ARTICLE pubs.acs.org/JPCC
Visible Light Photocatalysis of BiOI and Its Photocatalytic Activity Enhancement by in Situ Ionic Liquid Modification Yunan Wang,† Kejian Deng,‡ and Lizhi Zhang*,† †
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China ‡ Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan 430074, People’s Republic of China ABSTRACT: In this study, we investigate the photocatalysis mechanism of BiOI in detail and therefore demonstrate that the photocatalytic activity of BiOI could be enhanced greatly by in situ modification of an ionic liquid [Bmim]I (1-buty-3-methylimidazolium iodide). The ionic liquid modified BiOI (IL-BiOI) was prepared by reacting bismuth nitrate with [Bmim]I in water at 70 °C, where the ionic liquid could act as both iodine source and surface modified agent. On degradation of methyl orange (MO) under visible light irradiation (λ > 420 nm), IL-BiOI photocatalyst exhibited superior photocatalytic activity to the unmodified counterpart synthesized in the absence of [Bmim]I. The effects of ionic liquid modification on the photocatalytic activity enhancement were systematically investigated. We found that IL modification could trap the photoexcited electron at conduction band of BiOI to inhibit the recombination of photoinduced electronhole pairs and thus enhance its photocatalytic activity on the degradation of organic pollutants.
1. INTRODUCTION The photocatalytic degradation of pollutants is attracting considerable attention as one of the most promising methods for solving environmental problems.14 Titanium dioxide (TiO2) has been used to degrade many organic pollutants in water and air. It was confirmed by Ollis that •OH radicals predominantly resulted from the oxidation of H2O and OH by positive holes in the valence band were the primary active substance in TiO2 photocatalytic mechanism of the degradation of pollutants under UV irradiation.5 However, TiO2 photocatalysis is effective only upon irradiation of UV-light because of its relatively wide band gap. To overcome this shortcoming, some strategies have been exploited to increase the visible light photocatalytic activity of TiO2, including metal or nonmetal elements doping, sensitization, surface modification and coupling with other semiconductors, etc.14 BiIIIVIA-VIIA compounds bismuth oxyhalides BiOX (X = Cl, Br, and I) evoked great interest recently owing to their unique and excellent electrical, optical, magnetic, and photoluminescence properties. Many methods were developed for the synthesis of various BiOX nanostructures as photocatalysts. For example, three-dimensional (3D) flower-like BiOI hierarchical structures were synthesized by a solution route at room temperature by Zhang and his co-workers.6 Kaskel’s group synthesized BiOX nanoparticles in reverse micelles.7 Yu and his coworkers prepared BiOI with NaBiO3 3 2H2O and HI as the precursors at room temperature in the waterethanol solution.8 Our group reported a general one-pot nonaqueous solgel r 2011 American Chemical Society
method to prepare BiOX (X = Cl, Br, and I) nanoplate micropheres,9 and recently found the photocatalytic activity of BiOI could be significantly enhanced by iodine self-doping.10 Self-assembled three-dimensional BiOI microspheres composed of nanoplatelets were synthesized at low temperature by using ethanolwater mixed solvent as reaction media in the presence of NH3 3 H2O in Zhang’s group.11 Huang and his co-workers synthesized the two coupled photocatalysts xBiOI(1 x)BiOCl and xBiOBr(1 x)BiOI by soft chemical methods.12,13 Yu and his co-workers reported Pt/BiOI nanoplates via a solution combination with photodeposited method.14 Although the preparation methods of BiOX were studied so largely, their photocatalysis mechanism was rarely studied. Yu et al thought that the degradation of PCP-Na could be attributed to the reaction with the photogenerated hole directly rather than • OH radicals and •O2 species during the photocatalysis.8 Obviously, the lacking of detailed photocatalysis mechanism of BiOX strongly hinders the development of their activity enhancement methods. Ionic liquids (IL) have been widely employed as a new kind of reaction media in organic reactions and electrochemical devices due to their unique properties such as extremely low volatility, high ionic conductivity, good dissolving ability, designable structures, and a large electrochemical window.15,16 They have Received: May 6, 2011 Revised: June 21, 2011 Published: June 22, 2011 14300
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The Journal of Physical Chemistry C been already used as solvents, templates, or reactants for the synthesis of inorganic nanomaterials with novel morphologies and improved properties.17 For instance, Wang and his coworkers reported that hydrothermally synthesized ionic liquid [Bmim]OH-modified TiO2 nanoparticles exhibited enhanced photocatalytic activity under visible light.18 In this study, we investigate the photocatalysis mechanism of BiOI in detail and therefore demonstrate that the photocatalytic activity of BiOI could be enhanced by the in situ modification of an ionic liquid (1-buty-3-methylimidazolium iodide) during the synthesis according to its photocatalysis mechanism for the first time. The enhancing effects of ionic liquids are investigated and discussed in detail on the basis of photocatalysis mechanism of BiOI.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. All chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. 1.0 mmol of [Bmim]I (AR, 99.0%) was added slowly into 30 mL of deionized water. The mixture was stirred for 0.5 h to dissolve [Bmim]I completely at 70 °C in the water bath. Subsequently, 1.0 mmol of Bi(NO3)3 3 5H2O (AR, 99.0%) was added slowly into the above-mentioned solution. The resulting solution was kept at 70 °C for 0.5 h, and then naturally cooled to room temperature. The resulting precipitate (IL-BiOI) was collected and washed with deionized water and ethanol thoroughly and dried at 50 °C in air. For the purpose of comparison, BiOI bulk powders were also prepared by a similar chemical precipitation method in the absence of ionic liquid and denoted as P-BiOI. Hierarchical BiOI nanoplate microspheres were synthesized by a simple soft chemical method according to our previous report and denoted as M-BiOI.9 Carbon-doped TiO2 was synthesized by a hydrothermal method previously reported by our group and denoted as C-TiO2.19 2.2. Analysis of Photocatalysis Mechanism. Triethanolamine (TEOA) is an effective hole scavenger, and tert-butyl alcohol (TBA) was chosen as •OH scavenger because it reacts with 3 OH radicals with a high rate constant (k = 6 108).20,21 Typically, 0.1 g catalyst with TEOA or TBA (10 mM) was dispersed in MO aqueous solution (100 mL, 10 mg L1) before visible light irradiation. The concentration of MO during photocatalytic reaction was determined by measuring the absorption of MO solution at 464 nm. The nitroblue tetrazolium (NBT), exhibiting an absorption maximum at 259 nm, was used to determine the amount of •O2 generating from BiOI photocatalytic system.22,23 Photocatalytic reactions were carried out in beakers containing 2 g L1 BiOI aqueous suspensions and 5 105 mol L1 NBT solution. Before the reactions, the suspensions were stirred for 2 h in dark. The production of •O2 in BiOI suspensions was quantitatively analyzed by detecting the concentration of NBT in the BiOI suspensions with UVvis spectrophotometer. 2.3. Characterization. X-ray powder diffraction (XRD) measurements of the samples were performed in the reflection mode (Cu KR radiation, λ = 1.54178 Å) on a Rigaku Ultima III X-ray diffractometer. The morphologies were determined by a scanning electron microscopy (SEM, JEOL 6700-F). The samples for high-resolution transmission electron microscopy (HRTEM) were prepared by dispersing the final powders in ethanol and the dispersion was dropped on carboncopper grids. Then, the obtained powders deposited on a copper grid were observed by a
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high-resolution transmission electron microscope (JEOL JSM2010) operating at 200 kV. FT-IR spectra of the as-prepared samples were collected with a Thermo Nicolet Nexus 670 Spectrometer with a resolution of 4 cm1. Surface electronic states were analyzed by XPS (Perkin-Elmer PHI 5000C, Al KR). All binding energies were referenced to the C1s peak (284.6 eV) of the surface adventitious carbon. UVvis diffuse reflectance spectra (DRS) were obtained using a UVvis spectrometer (Shimadzu UV-2550) and converted from reflection to absorbance by the KubelkaMunk method.24 Nitrogen adsorption desorption isotherms were collected on a Micromeritics Tristar3000 surface area and porosity analyzer at 77 K after the sample had been degassed in the flow of N2 at 180 °C for 5 h. The BET surface area was calculated from the linear part of the BET plot (P/P0 = 0.10.25). 2.4. Photocatalytic Activity Tests. The photocatalytic activities of the as-prepared samples were evaluated by the degradation of MO in an aqueous solution. The initial pH of the MO solution was about 6.4. A 500 W tungsten halogen lamp was positioned inside a cylindrical vessel and surrounded by a circulating water jacket to cool it. A total of 0.1 g of photocatalyst was suspended in a 100 mL aqueous solution of 10 mg L1 MO. The solution was continuously stirred for about 2 h in the dark to ensure the establishment of an adsorptiondesorption equilibrium among the photocatalyst, MO and water before irradiation, then the solution was irradiated under visible light (λ > 420 nm). At irradiation time intervals of 1 h, about 5 mL of suspension was collected, and then the photocatalyst and the MO solution were separated by centrifugation. The concentration of MO was monitored by colorimetry with a Hitachi U-3310 UVvisible spectrophotometer. To rule out the effect of MO dye self-sensitization on the BiOI photocatalysis, four 100 W monochromatic lights (λ = 590 nm) were used as the light source instead of the 500-W tungsten halogen lamp for the degradation of 10 mg L1 MO aqueous solution over BiOI photocatalysts. Moreover, the photocatalysts were also used to degrade 10 mg L1 salicylic acid (SA) in an aqueous solution with a 500 W Xe lamp combined with a cutoff filter (λ > 420 nm) as a visible light source. The initial pH of the SA solution was about 6.0. 2.5. Photoelectrochemical Experiments. Photocurrents collected on an inert electrode (Pt) immersed in aqueous suspensions of photocatalysts were measured as described in Choi’s paper.25 Typically, 0.5 g L1 of IL-BiOI (or P-BiOI), 0.2 M of acetate (electron donor), and 0.5 mM of Fe3+ (reversible electron shuttle) were added in distilled water in a 100-mL glass breaker under visible light illumination. A platinum plate (1 1 cm2), a saturated calomel electrode (SCE), and a graphite rod were immersed in the reactor as the working (collector), reference, and counter electrodes, respectively. Nitrogen gas was continuously purged though the suspension. Photocurrents were collected in the suspension by applying a potential (+0.6 V vs SCE) to the Pt working electrode using a potentiostat (CHI-660C) connected to a computer. A 500 W Xe lamp with a 420 nm cutoff filter was a visible light source. The suspension was magnetically stirred throughout the photocurrent measurements.
3. RESULTS AND DISCUSSION The photocatalysis mechanism of Bi2WO6 was previously reported by Zhu et al.26 They thought that the degradation of RhB on Bi2WO6 did not involve •OH radicals, but depended on 14301
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Figure 1. (a) Photodegradation efficiencies of MO on P-BiOI in the presence of different scavengers under visible light irradiation. (b) Photodegradation of nitroblue tetrazolium (NBT) under visible light irradiation. (c) Schematic illustration of MO photodegradation over P-BiOI.
the direct hole transfer. In the upshift valence band of Bi2WO6 normally formed by the Bi 6s orbital hybridizing with the O 2p orbital,27 holes generated by photoexcitation of Bi3+ are regarded as Bi5+ (or Bi4+). Therefore, the standard redox potential (E° = +1.59 V at pH 0) of Bi2O4/BiO+ (BiV/BiIII) could be thought as a roughly estimated oxidation potential of hole photogenerated in the Bi2WO6 photocatalyst.26 Regarding BiOI shares a similar compound structure with Bi2WO6, the photogenerated holes may also play a major role in BiOI photocatalysis system. From the theoretical viewpoint, the broad valence band of BiOI was composed of the Bi 6s, Bi 5d, O 2s, O 2p, I 5s, and I 5p orbits.28 Therefore, the oxidation potential of hole photogenerated in BiOI photocatalyst (BiV/BiIII) could also be estimated as +1.59 V, higher than the redox potential of MO (1.48 V vs NHE).12 This suggests that the direct hole oxidation process is energetically possible. It is known that the 3 OH radicals could be generated via the hydroxide oxidation by the holes (eq 1)29 or the photogenerated electron-induced multistep reduction of O2 (eqs 24).30 hþ þ OH f • OH
ð1Þ
O2 þ e f • O 2
ð2Þ
•
O 2 þ e þ 2Hþ f H2 O2
ð3Þ
H2 O2 þ e f • OH þ OH
ð4Þ V
III
The standard redox potential of Bi /Bi is more negative than that of •OH/OH (+1.99 V),31 suggesting that the photogenerated holes are incapable of oxidizing adsorbed hydroxyl groups into •OH radicals. Therefore, •OH radicals might only be generated from the photogenerated electron-induced multistep reduction of O2 during BiOI photocatalysis.
To investigate the photocatalysis mechanism of BiOI in detail, we designed and carried out different radicals and holes trapping experiments for P-BiOI. First, triethanolamine (TEOA) was used to trap holes photogenerated in the P-BiOI. As shown in Figure 1a, the degradation efficiency of MO on P-BiOI decreased significantly from 27% to 0% when TEOA was added into the photocatalytic reaction solution, confirming photogenerated holes is mainly responsible for MO photodegradation on P-BiOI. Differently, the photodegradation efficiency of MO was just reduced from 27% to 19% with the addition of •OH scavenger tert-butyl alcohol, revealing that •OH radicals play a minor role on the degradation of MO in comparison with photogenerated holes. It is well-known that the photogenerated electrons are another key factor in photocatalytic process because they could produce superoxide and hydroxyl radicals via eqs 24. We therefore used the degradation of nitroblue tetrazolium (NBT) to determine the amount of •O2 generating from BiOI suspensions (Figure 1b). After 3 h, about 20% of NBT was degraded over P-BiOI under visible light irradiation, confirming the generation of photoelectrons and •O2 in under visible light irradiation. Therefore, we think that •OH radicals generated in BiOI suspensions under visible light are not from photogenerated holes, but from photogenerated electrons. It is known that the power for photogenerated electrons to generate •OH radicals is much weaker than that of photogenerated holes, resulting in small amount of •OH radicals produced in the P-BiOI suspensions under visible light. This is why the photodegradation efficiency of MO just slightly decreased in the presence of •OH scavenger TBA. Therefore, we conclude that direct hole oxidation and the oxidation of •OH radicals produced via e f •O2 f H2O2 f •OH route (eqs 24) are the major and minor ways for the photocatalytic degradation of MO on BiOI (Figure 1c), respectively. Obviously, this photocatalysis mechanism is differently from that proposed by Yu and his co-workers.8 They thought the CB potential of BiOI semiconductor was not 14302
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Figure 2. SEM images, XRD patterns, and HRTEM images of as-prepared BiOI powders. (a, b, e, and f) P-BiOI and (c, d, e, and g) IL-BiOI. (h) Schematic representation of the crystal structure of BiOI.
sufficient negative to reduce the O2 molecules by the photoexcited electron and therefore •OH radicals and •O2 species did not involve the photocatalytic degradation of PCP-Na. The band gap energy of BiOI samples was previously calculated to be 1.75 eV according to experimental lattice parameters.32 Although this narrow band gap character enables BiOI to absorb visible light effectively, it inevitably results in faster recombination of photoinduced electronhole pairs on BiOI than that on wide bang gap semiconductor such as TiO2. According to the above photocatalysis mechanism study results, it is possible to enhance the photocalytic activity of BiOI through trapping photogenerated electron via surface modification to inhibit the recombination of photoinduced electronhole pairs. We therefore develop an in situ ionic liquid method modification to enhance photocatalytic activity of BiOI and choose 1-buty-3methylimidazolium iodide [Bmim]I as the ionic liquid because it could act as both iodine source and surface modified agent. The resulting samples were first examined by SEM. Both P-BiOI and IL-BiOI samples were composed of abundant platelike nanostructures from SEM images (Figure 2ad). The ILBiOI nanoflakes (about 40 nm) were obviously much thinner than those of P-BiOI (about 100 nm), suggesting the presence of ionic liquid could inhibit the growth of BiOI nanoflakes. The
XRD analysis showed that the as-prepared IL-BiOI was better crystallized than P-BiOI at 70 °C (Figure 2e), while the relative intensity of the (002) plane peak in IL-BiOI’s XRD pattern exhibited noticeable reduction comparing with that of P-BiOI. Recently, Zheng and his co-workers synthesized ultrathin BiOCl nanoflakes of about 50 nm by using an ionic liquid [C16Mim]Cl as solvent.33 They found the ab plane favors the preferred adsorption of [C16Mim]+ cations, resulting retarding crystal growth in the [001] direction during BiOCl synthesis and thus forming thinner BiOCl nanoflakes, similar to the case in this study. All the clear lattice fringes of the interplane were about 0.28 nm in the HRTEM images of IL-BiOI and P-BiOI, which were in accordance with {110} planes of the tetragonal system of BiOI (Figure 2fg). As shown in Figure 2h, BiOI contains a layered [IBiOBiI] structure of alternate [Bi2O2] sheets and the I slabs, stacking together by the nonbonding interaction through the I atoms to form a [Bi2O2I2] layer along the c axis.34,35 In unit cell of BiOI, each Bi atom center is eight-coordinated by four O atoms and four I atoms in the form of an asymmetric decahedral geometry. Decahedra cells are linked to each other by common OI edges along the a and b axes forming infinite layers (001) and neighboring layers of decahedron are connected by common OO or II edges.33 The strong intralayer bonding 14303
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Figure 3. Plots of (Rhν)1/2 vs the energy of absorbed light and UVvis diffuse reflectance spectra (inset).
Figure 4. FT-IR spectra of P-BiOI, IL-BiOI and ionic liquid.
and the weak interlayer van der Waals interaction results in the formation of platelet morphology.11 The HRTEM observation also confirmed that these BiOI nanoplates were perpendicular to the c axis, suggesting that [Bmim]+ cations could be adsorbed on the (001) plane of BiOI via the coulomb coupling force with the iodine to effectively inhibit crystalline growth in the [001] direction and produce much thinner BiOI nanoflakes, as revealed by the XRD patterns and SEM images. UVvis diffuse reflectance spectra showed that IL-BiOI had an absorption edge up to about 650 nm (Figure 3). Interestingly, it possessed a higher UVvis absorbance than P-BiOI in the range of 200 to 600 nm, which is similar to fluorinated TiO2 reported by Choi et al.36 The band gap energy of IL-BiOI was approximately 1.77 eV, slightly larger than 1.72 eV for P-BiOI. The slight enlarged band gap energy of IL-BiOI might be attributed to small size effect because its nanoflake thickness is much thinner than that of P-BiOI.37 In order to verify the existence of IL on the surface of BiOI, ILBiOI was further characterized by means of FT-IR spectroscopy (Figure 4). A band corresponding to valent symmetrical A2u-type vibrations of the BiO bond (505 cm1)38 in P-BiOI and ILBiOI was observed. In the spectra of P-BiOI, IL-BiOI, and IL, the peaks at around 1630 and 3450 cm1 could be attributed to the bending and stretching vibrations of the hydroxyl group, respectively.39 Moreover, these peaks of IL-BiOI were smaller than those of P-BiOI, implying that there were less hydroxyl groups adsorbed on the surface of IL-BiOI than that of P-BiOI. This suggests ionic liquid in situ modified on IL-BiOI could restrict the adsorption of surface hydroxyl groups. Interestingly, several peaks were only observed in the IR spectra of IL-BiOI but absent in the spectrum of P-BiOI. Among these peaks, the one at 3146 cm1 corresponded to the CC bond in positions four and five of the imidazolium ring.40 The other two peaks around 2859 and 2926 cm1 could be assigned to the symmetric and asymmetric stretch of the HCH bond in butyl group. The strong peak in the 1380 cm1 region was consistent with the typical stretching modes of CN heterocycles.41 Meanwhile, the peaks at 1170, 815, and 714 cm1 were observed in the spectra of IL-BiOI and [Bmim]I but not found in that of P-BiOI, which are due to in plane deformation and out of plane deformation of CH bond.42
These IR spectrum difference confirmed that ionic liquid was modified onto the surface of IL-BiOI. The surface element composition of the IL-BiOI sample was then studied by X-ray photoelectron spectroscopy (XPS; Figure 5). The XPS survey spectrum revealed the existence of Bi, O, I, and C elements in IL-BiOI (Figure 5A). In Figure 5B, the binding energies at 158.4 and 163.7 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, which was characteristic of Bi3+ in BiOI.42 The C 1s of IL-BiOI was composed of two peaks at 284.6 and 286.2 eV (Figure 5C). They were respectively attributed to the surface adventitious carbon and CN groups of imidazolium, respectively.43,44 The binding energies of I 3d3/2 (629.2 eV) and I 3d5/2 (617.7 eV) of IL-BiOI shifted slightly comparing with those of P-BiOI (Figure 5D), which displayed binding energy of around 629.6 eV (I 3d3/2) and 618.1 eV (I 3d5/2) in good agreement with the state of I in the sample which is 1 valence.14 This slight shift might result from the coulomb coupling force between [Bmim]+ adsorbed on the BiOI surface and I, causing the electrons to transfer from Bi to I. The O1s core level spectra of both P-BiOI and IL-BiOI could be fitted by two peaks at binding energies of around 529.7 and 531.3 eV (Figure 5E), respectively. The dominant peak at about 529.7 eV can be attributed to lattice oxygen in BiOI,45 and the other peak at 531.5 eV may due to the CO, COO, and OH bonds on the surface.46 We assumed that the peak around 531.3 eV in the O 1s region was mainly attributed to hydroxyl groups adsorbed on the surface of BiOI and calculated abundance of surface hydroxyl groups by using the ratio of peak areas (SOH/S, S stands for the total peak area of O 1s region, and SOH expresses the area of surface hydroxyl groups). The SOH/S ratios were found to be 55.3% and 42.7% for P-BiOI and IL-BiOI, respectively, confirming less surface hydroxyl groups on IL-BiOI. This is consistent with the analysis of the FT-IR spectra. It was reported that TiO2 functionalized with ascorbic acid and IL caused a decrease of the surface hydroxyl groups.18,47 We conclude that imidazolium attached to the surface of IL-BiOI could occupy the sites for surface hydroxyl groups, resulting in less surface hydroxyl groups on IL-BiOI. We evaluated photocatalytic activities of the as-prepared P-BiOI and IL-BiOI samples by degradation of MO under visible 14304
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Figure 5. XPS survey spectrum of the as-prepared BiOI samples (A). The high-resolution XPS spectra of (a) P-BiOI and (b) IL-BiOI samples in the regions of Bi 4f (B), C 1s (C), I 3d (D), and O 1s (E).
light irradiation (λ > 420 nm) and compared them with M-BiOI and C-TiO2. Figure 6a showed the variation in adsorption of MO at 464 nm with the irradiation time and self-degradation of MO under visible light irradiation. It was found that the self-degradation of MO was negligible under visible light irradiation. However, the degradation of MO became obvious in the presence of photocatalysts. After 3 h of degradation under visible light irradiation, 6%, 27%, 78%, and 87% of MO were photocatalytically degraded on the samples of C-TiO2, P-BiOI, M-BiOI, and IL-BiOI, respectively. The photocatalytic degradation of MO was found to fit pseudofirst-order kinetics. The apparent reaction rate constants were calculated to be 0.569, 0.507, and 0.081 h1 for
IL-BiOI, M-BiOI, and P-BiOI, respectively. Although the BET surface area of IL-BiOI nanoflakes (8.2 m2 g1) was much lower than that of BiOI nanoplate microspheres (22.7 m2 g1) (Table 1), it exhibited a higher photocatalytic activity. In order to rule out the effect of surface areas of the photocatalysts on their photcatalytic activities, we normalized the photocatalytic degradation rates by the surface areas and found that the order for the normalized rates was similar with that of the original rates (Table 1). We therefore conclude that the enhanced photcatalytic activity of IL-BiOI is mainly attributed to its low recombination of photoinduced electronhole pairs by trapping photogenerated electrons via surface ionic liquid modification. 14305
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Figure 6. (a) Photocatalytic degradation of MO in the presence of different photocatalysts under visible light irradiation. (b) Effects of IL on the photocatalytic activity of BiOI. (c) Comparison of UVvis diffuse reflectance spectra of IL-BiOI and MO. (d) Photodegradation of MO on IL-BiOI under monochromatic light source (λ = 590 nm). (e) Photocatalytic degradation of SA in the presence of photocatalysts under visible light irradiation. (f) Photodegradation of MO on IL-BiOI in the presence of different scavengers under visible light irradiation. (g) Photodegradation of nitroblue tetrazolium (NBT) under visible light irradiation. (h) Schematic illustration of MO photodegradation over IL-BiOI.
To evaluate the role of IL on the photocatalysis of BiOI, we compared the photocatalytic performances of IL-BiOI and pure BiOI (P-BiOI) with adding equally excess IL (IL + BiOI) in the solution on the degradation of MO under the same conditions (Figure 6b). It is found that IL + BiOI showed similar photocatalytic performance with P-BiOI, which was much lower than that of IL-BiOI. This suggests that excess IL in the solution has no
positive effect on the photocatalytic activity of BiOI. In other words, the photocatalytic activity enhancement of IL-BiOI is attributed to the in situ ionic liquid modification, not the electrostatic attraction between [Bmim]+ and the negatively charged dye (MO). MO has a series of absorption bands in the 200540 nm region, and UVvis diffuse reflectance spectra showed that BiOI 14306
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Table 1. Summary of Synthetic Approach, Geometry, Textural Properties, Photocatalytic Activities, and Band Gap of BiOI Powders BiOI
methoda
geometry
P-BiOI
CP with KI
nanoplates
IL-BiOI M-BiOI e
CP with IL solgel with KI
nanoflakes nanoplate micropheres
ABET [m2.g1]
k b [h1]
k0 c [g 3 h1m2]
Egd [eV]
3.1
0.081
2.6 102
1.72
8.2 22.7
0.569 0.507
6.94 102 2.23 102
1.77 1.77
a
CP means chemical precipitation. b The reaction kinetics of MO degradation were analyzed with the pseudofirst-order model as expressed by ln(C0/C) = kt, and the k values were normalized with the surface areas, respectively. c The k0 values were k values normalized with the surface areas. d Band gap energy was estimated from the main absorption edges of the UVvis diffuse reflectance spectrum. e The sample was prepared according to ref 9.
had an absorption edge up to about 650 nm (Figure 6c). It is thought that the irradiation of visible light with wavelengths between 540 and 650 nm could eliminate the contribution of MO dye self-sensitization to the overall photocatalytic activity of BiOI. We therefore chose four 100 W monochromatic lights (λ = 590 nm) as the light source instead of visible light with λ > 420 nm for the degradation of 10 mg L1 MO aqueous solution over IL-BiOI photocatalysts and found that about 70% of MO was photocatalytically degraded in 3 h (Figure 6d). The fact of slight lower photocatalytic performance (70%) under monochromatic lights (λ = 590 nm) than that (87%) under visible light with λ > 420 nm revealed that the photocatalytic activity of ILBiOI was mainly attributed to photocatalysis, not to dye selfsensitization. Meanwhile, a colorless organic substance salicylic acid (SA) was also photocatalytically degraded on BiOI photocatalysts under visible light (λ > 420 nm) to confirm their visible light photocatalytic performance (Figure 6e). It was found that the self-degradation of SA was negligible under visible light irradiation. After 4 h of visible light irradiation (λ > 420 nm), about 15% and 46% of SA were photocatalytically degraded on P-BiOI and IL-BiOI, respectively. This further confirms enhanced visible light photocatalytic performance of IL-BiOI. The radicals and holes trapping experiments were further used to probe the photocatalysis mechanism of IL-BiOI. The degradation efficiency of MO also decreased significantly when TEOA (hole scavenger) was added (Figure 6f), confirming that photogenerated hole is main oxidation species for the degrading MO, similar to the case of P-BiOI. Interestingly, the addition of •OH scavenger (TBA) could just decrease the MO degradation efficiency from 81% to 73% on IL-BiOI, This means the effect of the addition of •OH scavenger (TBA) on the MO photodegradation in the presence of IL-BiOI became much weaker than the case of P-BiOI, suggesting that the role of •OH on photocatalytic degradation of MO on IL-BiOI was much less than that on P-BiOI. Meanwhile, we almost could not detect the generation of •O2 during the photocatalysis in the presence of IL-BiOI, confirming that IL modification could inhibit the production of • O2 via the reduction of oxygen molecules by the photogenerated electrons because most of these electrons could be trapped surface modified IL molecules via coulomb coupling force (Figure 6h). It is known that •OH radicals could be generated from the photogenerated electron-induced multistep reduction of O2 in BiOI photocatalytic systems. The trapping of photogenerated electron directly eliminates the contribution of •OH to the photodegradation of MO on IL-BiOI but enhances the photocatalytic activity of IL-BiOI by inhibiting the recombination of photoinduced electronhole pairs. We also compared the photocurrent collected in the suspension of P-BiOI and IL-BiOI by using Fe3+ an electron shuttle (Figure 7) to further investigate the effect of in situ IL
Figure 7. Comparison of Fe3+-mediated photocurrents collected on a Pt electrode in deaerated suspensions of different photocatalysts.
modification on photocatalysis. The photocurrent collected in IL-BiOI suspension was lower than that in the P-BiOI suspension. This is because the [Bmim]+ of IL-BiOI surface could function as a electron withdrawing group to restrain the rate of interfacial electron transfer by means of trapping conduction band electrons, confirming surface modification of ionic liquid could inhibit the recombination of photoinduced electronhole pairs by trapping the photogenerated electrons and thus enhance the photocatalytic activity of BiOI.
4. CONCLUSIONS In summary, we have investigated the photocatalysis mechanism of BiOI in detail and demonstrated that the photocatalytic activity of BiOI could be enhanced by the in situ modification of an ionic liquid (1-buty-3-methylimidazolium iodide) during the synthesis for the first time. We found that IL modification could trap the photoexcited electron at conduction band of BiOI to inhibit the recombination of photoinduced electronhole pairs, and thus enhance its photocatalytic activity on the degradation of organic pollutants. This study provides a facile method to enhance the photocatalytic activity of novel BiOX photocatalysts. ’ AUTHOR INFORMATION Corresponding Author
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dx.doi.org/10.1021/jp2042069 |J. Phys. Chem. C 2011, 115, 14300–14308
The Journal of Physical Chemistry C
’ ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grants 21073069 and 91023010), Program for Innovation Team of Hubei Province (2009CDA048), Self Determine Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grant CCNU09C01009), Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT0953), and Open Fund of Key Laboratory of Catalysis and Materials Science of Hubei Province (CHCL09001). ’ REFERENCES (1) Yamashita, H.; Harada, M.; Misaka, J.; Takeuchi, M.; Ikeue, K.; Anpo, M. J. Photochem. Photobiol. A 2002, 148, 257. (2) Bae, E. Y.; Choi, W. Y. Environ. Sci. Technol. 2003, 37, 147. (3) Jia, H. M.; Xiao, W. J.; Zhang, L. Z.; Zheng, Z.; Zhang, H. L.; Deng, F. J. Phys. Chem. C 2008, 112, 11379. (4) Vinodgopal, K.; Kamat, P. V. Environ. Sci. Technol. 1995, 29, 841. (5) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (6) Lei, Y. Q.; Wang, G. H.; Song, S. Y; Fan, W. Q; Pang, M.; Tang, J. K.; Zhang, H. G. Dalton Trans. 2010, 39, 3273. (7) Henle, J.; Simon, P.; Frenzel, A.; Scholz, S.; Kaskel, S. Chem. Mater. 2007, 19, 366. (8) Chang, X. F.; Huang, J.; Tan, Q. Y.; Wang, M.; Ji, G. B.; Deng, S. B.; Yu, G. Catal. Commun. 2009, 10, 1957. (9) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. J. Phys. Chem. C 2008, 112, 747. (10) Zhang, X.; Zhang, L. Z. J. Phys. Chem. C 2010, 114, 18023. (11) Xiao, X.; Zhang, W. D. J. Mater. Chem. 2010, 20, 5866. (12) Wang, W. D.; Huang, F. Q.; Lin, X. P. Scripta Mater. 2007, 56, 669. (13) Wang, W. D.; Huang, F. Q.; Lin, X. P.; Yang, J. H. Catal. Commun. 2008, 9, 8. (14) Yu, C. L.; Yu, J. C.; F, C. F.; Wen, H. R.; Hu, S. J. Mater. Sci. Eng., B 2010, 166, 213. (15) Caruso, F. Adv. Mater. 2001, 13, 11. (16) Katoh, R.; Hara, M.; Tsuzuki, S. J. Phys. Chem. B 2008, 112, 15426. (17) Guo, S. j.; Dong, S. J.; Wang, E. K. Adv. Mater. 2010, 22, 1269. (18) Hu, S. Z.; Wang, A. J.; Li, X.; Wang, Y.; L€owe, H. Chem. Asian J. 2010, 5, 1171. (19) Ren, W. J.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z.; Fan, X. X.; Zou, Z. G. Appl. Catal. B Environ. 2007, 69, 138. (20) Yan, S. C.; Li, Z. S.; Zou, Z. G. Langmuir 2010, 26, 3894. (21) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (22) Blelskl, B. H.; Shlue, G. G.; Bajuk, S. J. Phys. Chem. 1980, 84, 830. (23) Xu, X. L.; Duan, X.; Yi, Z. G.; Zhou, Z. W.; Fan, X. M.; Wang, Y. Catal.Commun. 2010, 12, 169. (24) Kubelka, P.; Munk, F.; Tech., Z. P. Z. Technische. Phys. Tech. Phys. 1931, 12, 593. (25) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086. (26) Fu, H. B.; Pan, C. S.; Yao, W. Q.; Zhu, Y. F. J. Phys. Chem. B 2005, 109, 22432. (27) Sleight, A. W.; Chen, H. Y.; Ferretti, A.; Cox, D. E. Mater. Res. Bull. 1979, 14, 1571. (28) Huang, W.; Zhu, Q. Comput. Mater. Sci. 2008, 43, 1101. (29) Yoon, S. H.; Lee, J. H. Environ. Sci. Technol. 2005, 39, 9695. (30) Liu, G. G.; Li, X. Z.; Zhao, J. C.; Horikoshi, S.; Hidaka, H. J. Mol. Catal. A: Chem. 2000, 153, 221. (31) Kim, S.; Choi, W. Environ. Sci. Technol. 2002, 36, 2019. (32) Huang, W. L.; Zhu, Q. J. Comput. Chem. 2009, 30, 183. (33) Ma, J. M.; Liu, X. D.; Lian, J. B.; Duan, X. C.; Zheng, W. J. Cryst. Growth Des. 2010, 10, 2511.
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