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Original article
13 (
1
); 3082-3091
doi:
10.1016/j.arabjc.2018.08.016

TiO2/carbon dots decorated reduced graphene oxide composites from waste car bumper and TiO2 nanoparticles for photocatalytic applications

,
 Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982 Dammam 31441, Saudi Arabia

⁎Corresponding author. hhbesisa@uod.edu.sa (Hanan H. Mohamed)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Upcycling waste car bumper to value added carbon based nanomaterials has been achieved through facile catalytic pyrolysis. As synthesized TiO2 nanoparticles has been used for the first time as catalyst during pyrolysis process of the plastic waste. The quality and the performance of the produced carbon nanomaterials have been evaluated through different characterization techniques. The results show that, while rGO were produced from the thermal pyrolysis of waste car bumper in absence of TiO2 catalyst, rGO decorated with carbon dots have been produced in presence of TiO2 catalyst. Interestingly, simultaneous TiO2 rutile to anatase phase transformation and carbon nanomaterials formation has been occurred upon the catalytic transformation of the waste car bumper, however, an expected anatase to rutile transformation has been observed upon thermal treatment of TiO2 in the absence of the bumper. The photocatalytic performance of the created TiO2/CDs/rGO has been tested for the photocatalytic degradation of methylene blue. This paper provides a first attempt as an innovative approach to the upcycling waste into novel and high performance carbon nanomaterials for the application in photocatalysis. This innovative approach can be utilized to create various carbon nanocomposites utilizing metal oxides as catalyst and different plastic waste system as a carbon resource.

Keywords

Waste car bumper
TiO2
Catalytic pyrolysis
TiO2/CDs/rGO
Photocatalytic activity
1

1 Introduction

Due to rapid economic development of industries and improvement of living standards, non-biodegradable man made materials such as polymer waste has increased dramatically. The polymer based materials including but not limited to, automobile parts, food packaging materials, electric component and building materials. The world production of polymer based waste reached 280 million tons in 2011 (PlasticEurope, 2012). Disposal of such huge quantities of waste into the environment is of great concern. Therefore, management of polymer waste is of crucial to reduce their environmental impact.

The polymer waste can be managed by recycling process in which waste materials are collecting, processing or re-manufacturing (either biologically, mechanically, chemically or thermally) into new products, and to market those products (making the wastes recycled) (Nishida, 2011; Kaufman and Themelis, 2009; Encyclopædia Britannica, 2013; Mueller, 2006; Shah et al., 2008; Sivan, 2011; Butler et al., 2011). Recycling is one of the favorite methods in the waste management hierarchy. However, the rate of global recycling is still not as high as it would be economically and environmentally desirable. Therefore, upcycling process of waste management has been developed in which the quality as well as the value of the recycled products has been upgraded.

Due to the fact that carbon is the major constituent of polymer, the polymer waste can therefore provide a carbon source for carbon-based value-added products. Such products include light hydrocarbons (Lovett et al., 1997; Buekens and Huang, 1998; Yang and Lin, 2009; Rasul et al., 2013), carbon black/activated carbon (Esfandiari et al., 2012), carbon fibers (Liu and Kumar, 2012; Norberg et al., 2013), fullerenes (Howard et al., 1991; Howard et al., 1994; Richter et al., 1997), carbon nanotubes (Zhuo and Levendis, 2014; Dai, 2002; Popov, 2004; De Volder et al., 2013; Dervishi et al., 2009), nanodiamond and graphene (Geim and Novoselov, 2007). Therefore converting waste plastics into such value-added products is an excellent example of upcycling. Amongst carbon based nanomaterials, reduced graphene oxide (rGO) and carbon nanotubes (CNTs) are promising carbon nanomaterial, possesses extraordinary electrical property, superior carrier transport, a high specific surface area and excellent thermal/chemical stability (Gao et al., 2010; Williams et al., 2008; Luo et al., 2012). In addition, carbon quantum dots (CDs) have been attracting more attention due to their chemical inertness, optical stability, biocompatibility and low toxicity (Baker and Baker, 2010; Liu et al., 2009; Li et al., 2012).

Amongst current technologies to transform plastic waste to value added carbon based nanomaterials, pyrolysis has shown the promise. Pyrolysis is a thermal decomposition in inert atmosphere (Miskolczi et al., 2009). Various products can be obtained from the pyrolysis of plastic waste including liquid phases (e. g., gasoline, liquid oils) and solid phases (carbon nanomaterials) in addition to the generating gases (Kongkarat et al., 1823; Yoshioka et al., 2004). The composition, quality and yield of the pyrolyzed polymer waste product is affected by a number of factors such as temperature (Yoshioka et al., 2004), retention time (Lopez et al., 2009), feedstock composition (Tröger et al., 2013), use of catalyst (Lerici et al., 2015; Miskolczi et al., 2009), moisture content (Chen et al., 2014), heating rate (Sharma et al., 2014) and particle size (Luo et al., 2010).

The use of catalyst is an important parameter for the pyrolysis of polymer waste. Catalysts can improve the quality and yield of the pyrolysis product. Different catalysts have been used in the catalytic pyrolysis of plastic wastes such as, Ni (Tang et al., 2005; Mishra et al., 2012), NiO (Gong et al., 2014), Ni/Mo/MgO (Bajad et al., 2015), zeolites (Syamsiro et al., 2014; Lopez et al., 2011), Fe2O3 (Sarker and Rashid, 2013), and Ca(OH)2 (Sarker et al., 2011). The catalyst effect on the polymer waste pyrolysis depends on the catalyst composition, electronic structure, catalyst surface area, porosity and pH.

This study focuses on the processing of innovative approach for transforming polymer waste into value added carbon based nanomaterials. Automobile waste particularly car bumper has been exploited as resources for carbon nanomaterials using catalytic pyrolysis process. To the best of my knowledge, first attempt has been made to use TiO2 nanoparticles as catalyst for pyrolysis of polymer waste. The quality and performance of the product have been evaluated through different characterizations and tests of nanostructure and physical properties. The characterization methods include XRD, SEM, TEM, TGA, Raman spectroscopy and BET surface area and N2 adsorption desorption measurements. The results shown that, the morphology of the produced carbon nanomaterials changed upon using TiO2 as a catalyst in pyrolysis process. While porous reduced graphene oxide sheets were produced from the thermal pyrolysis of waste car bumper in absence of TiO2 catalyst, reduced graphene oxide sheets decorated with carbon dots have been produced from the catalytic pyrolysis of waste car bumper in presence of TiO2 catalyst. Interestingly, TiO2 phase transformation from rutile to anatase has been occurred upon thermal treatment of TiO2 in presence of waste car bumper. In addition, the photocatalytic activity of the created carbon nanocomposites has been evaluated by measuring the photocatalytic degradation of methylene blue dye.

2

2 Experimental method

2.1

2.1 Synthesis of TiO2 nanoparticles

TiO2 nanoparticles were synthesized by hydrolysis of TiCl4 followed by hydrothermal treatment. In a typical synthesis, 10 ml of pre chilled TiCl4 was slowly added to 250 ml distilled water under ice bath and vigorous stirring. After that, 5 g urea was dissolved to the resulted suspension and then allowed to stirring for further 1 h. The resulted suspension was then transferred to 500 ml Teflon-lined autoclave and heated in the air oven at 120 °C for 15 h. After the hydrothermal treatment white precipitate was obtained. The product was collected and washed with deionized water several times by centrifugation and then dried at 70 °C for 2 h.

2.2

2.2 Catalytic transformation of waste car bumpers to carbon based nanomaterials

Waste car bumper has been transformed to carbon-based nanomaterials by catalytic thermal decomposition method. In a typical method, a piece of waste car bumper was first cut into small pieces and then washed with distilled water and dried in air. 10 g of the bumper pieces was transferred to Pyrex dish and placed in stainless steel utensil. A desired amount of the as synthesized TiO2 nanoparticles (0.5, 1 g) was added to the bumper pieces. After that, the utensil was tightly closed and then transferred into muffle furnace and heated at 700 °C for 1 h to afford shiny black powder. For comparison the waste bumper was thermal degraded in absence of TiO2 nanoparticles. The samples symbolized as PS700, PST0.5 and PST1 respectively with TiO2 content of 0 g, 0.5 g and 1 g respectively and TiO2 700 for thermally treated TiO2 at 700 °C. Catalytic transformation of waste car bumper to carbon-based nanomaterials is represented in Fig. 1. The mass balance for the catalytic pyrolysis of waste car bumpers at different concentration of TiO2 nanoparticles at 700 °C is represented in Table 1.

Schematic representation of catalytic transformation of waste car bumpers to carbon based nanomaterials.
Fig. 1 Schematic representation of catalytic transformation of waste car bumpers to carbon based nanomaterials.
Table 1 Mass balance for the catalytic pyrolysis of waste car bumpers at different concentration of TiO2 nanoparticles at 700 °C.
Sample name Input mass (g) Output mass (g) Pyrolysis yield of carbon nanomaterials (wt%)
PS 700 PS 700:
TiO2: 0 g, Car Bumpers: 10 g		 1.19 11.9
PST 0.5 PST 0.5:
TiO2:0.5 g, Car Bumpers:10 g		 1.76 12.6
PST 1 PST 1:
TiO2:1 g, Car Bumpers:10 g		 2.34 18.4

2.3

2.3 Characterization

Different characterization techniques have been used to evaluate the quality, performance and physical properties of the materials. The characterization methods include XRD, SEM, TEM, Raman spectroscopy; TGA and BET surface area measurements. The characterization measurements have been performed for raw car bumper before thermal treatment (PS0), the as prepared TiO2 nanoparticles and the TiO2 nanoparticles after thermal treatment at 700 °C (TiO2 700), the bumper after thermal treatment in the absence of TiO2 (PS700) and in the presence of TiO2 of different percent (PST0.5 and PST1). The washed and dried raw car bumper pieces were grinded to fine particles for different characterization. X-ray diffraction measurements were performed using Shimadzu’s-XRD Powder diffractometer (xrd-700, Maxima) with a Cu radiation source, at a scanning speed of 2° min−1, 40 kV tube voltage, and 30 mA tube current, the scanning range is 5–90°. The scanning electron microscope (SEM) (FEI, ISPECT S50, Czec Republic) was used to examine the specimens. SEM was performed using the following parameters: acceleration voltage = 20 kV, working distance about 10 mm and spot size 3. The powder specimens were mounted onto a metallic stub with a double-sided adhesive tape. Images were captured at different magnifications. For transmission electron microscopy (TEM), the powder was dispersed in ethanol, sonicated for 5 min and deposited onto TEM grid having carbon support film. The grids were dried before mounting into the TEM. A TEM, FEI, Morgagni, Czec Republic at 80 kV was used to record the images. The thermal decomposition of the materials was recorded with a DTA-TGA thermal analyzer Shimadzu’s (DTG-60H) in the temperature range 25–1000 °C. The specific surface area and pore diameter of the samples were measured by N2 adsorption isotherm using BET surface area of sample was measured on Quantachrome NOVA 1200e analyzer at 77 k. Raman spectrum was obtained using (Thermo Scientific) with a 532-nm laser as an excitation source. The thermal treatment of the samples was performed using Barnstead Thermolyne 6000 Muffle Furnace, burning at 700 °C in air atmosphere.

2.4

2.4 Photocatalytic activity

The photocatalytic performance of the carbon nanomaterials produced from thermal treatment of waste car bumper in the absence or presence of TiO2 nanoparticles has been determined by measuring the degradation of methylene blue dye (MB) as a model pollutant. The photocatalytic experiments were carried out in a 250 ml column glass reactor irradiated from the top with UV lamp (254 nm, 16 W). The photoreactor was open from the top to the atmosphere. In all experiments an optimized amount of 1 g/l of the catalyst (PS700, PST0.5, PST1 and pure TiO2) was homogeneously dispersed in 50 ml of ultra-high pure water. After that, 50 ml of 100 mg/l of an aqueous dye solution was added to the previous suspension. TiO2 Degussa P25 has been used as benchmark for the photocatalytic degradation of MB under the same experimental conditions. All experiments have been carried out at natural pH (6.6). Prior to the photocatalysis, the suspensions containing the catalyst and the dye were magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium. The irradiation was performed under magnetic stirring at room temperature. Sample aliquots (5 ml) were taken at the desired time and then filtered to separate the solid catalyst. The degradation efficiency of the samples was defined in terms of the C/C0 ratio, where C0 and C represent the initial and final concentrations of the dye at t0 and t respectively.

3

3 Results and discussion

3.1

3.1 Materials characterization

3.1.1

3.1.1 XRD

Fig. 2 shows the XRD patterns of the raw car bumper before thermal treatment (PS0), the as prepared TiO2 nanoparticles, and the car bumper after thermal treatment in the absence of TiO2 (PS700) and in the presence of TiO2 of various content (PST0.5 and PST1). The XRD of the raw car bumper shows a sharp peak at 19.5° characteristic of polystyrene (Adams et al., 1978). The XRD pattern of the as prepared TiO2 nanoparticles shows the diffraction peaks of both anatase and rutile phases with peaks can be well indexed to the (1 0 1), (0 0 4) and (2 0 0) reflections of anatase phase (JCPDS no. 21-1272) (Xiong and Zhao, 2012), and the (1 1 0), (1 0 1), (1 1 1) and (2 1 1) reflections of rutile phase (JCPDS no. 21-1276) (Xie et al., 2009). The percentages of anatase and rutile phase in the sample have been calculated from the integrated intensities of anatase (1 0 1) and rutile (1 1 0) to be 20.5% and 79.5% for anatase and rutile respectively. The XRD pattern of the car bumper after thermal treatment in the absence of TiO2 nanoparticles (PS700) shows a broad peak at 24.5° which is attributed to reduced graphene oxide sheets (rGO) (Stobinski et al., 2014). The XRD patterns of the bumper after thermal treatment in the presence of TiO2 nanoparticles (i.e., PST0.5, PST1) show the diffraction peaks of both anatase and rutile phases of TiO2 with 87% anatase, 13% rutile for PST1 and 85% anatase, 15% rutile for PST0.5. Interestingly, the presence of the car bumper hinder the anatase to rutile phase transformation at elevated temperature but a rutile to anatase phase transformation have been rather took place. For comparison, the XRD of TiO2 nanoparticles after thermal treatment in the absence of the waste car bumper at 700 °C (TiO2 700) is measured. It is shown that, the XRD pattern of the thermal treated TiO2 shows the diffraction peaks of pure rutile phase confirming the phase transformation of TiO2 anatase/rutile mixed phase to pure rutile under thermal treatment at 700 °C. Moreover, the characteristic peaks assigned to rGO did not appear for PST0.5 and PST1 samples. This can be explained by the overlapping of the rGO peaks with TiO2 anatase peaks.

XRD patterns of raw waste car bumper (PS0), waste car bumper after thermal treatment (PS700), as prepared TiO2, TiO2 after thermal treatment (TiO2700) and waste car bumper after thermal treatment in the presence of TiO2 of different percentage (PST0.5, PST1).
Fig. 2 XRD patterns of raw waste car bumper (PS0), waste car bumper after thermal treatment (PS700), as prepared TiO2, TiO2 after thermal treatment (TiO2700) and waste car bumper after thermal treatment in the presence of TiO2 of different percentage (PST0.5, PST1).

3.1.2

3.1.2 Raman

Fig. 3 shows the Raman spectra of the raw car bumper before thermal treatment (C0), the as prepared TiO2 nanoparticles and the TiO2 nanoparticles after thermal treatment at 700 °C (TiO2 700), the car bumper after thermal treatment in the absence of TiO2 (PS700) and in the presence of TiO2 (PST0.5 and PST1). The Raman spectra of raw bumper before thermal treatment (PS0) and after thermal treatment in absence of TiO2 (PS700) show two sharp peaks at 1341 and 1603 cm−1, which can be attributed to the disorder carbon (D band) and graphitic carbon (G band), respectively (How et al., 2014). An increase in the intensity of the D band as compared to the G band is observed for PS700 sample which indicates the removal of oxygen and thus confirming the transformation of polymer waste (polystyrene) to reduced graphene oxide. The Raman spectra of TiO2 before thermal treatment show the bands at 234 cm−1 (Eg), 432 cm−1 (Eg), and 612 cm−1 (A1g) due to TiO2 rutile phase (Porto et al., 1967). The Raman spectra of the bumper after thermal treatment in the presence of TiO2 (PST0.5 and PST1) show the D and G bands of graphitic carbon in addition to week peaks at 390 cm−1 (B1g), 510 cm−1 (A1g) and 620 cm−1 (Eg) and an intense peak at 140 cm−1 corresponding to the characteristic modes of anatase phase of TiO2 (Ohsaka et al., 1978).

Raman spectra of raw waste car bumper (PS0), waste car bumper after thermal treatment (PS700), as prepared TiO2 and waste car bumper after thermal treatment in the presence of TiO2 of different percentage (PST0.5, PST1).
Fig. 3 Raman spectra of raw waste car bumper (PS0), waste car bumper after thermal treatment (PS700), as prepared TiO2 and waste car bumper after thermal treatment in the presence of TiO2 of different percentage (PST0.5, PST1).

3.1.3

3.1.3 SEM

Fig. 4 shows the SEM images of the car bumper after thermal treatment in the absence of TiO2 (PS700) or in the presence of TiO2 (PST0.5 and PST1). The SEM image of PS700 shows dense layers of carbon materials. The SEM images of PST0.5 and PST1 show agglomeration of TiO2 nanoparticles on carbon layers.

(a) SEM image of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the absence of TiO2 (PS700). (b) SEM images of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the presence of TiO2 nanoparticles of different content (PST0.5, PST1).
Fig. 4 (a) SEM image of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the absence of TiO2 (PS700). (b) SEM images of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the presence of TiO2 nanoparticles of different content (PST0.5, PST1).

3.1.4

3.1.4 TEM

Fig. 5 shows the TEM images of the car bumper after thermal treatment in the absence of TiO2 (PS700) or in the presence of TiO2 (PST0.5 and PST1). The TEM images of car bumper after thermal treatment in the absence of TiO2 nanoparticles (Fig. 5(a) show porous carbon nanomaterial sheets. The TEM images of composite nanomaterials produced from thermal treatment of waste car bumper in the presence of TiO2 (Fig. 5(b and c)) show the agglomerated TiO2 nanoparticles anchored on porous carbon nanomaterials sheets. The images (d and e) show focused area of TiO2 agglomeration in PST0.5 sample indicating that, the TiO2 agglomeration composed of nanoparticles with an average size of ≈50 nm. The images (f–h) show the focused area of carbon sheets in PST0.5 sample. Thin sheets of reduced graphene oxide decorated with carbon dots are clearly observed. The results confirm that, the morphology of the produced carbon nanomaterials has been changed upon using TiO2 as a catalyst in the pyrolysis process. While porous reduced graphene oxide sheets were produced from the thermal pyrolysis of waste car bumper in absence of TiO2 catalyst, reduced graphene oxide sheets decorated with carbon dots have been produced from the catalytic pyrolysis of waste car bumper in presence of TiO2 catalyst.

(a): TEM image of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the absence of TiO2 (PS700). (b–h): TEM images of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the presence of TiO2 nanoparticles of different content (PST0.5, PST1).
Fig. 5 (a): TEM image of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the absence of TiO2 (PS700). (b–h): TEM images of waste car bumper after pyrolysis at 700 °C in inert atmosphere in the presence of TiO2 nanoparticles of different content (PST0.5, PST1).

3.1.5

3.1.5 TGA

Fig. 6 shows the thermal gravimetric analysis of PS700, PST0.5 and PST1. The weight loss consisted of two main stages. The first stage is from room temperature to 120 °C which is mainly due to the loss of adsorbed water. The second stage is at approximately 450 °C can be attributed to the thermal decomposition of the carbon nanomaterials. The TGA results show the excellent thermal stability of the produced nanocomposite. Furthermore, the thermal stability of the nanocomposite increases with increasing the TiO2 percentage.

TGA of waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in presence of TiO2 of different percentage (PST0.5, PST1).
Fig. 6 TGA of waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in presence of TiO2 of different percentage (PST0.5, PST1).

3.1.6

3.1.6 N2 adsorption desorption and surface area measurements

Fig. 7 shows the nitrogen adsorption-desorption measurements of the bumper after thermal treatment at 700 °C in the absence of TiO2 (PS700) and in the presence of TiO2 (PST0.5 and PST1). The N2 adsorption-desorption measurements confirm the porous structure of the produced carbon nanomaterials and TiO2@QDs-rGO nanocomposite materials. The BET surface area and the average pore size of the carbon nanomaterials produced from the thermal treatment of car bumpers in absence of TiO2 is higher than those in the presence of TiO2 nanoparticles. Furthermore, the surface area and pore size decrease with increase of TiO2 percent in the samples. The results can be readily explained by the blocking of the porous carbon surfaces by TiO2 nanoparticles. The measured N2 adsorption characteristic values of all samples are given in Table 2.

N2 adsorption desorption isotherms of waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in the presence of TiO2 of different percentage (PST0.5, PST1).
Fig. 7 N2 adsorption desorption isotherms of waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in the presence of TiO2 of different percentage (PST0.5, PST1).
Table 2 N2 adsorption-desorption characteristics of PS700, PST0.5 and PST1.
Sample SBET (m2 g−1) Pore volume (cm3 g−1) Average pore size (nm)
PS700 120.3 0.125 18.56
PST0.5 96.20 0.062 17.66
PST1 57.88 0.044 17.32

3.1.7

3.1.7 UV–vis diffuse reflectance measurements

The UV–vis diffuse reflectance spectra of the samples are shown in Fig. 8. The reflectance spectra of TiO2 exhibit the typical UV absorption of TiO2 nanoparticles with broad reflection in the visible region of the spectrum. The reflectance value is very much lower for PST 0.5 and PST 1 samples; indicating high absorption of light and evidencing that the produced carbon nanomaterials (i. e., Graphene Oxide) with TiO2 extend the absorbance into visible region (380–800 nm).

UV–vis spectra of pure TiO2, waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in the presence of TiO2 of different percentage (PST0.5, PST1).
Fig. 8 UV–vis spectra of pure TiO2, waste car bumper after thermal treatment in absence of TiO2 nanoparticles (PS700) and in the presence of TiO2 of different percentage (PST0.5, PST1).

3.2

3.2 Photocatalytic performance

Fig. 9(a) shows the UV–vis absorption spectra of an aqueous solution of MB during the UV illumination in the presence of PST0.5. Obviously, the absorption peak intensity of MB decrease with illumination time and almost vanish at around 120 min. Fig. 9(b) shows the photocatalytic degradation efficiency of MB as the variation of C/C0 with irradiation time in the presence of carbon nanomaterials produced from waste car bumpers in the absence of TiO2 (PS700), in the presence of TiO2@carbon nanomaterials (PST0.5, PST1) and the presence of pure TiO2 nanoparticles. TiO2 Degussa P25 has been used as a benchmark for comparison of the photocatalytic activity with the created carbon nanomaterials. The results shown in Fig. 9 indicate that the photocatalytic activity was greatly enhanced in the presence of TiO2 nanoparticles. The as-obtained TiO2/Carbon Dots/reduced graphene oxide composites (PS0.5 and PS1) exhibited an enhanced photocatalytic activity for the degradation of MB dye as compared to pristine TiO2 (TiO2 or P25). The enhancement of photocatalytic activity can be attributed to the high electron pair separation achieved by the electronic interaction between TiO2 and porous rGO/CDs carbon nanomaterils produced from waste car bumper. Furthermore, the highest photocatalytic activity was obtained for PST0.5 with Carbon: TiO2 ratio of 2:1 compared to PST1 with Carbon: TiO2 ratio of 1:1 indicating the increasing of photocatalytic activity with increasing the rGO content in the samples. The hyperdization of TiO2 with rGO will greatly enhance the photocatalytic activity of TiO2 which is owing to the excellent electronic conductive properties of the resulting TiO2/rGO nanocomposite, as well as increasing the specific surface area. Furthermore, rGO can be used as an electron transfer medium, which can restrain the recombination of photogenerated electrons and holes, and extend the lifetime of photogenerated electrons (Lightcap et al., 2010). Various studies showed an enhancement in the photocatalytic degradation of methylene blue using TiO2/rGO composites than using pure TiO2 nanoparticles. Yang et al, showed that TiO2/graphene porous composites exhibit better adsorption capacity and photocatalytic activity in degrading methylene blue than P25 (Yang et al., 2016). Wu et. al. showed a superior photocatalytic activity of TiO2 nanospindle upon conjugation with rGO for MB degradation (Wu et al., 2015). Mohamed H. H. reported an excellent photocatalytic activity for MB and CV dyes using biphasic TiO2(anatase and rutile) microspheres/reduced graphene oxide that was attributed to the synergetic effect through anatase, rutile and reduced graphene oxide multifunction (Mohamed, 2017).

(a) The change in the UV–vis absorbance spectrum of aqueous solutions of MB dye during UV- irradiation in the presence of PST0.5, (b) Photocatalytic degradation of MB (20 ppm) as the variation of C/C0 with irradiation time in the presence of different photocatalysts, (c) −ln(C/C0) with irradiation time in the presence of different photocatalysts.(d) The change in the TOC with irradiation time in the presence of different photocatalysts, (e) −ln(TOC/TOC0) with irradiation time in the presence of different photocatalysts.
Fig. 9 (a) The change in the UV–vis absorbance spectrum of aqueous solutions of MB dye during UV- irradiation in the presence of PST0.5, (b) Photocatalytic degradation of MB (20 ppm) as the variation of C/C0 with irradiation time in the presence of different photocatalysts, (c) −ln(C/C0) with irradiation time in the presence of different photocatalysts.(d) The change in the TOC with irradiation time in the presence of different photocatalysts, (e) −ln(TOC/TOC0) with irradiation time in the presence of different photocatalysts.

Furthermore, the change of Total Organic Carbon (TOC) during photocatalytic degradation of MB in the presence of P25, TiO2, PS700, PST0.5 and PST1 under UV light irradiation have been measured (Fig. 9(d). The TOC measurements indicated the disappearance of the organic carbon when MB and the photocatalyst (TiO2, PST0.5 or PST1) are exposed to UV light. The results showed that 99.93%, 72.10%, or 39.71% of TOC decrease was obtained after UV light irradiation for 60 min when PST0.5, PST1 or TiO2 was used as photocatalyst. Fig. 9(c) and (e), respectively, show the plots of −ln(C/C0) and −ln(TOC/TOC0) with irradiation time, respectively, for the photocatalytic degradation and mineralization of MB. The kinetic parameters of the photocatalytic degradation as well as mineralization of MB according to the data from Fig. 9(c) and (e) are presented in Table 3. An increase in the pseudo first order rate constants for MB degradation (KC) compared to the rate constants of MB mineralization (KTOC) indicating that, intermediates may be formed during photocatalytic degradation of MB (see Table 3).

Table 3 Kinetic parameters for the degradation of MB according to the data shown in Fig. 9(c) and (e).
Photocatalyst KC (min−1) R2 KTOC (min−1) R2
P25 0.0109 0.92 0.0071 0.942
TiO20 0.0075 0.89 0.0052 0.921
PST0.5 0.0678 0.992 0.0462 0.968
PST1 0.0308 0.993 0.018 0.982

It is very important to evaluate the recyclability of the created waste based nanocomposite from the environmental application point of view. The recyclability has been evaluated by carrying out the photocatalytic experiment three times using the PST0.5 nanocomposite. After each run the nanocomposite particles were collected and then washed by distilled water 3 times by centrifugation and then dried at 60 for 1 hr for the use in the next run. Fig. 10 shows the results of the recycle use of PST0.5 (1 g/l) for the degradation of MB (20 ppm). It is obvious that there is almost no change in the photocatalytic efficiency of the nanocomposite after the three-recycle use evidencing the excellent stability of the nanocomposite-based waste.

Recycle use of PST0.5 (1 g/l) for the photocatalytic degradation of MB (20 ppm).
Fig. 10 Recycle use of PST0.5 (1 g/l) for the photocatalytic degradation of MB (20 ppm).

4

4 Conclusions

In summary, a novel approach for transforming industrial waste particularly waste car bumper into carbon nanomaterials has been developed. As a first attempt, TiO2 nanoparticles were introduced as a catalyst for the upcycling of the waste car bumper through an economical thermal decomposition creating novel active carbon based nanomaterial namely TiO2/Carbon Dots/reduced graphene oxide. The as-obtained TiO2/Carbon Dots/reduced graphene oxide composites (TiO2/CDs/rGO) exhibited high photocatalytic activity for the degradation of MB dye as compared to pristine TiO2 under the same conditions. A synergetic effect between TiO2 and the incorporated carbon nanomaterials in (TiO2/CDs/rGO) is responsible of the enhancement of the photocatalytic activity. Moreover, the enhancement of the photocatalytic activity of the created nanomaterials is attributed to the role of CDs/rGO in enhancing the electron transport and light harvesting properties as well as decreasing the electron–hole pair recombination rate. The present study provides new insights into the upcycling of the industrial waste in a more sustainable manner, as well as useful information for large-scale fabrication of graphene-based carbon nanomaterials at low cost for multipurpose applications.

Acknowledgement

The Authors gratefully acknowledge the research units at College of Science, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia, and IRMC, TEM and SEM unit.

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