Sustainable approach toward synthesis of green functional carbonaceous 3-D micro/nanostructures from biomass
© Tavangar et al.; licensee Springer. 2013
Received: 24 April 2013
Accepted: 1 August 2013
Published: 8 August 2013
This study proposes a novel technique to synthesize functional carbonaceous three-dimensional (3-D) micro/nanocompounds from agricultural by-products using femtosecond laser irradiation. Biowastes of rice husk and wheat straw are value-engineered to carbonaceous structures in a single-step process under ambient conditions. Our results demonstrate that by controlling the laser fluence, structures with a variety of different morphologies from nanostructures to microstructures can be achieved. Also, the results indicate that altering the laser processing parameters influences the chemical composition of the synthesized structures. This sustainable approach presents an important step towards synthesizing 3-D micro/nanofibrous compounds from biowaste materials. These structures, as-synthesized or as nanocomposite fillers, can have practical uses in electronic, sensing, biological, and environmental applications.
Functional carbonaceous micro/nanostructures have drawn considerable attention in the past few years and are considered one of the most promising materials of the human future life . They have been broadly used in technological applications in different areas such as nanoelectronics, efficient energy storage, catalysis, sustainable chemical technology, and biomedical and environmental sciences [1, 2]. Functional nanostructured carbon materials have been prepared in a wide range of morphologies and structures either in form of different carbon allotropes or in complex compound structures, e.g., carbon nanotubes , nanospheres , nanodiamond , carbon nanofibers , and carbon-based hybrid nanostructures [7–10]. Thus far, several fabrication approaches such as hydrothermal carbonization , carbonization , and arc discharge  have been reported for the preparation of carbonaceous nanostructures. A special interest has been directed toward approaches that synthesize carbonaceous micro/nanostructures from renewable resources not only with regards to the economic point of view but also with respect to their sustainability and green, nontoxic routes.
Biomass, particularly agricultural by-products, is an abundant low-cost carbon source that can be processed to synthesize functional carbonaceous materials. Rice husk and wheat straw are lignocellulosic materials containing high-concentrated carbon. They possess several potential advantages such as low price, copious renewable source, biodegradability, and high specific strength and stiffness .
Although numerous studies have reported the synthesis of carbonaceous nanomaterials from pure xylose, glucose, cyclodextrin, sucrose, starch, etc., only few researches have been conducted to produce carbonaceous micro/nanostructures from natural resources . Most of the previous studies employed hydrothermal carbonization process, which requires catalysts and high temperatures and pressures . Therefore, a sustainable green approach to synthesize carbonaceous nano/microstructures from biomass in a single-step process under ambient conditions would be of great interest.
In this work, we have proposed a novel technique to engineer carbonaceous nano/microstructures from rice husks and wheat straws using femtosecond laser processing. To the best of the authors' knowledge, this is the first time that 3-D nano/microstructures have been synthesized from rice husks and wheat straws using laser ablation. The laser pulses hit rice husk and wheat straw powders and generate a mass quantity of nanoparticles, leading to interwoven micro/nanostructures after further nucleation and collision. The morphology of the structures has been studied using scanning electron microscopy (SEM). The chemical composition of the structures has been analyzed using energy-dispersive X-ray spectroscopy (EDS) analysis.
Results and discussion
Given a constant number of pulses, the laser fluence is reduced by decreasing the laser pulse energy. The laser-induced structures are results of particle aggregation. Particle aggregation takes place as part of vapor condensation by the collision of nucleus. To generate nanofibrous structures, an immense amount of nanoparticle aggregation is required. Therefore, continuous arrival of the laser pulses is needed in order to ablate the target material great enough to maintain the plume nucleus density at the critical level. Hence, critical amount of laser fluence should be transferred to the substrate in order to initiate the plume and keep it at the certain level. As a result, the formation of nanofibrous structures is not possible in lower laser pulse energies, and instead, microstructures would be generated.
Here, Pavg is the average power (in W), measured directly from incident laser pulse, Rrep (in s−1) is the laser pulse repetition rate, Ppulse = Pavg/Rrep is the laser pulse energy, and Afoc (in cm2) is the irradiation focal spot area. It can be obtained by calculating the theoretical laser minimum spot diameter (D0) as where λ0 is the wavelength of the laser, f is the effective focal length of the lens, and D denotes the laser beam diameter.
As Equation 1 suggests, increasing the laser average power results in a rise in the total laser energy flux transferred to the irradiated spot. The higher transferred laser energy flux for the optimum evaporation regime leads to an increase in the number of evaporated particles; then, the deposition rate of synthesized structures will be analogous to the number of evaporated particles.
Here, Npulse is the number of consecutive pulses hitting the target, and Revp is evaporation rate. After irradiation, plume temperature and pressure start to decrease leading to condensation and nucleation. The great amount of nuclei leads to the growth of particles, which will aggregate into interwoven structures after further collision. Since the rate of deposition of generated structures is proportional to the number of evaporated particles, denser structures are synthesized when specimens are targeted by higher energy laser pulses. This is in agreement with our experimental results where denser micro/nanostructures were observed when the targets were processed at higher energy pulses.
The proposed method suggests considerable promise for the synthesis of 3-D micro/nanostructures from green materials to develop new functional compound materials for various applications.
This work presented a laser-based approach to synthesize carbonaceous micro/nanofibrous structures from rice husks and wheat straws. To the best of our knowledge, this is the first time that synthesizing 3-D micro/nanofibrous structures generated from rice husks and wheat straws using femtosecond laser have been reported. The morphological analyses by SEM confirmed that fabricated structures were composed of approximately uniform 3-D structure at micro and nano sizes. Further experiments showed that by altering the laser pulse energy and the number of laser pulses, different structures from micro- to nanoarchitectures with different porosities and features could be achieved. The EDS analyses confirmed that laser irradiation affected the chemical composition as well; part of the organic matter is believed to be burned away owing to the laser irradiation. This approach suggests a promising step towards engineering green 3-D platforms from sustainable materials. The as-engineered carbonaceous materials would have very broad practical applications in a variety of areas, such as environmental, catalytic, electronic, sensing, and biological applications. They can also be utilized to form biodegradable nanocomposites with other materials, e.g., polymers.
This research is funded by the Natural Science and Engineering Research Council of Canada.
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