specific ion effects directed noble metal aerogels ... · vorable for handling and additional...

SC I ENCE ADVANCES | R E S EARCH ART I C L E

MATER IALS SC I ENCE

1Physical Chemistry, Technische Universität Dresden, Bergstr. 66b, 01062 Dresden,Germany. 2College of Chemistry and Materials Engineering, Wenzhou University,Wenzhou 325000, China. 3Helmholtz-Zentrum Dresden-Rossendorf, Institute of IonBeam Physics and Materials Research, Bautzner Landstrasse 400, 01328 Dresden,Germany. 4Theoretische Chemie, Fakultät für Chemie und Lebensmittelchemie,Technische Universität Dresden, 01062 Dresden, Germany.*Corresponding author. Email: [email protected] (A.E.);[email protected] (Y.H.)

Du et al., Sci. Adv. 2019;5 : eaaw4590 24 May 2019

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Specific ion effects directed noble metal aerogels:Versatile manipulation for electrocatalysis and beyondRan Du1, Yue Hu2*, René Hübner3, Jan-Ole Joswig4, Xuelin Fan1,Kristian Schneider1, Alexander Eychmüller1*

Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals andmonolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods,the sol-gel approachmanifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noblemetal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms re-tard structure/compositionmanipulation, hampering on-demand design for practical applications. Here, highly tun-able NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustablecomposition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superiorperformance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated.This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework forunderstanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance relationships for versatile applications.

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INTRODUCTIONInvestigation of functional porous materials (FPMs) is an everlastingtopic standing at the cutting edge of materials science because thecombined porous structures and versatile compositions of FPMs war-rant their remarkable performance in widespread fields (1). Foams,which are equal to aerogels in certain cases, are one type of widelystudied FPMs. They feature macroscopically monolithic structures fa-vorable for handling and additional applications and can be structuredfrom nearly any unit ranging from silica, nanocarbons, polymers, andtwo-dimensional (2D) materials to inorganic nanocrystals (2–6). As arising star in the foam family, noble metal foams (NMFs) raised tre-mendous interest upon their debut (7–9). Imparting 3D gel networkswith features of noble metals (e.g., high catalytic activity, high electricalconductivity, and plasmonic properties) has endowed NMFs with wideapplication potentials. However, their developments are still at the in-fant stage, where the fabrication strategies are more or less limited andstructures/properties cannot be well manipulated.

NMFs are typically fabricated by four classes of methods, i.e., deal-loying (9), templating (10, 11), direct freeze-drying (12, 13), and sol-gelprocess (14). Since only the sol-gel method has obtained great successin producing nanostructured and high–surface area NMFs under mildconditions, it immediately became the most popular synthetic strategyupon its introduction (14–19). Because the sol-gel–derived foams arein good agreement with the definition for aerogels, the termnoblemetalaerogels (NMAs) will be used in this manuscript afterward. Conven-tionally, NMAs were prepared by ultracentrifugation and subsequentdestabilization of the nanoparticles (NPs) solution to obtain noblemetalhydrogels (NMHs) in 1 to 2 weeks, followed by supercritical drying toafford the corresponding aerogels (14). Afterward, considerable effortswere made to enhance gelation kinetics, modulate microstructure, and

simplify fabrication procedures. Appropriate destabilizers (e.g., CaCl2)(15) or elevated temperature (333 K) (16) have been adopted to reducethe gelation time to several hours or several minutes depending on theprecursor concentrations (0.3 to 10mM). Using Ag nanoshells (17) orPdNi hollow spheres (18) as building blocks, hierarchically structuredNMAs with unique optical/electrochemical properties were obtained.To avoid the considerable costs incurred by the concentration process,Liu et al. (19) substantially simplified the gelation to one step, directlyinitiating Pd/Pt gels using specific amounts of NaBH4.

Despite substantial progress, the sol-gel method for preparation ofNMAs is still at its infant stage, and numerous mysteries remain in thisprocess, which largely constrains the available systems, impedes theunderstanding of the gelation mechanism, and blocks on-demand ma-nipulation. For example, regardless of the gelation method, ligamentsizes usually fall in the range of 3 to 6 nm for Pd/Pt and >100 nm forAu (14–16, 19, 20). Many strategies work well with the Pd/Pt/alloysystems but fail in obtaining nanostructured gold gels (14–16, 19). Ap-pealing core-shell–structured NMAs are obtained only in the form ofpowders, not to mention the tedious procedures involved (21). More-over, until now, the systematic modulation of the ligament size andcorresponding physicochemical properties have not been realized. In ad-dition, investigations of applications/properties of NMAs are almost on-ly restricted to electrocatalysis. Therefore, from both fundamental andpractical considerations, it is high time to develop distinct strategies thatnot only could fabricate and flexibly manipulate NMAs but also couldserve as a platform to study the gelation mechanism.

Salting out is a long-known method to destabilize colloidal parti-cles from solutions. This process is usually explained on the basis ofthe DLVO (Derjaguin-Landau-Vervey-Overbeek) theory, where intro-duced “undifferentiated” electrolyte ions raise the ionic strength, thuselectrostatically screening charged particles and inducing aggre-gation. However, specific ion effects emerge with increasing con-centration (e.g., ≥100 mM), and the Hofmeister series was proposedthereafter to arrange ions following their ability to salt out proteins(22–24). Thus, if the chemical diversity of different ions could be ac-tivated, unprecedented opportunities will be unlocked to engineering thegelation process and the corresponding gels. Although CaCl2 (≤1 mM)and NaCl (8 to 250 mM) have been used to prepare NMAs (15, 25),

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the adopted concentration is not suitable for unlocking specific ioneffects, and the systematic study of the specific ion effects is in absence.A handful of works found the role of specific anions in regulating thegelation behavior of supramolecular and polymer gels (26, 27), whilethe role of cations was scarcely considered. In this study, we presentthe rapid fabrication and flexible manipulation of NMAs by activatingand designing specific ion effects. By in-depth studying the gelation pro-cess experimentally and complementing DFT calculations, the specificion–directedmechanism is proposed, and the overall reaction process isoutlined. On this basis, versatile manipulations including compositions(Au, Ag, Pd, Pt, and alloys), ligament sizes (6.9 to 113.7 nm for gold and3.1 to 142.0 nm for others), specific surface areas (2.5 to 29.7 m2 g−1 forgold and 3.4 to 122.7m2 g−1 for others), and spatial element distribution(e.g., core-shell) are realized. The enormous ion library and the generalityof the presented method will offer unprecedented opportunities for fur-ther manipulation of NMAs and extending to diverse colloidal solutionsystems. Last, the interesting compression-induced dark-to-shiningtransition phenomenon, the programmable self-propulsion behavior,and the remarkable activity in electrocatalytic alcohol oxidation of as-prepared NMAs are demonstrated, suggesting their huge potentials inextensive fields.


RESULTS AND DISCUSSIONOverall gelation processBriefly, we added the as-prepared gold NP solution with specificsalts and grounded it 4 to 12 hours to yield the hydrogel, which wasfurther freeze-dried to obtain the corresponding aerogel (Fig. 1A,fig. S1, and movie S1). Here, the concentration of the metal pre-cursors is denoted as cM. Notably, the gel forms across wide cM(0.02 to 2 mM). The lowest cM (0.02 mM, ~5 days for gelation)was one to four order of magnitude lower than that of all reportedNMAs to date (0.2 mM to above 100 mM) (14–21, 25), indicating arobust gelation ability and completely eliminating needs for expensiveconcentration processes. In contrast, it is impossible to destabilize thegold NP solution (0.2 mM) by frequently used methods (14–16, 18)even with prolonged time (e.g., 3 days), such as addition of oxidantsor adopting elevated temperature (e.g., 348 K). The long gelation timeis a common issue for preparing NMHs. Although it has been cutdown to several minutes or several hours (15, 16), the expensive con-centration process or the elevated temperature involved can incur highcosts or unfavorable structures. Hence, our approach displays uniqueadvantages for rapid gelation at simultaneously low cM and ambienttemperature.

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Fig. 1. Analysis of the overall gelation process of gold NPs. (A) Digital photos of the gel preparation process. (B) Schematic demonstration of the gelation processand a corresponding force analysis. (C) The gradient distribution during the gelation characterized by ultraviolet-visible (UV-vis) absorption spectra. a.u., arbitrary units.(D) Several pieces of as-prepared hydrogels can assemble to one piece. (E to H) Time-lapse (E) UV-vis absorption spectra, (F) hydrodynamic size, (G) transmissionelectron microscopy (TEM), and (H) optical microscopy characterization during gelation. The inset in (E) shows the time-lapse UV-vis absorption evolution at 510 nm,which was recorded during the first minute upon reaction. (Photo credit: Ran Du.)

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More formidable issues remain in regulating multiscale struc-tures and properties of NMAs. These challenges are largelyattributed to the ambiguous understanding of the gelation process,which has only rarely been studied previously. To address theseissues, we thoroughly study the gelation process, which is pivotalin determining the as-obtained NMAs. As illustrated in Fig. 1Aand fig. S1, the red gold NP solution immediately turned blackupon reaction and displayed a vertical color gradient afterward, lastlyforming an extremely flexible hydrogel film at the bottom. This phe-nomenon is in sharp contrast with most systems of other materials(2, 4, 5, 28), where only the concentrated solution can afford free-standing gels, which are of similar size as the original volume. To ex-plain this unconventional phenomenon, we propose a gravity-drivenassembly model. As seen from Fig. 1B, salt-initiated aggregates grad-ually grow and settle down driven by gravity (see the SupplementaryMaterials) and lastly concentrate and evolve into the hydrogel at thebottom. To support the above model, we recorded ultraviolet-visible(UV-vis) absorption spectra of a “halfway” gel system at differentheights (Fig. 1C), unambiguouslymanifesting a gradient distributionalong the gravity direction. In addition, we demonstrate that aggregatesare active during the entire gelation process, which is evident from thespontaneous assembly of several fresh hydrogels into onemonolithic gel(Fig. 1D). Hydrogels can repair themselves after destruction (fig. S2),displaying promising self-healing properties in diverse environmentswithout external energy input. This remarkable self-healing behaviormay account for the observed monolithic structure of NMHs despitetheir fragility.

We further reveal the holistic picture of the gelation process byperforming several time-lapse characterizations. In situ UV-vis ab-sorption spectra show an instant change from the characteristic sur-face plasmon resonance (SPR) absorption of gold NPs (~514 nm) tobroadband absorption upon reaction (Fig. 1E). We performed thesingle-wavelength test within the first minute upon reaction as illus-trated in the inset of Fig. 1E. It shows that the SPR absorption of goldNPs disappeared within seconds, characterizing extremely fast forma-tion of aggregates with multiscale microstructures upon addition ofthe salts (29). The following intensity decrease agrees with the pro-posed gravity-driven sedimentation process. The in situ dynamic lightscattering (DLS) measurement indicated that the hydrodynamic size(dh) of NPs/aggregates rapidly increases from 5.0 ± 0.2 to 589 ± 11 nmwithin 2 min and achieves a maximum of 1131 ± 126 nm at ~60 min(Fig. 1F). Because of a positive correlation between the sedimentationspeed and the particle size (see the Supplementary Materials), largeraggregates fall down quickly and leave smaller ones in the solutionphase, resulting in decreasing aggregate sizes with prolonged time.In this way, we obtained a volcano-shaped dh-time curve due to thecompetition between growth and sedimentation of aggregates. Time-lapse transmission electron microscopy (TEM) and in situ optical testsfurther reveal the evolution footprints of 3D networks directly at dif-ferent scales (Fig. 1, G and H). The anisotropic growth of 0D NPs to1D nanowires can be attributed to the anisotropic character of electro-static repulsion. After the formation of gold NP dimers, the additionalNPs will preferentially attach to the dimers from their ends ratherthan from their sides due to less energy costs (30). By repeating thisprocess, NPs will grow mainly along the axial directions and eventu-ally form nanowire-structured networks. Thus far, we present anoverall picture of the sol-gel process by combining multiscale imag-ing techniques (10−9 to10−1 m), spectra analysis, and light scatteringmeasurements.


Specific ion effects in gelationTo manipulate NMAs, the microscale mechanism accounting for theNP growth needs to be studied in depth. In the present system, gelationwas induced by high-concentration salts (~102 mM) compared to pre-vious reports (15), satisfying the requirements to activate specific ioneffects dictated by theHofmeister series (23). Therefore, the current sys-temmay not only serve as an excellent platform to study the specific ioneffects in fabricating noblemetal gels but also provide an efficient tool toengineer the structures and the corresponding properties of NMAs.Here, 24 salts were selected and arranged according to the Hofmeisterseries, where the salting-out effect decreased from SO4

2− to SCN− foranions and from NH4

+ to Ca2+ for cations (table S1) (22, 24). As sum-marized in Fig. 2A, different ions imposed prominent effects on thecolor and the form of the final products (fig. S3 and S4), implicatingsuccessful activation of specific ion effects. The change of the color sug-gests the variation of the characteristic size of gold, while the differencein forms (powders or gels) implicates the disparate interactions of thebuilding blocks inside the products. These changes unambiguously in-dicate that the features of specific ions have been activated, enabling themodulation of the reaction pathways and thus the eventual structures.Generally, products changed fromblack to brown, from gels to powdersinduced by salts from the top left to the bottom right, roughly obeyingtheHofmeister series. This trend ismore evident for cations (along thex axis), which might be due to the enhanced cation-NP interaction re-sulting from small cation-NP distances, which are induced by electro-static attraction. The partially inversed order for double-charged Mg2+

and Ca2+ is presumably attributed to quite different surface charge en-vironments of different valance ions, which is common for the anom-alous Hofmeister series presented in specific cases (24). In contrast,anionsmay display complicated effects, involving both salting-out effectand competitive adsorption with citrates, thus leading to the gelationbehavior that deviates from the Hofmeister series (7). It is unexpectedto find that Cl− played an unusual role in boosting the ligament sizes,while the underlying mechanism is still under investigation.

On the basis of the observed phenomenon, the question ariseshow ions affect the color and form of the products. As dictated bythe Hofmeister series, gold NPs should be salted out less effectivelyfollowing the decreasing salting-out ability from NH4

+ to H+, presum-ably leading to gradually incomplete 3D networks, i.e., a gel-to-powdertransition. On the other hand, the color could reflect the ligament sizeto some extent. The black color often suggests small and hierarchicalmicrostructures resulting from multiple absorption and scattering oflight between numerous small-sized grains, while increasing ligamentsizes will direct the color of the materials to that of bulk gold (29). Thereason behind the ligament size evolution will be explained later. Wefurther correlate the gel status with the zeta potential and the DLS data(Fig. 2, B and C, and figs. S5 and S6). The absolute value of the zetapotential increased, and the maximum dh decreased following the blackgel, black powder, brown gel, and brown powder, suggesting increasedsolution stability and decreased network development, in line with theanalysis discussed before. In addition, we characterized the salting-outeffectiveness by the low-threshold gelation concentration (cs) of saltsand proved it to follow the Hofmeister series (Fig. 2D and fig. S7). Allexperiments above suggest that both the form (gel to powder) and color(black to brown) variation trends are strongly correlated to the salting-out effect of specific cations dictated by the Hofmeister series.

Aside from the salting-out function, cations also play a role in re-moving ligands. It is found that weakly bound citrates leave the NPsduring gelation, resulting in negligible residues in the final NP-fused

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3D networks (fig. S8). This phenomenon can only be caused by theintroduction of salts since other conditions remained unchanged.Considering the opposite charges between cations and citrates, oneexplanation might be that cations strip citrates away from NPs to al-low the development of gel networks. For verification, we performeddensity functional theory (DFT) calculations focusing on the citrate-cation interaction, so as to compare stripping ability of different cations(see the Supplementary Materials). Binding energies (Eb) are obtainedby calculating the energy change of the following reaction

RCOO� þ Xþ → RCOOX ð1Þ

where RCOO− and X+ denote citrate and cation, respectively. We con-sidered the solvent effects using the conductor-like screening modeldetailed in the Supplementary Materials. Eb represents the energy de-crease of the cation-ligand binding process and reflects the citrate re-moval efficiency of the different cations. As shown in fig. S9, thebinding energy of single-charged cations exactly followed theHofmeisterseries of NH4

+(0.44 eV) < K+(0.57 eV) < Na+(0.85 eV) < H+(6.59 eV).However, the order of H+ and double-charged cations following


Ca2+(2.27 eV) < Mg2+(2.96 eV) < H+(6.59 eV), which was inversed incomparison to the Hofmeister series. Although the order of bindingenergies given by the calculation partially inversed compared to theHofmeister series, it well conforms to the order of increasing chargedensity and suits the experimental phenomenon (Fig. 2A and table S1).Following the order of NH4

+, K+, Na+, Ca2+, Mg2+, and H+, the as-prepared products roughly evolved from gels to powders and fromblack to brown regardless of anions. The plotting of Eb versus ligamentsizes (fig. S9) further quantifies the trend of size evolution, generally dis-playing a positive relationship, which accords with the trend of colorchanges. It can be deduced that with increasing Eb from NH4

+ to H+,citrates will be more efficiently removed andmore active sites would beexposed. In this way, the isotropic growth from the enhanced van derWaals attraction between surfaces of gold NPs is promoted and over-whelms the anisotropic growth induced by anisotropic electrostaticrepulsion, which might be responsible for larger ligament sizes (30).Meanwhile, for high charge density cations (e.g., Mg2+, Ca2+, and H+),the very strong cation-citrate interaction can induce the fast formationof aggregates. Usually, too fast reaction kinetics is not preferred forgelation because the unstable environment may retard the intermedi-ates to develop intact interconnected networks. Whereas, by lowing

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Fig. 2. Analysis of the specific ion effects on the gelation behavior and ligament size. (A) Summary of the status of gels induced by different ions. The invertedtriangle and the diffused circle indicated the gel and powder, and black and brown indicated the color of the products. (B) Zeta potential upon reaction and (C) dhversus the color and form of products. The data were obtained by averaging detailed values from the inset diagram. (D) The low-threshold gelation concentration ofsalts (cs) versus the used cations. (E) The ligament size (averaged over the anions used as in the inset diagram) of as-synthesized gold aggregates versus cations.(F) Time-lapse ligament size evolution of gold aggregates induced by three typical salts. (G) Proposed mechanism for gel formation.

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the salt concentration from 33 to 3.3 mM (e.g., for MgCl2 or CaCl2),self-supporting gels can be derived, which is presumably attributed tothe decelerated reaction kinetics. Hence, in general, we observeincreased ligament size and unsupported products with higher Eb.

In virtue of specific ion effects analyzed above, we control the lig-ament size from 9.4 nm for NH4SCN to 199.6 nm for HCl (Fig. 2E).Compared to the Hofmeister series, certain inversions (K+ versus Na+)in ligament sizes might be attributed to similarities of certain cationsand to possible effects from anions. Statistics from time-lapse TEMimaging further revealed the growth mode of the NPs (Figs. 1G and2F), where the network development and the ligament size increaseoccurred simultaneously. Combining the above results, we proposea possible gelation mechanism (Fig. 2G and fig. S10): (i) The originalNPs instantly approach each other upon the addition of salts due toelectrostatic screening; (ii) the ligands are partially stripped away fromthe NPs by opposite-charged cations; (iii) the NPs fuse together to formaggregates driven by the raised surface energy of uncappedNPs; (iv) theaggregates repeat the above process and grow along both the axial andradial directions; and (v) the gravity-driven settlement of aggregatesfacilitates the hydrogel formation at the bottom. During gelation, thecations not only can precipitate the NPs by salting-out effects but alsocan strip ligands away from the NPs by electrostatic attraction. Thesecombined effects direct the cations to roughly follow the Hofmeisterseries, changing the products from gels to powders and from small tolarge ligament sizes. Further efforts, from both experimental and theo-retical sides, to verify the as-proposed mechanism and reveal the reac-tion process will be made in the near future.

Flexible manipulation of NMAsThe systematic manipulation of the ligament size and the corre-sponding physical properties of NMAs are the foundation for tailoringtheir specific applications, while it has not been realized previously.After a deep understanding of the gelation process and unlocking ofspecific ion effects, we devise the manipulation strategies in severalmanners. First, we deliberately selected specific salts (NH4SCN,NH4NO3, and KCl) as initiators based on the previous results (figs.S11 and S12), adjusting the ligament size from 8.9 ± 2.5 nm (forNH4SCN) to 80.3 ± 9.2 nm (for KCl) as characterized by TEM,scanning electron microscopy (SEM), and x-ray diffraction. In addi-tion, we achieved an even larger ligament size of 113.7 ± 16.2 nm byMgCl2-induced gels. The color of KCl-induced aerogel is brown andquite close to that of bulk gold, while the other two aerogels withsmaller ligament sizes appear black due to strong light absorption/scattering between nanosized domains (29). The change of the ligamentsize further tunes their density, specific surface area, and pore volumefrom83.0 to 212.8mgcm−3, 29.7 to 2.5m2 g−1 (5843.4 to 492.5m2mol−1),and 0.218 to 0.010 cm3 g−1, respectively. Second, we used a double-salt system to continuously adjust the ligament size between thevalues given by the two single-salt systems. As shown in Fig. 3A,NaCl/NaOH hybrid salts quasi-continuously modulate the ligamentsize from 64.0 ± 13.3 to 8.9 ± 1.8 nm. Instead of a linear variation, theligament size changed sharply by introducing a small amount of thesecond salt (2 to 10%) while keeping nearly unchanged in the broadmiddle region (NaOH% = 20 to 80%). Third, we modulated the liga-ment size by altering the concentrations of the used salts (NH4F wastaken as example) or gold precursors, ranging from 8.0 ± 1.1 to17.2 ± 4.6 nm and 6.9 ± 1.5 to 17.2 ± 3.9 nm, respectively (fig. S13).In virtue of the above strategies, the ligament sizes of the gold gelscould be designed from 6.9 to 113.7 nm, covering a wide region not


realized in previous reports (Fig. 3B) (9, 13, 14, 31). Moreover, thereis still ample space to extend the current systems and devise the gelparameters by elaborately selecting appropriate ions from the enor-mous ions library.We found that the used ligand, i.e., trisodium citrate,displayed—beyond its old role as only ligand or reductant—a newface as the gelation agent when its concentration is sufficiently high(fig. S14).

On the basis of the proposed gelation mechanism, the current sys-tem could be easily expanded to diverse noble metals (Ag, Pd, and Pt)and their alloys (fig. S15 and tables S2 and S3). Introducing a secondmetal to the gold system always leads to a reduction in ligament size,enabling fine-tuning the size from, e.g., 10.3 ± 2.1 to 3.7 ± 0.5 nm forthe Au/Pd system (Fig. 3C and fig. S16). Further investigations showedthat theAu,Ag, andAu/M (M=Pd, Pt) systemswith higherAu contentgive relatively large ligament sizes and are easier to bemodulated, whilePd and alloy systems with lower Au ratios resulted in small-sized gels(Fig. 3D and fig. S16). This phenomenon may partially result fromthe cohesive energy difference of metals, which positively correlateswith their enthalpy of vaporization (DHv/kJ mol−1), following Ag(258) < Au(324) < Pd(362) < Pt(469) (32). A lower DHv (Ag, Au) indi-cates weaker NP interactions, so that the NPsmay needmore coordinat-ing neighbors to stabilize the structure, which leads to large ligament sizes(33), and vice versa. Summarizing the data from all prepared NMAs, weobtain ligament sizes from 3.1 to 142.0 nm, densities from 44.0 ± 3.6 to212.8 ± 9.6mg cm−3, Brunauer-Emmett-Teller (BET) surface areas from122.7 to 2.5 m2 g−1 (15918.8 to 492.5 m2 mol−1), and Barrett–Joyner–Halenda (BJH) pore volumes from 0.704 to 0.013 cm3 g−1, demonstrat-ing the superiormanipulation capacity of the current strategy comparedto reportedmethods to date (table S4). Correlating the ligament sizewith diverse parameters/properties showed that, generally, the density/mechanical bending strength increase with increasing ligament size,while the BET surface area/pore volume decrease (Fig. 3, E and F). Thisprovides certain guidelines to engineer the physical parameters ofNMAs. Notably, because of the weak mechanical strength, softness,and irregular shape of the as-prepared NMAs, quantitative characteri-zation of theirmechanical properties is still a great challenge and cannotbe realized at this stage.

Among various NMAs, alloy aerogels always display a large ap-plication potential due to synergistic effects endowed by the multiplecomponents, especially when deliberately designed secondary struc-tures, e.g., hollow or core-shell architectures are included (16–19, 21).As seen from energy-dispersive x-ray spectroscopy (EDX) analysis inthe TEM, the spatial element distributions are either homogeneous[Pd-Pt (Fig. 3G)] or inhomogeneous [Au-Pd (fig. S17B) and Au-Pd-Pt(fig. S17D)] or show an imperfect core-shell structure [Au-Ag (fig. S17A)].To control the spatial distribution of elements, we develop a straight-forward dynamic shelling approach in the framework of the presentedstrategy. By simply introducing the second metal precursor during theinitial gelation stage of the firstmetal, the secondmetal can nucleate andgrow on the as-formed partially developed networks owing to the re-duced energy cost by the heterogeneous nucleation process, eventuallyresulting in core-shell–structured gels. This approach not only can yieldvarious bi- or trimetallic gelswithwell-defined core-shell structures (e.g.,Au-Ag, Au-Pd, Au-Pt, and Au-Pd-Pt) but also can modulate the shellthickness (0.5 to 2.5 nm) by simply adjusting the ratios of the differentcomponents (Fig. 3,H and I, and fig. S18). In comparisonwith previouslyreported core-shell–structured NMAs fabricated either via the under-potential deposition followed by a galvanic replacement reaction(PdxAu-Pt) (21) or via one-pot synthesis using a special reductant

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(PdPb-Pd) (34), the presented approach not only provides intactNMAswith well-defined and tunable core-shell structures but also manifestsits considerable generality and simplicity owing to its straightforwardmechanism.

Properties and applications of NMAsWe study certain properties and potential applications of as-preparedNMAs to demonstrate their practical values. Most bulk metals appearlustrous and white. In contrast, nanostructured NMAs appear black,which has been explained to be due to light trapping in hierarchical mi-crostructures because of multiple absorption and scattering by nearbygrains (29). On the other hand,mostmetals exhibit remarkable ductility(i.e., the plasticity), which is explained by their strong dislocation emis-sion ability (35). Therefore, it might be possible to induce a dark-to-shining transition by rearranging NMAs manually. As illustrated inFig. 4 (A to C), figs. S19 and S20, and movie S2, various NMAs couldbe easily pressed from a height ofmillimeters tomicrometers and there-by to regain metallic gloss via compacted nanostructured “mirrorsurfaces.” In addition, different aerogels could be welded togetherto form macroscopic heterostructures. Because of the extraordinary


plasticity, NMAs could be arbitrarily shaped and encased in elastomers(e.g., polydimethylsiloxane) for potential use as flexible conductors.

Using catalytic oxygen evolution by decomposingH2O2 (36), we usea high–surface area silver aerogel to serve as a powerful self-propulsiondevice as an alternative to expensive Pt-based materials (Fig. 4D). Theintensive catalytic reaction enables a maximum speed of 1.2 cm s−1 in1.5 weight % H2O2 solution, comparable to that of Ag micromotors(>0.1 cm s−1) andbioelectrochemical self-propulsion devices (~1.0 cm s−1)(36, 37). The motion form could be programmed using a heterostruc-tured Au-Ag gel (Fig. 4E and movie S3). Upon reaction, it rotated au-tomatically and displayed an angular speed of up to 168 rpm, which isdue to its nonsymmetrical structure where only the Ag part can catalyzeH2O2 for propulsion.

Last, we test the potential of NMAs in electrocatalysis by the alco-hol electro-oxidation reactions. We performed cyclic voltammogramsin the presence of ethanol, where the forward peak (i.e., the anodicpeak) represents the oxidation of freshly adsorbed ethanol, while thebackward peak indicates the removal of carbonaceous intermediatesproduced in the forward scan (20, 38). Hence, the peak current densityof the forward scan (If) and the ratio of the peak current densities of



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Fig. 3. Versatile manipulation of NMAs. (A) Tailor the ligament size of gold gels by introducing NaOH/NaCl hybrid salts. (B) Ligament size of gold aerogels fromdifferent references [references (Ref) a to d corresponds to (13), (9), (31), and (14), respectively]. (C) The ligament size variation with Au/Pd ratio. (D) Ligament sizemodulation of Au-Pd, Au-Pt, Pd, and Ag gels using different salts. (E) The dependence of density, Brunauer-Emmett-Teller (BET) surface area, and Barrett–Joyner–Halenda (BJH) pore volume of aerogels versus ligament size. (F) Demonstration of the size-dependent mechanical properties of aerogels by bending with a tweezer. Fromleft to right are Au-Ag-NH4F (5.8 ± 0.7 nm), Au-NH4SCN (8.9 ± 2.5 nm), Au-NH4NO3 (18.2 ± 4.0 nm), and Au-NaCl (64.0 ± 13.3 nm), respectively. (G to I) STEM–energy-dispersivex-ray spectroscopy (EDX) of three alloy gels with (G) homogeneous and (H and I) core-shell architectures.

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forward/backward scans (If/Ib) can serve as indicators to evaluate thecatalytic performance. As seen from Fig. 4, F and G, it is unexpectedthat high–surface area Pd and Pd-Pt aerogels (122.7 and 58.0 m2 g−1,respectively) show a lower performance compared to that of commer-cial Pd/C and Pt/C catalysts, presumably due to their less-continuousnetworks (fig. S15B). In contrast, Au-Pd and Au-Pd-Pt aerogels showedsubstantially higher performance with If of 2.65 and 4.82 A mgPd+Pt

−1,which are 2.8 to 6.1 times higher than that of commercial Pd/C orPt/C catalysts and higher than most reported NMAs such as Pd-Cu,Pd-Ni, and Au-Ag-Pd aerogels (ca. 2.0 to 5.6 times compared to Pd/C)(16, 18, 20, 38). Moreover, an If/Ib of 1.37 is achieved for Au-Pd-Ptaerogels, displaying advantage over that of commercial (0.90 to 1.15)and most reported NMAs catalysts (0.90 to 1.28) (16, 18, 20). However,we observed the considerably current decay of Au-Pd and Au-Pd-Ptaerogels during a long-term test (Fig. 4H and fig. S21), which is a com-mon issue for either commercial catalysts or reported Pd-based cata-lysts (16, 18, 20, 38). In addition, Au-Pd and Au-Pd-Pt aerogelsdelivered superior electrocatalytic performance for the methanol oxida-tion reaction (see fig. S21), displaying high If of 1.22 and 2.06 mgPd+Pt

−1

compared to that of Pd/C and Pt/C (<0.5 mgPd+Pt−1). This capability

may enable the possible use of aerogels as anodic catalysts for variousfuel cells. Aside from abundant active sites provided by the large spe-cific surface area, the exceptional performance of NMAs may also relyon the intact and highly conductive 3D networks provided by the goldcomponent, which can enhance the electrical conductivity, and thusfacilitating efficient electron transfer in electrocatalysis.


To sum up, we have developed a specific ion–directed gelationstrategy to rapidly fabricate and flexibly manipulate NMAs at roomtemperature from their NP solution. By activating specific ion effectsand subtly regulating the NP-ion interactions, diverse single-/multi-NMAswithwidely tunable compositions (Au,Ag, Pd, and Pt), ligamentsizes (3.1 to 142.0 nm), specific surface areas (2.5 to 122.7 m2 g−1), andspatial element distribution (e.g., core-shell) are obtained. Com-bining experimental results and DFT calculations, the gel status isfound to strongly depend on cation-ligand interactions, roughlyfollowing the Hofmeister series. On this basis, an overall picture ofthe sol-gel process is proposed, comprising electrostatic screening–induced aggregation, ligand stripping–directed NPs fusion, andgravity-driven sedimentation and gelation process. Last, severalintriguing properties/applications of NMAs are demonstrated, in-cluding compression-induced dark-to-shining transition, devisableself-propulsion behavior, and high electrocatalytic activity towardethanol/methanol oxidation. This study provides a conceptuallynew, general approach to fabrication of diverse NMAs, where flexi-ble manipulations of chemical composition, ligament size, specificsurface area, and spatial element distribution have been realized. Inaddition, certain perspectives in understanding the gelation processand underlying mechanism are proposed on the basis of experimen-tal and theoretical results. Therefore, this work may pave the way foron-target designing versatile NMFs for various applications andstudying the structure-performance relationship. Because of theenormous ion library and the generality of the gelation mechanism,


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Fig. 4. Plasticity and versatile applications of NMAs. (A) A piece of gold aerogel is manually pressed by ca. 98.5%. Scale bar, 30 mm (inset SEM image). (B) Originaland compressed NMFs made of Au, Ag, Pd, and Pt. (C) Heterostructured welded Au-Ag aerogel by pressing, the logo “TUD”made by compressing Au, Ag, and Au-Pd-Ptalloy aerogels, and a gold TUD logo encased in PDMS. (D and E) Self-propelling behavior of (D) Ag aerogel and (E) compressed Au-Ag aerogel in H2O2 solution. wt. %,weight %. Scale bars, 5 mm. (F to H) Ethanol electro-oxidation performance, (F) cyclic voltammetry curves, (G) summarized If and If/Ib, and (H) chronoamperometrycurves of different catalysts. (Photo credit: Ran Du.)

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the method we have introduced may also be adapted to versatilecolloidal solution systems for on-demand manipulation for desir-able applications.

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MATERIALS AND METHODSFabrication of NMHs and NMAsHydrogels were synthesized by the two-step method at ambient tem-perature (~293 K), i.e., the preparation of NP solutions and the follow-ing gelation. As an example, the aqueous solution of trisodium citratedehydrate (400.0 mM, 25.0 ml), HAuCl4·3H2O (32.5 mM, 30.8 ml), andNaBH4 (200.0 mM, 20.0 ml) were added successively to 4.93 ml of waterand stirred for 30 to 60min.After aging for ~1 day,NH4F (1.0M, 555.0 ml)solution was added, and the mixture was subsequently grounded for4 to 12 hours to acquire the hydrogel. After washing with water andexchanging with tert-butanol, the corresponding aerogel was ob-tained by freeze-drying for ~24 hours.

Electrochemical measurementsAll electrochemical tests were performed with a three-electrode system.The catalyst inkwas prepared by dispersing a specific amount of catalystin 2-propanol by sonication, and the accurate concentration of activecomponents (Pd andPt)was determined by inductively coupled plasmaoptical emission spectrometry. For electrocatalysis of ethanol andmeth-anol oxidation, the loading of active components was ~20 mg cm−2, andelectrochemical tests were performed in N2-saturated 1.0 M KOHaqueous solution containing 1 M ethanol or methanol.


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SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaaw4590/DC1Supplementary Materials and MethodsFig. S1. Characterizations of NP precursors.Fig. S2. Self-healing behavior of NH4F-induced gold hydrogels.Fig. S3. Digital photos of gelation behavior of gold NPs induced by different salts.Fig. S4. Digital photos of gelation behavior of gold NPs induced by four typical salts.Fig. S5. Zeta potential of gold NP solution after addition of different salts.Fig. S6. Time-lapse hydrodynamic size evolution during gelation.Fig. S7. The low-threshold gelation concentration and ligament sizes versus anions.Fig. S8. Residual analysis of as-prepared gold aerogels.Fig. S9. Energies derived by DFT calculations.Fig. S10. Proposed nanoscale force analysis and gelation mechanism.Fig. S11. Demonstration of the ligament size manipulation of gold aerogels usingspecific salts.Fig. S12. Nitrogen adsorption tests of different gold aerogels.Fig. S13. The relation of ligament size and precursors concentration.Fig. S14. Digital photos of gold gels initiated by other salts.Fig. S15. Demonstration and characterizations of diverse NMAs.Fig. S16. Ligament size manipulation of NMAs.Fig. S17. Scanning TEM–EDX analysis of different alloy gels prepared by one-step method.Fig. S18. High-angle annular dark-field scanning transmission electron microscopy imagingand EDX analysis of core-shell structured alloy gels.Fig. S19. SEM images of uncompressed aerogels.Fig. S20. Cross-sectional SEM images of compressed aerogels.Fig. S21. Electrocatalytic performance of different commercial and gel catalysts.Table S1. Summary of the gelation behavior of gold induced by different salts.Table S2. Summary of nitrogen adsorption data and ligament sizes of as-preparedaerogels.Table S3. Elemental analysis of different alloy aerogels.Table S4. Comparison of parameters of NMFs in literature.Movie S1. Demonstration of as-prepared black gels, brown gels, and black powders.Movie S2. Demonstration of pressing original aerogels into shining materials.Movie S3. Demonstration of self-propelled rotation of compressed Au-Ag aerogel.References (39–50)


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Acknowledgments: R.D. acknowledges the support from the Alexander von HumboldtFoundation. We thank M. Yu and I. Senkovska for gas adsorption tests; M. Yu for Fouriertransform infrared tests; S. Goldberg, C. Damm, and K. Nielsch for SEM and TEMmeasurements or training; L. Wang and J. Simmchen for optical imaging; X. Gao forx-ray photoelectron spectroscopy (XPS) measurements; and B. Cai, J. Wang, N. Mao,S. Deng, W. Liu, Y. Chen, and Y. Jiang for helpful discussions. X.F. acknowledges thefinancial support from the Chinese Scholarship Council. Funding: Furthermore, the useof HZDR Ion Beam Center TEM facilities and the funding of TEM Talos by the GermanFederal Ministry of Education of Research (BMBF), grant no. 03SF0451, in the framework ofHEMCP are acknowledged. This work was supported by the ERC AdG AEROCAT, theNational Natural Science Foundation of China (51602226), and the Natural ScienceFoundation of Zhejiang Province (LY19E020008). J.-O.J. acknowledges financial support bythe Deutsche Forschungsgemeinschaft for M-ERA.NET project ICENAP. Authorcontributions: R.D. conceived and designed experiments. R.D. and X.F. performed materialssynthesis. Y.H. carried out the SEM, XPS, and thermogravimetric (TGA) analyses. R.H. performedthe scanning TEM analysis. R.D. performed all other experimental characterizations andmeasurements. J.-O.J. carried out the computational part. R.D. and Y.H. performed the dataanalysis and mechanism explanation. R.D., A.E., Y.H., X.F., and K.S. discussed the manuscriptorganization. R.D. drafted the manuscript, and Y.H., A.E., R.H., and J.-O.J. revised it. All authorscommented on the manuscript. Competing interests: The authors declare that they have nocompeting interests. Data and materials availability: All data needed to evaluate theconclusions in the paper are present in the paper and/or the Supplementary Materials.Additional data related to this paper may be requested from authors.

Submitted 21 December 2018Accepted 15 April 2019Published 24 May 201910.1126/sciadv.aaw4590

Citation: R. Du, Y. Hu, R. Hübner, J.-O. Joswig, X. Fan, K. Schneider, A. Eychmüller, Specific ioneffects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond.Sci. Adv. 5, eaaw4590 (2019).

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and beyondSpecific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis

Ran Du, Yue Hu, René Hübner, Jan-Ole Joswig, Xuelin Fan, Kristian Schneider and Alexander Eychmüller

DOI: 10.1126/sciadv.aaw4590 (5), eaaw4590.5Sci Adv

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