Hexagonal TiO 2 /SiO 2 Porous Microplates for Methylene Blue Photodegradation

Hexagonal TiO 2 /SiO 2 Porous Microplates have been successfully synthesized by incorporation of Ti precursors into SiO 2 synthesized from Si precursors in a gelatin-CTAB mixture via the hydrothermal method. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX, nitrogen adsorption-desorption and Fourier transform infrared spectroscopy (FTIR). The sample has a surface area of 735 m 2 /g, pore volume of 0.67 cc/g, and pore diameter of 3.2 nm, according to the results of the characterization of hexagonal TiO 2 /SiO 2 porous microplates. The transformation of SiO 2 microspheres into hexagonal TiO 2 /SiO 2 porous microplates is revealed by a microparticle size increase of 84% and the transition of Si−O bonds into Ti−O and Si−O as measured by FTIR. The photocatalytic activity of hexagonal TiO 2 /SiO 2 porous microplates resulted in 81.15% photodegradation of methylene blue under UV light irradiation within 60 min, which was 21 % better than SiO 2 .


Introduction
The increase of carcinogenic organic dye compounds, such as methylene blue, which are detrimental to human health, aquatic and nonaquatic organisms is boosting the growth of textile industry waste [1,2].For the treatment of methylene blue waste by the formation of reactive chemical species, semiconductor-based photocatalyst materials were developed because they are environmentally acceptable, energy efficient, and tolerate mild reaction conditions without secondary contamination [3,4].Under light irradiation, semiconductor photocatalysts such as titanium dioxide (TiO2) [4,5], zinc oxide (ZnO) [6,7], copper oxide (CuO) [8,9] and cerium oxide (CeO2) [10,11] have been employed extensively for the degradation of dyes such as methylene blue.TiO2 has been widely utilized as a low-cost photocatalyst for the degradation of methylene blue in water due to its high electron mobility [12,13], long carrier life [14], chemical stability [15], wide radiation absorption range [16], high photosensitivity [17], and non-toxicity [18].TiO2 with a band gap energy of 3.3 eV at 300 K is common in the earth's crust and acts as a photocatalyst when exposed to ultraviolet light [19].
Tuning the photocatalytic performance of a TiO2 structure by adjusting its size, shape, surface area, defect concentration, and exposed crystalline sites [20].In addition, TiO2 is photocatalytic because OH − ions can preferentially adsorb on its positively charged surface, resulting in a higher generation rate of •OH radicals and enhanced photodegradation [21].Nevertheless, the possibility for TiO2 agglomeration is strong enough to necessitate material support for particle dispersion [22].SiO2 is one of the materials that has a substantial impact as a catalyst support material.Numerous researchers have researched the synthesis of SiO2 by various methods, but the topic of highsustainability green precursors derived from natural materials has assumed a prominent role in the last ten years [23].Gelatin is a naturally occurring substance with a high level of durability that has been utilized in several research to synthesize diverse morphologies of porous materials [24].However, no study has yet been undertaken on coupling gelatin with other templates to create SiO2 that is subsequently combined with Ti precursor.
In this study, hexagonal TiO2/SiO2 porous microplates were produced by incorporating titanium tetraisopropoxide (TTiP) as a Ti precursor onto SiO2 microspheres as a supporting material.These microspheres were synthesized using the hydrothermal method in a solution containing gelatin and cetyltrimethylammonium bromide (CTAB).This work examined the alteration of shape, porosity, and photocatalytic activity for the degradation of methylene blue from SiO2 to TiO2/SiO2 under UV light irradiation.

Synthesis of SiO2 Microspheres
First, CTAB and gelatin are mixed with NaOH in water with a molar ratio of gelatin: CTAB : NaOH : water : TEOS = 6.6×10 −5 : 0.27 : 0.087 : 26 : 0.02.The mixture was stirred at 80 °C for 2 h to form a white slurry.The resulting mixture was filtered, washed, and then dried at 70 °C for 24 h.The dried sol id was then calcified at 550 °C for 5 h at a rate of 2 °C/min to remove any residual surfactant.The results of the synthesis are labeled as SiO2 microspheres (SM).

Synthesis of Hexagonal TiO2/SiO2 Porous Microplates (HTSMP)
First TEOT in hexane (1:800 v/v) is mixed in 1 gram of SiO2 microsphere while stirring at 45 °C for 16 h.The mixture was then dried at 160 °C for 2 h and calcined annealed at 550 °C for 5 h.The results of the synthesis are labeled as hexagonal TiO2/SiO2 porous microplates (HTSMP).

Characterizations of the Material
The instrument used to test the sample included the Pan Analytical brand X-Ray Diffraction (XRD) (Version PW3050/60).From the diffractogram, crystal size is calculated by the Debye Scherrer formula as in the equation below of Equation (1).
where, D is the crystal size in Ǻ, λ is the wavelength used in the XRD test, which is 1.54056 Ǻ, and B is the width of half the peak in radians.θ is the angular position of the peak formation.XRD results also show FWHM to be able to determine the value of B (rad).The crystallinity of the sample can also be calculated through the X-Ray Diffraction characterization data using Equation (2).

Crystallinity crystalline peak area crystalline and amorphous peak areas
The Shimadzu 21 brand Fourier Transform Infrared (FTIR) spectrophotometer with 0.5 cm −1 resolution, Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM-EDX) with images taken with a JEOL JSM-700 microscope at a voltage speed of 15.0 kV, and Brunauer -Emmett-Teller (BET) brand Quantacrome Nova 1200e.a tool for testing samples using a Shimadzu UV-3600 Ultraviolet Visible (UV-Vis) Spectrophotometer with a wavelength of 665 nm.

Photodegradation of Methylene Blue
200 mL of 5 mg/L methylene blue solution plus 50 mg of photocatalyst.The photocatalyst process begins with 30 min of dark adsorption beneath a dark-closed shaker reactor.UV light irradiation then follows.Using a UV-Vis spectrophotometer with a wavelength range of 670 nm, the absorbance of 5 ml of solution was measured every 10 min by Equations ( 3)- (5).
where, %ED = Photodegradation efficiency of MB, C0 = initial concentration of MB, Ct = concentration of MB at t minutes.

Results and Discussion
Figure 1 demonstrates that all samples exhibit an amorphous SiO2 phase as indicated by the broad diffraction peak centered at 2θ = 23.08°according to the JCPDS file.29-0085 [25].Hexagonal TiO2/SiO2 porous microplates revealed that not only the SiO2 peak but also the low TiO2 peak at 2θ =25.5° were in good agreement with the JCPDS card number JCPDS No. 21-1276 as anatase [26].Except for the successful identification of the amorphous SiO2 phase in both samples, no significant changes were identified.
The existence of differences in composition before and after Ti impregnation apart from XRD can also be proven from EDX data.The SM has a percentage content of 72.69% Si, 25.15% O, and 2.16% C, but the composition changes after impregnation, i.e. 46.40% Si, 51.07%O, 2.14% C, and 0.32% Ti.The XRD data is strengthened by EDX data which shows that there is Ti content in mesoporous silica, which shows that the Ti impregnation process into Mesoporous Silica has been successfully carried out.Another phenomenon shows a decrease in the amount of Si after impregnation due to some of the Si in the framework being replaced by TiO2 which is explained by the appearance of the Ti content and an increase in the amount of oxygen.Meanwhile, the amount of carbon tends to be stable, which indicates that there is a number of carbons from gelatin that bond strongly to silica during synthesis to form C−Si bonds through the strong affinity between −HC−NH3 gelatin and Si−OH to bulid formation NH3−HC-------Si−OH from silica precursors.C−Si bonds are formed after calcination at 550 °C due to insufficient heat energy to decompose all the carbon.This also illustrates that TiO2 impregnation on mesoporous silica only blocks the silica surface as the dominant surface and has no effect on the carbon component.Figure 2 illustrates the morphology of microsized SiO2 microspheres and hexagonal TiO2/SiO2 porous microplates (HTSMP), respectively.This modification of the particle shape is predicted due to the employment of the sol-gel process in which there are two primary steps, namely hydrolysis and polycondensation [27].In the CTAB hydrolysis stage, gelatin and NaOH produce a sol in the form of micro-sized particles suspended in the solution.The subsequent step is the gel, which is the polycondensation reaction in the form of a transition from the sol to a gel, during which the micro solids begin to condense into a more complicated network [28].In the polycondensation stage, the interaction of CTAB, gelatin and alkaline solution drives droplet size so that the gel formed is a collection of semi-solid microdroplets.As a crystal core embryo, the microdroplet will undergo reorganization to form a larger and more regular agglomeration arrangement.The equilibrium between the crystal core embryo, the amorphous gel, and the remaining silica supply permits the expansion of the crystal nucleus into larger particles, which will continue to occur until the residual is depleted and crystals form in a stable condition, generating 0.19 µm silica microspheres.
Figure 2 also show that there are some particles that imperfect as hexagonal-like because the amount of Ti impregnated is small so the attraction between Ti−O−Si forming a hexagonal is also minimal.This small Ti give strong impact to form perfect hexagonal part by good interaction between silica and the Ti precursor during impregnation process.The part that does not interact with Ti is due to the minimal amount of Ti dispersed on the surface so nothing center electrostatic force to direct the hexagonal structure but Ti attraction only causes elongation and enlargement of the size of the silica on various surfaces .The result on the absence of interaction stimulants is stable spherical silica shape due to as previous research explnataion [29][30][31].
After incorporating Ti precursors into the surface of SiO2 microspheres in hexane solvent, an unexpected result was discovered.When reacting with Ti precursor in hexane, the Si−OH surface of silica produces a high electrostatic attraction for Ti (Figure 3).This electrostatic force permits Ti to become the center of attraction of the silica framework, as a result of the difference in electronegativity between Ti, Si, and O (1.5; 1.8; 3.5, respectively).Ti−O−Si interaction is believed to be the source of primary elongation since the    As a result, there are changes in the size and shape of silica, particularly the tetragonal anatase, based on the original characteristics of titanium.After organic compund removal from TEOT as Ti precursor, interaction with Ti and Si produces hexagonal plate particles measuring 1.21 µm with TiO2 scattered over the surface of Silica.Figure 4 indicates that the particle size distribution appears to increase significantly with the addition of TiO2, indicating that TiO2 improves the quality of uniformity of silica particles in general, while simultaneously increasing the particle size by up to 84% with the shape transformation from microspheres to hexagonal microplates.XRD confirms that the addition of 0.42% Ti to SiO2 enhances the crystallinity from 64% to 67% (Table 1).The 15% decrease in Si due to the absorption of fresh Ti and O into the framework demonstrates the success of the incorporation process, which must be evaluated in terms of porosity.

Bulletin of Chemical Reaction
Table 1.Physicochemical properties of samples.Figure 5 depicts the isotherms of all samples, which are predominantly type IV with a prominent hysteresis loop, indicating that the material is mesoporous [32].Table 1 displays the related pore structure parameters, which include specific surface area (SBET), total pore volume (Vt), mesopore surface area and average pore diameter, mesopore volume, and micropore volume.Capillary condensation in uniform pores is characterized by a sharp inflection of SiO2 microsphere isotherms in the P/P0 range of 0.3 to 0.7.Due to its huge homogeneous mesopores and narrow pore size distribution (PSD) centered on 2.92 nm, this mesoporous silica has a steep nitrogen absorption at relatively high P/P0 (Figure 6).Hysteresis loop of Hexagonal TiO2/SiO2 Microplates subjected to a greater pressure than SiO2 microspheres exhibited a larger sample pore size than before TiO2 incorporation.This is likely owing to the creation of relatively large new pores in the sample after integration because of particle overlapping between TiO2 and the silica surface or between TiO2 and silica [33].Table 1 demonstrates the change in surface area resulting from the transformation of SiO2 microspheres to hexagonal microplates made of TiO2/SiO2 due to the pore blocking effect of TiO2 particles on the SiO2 surface, the specific surface area, total pore volume, mesopore surface area, and mesopore volume fell by 8-9%.Due to the creation of new gaps between stacked TiO2 particles on the SiO2 surface, the pore width increased by 7% from 2.9 nm to 3.23 nm.
Examining the FTIR spectra of microsphere SiO2 and hexagonal TiO2/SiO2 microplates is depicted in Figure 6.Broad bands at 3400 cm −1 and 1600 cm −1 in both spectra are attributable to the stretching and bending vibrations of hydroxyl groups and water adsorbed on the SiO2 surface [34].TiO2/SiO2 samples exhibit characteristic peaks at 500 cm −1 , 1100 cm −1 , and 930 cm −1 , which correspond to the stretching vibrations of Ti−O, Si−O−Si, and Ti−O−Si, respectively [35].The spectrum around 820-850 cm −1 is characteristic of Si−O−Si and is displayed by both substances [36].Vibration appears to be a sign of molecular-level Ti−Si interaction.
Figures 7-8 exhibit the photodegradation of methylene blue using TiO2 as the photoactive material and SiO2 as the supporting material.When a photon strikes a photoactive semiconductor material, the valence band electrons will be excited into the conduction band.The presence of dark adsorption contributes to homogenization and achieving equilibrium between methylene blue when interacting on the surface of the material, allowing the formation of radicals during UV irradiation to occur more rapidly in the system and resulting in the oxidation of organic compounds occurring more   rapidly.5 mg/L of methylene blue was photodegraded by adding 50 mg of catalyst to 200 mL of methylene blue solution that was placed in a solution chamber with a 20 W UV lamp as the photon source.Prior to light irradiation, the suspension was magnetically agitated for 30 min to establish an equilibrium between adsorption and desorption.At a predetermined time, samples were taken, and the treated solution was evaluated using the spectrophotometric method.The pattern of color changes following the photocatalytic breakdown of methylene blue is depicted in Figure 8.It was determined that the presence of all elements contributed to the decolorization of methylene blue and significantly changed the concentration of methylene blue.The sample after impregnation contains silica, titania and oxygen which have different roles because silica with a large surface area increases the homogenization process of methylene blue on the silica surface during the dark adsorption process while titania acts as an active center for photodegradation.The band gap of TiO2 in subsequent studies is approximately 3.3 eV [37] and changed after incorporation in SiO2 to 3.21 eV [38], which is the material for confirming the cause of faster degradation in the presence of UV light compared to the material without a photocatalyst under UV light irradiation, which indicates that the SiO2 material without TiO2 only adsorbs methylene blue [39].Initial rate estimates demonstrated that the first 30 min of  dark adsorption doubled the rate of degradation relative to the absence of dark adsorption.Significantly faster degradation of methylene blue when SiO2 contains Ti relates to faster radical production accompanied by higher interactions between methylene blue and holes.Under the photooxidation process, all TiO2/SiO2 photocatalyst samples degraded around 81% of methylene blue in approximately 60 min (Figure 9).It is also assumed that the total decolorization of methylne blue comes from the presence of Ti as a photocatalyst and Si as a supporting material which functions to disperse the catalyst.The greater the surface area of silica as a supporting material, the better the Ti dispersion will be.Ti on the silica surface has band gap energy, resulting in higher decolorization compared to silica alone because the homogenization process during dark adsorption and photodegradation takes place optimally.Another assumption is that silica only decolorizes with an adsorption step starting from the first step in dark conditions until the next step in the last 90 min [40].This study was compared with other studies (Table 2) and proved that the degradation was more than other catalyst (under 60 min).

Conclusion
The integration of a Ti precursor into SiO2 generated by the hydrothermal technique in a gelatin-CTAB template led to the effective synthesis of a highly efficient TiO2/SiO2 photocatalyst.The findings of the analysis indicate that the product consists of hexagonal TiO2/SiO2 porous microplates with a crystallinity of approximately 67%, a surface area of 735 m 2 /g, a pore volume of 0.67 cm 3 /g, and a pore diameter of 3.2 nm.Hexagonal TiO2/SiO2 porous microplates as a photocatalyst with photocatalytic efficiency of 81% under UV light irradiation with a 30 min dark adsorption process concludes that this photocatalyst is very likely to be a future photocatalyst candidate due to the high sustainability of the SiO2 supporting material, which is created using gelatin as a natural template.
difference in electronegativity between Ti−O and Si−O is around 2.0 and 1.7 then 16 h of constant electrostatic attraction between Ti and silica is what triggers the expansion of the Si framework.