Chitosan-Templated lightweight spinel CoAl2O4 Nanofibril aerogels - Chau The Lieu Trang

Vietnam Journal of Science and Technology 56 (1A) (2018) 135-145 135 CHITOSAN-TEMPLATED LIGHTWEIGHT SPINEL CoAl2O4 NANOFIBRIL AEROGELS Chau The Lieu Trang 1, * , Nguyen Duc Cuong 1, 3 , Dang Thi Thanh Nhan 1, 2 , Do Dang Trung 4 1 Department of Chemistry, University of Sciences, Hue University, 77 Nguyen Hue, Hue City 2 Department of Chemistry, University of Education, Hue University, 34 Le Loi, Hue City 3 School of Hospitality and Tourism, Hue University, 22 Lam Hoang, Hue City 4 Department of Basic Sciences, University of Fire Fighting and Prevention, 243 Khuat Duy Tien, Thanh Xuan, Ha Noi * Email: chauthelieutrang@gmail.com Received: 15 August 2017; Accepted for publication: 21 March 2018 ABSTRACT We present lightweight macro-mesoporous spinel CoAl2O4 nanostructured aerogels derived from water-soluble aluminum-chitosan complexes. Chitosan nanofibrils interact with aluminum ions to swell into hydrogels. The aluminum-induced swelling is extended to dissolve the hydrogels in water to form a homogeneous aluminum-chitosan aqueous solution. The addition of cobalt ions in the aluminum-chitosan liquids which are solidified by lyophilization to generate cotton-like aerogel composites. Uniform incorporation of cobalt-aluminum hydroxide ions onto chitosan leads nanofibrils to serve as a hierarchical template to support mixed metal hydroxides in the aerogel composites. We investigated the thermal removal of chitosan template in the composites to obtain spinel CoAl2O4 aerogels that truly replicate spider web-like fibrillar networks of chitosan template. Enlarged porosity, high crystallinity, and lightweight make the CoAl2O4 aerogels useful as a colorful nano-pigment for magnetic ceramics. Keywords: chitosan nanofibrils, biotemplating, spinel CoAl2O4 aerogels, lightweight materials. 1. INTRODUCTION Ceramic aerogels with lightweight and thermal characteristics are an exciting class of highly porous materials for applied technologies [1]. These materials can be used as functional supports and insulators [2]. The goal to obtain lightweight, macro-mesoporous materials is being the integration of open network and low-density composition into a solid. Among different methods, templated synthesis is an outstanding method used to construct these materials [3-5]. The structural orientation of precursors by hierarchical templates leads to the formation of interconnected porous networks, often affording lightweight aerogel materials. Cobalt aluminates (CoAl2O4) is an important ceramic of the normal spinel aluminate family Chau The Lieu Trang, et al. 136 with aluminum in octahedral sites and cobalt in tetrahedral ones [6]. Porous CoAl2O4 nanomaterial is used as a blue pigment component for preparing colorful magnetic ceramics [7] and catalyst supports [8]. The crystallinity, nanoscale, and porosity of alumina are the most critical structured features to determine its reaction performance. Many examples of CoAl2O4 materials including nanocolloids [9] and powders [10] have been synthesized to investigate their unique magnetic and color properties for applications. To the best of our knowledge, the scalable fabrication of lightweight CoAl2O4 nanostructured aerogels is virtually unexplored. It is thus vital to find new, efficient strategies for CoAl2O4 materials with these novel aerogel features for exploring their potential uses [11, 12]. Chitosan is a deacetylated form of chitin mostly extracted from naturally abundant crustacean shells [13]. The nanostructural hierarchy of flexible chitosan fibrils makes it a novel substrate to develop new materials. The surface deacetylation leads chitosan fibrils to disrupt hydrogen bonding and decreased crystallinity. The exposed surface amino groups can be easily protonated to dissolve chitosan in acidic media and bind to metal ionic additives to form functional complexes. These chemical properties make chitosan nanofibrils attractive for many applications in different fields, dependent on the ways to design their structural forms [14]. It is of great interest to exploit the aqueous solubility and complexity of chitosan nanofibrils for the material synthesis. Previous studies have shown that chitosan is a fibril precursor for cotton fabrics [15], a matrix for photonic mineralization [16], a crosslinker for chitin hydrogels [17], and a photonic template for responsive hydrogels [18]. However, soft templating of tunable crystalline cobalt aluminate aerogels with water-soluble chitosan is almost unknown. Herein, we report a new method to produce lightweight CoAl2O4 aerogels from metal (cobalt-aluminum)-complexed chitosan aqueous solutions. We initially experimented the solubility of chitosan fibrils in water in the presence of cobalt-aluminum ions to form a neutral aqueous solution. The complexity of cobalt-aluminum ions with chitosan led nanofibrils to function as an aerogel template to recover lightweight spinel CoAl2O4 nanofibril aerogels. 2. EXPERIMENTAL Chemicals: Crustacean shells were collected from discarded sources of the fish processing industry of local seafood manufacturers in Vietnam. These shell sources were used as starting materials for preparing chitin and chitosan. Chitosan obtained by hot alkali treatment of natural chitin has a molecular weight similar to that of natural chitin (~203,1925 g/mol), due to negligible influence of the deacetylation on biopolymer hydrolysis[19]. Metal precursor (cobalt nitrate (Merk), aluminum nitrate (Merk)) and other chemicals (sodium hydroxide, hydrochloric acid, hydrogen peroxide, and ethanol purchased from China) were obtained from standard suppliers. Materials synthesis: Preparation of cobalt-aluminum hydroxide/chitosan composites and spinel CoAl2O4 aerogels: Chitosan nanofibers with degree of deacetylation of >90% prepared from crustacean shells prepared by our groups (~400 mg) were added to 60 mL of deionized water containing ~250 mg Al(NO3)3 and sonicated for 2 h and then stirred at room temperature for 48 h to generate obtain a homogeneous Al 3+ /chitosan aqueous solution. Cobalt nitrate (~133 mg) was added to 60 mL of Al 3+ /chitosan aqueous solution followed by ~260 mg of NaOH under stirring for 2 h to precipitate into cobalt hydroxide and aluminum hydroxide, affording a highly dispersed Co-Al hydroxide/chitosan gel aqueous mixture. This gel mixture was freeze-dried to Chitosan - templated lightweight CoAl2O4 nanofibril aerogels 137 form Co-Al hydroxide /chitosan aerogel composites (~200 mg) on solidification. The Co-Al hydroxide /chitosan aerogel composites (~1 g) were calcined in air 100 o C for 2 h and then heated to 550 °C for 6 h with a heating rate of 5 °C min -1 to remove chitosan template and generated ~200 mg of spinel CoAl2O4 aerogels. Materials characterization: Powder X-ray diffraction patterns of the samples were recorded on an Advance Bruker D8 X-ray diffractometer. Scanning electron microscopy images of the samples were obtained on a JSM-5300LV electron microscope. Samples were prepared by attaching them to aluminum stubs using double-sided adhesive tape and sputter coating with Au (8 nm). Transmission electron microscopy images of the samples were obtained on a JEOL-JEM 1010 electron microscope. Energy-dispersive X-ray analysis was collected using a JEOL-6490- JED-2000 scanning electron microscope. Thermogravimetric analyses of the samples (~10 mg) were conducted at a heating rate of 20 °C min -1 under oxygen atmosphere to 800 °C using a Labsys TG/DSC-SETARAM thermogravimetric analyser. Infrared spectra were obtained on neat samples using a IR-Prestige-21 spectrometer. Gas adsorption experiments were conducted on a Micromeritics system. Samples (~100 mg) were degassed at 50 o C in a vacuum for 8 h before measurements. 3. RESULTS AND DISCUSSION We explored that chitosan nanofibrils can swell and dissolve in water in the presence of aluminum cations to form a neutral, homogeneous solution. This aqueous solubility depends on the amount of aluminum cation loading and the degree of surface deacetylation of chitosan and mostly occurs towards different chitosan sources. Visibly, the chitosan nanofibrils first swelled and then dissolved in the Al 3+ aqueous solution to form homogeneous Al 3+ -chitosan aqueous solutions (Figure 1a). The chitosan solution is transparent and has a neutral pH of ~7. The homogeneity of the chitosan solution was maintained at different concentrations. The concentrated chitosan solution became viscous and then gelly by air-drying. Unlike acidic chitosan solutions that often precipitate in basic media, the neutral chitosan maintains well its liquid state. Due to the potential of chitosan solution for applied biopolymer technology, many efforts have been made to prepare chitosan aqueous solutions in the past decades. In terms of biomedicine applications, neutral chitosan aqueous solutions favor to be compatible with bio- systems. However, the acidic solutions are still a common form of chitosan for industrial applications. Recent studies have reported the acetylation of chitosan to be a facile method to obtain water-soluble fibrils [20, 21], but the chitosan aqueous solution is still slightly acidic. The preparation of actually neutral chitosan solution is almost unknown. Our finding presents a new, simple method for producing the neutral chitosan aqueous solution that is the first example of chitosan fibrils soluble in water in the presence of Al 3+ . This water-soluble chitosan may be a novel precursor to investigate the self-assembly of durable bioplastics for food packaging. The great potential is able to use the neutral chitosan solutions as a safe protective coating agent to fruits and vegetables [22-26]. Chau The Lieu Trang, et al. 138 Figure 1. Preparation of Al-chitosan liquids and their sponge-like aerogels. (a) Photo of neutral Al 3+ /chitosan aqueous solution, (b) Photo of Al 3+ /chitosan sponge-like aerogels, (c) Photo of lightweight Al 3+ /chitosan aerogels mounted on a natural flower, (d) SEM image of Al 3+ /chitosan aerogels, (e) PXRD patterns of pristine chitosan nanofibrils and Al 3+ /chitosan aerogels, and (f) EDX spectrum of Al 3+ /chitosan aerogels. Chitosan fibers are a form of promising cotton fabrics for practical applications in antibacterial bandage and tissue engineering [24]. Because the concentration of chitosan in the prepared aqueous solution is high, thus enabling to recover to solidified fibers. Very interesting, we carried out the lyophilization of the water-soluble chitosan solutions to obtain white fibrillar aerogels (Figure 1b). The yield of the fibrillar aerogel production is very high as ~400 mg solid products can be obtained from 60 mL of the chitosan aqueous solution (~6.6 wt%). The aerogels retain the shape of the pristine container with structural shrinkage. Naturally homogeneous fibers can be visualized in the aerogels. The aerogels possess large interspaces between uniform, thin fibrils highly distributed within open networks without any phase separation. The chitosan-based fibril aerogels look like natural sponges, which are as soft and lightweight as cotton fabrics (Figure 1c). The white chitosan cotton fabrics are stable in air atmosphere over the time and no significant change in color and shape could be observed. These indicate the physical and chemical durability of the prepared materials at ambient conditions. These visible observations assume that this is a reliable way to produce interesting cotton fabrics from native chitosan. Scanning electron microscopy (SEM) was used to analyze the structural organization of fibrils in the aerogels (Figure 1d). The aerogels are a highly porous fibrillar open network with macro-scale pores that is different from the chitosan structure. The fibrils highly interconnected with each other in all directions to form macroporous spider web-like nets through aerogels. This suggests that the enlarged porosity and low density of chitosan led to a lightweight fibrillar Al O C a) b) c) 2 cm d) e) Freeze drying 4 µm (110) (020) Al-chitosan Chitosan f) Chau The Lieu Trang 139 material. These results reveal that the prepared aerogels are a lightweight cotton fabric material. Structural analyses were performed for the fibrillar aerogels. Powder X-ray diffraction patterns (PXRD) (Figure 1e) reveal a chitosan crystal in the aerogels and no other crystal components could be detected. The chitosan fibrils in the aerogels exhibit noticeably lower intensity than those in the pristine chitosan sample, indicating the decrease in its crystallinity. We were surprised to realize the presence of aluminum species in the chitosan aerogels as evidenced by energy dispersive X-ray (EDX) spectroscopy (Figure 1f). The aluminum species did not diffract by X-ray (Figure 5a), indicating the aluminum-based species in the chitosan composites is in an amorphous form of hydroxyl substances. Figure 2. Preparation of cobalt-aluminum hydroxide/chitosan sponge-like aerogels. (a) Photo of neutral cobalt-aluminum hydroxide/chitosan aqueous solution, (b) Photo of cobalt-aluminum hydroxide/chitosan aerogels, (c) Photo of lightweight cobalt-aluminum hydroxide/chitosan sponge-like aerogels mounted on a natural flower, (d) FTIR spectra of cobalt-aluminum hydroxide/chitosan aerogels and pure chitosan. The solubility of chitosan fibers in Al 3+ -contained water may be due to the complexation between amino and hydroxyl groups of chitosan macromolecules and Al 3+ additives. This binding interactions often lead to the disruption of hydrogen bonding between chains in fibrils with a consequent strong decrease in crystallinity. This may be a main reason for forming the Al-chitosan aqueous solution under such experimental conditions [27]. Since the gelation of chitosan fibrils is mostly determined by the low degree of crystallinity. The decreased crystallinity induced by aluminum-chitosan interactions is one of crucial factors to make chitosan first swelling in water and then dissolving to form an aqueous solution. The chemical complexity of chitosan with aluminum ions is possible to be occurred in such experimental conditions as the chitosan aqueous solutions still presented aluminum after cleaning of unreacted residues by treating with dialysis. This complexity is uniform as no phase separation in the hybrid fibrils could be observed by SEM. The aluminum-chitosan complexes are strong to occur a) b) c) d) Freeze drying OH CH amide I amide II Nitrate C-O Co/Al-O Al-chitosan Chitosan Chau The Lieu Trang, et al. 140 aqueous dissolution of all crystalline fibrils to chitosan polymeric macromolecules. These analyzed results confirm that the water-soluble solution contained aluminum-chitosan complexes rather than chitosan alone. These soluble aluminum-chitosan macromolecular complexes highly distributed in water, which was reflected from the homogeneity of solidified fibrils in the aerogels obtained after lyophilization. Figure 3. (a,b) Photos of dark blue pigment of lightweight spinel CoAl2O4 aerogels with nanofiber spinel structure derived from the thermal removal of chitosan template by calcination and (c) TEM image of calcined spinel CoAl2O4 aerogels. The most studies on the aerogel materials have been reported for chitin and limited description of chitosan [28, 29]. The alkali-dissolved chitin dispersions were recently noticed as a novel solution to prepare chitin fibril aerogels. Unlike this, it is not still easily to obtain highly porous aerogels from the conventional acidic chitosan solutions owing to strong structural shrinkage under freeze drying. Some other chitosan solutions have been recently prepared as organic dissolution and acetylation, but aerogel structures from these solutions are virtually unexplored. By inventing a new method of dissolving chitosan in the aluminum aqueous solution, we initially present an interesting way to produce the chitosan sponges that are useful as cotton fabrics for the design of antibacterial bandages and others. We came up with an idea that the simultaneous incorporation of cobalt and aluminum ions into chitosan leads the nanofibrillar network to serve as a template for fabricating spinel CoAl2O4 aerogel replicas. In a typical synthesis, the addition of cobalt ions to the Al 3+ /chitosan aqueous solution allowed to obtain Co-Al hydroxide/chitosan aerogels after freeze drying of the resulting dispersions. We interestingly found that the calcination of the Co-Al hydroxide/chitosan composites under air afforded dark blue CoAl2O4 replicas after removal of chitosan template. The calcined products exhibit structural integrity of the aerogel morphology as they are still a lightweight fibril material. The unique structured features of fibril uniformity, large interspace, and cotton-like shape were originally conserved in the CoAl2O4 aerogels (Figure 3a,b). It is worthy to realize that CoAl2O4 is a ceramic, but the CoAl2O4 aerogels templated by chitosan are a soft material that looks like a natural fibril sponge. Infrared spectra (Figure 2d) of Co-Al hydroxide/chitosan composites show an amide I stretch at ~1640 cm -1 and a carbonyl stretch at ~1025 cm -1 of chitosan. The spectrum also shows a strong N-O stretching band at ~1315 cm -1 of nitrate and a sharp peak at ~830 cm -1 of metal (Co,Al)-O vibration that are absent in the pure chitosan. These comparative IR bands verify the prepared composites are a mixture of chitosan and Co-Al nitrates. The disappearance of an amide II stretch at ~1560 cm -1 in the composites may be due to bonding interactions between amino groups and mixed metal (Co,Al) ions. a) b) 2 cm 100 nm c) Chau The Lieu Trang 141 Figure 4. Structural network of spinel CoAl2O4 aerogels. (a-d) Differently magnified SEM images of CoAl2O4 aerogels prepared by calcining co-Al hydroxide/chitosan aerogel composites at 550 o C for 6 h under oxygen atmosphere to burn off chitosan template and crystalized mixed metal hydroxides to spinel mixed oxide replicas. Electron microscopy studies show that the overall aerogel morphology was preserved in the CoAl2O4 aerogels (SEM, Figure 4). It is very interesting to know that the structural integrity of the spider web-like networks of interconnected fibrils proceeded at nanoscale. The calcined CoAl2O4 aerogels still have the natural shapes of micro-sized interspaces and nanofibrils as those in the composites, but with smaller sizes by structural shrinkage. The CoAl2O4 fibrils appear rougher surfaces likely caused by particle aggregation. The average diameter of the CoAl2O4 fibrils is ~100 nm that is very close to that of chitosan fibrillar bundles (~200 nm in diameter). This supports that the uniform templating of cobalt-aluminum by chitosan occurred in the composites to replicate the fibril networks in the CoAl2O4 after template removal. Transmission electron microscopy (TEM) image of the calcined CoAl2O4 aerogels also show nano-sized fibril features in the porous networks (Figure 3c). These observations assume that the cobalt-aluminum ionic guests first complexed with chitosan hosts to form the fibril hybrid composites. Under calcination, the oxidized guests agglomerated into nanoparticles and then fused into CoAl2O4 fibrils oriented by chitosan template. The calcined CoAl2O4 aerogels exhibits distinct diffraction peaks at 31, 37, 45, 55, 59, 65 o (2) indexed to respective (220), (311), (400), (422), (511), (440) planes (PXRD, Figure 5a). These diffraction signals match those of standard spinel CoAl2O4, proving that the calcined product is spinel CoAl2O4 crystal structure [9]. The diffraction analyses confirm that the crystalline aerogels mostly contained spinel CoAl2O4 phase and no other impurities can be detected. The diffraction peaks are intense and broad to indicate the high degree of crystallinity and small size of the CoAl2O4 aerogels. No diffraction signals of chitosan crystals can be detected, further confirming the complete removal of chitosan in these CoAl2O4 aerogels. Energy dispersive X-ray (EDX) analyses (Figure 5b) show only cobalt and aluminum and 20 µm a) b) c) d) 3 µm 1 µm 1 µm Chau The Lieu Trang, et al. 142 oxygen elements in the calcined products. Elemental analyses confirm the absence of carbon element in the calcined products compared with the presence of carbon in the chitosan-templated composites. These results prove that the calcination of the composites simultaneously led to the complete removal of chitosan and transformed amorphous cobalt-aluminum hydroxide species to spinel CoAl2O4 polycrystals. Figure 5. Structural analyses of CoAl2O4 aerogels. (a) PXRD patterns of Co-Al hydroxide/chitosan aerogel composites and calcined CoAl2O4 aerogels, (b) EDX spectrum, (c) TGA curve (running at 20 o C min -1 under oxygen atmosphere), and (d) Nitrogen adsorption-desorption isotherms with inset of the pore size distribution of calcined CoAl2O4 aerogels. Nitrogen adsorption desorption isotherm studies (Figure 5c) of the CoAl2O4 aerogels show characteristic features of the type IV isotherm, presenting mesoporosity in macroporous networks of the spinel CoAl2O4 crystals. The CoAl2O4 aerogels have the BET surface area of ~200 m 2 g -1 and pore size distribution in the range of 50-100 nm. The broad pore sizes of the CoAl2O4 aerogels indicate macro-mesoporous ceramic structure formed after the thermal removal of chitosan template. This porous size range is also consistent with the value evaluated by SEM (Figure 5d). Due to the aerogels network constructed by chitosan nanofibril assemblies, the aerogel templating creates mesopores by chitosan nanofibrils and macropores by spider-wed networks. This analysis additionally proves that the lightweight characteristic of the spinel CoAl2O4 pigment ceramic aerogels is due to the highly porous networks with a low density feature. O Al Co Co a) b) c) (220) (311) (400) (511) (422) (440) ~40 nm (nm) CoAl2O4 JCPDS 44-0160 Co d) Prepared CoAl2O4 Prepared Co-Al hydroxide/chitosan Chau The Lieu Trang 143 As aforementioned, CoAl2O4 is a novel material for colorful magnetic pigments and catalyst supports. The vast majority of studies have made to prepare CoAl2O4 materials in the past decades [9, 11, 30, 31]. The difficulty of preparing CoAl2O4 often obtains conventional forms of nanoparticles and micropowders with limited success of porous structures, especially aerogels. As a result, our successful production of highly purified spinel CoAl2O4 aerogels using the new, facile method paves a way to investigate their unique structural properties for applications in colorful magnetic pigments, thermal insulation, and catalysis. The large space of the CoAl2O4 aerogels was occupied by air leads to a lightweight thermal material, which has the potential to thermal insulation. Due to the abundant availability of active catalytic sites of aluminum and cobalt in the oxide composites, the CoAl2O4 aerogels may be a promising catalyst support for various oxidation and reduction reactions [8]. High surface area and large porosity can easily access secondary components to the aerogel networks to accelerate the reaction performance of magnetic CoAl2O4-based nanocomposites [6]. Beyond the potential of the lightweight CoAl2O4 aerogels, the cobalt-aluminum/chitosan composites may be carbonized and selectively etched cobalt-aluminum away to prepare new nitrogen-doped carbon fibrillar aerogels for supercapacitors. 4. CONCLUSION We have shown for the first time the liquid templating of lightweight CoAl2O4 nanostructured aerogels by chitosan fibrils. A new phenomenon of swelling chitosan nanofibrils in Al 3+ -contained water was exploited to obtain a neutral aluminum-chitosan aqueous solutions. Our interesting finding is that the lyophilization of the cobalt-aluminum hydroxide/chitosan solution recovered cotton-like sponges that are the aerogel open networks of cobalt-aluminum hydroxide-templated chitosan fibrils. 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