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Enhanced photocatalytic performance of TiO2@C nanosheets derived from two-dimensional Ti2CTx
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Enhanced photocatalytic performance of TiO2@C nanosheets derived from two-dimensional Ti2CTx ⁎
Jingxiao Lia,b, Shun Wangc, Yulei Dua, , Wenhe Liaoa a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c School of Materials Science and Engineering, Henan University of Technology, Zhengzhou 450001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: MXene TiO2@C nanosheets High-energy ball milling Photocatalyst
In the present work, TiO2@C nanosheets are fabricated by high-energy ball milling of two-dimensional (2D) Ti2CTx. Here, Ti2CTx not only serves as titanium and carbon source, but also acts as structure-directing agent. It is found that TiO2 nanoparticles are uniformly and closely dispersed on the single or few-layered carbon sheets in the as-prepared TiO2@C nanosheets. Compared to titanium dioxide P25, the TiO2@C nanosheets exhibit enhancing photocatalytic degradation of methylene blue (MB) dye in aqueous solution under UV-irradiation. In addition, the TiO2@C nanosheets also possess high photocatalytic stability.
1. Introduction Titania-carbon (TiO2@C) composites are promising materials for photocatalytic applications, because carbon can effectively enhance the photocatalytic ability of TiO2 through decreasing recombination of photoexcited e-/h+ pairs [1–3]. To date, varieties of synthetic techniques including atomic layer deposition (ALD) [4], UV-assisted photoreduction [5,6], sol-gel [7], and solvothermal method [8,9] have been reported to prepare TiO2@C composites. High-temperature treatment is generally required in most of the aforementioned synthesis processes, which may result in aggregation of TiO2@C leading to decrease of the catalytic efficiency [10,11]. Recently, nanocrystalline TiO2 on thin flakes of disordered graphitic carbon has been fabricated through the oxidation of so-called “MXenes” (a new group of two-dimensional (2D) transition metal carbides and carbonitrides, exfoliated from their bulk counterparts MAX phases, where the M, A, and X represent early transition metal, A-group elements, and carbon/nitrogen, respectively) in air, CO2 or deionized water [12]. These synthesis methods of TiO2@C composites are facile and the specific surface area is enlarged owing to the growth of TiO2 nanoparticles onto every sheet. However, there still exist some problems with these synthesis methods. For example, the sizes and morphologies of TiO2 nanoparticles were difficult to be controlled when the Ti3C2Tx (T stands for terminated O and OH groups) powders were flash oxidized at 1150 °C for 30 s in air [12]. In addition, the Ti3C2Tx could not completely convert into TiO2 nanoparticles and carbon sheets by the isothermal treatment in pure CO2, or hydrothermal oxidation [12]. Li et al. also found at the temperature of 200 °C the
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Ti3C2Tx could be partially oxidized by oxygen [13]. Similar phenomenon was also found in heat treatment or post-etch annealing of Ti2CTx under different gas conditions [14,15]. In addition, Ahmed et al. showed that the Ti2CTx could be completely oxidized when it was immersed in hydrogen peroxide (H2O2) for 5 h at room temperature [16]. By means of in situ TEM analysis, Ghassemi et al. pointed out the formation of TiO2 nanoparticles during the flash oxidization of Ti3C2Tx is starting from top and bottom of the titanium layers [17]. As reported, the layers of the as-synthesized MXenes are usually bounded together, which hinders the oxidization process. In the present work, we introduce high-energy ball milling technique to promote the oxidization of Ti2CTx MXene and hence to produce TiO2@C composites. The schematic of high-energy ball milling of Ti2CTx is shown in Fig. 1. The mechanical forces during ball milling can easily break the van der Waals force between interlayers of stacked Ti2CTx, resulting in the formation of single or few layered Ti2CTx nanosheets. The decreased thickness makes the oxygen traveling more facilely through the Ti2CTx layers. In this work, TiO2@C nanosheets were successfully prepared by high-energy ball milling of Ti2CTx in air. The structural and photocatalytic properties of the as-prepared TiO2@C nanosheets were studied. 2. Experimental procedure Here, all chemicals (Ti powder, purity 99.9%; Al powder, purity 99.5%, Carbon powder, purity 99%; methyl blue (MB), purity 98.5%; and hydrofluoric acid (HF, 40%)) were bought from Alfa Aesar. The
Corresponding author. E-mail address: [email protected] (Y. Du).
https://doi.org/10.1016/j.ceramint.2018.01.139 Received 19 December 2017; Received in revised form 16 January 2018; Accepted 17 January 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.01.139
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60 min to analyze the concentration of MB with UV–vis absorption spectroscopy (Shimadzu UV2450). The cycling runs of the degradation of MB were conducted for three consecutive cycles. 10 mg of TiO2@C nanosheets were dispersed in 50 mL MB solution (20 mg L−1) in a 200 mL cylindrical quartz vessel. A 500 W Hg lamp was used as UV light source. The TiO2@C nanosheets were recycled by washing and centrifugation after every cycle. The concentration of MB was determined by measuring the absorption intensity at its maximum characteristic absorbance wavelength of 663 nm by using UV–vis spectrophotometer [18]. The photocatalytic performance was represented by degradation of the MB, which was defined as: Degradation efficiency = 1 − C / C0 = 1 − A / A0, where C0 (mg L−1) is the initial concentration of MB, C (mg L−1) is the MB concentration at time t (min), A0 is the UV–vis absorption of the original solution and A is the UV–vis absorption of degraded solution at time t (min).
Fig. 1. Schematic of high-energy ball milling of Ti2CTx to form TiO2@C.
preparation of Ti2CTx powders was illustrated in our previous work [15]. A planetary ball mill (QM-3SP2, Nanjing University Instrument Plant, China) with agate jars and agate balls was used. The internal volume of the milling agate jar is 250 mL. The agate balls of different sizes (Φ9.8 mm balls and Φ6.45 mm balls with a ratio of 24:31) were used in the present work. In the typical milling experiment, 500 mg Ti2CTx powders were put in the agate jar and then milled with agate balls (weight ratio of 1:55) in air atmosphere for 1.5 h with a speed of 200 r/min. The as-milled powders were cold-pressed under a pressure of about 1 GPa to form small pellets for further structural characterization. X-ray diffraction (XRD, Bruker-AXS D8 Advance, Cu-Kα radiation, λ = 1.54178 Å, operated at 40 mA and 40 kV), scanning electron microscope (Quant 250F), transmission electron microscope (JEM-200CX, operated at 200 kV), X-ray photoelectron spectroscopy (PHI 5000 VersaProbe, Al-Kα radiation) and Raman (JY HR800, λ = 514 nm) were employed to characterize the as-prepared samples. The photocatalytic properties of the as-prepared TiO2@C nanosheets were evaluated by the degradation of methyl blue (MB). In a typical run, a specified amount (10 mg) of TiO2@C nanosheets was suspended in 50 mL of methyl blue (20 mg L−1) aqueous solution in a cylindrical quartz vessel and magnetically stirred in the dark for 2 h to achieve the adsorption/desorption equilibrium. Then the mixture was irradiated by a 500 W Hg lamp under magnetically stirring. During the course of photocatalytic reaction, the temperature of the suspension was maintained at 25.0 °C. 3 mL of MB solution was taken out every
3. Results and discussion Fig. 2a and b shows SEM of Ti2CTx powders before and after highenergy ball milling, respectively. As can be seen from Fig. 2a, most of the as-synthesized Ti2CTx MXenes stack together though Al layers have been etched out. The high-energy ball milling destroys the multistacked structure, leading to the formation of free-standing nanosheets with much smaller surface area. Further phase identification is conducted by XRD and Raman spectra. It can be seen from XRD patterns (Fig. 2c) when the as-synthesized Ti2CTx MXenes are milled after 1.5 h, the typical wide low-angle MXene peaks disappear and some small peaks locating at the two theta values of 25.2°, 37.9°, 48.1° and 55.1° appear. According to JCPDS No. 21-1272, these peaks corresponds to d101, d004, d200 and d105 of the anatase phase respectively, indicative of the formation of titania. However, no obvious diffraction peaks of carbon can be observed. It is suspected that the carbon species are most likely poorly crystallized, similar to previous investigations of C-modified TiO2 composites [19,20]. From Fig. 2d, the Raman spectra of asmilled Ti2CTx shows six peaks locating at 144, 399, 519, 639, 1356 and 1593 cm−1. The peaks of 144, 399, 519 and 639 cm−1 are well matched with vibrational modes Eg(1), B1g(1), A1g & B1g(2) and Eg(3) of anatase, respectively [21,22]. The bands of 1356 and 1593 cm−1 Fig. 2. Characterization of Ti2CTx powders before and after high-energy ball milling for 1.5 h. a) SEM image of typical multilayers of Ti2CTx. b) SEM image of Ti2CTx after high-energy ball milling. c) XRD patterns of Ti2CTx before and after high-energy ball milling. The sign “#” stands for TiO2 and d) Raman spectra of Ti2CTx before and after high-energy ball milling. The sign “*” stands for TiO2.
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Fig. 3. a) TEM image of TiO2@C. b) HRTEM image of TiO2@C. Bottom right inset is SAED of this area. c) XPS patterns of Ti 2p. d) XPS patterns of C 1s. (e) XPS patterns of O 1s.
belong to the alleged D and G bands owing to sp2-bonded carbon atoms and presence of defects in the graphitic layers respectively, which directly confirms the formation of carbon. The FWHM of D peak is 85 cm−1 and the corresponding value of G peak is 125 cm−1. The ratio of the D and G band intensities, ID/ IG, is 1.15, a value higher than that has been reported for oxidation of MXenes at high temperature and carbide-derived carbons (CDCs) formed at 200 °C [12,23]. The ratio of D and G band intensities reveals the presence of highly disordered carbon during high-energy ball milling. More detailed structural studies of the as-prepared TiO2@C were performed by Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED). It is clear from Fig. 3a that the as-milled samples consist of monodispersed particles and show different morphological structures, with the size ranging from a few nanometers to tens of nanometers. Fig. 3b shows detailed structure including of a mixture of crystalline grains and amorphous regions. The lattice fringes in d-spacing were measured to be 0.351 nm, which is in agreement with the (101) plane of anatase TiO2 (JCPDS file No. 89-4921). From the observed SAED ring patterns, the lattice distance of the nanoparticles was calculated to be 0.356 nm. The presence of the amorphous carbon can be identified by the broad Debye ring. It can be seen that the TiO2 nanoparticles are dispersed uniformly and closely on amorphous carbon, leading to the formation of intimate contact between TiO2 and C. Notably, some surface defects also exist in TiO2 nanoparticles as indicated with dotted line circles. On one hand, a lot of similar defects in Ti2CTx are produced during exfoliation of Ti2AlC [15]. These defects are considered as nucleation sites for TiO2 growth when Ti2CTx is oxidized [16]. On the other hand, a part of electrons in the TiO2 crystal lattices could be excited by the milling mechanical forces, resulting in the original structures broken or cracked. The structural characteristics of the as-prepared TiO2@C composites are strongly related to the highenergy ball milling process. Firstly, the multi-layered stacked structure of Ti2CTx MXenes could be cracked into single or few-layered structure. Thus, it provides an easy way for oxygen to throughout the layered sample quickly. Secondly, the inner Ti atoms of the Ti2CTx structure
getting enough energy migrated outward to be oxidized into TiO2. The leaving C atoms at different locations with different energy could relocate, creating highly disordered graphitic carbon. Because there is no gradient of oxygen concentration, the TiO2 particles are extremely thin and dispersed uniformly on the disordered graphitic carbon. As can be seen from the Ti 2p XPS patterns (Fig. 3c), two peaks located at 459.9 eV (Ti 2p3/2) and 465.6 eV (Ti 2p1/2) with the spitorbital splitting of 5.7 eV were observed, indicating a normal state of Ti4+ in the as-milled sample [24,25]. The Ti 2p3/2 and Ti 2p1/2 peaks centered at 457.6 eV and 463.6 eV, respectively, are ascribed to Ti3+ [26,27]. Fig. 3d shows the C 1s spectra of as-milled sample. The peaks at 284.8 and 286.3 eV can be assigned to elemental carbon and C-O in epoxy groups or hydroxyl, proving the formation of carbon [28]. The peak at 282.3 eV, a position close to C 1s peak, usually can be considered as carbon replacing the oxygen atom in the structure of TiO2, indicative of the formation of the O–Ti–C bond [29,30]. The result provides direct evidence that chemical bonds exist between carbon and TiO2 in the as-prepared sample. In addition, the doping of C element could result in lattice distortion and generate oxygen vacancies in TiO2 [31]. The O 1s spectra of TiO2@C were observed in Fig. 3e. Besides the binding energy at 532 and 534.9 eV owing to absorbed oxygen and other functional groups, the peak at 530.6 eV can be assigned to Ti-O-Ti of TiO2 [32,33]. Methylene blue (MB) photocatalytic degradation experiment was performed to investigate the photocatalytic activity of the as-prepared TiO2@C nanosheets under UV light illumination (λ < 400 nm). As a reference, the photocatalytic performance of P25 was also measured under the same conditions. Fig. 4a presents changes in the relative concentrations of MB with and without irradiation. The adsorptiondesorption equilibrium of MB in the dark was established within 120 min. As can be seen from Fig. 4b, the adsorption capacity of TiO2@C is higher than that of P25. The degradation efficiency of MB is 85.7% and 68.5% for TiO2@C and P25, respectively, after 360 min of UV-illumination. Clearly, the enhanced photocatalytic activity of asprepared TiO2@C hybrid is 1.25 times higher than that of P25. The enhanced photocatalytic activity of the TiO2@C synthesized by high-
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Fig. 4. a) UV–vis adsorption spectrum of MB degraded by TiO2@C nanosheets within 360 min. b) Changes in the relative concentrations of MB with/ without UV-irradiation (λ < 400 nm) catalyzed by TiO2@C nanosheets. P25 as reference catalyst was also performed its photocatalytic capacity. c) The cycling runs of the degradation of MB under UV-light illumination. d) HRTEM image of TiO2@C nanosheets after photocatalytic degradation of MB. Top right inset is SAED of this area.
synthesize TiO2@C nanosheets. It was found that in the as-prepared TiO2@C nanosheets, the TiO2 nanoparticles were uniformly dispersed on the disordered graphitic carbon layer. The formation of intimate contact between TiO2 and C is surely beneficial to suppress the recombination of photo-induced electron-hole pairs. Defects, Ti3+ ions and C–Ti–O bonds were found in the as-prepared TiO2@C nanosheets. The as-prepared TiO2@C nanosheets exhibited better photocatalytic properties than P25, which could be attributed to its unique structural characteristics, existence of C–Ti–O bonds, Ti3+ ions and oxygen vacancy defects. In addition, the TiO2@C nanosheets possess high photocatalytic stability.
energy ball milling is ascribed the following reasons. Firstly, the unique structural characteristics of the as-prepared TiO2@C nanosheets are beneficial to improve its photocatalytic activity. As mentioned above, the as-prepared TiO2@C nanosheets exhibit a structure of uniform and close immobilization of TiO2 nanoparticles on the disordered graphitic carbon nanosheets. Owing to its high electric conductivity, disordered graphitic carbon could serve as an electron acceptor. The formation of the O–Ti–C bond in TiO2@C nanosheets can promote the photo-excited electron transfer from the conduction band of anatase TiO2 to carbon nanosheets. Thus, the recombination of e-/h+ pairs could be effectively suppressed. The relatively small size of the TiO2 can provide more active sites and trap more reactive species that can improve the photocatalytic reactivity. Secondly, the Ti 2p XPS patterns show the existence of Ti3+ ions that can provide donor sites and/or electron traps in the TiO2 [34,35]. Photoexcitation could activate the electrons in these sites to conduction band, and thus inducing the higher density of charge carrier in photocatalyst [36]. The activated Ti3+ states unoccupied can trap electrons and therefore suppressing the recombination of photogenerate electron-hole pairs [37]. Thirdly, as can be seen from HRTEM image, many defects exist in TiO2 lattice. Among all the defects, oxygen vacancies could narrow the band gap of the TiO2, which is beneficial to decrease the probability of electron-hole recombination. The cycling runs of the degradation of MB under UV-light illumination were carried out to reveal the photocatalytic stability of the asprepared TiO2@C (Fig. 4c). After 24 h irradiation for 3 runs of degradation, the photocatalytic activity of TiO2@C is almost maintained. The slightly decrease in photocatalytic activity might be partly caused by the inevitable loss of TiO2@C nanosheets during the washing and centrifugation process. The HRTEM image and SAED (Fig. 4d) show that the morphological structures of TiO2@C nanosheets remain almost unchanged after photocatalytic degradation reaction of MB. This stable structure may be attributed to the formation of O–Ti–C chemical bonds in the TiO2@C nanosheets. It is clear that the as-prepared TiO2@C nanosheets have high structural and photocatalytic stability.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51571116) and the Fundamental Research Funds for the Central Universities (No. 30915014101). References [1] M.H. Baek, W.C. Jung, J.W. Yoon, J.S. Hong, Y.S. Lee, J.K. Suh, Preparation, characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon, J. Ind. Eng. Chem. 19 (2013) 469–477. [2] K. Ullah, Z.D. Meng, S. Ye, L. Zhu, W.C. Oh, Synthesis and characterization of novel PbS-graphene/TiO2 composite with enhanced photocatalytic activity, J. Ind. Eng. Chem. 20 (2014) 1035–1042. [3] R.M. Mohamed, I.A. Mkhalid, Visible light photocatalytic degradation of cyanide using Au-TiO2/multi-walled carbon nanotube nanocomposite, J. Ind. Eng. Chem. 22 (2015) 390–395. [4] X. Meng, D. Geng, J. Liu, R. Li, X. Sun, Controllable synthesis of graphene-based titanium dioxide nanocomposites by atomic layer deposition, Nanotechnology 22 (2011) 165602–165612. [5] W. Fan, Q. Lai, Q. Zhang, Y. Wang, Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution, J. Phys. Chem. C 115 (2011) 10694–10701. [6] G. Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano 2 (2008) 1487–1491. [7] W. Li, F. Wang, S. Feng, J. Wang, Z. Sun, B. Li, Y. Li, J. Yang, A.A. Elzatahry, Y. Xia, D. Zhao, Sol-Gel design strategy for ultradispersed TiO2 nanoparticles on graphene for high-performance lithium ion batteries, J. Am. Chem. Soc. 135 (2013) 18300–18303. [8] K. Li, J. Xiong, T. Chen, L. Yan, Y. Dai, D. Song, Y. Lv, Z. Zeng, Preparation of graphene/TiO2 composites by nonionic surfactant strategy and their simulated sunlight and visible light photocatalytic activity towards representative aqueous
4. Conclusions In conclusion, the structure-directing agent Ti2CTx MXene could be employed as a precursor containing both Ti and C elements to 4
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[23] G.N. Yushin, E.N. Hoffman, A. Nikitin, H. Ye, M.W. Barsoum, Y. Gogotsi, Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide, Carbon 44 (2005) 2075–2082. [24] Y. Fu, H. Du, S. Zhang, W. Huang, XPS characterization of surface and interfacial structure of sputtered TiNi films on Si substrate, Mater. Sci. Eng. A 403 (2005) 25–31. [25] J. Yang, J. Dai, J. Li, Synthesis, characterization and degradation of Bisphenol A using Pr, N co-doped TiO2 with highly visible light activity, Appl. Surf. Sci. 257 (2011) 8965–8973. [26] J. Cai, Z. Huang, K. Lv, J. Sun, K. Deng, Ti powder-assisted synthesis of Ti3+ selfdoped TiO2 nanosheets with enhanced visible-light photoactivity, RSC Adv. 4 (2014) 19588–19593. [27] H. Cui, W. Zhao, C. Yang, Y. Hao, T. Lin, Y. Shan, Y. Xie, H. Gu, F. Huang, Black TiO2 nanotube arrays for high-efficiency photoelectrochemical water-splitting, J. Mater. Chem. A 2 (2014) 8612–8616. [28] S. Sakthivel, H. Kisch, Daylight photocatalysis by carbon-modified titanium dioxide, Angew. Chem. Int. Ed. 42 (2003) 4908–4911. [29] Y. Huang, W.K. Ho, S.C. Lee, L. Zhang, G. Li, J. Yu, Effect of carbon doping the mesoporous structure of nanocrystalline titanium dioxide and its solar-light-driven photocatalytic degradation of NOx, Langmuir 24 (2008) 3510–3516. [30] D. Gu, Y. Lu, B. Yang, Y. Hu, Facile preparation of micro-mesoporous carbon-doped TiO2 photocatalysts with anatase crystalline walls under template-free condition, Chem. Commun. 21 (2008) 2453–2455. [31] J. Zhang, Z. Xiong, X. Zhao, Graphene-metal-oxide composites for the degradation of dyes under visible light irradiation, J. Mater. Chem. 21 (2011) 3634–3640. [32] J. Hu, H. Li, Q. Wu, Y. Zhao, Q. Jiao, Synthesis of TiO2 nanowire/reduced graphene oxide nanocomposites and their photocatalytic performances, Chem. Eng. J. 263 (2015) 144–150. [33] J. Yu, G. Wang, B. Cheng, M. Zhou, Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders, Appl. Catal. B: Environ. 69 (2007) 171–180. [34] M. Liu, X. Qiu, M. Miyauchi, K. Hashimoto, Cu (II) oxide amorphous nanoclusters grafted Ti3+ self-doped TiO2: an efficient visible light photocatalyst, Chem. Mater. 23 (2011) 5282–5286. [35] D. Eder, R. Kramer, Stoichiometry of “titanium suboxide”. Part 2. Electric properties, Phys. Chem. Chem. Phys. 5 (2003) 1314–1319. [36] C. Mao, F. Zuo, Y. Hou, X. Bu, P. Feng, In situ preparation of a Ti3+ self-doped TiO2 film with enhanced activity as photoanode by N2H4 reduction, Angew. Chem. Int. Ed. 53 (2014) 10653–10657. [37] Y. Zhou, Y. Liu, P. Liu, W. Zhang, M. Xing, J. Zhang, A facile approach to further improve the substitution of nitrogen into reduced TiO2−x with an enhanced photocatalytic activity, Appl. Catal. B: Environ. 170 (2015) 66–73.
POPs degradation, J. Hazard. Mater. 250–251 (2013) 19–28. [9] M.S.A.S. Shah, A.R. Park, K. Zhang, J.H. Park, P.J. Yoo, Green synthesis of biphasic TiO2-reduced graphene oxide nanocomposites with highly enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 3893–3901. [10] J. Yu, S. Liu, H. Yu, Microstructures and photoactivity of mesoporous anatase hollow microspheres fabricated by fluoride-mediated self-transformation, J. Catal. 249 (2007) 59–66. [11] J. Yu, Y. Su, B. Cheng, Template-free fabrication and enhanced photocatalytic activity of hierarchical macro-/mesoporous titania, Adv. Funct. Mater. 17 (2007) 1984–1990. [12] M. Naguib, O. Mashtalir, M.R. Lukatskaya, B. Dyatkin, C. Zhang, V. Presser, Y. Gogotsi, M.W. Barsoum, One-step synthesis of nanocrystalline transition metal oxides on thin sheets of disordered graphitic carbon by oxidation of MXenes, Chem. Commun. 50 (2014) 7420–7423. [13] Z. Li, L. Wang, D. Sun, Y. Zhang, B. Liu, Q. Hu, A. Zhou, Synthesis and thermal stability of two-dimensional carbide MXene Ti3C2, Mater. Sci. Eng. B 191 (2015) 33–40. [14] R.B. Rakhi, B. Ahmed, M.N. Hedhili, D.H. Anjum, H.N. Alshareef, Effect of postetch annealing gas composition on the structural and electrochemical properties of Ti2CTx MXene electrodes for supercapacitor applications, Chem. Mater. 27 (2015) 5314–5323. [15] J. Li, Y. Du, C. Huo, S. Wang, C. Cui, Thermal stability of two-dimensional Ti2C nanosheets, Ceram. Int. 41 (2015) 2631–2635. [16] B. Ahmed, D.H. Anjum, M.N. Hedhili, Y. Gogotsi, H.N. Alshareef, H2O2 assisted room temperature oxidation of Ti2C MXene for Li-ion battery anodes, Nanoscale 8 (2016) 7580–7587. [17] H. Ghassemi, W. Harlow, O. Mashtalir, M. Beidaghi, M.R. Lukatskaya, Y. Gogotsi, M.L. Taheri, In situ environmental transmission electron microscopy study of oxidation of two-dimensional Ti3C2 and formation of carbon-supported TiO2, J. Mater. Chem. A 2 (2014) 14339–14343. [18] H. Hu, X. Wang, F. Liu, J. Wang, C. Xu, Rapid microwave-assisted synthesis of graphene nanosheets-zinc sulfide nanocomposites: optical and photocatalytic properties, Synth. Met. 161 (2011) 404–410. [19] Y. Huang, W.K. Ho, S.C. Lee, L.Z. Zhang, G.S. Li, J.C. Yu, Effect of carbon doping on the mesoporous structure of nanocrystalline titanium dioxide and its solar-lightdriven photocatalytic degradation of NOx, Langmuir 24 (2008) 3510–3516. [20] D. Gu, Y. Lu, B. Yang, Y. Hu, Facile preparation of micro-mesoporous carbon-doped TiO2 photocatalysts with anatase crystalline walls under template-free condition, Chem. Commun. 0 (2008) 2453–2455. [21] O. Toshiaki, I. Fujio, F. Yoshinori, Raman spectrum of anatase, TiO2, J. Raman Spectrosc. 7 (1978) 321–324. [22] H. Berger, H. Tang, F. Kevy, Growth and Raman spectroscopic characterization of TiO2 anatase single crystals, J. Cryst. Growth 130 (1993) 108–112.
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Enhanced photocatalytic performance of TiO2@C nanosheets derived from two-dimensional Ti2CTx ⁎
Jingxiao Lia,b, Shun Wangc, Yulei Dua, , Wenhe Liaoa a
School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c School of Materials Science and Engineering, Henan University of Technology, Zhengzhou 450001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: MXene TiO2@C nanosheets High-energy ball milling Photocatalyst
In the present work, TiO2@C nanosheets are fabricated by high-energy ball milling of two-dimensional (2D) Ti2CTx. Here, Ti2CTx not only serves as titanium and carbon source, but also acts as structure-directing agent. It is found that TiO2 nanoparticles are uniformly and closely dispersed on the single or few-layered carbon sheets in the as-prepared TiO2@C nanosheets. Compared to titanium dioxide P25, the TiO2@C nanosheets exhibit enhancing photocatalytic degradation of methylene blue (MB) dye in aqueous solution under UV-irradiation. In addition, the TiO2@C nanosheets also possess high photocatalytic stability.
1. Introduction Titania-carbon (TiO2@C) composites are promising materials for photocatalytic applications, because carbon can effectively enhance the photocatalytic ability of TiO2 through decreasing recombination of photoexcited e-/h+ pairs [1–3]. To date, varieties of synthetic techniques including atomic layer deposition (ALD) [4], UV-assisted photoreduction [5,6], sol-gel [7], and solvothermal method [8,9] have been reported to prepare TiO2@C composites. High-temperature treatment is generally required in most of the aforementioned synthesis processes, which may result in aggregation of TiO2@C leading to decrease of the catalytic efficiency [10,11]. Recently, nanocrystalline TiO2 on thin flakes of disordered graphitic carbon has been fabricated through the oxidation of so-called “MXenes” (a new group of two-dimensional (2D) transition metal carbides and carbonitrides, exfoliated from their bulk counterparts MAX phases, where the M, A, and X represent early transition metal, A-group elements, and carbon/nitrogen, respectively) in air, CO2 or deionized water [12]. These synthesis methods of TiO2@C composites are facile and the specific surface area is enlarged owing to the growth of TiO2 nanoparticles onto every sheet. However, there still exist some problems with these synthesis methods. For example, the sizes and morphologies of TiO2 nanoparticles were difficult to be controlled when the Ti3C2Tx (T stands for terminated O and OH groups) powders were flash oxidized at 1150 °C for 30 s in air [12]. In addition, the Ti3C2Tx could not completely convert into TiO2 nanoparticles and carbon sheets by the isothermal treatment in pure CO2, or hydrothermal oxidation [12]. Li et al. also found at the temperature of 200 °C the
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Ti3C2Tx could be partially oxidized by oxygen [13]. Similar phenomenon was also found in heat treatment or post-etch annealing of Ti2CTx under different gas conditions [14,15]. In addition, Ahmed et al. showed that the Ti2CTx could be completely oxidized when it was immersed in hydrogen peroxide (H2O2) for 5 h at room temperature [16]. By means of in situ TEM analysis, Ghassemi et al. pointed out the formation of TiO2 nanoparticles during the flash oxidization of Ti3C2Tx is starting from top and bottom of the titanium layers [17]. As reported, the layers of the as-synthesized MXenes are usually bounded together, which hinders the oxidization process. In the present work, we introduce high-energy ball milling technique to promote the oxidization of Ti2CTx MXene and hence to produce TiO2@C composites. The schematic of high-energy ball milling of Ti2CTx is shown in Fig. 1. The mechanical forces during ball milling can easily break the van der Waals force between interlayers of stacked Ti2CTx, resulting in the formation of single or few layered Ti2CTx nanosheets. The decreased thickness makes the oxygen traveling more facilely through the Ti2CTx layers. In this work, TiO2@C nanosheets were successfully prepared by high-energy ball milling of Ti2CTx in air. The structural and photocatalytic properties of the as-prepared TiO2@C nanosheets were studied. 2. Experimental procedure Here, all chemicals (Ti powder, purity 99.9%; Al powder, purity 99.5%, Carbon powder, purity 99%; methyl blue (MB), purity 98.5%; and hydrofluoric acid (HF, 40%)) were bought from Alfa Aesar. The
Corresponding author. E-mail address: [email protected] (Y. Du).
https://doi.org/10.1016/j.ceramint.2018.01.139 Received 19 December 2017; Received in revised form 16 January 2018; Accepted 17 January 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Li, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.01.139
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60 min to analyze the concentration of MB with UV–vis absorption spectroscopy (Shimadzu UV2450). The cycling runs of the degradation of MB were conducted for three consecutive cycles. 10 mg of TiO2@C nanosheets were dispersed in 50 mL MB solution (20 mg L−1) in a 200 mL cylindrical quartz vessel. A 500 W Hg lamp was used as UV light source. The TiO2@C nanosheets were recycled by washing and centrifugation after every cycle. The concentration of MB was determined by measuring the absorption intensity at its maximum characteristic absorbance wavelength of 663 nm by using UV–vis spectrophotometer [18]. The photocatalytic performance was represented by degradation of the MB, which was defined as: Degradation efficiency = 1 − C / C0 = 1 − A / A0, where C0 (mg L−1) is the initial concentration of MB, C (mg L−1) is the MB concentration at time t (min), A0 is the UV–vis absorption of the original solution and A is the UV–vis absorption of degraded solution at time t (min).
Fig. 1. Schematic of high-energy ball milling of Ti2CTx to form TiO2@C.
preparation of Ti2CTx powders was illustrated in our previous work [15]. A planetary ball mill (QM-3SP2, Nanjing University Instrument Plant, China) with agate jars and agate balls was used. The internal volume of the milling agate jar is 250 mL. The agate balls of different sizes (Φ9.8 mm balls and Φ6.45 mm balls with a ratio of 24:31) were used in the present work. In the typical milling experiment, 500 mg Ti2CTx powders were put in the agate jar and then milled with agate balls (weight ratio of 1:55) in air atmosphere for 1.5 h with a speed of 200 r/min. The as-milled powders were cold-pressed under a pressure of about 1 GPa to form small pellets for further structural characterization. X-ray diffraction (XRD, Bruker-AXS D8 Advance, Cu-Kα radiation, λ = 1.54178 Å, operated at 40 mA and 40 kV), scanning electron microscope (Quant 250F), transmission electron microscope (JEM-200CX, operated at 200 kV), X-ray photoelectron spectroscopy (PHI 5000 VersaProbe, Al-Kα radiation) and Raman (JY HR800, λ = 514 nm) were employed to characterize the as-prepared samples. The photocatalytic properties of the as-prepared TiO2@C nanosheets were evaluated by the degradation of methyl blue (MB). In a typical run, a specified amount (10 mg) of TiO2@C nanosheets was suspended in 50 mL of methyl blue (20 mg L−1) aqueous solution in a cylindrical quartz vessel and magnetically stirred in the dark for 2 h to achieve the adsorption/desorption equilibrium. Then the mixture was irradiated by a 500 W Hg lamp under magnetically stirring. During the course of photocatalytic reaction, the temperature of the suspension was maintained at 25.0 °C. 3 mL of MB solution was taken out every
3. Results and discussion Fig. 2a and b shows SEM of Ti2CTx powders before and after highenergy ball milling, respectively. As can be seen from Fig. 2a, most of the as-synthesized Ti2CTx MXenes stack together though Al layers have been etched out. The high-energy ball milling destroys the multistacked structure, leading to the formation of free-standing nanosheets with much smaller surface area. Further phase identification is conducted by XRD and Raman spectra. It can be seen from XRD patterns (Fig. 2c) when the as-synthesized Ti2CTx MXenes are milled after 1.5 h, the typical wide low-angle MXene peaks disappear and some small peaks locating at the two theta values of 25.2°, 37.9°, 48.1° and 55.1° appear. According to JCPDS No. 21-1272, these peaks corresponds to d101, d004, d200 and d105 of the anatase phase respectively, indicative of the formation of titania. However, no obvious diffraction peaks of carbon can be observed. It is suspected that the carbon species are most likely poorly crystallized, similar to previous investigations of C-modified TiO2 composites [19,20]. From Fig. 2d, the Raman spectra of asmilled Ti2CTx shows six peaks locating at 144, 399, 519, 639, 1356 and 1593 cm−1. The peaks of 144, 399, 519 and 639 cm−1 are well matched with vibrational modes Eg(1), B1g(1), A1g & B1g(2) and Eg(3) of anatase, respectively [21,22]. The bands of 1356 and 1593 cm−1 Fig. 2. Characterization of Ti2CTx powders before and after high-energy ball milling for 1.5 h. a) SEM image of typical multilayers of Ti2CTx. b) SEM image of Ti2CTx after high-energy ball milling. c) XRD patterns of Ti2CTx before and after high-energy ball milling. The sign “#” stands for TiO2 and d) Raman spectra of Ti2CTx before and after high-energy ball milling. The sign “*” stands for TiO2.
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Fig. 3. a) TEM image of TiO2@C. b) HRTEM image of TiO2@C. Bottom right inset is SAED of this area. c) XPS patterns of Ti 2p. d) XPS patterns of C 1s. (e) XPS patterns of O 1s.
belong to the alleged D and G bands owing to sp2-bonded carbon atoms and presence of defects in the graphitic layers respectively, which directly confirms the formation of carbon. The FWHM of D peak is 85 cm−1 and the corresponding value of G peak is 125 cm−1. The ratio of the D and G band intensities, ID/ IG, is 1.15, a value higher than that has been reported for oxidation of MXenes at high temperature and carbide-derived carbons (CDCs) formed at 200 °C [12,23]. The ratio of D and G band intensities reveals the presence of highly disordered carbon during high-energy ball milling. More detailed structural studies of the as-prepared TiO2@C were performed by Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED). It is clear from Fig. 3a that the as-milled samples consist of monodispersed particles and show different morphological structures, with the size ranging from a few nanometers to tens of nanometers. Fig. 3b shows detailed structure including of a mixture of crystalline grains and amorphous regions. The lattice fringes in d-spacing were measured to be 0.351 nm, which is in agreement with the (101) plane of anatase TiO2 (JCPDS file No. 89-4921). From the observed SAED ring patterns, the lattice distance of the nanoparticles was calculated to be 0.356 nm. The presence of the amorphous carbon can be identified by the broad Debye ring. It can be seen that the TiO2 nanoparticles are dispersed uniformly and closely on amorphous carbon, leading to the formation of intimate contact between TiO2 and C. Notably, some surface defects also exist in TiO2 nanoparticles as indicated with dotted line circles. On one hand, a lot of similar defects in Ti2CTx are produced during exfoliation of Ti2AlC [15]. These defects are considered as nucleation sites for TiO2 growth when Ti2CTx is oxidized [16]. On the other hand, a part of electrons in the TiO2 crystal lattices could be excited by the milling mechanical forces, resulting in the original structures broken or cracked. The structural characteristics of the as-prepared TiO2@C composites are strongly related to the highenergy ball milling process. Firstly, the multi-layered stacked structure of Ti2CTx MXenes could be cracked into single or few-layered structure. Thus, it provides an easy way for oxygen to throughout the layered sample quickly. Secondly, the inner Ti atoms of the Ti2CTx structure
getting enough energy migrated outward to be oxidized into TiO2. The leaving C atoms at different locations with different energy could relocate, creating highly disordered graphitic carbon. Because there is no gradient of oxygen concentration, the TiO2 particles are extremely thin and dispersed uniformly on the disordered graphitic carbon. As can be seen from the Ti 2p XPS patterns (Fig. 3c), two peaks located at 459.9 eV (Ti 2p3/2) and 465.6 eV (Ti 2p1/2) with the spitorbital splitting of 5.7 eV were observed, indicating a normal state of Ti4+ in the as-milled sample [24,25]. The Ti 2p3/2 and Ti 2p1/2 peaks centered at 457.6 eV and 463.6 eV, respectively, are ascribed to Ti3+ [26,27]. Fig. 3d shows the C 1s spectra of as-milled sample. The peaks at 284.8 and 286.3 eV can be assigned to elemental carbon and C-O in epoxy groups or hydroxyl, proving the formation of carbon [28]. The peak at 282.3 eV, a position close to C 1s peak, usually can be considered as carbon replacing the oxygen atom in the structure of TiO2, indicative of the formation of the O–Ti–C bond [29,30]. The result provides direct evidence that chemical bonds exist between carbon and TiO2 in the as-prepared sample. In addition, the doping of C element could result in lattice distortion and generate oxygen vacancies in TiO2 [31]. The O 1s spectra of TiO2@C were observed in Fig. 3e. Besides the binding energy at 532 and 534.9 eV owing to absorbed oxygen and other functional groups, the peak at 530.6 eV can be assigned to Ti-O-Ti of TiO2 [32,33]. Methylene blue (MB) photocatalytic degradation experiment was performed to investigate the photocatalytic activity of the as-prepared TiO2@C nanosheets under UV light illumination (λ < 400 nm). As a reference, the photocatalytic performance of P25 was also measured under the same conditions. Fig. 4a presents changes in the relative concentrations of MB with and without irradiation. The adsorptiondesorption equilibrium of MB in the dark was established within 120 min. As can be seen from Fig. 4b, the adsorption capacity of TiO2@C is higher than that of P25. The degradation efficiency of MB is 85.7% and 68.5% for TiO2@C and P25, respectively, after 360 min of UV-illumination. Clearly, the enhanced photocatalytic activity of asprepared TiO2@C hybrid is 1.25 times higher than that of P25. The enhanced photocatalytic activity of the TiO2@C synthesized by high-
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Fig. 4. a) UV–vis adsorption spectrum of MB degraded by TiO2@C nanosheets within 360 min. b) Changes in the relative concentrations of MB with/ without UV-irradiation (λ < 400 nm) catalyzed by TiO2@C nanosheets. P25 as reference catalyst was also performed its photocatalytic capacity. c) The cycling runs of the degradation of MB under UV-light illumination. d) HRTEM image of TiO2@C nanosheets after photocatalytic degradation of MB. Top right inset is SAED of this area.
synthesize TiO2@C nanosheets. It was found that in the as-prepared TiO2@C nanosheets, the TiO2 nanoparticles were uniformly dispersed on the disordered graphitic carbon layer. The formation of intimate contact between TiO2 and C is surely beneficial to suppress the recombination of photo-induced electron-hole pairs. Defects, Ti3+ ions and C–Ti–O bonds were found in the as-prepared TiO2@C nanosheets. The as-prepared TiO2@C nanosheets exhibited better photocatalytic properties than P25, which could be attributed to its unique structural characteristics, existence of C–Ti–O bonds, Ti3+ ions and oxygen vacancy defects. In addition, the TiO2@C nanosheets possess high photocatalytic stability.
energy ball milling is ascribed the following reasons. Firstly, the unique structural characteristics of the as-prepared TiO2@C nanosheets are beneficial to improve its photocatalytic activity. As mentioned above, the as-prepared TiO2@C nanosheets exhibit a structure of uniform and close immobilization of TiO2 nanoparticles on the disordered graphitic carbon nanosheets. Owing to its high electric conductivity, disordered graphitic carbon could serve as an electron acceptor. The formation of the O–Ti–C bond in TiO2@C nanosheets can promote the photo-excited electron transfer from the conduction band of anatase TiO2 to carbon nanosheets. Thus, the recombination of e-/h+ pairs could be effectively suppressed. The relatively small size of the TiO2 can provide more active sites and trap more reactive species that can improve the photocatalytic reactivity. Secondly, the Ti 2p XPS patterns show the existence of Ti3+ ions that can provide donor sites and/or electron traps in the TiO2 [34,35]. Photoexcitation could activate the electrons in these sites to conduction band, and thus inducing the higher density of charge carrier in photocatalyst [36]. The activated Ti3+ states unoccupied can trap electrons and therefore suppressing the recombination of photogenerate electron-hole pairs [37]. Thirdly, as can be seen from HRTEM image, many defects exist in TiO2 lattice. Among all the defects, oxygen vacancies could narrow the band gap of the TiO2, which is beneficial to decrease the probability of electron-hole recombination. The cycling runs of the degradation of MB under UV-light illumination were carried out to reveal the photocatalytic stability of the asprepared TiO2@C (Fig. 4c). After 24 h irradiation for 3 runs of degradation, the photocatalytic activity of TiO2@C is almost maintained. The slightly decrease in photocatalytic activity might be partly caused by the inevitable loss of TiO2@C nanosheets during the washing and centrifugation process. The HRTEM image and SAED (Fig. 4d) show that the morphological structures of TiO2@C nanosheets remain almost unchanged after photocatalytic degradation reaction of MB. This stable structure may be attributed to the formation of O–Ti–C chemical bonds in the TiO2@C nanosheets. It is clear that the as-prepared TiO2@C nanosheets have high structural and photocatalytic stability.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51571116) and the Fundamental Research Funds for the Central Universities (No. 30915014101). References [1] M.H. Baek, W.C. Jung, J.W. Yoon, J.S. Hong, Y.S. Lee, J.K. Suh, Preparation, characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon, J. Ind. Eng. Chem. 19 (2013) 469–477. [2] K. Ullah, Z.D. Meng, S. Ye, L. Zhu, W.C. Oh, Synthesis and characterization of novel PbS-graphene/TiO2 composite with enhanced photocatalytic activity, J. Ind. Eng. Chem. 20 (2014) 1035–1042. [3] R.M. Mohamed, I.A. Mkhalid, Visible light photocatalytic degradation of cyanide using Au-TiO2/multi-walled carbon nanotube nanocomposite, J. Ind. Eng. Chem. 22 (2015) 390–395. [4] X. Meng, D. Geng, J. Liu, R. Li, X. Sun, Controllable synthesis of graphene-based titanium dioxide nanocomposites by atomic layer deposition, Nanotechnology 22 (2011) 165602–165612. [5] W. Fan, Q. Lai, Q. Zhang, Y. Wang, Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution, J. Phys. Chem. C 115 (2011) 10694–10701. [6] G. Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano 2 (2008) 1487–1491. [7] W. Li, F. Wang, S. Feng, J. Wang, Z. Sun, B. Li, Y. Li, J. Yang, A.A. Elzatahry, Y. Xia, D. Zhao, Sol-Gel design strategy for ultradispersed TiO2 nanoparticles on graphene for high-performance lithium ion batteries, J. Am. Chem. Soc. 135 (2013) 18300–18303. [8] K. Li, J. Xiong, T. Chen, L. Yan, Y. Dai, D. Song, Y. Lv, Z. Zeng, Preparation of graphene/TiO2 composites by nonionic surfactant strategy and their simulated sunlight and visible light photocatalytic activity towards representative aqueous
4. Conclusions In conclusion, the structure-directing agent Ti2CTx MXene could be employed as a precursor containing both Ti and C elements to 4
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[23] G.N. Yushin, E.N. Hoffman, A. Nikitin, H. Ye, M.W. Barsoum, Y. Gogotsi, Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide, Carbon 44 (2005) 2075–2082. [24] Y. Fu, H. Du, S. Zhang, W. Huang, XPS characterization of surface and interfacial structure of sputtered TiNi films on Si substrate, Mater. Sci. Eng. A 403 (2005) 25–31. [25] J. Yang, J. Dai, J. Li, Synthesis, characterization and degradation of Bisphenol A using Pr, N co-doped TiO2 with highly visible light activity, Appl. Surf. Sci. 257 (2011) 8965–8973. [26] J. Cai, Z. Huang, K. Lv, J. Sun, K. Deng, Ti powder-assisted synthesis of Ti3+ selfdoped TiO2 nanosheets with enhanced visible-light photoactivity, RSC Adv. 4 (2014) 19588–19593. [27] H. Cui, W. Zhao, C. Yang, Y. Hao, T. Lin, Y. Shan, Y. Xie, H. Gu, F. Huang, Black TiO2 nanotube arrays for high-efficiency photoelectrochemical water-splitting, J. Mater. Chem. A 2 (2014) 8612–8616. [28] S. Sakthivel, H. Kisch, Daylight photocatalysis by carbon-modified titanium dioxide, Angew. Chem. Int. Ed. 42 (2003) 4908–4911. [29] Y. Huang, W.K. Ho, S.C. Lee, L. Zhang, G. Li, J. Yu, Effect of carbon doping the mesoporous structure of nanocrystalline titanium dioxide and its solar-light-driven photocatalytic degradation of NOx, Langmuir 24 (2008) 3510–3516. [30] D. Gu, Y. Lu, B. Yang, Y. Hu, Facile preparation of micro-mesoporous carbon-doped TiO2 photocatalysts with anatase crystalline walls under template-free condition, Chem. Commun. 21 (2008) 2453–2455. [31] J. Zhang, Z. Xiong, X. Zhao, Graphene-metal-oxide composites for the degradation of dyes under visible light irradiation, J. Mater. Chem. 21 (2011) 3634–3640. [32] J. Hu, H. Li, Q. Wu, Y. Zhao, Q. Jiao, Synthesis of TiO2 nanowire/reduced graphene oxide nanocomposites and their photocatalytic performances, Chem. Eng. J. 263 (2015) 144–150. [33] J. Yu, G. Wang, B. Cheng, M. Zhou, Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders, Appl. Catal. B: Environ. 69 (2007) 171–180. [34] M. Liu, X. Qiu, M. Miyauchi, K. Hashimoto, Cu (II) oxide amorphous nanoclusters grafted Ti3+ self-doped TiO2: an efficient visible light photocatalyst, Chem. Mater. 23 (2011) 5282–5286. [35] D. Eder, R. Kramer, Stoichiometry of “titanium suboxide”. Part 2. Electric properties, Phys. Chem. Chem. Phys. 5 (2003) 1314–1319. [36] C. Mao, F. Zuo, Y. Hou, X. Bu, P. Feng, In situ preparation of a Ti3+ self-doped TiO2 film with enhanced activity as photoanode by N2H4 reduction, Angew. Chem. Int. Ed. 53 (2014) 10653–10657. [37] Y. Zhou, Y. Liu, P. Liu, W. Zhang, M. Xing, J. Zhang, A facile approach to further improve the substitution of nitrogen into reduced TiO2−x with an enhanced photocatalytic activity, Appl. Catal. B: Environ. 170 (2015) 66–73.
POPs degradation, J. Hazard. Mater. 250–251 (2013) 19–28. [9] M.S.A.S. Shah, A.R. Park, K. Zhang, J.H. Park, P.J. Yoo, Green synthesis of biphasic TiO2-reduced graphene oxide nanocomposites with highly enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 3893–3901. [10] J. Yu, S. Liu, H. Yu, Microstructures and photoactivity of mesoporous anatase hollow microspheres fabricated by fluoride-mediated self-transformation, J. Catal. 249 (2007) 59–66. [11] J. Yu, Y. Su, B. Cheng, Template-free fabrication and enhanced photocatalytic activity of hierarchical macro-/mesoporous titania, Adv. Funct. Mater. 17 (2007) 1984–1990. [12] M. Naguib, O. Mashtalir, M.R. Lukatskaya, B. Dyatkin, C. Zhang, V. Presser, Y. Gogotsi, M.W. Barsoum, One-step synthesis of nanocrystalline transition metal oxides on thin sheets of disordered graphitic carbon by oxidation of MXenes, Chem. Commun. 50 (2014) 7420–7423. [13] Z. Li, L. Wang, D. Sun, Y. Zhang, B. Liu, Q. Hu, A. Zhou, Synthesis and thermal stability of two-dimensional carbide MXene Ti3C2, Mater. Sci. Eng. B 191 (2015) 33–40. [14] R.B. Rakhi, B. Ahmed, M.N. Hedhili, D.H. Anjum, H.N. Alshareef, Effect of postetch annealing gas composition on the structural and electrochemical properties of Ti2CTx MXene electrodes for supercapacitor applications, Chem. Mater. 27 (2015) 5314–5323. [15] J. Li, Y. Du, C. Huo, S. Wang, C. Cui, Thermal stability of two-dimensional Ti2C nanosheets, Ceram. Int. 41 (2015) 2631–2635. [16] B. Ahmed, D.H. Anjum, M.N. Hedhili, Y. Gogotsi, H.N. Alshareef, H2O2 assisted room temperature oxidation of Ti2C MXene for Li-ion battery anodes, Nanoscale 8 (2016) 7580–7587. [17] H. Ghassemi, W. Harlow, O. Mashtalir, M. Beidaghi, M.R. Lukatskaya, Y. Gogotsi, M.L. Taheri, In situ environmental transmission electron microscopy study of oxidation of two-dimensional Ti3C2 and formation of carbon-supported TiO2, J. Mater. Chem. A 2 (2014) 14339–14343. [18] H. Hu, X. Wang, F. Liu, J. Wang, C. Xu, Rapid microwave-assisted synthesis of graphene nanosheets-zinc sulfide nanocomposites: optical and photocatalytic properties, Synth. Met. 161 (2011) 404–410. [19] Y. Huang, W.K. Ho, S.C. Lee, L.Z. Zhang, G.S. Li, J.C. Yu, Effect of carbon doping on the mesoporous structure of nanocrystalline titanium dioxide and its solar-lightdriven photocatalytic degradation of NOx, Langmuir 24 (2008) 3510–3516. [20] D. Gu, Y. Lu, B. Yang, Y. Hu, Facile preparation of micro-mesoporous carbon-doped TiO2 photocatalysts with anatase crystalline walls under template-free condition, Chem. Commun. 0 (2008) 2453–2455. [21] O. Toshiaki, I. Fujio, F. Yoshinori, Raman spectrum of anatase, TiO2, J. Raman Spectrosc. 7 (1978) 321–324. [22] H. Berger, H. Tang, F. Kevy, Growth and Raman spectroscopic characterization of TiO2 anatase single crystals, J. Cryst. Growth 130 (1993) 108–112.
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