Ternary BiOBr/TiO₂/Ti₃C₂T_x MXene nanocomposites with heterojunction structure and improved photocatalysis performance

With the growing problems of energy crisis and environmental pollution, solar energy utilization and conversion is crucial in the sustainable development of human society. Compared to the conventional treatment methods, photocatalytic oxidation technology using eco-friendly semiconductors has been regarded as a promisingway to alleviate theworsening situationin environmental and energy, especially for treating pollutants in water [1-3]. Due to its natural abundance, non-toxicity, excellent chemical stability and photocatalytic activity, TiO₂ has been widely used as a semiconductor photocatalyst for organic pollutant degradation. However, the intrinsic wide band gap (~3.2 eV for anatase) leads to its activation only by ultraviolet light (λ < 400 nm), which greatly limits the practical applications of pure TiO₂ [4]. Therefore, many strategies have been developed to extend the light absorption range of TiO₂ into visible region, such as coupling TiO₂ with narrow bad gap semiconductors to form heterostructure. BiOBr is a good candidate for TiO₂ to form heterojunctions due to its suitable band gap (~2.7 eV) and chemical stability under visible light irradiation [5-7]. However, the general issue of rapid photo-generated electron-hole recombination still limits the further enhancement of photoactivity for BiOBr.

MXenes, an emerging two-dimensional materials family of early transition metal carbides and carbonitrides, have drawn widespread attention in the field of energy storage, environment remediation and biomedicine. Among the big MXene family, Ti₃C₂ MXene possesses the most in-depth etching chemistry and detailed theoretical research. Typically, the obtained multilayer Ti₃C₂ MXene is terminated by -OH, —O and/or —F functional groups (marked as T_x) and the formula is written as Ti₃C₂T_x [8-10]. Owing to its high electrical conductivity, hydrophilicity and layered structure, Ti₃C₂T_x is a promising cocatalyst in photocatalysis. Furthermore, Ti₃C₂T_x can be used as an ideal precursor for the preparation of TiO₂/Ti₃C₂T_x hybrid materials attributed to the uniformly dispersed titanium atoms [11-14]. But the multilayer morphology of Ti₃C₂T_x affects the absorption of visible light and hinders the light capture by the photocatalyst, resulting in limited utilization of light and low photocatalytic activity [15]. Therefore, it is significant to reduce the thickness of Ti₃C₂T_x and avoid preventing the light absorption of TiO₂.

Herein, 2D titanium carbide (Ti₃C₂T_x) nanosheets were firstly obtained by chemical exfoliation of commercially available Ti₃AlC₂ with lithium fluoride and hydrochloric acid. A series of ternary BiOBr/TiO₂/Ti₃C₂T_x nanocomposites were successfully prepared by a facile one-step hydrothermal procedure (presented in the supporting information), which were recorded as BTM-x (x is the theoretical mass ratio of Ti₃C₂T_x to BiOBr). For comparison, TiO₂/Ti₃C₂T_x hybrids were synthesized via an in-situ oxidation of the delaminated Ti₃C₂T_x nanosheets and pure BiOBr was also obtained via the similar hydrothermal process. Detailed synthesis procedure, characterization and photocatalytic evaluation were given in the supplementary information. The morphology, crystal structure, optical property and visible-light photocatalytic activity of the as-synthesized composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), highresolution transmission electron microscopy (HRTEM), UV–vis diffuse reflectance spectra (UV–vis DRS), photoluminescence spectra (PL), respectively. Moreover, a possible photocatalytic mechanism of BiOBr/TiO₂/Ti₃C₂T_x composites was proposed.

The X-ray diffraction patterns of pristine Ti₃AlC₂, Ti₃C₂T_x, TiO₂/Ti₃C₂T_x, BiOBr and BiOBr/TiO₂/Ti₃C₂T_x composites were recorded in the 2θ range of 5°~90° as illustrated in Fig. 1. After etching with lithium fluoride and hydrochloric acid, the characteristic diffraction peaks of Ti₃AlC₂ disappear. Meanwhile, the characteristic peaks according to (002) crystal plane is broadened and shifted from 9.8° to 6.2°, suggesting the successful exfoliation of Al atoms from the interlayers and the formation of Ti₃C₂T_x MXene [16]. After in situ oxidation via hydrothermal process, several new diffraction peaks at 2θ = 25.4°, 37.9°, 48.2°, 54.1°, 55.2°, 62.9° emerge in TiO₂/ Ti₃C₂T_x, which are in good accordance with the (101), (004), (200), (105), (211) and (204) planes of the anatase phase of TiO₂ (JCPDS No. 73-1764), respectively. Besides, the diffraction peak ascribed to the (002) crystal plane of Ti₃C₂T_x is still observable but presents relatively lower intensity than pristine Ti₃C₂T_x, which demonstrates the residue of Ti₃C₂T_x MXene and part transformation to TiO₂ [17, 18]. As shown in Fig. 1, typical peaks of the pure BiOBr were indexed as the tetragonal BiOBr (JCPDS No. 09-0393) without any crystalline impurity. For the BiOBr/TiO₂/Ti₃C₂T_x composites, it can be observed that the co-exist of the diffraction peaks of BiOBr and TiO₂ in all the composites [19-21]. However, it is hard to observe any peaks of Ti₃C₂T_x, which may be explained by the relatively low content of Ti₃C₂T_x.

Figure 1

Figure 1.  XRD patterns of Ti₃AlC₂, Ti₃C₂T_x, TiO₂/Ti₃C₂T_x, BiOBr and BiOBr/TiO₂/Ti₃C₂T_x composites.

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The morphology of the Ti₃C₂T_x, TiO₂/Ti₃C₂T_x, BiOBr and BTM-20% composites were characterized by SEM and HRTEM. Fig. 2a reveals the typical lamellar morphology of Ti₃C₂T_x MXene, confirming the Ti₃AlC₂ was successfully exfoliated to form Ti₃C₂T_x nanosheets. After the hydrothermal treatment, plenty of TiO₂ nanoparticles anchor on the Ti₃C₂T_x sheets (Fig. 2b). As demonstrated in Figs. 2c and d, the smooth nanoplates of BiOBr are evolved into rough and covered with numerous TiO₂ nanoparticles on the surface of the BiOBr. The lamellar structureof BiOBr is retained during its coupling with TiO₂/Ti₃C₂T_x, which is consistent with the XRD results. HRTEM images of BTM-20% composites in Figs. 3a and b further show the distribution of TiO₂ and BiOBr in the hybrid composite₂. Combining with the HRTEM images and EDS elemental mappings of BiOBr/TiO₂/Ti₃C₂T_x composites (Fig. S1 in Supporting information), it can be confirmed that the TiO₂ particles accompanied with residual Ti₃C₂T_x lamella are distributed over the BiOBr lamella uniformly. As exhibition in Figs. 3c and d, the corresponding lattice fringe value of 0.35 nm and 0.28 nm coincides well with the (101) anatase crystal plane of TiO₂ and the (110) plane of BiOBr, respectively [22]. Combining with XRD results, it proves that there have realized ideal interfacial contact and construction of heterojunction between TiO₂ and BiOBr.

Figure 2

Figure 2.  SEM images of (a) Ti₃C₂T_x (b) TiO₂/Ti₃C₂T_x (c) BiOBr and (d) BTM-20%.

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Figure 3

Figure 3.  (a, b) TEM images of BTM-20 %; (c, d) HRTEM of BTM-20%.

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The UV–vis diffuse reflectance spectra (UV–vis DRS) and photoluminescence (PL) spectra were performed to analyze the optical properties of the as-synthesized nanocomposite. As shown in Fig. 4a, pure BiOBr exhibits a strong light response in ultraviolet region and its absorption edge for visible light is around 460 nm which corresponds to a band gap of 2.7 eV [23]. Meanwhile, TiO₂/Ti₃C₂T_x shows higher visible absorption due to the essentially dark Ti₃C₂T_x. Compared with BiOBr, the UV–vis absorption edges of BiOBr/TiO₂/Ti₃C₂T_x composites occur an obvious red shift and show enhanced absorption for visible light due to the synergistic effect between BiOBr and TiO₂. This means BiOBr/TiO₂/Ti₃C₂T_x composites would be more effectively utilize solar energy than pure BiOBr, which is beneficial for improving photocatalytic efficiency.

Figure 4

Figure 4.  (a) UV–vis DRS and (b) PL spectra of BiOBr, TiO₂/Ti₃C₂T_x and BiOBr/TiO₂/Ti₃C₂T_x composites. (c) Visible-light photocatalytic degradation of RhB solution over BiOBr, TiO₂/Ti₃C₂T_x and BiOBr/TiO₂/Ti₃C₂T_x composites. (d) The proposed charge separation and transfer pathways in the BiOBr/TiO₂/Ti₃C₂T_x system under visible-light irradiation.

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To analyze the recombination details of photo-generated electrons and holes, room temperature PL spectra were carried out with an excitation wavelength of 325 nm. Fig. 4b shows the PL spectra of BiOBr, TiO₂/Ti₃C₂T_x and BiOBr/TiO₂/Ti₃C₂T_x composites. There is no doubt that TiO₂/Ti₃C₂T_x and BiOBr/TiO₂/Ti₃C₂T_x composites display much lower PL emission intensities than BiOBr, indicating their recombination of photo-generated electron-hole pairs is effectively inhibited [24]. It is rationally attributed by the remarkable conductivity of Ti₃C₂T_x and band structure matching between BiOBr and TiO₂.

As shown in Fig. 4c, the photocatalytic activities of the asprepared samples were evaluated by the degradation of rhodamine B solution(RhB, 100 mg/L)under visible light irradiation(λ≥420 nm). For eliminating the effect of absorption, all samples experienced a control test to get adsorption/desorption equilibrium in a dark condition before irradiation with visible light. It is obvious that TiO₂/Ti₃C₂T_x hybrids show barely photocatalytic activity for RhB under visible light irradiationwithin 30 min. Moreover, BiOBr/TiO₂/Ti₃C₂T_x nanocomposites present higher photocatalytic activity than pure BiOBr and TiO₂/Ti₃C₂T_x, especially BTM-20% possesses 99.8% degradation rate for RhB within 30 min. Besides, the x value has a conspicuous effect on the photocatalytic activity and there is an optimal value. This is mainly due to the synergistic effects in the BiOBr/TiO₂/Ti₃C₂T_x nanocomposites and ternary components play different roles in photocatalytic process. The recycling stability of BTM-20% during degradation of rhodamine B is presented in Fig. S3 (Supporting information). After five consecutive cycles, BTM-20% could keep 95.6% degradation rate, which indicates its good recyclability of the photocatalyst.

Based on the above results and discussion, a possible mechanism to explain the outstanding photocatalytic activity of BiOBr/TiO₂/Ti₃C₂T_x nanocomposites is elucidated. On the one hand, the absorption edges of BiOBr/TiO₂/Ti₃C₂T_x nanocomposites further extend to visible-light region and their absorption intensities enhance compared with BiOBr, which facilitates to harvest solar energy. On the other hand, the ternary heterojunction structure may generate a remarkable synergistic effect, leading to the efficient separation and transfer of photo-generated electrons and holes. As illustrated in Fig. 4d, the band gap of TiO₂ (~3.2 eV) is wider than that of BiOBr (~2.7 eV) and the corresponding conductive band (CB) and valence band (VB) positions of TiO₂ are both more negative than those of BiOBr. The intimate contact between BiOBr and TiO₂ may construct a heterojunction. As mentioned, TiO₂ shows only little visible light response due to its larger band gap, BiOBr can be excited easily by visible light and produce photo-induced electron-hole pairs. The construction of heterojunction between BiOBr and TiO₂ makes it rational that the photo-generated holes will rapidly migrate from the VB of BiOBr to VB of TiO₂ and the excited electrons will also transfer from the CB of TiO₂ to BiOBr under visible-light irradiation, leading to an efficient separation and transfer of photo-generated electrons and holes [25]. What is more, the remaining Ti₃C₂T_x in the ternary BiOBr/TiO₂/Ti₃C₂T_x hybrids can serve as an efficient electron trap due to its intrinsic conductivity to further inhibit the photoinduced charge recombination [26]. The photo-generated electrons on the CB of BiOBr and TiO₂ will fast transfer to Ti₃C₂T_x. The holes accumulated in the valence band of TiO₂ possess outstanding ability to directly oxidize the pollutants adsorbed on the photocatalyst surface. Meanwhile, abundant holes can also react with OH^- groups and H₂O molecules to produce ^•OH radicals. Superoxide radical anions (^•O₂^-) may also emerge from the reaction of the electrons in the CB of BiOBr with oxygen molecules absorbed on the surface of materials or dissolved in water [27-30]. Subsequently, both ^•OH and ^•O₂^- could act as active species to effectively degrade RhB.

In this work, a series of ternary BiOBr/TiO₂/Ti₃C₂T_x composites with heterojunction structure have been designed and constructed by a facile one-step hydrothermal process. The as-prepared ternary composites show higher visible-light photocatalytic activities for the degradation of RhB than pure BiOBr and TiO₂/ Ti₃C₂T_x due to the optimized synergetic effects of BiOBr, TiO₂ and Ti₃C₂T_x. It is found that there is a contact interface of heterojunction between BiOBr nanoplates and TiO₂ nanoparticles. The unique heterojunction structure not only enhance the absorption for visible light, but also promote the transfer of photo-excited electrons and holes due to the well-matching of energy band position between BiOBr and TiO₂. The introduction of Ti₃C₂T_x serves as an efficient electron trap and accelerates the migration and transfer of photo-excited electrons. This work may provide reference of utilizing MXene as novel co-catalytic material to achieve highly efficient, steady and cost-effective semiconductor photocatalysts.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 51472186, 51902232, 51402221) and the China Scholarship Council Fund (No. 201708420210).

Appendix A. Supplementary data

Supplementary material related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.11.038.

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