Physicochemical and Antibacterial Evaluation of TiO2/CNT Mesoporous Nanomaterials Prepared by High-Pressure Hydrothermal Sol–Gel Method under an Ultrasonic Composite Environment

TiO2 has attracted significant research interest, principally due to its nontoxicity, high stability, and abundance. Carbon-doped TiO2 can improve light absorption efficiency. In order to prepare high-efficiency photocatalysts, carbon-doped composites were prepared by hydrothermal reaction in a high-pressure reactor, and then TiO2/CNT mesoporous composites were prepared by the sol–gel method in an ultrasonic environment. Characterized by SEM and TEM, the composite materials contained TiO2 nanoparticles as well as CNT. After phase analysis, it was the anatase-doped phase. The following infrared light absorption performance and Escherichia coli bactericidal performance tests showed that it had better infrared and visible light absorption performance than pure TiO2. The TiO2/CNT mesoporous nanomaterials synthesized in this work are possible for clean industrial productions.


Introduction
With the advent of the steam engine revolution, electrical revolution, and information technology revolution, human beings have advanced by leaps and bounds in intelligent manufacturing, life sciences, and information technology [1][2][3][4][5]. However, with the development of science and technology, human beings also consume a lot of natural resources. In particular, the depletion of fossil non-renewable energy and environmental pollution poses new challenges to the development of science and technology [6][7][8][9][10][11].
Global climate change has prompted researchers from across the world to work together to search for new technologies for producing clean and renewable energy [12][13][14][15]. Photocatalytic reactions on semiconductive TiO 2 are widely used and investigated for (1) water splitting in view of H 2 production and (2) the destruction of environmental pollutants from water and air [16]. Starting from a point of practical engineering application, energy-efficient photocatalysts are highly demanded to effectively utilize the visible light that constitutes 43% of the total sunlight [17]. Hence, it is important to develop a visible light-responsive photo-catalyst, and some efforts have been devoted to developing "secondgeneration" TiO 2 and other narrow band-gap semiconductors that can absorb visible light [18].
Numerous scientists have made gratifying achievements in the field of semiconductor materials. In particular, the large-scale and low-cost production of titanium dioxide P25 Molecules 2023, 28, 3190 2 of 9 has the advantage of integrating various excellent properties in terms of material activity, specific surface area, and photoelectric performance [19][20][21][22][23]. The discovery of graphene is a gratifying achievement for mankind. The excellent properties of graphene have attracted research by scientists in various fields around the world and have made considerable achievements. However, the manufacturing cost of graphene has always been the bottleneck limiting its development. Therefore, many scientists have conducted research on carbon materials. From diamond and graphite, to fullerenes, carbon nanotubes, etc., carbon materials are full of magic. The development of 3D carbon-based nanocomposite multifunctional membranes has attracted researchers all over the world. The understanding of cavity quantum effect diffusions, reaction/inter-reaction, surface-area-to-volume ratio, and reaction kinetics is still at a preliminary level. Although we are able to produce a 3D nanotube filtration membrane at a thickness of around 200 nm, the further reduction in the thickness of this membrane for the fabrication of the membrane still remains a challenge. Therefore, it is important to search for new materials to fabricate the membrane. It has been reported in the literature that the combination of carbon materials and titanium dioxide can prepare excellent nanocomposite functional materials and exhibit various excellent properties. Especially in the fields of energy and environment, human beings urgently need to find sustainable energy materials and environmental governance materials [18,[24][25][26]. However, the interfacial bonding performance of composite materials has always been a key factor affecting their development, and the composite interface of titanium dioxide and carbon materials in particular has always attracted the research of scientists.
In this paper, the preparation of titanium dioxide nanoparticles coupled with carbon nanotube composites by the high-pressure hydrothermal reaction was studied. Nanomesoporous thin-film materials with a high specific surface area were prepared by the sol-gel method in the environment of ultrasonic radiation. The photoelectric properties and bactericidal properties of the composites were tested. The interface structure of the material was characterized by STEM in order to clarify the interfacial recombination mechanism of titanium dioxide and carbon nanotubes.
Tetra-n-butylorthotitanate [Ti(O-i-C4H9) 4] (TB) and ethanol absolute (EtOH) were used to prepare the Ti-bases sol. The addition of HCl was added to facilitate the hydrolysis rate of the mixture. The mole ratio of the ingredients was optimized at TB/EtOH = 1:50 with respect to the size of the nanoparticle produced by the sol.
The carbon-doped composite was dispersed into the Ti-based sol at 0.2 g/mL under vigorous magnetic stirring and subsequently placed in the ultrasonic bath for 5 h. The temperature was maintained at 80 • C. The sonicated mixture was then dried overnight at 80 • C and then calcined in an argon furnace (450 • C, 6 h) for converting the amorphous TiO 2 into the anatase phase.
The schematic diagram for material preparation is shown in Scheme 1. Molecules 2023, 28, x FOR PEER REVIEW 3 of 10 Scheme 1. Schematic illustration of the formation of mesoporous TiO2/CNT nanomaterials membrane by sol-gel ultrasonic irradiation method.

Performance and Characterization
Escherichia coli was selected for bactericidal performance. An amount of 10 mg of the sample was added to 10 mL of deionized water (18.2 M Ohm) and sterilized in an automatic high-temperature sterilizer. After 1.15 g/L of the bacterial solution was diluted to 0.01%, 100 μL was added to TiO2 sterilization solution and irradiated under 34.4 klux sunlight for 1 h to prepare solid medium (trypsin 10 g, yeast powder 5 g, NaCl 10 g, agar 20 g and deionized water to prepare 1 L solution). Then, the sterilizing solution after illumination was applied to the medium, and the sterilization results were observed after 8 h.
A scanning electron microscope (SEM, JSM-7800F, JEOL, Tokyo, Japan) equipped with an electron backscattered diffraction system was employed for surface characterization. A transmission electron microscope (TEM, JEOL-2100F, Tokyo, Japan) was used for the structural analysis of TiO2. Phase analysis was carried out on X-ray diffraction (XRD, XPERT-PRO, Hawaii, HI, USA).

Results and Discussion
SEM images of the TiO2 and TiO2 + CNT mesoporous composite are presented in Figure 1. The TiO2 prepared by the sol-gel method under ultrasonic irradiation was a mesoporous material ( Figure 1a). Carbon nanotubes were rolled sheets of carbon atoms that form cylinders. After adding CNTs, TiO2 aggregated grow on the surface of carbon fibers and TiO2 nanoparticles were bonded around CNTs to form nanocomposites ( Figure  1b). Moreover, the CNTs were exposed, confirming their successful incorporation into the CNTs. According to the energy dispersive spectroscopy (EDS) mapping in Figure 2. The results confirmed the presence and distribution of C, O, and Ti. The aggregated particles consisted of TiO2, and the strip structures consisted of CNTs. The green (O)-and yellow (Ti)-colored particles represented the TiO2 nanoparticles; the red (C)-colored structure represented CNTs. TiO2 nanoparticles were uniformly distributed on carbon nanotubes, which indicated the bonding between TiO2 nanoparticles and CNTs. It can be concluded that the one-dimensional CNTs were interlaced to form nanoporous structures, and zero-Scheme 1. Schematic illustration of the formation of mesoporous TiO 2 /CNT nanomaterials membrane by sol-gel ultrasonic irradiation method.

Performance and Characterization
Escherichia coli was selected for bactericidal performance. An amount of 10 mg of the sample was added to 10 mL of deionized water (18.2 M Ohm) and sterilized in an automatic high-temperature sterilizer. After 1.15 g/L of the bacterial solution was diluted to 0.01%, 100 µL was added to TiO 2 sterilization solution and irradiated under 34.4 klux sunlight for 1 h to prepare solid medium (trypsin 10 g, yeast powder 5 g, NaCl 10 g, agar 20 g and deionized water to prepare 1 L solution). Then, the sterilizing solution after illumination was applied to the medium, and the sterilization results were observed after 8 h.
A scanning electron microscope (SEM, JSM-7800F, JEOL, Tokyo, Japan) equipped with an electron backscattered diffraction system was employed for surface characterization. A transmission electron microscope (TEM, JEOL-2100F, Tokyo, Japan) was used for the structural analysis of TiO 2 . Phase analysis was carried out on X-ray diffraction (XRD, XPERT-PRO, Hawaii, HI, USA).

Results and Discussion
SEM images of the TiO 2 and TiO 2 + CNT mesoporous composite are presented in Figure 1. The TiO 2 prepared by the sol-gel method under ultrasonic irradiation was a mesoporous material (Figure 1a). Carbon nanotubes were rolled sheets of carbon atoms that form cylinders. After adding CNTs, TiO 2 aggregated grow on the surface of carbon fibers and TiO 2 nanoparticles were bonded around CNTs to form nanocomposites (Figure 1b). Moreover, the CNTs were exposed, confirming their successful incorporation into the CNTs. According to the energy dispersive spectroscopy (EDS) mapping in Figure 2. The results confirmed the presence and distribution of C, O, and Ti. The aggregated particles consisted of TiO 2 , and the strip structures consisted of CNTs. The green (O)-and yellow (Ti)-colored particles represented the TiO 2 nanoparticles; the red (C)-colored structure represented CNTs. TiO 2 nanoparticles were uniformly distributed on carbon nanotubes, which indicated the bonding between TiO 2 nanoparticles and CNTs. It can be concluded that the one-dimensional CNTs were interlaced to form nanoporous structures, and zero- dimensional TiO 2 particles were adhered to one-dimensional CNTs through gelation and sintering to form mesoporous composites. The agglomeration of the mesoporous materials was reduced by the ultrasonic process.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 10 dimensional TiO2 particles were adhered to one-dimensional CNTs through gelation and sintering to form mesoporous composites. The agglomeration of the mesoporous materials was reduced by the ultrasonic process.  The microstructure of TiO2 on CNTs was examined by TEM, which revealed different crystal planes of the composite (Figure 3a). After magnifying the red-circle area in Figure  3a, more detailed information is presented in Figure 3b. The (101) interplanar spacing was dimensional TiO2 particles were adhered to one-dimensional CNTs through gelation and sintering to form mesoporous composites. The agglomeration of the mesoporous materials was reduced by the ultrasonic process.  The microstructure of TiO2 on CNTs was examined by TEM, which revealed different crystal planes of the composite (Figure 3a). After magnifying the red-circle area in Figure  3a, more detailed information is presented in Figure 3b. The (101) interplanar spacing was The microstructure of TiO 2 on CNTs was examined by TEM, which revealed different crystal planes of the composite (Figure 3a). After magnifying the red-circle area in Figure 3a, Molecules 2023, 28, 3190 5 of 9 more detailed information is presented in Figure 3b. The (101) interplanar spacing was 0.375 nm (Figure 3b), indicating that TiO 2 was an anatase phase [27,28], and the TiO 2 semiconductor composite on CNT improved the light absorption efficiency of the semiconductor composite [29].
Molecules 2023, 28, x FOR PEER REVIEW 5 of 10 0.375 nm (Figure 3b), indicating that TiO2 was an anatase phase [27,28], and the TiO2 semiconductor composite on CNT improved the light absorption efficiency of the semiconductor composite [29]. According to the XRD results of the TiO2/CNTs mesoporous composite (Figure 4a), the XRD PDF cards of TiO2 and C were added for comparison. The TiO2/CNT mesoporous composite was an obvious anatase phase and C phase, which was conducive to improving the photoelectric properties of the material [30][31][32]. This result corresponds with the observation in SEM and TEM. As shown in Figure 4b, the BET specific surface area of TiO2/CNTs materials was 47.9 cm 3 /g, indicating the mesoporous structure. The high specific surface area was due to less agglomeration. When the relative pressure P/P0 is between 0.4 and 1.0, there is a hysteretic loop due to mesoporous adsorption, but the latter part of the curve bulged again, which corresponds to the porous adsorption system. Therefore, the dispersibility of TiO2 on CNTs is good. FT-IR was employed to analyze the functional groups on the film. The FTIR spectra in Figure 5 showed characteristic bands at 3400 cm −1 and 1630 cm −1 correspond to the surface water and hydroxyl group [33,34]. TiO2 anatase has stable ultraviolet light absorption According to the XRD results of the TiO 2 /CNTs mesoporous composite (Figure 4a), the XRD PDF cards of TiO 2 and C were added for comparison. The TiO 2 /CNT mesoporous composite was an obvious anatase phase and C phase, which was conducive to improving the photoelectric properties of the material [30][31][32]. This result corresponds with the observation in SEM and TEM. As shown in Figure 4b, the BET specific surface area of TiO 2 /CNTs materials was 47.9 cm 3 /g, indicating the mesoporous structure. The high specific surface area was due to less agglomeration. When the relative pressure P/P 0 is between 0.4 and 1.0, there is a hysteretic loop due to mesoporous adsorption, but the latter part of the curve bulged again, which corresponds to the porous adsorption system. Therefore, the dispersibility of TiO 2 on CNTs is good.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 10 0.375 nm (Figure 3b), indicating that TiO2 was an anatase phase [27,28], and the TiO2 semiconductor composite on CNT improved the light absorption efficiency of the semiconductor composite [29]. According to the XRD results of the TiO2/CNTs mesoporous composite (Figure 4a), the XRD PDF cards of TiO2 and C were added for comparison. The TiO2/CNT mesoporous composite was an obvious anatase phase and C phase, which was conducive to improving the photoelectric properties of the material [30][31][32]. This result corresponds with the observation in SEM and TEM. As shown in Figure 4b, the BET specific surface area of TiO2/CNTs materials was 47.9 cm 3 /g, indicating the mesoporous structure. The high specific surface area was due to less agglomeration. When the relative pressure P/P0 is between 0.4 and 1.0, there is a hysteretic loop due to mesoporous adsorption, but the latter part of the curve bulged again, which corresponds to the porous adsorption system. Therefore, the dispersibility of TiO2 on CNTs is good. FT-IR was employed to analyze the functional groups on the film. The FTIR spectra in Figure 5 showed characteristic bands at 3400 cm −1 and 1630 cm −1 correspond to the surface water and hydroxyl group [33,34]. TiO2 anatase has stable ultraviolet light absorption FT-IR was employed to analyze the functional groups on the film. The FTIR spectra in Figure 5 showed characteristic bands at 3400 cm −1 and 1630 cm −1 correspond to the surface water and hydroxyl group [33,34]. TiO 2 anatase has stable ultraviolet light absorption performance, but it has low light absorption efficiency in visible light and infrared light [35][36][37]. In order to improve the light absorption efficiency, carbon-doped TiO 2 nanomaterials were prepared [38,39]. It can be seen from Figure 5 that the infrared absorption efficiency of TiO 2 /CNT mesoporous composite was higher than that of pure TiO 2 , especially in the 600-800 nm band. Therefore, the carbon-doped TiO 2 nanomaterials behaved with a better light adsorption efficiency than the sample without carbon-doped TiO 2 nanomaterials.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 10 performance, but it has low light absorption efficiency in visible light and infrared light [35][36][37]. In order to improve the light absorption efficiency, carbon-doped TiO2 nanomaterials were prepared [38,39]. It can be seen from Figure 5 that the infrared absorption efficiency of TiO2/CNT mesoporous composite was higher than that of pure TiO2, especially in the 600-800 nm band. Therefore, the carbon-doped TiO2 nanomaterials behaved with a better light adsorption efficiency than the sample without carbon-doped TiO2 nanomaterials. Biofouling including organic fouling on the conventional membrane surface is inevitable, which is caused by the nature of biological system, where microorganisms and bioparticles are the main components. The literature revealed that the structure/matrix of biofouling layer (biofilm) was highly related to the extracellular polymeric substances (EPS) compound. EPS is considered as the key component that determines the structural and functional integrity of microbial aggregates. It forms a three-dimensional, gel-like, highly hydrated and locally charged biofouling layer matrix, in which the microorganisms are more or less immobilized. In addition, EPS has also been reported to be the most significant foulant toward the conventional membrane fouling problems. Research has helped to address the problem, but it is still unclear what the major mechanism flaw is that causes biofouling, particularly on conventional membrane surfaces during the water treatment process. Currently, the industry practice to get rid of the biofilm is to take out the fouled membrane from the system and then soak it in chemicals such as sodium hy- Biofouling including organic fouling on the conventional membrane surface is inevitable, which is caused by the nature of biological system, where microorganisms and bioparticles are the main components. The literature revealed that the structure/matrix of biofouling layer (biofilm) was highly related to the extracellular polymeric substances (EPS) compound. EPS is considered as the key component that determines the structural and functional integrity of microbial aggregates. It forms a three-dimensional, gel-like, highly hydrated and locally charged biofouling layer matrix, in which the microorganisms are more or less immobilized. In addition, EPS has also been reported to be the most significant foulant toward the conventional membrane fouling problems. Research has helped to address the problem, but it is still unclear what the major mechanism flaw is that causes biofouling, particularly on conventional membrane surfaces during the water treatment process. Currently, the industry practice to get rid of the biofilm is to take out the fouled membrane from the system and then soak it in chemicals such as sodium hypochlorite (NaOCl) and citric acid. This process causes three major problems: the generation of waste water, which requires further treatment; the interruption of the membrane treatment system, which reduces the production rate significantly; the reduction in membrane lifespan, which in turn increases the total cost of clean water production. The sterilization experiment embodies the photoelectric effect charge separation experiment. In order to test the visible light absorption and bactericidal performance of TiO 2 /CNT mesoporous composite, a mixture of Escherichia coli and different composites was made [40,41]. The mixture was spread on the agar medium after simulating sunlight irradiation, and the bactericidal results are shown in Figure 6. Figure 6(1) shows the blank group, and Figure 6(2) shows the bactericidal effect of TiO 2 mesoporous material. TiO 2 mesoporous material had a certain bactericidal effect, but TiO 2 /CNT mesoporous nanomaterials (Figure 6(3)) had a stronger bactericidal effect. The main reason was that the absorption efficiency of visible light was improved after doping CNT, and the stable composite interface is prepared under a high-pressure hydrothermal environment. Therefore, according to the results of bactericidal experiments, the TiO 2 /CNT mesoporous nanomaterials behaved with an excellent bactericidal performance, which is possible for clean industrial productions. pochlorite (NaOCl) and citric acid. This process causes three major problems: the generation of waste water, which requires further treatment; the interruption of the membrane treatment system, which reduces the production rate significantly; the reduction in membrane lifespan, which in turn increases the total cost of clean water production. The sterilization experiment embodies the photoelectric effect charge separation experiment. In order to test the visible light absorption and bactericidal performance of TiO2/CNT mesoporous composite, a mixture of Escherichia coli and different composites was made [40,41]. The mixture was spread on the agar medium after simulating sunlight irradiation, and the bactericidal results are shown in Figure 6. Figure 6(1) shows the blank group, and Figure 6(2) shows the bactericidal effect of TiO2 mesoporous material. TiO2 mesoporous material had a certain bactericidal effect, but TiO2/CNT mesoporous nanomaterials (Figure 6(3)) had a stronger bactericidal effect. The main reason was that the absorption efficiency of visible light was improved after doping CNT, and the stable composite interface is prepared under a high-pressure hydrothermal environment. Therefore, according to the results of bactericidal experiments, the TiO2/CNT mesoporous nanomaterials behaved with an excellent bactericidal performance, which is possible for clean industrial productions.

Conclusions
Preparation and stabilization of TiO2/CNT nanocomposites at the composite interface by a high-pressure hydrothermal method. TiO2/CNT mesoporous composite was prepared by the sol-gel method under an ultrasonic environment. Characterized under SEM and TEM, the composite materials contained TiO2 nanoparticles as well as CNT. After phase analysis, it was the anatase-doped phase. Infrared absorption performance and Escherichia coli bactericidal performance tests under visible light showed that TiO2/CNT

Conclusions
Preparation and stabilization of TiO 2 /CNT nanocomposites at the composite interface by a high-pressure hydrothermal method. TiO 2 /CNT mesoporous composite was prepared by the sol-gel method under an ultrasonic environment. Characterized under SEM and TEM, the composite materials contained TiO 2 nanoparticles as well as CNT. After phase analysis, it was the anatase-doped phase. Infrared absorption performance and Escherichia coli bactericidal performance tests under visible light showed that TiO 2 /CNT mesoporous composite had better infrared and visible light absorption performance than pure TiO 2 .