TiO2 nanoparticles were prepared using a sol–gel process in combination with a novel cationic gemini surfactant (CGS) with amide functional groups at low temperatures. Titanium (IV) isopropoxide (TIP) and CGS were used as the starting materials and as effective agents, respectively, to orient the nanoparticles during the sol–gel synthesis. To reveal both the structural and morphological properties of the nanopowders prepared in this work, they were characterized using X-ray diffraction (XRD) analysis, scanning electron microscope (SEM), and Brunauer–Emmett–Teller (BET) surface area apparatus. The pore volume and pore size were calculated using the Barrett–Joyner–Halenda (BJH) model on the desorption branch. The experimental results show that the surface area and average crystallite size of the obtained TiO2 nanopowders vary between 160–203 m2/g and 27–49 nm, respectively. It was observed that the N2 adsorption–desorption isotherms for almost all samples of TiO2X% CGS (X: mass of CGS) show the typical Type I with a hysteresis loop of H4. The photocatalytic activities of the CGS-modified nanocomposites are evaluated not only by the photocatalytic degradation of methyl orange (MO) but also by the reduction of Cr(VI) as model pollutants in the presence of visible light.

HIGHLIGHTS

  • Excitable TiO2/CGS composites were prepared by the sol-gel method.

  • XRD, SEM, BET, and N2 adsorption-desorption measurements were done for structural properties.

  • Photocatalytic activity of TiO2/CGS nanocomposite was evaluated on both the photoreduction of Cr(VI) ions and photooxidation of methyl orange under irradiation from a 365 nm lamp.

  • Utilized in this survey is a cationic gemini surfactant that has a similar CTAB structure and has two long chains and cationic head groups and is a preferred morphology agents. Compared to one head group with CTAB, a novel synthesized cationic surfactant with two long chains is anticipated to positively affect both the properties of catalyst structures and data of photocatalytic degradation of model pollutants.

Titanium dioxide (TiO2) is used for the remediation of polluted aquatic systems because of its excellent photocatalytic properties. Pollutant derivatives do not transfer from aqueous to another phase during photocatalysis performed based on TiO2 and do not cause the secondary pollution trouble contrary to conventional methods such as adsorption, biological methods, chlorination, ozonation, and their combinations widely used for treating the dye/Cr(VI)-containing wastewaters. Ultimately, it is also used in innovative techniques, including less time-consuming, more effective, and inexpensive methods. However, there are some general problems for the effective use of TiO2. First, TiO2 can only be applied with UV irradiation due to the wide bandgap being ∼3.2 eV; thereby, no visible light response is obtained. Second, a high recombination rate of photogenerated charge carriers occurred on the surface of TiO2 semiconductor, decreasing the photocatalytic efficiency. Third, the separation of the TiO2 nanoparticles from water. To overcome these problems, many technical approaches such as modification with metal ions, and non-metals, coupling with other semiconductor, dye sensitization, and support materials such as glass slides, metal or ceramic substrates, polymer films, optical fibers have been developed to improve the photocatalytic activity (Altın et al. 2016b; Hoang et al. 2021). The surface area of the photocatalyst is another crucial factor in photocatalytic operations. A large surface area and high photoactivity are the desired properties of an effective photocatalyst. However, in the sol–gel technique, it is very troublesome to control both these properties and the agglomeration of TiO2 nanoparticles because of the high surface energy of the nanometer materials.

Recently, surfactants have been used to eliminate these drawbacks of the photocatalyst and improve the catalytic performance. Their molecular structures and amphiphilic properties determine their orientation and formation of micelles on the surface at the stage of early preparation in the synthesis of the nanopowders, and thus can be used as stabilizers and dispersant agents. The dispersing effect of surfactants in the formation of TiO2 particles provides a larger surface area and higher porosity. Sol–gel preparation of TiO2 nanoparticles is the preferred method to obtain smaller particles and a large surface area. Surfactants provide a unique medium by keeping the nanoparticles apart from each other during nanoparticle formation. Therefore, the presence of a surfactant improves the photocatalytic efficiency of the catalyst by increasing the homogeneity of the sol–gel medium during nanoparticle formation.

Surfactants are known as amphiphilic molecules and are used in a wide range of industrial applications (Akbaş et al. 2012; Mohd Noh et al. 2017). Due to their great potential, they are used in many fields (Negm et al. 2013; Rosen et al. 2015; Nwidee et al. 2017; Sarıkaya et al. 2021). The application of surfactants and the surfactant-mediated production of nanoparticles have attracted great interest (Nogueira et al. 2015; Xu et al. 2015). Nanoparticle fabrication requires different strategies to bulk material fabrication and should be conducted (Khamhaengpol & Siri 2016; Shaban et al. 2019). It has been detected that SDBS (sodium dodecyl benzene sulfonate), CTAB, and TX-100 used to control the morphologies and structures of TiO2/reduced grapheme oxide (TiO2/RGO) as anionic, cationic, and non-ionic surfactants enhanced the catalyst surface area from 153 to 277 m2/g. The enhanced surface area of the (TiO2/RGO) samples contributed a considerable extent to the photocatalytic degradation of methylene blue due to not only high surface area but also to its inhibiting the reunion of the (e/h+) pairs that occurred on the catalyst surface in the presence of irradiation (Hu et al. 2017). Choi et al. (2006) used the surfactant polyethylene glycol sorbitan monooleate to prepare TiO2 nanoparticles and thin films. It was reported that the material has better structural and catalytic properties due to the anatase crystal phase, in addition to the high surface area, large pore volume and controlled pore size. The crystallite size is reported to be smaller with high crystallinity. A thermally stable spherical porous structure has been reported to exhibit four times higher photocatalytic activity, possibly due to a highly interconnected network in the crystal structure. Sodium dodecyl sulphate (SDS) surfactant was used as a capping agent to tailor the morphology of the K2V3O8 by Basu et al. (2022). It was reported that the nanocomposites prepared by adding 0.2 g SDS reflected the highest BET surface area (23.6 m2/g) compared to pure K2V3O8 and 0.1-SDS/K2V3O8. Moreover, it was determined that the average of K2V3O8 samples decreased by 75 and 50 nm for particle size 0.1-SDS/K2V3O8 and 0.2-SDS/ K2V3O8, respectively. CTAB, SDS, and PVP-modified TiO2 particles were evaluated in photocatalytic degradation of MB. TiO2 with surfactants photocatalytically indicated degradation efficiency due to the properties of the surface in an hour and less time (Alphas Jebasingh et al. 2020). Another study was conducted by Anderson and Russell using the Tween surfactant to prepare TiO2 thin films (Anderson & Russell 2014). Sol–gel TiO2 production was modified by the addition of the non-ionic surfactant Triton X-100 (Stathatos et al. 2004; Altın et al. 2016a). It can be easily removed from the reaction medium. Similarly, the TiO2 catalyst was prepared by the sol–gel method using polyethyleneimine or Pluronic P-123 as the surfactant (Galkina et al. 2011). Structural differences of the cationic surfactants, e.g., chain lengths, counterions, etc., provide different properties for the sol–gel preparation of TiO2. Cetyl-trimethylammonium bromide (CTAB), cetyl-trimethylammonium chloride (CTAC), benzalkonium chloride (BC), and octadecyl-trimethyl ammonium bromide (C18TAB) were employed by Casino et al. (2014) and the obtained nanoparticles demonstrated the desired porous properties and enhanced large specific surface area. MgO/g-C3N4 nanocomposite generated by the surfactant-assisted hydrothermal method was analyzed in both degradations of aqueous Malachite Green solution (25 ppm) and assessment of antimicrobial activity against the Gram-positive and Gram-negative bacteria. They explained that increasing specific surface area of MgO/g-C3N4 nanocomposite enhanced the number of active sites for absorption, improving the contact with the dye pigments, which in turn causes enhancement of photocatalytic performance (Madona & Sridevi 2022). In another study, nano-sized TiO2 particles were prepared using a microwave-assisted sol–gel reaction. Jang et al. (2016) used polyvinylpyrrolidone (PVP, Mw: ∼55,000), Triton X-100 (laboratory grade) and poly (ethylene glycol) triblock copolymer (P123, Mn: 5,800) to prepare TiO2 spheres with sizes ranging from 105 to 380 nm. It has been reported that Triton X-100 and P123 have higher performance in the photocatalytic removal of methylene blue. Triton TX-100 is the most common surfactant and yields TiO2 nanoparticles of very good quality (Altın et al. 2016a; Jang et al. 2016). It is a non-ionic surfactant containing a hydrophilic polyethylene oxide chain and a hydrophobic p-(2, 4, 4-trimethylpentan-3-yl) phenyl ether group. Modification of the surfactant structure can be a good alternative for photocatalytic applications. First, the novel dicationic surfactants with functional groups rich in nitrogen atoms that provide increased electronic charge density and length of hydrophobic tail create the TiO2 particles' resistance to aggregation in time. On the other hand, increasing the number of head groups in surfactant structure leads to electrostatic interaction with the prepared TiO2 and steric barriers to protect the nanoparticles from aggregation (Shaban et al. 2019). Thus, monodispersed TiO2 nano materials can produce high homogenous surface area with more active sites in the wastewater treatment. Not only the type of surfactant but also the conditions of sol–gel synthesis, such as pH, are important for the preparation of TiO2 (Loosli & Stoll 2017; Tsega & Dejene 2017).

The F–N co-doped TiO2 photocatalyst was prepared using CTAB, a cationic surfactant, and showed six times higher photoactivity than pure TiO2. It was reported that the intensity of XRD peaks increased with increasing CTAB concentration and crystallization (Xie et al. 2008). Tashiro & Yamazaki (2019) constructed ZnTPyP fibers by the surfactant-assisted method exploring CTAB and kinetic study on the degradation of Rhodamine B (RhB). In another study, CTAB was used as an agent to control the catalyst surface, which exhibited different properties depending on the amount of surfactant and directly affected the photocatalytic activity (Wang et al. 2020). It is also known that CTAB as a cationic surfactant can form a cationic C19H42N+ group, which is hydrophobic and can affect the growth of nanoparticles through electrostatic and steric effects. It has been noted that the ZnWO4 nanomaterials fabricated a simple co-precipitation method using three different surfactants (PVP, SDS, and HMTA) and exhibited successful photocatalytic activity to degrade ciprofloxacin (CIP) antibiotic due to hampering the coagulation (Sivaganesh et al. 2020). In another study, surfactants were used to synthesize the nano-micro AgCl crystals in the growth of AgCl crystals (Zhou et al. 2022). In addition, the morphology of the manufactured products may have a specific structure because the CTAB molecules restrict the agglomeration of the particles. Our material, a cationic gemini surfactant (CGS) (C40H84O2N4Br2), which is similar to the CTAB structure used in this study, has two long chains and cationic head groups, and is preferred as a morphology agent. It was proved that the critical micelle concentration (CMC) of CGS, whose physicochemical properties at different temperatures were determined in the previous literature, is lower than the concentration of CTAB (Sarıkaya et al. 2019). Due to the fact that CGS with much lower CMC values has higher surface activity, CGS was selected as a surfactant. It could be intriguing to investigate how the double chain structure together with the cationic head groups affects the structure of the catalyst in sol–gel synthesis and what contribution this CGS could make to the photocatalytic activity. As shown in Figure 8, the surface of TiO2 nanoparticles is positively charged at inherent pH (pH < 6.8), since the zero point of charge (pzc) of TiO2 (Degussa P25) is at pH 6.8, while it is negatively charged under alkaline conditions (pH > 6.8). As indicated, C40H84O2N4Br2 is a cationic surfactant with a positive charge throughout and TiO2 is adsorbed on the surface of gemini surfactant, which exerts a strong electrostatic repulsive force between them, which helps to hinder the aggregation of nanoparticles and enhance the more homogeneous surface.

In this study, TiO2 nanoparticles were prepared by the sol–gel process in combination with a novel CGS with amide functional groups. N, Nʹ-bis [3-(dodecanoylamino) propyl]-N, N, Nʹ, Nʹ-tetramethylhexane-1, 6-diammonium dibromide is the full chemical name, and the structure of the molecule is given below in Figure 1.
Figure 1

N, N′-bis [3-(dodecanoylamino) propyl]-N, N, N′, N′-tetramethylhexane-1,6-diammonium dibromide (Sarıkaya et al. 2019).

Figure 1

N, N′-bis [3-(dodecanoylamino) propyl]-N, N, N′, N′-tetramethylhexane-1,6-diammonium dibromide (Sarıkaya et al. 2019).

Close modal

This CGS has been prepared previously and its preparation and chemical properties have been described in detail (Sarıkaya et al. 2019). Due to their amphiphilic nature, they form micelles at a certain concentration, which is called the CMC. The CMC value of CGS was determined by conductivity measurements of pure surfactant solutions, and is 7.41 × 10−4 M at 303.15 K. The structure may be an advantage for the preparation of TiO2 nanoparticles. As can be seen from the molecular structure of CGS, it is a dicationic surfactant and can be an effective agent for nanoparticle alignment. TiO2 nanoparticles were prepared as both naked TiO2 and TiO2–CGS during the sol–gel process. This is the first study in which CGS was used as a surfactant. The calcination temperature was kept low (300 °C) to maintain the organic carbon and nitrogen after the formation of the nanoparticles. This is particularly important to obtain a nanoparticle surface attractive for strong absorption of organic molecules. The crystal structure and optical properties of the nanoparticles were investigated by the SEM, BET, XRD, and UV-Vis DRS methods. The photocatalytic properties of the characterized nanoparticles were tested for aqueous Cr(VI) or methyl orange (MO) (10 mg/L) as inorganic and organic model pollutants under visible light exposure. If their discharges to the aquatic environment from the metallurgical industry and leather industry are above the limit values, they will be malefic for all living species (Liu et al. 2022; Zhang et al. 2022). High concentrations of hexavalent chromium ion Cr(VI) in systems can heavily damage the immune system of aquatic animals and induce human respiratory nervous system, cardiovascular and cerebrovascular diseases because of the rich mobility of Cr(VI). Thus, the effective way is the reduction of Cr(VI) to Cr(III), which have poor mobility and low toxicity. On the other hand, the highly soluble synthetic MO that comprises azo groups such as auxochrome and chromophore discharges to underground or aboveground from textile, paint, plastic, and cosmetic with rapid industrialization, and consequently, wastewater containing MO dye may be carcinogenic and mutagenic if discharged without treatment and negatively affect all live species (Al-Mamun et al. 2022). So, TiO2 based on photocatalysis is one of the impressive solutions for degradation, mineralization of organic dyes, and reduction of inorganic materials.

Synthesis of CGS

In the first step, 3-(dimethylamino)-1-propylamine (0.1 mol) was weighed into a round-bottom flask and dissolved in chloroform under an inert atmosphere. Dodecanoyl chloride (0.1 mol) was slowly added dropwise while stirring. The chloroform layers were extracted with saturated potassium carbonate solution, water, and brine, respectively. It was then dried with anhydrous calcium chloride. Finally, the chloroform was removed under a reduced pressure. In the second step, N-[3-(dimethylamino)propyl]dodecanamide (0.08 mol) and 1,6-dibromohexane (0.04 mol) were mixed in a flask and the mixture was refluxed in acetone through stirring at 60 °C for 6 h under nitrogen atmosphere. This product was washed and purified three times by recrystallization in cold acetone. The CGS was obtained as a white powder (Sarıkaya et al. 2019).

Preparation of TiO2–CGS photocatalysts by the sol–gel method

Titanium (IV) isopropoxide (TIP) was used as a precursor, and all solvents and additional chemicals were of analytical grade. The details of the method are given in Figure 2 (Altın et al. 2016a).
Figure 2

Preparation process of TiO2–CGS via the sol–gel method.

Figure 2

Preparation process of TiO2–CGS via the sol–gel method.

Close modal

First, TIP (8.4 mL) was dissolved in 20 mL of absolute ethanol. The CGS was added to the solution at different percentages (10.71; 21.56; 53.90; 107.80; 161.70 and 215.6 mg CGS, corresponding to 0.5; 1.0; 2.5; 5.0; 7.5 and 10% of TiO2 mass, respectively) and keft stirred (1 h). On the other hand, 1 mL of concentrated HNO3 and 1 mL of distilled water were added to 10 mL of absolute ethanol and this mixture was added dropwise (extremely slow) to the first solution with vigorous mixing.

The mixture was stirred overnight and at the end of stirring, the mixture was dried at 76 °C for 12 h. The dried solid powder was calcined at 300 °C for 4 h. After cooling to room temperature, it was kept in dark bottles until use. TiO2 powder was also prepared by the same procedure without surfactant. The hydrolysis step was quite slow, especially under vigorous stirring. We have reported the sol–gel method and the calcination temperature applied to prepare the desired photoactive TiO2 nanoparticle (Altın et al. 2016a).

Characterization of TiO2–CGS photocatalysts

Structural characterization of the produced nanoparticles was performed. XRD analysis of the catalysts was carried out using a Rigaku Smartlab diffractometer with CuKα radiation (λ = 1.5406 A°) over the range 2θ = 20°–60° at room temperature. The surface morphology of the photocatalysts was studied using an SEM (JEOL, JSM 6610). The surface area, pore volume, pore size and crystallite size were determined by BET analysis (Oantachrome Autosorb IQ2).

Photocatalytic tests

Details of the photocatalytic assays have been described in previous work (Koç Keşir et al. 2020). A portion of 10 mg/L of Cr(VI) solution (250 mL) was placed in a quartz photoreactor and the pH was adjusted to 2.0 with H3PO4. This was preferred because the pH at the zero point of charge (pHpzc) of the TiO2 is 6.0–7.5, and the TiO2 surface is positively charged at a lower pH. The surface absorption of the anionic Cr(VI) ions on the catalyst allow better contact between the surface and the Cr(VI) ions. The studied concentration of Cr(VI) solution was quite high compared to the values found in the real environment. The produced photocatalyst (1.0 g/L) was added to the test solution with magnetic stirring. The suspension was illuminated with a 365-nm lamp (6 W, Spectroline ENF-260; the light intensity of 350 μW/cm2) for a specified exposure time. Every 30 min, a 2.0-mL sample of the suspension was taken to determine the remaining Cr(VI) ion concentration. The sample was centrifuged for 5 min to separate the solid catalyst (3,000 rpm, Eppendorf Centrifuge 5180) and filtered (0.45 μm membrane). The supernatant was used for further measurements. The photocatalytic tests were repeated with TiO2 nanoparticles prepared by the same sol–gel procedure without surfactant. All experiments were repeated in the presence and absence of light (light and dark experiments). The remaining Cr(VI) ions were determined by the standard method EPA (7196A) (Environmental Protection Agency) after staining with diphenylcarbazide. Percent removal was calculated using the remaining Cr(VI) ion concentration after treatment. The same procedure was performed for the degradation of MO at pH = 6 (the natural pH of the aqueous solution of MO). The photocatalytic efficiency of the catalysts was measured by spectrophotometric monitoring of the absorbance of MO at 464 nm (Unicam UV-2 spectrometer) in the presence and absence (darkness) of light. A 250-mL portion of the aqueous solution of MO (10 mg/L) and 0.25 g of catalyst (1 g/L) were added to the photoreactor. The same sampling and separation steps were applied. The initial concentration of MO was also higher than the actual values found in the wastewater stream. These high concentrations were used specifically to prove that the catalyst was effective under severe conditions. Removal rates were calculated using the remaining MO concentration after treatment. Each test (for removal of MO or Cr(VI) ions) was repeated three times and the relative standard deviations were generally less than 1%.

Structural analysis

As is well known, the nanocrystalline structure of TiO2 is controlled by the method of preparation and subsequent calcination (Loosli & Stoll 2017; Tsega & Dejene 2017). The presence of a surfactant and its concentration can affect the fine structure of the catalyst. Calcination at low temperature such as 300 °C is sufficient to obtain the most active anatase crystal structure (Koç Keşir et al. 2020) because it has been found that as the temperature increases, the probability of rutile content increases as well as the anatase phase (Stathatos et al. 2004). Therefore, this temperature is suitable to keep the remaining carbon and nitrogen as well as the organic-mediated surface of the catalyst.

The typical XRD patterns of the anatase TiO2 are shown in Figure 3. The XRD pattern of TiO2 and their derivatives were in agreement with the JCPDS Card no. 21-1272, presenting the characteristic diffraction peaks (approximately centered at 2θ: 25.4°, 38°, 48° and 54.4°) that belong to the (1 0 1), (2 0 0), (0 0 4) and (1 0 5) planes of the anatase structure of synthesized nanoparticles. No impurity was observed. The peak positions of nanoparticles did not have any changes in the presence of CGS. The XRD data indicated that the CGS loaded onto the surface of TiO2 had no effect on the crystal structure of TiO2 nanoparticles, which means that anatase TiO2 is obtained when it is calcined at this temperature. This is because the melting point of CGS is 196.0–197.0 °C, so CGS probably decays when calcined at 300 °C. The crystalline structure has changed from anatase to amorphous by increasing the surfactant (e–f). It should be kept in mind that all catalysts were calcined at low temperature (300 °C). If the calcination temperature had been higher than 300 °C, the anatase form would have been obtained. It is known that photocatalytic activity is correlated with the crystallinity of photocatalysts. Thus, the following average crystallite sizes: TiO2 TiO2–0.5%CGS; TiO2–1%CGS; TiO2–2.5%CGS; TiO2–5%CGS; TiO2–7.5%CGS; TiO2–10%CGS by using the Scherrer formula based on the (101) peak were evaluated. The average crystallite sizes of TiO2–CGS samples were calculated to be 49.4, 43.4, 37.5, 26.6, 36.2, 27.5, and 31.5 nm for TiO2 and TiO2–CGS (0.5–1.0–2.5–5.0–7.5–10%, resp.), which indicated that crystallite sizes of TiO2 nanoparticles were decreased in the presence of CGS up to a certain concentration of CGS. It is conceivable that CGS used in TiO2 nanoparticles synthesis via the sol–gel process did not change the peak positions of the anatase structure, but CGS molecules have a particular effect on the crystallite size of TiO2 particles.
Figure 3

XRD patterns of (a) TiO2, (b) TiO2–0.5%CGS, (c) TiO2–1%CGS, (d) TiO2–2.5%CGS, (e) TiO2–5%CGS, (f) TiO2–7.5%CGS and (g) TiO2–10%CGS.

Figure 3

XRD patterns of (a) TiO2, (b) TiO2–0.5%CGS, (c) TiO2–1%CGS, (d) TiO2–2.5%CGS, (e) TiO2–5%CGS, (f) TiO2–7.5%CGS and (g) TiO2–10%CGS.

Close modal
The images of SEM are given in Figure 4. The structural properties were similar, and no significant differences were observed. This could be because the organic template either could not be removed during heat treatment or covered the active pores of the TiO2 surface or broke the pore structure (Choi et al. 2007). It could also be because either the realignment of the particles within the sample results in poor dispersibility and aggregation of the particles when the template is used, or it does not help the particles to move away from each other (Wu et al. 2018). However, the analyses of BET revealed more supportive structural properties.
Figure 4

SEM images of the TiO2 catalysts at a 2-μm scale. (a) TiO2, (b) TiO2–0.5% CGS, (c) TiO2–1%CGS, (d) TiO2–2.5%CGS, (e) TiO2–5%CGS, (f) TiO2–7.5%CGS and (g) TiO2–10%CGS.

Figure 4

SEM images of the TiO2 catalysts at a 2-μm scale. (a) TiO2, (b) TiO2–0.5% CGS, (c) TiO2–1%CGS, (d) TiO2–2.5%CGS, (e) TiO2–5%CGS, (f) TiO2–7.5%CGS and (g) TiO2–10%CGS.

Close modal
N2 adsorption–desorption measurement at a temperature of 77.4 K for liquid N2 was also used to measure structural properties. The nitrogen sorption–desorption isotherm curves are shown in Figure 5.
Figure 5

(a) Nitrogen sorption–desorption isotherms and (b) pore size distribution curves of pure and TiO2–0.5–10%CGS.

Figure 5

(a) Nitrogen sorption–desorption isotherms and (b) pore size distribution curves of pure and TiO2–0.5–10%CGS.

Close modal

The isotherms for almost all samples of TiO2X% CGS (X: mass of CGS) exhibited typical type I with an H4 type hysteresis loop (defined by IUPAC) expressing slit pores, including pores in the micropore region. Type I isotherms are that adsorption is be restricted to the depletion of a single monolayer (Langmuir adsorption) of pollutants on the photocatalyst surface. Type H4 hysteresis is also often associated with narrow slit pores. The hysteresis loop in the relative pressure (P/P0) region of 0.1–0.99 may be due to the fact that once filled with adsorbate, the pores leave little or no external surface for further adsorption. Incorporating CGS to the microporous TiO2 lattice in the synthesis step does not collapse the TiO2 micropores structure and thus does not alter the isotherms’ behaviors. Figure 5(b) shows that the BJH pore size distribution curves obtained from the N2 adsorption–desorption measurements of the different samples are too narrow and the majority aperture is centered at about 10 Å. This indicates that all TiO2 including CGS molecules have a microporous structure. CGS may be partially covered the surface of TiO2 and blocked its pore channels. Also, as the mass ratio of CGS increase to 2.5% CGS, BET surface area and pore volumes of TiO2X% CGS nanoparticles enhance regularly. Yet, when surfactant used more than 2.5% CGS, the rise in both surface area and pore volume values is greater than naked TiO2 sample and smaller than mass ratio of 0.5–2.5% CGS. The increase in the surface area corresponds to crystallite size, thus validating that adding CGS improves the TiO2 surface properties to prevent particle aggregation, resulting in the decrease of crystallite size. The low surface area and pore volume may affect photocatalytic efficiency depending on structural, morphological incides and nature of pollutants. Detailed values of BET such as surface area, total pore volume, and pore size parameters are shown in Table 1.

Table 1

BET analyses of prepared TiO2 catalysts

Surface areaaPore volumebPore sizecCrystallite sized
(a) 160.0 0.0703 11.320 49.3 
(b) 203.3 0.0859 9.973 43.4 
(c) 220.0 0.1047 10.47 37.5 
(d) 201.3 0.0883 9.795 26.6 
(e) 180.8 0.0768 10.36 36.2 
(f) 177.5 0.0714 9.871 27.5 
(g) 170.2 0.0742 9.708 31.5 
Surface areaaPore volumebPore sizecCrystallite sized
(a) 160.0 0.0703 11.320 49.3 
(b) 203.3 0.0859 9.973 43.4 
(c) 220.0 0.1047 10.47 37.5 
(d) 201.3 0.0883 9.795 26.6 
(e) 180.8 0.0768 10.36 36.2 
(f) 177.5 0.0714 9.871 27.5 
(g) 170.2 0.0742 9.708 31.5 

Note: (a) TiO2, (b) TiO2–0.5%CGS, (c) TiO2–1%CGS, (d) TiO2–2.5%CGS, (e) TiO2–5%CGS, (f) TiO2–7.5%CGS, (g) TiO2–10%CGS.

aMultipoint BET, BJH method cumulative desorption surface area (m2/g).

bThe BJH method cumulative desorption pore volume (cc/g).

cThe BJH method desorption pore diameter (mode) (Å).

dCalculated by the Scherrer equation (nm).

As seen from Table 1, the prepared TiO2 has quite a large surface area with or without CGS. The surface area and pore size of both pure TiO2 particles and TiO2–X% CGS are in the range of 160–220 m2/g and 9.7–11.3 Å, respectively.

The surface area of pure TiO2 was 160.0 m2/g, and it was increased to 160.0–220.0 m2/g in the presence of CGS. These values are almost 10 times higher than those of TiO2 produced in the presence of surfactant Triton-TX100. It should be noted that very slow hydrolysis and vigorous mixing are applied during the sol–gel process. This could have an important impact on the production of small but porous particles with a large surface area. The cumulative desorption surface area increases by increasing the CGS concentration used during the sol–gel process. While TiO2 prepared without surfactant had a surface area of 160.0 m2/g, it increased up to 220 m2/g and then remained almost stable. The highest surface area was obtained when 1.0% CGS was used in the sol–gel process. Other parameters such as pore volume, pore size and crystallite size exhibited the same trend. Pore volume increased, pore size and crystallite size decreased with increasing CGS concentration. This result directly confirms that the CGS micelles play a major role in the increasing of surface area, which enhances the synergistic effect between adsorption and photocatalysis with the help of dicationic CGS molecules. Thus, more active adsorption and photocatalytic reaction can be provided by an enlarged surface area of synthesized TiO2 samples with CGS. Moreover, repulsive forces such as electrostatic and steric can be formed on the surface of TiO2 nanoparticles because of dispersing CGS with double cationic head groups. This means that dispersibility of the nanoparticles in synthesis solution was enhanced and the aggregation rate of particles was reduced. The contribution of CGS to the physical and chemical structure of TiO2 enables control of both the crystallite size and surface features of nanoparticles.

Photocatalytic removal of Cr(VI) and MO

All TiO2 catalysts were tested separately for photoreduction for Cr(VI) ions and photooxidation for MO. Prior to light exposure, the catalysts were tested for the removal of the target pollutants in the dark. The percentages of removal of Cr(VI) ions and MO are shown in Figures 6 and 7, respectively.
Figure 6

The photocatalytic removal of Cr(VI) ions after visible light irradiation ([C]o = 10 mg/L, catalyst mass = 1 g/L, pH = 2, λ = 365 nm).

Figure 6

The photocatalytic removal of Cr(VI) ions after visible light irradiation ([C]o = 10 mg/L, catalyst mass = 1 g/L, pH = 2, λ = 365 nm).

Close modal
Figure 7

The photocatalytic removal of MO after visible light irradiation ([MO]o = 10 mg/L, catalyst mass = 1 g/L, pH = 6, λ = 365 nm).

Figure 7

The photocatalytic removal of MO after visible light irradiation ([MO]o = 10 mg/L, catalyst mass = 1 g/L, pH = 6, λ = 365 nm).

Close modal
Figure 8

Orientation of TiO2 nanoparticles in the sol–gel medium.

Figure 8

Orientation of TiO2 nanoparticles in the sol–gel medium.

Close modal

The removal process depends on the surface area and crystallite size of the catalyst. As the surface area increases and the crystallite size decreases, the number of active sites responsible for photocatalytic degradation increases. Therefore, more Cr(VI) and MO molecules are adsorbed and the degradation increases. Both degradation data of Cr(VI) were in good agreement with the results of BET surface area and particle size. As seen in Table 1, the maximum specific surface area and crystallite size were found to be 220 m2/g and 27 nm for TiO2–1.0% CGS nanoparticles. Furthermore, in the photocatalytic reduction of Cr(VI) with TiO2–1.0% CGS nanoparticles, it was calculated that the reduction efficiency was 77.22% at an initial concentration of 10 mg/L of Cr(VI) after 150-min irradiation (Figure 6). The capacity of the catalyst to remove Cr(VI) was higher than that of pure TiO2 (33.42%) and TiO2 prepared in the presence of TX-100 (40%) under the same experimental conditions (Altın et al. 2016a). CGS played a significant role in increasing the dispersion of titanium dioxide particles in the sol–gel reaction media, and CGS surfactant at a ratio of 1.0% greatly increased the removal efficiency of Cr(VI). This could be due to the sufficient mass of surfactant molecules, which increased the morphological and structural properties to enhance Cr(VI) removal. It appeared that the Cr(VI) removal efficiency was decreased steadily with increasing CGS content. The decrease in removal efficiency was due to the aggregation of CGS on the TiO2 surface after the TiO2 surface reached saturation with respect to the surfactant. In contrast to the photocatalytic reduction of Cr(VI), MO was quite resistant to photochemical degradation and the removal percentage was 59.27% with TiO2–1.0% CGS for a 150-min irradiation period which is still higher than TiO2 prepared in the presence of TX-100 (36.84%) (Altın et al. 2016a). The highest percentage of MO removal was obtained with TiO2–7.5% CGS (69.89%) (Figure 7).

To calculate the apparent rate constant for the study of kinetic degradation of MO and Cr(VI), the loss of both pollutants was observed as a function of irradiation time, and the data were fitted to a first-order and zero-order rate model for Cr(VI) and MO, respectively, using the following equation:

The pseudo first-order equation is represented by
formula
Also, the zero-order equation is represented by
formula
where C0 and C represent the concentrations of organic and inorganic pollutants at the initial stage (0 min) and after irradiation for some time (t: 150 min), and Kapp is the pseudo first-order rate constant (min−1) for photocatalytic degradation of pollutants. The kinetics parameters for the photocatalytic efficiency of the catalysts are presented in Table 2.
Table 2

Kinetic parameters of pseudo first-order rate constants and zero-order of synthesized TiO2 with various amounts of CGS after 150-min irradiation

PhotocatalystPseudo first-order [Cr(VI)]
Zero-order (MO)
kapp (min−1)R2kappR2
TiO2  0.89  0.98 
TiO2–0.5% CGS  0.99  0.97 
TiO2–1.0% CGS  0.97  0.98 
TiO2–2.5% CGS  0.96  0.96 
TiO2–5.0% CGS  0.99  0.99 
TiO2–7.5% CGS  0.99  0.98 
TiO2–10% CGS  0.97  0.96 
PhotocatalystPseudo first-order [Cr(VI)]
Zero-order (MO)
kapp (min−1)R2kappR2
TiO2  0.89  0.98 
TiO2–0.5% CGS  0.99  0.97 
TiO2–1.0% CGS  0.97  0.98 
TiO2–2.5% CGS  0.96  0.96 
TiO2–5.0% CGS  0.99  0.99 
TiO2–7.5% CGS  0.99  0.98 
TiO2–10% CGS  0.97  0.96 

Logarithmic relationship of the ratio between the original concentration of pollutants (C) and the concentration after photocatalytic degradation (C0) against the equaled irradiation time (min) yields a linear line. The slope of linear regression equals to the apparent first-order rate constant Kapp (Riaz et al. 2020). The removal profiles of the Cr(VI) by the TiO2–CGS derivatives fitted well to the pseudo first-order kinetics, the first-order kinetics defining chemical reaction is based on a hypothesis in which the reaction rate is linearly dependent on the concentration of only one reactant. In contrast, the MO degradation profile obeys zero-order kinetics, indicating that the catalytic sites (e−/h+ pairs) on the surface of TiO2–CGS can oxidize the MO molecules that diffuse to the surface of the photocatalyst in the irradiation time, and zero-order reactions are independent from reactant concentration.

As shown in Table 2, it was found that the rate constant (Kapp value) of as-prepared TiO2–7.5% CGS is 1.2 min−1 with the value of correlation coefficient (R2: 0.98), exhibiting the fastest degradation rate for MO among the TiO2 samples synthesized using CGS by a modified sol–gel method, which is greater than that of approximately 4.1, 2.5, 1.5, 1.3, 1.2 and 1.01 times belonging to TiO2–0.5% CGS (0.29 min−1), TiO2 (0.48 min−1), TiO2–2.5% CGS (0.79 min−1), TiO2–1.0% CGS (0.89 min−1), TiO2–5.0% CGS (0.98 min−1) and TiO2–10% CGS (1.1 min−1), respectively.

As for Cr(VI), the TiO2–0.5% CGS and TiO2–1.0% CGS samples were found to have a similar pseudo first-order rate constant of 1.48 (R2: 0.99) and 1.49 min−1 (R2: 0.97). However, among the produced nanoparticle samples, the TiO2–1.0% CGS was preferred for comparing to first-order rate constant, because they have high removal percentage of Cr(VI) due to the biggest surface area and smallest crystallite size (Table 1). It was seen that the Kapp values were in a descending sequence of 2.5% (1.24 min1) > 7.5% (1.2 min1) > 5.0% (1.1 min1) > 10% (0.64 min1) CGS > TiO2 (0.41 min1) when the amount of CGS exceeded the 1.0%. The pseudo first-order rate constant of TiO2–1.0% CGS particles is nearly higher than 3.66 (TiO2), 2.40 (TiO2–10% CGS), 1.34 (TiO2–5.0% CGS), 1.24 (TiO2–7.5% CGS) and 1.20 (TiO2–2.5% CGS) times. These results could be ascribed to the blocking of active pores on the surface of the nanoparticles if there were undecayed surfactant molecules in the calcination stage. This is because all the surfactants cannot be removed in the calcination stage, and the remaining CGS, which cannot be removed, may hamper the interaction of the excitation light with the active pores on the TiO2 catalyst, causing a decrease in photoactivity, and may cause slower removal reaction. It can be surmised that the photo decolorization efficiency enhances with increasing Kapp value. Almost all catalysts prepared with CGS were more effective than TiO2 prepared without CGS in both dark and light conditions.

In the sol–gel production step (acid hydrolysis), the adsorption layer between CGS and TiO2 nanoparticles can be formed by the electrostatic interactions (cationic CGS and positively charged TiO2) as the TiO2 particles are positively charged under the acid hydrolysis conditions and CGS is a dicationic surfactant. In this way, homogeneously dispersed TiO2 nanomaterials can be obtained. This is because of the repulsive forces between the TiO2 particles and CGS, which effectively prevents the agglomeration of particles. Therefore, surface interactions between TiO2 and CGS are not expected to be much stronger than negatively charged surfactants. Thus, dicationic surfactant CGS can better generate repulsive forces than mono cationic surfactants. Moreover, CGS is a large molecule and provides a positively charged barrier between the TiO2 nanoparticles formed right after extremely slow hydrolysis. The possible repulsive forces between TiO2 particles and CGS molecules are schematically shown in Figure 8.

Clarification of photocatalytic mechanism of Cr(VI) and MO

The photocatalytic mechanism of MO degradation and Cr(VI) reduction may be stated as follows. When suspended TiO2 nanoparticles in the aqueous Cr(VI) and MO solution were excited, photons with energy (hʋ) higher than or equal to the semiconductor bandgap (3.2 eV) cause electronic excitation for migration of electrons from valance band to conduction band (CB), and thus, electron and positive hole pairs (e/h+) are generated, respectively, in the CB and valence band (VB) (1). These free electrons commonly scavenged by O2 absorbed on the photocatalyst's surface produce superoxide radical anion () (3), which can pave the way for reactive oxygen species (ROS) (i.e., , , H2O2, , etc.) It is universally acknowledged that the ROS are active radical species of catalytic processes. In addition, h+ reacts with OH or H2O to produce powerful unselective hydroxyl radicals () (2) as oxidants (with a redox potential of +2.80 V) (Kordouli et al. 2015).
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)

While h+, and subsequently decompose the dye molecules into mineral salts, CO2 and H2O as the end products (9) (Al-Mamun et al. 2022), electron (e) on the CB provides reduction to Cr(III) from Cr(VI) species in the pollutant solutions (4–8) (Djellabi et al. 2016).

In this work, a novel method for the preparation of TiO2 nanoparticles by the sol–gel method employing a CGS with amide functional groups was described. The morphology of the catalyst was characterized by using XRD, SEM, and BET analyses, and the photocatalytic effect was evaluated using model pollutants. A surfactant with a double chain structure and cationic head groups was chosen for the sol–gel synthesis. The cationic CGS could not only improve the crystallinity but also enhance the morphological properties such as surface area, pore volume, pore size, and thus develop the photocatalytic performance. As a result of these findings, CGS could be recommended as a new surfactant for easy TiO2 production. The amphiphilic nature and dicationic structure of this novel CGS with amide functional groups confer additional structural and surface properties to the well-known TiO2. However, further mechanistic evaluation is needed.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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