Large-scale fabrication of silk fibroin fibers with aligned porous microstructure for thermal insulation textiles

Polar bears living in extremely cold environments such as the Artic circle are gifted with a natural capability of keeping them warm. The effective thermal insulation is provided by their thick fat fur covered by hollow hairs consisting of a unique microstructure of hollow core and aligned shells with large pore volume. Mimicking such characteristics in synthetic fibers could make a huge impact in the development of smart textiles for thermal insulation. Researchers at Zhejiang University, China lead by Prof. H. Bai have used a “freeze-spinning” method to convert silk fibroin to continuous and large-scale fabrication of fibers with aligned porous microstructure, mimicking the structural and functional features of the hair of a polar bear.

The “freeze-spinning” method involves a combination of “directional freezing” and “solution spinning” (Fig. 1). A well-dispersed viscous aqueous silk fibroin solution (50 mg/ml) with a small amount of chitosan (w_{silk fibroin} : w_chitosan = 9:1), is extruded using a syringe at a constant speed to form a stable liquid wire. When the wire slowly passes through a cold copper ring (green colour ring in Fig. 1), ice crystals grew directionally with a lamellar pattern within the wire that enables expelling and assembling of the solutes to template the ice morphology. When the extrusion speed becomes equal to the freezing speed, a stable solid–liquid interface is formed above the cold copper ring. The collected frozen fiber is freeze dried to preserve its porous microstructure. Subsequently, these fibers are woven into a textile.

Fig. 1 Schematic illustration of the “freeze-spinning” technique, combining “directional freezing” with “solution spinning” to realize continuous and large-scale fabrication of biomimetic fibers with aligned porous structure (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

The microstructure of the fibers can be controlled by a careful choice of parameters such as solution concentration/viscosity, extrusion speed and freezing temperature. Scanning electron micrographic images acquired at the axial cross-section of fibers prepared at −40, −60, −80, and −100 °C indicate an aligned porous structure while those prepared at −196 °C possess a random porous structure (Fig. 2). The degree of variation in porosity of these fibers suggests that it would be possible to prepare fibers with different pore size by simply varying the freezing temperature. The aligned porous microstructure imparts a better strength and modulus for fibers prepared at −40, −60, −80, and −100 °C than the one with random pores obtained at −196 °C.

Fig. 2 Radial cross-sectional SEM images showing different porous structures of biomimetic fibers prepared at different freezing temperatures (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

Assessment of ability of the fibers with different pore size for thermal insulation, evaluated by the change in surface temperature of the fibers using infrared images and a measure of the temperature difference (|ΔT |) between the fiber surface and the stage indicates that for a given stage temperature, the smaller the pore size of fiber, the better is its insulating property. The insulation ability of the woven textiles with different layers (1, 3 and 5 layers) indicate that the one with more layers offer better thermal insulation property (Fig. 3(a)). In spite of the free fibers, a better insulation property is also observed for textiles woven using fibers with a smaller pore size, as evidenced by the infrared images and a higher |ΔT | (Fig. 3(b)). A comparison of the infrared images of rabbits covered with a single layer (~ 0.4 mm thick) of polyester and the woven textile clearly demonstrate the better thermal insulation ability of the latter. The small difference between the surface temperature and the background makes the rabbit covered with the woven textile almost invisible to the infrared camera (Fig 4(a)). The ability of the woven textiles to demonstrate this effect over a wide range of temperature from −10 to 40 °C (Fig. 4(b)) suggest that they can very well be explored as a thermal sheath material for military applications.

Fig. 3 (a) Infrared images of textiles woven from different porous fibers. Temperature of the textile surface is measured based on the infrared images when changing the stage temperature from −20 to 80 °C; (b) Temperature difference (|ΔT|) between the textile surface and the stage against the stage temperature for different textiles (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

Fig. 4 (a) Photographic and infrared images of a rabbit before and after wearing the commercial polyester textile and the textile woven with biomimetic porous fibers; (b) Rabbit wearing the biomimetic thermal stealth textile becomes invisible by the infrared camera, regardless of the background temperature (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

When carbon nanotubes (CNTs) are dispersed along with the silk fibroin solution, it would be possible to impart a conductive network for the fiber without damaging its aligned porous structure (Fig. 5(a)). The incorporation of CNTS helps to induce electrical conductivity (~1.1 S/m) and upon impressing an applied voltage of 5 V using a portable power source, the surface temperature of the CNT-doped textile can be increased from ~24 to 36.1 °C within 45 s (Fig. 5(b)). The temperature of the CNT-doped textile can be easily manipulated by an appropriate choice of applied voltage (Fig. 5(c)). By combining two layers of textiles, one with CNTs (for electrical heating) and another one without CNTs (for thermal insulation) it would be possible to develop a hybrid textile.

Fig. 5 (a) Photographic and SEM images of the CNT-doped textile; (b) Infrared images of a CNT-doped textile during the heating process at an applied voltage of 5 V; and (c) Extent of  increase in temperature versus time after applying a voltage of 1, 3, and 5 V to a 5 × 2 cm CNT-doped textile (Image credit: Cui et al., Adv. Mater. 2018, 1706807; DOI: 10.1002/adma.201706807)

The excellent thermal insulation property of the silk fibroin fibers and the woven textiles using them, the feasibility to impart electrical heating by incorporating CNT along with the fibers, good breathability and comfort in wearing the woven textiles seems to be promising towards the development thermal sheath materials for military applications and materials for personal thermal management.

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer Y. Cui et al., A Thermally Insulating Textile Inspired by Polar Bear Hair, Adv. Mater. 2018, 1706807, DOI: 10.1002/adma.201706807

 

Freeze-drying of TiO2 nanorod arrays improves charge transport properties and performance of solar cell

TiO₂ nanorod arrays (NRAs) prepared by hydrothermal treatment, anodic oxidation and sol-gel synthesis have received considerable attention in solar cells, photoelectrochemical water splitting and gas sensors. Conventional air drying of the TiO₂ NRAs could cause aggregation of neighboring nanostructures and distortion of their morphological features, which deleteriously influence their charge transport properties and surface area. In addition, chemically adsorbed halides and alkyl chains might change the surface properties and  influence the interfacial charge transfer process and hence the overall performance of the device. It is important to preserve the vertical alignment of the nanostructures as well as to avoid chemically adsorbed impurities to achieve fast electron transportation, conformal heterojunctions with guest materials and enhanced light scattering. Researchers at School of Advanced Materials and Nanotechnology and Key Lab of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, People’s Republic of China have employed a freeze drying method to dry the TiO₂ nanorod arrays obtained by a two-step hydrothermal process. The freeze-drying post-treatment renders a clean TiO₂ surface and preserves the vertically-aligned nanostructures.

The TiO₂ NRAs were synthesized on FTO (SnO₂:F) substrates by a two-step hydrothermal method. In the first step, a mixture of 1.5 g TiCl₄, 15 ml DI water and 15 ml HCl (36.5 wt%) was subjected to hydrothermal treatment at 150 °C for 6 h. The resultant powder obtained from the first step was treated with a similar solution without TiCl₄ at 150 °C for 3 h in the second step. The air-dried TiO₂ NRAs were directly collected by air-gun blowing, while the freeze-dried samples were obtained after freeze-drying them for 5 h.

The morphological features of air- and freeze-fried TiO₂ NRAs are shown in Fig. 1(a) and Fig. 1(b), respectively. In spite of the method of drying, there is not much difference in the surface morphologies of the TiO₂ NRAs. Nevertheless, the cross-sectional morphology of the air-dried TiO₂ NRAs indicates collapse of the nanorods and destruction of morphology, probably induced by surface tension effect. In contrast, the vertically-aligned nanorod arrays are well preserved for the freeze-dried TiO₂ NRAs since this methodology enables removal of water by sublimation and desorption under vacuum, which avoids solid/liquid interfaces and eliminates the surface tension effect. During freeze-drying, localized energy generated through intermolecular heat transfer enables breaking of the Ti-Cl bonds, desorption and collision of chlorine atoms, resulting in the formation of molecular chlorine. Thus, the freeze-drying post-treatment leads to a “clean” TiO₂ surface with a well-defined morphology of NRAs. The length of the air- and freeze-dried TiO₂ NRAs are ~7.1 μm and ~7.8 μm, respectively. In spite of a slight difference in their colour shade, no apparent difference is observed in their crystallinity.

Fig. 1 Surface and cross-sectional (top insets) SEM images of TiO₂ NRAs obtained by: (a) air-drying; and (b) freeze drying methods (bottom insets: optical images)

The air- and freeze-dried TiO₂ NRAs show a similar UV-visible absorption spectra before loading the dyes. However, after loading the dyes, the freeze-dried TiO₂ NRAs exhibits a decrease in absorption when compared to that of the air-dried ones (Fig. 2(a)). The amount of dye loaded in air-dried TiO₂ NRAs is 69.6 nmol/cm² while for the freeze-dried TiO₂ NRAs it is decreased to 44.3 nmol/cm², following a decrease in its surface area by ~ 36%. However, the perseverance of the ordered nanostructures enables the freeze-dried TiO₂ NRAs to exhibit an enhanced visible-NIR light-scattering performance when compared to that of the air-dried ones (upper inset of Fig. 2(a)). The band gaps of freeze- and air-dried TiO₂ NRAs are 2.91 eV and 2.95 eV, respectively (bottom inset of Fig.2(a)). The flatband potential (E_fb) of the freeze-dried TiO₂ NRA shows a negative shift (~ 0.05 V) when compared to that of the air-dried one. The donor density (N_d) of the freeze-dried TiO₂ NRA is slightly increased when compared to that of the air-dried TiO₂ NRA` from 0.53×10¹⁷/cm³ to 0.6×10¹⁷/cm³. The schematic energy level diagram (Fig.2(b)) indicates that the negative shift in CB and E_F as well as disappearance of the deep acceptor level of freeze-dried TiO₂ NRAs are due to the removal of the adsorbed species that could facilitate charge separation and transport.

Fig. 2 (a) UV-Vis-NIR absorption of the TiO₂ NRAs formed on FTO substrates, (top inset: diffused reflectance spectra; bottom inset: Kubelka-Munk function vs. energy); and (b)  schematic energy level diagrams

The sharp decay in photocurrent with in a second observed for the freeze-dried TiO₂ NRAs when compared to the prolonged duration of decay of photocurrent over 100 s (Fig. 3(a)) observed for the air-dried one suggests the occurrence of an efficient charge extraction across the freeze-dried TiO₂/electrolyte interface. The model dye sensitized solar cells device (top inset of Fig. 3(b)) fabricated using air- and freeze-dried TiO₂ NRAs and the corresponding J-V curves are shown in Fig. 3(b). The PV parameters of the device fabricated using air-dried TiO₂ NRAs are as follows: open-circuit voltage (V_oc) = 0.70 V, short-circuit current density (J_sc) = 6.68 mA/cm², fill factor (FF) = 66%, and power conversion efficiency (PCE) = 3.08%. For the device fabricated using freeze-dried TiO₂ NRAs, the PV parameters are improved as follows: V_oc= 0.68 V, J_sc=8.11 mA/cm², FF= 65%, and PCE=3.60%. The improvement in the PCE of freeze-dried TiO₂ NRAs by ~20% is mainly due to an increase in its J_s.

Fig. 3 (a) Photocurrent decay curves (inset: parallel capacitance vs. applied potential plots); (b) J-V curves (top inset: model of DSSCs using TiO₂ NRAs)

Freeze-drying post-treatment of TiO₂ NRAs preserves the vertically-aligned nanoarchitecture and provides a clean surface, which helps to improve the electronic properties and the performance of the solar cell.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: P. Zhong et al., Freeze-drying as a novel approach to improve charge transport in titanium dioxide nanorod arrays, ChemElectroChem 10.1002/celc.201700572