A F-ion assisted preparation route to improve the photodegradation performance of a TiO₂@rGO system-how to efficiently utilize the photogenerated electrons in the target organic pollutants

Abstract

A comparison of the work function of reduced graphene oxide (rGO) and the conduction band position of TiO₂ reveals that the density of TiO₂ particles grown on rGO could affect the photodegradation efficiency of a TiO₂@rGO heterojunction. Herein, with the introduction of F-ions into the preparation route, F-doped shaped TiO₂ nanocrystals are densely and uniformly decorated on rGO sheets via the ice bath hydrolyzation method. Thus, more dye molecules are adsorbed on the surface of TiO₂ and the photogenerated electrons in the excited dye molecules could be efficiently utilized to improve the overall photodegradation efficiency. The as-prepared F-doped TiO₂@rGO heterojunction showed extremely high photocatalytic efficiency under UV-vis light irradiation compared with that of the commercial P25 and the mixture of F-TiO₂ and rGO. It is proved that the ice bath hydrolyzation preparation route is crucial to improve the photodegradation efficiency of the final product since the pure TiO₂@rGO heterojunction is also much more efficient than the mixture of F-TiO₂ and rGO. The present work provides new insights into efficiently utilizing the photogenerated electrons in the target organic pollutants.

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1. Introduction

Photodegradation of organic pollutants has attracted extensive attention during the past few decades due to their great potential in confronting energy and environmental challenges.^1–7 TiO₂ is regarded as one of the most promising candidate materials in the field of photocatalysis for its low toxicity, strong ultraviolet (UV) light absorption, excellent chemical and thermal stability and resistance to photocorrosion.^8–10 Up to now, a series of TiO₂ and TiO₂-based materials have been widely applied in the photodegradation and detection of pollutants in air or water.^11–15 Especially, the composites of TiO₂ and carbon (TiO₂–C) are currently being considered as potential photocatalysts in the purification of air and water.^2,16–25

RGO, owing to the large specific surface area, excellent conductivity, mechanical and chemical stability,²⁶ has received considerable attention in many potential applications.^27–35 Moreover, the functional groups of rGO could provide reactive sites for surface chemical modification, which facilitates its use in composites materials.^2,36,37 Therefore, the combination of TiO₂ and rGO (TiO₂@rGO) could be much more promising to improve the photocatalytic performance of TiO₂ due to the vectorial transfer of photogenerated electrons from TiO₂ to rGO sheets which effectively suppresses the charge recombination, leaving more hole carriers and promoting the degradation of dyes.^38–41 Up to now, there are many reports on the photocatalytic application of the TiO₂@rGO-based materials.^42–44 Though the photocatalytic activity has been greatly enhanced compared with pure TiO₂, the photodegradation is generally realized by utilizing the photogenerated electron–hole pairs in TiO₂. Thus, the importance of efficient utilization of the photogenerated electrons in the target degradation dye molecules is always ignored. It is known that the work function of rGO (−4.42 eV) is larger than the conduction band position of TiO₂ (−4.40 eV), and the injection rate of the electrons from the excited dyes* into the rGO sheets is even faster than that into TiO₂.^44,45 However, the injected electrons in rGO could hardly promote the subsequent degradation efficiency.^44,45 Only those excited dyes adsorbed on the surface of TiO₂ could directly inject electrons into TiO₂, forming reactive oxygen species (ROSs) on TiO₂. The dyes which lost electrons are subsequently degraded by these ROSs, thus greatly enhancing the degradation efficiency. Rhodamine B (RhB) is a typical simulating organic pollutant molecule frequently used for evaluating the photodegradation efficiency of a photocatalyst. The work function of RhB and excited RhB* are −5.45 and −3.08 eV, respectively.^44,45 When RhB is used to evaluate the photodegradation efficiency of a TiO₂@rGO system, the energy band diagram can be expressed by Scheme 1. It is shown that if the surface of rGO sheets is not fully covered with TiO₂ nanoparticles, most of the RhB molecules will be adsorbed on rGO, the photogenerated electrons in the excited RhB* cannot flow to TiO₂ (line 1 in Scheme 1) and have little contribution to the photocatalytic activity. On the contrary, if most of the RhB molecules are adsorbed on TiO₂ nanoparticles, the excited RhB* could directly inject electrons into TiO₂ (line 2 in Scheme 1), thus the photogenerated electrons in RhB* could be efficiently utilized. Therefore, the dense and uniform growth of TiO₂ on rGO sheets, which is beneficial for the adsorption of the majority of dyes on the surface of TiO₂, is crucial to improve the photodegradation efficiency of the TiO₂@rGO system. However, this critical issue has been ignored for a long time and the realization and control of dense and uniform growth of TiO₂ on rGO sheet is still a great challenge.⁴⁶

Scheme 1 The energy band diagram of RhB and a TiO₂@rGO system (lines 1, 2 and 3 stand for the transferring route of the photogenerated electrons in RhB*; lines 4 and 5 stand for the recombination process of the electrons with RhB˙⁺).

Herein, the ice bath hydrolyzation method is adopted to achieve the dense and uniform growth of anatase TiO₂ nanoparticles on rGO sheets. Furthermore, F-ions are introduced to further improve the crystallinity of TiO₂ nanoparticles. The F-doped TiO₂@rGO can completely photodegrade the RhB solution with a concentration of 30 mg L⁻¹ within 6 min under UV-vis light irradiation, which is in sharp contrast with the F-doped TiO₂–rGO mixture, which can only degrade 60% of the RhB. It is proved that the dense cover of TiO₂ nanoparticles on rGO sheets, no matter with or without the doping of F-ion, can help to efficiently utilize the photogenerated electrons in the target RhB molecules to enhance the final photodegradation efficiency.

2. Experimental

2.1. Catalyst preparation

Graphene oxide (GO) was prepared using a modified Hummers' method.^47,48 0.55 mL of TiCl₄ kept in a refrigerator was extracted using a 1 mL syringe and rapidly injected into the bottom of a parafilm sealed 100 mL bottle with 50 mL of deionized (DI) water in the form of ice. After that, the bottle was firmly covered with the bottle cap and shaken continuously until the ice was dissolved. In the whole process, no white precipitation was observed. Then 9.57 mL of GO solution (4.6 mg mL⁻¹) was injected into the above bottle under stirring to disperse it well. After that, 0.21 g of NaF was added to the above TiO₂/GO solution to prepare the F-doped rGO@TiO₂ heterojunction. Then the solution was kept stirring in a water bath at 50 °C for 48 h. The color of the solution turned from transparent to gray slowly. The precipitate was collected by centrifugation after being washed with DI water and ethanol several times and then was dried completely at 70 °C. Finally, the product was kept in a ceramic crucible and annealed in a tubular oven at 500 °C for 120 min with nitrogen gas protection. For the preparation of pure TiO₂@rGO heterojunction: all the procedures were the same with that of F-doped TiO₂@rGO heterojunction without the addition of NaF in the precursor solution. The weight ratio of rGO in these two samples is 11%. The other TiO₂@rGO heterojunctions with different weight ratios of rGO were obtained by tuning the volume of the GO solution.

2.2. Catalyst characterization

X-ray diffraction (XRD) measurement was carried out using powder XRD (Bruker D8 Advance, with Cu K_α radiation operating at 40 kV and 40 mA, scanning from 2θ = 10° to 80°). Transmission electron microscope (JEM-2011 TEM, 200 kV) and Raman spectroscopy (LabRAM HR Evolution, 532 nm) were used to characterize the samples. The X-ray photoelectron spectroscopy (XPS) measurement was conducted on a EscaLab 250Xi spectrometer (Thermo Scientific) using monochromatic Al K_α X-ray source (anode HT = 15 kV) operating at a vacuum higher than 2 × 10⁻⁹ mbar. Nitrogen adsorption–desorption isotherms were measured on the system (Quantachrome, USA) at −196 °C. The specific surface area and porosity property analysis was calculated by the Braunauer–Emmett–Teller (BET) method. The optical property of the present nanocomposites was measured with diffuse reflectance UV-vis spectrometer (Shimadzu, SolidSpec-3700DUV) using an integrating sphere and adopting BaSO₄ as a reference.

2.3. Photocatalytic measurement

The photodegradation of RhB was measured under UV-vis light irradiation (300 ≤ λ ≤ 2500 nm) at room temperature. The light source is a 300 W Xe-arc lamp (PLS-SXE300, Beijing). In a typical process, 30 mg of catalyst was firstly dispersed in 42 mL DI water, and then mixed with 18 mL of 100 mg L⁻¹ RhB solution. Thus, the concentration of the final RhB solution is 30 mg L⁻¹. Before irradiation, the suspension was stirred in dark for 40 min to ensure the adsorption–desorption equilibrium. At a given time interval of 1 min, 2 mL solution was taken out and immediately centrifuged to remove the catalyst completely. In the durability test of the F-doped TiO₂@rGO heterojunction in the photodegradation of RhB under UV-vis light, three consecutive cycles were tested. After each cycle, the F-doped TiO₂@rGO heterojunction was filtered, washed thoroughly with water and annealed at 400 °C for 2 h. The concentration of the RhB solution was analyzed on a UV-vis spectrophotometer (UV-1800, Shimadzu). The pH value of the RhB solution was measured by a pH-meter (METTLER TOLEDO, PE20k) after completely removing the catalyst from solution (5 mL) by centrifugation.

3. Results and discussion

The preparation route of the F-doped TiO₂@rGO heterojunction is illustrated in Scheme 2. Generally, ice bath is adopted to ensure a slow hydrolyzation of TiCl₄ to form TiO₂ nanocrystal seeds with extremely small size on rGO sheets. The low temperature (50 °C) growth method could retard the rapid growth of TiO₂ nanocrystals in the solution. Thus, the further growth of TiO₂ on the seeds is expected to result in the dense and uniform growth of TiO₂ nanoparticles on rGO sheets. Moreover, the introduction of F-ion could enhance the photocatalytic activity of TiO₂ by improving the crystallinity of TiO₂.

Scheme 2 The preparation route of the F-doped TiO₂@rGO heterojunction.

To investigate the crystal phases of the F-doped TiO₂@rGO heterojunction, X-ray diffraction (XRD) analysis was performed (Fig. 1a). All the diffraction peaks are sharp, and can be indexed to anatase TiO₂ (JCPDS, 84-1286), suggesting a good crystallinity of TiO₂ nanoparticles. According to the Scherrer formula:

D = 0.89λ/β cos(θ) (1)

where D is average grain size, λ is the X-ray wavelength employed, θ is the diffraction angle, and β is the line broadening at half the maximum intensity after subtracting the instrumental broadening. The particle size of the decorated TiO₂ nanoparticles could be calculated by typical XRD peaks (Table S1†). Taking the (101) diffraction peak as an example, the size of the TiO₂ nanoparticles in the F-doped TiO₂@rGO heterojunction is around 12 nm. From the Raman spectra (Fig. 1b), the graphitized structures (D band (∼1341 cm⁻¹) and G band (∼1580 cm⁻¹)) were observed in both the F-doped TiO₂@rGO heterojunction and GO. It is shown that there is no peak position shift of the F-doped TiO₂@rGO heterojunction compared with that of GO. Furthermore, the intensity ratio of D band over G band (I_D/I_G) increases from 1.05 to 1.09, indicating the reduction of GO with the hydrothermal treatment.^28,49,50 X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. 1c) of the F-doped TiO₂@rGO shows that all the peaks of C, O, F and Ti can be observed, suggesting the presence of F, C and Ti species. The C 1s XPS signal shows that the three peaks at 284.6, 286.4, and 289.4 eV could be attributed to the C–C, C–O and C=O bonds in rGO-based composites (Fig. S1a†). Two peaks centered at 459.0 and 465.1 eV in the Ti 2p spectrum (Fig. S1b†) can be assigned to Ti 2p1/2 and Ti 2p3/2, respectively, which is in good agreement with the binding energy values of Ti⁴⁺ in pure anatase TiO₂, suggesting the formation of TiO₂ on the rGO sheets.⁵¹

Fig. 1 (a) X-ray diffraction (XRD) spectrum of the F-doped TiO₂@rGO heterojunction; (b) Raman spectra of the F-doped TiO₂@rGO heterojunction and GO; (c) XPS survey spectrum of the F-doped TiO₂@rGO heterojunction; (d) TEM, (e) enlarged TEM and (f) HRTEM images of the F-doped TiO₂@rGO heterojunction.

Owing to the small particle size, the supported TiO₂ nanoparticles can be hardly observed in the scanning electron microscopy (SEM) image (Fig. S2†). Here, the TEM image of the F-doped TiO₂@rGO heterojunction confirms that the TiO₂ nanoparticles were densely and uniformly decorated on rGO sheets (Fig. 1d). Compared with the previously reported TiO₂@rGO systems,^2,17,52 the decorated TiO₂ nanoparticles are much more dense and uniform in the present system. It is supposed that the formation of seeds in ice bath method greatly helps the in situ growth of the TiO₂ nanoparticles on rGO sheets, favouring the photoinduced electron transfer from TiO₂ to rGO. Both the edge of rGO and the structure of the TiO₂ nanoparticles were shown in an enlarged TEM image (Fig. 1e). RGO sheet behaves as a support for TiO₂ nanoparticles, and the TiO₂ nanoparticles are shaped due to the introduction of F-ion. The average size of the TiO₂ nanoparticles in F-doped TiO₂@rGO heterojunction is 13 nm (Fig. S3†), which is very consistent with the above calculated crystal size along (101) diffraction peak. All the observed lattice fringes of the shaped TiO₂ nanoparticles show a d-spacing of 0.357 to 0.360 nm (Fig. 1f), which can be well assigned to the (101) lattice fringes of anatase TiO₂.

Fig. 2a shows the photodegradation performance of the F-doped TiO₂@rGO heterojunction towards RhB with time under UV-vis light irradiation as a function of time. Here, C and C₀ represent the remaining concentration of dye solution with certain degradation time and after dark adsorption. And for comparison, the photodegradation performances of rGO, F-TiO₂, the mixture of F-TiO₂ and rGO, and P25 under the same condition are also shown. Before light irradiation, the RhB solutions with catalysts were maintained in dark for 40 min under stirring, this time period is proved to be long enough to reach the adsorption equilibrium as shown in Fig. S4.† And the remaining concentration of RhB was calibrated via the fitting curve (Fig. S5†). It is clearly shown that the F-doped TiO₂@rGO heterojunction possesses much better adsorption ability towards RhB compared with F-TiO₂, the mixture (F-TiO₂ and rGO), or P25 due to the present preparation route induced large surface area (Fig. S6 and S7 and Table S2†), which can be attributed to the dense and uniform growth of TiO₂ on rGO and the little ratio of bare rGO between covered TiO₂ nanocrystals. The F-doped TiO₂@rGO heterojunction shows much better photocatalytic efficiency than F-doped TiO₂, suggesting the formation of heterojunction between F-doped TiO₂ and rGO. The photodegradation efficiency of F-doped TiO₂ increased from 25% to 100% with the formation of the heterojunction structure since it can greatly suppress the electron–hole recombination. Similarly, the performance of the F-doped TiO₂@rGO heterojunction is more efficient than P25 for the same reason. Moreover, the performance of the mixture of the same amount of F-doped TiO₂ and rGO was much lower than that of the F-doped TiO₂@rGO heterojunction. The remaining amount of RhB at 6 min was still 44.9% when the mixture was used as the photocatalyst. It should be noted that in the whole adsorption and degradation process, the pH value maintains at a constant value of around 5.8 (Fig. S8†), thus, the pH value has no effect on the RhB degradation rate during the whole process. Moreover, there is no obvious change in band gap of TiO₂ by the introduction of F-ion (Fig. S9†), confirming that the influence of light absorption by TiO₂ can be ignored. Thus, it is considered that the dense and uniform growth of TiO₂ nanocrystals on rGO greatly favours the adsorption of the majority of dyes on the surface of TiO₂. As a result, the final photodegradation efficiency of the F-doped TiO₂@rGO heterojunction is much higher than the simple mixture. This conclusion could be further supported by the much better photocatalytic performance of F-doped TiO₂@rGO heterojunction than those of F-TiO₂ and the mixture under visible light (Fig. 2b). The photodegradation efficiencies of F-doped TiO₂@rGO and the mixture under visible light are 45% and 6% at 30 min, respectively. Considering that RhB solution is stable even under UV-vis light illumination (Fig. S10†), and holes in TiO₂ could not be generated, the efficient utilization of the photogenerated electrons in RhB molecules by the present F-doped TiO₂@rGO catalyst can be well confirmed. One should note that the photodegradation efficiency of F-doped TiO₂@rGO under visible light is only 7.2% at 6 min, which falls far behind that under UV-vis light illumination (100%), indicating that holes produced by the band-gap excitation of TiO₂ still plays a dominant role in the photodegradation process, while the dense and uniform growth of F-doped TiO₂ nanocrystals on rGO further enhances the photocatalytic efficiency by the efficient utilization of the photogenerated electrons in the target RhB molecules. Recyclability is another key factor for photocatalysts. Fig. 2c shows 3 cycles of photodegradation of the F-doped TiO₂@rGO heterojunction towards RhB. It is proved that the good photocatalytic activity could be maintained perfectly after 3 cycles, implying the promising practical application potential of the F-doped TiO₂@rGO heterojunction. RhB solutions with high concentrations (40 and 50 mg L⁻¹) can also be photodegradated in 12 and 16 min with 30 mg F-doped TiO₂@rGO heterojunctions under UV-vis light irradiation (Fig. 2d), which further confirms the practical application capability of the present F-doped TiO₂@rGO heterojunctions.

Fig. 2 (a) The liquid-phase photodegradation performances of rGO, F-doped TiO₂, the physical mixture (F-doped TiO₂ and rGO) and F-doped TiO₂@rGO heterojunction towards RhB with time under UV-vis light irradiation (300 ≤ λ ≤ 2500 nm); (b) the liquid-phase photodegradation performance of the F-doped TiO₂, mixture, F-doped TiO₂@rGO towards RhB with time under visible light irradiation (400 ≤ λ ≤ 2500 nm); (c) cycling photodegradation towards RhB; (d) the liquid-phase photodegradation performance of F-doped TiO₂@rGO heterojunction towards RhB (40 and 50 mg L⁻¹) with time under UV-vis light irradiation.

To confirm that the dense and uniform growth of TiO₂ nanoparticles on rGO can efficiently utilize the photogenerated electrons in the target RhB molecules, pure TiO₂@rGO heterojunction was also prepared to exclude the influence of F-ion on the crystallinity and morphology of TiO₂.^53,54 The TEM and HRTEM images of the pure TiO₂@rGO heterojunction were shown in Fig. 3a and b, respectively. From the Fig. 3a, it can be found that TiO₂ nanoparticles also were densely and uniformly decorated on rGO sheets, suggesting the crucial role of the present preparation route to the dense growth of TiO₂ on rGO. And TiO₂ nanoparticles are spherical (compared with the shaped TiO₂ nanocrystals in Fig. 1e). From the HRTEM image of the pure TiO₂@rGO heterojunction (Fig. 3b), it can be found that the lattice fringes of TiO₂ show a d-spacing of 0.358 to 0.364 nm, which also can be well assigned to the (101) lattice fringes of TiO₂. It can be concluded that the introduction of F-ion could modulate the morphology of TiO₂. The corresponding average size of the spherical TiO₂ nanoparticles in the pure TiO₂@rGO heterojunction is 7 nm (Fig. S11a†), which agrees well with the surface and pore measurement results (Fig. S11b and Table S2†). All the diffraction peaks (Fig. 3c) can also be indexed to anatase TiO₂ (JCPDS, 84-1286). However, the diffraction peaks are not as sharp as those of the F-doped TiO₂@rGO heterojunction, suggesting the crucial role of F-ion in improving the crystallinity of TiO₂ nanocrystals. The Fig. 3d shows the photodegradation performance of the pure TiO₂@rGO heterojunction towards RhB under UV-vis light irradiation. When the photodegradation time is 6 min, the remaining amount of RhB is 19%, which is still more efficient than the mixture. It is further proved that the dense cover of TiO₂ nanocrystals on rGO sheets, no matter with or without the doping of F-ion, can help to efficiently utilize the photogenerated electrons in RhB and enhance the final photodegradation efficiency. However, with the help of F-ion, the photodegradation efficiency is even better considering the higher surface area of the pure TiO₂@rGO heterojunction (263 m² g⁻¹, Table S2†).

Fig. 3 (a) The TEM image, (b) HRTEM image and (c) XRD pattern of the pure TiO₂@rGO heterojunction; (d) the liquid-phase photodegradation performance towards RhB.

To further confirm the influence of the density and uniformity of TiO₂ on the photocatalytic performance, F-doped TiO₂@rGO heterojunctions with different weight ratios of rGO (1, 5, 13 and 15%) were prepared, and the surface areas of them are 53, 101, 203 and 182 m² g⁻¹ (Fig. S12 and Table S2†), respectively. It is found that these samples could degrade 64%, 29%, 95% and 88% of RhB within 6 min as shown in Fig. 4, respectively. Although all these samples are much more efficient than the mixture (Fig. 2a), the content of rGO can affect the photocatalytic activity of F-TiO₂@rGO remarkably. A lower content of rGO would not favour the growth of TiO₂ on rGO and there is free TiO₂ nanocrystals in the catalyst, thus, the photodegradation efficiency is low due to the inefficient separation of the electron–hole pairs. The excessive content of rGO would induce more RhB molecules adsorpted on rGO rather than on the surface of TiO₂, which would not benefit the efficient utilization of the photogenerated electrons in RhB molecules. Thus, it is considered that a proper content of rGO could realize the dense and uniform growth of TiO₂ on rGO sheets and to ensure the efficient separation of the electron–hole pairs and the sufficient adsorption of RhB molecules on TiO₂.

Fig. 4 The liquid-phase photodegradation performances of F-doped TiO₂@rGO heterojunction with different weight ratios of rGO (1, 5, 13 and 15%) towards RhB under UV-vis light irradiation.

4. Conclusions

In conclusion, an ice bath hydrolyzation method is adopted to grow dense and uniform F-doped TiO₂ nanocrystals on rGO sheets. The as-prepared F-doped TiO₂@rGO heterojunction showed extremely high photocatalytic efficiency under UV-vis light irradiation compared with that of the commercial P25 and the mixture of F-TiO₂ and rGO. It is proved that the dense and uniform growth of TiO₂ nanocrystals on rGO greatly favors to utilize the photogenerated electrons in RhB. The pure TiO₂@rGO heterojunction is also much more efficient than the mixture of F-TiO₂ and rGO, proving that the ice bath hydrolyzation preparation route is crucial to ensure the dense and uniform growth of TiO₂ nanocrystals on rGO and improve the photodegradation efficiency of the final product. The present work presents a new idea on how to efficiently utilize the photogenerated electrons in the target organic pollutants.

Acknowledgements

We thank the financial supports from the National Natural Science Foundation of China (51372273), Xinjiang Program of Introducing High Level Talents, “Hundred Talents Program” of the Chinese Academy of Sciences.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: XPS, SEM, UV-vis absorption spectrum, and the size distribution. See DOI: 10.1039/c5ra21948e

‡ Linjuan Guo and Zheng Yang contribute equally to this work.

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