Aerosol-Assisted Self-Assembly of Aluminum Borate (Al18B4O33) Nanowires into Three Dimensional Hollow Spherical Architectures Jun Zhang,† Ammar Elsanousi,† Jing Lin,† Yang Huang,† E. M. Elssfah,† Dongfeng Chen,‡ Jianming Gao,† Zhixin Huang,† Xiaoxia Ding,† and Chengcun Tang*,†
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2764–2767
College of Physical Science and Technology, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and China Institute of Atomic Energy, Beijing 102413, People’s Republic of China ReceiVed May 29, 2007; ReVised Manuscript ReceiVed August 2, 2007
ABSTRACT: In this work, we report a new scheme for the generation of 3D hollow spherical architectures with single-crystal aluminum borate nanowires as building blocks, which are in situ formed in liquid droplets by a spray pyrolysis technique. The hollow structures were investigated by SEM and TEM microscopy observations in detail. Our comparative experiments show that the morphologies of the products depended less on the reaction conditions, whereas the crystal structures are related to the reaction temperature. The possible formation mechanism for the 3D hollow architectures has been discussed. As such, with a judicious choice of suitable materials, this technique should be extendable to other systems to directly grow curved, 3D-ordered assemblies built from 1D nanowires. The as-obtained aluminum borate (Al18B4O33) nanomaterials were used as the supports of nickel oxide as the photocatalyst on the degradation of dye brilliant red (X-3B) and showed an excellent ability to improve the photocatalytic activities compared to Al18B4O33 nanorods as supports. Introduction For novel technologies based on nanoscale devices, we need not only to prepare 1D nanomaterials but also try to organize them into designed patterns of configurations, which is of great interest in the areas of chemistry and materials science.1 The simplest synthetic route to the architectures is a self-assembly process based on different driving mechanisms,2 in which ordered aggregates are formed in a spontaneous process. Once such 1D nanostructured building blocks can be ordered and rationally assembled into appropriate two- or three-dimensional architectures, they would provide possibilities to better understand the concept of self-assembly. Moreover, they would offer fundamental scientific opportunities to probe brand-new properties and applications resulting from the spatial orientation and arrangement of the nanocrystals.3 However, compared with this great success in the spatial orientation of nanocrystals, little attention has been devoted to the controlled organization of primary building units into curved hollow nanostructures.4–6 Among the reported methods, a softchemistry route is mostly adopted, so it is difficult to obtain high crystal quality materials under such mild reaction conditions. Specifically, it should be noticed that comparatively little work has been performed on the fabrication of technologically important ternary metal oxide with the curved hollow structures.7 Although spray pyrolysis has been widely used to produce ceramic powders from solutions,8 there is no report about the preparation of spherical architectures assembled from nanowires units via this technology. The present work suggests that it is possible to directly grow curved, hollow, and 3D-ordered assemblies built from 1D nanocrystals through this simple process. Aluminum borate is selected as an example to illustrate our strategy, because aluminum borate is an important ceramic material with a high elastic modulus, low thermal expansion, high melting point, and excellent oxidation-resistant ability,9 * To whom correspondence should be addressed. Fax: 86-27-67861185. E-mail:
[email protected]. † Central China Normal University. ‡ China Institute of Atomic Energy.
as well as a typical self-catalytic growth phenomenon commonly observed in the process of nanocrystal growth.10 Experimental Section Solution Preparation. All chemicals were reagent grade and were used without further purification. The starting solutions were prepared by dissolving the same molar amounts of aluminum nitrate (Al(NO3)3 · 9H2O) and boron trioxide (B2O3) in methanol solution. The concentration of the aluminum nitrate was set to 0.2 M. The solutions were stirred for 5 h using a magnetic stirrer. Preparation of Aluminum Borate Architectures. The solutions were nebulized using an ultrasonic nebulizer operated at 1.7 MHz and then carried into a furnace heated to 1200 °C by an Ar flow. The residence time of the particles in the hot region of the furnace was around 2 s. The product was collected in a percolator with distilled water and then filtered by a fritted glass funnel, washed thoroughly with distilled water and ethanol, and finally dried in a vacuum at 60 °C. Characterization. The crystal structure and phase purity of the product were examined by means of X-ray diffraction (XRD, D/maxrB, Cu KR radiation) analysis. The overview of the sample morphology was checked by scanning electron microscopy (SEM, JSM-6700F, JEOL). The powder for SEM was first dispersed in absolute ethanol by manual shaking and dropped on the copper grid. Then, the sample was covered by the steamy gold after ethanol was volatilized completely. Sample powder was ultrasonically dispersed in absolute ethanol and dropped onto a carbon coated copper grid for transmission electron microscopy (TEM, JEM-2010F, JEOL) measurement. Photocatalytic Experiments. NiO/Al18B4O33 composites were prepared by adding a calculated amount of aluminum borate spherical architectures or nanorods (10 wt % to NiO) to the desired amount of nickel nitrate (Ni(NO3)2 · 6H2O) solutions with the concentration of 0.01 M, which were prepared as follows. The obtained mixed solution was magnetically stirred for 5 h, then dried at 120 °C for 4 h, and finally calcined for 12 h at 550 °C. The photocatalytic reactor consists of two parts: a quartz cell with a circulating water jack and a 500 W highpressure mercury lamp placed inside the quartz cell. In all experiments, the reaction temperature was kept at room temperature to prevent any thermal catalytic effect by using the circulating water jack. The photocatalytic property of NiO/Al18B4O33 was evaluated by decomposition of 50 mg/L of dye brilliant red (X-3B) in aqueous medium. A 0.2 g portion of powder was suspended in 200 mL of X-3B solution. UV illumination was conducted after the suspension was strongly magnetically stirred in the dark for 50 min to reach the adsorption–
10.1021/cg070492s CCC: $37.00 2007 American Chemical Society Published on Web 11/20/2007
Self-Assembly of Al18B4O33 Nanowires
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Figure 1. (a) Low-magnification SEM image of the aluminum borate architectures. (b and c) High-magnification SEM images of special architectures with (b) compactly and (c) loosely arranged nanowire building blocks. (d) SEM image in the presence of a broken spherical shell of a hollow microsphere. desorption equilibrium of X-3B on catalyst surfaces. During irradiation, about 3 mL of suspension was continually taken from the reaction cell at given time intervals. After a specified time, the solid and liquid were separated and a U-3310 UV–vis spectrometer (Hitachi) was used to measure the X-3B concentration in liquid. The total concentrations of all X-3B species were simply determined by the maximum absorption measurement.
Figure 2. XRD pattern of the as-prepared (a) Al18B4O33 and (b) Al4B2O9 spheres.
Results and Discussions Al18B4O33 nanowire-based hollow microsphere assemblies were obtained by the spray pyrolysis of a methanol solution of aluminum nitrate and boron trioxide at 1200 °C. The lowmagnification SEM image (Figure 1a) shows that the as-obtained product is composed of a large amount of well-dispersed spherical particles in high quantity but with a somewhat nonuniform size distribution. The diameters range from several hundred nanometers to several micrometers. Interestingly, the surface of the spheres is covered by many radial nanorods. Higher-magnification SEM observations clearly reveal that the entire structure of these architectures is built from nanowires with diameters in the range of 20-30 nm, self-knitting to form a sphere. The dominant morphology of the architectures is the clewlike sphere, in which the nanowires integrate tightly and few nanowires protrude (Figure 1b). A small part of the spherical architectures is constructed by the low density of nanowires protruding out of the surface (Figure 1c) also accompanying the space with small particles probably due to a relative short duration time in the process of spray pyrolysis. Furthermore, the hollow structure of the architectures could also be confirmed by the SEM observation for an occasionally broken spherical shell as shown in Figure 1d. Figure 2a shows the XRD pattern of the as-prepared product, which can be indexed as an orthorhombic structure with the lattice parameters of a ) 0.769, b ) 1.502, and c ) 0.565 nm, respectively. The pattern matches well with those assigned to Al18B4O33 concerning both the reflection profile and intensity (JCPDS 32-0003). No diffraction peaks from any impurity were observed in the XRD pattern, indicating a high purity and crystallization of the final sample. Figure 3a and b shows the TEM images of the spheres with diameters of about 0.2-1 µm. A relatively dark contrast between the surface and center of the sphere is usually observed. A typical morphology is shown
Figure 3. (a and b) Typical TEM images of the aluminum borate 3D architectures. (c) TEM image of the hollow spherical structure. (d) Highmagnification SEM image of the construction of the nanowires on the surface of a sphere. (e) HRTEM image of a single nanowire extruded from a sphere and the corresponding SAED pattern recorded along the [-210] zone axis.
in Figure 3c, indicating a hollow structure with the shell consisting of the nanowires array. This is in agreement with the SEM observation for the broken spherical shell. The detailed construction of the nanowires on the spherical surface was investigated by a high-magnification SEM image as shown in
2766 Crystal Growth & Design, Vol. 7, No. 12, 2007
Figure 4. (a) Comparison of the photocatalytic degradation of X-3B in the presence of different catalyzers. (b) SEM image of aluminum borate architectures when the concentration of aluminum nitrate was set to 0.5 M. (c–e) SEM images of NiO/Al18B4O33 nanorods (c) and NiO/Al18B4O33 spheres (d and e).
Figure 3d. The nanowires are knitted compactly with each other, exhibiting the integrality and consistency of these spherical architectures generated by a mechanical interaction between nanowires. A typical high-resolution (HR) TEM image taken from an individual nanowire is shown in Figure 3e, displaying the single-crystal nature of Al18B4O33 nanowires. Combined with the selective area electron diffraction (SAED) technique, the axis direction of the Al18B4O33 nanowires is along [001]. The lattice fringes are clearly visible with an interplanar spacing of 0.54 nm along the axis direction, which is in good agreement with the spacing of the (120) planes of Al18B4O33. Further experiments indicate that the formation of the spherical architectures is weakly dependent on the reaction conditions. In terms of the phase diagram of Al2O3-B2O3,11 there are only two stable compounds at ambient pressure, namely Al4B2O9 and Al18B4O33. The Al4B2O9 is a low-temperature stable phase and will be converted to Al18B4O33 at high temperature. The same morphology of spherical architectures assembled from nanowires was obtained when the temperature was decreased to 1050 °C, while the crystal structure was turned to Al4B2O9 (JCPDS 29-10) established by the XRD pattern (Figure 2b). Besides the temperature, the concentration of the precursors also has little influence on the morphology of this kind of architecture. Figure 4b shows the SEM micrograph of the product obtained at 1050 °C when the concentration of aluminum nitrate was changed to 0.5 M. We found that the nanowire-based spheres also could be formed, while there were many impurities on the surface of the spheres. A low yield of powder with the same spherical microstructure could be obtained when the concentration was decreased to 0.05 M.
Zhang et al.
In a typical spray pyrolysis process, the precursor solution is first nebulized by an ultrasonic nebulizer and the stream consisting of droplets suspended in a carrier gas is passed through a tubular furnace. In the furnace, several reactions, such as solvent evaporation and atomic rearrangement, take place in a continuous flow process. Solution aerosol techniques take advantage of many of the available solution chemistries and uniquely control the particle formation environment by compartmentalizing the solution into droplets. Unlike most solution processes, the method integrates the precipitation, calcinations, and sintering stages of powder synthesis into a single continuous process. On the basis of the above discussion, together with our previous works on the growth mechanism of aluminum borate, a possible formation mechanism for this 3D spherical architecture is proposed as follows. It is widely believed that the droplets, when sprayed into a tubular reactor under pyrolysis conditions, serve as microreactors and yield one particle per droplet, whose size determines the product dimension. For a certain atomizer, the droplet characteristics depend on the quality of the atomizer itself, the solution density, surface tension, and viscosity, which directly affect the droplet size and size distribution during atomization. When the droplets are transported into the furnace tube, the surface precipitation of dissolved substances within the droplet occurs during the particle formation, which causes the formation of the hollow structure. Then, abundant nanosized aluminum borate crystals with the phase of Al18B4O33 are formed and interwoven in the surface volume of each droplet. On the basis of the growth feature of aluminum borate nanowires, the formed aluminum borate nanocrystals have the ability to grow into nanowires with the framework of the self-catalytic growth mechanism.10 The nanowires grow and intervein each other at the same time in the confined droplet area, leading to the formation of 3D hollow spherical architectures. As known, photocatalytic degradation attracts increasing attention as a promising technology for the removal of toxic organic and inorganic contaminants from water. In fact, the practical photocatalyst with particle morphology is usually fixed to a support substrate. The substrate not only affords a reasonably high surface area and accessibility of the immobilized catalyst for photodegradation of the contaminants but also solves the problem of the separation of the fine catalyst powder from the treated solution. Taking account of the novel 3D hollow structure and the improved catalytic efficiency using aluminum borate whisker supports for metals or metal oxides,12 we expected that the obtained aluminum borate architectures would be used as a support in photocatalytic reaction. Additionally, the supported catalyst with the size of near micrometers can help the solid/ liquid separation. Herein, we used the aluminum borate materials to investigate their applications as photocatalytic supports of nickel oxide, examined by the degradation of dye brilliant red (X-3B) under irradiation. Comparative experiments were carried out to investigate the photocatalytic activity of the NiO particles with the supports of the as-obtained Al18B4O33 architectures and aluminum borate (Al18B4O33) nanorods prepared by the similar method reported in the literature10b. The results are illustrated in Figure 4a. The states of NiO grains in different supports are shown in Figure 4c–e. It is apparent that under identical conditions, the NiO/Al18B4O33 architecture composites demonstrate much more effective photocatalytic activity in degradading X-3B than the NiO/Al18B4O33 nanorods. The enhancement of the photodegradation rate originates from the Al18B4O33 spheri-
Self-Assembly of Al18B4O33 Nanowires
cal supports which provide a high concentration environment of organic molecules around the catalysts. This means that more effective adsorption sites are obtained and an increase the transfer rate of electron (e-) and hole (h+) to solution species occurs. In addition, the introduction of aluminum borate architectures greatly restrains the growth and conglomeration of NiO nanocrystals and consequently improves the photocatalytic activity. Therefore, the enhanced performance should be attributed to the distinct morphology construction with hollow sphere structure. Conclusions In summary, aluminum borate particles exhibiting a 3D hollow spherical architecture constructed by numerous singlecrystal nanowires have been obtained by the technique of spray pyrolysis. The architecture is principally a consequence of the radial concentration gradients established during sintering of multicomponent spherical droplets and the self-catalytic growth feature. This technique should be extendable to other systems to directly grow curved, 3D-ordered assemblies built from 1D nanowires through such a one-step aerosol-assisted chemical vapor deposition route. The processes are relatively simple, safe, and scalable for mass production. The resulting assemblies not only enrich the family of morphology in the field of nanoscience and nanotechnology but also show an excellent ability as photocatalytic supports. Acknowledgment. The authors gratefully acknowledge the financial support for this project from the Fok Ying Tong Education Foundation (grant no. 91050) and NNSF of China (grant no. 50202007).
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