Orthorhombic Bi2GeO5 Nanobelts: Synthesis, Characterization, and Photocatalytic Properties Ruigen Chen,† Jinhong Bi,† Ling Wu,*,‡ Zhaohui Li,† and Xianzhi Fu*,† Research Institute of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, P. R. China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1775–1779
ReceiVed July 31, 2008; ReVised Manuscript ReceiVed December 4, 2008
ABSTRACT: One-dimensional Bi2GeO5 nanobelts were first directly prepared by a surfactant-templated hydrothermal process. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy were used to characterize the obtained samples. By use of cetyltrimethyl ammonium bromide as the structure-directing template, Bi2GeO5 nanobelts with widths of about 70-180 nm were obtained. The mechanisms related to the morphology control of Bi2GeO5 are proposed and discussed. The UV-vis absorption spectra show that the as-prepared nanomaterials have a strong absorption edge in UV light and that their band gaps are somewhat relevant to the size and morphology. As a novel photocatalyst, the prepared Bi2GeO5 samples exhibit relatively high photocatalytic activity for the decomposition of azo dye methyl orange under UV irradiations. The samples obtained under different amounts of cetyltrimethyl ammonium bromide exhibited different photocatalytic performances. The effects of the crystallinity, specific surface area and morphology of the samples on the photocatalytic activities are also discussed.
1. Introduction It is known that low-dimensional nanostructured materials exhibit interesting and useful characteristics owing to shapespecific and quantum size effects. As a new family of onedimensional (1D) nanostructures with unique properties, inorganic nanobelts have recently attracted a lot of research interest since their discovery in 2001,1 because these distinctive geometric structures with a rectangular cross-section and welldefined faceted surfaces make the belts an ideal system not only for fully understanding dimensionally confined transport phenomena on the nanometer scale but also for building functional devices along individual nanobelts. The prospect of using nanostructured objects as components in nanotechnology has led researchers to explore the design of a great variety of interesting materials with tailored morphologies.1-3 Recently, numerous synthetic strategies have been developed for the fabrication of beltlike nanostructures. BaWO4 nanobelts were obtained in cationic reverse micelles.4 Monoclinic NH4V3O8 single-crystalline nanobelts have been synthesized at large scale in an ammonium metavanadate solution by a templates/catalysts-free route.5 Ga2O3 nanoribbons,6 SnO2 nanobelts7 were successfully synthesized by a simple thermal evaporation process. Helical titanium dioxide nanoribbons were prepared by using an organogel template.8 In addition, wet chemical methods, due to the low cost and potential for scaleup, have been developed to prepare beltlike nanostructures, such as Te9 and MoO3.10 Aurivillius-phase Bi2GeO5 (BGO) belonging to a Bi2O3GeO2 system has recently received much attention. It belongs to the orthorhombic system and space group Cmc21. The common structural peculiarity for this compound is [Bi2O2] layers separated from each other by [GeO4] tetrahedrons, and has been applied as promising catalysts for oxidative coupling of methane (OCM).11 Moreover, BGO is a material with * Corresponding author. E-mail:
[email protected] (L.W.);
[email protected] (X.F.). Tel/Fax: 86-591-83738608. † Fuzhou University. ‡ Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.
relatively good dielectric and ferroelectric properties. Despite these interesting properties, few reports have dealt with nanosized BGO materials. To date, the synthesis method for BGO has been focused on spontaneous crystallization of the overcool melt having the Bi2O3-GeO2 composition in platinum crucibles.12 Unfortunately, some impurities, such as Bi4(GeO4)3, Bi2Ge3O9, were obtained spontaneously as the stable phases because of the metastable property of BGO. This drawback made it difficult for the large-scale industrial applications. Therefore, methods for the synthesis of pure and nanosized Bi2GeO5 proves to be intriguing and valuable. Up to now, environmentally friendly aqueous processes have received much attention.13-15 This includes the processes in which materials are synthesized under mild conditions in nonharmful solvents such as water. This process has been applied to the syntheses of metastable compounds in aqueous solutions and the fabrication of thin films of various singlecrystal compounds.16-18 Taking hydrothermal methods as an example, this approach offers many advantages over conventional solid-state methods, such as mild synthesis conditions, highly crystallized and highly pure powders with narrow sizedistribution.19 Also, it has long been established that surfactant molecules which are useful as versatile soft templates can form different conformations by self-assembly and lead to the formation of different nanostructures.20,21 By taking advantages of these fascinating structures, designing of the different nanoscale quantum devices becomes feasible. Here we report on a shape-controlled synthesis of BGO through a simple solution chemistry route. Using a facile hydrothermal method and surfactant template technique, one-dimensional BGO nanobelts are obtained by controlling the reaction conditions. In addition, photocatalysis, as one of the most advanced oxidation technologies, has been the focus of numerous investigations because of its application for the quantitative destruction of undesirable chemical contaminants in water and air.22-25 However, most of researchers pay attention to traditional photocatalysts, such as TiO2. The search for novel heterogeneous photocatalysts with high performance is limited. Recently, the reports of some new photocatalysts with various morphologies have attracted much attention, for example, Bi2WO626,27 and
10.1021/cg800842f CCC: $40.75 2009 American Chemical Society Published on Web 02/02/2009
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Bi2MoO628 nanoplates, Zn2GeO429 nanorods and NiGa2O430 microcubes. Therefore, it is rather valuable to evaluate the photocatalytic activity of the BGO nanostructures. In the present work, methyl orange photodegradations were employed as probe reactions to evaluate the photocatalytic activity of the asprepared BGO nanostructures under the irradiation of UV light. As far as we know, this is the first fruitful attempt to observe the photoactivity of BGO catalyst.
2. Experimental Procedures 2.1. Catalyst Preparation. All of the reagents were analytical grade and used without further purification. In a typical procedure, a solution of 75 mL was prepared by dissolving 4.2 mmol of Bi(NO3)3 · 5H2O, 2.1 mmol of GeO2, and 0.4 mmol of cetyltrimethyl ammonium bromide (CTAB) in distilled water. Under stirring, the pH value of the mixture was adjusted to 9 by dripping 5% NH3 · H2O. After being stirred for 20 min, the mixture was transferred into a Teflon-lined autoclave with a capacity of 100 mL. Then, the autoclave was sealed into a stainless steel tank and was heated under autogenous pressure at 180 °C for 48 h. As the autoclave cooled to room temperature, the precipitation was separated by centrifugation, washed with distilled water and absolute ethanol, and dried at 80 °C for 6 h. The other samples were prepared by a similar procedure, except for the different CTAB concentrations, and the samples obtained under various amounts of CTAB (x ) 0.0, 0.4, 0.7, 1.0 mmol) were labeled for samples S0, S1, S2, and S3, respectively. 2.2. Characterization. The as-prepared samples were characterized by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer with Ni-filtered Cu KR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA. Data were recorded at a 2θ scan rate of 0.02° s-1 in the 2θ range of 10° to 80°. The crystallite size was calculated from X-ray line broadening via the Scherrer equation: D ) 0.89λ/β cos θ, where D is the crystal size in nm, λ the Cu KR1 wavelength (0.15406 nm), β the half-width of the peak in radians, and θ is the corresponding diffraction angle. The Brunauer-Emmett-Teller (BET) surface areas were determined by nitrogen adsorption-desorption isotherm measurements at 77 K with a ASAP2020 M apparatus (Micromeritics Instrument Corp.). Morphology of the samples was characterized by field emission scanning electron microscopy (SEM) (JSM-6700F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed using a JEOL JEM 2010 EX instrument at an accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated, and a drop was dripped onto the support film. UV-vis diffuse reflectance spectra (UV-vis DRS) of the samples were recorded between the 200 and 800 nm range at room temperature on a UV-vis spectrophotometer (Lambda-900, Perkin-Elmer). Barium sulfate was used as a referent. 2.3. Photocatalytic Activity Evaluation. Photocatalytic reactions were performed in a quartz tube with 4 cm inner diameter and 17.5 cm length. Three 4W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4T5) were used as the illuminating source. 160 mg of powdered photocatalysts was suspended in 160 mL of MO aqueous solution (20 mg/L) and stirred for 2 h before irradiation to ensure the reach of the adsorption/desorption equilibrium. At given irradiation time intervals, 3 mL of the suspension was collected at 20 min intervals during the experiment, and then centrifuged to remove the photocatalyst. The degraded solution was analyzed using a Varian Cary 50 Scan UV-vis spectrophotometer and the absorption peak at 464 nm was monitored. The percentage of degradation is reported as C/C0. C was the absorption of MO at each irradiated time interval of the main peak of the absorption spectrum at wavelength 464 nm; the band that is associated with the azo bond (-NdN-) was used to monitor the effect of the photocatalysis on the degradation of MO. C0 was the absorption of the starting concentration when adsorption/desorption equilibrium was achieved.
Figure 1. XRD patterns of Bi2GeO5 samples. Table 1. Crystallite Size and BET Surface Area of Bi2GeO5 Samples Synthesized by the Facile Hydrothermal Process sample name
CTAB (mmol)
crystallite size (nm)
BET (m2 g-1)
K (min-1)
R
S0 S1 S2 S3
0.0 0.4 0.7 1.0
46 42 39 57
4.3 6.1 6.8 2.1
0.0028 0.0079 0.0055 0.0048
0.9889 0.997 0.9994 0.9981
3. Results and Discussion 3.1. XRD and BET Analyses. The phase and crystallographic structure of the products were determined by powder X-ray diffraction (XRD), as shown in Figure 1. It can be found that the XRD patterns of all prepared samples present similar profiles. Eight distinctive peaks at 11.28°, 23.86°, 28.88°, 32.65°, 33.34°, 34.33°, 47.35°, and 58.15° were observed. These peaks match well with the (200), (1j11), (3j11), (020), (002), (600), (022), and (8j02) crystal planes of orthorhombic Bi2GeO5 (JCPDS 78-1334). These indicate that the samples exist in the orthorhombic phase. It is noted that the corresponding intensities of a few diffraction peaks among the products are dissimilar in some cases. For example, the peaks of (200) and (3j11) for sample S3 are stronger than those of any others. This may be attributed to their differences in morphology and microstructure. Furthermore, the intensity of the (600) peak in the XRD pattern of sample S2 is particularly strong. The average crystallite sizes calculated via the Scherrer equation from the (131) peak (2θ ) 28.3°) and the specific surface areas for different samples are shown in Table 1. It is observed that the crystallite sizes and the specific surface areas could be changed by adjusting the concentration of CTAB. For samples with different CTAB content, the crystallite sizes are on the order of S3 > S0 > S1 > S2, and the corresponding specific surface areas are on the contrary. It is assumed that a small amount of CTAB species is responsible for the lowering the crystallite sizes and increasing the specific surface areas. 3.2. Morphology. The morphologies of the synthesized products were studied by SEM, which are shown in Figure 2a-d. To investigate the effects of surfactants on the structure and morphology of the products, contrastive experiments were conducted. When the experiment was carried out in the absence of CTAB, the resulting products consist of a large quantity of nanoflakes, as shown in Figure 2a. From Figure 2b,c, nanoflakes morphology with nanobelts are formed with the addition of 0.4 mmol of CTAB, while the products consist almost entirely of nanobelts when 0.7 mmol of the surfactant is used. The typical widths of the nanobelts are in the range of about 70-180 nm. Moreover, when the CTAB addition is over
Orthorhombic Bi2GeO5 Nanobelts
Figure 2. SEM images of as-prepared samples: (a) S0; (b) S1; (c) S2; (d) S3.
Figure 3. The structure characterizations of sample S2: (a) TEM; (b) HRTEM image and (inset) its SAED pattern; (c) EDX spectrum.
1.0 mmol, as the Figure 2d shown, the perfect beltlike nanostructures have been destroyed, but several thicker and shorter nanobelts are obtained, accompanied by some misshapen flakes. All of these results suggest that the relative concentrations of CTAB need to be controlled to optimize the growth of nanobelts with a particular size range. The structure of nanobelts synthesized in the presence of CTAB (sample S2) was further examined with transmission electron microscopy (TEM) and high-resolution TEM. Figure 3a shows the typical TEM image of the obtained nanobelts with uniform size. The nanobelts have widths of around 150 nm and lengths of several micrometers. The rectangle-like cross section of the materials is also clearly observed. Figure 3b shows a HRTEM image of a nanobelt. The fringes of d ) 0.26 nm match
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that of the (600) crystallographic plane. The selected area electron diffraction (SAED) pattern (inset of Figure 3b) can be attributed to a single-crystalline nature of BGO nanobelts with growth direction along [110]. Energy-dispersive X-ray (EDX) analysis shows that only Bi, Ge, and O are contained in the samples. The absence of bromide and nitrogen in the resulting product indicates nearly complete removal of the surfactant. The elements of Cu and C are generated from the supporting carboncoated copper meshes. On the basis of previous work31,32 and our experimental observation, possible functions for CTAB in the present synthetic method can be explained as follows: the surfactant generates large numbers of rodlike micellar in aqueous solution, which may act as soft-templates for the formation of 1D nanostructures as well as stabilize the 1D nanostructures.33 3.3. Possible Formation Mechanism of the Bi2GeO5 Nanobelts. Throughout the whole experiments, a schematic diagram of the proposed growth mechanism is shown in Figure 4. In the hydrothermal process without CTAB, there are no active sites around the circumference of BGO nuclei. So, these BGO nanoparticles will attach to each other to lower their surface energy. Consequently, irregular BGO nanoflakes are obtained. Under the low CTAB condition, a small amount of capsules of CTAB are generated in the solution. Because of the coulomb force action, the strong attractive interactions between CTAB and the inorganic surface can arrest nucleation and change the shape and size of the primary clusters.34 The hydrophobic interactions and van der Waals attraction between surfactant molecules on adjacent nuclei then drive the assembly among these nanoparticles. Thus, longitudinal growth is dominant, and the BGO nucleus subsequently grows by aggregation in a certain direction to form nanobelts, except for the smaller nanoflakes. When CTAB was introduced in appropriate amounts, the initially formed nanoparticles were relatively small in dimension, and well dispersed in the solution. The selective adsorption of CTAB on specific crystal faces promotes BGO crystals to grow along the [110] direction forming the nanobelts. If a large enough amount of CTAB is added, small exchangeable water molecules in the emulsions may make the fuse rates between two adjacent nuclei very slow, and the growth along the [110] plane is impeded. At the same time, the higher precursor concentration will aggravate the agglomeration of the BGO nuclei, leading to the increase in thickness for the nanobelts. Moreover, particles with irregular morphology are also obtained. It has been demonstrated that experimental parameters including kinetic energy barrier, reaction temperature, time, reactant concentration, and solvent or precursor, could be manipulated to influence the growth pattern of nanocrystals. The present work is mainly focused on the control of BGO crystals by adjusting the concentration of surfactant. So, the detailed formation mechanism of BGO nanobelts in this hydrothermal process needs further investigation. 3.4. UV-vis DRS. Diffuse reflectance spectroscopy gives information about the energy structures and optical properties of semiconductor nanocrystals, and, therefore, the band gap energy of the semiconductor can be estimated. The photoabsorption ability of as prepared samples detected by UV-vis DRS are shown in Figure 5. The wavelength at the absorption edge, λ, was determined as the intercept on the wavelength axis for a tangential line drawn on the absorption spectra. All the samples show strong absorption in the UV light region around 350 nm. The steep shape of the spectra indicated that the light absorption was not due to the transition from the impurity level
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Figure 4. UV-vis diffuse reflectance of as-prepared Bi2GeO5 samples.
Figure 5. Schematic illustration of the possible growth mechanism of Bi2GeO5 nanostructure.
Figure 6. Concentration changes of MO at 464 nm as a function of light irradiation time in the presence of photocatalysts.
but was due to the band gap transition. From Figure 5, three band gap values for the samples of S1, S2, and S3 are slightly greater than the value for the sample of S0. The blue shift in the band gap can be attributed to quantum confinement effects because of the small size regime.35 3.5. Photocatalytic Activities. Figure 6 displays the concentration changes of MO at 464 nm as a function of irradiation time during the degradation process in aqueous solution in the
presence of BGO samples. In addition to experiments with BGO, a blank experiment in the absence of the photocatalyst but under UV irradiation showed that only a small quantity of MO was degraded. Obviously, in comparison with the sample S0, all the BGO samples modified with CTAB increase the percentage of degradation (for S0, 33% MO degradation, whereas after the modification of CTAB, 99% MO degradation was observed for S1, 62% for S2, and 55% for S3 under identical conditions) after 140 min UV irradiation, although they had similar band gap energies with the sample of S0. This may be due to the addition of a small amount of CTAB that decreases the crystallite size of Bi2GeO5, which facilitates a higher percentage of degradation. The reason may be the so-called particle size quantization effect.36 Moreover, the nanobelt structures favor the movement or transfer of electrons and holes generated inside the crystal to the surface,37 which also helps to enhance the photocatalytic activity of Bi2GeO5 nanobelt arrays. It can be revealed that the samples with different CTAB concentrations are dissimilar in photocatalytic performances. This difference may be explained in terms of the structural and physicochemical properties of the materials, such as nanostructures, band structures, crystallinity, surface areas, and so on. Sample S1 has the highest photocatalytic activity, which is about three times than that of S0. This may be due to the relatively large surface area and better crystallization. However, sample S2 also shows a relatively large surface area, but its crystallization is imperfect. This imperfect crystallization is considered to favorably increase the probability of electron-hole recombination.38 Samples S3 possess relatively highest perfect crystallization, but its surface area is smaller than others; therefore, its photocatalytic activity is lower than that of samples S1and S2. Therefore, as an overall effect of surface area and appropriate crystallinity, sample S1 prepared by the modification of 0.4 mmol of CTAB shows the most promising photoactivity. It is interesting to find that the sample S1, which has mixed nanofalkes and nanobelts, has relatively higher photocatalytic activity than the sample S2 with pure nanobelts morphology. Some unknown positive effects may exist on the interface between individual nanoflake and nanobelt. In addition, compared with sample S0 with pure nanoflakes morphology, the higher activity is also achieved on sample S3 with mixed morphology, although it has much smaller BET surface areas. This further confirms the existence of inherent interaction between nanoflakes and nanobelts. Further research is underway to clarify this kind of hybrid effect.
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Acknowledgment. The work was supported by National Natural Science Foundation of China (20571015, 20777011, and 20537010), National Key Basic Research Program of China (973 Program: 2007CB613306 and 2008CB617507), and grants from Natural Science Foundation of Fujian Province (E0510011) and Scientific Project of Fujian Province (2005HZ1008), China.
References
Figure 7. Kinetics of MO decolorization in solutions.
To detail analysis of the photocatalysis kinetics of the MO degradation in our experiments, we applied the pseudo-firstorder model as expressed by eq 1, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low.39
ln(C0/C) ) kt
(1)
where C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. Figure 7 is the photocatalytic reaction kinetics of MO degradation in solution on the basis of the data plotted in Figure 6. The rate constants obtained from the regression lines in Figure 7 are also listed in Table 1. As it can be seen, a rather good correlation to the pseudo-first-order reaction kinetics (R > 0.98) was found. It shows more clearly that the S1 sample has the highest MO removal rate.
4. Conclusions In summary, one-dimensional Bi2GeO5 nanobelts with layered structures were successfully synthesized by a surfactanttemplated hydrothermal process for the first time. A phase formation mechanism and morphology-controlled process also were proposed and discussed on the basis of experimental data. The UV-vis absorption spectra show that the as-prepared nanomaterials have strong absorption in UV light and that their band gaps are somewhat relevant to the size and morphology. Photocatalytic evaluation, via the decomposition of MO (20 mg/ L) under UV-light irradiation (λ ) 254 nm), reveals that Bi2GeO5 samples modified by CTAB exhibit better photocatalytic performances than those without the modification of CTAB. Among them, sample S1 with the morphology of nanobelts shows optimal photocatalytic activity because of the relatively large surface area, better crystallinity, and favorable transfer of electrons and holes generated inside the crystal to the surface. The results obtained in the present study indicate that the novel Bi2GeO5 material may serve as a promising efficient photocatalyst in water purification and may find potential application in related fields. Further work is under way to study the ferroelectric and dielectric properties of BGO nanobelts, as well as the possibility of synthesizing other nanobelts.
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CG800842F
