Preparation of strontium doped mesoporous ZnO nanoparticles to investigate their dye degradation efficiency

Mesoporous strontium doped ZnO nanoparticles are synthesized as photocatalyst by using zinc nitrate hexahydrate, surfactant P123, strontium nitrate hexahydrate via the hydrothermal process. X-ray diffraction (XRD), UV-visible spectroscopy (UV), Fourier transform infrared (FTIR), Photoluminescence (PL), Energy-dispersive x-ray (EDX), Scanning electron microscopy (SEM), Transmission Electron Microscopy (TEM), and Brunauer–Emmett–Teller (BET) characterizations are used for the analysis of all the samples. XRD spectra disclose the disparity in the crystal size 14.98 to 22.74 nm. The study of UV spectroscopy revealed the energy bandgap difference between 3.3–2.92 eV. PL spectroscopy shows the effect of doping on the electron-hole recombination rate of the sample. FTIR analysis has utilized to determine the functional groups such as –OH, C=O=C, and –C–O present in the sample. EDX spectra show the elemental compositions of the sample. SEM images show the agglomerated morphology and TEM images show the different shape morphology of the sample. BET analysis shows the occurrence of 39.9 m2 g−1 surface area with mesoporous morphology. The effect of the increasing percentage of strontium on the photocatalytic capability of ZnO is checked against methylene blue and congo red dyes with 75% and 80% degradation.


Introduction
In today's world, there are many environmental problems. One of them is water pollution. A lot of polluted water comes out from the industries which contain dyes [1]. Due to modern civilization, there is an increment in the formation of dyes. There are various semiconductor metal oxides nanostructures such as SnO 2 , ZnO, and TiO 2 which are used to treat wastewater via the eco-friendly process called solar photocatalysis [2][3][4]. To treat the waste-water, ZnO (n-type semiconductor) is preferred over other semiconductor metal oxides just because of its excellent electron mobility and 3.37 eV of band gap energy with chemically stable nature and ability to degrade a variety of dyes with antifungal and anti-corrosive property. Doping is a productive and rapid technique to make the photocatalytic properties better. Here we have used strontium as a doping element. The selection of a doping element having a bigger radius as compared to the Zn 2+ causes major lattice defects due to charge compensation. The difference in ionic radius between Sr 2+ (2.45 Å) and Zn 2+ (0.74 Å) influences the optical characteristics of ZnO. The role of strontium doping is to diminish the energy bandgap and electron-hole recombination of the pure ZnO nanoparticles. As the percentage of strontium doping increases, the energy bandgap of the ZnO nanoparticles is reduced progressively. K Pradeep raj et al synthesized strontium doped ZnO nanoparticles to quest their structural, optical, photoluminescence, and photocatalytic activity [5]. S Lakshmana Perumal et al investigated the structural, optical, and photocatalytic properties of strontium doped ZnO nanoparticles [6]. S Salvi et al synthesized strontium doped ZnO photocatalyst [7]. Linhua Xu et al investigate the structural and optical properties of strontium doped ZnO thin films [8]. RaminYousefi et al investigate the improved visible-light photocatalytic activity of strontium doped ZnO nanoparticles [9]. There are various techniques for the synthesis of pure and strontium doped ZnO nanoparticles just like the sol-gel Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. method, thermal deposition, co-precipitation method, hydrothermal method, etc [10]. In the present research, samples are prepared by the hydrothermal method. There is sufficient water available on earth [11]. Every chemical is easily soluble in water. Another advantage of using the hydrothermal method above diverse forms is its capability to build crystalline phases which are unsteady at the melting point [12]. The hydrothermal method is performed in a pressure vessel also defined as an autoclave, whereas processing conditions are controlled by adjusting the temperature/pressure.
The purpose of the experiment is to notice the influence of the strontium doping on the photocatalytic performance of ZnO nanoparticles with mesoporous morphology against Methylene Blue (MB) and Congo Red (CR) Dyes. . Hydrothermal treatment is provided to solution 3 (gel) by autoclave adjusting temperature for hours and after that, all the solutions were filtered and washed to obtain dry and white powder. For drying the samples were placed into the electric oven at 120°C and for calcination placed in a muffle furnace at 450°C (to remove the surfactant).

Experimental
To the synthesis of doped samples required the amount of strontium nitrate hexahydrate is added to solution 3 for the different molar percentages (1%, 2%, and 3%), (table 1).

Characterization
Instrument Quantochrome is used to determine the type-IV Nitrogen adsorption-desorption plot of the powder samples at 250°C of outgas temperature. PL spectra are determined by the Aligent Cary Eclipse spectrophotometer. Instrument Perkin Elmer is used to confirm the FTIR spectra. Powder samples at room temperature are used without any specific conditions of the sample preparation in FTIR, PL, XRD, and UV-vis. PANalytical x-ray diffractometer and Perkin Elmer LAMBDA 650 instruments are used to determine XRD as well as UV-vis spectra. Ethanol with sonication is used to prepare a suspension of powder sample to obtain the TEM images via the Tecnai™ G2 20 instrument. SEM images are obtained by using Hitachi instrument.

Photocatalytic degradation
The photocatalytic performance of the ZnO nanoparticles is performed by the use of Methylene Blue (MB) and Congo Red (CR) dyes. The calculated amount of dye (10 −5 M) is added in 100 ml of DI water. Now, put the 100 ml solution with 0.05 gm of photocatalyst on the sonication for the 1 h in the dark region before displaying to the visible light (mercury vapor lamp, 420-520 nm, placed 1 cm above in vertical position). At a regular interval of 20 min, drop out 10 ml from the solution to check the percentage degradation through the UVvisible spectrophotometer. A similar process is performed for the degradation of congo red dye and degradation is calculated.

Morphology and structural analysis
It is noticed that with the addition of strontium doping the agglomerated morphology of the ZnO nanoparticles increases as per displayed in figure 3. TEM images are presented in figure 4. TEM pictures show the exact shape and size of ZnO nanoparticles, mesoporous assembly of nanoparticles is visible in all the images. EDX reveals the presence of all elements with ZnO as shown in figure 5.
The study of BET analysis is used to investigate the pore diameter, pore-volume, and surface area. Three images associated with isothermal (figure 6), BJH (figure 7) and multi-point BET analysis ( figure 8). The pore diameter of 6.5 nm with 0.08 cc g −1 of pore volume was analyzed through BJH analysis. The multipoint BET shows the surface area of 39.9 m 2 g −1 . The presence of type-IV hysteresis in the isothermal displays that there is the existence of the mesoporous nature of sample A having slit-like pores [14].

UV-visible spectroscopy analysis
The key step to observe the change in the energy band gap of pure and doped samples is UV-visible spectroscopy

Fourier transform infrared spectroscopy analysis (FTIR)
As shown in figure 11, the results found from the study of FTIR spectra show that the functional groups lay in the region between 4000 cm −1 -400 cm

Photoluminescence spectroscopy (PL) analysis
Analysis of photoluminescence spectra describes that the electron-hole pair recombination decreases concerning the doping. The minimum intensity is observed from the maximum doping concentration (3%) [19]. Following their luminescence color, their corresponding defects were obtained and shown in figure 12. The PL-emission is obtained at 441 nm, 452 nm, and 470 nm shows the blue emission with the existence of Zinc vacancy, at 481 nm the blue-green emission with Zinc interstitial, at 494 nm green emission with oxygen antisite and oxygen interstitial [20,21].

Photocatalytic degradation activity
The deterioration of methylene blue (MB) and congo red (CR) at a pH value of 8, has been performed by the pure and strontium doped samples to illustrate their photocatalytic competence. The electrons in the valance band were shifted to the conduction band when exposed to the visible light [22]. The holes in the valence band break down the water particles to custom the hydroxyl radicals and electrons in the conduction band act in response with an oxygen particle to custom superoxide anion. As we increase the doping concentration the efficiency of photocatalysis increases and shifted toward visible light due to lower bandgap energy and electron-hole recombination speed as shown in figures 13 and 14. In the visible light, the maximum degradation of dye is shown by sample D against congo red (80%), table 3. The pattern of degradation is the same for both the dyes i.e. degradation increases with doping percentage.

Conclusion
The pure and strontium doped ZnO nanoparticles are magnificently characterized following the hydrothermal method. XRD pattern shows that entire synthesized samples contain the hexagonal wurtzite structure and the crystalline dimension of nanoparticles varies from 14.98-22.74 nm. UV-visible spectroscopy analyses that the energy band gap shrinks (3.3-2.92 eV) as the doping concentration increase. In the FTIR spectra, the presence of diverse functional groups is found in the sample. PL spectra show that the electron-hole recombination drops with an increase in the doping percentage. The agglomerated morphology of the samples in the nanometre range is displayed in the SEM images. TEM images show the exact shape and size of the crystal. The existence of all  elements (Zn, O, and Sr) that exist in the sample is shown by the EDX results. BET analysis shows the existence of a mesoporous structure along a high surface area. Thus, all the characterizations show that the strontium doped ZnO nanoparticles as an efficient photocatalyst are executed successfully by the hydrothermal method.

Conflict of interest
Author has no conflict of interest.