ARTICLE pubs.acs.org/JPCC
Formation of New Structures and Their Synergistic Effects in Boron and Nitrogen Codoped TiO2 for Enhancement of Photocatalytic Performance Ming-Yang Xing,† Wei-Kun Li,‡ Yong-Mei Wu,† Jin-Long Zhang,*,†,§ and Xue-Qing Gong*,‡ †
Key Lab for Advanced Materials and Institute of Fine Chemicals, and ‡Laboratories for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China § School of Chemistry and Materials Science, Guizhou Normal University, Guiyang 550001, People’s Republic of China ABSTRACT: A novel double hydrothermal method to prepare the boron and nitrogen codoped TiO2 is developed. Two different ways have been used for the synthesis of the catalysts, one through the addition of boron followed by nitrogen, and the other through the addition of nitrogen first and then by boron. The X-ray photoelectron spectroscopy analysis indicates the synergistic effect of boron and nitrogen with the formation of TiBNTi and TiNBO compounds on the surface of catalysts when nitrogen is introduced to the materials first. When the boron is added first, only TiNBO species occurs on the surface of catalysts. The above two compounds are all thought to enhance the photocatalytic activities of codoped TiO2. Density functional theory simulations are also performed to investigate the BN synergistic effect. For the (101) surface, the formation of TiBNTi structures gives rise to the localized states within the TiO2 band gap.
1. INTRODUCTION Nanosized TiO2 is used as the prototype photocatalyst and functional nanomaterial in a wide range of areas such as photo decomposition of water,1,2 dye-sensitized solar cells,3,4 antibecteria and antivirus materials,5,6 and photo degradation of pollutants in water and air.711 However, the large band gap of titania restricts its photocatalytic applications within the UV range. Many efforts have been devoted to extending its activity to the visible range.12 Nonmetal elements, such as nitrogen, carbon, boron, and sulfur, are usually used as dopant, and by replacing the oxygen in TiO2, they can bring the photocatalytic activity to the visible range.1317 Among them, nitrogen has attracted much attention and been widely studied. In some of our recent work on nitrogen-doped TiO2,13,14,18 we developed a new approach to synthesize the nitrogen-doped TiO2 nanocatalysts.14 We found the photocatalytic performance of nitrogen-doped TiO2 could be improved by the nitrogen doped into TiO2 lattice, but reduced by the nitrogen species only chemically adsorbed on the TiO2 surface. Moreover, by combining nitrogen doping and adsorption of ferric ions, we were also able to obtain TiO2 catalysts with rather high photocatalytic performance and other unique characteristics.18 However, as the dopant, boron has been less studied, and it is still controversial as to how it can affect the photoactivity of TiO2. In some studies, it was suggested that boron doping into TiO2 leads to a red shift of the absorption band to the visible region due to the overlapping of the impurity states of boron with the 2p r 2011 American Chemical Society
electronic states of oxygen.19,20 By contrast, it was reported in some other studies that the boron incorporation into TiO2 can induce a blue shift due to the decrease of crystal size (quantization effect).21,22 Meanwhile, boron has also been used as an important codopant together with nitrogen for modification of TiO2.2327 In et al.23 and Liu et al.24 proposed that TiO2 codoped with boron and nitrogen shows high photocatalytic activity under UV and visible lights, probably due to the existence of a synergistic effect between boron and nitrogen by forming TiBN structures at the surface. However, there is still no detailed illustration of the BN synergistic effect and its effect on the photocatalytic activity of TiO2. In this work, aiming at illustrating the exact role of synergistic effect in optical absorbance and photocatalytic activity of boron and nitrogen codoped TiO2, we systematically prepared various codoped TiO2 by using the double hydrothermal method. Different new bonds were determined to form on the surface of TiO2 when we changed the order of boron and nitrogen addition, and they can significantly affect the photoactivities of the materials. To provide deep insight into the BN synergistic effect and explain its influence on the photoactivity of catalysts, we also performed systematic density functional theory (DFT) calculations on the structures of various codoped TiO2 surfaces Received: November 21, 2010 Revised: February 18, 2011 Published: April 06, 2011 7858
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The Journal of Physical Chemistry C and their corresponding electronic configurations. Combining the calculated band structures and experimental measurements, we were able to obtain a clear relationship between the codoping structures and the photoactivity.
2. EXPERIMENTAL SECTION 2.1. Preparation of Different Dopant Orders of Boron and Nitrogen Codoped TiO2. Tetrabutyl titanate (23.44 mmol of
TBOT) was added to the ethanol (32 mL) with continuous stirring for 0.5 h to obtain solution A. H3BO3 (2.33 mmol) was dissolved in the ethanol (36 mL), and then deionized water (2.2 mL) and HNO3 (0.56 mL) were successively added to the solution under vigorous stirring to obtain solution B. Urea (5.86, 11.92, 23.44, 35.16 mmol corresponding to 0.5, 1.0, 2.0, 3.0 of N/ Ti molar ratios) was dissolved in the ethanol (8 mL) to obtain solution C. Solution A was added dropwise to solution B, while the mixture was stirred for 4 h. The resultant mixture was then transferred into a 100 mL Teflon-inner-linear stainless steel autoclave, which was kept under 393 K for 12 h. After this hydrothermal treatment, solution C was added to the above mixture, which was kept under 453 K for 12 h for the secondary hydrothermal treatment. After double hydrothermal treatment, the precipitate was washed, dried, and ground to obtain the nanoparticles. The samples doped first with boron and then nitrogen were denoted as (B, mN)TiO2, where m describes the molar ratio of nitrogen to titanium (m = 0.5, 1.0, 2.0, 3.0). BTiO2 was prepared by a similar procedure without the addition of urea. In the preparation process of BTiO2, nitric acid is only used to catalyze the hydrolysis of TBOT without introducing N into the TiO2 structure, and there is much previous literature reporting on using a nitric acid and boric acid mixture to achieve boron single doping TiO2.27,28 Urea instead of boric acid was added in solution B and boric acid instead of urea was added in solution C to prepare samples doped first with nitrogen, and then boron, which were denoted as (mN, B)TiO2, where m describes the N/Ti molar ratio. The amount of urea was 5.86, 11.92, 23.44, and 35.16 mmol corresponding to 0.5, 1.0, 2.0, and 3.0 of N/Ti molar ratios, respectively. NTiO2 (N/Ti molar ratio is 1.0) was prepared by a similar method without the addition of boric acid. The nature of the nitrogen species in the resulting nitrogen-doped TiO2 materials, however, is a matter of controversial discussions.29 Depending on the preparation methods, the resulting nitrogendoped samples most likely contain diverse nitrogen species and therefore may have different photocatalytic activities. A significant example is the unique difference between NTiO2 obtained from ammonia or urea as nitrogen source.30,31 The latter material photocatalyzes the visible-light mineralization of formic acid to carbon dioxide and water, whereas the former is inactive. Therefore, some literature pertaining to the use of urea in nitrogen doping of TiO2 has been published in recent years,29,31,32 and we also choose urea as the nitrogen source in this literature. Pure TiO2 was also prepared by a similar procedure without the addition of urea and boric acid. 2.2. Characterization. X-ray diffraction (XRD) patterns of all samples were collected in the range 2080° (2θ) using a Rigaku D/MAX 2550 diffractometer (Cu K radiation, λ = 1.5406 Å), operated at 40 kV and 100 mA. The crystallite size was estimated by applying the Scherrer equation to the full width at halfmaximum (fwhm) of the (101) peak of anatase and the (110) peak of rutile, with R-silicon (99.9999%) as a standard for the
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instrumental line broadening. The instrument employed for XPS studies was a Perkin-Elmer PHI 5000C ESCA system with Al KR radiation operated at 250 W. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 284.6 eV as an internal standard. The UVvis absorbance spectra were obtained for the dry-pressed disk samples using a Scan UVvis spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly, using BaSO4 as the reflectance sample. The spectra were recorded at room temperature in air within the range 200800 nm. 2.3. Measurements of Photocatalytic Activities. The photocatalytic activity of each sample was evaluated in terms of the degradation of methyl-orange (MO, 20 mg/L). The photocatalyst (0.07 g) was added into a 100 mL quartz photoreactor containing 70 mL of a 20 mg/L MO solution. The mixture was stirred for 30 min in the dark to reach the adsorptiondesorption equilibrium. A 1000 W tungsten halogen lamp equipped with a UV cutoff filters (λ > 420 nm) was used as a visible light source (the average light intensity was 60 mW cm2), and a 300 W high-pressure Hg lamp for which the strongest emission wavelength is 365 nm was used as a UV light source (the average light intensity was about 1230 μW cm2). The lamp was cooled with flowing water in a quartz cylindrical jacket around the lamp, and ambient temperature was maintained during the photocatalytic reaction. At the given time intervals, the analytical samples were taken from the mixture and immediately centrifuged, and then filtered through a 0.22 μm Millipore filter to remove the photocatalysts. The filtrates were analyzed by recording variations in the absorption in UVvis spectra of MO using a Cary 100 ultraviolet visible spectrometer. 2.4. Computational Details. The total energy spin-polarized DFT calculations have been carried out within the generalized gradient approximation (GGA) using the PWScf code included in the Quantum-Espresso package.33 Electronion interactions were described by ultrasoft pseudopotentials, with electrons from B, C, N, O 2s, 2p and Ti 3s, 3p, 3d, 4s shells explicitly included in the calculations.34 Plane-wave basis set cut-offs for the smooth part of the wave functions and the augmented density were 25 and 200 Ry, respectively. We used surface cells of 1 2 for anatase TiO2 (101) and 2 2 for anatase TiO2 (001), respectively. The anatase TiO2 (101) and (001) surfaces were modeled as periodic slabs with 3 and 6 trilayers of TiO2, respectively, and the vacuum between slabs was more than 10 Å. Only 1 1 1 k-point mesh was used in the calculations. The B and N atoms were doped on one side of the slab only, and during structural optimizations, all of the atoms, except those in the bottom OTiO layer of the slab, were allowed to move (force threshold was 0.05 eV/Å).
3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. The XRD patterns of the various samples are shown in Figure 1. The diffraction peaks of all samples are ascribed to the TiO2 anatase phase. These observations indicate that there has been virtually no phase change in TiO2 in the process of doping or codoping, regardless of the amounts of dopants. Furthermore, due to the small amount of boron used for doping, no significant characteristic peak of boron oxide was found in B-doped TiO2. The crystal sizes of all of the samples were estimated using the Scherrer 7859
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Figure 1. XRD patterns of BTiO2, (B, 0.5N)TiO2, (B, 1.0N)TiO2, and (B, 2.0N)TiO2 (a), and NTiO2, (0.5N, B)TiO2, (1.0N, B)TiO2, and (2.0N, B)TiO2 (b).
Table 1. Measured Structural Characteristics of Different Samples crystallite size (nm)
d-spacing (Å)
BTiO2
11.2
3.5
(B, 0.5N)TiO2
10.6
3.5
(B, 1.0N)TiO2 (B, 2.0N)TiO2
11.0 10.8
3.5 3.5
NTiO2
10.0
3.5
(0.5N, B)TiO2
9.8
3.5
(1.0N, B)TiO2
9.5
3.5
(2.0N, B)TiO2
9.9
3.5
equation:35 D¼
Kλ β cos θ
ð1Þ
where β is the half-height width of the diffraction peak of anatase, K = 0.89 is a coefficient, θ is the diffraction angle, and λ is the X-ray wavelength corresponding to the Cu KR irradiation. The estimated data are listed in Table 1. The crystal size reduces with both doping and codoping as indicated by the peak broadening in the XRD patterns of BTiO2, NTiO2, (B, mN)TiO2, and (mN, B)TiO2 as a function of N contents (Table 1). The size decrease could be caused by the lattice deformation and associated with oxygen vacancies in the anatase crystallites, which may hinder crystallite growth.36 In addition, there is no change in the “d” space values, which implies that the boron and nitrogen modification likely does not change the average unit cell dimension in doped and codoped samples.37 3.2. UVVis Diffuse Reflectance Spectra. The UVvis diffuse reflectance spectra of different samples are presented in Figure 2. As compared to the doped catalysts of BTiO2 and
NTiO2, the codoped samples of (B, mN)TiO2 and (mN, B)TiO2 give absorption intensities that increase with the amount of nitrogen. This is consistent with what Liu et al.24 have found that the absorption intensity of TiO2 gradually increased as the amount of doped B was increased and further enhanced upon additional N doping. 3.3. XPS Spectra. Figure 3a shows the XPS spectra for the B1s region of BTiO2 and (B, mN)TiO2. For B-doped TiO2, the peaks at 189.4 and 191.0 eV are attributed to the formation of TiB and TiOB, respectively.21,38,39 After nitrogen doping, there appeared a new peak at proximity 189.9190.2 eV due to the formation of BN structures on the surface of TiO2.24 Moreover, the peaks above 191.0 eV are attributed to boron oxides.21 It is expected that there are two possible structures for the formation of BN bonds, TiNB formed by substituting O in TiOB bonds with N and TiBN by replacing exposed B in TiB during nitridation.40 To verify the formation of BN bonds, we also measured the XPS spectrum of N1s, which is shown in Figure 3c. In their paper, Liu et al.24 proposed the formation of a TiBN on the basis of the N1s peak at 398.1 eV. However, in our present work, the N1s peak is not at 398.1 but at 399.1 eV (Figure 3c), which indicates that TiBN may not occur on the surface of catalysts. In fact, the peak at 399.1 eV could suggest the presence of TiNBO because in this structure the N is closer to the oxygen as compared to that in TiBNTi, which causes the N electron density to decrease due to the high electronegativity of oxygen. This can be also confirmed by the DFT calculations reported in the following sections. In the current work, the catalysts of (mN, B)TiO2 were also prepared by adding nitrogen first followed by boron. B1s XPS spectra of various (mN, B)TiO2 catalysts are shown in Figure 3b. The peaks at proximity 187.4188.3 and 191.0192.1 eV are ascribed to the structures of TiB and TiOB, respectively, similar to those of (B, mN)TiO2. The peaks at the higher bonding energy are ascribed to the boron oxides. The peak at proximity 189.2190.0 eV is attributed to the bond of BN, which can be due to the formation of TiNBO. It needs to be noted that the B1s peaks of BN in (mN, B)TiO2 are slightly lower than that in (B, mN)TiO2, because the structure of TiBNTi is also generated on the surface of (mN, B)TiO2. To verify the existence of these two structures, the XPS spectrum of N1s in (2.0N, B)TiO2 was measured, and it is shown in Figure 3d together with that of (B, 2.0N)TiO2 for comparison. For (2.0N, B)TiO2, there also exists a peak at 399.1 eV, which is characteristic for N1s in TiNB. Moreover, a new peak is obvious at 398.2 eV. This XPS result further indicates that, in addition to TiNB, TiBN also exists on the surface of (2.0N, B)TiO2. To confirm the new structures formation, Ti2p XPS spectra are also shown in Figure 4. When compared to the pure TiO2, the characterization peak shifts from 458.2 to 458.6 and 458.9 eV after boron and nitrogen codoping.24 The shifting to higher binding energy is resulting from the high electronegativity of O in TiNBO. Moreover, the lower binding energy of Ti2p for (1.0N, B)TiO2 than for (B, 1.0N)TiO2 maybe results from the additional formation of TiBNTi on the (1.0N, B)TiO2 surface due to its higher electropositivity of Ti than the O in TiNBO structures. 3.4. Evaluation of Photocatalytic Activity. Our experimental measurements reported in the above suggest that different orders of adding boron and nitrogen can cause different bonds to form on the TiO2 surface. In particular, structures of TiNBO 7860
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Figure 2. UVvis diffuse reflectance spectra of BTiO2, (B, mN)TiO2, NTiO2, and (mN, B)TiO2 (m = 0.5, 1.0, 2.0).
Figure 3. Fitted XPS spectra for the B1s region of B-doped TiO2, (B, 1.0N)TiO2, and (B, 2.0N)TiO2 (a), and (0.5N, B)TiO2, (1.0N, B)TiO2, and (2.0N, B)TiO2 (b). Fitted XPS spectra for the N1s region of (B, 2.0N)TiO2 (c) and (2.0N, B)TiO2 (d).
and TiBNTi occur at the (mN, B)TiO2, and only TiNBO occurs on the surface of (B, mN)TiO2. To illustrate the relationship between the synergistic effect and photocatalytic activity of TiO2, we measured the photocatalytic activities of codoped TiO2 with different orders of boron and nitrogen addition. In this work, all of the catalysts were prepared by using double hydrothermal method without high-temperature calcinations, which suggests that the nitrogen and boron species adsorbed on the surfaces may play a more important role in their photocatalytic performance. The (B, mN)TiO2 and (mN, B) TiO2 catalysts all exhibited higher photocatalytic activities than did pure TiO2, BTiO2, and NTiO2 under visible light as shown in Figure 5. This is clearly due to the synergistic effects between boron and nitrogen.23,24 In our XPS investigation, the bond of TiNBO is demonstrated to be formed on the surface of (B, mN)TiO2 when the boron is first added in the solution. On the other hand, when the nitrogen is added first, the synergistic effect exhibits as the formation of TiBNTi and TiNBO on the surface of catalysts. As we can see from
the data presented in Figure 5, both of the catalysts can improve the degradation rate of MO under visible light. Noteworthy, the photocatalytic activities of (mN, B)TiO2 are better than that of (B, mN)TiO2, which suggests the synergistic effect of TiBNTi together with TiNBO to be more favorable for the degradation than the TiNBO structure only. From Figure 5, we can also find that the degradation rates of MO increase with the amount of nitrogen, and the catalysts with 1.0 N/Ti molar ratio show the highest photocatalytic activities. This could be because the catalysts present a rather favorable synergistic effect under the N/Ti ratio of 1.0, in which case the catalysts give a relatively high BN peak at 190.0 eV (Figure 3a,b). The catalysts doped with nitrogen and boron also present high photocatalytic activities under the UV light as shown in Figure 6. Moreover, (mN, B)TiO2 again gives better performance than does (B, mN)TiO2 under such condition, largely due to the better synergistic effects of both TiBNTi and TiN BO structures. Different from that under visible light, the catalysts with 2.0 N/Ti molar ratios present the highest photo activities under UV light. As we have discussed early, the catalysts 7861
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Figure 4. Fitted XPS spectra for the Ti2p region of pure TiO2 (a), NTiO2 (b), BTiO2 (c), (1.0N, B)TiO2 (d), and (B, 1.0N)TiO2 (e).
Figure 5. Fractional removal of MO after visible light illumination for 5 h on TiO2, BTiO2, NTiO2, (B, mN)TiO2, and (mN, B)TiO2 (m = 0.5, 1.0, 2.0).
Figure 6. Fractional removal of MO after UV light illumination for 60 min on (B, mN)TiO2 and (mN, B)TiO2 (m = 0.5, 1.0, 2.0, 3.0).
with 1.0 N/Ti molar ratio have the highest photocatalytic activities under visible light because they contain a large amount
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of BN species formed on the surface. Under UV light, quite a lot of photoexcited electrons and holes can be generated. Although the synergistic effect also plays an important role in photo performance of catalysts under UV light irradiation, the large excess amount of BN species on the surface of catalyst could easily act as a recombination center for the photoexcited electrons and holes.41 3.5. DFT Calculations. Our XPS and photodegradation results of the codoped catalysts indicate that the codoping structures are responsible for their higher photoactivity under both visible and UV light. To provide deep insight into the influence of the BN synergistic effect on the photoactivity of catalysts, density functional theory calculations of the detailed structures and corresponding electronic properties of these surface bonds have been performed. The (101) facet is predominant on the exposed area of anatase minerals and polycrystalline powders, and theory calculations also showed that it has the lowest surface energy.42,43 On the other hand, the minor (001) facet contains surface Ti atoms with lower coordination number and is less stable as compared to (101).24 In this study, both (101) and (001) surfaces codoped with N and B were considered in the calculations. On the stoichiometric (101) surface, there exist two-coordinated (O2c) and three-coordinated (O3c-up and O3c-down) oxygen atoms (Figure 7a). By using DFT calculations, Finazzi et al.44 have shown that doped nitrogen prefers the substitutional site of 3-fold coordinated oxygen in the subsurface of (101) (O3cdown). However, the coexistence of doped N at the less stable two-coordinated positions cannot be ruled out. Their calculation results suggest that the relative stabilities of the doped TiO2 (101) with different surface O being replaced by N follow: O3cdown > O2c > O3c-up. Accordingly, in our work, we only considered two types of O atoms (O3c-down and O2c) to be replaced by N on the (101) surface. In addition, for TiO2 (001), there exist two types of surface O atoms, O2c and O3c (Figure 7b), which were both considered to be replaced by doped atoms in this work. Almost all of the possible codoping configurations were tested on TiO2 with one N (B) replacing a surface O and the B (N) sticking to it. The optimized structures of boron and nitrogen codoped anatase TiO2 (101) and (001) are presented in Figure 7c, d. Considering that in many cases the optimized structures (opt) exhibit a dramatic difference with respect to the input structures (IS), or different IS’s converge to the same opt, we also present the corresponding IS together with each optimized structure. It can be seen that on the (101) surface, when the B replaces the O2c first, a TiBNTi configuration can form (Figure 7c,a0 , a00 ); however, when the N replaces the O2c first, only a TiNB configuration can occur (Figure 7c,b0 ,b00 ). When O3c was replaced, no matter by a B or N first in the input configurations, the optimized structures always turned out to be a TiNBO structure with a N at the site of the missing O3c and the B binding with N and nearby two-coordinated oxygen atoms (Figure 7c,c00 , d00 ). With respect to the anatase TiO2 (001) surface, when the O2c was first replaced by either B or N, a TiBNTi structure was formed after optimization (Figure 7d,e00 ,f00 ), and when the O3c was first replaced by either B or N, a TiNBO structure was formed after optimization (Figure 7d,g00 ,h00 ). To obtain a deep understanding of the synergistic effect of BN codoping, the electronic structures of various BN substitutional configurations were calculated together with those of clean anatase TiO2 (101) and (001) surfaces as reference. It 7862
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Figure 7. Calculated structures for oxide nanoparticles. TiO2 anatase (101) (a) and (001) (b) surfaces (gray balls are for the Ti atoms, red balls are for the O atoms, and purple balls are for the two different surface oxygen atoms). Input (IS) and optimized (opt) structures of boron and nitrogen codoped TiO2 (101) (c) and (001) (d) surfaces (blue balls are for the N atoms, and brown balls are for the B atoms).
should be noted that the very small peaks at the edges of valence and conduction bands of TiO2 (101) and (001) come from the atoms at the fixed bottom layer as shown in Figure 8. They were omitted in estimating the band gaps. The DOS’s of various TiO2 (101) (or (001)) slabs were aligned with respect to the 3s states of the Ti6c at the fixed bottom layer. From the calculated density of states (DOS) of clean (101) surface and the various N and B codoped ones (see Figure 8ac), we can see that (i) an obvious localized state is introduced between valence band (VB) and conduction band (CB) of the TiO2 surface as the result of O2c being replaced by B and the formation of a TiBNTi structure (Figure 8b); and (ii) for the calculated DOS of the surface with O3c being replaced by either B or N (Figure 8c), because the antibonding states move to low-energy area, the band gap was reduced by ∼0.2 eV although there is no obvious gap state. From the calculated DOS of the clean and codoped (001) surfaces (Figure 8df), we can find that BN species induce slight reduction of the energy gaps between highest occupied and lowest unoccupied states without introducing the gap states. By comparing the calculated DOS of (101) and (001) surfaces, we can clearly see that the codoping of B and N has a more significant influence on the (101) surface. The calculated
energetic and electronic properties of the various anatase TiO2 (101) and (001) surfaces reported above are summarized in Table 2. According to the data listed in Table 2, we can expect that for the catalysts of (mN, B)TiO2, which were prepared by adding N first, it may prefer to replace O3c, and the configuration of TiNB-O that gives the highest stability would be more likely to occur. However, the amount of N in our experiment is rather large (N/Ti molar ratio of 0.53.0), which suggests the presence of the less stable codoping structures with the O2c being replaced by N. Moreover, because the TiBNTi structure is significantly more stable than the TiNB structure (Table 2), we may also expect that when the B is added following N, the TiBNTi structure would occur predominantly. These results are consistent with our XPS measurements (Figure 3d) that there are two peaks corresponding to the generation of TiNBO and TiBNTi. It should be mentioned that, in this work, we also calculated the TiO2 (101) surface doped by B only. It was determined that the structure with one O3c replaced by B is much more stable (>3.0 eV) than that with an O2c being replaced. Thus, we can expect that for the catalysts of (B, mN)TiO2, which was prepared by adding a low 7863
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Figure 8. Calculated DOS. Clean anatase TiO2 (101) (a), codoped (101) with O2c replaced by the B (TiBNTi, Figure 7c,a0 ) (b), and O3c replaced by either B or N (TiNBO, Figure 7c,c00 ,d00 ) (c). Clean anatase TiO2 (001) (d), codoped (001) with O2c replaced by either B or N (TiBNTi, Figure 7d,e00 ,f00 ) (e), and O3c replaced by either B or N (TiNBO, Figure 7d,g00 ,h0 ) (f).
Table 2. Calculated Energetic and Electronic Properties of Possible Structures on TiO2 Anatase (101) and (001) Surfacesa
(101)
(001)
E/eV
Eb/eV
O2cBN
þ2.77
0.78
O2cNB
þ3.08
clean surface
substitutional structure
1.63 TiBNTi TiNB
O3cBN
0
1.46
TiNBO
O3cNB
0
1.46
TiNBO
þ2.26 þ2.26
1.16 1.16
clean surface O2cBN O2cNB
1.25 TiBNTi TiBNTi
O3cBN
0
1.19
TiNBO
O3cNB
0
1.19
TiNBO
“E” represents the relative stabilities of different structures with E = 0 eV for the most stable ones, and “Eb” represents the band gap. a
amount of B first (B/Ti molar ratio no more than 0.1), almost all of the B would take the O3c site. According to our calculation results, when N was introduced following B, the structure of TiNBO could occur at the O3c sites, and the excess N may replace other O3c and O2c. These results are also consistent with our XPS measurements that there is only one peak corresponding to TiNBO (Figure 3c) for (B, mN)TiO2.
In addition, the above DFT calculations can help to explain our photoactivity experimental measurement results. As we have explained above, our XPS and DFT investigations show that TiBNTi and TiNBO structures are generated on the surface of codoped catalysts with different orders of adding boron and nitrogen. Both of these structures can increase the degradation rate of MO on codoped TiO2 under visible light, as shown in Figure 5. This is in line with the calculation results that for the B and N codoped TiO2, localized states in the band gap or lowenergy antibonding states are generated, which causes the band gap to decrease significantly (Figure 8 and Table 2). Considering that the band gap of TiO2 narrowed by TiBNTi more significantly than that by TiNBO (Table 2), one may also find it easy to understand why the photocatalytic activity of (mN, B)TiO2 is higher than that of (B, mN)TiO2 under both visible and UV light. Moreover, because the TiBNTi structure has a drastic effect on the band gap of TiO2 (101) surface by decreasing it from 1.63 to 0.78 eV (Table 2), it could exhibit the central synergistic effect with respect to the (mN, B)TiO2.
4. CONCLUSIONS In summary, the nitrogen and boron codoped TiO2 with different BN adding orders is successfully synthesized by using the double hydrothermal method. Both (B, mN)TiO2 and (mN, B)TiO2 catalysts with 1.0 N/Ti molar ratios show the highest photocatalytic activities under visible light, while the 7864
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The Journal of Physical Chemistry C catalysts with 2.0 N/Ti molar ratios present the highest photocatalytic activities under the irradiation of UV light. The BN synergistic effect works through the formation of TiBNTi and TiNBO structures on the surface of codoped catalysts. When nitrogen is added first ((mN, B)TiO2), TiBNTi and TiNBO compounds are generated on the surface. By contrast, when boron is introduced to the materials first ((B, mN)TiO2), only TiNBO species occurs on the surface. TiBNTi and TiNBO structures are both responsible for the improved photoactivity of codoped catalysts under both visible and UV light. According to the results of DFT calculations, the BN synergistic effect at the (101) surface can largely reduce the band gap, while such effect at the (001) surface is not so obvious. To the catalyst of (mN, B)TiO2, localized state is generated inside the TiO2 band gap, giving rise to a significant band gap decrease, which accounts for its photoactivity being better than that of (B, mN)TiO2.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (J.-L.Z.);
[email protected] (X.-Q.G.).
’ ACKNOWLEDGMENT This work has been supported by the National Nature Science Foundation of China (20703017, 20773039, 20977030), the National Basic Research Program of China (973 Program, 2007CB613301, 2010CB732306), the Science and Technology Commission of Shanghai Municipality (10520709900; 09QA1401300 (Shanghai Rising-Star Program)), and the Fundamental Research Funds for the Central Universities. ’ REFERENCES (1) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (2) Takabayashi, S.; Nakamura, R.; Nakato, Y. J. Photochem. Photobiol., A 2004, 166, 107. (3) Ngamsinlapasathian, S.; Sreethawong, T.; Suzuki, Y.; Yoshikawa, S. Sol. Energy Mater. Sol. Cells 2005, 86, 269. (4) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Chem. Commun. 2002, 1464. (5) Rinc’n, A.-G.; Pulgarin, C. Sol. Energy 2004, 77, 635. (6) Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Environ. Sci. Technol. 1998, 32, 726. (7) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (8) Wolfrum, E. J.; Huang, J.; Blake, D. M.; Maness, P.-C.; Huang, Z.; Fiest, J.; Jacoby, W. A. Environ. Sci. Technol. 2002, 36, 3412. (9) Hu, C.; Yu, J. C.; Hao, Z.; Wong, P. K. Appl. Catal., B 2003, 42, 47. (10) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (11) Yu, J. C.; Ho, W.; Yu, J.; Yip, H.; Wong, P. K.; Zhao, J. Environ. Sci. Technol. 2005, 39, 1175. (12) Chatterjee, D.; Dasgupta, S. J. Photochem. Photobiol., C 2005, 6, 186. (13) Cong, Y.; Zhang, J.; Chen, F.; Anpo, M.; He, D. J. Phys. Chem. C 2007, 111, 10618. (14) Xing, M.; Zhang, J.; Chen, F. Appl. Catal., B 2009, 89, 563. (15) Ohno, T.; Miyamoto, Z.; Nishijima, K.; Kanemitsu, H.; Xueyuan, F. Appl. Catal., A 2006, 302, 62.
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