The authors are grateful to Stephen Steiner at Aerogel Technologies for his inspiration and technical insights, and to Dr. Deryn Chu at the Army Research Laboratory-Sensors and Electron Devices Directorate, Dr. Christopher Haines at the Armament Research, Development and Engineering Center, U.S. Army RDECOM-ARDEC, and Dr. Stephen Bartolucci at the U.S. Army Benet Laboratories for their assistance. This work was supported by a Faculty Development Research Fund grant from the United States Military Academy, West Point.

CAUTION: Consult all relevant safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment. Rapid hydrogen gas evolution can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tube caps remain open as specified in the protocol.
1. Metal Gel Preparation
<ol>
	<li>Preparation of metal ion solutions. 
	<ol>
		<li>Prepare 2 mL of 0.1 M solutions of the following salts: HAuCl4&#8226;3H2O and Na2PdCl4 in deionized water. Prepare 2 mL of 0.1 M K2PtCl6 in a 1:1 (v/v) water and ethanol solvent. Vigorously shake and vortex solutions to aid in the dissolution of the salts.</li>
	</ol>
	</li>
	<li>Preparation of reducing agent solutions. 
	<ol>
		<li>Prepare 10 mL of 0.1 M solutions of the following reducing agents: dimethylamine borane (DMAB) and NaBH4 (sodium borohydride).</li>
	</ol>
	</li>
	<li>Preparation of Au gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M HAuCl4&#8226;3H2O solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with gold solution to ensure a rapid mix of salt and reducing agent solutions. Once the solutions are mixed, place the microcentrifuge tube vertically in a tube rack with the tube cap open. 
		Note: If the tube cap is left closed, hydrogen gas evolution will cause the pressure inside to force the cap to pop open and potentially spray the reduction mixture.</li>
	</ol>
	</li>
	<li>Preparation of Pd gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M Na2PdCl4 solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of NaBH4 into the microcentrifuge tube with palladium solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.</li>
	</ol>
	</li>
	<li>Preparation of Pt gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M K2PtCl6 solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with platinum solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.</li>
	</ol>
	</li>
	<li>Tube inversion. 
	<ol>
		<li>At approximately 5 min, cap the microcentrifuge tubes and gently invert 3-5 times to aid in the coalescence of metal particles not part of the metal gel. Ensure that the tube caps are immediately uncapped after inverting tubes, and replace tubes in a rack to maintain the vertical orientation of the tube.</li>
	</ol>
	</li>
	<li>Equilibration.
	<ol>
		<li>While Au, Pd, and Pt gels initially form within minutes, leave nascent gels in reducing agent solution for 3 &#8211; 6 h to allow for complete reduction of metal ions and for surface free energy minimization to occur. 
		Note: Metal gels occupy a smaller volume than the initial volume of mixed metal ion and reducing agent solution. Some additional slight volume contraction may be observed during equilibration time, and is more pronounced for gold gels and believed to be due to Ostwald ripening25.</li>
	</ol>
	</li>
	<li>Gel rinsing.
	<ol>
		<li>For Au, Pd, and Pt gels after the equilibration period, remove excess reducing agent solution, but leave enough solution volume so that the metal gel remains submerged. Ensure that the solution meniscus does not come in contact with the metal gel. 
		Note: Although the metal gels are stable enough to transfer between solutions with a spatula, capillary forces due to contact with the solution meniscus will deform and compress the gels resulting in an increase in the final aerogel density. This requires that some reducing agent solution remains in the tube with the gel submerged when transferring to deionized water.</li>
		<li>Slowly pipette deionized water to the top of the reaction microcentrifuge tubes. Submerge the microcentrifuge tube in a 50 mL conical tube full of deionized water and allow the gel to slide out of the microcentrifuge tube.</li>
		<li>Leave the gel in the deionized water for 24 h, and replace the water at 12 h. Do not to allow a liquid meniscus to come in contact with the gel.</li>
	</ol>
	</li>
</ol>
2. Electrochemical Surface Area (ECSA) Characterization of Wet Metal Gels
Note: Electrochemical characterization is performed on wet metal gels prior to conducting freeze drying. The resulting ECSA is then an estimate of the surface of the final aerogel structure. Nitrogen adsorption measurements are used to estimate the surface area of the dried aerogels.
<ol>
	<li>Solvent exchange.
	<ol>
		<li>Remove as much of the deionized water from the Au, Pd, and Pt rinse solutions as possible and ensure that the liquid meniscus does not come in contact with the gel.</li>
		<li>Add 50 mL, 0.5 M KCl to the conical tubes in order to exchange the deionized water with supporting electrolyte within the gel pores. Leave the gels in KCl solution for 24 h.</li>
	</ol>
	</li>
	<li>Working electrode preparation.
	<ol>
		<li>Coat a 1 mm platinum wire electrode with non-reactive lacquer using a fine bristle brush or other application device leaving a 4-5 mm length of the wire tip exposed.</li>
		<li>Allow 20 minutes for the lacquer to dry.</li>
		<li>Apply at least two coats of lacquer.</li>
	</ol>
	</li>
	<li>3-electrode cell set-up.
	<ol>
		<li>Use a 3-electrode cell set-up with an Ag/AgCl (3 M saturated) reference electrode, a 0.5 mm diameter Pt wire auxiliary/counter electrode, and the lacquer coated working electrode.</li>
		<li>Cut a plastic 50 mL conical tube in half and use as an electrochemical vial.</li>
		<li>Contact the gel with the working electrode with one of two methods: 1) impaled gel, or 2) contact mode.
		<ol>
			<li>Working electrode - impaled gel.
			<ol>
				<li>With the gel at the bottom of the modified 50 mL conical tube, gently insert the lacquer coated electrode into the gel. 
				Note: The impaled gel method proves more effective with Au gels, whereas Pd and Pt gels fracture more frequently upon electrode insertion.</li>
			</ol>
			</li>
			<li>Working electrode - contact mode.
			<ol>
				<li>Insert the lacquer coated working electrode into the conical tube along the inner surface and rest the metal gel on the top of the exposed Pt wire of the working electrode.</li>
			</ol>
			</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Electrochemical impedance spectroscopy (EIS).
	<ol>
		<li>Perform potentiostatic EIS scans with frequencies between 100 MHz and 1 mHz using a 10 mV amplitude sine wave. In the event of current overflows, use galvanostatic EIS with the same frequency range and a 100 &#8211; 200 mA amplitude sine wave.</li>
	</ol>
	</li>
	<li>Determination of electrochemical surface area (ECSA) from EIS data.
	<ol>
		<li>For Z&#34;, the imaginary component of impedance, at the lowest EIS frequency f of 1 mHz, and dividing by the sample mass, m, use the following equation to determine specific capacitance, Csp: 
		Csp = 1/(2&#960;fZ&#34;m)&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(1) 
		Note: Given that the ECSA is determined from a wet gel prior to freeze drying in Step 3 below, determine mass by assuming all of the metal ions in solution are reduced to form the gel. Based on this assumption, any actual yield less than 100% will result in underestimating Csp.</li>
	</ol>
	</li>
	<li>Cyclic voltammetry (CV).
	<ol>
		<li>Use scan rates of 100, 75, 50, 25, 10, 5, and 1 mV/s for CV measurements. Use voltage ranges of -0.2 to 0.2 V (vs Ag/AgCl) for Au gels, and select 0.1 to 0.4 V for Pd and Pt gels to avoid hydrogen adsorption and desorption, and oxidation-reduction of the metals.</li>
	</ol>
	</li>
	<li>Determination of electrochemical surface area (ECSA) from CV data.
	<ol>
		<li>Use the slowest CV scan rate of 1 mV/s, and calculate specific capacitance with the equation: 
		Csp = (&#8747;ivdv) / (2&#956;m&#916;V)&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(2) 
		Note: Here i and v are the current and potential in the CV scan (A and V), scan rate is &#956; (V/s), mass of the gel is m (g), and &#916;V is the potential window of discharge (vs Ag/AgCl).</li>
	</ol>
	</li>
</ol>
3. Aerogel Preparation and Characterization.
<ol>
	<li>Remove the deionized rinse water for Au, Pd, and Pt gels in Step 1.8 and ensure that the water meniscus does not come in contact with the metal gels.</li>
	<li>Place the gels in a -80 &#176;C freezer for no less than 30 min. Transfer the frozen metal gels to a freeze dryer with a set point pressure of 4 Pa or lower.</li>
</ol>

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The authors have nothing to disclose.

The noble metal aerogel synthesis method presented here results in the rapid formation of porous, high surface area monoliths that are comparable to slower synthesis techniques. The 1:1 (v/v) metal ion solution to reducing agent solution ratio is critical in facilitating the proposed gel formation model. The rapid hydrogen gas evolution as a by-product of the electrochemical reduction of metal ions serves as a secondary reducing agent and facilitates the densification, and fusion of growing nanoparticles during gel formation. Selection of the optimal combinations of metal ion types and reducing agents is also important given that many synthesis combinations shown in Table 1 do not result in gel formation.
Critical to preserving the gel structure subsequent to chemical reduction is to ensure that gel contact with the solution liquid-air surface is minimized in order to prevent compaction of the gel due to surface tension and capillary forces from water. Extended reduction and rinse times beyond those specified in the protocol may result in further compaction of the metal gels due to continued surface free energy minimization.
The presented synthesis method&#39;s primary benefit is the rapid formation of noble metal aerogels with feature sizes commensurate with slower synthesis techniques. The gel feature sizes are achieved without the use of preformed nanoparticles or capping agents during synthesis. Further the experimental correlation of pore and ligament sizes, and surface areas between SEM, XRD, nitrogen gas adsorption, EIS, and CV measurements suggests meaningful and reproducible values.
The protocol may be varied to scale up for the formation of larger aerogels by the use of 15 mL conical tubes with a 3 mL total reaction volume. However, increased reaction volumes are observed to yield increasing variability of final aerogel shape. This increased variability suggests that the aspect ratio of the reaction vessel relative to reaction volume is an important synthesis method consideration. While the primary benefit of the method is rapid gel formation, the lack of shape control reflects the most significant shortcoming for both small and large scale reactions. Future work includes the use of biotemplating and carbon composites as possible approaches to better achieve shape control35,36. As a rationally designed scaffold for metal reduction, biotemplated and composite materials may provide further control over ligament length, diameter, and aerogel shape. The direct and rapid synthesis method presented here offers an advancement in decreasing synthesis steps and times to achieve high specific surface areas, and offers a material approach for energy, catalysis, and sensor applications.

A wide range of energy storage and conversion, catalysis, and sensor applications benefit from three-dimensional metallic nanostructures which provide control over chemical reactivity, and mass transport properties1,2,3,4,5. Such 3-dimensional metallic nanostructures further enhance conductivity, ductility, malleability, and strength8,9. Integration into devices necessitates that materials be free-standing or combined with support materials. Incorporation of nanomaterials onto support structures provides a means of minimizing active material, but may suffer from weak adsorption and eventual agglomeration during device operation10,11.
While there are a variety of synthesis methods to control individual nanoparticle size and shape, few approaches enable control over contiguous 3-dimensional nanomaterials12,13,14. Noble metal 3-dimensional nanostructures have been formed through dithiol linkage of monodisperse nanoparticles, sol-gel formation, nanoparticle coalescence, composite materials, nanosphere chains, and biotemplating15,16,17,18. Many of these approaches require synthesis times on the order of days to weeks to yield desired materials. Noble metal nanofoams synthesized from the direct reduction of precursor salt solutions have been prepared with a faster synthesis timescale and with short-range order of hundreds of micrometers in length, but require mechanical pressing for device integration19,20.
First reported by Kistler, aerogels provide a synthesis route to achieve porous structures with high specific surface areas that are orders of magnitude less dense than their bulk material counterparts21,22,23. Extending 3-dimensional structures to the macroscopic length scale of bulk materials offers an advantage over nanoparticle aggregates or nanofoams that require support materials or mechanical processing. While aerogels provide a synthesis route to control porosity and particle feature size, however, extended synthesis times, and in some cases the use of capping agents or linker molecules, increases overall processing steps and time.
Here a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented24. Combining various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. The use of a microcentrifuge tube or small volume conical tube takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step facilitating a proposed nucleation, growth, densification, fusion, equilibration model for gel formation. A close correlation in aerogel nanostructure feature sizes is determined with scanning electron microscopy image analysis, X-ray diffractometry, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

<table border="0" cellpadding="0" cellspacing="0" width="691">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:215px;">HAuCl4&#376;&#8226;3H2O&#160;</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">16961-25-4</td>
			<td style="width:229px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:215px;">Na2PdCl4</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">13820-40-1</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:215px;">K2PtCl6</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">16921-30-5</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Pd(NH3)4Cl2</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">13933-31-8</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">K2PtCl4</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">10025-99-7</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Pt(NH3)4Cl2&#376;&#8226;H2O</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">13933-31-8</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">dimethylamine borane (DMAB)</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">74-94-2</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:215px;">NaBH4</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">16940-66-2</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:215px;">NaH2PO2&#376;&#8226;H2O</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td style="width:119px;">10039-56-2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Ethanol</td>
			<td style="width:128px;">Sigma-Aldrich</td>
			<td>792780</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Snap Cap Microcentrifuge Tubes, 2.0 mL</td>
			<td style="width:128px;">Cole Parmer</td>
			<td>UX-06333-70</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Snap Cap Microcentrifuge Tubes, 1.7 mL</td>
			<td style="width:128px;">Cole Parmer</td>
			<td>UX-06333-60</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Conical Centrifuge Tubes 15mL</td>
			<td style="width:128px;">Stellar Scientific</td>
			<td>T15-101&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Ag/AgCl Reference Electrode</td>
			<td style="width:128px;">BASi</td>
			<td style="width:119px;">MF-2052</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Pt wire electrode</td>
			<td style="width:128px;">BASi</td>
			<td style="width:119px;">MF-4130</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Miccrostop Lacquer</td>
			<td style="width:128px;">Tober Chemical Division</td>
			<td style="width:119px;">NA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Potentiostat</td>
			<td style="width:128px;">Biologic-USA</td>
			<td style="width:119px;">VMP-3</td>
			<td>Electrochemical analysis-EIS, CV</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">Freeze Dryer</td>
			<td style="width:128px;">Labconco</td>
			<td style="width:119px;">Freezone 2.5 Liter</td>
			<td>Aerogel freeze drying</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">XRD</td>
			<td style="width:128px;">PanAlytical</td>
			<td style="width:119px;">Empyrean</td>
			<td>X-ray diffractometry</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:215px;">Surface and Pore Analyzer</td>
			<td style="width:128px;">Quantachrome</td>
			<td style="width:119px;">NOVA 4000e</td>
			<td>Nitrogen gas adsorption</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:215px;">ImageJ, Image analysis software</td>
			<td style="width:128px;">National Institute of Health</td>
			<td style="width:119px;">NA</td>
			<td>SEM image analysis</td>
		</tr>
	</tbody>
</table>

a rapid synthesis method for au, pd, and pt aerogels via direct solution-based reduction

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

The addition of metal ion and reducing agent solutions together results in solutions immediately turning a dark black color with vigorous gas evolution. Observation of the reaction progress suggests the proposed gel formation mechanism shown in Figure 1. Gel formation proceeds through five steps of 1) nanoparticle nucleation, 2) growth, 3) densification, 4) fusion, and 5) equilibration. The first four steps are observed to occur during the first few minutes of the reaction, with the equilibration fifth step proceeding during the 3-6 h while the gel remains in the reducing agent solution, and continues during the deionized water rinse. Figure 2 shows representative Au, Pd, and Pt aerogels floating on the surface of water indicating a characteristic aerogel hydrophobicity. Pd and Pt gels remained black in color from the initial combination of metal ion and reducing agent solutions, whereas gold gels progressed from black at initial reduction to presenting a red-gold hue during the equilibration phase.
Figure 3 photo images depict wet metal gels after reduction with reducing agent solution replaced with deionized water. A range of metal ion solution concentrations less than the 0.1 M presented in Step 1 of the Protocol section were reduced with the same 0.1 M reducing agent concentrations. For the combinations of HAuCl4&#8226;3H2O with DMAB, Na2PdCl4 with NaBH4, and K2PtCl6 (in 50% ethanol) with DMAB, and metal ion concentrations of 5, 10, 25, 50, and 100 mM, gel size was found to decrease with decreasing metal ion concentration. The synthesis method presented here provides the significant advantage of rapid time scales to achieve the aerogel monoliths. However, the final gel size for 0.1 M metal concentrations is seen to be approximately &#188; to &#8531; of the synthesis solution volume. The gel formation mechanism facilitated by rapid hydrogen gas evolution results in fast gel formation, but ultimately results in a lack of shape control for this method.
To test the range of possible gel formation outcomes, various combinations of noble metal ions with three reducing agents were tested using the method presented in Protocol Step 1. The three reducing agents used were DMAB, NaBH4, and sodium hypophosphite (NaH2PO2). All reducing agents were used at 0.1 M concentration. The metal salts used were HAuCl43H2O, Na2PdCl4, Pd(NH3)4Cl2, K2PtCl4, Pt(NH3)4Cl2, and Na2PtCl6. Na2PtCl6 was prepared with deionized water and separately with a 1:1 deionized water to ethanol solvent. Metal ion concentrations were varied across a range of 100, 50, 25, 10, 5, 1, 0.5, and 0.1 mM. Table 1 indicates the minimum metal ion concentration at which gel formation was observed to occur. The metal ions and reducing agent combinations presented in Protocol step 1 resulted in the most reproducible and stable gels. While Au gels formed with the use of NaBH4, gel shapes were more varied and presented a rougher macroscopic surface texture. Pd gels formed with Na2PdCl4 and Pd(NH3)4Cl2 yielded similar results. Stable Pt gels were difficult to achieve using Pt(NH3)4Cl2 and K2PtCl4 likely owing to the thermodynamic stability of the square planar platinum ions. Large gel aggregates formed with Na2PtCl6 prepared in deionized water solvent, whereas the use of Na2PtCl6 prepared in a 1:1 deionized water to ethanol solvent more consistently resulted in monolithic gels. The addition of ethanol as a solvent was based on reports of Pt nanoparticle instability in ethanol to drive particle aggregation and fusion. In general, gel shape became more variable for decreasing precursor salt concentrations for Au, Pd, and Pt.
Scanning electron microscope (SEM) analysis of the gels reveals a hierarchical pore structure for Au, Pd, and Pt aerogels as seen in Figure 4. The range of pore and ligament sizes were determined from SEM image analysis with 200 - 300 measurements for each value. Au aerogels present large macropores with a range of 50 - 600 nm, and smooth ligaments with a diameter range of 18 - 280 nm with an average of 63.7 &#177; 36.0 nm. Pd and Pt aerogels differ in their structure from Au aerogels, in that they present a &#34;beads-on-a-string&#34; structure with fused nanoparticles. Pd macropores ranged from 50 - 340 nm, with ligament diameters ranging from 12 - 65 nm, with an average of 34.5 &#177;&#160;9.5 nm. Pt macropores range from 50 - 470 nm, with ligament diameters ranging from 13 - 60 nm, with an average of 29.7 &#177;&#160;9.0 nm24. The greater range in Pt macropore structure is attributed to Pt nanoparticle stability and the need for ethanol to drive coalescence, and consequent ease of large hydrogen gas bubble evolution during the gel formation process. X-ray diffraction (XRD) spectra from 20 - 70 &#176; in Figure 5 indicate characteristic peaks for Au, Pd, and Pt aerogels with no detectable oxides.
Nitrogen gas physisorption isotherms are shown for Au, Pd, and Pt aerogels in Figure 6a, 6c, and 6e, and indicate a Type IV isotherm characteristic of mesoporous materials with pores predominantly ranging from 2 &#8211; 50 nm28 . The Barrett&#8211;Joyner&#8211;Halenda (BJH) model for desorption curves was used for Figure 6b, 6d, and 6f to show the cumulative pore volume (cm3/g) and pore size distribution (dV/dd) for Au, Pd, and Pt aerogels with pore sizes in the 2 &#8211; 50 nm mesoporous range29 . To determine the aerogel specific surface areas, a multi-point Brunauer-Emmett-Teller (BET) model was used to analyze adsorption isotherms in Figure 6.30 For Au, Pd, and Pt aerogels this resulted in values of 3.06, 15.43, and 20.56 m2/g respectively. Noble metal aerogels synthesized from sol-gels of preformed nanoparticles have achieved similar specific surface areas31. Idealized ligament diameters based on the specific surface areas are 85.4, 33.1, and 13.6 nm for Au, Pd and Pt aerogels and generally correlate to feature sizes determined with SEM image analysis.
Electrochemical impedance spectra for Au, Pd, and Pt gels are shown in Figure 7a. Specific capacitance is plotted as a function of EIS frequency in Figure 7b with estimates of 2.18. 4.13, and 4.20 F/g for Au, Pd, and Pt gels. Based on a nominal 30 &#181;F/cm2 for metal surfaces, Au, Pd, and Pt specific surface areas are 7.27, 13.77, and 14.00 m2/g32. EIS spectra were fit with a transmission line model (TLM) based on a modified Randle&#39;s equivalent circuit model shown in Figure 7c. In this model, resistors (R), capacitors (C) or constant phase elements (CPE), and restricted diffusion elements (Zdiff) are connected in parallel and series. Electrolyte resistance and contact resistance with the working electrode at high frequency is represented by R1. Double layer capacitance, charge transfer, material resistance, and restricted ion diffusion through the hierarchically distributed pore network is represented by the parallel arrangement of CPE or C elements with serially connected R and Zdiff33,34. The TLM model effectively modeled EIS spectra for Au, Pd, and Pt gels.
Cyclic voltammetry scans are shown in Figure 8a-8c for Au, Pd, and Pt gels respectively. Using the 1 mV/s CV scan rate, specific capacitances for Au, Pd, and Pt gels were calculated to be 2.67, 7.99, and 5.12 F/g, and using the nominal value of 30 &#181;F/cm2, the same as for EIS capacitances, yields specific surface areas 8.90, 26.63, and 17.07 m2/g.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td colspan="3">Reducing Agent</td>
		</tr>
		<tr>
			<td rowspan="2">Salt&#160;</td>
			<td>DMAB</td>
			<td>NaBH4</td>
			<td>NaHPO2</td>
		</tr>
		<tr>
			<td>100 mM</td>
			<td>100 mM</td>
			<td>100 mM</td>
		</tr>
		<tr>
			<td>[AuCl4]-</td>
			<td>10 mM</td>
			<td>5 mM</td>
			<td>No Gel</td>
		</tr>
		<tr>
			<td>[Pd(NH3)4]2+</td>
			<td>25 mM</td>
			<td>5 mM</td>
			<td>No Gel</td>
		</tr>
		<tr>
			<td>[PdCl4]2-</td>
			<td>25 mM</td>
			<td>5 mM</td>
			<td>50 mM</td>
		</tr>
		<tr>
			<td>[Pt(NH3)4]2+</td>
			<td>No Gel</td>
			<td>100 mM</td>
			<td>No Gel</td>
		</tr>
		<tr>
			<td>[PtCl4]2-</td>
			<td>No Gel&#160;</td>
			<td>100 mM</td>
			<td>No Gel</td>
		</tr>
		<tr>
			<td>[PtCl6]2-</td>
			<td>25 mM</td>
			<td>No Gel</td>
			<td>No Gel</td>
		</tr>
		<tr>
			<td>[PtCl6]2-</td>
			<td rowspan="2">10 mM</td>
			<td rowspan="2">No Gel</td>
			<td rowspan="2">No Gel</td>
		</tr>
		<tr>
			<td>50% EtOH</td>
		</tr>
	</tbody>
</table>
Table 1. Concentration thresholds for gel formation for different combinations of salt type and reducing agents. Reproduced from reference 24 with permission.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57875/57875fig1.jpg" /> 
Figure 1. Proposed mechanism of noble metal gel formation. Synthesis proceeds via (a) initial nucleation of nanoparticles, (b) rapid growth of nanoparticles, (c) densification of nanoparticles due to hydrogen gas evolution, (d) fusion of nanoparticles, and finally (e) surface free energy minimization and equilibration of resulting gel. Reproduced from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57875/57875fig2.jpg" /> 
Figure 2. (a) Gold, (b) palladium, and (c) platinum aerogels floating on water. Modified from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57875/57875fig3.jpg" /> 
Figure 3. Noble metal gel synthesis across a range of salt concentrations from right to left of 100, 50, 25, 10, and 5 mM. (a) [AuCl4]- reduced with 100 mM DMAB. (b) [PdCl4]2- reduced with 100 mM NaBH4. (c) [PtCl6]2- prepared in 50% ethanol, reduced with 100 mM DMAB. Modified from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57875/57875fig4.jpg" /> 
Figure 4. Scanning electron micrographs of (a)-(b) gold, (c)-(d) palladium, and (e)-(f) platinum aerogels. Reproduced from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57875/57875fig5.jpg" /> 
Figure 5. XRD spectra for platinum (top), palladium (middle), and gold (bottom) aerogels. Modified from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57875/57875fig6.jpg" /> 
Figure 6. Nitrogen adsorption-desorption isotherms, and pore size distribution with cumulative pore volume for (a)-(b) gold, (c)-(d) palladium, and (e)-(f) platinum aerogels. Reproduced from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/57875/57875fig7.jpg" /> 
Figure 7. (a) Electrochemical impedance spectra for gold, palladium, and platinum gels performed in 0.5 M KCl vs Ag/AgCl reference electrode. (a) (inset) Low frequency EIS spectra from (a). (b) Specific capacitance (Csp) for gels calculated from low frequency EIS spectra in (a). (b) (inset) Low frequency Csp values. (c) RLC fitted transmission line model (TLM) for EIS spectra. Reproduced from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/57875/57875fig8.jpg" /> 
Figure 8. Cyclic voltammetry at scan rates of 50, 25, 10, 5, and 1 mV/s for (a) gold, (b) palladium, and (c) platinum gels. Voltage windows are (a) -0.2 V to 0.2 V, and (b)-(c) 0.1 V to 0.4 V (vs Ag/AgCl). Reproduced from reference 24 with permission. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57875/57875fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction at JoVE.com

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...

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8.6K Views.  United States Military Academy, West Point. A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Video: A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10-3 molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

Synthesis of Protein Bioconjugates via Cysteine-maleimide Chemistry

The chemical linking or bioconjugation of proteins to fluorescent dyes, drugs, polymers and other proteins has a broad range of applications, such as the development of antibody drug conjugates (ADCs) and nanomedicine, fluorescent microscopy and systems chemistry. For many of these applications, specificity of the bioconjugation method used is of prime concern. The Michael addition of maleimides with cysteine(s) on the target proteins is highly selective and proceeds rapidly under mild conditions, making it one of the most popular methods for protein bioconjugation.
We demonstrate here the modification of the only surface-accessible cysteine residue on yeast cytochrome c with a ruthenium(II) bisterpyridine maleimide. The protein bioconjugation is verified by gel electrophoresis and purified by aqueous-based fast protein liquid chromatography in 27% yield of isolated protein material. Structural characterization with MALDI-TOF MS and UV-Vis is then used to verify that the bioconjugation is successful. The protocol shown here is easily applicable to other cysteine - maleimide coupling of proteins to other proteins, dyes, drugs or polymers.

Synthesis of Protein Bioconjugates via Cysteine-maleimide Chemistry

This protocol details the important steps required for the bioconjugation of a cysteine containing protein to a maleimide, including reagent purification, reaction conditions, bioconjugate purification and bioconjugate characterization.

The chemical linking or bioconjugation of proteins to fluorescent dyes, drugs, polymers and other proteins has a broad range of applications, such as the ...

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica glass ampules). Reduction of such phases by elemental alkaline metals in amines affords crystalline chalcogenidoplumbate(II) salts comprised of [PbTe3]2- or [Pb2Ch3]2- anions, depending upon which sequestering agent for the cations is present: crown ethers, like 18-crown-6, or cryptands, like [2.2.2]crypt. Reactions of solutions of such anions with transition-metal compounds yield (poly-)chalcogenide anions or transition-metal chalcogenide clusters, including one with a &#181;-PbSe ligand (i.e., the heaviest-known CO homolog).
In contrast, the solid-state synthesis of a phase of the nominal composition &#34;K2PbSe2&#34; by successive reactions of the elements and by the subsequent solvothermal treatment in amines yields the first non-oxide/halide inorganic lead(IV) compound: a salt of the ortho-selenidoplumbate(IV) anion [PbSe4]4-. This was unexpected due to the redox potentials of Pb(IV) and Se(-II). Such methods can further be applied to other elemental combinations, leading to the formation of solutions with binary [HgTe2]2- or [BiSe3]3- anions, or to large-scale syntheses of K2Hg2Se3 or K3BiSe3 via the solid-state route.
All compounds are characterized by single-crystal X-ray diffraction and elemental analysis; solutions of plumbate salts can be investigated by 205Pb and 77Se or 127Te NMR techniques. Quantum chemical calculations using density functional theory methods enable energy comparisons. They further allow for insights into the electronic configuration and thus, the bonding situation. Molecular Rh-containing Chevrel-type compounds were found to exhibit delocalized mixed valence, whereas similar telluridopalladate anions are electron-precise; the cluster with the &#181;-PbSe ligand is energetically favored over a hypothetical CO analog, in line with the unsuccessful attempt at its synthesis. The stability of formal Pb(IV) within the [PbSe4]4- anion is mainly due to a suitable stabilization within the crystal lattice.

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of ChalcogenidoplumbatesII or IV

The synthesis of chalcogenidoplumbates(II,IV) via the in situ reduction of nominal "PbCh2" (Ch = Chalcogen) and via a solid-state reaction and subsequent solvothermal reactions is presented. Additionally, reactivities of plumbate(II) solutions are portrayed, which yield the heaviest-known CO homolog known to date: the &#181;-PbSe ligand.

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica ...

Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are demonstrated here. The catalyst forms a substitutional solid solution structure, with a small and uniform particle size around 2 nm. This is realized by careful control over the impregnation, calcination, and reduction steps during catalyst preparation and is identified by advanced in situ synchrotron techniques. The catalyst propane dehydrogenation performance continuously improves with increasing Cu:Pt atomic ratio.

A convenient method for the synthesis of 2 nm supported bimetallic nanoparticle Pt-Cu catalysts for propane dehydrogenation is reported here. In situ synchrotron X-ray techniques allow for the determination of the catalyst structure, which is typically unobtainable using laboratory instruments.

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are ...

Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 &#176;C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 &#176;C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

&#160; The protocol describes the synthesis and electrochemical testing of platinum-nickel nanowires. Nanowires were synthesized by the galvanic displacement of a nickel nanowire template. Post-synthesis processing, including hydrogen annealing, acid leaching, and oxygen annealing were used to optimize nanowire performance and durability in the oxygen reduction reaction.

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen ...

Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three preparation methods are described: (a) the incorporation of metal salts into silica or alumina wet gels using an impregnation method; (b) the incorporation of metal salts into alumina wet gels using a co-precursor method; and (c) the addition of metal nanoparticles directly into a silica aerogel precursor mixture. The methods utilize a hydraulic hot press, which allows for rapid (&#60;6 h) supercritical extraction and results in aerogels of low density (0.10 g/mL) and high surface area (200-800 m2/g). While the work presented here focuses on the use of copper salts and copper nanoparticles, the approach can be implemented using other metal salts and nanoparticles. A protocol for testing the three-way catalytic ability of these aerogels for automotive pollution mitigation is also presented. This technique uses custom-built equipment, the Union Catalytic Testbed (UCAT), in which a simulated exhaust mixture is passed over an aerogel sample at a controlled temperature and flow rate. The system is capable of measuring the ability of the catalytic aerogels, under both oxidizing and reducing conditions, to convert CO, NO and unburned hydrocarbons (HCs) to less harmful species (CO2, H2O and N2). Example catalytic results are presented for the aerogels described.

Here we present protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms. Methods for preparing materials using copper salts and copper-containing nanoparticles are featured. Catalytic testing protocols demonstrate the effectiveness of these aerogels for three-way catalysis applications.

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three ...

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity in aqueous CO2 reduction. Different from traditional metal nanocrystals, the synthesis of metal single atoms involves a matrix material that can confine those single atoms and prevent them from aggregation. We report an electrospinning and thermal annealing method to prepare Ni single atoms dispersed and coordinated in a graphene shell, as active centers for CO2 reduction to CO. During the synthesis, N dopants play a critical role in generating graphene vacancies to trap Ni atoms. Aberration-corrected scanning transmission electron microscopy and three-dimensional atom probe tomography were employed to identify the single Ni atomic sites in graphene vacancies. Detailed setup of electrochemical CO2 reduction apparatus coupled with an on-line gas chromatography is also demonstrated. Compared to metallic Ni, Ni single atom catalyst exhibit dramatically improved CO2 reduction and suppressed H2 evolution side reaction.

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

Here, we present a protocol for the synthesis and electrochemical testing of transition metal single atoms coordinated in graphene vacancies as active centers for selective carbon dioxide reduction to carbon monoxide in aqueous solutions.

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity ...

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another &#34;bad&#34; ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.