Published: September 06, 2011

r 2011 American Chemical Society 4386 dx.doi.org/10.1021/nl202538q |Nano Lett. 2011, 11, 4386–4392

LETTER

pubs.acs.org/NanoLett

Monitoring of Galvanic Replacement Reaction between SilverNanowires and HAuCl4 by In Situ Transmission X-ray MicroscopyYugang Sun*,† and Yuxin Wang‡

†Center for Nanoscale Materials and ‡X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne,Illinois 60439, United States

bS Supporting Information

Galvanic replacement reactions between solid metal nano-particles with oxidizing metal precursors represent a versa-

tile strategy for synthesizing hollow metal nanostructures withtailored properties that are difficult (or even impossible) toachieve from their solid counterparts.1 For example, reac-tions of silver nanocubes with aqueous solutions of HAuCl4enable the synthesis of hollow nanoboxes with seamless wallsand nanocages/nanoframes with porous walls that exhibitabsorption bands tunable in the range of both visible andnear-infrared (NIR) regions2,3 and have been used in bio-logical imaging,4 medical therapy,5,6 drug delivery,7 catalysis,8

sensing,9 etc. Similar reactions can also convert silver nano-wires into nanotubes made of other metals with uniqueproperties.3,10�12 Extensive studies have been done to inves-tigate the mechanism involved in the nanoscale galvanicreplacement reactions,3 but direct observation of morpholo-gical evolution of the nanoparticles has not been reported dueto the lack of in situ imaging techniques that are compatiblewith solution-based reactions. Most recently, in situ transmis-sion electron microscopy (TEM) combined with a special thincell has been used to study the growth of Pt nanoparticles froma solution of Pt(acetylacetonate)2 in a mixture of o-dichlor-obenzene and oleylamine through the reduction of Pt(II) withsolvated free electrons generated from inelastic scattering ofthe incident electron beam.13 However, the in situ TEM cell isnot suitable for studying the galvanic replacement reactionbetween silver nanowires and HAuCl4 solution because mix-ing them together in the cell immediately initiates the reactionbefore the cell can be loaded in the TEM chamber. In addition,the spacing between the two window membranes of the TEMcell cannot exceed 200 nm in order to allow the electron

beams to pass through the loaded solution for imaging.The small reaction chamber makes it difficult to load silvernanowires with diameters larger than 100 nm into the cell.Herein, we report the use of in situ transmission X-raymicroscopy (TXM) in combination with a flow cell for real-time monitoring the reaction between silver nanowires andHAuCl4. The use of a flow cell allows us to trigger the reactionby delivering a HAuCl4 solution into the cell when the TXM isready for imaging. The large penetration length of hard X-raysin solution makes it possible to use reaction chambers as thickas hundreds of micrometers that allow easy manipulation ofnanowires as well as potential in situ three-dimensional (3D)imaging with computed tomography (CT) technique. Theflow cell TXM technique can overcome the limitations ofin situ TEM for successfully observing the real-time morpho-logical evolution of silver nanowires in the course of a galvanicreplacement reaction with an aqueous solution of HAuCl4 atroom temperature.

A transmission X-ray microscope system works similarly toa conventional optical microscope. Figure 1 gives the sche-matic configuration of a typical transmission X-ray micro-scope that consists of a condenser lens, a beam stop, apinhole, an objective lens (i.e., Fresnel zone plate), and acharge-coupled device (CCD) detector. The diffraction lim-ited resolutions (R) is determined by 0.61λ/NA, where λ isthe X-ray wavelength and NA is the numerical aperture of theobjective lens.14 With current lens fabrication technology, the

Received: July 25, 2011Revised: August 17, 2011

ABSTRACT:Galvanic replacement reaction between silver nanowires and anaqueous solution of HAuCl4 has been successfully monitored in real time byusing in situ transmission X-raymicroscopy (TXM) in combination with a flowcell reactor. The in situ observations clearly show the morphological evolutionof the solid silver nanowires to hollow gold nanotubes in the course of thereaction. Careful analysis of the images reveals that the galvanic replacementreaction on the silver nanowires involves multiple steps: (i) local initiation ofpitting process; (ii) anisotropic etching of the silver nanowires and uniformdeposition of the resulting gold atoms on the surfaces of the nanowires; and (iii) reconstruction of the nanotube walls via anOstwaldripening process. The in situ TXM represents a promising approach for studying dynamic processes involved in the growth andchemical transformation of nanomaterials in solutions, in particular for nanostructures with dimensions larger than 50 nm.

KEYWORDS: Transmission X-ray microscopy, in situ TXM, liquid TXM, flow cell TXM, galvanic replacement reactionmechanism, silver nanowires

4387 dx.doi.org/10.1021/nl202538q |Nano Lett. 2011, 11, 4386–4392

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achievable NA is in the range of several mrad with a multi-keVhard X-ray radiation, leading to spatial resolution of 10�15 nmbut extended depth of focus of tens of micrometers.14,15

Although this resolution scale is lower than that of elec-tron microscopy techniques,13,16 the TXM approach offers

several unique advantages for in situ studies with solution-phase reactions:(1) The entire system can be operated in ambient environment

due to the strongpenetrationofhardX-ray in air and solvents,leading to the compatibility with solution-phase reactions.

Figure 1. Schematic illustration of the typical configuration of a transmission X-ray microscope. Incorporation of a flow cell provides the capability forreal-time monitoring the morphological evolution of solid Ag nanowires into hollow nanotubes in the course of galvanic replacement reaction of eq 1.

Figure 2. TXM images of three Ag nanowires (with different diameters) before (A) and (B�D) after they reacted withHAuCl4 at room temperature fordifferent times: (B) 12 s, (C) 28 s, and (D) 38 s. The time was normalized against the time when the image (A) was recorded. The black arrows highlighta position at which the thin nanowire was fragmented during the galvanic replacement reaction. The blue arrows highlight the positions at which theholes formed in the thick nanowires were eventually sealed via an Ostwald ripening process. The scale bar in (A) also applies to (B�D).

4388 dx.doi.org/10.1021/nl202538q |Nano Lett. 2011, 11, 4386–4392

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(2) The TXM has the capability to control image contrastof different elements by tuning X-ray energy. Forinstance, individual elemental compositions can bemapped by using their characteristic absorption edges,and furthermore the X-ray absorption near-edge struc-ture (XANES) spectra can be used to map the chemicalstates of individual elements.

(3) The large working distance (e.g., over 2 cm in the systemreported in this work) and open system design enable easyintegration of reaction vessels and easy access to the reactionvessels to manipulate complicated reaction conditions.

In this work, a flow cell (Figure S1, Supporting Information) isplaced in the X-ray beam path of the TXM system to study thegalvanic replacement reaction between Ag nanowires and anaqueous solution of HAuCl4:

3Ag þ AuCl4� f 3Agþ þ Au þ 4Cl� ð1Þ

The flow cell chamber is formed with two parallel Si3N4

membranes (with thickness of 100 nm) that are supported withtwo stainless steel disks sealed by a rubber O-ring. Two Teflontubes are connected to the cell for delivering the HAuCl4solution into the cell and discharging the waste solution out of

Figure 3. Magnified TXM images of the thin nanowires highlighted in box I, Figure 2B, before and after they reacted with HAuCl4 for different times.Similar to those in Figure 2, the black arrows highlight the positions at which the nanowire was fragmented, while the blue arrows highlight the positionsat which the openings were sealed to form a hollow nanotube. The scale bar applies to all images.

4389 dx.doi.org/10.1021/nl202538q |Nano Lett. 2011, 11, 4386–4392

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the cell, respectively. In a typical experiment, the Ag nanowiressynthesized through the well-established polyol process17 arefirst centrifuged and redispersed in ethanol to achieve a disper-sion solution with appropriate concentration. Placing a drop ofthe solution on the surface of one Si3N4 membrane followed bydrying in air at room temperature results in the deposition of theAg nanowires on the surface of themembrane. The van derWaalsforces between the nanowires and the Si3N4 membrane arestrong enough to prevent the nanowires from moving duringreaction, enabling the in situ monitoring of morphologicalvariation of single nanowires as the galvanic reaction proceeds.In the next step, the flow cell with Ag nanowires is assembled, as

shown in Figure S1, Supporting Information, and then placed inthe TXM system. The imaging process was initiated immediatelybefore an aqueous HAuCl4 solution with a concentration of1 mM is continuously injected into the flow cell by a syringepump to trigger the galvanic replacement reaction, and thenanowires in the imaging field are continuously monitored inreal time.

Figure 2 compares the images of the Ag nanowires and thecorresponding products obtained at different reaction times.The two wires at the top left region of Figure 2A are thin withdiameters of 80�90 nm and short with lengths of 3�7 μm. Incontrast, the wire at the bottom right region is thick with

Figure 4. Magnified TXM images of the thick nanowire highlighted in box II, Figure 2B before and after they reacted with HAuCl4 for different times.The scale bar applies to all images.

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diameter of ∼160 nm and long with length of ∼13 μm. Thisbimodal distribution of the Ag nanowires is consistent withtheir scanning electron microscopy (SEM) images shown inFigure S2, Supporting Information. Most of the as-synthesizedAg nanowires exhibit diameters in the narrow range of70�90 nm, while a very small fraction (<1%) of the nanowiresexhibit larger diameters in the range of 100�200 nm. Theimages in Figure 2 clearly show the morphological evolution ofsolid Ag nanowires into hollow nanotubes (Figure 2A versus2D) as the reaction proceeds. At the beginning of the reaction(e.g., 12 s as shown in Figure 2B), the replacement reactioninitiates only at some spots (such as those highlighted witharrows) that have surface energy higher than other surface areasof the nanowires, leading to the formation of pits at these spots.The reaction continues to etch the Ag nanowires from these pitsand simultaneously deposit Au on the surfaces of the nanowiresbecause the newly formed surfaces of the pits have an evenhigher surface energy and thus higher reactivity. When thereaction lasts long enough, the Ag nanowires are completelydissolved to leave tubes of Au (or Au�Ag alloy due to thepossible alloying of Au with the underneath silver nanowires)(Figure 2D). As shown in Figure S3, Supporting Information,the tubes detach from the window membrane into the wastesolution after reaction, leading to a difficulty for furtherchemical analysis of the resulting tubes with techniques likeenergy dispersive X-ray scattering (EDS). However, the depos-ited material should be Au according to eq 1 and the previousex-situ studies on the galvanic replacement reactions.3 Ashighlighted by the arrows, the surface morphologies near theinitially formed pits (as highlighted by the arrows in Figure 2B)are slightly different depending on the diameters of the Agnanowires.

For example, the replacement reaction at the pits formed on thethin nanowires (highlighted by the black arrow in box I, Figure 2B)tends to break the nanowires into segments (highlighted by theblack arrows in Figure 2C and D). Figure 3 shows a series of imagesof the thin nanowires that have been obtained at a time interval of 2 sto track their morphological evolution in details. At the very earlystage of the reaction (e.g., 6 s), a number of tiny bright spotscorrelated to material loss (highlighted by the black arrows) start toappear on the surface of the left thinner nanowire (with a diameterof ∼80 nm), indicating the initiation of pitting process. The pitsincrease in their dimensions and eventually break the nanowire intosegments as the reaction proceeds (highlighted by the black arrowsin the image obtained at 20 s). During the period of reaction, morepits are also formed on the surface of the nanowire to induce the

similar fragmentation (Figure S4, Supporting Information). As aresult, the product derived from this thin Ag nanowire exhibits aquasi-linear assembly of very short hollow tube segments andclusters of nanoparticles (e.g., the image obtained at 30 s). Incontrast, another Ag nanowire on the right side has been convertedinto a continuous hollow nanotube, although this nanowire exhibitsnonuniform diameters along its longitudinal axis. Its tip had adiameter of∼80 nm that is similar to the diameter of the nanowireon the left side, and its stemhas a slightly larger diameter of∼90 nm.As highlighted by the blue arrows, the replacement reaction alsoinitiates with the formation of pits at both the end and the stem, andthe complete reaction results in the formation of a continuousnanotube with a closed end. The difference of the two nanowiresshown in the image obtained at 30 s indicates that the lateraldimension (i.e., diameter) and the surface geometry/crystallineorientation of a Ag nanowire play important roles in determiningthemorphological evolution of the Ag nanowire in the course of thegalvanic reaction of eq 1. In spite of the low solubility of AgCl,18 noAgCl solids are observed during the reaction because the concen-trations of Ag+ and Cl� ions released from the reaction of a smallnumber of Ag nanowires with HAuCl4 are low enough to preventthe precipitation of AgCl (see Supporting Information for details).When the reaction time is too long (e.g., >32 s), additional Aunanoparticles are deposited on the outside surfaces of the hollownanotubes (Figure S3 and Movie S1, Supporting Information) dueto the reduction of HAuCl4 by reducing species that are generatedfrom the high-dose X-ray irradiation of water.19

As pointed out previously, increasing the diameter of Ag nano-wires is beneficial to the formation of nanotubes with continuouswalls. Figure 4 shows a series of images of the thick Ag nanowirewith a diameter of∼160 nm (highlighted in box II, Figure 2B) inthe course of the replacement reaction, clearly showing thetransformation of the solid nanowire into a continuous nanotube.Similar to the thin nanowires shown in Figure 3, the reaction alsoinitiates the formation of pits at some spots on the surface of thenanowire (as highlighted by the blue arrows). Continuousreaction etches out the thick Ag nanowire along its longitudinalaxis and deposits a Au layer on the surface of the nanowire,eventually resulting in the formation of a hollow nanotube at 38 s.Apparently, the hollowing process of the thick nanowire takes alonger time than the thin wires because more silver has to beetched during the reaction. The holes formed at the pits remainopen until the Ag nanowire had been completely dissolved(as highlighted by the blue arrows in the image obtained at 34 s).Further reaction leads to an Ostwald ripening process that canreconstruct the morphology of the nanotube to smoothen its

Figure 5. Schematic illustration of major steps involved in themorphological evolution associated with the galvanic replacement reaction between silvernanowires and an aqueous solution of HAuCl4. Depending on the diameter of the nanowires, the etching anisotropy is different: the reaction etches outsilver in a preference (A) along the transverse direction (highlighted by the red arrows) in thin nanowires and (B) along the longitudinal axis (highlightedby the white arrows) in thick nanowires. The inset shows how the galvanic replacement reaction occurs in the Ag nanowires.

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surface with a decrease in surface energy. As a result, the holes aresealed to form a continuous nanotube (as highlighted by the bluearrows in the image obtained at 38 s) because no additional Ag+

ions will be released inside the nanotube to diffuse out.The results obtained from the in situ TXM observations

clearly show the morphological evolution of single Ag nanowiresin the course of the galvanic replacement reaction between thenanowires and aqueous solution of HAuCl4 at room tempera-ture. Figure 5 summarizes the pathways for hollowing the Agnanowires with different diameters. Contacting HAuCl4 with Agnanowires initiates the galvanic reaction between them at somespecific spots rather than uniformly over the whole surfaces of thenanowires because these spots usually exhibit higher surfaceenergy originating from crystalline defects such as steps, disloca-tions, etc. Once the reaction starts, pits with irregular shapes andsurfaces are then formed at these spots. The pitting processlocally etches the Ag nanowires, while the resulting Au atoms areuniformly deposited on the intact surfaces of the nanowires toform Au sheaths. Continuous reaction etches out more Ag fromthe surfaces of the pits and deposits more Au on the outsidesurfaces of the nanowires, leading to the formation of hollowtubes. Depending on the diameter of the Ag nanowires, therelative etching rates along the longitudinal and transversedirections are different. As shown in Figure 5A, the reactionetches thin nanowires along their transverse direction faster thanalong their longitudinal direction when the diameters of nano-wires are less than 80 nm, leading to fragmentation of thenanowires. In contrast, the reaction etches thick nanowires(with diameters larger than 90 nm) with a preference along theirlongitudinal axes, resulting in the formation of nanotubes withcontinuous walls (Figure 5B). On the other hand, if the reactioninitiates the formation of pits at the end of a nanowire, thencontinuous reaction tends to etch the nanowire along its long-itudinal axis regardless of its diameter (Figure 5A). During thisprocess, the reaction occurs similar to the electrochemistry in abattery (inset, Figure 5). Ag atoms on the surfaces of the pits areoxidized to release Ag+ ions that can diffuse out through theopenings of the pits, while the leftover electrons diffuse to thesurfaces of the nanowires to reduce AuCl4

� ions into Au atomsthat are directly deposited on the surfaces to thicken the Ausheaths. Once the Ag nanowires are completely dissolved, thereplacement reaction stops. In the next step, an Ostwald ripeningstarts to reconstruct the walls of the Au nanotubes to seal theholes, leading to the formation of continuous nanotubes(Figure 5B). It is worthy to note that most of the poly(vinylpyrrolidone) (PVP) molecules adsorbed on the surfaces of theAg nanowires are removed during the washing/centrifugeprocesses.20 The residual PVP molecules do not influence thegalvanic replacement reaction between the Ag nanowires andHAuCl4, and eventually they are desorbed from the surfaces ofthe nanotubes.3

In conclusion, we have observed the morphological evolutionof Ag nanowires involved in the galvanic reaction betweennanowires and a HAuCl4 solution in real-time by combiningTXM imaging with a flow cell reactor. The in situ results clearlyreveal the multiple-step process associated with the chemicaltransformation of solid Ag nanowires into hollow Au (or Au�Agalloy) nanotubes at room temperature: (i) local initiation of thereaction at certain spots with high surface energy; (ii) formationof pits at these spots; (iii) dissolution of the Ag nanowiresthrough these pits and deposition of Au sheaths outside of thenanowires; and (iv) reconstruction of the nanotubes via an

Ostwald ripening process. The information provides a betterunderstanding of galvanic replacement reactions on the nan-ometer scale and helps us to better control the synthesis ofhollow nanostructures with tailored properties. Although thespatial resolution of TXM imaging is∼25 nm (with pixel size of∼10 nm) in this study, it can be potentially improved whenadvanced lithographic techniques are available for fabricatingzone plates (objective lenses) with better performance. Theenergy of incident X-rays can be finely tuned to achieve spatiallymapped XANES and electron energy loss spectroscopy that offerthe capability to study the dynamic evolution of elemental andchemical composition of the samples.21�23Many other detectorsare possibly integrated to the TXM system for simultaneouslymonitoring variations of crystalline structures by X-ray diffrac-tion, local chemical states by X-ray Raman and fluorescence, etc.As a result, in situ TXM is complementary to in situ TEM and ispromising to study the growth and transformation of nanoma-terials in solutions.

’ASSOCIATED CONTENT

bS Supporting Information. Experimental procedures, Fig-ures S1�S4 with corresponding figure captions, and Movie S1caption. This material is available free of charge via the Internet athttp://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: ygsun@anl.gov.

’ACKNOWLEDGMENT

Use of the Center for Nanoscale Materials, Advanced PhotonSource, and Electron Microscopy Center for Materials Researchat Argonne National Laboratory was supported by the U.S.Department of Energy, Office of Science, Office of Basic EnergySciences, under contract no. DE-AC02-06CH11357. Help fromDrs. Wenge Yang and Yang Ren is appreciated.

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