Photodegradation of Rhodamine B and Methyl Orange over Boron

Graphitic carbon nitride (g-C3N4) and boron-doped g-C3N4 were prepared by heating melamine and the mixture of melamine and boron oxide, respectively. ...
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Photodegradation of Rhodamine B and Methyl Orange over Boron-Doped g-C3N4 under Visible Light Irradiation S. C. Yan,†,§ Z. S. Li,*,†,‡,§ and Z. G. Zou†,‡,§ †

Eco-Materials and Renewable Energy Research Center (ERERC), Department of Physics, ‡Department of Materials Science and Engineering, and §National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China Received August 19, 2009. Revised Manuscript Received February 14, 2010

Graphitic carbon nitride (g-C3N4) and boron-doped g-C3N4 were prepared by heating melamine and the mixture of melamine and boron oxide, respectively. X-ray diffraction, X-ray photoelectron spectroscopy, and UV-vis spectra were used to describe the properties of as-prepared samples. The electron paramagnetic resonance was used to detect the active species for the photodegradation reaction over g-C3N4. The photodegradation mechanisms for two typical dyes, rhodamine B (Rh B) and methyl orange (MO), are proposed based on our comparison experiments. In the g-C3N4 photocatalysis system, the photodegradation of Rh B and MO is attributed to the direct hole oxidation and overall reaction, respectively; however, for the MO photodegradation the reduction process initiated by photogenerated electrons is a major photocatalytic process compared with the oxidation process induced by photogenerated holes. Boron doping for g-C3N4 can promote photodegradation of Rh B because the boron doping improves the dye adsorption and light absorption of catalyst.

Introduction In recent years, carbon materials with graphite-like structure, such as graphene,1-3 boron carbonitride (BCN),4,5 and carbon nitride (C3N4),6,7 have received considerable attention due to the wide applications. Recently, photocatalytic performance of graphite phase carbon nitride (g-C3N4) was found by Wang et al.8 The optical band gap of this organic semiconductor was determined to be 2.7 eV. This simple compound possesses the performance of hydrogen or oxygen production from water splitting under visible light irradiation. However, the quantum efficiency for hydrogen production is about 1% at 420 nm. In our previous report,9 the photodegradation performance of g-C3N4 obtained by heating low-cost melamine was investigated. Our results indicated that for photodegrading dye the photocatalytic activities of the g-C3N4 were higher than those of commercial nitrogen-doped TiO2. Interestingly, the photocatalytic activities of g-C3N4 can be significantly improved by loading Ag as a cocatalyst, suggesting that the polymer semiconductor has huge potentials in photocatalysis fields. More recently, the protonation treatment of assynthesized g-C3N4 showed a reversible band gap and significantly improved performance in photocatalytic H2 production *Corresponding author: e-mail [email protected]; Ph 86-25-83686630; Fax 86-25-83686632. (1) Wang, X. R.; Li, X. L.; Zhang, L.; Yoon, Y. K.; Weber, P. K.; Wang, H. L.; Guo, J.; Dai, H. J. Science 2009, 324, 768. (2) Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312. (3) Zhang, Y. B.; Tang, T. T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Shen, Y. R.; Wang, F. Nature 2009, 459, 820. (4) Liu, A. Y.; Wentzovitch, R. M.; Cohen, M. L. Phys. Rev. B 1989, 39, 1760. (5) Azevedo, S.; de Paiva, R. Europhys. Lett. 2006, 75, 126. (6) Liu, A. Y.; Cohen, M. L. Science 1989, 245, 841. (7) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; M€uller, J.; Schl€ogl, R.; Carlsson, J. M. J. Mater. Chem. 2008, 18, 4893. (8) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76. (9) Yan, S. C.; Li, Z. S.; Zou, Z. G. Langmuir 2009, 25, 10397. (10) Zhang, Y. J.; Thomas, A.; Antonietti, M.; Wang, X. C. J. Am. Chem. Soc. 2009, 131, 50.

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from water splitting.10 However, from the practical applications of view, it is expected to further improve the photocatalytic performance of g-C3N4 for obtaining more effective catalyst. Foreign elements such as boron and nitrogen can be introduced into the structural framework of graphite, leading to tunable electric properties from highly conductive (graphene) to semiconducting (BCN and C3N4). Among these carbon-containing semiconductors, the graphite-like BCN has a narrow band gap (0-2.45 eV, depending on the stoichiometry).5 This suggests that introducing boron into structural framework of graphite-like CNx compound (for example, g-C3N4) will narrow its band gap to absorb more visible light. Here, we report the boron doping for improving photocatalytic activity of g-C3N4 and photodegradation mechanisms of rhodamine B (Rh B) and methyl orange (MO) over g-C3N4.

Experiments Synthesis and Characterization of Boron-Doped g-C3N4. The photocatalyst of boron-doped g-C3N4 (denoted as B-doped g-C3N4) was prepared by heating the mixture of melamine and boron oxide in the semiclosed system for preventing sublimation of melamine. In a typical run, boron oxide powders (0.1 g) were dissolved in 20 mL of ethanol solution, and then 4 g of melamine powders was added into the solution. The mixture via drying at 80 °C for 3 h was put into an alumina crucible with a cover and then heated to 500 °C in a muffle furnace for 2 h with a heating rate of 20 °C/min; further heat treatment was performed at 520, 550, 580, and 600 °C for 2 h. The final product was washed by ethanol many times for removing any possible unreacted boron oxide. Using the same heating procedures, the pure g-C3N4 also was prepared by heating the melamine. The samples were characterized by the X-ray diffraction (XRD) for phase identification on the Rigaku Ultima III X-ray diffractometer. Ultravioletvisible (UV-vis) diffuse reflection spectra were measured using a UV-vis spectrophotometer (Shimadzu UV-2550, Japan) and converted from reflection to absorbance by the Kubelka-Munk method. The specific surface area was determined with the Brunauer-Emmett-Teller (BET) equation at 77 K by using an

Published on Web 02/23/2010

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Figure 2. XRD patterns for g-C3N4 (a) and the samples obtained by heating the mixture of melamine and boron oxide at 520 (b), 550 (c), 580 (d), and 600 °C (e).

Figure 1. Chemical structures of the Rh B and MO. adsorption apparatus (Micromeritics TriStar). X-ray photoelectron spectroscopy (XPS) was carried out on Thermo ESCALAB 250 spectrometer using monochromated Al KR (1486.6 eV) source operated at 110 W, and the photoelectrons were detected by a hemispherical analyzer operating at a pass energy of 20 eV. The binding energy was calibrated on the reference C 1s peak at 284.8 eV, and XPS curve fitting was performed with a nonlinear least-squares fitting program using a mixed GaussianLorentzian product function. For understanding the properties of g-C3N4 and B-doped g-C3N4, the photoluminescence (PL) spectroscopy was obtained by using the Cary eclipse fluorescence spectrophotometer. Photocatalytic Activity Test. The Rh B and MO (see Figure 1) dyes were chosen as the degrading pollutions to test the photocatalytic activities of the as-prepared samples. Photocatalytic reaction was preformed in a Pyrex reactor. The catalyst of 0.2 g was dispersed in Rh B or MO aqueous solution (100 mL, 4 mg L-1). The light irradiation system contains a 300 W Xe lamp with cutoff filter L42 for visible light and a water filter to remove the heating effects. The degradation efficiencies of MO and Rh B were evaluated using the UV-vis absorption spectra to measure the peak value of a maximum absorption of MO and Rh B solution. During the irradiation, about 5 mL of suspension was continually taken from the reaction cell at given time intervals for subsequent target dye concentration analysis after centrifuging. The investigated Rh B or MO solution shows a similar pH value of around 6.8, which does not affect the light absorption of dye. The degradation efficiency (%) can be calculated as c0 - c efficiency ð%Þ ¼  100% c0 where c0 is the initial concentration of dye and c is the revised concentration considering dye adsorption on the catalyst after photoirradiation. The concentration of the target dye is calculated by a calibration curve. The maximum absorption of MO and Rh B was at wavelengths of 463 and 552 nm, respectively. Total organic carbon (TOC) concentration during photolysis was carried out on a Shimadzu TOC-5000 analyzer to evaluate the mineralization of target dyes. The active species for photodegrading dyes were trapped by a spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The preparation procedure of DMPO solution was as follows: for removing oxygen dissolved in water, the deionized water was first heated at 100 °C for 30 min. Then the thus-treated deionized water was used to make the 50 mM DMPO aqueous solution in nitrogen atmosphere in a vacuum glovebox. The obtained DMPO solution with an appropriate content of catalyst was loaded into a quartz capillary and irradiated for a given time by using the same light Langmuir 2010, 26(6), 3894–3901

source used in the above photocatalytic reaction. The electron paramagnetic resonance (EPR) spectra were obtained using a Bruker (model EMX-10/12 X-band) electron paramagnetic resonance spectrometer. The settings were center field, 3480.0 G; microwave frequency, 9.2-9.8 GHz; power, 19.97 mW.

Results and Discussion The XRD patterns of the boron-doped samples are shown in Figure 2. The pattern of the undoped g-C3N4 sample that was prepared by heating melamine at 500 °C for 2 h and then at 520 °C for 2 h is also shown here for comparison. We can see that two peaks are found in all the samples. The small-angle peak at 13.08°, which corresponds to a distance d = 0.676 nm and is indexed as (100), is associated with an in-plane structural packing motif. The strongest peak at 27.41° for g-C3N4 is a characteristic interlayer stacking peak of aromatic systems, indexed as the (002) peak for graphitic materials. The calculated interplanar distance of aromatic units is d = 0.325 nm. For the boron-doped samples if the heat treatment temperature is higher than 520 °C, the (002) peak moved to 27.58°, which corresponds with the calculated d = 0.323 nm. The slight change of interplanar distance means that the boron element can be incorporated into the crystal structure of g-C3N4. In order to determine the oxidation state of B dopant in these photocatalysts, we examined the XPS spectra of these compounds. No XPS peak for B species was observed in the B-doped g-C3N4 heated at 520 °C; however, for the samples obtained at heating temperatures above 520 °C, a XPS speak of B 1s was detected. For example, the XPS spectra of B 1s, N 1s, and C 1s for the B-doped g-C3N4 obtained at 580 °C are shown in Figure 3. The B 1s spectrum in Figure 3a shows the peak at 192.4 eV is basically consistent with the reported binding energy of the C-NB group (192.1 eV) for BCN compound,11 which is higher than 190.1 eV for h-BN and lower than 194 eV for B2O3. This indicates that some of the boron atoms are introduced into the g-C3N4 frame. The C 1s binding energy of g-C3N4 shows mainly one carbon species with a binding energy of 288.2 eV, corresponding to a C-N-C coordination. In the N 1s spectrum several binding energies can be separated. The main signal shows occurrence of C-N-C groups (398.7 eV) and tertiary nitrogen N-(C)3 groups (400.7 eV). The peak at 404.6 eV is attributed to the charging effects. As demonstrated in graphite-like BCN materials, the N 1s peak at 399.2 eV is indicative of C-NB2 groups.11,12 Generally, the g-C3N4 product prepared by pyrolysis of melamine (11) Kawaguchi, M.; Kawashima, T.; Nakajima, T. Chem. Mater. 1996, 8, 1197. (12) Komatsu, T.; Goto, A. J. Mater. Chem. 2002, 12, 1288.

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Yan et al. Scheme 1. Boron Doping Route via Melem (2,5,8-Triaminotris-triazine), an Important Intermediate during Condensation of Melamine Rings to g-C3N4

Figure 3. XPS spectra of B 1s (a), C 1s (b), and N 1s (c) for borondoped g-C3N4.

was incomplete condensation, which was demonstrated by many authors by elemental analysis and Fourier transform infrared spectroscopy.13-16 The residual hydrogen atoms bind to the edges of the graphene-like C-N sheet in the form of C-NH2 and 2C-NH bonds. Recently, great efforts have been devoted to the preparation of BCN compound which combines the advantages of the hardness of diamond and the thermal stability of c-BN. Li et al.17 used the (13) Zhao, Y. C.; Yu, D. L.; Zhou, H. W.; Tian, Y. J. J. Mater. Sci. 2005, 40, 2645. (14) Montigaud, H.; Tanguy, B.; Demazeau, G.; Demazeau, G.; Alves, I.; Courjault, S. J. Mater. Sci. 2000, 35, 2547. (15) Terrones, M.; Redlich, P.; Grobert, N.; Trasobares, S.; Hsu, W. K.; Terrones, H.; Zhu, Y. Q.; Hare, J. P.; Reeves, C. L.; Cheetham, A. K.; R€uhle, M.; Kroto, H. W.; Walton, D. R. M. Adv. Mater. 1999, 11, 655. (16) Bojdys, M. J.; M€uller, J. O.; Antonietti, M.; Thomas, A. Chem.;Eur. J. 2008, 14, 8177. (17) Li, X. F.; Zhang, J.; Shen, L. H.; Lei, W. W.; Yang, D. P.; Cui, Q. L.; Zou, G. T. J. Phys.: Condens. Matter 2007, 19, 425235 (6 pp).

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mixture of boron oxide and melamine as precursors; the amorphous BCN was successfully prepared at 727 °C for 2 h under vacuum condition. Their experiments indicated that the chemical reaction of B2O3 þ C3N6H6 f BCN þ H2O can take place during heating. In our case of B-doped g-C3N4, the XPS result indicates that the reaction between B2O3 and C3N6H6 cannot occur at lower temperatures than 520 °C under ambient pressure; however, the g-C3N4 can form at 500 °C from the condensation of melamine.9 In addition, heating the mixture of B2O3 and the g-C3N4 obtained by heating the melamine at 500 °C for 2 h to above 520 °C will produce a same product with the heat treatment of melamine and B2O3 in the corresponding temperatures, as demonstrated by thermal analysis (see Figure S1a in the Supporting Information). Therefore, a two-step reaction was proposed for understanding the preparation of B-doped g-C3N4. The mixture of boron oxide and melamine were first treated at 500 °C for 2 h. In this stage the pyrolysis of melamine takes place to form g-C3N4 with residual C-NH2 and 2C-NH. As shown in Scheme 1, further increasing the heat treatment temperature above 520 °C, the C-NH2 and 2C-NH react with B2O3, and an expected product was obtained. The optical properties of the undoped and boron-doped g-C3N4 samples were investigated by UV-vis diffuse reflectance spectroscopy, and the results are shown in Figure 4. From Figure 4a, the main absorption of the undoped g-C3N4 heated at 520 °C, which exhibits a band gap of ca. 2.75 eV, shifts blue compared with that of the samples heated above 520 °C with a band gap of 2.7 eV. Obviously, the absorption tail gradually rises with increasing heating temperature from 520 to 600 °C. This probably is attributed to the structure defects formed in the samples treated at high temperatures, which improve the optical absorption of materials. In our previous report,9 the thermal analysis indicated that the decomposition of g-C3N4 occurred when heat treatment temperature is above 570 °C. In order to further demonstrate the origin of this absorption tail, the g-C3N4 was obtained by heating melamine at 680 °C for 2 h. At this temperature, the absorption tail was increased significantly, and the BET specific surface area of the sample increases largely from 10 m2/g of g-C3N4 obtained at 600 °C to 30 m2/g, indicating that the sample underwent a heavily decomposition process. The XRD presents the characteristic peaks of g-C3N4 phase (not shown here), but the peak intensity decreased significantly, suggesting that the structural integrality of g-C3N4 was broke partly at such high temperature. These evidence may support our argument that the large absorption tail originated from the structure defects. From Figure 4b, absorption edges of the borondoped samples shifted remarkably to longer wavelengths when the heat temperature was above 520 °C. The optical band gap of boron-doped sample prepared at 520 °C was the same as 2.75 eV of undoped g-C3N4. The boron-doped samples fabricated at above 520 °C possess the same band gap which was estimated to be 2.66 eV. Compared with the undoped g-C3N4, a slight decrease in band gap means that the B-doped g-C3N4 can absorb more visible light which is in favor of the improvement Langmuir 2010, 26(6), 3894–3901

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g-C3N4 (b) samples obtained at various temperatures.

Figure 5. Comparison of photocatalytic activities for Rh B (a) and MO (b) photodegradation over g-C3N4 and B-doped g-C3N4 samples prepared at various temperatures.

of photocatalytic activity. On the other hand, the B-doped g-C3N4 presents a weak absorption tail, indicating that some boron atoms in the g-C3N4 network increase thermal stability of g-C3N4. Indeed, the thermal analysis indicates that the decomposition temperature of B-doped g-C3N4 is about 30 °C higher than that of g-C3N4 (see Figure S1b in the Supporting Information). The photocatalytic activities for Rh B degradation over the undoped and B-doped g-C3N4 samples prepared at various temperatures are shown in Figure 5. The UV-vis spectroscopic changes of Rh B or MO over catalysts are shown in Figures S2 and S3 in the Supporting Information. The rate of color fading of Rh B over g-C3N4 gradually increased with increasing the heating temperature from 520 to 600 °C. The BET results indicate that these samples have a similar specific surface area of ca. 8-10 m2/g. However, 5, 7, 8, and 12.5% Rh B before the photoreaction was adsorbed on the surface of g-C3N4 obtained at 520, 550, 580, and 600 °C, respectively. This indicates that increasing the heating temperature can improve the adsorption of Rh B on g-C3N4. Indeed, as observed by UV-vis spectra and thermal analysis,9 increasing the heating temperature will induce the structure defects due to the decomposition of g-C3N4 at high temperature. The structure defects may cause a strong adsorption of Rh B on catalyst and therefore accelerate the photodegrading process. At 520 °C, the B doped and undoped g-C3N4 showed a similar behavior for photodegrading Rh B due to the noneffective boron doping at as low temperature as 520 °C, as demonstrated by XPS. Increase of temperature to 600 °C leads to a big drop in the photocatalytic activity of the B-doped g-C3N4. However, the B-doped

g-C3N4 samples prepared at 550 and 580 °C exhibited the better photodegradation activity for photodegrading Rh B than the undoped g-C3N4. Interestingly, the boron-doped sample obtained at 580 °C possesses the highest degrading rate of Rh B which is 1.5 times faster than Rh B photodegrading over the pristine g-C3N4 prepared at the same temperature. This probably implies that, in our case of B-doped g-C3N4 preparation, 580 °C is an appropriate heat treatment temperature. Checking the dye adsorption over the two kinds of samples, it is found that (see Figure S2 in the Supporting Information), after reaching the adsorptiondesorption equilibrium of Rh B on catalysts, all the B-doped g-C3N4 samples exhibit a similar adsorption capacity of 13% Rh B before the photoreaction. This indicates that the boron doping also can induce a strong adsorption for Rh B on g-C3N4. The photocatalytic activity of B-doped g-C3N4 obtained at 580 °C was much higher than the best activity of the g-C3N4 samples. Obviously, it indicated that something apart from the dye adsorption also accelerated the degradation of Rh B in the B-doped g-C3N4 system. The PL spectra for catalyst suspension in water and Rh B solution (4 mg L-1) were obtained, and the results are shown in Figure 6. For reaching the adsorption-desorption equilibrium, the Rh B solution containing catalyst was put in dark for 40 min before photoexcitation. Upon photoexcitation at 380 nm, the g-C3N4 exhibits the PL band at 470 nm. The energy of the PL peak (2.64 eV) basically agrees with the optical band gap energy (2.7 eV); therefore, the luminescence could be associated with the band-edge emission. As a further demonstration, the PL

Figure 4. UV-vis absorption spectra for g-C3N4 (a) and B-doped

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Figure 6. Photoluminescence spectra for g-C3N4 and B-doped g-C3N4 before and after dye adsorption. Excitation wavelength: 380 nm.

intensity for g-C3N4 suspension in Rh B solution was lower than that for the g-C3N4 suspension in water. This evidence probably means that the recombination process of photogenerated electronhole pairs was partially prevented at the presence of Rh B. The valence band XPS of g-C3N4 indicates that oxidation potential of g-C3N4 is 1.53 V (see Figure S4 in the Supporting Information), which is more positive than the redox potential of Rh B (1.43 V).18 This ascertains that for photodegrading Rh B in the g-C3N4 system direct photogenerated holes oxidation is energetically possible. The photogenerated holes react with Rh B rapidly that prevents the recombination for photogenerated electrons-holes, resulting in the lower PL intensity. Clearly, compared with the g-C3N4 the PL band of B-doped g-C3N4 shifts toward longer wavelengths by about 10 nm, and this shift is associated with the 0.04 eV decrease in band gap for g-C3N4 by boron doping. Interestingly, the PL intensity of g-C3N4 was significantly stronger than that of B-doped g-C3N4. This probably suggests that the boron doping can improve the surface state of g-C3N4, which is useful for improving the catalyst’s photocatalytic activity. However, for the degrading MO (see Figure 5b), the photodegrading rate decreases with increasing the heating temperature. The g-C3N4 obtained at 520 °C shows the highest photocatalytic activity. As mentioned above, the high temperature may induce the structure defects from the g-C3N4 decomposition. Therefore, the decreased activity indicates that the structure defects in g-C3N4 are not beneficial for the MO photodegradation. A similar photodegrading behavior was found in the MO degrading over the g-C3N4 and B-doped g-C3N4. MO adsorption behavior on the g-C3N4 is the same as that on B-doped g-C3N4 (see Figure S3 in the Supporting Information). This suggests that for degrading MO the effect of dye adsorption on the activity differences between two catalysts could be neglected. The kinetics of MO or Rh B photodecomposition on the catalyst surface can be described by the first-order equation   c0 ¼ kt ln c where k is the rate constant (min-1), c0 is the initial concentration of target dye, and c is the actual concentration of target dye at light irradiation time t. Parts a and b of Figure 7 present the linear relationship between ln (c0/c) and the irradiation time for Rh B and MO, respectively. Rate constant k is determined from the (18) Shen, T.; Zhao, Z. G.; Yu, Q.; Xu, H. J. J. Photochem. Photobiol. A: Chem. 1989, 47, 203.

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Figure 7. Relationship between the dye (Rh B (a) and MO (b)) degradation efficiency and the light irradiation time for the g-C3N4 and B-doped g-C3N4 photocatalysis system.

slope of the linear relationship of the natural logarithm of the ratio between the initial concentration of dye and the concentration after photocatalytic degradation versus the corresponding irradiation time. The value of k gives an indication of the activity of the photocatalyst. Apparently, for the Rh B photodegradation the rate constants for B-doped g-C3N4 sample heat-treated at 580 °C exhibits the highest rate constant of about 0.199 min-1, which is about twice faster than the 0.065 min-1 for the highest efficiency of g-C3N4 that was obtained at 600 °C. However, for photodegrading MO, the rate constant is basically similar for the g-C3N4 and B-doped g-C3N4. For example, the two kinds of samples obtained at 580 °C have the same rate constant that was determined to be 0.004 min-1. Generally, decolorization does not mean that the dyes have been completely oxidized into harmless final products such as H2O and CO2. Therefore, it is necessary to study TOC removal of dyes. From the experimental results (Figure 8), it was found that decolorization process is more beneficial than TOC removal. For example, after 40 min of irradiation over 98% color removal was achieved for photodegrading Rh B over g-C3N4 while TOC removal was just 46.5%. Similarly, TOC removal for discolored MO is about 80% after 5 h light irradiation. It can be seen that for photodegrading Rh B the B-doped g-C3N4 presents an increasing efficiency compared with the g-C3N4; the TOC removal reaches to 60% after 40 min light irradiation. This further indicates that the boron doping can improve the photodegrading activity of Rh B over g-C3N4. In order to understand the differences in activity for degrading Rh B and MO over B-doped g-C3N4, we should first explore the Langmuir 2010, 26(6), 3894–3901

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Figure 8. TOC removals as a function of light irradiation time for the solution of Rh B and MO in the g-C3N4 and B-doped g-C3N4 photocatalysis system.

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Figure 9. EPR spectra for the DMPO-containing aqueous suspensions of undoped and boron-doped g-C3N4, recorded after 15 min visible light irradiation were initiated.

mechanism of Rh B and MO photodegradation over the undoped g-C3N4. The active species, which formed in the undoped g-C3N4 and B-doped g-C3N4 photocatalysis system during light irradiation, were detected by EPR, and the spectroscopies are shown in the Figure 8. In general, the two reactive oxidation species, hydroxyl radicals (•OH) and superoxide (O2• or HOO•), are formed during the photocatalytic reaction induced by the light irradiation. In our cases of the two photocatalysis systems, visible light irradiation of DMPO-containing aqueous suspensions of undoped and B-doped g-C3N4 gives rise to exclusively a 1:2:2:1 quartet signal in the EPR spectra. The EPR parameters (g = 2.0056, aN = 15.0 G, and aH = 15.0 G) are characteristic of DMPO-OH• adducts formed upon trapping of OH• radicals by the DMPO molecules,19 indicating that hydroxyl radical is the active species for undoped and B-doped g-C3N4 system. The •OH is generated via the direct hole oxidation20 or photogenerated electron-induced multistep reduction of O2 (O2 þ e f O2•, O2• þ e þ 2Hþ f H2O2, H2O2 þ e f •OH þ OH-).21 Generally, the photodegradating activity of semiconductor photocatalyst is associated with oxidation ability of photogenerated holes in valence band and reduction ability of photogenerated electrons in conduction band. As demonstrated by VB XPS, the oxidation potential of g-C3N4 is 1.53 V, which means that the photogenerated holes are incapable of directly oxidizing adsorbed hydroxyl groups generating hydroxyl radicals (2.7 V vs NHE). For checking the formation of •OH, the hydrogen peroxide (H2O2) test strip was used to examine the existence of H2O2 during the photocatalytic reaction. However, H2O2 cannot be detected in the g-C3N4 photocatalysis system. This probably is because the H2O2 concentration is lower than 1 mg L-1, which is the minimum detectability of the hydrogen peroxide test strip. An interesting fact (see Figure 10) is that the photodegrading rates of MO and Rh B increase largely when the 1 mg L-1 H2O2 was added into the g-C3N4 photocatalysis system. No photodegradation of MO or Rh B was observed during light irradiating the aquatic solutions of H2O2 and dye without g-C3N4 photocatalyst. Clearly, the improved reaction activity is due to the introduction of H2O2, which accelerates the reaction of H2O2 þ e f •OH þ OH-. It is wellknown that hydroxyl radical reactions are nonselective and will virtually react with almost all the organic compounds by either H atom

abstraction, direct electron transfer, or insertion22 due to its high redox potential of 2.7 V vs NHE. Therefore, it seems a reliable speculation that the •OH formed in the g-C3N4 photocatalysis system under light irradiation is from the multistep reduction of O2. Usually, the degrading Rh B over TiO2 photocatalyst would occur via photosensitization pathway.23 The MO photodegration originated from the photocatalytic reaction.24 In our case of

(19) Wu, T.; Lin, T.; Zhao, J.; Hidaka, H.; Serpone, N. Environ. Sci. Technol. 1999, 33, 1379. (20) Yoon, S. H.; Lee, J. Environ. Sci. Technol. 2005, 39, 9695. (21) Liu, G. G.; Li, X. Z.; Zhao, J. C.; Horikoshi, S.; Hidaka, H. J. Mol. Catal. A: Chem. 2000, 153, 221.

(22) Zhou, H.; Smith, D. W. J. Environ. Eng. Sci. 2002, 1, 247. (23) Wu, T. X.; Liu, G. M.; Zhao, J. C.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845. (24) Chen, F.; Deng, Z. G.; Li, X. P.; Zhang, J. L.; Zhao, J. C. Chem. Phys. Lett. 2005, 415, 85.

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Figure 10. Comparison of photocatalytic activities for Rh B (a) and MO (b) degradation in different photocatalysis systems under visible light irradiation.

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g-C3N4, a single-wavelength (at 420 nm) irradiation was performed in order to avoid the strong light absorption by Rh B (450 nm < λ < 600 nm). By this illumination the Rh B photodegradation took place. This indicates that the degrading Rh B over g-C3N4 should be attributed to the photocatalysis, not the photosensitization process induced by dye self-sensitized photocatalytic oxidation. Several experiments were carried out to explore the photodegradation of Rh B and MO, and the results are shown in Figure 10a for Rh B and in Figure 10b for MO. The triethanolamine (TEOA) is an effective hole scavenger for the g-C3N4 photocatalysis system.8 In our cases of photodegrading MO and Rh B, the degrading rate of Rh B decreased significantly when the TEOA (10 vol %) was added into the reaction solution; just 17% Rh B was degraded after 80 min light irradiation. The photodegradation of MO was slightly depressed with the addition of TEOA. In order to further check the key species involved in the Rh B degradation, we examined the effects of the addition of a hydroxyl radical scavenger on the rate of Rh B photocatalytic degradation over g-C3N4. tert-Butyl alcohol (TBA) was chosen as • OH scavenger because it reacts with •OH radicals with a high rate constant (k = 6  108).25 As shown in Figure 8b, no rate change for Rh B photodegradation was observed in the presence of TBA. These results might support the argument that the photogenerated hole is main oxidation species for the degrading Rh B, but a minor factor for the degrading MO. The redox potential of Rh B and MO was determined by cyclic voltammetry to be 1.43 V18 and 1.48 V24 vs NHE, respectively, both of which are lower than oxidation potential of g-C3N4 (1.53 V). This indicates that the oxidation process is energetically possible. A further investigation is to discover the effect of reduction process induced by photogenerated electrons on photodegradation of Rh B and MO. The O2 is a key factor in photoreduction process that producing the superoxide and hydroxyl radicals. The additional experiment, the effect of O2 on photodegradation of target dyes, was preformed in argon atmosphere. A closed reaction setup was vacuum-treated several times in order to eliminate O2, and then, high-purity argon was followed into the reaction setup for obtaining ambient pressure. As a result, the degrading rate of MO depressed clearly; after 5 h visible light irradiation just 43% MO was degraded in the absence of O2 (see Figure S5 in the Supporting Information). It means that O2 is a main factor for MO photodegradation in our case of g-C3N4 photocatalyst, which affects the formations of the superoxide via direct reduction of O2 and the hydroxyl radicals via multistep reduction of O2. As demonstrated by Zhao’s group that the surperoxide radical is one of the main active species for MO photodegradation, which was also confirmed by the seriously depressed photodegradation of MO in the absence of oxygen.24 The O2 almost did not affect the degrading rate of Rh B (see Figure S6 in the Supporting Information); the main absorption peak at 552 nm disappeared completely after 80 min visible light irradiation. A weak residual absorption peak is found at 498 nm, which was presumed to be the rhodamine.26 These results indicate that reduction process initialed by photogenerated electron is a main reaction process for the degrading MO, which has a weak influence for the degrading Rh B. In fact, in our g-C3N4 system, even in the absence of oxygen, the photodegradation of MO was still carried out to some extent under the visible irradiation. It indicated that something apart from the active oxygen species also induced the degradation of MO in this scene. Taking account of the addition of TEOA as hole (25) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (26) Watanabe, T.; Takirawa, T.; Honda, K. J. Phys. Chem. 1977, 81, 1845.

3900 DOI: 10.1021/la904023j

Yan et al. Scheme 2. Photodegradation Pathways of MO and Rh B over g-C3N4a

a The Rh B degradation mainly originated from hole oxidation, and the MO degradation can be attributed to an overall reaction, but the photoreduction process is more advantaged than the photooxidation.

scavenger which leads to the slight decrease of MO photodegradation, we could attribute the MO photodegradation to the overall reaction process. In addition, the MO photodegradation in the presence of TEOA is twice as fast as that in the absence of O2. This observation strengthened our conclusion that the superoxide is an effective active species for degrading MO, but not for Rh B. It also means that the degrading MO over g-C3N4 is dominated by the photoreduction process compared with the photooxidation process. Aforementioned results can help us to discuss the activity differences between degrading Rh B and MO over B-doped g-C3N4. As shown in Scheme 2, our photodegradation experiments clearly indicate the photodegradation of Rh B and MO is mainly attributed to the photogenerated hole oxidation and photoreduction process, respectively. Compared with the undoped g-C3N4, however, the B-doped g-C3N4 exhibited the increased activity for degrading Rh B and the negligible effect for degrading MO. In the B-doped g-C3N4, as observed by UV-vis spectra, the boron doping gives rise to a small decrease in band gap from 2.7 eV of g-C3N4 to 2.66 eV of B-doped g-C3N4. Density functional theory calculations suggest that the visible light response of g-C3N4 originates from the electron transfer from the valence band populated by N 2p orbitals to the conduction band formed by C 2p orbitals. The XPS results indicate the boron is incorporated into the N sites by the form of 2C-NB or C-NB2 in g-C3N4. This suggests that the small decrease in band gap by boron doping originates from an increase in the top of valence band of g-C3N4. Indeed, as reported, the graphite-like BCN semiconductor has a narrow band gap of 1.6 eV.27 As a result, the EPR spectral intensity of B-doped g-C3N4 was clearly lower than that of g-C3N4 (see Figure 9). This means that the oxidation ability of photogenerated hole was slightly decreased in the B-doped g-C3N4. Indeed, during the photodegradation of MO over B-doped g-C3N4, the peak shift in the UV-vis absorption spectra was observed with increasing the light irradiation time (see Figure S3 in the Supporting Information), indicating that the intermediate products formed during the photodegradation reaction process due to the slightly decreasing photooxidation ability. However, the photodegrading rates for MO over g-C3N4 and B-doped g-C3N4 are basically similar. This is because the MO photodegradation over g-C3N4 is mainly from the photoreduction process. The PL results have indicated that the boron doping improves the adsorption of Rh B on g-C3N4, which is useful for accelerating the photodegradation rate of Rh B. In addition, the small decrease of 0.04 eV in band gap for B-doped g-C3N4 means an increased light absorption which also is beneficial for the photodegradation reaction. This probably suggests that the two (27) Liu, A. Y.; Wenrcovitch, R. M.; Cohen, M. L. Phys. Rev. B 1989, 39, 1760.

Langmuir 2010, 26(6), 3894–3901

Yan et al.

Article

advantaged factors from boron doping overcome the slight decrease in photooxidation ability, and therefore the B-doped g-C3N4 presents the enhanced activity for photodegrading Rh B. The stability of catalyst is a crucial factor for the practical applications. The stability tests for g-C3N4 and B-doped g-C3N4 obtained at 580 °C were measured by Rh B photodegradation; the results are shown in Figure S7 in the Supporting Information. No activity decrease was observed in the three reaction cycles, indicating that the two catalysts have good stability in the photoreaction. This is also demonstrated by XRD results for the undoped and B-doped g-C3N4 (see Figure S8 in the Supporting Information).

originated from the photogenerated hole oxidation. The degrading of MO can be attributed to the overall reaction, but the photoreduction process is more advantaged than the photooxidation. Introducing the boron element into the g-C3N4 would improve the photocatalytic activity for photodegrading Rh B due to the improvement of dye adsorption and light absorption of catalyst.

Summary In summary, we have successfully fabricated the B-doped g-C3N4 photocatalyst by heating the mixture of melamine and boron oxide. The EPR analysis and various comparison experiments indicated that the degrading of Rh B over g-C3N4 mainly

Supporting Information Available: UV-vis spectroscopic changes for MO and Rh B degradation, VB XPS of undoped and boron-doped g-C3N4, and the stability test for g-C3N4 and boron-doped g-C3N4. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(6), 3894–3901

Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20528302) and the National Basic Research Program of China (973 program, 2007CB613301 and 2007CB613305).

DOI: 10.1021/la904023j

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