Luminescent Polymer Composite Films Containing Coal-Derived

Nov 9, 2015 - Luminescent polymer composite materials, based on poly(vinyl alcohol) (PVA), as a matrix polymer and graphene quantum dots (GQDs) ...
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Luminescent Polymer Composite Films Containing Coal-Derived Graphene Quantum Dots Anton Kovalchuk,‡ Kewei Huang,‡ Changsheng Xiang,‡ Angel A. Martí,‡,∥,† and James M. Tour*,‡,∥,† ‡

Department of Chemistry, ∥Department of Materials Science and NanoEngineering, and †Smalley Institute for Nanoscale Science and Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: Luminescent polymer composite materials, based on poly(vinyl alcohol) (PVA), as a matrix polymer and graphene quantum dots (GQDs) derived from coal, were prepared by casting from aqueous solutions. The coalderived GQDs impart fluorescent properties to the polymer matrix, and the fabricated composite films exhibit solid state fluorescence. Optical, thermal, and fluorescent properties of the PVA/GQD nanocomposites have been studied. High optical transparency of the composite films (78 to 91%) and excellent dispersion of the nanoparticles are observed at GQD concentrations from 1 to 5 wt %. The maximum intensity of materials photoluminescence has been achieved at 10 wt % GQD content. These materials could be used in light emitting diodes (LEDs), flexible electronic displays, and other optoelectronic applications. KEYWORDS: fluorescent polymer composites, graphene quantum dots, nanocomposites, poly(vinyl alcohol), luminescent polymer composites, light emitting diodes

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INTRODUCTION Due to their unique size-dependent electro-optical properties, colloidal semiconductor quantum dots (QDs) have numerous potential applications in solar cells,1 light emitting diodes,2,3 bioimaging,4 electronic displays,5 and other optoelectronic devices and have thus been of significant research interest. However, due to the high market cost of inorganic QDs,6 on the order of thousands of US dollars per gram, their industrial use has been slow and limited. As a promising cost-effective alternative, carbon quantum dots (CQDs) and graphene quantum dots (GQDs) recently emerged as a new class of QD materials.7−12 CQDs and GQDs have advantages of nontoxicity, good solubility, stable photoluminescence, and better surface grafting, thus making them promising candidates for replacing inorganic QDs.11,12 Moreover, the recent discovery of a one-step multigram synthesis of GQDs from coal13,14 opens up the possibility of their large-scale industrial production. Previous methods of GQD synthesis7−12,15,16 involved high-cost raw materials such as graphene8,15 or photonic crystals10 and fairly low-yield and expensive methods such as laser ablation,7 electron beam lithography,11 or electrochemical synthesis.15 That made GQDs virtually unavailable for commercial applications. More recent research reports the preparation of GQD from fairly inexpensive organic sources such as an ascorbic acid12 and citric acid/urea16 that offers product cost reduction and availability on a larger scale. However, the synthesis of GQDs from coal (the least expensive material known) increases the possibility of the use of GQDs in future commercial products. Here we are investigating the © 2015 American Chemical Society

breadth of applications of coal-derived GQDs. The goal of this work is to explore solid-state fluorescent materials from the coal-derived GQDs that first exhibited fluorescence in aqueous solutions and study the properties of these materials. The incorporation of QDs in a transparent polymer matrix is one of the main approaches for their utilization in numerous photonic and optoelectronic applications and integration in real devices.16−20 In addition to serving the role of the matrix, polymers provide mechanical and chemical stability to the nanocomposite. Additionally, the introduction of polymers can slow QD agglomeration, thereby maintaining their emission properties. Polymer nanocomposites can be formed into technologically relevant structures such as thin films. Due to their high solubility in water, one of the most attractive methods for blending coal-derived GQDs with polymers is solution casting. Poly(vinyl alcohol) (PVA) was chosen as a matrix polymer because of its hydrophilic properties, solubility in water, high optical transparency, good chemical resistance, easy processability, and good film forming properties.21−24 GQDs obtained from bituminous coal13 were used as filler particles for the PVA-based nanocomposites. Because of the natural abundance of polar functional groups at the edges of GQDs synthesized from coal, they were used in polymer composites without any additional surface modification. In order to fabricate PVA/GQD composite films, both Received: July 6, 2015 Accepted: November 9, 2015 Published: November 9, 2015 26063

DOI: 10.1021/acsami.5b06057 ACS Appl. Mater. Interfaces 2015, 7, 26063−26068

Research Article

ACS Applied Materials & Interfaces components were dissolved in water and, after casting from solution, the water was evaporated leading to the film formation. Composites with 1 to 25 wt % GQD concentrations were prepared. Polymer composites containing these GQDs have not yet been reported.

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EXPERIMENTAL SECTION

Materials. Poly(vinyl alcohol) (99+% hydrolyzed, Mw 89000− 98000, Sigma-Aldrich), bituminous coal (Fisher Scientific), sulfuric acid (95−98%, Sigma-Aldrich), and nitric acid (70%, Sigma-Aldrich) were used as received. Dialysis bags (Membrane Filtration Products, Inc. Product number 1-0150-45) were used to purify the GQDs. GQD Synthesis. GQDs were synthesized from the bituminous coal using oxidative treatment in the mixture of sulfuric and nitric acid according to the previously reported procedure13 and purified using dialysis in DI water. Fabrication of the Composite Films. To make the composite films, the PVA powder (750−990 mg, depending on GQD loading) and various amounts of GQDs (from 10 mg for 1 wt % concentration to 250 mg for 25 wt % concentration) were dissolved in 20 mL of water using magnetic stirring and heating at 80 °C for 8 h to completely dissolve the powdered polymer; the GQDs dissolved almost instantly. Additional bath sonication for 10 min was used to ensure good dispersion of GQDs. Thereafter, 3 mL of each polymer/ GQD solution was placed into a glass Petri dish and dried under vacuum (∼12 Torr pressure) in a desiccator for 24 h at room temperature. The film formation takes place with the evaporation of water. Characterization. FT-IR spectra were acquired on a Nicolet FTIR infrared microscope with an attenuated total reflectance (ATR) attachment. Transmission electron microscopy (TEM) observations of the GQDs and PVA/GQD composites were conducted using JEOL1230 high contrast transmission electron microscope at 120 kV. For the TEM imaging of the composite films, small droplets of PVA/GQD aqueous solutions were deposited on TEM grids and dried in a desiccator to form ultrathin films transparent to the electron beam. High-resolution TEM (HR-TEM) images of the GQDs were collected using JEOL 2100 field emission gun transmission electron microscope at 200 kV. Ultraviolet−visible (UV/vis) spectra were recorded on a Shimadzu UV-2450 ultraviolet−visible spectrophotometer. Differential scanning calorimetry (DSC) analysis of the materials was performed using DSC Q10 calorimeter (TA Instruments) at 10 °C/min heating rate in the temperature range 25 to 250 °C followed by the cooling at 5 °C/min rate down to 25 °C. Thermogravimetric analysis (TGA) was performed on a TGA Q50 instrument (TA Instruments) at a heating rate of 10 °C/min, from room temperature to 600 °C. The experiments were carried out under an air atmosphere at a flow rate of 50 mL/min. Photoluminescence spectroscopy experiments were conducted using Jobin Yvon HORIBA NanoLog spectrofluorometer at 345 nm excitation and scanning from 370 to 660 nm (using front face modality). The quantum yield of the 10 wt % film was calculated following Wrighton et al.25 BaSO4 was used as a reference material, and the films’ photoluminescence and reflectivity were measured over BaSO4.

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Figure 1. TEM images of the (a, b) GQDs and PVA/GQD composites containing (c) 1 wt %, (d) 3 wt %, (e) 5 wt %, and (f) 10 wt % GQDs.

TEM images of the GQD distribution in the thin PVA/GQD composite films. The images at lower loadings support the achievement of a homogeneous GQD dispersion in the polymer matrix. The composite with the lowest GQD loading (1 wt %) showed almost no aggregation of the filler nanoparticles. At 3 wt % (Figure 1d), the particle sizes appear larger, possibly due to aggregation. An increase in the GQD concentration, up to 5 to 7 wt %, leads to moderate particle aggregation, with typical cluster size below 100 nm. At GQD concentrations approaching 10 wt % and above, considerable nanoparticle agglomeration was observed (Figure 1f) with a formation of loose agglomerates having dimensions > 500 nm. The GQDs demonstrate excellent dispersibility in the PVA matrix without any additional surface modification of these nanoparticles. This is a notable advantage of coal-derived GQDs over inorganic QDs that normally require surface treatment in order to prevent agglomeration.16,17,27 An important insight into both the structure and optical properties of the composite films can be provided by UV/vis spectroscopy. Figure 2 shows UV/vis spectra of the neat PVA and PVA/GQD films. The thickness of the analyzed film samples was ∼10 μm. The dependence of the film’s optical transparency (light transmittance at 550 nm wavelength) on the GQD content is plotted in Figure 3. Because of the fine

RESULTS AND DISCUSSION

FT-IR spectra for the GQDs, PVA, and PVA/GQD composites are shown in Supporting Information Figure S1. The spectra for the pure PVA and PVA/GQD composites (at different GQD concentrations) are similar to the additive bands of PVA and GQD.26 The intensity of the GQD peak (∼1570 cm−1) increases with the increased GQD loading. Figure 1a,b shows TEM and HR-TEM images of the GQDs synthesized from bituminous coal. The GQDs have irregular spherical-like shapes with a typical size of 15 to 50 nm. Figure 1c−f shows typical 26064

DOI: 10.1021/acsami.5b06057 ACS Appl. Mater. Interfaces 2015, 7, 26063−26068

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Figure 2. UV/vis spectra for the (a) neat PVA film and PVA/GQD composites with (b) 3 wt %, (c) 5 wt %, (d) 15 wt %, and (e) 25 wt % GQD concentration.

Figure 4. DSC thermograms (first heating cycles) for the (a) blank PVA and PVA/GQD composite films with (b) 3 wt %, (c) 7 wt %, (d) 15 wt %, (e) 20 wt %. and (f) 25 wt % filler content.

increases in the polymer melting peak temperature (Tm) from 227 °C for the neat PVA to 228 to 230 °C for the composites takes place upon the incorporation of 1 to 20 wt % of GQDs. The melting enthalpy (ΔHm) of the composites demonstrates a gradually declining trend with the increase in GQD loading (Table 1). Thus, GQDs reduce the crystallinity degree (Xc) of Table 1. Thermal Properties of the PVA/GQD Composites material PVA PVA/GQD PVA/GQD PVA/GQD PVA/GQD PVA/GQD PVA/GQD

3 wt % 5 wt % 7 wt % 15 wt % 20 wt % 25 wt %

Tm, °C

ΔHm, J/g

Xc,a %

Tc, °C

227 229 230 230 228 230 226

65.08 62.38 54.25 41.69 37.47 35.49 37.12

47.0 46.4 41.2 32.3 31.8 32.0 35.7

202 205 206 197 204 202 195

Xc calculated from the ratio ΔHm/ΔH0, where ΔHm is the measured and the ΔH0 is 100% crystalline melting enthalpy of PVA, respectively. Here ΔH0 is taken as 138.6 J/g,27 and ΔHm is normalized to the PVA content in the material. a

Figure 3. Optical transparency (measured at 550 nm) vs GQD concentration in the PVA/GQD composite films.

nanoscale dispersion of the GQDs up to 3 wt % loadings, the composite films retain very high optical transparency (∼91%) that is on the same level with the baseline polymer (91.4%). Further increase in the GQD loading leads to nanoparticle agglomeration that is evidenced by the considerable drop in the optical transparency (to 78% and below) at GQD concentrations from 5 wt %. These findings are consistent with the TEM observations. The film transparency is maintained at almost the same level (∼65%) in a wide range of the GQD concentrations from 7 to 15 wt %. On the basis of this data, the composites have comparable levels of nanoparticle agglomeration, near their volumetric saturation, at these filler loadings. The further drop in the optical transparency below 40% at 20 wt % concentration is consistent with a critical level of GQD agglomeration above the saturation point. At this point we might expect significant degradation of the materials’ optoelectronic properties. DSC thermograms (first heating cycles) for the PVA and PVA/GQD nanocomposites are shown in Figure 4. Only small

the host polymer; this effect can be attributed to strong molecular interactions, such as hydrogen bonding, between the system components as previously reported for the structurally similar PVA/reduced graphene oxide composites.28,29 It is known that the broad band between 3000 and 3500 cm−1 in the FT-IR spectra (Figure S1), involving the strong hydroxyl band for free and hydrogen-bonded alcohols, can indicate hydrogen bonding, possibly between the polymer matrix and nanoparticle filler.28 However, from the observed small shift in this band to lower wavenumbers for the PVA/GQD composites (from 3264 cm−1 for PVA to 3261 cm−1 for PVA/7 wt % GQD and 3253 cm−1 for PVA/20 wt % GQD), we cannot make clear conclusions about the interaction between the polymer chains and nanoparticle filler. The decrease of the polymer crystallinity was therefore probably caused by a combination of several factors, primarily steric effects and structural disorder induced by the incorporation of GQDs, with some influence from the hydrogen bonding. 26065

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ACS Applied Materials & Interfaces Crystallization temperature (Tc) of the PVA slightly increases by 3 to 4 °C at GQD concentrations of 3 to 5 wt % (Table 1) showing very small nucleation effect induced by the filler nanoparticles. The further Tc decrease at higher GQD loadings is apparently caused by the nanoparticle agglomeration at these concentrations. On the basis of the TGA data (Figure 5), the residual water content in the PVA and PVA/GQD films is ∼5 to 10 wt %; the

Figure 6. Photograph of the (a) PVA and PVA/GQD films containing (b) 3 wt %, (c) 5 wt %, and (d) 10 wt % GQDs under UV lamp (365 nm wavelength). Film width/length is 25 × 25 mm and thickness is ∼10 μm. The focus plane is on the top end of the glass slide substrates where the films are placed.

is evidenced by the increase of the film brightness in Figure 6b−d. The color of the emitted light appears to be white. The PVA film (Figure 6a) shows no emission. For comparison, Supporting Information Figure S4 shows an image of PVA/ GQD film in a dark room without any UV illumination. Since no luminescent light is emitted, the film cannot be detected and appears totally dark. Images of other films taken without UV light look the same. In order to explore the luminescence properties of the PVA/GQD nanocomposites, photoluminescent spectroscopy measurements in the solid state were done; the corresponding spectra are shown in Figure 7. The Figure 5. TGA profiles for the PVA and PVA/GQD composite films in air.

removal of water from the films takes place between 50 and 150 °C. As seen from Figure 5, the incorporation of GQDs in the PVA matrix changes the decomposition behavior of the polymer. The maximum weight loss temperature decreases from ∼366 °C for PVA to ∼280 °C for the composites. This result is an indication of a change in the polymer degradation mechanism, possibly induced by GQDs, that needs further study since it remains unclear. The amount of residue formed in the process of PVA decomposition increases with the addition of GQDs. While the neat PVA decomposes almost completely before 600 °C, considerable amounts of black carbonized residues (up to 20%) remain after the composites’ burnout. Formation of these carbonized residues can be explained as a result of GQD thermal reduction by the polymer, a known process for graphene oxide reduction to graphene.30,31 GQDs are chemically similar to graphene oxide and the same effect could be operating here in the case of PVA/ GQD nanocomposites. Supporting Information Figure S2 demonstrates luminescence emitted by dilute aqueous solution of GQDs (0.125 mg/ mL) under UV light; strong bright fluorescence is noted. The corresponding photoluminescence spectrum for GQDs in solution is shown in Supporting Information Figure S3. It was found that the incorporation of GQDs in the PVA matrix imparts luminescence properties to the resultant composites. The luminescence behavior of the PVA/GQD composite films was first noted in a photograph (Figure 6) taken under a UV lamp. The films were placed on the glass slides as substrates, and all the samples were uniformly lit with the UV light; only the top ends of the slides covered with the GQD-containing films return visible light due to the film photoluminescence. An increase in the composite emission intensity with GQD loading

Figure 7. Photoluminescence spectra of PVA and PVA/GQD composite films.

integrated areas of the photoluminescence spectra of the films vs the corresponding GQD concentrations are plotted in Figure 8. Acccording to this data, the photoluminescence intensity of the composites is concentration dependent and has a tendency to increase with increasing GQD content within the 1 to 10 wt % concentration range. The maximum intensity was observed at 10 wt % loading, at which the saturation point was apparently reached. Higher GQD concentrations (15 to 25 wt %) show similar integrated areas. Given that the maximum is reached at 10 wt %, we estimated the quantum yield of photoluminescence of this composite film to be 0.5%. This quantum yield does not take into account light reabsorption, which is likely to occur as the concentration of GQDs increases, 26066

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efficiently dispersed in a polymer phase. Luminescence was successfully achieved in PVA/GQD composites, and the materials exhibited concentration-dependent behavior, with photoluminescence intensity progressively increasing as the GQD content increased; the photoluminescence reached its maximum at 10 wt % loading. The PVA/GQD nanocomposites exhibit broad photoluminescence emission spectra covering most of the visible range, opening up the possibility of using these materials as luminophores in white light LEDs. Due to their low production cost, the coal-derived GQDs are feasible for large-scale industrial applications and might be successfully used as a cost-effective and eco-friendly alternative to conventional inorganic quantum dots. In future research we are planning to explore the alkyl and aryl edge functionalization of the GQDs to enhance their dispersion in hydrophobic polymers.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Integrated areas of the photoluminescence spectra of the PVA/GQD films vs the corresponding GQD concentrations.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06057. Additional figures with spectra and photographs (PDF)

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and therefore it must be taken as a lower limit. Furthermore, due to the limitations and uncertainties associated with calculating the quantum yield of this film (see Experimental Section), this value is more accurately an indicator of the order of magnitude of the quantum yield, rather than an absolute value. Nonetheless, this quantum yield is in aggrement with previously calculated quantum yields from GQDs in solution.14 The increase in photoluminescence with GQD concentration up to 10 wt % is consistent with an increase in the amount of photoluminescent species in the films. After 10 wt %, the photoluminescence reaches a maximum due to a variety of interrelated factors such as inner filter effect, photoluminescence reabsorption, and partial quenching due to particle agglomeration. Accordingly, in order to achieve the maximum output efficiency of the PVA/GQD composites in terms of their luminescence level, the recommended concentration range of GQDs is ∼10 wt %. Because these materials have a broad emission spectrum covering most of the visible range, PVA/GQD composites could potentially be used in white light LEDs as luminophores. These materials might be integrated into commercial LEDs operating in the UV range to produce white light from the GQDs composite. Normally, white light emission is achieved in LEDs by combining different emission sources, for example, blue and yellow QDs.32 However, the same effect might be achieved more easily with the use of the coal-derived GQDs, as seen from the above results, due to the wide emission spectra of the GQD composites. The broad spectrum of the composite films here could be caused by the variation in the GQD diameter (from 15 to 50 nm) as evidenced by TEM. Since QD emission properties are size-dependent, combining GQDs of different sizes in one system is expected to give a broad resultant spectrum.

AUTHOR INFORMATION

Corresponding Author

*(J.M.T.) E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): J.M.T. has interest in a company that has licensed technology on graphene quantum dot manufacture and applications. This is reported to and overseen by the Rice University of Research Compliance.

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ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research (FA9550-14-1-0111) and the Air Force Office of Scientific Research MURI (FA9550-12-1-0035). A.A.M. thanks the Welch foundation (C-1743) for support.

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CONCLUSION In conclusion, the coal-derived GQDs have been successfully blended with PVA using a simple and environmentally friendly solution method with water as the solvent. The GQDs show excellent dispersibility without any additional surface modification. This is an important advantage of the coal GQDs over inorganic QDs that typically require modification to be 26067

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