Thi Bang

Tam Dao

Thi Thu

Trung Loan Ha

Hon Do Nguyen

Nhien Le

Chi Nhan

Thi Mai

Loan Nguyen

Patrick Perré

Mao Dang

Nguyen

Trung Do Nguyen

Hon Nhien Le

Patrick Perre
email: patrick.perre@centralesupelec.fr

Nguyen Van

Cu Street

Effectiveness of photocatalysis of MMT-supported TiO2 and TiO2 nanotubes for rhodamine B degradation

Keywords: Photocatalysts, UV-C irradiation, TiO2 nanotubes, MMT/TiO2 and MMT/TiO2-NTs, and rhodamine B

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Synthetic dyes are widely used in various applications in the industries as paper, leather, and textile due to their color-giving properties [START_REF] Gupta | Application of low-cost adsorbents for dye removal -A review[END_REF][START_REF] Abdi | Synthesis of metalorganic framework hybrid nanocomposites based on GO and CNT with high adsorption capacity for dye removal[END_REF][START_REF] Katheresan | Efficiency of various recent wastewater dye removal methods: A review[END_REF][START_REF] Wang | Graphdiyne-modified TiO2 nanofibers with osteoinductive and enhanced photocatalytic antibacterial activities to prevent implant infection[END_REF]. They are responsible for environmental pollutions and human health [START_REF] Charumathi | Packed bed column studies for the removal of synthetic dyes from textile wastewater using immobilised dead C. tropicalis[END_REF] due to the presence of dye in effluents. It was stated that some of the following industries are mainly responsible for environmental pollution from dye effluent such the textile (54%), the dyeing industry (21%), paper and pulp (10%), tannery and paint (8%) and the dye manufacturing (7%) [START_REF] Rauf | Survey of recent trends in biochemically assisted degradation of dyes[END_REF] and other activities as dyes degradation [START_REF] Dos Santos | Review paper on current technologies for decolourisation of textile wastewaters: Perspectives for anaerobic biotechnology[END_REF]. Rhodamine B is a xanthene dye, which is widely used as a colorant in textiles industry, wool, silk, dye cells in biotechnology and food stuffs. However, it is harmful to human and animal as skin, eyes and respiratory tract irritation, carcinogenicity as well as reproductive and developmental toxicity [START_REF] Kingsley | Combustion synthesis of fine particle rare earth orthoaluminates and yttrium aluminum garnet[END_REF][START_REF] Nagaraja | Photocatalytic degradation of Rhodamine B dye under UV/solar light using ZnO nanopowder synthesized by solution combustion route[END_REF].

Therefore, the removal of rhodamine B from dyes effluents must be given priority before they are released into environment. Until now, there was some methodologies of removing of the dyes from wastewaters including chemical, physical methods and biological processes, however, they still have many disadvantages such as low efficiency and high cost [START_REF] Robinson | Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative[END_REF]. Photocatalyst approaches are currently being investigated for the removal of impurities in wastewater. Photocatalytic is a new and potential discovery that uses renewable energy sunlight to decompose organic pollutants into low-cost but highly efficient fuels and removal of trace contaminant from water [START_REF] Malato | Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends[END_REF]. This approach is also environmentally friendly, compared to previous traditional methods such as advanced oxidation, filters, and solar evaporation. In recent years, the use of semiconductor photocatalytic materials for treatment of organic compounds has achieved remarkable achievements and attracted from basic research to application. Some of them have been studied and used as photocatalysts such as ZnO, CdS, and Fe2O3 (Hassani et al., 2017;[START_REF] Pannak | Homogeneous distribution of nanosized ZnO in montmorillonite clay sheets for the photocatalytic enhancement in degradation of Rhodamine B[END_REF]. Titanium dioxide (TiO2) is known as a potential photocatalyst in the field of environmental cleaning because of its chemical and biological inertness, ease of fabrication, non-toxicity, abundance in availability and low cost [START_REF] Hassani | Sonocatalytic degradation of ciprofloxacin using synthesized TiO2 nanoparticles on montmorillonite[END_REF][START_REF] Chinh | XPS Spectra Analysis of Ti2+, Ti3+ Ions and Dye Photodegradation Evaluation of Titania-Silica Mixed Oxide Nanoparticles[END_REF][START_REF] Chinh | Effect of Carbon Nanotubes and Carbon Nanotubes/Gold Nanoparticles Composite on the Photocatalytic Activity of TiO2 and TiO2-SiO2[END_REF]. In particular, TiO2 has been widely studied for photocatalysis [START_REF] Fan | Single-site nickel-grafted anatase TiO2 for hydrogen production: Toward understanding the nature of visible-light photocatalysis[END_REF][START_REF] Huang | Nickel sulfide modified TiO2 nanotubes with highly efficient photocatalytic H2 evolution activity[END_REF][START_REF] Tahir | Performance analysis of nanostructured NiO-In2O3/TiO2 catalyst for CO2 photoreduction with H2 in a monolith photoreactor[END_REF]. However, TiO2 has limitations in photocatalytic applications due to the rapid recombination between electron and holes pairs, which cause to reduce photocatalytic efficiency. Moreover, it also has a wide band gap that only active photocatalyst in the ultraviolet light (less than 5% of the entire solar energy), inducing a lower photocatalytic activity [START_REF] Leong | TiO2 based photocatalytic membranes: A review[END_REF][START_REF] Diamond | Assessment of the potential hazard of nano-scale TiO2 in photocatalytic cement: application of a tiered assessment framework[END_REF]. To overcome the above limitations, some approaches are proposed to improve the photocatalytic activity. Specifically, TiO2 nanosheets were used as an electron mediator to prepare the CdSe/graphene/TiO2 nanosheet composite photocatalysts. They provided a great efficient charge separation, extended visible-light absorption range and stability for the degradation of methylene blue in aqueous solutions [START_REF] Ma | Visible-light-driven CdSe quantum dots/graphene/TiO2 nanosheets composite with excellent photocatalytic activity for E. coli disinfection and organic pollutant degradation[END_REF]. Moreover, the TiO2 nanosheets were combined with small plasmonic Au nanoparticles to improve the photocatalytic rate of hydrogen production through the large surface adsorptive sites for reactant adsorption and enhancing the charge separation through the Schottky transfer hub to neighboring TiO2 nanosheets [START_REF] Cheng | Constructing functionalized plasmonic gold/titanium dioxide nanosheets with small gold nanoparticles for efficient photocatalytic hydrogen evolution[END_REF]. It could be combined with different semiconducting materials [START_REF] Zhang | Boosted photocatalytic degradation of Rhodamine B pollutants with Z-scheme CdS/AgBr-rGO nanocomposite[END_REF] to prevent reuniting the photogenerated electron-hole pairs [START_REF] Dao | Natural core-shell structure activated carbon beads derived from Litsea glutinosa seeds for removal of methylene blue: Facile preparation, characterization, and adsorption properties[END_REF] and improve the photocatalytic activities [START_REF] Low | TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity[END_REF]. TiO2-NTs are synthesized through hydrothermal method from TiO2. They can improve the photocatalytic activity due to large specific surface area, ion exchange capacity, and fast electronic transfer over a long distance, and high adsorption of light due to a large ratio between the length and diameter of the tubes. Specially, all these properties could be obtained and controlled through the synthesis method [START_REF] Casu | Crystallization of TiO₂ Nanotubes by In Situ Heating TEM[END_REF][START_REF] Niu | Fabrication and antibacterial properties of cefuroxime-loaded TiO(2) nanotubes[END_REF]. Unfortunately, similar to most semiconductor materials used as photocatalyst, TiO2-TNs are also difficult in separating the electron-hole pairs, leading to reduce photocatalytic activity. Therefore, some strategies have been proposed to extend the lifetime of photoinduced charge carriers and enhance photocatalytic activities including doping metal [START_REF] Park | Photoluminescence of samarium-doped TiO2 nanotubes[END_REF][START_REF] Sasani | Structural and electronic properties of Mg and Mg-Nb co-doped TiO2 (101) anatase surface[END_REF], metal oxides [START_REF] Xu | Electrodeposition synthesis of MnO2/TiO2 nanotube arrays nanocomposites and their visible light photocatalytic activity[END_REF][START_REF] Habila | Synthesis and application of Fe3O4@SiO2@TiO2 for photocatalytic decomposition of organic matrix simultaneously with magnetic solid phase extraction of heavy metals prior to ICP-MS analysis[END_REF] and non-metals such as Zeolite, and Carbon and Montmorillonite (MMT) [START_REF] Yin | Effect of MMT content on structure of polyimide/(TiO2+MMT) nanocomposite films[END_REF][START_REF] Hassani | Heterogeneous photocatalytic ozonation of ciprofloxacin using synthesized titanium dioxide nanoparticles on a montmorillonite support: parametric studies, mechanistic analysis and intermediates identification[END_REF]. MMT is purified from bentonite and available in large reserves from natural mineral sources [START_REF] Thuc | Separation and purification of montmorillonite and polyethylene oxide modified montmorillonite from Vietnamese bentonites[END_REF].

Specially, it combines a low cost with interesting properties due to its layered structure, large surface area, cation exchange ability and high heat resistance. It is considered as potential material to be combined with TiO2 and TiO2-NTs to create novel materials, which could be used to degrade and store pollutants and toxic agents in wastewater [START_REF] Hassani | Sonocatalytic degradation of ciprofloxacin using synthesized TiO2 nanoparticles on montmorillonite[END_REF]Szczepanik, 2017a;[START_REF] Shan | Photocatalytic degradation of methyl orange by Ag/TiO2/biochar composite catalysts in aqueous solutions[END_REF]. In the literature, a study has successfully synthesized MMT/TiO2 from Tetraethyl orthotitanate for Ciprofloxacin (CIP) treatment.

The results showed that the CIP degradable efficiency of MMT/TiO2 nanocomposites reached up to 65.01% after 120 min adsorption time, which was much higher than that of MMT or TiO2 only [START_REF] Hassani | Sonocatalytic degradation of ciprofloxacin using synthesized TiO2 nanoparticles on montmorillonite[END_REF]. The photocatalytic effect of TiO2/MMT doping with Y 3+ (Yttrium) into TiO2 for Methyl Orange (MO) treatment was reported [START_REF] Bing | Photocatalytic Degradation of Methyl Orange Over Y3+ Doped TiO2 Pillared Montmorillonite[END_REF]. Thus, Y 3+ reduces the band gap to increase the photocatalytic capacity of TiO2, leading to increase the efficiency of MO degradation. In addition, MMT/TiO2 nanocomposite was produced by hydrothermal method to obtain the TiO2 particles range about 40-60 nm to treat Basic Blue 3 (BB3) [START_REF] Khataee | Sonocatalytic removal of an organic dye using TiO2/Montmorillonite nanocomposite[END_REF]. The results showed that the photocatalytic performance of MMT/TiO2 was much higher than that of MMT and TiO2 under different pH and BB3 concentration. In addition, it was reported that rhodamine B was degraded by various photocatalysts, which was clearly described in previous literature [START_REF] Natarajan | Study on UV-LED/TiO2 process for degradation of Rhodamine B dye[END_REF][START_REF] Shi | Highly efficient visible light-driven Ag/AgBr/ZnO composite photocatalyst for degrading Rhodamine B[END_REF][START_REF] Wang | Graphdiyne-modified TiO2 nanofibers with osteoinductive and enhanced photocatalytic antibacterial activities to prevent implant infection[END_REF]. The degradation process could be explained by the fact that the active radicals such as hydroxyl free radicals (•OH), and free superoxide radicals (•O2 -), attack the central carbon in the rhodamine B molecules and oxidize them to low-weight intermediates. In the opening-ring process, the active radicals attack the simple intermediates that are produced, which results in the formation of small and broken-ring compounds. The smaller compounds are degraded to CO2 and H2O.

In this paper, we will synthesis of MMT/TiO2 and MMT/TiO2-NTs photocatalysts with a low-cost hydrothermal method. These combinations could maximize the photocatalytic capabilities under dark and UV-C irradiation conditions for rhodamine B degradation at different concentrations from dyeing wastewater compared to TiO2 and TiO2-TNs only, because they are likely to slowdown of electron and holes pairs recombination.

Moreover, the reusability of MMT/TiO2 and MMT/TiO2-NTs for the rhodamine B degradation was repeated three time under the same conditions to evaluate the stability of the photocatalytic activities.

Materials and experiment

Materials

Rhodamine B was purchased from Sigma-Aldrich in highest purity. Montmorillonite (purity > 95%) was purified from bentonite in Lam Dong Province, Vietnam. Acid nitric, Sodium hydroxide, Ethanol, Sodium Chloride were purchased from Merck (Germany).

MMT preparation

200 g Bentonite and 5L distilled water were stirred for 24 hours and settled for 5 hours. 1 g of NaCl was added and mechanical stirred for 3 hours and settled for 24 hours. The suspension obtained was centrifuged for 30 min with a speed of 3500 rpm. The collected solids were washed with distilled water for 4 hours and settled for 8 hours. Ethanol (30%) was added to the above solution and stirred for 30 min before being settled for 24 hours.

Then, the suspension was centrifuged for 30 min at 3500 rpm to obtain high purity MMT.

Finally, MMT was dried at 60 °C for 72 hours. The purification of MMT from Bentonite could be referenced in previous studies [START_REF] Thuc | Separation and purification of montmorillonite and polyethylene oxide modified montmorillonite from Vietnamese bentonites[END_REF][START_REF] Nguyen | The starch modified montmorillonite for the removal of Pb(II), Cd(II) and Ni(II) ions from aqueous solutions[END_REF].

Preparation of TiO2-NTs

TiO2-NTs were synthesized by hydrothermal method from TiO2 powder with particle size in the range of 50-60 nm. 4.2 g of TiO2 powder was added slowly into 10M NaOH solution with TiO2/NaOH molar ratio of 1/30 and stirred for 4 hours at 50 o C. The suspension solution obtained was poured into a Teflon flask, which was then inserted into a stainless-steel jar and sealed. Hydrothermal process was then performed at 130°C for 24 hours in an autoclave before cooling to the room temperature. The product after hydrothermal process was filtered and washed with deionized water to reach a pH = 9.

The solid product was then dispersed in deionized water and adjusted to pH = 7 with 2M HNO3 solution and stirring for 2 hours before centrifuging to obtain solid product. Once again, the product was dispersed into deionized water at 80 o C and stirred to remove the salt formed during the above reaction. The solid product was then obtained by centrifugation for 15 min and dried at 60°C for 24 hours. Finally, the product was heated at 400 o C with a heating rate of 5 o /min and maintained at the final temperature for 2 hours to obtain TiO2-TNs [START_REF] Wong | A review on the formation of titania nanotube photocatalysts by hydrothermal treatment[END_REF][START_REF] Farghali | Hydrothermally synthesized TiO2 nanotubes and nanosheets for photocatalytic degradation of color yellow sunset[END_REF].

Preparation of MMT/TiO2 and MMT/TiO2-TNs photocatalysts

2.00 g of MMT was slowly added to 100 mL of deionized water and mechanical stirred 500rpm for 24 hours to allow MMT to expand completely. Then, 0.206 g TiO2 was added slowly to the above MMT suspension and stirred 500 rpm for 4 hours. This suspension solution was then treated by ultrasound (UPH100) at 100 W and frequency 30 kHz for 1 hour . The product was obtained through centrifugation process with a rotating speed of 3500 rpm for 15 min. Finally, the product was dried at 80°C in an oven for 24 hours to obtain MMT/TiO2 photocatalyst. MMT/TiO2-NTs photocatalyst was synthesized with similarly process and proportion for MMT/TiO2 photocatalysts. The contents of MMT/TiO2-NTs and MMT/TiO2 were calculated based on the cation exchange capacity of montmorillonite clay (CECMMT = 86 mmol/100g).

Photocatalysts characterizations

The characteristic vibration of functional groups in photocatalysts was characterized using a Fourier transform infrared (FT-IR) spectrometer Nicolet iS 50 (FT-IR, Thermo, USA) in the range 4000 to 400 cm -1 with a resolution of 2 cm -1 . The structure of TiO2, TiO2-NTs, MMT and nanocomposites was evaluated by X-ray diffraction using D2-PHASER diffractometer (Bruker, Germany) with a Ni-filtered CuK radiation (λ=1.54 Å) at 40 kV, 40 mA, scan rate 0.030 o /sec, and scan angle 2θ = 5-80 o . The SEM technique was used to evaluate the surface morphology of materials using a S4800 (Hitachi co., Japan). Samples were fixed on an aluminum stub for scanning electronic microscope using a carbon double-side tape. The following conditions were applied to obtain the images: secondary electron detector, high vacuum mode, an accelerating voltage of 10 kV, a spot size of 3 and a work distance of approximate 9 mm. The non-conductive samples were first coated with gold before being scanned. Finally, the structure of TiO2-NTs and the state of TiO2-TNs intercalation into MMT sheets in MMT/TiO2-TNs photocatalyst are observed by JEM-1400 Transmission Electron Microscope (TEM) equipment (JEOL, Japan) with an acceleration voltage 120kV. Energy-dispersive X-ray (EDX) spectroscopy (HORIBA H-7593) was used to detect and quantify the metallic elemental compositions in materials. The X-ray peak intensity is proportional to the percentage of elements present in the test sample.

Evaluation of the photocatalytic activity of materials

The adsorption ability and degradation of rhodamine B from dyeing wastewater of nanocomposites was evaluated in dark condition and irradiation condition by UV-C lamp, respectively. In the dark condition, 10 mg of each photocatalyst was slowly added to 100 mL of rhodamine B solution at an initial concentration of 10 ppm (Co) and stirred for 60 min. Then, every 30 min, 5 mL of the solution was extracted and centrifuged at 3500 rpm for 15 min to separate the material from the dye solution. The portion of the obtained solution was used to measure UV -Vis and calculate the decomposition efficiency.

For experiment with UV-C irradiation, a light source at a wavelength of 254 nm with 15W was used. Similar to the dark condition, 10 mg of test materials were added to 100 mL of rhodamine B solution. The solution was first stirred in the dark condition for 60 min. Then, UV-C lamp (UVC 16W -Philip) was used to light the solution. Every 30 min, 5 ml of solution was extracted and centrifuged at 3500 rpm for 15 min to eliminate the solid to remove the solid. The obtained solutions in dark and UV-C irradiation conditions were then measured with UV-Vis spectrometer (Jasco V-670) with a range of 400 to 700 nm, step size of 0.5 nm, and scanning speed of 400 nm/min.

Results and discussion

SEM and TEM images analysis of photocatalysts

The mechanism of TiO2-NTs formation is assumed that under hydrothermal condition, the Ti-O-Na and Ti-OH bonds are gradually formed under the influence of high concentrated NaOH solution. Then, the high surface energies of Ti-O-Na and Ti-OH lead to the coiling and formation of TiO2-TNs. The Ti-O-Ti bonds are created after water removal and heated for 2 hours at 400°C [START_REF] Cui | Facile microwave-assisted hydrothermal synthesis of TiO2 nanotubes[END_REF]. TEM image of TiO2-NTs is presented in Figure 1a.

The TiO2-TNs are formed from different directions with relatively uniform size as an average tube diameter of about 5 nm and tube length at least about 110 nm. The SEM analysis of pure MMT, MMT/TiO2 and MMT/TiO2-NTs photocatalysts is displayed in Figure 1 (b-d), respectively. As showed in Figure 1b, the MMT is stacked by sheets with different sizes and shapes. Figure 1c clearly displays the morphology of MMT immobilized TiO2 particles. The TiO2 particles are evenly dispersed on the surface of MMT sheets with nanosized range from 30 to 45 nm. This result is suitable with previous observations [START_REF] Yin | Effect of MMT content on structure of polyimide/(TiO2+MMT) nanocomposite films[END_REF][START_REF] Miao | Synthesis and characterization of TiO2-montmorillonite nanocomposites and their application for removal of methylene blue[END_REF][START_REF] Mishra | Microwave Treated Bentonite Clay Based TiO2 Composites: An Efficient Photocatalyst for Rapid Degradation of Methylene Blue[END_REF]. However, the dispersion and morphology of MMT/TiO2-NTs is much differential to MMT/TiO2, where TiO2-NTs are intercalated and dispersed randomly between MMT sheets to produce heterogeneous structure. The nanotubes morphology can be observed as presented in Figure 1d. In addition, the clearly structure and morphology of MMT/TiO2-NTs photocatalyst is described through the TEM analysis in Figure 2. The XRD pattern of MMT/TiO2-NTs nanocomposite shows also the existence in three main phases as MMT/TiO2 nanocomposite. However, the peak A (101) has a low intensity compared to that of TiO2 sample, corresponding to a lower crystallization. This

shows that during the hydrothermal process at 130 °C, the crystal structure of TiO2-NTs has been changed to reduce the anatase phase in TiO2. Moreover, the d001 spacing of MMT was reduced to 9.06 Å corresponding to a larger angle (2θ) 9.2° compared to pure MMT and MMT/TiO2 nanocomposite. This could be explained by the TiO2-TNs having entered the layer structure of MMT, leading to decrease the d001 spacing of MMT.

FTIR spectroscopy analysis of photocatalyst

Characterization of variations on functional groups and chemical bonds of TiO2, TiO2-NTs, pure MMT and their nanocomposites were evaluated through FTIR spectrum as shown in [START_REF] Khataee | Sonocatalytic removal of an organic dye using TiO2/Montmorillonite nanocomposite[END_REF]. TiO2-NTs has also a peak at wavenumber of 1641 cm -1 could be attributed to interlayer H-O-H molecules [START_REF] Wang | Photocatalytic degradation of 2,4dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO2 composite in aqueous solution under solar irradiation[END_REF]. A wide-bulb peak at wavenumber of 3390 cm -1 characterizes for the stretching vibration of -OH group on surface of TiO2-TNs.

The pure MMT has absorbance peaks at wavenumbers 3388 cm -1 and 3621 cm -1 that characterize the symmetrical stretching vibration of the -OH group of H2O molecules absorbed in MMT. A vibration at wavenumber of 1639 cm -1 characterizes for deformation of interlayer H-O-H molecules. A strong peak at wavenumber of 987 cm -1 characterizes asymmetrical stretching vibration of -Si-O group. A peak at wavenumber of 789 cm -1 characteristics of Al-O stretching vibration, a peak at wavenumber of 524 cm -1 represents the Al-O-Si deformation vibration and a peak at wavenumber of 462 cm -1 characteristics of Si-O-Si deformation [START_REF] Liu | Preparation and photocatalytic activity of silver and TiO2 nanoparticles/montmorillonite composites[END_REF][START_REF] Aber | Optimization of activated carbon fiber preparation from Kenaf using K2HPO4 as chemical activator for adsorption of phenolic compounds[END_REF][START_REF] Khataee | Sonocatalytic removal of an organic dye using TiO2/Montmorillonite nanocomposite[END_REF].

In comparison to FTIR spectra of pure TiO2, TiO2-NTs, MMT and their nanocomposites, it seems that component peaks are present in the FTIR spectra of nanocomposites. It is also confirmed that the wide absorbance band of 462 -789 cm -1 correspond to the adsorption of Ti-O. However, they did not show the stretching vibration of Si-O-Ti at wavenumber around 935 cm -1 [START_REF] Iwasaki | Preparation of high homogeneity BaO-TiO2-SiO2 gel[END_REF] as expected or this peak is overlapped with the strong peak of MMT or this might be due to the small proportion of TiO2 in MMT. Similarly, it is recorded about 85% (C/C0 = 0.15) compared to 73% (C/C0 = 0.27) in dark condition for MMT/TiO2-NTs photocatalyst. Thus, the results showed that under UV-C irradiation, the photocatalytic effective of TiO2 nanoparticles is increased through more production of free radicals ( • OH), leading to capturing and degrading of rhodamine B molecules. In addition, efficiency of MMT/TiO2-NTs photocatalyst is lower than that of MMT/TiO2. This is due to TiO2-NTs in tubular form, which was intertwined in layer structure of MMT sheets. This limits photon energy absorption of TiO2-TNs, resulting in the less •OH free radical production, resulting in lower rhodamine B degradation efficiency. The reusability of MMT/TiO2 and MMT/TiO2-NTs for the rhodamine B degradation was performed by repeating the reaction for three cycles using the recovered photocatalyst under the same conditions, as shown in Figure 8. After three consecutive runs, the photocatalytic activity of MMT/TiO2 was decreased by 18% and the photocatalytic activity of MMT/TiO2-NTs was decreased by 23.5%. This might be ascribed to the accumulation of photodegradation products and rhodamine B molecules on the catalyst surface which resulted in decreasing the active sites [START_REF] Pannak | Homogeneous distribution of nanosized ZnO in montmorillonite clay sheets for the photocatalytic enhancement in degradation of Rhodamine B[END_REF].

The mechanism of rhodamine B degradation of photocatalyst

The mechanism of rhodamine B degradation by photocatalysts is proposed in Figure 10.

The TiO2 components of MMT/TiO2 act as photocatalyst. Initially, the UV-C irradiation source produced high-energy photons activates to create photoinduced electrons (e -) and leave holes (h + ) of the TiO2. The negatively charged electrons are excited to cross the band gap (Eg) and jump into the conduction band (CB) leaving holes with positively charged charges in the valence region (VB). In the presence of MMT, the generated electrons move to the empty d-orbital of the metals in MMT structure. This electron transfer process can prevent the e --h + pair from recombination. Then, electrons trapped in MMT react with O2 to produce free superoxide radicals ( • O2 -) [START_REF] Xu | Construction of hierarchical 2D/2D Ti3C2/MoS2 nanocomposites for high-efficiency solar steam generation[END_REF]. At the same time, photogenerated holes react with H2O or the adsorbed hydroxyl ions (OH -) molecules to produce hydroxyl radicals ( • OH) [START_REF] Sun | The fabrication of 1D/2D CdS nanorod@Ti3C2 MXene composites for good photocatalytic activity of hydrogen generation and ammonia synthesis[END_REF]. Finally, the produced free radicals ( • OH, • O2 -, h+) with strong oxidizing ability will degrade rhodamine B molecules into degradation products [START_REF] Wang | Graphdiyne-modified TiO2 nanofibers with osteoinductive and enhanced photocatalytic antibacterial activities to prevent implant infection[END_REF][START_REF] Xiao | A new synthesizing method of TiO2 with montmorillonite: Effective photoelectron transfer to degrade Rhodamine B[END_REF]. Thus, the free radicals (•OH and •O2 -) attack the central carbon in the rhodamine B molecules and oxidize them to low-weight intermediates, which results in the formation of small and broken-ring compounds. In addition, the smaller compounds are degraded to CO2 and H2O [START_REF] Isari | Photocatalytic degradation of rhodamine B and real textile wastewater using Fe-doped TiO2 anchored on reduced graphene oxide (Fe-TiO2/rGO): Characterization and feasibility, mechanism and pathway studies[END_REF]. All steps of photocatalytic process to degrade rhodamine B are summarized in following equations (1-5):

TiO2 + hv TiO2 (e -+ h + ) (1)

TiO2 (e -+ h + ) + MMT TiO2 (h + ) + MMT (e -) (2)

TiO2 (h + ) + OH -TiO2 + • OH (3) MMT (e -) + O2 MMT + • O2 - (4) •O2 -+ H2O • OH + H + (5) h + + OH -• OH (6)
• OH/ • O2 -/h + radicals + rhodamine B degradation products (7) In addition, under the UVC irradiation, TiO2-NTs exhibits a higher photocatalytic capacity than TiO2. However, the decomposition efficiency of MMT/TiO2-NTs is lower than that of MMT/TiO2. This suggests that the addition of TiO2-NTs into MMT leads to decrease the photocatalytic activity of TiO2-NTs. This is due to the MMT has layer structure, when TiO2-NTs are intercalated into MMT layers, the inner surface is not adequately illuminated by UV-C irradiation as described in Figure 12. Therefore, the number of absorbed photons of TiO2-NTs inside is less than the outer surface, affecting the chemical transformation of the photocatalytic process. Therefore, the photocatalytic ability of TiO2-NTs is limited in incorporation with MMT sheets although they have better photocatalytic effectively than TiO2 nanoparticles.

Conclusions

In this study, bentonite mineral is purified to produce MMT clay that has good property of organic dye absorption but no photocatalytic activity. In addition, random-orientation TiO2-NTs were successfully synthesized by a low-cost hydrothermal method. Assynthesized TiO2-NTs were demonstrated to present much more efficient photodegradation of rhodamine B dye in comparison with commercial TiO2 nanoparticle photocatalyst.

However, the efficiency of rhodamine B degradation under UVC light of MMT/TiO2 is higher than that of MMT/TiO2-NTs. This is due to the immobilization of TiO2 Declarations of interest: none.

Figure 1 .Figure 2 :

 12 Figure 1. The TEM image of TiO2-NTs structure synthesized through hydrothermal method (a); and the SEM analysis of pure MMT (b), MMT/TiO2 (c) and (d) MMT/TiO2-NTs. This Figure highlights that TiO2-NTs and MMT sheets are intercalated in MMT/TiO2-NTs photocatalyst compared to MMT sheets immobilized TiO2 particles in the MMT/TiO2 photocatalyst. The MMT sheets are presented as the red-edged black blocks are surrounded by TiO2-NTs. Moreover, the TEM images also presents the interface between MMT sheets and TiO2-NTs as well as interface between TiO2-NTs, which also allow to evaluate the connection, alignment and orientation of the TiO2-NTs in the MMT/TiO2-NTs photocatalyst.

Figure 3 .

 3 Figure 3. XRD diffraction patterns of different samples: (a) MMT; (b) nanocomposite MMT/TiO2; (c) nanocomposite MMT/TiO2-NTs; and (d) nano TiO2

Figure 4 .

 4 Figure 4. TiO2 nanoparticles has only one peak at wavenumbers of 430 cm -1 , corresponding to stretching vibration of Ti-O, meanwhile TiO2-NTs have adsorption band at 460-908 cm -1 corresponding to stretching vibration of Ti-O band[START_REF] Pang | Effect of carbon and nitrogen co-doping on characteristics and sonocatalytic activity of TiO2 nanotubes catalyst for degradation of Rhodamine B in water[END_REF][START_REF] Khataee | Sonocatalytic removal of an organic dye using TiO2/Montmorillonite nanocomposite[END_REF]. TiO2-NTs has also a peak at wavenumber of 1641 cm -1 could

Figure 4 .

 4 Figure 4. FT-IR spectra of different samples: (a) MMT, (b) nanocomposite MMT/TiO2, (c) nanocomposite MMT/TiO2-NTs, (d) nano TiO2 and (e) TiO2-NTs

Figure 5 :

 5 Figure 5: The EXD spectra of MMT, MMT/TiO2, and MMT/TiO2-TNs photocatalysts

Figure 6 :

 6 Figure 6: UV-Vis adsorption spectra of Rhodamine B: (a) 10 ppm rhodamine B with different photocatalysts under dark condition and (b) 10 ppm rhodamine B with different photocatalysts under UV-C irradiation condition after 210 min. Accordingly, the photocatalytic activities and efficiencies of photocatalysts are calculated and exhibited in Figure 7. The photocatalytic activities in rhodamine B degradation of

Figure 7 :

 7 Figure 7: Photocatalytic degradation of rhodamine B with 10 ppm using different photocatalyst: (a) in the dark condition, (b) in the UV-C irradiation condition.In addition, the photocatalytic performance of MMT/TiO2 and MMT/TiO2-NTs to rhodamine B degradation is compared to that of the TiO2 based photocatalyst in the literature as exhibited in Table1. Thus, the photocatalytic performance of MMT/TiO2 and MMT/TiO2-NTs have shown a superior efficiency in rhodamine B degradation compared to other TiO2 based photocatalysts in the literature. Basically, the efficiency of rhodamine B degradation is dependent on many factors such as light source, rhodamine B concentration, and catalyst amount, etc. Indeed, these results suggest that the combination

Figure 8 :

 8 Figure 8: Photocatalytic activity of (a) MMT/TiO2 and (b) MMT/TiO2-NTs photocatalyst for three consecutive runs with10 ppm rhodamine B.

Figure 9 .

 9 Figure 9. Degradation effectiveness of MMT/TiO2 and MMT/TiO2-NTs photocatalysts at different concentrations of rhodamine B under UV-C irradiation.

Figure 10 :

 10 Figure 10: The photocatalytic mechanism of MMT/TiO2 (left) and MMT/TiO2-NTs (right) photocatalysts for rhodamine B degradation.

Figure 11 .

 11 Figure 11. LC-MS spectrum of degradation products with Rh-B for at 30 min during photocatalytic process

Figure 12 :

 12 Figure 12: The illustration of photocatalytic activities of MMT/TiO2 and MMT/TiO2-NTs for rhodamine B degradation under UV-C irradiation.

  nanoparticles at the surface of MMT sheets, so they are available for UVC light absorption. Besides, the effectiveness of TiO2-NTs is lost in the presence of MMT because TiO2-NTs intercalate into the MMT layers, leading to the inhibition of UVC light absorption. Moreover, MMT/TiO2-NTs nanocomposite still shows a better rhodamine B removal effectiveness in the initial period of 1 hour under UVC light irradiation. As a result, the combination of MMT and TiO2-NTs can be applied for water treatment in short time. The photocatalytic performance of MMT/TiO2 reached to 100% for 3ppm and 90% at 10ppm of rhodamine B, while these values are 97.5% and 85.5%, respectively, recorded for MMT/TiO2-NTs.

Table 1 .

 1 Photocatalytic performance of TiO2 and TiO2-NTs based photocatalyst to

	rhodamine B degradation compared to previous studies
	Catalyst	Light source	Concentration	Catalyst	Degradation	Exposed	Ref
			(mg/L)	of	amount	(%)	time
			rhodamine B	(mg)		(min)
	TiO2	UV-C lamp	10		10	20.0	210	this
								study
	TiO2-NTs	UV-C lamp	10		10	65.3	210	this
								study
	MMT/TiO2	UV-C lamp	10		10	90.0	210	this
								study
	MMT/TiO2	UV-C lamp	3		10	100	210	This
								study
	MMT/TiO2-NTs	UV-C lamp	10		10	85.5	210	this
								study
	MMT/TiO2-NTs	UV-C lamp	3		10	97.5	210	this
								study
	10%MWCNT/TNT HPMV lamp	50		50	89	60	(Natar
								ajan et
								al.,
								2017)
	V2O5/TiO2	Visible	10		1.6 g/L	70	240	(Wang
								et al.,
								2012)
	BiVO4/TiO2	Visible	1		-	85	120	(Hu et

Acknowledgments:

This research was supported by Vietnam National University Ho Chi Minh City (VNU-HCM) [grant number C2021]. The authors would like to thank Departement de la Marne, Greater Reims, Region Grand Est and European Union with European Regional Development Fund (ERDF Champagne Ardenne 2014-2020) for their financial support to the Chair of Biotechnology of CentraleSupélec.