Research Article www.acsami.org
Chitin Liquid-Crystal-Templated Oxide Semiconductor Aerogels Trang The Lieu Chau,† Dung Quang Tien Le,‡ Hoa Thi Le,† Cuong Duc Nguyen,*,†,‡,§ Long Viet Nguyen,*,∥,⊥ and Thanh-Dinh Nguyen*,# †
Department of Chemistry, Hue University of Sciences, Hue University, 77 Nguyen Hue, Hue City, Vietnam Department of Physics, Hue University of Sciences, Hue University, 77 Nguyen Hue, Hue City, Vietnam § Faculty of Hospitality and Tourism, Hue University, 22 Lam Hoang, Hue City, Vietnam ∥ Ceramics and Biomaterials Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam ⊥ Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam # Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada ‡
S Supporting Information *
ABSTRACT: Chitin nanocrystals have been used as a liquid crystalline template to fabricate layered oxide semiconductor aerogels. Anisotropic chitin liquid crystals are transformed to sponge-like aerogels by hydrothermally cross-linked gelation and lyophilization-induced solidification. The hydrothermal gelation of chitin aqueous suspensions then proceeds with peroxotitanate to form hydrogel composites that recover to form aerogels after freeze-drying. The homogeneous peroxotitanate/chitin composites are calcined to generate freestanding titania aerogels that exhibit the nanostructural integrity of layered chitin template. Our extended investigations show that coassembling chitin nanocrystals with other metal-based precursors also yielded semiconductor aerogels of perovskite BaTiO3 and CuOx nanocrystals. The potential of these materials is great to investigate these chitin sponges for biomedicine and these semiconductor aerogels for photocatalysis, gas sensing, and other applications. Our results present a new aerogel templating method of highly porous, ultralight materials with chitin liquid crystals. KEYWORDS: chitin nanocrystals, liquid crystalline templating, lightweight semiconductors, hydrogels, aerogels including inorganic (e.g., silica microspheres11 and anodic alumina12) and organic (e.g., block copolymers,13 resin microspheres,14 or biomacromolecules15) substances are used to construct highly porous materials. Some beautiful examples of the templated synthesis are TiO2 opals,16 CuO−ZnO opals,17 cellulose hollows,18 and metallic sponges19 with exceptional properties for many applications. These templating processes often afford lightweight materials due to the formation of highly porous networks. Biomacromolecules have hierarchical fibrillar structures made up of primary chains at the molecular and nanoscale levels.20 Complex surface properties of polymeric biomolecules often lead to the evolution of their fibrous networks induced by hydrogen bond cross-linking. High porosity and low density make biopolymer networks interesting as lightweight materials.21 These porous biopolymer networks can be used as an alternative template to fabricate porous materials.22,23 Chemical transformation of biomacromolecules also proceeds by synthetic reactions to self-assemble into new structural forms such as hydrogels,24 liquid crystals,25−27 and fibers.28 These
1. INTRODUCTION The performance and applications of materials are dominated by their structural properties.1 An open network, enlarged porosity, and nanosized materials are crucial structural features to access and discover technology.2 Aerogels are an ultralight porous material with high porosity, large surface area, and a low-density network.3 In aspects of sustainable and optoelectronic applications, functional aerogel materials of semiconductors and biopolymers appear to be appropriate substances for specific device fabrications.4 There has been an increasing demand for these materials for emerging applications. Noticeable studies on this subject discuss titania-based catalysts5 and solar cells,6 CuO/carbon sensors,7 and chitin-like cotton fabrics.8 The greatest importance of these scientific contributions is that synthetic methods have been successfully devoted to the manipulation of the organized structures of the materials at multiple size levels. The templated synthesis is a powerful routine for the construction of porous materials with tunable structures and geometries ranging from macro- to nanosizes.9,10 Morphological features of the templated materials are often determined by structure-oriented agents. The control of the morphological organization of templates in synthetic reactions is an important step to design structures of materials. Two classes of templates © 2017 American Chemical Society
Received: May 30, 2017 Accepted: August 15, 2017 Published: August 28, 2017 30812
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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ACS Applied Materials & Interfaces chemically modified biomacromolecules can access solvents to investigate the assembled templating of architectured materials. This strategy is actually of interest to develop new materials as recent efforts have been reported dealing with the use of biomacromolecules (e.g., peptides, DNA, carbohydrates) to template porous semiconductors (e.g., TiO2,29 BaTiO3,30 CuOx31). Chitin is a major component in the shells of crustaceans and the cell walls of fungi mushrooms. As the most abundant biopolymer after cellulose, chitin is a novel biofiber source to develop sustainable materials for biomedicine.32 The discovery of new materials from chitin dates back to the 1990s. In 1993, Revol and Marchessault pioneered the preparation of chitin liquid crystals by acid-catalyzed hydrolysis of chitin fibrils.33 The acid hydrolysis of native chitin fibrils breaks down amorphous regions and transforms crystalline regions to rodshaped chitin nanocrystals (NChs). Chitin nanoparticle colloids can be stable in water and self-organize into anisotropic liquid crystals (LCs) at above the critical concentration. In 1992, Kresge et al.34 reported a breakthrough in LC-templated synthesis of periodic porous materials. The most interesting aspect of this LC-templated synthesis is the ability to capture the ordered organization of LCs on solidification to create new materials. With this outstanding concept, NChs have been used as an LC template by scientists to prepare porous materials. LC templating with NChs had been previously studied by Alonso and co-workers to prepare mesoporous silica35 and alumina36 and later extended to other components by others.37−42 Since most previous studies on chitin nanocrystals have obtained thin films, hydrogels, or mesoporous structures of the materials, aerogel forms of chitin sponges and semiconductor nanocrystals are virtually unexplored. Herein we presented LC templating of porous metal oxide semiconductor aerogels with NChs. Hydrothermal gelation of chitin LCs with formaldehyde yielded hydrogels that freezedried to form chitin sponge-like aerogels with a layered structure. We found that water-soluble peroxotitanate is highly compatible with chitin LCs to perform a chitin-templated synthesis of freestanding, ultralight layered titania aerogels via hydrothermal gelation, lyophilization, and then template removal. This concept was extended to BaTiO3 and CuOx aerogels, presenting a generalizable method for highly porous semiconductors.
Figure 1. Chitin sponge-like aerogels prepared from chitin LCs: (a) photo of chitin LCs, (b) photo of chitin hydrogels, (c) photo of chitin aerogels prepared by freeze-drying of the hydrogels, (d) TEM image of chitin nanocrystals, and (e) SEM image of chitin aerogels.
structural organization of chitin LCs, the chitin aqueous suspensions (∼6.6 wt %, pH ∼ 4) were dried at ambient conditions to form solid films. A scanning electron microscopy (SEM) image (Figure S3) of these chitin films shows a layered nematic structure.35,41 This indicates that chitin LCs have a nematic ordered structure that was preserved in the films under drying. The availability of surface amino groups on NChs makes its LC phase possible to be chemically reactive with additives for gelation. Taking this benefit into account, we investigated sequential gelation and solidification of chitin LCs into fibrillar networks that serve as an alternative template for fabricating oxide semiconductor aerogels. We first attempted to prepare metal/chitin composite gels by adding metal ions to the chitin aqueous suspension, but these gels were strongly shrunk to form solids rather than aerogels. We later found that the gelation of chitin LCs can be achieved by a hydrothermolysis of the chitin aqueous suspensions in the presence of aldehyde crosslinkers. In a typical preparation, the chitin aqueous suspensions (∼6.6 wt %, pH ∼ 4) were mixed with formaldehyde and then treated hydrothermally at 70 °C for 20 h to form homogeneous hydrogels. Interestingly, these gels retained the shape of the chitin aqueous suspensions (Figure 1b). Under freeze-drying, the hydrogels were recovered to chitin aerogels with shape and size retention without negligible shrinkage (Figure 1c). The chitin aerogels are an ultralight, soft fibrillar material that looks like chitin marine sponges.44 Structural analyses reveal the retention of α-chitin nanocrystals in the aerogels (Figure S5). The thermal stability of the chitin aerogels is higher than the nanocrystalline chitin powders, possibly due to cross-linking of functional groups of NChs with formaldehyde (Figure S5c). SEM images (Figure 1e) of the chitin aerogels show highly porous networks with hierarchically layered organization of chitin nanorod assemblies. This assumes that the anisotropic structure of chitin LCs is blocked by hydrothermal gelation. Under freeze-drying, this organized structure was basically conserved as the hydrogels released water to recover the layered nematic chitin aerogels. To our knowledge, this is the first transformation of the anisotropic chitin LCs into the ultralight sponge-like aerogels.
2. RESULTS AND DISCUSSION Chitin LCs were prepared by sequential deacetylation and hydrolysis of native chitin fibrils extracted from crustacean shells. The crab shells were deproteinized and decalcified to obtain chitin flakes. The purified chitin fibrils were partially deacetylated by a hot alkali treatment and then thermally hydrolyzed in a concentrated HCl solution to form NChs. NCh colloids were dispersed in water at pH ∼ 4 to form a stable chitin aqueous suspension. These homogeneous colloidal suspensions were anisotropic LCs when the concentration of chitin reached the critical level at ∼6.6 wt % (Figure 1a). Structural and elemental analyses of the prepared NCh samples confirm that they are pure chitin crystals with an elemental ratio of 6.19% nitrogen, 42.49% carbon, 6.47% hydrogen (Figure S1).43 Transmission electron microscopy (TEM) images (Figure 1d) of the prepared aqueous suspensions show that chitin crystals are spindle-shaped nanoparticles with diameters of 10−20 nm and lengths of 300−500 nm that are similar to results reported in the literature.33 To clarify the 30813
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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chitin aerogel composites. The anisotropic structure with birefringence of the peroxotitanate/chitin aerogel composites is observed under crossed polarizers, supporting the conservation of the nematic structure of chitin LCs during aerogel templating (Figure S11). Sequential removal of chitin template in the peroxotitanate/ chitin composites was accomplished by calcination. The most interesting appearance of the template removal is that the calcined products originally retained the fibrillar aerogel morphology of the composites (Figure 2c). The calcined products are white ceramic aerogels with dimensions of several centimeters. After chitin removal, the calcined aerogels are freestanding and lightweight as they can be mounted on natural spider webs without damage (Figure S12). Powder X-ray diffraction (PXRD) patterns (Figure 2e) of the calcined products show only diffraction peaks characteristic of anatase TiO2 crystals, and the signals of chitin crystals are absent. Thermogravimetric analysis (TGA) profiles (Figure 2f) show a very low content of TiO2 (∼8 wt %) loaded in the chitintemplated composites. EDX spectra (Figure S13) also show only titanium and oxygen elements present in the calcined samples that do not contain carbon and nitrogen elements confirmed by elemental analyses. These identify that the calcination of the peroxotitanate/chitin aerogel composites simultaneously led to the complete removal of chitin and crystallized the amorphous peroxotitanate into its anatase polymorph. Structural organization of TiO2 aerogels was assessed using SEM (Figure 3). At low magnification, we see cracked flakes randomly arranged into an open network with an interspace of ∼1−4 μm. Looking at cross sections of ∼200 nm thick flakes, a layered structure is organized throughout its entire thickness. The flakes also possess ∼50−100 nm diameter holes confined within interlayers. From the top view of the layered flakes at higher magnifications, we see spindle-like features organized with long-range alignment in each layer within flakes. The morphology of these nanospindles resembles that of NChs but with a little smaller size (∼6 nm in diameter) because of structural shrinkage by calcination. The spindle-like features distributed through TiO2 aerogels, and no aggregation or phase separation could be observed in the structure. Overall, TiO2 aerogels have an open network of nematically layered nanorod assemblies resembling chitin aerogels. These electron microscopy images reveal that peroxotitanate precursors uniformly combined with chitin template to form the homogeneous composites. The hydrothermal gelation of these aqueous mixtures led water to be blocked in the layered hydrogel composites. Under freeze-drying of the hydrogels, water was released to leave large interspaces confined inside layered structures of the aerogel composites. The calcination of the composites led cylindrical mesoporous structures to be formed by chitin template removal and maintained macrosized interspaces, producing freestanding TiO2 aerogels. Altogether we can conclude that TiO2 aerogels accurately replicated the layered nematic aerogel organization of chitin template. Nitrogen sorption studies show that TiO2 aerogels have typical type-IV isotherms with type-H2 hysteresis characteristic of a macro-mesoporous structure (Figure 2g). This indicates that the aerogel networks of the chitin template created a macroporous structure in the mesoporous TiO2 replicas. TiO2 aerogels have the surface area of ∼80 m2 g−1 and pore volume of ∼0.48 cm3 g−1. Noticeably, TiO2 aerogels have multimodel pore size distributions centered at ∼9, ∼15, and ∼40 nm. The
These chitin aerogels have promise for applications in absorbents, filters, tissue engineering, and antibacterial bandages.32 Also of particular interest is the ability to investigate the surface modification of chitin to hydrophobic aerogels attractive for oil separation.45 The successful preparation of the chitin aerogels encouraged us to capture the anisotropic fibrillar networks of gelatinized chitin suspensions in semiconductors by means of LC templating. The potential of TiO2 , BaTiO3 , and CuO x semiconductors is vast as they are extensively used for photocatalysis, photovoltaics, and sensing.46−48 We applied the chitin LC templating strategy for these representative oxide semiconductors but no limitation to others. The synthetic method is the homogeneous combination of different components into chitin-templated oxide aerogel composites, and later, the chitin template in the composites was thermally removed to obtain porous semiconductor replicas. Chitin LC-templated synthesis was first examined with titanium elements to prepare titania materials (Scheme 1). A Scheme 1. Schematic of Liquid Crystalline Templating of Lightweight TiO2 Aerogels with Chitin Nanocrystals
titanium alkoxide such as titanium isopropoxide is a typical precursor used for preparing titania. However, this precursor hydrolyzes very quickly in water to control organized structures of reacted products as it often forms large aggregates that are phase-separated from others. To gain homogeneous titanium/ chitin aerogel composites, we prepared peroxotitanate compounds to use as a new titanium precursor soluble in water to be compatible with chitin LCs (Figure S7).49 The chitin aqueous suspensions (∼6.6 wt %, pH ∼ 4) were mixed sequentially with formaldehyde and peroxotitanate aqueous solution to form composite mixtures. The resulting mixed suspensions retained structural homogeneity as we can see a uniform orange color for the samples resulting from a good distribution of orange peroxotitanate precursors within the chitin template (Figure S8). These aqueous mixtures were hydrothermally treated at 70 °C for 20 h to obtain homogeneous hydrogel composites (Figure 2a). The freezedrying of the hydrogels afforded intact orange aerogel composites (Figure 2b). Similar to the chitin aerogels, the structured features of fibrillar layers, interspaces, and lightweight characteristics were visibly observed in the aerogel composites (Figure 2d). The bulk density of this peroxotitanate/chitin aerogel composite determined by the mass-tovolume ratio is found to be approximately 0.014 g cm−3. We also note that the aerogel solidification of the peroxotitanate/ chitin composites is not reversible by redissolving some peroxotitanate precursors in water. It is interesting to realize that the uniform combination of peroxotitanate precursors with chitin template led to the structural integrity of the fibril softness and component homogeneity in the peroxotitanate/ 30814
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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Figure 2. LC templating of TiO2 aerogels with chitin nanocrystals. Photos of (a) peroxotitanate/chitin hydrogel composites, (b) peroxotitanate/ chitin aerogel composites, and (c) TiO2 aerogels. (d) Photo of peroxotitanate/chitin aerogel composites placed on a natural flower to show lightweight characteristics of the as-prepared materials. (e) TGA curves of peroxotitanate/chitin aerogel composites in comparison with chitin aerogels. PXRD pattern (f) and nitrogen adsorption−desorption isotherms (g) of TiO2 aerogels; inset of part g is BJH pore size distribution.
pore sizes at ∼9 and ∼15 nm are more dominant than that at ∼40 nm. TEM images (Figure S15) also show cylindrical mesopores in the TiO2 aerogels. The pore size range of ∼9−15 nm and ∼40 nm is close to the diameters of NChs (∼10−18 nm) and of the large interspaces within the aerogel networks, respectively. These pore sizes are consistent with the values observed by SEM, which again confirm multiple-sized porous structures in the layered TiO2 aerogels. These analyses prove that LC templating created the homogeneous peroxotitanate/ chitin composites. These composites released chitin nanospindles by calcination to leave cylindrical mesopores in TiO2 aerogels. The thermal removal of the template caused structural shrinkage of the material, leading the pore size range to be smaller than the diameters of NChs. Despite the templating of porous titania-based materials with chitin having been reported by some researchers,42,50,51 these freestanding titania aerogels are conceptually new.
We also extended this method to synthesize perovskite BaTiO3 semiconductors. The hydrothermal gelation of bariumperoxotitanate/chitin aqueous mixtures was carried out to form hydrogel composites. The hydrogels were lyophilized to generate aerogel composites that contained approximately 10 wt % inorganic components (Ti and Ba). Similarly to the peroxotitanate/chitin composites, we also obtained lightweight barium-peroxotitanate/chitin aerogel composites (Figure 4a). SEM images (Figure S17) of barium-peroxotitanate/chitin composites show a homogeneous aerogel structure of layered nanofibrillar assemblies. This assumes that the incorporation of Ba ionic additives into peroxotitanate/chitin composites mostly did not disrupt the layered alignment of chitin nanocrystals. The barium-peroxotitanate/chitin composites were calcined at 900 °C in air to remove chitin template to generate mixed oxide replicas. The calcined products were a little more shrinkable than the TiO2 samples owing to the higher 30815
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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gelation of Cu2+/formaldehyde/chitin aqueous mixtures at 70 °C for 20 h afforded homogeneous aerogel composites after freeze-drying of the resulting hydrogels (Figure S20). The green composites were layered fibrillar aerogels with the original shape of the hydrogels (Figure 5a). The proper loading of Cu-based precursors in chitin LCs was ∼17 wt % to obtain homogeneous aerogel composites (TGA, Figure S22). SEM images (Figure 5c) clearly show ∼40 nm sized CuO particles embedded within layered nematic chitin assemblies in the aerogels. This good combination was further confirmed by TEM to reflect the fact that CuO particles decorated chitin spindles at the nanoscale in the aerogel composites (Figure S25). Calcination of the Cu2+/chitin composites in air generated CuO replicas. The calcined aerogels appeared with a little shrinkage and darkness but mostly retained the overall shape of the composites. PXRD patterns (Figure 5e) of the calcined products show intense diffraction peaks characteristic of cubic CuO crystal, and no signals of chitin are detectable, confirming the complete removal of chitin template in the composites. Electron microscopy images (Figure 5d and Figure S27) of CuO aerogels show the retention of open networks made up of CuO rod-shaped nanoparticles. The thermal overgrowth of CuO nanocrystals diminished the layered structure of the aerogels as tiny particles templated by chitin nanospindles fused together by calcination to form ∼20 nm diameter CuO nanorods interconnected in the aerogels. Like the TiO2 aerogels, the CuO aerogels have also a macro-mesoporous structure with different pore sizes that range from ∼8 to ∼19 nm and a surface area of ∼70 m2 g−1 (N2 sorption isotherms, Figure 5f). The CuO aerogels possess more macropores, dominant as compared to the mesopores, reflecting the role of chitin nanocrystals in the aerogel templating. Furthermore, we also prepared Cu/CuO aerogels by thermal reduction of the CuO samples under 10% H2/90% N2 at 400 °C for 4 h. The reduced products are dark brown and more rigid than CuO aerogels, resulting from the surface reduction of the CuO aerogels by H2 (Figure S28a). Remarkably, the surface reduction of CuO by H2 retained the aerogel structure and seemed to form smaller Cu nanoparticles on the CuO aerogel networks (SEM in Figure S28b). These hierarchical CuOx nanostructured aerogels may be a novel catalyst support for investigating electrocatalysis of CO2 reduction54 and glucose detection.55 In other words, these metal (Ti, Ba−Ti, Cu)/chitin composites may also be novel precursors to prepare functionally carbonized aerogels for supercapacitors and electrocatalysis.
Figure 3. Layered nematic organization of TiO2 nanostructured aerogels: (a) SEM image of TiO2 aerogels viewed at low magnification and (b−d) SEM images of TiO2 aerogels viewed along top surfaces of flakes interconnected within open networks of semiconductor aerogels at different magnifications.
temperature calcination, but the aerogel integrity was in the lightweight material after chitin removal (Figure 4b). PXRD patterns (Figure 4c) reveal that the calcined products contained a major tetragonal phase characteristic of BaTiO3 crystals alongside minor components of Ba4Ti2O27, anatase TiO2, and brookite TiO2 crystals.52 Diffraction peaks of α-chitin nanocrystals could not be observed in the calcined composites whereas they are present in the as-prapared composites. EDX analyses (Figure 4d) also confirm the presence of barium elements in the calcined products. This indicates that the calcination of the barium-peroxotitanate/chitin composites simultaneously led to the complete removal of chitin and the incorporation of some Ba ions into TiO2 to generate perovskite BaTiO3 crystals. Electron microscopy images (Figures 4e,f) show the conservation of the overall morphology of nanofibril aerogels in BaTiO3 aerogels. The BaTiO3 samples still retained structural homogeneity throughout aerogel networks. Highly magnified SEM images reveal that ∼100 nm diameter fibrils within aerogel networks were actually constructed from ∼5 nm sized nanoparticles. Unlike the TiO2 aerogels with spindleshaped features, the BaTiO3 aerogels are made up of particleaggregated nanofibrils, possible due to the formation of the structure at high temperature (at 900 °C). Nitrogen sorption studies (Figure S18) showed that BaTiO3 aerogels have a macroporous structure with a pore size distribution at ∼40 nm and low surface area of ∼50 m2 g−1. These highly porous TiO2 and BaTiO3 aerogels are novel semiconductors to investigate unique optoelectronic properties for photocatalysis and photovoltaics. Our qualitative evaluations of photocatalysis showed that the TiO2 and BaTiO3 aerogels both exhibit photocatalytic activity higher than Degussa P25 for the UV light photodegradation of methylene blue (Figure S19). This enhanced activity could be due to the enlarged porosity and nanoscale structure of the semiconductor aerogels. These preliminary photocatalytic results are necessary to further investigate catalytic improvement by doping cocatalysts onto these titania-based aerogels.53 To generalize our synthetic method, the chitin LC templating was also applied for CuOx. The hydrothermal
3. CONCLUSIONS In summary, we have shown for the first time the liquid crystalline templating of oxide semiconductor aerogels with chitin nanocrystals. Hydrothermal gelation of chitin liquid crystals followed by lyophilization afforded chitin sponge-like aerogels attractive for absorbent, tissue engineering, and antibacterial studies. Under liquid crystalline templating, the homogeneous combination of chitin nanocrystals with peroxotitanate yielded aerogel composites. After thermal removal of chitin template in the composites, we obtained freestanding lightweight TiO2 aerogels that replicated the layered aerogel networks of chitin nanocrystals. By coassembling barium, peroxotitanate, and chitin nanocrystals into composites, perovskite BaTiO3 nanostructured aerogels were obtained from the calcination of the resulting composites. This chitin-templated synthesis was also extended to CuOx aerogels, 30816
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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Figure 4. LC templating of BaTiO3 aerogels with chitin nanocrystals: photos of (a) barium-peroxotitanate/chitin aerogel composites and (b) BaTiO3 aerogels, PXRD patterns (c) of BaTiO3 aerogels (red) and barium-peroxotitanate/chitin aerogel composites (black), and (d) EDX spectrum and (e, f) differently magnified SEM images of BaTiO3 aerogels. partial surface deacetylation. The deacetylated chitin samples were collected and washed thoroughly with water, and were then treated with a HCl aqueous solution (100 mL, 4 M) at 104 °C for 18 h for hydrolysis to form chitin nanocrystals. The dark brown colors in the reaction mixtures were oxidized by H2O2 (25 mL, 30 vol %) at 90 °C for 30 min to obtain cloudy chitin solutions. After acid hydrolysis, the reaction mixtures were diluted with 100 mL water and then centrifuged to remove soluble chitin species. This purification was repeated three times to obtain chitin aqueous suspensions. The chitin nanocrystal colloids were adjusted to a concentration of ∼6.6 wt % and a pH value of ∼4 to obtain a stable chitin aqueous suspension. Peroxotitanate Aqueous Solution. Peroxotitanate was prepared following the previously reported procedure.49 Typically, an aqueous solution containing commercial TiO2 powders and 10 mol L−1 NaOH was sonicated by an ultrasonic transducer (32 kHz, 100 W) and then transferred to a Teflon-lined autoclave, sealed, and heated to 130 °C in an oven for 10 h. After hydrothermal treatment, precipitates were
demonstrating a general synthetic method for highly porous semiconductors using a chitin template from discarded crustacean shells. These oxide aerogels could be novel semiconducting supports for investigating photocatalysis and gas sensing from optoelectronic device fabrications.
4. EXPERIMENTAL SECTION Chitin Aqueous Suspension. Dried crab shells (∼50 g) were deprotenized with a NaOH aqueous solution (5 wt %, 1000 mL) at 80 °C for 6 h and then decalcified with a HCl aqueous solution (7 vol %, 1000 mL) twice at room temperature for 24 h to obtained chitin shells after copious water washing. Pigments were bleached with a H2O2 aqueous solution (5 vol %, 1000 mL) at 90 °C for 3 h to obtain pure white chitin with an extraction yield of ∼13 wt %. For preparation of chitin liquid crystals, the purified chitin flakes (∼5 g) were treated with a NaOH aqueous solution (80 mL, 33 wt %) at 90 °C for 2 h for 30817
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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Figure 5. LC templating of CuO aerogels with chitin nanocrystals: photos of (a) Cu2+/chitin aerogel composites (inset showing layered fibrillar textures observed from top surfaces of aerogel composites) and (b) CuO aerogels, (c) SEM image of Cu2+/chitin aerogel composites, (d) TEM image of CuO aerogels, (e) PXRD pattern of CuO aerogels, and (f) N2 adsorption−desorption isotherms of CuO aerogels; inset of BJH pore size distribution. collected and washed thoroughly with deionized water and 0.1 mol L−1 HCl aqueous solution and then dried under vacuum at 80 °C for 2 h. The reacted product was reacted with H2O2 at 90 °C under stirring for 1 h to form a homogeneous yellow peroxotitanate aqueous solution that can be stable for several weeks at temperatures lower than 10 °C. Chitin Sponge-like Aerogels. Formaldehyde (0.2 mL) was added to a chitin aqueous suspension (∼10 g, ∼6.6 wt %, pH ∼ 4) under stirring and sonication. These mixed suspensions were then treated hydrothermally at 70 °C for 20 h to form homogeneous chitin hydrogels. The hydrogels were freeze-dried to form chitin sponge-like aerogels (∼500 mg). TiO2 Aerogels. A peroxotitanate aqueous solution (5 mL, 10 g L−1) was added slowly to a chitin aqueous suspension (∼10 g, ∼6.6 wt %, pH ∼ 4) under vigorous stirring, and then formaldehyde (0.2 mL) was added to this mixture. The reaction mixtures were sonicated in 15
min to form stable mixed suspensions. These mixed suspensions were then treated hydrothermally at 70 °C for 20 h to form yellow, semitransparent hydrogel composites. The hydrogels were freeze-dried to form peroxotitanate/chitin aerogel composites (∼500 mg). For removal of chitin, the yellow composites were calcined in air to 100 °C at 2 °C min−1, held at 100 °C for 2 h, then heated to 500 °C at 2 °C min−1, and held at 500 °C for 6 h to generate white TiO2 aerogels (∼50 mg). BaTiO3 Aerogels. The preparation of BaTiO3 aerogels was carried out via the exact same procedure for TiO2 aerogels except for different chemical compositions and calcination temperatures. A typical preparation of barium-peroxotitanate/chitin aerogel composites used BaCl2 aqueous solution (3 mL, 4 g L−1), peroxotitanate solution (7 mL, 10 g L−1), chitin aqueous suspension (10 g, 6.6 wt %, pH ∼ 4), and formaldehyde (0.2 mL). The barium-peroxotitanate/chitin aerogel 30818
DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
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ACS Applied Materials & Interfaces composites were calcined in air to 100 °C at 2 °C min−1, held at 100 °C for 2 h, then heated to 900 °C at 2 °C min−1, and held at 900 °C for 6 h to generate BaTiO3 aerogels (∼100 mg). CuOx Aerogels. A CuSO4 aqueous solution (2 mL, 0.3 g) was added slowly to a chitin aqueous suspension (∼10 g, ∼6.6 wt %, pH ∼ 4) under stirring. Formaldehyde (0.2 mL) was then added to this mixture to form a gel-like suspension. These mixed suspensions were then treated hydrothermally at 70 °C for 20 h to form green, homogeneous hydrogel composites. The hydrogels were freeze-dried to form Cu2+/chitin aerogel composites (∼700 mg). The composites were calcined in air to 100 °C at 2 °C min−1, held at 100 °C for 2 h, then heated to 500 °C at 2 °C min−1, and held at 500 °C for 6 h to remove chitin. Black CuO aerogels (∼68 mg) were obtained. The surface reduction of CuO was carried out by calcining the samples under 10% H2/90%N2 at 400 °C for 4 h to form dark-brown Cu/CuO aerogels. Overall, the Cu2+/chitin weight ratio in the composites can be varied to optimize the highly porous network aerogels.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07680. Details of materials synthesis, structural analyses, photographs, extended electron microscopy images, and photocatalytic activity (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (C.D.N.). *E-mail:
[email protected] (L.V.N.). *E-mail:
[email protected] (T.-D.N.). ORCID
Thanh-Dinh Nguyen: 0000-0003-2226-048X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful to the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant 103.02-2016.41 for financial support of this research.
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.7b07680 ACS Appl. Mater. Interfaces 2017, 9, 30812−30820
