Research Article pubs.acs.org/journal/ascecg
Novel Synthesis of Silicon Carbide Nanowires from e‑Waste Samane Maroufi,* Mohannad Mayyas, and Veena Sahajwalla Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia ABSTRACT: In this study a novel new process for the synthesis of silicon carbide nanowires from a voluminous and problematic waste stream, e-waste, is verified. Silicon carbide nanowires (SiC NWs) with diameters of 30−200 nm and length up to 10 μm were synthesized via the carbothermal reduction of two electronic waste (e-waste) components. The glass fraction of an obsolete computer monitor (GCM) was used as a source of silica and the computer’s plastic shell (CPS) as a carbon source. After identifying the composition of the GCM and the CPS, a stoichiometric mixture of both components was prepared and subsequently heat-treated at 1550 °C in an inert atmosphere. The resulting product was characterized, and results confirmed the formation of three distinct morphologies of SiC NWs; smooth surface NWs, bamboo-like NWs, and hexagonal prism NWs with high-density stacking faults (SF). According to the N2 physisorption analysis, the SiC NWs showed mesoporous characteristics with a specific surface area of 51.4 m2/g and pore size distribution ranging from 2 to 15 nm. Given the growing global burden of e-waste, we have demonstrated a new low cost means of synthesizing SiC NWs and the use of a problematic waste, delivering potential economic and environmental benefits. KEYWORDS: Carbothermal reduction, SiC nanowires, e-Waste, Bamboo-like/hexagonal prism, Photocatalyst
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mental issue,6−8 E-waste recycling and the recovery of values from that9,10 have been explored by large number of researchers. Despite the potential value of e-waste, recycling rates remain relatively low worldwide. Formal, industrial scale processing accounts for some 20−25% of e-waste in advanced economies in Europe and the U.S., although recycling rates in many nations, such as the U.K., are lower (14%).1 In Australia, only 10% of e-waste is recycled. The remainder of the world’s ewaste is currently destined for landfill or for informal processing in developing nations where the manual separation of metallic components for recovery exposes poor communities and workers to serious environmental and health risks.11 Obsolete computers are of particular concern because of huge production volumes, short replacement cycles, and harmful components. In Australia, 88% of discarded computers are sent to the landfill, 10% are held in storage awaiting disposal and only 1.5% are recycled.12 Also, as both formal and informal e-waste processing focuses mainly on metals recovery, the bulk of other materials, including the glass and plastics, is often incinerated or landfilled due to the lack of an efficient and cost-effective means of processing this waste fraction.13 As such, the large volumes of glass and plastic within e-waste represent both a significant global waste burden and a potentially valuable, untapped resource. The novel process described here presents a new
INTRODUCTION Electronic waste, or e-waste, is the fastest growing waste stream in the world.1 With high consumer demand, ubiquitous digital connectivity, increasingly short production cycles, and the early obsolescence of products in both affluent and developing country markets, the manufacturing and sales of equipment in the electronic industry is now worth ∼U.S. $1 trillion annually, yielding some 30−50 million tonnes of obsolete equipment worldwide each year. The average lifetime of a computer, for example, decreased from 4.5 years in 1992 to about 2 years in 2005.1 The U.S. Environmental Protection Agency estimates the average American household uses some 28 devices,2 and in 2012, Americans generated an average of 30 kgs of e-waste per capita.3 Embedded within e-waste are many valuable materials with a positive market value for recovery and resale such as precious metals, including gold, silver, platinum, palladium, and copper, and strategic metals, like rare earth metals and other nonferrous metals. However, e-waste is particularly complex as it also contains a range of hazardous substances and contaminants as well as large volumes of other waste materials, including potentially valuable glass and plastics. This means ewaste is technically and economically challenging to recycle and, hence, problematic. A 2015 report by the United Nations Environment Program estimated some 42 million tonnes of electronic waste was thrown away worldwide in 2014, at a cost to global economy, in terms of lost embedded resources, of as much as USD52 billion.4 As a global, interregional and domestic problem, E-waste has become an important topic and aroused researcher’s attention worldwide. The problem associated with discarded E-waste,5 its health and environ© 2017 American Chemical Society
Received: January 17, 2017 Revised: March 23, 2017 Published: April 13, 2017 4171
DOI: 10.1021/acssuschemeng.7b00171 ACS Sustainable Chem. Eng. 2017, 5, 4171−4178
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agricultural wastes as a source material.47 Waste PVB, Poly(vinyl butyral) has also been reported as an input in the synthesis of SiC nanocrystals (SiC NCs). However, no study has, to be best of our knowledge, achieved the synthesis of SiC nanowires from two problematic, interlinked waste streams; waste glass and plastics from e-waste. As the raw materials for the synthesis of SiC NWs are costly and cannot be produced in sufficient quantities for commercial purposes, waste streams as inputs are attractive alternatives. Consequently, the novel process reported here opens up a sustainable new, cost-effective approach for the production of advanced materials while, at the same time, potentially reducing the volumes of globally significant waste discarded into the environment.
opportunity to transform a high volume, low value waste stream into value-added advanced materials. One-dimensional (1D) semiconductor nanostructures (wires, rods, belts, and tubes) have become the focus of intensive research because of their unique application in electronic, optoelectronic, and sensor devices at the nanoscale. They possess novel properties intrinsically associated with low dimensionality and size confinement, which make “bottom-up” construction of nanodevices possible. Among these semiconductor nanowires, SiC NWs have particularly attractive properties, as they combine the well-established chemical and mechanical properties of SiC, such as wide band gap, excellent thermal conductivity, chemical inertness, high electron mobility, and biocompatibility, with the size dependent advantages/characteristics of quasi one-dimensional structures.14 This makes SiC NWs especially promising for applications in microelectronics and optoelectronics, attracting considerable interest from materials and devices researchers.15 A wide range of nanostructured SiC, including nanoparticle/ whisker composite,16 nanoribbon,17 nanowire (NWs),18 nanowhiskers,19 nanochains,20 and fibers,21,22 have been synthesized via several methods such as chemical vapor deposition (CVD),23 arc discharge,24 carbothermal reduction. Owing to its efficiency and simplicity, carbothermal reduction technique is considered as the most economically viable method. In addition, this method does not rely on hazardous precursors unlike other techniques. Metallothermic reduction is an alternative method to the conventional production of SiC, but production cost in this technique is high depending on the used metal. The metallothermic reduction also does not provide a full conversion of SiO2 and involves a formation of secondary phases.25−27 In the carbothermal reduction technique, silica is reduced by a carbon source; the carbon source is believed to significantly affect the reaction rate, atomic diffusion, and subsequently the morphology of the synthesized SiC. Several researchers have utilized this technique and synthesized SiC using different types of carbon sources.28−30 Sun et al.31 used multiwall carbon nanotubes and synthesized multiwall silicon carbide nanotubes. Kudrenko et al.32 synthesized SiC nanowires with diameters of 20−200 nm and lengths from tens to hundreds of micrometres using carbon fibers. In other works, carbon nanotubes,28 bamboo-like nanorods,29 and activated carbon30 were also utilized as reducing agents for the synthesis of SiC NWs. In this study, we have produced SiC NWs from two alternative silica and carbon sources using abundant fractions of e-waste, GCM as a source of silica and CPS as a source of carbon. In recent years, considerable research has focused on the utilization of waste glass as a coarse and fine aggregate for alkali-silica reactions, decorative material in concrete,33−41 and as a foam for thermal and acoustic insulation applications.42−46 However, despite research efforts to produce value-added products, the use of waste glass in synthesizing highly advanced materials such as nanostructured SiC has not been reported previously. There have, however, been several reports in the literature focusing on the synthesis of SiC nanoparticles, such as nanowires, nanorods, or spherical colloids, through the high temperature treatment of agricultural waste products. Using a mixture of oil palm fibers and rice husk ash, the formation of 3C-SiC nanowires and nanocones have been reported via pyrolysis at different temperatures and using variations in constituent concentrations. Their study47 has shown that SiC nanoparticles can be grown in large quantities using low cost,
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MATERIALS AND EXPERIMENTAL PROCEDURE
Samples of GCM and CPS were collected from the Reverse E-waste Company, Sydney, Australia. The GCM was pulverized in a ring mill to a fine powder. The CPS was first pyrolyzed at 1550 °C for 20 min and then ground using a mortar; the resulting powder was denoted as P-CPS. Powders of GCM and P-CPS were subsequently mixed in a stoichiometric ratio and then hot-pressed into pellets using a uniaxial hydraulic press by applying 3 bar of pressure at 180 °C for 20 min. The resulting pellet was placed in an alumina boat, covered with an alumina lid and heated isothermally at 1550 °C in a tubular furnace (100 cm length × 5 cm diameter) under argon purge (1 L min−1) for 150 min. The sample was then removed from the hot zone of the furnace to cold zone (i.e., furnace mouth) and left to cool down for 10 min under argon purge. The resulting material was denoted as GCM/ P-CPS@1550. The composition and morphology of the resulting product were characterized by X-ray photoelectron spectroscopy (XPS), X’pert PRO multipurpose XRD (MPD system), Fourier transform infrared spectroscopy (FTIR), FEI Nova Nano-SEM 230 (FE-SEM), transmission electron microscopy (TEM), and N2 isothermal adsorption method.
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RESULTS AND DISCUSSION Characterization of Raw Materials. The elemental composition of the GCM was identified by XRF analysis. As shown in Table 1, GCM consisted primarily of SiO2 (70.3%) Table 1. Chemical Composition of GCM oxides
Na2O
SiO2
Al2O3
CaO
MgO
MnO
Fe2O3
weight percent
13.9
70.3
1.8
7.8
4.6
0.01
0.1
and some oxide minerals such as Na2O (13.9%), Al2O3, CaO, and MgO. The presence of such oxides is very important for the purpose of SiC production as they provide control over the morphology and particle size of the resulting SiC. Na2O liberates from the system during the process of heating up to 1550 °C. The evolution of Na2O and other gases from plastics physically micronize the particles and prevent their agglomeration.48 The presence of other oxides (i.e., Al2O3, CaO, and MgO) is favorable as they affect the particle morphology according to Zener pinning phenomenon.49,50 Oxide particles control the grain growth by exerting a pinning pressure which counteracts the driving force pushing the boundaries.51,52 For high purity product, these oxides can be removed by selective dissolution using acids. The thermal degradation behavior of the CPS was studied by thermogravimetric analysis (TGA); samples were heated from room temperature to 1200 °C at heating rates of 5 °C/min and 20 °C/min under nitrogen purge of 20 mL min−1. The result is shown in Figure 1. The figure shows only one degradation step 4172
DOI: 10.1021/acssuschemeng.7b00171 ACS Sustainable Chem. Eng. 2017, 5, 4171−4178
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Figure 1. Mass loss and Gaussian fit of derivative thermogravimetric (DTG) curves of waste CPS at heating rates of 5 and 20 mL min−1 under nitrogen purge of 20 mL min−1.
Figure 3. FTIR spectra of waste CPS.
889 cm−1 are also related to the CH out-of-plane deformation of meta and ortho disubstituted benzenes in polycarbonate. Antisymmetrical stretching of Si−O−Si of silica impurity in the CPS can be observed at around 1014 cm−1. The product of the isothermal pyrolysis of CPS at 1550 °C (i.e., P-CPS) was obtained and characterized. The XRD pattern of the P-CPS, in Figure 4, shows the strong diffraction peaks
starting around 400 °C at a heating rate of 5 °C/min. The main decomposition stage shifted toward the higher temperature range when the heating rate increased from 5 °C/min to 20 °C/min. The CPS continued to degrade until 510 and 590 °C was reached, at 5 °C/min and 20 °C/min, respectively. Around 12% of more thermally stable residues remained in the end at both heating processes. According to LECO analysis, the remaining material was composed of ∼95.6 wt % carbon. The chemical bonding of elements in the CPS was identified using XPS analysis. As shown in Figure 2, several peaks that belong to the photoejected electrons from the orbital 1s of C, O, and Fe were detected. Peaks of auger electrons, marked as OKLL, can be also observed. The high-resolution XPS spectrum of the C 1s region is also displayed in Figure 2. The region clearly shows two peaks at 284.8 and 286.4 eV corresponding to C−C and C−O bonds. The FTIR spectrum of the CPS was also obtained. As shown in Figure 3, it is similar to the spectrum of polycarbonate polymer. IR adsorption bands at around 2970 cm−1 and 2920 −1 are, respectively, assigned to the antisymmetrical stretching of −CH3 and the symmetrical stretching of −CH2− in polycarbonate. The band at 1770 cm−1 is related to the C O stretching modes of the functional group while the peak at 1506 cm−1 results from the intense aromatic breathing modes. Polycarbonate typically has a distinguished IR band at 1499 cm−1 corresponding to the stretching mode of the benzene ring. The triplet in the range between 1165 cm−1 to 1228 cm−1 belongs to the C−O−C stretching mode. The IR band at 962 cm−1 results from the CH out of plane deformation of transdisubstituted alkenes. The bands at 696 cm−1, 760, 832, and
Figure 4. XRD spectra of waste P-CPS.
that belongs to graphite. The XRD pattern also shows other diffraction peaks corresponding to FeSi and Fe. The presence of these compounds in the P-CPS is related to the FeOx and SiO2 impurity in the CPS; during pyrolysis of CPS at 1550 °C, the carbothermal reduction of FeOx and SiO2 occurred leading to the formation of Fe and FeSi alloy.53 Synthesis and Characterization of SiC. The composition of GCM/P-CPS@1550 was identified using XRD and FTIR analyses. The XRD pattern of GCM/P-CPS@1550, Figure 5a,
Figure 2. (a) General XPS spectrum of waste CPS and (b) high-resolution XPS spectra of carbon region (right side). 4173
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Figure 5. (a) XRD and (b) FTIR spectra of synthesized SiC NWs.
Figure 6. FE-SEM images of synthesized SiC at 1550 °C.
Figure 7. High magnification of FE-SEM images of synthesized SiC at 1550 °C.
shows strong peaks of β-SiC and small peaks, marked with SF. These small peaks are typical characteristics of β-SiC whiskers which resulted from the stacking faults in the whisker structure. The FTIR spectrum of GCM/P-CPS@1550, Figure 5b, shows only one strong peak at ∼807 which corresponds to the stretching mode of Si−C bond, further confirming the formation of SiC. It is worth noting that the FTIR of the SiC sample only shows a large peak for SiC; no peaks for SiO2 are observed which provides evidence on the full conversion of SiO2.
The structure and morphology of the produced SiC was investigated using FE-SEM analysis. The FE-SEM image of the resulting SiC, in Figure 6, indicates that the SiC consisted primarily of NWs that appeared in different morphologies; NWs with smooth surfaces (from 30 to 200 nm diameter and up to 10 μm length), NWs with hexagonal prism morphology, and bamboo-like NWs. The high magnification FE-SEM images, in Figure 7, clearly show the nanostructural features of the hexagonal prismatic and bamboo-like NWs. The mechanisms of SiC NWs formation has been investigated by 4174
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Figure 8. TEM images of SiC NWs.
Figure 9. (a) N2 adsorption/desorption isotherm and (b) BJH adsorption pore size distribution of the SiC NWs.
several researchers.29,54−59 The formation of SiC NWs occurs through two stages of nucleation and growth. Silicon monoxide gas primarily forms according to the following reaction:
line), can be observed in the structure of the bamboo-like and the hexagonal prismatic NWs. This nonuniform structure indicates the presence of a high concentration of stacking faults and planar defects that results from the irregular stacking of small units of truncated rectangular sheets of β-SiC. The high density of stacking faults, observed in the structure of bamboolike NWs and the hexagonal prismatic NWs, is attributed to the low energy required for stacking fault formation.32 The concentration of stacking faults throughout the NWs length depends on the growth kinetics and is believed to originate from the thermal stress during the growth process.19,56 The difference in the energy level of the Si (111) and C (111) planes is believed to induce surface tension that eventually causes stacking faults in the structure of the NWs.32 In bamboo-like NWs, Figure 7, several knots (green arrow) are observed along the axis of the NWs (red line) with a high density of stacking faults. Such irregularities are attributed to the variations in the concentration of CO in different regions along the NWs. The growth of SiC NWs occurs along the [111] direction through the reaction of CO and SiO (reaction 3). If the adsorption rates of gaseous SiO and CO are approximately equal to the precipitation rate of SiC, smooth SiC nanowires are expected to be formed (Figure 6c,d). However, the concentration of CO in some regions could reach a very high degree of supersaturation, which may cause structural imperfections or defects. The narrow regions of bamboo-like NWs (yellow arrows in Figure 7) are believed to grow under conditions of high supersaturation of CO while the growth of wide regions result from a moderate supersaturation. TEM images shown in Figure 8 also confirm the presence of a high density of stacking faults and planar defects perpendicular to the SiC NW axis. From TEM images,
SiO2 (solid) + C(solid) → SiO(vapor) + CO(vapor) ΔG° = 668.07 − 0.3288T (kJ)
The generated silicon monoxide reacts with the solid carbon particles (reaction 2), which results in the formation of SiC particulates that serve as nuclei. SiO deposits, intermediately, on the nuclei and reacts with the carbon to form SiC with preferential unidirectional growth on the [111] crystal plane of SiC. SiO(vapor) + 2C(solid) → SiC(nanoparticles) + CO(vapor) ΔG° = −78.89 + 0.0010T (kJ)
(2)
Under the condition of CO vapor saturation, NWs grow via reaction 3: SiO(vapor) + 3CO(vapor) → SiC(solid) + 2CO2 (vapor) ΔG° = −403.51 + 0.339T (3)
The CO2 gas formed by reaction 3 returns back to carbon particles and reacts with them to form CO through reaction 4: CO2 (vapor) + C(solid) → 2CO(vapor) ΔG° = 162.31 − 0.1690T (kJ)
(4)
Looking closely at the FE-SEM images of SiC at high magnification, a striped pattern, highlighted in Figure 7 (yellow 4175
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Figure 10. (a) UV−vis spectra of the MB solution after photodegradation tests, 20 mg sample charge in 15 mL of 10−5 M MB solution irradiated with 365 nm UV lamp and (b) liquid-phase photocatalytic degradation of MB under the irradiation of UV light.
powder photoctalysts,64,65 even though this SiC was synthesized from waste materials.
bamboo-like SiC with a diameter of about 150 nm with a knot along the length of NWs can be clearly observed. The pore size distribution of the SiC NWs is shown in Figure 9b. The SiC NWs shows micropores with a dominant pore radius smaller than 2.5 nm. The structural defects discussed earlier are believed to result in such pore size distribution. A BET surface area of 51.4 m2/g was measured using N2 adsorption/desorption analysis. Owing to its suitable band gap (2.3−3.3 eV) and other excellent properties of high thermal conductivity and mechanical strength, SiC could be considered a promising photocatalyst under visible light irradiation. In work done by Liu et al.,55 SiC NWs were shown as having a good photoelectrochemical hydrogen evolution performance under UV irradiation. As the SiC NWs synthesized in this work exhibited high surface areas, they are promising candidates for photocatalytic applications. SiC NWs synthesized here were used as photocatalyst for the photodegradation of methylene blue (MB) in aqueous solution under UV−vis irradiation. Photodegradation tests were conducted by irradiating a mixture of 20 mg of SiC NWs and 15 mL of 10−5 M MB solution with 325 nm UV light for 10, 20, and 30 min. The transmission change in MB solutions after irradiation at 664 nm was observed by UV−vis, PerkinElmer Lambda 35 UV−visible spectrometer. A sample of SiC nanoparticles was also tested as a reference and its photocatalytic behavior was compared to that of SiC NWs. Figure 10a shows the UV−vis spectra of the MB solution after photodegradation tests. The red dot line indicates the characteristic peak of MB ultraviolet adsorption at 664 nm. It clearly shows that different samples have different photocatalytic effect on the MB solutions. Figure 10b presents the MB degradation results by the powders under different irradiation time. It can be seen that the UV irradiation almost has no effect on the MB concentration when photocatalyst was not added. However, the synthesized SiC NWs can significantly decompose the MB solution. With the increase of the irradiation time, the sample shows consistent trend of decomposing MB solution which indicates that the SiC NWs maintain their photocatalytic activity even after 30 min. Compared to SiC nanoparticles, the synthesized SiC NWs have significantly higher efficiency. This can be attributed to the fact that different morphologies have different crystal facets exposed on the particle surface. Crystal facets in a crystal have different surface energies which may subsequently lead to different photocatalytic behavior.60,61 Furthermore, it should be noted that the synthesized SiC NWs possess higher photocatalytic activity than most of the previous reported thin films62,63 and also have comparable efficiency with most of the
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CONCLUSIONS Bamboo-like SiC NWs and SiC NWs with hexagonal prisms with a diameter of 30−100 nm and length of up to10 μm were synthesized using a novel, sustainable process. The glass fraction of an obsolete computer monitor (GCM) and the computer’s plastic shell (CPS) were utilized as silica and carbon sources, respectively. Pyrolysis of CPS was carried out at 1550 °C for 20 min and the resulting residue was subsequently used for SiC synthesis. The carbothermal reduction of GCM occurred at 1550 °C in argon under atmospheric pressure using CPS char. A striped pattern was observed as a nanostructural feature of “bamboo-like” NWs and NWs with hexagonal prisms, which was attributed to the high concentration of stacking faults (SF). The BET specific surface area of the SiC NWs, measured using nitrogen adsorption/desorption isotherm technique, was around 51.4 m2/g. The synthesized SiC NWs also possessed high photocatalytic activity. Given the growing global burden of e-waste, the potential of SiC NWs and the suggested novel sustainable process could simultaneously demonstrate a new low cost means of synthesizing SiC NWs and the benefits of transforming waste to value.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +61(2)9385 4471. E-mail: s.maroufi@unsw.edu.au. ORCID
Samane Maroufi: 0000-0001-5553-8519 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support for this research was provided by the Australian Research Council through Laureate Fellowship FL140100215.
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DOI: 10.1021/acssuschemeng.7b00171 ACS Sustainable Chem. Eng. 2017, 5, 4171−4178
