Article pubs.acs.org/IECR
Visible-Light Photocatalytic Removal of NO in Air over BiOX (X = Cl, Br, I) Single-Crystal Nanoplates Prepared at Room Temperature Wendong Zhang,† Qin Zhang,† and Fan Dong*,‡ †
College of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400045, China Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China
‡
S Supporting Information *
ABSTRACT: Well-crystallized layered two-dimensional bismuth oxyhalide (BiOX; X = Cl, Br, I) single-crystal nanoplates were synthesized via a facile and low-cost method at room temperature. The as-synthesized samples were analyzed by various characterization techniques. The photocatalytic activity of the samples was evaluated by the removal of NO at the indoor air level under visible-light irradiation. The band gap and thermal stability of bismuth oxyhalides decreased with increased X atomic numbers. The as-synthesized BiOBr nanoplates exhibited highest photocatalytic activity due to the favorable factors of ultrathin nanoplates, layered structures, relatively high surface area, and suitable band structure, exceeding that of BiOCl and BiOI. The present work could provide new insight into the low-temperature preparation and appropriate selection of visible-light photocatalysts for environmental application.
1. INTRODUCTION Nitrogen oxides (NOx) are major air pollutants generated from automotive engines and power plants that have caused serious environmental pollution with devastating effects on human health and ecosystem, such as photochemical smog, tropospheric ozone, eutrophication, acid rain, and fine particulate matter (PM10/PM2.5).1,2 Thus, it is highly desirable to develop novel and efficient strategies for the removal of NOx in air. In recent years, photocatalytic technology has received considerable attention because of its wide application in environmental remediation and alternative clean energy,3−7 such as air/water purification,8,9 solar cells,10 and hydrogen evolution.11,12 Photocatalysis, as one of the most promising technologies, could utilize natural sunlight or artificial indoor illumination to remove typical pollutants. During the past 40 years, many efforts have been devoted to fabricating different types of photocatalysts such as metal oxides, metal nitrides, metal sulfides, and heterojunctioned materials. We were inspired to search for new preparation methods and photocatalysts. Bismuth oxyhalides (BiOX; X = Cl, Br, I), as a new family of promising photocatalysts, have attracted intensive attention because of their interesting physical and chemical properties,13−15 such as special micro/nanostructures, band gaps, electrical and optical properties, etc. Ai et al. reported that BiOBr nanoplate microspheres exhibited efficient photocatalytic removal of NO under visible light.16 Dong et al. prepared porous BiOI/BiOCl composite nanoplate microflowers with visible-light photocatalytic activity for the removal of NO.8 In the presence of as-synthesized photocatalytic materials, NO reacted with the photogenerated reactive radicals and produced HNO2 and HNO3, which involved four reactions displayed in eqs 1−4.8,16,17 NO + 2•OH → NO2 + H 2O © 2013 American Chemical Society
NO2 + •OH → NO3− + H+
(2)
NO + NO2 + H 2O → 2HNO2
(3)
NO + •O2− → NO3−
(4)
However, there is still dearth of exploring bismuth oxyhalides with various morphologies such as visible-light-driven photocatalysts for the removal of NOx. Various micro/nanostructures of bismuth oxyhalides including nanoflakes,18 nanosheets,19 and three-dimensional (3D) hierarchical micro/nanostructures20−23 have been synthesized via different methods. Some studies showed that twodimensional (2D) BiOX (X = Cl, Br, I) nanostructures favored transfer and separation of the photogenerated electron and hole, which could improve the photocatalytic activity.8 2D BiOX (X = Cl, Br, I) structures can be synthesized by hydrothermal, solvothermal, sol−gel, ionic-assistant, and reverse microemulsion methods.24,25 However, the high temperature and pressure (for hydrothermal and solvothermal methods), time (for the sol−gel method), and high cost and time (for ionic-assistant and reverse microemulsion methods) limited their large-scale applications in pollution control and solar energy conversion. Therefore, it is important to develop facile and low-cost methods to prepare BiOX and improve the photocatalytic activity for practical applications. In this paper, well-crystallized layered 2D BiOX (X = Cl, Br, I) single-crystal nanoplates have been synthesized by a facile and low-temperature method. The photocatalytic activity of 2D BiOX has been explored in the photodegradation of NO at the indoor air level under visible-light irradiation. The microReceived: Revised: Accepted: Published:
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February 25, 2013 May 3, 2013 May 3, 2013 May 3, 2013 dx.doi.org/10.1021/ie400615f | Ind. Eng. Chem. Res. 2013, 52, 6740−6746
Industrial & Engineering Chemistry Research
Article
For each photocatalytic activity test, two sample dishes (with a diameter of 12.0 cm) containing a photocatalyst powder were placed in the center of the reactor. The weight of the photocatalyst used for each dish was kept at 0.1 g. A BiOX sample was added into 30 mL of H2O and sonicated for 10 min, and then photocatalyst samples were prepared by coating aqueous suspensions of the samples onto the glass dishes. The coated dish was pretreated at 60 °C to remove water in the suspension and then cooled to room temperature before photocatalytic testing. NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance, BOC gas) with a National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 600 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc., model 111). The relative humidity (RH) at indoor environmental conditions is 40− 80%. The desired RH in the present system is controlled at 50% in the gas flow, which could simulate the indoor environmental conditions. The desired RH level of the NO flow was controlled at 50% 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 2.4 L min−1 by a MFC. After adsorption− desorption 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−1. The removal ratio (η) of NO was calculated as (1 − C/C0) × 100%, where C and C0 are the concentrations of NO in the outlet and feeding streams, respectively. The kinetics of a photocatalytic NO removal reaction is a pseudo-first-order at low NO concentration as ln(C0/C) = kt, where k is the Arrhenius rate constant.
structure, morphology, specific surface area, thermal stability, and spectral properties of the as-synthesized BiOX (X = Cl, Br, I) were analyzed by various characterization techniques. The band gap and thermal stability of the as-synthesized BiOX (X = Cl, Br, I) decreased with increased X atomic numbers. BiOCl and BiOBr exhibited remarkable photocatalytic activity, and BiOI has almost no activity. The BiOBr nanoplates exhibited the highest photocatalytic activity because of the favorable factors of ultrathin nanoplates, layered structures, relatively high surface area, suitable band structure, and high visible-light absorption. The present work demonstrated that the BiOBr single-crystal nanoplates were efficient photocatalytsts for potential large-scale environmental application.
2. EXPERIMENTAL SECTION 2.1. Synthesis of BiOX (X = Cl, Br, I). All chemicals used in this study were analytical grade and were used without further treatment. The preparation process was conducted at room temperature. In a typical synthesis, 0.01 mol of KX (X = Cl, Br, I) was dissolved in 100 mL of H2O, and 0.01 mol of Bi(NO3)3·5H2O was dissolved in 100 mL of an aqueous solution containing 9 mL of acetic acid (HAc) and vigorously stirred for 30 min. The Bi(NO3)3 solution was added dropwise into the KX (X = Cl, Br, I) solution and stirred magnetically for 1 h at room temperature. The resulting suspension was aged for 4 h. The precipitates (BiOCl, BiOBr, and BiOI) were collected by filtration, washed thoroughly four times with distilled water and ethanol, and then dried at 60 °C overnight to get the final samples. The weights of BiOCl, BiOBr, and BiOI were 2.42, 2.85, and 3.31 g, respectively. 2.2. Characterization. The crystal phases of the samples were analyzed by X-ray diffraction with Cu Kα radiation (XRD; model D/max RA, Rigaku Co., Japan). The morphologies of the obtained products were characterized by scanning electron microscopy (SEM; JEOL model JSM-6490, Japan). The morphologies and structures of the samples were further examined by transmission electron microscopy (TEM; JEM2010, Japan). The UV−vis diffuse-reflectance spectra were obtained for the dry-pressed disk samples using a Scan UV−vis spectrophotometer (UV−vis DRS; UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly, using BaSO4 as the reflectance sample. Nitrogen adsorption−desorption was conducted on a nitrogen adsorption apparatus (ASAP 2020, USA). The thermal stability was detected using thermogravimetric analysis (TG-DSC; Netsch STA 449F3) under a N2 gas atmosphere. All of the samples were degassed at 50 °C prior to the measurements. 2.3. Visible-Light Photocatalytic Performance. The photocatalytic activity was investigated by the removal of NO at ppb levels in a continuous-flow reactor at ambient temperature. The volume of the rectangular reactor, which was made of poly(methyl methacrylate) plastics and covered with QuartzGlass, was 4.5 L (30 cm × 15 cm × 10 cm). A 100 W commercial tungsten halogen lamp (General Electric) was vertically placed outside and above the reactor. Four minifans were used to cool the flow system. An adequate distance was also kept from the lamp to the reactor for the same purpose, to keep the temperature at a constant level. For the visible-light photocatalytic activity test experiment, a UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. The schematic illustration of the reacting apparatus is shown in Figure S1 in the Supporting Information (SI; MFC = mass flow controller).
3. RESULTS AND DISCUSSION 3.1. Phase and Morphology. The purity and crystallinity of the as-synthesized BiOCl, BiOBr, and BiOI were investigated by powder XRD analysis. Figure 1 shows that all of the diffraction peaks for each sample can be perfectly indexed to the tetragonal phase of BiOCl (JCPDS 06-0249), BiOBr (JCPDS 73-2061), and BiOI (JCPDS 73-2062), respectively.
Figure 1. XRD patterns of the as-synthesized BiOCl (a), BiOBr (b), and BiOI (c). 6741
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No other diffraction peaks were detected, indicating that all of the as-synthesized samples have high purity. The intensive and sharp diffraction peaks imply that the as-synthesized samples are well-crystallized. The strongest diffraction peaks of BiOCl (Figure 1a), BiOBr (Figure 1b), and BiOI (Figure 1c) correspond to the (001) planes. The formation of BiOX (X = Cl, Br, I) crystals originates from the chemical reactions between the starting compounds following eqs 5−7.26 Bi(NO3)3 + H 2O → BiONO3 + 2H+ + 2NO3−
(5)
KX → K+ + X−
(6)
−
BiONO3 + X → BiOX +
NO3−
(7)
For BiOCl, the layered structures are composed of a large quantity of smooth nanoplates stacked together (Figure 2a). Figure 3. SEM (a and b), TEM (c), and HRTEM (d) images and SAED patterns (inset of c) of the as-synthesized BiOBr.
Figure 2. SEM (a and b), TEM (c), and HRTEM (d) images and SAED patterns (inset of c) of the as-synthesized BiOCl.
Further observation (Figure 2b) indicates that the nanoplates are 72−108 nm in thickness. Figure 2c shows that the size of nanoplates is between 210 and 570 nm across. The SAED pattern (inset in Figure 2c) indicates good crystalline and single-crystal nanoplates. The high-resolution TEM (HRTEM) image (Figure 2d) reveals clear lattice fringes of a single nanoplate with a d spacing of 0.26 nm, which is consistent with the (200) plane. For BiOBr, the aggregation of smooth nanoplates contributed to formation of the layered structure (Figure 3a), and the thickness of a single nanoplate is about 58 nm (Figure 3b). A typical image (Figure 3c) implies that there are a large number of relatively regular nanoplates, with the sizes of nanoplates being 220−620 nm. The SAED pattern (inset in Figure 3c) confirms the single-crystal nanoplate and wellcrystallized structure. Further observation in Figure 3d shows clearly that the interplanar distance of the lattice is 0.34 nm, corresponding to the (011) plane of BiOBr. For BiOI, a large number of ultrathick smooth nanoplates constructed the layered BiOI morphology (Figure 4a) and are 158−216 nm in thickness (Figure 4b). The TEM image reveals the BiOI nanoplates to be ultrathick and ultrabig (the size is between 900 and 1200 nm), in agreement with SEM observation. The SAED pattern of an individual BiOI nanoplate (inset in Figure 4c) proves the single-crystal and good crystalline nature. Further observation by an HRTEM image
Figure 4. SEM (a and b), TEM (c), and HRTEM (d) images and SAED patterns (inset of c) of the as-synthesized BiOI.
(Figure 4d) indicates that the d spacing of a single nanoplate is 0.27 nm, which corresponds well with the lattice spacing of the (110) plane. The internal structure of (BiO)22+ layers interleaved by double slabs of X− atoms guided the growth of BiOX at a certain axis to form 2D nanoplate morphology.8 The assynthesized BiOCl, BiOBr, and BiOI samples are composed of single-crystal nanoplates with good crystallinity. The theoretical values of the lattice parameters (V/Å3) for BiOCl, BiOBr, and BiOI were 0.907, 0.920, and 0.926, respectively.27 The increase in the size of the nanoplates can be ascribed to the increase in the lattice parameters, bond angles, and interatomic distances with the growth of the X atomic number.28 3.2. Band-Gap Structure. The UV−vis DRS spectra of assynthesized BiOCl, BiOBr, and BiOI are shown in Figure 5a; BiOCl exhibits strong absorption in the ultraviolet region, and BiOBr and BiOI show visible-light absorption. The band gaps of as-synthesized BiOCl, BiOBr, and BiOI (Eg) estimated from the intercept of the tangents to the plots of (Ahν)1/2 versus photoenergy (Figure 5b) are 3.20, 2.76, and 1.77 eV, respectively. This fact indicates that the band gap of BiOX (X = Cl, Br, I) decreases with increasing X atomic number. 6742
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Figure 5. UV−vis DRS (a) and plots of (Ahν)1/2 versus photoenergy (b) of the as-synthesized BiOCl, BiOBr, and BiOI.
However, the band gap of the BiOCl nanoplates is narrower than the theoretical value (3.46 eV).29 It is well-known that the micro/nanostructure and morphology of the semiconductor have an important influence on the optical properties. In order to further investigate the effect of the band structure on the activity of the as-synthesized samples, the positions of the conduction band (CB) and valence band (VB) edges were calculated using a simple method. The CB edge (ECB) of a semiconductor at the point of zero charge (pHzpc) can be predicted by the equation ECB = X − EC − 1/2Eg,30 where X is the absolute electronegativity of the semiconductor, EC is the energy of free electrons on the hydrogen scale (∼4.5 eV), and Eg is the band-gap energy of the semiconductor. The calculated bottom of the CBs and top of the VBs of BiOCl, BiOBr, and BiOI are listed in Table 1. According to this result, a schematic Figure 6. Schematic illustration of the band-gap structures of BiOCl, BiOBr, and BiOI.
Table 1. Absolute Electronegativity, Calculated CB Edge, Calculated VB Position, and Band-Gap Energy for BiOCl, BiOBr, and BiOI at the pHzpc semiconductor
absolute electronegativity X (eV)
calcd CB position (eV)
calcd VB position (eV)
band-gap energy Eg (eV)
BiOCl BiOBr BiOI
6.34 6.18 5.99
0.24 0.30 0.61
3.44 3.06 2.38
3.20 2.76 1.77
exothermic peak at 852.3 °C may be caused by further decomposition of the generated products.31 This fact indicates that the thermal stability of BiOX (X = Cl, Br, I) decreases with increasing X atomic number (see Table 2), which might originate from the cocontribution of the lattice parameters and interatomic binding energies. 3.4. Visible-Light Photocatalytic Removal of NO in Air. The photocatalytic performance of the as-synthesized samples was evaluated by the removal of NO in the gas phase in order to demonstrate their potential ability for indoor air purification under visible-light irradiation at room temperature (Figure 8). Figure 8a shows variation of the NO concentration (C/C0 %) with the irradiation time over the samples under visible-light irradiation (λ > 420 nm). Here, C0 is the initial concentration of NO and C is the concentration of NO after photocatalytic reaction for time t. As shown in Figure 8a, the NO concentration for all samples decreased rapidly because of photocatalytic degradation in 5 min. However, the reaction intermediates and final products generated during irradiation may occupy the active sites of the photocatalyst, which results in a decrease in activity. As shown in eqs 1−4, the photocatalytic oxidation of NO to NO3− was the major process with NO2 as an intermediate oxidation product, and most of the NO was oxidized to the final product HNO3. The NO2 concentration during photocatalytic oxidation is monitored, as shown in Figure S2 in the SI. Figure S2 in the
illustration of the band-gap structures for the samples can be drawn and shown in Figure 6. As can be seen, the VB edge potential of BiOCl is 3.44 eV, which means that it possesses strong oxidation ability. BiOBr has a band gap of 2.76 eV, which is suitable for visible-light excitation. However, the VBedge potential of BiOX (X = Cl, Br, I) decreases from 3.44 to 2.38 eV, indicating that the oxidation ability is becoming poorer. 3.3. Thermal Stability. Figure 7a shows that no significant weight loss of BiOCl is recorded when the temperature is below 675 °C; the mass of BiOCl sharply decreases from 675 °C with an exothermic peak on the DSC curve, indicating decomposition of BiOCl. As shown in Figure 7b, BiOBr becomes unstable above 585 °C; the DSC curve reveals that decomposition of BiOBr occurred between 585 and 730 °C with an exothermic peak at 680.5 °C. Figure 7c suggests that BiOI starts to decompose at 525 °C with an exothermic peak at 668.5 °C. Further observation illustrates that the second 6743
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Figure 7. TG-DSC curves of the as-synthesized BiOCl (a), BiOBr (b), and BiOI (c).
SI shows that NO can be oxidized to NO2 by the as-synthesized BiOX (X = Cl, Br, I). Further observation indicates that BiOBr exhibits higher oxidation ability for NO2 than BiOCl and BiOI. This fact implies that the diffusion rate of the reaction intermediate of BiOBr is faster than that of BiOCl and BiOI because of the relatively high surface area, which promotes NO2 to be converted to the final HNO3. After 40 min of irradiation, the ratio of exit-to-feed NO concentration of BiOCl is 11.8%. It is well-known that BiOCl
Table 2. Physicochemical Properties of the As-Synthesized BiOCl, BiOBr, and BiOI sample
SBET (m2 g−1)
pore volume (cm3 g−1)
thermal stability (°C)
k (min−1)
k′ min−1/(m2 g−1)
BiOCl BiOBr BiOI
5 11 9
0.015 0.023 0.029
