1. Emerging Noble Metal Aerogels: State of the Art and a Look Forward
Ran Du, Xuelin Fan, Xinyi Jin, René Hübner, Yue Hu, Alexander Eychmüller 
Matter 
Volume 1, Issue 1, Pages (July 2019)
DOI: /j.matt
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2. Matter 2019 1, 39-56DOI: ( /j.matt )
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3. Figure 1 An Overall Picture of Noble Metal Aerogels
(A–D) A gold aerogel presented on multiple scales: digital photo (A), optical image (B), SEM image (C), and TEM image (D).
(E) The typical chemical compositions of NMAs.
(F) Key progress of NMAs fabricated by sol-gel methods and non-sol-gel methods.
Reprinted from Erlebacher et al.,6 Walsh et al.,10 and Biener et al.,11 with permission. Copyright 2001&2003&2009, Springer Nature. Reprinted from Gao et al.,12 Freytag et al.,13 Bigall et al.,7 Liu et al.,14 and Zhu et al.,15 with permission. Copyright 2009&2013&2014&2016, Wiley-VCH Verlag GmbH. KGaA, Weinheim. Reprinted from Gao et al.,16 with permission. Copyright 2014, American Chemical Society. Reprinted from Du et al.,17 with permission. Copyright 2019, American Association for the Advancement of Science.
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4. Figure 2
The Strategies for Fabricating Noble Metal Gels via Sol-Gel Methods
Single-level structured gels can be created by routes 1 and 2 (one-step and two-step methods), while multi-level structured gels are prepared by routes 3–6 (bottom-up or post engineering). NPs denote nanoparticles and SE-NPs denote specially engineered nanoparticles.
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5. Figure 3 Noble Metal Aerogels Derived by Diverse Sol-Gel Tactics
(A) The formation of gold gels induced by the destabilization of Au NPs with dopamine. Reprinted from Wen et al.,36 with permission. Copyright 2016, American Chemical Society.
(B) The Pd-Pt alloy gel obtained by one-step reduction of the corresponding metal salts precursors with a specific amount of NaBH4. Reprinted from Liu et al.,14 with permission. Copyright 2013, Wiley-VCH Verlag GmbH. KGaA, Weinheim.
(C and D) The Pd-Ni hollow-structured alloy gel derived by a two-step sol-gel method. Reprinted from Cai et al.,37 with permission. Copyright 2015, Wiley-VCH Verlag GmbH. KGaA, Weinheim.
(E) The Pd-Pb core-shell-structured gel derived by a one-step sol-gel method. Reprinted from Zhu et al.,38 with permission. Copyright 2018, The Royal Society of Chemistry.
(F) The formation of Pt gels induced by the destabilization of Pt cubes with hydrazine in the organic phase. Reprinted from Naskar et al.,39 with permission. Copyright 2017, American Chemical Society.
(G) The Ag-Pt hierarchical-structured gel derived by post-galvanic replacement reaction. Reprinted from Liu et al.,40 with permission. Copyright 2016, American Chemical Society.
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6. Figure 4 The Proposed Model and the Mechanism of the Gelation Process
(A and B) Digital photos (A) and the gravity-driven model (B) for the gelation process of a gold gel. Reprinted from Du et al.,17 with permission. Copyright 2019, American Association for the Advancement of Science.
(C) The electrostatic crosslinking mechanism of the Ca2+-induced gelation of palladium NPs. Reprinted from Wen et al.,45 with permission. Copyright 2014, American Chemical Society.
(D) The possible gelation mechanism induced by the salting-out effect. Reprinted from Du et al.,17 with permission. Copyright 2019, American Association for the Advancement of Science.
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7. Figure 5 Diverse Applications of Noble Metal Aerogels
(A) OER reactions catalyzed by Ir-Cu aerogels in O2-saturated 0.1 M HClO4 aqueous solution. The linear sweep voltammetry (LSV) curves are obtained at a rotation rate of 1,600 rpm at a scan rate of 10 mV s−1. The inset shows a high-angle annular dark-field scanning TEM image of the Ir-Cu aerogel. Reprinted from Shi et al.,58 with permission. Copyright 2018, American Chemical Society.
(B and C) CO2-RR catalyzed by Pd-Cu aerogels in [Bmim]BF4 aqueous solution with 25 mol % [Bmim]BF4 and 75 mol % water. (B) LSV curves for PdxCuy aerogels: (a) Pd67Cu33, (b) Pd71Cu29, (c) Pd83Cu17, (d) Pd87Cu13, and (e) Pd89Cu11. The inset shows an SEM image of the aerogel. (C) Faradic efficiency for CH3OH over PdxCuy aerogels. Reprinted from Lu et al.,50 with permission. Copyright 2018 Wiley-VCH Verlag GmbHa. KGaA, Weinheim.
(D and E) The setup of chemically driven actuation by gold foams (D), and the strain versus time curve under alternate exposure of 7% O3 (in O2) and pure CO (E). Reprinted from Biener et al.,11 with permission. Copyright 2009, Springer Nature.
(F) SERS detection of Rd-101 by using an Au-Ag alloy aerogel as substrate. The inset shows the corresponding Raman spectra and the molecular structure of Rd-101. Reprinted from Gao et al.,43 with permission. Copyright 2016, American Chemical Society.
(G) Hydrogen storage by using nanostructured palladium foams. The inset shows the hydrogen absorption/desorption isotherms of palladium foams and nanoparticles. Reprinted from Gilbert et al.,28 with permission. Copyright 2017, American Chemical Society.
(H) Self-propulsion by using compressed gold-silver aerogels. Compressed gold/silver aerogels rotated in H2O2 solution. Reprinted from Du et al.,17 with permission. Copyright 2019, American Association for the Advancement of Science.
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