Bi2O3, BiVO4, and Bi2WO6: Impact of Surface Properties on

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Bi2O3, BiVO4, and Bi2WO6: Impact of Surface Properties on Photocatalytic Activity under Visible Light Tamar Saison,†,‡ Nicolas Chemin,‡ Corinne Chaneac,*,† Olivier Durupthy,† Valerie Ruaux,§ Laurence Mariey,§ Franc- oise Mauge,§ Patricia Beaunier,|| and Jean-Pierre Jolivet† †

)

UPMC Univ Paris 06, CNRS, UMR 7574, Chimie de la Matiere Condensee de Paris, College de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡ Saint-Gobain Recherche, 39 quai Lucien Lefranc, BP 135, 93303 Aubervilliers, France § Laboratoire de Catalyse et Spectrochimie de Caen, EnsiCaen, Universite de Caen, CNRS, 6 boulevard Marechal Juin, 14050 Caen, France UPMC Univ Paris 06, CNRS, UMR 7197, Laboratoire de Reactivite de Surface, 4 place Jussieu, 75252 Paris Cedex 05, France

bS Supporting Information ABSTRACT: Bismuth-based oxides have attractive photocatalytic properties under visible light. A better understanding of the origin of that good photocatalytic activity should allow its control and its optimization. In this Article, we have studied the impact of surface properties on photocatalytic activity for three bismuth-based oxides Bi2O3, BiVO4, and Bi2WO6. The surface acidity of particles was investigated by pyridine adsorption using infrared spectroscopy and the photocatalytic activity was investigated by the degradation of an aqueous solution of rhodamine B (RhB) and of stearic acid deposited on films under visible light. From the results obtained for the three bismuthbased oxides, we established a relation between surface acidity and photocatalytic mechanism of RhB degradation. The best photocatalytic efficiency for RhB and stearic acid degradation has been obtained with Bi2WO6 that also exhibits the highest surface acidity. The most acid sites promote a strong interaction with the pollutant, implying a short distance between the pollutant and the photocatalyst. Consequently, the photogenerated electrons, holes, and radicals can reach more easily the pollutant, leading to an efficient degradation under visible light. The development of strong acid solids such as Bi2WO6 emerged as promising materials for the degradation of pollutants. Because the good photocatalytic properties are related to the high acidity of Bi2WO6, we are interested in its origin using the multi site complexation model (MUSIC model). We found high acidity sites that are located on the lateral faces of the Bi2WO6 platelets (crystallographic planes (101), (101), (100), and (001)).

1. INTRODUCTION In the past decades, the development of visible-light-driven photocatalysts has attracted much attention. A large number of investigations have focused on semiconductor-based photocatalysts involving mainly water splitting1-3 and organic degradation4-6 under ultraviolet or visible-light irradiation. Titanium dioxide was found to be the most efficient photocatalyst under UV irradiation and therefore is the most commercialized.7 However, TiO2 can be only excited by ultraviolet irradiation that corresponds to 3-4% of the solar spectrum. The use of visible-light-driven photocatalysts will not only increase the outdoor photocatalytic activity but also enable the extension of the indoor applications where there is almost no UV irradiation. There are two main strategies to develop new visible photocatalysts. The first one is the doping of titanium dioxide with aims to extend its absorption to visible range. The origin of the visible absorption has not been clearly elucidated; it could come from an interaction between the orbitals 2p of dopant with 2p O orbitals in the newly formed valence band or the creation of dopant isolated states above the valence band maximum.8 TiO2 doped with carbon,9 nitrogen,10 fluorine,11 or sulfur12 have r 2011 American Chemical Society

shown in some cases a photocatalytic activity in the visible range. However, the methods used for the doping element introduction are not fully controlled, and the low thermal stability of these compounds limits their applications. Moreover, these dopants act as recombination centers between photogenerated electrons and holes and consequently decrease photocatalyst efficiency. The second strategy is to use others materials that are absorbing in the visible range. Bismuth-based oxides appear to be good candidates because most of them have a band gap in visible range thanks to the interaction between 6s Bi and 2p O orbitals at the top of the valence band.13-15 Moreover, they are chemically and thermally stable and nontoxic.16 BiVO4 has been already prepared by solid-state reaction,17 solution processes at room temperature,16 hydrothermal treatment,16 and metal organic decomposition,18 and Bi2O3 has been prepared by solid-state reaction, sonochemical route,14 thermolysis,19 or hydrothermal treatment.20 For these two compositions, among the various Received: September 24, 2010 Revised: February 13, 2011 Published: March 10, 2011 5657

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The Journal of Physical Chemistry C possible crystalline structures, only monoclinic structure presents a photocatalytic activity. BiVO4 monoclinic photodegrades organic pollutants21-23 and splits H2O into O2 and H2 from an aqueous solution of AgNO3 under visible-light irradiation.1-3 The photodegradation of rhodamine B,24 methyl orange,14 and peroxomonosulfate25 has been observed in the presence of Bi2O3 monoclinic. Bi2WO6 orthorhombic structure obtained via a hydrothermal route showed a photocatalytic activity under visible light for organic molecules degradation as rhodamine B15 and acetaldehyde26,27 and also for O2 evolution.28 In fact, the photocatalytic activity of a material is largely influenced by various characteristics of solid such as crystal structure, crystal size, surface area, and morphology. Although electrons and holes must be efficiently photogenerated within the bulk material, the chemical degradation of a pollutant must proceed at the surface of the catalyst, and consequently the surface properties appear as a key parameter. Devi et al. showed that the constant rate of cationic and anionic dyes degradation in the presence of TiO2 doped with Mo6þ increases with the dye adsorption capacity.29 Indeed, a strong adsorption will reduce the recombination of photogenerated charge carriers by facilitating the interfacial charge transfer process. Moreover, TiO2-based solid acids have been found as efficient photocatalysts for the elimination of volatile organic compounds.29-32 For example, Wang et al. showed that the sulfation of TiO2 photocatalyst induced high surface acidity, implying a very effective degradation of pyridine.30 The control of surface properties thus allows the increase of the photocatalytic efficiency of the material. In this Article, we report the study of three bismuth-based oxides, Bi2O3, BiVO4, and Bi2WO6, which have been synthesized by simple aqueous process, allowing one to obtain nanocrystals. Their photocatalytic activity has been studied through the degradation of rhodamine B (RhB) in an aqueous solution and the decomposition of stearic acid deposited on films under visible light. To understand the difference in photocatalytic properties, the surface acidity of the three samples has been evaluated by the adsorption and desorption of pyridine followed by IR spectroscopy. For the first time on these materials, the link between the photocatalytic activity and the surface acidity has been established. The findings in this study help to select Bi2WO6 as the most efficient material for visible-light photocatalysis in comparison to Bi2O3 and BiVO4. The main exposed surfaces of Bi2WO6 have been determined by electron microscopy and electron diffraction, and their acidic properties have been studied in details using multi site complexation model, which allows the prediction of the nature and charge of the different surface groups.

2. EXPERIMENTAL SECTION 2.1. Syntheses. All samples were synthesized by precipitation in aqueous solution. All chemicals were analytic grade reagents used without further purification. A bismuth stock solution with a concentration of 0.2 mol L-1 was obtained by dissolution of Bi(NO3)3 3 5H2O in 1.5 mol L-1 nitric acid. BiVO4: 25 mL of bismuth stock solution was diluted with 25 mL of water. Powder of V2O5 was then added with a molar ratio of Bi/V = 1. The mixture was left stirring for 3 days at room temperature. Bi2O3: Concentrated sodium hydroxide was added to bismuth stock solution up to a pH of 11.5. The solution was then stirred at room temperature for 24 h. Bi2WO6: Tungstate solution was prepared by the dissolution of H2WO4 in 1.5 mol L-1 nitric acid. Bismuth

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and tungstate stock solutions were mixed with a molar ratio of Bi/W = 2. The solution was transferred into a 20 mL Teflonlined autoclave and then heated at 200 °C for 24 h under autogenous pressure. The autoclave was left to cool at room temperature. The collected precipitates in the different syntheses were centrifuged, washed three times with water, and dried under nitrogen flux. According to energy-dispersive X-ray spectroscopy analysis conducted by means of a scanning electron microscope (PGT Imix equipped with a Germanium detector), no sample contains sodium below the detection limit of 0.5 atomic %. Films of bismuth-based oxides have been processed by spin-coating deposition of 0.1 g of photocatalyst dispersed in 10 mL of tetrahydrofuran on a glass substrate. Films were dried at room temperature under air. 2.2. Characterizations. UV-visible diffuse reflectance spectra (DRS) of the samples were recorded on a Varian-Cary 5E spectrometer with integrating sphere from 300 to 800 nm. Powder X-ray diffraction (XRD) measurements were performed with a Br€ucker D8 X-ray diffractometer operating in the BraggBrentano reflection mode at Cu KR radiation. The data were collected in the 20-80° 2θ range with 0.02° steps and a counting time of 5 s per step. Particles have been observed by field emission gun scanning electron microscopy using a LEO DSM 982 Gemini at a voltage of 15 kV. The samples were mounted on SEM stubs and coated with platinum using a standard procedure. Nitrogen adsorption-desorption measurements were conducted at 77 K on a Micromeritics Tristar apparatus. Specific surface areas were determined following the BrunauerEmmet-Teller analysis. The particles morphology and orientation were studied by transmission electron microscopy using JEOL JEM 100 CX (100 kV) apparatus. Samples were dispersed in absolute ethanol, and the suspension was ultrasonicated. A drop of this suspension was deposited on a carbon-coated copper grid and air-dried. The d-spacings obtained from the selected area electron diffraction (SAED) pattern were calibrated using the Au pattern. 2.3. Photocatalytic Test. First, photocatalytic activity of the samples was evaluated by degradation of aqueous RhB under UV-A or visible light irradiation. A neon lamp with a maximum at 365 nm was used as UV-A irradiation, whereas for visible irradiation, an artificial solar irradiation (Atlas Suntest model XLSþ) with the 400 nm cutoff filter at 400 W/m2 has been taken. In each experiment, 0.5 g of photocatalyst was poured into 100 mL of a RhB solution with an absorbance of 1.1. Prior to irradiation, the suspension was magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium of the dye on the photocatalyst. Next, the solution was illuminated during 2 h. Every 20 min, 3 mL aliquots were taken and centrifuged to remove the particles. The filtrates were analyzed by visible spectrophotometry (400-800 nm) using Ocean Optics HR4000 high-resolution spectrometer. The photocatalytic activity has also been studied by degradation of stearic acid under visible light. A thin film of stearic acid was deposited on photocatalyst film by spin-coating. The degradation of stearic acid has been followed by infrared spectroscopy on a Nicolet Nexus 6700 with a deuterated triglycine sulfate (DTGS) detector. Stearic acid shows the characteristic vibration bands of the CH bound at 2923 and 2853 cm-1 (asymmetric and symmetric stretching modes of the CH2 groups, respectively) and 2957 cm-1 (asymmetric in-plane stretching mode of the CH3 group). Its concentration has been evaluated by integrating the area from 2800 to 3000 cm-1. 5658

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The Journal of Physical Chemistry C 2.4. FT-IR Spectroscopy. The acidity of the photocatalysts was studied by IR spectroscopy using adsorbed pyridine as spectroscopic probe molecule. Infrared spectra were recorded on self-supported discs (2 cm2, ∼50 mg), which were placed into an infrared quartz cell (KBr windows) connected to a vacuum line. The FT-IR spectrometer was a Nicolet Nexus apparatus equipped with an extended KBr beam splitter and a mercury cadmium telluride (MCT) detector. Spectra were recorded at room temperature from 4000 to 400 cm-1 with an accumulation of 256 scans at 4 cm-1 resolution. Prior to the adsorption experiment, the samples were activated by heating (3 K min-1) to 473 K under secondary vacuum (P ≈ 10-4 Pa) and kept at this temperature for 2 h. After the activation, pyridine was introduced into the cell via the vacuum line at equilibrium pressure (133 Pa), and the adsorbed pyridine was evacuated under vacuum at increasing temperatures (273, 323, 373, 423 K). After each step, an infrared spectrum was recorded. Pyridine was dried on molecular sieves prior to its use. 2.5. Modeling Multi Site Complexation. Multi site complexation (MUSIC) model, developed by Hiemstra et al., predicts the proton affinity of individual surface groups on oxide surface.33,34 The first version of MUSIC model uses the local electrostatic neutralization of combined atoms based on Pauling’s valence bond principle.33,35 The formal bond valence (ν) of a cation was defined as the ratio of its formal charge (z) to its coordination number (CN): ν = z/CN. However, this definition of formal bond valence assuming equal bond lengths around the metal ion is not valid for most of the oxides, and MUSIC model was improved in taking into account the differences in metaloxygen distances in the coordination sphere.34 The effective bond valence s is defined as s = e(R-R0)/b in which R is the metaloxygen distance, R0 is an element specific distance (R0 = 0.2094 nm for bismuth and 0.1917 nm for tungsten), and b is a constant (b = 0.037 nm).36 The model takes also into account the interactions with water molecules through hydrogen bond framework. So, the effective oxygen charge is given as q0 = ∑sj þ msH þ n(1 - sH) þ V, where ∑sj is the sum of effective bond valences of metal cations bonded to oxygen, m is the number of donating hydrogen bonds, n is the number of accepting hydrogen bonds, sH is the bond valence for an adsorbed proton (sH = 0.8), and V is the valence of oxygen (V = -2). The total number of hydrogen bonds of a singly coordinated oxygen to the metal is expected to be two for steric reasons (m þ n = 2). For doubly coordinated surface oxygen, we take in our study m þ n = 1. For triply coordinated oxygen, only one orbital is available for proton interaction, that is, m þ n = 1. Hiemstra et al. proposed a linear relationship between the intrinsic proton affinity constant log K and the charge of surface oxygen q0: log K= -A* q0 in which A is a constant equal to 19.8.34 This relation allows one to calculate the proton affinity constants K1 (oxo) and K2 (hydr) of various surface groups:

Mn -Oðns - 2Þ þ Hþ S Mn -OHðns - 1Þ K 1 ðoxoÞ Mn -OHðns - 1Þ þ Hþ S Mn -OH2 ðnsÞ K 2 ðhydrÞ and consequently to know the nature of the group (Mn-O, Mn-OH, or Mn-OH2) as a function of pH. Moreover, the effective charge of the surface group (an integer charge is not mandatory) can be calculated from the charge of oxygen and by taking into account the contribution of donating and accepting hydrogen bonds.34

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Figure 1. Powder XRD patterns of Bi2O3 (a), BiVO4 (b), and Bi2WO6 (c).

Figure 2. Diffuse reflectance spectra of Bi2O3 (a), BiVO4 (b), and Bi2WO6 (c).

3. RESULTS AND DISCUSSION 3.1. Sample Characteristics. Crystalline Structure. All synthesized samples contain a unique well-crystallized phase (Figure 1). The Bi2O3 and BiVO4 powders display a monoclinic structure according to JCPDS 41-1449 and 83-1699, respectively (Figure 1a,b). Bismuth tungstate exhibits an orthorhombic structure with lattice parameters a = 0.5457 nm, b = 1.6436 nm, and c = 0.5438 nm (JCPDS 39-0256) (Figure 1c). Optical Band Gap. Optical absorptions of samples are shown in Figure 2. The steep shape of the spectra is characteristic of a band gap transition and cannot be attributed to the transition from the impurity level.37 For a crystalline semiconductor, the optical absorption near the band edge is expressed by the formula Rhν = A(hν - Eg)n/2, where R, h, ν, Eg, and A correspond to absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively.38 The last parameter n depends on the characteristics of the transition in a semiconductor, for a direct transition, n = 1, whereas for an indirect transition, n = 4. The absorption coefficient is proportional to the absorbance. For the three bismuth-based oxides, the transition was shown to be direct.14,22,37 The band gap energy is estimated on the plot (Ahν)2 = f(hν) by the intercept of the tangent to the plot with abscissa (see Figure S1 in the Supporting Information). The measured band gap of Bi2O3 is 2.85 ( 0.01 eV, and its color is pale yellow. BiVO4 is vivid yellow, and its measured band gap is 5659


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Figure 3. SEM photographs of Bi2O3 (a), BiVO4 (b), and Bi2WO6 (c).

Table 1. Characteristics of the Different Bismuth-Based Oxidesa crystalline structure

band gap

surface area

morphology

Bi2O3 BiVO4

monoclinic monoclinic

2.85 eV 2.44 eV

14.6 m2 g-1 3.0 m2 g-1

fibers faceted particles

Bi2WO6

orthorhombic

2.84 eV

13.5 m2 g-1

nanoplates

a

Measurement error on band gap value is 0.01 eV and on surface area is 0.5 m2 g-1.

2.44 ( 0.01 eV. Bismuth tungstate is very light green and has a band gap of 2.84 ( 0.01 eV. Morphology and Surface Area. The synthesis by soft chemistry allowed one to produce submicronic particles with high

surface area as compared to the conventional solid-state technique (Figure 3).39,40 Bi2O3 with a surface area of 14.6 ( 0.5 m2 g-1 is composed by thin fibers, which tend to agglomerate each other to form bigger particles (Figure 3a). BiVO4 has a surface area of 3.0 ( 0.5 m2 g-1 and consists of agglomerated faceted particles with sizes from 200 to 800 nm exhibiting flat surfaces (Figure 3b). Bismuth tungstate presents nanoplatelets morphology with a surface of 13.5 ( 0.5 m2 g-1 (Figure 3c). Table 1 summarizes the main characteristics of the different samples. 3.2. Photocatalytic Properties. The photocatalytic properties of the three materials were studied under visible light using two different tests: the degradation of RhB in aqueous solution and the decomposition of stearic acid deposited on films. Rhodamine B Photodegradation. Photocatalytic activity was investigated by degradation of RhB in aqueous solution under visible light. RhB is not degraded under illumination in the absence of photocatalyst, nor in the dark in the presence of the photocatalyst. As shown in Figure 4, all samples photodecompose RhB under visible light. In the case of Bi2O3 and BiVO4, there is a decrease of the absorption band without shifting of the maximum absorbance wavelength at 555 nm during the photocatalytic test. On the contrary, in the presence of Bi2WO6, the absorption peak is blue-shifted in addition to its global decrease. These two different phenomena show that RhB is degraded via two different pathways. The decrease of RhB maximum absorbance with Bi2O3 and BiVO4 results from the degradation of the conjugated structure due to a photocatalytic process.41-43 For Bi2WO6, the degradation occurs via the N-de-ethylation of RhB in a stepwise process.15,43-45 Indeed, RhB initially tetra-ethylated (λmax = 555 nm) becomes tri, di-, mono-, and no-ethylated (λmax is, respectively, 539, 522, 510, and 498 nm). The molar absorptivity of the fully de-ethylated RhB at λmax = 498 nm is around 70% of the one of the pristine RhB at λmax = 555 nm. Consequently, the measured peak intensity of the solution after 120 min of irradiation, which is only 12% of that at the beginning of the experiment, indicates that the degradation of conjugated structure occurs at the same time as N-de-ethylation reactions. According to published studies on TiO2 and Bi2WO6, the Nde-ethylation process can be assigned to a photosensitized process where the RhB transfers its electron to the conduction band of the photocatalyst.41-43,46 After the transfer, RhB is degraded by successive N-de-ethylation reactions. A similar test performed under UV-A irradiation has shown no wavelength shift of maximum absorbance. This confirms that the Nde-ethylation reactions observed under visible light come from a photosensitized process. In summary, in the presence of Bi2O3 and BiVO4, RhB is degraded only via the photocatalytic process, that is, by degradation of its conjugated ring structure. With Bi2WO6, degradation of RhB occurs by two processes: the photocatalytic process (degradation of the conjugated structure) and the photosensitized process (N-de-ethylation reactions). Reaction Mechanism. To understand the origin of the two different RhB degradation pathways, the surface acidity of particles has been characterized. Pyridine is commonly used as a probe molecule for the infrared characterization of the acid sites of particles.47-49 Because of the presence of strong absorption bands on Bi2O3 and Bi2WO6 below 1570 cm-1, only the 17001570 cm-1 zone has been used to study the adsorption mode of pyridine on bismuth-based oxides. Brønsted acid sites are characterized by a band at 1640 cm-1 (ν8a), whereas Lewis acid 5660



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Figure 4. Visible spectra changes of RhB in aqueous solution in the presence of Bi2O3 (a), BiVO4 (b), and Bi2WO6 (c) with irradiation time (20 min interval) under visible light.

sites are characterized by a band in the 1620-1590 cm-1 wavenumber range (ν8a) and at 1574 cm-1 (ν8b). The wavenumber of the ν8a mode of the coordinated molecule is also indicative of the strength of the Lewis acidic sites: the higher is the ν8a wavenumber, the greater is the acidic strength. Figure 5 presents the spectra of the pyridine adsorbed on the different samples. The first spectrum represents the pyridine adsorbed at equilibrium at room temperature. For Bi2O3, the ν8a band at 1590 cm-1 and the ν8b band at 1574 cm-1 are assigned to pyridine adsorbed on very weak Lewis acid sites (Figure 5a). No band ascribed to pyridinium species has been detected, indicating that no Brønsted sites are present on the Bi2O3 surface. Moreover, the absence of OH group detection before probe adsorption allows us to discard the assignment of the band at 1590 cm-1 to H-bond pyridine species. Once the equilibrium is established, the sample has been evacuated under vacuum at room temperature and then heated at different temperatures (323, 373, and 423 K) to follow the desorption of the pyridine. In the case of Bi2O3, all pyridine is desorbed at 323 K (Figure 5a, spectrum (n)). One can note the presence of a band at 1607 cm-1 that is completely eliminated from evacuation at room temperature. It is ascribed to the combination ν1þν6a associated with the weak Lewis acid sites.48 For BiVO4, the ν8a bands at 1602-1593 cm-1 and the ν8b at 1574 cm-1 are characteristics of Lewis acid sites (Figure 5b). The ν8a band shifts from 1593 to 1602 cm-1 during the successive steps of the thermodesorption. This reveals the heterogeneity of the Lewis acid sites; the higher component at 1602 cm-1 is already present at room temperature but is partially hidden by a larger fraction of weaker Lewis acid sites at 1593 cm-1. No band was observed at 1640 cm-1, indicating the absence of Brønsted sites. The temperature of total desorption of pyridine is 423 K.

On Bi2WO6 surface, the Lewis acid sites are characterized by ν8a bands at 1607-1611 cm-1 and ν8b mode at 1574 cm-1 (Figure 5c). Similarly to the BiVO4 sample, the wavenumber of the ν8a vibrational mode shifts toward higher wavenumber during thermodesorption. The strongest Lewis sites have the ν8a band at 1611 cm-1. Moreover, Brønsted acid sites characterized by the ν8a band at 1640 cm-1 have been identified. Bands at 1595 cm-1 that disappear under vacuum at room temperature are also detected. They can indicate the presence of very weak Lewis acid sites. One cannot exclude their assignment to pyridine H-bonded with OH groups not enough acidic to protonate pyridine. Finally, there is still adsorbed pyridine at 423 K. Therefore, the temperature of total desorption of pyridine is higher than 423 K. The pyridine is eliminated at higher temperature (>423 K) on Bi2WO6 surface than on Bi2O3 (323 K) and BiVO4 (423 K), indicating stronger interaction between Bi2WO6 and pyridine. Strong Lewis acid sites have been detected for Bi2WO6 photocatalysts (ν8a = 1611 cm-1) as compared to Bi2O3 (ν8a = 1590 cm-1) and BiVO4 (ν8a = 1602 cm-1). No pyridinium ions have been identified on Bi2O3 and BiVO4 surfaces, meaning the absence of Brønsted sites, while in the case of Bi2WO6, Brønsted acid sites have been characterized by the presence of the ν8a mode band at 1640 cm-1. This comparison of the temperature of pyridine total desorption, the strength of Lewis acid sites, and the presence of Brønsted sites leads to the distinction between Bi2O3 and BiVO4, weak acid solids, and Bi2WO6, a strong acid oxide. We observed different pathways of rhodamine B degradation for the different photocatalysts: Bi2O3 and BiVO4 degrade RhB only via the photocatalytic process, whereas Bi2WO6 decomposes it via the photocatalytic and photosensitized process. The 5661



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Figure 5. Infrared spectra of pyridine adsorbed at 293 K [133 Pa at equilibrium (i)], after primary vacuum at room temperature (j), after secondary vacuum at room temperature [5 min (k), 10 min (l), 15 min (m)], at 323 K [15 min (n)], 373 K [15 min (o)], 423 K [15 min (p)] on Bi2O3 (a), BiVO4 (b), and Bi2WO6 (c).

photosensitized process is only possible if the interaction between RhB and the surface is enough strong. Actually, the electron injection rate decreases with the distance between the sensitizer and the metal oxide surface due to a lower coupling between the electronic levels of the dye and the semiconductor.50-52 That implies a stronger interaction between RhB and Bi2WO6 as compared to Bi2O3 and BiVO4. This interaction RhB-photocatalyst can be correlated to the surface acidity properties: (i) For weak acid solids as Bi2O3 and BiVO4, the interaction between the RhB and the surface is too weak to have the electron transfer from the RhB to the photocatalyst. Therefore, the degradation occurs only via the photocatalytic process. (ii) For strong acid solids such as Bi2WO6, the interaction between the RhB and acidic sites is strong enough to have the electron transfer implying the presence of the photosensitized process, in addition to the photocatalytic one. These two processes lead to an efficient degradation involving the total destruction of the chromophore after 80 min, while 55% of initial RhB is degraded in the presence of BiVO4 and 80% for Bi2O3 (Figure 4). The interaction between acidic sites of Bi2WO6 and RhB has been confirmed by changing the pH of RhB/Bi2WO6 solution: (i) At more acidic pH (pH = 2), all of RhB is adsorbed due to the protonation of more acidic sites. (ii) At more basic pH (pH = 12), no adsorption of RhB has been observed because of the deprotonation of all acidic sites. Moreover, we showed that the photosensitized process leads to the N-de-ethylation of RhB,; this supposes that the interaction occurs through the diethylamino group.53,54 Therefore, RhB is probably adsorbed on acid sites in the same way as pyridine, that is, through a

Figure 6. Photocatalytic degradation under visible light of stearic acid deposited on films of Bi2O3 (]), BiVO4 (gray 0), and Bi2WO6 (light gray O).

nitrogen lone pair. Some publications attribute the feasibility of the photosensitized process to the adsorption of large amounts of RhB.55,56 However, the primordial parameter is not the amount of adsorbed RhB but the strength of the interaction between RhB and the photocatalyst surface. Indeed, Bi2O3 adsorbs more RhB than Bi2WO6 probably due to its higher surface area (14.6 m2 g-1 as compared to 13.5 m2 g-1 for Bi2WO6), but the interaction is too weak to have the photosensibilization effect. Through this study, we have highlighted the impact of surface acidity of the 5662



Figure 7. Fourier transform infrared spectra of stearic acid (a), Bi2WO6 (b), and stearic acid adsorbed on Bi2WO6 (c).

photocatalyst on the RhB degradation mechanism under visible light. Stearic Acid Photodegradation. Because of the competition between photosensitization and photocatalytic processes taking place during dye degradation under visible light irradiation, the photocatalytic activity of BiVO4, Bi2O3, and Bi2WO6 particulate films has been monitored using stearic acid as probe molecule. Stearic acid was chosen for two main reasons: (i) it does not absorb in the visible range, although also its degradation is evidence of photocatalytic activity under visible light, and (ii) it is representative of sebum from the skin of a finger, and therefore its degradation on films is close to the application of indoor selfcleaning glasses. As shown in Figure 6, all films degrade stearic acid, meaning that the three samples (Bi2O3, BiVO4, and Bi2WO6) are visible-light-driven photocatalysts. However, bismuth tungstate has the highest photocatalytic activity. This seems to be due to a stronger adsorption of stearic acid on the high acidic surface of Bi2WO6 as compared to Bi2O3 and BiVO4. The photogenerated species (electrons, holes, and radicals) could reach the pollutant easier and therefore make the degradation more efficient. The interaction between stearic acid and Bi2WO6 has been studied by infrared spectroscopy. Bi2WO6 was mixed with stearic acid in ethanol in the dark during 24 h. After centrifugation of the powder, the product named stearic acidBi2WO6 was analyzed by FTIR by putting the powder on a Si wafer. The presence of stearic acid on Bi2WO6 surface was confirmed by the observation of CH2 bands at 2917 and 2850 cm-1 (see Figure S2 in the Supporting Information). Pure stearic acid spectrum is characterized by the absorption band at 1699 cm-1, which is assigned to the COOH group (Figure 7a). For stearic acid-Bi2WO6, the COOH vibration peak of stearic acid at 1699 cm-1 disappeared, and new peaks at 1567 and 1415 cm-1 appeared (Figure 7c). This indicates a strong interaction between stearic acid and Bi2WO6 through the carboxylate function of stearic acid.57,58 In conclusion, the best photocatalytic efficiency for RhB and stearic acid degradation under visible irradiation has been obtained with Bi2WO6 that exhibits the highest surface acidity. We believe that the highly acid sites attract very strongly the studied organic compounds. The compounds are located closer from the surface of the catalyst, allowing an easy interfacial charge transfer in limiting the recombination of photogenerated charge carriers.

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Figure 8. TEM image of Bi2WO6 platelet (a), the corresponding SAED pattern (b), and the orientation of crystal deduced from SAED pattern (c) (legend: bismuth in gray, tungsten in white, and oxygen in black).

Figure 9. Representation of Bi2WO6 platelet showing the different crystallographic planes.

3.3. Origin of Bi2WO6 Acidity. As the good photocatalytic properties of Bi2WO6 seem to be due to its high surface acidity, we were interested in its origin. Bi2WO6 is obtained as platelets of about 25 nm in thickness and 0.2-2 μm in length (Figure 3c). First, we identified by electron diffraction the crystallographic planes that bound the platelets. Next, we did calculations of pKa of the different sites contained in each plane. Identification of Crystallographic Planes. The selective area electron diffraction (SAED) pattern of an individual platelet shows well-defined ED spots, indicating the single-crystal nature of the platelet (Figure 8a,b). The different spots have been indexed by measuring the diffracted distances and the interplanar angles. The SAED can be associated with the diffraction pattern of [010] zone axis, indicating that the basal face of the plate is (010). The preferred (010) surface orientation of Bi2WO6 platelets has already been observed in the case of individual plates and aggregated plates, forming flower-like Bi2WO6 superstructure.37,59-61 Moreover, the orientation of the crystal can be deduced from the orientation of the SAED pattern (Figure 8c). The correlation between the crystal orientation (Figure 8c) and the image of the platelet in the real space (Figure 8a) leads to the identification of lateral faces. The large edges of platelets correspond to the faces (101) and (101), whereas the small edges are the faces (100) and (001) (Figure 8a and c). In conclusion, by the analysis of TEM image and its electron diffraction pattern, we identified all crystallographic planes of the plates (Figure 9). 5663



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Table 2. Face (010): Proton Affinity Constants and Charge of the Surface Groups surfacea

group

Ns (nm-2)b

log K1

log K2c

charge Mn-Od

(010)y=0.25

Bi2-O

6.7

23.1

(010)y=0.75

Bi2-O

6.7

21.7

(010)y=0.42

W1-O

6.7

9.44

W2-O

6.7

-2.49

W2-O

6.7

-5.87

W1-O

6.7

10.1

W2-O

6.7

-2.49

-0.07

W2-O

6.7

-5.87

þ0.10

(010)y=0.92

charge Mn-OHd -0.37 -0.29

-2.44

-0.88

þ0.12

-0.07 þ0.10 -1.73

-0.92

þ0.09

The surface has been cut from y = 0.25, 0.42, 0.75, and 0.92. b Ns represents the site density. c K2 is not calculated for M2-O because we considered that the presence of M2-OH2(2s) is unlikely due to its high acidity. d The charge of surface group is only calculated when its existence is possible in the 2-12 pH range. a

Multi Site Complexation Model. To locate the high acid sites on the platelet, we calculated the constants of acidity of all of the sites of the various planes. Multi site complexation model (MUSIC model) developed by Hiemstra et al. has been used to calculate the proton affinity constants K1 (oxo) and K2 (hydr) of surface groups Mn-O(ns-2) and Mn-OH(ns-1) (see details in the Experimental Section).33,34 The calculations have been performed on the five main surfaces of the platelets ((010), (101), (101), (100), and (001)). The nature of the atoms exposed at the surface of a crystal depends, of course, on the direction of the cleavage of the solid but also on the level where the crystal was cut. This is even more important for materials such as Bi2WO6 where the structure corresponds to the alternative stacking of the different cations between oxygen layers. Consequently, the (010) surface may display either cornershared WO6 octahedral layer or bismuth layer depending on where the cell is cut along the b axis. We then considered four different cleavage heights of the unit cell in the (010) direction: (010)y=0.25, (010)y=0.42, (010)y=0.75, and (010)y=0.92 (see Figure S3 in the Supporting Information). The surfaces (010)y=0.25 and (010)y=0.75 have a bismuth oxygen layer at their surface, whereas the two others (010)y=0.42 and (010)y=0.92 exposed a WO6 layer. The proton affinity constants K1 and K2 and the charge of groups Mn-O and Mn-OH are exposed in Table 2. The surfaces (010)y=0.25 and (010)y=0.75 only have one type of sites, Bi2-OH negatively charged. The surfaces composed by WO6 layer exhibit two different sites of W2-O with a charge of -0.07 and þ0.10 and W1-O site, which is protonated in W1-OH for a pH > 9 with a charge of þ0.09 for (010)y=0.42 and þ0.12 for (010)y=0.92. To compare the acidity of the different surfaces, we define σþ, the positive charge of the surface, which is the sum of the charge of acid sites multiplied by their density: σþ = ∑σ*Ns. We considered as acid sites the sites Mn-OH and Mn-OH2 positively charged. Because the photocatalytic tests are performed in deionized water, we fixed the pH at 5 for charge calculations. As we can see in Table 3, the face (010) has no σþ when Bi-O layers are exposed due to the absence of positively charge sites. When the face (010) is composed of WO6 octahedra, σþ equals 0.60 or 0.80, meaning a relatively low Brønsted acidity. The same calculations have been performed on lateral faces; the affinity constants and charge of the surface groups are exposed in the Supporting Information (Tables S1-S4). According to these results, the positive charge, σþ, has been calculated for all lateral faces (Table 3). The long edges of the platelets have a σþ value equal to 1.7 for (101) face and 3.6 for

Table 3. Positive Charge σþ Value of the Different Faces of Bi2WO6 Platelets basal face (010)y=0.25a (010)y=0.75a (010)y=0.42a (010)y=0.92a

σþ lateral faces long edges σþ lateral faces short edges σþ 0

(101)xþz=1.01b

1.6 (100)

2.8

0

(101)xþz=0.95c

3.6 (001)zg0.29d

4.0

(001)ze0.79d

3.6

0.80 0.60

a The surface has been cut from y = 0.25, 0.42, 0.75, and 0.92. b The surface corresponds to atoms in the units below the plane x þ z = 1.01 (expressed in fractional coordinates). c The surface corresponds to atoms in the units below the plane x þ z = 0.95 (expressed in fractional coordinates). d The surface has been cut below z = 0.29 or above 0.79.

(101). The σþ value of the short edges is 2.8 for (100) and 3.6 or 4.0 depending where the surface is cut for the (001) face. All lateral faces have a higher positive charge σþ than the basal face, meaning that they are more acidic. By taking into account the comparison of the positive charge of the surface and the fact that the basal face has almost no acid sites, we can conclude that the high acidity of platelets comes from mainly the lateral faces of the platelets (faces (101), (101), (100), and (001)).

4. CONCLUSIONS Under visible light, bismuth-based oxides such as Bi2O3, BiVO4, and Bi2WO6 show a photocatalytic activity for the degradation of RhB in solution and decomposition of stearic acid deposited on films. We demonstrated that the photocatalytic mechanism is tightly linked to the surface properties of the compounds. For weak acid solids, Bi2O3 and BiVO4, the interaction with some pollutants such as RhB and stearic acid is somewhat weak and does not lead to an efficient degradation. On the contrary, for the solid Bi2WO6 bearing strong acid sites, the adsorption of these pollutants is very strong. The short distance between the pollutant and the photocatalyst surface allows the photogenerated electrons, holes, and radicals to reach the pollutant more easily, implying an efficient degradation under visible light. For example, Bi2WO6 degrades RhB through the photocatalytic and the photosensitized processes. The former process is possible because of a strong interaction between RhB and the photocatalyst. The good photocatalytic efficiency of Bi2WO6 for stearic acid decomposition is also attributed to the strong interaction between stearic acid and its acid sites. The development of strong acid solids for photocatalysis is a promising approach for degradation of pollutants containing 5664


The Journal of Physical Chemistry C a basic function to increase the strength of the interaction photocatalyst-pollutant. On the basis of the discussed theoretical considerations, the origin of the high acidity of Bi2WO6 was attributed to the presence of specific sites located onto the lateral faces of platelets. Future work has to focus to control the morphology of platelets to increase the proportion of lateral faces.

’ ASSOCIATED CONTENT

bS

Supporting Information. For more details about MUSIC calculations, the affinity constants and charge of surface groups for planes (100), (001), (101), and (10-1) are exposed here. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ33144271529. Fax: þ33144271504. E-mail: corinne. [email protected].

’ ACKNOWLEDGMENT This research work was supported by Saint-Gobain Recherche. We also wish to thank Sandrine Clary-Lespinasse and Sebastien Aiello for technical support, Corinne Papret for scanning electron microscopy, and Pierre Gras for his help with experiments and for useful scientific discussions. ’ REFERENCES (1) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459–11467. (2) Yu, J.; Kudo, A. Adv. Funct. Mater. 2006, 16, 2163–2169. (3) Tokunaga, S.; Kato, H.; Kudo, A. Chem. Mater. 2001, 13, 4624–4628. (4) Li, G.; Zhang, D.; Yu, J. C. Chem. Mater. 2008, 20, 3983–3992. (5) Xie, B.; Zhang, H.; Cai, P.; Qiu, R.; Xiong, Y. Chemosphere 2006, 63, 956–963. (6) Zheng, Y.; Wu, J.; Duan, F.; Xie, Y. Chem. Lett. 2007, 36, 520–521. (7) Fujishima, A.; Zhang, X. C.R. Chim. 2006, 9, 750–760. (8) Liu, G.; Wang, L.; Yang, H. G.; Cheng, H.-M.; Lu, G. Q. J. Mater. Chem. 2010, 20, 831–843. (9) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908–4911. (10) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (11) Ho, W.; Yu, J. C.; Lee, S. Chem. Commun. 2006, 1115–1117. (12) Ohno, T.; Mitsui, T.; Matsumura, M. Chem. Lett. 2003, 32, 364–365. (13) Stoltzfus, M. W.; Woodward, P. M.; Seshadri, R.; Klepeis, J.; Bursten, B. Inorg. Chem. 2007, 46, 3839–3850. (14) Zhang, L.; Wang, W.; Yang, J.; Chen, Z.; Zhang, W.; Zhou, L.; Liu, S. Appl. Catal., A 2006, 308, 105–110. (15) Fu, H.; Pan, C.; Yao, W.; Zhu, Y. J. Phys. Chem. B 2005, 109, 22432–22439. (16) Liu, J.; Wang, H.; Wang, S.; Yan, H. Mater. Sci. Eng., B 2003, 104, 36–39. (17) Roth, R. S.; Waring, J. L. Am. Mineral. 1963, 48, 1348–1356. (18) Galembeck, A.; Alves, O. L. Thin Solid Films 2000, 365, 90–93. (19) Zhou, L.; Wang, W.; Xu, H.; Sun, S.; Shang, M. Chem.-Eur. J. 2009, 15, 1776–1782. (20) Yang, Q.; Li, Y.; Yin, Q.; Wang, P.; Cheng, Y. Mater. Lett. 2002, 55, 46–49.

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Bi2O3, BiVO4, and Bi2WO6: Impact of Surface Properties on

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