Bacterial cellulose/graphene oxide aerogels with enhanced dimensional and thermal stability
Introduction
Cellulose is the most abundant biopolymer in earth, with an annual production of about 1.5 trillion tons, mostly for paper and textile industries (Du, Zhang, Liu, & Deng, 2017). Indeed, humans have been exploring cellulose since ancient times due to its high availability (e.g. cotton, wood, hemp) and easy processability for daily routine materials (Klemm, Heublein, Fink, & Bohn, 2005). Recently, nanotechnology is having a major impact on the development of several new approaches for processing cellulose (bacterial or vegetal). Nanocellulose arises as one of the most sustainable nanomaterial due to its high availability, biodegradability and biocompatibility, with promising applications in areas of environmental remediation, medical care, cosmetics and beyond (Wang, Urbas, & Li, 2018). This nanomaterial has been intensely explored for the development of multifunctional bio-based nanomaterials with several different architectures, including highly porous aerogels and mechanically strong paper or films (Laromaine et al., 2018; Lavoine & Bergstrom, 2017).
Beside the potential applications of nanocellulose by itself, the incorporation of an additional nanophase can provide the appearance/improvement of outstanding properties due to the establishment of synergistic effects (Lavoine & Bergstrom, 2017; Vilela et al., 2018). The application range of nanocellulose-based nanocomposites has thus been extended to the fields of antibacterial materials, sensors, catalysis and energy (Alonso-Díaz et al., 2019; Wei, Rodriguez, Renneckar, & Vikesland, 2014).
Recently, graphene oxide (GO) outstands as one of the most appealing nanomaterial for applications in environmental science, energy storage, and medical science (Li, Wang, Li, Feng, & Feng, 2019). Moreover, it has been widely explored as a reinforcing agent for cellulose macrostructures for the development of multifunctional materials (Zhou et al., 2019). GO nanosheets are graphene derivatives decorated with several oxygenated functional groups (carboxylic, hydroxyl and epoxy) on their basal planes and edges, resulting in a hybrid carbon nanostructure comprising a mixture of sp2 and sp3 domains (Bianco et al., 2013; Dreyer, Park, Bielawski, & Ruoff, 2010). Concerning the compatibility between cellulose and GO, it is reported that the highly oxygenated GO interacts with cellulose hydroxyl groups by the establishment of hydrogen bonds (Song et al., 2016; Song et al., 2017) resulting in the formation of mechanical stable nanocomposites (Wei et al., 2019) with an effective 3D interconnected network (Jiang, Cui, Song, Shi, & Ding, 2018). Many studies give insights about the fabrication of cellulose/GO nanocomposite structures for several applications, e.g., to remove heavy metals, organic dyes, oils or pesticide residues from fluids and for adsorption of air pollutants (Chen, Zhou, Zhang, You, & Xu, 2016; Luo et al., 2018; Mi et al., 2018; Wei et al., 2017; Yakout, El-Sokkary, Shreadah, & Abdel Hamid, 2017; Yao et al., 2017a); in energy devices, such as supercapacitors (Wan, Jiao, & Li, 2017; Zhang, Wang et al., 2017; Zheng, Cai, Ma, & Gong, 2015), in electro-magnetic interference shielding field (Wan & Li, 2016), and for biomedical applications with particular relevance in the field of tissue engineering (Ege, Kamali, & Boccaccini, 2017; Ramani & Sastry, 2014; Shao, Liu, Liu, Wang, & Zhang, 2015).
The inherent interfacial compatibility of polar nanocellulose with GO provides a stimulating starting point to explore solution-based methodologies for the preparation of homogenous dispersions, in which the structural integrity of the biopolymer matrix is improved. Innovative ways should be developed to maximize the interactions between the matrix and filler, since this is the dominant and key factor in the enhancement of the specific surface area and mechanical performance. For example, the abundant oxygen containing groups in GO likely interact with the hydroxyl groups in cellulose through hydrogen bonds. This has been proven by Yao et al. (2017a) (2017b) by using ultrasonic treatment for composites preparation, in which the high energy supply of ultrasounds induced fast formation of hydrogen bonds. However, the ultrasound method is not the most adequate to prepare three-dimensional structures, since cavitation can easily break the delicate gel framework. The most popular fabrication methods of cellulose/GO in the form of aerogels/foams include freeze-casting, supercritical fluids (ethanol or carbon dioxide) and Pickering (Borrás et al., 2018; Lavoine & Bergstrom, 2017; Martoïa et al., 2016). Particularly, the directional freeze-casting method could create hierarchical materials with aligned porous structures and high mechanical robustness (Mi et al., 2018; Wicklein et al., 2014). Importantly, this synthetic methodology offers a high control over the pore size and density of nanocellulose-based nanocomposites.
The establishment of chemical crosslinking between nanocellulose and GO is another important strategy explored for the improvement of the dimensional stability of nanocomposite foams. Wicklein et al. (Wicklein et al., 2014) developed ultralight and anisotropic porous foams of cellulose nanofibers with GO by studying the combination of a crosslinking agent (boric acid) addition and directional freeze-casting. The foams revealed excellent combustion resistance, thermal conductivity and high radial mechanical resistance (even after exposure to 85 % rh). Recently, Ge et al. (Ge et al., 2018) reported the development of an ultra-strong aerogel, based on carboxymethyl cellulose and GO crosslinked also resourcing/using boric acid. This composite material produced attained fairly good compressive strength and Young modulus of 349 and 1029 kPa, respectively compatible with different applications. Other examples reporting the covalent linkage between nanocellulose and GO for the formation of macrostructures with improved properties can be easily found in the literature (Liu, Zhou, Zhu et al., 2015). However, this strategy has some clear limitations, including restrained stability, complex surface chemistry, restrictions for geometrical confinement, the use of organic solvents and the need of purification of the final materials.
A simple way to improve the dimensional and mechanical stability of lightweight and hydrophilic GO based materials, especially in wet environments, consists on GO reduction, thus removing oxygen containing functional groups and, consequently, decreasing its original hydrophilicity (Wen, Wu, Zhang, Li, & Shi, 2017; Xiong et al., 2018). Besides, the tailoring of GO surface chemistry by reduction could help to increase its reactivity for amphiphilicity-driven assembly strategy. Recently, Xiong et al. (Xiong et al., 2018) reported an efficient approach for constructing hybrid materials based on a net of 1D cellulose nanofibers wrapped in GO nanosheets. Their findings showed that the interface-driven assembly between the two components is mainly governed by the level of reduction of GO nanosheets, where highly reduced GO are tightly surrounded by a dense conformal nanocellulose network (Xiong et al., 2018). Therefore, the morphology and dimensional stability of 3D bacterial cellulose/ graphene oxide (BC/GO) aerogels can be tailor-made by exploring different reduction strategies for improved interfacial hydrophobic-hydrophilic interactions between the individual nanoelements.
This work can represent an important step forward to the development of novel multifunctional BC-based aerogels reinforced with GO for improved performance in different environments, envisaging lightweight structures for packaging, filters for atmosphere and water treatment, or energy applications. Taking these considerations into account, in this study, it is presented a simple, fast, and environmentally friendly preparation method of BC/GO aerogels with different ratios, e.g., 90/10, 75/25 and 50/50. Herein, special focus was directed towards the effect of the addition of a residual amount of dimethyl sulfoxide (DMSO) to the BC/GO aqueous suspension on the structure of the resulting aerogels. Moreover, the impact of two different types of reduction treatments applied to the aerogels, either with an aqueous solution of hydrazine (N2H4) or with ammonia (NH3) gas, in their chemical and structural features was studied.
Section snippets
Synthesis of BC/GO aerogels
Bacterial cellulose (BC) aqueous suspension nano-fibrils with 1 % of solids was used as the cellulose source in this work (kindly supplied by BCTECHNOLOGIES, Lda.). BC and GO (Graphenea® in aqueous suspension of 4 mg/mL) were thoroughly mixed for 1 h at room temperature (RT). Four BC/GO sample compositions were prepared (100, 90, 75 and 50 wt. % of BC with respect to the GO dry mass), which are designated hereafter by BC, BC/GO10, BC/GO25 and BC/GO50, respectively. The composites were prepared
Effect of solvent composition on BC/GO aerogels
The BC/GO aerogels were prepared by vigorous stirring of mixtures of the aqueous suspensions of both individual components. In this process, BC nanofibers were dispersed and self-assembled on the surface of GO (Liu, Zhou, Tang, & Tang, 2015). The synthesis of a BC/GO aerogel is schematically illustrated in Fig. 1. The SEM observation of the as-prepared BC/GO aerogels without DMSO addition confirms the porous structure and the homogenous mixture between the cellulose fibrils and the GO sheets,
Conclusions
In summary, this work reports a new methodology for the development of multifunctional BC/GO aerogel materials with improved properties. The internal structural morphology of BC/GO aerogels can be easily tuned by the addition of DMSO to aqueous suspensions during the gelification process. The addition of DMSO promotes the formation of small and elongated pores with axial arrangement. Thermal treatment of BC/GO samples with NH3 revealed to be a less invasive and more efficient treatment for
Acknowledgements
The present study was supported by the Portuguese Foundation for Science and Technology (FCT) under the Grant SFRH/BD/111515/2015 (S. Pinto), the Contract CEECIND/01913/2017 (G. Gonçalves) and UID/EMS/00481/2019-FCT for the TEMA Research Unit. We acknowledge CENTRO-01-0145-FEDER-022083 - Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund. The Spanish National Plan of Research with projects
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