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Construction of TiO2-pillared multilayer graphene nanocomposites as efficient photocatalysts for ciprofloxacin degradation

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    Jian-sheng Wang    E-mail:

  • Received: 23 July 2020Revised: 9 September 2020Accepted: 11 September 2020Available online: 12 September 2020
  • We successfully constructed TiO2-pillared multilayer graphene nanocomposites (T-MLGs) via a facile method as follows: dodecanediamine pre-pillaring, ion exchange (Ti4+ pillaring), and interlayer in-situ formation of TiO2 by hydrothermal method. TiO2 nanoparticles were distributed uniformly on the graphene interlayer. The special structure combined the advantages of graphene and TiO2 nanoparticles. As a result, T-MLGs with 64.3wt% TiO2 showed the optimum photodegradation rate and adsorption capabilities toward ciprofloxacin. The photodegradation rate of T-MLGs with 64.3wt% TiO2 was 78% under light-emitting diode light irradiation for 150 min. Meanwhile, the pseudo-first-order rate constant of T-MLGs with 64.3wt% TiO2 was 3.89 times than that of pristine TiO2. The composites also exhibited high stability and reusability after five consecutive photocatalytic tests. This work provides a facile method to synthesize semiconductor-pillared graphene nanocomposites by replacing TiO2 nanoparticles with other nanoparticles and a feasible means for sustainable utilization of photocatalysts in wastewater control.
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Construction of TiO2-pillared multilayer graphene nanocomposites as efficient photocatalysts for ciprofloxacin degradation

  • Corresponding author:

    Jian-sheng Wang    E-mail:

  • 1. Hebei Provincial Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology, Tangshan 063210, China

Abstract: We successfully constructed TiO2-pillared multilayer graphene nanocomposites (T-MLGs) via a facile method as follows: dodecanediamine pre-pillaring, ion exchange (Ti4+ pillaring), and interlayer in-situ formation of TiO2 by hydrothermal method. TiO2 nanoparticles were distributed uniformly on the graphene interlayer. The special structure combined the advantages of graphene and TiO2 nanoparticles. As a result, T-MLGs with 64.3wt% TiO2 showed the optimum photodegradation rate and adsorption capabilities toward ciprofloxacin. The photodegradation rate of T-MLGs with 64.3wt% TiO2 was 78% under light-emitting diode light irradiation for 150 min. Meanwhile, the pseudo-first-order rate constant of T-MLGs with 64.3wt% TiO2 was 3.89 times than that of pristine TiO2. The composites also exhibited high stability and reusability after five consecutive photocatalytic tests. This work provides a facile method to synthesize semiconductor-pillared graphene nanocomposites by replacing TiO2 nanoparticles with other nanoparticles and a feasible means for sustainable utilization of photocatalysts in wastewater control.

    • Ciprofloxacin (CIP), a synthetic third-generation fluoroquinolone antibiotic, has widely been used for the efficient treatment of a number of bacterial infections [1]. However, the bio- and photo-degradation of CIP are difficult [2]. Hence, the investigation of the photocatalytic degradation of CIP by solar energy as a potential way to solve water pollution has attracted extensive research attention [38].

      TiO2 is the most widely studied and used as a photocatalyst in commercial applications because of its advantages [910]. However, the wide band gap and high photoelectron–hole recombination probability hinder the improvement of the photocatalytic performance of TiO2 [11]. Strategies, including doping, nanostructuring, surface modification, and heterojunction construction, have been proposed to enhance the semiconductor properties of TiO2 [7, 1216]. Graphene possesses a large specific surface area, excellent conduction, and chemical stability [17], resulting in good electron transfer and electron holding capacity. When TiO2 combines with graphene, the photocatalytic activity of graphene–TiO2 compositions is higher compared with pure TiO2 [1821] because of the effective charge transfer. Graphene–TiO2 nanocomposites can be used not only in photocatalysis but also in dye-sensitized solar cells, ultraviolet photodetectors, and supercapacitors [2226]. In addition, the good performances of these nanocomposites require a high interfacial contact [2730] between graphene and TiO2, which is the key to improve electron transfer and reducing electron–hole recombination, especially for photocatalysts [31]. TiO2 nanoparticles anchoring onto graphene sheets by chemical interaction has been verified by a significantly enhanced photocatalytic activity [32].

      To date, TiO2 is usually loaded on the surface of multilayer or single-layer graphene in TiO2–graphene nanocomposites [2324, 26, 3337]. Graphene–TiO2 nanocomposite photocatalytic degradation rates differ, as will be compared later. To the best of our knowledge, TiO2-pillared multilayer graphene nanocomposites (T-MLGs), which are formed through the insertion of TiO2 nanoparticles into the interlayer spacing of multilayer graphene, have rarely been reported. Their special structure (pillared structure) can improve the photocatalytic efficiency by reducing photoelectron–hole recombination probability, which was verified by intercalating TiO2 into the interlayer regions of layered materials, e.g., HTaWO6 and K2Ti4O9 [16, 38]. Therefore, to enhance the photocatalytic activity of TiO2, we synthesized T-MLGs via a facile method as follows: dodecanediamine pre-pillaring, ion exchange (Ti4+ pillaring), and interlayer in-situ formation of TiO2 by hydrothermal method. The preparation and photocatalytic activity for CIP degradation of T-MLGs were investigated in detail in this work.

    2.   Experimental
    • All chemicals were of analytical reagent grade and used as received without any further purification.

    • Dodecanediamine pre-pillared graphene oxide (GO) was prepared by Margarita’s method [39]. First, 100 mg GO was dispersed in 16.5 mL deionized water via ultrasonication. Then, 4.16 mmol 1,12-dodecanediamine was dissolved in 17.5 mL ethanol, and the mixture was added dropwise to GO suspension under continuous stirring at room temperature for at least 48 h. Then, GO/dodecanediamine was centrifuged, washed with deionized water and absolute ethanol for several times, and dried at 80°C for 24 h. Finally, dodecanediamine pre-pillared GO was prepared successfully. GO with thickness of 0.8–1.2 nm was purchased from Nanjing XianFeng Nanomaterials Technology Co. Ltd., China.

    • The TiO2 sol was prepared before the Ti4+ exchange with dodecanediamine, which was prepared in accordance with the previously procedure reported by Chen et al. [40]. TiO2 sol was adjusted to pH of 4–5 by 0.1 M ammonia solution. Then, the dodecanediamine pre-pillared GO was scattered dropwise into mixture mentioned above under magnetic stirring for 24 h to complete the Ti4+ exchange with dodecanediamine. After filtering, washing, and drying at 65°C for 24 h, the precursor powder was obtained.

    • First, the precursor powder was dispersed in 30 mL deionized water via ultrasonication. Then, the mixed solution was transferred to a 50 mL Teflon-lined autoclave and reacted at 200°C for 4 h and cooled to 25°C naturally. Afterward, the precipitates were centrifuged, washed with deionized water and absolute ethanol until the filtrate was neutral, and dried at 80°C for 4 h. Finally, the T-MLGs were prepared successfully.

      By controlling the amount of TiO2 sol, T-MLGs with different TiO2 weight percentages of 10.4wt%, 64.3wt%, and 94.1wt% were obtained and denoted as T-MLG-10.4, T-MLG-64.3, and T-MLG-94.1, respectively. TiO2 as a control was prepared in the same experiment condition by hydrothermal method.

      Fig. 1 illustrates the preparation process of T-MLGs.

      Figure 1.  Schematic diagram for synthesis process of T-MLGs.

    • The crystal structures were analyzed by X-ray diffraction (XRD, D/MAX2500PC Rigaku, Cu Kα). The surface morphologies were investigated by scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM-2010). Raman spectrum was detected by DXR laser Raman spectroscopy. Fourier transform infrared spectra (FTIR) were measured on a Nicolet 470 FT-IR spectrometer. The photoluminescence (PL) spectra were obtained using a fluorescence spectrometer (PE, LS-S5). The excitation wavelength was 380 nm.

    • A total of 20 mg TiO2 or 20 mg T-MLGs were dispersed in 40 mL CIP aqueous solution (15 mg/L). First, the mixtures were magnetically stirred for 30 min in the dark to establish the adsorption–desorption equilibrium of CIP on the sample surface. A 5 W light-emitting diode (LED) lamp (white light, wavelength λ > 420 nm) was employed as the light source. The CIP solution (3 mL) was extracted and centrifuged at a rate of 8000 r/min for 2 min every 30 min. The concentration change of CIP was determined using the spectrophotometric method (CE3021, Shanghai Precision & Scientific Instrument Co. Ltd., China) at 276 nm.

      where C0 and Ct are the concentrations of the initial CIP and CIP after t min irradiation, respectively.

    3.   Results and discussion
    • Fig. 2 shows the XRD patterns of GO, GO/dodecanediamine, GO/TiO2 precursor, and T-MLGs with different TiO2 contents. Compared with GO, the (001) peaks of GO/dodecanediamine and GO/TiO2 precursor shifted toward lower angles (from 11.1° to 7.3° and 11.1° to 7.67°, respectively), indicating the enlargement of interplanar distance as shown in Fig. 2(a). The large interplanar distance indicated that dodecanediamine or TiO2 sol had intercalated into the lamellar GO sheets [41]. Fig. 2(b) shows the XRD patterns of T-MLG-10.4, T-MLG-64.3, T-MLG-94.1, and pure TiO2. All samples showed a similar crystal structure, suggesting the formation of T-MLG nanocomposites after 200°C hydrothermal treatment. The intensity of the diffraction peaks increased as the TiO2 content increased. The diffraction patterns had five broad peaks at 25.3°, 37.8°, 48.0°, 54.0°, and 62.7°, corresponding to the (101), (004), (200), (211), and (204) planes of TiO2 (JCPDS No. 21-1272), respectively. However, no typical peak of graphene was observed in T-MLGs by XRD. Raman scattering was performed to confirm the existence of graphene in the nanocomposites.

      As shown in Fig. 3(a), the Raman characteristic peaks at 160, 409, 531, and 654 cm−1, which belong to TiO2, were observed in all T-MLGs, thus confirming the existence of TiO2 and consistent with the observation from the XRD in Fig. 2(b) [20]. Furthermore, two main peaks were observed at 1363 (peak D) and 1612 (peak G) cm−1 for all T-MLG samples, signifying the existence of graphene in T-MLGs. The intensity of graphene significantly decreased as the TiO2 content increased (Fig. 3(a)). The reason may be the replacement of the graphene surface by TiO2 nanoparticles [42].

      Figure 2.  XRD patterns of (a) GO, GO/dodecanediamine, and GO/TiO2 precusor and (b) T-MLGs with different TiO2 contents.

      Figure 3.  Raman spectra of (a) pure TiO2 and T-MLGs; (b) FTIR spectra of GO, T-MLG-64.3 and pure TiO2. Inset in (a) shows partial enlarged drawing of T-MLG-10.4.

      Although the existence of graphene and TiO2 has been confirmed by Raman spectra, the connection between TiO2 and graphene should be studied further by FTIR spectra. Fig. 3(b) displays the FTIR spectra of GO, T-MLG-64.3, and TiO2. The absorption bands at 3401, 1622, and 1371 cm−1 in GO and T-MLG-64.3 indicated the presence of O–H stretching vibrations, skeletal ring vibrations of C=C, and tertiary C–OH groups [43], respectively. The absorption bands at 1722 and 1049 cm−1 showed the C=O and C–O stretching vibrations of COOH groups, respectively. However, both adsorption bands disappeared on T-MLG-64.3, indicating the reduction of GO after hydrothermal treatment [4446]. Meanwhile, the peaks at 3423, 1629, and 461 cm−1 in TiO2 nanoparticles and T-MLGs corresponded to the stretching and bending vibrations of –OH and Ti–O stretching mode, respectively. TiO2 nanoparticles evidently existed in the T-MLGs. The broad absorption at 461 cm−1 in T-MLG-64.3 was considerably plumper than that in TiO2, and this result was ascribed to the formation of Ti–O–C vibration in T-MLG-64.3 [44]. The presence of Ti–O–C bonds indicated the chemical interaction between TiO2 and graphene [47].

      Figs. 4(a)4(c) show the SEM micrographs of T-MLG-10.4, T-MLG-64.3, and T-MLG-94.1, respectively. T-MLGs with low TiO2 content exhibited a plate-like morphology (Fig. 4(a)). With the increase in TiO2 content, TiO2 uniformly decorated and existed on the interlayer of graphene with high density (Figs. 4(b) and 4(c)). Moreover, the alveolate morphology of T-MLGs may be mainly attributed to the dehydroxylation and dehydration of TiO2 gel precursor to oxide pillars after hydrothermal treatment [48]. The morphology of T-MLG-64.3 was further investigated by transmission electron microscopy (TEM). As shown in Figs. 4(d) and 4(e), TiO2 nanoparticles were embedded in the graphene sheets. The HRTEM image of T-MLGs in Fig. 4(f) demonstrated that the TiO2-pillared structure was obtained successfully. The evidence was the enlarged interplanar distance of graphene sheets from 0.34 to 0.516 nm (inset in Fig. 4(f)).

      Figure 4.  SEM micrographs of (a) T-MLG-10.4, (b) T-MLG-64.3, and (c) T-MLG-94.1; (d)–(f) TEM images of T-MLG-64.3. Inset in (f) shows partial enlarged drawing.

      T-MLGs were prepared successfully via a facile method as follows: dodecanediamine pre-pillaring, ion exchange (Ti4+ pillaring), and interlayer in-situ formation of TiO2 by hydrothermal method based on the above analysis.

      The photocatalytic degradation of CIP (natural pH of 5.8) with 0.5 g/L photocatalysts was carried out in accordance with the preliminary experiment [49]. The adsorption property is an important factor affecting the photocatalytic performance. As shown in Fig. 5(a), the adsorption capacity of photocatalysts toward CIP after dark stirring for 30 min followed the order: T-MLG-64.3 > T-MLG-10.4 > T-MLG-94.1 > TiO2. T-MLG-64.3 exhibited the best adsorption property. A blank without photocatalyst was also prepared under the same conditions to eliminate the effect of photolysis. Self-degradation rate was 3% after 150 min due to negligible photooxygenation reactions in the presence of molecular oxygen (Fig. 5(b)) [50]. To verify the enhanced photocatalytic performance of T-MLG-64.3, we performed comparative experiment for the degradation of CIP by using pure TiO2 under the same conditions. The degradation efficiency of T-MLG-64.3 reached 78%, which was notably better than that of pure TiO2 (42%) (Fig. 5(b)). Fig. 5(c) shows the photocatalytic degradation of CIP kinetics and the fitting results, which followed pseudo-first-order kinetics. The apparent reaction rate constant of T-MLG-64.3 was as much as 3.89 times than that of pure TiO2. Five cycles of the photocatalytic experiments were conducted to examine the photostability and structure stability of T-MLGs, and one cycle lasted for 150 min. Fig. 5(d) offered the evidence that pillared structure nanocomposites are reusable and stable in recycling photocatalytic degradation experiments. After each cycle, the photocatalyst was washed and centrifuged for several times and finally irradiated under simulated sunlight for 60 min to ensure that the CIP molecules were removed completely. For comparison, the degradation rates of different organic pollutants over different kinds of graphene/TiO2 composites and TiO2 are listed in Table 1.

      Organic pollutantIrradiation systemGraphene content in
      graphene/TiO2 composite
      Degradation rate of pollutant / %Ref.
      Graphene/TiO2 compositePure TiO2
      Methylene blueSunlight8wt%98.67 (45 min)52 (2 h)[51]
      CiprofloxacinSunlight8wt%96.73 (60 min)65.52 (60 min)[51]
      Rhodamine-BSolar light0.09 g98 (60 min)42 (60 min)[18]
      Methylene blueUV lamp (λ = 365 nm)0.01 g~97 (150 min)~58 (150 min)[32]
      Methylene blueLED lamp (λ > 420 nm)0.01 g~67 (150 min)~58 (150 min)[32]
      Methylene blueUV lamp75wt%~61 (70 min)~35 (70 min)[52]
      CiprofloxacinLED lamp (λ > 420 nm)35.7wt%78 (150 min)42 (150 min)This work

      Table 1.  Photocatalytic properties of graphene/TiO2 composites in comparison with pure TiO2

      Figure 5.  (a) Adsorption of CIP in presence of different photocatalysts; (b) photocatalytic degradation of CIP; (c) pseudo-first order kinetics curves of the photocatalytic degradation of CIP (k—Apparent reaction rate constant; R2—Coefficient of determination); (d) photo-degradation rates of CIP after repeated cycles (irradiation time of 150 min).

      The PL spectroscopy intensity of T-MLG-64.3 was lower than that of pure TiO2 (Fig. 6). A high PL intensity indicates the easy recombination of electrons and holes [49, 53]. Thus, when TiO2 intercalated into the interlayer of graphene to form a TiO2-pillared structure, graphene instantly accepted the photoinduced electron to reduce the recombination of electrons and holes and boost the electron–hole separation in TiO2.

      Figure 6.  PL spectra of pure TiO2 and T-MLG-64.3 (excitation: 380 nm).

      The active species capture experiment was conducted in this study to determine the active species that play the main role in the photocatalytic process of CIP degradation. 1 mmol·L−1 isopropyl alcohol (IPA), 1 mmo·L−1 triethanolamine (TEOA), and 0.1 mmol·L−1 benzoquinone (BQ) were added as scavengers for hydroxyl radical ($ \cdot {\rm{OH}}$), hole (h+), and superoxide radical ($ \cdot {\rm{O}}_{\rm{2}}^ - $) to CIP solution in the presence of T-MLG-64.3 photocatalyst, respectively. Other experimental conditions were the same as given for the photocatalytic degradation of CIP. Fig. 7 shows the different degradation rates after adding scavengers and the significant role of active species during photocatalysis. The degradation rates of CIP decreased from 78% of the blank to 63.8%, 16.9%, and 33.8% with IPA, TEOA, and BQ, respectively. Therefore, the hole provided the greatest contribution to CIP decomposition, followed by superoxide radical and then hydroxyl radical. The major photocatalytic process for CIP degradation using T-MLG-64.3 was proposed with the following reactions (2)–(9).

      Figure 7.  Photocatalytic degradation rates of CIP for T-MLG-64.3 at presence of different scavengers after 150 min irradiation.

    4.   Conclusion
    • In summary, we constructed a new pillared structure of T-MLGs via a facile method as follows: dodecanediamine pre-pillaring, ion exchange (Ti4+ pillaring), and interlayer in-situ formation of TiO2 by hydrothermal method. We proved through various testing means that TiO2 nanoparticles had intercalated into the interlayer of graphene to form a TiO2-pillared structure. T-MLG with 64.3wt% TiO2 showed the optimum photodegradation rate and adsorption capabilities toward CIP. The photodegradation rate of T-MLG with 64.3wt% TiO2 was 78% under LED light irradiation for 150 min. Meanwhile, the pseudo-first-order rate constant of T-MLG with 64.3wt% TiO2 was 3.89 times that of pristine TiO2. Such result was mainly attributed to the special structure with enhanced photoinduced electron–hole separation in TiO2. Furthermore, the material exhibited high stability and reusability after five consecutive photocatalytic tests. The hole and superoxide radical were determined as the main contributors of CIP degradation. This work provides a facile method to synthesize semiconductor-pillared graphene nanocomposites by replacing TiO2 nanoparticles with other nanoparticles and a feasible technical for the sustainable utilization of photocatalysts in wastewater control.

    • This work was financially supported by the Youth Fund of Hebei Province Education Department, China (No. QN2017117) and the Hebei Natural Science Funds for the Joint Research of Iron and Steel, China (Nos. E2019209374 and E2015209278).

Reference (53)



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