Visible Light-Induced Efficient Contaminant Removal by Bi5O7I

Feb 13, 2009 - A new visible light-driven photocatalyst, Bi5O7I, prepared by a hydrothermal method was studied. The as-prepared Bi5O7I exhibited effic...
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Environ. Sci. Technol. 2009, 43, 2005–2010

Visible Light-Induced Efficient Contaminant Removal by Bi5O7I SONGMEI SUN, WENZHONG WANG,* LING ZHANG, LIN ZHOU, WENZONG YIN, AND MENG SHANG State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China

Received November 22, 2008. Revised manuscript received January 15, 2009. Accepted January 21, 2009.

A new visible light-driven photocatalyst, Bi5O7I, prepared by a hydrothermal method was studied. The as-prepared Bi5O7I exhibited efficient photocatalytic activity in the decomposition of a widely used dye, tetraethylated rhodamine (RhB), in water and acetaldehyde (CH3CHO) in air under visible light irradiation. Besides decoloring, the reduction of chemical oxygen demand concentration was also observed in the degradation of RhB, further demonstrating the photocatalytic performance of Bi5O7I. The results of density functional theory calculations indicated that the conduction band bottom of Bi5O7I is mainly composed of Bi 6p orbits, and the valence band top primarily consists of I 5p and O 2p orbits. The as-prepared Bi5O7I exhibited much higher photocatalytic activity than Bi2O3, which may be ascribed to the hybrid states of the valence bands as well as the internal electric fields between Bi5O7 and I slabs. According to experimental results, a possible photocatalytic mechanism of Bi5O7I was proposed.

1. Introduction In recent years, a large number of investigations have focused on the semiconductor-based photocatalysts because of their wide applications in solar energy conversion and environmental purification (1-3). In view of the efficient utilization of visible light, which accounts for a large proportion of the solar spectrum and artificial light sources, the discovery of an active visible light-driven photocatalyst often attracts much attention. As is well-known, the widely used photocatalyst TiO2 is only active in the ultraviolet (UV) light range. Although modification of TiO2 by doping or ion-implanting methods has been used to obtain visible light-driven photocatalysts (4-8), dopants usually also act as recombination centers between the photogenerated electrons and holes (9). Furthermore, doped materials often suffer from thermal instability (4), and the doping process may require expensive ion implantation equipment (10). Accordingly, many researchers have focused their efforts on the design and development of undoped, single-phase oxide photocatalysts working under visible light illumination (11-13). The fundamental principles of photocatalysis, which include the generation of photogenerated electron-hole pairs in the semiconductor and the separation and utilization of charge carriers, have been extensively studied to guide the exploration of visible light-driven photocatalysts. It was reported * Corresponding author fax: +86-21-5241-3122; e-mail: wzwang@ mail.sic.ac.cn. 10.1021/es8032814 CCC: $40.75

Published on Web 02/13/2009

 2009 American Chemical Society

that the generation of electron-hole pairs is due to the excitation of photoelectrons from the valence band to the conduction band in the semiconductor. This process is strongly associated with electronic structures of the semiconductors (14, 15). Some active visible light-driven photocatalysts with special electronic structures were recently reported (11, 16-19), which indicate the electronic structure of a photocatalyst plays an important role in determining photoabsorption and photocatalytic activity. For example, oxide semiconductors usually have low visible light-driven photocatalytic activity for their deep and localized valence bands, which were mainly formed by O 2p levels. Recently, Kudo et al. found that bismuth-based oxide semiconductors are potential candidates for highly active photocatalysts because the Bi 6s and O 2p levels form largely dispersed hybridized valence bands (11, 20) that favor the mobility of photoproduced holes and are beneficial to the oxidation reaction (21, 22). These findings demonstrate that, with the addition of certain elements into crystal structures to obtain highly active visible light-driven photocatalysts, the valence band control method is an experimentally feasible and interesting research subject. In the present paper, we report a new photocatalyst, Bi5O7I, which is active in the photocatalytic oxidative decomposition of a widely used dye, tetraethylated rhodamine (RhB), in water, and acetaldehyde (CH3CHO) in air under visible light irradiation. Reduction of chemical oxygen demand (COD) concentration was observed in the degradation of RhB, further demonstrating the photocatalytic performance of Bi5O7I. The photodegradation rate of RhB on Bi5O7I is much higher than that on Bi2O3 and the widely used photocatalyst TiO2 under the same conditions. Density functional theory (DFT) calculations revealed that the highly active photocatalytic performance is closely related to the introduction of I 5p orbits to the valence bands, which largely dispersed the valence bands and increased the mobility of photogenerated holes. This successful example of the valence band control method to obtain highly active photocatalysts using Bi5O7I may inspire studies of other highly active photocatalysts in oxide semiconductors.

2. Experimental Section 2.1. Sample Preparation. Bi5O7I was prepared by a hydrothermal method. All chemicals are of analytic purity and used without further purification. In a typical process, 2.425 g of Bi(NO3)3 · 5H2O was dissolved in 5 mL of HNO3 (4 M). A 2 M NaOH aqueous solution was added dropwise to adjust the pH value of the solution to 11.5 under vigorous stirring, and a white suspension formed. Then 2 mL of 1 M KI aqueous solution was added to the suspension. After being stirred for 30 min, the suspension was added to a 50 mL Teflon-lined autoclave up to 70% of the total volume. The autoclave was sealed in a stainless steel tank and heated at 150 °C for 18 h. Subsequently, the reactor was cooled to room temperature naturally. The resulting sample was collected, washed with deionized water, and dried at 50 °C for 24 h in air. For comparison, monoclinic nanocrystalline Bi2O3 was prepared by a sonochemical method according to our previous study (23). P25 (nanoscale TiO2 powder, surface area 50 m2 g-1) was purchased from Degussa AG of Germany. 2.2. Characterization. The purity and crystallinity of the as-prepared samples were characterized by powder X-ray diffraction (XRD) on a Japan Rigaku Rotaflex diffractometer using Cu KR radiation, while the voltage and electric current were held at 40 kV and 100 mA, respectively. Scanning electron microscope (SEM) characterizations were performed VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XRD pattern of the as-prepared Bi5O7I sample. on a Shimadzu EPMA-8705QH2 electron probe microanalyzer. Transmission electron microscope (TEM) analyses were performed by a JEOL JEM-2100F field emission electron microscope. The UV-visible diffuse reflectance spectrum (DRS) of the sample was measured using a Hitachi U-3010 UV-visible spectrophotometer. Nitrogen adsorption-desorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer. The Brunauer-EmmettTeller (BET) surface area was estimated using adsorption data. 2.3. Electronic Structure Calculation. First-principles calculations were carried out using the all electron Blo¨chl’s projector augmented wave (PAW) approach (24, 25) within the generalized gradient approximation (GGA), as implemented in the highly efficient Vienna ab initio simulation package (VASP) (26, 27). The k-point meshes for Brillouin zone sampling were constructed using the Monkhorst-Pack scheme (28). A plane wave cutoff energy of 450 eV was used. 2.4. Photocatalytic Test. Photocatalytic activity of the Bi5O7I sample was evaluated by the degradation of RhB and acetaldehyde under visible light irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. For the degradation of RhB, the reaction cell was placed in a sealed black box of which the top was opened, and the cutoff filter was placed to provide visible light irradiation. In each experiment, 0.1-0.2 g of photocatalyst was added into 100 mL of RhB solution (1 × 10-5 to 2 × 10-4 mol/L). Before illumination, the solution was stirred for 120 min in the dark in order to reach adsorption-desorption equilibrium between the photocatalyst and RhB. At 10 min intervals, a 4 mL suspension was sampled and centrifuged to remove the photocatalyst particles. Then the adsorption UV-visible spectrum of the centrifugated solution was recorded using a Hitachi U-3010 UV-visible spectrophotometer. Chemical oxygen demand (COD) was estimated before and after the treatment using the K2Cr2O7 oxidation method. For the degradation of CH3CHO, 0.25 g of the as-prepared photocatalyst was placed at the bottom of a gas-closed reactor at room temperature (capacity 1 L). This reactor is made of glass and has a quartz window. The reaction gas mixture (1 atm) consisted of 100 ppm CH3CHO and N2 balance gas. Prior to irradiation, the reaction system was equilibrated for about 120 min until no changes in the concentrations of acetaldehyde and CO2 were monitored. Gaseous samples (1 mL) were periodically extracted and analyzed by a gas chromatograph (GC) equipped with a flame ionization detector (N2 carrier) and a catalytic conversion furnace.

3. Results and Discussion 3.1. Characterizations of the Bi5O7I Sample. Figure 1 shows the XRD pattern of the Bi5O7I photocatalyst. It is found that the photocatalyst is well-crystallized in a single phase. All of the diffraction peaks can be well-indexed to JCPDS 40-0548, 2006

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FIGURE 2. Typical diffuse reflection spectrum of the as-prepared Bi5O7I sample. The inset is the ahν-hν curve. which is the most stable phase among bismuth iodide oxides (29, 30). No other possible impurities can be detected. After refinement, the cell constants of Bi5O7I were calculated to be a ) 16.26 Å, b ) 5.35 Å, and c ) 11.50 Å, which are consistent with JCPDS 40-0548. The as-prepared Bi5O7I crystallizes in space group C with the monoclinic crystal structure, as shown in Figure S1 of the Supporting Information. The structure exhibits a layered configuration. The Bi5O7 and I slices are orderly and piled up one-by-one along the c-axis into the unique layered structure. Permanent static electric fields between Bi5O7 and I layers may work as the accelerators for the separation of electron-hole pairs upon photoexcitation and may favor the highly photocatalytic efficiency of Bi5O7I. The morphology of the sample was investigated by SEM and TEM (Figure S2 of the Supporting Information). Images reveal the as-prepared Bi5O7I exhibits a rodlike morphology, where the length and diameter of the rods varied over the ranges of 7-25 and 0.3-1 µm, respectively. The selected area electron diffraction (SAED) pattern recorded at one microrod exhibits a regular and clear square diffraction spot array revealing the single-crystal nature of the microrod. Optical absorption of the as-prepared Bi5O7I sample was measured using an UV-visible spectrometer. As shown in Figure 2, the Bi5O7I sample has photoabsorption from UV light to visible light, and the wavelength of the absorption edge is 430 nm. For a crystalline semiconductor, the optical band gap is determined by the following equation using the optical absorption data near the band edge (15, 31) ahν ) A(hν - Eg)n ⁄ 2

(1)

where a, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, n decides the characteristics of the transition in a semiconductor, i.e., direct transition (n ) 1) or indirect transition (n ) 4). The values of n and Eg were determined using the following steps: (1) plot ln(ahν) versus ln(hνEg), using an approximate value of Eg, and then determine the value of n with the slope of the straightest line near the band edge, (2) plot (ahν)2/n versus hν, and then evaluate the band gap Eg by extrapolating the straightest line to the hν axis intercept. Following this method, the value of n for Bi5O7I was estimated to be 2. This means that the optical transitions for Bi5O7I are indirectly allowed, which is in good agreement with the following discussion on band structure, which states that Bi5O7I is an indirect gap semiconductor. The value of the band gap for the catalyst was estimated to be 2.87 eV. 3.2. Photocatalytic Performance. RhB, which showed a major absorption band at 553 nm, was chosen as a repre-

FIGURE 3. (a) UV-visible spectral changes of RhB (1 × 10-5 M) in an aqueous Bi5O7I dispersion as a function of irradiation time under visible light illumination (catalyst, 0.1 g). (b) Photodegradation efficiencies of RhB (1 × 10-5 M) as a function of irradiation time by different photocatalysts (catalyst, 0.1 g). (c) Kinetic linear simulation curve of RhB (1 × 10-5 M) photocatalytic degradation with Bi5O7I, Bi2O3, and TiO2. (d) Variation of COD and transmittance of RhB (2 × 10-4 M) aqueous solutions with irradiation time (catalyst, 0.2 g). Inset: UV-visible spectral changes of a RhB (2 × 10-4 M) aqueous solution as a function of irradiation time. sentative model pollutant to evaluate the photocatalytic performance of as-prepared Bi5O7I. Visible light irradiation of the aqueous RhB/Bi5O7I suspension led to an apparent decrease of RhB. Figure 3a displays the temporal evolution of the spectral changes during the photodegradation of RhB over the Bi5O7I sample. A rapid decrease of RhB absorption at a wavelength of 553 nm was observed, along with absorption band shifts to shorter wavelengths. Similar hypsochromic shifts, which are caused by N-demethylation of RhB during the photodegradation, had been observed in the RhB/TiO2 system reported by Zhao et al. (32). The sharp decrease and shift of the major absorption band within 20 min indicate that the as-prepared Bi5O7I sample exhibits high photocatalytic activity in the degradation of RhB. The photodegradation efficiencies of RhB mediated by different photocatalysts, as well as without photocatalysts (photolysis of RhB), under visible light illumination with otherwise identical conditions are displayed in Figure 3b. It demonstrates that photolysis of RhB is extremely slow without a photocatalyst under visible light illumination. However, with Bi5O7I as the photocatalyst, 100% of RhB is decolorized after 60 min, showing the excellent photocatalytic activity of Bi5O7I under visible light irradiation. The adsorption of RhB on the Bi5O7I sample in the dark was also checked. After 60 min, the concentration of RhB decreased 4% only, suggesting the decolorizing of RhB by Bi5O7I microrods is mainly caused by photodegradation but not adsorption. For comparison, the photocatalytic properties of Bi2O3 and TiO2 were also tested. After irradiation by visible light for 60 min, the degradation rate of RhB by P25 was only 14% (Figure 3b), which is the lowest among these photocatalysts. Under the same conditions, the degradation rate of RhB by Bi2O3 was 35%, also less efficient than that of Bi5O7I. The BET surface area of asprepared Bi5O7I and Bi2O3 was 2.87 and 8.64 m2 g-1,

respectively. Although the BET surface area was much lower than that of Bi2O3, the Bi5O7I sample exhibits much higher photocatalytic activity. This directly demonstrates the iodine ion in the crystal structure is advantageous for high photocatalytic activity. To quantitatively understand the reaction kinetics of the RhB degradation in our experiments, we applied the Langmuir-Hinshelwood model as expressed by eq 2, which is well-established for photocatalysis experiments when the pollutant is in the millimolar concentration range (33, 34). R)-

krKC dC ) krθ ) dt (1 + KC)

(2)

where R is the reaction rate, kr is the reaction rate constant, θ is the surface coverage, K is the adsorption coefficient of the reactant, and C is the reactant concentration. When C is very small, the product KC is negligible with respect to unity so that eq 2 describes first-order reaction kinetics. Setting eq 2 at the initial conditions of the photocatalytic procedure, t ) 0, the concentration transforms to C ) C0, which gives eq 3. -ln

C ) kt C0

(3)

where C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the apparent first-order rate constant. A kinetic curve of RhB photocatalytic degradation based on the data plotted in Figure 3b is shown in Figure 3c. It is clear that the curve with time (t) as abscissa and ln(C0/C) as the vertical ordinate is close to a linear curve, which indicates the photocatalytic degradation of RhB using Bi5O7I follows first-order reaction kinetics. The values of k for Bi5O7I, Bi2O3, and TiO2 are 0.0547, 0.0085, and 0.0026 VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Photocatalytic activity of Bi5O7I for the degradation of acetaldehyde in air under visible light. min-1, respectively, indicating a preferable photocatalytic performance of Bi5O7I. As the reduction of COD reflects the extent of degradation or mineralization of an organic species, the percentage change of COD in the photodegradation of RhB was studied as a function of irradiation time of visible light, as shown in Figure 3d. The initial COD concentration of the RhB solution (95.8 mg/L) is 174.6 mg/L, and the T% (measured at 500 nm) is 0.8%. After visible light irradiation for 4 h, the COD concentration decreased to 92.76 mg/L, and the T% at 500 nm reached 91%. The reduction of COD and the increase of the T% further confirm that RhB was truly photodegraded by photocatalyst Bi5O7I. In the present study, Bi5O7I also showed poor photocatalytic activity when the light wavelength was longer than 500 nm. As shown in Figure S3 of the Supporting Information, about 20% of RhB was photodegraded after 60 min of irradiation, although it could not be activated by such light as revealed by the UV-visible diffuse reflection spectrum (Figure 2). Under this condition, the degradation of RhB may be ascribed to a photosensitization process, which has been reported in the study of photocatalysts TiO2 (35, 36), Bi2WO6 (18), etc. Although the degradation of RhB resulted from photocatalysis with the assistance of photosensitization, the former is the predominant process, which could be concluded from Figure S3 of the Supporting Information. To further confirm and reveal the photocatalytic properties of as-prepared Bi5O7I, acetaldehyde (CH3CHO), which has no light absorption, was selected to evaluate the photocatalytic activity. It was found that CH3CHO was degraded by Bi5O7I with an obvious production of CO2 under visible light irradiation, as shown in Figure 4. Different from RhB, CH3CHO does not absorb light; thus, the photosensitization process did not exist in such a photodegradation process. As a result, the degradation of CH3CHO was fully attributed to the photocatalytic process, indicating that Bi5O7I is indeed a visible light-induced photocatalyst. 3.3. Electronic Structure. Ab initio calculations were performed to evaluate the electronic structures of Bi5O7I. Panels a and b of Figure 5 show the densities of states and band structures of Bi5O7I calculated by VASP. As shown in Figure 5a, the lowest unoccupied state lies at the G point, while the highest occupied state is at the F point. This means that Bi5O7I is an indirect gap semiconductor. In other words, the excited electrons have to travel a certain k-space distance to be emitted to the valence band. This reduces the recombination opportunity of the excited electrons and holes, which benefits the hole-electron separation and the charge transport. The second highest occupied state is at the F point. 2008

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FIGURE 5. (a) Calculated band structures for Bi5O7I by GGA along the high-symmetry axes of the Brillouin zone. (b) Total DOS and partial DOS of Bi5O7I obtained by GGA.

SCHEME 1. Photocatalytic Mechanism for Bi5O7I

This means that direct transition from F to F is allowed, which is the reason for n ) 2 in eq 1. The band gap of Bi5O7I was estimated to be 1.68 eV. Generally, the band gap calculated by DFT is smaller than that obtained experimentally, which is frequently pointed out as a common feature of DFT calculations (37). Figure 5b shows the partial and total densities of states of Bi5O7I. It is clear that the conduction band bottom mainly consists of Bi 6p orbits, while the valence band top mainly consists of I 5p, O 2p, and Bi 6s orbits. Compared with Bi2O3, of which the valence band top consists of Bi 6s and O 2p orbits (23), the valence band of Bi5O7I is largely more dispersed by I 5p orbits, which means that the photogenerated holes have a small effective mass. This probably explains the higher photocatalytic activity in the

presence of Bi5O7I compared with that in the presence of Bi2O3, although Bi2O3 has a smaller band gap of about 2.85 eV (23). 3.4. Visible Light-Induced Degradation Mechanism. According to the above experimental results, without regard to the photosensitization and photolysis process, a possible photocatalytic mechanism (Scheme 1) of the catalyst was proposed as follows. The first step: Bi5O7I + hν f Bi5O7I (e- + h+)

(4)

The second step: O2- was produced through the oxidation process of catalyst (Bi5O7I) combined with molecular oxygen dissolved in the aqueous solution or in the atmosphere. •

Bi5O7I (e-) + O2 f Bi5O7I + •O2-

(5)

The third step: • O2- can be transformed into H2O2 in aqueous solution or in the atmosphere by the water molecule adsorbed on the catalyst surface. 2•O2- + 2H2O f 2OH- + H2O2 + O2

(6)

The forth step: H2O2 + Bi5O7I (e-) f •OH + OH- + Bi5O7I

(7)

The fifth step: RhB (acetaldehyde) + •OH f degradation product (CO2) (8) It has been reported that hydroxyl radicals are the most important oxidizing species (38-41). Oxidation of RhB and CH3CHO was completed via successive attacks by •OH. Figure S4a of the Supporting Information shows the photodegradation of RhB was significantly restrained by the addition of the radical scavenging agent sodium hydroxide, possibly caused by eliminating radicals. Photodegradation of RhB after 60 min of reaction was 100% without a radical scavenging agent (Figure 3b), while the degradation was restrained to 24% by the addition of sodium hydroxide into the RhB solution. In another experiment, it was found the addition of isopropanol, a well-known scavenger of •OH radicals (42), into the photoreaction system also caused an apparent decrease in the degradation rate of RhB (Figure S4b of the Supporting Information). The results from these studies demonstrated that photocatalytic reaction takes place through the •OH radical mechanism. Figure S4c of the Supporting Information shows the photodegradation of RhB was greatly accelerated by adding H+ to the solution that was adjusted to pH 2.8 with HNO3. The degradation reached 100% within 10 min by adding H+, indicating that most of •O2- was transformed into H2O2 (eq 6).

Acknowledgments This work is financially supported by National Natural Science Foundation of China Grant 50732004, National Basic Research Program of China (973 Program, 2007CB613302), and Nanotechnology Programs of Science and Technology Commission of Shanghai Municipality (0852nm00500).

Supporting Information Available Figure S1 shows the schematic layered structure of Bi5O7I. Figure S2 demonstrates (a) low-magnified and (b) highmagnified SEM images of the as-prepared Bi5O7I sample and (c) low-magnified and (d) high-magnified TEM images of the as-prepared Bi5O7I sample. Inset of panel d of Figure S2

shows a recorded SAED pattern. Figure S3 reveals the photocatalytic degradation of RhB (1 × 10-5 M) over Bi5O7I under different irradiation lights (catalyst, 0.1 g). Figure S4 reveals the photocatalytic degradation of RhB (1 × 10-5 M) over Bi5O7I under different solutions. This information is available free of charge via the Internet at http://pubs.acs.org.

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