One-Step Preparation of High Performance TiO₂/CNT/CQD Nanocomposites Bactericidal Coating with Ultrasonic Radiation

Abstract

As an environmental semiconductor material, TiO₂ has important applications in the fields of environmental protection and water treatment. The preparation of P25 particles into nano-functional material films with a high specific surface area has always been a bottleneck limiting its large-scale application. In this paper, a one-step method of preparing TiO₂ nanocomposites by doping carbon nanotube (CNT) and carbon quantum dots (CQD) with tetrabutyltitanate and P25 TiO₂ under ultrasonic radiation is proposed to synthesize a novel antifouling material, which both eliminates the bacterium of Escherichia coli and shows good photoelectric properties, indicating a great value for the industrial promotion of TiO₂/CNT. This mesoporous composite exhibits a high specific surface area of 78.07 M²/g (BET) and a tested pore width range within 10–120 nm. The surface morphology of this composite is characterized by TEM and the microstructure is characterized through XRD. This preparation method can fabricate P25 particles into a nano-functional material film with a high specific surface area at a very low cost.

1. Introduction

Photosynthesis in nature achieves a highly efficient solar energy conversion mainly through combining several different molecules to arrange the nanoscale, which indicates the importance of a technique for the site-selective coupling of different materials to realize artificially efficient devices [1,2,3]. TiO₂ is a kind of safe, stable, and cheap material, which has attracted significant research interest [4,5,6,7,8,9]. Photo-catalytic reactions on the semi-conductive surfaces of TiO₂ materials are widely investigated and reported for the purposes of (1) water splitting to produce H₂ and (2) eliminating pollutants from water and air [10,11,12,13,14,15]. Obviously, the direct splitting of water molecules through Photo-electrochemical (PEC) processes is a sustainable green approach to convert water and sunlight to hydrogen and oxygen to achieve [16,17]. Much effort has been made in the selection of efficient stable semiconductor materials to build PEC cells, thus bringing silicon (Si), III–V compounds, and various oxides into sight [18,19,20,21,22,23]. In the first-row of transition metal oxides, TiO₂ is a poor water oxidation catalyst with a large over potential for the sluggish kinetics and oxygen evolution reaction [14]. Hence, it is essential to develop a TiO₂-based visible light responsive photo-catalyst, and much work has been done to develop “second-generation” TiO₂ and other narrow band gap semiconductors to absorb visible light [17,24,25,26,27]. Though nano-sized particles could be prepared by the sol-gel method for the film’s preparation, the size of the particles is difficult to control [28,29,30,31,32]. The introduction of the graphene/graphene derivatives (including graphene oxide (GO) and other forms of functionalized graphene), the 3D single atom thick carbon layer, as the framework of the membrane, enables the morphology of the nanostructured materials to be extended from 1D to 0D (nanoparticles). The range of the nanostructured materials chosen is extended from only the photocatalytic materials (TiO₂, ZnO, CoS, CdS, CdSe, MoO_x, etc.) to other functional materials (MnO₂ for oxidation, Ag for disinfection, Pt for catalysis, etc.). Applying the 3D graphene-based nanocomposite multicomposite membrane with an expected thickness < 100 nm for the desalination applications will potentially create a new and promising industrial sector, and these applications are still undeveloped.

It is known that TiO₂ is a wide bandgap semiconductor, with a low-to-moderate response rate to sunlight, and research has focused on reducing the absorption edge energy to improve the utilization of sunlight [33,34]. Moreover, the large specific area of mesoporous TiO₂ nanoparticles is another favorable factor for generating large quantities of active sites for a photocatalytic reaction, which has taken place on the surface of the photocatalyst [35]. Additionally, the preparation of P25 particles into nano-functional material films with a high specific surface area has always been a bottleneck limiting its large-scale application.

In this work, sol was prepared with tetrabutyl titanate; commercial P25 TiO₂ was added to ensure the uniformity of the nanoparticles; and CNT and CQD were doped to improve the light absorptivity of the semi-conductive materials. CQD also contributed to the gel’s formation.

2. Experimental Section

2.1. Synthesis

Ti(O-i-C₄H₉)₄ (TB, Aladdin, AR) (≥98% Ti) and ethanol absolute (EtOH, Aladdin, Nanjing, China, AR) were blended with a 5% (wt.) multi-wall carbon nanotube (Nano-Com Shanghai, China,) average particle size 10–20 nm) to make a mixture. Hydrochloride acid was added as a catalyst to enhance the hydrolysis rate. Commercial P25 TiO₂ (Innochem) particles with an average particle size of 25 nm were dispersed into the Ti-based sol at 0.2 g/mL in vigorous magnetic stirring for 30 min. GQD nanomaterials with a diameter of 15 nm were purchased from Aladdin (G196610-100 mL). GQDs were dispersed into the tetra-n-butyl orthotitanate at 80 mg/mL in vigorous magnetic stirring for more than 5 min. The mixture was then stirred via vigorous mechanical stirring and subsequently placed in an 80 °C water bath with an ultrasonic (Dakou, Kongshan, China, KQ-300VDE, 45 kHz) for 5 h. After that, the sonicated mixture was dried overnight at a temperature of 80 °C and then calcined in an Ar atmosphere at 450 °C for 6 h before the grounding procedure to obtain an anatase phase. The schematic of the formation of the mesoporous TiO₂/CNT/CQD nanocomposites membrane is shown in Scheme 1.

2.2. Characterization

Various characterizing techniques, including a transmission electron microscope (FEI Talos-F200S TEM), pore structure determination (Brunauer–Emmett–Teller, BET method), and X-ray diffraction (XRD), were applied to analyze the material. A transmission electron microscope (JEOL, JEOL-2100F, Tokyo, Japan) was used to characterize the microstructure. UV–vis DRS was performed on a spectrophotometer (Lambda750, PerkinElmer, Inc., Waltham, MA, USA). The absorbance was tested using a UV–vis spectro-photometer (722, Weimipai Technology Co., Ltd., Hangzhou, China). The light absorption properties were measured using a Fourier Infrared Instrument (PerkinElmer Spectrum, Akron, OH, USA). The absorbance properties were measured using the Zolix SS150 (Zolix Instruments Co., Ltd., Beijing, China). The light intensity and photoluminescence spectrum’s structure were determined using a fluorescence spectrometer (F-4500, Hitachi, Ibaraki, Japan). The photo-electrochemical properties were measured on an electrochemical workstation (Chenhua, CHI660E, Shanghai, China).

2.3. Antibacterial Tests

To test the materials with the bacterium of Escherichia coli, typically, the sample (10 mg) was added to 10 mL of pure water to make a TiO₂ solution (solution T) with TiO₂ suspended within. To remove the microorganisms from the system, the whole set was sterilized in an automatic high-temperature sterilization pot. Then, using a conventional bacterial solution with its original concentrate of 1.15 g/L, a 0.01% diluted solution was prepared and 100 μL of this solution was added into the TiO₂ solution (solution T) as a source of Escherichia coli. The mixture was irritated under 34.4 Klux solar light for 1 h. After lighting, the antibacterial solution was coated on the prepared solid medium (10 g of pancreatic protein, 5 g of yeast powder, 10 g of NaCl, and 20 g of agar diluted with 1 L of pure water) and was tested after 8 h for its anti-bacterial performance.

3. Results and Discussion

Figure 1 presents the results of the BET test of the TiO₂/CNT/CQD nanocomposites. The surface area of TiO₂/CNT/CQD is as high as 78.07 m²/g and the pore width range is 10–120 nm, indicating that an ultrasonic radiation treatment is a useful technique to prepare mesoporous materials [36,37,38,39]. Bai has reported that the surface area of the TiO₂/CNT mesoporous composite material is 42.90 m²/g [40]. Therefore, the doping of CQD is beneficial to increase the surface of the TiO₂/CNT/CQD nanocomposites. It attributes to the small particle size.

Figure 2 shows the energy disperse spectroscopy (EDS) mapping results of the TiO₂/CNT/CQD mesoporous nanocomposites prepared under ultrasonic radiation. Small TiO₂ particles appear to cluster around the carbon nanotube, indicating that the nanotube is a sufficient support material for a better TiO₂/CNT dispersion in this specific preparation. Although the composite interface between TiO₂ and the carbon nanotube, which appears to be a loose one, needs to be further improved, the doping of TiO₂ with CNT and CQD obviously improves the dispersion of TiO₂ as well as promotes the formation of hetero junctions at the TiO₂-CNT interface, which favors the improvement of the absorption efficiency of visible light [41,42].

The TEM results of TiO₂/CNT/CQD prepared with the ultrasonic treatment are shown in Figure 3. Figure 3a depicts the agglomeration of small particles in the TiO₂/CNT/CQD mesoporous nanocomposites. In Figure 3b, the interface between the two main phases, namely the TiO₂ and the CNT, has been remarkably enhanced through a high temperature. The high-resolution diagrams, Figure 3c,d, correspond to anatase TiO₂ [43,44], which clearly represent the relevant information at the atomic scale. Figure 3e,f are high-resolution TEM diagrams of CQD. From Figure 4, it can be seen that the morphology and spacing between the crystal planes are CQD, and CQD is successfully doped into P25 and CNT by sol-gel, but there is only a small amount of reunion.

The FTIR spectra have indicated that the characteristic bands at 3400 cm⁻¹ and 1630 cm⁻¹ correspond to the surface water and hydroxyl group [45]. Figure 4 shows the XRD results of the TiO₂/CNT/CQD mesoporous composite. As shown in Figure 4, it can be found that the TiO₂/CNT/CQD mesoporous composite is an obvious anatase phase and C phase, which is conducive to improving the photoelectric properties of the material. TiO₂ nanoparticles have attracted great interest because of their special physical and chemical properties, especially as photocatalytic oxidation catalysts in corresponding device materials and environmental pollution control [46].

Marine fouling is a critical issue in modern marine science and technology, which strictly determines marine transportation and farming. The existing antifouling approaches, such as antifouling hydrogel coatings and organic tin coating [47,48,49], always demonstrate an unsatisfactory performance. In our system, the material shows the CV curves and anti-bacterial properties of the control, TiO₂/CNT and TiO₂/CNT/CQD, using Escherichia coli bacteria (Figure 5). Comparing the bacteriostasis processes uses TiO₂/CNT (Figure 5c) and TiO₂/CNT/CQD (Figure 5d) as anti-bacterium agents, respectively. In this experiment, obviously, the number of Escherichia coli bacterial colonies in Petri dishes all increased with time. The sample (c) with TiO₂/CNT/CQD as an anti-bacterium agent got the bacterium of Escherichia coli under control, while the sample using TiO₂ as an anti-bacterium agent just grew more bacterial colonies, indicating that doping TiO₂ with CNT and CQD contributes to boosting the TiO₂s performance in an environmental pollution control.

4. Conclusions

The main conclusions were as follows:

(1)

The composites prepared exhibit a large specific surface area of 78.07 m²/g and a pore width of 10–120 nm, indicating that an ultrasonic radiation treatment contributes to forming nanocomposites with a high specific surface area.

(2)

Commercial P25 particles can be prepared into gel films at a low cost by sol-gel ultrasonic radiation.

(3)

TiO₂/CNT/CQD nanocomposites prepared by doping CNT/CQD can significantly improve the visible light absorption efficiency and bactericidal efficiency.