Gemini surfactant assisted synthesis of mesoporous Mn/Mg bimetal doped TiO2 nanomaterial: characterization and photocatalytic activity studies under visible light irradiation

The present work mainly aimed to synthesize different weight percentages (0.25–1.00 wt%) of Manganese (Mn2+) and Magnesium (Mg2+) bimetal ions doped TiO2 nanomaterial assisted with different weight percentages (5–15 wt%) of Gemini surfactant (GS) using sol-gel method. The bimetal doped and undoped TiO2 photocatalysts were characterized by X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), UV-Visible Diffused Reflectance Spectroscopy, Transmission Electron Microscopy, Brunauer-Emmett-Teller surface area analyzer, and Photoluminescence Spectroscopy. Characterization results revealed that mesoporous multi-particle anatase TiO2 nanoparticles with a narrowed band gap, small particle size, and high surface area were formed due to the combined effect of Mn2+/Mg2+ bimetal ions doping and effective encapsulation of GS over the initially formed TiO2 nanoparticles. The surface elemental composition of the 0.25 wt% Mn2+ and 1.00 wt% Mg2+ bimetal doped TiO2 in the presence of 10 wt% of GS (after calcination) revealed the presence of both the metal dopants Mn2+ and Mg2+ along with the Ti and O and their chemical interactions were further confirmed by FT-IR results. The photocatalytic activity of these catalysts was assessed by the degradation of Methyl Red using visible light irradiation. To understand the effect of different reaction parameters on the photocatalytic activity of the nanocatalysts such as the dopant concentration, surfactant concentration, catalyst dosage, solution pH, and initial dye concentration were investigated and optimized to achieve the best performance. The photoluminescence results conclude that OH radicals are the crucial reactive species responsible for oxidative photocatalytic degradation of Methyl Red.


Introduction
It is worth noting that among the different semiconductors that are being studied from the past few decades for the photocatalytic degradation of various organic pollutants from wastewater, TiO 2 stands as the most preferable one due to its non-toxicity, non-corrosive, photostability, and low cost. To make it suitable for visible light activity, many attempts are being made to modify the TiO 2 electronic structure to overcome its disadvantages of high bandgap energy and high rate of electron-hole recombination. The recent literature findings revealed that bimetal ions doping into TiO 2 crystal lattice had overcome the above drawbacks and significantly enhanced its visible light activity [1][2][3]. Among the various metal ion dopants studied so far, transition metal ion dopants can greatly reduce the bad gap and electron-hole recombination by forming an extra energy level below the conduction band of TiO 2 due to the interaction of their 3d or 4d states with Ti 3d states [4]. Recently, Divya Lakshmi et al. [5] reported that Manganese (Mn 2+ ) is most preferable among the transition metals owing to the presence of t 2 g orbital of d is very close to the conduction band of TiO 2 by which the absorption can be possibly shifted to the visible part of the spectrum. On the other hand, others [6][7][8] investigated the effect of Magnesium (Mg 2+ ), an alkaline earth metal, on the crystalline structure and catalytic activity of TiO 2 and found its absorption band was shifted more towards the visible region, which resulted in performing a high photocatalytic activity. Hence, Mn 2+ and Mg 2+ ions could be more effective as metal ion dopants to modify the TiO 2 for better photocatalytic performance under visible light irradiation.
As the size, shape, and surface area of the photocatalyst play a major role in the photocatalytic degradation process of organic pollutants from wastewater, surfactants were the best-reported species to control the growth and agglomeration of nanoparticles during the synthesis process. Among those surfactants, more surface-active anionic Gemini surfactant, 1,4-Butane sultone (GS) plays an important role in effective encapsulation of initially formed TiO 2 particles to synthesize the spherical shaped bimetal doped TiO 2 nanoparticles with small grain size and large surface area [9]. Hence, in the present study, we aimed to synthesize the Mn 2+ and Mg 2+ bimetal doped TiO 2 nanoparticles in the presence of GS using the sol-gel process through which we can achieve high purity and homogeneous materials at room temperatures under stoichiometry control [10].
Compared to bulk materials, nanomaterials possess unique physical, chemical, and mechanical properties. To understand overall properties, it is important to characterize the structural, optical, and morphological properties of nanomaterials prior to proceed for different applications to obtain better results [11]. Hence, several advanced sophisticated techniques were used in the present study to characterize the crystalline phase, surface morphology, elemental composition, visible light absorption capacity, particle size, and surface area of assynthesized bimetal doped TiO 2 nanoparticles to understand its suitability for photocatalytic degradation applications.
Due to the widespread industrial activities, various organic dye pollutants are being discharged into water bodies causing severe water pollution. Almost, 15% of the dyes produced worldwide are lost during synthesis and processing with wastewater. Moreover, 50% of all the dyes used in various industries are carcinogenic azo dyes, which are characterized by one or more azo bonds (−N=N-) [12] among which, Methyl Red (MR) is a familiar azo dye and is widely used in paper printing, textile dyeing, and leather industries [13]. It causes skin and eye irritation, irritation of the respiratory and digestive tract and is a suspected carcinogen, mutagen, and mitotic poison [14].
Hence, the removal of these pollutants from wastewaters in an eco-friendly way stands as a present major research concern. Therefore, in the present study, we attempted to remove this model azo dye pollutant from the contaminated water using as-synthesized Mn 2+ / Mg 2+ bimetal doped TiO 2 nanocatalysts, and their photocatalytic efficiency was tested by optimizing the important reaction parameters like dopant concentration, surfactant concentration, solution pH, catalyst dosage and initial dye concentration for the better water pollution abatement.

Materials
All the chemicals used in this research were of reagent grade and used without any further purification. Titanium tetra-nbutoxide (Ti(OBu) 4

Synthesis of nanocatalyst
Mn 2+ /Mg 2+ bimetal ions doped nanotitania and GS assisted Mn 2+ /Mg 2+ bimetal doped TiO 2 samples were prepared by sol-gel method. In this method, Ti(OBu) 4 (10 mL) was added to ethanol (20 mL) and acidified with 1.4 mL of HNO 3 taken in a pyrex glass vessel (Solution-I) and stirred for 20 min. Desired wt% (0.25-1.0 wt%) of Mn 2+ and Mg 2+ dopants proportionate to TiO 2 weight were taken in ethanol (20 mL) in another beaker and added 4.5 mL of Milli-Q water for effective hydrolysis purpose (Solution-II). Then the solution-II was slowly added to solution-I dropwise with continuous stirring. A colloidal solution that formed after complete mixing is further stirred for 90 min and aged for 48 h to obtain the gel. The formed gel was dried (at 70°C), ground, and calcined at 450°C for about 5 h in a furnace. To get homogeneous powder the calcined samples were well ground after cooling.
To optimize the best dopant concentrations, the photocatalytic efficiency of the as-synthesized bimetal doped TiO 2 samples was tested by degrading MR dye and 0.25 wt% Mn 2+ and 1.00 wt% Mg 2+ bimetal doped TiO 2 (MMT5) was found to be the best. For further enhancement in the photocatalytic activity of the MMT5 catalyst, GS assisted MMT5 nanocatalysts were synthesized by following the same procedure but different wt% (5, 10, and 15 wt%) of GS was added after the mixing of solution-I and solution-II. To study and compare the effect of dopants and surfactant on the photocatalytic properties of bimetal doped TiO 2 , an undoped TiO 2 sample was prepared using the same procedure without adding dopants and surfactant precursors. Details and nomenclature of all the synthesized catalysts are presented in Table 1.

Characterization techniques
The crystalline structure of photocatalysts was studied by powder X-ray diffraction (XRD) spectra using PANanalytical diffractometer with CuKα radiation (λ = 0.15405 nm) operated at 45 kV, 40 mA, and 0.2θ min − 1 scan rate. Based on the Scherrer equation using full width at half maximum (FWHM), the crystallite size of TiO 2 nanocatalysts was determined. The UV-Visible Diffuse reflectance spectra (UV-Vis DRS) of the catalysts were recorded in the range of 200-800 nm with a Shimadzu 3600 UV-Vis DRS Spectrophotometer using BaSO 4 as reference. Scanning electron microscopy (SEM) JSM 6300, Japan equipped with energy dispersive X-Ray (EDX) spectroscopy was used to investigate the surface morphology and elemental analysis, respectively. The microstructure and particle size of the samples were analyzed by highresolution transmission electron microscopy (HRTEM) JEM-2100 JEOL at an operating voltage of 200 kV. Fourier transform infrared spectra (FT-IR) of the nanocatalysts were recorded on an FT-IR spectrometer (Nicolet Avatar360). Brunauer-Emmet-Teller (BET) surface area, pore size, and pore volume of the nanocatalysts were determined using Quanta Chrome Nova 2200E system by recording N 2 adsorptiondesorption isotherms at 77.3 k. Photoluminescence (PL) analysis was performed by Hitachi F-7000 fluorescence spectrophotometer. The pH of the reaction mixture and the amount of MR degraded during the photocatalytic process were adjusted and monitored using Elico Digital pH meter (Model IIIE, EI) and UV-vis spectrophotometer (Shimadzu 1601), respectively.
Experimental procedure for the determination of visible light photocatalytic activity of nanocatalyst The photocatalytic activity of surfactant-assisted Mn 2+ /Mg 2+ bimetal ions doped TiO 2 nanocatalyst was examined by the degradation of a model azo dye pollutant, MR in a visible light photoreactor as shown in Fig. 1. For visible light illumination, a high-pressure metal halide lamp (400 W, 35,000 lm, and 436-546 nm output) was used and placed 20 cm away from the pyrex glass vessel containing 100 mL of MR dye solution of desired concentration (1-10 mg L − 1 ) with sufficient amount of the catalyst. To screen the infrared radiation effect and to keep the constant temperature (25°C) during the degradation experiments, water was passed continuously around the reaction vessel. Prior to visible light irradiation, the pH of the reaction mixture was adjusted to the desired value by the addition of 0.1 N NaOH/0.1 N HCl and the reaction mixture was stirred in dark for 30 min to establish the adsorption-desorption equilibrium between MR dye and surface of the catalyst [15]. After the visible light irradiation, 5 mL aliquots of samples were collected at regular time intervals using a Millipore syringe (0.45 μm) and change in the concentration of MR dye during the degradation process was studied with a UV-Vis spectrophotometer (Shimadzu 1601) by measuring its absorbance at 525 nm. The percentage degradation of MR was determined by using the Eq. (1) [16].
Where C 0 is the initial concentration of MR dye solution before degradation and C t is the concentration at time t. For better photocatalytic degradation, the effect of reaction parameters like dopant concentration, surfactant concentration, solution pH, catalyst dosage, and initial dye concentration was studied and optimized by varying the measured parameter while keeping other parameters constant and the results are discussed in section 4.1-4.5.
As the photocatalytic degradation of organic pollutants is mainly initiated by highly oxidative hydroxyl radicals (•OH), which can be easily captured by fluorescent probe molecules, the formation of •OH in the photocatalysis process was studied by PL technique using coumarin as a probe molecule [16].

Results and discussion
XRD study   [17,18]. And it is also known that as the Mn 2+ and Mg 2+ are more electropositive, the electronic cloud in each TiO 2 might be loosely held, favoring the formation of less dense anatase phase [19]. The average crystallite sizes of the undoped, bimetal-doped (MMT) and surfactant-assisted bimetal doped (MMT-GS) catalysts were calculated based on the FWHM of the characteristic high intensified peak using Scherrer eq. [1] (d = kλ/β Cosθ), where d is the average crystallite size, k is 0.9 (Scherrer constant), λ is 1.5406 (X-ray wavelength), β is the FWHM and θ is the diffraction angle and are tabulated in Table 2. From the table, the average crystallite size of the catalysts was found to be ranging from 7.21-10.22, 10.86-12.87, and 18.30 nm for MMT-GS, MMT, and undoped TiO 2 nanocatalysts, respectively. The substitutional doping of metal ions into the TiO 2 lattice inhibited the grain growth by formation of Ti-O-Mn and Ti-O-Mg due to which the crystallite size was decreased in MMT catalysts. Further decrease in crystallite size was observed for the catalyst prepared in presence of surfactant, MMT-GS, which can be attributed to the effective capping nature of the GS which controls the nucleation and minimizes the agglomeration of TiO 2 nanoparticles during the synthesis process [20].

UV-Vis DRS study
From the UV-Vis DRS of the undoped TiO 2, MMT, and MMT-GS nanomaterials shown in Fig. 3a, it is observed that absorbance bands are shifted more towards the higher wavelengths, i.e., red-shifted in MMT and MMT-GS catalysts compared to undoped TiO 2 , which is possibly due to the decreased bandgap by co-doping of Mn 2+ and Mg 2+ into the TiO 2 matrix. The decrease in bandgap can be ascribed to the joint effect of these two metal dopant ions (Mn 2+ and Mg 2+ ) in MMT and MMT-GS system, where these formed the extra energy levels below the conduction band of TiO 2 and thus reduces the electron-hole recombination by trapping electrons and enhances the visible light absorbance. From Fig. 3b it is further confirmed by bandgap energies obtained for all the synthesized catalysts using Kubelka-Monk formalism and Tauc plot method [21]. The corresponding band gap energy values are presented in Table  2. Before proceeding to further characterization, we have conducted trial photocatalytic degradation experiments for all the synthesized catalysts using MR dye. From the results, it was noticed that among all the MMT catalysts, MMT5 exhibited better photocatalytic activity and moreover, MMT5 assisted with 10 wt% of GS, MMT5-GS2 has shown some more enhanced photocatalytic activity. Hence, we selected these two particular catalysts for further characterization with SEM-EDX, TEM, BET, FT-IR, and PL study.
SEM and EDX studyThe SEM images shown in Fig. 4 illustrate the surface morphology of undoped TiO 2 , MMT5, and MMT5-GS2, respectively. In comparison, it can be clearly seen that the morphology of the TiO 2 has been changed from large aggregates with a scratchy surface in undoped TiO 2 (Fig. 4a) to multiparticle  agglomerated irregular shaped particles with rough surface in MMT5 (Fig. 4b) and pseudospherical less agglomerated nanoparticles with smooth surface area and small particle size in MMT-GS2 (Fig. 4c). This is clearly indicating that Mn 2+ and Mg 2+ bimetal ions doping and capping action with the GS have a significant impact on the morphology of TiO 2 nanoparticles which controls the grain growth and particle nucleation. Also, the chemical composition of MMT5-GS2 sample was investigated by EDX with the results depicted in

TEM study
The TEM images of MMT5 and MMT5-GS2 photocatalysts are shown in Fig. 6a and b respectively. The TEM image (Fig. 6a) of MMT-5 shows pseudospherical shape with multiparticle agglomeration and an average particle size of 6.6 nm. Whereas in MMT5-GS2 (Fig. 6b), most of the TiO 2 particles are well dispersed with very low agglomeration compare to Fig. 6a and the particle size is in the range of 2.3-5.4 nm, with an average particle size of 3.8 nm. Thus, it clearly indicates that the GS effectively inhibited the particle overgrowth and aggregation and resulted in small particle size with increased surface area, which is well correlated with BET results given in Table 2. The selected area electron diffraction (SAED) patterns shown in Fig. 6c confirmed the anatase phase with good crystallinity indexed by the concentric rings, which is in good agreement with XRD diffraction patterns. In addition to SAED, from the HRTEM image of MMT5-GS2 (Fig. 6d) the observed lattice fringes with dspacing of 0.352 nm correspond to (101) plane of anatase and further confirms the single-crystal nature and high crystallinity of anatase TiO 2 [22].

FT-IR study
The incorporation of metal ion dopants, Mn 2+ and Mg 2+ into TiO 2 lattice was further confirmed by FT-IR results. The FT-IR spectra of the undoped TiO 2 , MMT5, and MMT5-GS2 (before and after calcination) are shown in Fig. 7 (Fig. 7a) were deformed/shifted to 620.3 and 1300.1 cm − 1 in MMT5 (Fig. 7b) and 540.0 and 1182.0 cm − 1 in MMT5-GS2 ( Fig. 7c and d), respectively, which can be attributed to the presence of dopants in TiO 2 lattice. Hence, the FT-IR study confirms that Mn 2+ and Mg 2+ are substitutionally doped into TiO 2 lattice by replacing Ti 4+ and formed a new network, i.e., Ti-O-Mn and Ti-O-Mg which are in good agreement with the previous reports [7,24]. The FT-IR spectra of GS are shown in Fig. S1 Fig. 7c which confirms the existence of strong electrostatic interaction between GS and surface of catalyst in MMT5-GS2 before calcination [9]. From Fig. 7d, the absence of these peaks confirms that there is no surfactant remaining in the synthesized catalyst, MMT5-GS2 after calcination. This indicates that due to calcination at 450°C, the surfactant is completely eliminated from nanocatalyst.

BET surface area analysis
To study the effect of bimetal doping and GS on the surface area and porosity nature of the as-synthesized TiO 2 nanoparticles of undoped TiO 2 , MMT5 and MMT5-GS2, N 2 adsorption-desorption isotherms and their corresponding Berret-Johner-Halenda (BJH) pore size distribution plots were recorded and presented in Fig. S2a and S2b. From Fig. S2a, it results in a type -IV isotherm with H 2 hysteresis loop, characteristic of the ordered mesoporous structure of the catalyst [25]. The average surface area, A BET (m 2 g − 1 ) of all prepared catalysts was determined and tabulated in Table 2.
From Table 2 the MMT5 has shown increased surface area (112 m 2 g − 1 ) compared to undoped TiO 2 (64 m 2 g − 1 ) due to crystal growth suppression by dopants. Whereas catalyst prepared in the presence of GS surfactant, MMT5-GS2 showed a higher surface area (230 m 2 g − 1 ) compared to both MMT5 and undoped TiO 2 . It could be strong evidence for the decreased particle size of the TiO 2 resulting from the effective capping ability of the surfactant, which restricts the particle growth and nucleation during the synthesis process. To better compare the effect of bimetal doping and capping of GS on as-synthesized TiO 2 nanoparticles over single Mn 2+ and Mg 2+ doped TiO 2 , comparative results are tabulated by referencing Mn [26] and Mg [27] single doped literature reports in Table 3.

PL spectra
One of the essential reactive species in the process of photocatalysis is •OH which is responsible for oxidation reactions. Because of the high reactivity and short life of •OH, it is impossible for direct detection. To inspect the production of •OH from the catalyst during the photocatalysis reaction, a PL technique has been adopted using coumarin as a fluorescent probe molecule, which on reaction with •OH yields the 7-hydroxy coumarin [16]. In this technique, 0.1 g of catalyst is dispersed in 100 mL of 10 ppm coumarin solution in acidic conditions and illuminated to visible light irradiation. Small aliquots of reaction solution samples were withdrawn for every 30 min, filtered and PL intensity was measured in the range of 350-600 nm with excitation fixed at a wavelength of 435 nm. It has been observed from Fig. 8 that between 440 and 460 nm there exist PL spectra of the generating 7-hydroxy coumarin with maximum emission at 450 nm. A linear increase in PL intensity was observed with increasing irradiation time. However, in the absence of irradiation, no excitation was observed for the sample (0 min in Fig. 8).
From the above results, it could be understood that the number of •OH's produced on the catalyst surface was directly proportional to the irradiation time. The results further assure that the synthesized nanomaterial shows the enhanced rate of formation of •OH's under irradiation to visible light. This is due to the fact that the already formed photo holes in the valence band of bimetal doped TiO 2 could directly react with H 2 O/OH − to produce •OH [28].

Evaluation of the photocatalytic activity of synthesized MMT-GS nanocatalysts by the degradation of MR
Visible light assisted catalytic activity of the synthesized MMT-GS photocatalysts was evaluated by degradation of MR. For better photocatalytic degradation, the effect of reaction parameters like concentration of dopants, surfactant concentration, solution pH, catalyst dosage, and initial dye concentration was studied and optimized as follows.

Effect of metal ion dopant (Mn 2+ and Mg 2+ ) concentration
In order to find out and optimize the best dopant concentration of bimetal doped photocatalyst among all the synthesized MMT photocatalysts, degradation experiments were carried out with each photocatalyst under visible light irradiation by taking catalyst dosage 0.05 g, solution pH 4, and initial MR concentration 10 mg L − 1 . The rate of degradation of MR has been studied by measuring the MR absorbance, which is inserted in Fig. 9a. It can be seen from the figure that photocatalytic performance was found better for all the bimetal doped TiO 2 photocatalysts than undoped one and among all the synthesized photocatalysts, the best catalytic performance is shown by MMT5. This can be attributed to its narrowed bandgap resulting from the impurity energy levels between the valence band and conduction band that is created by the presence of both metal dopants which helps in the generation of more electron-hole pairs. Also, its small crystallite size and high surface area facilitate the adsorption of a greater number of dye molecules on its surface for degradation [7]. Therefore, based on the above results MMT5 was selected for optimizing other parameters.

Effect of GS concentration
To find out the optimal concentration of GS in MMT5 photocatalyst for efficient photocatalytic performance, as synthesized MMT5-GS catalysts with various wt% of GS (5, 10, and 15 wt%) along with MMT5 were tested by the degradation of MR under visible light irradiation by maintaining other parameters constant, i.e., catalyst dosage is 0.05 g, solution pH 4 and initial MR concentration 10 mg L − 1 . It is observed from Fig. 9b that all MMT5 nanocatalysts prepared in the presence of GS degraded the MR in less time compared to MMT5 (Fig. 9a) which clearly indicates the significant role of the capping nature of the GS on the photocatalytic activity of MMT catalysts. From the apparent rate constants obtained by slopes of the individual curves (inset of Fig. 9b), it can be clearly seen that the degradation rate was increased up to 10 wt% GS concentration and then decreased on further loading. It could be due to the fact that increased GS concentration may restrict the effective doping of the metal ions into TiO 2 crystal lattice. Hence, MMT5-GS2 had been taken as a catalyst for further studies.

Effect of solution pH
According to Tang et al. [29], for charged substrates like TiO 2 , there is a significant dependency on the pH value for the effective photocatalytic degradation, because the overall surface charge and adsorptive properties of TiO 2 nanomaterial depend strongly on solution pH. Hence, in this study, the effect of pH on the catalytic performance of the MMT5-GS2 was carried out by ranging the pH from 2 to 8 and keeping other parameters constant, i.e., catalyst dosage 0.1 g and initial MR concentration 10 mg L − 1 . From Fig. 9c, it is observed that the degradation of MR is high in the acidic medium compared to the basic medium. This may be due to the strong electrostatic interaction between the positively charged surface (H + ions) of the catalyst and negatively charged dye molecules. When the pH increased to basic medium the catalyst surface acquires a negative charge and electrostatically repels the likecharged dye molecules. In specific, the degradation of MR is high at solution pH 4, because as the positive charge (H + ions) on TiO 2 surface increases, negatively charged MR can easily be adsorbed on the surface of the catalyst.
And a further increase in pH greater than 4 will facilitate the hydrolysis of metal ions on the surface of TiO 2 which seizes its photocatalytic activity.

Effect of catalyst dosage
To avoid the wastage of catalyst and minimize the screening effect, a number of experiments were carried out to get the optimum photocatalyst dosage by varying its amount ranging from 0.025 to 0.1 g using 10 mg L − 1 of 100 mL of MR solution at solution pH 4. It was observed from Fig. 9d that the degradation of the MR increased linearly with the increase of catalyst loading up to 0.05 g because of the increased total surface area for dye adsorption. But after increasing the photocatalyst beyond 0.05 g, it creates turbidity and agglomeration of the catalyst particles due to excess photocatalyst and reduces the light transmission through the solution which leads to less photocatalytic activity [18]. And also, collisions between active molecules and ground-state molecules of bimetal doped TiO 2 results in the deactivation of the catalyst particles [16].

Effect of initial dye concentration
To study the effect of the initial dye concentration at a fixed weight of catalyst (0.05 g) and solution pH 4, the experiments were carried out with different concentrations of MR dye from 5 to 15 mg L − 1 with results presented in Fig. 9e. The results demonstrate that the rate of degradation of MR dye increased up to 10 mg L − 1 . But, a further increase in dye concentration causes the deactivation of the catalyst due to the blanket effect [30]. And because of the fixed catalyst dosage (0.05 g), there is no possibility to produce a sufficient number of •OH to counteract the increased number of dye molecules. Hence, the rate of degradation decreases.

Conclusions
GS abetted mesoporous Mn 2+ /Mg 2+ bimetal doped TiO 2 nanocatalysts were successfully synthesized by the solgel method without a change in the crystallinity of TiO 2 nanoparticles. The effective charge separation and narrowed band gap created by Mn 2+ /Mg 2+ bimetal ions doping and reduced particle size and increased surface area resulting from capping of GS had significantly enhanced the photocatalytic performance of MMT5-GS2 nanocatalyst towards the degradation of MR dye under visible light irradiation. The PL results exhibited that •OHs are the crucial reactive species responsible for oxidative photocatalytic degradation of MR. The highest degradation of MR was achieved in 60 min at optimized reaction conditions of 10 wt% GS on 0.25 wt% Mn 2+ / 1.00 wt% Mg 2+ bimetal ions doped TiO 2 , solution pH 4, catalyst loading of 0.05 g, and initial MR concentration 10 mg L − 1 . Based on its effective photocatalytic activity,