Abstract
One-dimensional composite - nanowires were successfully fabricated by a simple microwave-assistant hydrothermal method. The successful fabrication of one-dimensional - nanowires can only be realized at pH value range of 1–5 of the starting solution. The fabricated nanowires were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectra (UV-Vis DRS), and photoluminescence (PL), respectively. The diameter of the nanowires varies from 30 to 100 nm; the length is a few micrometers. Methylene blue (MB) solutions were used to evaluate the visible light photocatalytic activities of the fabricated samples. Compared with , the fabricated - nanowires show enhanced efficiency in oxidization of MB under visible light.
1. Introduction
One-dimensional nanostructures with distinctive structures and properties play important roles in nanotechnology for their use as building blocks in nanoscale circuits, optoelectronic, electrochemical, and electromechanical devices [1, 2]. The synthesis of one-dimensional (1D) nanostructures [3–7] has gained tremendous amount of attention in recent years due to their fascinating sizes, shapes, and material-dependent properties. Nanotubes, nanobelts, nanorods, and semiconducting nanowires have been fabricated through a number of advanced nanolithographic techniques, thermal evaporations, and solution-based methods [8–11]. Among all the one-dimensional nanostructures, the composite nanowires with two different constitutions are widely investigated [12, 13]. Vomiero et al. produced radial and longitudinal nanosized In2O3-SnO2 by applying a suitable methodology of transport and condensation [14]. Bachas’s group has synthesized a new class of bimetallic nanotubes based on Pd/Fe and demonstrated their efficacy in the dechlorination of PCB 77 (a polychlorinated biphenyl) one-dimensional iron metal nanotubes of different diameters were prepared by electroless deposition within the pores of PVP-coated polycarbonate membranes using a simple technique under ambient conditions. The longitudinal nucleation of the nanotubes along the pore walls was achieved by mounting the PC membrane between two halves of a U-shape reaction tube [15]. Coaxial SnO2@TNTs, with two-fold tubular structures, are assembled by electrochemical and solvothermal embedments of SnO2 nanolayers inside the pristine TiO2 nanotube arrays. The excellent electrochemical properties originated from the synergistic effect with improved electronic conductivity and dual lithium storage mechanism, demonstrating that the coaxial SnO2@TNT hybrid is a promising candidate for electrochemical energy storage [16]. In case of our research, microwave hydrothermal method was employed in the fabrication process. Microwave irradiation provides rapid and uniform heating of reagents and solvents [17–19]. The rapid microwave heating provides uniform nucleation and growth conditions, which conduce to homogeneous and well crystalline nanomaterials. We demonstrate a simple and efficient approach to synthesize one-dimensional nanowires within 60 min of microwave irradiation. During the process, no organic surfactant and catalyst were used. The time and energy consumption were saved with the microwave route of syntheses. Comparative experiment was carried out the same reaction time by common hydrothermal method while keeping other synthetic parameters in same. No nanowire was obtained.
Bismuth-containing oxides have been extensively studied because of their interesting properties, including ferroelectricities [20], ionic conductivities [21], superconductivities [22], and catalytic activities [23]. As a photocatalyst [24, 25], the conduction and valence band edges of Bi2O3 are +0.33 and +3.13 V relative to NHE, respectively. The band structure of Bi2O3 accounts for its ability to oxidize organic pollution and generate highly reactive species, such as and OH radicals. Although the Bi2O3 have the proper band structure, the efficiency to decompose organic pollution is still low. In order to improve the photocatalytic ability of Bi2O3, Bi2CrO6 was connected to Bi2O3. Bi2CrO6 is a novel material with narrow band gap. Upon connection of Bi2CrO6 to Bi2O3 nanowires, a unique semiconductor—semiconductor nanoscale contact can be prepared, which shows different properties from Bi2O3. In this paper, the Bi2O3 and Bi2CrO6 were synthesized synchronously in the same Teflon autoclave, Bi2O3 and Bi2CrO6 are in well contact, and the photo-generated electron and holes can migrate from each other freely. Compared with Bi2O3, the Bi2O3-Bi2CrO6 nanowires show enhanced efficiency in photodegradation of methylic blue under visible light.
2. Experimental Section
Analytic grade Bismuth nitrate (Bi(NO3)3), sodium chromate (Na2CrO4), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were used as received without any treatment. In a typical procedure, solutions of Bi(NO3)3 and solutions of Na2CrO4 in water were prepared as stock solutions in advance. 10 mL of 0.2 M Bi(NO3)3 stock solution was mixed with 10 mL of 0.1 M Na2CrO4 aqueous solution, and 1 M HCl solution was added to adjust the pH value of the mixed solution to 3. The solution was magnetically stirred for about 0.5 h, then transferred into a special Teflon autoclave, and heated at 180°C for 1 h under microwave radiation. The resulting precipitate was collected, washed with dilute HCl aqueous solution and deionized water until the pH value of the washing solution was about 7, and dried in air at 80°C for 8 h.
X-ray powder diffraction (XRD) patterns were obtained by a Bruker AXS D8 advance powder diffractometer. The scanning electron microscope (SEM) image of the products was examined by a Hitachi, S-4800 microscope. Transmission electron microscopy (TEM) was performed with a HITACH H-600 Electron Microscope. The UV-visible diffuse reflectance spectrum was obtained by Shimadzu UV-2550. PL spectrum measurement was performed in a fluorescence spectrophotometer (Edinburgh, FL920) at room temperature. A reference photocatalyst Bi2O3 was prepared using the method reported in the literature [24]. Photocatalytic activities of Bi2O3-Bi2CrO6 were evaluated by studying degradation of methylic blue (MB) dye. The photocatalytic degradation of MB dye was carried out with 0.2 g of the powdered photocatalyst suspended in 100 mL solution of MB dye prepared by dissolving 20 mg of MB powder in 1 L of distilled water in a Pyrex glass cell at room temperature under air. The optical system for detecting the catalytic reaction included a 300 W Xe arc lamp (focused through a shutter window, Chang Tuo, Beijing) with UV cutoff filter (providing visible light nm).
3. Results and Discussion
A powder XRD pattern (Figure 1) of the product initially suggests the coexistence of Bi2O3 (JCPDS file 16-654) and Bi2CrO6 (JCPDS file 1-738). The peaks in this figure can be indexed to a cubic phase of Bi2O3 (space group Fm-3 m (no. 225)) with lattice constants = 5.66 Å. The system and lattice constants of Bi2CrO6 are not shown in the Joint Committee on Powder Diffraction Standards card. Figure 2 shows SEM images of Bi2O3-Bi2CrO6 nanowire. The diameter of the nanowires is about 30–100 nm, and the length is in the range of micrometers. The morphology of the nanowire is uniform.
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Typical TEM images of Bi2O3-Bi2CrO6 nanowires are shown in Figures 3(a) and 3(b). According to Figure 3(a), the diameter of the nanowires varies from 30 to 100 nm, the length is a few micrometers, which is consistent with SEM results. Figure 3(b) shows TEM micrograph of a typical nanowire junction. The knots can be clearly seen in the nanowire structures. The interval between two knots is not equal and ranges from several ten to hundred of nanometers.
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UV-visible diffuse reflectance spectra of commercial Bi2O3, fabricated Bi2O3 and the composite Bi2O3-Bi2CrO6 nanowires are shown in Figure 4. It can be seen that the absorbance of Bi2O3-Bi2CrO6 nanowire extends to the visible light region. The onset of the adsorption edge is 515 nm in the UV-vis diffuse reflectance spectrum. The nanowire can absorb light with wavelength < 515 nm, covering the region from UV through near visible light in the sunlight, and can be used as a visible light photocatalyst to decompose the organic pollution. Compared with commercial and fabricated Bi2O3, the light absorption of Bi2O3-Bi2CrO6 in 440 nm to 540 nm is strongest. The PL spectrum of the sample is presented in Figure 5. Two peaks, 535 and 651 nm in the PL spectrum can be seen, which meant two recombined semiconductors. Both of them are in visible light region; one is attributed to Bi2O3, and the other is due to Bi2CrO6. The excitation wavelength was set at 430 nm.
To probe the photocatalytic activity of the composite Bi2O3-Bi2CrO6 nanowire in visible part of the solar spectrum, the bleaching of MB was carried out under irradiation of a Xe arc lamp with UV cutoff filter (providing visible light nm). An aliquot (2 mL) of MB solution was removed at interval times and placed in the UV-vis spectrometer for analysis. The results were shown in Figure 6. As shown in Figure 6, the photocatalytic activity of Bi2O3-Bi2CrO6 nanowires is higher than fabricated Bi2O3 and the commercial available Bi2O3; more than 82% of MB molecules were decomposed in 6 h, while the reference photocatalysts Bi2O3 and the commercial available Bi2O3, only 33% and 17% of the MB dye, were decomposed in 6 h, respectively. (As pure Bi2CrO6 phase cannot be obtained, so we just choose the Bi2O3 as the reference photocatalysts.) The valence band of Bi2O3 is composed of O2p, and the conduction band is consisted of Bi5d. The import of Cr element leads to the band positions of Bi2CrO6 different from the band positions of Bi2O3. (Since the pure Bi2CrO6 phase and the crystal structure and lattice constants of Bi2CrO6 cannot be obtained, we cannot use the plane-wave-based density functional method to clarify whether Cr6+ contributed to valence- or conduction-band formation in Bi2CrO6). The photogenerated electrons and holes can transfer between Bi2O3 and Bi2CrO6, the different band positions of Bi2O3 and Bi2CrO6, enhance the separation of photogenerated electrons and holes, leading to higher efficiency in degradation of MB.
The successful fabrication of one-dimensional composite Bi2O3-Bi2CrO6 nanowires can only be realized at the pH value range of 1–5 of starting solution. Comparative experiments were made by adjusting the starting solution’s pH value 6, 7, and 11 while keeping other synthetic parameters in the same condition. Figure 7 shows the XRD pattern of as-synthesized products. The XRD measurements show that the composition and structure changed drastically along with the increasing of the pH value. Below pH 5, the XRD pattern exhibits the product is composed of Bi2O3 (JCPDS file 16–654) and Bi2CrO6 (JCPDS file 1-738). The peaks belonged to Bi2O3 and Bi2CrO6 become weakened till disappeared, and subsequent peaks of newly formed crystals structure start to emerge above pH 5. At pH 11, the sample is mainly composed of Bi2O2.33 (JCPDS file 27-51), Bi(OH)3 (JCPDS file 1-898), and CrOOH (JCPDS file 70-1115). Seen from Figure 7, the pH values of the starting solution greatly affect the fabrication of composite Bi2O3-Bi2CrO6 nanowires, and the composite Bi2O3-Bi2CrO6 nanowires can be obtained at starting solution’s pH value below 5.
In conclusion, one-dimensional Bi2O3-Bi2CrO6 nanowires were synthesized by a simple microwave-assistant hydrothermal method. The diameter of the wires is about 30–100 nm, and the length is in range of micrometers. The sample can absorb visible light ( nm), covering the region from UV through near visible light in the sunlight, showing high efficiency in degradation of MB under visible light irradiation. The work of Bi2O3-Bi2CrO6 nanowires on water splitting is still in process.
Acknowledgments
This work is financially supported by the National Basic Research Program of China (973 Program, Grant 2007CB613302), the National Natural Science Foundation of China under Grants 20973102, 51021062, 51002091, and 21007031. The authors also express thanks for editor’s patience.