Journal of Colloid and Interface Science 430 (2014) 100–107
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The composite of nitrogen-doped anatase titania plates with exposed {0 0 1} facets/graphene nanosheets for enhanced visible-light photocatalytic activity Jian-Wen Shi, Hui-Ying Ai, Jian-Wei Chen, Hao-Jie Cui, Ming-Lai Fu ⇑ Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China
a r t i c l e
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Article history: Received 19 December 2013 Accepted 14 May 2014 Available online 23 May 2014 Keywords: TiO2 Graphene Photocatalysis Doping Visible light
a b s t r a c t Composite photocatalysts composed of nitrogen-doped anatase TiO2 plates with exposed {0 0 1} facets (NTS) and graphene nanosheets (G) were firstly synthesized by a facile one-pot hydrothermal process. The morphologies, structural properties, and photocatalytic activities of the resultant NTS/G composites were investigated in detail. Graphene nanosheets were demonstrated play three important roles in the NTS/G composites, as transporter of photo-excited electrons, extender of light absorption range and enhancer of adsorptive capacity, respectively. Due to the effective charge anti-recombination, the efficient utilization of the visible light and the high adsorptive capacity to target pollutants, the composites exhibited significant improvement in photocatalytic degradation of methylene blue under visible light irradiation. Based on the results, the mechanism of enhanced visible-light photocatalytic activity on NTS/G composites was proposed. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Owing to potential applications in environmental clean-up and energy conversion, heterogeneous photocatalysis employing semiconductors has attracted extensive interest [1,2]. As a dominant semiconductor, TiO2 has been most widely investigated in the past decades due to its excellent properties such as nontoxicity, cheapness, and chemical stability [3,4]. However, the photocatalytic activity of TiO2 is still far below the level of practical application for two main unfavorable factors, restrictive light absorption (only responsive to ultraviolet light with wavelength below 387 nm due to its wide band-gap) and high charge recombination rate [5,6], which severely restricts its practical applications. Graphene, a two dimensional carbonaceous material, has attracted much attention recently for its high thermal conductivity (5000 W m 1 K 1), excellent mobility of charge carriers (200,000 cm2 V 1 s 1), a large specific surface area (calculated value 2630 m2 g 1) and good mechanical stability [7,8]. Considering its excellent mobility of charge carriers, graphene may be an excellent candidate to incorporate with TiO2 for the effective charge anti-recombination. To date, many publications have ⇑ Corresponding author. Address: Institute of Urban Environment, Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen, Fujian 361021, China. Fax: +86 592 6190977. E-mail address: [email protected] (M.-L. Fu). http://dx.doi.org/10.1016/j.jcis.2014.05.027 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
confirmed that graphene act the transporter of electrons in TiO2/ Graphene composites, which significantly inhibits the recombination of photo-generated electrons and holes. For example, Zhang et al. [9] reported that P25/Graphene composite showed higher photocatalytic activity for the degradation of methylene blue (MB) in comparison with bare P25 because the composite has a higher charge separation rate. Liu et al. [10] reported selfassembled TiO2 nanorods on large graphene oxide exhibited an improved photocatalytic activity due to the effective charge anti-recombination on graphene oxide. Lightcap and co-workers [11] even demonstrated the feasibility of using graphene as an electron-transfer medium in a TiO2/Graphene composite. Notably, the large specific surface area of graphene is hopeful to improve the adsorption capacity of TiO2/Graphene composites. Therefore, it is expected that the combination of TiO2 and graphene is promising to possess simultaneously excellent electron transferability, adsorbability and stability, which can be suitable for improving the photocatalytic activity of TiO2. On the other hand, the photocatalytic activity of TiO2 also intrinsically depends on the surface atomic structure and the crystallinity. In particular, the favorable surface atomic structure such as high reactive facets is highly expected to effectively enhance photocatalytic activity [12,13]. Since the pioneering work on anatase TiO2 plates with exposed {0 0 1} facets reported by Yang and co-workers [14], many publications have confirmed that the photocatalytic activity of anatase TiO2 crystals could be significantly
J.-W. Shi et al. / Journal of Colloid and Interface Science 430 (2014) 100–107
improved because of the exposure of high reactive {0 0 1} facets [15,16]. These successful examples inspire researchers to combine anatase TiO2 plates with exposed {0 0 1} facets with graphene for higher photocatalytic activity, and some positive results have been achieved [17,18]. For example, Sun et al. [18] prepared nano-sized TiO2 plates with exposed {0 0 1} facets on the platform of graphene nanosheets, and found that the composite exhibited significant improvement in photocatalytic degradation of Rhodamine B, and the enhancement was attributed to the synergistic effect of the high catalytic activity of {0 0 1} facets and the effective charge anti-recombination of graphene. Nevertheless, in available cases, anatase TiO2 plates may have no visible-light response due to its large band gap (3.2 eV). Inspiringly, nonmetal doped (e.g. N, S, C-doped) anatase TiO2 plates with exposed {0 0 1} facets and visible-light response have been successfully prepared in recent years [19–22]. Therefore, combining nonmetal doped anatase TiO2 plates with exposed {0 0 1} facets and graphene may be a promising attempt to solve aforementioned two unfavorable factors of TiO2 and to obtain a new composite photocatalyst with enhanced visible-light photocatalytic activity. In our previous paper [23], N-doped anatase TiO2 plates with exposed {0 0 1} facets (abbreviated as NTS) were successfully prepared, and the optimal prepared conditions were ascertained. The resulting photocatalyst was demonstrated to be visible-light photocatalytic activity for the decoloration of MB. Encouraged by this work, here, we attempt to synthesize the composite photocatalyst consisted of NTS and graphene by a facile one-pot process. The formation of NTS and the following combination of NTS and graphene were successfully achieved. Our results demonstrated the visible-light photocatalytic activity of NTS/Graphene (abbreviated as NTS/G) composite was extremely improved due to the introduction of graphene. To the best of our knowledge, this work is the first report on the combination of NTS and graphene, and is also the first report on one-pot process to synthesize NTS/Graphene composite photocatalyst with enhanced visible-light photocatalytic activity. 2. Experimental section All the reagents used in this work were of analytical grade and were used without any further purification: TiN and graphite were purchased from Aladdin Reagent Company, and all the other reagents, such as methylene blue, KMnO4, H2SO4, HCl, HF and H2O2 were purchased from Sinopharm Chemical Reagent Company, Ltd., China. 2.1. Synthesis of graphene oxide Graphene oxide (GO) was synthesized by the modified Hummers’ method [24]. The detailed process is described as below: 1.0 g of graphite and 6.0 g of KMnO4 were mixed with 100 mL of H2SO4 (98%) in a beaker and placed in an ice bath. One hour later, the mixture was heated to 35 °C and kept stirring for 5 h in an oil bath. Then the mixture was diluted with 200 mL of deionized water under vigorous agitation. Thereafter, 100 mL of H2O2 was added to the mixture drop by drop. Then the mixture was rinsed and centrifuged with 5% HCl and deionized water for several times. After filtration and drying under vacuum at 60 °C, GO was obtained as a gray powder. 2.2. Synthesis of NTS/G composites NTS/G composites were synthesized by a facile one-pot hydrothermal process. In a typical synthesis route, 6 mL of GO aqueous solution with the concentration of 1.53 mg/mL, 2.5 mL of HF
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(40%) and 21.5 mL of deionized water were mixed under vigorous stirring to form well-distributed solution. Then, 0.9 g of TiN solid powder (particle size: 2–10 lm) was added into the solution. After stirring for 20 min, the suspension was transferred to an 80 mL Teflon-lined autoclave and heated at 180 °C for 20 h. By this hydrothermal treatment, the reduction of GO to graphene and the formation of N-doped TiO2 plates with exposed {0 0 1} facets were simultaneously achieved. After the hydrothermal process, the products were collected by centrifugation and washed with deionized water for several times, and then were dried overnight at 80 °C in air. In order to investigate the effect of graphene content on the photocatalytic activity of NTS/G composites, the percentages of graphene to TiO2 were varied by the added volume of GO aqueous solution from 0 to 10 mL (0, 2, 4, 6, 8 and 10 mL), and the resulting samples were labeled as NTS/G-x, where x = 0, 2, 4, 6, 8 and 10, respectively. Additionally, NTS/G-0 was abbreviated as NTS for convenience.
2.3. Characterization Scanning electron microscopy (SEM) images were obtained by an S-4800 (Hitachi, Japan) equipment. Transmission electron microscope (TEM) tests were carried out on JEM 2100 (JEOL, Japan). The elemental composition over the desired region was detected by an energy dispersive X-ray spectrometer (EDS) attached to the TEM. X-ray diffraction (XRD) patterns were recorded at room temperature with an X’pert PROMPD diffractometer (PANalytical, Holland) with copper Ka1 radiation. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB250 (Thermo Scientific, America) with aluminum Ka radiation. UV–Vis diffuse reflectance spectroscopies (DRS) were recorded by a UV-2450 spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere, and the baseline correction was done using a calibrated sample of barium sulfate. The nitrogen adsorption measurements were taken at 77 K using an ASAP 2010 analyzer (Micromeritics, USA), and the Barrett–Joyner–Halenda (BJH) pore diameter distribution curves were obtained from the desorption branch and specific surface areas were obtained according to the Brunauer–Emmett–Teller (BET) model. Photoluminescence (PL) spectra were measured at room temperature using a fluorescence spectrophotometer (Hitachi, F-4600) with an excitation wavelength of 300 nm.
2.4. Experimental procedures of photocatalytic decoloration The photocatalytic experiment was carried out in a photo reaction system (as illustrated in our previous publication [25]) by using MB as a model pollutant. A 1000 W Xe lamp equipped with a 420 nm glass filter (removing the UV irradiation below 420 nm), positioned in the center of a water-cooled quartz jacket, was used to offer visible light irradiation. At the side of quartz jacket, a 50-mL cylindrical vessel was used as the reactive bottle to load reaction solution. The distance between lamp and reactive bottle was 40 mm. In the bottom of the reactive bottle, a magnetic stirrer was equipped to achieve effective dispersion. The temperature of the reaction solution was maintained at 30 ± 0.5 °C by cooling water. Photocatalyst powder (50 mg) was added into 50 mL of 10 mg/L MB solution to form suspension. Then, the suspension was irradiated with visible light. During the irradiation, the suspension was stirred continuously. At given interval, 3 mL of suspension was taken out and immediately centrifuged to eliminate the solid particles. The absorbance of the filtrate was measured by a spectrophotometer at the maximum absorbance peak 665 nm.
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3. Results 3.1. TEM and SEM Fig. 1a shows the TEM images of GO with different magnification. The satin-like GO nanosheet can be clearly observed from the inset in Fig. 1a, and there are plenty of wrinkles on the transparent sheet due to the two dimensional nature of GO. The highmagnification TEM image (Fig. 1b) reveals the GO sheet consists of three or four layers. The inset in Fig. 1b presents the corresponding selected area electron diffraction pattern (SAED) of GO. The diffraction rings can be indexed as a hexagonal structure similar to that of graphene, despite the deformation resulted from locally induced sp3 hybridization [26]. After TiN particles, irregular bulks with relatively smooth surface (Fig. A1a in Appendix), were treated for 20 h in the present of HF under hydrothermal conditions, TiO2 plates with dominant {0 0 1} facets could be formed. As shown in Fig. 1c that the products consist of well-defined plate-shaped structures with a square outline, and the two flat and square surfaces can be ascribed to {0 0 1} and the eight isosceles trapezoidal surfaces are {1 0 1} facets of the anatase TiO2 crystal [14], as illustrated in the inset. The side length of TiO2 sheets is about 2.5 lm and the percentage of {0 0 1} facets is estimated to be as high as ca. 70%. From the SEM images of NTS/G composites (Figs. 1d and A1b–d in Appendix), it can be clearly observed that graphene nanosheets with micrometers-long wrinkles are intimately grafted on the TiO2 plates. This intimate interaction enables the photogenerated electrons to more easily transfer from TiO2 plates to graphene nanosheets during the photocatalytic process, which induces much higher photocatalytic activity. 3.2. XRD The XRD patterns of GO, TiN, NTS and NTS/G-6 are displayed in Fig. 2. It can be seen from the XRD pattern of GO that a
well-defined peak at a lower diffraction angle of 2h = 9.8°, indicative of good layer regularity with a repeating interlayer distance of 0.80 nm, which is related to the accommodation of various oxygen species (mostly ether-ring oxygen and hydroxyl) and water molecules, and related to the changes in the carbon hexahedron grid plane due to the oxidation of graphite [27,28]. The characteristic (0 0 2) diffraction of graphite at 2h = 26.4° did not appear, which confirms that complete oxidation had taken place, resulting in the formation of a new well ordered, lamellar structure. Five peaks appeared in the XRD pattern of TiN at 2h = 36.64°, 42.57°, 61.77°, 74.02° and 77.91°, which are corresponding to (1 1 1), (0 0 2), (0 2 2), (1 1 3) and (2 2 2) planes of cubic TiN, respectively (Ref. No. 03-065-0715). After hydrothermal treatment for 20 h, the peaks at 2h = 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69° (Ref. No. 00-021-1272), which all belong to anatase TiO2, appeared in the XRD pattern of NTS, and only one weak peak (2h = 42.57°) belonging to TiN can be hardly observed, implying that the vast majority of anatase TiO2 was formed. Unfortunately, no typical diffraction peaks of the separate graphene can be observed in the XRD pattern of NTS/G-6, except that those diffraction peaks matched well with the crystal structure of the anatase-phase TiO2 and the weak peaks belonging to TiN. The XRD patterns of other NTS/G composites (Fig. A2 in Appendix), such as NTS/G-2, NTS/G-4, NTS/G-8, NTS/G-10, also confirmed the same rules. The reason can be attributed to the fact that the main peak of anatase TiO2 at 25.28° maybe shields the main characteristic peak of graphene at 24.5° [29].
3.3. XPS Fig. 3 shows the XPS full survey spectra of GO, NTS and NTS/G-6. As reported [30], GO contains only C, O, and very small amounts of S and Si, which may be introduced in the prepared process. Both NTS and NTS/G-6 contain Ti, O, N, C and F elements. The peak of N1s implies that N-doping has been realized [19,23], and the peak
Fig. 1. TEM (a) and HR-TEM (b) of GO and SEM of NTS (c) and NTS/G-6 (d).
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Fig. 2. The XRD patterns of GO, TiN, NTS and NTS/G-6.
of F1s can be attributed to the grafted F ions [19,23]. The elaborated analysis of N1s can be found from our previous paper [23]. Fig. 4 displays the C1s high-resolution XPS spectra of GO and NTS/G-6. As reported in early publication [31], two typical peaks of GO located at 285.0 and 287.0 eV can be fitted, which are usually assigned to adventitious carbon and sp2-hybridized carbon from the GO, and the oxygen-containing carbonaceous bands (CAOH), respectively. The presence of oxygen-containing carbon in GO could provide active sites for directed connection with the surface of NTS. The high-resolution XPS spectra of C1s from NTS/G-6 can be fitted as three peaks at binding energies of 285.0, 286.7 and 288.7 eV, respectively. As confirmed in previous publications [32,33], the peak centered at the binding energy of 285.0 eV can be assigned to the CAC, C@C, and CAH bonds (sp2) of graphene, the peaks at the binding energy ranges of 286.0–287.0 eV can be attributed to the CAOH, and the peaks at 288.4–288.8 eV can be ascribed to O@CAOH bands. The appearance of O@CAOH bands indicates that the AOH groups on the NTS possibly react with the ACOOH groups on the GO surface through esterification to form O@CAOATi bonds. It should be noted the intensity of peak at 286.7 eV for CAOH group on GO depresses remarkably in the C1s XPS spectrum of NTS/G-6, which can be attributed to the reduction of GO to graphene during the hydrothermal treatment [27]. 3.4. UV–Vis adsorption spectra Fig. 5 shows the UV–Vis adsorption spectra of all samples. In order to highlight the effect of N-doping, reference un-doped anatase TiO2 plates (abbreviated as Ref) with a comparable particle size and percentage of {0 0 1} facets to nitrogen doped anatase TiO2 plates were prepared by a similar procedure where TiB2 was used as precursor [19], and its UV–Vis adsorption spectrum was also examined and compared. It can be seen that TiN powder, with the color of taupe, exhibits strong absorption in whole range of wavelength employed. Due to the formation of anatase, the characteristic absorption edge at around 350–390 nm of semiconductor
TiO2 can be clearly observed from the spectra of other samples (NTS, NTS/G-6 and Ref). Compared with Ref, NTS exhibit strong absorption in the visible region due to N-doping, indicating that it can be excited by visible light. The reason may be attributed to the appearance of some localized states in the band gap of TiO2 due to N-doping and the narrowed gap of charge transfer, allowing visible light absorption [34,35]. It is widely accepted that the nitrogen incorporation into the crystal matrix of TiO2 modifies the electronic band structure of TiO2, leading to a new substitution N 2p band formed above O 2p valance band, which narrows the band gap of TiO2 and shifts optical absorption to the visible light region [36,37]. Compared with NTS, both the light absorption in the visible region and the redshift in the absorption edge of NTS/ G-6 are further improved due to the introduction of graphene, which indicates that a more efficient utilization of the solar spectrum could be achieved [18]. 3.5. BET In order to gain the information about specific surface area and porous property, nitrogen adsorption–desorption experiments were carried out. Fig. 6 shows the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves (inset in Fig. 6) of NTS and NTS/G-6. It can be seen that the two samples are similar and present hysteresis loops, indicating the presence of large mesopores and macropores, which can be classified as type IV according to IUPAC classification. These isotherms exhibit H3 hysteresis loops associated with the presence of slit-like pores, which can be attributed the pores formed from the stack of NTS. The pore-size distributions (inset in Fig. 6) demonstrate the existence of mesopores and macropores. The specific surface area of the NTS/G-6 was calculated to be 2.75 m2 g 1 based on the BET analysis, which is higher than that of NTS (1.76 m2 g 1). The possible reason is the presence of graphene, which has an extremely high surface area (theoretical value of 2600 m2 g 1). The higher surface area can provide the possibility for the efficient diffusion and transportation of the degradable organic molecules
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and hydroxyl radicals in photocatalytic reaction, which will lead to the enhanced photocatalytic activity. 3.6. Photocatalytic activity It is well-known that MB is one of the most important representative organic dye and has been widely applied in the industrial production, which often contaminates environment. So MB was chosen as a pollutant model to evaluate photocatalytic activity of the samples. Before the photocatalysis, the solution including MB and catalyst is stirred in dark for one hour for establishing the adsorption equilibrium. The decoloration of MB solution versus photocatalytic time under visible light irradiation (k > 420 nm) is shown in Fig. 7a. It can be seen that NTS/G composites show higher photocatalytic activity in comparison with bare NTS, implying
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Wavelength (nm) Fig. 5. UV–Vis diffuse reflectance spectroscopies of samples.
graphene plays important roles on the improvement of photocatalytic activity. Among these NTS/G composites, NTS/G-6 displays the highest photocatalytic activity, indicating there is an optimal ratio between graphene and NTS for the maximum of photocatalytic activity.
3.7. Kinetics analysis To obtain a quantitative understanding on the reaction kinetics of the MB degradation, we applied the pseudo-first order model as expressed by the equation ln(C0/C) = kt, which is generally used for photocatalytic degradation process [38,39] (where C0 and C are the
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4. Discussion
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Compared with NTS, NTS/G composites show higher photocatalytic activity. So it can be deduced that graphene must play important roles to enhance the photocatalytic activity of NTS. From above-mentioned results and analysis, we attempt to discuss the role of graphene.
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(1) Transportation of photo-excited electrons. Graphene has high charge carrier mobility, so the electrons in conduction band (CB) of TiO2, which are photo-excited from valence band (VB) of TiO2, can be rapidly transported on the graphene nanosheets. The rapid transport of electrons helps to inhibit the recombination of electrons and holes, which in turn results in enhanced photocatalytic activity in NTS/G composites. In order to confirm the charge anti-recombination of graphene, the photoluminescence (PL) spectra of samples were measured. For semiconductors, the PL spectrum is related to the transfer behavior of photo-generated electrons and holes, therefore it can reflect the separation and recombination of photo-generated charge carriers [40]. As shown in Fig. 8, PL intensity of NTS is much higher than those samples of NTS/G, indicating that graphene can efficiently inhibit the recombination of photogenerated electrons and holes, which can be ascribed to the fact that graphene can transport the electrons rapidly and prevent the direct recombination of photo-generated charge carriers [41]. Further inspection of PL spectra reveals that NTS/G-6 has the lowest PL intensity compared to other samples, which shows that optimal loading of graphene has reduced the recombination of excited electrons and holes. (2) Extension of light absorption range. As shown in Fig. 5, there is an obvious red shift in the absorption edge of NTS/G-6 compared with NTS. Furthermore, the light absorption of NTS/G-6 in the visible region is stronger than that of NTS. The absorption range of light plays an important role in the photocatalysis, especially for the visible light inducing photo-degradation of organic pollutants. The red shift of light absorption edge and the stronger light absorption in visible region are very helpful to the improvement of photocatalytic activity under visible light irradiation. (3) Enhancement of adsorptive capacity. As indicated in Fig. 7, the adsorptive capacity of NTS/G composites (after equilibrium in the dark for 1 h) gradually enhances with the increase content of graphene (in order to be observed more
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(b) Fig. 7. (a) photocatalytic decoloration of MB solution over different photocatalysts under visible light irradiation (k > 420 nm), and (b) photocatalytic reaction kinetics of MB decomposition (ln(C0/C)–time curves).
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concentrations of MB in solution at time 0 and t, respectively, and t is the reaction time; k is the reaction rate constant (min 1)). It can be seen from Fig. 7b that the k has a maximum value (0.085 min 1) when NTS/G-6 was used as photocatalyst, indicating that NTS/G-6 has the highest photocatalytic activity among all as-prepared samples.
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clearly, histogram is shown in Fig. A3 in Appendix), which can be attributed to the large specific surface area of graphene. Besides the simple physical adsorption, the p–p stacking between MB and aromatic regions of graphene is another reason for the enhancement of adsorptive capacity [9]. It is well known that the photocatalytic reaction occurs on the surface of the catalysts, and recombination of the photogenerated electron and hole is very fast, so interfacial charge carrier transfer is possible only when the donor or acceptor is pre-adsorbed before the photocatalytic reaction. The preliminary adsorption of the substrates and the adsorption capacity of samples are very important pre-requisites for highly efficient degradation [34]. Although graphene nanosheets play so many positive roles in enhancing the photocatalytic activity of NTS, it does not mean that the photocatalytic activity of NTS/G composite always increases with the increase in graphene nanosheet content. There is a competition for light absorption between NTS and graphene because graphene does absorb a certain amount of light. Therefore, too much graphene in NTS/G composite is disadvantaged to photocatalytic activity. This is the reason why NTS/G-8 and NTS/G-10 presented worse photocatalytic activity in comparison with NTS/ G-6. Additionally, the separation of photo-generated electrons and holes is also accelerated by high-energy {0 0 1} facets. In general, the interfacial electron transfer is mediated by the surface defects. Typically, high-energy {0 0 1} facets are composed of high densities of under-coordinated Ti atoms and a very large TiAOATi bond angle, forming abundant oxygen deficiency. Thus, it can be deduced that there are synergistic effects between high reactive {0 0 1} facets and graphene nanosheets to accelerate the separation of photo-generated electrons and holes, which significantly enhance the photocatalytic activity of NTS/G composites. 4.2. Proposed photocatalytic mechanism Based on previous publications [42–44] and our experimental results, we attempt to propose the reaction mechanism for the decoloration of MB on NTS/G composites under visible light irradiation as follows, which is schematically shown in Fig. 9. Firstly, electrons located in the localized N states are excited to the CB of TiO2 under visible light irradiation and correspondingly left holes in the localized N states, and then these electrons can
be rapidly transported by graphene, which helps to inhibit the recombination of electrons and holes. After that, these electrons are scavenged by molecular oxygen to produce the superoxide radical anion and hydrogen peroxide. These new formed intermediates can interact to produce hydroxyl radicals (OH). In addition, the photo-generated holes formed in the localized N states react with hydroxyl groups and H2O molecule to produce hydroxyl radicals. Finally, MB molecules pre-adsorbed on photocatalyst are oxidized by these oxidants and are mineralized into final products step by step. 5. Conclusions Composites composed of nitrogen-doped anatase TiO2 plates with exposed {0 0 1} facets and graphene nanosheets were firstly synthesized by a facile one-pot hydrothermal process. Graphene nanosheets play three important roles, transportation of photo-excited electrons, extension of light absorption range and enhancement of adsorptive capacity, to improve the visible-light photocatalytic activity of NTS/G composites. Due to the high charge carrier mobility of graphene, the recombination of photo-generated electrons and holes can be effectively inhibited. Due to the redshift of light absorption edge and the stronger light absorption in visible region resulted from graphene, more efficient utilization of the visible light could be achieved. Due to the large specific surface area of graphene and the p–p stacking between MB and aromatic regions of graphene, more MB molecules can be preliminarily adsorbed for highly efficient degradation. Additionally, the synergistic effects between high reactive {0 0 1} facets and graphene nanosheets further accelerate the separation of photo-generated electrons and holes, significantly enhancing the photocatalytic activity of NTS/G composites. It is expected that such composite materials will be useful in environment remediation and energy conversion such as water splitting and solar cell, and may shed light on the deliberately designed material toward more efficient visible-light photocatalysis. Acknowledgments This work was sponsored by the Key Project of Science and Technology Plan of Fujian Province (2012Y0066), the National High Technology Research and Development Program (‘‘863’’ Program) of China (No. 2012AA062606), the International Science & Technology Cooperation Program of China (No. 2011DFB91710), Xiamen Distinguished Young Scholar Award (No. 3502Z20126011) and Xiamen Science & Technology Major Program (No. 3502Z20131018). The valuable comments of anonymous reviewers are greatly appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.05.027. References
Fig. 9. Photocatalytic mechanism on the NTS/G composite under visible light irradiation.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
The composite of nitrogen-doped anatase titania plates with exposed {0 0 1} facets/graphene nanosheets for enhanced visible-light photocatalytic activity Jian-Wen Shi, Hui-Ying Ai, Jian-Wei Chen, Hao-Jie Cui, Ming-Lai Fu ⇑ Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China
a r t i c l e
i n f o
Article history: Received 19 December 2013 Accepted 14 May 2014 Available online 23 May 2014 Keywords: TiO2 Graphene Photocatalysis Doping Visible light
a b s t r a c t Composite photocatalysts composed of nitrogen-doped anatase TiO2 plates with exposed {0 0 1} facets (NTS) and graphene nanosheets (G) were firstly synthesized by a facile one-pot hydrothermal process. The morphologies, structural properties, and photocatalytic activities of the resultant NTS/G composites were investigated in detail. Graphene nanosheets were demonstrated play three important roles in the NTS/G composites, as transporter of photo-excited electrons, extender of light absorption range and enhancer of adsorptive capacity, respectively. Due to the effective charge anti-recombination, the efficient utilization of the visible light and the high adsorptive capacity to target pollutants, the composites exhibited significant improvement in photocatalytic degradation of methylene blue under visible light irradiation. Based on the results, the mechanism of enhanced visible-light photocatalytic activity on NTS/G composites was proposed. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Owing to potential applications in environmental clean-up and energy conversion, heterogeneous photocatalysis employing semiconductors has attracted extensive interest [1,2]. As a dominant semiconductor, TiO2 has been most widely investigated in the past decades due to its excellent properties such as nontoxicity, cheapness, and chemical stability [3,4]. However, the photocatalytic activity of TiO2 is still far below the level of practical application for two main unfavorable factors, restrictive light absorption (only responsive to ultraviolet light with wavelength below 387 nm due to its wide band-gap) and high charge recombination rate [5,6], which severely restricts its practical applications. Graphene, a two dimensional carbonaceous material, has attracted much attention recently for its high thermal conductivity (5000 W m 1 K 1), excellent mobility of charge carriers (200,000 cm2 V 1 s 1), a large specific surface area (calculated value 2630 m2 g 1) and good mechanical stability [7,8]. Considering its excellent mobility of charge carriers, graphene may be an excellent candidate to incorporate with TiO2 for the effective charge anti-recombination. To date, many publications have ⇑ Corresponding author. Address: Institute of Urban Environment, Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen, Fujian 361021, China. Fax: +86 592 6190977. E-mail address: [email protected] (M.-L. Fu). http://dx.doi.org/10.1016/j.jcis.2014.05.027 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
confirmed that graphene act the transporter of electrons in TiO2/ Graphene composites, which significantly inhibits the recombination of photo-generated electrons and holes. For example, Zhang et al. [9] reported that P25/Graphene composite showed higher photocatalytic activity for the degradation of methylene blue (MB) in comparison with bare P25 because the composite has a higher charge separation rate. Liu et al. [10] reported selfassembled TiO2 nanorods on large graphene oxide exhibited an improved photocatalytic activity due to the effective charge anti-recombination on graphene oxide. Lightcap and co-workers [11] even demonstrated the feasibility of using graphene as an electron-transfer medium in a TiO2/Graphene composite. Notably, the large specific surface area of graphene is hopeful to improve the adsorption capacity of TiO2/Graphene composites. Therefore, it is expected that the combination of TiO2 and graphene is promising to possess simultaneously excellent electron transferability, adsorbability and stability, which can be suitable for improving the photocatalytic activity of TiO2. On the other hand, the photocatalytic activity of TiO2 also intrinsically depends on the surface atomic structure and the crystallinity. In particular, the favorable surface atomic structure such as high reactive facets is highly expected to effectively enhance photocatalytic activity [12,13]. Since the pioneering work on anatase TiO2 plates with exposed {0 0 1} facets reported by Yang and co-workers [14], many publications have confirmed that the photocatalytic activity of anatase TiO2 crystals could be significantly
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improved because of the exposure of high reactive {0 0 1} facets [15,16]. These successful examples inspire researchers to combine anatase TiO2 plates with exposed {0 0 1} facets with graphene for higher photocatalytic activity, and some positive results have been achieved [17,18]. For example, Sun et al. [18] prepared nano-sized TiO2 plates with exposed {0 0 1} facets on the platform of graphene nanosheets, and found that the composite exhibited significant improvement in photocatalytic degradation of Rhodamine B, and the enhancement was attributed to the synergistic effect of the high catalytic activity of {0 0 1} facets and the effective charge anti-recombination of graphene. Nevertheless, in available cases, anatase TiO2 plates may have no visible-light response due to its large band gap (3.2 eV). Inspiringly, nonmetal doped (e.g. N, S, C-doped) anatase TiO2 plates with exposed {0 0 1} facets and visible-light response have been successfully prepared in recent years [19–22]. Therefore, combining nonmetal doped anatase TiO2 plates with exposed {0 0 1} facets and graphene may be a promising attempt to solve aforementioned two unfavorable factors of TiO2 and to obtain a new composite photocatalyst with enhanced visible-light photocatalytic activity. In our previous paper [23], N-doped anatase TiO2 plates with exposed {0 0 1} facets (abbreviated as NTS) were successfully prepared, and the optimal prepared conditions were ascertained. The resulting photocatalyst was demonstrated to be visible-light photocatalytic activity for the decoloration of MB. Encouraged by this work, here, we attempt to synthesize the composite photocatalyst consisted of NTS and graphene by a facile one-pot process. The formation of NTS and the following combination of NTS and graphene were successfully achieved. Our results demonstrated the visible-light photocatalytic activity of NTS/Graphene (abbreviated as NTS/G) composite was extremely improved due to the introduction of graphene. To the best of our knowledge, this work is the first report on the combination of NTS and graphene, and is also the first report on one-pot process to synthesize NTS/Graphene composite photocatalyst with enhanced visible-light photocatalytic activity. 2. Experimental section All the reagents used in this work were of analytical grade and were used without any further purification: TiN and graphite were purchased from Aladdin Reagent Company, and all the other reagents, such as methylene blue, KMnO4, H2SO4, HCl, HF and H2O2 were purchased from Sinopharm Chemical Reagent Company, Ltd., China. 2.1. Synthesis of graphene oxide Graphene oxide (GO) was synthesized by the modified Hummers’ method [24]. The detailed process is described as below: 1.0 g of graphite and 6.0 g of KMnO4 were mixed with 100 mL of H2SO4 (98%) in a beaker and placed in an ice bath. One hour later, the mixture was heated to 35 °C and kept stirring for 5 h in an oil bath. Then the mixture was diluted with 200 mL of deionized water under vigorous agitation. Thereafter, 100 mL of H2O2 was added to the mixture drop by drop. Then the mixture was rinsed and centrifuged with 5% HCl and deionized water for several times. After filtration and drying under vacuum at 60 °C, GO was obtained as a gray powder. 2.2. Synthesis of NTS/G composites NTS/G composites were synthesized by a facile one-pot hydrothermal process. In a typical synthesis route, 6 mL of GO aqueous solution with the concentration of 1.53 mg/mL, 2.5 mL of HF
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(40%) and 21.5 mL of deionized water were mixed under vigorous stirring to form well-distributed solution. Then, 0.9 g of TiN solid powder (particle size: 2–10 lm) was added into the solution. After stirring for 20 min, the suspension was transferred to an 80 mL Teflon-lined autoclave and heated at 180 °C for 20 h. By this hydrothermal treatment, the reduction of GO to graphene and the formation of N-doped TiO2 plates with exposed {0 0 1} facets were simultaneously achieved. After the hydrothermal process, the products were collected by centrifugation and washed with deionized water for several times, and then were dried overnight at 80 °C in air. In order to investigate the effect of graphene content on the photocatalytic activity of NTS/G composites, the percentages of graphene to TiO2 were varied by the added volume of GO aqueous solution from 0 to 10 mL (0, 2, 4, 6, 8 and 10 mL), and the resulting samples were labeled as NTS/G-x, where x = 0, 2, 4, 6, 8 and 10, respectively. Additionally, NTS/G-0 was abbreviated as NTS for convenience.
2.3. Characterization Scanning electron microscopy (SEM) images were obtained by an S-4800 (Hitachi, Japan) equipment. Transmission electron microscope (TEM) tests were carried out on JEM 2100 (JEOL, Japan). The elemental composition over the desired region was detected by an energy dispersive X-ray spectrometer (EDS) attached to the TEM. X-ray diffraction (XRD) patterns were recorded at room temperature with an X’pert PROMPD diffractometer (PANalytical, Holland) with copper Ka1 radiation. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB250 (Thermo Scientific, America) with aluminum Ka radiation. UV–Vis diffuse reflectance spectroscopies (DRS) were recorded by a UV-2450 spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere, and the baseline correction was done using a calibrated sample of barium sulfate. The nitrogen adsorption measurements were taken at 77 K using an ASAP 2010 analyzer (Micromeritics, USA), and the Barrett–Joyner–Halenda (BJH) pore diameter distribution curves were obtained from the desorption branch and specific surface areas were obtained according to the Brunauer–Emmett–Teller (BET) model. Photoluminescence (PL) spectra were measured at room temperature using a fluorescence spectrophotometer (Hitachi, F-4600) with an excitation wavelength of 300 nm.
2.4. Experimental procedures of photocatalytic decoloration The photocatalytic experiment was carried out in a photo reaction system (as illustrated in our previous publication [25]) by using MB as a model pollutant. A 1000 W Xe lamp equipped with a 420 nm glass filter (removing the UV irradiation below 420 nm), positioned in the center of a water-cooled quartz jacket, was used to offer visible light irradiation. At the side of quartz jacket, a 50-mL cylindrical vessel was used as the reactive bottle to load reaction solution. The distance between lamp and reactive bottle was 40 mm. In the bottom of the reactive bottle, a magnetic stirrer was equipped to achieve effective dispersion. The temperature of the reaction solution was maintained at 30 ± 0.5 °C by cooling water. Photocatalyst powder (50 mg) was added into 50 mL of 10 mg/L MB solution to form suspension. Then, the suspension was irradiated with visible light. During the irradiation, the suspension was stirred continuously. At given interval, 3 mL of suspension was taken out and immediately centrifuged to eliminate the solid particles. The absorbance of the filtrate was measured by a spectrophotometer at the maximum absorbance peak 665 nm.
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3. Results 3.1. TEM and SEM Fig. 1a shows the TEM images of GO with different magnification. The satin-like GO nanosheet can be clearly observed from the inset in Fig. 1a, and there are plenty of wrinkles on the transparent sheet due to the two dimensional nature of GO. The highmagnification TEM image (Fig. 1b) reveals the GO sheet consists of three or four layers. The inset in Fig. 1b presents the corresponding selected area electron diffraction pattern (SAED) of GO. The diffraction rings can be indexed as a hexagonal structure similar to that of graphene, despite the deformation resulted from locally induced sp3 hybridization [26]. After TiN particles, irregular bulks with relatively smooth surface (Fig. A1a in Appendix), were treated for 20 h in the present of HF under hydrothermal conditions, TiO2 plates with dominant {0 0 1} facets could be formed. As shown in Fig. 1c that the products consist of well-defined plate-shaped structures with a square outline, and the two flat and square surfaces can be ascribed to {0 0 1} and the eight isosceles trapezoidal surfaces are {1 0 1} facets of the anatase TiO2 crystal [14], as illustrated in the inset. The side length of TiO2 sheets is about 2.5 lm and the percentage of {0 0 1} facets is estimated to be as high as ca. 70%. From the SEM images of NTS/G composites (Figs. 1d and A1b–d in Appendix), it can be clearly observed that graphene nanosheets with micrometers-long wrinkles are intimately grafted on the TiO2 plates. This intimate interaction enables the photogenerated electrons to more easily transfer from TiO2 plates to graphene nanosheets during the photocatalytic process, which induces much higher photocatalytic activity. 3.2. XRD The XRD patterns of GO, TiN, NTS and NTS/G-6 are displayed in Fig. 2. It can be seen from the XRD pattern of GO that a
well-defined peak at a lower diffraction angle of 2h = 9.8°, indicative of good layer regularity with a repeating interlayer distance of 0.80 nm, which is related to the accommodation of various oxygen species (mostly ether-ring oxygen and hydroxyl) and water molecules, and related to the changes in the carbon hexahedron grid plane due to the oxidation of graphite [27,28]. The characteristic (0 0 2) diffraction of graphite at 2h = 26.4° did not appear, which confirms that complete oxidation had taken place, resulting in the formation of a new well ordered, lamellar structure. Five peaks appeared in the XRD pattern of TiN at 2h = 36.64°, 42.57°, 61.77°, 74.02° and 77.91°, which are corresponding to (1 1 1), (0 0 2), (0 2 2), (1 1 3) and (2 2 2) planes of cubic TiN, respectively (Ref. No. 03-065-0715). After hydrothermal treatment for 20 h, the peaks at 2h = 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69° (Ref. No. 00-021-1272), which all belong to anatase TiO2, appeared in the XRD pattern of NTS, and only one weak peak (2h = 42.57°) belonging to TiN can be hardly observed, implying that the vast majority of anatase TiO2 was formed. Unfortunately, no typical diffraction peaks of the separate graphene can be observed in the XRD pattern of NTS/G-6, except that those diffraction peaks matched well with the crystal structure of the anatase-phase TiO2 and the weak peaks belonging to TiN. The XRD patterns of other NTS/G composites (Fig. A2 in Appendix), such as NTS/G-2, NTS/G-4, NTS/G-8, NTS/G-10, also confirmed the same rules. The reason can be attributed to the fact that the main peak of anatase TiO2 at 25.28° maybe shields the main characteristic peak of graphene at 24.5° [29].
3.3. XPS Fig. 3 shows the XPS full survey spectra of GO, NTS and NTS/G-6. As reported [30], GO contains only C, O, and very small amounts of S and Si, which may be introduced in the prepared process. Both NTS and NTS/G-6 contain Ti, O, N, C and F elements. The peak of N1s implies that N-doping has been realized [19,23], and the peak
Fig. 1. TEM (a) and HR-TEM (b) of GO and SEM of NTS (c) and NTS/G-6 (d).
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of F1s can be attributed to the grafted F ions [19,23]. The elaborated analysis of N1s can be found from our previous paper [23]. Fig. 4 displays the C1s high-resolution XPS spectra of GO and NTS/G-6. As reported in early publication [31], two typical peaks of GO located at 285.0 and 287.0 eV can be fitted, which are usually assigned to adventitious carbon and sp2-hybridized carbon from the GO, and the oxygen-containing carbonaceous bands (CAOH), respectively. The presence of oxygen-containing carbon in GO could provide active sites for directed connection with the surface of NTS. The high-resolution XPS spectra of C1s from NTS/G-6 can be fitted as three peaks at binding energies of 285.0, 286.7 and 288.7 eV, respectively. As confirmed in previous publications [32,33], the peak centered at the binding energy of 285.0 eV can be assigned to the CAC, C@C, and CAH bonds (sp2) of graphene, the peaks at the binding energy ranges of 286.0–287.0 eV can be attributed to the CAOH, and the peaks at 288.4–288.8 eV can be ascribed to O@CAOH bands. The appearance of O@CAOH bands indicates that the AOH groups on the NTS possibly react with the ACOOH groups on the GO surface through esterification to form O@CAOATi bonds. It should be noted the intensity of peak at 286.7 eV for CAOH group on GO depresses remarkably in the C1s XPS spectrum of NTS/G-6, which can be attributed to the reduction of GO to graphene during the hydrothermal treatment [27]. 3.4. UV–Vis adsorption spectra Fig. 5 shows the UV–Vis adsorption spectra of all samples. In order to highlight the effect of N-doping, reference un-doped anatase TiO2 plates (abbreviated as Ref) with a comparable particle size and percentage of {0 0 1} facets to nitrogen doped anatase TiO2 plates were prepared by a similar procedure where TiB2 was used as precursor [19], and its UV–Vis adsorption spectrum was also examined and compared. It can be seen that TiN powder, with the color of taupe, exhibits strong absorption in whole range of wavelength employed. Due to the formation of anatase, the characteristic absorption edge at around 350–390 nm of semiconductor
TiO2 can be clearly observed from the spectra of other samples (NTS, NTS/G-6 and Ref). Compared with Ref, NTS exhibit strong absorption in the visible region due to N-doping, indicating that it can be excited by visible light. The reason may be attributed to the appearance of some localized states in the band gap of TiO2 due to N-doping and the narrowed gap of charge transfer, allowing visible light absorption [34,35]. It is widely accepted that the nitrogen incorporation into the crystal matrix of TiO2 modifies the electronic band structure of TiO2, leading to a new substitution N 2p band formed above O 2p valance band, which narrows the band gap of TiO2 and shifts optical absorption to the visible light region [36,37]. Compared with NTS, both the light absorption in the visible region and the redshift in the absorption edge of NTS/ G-6 are further improved due to the introduction of graphene, which indicates that a more efficient utilization of the solar spectrum could be achieved [18]. 3.5. BET In order to gain the information about specific surface area and porous property, nitrogen adsorption–desorption experiments were carried out. Fig. 6 shows the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves (inset in Fig. 6) of NTS and NTS/G-6. It can be seen that the two samples are similar and present hysteresis loops, indicating the presence of large mesopores and macropores, which can be classified as type IV according to IUPAC classification. These isotherms exhibit H3 hysteresis loops associated with the presence of slit-like pores, which can be attributed the pores formed from the stack of NTS. The pore-size distributions (inset in Fig. 6) demonstrate the existence of mesopores and macropores. The specific surface area of the NTS/G-6 was calculated to be 2.75 m2 g 1 based on the BET analysis, which is higher than that of NTS (1.76 m2 g 1). The possible reason is the presence of graphene, which has an extremely high surface area (theoretical value of 2600 m2 g 1). The higher surface area can provide the possibility for the efficient diffusion and transportation of the degradable organic molecules
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282
280
Binding energy (eV) NTS
Intensity (a.u.)
N1s
Ti3s Ti3p
C1s
F1s
Ti2p
OKLL
Intensity (a.u.)
O1s
NTS/G-6
1000
800
600
400
200
0
294
292
288
286
284
400
Absorbance (a.u.)
Ti3s
N1s
Ti3p
C1s
F1s
600
280
1.4 TiN NTS NTS/G-6 Ref
1.2
800
282
Fig. 4. The high-resolution C1s spectra of GO and NTS/G-6.
NTS/G-6
Ti2p
1000
290
Binding energy (eV)
OKLL
Intensity (a.u.)
O1s
Banding energy (eV)
200
0
Banding energy (eV)
1.0 0.8 0.6 0.4 0.2 0.0 300
Fig. 3. The XPS full survey spectra of GO, NTS and NTS/G-6.
and hydroxyl radicals in photocatalytic reaction, which will lead to the enhanced photocatalytic activity. 3.6. Photocatalytic activity It is well-known that MB is one of the most important representative organic dye and has been widely applied in the industrial production, which often contaminates environment. So MB was chosen as a pollutant model to evaluate photocatalytic activity of the samples. Before the photocatalysis, the solution including MB and catalyst is stirred in dark for one hour for establishing the adsorption equilibrium. The decoloration of MB solution versus photocatalytic time under visible light irradiation (k > 420 nm) is shown in Fig. 7a. It can be seen that NTS/G composites show higher photocatalytic activity in comparison with bare NTS, implying
400
500
600
700
800
Wavelength (nm) Fig. 5. UV–Vis diffuse reflectance spectroscopies of samples.
graphene plays important roles on the improvement of photocatalytic activity. Among these NTS/G composites, NTS/G-6 displays the highest photocatalytic activity, indicating there is an optimal ratio between graphene and NTS for the maximum of photocatalytic activity.
3.7. Kinetics analysis To obtain a quantitative understanding on the reaction kinetics of the MB degradation, we applied the pseudo-first order model as expressed by the equation ln(C0/C) = kt, which is generally used for photocatalytic degradation process [38,39] (where C0 and C are the
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4. Discussion
10
0.0012
9
0.0010
4.1. The role of graphene
8 dV/dD (cm /g)
7
NTS NTS/G-6
0.0008
Compared with NTS, NTS/G composites show higher photocatalytic activity. So it can be deduced that graphene must play important roles to enhance the photocatalytic activity of NTS. From above-mentioned results and analysis, we attempt to discuss the role of graphene.
2
Volume adsorbed (cm3/g)
11
6 5
0.0006 0.0004
4
0.0002
3
0.0000 0
2
4
2
6
8
10
12
14
16
18
20
Pore diameter (nm)
1 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0) Fig. 6. Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) of NTS and NTS/G-6.
1.0 NTS/G-2 NTS/G-6
0.9
NTS/G-4 NTS/G-8
NTS/G-10 NTS
TiN
0.8
C/C0
0.7 0.6 0.5 0.4 light off light on
0.3 0.2
-60
0
20
40
60
80
100
120
Time (min)
(a) 1.0 NTS NTS/G-2 NTS/G-4 NTS/G-6 NTS/G-8 NTS/G-10 TiN
ln(C 0 /C)
0.8
0.6
0.4
(1) Transportation of photo-excited electrons. Graphene has high charge carrier mobility, so the electrons in conduction band (CB) of TiO2, which are photo-excited from valence band (VB) of TiO2, can be rapidly transported on the graphene nanosheets. The rapid transport of electrons helps to inhibit the recombination of electrons and holes, which in turn results in enhanced photocatalytic activity in NTS/G composites. In order to confirm the charge anti-recombination of graphene, the photoluminescence (PL) spectra of samples were measured. For semiconductors, the PL spectrum is related to the transfer behavior of photo-generated electrons and holes, therefore it can reflect the separation and recombination of photo-generated charge carriers [40]. As shown in Fig. 8, PL intensity of NTS is much higher than those samples of NTS/G, indicating that graphene can efficiently inhibit the recombination of photogenerated electrons and holes, which can be ascribed to the fact that graphene can transport the electrons rapidly and prevent the direct recombination of photo-generated charge carriers [41]. Further inspection of PL spectra reveals that NTS/G-6 has the lowest PL intensity compared to other samples, which shows that optimal loading of graphene has reduced the recombination of excited electrons and holes. (2) Extension of light absorption range. As shown in Fig. 5, there is an obvious red shift in the absorption edge of NTS/G-6 compared with NTS. Furthermore, the light absorption of NTS/G-6 in the visible region is stronger than that of NTS. The absorption range of light plays an important role in the photocatalysis, especially for the visible light inducing photo-degradation of organic pollutants. The red shift of light absorption edge and the stronger light absorption in visible region are very helpful to the improvement of photocatalytic activity under visible light irradiation. (3) Enhancement of adsorptive capacity. As indicated in Fig. 7, the adsorptive capacity of NTS/G composites (after equilibrium in the dark for 1 h) gradually enhances with the increase content of graphene (in order to be observed more
0.2 500 0.0 20
40
60
80
100
120
Time (min)
(b) Fig. 7. (a) photocatalytic decoloration of MB solution over different photocatalysts under visible light irradiation (k > 420 nm), and (b) photocatalytic reaction kinetics of MB decomposition (ln(C0/C)–time curves).
400
Intensity (a.u.)
0
NTS NTS/G-2 NTS/G-4 NTS/G-6 NTS/G-10
300 200 100
concentrations of MB in solution at time 0 and t, respectively, and t is the reaction time; k is the reaction rate constant (min 1)). It can be seen from Fig. 7b that the k has a maximum value (0.085 min 1) when NTS/G-6 was used as photocatalyst, indicating that NTS/G-6 has the highest photocatalytic activity among all as-prepared samples.
0 640
660
680
700
720
740
760
780
Wavelength (nm) Fig. 8. Photoluminescence spectra of NTS and NTS/G excited at the wavelength of 300 nm.
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clearly, histogram is shown in Fig. A3 in Appendix), which can be attributed to the large specific surface area of graphene. Besides the simple physical adsorption, the p–p stacking between MB and aromatic regions of graphene is another reason for the enhancement of adsorptive capacity [9]. It is well known that the photocatalytic reaction occurs on the surface of the catalysts, and recombination of the photogenerated electron and hole is very fast, so interfacial charge carrier transfer is possible only when the donor or acceptor is pre-adsorbed before the photocatalytic reaction. The preliminary adsorption of the substrates and the adsorption capacity of samples are very important pre-requisites for highly efficient degradation [34]. Although graphene nanosheets play so many positive roles in enhancing the photocatalytic activity of NTS, it does not mean that the photocatalytic activity of NTS/G composite always increases with the increase in graphene nanosheet content. There is a competition for light absorption between NTS and graphene because graphene does absorb a certain amount of light. Therefore, too much graphene in NTS/G composite is disadvantaged to photocatalytic activity. This is the reason why NTS/G-8 and NTS/G-10 presented worse photocatalytic activity in comparison with NTS/ G-6. Additionally, the separation of photo-generated electrons and holes is also accelerated by high-energy {0 0 1} facets. In general, the interfacial electron transfer is mediated by the surface defects. Typically, high-energy {0 0 1} facets are composed of high densities of under-coordinated Ti atoms and a very large TiAOATi bond angle, forming abundant oxygen deficiency. Thus, it can be deduced that there are synergistic effects between high reactive {0 0 1} facets and graphene nanosheets to accelerate the separation of photo-generated electrons and holes, which significantly enhance the photocatalytic activity of NTS/G composites. 4.2. Proposed photocatalytic mechanism Based on previous publications [42–44] and our experimental results, we attempt to propose the reaction mechanism for the decoloration of MB on NTS/G composites under visible light irradiation as follows, which is schematically shown in Fig. 9. Firstly, electrons located in the localized N states are excited to the CB of TiO2 under visible light irradiation and correspondingly left holes in the localized N states, and then these electrons can
be rapidly transported by graphene, which helps to inhibit the recombination of electrons and holes. After that, these electrons are scavenged by molecular oxygen to produce the superoxide radical anion and hydrogen peroxide. These new formed intermediates can interact to produce hydroxyl radicals (OH). In addition, the photo-generated holes formed in the localized N states react with hydroxyl groups and H2O molecule to produce hydroxyl radicals. Finally, MB molecules pre-adsorbed on photocatalyst are oxidized by these oxidants and are mineralized into final products step by step. 5. Conclusions Composites composed of nitrogen-doped anatase TiO2 plates with exposed {0 0 1} facets and graphene nanosheets were firstly synthesized by a facile one-pot hydrothermal process. Graphene nanosheets play three important roles, transportation of photo-excited electrons, extension of light absorption range and enhancement of adsorptive capacity, to improve the visible-light photocatalytic activity of NTS/G composites. Due to the high charge carrier mobility of graphene, the recombination of photo-generated electrons and holes can be effectively inhibited. Due to the redshift of light absorption edge and the stronger light absorption in visible region resulted from graphene, more efficient utilization of the visible light could be achieved. Due to the large specific surface area of graphene and the p–p stacking between MB and aromatic regions of graphene, more MB molecules can be preliminarily adsorbed for highly efficient degradation. Additionally, the synergistic effects between high reactive {0 0 1} facets and graphene nanosheets further accelerate the separation of photo-generated electrons and holes, significantly enhancing the photocatalytic activity of NTS/G composites. It is expected that such composite materials will be useful in environment remediation and energy conversion such as water splitting and solar cell, and may shed light on the deliberately designed material toward more efficient visible-light photocatalysis. Acknowledgments This work was sponsored by the Key Project of Science and Technology Plan of Fujian Province (2012Y0066), the National High Technology Research and Development Program (‘‘863’’ Program) of China (No. 2012AA062606), the International Science & Technology Cooperation Program of China (No. 2011DFB91710), Xiamen Distinguished Young Scholar Award (No. 3502Z20126011) and Xiamen Science & Technology Major Program (No. 3502Z20131018). The valuable comments of anonymous reviewers are greatly appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.05.027. References
Fig. 9. Photocatalytic mechanism on the NTS/G composite under visible light irradiation.
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