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graphene oxide composite films
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A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films Q1
L. Valentini a,n, M. Cardinali a, E. Fortunati a, L. Torre a, J.M. Kenny a,b a b
Dipartimento di Ingegneria Civile e Ambientale, Università di Perugia, UdR INSTM, Pentima 4, 05100 Terni, Italy Institute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 16 January 2013 Accepted 6 April 2013
In this study nanocrystalline cellulose (CNC)/graphene oxide (GO) composite films were prepared by drop casting water dispersion of GO in the presence of CNC and their nanostructures, surface and electrical properties were investigated. It was found that pristine hydrophilic GO presents a good dispersion when mixed with CNC along with a decrease of the composite electrical resistivity. The surface properties of the composite film indicated a poorer wettability with respect to that measured for separated materials. By applying an electric current through the CNC/GO composite a transition from an electrically insulating material to a conductive one was observed along with an improved wettability. The obtained results open an easy route for paper electronic based on the integration of nanocrystalline cellulose onto graphene devices. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Coatings Nanocrystalline cellulose Graphene oxide Electrical properties
1. Introduction Cellulose, one of the world's most abundant, natural and renewable biopolymer resources, is widely present in various forms of biomasses, such as trees, plants, tunicate and bacteria and it finds applications in many spheres of modern industry. Cellulose has been shown to be a long-chain polymer with repeating units of D-glucose, a simple sugar [1]. In plant cell walls, approximately 36 individual cellulose molecule chains connect with each other through hydrogen bonding to form larger units known as elementary fibrils, which are packed into larger microfibrils with 5–50 nm in diameter and several micrometers in length. These microfibrils have disordered (amorphous) regions and highly ordered (crystalline) regions. When the start materials, as lignocellulosic biomass, fiber plant, trees, tunicates, etc., are subjected to pure mechanical shearing, and a combination of chemical, mechanical and/or enzymatic treatment, the amorphous regions of cellulose microfibrils are selectively hydrolyzed under certain conditions [2,3]. Consequently, these microfibrils break down into shorter crystalline parts with high crystalline degree, which are generally referred to as nanocrystalline cellulose or cellulose nanocrystals (CNC) [4]. Nanocrystalline cellulose, is typically a rigid rod-shaped monocrystalline domain with 1–100 nm in diameter and from 10 to 100 nm in length depending on the resources of cellulose. CNC have a crystalline structure, a very high aspect ratio (length/diameter around 70), and
Q2
n
Corresponding author. Tel.: +39 0744492924. E-mail address: [email protected] (L. Valentini).
a large surface area (ca. 150 m2/g) [5]. During the past decade, CNC have attracted considerable attention attributed to their unique features. First, CNC have nanoscale dimensions and excellent mechanical properties. The theoretical value of Young's modulus along the chain axis for perfect native CNCs is estimated to be 167.5 GPa [6]. Due to abundance of hydroxyl groups on surface of CNC, reactive CNC can be modified with various chemical groups to accomplish expected surface modification, such as esterification, etherification, oxidation, silylation, or polymer grafting, which could successfully functionalize the CNC and facilitate the incorporation and dispersion of CNC into different polymer matrices [4]. In addition, high aspect ratio, low density, low energy consumption, inherent renewability, biodegradability and biocompatibility are also the advantages of environmentally-friendly CNC [7,8]. Graphene oxide (GO) is a water soluble insulating material that can be obtained by oxidizing graphite [9]. The main disadvantage of this material is that the charge carrier transport (i.e. electrons) observed in nearly ideal graphene is absent in GO, but at the same time the easy processing and the versatile properties of GO make the reduction methods for such material attractive for fundamental research as well as for applications. Paper, an organic-based material, is universally available: the high demand and the mass production of paper has made it one of the cheapest materials. The possibility to integrate electronic and optoelectronic functions within the production methods of the paper industry is of current interest to enhance and to add new functionalities to paper. Thus, composites prepared from aqueous GO solution with CNC represents a method for manufacturing cellulose nanocrystal/ graphene oxide composites. Nevertheless, an unavoidable drawback of this approach is that the resistance of such composite films is
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.04.034
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101
L. Valentini et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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exponentially higher than that of graphene based composites. Here we report on the preparation of nanocrystalline cellulose/graphene oxide composite films and their exposure to an external electric field. It was found that the application of an external electric field leads to the reduction of the electrical resistivity of the composite film. 2. Experimental part Graphene oxide sheets produced by the Hummers method were purchased from Cheaptubes. Water solution (1 mg/1 mL) of GO was prepared by an ultrasonication bath for 8 h to yield a stable brown solution. After sonication, the solution was transferred to a vial and it was centrifuged for 30 min at 600 rpm. The supernatant of the dispersion (ca. 10 mL) was carefully extracted and separated from the residual visible at the bottom of the vial. This procedure was adopted because during reaction and processing, the graphene sheets are not only derivatized with oxygencontaining groups but also disaggregated into smaller pieces. Cellulose nanocrystal (CNC) suspension was prepared from microcrystalline cellulose (MCC, supplied by Sigma Aldrich) by sulfuric acid hydrolysis following the recipe used by Cranston and Gray [10]. Hydrolysis was carried out with 64% (w/w) sulfuric acid at 45 1C for 30 min with vigorous stirring. After removing the acid, dialysis, and ultrasonic treatment were performed. The resultant cellulose nanocrystal aqueous suspension was approximately 0.5% (w/w) by weight and the yield was ca. 20%.
The obtained CNC showed the typical acicular structure with the dimensions ranging from 100 to 200 nm in length and 5–10 nm in width as previously [11,12]. CNC suspension was added to the GO solution (1 mL/1 mL). The mixing process was performed in an ultrasonication bath for 20 min at room temperature. The samples for the characterizations were prepared by drop casting onto glass substrates two different quantities of the final solution (200 μL and 10 μL). The substrates were previously cleaned in an ultrasonic bath of acetone and dried under nitrogen. The films were annealed in vacuum for 1 h at 100 1C in order to remove water moisture. The morphology of the prepared samples was observed by field emission scanning electron microscopy (FESEM). Fourier
Table 1 Contact angle values for the prepared samples. (*) Indicates the contact angle value for the CNC/GO composite film after the application of the external electric field. Sample
Contact angle [deg]
Glass substrate GO CNC CNC/GO CNC/GOn
32 45 34 59 49
Fig. 1. FESEM images of (a) CNC and (b) CNC/GO films. In the panel (b) the arrows indicate CNC aggregation. (c) Cross section FESEM images of GO (left) and CNC/GO (right) composite films. All scale bars indicate 200 nm.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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transform infrared spectroscopy (FTIR), in the 500–4000 cm−1 range, was carried out in transmission mode by drop casting the prepared solutions onto silicon substrates. The wettability of GO, CNC and CNC/GO films, respectively, was investigated by contact angle measurements. Contact angles were measured with a FTA 1000 Series instrument equipped with a CCD camera. Deionized water droplets were dropped carefully onto the surfaces and the contact angle was monitored statically as a function of time.
3
For electrical characterization, Al electrodes (1 mm 5 mm) were deposited on the top of a glass substrate by vacuum evaporation with an optimized thickness of 70 nm and with a spacing of 1 mm and 4 mm, respectively. The GO solution as well as the GO/CNC suspensions were drop-cast, in the two different quantities reported above, between the Al electrodes and left to evaporate in air at room temperature. The current–voltage characteristic was performed using a 2-probe method by computercontrolled Keithley 4200 Source Measure Unit. The electrical conductivity of the samples was monitored, at room temperature, by applying a sweeping DC electric voltage from 0 V to 30 V between the aluminum electrodes.
3. Results and discussion
Fig. 2. FTIR spectra of pure CNC (top curve) and CNC/GO composite films (bottom curve).
Fig. 1 shows FESEM images of cellulose nanocrystals and CNC/ GO films, drop cast from 10 μL solution onto glass substrate. Visibly separated pure cellulose nanocrystals appear on glass (Fig. 1a) whereas CNC aggregation (Fig. 1b) is prevalent on CNC/GO sample. In this regard it is known that the cellulose nanocrystals are weakly anionic in aqueous suspension because of the organic sulfate groups on the surface of the crystals, caused by the sulfuric acid hydrolysis in their preparation [13]. The electrostatic repulsion between anionic GO and weakly anionic cellulose nanocrystals is likely to cause the surface aggregation of nanocrystals which are forced to the repulsive interaction with GO.
Fig. 3. (a) Current–voltage curves of GO, CNC and CNC/GO films. (b) Current–voltage curve of GO film. (c) Current–voltage curve of CNC/GO composite film. (d) Photographs and UV–vis spectra of CNC/GO composite film before (left) and after (right) the application of the external electric filed.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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L. Valentini et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The morphology of the samples was found to influence the wettability behavior of GO, CNC and CNC/GO films, respectively, as reported in Table 1. The contact angle measurements of GO as well as of CNC films indicate, as expected, an hydrophilic behavior while an increase of the contact angle value was observed for the CNC/GO composite. The abundant oxygen containing groups decorated in GO sheet could interact with the hydroxyl groups and oxygen atoms in CNC chain by hydrogen bonds, which is beneficial to homogeneously disperse the GO and the CNC chains as reported in Fig. 1c. In this regard Fig. 2 shows the FTIR spectra of CNC and CNC/GO composite films. In the IR spectrum of pure CNC, the peaks at 2895 and 1372 cm−1 are assigned to C–H stretching and bending vibrations [14]. The peaks at 1423 and 1060 cm−1 are assigned to the vibrations of O–H and C–O, respectively. The broad band at 3422 cm−1 is associated with the hydroxyl groups. The peak at 1649 cm−1 is associated with intramolecular hydrogen bonds. The characteristic absorption peaks in the spectra of CNC/GO composite film are similar to the curve of pure CNC, suggesting the homogeneous dispersion of stacked GO layers in CNC as reported in Fig. 1c. The formation of a percolative network of separated and negatively charged GO layers in presence of the nanocrystalline cellulose (Fig. 1c) is confirmed by the current–voltage characterization where the poor electrical conductivity of densely packed GO was slightly enhanced by the presence of CNC in the composite (Fig. 3a). The structure of GO is characterized by sp2 carbon clusters separated by sp3 carbon sites that act as barriers for charge carriers [9]. It was demonstrated that deoxygenation could make GO recover its electrical conductivity to some extent. In this regard, recently, it has been reported that the heat produced by the Joule effect will be generated when applying a large current through an insulating graphene oxide layer [15]. This makes possible the thermal reduction of GO due to Joule effect. Accordingly to these results, we investigated the effect on the composite conductivity of an electrical current passing thorough the film. As shown in Fig. 3b and c, it is found that the current increases rapidly when a voltage sweep was applied to the GO and composite films. The same effect was observed for the pure GO film while the conductivity of pure CNC did not change (data not shown). Mixing insulating GO layers in CNC and applying a voltage, the system becomes conducting at about 25 V. This means
that reduced GO conducting pattern is formed in the composite at about this voltage leading to a conductive state. The optical energy gap (electrical conductivity) of the GO sp2 cluster is inversely (directly) proportional to its size. The optical images and the UV–vis spectra, respectively, of the CNC/GO composite film before and after the exposure to the external electric voltage are reported in Fig. 3d; after the application of the electric voltage the sample appears less transparent. Associating the increase of the CNC/GO electrical conductivity after the exposure to the electric field (Fig. 3c) to the restoration of the GO sp2 clusters, it is possible to conclude that the electrical current passing through the composite film induces a local heating that results in a decrease of the optical gap of the sample. 4. Conclusions In summary homogeneous and conductive CNC/GO composite films were prepared. We have investigated the effect of graphene oxide layers on the arrangement of cellulose nanocrystals. It has been reported how the application of an electric current through the composite leads to the formation of a conductive CNC/GO film. Such results could lead to many exciting functional properties of graphene-based nanocellulose composites. References [1] Kalia S, Dufresne A, Cherian BM, Kaith BS, Averous L, Njuguna J, et al. Int J Polym Sci 2011:35 Article ID 837875. [2] Beck-Candanedo S, Roman M, Gray DG. Biomacromol 2005;6:1048–54. [3] Bondeson D, Mathew A, Oksman K. Cellulose 2006;13:171–80. [4] Habibi Y, Heim T, Douillard RJ. Polym Sci Pol Phys 2008;46:1430–6. [5] Cavaille JY, Ruiz MM, Dufresne A, Gerard JF, Graillat C. Compos Interface 2000;7(2):117–31. [6] Tashiro K, Kobayashi M. Polymer 1991;32:1516–30. [7] Siro I, Plackett D. Cellulose 2010;17:459–94. [8] Fortunati E, Peltzer M, Armentano I, Torre L, Jimenez A, Kenny JM. Carbohydr Polym 2012;90:948–56. [9] Eda G, Chhowalla M. Adv Mater 2010;22:2392–415. [10] Cranston ED, Gray DG. Biomacromol 2006;7:2522–30. [11] Fortunati E, Armentano I, Zhou Q, Iannoni A, Saino E, Visai L, et al. Carbohydr Polym 2012;87:1596–605. [12] Fortunati E, Armentano I, Zhou Q, Puglia D, Terenzi A, Berglund LA, et al. Polym Degrad Stab 2012;97:2027–36. [13] de Souza Lima MM, Borsali R. Macromol Rapid Commun 2004;25:771. [14] Titelman GI, Gelman V, Bron S, Khalfin RL, Cohen Y, Bianco-Peled H. Carbon 2005;43:641–9. [15] Jung I, Dikin DA, Piner RD, Ruoff RS. Nano Lett 2008;8:4283–7.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
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Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films Q1
L. Valentini a,n, M. Cardinali a, E. Fortunati a, L. Torre a, J.M. Kenny a,b a b
Dipartimento di Ingegneria Civile e Ambientale, Università di Perugia, UdR INSTM, Pentima 4, 05100 Terni, Italy Institute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 16 January 2013 Accepted 6 April 2013
In this study nanocrystalline cellulose (CNC)/graphene oxide (GO) composite films were prepared by drop casting water dispersion of GO in the presence of CNC and their nanostructures, surface and electrical properties were investigated. It was found that pristine hydrophilic GO presents a good dispersion when mixed with CNC along with a decrease of the composite electrical resistivity. The surface properties of the composite film indicated a poorer wettability with respect to that measured for separated materials. By applying an electric current through the CNC/GO composite a transition from an electrically insulating material to a conductive one was observed along with an improved wettability. The obtained results open an easy route for paper electronic based on the integration of nanocrystalline cellulose onto graphene devices. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Coatings Nanocrystalline cellulose Graphene oxide Electrical properties
1. Introduction Cellulose, one of the world's most abundant, natural and renewable biopolymer resources, is widely present in various forms of biomasses, such as trees, plants, tunicate and bacteria and it finds applications in many spheres of modern industry. Cellulose has been shown to be a long-chain polymer with repeating units of D-glucose, a simple sugar [1]. In plant cell walls, approximately 36 individual cellulose molecule chains connect with each other through hydrogen bonding to form larger units known as elementary fibrils, which are packed into larger microfibrils with 5–50 nm in diameter and several micrometers in length. These microfibrils have disordered (amorphous) regions and highly ordered (crystalline) regions. When the start materials, as lignocellulosic biomass, fiber plant, trees, tunicates, etc., are subjected to pure mechanical shearing, and a combination of chemical, mechanical and/or enzymatic treatment, the amorphous regions of cellulose microfibrils are selectively hydrolyzed under certain conditions [2,3]. Consequently, these microfibrils break down into shorter crystalline parts with high crystalline degree, which are generally referred to as nanocrystalline cellulose or cellulose nanocrystals (CNC) [4]. Nanocrystalline cellulose, is typically a rigid rod-shaped monocrystalline domain with 1–100 nm in diameter and from 10 to 100 nm in length depending on the resources of cellulose. CNC have a crystalline structure, a very high aspect ratio (length/diameter around 70), and
Q2
n
Corresponding author. Tel.: +39 0744492924. E-mail address: [email protected] (L. Valentini).
a large surface area (ca. 150 m2/g) [5]. During the past decade, CNC have attracted considerable attention attributed to their unique features. First, CNC have nanoscale dimensions and excellent mechanical properties. The theoretical value of Young's modulus along the chain axis for perfect native CNCs is estimated to be 167.5 GPa [6]. Due to abundance of hydroxyl groups on surface of CNC, reactive CNC can be modified with various chemical groups to accomplish expected surface modification, such as esterification, etherification, oxidation, silylation, or polymer grafting, which could successfully functionalize the CNC and facilitate the incorporation and dispersion of CNC into different polymer matrices [4]. In addition, high aspect ratio, low density, low energy consumption, inherent renewability, biodegradability and biocompatibility are also the advantages of environmentally-friendly CNC [7,8]. Graphene oxide (GO) is a water soluble insulating material that can be obtained by oxidizing graphite [9]. The main disadvantage of this material is that the charge carrier transport (i.e. electrons) observed in nearly ideal graphene is absent in GO, but at the same time the easy processing and the versatile properties of GO make the reduction methods for such material attractive for fundamental research as well as for applications. Paper, an organic-based material, is universally available: the high demand and the mass production of paper has made it one of the cheapest materials. The possibility to integrate electronic and optoelectronic functions within the production methods of the paper industry is of current interest to enhance and to add new functionalities to paper. Thus, composites prepared from aqueous GO solution with CNC represents a method for manufacturing cellulose nanocrystal/ graphene oxide composites. Nevertheless, an unavoidable drawback of this approach is that the resistance of such composite films is
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.04.034
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101
L. Valentini et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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exponentially higher than that of graphene based composites. Here we report on the preparation of nanocrystalline cellulose/graphene oxide composite films and their exposure to an external electric field. It was found that the application of an external electric field leads to the reduction of the electrical resistivity of the composite film. 2. Experimental part Graphene oxide sheets produced by the Hummers method were purchased from Cheaptubes. Water solution (1 mg/1 mL) of GO was prepared by an ultrasonication bath for 8 h to yield a stable brown solution. After sonication, the solution was transferred to a vial and it was centrifuged for 30 min at 600 rpm. The supernatant of the dispersion (ca. 10 mL) was carefully extracted and separated from the residual visible at the bottom of the vial. This procedure was adopted because during reaction and processing, the graphene sheets are not only derivatized with oxygencontaining groups but also disaggregated into smaller pieces. Cellulose nanocrystal (CNC) suspension was prepared from microcrystalline cellulose (MCC, supplied by Sigma Aldrich) by sulfuric acid hydrolysis following the recipe used by Cranston and Gray [10]. Hydrolysis was carried out with 64% (w/w) sulfuric acid at 45 1C for 30 min with vigorous stirring. After removing the acid, dialysis, and ultrasonic treatment were performed. The resultant cellulose nanocrystal aqueous suspension was approximately 0.5% (w/w) by weight and the yield was ca. 20%.
The obtained CNC showed the typical acicular structure with the dimensions ranging from 100 to 200 nm in length and 5–10 nm in width as previously [11,12]. CNC suspension was added to the GO solution (1 mL/1 mL). The mixing process was performed in an ultrasonication bath for 20 min at room temperature. The samples for the characterizations were prepared by drop casting onto glass substrates two different quantities of the final solution (200 μL and 10 μL). The substrates were previously cleaned in an ultrasonic bath of acetone and dried under nitrogen. The films were annealed in vacuum for 1 h at 100 1C in order to remove water moisture. The morphology of the prepared samples was observed by field emission scanning electron microscopy (FESEM). Fourier
Table 1 Contact angle values for the prepared samples. (*) Indicates the contact angle value for the CNC/GO composite film after the application of the external electric field. Sample
Contact angle [deg]
Glass substrate GO CNC CNC/GO CNC/GOn
32 45 34 59 49
Fig. 1. FESEM images of (a) CNC and (b) CNC/GO films. In the panel (b) the arrows indicate CNC aggregation. (c) Cross section FESEM images of GO (left) and CNC/GO (right) composite films. All scale bars indicate 200 nm.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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transform infrared spectroscopy (FTIR), in the 500–4000 cm−1 range, was carried out in transmission mode by drop casting the prepared solutions onto silicon substrates. The wettability of GO, CNC and CNC/GO films, respectively, was investigated by contact angle measurements. Contact angles were measured with a FTA 1000 Series instrument equipped with a CCD camera. Deionized water droplets were dropped carefully onto the surfaces and the contact angle was monitored statically as a function of time.
3
For electrical characterization, Al electrodes (1 mm 5 mm) were deposited on the top of a glass substrate by vacuum evaporation with an optimized thickness of 70 nm and with a spacing of 1 mm and 4 mm, respectively. The GO solution as well as the GO/CNC suspensions were drop-cast, in the two different quantities reported above, between the Al electrodes and left to evaporate in air at room temperature. The current–voltage characteristic was performed using a 2-probe method by computercontrolled Keithley 4200 Source Measure Unit. The electrical conductivity of the samples was monitored, at room temperature, by applying a sweeping DC electric voltage from 0 V to 30 V between the aluminum electrodes.
3. Results and discussion
Fig. 2. FTIR spectra of pure CNC (top curve) and CNC/GO composite films (bottom curve).
Fig. 1 shows FESEM images of cellulose nanocrystals and CNC/ GO films, drop cast from 10 μL solution onto glass substrate. Visibly separated pure cellulose nanocrystals appear on glass (Fig. 1a) whereas CNC aggregation (Fig. 1b) is prevalent on CNC/GO sample. In this regard it is known that the cellulose nanocrystals are weakly anionic in aqueous suspension because of the organic sulfate groups on the surface of the crystals, caused by the sulfuric acid hydrolysis in their preparation [13]. The electrostatic repulsion between anionic GO and weakly anionic cellulose nanocrystals is likely to cause the surface aggregation of nanocrystals which are forced to the repulsive interaction with GO.
Fig. 3. (a) Current–voltage curves of GO, CNC and CNC/GO films. (b) Current–voltage curve of GO film. (c) Current–voltage curve of CNC/GO composite film. (d) Photographs and UV–vis spectra of CNC/GO composite film before (left) and after (right) the application of the external electric filed.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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L. Valentini et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The morphology of the samples was found to influence the wettability behavior of GO, CNC and CNC/GO films, respectively, as reported in Table 1. The contact angle measurements of GO as well as of CNC films indicate, as expected, an hydrophilic behavior while an increase of the contact angle value was observed for the CNC/GO composite. The abundant oxygen containing groups decorated in GO sheet could interact with the hydroxyl groups and oxygen atoms in CNC chain by hydrogen bonds, which is beneficial to homogeneously disperse the GO and the CNC chains as reported in Fig. 1c. In this regard Fig. 2 shows the FTIR spectra of CNC and CNC/GO composite films. In the IR spectrum of pure CNC, the peaks at 2895 and 1372 cm−1 are assigned to C–H stretching and bending vibrations [14]. The peaks at 1423 and 1060 cm−1 are assigned to the vibrations of O–H and C–O, respectively. The broad band at 3422 cm−1 is associated with the hydroxyl groups. The peak at 1649 cm−1 is associated with intramolecular hydrogen bonds. The characteristic absorption peaks in the spectra of CNC/GO composite film are similar to the curve of pure CNC, suggesting the homogeneous dispersion of stacked GO layers in CNC as reported in Fig. 1c. The formation of a percolative network of separated and negatively charged GO layers in presence of the nanocrystalline cellulose (Fig. 1c) is confirmed by the current–voltage characterization where the poor electrical conductivity of densely packed GO was slightly enhanced by the presence of CNC in the composite (Fig. 3a). The structure of GO is characterized by sp2 carbon clusters separated by sp3 carbon sites that act as barriers for charge carriers [9]. It was demonstrated that deoxygenation could make GO recover its electrical conductivity to some extent. In this regard, recently, it has been reported that the heat produced by the Joule effect will be generated when applying a large current through an insulating graphene oxide layer [15]. This makes possible the thermal reduction of GO due to Joule effect. Accordingly to these results, we investigated the effect on the composite conductivity of an electrical current passing thorough the film. As shown in Fig. 3b and c, it is found that the current increases rapidly when a voltage sweep was applied to the GO and composite films. The same effect was observed for the pure GO film while the conductivity of pure CNC did not change (data not shown). Mixing insulating GO layers in CNC and applying a voltage, the system becomes conducting at about 25 V. This means
that reduced GO conducting pattern is formed in the composite at about this voltage leading to a conductive state. The optical energy gap (electrical conductivity) of the GO sp2 cluster is inversely (directly) proportional to its size. The optical images and the UV–vis spectra, respectively, of the CNC/GO composite film before and after the exposure to the external electric voltage are reported in Fig. 3d; after the application of the electric voltage the sample appears less transparent. Associating the increase of the CNC/GO electrical conductivity after the exposure to the electric field (Fig. 3c) to the restoration of the GO sp2 clusters, it is possible to conclude that the electrical current passing through the composite film induces a local heating that results in a decrease of the optical gap of the sample. 4. Conclusions In summary homogeneous and conductive CNC/GO composite films were prepared. We have investigated the effect of graphene oxide layers on the arrangement of cellulose nanocrystals. It has been reported how the application of an electric current through the composite leads to the formation of a conductive CNC/GO film. Such results could lead to many exciting functional properties of graphene-based nanocellulose composites. References [1] Kalia S, Dufresne A, Cherian BM, Kaith BS, Averous L, Njuguna J, et al. Int J Polym Sci 2011:35 Article ID 837875. [2] Beck-Candanedo S, Roman M, Gray DG. Biomacromol 2005;6:1048–54. [3] Bondeson D, Mathew A, Oksman K. Cellulose 2006;13:171–80. [4] Habibi Y, Heim T, Douillard RJ. Polym Sci Pol Phys 2008;46:1430–6. [5] Cavaille JY, Ruiz MM, Dufresne A, Gerard JF, Graillat C. Compos Interface 2000;7(2):117–31. [6] Tashiro K, Kobayashi M. Polymer 1991;32:1516–30. [7] Siro I, Plackett D. Cellulose 2010;17:459–94. [8] Fortunati E, Peltzer M, Armentano I, Torre L, Jimenez A, Kenny JM. Carbohydr Polym 2012;90:948–56. [9] Eda G, Chhowalla M. Adv Mater 2010;22:2392–415. [10] Cranston ED, Gray DG. Biomacromol 2006;7:2522–30. [11] Fortunati E, Armentano I, Zhou Q, Iannoni A, Saino E, Visai L, et al. Carbohydr Polym 2012;87:1596–605. [12] Fortunati E, Armentano I, Zhou Q, Puglia D, Terenzi A, Berglund LA, et al. Polym Degrad Stab 2012;97:2027–36. [13] de Souza Lima MM, Borsali R. Macromol Rapid Commun 2004;25:771. [14] Titelman GI, Gelman V, Bron S, Khalfin RL, Cohen Y, Bianco-Peled H. Carbon 2005;43:641–9. [15] Jung I, Dikin DA, Piner RD, Ruoff RS. Nano Lett 2008;8:4283–7.
Please cite this article as: Valentini L, et al. A novel method to prepare conductive nanocrystalline cellulose/graphene oxide composite films. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.04.034i
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