Gemini surfactant abetted 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 (Mn 2+ ) and Magnesium (Mg 2+ ) bimetal ions doped TiO 2 nanomaterial assisted with different weight percentages (5-15 wt%) of Gemini Surfactant (GS) using sol-gel method. The bimetal doped and undoped TiO 2 photocatalysts were characterized by X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), UV-Visible Diffused Reectance Spectroscopy, Transmission Electron Microscopy, Brunauer-Emmett-Teller surface area analyzer, and Photoluminescence Spectroscopy. Characterization results evinced that Mn 2+ /Mg 2+ bimetal ions doping and encapsulation of GS on TiO 2 nanoparticles promote the formation of mesoporous multi-particle anatase TiO 2 nanocatalysts with a narrowed band gap, less particle size, and high surface area. The surface elemental composition of the 0.25 wt% Mn 2+ & 1.00 wt% Mg 2+ bimetal doped TiO 2 in presence of 10 wt% of Gemini surfactant (after calcination) revealed the presence of both the metal dopants Mn 2+ and Mg 2+ along with the Ti and O and their chemical interactions were further conrmed by FT-IR results. The photocatalytic activity of these catalysts was assessed by the degradation of Methyl Red (MR) using visible light irradiation. To understand the effect of different reaction parameters on the photocatalytic activity of the nanocatalysts such as the dopant concentration, GS concentration, catalyst dosage, solution pH, and initial dye concentrations 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 MR.


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 ndings revealed that bimetal ions doping into TiO 2 crystal lattice had overcome the above drawbacks and signi cantly 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, K.V. Divya Lakshmi et al. reported that Manganese 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 [5]. On the other hand, Changneng Zhang et al., Desta shumuye et al. and So aou et al. were investigated the effect of Magnesium, 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 high photocatalytic activity [6][7][8]. 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 plays 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) is the most suitable as a capping agent to fabricate 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 Gemini surfactant 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 possesses 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 as-synthesized 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 is a familiar azo dye and is widely used in paper printing, textile dyeing, and leather industries [13]. It causes skin & eye irritation and irritation of the respiratory & 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 e ciency was tested by optimizing the important reaction parameters like dopant concentration, surfactant concentration, pH, catalyst dosage and initial concentration of the dye for the better water pollution abatement.

Materials
All the chemicals used in this research were of reagent grade and used without any further puri cation.
Titanium tetra-n-butoxide (Ti(OBu) 4   To optimize the best dopant concentrations, the photocatalytic e ciency of the as-synthesized bimetal doped TiO 2 samples were tested by degrading Methyl red dye and 0.25 wt% Mn 2+ and 1.00 wt% Mg 2+ doped TiO 2 (MMT5) was found to be the best. For further enhancement in the photocatalytic activity of the MMT5 catalyst, Gemini surfactant-assisted MMT5 nanocatalysts were synthesized by following the same procedure but different wt% (5 wt%, 10 wt% and 15 wt%) of Gemini surfactant 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 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  [15]. For visible light illumination, a high-pressure metal halide lamp (400 W, 35,000 lm, 436-546 nm output) has been used and placed 20 cm away from the pyrex glass vessel containing 100 mL of MR dye solution of desired concentration (1-10 mgL -1 ) with su cient amount of the catalyst. To screen the IR 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 a desired value by 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 [16]. After the visible light irradiation, 5 mL aliquots of samples were collected at regular time intervals using 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) [17].

(see Equation 1 in the Supplementary Files)
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 concentration of dopants, surfactant concentration, pH of the solution, 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 OH radicals, which can be easily captured by uorescent probe molecules, the formation of •OH in the photocatalysis process was studied by photoluminescence (PL) technique using coumarin as a probe molecule [17].

X-ray Diffraction Study
From the XRD patterns of all the synthesized catalysts shown in Fig. 1a  matrix which is con rmed by the absence of any diffraction peaks related to Mn and Mg oxides or other compounds [18,19]. 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 [20]. The average crystallite sizes of the undoped, bimetal-doped (MMT) and surfactant-assisted codoped (MMT-GS) catalysts were calculated based on the FWHM of the characteristic high intensi ed peak using Scherrer equation [1] (d = kλ / β Cos θ), where d is the average crystallite size, k is 0.9 (Scherrer constant), λ is 1.5406 Ǻ (X-ray wavelength), β is the full width at half maximum (FWHM) and θ is the diffraction angle and are tabulated in

UV-Visible Diffuse Re ectance Spectra Study
From the UV-visible diffuse re ectance spectra of the undoped TiO 2, MMT and MMT-GS nanomaterials shown in Fig. 2a, 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 possible due to the  Fig. 2b it is further con rmed by bandgap energies obtained for all the synthesized catalysts using Kubelka-Monk formalism and Tauc plot method [22]. The corresponding band gap energy values are presented in Table 2 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 photoluminescence study.

Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy
The FESEM images shown in Fig. 3a, b and c illustrates the surface morphology of undoped TiO 2 , MMT5 and MMT5-GS2, respectively. On 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. 3a) to multiparticle agglomerated irregular shaped particles with rough surface in MMT5 and pseudospherical less agglomerated nanoparticles with smooth surface area and less particle size in MMT-GS2. This is clearly indicating that Mg 2+ and Mn 2+ bimetal ions doping and capping action with Gemini surfactant have a signi cant impact on the morphology of TiO 2 nanoparticles which controlled the grain growth and particle nucleation.

Transmission Electron Microscope
The TEM images of MMT5 and MMT5-GS2 photocatalysts are shown in Fig. 5a and b respectively. The TEM image (Fig. 5a) of MMT-5 shows pseudospherical shaped with multiparticle agglomeration and an average particle size of 6.6 nm. Whereas in MMT5-GS2 (Fig. 5b), most of the TiO 2 particles are well dispersed with very low agglomeration compare to Fig. 5a 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 was effectively inhibited the particle overgrowth and aggregation and resulted in less 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. 5c 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 HR-TEM image of MMT5-GS2 (Fig. 5d) the observed lattice fringes with d-spacing of 0.352 nm correspond to (101) plane of anatase and further con rms the single-crystal nature and high crystallinity of anatase TiO 2 [23].

Fourier Transform Infrared Spectroscopy
The incorporation of metal ion dopants, Mn 2+ and Mg 2+ into TiO 2 lattice was further con rmed by FT-IR results. The FT-IR spectra of the undoped TiO 2 , MMT5 and MMT5-GS2 (before and after calcination) were shown in Fig. 6a- (Fig. 6a) were deformed/shifted to 620.32 and 1300.12 cm -1 in MMT5 (Fig. 6b) Fig. 6c which con rms the existence of strong electrostatic interaction between GS and surface of catalyst in MMT5-GS2 before calcination [27].
From Fig. 6d, the absence of these peaks con rms that there was no surfactant remained in the synthesized catalyst, MMT5-GS2 after calcination (AC). This indicated that due to calcination at 450 °C, the surfactant is completely eliminated from nanocatalyst.

Brunauer-Emmett-Teller surface area analysis
To study the effect of bimetal doping and GS on the surface area and porosity nature of the assynthesized 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. 7a-b. From Fig. 7a, it results in a type -IV isotherm with H 2 hysteresis loop, characteristic of the ordered mesoporous structure of the catalyst [28]. 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 MMT has shown increased surface area (112.03 m 2 g -1 ) compared to undoped TiO 2 (64.09 m 2 g -1 ) due to crystal growth suppression by dopants. Whereas catalyst prepared in the presence of GS surfactant, MMT5-GS2 showed higher surface area (230.20 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 resulted 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  Table 3.

Photoluminescence Spectra
One of the essential reactive species in the process of photocatalysis is hydroxyl radical (•OH) and is responsible for oxidation reactions. Because of the high reactivity and short life of hydroxyl radical, it is impossible for direct detection. To inspect the production of hydroxyl radicals from the catalyst during the photocatalysis reaction, photoluminescence technique has been adopted using coumarin as a uorescent probe molecule, which on reaction with hydroxyl radicals yields the 7-hydroxy coumarin [17]. In this technique, 0.10 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, ltered and photoluminescence intensity was measured in the range of 350-600 nm with excitation xed at a wavelength of 435 nm. It has been observed from Fig. 8 that in-between of 440-460 nm there exist photoluminescence spectra of the generated 7-hydroxy coumarine with maximum emission at 450 nm. A linear increase in photoluminescence 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 hydroxyl radicals produced on the catalyst surface was directly proportional to the irradiation time. The results further assure that the synthesized nanomaterial showed the enhanced rate of formation of hydroxyl radicals 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/OHto produce hydroxyl radical [31].

Evaluation Of Photocatalytic Activity Of Synthesized Mmt-gs Nanocatalysts By Degradation Of Methyl Red
Visible light assisted catalytic activity of the synthesized MMT-GS photocatalysts were evaluated by degradation of MR. For better photocatalytic degradation, the effect of reaction parameters like concentration of dopants, surfactant concentration, pH of the solution, catalyst dosage, and initial dye concentration was studied and optimized as follows.

Effect of Metal ion dopant (Mn 2+ & Mg 2+ ) Concentration
In order to nd 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, pH 4 and initial MR concentration 10 mgL -1 . The rate of degradation of MR has been studied by measuring the MR absorbance, which is inserted in Fig. 9. It can be seen from the gure that photocatalytic performance was found better for all the bimetal doped TiO 2 photocatalysts than undoped one and in speci c among all the synthesized photocatalysts, the best catalytic performance was shown by 0.25 wt% of Mn 2+ & 1.00 wt% of Mg 2+ bimetal doped TiO 2, MMT5. This can be attributed to its narrowed bandgap resulted from the impurity energy levels between the valence band and conduction band that were created by the presence of both metal dopants which helps in the generation of more electron-hole pairs. And also, its less crystallite size and high surface area facilitate the adsorption of a greater number of dye molecules on its surface for the degradation [7]. Therefore, based on the above results MMT5 was selected for optimizing other parameters.

Effect of Gemini surfactant 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, pH 4 and initial MR concentration 10 mgL -1 . It is observed from Fig.10 that all MMT5 nanocatalysts prepared in presence of GS degraded the MR in less time compared to MMT5 (Fig. 9) which clearly indicated the signi cant 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. 10) 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.

Effect of pH
According to Tang et al., [32] for charged substrates like TiO 2 , there is a signi cant 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 dye concentration 10 mgL -1 . From Fig. 11 it was 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 negative charge and electrostatically repels the like-charged dye molecules. In speci c, the degradation of MR was high at 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 mgL -1 of 100 mL of MR solution at pH 4. It was observed from Fig. 12 that the degradation of the dye 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 [33]. And also, collisions between active molecules and ground-state molecules of bimetal doped TiO 2 results in deactivation of the catalyst particles [17].

Effect of initial concentration of dye
To study the effect of the initial concentration of dye at a xed weight of catalyst (0.05 g) and pH 4, the experiments were carried out with different concentrations of MR dye from 5 to 15 mgL -1 and results are presented in Fig. 13. The results demonstrate that the rate of degradation of MR dye increased up to 10 mgL -1 . But, further increase in dye concentration causes deactivation of the catalyst due to the blanket effect [34]. And because of xed catalyst dosage (0.05 g) there is no possibility to produce a su cient number of OH radicles to counteract the increased number of dye molecules. Hence, the rate of degradation decreases.

Conclusions
Gemini surfactant abetted mesoporous Mn 2+ /Mg 2+ bimetal doped TiO 2 nanocatalysts were successfully synthesized by the sol-gel method without 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 resulted from capping of GS had signi cantly enhanced the photocatalytic performance of MMT5-GS2 nanocatalyst towards the degradation of MR dye under visible light irradiation. The photoluminescence results exhibited that OH radicals 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 mgL -1 . Based on its effective photocatalytic activity, MMT5-GS2 nanocatalyst could be used as a promising material for the abatement of water pollution caused by organic dyes.         The effect of pH on the rate of degradation of MR dye by MMT5-GS2. Here, catalyst dosage 0.1 g and [MR]=10 mgL-1.