Article pubs.acs.org/JPCC
Facile Synthesis of Surface-Modified Nanosized α‑Fe2O3 as Efficient Visible Photocatalysts and Mechanism Insight Wanting Sun, Qingqiang Meng, Liqiang Jing,* Dening Liu, and Yue Cao Key Laboratory of Functional Inorganic Materials Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China S Supporting Information *
ABSTRACT: In this study, α-Fe2O3 nanoparticles with high visible photocatalytic activity for degrading liquid-phase phenol and gas-phase acetaldehyde have been controllably synthesized by a simple one-pot water-organic two-phase separated hydrolysis-solvothermal (HST) method. Further, the visible photocatalytic activity is enhanced greatly after modification with a proper amount of phosphate. The enhanced activity is attributed to the increased charge separation by promoting photogenerated electrons captured by the adsorbed O2 by means of the atmosphere-controlled surface photovoltage spectra, along with the photoelectrochemical I−V curves. On the basis of the O2 temperature-programmed desorption measurements, it is suggested for the first time that the promotion effect results from the increase in the amount of O2 adsorbed on the surfaces of Fe2O3 by the partial substitution of −Fe−OH with −Fe−O−P−OH surface ends. Expectedly, the positive strategy would be also applicable to other visible-response nanosized oxides as efficient photocatalysts. This work will provide us with a feasible route to synthesize oxide-based nanomaterials with good photocatalytic performance. amounts of the incident visible solar spectrum,19,20 has attracted tremendous interest in its potential application for photocatalytic degradation of organic contaminations in both water and air,21−23 and for photoelectrochemical water splitting to produce H2 as a popular solar fuel.24−26 However, its performance is not ideal for practical application. To improve the performance of α-Fe2O3, a great deal of effort has been made up to day. For example, α-Fe2O3/SnO2 incorporating semiconductors,27,28 Fe3O4@Fe2O3 core/shell nanoparticles,29 hybrid Fe2O3−Pd nanoparticles,20 and Fe2O3 doped with metallic and nonmetallic elements,30,31 have been carried out to enhance the photocatalytic activity for degrading organic pollutants and water splitting with certain successes. Generally speaking, because photocatalytic reactions typically occur at the surfaces of oxide photocalalysts,32 surface modification would influence the photocatalytic performance by altering the electron- or hole-induced reaction paths. Inorganic nonmetal redox-inert anions, often used as surface modifiers, have been reported to improve the photocatalytic activity of TiO2 under ultraviolet illumination.33−36 Our group have recently demonstrated that the activity of TiO2 for degrading pollutants and water splitting is enhanced obviously after phosphate modification.37,38 The phosphate anions are abundant in nature with several advantages,39,40 such as strong
1. INTRODUCTION Hematite (α-Fe2O3), a kind of thermodynamically stable iron oxide phase under ambient conditions with virtues of low cost, good corrosion resistance, and excellent environmental compatibility, has become the focus of intensive research for widespread potential applications in many fields including catalysis, pigments, gas sensors, field emission, and lithium ion battery electrodes.1−8 As stimulated by the aforementioned promising applications, much attention has been paid to the controlled synthesis of α-Fe2O3 by various methods, including vapor−solid growth technique, high-energy ball milling, chemical precipitation, sol−gel process, hydrothermal approach and so forth.9−14 These techniques, however, usually exhibit marked shortcomings, such as weak crystallinity, poor monodispersity, needed complicated synthesis, and posttreatment procedures. The shortcomings would greatly influence the widely practical applications of Fe2O3. Therefore, it is desirable to develop a facile approach to controllably synthesize α-Fe2O3 with ideal performance. Semiconductor photocatalysis has attracted much attention in recent years owing to its applications to environmental purification and to produce solar fuel as sustainable energy resource, and TiO2 is taken as one of the ideal photocatalysts due to its virtues.15−18 However, one critical drawback of TiO2 is that its band gap is so large that it is active only under UV light as a small portion of the solar spectrum. Presently, αFe2O3, as an n-type semiconductor with an indirect band gap of 2.0−2.2 eV that allows for the absorption of substantial © 2013 American Chemical Society
Received: September 27, 2012 Revised: December 24, 2012 Published: January 2, 2013 1358
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crystallization process would happen in the organic phase, according to the boiling point of n-butanol, which is higher than that of water. Thus, the resulting Fe2O3 is collected in the nbutanol after the autoclave was allowed to cool naturally to room temperature, and subsequently washed with distilled water and absolute ethanol in turn, and dried at 80 °C in air. The samples obtained are represented by a-N-b, in which N means NH3·H2O, a is the reaction temperature, and b is the molar ratio of NH3·H2O to Fe3+. To compare, a conventional precipitation-hydrothermal route (PHR) was used to synthesize Fe2O3 as follows. A certain amount of NH3·H2O (the molar ratio of NH3·H2O to Fe3+ was 3) was dropped into 0.2 mol/L Fe(NO3)3 aqueous solution gradually and then was kept at 140 °C for 6 h in a Teflon-lined stainless-steel vessel to carry out hydrothermal reactions. After cooling naturally to room temperature, washing with distilled water and absolute ethanol in turn, and drying at 80 °C in air, Fe2O3 was produced. For the Fe2O3, it is denoted as Fe2O3−PHR. The resulting Fe2O3 was further modified with different amount of phosphate as follows. A 0.2 g sample of 140-N-3 powders was impregnated in a 50 mL of planned-concentration orthophosphoric acid solution under violent stirring for 2 h. Subsequently, the resulting suspension was centrifuged, and then washed with water for several times and dried at 80 °C, along with the thermal treatment at 400 °C for 1 h. The phosphate-modified sample is defined as XP-F, in which P means phosphoric acid, F is used to represent Fe2O3 and X indicates the concentration of phosphoric acid solution used. To carry out photoelectrochemical (PEC) measurements, unmodified and phosphate-modified Fe2O3 film electrodes were prepared. First, the nanocrystalline Fe2O3 paste was prepared as follows. A 0.5 g sample of α-Fe2O3 powders was dispersed in 2 mL of isopropyl alcohol and then treated by an ultrasonic process for 30 min and stirred for 30 min. After that, 0.25 g of Macrogol-6000 was added to the diluted powders, and then the mixture had an ultrasonic treatment and was stirred for 30 min. Finally, 0.1 mL of acetylacetone was introduced to the mixture above, which still had an ultrasonic treatment and was stirred for 1 day. Conductive fluorine-doped tin oxide (FTO)coating glasses, used as the substrates for the Fe2O3 films, were cleaned by ultrasonic processing in acetone for 0.5 h and then in deionized water for another 0.5 h prior to use. The Fe2O3 film was prepared by the doctor blade method using Scotch tape as the spacer. After being dried in air for 0.5 h, the film was sintered at 450 °C for 0.5 h. Subsequently, the Fe2O3 film was immersed into a planned-concentration orthophosphoric acid solution for 1 h. After that, the film was rinsed with deionized water, dried naturally in air for 0.5 h, and then sintered at 450 °C for 0.5 h. The film on FTO glass was cut into 1.7 × 3.0 cm2 pieces with an exposed Fe2O3 surface area of 1.7 × 1.5 cm2. To make a photoelectrode, an electrical contact was made with the FTO substrate by using silver conducting paste connected to a copper wire which was then enclosed in a glass tube. The unmodified and phosphate-modified Fe2O3 films are designated as FF and YP-FF, respectively, in which FF means Fe2O3 film and Y indicates the concentration of ortho-phosphoric acid solution used. 2.2. Characterization of the Samples. The crystal structure of the samples were characterized by X-ray powder diffraction (XRD) with a Rigaku D/MAX-rA powder diffractometer (Japan), using Cu Kα radiation (λ = 0.154 18 nm), and an accelerating voltage of 30 kV and emission current of 20 mA were employed. Transmission electron microscopy
bonding ability, high negative-charge, easy formation of hydrogen bond, and chemical redox-inertness toward photogenerated electrons and holes. Thus, modified phosphate groups would significantly influence the surface chemistry of nanosized oxides. Although surfactant phosphate anion as an efficient banding ligand has been found to determine the shape of Fe2O3 nanoparticles,41,42 to the best of our knowledge, few studies are involved with the phosphate modification to improve the visible photocatalytic activity of Fe2O3. In addition, it is widely accepted that the step that the photogenerated electrons are captured by the adsorbed O2 is very crucial for efficient photocatalytic reactions by preventing the buildup of negative charges,37 which is very meaningful for us to understand the mechanisms on the enhanced photocatalytic activity and to design high-activity oxide-based visible photocatalytic nanomaterials by surface nanoengineering strategies. Surprisingly, this step is often neglected. This greatly spurs us to carry out this work. Herein, we first have successfully developed a one-pot waterorganic two-phase separated hydrolysis-solvothermal (HST) approach to controllably prepare α-Fe2O3 nanoparticles with high photocatalytic activity under visible illumination. Then, the visible activity of the resulting α-Fe2O3 is further enhanced by modification with a proper amount of phosphate. On the basis of the atmosphere-controlled surface photovoltage spectra, time-resolved surface photovoltage spectra, O2 temperatureprogrammed desorption measurement and photoelectrochemical I−V curves, it is clearly demonstrated for the first time that the surface modification with an appropriate amount of phosphate improves the adsorption of O2 so as to promote the photogenerated electrons captured, leading to the increase in the charge separation and then in the visible photocatalytic activity. Moreover, the enhanced adsorption of O2 is closely related to the substitution of −Fe−OH with −Fe−O−P−OH surface ends. This work will provide us with a feasible route to synthesize oxide-based nanomaterials with good photocatalytic performance.
2. EXPERIMENTAL SECTION All of the reagents were of analytical grade and used as received without further purification, and deionized water was employed throughout. 2.1. Synthesis of Materials. Similar to the HST method by which nanocrystalline anatase TiO2 with high photocatalytic activity under UV illumination has been synthesized using Ti(OBu)4 and toluene as Ti resource and organic solvent in our group, respectively.43 A modified HST method is developed to controllably synthesize nanosized α-Fe2O3 by choosing Fe(NO3)3·9H2O as Fe resource and n-butanol as the organic phase. The key of the controlled synthetic method is to select n-butanol with a little higher boiling point than water as organic phase to dissolve Fe(NO3)3 and also as hydrothermal solvent, and to introduce volatile ammonia into the water system to modulate the hydrolysis of Fe ions in the organic phase. In a typical experiment, 10 mL of water phase containing a planned amount of ammonia and 8 mL of n-butanol phase containing 0.8 g of Fe(NO3)3·9H2O were respectively placed in a 30 mL Teflon lined stainless-steel vessel, in which a 10 mL of weighing bottle is installed to contain the organic n-butanol. Then, the sealed device is kept at a certain temperature (120− 160 °C) for 6 h. Under the solvothermal conditions, the hydrolysis and nucleation process would take place at the interfaces between the water and n-butanol, and the subsequent 1359
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a planned concentration of acetaldehyde gas. The reactor was placed horizontally and irradiated from the top side by using a 150 W xenon lamp with a cutoff filter (λ > 420 nm). In a typical photocatalytic process, 0.15 g of photocatalyst was used, and a premixed gas system, which contained 810 ppm acetaldehyde, 20% of O2, and 80% of N2, was introduced into the reactor. To reach adsorption saturation, the mixed gas continuously moved through the reactor for 0.5 h prior to the irradiation. The determination of acetaldehyde concentrations at different time intervals in the photocatalysis was performed with a gas chromatograph (GC-2014, Shimadzu) equipped with a flame ionization detector. 2.4. Photoelectrochemical (PEC) Experiments. PEC experiments were performed in a glass cell using a 150 W xenon lamp with a cutoff filter (λ > 420 nm) and a stabilized current power supply as the illumination source, and 0.5 mol/L NaClO4 solution as the electrolyte. The working electrode was the prepared Fe2O3 film (1.7 × 1.5 cm2), vertically illuminated from the FTO glass side. Platinum wire (99.9%) was used as the counter electrode, and an Ag/AgCl (saturated KCl) electrode was used as the reference electrode to which all the potentials in the paper were referred at 25 °C. For the measurement in the presence and absence of O2, oxygen and oxygen-free nitrogen gas were bubbled through the electrolyte before and during the experiments, respectively. Applied potentials were controlled by a commercial computercontrolled potentiostat (LK2006A made in China). For comparison, the I−V curves were also measured in the dark.
(TEM) observation was carried out on a JEOL JEM-2010EX instrument operated at 200 kV accelerating voltage. The UV− vis diffuse reflectance spectra of the samples were recorded with a Model Shimadzu UV-2550 spectrometer. The specific surface areas of the samples were tested by BET instrument (Micromeritics automatic surface area analyzer Gemini 2360, Shimadzu), with nitrogen adsorption at 77 K. The Fourier transform infrared spectra (FT-IR) of the samples were collected with a Bruker Equinox 55 spectrometer, using KBr as diluents. X-ray photoelectron spectroscopy (XPS) was equipped with a Kratos-AXIS ULTRA DLD apparatus with Al (Mono) X-ray source to gain further insight into the surface composition and elemental chemical state of the samples, and the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). The surface photovoltage spectroscopy (SPS) measurements of samples were conducted with a home-built apparatus that had been described in detail elsewhere,44−46 the powder sample was sandwiched between two ITO glass electrodes by which the outer electric field could be employed, and the sandwiched electrodes could be arranged in an atmosphere-controlled container with a quartz window. A study on O2 temperature-programmed desorption (TPD) is available to probe the interaction of O2 with oxide surfaces, which is carried out in a flow apparatus built by ourselves. In a typical O2-TPD experiment, the sample (about 50 mg) was placed in a quartz tube (i.d. 6 mm) with a small amount of quartz wool plugging at two sides and pretreated at 275 °C for 30 min in an ultrahigh-purity He flow of 20 mL/min. After the sample was cooled to ambient temperature, ultrahigh-purity O2 was continuously passed over the sample for 90 min. Subsequently, the sample was flushed with the He flow for removal of residual O2 in the quartz tube and a part of O2 adsorbed physically on the sample. Finally, the O2-TPD profile of the sample was recorded by increasing the temperature from 30 °C to the desired temperature at a heating rate of 10 °C/ min under 20 mL/min of He flow, using a gas chromatograph (GC-2014, Shimadzu) with a TCD detector to monitor the desorbed O2 amount. 2.3. Evaluation of Visible Photocatalytic Activities. Phenol is a typical recalcitrant contaminant without sensitizing as a dye, and acetaldehyde, as a kind of volatile toxic organic compounds widely existing in industrial production, is harmful to our health and environment. Thus, phenol and acetaldehyde are taken as liquid-phase and gas-phase pollutant representatives to evaluate the photocatalytic activity of the synthesized Fe2O3-based samples under visible light irradiation, respectively. The liquid-phase photocatalytic experiments were carried out in a 100 mL of open photochemical glass reactor equipped with an optical system provided from a side of the reactor by using a 150 W GYZ220 high-pressure Xenon lamp made in China with a cutoff filter (λ > 420 nm), which was placed at about 10 cm from the reactor. During the evaluation of photocatalytic degradation of phenol, 0.15 g of photocatalyst and 60 mL of 10 mg/L phenol solution were mixed by a magnetic stirrer for 2 h under visible light irradiation. After photocatalytic reactions, the phenol concentrations were analyzed by the colorimetric method of 4-aminoantipyrine at the characteristic optical adsorption of 510 nm with a Model Shimadzu UV2550 spectrophotometer after centrifugation. Photocatalytic degradation of gas-phase acetaldehyde was conducted in 640 mL of cylindrical quartz reactor for 3 mouths for introducing a planned amount of photocatalyst powders and
3. RESULTS AND DISCUSSION 3.1. Characterization and Photocatalytic Activity of the Resulting Fe2O3. The X-ray diffraction (XRD) is used to analyze the crystal structure and crystallization degree of samples. Figure 1 displays XRD patterns of Fe2O3 samples synthesized by the HST method with different experimental conditions. All diffraction peaks of the samples can be indexed as the hexagonal-phase α-Fe2O3 (hematite) according to the standard card JCPDS 33-0664. It can be seen that, as the ammonia concentration and hydrothermal temperature increase, the intensities of XRD peaks gradually become strong, indicating that the sample’s crystallization and corresponding crystallite size increase gradually according to the Scherrer formula.47 However, as the reaction temperature rises to 160 °C and the ammonia concentration is greater than 3 (the molar ratio of NH3·H2O to Fe3+), the diffraction peak intensities of the samples do not nearly change a little. Thus, when the reaction temperature is lower than 160 °C and the ammonia concentration is lower than 3, there are suitable conditions to controllably synthesize nanosized α-Fe2O3. In addition, compared with the α-Fe2O3 prepared by the PHR method, as shown in the Supporting Information (Figure S1), the 140-N-3 sample exhibits wide and weak diffraction peaks, indicating that it has small crystallite size. On the basis of the TEM images shown in Figure S2, Supporting Information, one can see that the Fe2O3−PHR with an irregular sphere form has an average diameter of about 80 nm, whereas the 140-N-3 with a similar sphere form does an average size of about 15 nm. This is responsible for the about 4-time larger BET surface area (83.98 m2·g−1) of the 140-N-3 than that of the Fe2O3−PHR (18.42 m2·g−1). Expectedly, as the nanoparticle size decreases, the blue shift in the DRS absorption spectra (Figure S3, Supporting Information) can be seen. From the above analysis, it is demonstrated that α-Fe2O3 nanoparticles with small size and 1360
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Figure 1. XRD patterns of different Fe2O3 samples. (A) (a) 120-N-0; (b) 120-N-1; (c) 120-N-3; (d) 120-N-5; (e) 120-N-7. (B) (a) 140-N0; (b) 140-N-1; (c) 140-N-3; (d) 140-N-5; (e) 140-N-7. (C) (a) 160N-0; (b) 160-N-1; (c) 160-N-3; (d) 160-N-5; (e) 160-N-7.
Figure 2. TEM images of unmodified and phosphate-modified Fe2O3 samples: (A) F; (B) 0.15P-F.
large surface area could be controllably synthesized by the HST method, much superior to the traditional PHR one. In particular, it is seen from Figure S4 (Supporting Information) that the photocatalytic activity of the Fe2O3− PHR for degrading liquid-phase phenol and gas-phase acetaldehyde is very low under visible light illumination, whereas the Fe2O3 obtained by the HST process exhibits much high activity, which is attributed to its small nanoparticle size and high surface area. Moreover, among the as-prepared Fe2O3 samples, the 140-N-3 one displays the highest activity, which depends on the comprehensive results of nanoparticle size, surface area, and crystallinity. Thus, we choose the 140-N-3 for further modification with phosphate in the next work. 3.2. Structural Characterization and Surface Composition of Modified Fe2O3. The XRD patterns of unmodified and phosphate-modified Fe2O3 are shown in Figure S5, Supporting Information. It is confirmed that all the samples are pure hematite, demonstrating that the phosphate modification has nearly no effect on the crystalline-phase composition, crystallite size, and crystallinity of Fe2O3. And also, one can see from the representative TEM photographs of different Fe2O3 samples shown in Figure 2 that the spherical morphology and nanoparticle size (about 20 nm) do not change after phosphate modification. Compared with the unmodified Fe2O3, the modified one exhibits a good dispersion,
which is attributed to the roles of the phosphate modification effectively inhibiting the agglomerations and contacts among Fe2O3 nanoparticles.35,36 Expectedly, the phosphate modification does not influence the optical absorption of α-Fe2O3 on the basis of the DRS spectra (Figure S6, Supporting Information). As seen from the FT-IR spectra of different Fe2O3 samples shown in Figure S7 (Supporting Information), the strong peak ranging from 460 to 570 cm−1 corresponds to the Fe−O stretching vibration mode in crystal α-Fe2O3.42,48 The IR peaks at about 1630 and 3400 cm−1 are generally assigned to hydroxyl groups and adsorbed water molecules, respectively.49,50 A new IR band at 940−1108 cm−1 emerges in the modified Fe2O3 and its intensity is proportional to the used phosphate amount, which is attributed to the characteristic absorption peaks of phosphate groups,51−53 or to the characteristic frequencies of antisymmetric/symmetric stretching of −P−O and −P−OH groups,54,55 suggesting that the formation of P−OH groups on the surfaces of modified Fe2O3. Compared with the unmodified Fe2O3, the characteristic stretching vibration band of modified α-Fe2O3 displays a slight shift to high wavenumber and the stretching vibration absorption peak of Fe−O−P centering at 1048 cm−1 appears,56 indicating that the phosphate groups are modified on the surfaces of Fe2O3 via chemical bonding. This is 1361
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further supported by the XPS spectra shown in Figure S8, Supporting Information. As seen from the Fe2p XPS, one can see that there are two main peaks at about 711 and 725 eV, corresponding to Fe2p3/2 and Fe2p1/2, respectively. Noticeably, the shakeup satellite structures as the fingerprints of electronic structure of Fe3+ are also observed at the higher binding energy sides of the main peaks.57,58 This is in agreement with the XRD result that the resulting samples are pure α-Fe2O3 phase. Compared with the unmodified Fe2O3, the phosphate-modified one exhibits a slight high binding energy for Fe2p, demonstrating that the phosphate modification would have certain effects on the surface properties of Fe2O3. For the modified Fe2O3, it is seen that the binding energy of P2p is centered at about 133 eV, which is characteristic for P element in the phosphate.59 This is in good agreement with the IR results. In addition, the isoelectric point (IEP) of Fe2O3 is changed from pH 8.5 to about pH 6.5 after phosphate modification shown in Figure S9 (Supporting Information), implying that the −P−OH groups exist on the modified Fe2O3 surfaces.60 3.3. Photogenerated Charge Properties of Modified Fe2O3. The photocatalytic activity of material is closely related to the behavior of photogenerated charges.61 And, also, the surface photovoltaic spectroscopy (SPS), with its very high sensitivity, is a well-suited and direct method to explore the properties of photogenerated charges of solid semiconducting materials.62 In light of the SPS principle,44,63 the surface photovoltage signal of semiconductor materials mainly originates from the creation of electron−hole pairs, followed by the separation under the built-in electrical filed in the space charge region and/or at the aid of the diffusion process. Nevertheless, for nanoparticles, band bending in bulk semiconductors would not occur due to the limited size. In this case, the SPS response should mainly derive from the photogenerated charge separation via the diffusion process because the built-in electric fields are neglected.29 Figure 3 shows the SPS responses of unmodified and modified Fe2O3 in different O2-concentration atmosphere. For F and 0.15P-F, if there is no oxygen (in pure N2 atmosphere), the photoelectrons and photoholes would easily recombine, leading to no SPS response. And, the SPS response gradually becomes strong as O2 content increases, indicating that the presence of O2 is an essential condition for the SPS occurrence of α-Fe2O3 because of its ability to capture photogenerated electrons. Thus, it is deduced that the positive photogenerated holes can preferentially diffuse to the surfaces of testing electrode in the presence of O2, leading to an obvious SPS response. This is further confirmed by the increased SPS responses at the aid of outer positive field in air shown in Figure S10, Supporting Information. Although the phosphate modification would not change the SPS attributes, it could greatly affect the SPS intensity as shown in Figure 4. One can see that the SPS response gradually becomes strong as the used phosphate amount increases, indicating that the separation of photogenerated charges of Fe2O3 is enhanced in the presence of O2 after phosphate modification. In fact, this is also preliminarily supported by the time-resolved photovoltage spectroscopy, as shown in Figure S11 (Supporting Information), indicating that phosphate modification could be beneficial to promote the separation of photogenerated carries. However, if the phosphate amount is too large, the SPS intensity begins to go down, even lower than that of unmodified Fe2O3, such as 0.5P-F. This is possibly
Figure 3. SPS responses of unmodified (A) and phosphate-modified Fe2O3 samples (B) in N2 (a), air (b), and O2 (c).
Figure 4. SPS responses of unmodified (a) and phosphate-modified Fe2O3 (b)−(f) in air. Concentration (mol/L) of phosphoric acid solution used: (b) 0.05, (c) 0.1, (d) 0.15, (e) 0.3, and (f) 0.5.
because that the excess phosphate would be unfavorable to transport photogenerated charges so as to influence the charge separation, which is also supported by the following PEC results. Therefore, it is clearly demonstrated that the modification with an appropriate amount of phosphate would obviously enhance the SPS response of nanosized Fe2O3 in the presence of O2, leading to the marked increase in the photogenerated charge separation. 3.4. Visible Photocatalytic Activities of Modified Fe2O3. The photocatalytic activities for degrading liquidphase phenol solution and gas-phase acetaldehyde have been evaluated, as shown in Figure 5. As seen here, the photocatalytic activity of as-prepared unmodified Fe2O3 is very low, whereas the Fe2O3 modified with an appropriate amount of phosphate exhibits remarkably high activity. Among the phosphate-modified Fe2O3 samples, 0.15P-F with surface 1362
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Figure 6. Curves of O2 temperature-programmed desorption on unmodified and phosphate-modified Fe2O3 samples.
it begins to come down when the temperature goes up to about 390 °C, until no desorbed O2 at 500 °C. Compared with the unmodified Fe2O3, the phosphate-modified one shows much slow desorption of O2, especially at the high temperature (over 350 °C) corresponding to the chemically adsorbed form.65 And also, for the same temperature, the amount of desorbed O2 of the phosphate-modified Fe2O3 is much larger than that of F, demonstrating that the phosphate modification remarkably promotes the adsorption of O2 on the surfaces of Fe2O3. Naturally, the increase in the amount of adsorbed O2, especially for the chemically adsorbed form, would be beneficial for capturing the photogenerated electrons of Fe2O3, leading to the increased SPS response. Thus, it is expected that the phosphate modification would be favorable for the photoelectrochemical reduction of O2. The photoelectrochemical properties of unmodified and phosphate-modified Fe2O3 photoanodes were studied by measuring I−V plots in the absence or presence of O2 systems, as shown Figure S12, Supporting Information. It can be seen that the I−V curve of the unmodified FF under visible illumination nearly overlaps with the one in the dark, indicating that the photogenerated electron−hole recombination of Fe2O3 easily take place in the absence of O2 in itself, which is in agreement with the literature.19,20,31 This is greatly different from TiO2.38 Similar to the FF, the phosphate-modified FF exhibits nearly the same current under irradiation as that in the dark, implying that the phosphate modification would not influence the photogenerated charge separation of Fe2O3 in the absence of O2 in the neutral (pH7) system, which is also unlike the phosphate-modified TiO2. For TiO2, the photocurrent density could be enhanced markedly after phosphate modification, which is attributed to the formed strong negative electric field at the surfaces of TiO2.35,38,66 It is different from TiO2 that the surfaces of Fe2O3 should be charged by a small amount of negative charges after phosphate modification, because its IEP is changed from pH 8.5 to about pH 6.5, which is in agreement with the literatures.67,68 Thus, it is speculated that the phosphate modification would display weak effects on the photogenerated charge separation of Fe2O3. Although the photocurrent of unmodified or phosphate-modified FF is the same as its current in the dark in the presence of O2 (as shown in Figures S13 and S14, Supporting Information), the modification with a proper amount of phosphate could enhance the current of FF as shown in Figure 7. However, if the amount of used phosphate is excess, the current of modified FF begins to go down, even lower than the unmodified FF, which is
Figure 5. Photocatalytic degradation rates of liquid-phase phenol (A) and gas-phase acetaldehyde (B) on different Fe2O3 samples.
atomic number ratio of P to Fe is 0.06 based on the XPS result displays the highest activity. Noticeably, the high photocatalytic activity of the as-prepared Fe2O3 corresponds to its strong SPS response. Widely accepted, the separation and recombination of photoinduced charge carriers are in competitive processes, and the photocatalytic reaction is effective only when photoinduced electrons and holes are separated.61,64 And, also, the step that the photogenerated electrons are captured by the adsorbed O2 is crucial for charge separation and further for photocatalytic reactions. In this case, it is reasonable that the photocatalytic activity is consistent with the SPS response. On the basis of the above SPS results, it is confirmed that the modification with a proper amount of phosphate greatly improve the charge separation of the as-prepared Fe2O3, which is very responsible for the enhanced activity. Because the phosphate modification would not influence the crystal structure, nanoparticle size and optical adsorption of Fe2O3, it is assumed that the phosphate groups should play important roles in the production of SPS response and then in the photocatalytic reactions in the presence of O2. In addition, it is worth noting that the modification with excess phosphate is unfavorable for the SPS response and the photocatalytic reactions. This is possibly attributed to the point that the excess phosphate modified on the surfaces would suppress the charge transportation or transfer.37 3.5. Mechanism Insight. Although the phosphate modification does not change the SPS attribute of Fe2O3, it could greatly influence its SPS intensity in the presence of O2. Thus, it is assumed that the phosphate modification is favorable for the adsorption of O2. To prove this assumption, the curves of O2 temperature-programmed desorption (TPD) of unmodified and phosphate-modified Fe2O3 were recorded, as shown in Figure 6. For the F, as the desorption temperature rises, the amount of desorbed O2 gradually increases; however, 1363
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phosphate modification is attributed to the effective separation of photogenerated charge carriers resulting from the increase in the amount of adsorbed O2. And also, it is suggested for the first time that the change of surface ends by the substitution of −Fe−OH with −Fe−O−P−OH groups after phosphate modification would greatly promote the adsorption of O2, which is responsible for the enhanced charge separation and then visible photocatalytic activity, and the increase in the surface acidity would be favorable to promote the adsorption of O2. Naturally, it is speculated that the surface acidity-increased modification method would be applicable to improve the photocatalytic activity of other oxide-based semiconductor photocatalysts greatly, such as WO3, BiVO4, LaFeO3, and so on. This work would facilitate our deep understanding about the mechanism of enhanced photocatalytic activity on surface modification and might provide feasible strategies to further design and synthesize oxide-based semiconductors with excellent photocatalytic performance.
Figure 7. I−V curves of different Fe2O3 electrodes in the dark. Potentials are measured against a Ag: AgCl (saturated KCl solution) reference electrode in 0.5 mol/L NaClO4 solution under O2.
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because that the excess phosphate groups are unfavorable for charge transportation or transfer processes. This further proves the previous expectation. Obviously, the increase in the amount of chemically adsorbed O2 after phosphate modification, which would contribute to capturing the photogenerated electrons, should result from the surface state with surface-bound phosphate groups. According to the above discussion, there are a certain amount of phosphate groups (−Fe−O−P−OH) on the surfaces of phosphate-modified Fe2O3 besides plentiful hydroxyl groups (−Fe−OH) in comparison with naked Fe2O3. Thus, it is expected that the enhanced amount of O2 adsorption should be attributed to the partial substitution of −Fe−OH with −Fe− O−P−OH groups, especially for the attribute change of H in the −OH group. That is to say that the −Fe−O−P−OH groups, which can act as acid sites due to the acidic character of phosphate groups, are possibly favorable for O2 adsorption compared with −Fe−OH groups. For this, a detailed comparative experiment is designed to carry out, as shown in Figure S15, Supporting Information. It can be seen that the SPS response of the phosphatemodified Fe 2 O 3 becomes weak after subsequent KNO 3 treatment, however, still stronger than that of the unmodified one. This is in good agreement with the amount of adsorbed O2 on the basis of the O2-TPD curves. As expected, the visible photocatalytic activity of the phosphate-modified Fe2O3 for degrading acetaldehyde is decreased after the KNO3 treatment, whereas still higher than that of the unmodified one. Because the substitution of the H in the −Fe−O−P−OH group with K based on the XPS results, it is concluded that the H change in the surfaces of Fe2O3 would greatly influence the adsorption of O2, and then charge separation and photocatalytic activity.
ASSOCIATED CONTENT
S Supporting Information *
XRD patterns, TEM images, DRS spectra and photocatalytic degradation data of the Fe2O3−PHR and 140-N-3 samples; XRD patterns, DRS spectra, FT-IR spectra, XPS spectra, ζ potential, SPS responses, time-resolved SPS response and I−V curves of unmodified Fe2O3 and phosphate-modified Fe2O3 samples; XRD patterns, XPS spectra, SPS responses, O2-TPD curves and photocatalytic degradation data of H2O−F, H2O− 0.15P−F and KNO3−0.15P−F samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Nature Science Foundation of China (21071048), the Program for Innovative Research Team in University, the Chang Jiang Scholar Candidates Programme for Provincial Universities in Heilongjiang (2012CJHB003), the Science Foundation of Harbin City of China (No. 2011RFXXG001), and the Program for Innovative Research Team in Heilongjiang University (Hdtd2010-02), for which we are very grateful.
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4. CONCLUSIONS In this paper, we report a facile one-pot water−organic twophase separated hydrolysis-solvothermal method for synthesis of α-Fe2O3 nanoparticles with high photocatalytic activity for degrading liquid-phase phenol and gas-phase acetaldehyde under visible illumination for the first time. Moreover, the photocatalytic activity of the resulting α-Fe2O3 nanoparticles is greatly improved by modification with a proper amount of phosphate. Mainly based on the atmosphere-controlled SPS responses, time-resolved photovoltage spectra and the O2-TPD curves, along with the photoelectrochemical reduction of O2, it is confirmed that the enhanced activity of α-Fe2O3 after 1364
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