g-C3N4

Nov 13, 2013 - photocatalytic reaction, the suspension was stirred in darkness for 30 min to ..... absorption edge of the pure g-C3N4 occurs at ca. 45...
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In Situ Microwave-Assisted Synthesis of Porous N‑TiO2/g‑C3N4 Heterojunctions with Enhanced Visible-Light Photocatalytic Properties Xiao-jing Wang, Wen-yan Yang, Fa-tang Li,* Ya-bin Xue, Rui-hong Liu, and Ying-juan Hao College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, China S Supporting Information *

ABSTRACT: An in situ microwave-assisted synthesis approach has been developed to prepare N-TiO2/g-C3N4 composites using H2TiO3 as the reactant and NH3·H2O as the N-doping source. In this way, the N-TiO2/g-C3N4 composite catalysts have a porous structure and large surface areas, which increase the contact area of pollutants. Degradation of rhodamine B (Rh B) and methylene blue (MB) were carried out to evaluate the photocatalytic activity of samples under visible light irradiation. N-TiO2/gC3N4 composite with 40 wt % N-TiO2 exhibits the highest photocatalytic activity and the optimal temperature is 400 °C. The increased photocatalytic activity of N-TiO2/g-C3N4 composites can be attributed to the formation of the heterojunction between N-TiO2 and g-C3N4, which suppresses the recombination of photoinduced electron−hole pairs. The tests of radical scavengers confirmed that •O2− was the main reactive species during the photocatalytic process. doped NaNbO330 and N-doped H2Ta2O6,31 etc. The results show that building C3N4-based heterostructures is an effective method to enhance its photocatalytic activity. On the other hand, N-doped TiO2 is one of the most studied visible-light photocatalysts due to its inexpensive, stable, and high activity since Asahi et al. reported it.32 N-doped TiO2 is also used to combine with C3N4 to form hybrid material by Yang et al.33 They prepared N-doped TiO2/C3N4 composite samples by heating the mixture of the hydrolysis product of TiCl4 and C3N4. However, the specific surface areas of the composites were not mentioned and the electrons/holes transfer mechanism between the two components was not discussed. Herein, an in situ microwave-assisted synthesis approach has been developed to prepare N-TiO2/g-C3N4 composites. Ndoped TiO2 was synthesized using inexpensive H2TiO3 as the reactant and NH3·H2O as the N-doping source, without any surfactants and special gas atmosphere. It is known that several approaches have been proposed to prepare N-doped TiO2, including the sol−gel method,34 high-temperature calcination in a NH3 atmosphere,35 hydrothermal synthesis,36 precipitation,37 compound pyrolysis,38 and a microwave-assisted process.39 Among them, the microwave-assisted synthesis is a facile, highly efficient and environmentally friendly method.40 For example, Ou et al.39 has prepared N-doped TiO2 by thermally treating microwave-assisted titanate nanotubes (TNTs, NaxH2−xTi3O7) in an Ar/NH3 atmosphere. In the above methods, most of the Ndoped TiO2 have been prepared using expensive organic titanium sources as raw materials, which made the prepared TiO2 have a relative high price. In this study, based on a facile method synthesizing porous Ndoped TiO2 via a microwave-assisted route, a series of porous N-

1. INTRODUCTION Photocatalysis is highly expected to be a green technology for environmental purification due to its potential utilization to solar energy for purification of water and air.1 Nevertheless, most of the widely used photocatalysts, such as TiO2 and ZnO, have two main drawbacks: one is the low utilization efficiency for solar energy due to their wide band gaps,2 and the other is high recombination rate of photoinduced electrons and holes. Therefore, searching for highly active visible-light photocatalysts will always be a worldwide striving direction. Recently, a novel stable metal-free photocatalyst, polymeric graphite-like carbon nitride (g-C3N4) was reported by Wang et al.,3 which has attracted intense interest for its applications in photocatalytic hydrogen production and photocatalytic degradation of organic pollutants under visible light because of its narrow band gap of 2.7 eV.4 However, the photocatalytic efficiency of the pure g-C 3 N 4 is limited by the high recombination rate of its photogenerated electron−hole pairs.5 To date, continuous attempts have been carried out to improve the quantum yield of g-C3N4, for example, designing nanoporous structures,6,7 chemical doping with nonmetal8−11 or metal elements,12,13 coupling with grapheme,14,15 protonation,16 building heterostructures,17−25 etc. Among them, constructing heterostructures by means of combining g-C3N4 with other appropriate semiconductors is an effective method to improve the separation rate of photogenerated carriers, resulting in an enhanced quantum yield. It is noteworthy that many of the reported heterojunction semiconductors are composites of an ultraviolet-light-driven photocatalyst, such as ZnO/g-C3N4,18,26 or BiPO4/g-C3N423 composite, which means that the ultravioletlight-driven photocatalyst cannot be excited by the visible light and has the sole effect of transferring electrons or holes. To use the visible light more effectively, recently, some visiblelight-driven composite photocatalysts were combined with gC3N4 to form composites, such as ZnWO6,2 WO3,5 TaON,17 BiOBr,20 Bi2WO6,21 Ag3PO4,22 SmVO4,27 CdS,28 DyVO4,29 N© 2013 American Chemical Society

Received: Revised: Accepted: Published: 17140

August 27, 2013 October 30, 2013 November 13, 2013 November 13, 2013 dx.doi.org/10.1021/ie402820v | Ind. Eng. Chem. Res. 2013, 52, 17140−17150

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The X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 1600 ESCA XPS system. 2.3. Photocatalytic Activity Tests. The photocatalytic activities of the catalysts were tested by the degradation of RhB and MB under visible light irradiation. A 200 mL of aqueous solution with the concentration of 10 mg/L was mixed with 0.20 g of photocatalyst powder in a 500 mL beaker. Prior to photocatalytic reaction, the suspension was stirred in darkness for 30 min to reach adsorption/desorption equilibrium. The irradiation was performed with a 300 W xenon arc lamp that was installed in a light-condensing lamp housing, and a 400 nm cutoff filter was placed in front of the reaction vessel so as to obtain the visible light. The light intensity of the lamp was 0.16 kW/m2 at 400 nm, which was measured by a light-intensity meter. At given time intervals, 5 mL of the suspension was sampled, centrifuged, and filtered to remove the photocatalyst. Then the concentration of pollutants was analyzed using a UV−vis spectrophotometer. The decoloration rate was reported as C/C0, where C was the pollutants concentration after adsorption or photocatalysis and C0 was initial concentration. The total organic carbon (TOC) content of the samples after degradation (60 min) was measured with a Shimadzu TOC-VWS analyzer. 2.4. Determination of Reactive Species. To detect the active species generated in the photocatalytic system, various scavengers, including 2-propanol (IPA, 10 mmol/L), AgNO3 (6 mmol/L), EDTA-2Na (6 mmol/L), 1,4-benzoquinone (C6H4O2, 6 mmol/L), and N2 (0.2 L/min) were introduced into the solution of RhB. Nitrotetrazolium blue chloride (NBT, 2.5 × 10−5 mol/L) was used to determine the amount of •O2− generated in the photocatalytic process because NBT can be detected by a UV−vis spectrophotometer, which exhibits an absorption maximum at 260 nm whereas the product of •O2− and NBT does not.41 The experimental procedures were as follows: NBT was dissolved in H2O to form a 2.5 × 10−5 mol/L solution. Then, 0.20 g of the photocatalyst was dispersed in 200 mL of the NBT aqueous solution. The mixture was irradiated under visible light for 60 min. Then, 5 mL of the suspension were sampled, centrifuged, filtered, and measured on Shimadzu UV-2550 spectrophotometer.

doped TiO2/g-C3N4 heterostructures were prepared by mixing the as-prepared g-C3N4 with the precursor of the TiO2 and then microwave irradiation. The adopted synthesis route ensures not only the successful growth of N-TiO2 nanostructures on g-C3N4 lamellar substrate but also the high dispersion of N-TiO2 nanostructures on g-C3N4 without aggregation. Moreover, in this way, the N-TiO2/g-C3N4 composite catalysts have porous structure and large surface areas, increasing the contact area for the objective pollutants. The photodegradation behaviors of rhodamine B (RhB) and methylene blue (MB) over the prepared photocatalysts were measured under visible light (λ > 400 nm) irradiation. Based on the relative band positions of the two materials and the experimental results, the photocatalytic mechanism of N-TiO2/g-C3N4 heterojunctions was presented.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. All chemicals for synthesis and analysis were commercially available and used without further treatments. The g-C3N4 powder was synthesized by thermal polycondensation of melamine.3 Typically, 6 g of melamine was put into a crucible with a cover and calcined at 550 °C for 2 h in a muffle furnace, with a heating rate of 20 °C/min. The resulting yellow product was collected and ground into powders for further use. The objective N-TiO2 was formed by peroxotitanate. Peroxotitanate was prepared by adding 1.0 g of H2TiO3 into an ice-cooled solution, which was composed of purified water, H2O2 (30%), and NH3·H2O (28%, its role is to ensure the formation of peroxotitanate). Then the mixture was stirred for 30 min and a homogeneous pale yellow-green peroxotitanate solution was obtained. In the meanwhile, appropriate amount of g-C3N4 was added into 25 mL of purified water to be ultrasonically stirred for 30 min to completely disperse the g-C3N 4. Then, the peroxotitanate solution was mixed with the g-C3N4 suspension. After being stirred for 3 h, the mixture was transferred into a flask and microwave-irradiated for 1 h. In the reaction process, the power and the temperature of the microwave oven (XH-100B, Beijing Xianghu Co., Ltd.) were set at 500 W and 80 °C, respectively. After irradiation, the products were separated by filtration and then dried at 80 °C. The obtained samples were calcinated at 300−500 °C in a muffle furnace for 2 h. Pure NTiO2 was prepared under the same conditions in the absence of g-C3N4. As a reference, a mechanically mixed N-TiO2/g-C3N4 (40 wt % N-TiO2) sample was obtained by grinding N-TiO2 and g-C3N4 powders together. 2.2. Characterizations. The thermogravimetry and differential thermal analysis were performed using a Shimadzu TGA60H thermal analyzer. The atmosphere was air and the heating rate was 10 °C/min. X-ray diffraction (XRD) analysis was carried out with Rigaku D/MAX 2500 X-ray diffractometer using Cu Kα radiation. Fourier transform infrared (FT-IR) spectra were obtained using a Shimadzu IR Prestige 21 spectrometer. The Brunauer−Emmett−Teller (BET) specific surface areas (SBET) were measured using a Quantachrome NOVA2000 nitrogen adsorption/desorption apparatus. Transmission electron microscopy (TEM) and high resolution electron microscopy (HRTEM) images were taken from a JEOL JEM-2010 electron microscope. The UV−vis diffuse reflectance spectra (DRS) were recorded between 200 and 800 nm using a Shimadzu UV2550 spectrophotometer. The photoluminescence (PL) measurements were carried out using a Hitachi F-4600 fluorescence spectrophotometer with an excitation wavelength of 400 nm.

3. RESULTS AND DISCUSSION 3.1. TG-DSC Analysis. To observe the thermostability of the composites, the TG-DSC experiment was carried out from room temperature to 800 °C at a heating rate of 10 °C/min. As can be seen in Figure 1, the g-C3N4 phase in the as-prepared composites becomes unstable when the heat temperature is above 500 °C and there is an endothermal peak between 600 to 730 °C, which can be attributed to the demoposition of g-C3N4. The weight of the 40 wt % N-TiO2/g-C3N4 composites decreases rapidly in the temperature range 550−700 °C, indicating that the demoposition of g-C3N4 occurred in this temperature range. This phenomenon shows that g-C3N4 in the composite is stable below 500 °C and can be used to construct composite photocatalysts at a moderate temperature. 3.2. Crystal Structure and Specific Surface Area. XRD patterns of N-TiO2, g-C3N4, and N-TiO2/g-C3N4 composite with different N-TiO2 content are shown in Figure 2a. The results indicate that the diffraction peaks of pure N-TiO2 correspond to the anatase phase of TiO2 [JCPDS No. 211272]. The diffraction peaks of pure g-C3N4 appearing at 27.4° and 13.1° correspond to the (002) and (110) planes, which correspond to the characteristic interplanar staking peaks of aromatic systems and the interlayer structural packing, 17141

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macroporous materials. These macropores are believed to be produced by interaggregated g-C3N4 flakes. N-TiO2 and NTiO2/g-C3N4 composites have typical type IV nitrogen isotherms with H1 hysteresis, indicative of mesoporous structure. These mesoporous structures are thought to be produced by the release of NH3 during calcination. The specific surface areas of various samples are shown in Table 1. The SBET value of N-TiO2 is 87.85 m2/g, whereas pure g-C3N4 displays a SBET value of 14.11 m2/g. The high SBET value of N-TiO2 is most likely formed by the release of NH3 during calcination. The NTiO2/g-C3N4 heterostructures have surface areas between 20.13 and 45.36 m2/g. These results indicate that the in situ chemical synthesis of N-TiO2 on the surface of g-C3N4 maintains a large specific surface area. 3.3. SEM and TEM Analysis. The particle sizes and the morphologies of the representative samples were examined by SEM, as shown in Figure 3. The g-C3N4 sample (Figure 3a) displays an aggregated morphology with a large size and lamellar structure. The surface of the aggregation is very smooth, showing the layer structure of g-C3N4. From Figure 3b, it can be seen that the obtained pure TiO2 is made up of regularly round particles with sizes around 15 nm. Figure 3c shows the SEM morphology of the 40 wt % N-TiO2/g-C3N4 composite. It is obvious that the surface of g-C3N4 becomes rough after loading TiO2 particles. The dispersion state and the structure of g-C3N4, N-TiO2, and NTiO2/g-C3N4 composite were obtained by TEM and HRTEM in Figure 4. It is observed from Figures 4a and 5b that the g-C3N4 displays a lamellar shape and has an amorphous structure, whereas the N-TiO2 exhibits uniform particles and the size is almost consistent with SEM results. It is also seen from Figure 4c that there are dark particles and gray areas. The particles with dark color can be assigned to N-TiO2, whereas the gray area can be assigned to g-C3N4. This shows that the N-TiO2 particles disperse on the surface of g-C3N4 well. Although having undergone the ultrasonic treatment before TEM observation, N-TiO2/g-C3N4 displays a firm connection between N-TiO2 and g-C3N4, showing the formation of heterojunction. Figure 4d shows that the lattice fringe of 0.234 and 0.353 nm, corresponding to the (112) and (101) planes of anatase TiO2, is clearly observed in the N-TiO2/g-C3N4 composite and that the interfaces between the N-TiO2 and g-C3N4 are smooth, which further verifies the formation of an N-TiO2/g-C3N4 heterojunction. 3.4. XPS Analysis. The XPS were obtained to analyze the oxidation state and the surface chemical composition of N-TiO2/

Figure 1. TG-DSC thermograms for heating the 40 wt % N-TiO2/gC3N4 composite.

respectively.3 For N-TiO2/g-C3N4, the XRD patterns reveal a coexistence of N-TiO2 and g-C3N4. The peak intensities of gC3N4 increase gradually with the increasing g-C3N4 content in the composites. No other impurity phases are found, indicating these N-TiO2/g-C3N4 samples are two-phase composites. Figure 2b shows the XRD patterns of N-TiO2/g-C3N4 sample with 40 wt % N-TiO2 treated at different temperature. The diffraction peaks at 27.4° are characteristics of (002) planes of gC3N4. An obviously peak at 25.4° for the composite treated at 300 °C indicates the formation of crystal structure of anatase. As calcination temperature increased from 300 to 500 °C, the peak at 25.4° becomes narrower and higher, implying that the TiO2 crystal tends to be perfect and grows into larger particles. On the other hand, no g-C3N4 peak is found in the sample calcinated at 500 °C, suggesting that the demoposition of some g-C3N4 in NTiO2/g-C3N4 occurs at this temperature. However, g-C3N4 is stable at 500 °C according to the TG measurment. The reason is that the TG test was carried out at a high rate of 10 °C/min, and then the decomposition tempture of C3N4 may increase. On the other hand, the calcination process of the N-TiO2/g-C3N4 composite was 2 h. There was enough time for the decomposition of g-C3N4 in this circumstances. So g-C3N4 became unstable at 500 °C. Figure S1 (Supporting Information) exhibits the adsorption− desorption isotherms of N-TiO2, g-C3N4 and 40 wt % N-TiO2/gC3N4 composite. As can be observed, g-C3N4 exhibits a type-II nitrogen isotherm, which is typically a characteristic of

Figure 2. XRD patterns of (a) N-TiO2, g-C3N4, and N-TiO2/g-C3N4 with various TiO2 content calcined at 400 °C and (b) 40 wt % N-TiO2/g-C3N4 composite calcined at different temperatures. 17142

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Table 1. SBET Values and Photocatalytic Activities of N-TiO2/g-C3N4 Heterojunctions sample

SBET (m2/g)

reaction rate constant k (min−1) for Rh B

ln C0/C1

TOC removal at 60 min (%) for RhB

reaction rate constant k (min−1) for MB

ln C0/C1

TOC removal at 60 min (%) for MB

g-C3N4 N-TiO2 15%N-TiO2/g-C3N4 30%N-TiO2/g-C3N4 40%N-TiO2/g-C3N4 50%N-TiO2/g-C3N4 40%N-TiO2+g-C3N4

14.11 87.85 20.13 31.60 40.85 45.36 42.39

0.045 0.034 0.055 0.072 0.091 0.072 0.056

0.0745 0.1257 0.0868 0.1035 0.0817 0.0786 0.1150

41 35 58 63 77 72 49

0.040 0.007 0.053 0.062 0.070 0.065 0.012

0.2514 0.1012 0.3545 0.3344 0.4947 0.4190 0.1947

32 5 55 57 69 66 15

Figure 3. SEM images of (a) g-C3N4, (b) N-TiO2, and (c) the 40 wt % N-TiO2/g-C3N4 composite.

Figure 4. TEM images of (a) g-C3N4, (b) N-TiO2, (c) the 40 wt % N-TiO2/g-C3N4 composite and (d) HRTEM images of the 40 wt %N-TiO2/C3N4 composite.

g-C3N4 heterojunctions and to further study the interaction of NTiO2 with g-C3N4 support. Figure 5 provides the XPS spectra of

Ti 2p, C1s, , and O1s for N-TiO2, g-C3N4, and N-TiO2/g-C3N4 composite. Ti2p3/2 core level appears at 459.0 and 458.7 eV for 17143

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Figure 5. X-ray photoelectron spectra for (a) Ti 2p, (b) C1s, (c) N 1s, and (d) O1s of representative samples.

peak intensity of N-TiO2/g-C3N4 composites at 530.3 eV is weaker than that of N-TiO2 sample, which can be due to the lower concentration of N-TiO 2 in the N-TiO 2 /g-C 3 N 4 composites. 3.5. UV−Vis Diffuse Reflectance Spectroscopy. Figure 6a shows the UV−vis DRS of g-C3N4, N-TiO2, and N-TiO2/gC3N4 composites with different N-TiO2 content. As revealed from Figure 6a, the absorption threshold values of the N-TiO2 are extended up to the visible light region, and the main absorption edge of the pure g-C3N4 occurs at ca. 450 nm. The band gap energies of the direct transition semiconductor can be calculated by a plot of (αhν)2 versus the photon energy (hν). The absorption coefficient α and direct band gap Eg of N-TiO2 and gC3N4 are related through the following equation:46

N-TiO2 and N-TiO2/g-C3N4 composites, respectively, as presented in Figure 5a. The shift of the Ti2p3/2 value of NTiO2/g-C3N4 composites can confirm the interaction between N-TiO2 and g-C3N4 in heterojunctions because the interaction between N-TiO2 and g-C3N4 can change the electronic structure of Ti species in the catalyst, and increase the effective positive charge on the Ti species.42 Figure 5b shows the C1s XPS spectra of the samples. Only one peak in the N-TiO2 sample locates at 284.7 eV, which is derived from carbon contamination. For the gC3N4 sample, in addition to the polluted carbon peak, another peak at 288.2 eV is observed, which can be assigned to the N CN coordination in graphitic carbon nitride. The N-TiO2/gC3N4 composite also displays two C1s peaks at 284.7 and 288.2 eV, indicating the existence of g-C3N4. The N 1s features are observed in Figure 5c. A broad peak extending from 393 to 406 eV is observed for all of the samples. The main N1s peak at a binding energy of 398.7 eV in g-C3N4 can be assigned to sp2hybridized nitrogen (CNC),8 thus confirming the presence of sp2-bonded graphitic carbon nitride. The two peaks at 400.5 and 404.8 eV are attributed to tertiary nitrogen (NC3) groups and the charging effects.8 There are two peaks obtained at 401.1 and 399.6 eV for N-TiO2. The peak approximately 401 eV is attributed to the OTiN sites substitutionally incorporated into the TiO2 lattice.43,44 The peak at 399.5 eV can be attributed to the N atoms located at the interstitial sites of the TiO2 lattice, such as Ti−N−O and Ti−O−N.43,44 However, the binding energy of tertiary nitrogen (NC3) groups at 404.8 eV for NTiO2/g-C3N4 composites disappeared, indicating that the gC3N4 structure has changed feebly after interaction with N-TiO2 to form the heterojunction. The O1s peak (Figure 5d) centered at 530.3 eV is associated with the O2− in the N-TiO2. The other O1s peak at 532.1 is associated with the presence of an −OH group of water molecules on the surface of photocatalysts.45 The

(αhν)2 = A(hν − Eg )

(1)

where α represents the absorption coefficient, h is Planck’s constant, ν is the light frequency, and A is a constant. According to eq 1, the band gap energy (Eg) of the resulting samples can be estimated from a plot of (αhν)2 versus energy (hν). The interception of the tangent to the X axis would give a good approximation of the Eg of the samples (Figure 6b). Thus, the band gaps of as-prepared N-TiO2 and g-C3N4 are estimated to 2.90 and 2.73 eV, respectively. All N-TiO2/g-C3N4 composites exhibited a mixed absorption property of N-TiO2 and g-C3N4. 3.6. FTIR Spectroscopy Analysis. The FT-IR spectra of various samples are shown in Figure S2 (Supporting Information). In the FT-IR spectrum of N-TiO2, the broad band at around 500 cm−1 is due to Ti−O−Ti vibration.47 The FTIR peaks of the as-prepared g-C3N4 are very similar to those of the published result.20 The absorption peaks from 1200 to 1640 cm−1 is attributed to stretching modes of C−N heterocycles. A broad peak near 3150 cm−1 corresponds to the stretching modes 17144

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structure and large surface area because of the release of NH3 gas from peroxotitanate during calcination. The in situ growth strategy can avoid the particle agglomeration of N-TiO2 on the gC3N4 sheet, resulting in a uniform distribution of porous N-TiO2 nanoparticles on the g-C3N4 surface. 3.8. Photocatalytic Activity. The photocatalytic activities of various samples were evaluated by decomposing RhB and MB under visible light, as shown in Figure 8a,b. A mechanically

Figure 6. (a) UV−vis DRS of N-TiO2, g-C3N4, and N-TiO2/g-C3N4 composites with different TiO2 content. (b) Plots of (αhν)2 versus photon energy (hν) for the band gap energies of N-TiO2 and g-C3N4..

of terminal NH2 of NH groups at the defect sites of the aromatic ring. The band near 810 cm−1 is ascribed to the breathing mode of s-triazine.20 The characteristic peaks for g-C3N4 in of N-TiO2/ g-C3N4 composite still remain, indicating the maintenance of gC3N4 structure during the combination between N-TiO2 and gC3N4. 3.7. In Situ Growth Strategy of N-TiO2/g-C3N4 Heterojunctions. Direct growth is widely used to prepare different kinds of composites such as grapheme-based metal compounds48 and bismuth oxyhalide-based heterojunctions.49 In the present work, the formation process of a N-TiO2/g-C3N4 hybrid composite is shown in Figure 7. At the early stage of the reaction

Figure 8. Photocatalytic activities of N-TiO2, g-C3N4, and N-TiO2/gC3N4 composites on the degradation of (a) RhB and (b) MB under visible light irrdiation.

blended N-TiO2 and g-C3N4 sample with 40 wt % N-TiO2 content was used for comparison. It is known that when the initial dye concentrations of the reactants are low, the photocatalytic reactions follow the Langmuir−Hinshelwood pseudo-first-order kinetics model. The kinetics equation can be expressed as follows:51 ln C0/C = kt + ln C0/C1

where k is the pseudo-first-order rate constant, C0 is the original RhB or MB concentration (10 mg/L), C1 is the concentration after adsorption, and C represents the concentration at reaction time t. The reaction rate constants k and adsorption rate ln C0/C1 are calculated and shown in Table 1 to compare the photocatalytic abilities of these samples quantitatively. In the absence of a photocatalyst, the self-degradation of RhB can be negligible. The pristine N-TiO2 and g-C3N4 show certain ability in RhB degradation under visible light irradiation in the present work. It is seen from Figure 8a that the photocatalytic activity is enhanced gradually with the content of N-TiO2

Figure 7. Shematic representation for the in situ deposition of N-TiO2 nanoparticles on g-C3N4 sheets.

process, H2TiO3 reacts with the mixture of H2O2 and NH3·H2O and then is converted to peroxotitanate.50 After peroxotitanat solution was mixed with the g-C3N4 sheet, the peroxotitanate can be adsorbed on the surface of the g-C3N4 and be converted into amorphous TiO2 nanoparticles under the irradiation of microwave. After calcination at 400 °C, N-TiO2 crystals are obtained. In this way, the N-TiO2/g-C3N4 composite catalyst has a porous 17145

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that the 400 °C calcination sample exhibits the best photocatalytic activity. It is known that both the crystal sizes and crystalline have influence on photocatalytic ability. As shown in Figure 2b, the sample treated at 300 °Cexhibits low crystallinity, which is not beneficial to the transfer of electrons and holes. As the calcination temperature increases, the peak at 25.4 became narrower and higher, implying that TiO2 grows up into larger particle. However, when the temperature reaches 500 °C, there is an obvious decrease in the photocatalytic activity. There are two main reasons. First, when the sample is calcinated at 500 °C, no g-C3N4 peak is found (Figure 2b). That shows that the g-C3N4 in N-TiO2/g-C3N4 sublimates. Second, as the temperature is up to 500 °C, the color of N-TiO2/g-C3N4 changes from yellow to white, indicating the decrease of the N elements in the TiO2. The stability of a photocatalyst is also very important for point of view of its practical application. Hence, the stability of the 40 wt % N-TiO2/g-C3N4 composite was further investigated by recycling the photocatalyst for degradation of RhB. As shown in Figure S4 (Supporting Information), after six cycles, there is only a slight loss of activity. Therefore, the N-TiO2/g-C3N4 composite can be used as high-performance and stable visible-light photocatalysts. 3.9. Possible Photocatalytic Mechanism of N-TiO2/gC3N4 Heterojunctions. 3.9.1. Conduction and Valence Band Levels of N-TiO2 and g-C3N4. For constructing heterostructures, considering the suitable conduction band (CB) and valence band (VB) levels of the two individual semiconductors is necessary.23 On the basis of the results of the DRS, the CB and VB edge positions of N-TiO2 can be calculated according to the Mulliken electronegativity theory. Herein, the electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy. The VB edge potential of a semiconductor at the point of zero charge can be calculated by the following empirical equation:53

increasing from 15 to 40 wt %. However, further increasing the content of N-TiO2 in the composites leads to a decrease in the degradation rate. This result may be attributed to the agglomeration of N-TiO2 in the g-C3N4 sheet, which can weaken the heterojunction structure and decrease the catalytic activity.52 Therefore, a suitable ratio and well dispersion of N-TiO2 in the composites are necessary for heterojunctions. On the other hand, the photocatalytic activity of the blend sample is appreciably higher than that of pure N-TiO2 and gC3N4 samples, whereas most of the N-TiO2/g-C3N4 composites exhibited higher photocatalytic activity than the blend sample under visible light irradiation, except for 15 wt % N-TiO2/gC3N4. This implies that there should be some interaction between N-TiO2 and g-C3N4 in the in situ growth process, which plays an important role in improvement the photocatalytic activity. Moreover, MB was also chosen as another model organic pollutant to further evaluate photocatalytic activity. The results are shown in Figure 8b. Only feeble photolysis of MB is observed after 1 h visible light irradiation with the presence of pure NTiO2. The pure g-C3N4 and mixture of N-TiO2 and g-C3N4 have certain effects in MB degradation. Similar to the RhB degradation results, the in situ coupling of N-TiO2 and g-C3N4 results in an increase of MB degradation. The N-TiO2/g-C3N4 composite with 40 wt % N-TiO2 content also shows the best performance, with reaction rate constants k = 0.070 min−1, indicating that the N-TiO2/g-C3N4 composite is an efficient visible-light photocatalyst. Figure S3a and S3b (Supporting Information) reveals a gradual decrease in the RhB and MB aqueous absorption under visible light irradiation over 40 wt % N-TiO2/g-C3N4 composite. There is a blue shift of its maximum absorption wavelength for both RhB and MB, showing there are intermediate products formed during the degradation process of RhB and MB. After illumination for 60 min, these absorption peaks disappear, suggesting that a complete decolorization of RhB and MB solution is achieved. The corresponding TOC removals of RhB and MB after 60 min irradiation are shown in Table 1. The TOC removal is lower than the decoloration yield, revealing that the mineralization is not complete and some organic molecules may be transformed into intermediate products. The influence of the calcination temperature on the degradation yield of RhB is shown in Figure 9. It is evident

E VB = Χ − E c + 0.5Eg

where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ec is the energy of free electrons on the hydrogen scale (approximately 4.5 eV), and Eg is the band gap energy of the semiconductor. The CB edge potential (ECB) can be determined by ECB = EVB − Eg. The value of X for N-TiO2 is 5.61 eV, and the EVB of N-TiO2 was calculated to be 2.54 eV. Thus, the ECB of N-TiO2 was estimated to be −0.34 eV. According to the previous reports,27 the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) potentials of g-C 3 N4 were determined to be −1.2 and 1.5 eV, respectively. Thus, N-TiO2 and g-C3N4 have the suitable conduction and valence band levels for promoting charge separation at the heterojunction interfaces. 3.9.2. PL Analysis. PL emission measurement has been widely used to investigate the fate of electron−hole pairs in semiconductor particles because PL emission is known to result from the recombination of excited electrons and holes for some semiconductors. The higher the PL emission intensity, the higher the recombination efficiency of the photogenerated carriers and the lower the photocatalytic activity.54 Figure 10 shows the PL spectra of the g-C3N4, N-TiO2/g-C3N4 heterojunction and mechanically mixed N-TiO2 and g-C3N4 sample. The main emission peak for pure g-C3N4 is centered at about 460 nm, which is approximately equal to the band gap energy of pure gC3N4. Pure g-C3N4 exhibits the highest intensity among these samples. After the N-TiO2 is introduced, the intensity of the PL

Figure 9. Photocatalytic degradation of RhB under visible irradiation with 40 wt % N-TiO2/g-C3N4 composites calcinated at different temperatures. 17146

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Figure 10. Photoluminescence spectra ofthe g-C3N4, N-TiO2/g-C3N4 heterojunction and mechanically mixed N-TiO2 with g-C3N4 samples.

emission decreases, which can be attributed to the decrease of the concentration of g-C3N4 in the mixture or the heterojunction effect. The emission intensity of the N-TiO2/g-C3N4 heterojunction is lower than that of the mechanically mixed N-TiO2 with g-C3N4, which indicates that the recombination rate of photogenerated charge carriers is lower in the N-TiO2/g-C3N4 heterojunction. The PL results confirm the importance of the heterojunctions in hindering the recombination of electrons and holes. 3.9.3. Detection of Reactive Species. The radicals and holes trapping experiments are designed to elucidate the photocatalytic oxidation process. Figure 11a displays the effects of different scavengers on the reaction rate constant (k) of RhB degradation over the 40 wt % N-TiO2/g-C3N4 composite. The more k is reduced, the more important role the oxidizing species play in the reaction.55 As shown in Figure 11a, there is no conspicuous changes for k values when a •OH scavenger, 2-propanol (IPA), and a h+ scavenger, EDTA-2Na(18), were added. This indicates that •OH and h+ are not the main reactive species involved in the RhB photocatalytic oxidation process. However, the k decreases obviously from 0.091 to 0.063 min−1 in the presence of AgNO3 (e− scavenger),56 which suggests that e− play an important role in the decoloration of RhB. Because the photoinduced electrons are the reactant of generating •O2−, a •O2− scavenger, benzoquinone (BQ)57 was added. As shown in Figure 11a, the k decreases greatly to 0.021 min−1, indicating that •O2− is the main reactive species. Furthermore, because O2 adsorbed on the surface of the catalyst reacts with e− to generate •O2−, the role of O2 must be verified to further clarify whether there are •O2− radicals. The results shown in Figure 11a indicate that the k decreases to 0.036 min−1 under the anoxic suspension (N2-saturated condition), exhibiting that O2 acts as an efficient electron trap and the •O2− is actually the reactive species.58,59 The above results indicate that • 2− O is the most important oxidizing species during the RhB photocatalytic process, and that h+ and •OH in the solution are not the main active species. To further determine the more generation of •O2− in the composite, NBT was used to determine the amount of •O2− generated from N-TiO2, g-C3N4, mixed N-TiO2 with g-C3N4, and the N-TiO2/g-C3N4 photocatalytic system. Figure 11b shows the transformation percentage of NBT with 1 h visible light irradiation during the photocatalytic reaction. For N-TiO2, the transformation percentage of NBT can be neglected even though the CB edge potential of N-TiO2 (−0.34 eV) is more

Figure 11. (a) K values of 40 wt % N-TiO2/g-C3N4 composite with different scavengers and under N2 saturated condition. (b) Transformation percentage of NBT concentration after 1 h visible light irradiation.

negative than the potential of O2/•O2− (−0.28 V vs SHE).29 It indicates the high recombination rate of its photogenerated electron−hole pairs. For g-C3N4, there are obviously transformation percentage of NBT, indicating that part of the photogenerated electrons in the CB of g-C3N4 (−1.2 eV) would react with O2 to produce •O2−. For mixed N-TiO2 with g-C3N4 and the N-TiO2/g-C3N4 composite photocatalytic system, the N-TiO2/g-C3N4 composite shows the maximum transformation percentage of NBT, even higher than that for pure g-C3N4. It indicates that part of the photogenerated electrons in the CB of g-C3N4 may transfer to the CB of N-TiO2. Then, the separation efficiency of photogenerated electrons and holes increases and electrons in the CB of N-TiO2 react with O2 to produce more • O2−. 3.9.4. Photocatalytic Mechanism of N-TiO2/g-C3N4 Heterojunctions. On the basis of the above experimental results, a schematic diagram of the N-TiO2/g-C3N4 heterojunctions is proposed in Scheme 1. g-C3N4 absorbs visible light to induce π−π* transitions, which results in the excitation of the electrons from the HOMO to the LUMO. Because the LUMO level of gC3N4 (−1.2 eV) is more negative than the CB edge of N-TiO2 (−0.34 eV), the excited electrons on the LUMO of g-C3N4 can inject into the CB of N-TiO2. The electrons on the CB edge of NTiO2 and the untransferred electrons on the LUMO of g-C3N4 are good reductants that can capture O2 to generate •O2− because the CB edge potential of N-TiO2 and the LUMO level are more negative than EO2/•O2− (−0.28 V vs SHE).29 17147

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Scheme 1. Schematic Diagram of the Separation and Transfer of Photogenerated Charges in the N-TiO2/g-C3N4 Heterojunctions Combined with the Possible Reaction Mechanism

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21376061, 21076060, 20806021), the Program for New Century Excellent Talents in University (No. NCET-12-0686), and the Doctoral Science Foundation of Hebei University of Science and Technology (No. QD201049).

Meanwhile, N-TiO2 can also be excited by visible light and generate electrons to form •O2−. These •O2− radicals are the most important oxidizing species responsible for the degradation of RhB.

4. CONCLUSIONS A novel method for preparation of N-doped TiO2 nanopowders was devoloped, and the doping function of NH3·H2O in the microwave process for N-doping was found. On the basis of the technique, N-TiO2/g-C3N4 heterostructures have been prepared by a novel in situ microwave-assisted route. The adopted synthesis route ensures not only the successful growth of N-TiO2 nanostructures on g-C3N4 lamellar substrate but also the high dispersion of N-TiO2 nanostructures on g-C3N4 without aggregation. The optimal content of N-TiO2 was found to be 40 wt %, and the optimal calcination temperature is 400 °C. By the release of NH3 from peroxotitanate during calcination, the NTiO2/g-C3N4 composites catalyst had a porous structure and large surface areas. The heterostructures had better performance for RhB and MB photocatalytic degradation compared with the pure N-TiO2 + g-C3N4. The efficient separation of electrons and holes originated from the formation of N-TiO 2 /g-C 3 N 4 heterostructure is beneficial to the photocatalytic activity. In the degradation process of RhB, •O2− played a major role whereas •OH and h+ can be negligible. This work provides a facile and efficient way to design composite heterostructured photocatalysts through the in situ growth strategy.

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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen adsorption−desorption isotherms, FT-IR spectra, UV−vis spectral changes of RhB and MB, and photocatalytic recycling tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*F. Li: tel/fax, +86 311 81668528; e-mail, [email protected]. Notes

The authors declare no competing financial interest. 17148

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