New Insights Into BiVO4 Properties as Visible Light Photocatalyst

Article pubs.acs.org/JPCC

New Insights Into BiVO4 Properties as Visible Light Photocatalyst Tamar Saison,†,‡ Nicolas Chemin,‡ Corinne Chanéac,† Olivier Durupthy,*,† Laurence Mariey,§ Françoise Maugé,§ Vlasta Brezová,∥ and Jean-Pierre Jolivet† †

Laboratoire de Chimie de la Matière Condensée de Paris, Collège de France, UPMC Université Paris 06, Sorbonne Universités, CNRS, 11 place Marcelin Berthelot, 75005 Paris, France ‡ Service Produits Composites et Revêtement de Surfaces, Saint-Gobain Recherche, 39 quai Lucien Lefranc, BP 135, 93303 Aubervilliers, France § Laboratoire de Catalyse et Spectrochimie de Caen, ENSICAEN, Université de Caen, CNRS, 6 boulevard Maréchal Juin, 14050 Caen, 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 S Supporting Information *

ABSTRACT: Bismuth vanadate has attractive photocatalytic properties under visible light. The influence of structure and morphology of BiVO4 nanomaterials on its photocatalytic properties in the UV and the visible domain was investigated. The selection of different sets of synthetic parameters in aqueous solutionpH or the use of organic additivesallowed the formation of tetragonal zircon, tetragonal scheelite, and monoclinic scheelite structure and different morphologies of that last phase. First, the tetragonal zircon was found to be the only inactive structure. Then, the best material for photocatalytic degradation of rhodamine B in solution and stearic acid deposited directly on the photocatalyst is the core−shell tetragonal zircon−monoclinic scheelite system prepared in the presence of sodium dodecyl sulfate. The enhanced properties are explained by the presence of strong surface acidic sites corresponding to the presence of surface sulfate residues rather than to the specific morphology of the material. Additionally, an EPR study on the ability of BiVO4 to generate active surface radical showed that hydroxyl radicals are not generated and that superoxide ion concentration under irradiation is close to the detection threshold. Depending on the selected irradiation wavelength, bismuth vanadate may present a better photocatalytic activity than titanium oxide. It is shown to be equivalent to bismuth tungstate under blue light.

1. INTRODUCTION Titanium oxide is the most studied and commercialized photocatalytic material for air and water decontamination under UV irradiation.1−3 However, TiO2 only harvests ultraviolet irradiation that is to say less than 5% of the solar spectrum and cannot be used for indoor applications where no UV irradiation is present. Alternatively to the surface or bulk doping strategies aiming to extend the TiO2 light absorption domain to the visible region,4−7 an emerging pathway is to the design completely different materials absorbing photons in the visible range.8−11 With an appropriate band gap energy due to a 6s Bi and 2p O orbitals hybridation at the valence band edge,12−16 bismuth based oxides appear to be good candidates for visible light pollutant photodegradation. In a previous study we have compared the photocatalytic efficiency of three of these bismuth based oxides, BiVO4, Bi2O3 and Bi2WO6 prepared from aqueous solution syntheses.9 The focus was put on Bi2WO6 surfaces and on the characteristics required to prepare an efficient photocatalyst for photodegradation of rhodamine B (RhB) in aqueous solution and stearic acid on a film.17 The choice of Bi2WO6 was motivated by its better photocatalytic activity. We focus here on the © 2015 American Chemical Society

BiVO4 structure that presents an interesting polymorphism and that is intensively studied for water-splitting applications.18 The requirements to get a good photocatalyst for depollution are slightly different from those for water splitting and the comparative efficiency of the different polymorphs was properly studied only once.19 As the tetragonal scheelite polymorph was first declared to be a poor photocatalyst, very few photocatalytic studies are devoted to that material.20−22 For the monoclinic scheelite phase, the efficiency as electrode for the water splitting is now well described because the valence and conduction bands are well positioned toward the water redox couples. 23,24 Oppositely, less is known in the photocatalytic domain. Is that structure able to produce active radical species such as hydroxyl radicals •OH and superoxide ions O2•− or does it operate only via photogenerated holes and electrons trapped on surface sites?25 Indeed, the degradation of pollutants such as phenol that poorly adsorb on photocatalyts surface requires Received: February 12, 2015 Revised: May 20, 2015 Published: May 21, 2015 12967

DOI: 10.1021/acs.jpcc.5b01468 J. Phys. Chem. C 2015, 119, 12967−12977

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The Journal of Physical Chemistry C

BiVO4-pH4. A 25 mL aliquot of vanadate stock solution was added to 25 mL of bismuth stock solution in order to reach a ratio of Bi/V = 1. The pH of the solution was then set to 4 using potassium carbonate. The mixture was left under stirring for 3 weeks at room temperature. The washing of collected precipitates were done three times with water and dried under nitrogen flux. Films of bismuth vanadate have been processed by spincoating deposition on a glass substrate of 0.1 g of photocatalyst dispersed in 10 mL of tetrahydrofuran. Films were dried at room temperature under air. Titanium dioxide Aeroxide P25 (Evonic Degussa, Germany) and Bi2WO3-MW-SDS (prepared according to ref 17) were used for the comparison of photocatalysts activity. 2.2. Characterizations. UV−visible diffuse reflectance spectra (DRS) of the samples were recorded on a VarianCary 5E spectrometer in the 300−800 nm range using an integrating sphere. Powder X−ray diffraction (XRD) measurements were performed with a Bruker 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. The Raman spectra were recorded on a ThermoNicolet Raman spectrometer in the range 95 to 1274 cm−1 with a 2.5 cm−1resolution with a laser working at 785 nm. Platinum coated particles have been observed by field emission gun scanning electron microscopy using a LEO DSM 982 Gemini at a voltage of 15 kV. Nitrogen adsorption−desorption measurements were conducted on a Micromeritics Tristar apparatus at 77 K. Specific surface areas were determined following the Brunauer− Emmet−Teller analysis. The particles morphology and orientation were studied by transmission electron microscopy using JEOL JEM 100 CX (100 kV) apparatus. Samples were ground and 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. A calibration using the Au pattern was done prior to d-spacing measurements on the selected area electron diffraction (SAED) patterns. 2.3. Photocatalytic Tests. The aqueous degradation of a RhB solution under visible light irradiation was used as first photocatalytic test. An artificial solar irradiation (Atlas Suntest Model XLS+) with the 400 nm cutoff filter at 400 W/m2 has been taken. Typically, 0.5 g of photocatalyst was poured into 100 mL of a RhB solution with an initial absorbance value at 555 nm of A = 1.1. Alternatively for the most active material BiVO4-SDS, a second set of experimental conditions was used: 0.1 g of photocatalyst was poured into 100 mL of a RhB solution with an initial absorbance at 555 nm of 1.8. The adsorption−desorption equilibrium of the dye on the photocatalyst’s surface was obtained through the stirring of the suspension in the dark during 1 h prior to the beginning of the 2 h irradiation. Then, 3 mL aliquots were withdrawn and centrifuged to remove the particles every 20 min. The filtrates were analyzed by visible spectrophotometry (400−800 nm) using Ocean Optics HR4000 high-resolution spectrometer. A second photocatalytic test of the stearic acid degradation under visible light was done using the same light source as the previous test. A thin film of stearic acid in ethanol was deposited on photocatalyst film by spin-coating. The stearic acid degradation has been followed using infrared (IR) spectroscopy on a Nicolet Nexus 6700 with a deuterated triglycine sulfate (DTGS) detector. Its concentration has been evaluated by integrating area from 2800 to 3000 cm−1. Finally,

such radicals and the reported degradation of that molecule by BiVO 4 is always done in the presence of hydrogen peroxide.26,27 Moreover, the surface chemistry of bismuth vanadate is not, to the best of our knowledge, properly described in the literature, despite it having a tremendous impact both on photocatalytic processes and on water splitting reactions. The effect of the morphology on the BiVO4 has already been discussed in several studies,28−33 but it has been correlated to the ability of charge carriers to migrate to specific surface rather than to surface properties. In the present paper, we report several syntheses for the three polymorphs of BiVO4 as pure phase, the monoclinic scheelite structure with different exposed faces and even a core−shell BiVO4 sample with inorganic surface additive. The photocatalytic activity of all those samples 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. For that second test a comparison was also performed between the best BiVO4 sample, a Bi2WO6 sample and TiO2 P25 under blue light. The different types of active radicals photogenerated in aerated suspensions by the synthesized bismuth vanadate materials was analyzed using electron paramagnetic resonance (EPR) spectroscopy coupled with spin trapping technique. The surface acidity of the different synthesized morphologies of the monoclinic scheelite structure of BiVO4 was also studied in gas phase with pyridine adsorption and correlated to the photodegradation of rhodamine B. The calculated positions of both the valence and conduction bands were confirmed, and the fair (but still limited in comparison with TiO2) photocatalytic activity of BiVO4 was explained. The increased efficiency obtained through structure, morphology, and surface additive control was also discussed.

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. A vanadate stock solution with a concentration of 0.2 mol·L−1 was prepared by dissolution of NaVO3 in distilled water. BiVO4-TZ. A 25 mL aliquot of vanadate stock solution was added to 25 mL of bismuth stock solution in order to reach a ratio of Bi/V = 1. The mixture was left under stirring 30 min at room temperature. BiVO4-TS. The preparation mode is very similar to that of BiVO4-TZ, with only the aging time of the mixture prolonged to 2 h. BiVO4-MS. A 25 mL aliquot of bismuth stock solution was diluted with 25 mL of water. A powder of V2O5 was then added with a molar ratio of Bi/V = 1. The mixture was left under stirring 3 days at room temperature. BiVO4-X. A 78 mg sample of xylitol was added under constant stirring to 25 mL of bismuth stock solution in order to reach a ratio of Bi/Xylitol =10. Then 25 mL of vanadate stock solution was added. The mixture was left under stirring for 5 weeks at room temperature. BiVO4-SDS. 146 mg of sodium dodecyl sulfate (SDS) were added under constant stirring to 25 mL of bismuth stock solution in order to reach a ratio of Bi/SDS = 10. Then 25 mL of vanadate stock solution were added. The mixture was left under stirring 1 week at room temperature. 12968


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parameters and the reference PDF cards of the three structures are reported in Table 1. The diffraction lines in the three diffraction patterns are very narrow and roughly correspond to the instrumental broadening. Consequently, the Sherrer equation for the determination of crystalline particles size is hardly applicable and crystallites size must be larger than 100 nm. Since the MS structure was targeted in the presence of organic additives or under specific pH conditions, long aging durations in solution were done. In the three syntheses under such conditions, the MS structure was obtained as the major phase (Figure 1d−f). In the XRD data of BiVO4-X and BiVO4pH4, very small diffraction lines corresponding to the TZ phase can be observed (Figure 1d,f). This additional TZ phase can be neglected in these two samples and could not be detected by the electron diffraction experiments. However, this secondary structure is much more evident in the BiVO4-SDS sample (Figure 1e), and its presence is discussed in the following paragraphs. Size, Morphology, and Surface Area. Under the abovedescribed experimental conditions, the mixing of the bismuth and vanadium sources in aqueous solution immediately induces the precipitation of the mixed composition BiVO4 with a micron-sized particles as reported in Figure 2. The SEM images of the reference particles associated with the three structures display roughly round shape particles with 2−5 μm in size that differ in their surface texture (Figure 2a−c). Indeed the BiVO4TZ particles surface is rough but disordered (Figure 2d), while that of BiVO4-TS is composed of small nanometric (50 nm) subunits with an almost square section perfectly oriented in the same direction (Figure 2e). As for BiVO4-MS, the surface seems to be composed of aggregated truncated pyramids more around 200 nm in size (Figure 2f). The morphology of BiVO4X particles is closer to well-shaped octahedrons (Figure 2g) of about 100 nm in size. The particles of BiVO4-SDS are micrometric but seem to result from the stacking of flat triangle tiles rather than cubes of pyramids (Figure 2h). The last BiVO4pH4 sample consists of micrometric platelets made of the oriented aggregation of ideally 50 nm cubic subunits (Figure 2i). This last morphology is in good agreement with the description given in the literature for similar syntheses. In order to give more insights into particles shape and orientation of the particles displaying the MS structure, TEM images of well-dispersed particles or ultrathin slices of the corresponding particles are reported in Figure 3. The shape of BiVO4-MS and BiVO4-X (Figure 3, parts a and b) particles are strikingly similar except for the size which is significantly smaller in the presence of xylitol. As for the nature of the exposed faces, two different kinds of facets seem to be exposed namely the lateral faces of the octahedrons and their truncated summits. However, the exact nature of exposed faces could not be determined for the selected area electron diffraction (SAED) images of octahedrons randomly deposited on a carbon grid gave hardly usable zone axis such as that reported in inset. In spite of this, such a truncated octahedron morphology was already observed in other studies28,31 and the attributed crystal facets are {110} for the octahedron faces and {010} for the truncated top. The faces of the BiVO4-pH4 sample could more easily be assigned thanks to the TEM image reported in Figure 3c and the corresponding SAED in inset. The basal face {001} of the aggregated platelets corresponds to the [001] zone axis of the SAED image. The lateral faces can then be assigned to {100} faces. The preparation of the BiVO4-SDS sample for

stearic acid degradation tests were performed using different LED set with different maximum wavelength (UV λmax = 385 nm, blue λmax = 445 nm, cyan λmax = 501 nm, amber λmax = 592 nm) as photon source. 2.4. EPR Study of Radical Formation. The formation of hydroxyl radicals and superoxide radical anions upon irradiation of BiVO4 and TiO2 aqueous suspensions was investigated by EPR spin trapping technique using a spin trapping agent, 5,5dimethyl-1-pyrroline N-oxide (DMPO; Sigma-Aldrich). Experimental details on the method are reported in a previous article.17 Additional details can also be found in the Supporting Information 2.5. FT-IR Spectroscopy. The acidity of the photocatalysts was studied by IR spectroscopy using adsorbed pyridine as spectroscopic probe molecule. IR spectra were recorded on selfsupported disks (2 cm2, ∼50 mg precisely weighed) which were placed into an IR 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) up 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 (Aldrich, >99%) was dried on molecular sieves prior to its use.

3. RESULTS AND DISCUSSION 3.1. Sample Characteristics. Crystalline Structure. As stated in the literature the monoclinic scheelite (MS) structure of BiVO4 is the thermodynamically stable structure. Consequently, the other two known polymorphs, namely the tetragonal scheelite (TS) and tetragonal zircon (TZ) structures may only be obtained after short aging times in solution to avoid their evolution to the MS structure. Under the conditions of the first three syntheses, a pure and well crystallized phase (Figure 1a−c) of bismuth vanadate was obtained with the TZ, TS, and MS structures, respectively. Details on the lattices

Figure 1. Powder XRD patterns of BiVO4-TZ (a), BiVO4-TS (b), BiVO4-MS (c), BiVO4-X (d), BiVO4-SDS (e), and BiVO4-pH4 (f). 12969


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The Journal of Physical Chemistry C Table 1. Structural Data on the BiVO4 Polymorphs Extracted from the Quoted JCPDS Files structure

tetragonal zircon (TZ)

tetragonal scheelite (TS)

monoclinic sheelite (MS)

[a, b, c] (Å) [α, β, γ] (deg) JCPDS Bi−O (Å)

[7.300, 7.299, 6.457] [90, 90, 90] 14−0133 4 × 2.414; 4 × 2.549

[5.147, 5.147, 11.721] [90, 90, 90] 75−2481 4 × 2.453; 4 × 2.499

[5.194, 5.090, 11.697] [90, 90.38, 90] 10−5711 2 × 2.354; 2 × 2.372 2 × 2.516; 2 × 2.628

Figure 2. SEM images of BiVO4-TZ (a, d), BiVO4-TS (b, e), BiVO4-MS (c, f), BiVO4-X (g), BiVO4-SDS (h), and BiVO4-pH4 (i).

Figure 3. TEM images of BiVO4-MS (a), BiVO4-X (b), BiVO4-pH4 (c), and BiVO4-SDS (d−f) with SAED in inset. Image d corresponds to core crystallites with TZ structure; image e corresponds to shell crystallites with MS structure; image f is a microtome slice of both core and shell crystallites.

TEM analysis according to the procedure described in the Experimental Section led to the breakage of the particles and the observation of two distinct particles populations reported in Figure 3, parts d and e. Small ellipsoidal nanoparticles of about 20 nm form oriented aggregates about one micron in size are observed. SAED analyses confirmed the oriented attachment of those particles with the TZ structure. Additionally, the bigger triangular of diamond shaped nanoparticles of about 200 nm are observed. The SAED pattern indicate a MS structure for those particles and the {001} face as the main exposed surface.

In order to determine the relative position of these two sets of particles we have performed ultrathin slices of the initial aggregate trapped in a resin. The TEM images reported in Figure 3f (enlargement in the inset) show a core−shell structure of the aggregates with small nanoparticles presenting the TZ structure surrounded by larger nanoparticles with the MS structure. These data are in agreement with the SEM and XRD characterizations. An indirect way to study the particles dimensions (combine with their aggregation state) is the determination of their 12970


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The Journal of Physical Chemistry C Table 2. Characteristics of the Different Bismuth Vanadates and Two Reference Samplesa RhB degradation after 2 h of irradiation (%) BiVO4-TZ BiVO4-TS BiVO4-MS BiVO4-X BiVO4-SDS BiVO4-pH4 Bi2WO6-MW-SDS TiO2 P25

stearic acid degradation formal constant (×10−2 min−1)

surface area (m2·g−1)

band gap (eV)

visible light

visible light

UV LED

blue LED

3 1 3 10 11 11 21 49

2.36 2.30 2.46 2.51 2.44 2.47 2.84 3.15

0 28 38 29 100b 0 − −

− − 0 0 1.5 0 8.5 −

− − − − 110 ± 10 − 140 ± 10 300 ± 40

− − − − 9±1 − 8±1 0

Note: Measurement error on surface area is 0.5 m2·g−1 and on band gap value is 0.01 eV. bRhB degradation measured at λ = 555 nm; N-deethylation mechanism is discussed below. UV LED: λmax = 385 nm. Blue LED: λmax = 445 nm.

a

Figure 4. (a) Relative concentration of RhB monitored in the aqueous BiVO4 suspensions upon visible light irradiation (maximum relative error ±3%): BiVO4-TZ (cross), BiVO4-TS (open circle) BiVO4-MS (triangle), BiVO4-X (circle), BiVO4-SDS (square), and BiVO4-pH4 (diamonds) (experimental conditions 0.5 g of photocatalyst in 100 mL of RhB with [Ai] = 1.1 except BiVO4-SDS with 0.1 g of photocatalyst in 100 mL of RhB with [Ai] = 1.8 measured at λ = 555 nm). (b) Changes in the corresponding visible spectra of RhB in aqueous suspensions of BiVO4-SDS monitored before exposure and in 20 min intervals during 120 min of irradiation.

visible light using the degradation of RhB in aqueous solution test and then the decomposition of stearic acid deposited on films was used to discriminate the samples based on the MS structure. Rhodamine B Photodegradation. Initial tests in the absence of photocatalyst, or in the dark confirmed that presence of light and photocatalyst is crucial for RhB degradation in aqueous solution. As shown in Figure 4, all the samples do not photodecompose RhB under the visible light exposure. Indeed, the BiVO4-TZ and BiVO4-pH4 samples display no change in RhB concentration in solution even after 2 h of irradiation. For BiVO4-TS, BiVO4-MS, and BiVO4-X, the degradation happens without a significant shift of the maximum wavelength absorption of RhB (indicating a direct degradation of the conjugated system41−43 as major degradation mechanism) and does not exceed 40% of degradation after 2 h of irradiation. In the specific case of BiVO4-SDS the degradation was much too fast under the initial degradation conditions and the test was rerun using lower photocatalyst and higher dye concentrations (Figure 4). The evolution of absorption spectra upon exposure clearly show a shift of the absorbance maximum attributed to the degradation via N-de-ethylation of RhB in a stepwise process.43−46 Indeed, the completely de-ethylated molecules present a maximum absorbance at 498 nm for instance. According to previous published studies on TiO2 and Bi2WO6, the N-de-ethylation process is due to a photosensitized process where the RhB molecule transfers electron to the conduction band of the photocatalyst.41−43,47 The transfer

specific area. The values measured are reported in Table 2, along with other particles characteristics determined in our study. The surface area values obtained for BiVO4 samples synthesized in the presence of organic additives or at given pH are significantly higher than those of the three reference materials. This is in good agreement with the previous studies on the effect of the pH on BiVO431,33,34 or of the additives on other bismuth-based materials17 or oxides.35−37 The use of additives allowed both the variation of the exposed faces and the increase of the amount of exposed surfaces. The counterpart may be a “contamination” of the surface with organic residues. In the case of SDS, sulfur containing species are observed at the surface of the particles after the different washings through X-photoelectron spectroscopy and thermogravimetric analysis coupled with mass spectrometry as shown in Figure SI-1 (Supporting Information). In contrast, xylitol was efficiently removed from the surface. Energy Band Gap. The method used to determine the optical band gap from the absorption spectrum is reported in a previous article.9 The direct38,39 band gap energy values of BiVO4 are reported in Table 2. The obtained values for the three structures are in good agreement with data found in the literature19,40 and the four materials mainly composed of the MS structure only differ of 0.07 eV which is not significant to impact on photon absorption. All the obtained samples are bright yellow. 3.2. Photocatalytic Properties. The photocatalytic properties of all the BiVO4 samples were first studied under 12971


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The Journal of Physical Chemistry C is followed by successive N-de-ethylation degradation steps of RhB. Stearic Acid Photodegradation. The photocatalytic activity was also studied on films during the degradation of stearic acid under visible light. This molecule correctly represents the skin sebum of fingers and cannot be photosensitized in the visible range. consequently, the study of its degradation on film is close to the application of indoor self-cleaning glasses. Among all the BiVO4 samples containing the MS structure, only the BiVO4SDS significantly degrades stearic acid under visible light, as shown in Figure 5. The decline in stearic acid concentration

Figure 6. Optical absorption spectra of BiVO4-SDS (light gray), Bi2WO6-MW-SDS (gray), and TiO2 P25 (black) superimposed on emission spectra of UV LED (dotted line) and blue LED (dash line) sources.

BiVO4-SDS sample is now as efficient as Bi2WO6 under blue LED. As Bi2WO6 absorbs light significantly less than BiVO4 in the blue LED emission range as shown in Figure 6, the bismuth tungstate is still more efficient to use light for photocatalysis than bismuth vanadate. Consequently, BiVO4-SDS is more efficient than TiO2 under blue light due to the huge difference in photon harvesting but it is still not really better that Bi2WO6. The use of LED with emission wavelength higher than 450 nm is necessary to have a better efficiency with BiVO4 compared to Bi2WO6. However, with cyan LED (λmax = 501 nm) the photocatalytic degradation is too low, even with BiVO4-SDS, to be detected. From the different tests, we may first state that the TZ structure of BiVO4 is inactive for photocatalytic degradation as mentioned in the literature. Indeed, DFT results have shown that the two regular BiO8 polyhedrons (D2d point symmetry) prevent a strong interaction between Bi 6s and O 2p that favors the holes mobility in the valence band.13 The TS polymorph is really photocatalytically active for RhB degradation which goes against other published results.19,21 In that structure the BiO8 polyhedron is also very symmetric (D2d point symmetry) when considering the crystallographic data from JCPDS file 10-5711 correlated to a Td geometry of the associated VO4 site. However, Raman experiments reported in Figure SI-2 (in Supporting Information) clearly display more Raman active vibration modes for BiVO4-TS than the four expected for a truly Td VO4 geometry. Consequently, the BiO8 polyhedron must be more distorted than proposed in the crystallographic data. In order to check the compatibility of the Raman data on the TS structure and the XRD patterns we have simulated XRD patterns of slightly distorted TS cell: the lattice parameters of the cell and the metals positions were not changed, only one V−O bond was changed from 1.727 to 1.850 Å and most of the symmetry elements were preserved. As the unit cells only differ on the position of some of the lightest atoms of the structure, the experimental and simulated diffractograms reported in Figure SI-3 (in Supporting Information) can be superimposed. This means that the Raman and XRD results are compatible. In the TS structure the VO4 and BiO8 polyhedrons present a lower symmetry (C3v) than in the TZ structure (Td). Consequently, the interactions between Bi 6s and O 2p orbitals are effective. We may then conclude that the TS and MS structure are both active because the Bi 6s lone pair contributes to the top of the valence band.

Figure 5. Photocatalytic degradation under visible light exposure of stearic acid deposited on films of BiVO4-MS (triangle), BiVO4-X (circle), BiVO4-SDS (square), and BiVO4-pH4 (diamonds) monitored by IR spectroscopy integrating area in the range 2800−3000 cm−1.

A(t) monitored by IR spectroscopy on the exposure can be mathematically (although it has not a truly chemical background) described by a formal zero order kinetics: A(t) = A(t = 0) − k × t. The obtained values of formal zero-order constant k are reported in Table 2. In order to determine more precisely the efficiency of our best BiVO4 photocatalyst namely BiVO4-SDS in the UV and the visible range it was compared to reference catalyst TiO2 P25 for the degradation of stearic acid under the irradiation of different LED sources, as well as to an optimized visible light photocatalyst Bi2WO6-MW-SDS (characteristics in Table 2 and in17). Optical absorptions of BiVO4-SDS, Bi2WO6-MW-SDS, and TiO2 P25 are superimposed on the emission spectra of the two LED used for the degradation tests in Figure 6. The formal kinetic constants for stearic acid degradation using the blue and UV LED sources are reported in Table 2. Under UV irradiation (λmax = 385 nm), TiO2 P25 reference degrades stearic acid twice as fast as BiVO4 and Bi2WO6 samples despite a smaller overlapping of the absorption band with the UV lamp emission spectrum as shown in Figure 6. This evidence the better photon harvesting efficiency of TiO2 P25 in the 360−400 nm wavelength range as well as an improved conversion of the generated charge carriers into active radicals. Under blue light, the TiO2 photocatalyst is now inactive due to the absence overlapping between the semiconductor absorption band and the LED emission spectrum. Oppositely, the bismuth vanadate is still active with a value of formal kinetic constant 10 times smaller when compared to the UV LED experiment (Table 2) while the irradiance only decreased of 10%. Moreover the 12972


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Figure 7. IR 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)] and at 323 [15 min (n)], 373 [15 min (o)], and 423 K [15 min (p)] on BiVO4-MS (a), BiVO4-X (b), BiVO4-SDS (c), and BiVO4-pH4 (d).

indicating the absence of Brønsted sites. After that first adsorption step, the sample has been evacuated under vacuum at room temperature and then heated at different temperatures (323, 373, and 423 K) in order to characterize the pyridine desorption from the weakest acidic sites. The ν8a band shifts from 1593 to 1602 cm−1 during the successive thermodesorption steps. This shift is indicative of the Lewis acidic sites heterogeneity: the component at 1602 cm−1 being partially hidden at room temperature by the larger fraction of weaker Lewis acidic sites at 1593 cm−1. The total desorption temperature of pyridine is quite low, i.e., T = 423 K, indicating the weakness of the Lewis acidity (Figure 7a, spectrum p). The surface acidity of BiVO4-X is very similar to BiVO4-MS with the ν8a bands at 1594 cm−1 and the ν8b at 1574 cm−1 characteristic of Lewis acidic sites and no Brønsted sites detected (Figure 7b). Again, the ν8a band shifts from 1594 to 1601 cm−1 during the thermodesorption procedure and the temperature of pyridine total desorption is also 423 K. The acidity of the BiVO4-SDS sample differs from the first two samples because it presents Brønsted sites in addition to stronger Lewis sites. The Lewis acidic sites are characterized by ν8a bands at 1608 cm−1 and ν8b mode at 1574 cm−1 (Figure 7c) and the wavenumber of the ν8a vibrational mode is shifted up to 1613 cm−1 during thermodesorption. Brønsted acidic sites characterized by the ν8a band at 1640 cm−1 are clearly identified. Band at 1595 cm−1 that disappears under vacuum at room temperature, is also detected. This latter likely indicates the presence of weakly acidic OH groups whose acidity is not sufficient to protonate pyridine. Finally, there is still adsorbed pyridine at 423 K on both Brønsted and Lewis acidic sites indicating that the pyridine total desorption temperature is higher than 423 K. This point out that BiVO4-SDS presents stronger acidic properties than BiVO4-MS and BiVO4-X.

Among the BiVO4 materials presenting the same MS structure, significant photocatalytic efficiency differences were observed that could not be correlated with the modification of specific surface area. Indeed, the BiVO4-MS is more efficient than the BiVO4-X samples for RhB degradation under visible light whereas the later exposes three times more surface for the same weight of photocatalyst. The differences in the morphology of the particles may explain the enhanced efficiency of the BiVO4-SDS sample but in that sample the {001} faces exposed are also observed on the BiVO4-pH4 sample that is inactive. Consequently the origin of the good photocatalytic activity of BiVO4-SDS must be due either to its specific core−shell structure or to specific “sulfate” surface sites formed during the synthesis. In order to determine more precisely the relative impact of the particles morphologies and of specific surface sites on the photocatalytic efficiency, a molecular probe of the acidic sites was used, namely pyridine. 3.3. Surface Acidity Study through Pyridine Thermodesorption. Pyridine is commonly used as a probe molecule for the IR characterization of the surface acidic sites of oxides.48 The IR spectra of adsorption−desorption of pyridine allows identification of the nature and strenght of acidic sites. The pyridine adsorption on BiVO4 samples was studied in the 1700−1500 cm−1 wavenumber domain. Brønsted acidic sites are characterized by bands at 1640 cm−1 (ν8a) and at 1540 cm−1 (ν19b) and Lewis acidic sites by bands in the 1620−1590 cm−1 wavenumber range (ν8a) and at 1574 cm−1 (ν8b). The wavenumber value of the ν8a mode is proportional to the strength of the Lewis acidic sites. The IR spectra of the pyridine adsorbed on the different samples are reported in Figure 7. The top spectrum represents room temperature adsorption equilibrium of pyridine. For BiVO4-MS, the ν8a bands at 1593 cm−1 and the ν8b at 1574 cm−1 characterize Lewis acidic sites (Figure 7a). No band was observed at 1640 cm−1 12973


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The Journal of Physical Chemistry C The surface acidity of the BiVO4-pH4 sample seems to be the weakest of all the studied samples. Only Lewis acidic sites are observed with the ν8a bands at 1595 cm−1 and the ν8b at 1575 cm−1 (Figure 7d). The additional small band at 1607 cm−1 that disappears only by applying a secondary vacuum, is assigned to a combination band of ν1 + ν6a vibrations of physisorbed pyridine species.49 Such band was hidden by other contributions in the previous cases. Presence of physisorbed species is also evidenced by the band at 1588 cm−1. The ν8a band of Lewis acidic sites is only shifted from 1595 to 1600 cm−1 during the vacuum steps and the total desorption temperature of pyridine is 373 K. All these features reveal a very weak interaction between pyridine and the BiVO4-pH4. From the analysis of the four samples we may conclude that the BiVO4-SDS sample present the more acidic surfaces with both strong Lewis and Brønsted acidic sites. The BiVO4-MS and BiVO4-X samples present an intermediate and rather similar surface acidity, while that of the BiVO4-pH4 sample displays the weakest acidic sites. The strong acidity of BiVO4SDS sample that exposes the same {001} face as BiVO4-pH4 may be attributed to the presence of surface sulfate groups. The chemical analysis of that sample gave an amount of sulfur of 0.04 at. % and the XPS analysis of the surface confirmed the presence of sulfur as sulfate groups (the S 2s binding energy was found at 232.9 eV). Consequently a very low amount of sulfate residues is able to significantly increase surface acidity. Indeed, it was shown that on titanium dioxide, surface sulfate groups can attract electrons from Lewis and Brønsted acidic sites in its vicinity and consequently increase their strength.50 The differences in surface acidity may explain both the differences in photocatalytic efficiency to degrade RhB and the changes in the degradation mechanism itself. Indeed, The BiVO4-SDS sample is the only photocatalyst to display strong Brønsted acidic sites in addition to Lewis ones and is also the only one to perform the two mechanisms of degradation through direct photocatalysis or through photosensitization. That second mechanism requires a strong interaction probably between the −N(C2H5)2 groups and the strong surface acidic sites (in water the Lewis acidic sites can be transformed into strong Brønsted acidic sites)50 that is favored in mild acidic medium. On the contrary, the weakness of BiVO4-pH4 surface acidity does not allow a strong RhB adsorption and is even insufficient to keep the dye in vicinity to allow the photogenerated species in solution and to degrade it before recombination. Another explanation for the differences in photocatalytic efficiency that must be checked is the ability of all the synthesized materials to produce the same amount of photogenerated electrons and holes and to use them to produced active radicals at the surface such as hydroxyl radicals (•OH) or superoxide radical anions (O2•−). In order to define more precisely the amount of reactive radicals generated by the bismuth vanadate systems, EPR experiments were performed. 3.4. Determination of Generated Radicals. The lifetimes of both hydroxyl radicals (•OH) or superoxide radical anions (O2•−) are too short for detection using direct EPR technique. Consequently an EPR spin trapping method was developed to allow a selective and easy detection of photogenerated radicals even at low concentration.51 The methodology of the technique is reported in Supporting Information (SI-4). All the samples containing a BiVO4 suspension reveal under UV irradiation a •DMPO−OH concentration of the same order of magnitude as the blank experiment as reported in Figure 8.

Figure 8. Concentration of •DMPO−OH spin adduct after UV irradiation (λmax = 365 nm) of aerated aqueous suspensions of different photocatalysts (UV-radiation dose 3 J·cm−2). (Photocatalyst concentration 0.25 g·L−1, [DMPO]i = 0.025 mol·L−1).

The TiO2 sample is the only one presenting the appropriate valence band position and quantum yield to produce significant amount of •DMPO−OH spin adduct. This leads us to the conclusion that BiVO4 does not produce hydroxyl radicals or at a concentration below the detection threshold of the technique. This result is in agreement with a study that was unable to detect hydroxyl radical using photoluminescence in the presence of terephthalic acid.52 The detection of superoxide radical anions (O2•−) must be done in DMSO as explained in Supporting Information and the EPR results reported there show that the only photocatalyst that produced significant amounts of O2•− is TiO2 P25 which is in good agreement with previous studies.53,54 Consequently, due to technical limitations, we cannot definitely conclude whether the BiVO4 samples are likely to produce the superoxide radical anions and to use them in photodegradation reactions but in any cases their role is very limited. 3.5. Insights in BiVO4 Properties as Photocatalyst. The photocatalytic efficiency of BiVO4 was tested on several model pollutants such as 2-propanol,55 acetaldehyde,56 phenol,26,27 methyl orange,57 methyl blue,31 and rhodamine B.26,31 Titanium dioxide is known to be active in the degradation of all those organic molecules under UV irradiation because it is able at the same time to produce a high amount of active radicals from the harvested photons and the surface OH groups or adsorbed oxygen molecules and to use directly the surface trapped holes and electrons on the adsorbed pollutant. In the case of BiVO4, the generation of reactive radicals close to the surface from photogenerated charge carriers is very scarce and the use of additives such as hydrogen peroxide H2O2 is mandatory to reach good degradation rate for molecules like phenol that do not adsorb on photocatalysts.27 The explanation may be found in the values of the corresponding redox couples • OH/H2O and O2/O2•− that display a potential vs SHE of +2.38 and −0.33 V, respectively in water.58,59 These two potentials values correspond then to lower and higher values of the conduction and the valence bands edges positions respectively if the radical production is observed. Consequently the gap of the photocatalyst should be bigger than 2.71 V which is fulfilled in the case of TiO2 but not for BiVO4. Alternatively, this visible light photocatalyst may at least generate one radicals. The positions of the valence and conduction bands of BiVO4 were estimated from the Mulliken electronegativity and 12974


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involving molecular oxygen is the two step reaction involving at each step only one electron as described below:

the experimental band gap value using the following eqs 1 and 260 E VB = χ − E e +

Eg 2

O2 + H+ + e− → •O2 H •

e

ECB = χ − E −

O2 H + H+ + e− → H 2O2

Eg 2

(3)

(1) (4)

The redox potentials of these two reactions are −0.15 and +1.5 V vs SHE respectively62 From the various measurements performed on the different morphologies of the monoclinic sheelite structure of BiVO4 we may deliver the following statements. (i) A high specific surface is a good characteristic for a photocatalyst but it is not necessary the most important one. Indeed, more interfaces may imply more catalytic acts on the condition that the created interfaces are active. (ii) The morphology of a photocatalyst nanoparticle may impact on its efficiency in different ways. The change in activity may be due to a better surface interaction with pollutants thanks to the appropriate surface sites. It may also be attributed to the presence of more charge carriers on the selected surfaces.29−31 (iii) The relatively low activity of a surface may be compensated by the presence of selected surface additives depending on the degradation mechanism of the pollutant. For a degradation mechanism that implies a surface adsorption, any additives that favor that adsorption are wanted. On the contrary, the covering of the photocatalyst surface with an additive that blocks the hydroxyl radical generation may negatively impact on the degradation of poorly adsorbed pollutants such as phenol. These statements show that there are several ways to improve an already active photocatalytic structure depending on the targeted pollutant.

(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 = 2.5 V). The equations gave for BiVO4 values of EVB = 2.79 V and ECB = 0.29 V vs SHE. These values are in fair agreement with those of a recent study combining a DFT approach with X-ray spectroscopies that proposed a CB position in the 0.1−0.3 V vs SHE range23 while Mott−Schottky analyses in electrolyte propose a CB maximum at 0.02 V vs SHE.24,61 These results are summarized in Figure 9 with those calculated for TiO2.

4. CONCLUSIONS Two of the three commonly found structures of BiVO4, namely the tetragonal sheelite (TS) and monoclinic sheelite (MS) structures, are photocatalytic active for the degradation of RhB in solution under visible light. This was already known for the MS structure but it is new for the TS one. The BiO8 and VO4 polyhedrons in the TS structure are in fact more distorted that what is proposed in the crystallographic data and the Bi 6s “lone pair” is present and may explain the observed photocatalytic activity. We have prepared a set of bismuth vanadate samples presenting mainly the MS structure but with different particles sizes, morphologies and surface dopants. The comparison of photodegradation efficiency of RhB and stearic acid under visible and UV light respectively lead us to conclude that the {001} surface of BiVO4 is not very active unless sulfate surface group are present. Indeed those sulfate groups were shown to enhance the Lewis and Brønsted acidic sites of the bismuth vanadate surface. Thanks to that surface modification, bismuth vanadate becomes more efficient than titanium dioxide and even bismuth tungstate under blue LED. Moreover, we confirmed that BiVO4 is unable to generate active radicals and we correlated that with the calculated band diagram of that semiconductor. Bismuth vanadate is not a very efficient photocatalyst for pollutants degradation unless used with appropriate light source but is still very interesting for water splittingin the presence of reducing agent. This application also requires a good extraction of the charge carriers from the photoactive materials and the design of the material presented here may be useful to optimize that aspect. It also would be interesting to study the tetragonal sheelite structure for that specific application.

Figure 9. Calculated valence and conduction band positions for TiO2 and BiVO4 and the potential of the radicals’ redox couples •OH/H2O and O2/O2•− in water vs SHE.

This figure clearly demonstrates that the visible light photocatalyst is unable to convert photogenerated electron into reactive superoxide anions. Oppositely, calculated valence band maximum potential should allow the generation of hydroxyl radicals. However, this model does not take into account two important parameters: the curvature of valence and conduction bands at the solid−electrolyte interface and the impact of electrolyte pH on the involved redox couples. It is still possible to use the bismuth vanadate structure to photogenerate reactive radicals in the solution but an additional radical source such as H2O2 whose redox couples are in the 0.5−2 V vs SHE range58 is needed. Consequently some holes directly injected from the solid can be used in the photocatalytic oxidation of the pollutants on the condition they did not recombine with electrons. The electron−hole recombination is not immediate as the hole diffusion length in BiVO4 was determined to be about 100 nm so the material can be used as photocatalyst through direct hole reaction with adsorbed species but the photogenerated electrons must have reacted in some way. A possible consumption of the electrons still 12975


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ASSOCIATED CONTENT

S Supporting Information *

Additional data giving more details about the XPS spectra of BiVO4-MS and BiVO4-SDS samples, TGA-MS data of BiVO4SDS, Raman spectra of BiVO4-TZ, BiVO4-TS and BiVO4-MS samples, and modeled X-ray diffractograms of distorted BiVO4TS structure. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b01468.

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AUTHOR INFORMATION

Corresponding Author

*(O.D.) Telephone: +33144271543. Fax: +33144271504. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by Saint-Gobain Recherche. We also wish to thank Sandrine Clary-Lespinasse for technical support and Corinne Papret for scanning electron microscopy. We are also grateful to Eric Picquenard (MONARIS, UPMC, France) for the acquisition of the Raman spectra, to Valérie Ruaux (LCS, U-Caen, France) for implementation and help in the IR experiments, to Eric Puzenat (IRCELyon, U. Lyon 1, France) for the LED photon emission quantification, and to Guillaume Maurin (Laboratoire de physicochimie de la matière condensée, U. Montpellier, France) for the modelled X-ray diffraction patterns of BiVO4-TS. V.B. is acknowledges financial support by the Scientific Grant Agency of the Slovak Republic (VEGA Project 1/0041/15).

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New Insights Into BiVO4 Properties as Visible Light Photocatalyst

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