Surface Modification of Bi2O3 with Fe(III) Clusters toward Efficient

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Surface Modification of BiO with Fe(III) Clusters Toward Efficient Photocatalysis in a Broader Visible Light Region: Implications of the Interfacial Charge Transfer Wenbin Sun, Huiyu Zhang, and Jun Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503845g • Publication Date (Web): 14 Jul 2014 Downloaded from http://pubs.acs.org on July 15, 2014

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The Journal of Physical Chemistry

Surface Modification of Bi2O3 with Fe(III) Clusters Toward Efficient Photocatalysis in a Broader Visible Light Region: Implications of the Interfacial Charge Transfer

Wenbin Sun, Huiyu Zhang, and Jun Lin* Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China ABSTRACT: In this work, the surface modification of Bi2O3 with Fe(III) ions has been achieved through an impregnation technique. The Fe(III)-modified Bi2O3 exhibits a much higher photocatalytic activity for the degradation of orange II than bare Bi2O3 in a broader visible light region. Based on various characterization results and ESR measurements, it was revealed that in the visible light beyond the bandgap energy of Bi2O3, the surface Fe(III) modification induces a direct interfacial charge transfer (IFCT) from the valence band of Bi2O3 to the surface Fe(III) clusters, where Fe(III) can serve as an oxygen reduction site. This IFCT process has been clearly demonstrated to play a critical role in the higher photocatalytic performance of the Fe(III)-modified Bi2O3 by offering a new channel for the efficient generation of the hole in the valence band of Bi2O3 substrate. The photocatalytic mechanisms of the Fe(III)/Bi2O3 in different visible light regions were also discussed in details.

Keywords: Photocatalysis, Bi2O3, Fe(III) modification, Interfacial charge transfer.

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1. INTRODUCTION In recent years, photocatalysis with a primary focus on a number of narrow band gap semiconductors as photocatalysts has gained increasing attention because of its potential application in environmental pollution remediation and energy conversion using solar energy.1-6 Among various narrow band gap semiconductors, bismuth oxide (Bi2O3) in α form is an attractive material with some considerable merits such as thermal stability, non-toxicity as TiO2, chemical inertness in neutral water, dielectric permittivity, and wide applications in gas sensors, optical coatings, and photocatalysis.3,7-10 As compared to TiO2, moreover, Bi2O3 owns a deeper valence band (VB) maximum (~+3.13 V vs NHE), which infers a higher oxidation power of the holes in its VB. In the view of these special properties, Bi2O3 can be regarded as a promising candidate for an efficient visible light photocatalyst. Bi2O3 alone, however, shows a low photocatalytic performance since its conduction band (CB) electron (~ +0.33 V vs NHE) cannot be efficiently consumed by oxygen molecule through one-electron reduction process (Eo(O2/O2-•) =－0.33 V vs NHE). To overcome the drawback, several efforts have been made to improve the photocatalytic activity of Bi2O3 recently, including the microstructure control,10 surface modification with noble metal and fluorine,11-13 and combination of Bi2O3 with other semiconductors such as ZnO and NiO.14,15 Nevertheless, all these Bi2O3-based materials exhibit efficient photocatalytic performances in a limited visible region only due to the limitation in the band gap energy of Bi2O3 substrate. For the full utilization of solar light, it is desirable and meaningful to develop Bi2O3 with a deep valence band as an efficient photocatalyst in a broader visible light region. According to the literature that suggested the presence of the photo-induced interface charge transfer (IFCT) from semiconductor to an adsorbed molecular species,16,17 Hashimoto group successfully designed Cu(II) or Fe(III) clusters-grafted TiO2 and SrTiO3 as visible light photocatalysts recently.18-24 It was revealed, in these systems, the electrons in the valence band of TiO2 can be directly transferred to the surface-grafted clusters by photoinduced interfacial charge transfer (IFCT) process, leaving the holes in the valence band to drive the complete oxidation of organic substances. These systems not only achieve the efficient consumption of the photo-generated electron via the reduction of the grafted metallic ions, taking advantage of the high oxidation power of the hole in the valence band for the complete oxidation of organic substances, but also extend the photocatalyst’s response from UV to visible light region. Inspired 2


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by this strategic approach, we attempt to develop Bi2O3 to be a long wavelength visible light responsive photocatalyst by the surface Fe(III) modification. In the current work, it was clearly demonstrated that the surface Fe(III) modification can enable Bi2O3 to become an efficient photocatalyst even in the visible light with the wavelengths longer than that corresponding to the band gap of Bi2O3. Furthermore, the photoinduced interfacial charge transfer (IFCT) occurring in the photocatalysis of Fe(III)-modified Bi2O3 was well clarified in details according to the various characterization results and electron spin resonance (ESR) measurements.

2. EXPERIMENTAL SECTION 2.1. Preparation of Bi2O3 Samples. All reagents used in the study were of analytic grade without any further purification. Bi2O3 samples were prepared using a facile precipitation method, as reported in our previous works.11,13 In a typical procedure, the amount of 10.78g of Bi(NO3)3·H2O was dissolved into a 30 ml aqueous solution of nitric acid (1.5 M), followed by the slow addition of NaOH (50% V/V) under vigorously stirring. A yellow suspension formed immediately when the pH value rose to 13. Afterward, the resulting suspension was heated with stirring at 80oC for 2 hrs. The precipitate was collected by centrifugation and washed with deionized water and ethanol several times before dried at 120oC for 5hrs. Finally, the obtained precipitate was further calcined at 450oC for 5 hrs to form crystalline Bi2O3 powder. 2.2. Modification of Bi2O3 Samples with Fe(III) Ions. The fabrication of Fe(III) ions on the surface of Bi2O3 samples was conducted by an impregnation technique.19 Briefly, 1.0 g of the Bi2O3 sample was dispersed in the 15 ml aqueous solution with the desired amount of FeCl3·6H2O as the Fe(III) precursor. The different amounts of FeCl3·6H2O were used to give the weight fractions of Fe to Bi2O3 in the dispersions at 0.05%, 0.1%, and 0.3%, respectively. The dispersion was heated at 90oC for 1 hr in a sealed glass reactor and then filtered and washed with a sufficient amount of deionized water. The resulting residue was dried at 110oC for 24 hrs and subsequently ground into a fine powder. The obtained Bi2O3 samples modified with Fe(III) ions were denoted as 0.05%, 0.1%, and 0.3% Fe(III)/Bi2O3, respectively, based on the amounts of FeCl3·6H2O used in the impregnation process. For the comparison of the following physiochemical and catalytic properties, bare Bi2O3 unmodified with Fe(III) ions was also treated in the same manner in the absence of FeCl3·6H2O. 3



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2.3. Characterization. The crystal phases of these samples were identified at room temperature with an X-ray diffractometer (XRD) (Shimadzu, XRD-7000) using Cu Kα as X-ray radiation under 40 kV and 30 mA. The UV-vis diffuse reflectance spectra of the samples were recorded on a spectrometer (Hitachi U-4100) with BaSO4 as reflectance standard. The surface chemical states of the compositional elements in the samples were studied by X-ray photoelectron spectroscopy (XPS) (Kratos AXIS Ultra) using 300 W Al Kα radiation. All binding energies were referenced to the C1s peak (284.6 eV) of the surface adventitious carbon. The morphologies were characterized by field-emission scanning electron microscopy (FESEM) (JEOL JSM-7401F) and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010), respectively. 2.4. Evaluation of Photocatalytic Activity. The degradation of Orange II was carried out to evaluate the photocatalytic activities of the samples under the visible light irradiation. Typically, 100 mg of the catalyst sample was suspended in 100 mL of Orange II aqueous solution (~ 5.6×10-5 M). The light source was a 300 W Xe arc lamp (CHF-XM150, Beijing Trusttech) equipped with a wavelength cutoff filter (λ > 420nm, λ > 450nm, or λ > 470nm). Prior to the irradiation, the suspension was magnetically stirred in dark for an hour to ensure the establishment of an adsorption-desorption equilibrium between the catalyst surface and Orange II. Afterward, the suspension was exposed to the visible light under magnetic stirring and bubbled with O2 gas. At the given time interval during the photoreaction, 3 mL of the suspension was sampled, followed by centrifugation to remove the catalyst particles. The evolution of the Orange II concentration during the photoreaction was monitored by measuring the absorbance of the filtrate at λ = 483 nm on a UV-vis spectrophotometer (Hitachi U-3310). Moreover, four 3W monochromatic lights (λ = 420 nm) instead of the 300 W Xe arc lamp were also used as light source for the degradation of orange II over the samples. 2.5. ESR Measurement. To clearly verify the interface charge transfer (IFCT) process involved in the Fe(III)/Bi2O3 photocatalysis, the spin trap method was employed by using diamagnetic reagent 5,5’-dimethyl-1-pirroline-N-oxide (DMPO) to produce the stable paramagnetic spin-adduct with •

CH2OH radical formed in the visible light irradiated methanol suspension of the catalysts in the

absence of soluble oxygen. The measurements on the DMPO-•CH2OH spin adducts were performed at room temperature by an ESR spectrometer (JEOL JES-FA200) under irradiation with a 500 Xe lamp, in front of which, three kinds of glass cutoff filters, λ > 420nm, λ > 450nm, and 4


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λ > 470nm, were used, respectively. The ESR spectrometer was operated at the conditions of centerfield = 322.83 mT, microwave power = 4.00 mW, microwave frequency = 9053 MHz, and sweep width = 5 mT. Furthermore, ESR measurements were also conducted at liquid nitrogen temperature for the solid powder of Fe(III)/Bi2O3 sample irradiated by the visible light. The settings for the latter were centerfield = 250.00 mT, microwave power = 4.00 mW, microwave frequency = 9068.59, and sweep width = 2.5×100 mT, respectively.

3. RESULTS AND DISCUSSION 3.1. Phase Structure and Morphology. Figure 1 displays the XRD patterns of bare Bi2O3 and Bi2O3 modified with the different Fe(III) contents. All samples maintain a single characteristic monoclinic phase of well-crystallized α-Bi2O3 (JCPDS No. 41-1449), indicating the modification with Fe(III) ions doesn’t affect the phase structure of Bi2O3. No any characteristic XRD peaks associated with iron oxide and hydroxide are observed in the modified samples. According to Bragg’s law, the diffraction peak would obviously shift toward a higher 2θ value if the foreign Fe(III) ion with a smaller radius (0.064nm) substitutes for the host Bi3+ ion with a larger radius (0.103nm) in Bi2O3 lattice. Upon the careful comparison of the (120) diffraction peaks of all samples in the range of 2θ = 26.5-28.0o, as shown in the insert of Figure 1, it is clear that there is no change in the (120) peak position after the Fe(III) modification. The results suggest that the Fe(III) ions exist on the surface of the Bi2O3. Since the amount of Fe(III) used in the modification is too low to be detected by XRD, its presence and present form will be further confirmed by HRTEM observation, XPS and ESR analysis.

Figure 1. X-ray diffraction patterns of bare Bi2O3 and Bi2O3 modified with different Fe(III) 5



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contents. The insert is the diffraction peak positions of crystal plane (120) in the range of 2θ = 26.5~28.0o. The morphologies and microstructures of the samples were studied by FESEM and TEM analysis, respectively. The FESEM observations (see Figure S1 in the Supporting Information) show that both bare Bi2O3 and 0.1% Fe/Bi2O3 consist of highly dispersed microrods with the diameters ranging from 500-800 nm and several micrometers in length, which is similar to the α-Bi2O3 in our previous reports.11,13 No apparent changes in FESEM morphology are observed after the modification. Figure 2 shows the HRTEM image of 0.1% Fe(III)/Bi2O3. The clear lattice fringes of the substrate demonstrate that the substrate Bi2O3 is highly crystallized, which is in a good agreement with the XRD result above. The measured lattice fringes of 0.26 nm and 0.33 nm well match the (022) and (120) crystallographic planes of α-Bi2O3, respectively.11,25,26 In particular, it can be well confirmed in the HRTEM image that the amorphous Fe(III) nanoclusters with the size of about 2~4 nm are dispersed on the surface of substrate α-Bi2O3. A good attachment of the Fe(III) nanoclusters to the Bi2O3 surface is also observed in the image.

Figure 2. HRTEM image of 0.1% Fe(III)/Bi2O3 sample

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Figure 3. (a) Bi4f core-level spectra of Bi2O3 before and after surface Fe(III) modification. (b) Fe2p core-level spectra of 0.1% and 0.3% Fe(III)/Bi2O3. 3.2. XPS Analysis and Optical Property. XPS analysis was carried out to study the chemical states of the surface elements. Figure 3a shows the high-resolution XPS spectra of Bi4f regions of the Bi2O3 before and after the surface Fe(III) modification. In the Bi4f core-level spectra of the bare Bi2O3, two peaks appear at the binding energies of 158.8 eV and 164.1 eV, ascribed to Bi4f7/2 and Bi4f5/2, respectively. Both the peak position and separation (∆ = 5.3 eV) are well consistent with a characteristic of Bi3+ in α-Bi2O3 according to the previous reports.27 Notably, the binding energies of Bi4f7/2 and Bi4f5/2 obviously shift to higher values after the Fe(III) modification in the cases of 0.1%, and 0.3% Fe(III)/Bi2O3. This observed shift to a higher binding energy indicates a strong interfacial interaction between Bi2O3 substrate and Fe(III) clusters, confirming the HRTEM observation above. The Fe2p core-level spectra clearly demonstrate the existence of Fe species in the Fe(III)-modified Bi2O3 samples. As shown in Figure 3b, the Fe species exist in both oxidation states of +3 and +2, and the ferric state is the major oxidation state. The binding energy of the 7



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Fe2p3/2 XPS peak at about 711.2 eV for Fe(III) in both samples lies between the two values that correspond to Fe(III) in crystalline Fe2O3 and FeOOH, respectively.28 Taking account into the modification process used and the related literatures,19,24 it can be reasonably considered that the Fe(III) ions are present as distorted FeOOH-like amorphous nanoclusters on the surface of the Bi2O3. The observed Fe (II) species might originate in the XPS measurement, in which the Fe(III) could be reduced to Fe(II) under high vacuum (10-10-10-9 Torr) XPS chamber. The presence of the Fe(II) species in the Fe(III)-modified samples also supports the formation of distorted FeOOH-like amorphous clusters since the reduction of Fe(III) to Fe(II) scarcely occurs in the case of Fe(III) present in the form of either crystalline Fe2O3 or FeOOH.19 A similar phenomenon was also reported for Fe(III)/TiO2 and Cu(II)/TiO2 systems.18,19,29

Figure 4. UV-vis diffuse reflectance spectra of Bi2O3 before and after surface Fe(III) modification.

The optical properties of the Bi2O3 before and after the surface Fe(III) modification were measured by UV-vis diffuse reflectance spectroscopy at room temperature (Figure 4). As shown in Figure 4, bare Bi2O3 presents an intense absorption with the steep edge at the wavelength near 450 nm, corresponding to its characteristic band gap value of approximately 2.8 eV. The surface Fe(III) modification significantly extends the absorption tail to above 600 nm. Based on the previous reports,19,24 the additional absorption can be partially attributed to the interface charge transfer (IFCT) from the valence band of Bi2O3 to the surface Fe(III) nanoclusters. The intensity of the additional absorption band increases with the increase of Fe(III) contents. It was reported that the crystalline iron (hydr)oxides in various forms have a strong absorption starting at the wavelength more than 600 nm due to their shorter band gaps.30,31 Thus, the presence of the FeOOH-like 8


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amorphous nanoclusters, as confirmed above, should also contribute to the observed additional absorption. 3.3. Photocatalytic Activity and Mechanism. To investigate the effect of the surface Fe(III) modification on photocatalytic performance, the photocatalytic activities for the degradation of Orange II over bare Bi2O3 and Fe(III)-modified Bi2O3 were evaluated under the visible light irradiation at λ > 420 nm, as shown in Figure 5a. The control experiment shows the self-degradation of Orange II is negligible under the same irradiation in the presence of no catalysts. All Fe(III)-modified samples exhibit higher photocatalytic activities for the degradation of orange II than bare Bi2O3, indicating the surface Fe(III) modification is an efficient approach for enhancing the photocatalysis of Bi2O3. The highest photocatalytic activity is observed for the 0.1% Fe(III)/Bi2O3, over which approximately 98% degradation of orange II is achieved under the visible light irradiation at λ > 420 nm for 3 h.

Figure 5. (a) Photocatalytic degradation of Orange II over bare Bi2O3 and Fe(III)-modified Bi2O3 under visible light irradiation (λ > 420 nm). The insert is the chemical structure of Orange II. (b).

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Photocatalytic degradation of orange II over bare Bi2O3 and 0.1% Fe(III)/Bi2O3 under visible light irradiation (λ > 450 nm and λ > 470nm). The various characterization results above indicate that Fe(III) ions exist in the form of distorted FeOOH-like amorphous nonaclusters closely dispersed on the surface of Bi2O3, and that no changes in the phase structure and morphology of the Bi2O3 substrate are observed after the surface Fe(III) modification. Since the potential energy of Fe(III) ions (Eo (Fe3+/Fe2+) = 0.771 V vs NHE) locates below the that of the conduction band bottom of Bi2O3, a direct interfacial charge transfer (IFCT) from the valence band to the surface Fe(III) to form Fe(II) would be induced by the irradiation of the visible light beyond the bandgap energy of Bi2O3 based on the earlier reports.19,24 the strong interaction between the Bi2O3 substrate and surface Fe(III) clusters, as evidenced by XPS analysis and HRTEM observation, is favorable for the photoinduced IFCT process effectively. Herein, we speculate the IFCT process would be responsible for the higher photocatalytic activity over Fe(III)/Bi2O3 irradiated by the visible light at λ > 420 nm. To clarify the IFCT process in the photocatalysis of Fe(III)/Bi2O3, the degradation of Orange II over bare Bi2O3 and 0.1% Fe(III)/Bi2O3 was also carried out under the visible light irradiation at λ > 450 nm and λ > 470 nm, respectively. As shown in Figure 5b, bare Bi2O3 exhibits very poor photocatalytic activities. This is attributed to its very low or no absorption in the two visible light regions, as observed in Figure 4. Amazingly, the surface Fe(III) modification still allows the Bi2O3 to efficiently degrade orange II in the two light regions, approximately 60% and 40% degradation of orange II are achieved over 0.1% Fe(III)/Bi2O3 after carrying out the reaction for 3 h, respectively. In the two visible light regions that the substrate Bi2O3 almost has no absorption, IFCT process is only possible way for the electron transfer occurring in the 0.1% Fe(III)/Bi2O3. In the IFCT process, the photogenerated electrons are directly transferred to the surface Fe(III) ions to form Fe(II) ions, leaving the holes in the valence band of the Bi2O3 for the degradation of Orange II.

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e CB

O2/H2O2 0.68V Fe3+/Fe2+ 0.771V

e

Visible light

T

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IFC

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Fe(III) cluster Orange II

VB

h h

Degraded products

Fe(III)/Bi2O3 Scheme 1. Schematic Illustration of the Photocatalysis over Fe(III)-Modified Bi2O3 According to the above photoreaction results, therefore, we are allowed to propose a reasonable mechanism for the photocatalysis over Fe(III)-modified Bi2O3 as follows. In the case of the irradiation at λ > 420 nm, the holes over Fe(III)/Bi2O3 can be generated through both the direct electron transfer to the surface Fe(III) clusters by IFCT and the direct electron excitation into the conduction band. The electrons transferred to Fe(III) clusters by IFCT would reduce Fe(III) to form Fe(II), while the formed Fe(II) easily becomes back to Fe(III) through the multi-electron reduction of oxygen at ambient conditions.32 In other words, the surface Fe(III) can serve as an oxygen reduction site here. In contrast, the fate of the electrons excited to the conduction band is similar to that over bare Bi2O3, probably forming H2O2 through a two-electron reduction process in addition to recombining with the holes in the valence band. As a result, the IFCT process offers an additional channel for the efficient generation of the hole over the Fe(III)/Bi2O3 catalyst, leading to the higher photocatalytic activity compared to bare Bi2O3 in Figure 5a. In the case of the irradiation at λ > 450 nm or λ > 470 nm, the IFCT becomes sole channel for the efficient generation of the hole over the Fe(III)/Bi2O3 because of almost no bandgap excitation. It was observed in Figure 5b that the holes generated by IFCT only can also drive an efficient degradation of Orange II. Furthermore, as shown in Figure S2 (Supporting Information), approximately 41% and 48% degradation of Orange II are observed over bare Bi2O3 and 0.1% Fe(III)/Bi2O3, respectively, when four 3W monochromatic lights (λ = 420 nm) are used as light source. This result indicates that under only bandgap excitation at λ = 420 nm, the surface Fe(III) clusters also have contribution to the photocatalytic degradation of Orange II, and suggests that the a part of electrons excited to the conduction band transfer to the surface Fe(III) clusters, resulting in an increase in the charge separation to some extent. Therefore, in addition to the IFCT, the electrons from the conduction band to the surface Fe(III) clusters should also contribute to the 11



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higher photocatalytic activity over the 0.1% Fe(III)/Bi2O3 irradiated at λ > 420 nm (Figure 5a). The above understanding of the photocatalysis over the Fe(III)/Bi2O3 is schematically illustrated in Scheme 1. These photoreaction experiments above clearly demonstrate that the surface Fe(III) modification can effectively enhance the photocatalytic activity of Bi2O3 even in the visible light beyond the bandgap energy of Bi2O3 substrate. It is noteworthy, furthermore, that the enhancement in photocatalytic activity gets weakened over 0.3% Fe(III)/Bi2O3, as shown in Figure 5. This is probably due to the presence of excess Fe(III) clusters, which shields the substrate Bi2O3 from light and diminishes active sites for Orange II absorption.

Figure 6. ESR spectra of Fe(III) ions in 0.1% Fe(III)/Bi2O3 measured at liquid nitrogen temperature in vacuum. 3.4. ESR Evidence for Mechanism. ESR measurements were carried out to clarify the interfacial charge transfer (IFCT) process involved in the photocatalysis of Fe(III)-modified Bi2O3. As shown in Figure 6, a signal at low magnetic field can be clearly searched in the ESR spectra of 0.1% Fe(III)/Bi2O3 before light irradiation. A very similar signal with the same parameter of g-tensor was also reported for Fe(III) species in Fe(III)/TiO2 system and Fe-exchanged zeolites, respectively.33,34 Based on the related reports and the characteristic of the parameter, the observed signal at g = 4.29 can be identified to be isolated Fe(III) ions with a strongly distorted rhombic environment,33,35,36 which well supports the XPS analysis and HRTEM observation. As proposed above, the direct IFCT from the valence band of the Bi2O3 to the surface Fe(III) to form Fe(II) occurs in the Fe(III)/Bi2O3 upon the visible light irradiation. Then, we examined the changes in 12


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the intensity of the signal of Fe(III) located at g = 4.29 in ESR spectra as the effect of visible light irradiation. The signal intensity of Fe(III) appears to be slightly decreased by the light irradiation at λ > 470 nm, as seen in Figure 6. The observed decrease infers a reduction in the Fe(III) species after the light irradiation, probably ascribed to the reduction of Fe(III) to Fe(II) by IFCT. Since the signal of Fe(III) itself is quite low due to its small amount, its presence can be used to identify the type of Fe(III) species in our sample, but its slight change in intensity after the light irradiation is not enough to support the occurrence of IFCT process.

Figure 7. ESR signals of DMPO-•CH2OH spin adducts in the methanol suspension of (a) bare Bi2O3, (b) 0.1% Fe(III)/Bi2O3 before and after irradiation at λ > 420 nm, (c) 0.1% Fe(III)/Bi2O3 before and after irradiation at λ > 450 nm, and (d) 0.1% Fe(III)/Bi2O3 before and after irradiation 13



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at λ > 470 nm. To provide more convincing evidences for IFCT in the Fe(III)-modified Bi2O3, the DMPO-•CH2OH spin adducts formed in the visible light irradiated methanol suspensions of bare Bi2O3 and 0.1% Fe(III)/Bi2O3 in the absence of oxygen were detected by ESR spectroscopy. In the case of the ESR detection, the photocatalytic oxidation of the methanol by the hole photoexcited in the valence band of the Bi2O3 can lead to the production of hydroxymethyl radical (•CH2OH) through the reaction 1:37-39 CH3OH + hVB+ → •CH2OH + H+

(1)

•CH2OH + DMPO → DMPO-•CH2OH

(2)

According to the earlier reports, the conversion of methanol to •CH2OH radicals proceeds rapidly with a rate constant at k = 9.7 × 108 M-1 s-1.40 Trapping of •CH2OH radicals by DMPO (reaction 2) is also a relatively efficient process with a rate constant at k = 2.2 × 107 M-1 s-1.41 Here, we attempted to detect the DMPO-•CH2OH spin adducts to evaluate the generation of the photoexcited holes in the catalysts. Figure 7 illustrates the ESR spectra of DMPO-•CH2OH spin adducts formed in bare Bi2O3 and 0.1% Fe(III)/Bi2O3 before and after the irradiation in different visible light regions. In the case of the irradiation at λ > 420 nm (Figure 7a), no any ESR signals are observed over bare Bi2O3 even after the irradiation for 18 min. This is due to the fast recombination of the photoexcited electron-hole pair in the absence of oxygen scavenger. In contrast, in Figure 7b, the characteristic ESR sextet peaks of DMPO-•CH2OH spin adducts clearly appear over the 0.1% Fe(III)/Bi2O3 after the irradiation at λ > 420 nm for 6 min, and the intensities of the sextet peaks increase with the irradiation time. This indicates that the photoexcited hole can be effectively generated over 0.1% Fe(III)/Bi2O3 in the absence of oxygen, attributed to the photoinduced IFCT process to great extent, and also to that the electron excited to the conduction band transfers to the surface Fe(III). The photoinduced IFCT is further revealed by the generation of the ESR sextet peaks of DMPO-•CH2OH spin adducts over the 0.1% Fe(III)/Bi2O3 irradiated by the visible light beyond the bandgap energy of the Bi2O3. As shown in Figure 7c and d, the irradiation at λ > 450 nm or λ > 470 nm also produces clearly visible sextet peaks of DMPO-•CH2OH spin adducts. In the two visible regions in which no bandgap excitation occurs, the generation of the photoexcited holes over the 0.1% Fe(III)/Bi2O3 originates from the IFCT

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process only. Moreover, within the same irradiation time, the intensity of the ESR peaks of DMPO-•CH2OH spin adducts is stronger under the irradiation at λ > 420 nm than under the irradiation at λ > 450 nm (Figure 7b and c). This is consistent with the photocatalytic performances of bare Bi2O3 and 0.1% Fe(III)/Bi2O3 irradiated by the visible light at λ = 420 nm (Figure S2), and indicates that the a part of the electrons excited to the conduction band also can transfer to the surface Fe(III) clusters, resulting in the generation of the holes in the valence band. The above ESR measurements shown in Figure 7 well parallel the photocatalytic activities of bare Bi2O3 and 0.1% Fe(III)/Bi2O3, and nicely support the mechanism illustrated in Scheme 1. In summary, the Fe(III)-modified Bi2O3 has been successfully synthesized through a precipitation method, followed by an impregnation technique. The Fe(III)-modified Bi2O3 exhibits a higher photocatalytic activity than bare Bi2O3 for the degradation of Orange II in a broader visible light regions. It was well demonstrated that a direct interfacial charge transfer from the valence band of Bi2O3 to the surface Fe(III) clusters occurs in the Fe(III)-modified Bi2O3 irradiated by the visible light beyond the bandgap energy of the Bi2O3 substrate. This IFCT offers a new channel for the efficient generation of the holes in the valence band, which is mainly responsible for the higher photocatalytic activity.

ASSOCIATED CONTENT Supporting Information FESEM of bare Bi2O3 and 0.1% Fe(III)/Bi2O3, photocatalytic degradation of orange II over bare Bi2O3 and 0.1% Fe(III)/Bi2O3 under the irradiation of four 3W monochromatic lights (λ = 420 nm), and complete author list for reference 5. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel: +8610-62514133. Fax: +8610-62516444. E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to acknowledge the financial support from the National Natural Science 15



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Table of Content (TOC) Image

e e

λ>420nm IF C T

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O2/H2O2 Fe3+/Fe2+ Red

h h

Ox Fe(III)/Bi2O3 At λ>450 or >470nm IFCT only

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Surface Modification of Bi2O3 with Fe(III) Clusters toward Efficient

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