Preparation and Photocatalytic Performance of Ti3C2/TiO2/CuO Ternary Nanocomposites
Ti3C2/TiO2/CuO nanocomposites were synthesized via the decomposition of a mixture of Ti3C2 (a novel two-dimensional carbide) and cupric nitrate under an argon atmosphere. The morphology and structures of the obtained samples were characterized. X-ray diffraction and energy dispersive spectrometer analysis indicate that the sample is composed of Ti3C2, anatase-TiO2, and CuO. Scanning electron microscopy images show that CuO and TiO2 nanoparticles were evenly distributed on the surface of Ti3C2. The particles size increased with an increase in the cupric nitrate content. Photocatalytic degradation of methyl orange shows that the Ti3C2/TiO2/CuO nanocomposite has good photocatalytic degradation efficiency. A possible photocatalytic mechanism of the Ti3C2/TiO2/CuO nanocomposites was proposed. The data indicated that CuO and Ti3C2 effectively promote the separation of photoelectrons from vacancies.
Industrial wastewater has become one of the major factors in water pollution, and in particular, wastewater from dye industries is difficult to treat. Of the various techniques for cleaning the environment, photocatalytic degradation has been widely researched to treat pollutants that are difficult to degrade. TiO2 and ZnO are the most widely studied photocatalysts [1, 2]. To improve the photocatalysis efficiency of TiO2, binary nanocomposites, such as TiO2/metal (Ag [3, 4], Au [5, 6], and Pt ), TiO2/graphene [8–10], TiO2/ZnO , TiO2/CuO , TiO2/(CdSe, CdS, and CdSeS) [13, 14], and TiO2/SnO2 [15, 16], have been extensively studied, and the results show that the binary nanocomposites perform better than the single components. Less research has been done on ternary nanocomposites  than on binary nanocomposites. Ti3C2 is a member of a family of novel two-dimensional carbides or carbonitrides called MXenes, which are synthesized by etching an A-group element from the corresponding MAX phase using hydrofluoric acid . Because of its special laminated structure, Ti3C2 has been widely researched in many fields [19–23]. However, the resistance to oxidization of Ti3C2 is not good. Under hydrothermal conditions or in an oxygen atmosphere, the surface of Ti3C2 easily oxidizes to form TiO2 at temperatures above 100°C [18, 24, 25]. Hence, Ti3C2 can be used as a precursor for TiO2 and as a carrier for other oxides.
In this study, Ti3C2/TiO2/CuO ternary nanocomposites were successfully fabricated using cupric nitrate decomposition with Ti3C2 under an argon atmosphere. The photocatalytic performance of Ti3C2/TiO2/CuO was studied using MO as a simulated pollutant. The results show that the Ti3C2/TiO2/CuO ternary composite has good catalytic activity for MO photodegradation.
In these experiments, 0 g, 0.005 g, 0.01 g, 0.02 g, and 0.04 g of cupric nitrate were each dissolved in 1 mL of deionized water, and then 1 g of Ti3C2 powder (prepared according to the literature ) was added. The mixture was kept at room temperature for 24 h and then dried at 70°C in a vacuum. The Ti3C2/TiO2/CuO nanocomposites were synthesized by annealing the mixture under an argon atmosphere at 500°C for 30 min. The heating rate was set at 10°C/min. The obtained samples were denoted as sample a, sample b, sample c, sample d, and sample e according to cupric nitrate masses, respectively.
The detailed procedure is represented schematically in Figure 1. In this process, the chemical reaction for the formation of Ti3C2/TiO2/CuO may be expressed as follows:
The crystal structure of the obtained samples was characterized using powder X-ray diffraction (XRD, Philips X’Pert-MPD system, Cu Kα radiation). The morphology and microstructures of the samples were examined using a Quanta 250 field emission gun scanning electron microscope (FEI Quanta 250 FEG-SEM) equipped with a Bruker Nano GmbH with an X-Max 30 mm2 detector energy dispersive X-ray spectrometer (Bruker Quantax 200 XFlash EDS).
The photocatalytic activity of the samples was evaluated for the photodegradation of MO (20 mg/L) under UV illumination. First, the obtained powder catalyst (100 mg) was dispersed in a solution of MO (100 mL) and was magnetically stirred in the dark for 30 min to establish the adsorption equilibrium. The whole photocatalytic reaction was carried out at 10°C. The solution was then irradiated with a 175 W mercury lamp light (GGZ175, Shanghai Jiguang Special Lighting Electrical Appliance Factory, China). 5.0 mL of solution was transferred every 10 min, centrifuged to remove the catalyst, and then analyzed using a UV-vis spectrophotometer (TU-1900, Beijing Purkinje General Instrument Co. Ltd., China).
3. Results and Discussion
XRD results of the samples are shown in Figure 2. The peaks detected at 2θ = 9.02°, 18.2°, and 27.5° were assigned as the diffraction peaks of Ti3C2 . The peaks at 25.34°, 37.90°, 48.17°, 54.02°, and 55.16° were indexed to the (101), (004), (200), (105), and (211) planes, respectively, of anatase-TiO2 (JCPDF Number 21-1272). This means Ti3C2 was partially oxidized to TiO2 after treatment at 500°C for 30 min. The oxygen came from the adsorbed water and the thermal-decomposition of cupric nitrate. Because of the low content and the small crystalline size of CuO in the nanocomposites, the XRD signal was weak and may have been covered by the robust signal of TiO2 and Ti3C2. Thus, the diffraction peaks of CuO were not observed.
SEM images of samples a–e are shown in Figures 3(a)–3(e). Figure 3(a) is the SEM image of the sample which was obtained by Ti3C2 that was kept at 500°C for 30 min under an argon atmosphere. It can be observed that the sample had a smooth surface with small nanoparticles, which may be TiO2. Figures 3(b)–3(e) show the SEM image of the Ti3C2/TiO2/CuO ternary nanocomposites. From these figures, it can be seen that a large number of CuO and TiO2 nanoparticles were densely connected to one another on the surface of the Ti3C2 nanosheets. With an increase in the cupric nitrate content, the density of the particulates on Ti3C2 increased. The increase of the size of the nanoparticles (see Figures 3(d) and 3(e)) indicates an agglomeration of nanoparticles. It can be also observed that the thickness of the layer increased. In order to observe the distribution of different elements, sample c was further identified using EDS mapping analysis of Ti, C, O, Cu, Al, and F in SEM from the selected areas in Figure 3(f). The results show that CuO was distributed evenly on the surface of Ti3C2.
The photocatalytic activities of the samples were measured, and the results are shown in Figure 4. It can be seen that the Ti3C2/TiO2/CuO nanocomposites have good photocatalytic activity. The MO quickly degraded to about 99% in 80 minutes with sample c as the catalyst. The photocatalytic activity of the nanocomposites was enhanced with an increase in the cupric nitrate content and was best when 0.01 g of cupric nitrate was added. After that, with an increase in the mass of cupric nitrate, agglomeration occurred and led to a decrease in the photocatalytic performance.
A possible photocatalytic mechanism of the Ti3C2/TiO2/CuO nanocomposites is shown in Figure 5. The band gap between the TiO2 valance band (VB) and the conduction band (CB) is 3.2 eV. Under UV radiation, an electron and hole are generated on the surface of TiO2 and CuO. Because the CB of CuO is situated below that of TiO2, the electrons in the CB of TiO2 are easily transferred to the CB of CuO. On the other hand, the photoelectrons of TiO2 and CuO are transferred promptly by Ti3C2 because of its good electron conductivity which lowers the electron-hole recombination rate. Analysis indicates that Ti3C2/TiO2/CuO exhibited a more effective electron-hole separation under UV radiation. Thus, the surface redox process by photogenerated electrons and holes occurs more easily, and, correspondingly, the photocatalytic activity of the samples significantly improved.
Ti3C2/TiO2/CuO ternary nanocomposites were fabricated via cupric nitrate decomposition on the surface of Ti3C2. Analysis indicates that the samples were composed of Ti3C2 nanosheets, TiO2, and CuO nanoparticles. The photocatalytic degradation experiments showed that the Ti3C2/TiO2/CuO nanocomposite has a better efficiency of electron-hole separation than TiO2, and this significantly improved the photocatalytic activity of the nanocomposites.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors gratefully acknowledge the National Nature Science Foundation of China (51205111, 51472075), the Plan for Scientific Innovation Talent of Henan Province (134100510008), the Program for Innovative Research Team of Henan Polytechnic University (T2013-4), and Key Project of Henan Educational Committee (15A440006).
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