Thermal Nanowiring of E-Waste: A Sustainable Route for Synthesizing

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Thermal Nanowiring of E‑Waste: A Sustainable Route for Synthesizing Green Si3N4 Nanowires Samane Maroufi,* Mohannad Mayyas, Rasoul Khayyam Nekouei, Mohammad Assefi, 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: This paper details a sustainable process for synthesizing green Si3N4 nanowires (NWs) from high volume fractions of electronic waste (e-waste). Obsolete computers were manually dismantled, and the glass fraction of the monitors (GCM) and their plastic shells (PS) were separated and used as silica and carbon sources, respectively. E-waste is the world’s fastest growing waste stream, and although considerable research effort is currently focused on metals recovery, there are few options for the huge volumes of glass and plastic waste left behind. Using a blend of GCM and PS, we applied a novel route, thermal nanowiring at 1550 °C under nitrogen purge in atmospheric pressure. FE-SEM studies revealed the resulting Si3N4 NWs, with diameters of 75−250 nm, mostly appeared to turn in random directions and exhibited twisted pattern along their axes, indicating the Si3N4 crystalline grew through a screw-dislocation-driven mechanism. In a comparison experiment, waste toner powder with iron oxide (∼38 wt %) was added as a source of Fe to the GCM-PS blend to enable the investigation of its catalytic effect on the morphology of the Si3N4 synthesized. The addition of the toner powder resulted in long Si3N4 nanobelts (up to 40 μm) with traces of liquid droplets on their tips, indicating the Fe had promoted the formation of the long and straight nanobelts via the vapor−liquid−solid VLS mechanism. The novel route, thermal nanowiring described here, confirms a new opportunity to transform a globally significant waste burden into value-added 1D materials, thereby simultaneously delivering economic and environmental benefits. KEYWORDS: Silicon nitride nanowires, E-waste, Glass fraction, Plastic shell, Carbothermal nitridation technique

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INTRODUCTION As the global electronics industry grows, with sales exceeding 1 trillion USD/year, so too does the vast volume of electronic waste, or e-waste, being thrown away. Driven by high consumer demand in both developed and developing economies, short replacement cycles, and the early obsolescence of many digital devices, e-waste is now the world’s fastest growing waste stream, with some 30−50 million tonnes of e-waste discarded every year. It is also one of the most challenging. Although many valuable materials such as precious metals (gold, silver, platinum, palladium, and copper), strategic metals (rare Earth metals), and other nonferrous metals are embedded within ewaste, it is a complex mix. In addition to these potentially valuable resources, e-waste contains hazardous substances and contaminants as well as large volumes of other waste materials, like plastics and glass. As such, it is technically difficult, and often uneconomical, to recycle. Consequently, a significant proportion of e-waste is sent to be landfilled, stockpiled, or illegally exported to developing nations, where informal processing exposes poor communities to serious risks of contamination.1−3 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 © XXXX American Chemical Society

to the global economy, in terms of embedded resources lost, of as much as 52 billion USD.4 Given the global, interregional, and local impacts of this ewaste burden, researchers are focusing considerable attention on the health and environmental issues associated with ewaste5−7 and on e-waste recycling and the recovery of valuable resources.8,9 Research into e-waste processing and/or recycling in recent years had yielded various different approaches. All potential solutions require at least some separation of e-waste into its various components (plastics, glass, metal, etc.). As both formal and informal e-waste processing focuses mainly on metals recovery, the bulk of the remaining waste materials, including the glass and plastics, is often incinerated or landfilled due to the lack of an efficient and cost-effective means of processing these waste fractions.10 The large volumes of glass and plastic within e-waste, therefore, represent both a significant global waste burden and a potentially valuable, untapped resource. Received: November 8, 2017 Revised: January 2, 2018 Published: February 2, 2018 A

DOI: 10.1021/acssuschemeng.7b04139 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

catalysts,16 solvothermal synthesis,57 and the nitridation of an Fe−Si catalyst.58 Despite above-mentioned works, no study has, to the best of our knowledge, achieved the synthesis of Si3N4 nanomaterials from two waste streams. Owing to its simplicity and adaptability with feed materials derived from waste, the carbothermal nitridation technique seems to be a worthstudying technique for the sustainable production of Si3N4. In addition, this method does not rely on hazardous precursors unlike other techniques. The use of waste streams as inputs using this technique would, therefore, provide an attractive pathway for synthesis of high-quality Si3N4 nanostructures. The novel route described here offers a sustainable, cost-effective approach for the production of advanced materials while, at the same time, diverting a globally significant waste stream away from landfill. In this study, we aimed to use the glass fraction of obsolete computer monitors (GCM) (as a source of silica) and the computer’s plastic shell (PS) (carbon source) for the synthesis of Si3N4 NWs via a sustainable route, thermal nanowiring. The effect of iron as a catalyst on the morphology of the synthesized Si3N4 was investigated by adding waste toner powder to the primary materials.

Computer monitors, televisions, and other devices with glass screens are a major component of e-waste destined for landfill. In Australia, for example, around 88% of the four million computers and three million TVs purchased annually end up in landfills.11 Glass contains a meaningful amount of SiO2 which can be a valuable and rich alternative source of silicon in the fabrication of valuable products, such as SiC12,13 and Si3N4. Waste toner cartridges from printers and other copying devices are, likewise, difficult to recycle, despite their potential value. Only in Australia, over 5000 tons of toner (i.e., laser toner cartridge, photocopier toner, etc.) are generated annually. The residual toner powder in waste toner cartridges contains 34% iron oxide. In Australia, this is currently downcycled through its incorporation into road base; a waste of valuable resource. Silicon nitride is an advanced ceramic compound with outstanding thermomechanical properties, chemical inertness,14,15 high temperature/corrosive durability,16 outstanding oxidation and corrosion resistance, and high thermal conductivity.17 It has received considerable attention as one of the most important engineering ceramics for structural applications18,19 such as in chemical reactors, bearings for heat exchangers and in gas turbine components, 20−22 and automobile and truck engines.23 Si3N4 is also a wide band gap (5.3 eV)14 semiconductor24 and an excellent host material with high doping levels in which midgap levels can be introduced to tailor its electronic/optic properties by proper doping.25,26 As Si3N4 could prove an excellent host material, in terms of its mechanical strength, thermal/chemical stability, and high dopant concentration,25,26 it is considered a promising candidate for high-temperature microelectronic and optoelectronic applications,27,28 and for use in high-radiation environments.24 In its various one-dimensional (1D) forms, such as nanowires (NWs) and nanobelts (NBs), silicon nitride (Si3N4)29,30 not only possesses the outstanding properties of its bulk counterpart but also displays its own unique properties.31−33 These include superior photoelectric and mechanical properties, making Si3N4 nanostructures potentially useful in many important areas, such as nanoelectronics, energy conversion and storage, lasers, chemical sensing and catalysis, and light/ field emission devices,25 nanodevices, nanocomposites,34,35 and reinforcement materials.30 A range of methods to synthesize Si3N4 1D nanostructures with various morphologies (e.g., nanobelts, nanodendrites, and nanosheets)36 have been attempted and investigated. Traditionally, 1D Si3N4 nanomaterials are prepared via chemical vapor deposition (CVD),37,38 plasma enhanced CVD (PECVD),39 microwave plasma heating,40,41 carbothermal reduction,42 catalytic pyrolysis of a polymer precursor,16,43 sol−gel,44 and combustion techniques.45 The synthesis of Si3N4 nanobelts has been achieved via the following: vapor−solid thermal reactions between NH3 and SiO;46 FeCl2-catalyzed pyrolysis of a polysilazane precursor;24 NiCl2-catalyzed pyrolysis of amorphous silicon carbonitride precursors;47 and catalytic-thermal chemical vapor deposition (CVD) on carbon felt substrate using silicon.48 Si3N4 NWs also have been synthesized using various methods, such as carbothermal reduction and nitriding reactions at high temperatures,49−54 confined reactions using carbon nanotubes as templates,25 nitridation of silicon powders,49−51 oxideassisted growth,55 combustion under a high N2 pressure,56 hotfilament CVD or microwave plasma heating method,40,41 thermal decomposition of a polymer using FeCl2 powders as

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MATERIALS AND EXPERIMENTAL PROCEDURE

Waste computer monitors were collected from the Reverse E-waste company, Sydney, Australia. Residual (waste) toner powder from discarded toner cartridges was supplied by TES-AMM. We dismantled the waste computer monitors into their two main components, the glass computer monitor (GCM) screens and their plastics shells (PS), so we could use these fractions as sources of silica and carbon, respectively. After pulverizing the GCM sample using a ring mill, the resulting fine powder was analyzed using XRF. The PS was characterized in two ways. First, the PS was ground using a cryomill and then characterized using XPS. Second, an additional sample of PS was pyrolyzed at 1550 °C for 20 min under argon purge (1 L min−1) at atmospheric pressure. The pyrolyzed sample was then ground using a mortar, and the resulting powder was denoted as P-PS. The P-PS was then characterized using XRD. Synthesis of Si3N4 Nanowires. The crushed GCM powder and P-PS samples were mixed in the ratio of 3 to 2, with a total mass of 1 g. The resulting mixture was heated to 1550 °C in a hot tubular furnace (100 cm length × 5 cm diameter) under nitrogen purge (1 L min−1) for 130 min. After heat treatment, the sample was kept in the cold zone (i.e., furnace mouth) in nitrogen atmosphere for 10 min. The composition and elemental distribution of the resulting phases were further examined by X’pert PRO multipurpose XRD (MPD system), Fourier transform infrared spectroscopy (FTIR), FEI Nova Nano-SEM 230 (FE-SEM), transmission electron microscopy (TEM), and the N2 isothermal adsorption method. Synthesis of Si3N4 NWs with an Iron Catalyst. As above, the crushed GCM powder and P-PS samples were mixed in the ratio of 3 to 2. To investigate the effect of an iron catalyst on the morphology of the final product, waste toner powder was added to GCM and P-PS mixture. The resulting ratio of GCM, P-PS, and toner was 3:2:1. Likewise, the total mass of the mix was 1 g. The experiment was repeated. As above, the mixture was heated up to 1550 °C in a hot tubular furnace (100 cm length × 5 cm diameter) under nitrogen purge (1 L min−1) for 130 min. After heat treatment, the sample was kept in the cold zone (i.e., furnace mouth) in nitrogen atmosphere for 10 min. The composition and elemental distribution were then also investigated using the methods previously listed. B


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Figure 1. (a) General XPS spectrum of waste PS. (b) High-resolution XPS spectra of carbon region. (c) XRD spectra of P-PS.

Table 1. Chemical Composition of GCM oxides weight percent

Na2O 13.9

SiO2 70.3

Al2O3 1.8

CaO 7.8

MgO 4.6

MnO 0.01

Fe2O3 0.1

Figure 2. (a) XRD and (b) FTIR spectra of toner powder sample.

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RESULTS AND DISCUSSIONS Characterization of Materials and the As-Synthesized Si3N4 NWs. Materials (Waste-Derived Inputs). Plastic Shells (PS). Using XPS analysis, we identified the chemical bonding of the elements in the PS. As can be seen in Figure 1a, there are peaks which belong to the photoejected electrons from the orbital 1s of C, O, and Fe. 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 1b. The region clearly shows two peaks at 284.8 and 286.4 eV, corresponding to C−C and C−O bonds. Pyrolyzed Plastic Shells (P-PS). XRD was used. As seen in Figure 1c, the diffraction pattern of P-PS powder shows a sharp peak at around 43° and other small peaks corresponding to carbon. Several peaks are also detected in the XRD spectrum which belong to Fe−C and FeSi phases. The presence of such phases is related to the FeOx and SiO2 impurity in the PS. During pyrolysis of the PS at 1550 °C, the carbothermal reduction of FeOx and SiO2 occurred leading to the formation of Fe and FeSi alloy59 which can be easily separated from carbon as they are in the form of large solid spherical particles. Glass Computer Monitor (GCM). The elemental composition of the GCM was identified by XRF analysis. As shown in Table 1, the GCM consisted primarily of SiO2 (70.3%) and some oxide minerals such as Na2O (13.9%), Al2O3, CaO, and MgO. During heating to 1550 °C to synthesize the Si 3 N 4 nanostructures, the evolution of Na2O and other gases from

the plastics in the mix under investigation physically micronized the particles and prevented their agglomeration.60 The presence of other oxides (i.e., Al2O3, CaO, and MgO) promotes grain growth to rodlike b-Si3N4 from a-Si3N4 through solution-reprecipitation process.61 It has been shown that the presence of metal oxide additives has a crucial influence upon grain size and shape of Si3N4.62−64 For a high-purity product, these oxides can be removed by selective dissolution using acids. Toner Cartridge Powder. The X-ray diffraction pattern of the toner powder sample, Figure 2a, shows several large peaks corresponding to Fe3O4 and small peaks belonging to SiO2 and Fe2O3. The toner powder sample obtained from cartridge was used as a source of Fe. It was added to the starting materials (GCM + P-PS) to study its effect as a catalyst on the morphology of resulting Si3N4. The FTIR spectrum of the toner powder was also obtained, as shown in Figure 2b. The band at 699 cm−1 is related to the aromatic CC stretching modes of the functional group while the peaks at 728, 757, 830, and 1099 cm−1 are the result of the aromatic CH out of plane bonds, CH out of plane deformation, p-disubstituted benzenes, and COC asymmetric stretching mode, respectively. The band at around 1155 cm−1 belongs to the COC stretching vibration. IR adsorption bands at around 1243 and 1364 cm−1 are, respectively, assigned to amide III (CN stretching + amide CO in plane bonding) and CH bond (CH2 twisting). Presence of a bisphenol A-based polymer, such as polyester can be confirmed at bands around 1512 and 1610 cm−1. The bands C


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Figure 3. (a) XRD and (b) FTIR spectra of synthesized Si3N4 NWs.

Figure 4. SEM images of Si3N4 NWs from 3 μm to 500 nm magnification.

Figure 5. TEM images of Si3N4 NWs. (a) Diffraction contrast Si3N4 NW showing that dislocation is present in the trunk. (b) Branched Si3N4. (c) Illustration of the Eshelby twist in a dislocated NW.72

at 1720, 2849, and 2914 cm−1 can be attributed to CO carbonyl (stretch) and CH2 asymmetric and symmetric stretching modes, respectively.65 Characterization of the Synthesized Si3N4 NWs. The assynthesized sample was characterized using XRD and FTIR. The X-ray spectrum of the final product (Figure 3a) shows several sharp diffraction peaks corresponding to α-Si3N4 and β-

Si3N4, with β-Si3N4 the predominant crystalline phase. No diffraction peaks corresponding with unreduced SiO2 or unreacted carbon were found in the XRD patterns, indicating that the SiO2 was fully reduced and that no excessive carbon remained in the final product. Figure 3b shows the FTIR absorbance of the as-synthesized sample in a broad band, in the range 500−4000 cm−1. The peak D


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Figure 6. (a) SEM image of NWs with iron droplet on the tip. (b) Higher magnification. (c) Growth mechanism of the as-synthesized Si3N4 with waste toner powder: (I) Diffusion of SiO intermediate vapor and N2 into Fe droplet and formation of Fe−Si−N liquid droplet; (II) removal of CO and nucleation of Si3N4; (III) VLS-tip growth for Si3N4 nanobelt; (IV) final long Si3N4 nanobelt with iron droplet on the tip.

at approximately 684 cm−1 was ascribed to a Si−Si stretching mode.14,42,66 Two absorption peaks at approximately 807 and 1037 cm−1 could be attributed to Si−N stretching vibration mode of β-Si3N4, based on previous research.67 Notably, the FTIR result of the Si3N4 sample shows only a large peak for Si− N; no peaks for SiO2 are observed, providing further evidence of the full conversion of SiO2. The microstructure of as-synthesized Si3N4 product was examined by FE-SEM. The low-magnification SEM image (Figure 4) indicates the Si3N4 produced consists primarily of NWs structures. High-magnification SEM images (Figure 4) further reveal that most of NWs have twisted wirelike morphology (from 75 to 250 nm diameter and up to 2 μm length). Each of the NWs appeared to turn in a random direction and possessed branches which grew outward from the main stem, exhibiting T-shaped and Y-shaped morphologies. Moreover, the NWs do not show a smooth surface morphology; they have a twisted surface morphology. TEM images of the as-synthesized Si3N4 nanowires, Figure 5, reveal the Si3N4 NWs produced have a twisted pattern along their axis. This observation is helpful in developing a fundamental understanding of the NWs’ growth mechanism, as this particular morphology has been attributed to a screwdislocation-driven NW growth mechanism,68−70 in which axial screw dislocations provide the self-perpetuating steps to enable 1D nanomaterials.71 In low supersaturation conditions, the typical “layer-by-layer” (LBL) crystalline growth pattern is not energetically favorable. With screw dislocation, atoms are accommodated in an efficient manner which is energetically favorable.70 In 1953,72 Eshelby conducted a theoretical analysis of a screw dislocation lying parallel to the axis of a thin rod. He showed that the axial screw dislocation induces a torque in a rod of finite length which, in turn, leads to an elastic twist of the crystalline lattice and thus the formation of a chiral pattern if at least one end of the rod is free to rotate. This phenomenon has since been called the Eshelby twist (Figure 5c). In the examples of dislocation-driven NW growth detailed here, the screw dislocations that propagate 1D growth originate from spontaneously formed highly defective “seed” crystals.73 An initial spike in supersaturation of SiO triggered by the onset of N2 flow then provides a favorable low superstation of SiO that ensures dislocation-driven NW growth dominates over (or at least is not overwhelmed by) other competing growth mechanisms. The dislocations then propagate anisotropically to form Si3N4 NW trunks.71 This “Eshelby twist” is mathematically expressed as72

α=

cb π (R + r 2 ) 2

(1)

in which α is the twist rate (the twist of lattice in radians per unit length) in a thin cylindrical structure in terms of its geometric parameters (outer and inner radii, R and r), and b is the screw component of Burgers vector. c is a constant factor that depends on the shape of the cross-section; for a circular cross-section c = 1. In a work reported by Milhet et al.74 the dislocation Burger vector magnitude along the Si3N4 screw direction was reported as 0.29 nm. Assuming the average diameter of the as-synthesized Si3N4 in this work is 100 nm, the Eshelby twist will be 2.1° μm−1 or 171.4 μm per a full 360° rotation. It is worth noting the immediate branched NWs are perpendicular to the central NWs, suggesting the existence of epitaxial conditions with the growth being along the [001] or equivalent direction.65 The mechanisms of Si3N4 NWs formation have been investigated by several researchers.75−77 Si3N4 is fabricated via the carbothermal nitridation of SiO2, as per the following overall reaction: 3SiO2 (s) + 6C(s) + 2N2(g) → Si3N4(s) + 6CO(g)

(2)

Reaction 2 occurs via multiple intermediate steps. The initial physical contact between SiO2 and carbon particles results in the formation of SiO and CO gases (Reaction 3). SiO2 (s) + C(s) → SiO(g) + CO(g)

(3)

CO gas formed in reaction 3 in turn reacts with SiO2 and reduces it to SiO gas (as per reaction 4). The produced CO2 gas participates in Boudouard reaction (reaction 5) which takes place simultaneously with reaction 4, and CO is formed. SiO2 (s) + CO(g) → SiO(g) + CO2 (g)

(4)

C(s) + CO2 (g) ↔ 2CO(g)

(5)

This process has been reviewed through different mechanisms with different reaction pathways. However, the general opinion seems to be that SiO, as a volatile intermediate, plays a key role in producing Si3N4. With SiO present in system, αSi3N4 is formed via heterogeneous nucleation according to reaction 6, and further growth of Si3N4 nuclei occurs through a gas-phase reaction process, most likely the reaction of CO, SiO, and N2 shown in reaction 7.76 3C(s) + 3SiO(g) + 2N 2(g) → Si3N4(s) + 3CO(g) E

(6)


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cartridges was used as a source of Fe in a comparison experiment to investigate its catalytic effect on the morphology and structure of the Si3N4 synthesized. The addition of waste toner powder resulted in long Si3N4 nanobelts (up to 40 μm) with traces of liquid droplets on their tips, indicating the Fe had promoted the formation of the long and straight nanobelts via a novel route, thermal nanowiring. The Si3N4 NWs synthesized in this work can be considered promising candidates for heterocatalysis applications. The proposed sustainable process could simultaneously demonstrate a new low-cost means of synthesizing high-quality 1D Si3N4 NWs and the benefits of transforming waste to value. Such sustainable, cost-effective approaches to transforming waste into secondary resources and products can help address the challenge of global resource depletion.

3SiO(g) + 3CO(g) + 2N2(g) → Si3N4(s) + 3CO2 (g) (7)

The as-synthesized Si3N4 structures described here are promising candidates for photocatalysis and heterocatalysis applications. Such Si3N4 in combination with TiO2 (TiO2 loaded on Si3N4) is expected to exhibit higher photocatalytic activity than a TiO2 photocatalyst loaded onto other supports (SiO2, Al2O3, and SiC).78 Effect of Fe on the Formation of Si3N4 NWs. Fe is known to be an effective catalyst that plays a significant role in the growth mechanism of Si3N4. To examine the catalytic effects of iron on the formation of Si3N4 NW structures, a comparison experiment was run. Waste toner powder, as an impurity, was added to the original starting materials, GCM and P-PS, in a ratio of 3 GMC/2 P-PS/1 waste toner. Figure 6 shows the FE-SEM studies of the morphology of the final product. The experiment without waste toner added (Figure 4) generated fewer and shorter Si3N4 nanowires, which twisted and turned in random directions. However, adding waste toner resulted in (Figure 6a,b) the formation of long Si3N4 nanobelts with traces of liquid droplets on their tips, indicating the Fe had promoted the formation of these long and straight nanobelts. Iron catalysts were detected using energy dispersive X-ray spectroscopy (EDS) at tips of the nanowires. The traces of liquid droplets seen on the tips of nanobelts may indicate that the growth of the nanobelts was dominated by the well-established VLS mechanism, shown in Figure 6c. In this mechanism, the role of the impurity is to form a liquid alloy droplet of relatively low freezing temperature. The liquid droplet is a preferred site for deposition from the vapor, which causes the liquid to become supersaturated with them.14 The iron particles in the waste toner powder became molten owing to the diffusion of carbon. Liquid Fe acts as a preferred sink for arriving gases. Intermediate SiO vapor, generated according to eqs 2 and 3, along with N2 reached the surface of Fe droplet and diffused into it (Figure 6cI), forming eutectic Fe−Si−N liquid droplets. SiO is reduced by the carbon dissolved in the Fe, and Si and CO gas are formed (eq 8). When the concentration of Si−N in the Fe−Si−N liquid droplets exceeded saturation level, Si3N4 seeds began to precipitate from the supersaturated droplets (Figure 6cII). As the crystals successively grew, they lifted the liquid catalysts up and eventually grew to straight nanobelts (Figure 6cIII). By a continuation of this process the alloy droplets “ride” atop the growing nanobelts (Figure 6cIV).

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Corresponding Author

*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 This research was supported under the Australian Research Council’s Industrial Transformation Research Hub funding scheme (project IH130200025).

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Fe − Si − N(liquid)

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Si(l) + N(l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Si3N4(s)(tip − growth)

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SiO(g) + N2(g) + Fe(l) + C(s)dissolved → Fe−Si−N(l) + CO(g)

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CONCLUSIONS Si3N4 nanowires (NWs) were successfully synthesized via the carbothermal nitridation of samples of two problematic components of e-waste: the glass fraction of obsolete computer monitors and computers’ plastic shells (PS). The resulting Si3N4 NWs, with diameters of 75−250 nm, exhibited a twisting structure, turning in random directions with a twisted pattern along their axes which is attributable to a screw-dislocationdriven growth mechanism. Waste toner powder from discarded F


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