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Graphene oxide capturing surface-fluorinated TiO2 nanosheets for advanced photocatalysis and the reveal of synergism reinforce mechanism Kai Dai,a Luhua Lu,*b Qi Liu,*c Guangping Zhu,a Qinzhuang Liua and Zhongliang Liua Surface-fluorinated TiO2 (F–TiO2) nanosheets with exposed (001) facets were synthesized from a scalable hydrothermal treatment assisted by a specific stabilization effect of fluorine ions on the (001) facets. Assembly of F–TiO2 on graphene oxide (GO) sheets into GO/F–TiO2 hybrid in aqueous solution was further achieved by making use of the surfactant role of GO. Photocatalytic properties of GO/F–TiO2 hybrid were evaluated under 365 nm UV light irradiation for photodegradation of methylene blue (MB). An optimal GO content has been determined to be 3 wt%, and the corresponding apparent pseudo-firstorder rate constant Kapp is 0.0493 min−1, 3 times and 4 times more than that of pure TiO2 nanosheets and commercial P25 photocatalyst, respectively. To reveal the synergism reinforce mechanism of GO/F–TiO2 hybrid, photo absorption, surface absorption, and the photoelectrochemical current properties have been intensively studied. We found that enhanced electron–hole separation has been the key issue for the reinforcement of photocatalytic performance. F–TiO2 with exposed (001) facet has stronger electron–
Received 15th September 2013, Accepted 14th October 2013 DOI: 10.1039/c3dt52542b www.rsc.org/dalton
1.
hole separation resulting in a higher photoelectrochemical current than that of P25 photocatalyst. Moreover, hybridization of F–TiO2 with GO could further increase the photoelectrochemical current and the largest current for the optimal weight fraction of GO is in good accordance with the photocatalysis performance. The GO/F–TiO2 interface junction that favors the electron–hole separation was attributed to the photoelectrochemical current enhancement.
Introduction
Titanium dioxide (TiO2) has been the most famous photocatalyst in a variety of applications such as pollutants degradation, water splitting, fuel cells and solar cells for its wide bandgap, highly stable performance, environmental compatibility and scalable industrial production.1 TiO2 exists in three crystallographic forms in nature: brookite, anatase and rutile. Anatase plays a dominant role in photocatalysis applications due to a generally observed superior photocatalytic efficiency as well as facile synthesis.2 Scientists have demonstrated that the order of the average surface energies of anatase TiO2 is 0.90 J m−2 for {001} > 0.53 J m−2 for {100} > 0.44 J m−2 for {101}.3 Increasing attention has thus focused on the synthesis
a
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, P.R. China b State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P.R. China. E-mail: [email protected] c Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, P.R. China. E-mail: [email protected]
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of anatase TiO2 nanostructure with dominant {001} facets. Dai et al. recently demonstrated a method to prepare anatase TiO2 nanocrystals with exposed {001} facets with the aqueous acetic acid solution, but only with 9.6% of the surface being enclosed by {001} facets.4 Yang et al. recently reported the synthesis of micro-sized anatase TiO2 crystallites with highly energetic {001} facets exposed with the help of hydrofluoric acid (HF). However, the percentage of exposed (001) facets was only 47%, and the crystallite size was relatively large.5 Han et al. made an important progress on the synthesis of anatase TiO2 nanosheets, and the percentage of (001) facets in the sheets is surprisingly as high as 80%.6 Gordon et al. reported a method to engineer the percentage of {001} and {101} facets of uniform anatase TiO2 nanocrystals through the choice of cosurfactant and titanium precursor.7 However, the photocatalytic efficiency on a bare TiO2 remains quite limited, mainly due to the rapid recombination rate of photogenerated electron–hole pairs within TiO2 particles.8 Of special interests are the recent attempts to modify TiO2 with nanostructured carbonaceous materials such as carbon fiber, activated carbon, amorphous carbon, carbon nanotubes and fullerenes.9 Within all these materials, graphene, an sp2-bonded carbon atoms with monolayers of carbon atoms
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arranged in a honeycombed network, has recently received the most attention in the field of material science because of its unique qualities, which include unique optical, electrical and electrochemical properties resulting from long-rangeconjugation.10 Graphene/TiO2 composites have been reported to exhibit interesting photocatalytic activities. Wang et al. reported the enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air.11 Zhang et al. reported graphene–P25 composites synthesized under hydrothermal conditions and showed their enhanced photocatalytic activity for degradation of methylene blue (MB) in aqueous solutions.12 Their hybridization has been found to enhance the photocatalytic activity in the degradation of organic pollutions, the water photocatalytic splitting, and antibacterial application.13 They have exhibited somehow enhanced photocatalytic performance than pure TiO2 while new problems have arisen such as lacking of scalability for complicated materials’ preparation methods and the variety of unconfirmed mechanisms (e.g. enhanced surface absorption, light absorption or electron–hole separation) for photocatalyst performance enhancement.14 All these problems need a good resolution for further development. Graphene’s derivative graphene oxide (GO) has received a great deal of attention for environmental applications because of the large surface area, light weight and it possesses a large amount of delocalized electrons.15 The coexisting of hydrophobic aromatic carbon and hydrophilic oxygen groups endow GO with character of normal surfactant.16 Moreover, light absorption of GO is largely reduced than graphene, which could favor the light irradiation on TiO2 surface.17 Thus, designing a hybrid composed of GO and TiO2 with well exposed {001} facets for highly enhanced photocatalytic performance by a scalable solution approach is reasonable and attractive. In this work, surface-fluorinated TiO2 nanosheets (F–TiO2) hybridized with GO were prepared on a scalable approach without any catalysts or templates. The high-quality F–TiO2 nanosheets prepared from hydrothermal reaction were found to be capable of dispersed by GO and well anchored on the GO surface, endowing the hybrid with interfacial junction and solution processability. The as-prepared GO/F–TiO2 nanosheet composites possess well exposed (001) high energy facets, which has shown stronger photocatalytic performance than pure TiO2 nanosheet and the well known commercial P25. To reveal the mechanism of the reinforced photocatalyst performance, hybrids of different GO weight fraction have been prepared and their photocatalyst performance along with light absorption, specific surface area and photoelectrochemical current have been characterized. The synthesized sample exhibits attractive recyclable photocatalytic properties and high potential for practical applications. Photocurrent test has indicated that the existence of GO has obviously reinforced the photoelectron–hole separation, which favors the enhancement of the photocatalytic performance. It is found that the enhanced electron–hole separation was the key issue for the reinforcement of photocatalytic performance except the bulk
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light absorption of hybrid and dye absorption on the hybrid surface. We think that this simple approach as a highly efficient photocatalyst preparation method and clarification of the mechanism for performance enhancement through hybridization of semiconductor nanomaterials with GO could be of value for their further development.
2. Experimental 2.1
Materials
Natural graphite powder (325 mesh) was commercially obtained from Alfa-Aesar, commercial TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. MB, tetrabutyl titanate (Ti(OC4H9)4), concentrated sulphuric acid (98%, H2SO4), and potassium permanganate (KMnO4) and polyethylene glycol (PEG) were provided from Shanghai Chemical Reagent Co. Ltd, China. Aqueous hydrogen peroxide solution (H2O2, 30%), sodium sulphate (Na2SO4), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), HF (40 wt%) and ammonia solution (NH4OH, 25%) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Double distilled water was used for solution preparation. All reactants were of analytical purity and used as received without further purification. 2.2
Preparation of GO
GO was synthesized by the modified Hummers’ method.18 In a typical preparation procedure, 3.0 g of graphite was put into a mixture of 12 mL of concentrated H2SO4, 2.5 g of K2S2O8, and 2.5 g of P2O5. The solution was heated to 80 °C and kept under stirring for 4.5 h in oil bath. Then the mixture was diluted with 500 mL of deionized water, and the product was obtained by filtering using a 0.2 μm nylon film and dried under ambient condition. Thereafter, 15 g KMnO4 was added gradually with stirring, to prevent the temperature of the mixture from exceeding 10 °C. The ice bath was then removed and the mixture was stirred at 35 °C for 2 h. The reaction was terminated by adding 700 mL of distilled water and 20 mL of 30% H2O2 solution. Finally, the GO was recovered by filtration and drying. 2.3
Preparation of F–TiO2 nanosheet
25 mL of Ti(OC4H9)4, 3 mL HF were initially vigorously stirred for 0.5 h to form the mixture. Then, the mixture was transferred into a 50 mL Teflon-lined autoclave and subsequently heated at 180 °C for 24 h. After hydrothermal reaction, the white precipitates were harvested by centrifugation and throughout washing with water and ethanol, and then dried in an oven at 60 °C for 6 h obtaining F–TiO2. 2.4
Preparation of GO/F–TiO2 composites
A certain amount of GO and 1 g of the as-prepared F–TiO2 were added in 100 mL water, and then ultrasonicated at 30 °C for 40 min to obtain a uniform mixture. After filtering with double distilled water and drying at 60 °C for 6 h, GO/F–TiO2
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composite photocatalysts were obtained. To investigate the effect of the GO content on the photocatalytic performance rates of the GO/F–TiO2 composites, the weight percentages of GO to F–TiO2 were varied from 0 to 5 (1, 2, 3, 4, and 5 wt%) by varying the weight of GO, and the samples are presented as x% GO/TiO2, where x is the weight content of GO. 2.5
Analytical and testing instruments
The structure of the samples was observed on a Tecnai G2 F20 S-Twin HRTEM at an accelerating voltage of 100 kV with Inca Energy-dispersive X-ray spectroscopy (EDX). The X-ray photoelectron spectra (XPS) of the samples were measured using a Thermo ESCALAB 250 with an Al Kα X-ray photoelectron spectrometer at 150 W. X-ray diffraction (XRD) data for samples were collected using a Rigaku D/MAX 24000 diffractometer at room temperature with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 5° to 70°, operated at 40 kV and 40 mA, and a scanning speed of 10° min−1. The infrared absorption spectra were recorded over the frequency range from 400 to 4000 cm−1 using a Nicolet 6700 FT-IR spectrophotometer. The spectra were measured after the spectrum scan of the blank pure KBr pellet. The Brunauer–Emmett–Teller (BET) specific surface area values were determined by using nitrogen adsorption data at 77 K obtained by a Micromeritics ASAP 2010 system with multipoint BET method. UV-vis diffuse reflectance spectroscopy (DRS) measurements were carried out using a Hitachi UV-3600 UV-vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 250 to 600 nm, and BaSO4 was used as a reflectance standard. The total organic carbon (TOC) was determined by using an Elementar Liqui TOC II analyzer. The photoelectrochemical measurements were measured on a CHI-660D electrochemical system, using a conventional three-electrode cell. The counter and the reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The electrolyte solution was 1 M Na2SO4. The working electrodes were prepared as follows: 0.1 g ground sample was mixed with 0.02 g PEG and 0.5 ml distilled water was added to make a slurry. The slurry was then injected onto a 1.0 cm × 1.0 cm ITO glass electrode and these electrolytes were dried at 60 °C for 2 h and then calcined at 250 °C for 4 h. 2.6
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concentration was about 664 nm. The degradation efficiency was calculated as follows:19 η¼
C0 C 100% C0
ð1Þ
where C0 is the absorbance of the original MB solution and C is the absorbance of the MB solution after UV light irradiation. According to the Langmuir–Hinshelwood kinetics model, the photocatalytic process of MB can be expressed as the following apparent pseudo-first-order kinetics equation:20 ln
C0 ¼ kapp t C
ð2Þ
where kapp is the apparent pseudo-first-order rate constant, C0 is the original MB concentration and C is the MB concentration in aqueous solution at time t.
3. Results and discussion Fig. 1 shows the XRD patterns of TiO2-based samples in comparison to the pattern of GO. For GO, the XRD peak at 10° for the (001) facets corresponds to 0.87 nm interlayer spacing, which is much larger than that of pristine graphite (0.34 nm) due to the introduction of oxygen-containing functional groups on the graphite sheets.21 As can be seen from Fig. 1, only anatase phases of TiO2 (JCPDS no. 21-1272, space group: I41/amd (141), a = 0.379 nm and c = 0.951 nm) was observed in pure TiO2. Notably, no characteristic diffraction peaks of GO were observed in all the GO/F–TiO2 hybrids. And our results correlate well with the previous studies that the diffraction peaks become weakened or even disappear whenever the regular stacks of GO are exfoliated.22 A possible explanation is that, with the help of ultrasonic, TiO2 nanocrystals are intercalated into stacked GO layer,23 leading to the exfoliation of GO. Fig. 2a and 2b show typical SEM and TEM images for the as-prepared F–TiO2 nanosheets. A large amount of F–TiO2 nanosheets with side length of 50–60 nm and thickness of 10–15 nm can be easily observed. As indicated in Fig. 2b, HRTEM image directly shows that the lattice spacing parallel
Photocatalytic experiment
The photocatalytic activity of the samples was evaluated by the photocatalytic degradation of MB under 250 W 365 nm Hg lamp irradiation. The photocatalytic experiments were carried out in a reactor containing a 100 mL 10 mg L−1 aqueous solution of MB and 0.1 g photocatalysts. The distance between the lamp and the reactor was 10 cm. Before irradiation, the suspension was magnetically stirred in the dark for 1 h to establish an adsorption/desorption equilibrium under ambient conditions. Then, the mixture was exposed to the UV light irradiation. At the given irradiation time, the concentration of MB was quantified by the absorbance, and the absorbance wavelength used in the spectrophotometer to determine MB
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Fig. 1
XRD patterns of GO, F–TiO2 and GO/F–TiO2 composites.
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Fig. 2 SEM image of (a) F–TiO2 nanosheets and TEM images of (b) F–TiO2 nanosheets, (c) GO and (d) 3% GO/F–TiO2. The insets of (b) and (d) show HRTEM images of F–TiO2 nanosheets and 3% GO/F–TiO2.
to the top and bottom facets is ca. 0.235 nm, corresponding to the (001) planes of anatase F–TiO2, which indicates the top ˉ) and bottom facets of the nanosheets are the (001) and (001 planes, respectively. Another set of the lattice fringes with spacing of 0.35 nm, corresponding to the (101) planes of anatase, can be also clearly revealed from the sheets lying on the TEM grid. On the basis of the TEM and HRTEM results, we can calculate the percentage of exposed {001} facets on the F–TiO2 nanosheets is 62%–71%. The fluorine ions will play a key role in the formation of F–TiO2 nanosheets with high percentage of exposed {001} facets.6 All the prepared F–TiO2 samples display good crystallinity. The TEM image in Fig. 2d presents well-dispersed F–TiO2 nanosheets on the surface of GO that served as support and electron donor. The surface attachment of F–TiO2 nanosheets, instead of complete or partial wrapping by GO, assured full exposure of both GO sheets and F–TiO2 nanosheets to contact the solution, which will improve the photocatalytic activity. Fig. 3a and 3b show the EDX spectrum of the as-synthesized GO and 3% GO/F–TiO2 nanosheets, respectively. The pattern indicates that GO only contain elements of C and O, and 3% GO/F–TiO2 nanosheets only contain elements of C, O, Ti and F, without any other impurities.
Fig. 3 EDX spectrum of (a) as-synthesized GO and (b) 3% GO/F–TiO2 nanosheets.
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Fig. 4 The overview (a) and the corresponding high-resolution XPS spectra (b) C1 s, (c) O1 s, (d) Ti2p and (e) F1 s of the as-prepared 3% GO/ F–TiO2.
The XPS spectrum including signals for C1 s, Ti2p, O1 s and F1 s of 3% GO/F–TiO2 nanosheets to probe the chemical environment of the elements in the near surface range is shown in Fig. 4a. As indicated in Fig. 4b, the asymmetrical and broad features of the observed C1 s peaks suggest the co-existence of distinguishable models. A deconvolution core level spectra at about 284.8, 286.8 and 288.7 eV has been given. The sharp peak located at 284.8 eV is attributed to sp2-hybridized carbons (C–C).24 While the peak at 286.8 eV was ascribed to the existence of C–OH bonds, and the relatively weak peak at 288.7 eV was ascribed to the existence of C–OOH bonds.25 Fig. 4c shows the high-resolution spectra of O1s. In the case of GO/F–TiO2 hybrid, the curve fitting of O1s spectra basically indicates three components centered at 530.4, 532.5 and 535.8 eV. The peak at 530.4 eV is due to oxygen in the TiO2 crystal lattice.26 The latter two peaks are commonly ascribed to the surface oxygen complexes of carbon phase.27 This clearly indicates that all the titanium cations in the hybrid material are in the oxidized state. Hydroxyl oxygen on the GO/F–TiO2 will not only form OvC–O–Ti bonds, but also lead to an increase of photocatalytic activity because the surface hydroxyl is an active species and plays an important role in semiconductor photocatalysis. As indicated in Fig. 4d, it is observed that the binding energies of Ti2p3/2 and 2p1/2 are centered at
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Fig. 5
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FTIR spectra of TiO2, GO and 3% GO/TiO2.
459.5 and 465.1 eV, respectively, in agreement with those of pure TiO2.28 Fig. 4e shows the F1 s centered at 684.2 eV, which is a typical value for F–TiO2 systems such as uTi–F species on the F–TiO2 crystal surface.29 No signal for F− in the lattice of F–TiO2 (BE = 688.5 eV) is found. It is well known that the fluorination on the surface of F–TiO2 can accelerate the photocatalytic degradation of a wide range of organic pollutants since the •OH radicals generated on the surface of F–TiO2 are more mobile than those generated on pure TiO2 under UV irradiation. Fig. 5 shows the FT-IR spectra of TiO2, GO and 3% GO/ TiO2. The broad peak at 3000–3500 cm−1 in the spectra of GO corresponds to the stretching mode of –OH, and the physically adsorbed H2O also contributes to this broad peak. The peak at 1656 cm−1 is attributed to the CvO stretching mode of COOH groups. The peaks at 1391 cm−1, 1201 cm−1, 1064 cm−1 are ascribed to C–O in C–OH and C–O–C functional groups, respectively. Furthermore, an absorption band at 1534 cm−1 was clearly observed for 3% GO/TiO2 composites, this band can be attributed to the skeletal vibration of the GO.30 The photocatalytic activity of GO/F–TiO2 composites was studied by degradation of MB under 365 nm UV light irradiation sources. As a comparison, MB degradation with pure TiO2, Degussa P25 and no catalyst was also carried out under identical conditions. As shown in Fig. 6a, about 6%, 8%, 14%, 18%, 23%, 25% and 30% of MB were absorbed on the Degussa P25, TiO2, 1% GO/TiO2, 2% GO/TiO2, 3% GO/ TiO2, 4% GO/TiO2 and 5% GO/TiO2, respectively. The absorption of MB on the photocatalyst increases as GO weight ratio increases for the strong surface absorption ability of GO. The degradation of MB in Degussa P25, TiO2, 1% GO/TiO2, 2% GO/ TiO2, 3% GO/TiO2, 4% GO/TiO2 and 5% GO/TiO2 was 55%, 64%, 82%, 96%, 89%, 86% and 83%, respectively. Fig. 6b shows that there is a linear relationship between ln C0/C and t, confirming that the photodegradation reaction is indeed pseudo-first-order. According to eqn (2), kapp of the photodegradation of MB are 0.0112 min−1, 0.0157 min−1, 0.0264 min−1, 0.0339 min−1, 0.0493 min−1, 0.0390 min−1, 0.0242 min−1 for Degussa P25, TiO2, 1% GO/TiO2, 2% GO/ TiO2, 3% GO/TiO2, 4% GO/TiO2 and 5% GO/TiO2, respectively. An optimal degradation performance of 96% MB was found for 3% GO/TiO2, 3% GO/TiO2, showing superior catalytic activity to commercial Degussa P25, pure TiO2 and other GO/
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Fig. 6 (a) Photocatalytic degradation of MB under UV light irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB degradation and (c) TOC removal by different catalysts.
TiO2 composites. This means that the enhanced photocatalytic performance of GO/TiO2 is not mainly determined by surface absorption of MB. Moreover, in order to embody completely the mineralization of MB, the reduction of the TOC was also presented to show the complete mineralization efficiency of MB by GO/TiO2 nanosheets and P25. As indicated in Fig. 6c, 3% GO/TiO2 showed much higher photocatalytic performance than P25 and other GO/TiO2 composites. For photocatalysis process, four factors are crucially important, that is, the structure of the catalyst, the adsorption of the contaminant molecules, the light absorption, and the electron–hole transportation and separation. The exposure of highly reactive {001} facets of the nanosheets and their surface fluorination are the dominant reasons for photocatalytic oxidation reactions. Photocatalytic activity of F–TiO2 nanosheets
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Fig. 7 The production of GO/TiO2 and the proposed photocatalytic mechanism of GO/TiO2 photocatalyst for the degradation of MB under UV light irradiation.
has been found to be much higher than that of P25. Compared with pure TiO2 nanosheets, the GO/TiO2 composites demonstrated a remarkable enhancement of photocatalytic properties. A schematic illustration of the mechanism of enhanced photocatalytic performance of the GO/TiO2 composites is shown in Fig. 7. The major reaction steps in this mechanism under UV-light irradiation are described in the following equations:31 TiO2 þ hν ! ðeCB Þ þ ðhVB þ Þ ! GOðe Þ þ hþ H2 O
ð3Þ
TiO2 ðeCB Þ þ O2;ads ! • O2 þ TiO2 ! • OH þ TiO2
ð4Þ
TiO2 ðhVB Þ þ OHads ! •OH þ TiO2
ð5Þ
GOðe Þ þ O2;ads ! O2 þ GO ! • OH þ GO
ð6Þ
MB þ •OH ! CO2 þ H2 O
ð7Þ
H2 O
When the as-prepared samples were irradiated by a photon of sufficient energy, equal or larger than band gap, the valence electrons (e−) of anatase are excited to the CB, creating holes (h+) in the valance band VB. Normally, these charge carriers quickly recombine and only a fraction of electrons and holes participate in the photocatalytic reaction. However, when an optimal amount of GO was incorporated into the TiO2, the CB of anatase TiO2 is −0.24 V (vs. SHE),11 while the potential of GO is −0.08 V (vs. SHE).32 Thus, the photoinduced electrons on the CB of TiO2 can be smoothly transferred to GO sheets under UV irradiation, which effectively retards the recombination of photoinduced e− and h+. the photogenerated h+ left in TiO2 VB can react with adsorbed water molecules or surface hydroxyl groups to form hydroxyl radicals •OH, and the excited e− stored in GO are trapped by O2 molecules to form reactive superoxide radical ion •O2−. Both •O2− and •OH are highly reactive toward MB degradation. Owing to the high specific surface area and superior electron mobility of GO, an appropriate integration of GO and TiO2 would give rise to a hybrid nanocomposite to achieve high photodegradation activity. However, when the content of GO is further increased above its optimum value, the photocatalytic performance deteriorates. The reasons can be explained as follows: (a) the large amount of GO can absorb some UV light and thus there exists UV light lost on TiO2; (b) some GO will act as a kind of recombination center instead of providing an electron pathway. Thus, 3% GO is the most suitable ratio for GO/TiO2 catalyst in this study.
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Fig. 8 UV-vis diffuse reflection spectra of pure TiO2 and GO/TiO2 composites.
To reveal the key issues within these four factors, light absorption, specific surface area and photoelectrochemical current for related sample have been characterized. UV-Vis DRS of the samples are shown in Fig. 8. All the samples studied display typical absorption with an intense transition in the UV region, mediated by the intrinsic band gap absorption of TiO2 resulting from the electron transitions from the valence band (VB) to conduction band (CB). As indicated in Fig. 8, the fundamental absorption edge of anatase TiO2 nanosheets is around 400 nm. With the introduction of GO, the GO/TiO2 composites display the same absorption edge as pure TiO2, implying that carbon was not incorporated to the lattice of TiO2 and the layered GO sheet was only a substrate for immobilization of TiO2 nanosheets. In contrast, GO/F–TiO2 composites extend their broad background absorption in the visible-light region. Also, the composite samples show a stronger broad background absorption with increasing GO content. This can be attributed to the presence of GO in the GO/TiO2 composites. Fig. 9 shows the nitrogen adsorption–desorption isotherms for TiO2 nanosheets and the as-prepared GO/TiO2 nanocomposites at 77 K. The data of BET surface area and pore specific volume of samples are listed in Table 1. The nitrogen sorption isotherms for the aforementioned two samples are similar and display hysteresis loops at relative pressures (P/P0) close to
Fig. 9
Isotherms for nitrogen adsorption–desorption.
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Table 1 BET surface area and pore volume data of the as-prepared samples
Samples
BET surface area (m2 g−1)
Pore volume (cm3 g−1)
P25 TiO2 1% GO/TiO2 2% GO/TiO2 3% GO/TiO2 4% GO/TiO2 5% GO/TiO2
50 89 126 148 160 176 204
— 0.07 0.11 0.13 0.16 0.20 0.23 Fig. 11 Comparison of photodegradation performance within five cycles for 3% GO/TiO2 and pure TiO2.
Fig. 10 Transient photocurrent responses of P25, TiO2 nanosheets, 1% GO/TiO2, 2% GO/TiO2, 3% GO/TiO2 and 4% GO/TiO2.
unity, indicating the presence of large mesopores and macropores. Table 1 shows that all GO/TiO2 composite samples have larger specific surface areas than pure TiO2. This is due to the presence of GO in the composites, which has an extremely high surface area. It is reasonable to conclude that the BET specific surface area increases with increasing amount of GO. Similar to that of light absorption character of these sample, increasing the weight fraction of GO in the hybrid could increase the value of specific surface area, which is not in accordance with the photocatalyst performance variation of GO weight fraction. The transient photocurrent responses of P25, pure TiO2 nanosheets and different GO/TiO2 samples were investigated for several on–off cycles of irradiation to give further evidence to support the proposed photocatalytic mechanism. As indicated in Fig. 10, the photocurrent comes back to stable values when the Hg lamp is turned on, and the photocurrent value rapidly decreases to zero as soon as the light turns off. This phenomenon indicates that the photogenerated electrons migrate to the ITO substrates to produce photocurrent under UV light irradiation. In the present study, pure TiO2 nanosheets showed higher photocurrent intensity than P25, and the stable photocurrent value of 3% GO/TiO2 showed the highest photocurrent intensity, which is nearly 3 times as high as that of pure the TiO2 nanosheets. This obvious enhancement of photocurrent indicates smaller recombination and more efficient separation of photogenerated e−–h+ pairs at the 3% GO/TiO2 interface, which is in accordance with that of enhanced photocatalytic performance.
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As discussed above, the stability of the photocatalyst is crucially important for the practical applications of a photocatalyst. Though some particles are of very high photocatalytic efficiency under experimental conditions, they are not suitable for efficient large scale practical applications. The durability of the photocatalytic activity was studied by re-using the catalysts in fresh MB under identical conditions. The irradiation time for each test is 60 min. As shown in Fig. 11, the photocatalytic efficiency of 3% GO/TiO2 shows no observable change even after five recycles. The results indicated that 3% GO/TiO2 photocatalyst has a good reusable performance. By comparison, the performance of TiO2 nanosheets was also investigated. As indicated in Fig. 11, the degradation efficiency of MB is about 64% in the first cycle for TiO2 nanosheet catalyst, and 51% after five cycles. The decrease in the photocatalytic efficiency of TiO2 nanosheets should be related to the weight loss of collection. The 3% GO/TiO2 could be easily collected by low speed centrifugation or simple filtration over a short time, while TiO2 nanosheets have to be collected by long time high speed centrifugation. This is beneficial for the separation and reuse of a catalyst. It is obvious that 3% GO/TiO2 hybrid is a suitable photocatalyst due to its high activity and excellent recycled performance accompanied by easy separation from the reaction system.
4.
Conclusions
In summary, surface-fluorinated TiO2 nanosheet on the surface of GO sheets were successfully synthesized by a simple hydrothermal and colloidal self assemble approach. F–TiO2 nanosheets were uniformly anchored on the surface of the GO sheets. Photocatalytic activity of the as-synthesized catalysts was examined by the photodegradation of MB under UV light irradiation. The results showed that F–TiO2 nanosheets with high percentage of exposed {001} facets exhibit higher photocatalytic activity than P25, and the GO/F–TiO2 nanocomposite is able to exhibit much higher stability and activity than bare F–TiO2. The influence of GO on the photocatalytic activity of GO/F–TiO2 nanocomposites has been examined systematically by considering different weight addition ratios of GO. The MB
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dye can be decomposed by 96% using 3% GO/F–TiO2 within 60 min, but more than 3% GO addition to the GO/F–TiO2 nanocomposite leads to a decreased photocatalytic activity. Combined with the light absorption, specific surface area and photoelectrochemical current evaluation, it has been confirmed that the reinforced electron–hole separation of existed GO is the key issue for photocatalyst performance enhancement. We think this simple method for highly efficient photocatalyst preparation method and clarification on the mechanism for performance enhancement through hybridization of semiconductor nanomaterials with GO could be of value for their further development.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51302101, 21303129), the key Foundation of Educational Commission of Anhui Province (KJ2012A250), and the Huaibei Science and Technology Development Funds (20110305).
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25 H. S. Jang, J. M. Yun, D. Y. Kim, D. W. Park, S. I. Na and S. S. Kim, Electrochim. Acta, 2012, 81, 301. 26 S. Södergren, H. Siegbahn, H. Rensmo, H. Lindström, A. Hagfeldt and S. E. Lindquist, J. Phys. Chem. B, 1997, 101, 3087. 27 J. Zhang, J. Jiang and X. S. Zhao, J. Phys. Chem. C, 2011, 115, 6448; G. Eda, G. Fanchini and M. Chhowalla, Nat. Nanotechnol., 2008, 3, 270. 28 E. Barborini, A. M. Conti, I. Kholmanov, P. Piseri, A. Podestà, P. Milani, C. Cepek, O. Sakho, R. Macovez and M. Sancrotti, Adv. Mater., 2005, 17, 1842. 29 K. Lv, Q. Xiang and J. Yu, Appl. Catal., B, 2011, 104, 275. 30 W. Fan, Q. Lai, Q. Zhang and Y. Wang, J. Phys. Chem. C, 2011, 115, 10694. 31 D. Zhao, G. Sheng, C. Chen and X. Wang, Appl. Catal., B, 2012, 111–112, 303. 32 Q. J. Xiang, J. G. Yu and M. Jaroniec, Nanoscale, 2011, 3, 3670.
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PAPER
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Graphene oxide capturing surface-fluorinated TiO2 nanosheets for advanced photocatalysis and the reveal of synergism reinforce mechanism Kai Dai,a Luhua Lu,*b Qi Liu,*c Guangping Zhu,a Qinzhuang Liua and Zhongliang Liua Surface-fluorinated TiO2 (F–TiO2) nanosheets with exposed (001) facets were synthesized from a scalable hydrothermal treatment assisted by a specific stabilization effect of fluorine ions on the (001) facets. Assembly of F–TiO2 on graphene oxide (GO) sheets into GO/F–TiO2 hybrid in aqueous solution was further achieved by making use of the surfactant role of GO. Photocatalytic properties of GO/F–TiO2 hybrid were evaluated under 365 nm UV light irradiation for photodegradation of methylene blue (MB). An optimal GO content has been determined to be 3 wt%, and the corresponding apparent pseudo-firstorder rate constant Kapp is 0.0493 min−1, 3 times and 4 times more than that of pure TiO2 nanosheets and commercial P25 photocatalyst, respectively. To reveal the synergism reinforce mechanism of GO/F–TiO2 hybrid, photo absorption, surface absorption, and the photoelectrochemical current properties have been intensively studied. We found that enhanced electron–hole separation has been the key issue for the reinforcement of photocatalytic performance. F–TiO2 with exposed (001) facet has stronger electron–
Received 15th September 2013, Accepted 14th October 2013 DOI: 10.1039/c3dt52542b www.rsc.org/dalton
1.
hole separation resulting in a higher photoelectrochemical current than that of P25 photocatalyst. Moreover, hybridization of F–TiO2 with GO could further increase the photoelectrochemical current and the largest current for the optimal weight fraction of GO is in good accordance with the photocatalysis performance. The GO/F–TiO2 interface junction that favors the electron–hole separation was attributed to the photoelectrochemical current enhancement.
Introduction
Titanium dioxide (TiO2) has been the most famous photocatalyst in a variety of applications such as pollutants degradation, water splitting, fuel cells and solar cells for its wide bandgap, highly stable performance, environmental compatibility and scalable industrial production.1 TiO2 exists in three crystallographic forms in nature: brookite, anatase and rutile. Anatase plays a dominant role in photocatalysis applications due to a generally observed superior photocatalytic efficiency as well as facile synthesis.2 Scientists have demonstrated that the order of the average surface energies of anatase TiO2 is 0.90 J m−2 for {001} > 0.53 J m−2 for {100} > 0.44 J m−2 for {101}.3 Increasing attention has thus focused on the synthesis
a
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, P.R. China b State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P.R. China. E-mail: [email protected] c Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, P.R. China. E-mail: [email protected]
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of anatase TiO2 nanostructure with dominant {001} facets. Dai et al. recently demonstrated a method to prepare anatase TiO2 nanocrystals with exposed {001} facets with the aqueous acetic acid solution, but only with 9.6% of the surface being enclosed by {001} facets.4 Yang et al. recently reported the synthesis of micro-sized anatase TiO2 crystallites with highly energetic {001} facets exposed with the help of hydrofluoric acid (HF). However, the percentage of exposed (001) facets was only 47%, and the crystallite size was relatively large.5 Han et al. made an important progress on the synthesis of anatase TiO2 nanosheets, and the percentage of (001) facets in the sheets is surprisingly as high as 80%.6 Gordon et al. reported a method to engineer the percentage of {001} and {101} facets of uniform anatase TiO2 nanocrystals through the choice of cosurfactant and titanium precursor.7 However, the photocatalytic efficiency on a bare TiO2 remains quite limited, mainly due to the rapid recombination rate of photogenerated electron–hole pairs within TiO2 particles.8 Of special interests are the recent attempts to modify TiO2 with nanostructured carbonaceous materials such as carbon fiber, activated carbon, amorphous carbon, carbon nanotubes and fullerenes.9 Within all these materials, graphene, an sp2-bonded carbon atoms with monolayers of carbon atoms
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arranged in a honeycombed network, has recently received the most attention in the field of material science because of its unique qualities, which include unique optical, electrical and electrochemical properties resulting from long-rangeconjugation.10 Graphene/TiO2 composites have been reported to exhibit interesting photocatalytic activities. Wang et al. reported the enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air.11 Zhang et al. reported graphene–P25 composites synthesized under hydrothermal conditions and showed their enhanced photocatalytic activity for degradation of methylene blue (MB) in aqueous solutions.12 Their hybridization has been found to enhance the photocatalytic activity in the degradation of organic pollutions, the water photocatalytic splitting, and antibacterial application.13 They have exhibited somehow enhanced photocatalytic performance than pure TiO2 while new problems have arisen such as lacking of scalability for complicated materials’ preparation methods and the variety of unconfirmed mechanisms (e.g. enhanced surface absorption, light absorption or electron–hole separation) for photocatalyst performance enhancement.14 All these problems need a good resolution for further development. Graphene’s derivative graphene oxide (GO) has received a great deal of attention for environmental applications because of the large surface area, light weight and it possesses a large amount of delocalized electrons.15 The coexisting of hydrophobic aromatic carbon and hydrophilic oxygen groups endow GO with character of normal surfactant.16 Moreover, light absorption of GO is largely reduced than graphene, which could favor the light irradiation on TiO2 surface.17 Thus, designing a hybrid composed of GO and TiO2 with well exposed {001} facets for highly enhanced photocatalytic performance by a scalable solution approach is reasonable and attractive. In this work, surface-fluorinated TiO2 nanosheets (F–TiO2) hybridized with GO were prepared on a scalable approach without any catalysts or templates. The high-quality F–TiO2 nanosheets prepared from hydrothermal reaction were found to be capable of dispersed by GO and well anchored on the GO surface, endowing the hybrid with interfacial junction and solution processability. The as-prepared GO/F–TiO2 nanosheet composites possess well exposed (001) high energy facets, which has shown stronger photocatalytic performance than pure TiO2 nanosheet and the well known commercial P25. To reveal the mechanism of the reinforced photocatalyst performance, hybrids of different GO weight fraction have been prepared and their photocatalyst performance along with light absorption, specific surface area and photoelectrochemical current have been characterized. The synthesized sample exhibits attractive recyclable photocatalytic properties and high potential for practical applications. Photocurrent test has indicated that the existence of GO has obviously reinforced the photoelectron–hole separation, which favors the enhancement of the photocatalytic performance. It is found that the enhanced electron–hole separation was the key issue for the reinforcement of photocatalytic performance except the bulk
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light absorption of hybrid and dye absorption on the hybrid surface. We think that this simple approach as a highly efficient photocatalyst preparation method and clarification of the mechanism for performance enhancement through hybridization of semiconductor nanomaterials with GO could be of value for their further development.
2. Experimental 2.1
Materials
Natural graphite powder (325 mesh) was commercially obtained from Alfa-Aesar, commercial TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. MB, tetrabutyl titanate (Ti(OC4H9)4), concentrated sulphuric acid (98%, H2SO4), and potassium permanganate (KMnO4) and polyethylene glycol (PEG) were provided from Shanghai Chemical Reagent Co. Ltd, China. Aqueous hydrogen peroxide solution (H2O2, 30%), sodium sulphate (Na2SO4), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), HF (40 wt%) and ammonia solution (NH4OH, 25%) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Double distilled water was used for solution preparation. All reactants were of analytical purity and used as received without further purification. 2.2
Preparation of GO
GO was synthesized by the modified Hummers’ method.18 In a typical preparation procedure, 3.0 g of graphite was put into a mixture of 12 mL of concentrated H2SO4, 2.5 g of K2S2O8, and 2.5 g of P2O5. The solution was heated to 80 °C and kept under stirring for 4.5 h in oil bath. Then the mixture was diluted with 500 mL of deionized water, and the product was obtained by filtering using a 0.2 μm nylon film and dried under ambient condition. Thereafter, 15 g KMnO4 was added gradually with stirring, to prevent the temperature of the mixture from exceeding 10 °C. The ice bath was then removed and the mixture was stirred at 35 °C for 2 h. The reaction was terminated by adding 700 mL of distilled water and 20 mL of 30% H2O2 solution. Finally, the GO was recovered by filtration and drying. 2.3
Preparation of F–TiO2 nanosheet
25 mL of Ti(OC4H9)4, 3 mL HF were initially vigorously stirred for 0.5 h to form the mixture. Then, the mixture was transferred into a 50 mL Teflon-lined autoclave and subsequently heated at 180 °C for 24 h. After hydrothermal reaction, the white precipitates were harvested by centrifugation and throughout washing with water and ethanol, and then dried in an oven at 60 °C for 6 h obtaining F–TiO2. 2.4
Preparation of GO/F–TiO2 composites
A certain amount of GO and 1 g of the as-prepared F–TiO2 were added in 100 mL water, and then ultrasonicated at 30 °C for 40 min to obtain a uniform mixture. After filtering with double distilled water and drying at 60 °C for 6 h, GO/F–TiO2
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composite photocatalysts were obtained. To investigate the effect of the GO content on the photocatalytic performance rates of the GO/F–TiO2 composites, the weight percentages of GO to F–TiO2 were varied from 0 to 5 (1, 2, 3, 4, and 5 wt%) by varying the weight of GO, and the samples are presented as x% GO/TiO2, where x is the weight content of GO. 2.5
Analytical and testing instruments
The structure of the samples was observed on a Tecnai G2 F20 S-Twin HRTEM at an accelerating voltage of 100 kV with Inca Energy-dispersive X-ray spectroscopy (EDX). The X-ray photoelectron spectra (XPS) of the samples were measured using a Thermo ESCALAB 250 with an Al Kα X-ray photoelectron spectrometer at 150 W. X-ray diffraction (XRD) data for samples were collected using a Rigaku D/MAX 24000 diffractometer at room temperature with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 5° to 70°, operated at 40 kV and 40 mA, and a scanning speed of 10° min−1. The infrared absorption spectra were recorded over the frequency range from 400 to 4000 cm−1 using a Nicolet 6700 FT-IR spectrophotometer. The spectra were measured after the spectrum scan of the blank pure KBr pellet. The Brunauer–Emmett–Teller (BET) specific surface area values were determined by using nitrogen adsorption data at 77 K obtained by a Micromeritics ASAP 2010 system with multipoint BET method. UV-vis diffuse reflectance spectroscopy (DRS) measurements were carried out using a Hitachi UV-3600 UV-vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 250 to 600 nm, and BaSO4 was used as a reflectance standard. The total organic carbon (TOC) was determined by using an Elementar Liqui TOC II analyzer. The photoelectrochemical measurements were measured on a CHI-660D electrochemical system, using a conventional three-electrode cell. The counter and the reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The electrolyte solution was 1 M Na2SO4. The working electrodes were prepared as follows: 0.1 g ground sample was mixed with 0.02 g PEG and 0.5 ml distilled water was added to make a slurry. The slurry was then injected onto a 1.0 cm × 1.0 cm ITO glass electrode and these electrolytes were dried at 60 °C for 2 h and then calcined at 250 °C for 4 h. 2.6
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concentration was about 664 nm. The degradation efficiency was calculated as follows:19 η¼
C0 C 100% C0
ð1Þ
where C0 is the absorbance of the original MB solution and C is the absorbance of the MB solution after UV light irradiation. According to the Langmuir–Hinshelwood kinetics model, the photocatalytic process of MB can be expressed as the following apparent pseudo-first-order kinetics equation:20 ln
C0 ¼ kapp t C
ð2Þ
where kapp is the apparent pseudo-first-order rate constant, C0 is the original MB concentration and C is the MB concentration in aqueous solution at time t.
3. Results and discussion Fig. 1 shows the XRD patterns of TiO2-based samples in comparison to the pattern of GO. For GO, the XRD peak at 10° for the (001) facets corresponds to 0.87 nm interlayer spacing, which is much larger than that of pristine graphite (0.34 nm) due to the introduction of oxygen-containing functional groups on the graphite sheets.21 As can be seen from Fig. 1, only anatase phases of TiO2 (JCPDS no. 21-1272, space group: I41/amd (141), a = 0.379 nm and c = 0.951 nm) was observed in pure TiO2. Notably, no characteristic diffraction peaks of GO were observed in all the GO/F–TiO2 hybrids. And our results correlate well with the previous studies that the diffraction peaks become weakened or even disappear whenever the regular stacks of GO are exfoliated.22 A possible explanation is that, with the help of ultrasonic, TiO2 nanocrystals are intercalated into stacked GO layer,23 leading to the exfoliation of GO. Fig. 2a and 2b show typical SEM and TEM images for the as-prepared F–TiO2 nanosheets. A large amount of F–TiO2 nanosheets with side length of 50–60 nm and thickness of 10–15 nm can be easily observed. As indicated in Fig. 2b, HRTEM image directly shows that the lattice spacing parallel
Photocatalytic experiment
The photocatalytic activity of the samples was evaluated by the photocatalytic degradation of MB under 250 W 365 nm Hg lamp irradiation. The photocatalytic experiments were carried out in a reactor containing a 100 mL 10 mg L−1 aqueous solution of MB and 0.1 g photocatalysts. The distance between the lamp and the reactor was 10 cm. Before irradiation, the suspension was magnetically stirred in the dark for 1 h to establish an adsorption/desorption equilibrium under ambient conditions. Then, the mixture was exposed to the UV light irradiation. At the given irradiation time, the concentration of MB was quantified by the absorbance, and the absorbance wavelength used in the spectrophotometer to determine MB
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Fig. 1
XRD patterns of GO, F–TiO2 and GO/F–TiO2 composites.
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Fig. 2 SEM image of (a) F–TiO2 nanosheets and TEM images of (b) F–TiO2 nanosheets, (c) GO and (d) 3% GO/F–TiO2. The insets of (b) and (d) show HRTEM images of F–TiO2 nanosheets and 3% GO/F–TiO2.
to the top and bottom facets is ca. 0.235 nm, corresponding to the (001) planes of anatase F–TiO2, which indicates the top ˉ) and bottom facets of the nanosheets are the (001) and (001 planes, respectively. Another set of the lattice fringes with spacing of 0.35 nm, corresponding to the (101) planes of anatase, can be also clearly revealed from the sheets lying on the TEM grid. On the basis of the TEM and HRTEM results, we can calculate the percentage of exposed {001} facets on the F–TiO2 nanosheets is 62%–71%. The fluorine ions will play a key role in the formation of F–TiO2 nanosheets with high percentage of exposed {001} facets.6 All the prepared F–TiO2 samples display good crystallinity. The TEM image in Fig. 2d presents well-dispersed F–TiO2 nanosheets on the surface of GO that served as support and electron donor. The surface attachment of F–TiO2 nanosheets, instead of complete or partial wrapping by GO, assured full exposure of both GO sheets and F–TiO2 nanosheets to contact the solution, which will improve the photocatalytic activity. Fig. 3a and 3b show the EDX spectrum of the as-synthesized GO and 3% GO/F–TiO2 nanosheets, respectively. The pattern indicates that GO only contain elements of C and O, and 3% GO/F–TiO2 nanosheets only contain elements of C, O, Ti and F, without any other impurities.
Fig. 3 EDX spectrum of (a) as-synthesized GO and (b) 3% GO/F–TiO2 nanosheets.
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Fig. 4 The overview (a) and the corresponding high-resolution XPS spectra (b) C1 s, (c) O1 s, (d) Ti2p and (e) F1 s of the as-prepared 3% GO/ F–TiO2.
The XPS spectrum including signals for C1 s, Ti2p, O1 s and F1 s of 3% GO/F–TiO2 nanosheets to probe the chemical environment of the elements in the near surface range is shown in Fig. 4a. As indicated in Fig. 4b, the asymmetrical and broad features of the observed C1 s peaks suggest the co-existence of distinguishable models. A deconvolution core level spectra at about 284.8, 286.8 and 288.7 eV has been given. The sharp peak located at 284.8 eV is attributed to sp2-hybridized carbons (C–C).24 While the peak at 286.8 eV was ascribed to the existence of C–OH bonds, and the relatively weak peak at 288.7 eV was ascribed to the existence of C–OOH bonds.25 Fig. 4c shows the high-resolution spectra of O1s. In the case of GO/F–TiO2 hybrid, the curve fitting of O1s spectra basically indicates three components centered at 530.4, 532.5 and 535.8 eV. The peak at 530.4 eV is due to oxygen in the TiO2 crystal lattice.26 The latter two peaks are commonly ascribed to the surface oxygen complexes of carbon phase.27 This clearly indicates that all the titanium cations in the hybrid material are in the oxidized state. Hydroxyl oxygen on the GO/F–TiO2 will not only form OvC–O–Ti bonds, but also lead to an increase of photocatalytic activity because the surface hydroxyl is an active species and plays an important role in semiconductor photocatalysis. As indicated in Fig. 4d, it is observed that the binding energies of Ti2p3/2 and 2p1/2 are centered at
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Fig. 5
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FTIR spectra of TiO2, GO and 3% GO/TiO2.
459.5 and 465.1 eV, respectively, in agreement with those of pure TiO2.28 Fig. 4e shows the F1 s centered at 684.2 eV, which is a typical value for F–TiO2 systems such as uTi–F species on the F–TiO2 crystal surface.29 No signal for F− in the lattice of F–TiO2 (BE = 688.5 eV) is found. It is well known that the fluorination on the surface of F–TiO2 can accelerate the photocatalytic degradation of a wide range of organic pollutants since the •OH radicals generated on the surface of F–TiO2 are more mobile than those generated on pure TiO2 under UV irradiation. Fig. 5 shows the FT-IR spectra of TiO2, GO and 3% GO/ TiO2. The broad peak at 3000–3500 cm−1 in the spectra of GO corresponds to the stretching mode of –OH, and the physically adsorbed H2O also contributes to this broad peak. The peak at 1656 cm−1 is attributed to the CvO stretching mode of COOH groups. The peaks at 1391 cm−1, 1201 cm−1, 1064 cm−1 are ascribed to C–O in C–OH and C–O–C functional groups, respectively. Furthermore, an absorption band at 1534 cm−1 was clearly observed for 3% GO/TiO2 composites, this band can be attributed to the skeletal vibration of the GO.30 The photocatalytic activity of GO/F–TiO2 composites was studied by degradation of MB under 365 nm UV light irradiation sources. As a comparison, MB degradation with pure TiO2, Degussa P25 and no catalyst was also carried out under identical conditions. As shown in Fig. 6a, about 6%, 8%, 14%, 18%, 23%, 25% and 30% of MB were absorbed on the Degussa P25, TiO2, 1% GO/TiO2, 2% GO/TiO2, 3% GO/ TiO2, 4% GO/TiO2 and 5% GO/TiO2, respectively. The absorption of MB on the photocatalyst increases as GO weight ratio increases for the strong surface absorption ability of GO. The degradation of MB in Degussa P25, TiO2, 1% GO/TiO2, 2% GO/ TiO2, 3% GO/TiO2, 4% GO/TiO2 and 5% GO/TiO2 was 55%, 64%, 82%, 96%, 89%, 86% and 83%, respectively. Fig. 6b shows that there is a linear relationship between ln C0/C and t, confirming that the photodegradation reaction is indeed pseudo-first-order. According to eqn (2), kapp of the photodegradation of MB are 0.0112 min−1, 0.0157 min−1, 0.0264 min−1, 0.0339 min−1, 0.0493 min−1, 0.0390 min−1, 0.0242 min−1 for Degussa P25, TiO2, 1% GO/TiO2, 2% GO/ TiO2, 3% GO/TiO2, 4% GO/TiO2 and 5% GO/TiO2, respectively. An optimal degradation performance of 96% MB was found for 3% GO/TiO2, 3% GO/TiO2, showing superior catalytic activity to commercial Degussa P25, pure TiO2 and other GO/
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Fig. 6 (a) Photocatalytic degradation of MB under UV light irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB degradation and (c) TOC removal by different catalysts.
TiO2 composites. This means that the enhanced photocatalytic performance of GO/TiO2 is not mainly determined by surface absorption of MB. Moreover, in order to embody completely the mineralization of MB, the reduction of the TOC was also presented to show the complete mineralization efficiency of MB by GO/TiO2 nanosheets and P25. As indicated in Fig. 6c, 3% GO/TiO2 showed much higher photocatalytic performance than P25 and other GO/TiO2 composites. For photocatalysis process, four factors are crucially important, that is, the structure of the catalyst, the adsorption of the contaminant molecules, the light absorption, and the electron–hole transportation and separation. The exposure of highly reactive {001} facets of the nanosheets and their surface fluorination are the dominant reasons for photocatalytic oxidation reactions. Photocatalytic activity of F–TiO2 nanosheets
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Fig. 7 The production of GO/TiO2 and the proposed photocatalytic mechanism of GO/TiO2 photocatalyst for the degradation of MB under UV light irradiation.
has been found to be much higher than that of P25. Compared with pure TiO2 nanosheets, the GO/TiO2 composites demonstrated a remarkable enhancement of photocatalytic properties. A schematic illustration of the mechanism of enhanced photocatalytic performance of the GO/TiO2 composites is shown in Fig. 7. The major reaction steps in this mechanism under UV-light irradiation are described in the following equations:31 TiO2 þ hν ! ðeCB Þ þ ðhVB þ Þ ! GOðe Þ þ hþ H2 O
ð3Þ
TiO2 ðeCB Þ þ O2;ads ! • O2 þ TiO2 ! • OH þ TiO2
ð4Þ
TiO2 ðhVB Þ þ OHads ! •OH þ TiO2
ð5Þ
GOðe Þ þ O2;ads ! O2 þ GO ! • OH þ GO
ð6Þ
MB þ •OH ! CO2 þ H2 O
ð7Þ
H2 O
When the as-prepared samples were irradiated by a photon of sufficient energy, equal or larger than band gap, the valence electrons (e−) of anatase are excited to the CB, creating holes (h+) in the valance band VB. Normally, these charge carriers quickly recombine and only a fraction of electrons and holes participate in the photocatalytic reaction. However, when an optimal amount of GO was incorporated into the TiO2, the CB of anatase TiO2 is −0.24 V (vs. SHE),11 while the potential of GO is −0.08 V (vs. SHE).32 Thus, the photoinduced electrons on the CB of TiO2 can be smoothly transferred to GO sheets under UV irradiation, which effectively retards the recombination of photoinduced e− and h+. the photogenerated h+ left in TiO2 VB can react with adsorbed water molecules or surface hydroxyl groups to form hydroxyl radicals •OH, and the excited e− stored in GO are trapped by O2 molecules to form reactive superoxide radical ion •O2−. Both •O2− and •OH are highly reactive toward MB degradation. Owing to the high specific surface area and superior electron mobility of GO, an appropriate integration of GO and TiO2 would give rise to a hybrid nanocomposite to achieve high photodegradation activity. However, when the content of GO is further increased above its optimum value, the photocatalytic performance deteriorates. The reasons can be explained as follows: (a) the large amount of GO can absorb some UV light and thus there exists UV light lost on TiO2; (b) some GO will act as a kind of recombination center instead of providing an electron pathway. Thus, 3% GO is the most suitable ratio for GO/TiO2 catalyst in this study.
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Fig. 8 UV-vis diffuse reflection spectra of pure TiO2 and GO/TiO2 composites.
To reveal the key issues within these four factors, light absorption, specific surface area and photoelectrochemical current for related sample have been characterized. UV-Vis DRS of the samples are shown in Fig. 8. All the samples studied display typical absorption with an intense transition in the UV region, mediated by the intrinsic band gap absorption of TiO2 resulting from the electron transitions from the valence band (VB) to conduction band (CB). As indicated in Fig. 8, the fundamental absorption edge of anatase TiO2 nanosheets is around 400 nm. With the introduction of GO, the GO/TiO2 composites display the same absorption edge as pure TiO2, implying that carbon was not incorporated to the lattice of TiO2 and the layered GO sheet was only a substrate for immobilization of TiO2 nanosheets. In contrast, GO/F–TiO2 composites extend their broad background absorption in the visible-light region. Also, the composite samples show a stronger broad background absorption with increasing GO content. This can be attributed to the presence of GO in the GO/TiO2 composites. Fig. 9 shows the nitrogen adsorption–desorption isotherms for TiO2 nanosheets and the as-prepared GO/TiO2 nanocomposites at 77 K. The data of BET surface area and pore specific volume of samples are listed in Table 1. The nitrogen sorption isotherms for the aforementioned two samples are similar and display hysteresis loops at relative pressures (P/P0) close to
Fig. 9
Isotherms for nitrogen adsorption–desorption.
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Table 1 BET surface area and pore volume data of the as-prepared samples
Samples
BET surface area (m2 g−1)
Pore volume (cm3 g−1)
P25 TiO2 1% GO/TiO2 2% GO/TiO2 3% GO/TiO2 4% GO/TiO2 5% GO/TiO2
50 89 126 148 160 176 204
— 0.07 0.11 0.13 0.16 0.20 0.23 Fig. 11 Comparison of photodegradation performance within five cycles for 3% GO/TiO2 and pure TiO2.
Fig. 10 Transient photocurrent responses of P25, TiO2 nanosheets, 1% GO/TiO2, 2% GO/TiO2, 3% GO/TiO2 and 4% GO/TiO2.
unity, indicating the presence of large mesopores and macropores. Table 1 shows that all GO/TiO2 composite samples have larger specific surface areas than pure TiO2. This is due to the presence of GO in the composites, which has an extremely high surface area. It is reasonable to conclude that the BET specific surface area increases with increasing amount of GO. Similar to that of light absorption character of these sample, increasing the weight fraction of GO in the hybrid could increase the value of specific surface area, which is not in accordance with the photocatalyst performance variation of GO weight fraction. The transient photocurrent responses of P25, pure TiO2 nanosheets and different GO/TiO2 samples were investigated for several on–off cycles of irradiation to give further evidence to support the proposed photocatalytic mechanism. As indicated in Fig. 10, the photocurrent comes back to stable values when the Hg lamp is turned on, and the photocurrent value rapidly decreases to zero as soon as the light turns off. This phenomenon indicates that the photogenerated electrons migrate to the ITO substrates to produce photocurrent under UV light irradiation. In the present study, pure TiO2 nanosheets showed higher photocurrent intensity than P25, and the stable photocurrent value of 3% GO/TiO2 showed the highest photocurrent intensity, which is nearly 3 times as high as that of pure the TiO2 nanosheets. This obvious enhancement of photocurrent indicates smaller recombination and more efficient separation of photogenerated e−–h+ pairs at the 3% GO/TiO2 interface, which is in accordance with that of enhanced photocatalytic performance.
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As discussed above, the stability of the photocatalyst is crucially important for the practical applications of a photocatalyst. Though some particles are of very high photocatalytic efficiency under experimental conditions, they are not suitable for efficient large scale practical applications. The durability of the photocatalytic activity was studied by re-using the catalysts in fresh MB under identical conditions. The irradiation time for each test is 60 min. As shown in Fig. 11, the photocatalytic efficiency of 3% GO/TiO2 shows no observable change even after five recycles. The results indicated that 3% GO/TiO2 photocatalyst has a good reusable performance. By comparison, the performance of TiO2 nanosheets was also investigated. As indicated in Fig. 11, the degradation efficiency of MB is about 64% in the first cycle for TiO2 nanosheet catalyst, and 51% after five cycles. The decrease in the photocatalytic efficiency of TiO2 nanosheets should be related to the weight loss of collection. The 3% GO/TiO2 could be easily collected by low speed centrifugation or simple filtration over a short time, while TiO2 nanosheets have to be collected by long time high speed centrifugation. This is beneficial for the separation and reuse of a catalyst. It is obvious that 3% GO/TiO2 hybrid is a suitable photocatalyst due to its high activity and excellent recycled performance accompanied by easy separation from the reaction system.
4.
Conclusions
In summary, surface-fluorinated TiO2 nanosheet on the surface of GO sheets were successfully synthesized by a simple hydrothermal and colloidal self assemble approach. F–TiO2 nanosheets were uniformly anchored on the surface of the GO sheets. Photocatalytic activity of the as-synthesized catalysts was examined by the photodegradation of MB under UV light irradiation. The results showed that F–TiO2 nanosheets with high percentage of exposed {001} facets exhibit higher photocatalytic activity than P25, and the GO/F–TiO2 nanocomposite is able to exhibit much higher stability and activity than bare F–TiO2. The influence of GO on the photocatalytic activity of GO/F–TiO2 nanocomposites has been examined systematically by considering different weight addition ratios of GO. The MB
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dye can be decomposed by 96% using 3% GO/F–TiO2 within 60 min, but more than 3% GO addition to the GO/F–TiO2 nanocomposite leads to a decreased photocatalytic activity. Combined with the light absorption, specific surface area and photoelectrochemical current evaluation, it has been confirmed that the reinforced electron–hole separation of existed GO is the key issue for photocatalyst performance enhancement. We think this simple method for highly efficient photocatalyst preparation method and clarification on the mechanism for performance enhancement through hybridization of semiconductor nanomaterials with GO could be of value for their further development.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51302101, 21303129), the key Foundation of Educational Commission of Anhui Province (KJ2012A250), and the Huaibei Science and Technology Development Funds (20110305).
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