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Preparation and Characterization of Mo Doped in BiVO4 with Enhanced Photocatalytic Properties Bitao Liu 1 , Xuelian Yan 1 , Hengqing Yan 1 , Yucen Yao 1 , Yanhua Cai 1 , Jumeng Wei 2, *, Shanyong Chen 1, *, Xuhui Xu 3 and Lu Li 1, * 1

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Chongqing Key Laboratory of Environmental Materials and Remediation Technology, Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Yongchuan, Chongqing 402160, China; [email protected] (B.L.); [email protected] (X.Y.); [email protected] (H.Y.); [email protected] (Y.Y.); [email protected] (Y.C.) College of Chemistry and Materials Engineering, Anhui Science and Technology of University, Fengyang 233100, China School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] Correspondence: [email protected] (J.W.); [email protected] (S.C.); [email protected] (L.L.); Tel.: +86-0550-6733359 (J.W.); +86-023-49891752 (S.C. & L.L.)

Received: 3 July 2017; Accepted: 17 August 2017; Published: 21 August 2017

Abstract: Molybdenum (Mo) doped BiVO4 was fabricated via a simple electrospun method. Morphology, structure, chemical states and optical properties of the obtained catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), N2 adsorption–desorption isotherms (BET) and photoluminescence spectrum (PL), respectively. The photocatalytic properties indicate that doping Mo into BiVO4 can enhance the photocatalytic activity and dark adsorption ability. The photocatalytic test suggests that the 1% Mo-BiVO4 shows the best photocatalytic activity, which is about three times higher than pure BiVO4 . Meanwhile, 3% Mo-BiVO4 shows stronger dark adsorption than pure BiVO4 and 1% Mo-BiVO4 . The enhancement in photocatalytic property should be ascribed to that BiVO4 with small amount of Mo doping could efficiently separate the photogenerated carries and improve the electronic conductivity. The high concentration doping would lead the crystal structure transformation from monoclinic to tetragonal phase, as well as the formation of MoO3 nanoparticles on the BiVO4 surface, which could also act as recombination centers to decrease the photocatalytic activity. Keywords: BiVO4 ; MoO3 ; photocatalyst; visible light

1. Introduction Semiconductor photocatalysis based on solar energy has been demonstrated to be one of effective techniques to eliminate organic pollutants in wastewater [1]. Conventional TiO2 is considered as an efficient UV light-driven photocatalyst. However, the UV light is only 4% in the solar light spectrum, thereby limiting its application [2]. In this regard, researchers have focused on exploring new photoactive materials, which could be driven by considerable and cost-efficiently visible light. Monoclinic bismuth vanadate (BiVO4 ) with narrow band gap (Eg = 2.4 eV) is one of the excellent visible-light-driven photocatalysts [3]. Theoretically, its solar-to-hydrogen conversion efficiency can reach up to 9.2%. However, the actual photocatalytic efficiency of pristine BiVO4 is far below what is expected, because it suffers from poor electronic transportation and short carrier diffusion length, resulting in numerous bulk recombination of electron–hole pairs [4,5]. Therefore, effectively inhibiting

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the recombination of photo-induced electron–holes is the key factor to improve the photocatalytic activity of monoclinic BiVO4 . By the incorporation with inorganic nanostructures, such as Bi2 WO6 [6], MoS2 [7] and g-C3 N4 [8], the photocatalytic activity can be efficiently improved. However, the synthesis process of such composite is usually complex. Another useful way to increase the photocatalytic activity is doping metal/non-metallic elements into catalyst, which can easily modify its electronic properties and thus enhance the photocatalytic activity. In general, doping could extend the optical absorption, enhance the surface adsorption for dyes molecules and improve the electrical conductivity [9]. Various studies have shown that the incorporation of molybdenum (Mo) into BiVO4 can bring out significant improvement in electronic properties (e.g., carrier concentrations or CB level) [4,10]. For example, Parmar and coworkers [9] concluded that the estimated carrier concentration of Mo-doped BiVO4 photoanode was about two times higher than that of undoped BiVO4 . The VBs and CBs of Mo-doped BiVO4 were broadened because of the overlapped orbitals. Ding et al. [11] stated that the substitute of Mo6+ in V5+ sites could facilitate the separation of carriers. However, all of the experimental studies and theoretical calculations were based on photocatalytic water-splitting. The limitation of the systematic research on the structure and morphology evolution can hinder the improvement of the photocatalytic degradation of organic pollutants, especially for the BiVO4 . In this paper, we report a simple and good repeatability method to fabricated Mo doped BiVO4 . The application in dye degradation for MB under visible light was performed. The details of the photocatalytic properties are also discussed. 2. Experimental and Characterization 2.1. Synthesis of BiVO4 and Mo-Doped BiVO4 All chemicals used in this work are of analytical-reagent grade without further purification. The procedure for spinning precursor with same stoichiometric ratio of bismuth and vanadium source is followed by our previous paper [12]. Then, different amounts of ammonium molybdate (0.4, 1, 1.5, 3 mol %) were added into these solutions, while stirring for 12 h to obtain homogeneous precursors. During the electrospinning process, the electrospun temperature was fixed at 60 ◦ C with 15 kV voltages. The collected composites were first dried in a vacuum oven at 70 ◦ C for 40 min followed by calcination at 450 ◦ C for 2 h with a heating rate of 1 ◦ C/min. 2.2. Characterization Powder X-ray diffraction was carried on TD-3500 X-ray diffractometer (Tongda, Dandong, Liaoning, China) using Cu Kα radiation, operating at 30 kV and 20 mA. An ESCA Lab MKII (VG, London, UK) was employed for X-ray photoelectron spectroscopy, to investigate surface electronic properties of BiVO4 and Mo-doped BiVO4 . Scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM,) were taken with a JEM-2010 (JEOL, Tokyo, Japan) operating at 20 kV. The surface area (BET) was measured on a BELSORT-max (MicrotracBEL, Osaka, Japan) instrument. The UV-vis diffuse reflectance spectra (DRS) of the materials were obtained on a UV-vis spectrophotometer with an integrating sphere (U-3900, Hitachi, Tokyo, Japan). Photoluminescence spectra were registered on OmniPL-LF325 spectrofluorometer (Zolix, Beijing, China) with 325 nm laser as a radiation source. 2.3. Photocatalytic Test and PEC Measurements During the photodegradation process, 0.05 g of the catalyst was suspended in 50 mL of methylene blue (MB) solution with the concentration of 10 ppm. After stirring for 30 min in dark to establish the adsorption–desorption equilibrium, the solution was illuminated to visible light with a cut filter of 420 nm using 500 W Xe arc lamp as light source, under continuous stirring. At a given time interval, 4 mL of suspension were taken out and centrifuged for 10 min to remove the photocatalyst.

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The centrifuged solution was analyzed by recording the variations of the absorption peak at 664 nm Electrochemical tests were performed on an AUTOLAB PGSTAT302N station (Methrohm, using a UV-vis spectrometer (Cary 50, Varian, Shanghai, China). Herisau, Switzerland). A saturated calomel electrode (SCE) and Pt wire were used as reference and Electrochemical tests were performed on an AUTOLAB PGSTAT302N station (Methrohm, Herisau, counter electrode, respectively. ITO glasses covered with 2 mg of the prepared powders were used Switzerland). A saturated calomel electrode (SCE) and Pt wire were used as reference and counter as work electrode. A 500 W Xe arc lamp with a 420 nm cutoff filter was employed as visible light electrode, respectively. ITO glasses covered with 2 mg of the prepared powders were used as work source. The photoanode current was measured in 0.1 M Na2SO4 electrolyte, with a fixed applied electrode. A 500 W Xe arc lamp with a 420 nm cutoff filter was employed as visible light source. potential of 0.0 V. Electrochemical impedance spectra (EIS) were measured in 1 M KOH electrolyte. The photoanode current was measured in 0.1 M Na2 SO4 electrolyte, with a fixed applied potential of 0.0 V. Electrochemical impedance spectra (EIS) were measured in 1 M KOH electrolyte. 3. Results and Discussion 3. Results and Discussion The photocatalytic degradation for MB solution is shown in Figure 1a. In dark adsorption process, it is clearly seen that all Mo-BiVO4 samples exhibit stronger adsorption property for MB The photocatalytic degradation for MB solution is shown in Figure 1a. In dark adsorption process, molecules compared with the undoped BiVO4. Of note, dark adsorption performance of 1.5% it is clearly seen that all Mo-BiVO4 samples exhibit stronger adsorption property for MB molecules Mo-BiVO4 (30.66%) and 3% Mo-BiVO4 (31.93%) is higher than that of 0.4% Mo-BiVO4 (18.36%) and compared with the undoped BiVO4 . Of note, dark adsorption performance of 1.5% Mo-BiVO4 (30.66%) 1% Mo-BiVO4 (17.48%). However, the 1.5% Mo-BiVO4 and 3% Mo-BiVO4 show a lower and 3% Mo-BiVO4 (31.93%) is higher than that of 0.4% Mo-BiVO4 (18.36%) and 1% Mo-BiVO4 (17.48%). photocatalytic activity. After 120 min illumination, photocatalytic activity order of different catalysts However, the 1.5% Mo-BiVO4 and 3% Mo-BiVO4 show a lower photocatalytic activity. After 120 min is as follows: 1% Mo-BiVO4 (88.57%) > 0.4% Mo-BiVO4 (85.46%) > 1.5% Mo-BiVO4 (51.32%) > 3% illumination, photocatalytic activity order of different catalysts is as follows: 1% Mo-BiVO4 (88.57%) > Mo-BiVO4 (43.35%) > BiVO4 (29%). 0.4% Mo-BiVO4 (85.46%) > 1.5% Mo-BiVO4 (51.32%) > 3% Mo-BiVO4 (43.35%) > BiVO4 (29%). Additional adsorption test in dark was measured to evaluate the MB degradation during the Additional adsorption test in dark was measured to evaluate the MB degradation during the above irradiation process. As presented in Figure 1b, the concentration of MB solution is observably above irradiation process. As presented in Figure 1b, the concentration of MB solution is observably decreased within the first 20 min for all five samples. In addition, the adsorption performance order decreased within the first 20 min for all five samples. In addition, the adsorption performance order for the five samples is in accordance with the above. In the next 100 min, the concentration of MB for the five samples is in accordance with the above. In the next 100 min, the concentration of MB solution for five samples is almost equal to the results of first 20 min. solution for five samples is almost equal to the results of first 20 min. Based on above analysis, it is indicated that suitable doping of Mo would be the key point to Based on above analysis, it is indicated that suitable doping of Mo would be the key point enhance the photocatalytic activity or dark adsorption ability. In view of similar photocatalytic to enhance the photocatalytic activity or dark adsorption ability. In view of similar photocatalytic properties of 0.4% Mo-BiVO4 and 1% Mo-BiVO4 (as well as 1.5% Mo-BiVO4 and 3% Mo-BiVO4), we properties of 0.4% Mo-BiVO4 and 1% Mo-BiVO4 (as well as 1.5% Mo-BiVO4 and 3% Mo-BiVO4 ), we choose BiVO4, 1% Mo-BiVO4 and 3% Mo-BiVO4 for further characterization, and then investigate the choose BiVO4 , 1% Mo-BiVO4 and 3% Mo-BiVO4 for further characterization, and then investigate the effect of Mo-doping on the photocatalytic activity for BiVO4. effect of Mo-doping on the photocatalytic activity for BiVO4 .

Figure (a) Photocatalytic Photocatalytic degradation degradation of of MB MB using using undoped and different different amount Figure 1. 1. (a) undoped BiVO BiVO44 and amount of of Mo-doped BiVO under visible light (λ > 420 nm) illumination; and (b) adsorption properties of 4 Mo-doped BiVO4 under visible light (λ > 420 nm) illumination; and (b) adsorption properties of the the products above. fivefive products above.

presents the the typical typical XRD XRD patterns patterns of ofbare bareBiVO BiVO44and andMo-BiVO Mo-BiVO44 samples. samples. The The BiVO BiVO44 Figure 2 presents and 1% Mo-BiVO44 show relatively broad peaks and low intensity, indicating their low crystallinity. show relatively broad peaks and low intensity, indicating their low crystallinity. 3% Mo-BiVO Mo-BiVO44 exhibits sharp peaks with strong intensity because of their high crystallinity crystallinity The 3% (Figure 2a) 2a)[13], [13],indicating indicatingthat thathigher higher doping concentration could promote the grain growth and (Figure doping concentration could promote the grain growth and lead lead to an increased crystallinity. Numerous studies have demonstrated that appropriate crystal to an increased crystallinity. Numerous studies have demonstrated that appropriate crystal defects such defects such as surface oxygen vacancies could act as electrons or holes, as surface oxygen vacancies could act as scavengers forscavengers electrons orfor holes, promoting the promoting separation the separation efficiency of carriers [1,14,15]. Especially, the4 and peaks BiVO4 and 1%well Mo-BiVO 4 are efficiency of carriers [1,14,15]. Especially, the peaks of BiVO 1%ofMo-BiVO indexed to 4 are well indexed to monoclinic (JCPDS 3% Mo-BiVO 4 structure, however, monoclinic structure (JCPDS structure NO.14-0688). The NO.14-0688). 3% Mo-BiVO4The structure, however, progresses toward progresses toward a mixture of monoclinic and tetragonal phase, which can be observed from the sharp characteristic tetragonal peaks located at 34.5°, 46.9°, and 58.6° (JCPDS NO.48-0744). Similar

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a mixture of monoclinic and tetragonal phase, which can be observed from the sharp characteristic tetragonal peaks located at 34.5◦ , 46.9◦ , and 58.6◦ (JCPDS NO.48-0744). Similar results have also been Materials 976 and coworkers [16]. Tetragonal structure BiVO4 could decrease the photocatalytic 4 of 9 found by 2017, Bard10,A.J. activity due to its regular electronic arrangement, which was consistent with the photocatalytic test resultsThe have alsotransformation been found by in Bard A.J. and coworkers [16]. Tetragonal structure BiVO4 could results. phase 1.5% Mo-BiVO 4 results in different photocatalytic performance decrease the photocatalytic activity due to its regular electronic arrangement, which was consistent compared to 1% Mo-BiVO4 . Figure 2b shows the magnified XRD peaks between 46◦ and 49◦ . It can be with the photocatalytic test results. The phase transformation in 1.5% Mo-BiVO4 results in different clearly seen that two peaks at 47◦ could be observed. These two peaks are well indexed to (024) and photocatalytic performance compared to 1% Mo-BiVO4. Figure 2b shows the magnified XRD peaks (022) planes for tetragonal BiVO4 and MoO3 (JCPDS NO. 47-1081), respectively. The XPS technique was between 46° and 49°. It can be clearly seen that two peaks at 47° could be observed. These two peaks employed to investigate the surface electronic properties of samples. The high-resolution XPS spectral are well indexed to (024) and (022) planes for tetragonal BiVO4 and MoO3 (JCPDS NO. 47-1081), of respectively. BiVO4 , 1% Mo-BiVO and 3% Mo-BiVO are presented Figure 3. The element spectral 4 samples The XPS4 technique was employed to investigate the in surface electronic properties of clearly indicates typical spin-orbit separation for 4Oand 1s. 3% ForMo-BiVO bare BiVO main 4 , two are samples. The high-resolution XPS doublet spectral of BiVO4, 1%except Mo-BiVO 4 samples asymmetric at 159.2 164.4 spectral eV are ascribed to Bi 4f7/2 and Bi 4f5/2 , which areseparation the features presented peaks in Figure 3. Theand element clearly indicates typical spin-orbit doublet of except Bi3+ infor BiVO , and the two asymmetric peaks at 516.7 and 524.3 eV are corresponded to Vto2p 4 For bare BiVO4, two main asymmetric peaks at 159.2 and 164.4 eV are ascribed O 1s. Bi3/2 5+ and V and 2p1/2Bi, which should in BiVO For two Mo-BiVO there 4 [9,17]. 4 samples, 4f7/2 4f5/2, which arebe theascribed featuresto ofthe Bi3+Vin BiVO 4, and the two asymmetric peaks at 516.7 and is a524.3 gradual toward higher binding energies in the characterized V 2p, and O eV areshift corresponded to V 2p 3/2 and V 2p1/2, which should be ascribedpeaks to theof V5+Biin4f, BiVO 4 [9,17]. 1s For withtwo the Mo-BiVO increase 4ofsamples, Mo concentration. The specific of elements are in listed there is a gradual shift binding toward energies higher binding energies the in characterized peaks higher of Bi 4f,binding V 2p, and O 1s with increase of5+Mo concentration. The specific Table 1. The slightly energies of allthe ions (Bi3+ , V and O2− ) in Mo-doped BiVO4 bindingare energies elements are listed Table 1. The slightly higher bindingofenergies of all ions samples mainlyofassociated with theinrelatively higher electronegativity the dopants (Mo6+ : 3+, V 5+ and O2−) in Mo-doped BiVO4 samples are mainly associated with the (Bi> relatively higher 2.16 V5+ : 1.63) [9]. As for the 3% Mo-BiVO4 , the binding energies of Mo6+ is still higher than 6+ 5+ of the dopants (Mo : 2.16 > V :it1.63) [9]. As forthat the this 3% Mo-BiVO 4, the should binding be 1%electronegativity Mo-BiVO4 . Combined with the XRD results, is concluded phenomenon 6+ is still higher than 1% Mo-BiVO4. Combined with the XRD results, it is concluded energies Mo ascribed toofthe existence of MoO3 on BiVO4 surface. The Mo concentration can be calculated from that this The phenomenon should be ascribed the increasing existence ofthe MoO on BiVO(from 4 surface. The Mo XPS data. atomic ratio of Mo:V increasetowith Mo3 doping 1.76%:8.78% to concentration can be calculated from XPS data. The atomic ratio of Mo:V increase with increasing the 4.47%:8.95%), indicating the success doping. Mo doping (from 1.76%:8.78% to 4.47%:8.95%), indicating the success doping. Table 1. Binding energies of elements in the samples determined by high-resolution XPS analysis. Table 1. Binding energies of elements in the samples determined by high-resolution XPS analysis. Binding Energy (eV) Samples 4f7/2 5/2 2p3/2 3/2 V OO1s1s MM 3d3d 5/2 MoMo 3d3/23d3/2 Samples Bi Bi 4f7/2 BiBi 4f4f VV2p V 2p 2p1/2 5/2 1/2 5/2 BiVO4 159.2 ± 0.1 164.4 ± 0.1 516.7 ± 0.1 524.3 ± 0.1 529.8 ± 0.1 BiVO4 159.2 ± 0.1 164.4 ± 0.1 516.7 ± 0.1 524.3 ± 0.1 529.8 ± 0.1 Mo-BiVO4159.3 159.3 ± 0.1 164.6 164.6 0.1 516.8 516.8±± 0.1 0.1 524.5 0.10.1 232.3 ± 0.1 ± 0.1 1% 1% Mo-BiVO ± 0.1 ±±0.1 524.5 ±±0.1 0.1 530.0 530.0± ± 232.3 ± 0.1235.5 235.5 ± 0.1 4 4159.5 159.5 ± 0.1 164.8 164.8 0.1 517.0 517.0±± 0.1 0.1 524.6 0.10.1 232.5 ± 0.1 ± 0.1 Mo-BiVO 3% 3% Mo-BiVO ± 0.1 ±±0.1 524.6 ±±0.1 0.1 530.2 530.2± ± 232.5 ± 0.1235.7 235.7 ± 0.1 4

Figure 2. 2. (a)(a) XRD and3% 3%Mo-BiVO Mo-BiVO ; (b) the magnified Figure XRDpatterns patternsofofundoped undopedBiVO BiVO44,, 1% 1% Mo-BiVO Mo-BiVO44and 4; 4(b) the magnified ◦ XRD peaks at at 4747°ofofallallsamples. XRD peaks samples.

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Figure High-resolutionXPS XPSspectra spectraof ofundoped undoped BiVO BiVO44,, 1% 4: (a) Bi 4f; Figure 3. 3. High-resolution 1%Mo-BiVO Mo-BiVO4 4and and3% 3%Mo-BiVO Mo-BiVO 4 : (a) Bi 4f; (b) V 2p; (c) O 1s; and (d) Mo 3d. (b) V 2p; (c) O 1s; and (d) Mo 3d.

It is generally accepted that the morphologies greatly influence the photocatalytic performance It is generally accepted that the morphologies greatly influence the photocatalytic [18,19]. SEM images of BiVO4, 1% Mo-BiVO4 and 3% Mo-BiVO4 are displayed in Figure 4a–c. For the performance of BiVO , 1% Mo-BiVO4 and Mo-BiVO 4 are displayed reference of[18,19]. undopedSEM BiVOimages 4, the diameter is4 less than 200 nm, with3% many pores presenting on the in Figure 4a–c. For 4a). the With reference of undoped diameter is less than 200 nm, with many 4 , the surface (Figure the addition of 1%BiVO of (NH 4)6Mo7O24·4H2O, there is apparent change in pores presenting on the surface (Figure 4a). With the addition of (Figure 1% of (NH )6 Mo 4H2 O, there 7 O24 ·increasing morphologies, which are composed of closely arranged particles 4b). 4By further is apparent change in morphologies, which are composed of closely arranged particles (Figure the amount of (NH4)6Mo7O24·4H2O to 3%, some are broken and grown into large particles (Figure 4c).4b). ByThe further increasing thethree amount of (NHdecreased ·4H2 Oincreasing to 3%, some broken and grown surface area of samples Moareconcentration, whichinto is large in 4 )6 Mo7 O24with particles (Figure The surface(Figure area of5). three decreased with increasingindicates Mo concentration, agreement with4c). SEM analyses The samples above morphology transformation that the introduction of Mo ions beneficial for (Figure particle5). growth, which is in agreement with XRD results which is in agreement withisSEM analyses The above morphology transformation indicates [20]. breakage of structure for 3% for Mo-BiVO 4 suffers from pooriselectron transportation, that theThe introduction of some Mo ions is beneficial particle growth, which in agreement with XRD resulting enhancement. To further analyze the microstructure of Mo results [20]. in Thedecreased breakage photocatalytic of some structure for 3% Mo-BiVO from poor electron transportation, 4 suffers doped BiVO 4 catalysts in detail, Figure 4d–f presents the TEM images of three products. When is resulting in decreased photocatalytic enhancement. To further analyze the microstructure of MoMo doped doped into BiVO 4 , the pores disappeared and the particles become bigger, which is in agreement BiVO4 catalysts in detail, Figure 4d–f presents the TEM images of three products. When Mo is doped with above results. Figure 4h–j shows the HR-TEM patterns of those samples. From the image of into BiVO 4 , the pores disappeared and the particles become bigger, which is in agreement with above Figure 4h, the observation of HR-TEM clear lattice fringe spacing 0.467 nm is indexed to the crystal results. Figure 4h–j shows the patterns of thoseof samples. From the image of (011) Figure 4h, the plane of BiVO4. As for 1% Mo-BiVO4 (Figure 4i), the obtained lattice fringes of d = 0.317 nm, 0.467 observation of clear lattice fringe spacing of 0.467 nm is indexed to the (011) crystal plane of BiVO4 . nm, and 0.307 nm are corresponded to the (130), (011) and (121) crystallographic planes of BiVO4, As for 1% Mo-BiVO4 (Figure 4i), the obtained lattice fringes of d = 0.317 nm, 0.467 nm, and 0.307 nm6+are respectively. No Mo element fringes can be observed. This could be ascribed to the substitution Mo corresponded to the (130), (011) and (121) crystallographic planes of BiVO , respectively. No Mo element ions in lieu of V5+, resulting in the formation of solid solution because of 4the similar ionic radius [21]. fringes can be observed. This could be ascribed to the substitution Mo6+ ions in lieu of V5+ , resulting However, for 3% Mo-BiVO4 (Figure 4j), the lattice distance of d = 0.336 nm ascribed to (111) plane of in the formation of solid solution because of the similar ionic radius [21]. However, for 3% Mo-BiVO4 MoO3 is measured near the edge of BiVO4, which coincides with XRD and XPS analysis. In effect, (Figure 4j), the latticeisdistance of d = 0.336 nm ascribed (111) plane MoO3 ishigh measured near the edge this phenomenon very common in impurity dopingtoresearch [22].ofNamely, concentrations of of doped BiVO4 ,impurity which coincides with XRD and XPS analysis. In effect, this phenomenon is very common elements easily combine with oxygen to form secondary oxides and then hinder the in impurity doping activity. researchThe [22].formation Namely, high concentrations of doped impurity elements easily combine photocatalytic of MoO 3 at the BiVO4 surface from excess dopants can explain with oxygen to form secondary oxides and then hinder the photocatalytic activity. The formation the contradiction of higher doping concentration and higher crystallinity, which coincides with XRD of analysis. MoO3 at the BiVO4 surface from excess dopants can explain the contradiction of higher doping concentration and higher crystallinity, which coincides with XRD analysis.

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Figure 4. (a–c); TEM TEM (d–f) and HRTEM (h–j) images BiVO44, 1% 44 and 3% Mo-BiVO 44, Figure 4.SEM SEM (a–c); (d–f) and HRTEM (h–j)ofimages of Mo-BiVO BiVO4 , 1% Mo-BiVO 4 and 3% Mo-BiVO4 , respectively. respectively.

Figure5.5.N N222adsorption–desorption adsorption–desorption isotherms isotherms of of BiVO BiVO444,, 1% 1% Mo-BiVO Mo-BiVO444 and Figure and 3% 3% Mo-BiVO Mo-BiVO444..

Based on the above analysis, we proposed a mechanism to explain the formation of the Based on the above analysis, we proposed a mechanism to explain the formation of the Mo-BiVO4 , Mo-BiVO44, as shown in Scheme 1. Bare BiVO44 consists of homogeneous dispersed sol-gel particles as shown in Scheme 1. Bare BiVO4 consists of homogeneous dispersed sol-gel particles with many with many pores. With a small amount of Mo ions doped into BiVO44 lattices, the BiVO44 particles pores. With a small amount of Mo ions doped into BiVO4 lattices, the BiVO4 particles become bigger become bigger while the pores disappear. No lattice fringe about Mo compounds can be detected, while the pores disappear. No lattice fringe about Mo compounds can be detected, indicating the indicating the formation of solid solution. When the doping concentration reaches 3%, the saturated formation of solid solution. When the doping concentration reaches 3%, the saturated Mo ions Mo ions preferably form MoO33 particles near the BiVO44 surface. preferably form MoO3 particles near the BiVO4 surface. Figure 6a shows the UV-vis diffuse reflectance spectra of all three samples. Distinctly, all three Figure 6a shows the UV-vis diffuse reflectance spectra of all three samples. Distinctly, all three samples exhibit sharp absorption edges, in agreement with typically reported monoclinic BiVO44 samples exhibit sharp absorption edges, in agreement with typically reported monoclinic BiVO4 [23,24]. [23,24]. Moreover, the UV-vis diffuse reflectance spectra curves of undoped BiVO44 and 1% Moreover, the UV-vis diffuse reflectance spectra curves of undoped BiVO4 and 1% Mo-BiVO4 almost Mo-BiVO44 almost completely overlap, there is little shift towards longer wavelength (red-shift) for completely overlap, there is little shift towards longer wavelength (red-shift) for 3% Mo-BiVO4 , 3% Mo-BiVO44, indicating that Mo dopants have no significant effect on the light absorption for indicating that Mo dopants have no significant effect on the light absorption for BiVO4 structure. BiVO44 structure. For a deep investigation, the PL spectra of all three samples were obtained to For a deep investigation, the PL spectra of all three samples were obtained to characterize the characterize the separation efficiency of the photogenerated electrons and holes. As shown in Figure 6b, both BiVO44 and 3% Mo-BiVO44 possess strong emission intensity in the range 375–550 nm. In

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theory, theefficiency PL emission is generated from the recombination electrons and holes.6b, However, the separation of the photogenerated electrons and holes.of shown in Figure both BiVO 4 theory, the PL emission is generated from the recombination ofAs electrons and holes. However, the emission intensity4 possess of 1% strong Mo-BiVO 4 is very weak inand almost negligible. ItInistheory, well known and 3% Mo-BiVO emission intensity the range 375–550 nm. the PL emission intensity of 1% Mo-BiVO4 is very weak and almost negligible. It is well known that lower PL intensity higher separation could lead to aintensity higher emission is generated fromindicates the recombination of electronsefficiency, and holes. which However, the emission that lower PL intensity indicates higher separation efficiency, which could lead to a higher photocatalytic activity [25]. of 1% Mo-BiVOactivity weak and almost negligible. It is well known that lower PL intensity indicates 4 is very[25]. photocatalytic higher separation efficiency, which could lead to a higher photocatalytic activity [25].

Scheme Scheme 1. 1. Illustration of the the formation formation of of different different concentration concentration Mo-BiVO Mo-BiVO4. Scheme 1. Illustration of the formation of different concentration Mo-BiVO44.

Finally,the thephotoelectrochemical photoelectrochemical behaviors of undoped 4, 1% Mo-BiVO4 and 3% Finally, behaviors of undoped BiVO4 , BiVO 1% Mo-BiVO 4 and 3% Mo-BiVO 4 Finally, the photoelectrochemical behaviors of undoped BiVO 4, 1% Mo-BiVO 4 and 3% Mo-BiVO 4 samples were performed. The photocurrent response results of three samples set applied samples performed. The photocurrent response results of three samples appliedset potential Mo-BiVOwere 4 samples were performed. The photocurrent response results of threeset samples applied potential as 0 are shown7a.inObviously, Figure 7a. Obviously, Mo-BiVO 4 exhibits higher photocurrent as 0 are shown Mo-BiVO41% exhibits higher photocurrent than potential as 0 in areFigure shown in Figure 7a.1% Obviously, 1% Mo-BiVO 4 exhibits higherintensity photocurrent intensity than pure BiVO 4 and 3% Mo-BiVO4. The photocurrent of 1% Mo-BiVO4 is 3.5 times and 2.3 pure BiVO and 3% Mo-BiVO . The photocurrent of 1% Mo-BiVO is 3.5 times and 2.3 times higher intensity than pure BiVO4 and4 3% Mo-BiVO4. The photocurrent of41% Mo-BiVO4 is 3.5 times and 2.3 4 timesthose higher thoseBiVO of undoped BiVO 4 and 3% Mo-BiVO4, indicating that lower concentration of than of than undoped Mo-BiVO lower concentration of Mo doped times higher than those of undoped BiVO 4 and 3% Mo-BiVO4that , indicating that lower concentration of 4 and 3% 4 , indicating Mo doped BiVO 4 can obtain stronger ability in separation and transfer of photogenerated BiVO can obtain stronger ability in separation and transfer of photogenerated electron–hole pairs Mo doped BiVO4 can obtain stronger ability in separation and transfer of photogenerated 4 electron–hole pairs comparing withMo undoped higher Mo doped BiVO4. Electronic impedance comparing with undoped or higher doped or BiVO impedance spectroscopy (EIS) is electron–hole pairs comparing with undoped or higher Mo doped BiVO4. Electronic impedance 4 . Electronic spectroscopy (EIS) is another important means to characterize the ability of charge transfer. As another important to characterize ability of charge transfer. As shown in Figure 7b, the spectroscopy (EIS) means is another important the means to characterize the ability of charge transfer. As shown in Figure 7b, the order of arc radii is as follows: 1% Mo-BiVO 4 > 3% Mo-BiVO4 > undoped order of arc radii is as follows: 1% Mo-BiVO > 3% Mo-BiVO > undoped BiVO . It is well-known that shown in Figure 7b, the order of arc radii4is as follows: 1% 4 Mo-BiVO4 > 3%4Mo-BiVO4 > undoped 4. It is well-known that a lower arc radius implies a higher efficient charge carrier transfer at the aBiVO lower arc implies a higher charge carrier transfer at the charge electrolyte andtransfer photoanode BiVO 4. It is radius well-known that a lowerefficient arc radius implies a higher efficient carrier at the electrolyte andTherefore, photoanode [26]. Therefore, 1% Mo-BiVO 4 shows the strongest separation interface [26]. 1%interface Mo-BiVO the strongest separation efficiency of electron–hole electrolyte and photoanode interface [26]. Therefore, 1% Mo-BiVO 4 shows the strongest separation 4 shows efficiency of is electron–hole which is a key factor for high photocatalytic photocatalyst. pairs, which a key factor pairs, for high photocatalytic photocatalyst. efficiency of electron–hole pairs, which is a key factor for high photocatalytic photocatalyst.

Figure 6. The TheUV-vis UV-visdiffuse diffusereflectance reflectancespectra spectra(a);(a); and spectra of undoped BiVO and PLPL spectra (b) (b) of undoped BiVO 4, 1% 4, Figure 6. The UV-vis diffuse reflectance spectra (a); and PL spectra (b) of undoped BiVO4, 1% 1% Mo-BiVO Mo-BiVO 4 and 3% 3% Mo-BiVO 4. Mo-BiVO 4 and 4. Mo-BiVO4 and 3% Mo-BiVO4.

The foregoing foregoing analysis photocatalytic forfor Mo doped in The analysis shows shows that thatthe theenhancement enhancementof photocatalyticactivity activity Mo doped The foregoing analysis shows that the enhancement ofofphotocatalytic activity for Mo doped in BiVO 4 is is ascribed to tothetheimproved of in BiVO ascribed improvedelectronic electronicconductivity conductivity and and the the efficient efficient separation separation 4 ascribed BiVO 4 is to the improved electronic conductivity and the efficient separation of of photogeneratedcarries. carries. However, increase in doped-Mo species is not accompanied by an photogenerated an an increase in doped-Mo species is not accompanied by an increase photogenerated carries.However, However, an increase in doped-Mo species is not accompanied by an increase in photocatalytic activities and photocurrent intensity. Wethree proposed three main reasons to in photocatalytic activities and photocurrent intensity. We proposed main reasons to explain this increase in photocatalytic activities and photocurrent intensity. We proposed three main reasons to explain this phenomenon: (1) the occurrence of tetragonal phase for 3% Mo-BiVO4 can hinder the explain this phenomenon: (1) the occurrence of tetragonal phase for 3% Mo-BiVO4 can hinder the photocatalytic activity; (2) the formation of MoO3 at the surface of BiVO4 in 3% Mo-BiVO4 can photocatalytic activity; (2) the formation of MoO3 at the surface of BiVO4 in 3% Mo-BiVO4 can

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phenomenon: (1) the occurrence of tetragonal phase for 3% Mo-BiVO4 can hinder the photocatalytic increase(2) thethe crystallinity and thereby reduce the crystal defects; and (3) the excess dopants may act activity; formation of MoO 3 at the surface of BiVO4 in 3% Mo-BiVO4 can increase the crystallinity as recombination [27].defects; and (3) the excess dopants may act as recombination centers [27]. and thereby reducecenters the crystal

Figure 7. Photocurrent Photocurrentresponse response(a); (a);and andEIS EISspectra spectra(b)(b)ofof undoped BiVO Mo-BiVO undoped BiVO 4, 1% Mo-BiVO 4 and 3% 4 , 1% 4 and 3% Mo-BiVO . Mo-BiVO4. 4

4. Conclusions 4 were successfully fabricated. The into BiVO In summary, summary, photocatalysts photocatalysts with withMo Modoping doping into BiVO successfully fabricated. 4 were photocatalytic teststests indicate thatthat thethe 1%1%Mo-BiVO 4 4 exhibits The photocatalytic indicate Mo-BiVO exhibitsthe thestrongest strongest photocatalytic photocatalytic activity whichcan candegrade degrade 88.57% MB 120 within min. Mo-BiVO For 1.5% Mo-BiVO 4 and 4 3% Mo-BiVO4 which 88.57% of MBofwithin min.120 For 1.5% 3% Mo-BiVO photocatalysts, 4 and photocatalysts, theirperformance photocatalytic performance is similar BiVO to the4 undoped BiVO4 with their photocatalytic is similar to the undoped with the exception of the the exception strongest of theadsorption strongest dark adsorption ability. This phenomenon should be due to transformation phase dark ability. This phenomenon should be due to transformation phase and the newly and theMoO newly formed on MoO 3 particles onThe BiVO 4 surface. The optical performance results indicate formed BiVO optical performance results indicate that the obvious 3 particles 4 surface. that the obvious enhancement activity of photocatalytic activity for 1% Mo-BiVO 4 should be attributed to the enhancement of photocatalytic for 1% Mo-BiVO should be attributed to the efficient separation 4 efficient separation of photogenerated carries and improved electronic conductivity. of photogenerated carries and improved electronic conductivity.

Acknowledgments: National and and International International Cooperation Cooperation Special Special Project Project in Science and Technology Acknowledgments: National (2014DFR50830); Chongqing Natural Science Foundation (cstc2016shmszx20002, (2014DFR50830); Chongqing Natural Science Foundation (cstc2016shmszx20002, cstc2017 cstc2017 jcyjAX0259 jcyjAX0259 and and jcyjAX0141); Scientific and Technological Research Program of Chongqing Municipal Education Commission jcyjAX0141); Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1711272); and Chongqing high school youth backbone teacher funding scheme are gratefully acknowledged; (KJ1711272); and highLaboratory school youth backbone teacher funding are gratefully acknowledged; Open Pprojects of Chongqing Chongqing Key of Environmental Materials & scheme Remediation Technologies (CEK1502). Open Pprojects of Chongqing Key Laboratory of Environmental Materials & Remediation Technologies Author Contributions: Bitao Liu designed the experiments and writing; Xuelian Yan performed the experiments; (CEK1502).Yan contributed characterization; Yucen Yao contributed paper modification; Shanyong Chen analyzed Hengqing the data; Contributions: Jumeng Wei contributed search; Cai and Xu gave data interpretation; Li Author Bitao Liuliterature designed the Yanhua experiments andXuhui writing; Xuelian Yan performedLuthe contributed figures. experiments; Hengqing Yan contributed characterization; Yucen Yao contributed paper modification; Conflicts Interest: The authors declare no Wei conflict of interest. ShanyongofChen analyzed the data; Jumeng contributed literature search; Yanhua Cai and Xuhui Xu gave data interpretation; Lu Li contributed figures.

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