Preparation and Photocatalytic Performance of Ti3C2/TiO2/CuO Ternary Nanocomposites

Ti 3 C 2 /TiO 2 /CuO nanocomposites were synthesized via the decomposition of amixture of Ti 3 C 2 (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 Ti 3 C 2 , anatase-TiO 2 , and CuO. Scanning electron microscopy images show that CuO and TiO 2 nanoparticles were evenly distributed on the surface of Ti 3 C 2 . The particles size increased with an increase in the cupric nitrate content. Photocatalytic degradation of methyl orange shows that the Ti 3 C 2 /TiO 2 /CuO nanocomposite has good photocatalytic degradation efficiency. A possible photocatalytic mechanism of the Ti 3 C 2 /TiO 2 /CuO nanocomposites was proposed. The data indicated that CuO and Ti 3 C 2 effectively promote the separation of photoelectrons from vacancies.


Introduction
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.TiO 2 and ZnO are the most widely studied photocatalysts [1,2].To improve the photocatalysis efficiency of TiO 2 , binary nanocomposites, such as TiO 2 /metal (Ag [3,4], Au [5,6], and Pt [7]), TiO 2 /graphene [8][9][10], TiO 2 /ZnO [11], TiO 2 /CuO [12], TiO 2 /(CdSe, CdS, and CdSeS) [13,14], and TiO 2 /SnO 2 [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 [17] than on binary nanocomposites.Ti 3 C 2 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 [18].Because of its special laminated structure, Ti 3 C 2 has been widely researched in many fields [19][20][21][22][23].However, the resistance to oxidization of Ti 3 C 2 is not good.Under hydrothermal conditions or in an oxygen atmosphere, the surface of Ti 3 C 2 easily oxidizes to form TiO 2 at temperatures above 100 ∘ C [18,24,25].Hence, Ti 3 C 2 can be used as a precursor for TiO 2 and as a carrier for other oxides.
In this study, Ti 3 C 2 /TiO 2 /CuO ternary nanocomposites were successfully fabricated using cupric nitrate decomposition with Ti 3 C 2 under an argon atmosphere.The photocatalytic performance of Ti 3 C 2 /TiO 2 /CuO was studied using MO as a simulated pollutant.The results show that the Ti 3 C 2 /TiO 2 /CuO ternary composite has good catalytic activity for MO photodegradation.

Experimental
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 Ti 3 C 2 powder (prepared according to the literature [18]) was added.The mixture was kept at room temperature for 24 h and then dried at 70 ∘ C in a vacuum.The Ti 3 C 2 /TiO 2 /CuO nanocomposites were synthesized by annealing the mixture under an argon atmosphere at 500 The detailed procedure is represented schematically in Figure 1.In this process, the chemical reaction for the formation of Ti 3 C 2 /TiO 2 /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 K 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 mm 2 detector energy dispersive Xray spectrometer (Bruker Quantax 200 XFlash 6 | 30 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).

Results and Discussion
XRD results of the samples are shown in Figure 2. The peaks detected at 2 = 9.02 ∘ , 18.2 ∘ , and 27.5 ∘ were assigned as the diffraction peaks of Ti 3 C 2 [20].The peaks at 25.34 ∘ , 37.90 ∘ , 48.17 Intensity oxygen came from the adsorbed water and the thermaldecomposition 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 TiO 2 and Ti 3 C 2 .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 Ti 3 C 2 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 TiO 2 .Figures 3(b)-3(e) show the SEM image of the Ti 3 C 2 /TiO 2 /CuO ternary nanocomposites.From these figures, it can be seen that a large number of CuO and TiO 2 nanoparticles were densely connected to one another on the surface of the Ti 3 C 2 nanosheets.With an increase in the cupric nitrate content, the density of the particulates on Ti 3 C 2 increased.The increase of the size of the nanoparticles (see Figures 3(d  1.8 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 Ti 3 C 2 .
The photocatalytic activities of the samples were measured, and the results are shown in Figure 4.It can be seen that the Ti 3 C 2 /TiO 2 /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 Ti 3 C 2 /TiO 2 / CuO nanocomposites is shown in Figure 5.The band gap between the TiO 2 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 TiO 2 and CuO.Because the CB of CuO is situated below that of TiO 2 , the electrons in the CB of TiO 2 are easily transferred to the CB of CuO.On the other hand, the photoelectrons of TiO 2 and CuO are transferred promptly by Ti 3 C 2 because of its good electron conductivity which lowers the electron-hole recombination rate.Analysis indicates that Ti 3 C 2 /TiO 2 /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.nanocomposite has a better efficiency of electron-hole separation than TiO 2 , and this significantly improved the photocatalytic activity of the nanocomposites.

Figure 2 :
Figure 2: XRD patterns of the as-prepared samples.
) and 3(e)) indicates an agglomeration of nanoparticles.It can be also observed that the thickness of

Ti 3 C 2 /Figure 4 :Figure 5 :
Figure 4: Curves of / 0 versus time for the photodegradation of MO with samples a-e.