Efficient visible light-induced degradation of rhodamine B by W(NxS1−x)2 nanoflowers

Here, W(NxS1−x)2 nanoflowers were fabricated by simple sintering process. Photocatalytic activity results indicated our fabricated N-doped WS2 nanoflowers shown outstanding photoactivity of degradating of rhodamine B with visible light. Which is attributed to the high separation efficiency of photoinduced electron–hole pairs, the broadening of the valence band (VB), and the narrowing of energy band gap. Meanwhile, our work provided a novel method to induce surface sulfur vacancies in crystals by introduing impurities atoms for enhancing their photodegradation.

In the past decades, there has been a great interest in developing semiconductor-based photocatalysts due to its high catalytic efficiency and good stability for water splitting and removal of hazardous organic compounds in industrial wastewater using solar energy [1][2][3][4][5][6] . TiO 2 , a typical traditional photocatalyst, has many merits, including its low cost, high efficiency and excellent stability 7 . However, it can't absorb visible light and suffers from fast recombination rate of the photogenerated charge carriers 8 . In order to overcome these drawbacks, numerous investigations have been devoted to give new types of photocatalysts, where two-dimensional (2D) nanomaterials with exotic electronic properties and high specific surface areas are considered to be the good candidates 9,10 , as well as, they have attracted tremendous attention in heterogeneous catalysis [11][12][13] , sensors 14 , energy storage 15,16 and electronics [17][18][19] .
Recently, transition metal sulfide has attracted intensive attention for their graphene-like structure. Tungsten disulfide (WS 2 ), belonging to layered transition-metal dichalcogenides family, exhibits extraordinary electrical 19 and photonic properties 20,21 . WS 2 possesses hexagonal crystal structure with space group P63/mmc and each WS 2 monolayer contains an individual layer of W atoms with 6-fold coordination symmetry, which are then hexagonally packed between two trigonal atomic layers of S atoms 22 . Generally, bulk WS 2 has an indirect band gap of 1.35 eV, and when it is thinned to a single layer it becomes direct band gap semiconductor with a gap of 2.05 eV 23,24 . Hence, fewer layers WS 2 nanosheets are the promising candidates for photocatalyst because of the number of active sites increases with the specific surface area at the nanoscale and the sites promote interfacial charge transfer for photo-induced electron-hole pairs 25,26 . Nitrogen (N) doping is widely used in traditional semiconductor industry for effectively controlling their electronic properties. Recently, results indicated that the N doped graphene had the improved photocatalytic performance of photocatalysts than the bare graphene. Sacco et al. found that the N-doped TiO 2 showed a higher photocatalytic activity for photodegradation of phenol under visible light irradiation than the TiO 2 and titanium dioxide (P25) 8 . Meng et al. also reported that the photocatalytic the MO (methyl orange) evolution of N-La 2 Ti 2 O 7 could be effectively improved by N doping 27 . In addition, many other researchers also demonstrated that various photocatalysts such as N-ZrO 2 28 , N-(BiO) 2 CO 3 29 , N-BiVO 4 30 and N-ZnO 31 showed a higher photocatalytic performance compared to their pure phase.
In this paper, we reported a different approach for the synthesis of WS 2 nanoflowers with in-suit nitrogen-doping by a simple sintering process. Results indicated that the fabricated N-doped WS 2 nanoflowers showed a BET area as high as 58.87 m 2 /g, which was 19.3 times than that of bare WS 2 nanosheets (BET area 3.05 m 2 /g) 32 . In addition, we reported the excellent visible light-induced degradation of rhodamine B by N-doped WS 2 nanoflowers. Results indicated that 20 mg of our photocatalysis could completely degrade 50 ml of 20 mg L −1 RhB in 70 minutes with excellent recycling and structural stability.

Results and Discussion
Characterization. The obtained product of S1.2 and the Used sample (N-doped WS 2 nanoflowers were used by the photocatalytic activity testing) were first measured by XRD and the results are illustrated in Fig. 1a. As can be seen that the five distinct peaks correspond to (002), (012), (104), (110), and (202) diffraction peacks of hexagonal WS 2 (JCPDF 84-1399). For the Used sample, all the diffraction peaks exist and no other new phase appear, indicating that our sample has a stable structure in the photocatalytic process, which is further proved by Raman spectrum (Fig. 2b). The Raman spectrum shows typical features of layered WS 2 where the E 2g 1 and A 1g modes are, located around 350 and 417 cm −1 33,34 . For the Used sample, the two distinct peaks were similar to the primitive product, providing more stable evidence for the property. To investigate the morphology of samples, the SEM measurement was considered and the result for sample S1.2 are presented in Fig. 2c and d. It can be seen from Fig. 2c and d that the sample show the flower-like structure and each of the component shows nanosheet feature. It can be seen that the morphology of our sample didn't change obviously after photocatalytic, which also reveal the obtained product has a stable structure. Besides, energy-dispersive X-ray spectroscopy (EDS) analysis was carried out to verify the element-composition of the sample. As shown in the inset of Fig. 2c, EDS result clearly shows the presence of elements W, S and N in our fabricated sample.
To further verify the morphology of the as prepared sample, the TEM measurement was employed. As illustrated in Fig. 3a and c, the results also indicate our sample (S 1.2) shows the nanoflower-structure. From the high-resolution TEM (HRTEM) of N-doped WS 2 nanoflowers (S1.2, shown in Fig. 3b), it can be intuitively seen that the sample reveals prefect lattice features, meanwhile, the interlayer spacing of ≈ 1.9 nm agrees well with the (012) planes of WS 2 . The inset of Fig. 3b shows the outstanding layered structure of the N-doped WS 2 nanoflowes. Figure 3c shows the HAADF-STEM (High-angle annular dark-field scanning transmission electron microscopy) image of S1.2. In addition, N, W and S element mapping are shown in Fig. 3d to f respectively, where the result indicates N element is evenly distributed in the sample. To study the composition and chemical nature of the as-prepared N-doped WS 2 nanoflowers, XPS spectrum was exployed. As shown in Fig. 4a, it can be clearly seen that the full range XPS spectrum of the N-doped WS 2 nanoflowers (S1.2) only contains N, S, and W elements, indicating there is no impurity elements in the sample. The high-resolution XPS spectrum of W 4f 7/2 and W 4f 5/2 are located at 32.7 and 34.8 eV, as shown in Fig. 4b. The XPS spectrum of W 4 f for S 1.2 can be deconvoluted into four peaks, which are attributed to the following functional groups: W-N bonds (33.2 eV and 35.3 eV) and W-S bonds (32.6 eV and 34.7 eV), indicating parts of S sites were replaced by N in WS 2 . Meanwhile, Fig. 4c shows the S 2p XPS spectrum, which can be separated into two peaks at 162.4 eV and 163.5 eV, corresponding with S-W bonds of S 2p 3/2 and S 2p 1/2 . In order to further prove that the parts of S sites are replaced by N in WS 2 , the XPS spectrum of N 1s is fitted. As shown in Fig. 4d, two well-defined peaks can be distinguished, which indicated the N 1s binding energies were 397.4 eV and 399.5 eV, respectively. Generally, the peaks at 400 eV can be assigned to N that is surface bond with N or O, which is in agreement with other previous results 35 . Another peaks at 397.2 eV can be assigned to N-W band 36 , further indicating the parts of S sites are replaced by N on WS 2 . In addition, the nitrogen adsorption-desorption curves were performed to further study the specific surface area of the samples and the result of the representive sample S 1.2 are presented in Fig. 4e and f, revealing the sample has a larger BET area of representive sample S 1.2 (58.87 m 2 /g), which is lager than report results of Wu et al. (1.6 m 2 /g) 37  Evaluation of photocatalytic Reaction. The photocatalytic performances of the as-prepared samples were evaluated by degrading of RhB aqueous solution at room temperature under visible light irradiation, as shown in Fig. 5. As shown in Fig. 5a, the sample S 1.2 and its bulk were used in degrading the RhB under visible light irradiation, which can be clearly seen that the degrading rate of RhB of S 1.2 is larger than its bulk in a visible light irradiation although the absorbed rate of RhB of sample S 1.2 shows the similar value with its bulk in a dark condition (Table 1) (Supporting Information S1), which may be corresponding with its bandgap (S 1.2 1.68 eV, bulk 1.82 eV) and BET area (S 1.2 58.87 m 2 /g, bulk 24.64 m 2 /g), as shown in Figure S1 (Supporting Information). Meanwhile, plots of the absorbance versus wavelength for degradation of RhB for N-doped WS 2 nanoflowers at various irradiation times is shown in Fig. 5b. It can be seen that the intensity of the absorption peaks continuously decreases without any changes in their position during the degradation reactions, and its intensity sharp decreases in 10 minutes and then disappear gradually in 70 minutes. For the purpose of practical use, the stability of S 1.2 was also investigated by the degradation of RhB under visible-light irradiation (Fig. 5c). It can be clearly seen that the as-prepared N-WS 2 nanoflowers does not exhibit obvious loss in photocatalytic activity even after using for 4 cycles, revealing its excellent recycling and structural stability (previous XRD and Raman results). In addition, to study the influence of the N concentration on the photocatalytic activity of N-doped WS 2 nanofloweres, a series of photocatalytic experiments were carried out for the N-doped WS 2 nanoflowes with different N concentration. As can be clearly seen from Fig. 5d that the degrading rate of RhB with the photocatalysts followed the order of S 1.2 (1.2 at% N) > S 0.6 (0.6 at% N) > S 0.3 (0.3 at% N), indicating the degradation rate is gradually increases with the increasing of the N concentration.  can be confirmed by the photocurrent measurement 39,40 . Actually, larger magnitude of photocurrent suggests higher charge collection efficiency of the electrode surface, indicating higher separation efficiency of electron-hole pairs. Figure 6a shows the photocurrent results of sample S 1.    2 , and holes (h + ) influences the photocatalytic degradation process, the degradation of RhB over S 1.2 with various scavengers were explored. As shown in Fig. 6b, for our N-doped WS 2 system, the photocatalytic performance decreased greatly by addition of TBA or t-BuOH (Supporting Information S4), but changed very slightly by addition of others scavengers, suggesting that the hydroxyl radical is the domination oxidative species of N-doped WS 2 and others only play an assistant roles.

Mechanism of Enhanced
The band-gap energy of all the samples are estimated from the plot of (ahv) n versus hv by extrapolating the straight line to the X axis intercept, as shown in Fig. 7a-c. The band-gap energies of S 0.3, S 0.6 and S 1.2 are found to be 2.0, 1.75 and 1.68 eV, respectively. Results indicate the band-gap energy is gradually decreased with the increasing of the N concentration 45 . In addition, to further study the influence of the N concentration on the band gap, the density of states (DOS) of the valence band of N-doped WS 2 photocatalysts were measured by the valance band XPS. As shown in Fig. 7d-f, it can be clearly seen that the edge of the valance band energy with the photocatalysts followed the order of S 0.3 > S 0.6 > S 1.2, indicating the valance band maximum rise with low density of states 46 . The band gap shift is attributed to lattice defects such as those arising from interstitial nitrogen 47 .
Based on the above results, a schematic diagram for the density of states of pure WS 2 and N-doped WS 2 nanoflowers has been proposed shown in Fig. 8 to give the mechanism of enhanced Photocaptalytic activity and efficiency in N doped WS 2 nanoflowers. The forbidden gap of pure WS 2 (2.49 eV) was reported by Hong et al. 48 ,  which can only absorb light wavelength less than 498 nm. In recent reports, numerous investigations have been enhanced photocatalysis efficiency by introduced surface oxygen vacancies in several semiconductors, such as, BiPO 4 49 , CeO 2 50 and Bi-component Cu 2 O-CuCl 51 , which could be demonstrated to be conductive to band gap narrowing and photoactivity. Compared to the surface oxygen vacancies, the introduction of surface sulfur vacancies by doping N in our research narrows band gap and many shallow surface sulfur vacancies appear at the valance band (VB), as well as, N doping could introduce an impurity band. Furthermore, the introduction of surface sulfur vacancies can expand the VB width, which contributes to increasing the separation efficiency of photoinduced electron-hole pairs, leading to enhancement of photocatalytic activity. Moreover, N doping can extend the visible light absorption edge and the electrons are excited from the N impurity level to the conduction band, guaranteeing higher activity in degrading RhB 52 . Therefore, our sample possesses a high photocatalytic efficiency.
In addition, although as-prepared N-doped WS 2 nanoflowers show obvious photocatalysis, it is not so easy to recycle. Catalysts with magnetic properties, namely magnetic catalysts could overcome this problem 53 . Therefore,  it is gratifying to find a strategy for fabricating magnetic photocatalysts. Recently, magnetically separable semiconductor materials have attracted increasing attention because of their efficient recycle in water treatment, such as, Ni-Au-Zn 54 , NiO nanosheets 55 , Ag@AgCl 56 , r-Fe 2 O 3 @TiO 2 57 and etc. Here, α -Fe 2 O 3 @N-doped WS 2 heterostructure with strong magnetic property was prepared and employed to magnetically separate our catalysts from the solution of RhB. The SEM and TEM results of α -Fe 2 O 3 @N-doped WS 2 heterostructure is shown in Figure S3 (Supporting Information). As shown in Fig. 9, the degradation rate of RhB is almost 50% in 70 minutes, and it can be magnetically separation in 30s (shown in the upper right of Fig. 9). These results indicate that α -Fe 2 O 3 @N-doped WS 2 heterostructure can not only serve as efficient photocatalysts but also easy separate from organic pollutants.

Conclusions
In summary, we fabricated a series of W(N x S 1−x ) 2 nanoflowers via regulation of the mass ratio between tungsten pentachloride and thiourea in a mixed solvent system, as well as, fabricated the α -Fe 2 O 3 @N-doped WS 2 heterostructure. Under visible light irradiation, N doping can significantly increase the photocatalytic performance of WS 2 with the best efficiency obtained for 1.2 at% nitrogen doping. The expanded the utilization of visible light and the enhanced photoccatalytic activity both are resulted from the production of the surface sulfur vacancies by N doping. Meanwhile, we also demonstrated that the α -Fe 2 O 3 @N-doped WS 2 heterostructure can be easily separated from the organic pollutants, which improves the actual utilization rate of our sample.