Investigating the enhanced photocatalytic degradation of bromophenol blue using Ni/Zn co-doped Strontium Oxide nanoparticles synthesized via hydrothermal method

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Introduction
Dyes are of utmost importance in several sectors of the dyeing and textile industries.More than 100,000 commercially accessible synthetic dyes are commonly utilised in various industries.The dyes commonly utilised are typically obtained from two primary sources, namely coal tar and petroleum intermediates.The combined annual production of these dyes exceeds 7 × 10 5 tons (Sahu;Singh, 2019, Robinson et al., 2001).Each year, approximately 15% (equivalent to one thousand tons) of non-biodegradable textile dyes are released into natural streams and aquatic bodies via textile waste effluents (Javaid et al., 2021).
In the dyeing and finishing factories, it is customary for approximately 120-280 liters of water to be consumed per kilogram of cloth during the processing phase.Dyes are frequently the first chemical to be acknowledged in industrial waste-water because of their visibility even in a minute quantity (<1 ppm) (Chakraborty et al., 2022).
In addition to their adverse physical properties and poisonous character, the escalating output of dyes resulting from the rapid growth of industrialisation has necessitated the development of efficient treatment methods.Hence, several technologies have been examined to mitigate the significant and complex pollutants, with the aim of minimising their detrimental effects on the environment.Various conventional physical approaches, including activated carbon adsorption, reverse osmosis, and, have been employed for the purpose of dye removal (Tansakul et al., 2011, Oyarce et al., 2021).Nevertheless, these mechanisms just facilitate the movement of pollutants from one medium to another, hence resulting in the occurrence of secondary contamination.Typically, additional measures are necessary for the treatment of solid wastes and renewal of the adsorbent, resulting in an increased financial burden on the whole process.Various chemical processes, including are frequently employed for the purpose of removing synthetic dyes (Epold et al., 2012, Arslan et al., 2000).Nevertheless, the production of toxic and unstable metabolites during these processes has detrimental impacts on the health of both animals and humans.
In recent times, there has been a notable surge in the interest surrounding the utilisation of ZnO nanoparticles in the field of photocatalysis (Sakthivel et al., 2003, Jadhav et al., 2014).This heightened attention can be attributed to the remarkable catalytic activity and quantum efficiency exhibited by ZnO nanostructures, particularly when compared to the extensively employed TiO2 nanomaterials (Sun et al., 2008).To date, a range of ZnO nanostructures have been employed for the purpose of catalysing the degradation of noxious dyes into less dangerous chemical compounds by the process of photocatalysis under the influence of UV light irradiation.Sun et al. and Kuo et al. conducted experiments to investigate the photo-catalytic degradation of methylene blue and rhodamine-B using ZnO nanobelts and ZnO nanowires on a silicon wafer with the support of SnO2/Sn, respectively.
The results showed that methylene blue was degraded by approximately 94% after 5 hours, while rhodamine-B was degraded by approximately 94% within 300 minutes under UV light illumination (Wang et al., 2009, Sun et al., 2008).In recent times, there has been an increasing use of metal oxides-based materials, such as ZnO (Zada et al., 2022), V2O5 (Jenifer et al., 2021), CuO (Sasikala et al., 2016), TiO2 (Al-Mamun et al., 2022), WO3 (Verma et al., 2020), Fe2O3 (Subaihi;Naglah, 2022), SnO2 (Mishra et al., 2022), Al2O3 (Anbarasu et al., 2020), ZrO2 (Dawoud et al., 2020) , and others, in the field of photocatalysis.Although titanium dioxide (TiO2) demonstrated efficacy as a catalyst for the degradation of dye compounds when exposed to UV/Visible light irradiation.One significant limitation of the TiO2 catalyst is its relatively high cost, which restricts its widespread implementation in commercial settings.Therefore, the development of a cost-effective catalyst for the elimination of stubborn chemicals with enhanced adsorption, efficient degradation, and outstanding light absorption characteristics was of utmost significance (Ahmad et al., 2022).Hence, ZnO has garnered significant attention as a potential photocatalyst due to its advantageous attributes such as cost-effectiveness, abundance, chemical stability, affordability, non-toxicity, environmental friendliness, extended shelf-life, high electron mobility, and favourable quantum yields (Ahmad et al., 2022).
It should be noted that ZnO has the ability to exist in three distinct phases, specifically hexagonal wurtzite, cubic sphalerite, and cubic rock salt.The hexagonal wurtzite phase, which was observed to be highly stable at room temperature, was characterised by the presence of interpenetrating face-centered cubic (FCC) lattices composed of Zn and O (Prasad;Karthikeyan, 2017).In addition, it has been demonstrated that the ZnO semiconductor exhibits exceptional photocatalytic properties and can be synthesised in a wide range of morphologies such as nanorods, nanobowls, nanospheres, nanofilms, nanoparticles, nanodisks, nanotubes, nanoneedles, nanosheets, nanowires, nanowhiskers, and others (Alam et al., 2021, Saleh, 2019).
The present study focuses on the preparation of Ni and Zn co-doped Strontium oxide by hydrothermal method, their characterizations and using for the photocatalytic degradation of Bromophenol Blue dye which is present in various industrial effluent wastes.

Chemicals and equipments
Nickel Nitrate and Stronsium Nitrate were purchased from Daejung, S. Korea.Zinc Nitrate, Sodium Hydroxide ang Dye were purchased from Sigm-Aldrich, Schnelldorf, Germany. 100 mL Teflon lined autoclave (Model: Tob new energy, Item No: Tob reactor, China) made of stainless steel with operating temperature utpo 240 °C, Microwave oven (Dawlance Microwave Oven -MD4 N, Output Power 1000W) and Muffle furnace (temperature upto 1300 °C, made by Across International) were used for heating, drying and calcination of samples.

Synthesis
The synthesis of Strontium oxide nanoparticles was conducted using a hydrothermal method (Figure 1).A solution was prepared by dissolving 1 g of Strontium nitrate hexahydrate in 60 mL of water, which was afterwards stirred.Simultaneously, three distinct solutions comprising Nickel nitrate and Zinc nitrate were created and afterwards introduced into three separate solutions of Strontium nitrate hexahydrate.The mixes were continuously agitated at ambient temperature.
The pH of the mixture was maintained at a basic level by gradually adding a sodium hydroxide solution.
Following a specific time period, a white precipitate was observed to have been generated.The precipitate was afterwards dissolved again in the solution, which was then transferred to a Teflon autoclave made of stainless steel.The autoclave was placed in a microwave oven and maintained at a temperature of 150 °C for a duration of 24 h, resulting in the formation of a crystalline precipitate.The precipitate was subsequently extracted from the Teflon autoclave and subjected to a thorough washing process in order to eliminate any contaminants.The precipitate was subsequently subjected to microwave drying at a temperature of 110 °C, followed by calcination at 450 °C for a duration of 1 h.This process resulted in the formation of Strontium oxide, as well as Strontium oxide doped with Nickel and Zinc.
Figure 1.Schematic synthesis of Strontium oxide nanoparticles was conducted using a hydrothermal method.

Dye degradation
A stock solution of bromophenol blue with a concentration of 10 parts per million (ppm) was generated by dissolving 0.01 g of dry powder dye in 1000 milliliters of distilled water with vigorous agitation.Subsequently, aliquots of working solutions with varying concentrations were extracted from the stock solution and utilised for the purpose of conducting photocatalytic degradation experiments.The dye's photocatalytic degradation was carried out utilising a 100 W incandescent light.
Approximately 0.2 g of photocatalysts were distributed in individual working solutions, which were obtained from a stock solution.These solutions were subsequently left in darkness for a duration of 30 min to allow for the establishment of adsorption/desorption equilibrium prior to the commencement of the photo degradation reaction.Following this, the light bulb was switched on.Various solutions were employed at different time intervals, and the catalysts were subsequently eliminated by the filtration process.The measurement of absorbance for the filtered solution was conducted using a UV-Vis spectrometer.The percentage of photodegradation of bromophenol blue was determined by employing the following equation.The degradation percentage can be calculated using the formula: In this context, the symbol "Co" represents the starting concentration of the dye, while "Ct" represents the concentration of the dye at a given time "t".

Characterizations
Different characterization methods were entertained to analyze and understand the properties of samples.UV-Vis spectroscopic study was conducted using a Shimadzu UV-1800 spectrometer, wavelength range from 190-1100 nm, assisting absorbance measurements.The FT-IR (V.640, USA) ranges from 400 to 4000 cm -1 was used to study functional groups by building spectra from a 0.09 g sample.Photoluminescence (PL) analysis, executed at Abdul Wali Khan University Mardan, played a vital role in evaluating material purity with a spectral range of 230 to 1000 nm and a non-destructive methodology.X-ray diffraction difratometer (D-2 Phaser, Burker, Denver, CO, USA) was used by applying 20 degree per minute, and a Cu kα radiation source was used, providing understandings into the crystalline structure of samples.The shape evaluation of samples was conducted through scanning electron microscope (model, JSM5910, JEOL.Kyoto, Japan) was used with accelarartion voltge of 30 KV was used to analyxe the suface morphology of the samples.

UV/Vis specto study
Figure 2 illustrates the UV-Visible spectrum of the samples, with a wavelength range of 230 to 700 mn.The synthesis of the samples is substantiated by the existence of a significant excitonic absorption band at a wavelength of 294 nm.The manufacture of Strontium oxide leads to the creation of monodispersed colloidal particles, which in turn gives rise to the manifestation of quantum size effects (Athar, 2013).The band gap of the Strontium oxide nanoparticles that were synthesised was determined to be approximately 4.21 eV.
The introduction of Zn and Ni as dopants leads to a gradual extension of the absorption edges towards longer wavelength regions.As a consequence, there is a decrease in the band gaps.Interestingly, the absorption edges of SrO nanocrystals doped with various amounts of Ni/Zn co-dopants (namely, 2%, 3%, and 4% of Zn and Ni) occur at roughly 306 nm, 311 nm, and 318 nm, respectively.The Tauc plot was employed to determine the band gaps.The band gaps of Nickel/Zinc-doped Strontium oxide nanocrystals, namely those with 2%, 3%, and 4% nickel/zinc co-doping, have been measured and found to be 4.05 eV, 3.98 eV, and 3.89 eV, respectively.Based on the obtained data, it is evident that the band gap energies of Strontium oxide nanoparticles exhibit a consistent decrease with an increasing quantity of Ni/Zn in the samples.

FT-IR
Figure 3 displays the FTIR spectrum of the synthesised nanostructures.The spectra were collected in the 400 to 4000 cm -1 range.Figure 3 depicts the FTIR spectrum of Strontium oxide nanoparticles, which have a band at 705 cm -1 and 856 cm -1 .Undoped Strontium oxide stretching produced this band.A noticeable peak can be seen in the H-O-H bending at a range of 1452 cm -1 .Amino acid C-O stretching occurs in the band at 1033 cm -1 .This peak, which ranges from 3000 to 3600 cm -1 , demonstrates the presence of both interstitial water molecules and the -OH group.The peak serves as a sign that these two components are present.The apex of the Zn-O and Ni-O stretches, as well as the Ni/Zn-O-H bending vibrations, may arise at around 416 cm -1 and 588 cm -1 , respectively (Wiercigroch et al., 2017;Akpomie et al., 2022).
Figure 3. FTIR spectra of Ni/Zn co-doped SrO nanoparticles and pure SrO.
The samples in question are crystalline materials, as shown by the samples' extremely distinct XRD diffraction peaks.It's fascinating to see that adding Zn and Ni as dopants promotes the growth of the ( 020), ( 112), and ( 114) crystal planes at 14.18, 26.93, and 28.31 degrees while also enhancing the intensity of the XRD peak at 18. 01, 24.76, 37.98, and 40.93 degrees.To obtain a precise measurement of the nano-catalyst's crystal size, Scherrer's equation was used.
The average crystallite diameters of 2Ni/Zn SrO, 3Ni/Zn SrO, and 4Ni/Zn Strontium oxide nanoparticles, as well as undoped SrO and Ni/Zn co-doped SrO, are measured in this work.These nanoparticles were measured to have diameters of 30.50, 35.72, 35.81, and 36.97 nm, respectively.The lack of a discernible peak shift in the XRD pattern suggests that the sizes of all the samples are very similar to one another.

EDX
Figure 5 displays the EDX results as well as the sample's elemental makeup, which is 2Ni/Zn SrO.The graph demonstrates that zinc, nickel, carbon, and copper have sharp peaks while Strontium and oxygen have high peaks.The samples' exceptional purity is demonstrated by the abundantly obvious inclusion of all required components.Additionally, the sample 2Ni/Zn SrO has considerable amounts of Strontium and oxygen (82.02%), although the amount of doped zinc and nickel is only about 3.21 percent as given EDX table.The final 15% is made up of carbon and copper.
Figure 5. EDX of the sample.

PL study
According to the PL spectra, pure Strontium oxide displays peaks that are significantly more intense than those of the other co-doped SrO at 427, 446, 467 nm, 512, and 570 nm.Since PL intensity is a direct indicator of electron-hole recombination, the fact that pure SrO displays a peak that is more intense than that of other samples shows that it undergoes electron-hole recombination often.Because the amount of charge carriers produced by photons directly affects the photocatalytic activity of semiconductors (Martha et al., 2012, Mukherji et al., 2011), pure SrO exhibits less photocatalytic activity.However, all of the Ni/Zn co-doped SrO samples have lower PL intensities when compared to pure SrO.The 3Ni/Zn SrO sample is determined to have the lowest PL intensity.Due to the low PL intensity, it seems as though more excited electrons are trapped and are stably transmitting over the contact (Shah et al., 2013).As a consequence, there is hardly any charge carrier recombination in the case of 3Ni/Zn SrO, which is proof of the materials' improved photocatalytic activity.
Figure 6.PL spectra of pure SrO and Ni/Zn co-doped SrO NPs.

Photocatalytic activity
The photocatalytic activity of the produced photocatalysts was investigated while BPB was utilised as a model dye under visible light irradiation in an aqueous medium.Figure 7 shows the UV-vis absorbance spectra of bromophenol blue in an aqueous solution before and after the material was exposed to UV light for intervals ranging from 0 to 210 minutes.To achieve equilibrium between the adsorption and desorption of the dye molecules, the mixture was magnetically stirred for 30 min while left in the dark.At a distance of 7 centimeters from the material being studied, the combination was exposed to visible light generated by a Tungsten filament lamp.At regular intervals of 30 minutes apart, 10 mL aliquots were taken while the reaction mixture was continually stirred.The findings are presented in figure7 after the mixture was centrifuged for five minutes at a speed of 3000 rpm.
The results showed that as the irradiation duration grew from 0 to 210 min, the absorbance of the BPB solution at its maximum wavelength of 591 nm rapidly dropped.The experiment was conducted with this tendency in mind.This shows that bromophenol blue's photodegradation is time-dependent and that it increases with longer exposure to light.The results show that each photo catalyst has outstanding photocatalytic activity against bromophenol blue in the aqueous medium.In the present study, photocatalysts made of pure SrO and Ni/Zn co-doped SrO are responsible for the photocatalytic degradation of bromophenol blue in the aqueous medium.
It certainly shouldn't come as much of a surprise to hear that the catalytic activity of un-doped Strontium oxide is on the lower side of the spectrum given the large band gap and speedy charge recombination that this chemical features.However, when Ni/Zn is used as a dopant, there is a significant rise in the photocatalytic activity.The quantity of Ni/Zn dopant added also affects how well the photocatalytic degradation works, and the material made of 3Ni/Zn SrO has the maximum degradation effectiveness.However, when the Ni/Zn ratio is increased even further (4 Ni/Zn SrO), the photocatalytic activity is decreased.This is as a result of the shadow effect caused by Ni/Zn.

Conclusions
Using the hydrothermal technique, SrO and Ni/Zn co-doped SrO nanoparticles were successfully produced.
As-synthesized samples were put through a number of tests, including UV-Vis, FTIR, XRD, SEM, PL, and EDX spectroscopy, to determine form, optical properties, and phase purity.The XRD analysis amply demonstrated the crystalline character of the generated systems.
The modes of vibration present in Strontium oxide deposits were identified using FTIR spectroscopy.We were able to examine the optical properties of Strontium oxide and Ni/Zn co-doped Strontium oxide using spectroscopy.Bromophenol blue removal from water using Strontium and Ni/Zn co-doped Strontium oxide photo-catalysts was studied.Strontium oxide nanoparticles only destroyed about 85% of the BPB dye, whereas Ni/Zn co-doped Sr-O photo catalysts degraded more than 97% of it.

Acknowledgments
The author is greatly acknowledging the Department of Chemistry, Abdul wali khan university Mardan and University of Haripur, Haripur, Khyber Pakhtunkhwa, Pakistan for providing experimental facilities.

Authors' Contributions
Shahid Zaman: conceptualization, methodology, and writing -original draft preparation.Muhammad Kashif: investigation, data curation, and formal analysis.Muffirah Shah: synthesis of Ni/Zn co-doped Strontium oxide nanoparticles via hydrothermal method and photocatalysis.Abdul Hameed: characterization and analysis of photocatalytic properties.Noor Majeed: experimental design and data interpretation.Muhammad Ismail: supervision, funding acquisition, and project administration.Ilyas Khan: manuscript review and editing, contribution to theoretical insights.Saif Ullah: validation of results and statistical analysis.Naqash Khan: literature review, background research, and manuscript review.All authors have read and approved the final manuscript.

Conflicts of Interest
No conflicts of interest.

Ethics Approval
Not applicable.

Figure 4 .
Figure 4. XRD pattern of pure SrO and Ni/Zn co-doped Strontium oxide nanoparticles.

Table 1 .
Time and percent degradations shown by the samples.