g-C3N4

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Oxygen-Doped MoS2 Nanospheres/CdS Quantum Dots/g‑C3N4 Nanosheets Super-Architectures for Prolonged Charge Lifetime and Enhanced Visible-Light-Driven Photocatalytic Performance Tianyu Zhao,† Zipeng Xing,*,† Ziyuan Xiu,† Zhenzi Li,‡ Shilin Yang,† and Wei Zhou*,† †

Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R. China ‡ Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, P. R. China ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/05/19. For personal use only.

S Supporting Information *

ABSTRACT: Oxygen-doped MoS2 nanospheres/CdS quantum dots/g-C3N4 nanosheets are synthetized through hydrothermal and chemical bath deposition− calcination processes. The prepared materials are characterized by X-ray diffraction transient-state photoluminescence spectra, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical experiment. These results show that the ternary composite material has longer lifetime of photogenerated carriers and more active sites, thereby enhancing photocatalytic performance. CdS quantum dots act as a bridge between the intermediate transport charges in the ternary composite. The oxygen defect engineering prolongs the lifetime of carriers obviously, which is confirmed by transient-state photoluminescence. Moreover, the photocatalytic H2 evolution and photodegradation of bisphenol A for MoS2/CdS/g-C3N4 is up to 956 μmol h−1 g−1 and 95.2% under visible-light irradiation, respectively. Furthermore, excellent photocatalytic activity can be ascribed to the synergistic effect of defect engineering and formation of ternary heterostructures, which is with broad-spectrum response, longer lifetime of photo-induced electron− holes, and more surface active sites. KEYWORDS: MoS2 nanosphere, g-C3N4, heterojunction, quantum dot, photocatalysis the surface.16 Therefore, the preparation of nanospheres with 2D-layered MoS2 can obtain more specific surface areas and active sites. Furthermore, it has been reported recently that the introduction of oxygen defects into the MoS2 structure can improve the photocatalytic performance, due to the introduction of defects improves the number of active sites.17,18 Among various semiconductor photocatalysts, cadmium sulfide (CdS) has been generally studied as visible-light photocatalysts, due to its narrow band gap, visible light response, and appropriate band gap position.12,19−21 CdS is considered as one of the most prominent photocatalytic nanomaterials for photocatalytic H2 evolution.22−25 However, the photocatalytic properties of nanosized CdS are usually limited due to agglomeration and the fast recombination of photo-induced carriers in the photocatalytic process. In addition, CdS has also another issue, which is photocorrosion in the photocatalytic process. In order to overcome these disadvantages, many efforts have been tried. The combination of CdS with other photocatalytic materials is a commonly used

1. INTRODUCTION Nowadays, energy shortage and environmental pollution are becoming progressively serious. To solve these problems effectively, photocatalytic technology is placed with great expectations.1−4 At present, plenty of photocatalysts have been synthetized and catholically studied. It has been applied to the photocatalytic H2 evolution, photodegradation of organic pollutants, and other challenges of environmental and energy.5−7 However, there are still many defects in the photocatalytic technology, which need further improvement. Improving the segregation and transmission efficiency of photo-induced electron−holes, expanding the range of light absorption response, and the construction of a reasonable band gap structure are the key points to be solved in the preparation of photocatalysts. Because of the unique structure and characteristics of transition metal sulfides, it has developed rapidly in recent years.8−10 Among them, molybdenum disulfide (MoS2) as a representative two-dimensional (2D)-layered nanomaterial of transition metal sulfides has attracted a lot of attention in photocatalysis.10−13 MoS2 has a similar graphene structure consisting of three layers of atomic layer (S−Mo−S) and linked by van der Waals forces.14,15 It is reported that the active site at the edge of MoS2 has more catalytic activity than © XXXX American Chemical Society

Received: December 4, 2018 Accepted: January 24, 2019

A

DOI: 10.1021/acsami.8b21131 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Diagram for the Formation of MoS2 Nanospheres/CdS Quantum Dots/g-C3N4 Nanosheets

modification method.26−28 In addition, another effective method is to prepare CdS with different morphologies. In the recent years, organic semiconductors have been developed as catalysts for photocatalysis. As one of the organic semiconductor materials, graphitic carbon nitride (g-C3N4) as a polymer photocatalyst has been catholically utilized in the field of photocatalysis.29−32 In 2009, Wang et al. prepared the g-C3N4 to use in the field of photocatalysis, which showed excellent visible-light response and photocatalytic performance.33 Furthermore, Wang et al. synthetized a novel polymeric C3N4 foam with large surface area and mesoporous structure. The polymeric C3N4 foam can effectively remove pollutants and generate hydrogen in wastewater.34 However, the band gap of g-C3N4 is about 2.8 eV, which only absorbs visible-light up to about 460 nm. In addition, the recombination of photoinduced electron−holes and the transmission problem for gC3N4 are the reasons affecting their photocatalytic properties. Recently, in order to solve the above issues, combination with g-C3N4 and other photocatalysts has become a way to improve the photocatalytic performance.35,36 Wu et al. prepared petallike CdS/S doping C3 N 4 composite via the one-pot solvothermal method. The composite photocatalysts exhibit enhanced photocatalytic performance due to the heterojunctions with CdS and C3N4 that prolong the lifetime and enhance the separation efficiency of photo-induced electron− holes.37 Therefore, the heterostructure formed by combining g-C3N4 with other semiconductor materials can effectively improve the lifetime of photo-induced electron−holes and enhance the photocatalytic performance. In this paper, reasonable structure and band gap design are applied to the preparation of photocatalysts. MoS2 nanospheres/CdS quantum dots/g-C3N4 nanosheets with hierarchical structure are prepared through a facile method. The performance of photocatalysts is tested by photocatalytic H2 evolution and photodegradation under visible-light irradiation. In addition, the as-prepared photocatalysts are characterized in detail and the possible mechanism of photocatalysis is also proposed.

Figure 1. XRD patterns of MoS2, g-C3N4, CdS, MoS2/CdS, and MoS2/CdS/g-C3N4 (a). SEM (b) and TEM images (c−e) of MoS2 nanospheres. TEM images (f−h) of MoS2/CdS/g-C3N4.

2. EXPERIMENTAL SECTION Detailed experiments are included in the Supporting Information. The synthesis of MoS2 nanospheres/CdS quantum dots/g-C3N4 nanosheets is shown in Scheme 1. First, MoS2 nanospheres and g-C3N4 nanosheets were prepared. Then, the precursor of CdS quantum dots was added to the suspension containing MoS2 nanospheres and gC3N4 nanosheets. Finally, the final product was obtained by calcination.

According to previous reports, oxygen is doped into the lattice during low temperature preparation.17,18 For g-C3N4, there are two characteristic peaks at 27.4° and 13.2°, corresponding to (002) and (110) diffraction plane, respectively. CdS shows three characteristic peaks at 51.8°, 43.8°, and 26.6°, respectively, corresponding to (311), (220), and (111) crystal planes of cubic phase CdS (JCPDS #10-0454). For MoS2/CdS composites, it shows the characteristic peaks of MoS2 and CdS, which confirms the successful combination of MoS2 and CdS. For MoS2/CdS composites, the characteristic peaks of g-C3N4, CdS, and MoS2 show the successful formation for the ternary composite structure. Furthermore, the (002) characteristic

3. RESULTS AND DISCUSSION As shown in Figure 1a, there are three characteristic X-ray diffraction (XRD) peaks for MoS2 nanospheres at 12.1°, 33°, and 59° respectively, which can be assigned to (002), (100), and (110) planes. It is worth noting that the (002) crystal plane of the three-dimensional nanospheres has been shifted compared with the traditional 2D nanosheet structure. B


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Figure 2. High-resolution O 1s XPS spectra of MoS2 nanospheres and MoS2/CdS/g-C3N4 (a), and high-resolution XPS spectra of N 1s (b), C 1s (c), Cd 3d (d), Mo 3d (e), and S 2p (f) for MoS2/CdS/g-C3N4, respectively.

Furthermore, the CdS quantum dots are observed in Figure S3. The diameter of CdS quantum dots is about 10 nm. The existence of MoS 2 nanospheres and thin layer g-C3 N4 nanosheets can be clearly observed in Figure S4. However, CdS was not observed due to its small particle size and difficult to detect. As shown in Figure 1f, MoS2 nanospheres, g-C3N4 nanosheets, and CdS quantum dots can be clearly observed; quantum dots are attached to the surface of MoS2 nanospheres; and MoS2 nanospheres are coated with g-C3N4 nanosheets. Furthermore, the quantum effect of CdS quantum dots can greatly accelerate separation and suppress the recombination of photogenerated carriers. CdS quantum dots are dispersed on the surface of MoS2 nanospheres and effectively contact with g-C3N4 nanosheets as intermediate charge transport channels (Figure 1g). As shown in the Figure 1h, the lattice stripe of CdS and MoS2 can be seen obviously. Moreover, the lattice stripe of MoS2 is 0.67 nm corresponding to the (002) plane. In addition, the (111) plane of CdS with 0.336 nm can also be observed. The composites can be successfully prepared from the above SEM and TEM images. Furthermore, the elements mapping image of MoS2/CdS/gC3N4 is shown in Figure S5. It indicates the formation of OMoS2/CdS/g-C3N4 heterostructures. The surface chemistry and chemical composition of the composites are analyzed by X-ray photoelectron spectroscopy (XPS). In Figure S6, the full XPS spectrum shows that O, C, N, S, Cd, and Mo elements can be definitely observed, which

peaks of MoS2 and the (111) characteristic peaks of CdS are covered by the characteristic peaks of g-C3N4. From scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, the as-prepared MoS2 nanospheres are composed of lamellar MoS2 with a main diameter of about 400 nm. Furthermore, the nanospheres are very uniform as shown in Figure 1b. In Figure 1c, it is also obvious that the sizes of the MoS2 nanospheres are uniform. In addition, it can be unequivocally shown that the spherical surface of MoS2 nanospheres exposes a large number of edges of MoS2 nanosheets (Figure S1). According to previous studies, we can know that the edge of MoS2 nanosheets has more active sites and better catalytic activity. Therefore, the asprepared MoS2 nanospheres may have better performance of photocatalysis than conventional MoS2 nanosheets. As shown in Figure 1d,e, it can be clearly seen that oxygen-doped MoS2 nanospheres are assembled by lamellar MoS2 and the spheres are uniform. As shown in Figure 1e, the interlayer distance of oxygen-doped MoS2 nanospheres is 0.67 nm (002). It is noteworthy that the distance between the interlayers is significantly larger than that of the conventional layered MoS2 (0.62 nm), which is due to the O doped in the structure of MoS2 nanospheres to replace S at S sites.17,18 Compared with conventional layered MoS2, oxygen-doped MoS2 nanospheres may have more specific surface areas and active sites. The SEM image of g-C3N4 is shown in Figure S2. It can be seen that the as-prepared g-C3N4 has a wrinkled surface. C


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Figure 3. Steady-state PL spectra (a), TS-PL spectra with 325 nm laser excitation (b), EIS Nyquist plots (c), I−t curves (d), and photocatalytic H2 evolution (e). Stability test of MoS2/CdS/g-C3N4 during the photocatalytic H2 evolution for 12 h (f).

induced electron−holes in MoS2/CdS/g-C3N4 is the least. The low photo-induced carrier recombination rate can effectively improve the lifetime of photo-induced carriers and photocatalytic performance. Furthermore, the lifetime of photoinduced carriers is further investigated by transient-state PL spectra. The decay process of the photocatalyst can be well fitted with a bi-exponential function as follows, eq 1

proves that MoS2, CdS, and g-C3N4 are successfully combined. As shown in Figure 2a, it can be observed that Mo−O exists in XPS spectra of MoS2 nanospheres and MoS2/CdS/g-C3N4, which is consistent with the XRD analysis and further proves the doping of oxygen. In Figure 2b, the three characteristic peaks are located at 400.9, 399.3, and 398.4 eV, corresponding to C−N−H, N−(C)3, and CN−C, respectively.30,38 Furthermore, the binding energy of C−N, C−(N)3, and C− C at 288 and 284.6 eV form the C 1s spectrum of MoS2/CdS/ g-C3N4 in Figure 2c.39−41 Figure 2d shows that a pair of characteristic peaks in Cd 3d spectra of MoS2/CdS/g-C3N4 is located at 411.6 and 404.8 eV, corresponding to Cd 3d3/2 and Cd 3d5/2, respectively.42 The high-resolution Mo 3d spectra of MoS2/CdS/g-C3N4 display two peaks at 231.9 and 228.5 eV assigning to Mo4+ 3d3/2 and Mo4+ 3d5/2 (Figure 2e).43 Moreover, the peak centered at 226 eV ascribed to S 2s is also observed. As shown in Figure 2f, the S 2p spectra show that two characteristic peaks are located at 162.1 and 161.2 eV, corresponding to S 2p1/2 and S 2p3/2, respectively.44,45 To verify the separation and transmission efficiency of photo-induced carriers, a series of tests have been applied to verify the ternary composite. Lifetime of photo-induced carriers is a key factor in visible light photocatalytic reaction. Generally speaking, the longer carrier lifetime can enhance the photocatalytic performance. As shown in Figure 3a, steadystate photoluminescence (PL) spectra show that the peak intensity of MoS2/CdS/g-C3N4 is the weakest compared with other samples, which proves that the recombination of photo-

I(t ) = B1 e−t /τ1 + B2 e−t /τ2

(1)

Equation 2 shown below can be used to calculate the average lifetime τ of the photocatalyst. τ = (B1τ12 + B2 τ2 2)/(B1τ1 + B2 τ2)

(2)

The respective kinetic parameters of samples after the biexponential fitting are listed in Table S1. The transient-state PL further illustrates that MoS2/CdS/g-C3N4 has the longest carrier lifetime compared with other samples, and the increase of carrier lifetime will effectively improve the utilization of photo-induced carriers in the photocatalytic process (Figure 3b). In Figure 3c, the electrochemical impedance spectroscopy (EIS) spectra show that the minimum semicircle radius of MoS2/CdS/g-C3N4 represents the minimum impedance, indirectly confirming that the resistance of the charge transfer in the material is small, which is beneficial to the transfer of photo-induced carriers. In addition, the maximum photocurrent intensity of MoS2/CdS/g-C3N4 is observed in Figure 3d compared with other samples, which is due to the reasonable structure design of the ternary composite for the D


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Figure 4. UV−vis diffuse reflectance spectra (a), hydroxyl radical amount-related fluorescence spectra (b), photocatalytic degradation curve of BPA (c), and the recycling runs of MoS2/CdS/g-C3N4 for degrading BPA (d). Work function drawings of MoS2, CdS, and g-C3N4 (e). Schematic illustration of the photocatalytic process of MoS2/CdS/g-C3N4 composites (f).

photothermal effect. Furthermore, as shown in the Figure S7, the color of the photocatalyst corresponds to the result of the UV−vis diffuse reflection. The rate of generation of hydroxyl radicals corresponds to the separation of photo-induced carriers. In the process of photocatalysis, it also represents the photocatalytic performance. Because coumarin readily reacts with hydroxyl radicals to generate 7-hydroxy-coumarin, coumarin is often used to test the content of hydroxyl radicals. As shown in Figure 4b, the sequence of obtained fluorescence intensity is as follows: no photocatalysts < MoS2 < g-C3N4 < MoS2/CdS < MoS2/CdS/g-C3N4, which is well coincident with the above photophysical results. Photocatalytic degradation of Rh B is shown in Figure S8a. MoS2/CdS/g-C3N4 exhibits the best photocatalytic degradation ability than that of g-C3N4, MoS2, and MoS2/CdS. After 20 min of illumination, the degradation rate of MoS2/CdS/g-C3N4 is up to 99%, which is several times higher than that of MoS2 and g-C3N4. Furthermore, as shown in Figure S8b, the apparent reaction rate constants of MoS2, g-C3N4, MoS2/CdS, and MoS2/CdS/ g-C3N4 are 0.0113, 0.0265, 0.0374, and 0.2143 min−1, respectively. The apparent reaction rate constant of MoS2/ CdS/g-C3N4 is the highest, which is almost 19 and 8.1 times higher than that of MoS2 and g-C3N4. Photodegradation of bisphenol A (BPA) is shown in Figure 4c. Compared with others, MoS2/CdS/g-C3N4 has the highest degradation rate of

transmission and separation of the photo-induced carriers. Combined with the above results, it can be concluded that the ternary structure is beneficial to the separation and transmission of the photo-induced carriers, thus prolonging the lifetime of the photogenerated carriers and improving the photocatalytic activity. Figure 3e shows the photocatalytic H2 production rate of CdS, MoS2, g-C3N4, MoS2/CdS, g-C3N4/CdS, MoS2/g-C3N4, and MoS2/CdS/g-C3N4. It can be definitely observed that the sample MoS2/CdS/g-C3N4 has the highest H2 production rate of 956 μmol h−1 g−1. This is due to the longer lifetime of photo-induced carriers and more active sites in the ternary composite, which effectively improves the utilization of photoinduced carriers during the photocatalytic process. Furthermore, cyclic stability tests show that the prepared catalyst had good stability (Figure 3f), which is favorable for practical applications. Figure 4a shows the UV−vis diffuse reflectance spectra of MoS2, CdS, g-C3N4, MoS2/CdS, and MoS2/CdS/g-C3N4. Compared with CdS and g-C3N4, MoS2 has light response not only in the UV and visible regions, but also in the infrared region. It can be observed that after the three materials are compounded, the MoS2/CdS/g-C3N4 composite has optical absorption not only in the visible region, but also in the infrared region. This means that the composite has potential E


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performance may be attributed to efficient transfer and separation of photo-induced electron−holes, the massive active sites and adsorption sites on the photocatalyst surface, suitable energy band position, and the synergistic effect among MoS2 nanospheres, CdS quantum dots, and g-C3N4 nanosheets. Therefore, the ternary heterojunctions provide an interesting scheme for the synthesis and design of photocatalysts in the future.

95.2%. Because the ternary composites have more active sites and effectively prolongs the lifetime of photo-induced electron−holes, thus it greatly improves the performance of photocatalysis. Furthermore, the degradation cycle test confirms that the photocatalysts have good stability, which corresponds to the previous results (Figure 4d). Moreover, active groups are an important factor affecting photodegradation. Therefore, through the species trapping experiments, we can find out which active substance plays a major role in photodegradation. tert-Butyl alcohol (TBA), ethylenediaminetetraacetic acid (EDTA)-Na, and BQ can capture • OH, h+, and •O2− produced in photodegradation reaction, respectively. As shown in Figure S9, the photodegradation rate is decreased significantly when EDTA-Na and BQ are added, which proved that h+ and •O2− are the main active groups in the photodegradation reaction. It is noteworthy that the photodegradation performance does not change significantly when TBA is added, which indicates that TBA plays a secondary role in photodegradation reaction. The work function is a basic characteristic of the semiconductor material. It represents the characteristics of electron transfer to the surface of solid materials. The scanning Kelvin probe technique is used to characterize the work function of MoS2, CdS, and gC3N4. As shown in Figure 4e, the order of work functions is as follows: MoS2 > CdS > g-C3N4. These results indicate that the electron transfer is from g-C3N4 through CdS to MoS2. Furthermore, in order to further explore the process of electronic transmission, the Mott−Schottky curve is measured. As shown in Figure S10, the flat band potentials of g-C3N4, CdS, and MoS2 are estimated to be −1.08, −0.59, and −0.46 eV versus Ag/AgCl, respectively. In addition, as the tangent slope is positive, the three materials are n type semiconductors. This result further proves that the order of electron transfer is from g-C3N4 through CdS to MoS2. According to the above results, Figure 4f illustrates the photocatalytic mechanism of MoS2/CdS/g-C3N4 in detail. Furthermore, possible electron transfer pathways in energy band structures of MoS2/CdS/gC3N4 under irradiation have also been proposed (Figure S11). Under illumination, electrons generated on g-C3N4 are transported to MoS2 via CdS quantum dots with quantum effect, and holes on MoS2 are transmitted to g-C3N4 via CdS quantum dots. CdS quantum dots act as a bridge between the intermediate transport charges in the ternary composite, thereby prolonging the lifetime of the photo-induced electron−holes and improving the performance of photocatalysis. In addition, because the specific structure of the MoS2 nanospheres has more active sites, the photocatalytic reaction is promoted. In the process of photocatalytic reaction, water molecules react with electrons to form hydrogen and react with holes to form hydroxyl radicals. Hydroxyl radical as an oxidizing active substance can effectively oxidize organic pollutants. A reasonable construction of the ternary composite material effectively prolongs the lifetime of the photo-induced charge carriers and increases the active sites, thereby improving the photocatalytic reactivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b21131.

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Experimental section; SEM images of MoS2, g-C3N4, and MoS2/CdS/g-C3N4; TEM image of CdS quantum dots; XPS survey spectrum and mapping images of MoS2/CdS/g-C3N4; photographs of the as-prepared photocatalysts; photodegradation of Rh B for different photocatalysts; species trapping experiments; Mott− Schottky plots of g-C3N4, CdS, and MoS2; band structures of MoS2/CdS/g-C3N4 under irradiation; results of the exponential decay-fitted parameters for the fluorescence lifetime of the as-prepared photocatalysts; and comparison of photocatalytic H2 evolution rate and photocatalytic degradation rate between MoS2/ CdS/g-C3N4 and other tertiary catalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-451-8660-8616. Fax: +86-451-8660-8240 (Z.X.). *E-mail: [email protected] (W.Z.). ORCID

Zipeng Xing: 0000-0002-9429-5780 Wei Zhou: 0000-0002-2818-0408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21871078 and 51672073), the Natural Science Foundation of Heilongjiang Province (B2018010 and H2018012), the Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Postgraduate Innovative Science Research Project of Heilongjiang University (YJSCX2018-166HLJU), and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016018).

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4. CONCLUSIONS In summary, MoS2/CdS/g-C3N4 photocatalysts are synthetized through hydrothermal, chemical bath deposition, and calcination processes. MoS2/CdS/g-C3N4 photocatalysts show the most efficient photocatalytic H2 evolution. Moreover, the photodegradation rate of BPA for MoS2/CdS/g-C3N4 is the largest (as high as 95.2%). The excellent photocatalytic F


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