ARTICLE pubs.acs.org/Langmuir
Novel in Situ N-Doped (BiO)2CO3 Hierarchical Microspheres Self-Assembled by Nanosheets as Efficient and Durable Visible Light Driven Photocatalyst Fan Dong,†,‡ Yanjuan Sun,† Min Fu,† Wing-Kei Ho,|| Shun Cheng Lee,*,‡ and Zhongbiao Wu*,§ †
)
College of Environmental and Biological Engineering, Chongqing Key Laboratory of Catalysis Theory and Application Technology in Universities, Chongqing Technology and Business University, Chongqing 400067, P. R. China ‡ Department of Civil and Structural Engineering, Research Center for Environmental Technology and Management, The Hong Kong Polytechnic University, Hong Kong, P. R. China § Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, P. R. China Hong Kong Nano and Advanced Materials Institute Limited, hosted by The Hong Kong University of Science and Technology, Hong Kong, P. R. China
bS Supporting Information ABSTRACT: Novel N-doped (BiO)2CO3 hierarchical microspheres (N-BOC) were fabricated by a facile one-pot template free method on the basis of hydrothermal treatment of bismuth citrate and urea in water for the first time. The N-BOC sample was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UVvis diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorptiondesorption isotherms, and Fourier transform-infrared spectroscopy. The N-BOC was constructed by the self-assembly of single-crystalline nanosheets. The aggregation of nanosheets led to the formation of hierarchical framework with mesopores, which is favorable for efficient transport of reaction molecules and harvesting of photoenergy. Due to the in situ doped nitrogen substituting for oxygen in the lattice of (BiO)2CO3, the band gap of N-BOC was reduced from 3.4 to 2.5 eV, making N-BOC visible light active. The N-BOC exhibited not only excellent visible light photocatalytic activity, but also high photochemical stability and durability during repeated and long-term photocatalytic removal of NO in air due to the special hierarchical structure. This work demonstrates that the facile fabrication method for N-BOC combined with the associated outstanding visible light photocatalytic performance could provide new insights into the morphology-controlled fabrication of nanostructured photocatalytic materials for environmental pollution control.
1. INTRODUCTION Nitric oxide (NO), mainly produced from combustion process of fossil fuels and vehicle exhaust, is considered a primary air pollutant, since it is responsible for atmospheric environmental problems like photochemical smog, acid rain, tropospheric ozone, and ozone layer depletion.1 In living areas, long-term exposure to NO and the derivatives of NO during transformation cause adverse implications on human health.2 There are several routes to control the atmospheric pollution of NO, such as absorption, adsorption, thermal catalysis, or electrical discharge. However, the concentration of NO in indoor air and outdoor urban environment is very low, usually at ppb level. In addition, these methods have low removal efficiency for pollutants at such a low level.3 As people’s awareness for public environment and health is ever increasing, more and more attention has been paid to indoor air quality (IAQ), which is an important determinant of human health, comfort, and productivity. Thus, there is an urgent need to develop new technology to remove NO pollutants at ppb level effectively.4 r 2011 American Chemical Society
Semiconductor photocatalysis as a green technology has provided an alternative way for the treatment of pollutants and generation of hydrogen.57 The widely used, powerful TiO2 with band gap of 3.2 eV requires UV light (4% fraction of solar light) to initiate the photocatalytic oxidation.8,9 In order to utilize visible light in solar and indoor illumination, developing efficient and stable visible light driven photocatalysts is desirable and has become one of the most important topics in environmental photocatalysis.10 Doping metals/nonmetals, coupled with other lower band gap semiconductors and hydrogen treatment, have been successfully applied to enhance the visible light activity of TiO2.8,1114 The fabrication and self-assembly of micro/nanoscale functional materials with special morphology, orientation, and hierarchy have attracted intense attention due to their importance in basic scientific research and potential technological applications Received: July 18, 2011 Revised: November 8, 2011 Published: November 28, 2011 766
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H2O in a 100 mL autoclave Teflon vessel and stirred for 30 min. The resulted aqueous precursor suspension was then hydrothermally treated at 180 °C for 48 h. The obtained solid sample was filtered, washed with water and ethanol three times, and dried at 60 °C to get final N-doped (BiO)2CO3 hierarchical microspheres (N-BOC) with no further treatment. When urea was replaced by sodium carbonate (0.92 g) with other conditions identical, undoped (BiO)2CO3 (BOC) with particle morphology was produced. For comparison, C-doped TiO2 was prepared by a reported hydrothermal method and the commercial Degussa P25 was used as a reference sample.29 2.2. Structural Characterization Methods. The crystal phase was analyzed by X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Japan). X-ray photoelectron spectroscopy with Al Kα X-rays (hν = 1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties and to probe the total density of the state (DOS) distribution in the valence band. The shift of the binding energy was corrected using the C1s level at 284.8 eV as an internal standard. A scanning electron microscope (SEM, JEOL model JSM-6490, Japan) was used to characterize the morphology of the samples. The morphology and structure were examined by transmission electron microscopy (TEM: JEM-2010, Japan). The UVvis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UVvis spectrophotometer (UVvis DRS: UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly, using BaSO4 as reflectance sample. Nitrogen adsorptiondesorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP 2020, USA) with all samples degassed at 150 °C prior to measurements. FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. 2.3. Evaluation of Photocatalytic Activity. The photocatalytic activity was evaluated by removal of NO at ppb level in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm 15 cm 10 cm). A 300 W commercial tungsten halogen lamp (General Electric) was vertically placed outside the reactor. Four mini-fans were used to cool the lamp and flow system. For the visible light photocatalytic activity test, UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. For photocatalytic activity test under UVvis light, the UV cutoff filter was removed. Photocatalyst (0.15 g) was coated onto a dish with a diameter of 12.0 cm. The coated dish was then pretreated at 70 °C to remove water in the suspension. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance, BOC gas) with traceable National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 450 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc., model 111). The desired relative humidity (RH) level of the NO flow was controlled at 70% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 3.3 L/min by a mass flow controller. After the adsorptiondesorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The removal efficiency (η) of NO was calculated as η (%) = (1 C/C0) 100%, where C and C0 are concentrations of NO in the outlet steam and the feeding stream, respectively. The kinetics of photocatalytic NO removal reaction is a pseudo first-order reaction at low NO concentration as ln(C0/C) = kt, where k is the initial apparent rate constant.
Figure 1. XRD pattern of the as-prepared N-BOC.
(such as catalysis, pollution control, drug delivery, sensing, photonic crystals). Recently, alternative hierarchical bismuth-containing materials composed of 2D nanosheets or nanoplates have been developed and exhibited excellent visible light induced photocatalytic performance, such as Bi2WO6, Bi2MoO6, BiVO4, BiOX (X = Br, I), BiFeO3, Bi2Ti2O7, and Bi2Sn2O7.1522 It was also found that 2D nanostructure favored the transfer of electrons and holes generated inside the crystal surface and promoted charge separation, which helped to enhance the photocatalytic activity.10 Very recently, a new member of bismuth-containing materials named bismuth subcarbonate (BiO)2CO3 was synthesized by various methods and found to display promising antibacterial performance and photocatalytic activities in degradation of aqueous pollutants.2328 Owing to interesting multifunctional performance, nanostructured (BiO)2CO3 materials have received great research interest during the past several years. However, the various synthesized (BiO)2CO3 structures as photocatalyst were undoped with large band gap (3.13.5 eV), which can only be excited by UV light. Inspired by the widely investigated nonmetal TiO2 with high visible light activity,11 we believed it to be significant to develop a new method to dope (BiO)2CO3 with a nonmetal in order to make (BiO)2CO3 visible light active. In this research, we develop a facile one-pot and template free method for the fabrication of novel N-doped (BiO)2CO3 hierarchical microspheres (N-BOC) by hydrothermal treatment of aqueous bismuth citrate and urea for the first time. Nitrogen is found to be in situ doped in the lattice of (BiO)2CO3, making the as-prepared N-BOC visible light active. More importantly, as an outstanding photocatalyst, the N-doped (BiO)2CO3 hierarchical microspheres exhibit efficient visible (solar) light photocatalytic activity and high photochemical stability and durability during photocatalytic removal of NO in air, shedding new light on the fabrication of high-performance nanostructured photocatalysts driven by visible light for pollution control.
2. EXPERIMENTAL SECTION 2.1. Fabrication of N-Doped (BiO)2CO3 Hierarchical Microspheres. All chemicals used in this study were analytical grade (Sigma-
3. RESULTS AND DISCUSSION
Aldrich) and were used without further purification. Distilled water was used in all experiments. In a typical fabrication, appropriate amounts of bismuth citrate (1.60 g) and urea (0.72 g) were mixed with 75 mL of
3.1. Phase Structure. Figure 1 shows the XRD pattern of N-BOC compared with standard PDF card of (BiO)2CO3 767
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Figure 2. XPS spectra of N-BOC and BOC, (a) N1s, (b) C1s, (c) O1s, (d) Bi4f, and (e) VB.
(JCPDS-ICDD Card No. 411488). The XRD pattern of BOC is shown in SI Figure S1. It can be seen that all the diffraction peaks of N-BOC and BOC can be indexed to (BiO)2CO3. No peaks from other phases can be observed, implying the high phase purity of the products. Compared with the relative peak intensity shown in the standard PDF card, the enhanced (110) diffraction peak of N-BOC indicates that N-BOC have a preferred orientation along the (110) planes, which is further demonstrated below by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). 3.2. XPS Analysis. The XPS measurement was carried out to determine the chemical state of the elements and total density of states distribution (DOS) of the valence band in N-BOC and
BOC, as shown in Figure 2. As can be seen in Figure 2a, an obvious peak of N-BOC centered at 400.8 eV appears compared to BOC, which indicates that nitrogen was in situ doped into the (BiO)2CO3 during hydrothermal treatment. The doped nitrogen originates from the decomposition of urea during hydrothermal reaction. The N1s peak at around 400 eV has been frequently observed in N-doped TiO2, where nitrogen was substituted for oxygen in TiO2 and modified the band structure subsequently.3033 In our case, doped nitrogen also substitutes for oxygen atom in the lattice of (BiO)2CO3, which would change the band structure of (BiO)2CO3. The surface atomic concentration of the doped nitrogen is determined to be 1.17% by XPS. Elemental analysis further confirms the existence of N and the determined concentration is 0.83%. The concentration of N by elemental analysis is 768
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Figure 3. UVvis DRS and plots of (αhν)1/2 vs photon energy (inset) of N-BOC and BOC.
slightly lower than that by XPS, which can be ascribed to the fact that XPS is a measurement technique mainly for surface characterization. As the doped nitrogen element comes from the urea added in the precursor suspension, the concentration of doped nitrogen can be controlled quantitatively through the variation of the amount of urea added. This result will be reported in the future. A broad energy range from 291 to 282 eV can be observed in Figure 2b. The peaks at 284.8 eV can be assigned to adventitious carbon species from XPS measurement, while the peak at 288.8 eV can be ascribed to carbonate ion in (BiO)2CO3 for both samples. The O1s spectra are also recorded (Figure 2c), which can be fitted by three peaks at binding energies of 529.9, 531.1, and 532.2 eV, respectively. The peak at 529.9 eV is characteristic of BiO binding energy in (BiO)2CO3,34 and the other two peaks at around 531.1 and 532.2 eV can be assigned to carbonate species and adsorbed H2O (or surface hydroxyl groups) on the surface. The content of surface hydroxyl groups of N-BOC is much higher than that of BOC (Figure 2c), which is advantageous for promoting photocatalytic activity. Two strong peaks at 159.0 and 164.3 eV in Figure 2d are assigned to Bi4f7/2 and Bi4f5/2, respectively, which is characteristic of Bi3+ in (BiO)2CO3.4 The DOS of valence band (VB) is shown in Figure 2e. Interestingly, additional diffusive electronic states of N-BOC above the valence band maximum (VBM) can be observed above VB edge compared to BOC, indicating the existence of midgap above the VB.9,29,35 The formation of midgap can be ascribed to the in situ nitrogen doping, which modified the band structure of (BiO)2CO3. This result is consistent with the effect of nitrogen doping on the band structure of TiO2.9,11,35 The newly formed midgap between VB and conduction band (CB) can make the light absorption spectra of N-doped (BiO)2CO3 hierarchical microspheres shift to visible light. 3.3. UVvis DRS and Band Structure. Figure 3 shows the UVvis DRS spectra for N-BOC and BOC. The nitrogen doping makes the absorption edge of N-BOC shift from 360 nm to nearly 500 nm in the visible light region. The band gap energy of N-BOC estimated from the intercept of the tangents to the plots of (αhν)1/2 vs photon energy (inset in Figure 3) is 2.5 eV, which is significantly smaller compared to that of BOC of 3.4 eV.36 The shift in the absorption spectra of N-BOC was also indicated by the change in color of the samples from white (BOC) to bright
Figure 4. (a, b) SEM and (c, d) TEM images of N-BOC, SAED of a single nanosheet.
yellow. Utilizing visible light to drive photocatalytic reaction is a key challenge since many oxide photocatalytic materials only absorb UV light. In contrast to the undoped (BiO)2CO3, N-doped (BiO)2CO3 exhibits a large red shift in absorbance, which indicates nitrogen doping can also be extended to modify other photocatalyst with wide band gap. The steep absorption edge of N-BOC is significant due to the large reduction in band gap, consistent with additional diffusive electronic states (Figure 2e) caused by successful nitrogen doping. 3.4. Morphological Structure. The hydrothermal treatment of bismuth citrate and urea mixture produces large population of flower-like microspheres as shown in the typical SEM images in Figure 4a. The as-prepared N-BOC microspheres have an average diameter of 2.0 to 10.0 μm. The higher magnification SEM image (Figure 4b) reveals that each microsphere is constructed by many self-organized nanosheets with a thickness of about 20 nm. These nanosheets are arranged hierarchically to form microsphere superstructures with slightly different morphologies. The structural morphology of the product was further investigated by TEM. As shown in Figure 4c, the entire microsphere is composed of self-assembled nanosheets. The nanosheets are very thin and therefore relatively transparent to the electron beam. The HRTEM image of a single nanosheet on the edge of the microsphere is shown in Figure 4d. The lattice spacing is determined to be 0.272 nm, matching the spacing of the (110) crystal plane of (BiO)2CO3. The SAED pattern of one single nanosheet displays an array of clear and regular diffraction spots, indicating that the nanosheet is well-defined single crystalline in nature. Thus, the hierarchical N-BOC microspheres originate from the self-assembly of the single-crystalline nanosheets with preferred orientation. When urea was replaced by sodium carbonate, (BiO)2CO3 with particle morphology was produced (see SI Figure S2), which implied that the addition of urea plays a crucial role in formation of N-doped (BiO)2CO3 hierarchical microspheres. Combining with the XPS result for the N1s region, it can be inferred that urea acted as both structure directing factor and nitrogen doping source. The formation mechanism of N-doped 769
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Figure 5. (a) N2 adsorptiondesorption isotherms, and (b) pore-size distribution of N-BOC.
Table 1. SBET, Pore Volume, Band Gap Energy, and Initial Rate Constant k of N-doped (BiO)2CO3, (BiO)2CO3, C-doped TiO2, and P25
(BiO)2CO3 hierarchical microspheres is rather complicated and is under investigation. 3.5. Specific BET Surface Areas and Pore Structure. Figure 5 shows the nitrogen adsorptiondesorption isotherms and the corresponding pore size distribution curve of N-BOC. According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the majority of physisorption isotherms can be classified into six types. Typically, N-BOC has an isotherm of type IV, indicating the presence of mesopores.37 The shape of the hysteresis loops is of type H3, which implies that the formation of slit-like pores is due to the aggregations of the plate-like particles.37 It is consistent with the microstructure of N-BOC, which is self-assembled with nanosheets (see Figure 4). The existence of mesopores is further confirmed by the corresponding pore size distribution (Figure 5b). Besides the small mesopores (ca. 3.6 nm), large mesopores with a maximum pore diameter of ca. 6.1 nm can also be defined. As the nanosheets do not contain mesopores, the aggregation of nanosheets contributed to the formation of the observed mesopores. The BET surface area (SBET) and pore volume of the N-doped (BiO)2CO3 hierarchical microsphere are about 30.4 m2/g and 0.108 cm3/g, much higher than that of (BiO)2CO3 particles with SBET of 3.1 m2/g and pore volume of 0.009 cm3/g (Table 1). This hierarchical structure with mesopores can function as transport path for reactants and products and provide more active sites in photocatalysis.3,4,3840 The unique three-dimensional hierarchical framework is also well-suited for efficient photoenergy harvesting and introducing reactive molecules into the interior space of the hierarchical structure.38,4143 3.6. Evaluation of the Photocatalytic Activity and Photochemical Stability. Self-assembly of nanoscale building blocks into 3D complex architecture is a hot research topic currently.10 The novel N-doped (BiO)2CO3 hierarchical nanosheets microspheres were applied to photocatalytic removal of NO in air in order to demonstrate their potential ability for pollution control (Figure 6). Figure 6a shows the variation of NO concentration (C/C0 %) with irradiation time over N-BOC under visible light irradiation with BOC, C-doped TiO2, and P25 as references. Here, C0 is the initial concentration of NO, and C is the concentration of NO after photocatalytic reaction for t. As noted, NO could not be photolyzed under light irradiation without photocatalyst.4 In the presence of the as-prepared photocatalytic materials, the NO reacted with the photogenerated reactive radicals and produced HNO2 and HNO3,4,44 which involved four
SBET
Pore vol.
Band
Sample
(m2/g)
(cm3/g)
Gap (eV)
kv (min1)
ks (min1)
N-BOC
30.4
0.108
2.66/2.0
0.187
0.203
3.1
0.009
3.40
0.0048
0.104
122.5
0.248
2.90
0.0831
0.168
51.2
0.090
3.0
0.0397
0.174
BOC C-TiO2 P25
reactions displayed in eqs 14. NO þ 2•OH f NO2 þ H2 O
ð1Þ
NO2 þ •OH f NO3 þ Hþ
ð2Þ
NO þ NO2 þ H2 O f 2HNO2
ð3Þ
NO þ •O2 f NO3
ð4Þ
As shown in Figure 6a, after 60 min irradiation, (BiO)2CO3 particles show negligible visible light activity due to the large band gap of 3.4 eV. P25 exhibits weak visible light activity with a removal ratio of 10.6% due the existence of rutile TiO2 in the phase. The NO removal ratio reaches 20.3% over C-doped TiO2 as it is a well-known good visible light driven photocatalyst. Recent research showed that the visible light photocatalytic removal ratio of NO in air over N-doped TiO2 could reach 36%.45 Interestingly, the as-prepared N-doped (BiO)2CO3 hierarchical microspheres exhibit superior photocatalytic activity with an NO removal ratio of 49.4% under visible light, much higher than that of C-doped TiO2 and N-doped TiO2. Table 1 shows that the SBET and the pore volume of N-BOC are much lower than that of C-doped TiO2. Recent reports have proven that there are significant advantages of hierarchical and hollow structure for enhancement of the photocatalytic activity.45,46 Song et al. fabricated hollow NiO microspheres, which showed significantly enhanced photocatalytic activity over NiO rods due to the increasing number of surface active sites and the surface charge carrier transfer rate.46 Lu et al. prepared a novel ZnO hierarchical micro/nanoarchitecture with dense nanosheet-built networks, exhibiting excellent photocatalytic performance due to the inhibition of electron/hole pair recombination caused by special 770
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Figure 6. (a) Visible light photocatalytic activities of N-doped (BiO)2CO3, (BiO)2CO3, C-doped TiO2, and P25 and (b) repeated visible light photocatalytic activities of N-doped (BiO)2CO3 for the removal of NO in air.
structural features.47 Thus, the outstanding visible light photocatalytic activity of N-BOC could mainly be attributed to the special hierarchical structure. With nanosheet assembled hierarchical structure, the interconnected nanosheets allow multiple reflections of light, which enhances light-harvesting and thus increases the quantity of photogenerated electrons and holes available to participate in the photocatalytic reaction.4143 The porous hierarchical structure is also beneficial for enhancing the flow rate of the gas molecules.38 The hierarchical structure was favorable for the efficient utilization of photoenergy and the diffusion of reaction intermediates, thus enhancing the photocatalytic activity.38,4143 As shown in eqs 14 that the photocatalytic oxidation of NO to NO3 was the major process with NO2 as an intermediate oxidation product,4 most of the NO was converted to final product HNO3. The fact of enhanced activity for N-BOC is further proven by monitoring the concentration of reaction intermediate NO2 during photocatalytic oxidation of NO, as shown in SI Figure S3a. Figure S3a shows that the molar fraction of NO2 concentration in N-BOC is lower than that of all other reference samples, implying that the diffusion rate of the reaction intermediate is faster than that of other samples due the hierarchical structure, which promotes NO 2 to be oxidized to final NO3. The photochemical stability and durability of a photocatalyst under irradiation should be considered for its practical applications. An excellent photocatalyst should maintain stable structure and durable activity so that the catalyst can be used repeatedly.4 Some photocatalysts suffer from instability for air purification applications, because the intermediates and products generated during photocatalytic reaction could accumulate on the catalyst surface, which possibly in turn deactivates the photocatalyst.48 To test the stability and durability of N-BOC on photocatalytic NO in air removal, multiple runs of photocatalytic experiments were carried out (Figure 6b). It is significant to find that the novel N-BOC could maintain efficient and durable visible light photocatalytic activity after six cycles of repeated runs with no obvious deactivation. The great power of N-BOC for NO removal was further demonstrated by utilizing UVvis light, as shown in Figure 7. The reference samples (BiO)2CO3 particles, C-doped TiO2, and P25 exhibited decent photocatalytic activity, especially for P25 with a removal ratio of 45.9%, as a well-known efficient photocatalyst under UV irradiation. However, N-BOC was more powerful than all of them, reaching a high removal rate of 56.8%. Besides,
Figure 7. Photocatalytic activity of N-doped (BiO)2CO3, (BiO)2CO3, C-doped TiO2, and P25 under UVvis light irradiation.
evolution of NO2 intermediate was maintained at a low level for N-BOC sample (SI Figure S3b). The LangmuirHinshelwood model (L-H) was used to describe the initial rates of photocatalytic oxidation of NO in air, which was recognized to follow mass transfer-controlled firstorder kinetics as a result of low concentration of target pollutants.3,4 The linear plot of ln(C/C0) versus irradiation time t is shown in SI Figure S4. The calculated initial rate constants k of N-doped (BiO)2CO3, (BiO)2CO3, C-doped TiO2, and P25 under both visible (kv) and solar (ks) light irradiation were summarized in Table 1. To further test the stability and durability of N-BOC under irradiation, long-term photocatalytic activity over N-BOC under UVvis light irradiation was performed. It can be seen from Figure 8a that the photocatalytic activity is stable and durable during photocatalytic oxidation of NO without deactivation after more than 14 h irradiation. An SEM image of an N-doped (BiO)2CO3 hierarchical microsphere after long-term irradiation is shown in the inset in Figure 8a. The nanosheet self-assembled hierarchical morphology is almost the same as in the SEM image of fresh sample in Figure 4b, which indicates that the microstructure of N-BOC does not change after long irradiation. The stability of N-BOC is further confirmed by the FI-TR spectra of N-BOC before and after long-term irradiation, as shown in 771
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Figure 8. (a) Long-term photocatalytic activity of N-BOC under UVvis light irradiation (inset shows the SEM image of N-BOC after reaction), and (b) FT-IR spectra of N-BOC before and after reaction.
Figure 8b. Four characteristic band groups in FT-IR at 1067 cm1 (symmetric stretching mode ν1), 846 and 820 cm1 (out-ofplane bending mode ν2), 1468 and 1391 cm1 (antisymmetric vibration ν3), 670 cm1 (in-plane deformation ν4), and 1756 and 1730 cm1 (ν1 + ν4) are observed for both samples.49 It can also be seen from Figure 8b that the reaction intermediates and reaction products during photocatalytic oxidation of NO (such as HNO2 and HNO3) cannot be observed except for the vibration mode of (BiO)2CO3 for the sample used. This result suggests that the reaction intermediates and products could diffuse rapidly due to the hierarchical structure. As the solar light consists of both UV and visible light, our novel N-doped (BiO)2CO3 hierarchical microsphere could fully utilize the solar light or indoor visible light source to remove the NO in air by photocatalysis. The facile template fabrication method combined with the associated outstanding photocatalytic performance could provide new insights into the controlled fabrication of novel nanostructured visible light driven photocatalytic materials with high performance for environmental pollution control.
4. CONCLUSION In summary, we have successfully developed a one-pot and template-free method to fabricate novel N-doped (BiO)2CO3 hierarchical microspheres based on hydrothermal treatment of bismuth citrate and urea in water. SEM and TEM analysis revealed that the N-BOC was constructed by the self-assembly of single-crystalline nanosheets with thickness of about 20 nm. The formation of hierarchical framework with mesopores can be ascribed to the aggregation of numerous nanosheets, which is favorable for efficient transport of reaction molecules and harvesting of photoenergy. Nitrogen was found to be in situ doped in the lattice of (BiO)2CO3, substituting for oxygen atoms in N-BOC. Due to the nitrogen doping, the band gap of N-BOC was reduced to 2.5 eV compared to 3.4 eV for undoped (BiO)2CO3, making N-BOC visible light active. The N-BOC not only exhibited excellent visible light photocatalytic activity but also high stability and durability during repeated and long-term photocatalytic removal of NO in air due to the special hierarchical structure. This work demonstrates that the facile fabrication method for N-BOC combined with the associated outstanding visible light photocatalytic performance could provide new insights into the controlled fabrication of nanostructured visible
light driven photocatalytic materials with high performance for environmental pollution control.
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD pattern of BOC sample; SEM image of BOC sample with particle morphology; Evolution of NO2 during photocatalytic removal of NO under visible light and UVvis light irradiation over N-BOC, BOC, C-doped TiO2 and P25 and dependence of -ln(C/C0) on irradiation time under visible light and UVvis light irradiation over N-BOC. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel/Fax: + 852-27666011. E-mail:
[email protected] (S. C. Lee),
[email protected] (Zhongbiao Wu).
’ ACKNOWLEDGMENT This research is financially supported by the National Natural Science Foundation of China (51108487), the Program for Young Talented Teachers in Universities (Chongqing, 2011), Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), the National High Technology Research and Development Program (863 Program) of China (2010AA064905), the Program for Chongqing Innovative Research Team Development in University (KJTD201020), the Research Grants Council of Hong Kong (PolyU 5204/07E and PolyU 5175/ 09E) and The Hong Kong Polytechnic University (GU712, GYX75, and GYX0L). ’ REFERENCES (1) Skalska, K.; Miller, J. S.; Ledakowicz, S. Sci. Total Environ. 2010, 408, 3976–3989. (2) Fischer, S. L.; Koshland, C. P. Environ. Sci. Technol. 2007, 41, 3121–3126. (3) Li, G. S.; Zhang, D. Q.; Yu, J. C.; Leung, M. K. H. Environ. Sci. Technol. 2010, 44, 4276–4281. (4) Ai, Z. H.; Ho, W. K.; Lee, S. C.; Zhang, L. Z. Environ. Sci. Technol. 2009, 43, 4143–4150. (5) Mo, J. H.; Zhang, Y. P.; Xu, Q. J.; Lamson, J. J.; Zhao, R. Y. Atmos. Environ. 2009, 43, 2229–2246. 772
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dx.doi.org/10.1021/la202752q |Langmuir 2012, 28, 766–773
