A Versatile Strategy for Tailoring Noble Metal Supramolecular Gels

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A Versatile Strategy for Tailoring Noble Metal Supramolecular Gels/Aerogels and Their Application in Hydrogen Evolution Yan Zheng, Na Li, Somnath Mukherjee, Yingchao Yang, Junlin Yan, Jing Liu, and Yu Fang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00401 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Nano Materials

A Versatile Strategy for Tailoring Noble Metal Supramolecular Gels/Aerogels and Their Application in Hydrogen Evolution Yan Zheng, Na Li, Somnath Mukherjee, Yingchao Yang, Junlin Yan*, Jing Liu*, Yu Fang Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China

ABSTRACT: Self-assembly of noble metal nano-building blocks (NBBs) to hierarchical self-supported porous monoliths provides a great potential for achieving their wonderful properties along with practical applications. However, the self-assembly procedure for this kind of materials reported in template and sol-gel approach are limited to specific conditions and little interest was paid to the role of ligands in the self-assembly process in order to grasp the control of the structure and properties. Herein, two ligands (L1 and L2) containing thiol and glutamic acid moiety were synthesized and grafted to noble metallic nanowires and nanospheres. It had been shown that these kind of functional NBBs formed supramolecular gels in various solvents. Interestingly, these gels displayed stimulus-responsibility that has remained unreported with noble metal gels so far. In particular, the gels formed by Pt nanowires capping with L2 (PtNW-L2) demonstrated both thermo and shearing reversible gelation. Furthermore, aerogels can easily be prepared through supercritical drying and their density and porosity were regulated by adjusting the NBBs concentration. Surprisingly, the self-standing aerogels constructed by nanowires kept intact except a dramatic compression by applying external forces. Specially, the aerogel by PtNW-L2 displayed elastic nature for at least 10 cycles of loading and unloading external force, rarely reported in template-free noble metal aerogels. FT-IR, SEM and TEM measurements revealed that the basic unit with designed ligands aggregate to bunch of nanowires and reticular structures by hydrogen bonding. Owing to the 3-D network and hierarchical pores, Pt aerogels prepared by the present strategy exhibited low overpotential, more available active sites and long-term stability in hydrogen evolution reaction. Thus, this work provides a novel versatile strategy for the synthesis of porous noble metal monoliths and it would probably be widely used for

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fabricating other kinds of porous materials with remarkable properties. KEYWORDS:

Aerogel;

Porous

material;

Self-assembly

Supramolecular gel; Hydrogen evolution


of

nanoparticle;

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Introduction Nanoscale building blocks (NBBs) are believed to be the basic component of next generation devices. This has led to a substantial surge in research activities pertaining to the development and fundamental understanding of processes and assembly at the nanoscale level1-3. A range of well-defined NBBs of various chemical compositions, sizes, shapes with extraordinary physical and chemical properties has been prepared by top-down and/or bottom-up fabrication approaches4-6. Unfortunately, functional NBBs can rarely be used directly and the performances of novel hierarchical materials and devices assembled by NBBs (usually nanoparticles or films) are far away from expectation due to the drastic degradation of their active surface area and poor durability7,8. A promising alternative could be the use self-supported porous monolith that offers desirable combination of valuable properties of NBBs and characteristics of porous materials9-12. As distinguished porous nanomaterials, self-supported aerogels, having 3-D continuous bulk backbone, open cellular hierarchical pores, high specific surface area and porosity ratio, have attracted much attention for their relevant properties and applications in various areas13,14, including adsorption15, separation16, sensing17,18 and catalysis19,20. Besides, in recent years, self-supported noble metal aerogels, endowing outstanding functions of NBBs to aerogels, have caught the researchers’ interest due to their high available active sites, good mass transportation and great durability9,14,21-25. Analogy to the traditional aerogels (inorganic oxide and organic aerogels), noble metal aerogels were also prepared by self-assembly of NBBs through templates and sol-gel approach. Firstly, precursors (metal salts) to the noble metals were reduced by chemical or electrochemical means to develop specific NBBs. Then, the NBBs would either undergo a direct assembly to the templates or disperse in the sols. In template approach, self-supported noble metal aerogels could be obtained through corrosion of the non-noble metallic framework26,27 or transferred from other metal aerogels by selective dealloying28 or galvanic replacement29 techniques.



Hydrogen bubble dynamic templates have also been developed to prepare self-supported Au, Pd and Pt aerogels30. However, in template approach, it is still a great challenge to balance an efficient removal of the templates and keeping of a better control over the porosity and the texture. In sol-gel process, noble metal NBBs sols would transfer to gels spontaneously via in situ reduction of noble metal precursors in a single step31,32. On the other side, the transformation could also be induced by aging33,34 or adding an inductor of self-assembly in a two-step procedure. Of late, a destabilization process based on the removal of a part of capping ligands was widely used to prepare mono/multi-noble metal aerogels21,35-41. Colloidal noble metal NBBs can also self-assemble to gels activated by freezing42-45, hydrothermal processing46,47, changing the pH48 and the presence of salts49. These self-assembly methods in sol-gel procedure are highly dependent on specific ligands, solvents, destabilizing procedure and they are deficient in ensuring simultaneously the optimum porous structure, the morphology (shape and size) and the composition (phase composition and crystallinity) for the aerogel functions. Therefore, it is an urgent need to develop general and simple strategies to create varieties of noble metal aerogels in scalable ways and promote their wide applications in practical areas. Eychmüller et al. synthesized hybrid gels of CdTe/Au with controlled content through cross-linking the NPs by complexing the capping ligand 5-mercaptomethyltetrazole (5-HSCH2Tz) and Cd2+ ions50. In this report, the synthesis of Au NPs, the ligand exchange of capping ligand from citrate to 5-HSCH2Tz and gel formation was conducted individually. This kind of method provided an ideal opportunity to manage the synthesis of NBBs and the self-assembly process independently to attain optimum structures and properties of noble metal aerogels. Although self-supported noble metal gels were fabricated by efficient and innovative self-assembly methods based on supramolecular week interactions, the gels scarcely performed as a supramolecular gel. For example, the gels are devoid of some smart properties such as stimulus-responsiveness. Furthermore, little interest has been paid to the role of ligands in the self-assembly process to tune the structure and properties for specific physical and chemical attributes, in accordance with the need.


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The surfacial ligands play a decisive role in the self-assembly of NBBs prepared for novel nanomaterials and devices. Therefore, to design functional ligands, having a strong self-assembling ability, for the facilitation of the self-assembly of NBBs is, indeed, a good option. Supramolecular gels formed by the self-assembly of low-molecular-weight gelators (LMWGs) have gained much attention over the last few decades, owing to their potential applications in biomedical field and as advanced materials51,52. Several non-covalent interactions are responsible for the formation of supramolecular architectures in the gel state and the mechanisms have also been elucidated clearly53. Herein, in view of their strong self-assembling abilities through hydrogen bonding interactions, two glutamic acid derivatives with thiol group were synthesized as the ligands for noble metal NBBs. As expected, the noble metallic NBBs capped with designed ligands (ligand 1 and ligand 2) displayed effective gelation in several solvents and it is versatile for the self-assembly of noble metal NBBs with varied morphologies or chemical composition. Accordingly, the as-prepared

physical

gels

were

stimulus-responsive

(thermo-

and

shearing-reversibility) and their corresponding aerogels demonstrated certain degree of flexibility and tunable structure by adjusting the chemical composition and shape of NBBs or chemical structure of ligands. More interestingly, the aerogel assembled by Pt nanowires showed outstanding performance in electrochemical hydrogen evolution reaction (HER).

Scheme 1. Schematic illustration for the preparation of noble metal supramolecular gels/aerogels by self-assembly of capping ligands.



Results and discussion Ligand exchange for noble metal NBBs with LMWG ligands In order to endow NBBs with self-assembly behaviors as that of LMWGs, we planned to impart surfacial ligands with strong self assembling ability. Therefore, we designed and synthesized a special type of ligand containing glutamic acid residue and thiol (Scheme 1) in which the former was considered to be a versatile self-assembling skeleton and the later can anchor on the noble metal NBBs. As expected, L1 and L2 can gelatinize most of common organic solvents (Table S1 and Figure S1) and it is reasonable that they may import the great gel ability to inorganic NBBs. To test the efficacy of our desired strategy, firstly, platinum nanowires (PtNW) were chosen as a model of noble metal NBBs. The PtNW (~3 nm in diameter) with length up to 5 μm were synthesized following a reported method54 (Figure 1a). The original ligands (ethylene glycol) on the surface were replaced by the designed ligands (L1 and L2) through a ligand exchange procedure in tetrahydrofuran. TEM images clearly show that the bunch of the PtNW (about 20 nm in diameter) with

Figure 1. TEM images of PtNW before (a) and after ligand exchange by L1 (b) and L2 (c); XPS spectra (d) of PtNW-L2. Inset in (d): the high resolution XPS of S 2p of PtNW-L2.


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ethylene glycol as the capping ligand (Figure S2), are similar to that of the references. However, after the ligand exchange process reached completion, the nanowire bunches unbundled into single nanowire strands with about 3 nm-diameter (Figure 1b and 1c). X-ray photoelectron spectroscopy (XPS) detection revealed that there were obvious sulfur species in L1- and L2-capped PtNW (PtNW-L1 and PtNW-L2) (Figure 1d and Figure S3d). Furthermore, from the high resolution XPS spectra of mercaptan (S 2p1/2), it was found that the binding energy of S 2p1/2 at 163.2 eV shifted to 162.8 eV, owing to the interaction between mercaptan and Pt55. In addition, there was obvious peak of binding energy at 286.4 eV from high resolution XPS spectrum of C 1s in original PtNW (Figure S3a), corresponding to the C-OH in ethylene glycol. However, after encountering a ligand exchange process, the peak disappeared (Figure S3b and S3c), which indicated that the original capping agents (ethylene glycol) were completely replaced by the present designed ones. Accordingly, the grafting density of L1 and L2 on PtNW was calculated to be about 3.45 molecules/nm2 and 7.13 molecules/nm2 from the results obtained from themogravimetric analysis (TGA, Figure S4). Moreover, XRD results demonstrated that the functionalized PtNW possessed the same crystal structure as the original ones with a face-centered cubic (fcc) structure (Figure S5). Fabrication of noble metal supramolecular gels and aerogels To investigate the self-assembling behaviors of noble metal NBBs, they were dispersed in solvents by ultrasonication followed by heating, and then the dispersion was cooled to room temperature. Interestingly, several noble metal supramolecular wet gels formed in solvents (Table 1). As shown in Table 1, PtNW without designed ligands cannot form gels in any tested solvents (Figure S6). PtNW-L1 can gelate toluene with a critical gel concentration (CGC) of 4.5 w/v%, while, PtNW-L2 can congeal THF, CHCl2 and CHCl3 with a CGC of 1.5 w/v%. Noticeably, when gelation concentration was greater than 7.5 w/v%, the solvents were not enough to soak all the functionalized PtNW. To test the versatility of our strategy, alloy nanowires of Pt/Co with 0.7:0.3 (atomic ration, Pt/CoNW), gold nanowires (AuNW), gold nanoparticles



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(AuNP) were synthesized according to reported methods54,56,57 and It is clearly that the basic units were about 3 nm in diameter for nanowires and 15 nm in diameter for Au nanoparticle (Figure S7). According to the good gelation ability of PtNW-L2, the original capping ligands of these NBBs were exchanged to L2 by the referred ligand exchange procedure. For simplification, these NBBs are denoted as Pt/CoNW-L2, AuNW-L2 and AuNP-L2, and their grafting density were also calculated to be 6.73 molecules/nm2, 8.11 molecules/nm2 and 7.52 molecules/nm2 by TGA (Figure S4). Then, the gelation behaviors of these NBBs were tested by the same procedure as that for PtNW. From the gelation results in Table 1, Pt/CoNW-L2 can immobilize THF, CH2Cl2 and CHCl3; AuNW-L2 can gel CH2Cl2 and CHCl3, whereas AuNP-L2 can form gels in cyclohexane and CHCl3. It can found that NBBs with same ligands possess similar gelation ability and the shape can also affect the self-assembly behaviors slightly, therefore, it can conclude that the effectiveness of this strategy could be regulated by the nature (chemical composition or shape) of NBBs or the capping ligands, however, obviously, the later was the determinant and a subtle change in the ligand structure had critical impact on the overall gelation properties of functionalized NBBs. From all the above tests, it demonstrated that noble metal supramolecular gels could be prepared by capping ligands with strong self-assembling ability to noble metal NBBs, meanwhile, this novel self-assembly strategy provide a simple and adaptable way to regulate noble metal gels by adjusting the capping ligands and nature of NBBs. Table 1. Self-assembling behaviors of PtNW, PtNW-L1, PtNW-L2, Pt/CoNW-L2, AuNW-L2 and AuNP-L2 in various solvents NBBs Solvents

PtNW

Methanol Ethanol Acetonitrile Dimethyl sulfoxide Dimethylformamide Acetone Ethyl acetate Tetrahydrofuran

P C P P P P P P

PtNW-L1 PtNW-L2 Pt/CoNW-L2 AuNW-L2 AuNP-L2 P P P P P P P C

PG P P P P P P G

P P P P P P P G


P P P P P P P C

P P P C P P P C

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1,4-Dioxane Dichloromethane Chloroform Benzene Toluene Tetrachloromethane n-Hexane Cyclohexane

P P P P P P P P

P P P C G P P P

P G G C P P P P

P G G C P P P P

P G G P P P P P

C P G P P P P PG

C = Colloid, G = Gel, P = Precipitate, PG = Partial Gel.

Figure 2. The stimulus-responsibility of PtNW-L2 supramolecular gel in CHCl3. the photo of suspension (a), gel (b and c) and monolithic aerogel (d).

Interestingly, all the above-mentioned physical gels showed thermo-reversible transition between sol state and gel state. Specifically, the gels started flowing by heating up at 50 ºC for 5 min (Figure 2a and Figure S8a-d), and returning to gel state upon cooling (Figure 2b and Figure S8a1-d1). Besides thermo-reversibility, the gel of PtNW-L2 in CHCl3 was also responsive to sonication and shearing. The gel was transferred to a suspension by shaking, and it can revert to gel upon resting (Figure 2c). Interestingly, the same phenomenon was observed when that gel was treated under ultrasonication, suggesting to its multi-stimulus responsive behavior. Surprisingly, for all the gels, even though the solvent was removed by freeze-drying or supercritical CO2 drying, the powders could form gels again through standard gelation test procedure like LMWGs. To the best of our knowledge, such reversibility in noble metallic gels has not been reported so far. Owing to the good performance,



the stress sweep experiments of rheological studies for PtNW-L2 wet gels at different concentrations were conducted and shown in Figure S9. The storage modulus (G′) was found to be more than one order magnitude higher than the loss modulus (G″) pointing towards dominant elastic nature of these wet gels. At a definite stress value they crossed each other at which sharp decrease in moduli occurred. This definite value is known as yield stress and excitingly, for PtNW-L2 at 7.5 w/v%, the value was 7230 Pa. Such a high yield stress value indicated a high mechanical strength of PtNW-L2 wet gel which might be due to the stable self-assembled 3-D network achieved by effective cross-linking of nanowires. It should be noted that this kind of test has never been performed for template-free noble metal gels formed by self-assembly of NBBs As a well-known porous material, aerogel, especially the one constructed by functional building blocks, has received increasing attention in the last two decades. Luckily, all the above supramolecular gels can be transferred to monolithic aerogels without obvious shrinking by supercritical CO2 drying (Figure 2d and Figure S8a2-d2). From SEM images (Figure 3), the aerogel networks were characterized as 3D continuous reticular nanowires having micron pores. Investigating SEM images of Pt and Pt/Co aerogels carefully in different solvents and also at various gelation concentrations (Figure 3a-c and Figure S10), it can be observed that the basic component of networks is almost the same nanowire with 40 nm in diameter formed by the assembled original nanowires (about 3 nm in diameter, Figure 3a'-c'). From concentration-dependent SEM images (Figure S10), it showed that the ultrathin nanowires assembled from loose separated ones to compacted bunch of nanowires as concentration increasing. In case of AuNW-L2 and AuNP-L2 aerogels, the network was characterized by reticular nanowires were about 15 nm and 40 nm (Figure 3d and e), respectively, and there were no substructure for both of them from high magnification SEM images (Figure 3d' and e'), which were attributed to confusion of Au NBBs during ultrasonication, heating or supercritical drying. In addition, as a hierarchical porous material, pores of the Pt monolithic aerogel in this work were measured by


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nitrogen physisorption isotherms. As indicated by the Brunauer-Emmett-Teller plots (Figure S11), the PtNW-L1 and PtNW-L2 aerogels possessed a wide spread of both meso- and macropores with a specific surface area of the PtNW-L1 aerogel of 8.79 m2g-1, and the PtNW-L2 aerogel of 13.23 m2g-1. Ultralow density is a great feature of aerogel and the density of aerogels in the present work was calculated according to the weighing mass of NBBs and the volume of the gel. Clearly, another feature of our self-assembly strategy for noble metal aerogels is regulating the density easily. For example, the density of PtNW-L2 aerogel could be easily regulated from 0.023 g/cm3 to 0.066 g/cm3 by varying the NBBs concentration from 1.5% to 7.5% (w/v) respectively (see details in Section 12 of Supporting Information), far lower than the corresponding bulk metals (21.45 g/cm3). According to the density of aerogel and bulk metal, the porosity of PtNW-L2 aerogel calculated was found to be 99.7% at 7.5 w/v%.

Figure 3. Low and high magnification SEM images of PtNW-L1 (a, a'), PtNW-L2 (b, b'), Pt/CoNW-L2 (c, c'), AuNW-L2 (d, d') and AuNP-L2 (e, e') aerogels.

Usually, supramolecular xerogels and aerogels are fragile and split into pieces or powders when external forces are applied. Surprisingly, the self-standing aerogels constructed by nanowires (PtNW-L1, PtNW-L2, Pt/CoNW-L2 and AuNW-L2) can keep themself intact except a dramatic compaction of shape from a cylinder to a disk (Figure 4a' and 4b', and Figure S12a-c). AuNP-L2 aerogel constructed by nanospheres demonstrated brittle damage by external force (Figure S12d and d'). The totally



different ability in compressibility could be attributed to the various shapes of NBBs. In aerogels by nanowires, the network was formed by tangled nanowires, which is ductile. While in the aerogels by nanospheres, the backbone was stacked with nanospheres one by one, leading to a fragile network. Experientially, materials constructed by tangled wires are flexible, therefore, the PtNW-L2 aerogel was studied detailed by dynamic mechanical analyzer (DMA). It can be seen that even the compression pressure reached 100 kPa, the cylinder aerogel was just compacted to 36.3% of its original height, leaving an integral compacted disk integral (Figure 4a), attributed to the continuous network by tangling nanowires. A carefully study of the compression curve revealed that the strain-stress curve was linear at the beginning stage, indicative of the elastic nature of aerogel. Therefore, the PtNW-L2 aerogel (7.5 w/v%) was measured in reversible compression mode, which was conducted at a static force speed of 0.05 N/min to the terminal force of 1 N. From the curves of loading-unloading cycles (Figure 4b), it can be seen that the aerogel was compressed to 98.50% at 1 N, and it can be recovered to 99.40% by unloading the external force at first cycle, indicating the elastic nature of aerogel. The damage of elasticity could be due to the breakage of part of the network. Moreover, when the test was repeated 10 times, the Pt aerogel still can restore to certain extent, even though the elasticity was decreasing by increasing the compression. From the above DMA measurements, it can be concluded that the as-prepared aerogels possess amazing flexibility, rarely reported in template-free self-assembled porous metallic materials.

Figure 4. Strain vs stress curve of static (a) and reversible (b) compression of a cylinder aerogel by PtNW-L2 in CHCl3 at 7.5 w/v%. Inset: the photos of the cylinder aerogel before (a′) and after (b′) compression.


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Formation mechanism of noble metal supramolecular gel and aerogel It is a well-known fact that non-covalent interactions like hydrogen-bonding, dipole-dipole, π-π staking, electrostatic and van der Waals forces are responsible for the formation of 3-D network of supramolecular gel. FT-IR spectroscopy is a very effective tool for investigating the hydrogen bonding interaction between gelator molecules. FT-IR spectra of monomolecular L2 in CDCl3, PtNW-L2 and the corresponding aerogel revealed sharp peaks at 3296 cm-1, 3295 cm-1 and 3292 cm-1 respectively (Figure 5), which were assigned to hydrogen bonded N-H stretching vibration. It can be clearly seen that, in aerogel state, N-H stretching vibration was distinctly moved to lower wavenumber. The signals corresponding to the stretching vibration of C=O for L2, PtNW-L2 and aerogel appeared at 1635 cm-1 , 1634 cm-1 and 1631 cm-1 respectively with the peaks of the aerogel state moving to lower wavenumbers. Moreover, the signals assigned to the N-H bending vibration for L2, PtNW-L2 and aerogel were found at 1552 cm-1, 1553 cm-1 and 1555 cm-1 respectively with a shift to higher wavenumbers for aerogels compared to the ligand itself. The similar shift can be observed from the PtNW-L1 system (Figure S13). These results clearly indicate the formation of strong hydrogen bonding between amide C=O and N-H, resulting a strong self-assembly in gel and aerogel state. To elaborate formation process of the network of noble metal supramolecular gels, concentration-dependent TEM measurements were conducted on PtNW-L2 system (Figure S14). It can be seen that the basic unit of nanowires (~3 nm in diameter) were leaned loosely at 0.01% (w/v), then they got closer to a bunch of nanowires about 20 nm in diameter as the concentration increase to 0.1% (w/v), and they aggregated to compacted ones about 40 nm in diameter when the concentration is 1% (w/v). With the results of FT-IR spectra and morphology studies by SEM and concentration-dependent TEM in mind, the gelation process can be described as follows: by ultrasonication and heating, the NBBs wrapped with designed ligands were dispersed in solvents, then, as the suspension was cooled down, the basic NBBs self-assembled to bigger and closer bunches of nanowires based on hydrogen bonding



from wrapping ligands on nanowires. Meanwhile, the bunches of nanowires grew bigger and bigger. Finally, the nanowires formed intertwined 3-D continuous networks and they can absorb and immobilize the solvents to form gels due to surfacial gelatinizing ligands, which act as the same as that of supramolecular gelators.

Figure 5. FT-IR spectra of L2 (10 mM in CDCl3 solution), PtNW-L2 and PtNW-L2 aerogel (7.5 w/v%) from CHCl3.

Figure 6. LSV curves (a) of PtNW-L1 aerogel, PtNW-L2 aerogel, PtNW-L2-acetone and 20% Pt/C. Inset: histogram of their corresponding overpotential at 10 mA/cm2. The aerogels are from CHCl3 at 7.5 w/v%. LSV curves (b) of PtNW-L2 aerogel (7.5 w/v%) before and after 1000 CV cycles.


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SEM images of (c) PtNW-L1 aerogel, (d) PtNW-L2 aerogel and (e) PtNW-L2-acetone in situ electrodes.

Performance of PtNW-L2 aerogel in electrochemical hydrogen evolution reaction Porous noble metal nanomaterials have demonstrated good performance in electrocatalytic process, such as HER and ORR. As reported, the self-assembled aerogel containing self-supported 3-D continuous network would reduce the overpotential associated with the catalyst/support interface. Meanwhile, the nano-sized NBBs of platinum aerogel are responsible for high surface areas, whereas the micrometer-sized pores served as diffusion channels to enhance the mass transportation for both gas and water58. Therefore, the electrocatalytic properties of the self-assembled Pt aerogels from CHCl3 (at 7.5 w/v%) toward the HER were evaluated. For contrast, assemblies of PtNW-L2 without hierarchical pores (precipitating from poor solvent (acetone), PtNW-L2-acetone) and commercial 20% Pt/C were tested by the same procedure as that of Pt aerogels. As expected, the overpotential at 10 mA/cm2 of PtNW-L2-acetone, PtNW-L1 aerogels, PtNW-L2 aerogels and Pt/C emerged at 78 mV, 66 mV, 45 mV, and 52 mV, respectively (Figure 6a). Clearly, the overpotentials of PtNW-L1 and PtNW-L2 aerogels were much lower than that of PtNW-L2-acetone. Calculated from CV measurements, the electrochemical double-layer capacitance values (Cdl) of PtNW-L2-acetone assembly, PtNW-L1 aerogel, and PtNW-L2 aerogel, powerful parameters to estimate the electrochemical active surfaces of materials, were found to be 11.1 mF cm-2, 19.4 mF cm-2, and 24.6 mF cm-2 (Figure S15), respectively, indicating the availability of more electrochemical active surface in aerogels. This primary judgment was further proved by SEM measurements of in situ electrodes. From SEM images (Figure 6c-e), continuous network with shaggy hierarchical pores was found in the electrode with Pt aerogels, while in the electrode with PtNW-L2-acetone assembly, compacted film without obvious pores was demonstrated. Clearly, the former structure was more favorable in transporting water and gas and providing active sites. Moreover, the LSV curve of Pt-L2 aerogel exhibited nearly no difference compared to the initial one even after 1000 cycles (Figure 6b), presenting PtNW-L2 aerogel as a highly stable electrocatalyst for HER.



Considering the same chemical composition for controlling sample and aerogels and the unique hierarchical porous structure of aerogels, it can be concluded that the raising of electrochemical performance was stemmed from the self-supported networks and hierarchical pores. An interesting phenomenon is that the performance of PtNW-L2 aerogel is obviously better than that of PtNW-L1 aerogel, which demonstrated that a minor modification of ligand structure would change properties of aerogels dramatically. The above results clearly indicate the effectiveness of present strategy to improve electrochemical catalytic ability of the self-assembling NBBs to self-supported porous monolith and regulate their performance by tuning the structure of the ligands.

Conclusions In summary, we have successfully developed a simple and versatile strategy to prepare noble metal supramolecular gels on the basis of NBBs through self-assembly of capping ligands and the ligands had a decisive influent on the gelation behaviors. Interestingly, the gels are thermo- and shearing-responsive, falling under the category of smart gel. After supercritical drying, hierarchical porous noble metal aerogels were obtained and exhibited amazing elasticity and compressibility. Owing to the self-supported 3-D continuous networks and hierarchical pores, Pt aerogels demonstrated an excellent electrocatalytic activity towards the HER for their good mass transportation and huge available active sites. Importantly, the structure and properties of aerogel could be regulated by tuning the structure of the ligands, concentration and chemical composition and shapes of NBBs. We strongly believe that this new strategy could be proved to be very useful in the preparation of assemblies or aerogels constructed by NBBs feathered with outstanding properties for various applications.

Experimental Section Chemicals Boc-L-glutamic acid, n-dodecylamine, 4-dimethylamino pyridine (DMAP),


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trifluoroaceticacid, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydro-chloride (EDC•HCl, >98%), H2PtCl6•6H2O, HAuCl4•3H2O, CoCl2, oleylamine, sodium citrate ethylene glycol, and 11-mercaptoundecanoic acid were purchased from Macklin Co. Ltd. Nafion solution was purchased from Sigma-Aldrich Co. Ltd. Methylene chloride, and triethylamine were distilled from CaH2. The other solvents provided by Sinopharm Group of China were of analytical grade and were used without further purification. Water was purified by the Direct-Q water purification system (18.2 mΩ, Millipore Co.). Characterization 1H

NMR spectra were recorded on Bruker 600 MHz spectrometer with

tetramethylsilane (TMS) as an internal standard. FTIR spectra were acquired using a Fourier Transform Infrared Spectrometer (Bruker Tensor 27). ESI-MS spectra were collected on a Bruker maxis UHR-TOF mass spectrometer in the positive mode. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F20 field transmission electron microscope at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) images of the materials were taken by Hitachi SU8220 field transmission electron microscope. X-ray photoelectron spectra (XPS) were examined by an AXIS ULTRA X-ray photoelectron spectrometer (Kratos Analytical Ltd.) using Al Kα radiation for the chemical analysis. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffraction system in the reflection mode and all data were recorded using Cu Kα radiation. Thermogravimetric analysis (TGA) was conducted using a thermal analyzer with a heating rate of 10 °C/min in the air. The mechanical strength test of the gel was performed by using a dynamic mechanical analyzer (Q800DMA, TA Instrument, USA) at a compression model on 25 °C. Rheological experiments were performed on stress-controlled rheometer (TA Instruments AR-G2) equipped with a parallel plate (20 mm diameter). Nitrogen physisorption isotherms were obtained using ASAP 2020 HD88 surface analyzer. Porosity measurement of aerogels was obtained by using micromeritics instrument (AUTOPORE 9500, USA). Electrocatalyst for HER were



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taken by CHI Instruments 660e electrochemical workstation (Shanghai Chenhua Co. Ltd., China). Synthesis of ligands Synthesis of ligand 1. 50 mL dichloromethane solution of DMAP (0.21 g, 1.74 mmol), EDC•HCl (4.01 g, 20.88 mmol), and 11-mercaptoundecanoic acid (2.28 g, 10.44 mmol) was dropped into 70 mL dichloromethane solution of 1,5-bis (hexylamino)-1,5-dioxopentan-2-aminium chloride (5.40 g, 10.44 mmol), and triethylamine (1.40 mL, 10.06 mmol) under ice bath and stirred for 24 h in darkness. The mixture was evaporated under vacuum, and further purified by a silica gel column eluted with chloroform/methanol (20:1, v/v) to give the product as a white powder (4.61 g, yield: 61%). FT-IR (KBr, vmax/cm−1): 3294 (N-H), 2920 (H-C=O), 1636 (C=O). MS (m/z, ESI+): calculated for [C40H79N3O3S + Na]+, 704.5734, found: 704.5720; 1H NMR δH (600 MHz; CDCl3; Me4Si): 7.04 (1H, NHCOCH), 6.99 (1H, NHCOCH2), 6.21 (1H, CHNHCO), 4.29-4.36 (1H, NHCHCO), 3.20-3.26(4H, CH2NHCO), 2.50-2.52 (2H, CH2CH2SH), 1.75-2.45 (6H: 4H: CHCH2CH2CO, 2H: NHCOCH2),

1.40-1.65

(8H:

2H:

CH2CH2SH,

2H:

CHNHCOCH2,

4H:

CH2CH2NHCO), 1.25 (48H, b, methylene), 0.87-0.92 (6H, CH3CH2). Synthesis of tert-butyl (11-((1,5-bis(dodecylamino)-1,5-dioxopentan-2-yl) amino)-11-oxoundecyl)carbamate (compound 1). 40 mL dichloromethane solution of DMAP (0.06 g, 0.5 mmol), EDC•HCl (2.01 g, 10.50 mmol), and 11-mercaptoundecanoic acid (1.51 g, 5.01 mmol) was dropped into 70 mL dichloromethane

solution

of

1,5-bis

(hexylamino)-1,5-dioxopentan-2-aminium

chloride (2.37 g, 4.50 mmol), and triethylamine (1.00 mL, 7.19 mmol) under ice bath and stirred for 24 h. The mixture was evaporated under vacuum, and further purified by a silica gel column eluted with chloroform/methanol (10:1, v/v) to give the product as a white powder (2.72 g, yield: 70%). (600 MHz; CDCl3; Me4Si): 7.32 (1H, NHCOCH), 7.23 (1H, NHCOCH2), 6.64 (1H, CHNHCO), 4.61 (1H, NHCOO), 4.45 (1H, NHCHCO), 3.15-3.26 (4H, CH2NHCO), 3.05-3.15 (2H, CH2NHCOO),


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2.20-2.45 (4H, CHCH2CH2CO), 1.90-2.15 (2H: NHCOCH2), 1.40-1.65 (4H: b, methylene, 9H: t CCH3), 1.25 (52H, b, methylene), 0.87-0.92 (6H, CH3CH2). Synthesis of ligand 2. 50 mL dichloromethane solution of DMAP (0.04 g, 0.30 mmol), EDC•HCl (1.15 g, 6.00 mmol), and 11-mercaptoundecanoic acid (0.65 g, 3.00 mmol) was dropped into 70 mL dichloromethane solution of tert-butyl (11-((1,5-bis(dodecylamino)-1,5-dioxopentan-2-yl)amino)-11-oxoundecyl) carbamate (2.13 g, 3.00 mmol), and triethylamine (0.50 mL, 3.59 mmol) under ice bath and stirred for 24 h with darkness. The mixture was evaporated under vacuum,

and

further purified by a silica gel column eluted with chloroform/methanol (20:1, v/v) to give the product as white powder (1.61 g, yield: 58%). FT-IR (KBr, vmax/cm−1): 3296 (N-H), 2918 (H-C=O), 1635 (C=O). MS (m/z, ESI+): calculated for [C51H100N4O4S + Na]+, 887.7357, found: 887.7347; 1H NMR δH (600 MHz; CDCl3; Me4Si): 7.06 (1H, NHCOCH), 6.96 (1H, NHCOCH2), 6.27 (1H, CHNHCO), 5.49 (1H, NHCOCH2), 4.35 (1H, NHCHCO), 3.19-3.25 (6H, CH2NHCO), 2.30-2.72 (2H, CH2CH2SH), 1.75-2.25 (8H: 4H: CHCH2CH2CO, 4H: NHCOCH2), 1.35-1.65 (8H: 2H: CH2CH2SH, 6H: CH2CH2NHCO), 1.10-1.30 (64H, b, methylene), 0.87-0.92 (6H, CH3CH2). Ligand exchange with designed ligand for noble metal NBBs. The original NBBs were prepared by the reported methods and preserved in ethanol (PtNW and Pt/CoNW), oleylamine (AuNW) and aqueous solution (AuNP). The ligand exchange process was described as follows (taking Pt NWs as an example): 0.5 mL(nPt=0.19 mmol) ethanol solution of Pt nanowires was added into a 10 mL tetrahydrofuran solution of the ligand (0.07 mmol) and stirred for 24 h in darkness at 35 °C. The resultant products were isolated by centrifugation and washed with tetrahydrofuran. The above steps were repeated three times and the designed ligand-capped Pt NWs was obtained by freezing-dry from benzene. Preparation of noble metal supramolecular gels and aerogels. Required amounts of the designed ligand-capped NBBs were dispersed into measured amounts of selected pure solvent using sonication for 5 min and then heated at 50°C for 5 min to ensure that all ligand-capped NBBs were well-distributed. Subsequently the



dispersion was cooled to room temperature so that the supramolecular wet gels could form. Then, their corresponding aerogels were obtained by supercritical CO2 drying. Electrocatalyst for Hydrogen Evolution Reaction by Pt aerogels. All electrocatalytic activities of the as-synthesized catalysts were carried out at ambient environment on the CHI Instruments 660e electrochemical workstation (Shanghai Chenhua Co. Ltd., China) in a standard three-electrode system. Glassy carbon electrode (GCE, 3 mm in diameter), graphene and Ag/AgCl with saturated KCl electrode were used as the work electrode, the auxiliary electrode and the reference electrode, respectively. Potential was measured against Ag/AgCl with saturated KCl and presented versus reversible hydrogen electrode. Potential conversion was based on the Nernst equation: E (V vs. RHE) = E (V vs. Ag/AgCl) + 0.197 V + 0.0591 × pH. The sample was prepared by dispersing 4 mg of the catalyst into 1 mL of 2:1 v/v ethanol/water and 30 μL Nafion by ultrasonication to form a homogeneous ink. Then 5 μL of the ink was covered onto a GCE (loading amount of ~ 0.14 mg cm-2) and dried in room temperature for measurements. Commercial 20% Pt/C catalyst was used as a reference sample. The polarization curves were recorded by linear sweep voltammetry (LSV) mode at a scan rate of 5 mV/s. The long-term stability of the catalyst was performed by cycling the potential between -0.5 and -0.3 V vs Ag/AgCl at a scan rate of 5 mV/s. To evaluate the electrochemical active surface area (ECSA) of the catalyst, cyclic voltammograms (CV) were tested by measuring the double-layer capacitance values (Cdl) under the potential window of 0.015-0.115 V vs. Ag/AgCl with various scan rates from 10 to 200 mV/s. All the polarization curves were obtained without iR compensation.

Acknowledgments This research work was supported by the National Natural Science Foundation of China (Grant 21503128, 21573141), the Fundamental Research Funds for the Central Universities (Grant GK 201701004), and the Program of Introducing Talents of Discipline to Universities (Grant B14041). We thank Hanying Zhan and Xiangyang


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Yan for DMA and XPS measurements and discussion.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. Gelation results of L1, L2 and PtNW and their corresponding photos, TEM images and XPS spectra of Pt nanowires, XRD curve of PtNW-L1, PtNW-L2, TGA, TEM images, stimulus-responsibility and strain-stress curves of Pt/CoNW-L2, AuNW-L2 and AuNP-L2, rheological experiments of PtNW-L2 supramolecular gels, SEM images of PtNW-L2 aerogels from various solvents and concentration, calculation of pore and density of aerogels, FT-IR spectrum of PtNW-L1 aerogel, TEM images of PtNW-L2 in different concentrations, The Cdl of PtNW-L2-Acetone, PtNW-L1 aerogel and PtNW-L2 aerogel in HER, the synthetic route of ligands.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

REFERENCES (1) Tan, S. F.; Chee, S. W.; Lin, G.; Mirsaidov, U. Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution. Acc. Chem. Res. 2017, 50, 1303-1312. (2) Li, B.; Li, W.; Li H.; Wu, L. Ionic Complexes of Metal Oxide Clusters for Versatile Self-Assemblies. Acc. Chem. Res. 2017, 50, 1391-1399. (3) Zhang, J.; Li, Y.; Zhang, X.; Yang, B. Colloidal Self-Assembly Meets Nanofabrication: From Two-Dimensional Colloidal Crystals to Nanostructure Arrays. Adv. Mater. 2010, 22, 4249-4269. (4) Yao, S.; Zhu, Y. Nanomaterial-Enabled Stretchable Conductors: Strategies, Materials and Devices. Adv. Mater. 2015, 27, 1480-1511. (5) Ling, D.; Lee, N.; Hyeon, T. Chemical Synthesis and Assembly of Uniformly Sized Iron Oxide Nanoparticles for Medical Applications. Acc. Chem. Res. 2015, 48, 1276-1285.



(6) Chen, Y.; Fan, Z.; Zhang, Z.; Niu, W.; Li, C.; Yang, N.; Chen, B.; Zhang, H. Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications. Chem. Rev. 2018, 118, 6409-6455. (7) Liu, J-W.; Liang, H-W.; Yu, S-H. Macroscopic-Scale Assembled Nanowire Thin Films and Their Functionalities. Chem. Rev. 2012, 112, 4770-4799. (8) Vogel, N.; Retsch, M.; Fustin, C-A.; Campo, A. d.; Jonas, U. Advances in Colloidal Assembly: The Design of Structure and Hierarchy in Two and Three Dimensions. Chem. Rev. 2015, 115, 6265-6311. (9) Rechberger F.; Niederberger, M. Synthesis of Aerogels: From Molecular Routes to 3-Dimensional Nanoparticle Assembly. Nanoscale Horiz. 2017, 2, 6-30. (10) Tappan, B. C.; Steiner Ⅲ, S. A.; Luther, E. P. Nanoporous Metal Foams. Angew. Chem. Int. Ed. 2010, 49, 4544-4565. (11) Wang, X.; Ruditskiy, A.; Xia, Y. Rational Design and Synthesis of Noble-metal Nanoframes for Catalytic and Photonic Applications. Natl. Sci. Rev. 2016, 3, 520-533. (12) Gaponik, N.; Herrmann, A-K.; Eychmüller, A. Colloidal Nanocrystal-Based Gels and Aerogels: Material Aspects and Application Perspectives. J. Phys. Chem. Lett. 2012, 3, 8-17. (13) Cai, B.; Sayevich, V.; Gaponik, N.; Eychmüller, A. Emerging Hierarchical Aerogels: Self-assembly of Metal and Semiconductor Nanocrystals. Adv. Mater. 2018, 30, 1707518. (14) Jiang, S.; Agarwal, S.; Greiner, A. Low-Density Open Cellular Sponges as Functional Materials. Angew. Chem. Int. Ed. 2017, 56, 15520-15538. (15) Subrahmanyam, K. S.; Malliakas, C. D.; Sarma, D.; Armatas, G. S.; Wu, J.; Kanatzidis, M. G. Ion-exchangeable Molybdenum Sulfide Porous Chalcogel: Gas Adsorption and Capture of Iodine and Mercury. J. Am. Chem. Soc. 2015, 137, 13943-13948. (16) Cacho-Bailo, F.; Etxseberría-Benavides, M.; David, O.; Téllez, C.; Coronas, J. Structural Contraction of Zeolitic Imidazolate Frameworks: Membrane Application on Porous Metallic Hollow Fibers for Gas Separation. ACS Appl. Mater. Interfaces 2017, 9, 20787-20796. (17) Ali, I.; Chen, L.; Huang, Y.; Song, L.; Lu, X.; Liu, B.; Zhang, L.; Zhang, J.; Hou, L.; Chen, T. Humidity-Responsive Gold Aerogel for Real-time Monitoring of Human Breath. Langmuir 2018, 34, 4908-4913. (18) Wang, G.; Qin, J.; Zhou, X.; Deng, Y.; Wang, H.; Zhao, Y.; Wei, J. Self-template Synthesis of Mesoporous Metal Oxide Spheres with Metal-mediated Inner Architectures and Superior Sensing Performance. Adv. Funct. Mater. 2018, 28, 1806144. (19) Cai, B.; Hübner, R.; Sasaki, K.; Zhang, Y.; Su, D.; Ziegler, C.; Vukmirovic, M. B.; Rellinghaus, B.; Adzic, R. R.; Eychmüller, A. Core–Shell Structuring of Pure Metallic Aerogels towards Highly Efficient Platinum Utilization for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2018, 57, 2963-2966. (20) Wan, W.; Zhang, R.; Ma, M.; Zhou, Y. Monolithic Aerogel Photocatalysts: A Review. J. Mater. Chem. A 2018, 6, 754-775. (21) Liu, W.; Herrmann, A-K.; Bigall, N. C.; Rodriguez, P.; Wen, D.; Oezaslan, M.; Schmidt, T. J.; Gaponik N.; Eychmüller, A. Noble Metal Aerogels-Synthesis, Characterization, and Application as Electrocatalysts. Acc. Chem. Res. 2015, 48, 154-162. (22) Bigall, N. C.; Herrmann, A-K.; Vogel, M.; Rose, M.; Simon, P. W.; Carrillo-C.; Dorfs, D.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Hydrogels and Aerogels from Noble Metal Nanoparticles. Angew. Chem. Int. Ed. 2009, 48, 9731-9734. (23) Cai, B.; Wen, D.; Liu, W.; Herrmann, A-K.; Benad, A.; Eychmüller, A. Function-Led Design of Aerogels: Self-assembly of Alloyed PdNi Hollow Nanospheres for Efficient Electrocatalysis. Angew. Chem. Int. Ed. 2015, 54, 13101-13105. (24) Henning, S.; Ishikawa, H.; Kühn, L.; Herranz, J.; Müller, E.; Eychmüller, A.; Schmidt, T. J. Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes. Angew. Chem. Int. Ed. 2017, 56, 10707-10710. (25) Wen, D.; Liu, W.; Haubold, D.; Zhu, C.; Oschatz, M.; Holzschuh, M.; Wolf, A.; Simon, F.; Kaskel, S.; Eychmüller, A. Gold Aerogels: Three-dimensional Assembly of Nanoparticles and Their Use as Electrocatalytic Interfaces. ACS Nano 2016, 10, 2559-2567. (26) Yu, J.; Ding, Y.; Xu, C.; Inoue, A.; Sakurai, T.; Chen, M. Nanoporous Metals by Dealloying Multicomponent Metallic Glasses. Chem. Mater. 2008, 20, 4548-4550. (27) Wang, X.; Qi, Z.; Zhao, C.; Wang, W.; Zhang, Z. Influence of Alloy Composition and Dealloying Solution on the Formation and Microstructure of Monolithic Nanoporous Silver


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


through Chemical Dealloying of Al-Ag Alloys. J. Phys. Chem. C 2009, 113, 13139-13150. (28) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Evolution of Nanoporosity in Dealloying. Nature 2001, 410, 450. (29) Liang, H-W.; Cao, X.; Zhou, F.; Cui, C-H.; Zhang, W-J.; Yu, S-H. A Free-standing Pt-Nanowire Membrane as A Highly Stable Electrocatalyst for the Oxygen Reduction Reaction. Adv. Mater. 2011, 23, 1467-1471. (30) Burpo, F. J.; Nagelli, E. A.; Morris, L. A.; McClure, J. P.; Ryu, M. Y.; Palmer, J. L. Direct Solution-based Reduction Synthesis of Au, Pd, And Pt Aerogels. J. Mater. Res. 2017, 32, 4153-4165. (31) Liu, W.; Herrmann, A-K.; Geiger, D.; Borchardt, L.; Simon, F.; Kaskel, S.; Gaponik, N.; Eychmüller, A. High-performance Electrocatalysis on Palladium Aerogels. Angew. Chem. Int. Ed. 2012, 51, 5743-5747. (32) Krishna, K. S.; Suchand Sandeep, C. S.; Philip, R.; Eswaramoorthy, M. Mixing Does the Magic: A Rapid Synthesis of High Surface Area Noble Metal Nanosponges Showing Broadband Nonlinear Optical Response. ACS Nano 2010, 4, 2681-2688. (33) Liu, W.; Haubold, D.; Rutkowski, B.; Oschatz, M.; Hübner, R.; Werheid, M.; Ziegler, C.; Sonntag, L.; Liu, S.; Zheng, Z.; Herrmann, A-K.; Geiger, D.; Terlan, B.; Gemming, T.; Borchardt, L.; Kaskel, S.; Czyrska-Filemonowicz, A.; Eychmüller, A. Self-supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels as Highly Active and Durable Electrocatalysts. Chem. Mater. 2016, 28, 6477-6483. (34) Qin, G. W.; Liu, J.; Balaji, T.; Xu, X.; Matsunaga, H.; Hakuta, Y.; Zuo, L.; Raveendran, P. A Facile and Template-free Method to Prepare Mesoporous Gold Sponge and Its Pore Size Control. J. Phys. Chem. C 2008, 112, 10352-10358. (35) Heiligtag, F. J.; Rossell, M. D.; Süess, M. J.; Niederberger, M. Template-free Co-assembly of Preformed Au and TiO2 Nanoparticles into Multicomponent 3D Aerogels. J. Mater. Chem. 2011, 21, 16893. (36) Wen D.; Eychmüller, A. 3D Assembly of Preformed Colloidal Nanoparticles into Gels and Aerogels: Function-led Design. Chem. Commun. 2017, 53, 12608-12621. (37) Yao, Q.; Arachchige, I. U.; Brock, S. L. Expanding the Repertoire of Chalcogenide Nanocrystal Networks: Ag2Se Gels and Aerogels by Cation Exchange Reactions. J. Am. Chem. Soc. 2009, 131, 2800-2801. (38) Ziegler, C.; Wolf, A.; Liu, W.; Herrmann, A-K.; Gaponik, N.; Eychmüller, A. Modern Inorganic Aerogels. Angew. Chem. Int. Ed. 2017, 56, 13200-13221. (39) Heiligtag, F. J.; Cheng, W.; Mendonca, V. R. D.; Süess, M. J.; Hametner, K.; Günther, D.; Ribeiro, C.; Niederberger, M. Self-assembly of Metal and Metal Oxide Nanoparticles and Nanowires into A Macroscopic Ternary Aerogel Monolith with Tailored Photocatalytic Properties. Chem. Mater. 2014, 26, 5576-5584. (40) Gao, X.; Esteves, R. J; Luong, T. T. H.; Jaini, R.; Arachchige, I. U. Oxidation-induced Self-assembly of Ag Nanoshells into Transparent and Opaque Ag Hydrogels and Aerogels. J. Am. Chem. Soc. 2014, 136, 7993-8002. (41) Hendel, T.; Lesnyak, V.; Kühn, L.; Herrmann, A-K.; Bigall, N. C.; Borchardt, L.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Mixed Aerogels from Au and CdTe Nanoparticles. Adv. Funct. Mater. 2013, 23, 1903-1911. (42) Freytag, A.; Sánchez-Paradinas, S.; Naskar, S.; Wendt, N.; Colombo, M.; Pugliese, G.; Poppe, J.; Demirci, C.; Kretschmer, I.; Bahnemann, D. W.; Behrens, P.; Bigall, N. C. Versatile Aerogel Fabrication by Freezing and Subsequent Freeze-drying of Colloidal Nanoparticle Solutions. Angew. Chem. Int. Ed. 2016, 55, 1200-1203. (43) Gilbert, D. A.; Burks, E. C.; Ushakov, S. V.; Abellan, P.; Arslan, I.; Felter, T. E.; Navrotsky, A.; Liu, K. Tunable Low Density Palladium Nanowire Foams. Chem. Mater. 2017, 29, 9814-9818. (44) Yan, P.; Brown, E.; Su, Q.; Li, J.; Wang, J.; Xu, C.; Zhou, C.; Lin, D. 3D Printing Hierarchical Silver Nanowire Aerogel with Highly Compressive Resilience and Tensile Elongation through Tunable Poisson’s Ratio. small, 2017, 13, 1701756. (45) Qian, F.; Lan, P. C.; Freyman, M. C.; Chen, W.; Kou, T.; Olson, T. Y.; Zhu, C.; Worsley, M. A.; Duoss, E. B.; Spadaccini, C. M.; Baumann, T.; Han, T. Y-J. Ultralight Conductive Silver Nanowire Aerogels. Nano Lett. 2017, 17, 7171-7176. (46) Zhang, Y. X.; Zeng, H. C. Gold Sponges Prepared via Hydrothermally Activated



Self-assembly of Au Nanoparticles. J. Phys. Chem. C 2007, 111, 6970-6975. (47) Rajamathi M.; Seshadri, R. Oxide and Chalcogenide Nanoparticles from Hydrothermal/Solvothermal Reactions. Curr. Opin. Solid State Mater. Sci. 2002, 6, 337-345. (48) Liu, J.; Raveendran, P.; Qin, G.; Ikushima, Y. Self-assembly of β-D Glucose-stabilized Pt Nanocrystals ito Nanowire-like Structures. Chem. Commun. 2005, 2972-2974. (49) Ranmohotti, K. G. S.; Gao, X.; Arachchige, I. U. Salt-mediated Self-assembly of Metal Nanoshells into Monolithic Aerogel Frameworks. Chem. Mater. 2013, 25, 3528-3534. (50) Lesnyak, V.; Wolf, A.; Dubavik, A.; Borchardt, L.; Voitekhovich, S. V.; Gaponik, N.; Kaskel, S.; Eychmüller, A. 3D Assembly of Semiconductor and Metal Nanocrystals: Hybrid CdTe/Au Structures with Controlled Content. J. Am. Chem. Soc. 2011, 133, 13413-13420. (51) Jones, C. D.; Steed, J. W. Gels with Sense: Supramolecular Materials that Respond to Heat, Light and Sound. Chem. Soc. Rev. 2016, 45, 6546-6596. (52) Wei, Z.; Yang, J. H.; Zhou, J.; Xu, F.; Zrínyi, M.; Dussault, P. H.; Osada, Y.; Chen, Y. M. Self-healing Gels Based on Constitutional Dynamic Chemistry and Their Potential Applications. Chem. Soc. Rev. 2014, 43, 8114-8131. (53) Yu, G.; Yan, X.; Han, C.; Huang, F. Characterization of Supramolecular Gels. Chem. Soc. Rev. 2013, 42, 6697-6722. (54) Yu, B. Y.; Wu, H. B.; Yan, Y.; Lou, X. W.; Wang, X. Ultrathin and Ultralong Single-crystal Platinum Nanowire Assemblies with Highly Stable Electrocatalytic Activity. J. Am. Chem. Soc. 2013, 135, 9480-9485. (55) Liu, Z.; Qi, J.; Liu, M.; Zhang, S.; Fan, Q.; Liu, H.; Liu, K.; Zheng, H.; Yin, Y.; Gao, C. Aqueous Synthesis of Ultrathin Platinum/Non-noble Metal Alloy Nanowires for Enhanced Hydrogen Evolution Activity. Angew. Chem. Int. Ed. 2018, 57, 11678-11682. (56) Huo, Z.; Tsung, C-K.; Huang, W.; Zhang, X.; Yang, P. Sub-Two Nanometer Single Crystal Au Nanowires. Nano Lett. 2008, 8, 2041-2044. (57) Mao, B.; Liu, B.; Wang, Y.; Li, G.; Song, Y.; Ma, L.; Liu, G. Preparation of Au Colloid of Small Size in Aqueous Solution. Rare. Metal. Mat. Eng. 2009, 38, 516-518. (58) Lu, Q.; Hutchings, G. S.; Yu, W.; Zhou, Y.; Forest, R. V.; Tao, R.; Rosen, J.; Yonemoto, B. T.; Cao, Z.; Zheng, H.; Xiao, J. Q.; Jiao, F.; Chen, J. G. Highly Porous Non-precious Bimetallic Electrocatalysts for Efficient Hydrogen Evolution. Nat. Comm. 2015, 6, 6567.


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A Versatile Strategy for Tailoring Noble Metal Supramolecular Gels

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