Research Article pubs.acs.org/journal/ascecg
Sustainable Fabrication of Protective Nanoscale TiN Thin Film on a Metal Substrate by Using Automotive Waste Plastics Songyan Yin,† Ravindra Rajarao,*,† Charlie Kong,‡ Yu Wang,‡ Bill Gong,‡ and Veena Sahajwalla† Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering and ‡Mark Wainwright Analytical Centre, Kensington, UNSW Australia, Sydney, New South Wales 2052, Australia
ACS Sustainable Chem. Eng. 2017.5:1549-1556. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 07/01/18. For personal use only.
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ABSTRACT: Automotive plastics are heterogeneous and complex mixtures of various plastics incorporated with significant amounts of additives and fillers. Automotive shredded residue (ASR) waste plastics are difficult to recycle and hence contribute significantly to environmental problems. In this study, a sustainable approach to fabricate protective nanoscale TiN thin film on a metal surface by using automotive waste plastics as titanium and carbon sources is investigated. The synthesis of nanoscale thin film of titanium nitride on steel substrate was carried out by using carbothermal reduction and nitridation reaction in a nitrogen atmosphere. The formation of TiN nanofilm on steel substrate was confirmed by XPS, XRD, and EDS techniques. The TiN film was characterized by HRTEM, and thickness was found to be in the range of 18 nm. The uniform and highly crystalline TiN thin film could provide good oxidation resistance to steel substrate. This innovative approach of using automotive waste plastics as titanium and reductant source for fabricating TiN film on metal could be an alternative to conventional techniques. This novel route reduces the manufacturing cost and also provides a sustainable recycling solution for automotive waste plastic. KEYWORDS: Automotive waste plastics, Recycling, Titanium nitride, Nanofilm, Oxidation resistance
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INTRODUCTION Plastics are inexpensive, lightweight, durable, and resistant to chemicals, which can readily be molded into a variety of products for wide range of applications and hence have revolutionized the world by being widely used in all aspects of our lives.1 Consequently, its production and waste generation is accelerating at a staggering rate, reaching up to 311 million tons in annual production and over one billion tons in waste accumulation.2 This is alarming as the majority of plastic waste might persist for centuries or even longer owing to its nonbiodegradable nature.3 Plastics revolution in the automotive industry can be traced back to the 1950s; various polymers such as acrylonitrile-butadiene-styrene (ABS), polypropylene (PP), polyethylene (PE), polyamide (PA), polycarbonate (PC), and their copolymers have been widely used.4 Over the past decades, use of plastics in automotive industry has grown from 30 kg per unit to around 150 kg due to beneficial advantages in vehicle weight reduction, fuel efficiency, and greenhouse gas emission.5 During the recycling of end-of-life vehicles, major metal components are recovered for a further recycling process, and left over plastics from auto parts and components which are produced at a rate of approximately 10 million tons per year were generally discarded in landfills.6 The increase in persistent environmental pollution caused by automotive waste like greenhouse gas emissions and metal leaching has limited the amount of automotive waste plastics going to landfills.7 European Directive 2000/53/EC states that only 5 wt % of an end-of-life vehicle waste may be landfilled.8 Recycling automotive waste plastics is extremely difficult and challenging © 2016 American Chemical Society
due to its heterogeneous nature and variations in the polymer composition.9 Automotive waste plastics are a complex mixture of almost all kinds of thermoplastics, rubbers, and thermosets.10 These plastics have a strong mutual entanglement with each other and contain a wide range of inevitable additives (SiO2, MgO, TiO2) as well as extraneous materials. Hence automotive waste plastics are extremely difficult and challenging for recyclers and cannot be easily recycled by a chemical process.11 Despite extensive research dedicated to automotive shredded residue (ASR) waste plastics in recent years, there are still no appropriate recycling practices.12 Therefore, a cost-effective and sustainable approach to transform and thereby recycle automotive waste plastics is urgently needed for the current scenario to reduce landfill issues and environmental pollution and also to achieve the European Directive 2005/64/EC target.13 Titanium nitride (TiN) is an attractive transition metal nitride with a sodium chloride crystal structure and has been widely used as a superior coating material since the last midcentury.14 TiN has remarkable physical and chemical properties such as high hardness, excellent chemical stability, high electronic conductivity, and radiant golden color. Due to outstanding properties, TiN is widely used in various applications such as microelectronic devices, solar cells, mechanical tools, and in cosmetic industries.15 The fabrication Received: September 19, 2016 Revised: November 20, 2016 Published: December 5, 2016 1549
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gas was introduced at a flow rate of 1 L/min into the horizontal tube furnace for the nitriding reaction. The assembly was initially placed in the cold zone (∼300 °C) of the furnace to avoid thermal shock. After 5 min, assembly with the sample was pushed into the hot zone and maintained at a temperature of 1150 °C over 20 min. After 20 min, the sample was removed from the hot zone and placed in the cold zone for another 15 min to minimize sample oxidation. CH4, CO, and CO2 gases evolved during the process were monitored by the IR gas analyzer unit (Advance Optima model ABB AO2020). For a reference run, plain carbon steel was solely processed in the horizontal furnace under the same condition. The experiment was repeated more than 10 times to establish its reproducibility. After heat treatment, an automotive waste plastics treated sample, shiny gold layer coated steel substrate as shown in Figure 2a, and its reference − an isolated (without automotive waste plastic) steel sample after heat treatment as shown in Figure 2b, were collected and subjected for characterization. Characterization of Nano Scale Thin TiN Film on Steel Substrate. The chemical state and elemental composition of thin film deposited on steel substrate were investigated by an Xray photoelectron spectrometer (XPS, Thermo ESCALAB250Xi). The crystallographic characteristic of TiN film was identified by an X-ray diffraction technique (XRD, Bruker D8 TXS). The surface morphology of reference steel and a TiN coated steel sample was investigated by a field emission scanning electron microscope (FE-SEM, JEOL 7001). By a field emission transmission electron microscope (TEM, Philips CM200) in conjunction with energy dispersive spectroscopy (EDS, Bruker QUANTAX), microstructure characterization and relative elemental distribution were studied. A dual electron/focused ion-beam system (FEI Nova Nanolab 200) was used for preparation of cross-sectional TEM specimens.
routes such as chemical vapor deposition, physical evaporation, and magnetron sputtering are widely employed to coat TiN thin film for various applications.16 All these techniques require expensive titanium precursors (99.99% pure), which greatly restrict the application to a larger extent even though it can greatly enhance the corrosion and wear resistance of substrate materials.17 Automotive waste plastics generally are a mixture of various polymers such as polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyurethane (PUR), and acrylonitrile butadiene styrene (ABS) which is extensively used in car components and parts. These plastics are carbon-bearing materials and are mostly manufactured with a titanium dioxide additive for UV and thermal stability purposes. In view of its complex combination of various plastics incorporated with a significant amount of additives, like titanium dioxide, automotive waste plastics have the potential to be used as a good resource for generating thin TiN film on a metal surface. In this work, we report a novel and sustainable approach to synthesize TiN nanofilm by using automotive waste plastics as titanium and carbon reductant precursors. A thin layer of TiN film, with a thickness of approximate 18 nm, was successfully synthesized on steel substrate by heating with automotive waste plastics in a nitrogen atmosphere at elevated temperature. Detail characterization of the nanoscale TiN thin film deposited on steel substrate was carried out by using various analytical tools such as an X-ray photoelectron spectrometer (XPS), high resolution X-ray diffraction (HRXRD), a field emission scanning electron microscope (FE-SEM), focused ion beam (FIB), a high resolution transmission electron microscope (HRTEM) fitted with energy dispersive X-ray spectroscopy (EDS), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. This innovative approach of using automotive waste plastics as a titanium and carbon reductant resource for fabricating TiN film on a metal surface could be an alternative to conventional expensive titanium precursors.
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RESULTS AND DISCUSSION Characterization of Automotive Waste Plastics. The elemental analysis by using XRF spectroscopy, a LECO analyzer, and ash analysis was performed to determine the various elements present in automotive waste plastics. Proximate analysis was also performed to determine the moisture and volatile matter. The results of elemental and proximate analysis are shown in Table 2. The proximate and LECO analysis indicate the presence of carbon based polymers and small quantity of ash (5.28 wt %). XRF results of the ash derived from automotive waste plastics show the presence of titanium oxide and silica in significant quantity. Automotive waste plastics are composed of various types of plastics, and hence identifying the types of polymer in plastics FTIR measurements was carried out. The different types of an automotive waste plastics sample obtained from OneSteel Recycling were distinguished based on their color and hardness. The segregated plastics were subjected for FTIR measurements to identify the functional groups. Based on the FTIR characteristic peaks, polymers such as polypropylene, polyethylene, polycarbonate, polyamide, acrylonitrile-butadienestyrene, and copolymer (polycarbonate-acrylonitrile-butadiene-styrene) were identified. Figure 3 shows the FTIR spectra of major polymers present in automotive waste plastics. Characterization of Nanoscale Thin TiN Film on Steel Substrate. After heat treatment of steel substrate with automotive waste plastics at high temperature, a thin layer of golden color was seen on a steel surface (Figure 2a). The coated steel substrate was collected and characterized by
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EXPERIMENTAL SECTION Materials. Automotive Waste Plastics. Shredded automotive waste plastics of a size around 1−2 mm were supplied by OneSteel Recycling, Australia and were used as received in this study. Elemental analysis of obtained automotive waste plastics was investigated by using X-ray Fluorescence (XRF, PANalytical MINIPAL 4, EDXRF Spectrometer) and a LECO analyzer (TruSpec Analyzer: CN Module and S Module). An infrared spectroscopy study was also performed on automotive waste plastics to determine polymeric content by using Spectrum 100, PerkinElmer infrared spectroscopy. Steel Substrate. Sliced LECO carbon calibration steel of diameter 4 mm, thickness 1 mm was used as a metal substrate for TiN generation study. The carbon content of steel substrate was 0.39 wt %. Its basic chemical composition analyzed by ICPMS is given in Table 1. Experimental Method. The schematic diagram of the experimental setup used in this study is shown in Figure 1. Approximately 2.8 g of automotive waste plastics along with steel substrate was placed in a covered alumina cruciblegraphite sample holder assembly. High purity (99.9%) nitrogen Table 1. Alloy Composition for Steel unit
Ca
K
Mn
Ni
Zn
Fe
wt %
0.04
0.01
0.67
0.16
0.01
99.1 1550
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Figure 1. Schematic representation of carburization experiment.
Figure 2. (a) Shiny gold TiN coated steel and (b) a reference steel sample, heat treated under the same condition, in isolation.
Table 2. Composition Analysis for Waste ASR Plastics proximate analysis (wt % as received) moisture 0.08 ash 5.28 volatile matter 92.9 fixed carbon 2.22 ultimate analysis (wt % as received) C N S O elemental analysis (X-ray fluorescence, wt TiO2 SiO2 MgO CaO Al2O3 Fe2O3 SO3 BaO P2O5 ZnO CuO Na2O K2O Mn3O4 Cr2O3 SrO ZrO PbO HfO2 NiO
74.81 2.04 0.12 0.27 % of ash) 37.222 23.627 13.106 10.992 4.12 3.082 1.582 1.033 0.832 0.733 0.675 0.654 0.249 0.104 0.103 0.047 0.044 0.044 0.024 0.02
Figure 3. FTIR spectra of ASR waste plastics.
XPS Analysis. The XPS technique was used to characterize and to study the chemical structure of gold film deposited on steel substrate. The coated substrate sample was ultrasonically cleaned in acetone for 5 min to remove hydrocarbon contamination on the surface. XPS analysis was performed for the outmost surface and also on the etched surface (90s and 180s by argon ion beam of 3 keV) of gold film. The XPS survey spectra of three surfaces (nonetched, 90 and 180 s etched) of film indicate the presence of O 1s, Ti 2p, N 1s, and C 1s peaks (Figure 4a). The spectral results of the etched surface indicate the decrease in intensity of carbon and oxygen peaks and the increase in intensity of titanium and nitrogen peaks significantly compared to the nonetched outer surface. The atomic concentration (%) of carbon, oxygen, titanium, and nitrogen in all three surfaces of film is described in Figure 4c. The outermost layer was dominated with carbon and oxygen, while titanium and nitrogen dominate the inner etched layers of film. Hence the golden shiny thin film coat on steel substrate is predominantly composed of TiN with unavoidable contamination or oxidation occurring on its outmost surface. XPS spectra of the Ti peak can be deconvoluted to 4 components as shown in Figure 4b. For the outmost surface of
various techniques to determine the composition, chemical structure, and thickness of the golden layer. 1551
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Figure 4. (a) XPS survey spectrum of an ASR treated steel sample, top surface, etched 90 s and 180 s, (b) the core level of Ti 2p3, (c) C, O, N, Ti depth distribution on the surface of an ASR treated steel sample, and (d) the core level of N 1s.
film, the strong peak at 457.9 eV corresponds to titanium oxide, while the less intense peaks at 455.2 and 454.7 eV refer to stoichiometric titanium nitride, TiN.18 For the surface, etched after 90 and 180 s, the two strong peaks at 454.8 and 455.8 eV were observed and correspond to titanium nitride.19 This is in good agreement with the published XPS results for titanium nitride fabricated using traditional magnetron sputtering techniques.20 The deconvoluted nitrogen peak is shown in Figure 4d; the presence of 397.2 peak in the etched surface of film clearly indicates the formation of TiN film on steel substrate. The absence of peak 397.2 in the spectrum of the outermost film surface indicates the absence of TiN due to contamination of the surface. The XPS spectrum for N1 s peaks as given in Figure 4d also shows high consistency with that of Ti 2p3/2 and low intensity in the outmost surface, but significant peaks positioned at the binding energy of 397.2 eV are attributed to the formation of the Ti−N bond. PS results confirm the formation of titanium nitride film on steel substrate by using automotive waste plastics as resources. The presence of titanium oxide on the outmost surface layer indicates that oxidation occurred after the TiN film formation and took place during the removal of the sample from furnace to air atmosphere.
XRD Analysis. The crystallographic characteristic of TiN film coated on steel substrate was investigated by using X-ray diffraction. Figure 5a and Figure 5b show the XRD pattern of virgin steel substrate and TiN thin film coated steel substrate. As shown in Figure 5a, (110), (200), and (211) peaks at 2θ = 44.7, 65.0, and 82.3 correspond to iron indicating that the virgin steel sample was composed of pure iron. The XRD pattern of TiN coated steel substrate shows additional peaks at 2θ = 36.6, 42.6, 61.8, 74.1, and 78.2 which are characteristic (111), (200), (220), (311), and (222) peaks of TiN.21 TiN film coated on steel substrate belongs to the Fm3̅m space group with a lattice parameter of 0.425 nm, and no significant preferred orientation was observed. The steel sample was treated at the same conditions (20 min, 1150 °C) without automotive plastics to determine the extent of oxidation and oxidation products. As seen in Figure 5c, the XRD pattern of the treated steel sample shows both hematite (Fe3O4) and magnetite (Fe2O3) phases. The presence of both hematite and magnetite structures indicates that the steel was oxidized under a temperature lower than 570 °C. The oxidation would have occurred during the sample removal from nitrogen gas atmosphere to air atmosphere during removal from a furnace as aforementioned in the XPS analysis section. 1552
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Figure 7. (a) TEM-EDS for steel treated with ASR at 1150 °C for 20 min, (b) TEM-EDS for steel treated without auto waste plastics at 1150 °C for 20 min, (c) TiN nano-thin film on the steel surface treated with ASR at 1150 °C for 20 min, and (d) iron oxide layer formed on the surface for steel treated without auto waste plastics at 1150 °C for 20 min.
Figure 5. (a) XRD pattern for virgin steel without any treatment, (b) XRD pattern for steel treated with ASR at 1150 °C for 20 min, (c) XRD pattern for steel treated without auto waste plastics at 1150 °C for 20 min, and (d) reference pattern of iron oxide 01-089-2388 and 01-089-2810.
images and selected area diffraction pattern (SAED) of the TiN-steel coated sample and the reference heat treated sample. Figure 7a gives the composition variation across the interface between the reference steel sample and TiN coated steel substrate (between steel substrate and the deposited golden TiN-steel). The carbon, gold, and platinum strip was observed on the top of TiN thin film. The image also shows that the steel substrate surface was uniformly coated with a thin layer TiN film with thickness of around 18 nm. This TiN coating is dense and adheres to the steel substrate very well. The interface between steel substrate and TiN film is extremely sharp, and no discrete layer was observed. The SAED pattern of TiN grain of thin film is shown in Figure 7c. The distinct spot pattern and good background indicate the highly crystallized nature of TiN film. For selected TiN grain, tilting the specimen at the ⟨111⟩ beam direction, the interplanar angle between the principle spots was found to be 60°, which is in good agreement with the (110) reflections of NaCl type cubic structured TiN. The result is also consistent with the observed XRD and as referred to in ICSD collection code 01-087-0628 for TiN. The morphology of the reference heat treated steel sample without auto waste is shown in Figure 7b and Figure 7d; the TEM images clearly indicate the layer of oxide on the surface of the steel sample with thickness of approximately 500 nm. The observed oxide layer shows the porous nature and defects which exhibit a distinct seam adjacent to the steel substrate, which could negatively lower wear and corrosion resistance of the steel.23 The detailed XPS, XRD, FE-SEM, and TEM investigation of gold coated steel substrate proves the successful fabrication of protective TiN thin film on steel substrate by carbothermal reduction using automotive waste plastics as a titanium resource. The coated sustainable TiN film was dense and highly crystalline and can provide good oxidation resistance for the steel substrate. Mechanism. Automotive waste plastics were powdered by using a cryogenic grinder to use for X-ray diffraction analysis.
However, this oxidation of the steel sample can be avoided, as demonstrated in Figure 5b, when the steel was treated with auto waste plastics under a given condition, which clearly indicates that newly formed synthetic TiN film by using automotive waste plastics as a titanium source has an excellent oxidation resistance. FE-SEM and TEM Analysis. The surface morphology of both a reference steel sample and golden TiN thin film steel substrate was studied by using the FE-SEM technique. As shown in Figure 6, it can be observed from the images that the
Figure 6. (a) Surface morphology for steel treated with ASR at 1150 °C for 20 min and (b) surface morphology for steel solely treated at 1150 °C for 20 min.
surface of TiN steel substrate has a smooth and dense surface appearance compared to a nontreated steel sample. To further understand the thickness and nature of the TiN film, cross sections of film were characterized by using TEM. The procedure to prepare the TEM sample using a focused ion beam (FIB) technique was followed as described in the literature.22 A thin layer of carbon and sputter-coated gold film was separately deposited on the specimen surface, followed by platinum strip deposition in the FIB on the area of interest to promote uniform milling and protect the edge structure of samples from damage. Figure 7 shows the bright field TEM 1553
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Figure 8. (a) XRD patterns of ASR and (b) evolved gas analysis of ASR.
Reaction 1 is a sum of a more complex set of reactions. The initiation of reaction was started from the reduction of titanium dioxide to lower oxidation valence titanium oxides like Ti5O9, Ti4O7, and then Ti3O5. The last step is nitriding Ti3O5 to synthesize TiN through a substitution of oxygen atoms by nitrogen atoms. The complex set of reactions can be described by eqs 2−5 shown below:
The X-ray diffraction pattern (Figure 8a) revealed the existence of a significant amount of titanium dioxide by the presence of peaks at 27.7, 36.1, 41.4, 54.4, 56.8, 69.2, and 82.4, which is in good agreement with the XRD reference pattern of 01-0760649, TiO2 (rutile). In addition to titanium oxide peaks, intense peaks at 6.9, 18.7, and 21.5 corresponding to polypropylene were also present. The other small and broad peaks present may correspond to other polymers and oxides in automotive waste plastics. Automotive waste plastics were also subjected to 1150 °C, and evolved gases during pyrolysis were analyzed by using IR gas analyzer. As automotive plastics are predominantly composed of polypropylene and polyethylene, liberation of smaller molecules such as CO, CO2, CH4, and other gases is expected. The IR gas analysis data obtained by pyrolysis of automotive waste plastics is shown in Figure 8b. At the beginning of the pyrolysis reaction (0−3 min), the liberated gas was primarily composed of methane (CH4) with a maximum concentration of 1.0*105 ppm, which was in good agreement with the polypropylene literature data. A small quantity of CO2 and CO was also observed along with methane around 0−3 min of pyrolysis reaction. The concentration of methane and CO2 decreased with time and approached zero around 6 min of reaction, but CO gas was generated at steady rate with a slight decrease over time. The generation of higher molecular weight gases like C2H2 and C2H4 was not monitored by an IR gas analyzer. Methane is well-known as an efficient reducing agent for metal oxide reduction.24 Use of methane rather than graphite or charcoal as a reducing agent in the reduction of metal oxide could not only decrease the operating temperature but also diminish the emission of greenhouse gases.25 Application of methane as a reducing agent to produce TiN has been reported in recent work.26 The overall carbothermal reduction and nitration of titanium dioxide to produce TiN with standard Gibbs function at 1423 K can be written as
5TiO2 + CH4 → Ti5O9 + 2H 2 + CO ΔG = −88 kJ/mol (T = 1423 K)
4Ti5O9 + CH4 → 5Ti4O7 + 2H 2 + CO ΔG = −71 kJ/mol (T = 1423 K)
(3)
3Ti4O7 + CH4 → 4Ti3O5 + 2H 2 + CO ΔG = −62 kJ/mol (T = 1423 K)
(4)
2Ti3O5 + 10CH4 + 3N2 → 6TiN + 20H 2 + 10CO ΔG = −572 kJ/mol (T = 1423 K)
(5)
From the gas analysis mentioned above, the decomposition of waste plastics at high temperature occurs very fast, hence methane and other hydrocarbons from automotive waste plastics generate quickly after subjecting waste plastics to high temperature. It is clearly seen from the recorded IR gas analysis results that methane and CO2 generation reduces to zero ppm at around 6 min of pyrolysis; the generation of CO gas was at a steady rate, slightly decreasing over time, which establishes the continuation of carbothermal reduction and nitriding reactions after 6 min. It is well-known that using carbon black as a reducing agent to reduce titanium dioxide is a conventional route to synthesize titanium nitride through carbothermal reactions and nitridation in a nitrogen atmosphere.27,28 At high temperature (1150 °C), CH4 liberated from automotive plastics waste could be easily decomposed into C and H2, which can be written as
2TiO2 + 4CH4 + N2(g) → 2TiN + 4CO(g) + 8H 2 ΔG = −243 kJ/mol (T = 1423 K)
(2)
(1)
CH4 → C + 2H 2
ΔG = − 66 kJ/mol (T = 1423 K) (6)
In this study, the reaction temperature 1423 K was adopted for energy saving purposes; automotive waste plastics were the source of both TiO2 and reducing methane gas; nitrogen gas was introduced in this system with a flow rate of 1 L/min.
Similarly, other hydrocarbons like C2H2, C2H4, etc. would also crack into carbon black and hydrogen under high temperature. The relative reaction can be simplified as 1554
DOI: 10.1021/acssuschemeng.6b02253 ACS Sustainable Chem. Eng. 2017, 5, 1549−1556
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1 yH2 2
utilized to reduce titanium oxide to titanium nitride by using nitrogen for the nitridation process. (e) This innovative and sustainable approach of TiN film fabrication can be an effective solution to divert automotive waste plastics from landfills and also may reduce dependency on using expensive titanium as raw material for fabrication purposes.
ΔG is hydrocarbon dependent (7)
The presence of metals (like Fe substrate) can accelerate carbon black production by acting as a catalyst. Traditionally, the ideal temperature to synthesize TiN by solid carbon through a carbothermal reaction is in the range 1300−1800 °C as reported in the literature.29 The overall reaction, with standard Gibbs function at 1573 K, can be written as
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Corresponding Author
*E-mail:
[email protected].
2TiO2 + 4C + N2(g ) → 2TiN + 4CO(g ) ΔG = −55 kJ/mol (T = 1573 K)
ORCID
(8)
Ravindra Rajarao: 0000-0002-2846-4232
However, the reaction temperature could be decreased with the decrease of concentration of CO in this system.30 It can be deduced from the gas analysis mentioned above that the composition of CO gas in this system is less than 5% with a maximum of 6942 ppm; while N2, as a main carrier gas, could account for at least 50% of the whole gas system. Thus, the equation mentioned above can be modified into
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported under Australian Research Council’s Industrial Transformation Research Hub funding scheme (project IH130200025).
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2TiO2 + 4C + N2(g , 50%) → 2TiN + 4CO(g , 5%) ΔG = −112 kJ/mol (T = 1473 K)
AUTHOR INFORMATION
REFERENCES
(1) Hopewell, J.; Dvorak, R.; Kosior, E. Plastics recycling: challenges and opportunities. Philos. Trans. R. Soc., B 2009, 364, 2115−2126. (2) Plastics European, Plastics-the facts 2015, An analysis of European plastics production, demand and waste data, 2015. (3) Barnes, D. K. A.; Galgani, F.; Thompson, R. C.; Barlaz, M. Accumulation and fragmentation of plastic debris in global environments. Philos. Trans. R. Soc., B 2009, 364, 1985−1998. (4) Maxwell, J. Plastics in the automotive industry; Woodhead Publishing Limited: Cambridge, England, 1994. (5) Duval, D.; MacLean, H. L. The role of product information in automotive plastics recycling: a financial and life cycle assessment. J. Cleaner Prod. 2007, 15, 1158−1168. (6) Sakai, S.; Yoshida, H.; Hiratsuka, J.; Vandecasteele, C.; Kohlmeyer, R.; Rotter, V. S.; Passarini, F.; Santini, A.; Peeler, M.; Li, J.; Oh, G.; Chi, N. K.; Bastian, L.; Moore, S.; Kajiwara, N.; Takigami, H.; Itai, T.; Takahashi, S.; Tanabe, S.; Tomoda, K.; Hirakawa, T.; Hirai, Y.; Asari, M.; Yano, J. An international comparative study of end-of-life vehicle (ELV) recycling systems. J. Mater. Cycles Waste Manage. 2014, 16, 1−20. (7) Gonzalez-Fernandez, O.; Hidalgo, M.; Margui, E.; Carvalho, M. L.; Queralt, I. Heavy metals’ content of automotive shredder residues (ASR): Evaluation of environmental risk. Environ. Pollut. 2008, 153, 476−482. (8) Directive 2000/53/EC on end of life vehicles, UK Government Consultation Paper, Department of Trade and Industry: 2001. (9) Vermeulen, I.; Van Caneghem, J.; Block, C.; Baeyens, J.; Vandecasteele, C. Review: Automotive shredder residue (ASR): Reviewing its production from end-of-life vehicles (ELVs) and its recycling, energy or chemicals’ valorisation. J. Hazard. Mater. 2011, 190, 8−27. (10) Buekens, A.; Zhou, X. Recycling plastics from automotive shredder residues: a review. J. Mater. Cycles Waste Manage. 2014, 16, 398−414. (11) Nourreddine, M. Recycling of auto shredder residue. J. Hazard. Mater. 2007, 139, 481−490. (12) Cossu, R.; Lai, T. Automotive shredder residue (ASR) management: An overview. Waste Manage. 2015, 45, 143−151. (13) Directive 2005/64/Ec of the european parliament and of the council, Official Journal of the European Union: 2015. (14) Carmalt, C. J.; Newport, A.; Parkin, I. P.; Mountford, P.; Sealey, A. J.; Dubberley, S. R. Synthesis of TiN thin films from titanium imido complexes. J. Mater. Chem. 2003, 13 (1), 84−87. (15) Sen, K.; Banu, T.; Debnath, T.; Ghosh, D.; Das, A. K. Towards a comprehensive understanding of the chemical vapor deposition of
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Similarly, this reaction is also a sum of a more complex set of reactions initiated from the reduction of titanium dioxide to lower oxidation valence titanium oxide and then nitriding to synthesize TiN through a substitution of oxygen atoms by nitrogen atoms. Furthermore, the hydrogen generated in reactions 1−7 could facilitate the carbothermal reduction of titanium dioxide.31 When the reduction of titanium dioxide is carried out in a hydrogen containing gas, hydrogen could participate in reduction reactions to reduce titanium dioxide into its suboxides to accelerate the carbothermal reduction of titanium dioxide. Overall, synthesis of TiN by using automotive waste plastics is a chain of complicated reactions; it involves the decomposition of waste plastics and the hydrocarbon products, followed by carbothermal reduction as well as the nitriding of titanium oxide from TiN thin film on steel substrate. This innovative approach of using automotive plastic to fabricate TiN thin film on valuable metals has significant industrial potential and also could be used as a supplementary titanium source for TiN synthesis.
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CONCLUSIONS A novel approach of using automotive waste plastics as a titanium source was investigated to fabricate TiN thin film on steel substrate. The main conclusions established from the results are as follows: (a) The results revealed the feasibility and advantages of using automotive waste plastics as a titanium resource to fabricate TiN film on metal substrate. (b) The temperature of 1150 °C and 30 min duration was sufficient to effectively generate TiN nano thin film on steel substrate. (c) The XPS, XRD, SEM, and TEM results establish that TiN film was dense and highly crystalline, and thickness was around 18 nm. (d) The mechanism of TiN film involves a complex chain of reactions i.e. decomposition of waste plastics and the hydrocarbon products to form carbon; produced carbon is 1555
DOI: 10.1021/acssuschemeng.6b02253 ACS Sustainable Chem. Eng. 2017, 5, 1549−1556
Research Article
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DOI: 10.1021/acssuschemeng.6b02253 ACS Sustainable Chem. Eng. 2017, 5, 1549−1556
