Monodisperse α-Fe2O3 Mesoporous Microspheres: One-Step NaCl-Assisted Microwave-Solvothermal Preparation, Size Control and Photocatalytic Property
© Cao and Zhu. 2010
Received: 28 June 2010
Accepted: 5 August 2010
Published: 18 August 2010
A simple one-step NaCl-assisted microwave-solvothermal method has been developed for the preparation of monodisperse α-Fe2O3 mesoporous microspheres. In this approach, Fe(NO3)3 · 9H2O is used as the iron source, and polyvinylpyrrolidone (PVP) acts as a surfactant in the presence of NaCl in mixed solvents of H2O and ethanol. Under the present experimental conditions, monodisperse α-Fe2O3 mesoporous microspheres can form via oriented attachment of α-Fe2O3 nanocrystals. One of the advantages of this method is that the size of α-Fe2O3 mesoporous microspheres can be adjusted in the range from ca. 170 to ca. 260 nm by changing the experimental parameters. High photocatalytic activities in the degradation of salicylic acid are observed for α-Fe2O3 mesoporous microspheres with different specific surface areas.
The fabrication of mesoporous materials of transition metal oxides has attracted more and more attention in recent years for their unique catalytic, electrochemical, magnetic and adsorptive properties [1–4]. Among them, α-Fe2O3 mesoporous materials are of particular interest, because α-Fe2O3 is widely used in catalysis , photoelectrodes , sensors , the anode material for Li-ion batteries  and so on. As an important n-type semiconductor, α-Fe2O3 is also used as a photocatalyst [9, 10], especially in the degradation of salicylic acid [11–13]. Salicylic acid is a complexing agent that forms stable complexes with iron ions, and it is one of pollutants in waste effluent . Mesoporous structures will benefit the photocatalytic activity of α-Fe2O3 due to the high specific surface area and the redox activity of the surfaces and nanopores.
Although the preparation of mesoporous silica, aluminosilicates, aluminophosphates and related materials is already well established [15–18], however, the synthesis of mesoporous materials of transition metal oxides is much more difficult and less reported [19, 20]. Several mesoporous materials of transition metal oxides such as TiO2, ZrO2, Nb2O5, WO3 and MnOx [21–27] have been prepared owing to researchers' unremitting effort. α-Fe2O3 mesoporous structures were prepared using soft templating methods [1, 28–31], as well as using mesoporous silica as hard template . However, such methods suffer from some disadvantages. Soft templating methods usually lead to the formation of mesoporous α-Fe2O3 with amorphous walls, while the hard templating methods usually involve multistep processes and sometimes lead to the damage of pore structures during the removal of hard templates.
Monodisperse nanocrystals display novel properties thus to stimulate intensive researches on the synthesis of monodisperse nanocrystals for their fundamental and technological importance . However, challenges still arise, how to combine the mesoporous structure with monodisperse microspheres, for the enhancement of the structural stability and photocatalytic property of α-Fe2O3. Herein, we report a simple one-step NaCl-assisted microwave-solvothermal method for the preparation of monodisperse α-Fe2O3 mesoporous microspheres. In the present approach, monodisperse α-Fe2O3 mesoporous microspheres can form via oriented attachment of α-Fe2O3 nanocrystals in the presence of NaCl. One of the advantages of this method is that the size of α-Fe2O3 mesoporous microspheres can be adjusted in the range from ca. 170 to ca. 260 nm by changing the experimental parameters. High photocatalytic activities in the degradation of salicylic acid are observed for typical samples of α-Fe2O3 mesoporous microspheres with different specific surface areas.
Materials and Methods
Preparation of Monodisperse α-Fe2O3 Mesoporous Microspheres
Experimental parameters for the preparation of typical samples by the microwave-solvothermal method
0.404 g Fe(NO3)3 · 9H2O + 0.117 g NaCl + 0.111 g PVP + 15 ml H2O + 15 ml ethanol
0.404 g Fe(NO3)3 · 9H2O + 0.111 g PVP + 15 ml H2O + 15 ml ethanol
0.404 g Fe(NO3)3 · 9H2O + 0.117 g NaCl + 15 ml H2O + 15 ml ethanol
0.404 g Fe(NO3)3 · 9H2O + 0.117 g NaCl + 0.222 g PVP + 15 ml H2O + 15 ml ethanol
Same as sample 1
Same as sample 1
Photocatalytic Activity Measurements
The photocatalytic reactor consisted of two parts: a 70-ml quartz tube and a high-pressure Hg lamp. The Hg lamp was positioned parallel to the quartz tube. In all experiments, the photocatalytic reaction temperature was kept at about 35°C. The reaction suspension was prepared by adding the sample (20 mg) into 50 ml of salicylic acid solution with a concentration of 20 mg l-1. The suspension was sonicated for 15 min and then stirred in the dark for 30 min to ensure an adsorption/desorption equilibrium prior to UV irradiation. The suspension was then irradiated using UV light under continuous stirring. Analytical samples were withdrawn from the reaction suspension after various reaction times and centrifuged at 10,000 rpm for 5 min to remove the particles for analysis.
Characterization of Samples
The as-prepared samples were characterized using X-ray powder diffraction (XRD) (Rigaku D/max 2550V, Cu Kα radiation, λ = 1.54178 Å), scanning electron microscopy (SEM) (JEOL JSM-6700F) and transmission electron microscopy (TEM) (JEOL JEM-2100F). The Brunauer–Emmett–Teller (BET) surface area and pore size distribution were measured with an accelerated surface area and porosimetry system (ASAP 2010, USA). The photocatalytic reactions were carried out under irradiation of a 300-W high-pressure Hg lamp (GGZ300, Shanghai Yaming Lighting) with a maximum emission at about 365 nm. The salicylic acid concentrations were analyzed using a UV–vis spectrophotometer (UV-2300, Techcomp) at a wavelength of 297 nm.
Results and Discussion
The detailed preparation procedures for the samples are described in the experimental section, and the preparation conditions for typical samples are listed in Table 1.
In order to control the size of mesoporous microspheres, comparative experiments were performed by changing the experimental parameters. Sample 4 was obtained with increased PVP concentration (0.222 g), while the other conditions were unchanged. One can see that sample 4 is also composed of α-Fe2O3 microspheres with an average diameter of ca. 205 nm, as shown in Figure 3d, e. Although sample 4 has a similar morphology to that of sample 1, the average diameter of microspheres in sample 4 is larger than that of sample 1, indicating that the concentration of PVP has an effect on the size of as-prepared α-Fe2O3 microspheres.
Figure 3f, g shows TEM micrographs of sample 5 prepared when the microwave-solvothermal time was increased from 30 to 60 min, and the average diameter of α-Fe2O3 mesoporous microspheres increases from ca. 170 to ca. 225 nm. This experimental result indicates that longer microwave-solvothermal time results in larger mesoporous microspheres. Thus, the size of α-Fe2O3 mesoporous microspheres can be controlled by adjusting microwave-solvothermal time.
We have developed a simple one-step NaCl-assisted microwave-solvothermal method for the preparation of α-Fe2O3 monodisperse mesoporous microspheres formed by oriented assembly of nanocrystals. In this approach, Fe(NO3)3 · 9H2O is used as the iron source, and PVP acts as a surfactant in the presence of NaCl in mixed solvents of H2O and ethanol. NaCl is found to play an important role in the formation of α-Fe2O3 monodisperse mesoporous microspheres. One of the advantages of this method is that the size of α-Fe2O3 mesoporous microspheres can be adjusted in the range from ca. 170 to ca. 260 nm by changing the experimental parameters. High photocatalytic activities in the degradation of salicylic acid are observed for α-Fe2O3 mesoporous microspheres. The combination of the mesoporous structure with monodisperse microspheres is beneficial for the enhancement of the photocatalytic property of α-Fe2O3 in the degradation of salicylic acid by giving α-Fe2O3 mesoporous microspheres higher specific surface area and narrower pore size distribution.
Financial support from Science and Technology Commission of Shanghai (0852nm05800, 1052nm06200) and National Natural Science Foundation of China (50772124, 50821004) is gratefully acknowledged.
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