photocatalysis.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj05229h |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 |