Co-Reduction synthesis and Characterization of Ytterbium-substituted ZnS Nanoparticles and Investigation of Catalytic Performance

INTRODUCTION 
Recently, nano-crystalline semiconductors have been chiefly investigated as the most favorable photocatalyst for environmental remediation such as water refinement, air purification, hazardous waste remediation, and heavy metals degradation due to their non-secondary pollution and high functionality [1-5]. Due to the unpleasant influence of organic molecules on human safety, various treatment processes have been employed for treating colored effluents including biological treatment, flocculation/coagulation, chemical oxidation, advanced oxidation processes, membrane filtration, electrochemical oxidation, adsorption, and ion exchange [6-9]. Research of doping agents or impurity effects on the physical characteristics of semiconductors exist for both applied and basic studies. Rare-earth cations containing unoccupied 4f and empty 5d orbitals are frequently utilized as photo-catalysts or enhanced catalysis. In addition, doping with lanthanide ions with 4f electron configurations could exceptionally boost the isolation rate of photo-induced charge carriers in photocatalysts and significantly promote the catalytic ability [10-15].
Zinc sulfide is an essential II–VI semiconductor compound with a wide direct band-gap of Eg = 3.68 eV (bulk) [16]. ZnS has been examined owning to its vast potentials as catalysts and phosphors. ZnS is also appropriate for various technologies such as solar cells, electro-luminescent devices, and many other optoelectronic devices due to its stability in virtually all environments, high resistance to thermal shock, and extra-low bulk losses. Further, Zinc sulfide is a potent and promising catalyst for the photocatalytic removal of organic dyes as reported by several researchers [17-20]. In this study, a facile hydrothermal process was used for preparing undoped and Yb3+-substituted ZnS (YbxZn1−xS) particles. The photocatalytic performance of such particles towards the decolorization of Tartrazine (as a model organic dye) with visible-light radiation was evaluated. Table 1 provides the characteristics of Yellow 5. Considering the literature, there is no previous report related to the use of ZnS and YbxZn1-xS particles for the elimination of Tartrazine (Yellow 5). Furthermore, this study aims to assess the influence of inorganic ions on the degradation percentage of Yellow 5. 

MATERIALS AND METHODS
Chemicals
All chemicals utilized in this research were of analytical grade and used without further purification. NaOH, N2H4.H2O (99%), ZnSO4.6H2O (99.5%) and S (99%) were obtained from Merck; Yb (NO3)3.5H2O and ethanol (99%) were acquired from Aldrich, and Tartrazine from Zhejiang Yide Chemical Co. (China).

Synthesis of Yb3+-doped ZnS compounds
Yb3+-doped ZnS particles with different Yb3+ contents (0-6% mol) were synthesized employing hydrazine hydrate (N2H4· H2O) as the reducing factor using the hydrothermal method. In a normal preparation, appropriate molar ratios of Yb (NO3)3.5H2O, 2 mmol S, 1 mmol NaOH and ZnSO4.6H2O were first dissolved in 25 mL distilled water. Hydrazine hydrate (N2H4 ·H2O) was then added drop-wise to the above solution under middle-speed stirring. After constant stirring, the resulting solution was moved into a 50 ml Teflon-lined stainless-steel, placed in an oven at 160°C for 12h, and subsequently, the autoclave was cooled naturally to room temperature. As-obtained YbxZn1−xS particles were collected and washed with absolute ethanol and distilled water several times for eliminating residual impurities, and then vacuum-dried at 70 °C for 2 h. As a result, the final yellow–white powders were obtained.

Characterization Methods
The XRPD characterization was employed for determining the crystal phase composition of samples at room temperature by using a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with high-intensity monochromatic Cu Ka radiation (λ=1.5406 A˚), accelerating voltage of 40 kV, and emission current of 30 mA. The surface state and the morphology were observed via an electron microscope (SEM, S-4200, Hitachi, Tokyo, Japan), and the chemical state of the constituent elements as well as chemical identification were analyzed by using X-ray photoelectron spectroscopy, XPS (K-ALPHA, UK), and Thermo Scientific spectrophotometer. Cell parameters were measured through Celref program based on PXRD patterns, and reflections were determined and fitted by applying a profile-fitting procedure with the Winxpow program. The reflections observed in 2θ= 10–80° were employed for identifying the lattice parameter.

Photo-catalytic investigation
The catalytic performance of bare and Yb3+-doped ZnS nanoparticles were assessed in an aqueous solution under visible light by monitoring the degradation of Yellow 5. In a usual method, 0.1 g of the photocatalyst material was added to 100 ml Yellow 5 solution with a starting concentration of 5 mg/l. The suspended photocatalyst and Yellow 5 were magnetically stirred in a quartz photoreactor in the dark for 100 min to form an adsorption/desorption equilibrium of the dye. Then, a 40 W visible-light lamp was used for irradiating the solution. The degradation percentage (DE (%)) was stated as the ratio of decolorized dye concentration to that of the initial one multiplied by 100. During the photocatalytic process, 5 ml of the suspension was sampled at a desired time and after centrifugation; the removal of color was assessed through UV-Vis spectrophotometer by determining the absorbance of the solution at λ max = 428 nm.

RESULTS AND DISCUSSION
Physical properties and characteristics of as-prepared materials
Fig. 1 displays the XRPD patterns of the pure and Yb3+-substituted ZnS samples. The observed diffraction peaks of the as-prepared compounds can be attributed to the pure, typical, and well-crystallized cubic ZnS (JCPDS No. 05-0566)[21]. No peaks showing impurities were observed, confirming that the hydrothermal route applied in this study was successful in preparing the desired samples. Additionally, the sharp peaks in the XRPD patterns of the prepared samples indicate that the obtained products were high crystalline. Behind doping values of x = 0.06 for Yb3+, extra undisclosed phases were observed. There was a slight shift to the lower diffraction angles in the 6 % Yb3+-doped ZnS pattern. This result can be related to the expansion of ZnS lattice due to the presence of Yb3+ions which have larger radii (0.99˚A) compared with Zn2+ions (0.74˚A). Moreover, this shift to lower angles confirms the successful doping of Yb3+into the Zn2+lattice.  
The cell parameters of the as-obtained samples were calculated by XRPD patterns. With increasing dopant content (x), a parameter for Yb3+ increased (Figure 2). The shift for lattice constants can be related to the effective ionic radii of the Ln3+ ions, which led to larger lattice parameters for Yb3+ doped samples.
XPS analysis was utilized to examine the valence state and the chemical composition of Yb in the 6% Yb3+-doped ZnS particles (Fig. 3). The binding energy for the Zn2p region of Yb3+-doped ZnS is shown in Fig. 3a. The Zn 2p1/2 and 2p3/2 peaks were detected at around 1047.2 and 1022.4 eV which can be attributed to the Zn element in zinc sulfide, validating the oxidation state of 2 for Zn in the 6% Yb3+-doped ZnS sample [22, 23]. The S2p curve of ZnS manifested a strong peak at around 162.9 eV, which is ascribed to the coordination of Sulfur and Zn atoms (Zn–S–Zn) in the structure of ZnS (Fig. 3b) [24]. As shown in Fig. 3c, two peaks centered at 186.2 and 200.18 eV can be related to the Yb 4d5/2 and Yb 4d3/2, respectively [25].
TEM and SEM analyses were conducted for investigating the morphologies of the products. In Fig. 4a and b, spherical and uniform particles of about 10−80 nm in diameter with a little agglomeration can be observed. Through incorporating Yb3+ ions into the lattice of ZnS, the size and surface morphology of the sample had no clear changes. Fig. 4c and d show TEM image and SEAD pattern of the 6% Yb3+ sample which validates the SEM output and good crystallinity of materials.
Size distribution for Yb3+-doped ZnS particles was attained to be in the range of 10–80 nm, which is smaller than that of pure ZnS (Fig. 5).

Effect of operating conditions on the photocatalysis of Yellow 5
Effect of Yb3+ content of YbxZn1-xS nanoparticles
The degradation process of Tartrazine (Yellow 5) was examined under visible light irradiation by YbxZn1-xS with various mole fractions (x = 0.00, 0.01, 0.02, 0.04, 0.06) in order to explore the perfect conditions of photocatalytic performance. Fig. 6 demonstrates the degradation percentage of Yellow 5 over divergent Yb3+-doped ZnS catalysts during 100 min of the reaction. As indicated in Fig. 5, the nanomaterials doped with proper content of Yb3+ion enhanced the photocatalytic performance much more than bare ZnS did. Specifically, the sample with the Yb3+molar ratio of 0.06 manifested the best catalytic activity. The reason for high photocatalytic activity of Yb0.06Zn0.94S can be explained in the following way. Normally, rare-earth cations can perform either as a recombination center or as a mediator of interfacial charge in the crystalline structure of the photocatalyst [26, 27]. Therefore, based on the dopant value, the doped ZnS performance or efficiency is changeable. At low mole content of dopant, Yb3+ ions can retard electron/hole recombination rate by capturing photoinduced electrons, and, consequently, promoting the interfacial charge transfer for Yellow 5 degradation [28]. However, when the mole ratio of the doping agent is greater than the maximum value, the recombination rate may increase by decreasing the distance between the trapping sites in ZnS structure, leading to a reduction in the catalytic activity. Thus, the best content of Yb3+ is substantial in isolating the photo-induced electron/hole pairs and boosting the lifetime of charge carriers. 

Photo-catalytic decolorization mechanism of Yellow 5 on Yb0.06Zn0. 94S
In order to properly explain the photocatalytic performance of the Yb0.06Zn0.94S samples and assess the realizable mechanism of reaction, the UV–Vis absorption spectra of Yellow 5 at different irradiation times for the photocatalytic process are shown in Fig. 7. The decreasing concentration of Yellow 5 during the catalytic procedure is utilized to evaluate the potential of the catalyst.  
Here, the doping Yb3+ acts as an electron predator on the surface of ZnS, repressing the re-combination of electron-hole pairs and increasing their lifetime; therefore, the photo-catalytic activity of the catalyst is elevated. The realizable mechanism for the elevated photocatalysis of Sm0.04Zn0.96S is suggested as follow. The electrons (e−) are excited from the valence band to the conduction band of ZnS through visible light irradiation, and the holes (h+) are generated. Yb3+ content in ZnS can efficiently scavenge the electrons and hinder their recombination with h+ due to the existence of partially filled 4f-orbital [29, 30]. The decolorization mechanism for the Yb0.06Zn0. 94S is provided in Eqs 1-10.

Yb0.06Zn0.94S + visible light → Yb0. 06Zn0.94S (h + ads + e - ads)(1)
e – ads  + Yb3+ → Yb2+ (electron scavenging tread)                (2) 
e – ads  + O2 ads    → O2.- ads                                              (3)
O2.- ads + Yb3+ → Yb2+ (electron scavenging tread)         (4)
Yb2+ + O2 ads  → Yb3+ + O2.- ads (electron transferring tread)              (5)
H+ ads  + O2.- ads → .OOH ads                                                  (6)
.OOH ads + H+ ads + e – ads → H2O2 ads                                 (7)
H2O2 ads + e – ads → .OH ads + OH –ads                                (8)
h + ads + H2O ads → .OH ads + H+ ads                                    (9)
h + ads + OH –ads → .OH ads                                             (10)

The schematic illustration of the catalytic activity of as-prepared nanoparticles is presented in Fig. 8.

The outcome of catalyst concentration and reusability
The starting rate of photocatalytic degradation process relies on the catalyst concentration [31]. The effect of catalyst loading on the elimination percentage of Yellow 5 can be seen in Fig. 9. The color removal ratios were 47.85, 62.48, 81.29, 92.15 and 88.45 % at the concentrations of 0.25, 0.5, 0.75, 1.0, and 2 g/L, respectively. The DE% increased from 0.25 to 1.0 g/L, and then declined. This could be explained by the aggregation of catalyst beyond 1.0 g/L which resulted in reducing the number of active sites.
Reusability is one of the most substantial parts for a catalyst. Fig.10 shows the reusability assays of Yb0.06Zn0.94S catalyst in the degradation of Yellow 5, during the 5 round tests under the perfect condition of 100 min for irradiation time, 10 mg/L of tartrazine, 1.0 g/L of Yb0.06Zn0.94S photo catalyst. Following each degradation test, the catalyst was washed with distilled water, dried at 70 °C for 2h, and utilized in the next run. As shown in Fig. 10, Yb0.06Zn0.94S manifested outstanding chemical firmness without any significant decomposition or photo-corrosion during the 5 rounds of the catalytic reaction, suggesting that it is a superior catalyst for empirical applications. 

Effect of Initial Yellow 5 concentrations 
Various initial dye concentrations in the range of 10 to 30 mg/L were used in this research. According to Fig. 11, DE percentage reduced from 92.15 to 59.81% for the initial concentration of 10 to 30 mg/L. Filling the energetic sites on the catalyst surface by pollutant molecules at high concentrations results in an astounding decrease in the degradation effectiveness. Preventing the diffusion of light to the catalyst surface can lead to unsatisfactory performance at a dense concentration of dye solution. 

The role of radical scavengers on the photocatalytic performance
To evaluate the mechanism of decolorization process and to find the chief oxidative species, assays were conducted in the presence of proper scavengers of active species. As indicated by Fig. 12, by separately adding oxalate (a scavenger of h+VB), t-BuOH (a scavenger of hydroxyl radicals), and I- (scavenger of hole), the decolorization percentage reduces to 65.39%, 52%, and 31.67 %, respectively. In the case of benzoquinone (BQ) (a scavenger of superoxide radicals), the dye degradation was hindered extraordinarily. These results demonstrate that superoxide radicals and the h+VB were the principal oxidative species in decomposing dye structure. Moreover, the hydroxyl radicals influence the decolorization.

CONCLUSION
In this research, pure and Yb3+–doped ZnS were obtained by a simple hydrothermal approach and employed as a photocatalyst under visible light irradiation for removing Yellow 5. XRD analysis displayed well crystalline cubic structure of ZnS, and the substitution of Yb3+ions into the ZnS lattice was validated by the XPS analysis. In addition, the surface morphology and size of the samples had no obvious changes after incorporating Yb3+ into the lattice of ZnS. The results indicated that the decolorization efficiency of Yb3+-doped ZnS was higher than that of pure ZnS, and the degradation efficiency was significantly affected by the content of Yb dopant in ZnS. The promoted decolorization efficiency was found in the presence of 6% Yb3+-doped ZnS particles. The color removal percentages of Yb0.06Zn0.94S and undoped ZnS were 92.15 and 22.37% after 100 min of treatment, respectively. Benzoquinone led to the highest negative effect on the photocatalysis of Yellow 5. Generally, the application of Yb3+-doped ZnS particles can be a promising and effective approach for the elimination of colored effluents.

ACKNOWLEDGMENT
This work is funded by Sayyed Jamaleddin Asadabadi University Research Grant.