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Thermal and electrical properties of starch–graphene oxide nanocomposites improved by photochemical treatment
Accepted Manuscript Title: THERMAL AND ELECTRICAL PROPERTIES OF STARCH-GRAPHENE OXIDE NANOCOMPOSITES IMPROVED BY PHOTOCHEMICAL TREATMENT Author: Priscilla P. Peregrino Maria J.A. Sales Mauro F.P. da Silva Maria A.G. Soler Luiz F.L. da Silva Sanclayton G.C. Moreira Leonardo G. Paterno PII: DOI: Reference:
S0144-8617(14)00128-3 http://dx.doi.org/doi:10.1016/j.carbpol.2014.02.008 CARP 8574
To appear in: Received date: Revised date: Accepted date:
11-1-2014 1-2-2014 3-2-2014
Please cite this article as: Peregrino, P. P., Sales, M. J. A., Silva, M. F. P., Soler, M. A. G., Silva, L. F. L., Moreira, S. G. C., & Paterno, L. G.,THERMAL AND ELECTRICAL PROPERTIES OF STARCH-GRAPHENE OXIDE NANOCOMPOSITES IMPROVED BY PHOTOCHEMICAL TREATMENT, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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THERMAL AND ELECTRICAL PROPERTIES OF STARCH-GRAPHENE
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OXIDE
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TREATMENT
NANOCOMPOSITES
IMPROVED
BY
PHOTOCHEMICAL
4 Priscilla P. Peregrinoa, Maria J. A. Salesa, Mauro F. P. da Silvab, Maria A. G. Solerc,
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Luiz F. L. da Silvad, Sanclayton G. C. Moreirad, Leonardo G. Paternoa*
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PA 66075-900, Brazil
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Universidade de Brasília, Instituto de Química, Brasília DF 70910-900, Brazil
Pontifícia Universidade Católica de São Paulo, FCET, São Paulo SP 01303-050, Brazil
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Universidade de Brasília, Instituto de Física, Brasília DF 70910-900, Brazil
Universidade Federal do Pará, Instituto de Ciências Exatas e Naturais (ICEN), Belém
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13 *corresponding author.
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Tel.: +55 61 3107 3869; FAX: +55 61 3107 3900
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E-mail address: [email protected]
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Highlights for Review ! bionanocomposites of starch filled with graphene oxide
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! reagentless photochemical treatment to reduce graphene oxide and improve thermal and electrical properties of nanocomposites ! assessment of molecular-level interaction of starch and graphene oxide by ATR-FTIR and thermogravimetric analysis
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Bionanocomposite films have been prepared by casting an aqueous suspension of
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acetylated starch (ST) and poly(vinyl alcohol) (PVA) loaded with graphene oxide (GO).
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A photochemical and reagentless method has been successfully performed to convert
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the GO phase into reduced graphene oxide (RGO). The nanocomposites have displayed
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improved thermal and electrical properties when the amount of the GO phase is
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increased and properly converted to RGO. The molecular-level interactions between
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components are mainly hydrogen-bonding type according to attenuated total
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reflectance-Fourier transform infrared (ATR-FTIR) and Raman spectroscopies, as well
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as thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has
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confirmed the effective mixing between GO and the ST-PVA matrix. The thermal
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diffusivity and electrical resistivity of ST-GO nanocomposites have increased one order
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and decreased two orders of magnitude, respectively, after the photochemical treatment.
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These findings have confirmed the effectiveness of the proposed approach to produce
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starch-based nanocomposites with improved thermal and electrical properties.
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Key-words: bionanocomposites, starch, graphene oxide, thermal properties, electrical
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properties, photochemical reduction.
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1. Introduction
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Bionanocomposites represent today an environmentally friendly strategy for the
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production of novel materials while employing raw components from the plenty source
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provided by biomass. These systems are comprised by a continuous biopolymer matrix
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loaded with nanofillers, usually inorganic in nature. While the biopolymer matrix
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ensures the material’s biodegradability, nanofillers provide the required thermal,
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mechanical, and electrical properties, usually very poor in the biopolymer alone
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(Darder, Aranda, & Ruiz-Hitzky, 2007).
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Among several biopolymers for bionanocomposites starch is very appealing
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because of its low cost and availability from different natural sources such as vegetables
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and cereals (Paraginski et al., 2014). Besides the nourishing value, the film forming
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ability has given to starch a more remarkable role, for example in food packaging.
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Starch-based films are biodegradable and edible and, therefore, potential substitutes for
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many synthetic packaging materials and pose negligible harm to the environment (Tang
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et al., 2012).
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On the other hand, starch exhibits poor thermal and electrical properties, which
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restrict its widespread use. This apparent limitation can be, however, circumvented by
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adding nanofillers such as graphene oxide (GO) sheets. GO can be considered a
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precursor for the preparation of graphene-based materials (Zhu et al., 2010). Besides
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that, GO is highly hydrophilic and dispersible in water, features that readily enable
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mixing and compatibility of GO and starch, as well as with other polymeric matrixes
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(Jaber-Ansari & Hersam, 2012). In fact, this possibility has been recently verified by
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different research groups. For example, Li et al. (Li, Liu & Ma, 2011) and Ma et al. (Ma
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et al., 2012) have carried out a pioneer research on nanocomposites made of starch and
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GO when they have found that GO increases the Young modulus and the thermal
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stability of pure starch. A subsequent investigation has been conducted by He and
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colleagues (He et al., 2013) when they have observed that GO prevents starch from
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degradation in both acidic and alkaline environments. Rodriguez-Gonzalez et al.
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(Rodríguez-González et al., 2012) have observed that the mechanical properties (storage
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modulus) of a chitosan-starch matrix is substantially improved (about 900 %) with only
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0.5 wt % of GO. Ashori's group have introduced the melting processing of
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polypropylene composites with either sugar cane bagasse or wood flours, both enriched
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with graphene nanoplatelets (Sheshmani, Ashori & Fashapoyeh, 2013; Chaharmahali et
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al., 2014). They have observed that there is an optimum graphene content for the
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bionanocomposites to reaching the best mechanical properties.
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However, another main interest on introducing GO into common polymeric
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matrices is to provide or enhance their optoelectronic properties. GO is usually obtained
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after chemical exfoliation of graphite which is a more affordable and readily available
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carbon source (Jaber-Ansari & Hersam, 2012). GO resembles graphene by its
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monoatomic dimension but it also differs significantly from it because of oxygen
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bearing functionalities such as hydroxyl, carbonyl, carboxyl, and epoxy. These
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functionalities disrupt part of the sp2 conjugation and turn GO into an electrical 3 Page 3 of 30
insulating material. However, part of optical, electronic and many other properties of
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pristine graphene can be restored after proper reduction of GO (Stankovich, et al., 2007;
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Tung et al., 2009). Concerning that, we have produced starch-based nanocomposite
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films in which GO is introduced and photochemically converted into its reduced form,
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namely reduced graphene oxide (RGO). The reduced nanocomposite has thermal and
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electrical properties improved after the photochemical process, which is easy to perform
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and does not demand for hazardous chemicals such as hydrazine or sodium
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borohydride.
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Herein, we describe the preparation and investigate the properties of
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bionanocomposite cast films made of starch filled with GO sheets (ST-GO). UV-vis,
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attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and Raman
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spectroscopies are employed to determine the structure and infer about the molecular-
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level interaction between the starch matrix and GO sheets. An additional study
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performed with thermogravimetric analysis (TGA) provides further evidence for the
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interaction of bionanocomposites' components. In order to restore part of the
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outstanding properties of graphene we have submitted the bionanocomposite films to
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UV radiation (254 nm). After a proper irradiation time, the thermal diffusivity and
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electrical properties of ST-GO films are significantly improved, due to partial reduction
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of GO, as confirmed by Raman and UV-vis spectroscopies.
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2. Experimental
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2.1. Materials
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Graphite powder (Union Carbide, SP-1 grade, 100 μm), acetylated starch (Avebe Guaíra
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Amidos, Brazil), poly(vinyl alcohol)-PVA (Sigma-Aldrich, 89,000-98,000 g.mol-1),
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H2SO4 98%, HNO3 67%, KClO3, KHCO3, HCl 37%, NH4OH 47%, were purchased
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from Sigma-Aldrich, Merck and Carlo Erba, all analytical grade or better and used as
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received. Ultrapure water (18.2 M! .cm) purified by a Milli-Pore Milli-Q system was
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used throughout all procedures. Graphene oxide (GO) was obtained via a two step
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procedure. In the first, graphite powder was oxidized in a mixture of 1:2 H2SO4/HNO3
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(2:1 v/v) and KClO3 as originally described by Staudenmaier (Staudenmaier, 1898) with
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some modification as described elsewhere (Silva et al., 2010). In the second step, the as-
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obtained graphitic oxide was suspended in NH4OH aqueous solution (pH 10) under
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sonication (150 W, 1 h). The resulting GO suspension was centrifuged at 15,000 rpm
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for 20 min to remove remaining solids and stored for the nanocomposites’ preparation.
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2.2. Preparation of Starch-GO (ST-GO) nanocomposites. 2.5 g of acetylated starch (ST) and 0.15 g of PVA were suspended together in 40
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mL of deionized water. PVA was used as an additional plasticizer (besides water) for
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starch. The milky ST-PVA suspension was kept under magnetic stirring at 90 oC for 1 h
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in order to gelatinize starch and ensure their complete mixing. In another vessel,
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additional 0.05 g of PVA were dissolved in deionized water (70 oC) under magnetic
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stirring until reaching its complete dissolution. A fixed volume of the GO aqueous
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suspension was then added to this PVA solution under magnetic stirring at 70 oC until
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obtaining a homogenous solution. This procedure was adopted to enhance compatibility
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between GO and starch and to avoid GO flocculation. Four different PVA-GO solutions
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with different GO concentrations were prepared. A fifth PVA solution absent of GO
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was prepared and used for the preparation of the control film. For the preparation of ST-
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GO nanocomposite films, the PVA-GO solution was added slowly to the ST-PVA
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suspension under magnetic stirring at 60 oC. The stirring and temperature were kept for
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more 1 h after addition of the PVA-GO solution. The resulting pale yellow suspension
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was then cast onto Petri dishes (diameter: 10 cm) and left drying in oven (60 oC) for 24
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h. The control film (absent of GO) was prepared under the same procedure. Five
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nanocomposite samples were prepared, labeled as: ST-C (control), ST-GO-010, ST-
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GO-025, ST-GO-050, and ST-GO-075. The numbers refer to the weight percentage of
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GO in the nanocomposite (regarding the starch mass).
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2.3. UV treatment of ST-GO samples In order to induce the photochemical reduction of GO, ST-GO samples (1.0 x
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2.5 cm ) were exposed to UV light (254 nm) (Boitton Instrumentos, Brazil) for different
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periods of time up to 2 hours. UV-vis spectra of films were obtained after different
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irradiation time to monitor the photochemical effect. For comparison, the control film
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(absent of GO) underwent the same treatment and monitoring. The ATR-FTIR and
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Raman spectra, thermal diffusivity, and current vs. potential curves of un- and treated
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samples were measured in order to evaluate the effectiveness of the photochemical
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treatment.
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2.4. Film characterizations
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The chemical structure of nanocomposite films was assessed by UV-vis (Varian
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Cary 5000), ATR-FTIR (Varian 640IR) and Raman (Jobin Yvon T 6400, 514 nm laser
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line, 0.3 mW) spectroscopies. Film’s morphology (surface and cross-section areas) was evaluated by scanning
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electron microscope (JEOL 440A). Film’s cross-section was made after submitting
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films to liquid nitrogen and further cracking. Samples were attached to brass stubs with
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the aid of conductive carbon tape and gold-coated by sputtering.
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Thermogravimetry (TGA) and derivative thermogravimetry (DTG) analysis
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were conducted in a DTG-60H Shimadzu thermoanalyzer. Powdered samples (~ 5 mg)
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were filled in platinum pans and evaluated under nitrogen atmosphere (30 mL.min-1), at
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heating rate of 10 °C.min-1.
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The thermal diffusivity of film samples was determined by photopyroelectric
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spectroscopy (PPS) as described by Mandelis and Zver (Mandelis & Zver, 1985), with
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the aid of a home-made experimental setup. The PPS working principle bases on the
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irradiation of a solid sample with a monochromatic source and measurement of that part
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of optical energy that is converted into heat by non-radiative deexcitation processes
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within the solid. According to the model described by Mandelis and Zver, a non-
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uniform variation on temperature leads to a voltage drop in the pyroelectric transductor.
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The voltage drop is related to the sample’s thermal diffusivity, according to equation 1
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as follows:
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where ! s is the sample’s nonradiative conversion efficiency; !
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diffusivity of the pyroelectric transducer and sample, respectively; kp is the transducer’s
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thermal conductivity; bsp is the ratio between the product of sample’s and transducer’s
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thermal conductivity and diffusivity (= ks! s/kp! p), ! is the angular frequency of the
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electrical field, and Ls is the sample`s thickness. In addition, the parameter A (=
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pI0/2k! 0), where p is the pyroelectric coefficient of the transducer, I0 is the incident light
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source irradiance, k is the transducer’s dielectric constant, and ! 0 is the vacuum
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permittivity (= 8.854 x 10-12 C V-1m-1). In the present study, samples` thicknesses were
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evaluated with a Mitutoyo micrometer, model M110-25. The sample`s thicknesses were
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about 12.0 ± 0.5 ! m.
p
and ! s are the thermal
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The electrical properties of the ST-GO samples, before and after photochemical
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reduction, were determined by current versus potential curves obtained with the HP
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4156A Semiconductor Parameter Analyzer. The ST-GO and ST-RGO films (1.0 x 2.5
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cm2) were coated with four equidistant silver epoxy contacts (MG Chemicals 8331
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Conductive Epoxy Adhesive) through which gold microprobes were connected to
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perform electrical measurements. A fixed current was applied through the two external
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contacts and the voltage drop across the two inner contacts was thus measured.
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3.1. Structure and morphology of ST-GO nanocomposite films
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Figure 1 provides UV-vis absorption spectra along with digital snapshots of ST-GO
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nanocomposites with different amounts of GO. Similarly to the GO aqueous suspension
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(not shown), the spectra of nanocomposites (Figure 1a) exhibit two maxima absorption
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peaks located at 230 nm and 300 nm, which are ascribed to the ! ! ! * transition in
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aromatic C=C bonds and the n! ! * transition in carbonyl groups, respectively (Li et al,
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2008; Guardia et al., 2012). The nanocomposites also display the same brownish color
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of GO suspension and they become darker as more GO is incorporated (Figure 1b). In
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addition, the transparency as estimated by the film transmittance (400-700 nm) of
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nanocomposites drastically reduces with incorporation of GO. For example, the ST-GO-
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0.10 sample has a transmittance of 75%, which then reduces to 35 % in the ST-GO-
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0.25, and then to 20% (ST-GO-0.50) and less than 20% for the ST-GO-0.75.
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Figure 1. UV-vis absorption spectra (a) and digital snapshots (b) of ST-GO
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nanocomposites with different GO loadings (in weight percentage), as indicated.
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The ATR-FTIR spectra of control, pure GO and ST-GO-0.50 nanocomposite are
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presented in Figure 2. The band referred to the stretching of O-H bond is clearly seen in
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the three samples. While in the control and in the ST-GO-0.50 nanocomposite this band
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peaks at 3360 cm-1, in pure GO the same band is more asymmetric and exhibits a
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maximum at shorter wavenumber (3235 cm-1). In the range below 2000 cm-1, the
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fingerprints of starch component are readily recognized in both control and ST-GO-0.50
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nanocomposite. In fact, the ST-GO-0.50 nanocomposite spectrum (in green) is almost
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indistinguishable from the control (in black) and shows none of the bands seen in the
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GO spectrum (in red). The main vibration modes in the control sample (in black) have
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been ascribed in accordance to literature (Flores-Morales, Jiménez-Estrada & Mora-
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Escobedo, 2012) as follows: C=O stretching of the acetylated groups (1742 cm-1); O-H
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bending (1652 cm-1); C-O stretching (1154 cm-1); C-O-H stretching (1082 cm-1); C-O-H
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bending (1022 cm-1); and C-O-C bending of ! -1,4-glycosidic bond (998 cm-1). In the
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spectrum of the ST-GO-0.50 nanocomposite (in green) most of bands have been
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systematically shifted to shorter wavenumbers. This trend suggests the interaction of
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starch and GO via hydrogen bonding. The assignments for bands in the ST-GO-0.50
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sample have been made as follows: C=O stretching of the acetylated groups (1742 cm-
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1
); O-H bending (1640 cm-1); C-O stretching (1150 cm-1); C-O-H stretching (1078 cm-
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1
); C-O-H bending (1022 cm-1); and C-O-C bending of ! -1,4-glycosidic bond (997 cm-
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1
). The greatest shift has been observed for the O-H bending, which is about 12 cm-1.
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The spectrum of pure GO displays the oxygen-bearing functionalities that have been
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introduced after graphite oxidation. In accordance to literature (Hontoria-Lucas et al.,
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1995) the assignments for such bands have been made as follows: C=O stretching (1714
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cm-1); C=C stretching (1612 cm-1); C-O-H bending (1417 cm-1); C-O-C stretching
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(1060cm-1); and =C-H bending (980 cm-1). The spectra of other ST-GO nanocomposites
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have showed similar features and are not presented.
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Figure 2. ATR-FTIR spectra of control (black), pure GO (red), and ST-GO-0.50
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nanocomposite (green) samples.
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Raman spectra of control, pure GO, and ST-GO-0.50 nanocomposite samples are shown
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in Figure 3. As a general observation, GO bands screen any other bands from the starch
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+ PVA matrix. The same trend has been observed for all other ST-GO compositions and
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for that reason their corresponding spectra are not shown. This effect is ascribed to the
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use of 514 nm excitation that matches better GO vibrations than starch ones. The
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spectrum of the control sample features the C-O and C-C stretching as well as the C-O-
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C deformation modes typical of the glycosidic bonds in starch. In accordance to
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literature (Almeida et al., 2010; Rodriguez-Gonzalez et al., 2012) the main vibrations
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(wavenumbers assigned in the spectrum) have been tentatively ascribed as follows:
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bending of C-C-H and C-O-C bonds (858 and 925 cm-1); stretching of C-O and C-C
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bonds (1129 and 1209 cm-1); bending of C-C-H, O-C-H and C-O-H bonds (1266 cm-1); 9 Page 9 of 30
stretching of C-O and bending of C-O-H bonds (1340 cm-1); bending of C-O-H bond
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(1387 cm-1), and bending of C-H and C-O-H bonds (1462 cm-1). The distinct bands at
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925 and 858 cm-1 are referred to vibrations originated from α-1,4 glycosidic linkages
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and provide a fingerprint for the amylose component of starch. The spectrum of pure
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GO (in red) displays two intense bands at 1340 cm-1 and 1600 cm-1. The first, the D-
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band, corresponds to the breathing mode of k-point phonons of A1g symmetry while the
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second one, the G-band, corresponds to the first-order scattering of the E2g phonon of
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sp2 carbons. Both bands are regarded to the graphene structure according to literature
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(Saito et al., 2011). The spectrum of the ST-GO-0.50 nanocomposite (in green)
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exclusively features bands regarding GO. This screening effect has been discussed
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above. The relative Raman D/G intensity ratio (ID/IG) is inversely proportional to
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average size of the sp2 domains (Saito et al., 2011). In the case of our pure GO, ID/IG =
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1.19, while for the ST-GO-0.50 nanocomposite, ID/IG = 0.84. The slight decrease of
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ID/IG when going from pure GO to ST-GO-0.50 nanocomposite has no explanation so
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far. As to other nanocomposites compositions, we have found no correlation between
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ID/IG and GO content, which has remained almost constant: 0.86 (ST-GO-0.10); 0.81
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(ST-GO-0.25); 0.84 (ST-GO-0.50); and 0.87 (ST-GO-0.75). After the photochemical
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treatment, which will be discussed in detail at section 3.3, the ID/IG ratio in the
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nanocomposite sample with 0.50 % wt of GO and now labeled ST-RGO-0.50,
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underwent a slight increase to about 0.92. This increase corresponds to reduction
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reactions of the GO phase, as already proposed by Stankovich and coworkers
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(Stankovich et al., 2007).
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ip t cr us an Figure 3. Raman spectra of control (starch + PVA, in black), pure GO (in red), ST-GO-
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0.50 nanocomposite (in green), and ST-RGO-0.50 nanocomposite (in blue). The
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spectrum in blue refers to the one obtained after the ST-GO-0.50 has been submitted to
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UV exposition for 160 min., as described in section 2.3.
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Figure 4 presents SEM images of the surface (top) and cross section (bottom) of
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nanocomposites. While the surface of all samples, including that of the control, is
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relatively flat and smooth, the cross section after fracture is more irregular on the
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nanocomposites with GO. This is because GO is well dispersed and strongly adhered to
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the starch-PVA matrix.
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Figure 4. SEM images of surface (top images; scale bar: 100 ! m) and cross section
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(bottom images; scale bar: 10 ! m) of nanocomposites: control (a) and (d); ST-GO-0.10
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(b) and (e); and ST-GO-0.50 (c) and (f).
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3.2. Thermal properties
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Figure 5 shows TG curves of the control and GO-containing nanocomposites. In Figure
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5a, the thermal behavior of the ST-GO-0.50 nanocomposite is compared to that of the
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control and pure GO. For pure GO (curve in black), one observes the decomposition at
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200 oC associated to a mass loss of about 60 %. This event has been associated to the
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dehydration of oxygenated groups accompanied by the partial reestablishment of the pi-
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conjugated system (Silva et al., 2011). The latter event maintains the partially reduced
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GO stable so that the residue after reaching 600 oC is about 30 %, which is greater than
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that left after decomposition of pure starch or the ST-GO-0.50 nanocomposite. As to
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pure starch, the TG curve (in blue) presents two main thermal events, the first one at ~
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100 oC that corresponds to the water elimination and a second event, at 300 oC, which is
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regarded to elimination of polyhydroxyl groups and starch depolymerization
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(Schlemmer, Sales & Resck, 2010; Schlemmer & Sales, 2010). At 600 oC, almost all
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starch is decomposed. The nanocomposites’s TG curve (in red) displays the elimination
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of water as well as the decomposition of starch. However, the residual mass at 600 oC is
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slightly greater than the control, most likely due to the residual, more stable GO.
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In Figure 5b, TG curves of ST-GO nanocomposites are presented for evaluating
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the effect of GO on their thermal stability. As one notes, as more GO is incorporated,
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the onset for the main decomposition event occurs at lower temperature whereas the
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residual mass at 600
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decomposition is attributed to the intercalation of GO sheets in between starch chains.
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This certainly reduces the intermolecular forces in starch as well as provides more
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oxygen reactive species that contribute for the starch decomposition. This result has
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been shared by other investigators working on similar systems (Ma et al., 2013; He et
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al., 2013; Li et al., 2011). This hypothesis is corroborated further by data collected in
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Table 1 (second column) that show that the more GO is introduced the lower is the
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enthalpy for the water elimination. This observation along with ATR-FTIR data (Figure
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2) allows one to conclude that GO sheets replace water molecules in the nanocomposite
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while establishing hydrogen bonds with starch and PVA chains. Additionally, Table 1
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(third column) collects thermal diffusivities measured accross the samples. The data
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show that samples containing GO are better thermal conductors than the control sample.
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This improvement has been ascribed to the few graphene domains remaining in GO
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after graphite oxidation. The outstanding thermal conduction in deffect free graphene is
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ascribed to the in-plane scattering of phonons (in the diffusion limit regime) (Pop,
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Varshney & Roy, 2012).
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(b) ST-GO nanocomposites with different loadings of GO, as indicated.
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Table 1. Thermal properties of control and ST-GO nanocomposites. Thermal Diffusivity (x 10-4 cm2.s-1) Sample Enthalpy (J.g-1)
352
Before Reduction
After Reduction
Control
465.11
1.86
-
ST-GO-0.10
491.15
1.92
7.1
ST-GO-0.25
196.27
3.08
8.3
ST-GO-0.50
102.12
3.59
11.5
ST-GO-0.75
84.36
7.19
17.5
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353
3.3. Photochemical reduction of ST-GO nanocomposites
354
The network structure of GO sheets is highly attractive and potentialy promising for the
355
improvement of mechanical properties of biopolymers such as starch. However, its
356
optoelectronic properties are still very poor since the pi-conjugated system is partially
357
disrupted after the graphite oxidation. Thus, the restoring of such conjugated structure is
358
mandatory for the attainment of electrical conducting composites. Generally, reductive
359
chemicals such as hydrazine and sodium borohydride, have been successfully employed
360
for this task even though they involve severe health and environmental concerns. In this 14 Page 14 of 30
sense, milder reduction approaches have been pursued and photochemical procedures
362
have called special attention. The photochemical reduction of GO to RGO by UV light
363
has been made possible because GO sheets strongly absorb energy at this wavelength
364
and subsequent relaxation processes could trigger a local heating of the sheets that
365
favors their deoxygenation (Kumar et al., 2012; Guardia et al., 2012; Fabbri et al.,
366
2012). Such a reduction method would provide the means by which one can easily
367
produce RGO with relatively less impurity.
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361
Considering those aspects, we have exposed the ST-GO-0.50 nanocomposite to
369
an UV light source (354 nm) and followed by UV-vis spectroscopy the structural
370
changes caused by the irradiation (see section 2.3). According to Figure 6, the ratio of
371
UV absorbances at 250 and 300 nm, which are ascribed to n! ! * (C=O) and ! ! ! *
372
(C=C) transitions, respectively, decreases with the exposure time. Moreover, the
373
snapshopts in the inset show that the nanocomposite initially brown have become
374
deeply dark at the end of treatment (after 160 min. of exposure). A control experiment
375
performed in the same conditions with the control film has not showed any darkening
376
effect of the starch matrix.
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377 378 379
Figure 6. Monitoring of absorbance ratios (A250/A300) of the ST-GO-0.50
380
nanocomposite as a function of the exposure time to UV light (254 nm). Inset:
381
snapshots of nanocomposite sample at the beggining (0 min.) and after 160 minutes of
382
exposure. 15 Page 15 of 30
383 To confirm the effectiveness of the proposed photochemical reduction method we have
385
carried out Raman spectroscopy as well as measured the thermal diffusivity and
386
electrical properties of nanocomposites. As displayed by Figure 3, the ID/IG ratio
387
increases from 0.84 (ST-GO-0.50, in green) to 0.91 (ST-RGO-0.50, in blue) after the
388
photochemical treatment. It is known that the ID/IG ratio is inversely proportional to the
389
average size of sp2 domains (Tuinstra & Koenig, 1970). Thus, the increase that has been
390
observed herein suggests that new graphitic domains have been created and that they are
391
smaller in size to the ones present in GO before reduction, but more numerous. In
392
summary, part of the pi-conjugated system is recovered, which is consistent with the
393
GO ! RGO reduction. The photochemical treatment has increased thermal diffusivities
394
one order of magnitude, as quoted in Table 1 (fourth column), since RGO is a better
395
thermal conductor than GO.
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The reduction of GO phase within the nanocomposites was also probed by
397
electrical measurements. Figure 7 displays current versus voltage curves for the ST-GO-
398
0.50 nanocomposite before and after the photochemical treatment. For the same voltage
399
range, the current measured within the nanocomposite has increased two orders of
400
magnitude after the photochemical treatment. The different colors refer to curves
401
aquired through different contact pairs on the sample. It is possible to note that data drift
402
is very low when measurement contacts are changed because samples and contacts are
403
quite uniform. The overpotential seen for the sample before reduction (bottom curves)
404
has been credited to its highly insulating behavior. Based on these results, one can
405
conclude that the photochemical treatment is sufficient to reduce GO to RGO and to
406
improve the thermal and electrical properties of the ST-GO nanocomposite.
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16 Page 16 of 30
ip t cr us Figure 7. Current versus voltage curves for ST-GO-0.50 nanocomposite before and after
410
photochemical reduction. The different colors refer to curves aquired between two
411
different pairs of contacts on the sample.
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412 413 4. Conclusions
415
Starch-graphene oxide nanocomposites (ST-GO) exhibit better thermal and electrical
416
properties when the amount of the GO phase is increased and properly converted to
417
reduced graphene oxide (RGO), respectively. Because water molecules are readily
418
replaced by GO sheets, the latter interact with the ST matrix through hydrogen bonds,
419
the enthalpy for water release from the nanocomposites become significant lower as
420
more GO is introduced. However, GO sheets also reduce the interaction between ST
421
domains and induce their degradation at lower temperatures. A reagentless
422
photochemical method for conversion of the GO phase into RGO increases the thermal
423
diffusivity and lowers the electrical resistance of nanocomposites to about one and two
424
orders of magnitude, respectively. Raman and UV-vis absorption spectroscopies
425
provide strong evidence for the partial restoring of the pi-conjugated system after the
426
photochemical treatment. In summary, the ST-GO nanocomposites and the
427
photochemical approaches described herein enable one for the production of low cost
428
and environmentally friendly nanomaterials with potential thermal and electrical
429
properties.
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431
Acknowledgements
432
The financial support of Brazilian agencies CNPq (process # 308038/2012-6), CAPES,
433
and Finatec is greatfully acknowledged. The authors also wish to thank Dr. Roberto
434
Cavallari (PSI-EPUSP) for the electrical characterizations.
436
ip t
435 References
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Table 1. Thermal properties of control and ST-GO nanocomposites. Sample Enthalpy (J.g-1) Thermal Diffusivity (x 10-4 cm2.s-1) Before Reduction
After Reduction
465.11
1.86
-
ST-GO-0.10
491.15
1.92
7.1
ST-GO-0.25
196.27
3.08
8.3
ST-GO-0.50
102.12
3.59
ST-GO-0.75
84.36
7.19
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Control
11.5
17.5
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544
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Peregrino et al.
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22 Page 22 of 30
546
Figure Captions
547 548 549
Figure 1. UV-vis absorption spectra (a) and digital snapshots (b) of ST-GO nanocomposites
550
with different GO loadings (in weight percentage), as indicated.
ip t
551 552
Figure 2. ATR-FTIR spectra of control (black), pure GO (red), and ST-GO-0.50 nanocomposite
553
(green) samples.
cr
554
Figure 3. Raman spectra of control (starch + PVA, in black), pure GO (in red), ST-GO-
556
0.50 nanocomposite (in green), and ST-RGO-0.50 nanocomposite (in blue). The
557
spectrum in blue refers to the one obtained after the ST-GO-0.50 has been submitted to
558
UV exposition for 160 min., as described in section 2.4.
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555
559
Figure 4. SEM images of surface (top images; scale bar: 100 ! m) and cross section (bottom
561
images; scale bar: 10 ! m) of nanocomposites: control (a) and (d); ST-GO-0.10 (b) and (e); and
562
ST-GO-0.50 (c) and (f).
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Figure 5. TG curves of (a) pure starch, pure GO, and ST-GO-0.50 nanocomposite and (b) ST-
565
GO nanocomposites with different loadings of GO, as indicated.
te
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564 566
Figure 6. Monitoring of absorbance ratios (A250/A300) of the ST-GO-0.50
568
nanocomposite as a function of the exposure time to UV light (254 nm). Inset:
569
snapshots of nanocomposite sample at the beggining (0 min.) and after 160 minutes of
570
exposure.
571
Ac ce p
567
572
Figure 7. Current versus voltage curves for ST-GO-0.50 nanocomposite before and after
573
photochemical reduction. The different colors refer to curves aquired between two different
574
pairs of contacts on the sample.
575 576
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S0144-8617(14)00128-3 http://dx.doi.org/doi:10.1016/j.carbpol.2014.02.008 CARP 8574
To appear in: Received date: Revised date: Accepted date:
11-1-2014 1-2-2014 3-2-2014
Please cite this article as: Peregrino, P. P., Sales, M. J. A., Silva, M. F. P., Soler, M. A. G., Silva, L. F. L., Moreira, S. G. C., & Paterno, L. G.,THERMAL AND ELECTRICAL PROPERTIES OF STARCH-GRAPHENE OXIDE NANOCOMPOSITES IMPROVED BY PHOTOCHEMICAL TREATMENT, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
THERMAL AND ELECTRICAL PROPERTIES OF STARCH-GRAPHENE
2
OXIDE
3
TREATMENT
NANOCOMPOSITES
IMPROVED
BY
PHOTOCHEMICAL
4 Priscilla P. Peregrinoa, Maria J. A. Salesa, Mauro F. P. da Silvab, Maria A. G. Solerc,
6
Luiz F. L. da Silvad, Sanclayton G. C. Moreirad, Leonardo G. Paternoa*
7 8
a
9
b
10
c
11
d
12
PA 66075-900, Brazil
cr
Universidade de Brasília, Instituto de Química, Brasília DF 70910-900, Brazil
Pontifícia Universidade Católica de São Paulo, FCET, São Paulo SP 01303-050, Brazil
us
Universidade de Brasília, Instituto de Física, Brasília DF 70910-900, Brazil
Universidade Federal do Pará, Instituto de Ciências Exatas e Naturais (ICEN), Belém
an
13 *corresponding author.
15
Tel.: +55 61 3107 3869; FAX: +55 61 3107 3900
16
E-mail address: [email protected]
21 22 23 24 25 26 27 28
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29 30 31 32 33
Highlights for Review ! bionanocomposites of starch filled with graphene oxide
1 Page 1 of 30
34 35 36 37 38
! reagentless photochemical treatment to reduce graphene oxide and improve thermal and electrical properties of nanocomposites ! assessment of molecular-level interaction of starch and graphene oxide by ATR-FTIR and thermogravimetric analysis
39
ip t
40 ABSTRACT
42
Bionanocomposite films have been prepared by casting an aqueous suspension of
43
acetylated starch (ST) and poly(vinyl alcohol) (PVA) loaded with graphene oxide (GO).
44
A photochemical and reagentless method has been successfully performed to convert
45
the GO phase into reduced graphene oxide (RGO). The nanocomposites have displayed
46
improved thermal and electrical properties when the amount of the GO phase is
47
increased and properly converted to RGO. The molecular-level interactions between
48
components are mainly hydrogen-bonding type according to attenuated total
49
reflectance-Fourier transform infrared (ATR-FTIR) and Raman spectroscopies, as well
50
as thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has
51
confirmed the effective mixing between GO and the ST-PVA matrix. The thermal
52
diffusivity and electrical resistivity of ST-GO nanocomposites have increased one order
53
and decreased two orders of magnitude, respectively, after the photochemical treatment.
54
These findings have confirmed the effectiveness of the proposed approach to produce
55
starch-based nanocomposites with improved thermal and electrical properties.
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Key-words: bionanocomposites, starch, graphene oxide, thermal properties, electrical
58
properties, photochemical reduction.
59 60
1. Introduction
61
Bionanocomposites represent today an environmentally friendly strategy for the
62
production of novel materials while employing raw components from the plenty source
63
provided by biomass. These systems are comprised by a continuous biopolymer matrix
64
loaded with nanofillers, usually inorganic in nature. While the biopolymer matrix
65
ensures the material’s biodegradability, nanofillers provide the required thermal,
66
mechanical, and electrical properties, usually very poor in the biopolymer alone
67
(Darder, Aranda, & Ruiz-Hitzky, 2007).
2 Page 2 of 30
Among several biopolymers for bionanocomposites starch is very appealing
69
because of its low cost and availability from different natural sources such as vegetables
70
and cereals (Paraginski et al., 2014). Besides the nourishing value, the film forming
71
ability has given to starch a more remarkable role, for example in food packaging.
72
Starch-based films are biodegradable and edible and, therefore, potential substitutes for
73
many synthetic packaging materials and pose negligible harm to the environment (Tang
74
et al., 2012).
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On the other hand, starch exhibits poor thermal and electrical properties, which
76
restrict its widespread use. This apparent limitation can be, however, circumvented by
77
adding nanofillers such as graphene oxide (GO) sheets. GO can be considered a
78
precursor for the preparation of graphene-based materials (Zhu et al., 2010). Besides
79
that, GO is highly hydrophilic and dispersible in water, features that readily enable
80
mixing and compatibility of GO and starch, as well as with other polymeric matrixes
81
(Jaber-Ansari & Hersam, 2012). In fact, this possibility has been recently verified by
82
different research groups. For example, Li et al. (Li, Liu & Ma, 2011) and Ma et al. (Ma
83
et al., 2012) have carried out a pioneer research on nanocomposites made of starch and
84
GO when they have found that GO increases the Young modulus and the thermal
85
stability of pure starch. A subsequent investigation has been conducted by He and
86
colleagues (He et al., 2013) when they have observed that GO prevents starch from
87
degradation in both acidic and alkaline environments. Rodriguez-Gonzalez et al.
88
(Rodríguez-González et al., 2012) have observed that the mechanical properties (storage
89
modulus) of a chitosan-starch matrix is substantially improved (about 900 %) with only
90
0.5 wt % of GO. Ashori's group have introduced the melting processing of
91
polypropylene composites with either sugar cane bagasse or wood flours, both enriched
92
with graphene nanoplatelets (Sheshmani, Ashori & Fashapoyeh, 2013; Chaharmahali et
93
al., 2014). They have observed that there is an optimum graphene content for the
94
bionanocomposites to reaching the best mechanical properties.
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However, another main interest on introducing GO into common polymeric
96
matrices is to provide or enhance their optoelectronic properties. GO is usually obtained
97
after chemical exfoliation of graphite which is a more affordable and readily available
98
carbon source (Jaber-Ansari & Hersam, 2012). GO resembles graphene by its
99
monoatomic dimension but it also differs significantly from it because of oxygen
100
bearing functionalities such as hydroxyl, carbonyl, carboxyl, and epoxy. These
101
functionalities disrupt part of the sp2 conjugation and turn GO into an electrical 3 Page 3 of 30
insulating material. However, part of optical, electronic and many other properties of
103
pristine graphene can be restored after proper reduction of GO (Stankovich, et al., 2007;
104
Tung et al., 2009). Concerning that, we have produced starch-based nanocomposite
105
films in which GO is introduced and photochemically converted into its reduced form,
106
namely reduced graphene oxide (RGO). The reduced nanocomposite has thermal and
107
electrical properties improved after the photochemical process, which is easy to perform
108
and does not demand for hazardous chemicals such as hydrazine or sodium
109
borohydride.
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Herein, we describe the preparation and investigate the properties of
111
bionanocomposite cast films made of starch filled with GO sheets (ST-GO). UV-vis,
112
attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and Raman
113
spectroscopies are employed to determine the structure and infer about the molecular-
114
level interaction between the starch matrix and GO sheets. An additional study
115
performed with thermogravimetric analysis (TGA) provides further evidence for the
116
interaction of bionanocomposites' components. In order to restore part of the
117
outstanding properties of graphene we have submitted the bionanocomposite films to
118
UV radiation (254 nm). After a proper irradiation time, the thermal diffusivity and
119
electrical properties of ST-GO films are significantly improved, due to partial reduction
120
of GO, as confirmed by Raman and UV-vis spectroscopies.
121
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2. Experimental
123
2.1. Materials
124
Graphite powder (Union Carbide, SP-1 grade, 100 μm), acetylated starch (Avebe Guaíra
125
Amidos, Brazil), poly(vinyl alcohol)-PVA (Sigma-Aldrich, 89,000-98,000 g.mol-1),
126
H2SO4 98%, HNO3 67%, KClO3, KHCO3, HCl 37%, NH4OH 47%, were purchased
127
from Sigma-Aldrich, Merck and Carlo Erba, all analytical grade or better and used as
128
received. Ultrapure water (18.2 M! .cm) purified by a Milli-Pore Milli-Q system was
129
used throughout all procedures. Graphene oxide (GO) was obtained via a two step
130
procedure. In the first, graphite powder was oxidized in a mixture of 1:2 H2SO4/HNO3
131
(2:1 v/v) and KClO3 as originally described by Staudenmaier (Staudenmaier, 1898) with
132
some modification as described elsewhere (Silva et al., 2010). In the second step, the as-
133
obtained graphitic oxide was suspended in NH4OH aqueous solution (pH 10) under
134
sonication (150 W, 1 h). The resulting GO suspension was centrifuged at 15,000 rpm
135
for 20 min to remove remaining solids and stored for the nanocomposites’ preparation.
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136 137
2.2. Preparation of Starch-GO (ST-GO) nanocomposites. 2.5 g of acetylated starch (ST) and 0.15 g of PVA were suspended together in 40
139
mL of deionized water. PVA was used as an additional plasticizer (besides water) for
140
starch. The milky ST-PVA suspension was kept under magnetic stirring at 90 oC for 1 h
141
in order to gelatinize starch and ensure their complete mixing. In another vessel,
142
additional 0.05 g of PVA were dissolved in deionized water (70 oC) under magnetic
143
stirring until reaching its complete dissolution. A fixed volume of the GO aqueous
144
suspension was then added to this PVA solution under magnetic stirring at 70 oC until
145
obtaining a homogenous solution. This procedure was adopted to enhance compatibility
146
between GO and starch and to avoid GO flocculation. Four different PVA-GO solutions
147
with different GO concentrations were prepared. A fifth PVA solution absent of GO
148
was prepared and used for the preparation of the control film. For the preparation of ST-
149
GO nanocomposite films, the PVA-GO solution was added slowly to the ST-PVA
150
suspension under magnetic stirring at 60 oC. The stirring and temperature were kept for
151
more 1 h after addition of the PVA-GO solution. The resulting pale yellow suspension
152
was then cast onto Petri dishes (diameter: 10 cm) and left drying in oven (60 oC) for 24
153
h. The control film (absent of GO) was prepared under the same procedure. Five
154
nanocomposite samples were prepared, labeled as: ST-C (control), ST-GO-010, ST-
155
GO-025, ST-GO-050, and ST-GO-075. The numbers refer to the weight percentage of
156
GO in the nanocomposite (regarding the starch mass).
158 159
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2.3. UV treatment of ST-GO samples In order to induce the photochemical reduction of GO, ST-GO samples (1.0 x
2
160
2.5 cm ) were exposed to UV light (254 nm) (Boitton Instrumentos, Brazil) for different
161
periods of time up to 2 hours. UV-vis spectra of films were obtained after different
162
irradiation time to monitor the photochemical effect. For comparison, the control film
163
(absent of GO) underwent the same treatment and monitoring. The ATR-FTIR and
164
Raman spectra, thermal diffusivity, and current vs. potential curves of un- and treated
165
samples were measured in order to evaluate the effectiveness of the photochemical
166
treatment.
167 168
2.4. Film characterizations
5 Page 5 of 30
169
The chemical structure of nanocomposite films was assessed by UV-vis (Varian
170
Cary 5000), ATR-FTIR (Varian 640IR) and Raman (Jobin Yvon T 6400, 514 nm laser
171
line, 0.3 mW) spectroscopies. Film’s morphology (surface and cross-section areas) was evaluated by scanning
173
electron microscope (JEOL 440A). Film’s cross-section was made after submitting
174
films to liquid nitrogen and further cracking. Samples were attached to brass stubs with
175
the aid of conductive carbon tape and gold-coated by sputtering.
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Thermogravimetry (TGA) and derivative thermogravimetry (DTG) analysis
177
were conducted in a DTG-60H Shimadzu thermoanalyzer. Powdered samples (~ 5 mg)
178
were filled in platinum pans and evaluated under nitrogen atmosphere (30 mL.min-1), at
179
heating rate of 10 °C.min-1.
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The thermal diffusivity of film samples was determined by photopyroelectric
181
spectroscopy (PPS) as described by Mandelis and Zver (Mandelis & Zver, 1985), with
182
the aid of a home-made experimental setup. The PPS working principle bases on the
183
irradiation of a solid sample with a monochromatic source and measurement of that part
184
of optical energy that is converted into heat by non-radiative deexcitation processes
185
within the solid. According to the model described by Mandelis and Zver, a non-
186
uniform variation on temperature leads to a voltage drop in the pyroelectric transductor.
187
The voltage drop is related to the sample’s thermal diffusivity, according to equation 1
188
as follows:
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190
where ! s is the sample’s nonradiative conversion efficiency; !
191
diffusivity of the pyroelectric transducer and sample, respectively; kp is the transducer’s
192
thermal conductivity; bsp is the ratio between the product of sample’s and transducer’s
193
thermal conductivity and diffusivity (= ks! s/kp! p), ! is the angular frequency of the
194
electrical field, and Ls is the sample`s thickness. In addition, the parameter A (=
195
pI0/2k! 0), where p is the pyroelectric coefficient of the transducer, I0 is the incident light
196
source irradiance, k is the transducer’s dielectric constant, and ! 0 is the vacuum
197
permittivity (= 8.854 x 10-12 C V-1m-1). In the present study, samples` thicknesses were
198
evaluated with a Mitutoyo micrometer, model M110-25. The sample`s thicknesses were
199
about 12.0 ± 0.5 ! m.
p
and ! s are the thermal
6 Page 6 of 30
The electrical properties of the ST-GO samples, before and after photochemical
201
reduction, were determined by current versus potential curves obtained with the HP
202
4156A Semiconductor Parameter Analyzer. The ST-GO and ST-RGO films (1.0 x 2.5
203
cm2) were coated with four equidistant silver epoxy contacts (MG Chemicals 8331
204
Conductive Epoxy Adhesive) through which gold microprobes were connected to
205
perform electrical measurements. A fixed current was applied through the two external
206
contacts and the voltage drop across the two inner contacts was thus measured.
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207 3. Results and Discussion
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3.1. Structure and morphology of ST-GO nanocomposite films
211
Figure 1 provides UV-vis absorption spectra along with digital snapshots of ST-GO
212
nanocomposites with different amounts of GO. Similarly to the GO aqueous suspension
213
(not shown), the spectra of nanocomposites (Figure 1a) exhibit two maxima absorption
214
peaks located at 230 nm and 300 nm, which are ascribed to the ! ! ! * transition in
215
aromatic C=C bonds and the n! ! * transition in carbonyl groups, respectively (Li et al,
216
2008; Guardia et al., 2012). The nanocomposites also display the same brownish color
217
of GO suspension and they become darker as more GO is incorporated (Figure 1b). In
218
addition, the transparency as estimated by the film transmittance (400-700 nm) of
219
nanocomposites drastically reduces with incorporation of GO. For example, the ST-GO-
220
0.10 sample has a transmittance of 75%, which then reduces to 35 % in the ST-GO-
221
0.25, and then to 20% (ST-GO-0.50) and less than 20% for the ST-GO-0.75.
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Figure 1. UV-vis absorption spectra (a) and digital snapshots (b) of ST-GO
225
nanocomposites with different GO loadings (in weight percentage), as indicated.
226
The ATR-FTIR spectra of control, pure GO and ST-GO-0.50 nanocomposite are
227
presented in Figure 2. The band referred to the stretching of O-H bond is clearly seen in
228
the three samples. While in the control and in the ST-GO-0.50 nanocomposite this band
229
peaks at 3360 cm-1, in pure GO the same band is more asymmetric and exhibits a
230
maximum at shorter wavenumber (3235 cm-1). In the range below 2000 cm-1, the
231
fingerprints of starch component are readily recognized in both control and ST-GO-0.50
232
nanocomposite. In fact, the ST-GO-0.50 nanocomposite spectrum (in green) is almost
233
indistinguishable from the control (in black) and shows none of the bands seen in the
234
GO spectrum (in red). The main vibration modes in the control sample (in black) have
235
been ascribed in accordance to literature (Flores-Morales, Jiménez-Estrada & Mora-
236
Escobedo, 2012) as follows: C=O stretching of the acetylated groups (1742 cm-1); O-H
237
bending (1652 cm-1); C-O stretching (1154 cm-1); C-O-H stretching (1082 cm-1); C-O-H
238
bending (1022 cm-1); and C-O-C bending of ! -1,4-glycosidic bond (998 cm-1). In the
239
spectrum of the ST-GO-0.50 nanocomposite (in green) most of bands have been
240
systematically shifted to shorter wavenumbers. This trend suggests the interaction of
241
starch and GO via hydrogen bonding. The assignments for bands in the ST-GO-0.50
242
sample have been made as follows: C=O stretching of the acetylated groups (1742 cm-
243
1
); O-H bending (1640 cm-1); C-O stretching (1150 cm-1); C-O-H stretching (1078 cm-
244
1
); C-O-H bending (1022 cm-1); and C-O-C bending of ! -1,4-glycosidic bond (997 cm-
245
1
). The greatest shift has been observed for the O-H bending, which is about 12 cm-1.
246
The spectrum of pure GO displays the oxygen-bearing functionalities that have been
247
introduced after graphite oxidation. In accordance to literature (Hontoria-Lucas et al.,
248
1995) the assignments for such bands have been made as follows: C=O stretching (1714
249
cm-1); C=C stretching (1612 cm-1); C-O-H bending (1417 cm-1); C-O-C stretching
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8 Page 8 of 30
250
(1060cm-1); and =C-H bending (980 cm-1). The spectra of other ST-GO nanocomposites
251
have showed similar features and are not presented.
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Figure 2. ATR-FTIR spectra of control (black), pure GO (red), and ST-GO-0.50
255
nanocomposite (green) samples.
257
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253 254
258
Raman spectra of control, pure GO, and ST-GO-0.50 nanocomposite samples are shown
259
in Figure 3. As a general observation, GO bands screen any other bands from the starch
260
+ PVA matrix. The same trend has been observed for all other ST-GO compositions and
261
for that reason their corresponding spectra are not shown. This effect is ascribed to the
262
use of 514 nm excitation that matches better GO vibrations than starch ones. The
263
spectrum of the control sample features the C-O and C-C stretching as well as the C-O-
264
C deformation modes typical of the glycosidic bonds in starch. In accordance to
265
literature (Almeida et al., 2010; Rodriguez-Gonzalez et al., 2012) the main vibrations
266
(wavenumbers assigned in the spectrum) have been tentatively ascribed as follows:
267
bending of C-C-H and C-O-C bonds (858 and 925 cm-1); stretching of C-O and C-C
268
bonds (1129 and 1209 cm-1); bending of C-C-H, O-C-H and C-O-H bonds (1266 cm-1); 9 Page 9 of 30
stretching of C-O and bending of C-O-H bonds (1340 cm-1); bending of C-O-H bond
270
(1387 cm-1), and bending of C-H and C-O-H bonds (1462 cm-1). The distinct bands at
271
925 and 858 cm-1 are referred to vibrations originated from α-1,4 glycosidic linkages
272
and provide a fingerprint for the amylose component of starch. The spectrum of pure
273
GO (in red) displays two intense bands at 1340 cm-1 and 1600 cm-1. The first, the D-
274
band, corresponds to the breathing mode of k-point phonons of A1g symmetry while the
275
second one, the G-band, corresponds to the first-order scattering of the E2g phonon of
276
sp2 carbons. Both bands are regarded to the graphene structure according to literature
277
(Saito et al., 2011). The spectrum of the ST-GO-0.50 nanocomposite (in green)
278
exclusively features bands regarding GO. This screening effect has been discussed
279
above. The relative Raman D/G intensity ratio (ID/IG) is inversely proportional to
280
average size of the sp2 domains (Saito et al., 2011). In the case of our pure GO, ID/IG =
281
1.19, while for the ST-GO-0.50 nanocomposite, ID/IG = 0.84. The slight decrease of
282
ID/IG when going from pure GO to ST-GO-0.50 nanocomposite has no explanation so
283
far. As to other nanocomposites compositions, we have found no correlation between
284
ID/IG and GO content, which has remained almost constant: 0.86 (ST-GO-0.10); 0.81
285
(ST-GO-0.25); 0.84 (ST-GO-0.50); and 0.87 (ST-GO-0.75). After the photochemical
286
treatment, which will be discussed in detail at section 3.3, the ID/IG ratio in the
287
nanocomposite sample with 0.50 % wt of GO and now labeled ST-RGO-0.50,
288
underwent a slight increase to about 0.92. This increase corresponds to reduction
289
reactions of the GO phase, as already proposed by Stankovich and coworkers
290
(Stankovich et al., 2007).
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ip t cr us an Figure 3. Raman spectra of control (starch + PVA, in black), pure GO (in red), ST-GO-
293
0.50 nanocomposite (in green), and ST-RGO-0.50 nanocomposite (in blue). The
294
spectrum in blue refers to the one obtained after the ST-GO-0.50 has been submitted to
295
UV exposition for 160 min., as described in section 2.3.
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296
Figure 4 presents SEM images of the surface (top) and cross section (bottom) of
298
nanocomposites. While the surface of all samples, including that of the control, is
299
relatively flat and smooth, the cross section after fracture is more irregular on the
300
nanocomposites with GO. This is because GO is well dispersed and strongly adhered to
301
the starch-PVA matrix.
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302
Figure 4. SEM images of surface (top images; scale bar: 100 ! m) and cross section
304
(bottom images; scale bar: 10 ! m) of nanocomposites: control (a) and (d); ST-GO-0.10
305
(b) and (e); and ST-GO-0.50 (c) and (f).
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3.2. Thermal properties
309
Figure 5 shows TG curves of the control and GO-containing nanocomposites. In Figure
310
5a, the thermal behavior of the ST-GO-0.50 nanocomposite is compared to that of the
311
control and pure GO. For pure GO (curve in black), one observes the decomposition at
312
200 oC associated to a mass loss of about 60 %. This event has been associated to the
313
dehydration of oxygenated groups accompanied by the partial reestablishment of the pi-
314
conjugated system (Silva et al., 2011). The latter event maintains the partially reduced
315
GO stable so that the residue after reaching 600 oC is about 30 %, which is greater than
316
that left after decomposition of pure starch or the ST-GO-0.50 nanocomposite. As to
317
pure starch, the TG curve (in blue) presents two main thermal events, the first one at ~
318
100 oC that corresponds to the water elimination and a second event, at 300 oC, which is
319
regarded to elimination of polyhydroxyl groups and starch depolymerization
320
(Schlemmer, Sales & Resck, 2010; Schlemmer & Sales, 2010). At 600 oC, almost all
321
starch is decomposed. The nanocomposites’s TG curve (in red) displays the elimination
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12 Page 12 of 30
322
of water as well as the decomposition of starch. However, the residual mass at 600 oC is
323
slightly greater than the control, most likely due to the residual, more stable GO.
324
In Figure 5b, TG curves of ST-GO nanocomposites are presented for evaluating
325
the effect of GO on their thermal stability. As one notes, as more GO is incorporated,
326
the onset for the main decomposition event occurs at lower temperature whereas the
327
residual mass at 600
328
decomposition is attributed to the intercalation of GO sheets in between starch chains.
329
This certainly reduces the intermolecular forces in starch as well as provides more
330
oxygen reactive species that contribute for the starch decomposition. This result has
331
been shared by other investigators working on similar systems (Ma et al., 2013; He et
332
al., 2013; Li et al., 2011). This hypothesis is corroborated further by data collected in
333
Table 1 (second column) that show that the more GO is introduced the lower is the
334
enthalpy for the water elimination. This observation along with ATR-FTIR data (Figure
335
2) allows one to conclude that GO sheets replace water molecules in the nanocomposite
336
while establishing hydrogen bonds with starch and PVA chains. Additionally, Table 1
337
(third column) collects thermal diffusivities measured accross the samples. The data
338
show that samples containing GO are better thermal conductors than the control sample.
339
This improvement has been ascribed to the few graphene domains remaining in GO
340
after graphite oxidation. The outstanding thermal conduction in deffect free graphene is
341
ascribed to the in-plane scattering of phonons (in the diffusion limit regime) (Pop,
342
Varshney & Roy, 2012).
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ip t cr us Figure 5. TG curves of (a) pure starch, pure GO, and ST-GO-0.50 nanocomposite and
347
(b) ST-GO nanocomposites with different loadings of GO, as indicated.
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348 349
Table 1. Thermal properties of control and ST-GO nanocomposites. Thermal Diffusivity (x 10-4 cm2.s-1) Sample Enthalpy (J.g-1)
352
Before Reduction
After Reduction
Control
465.11
1.86
-
ST-GO-0.10
491.15
1.92
7.1
ST-GO-0.25
196.27
3.08
8.3
ST-GO-0.50
102.12
3.59
11.5
ST-GO-0.75
84.36
7.19
17.5
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3.3. Photochemical reduction of ST-GO nanocomposites
354
The network structure of GO sheets is highly attractive and potentialy promising for the
355
improvement of mechanical properties of biopolymers such as starch. However, its
356
optoelectronic properties are still very poor since the pi-conjugated system is partially
357
disrupted after the graphite oxidation. Thus, the restoring of such conjugated structure is
358
mandatory for the attainment of electrical conducting composites. Generally, reductive
359
chemicals such as hydrazine and sodium borohydride, have been successfully employed
360
for this task even though they involve severe health and environmental concerns. In this 14 Page 14 of 30
sense, milder reduction approaches have been pursued and photochemical procedures
362
have called special attention. The photochemical reduction of GO to RGO by UV light
363
has been made possible because GO sheets strongly absorb energy at this wavelength
364
and subsequent relaxation processes could trigger a local heating of the sheets that
365
favors their deoxygenation (Kumar et al., 2012; Guardia et al., 2012; Fabbri et al.,
366
2012). Such a reduction method would provide the means by which one can easily
367
produce RGO with relatively less impurity.
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Considering those aspects, we have exposed the ST-GO-0.50 nanocomposite to
369
an UV light source (354 nm) and followed by UV-vis spectroscopy the structural
370
changes caused by the irradiation (see section 2.3). According to Figure 6, the ratio of
371
UV absorbances at 250 and 300 nm, which are ascribed to n! ! * (C=O) and ! ! ! *
372
(C=C) transitions, respectively, decreases with the exposure time. Moreover, the
373
snapshopts in the inset show that the nanocomposite initially brown have become
374
deeply dark at the end of treatment (after 160 min. of exposure). A control experiment
375
performed in the same conditions with the control film has not showed any darkening
376
effect of the starch matrix.
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377 378 379
Figure 6. Monitoring of absorbance ratios (A250/A300) of the ST-GO-0.50
380
nanocomposite as a function of the exposure time to UV light (254 nm). Inset:
381
snapshots of nanocomposite sample at the beggining (0 min.) and after 160 minutes of
382
exposure. 15 Page 15 of 30
383 To confirm the effectiveness of the proposed photochemical reduction method we have
385
carried out Raman spectroscopy as well as measured the thermal diffusivity and
386
electrical properties of nanocomposites. As displayed by Figure 3, the ID/IG ratio
387
increases from 0.84 (ST-GO-0.50, in green) to 0.91 (ST-RGO-0.50, in blue) after the
388
photochemical treatment. It is known that the ID/IG ratio is inversely proportional to the
389
average size of sp2 domains (Tuinstra & Koenig, 1970). Thus, the increase that has been
390
observed herein suggests that new graphitic domains have been created and that they are
391
smaller in size to the ones present in GO before reduction, but more numerous. In
392
summary, part of the pi-conjugated system is recovered, which is consistent with the
393
GO ! RGO reduction. The photochemical treatment has increased thermal diffusivities
394
one order of magnitude, as quoted in Table 1 (fourth column), since RGO is a better
395
thermal conductor than GO.
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The reduction of GO phase within the nanocomposites was also probed by
397
electrical measurements. Figure 7 displays current versus voltage curves for the ST-GO-
398
0.50 nanocomposite before and after the photochemical treatment. For the same voltage
399
range, the current measured within the nanocomposite has increased two orders of
400
magnitude after the photochemical treatment. The different colors refer to curves
401
aquired through different contact pairs on the sample. It is possible to note that data drift
402
is very low when measurement contacts are changed because samples and contacts are
403
quite uniform. The overpotential seen for the sample before reduction (bottom curves)
404
has been credited to its highly insulating behavior. Based on these results, one can
405
conclude that the photochemical treatment is sufficient to reduce GO to RGO and to
406
improve the thermal and electrical properties of the ST-GO nanocomposite.
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410
photochemical reduction. The different colors refer to curves aquired between two
411
different pairs of contacts on the sample.
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412 413 4. Conclusions
415
Starch-graphene oxide nanocomposites (ST-GO) exhibit better thermal and electrical
416
properties when the amount of the GO phase is increased and properly converted to
417
reduced graphene oxide (RGO), respectively. Because water molecules are readily
418
replaced by GO sheets, the latter interact with the ST matrix through hydrogen bonds,
419
the enthalpy for water release from the nanocomposites become significant lower as
420
more GO is introduced. However, GO sheets also reduce the interaction between ST
421
domains and induce their degradation at lower temperatures. A reagentless
422
photochemical method for conversion of the GO phase into RGO increases the thermal
423
diffusivity and lowers the electrical resistance of nanocomposites to about one and two
424
orders of magnitude, respectively. Raman and UV-vis absorption spectroscopies
425
provide strong evidence for the partial restoring of the pi-conjugated system after the
426
photochemical treatment. In summary, the ST-GO nanocomposites and the
427
photochemical approaches described herein enable one for the production of low cost
428
and environmentally friendly nanomaterials with potential thermal and electrical
429
properties.
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431
Acknowledgements
432
The financial support of Brazilian agencies CNPq (process # 308038/2012-6), CAPES,
433
and Finatec is greatfully acknowledged. The authors also wish to thank Dr. Roberto
434
Cavallari (PSI-EPUSP) for the electrical characterizations.
436
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435 References
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Table 1. Thermal properties of control and ST-GO nanocomposites. Sample Enthalpy (J.g-1) Thermal Diffusivity (x 10-4 cm2.s-1) Before Reduction
After Reduction
465.11
1.86
-
ST-GO-0.10
491.15
1.92
7.1
ST-GO-0.25
196.27
3.08
8.3
ST-GO-0.50
102.12
3.59
ST-GO-0.75
84.36
7.19
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Control
11.5
17.5
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Figure Captions
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Figure 1. UV-vis absorption spectra (a) and digital snapshots (b) of ST-GO nanocomposites
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with different GO loadings (in weight percentage), as indicated.
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Figure 2. ATR-FTIR spectra of control (black), pure GO (red), and ST-GO-0.50 nanocomposite
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(green) samples.
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Figure 3. Raman spectra of control (starch + PVA, in black), pure GO (in red), ST-GO-
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0.50 nanocomposite (in green), and ST-RGO-0.50 nanocomposite (in blue). The
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spectrum in blue refers to the one obtained after the ST-GO-0.50 has been submitted to
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UV exposition for 160 min., as described in section 2.4.
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Figure 4. SEM images of surface (top images; scale bar: 100 ! m) and cross section (bottom
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images; scale bar: 10 ! m) of nanocomposites: control (a) and (d); ST-GO-0.10 (b) and (e); and
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ST-GO-0.50 (c) and (f).
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Figure 5. TG curves of (a) pure starch, pure GO, and ST-GO-0.50 nanocomposite and (b) ST-
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GO nanocomposites with different loadings of GO, as indicated.
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Figure 6. Monitoring of absorbance ratios (A250/A300) of the ST-GO-0.50
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nanocomposite as a function of the exposure time to UV light (254 nm). Inset:
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snapshots of nanocomposite sample at the beggining (0 min.) and after 160 minutes of
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exposure.
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Figure 7. Current versus voltage curves for ST-GO-0.50 nanocomposite before and after
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photochemical reduction. The different colors refer to curves aquired between two different
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pairs of contacts on the sample.
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