Nano-Micro Letters

, Volume 7, Issue 2, pp 183–193

Fe2O3-Modified Porous BiVO4 Nanoplates with Enhanced Photocatalytic Activity

Open Access
Article

Abstract

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.

Graphical Abstract

Fe2O3, as a cheap and efficient co-catalyst, could greatly enhance the photocatalytic activity of BiVO4 porous nanoplates in decomposing organic pollutants.

Keywords

Fe2O3 BiVO4 Nanostructures Co-catalyst Photocatalytic activity 

1 Introduction

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 [1]. To improve the photocatalytic activity of BiVO4, some typical strategies such as preparing monoclinic crystalline phase [2], 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 [7]. 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 [14]. 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 [15].

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 [18], 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 [21]. 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) [22]. 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 [29]. Similarly, nanoporous CdxZn1−xS nanosheets have been prepared through the cation exchange reaction of ZnS–amine nanosheets with cadmium ions [30]. 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 ππ 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.

In this study, we present a new method for synthesizing porous BiVO4 nanoplates via exchange reaction between ligand 1,4-benzenedicarboxylate (BDC) in 2D Bi2(BDC)3 nanoplates derived from direct self-assembly and VO43− anions under hydrothermal condition, schematically shown in Fig. 1. Subsequently, Fe2O3-modified porous BiVO4 nanoplates were obtained through a simple hydrothermal deposition–precipitation route. The as-obtained Fe2O3/BiVO4 products displayed excellent photocatalytic activities for degrading both dye and colorless organic pollutants. Based on UV–Visible diffuse-reflectance spectra (DRS), photoluminescence (PL) spectra, and a series of comparative experiments, Fe2O3 as a role of non-precious metal co-catalyst in the present composite system was proposed.
Fig. 1

Representation for the synthetic route to porous BiVO4 nanoplate

2 Experimental Section

2.1 Preparation of Porous BiVO4 Nanoplates

Bi2(BDC)3 nanoplates were prepared through our previous report [34]. 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.

2.3 Characterizations

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

Figure 2a shows XRD pattern of the products obtained through the direct exchange reaction of BDC molecules in Bi2(BDC)3 with VO43− anions under hydrothermal condition. All the diffraction peaks can be indexed to a monoclinic scheelite structure of BiVO4 (JCPDS No. 75-2480). No obvious reflection peaks from other impurities can be observed. When only 1 mmol of NaOH were used, some part of BiVO4 could be converted into Bi2O3 (Fig. 2e, marked with asterisks) and formed Bi2O3/BiVO4 composites under hydrothermal treatment, which is consistent with our previous report [10]. But when different amounts of Fe(NO3)3 (0.008, 0.02, 0.04 mmol) and 1 mmol of NaOH were simultaneously added into the reaction system, the resultant products contained no Bi2O3 (Fig. 2b, c, d). The results showed that Fe3+ ions would preferentially react with OH- ions in solution. But, no diffraction peaks of Fe2O3 could be observed because its content in the products is lower than the XRD limit of detection and/or poor crystalline degree. The existence of Fe2O3 could be confirmed by subsequent XPS, EDX spectra, and HRTEM images.
Fig. 2

XRD patterns of the products hydrothermally synthesized with Bi2(BDC)3 and NaVO3a BiVO4, b NaOH and 0.008, c NaOH and 0.02, d NaOH and 0.04, e NaOH and 0 mmol of Fe(NO3)3, respectively, and f BiVO4 and Fe(NO3)3

The survey XPS spectrum reveals that the products are composed of Bi, V, O, Fe, and C elements (Supporting Information, Fig. S1). Figure 3a, b, c and d shows high-resolution XPS spectra of Bi 4f, V 2p, Fe 2p, and O 1s of the products, respectively. In Fig. 3a, the peaks with binding energies of 159.2 and 164.4 eV correspond to Bi 4f7/2 and Bi 4f5/2 in BiVO4 [11], respectively. The values (516.7 and 524.5 eV) of the binding energies for V 2p are in agreement with previous report on V5+ in BiVO4 [35]. In the high-resolution Fe 2p spectrum (Fig. 3c), two distinct peaks located at 710.8 eV for Fe 2p3/2 and 724.2 eV for Fe 2p1/2 with a shake-up satellite at 719.3 eV were observed. This is characteristic of Fe3+ in Fe2O3 [19]. The O 1s peak is asymmetric, indicating that different oxygen species were present in the near surface region. The major component is attributed to the lattice oxygen (529.8 eV) in BiVO4 or Fe2O3. The second component centered at 532. 3 eV is associated with non-equivalent OH group on the surface [36]. Combined with the XRD results, it can be known that the products consist of BiVO4 and possible Fe2O3.
Fig. 3

Typical XPS high-resolution spectra of the products synthesized with BiVO4, NaOH, and 0.02 mmol of Fe(NO3)3: a Bi 4f, b V 2p, c Fe 2p, and d O 1 s

We chose Bi2(BDC)3 as precursor to synthesize BiVO4 because the former was easily self-assembled into nanostructures through ππ stacking of benzene ring structure in BDC molecule at room temperature. Moreover, the flexible BDC ligand was helpful to alleviate the strain caused by crystal lattice mismatch between Bi2(BDC)3 and BiVO4 and maintain the framework of nanoplate during chemical conversion process. Figure 4a represents a typical FE-SEM image of the as-obtained Bi2(BDC)3 precursor, which shows that the sample is nanoplates with average size of 100 nm in thickness, 500 nm in width, and 1.5 μm in length. After chemical transformation, Bi2(BDC)3 nanoplates were converted into porous BiVO4 nanoplates (Fig. 4b). These pores are linked together and distributed throughout the whole nanoplates. Magnified FE-SEM image shows that there are some tiny nanoparticles on the surfaces of the products obtained through hydrothermal deposition–precipitation route (Fig. S2). Further insight morphology and the microstructure of the products can be obtained from TEM and HRTEM images. In agreement with FE-SEM observations, TEM also shows that the products are porous nanoplates. Careful analyses show that there are some aggregates of tiny particles (marked with white arrowhead) closely integrating with the surfaces of nanoplates. Figure 4d represents HRTEM image recorded on the white rectangular area in Fig. 4c. The fringe spacing of 0.362 and 0.312 nm in the HRTEM image corresponds to the interplanar separation of (110) and (103) planes of monoclinic BiVO4, respectively. The HRTEM image of tiny particles displays the fringe spacing of 0.221 nm, which is consistent with the interplanar spacing of (113) plane of hexagonal-phase Fe2O3. The EDX spectroscopy recorded on a single nanoplate shows that the sample consists of Bi, V, Fe, Cu, C, and O elements (Fig. 4f). The Cu and C elements come from copper grid coated carbon for sample support. Bi, V, Fe and O elements correspond to BiVO4 and Fe2O3. The EDX spectrum further supports the results of HRTEM. The mapping images of an individual Fe2O3/BiVO4 porous nanoplate show that Fe2O3 nanoparticles scattered unequally on the surfaces of porous BiVO4 nanoplates (Fig. S3). The quantitative calculation on the EDX spectrum recorded on SEM (Fig. S4) shows that the content of Fe2O3 in the sample is ca. 3.8 mol %. All the above results indicate that the as-obtained products are Fe2O3-modified BiVO4 porous nanoplates.
Fig. 4

FE-SEM images of a Bi2(BDC)3 and b BiVO4. c TEM image of Fe2O3-modified BiVO4. HRTEM images recorded on the places marked with white rectangular aread and white arrowheade in Fig. 4c. f EDX spectrum performed on a single nanoplate

3.2 Photocatalytic Properties

The photocatalytic activities of the as-obtained Fe2O3/BiVO4 porous nanoplates were evaluated by the degradation of RhB and phenol, common pollutant models as dye and colorless organic compounds, respectively, in water under visible light irradiation (λ > 400 nm). As shown in Fig. 5a, the photodegradation efficiency of RhB over Fe2O3/BiVO4 is near 100 % within 80 min under visible light irradiation. However, RhB is stable and has a negligible degradation in the absence of any photocatalysts under visible light irradiation [37]. The photocatalytic activity of Fe2O3/BiVO4 is significantly superior to pristine BiVO4 and Bi2O3/BiVO4 as well. Considering most of the photocatalytic reactions follow a pseudo-first-order kinetic equation: ln(C0/C) = kt (where C0 and C are the equilibrium concentration of adsorption and the concentration of RhB at the irradiation time t, respectively, and k is the apparent rate constant), the plots of ln(C0/C) versus t were performed. The ln(C0/C) versus t displays a linear relationship (Fig. 5b), which shows that the photodegradation of RhB on these photocatalysts obeys the rules of first-order reaction kinetics. From the fitting results, the apparent reaction rate constants are 0.0013, 0.0057, and 0.0358 min−1 for pristine BiVO4, Bi2O3/BiVO4, and Fe2O3/BiVO4, respectively. As a result, the photocatalytic reaction rate of Fe2O3/BiVO4 is 27 and 6 times larger than that of pristine BiVO4 and Bi2O3/BiVO4, respectively. Fe2O3/BiVO4 also displayed excellent photocatalytic activity for degrading colorless phenol. As shown in Fig. 5c, the photocatalytic activity of BiVO4 was greatly enhanced after the modification of Fe2O3, which was in good agreement with the results of photocatalytic decomposition of RhB. The apparent reaction rate constants are 0.0006, 0.0009, and 0.019 min−1 for BiVO4, Bi2O3/BiVO4, and Fe2O3/BiVO4, respectively. As a result, the photocatalytic reaction rate of Fe2O3/BiVO4 is 31 and 21 times larger than that of unmodified BiVO4 and Bi2O3/BiVO4, respectively. Whether the pollutant model is RhB or phenol, the amount of Fe2O3 in the products has obvious influence on photocatalytic activity of Fe2O3/BiVO4. When too lower or too higher concentration of Fe(NO3)3 was used, less or more Fe2O3 was produced. It was found that the photocatalytic activity of Fe2O3/BiVO4 would decline in both cases. However, all the composites show higher photocatalytic activity than pure BiVO4. Therefore, finely adjusting the content of Fe2O3 will further improve the photocatalytic activity of Fe2O3/BiVO4. In addition, Fe2O3-modified BiVO4 porous nanoplates show good reusability (Fig. 6). After five times cycles for the photocatalytic degradation of RhB and phenol, the sample did not exhibit obvious loss of photocatalytic activity.
Fig. 5

a RhB concentration changes with irradiation time over no catalysts, BiVO4, Fe3+ ions modified BiVO4, Fe2O3 nanorods, Bi2O3/BiVO4, and Fe2O3/BiVO4 synthesized with different amounts of Fe(NO3)3. b Plots of ln(C0/C) versus t of BiVO4, Bi2O3/BiVO4 and Fe2O3/BiVO4. c Phenol concentration changes with irradiation time over no catalysts, Fe2O3 nanorods, BiVO4, Bi2O3/BiVO4, and Fe2O3/BiVO4 synthesized with different amounts of Fe(NO3)3. d Plots of ln(C0/C) versus irradiation time (t) of BiVO4, Bi2O3/BiVO4 and Fe2O3/BiVO4. 0.008, 0.04 and 0.02 represent the amount of Fe(NO3)3 used (the unit is mmol)

Fig. 6

The stability of Fe2O3 modified BiVO4 porous nanoplates after 5 cycles for photocatalytic degradation of RhB (a) and phenol (b)

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) [40]. 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.

The photocatalytic activity of the composite photocatalysts in decomposing organic pollutant is closely related to their adsorption capability, light energy utilization, separation efficiency of photogenerated carriers, and surface catalytic performance [42, 43, 44, 45]. Fe2O3/BiVO4 and pristine BiVO4 have similar adsorption behavior to both RhB and phenol (Fig. 7a, b). The introduction of Fe2O3 and Bi2O3 do not noticeably improve adsorption ability of BiVO4 to the organic pollutants (Fig. S10). Figure 7c represents UV–Vis DRS of Fe2O3/BiVO4 and pristine BiVO4. There are no obvious differences between them except that Fe2O3/BiVO4 displays slightly higher absorption in both UV and visible light region than BiVO4, which is possibly attributed to larger absorption coefficient of Fe2O3 (ca. 105 cm−1) than that of BiVO4 and its narrower band gap (2.2 eV) [46]. As we know, PL spectra can be used as an efficient means to evaluate the separation efficiency of the photogenerated carriers [11]. Figure 7d displays the room temperature PL emission spectra (excited at 400 nm) of pure BiVO4 and Fe2O3/BiVO4. The result shows that PL emission intensity of original BiVO4 decreases after it was combined with Fe2O3, which suggests the photogenerated electrons and holes are efficiently separated. In order to further understand how photogenerated electron and hole migrate, the photocatalytic activities for water splitting of pure BiVO4, 0.008Fe2O3/BiVO4, 0.02Fe2O3/BiVO4, and 0.04Fe2O3/BiVO4 were comparatively studied. The amount of O2 evolved in the first hour was evaluated. As shown in Fig. 7e, the photocatalytic O2 evolution activities of the samples gradually decline with increasing content of Fe2O3 and pure BiVO4 possesses the best performance. The results suggest that photogenerated holes not electrons were transferred to the surfaces of Fe2O3. Otherwise, more O2 will be produced on Fe2O3/BiVO4 because the composite material can more efficiently separate photogenerated carriers than pure BiVO4. Fe2O3/BiVO4 showed lower photocatalytic water splitting activity than pure BiVO4 because Fe2O3 had lower oxidation potential than BiVO4. When photogenerated holes were transferred to the surfaces of Fe2O3, their water oxidation performance correspondingly weakened. The tradeoff between the separation of carriers and decreased oxidation potential leads to declined photocatalytic water splitting performance of Fe2O3/BiVO4. Therefore, Fe2O3 can act as a hole scavenger and efficiently separates photogenerated carriers.
Fig. 7

The adsorption behavior of a RhB and b phenol on BiVO4 and Fe2O3/BiVO4. c UV–Vis DRS of pristine BiVO4, Fe2O3/BiVO4 and Co3O4/BiVO4. d PL spectra of BiVO4 and Fe2O3/BiVO4 excited at λ = 400 nm at room temperature. e The oxygen evolution rate of pure BiVO4, 0.008 Fe2O3/BiVO4, 0.02 Fe2O3/BiVO4, and 0.04 Fe2O3/BiVO4 under visible light irradiation (λ > 420 nm)

It is known that noble metals can act as co-catalysts on the surfaces of photocatalysts, which can greatly enhance the surface catalytic activity of the materials. Recent studies showed that some non-precious metal such as MoS2, graphene, and cobalt phosphate could serve as co-catalysts like noble metal as well [47, 48, 49]. In this study, the loading of small amounts of Fe2O3 also enhanced surface catalytic activity of BiVO4. To further support the role of a co-catalyst of Fe2O3 in our system, we compared the photocatalytic activity of Fe2O3/BiVO4 and Co3O4/BiVO4 in degrading phenol. Co3O4/BiVO4 was prepared following reported procedures [50], where Co3O4/BiVO4 possessed excellent photocatalytic activity for degrading phenol. As shown in Fig. 8, the photocatalytic activity of the present Fe2O3/BiVO4 greatly surpassed that of Co3O4/BiVO4. The photocatalytic reaction rate constant of the former (1.9 × 10−2 min−1) is six times larger than that of the latter (3 × 10−3 min−1). Compared with Fe2O3/BiVO4, Co3O4/BiVO4 shows much larger and wider absorption in both UV and visible light region, as shown in Fig. 7c. PL spectrum indicates that the emission intensity of BiVO4 was greatly reduced after the loading of Co3O4 (Fig. S11), which showed that photogenerated carriers were efficiently separated in Co3O4/BiVO4 system as well. In addition, Co3O4/BiVO4 and Fe2O3/BiVO4 has similar adsorption ability to phenol (Fig. S12). Considering all above facts, it can be understood that higher surface catalytic activity of Fe2O3/BiVO4 lead to its higher catalytic activity than Co3O4/BiVO4. Specifically, Fe2O3 played a key role as co-catalyst for enhanced photocatalytic activity of Fe2O3/BiVO4 due to its low content, efficient separation of photogenerated carriers and good surface catalytic activity. It should be highlighted here that the photocatalytic activity of the as-obtained Fe2O3/BiVO4 is among one of the best photocatalysts reported to date, and is superior to noble metal Pt loaded the same porous BiVO4 nanoplates (Only 40 % of phenol was degraded under the same conditions, Fig. S13).
Fig. 8

a Phenol concentration changes with irradiation time over Co3O4/BiVO4 and Fe2O3/BiVO4. b Plots of ln(C0/C) versus t of Co3O4/BiVO4 and Fe2O3/BiVO4

4 Conclusions

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.

Acknowledgments

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).

Supplementary material

40820_2015_33_MOESM1_ESM.doc (11.7 mb)
Supplementary material 1. Supplementary data associated with this article can be found in the online version at http://www.springer.com/40820(DOC 11948 kb)

Copyright information

© The Author(s) 2015

Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

Authors and Affiliations

  1. 1.Nanomaterials and Chemistry Key LaboratoryWenzhou UniversityWenzhouPeople’s Republic of China

Personalised recommendations