1 Introduction
Photocatalytic H2 production has been regarded as an ideal technology for clear energy [1–3]. The development of photocatalysts is the critical segment for this technology. In general, the photocatalytic activity of photocatalysts strongly depends on their absorption ability to light photons and separation ability of photoinduced electron-hole pairs [4,5]. Therefore, the screening of photocatalysts with narrow band gaps and further modification for promoting the separation efficiency of charge are the directions of efforts for the development of photocatalysts.
CdS, a visible-light-response photocatalytic material, can absorb light not longer than 520 nm, whose energy reaches nearly half of the entire visible light [6]. Moreover, CdS possesses very suitable conduction band (CB) and valence band (VB) potentials, and meets the thermodynamic requirement for H2 evolution reaction. However, the photocatalytic activity on pure CdS for H2 evolution is very low due to the lack of active sites on the surface of CdS as well as the rapid recombination of photoinduced electron-hole pairs. To overcome this deficiency, some noble metals, such as Pt and Pd as cocatalysts are loaded on the surface of CdS [7]. With their high work function, noble metals facilitate the transfer of electrons from CdS to their surface while holes remain on CdS, causing the separation of electrons and holes. Apparently, although noble metals have excellent performance for promoting photocatalytic activity, they are expensive and rare, which is detrimental to the practical application on a large scale. Recently, non-noble metal sulfides, such as NiS, CoS2, and MoS2 have been regarded as good substitutes for noble metal cocatalysts [8–11]. With the same type of characte-ristic as CdS, metal sulfide cocatalysts tend to have better interface contacts with CdS, thus facilitating the charge transfer. Among the many metal sulfide cocatalysts, the two-dimensional (2D) layered material MoS2 has received more attention due to the very low free energy of H adsorption on its edge sites [12]. Moreover, MoS2 with a single or few layers possesses a much better performance than bulk MoS2 because of the lower charge transfer resistance [13].
As a typical layered material, MoS2 stacked nanosheets are formed by the growth of three stacked atom layers (S-Mo-S). Therefore, ultrathin MoS2 nanosheets can be obtained by controlling the growth in the direction of the z axis or exfoliating bulk MoS2. Currently, the liquid phase exfoliation (LPE) method was widely used for obtaining mono- or several-layered MoS2 by ultrasonicating bulk MoS2 to destroy the van der Waals force between two adjacent layers [14–17]. Unfortunately, the yield of the ultrathin MoS2 was comparatively low and went against the particle application. Additionally, using MoO3 and S powder as precursors for deposition of MoS2 on substrates, the high quality ultrathin MoS2 could be prepared by utilizing a chemical vapor deposition (CVD) method [18]. However, its complex preparation process also limited its application on a large scale. Therefore, the development of a simple method for preparing ultrathin MoS2 to enhance activity is of great significance.
Herein, a simple method was used to load ultrathin MoS2 on the surface of CdS nanorod for promoting the photocatalytic activity. The in-situ growth of MoS2 on CdS would lead to a strong interface contact between them, which would facilitate the charge transfer and enhance the photocatalytic activity. First, nano Mo species were loaded and fastened on the CdS nanorod. Then, the S powder vulcanized Mo species to form ultrathin MoS2. The MoS2 was loaded on CdS due to its strong 2D orientation. The morphology of few-layered MoS2 on CdS nanorod was verified. The performance and roles of ultrathin MoS2 on enhancing photocatalytic activity were investigated.
2 Experimental
2.1 Material synthesis
Cadmium nitrate, thiourea, sulfur and 1,2-ethylenediamine (en) were purchased from Macklin (Shanghai, China). Ammonium molybdate and lactic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. China.
CdS nanorod was synthesized by utilizing a solvothermal method. 3.08 g of cadmium nitrate and 2.28 g of thiourea were dissolved into 48 mL of 1,2-ethylenediamine (en). After stirring for 0.5 h, the mixture was then transferred to a sealed Teflonlined autoclave and kept at 160°C for 48 h. The obtained product was cleaned by distilled water and anhydrous alcohol. Finally, CdS nanorods, were obtained in vacuum furnace by drying at 40°C for 24 h.
The growth of MoS2 on CdS nanorod was achieved by utilizing an impregnation method. 0.3 g of CdS nanorod was added into 30 mL of aqueous solution containing an appropriate amount of (NH4)6Mo7O24. Under continuous stirring, the suspension was dried by water bath at 60°C. The obtained yellow powder was mixed with 0.1 g of S powder and then vulcanized at 400°C. Finally, the powder was washed by alcohol to obtain MoS2/CdS. The mass ratios of MoS2 and CdS are m(MoS2): m(CdS) = 1: 100 (w(MoS2) = 1%, w is mass fraction), m(MoS2): m(CdS) = 1: 50 (w(MoS2) = 2%) and m(MoS2): m(CdS) = 1: 33 (w(MoS2) = 3%), respectively. A schematic diagram of the photocatalyst synthesis is shown in Fig. 1. MoS2 was also prepared by (NH4)6Mo7O24 and S powder using the same method.
2.2 Characterization
The crystal structure of materials was confirmed by X-ray diffraction (XRD, Bruker-AXS, Cu Kα, λ = 0.15406 nm). The morphology and energy dispersive X-Ray spectroscopy (EDX) analysis of samples were characterized by using high resolution transmission electron microscopy (HRTEM) at 200 kV (FEI Talos F200, USA). The UV-vis diffuse reflection spectra (DRS) were determined by a UV-vis spectrophotometer TU-1901 (Purkinje, China) and were converted to absorbance by using a Kubelka-Munk method. The surface electronic state was analyzed by using X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis UltraDLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C 1s= 284.6 eV) as a reference. Photoluminescence (PL) spectra were recorded using an Edinburgh FLS980 fluorescence spectrometer with an excitation light wavelength of 325 nm. Linear sweep voltammetry (LSV), photocurrent response (I-t), electrochemical impedance spectroscopy (EIS), and Mott-Schottky measurements were performed on a CH Instrument 760D electrochemical workstation (Chenhua, Shanghai) using a 0.5 mol/L Na2SO4 aqueous solution as the electrolyte.
2.3 Photocatalytic activity for H2 evolution
Photocatalytic H2 production was achieved in a 350 mL reactor, which contained 50 mg of CdS and 70 mL of lactic acid aqueous solution (V(lactic acid): V(water) = 1: 5, V is volume). Subsequently, the reaction was connected with a vacuum system to remove the air. A 300 W Xe lamp (λ>420 nm) was used as light source. The generated H2 was detected by a gas chromatography with a thermal conductivity detector (TCD) detector.
3 Results and discussion
Figure 2 demonstrates the XRD patterns of CdS, MoS2/CdS, and MoS2. The main diffraction peaks of CdS at 24.8°, 26.5°, 28.2°, and 43.7° correspond to (100), (002), (101), and (110) plane of CdS with a hexagonal structure, respectively. For MoS2, the diffraction peaks at 14.4°, 32.7°, and 39.6° correspond to the (002), (100), and (103) planes of hexagonal MoS2 (JPCDS 65-1951), respectively, indicating that MoS2 can be prepared by S powder as S source. No diffraction peak belonging to MoS2 is observed in MoS2/CdS samples, which should be attributed to the low loading amounts and/or the low diffraction intensity. Figure 3(a) presents the DRS spectra of pure CdS, MoS2 and m(MoS2): m(CdS) = 1: 50. The absorption edge of CdS is approximately 520 nm, corresponding to a band gap of 2.4 eV [19]. The performance of light absorption implies that the CdS nanorod has a hexagonal phase. MoS2 exhibits a strong absorption in the entire visible light region (400–800 nm), which should be attributed to its narrow bang gap (1.1 eV) [20]. From Fig. 3(b), it can be observed that the band gap of CdS does not change after MoS2 loading, suggesting that MoS2 only supports on CdS and does not change the crystal lattice of CdS. However, after MoS2 loading, m(MoS2): m(CdS) = 1: 50 exhibits a new light absorption longer than 520 nm, which should be attributed to the adsorption of MoS2. The adsorption peaks at approximately 620 nm and 670 nm originate from the K point of the Brillouin zone of MoS2 [21], which confirms that MoS2 has been loaded on the surface of CdS successfully.
The chemical composition and the state of the elements in m(MoS2): m(CdS) = 1: 100 were analyzed by XPS. A survey of the XPS spectrum of MoS2/CdS (Fig. 4(a)) indicates that Cd, S, Mo, and O elements exist in MoS2/CdS. Two peaks located at 411.8 and 405.1 eV can be observed in the XPS spectrum of Cd 3d (Fig. 4(b)), matching the binding energies of Cd 3d5/2Cd 3d3/2 and of CdS [22]. From Fig. 4(c), it can be noticed that the XPS spectrum of S 2p can be fitted to two peaks at 162.7 and 161.5 eV, corresponding to the binding energies of S 2p1/2 and S 2p3/2 of S2– [23]. In Fig. 4(d), the Mo 3d spectrum is assigned to three peaks. The two peaks at 232.2 and 229.0 eV can be matched with the Mo 3d3/2 and Mo 3d5/2 of MoS2, respectively [24]. In addition, the peak at 225.8 eV is related to the BE of S 2s of S2−. Therefore, the XPS analysis confirms the successful preparation of MoS2/CdS.
To presented the microstructure of MoS2 and CdS, the m(MoS2): m(CdS) = 1: 50 sample was investigated by HRTEM. Figure 5(a) shows that CdS has a rod-like structure with a length of 200–300 nm and a width of 30–50 nm. Figures 5(b) and 5(c) shows that the MoS2 loaded on the surface of CdS nanorod possesses an ultrathin nanosheet structure. The lattice spacing of 0.62 nm and 0.23 nm corresponds to the (002) and (103) of hexagonal MoS2, respectively, which is consistent with the XRD results [24]. A-few-layered MoS2 possesses more active sites, thus promoting the photocatalytic activity [25]. In addition, Fig. 5(c) shows that a-few-layered MoS2 nanosheets are in close contact with the CdS nanorods, which is greatly conducive to the charge migration between CdS and MoS2 [26]. The elemental mapping images (Fig. 5(d)) clearly reveal the ultrathin MoS2 loaded on the surface of CdS nanorod. Moreover, the EDX spectrum also indicates the existence of Mo, Cd, and S elements, suggesting that MoS2 has been loaded on the surface of CdS nanorod successfully.
The photocatalytic activity for H2 evolution on CdS and MoS2/CdS were examined under visible light (λ>420 nm) when lactic acid was used as a sacrificial reagent (Fig. 6(a)). From Fig. 6(a), it can be seen that pure CdS exhibits a low photocatalytic activity for H2 evolution (92 μmol/h) due to the rapid charge recombination on the surface of CdS. Under the same experimental conditions, after MoS2 loading, the photocatalytic hydrogen production rates were significantly increased. The hydrogen production rates of the MoS2/CdS catalysts loaded with w(MoS2) = 1%, w(MoS2) = 2% and w(MoS2) = 3% increases first and then decreases. The photocatalytic hydrogen production rate of m(MoS2): m(CdS) = 1: 50 reaches 542 μmol/h, which is 6 times that of pure CdS and is also higher than that of most of the studies conducted previously, as summarized in Table 1. The result suggests that the sulfide MoS2 is a good substitute for precious metals in photocatalytic hydrogen production. However, the H2 evolution rate decreases when the loading amount is more than m(MoS2): m(CdS) = 1: 50, probably as excessive MoS2 on surface of CdS would become the recombination center of electrons and holes. To examine the stability of photocatalyst, a cyclic experiment of photocatalytic reaction on m(MoS2): m(CdS) = 1: 50 was conducted. After each photocatalytic experiment was completed, the photocatalyst was washed with deionized water and dried for the next photocatalytic experiment. As manifested in Fig. 6(b), the hydrogen production performance of m(MoS2): m(CdS) = 1: 50 does not decrease during four experimental cycles, indicating the good stability of MoS2/CdS.
Tab.1 Comparison of H2 evolution rates of the current work and previous studies |
| Photo- catalyst | Catalyst dosage /mg | Cocatalyst/(mass fraction, %) | Preparation method | Sacrificial reagent | Light source | H2 evolution rate/(μmol∙h−1∙g−1) | Ref. |
|---|---|---|---|---|---|---|---|
| CdS | 50 | 2.0% MoS2 | Solid state (S powder) | Lactic acid | 300 W Xenon lamp, λ>420 nm | 10840 | This work |
| CdS | 50 | 5% MoS2 | Hydrothermal | Amoxicillin | 300 W Xenon lamp, λ>420 nm | 1760 | [10] |
| CdS | 100 | 3% MoS2 | Solid state (H2S) | Lactic acid | 300 W Xenon lamp, λ>420 nm | 5330 | [11] |
| CdS | 300 | 1.0% NiS | Hydrothermal | Lactic acid | 300 W Xenon lamp, λ>420 nm | 7000 | [8] |
| CdS | 200 | 2% MoS2 | Adsorption | Lactic acid | 300 W Xenon lamp, λ>420 nm | 12950 | [14] |
| CdS | 20 | 25% MoS2 | Hydrothermal | TEOA | 300 W Xenon lamp, λ>400 nm | 1145 | [19] |
| CdS | 10 | 6.39% MoS2 | Hydrothermal | Na2S+ Na2SO3 | 300 W Xenon lamp, λ>420 nm | 4540 | [22] |
To realize a photocatalytic reaction, photocatalysts need to have not only suitable energy band potentials but also good abilities to separate photogenerated electrons and holes. Although CdS has enough negative CB potentials to reduce protons (H+) to H2, the lack of active sites for H2 evolution on the surface of CdS results in a low H2 evolution efficiency. To probe the effect of MoS2 on enhancing the photocatalytic activity of CdS, a photoelectrochemical test was conducted. Figure 7(a) depicts the LSV curves of pure CdS and MoS2 loaded CdS. It can be seen that pure CdS exhibits a low reduction current density, indicating that the occurrence of H2 evolution reaction on the surface of CdS is difficult. After MoS2 loading, the reduction current density of CdS is obviously promoted, implying that the H2 evolution reaction occurs on the surface of MoS2 easily [27,28]. Similarly, compared with pure CdS, m(MoS2): m(CdS) = 1: 50 shows a smaller semicircle arc in EIS curves (Fig. 7(d)), suggesting that the charge transfer resistance between photocatalyst and water is reduced [29,30]. These results indicate that the H2 evolution reduction reaction is more likely to occur.
To examine the charge separation ability of MoS2, the transient photocurrent and PL measurements of CdS and MoS2 loaded CdS were performed. As exhibited in Fig. 7(c), pure CdS exhibits a very low photocurrent density, indicating the strong recombination of carriers on its surface. After MoS2 loading, the photocurrent density is significantly enhanced, indicating that MoS2 can promote the charge separation of carriers [31,32]. The m(MoS2): m(CdS) = 1: 50 has the highest photocurrent density, revealing the best charge separation efficiency. However, the photocurrent density becomes low when the loading amount exceeds w(MoS2) = 2%, suggesting that excessive MoS2 results in a low photoelectric conversion efficiency. A similar tendency also appears in the PL measured result (Fig. 7(d)), which represents the level of recombination of carriers [33,34]. Clearly, pure CdS possesses the strongest PL intensity. After MoS2 loading, the PL intensity decreases, implying the charge separation efficiency is enhanced. However, the PL intensity of m(MoS2): m(CdS) = 1: 33 becomes stronger than that of m(MoS2): m(CdS) = 1: 50, indicating that excessive MoS2 would become the center of recombination, thus reducing the photocatalytic activity [35,36].
Based on the analysis mentioned above, the mechanism of loaded MoS2 in enhancing the photocatalytic activity for H2 evolution can be explained as follows: Under visible light irradiation, the electrons are generated from the VB of CdS to its CB. However, for pure CdS, because of the lack of active sites, the electrons rapidly recombine with holes. After MoS2 loading, because the CB potential of MoS2 (−0.63 V versus Ag/AgCl, pH= 6.8) is lower than that of CdS (−1.25 V versus Ag/AgCl, pH= 6.8), the electrons from the CB of CdS are likely to transfer to the CB of MoS2, resulting in the separation of electrons and holes in space (Fig. 8). The ultrathin structure of MoS2 loaded on CdS shortens the migration distance of electron from CdS to MoS2, facilitating charge separation. Moreover, the reduction reaction of protons is more likely to happen on the surface of MoS2, further enhancing the photocatalytic activity. However, excessive MoS2 will cover the surface of CdS, which blocks the light absorption and becomes the recombination center of electron and holes, thus decreasing the photocatalytic activity.
4 Conclusions
In summary, a-few-layered MoS2 was loaded on CdS nanorod by utilizing a solid state method using sulfur powder as sulfur source for photocatalytic H2 production. Based on the difference of CB potential in CdS and MoS2, the electrons from CdS tend to transfer to MoS2, which results in a high charge separation efficiency. The ultrathin structure of MoS2 on CdS shortens the migration distance of electron from CdS to MoS2, facilitating charge separation. Moreover, the reduction reaction of proton is more likely to occur on the surface of MoS2 further enhancing photocatalytic activity. Compared with CdS, the photocatalytic activity of MoS2 loaded CdS is significantly improved. The hydrogen evolution rate of m(MoS2): m(CdS) = 1: 50 reaches 542 μmol/h, which is 6 times of that on pure CdS (92 μmol/h). This work gives a new design for photocatalysts with high photocatalytic activity and provides a deeper understanding of cocatalysts on enhancing photocatalytic activity.