A ternary Er3+-BiVO4/TiO2 complex heterostructure with excellent photocatalytic performance

A ternary Er 3+ -BiVO 4 /TiO 2 complex heterostructure with excellent photocatalytic performance S. Obreg´ on and G. Col´ on* Ternary erbium doped BiVO 4 /TiO 2 complexes are synthesized by means of a simple impregnation method with good photoactivities under sun-like excitation for the degradation of phenol. From the structural and morphological characterization it has been stated that the presence of Er 3+ induces a slight stabilization of the tetragonal phase probably due to its incorporation in the BiVO 4 lattice. Therefore a ternary heterostructured material has been obtained. The best photocatalytic performance was attained for the samples with 1 wt% of Er 3+ -doped BiVO 4 content with respect to TiO 2 . The occurrence of a complex structural mixture with the adequate band position leads to effective charge pair separation which induces higher photocatalytic activities. 1. Introduction Visible-light-induced semiconductor photocatalysts have revealed great potential applications in elimination of organic pollutants. 1 However, most of the proposed semiconductor photocatalysts are clearly restricted by their poor efficiency. Within this framework, bismuth vanadate (m-BiVO 4 ) has been proposed as a novel alternative for TiO 2 -based photocatalysis. Thus, in the last few years, many studies described the interesting performances of BiVO 4 for organic contaminants degradation as well as O 2 evolution reactions under visible-light irradiation. 2–6 It has been widely reported that the photocatalytic performance of BiVO 4 is strongly dependent on its morphology and microstructure. 7–10 Different synthetic routes leads to BiVO 4 in two crystalline phases: monoclinic (space group I2/b)and tetragonal (space group I4 1 /a) scheelite. 11,12 Among the above crystal phases, the monoclinic BiVO 4 is the best visible-light- driven photocatalyst. The results reported by Yu et al. demonstrated that the crystalline structure is the vital factor controlling MB degradation and O 2 evolution reactions. 13 In the same direction, Tokunaga et al. reported that monoclinic structure shows much higher activity than a tetragonal one for O 2 evolution reaction. 14 From DFT calculations it is proposed that the effective hybridization of Bi6s state and the O2p state at the top of the valence band would be the responsible of the lower band gap value exhibited by m-BiVO 4 (ca. 2.4 eV) (Scheme 1). 15,16 On the basis of the wide reported results, the photocatalytic activity of tetragonal BiVO 4 appears almost negligible 17–19 while the photocatalytic activity of m-BiVO 4 is still scarce due to the poor charge-transport characteristics and the weak surface adsorption properties of this material. 20,21 Therefore, it is necessary to develop effective strategies to improve the charge separation efficiency and enhance visible-light photocatalytic activity of BiVO 4 photocatalysts. For the photoactivity improvement sake, different approaches have been proposed which include heterojunction structure formation, 20,22,23 co-catalysts loading, 24,25 and impurity doping. 26,27 Among these approaches, the co-existence of monoclinic-tetragonal heterostructure appears as a new way to be considered. Recently, it has been stated that the existence of a mixed-phase BiVO 4 clearly show higher photocatalytic activity. 28 The improved photocatalytic performance of m–t heterostructured BiVO 4 has been associated to promotion photoinduced electron–hole pairs separation. 29 Within this framework, Obreg´ on et al. reported the unexpected enhanced photocatalytic activity of tetragonal stabilized Ln-doped BiVO 4 . 30,31 Moreover, by changing the precursor addition sequence in the synthesis route, we have obtained a monoclinic- Scheme 1 Schematic band structures of the t-BiVO 4 and m-BiVO 4 . Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, C/Am´ erico Vespucio, 49, 41092 Sevilla, Spain. E-mail: [email protected]; Tel: +34 954489536 Cite this: RSC Adv.,2014,4, 6920 Received 12th November 2013 Accepted 23rd December 2013 DOI: 10.1039/c3ra46603e www.rsc.org/advances 6920 |RSC Adv.,2014,4, 6920–6926 This journal is © The Royal Society of Chemistry 2014 RSC Advances PAPER Published on 06 January 2014. Downloaded by Centro de Investigaciones Cientificas Isla de la Cartuja (CICIC) on 12/06/2014 10:44:36. View Article Online View Journal | View Issue tetragonal heterostructured Er 3+ -BiVO 4 exhibiting notably a high photoactivity under sun-like irradiation. 32 For this system, we have stated that the presence of erbium provokes a signi- cantly higher photoactivity that could be related to a double complex mechanism. Firstly, the formation of the tight interface m–theterostructure could be the responsible of a more effective charge separation. And secondly, but not less important, the contribution of extra-photons generated by a cooperative luminescence process in the overall mechanism due an energy transfer process from erbium ions to t-BiVO 4 and m- BiVO 4 could enhance the photon efficiency of the photocatalytic process. Thus, as the formation of heterojunction can signicantly reduce the recombination and speed up the separation rate of photogenerated charge carriers, coupling BiVO 4 with other semiconductors are oen used as an effective modication method. Within this frame, Long et al. proposed the Co 3 O 4 / BiVO 4 composite photocatalyst ash a p–n heterojunction semiconductor. These authors showed that the composite photocatalyst presented much higher photoactivity than pure BiVO 4 in the degradation of phenol under visible light irradiation. 33 Moreover, other authors reported a V 2 O 5 /BiVO 4 composite exhibiting the enhanced photocatalytic performance. 34,35 Finally, the photocatalytic properties of coupled TiO 2 /BiVO 4 have been also recently investigated demonstrating the interest of BiVO 4 based heterojunctions. 36,37 However, in these later cases m-BiVO 4 has been considered. Taking into account m- BiVO 4 conduction and valence bands position with respect to TiO 2 , it is clear that an unfavourable situation is taking place. In the present paper we present a ternary complex heterostructure formed by TiO 2 anatase and monoclinic-tetragonal BiVO 4 by erbium doping. The photocatalytic activities were evaluated by the photodegradation of phenol under simulated sun-light irradiation and the possible electronic mechanism responsible of the enhanced photocatalytic activities of Er 3+ -doped BiVO 4 / TiO 2 composite photocatalysts was also discussed. 2. Experimental 2.1 Samples preparation TiO 2 sample was obtained by means of a hydrothermal method elsewhere described. 38 In brief, a TiO 2 colloidal solution was obtained by adding certain amount of Ti 4+ –isopropanol solution to 400 mL of distilled water at pH ¼2 achieved by means of acetic acid. Aer TTiP addition a white precipitate is obtained that upon stirring at room temperature for one week evolve to a milky homogeneous solution. A certain amount of triethylamine (TEA) was then added drop wise to the Ti-solution aliquot till the pH value was 9. Aerwards, the obtained white precipitate suspension was then placed in a Teon recipient inside of stainless steel autoclave reactor. The hydrothermal treatment was performed at 120 C, 20 hours. The as obtained precipitate was then ltered, repeatedly washed and dried overnight at 120 C. Then TiO 2 powder was submitted to a further calcination treatment at 300 C for 2 hours. On the other hand, for the preparation of Er 3+ doped BiVO 4 , we have followed the method previously reported. 32 Briey, 5 mmol of Bi(NO 3 ) 3 $5H 2 O was dissolved in 10 mL of glacial acetic acid at room temperature. A second aqueous solution was prepared by dissolving the corresponding stoichiometric amount of NH 4 VO 3 and the corresponding amount of Er(NO 3 ) 3 (0.75 at.%) in 60 mL of hot distilled water. A milky colloidal suspension is obtained which could indicate the formation of small ErVO 4 particles. Aerwards, the ammonium metavanadate solution was added to the bismuth nitrate aqueous solution and the process was accompanied with a vigorous stirring. The pH of the obtained suspension was adjusted to 9.0 by adding concentrated NH 4 OH (13 mol L 1 ). The slurry was encased in a Teon vessel and heated under microwave irradiation using a microwave reactor. The temperature was xed at 140 C with a maximum variable power of 195 W during 30 min. The obtained precipitate was then cooled until room temperature, ltered and repeatedly washed and dried overnight at 120 C. Aerwards, thus obtained samples were submitted to a further calcination treatment at 300 C for 2 h. Hybrid composites were achieved by simple impregnation method previously described. 39 In a typical procedure, the appropriate amounts of TiO 2 and Er-BiVO 4 were added into methanol and sonicated separately for 30 min. Then, these two solutions were mixed and stirred at room temperature for 24 h. Aerwards, the composite photocatalysts were obtained by evaporating the methanol at 80 C. Er-BiVO 4 contents ranged from 0.5 wt% to 5 wt% with respect to TiO 2 . 2.2 Materials characterization BET surface area and porosity measurements were carried out by N 2 adsorption at 77 K using a Micromeritics 2010 instrument. X-ray diffraction (XRD) patterns were obtained using a Siemens D-501 diffractometer with Ni lter and graphite monochromator. The X-ray source was Cu Karadiation (0.15406 nm). The diffraction patterns were recorded from 2q 10to 80with step of 0.05and 120 s per step. Crystallite sizes were obtained from Rietveld renement. The UV diffuse reectance spectra were measured using an UV-vis spectrophotometer equipped with an integrating sphere (JASCO V-570). The reference sample used was a BaSO 4 coated standard pattern. Micro-Raman measurements were performed using a Lab- RAM Jobin Yvon spectrometer equipped with a microscope. Laser radiation (l¼532 nm) was used as excitation source at 5 mW. All measurements were recorded under the same conditions (2 s of integration time and 30 accumulations) using a 100magnication objective and a 125 mm pinhole. Field emission scanning electron (FE-SEM) and Transmission electron microscopy (TEM) was performed by using a Hitachi S 4800 and Philips CM 200 microscopes, respectively. The samples were dispersed in ethanol using an ultrasonicator and dropped on a carbon grid. 2.3 Photocatalytic tests The photocatalytic activity of the samples was tested by means the degradation of phenol under simulated solar conditions. In This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4, 6920–6926 | 6921 Paper RSC Advances Published on 06 January 2014. Downloaded by Centro de Investigaciones Cientificas Isla de la Cartuja (CICIC) on 12/06/2014 10:44:36. View Article Online a typical procedure, 0.05 g of photocatalyst was placed in a batch reactor containing 50 mL of phenol whose initial concentration was 30 mg L 1 . The suspension was maintained under dark conditions for 15 min in order to achieve the adsorption–desorption equilibrium of the dye on the photocatalyst surface. Aerwards, the suspension was irradiated with simulated solar light by means a solar simulator PEC-L01, Peccell. Samples of 1 mL were taken at given interval times and the photocatalyst was separated using a nylon lter. The concentrations were monitored by checking the absorption spectrum of each sample through its absorption band maximum (270 nm) using an UV-vis spectrophotometer (MECASYS Optizen 2120UV). 3. Results and discussion We have prepared a simple heterostructure based on TiO 2 and Er-BiVO 4 systems. The structural characterization of both was already reported by us. 32,38 The sol–gel hydrothermal preparation of TiO 2 from colloidal suspension leads to anatase phase system with calculated crystallite size of ca. 15 nm (Table 1). On the other hand, it has been stated that the incorporation of Er 3+ in the BiVO 4 structure induces the stabilization of the tetragonal structure (Fig. 1). Such structural stabilization has been explaining by considering the formation of small ErVO 4 seeds during the synthetic procedure, previous to the BiVO 4 formation. 32 ErVO 4 is normally obtained in the tetragonal zircon structure. 40 These small seeds would act as structure-directing agent conditioning the formation of the tetragonal BiVO 4 . Thus, a mixture of monoclinic and tetragonal phase is obtained, being the tetragonal the predominant one (70% tetragonal vs. 30% monoclinic). The XRD patterns of composite systems denote the presence of former structures, anatase TiO 2 and the corresponding growing fraction of tetragonal-monoclinic mixture of Er-BiVO 4 (Fig. 1). The BET surface area of TiO 2 is found to be signicantly high, ca. 102 m 2 g 1 . On the contrary and as widely reported in the literature, for BiVO 4 the surface area values are considerably low, always below 1–2m 2 g 1 (Table 1). 41,42 The occurrence of the tetragonal phase clearly induces a certain increase in this feature. 30,32 Therefore, heterostructured composites show a slightly lower surface area with respect to pristine TiO 2 . FromRamanspectroscopyitispossibletoobtaininteresting information from the structural point of view (Fig. 2). For bare TiO 2 we have found the typical Raman bands associated to anatase structure: 150 (e.g. (1)), 196 (e.g. (2)), 396 (A 1g /B 1g ), 516 (A 1g ), and 640 cm 1 (e.g. (3)). Single BiVO 4 system shows Raman bands around 158, 208, 324, 362, 710, and 826 cm 1 were observed for all samples, which are typical vibrational bands of monoclinic BiVO 4 (inset in Fig. 2). 43,44 Raman bands at 324 and 366 cm 1 are assigned to the asymmetric and symmetric deformation modes of the VO 43 tetrahedron, respectively. Additionally, the Raman bands at 822 cm 1 corresponds to the symmetric V–O stretching mode with A g symmetry. When tetragonal phase is present, it can be observed a notably shiof this later band toward higher frequencies (ca. 850 cm 1 ). Thus in our case, in which BiVO 4 appears composed by a mixture of tetragonal and monoclinic phase, with the former as the main crystalline phase, only the 850 cm 1 is clearly observable. Table 1 Surface, structural and photocatalytic characterization for TiO 2 /Er-BiVO 4 heterostructured catalysts Samples Crystallite size a (nm) BET (m 2 g 1 ) Band gap (eV) TiO 2 14 (a) 102 3.14 Er-BiVO 4 0.5% 15 (a) 99 3.14 Er-BiVO 4 1.0% 15 (a) 91 3.13 Er-BiVO 4 2.0% 15 (a) 90 3.13 Er-BiVO 4 3.0% 15 (a) 92 3.07 Er-BiVO 4 4.0% 15 (a) 89 3.05 Er-BiVO 4 5.0% 15 (a) 90 2.80 Er-BiVO 4 52 (ts) 3 2.80 + 2.35 BiVO 4 48 (ms) 1 2.35 a (a: anatase, ts: tetragonal scheelite; ms: monoclinic scheelite). Fig. 1 XRD patterns for TiO 2 /Er-BiVO 4 heterostructures with different Er-BiVO 4 content. Fig. 2 Raman spectra for TiO 2 /Er-BiVO 4 heterostructures with different Er-BiVO 4 content. 6922 |RSC Adv.,2014,4, 6920–6926 This journal is © The Royal Society of Chemistry 2014 RSC Advances Paper Published on 06 January 2014. Downloaded by Centro de Investigaciones Cientificas Isla de la Cartuja (CICIC) on 12/06/2014 10:44:36. View Article Online In addition the above structural characterization, it can be stated a set of bands in the range 400–700 nm that could be attributed to the visible uorescence emission in Er-doped systems (inset in Fig. 2). Such visible luminescence upon green excitation has been widely reported for Er based phosphors materials. As we have previously shown, such emission is related to the presence of tetragonal phase. 30 The Er-doping onto BiVO 4 structure is achieved by an effective Er substitution in Bi 3+ sites of the tetragonal structure. Within this situation, a strong luminescence emission is observed. Therefore, the additional bands observed in the heterostructured composite spectra correspond to such visible luminescence process. The morphology of BiVO 4 has extensively reported to be dependent on the preparation route. 45 In our case, bare BiVO 4 shows a square rod-like morphology. 30 The morphology of Erdoped systems clearly denotes the tetragonal-monoclinic structural mixture (Fig. 3a). Thus, both morphologies (square rods and needle-like particles) clearly cohabit in this system. Heterostructured composites are formed by Er-BiVO 4 effectively covered by TiO 2 (Fig. 3b and c). Additionally, it can be noticed large TiO 2 aggregates formed by small particles of about 10–15 nm size (inset Fig. 3c and EDS spectra). Regarding to UV-vis absorption properties, the diffuse reectance UV-vis spectra of different composites are shown in Fig. 4. As it can be noticed three absorption edges can be found, corresponding to TiO 2 anatase and monoclinic and tetragonal mixture for Er-BiVO 4 (inset Fig. 4). The calculated band gap values (Table 1) are in accordance to the heterostructured assembly of TiO 2 and Er-BiVO 4 systems. It is worthy to note that due to the low doping level, it is not possible to detect the excitation bands for Er 3+ species. In spite of this, the strong uorescence bands observed in Raman spectra would indicate the important luminescence process which is taking place. The photocatalytic performance of the Er-doped systems was studied for phenol degradation under solar-like irradiation (Fig. 5). The rst point that is worthy to mention is that heterostructured composite systems lead to higher photoactivities than bare TiO 2 . Thus, as Er-BiVO 4 content increases it can be notice a progressive raise in the reaction rates till Er-BiVO 4 content reaches 1 wt% value (Fig. 5b). From this value, the Fig. 3 SEM and TEM study of heterostructured systems: (a) SEM image of Er-BiVO 4 ; (b) SEM image for TiO 2 /Er-BiVO 4 (1% Er-BiVO 4 ) heterostructures; (c) TEM image for TiO 2 /Er-BiVO 4 (1% Er-BiVO 4 ) heterostructure including EDS analysis for different positions in the heterostructure interface (inset: TEM image for pristine TiO 2 ). This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4, 6920–6926 | 6923 Paper RSC Advances Published on 06 January 2014. Downloaded by Centro de Investigaciones Cientificas Isla de la Cartuja (CICIC) on 12/06/2014 10:44:36. View Article Online reaction rate starts to decrease reaching a similar value as pristine TiO 2 for 5 wt%. The photocatalytic performance of the proposed complex heterostructure clearly overcomes the already published BiVO 4 – TiO 2 systems. 36,37 Thus, Hu et al. proposed a band conguration in which upon visible excitation BiVO 4 would act as sensitizer. Based on the position of BiVO 4 and TiO 2 conduction bands the photogenerated electron transfer from BiVO 4 to TiO 2 is energetically forbidden. So, a different electronic mechanism should be considered in this case. Moreover, Zhang et al. showed an interesting synergistic effect taking into account the double UV and visible photoactivity of the BiVO 4 /TiO 2 heterostructure. 37 However, these authors were able to discolorate Rhodamine B aer 6 hours of irradiation. Thus, the results presented here clearly surpass the above mentioned published ones. In order to envisage a possible mechanism, we have also performed the photoactivity studies for heterostructure composite formed by TiO 2 and undoped m-BiVO 4 (Fig. 6). Firstly, it is worthy to note that m-BiVO 4 showed a lower photoactivity than Er-BiVO 4 for which a mixture of tetragonal and monoclinic phase is present. This fact was already stated by us and explained by considering a double cooperative photonic and electronic mechanism involved. 30,32 Regarding to heterostructured composite systems, it is clear that the TiO 2 /m-BiVO 4 assembly, though increases the photoactivity with respect to TiO 2 , shows a reaction rate still fairly lower than TiO 2 /Er-BiVO 4 heterostructure. Taking into account the low Er doping level in the TiO 2 /Er-BiVO 4 composite (0.75 at.% Er-BiVO 4 is present in 1 wt% of the composite), for these heterostructure it would be expected that the up-conversion luminescent process might be almost negligible. So, the enhanced photocatalytic activity must be associated to the presence of an effective intimate heterojunction. Thus, an Fig. 4 Diffuse reflectance spectra for TiO 2 /Er-BiVO 4 heterostructures with different Er-BiVO 4 contents. Fig. 5 (a) Evolution of phenol concentration (C 0 ¼50 ppm) with photodegradation time; (b) calculated reaction rates for TiO 2 /Er-BiVO 4 heterostructures with different Er-BiVO 4 content. Fig. 6 Comparison of phenol photodegradation reaction rates for different bares and heterostructured materials. 6924 |RSC Adv.,2014,4, 6920–6926 This journal is © The Royal Society of Chemistry 2014 RSC Advances Paper Published on 06 January 2014. Downloaded by Centro de Investigaciones Cientificas Isla de la Cartuja (CICIC) on 12/06/2014 10:44:36. View Article Online effective spatial charge separation would be the responsible of the photocatalytic activity enhancement (Scheme 2). If we consider the band positions for different phases, the difference in the observed band gap values for monoclinic and tetragonal structures lays on the participation of Bi6s orbitals in the monoclinic valence band hybridization. 16 The participation of Bi6s orbital in the band hybridization would stabilize the valence band top through coupling Bi6s–O2p antibonding state with Bi6p state. 2,46 Moreover, the sterically active Bi6s 2 lone pair would play a key role in raising up the O2p states and reducing the band gap. 15 From the calculated energies for BiVO 4 valence and conduction bands, 47 the following scheme can be envisaged: m-BiVO 4 valence band top is positioned at slightly lower energy with respect to the TiO 2 one, while the corresponding for t-BiVO 4 is located at slightly higher energy position (Scheme 2). On this basis, it could be expected that upon UV excitation, t- BiVO 4 would act as electron sink while holes will be derived to TiO 2 . In the case of TiO 2 /m-BiVO 4 the charge pairs ow in the m-BiVO 4 direction. For this reason, the reaction rate for TiO 2 /m- BiVO 4 appears comparable than that obtained for TiO 2 . Thus, it can be assumed that the occurrence of a structural mixture in the BiVO 4 with predominant tetragonal phase would also help to enhanced charge separation. In addition to this proposed synergistic mechanism in a lower extent, the occurrence of a tetragonal-monoclinic heterostructured BiVO 4 has been also demonstrated to have better photocatalytic performances. 31 From the relative band positions for tetragonal and monoclinic structures an effective charge separation can be also considered. Therefore, this additional interactive process, though in lower level due to the small BiVO 4 content, might be also considered in the overall photocatalytic mechanism. 4. Conclusions We have obtained a complex heterostructured system by a simple impregnation method, showing improved photoactivity under solar-like irradiation. From this method we have obtained an effective coverage of large Er-BiVO 4 particles with 15 nm size TiO 2 . From the structural point of view we have stated that the existence of a phase mixture in BiVO 4 leads to better photocatalytic performance of the heterojunction. From the results obtained, it would be proposed that the concurrence of TiO 2 and Er 3+ -BiVO 4 complex heterostructure would induce an improved performance due to an optimized charge separation process. Acknowledgements The nancial support by projects P09-FQM-4570 and ENE2011- 24412 is fully acknowledged. S. Obreg´ on thanks CSIC for the concession of a JAE-Pre grant. References 1 A. Kubacka, M. Fern´ andez-Garc´ ıa and G. Col´ on, Chem. Rev., 2012, 112, 1555–1614. 2 A. Kudo, K. Omori and H. Kato, J. Am. Chem. Soc., 1999, 121, 11459–11467. 3 J. Yu and A. 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