Journal of Colloid and Interface Science 452 (2015) 24–32
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile preparation of ferric giniite hollow microspheres and their enhanced Fenton-like catalytic performance under visible-light irradiation Xuhong Zhang a,b, Yu Zhang a, Lin Gao a, Haitao Yu a,⇑, Yu Wei a,⇑ a b
College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
g r a p h i c a l a b s t r a c t
The me-dependent photo-degradaon of MB
a r t i c l e
i n f o
Article history: Received 18 December 2014 Accepted 25 March 2015 Available online 13 April 2015 Keywords: Ferric giniite Carbonaceous materials Fenton-like Photocatalytic activity
a b s t r a c t Ferric giniite hollow spheres with diameters of about 1.2–1.4 lm were successfully fabricated with a onepot hydrothermal process. All chemicals used were low-cost compounds and environmentally benign. The obtained products were characterized by field emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), nitrogen adsorption– desorption isotherms, Fourier-transform IR spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). It was found that as-prepared ferric giniite hollow spheres exhibited much enhanced photocatalytic activity (99.5%) for the degradation of methylene blue in the presence of visible light irradiation and H2O2. Experimental results indicate that the existence of the carbonaceous shells enhances the photocatalytic activity of ferric giniite hollow spheres via the synergistic effect between carbon and ferric giniite, such as improving the adsorption, absorbing more light and exhibiting high activity to produce hydroxyl radicals through catalytic decomposition of H2O2. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (Y. Wei). http://dx.doi.org/10.1016/j.jcis.2015.03.043 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
Fenton reactions have proven to be effective methods to treat organic pollutants in wastewater [1–3]. However, the application of traditional Fenton reaction is limited by the narrow working
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pH (2.5–3.5) [4], the formation of a significant amount of ferric hydroxide sludge in the course of Fenton treatment, and the requirements of further separation and disposal for the resulting sludge [5,6]. To overcome these drawbacks, heterogeneous Fenton-like catalysts, such as iron oxides [7–13], iron-immobilized zeolites [14], clay-based materials [15–19] and carbon materials [20,21] have been developed. These heterogeneous catalysts can be effectively used at neutral pH and can be easily separated by physical sedimentation after reaction completion [22]. Recently, several researchers have reported that iron hydroxyl phosphate can be used as active heterogeneous Fenton catalysts to degrade organic pollutants [23–25]. For example, Li et al. developed Fe5(PO4)4(OH)32H2O nanoparticles as photo-Fenton catalysts, which exhibited high photocatalytic activity for the degradation of methylene blue in the presence of visible light irradiation and H2O2 [23]. Duan et al. prepared ferric giniite microcrystals with controlled sizes and shapes by a facile one-pot approach employing ionic-liquid precursors [24]. The measurements indicated that the photocatalytic activity of as-prepared Fenton-like catalysts were highly dependent on the exposed facets. Zhang et al. reported that the single-crystal dendritic iron hydroxyl phosphate could be used as an effective heterogeneous Fenton-like catalyst for the degradation of phenol [25]. However, it was noticed that the adsorption capacities of these iron hydroxyl phosphate heterogeneous catalysts were relative low. Because the contaminant molecules have to be adsorbed on the surface of photocatalyst particles before being decomposed, the adsorption ability of the photo-catalyst is an important factor governing the photocatalytic activity. Therefore, it is important to modify the iron hydroxyl phosphate with other materials for enhancing its adsorption capability and photo-catalytic activity. In recent years, amorphous carbon, which can be obtained via inexpensive and environmentally benign hydrothermal processes using glucose as a precursor, have received considerable attention because of their wide applications as adsorbents, catalysts, and others [26–32]. The surface of the amorphous carbon is hydrophilic and has have many functional groups on their surfaces, such as carboxylic, aldehyde, and hydroxyl groups. Furthermore, the carbonaceous shells are highly porous, which could not only increase the surface area, but also facilitate the penetration of reactive species. Many inorganic materials modified with carbonaceous materials have been reported to represent enhanced photocatalytic properties. For example, Luo et al. fabricated magnetite/carboxylate-rich carbon spheres via hydrothermal carbonization (HTC) process [26]. Without using H2O2, magnetite/carboxylate-rich carbon spheres exhibit excellent photodegradation activity at neutral pH under visible light irradiation. Zhang et al. reported the synthesis of ZnO hybridized with graphite-like carbon by coating the glucose-derived HTC on the surface of ZnO nanoparticles [27]. It was found that the carbon coating functioned to impede the photocorrosion of ZnO and enhance the photostability of ZnO catalysts in UV-irradiation. Zhong et al. fabricated carbon-deposited TiO2 (TiO2@C) with a one-pot hydrothermal process by using glucose as a carbon source [33]. It was found that the photocatalytic activity of TiO2@C was greatly enhanced compared to noncarbon-TiO2 under visible irradiation. Wang et al. fabricated a carbon coated SnO2 photocatalyst by using sucrose as a carbon source and the microwave hydrothermal method [34]. The prepared carbon coated SnO2 exhibits high and stable photocatalytic activity for the degradation of Rhodamine B (RhB) under UV–Vis irradiation. Herein, based on the latest progress in hydrothermal preparation of carbon-coated inorganic materials, we fabricated ferric giniite hollow spheres modified with carbonaceous materials by a onepot hydrothermal method using glucose as carbon source at low temperature. When mixed with a small amount of H2O2, the asprepared ferric giniite hollow spheres exhibited high visible-light
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photocatalytic activity toward methylene blue (MB) in aqueous solution. Compared with the traditional photo-Fenton-like process that requires the assistance of UV irradiation, the as-prepared ferric giniite hollow spheres exhibited much enhanced photocatalytic activity for the degradation of methylene blue in the presence of visible-light irradiation and H2O2. 2. Experimental section 2.1. Preparation of the samples Sample 1 (S1) was prepared by one-step hydrothermal method. All reagents are analytical grade and used without any further purification. In a typical synthesis, 5 mmol FeCl36H2O, 0.6 mmol glucose and 0.6 mmol NaH2PO46H2O were dissolved in 75 ml distilled water. The solution was transferred to a 100 ml Teflon-lined autoclave. The autoclave was then sealed and maintained at 180 °C for 8 h. After cooling down naturally, the dark green precipitate was washed with distilled water three times before being dried at 60 °C for 24 h. Sample 2 (S2) was synthesized according to the method described in previous work [23]. In a typical synthesis procedure, Fe(NO3)36H2O (5 mmol) was completely dissolved in deionized water (14 ml). Under vigorous agitation, an aqueous solution (14 ml) containing Na3PO46H2O (5 mmol) was added into the above solution at room temperature. The pH value of the mixture was adjusted to 4 with 10 M NaOH and HNO3 (68%). Then, the mixture was transferred into a Teflon-lined steel autoclave of 40 ml, and the autoclave was heated under autogenous pressure at 180 °C for 24 h. Then, the autoclave was cooled to room temperature gradually. The yellowish green precipitate was washed with distilled water three times. Then, it was dried at 60 °C in air. 2.2. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8-ADVANCE X-ray diffractometer using Cu Ka radiation at 40 kV and 40 mA. The compositions of the powders were characterized at room temperature by Fourier-transform IR spectroscopy on an FTIR-8900 instrument in the range 400– 4000 cm1. Infrared spectra of the products were recorded by pelletizing a few milligrams of the sample with KBr. The morphology and the composition of the products were examined by fieldemission scanning electron microscopy (FESEM, HITACHI S-4800) equipped with an energy dispersive spectrometry (EDS). Selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) images of products were performed with a JEOL-2010 high resolution transmission electron microscopy (HRTEM, JEOL Ltd., Japan). Surface areas were determined by the BET method using N2 adsorption–desorption isotherms. N2 adsorption–desorption isotherms were measured on a gas sorption analyzer (Quantachrome, NOVA 4000e) at a liquid nitrogen temperature. XPS studies were performed on a Physical Electronics/PHI 5300 X-ray photoelectron spectrometer with the Al Ka X-ray source (hv 1486.6 eV). The position of the C1s peak was taken as a standard (with a banding energy of 284.6 eV). 2.3. Photo-Fenton and photocatalytic experiments Photocatalytic activities of the samples were evaluated by the photocatalytic decolorization of MB under visible light. In a typical process, 30 mg Fe5(PO4)4(OH)32H2O nanoparticles were dispersed into 100 ml aqueous solution of MB (10 mg l1) solution (photoFenton-like reaction was initiated by adding 0.5 ml H2O2 to the reactor). Before illumination, the suspension was sonicated for
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1 min and magnetically stirred in dark for 0.5 h to achieve the adsorption–desorption equilibrium among the photocatalyst, MB and water. The solution was then exposed to a 150 W tungstenhalogen lamp with a 420 nm cut off filter to provide visible-light irradiation. The reaction temperature was kept at room temperature by cooling water to prevent any thermal catalytic effect. The samples were collected by centrifugation every 30 min to measure methylene blue (MB) degradation by UV–Vis spectroscopy (Yoke UV752). To test the stability and recyclability of samples, the catalyst was sedimentated from an aqueous suspension for about 1 h, after the added MB was almost degraded. The supernatant was centrifuged to collect the residual catalyst in it. After the two parts of the catalyst were gathered together, it was washed with a mixture of deionized water and ethanol several times and then dried at 60 °C for 12 h. Then, the final products were suspended into a fresh solution of MB and H2O2, and the MB degradation was continued as the second cycle. This process was repeated several times.
3. Results and discussions Fig. 1a shows the XRD patterns of the samples prepared in typical synthesis process at 180 °C for 8 h. The diffraction peaks of all the samples could be easily indexed as a pure, orthorhombic crystalline phase Fe5(PO4)4(OH)32H2O, which is in good agreement with the standard card (JCPDS Card number: 45-1436). Fig. 1b presents the SEM image of the as-prepared sample. It can be seen that the product is composed of microspheres with diameters ranging from 1.2 to 1.4 lm. From the related SEM image in higher magnification (Fig. 1c), it can be clearly found that a lot of pores, with a diameter ranging from 20 to 60 nm, are evenly distributed on the surface of the microspheres. One broken Fe5(PO4)4(OH)32H2O microsphere clearly shows the hollow nature of the Fe5(PO4)4(OH)32H2O microspheres (Fig. 1d). Moreover, it can be seen that the microspheres are formed via the self-assembly of
the irregular nanocrystallines and there are interstitial spaces available among these nanocrystallines, which provide connected channels for mass exchange between the inner space of Fe5(PO4)4(OH)32H2O microspheres and the outer solution. These crystal strips are aligned perpendicularly to the spherical surface, pointing toward a common center. TEM was used to elucidate the interior structure of the microspheres. Fig. 2a shows that the spheres have pale center region in contrast to a dark edge, suggesting the spheres are hollow. The selected area electron diffraction (SAED) pattern (an inset in Fig. 2b) is composed of many separated bright spots instead of rings, which imply the single crystal-like feature of the hollow spheres. The HRTEM image of Fe5(PO4)4(OH)32H2O microstructures is shown in Fig. 2c and the clear lattice image indicates the high crystallinity and single-crystalline nature of the Fe5(PO4)4(OH)32H2O microstructures. In addition, an encapsulating amorphous layer can be easily observed (indicated by the two white arrows) through the HRTEM image, with a thickness of 2.5 nm. The measured lattice spacings of 0.228 and 0.32 nm are consistent with the d values of the (4 2 1) and (3 3 0) planes of Fe5(PO4)4(OH)32H2O, respectively (Fig. 2d). The porosity of the hollow spheres was substantiated by the measurement of nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface area. Fig. 3a shows the N2 adsorption–desorption isotherm and pore size distribution curve (inset in Fig. 3a) of the Fe5(PO4)4(OH)32H2O hollow spheres. The isotherm can be classified as type IV with an apparent hysteresis loop in the range 0.5–1.0 P/P0, indicating the presence of mesopores. The plot of pore size distribution determined by the Barrett–Joyner–Halenda (BJH) method shows that there is broad pore size distribution in the range 5–25 nm. The mesopores on the hollow sphere can be attributed to the interspaces of the constituent particles and the existence of amorphous carbonaceous layer. The BET surface area of the hollow sphere is 10.219 m2 g1 and the pore volume is 0.039 cm3 g1. The high surface area and large pore volume further support that the hollow
Fig. 1. (a) XRD patterns, (b) overall SEM image, (c) detailed view of an individual hollow sphere, (d) magnified image of a broken microsphere of Fe5(PO4)4(OH)32H2O (S1) prepared at 180 °C for 8 h.
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Fig. 2. (a) TEM image of Fe5(PO4)4(OH)32H2O (S1), (b) TEM image of the quarter of an Fe5(PO4)4(OH)32H2O hollow sphere. The corresponding SAED pattern of the product taken from the edge of the hollow sphere (inset), (c) HRTEM of Fe5(PO4)4(OH)32H2O hollow sphere (S1), (d) a magnified image of the square-marked area in (c).
Fig. 3. (a) The nitrogen adsorption–desorption isotherm and pore size distribution curve (inset), (b) the EDS spectrum of Fe5(PO4)4(OH)32H2O samples (S1).
spheres have porous structure. The EDS result shown in Fig. 3b demonstrates that the as-prepared sample contains C (12.30%, weight%), Fe (23.09%), O (53.07%), and P (11.54%), indicating the presence of carbonaceous shells on the Fe5(PO4)4(OH)32H2O microspheres. In order to confirm the surface oxidation state of synthesized iron oxide, the surface oxidation state of catalyst was measured by X-ray photoelectron spectrometer (XPS). Fig. 4a shows XPS spectra of ferric giniite (S1) and ferric giniite (S2). Compared with
S2, the binding energies of iron in S1 shifted to low energy states. XPS analysis was conducted to investigate the oxidation state of Fe on the surface of S1. The Fe 2p spectra (Fig. 4b) are split into two main peaks (corresponding to Fe 2p3/2 and Fe 2p1/2) by spin–orbit coupling, which can be further subdivided into peaks at 709.2 and 711.6 eV, and shake-up satellite peaks at 722.5 and 724.8 eV. The peak located at 711.6 eV corresponds unambiguously to oxygenbonded ferric ion (Fe(III)AO). The peak arising at 709.2 eV can be assigned to Fe(II)AO, which indicate existence of Fe(II) ions on
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Fig. 4. XPS and FT-IR spectra of Fe5(PO4)4(OH)32H2O samples: (a) Fe 2p of S1 and S2, (b) Fe 2p of S1, (c) C1s of S1, (d) FT-IR of S1.
the surface of S1 [35–37]. Fig. 4c displays the typical XPS spectrum of the C1s region. The carbon 1s peak was deconvoluted into three spectral bands at 284.6, 285.6 and 288.7 eV. The most intense peak at 284.6 eV attributed to the Cn, while the relatively small peaks at 285.6, and 288.7 eV represent CAOH, and OAC@O (and OAC@O) bonds, respectively [33,34,38]. The coating effect of glucose-derived, carbon-rich polysaccharide (GCP) on the surface of ferric giniite was further proved by typical FT-IR spectroscopy (Fig. 4d). The broad characteristic band from 3600 to 3100 cm1 could be assigned to OAH stretching vibration arising from hydroxyl groups on nanoparticles and adsorbed glucose and water. The peaks around 2920 and 2850 cm1 could be assigned to asymmetric and symmetric vibrations of CAH in ACH2A. The vibration bands located at 1620 and 1410 cm1 can be assigned to the asymmetric and symmetric stretching vibration of COO groups, confirming that a lot of carboxylate groups from carbon were coordinated strongly to the iron cations. The other bands at 1092, 1036, 960, 602 and 465 cm1 were assigned to the vibration in the PO3 4 group of the Fe5(PO4)4(OH)32H2O. From the FT-IR spectra of the ferric giniite hollow sphere, it can be observed that the carbonization was incomplete and ferric giniite particles were coated by carbonaceous polysaccharide. To investigate the formation mechanism of Fe5(PO4)4(OH)3 2H2O hollow microspheres, a detailed time-dependent evolution experiment was carried out. Fig. 5 shows the SEM images of samples obtained at the reaction times of 1, 2, 4 and 8 h, respectively. From Fig. 5a, it can be clearly seen that spherical particles with diameters of 1.0–1.4 lm were obtained within 1 h. It is worth noting here that the shells of the above Fe5(PO4)4(OH)32H2O spheres were relative smooth. With the reaction time increased to 2 h and
4 h, the sizes of primary particles (especially the particles on the surface) obviously became larger and larger (Fig. 5b and c). When the reaction time was increased to 8 h, the sizes of primary particles of microspheres grew larger continuously and there appeared to be an obvious porous structure, with a pore diameter of 20–60 nm, on the surface of most microspheres (Fig. 5d). Fig. 5a– d clearly illustrates the changes in the porosity of the nanostructures. The sizes of the microspheres remain relatively unchanged, but with prolonged hydrothermal treatment the interior of the microspheres is subjected to a dissolution process, leading to spheres of greater porosity. In addition, XRD measurement was further employed to characterize the intermediates at different reaction time intervals. It can be seen that tetragonal Fe5(PO4)4(OH)32H2O is formed after hydrothermal reaction for 1 h. All the diffraction peaks shown in Fig. 6 can be indexed as orthorhombic crystalline phase Fe5(PO4)4(OH)32H2O, which is consistent with the standard card (JCPDS Card number: 45-1436). No obvious diffraction peak for the carbon is observed, suggesting the carboxylate-rich carbon is amorphous carbon. Glucose plays an important role in the formation of Fe5(PO4)4(OH)32H2O. Several detailed experiments with different amounts of glucose were performed while keeping other parameters constant. As shown in Fig. 7a, when the reaction was carried out in the absence of glucose, the samples was composed of large-scale ellipsoids with diameters of 100–300 nm. The characteristic XRD pattern of synthesized samples was corresponded to the mixed phase of a-Fe2O3 (major) (JCPDS Card number: 33-0664) and Fe5(PO4)4(OH)32H2O (minor) (Fig. 8a). When a small amount of glucose (0.2 mmol) was introduced to the reaction solution, the synthesized samples was also corresponded to the mixed
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Fig. 5. The morphology evolution of the samples prepared at 180 °C for different reaction times: (a) 1 h, (b) 2 h, (c) 4 h, (d) 8 h.
Fig. 6. XRD patterns of samples obtained at 180 °C for different reaction times: (a) 1 h, (b) 2 h, (c) 4 h, (d) 8 h.
phase of Fe5(PO4)4(OH)32H2O and a-Fe2O3 (Fig. 8b). However, the peak intensity of the a-Fe2O3 became weak and the diffraction peaks of Fe5(PO4)4(OH)32H2O became strong. The SEM images (Fig. 7b) show that the product consists of well dispersed quasispheres with an average diameter of about 0.6–0.9 lm. These quasi-spheres tend to be angular in shape, indicating they are consisted of numerous densely packed irregular nanocrystals. When the concentration of glucose exceeded 0.4 mmol, the product was completely transformed to Fe5(PO4)4(OH)32H2O (Fig. 8c). From the SEM observation (Fig. 7c), the product consisted entirely of well-defined porous spheres with diameters in the range of 1.0–1.1 lm. When the glucose amount was further increased to 0.9 mmol, the obtained Fe5(PO4)4(OH)32H2O microspheres appeared to be larger and more porous. Based on the results of the control experiments, it can be seen the synthesized microspheres show insignificant morphology changes and size distribution improvements with the aid of glucose additives. Based on the above experimental results, the formation process can be divided into two steps as follows: At the early reaction
stage, the Fe3+ ions and H2PO were hydrolysed into 4 Fe5(PO4)4(OH)32H2O nanocrystals. The freshly formed nanocrystals are unstable due to their high surface energy and they evolved to Fe5(PO4)4(OH)32H2O aggregates through oriented attachment, driven by the minimization of surface energy. The new formed Fe5(PO4)4(OH)32H2O aggregates were loosely attached due to the glucose-coated surface of the Fe5(PO4)4(OH)32H2O nanocrystals. According to previous reports, at elevated temperatures, glucose can serve as a reductant in solution due to its hydroxyl group [29]. Then, part of Fe3+ in the Fe5(PO4)4(OH)32H2O were reduced to Fe2+ by glucose. As a result, many pores were left in the Fe5(PO4)4(OH)32H2O. It is noted that glucose plays a key role in inducing the formation of Fe5(PO4)4(OH)32H2O. During the reaction process, the glucose can carbonize to transform into glucosederived carbon-rich polysaccharide (GCP), which have many functional groups on their surfaces, such as carboxylic, aldehyde, and hydroxyl groups. These surface functional groups can bind metal cations through coordination or electrostatic interactions [39,40], which is favorable for the formation of Fe5(PO4)4(OH)32H2O nanoparticles. In the second stage, the Fe5(PO4)4(OH)32H2O mesoporous spheres consisted of numerous smaller nanocrystals gradually transform to the Fe5(PO4)4(OH)32H2O hollow microspheres composed of bigger irregular nanocrystallines. The reconstruction of Fe5(PO4)4(OH)32H2O microspheres can be easily realized via a simple ripening process. Compared with the large and wellcrystallized particles on the exteriors of the spheres, the inner small crystallites have higher surface energy, which provides them driving force for the Ostwald ripening. Therefore, the inner small crystallites of a sphere are undergoing mass relocation through dissolving and regrowth, whereas the outer larger ones serve as new starting growth sites. With the continuous mass transportation from the inner core to the outermost surface of the same sphere, the hollow interior occurred. Due to the Ostwald ripening process, large quantity pores and channels in the spheres are produced. Thus, it can be concluded that the formation of Fe5(PO4)4(OH)32H2O hollow microspheres involves cooperation of oriented aggregation and Ostwald ripening process. On the other hand, when the temperature is above 160 °C, glucose can be
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Fig. 7. SEM images of samples obtained at 180 °C for 8 h with the addition of glucose: (a) 0, (b) 0.2, (c) 0.4, (d) 0.9 mmol.
Fig. 8. XRD patterns of samples obtained at 180 °C for 8 h with the addition of glucose: (a) 0, (b) 0.2, (c) 0.4, (d) 0.9 mmol.
carbonized. The uniform amorphous carbonaceous coating is possibly formed by the carbonization of glucose around the Fe5(PO4)4(OH)32H2O nanoparticles involving the intermolecular cross-linking and dehydration of the glucose, oligosaccharides, and/or other macromolecules derived from glucose during the hydrothermal treatment [33,34]. The XRD patterns show that the crystallinity of the products is increased gradually with the reaction time prolonged, which also validates the above proposition that Ostwald ripening is the reasonable mechanism in the present system. Therefore, by simply controlling the reaction time, the microspheres could be effectively tuned from solid to porous structure. The forming process is similar to the previous reports of BiOI [41] and SnO2 [42] hollow or porous structures. The photocatalytic activity of the ferric giniite (S1) nanoparticles was evaluated by the degradation of MB in aqueous solution at room temperature. It can be observed from Fig. 10 that approximately 49.2% organic dye is adsorbed on the catalyst S1 after the dark treatment. The high adsorption ability of ferric giniite (S1)
should be attributed to the existence of the amorphous carbon on the composites surface. The simultaneous presence of ferric giniite (S1) nanoparticles and H2O2 removes 80.5% of MB within 120 min in the dark, which implies that ferric giniite nanoparticles (S1) can actually be regarded as heterogeneous Fenton-like catalyst. When visible-light illumination was introduced to the reaction system, it was found that the degradation efficiency was about 99.5% after 120 min. Additionally, an experiment was also performed in the presence of ferric giniite (S2) (The XRD pattern and SEM image of S2 was displayed in Fig. 9) with H2O2 under visible-light illumination (Fig. 10). It should be noted that the degradation efficiency of MB was only 23.3%. As can be seen, without any catalyst, nearly no degradation of MB was detected under visible light irradiation. These experimental results indicate that ferric giniite (S1) exhibited better catalytic activity than ferric giniite (S2) in the degradation of MB aqueous solution in the presence of H2O2 under visible-light irradiation. After four recycles for the photodegradation of MB, the catalyst did not exhibit any significant loss of activity, confirming that Fe5(PO4)4(OH)32H2O hollow spheres (S1) were not deactivated during the photocatalytic oxidation of the pollutant molecules (Fig. 11). As mentioned above, the simultaneous presence of ferric giniite (S1), H2O2 and visible-light provides the most effective conditions for the degradation of methylene blue (MB). The time-dependent photo-degradation of MB in the presence of the Fe5(PO4)4(OH)32H2O prepared in the precursor solution with different amount of glucose is shown in Fig. 12. As the amount of glucose during the hydrothermal process increases, the degradation efficiency of Fe5(PO4)4(OH)32H2O photocatalyst is enhanced, which is consistent with other photocatalyst modified with carbonaceous materials in previous reports. This can be explained by the fact that the carbonaceous materials obtained from the HTC process at lower temperature possessed numerous oxygen-containing groups on their surfaces and hence could improve adsorption capacity and visible light photoactivity of the products [34,39,43]. On the basis of our experimental results, mechanism of the photochemical process of the ferric giniite (S1) nanoparticles at neutral pH and under visible-light illumination is proposed. The fact that ferric giniite (S1) is capable of degrading MB in the dark
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Fig. 9. (a) XRD patterns, (b) SEM image of Fe5(PO4)4(OH)32H2O (S2).
Fe(II)–Fe(III)–bearing mineral, reactions involving both Fe(II) and Fe(III) may occur. Therefore, the presence of iron (II) in the ferric giniite (S1) nanoparticles, which is originated from reduction of Fe(III) by glucose under the hydrothermal treatment, can enhance the production rate of HO and enhance the efficiency of Fentonlike catalytic properties. Dark:
BFeðIIÞ þ H2 O2 ! BFeðIIIÞ þ OH þ OH
ð1Þ
BFeðIIIÞ þ H2 O2 ! BFeðIIÞ þ OOC þ OH
ð2Þ
BFeðIIIÞ þ OOC ! BFeðIIÞ þ O2 þ Hþ
ð3Þ
2 OOC ! O2 þ H2 O2
ð4Þ
Light: Fig. 10. The time-dependent photo-degradation of MB under different situations (Experimental conditions: initial MB concentration of 10 mg l1, solution pH 7, catalyst concentration of 300 mg l1).
can be attributed to the presence of adsorbed Fe(II) ions on the surface of ferric giniite (S1), which is evidenced by XPS (Fig. 4b). In mineral catalyzed reaction, the dominant reaction is a chain of reactions occurring on the mineral surface. If only Fe(III) is originally present, Fe(II) is slowly generated by reactions (2)–(4) initiating oxidation reaction (1) [24,44]. In the case of mixed
Fig. 11. MB removal efficiencies of recycled ferric giniite (S1) (Experimental conditions: initial MB concentration of 10 mg l1, solution pH 7, catalyst concentration of 300 mg l1).
Dye þ visible light ! Dye
ð5Þ
Dye þ BFeðIIIÞ ! Dyeþ þ BFeðIIÞ
ð6Þ
BFeðIIÞ þ H2 O2 ! BFeðIIIÞ þ OH þ OH
ð7Þ
Dyeþ þ OH ! products
ð8Þ
When visible-light illumination was introduced to the reaction system, it was found that the degradation efficiency was improved. In the Fe(II/III)/H2O2/dyes/visible light system, the visible light is absorbed by the dye pollutants. The visible light-excited dyes can
Fig. 12. The time-dependent photo-degradation of MB in the presence of the Fe5(PO4)4(OH)32H2O prepared in the precursor solution with different amount of glucose (a) 0.4, (b) 0.6, (c) 0.9 mmol.
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transfer electrons to Fe(III), which leads to regeneration of Fe(II) and an easy cycle of Fe(III)/Fe(II) (Eqs. (5) and (6)) [45,46]. As a result, much faster degradation and mineralization can be achieved in the photo-Fenton reaction of dyes under visible light irradiation. The exact mechanism needs to be further explored in the future studies. Encapsulating ferric giniite with amorphous carbon offers several advantages: Firstly, the hydrophilic groups of amorphous carbonaceous layer can act as grabbers to immobilize Fe(II) ions on their surface from solution and prevent the oxidation of the Fe(II) ions. Secondly, the amorphous carbonaceous layer can enhance the adsorption capability for organic dyes, which can enrich the dye molecules on the surface of photoactive ferric giniite particles, thus resulting in the acceleration of photocatalytic reactions. Thirdly, the carbon coating on the ferric giniite particles can effectively enhance visible light absorption. 4. Conclusions In summary, ferric giniite hollow spheres were synthesized through a simple one-step hydrothermal method. A carbonaceous layer was formed on the ferric giniite surface during the hydrothermal process via the dehydration of glucose. The prepared ferric giniite (S1) modified with carbonaceous materials exhibited higher activity to produce hydroxyl radicals through catalytic decomposition of H2O2 and could degrade highly concentrated MB solution under visible-light irradiation. These results indicate that the ferric giniite modified with carbonaceous materials provides more possibility to serve as an ideal Fenton-like catalyst under visiblelight irradiation. Acknowledgments We greatly appreciate the support of the National Natural Science Foundation of China (Nos. 21277040, 21272054, 21072043, 21173067 and 21203052), the Youth Foundation of Hebei provincial department of education (No. 2010142), Nature Science Foundation of Hebei Province (B2011205037) and the Key Project of Chinese Ministry of Education (No. 207012). References [1] J.J. Pignatello, E. Oliveros, A. MacKay, Crit. Rev. Environ. Sci. Technol. 36 (2006) 1. [2] F. Li, X. Li, X. Li, T. Liu, J. Dong, J. Colloid Interface Sci. 311 (2007) 481. [3] E. Brillas, I. Sires, M.A. Oturan, Chem. Rev. 109 (2009) 6570. [4] P. Bautista, A.F. Mohedano, J.A. Casas, J.A. Zazo, J.J. Rodriguez, J. Chem. Technol. Biotechnol. 83 (2008) 1323.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile preparation of ferric giniite hollow microspheres and their enhanced Fenton-like catalytic performance under visible-light irradiation Xuhong Zhang a,b, Yu Zhang a, Lin Gao a, Haitao Yu a,⇑, Yu Wei a,⇑ a b
College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
g r a p h i c a l a b s t r a c t
The me-dependent photo-degradaon of MB
a r t i c l e
i n f o
Article history: Received 18 December 2014 Accepted 25 March 2015 Available online 13 April 2015 Keywords: Ferric giniite Carbonaceous materials Fenton-like Photocatalytic activity
a b s t r a c t Ferric giniite hollow spheres with diameters of about 1.2–1.4 lm were successfully fabricated with a onepot hydrothermal process. All chemicals used were low-cost compounds and environmentally benign. The obtained products were characterized by field emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), nitrogen adsorption– desorption isotherms, Fourier-transform IR spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). It was found that as-prepared ferric giniite hollow spheres exhibited much enhanced photocatalytic activity (99.5%) for the degradation of methylene blue in the presence of visible light irradiation and H2O2. Experimental results indicate that the existence of the carbonaceous shells enhances the photocatalytic activity of ferric giniite hollow spheres via the synergistic effect between carbon and ferric giniite, such as improving the adsorption, absorbing more light and exhibiting high activity to produce hydroxyl radicals through catalytic decomposition of H2O2. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (Y. Wei). http://dx.doi.org/10.1016/j.jcis.2015.03.043 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
Fenton reactions have proven to be effective methods to treat organic pollutants in wastewater [1–3]. However, the application of traditional Fenton reaction is limited by the narrow working
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pH (2.5–3.5) [4], the formation of a significant amount of ferric hydroxide sludge in the course of Fenton treatment, and the requirements of further separation and disposal for the resulting sludge [5,6]. To overcome these drawbacks, heterogeneous Fenton-like catalysts, such as iron oxides [7–13], iron-immobilized zeolites [14], clay-based materials [15–19] and carbon materials [20,21] have been developed. These heterogeneous catalysts can be effectively used at neutral pH and can be easily separated by physical sedimentation after reaction completion [22]. Recently, several researchers have reported that iron hydroxyl phosphate can be used as active heterogeneous Fenton catalysts to degrade organic pollutants [23–25]. For example, Li et al. developed Fe5(PO4)4(OH)32H2O nanoparticles as photo-Fenton catalysts, which exhibited high photocatalytic activity for the degradation of methylene blue in the presence of visible light irradiation and H2O2 [23]. Duan et al. prepared ferric giniite microcrystals with controlled sizes and shapes by a facile one-pot approach employing ionic-liquid precursors [24]. The measurements indicated that the photocatalytic activity of as-prepared Fenton-like catalysts were highly dependent on the exposed facets. Zhang et al. reported that the single-crystal dendritic iron hydroxyl phosphate could be used as an effective heterogeneous Fenton-like catalyst for the degradation of phenol [25]. However, it was noticed that the adsorption capacities of these iron hydroxyl phosphate heterogeneous catalysts were relative low. Because the contaminant molecules have to be adsorbed on the surface of photocatalyst particles before being decomposed, the adsorption ability of the photo-catalyst is an important factor governing the photocatalytic activity. Therefore, it is important to modify the iron hydroxyl phosphate with other materials for enhancing its adsorption capability and photo-catalytic activity. In recent years, amorphous carbon, which can be obtained via inexpensive and environmentally benign hydrothermal processes using glucose as a precursor, have received considerable attention because of their wide applications as adsorbents, catalysts, and others [26–32]. The surface of the amorphous carbon is hydrophilic and has have many functional groups on their surfaces, such as carboxylic, aldehyde, and hydroxyl groups. Furthermore, the carbonaceous shells are highly porous, which could not only increase the surface area, but also facilitate the penetration of reactive species. Many inorganic materials modified with carbonaceous materials have been reported to represent enhanced photocatalytic properties. For example, Luo et al. fabricated magnetite/carboxylate-rich carbon spheres via hydrothermal carbonization (HTC) process [26]. Without using H2O2, magnetite/carboxylate-rich carbon spheres exhibit excellent photodegradation activity at neutral pH under visible light irradiation. Zhang et al. reported the synthesis of ZnO hybridized with graphite-like carbon by coating the glucose-derived HTC on the surface of ZnO nanoparticles [27]. It was found that the carbon coating functioned to impede the photocorrosion of ZnO and enhance the photostability of ZnO catalysts in UV-irradiation. Zhong et al. fabricated carbon-deposited TiO2 (TiO2@C) with a one-pot hydrothermal process by using glucose as a carbon source [33]. It was found that the photocatalytic activity of TiO2@C was greatly enhanced compared to noncarbon-TiO2 under visible irradiation. Wang et al. fabricated a carbon coated SnO2 photocatalyst by using sucrose as a carbon source and the microwave hydrothermal method [34]. The prepared carbon coated SnO2 exhibits high and stable photocatalytic activity for the degradation of Rhodamine B (RhB) under UV–Vis irradiation. Herein, based on the latest progress in hydrothermal preparation of carbon-coated inorganic materials, we fabricated ferric giniite hollow spheres modified with carbonaceous materials by a onepot hydrothermal method using glucose as carbon source at low temperature. When mixed with a small amount of H2O2, the asprepared ferric giniite hollow spheres exhibited high visible-light
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photocatalytic activity toward methylene blue (MB) in aqueous solution. Compared with the traditional photo-Fenton-like process that requires the assistance of UV irradiation, the as-prepared ferric giniite hollow spheres exhibited much enhanced photocatalytic activity for the degradation of methylene blue in the presence of visible-light irradiation and H2O2. 2. Experimental section 2.1. Preparation of the samples Sample 1 (S1) was prepared by one-step hydrothermal method. All reagents are analytical grade and used without any further purification. In a typical synthesis, 5 mmol FeCl36H2O, 0.6 mmol glucose and 0.6 mmol NaH2PO46H2O were dissolved in 75 ml distilled water. The solution was transferred to a 100 ml Teflon-lined autoclave. The autoclave was then sealed and maintained at 180 °C for 8 h. After cooling down naturally, the dark green precipitate was washed with distilled water three times before being dried at 60 °C for 24 h. Sample 2 (S2) was synthesized according to the method described in previous work [23]. In a typical synthesis procedure, Fe(NO3)36H2O (5 mmol) was completely dissolved in deionized water (14 ml). Under vigorous agitation, an aqueous solution (14 ml) containing Na3PO46H2O (5 mmol) was added into the above solution at room temperature. The pH value of the mixture was adjusted to 4 with 10 M NaOH and HNO3 (68%). Then, the mixture was transferred into a Teflon-lined steel autoclave of 40 ml, and the autoclave was heated under autogenous pressure at 180 °C for 24 h. Then, the autoclave was cooled to room temperature gradually. The yellowish green precipitate was washed with distilled water three times. Then, it was dried at 60 °C in air. 2.2. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8-ADVANCE X-ray diffractometer using Cu Ka radiation at 40 kV and 40 mA. The compositions of the powders were characterized at room temperature by Fourier-transform IR spectroscopy on an FTIR-8900 instrument in the range 400– 4000 cm1. Infrared spectra of the products were recorded by pelletizing a few milligrams of the sample with KBr. The morphology and the composition of the products were examined by fieldemission scanning electron microscopy (FESEM, HITACHI S-4800) equipped with an energy dispersive spectrometry (EDS). Selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) images of products were performed with a JEOL-2010 high resolution transmission electron microscopy (HRTEM, JEOL Ltd., Japan). Surface areas were determined by the BET method using N2 adsorption–desorption isotherms. N2 adsorption–desorption isotherms were measured on a gas sorption analyzer (Quantachrome, NOVA 4000e) at a liquid nitrogen temperature. XPS studies were performed on a Physical Electronics/PHI 5300 X-ray photoelectron spectrometer with the Al Ka X-ray source (hv 1486.6 eV). The position of the C1s peak was taken as a standard (with a banding energy of 284.6 eV). 2.3. Photo-Fenton and photocatalytic experiments Photocatalytic activities of the samples were evaluated by the photocatalytic decolorization of MB under visible light. In a typical process, 30 mg Fe5(PO4)4(OH)32H2O nanoparticles were dispersed into 100 ml aqueous solution of MB (10 mg l1) solution (photoFenton-like reaction was initiated by adding 0.5 ml H2O2 to the reactor). Before illumination, the suspension was sonicated for
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1 min and magnetically stirred in dark for 0.5 h to achieve the adsorption–desorption equilibrium among the photocatalyst, MB and water. The solution was then exposed to a 150 W tungstenhalogen lamp with a 420 nm cut off filter to provide visible-light irradiation. The reaction temperature was kept at room temperature by cooling water to prevent any thermal catalytic effect. The samples were collected by centrifugation every 30 min to measure methylene blue (MB) degradation by UV–Vis spectroscopy (Yoke UV752). To test the stability and recyclability of samples, the catalyst was sedimentated from an aqueous suspension for about 1 h, after the added MB was almost degraded. The supernatant was centrifuged to collect the residual catalyst in it. After the two parts of the catalyst were gathered together, it was washed with a mixture of deionized water and ethanol several times and then dried at 60 °C for 12 h. Then, the final products were suspended into a fresh solution of MB and H2O2, and the MB degradation was continued as the second cycle. This process was repeated several times.
3. Results and discussions Fig. 1a shows the XRD patterns of the samples prepared in typical synthesis process at 180 °C for 8 h. The diffraction peaks of all the samples could be easily indexed as a pure, orthorhombic crystalline phase Fe5(PO4)4(OH)32H2O, which is in good agreement with the standard card (JCPDS Card number: 45-1436). Fig. 1b presents the SEM image of the as-prepared sample. It can be seen that the product is composed of microspheres with diameters ranging from 1.2 to 1.4 lm. From the related SEM image in higher magnification (Fig. 1c), it can be clearly found that a lot of pores, with a diameter ranging from 20 to 60 nm, are evenly distributed on the surface of the microspheres. One broken Fe5(PO4)4(OH)32H2O microsphere clearly shows the hollow nature of the Fe5(PO4)4(OH)32H2O microspheres (Fig. 1d). Moreover, it can be seen that the microspheres are formed via the self-assembly of
the irregular nanocrystallines and there are interstitial spaces available among these nanocrystallines, which provide connected channels for mass exchange between the inner space of Fe5(PO4)4(OH)32H2O microspheres and the outer solution. These crystal strips are aligned perpendicularly to the spherical surface, pointing toward a common center. TEM was used to elucidate the interior structure of the microspheres. Fig. 2a shows that the spheres have pale center region in contrast to a dark edge, suggesting the spheres are hollow. The selected area electron diffraction (SAED) pattern (an inset in Fig. 2b) is composed of many separated bright spots instead of rings, which imply the single crystal-like feature of the hollow spheres. The HRTEM image of Fe5(PO4)4(OH)32H2O microstructures is shown in Fig. 2c and the clear lattice image indicates the high crystallinity and single-crystalline nature of the Fe5(PO4)4(OH)32H2O microstructures. In addition, an encapsulating amorphous layer can be easily observed (indicated by the two white arrows) through the HRTEM image, with a thickness of 2.5 nm. The measured lattice spacings of 0.228 and 0.32 nm are consistent with the d values of the (4 2 1) and (3 3 0) planes of Fe5(PO4)4(OH)32H2O, respectively (Fig. 2d). The porosity of the hollow spheres was substantiated by the measurement of nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface area. Fig. 3a shows the N2 adsorption–desorption isotherm and pore size distribution curve (inset in Fig. 3a) of the Fe5(PO4)4(OH)32H2O hollow spheres. The isotherm can be classified as type IV with an apparent hysteresis loop in the range 0.5–1.0 P/P0, indicating the presence of mesopores. The plot of pore size distribution determined by the Barrett–Joyner–Halenda (BJH) method shows that there is broad pore size distribution in the range 5–25 nm. The mesopores on the hollow sphere can be attributed to the interspaces of the constituent particles and the existence of amorphous carbonaceous layer. The BET surface area of the hollow sphere is 10.219 m2 g1 and the pore volume is 0.039 cm3 g1. The high surface area and large pore volume further support that the hollow
Fig. 1. (a) XRD patterns, (b) overall SEM image, (c) detailed view of an individual hollow sphere, (d) magnified image of a broken microsphere of Fe5(PO4)4(OH)32H2O (S1) prepared at 180 °C for 8 h.
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Fig. 2. (a) TEM image of Fe5(PO4)4(OH)32H2O (S1), (b) TEM image of the quarter of an Fe5(PO4)4(OH)32H2O hollow sphere. The corresponding SAED pattern of the product taken from the edge of the hollow sphere (inset), (c) HRTEM of Fe5(PO4)4(OH)32H2O hollow sphere (S1), (d) a magnified image of the square-marked area in (c).
Fig. 3. (a) The nitrogen adsorption–desorption isotherm and pore size distribution curve (inset), (b) the EDS spectrum of Fe5(PO4)4(OH)32H2O samples (S1).
spheres have porous structure. The EDS result shown in Fig. 3b demonstrates that the as-prepared sample contains C (12.30%, weight%), Fe (23.09%), O (53.07%), and P (11.54%), indicating the presence of carbonaceous shells on the Fe5(PO4)4(OH)32H2O microspheres. In order to confirm the surface oxidation state of synthesized iron oxide, the surface oxidation state of catalyst was measured by X-ray photoelectron spectrometer (XPS). Fig. 4a shows XPS spectra of ferric giniite (S1) and ferric giniite (S2). Compared with
S2, the binding energies of iron in S1 shifted to low energy states. XPS analysis was conducted to investigate the oxidation state of Fe on the surface of S1. The Fe 2p spectra (Fig. 4b) are split into two main peaks (corresponding to Fe 2p3/2 and Fe 2p1/2) by spin–orbit coupling, which can be further subdivided into peaks at 709.2 and 711.6 eV, and shake-up satellite peaks at 722.5 and 724.8 eV. The peak located at 711.6 eV corresponds unambiguously to oxygenbonded ferric ion (Fe(III)AO). The peak arising at 709.2 eV can be assigned to Fe(II)AO, which indicate existence of Fe(II) ions on
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Fig. 4. XPS and FT-IR spectra of Fe5(PO4)4(OH)32H2O samples: (a) Fe 2p of S1 and S2, (b) Fe 2p of S1, (c) C1s of S1, (d) FT-IR of S1.
the surface of S1 [35–37]. Fig. 4c displays the typical XPS spectrum of the C1s region. The carbon 1s peak was deconvoluted into three spectral bands at 284.6, 285.6 and 288.7 eV. The most intense peak at 284.6 eV attributed to the Cn, while the relatively small peaks at 285.6, and 288.7 eV represent CAOH, and OAC@O (and OAC@O) bonds, respectively [33,34,38]. The coating effect of glucose-derived, carbon-rich polysaccharide (GCP) on the surface of ferric giniite was further proved by typical FT-IR spectroscopy (Fig. 4d). The broad characteristic band from 3600 to 3100 cm1 could be assigned to OAH stretching vibration arising from hydroxyl groups on nanoparticles and adsorbed glucose and water. The peaks around 2920 and 2850 cm1 could be assigned to asymmetric and symmetric vibrations of CAH in ACH2A. The vibration bands located at 1620 and 1410 cm1 can be assigned to the asymmetric and symmetric stretching vibration of COO groups, confirming that a lot of carboxylate groups from carbon were coordinated strongly to the iron cations. The other bands at 1092, 1036, 960, 602 and 465 cm1 were assigned to the vibration in the PO3 4 group of the Fe5(PO4)4(OH)32H2O. From the FT-IR spectra of the ferric giniite hollow sphere, it can be observed that the carbonization was incomplete and ferric giniite particles were coated by carbonaceous polysaccharide. To investigate the formation mechanism of Fe5(PO4)4(OH)3 2H2O hollow microspheres, a detailed time-dependent evolution experiment was carried out. Fig. 5 shows the SEM images of samples obtained at the reaction times of 1, 2, 4 and 8 h, respectively. From Fig. 5a, it can be clearly seen that spherical particles with diameters of 1.0–1.4 lm were obtained within 1 h. It is worth noting here that the shells of the above Fe5(PO4)4(OH)32H2O spheres were relative smooth. With the reaction time increased to 2 h and
4 h, the sizes of primary particles (especially the particles on the surface) obviously became larger and larger (Fig. 5b and c). When the reaction time was increased to 8 h, the sizes of primary particles of microspheres grew larger continuously and there appeared to be an obvious porous structure, with a pore diameter of 20–60 nm, on the surface of most microspheres (Fig. 5d). Fig. 5a– d clearly illustrates the changes in the porosity of the nanostructures. The sizes of the microspheres remain relatively unchanged, but with prolonged hydrothermal treatment the interior of the microspheres is subjected to a dissolution process, leading to spheres of greater porosity. In addition, XRD measurement was further employed to characterize the intermediates at different reaction time intervals. It can be seen that tetragonal Fe5(PO4)4(OH)32H2O is formed after hydrothermal reaction for 1 h. All the diffraction peaks shown in Fig. 6 can be indexed as orthorhombic crystalline phase Fe5(PO4)4(OH)32H2O, which is consistent with the standard card (JCPDS Card number: 45-1436). No obvious diffraction peak for the carbon is observed, suggesting the carboxylate-rich carbon is amorphous carbon. Glucose plays an important role in the formation of Fe5(PO4)4(OH)32H2O. Several detailed experiments with different amounts of glucose were performed while keeping other parameters constant. As shown in Fig. 7a, when the reaction was carried out in the absence of glucose, the samples was composed of large-scale ellipsoids with diameters of 100–300 nm. The characteristic XRD pattern of synthesized samples was corresponded to the mixed phase of a-Fe2O3 (major) (JCPDS Card number: 33-0664) and Fe5(PO4)4(OH)32H2O (minor) (Fig. 8a). When a small amount of glucose (0.2 mmol) was introduced to the reaction solution, the synthesized samples was also corresponded to the mixed
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Fig. 5. The morphology evolution of the samples prepared at 180 °C for different reaction times: (a) 1 h, (b) 2 h, (c) 4 h, (d) 8 h.
Fig. 6. XRD patterns of samples obtained at 180 °C for different reaction times: (a) 1 h, (b) 2 h, (c) 4 h, (d) 8 h.
phase of Fe5(PO4)4(OH)32H2O and a-Fe2O3 (Fig. 8b). However, the peak intensity of the a-Fe2O3 became weak and the diffraction peaks of Fe5(PO4)4(OH)32H2O became strong. The SEM images (Fig. 7b) show that the product consists of well dispersed quasispheres with an average diameter of about 0.6–0.9 lm. These quasi-spheres tend to be angular in shape, indicating they are consisted of numerous densely packed irregular nanocrystals. When the concentration of glucose exceeded 0.4 mmol, the product was completely transformed to Fe5(PO4)4(OH)32H2O (Fig. 8c). From the SEM observation (Fig. 7c), the product consisted entirely of well-defined porous spheres with diameters in the range of 1.0–1.1 lm. When the glucose amount was further increased to 0.9 mmol, the obtained Fe5(PO4)4(OH)32H2O microspheres appeared to be larger and more porous. Based on the results of the control experiments, it can be seen the synthesized microspheres show insignificant morphology changes and size distribution improvements with the aid of glucose additives. Based on the above experimental results, the formation process can be divided into two steps as follows: At the early reaction
stage, the Fe3+ ions and H2PO were hydrolysed into 4 Fe5(PO4)4(OH)32H2O nanocrystals. The freshly formed nanocrystals are unstable due to their high surface energy and they evolved to Fe5(PO4)4(OH)32H2O aggregates through oriented attachment, driven by the minimization of surface energy. The new formed Fe5(PO4)4(OH)32H2O aggregates were loosely attached due to the glucose-coated surface of the Fe5(PO4)4(OH)32H2O nanocrystals. According to previous reports, at elevated temperatures, glucose can serve as a reductant in solution due to its hydroxyl group [29]. Then, part of Fe3+ in the Fe5(PO4)4(OH)32H2O were reduced to Fe2+ by glucose. As a result, many pores were left in the Fe5(PO4)4(OH)32H2O. It is noted that glucose plays a key role in inducing the formation of Fe5(PO4)4(OH)32H2O. During the reaction process, the glucose can carbonize to transform into glucosederived carbon-rich polysaccharide (GCP), which have many functional groups on their surfaces, such as carboxylic, aldehyde, and hydroxyl groups. These surface functional groups can bind metal cations through coordination or electrostatic interactions [39,40], which is favorable for the formation of Fe5(PO4)4(OH)32H2O nanoparticles. In the second stage, the Fe5(PO4)4(OH)32H2O mesoporous spheres consisted of numerous smaller nanocrystals gradually transform to the Fe5(PO4)4(OH)32H2O hollow microspheres composed of bigger irregular nanocrystallines. The reconstruction of Fe5(PO4)4(OH)32H2O microspheres can be easily realized via a simple ripening process. Compared with the large and wellcrystallized particles on the exteriors of the spheres, the inner small crystallites have higher surface energy, which provides them driving force for the Ostwald ripening. Therefore, the inner small crystallites of a sphere are undergoing mass relocation through dissolving and regrowth, whereas the outer larger ones serve as new starting growth sites. With the continuous mass transportation from the inner core to the outermost surface of the same sphere, the hollow interior occurred. Due to the Ostwald ripening process, large quantity pores and channels in the spheres are produced. Thus, it can be concluded that the formation of Fe5(PO4)4(OH)32H2O hollow microspheres involves cooperation of oriented aggregation and Ostwald ripening process. On the other hand, when the temperature is above 160 °C, glucose can be
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Fig. 7. SEM images of samples obtained at 180 °C for 8 h with the addition of glucose: (a) 0, (b) 0.2, (c) 0.4, (d) 0.9 mmol.
Fig. 8. XRD patterns of samples obtained at 180 °C for 8 h with the addition of glucose: (a) 0, (b) 0.2, (c) 0.4, (d) 0.9 mmol.
carbonized. The uniform amorphous carbonaceous coating is possibly formed by the carbonization of glucose around the Fe5(PO4)4(OH)32H2O nanoparticles involving the intermolecular cross-linking and dehydration of the glucose, oligosaccharides, and/or other macromolecules derived from glucose during the hydrothermal treatment [33,34]. The XRD patterns show that the crystallinity of the products is increased gradually with the reaction time prolonged, which also validates the above proposition that Ostwald ripening is the reasonable mechanism in the present system. Therefore, by simply controlling the reaction time, the microspheres could be effectively tuned from solid to porous structure. The forming process is similar to the previous reports of BiOI [41] and SnO2 [42] hollow or porous structures. The photocatalytic activity of the ferric giniite (S1) nanoparticles was evaluated by the degradation of MB in aqueous solution at room temperature. It can be observed from Fig. 10 that approximately 49.2% organic dye is adsorbed on the catalyst S1 after the dark treatment. The high adsorption ability of ferric giniite (S1)
should be attributed to the existence of the amorphous carbon on the composites surface. The simultaneous presence of ferric giniite (S1) nanoparticles and H2O2 removes 80.5% of MB within 120 min in the dark, which implies that ferric giniite nanoparticles (S1) can actually be regarded as heterogeneous Fenton-like catalyst. When visible-light illumination was introduced to the reaction system, it was found that the degradation efficiency was about 99.5% after 120 min. Additionally, an experiment was also performed in the presence of ferric giniite (S2) (The XRD pattern and SEM image of S2 was displayed in Fig. 9) with H2O2 under visible-light illumination (Fig. 10). It should be noted that the degradation efficiency of MB was only 23.3%. As can be seen, without any catalyst, nearly no degradation of MB was detected under visible light irradiation. These experimental results indicate that ferric giniite (S1) exhibited better catalytic activity than ferric giniite (S2) in the degradation of MB aqueous solution in the presence of H2O2 under visible-light irradiation. After four recycles for the photodegradation of MB, the catalyst did not exhibit any significant loss of activity, confirming that Fe5(PO4)4(OH)32H2O hollow spheres (S1) were not deactivated during the photocatalytic oxidation of the pollutant molecules (Fig. 11). As mentioned above, the simultaneous presence of ferric giniite (S1), H2O2 and visible-light provides the most effective conditions for the degradation of methylene blue (MB). The time-dependent photo-degradation of MB in the presence of the Fe5(PO4)4(OH)32H2O prepared in the precursor solution with different amount of glucose is shown in Fig. 12. As the amount of glucose during the hydrothermal process increases, the degradation efficiency of Fe5(PO4)4(OH)32H2O photocatalyst is enhanced, which is consistent with other photocatalyst modified with carbonaceous materials in previous reports. This can be explained by the fact that the carbonaceous materials obtained from the HTC process at lower temperature possessed numerous oxygen-containing groups on their surfaces and hence could improve adsorption capacity and visible light photoactivity of the products [34,39,43]. On the basis of our experimental results, mechanism of the photochemical process of the ferric giniite (S1) nanoparticles at neutral pH and under visible-light illumination is proposed. The fact that ferric giniite (S1) is capable of degrading MB in the dark
X. Zhang et al. / Journal of Colloid and Interface Science 452 (2015) 24–32
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Fig. 9. (a) XRD patterns, (b) SEM image of Fe5(PO4)4(OH)32H2O (S2).
Fe(II)–Fe(III)–bearing mineral, reactions involving both Fe(II) and Fe(III) may occur. Therefore, the presence of iron (II) in the ferric giniite (S1) nanoparticles, which is originated from reduction of Fe(III) by glucose under the hydrothermal treatment, can enhance the production rate of HO and enhance the efficiency of Fentonlike catalytic properties. Dark:
BFeðIIÞ þ H2 O2 ! BFeðIIIÞ þ OH þ OH
ð1Þ
BFeðIIIÞ þ H2 O2 ! BFeðIIÞ þ OOC þ OH
ð2Þ
BFeðIIIÞ þ OOC ! BFeðIIÞ þ O2 þ Hþ
ð3Þ
2 OOC ! O2 þ H2 O2
ð4Þ
Light: Fig. 10. The time-dependent photo-degradation of MB under different situations (Experimental conditions: initial MB concentration of 10 mg l1, solution pH 7, catalyst concentration of 300 mg l1).
can be attributed to the presence of adsorbed Fe(II) ions on the surface of ferric giniite (S1), which is evidenced by XPS (Fig. 4b). In mineral catalyzed reaction, the dominant reaction is a chain of reactions occurring on the mineral surface. If only Fe(III) is originally present, Fe(II) is slowly generated by reactions (2)–(4) initiating oxidation reaction (1) [24,44]. In the case of mixed
Fig. 11. MB removal efficiencies of recycled ferric giniite (S1) (Experimental conditions: initial MB concentration of 10 mg l1, solution pH 7, catalyst concentration of 300 mg l1).
Dye þ visible light ! Dye
ð5Þ
Dye þ BFeðIIIÞ ! Dyeþ þ BFeðIIÞ
ð6Þ
BFeðIIÞ þ H2 O2 ! BFeðIIIÞ þ OH þ OH
ð7Þ
Dyeþ þ OH ! products
ð8Þ
When visible-light illumination was introduced to the reaction system, it was found that the degradation efficiency was improved. In the Fe(II/III)/H2O2/dyes/visible light system, the visible light is absorbed by the dye pollutants. The visible light-excited dyes can
Fig. 12. The time-dependent photo-degradation of MB in the presence of the Fe5(PO4)4(OH)32H2O prepared in the precursor solution with different amount of glucose (a) 0.4, (b) 0.6, (c) 0.9 mmol.
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transfer electrons to Fe(III), which leads to regeneration of Fe(II) and an easy cycle of Fe(III)/Fe(II) (Eqs. (5) and (6)) [45,46]. As a result, much faster degradation and mineralization can be achieved in the photo-Fenton reaction of dyes under visible light irradiation. The exact mechanism needs to be further explored in the future studies. Encapsulating ferric giniite with amorphous carbon offers several advantages: Firstly, the hydrophilic groups of amorphous carbonaceous layer can act as grabbers to immobilize Fe(II) ions on their surface from solution and prevent the oxidation of the Fe(II) ions. Secondly, the amorphous carbonaceous layer can enhance the adsorption capability for organic dyes, which can enrich the dye molecules on the surface of photoactive ferric giniite particles, thus resulting in the acceleration of photocatalytic reactions. Thirdly, the carbon coating on the ferric giniite particles can effectively enhance visible light absorption. 4. Conclusions In summary, ferric giniite hollow spheres were synthesized through a simple one-step hydrothermal method. A carbonaceous layer was formed on the ferric giniite surface during the hydrothermal process via the dehydration of glucose. The prepared ferric giniite (S1) modified with carbonaceous materials exhibited higher activity to produce hydroxyl radicals through catalytic decomposition of H2O2 and could degrade highly concentrated MB solution under visible-light irradiation. These results indicate that the ferric giniite modified with carbonaceous materials provides more possibility to serve as an ideal Fenton-like catalyst under visiblelight irradiation. Acknowledgments We greatly appreciate the support of the National Natural Science Foundation of China (Nos. 21277040, 21272054, 21072043, 21173067 and 21203052), the Youth Foundation of Hebei provincial department of education (No. 2010142), Nature Science Foundation of Hebei Province (B2011205037) and the Key Project of Chinese Ministry of Education (No. 207012). References [1] J.J. Pignatello, E. Oliveros, A. MacKay, Crit. Rev. Environ. Sci. Technol. 36 (2006) 1. [2] F. Li, X. Li, X. Li, T. Liu, J. Dong, J. Colloid Interface Sci. 311 (2007) 481. [3] E. Brillas, I. Sires, M.A. Oturan, Chem. Rev. 109 (2009) 6570. [4] P. Bautista, A.F. Mohedano, J.A. Casas, J.A. Zazo, J.J. Rodriguez, J. Chem. Technol. Biotechnol. 83 (2008) 1323.
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