Effect of Ni doping on the structural, optical and photocatalytic activity of MoS2, prepared by Hydrothermal method

Transition metal dichalcogenides (TMDs) are promising materials for photocatalytic functions. In class of TMDs, MoS2 is comprehensively explored as a co-catalyst due to the extraordinary activity for photocatalytic activity of organic dye degradation. But the catalytic activities of MoS2 are generated through S ions on depiction edges. Also numerous of S ions existed on basal planes are catalytically inactive. The insertion of external metals in MoS2 organism is extensive way for activation of basal planes surface to enhance concentration of catalytically active sites. For this purpose, nanoparticles of Nickel (Ni) doped MoS2 are prepared by hydrothermal technique. Structural and morphological analysis are characterized by XRD and SEM, respectively. XRD results showed that Ni is completely doped into MoS2. SEM showed that pure MoS2 has sheet like structure and Ni doped MoS2 has mix disc and flower like structure. Band gap energy was observed in declining range of 2.30–1.76 eV. The photocatalytic activity of pure MoS2 and Ni doped MoS2 were evaluated by degrading MB and RhB dyes under UV light irradiation. MB dye degradation of MB was 71% for pure MoS2. For 1% to 5% Ni doping in MoS2, MB dye degradated from 85% to 96%. It means that MB dye degradation of MB was enhanced continuously by increasing the concentration of Ni doping. RhB dye degradation of RhB was 62% for pure MoS2. For 1% to 5% Ni doping in MoS2, the RhB dye degradated from 77% to 91%.


Introduction
Renewable energy, photocatalysts and nanoscience have attracted the interests of many researchers from last couple of decades [1][2][3][4][5]. Various nanoscale semiconductors can be developed through different synthesis techniques. Organic or inorganic pollutant materials can be removed by the application of ZnO, Fe 2 O 3 , TiO 2 , CdS and MoS 2 photocatalysis materials [6][7][8][9][10][11]. Currently, higher surface to volume ratio and quantum effects have boost the significance of nanomaterials in fields including pollution, smell prevention and renewable energies [12]. Compositional variations, structures, shapes and dimensions of semiconductors have defined such nanomaterials in technological applications [13]. Photo-oxidation and photo-reduction are numerous terms applied for oxidation and reduction executed in light. Band gap excitation appears by partitions of charges when light strikes in semiconductor materials. Oxidation and reductions on substrates can be initiated by the application of electrons and holes generated by light. Charge transformations amongst molecules at nanoscale, the materials have important part for photocatalysis development [14][15][16]. ZnO semiconductors have wide band gap, energetic under UV region of light and embrace just 5% of light [17]. Various researchers have reported the enrichment in photocatalytic performance through adjustment of hertrojunctions of ZnO with TiO 2 , Ag 2 O, Cu 2 O etc semiconductors [18][19][20][21]. Recently for photcatalysis applications, transition metal Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
sulphides have become potential candidate and have successfully substituted the noble metals [22][23][24]. P-type semiconductor molybdenum disulphide (MoS 2 ) having contracting band gap (1.7 eV) have drawn the attention of researchers. MoS 2 have powerful adsorption in visible range of solar spectra and is categorized as a stable layer metal dichalcogenides [25]. Feng-Jun Zhang et al have synthesized MoS 2 photocatalyst having stronger photcatalytic hydrogen generation efficiency [26]. Youzi Zhang et al have prepared the MoS 2 with 242% enhancement in production of H 2 gas [27]. Said Ridene has investigated improving results by fabricating MoS 2 and employing in laser emitting gadgets [28]. Adhimoorthy Saravanan et al have composed the MoS 2 nanomaterials for future gas sensor and storage employments [29]. Various metals can be doped into P-type semiconductor molybdenum disulphide (MoS 2 ) to explore the potential for various applications. The insertion of external metals in MoS 2 classification is an extensive way for activation of basal planes surface to enhance concentration of catalytically active sites. For this purpose, nanoparticles of Nickel (Ni) doped MoS 2 are prepared by hydrothermal technique to expand photocatalytic activity of MoS 2 nanomaterials to develop visible light state of solar spectra. In current research work MoS 2 and Ni 2+ doped MoS 2 have been developed for photocatalytic applications. Ni was inserted with different ratios of 1-5%. Hydrothermal technique was employed to fabricate such nanomaterials. After synthesis XRD, SEM, UV and photocatalytic characterizations are examined of MoS 2 and Ni (1-5%) doped MoS 2 .

Experimentation
MoS 2 solution was prepared by adding 0.72 g of MoO 3 and 0.38 g of thiourea into 30 ml of de-ionized water. Similarly, 2 and 4 wt% nickel nitrate was added in this solution for the synthesis of 2% and 4% Ni doped MoS 2 . At room temperature all the solutions were stirred for 20 min for homogenous mixing. And then these three solutions were shifted into 50 ml Teflon stainless steel autoclaves at 200 o C for 9 h. The autoclaves were cooled down at room-temperature. These four samples were cleaned three times with de-ionized water and ethanol and then dried in vacuum at 80 o C for 5 h. Structural and surface morphology of these specimens were characterized by XRD and SEM, correspondingly. Photo catalytic activity (PCA) of Ni doped MoS 2 was analyzed by degrading methylene blue (MB) and rhodamine blue dye (RhB). A 10 mg Ni doped MoS 2 was mixed 30 mL of dye solution (1 × 10 −5 mol l −1 ). To make sure adsorption-desorption equilibrium stirred the mixture for the period of 1 h in absence of light. 500 W Hg lamp was utilized as a resource UV light. 100 mW cm −2 was adjusted irradiation intensity of UV light source and cutoff filter was employed to exclude the visible light. The suspension was irradiated to UV light for 240 min. After predetermined time period, 2 ml specimen was withdrawn. Then filtered and left over dye concentrations were anticipated at 665 nm and 554 nm (CE Cecil 7200, UK) for MB and RhB, respectively.  [29]. It can be observed that more peaks with the increasing percentage of Ni as compared to the pure MoS 2 are appeared. XRD patterns of all products are well matched with crystal arrangements of MoS 2 JCPDS. Same kinds of MoS 2 peaks were detected by Youzi Zhang et al [27]. Results obtained from XRD pattern are well agreed with the results obtained from FESEM analysis.     spectroscopy absorption coefficient (α) can be computed. Following relation can be employed to inspect absorption coefficient as,
The maximum degradation of RhB dye for 5% Ni doped MoS 2 was observed 91% and maximum degradation of MB dye for 5% Ni doped MoS 2 was observed 96%. It means that degradation of RhB was slightly lower than the MB dye. Using stronger visible light PCA results indicate that photocatalytic properties of doped MoS 2 are larger than pure MoS 2 means significantly enhanced electron-hole pair's separation and increased the photocatalytic properties of MoS 2 because doping increases the active sites of MoS 2 [35]. Table 1 shows the summary of MD dye (%) and RhB dye (%) for pure MoS 2 and Ni-doped MoS 2 (1%, 2%, 3%, 4% and 5%). It can be examined that increasing trends are shown by all nano-composites. Figure 6 also demonstrates the behaviors shown by all compositions for MB dye (%) and RhB dye (%). The photodegradation presentation of MoS 2 can be improved by the increasing doping of Ni at appropriate treatment time. Such occurrences can be revealed by the generation of Ni nanoparticles in MoS 2 structure. The enhancements in  photodegradation for both MB and RhB dyes are illustrated in figure 6. Also band gap is decreasing with the increase of Ni contents. It means distance amongst valance band and conduction band is decreasing. Such phenomena are leading the excited electrons to move from valance to conduction band in limited period of time. Hence with the increment of Ni contents the photodegradation for both MB and RhB dyes are enriching [36,37]. Figure 7 shows plot of C/C o versus irradiation time (C and C o are the residual and initial concentration dyes) for MB and RhB dyes. Under light irradiation, the photocatalytic activity was enhanced and it was observed that the degradation rate was fast for first 45 min, which slowed down with time. Mg doped MoS 2 furnished the MB and RhB dyes degradation of 91% and 84% for 240 min UV irradiation

Conclusions
Transition metal dichalcogenides (TMDs) are potential materials for photocatalytic applications. MoS2 and Ni doped MoS 2 have been successfully prepared by hydrothermal technique. XRD analysis confirmed the formation of Ni doped MoS 2 nanoparticles. The doping of Ni has effected on the morphology of nanoparticles, according to FESEM. Band gap energy was observed in variable trends with the range of 2.30-1.76 eV with the addition percentage of Ni contents. The improvement in photocatalytic activity of MoS 2 has been observed when Ni was doped in it. MB dye and RhB dye percentages demonstrated increasing trends with the addition of nickel contents.