Fabrication and application of macroscopic nanowire aerogels

Yutong Niu a, Fuzhong Li a, Wuxi Zhao a and Wei Cheng *ab
aCollege of Materials, Xiamen University, 422 Siming South Road, Xiamen, Fujian 361005, China. E-mail: weicheng@xmu.edu.cn
bFujian Key Laboratory of Materials Genome, Xiamen University, China

Received 31st December 2020 , Accepted 22nd March 2021

First published on 23rd March 2021


Abstract

Assembly of nanowires into three-dimensional macroscopic aerogels not only bridges a gap between nanowires and macroscopic bulk materials but also combines the benefits of two worlds: unique structural features of aerogels and unique physical and chemical properties of nanowires, which has triggered significant progress in the design and fabrication of nanowire-based aerogels for a diverse range of practical applications. This article reviews the methods developed for processing nanowires into three-dimensional monolithic aerogels and the applications of the resultant nanowire aerogels in many emerging fields. Detailed discussions are given on gelation mechanisms involved in every preparation method and the pros and cons of the different methods. Furthermore, we systematically scrutinize the application of nanowire-based aerogels in the fields of thermal management, energy storage and conversion, catalysis, adsorbents, sensors, and solar steam generation. The unique benefits offered by nanowire-based aerogels in every application field are clarified. We also discuss how to improve the performance of nanowire-based aerogels in those fields by engineering the compositions and structures of the aerogels. Finally, we provide our perspectives on future development of nanowire-based aerogels.


image file: d0nr09236c-p1.tif

Wei Cheng

Dr Wei Cheng is currently an associate professor in the department of materials science and engineering at Xiamen University in China. He received his Ph.D. in materials science in 2016 from the Swiss Federal Institute of Technology in Zurich (ETH Zurich). After working as a postdoctoral research fellow at the University of British Columbia (UBC) in Canada, he joined Xiamen University as a principal investigator in 2019. His research interests are focused on inorganic nanomaterials for electrochromics, catalysis and energy storage.


1. Introduction

Aerogels, defined as solid three-dimensional (3D) networks whose pores are filled with air, show large surface area (>100 m2 g−1), low density (0.0011 to ∼0.5 g cm−3), and high porosity (>95%).1–3 The origin of aerogels dates back to 1931 when Kistler reported on the replacement of the liquid in inorganic/organic gels by gas with negligible shrinkage using supercritical drying.4 This original work did not attract too much research attention on aerogels until 1970s and '80s when silica aerogel found practical use as Cherenkov detectors in particle accelerators and superinsulators in double-pane windows.1 Since then, a variety of materials including oxides, metal sulfides, carbon materials, and polymers have been prepared in the form of aerogels by a sol–gel chemistry process followed by supercritical or freeze drying.1–3 These conventional aerogels are usually of amorphous and brittle nature and their compositions and morphologies are hard to control, leading to limited functionality which is a major roadblock for their wide range applications.

The assembly of crystalline nanoparticles into macroscopic porous 3D networks represents an effective approach to achieve crystalline aerogels with tunable compositions and morphologies, which enriches the properties and applications of aerogels.5–12 Brock's group pioneered the research on nanoparticle aerogels.5,13,14 In 2005, her group reported the first nanoparticle aerogels by gelling metal chalcogenide nanocrystals into 3D networks via the partial removal of coating ligands on the nanoparticles through chem/photo-oxidation of capping groups.13 The resultant macroscopic aerogel monoliths exhibit characteristic features of aerogels and also preserve the integrity as well as the optical properties of the original chalcogenide nanobuilding blocks. Since the discovery of nanoparticle aerogels, a vast amount of metal chalcogenide, metal, and metal oxide nanoparticle-based aerogels have been prepared by controllable destabilization of the corresponding concentrated nanoparticle suspensions.5,7,8 More importantly, the idea of assembling preformed nanoparticles into aerogels has made it possible to control the compositions and structures of aerogels by tuning the compositions, sizes and morphologies of primary nanobuilding blocks and by engineering the gel structures.10–12 As a result, the aerogels prepared from nanoparticles showed significantly enhanced optoelectronic, electro/photo catalytic, and sensing properties in comparison to primary nanobuilding blocks.10–12 Since 2007, many comprehensive reviews regarding nanoparticle aerogels have been published, covering topics on the preparation and application of metal chalcogenide, metal and metal oxide nanoparticles aerogels5,7–9,11,15,16 as well as structure and property control of nanocrystal aerogels at nano-, micro- and macroscales.10,12,17,18 However, most of the aerogels were produced by using zero-dimensional (0D) spherical nanoparticles as building blocks and the weak interparticle force throughout the nanoparticle networks rendered the resultant aerogels too fragile to be integrated into a practical device without destroying their structural integrity.8–12 In addition, the ligands coating on nanoparticles are essential for making stable nanoparticle dispersion that is critical for gelling the nanoparticle into 3D networks but are detrimental to the application of the nanoparticle aerogels because of the poor electrical conductivity of the organic ligands.8–12

In recent years, in order to address the brittleness and poor conductivity of the aerogels based on 0D nanoparticles, much more attention has been paid to construct aerogels using 1D anisotropic nanostructures with high aspect ratios such as nanowires/nanotubes/nanofibers as building blocks, because the high aspect ratios of these nanostructures may enable self-cross linking that could potentially improve the mechanical properties and electrical conductivity of the aerogels.19–27 In 2007, Bryning et al. reported creation of mechanically strong and conducting carbon nanotube aerogels by supercritical or freeze drying of aqueous nanotube gel precursors.19 This work has aroused intensive research interests on aerogels made of 1D carbonaceous nanomaterials such as carbon,28–32 cellulose,33,34 and amyloid nanofibers35,36 as well as non-carbonaceous inorganic nanowires.24,25,37–42 Due to the large volume of existing literature studying carbonaceous aerogels that has already been reviewed by many other groups,43,44 this article only focuses on aerogels made from non-carbon inorganic nanowires.

In 2012, Kong's group found that when the concentration of the nanowire dispersion reached a threshold value, the nanowires would self-cross into 3D networks, resulting in the formation of elastic nanowire aerogels with excellent electrical conductivity.24 This seminal work has inspired the development of a significant amount of nanowire aerogels including various ceramic, metal, metal oxide, metal phosphate nanowire aerogels.26,38,40,41,45–49 Different from the typical application of macroscopic-scale 2D assembled nanowires in thin-film based electronic devices such as transistors, sensors and photodetectors,50–52 the intrinsic properties of anisotropic nanowire building blocks combined with the structural characteristics of aerogels have promoted wide application of aerogels in many emerging fields including thermal insulator,53 energy storage and conversion,54 catalysis,27 pollutants removal,55 pressure sensor,37 and solar steam generation.56 As a matter of fact, the literature regarding nanowire aerogels is growing rapidly. However, there is a lack of a progress report summarizing the latest developments in nanowire aerogels and pointing out the future directions for nanowire aerogels.

Herein, our review article provides a comprehensive summary of recent advances in nanowire aerogels and formulates guidelines for the design and fabrication of mechanically strong nanowire aerogels for various applications. We summarize the methods that have emerged so far for gelling nanowires into 3D networks that were converted to aerogels by a drying step. We also discuss the gelation mechanism involved in each method and compare the advantages and disadvantages of each preparation method. We further provide a thorough review of the nanowire aerogels in a diverse range of applications and analyze how the nanowire aerogel can make a difference in each application field. In the end, we point out the challenges persisting in the way of further development of nanowire aerogels and work out guidelines that may significantly move forward the field of nanowire-based aerogels.

2. Fabrication of nanowire aerogels

Starting from pre-synthesized nanowires, there are typically two steps for the preparation of nanowire aerogels: gelation of nanowires into macroscopic 3D wet-gel monoliths and removal of solvents trapped in the wet gels without structural shrinkage (Fig. 1). Several gelation methods including template-based gelation,57 solvent-evaporation induced gelation,24in situ gelation,25 centrifugation induced gelation,58 and cross-linker induced gelation59 have been proposed to assemble nanowires into macroscopic wet-gels that can be converted to aerogels by drying methods such as freeze-drying and supercritical drying. In this section, we provide detailed discussions on the gelation methods and drying methods.
image file: d0nr09236c-f1.tif
Fig. 1 Typical procedures for fabrication of nanowire aerogels.

2.1 Template-based gelation

Template-based gelation method is a rational way for designing and constructing 3D nanowire aerogels with controlled porosities and structures, due to the possibility to choose and engineer the templates according to our demands. Several template-based methods including ice-templating, emulsion templating and biotemplating have been developed for making nanowire aerogels with controllable architectures.

Ice-templating gelation is the most frequently used template-based method for the assembly of nanowires into macroscopic 3D networks, owing to its simplicity, high efficiency, and controllability. This method typically involves the formation of ice as a template by freezing nanowire dispersion and sublimation of ice under vacuum conditions, which leads to the formation of a highly porous percolated network of nanowires connected by the van der Waals force. In 2013, Tang et al. successfully fabricated Cu nanowire aerogels with ultralow density by freezing the Cu nanowire aqueous dispersion followed by freeze-drying.23 The freezing process occurred in a freezer that provided a homogeneous cooling atmosphere under which the solidification of the solvent converges inward, resulting in the formation of aerogels with spherical pore structures. By tuning the concentration of the nanowire aqueous suspension, the density of the Cu nanowire aerogels can be optimized to be as low as 4.6 mg cm−3 that corresponds to 0.05% of the density of bulk copper and the porosity of the Cu nanowires can be as high as 99.91%. The same strategy has also been used by Cao's group for processing PVP-coated Cu nanowires into 3D aerogels with tunable porosities.60,61 These Cu nanowire@PVP aerogels can be converted to highly robust and stable Cu nanowire@graphene core–shell aerogels by an annealing step under an inert atmosphere. Importantly, pore sizes of the nanowire aerogels made by ice-templating are tunable by changing the solvent composition of the nanowire dispersion. For example, Qian et al. studied the effects of solvent composition on the porosity of gold nanowires made by the ice templating method.62 They found that the aerogels made with pure water as the solvent exhibited large pores with sizes in the range between several tens to hundreds of microns, while, addition of tert-butanol (TBA) to water led to the formation of aerogels with small pores in the size range of several microns. They claimed that the addition of TBA promoted the nucleation of ice but inhibited the crystallization of water, leading to the formation of more ice crystals with smaller crystallite sizes. By engineering the porosity, they were able to achieve gold nanowire aerogels with ultralow density of 6 mg cm−3 which is the lowest density value reported so far for gold materials.

The ice-templating method can also be used for the preparation of nanowires aerogels with highly anisotropic pore structures originating from the anisotropic growth of ice crystals caused by the gradient temperature that exists in the freezing process. When a nanowire suspension is frozen by placing on a precooled metal block in liquid nitrogen, the ice crystals rapidly nucleate at the bottom, then grow upward, forming parallel crystal pillars (Fig. 2a). The nanowires are squeezed into the interstitial places between the pillars, forming interconnected lamellar architectures. After lyophilization, the ice pillars are removed by sublimation, resulting in the formation of aerogels with anisotropic cellular pore structures (Fig. 2b and c). This directional free-casting method has been successfully used for producing light-weight freestanding Ag nanowire57 and TiO2 nanofiber aerogels27 with hierarchically ordered, cellular architectures (Fig. 2c). Based on the anisotropic growth of ice, Feng's group developed a bidirectional freeze-casting technique for achieving hierarchical Ag nanowire aerogels with two levels of structures: nanoribbons of interlacing Ag nanowires and 3D networks of the nanoribbons.63 Nyström's group reported the fabrication of Ag nanowire/nanocellulose aerogels with lamellar and honeycomb-like pore structures using bidirectional and unidirectional freeze-casting method respectively.64


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Fig. 2 (a) The processes for preparation of nanowire aerogels by a typical ice templating method. Photographs and SEM images of (b) Ag nanowire57 and (c) TiO2 nanowire27 aerogels prepared by the ice templating method.

In the conventional ice-templating method, the nanowire building blocks are inhomogeneously distributed in the aerogels, making the properties of the aerogels less reproducible and reliable. To address this issue, Wong's group developed an emulsion template technique for constructing nanowire aerogels with isotropic and homogeneous hierarchical architectures.42 The emulsion templates were produced by intensively shaking mixtures of immiscible oil (cyclohexane) and water. Then, Ag nanowires and the stabilizer polyvinyl alcohol (PVA) were added to the mixtures. Afterwards, the nanowires could self-assemble around the spherical oil droplets with uniform distributions, resulting in the formation of a stable emulsion. Freeze-drying was applied to the emulsion to get rid of oil and ice, eventually leading to the formation of highly porous percolated nanowires with homogeneous and isotropic microstructures. In addition to the emulsion template method, a bio-template method has been developed by MacLachlan's group for the fabrication of light-weight alumina nanofiber aerogels.65 In this method, the biomaterial chitosan nanofibrils as templates were mixed with Al3+ aqueous solution to produce Al-chitosan hydrogels that can be converted to an aerogel by freeze-drying. Thermal decomposition of the composite aerogels yielded Al2O3 aerogels that possessed similar structural organization as that of the chitosan nanofibril aerogels.

2.2 Solvent-evaporation induced gelation

Solvent-evaporation has been used as a facile and efficient strategy for self-assembly of nanoparticles into ordered micro- or macroscopic structures.66,67 Kong's group successfully used this method for the assembly of nanowires into disordered 3D networks.24 They found that continuous evaporation of the solvent in a nanowire dispersion would concentrate the dispersion to a point where the nanowires start interlinking together by van der Waals interaction and the liquid stops flowing, resulting in the formation of a wet-gel which can be converted to aerogel by supercritical drying (Fig. 3a). This solvent induced gelation method represents a generic approach for the assembly of pre-formed nanowires into 3D monolithic architectures and has successfully been used for making Ag, Si, and MnO2 nanowires and carbon nanotube aerogels (Fig. 3b).24 The key of this method lies in the formation of homogeneous and stable dilute nanowire dispersion and slow evaporation of the solvent. During the evaporation process, the nanowire dispersion should always be stable and the evaporation rate should be slow enough to avoid precipitation of nanowires from the suspension, before reaching the gelation concentration. Both factors are critical for the formation of evenly distributed 3D nanowire networks with well-dispersed pore structures.
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Fig. 3 (a) Schematic illustration of fabrication of nanowire aerogels by solvent-induced gelation followed by critical point drying. (b) Photographs of colloidal suspensions, wet gels, surfactant-removed gels and aerogels of Ag and MnO2 nanowires.24

2.3 In situ gelation

The in situ gelation method is a one-step approach that combines nanowire synthesis and self-assembly of nanowires into 3D networks, thus avoiding the tedious procedures to synthesize nanowires, make nanowire dispersions and gel nanowires into 3D networks. In this method, the reactions between molecular precursors yield nanowires that cross-link into 3D wet gels via van der Waals forces when the concentration of the formed nanowires reaches a threshold value. In 2013, Kong's group for the first time found that manganese oxide and titanium oxide nanowire hydrogels/aerogels can be produced by a one-step hydrothermal synthesis.25 During the hydrothermal treatment, the nanowires grew out from the homogeneous precursor solution and started connecting to each other to form hydrogels when the length and concentration of the nanowires reached a critical value (Fig. 4a). The resultant hydrogels were converted to aerogels by a supercritical drying step. The obtained aerogels are flexible and bendable (Fig. 4b). They are composed of highly crystalline nanowires with extremely high aspect ratios that enable the interconnection of the nanowire building blocks and formation of highly porous networks with pore sizes ranging from hundreds of nanometers to a few micrometers (Fig. 4c–e). The porosity and density of the aerogels are controllable by changing the initial concentrations of the precursors and reaction times. Inspired by this work, Yang's group68 and Feng's group69 independently produced monolithic Ag nanowire hydro/aerogels by a one-step solvothermal treatment of concentrated silver salt precursor ethylene glycol solutions. The concentration of the precursor solution should be much higher than for normal nanowire synthesis, otherwise the hydrogels cannot be formed. In addition, the one-step hydrothermal treatment method has also been used for synthesizing monolithic tungsten oxide nanowire aerogels.70
image file: d0nr09236c-f4.tif
Fig. 4 (a) A scheme showing in situ formation of nanowire wet-gel during a hydrothermal synthesis and the transformation of wet gel to aerogel by critical point drying. (b) Flexible cryptomelane-type manganese oxide (K2−xMn8O16) nanowire aerogel prepared by in situ gelation method.25 (c) SEM image of the nanowire aerogel. (d and e) TEM and HRTEM images of the nanowires.

Kong's group extended the idea of in situ gelation to realize the fabrication of Cu nanowire aerogels by heating the aqueous suspension of the precursor mixtures (NaOH[thin space (1/6-em)]:[thin space (1/6-em)]Cu(SO4)[thin space (1/6-em)]:[thin space (1/6-em)]ethylenediame:hydrazine) and the metallic copper nanowires started growing out of the suspension.40 With the reaction going on, the length and concentration of the nanowires increased, then the nanowires started connecting each other, and eventually formed 3D monolithic networks. The interconnected nanowire monoliths floated on top of the solution, which might be associated with the entrapped gases produced during the synthesis. The in situ formation of highly porous 3D copper nanowire networks can be explained by a “bubble-controlled assembly” mechanism proposed by Sun's group.71 They found the bubbles were generated from sonication of hydrazine and the reactions between Cu2+ and N2H4 that yielded a significant amount of N2. During the heating treatment, the bubbles floated upward due to the decreased viscosity caused by heating, meanwhile, the copper nanowires started to grow out of the suspension and self-assembled on the surface of the bubbles to form sol flocks which eventually stacked to form monolithic hydrogels at the solution–air interface. The porosities of the gels were determined by the bubble concentrations that can be adjusted by tuning the reaction parameters such as heating temperature and concentration of the reactants.

2.4 Centrifugation-induced gelation

The centrifugation induced gelation strategy exploits centrifugal force to move nanowires dispersed in solvent to the bottom of the centrifugation tube and enable efficient contact between the nanowires via the van der Waals force, forming 3D networks with the solvent trapped in the pores between the interlinked nanowires (Fig. 5a).58 The obtained nanowire wet gels can be detached from the tube by adding a solvent with higher density than that of the solvent used for dispersing the nanowires. By using this centrifugation induced gelation method, it is possible to fabricate monolithic tungsten oxide nanowire aerogels that display fluffy and porous networks composed of distinguishable nanofibers that are arranged into circular structures (Fig. 5b–d). These tungsten oxide nanowires show a high surface area of 157 m2 g−1 that is more than 200 times higher than that of the corresponding nanowire powders, which could potentially promote the application of tungsten oxides in areas such as photocatalysis and gas sensing where surface areas are critical. In addition, Liu et al. used the centrifugation method to concentrate manganese oxide octahedral molecular sieve nanowire dispersion to form a slurry at the bottom of the centrifugation tube. The slurry can be converted to 3D free-standing aerogels with robust mechanical properties by a freeze-drying step.72 The centrifugation induced gelation is not only effective for gelling 1D nanowires into 3D monolithic architectures but also able to assemble 2D nanosheets into highly porous 3D networks.73
image file: d0nr09236c-f5.tif
Fig. 5 (a) Production of nanowire aerogels by centrifugation induced gelation of nanowires followed by critical point drying. (b) A photograph and (c and d) SEM and TEM images of a macroscopic W18O49 nanowire aerogel monolith produced by centrifugation induced gelation method.58

2.5 Cross-linker induced gelation

The cross-linking reaction between the capping ligands on the surface of particles is a typical route towards connecting the nanoparticles to 3D networks in nanoparticle dispersions.5,74 Inspired by the concept of cross-liking, Liu et al. developed a universal strategy for gelling a 1D nanowire suspension using 2D graphene oxide and MXene nanosheets as physical cross-linkers.59 As shown in Fig. 6a, when the content of nanowires in solvent reaches a certain concentration, the addition of a small amount of the nanosheets can trigger the gelation by knotting-tying and wrapping around the junctions between the nanowires via van der Waals forces, leading to the formation of dynamically stable nanowire wet-gels. A freeze-drying step can convert the wet-gels to aerogels that show 3D networks composed of well-distributed and interconnected nanowires with intersections wrapped by flexible nanosheets. This cross-linker induced gelation method has been utilized for producing a variety of functional nanowire aerogels including conducting Ag nanowires, semiconducting ZnO nanowires, MnO2 nanowires and their mixtures (Fig. 6b).
image file: d0nr09236c-f6.tif
Fig. 6 (a) Schematic of gelation of nanowires into 3D networks by using graphene oxide nanosheets as cross-linkers. (b) SEM images of aerogels of Ag, ZnO, MnO2 nanowires and mixture of Ag and MnO2 nanowire produced by cross-linker induced gelation followed by freeze drying.59

2.6 Comparison of gelation methods

Table 1 provides a comprehensive comparison of all the aforementioned gelation methods in terms of ‘universality’, ‘simplicity’ and ‘scalability’ of every method by itself and the ‘porosities’ and ‘mechanical properties’ of the nanowire aerogels obtained by every gelation method. More details regarding the advantages and disadvantages of every method are summarized as follows:

(1) The template-based gelation method is a versatile strategy to prepare various nanowires into aerogels with tunable porosities and densities and is potentially a scalable method, but it typically involves a template removal step which may destroy the structures of the aerogels or introduce impurities due to the incomplete template removal.

(2) Solvent-induced gelation method is a straightforward, simple, but time-consuming method for gelling nanowires. It is difficult to use this method to control the porosities and microstructures of the nanowire aerogels. It also requires surfactants to be coated on nanowires to achieve a stable and homogeneous suspension, which might be detrimental to the application of aerogels.

(3) In situ gelation method is the only method that directly converts molecular precursors to desired nanowire gels, while all other methods used preformed nanowires as precursors. It is possible to prepare mechanically robust 3D nanowire networks with controllable porosity and density simply by tuning the in situ synthesis conditions. But, it is only suitable for preparing specific materials in the form of nanowire aerogels. Due to the possibility of unreacted precursors trapped in the 3D networks, the purity of the final aerogels cannot be guaranteed.

(4) Centrifugation-induced gelation is a simple and fast gelation method. This method does not require the preparation of a stable nanowire suspension, thus surface-coating of ligands or polymers on nanowires is not necessary, which guarantees the high purity of the final aerogels. But this method cannot be used for tuning the porosity and density of the aerogels. This method requires a high speed centrifuge that limits the preparation on a large scale.

(5) Cross-linker induced gelation method is a general way to gel nanowires. The obtained wet-gels are dynamically stable and suitable for post-processing such as 3D printing that allows for making nanowire aerogels with various shapes. But it is impossible to use this method to obtain pure nanowire gels, because it always needs additional linkers to connect the nanowires.

Table 1 Comparison of advantages and disadvantages of different gelation methods
Method Universality Simplicity Scalability Purity Controllable porosity Mechanical property
★, ★★, ★★★ represents the low/weak, medium and strong/high, respectively.
Template-based gelation ★★★ ★★ ★★★ ★★★ ★★★
Solvent-evaporation induced gelation ★★★ ★★ ★★ ★★
In situ gelation ★★★ ★★ ★★★
Centrifugation induced gelation ★★ ★★★ ★★★ ★★
Cross-linker induced gelation ★★★ ★★ ★★ ★★ ★★


Overall, these gelation methods provide a useful tool box for connecting the 1D nanowires into macroscopic 3D networks. Every gelation method has its own advantages and disadvantages. There is no such a perfect method that meets all the criteria listed in Table 1, indicating much more research efforts should be dedicated to searching for new gelation methods for the assembly of nanowires into robust 3D networks.

2.7 Drying methods

Freeze drying and critical point drying are two typical drying methods used for removing the solvents in the wet-gels without structural collapse.7 Freeze drying typically involves two steps: freezing the wet-gel at a temperature below the freezing point of the solvent trapped in the wet-gel and sublimation of the frozen solvent under vacuum conditions. The sublimation process avoids structural damage that is usually caused by capillary force originating from the surface tension of the liquid–air interface in a conventional drying process. Due to the easy availability and good understanding of the crystallization process, water is the most frequently used solvent in the freeze-drying method. We have already provided a detailed discussion on water-based freeze drying in Section of 2.1. Water-based freeze-drying is a low-cost and scalable drying method. But this method is time-consuming, owing to the slow sublimation of ice and the crystallization process of water may cause structural damage to the 3D networks of gels. In order to address these challenges, Ren et al. replaced water in the metal oxide wet-gels with organic solvents such as acetonitrile and tert-butanol that have relatively low surface tension, high freezing point and high vapor pressure, then directly dried the wet-gels under vacuum conditions.75 They found that during the drying process the solvent at the outer surface evaporated first and this endothermal evaporation process instantly led to the decrease of the internal temperature of the gel to a point below the freezing point of the solvent, resulting in the solidification of partial solvents that sublimated quickly. This continuous evaporation–solidification–sublimation process eventually removed all solvents in the wet gels, leading to the formation of final aerogels with integrity. This efficient drying method was named the “organic solvent sublimation drying (OSSD)” method. Compared with the water-based freeze drying method, the drying speed of OSSD method is faster and the effects of structural fraction are also less obvious. But the aerogels obtained by this method suffer from certain extent of structural shrinkage.75 In comparison with freeze drying, supercritical drying is a less invasive way to remove the solvent from the wet-gels.7,76 In this method, the pressure and temperature have to be raised to a supercritical point where the liquid phase becomes the supercritical fluid in which the liquid–gas phase boundary disappears, thereby eliminating the structural damaging effect caused by the capillary force. Among all the fluids that have suitable critical drying point for supercritical drying, CO2 is the most widely used one, because of its safety, low-cost and relatively low pressure and temperature for the critical point (31.1 °C and 73.8 bar). The CO2 supercritical drying method is the most efficient way to remove the solvent from the wet-gel with minimum volume changes, but this method typically involves sophisticated equipment and time-consuming solvent exchange process, limiting the preparation of aerogels on a large scale.

2.8 Other methods

In addition to the aforementioned preparation methods that typically involved the gelation of nanowire suspension followed by a drying step to achieve 3D aerogels, there are also some methods that can directly synthesize nanowire aerogels from precursors without experiencing the gelation step. For example, Wang et al. developed blow-spin techniques to prepare nanowire aerogels.41 In this method, the precursor solution of the metal–organic species mixed with polyvinyl pyrrolidone (PVP) was pumped into a nozzle, simultaneously, air flowed through a concentric outer nozzle, blowing the precursor solution out to form nanofibers with diameters in the range of hundreds of nanometers. The nanofibers were ejected to an air-permeable cage-like collector, forming highly porous metal–organic/PVP nanofiber aerogels that can be converted to ceramic metal oxide nanofiber aerogels by calcination. This technique is suitable for the fabrication of a variety of ceramic metal oxide nanofiber aerogels such as TiO2, ZrO2, yttria-doped ZrO2(YSZ) and BaTiO3. In addition, Lei Su et al. reported a chemical vapor deposition method for directly making SiC nanowire aerogels.38 This method contains three steps: nucleation of SiC on graphite substrates and growth into nanowires, continuous feeding of precursors that leads to the nucleation and growth of nanowires on pre-existing SiC nanowires to form porous 3D nanowire architectures, and detachment of the nanowire aerogels from the graphite substrates and removal of carbon residues by annealing in air.

3. Application of nanowire aerogels

3.1 Nanowire aerogels as thermal insulators

Thermal insulating materials play a very important role in energy conservation and thermal protection of buildings, vehicles, aerospace crafts, and industrial machinery.77–79 Aerogels are so far the most effective thermal insulating materials, due to their extremely low thermal conductivity originating from the high porosity and extremely low density.2 For example, the mostly studied and widely used SiO2 aerogels show thermal conductivity from 0.012 to 0.02 W m−1 K−1 that is even lower than the thermal conductivity of air (0.023 W m−1 K−1).2 However, conventional oxide ceramic aerogels prepared by sol–gel methods suffer from brittleness and volume shrinkage at elevated temperatures that limit the practical application of aerogels as thermal insulators under extreme conditions.80,81 In order to overcome this issue, recent research attentions have been attracted to construct heat-insulating aerogels composed of 1D ceramic materials with high aspect ratios. The interconnected 3D networks of highly robust and flexible nanowire/nanofiber building blocks could endow the aerogels with outstanding mechanical properties that can withstand reversible large-strain compression, meanwhile, tuning the compositions and pore structures of the aerogels could enhance the thermal stability and heat insulating properties.

Many types of 1D nanostructured ceramic materials including oxide ceramic nanofibers,26,41,45,82 SiC nanowires,38,53,83 Si3N4[thin space (1/6-em)]84 and BN85 nanobelts have been prepared to the form of aerogels that show temperature-invariant superelasticity, low thermal conductivity, and fire resistance. In 2018, Ding's group prepared ceramic nanofibrous aerogels (Fig. 7a) by freeze drying of mixtures of water, SiO2 nanofibers, polyacrylamide, and aluminoborosilicate (AlBSi) sol, followed by calcination in air at 900 °C.26 The calcination led to the decomposition of organic polyacrylamide and the formation of amorphous AlBSi that cemented the adjacent SiO2 fibers. The obtained ceramic aerogels show lamellar architectures that are composed of interconnected flexible SiO2 nanofibers cross-linked by AlBSi. The aerogels display a low thermal conductivity of 0.032 W m−1 K−1 (density = ∼0.5 mg cm−3), making them an excellent heat insulation material. As shown in Fig. 7b, when the aerogel was heated on a heating stage at 350 °C for 30 min, the top side of the aerogel monolith was maintained at a low temperature in the range from 50 to 63 °C. The aerogels also showed temperature-invariant superelasticity (elastic resilience >80% strain) up to 1100 °C (Fig. 7c). The same group also used a similar method to fabricate hierarchical cellular structured SiO2 nanofibrous aerogels that are composed of intertwined SiO2 nanofibers and SiO2 nanoparticles (the nanofibers and nanoparticles were bonded by SiO2 sol).82 This hierarchical aerogel shows temperature-invariant superelasticity in the wide temperature range from −196 to 1100 °C and thermal conductivity of (0.032 W m−1 K−1 (density = ∼0.2 mg cm−3). Due to the similar architectures, the mechanical stability is quite similar with that of the SiO2 nanofibrous aerogels mentioned above. The much lower thermal conductivity was due to the introduction of SiO2 nanoparticle aerogels into the SiO2 nanofibrous aerogels. On the one hand, phonon transportation along the nanoparticle networks was significantly hindered by the interface between nanoparticles, resulting in the reduction of the thermal conductivity of the solid phase. On the other hand, the small pore sizes in the nanoparticle aerogels enable more frequent collisions between molecules in air and solid networks, leading to the decrease of thermal conductivity of the gas phase.


image file: d0nr09236c-f7.tif
Fig. 7 (a) A photograph of a ceramic SiO2 nanofibrous aerogel on the tip of a feather.26 (b) Photograph and infrared images of a SiO2 nanofibrous aerogel on a heating stage at 350 °C for 30 min. (c) Photographs showing compression and recovery of nanofibrous aerogel in the flame of a butane blow torch. (d) A photograph of a monolithic SiC nanowire aerogel on a dandelion.38 (e) A photograph showing high compressibility of the SiC nanowire aerogel. (f) Photographs of a fresh petal on a 10 mm thick SiC aerogel heated on an alcohol lamp for 10 minutes. (g) Photographs of the as-prepared SiC@SiO2 nanowire aerogel before and after heating at 1000 °C for 30 min.53 (h) An infrared image showing the temperature distribution on the surface of the composite aerogel heated by a butane blow torch. (i) Images showing the compressibility of the composite aerogel during heating by the flame of a butane blow torch.

Because SiC has better high-temperature chemical stability and heat resistance than oxide ceramic materials, SiC nanowire aerogels have also been pursued as thermal insulator materials. Wang's group developed a chemical vapor deposition method for fabricating SiC nanowire aerogels (Fig. 7d) that are constructed of randomly interwoven nanowires with diameter in the range of 50–70 nm and length up to hundreds of micrometers.38 The obtained aerogels exhibit density of ∼5 mg cm−3, chemical stability at temperatures up to 1000 °C, large strain (>70%) recoverable compressibility (Fig. 7e) and low thermal conductivity of about 25 W m−1 K−1. The low thermal conductivity enables the SiC nanowire aerogel to prevent the fresh pedal from burning by an alcohol lamp (Fig. 7f). Very recently, the same group prepared SiC@SiO2 nanowire aerogels (Fig. 7g) that show anisotropic and hierarchical architectures with well aligned tubular pores (the side length is about 25 μm) using directional freeze casting method and subsequent heating at 1000 °C.53 The anisotropic microstructures give rise to anisotropic thermal conductivity of the aerogels. The aerogels show extremely low thermal conductivity of 14 W m−1 K−1 along the radial direction of the pores, in contrast to the value of 35 W m−1 K−1 along the axial direction. This is because the gas convection and solid thermal conduction along the axial direction favors heat transfer in the axial direction but decrease the thermal conductivity of gas phase in the radial direction, meanwhile, the interfacial thermal resistance is increased by the extra tortuous solid conduction path along the radial direction together with the large amount of phonon barriers generated at the SiC and a-SiO2 interface and at the stacking faults in the SiC core, thus resulting in the reduction of solid conduction in the radial direction. The anisotropic pore structures also lead to high stiffness (a specific modulus of 24.7 kN m kg−1) and recoverable radial compressibility (full recovery from 80% strain). The amorphous SiO2 coating on SiC improve the thermal stability of SiC so that the aerogel can withstand 1000 °C annealing in air (Fig. 7g) and 1200 °C in a fire-erosion environment (Fig. 7h and i).

3.2 Nanowire aerogels for energy storage and conversion

Nanowire-based aerogels are promising candidate electrode materials for energy storage and conversion devices like batteries because of their large surface area that provides a significant amount of electrochemically active sites, high porosities that facilitate the efficient contact between the electrolyte and electrode materials and tolerate the volume expansion and contraction caused by the insertion and extraction of ions, and nanosize of the primary nanobuilding blocks that shorten the diffusion length of charge carriers in the materials.46,69–73 In addition, the structural flexibility of the nanowire aerogels enables the integration of the aerogels into the device without deterioration of the integrity of the aerogels. For example, 3D free-standing Ag nanowire aerogels have been reported as effective host materials for encapsulating lithium by alloying between lithium and silver for a lithium metal-based battery.86 The high electrical conductivity of Ag nanowires and 3D porous architectures of the aerogels enabled homogeneous alloying, suppression of the infinite volume expansion of lithium during cycling, and prevention of the dendrite growth of lithium metal, making the Ag nanowire aerogels promising electrode materials for rechargeable lithium-metal based batteries. In spite of the great potential of nanowire-based aerogels for being electrode materials, it remains a challenge to incorporate electrochemically active metal oxide/sulfide nanowire-based aerogels into the electrodes of batteries for achieving high electrochemical performance because of their intrinsically poor electrical conductivity.

Cao's group proposed two strategies to tackle this issue.54,87,88 The first strategy is to directly coat electrochemically active electrode material on highly conductive pre-formed 3D carbon nanotube aerogels to achieve composite aerogels for electrode applications. They have successfully prepared coaxial carbon@MoS2 nanotube87 and carbon@TiO2 nanotube88 aerogels for anodes of lithium ion batteries. These free-standing composite aerogels are flexible and compressible. The robust mechanical properties together with the high electrical conductivity of the composite aerogels make it possible to integrate them into the anodes of the batteries without using polymer binder and conductive additives that are essential for conventional electrodes. Both composite aerogels showed high specific capacity with excellent cycling stability. The other strategy is to fabricate dual reticulate nanowire aerogels that are composed of electrochemically active nanowires and electrically conductive carbon nanotubes. They produced nanowire-based aerogels consisting of entwined FeS2 nanowires and carbon nanotubes.54 This composite aerogel showed electrical conductivity of 0.65 S cm−1 in comparison with the non-conductive nature of FeS2, maximum Young's modulus of 1.32 MPa that is almost 10 times the value of carbon nanotube aerogels (0.14 MPa). The high electrical conductivity and outstanding mechanical properties allowed them to compact the aerogels to produce free-standing electrodes for lithium ion batteries that displayed a high mass specific capacity of 1031 mA h g−1 at current density of 100 mA g−1 and high areal capacity of 10.0 mA h cm−2 at current density of 0.5 mA cm−2.

Decorating the nanowire aerogels with graphene or reduced graphene oxides represents another very important strategy to overcome the issue regarding the intrinsically poor electrical conductivity of the nanowire electrode materials. For example, flexible sodium titanate Na2Ti3O7(NTO)@reduced graphene oxide core–shell nanowire aerogels have been prepared for sodium ion storage (Fig. 8a).89 The composite nanowire aerogels show significantly enhanced capacity, rate performance, and cycling performance in comparison with the pristine NTO nanowires aerogels and the 3D NTO aerogels on graphene paper (Fig. 8b–d). Further studies indicate that a thin layer of reduced-graphene oxide wrapping on NTO nanotubes as well as the 3D open-pore architectures of the aerogels facilitate the rapid ion diffusion across the electrolyte and electrode materials (Fig. 8b) and prevents the volume expansion during the insertion and extraction of sodium ions into the NTO nanowires. In addition, V2O5 nanowire@graphene composite aerogels have been fabricated for magnesium ion storage, showing excellent Mg ion storage capability with a high specific capacity of 330 mA h g−1, and high cycling stability (80% percent of the original capacity was retained after 200 cycles).90 The outstanding Mg ion storage properties can be attributed to the synergistic effects provided by graphene decoration and 3D high porous structures of the aerogels.


image file: d0nr09236c-f8.tif
Fig. 8 (a) Photograph of sodium titanate nanowire@reduced graphene oxide core–shell (NTO@GCS) aerogel sitting on a dandelion.89 (b) Rate-capability and (c) Nyquist plots of NTO@GCS, NTO-GP (sodium titanate nanowire@graphene oxide paper) and pNTO (pristine sodium titanate) electrodes as anode materials for sodium ion batteries. (d) Long-term cycling stability of 3D NTO@GCS (red) and NTO-GP (blue) aerogels at current densities of 2 C and 4 C.

3.3 Nanowire aerogels as adsorbents

Air and water pollution have become a global environmental issue that seriously affects the health and life of mankind. Development of materials that can remove toxic species from water and air is one of the most effective strategies to solve this issue. Aerogels, due to their low density, large surface area and high porosities, represent a very promising type of adsorbing materials for purifying the waste water and polluted air.55,73,91 However, because of the brittleness, the capillary force generated during immersing aerogels into water could break the monolithic aerogels into smaller powders, making it hard to recover and reuse the aerogels for waste water treatment. It is also hard to integrate the brittle aerogels into an air-purification set-up without breaking its integrity. To tackle this issue, nanowire aerogels with robust mechanical properties have been proposed for purifying polluted air and water.

Nanowire aerogels have great potential for filtering polluted air and removing oil from the oil polluted water. It has been reported that hydrophobic hydroxyapatite (HAP) nanofiber aerogels prepared by freeze-drying (Fig. 9a–c) can filter PM2.5 and PM10 in polluted air with a removal efficiency of about 99% at high PM concentration up to 1800 μg cm−3, with a stable working life of 120 h (Fig. 9d–f) and a negligible pressure drop, suggesting HAP aerogels as a promising candidate material for air purifiers and breathing masks.55 In addition, the hydrophobic HAP aerogels exhibit high adsorption capacities (83 to 156 times the weight of the aerogels) for different non-water soluble solvents (oil) including chloroform, oleic acid, cyclohexane and so on. Most importantly, the elastic properties of the aerogel make it recyclable for oil-removal by adsorption-squeezing-adsorption. The authors also designed and made a separation device for continuous separation of oil–water by flowing the mixture of water and oil through the aerogels (Fig. 9g and h). Al2O3 nanotube aerogels have also been reported for the selective sorption of various oils from the oil–water mixtures, with adsorption capacities in the range of 31–73 g−1 depending on the oil type.92 Due to the outstanding mechanical properties of the nanotube aerogels, the aerogels can be reused after distillation and squeezing. Cu nanowire monolithic aerogels modified with (heptadecafluoro-1,1,2,2-tetradecyl)trimethoxysilane (FAS) exhibit superhydrophobic effect that enables the copper nanowire aerogels to quickly adsorb toluene with a maximum weight gain about 34 times the weight of the original aerogels, indicating that the FAS modified aerogel could be used as an adsorbent for oil-spill cleanup.23


image file: d0nr09236c-f9.tif
Fig. 9 (a) A photograph of hydroxyapatite (HAP) nanowire-based inorganic aerogel monolith. (b and c) SEM images of the aerogel. (d) Illustration of the filtration of PM2.5 in polluted air by using the HPA aerogel as a filer. (e) SEM image of the aerogel after filtration of PM2.5. (f) PM2.5 removal efficiencies of the HPA aerogel filter with a thickness of 9 mm for 120 h. (g) Illustration of the continuous oil–water separation set-up by using the HAP nanowire aerogel as the adsorbent. (h) Continuous separation of cyclohexane dyed with oil red from water.55

Nanowire aerogels have also been reported as recyclable absorbing materials for removing organic dyes and heavy metal ions from water. Wang's group reported that porous MnO2 nanowire hydrogels with negative surface charge can efficiently adsorb organic dyes such as methyl violet from water, due to the electrostatic forces between the macroscopic gels and the dyes.39 The monolithic hydrogel also shows selective adsorption to Pb2+ in contaminated water, possibly due to negative surface charge and the tunnel crystal structures of MnO2 fitting the size of the ionic radius of Pb2+. Kong's group made a water filter with the cryptomelane nanowire hydrogels.25 The filter can efficiently remove toxic pollutants such as organic dyes and heavy metal ions that are adsorbed by the hydrogels filter via electrostatic force between the filter and the dyes and metal ions when the contaminated water flowed through the filter.

3.4 Nanowire aerogels for electro/photocatalysis

Nanomaterial-based aerogels are of great importance to design high-performance electrocatalysts because they combine the beneficial features of the nanocatalyst (tunable electronic structures by engineering the structures of the nanobuilding blocks) and aerogels (more active sites and easier mass transfer provided by the large surface area and high porosity).11 In fact, 3D aerogels based on 0D metal nanoparticles have been developed for electrocatalyzing oxygen reduction reaction, fuel oxidation reaction, and water splitting reaction.11 All of these metal nanoparticle aerogels showed significantly enhanced electrocatalytic activities in comparison with the corresponding nanoparticles. However, due to the brittle nature and poor electrical conductivity, these nanoparticle aerogels have to be processed into electrodes using conductive supporting substrates and polymer binders, leading to the creation of an interface between the substrate and catalyst that limits electron transfer, reduces loading amount of the active catalyst, and complicates the procedures for preparing the electrodes.

These issues can be addressed by making nanowire aerogels with structural flexibility and mechanical strength that enables the preparation of self-supported catalysts without any additives. For instance, elastic TiO2 nanofibrous aerogels with hierarchically ordered and cellular architectures have been reported as self-supported electrocatalyst for catalyzing the electrochemical reduction of N2 to ammonia.27 The interconnected networks of the nanofibers together with the hierarchically ordered pore architectures ensure the superelasticity of the aerogels. Abundant oxygen vacancies (OVs) were introduced into the pristine TiO2 nanofiber aerogels by reacting the TiO2 aerogel with metallic lithium, significantly boosting the electrical conductivity of the TiO2 aerogels. The excellent conductivity and mechanical properties made it possible to integrate the TiO2 nanofiber aerogels into the electrolyzer without any conductive support. The DFT calculation indicated that the OVs facilitate the N2 adsorption and activation, thus tremendously improving the ammonia yield, faradaic efficiency, and long-term stability of the electrocatalyst.

Chang's group fabricated free-standing 3D Ag nanowire aerogels (Fig. 10a–c) and studied their application as self-supported electrocatalyst for H2O2 reduction reaction.93 Cyclic voltammetry (CV) measurements show the Ag nanowire aerogel delivers a cathodic current density more than 60 and 4 times higher than that of the Ag nanorods and Ag nanowires coated on carbon paper (CP) (Fig. 10d). The exchange current density of the Ag nanowire aerogel is about 4 and 2 times higher than that of Ag nanorods and Ag nanowires coated on CP (Fig. 10e). These results clearly prove that the porous 3D networks significantly improve the electrocatalytic activity of Ag for H2O2 reduction reaction. Further electrochemical impedance measurements show the Ag nanowire aerogel has a charge transfer resistance much lower than that of the Ag nanorods and Ag nanowires on CP (Fig. 10f), suggesting that the interconnected, continuous 3D networks promote the electron transport along the Ag nanowires and charge transfer from the Ag nanowires to H2O2 and the high porosity of the aerogels facilitate the H2O2 mass transport from the electrolyte and electrode.


image file: d0nr09236c-f10.tif
Fig. 10 (a) A photograph of an ultralight Ag nanowire monolith supported on a fern; (b) SEM characterization of the Ag nanowire aerogel and (c) TEM image of a single Ag nanowire. (d) CV curves, (e) Tafel polarization curves and (f) Nyquist plots of pristine carbon paper (CP, black), Ag rod (blue), Ag nanowire@CP (green), and Ag nanowire aerogel (red) electrodes in an aqueous electrolyte containing 0.1 M H2O2, 1.0 M NaOH and 20 g L−1 NaCl.93

In addition to the electrocatalytic applications, monolithic nanowire aerogels also show promising application for photocatalyst. For example, Ag doped MnO2 nanowire aerogels have been used by Sun's group for photodegradation of organic dyes in aqueous solution under irradiation of visible light.94 The doped aerogels show much faster photodecomposition rate than the well-dispersed nanowires and the Ag doped MnO2 nanowire pellets because the 3D porous networks facilitate sufficient mass transport between the catalyst and reactants, meanwhile, the doping improves the life time of the phonon generated charge carriers and the light adsorption in the visible light range and introduces the surface oxygen vacancies that serve as active catalytic sites. Importantly, monolithic nanowire aerogel photocatalysts with mechanical strength can be easily collected after fully decomposing the organic dyes, making them recyclable photocatalysts. In contrast, nanowire powder photocatalysts have to be cycled by complicated steps like centrifugation and filter. Composite aerogels of the W18O49 nanowires@reduced graphene oxide aerogels have also been reported as efficient photocatalysts towards degradation of organic dyes under visible light irradiation.95 The large surface area of the aerogels enables high adsorption of organic dyes and the high conductivity of the graphene oxides suppresses the electron–hole combination that occurrs in light-adsorber W18O49, thus significantly enhancing the photocatalytic performance of the composite aerogels.

3.5 Nanowire aerogels for piezoresistive pressure sensors

Piezoresistive pressure sensors can transduce pressure information to conductivity signal, which has shown widespread application in detecting human motion, creating artificial skin, and monitoring personal health and therapeutics.96–98 Metal nanowire aerogels, due to its high conductivity and mechanical flexibility, has emerged as a new type of candidate materials for piezoresistive sensor.32,58,82 Cheng's group prepared highly compressible monolithic copper nanowire-PVA aerogels by ice-templating method (Fig. 11a–c) and studied their piezoresistive properties.37 Under a compressive pressure, the resistance of the monolithic aerogels decreased very fast in the initial compression stage and then reached a plateau when the strain is above 30% (Fig. 11d). This unique piezoresistive property can be used for sensing the pressure. The quantitative resistive change of the nanowire aerogel-based sensor upon repeated loading and unloading of different pressures is shown in Fig. 11e. The sensitivity of the aerogel piezoresistive sensor was determined to be 0.036 kPa−1 (Fig. 11f). This aerogel can sustain 10[thin space (1/6-em)]000 times loading and unloading pressure of 400 Pa (Fig. 11g), indicating excellent cycling stability. Xu et al. found that the sensitivity of the copper nanowire aerogels is associated with their porosities.71 They managed to tune the sensitivity of the copper aerogels sensor from 0.02 kPa−1 to 0.7 kPa−1 by engineering the porosity of the aerogels. The optimum copper nanowire sensor with a sensitivity of 0.7 kPa−1 showed a response time of 80 ms and detection limit of 43 Pa. Cao's group reported that Cu nanowire@graphene core–shell aerogels showed a fast response time of 16 ms and a detection limit of about 640 Pa.61 Zhang's group prepared silver@polypyrrole core–shell nanowires for piezoresistive sensor.99 The composite aerogels exhibited outstanding sensing properties with a sensitivity of 0.33 kPa−1, short response time of 1 ms, and minimum detectable pressure of 4.93 Pa. The outstanding sensing property of the conductive composite aerogels can be attributed to their superelasticity originating from the rich pores and flexible networks of the composite aerogels as well as welded junctions enabled by the polypyrrole coating.
image file: d0nr09236c-f11.tif
Fig. 11 (a) A photograph of Cu nanowire–PVA composite aerogel monolith. (b) SEM image of the composite aerogel. (c) Snapshot photograph of the composite aerogel monolith under compressing and shape recovery. (d) Compressive stress and electrical resistance as a function of compressive strains for the composite aerogel. (e) Multicycles tests of resistance change as a function as time upon loading and unloading pressure in the range from 1 to 2.5 kPa. (f) Resistance change as a function of the loaded pressure. (g) Cycling stability test under loading and unloading pressure of 400 Pa.37

3.6 Nanowire aerogels for solar steam generation

Solar steam generation is a process in which solar energy is directly used to heat water to generate water vapor.100,101 This solar-driven steam process can be applied to purify wastewater and desalinate sea water, which could potentially solve the global issue regarding freshwater shortage.83,84 In order to build an efficient solar steam generation device, it is essential to develop photothermal materials with broad light adsorption, low thermal conductivity and high porosities for quick water transportation. Nanowire aerogels, due to their inherent porous structure, high thermal insulation and mechanical flexibility could meet the requirements of the photothermal materials for efficient solar evaporators. Deng's group fabricated elastic SiO2@carbon nanotube composite nanofibrous aerogels with a cellular architecture by ice-templating method and used them (Fig. 12a and b) as photothermal materials for solar steam generation in seawater.56 The convection and diffusion within the vertical cellular pores facilitate the transportation of salt and water from the surface of vapor-aerogels interface to bulk sea water, preventing the crystallization of salt on the aerogel. The carbon nanotube aerogels act as a light absorber that significantly enhances the light adsorption of the aerogels. The 3D interconnection of flexible SiO2 nanofibers endow the composite aerogels with robust mechanical properties (Fig. 12c) that ensure the stable evaporation efficiency for long-term utilization under harsh conditions. Due to the unique structural and compositional design, the solar evaporator based on this composite aerogel shows light absorbance of up to 98% (Fig. 12d) and excellent evaporation performance of 1.50 kg m−2 h−1 under 1 sun irradiation without decay during long term use under harsh conditions (Fig. 12e and f).
image file: d0nr09236c-f12.tif
Fig. 12 (a) Photograph and (b) SEM image of carbon nanotube@SiO2 nanofibrous aerogel (CNFAs). (c) Photographs showing the compressibility of the CNFAs in water. (d) Solar spectrum and UV-Vis-NIR adsorbing spectrum of the CNFAs in the wavelength ranging from 200 to 2500 nm. (e) Evaporation rate with and without the CNFAs in the evaporator under 1-sun irradiation. (f) Cycling performance of the evaporator under 1-sun irradiation.56

Li's group designed and synthesized monolithic polypyrrole-coated MnO2 nanowire@reduced graphene oxide aerogels (PNGA) for efficient solar vapor generation.102 The composite aerogel shows robust 3D interconnected porous networks that offer a pathway for water transport and have a thermal insulation effect that prevents heat transfer from the aerogels to bulk water. Polypyrrole coating improves the optical adsorption of the aerogels to ∼100% in the wavelength range between 200 to 2500 nm. The synergistic effects of strong light adsorption, convenient water transportation and thermal insulation enable the PNGA to deliver a water evaporation rate of up to1.587 kg m−2 h−1 with a solar steam efficiency of 93.8%, which is comparable to state-of-the-art photothermal materials for solar steam generation.

4. Conclusions and outlook

We systematically reviewed the approaches for making nanowire aerogels and their properties and applications. The fabrication procedures typically involve a gelation step for arranging nanowires into macroscopic 3D solid networks followed by a drying step. Both gelation and drying steps are critical for the formation of final aerogels. The prepared nanowire aerogels show unique structural features of aerogels as macroscopic bulk materials and keep the anisotropic nanosized features of primary nanowire building blocks. This type of hierarchical architecture combines the structural and functional benefits of both nanowires and aerogels, leading to wide applications of nanowire aerogels in a variety of areas. In spite of tremendous progress on nanowire-based aerogels, there are still several unresolved key questions: (1) how to prepare nanowire aerogels on a large scale; (2) how to tune the properties of nanowire aerogels by engineering the primary nanowire building blocks as well as the structural features of the whole aerogel body; (3) how to incorporate other nanomaterials into nanowire aerogels to achieve multifunctional composite aerogels. Future efforts should be devoted to addressing these questions to achieve the goal of designing nanowire aerogels with desired properties and promoting practical application of nanowire aerogels in various areas.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We dedicate this work to the centenary of Xiamen University. We are grateful to the Natural Science Foundation of Fujian Province of China (No. 2020J01037), Natural Science Foundation of Guangdong Province (No. 2021A1515010682), Fundamental Research Funds for the Central Universities (20720200075) and Nanqiang Youth Talented Program of Xiamen University.

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Footnote

These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2021