Fe2O3-Modified Porous BiVO4 Nanoplates with Enhanced Photocatalytic Activity
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As BiVO4 is one of the most popular visible-light-responding photocatalysts, it has been widely used for visible-light-driven water splitting and environmental purification. However, the typical photocatalytic activity of unmodified BiVO4 for the degradation of organic pollutants is still not impressive. To address this limitation, we studied Fe2O3-modified porous BiVO4 nanoplates. Compared with unmodified BiVO4, the Fe2O3-modified porous BiVO4 nanoplates showed significantly enhanced photocatalytic activities in decomposing both dye and colorless pollutant models, such as rhodamine B (RhB) and phenol, respectively. The pseudo-first-order reaction rate constants for the degradation of RhB and phenol on Fe2O3-modified BiVO4 porous nanoplates are 27 and 31 times larger than that of pristine BiVO4, respectively. We also found that the Fe2O3 may act as an efficient non-precious metal co-catalyst, which is responsible for the excellent photocatalytic activity of Fe2O3/BiVO4.
KeywordsFe2O3 BiVO4 Nanostructures Co-catalyst Photocatalytic activity
As BiVO4 is one of the most popular visible-light-responding photocatalysts, it has attracted much attention since Kudo and co-authors first reported its photocatalytic activity in 1998 . To improve the photocatalytic activity of BiVO4, some typical strategies such as preparing monoclinic crystalline phase , obtaining large specific surface area and high-energy facets [3, 4], constructing special architectures [5, 6], and combining two or several above methods together have been developed . However, until now, the photocatalytic activity of single-component BiVO4 is still not ideal yet for practical application. The rational design of composite photocatalysts could extend the spectral responsive range and promote the separation of photogenerated carriers and thus would improve photocatalytic activity dramatically compared to their host single-component materials [8, 9]. Based on this strategy, we fabricated n–p core–shell BiVO4@Bi2O3 and n–n Bi2S3/BiVO4 composite microspheres previously [10, 11]. Although these composite materials showed better photocatalytic activity than the pure BiVO4, their photocatalytic activity was still not impressive. The studies on photoelectrochemical water splitting of BiVO4 as photoanodes have showed that BiVO4 was poor catalyst for water oxidation. However, the appropriate modification of its surface with various oxygen evolution catalysts such as cobalt–phosphate, RhO2, and Pt could greatly improve its performance [12, 13]. These findings suggest that rationally loading co-catalyst on the surface of BiVO4 can improve its photocatalytic activity for the degradation of various organic pollutants. Recently, Li’s group reported that the photocatalytic activity of BiVO4 in oxidizing thiophene could be significantly enhanced through the modification with Pt and RuO2 co-catalysts . Nevertheless, Pt and Ru are noble metals and expensive reagents. Therefore, it would be highly desirable if we can achieve the enhanced photocatalytic activity of BiVO4 using earth-abundant elements instead of the rare and precious ones .
Fe2O3 consists of earth-abundant iron and oxygen elements and possesses the advantages of low cost, environmental friendliness and easy production, which has wide applications in many fields such as energy storage and conversion [16, 17], catalysis , sensing and biomedicine [19, 20]. As n-type semiconductor with a band gap of ca. 2.2 eV, Fe2O3 is also a potential visible-light-driven photocatalyst . Nevertheless, the photocatalytic activity of unmodified Fe2O3 is poor because of its low carrier mobility, short minority carrier life time (ca. 10 ps) and diffusion length (ca. 2–4 nm) . Therefore, there has been a lot of research focused on how to enhance photoelectrochemical and photocatalytic performance of Fe2O3 [23, 24]. On the other hand, the successful applications of Fe2O3 in many important organic catalytic reactions suggested that it had good catalytic reaction activity [18, 25]. However, to the best of our knowledge, Fe2O3 as a role of co-catalyst in photocatalytic system has never been reported until now.
On the other hand, the fabrication of porous two-dimensional (2D) nanostructures has drawn much interest because the porous 2D structure of these materials can increase materials’ surface area, facilitate the adsorption and diffusion of reactant molecules, and accelerate the transfer of photogenerated carriers from the interior to the surface of the material [26, 27, 28]. As a result, the photocatalysts with porous 2D nanostructures are expected to have good photocatalytic activity. Recently, Yu and co-authors have reported the synthesis of porous CuS/ZnS nanosheets with excellent photocatalytic activity through cation exchange reaction between performed inorganic–organic hybrid ZnS(en)0.5 nanosheets and Cu2+ ions . Similarly, nanoporous Cd x Zn1−x S nanosheets have been prepared through the cation exchange reaction of ZnS–amine nanosheets with cadmium ions . However, the above-mentioned methods were limited in a few inorganic–organic hybrid 2D nanomaterials. Comparatively speaking, 2D metal complex nanostructures are more easily prepared through self-assembly of ligand at room temperature, which is driven by various non-covalent interactions including π–π stacking, van der Waals bonding, hydrophobic interactions, etc. [31, 32, 33]. Subsequently, 2D porous compounds nanostructures can be obtained by exchange reaction between ligands in 2D metal complexes nanostructures and anions in the desired compounds under certain conditions. However, this kind of ligand-anion exchange route to synthesize 2D porous nanostructured material has not, so far, been developed.
2 Experimental Section
2.1 Preparation of Porous BiVO4 Nanoplates
Bi2(BDC)3 nanoplates were prepared through our previous report . For the preparation of porous BiVO4 nanoplates, 0.1504 g of the as-obtained Bi2(BDC)3 and 0.0732 g of NaVO3 were dispersed into 40 mL of ultrapure water under stirring. Then the solution was put into a Teflon® lined stainless steel autoclave with 50 mL of capability and heated at 180 °C for 10 h. After the autoclave was cooled to room temperature, the products were separated through centrifugation and washed three times with ultrapure water and absolute ethanol. Finally, the products were dried under vacuum at 60 °C for 4 h.
2.2 Preparation of Fe2O3-Modified BiVO4 Porous Nanoplates
In a typical procedure, 0.4 mmol of porous BiVO4 nanoplates, 1.0 mmol of NaOH, and different amounts of Fe(NO3)3 (0.008, 0.02, 0.04 mmol) were added into 40 mL of ultrapure water in sequence under stirring. After that, the solution was put into a Teflon® lined stainless steel autoclave with 50 mL of capability and heated at 160 °C for 12 h. After the autoclave was cooled to room temperature, the resultant products were separated via centrifugation and washed three times with ultrapure water and absolute ethanol, respectively. Finally, the products were dried under vacuum at 60 °C for 4 h.
Powder X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advanced X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 8°/min in the 2θ range of 10°–70°. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo ESCALAB 250 X-ray photoelectron spectrometer with an excitation source of Al Kα radiation (λ = 1,253.6 eV). Field emission scanning electron microscopy (FE-SEM) images and energy dispersed X-ray (EDX) spectra were taken on a Nova NanoSEM 200 scanning electron microscope. Transmission electron microscopy (TEM), EDX, high-resolution TEM (HRTEM) images and mapping images were taken on a JEOL 2010 microscope, using an accelerating voltage of 200 kV. The UV–Visible DRS were recorded on a UV2450 (Shimadzu) using BaSO4 as reference. The PL spectra were recorded on a Fluoromax-4 spectrofluorometer (HORIBA Jobin Yvon Inc.) equipped with a 150 W xenon lamp as the excitation source.
2.4 Photocatalytic Properties
The photocatalytic activity of Fe2O3/BiVO4 nanoplates was evaluated by the degradation of RhB and phenol under visible-light irradiation from 500 W Xe light (CHF-XM500, purchased from Beijing Trusttech Co., Ltd) equipped with a 400 nm cutoff filter, and water splitting using a 300 W Xe lamp and optical cutoff filter (λ > 420 nm). In a typical process, 100 mg of photocatalysts was added to 100 mL of rhodamine B (RhB) solution (10−5 mol L−1). Before illumination, the solution was magnetically stirred in the dark for 12 h to ensure an adsorption–desorption equilibrium between the photocatalysts and RhB. After that, the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 3 mL of solution was sampled and centrifuged to remove the photocatalyst particles. Then, the filtrates were analyzed by recording variations of the absorption band maximum (553 nm) in the UV–Vis spectra of RhB by using a Shimadzu UV2501PC spectrophotometer. The studies of photocatalytic activities for other samples adopted the same measurement process. For photocatalytic degradation of phenol, the initial concentration of phenol solution was 1 × 10−4 mol L−1 and kept other conditions unchanged. After visible light irradiation of different period time, the centrifugated solution was analyzed by recording variations of the absorption band maximum (270 nm) of phenol in the UV–Vis spectra by using a Shimadzu UV2450 PC spectrophotometer. As to photocatalytic water splitting, 0.1 g of photocatalysts was dispersed in 150 mL of 0.02 M aqueous AgNO3 solution in a Pyrex reaction cell and thoroughly degassed by evacuation in order to drive off the air inside. The amount of evolved O2 was measured by an online gas chromatograph.
3 Results and Discussion
3.1 Crystal Structure, Compositions and Morphology
3.2 Photocatalytic Properties
3.3 The Role of Fe2O3 and Photocatalysis Mechanism
The doping of Fe3+ ions in the lattice of some photocatalysts or its graft on the surfaces of certain photocatalysts would enhance the photocatalytic activities of the photocatalysts [38, 39]. In order to survey whether the enhancement of photocatalytic activity of BiVO4 stems from the doping or grafting effect of Fe3+ ions, the control experiment without using NaOH was carried out. The photocatalytic activities of the resultant products have no obvious change for the decomposition of RhB (Fig. 5a), compared with pristine BiVO4. The result shows that the doping and grafting of Fe3+ ions were not main reasons for the enhanced photocatalytic activity of Fe2O3/BiVO4. Previous studies indicated that single-component Fe2O3 nanocrystals showed poor and negligible photocatalytic activity for the degradation of RhB and phenol under visible light irradiation [40, 41], respectively. To further understand the role of Fe2O3 in the composite material, Fe2O3 nanorods were synthesized through modified method (Supporting Information, Fig. S5 and Fig. S6) . As shown in Fig. 5a and c, only a small quantity (<25 %) of RhB molecules were degraded on Fe2O3 nanorods within 90 min while almost no detectable decomposition was observed for phenol. It was observed that the size of Fe2O3 is very small in Fe2O3/BiVO4 composites. In order to exclude the possibility of nanosize effect of Fe2O3 for the enhanced photocatalytic activity, ultrasmall Fe2O3 nanoparticles supported by SiO2 nanospheres were synthesized and their photocatalytic activities were evaluated by the degradation of phenol (Fig. S7–S9). As shown in Fig. S9, Fe2O3/SiO2 showed very poor photocatalytic activity as well. The above results further indicate that Fe2O3 itself has very weak photocatalytic activity, which cannot account for the excellent photocatalytic activity in the present Fe2O3/BiVO4 system.
In conclusion, Fe2O3/BiVO4 nanoplates with porous structures have been prepared through a mild chemical conversion and subsequently hydrothermal deposition–precipitation routes. The as-prepared Fe2O3/BiVO4 showed excellent photocatalytic activity in degrading both RhB and phenol. After the modification of Fe2O3, the photocatalytic activity of pristine BiVO4 could be increased more than one order of magnitude. It is believed that Fe2O3 acts as an efficient co-catalyst, which contributes to the excellent photocatalytic activity of Fe2O3/BiVO4 porous nanoplates. The facile preparation method, low cost of raw materials, excellent photocatalytic activity, and good reusability of the Fe2O3/BiVO4 porous nanoplates make the material a promising photocatalyst for the application in the field of environmental remedy.
The authors would like to express acknowledge for partial financial support from NSFC (51372173, 51002107, and 21173159), NSFC for Distinguished Young Scholars (51025207), Research Climb Plan of ZJED (pd2013383), Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201409SIC), Xinmiao talent project of Zhejiang Province (2013R424060), and College Students Research Project of Wenzhou University (14xk193).
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