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Maize Phytoliths and Photoluminescent Silica Nanotubes Prepared from a Natural Silica Resource Congyun Zhang, Haitao Kang, Kai Lv, Hui Chen, and Shiling Yuan* Key Laboratory for Colloid and Interface Chemistry, Shandong University, No. 27, Shanda South Road, Jinan 250100, China ABSTRACT: Maize leaves and sheaths are cheap agricultural byproducts that contain an abundance of amorphous hydrated silica (named phytoliths). However, there have been no attempts at utilizing the phytoliths to synthesize silica nanotubes. In this paper, we describe the morphologies and microstructures of phytoliths in leaves and sheaths of maize, and synthesize hollow silica nanotubes using the phytoliths in cetyltrimethyl ammonium bromide (CTAB) /sodium dodecyl benzene sulfonate (SDBS) surfactant system. TEM and SEM images show that the phytoliths exist in cross and dumbbell shapes in maize. The silica nanotubes obtained from the naturally deposited phytoliths exhibit special blue photoluminescence. This optical characteristic indicates that the agro-based waste materials may have potential applications in the fields of light localization and other integrated optical devices.
1. INTRODUCTION Silicon is ubiquitous and the second most prominent element in the Earth's crust and soil. It is significant to maintain the process of plant growth. The aqueous silicic acid (Si(OH)4) transports along the transpiration stream, polymerizes, and then deposits in the cell lumens, cell walls, and intercellular space or external layers. Generally, they are all named as phytoliths. Rice and maize, which belong to the Gramineae family, contain substantial amounts of phytoliths deposited in their husks, stalks, and leaves. Rice husk ash contains ∼92% 97% amorphous silica after calcination,1,2 which can be considered as important highgrade amorphous silica, useful for preparation of nanostructured silica.3 Chiarakorn et al. synthesized mesoporous materials MCM-41 from rice husk ash and studied the effect of different functional silanes on modifying their surface.4 The mesoporous silica SBA-15 with highly ordered hexagonal pore-arrangement, as well as other mesoporous silica particles with controlled morphology, have also been prepared from phytoliths.5 7 Moreover, some researchers have attempted to biotransform the enormous amount of amorphous silica present in plants into high value crystalline silica in the form of 2 6-nm quasispherical nanoparticles at room temperature;8 some other reports showed that ZSM-5 and sodium X zeolite can be synthesized from rice husk ash.9,10 However, maize, containing an enormous amount of silica, has not been focused on yet due to geographical restrictions. Their ashes, which contain above 80 wt % silica and minor amounts of metallic elements, may serve as one of the most economic sources of nanoscale silica in the future.11,17 Recently, the fabrication of silica nanostructures has focused on the self-assembly method which involves interactions of silica precursors and soft templates. Silica nanotubes were prepared by addition of an ammonium hydroxide solution to ethanol/tetraethyl orthosilicate (TEOS) in an organic-crystalassisted self-assembly system. Jang and Banerjee reported that the reverse-microemulsion-mediated sol gel method and reverse-micelle sol gel method were explored to fabricate sizetunable silica nanotubes and photoluminescent silica nanomaterials, respectively.12,13 Wu synthesized chiral mesoporous silica nanotubes by the self-assembly of an achiral surfactant r 2011 American Chemical Society
sodium dodecyl sulfate (SDS).14 Basically, sol gel technique and TEOS (i.e., silica source) are widely used in these fabrication processes by the self-assembly methods. However, there have not been recent attempts at fabrication of silica nanotubes from phytoliths through the self-assembly method. One of our objectives is to investigate the arrangement, morphologies, and nanostructures of phytoliths in maize by the methods of scaning electron microscopy (SEM) and optical microscopy; the other objective is to apply a novel approach for the synthesis of nanostructure materials from phytoliths using selfassembly method and to investigate the photoluminescent (PL) nature of these nanotubes. The hollow nanotubes of silicon oxide were fabricated by utilizing the silica powder obtained from maize plant ashes as silica precursor instead of TEOS. The synthesis of silica nanotubes was performed in an aqueous catanionic surfactant system, composed of cetyltrimethyl ammonium bromide (CTAB)/sodium dodecyl benzene sulfonate (SDBS). The resultant samples possess luminescent nature due to some intrinsic or extrinsic defects. Compared with other approaches,22 the PL silica nanotubes were synthesized in mild reaction system. Maize leaves and sheaths, which are always treated as agricultural waste, can harbor abundant phytoliths. Therefore, to utilize the waste is very economic; the required chemicals, such as CTAB, SDBS, and hydrochloric acid, are cheap, and the prepared solutions are all at low concentration. Furthermore, such PL characteristics have great potential application in light localization or nanointerconnections for future integrated optical devices. More importantly, our novel approach provides a facile way to fabricate new inorganic nanomaterials with potential applications in the fields of optoelectronic devices, lower dimensional waveguide, and optical sensors.
2. EXPERIMENTAL SECTION 2.1. Raw Material Preparation. The maize for the present study was supplied by Cangzhou Region (Hebei province, Received: February 26, 2011 Accepted: September 29, 2011 Revised: September 24, 2011 Published: September 29, 2011 12521
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Industrial & Engineering Chemistry Research China). CTAB and SDBS were purchased from Tianjin Damao Chemical Reagents Factory (China) and Tianjin Kermel Chemical Reagent Co. Ltd. (China), respectively. Hydrochloric acid was obtained from Laiyang Chemical Reagent Co. Ltd. (China). All reagents were of analytical grade. The purified leaves and sheaths of maize samples were treated with 1 mol/L dilute hydrochloric to eliminate metallic impurities, at a ratio of 50 g of samples in 1 L of hydrochloric solution. After leaching, the samples were washed thoroughly with distilled water and then dried at 100 °C. The cleaned samples were treated with two methods: the samples were either burned in a porcelain crucible inside a muffle furnace at 550 °C for 4 h or subjected to acid treatment. After that, the samples were kept at 60 70 °C for 24 h in a 4:1 mixture of concentrated nitric sulphuric acids followed by centrifugation and washing with deionized water.15 2.2. Synthesis of Silica Nanotubes. The appropriate amount of ash was stirred with 10% w/v sodium hydroxide solution for 1 h to completely convert the phytoliths to sodium silicate.7 Sodium silicate solution was stirred mechanically and 1 mol/L hydrochloric acid was added to adjust pH to 9.0 10.0 and to produce soft gel (named Solution A). Then vesicular solutions were obtained by mixing the appropriate amounts of CTAB and SDBS surfactant solutions (named Solution B). Finally, Solution A was added to solution B with agitation and stirred for two days at room temperature. The obtained mixture was transferred to a polyethylene autoclave in which a hydrothermal reaction was carried out at 100 °C for 24 h. The resultant products were collected by centrifugation, washed several times with distilled water, thoroughly dried, and calcined at 550 °C for 5 h. 2.3. Characterization of Materials. The formed nanostructures were observed with an optical microscope (model CX31, Olympus, Japan) and scanning electron microscopy (SEM, JEOL JSM-7600F). Energy dispersive X-ray spectroscopy (EDX) was performed on the same SEM instrument with EDX spectroscopy (model Horiba EX-450), operating voltage was 15 kV. Transmission electron microscope (TEM) images were obtained on a JEOL 100CX-II microscope operating at 100 kV. For TEM observation, the samples were ultrasonically dispersed in ethanol and a drop of the dispersion was deposited on a Formvar-covered Cu grid. Thermal analysis was performed on an analytical balance (model AL204, Mettler Toledo, Switzerland). Approximately 2.36 mg of sample was placed in a porcelain crucible in a muffle furnace (model SL, Shanghai, China) for analyses at different temperatures. The heating rate was 2 °C/min under normal pressure. The nanotubes produced in different concentrations of surfactants systems were monitored by Fourier transform infrared spectroscopy (FTIR). Samples for FTIR were taken into KBr pellets and analyzed on a VERTEX-70 Bruker AG instrument. The emission spectrum of silica nanotubes was determined with a LS-55 fluorescence spectrophotometer. The excitation wavelength was 350 nm with a bandwidth of 5 nm.
3. RESULTS AND DISCUSSION 3.1. Thermal Gravimetric Analysis. The thermal gravimetric (TG) and differential thermogravimetry (DTG) analysis obtained at a heating rate of 2 °C/min (Figure 1) show the weight loss from 200 to 800 °C under air atmosphere.16 The thermal decomposition of maize leaves occurs through two degradation steps. The first reaction stage can be attributed to the loss of volatile substances. Celluloses hemicellulose and lignin are decomposed into intermediate, organic material of smaller molecular mass accompanied by the volatilization of gas product in the form of water and carbon
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Figure 1. TG analysis and DTG analysis curves of maize leaves (heating rate 2 °C/min; air flow rate 50 mL/min).
Figure 2. Optical micrograph of phytoliths in maize after combustion at 550 °C. The inset in A (a, b, c) correspond to high magnification images of phytoliths in leaves shown as the arrow. The illustration d is the magnification images of phytoliths in sheaths (text for details). In c and d: the bar is 10 μm.
dioxide. The corresponding DTG curve represents a maximum mass loss rate at 323 °C in the first stage reaction. The second stage of the thermal decomposition is ascribed to the further oxidation of carbon in the residual intermediate and organic material. The DTG indicates that the maximum decomposition rate of residual carbon appears around 510 °C in the second stage. The sample weight is stabilized after a total weight loss of 80% at 600 °C. The remaining 20% consists mostly of amorphous silica particles, a small quantity of metallic oxidation, and difficult volatile organic carbon. The following should be pointed out regarding formation of the difficult volatile organic carbon: Water-soluble silicic acid is transported along the transpiration stream and deposited in cells and intercellular spaces in the form of phytoliths. During the process of polymerization and deposit, some organic carbon such as hemicellulose and lignin is embedded in the phytoliths. Under the protection of silica shell, this kind of carbon cannot be removed thoroughly. In addition, it is worth mentioning that the TG curve is increased marginally in the initial stage due to the oxidation of metal impurities in the sample.3 Metal always enters the plant through its root in a form of soluble inorganic salt, and then moves to the outer surface accompanied with the soluble silicate acid by photosynthesis. 3.2. Structure of Phytoliths. Although the sample using acid treatment or heat treatment still contains a small amount of impurities such as metallic oxides and difficult volatile organic 12522
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Figure 3. SEM micrographs (images A and B) of leaves after acid treatment and corresponding EDX spectra from the field views shown at the cross.
carbon, most of the rest is high-purity amorphous silica according to our previous works.11,17 Figure 2 shows photomicrographs of phytoliths in the leaves (Figure 2A) and sheaths (Figure 2B). We note that the fragments of leaves and sheaths still retain the morphology of epidermal cells with a different morphology of phytoliths after calcination treatment, indicating that the microscopic particles of hydrated silica were deposited in intracellular or intercellular cells of living plant. In epidermal cells of leaves (Figure 2A), there are two types of cells. One is a long cell with a length of around 70 85 μm and a width of about 12 17 μm, indicating smooth and sinuous elongate forms, and the other is a short cell with 15 20 μm length and 5 10 μm width. Normally both kinds of cells are formed alternatively in the leaves (Figure 2). In fact, the short cell also can be considered as silica cell a type of phytoliths with characteristic morphology. In the plant, the morphology of
phytoliths is affected by variables such as the species of plant, climate, and biological environment. For example, the phytolith is characterized by a spherical shape in wheat straw,17 although the wheat and maize belong to the same Gramineae plant. Because of their specific morphological characteristics and stable resistant siliceous structures which are released from decayed, burned, or digested plant tissues, phytoliths analysis is important in archeology and palaeoenvieronmental studies.18,19 In addition, it has also emerged as a relevant tool applied in investigating plant and subsistence patterns in regions.20 In Figure 2A, the phytoliths have different morphologies in the epidermis of one leaf. Two dominant forms exist, i.e., dumbell shape (illustration a) and cross shape (illustration b). Furthermore, they appear independently along the direction of the long cells. There is much ambiguity about the dominant factor of the characteristic morphology. However, we think that the dumbbell 12523
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Figure 4. SEM (A and B) and TEM (C and D) images of calcined silica nanostructures synthesized with different CTAB/SDBS molar ratios. Samples A and C, 0.028 mol/L; samples B and D, 0.085 mol/L.
phytolith is the initial hydrated silica deposited in the short cell with the growth of plants, and the crossed phytolith is the mature short cell. In the epidermis of sheaths (Figure 2B), the major portion of the silica cells are cross shape, and most emerge together. Hence, the role of the sheaths is to support the leaves to avoid folding with the growth of plants, and the connected crosslike phytoliths can make sheaths stronger under different weather conditions. For the dumbbell or cross phytoliths in the epidermis of leaves and sheaths, we noted that most of cross and dumbbell phytoliths are transparent in leaves. These types of phytoliths commonly have an attachment scar on the dominantly psilate surface in the form of one or several pits (illustration a, b), while some of the phytoliths representing darker forms contain more carbon-filled inclusions, appearing as radial patterns (illustration d).21 Because the materials are found in most of the silica cells, it should be a universal phenomenon. As in our previous work about the sphere phytoliths in the wheat straw,17 we conclude that the dumbbell or cross phytoliths are typically showing a core shell structure. Under combustion or acid treatment, some organic materials are destroyed, however, other organic ones still remain inside the phytoliths under the protection of the outer silica shell. Thus, the core formed is about organic materials (i.e., the black materials in the illustration d in Figure 2B), and the shell is silica. To further investigate the morphology and microstructure composition in the maize, a leaf fragment was imaged by SEM and further confirmation was done by EDX analysis. Figure 3A and B show lower ( scale 10 μm, magnification 600) and higher magnification (scale 60 nm, magnification 15 000) SEM images recorded from the acid-treated maize leaves, respectively. In Figure 3A, the microstructure of phytoliths is clearly observed after the decomposition of organic impurity. Figure 3B shows that the phytoliths represent regular spherical shapes with the diameter of 20 30 nm in nanoscale. The silica cells (phytoliths with typical morphology: cross or dumbbell phytoliths) arrange alternatively with the long cells. The SEM images and corresponding EDX spectra are shown in Figure 3a d. The EDX spectra in Figure 3a d indicate the elements signal in cross-shape phytoliths, stomata, intercellular space, and cell wall (field views shown at the cross), respectively. The crossshape phytoliths and cell wall only consist of Si and O elements, (Figure 3a, 3d), while some organic carbon exists in stomata (Figure 3b) and at the cell borders (Figure 3c).
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Figure 5. FTIR spectra of silica nanotubes synthesized by using the CTAB/SDBS surfactant systems as templates after calcination at 550 °C.
From the SEM images and EDX spectra we can conclude that in addition to the silica-cells, silicon biomineralization also occurs in other tissues such as in epidermic long cells, stomata (as shown by an arrow), and so on. The silica extracted from the maize plants is abundant and of high-purity, available for commercial application widely. 3.3. Preparation of Nanotube. Figure 4 shows the samples produced in colloidal self-assemblies of surfactant H2O solution with different concentrations under hydrothermal condition. From the SEM and TEM images, it can be shown that all of these materials have a twisted rodlike morphology after calcinations. The nanotubes appear with smooth surface and ordered fringes. The outer diameter is approximately 70 100 nm, and the inner diameter ranges from 30 to 50 nm. (Image E is the magnified section of image B). They look like corals (Figure 4D), similar to the samples reported previously.22 The SEM images exhibit clearly that rodlike nanotubes with similar diameters can be formed at different concentrations of the mixtures of CTAB and SDBS solution, which are intertwining and interlacing to form a silica nanostructured network. Irregular spherical silica nanostructures were only observed at low concentrations, whereas no silica nanostructures were obtained below 0.085 mol/L concentration of surfactant mixtures in this stirring condition. Such results can be ascribed to the shear stress exerted on the micelle template under stirring. The shear force decreases the number of surfactant molecules participating in rodlike micelle formation. The initial hypothesis is to utilize vesicles as templates to synthesize silica nanosphere.23 The mixture of surfactant CTAB and SDBS appears transparent blue at certain molar proportion, which indicates vesicular spontaneous formation.24 Under the different experimental conditions, such as the temperature, the presence of metal cations, and the mechanical agitation, the selfassemblies of surfactant micelle solution adopt different configurations. On the basis of experimental results, a plausible mechanism for the formation of nanotubes is proposed. The determining factor resulting in the disappearance of vesicle in solution is the hydrothermal reaction. The hydrothermal temperature at 100 °C induces a phase transition from spherical micelles to interconnected cylinders: the spherical micelles grow in one direction and convert to wormlike micelles which are intertwining or forming network nanostructures with branches. To better understand the effect of synthetic variables on the formation of nanotubes, a series of 12524
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Figure 6. SEM micrograph (A) and corresponding EDX spectra (b and c).
experiments was undertaken with changing reaction parameters. It is noticed that no silica nanotubes were obtained without hydrothermal reaction. The initial step is that the surfactant molecules organize independently, and then the siliceous framework polymerizes around these preformed surfactant aggregates. The hydrothermal temperature strengthens the interaction between silica acid radical ions, and plays an intimate role in directing the polymerization of siliceous framework and self-assembly on the surface of preformed surfactant.25 In addition, the metal salt also plays an important role in the formation of nanotube structures. At the process of synthesis of silica nanotubes, a byproduct exists in the form of sodium chloride in solution A. The presence of the metal cations could compress the electrical double layer of micelle, enhance ionic strength, and induce more surfactants molecule to participate to the formation of wormlike micelles.12 When the metal cations were removed from solution A, no silica nanostructures were fabricated. In this study, templates of CTAB/ SDBS surfactant systems are used as directing and structuring agents for the preparation of hollow silica nanotubes. The growth of material from solution occurs at the interface between the template and the surrounding solution, and the process leads to coating of the template which plays an important role of spatial fillers. The hollow nanostructures are obtained after the removal of template. The silica gel produced from the maize plant was used as the silica precursor. When silica gel (solution A) was added into the CTAB/SDBS aqueous solution, the silica acid radical ions were absorbed at the interface of wormlike micelles, reacting with hydrocarbon chain by van der Waals force, and electrostatic forces. The resulting product was thoroughly calcinated at 550 °C to remove the templates, then the hollow nanotubes were obtained. Figure 5 shows the FTIR spectrum of nanotubes using surfactant self-assemblies as templates. The predominant absorption peaks around 1110 cm 1 were observed which can be assigned to the Si O Si antisymmetric stretching.8 This peak, the shoulder of which appeared at 800 cm 1, and the bending vibrations at 473 cm 1 indicate the presence of silica. The broad shoulder displayed at approximately 3437 cm 1 can be attributed to symmetric stretching vibration of Si OH groups. It shows no significant changes in the peak position and intensity while increasing the concentrations of surfactants. During the process of EDX analysis (Figure 6), the silica nanostructures were dispersed in HPLC grade ethanol and sonicated for 5 min. A drop of this dispersion was taken
Figure 7. Room-temperature PL spectra of silica nanotubes under excitation at 340 nm: (A) maize ash, (B) nanotubes obtained from maize ash.
on Cu grid. The sample was coated with gold when the solvent evaporated. The EDX spectra show that the synthesized compounds contain mostly Si and O with the mass percentage 22.84:61.74. The Cu and Pt signals are attributed to the Cu grid and gold coated the samples. The FTIR spectra results and EDX analysis display that most of the surfactant templates are removed and the synthesized nanostructures are present in the form of high-grade silica. The optical properties of the silica nanotubes at room temperature were also investigated. Figure 7 shows the PL spectra of maize ash and nanotubes (prepared in CTAB/SDBS mixtures of 0.085 mol/L concentrations) under excitation at 340 nm. The PL feature could be obtained from other nanotubes prepared from different concentrations of surfactant systems, such as CTAB/SDBS mixtures at 0.056 or 0.028 mol/L. A main, intense peak is observed at ∼420 nm (3.0 eV), with a shoulder at 460 nm (2.7 eV) and a weak peak at 530 nm (2.34 eV). A weaker emission is clearly distinguishable at 489 nm (2.55 eV). In general, the luminescence characteristic depends mainly on the silica intrinsic or extrinsic defects centers in nature, while other variables such as solvent used, water content,12 aging, annealing, gas flow, and heating13 also affect the PL properties. There are several kinds of defects responsible for these luminescence bands: 12525
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Industrial & Engineering Chemistry Research the Si OH groups on the inner and outer nonowires surfaces;26 nonbridging oxygen hole centers (NBOHCs),27 tSi O 3 in silica matrix, oxygen hole centers (ODCs), and self-trapped excitons (STEs). The NBOHCs which consist of the hydroxyl radicals (tSi—OH), the peroxy linkages (tSi—O—O—Sit) in oxygen-rich species and irradiation-inducing strained silicon oxygen can induce a red band at ∼620 690 nm.27 SETs refer to a peroxy linkage and an electron hole pair. In silica nanomaterials, ODCS often originates from neutral oxygen vacancies, tSi—Sit, and 2-fold coordinated silicon defects (tSi—O—Si—O—Sit). As shown in Figure 7, the absorption at 420 nm is distinguishable which corresponds to some intrinsic diamagnetic defect center: 2-fold coordinated silicon lone-pair center,22 while the blue light emission at 460 nm can be ascribed to the neutral oxygen vacancy (tSi—Sit). The strong peak at 489 nm is probably attributed to the hydroxyl radicals (tSi—OH) covered outer or inner surfaces of the silica nanotubes. This is consistent with the infrared absorption of symmetric stetching vibration of Si OH groups observed in Figure 5. In conclusion, the silica nanotubes prepared from nature phytoliths perform the visible luminescence characteristic while the PL emission curve is clearly distinct from other synthesized amorphous silica nanotubes.12,22,28 The blue emission from our silica nanotubes should chiefly originate from ODCs.
4. CONCLUSIONS Maize harbors abundant amorphous hydrated silica (phytoliths) which can be used as a novel silica resource. We have demonstrated that the phytoliths may be used to fabricate hollow silica nanotubes with smooth surface and ordered fringes using the templates of selfcolloidal assemblies. The synthesized nanotubes are intertwining to network nanostructure and display a visible photoluminescent characteristic. The PL spectrum clearly display three emission peaks especially the stable and strong blue light band at 420 nm. The luminescence property depends mainly on the silica structure, which is sensitive to environmental variables. We expect that the synthesis of photoluminescent silica nanotubes using phytoliths could lead to an energy conversion and economically viable green approach toward the optical device application. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by Natural Science Foundation of China (20873074 and 21043008) for financial support. ’ REFERENCES (1) Yalc-in, N.; Sevinc-, V. Studies on Silica Obtained from Rice Husk. Ceram. Int. 2001, 27, 219. (2) Della, V. P.; K€uhn, I.; Hotza, D. Rice Husk Ash as an Alternate Source for Active Silica Production. Mater. Lett. 2002, 57, 818. (3) Liou, T. H. Preparation and Characterization of Nano-Structured Silica from Rice Husk. Mater. Sci. Eng., A 2004, 364, 313. (4) Chiarakorn, S.; Areerob, T.; Grisdanurak., N. Influence of Functional Silanes on Hydrophobicity of MCM-41 Synthesized from Rice Husk. Sci. Technol. Adv. Mater. 2007, 8, 110. (5) Chareonpanich, M.; Nanta-ngern Limtrakul, A. Short-Period Synthesis of Ordered Mesoporous Silica SBA-15 Using Ultrasonic Technique. Mater. Lett. 2007, 61, 5153.
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