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Biodegradable blends of graphene quantum dots and thermoplastic starch with solid-state photoluminescent and conductive properties
International Journal of Biological Macromolecules 139 (2019) 367–376
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Biodegradable blends of graphene quantum dots and thermoplastic starch with solid-state photoluminescent and conductive properties Jie Chen a,b,c, Zhu Long a,b,⁎, Shuangfei Wang b, Yahui Meng a, Guoliang Zhang a, Shuangxi Nie b a b c
Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China Department of Mechanical & Electronic, Yuncheng University, Yuncheng 044000, China
a r t i c l e
i n f o
Article history: Received 14 February 2019 Received in revised form 16 July 2019 Accepted 30 July 2019 Available online 01 August 2019 Keywords: Thermoplastic starch Graphene quantum dots Solid-state photoluminescence Conductive property Film
a b s t r a c t Polymer composites based on blends of graphene quantum dots (GQDs) with thermoplastic starch (TPS) were prepared by melt-extrusion combined with hot pressing. The GQDs/TPS films were characterized as potential novel, high-performance, and ecofriendly composites replacing traditional non-biodegradable plastic packaging materials. GQDs stock solutions of different concentrations were incorporated into TPS matrices in order to analyze the solid-state fluorescent properties and conductive properties of GQDs/TPS films. The fluorescent, conductive, morphological, mechanical, and optical properties of the GQDs/TPS films were characterized by ultraviolet–visible spectroscopy, surface resistance measurement, scanning electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, tensile testing, and X-ray diffraction (XRD). FT-IR studies indicated hydrogen bonding between the oxygen-containing groups on GQDs surfaces and the –OH groups in the TPS. The mechanical testing results showed the optimum GQDs loading of 10.9 wt% in the blend. XRD and TEM studies indicated uniform graphene dispersions in the TPS matrix for ≤10.9 wt% GQDs loading; further increases in loading caused agglomeration. The maximum photoluminescence intensity and conductivity of the materials were obtained at 10.9 wt% GQDs loading. These materials have potential applicability in flexible optoelectronic packaging materials. © 2019 Published by Elsevier B.V.
1. Introduction In recent years, graphene has attracted increasing attention as a nanocarbon material because it offers excellent characteristics including a large specific surface area, high carrier mobility, excellent mechanical flexibility, good thermal/chemical stability, adjustable excitation and emission wavelengths, fluorescence stability, photobleaching, and other good biocompatible properties [1,2]. Compared to twodimensional graphene nanosheets (GNSs) and one-dimensional graphene nanoribbons (GNRs), zero-dimensional graphene quantum dots (GQDs) are characterized by their sizes of b10 nm [3,4]. Because GQDs show stronger quantum confinement effects and boundary effects than GNSs and GNRs, they have potential applicability in many fields, including solar photovoltaic devices, biopharmaceuticals, lightemitting diodes, and sensors [5,6]. Recent scientific studies have indicated that citric acid pyrolysis as a bottom-up GQDs preparation method yielded products with good dispersibility and high crystallinity by adjusting the degree of citric acid carbonization [7].
⁎ Corresponding author at: Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China. E-mail address: [email protected] (Z. Long).
https://doi.org/10.1016/j.ijbiomac.2019.07.211 0141-8130/© 2019 Published by Elsevier B.V.
Compared to other methods, such as electron beam lithography [8], electrochemical [9], microwave [10], chemical vapor deposition [11], pyrolysis (for blue luminescent graphene), and laser ablation [12], the citric acid method could permit the industrial-scale production of cheap renewable materials because it entails cheap raw materials, simple steps, low equipment requirements, and highly safe operation. In order to better apply GQDs to optoelectronic devices, GQDs electrical conductors and plastic substrates are necessary to form flexible transparent conductive films [13]. Because petroleum-based plastic products have negative environmental impacts via the accumulation of non-biodegradable wastes [14], biodegradable transparent films, derived from natural organic polymers with low aggregation indices have attracted worldwide attention, with excellent characteristics including biodegradability, biocompatibility, safety, and reproducibility [15]. Starch is considered a high-potential resource for the development of bio-polymers because it is abundant, low in cost, renewable, and biodegradable [16]. Starch can be converted to thermoplastic starch (TPS) under high temperature and high pressure in the presence of plasticizers (such as glycerin and water); it has properties similar to those of synthetic thermoplastics [17]. However, plasticized starch-based materials are easily affected by humidity and temperature and thus poor in mechanical strength and water resistance; this limits their potential applications [18].
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Commonly used methods for synthesizing GQDs involve high cost materials (such as graphite [19] or 6photonic crystal [20]), lower enthalpy and high cost methods (such as laser ablatin [12], electron beam lithography or electrochemical synthesis [9]). These factors limit the commercial application of GQDs. Recent studies have reported that the preparation of GQDs from relatively inexpensive organic sources, such as citric acid or urea [21], is beneficial in reducing production costs and expanding production scale. GQDs produced by citric acid pyrolysis are highly soluble in water and do not experience surface modification in polymer composites, which is ascribed to the abundant polar groups on the edges of the GQDs and on the substrate [22]. Therefore, GQDs could be used to reinforce TPS films [23]. During the melt extrusion process, the starch can be modified into TPS with a GQDs standard aqueous solution and plasticizer. Recently, GQDs/TPS films have been prepared by solution casting [24]. However, the preparation of GQDs/TPS films using GQDs as fillers and plasticizers has not yet been reported. The mechanical and optoelectronic properties of such films are studied here.
2. Materials and methodology 2.1. Materials Citric acid monohydrate (CA, weight-averaged molecular weight Mw = 54,000), glycerol, and sodium hydroxide (NaOH) were purchased as analytical-grade reagents from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China); corn starch was obtained from Shanghai Quanwang Biotechnology Co., Ltd. (Shanghai, China). The water used in the experiment was deionized. 2.2. Synthesis of GQDs GQDs were directly prepared by the citric acid pyrolysis method (see in Fig. 1). The citric acid was carbonized through pyrolysis to dehydrate and condense the citric acid molecules into GQDs. Following the conventional method, 2 g CA was placed in a 5-mL beaker and heated to 200 °C using a heating mantle. After 5 min, CA was partially liquefied. With increased heating time, the color of the CA liquid gradually
Fig. 1. Diagram for the synthesis of GQDs.
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changed from colorless to light yellow to orange after 30 min, indicating GQDs formation. The obtained orange liquid of GQDs was added dropwise to 40 mL, 20 mL, 10 mL, 5 mL, and 3 mL of a 10 mg/mL NaOH solution under vigorous stirring. These were neutralized to pH 7.0 with NaOH solutions to obtain 5%, 10%, 20%, 40%, and 60% stock aqueous solutions of GQDs (denoted as 5GQDs, 10GQDs, 20GQDs, 40GQDs, and 60GQDs, respectively). 2.3. Fabrication of the composite films TPS-based composite films, shown in Scheme 1, were prepared by melt-extrusion with the ratio of starch: glycerol: GQDs stock solution of 55:30:15 (wt%). After this preliminary step, the glycerol and GQDs stock solution were dissolved at 40 °C. Next, the dried starch was gradually added and thoroughly mixed for 2 h at 800 rpm with an overhead stirrer. Scheme 2 depicted the preparation mechanism of thermoplastic starch. The wet modified starch mixture was sealed in a plastic bag for 48 h. Afterward, the resulting blend was extruded through a co-rotating twin-screw extruder (Shanghai Xinshuo Precision Machinery Co., Ltd., China). The screw speed was adjusted to 100 rpm and the feeder-to-nozzle temperature was set to 140 °C. Subsequently, the obtained linear materials were air-cooled and granulated using a blade grinder equipped with a nominal 2-mm internal diameter. Finally, the plasticized materials were equilibrated for one week and then thermo-pressed for 15 min at 140 °C under a load of 5 tons in order to produce composite films with the approximate thickness of 0.3 mm.
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2.4.3. Transmission electron microscope (TEM) The dispersibility of GQDs in the stock solutions was surveyed by a TF20/2100F transmission electron microscope (TEM, JEOL, Japan) operating at 200 kV with a resolution ratio of 0.20 nm. 2.4.4. Scanning electron microscope (SEM) The surface morphologies of the composite films were observed with an SU 1510 scanning electron microscope (SEM, Hitachi, Japan) at an accelerating voltage of 5 kV after coating with gold. 2.4.5. Mechanical properties Mechanical properties including the tensile strength and elongation at break were investigated by a universal testing machine (KDII-0.05, Shenzhen KQL Test Instrument Co., Ltd., China) at room temperature. Samples were pre-conditioned at 23 ± 2 °C and relative humidity of 53 ± 2%. They were then cut into strips of 150 mm × 10 mm (length × width). The composite films were loaded at a constant cross-head speed of 50 mm/min and a clamping length of 100 mm. During the process, five measurements were performed in triplicate. 2.4.6. Transmittance of films The optical transmittances of the film samples were measured using a TU-1901 ultraviolet–visible (UV–vis) spectrophotometer (Beijing General Analysis Instrument Co., Ltd.) in the incident light wavelength range 280–800 nm.
2.4. Film characterization
2.4.7. Solid-state photoluminescence spectra of films The solid-state photoluminescence (PL) spectra of the GQDs/TPS films were obtained by an F-4600 fluorescence spectrophotometer (Hitachi High-tech Company, Japan).
2.4.1. FT-IR analysis The functional groups of the GQDs, TPS, and GQDs/TPS films were tested and analyzed by Fourier-transform infrared (FT-IR) spectroscopy (Nicolet Nexus, Thermo Electron Corporation, Waltham, MA, USA) in the range 4000–500 cm−1 with a resolution of 4 cm−1.
2.4.8. Surface resistance measurement The surface resistances of the composite films were measured using a LFY-406 material resistivity tester. The test principle is shown in Fig. 2. Using a two-probe method, a constant force of 10 N is applied to the two Cu electrodes, on which the probes are placed.
2.4.2. X-ray diffraction (XRD) The X-ray diffraction (XRD) patterns of the GQDs/TPS films were obtained using an X'Pert PRO MPD X-ray diffractometer (Panalytical, Poland) with Ni-filtered Cu (Kα) radiation at ambient temperature, 40 kV generator tension, and 30 mA generator current, in the scanned 2θ range 5–50° at the rate of 4°/min with a step size of 0.05°.
Rs ¼
R0 W L
ð1Þ
where, Rs is the surface resistance of the sample (Ω/square or Ω/sq), R0 is the resistance reading value of the multimeter (Ω), and W and L are the width and length, respectively, of the rectangle maintained by both probes. The average surface resistance of the three specimens
Scheme 1. The preparation process of GQDs/TPS composite films
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Scheme 2. The preparation mechanism of thermoplastic starch
prepared in the same batch was taken as the surface resistance value. In general, when the test area is a square, i.e., L = W, Rs = R is called the square resistance. In this study, L = W = 50 mm, and formula (2) i ρv ¼ Rs t
ð2Þ
Notes: results of thermal stability properties and hydrophilicity properties of composite films were shown in Fig. S1, Fig. S2 and Table S1. Meanwhile, the detailed information were described in the SI. 3. Results and discussion 3.1. FT-IR spectrum of composite films The FT-IR spectrum of the GQDs, neat TPS films, and GQDs/TPS composite films are shown in Fig. 3 (a, b). Fig. 3 (a) shows the characteristic peaks of GQDs, with –OH vibrations indicated at ~3285.14 cm−1, C_O stretching vibrations of carboxylic groups at 1641.07 cm−1, skeletal vibrations from unoxidized graphitic domains at 1557.51 cm−1, C–OH (hydroxyl) stretching vibrations at 1386.33 cm−1, C–O–C (epoxy) stretching vibrations at 1260.97 cm−1
(fullerenol), and C\\O stretching vibrations at 1095.12 cm−1. Similar FT-IR results from graphene have been reported [25]. Fig. 3 (b) shows the FT-IR spectra of pure TPS films and of GQDs/TPS composite films with different GQDs concentrations. The FT-IR spectra of the synthesized GQDs/TPS composite films are similar to the spectral curves of the neat TPS films, with peaks at ~1574.36 cm−1 from the GQDs. Meanwhile, the intensity of the GQDs peak is increased as the concentration of GQDs increases. The broad peak at 3200–3500 cm−1 is ascribed to the stretching vibration of hydroxyl groups; that at 2925.39 cm−1 is assigned to C\\H stretching, while the bump at 1716.53 cm−1 is attributed to the C_O functional group and that at 1647.30 cm−1 is attributed to bound water. The absorption at 1150.20 cm−1 corresponds to stretching in aliphatic alcohols. The absorption band over 3200–3500 cm−1 from the plasticizer/starch blend is assigned to both inter- and intramolecular hydrogen-bonded –OH groups in the glycerol and starch, which are unaffected at low GQDs loadings. However, higher GQDs loadings cause decreases in the intensity of this absorption band, indicating the occurrence of hydrogen-bonding interactions between the –OH groups present in TPS and oxygencontaining groups in the GQDs, at the cost of inter- and intramolecular hydrogen bonding in the TPS [26]. 3.2. XRD analysis
Fig. 2. The test principle of surface resistances of composite films.
Fig. 3(c) presents the XRD patterns of the pure TPS films and GQDs/ TPS composite films. The major characteristic peak of the TPS films appears at 2θ = 19.78°; minor peaks appear at 2θ = 12.93° and 22.49°, corresponding to the mixture of A- and V-type crystal structures present in the TPS film [27,28]. The XRD pattern trends for the synthesized GQDs/TPS composite films are similar to that for the neat TPS. The characteristic peak of the GQDs/TPS composite film appears at around 2θ = 26.67°, assigned to the limited ordering structure of the graphene quantum dot (GQDs) obtained by citric acid at high temperature [29]. The weak broad peaks are related to the thinness and disordered stacking of some GQDs. For the composite films, the diffraction patterns are dependent on the GQDs content. At the low loadings of 1.4 wt% GQDs and 2.7 wt% GQD, the diffraction patterns of GQDs/TPS, with the weak characteristic peak of GQDs, are similar to that of neat TPS. The absence of the characteristic GQDs peak indicates that the graphene is exfoliated and individually dispersed in the polymer matrix. As the GQDs loading is increased to 10.9 wt%, the characteristic peak of GQDs shows lower intensity than 5.5 wt% GQDs and 2.7 wt% GQDs. This is because of the agglomeration
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Fig. 3. (a) FT-IR of graphene quantum dots and (b) composite films; (c) XRD diagrams of films; and (d) Mechanical properties of films.
Fig. 4. The TEM images of GQDs dispersion in stock solutions.
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of GQDs in the composite films, which weakens the interfacial adhesion between the nanosheets and the TPS matrix. 3.3. Morphological analysis TEM images of GQDs dispersed in the GQDs stock solutions are shown in Fig. 4. Because the stock solution with the lowest concentration of 5GQDs shows very little aggregation, homogeneous GQDs dispersions are observed for low GQDs loading levels. With the aggregation of 20GQDs, the particle sizes appear to increase. Increases in particle concentration may improve the particle aggregation with normal cluster sizes for GQDs concentrations of 40%–60%. The GQDs display good dispersibility in the stock solutions without surface treatment. This is a major benefit of GQDs compared to inorganic quantum dots (QDs), which often require surface treatment to prevent agglomeration. The influence of GQDs content on the surface morphologies of the GQDs/TPS composite films was studied by SEM, as shown in Fig. 5. The TPS film has a dense and smooth surface from hydrogen bonding among the TPS molecules under high temperature and pressure. By contrast, the GQDs/TPS composite films have relatively rough surfaces, attributed to the incorporation of GQDs in the starch matrices [30]. The images obviously show that composite films with low GQDs loadings can have uniform GQDs distributions, and that GQDs aggregation occurs as the concentration of GQDs is increased. The surfaces of the GQDs/TPS films with low GQDs contents (i.e., 1.4, 2.7, and 5.5 wt%) show no agglomeration because the GQDs prepared from citric acid are amphiphilic with polar surface groups, yielding uniform GQDs distributions in the starch. When the content of GQDs in the starch matrix reaches 16.4 wt%, some large clustered particles appear on the composite surface from GQDs aggregation. Simultaneously, the uniform and flexible surface is destroyed by the spatial effect of the conversion of starch to TPS. The high GQDs contents affect the arrangement of starch molecular chains and causes composite morphology conversion [31]. For 10.9 wt% GQDs, fewer clusters are observed on the surface of the composite film; more serious agglomeration is detected in the 16.4 wt% film.
3.4. Mechanical properties The mechanical properties such as tensile strength and elongation at break of the TPS films and GQDs/TPS composite films are shown in Fig. 3 (d). As the content of GQDs increases from 1.4 wt% to 10.9 wt% in the starch matrix, the edges of adjacent one-dimensional GQDs sheets may join together and lead to efficient load transfer between the starch matrix and the GQDs filler, thus increasing the tensile strengths of the composite films because of the good GQDs dispersity [32]. Meanwhile, the GQDs/TPS composite films have adequate strength to absorb the applied force, as previously reported. However, at 16.4 wt% GQDs, the tensile strength of the composite is decreased. This is because single GQDs sheets tend to become restacked in agglomerates, thus weakening the interface between GQDs filler and TPS matrix [33]. The elongation at break indirectly reflects the material flexibility. The pure TPS film is obviously more flexible than the composite films. Although the elongations at break of the GQDs/ TPS composite films are slightly different, the films retain suitable flexibility. 3.5. Transparency Fig. 6 exhibits the transparency of the TPS films and GQDs/TPS composite films under visible light. Through the TPS films under ambient visible light, the blue-colored logo on the paper is clearly observed, indicating the good transparency of pure TPS in the visible range. However, the transparency of the GQDs/TPS composite films evolves with increasing the GQDs loading content from 1.4 wt% to 16.4 wt%. The blue logo beneath the composite films gradually becomes blurred with increased GQDs concentration. This is because the GQDs are brown; the GQDs/TPS composite films have greater scattering effects and some GQDs aggregation occurs in the films [34]. The series of photographs in Fig. 6 shows the differences in transparency of the film samples with different compositions. In order to accurately study the difference in transparency of film samples with different GQDs contents, the light transmissions of the composite films were measured with a UV–vis spectrophotometer in Fig. 7(a) that shows the variation in light transmittance of the different composite films as a function of incident light wavelength.
Fig. 5. SEM images of GQDs dispersion in the GQDs/TPS composite films.
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Fig. 6. The Transparency of composite films.
3.6. Solid-state photoluminescence Fig. 8 shows digital photographs of the luminescent emission of the composite films under UV irradiation at 365 nm, and Fig. 7(c–d) exhibit the solid-state PL spectrum of GQDs and UV absorption spectrum of GQDs. The digital photographs show the prepared composite films with diameters of 30 mm and thickness of 0.30 mm. The addition of GQDs to the TPS matrices produces PL properties; the prepared GQDs/ TPS composite films have PL characteristics and become brighter with UV irradiation. By increasing the GQDs content in the composite film matrix, the intensity of emitted light is enhanced and the color becomes brighter blue, which is ascribed to the lack of emission in the absence of excited photons. However, the pure TPS films show dark blue emitted light because they do not radiate emission. The solid-state PL spectra of GQDs/TPS composite films were tested in order to research the quantitative properties of the synthesized composite materials. As seen in Fig. 7(b), the PL spectral curves of the GQDs/TPS films are wider with splitting, unlike that of the GQDs in water. These are attributed to creation and amplification phenomena (i.e., intersystem transitions and internal and external transformation) and the self-absorption of the aggregated GQDs. For GQDs concentrations increasing from 1.4 wt% to 10.9 wt%, the PL intensity of the GQDs/TPS films increases, attributed to the increased PL spaces in the composite films [35]. However, for the film with 16.4 wt% GQDs, the PL intensity is decreased because different aggregates of GQDs are created by the inner filtering effect, PL reabsorption, and partial quenching.
3.7. Surface resistance measurements The influence of GQDs content on the electrical conductivity of the GQDs/TPS composite films is analyzed by measuring the surface
resistance (SR) and electrical resistivity (ER), as shown in Table 1. The pure TPS film has a very high resistance because of the insulating properties of the starch; thus, neat TPS is excluded from the comparison. GQDs are the only conductive ingredients in the composite films; they apparently improve the electrical conductivity (EC) of the TPS-based films and their dispersity determines the electrical properties of the composite films. With increasing GQDs content, the ER of the composite film is decreased and the conductivity is increased, showing strong interactions and increased dispersion of GQDs in the TPS matrix [36]. The ER is decreased with the addition of 0–2.7 wt% GQDs; while ER is decreased more slowly as the GQDs content increases from 5.5 to 16.4 wt%, attributed to the aggregation of high GQDs loads. High GQDs loading causes the colorless and transparent TPS films to turn brown and low in transparency, as shown. 4. Conclusion GQDs produced from citric acid were incorporated into TPS matrices to prepare GQDs/TPS composite films by melt-extrusion with glycerol and distilled water as plasticizers. The GQDs obtained from citric acid were distributed uniformly in aqueous GQDs stock solutions with little agglomeration in the absence of surface treatment. In the FT-IR spectra of the GQDs/TPS composite films, the characteristic peak of the GQDs was identified at 1574.36 cm −1 . The characteristic XRD peak of the GQDs in the GQDs/TPS composite films appeared at 2θ = 26.67°, indicating the GQDs chemical structure produced by the carbonization of citric acid. The tensile strength and surface resistances of the composites were first increased with increasing GQDs contents because of the good dispersion of low-content GQDs in the composite films. However, high GQDs loading affected the alignment of the starch molecular chains, thereby reducing the
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Fig. 7. (a) Light transmittance of composite films with the wavelength of incident light; (b) Integrals of the PL spectra versus the GQDs content; (c) Solid-state PL spectrum of GQDs; (d) UV absorption spectrum of GQDs.
Fig. 8. Photograph of the TPS and GQDs/TPS films under UV lamp (365 nm wavelength).
J. Chen et al. / International Journal of Biological Macromolecules 139 (2019) 367–376 Table 1 The electrical conductivity of GQDs/TPS composite films. Sample TPS 1.4 wt% GQDs/TPS 2.7 wt% GQDs/TPS 5.5 wt% GQDs/TPS 10.9 wt% GQDs/TPS 16.4 wt% GQDs/TPS
Thickness (mm)
SR (MΩ)
ER (Ω·m)
0.37 ± 0.02 0.30 ± 0.01 0.25 ± 0.01 0.28 ± 0.01 0.30 ± 0.03 0.23 ± 0.02
92.53 ± 4.83 32.47 ± 6.59 10.99 ± 0.85 8.78 ± 0.52 5.48 ± 0.27 3.86 ± 0.17
34.24 ± 1.79 × 103 9.74 ± 1.98 × 103 2.75 ± 0.21 × 103 2.46 ± 0.15 × 103 1.64 ± 0.08 × 103 0.89 ± 0.04 × 103
tensile strength and surface resistances of films. High GQDs loading reduced the film transparency because the GQDs themselves were brown, and because the loaded films had greater scattering effects. Increasing the content of GQDs in the composite film matrix increased the intensity of the solid-state PL emission. Photoluminescence was maximized at 10.9 wt% GQDs, suggesting the potential applicability of these materials as illuminants in white light-emitting diodes. The demonstrated environmentally friendly GQDs/TPS composite films prepared by melt extrusion have the advantages of low material and production costs, biodegradability, easy processing, and suitability for large-scale production. GQDs obtained by citric acid pyrolysis are thus suggested as a candidate for cost-effective and eco-friendly optical systems. Acknowledgments This work was supported by the National Natural Science Foundation of China [31270633]; Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control [KF201703]; The Lianyungang 555 Talents Project Program of China [2015-13] and the National Basic Research Program of China [No.2014CB460610]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.07.211. References
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Biodegradable blends of graphene quantum dots and thermoplastic starch with solid-state photoluminescent and conductive properties Jie Chen a,b,c, Zhu Long a,b,⁎, Shuangfei Wang b, Yahui Meng a, Guoliang Zhang a, Shuangxi Nie b a b c
Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China Department of Mechanical & Electronic, Yuncheng University, Yuncheng 044000, China
a r t i c l e
i n f o
Article history: Received 14 February 2019 Received in revised form 16 July 2019 Accepted 30 July 2019 Available online 01 August 2019 Keywords: Thermoplastic starch Graphene quantum dots Solid-state photoluminescence Conductive property Film
a b s t r a c t Polymer composites based on blends of graphene quantum dots (GQDs) with thermoplastic starch (TPS) were prepared by melt-extrusion combined with hot pressing. The GQDs/TPS films were characterized as potential novel, high-performance, and ecofriendly composites replacing traditional non-biodegradable plastic packaging materials. GQDs stock solutions of different concentrations were incorporated into TPS matrices in order to analyze the solid-state fluorescent properties and conductive properties of GQDs/TPS films. The fluorescent, conductive, morphological, mechanical, and optical properties of the GQDs/TPS films were characterized by ultraviolet–visible spectroscopy, surface resistance measurement, scanning electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, tensile testing, and X-ray diffraction (XRD). FT-IR studies indicated hydrogen bonding between the oxygen-containing groups on GQDs surfaces and the –OH groups in the TPS. The mechanical testing results showed the optimum GQDs loading of 10.9 wt% in the blend. XRD and TEM studies indicated uniform graphene dispersions in the TPS matrix for ≤10.9 wt% GQDs loading; further increases in loading caused agglomeration. The maximum photoluminescence intensity and conductivity of the materials were obtained at 10.9 wt% GQDs loading. These materials have potential applicability in flexible optoelectronic packaging materials. © 2019 Published by Elsevier B.V.
1. Introduction In recent years, graphene has attracted increasing attention as a nanocarbon material because it offers excellent characteristics including a large specific surface area, high carrier mobility, excellent mechanical flexibility, good thermal/chemical stability, adjustable excitation and emission wavelengths, fluorescence stability, photobleaching, and other good biocompatible properties [1,2]. Compared to twodimensional graphene nanosheets (GNSs) and one-dimensional graphene nanoribbons (GNRs), zero-dimensional graphene quantum dots (GQDs) are characterized by their sizes of b10 nm [3,4]. Because GQDs show stronger quantum confinement effects and boundary effects than GNSs and GNRs, they have potential applicability in many fields, including solar photovoltaic devices, biopharmaceuticals, lightemitting diodes, and sensors [5,6]. Recent scientific studies have indicated that citric acid pyrolysis as a bottom-up GQDs preparation method yielded products with good dispersibility and high crystallinity by adjusting the degree of citric acid carbonization [7].
⁎ Corresponding author at: Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China. E-mail address: [email protected] (Z. Long).
https://doi.org/10.1016/j.ijbiomac.2019.07.211 0141-8130/© 2019 Published by Elsevier B.V.
Compared to other methods, such as electron beam lithography [8], electrochemical [9], microwave [10], chemical vapor deposition [11], pyrolysis (for blue luminescent graphene), and laser ablation [12], the citric acid method could permit the industrial-scale production of cheap renewable materials because it entails cheap raw materials, simple steps, low equipment requirements, and highly safe operation. In order to better apply GQDs to optoelectronic devices, GQDs electrical conductors and plastic substrates are necessary to form flexible transparent conductive films [13]. Because petroleum-based plastic products have negative environmental impacts via the accumulation of non-biodegradable wastes [14], biodegradable transparent films, derived from natural organic polymers with low aggregation indices have attracted worldwide attention, with excellent characteristics including biodegradability, biocompatibility, safety, and reproducibility [15]. Starch is considered a high-potential resource for the development of bio-polymers because it is abundant, low in cost, renewable, and biodegradable [16]. Starch can be converted to thermoplastic starch (TPS) under high temperature and high pressure in the presence of plasticizers (such as glycerin and water); it has properties similar to those of synthetic thermoplastics [17]. However, plasticized starch-based materials are easily affected by humidity and temperature and thus poor in mechanical strength and water resistance; this limits their potential applications [18].
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Commonly used methods for synthesizing GQDs involve high cost materials (such as graphite [19] or 6photonic crystal [20]), lower enthalpy and high cost methods (such as laser ablatin [12], electron beam lithography or electrochemical synthesis [9]). These factors limit the commercial application of GQDs. Recent studies have reported that the preparation of GQDs from relatively inexpensive organic sources, such as citric acid or urea [21], is beneficial in reducing production costs and expanding production scale. GQDs produced by citric acid pyrolysis are highly soluble in water and do not experience surface modification in polymer composites, which is ascribed to the abundant polar groups on the edges of the GQDs and on the substrate [22]. Therefore, GQDs could be used to reinforce TPS films [23]. During the melt extrusion process, the starch can be modified into TPS with a GQDs standard aqueous solution and plasticizer. Recently, GQDs/TPS films have been prepared by solution casting [24]. However, the preparation of GQDs/TPS films using GQDs as fillers and plasticizers has not yet been reported. The mechanical and optoelectronic properties of such films are studied here.
2. Materials and methodology 2.1. Materials Citric acid monohydrate (CA, weight-averaged molecular weight Mw = 54,000), glycerol, and sodium hydroxide (NaOH) were purchased as analytical-grade reagents from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China); corn starch was obtained from Shanghai Quanwang Biotechnology Co., Ltd. (Shanghai, China). The water used in the experiment was deionized. 2.2. Synthesis of GQDs GQDs were directly prepared by the citric acid pyrolysis method (see in Fig. 1). The citric acid was carbonized through pyrolysis to dehydrate and condense the citric acid molecules into GQDs. Following the conventional method, 2 g CA was placed in a 5-mL beaker and heated to 200 °C using a heating mantle. After 5 min, CA was partially liquefied. With increased heating time, the color of the CA liquid gradually
Fig. 1. Diagram for the synthesis of GQDs.
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changed from colorless to light yellow to orange after 30 min, indicating GQDs formation. The obtained orange liquid of GQDs was added dropwise to 40 mL, 20 mL, 10 mL, 5 mL, and 3 mL of a 10 mg/mL NaOH solution under vigorous stirring. These were neutralized to pH 7.0 with NaOH solutions to obtain 5%, 10%, 20%, 40%, and 60% stock aqueous solutions of GQDs (denoted as 5GQDs, 10GQDs, 20GQDs, 40GQDs, and 60GQDs, respectively). 2.3. Fabrication of the composite films TPS-based composite films, shown in Scheme 1, were prepared by melt-extrusion with the ratio of starch: glycerol: GQDs stock solution of 55:30:15 (wt%). After this preliminary step, the glycerol and GQDs stock solution were dissolved at 40 °C. Next, the dried starch was gradually added and thoroughly mixed for 2 h at 800 rpm with an overhead stirrer. Scheme 2 depicted the preparation mechanism of thermoplastic starch. The wet modified starch mixture was sealed in a plastic bag for 48 h. Afterward, the resulting blend was extruded through a co-rotating twin-screw extruder (Shanghai Xinshuo Precision Machinery Co., Ltd., China). The screw speed was adjusted to 100 rpm and the feeder-to-nozzle temperature was set to 140 °C. Subsequently, the obtained linear materials were air-cooled and granulated using a blade grinder equipped with a nominal 2-mm internal diameter. Finally, the plasticized materials were equilibrated for one week and then thermo-pressed for 15 min at 140 °C under a load of 5 tons in order to produce composite films with the approximate thickness of 0.3 mm.
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2.4.3. Transmission electron microscope (TEM) The dispersibility of GQDs in the stock solutions was surveyed by a TF20/2100F transmission electron microscope (TEM, JEOL, Japan) operating at 200 kV with a resolution ratio of 0.20 nm. 2.4.4. Scanning electron microscope (SEM) The surface morphologies of the composite films were observed with an SU 1510 scanning electron microscope (SEM, Hitachi, Japan) at an accelerating voltage of 5 kV after coating with gold. 2.4.5. Mechanical properties Mechanical properties including the tensile strength and elongation at break were investigated by a universal testing machine (KDII-0.05, Shenzhen KQL Test Instrument Co., Ltd., China) at room temperature. Samples were pre-conditioned at 23 ± 2 °C and relative humidity of 53 ± 2%. They were then cut into strips of 150 mm × 10 mm (length × width). The composite films were loaded at a constant cross-head speed of 50 mm/min and a clamping length of 100 mm. During the process, five measurements were performed in triplicate. 2.4.6. Transmittance of films The optical transmittances of the film samples were measured using a TU-1901 ultraviolet–visible (UV–vis) spectrophotometer (Beijing General Analysis Instrument Co., Ltd.) in the incident light wavelength range 280–800 nm.
2.4. Film characterization
2.4.7. Solid-state photoluminescence spectra of films The solid-state photoluminescence (PL) spectra of the GQDs/TPS films were obtained by an F-4600 fluorescence spectrophotometer (Hitachi High-tech Company, Japan).
2.4.1. FT-IR analysis The functional groups of the GQDs, TPS, and GQDs/TPS films were tested and analyzed by Fourier-transform infrared (FT-IR) spectroscopy (Nicolet Nexus, Thermo Electron Corporation, Waltham, MA, USA) in the range 4000–500 cm−1 with a resolution of 4 cm−1.
2.4.8. Surface resistance measurement The surface resistances of the composite films were measured using a LFY-406 material resistivity tester. The test principle is shown in Fig. 2. Using a two-probe method, a constant force of 10 N is applied to the two Cu electrodes, on which the probes are placed.
2.4.2. X-ray diffraction (XRD) The X-ray diffraction (XRD) patterns of the GQDs/TPS films were obtained using an X'Pert PRO MPD X-ray diffractometer (Panalytical, Poland) with Ni-filtered Cu (Kα) radiation at ambient temperature, 40 kV generator tension, and 30 mA generator current, in the scanned 2θ range 5–50° at the rate of 4°/min with a step size of 0.05°.
Rs ¼
R0 W L
ð1Þ
where, Rs is the surface resistance of the sample (Ω/square or Ω/sq), R0 is the resistance reading value of the multimeter (Ω), and W and L are the width and length, respectively, of the rectangle maintained by both probes. The average surface resistance of the three specimens
Scheme 1. The preparation process of GQDs/TPS composite films
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Scheme 2. The preparation mechanism of thermoplastic starch
prepared in the same batch was taken as the surface resistance value. In general, when the test area is a square, i.e., L = W, Rs = R is called the square resistance. In this study, L = W = 50 mm, and formula (2) i ρv ¼ Rs t
ð2Þ
Notes: results of thermal stability properties and hydrophilicity properties of composite films were shown in Fig. S1, Fig. S2 and Table S1. Meanwhile, the detailed information were described in the SI. 3. Results and discussion 3.1. FT-IR spectrum of composite films The FT-IR spectrum of the GQDs, neat TPS films, and GQDs/TPS composite films are shown in Fig. 3 (a, b). Fig. 3 (a) shows the characteristic peaks of GQDs, with –OH vibrations indicated at ~3285.14 cm−1, C_O stretching vibrations of carboxylic groups at 1641.07 cm−1, skeletal vibrations from unoxidized graphitic domains at 1557.51 cm−1, C–OH (hydroxyl) stretching vibrations at 1386.33 cm−1, C–O–C (epoxy) stretching vibrations at 1260.97 cm−1
(fullerenol), and C\\O stretching vibrations at 1095.12 cm−1. Similar FT-IR results from graphene have been reported [25]. Fig. 3 (b) shows the FT-IR spectra of pure TPS films and of GQDs/TPS composite films with different GQDs concentrations. The FT-IR spectra of the synthesized GQDs/TPS composite films are similar to the spectral curves of the neat TPS films, with peaks at ~1574.36 cm−1 from the GQDs. Meanwhile, the intensity of the GQDs peak is increased as the concentration of GQDs increases. The broad peak at 3200–3500 cm−1 is ascribed to the stretching vibration of hydroxyl groups; that at 2925.39 cm−1 is assigned to C\\H stretching, while the bump at 1716.53 cm−1 is attributed to the C_O functional group and that at 1647.30 cm−1 is attributed to bound water. The absorption at 1150.20 cm−1 corresponds to stretching in aliphatic alcohols. The absorption band over 3200–3500 cm−1 from the plasticizer/starch blend is assigned to both inter- and intramolecular hydrogen-bonded –OH groups in the glycerol and starch, which are unaffected at low GQDs loadings. However, higher GQDs loadings cause decreases in the intensity of this absorption band, indicating the occurrence of hydrogen-bonding interactions between the –OH groups present in TPS and oxygencontaining groups in the GQDs, at the cost of inter- and intramolecular hydrogen bonding in the TPS [26]. 3.2. XRD analysis
Fig. 2. The test principle of surface resistances of composite films.
Fig. 3(c) presents the XRD patterns of the pure TPS films and GQDs/ TPS composite films. The major characteristic peak of the TPS films appears at 2θ = 19.78°; minor peaks appear at 2θ = 12.93° and 22.49°, corresponding to the mixture of A- and V-type crystal structures present in the TPS film [27,28]. The XRD pattern trends for the synthesized GQDs/TPS composite films are similar to that for the neat TPS. The characteristic peak of the GQDs/TPS composite film appears at around 2θ = 26.67°, assigned to the limited ordering structure of the graphene quantum dot (GQDs) obtained by citric acid at high temperature [29]. The weak broad peaks are related to the thinness and disordered stacking of some GQDs. For the composite films, the diffraction patterns are dependent on the GQDs content. At the low loadings of 1.4 wt% GQDs and 2.7 wt% GQD, the diffraction patterns of GQDs/TPS, with the weak characteristic peak of GQDs, are similar to that of neat TPS. The absence of the characteristic GQDs peak indicates that the graphene is exfoliated and individually dispersed in the polymer matrix. As the GQDs loading is increased to 10.9 wt%, the characteristic peak of GQDs shows lower intensity than 5.5 wt% GQDs and 2.7 wt% GQDs. This is because of the agglomeration
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Fig. 3. (a) FT-IR of graphene quantum dots and (b) composite films; (c) XRD diagrams of films; and (d) Mechanical properties of films.
Fig. 4. The TEM images of GQDs dispersion in stock solutions.
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of GQDs in the composite films, which weakens the interfacial adhesion between the nanosheets and the TPS matrix. 3.3. Morphological analysis TEM images of GQDs dispersed in the GQDs stock solutions are shown in Fig. 4. Because the stock solution with the lowest concentration of 5GQDs shows very little aggregation, homogeneous GQDs dispersions are observed for low GQDs loading levels. With the aggregation of 20GQDs, the particle sizes appear to increase. Increases in particle concentration may improve the particle aggregation with normal cluster sizes for GQDs concentrations of 40%–60%. The GQDs display good dispersibility in the stock solutions without surface treatment. This is a major benefit of GQDs compared to inorganic quantum dots (QDs), which often require surface treatment to prevent agglomeration. The influence of GQDs content on the surface morphologies of the GQDs/TPS composite films was studied by SEM, as shown in Fig. 5. The TPS film has a dense and smooth surface from hydrogen bonding among the TPS molecules under high temperature and pressure. By contrast, the GQDs/TPS composite films have relatively rough surfaces, attributed to the incorporation of GQDs in the starch matrices [30]. The images obviously show that composite films with low GQDs loadings can have uniform GQDs distributions, and that GQDs aggregation occurs as the concentration of GQDs is increased. The surfaces of the GQDs/TPS films with low GQDs contents (i.e., 1.4, 2.7, and 5.5 wt%) show no agglomeration because the GQDs prepared from citric acid are amphiphilic with polar surface groups, yielding uniform GQDs distributions in the starch. When the content of GQDs in the starch matrix reaches 16.4 wt%, some large clustered particles appear on the composite surface from GQDs aggregation. Simultaneously, the uniform and flexible surface is destroyed by the spatial effect of the conversion of starch to TPS. The high GQDs contents affect the arrangement of starch molecular chains and causes composite morphology conversion [31]. For 10.9 wt% GQDs, fewer clusters are observed on the surface of the composite film; more serious agglomeration is detected in the 16.4 wt% film.
3.4. Mechanical properties The mechanical properties such as tensile strength and elongation at break of the TPS films and GQDs/TPS composite films are shown in Fig. 3 (d). As the content of GQDs increases from 1.4 wt% to 10.9 wt% in the starch matrix, the edges of adjacent one-dimensional GQDs sheets may join together and lead to efficient load transfer between the starch matrix and the GQDs filler, thus increasing the tensile strengths of the composite films because of the good GQDs dispersity [32]. Meanwhile, the GQDs/TPS composite films have adequate strength to absorb the applied force, as previously reported. However, at 16.4 wt% GQDs, the tensile strength of the composite is decreased. This is because single GQDs sheets tend to become restacked in agglomerates, thus weakening the interface between GQDs filler and TPS matrix [33]. The elongation at break indirectly reflects the material flexibility. The pure TPS film is obviously more flexible than the composite films. Although the elongations at break of the GQDs/ TPS composite films are slightly different, the films retain suitable flexibility. 3.5. Transparency Fig. 6 exhibits the transparency of the TPS films and GQDs/TPS composite films under visible light. Through the TPS films under ambient visible light, the blue-colored logo on the paper is clearly observed, indicating the good transparency of pure TPS in the visible range. However, the transparency of the GQDs/TPS composite films evolves with increasing the GQDs loading content from 1.4 wt% to 16.4 wt%. The blue logo beneath the composite films gradually becomes blurred with increased GQDs concentration. This is because the GQDs are brown; the GQDs/TPS composite films have greater scattering effects and some GQDs aggregation occurs in the films [34]. The series of photographs in Fig. 6 shows the differences in transparency of the film samples with different compositions. In order to accurately study the difference in transparency of film samples with different GQDs contents, the light transmissions of the composite films were measured with a UV–vis spectrophotometer in Fig. 7(a) that shows the variation in light transmittance of the different composite films as a function of incident light wavelength.
Fig. 5. SEM images of GQDs dispersion in the GQDs/TPS composite films.
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Fig. 6. The Transparency of composite films.
3.6. Solid-state photoluminescence Fig. 8 shows digital photographs of the luminescent emission of the composite films under UV irradiation at 365 nm, and Fig. 7(c–d) exhibit the solid-state PL spectrum of GQDs and UV absorption spectrum of GQDs. The digital photographs show the prepared composite films with diameters of 30 mm and thickness of 0.30 mm. The addition of GQDs to the TPS matrices produces PL properties; the prepared GQDs/ TPS composite films have PL characteristics and become brighter with UV irradiation. By increasing the GQDs content in the composite film matrix, the intensity of emitted light is enhanced and the color becomes brighter blue, which is ascribed to the lack of emission in the absence of excited photons. However, the pure TPS films show dark blue emitted light because they do not radiate emission. The solid-state PL spectra of GQDs/TPS composite films were tested in order to research the quantitative properties of the synthesized composite materials. As seen in Fig. 7(b), the PL spectral curves of the GQDs/TPS films are wider with splitting, unlike that of the GQDs in water. These are attributed to creation and amplification phenomena (i.e., intersystem transitions and internal and external transformation) and the self-absorption of the aggregated GQDs. For GQDs concentrations increasing from 1.4 wt% to 10.9 wt%, the PL intensity of the GQDs/TPS films increases, attributed to the increased PL spaces in the composite films [35]. However, for the film with 16.4 wt% GQDs, the PL intensity is decreased because different aggregates of GQDs are created by the inner filtering effect, PL reabsorption, and partial quenching.
3.7. Surface resistance measurements The influence of GQDs content on the electrical conductivity of the GQDs/TPS composite films is analyzed by measuring the surface
resistance (SR) and electrical resistivity (ER), as shown in Table 1. The pure TPS film has a very high resistance because of the insulating properties of the starch; thus, neat TPS is excluded from the comparison. GQDs are the only conductive ingredients in the composite films; they apparently improve the electrical conductivity (EC) of the TPS-based films and their dispersity determines the electrical properties of the composite films. With increasing GQDs content, the ER of the composite film is decreased and the conductivity is increased, showing strong interactions and increased dispersion of GQDs in the TPS matrix [36]. The ER is decreased with the addition of 0–2.7 wt% GQDs; while ER is decreased more slowly as the GQDs content increases from 5.5 to 16.4 wt%, attributed to the aggregation of high GQDs loads. High GQDs loading causes the colorless and transparent TPS films to turn brown and low in transparency, as shown. 4. Conclusion GQDs produced from citric acid were incorporated into TPS matrices to prepare GQDs/TPS composite films by melt-extrusion with glycerol and distilled water as plasticizers. The GQDs obtained from citric acid were distributed uniformly in aqueous GQDs stock solutions with little agglomeration in the absence of surface treatment. In the FT-IR spectra of the GQDs/TPS composite films, the characteristic peak of the GQDs was identified at 1574.36 cm −1 . The characteristic XRD peak of the GQDs in the GQDs/TPS composite films appeared at 2θ = 26.67°, indicating the GQDs chemical structure produced by the carbonization of citric acid. The tensile strength and surface resistances of the composites were first increased with increasing GQDs contents because of the good dispersion of low-content GQDs in the composite films. However, high GQDs loading affected the alignment of the starch molecular chains, thereby reducing the
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Fig. 7. (a) Light transmittance of composite films with the wavelength of incident light; (b) Integrals of the PL spectra versus the GQDs content; (c) Solid-state PL spectrum of GQDs; (d) UV absorption spectrum of GQDs.
Fig. 8. Photograph of the TPS and GQDs/TPS films under UV lamp (365 nm wavelength).
J. Chen et al. / International Journal of Biological Macromolecules 139 (2019) 367–376 Table 1 The electrical conductivity of GQDs/TPS composite films. Sample TPS 1.4 wt% GQDs/TPS 2.7 wt% GQDs/TPS 5.5 wt% GQDs/TPS 10.9 wt% GQDs/TPS 16.4 wt% GQDs/TPS
Thickness (mm)
SR (MΩ)
ER (Ω·m)
0.37 ± 0.02 0.30 ± 0.01 0.25 ± 0.01 0.28 ± 0.01 0.30 ± 0.03 0.23 ± 0.02
92.53 ± 4.83 32.47 ± 6.59 10.99 ± 0.85 8.78 ± 0.52 5.48 ± 0.27 3.86 ± 0.17
34.24 ± 1.79 × 103 9.74 ± 1.98 × 103 2.75 ± 0.21 × 103 2.46 ± 0.15 × 103 1.64 ± 0.08 × 103 0.89 ± 0.04 × 103
tensile strength and surface resistances of films. High GQDs loading reduced the film transparency because the GQDs themselves were brown, and because the loaded films had greater scattering effects. Increasing the content of GQDs in the composite film matrix increased the intensity of the solid-state PL emission. Photoluminescence was maximized at 10.9 wt% GQDs, suggesting the potential applicability of these materials as illuminants in white light-emitting diodes. The demonstrated environmentally friendly GQDs/TPS composite films prepared by melt extrusion have the advantages of low material and production costs, biodegradability, easy processing, and suitability for large-scale production. GQDs obtained by citric acid pyrolysis are thus suggested as a candidate for cost-effective and eco-friendly optical systems. Acknowledgments This work was supported by the National Natural Science Foundation of China [31270633]; Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control [KF201703]; The Lianyungang 555 Talents Project Program of China [2015-13] and the National Basic Research Program of China [No.2014CB460610]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.07.211. References
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