New Insights into Bi2WO6 Properties as a Visible-Light Photocatalyst

Article pubs.acs.org/JPCC

New Insights into Bi2WO6 Properties as a Visible-Light Photocatalyst Tamar Saison,†,‡ Pierre Gras,† Nicolas Chemin,‡ Corinne Chanéac,† Olivier Durupthy,*,† Vlasta Brezová,§ Christophe Colbeau-Justin,∥ and Jean-Pierre Jolivet† †

UPMC Univ Paris 06, CNRS, UMR 7574, Chimie de la Matière Condensée de Paris, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡ Service Produits Composites et Revêtement de Surfaces, Saint-Gobain Recherche, 39 quai Lucien Lefranc, BP 135, 93303 Aubervilliers, France § Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia ∥ Univ Paris-Sud, CNRS UMR8000, Laboratoire de Chimie Physique, Orsay F-91405, France S Supporting Information *

ABSTRACT: Bismuth tungstate has attractive photocatalytic properties under visible light. A better understanding of the origin of that good activity should allow its control and its optimization. An improved photocatalytic activity to different pollutants was obtained using bismuth-based oxide obtained by microwave-assisted synthesis combined with the addition of sodium dodecylsulfate as a size tailoring agent. It is shown using electron paramagnetic resonance that bismuth tungstate is able to generate hydroxyl radicals in aqueous aerated solution, but no superoxide radical anions are formed. The catalytic efficiency relative to TiO2 could be associated to differences in the number of excitons generated, to their lifetimes as holes and electrons in the semiconductor, and to valence and conduction band positions. tungstate seems to be one of the more efficient.13 The Bi2WO6 orthorhombic structure obtained via a hydrothermal route showed a photocatalytic activity under visible light for organic molecule degradation as rhodamine B9 and acetaldehyde14,15 and also for O2 evolution.16 It is generally possible to improve the catalytic efficiency of a photocatalyst with a defined structure by adjusting other key characteristics such as crystal structure, crystal size, surface area, morphology, and surface additives. The huge versatility of the sol−gel methods and, when higher-energy input is necessary, the use of hydrothermal conditions allow that fine control and the preparation of adequate nanoparticles of the desired material.17−20 In the specific case of bismuth tungstate, different experimental parameters were studied to design nanoparticles such as the nature of the solvent or of the metal precursors,9,21,22 the acidity of the aqueous solution,23,24 the relative amounts of bismuth and tungsten,14,15,25 the heating temperature,15 and the use of polymers as organic additives.24,26,27 An interesting alternative to the conventional heating was also studied through the use of microwave-assisted heating. Indeed, the interactions between the electromagnetic field with centimeter wavelength and the solvent/reactant dipoles induce a fast and homogeneous heating of the solution. This consequently significantly

1. INTRODUCTION In the fast expanding domain of photocatalyst elaboration for air and water decontamination, titanium dioxide was found to be the most efficient photocatalyst under UV irradiation and therefore is the most commercialized.1−3 However, TiO2 can be only excited by ultraviolet irradiation that corresponds to 3−4% of the solar spectrum. This also prevents its indoor applications where there is almost no UV irradiation. Up to now, the most studied strategy to develop new visible photocatalysts is to extend the absorption of TiO2 in the visible range through the surface or bulk doping like TiO2 doped with carbon,4 nitrogen,5 fluorine,6 or sulfur.7 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 may act as recombination centers between photogenerated electrons and holes and consequently decrease photocatalyst efficiency.3 The expected improvements are somewhat limited, and drastic improvement of photocatalytic properties in the visible range is more likely to happen by changing completely the active material. That is why an emerging alternative strategy is to use others materials which are absorbing in the visible range.8 Bismuth-based oxides appear to be good candidates since most of them have a band gap in the visible range thanks to the interaction between 6s Bi and 2p O orbitals at the top of the valence band.9−11 In addition, they are chemically and thermally stable and nontoxic.12 Among the different compositions, bismuth © 2013 American Chemical Society

Received: May 16, 2013 Revised: October 8, 2013 Published: October 8, 2013 22656

dx.doi.org/10.1021/jp4048192 | J. Phys. Chem. C 2013, 117, 22656−22666

The Journal of Physical Chemistry C

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water. Amounts of 15 mL of the bismuth and tungstate acid stock solutions were mixed and transferred into a 100 mL Teflon-lined reactor and then heated in a microwave oven (Anton Paar, Multiwave 3000 rotor XF100) at 200 °C during 4 h. Bi2WO6-MW-SDS: The synthesis is similar to that of Bi2WO6-MW, but solid sodium dodecylsulfate (SDS) was added to the initial bismuth solution to obtain a final molar ratio Bi/W/SDS of 2/1/1 in the reacting medium. The collected precipitates in the different syntheses were centrifuged, washed three times with water, and dried under nitrogen flux. Films of bismuth tungstate have been processed by spincoating 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 VarianCary 5E spectrometer with integrating sphere from 300 to 800 nm. Powder X-ray diffraction (XRD) measurements were performed with a Brücker D8 X-ray diffractometer operating in the Bragg−Brentano reflection mode at Cu Kα 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 Brunauer−Emmet−Teller analysis. The particle morphology and orientation were studied by transmission electron microscopy using a 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 dspacings obtained from the selected area electron diffraction (SAED) pattern were calibrated using the Au pattern. 2.3. Photocatalytic Test. Photocatalytic activity of the samples was evaluated through the degradation of an aqueous RhB solution under visible light irradiation. An artificial solar irradiation (Atlas Suntest model XLS+) with the 400 nm cutoff filter at 400 W/m2 has been taken. In each experiment, 0.1 g of photocatalyst was poured into 100 mL of a RhB solution with an absorbance of 1.8. Prior to irradiation, the suspension was magnetically stirred in the dark during 1 h to ensure the establishment of an adsorption−desorption equilibrium of the dye on the photocatalyst. Then, 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 an Ocean Optics HR4000 high-resolution spectrometer. The photocatalytic activity has also been studied following the degradation of stearic acid under visible light (same light source as for the RhB degradation). A thin film of stearic acid was deposited on photocatalyst film by spin-coating. The degradation of stearic acid has been followed using 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. Finally, stearic

reduces the synthesis duration and generally yields smaller particles in comparison to conventional heating.28−30 Only a few studies are devoted to the preparation of bismuth tungstate under microwave irradiation,31−35 and none of them propose a direct comparison between a conventional and a microwaveassisted synthesis in the same experimental conditions. Moreover only one organic additive was proposed as a sizeand shape-controlling agent. The modification of various synthetic parameters may rapidly lead to the formation of tens of samples to be tested in photocatalysis. This catalytic test step is often time consuming, and it is of particular interest to estimate, prior to the test, the efficiency of a material from specific characteristics. Therefore, a better understanding of fundamental aspects of the photocatalytic process with Bi2WO6 is mandatory. The issues that have to be addressed are the following: Is there an optimal size and morphology of the particles for photocatalysis applications? What are the active radicals photogenerated in aqueous solution? What is the amount of exciton generated and their recombination behavior? The question of the morphology was treated in a previous publication13 through high-resolution microscopy techniques and well-defined catalytic tests coupled with the use of the MUSIC model to estimate the relative acidity of the Bi2WO6 surfaces.36,37 In the present paper, we report the study of Bi2WO6, which has been synthesized by a simple aqueous process allowing us to obtain nanocrystals. The effect of microwave heating on particle characteristics is described in detail as well as why the organic additive sodium dodecylsulfate was selected after the testing of more than ten other additives. The 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 go deeper into the difference in photocatalytic properties between the visible light photocatalyst Bi2WO6 and the UV one TiO2, a system of light-emitting diodes (LEDs) with narrow wavelength ranges was used for the decomposition of stearic acid. The ability of the synthesized bismuth tungstate materials to photogenerate excited electrons and holes was studied through time-resolved microwave conductivity (TRMC) experiments, and their ability to generate different types of radicals in aerated aqueous solutions was analyzed using electron paramagnetic resonance (EPR) spectroscopy via a spin trapping technique. From the different experiments it was possible to confirm the calculated positions of the valence and conduction bands of that semiconductor and to explain the good (but still limited in comparison with TiO2) photocatalytic activity of Bi2WO6 and to propose the methods of its improvement.

2. EXPERIMENTAL SECTION 2.1. Syntheses. All samples were synthesized by precipitation in aqueous solution. All chemicals were analytical 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)3·5H2O in 1.5 mol·L−1 nitric acid. Bi2WO6-Hydro: A tungstic acid solution (25 mL, 0.1 mol·L−1) was prepared by the dissolution of H2WO4 (0.63 g) in 1.5 mol· L−1 nitric acid. Amounts of 7.5 mL of the bismuth and tungstic acid stock solutions were mixed and transferred into a 20 mL Teflon-lined autoclave and then heated at 200 °C during 24 h. The autoclave was let to cool at room temperature. Bi2WO6MW: A sodium tungstate solution (25 mL, 0.1 mol·L−1) was prepared by the dissolution of Na2WO4·2H2O (0.63 g) in 22657



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acid degradation tests were performed using a different LED set with different maximum wavelength (UV λmax = 385 nm, blue λmax = 445 nm, cyan λmax = 501 nm, amber λmax = 592 nm) as the photon source. Additional experimental details on the photodegradation system, the LED used, and the measured irradiance on the photocatalysts are reported in the Supporting Information. 2.4. EPR Study of Radical Formation. The formation of hydroxyl radicals and superoxide radical anions upon irradiation of Bi2WO6 and TiO2 aqueous suspensions was investigated by the EPR spin-trapping technique using a spin-trapping agent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO; Aldrich). DMPO was freshly distilled before application and stored under argon at −18 °C. The TiO2 and Bi2WO6 suspensions containing DMPO were freshly prepared following a method depending on the radical targeted. For the hydroxyl radical trapping: 50 μL of photocatalyst suspension (1 g·L−1) in water + 25 μL of DMPO ([DMPO]i = 25 g·L−1) in water + 125 μL of water. For the superoxide radical anion trapping: 50 μL of photocatalyst suspension (1 g·L−1) in dimethylsulfoxide (DMSO; Merck Seccosolv max 0.05% H2O) + 25 μL of DMPO ([DMPO]i = 25 g·L−1) in DMSO + 125 μL of DMSO. The carefully mixed suspensions were immediately transferred to a quartz flat cell optimized for the TE102 cavity and were saturated in air with a syringe prior to irradiation. The samples were irradiated three minutes at 295 K directly in the EPR resonator, and EPR spectra were recorded in situ using an EMX X-band EPR spectrometer (Bruker, Germany). As an irradiation source an UV LED was used (λmax = 365 nm; Bluepoint LED, Hönle UV Technology, Germany). The radiation flux 160 W·m−2 was measured in the active part of TE102 cavity using an UVX radiometer (UVP, USA). The experimental EPR spectra acquisition and integration were carried out using WINEPR standard programs (Bruker); their simulations were calculated using the Winsim2002 software.38 The concentration of photogenerated adducts was determined from double-integrated spectra using aqueous solutions of 4-hydroxy-2,2,6,6tetramethylpiperidine N-oxyl (Tempol; Aldrich) as calibration standards measured under strictly identical EPR instrument settings. 2.5. Electronic Properties. The charge-carrier lifetimes in TiO2 after UV illumination have been determined by microwave absorption experiments using the time-resolved microwave conductivity (TRMC) method.39−41 TRMC measurements were carried out as previously described.39,42,43 The incident microwaves were generated by a Gunn diode in the Kα band (28−38 GHz). Experiments were performed at 31.4 GHz, the frequency corresponding to the highest microwave power. The pulsed light source was a Nd:YAG laser providing an IR radiation at 1064 nm. The full width at half-maximum of one pulse was 10 ns, and the repetition frequency of the pulses was 10 Hz. UV light (355 nm) was obtained by tripling the IR radiation. The light energy density received by the sample was 1.3 mJ·cm−2.

Figure 1. Powder XRD patterns of Bi2WO6-Hydro (a), Bi2WO6-MW (b), and Bi2WO6-MW-SDS (c).

diffraction patterns are in fact a composition of several diffraction lines, and the Sherrer equation for the determination of crystalline particles size is hardly applicable. Size, Morphology, and Surface Area. The synthesis by soft chemistry allowed us to produce submicronic particles with high surface area compared to the conventional solid-state technique.24,44 Moreover, microwave-assisted syntheses are generally known to yield smaller nanoparticles than their conventional heating counterpart.28,45 The size evolution with the preparation mode is directly observed in SEM reported in Figure 2 and in TEM in Figure 3. In any case the classical platelet shape of bismuth tungstate is obtained, but the platelets do not stack in particular habits. The most significant evolution observed is the reduction of particle width from Bi2WO6-Hydro (polydisperse from 500 nm to 2 μm) to Bi2WO6-MW (100− 400 nm), and the smallest is Bi2WO6-MW-SDS (40−100 nm). The thickness is more difficult to determine, but it is also decreasing from 30 to 40 nm for the sample obtained in an autoclave to 10−20 nm for those prepared in a microwave oven. A previous study on the first sample indicated that the more active surfaces for catalysis in Bi2WO6 are the lateral facets of the platelets rather than the basal ones.13 The TEM images of the different samples of the present study were carefully analyzed, and the nature of the lateral faces as well as their relative ratio seemed unchanged with the preparation mode. An indirect way to study the particle dimensions (combined with their aggregation state) is the determination of the specific surface. Bi2WO6-Hydro displays a surface of 9 ± 0.5 m2 g−1; Bi2WO6-MW is made of smaller nanoparticles and consequently a larger surface of 16 ± 0.5 m2 g−1; and Bi2WO6-MW-SDS displays the larger specific surface with 21 ± 0.5 m2 g−1. The choice of the microwave as a heating mode for the preparation of Bi2WO6 has already been studied previously, but surprisingly, the comparison with the same experiment performed in the same solvent in the autoclave was never proposed. Moreover, among the numerous organic additives tested to control the morphology of Bi2WO6 (polyols such as ethylene glycol, dicarboxylic acids such as oxalic, succinic, or tartric acid, and tensioactives such as cetyl trimethylammonium bromide), the sodium dodecylsulfate was found to be the most efficient in changing the particle

3. RESULTS AND DISCUSSION 3.1. Sample Characteristics. Crystalline Structure. All synthesized samples contain an unique well-crystallized phase (Figure 1) of bismuth tungstate exhibiting an orthorhombic structure with lattice parameters a = 0.5457 nm, b = 1.6436 nm, and c = 0.5438 nm (JCPDS 39-0256). The relative intensity of the different diffraction peaks seems unaltered by the reaction conditions. Moreover, most of the peaks observed in the 22658



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Figure 2. SEM images of Bi2WO6-Hydro (a,b), Bi2WO6-MW (c,d), and Bi2WO6-MW-SDS (e,f).

Figure 3. TEM images of Bi2WO6-Hydro (a), Bi2WO6-MW (b), and Bi2WO6-MW-SDS (c).

to change the growth kinetics of the different faces like it was shown on other metals or oxides.46−48 Unfortunately, none of the additives used could overcome the layered topology of the Bi2WO6 structure that forces the particle to adopt the platelet morphology. Only SDS could reduce the particle size of another factor two in comparison to the microwave-assisted

dimensions in the microwave-assisted synthesis. Indeed, the use of microwave heating allowed dividing by five the width of the particles. This may be due to a faster and more homogeneous heating in the microwave. However, the platelet morphology could not be modified. A selective adsorption of organic molecules on lateral faces of the Bi2WO6 platelets was expected 22659



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Table 1. Characteristics of the Different Bismuth-Based Oxidesa stearic acid degradation constant (× 10−2 min−1) Bi2WO6-Hydro Bi2WO6-MW Bi2WO6-MW-SDS TiO2 P25 a

−1

surface area (m ·g )

band gap (eV)

visible light

9 16 21 49

2.80 2.84 2.84

1.38 ± 0.04 5.7 ± 0.17 8.5 ± 0.7

2

UV LED

blue LED

140 ± 10 300 ± 40

8±1 0

Note: Measurement error on band gap value is 0.01 eV and on surface area is 0.5 m2·g−1 (UV LED, λmax = 385 nm, Blue LED, λmax = 445 nm).

synthesis without additives. Sulfur-containing species may remain at the surface of the particles after the different washings when using SDS as additive, but X-ray photoelectron spectroscopy and thermogravimetric analysis coupled with mass spectrometry confirm the absence of any leftovers. The use of microwave heating or that of additives may change other properties of the material and consequently its photocatalytic behavior. Optical Band Gap. Optical absorptions of samples are reported in the Supporting Information. The method used to determine the optical band gap from the absorption spectrum is reported in a previous article.13 In the case of Bi2WO6, the transition is known to be direct,49,50 and the obtained values are reported in Table 1. The three materials only differ by 0.04 eV which is very close to the experimental error. The values found are in good agreement with the data reported in the literature and with the very light green color of the powders. 3.2. Photocatalytic Properties. The photocatalytic properties of the three Bi2WO6 materials were first 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. The presence of light and photocatalyst is crucial here, as RhB is not degraded under illumination either in the absence of photocatalyst or in the dark in the presence of the photocatalyst. As shown in Figure 4, all samples photodecompose RhB under visible light. In the presence of Bi2WO6, the absorption peak is blue-shifted in addition to its global decrease upon exposure. Indeed, RhB is degraded via two different reaction pathways: degradation of the conjugated structure due to a photocatalytic process51−53 or degradation via N-de-ethylation of RhB in a stepwise process.21,53−55 According to published studies on TiO2 and Bi2WO6, the N-deethylation process can be assigned to a photosensitized process where the RhB transfers its electron to the conduction band of the photocatalyst.51−53,56 After the transfer, RhB is degraded by successive N-de-ethylation reactions. The combination of the two pathways leads to faster degradation with Bi2WO6-MWSDS and Bi2WO6-MW than with Bi2WO6-Hydro. Moreover the N-de-ethylation of RhB is favored with samples prepared in the microwave oven indicating a higher adsorption of the dye on the photocatalyst. The initial adsorption of the dye before the beginning of the photodegradation is even more pronounced for Bi2WO6-MW-SDS (with a decrease of 44% of Aλ=555 nm instead of 22% for Bi2WO6-Hydro). This can be directly correlated with the evolution in specific surface of the sample. The differences observed in the competition between the two proposed pathways may be related to a slight increase in the thickness/width ratio of the platelets that promote the presence of more acidic surfaces as demonstrated previously.13 The adsorption of RhB is then favored, and the N-de-ethylation

Figure 4. Visible spectra changes of RhB in aqueous solution in the presence of Bi2WO6-Hydro (a), Bi2WO6-MW (b), and Bi2WO6-MWSDS (c) with irradiation time (20 min interval between t = 0 min and t = 120 min) under visible light.

mechanism is more pronounced. A similar test performed under UV-A irradiation has shown no wavelength shift of RhB maximum absorbance. This confirms that the N-de-ethylation 22660



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reactions observed under visible light come from a photosensitized process.49 Stearic Acid Photodegradation. The photocatalytic activity was also studied on films during the degradation of stearic acid under visible light. Stearic acid was chosen for two main reasons: (i) It does not absorb in the visible range although 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 self-cleaning glasses. As shown in Figure 5a, all films degrade stearic acid under visible light, but Bi2WO6-

Figure 6. Optical absorption spectra of Bi2WO6-MW-SDS (light gray) and TiO2 P25 (black) superimposed on emission spectra of UV LED (dotted line) and blue LED (dash line) sources.

1. Using an UV LED with a maximum emission at λmax = 385 nm, the reference TiO2 catalyst is twice as efficient as Bi2WO6 despite a smaller overlapping of the absorption band with the UV lamp emission spectrum as shown in Figure 6. This evidences the better photon harvesting efficiency of TiO2 P25 in the 360−400 nm region and a better conversion of the generated holes and electrons into active radicals. With the blue LED, the TiO2 photocatalyst is no more active as confirmed by the absence of overlapping of the absorption band and the emission spectrum. On the contrary, the bismuth tungstate is still active under blue light, but with a value of kinetic constant divided by a factor of 20 when compared to the UV LED experiment, while the irradiance only decreased from 393 to 354 W m−2. As UV photons are more energetic than blue ones, the corresponding photon flux is not the same for the two experiments. Consequently, the photon flux for the two lamps was measured at different working intensity (results reported in the Supporting Information), and the intensity of each system was set to perform the degradation test with each lamp at a photon flux of 525 μmol s−1 m−2. In this case the kinetic constants with Bi2WO6-MW-SDS are kUV = 1.2 ± 0.01 min−1 and kblue = 0.08 ± 0.01 min−1. Bismuth tungstate is 15 times more active under UV light than under blue light. This significant difference may be first explained by the fact that the light absorption is lower for Bi2WO6 in the blue region than in the UV range. Moreover, the use of a less energetic photon may promote photogenerated electrons just at the lower edge of the conduction band, while that of a more energetic photon will create electrons and holes more profound in the valence and conduction bands that may react differently before being thermalized. To go more into detail on the efficiency of the different samples to generate electrons and holes from harvested photons, time-resolved microwave conductivity experiments were performed. 3.3. Charge Carrier Dynamics. In a TRMC experiment, the intensity of the obtained signal is proportional to the product of the amount of charge carriers and their mobility. A detailed description of the technique is provided elsewhere.42,43,57 It is commonly accepted that in titanium dioxide

Figure 5. Photocatalytic degradation under visible light of stearic acid deposited on films of Bi2WO6-Hydro (black circle), Bi2WO6-MW (gray triangle), and Bi2WO6-MW-SDS (light gray square). Inset: Formal zero-order kinetic constant of the different Bi2WO6 samples plotted against their specific surface.

MW-SDS has the highest photocatalytic activity. The degradation reaction can be mathematically (although it has not a truly chemical background) described by a formal zeroorder kinetics: A(t) = A(t = 0) − k × t with A(t) being the integrating area from 2800 to 3000 cm−1 corresponding to C− H bonds evaluated after irradiation time t. The k values obtained from the modeling of degradation curves are reported in Table 1. Moreover, these kinetic constants are plotted against the specific surface, obtaining good linear correlation (inset in Figure 5). This clearly demonstrates that the improved photocatalytic efficiency of Bi2WO6-MW-SDS is due to a larger catalyst surface and, consequently, that in the case of stearic acid the interface reactions are the limiting step of the catalytic process. To determine more precisely the efficiency of our best Bi2WO6 photocatalyst, namely, Bi2WO6-MW-SDS in the UV and the visible range it was compared to reference catalyst TiO2 P25 from Evonik for the degradation of stearic acid under the irradiation of different LED sources. Optical absorptions of Bi2WO6-MW-SDS and TiO2 P25 are superimposed on the emission spectra of the four LEDs used for the degradation tests in Figure 6. The emission wavelengths of the cyan and amber LEDs correspond to photon energies lower than the band gap values of both photocatalysts, and consequently no degradation of stearic acid could be detected even after 20 h. Using the blue and UV LED sources, kinetic constants could be calculated, and the corresponding values are reported in Table 22661



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(O2•−) are paramagnetic species, but their lifetimes are much too short for detection using the direct EPR technique in aqueous media. Consequently an EPR spin trapping method was developed to allow an easy detection of low concentrations of photogenerated radicals and to distinguish the different radicals.58 The spin trapping agent used, DMPO, may form corresponding paramagnetic spin adducts with both photogenerated species, i.e., •DMPO−OH and •DMPO−O2−. The stability of spin adducts depends on the nature of the solvents, and in aqueous systems pH value plays a dominant role. However, alternative nonradical reactions may also produce the detected spin adducts, e.g., formation of •DMPO−OH via hydrolysis of DMPO•+ originating from one-electron oxidation of DMPO.59 That is why the EPR experiments must be carefully done using blank samples and references to calibrate the concentration results. The formation of •DMPO−OH can be directly studied in water, and the hydroxyl radical trapped may either come from the interaction between water and photogenerated holes according to eq 1 or from the evolution of superoxide radical anions according to eqs 2−4.60

free charge carriers are the electrons rather than the holes42,57 and that the decrease of the TRMC signal corresponds to their disappearing by recombination. The maximum intensity of the signal is related to the photon conversion ability of the material. However, as the intensity of the TRMC is also proportional to a sensibility factor that depends on the microwave frequency and the material conductivity, it is impossible to compare with this technique two materials differing in the chemical composition, and sometimes an important difference in preparation may prevent a correct comparison. However, the analysis and comparison of signal decrease is still possible. The short time range (0−100 ns) decrease rate is correlated to the recombination rate of the charge carriers (free holes and electrons). The long time range (after 100 ns) decrease rate is due to recombination between electrons and relaxed trapped holes. The TRMC signals of the three Bi2WO6 samples as well as that of TiO2 P25 (with a scaling factor of 1/4 on the intensity) are reported in Figure 7.

H 2O + h+ → •OH + H+

(1)

O2•− + H+ → •O2 H

(2)

2•O2 H → H 2O2 + O2

(3)

H 2O2 + O2•− → •OH + O2 + HO−

(4)

Upon UV irradiation of the aerated TiO2 or Bi2WO6 suspensions in water in the presence of DMPO, typical fourline EPR spectra were monitored, characterized by spin Hamiltonian parameters aN = 1.505 mT, aHβ = 1.472 mT, and g = 2.0059 attributed to the hydroxyl radical added to DMPO (•DMPO−OH), as is shown in the inset of Figure 8. All the samples containing a photocatalyst suspension reveal under UV irradiation a •DMPO−OH concentration significantly higher than the blank experiment as reported in Figure 8. This clearly demonstrates that all the tested photocatalysts are able to generate hydroxyl radicals. This is widely accepted in the case of TiO2,61−65 but it is quite new for Bi2WO6. Indeed, there is only one study that unsuccessfully tried to detect those radicals in Bi2WO6.66 According to our experiment, the amount of hydroxyl radical adducts generated by Bi2WO6 is only 15% of that by TiO2 P25. The same EPR experiment was done again with another radiation LED source (λmax = 400 nm instead of λmax = 365 nm previously), and the radical concentrations measured with TiO2 P25, Bi2WO6-Hydro, Bi2WO6-MW, and Bi2WO6-MW-SDS are, respectively, 2.3, 1.3, 0.8, and 1.4 μmol· L−1. These values cannot be correlated with those determined with the first LED because of the difference of light irradiance in the EPR cavity. However, this new set of radical concentrations shows that TiO2 is only twice more efficient than Bi2WO6 to generate •OH radicals when absorbing a λmax = 400 nm irradiation. Such an evolution in radical generation can then be correlated with the differences in light absorption of TiO2 and Bi2WO6 photocatalysts. The second radical •DMPO-O2− cannot easily be detected in aqueous solvent for the following main reasons: the stability of a superoxide radical anion in water is limited; O2•− is only weakly reactive toward DMPO in aqueous media (the kinetic constant is 8 orders of magnitude lower than that with •OH); and the spin adduct is not very stable in water (t1/2 = 80 s).67,68 A good way to avoid such limitation is to replace water by an

Figure 7. TRMC signals of Bi2WO6-Hydro (black full line), Bi2WO6MW (gray line), Bi2WO6-MW-SDS (light gray line), and TiO2 P25 (dashed line; scaling factor of 1/4 on the intensity).

The maximum intensity of the signal with Bi2WO6-MW-SDS is about 30% higher than for the other two materials, but its decrease in the very first tens of nanoseconds after the pulse is also more pronounced. Consequently, in the smaller nanoparticles more free holes and electrons are generated but are not separated fast enough and are rapidly recombined. This is less pronounced with the other two bismuth tungstate samples. After that first rapid decrease, the slopes of the three Bi2WO6 are very similar and seem to be more pronounced than that of TiO2. Consequently, titanium dioxide may be more efficient in separating holes and electrons by trapping one of them long enough to avoid undesired recombination. When those charge carriers are close enough to the surface they may generate different kinds of radicals such as hydroxyl radicals (•OH) or superoxide radical anions (O2•−) depending, among other parameters, on the relative position of the valence and conduction band of the photocatalyst. To define more precisely the reactive radicals generated by the bismuth tungstate system, EPR experiments were performed. 3.4. Determination of Generated Radicals. Both hydroxyl radicals (•OH) and superoxide radical anions 22662



Article • DMPO-OCH3, produced via a photocatalytic reaction pathway involving the DMSO solvent and surface hydroxyl groups/ water molecules. (Details on the EPR spectrum of •DMPOOCH3 and the supposed reaction pathway are reported in the Supporting Information.) Consequently, the concentrations of the two spin adducts •DMPO-O 2 − and • DMPO-OCH 3 obtained upon UV irradiation (λmax = 365 nm) in the different photocatalyst suspensions are determined and reported in Figure 9. The only photocatalyst that produced significant amounts of spin adduct related to O2•− in DMSO is TiO2 which is in good agreement with previous studies.62,65 The measured concentration of those spin adducts in Bi2WO6 suspension is only slightly above the blank values and may not be undoubtedly attributed to the photogeneration of superoxide radical anions. Indeed, other phenomena (between DMPO and photogenerated electrons and holes or else) may produce few amounts of O2•−. Consequently, Bi2WO6 is not likely to produce the superoxide radical anions and to use them in photodegradation reactions. 3.5. Insights into Bi2WO6 Properties As a Photocatalyst. To degrade efficiently a wide range of pollutants, a good photocatalyst must produce a high amount of active radicals from the harvested photons. The direct use of surfacetrapped holes and electrons on the photocatalyst may induce a degradation mechanism for some molecules; however, for photo-oxidation the presence of hydroxyl radicals is more efficient, and that of superoxide radical anions may also help. The corresponding redox couples •OH/H2O and O2/O2•− display a potential vs SHE of 2.38 and −0.33 V, respectively, in water.69,70 This means that a photocatalyst that is able to form both radicals under UV irradiation should have these two potential values between the edges of the valence and the conduction bands. Consequently, the gap of the photocatalyst should be bigger than 2.71 V which is the case of TiO2 but very close for Bi2WO6. Depending on the relative band edge position, this visible light photocatalyst may generate one of two radicals, and it is •OH as demonstrated in the EPR study. This means that the photogenerated electrons are not energetic enough to react with molecular oxygen and are more likely recombined with holes. The increased recombination may be seen directly in TRMC experiments and indirectly in the catalytic tests and the concentration of •DMPO-OH spin adducts in EPR studies. A good way to improve the Bi2WO6 photocatalysts would be to trap the photogenerated electrons far from the active holes using heterostructured systems. The positions of the valence and conduction bands of Bi2WO6 were calculated from the band gap value and the Mulliken electronegativity using the following eqs 5 and 671

Figure 8. Measured •DMPO−OH spin adduct concentration in the presence of different photocatalysts under UV light (λmax = 365 nm). Inset represents experimental (solid line) and simulated (dotted line) EPR spectra of the •DMPO−OH spin adduct obtained upon 3 min irradiation of Bi2WO6-MW-SDS in aqueous aerated suspensions containing DMPO. (Magnetic field sweep 8 mT.)

aprotic solvent such as DMSO.65,67 Upon UV irradiation of the aerated TiO2 or Bi2WO6 suspensions in DMSO in the presence of DMPO, the twelve-line EPR signal dominated in the spectra, characterized by spin Hamiltonian parameters aN = 1.273 mT, aHβ = 1.034 mT, and aHγ = 0.138 mT; g = 2.0059 is attributed to the superoxide radical anion added to DMPO (•DMPOO2−), as is shown in the inset of Figure 9.68 Another spin adduct was observed in the experimental spectra, namely,

E VB = χ − E e + ECB = χ − E e −

Eg 2

(5)

Eg

(6) 2 with χ being the geometric mean value of Mulliken electronegativity of the different atoms of the material, Ee the free electron energy (Ee = 4.5 V vs SHE), and Eg the measured band gap of the photocatalyst (experimental results gave Eg = 2.9 V). The equations gave for Bi2WO6 values of EVB = 3.15 V and ECB = 0.25 V vs SHE. The conduction and valence band positions are reported in Figure 10 and compared to those calculated for TiO2 and to the radical couple positions.

Figure 9. Measured •DMPO-O2− and •DMPO-OCH3 spin adduct concentrations and their sum in the presence of different photocatalysts under UV light (λmax = 365 nm). Inset represents experimental EPR spectrum of DMPO spin adducts obtained upon 3 min exposure for TiO2 P25 in aerated DMSO suspensions. (Magnetic field sweep 7 mT.) 22663



Article

that are able to reduce the photocatalyst particle size. This decrease in particle size is the main explanation for an improved activity. It was however impossible to change drastically the morphology of the Bi2WO6 platelets to express more of the active faces in RhB degradation. We demonstrated using LED sources with selected irradiation wavelength that the photocatalytic activity is tightly linked to the amount of photons converted into photogenerated electrons and holes by the semiconductor. Moreover we characterized for the first time the nature of the active radicals generated by Bi2WO6, and we correlated that with the calculated band diagram of that semiconductor. This allowed us to explain the relatively low activity of bismusth tugstate when compared to TiO2 by the fact that fewer radicals are generated because less photons are harvested and more exciton pairs recombine rapidly because electrons are not reacting with dioxygen. The improvement pathways for that promising visible-light photocatalyst are to trap the electrons in a metal dispersed at its surface or to use it combined in a heterostructure with another semiconductor with the adequate band positions.

Figure 10. Calculated valence and conduction band positions for TiO2 and Bi2WO6 and the potential of the radicals’ redox couples •OH/ H2O and O2/O2•−.

■

S Supporting Information *

This figure clearly demonstrates that the main drawback of the visible-light photocatalyst is that photogenerated electrons and holes cannot be both converted into active radicals and that unless a good solution is found to segregate them they are prone to recombine more than in TiO2. However, as some holes were used in the photocatalytic oxidation of the pollutants, this means that they did not recombine with electrons and that these electrons must have reacted in some way. A possible consumption of the electrons still involving molecular oxygen is the two-step reaction involving at each step only one electron as described below O2 + H+ + e− → •O2 H •

O2 H + H+ + e− → H 2O2

ASSOCIATED CONTENT

To have more details about the optical absorption spectra of Bi2WO6 samples, the LED system used in that study, about the EPR spectrum of •DMPO-OCH3, and the mechanical pathway of its formation, additional data are exposed in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +33144271543. Fax: +33144271504. E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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(8)

ACKNOWLEDGMENTS This research work was supported by Saint − Gobain Recherche. We also wish to thank Sandrine Clary-Lespinasse for technical support, Corinne Papret for Scanning Electron Microscopy, and Patricia Beaunier for Transmission Electron Microscopy. We are also grateful to Dr. Eric Puzenat from IRCELYON for his precious help in LED photon emission determination. V.B. is acknowledged for financial support by Scientific Grant Agency of the Slovak Republic (VEGA project 1/0289/12).

The redox potentials of these two reactions are −0.15 and 1.5 V vs SHE, respectively,61 and the fact that the photocatalytic reaction is not performed at pH = 0 makes the two reactions theoretically possible. Indeed, theoretical and experimental features indicate that the relative positions of the conduction band edge of BiWO6 and that of the one-electron reduction molecular oxygen electrode potentials may not exactly be as described in Figure 10. First, according to the Nernst equation the conduction band edge of that semiconductor is shifted to negative direction. Then, an experimental study demonstrated the ability of bismuth tungstate to evolve H2 from aqueous methanol solutions72 indicating that photogenerated electrons at the bend edge may be more negative than 0 V vs SHE which is more negative than the calculated values from the simple model described above. Consequently, the simple scheme of Figure 10 must be refined using stronger experimental results such as XUPS analyses and pH-dependent photoelectrochemical studies with different amounts of molecular oxygen available in the system.

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REFERENCES

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4. CONCLUSIONS Under visible light, Bi2WO6 shows a photocatalytic activity for the degradation of RhB in solution and decomposition of stearic acid deposited on films that may be improved by using microwave-assisted syntheses of Bi2WO6 and organic additives 22664



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New Insights into Bi2WO6 Properties as a Visible-Light Photocatalyst

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