New Perspectives on Silver Nanowire Formation from Dynamic Silver

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New perspectives on silver nanowire formation from dynamic silver ion concentration monitoring and NO production in the polyol process David R. Whitcomb, Aaron R. Clapp, Philippe Buhlmann, Jeffrey C. Blinn, and Junping Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01289 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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Crystal Growth & Design

Cover page New perspectives on silver nanowire formation from dynamic silver ion concentration monitoring and NO production in the polyol process David R. Whitcomb,* Aaron R. Clapp, Philippe Bühlmann,a Jeffrey C. Blinn, and Junping Zhang Carestream Health, Inc., 1 Imation Way, Oakdale, MN 55128, United States a

Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis MN 55455, United States For the first time, previously unknown multiple reaction stages have been identified during the reduction of silver ion reduction in the high-temperature polyol process of synthesizing silver nanowires. New in-situ potentiometric silver ion measurements clearly show different phases of the crystal growth process along the reaction path as silver ions are reduced to metallic silver. Incorporation of nitric oxide monitoring of the reaction headspace gases provides additional insight into the reactions affecting the silver ion reduction process. The combination of monitoring the Ag+ ion concentration with evolved nitric oxide demonstrates that the reduction of silver ions to form crystalline silver of various morphologies in the polyol process is far more complicated than previously realized. These new approaches also provide powerful new tools to study other elevated temperature, metal reduction reactions. David R. Whitcomb Carestream Health 1 Imation Way DIS-4D-80 Oakdale, MN 55128 [email protected] 651.458.3024

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Title page New perspectives on silver nanowire formation from dynamic silver ion concentration monitoring and NO production in the polyol process David R. Whitcomb,* Aaron R. Clapp, Philippe Bühlmann,a Jeffrey C. Blinn, and Junping Zhang Carestream Health, Inc., 1 Imation Way, Oakdale, MN 55128, United States a

Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis MN

55455, United States * [email protected] Abstract

For the first time, previously unknown multiple reaction stages have been identified during the reduction of silver ion reduction in the high-temperature polyol process of synthesizing silver nanowires. New in situ potentiometric silver ion measurements clearly show different phases of the crystal growth process along the reaction path as silver ions are reduced to metallic silver. Incorporation of nitric oxide monitoring of the reaction headspace gases provides additional insight into the reactions affecting the silver ion reduction process. The combination of monitoring the Ag+ ion concentration with evolved nitric oxide demonstrates that the reduction of silver ions to form crystalline silver of various morphologies in the polyol process is far more complicated than previously realized. These new approaches also provide powerful new tools to study other elevated temperature, metal reduction reactions.


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Introduction The dramatic change in material properties (physical, mechanical, electrical, catalytic, chemical, optical, etc.) as physical size decreases into the nanoscale regime is the primary driving force for contemporary exploration and exploitation of nanomaterials for a wide range of new commercial applications.1 All of these applications require efficient, sustainable, safe, and large-scale (1 to 103 kg) controlled manufacturing of specific metal morphologies, from simple spheres to more complex shapes such as rods, wires, or cubes. Today the challenge is not so much generating specific morphologies, as all of these shapes have been reported in the literature, but cleanly producing those morphologies in sufficient quantity and purity. One major route to preparing metal nanomaterials is the “polyol” process that is particularly used for silver nanoparticles and specifically nanowires, which is the subject of hundreds of recent publications and has been extensively reviewed.2,3 Despite all of the published work on the polyol process, there remain significant questions on how the various morphologies are formed, which may now be attributed to the previously unrealized complexity of the overall reaction. In principle, the thermally induced reduction of silver (Ag+) via the polyol process4,5 is a relatively simple reaction: heating silver ions in a polyol, in the presence of a capping agent, produces silver particles, and the addition of a small amount of halide facilitates the formation of silver nanowires. Scheme 1. General reaction for the reduction of Ag+ to Ag0 in the polyol process. Ag+ + capping agent + X- + polyol → 1D Ag0 + Ag0 particles + organic byproducts The chemistry involved in the formation of the metal morphologies in the polyol reaction has been inferred largely from post-reaction TEM and SEM images and x-ray diffraction data. As


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useful as this information is, it cannot provide real-time in situ chemistry information during the reaction process. We can now show how potentiometric monitoring of the Ag+ ion concentration, [Ag+] (strictly speaking, silver ion activity), at high temperatures in a purely organic solvent with millisecond response times, provides a powerful new probe into this reaction. It is performed in this work by measurement of the electric potential between a silver rod as a probe and a specially designed reference electrode (described in detail in the Supporting Information). In addition, incorporating a second probe that quantitatively measures the nitric oxide (NO) produced from the nitrate anion provides, for the first time, the ability to real-time identify different processes occurring during previously unknown stages of this reaction and reveals how complex this reaction is.

Experimental Procedures Materials. All polyol solvents were the best commercially available for water content (60 minutes and then switched just prior to heating (the solution was not sparged during the reaction) to maintain a slightly positive N2 headspace pressure. The headspace gas passed through a dry ice trap to prevent the Horiba instrumentation from being contaminated with solvent. The


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solution was heated to temperature and 1.0 M AgNO3 stock solution (continuously sparged) was added at 0.25-1.0 mL/min via a syringe pump through a 12-gauge Teflon syringe needle. Typical ratios of PVP:Ag+:Cl- = 1.5:1:0.02. All reactions and sample withdrawals were carried out under complete N2 atmosphere. NO concentration in the N2 gas flow was determined with a Horiba VS3003 Multi-gas Sampling Unit coupled with a VA3002 Multi-Gas Analyzer Unit (total N2 flow rate = 1.5 L/min). Caution: NO exposure or gas pressure increases can occur in the polyol reaction from metal nitrate sources.

Results and Discussion A good starting point for this discussion is temperature, which is one of the most important parameters in the polyol reaction. There are numerous reports in the literature where the reaction temperature has ranged from as low as 90 °C (requiring at least 24 hours)6 to the polyol’s boiling point, around 190 °C or higher (depending on the specific polyol used).7,8,9 A wide range of silver nanowire aspect ratios were observed in the 110-190 °C range (where in ethylene glycol the aspect ratios generally increased with temperature), with minimum diameters at 170 °C and maximum lengths above 180 °C.10 Others have observed similar temperature-aspect ratio effects, with the lower temperatures favoring non-wire particles.11,12,13 As expected, the rate of Ag+ reduction increases with temperature, which is seen by the rate of change in the overall [Ag+], consistent with a drop in the measured potential with time. Of particular interest in the Ag+ reduction, and unknown without potentiometric monitoring, is a transition in reaction kinetics over the course of the reaction (beginning at reaction temperatures as low as 130 °C), which is difficult to track visually but is clearly detected potentiometrically (Figure 1).


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Figure 1. Effect of temperature on Ag+ reduction in the AgNO3/Cl-/PVP reaction system in propylene glycol, which is measured as potential vs. time (minutes from start of Ag+ addition). Reactions were run at 110, 120, 130, 140, and 150 °C. The end of Ag+ addition is indicated by the vertical dashed line for all cases.

Five temperatures for the Ag+ reduction to Ag0 reaction are shown in Figure 1: Ag+ reduction to Ag0 is slow at 110 °C (black curve), requiring over two hours to achieve the same level of Ag+ reduction in a fraction of the time at 150 °C (purple curve). Potentiometric monitoring easily tracks [Ag+] in any of these temperature ranges. Most literature reports utilize reactions at the higher temperatures, therefore, for simplicity we will focus on the 130-150 °C range. Thus, Figure 2 shows the buildup and subsequent reduction of Ag+ during the continuous addition of silver nitrate (AgNO3) into a solution of PVP/Cl- in which five phases are identified and individually discussed below.


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Figure 2. Individual phases (labeled 1-5) in the reduction reaction of Ag+ to Ag0 in the polyol process at 130 °C (a total of 30 mmol Ag+ added at 0.5 mL/min; MnCl2 is the halide source), potential (80 mV = 10-fold change in [Ag+]) vs. time.

Phase 1. All polyol reactions producing silver nanowires utilize a halide without which nanowires do not form.7,14 The role of the halide has been variously explained to be a crystal etchant,2 a redox-controlling agent15 as a source of Cl- for AgCl seeds,16 electrostatic stabilizer of silver seeds,17,18 to control silver ion concentration,19,20 and to inhibit large polycrystalline Ag0 formation.21 Recently, Schuette and Buhro reported that the AgCl crystals formed in these reactions serve as the seeds for silver nanowire growth,22 although the AgCl crystal size has no apparent influence on the final silver nanowire diameter.23 It should be noted that Schuette and Buhro also quantitatively measured [Ag+] at various stages of the reduction reaction using a gravimetric technique to determine the Ag0 content during the reaction. While successful, they


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recognized that the technique was cumbersome and assumed that the precipitate was pure Ag0, ignoring the small amount of collected AgCl. Potentiometrically, the immediate precipitation of AgCl can be detected from the initial addition of Ag+ to the hot polyol solution, which, on close inspection of the measured potential in the early stage of Ag+ addition (within the first few minutes in Figure 2), is indicated by a lag in voltage response during a constant Ag+ addition and the corresponding calculated [Ag+] (see below). This is supported by sample extracts from the hazy reaction mixture such as shown in Figure 3.

Figure 3. SEM image of representative AgCl crystals observed in Phase 1. Depending on the Ag+:Cl- ratio, temperature, and solvent, Phase 1 largely represents AgCl formation without any significant Ag0 formation. Expanding the timescale of the measured potential during the initial Ag+ addition essentially shows a Ag+ titration of the available Cl-. Calculated silver ion concentrations based on the electrode response in this regime provide additional support for this conclusion (see Figure 4 in the Phase 2 discussion below). At this stage of the reaction, the role of the halide is the simple formation of AgCl, which is consistent with the literature.19,12 In fact, the halides can be easily titrated potentiometrically in any polyol solvent (see Figure S3). Phase 2. A somewhat surprising part of the polyol reaction is Phase 2, in which very little reduction is occurring, even though the reaction is held at high temperatures. Simply stated, Phase 2 primarily builds the [Ag+] as the AgNO3 addition continues. This can be seen upon


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conversion of the measured potentials to concentrations, as shown in Figure 4 (see Supplemental Information for calculation details):

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Figure 4. Measured [Ag+] (open circles) and expected concentration (solid line) vs. time from the reaction shown in Figure 1. Phases 1-5 are separated by dashed lines (the end of the Ag+ addition coincides with the vertical line at 59 minutes). The former is determined by fitting E o such that the early concentration data points match the solid line’s slope (fixed as the constant AgNO3 injection rate).

The solid line in Figure 4 shows the expected concentration within the reactor (assuming no reaction), accounting for a constant AgNO3 injection rate and accompanying volume change. The early concentration data points should closely match this slope (assuming the early reaction rate is much less than the molar addition rate), which serves as a calibration standard. In this example at 130 °C, E o = 636.9 mV, where the initial slope of the fitted line is 2.50 mol L-1 min-1.


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Although the injection of AgNO3 started at t = 0 minutes, there is a short response lag due to the presence of the chloride ions. The initial amount of Ag+ goes toward the formation of insoluble AgCl until all of the free Cl- is exhausted and the [Ag+] rises abruptly. The fitting algorithm accounts for this effect and ensures that a proper region of the measured concentration curve matches the molar addition rate. While the solution color often changes during Phase 2, which is due to some Ag0 nanoparticle formation (as seen in the calculated [Ag+] above), the vast majority of the Ag+ addition in Phase 2 is simply adding to the total; there is significant Ag+ accumulation where the addition rate far exceeds the reaction rate. Extraction of samples during the Ag+ addition process typically shows AgCl crystals, a few silver nanoparticles (AgNP), and the earliest stage of silver nanowire formation (Figure 5), which is consistent with other reports.24

Figure 5. Darkfield optical microscope image of particles with preliminary 1D-Ag0 in Phase 2.

Phase 3. The [Ag+] buildup and the onset of Ag0 formation during Phase 2 may be also be described as an induction period leading into Phase 3, where rapid nucleation and silver nanowire growth begins in an autocatalytic process. This latter phase is where large numbers of


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silver nanowires are observed along with various levels of nano- and sub-micrometer Ag0 particles, as shown in Figure 6.

Figure 6. Darkfield optical microscope image of particles and silver nanowires in Phase 3. Rapid reduction of the Ag+ ion over the course of only 10-20 minutes at 150 °C in propylene glycol is occurring, as clearly demonstrated by the measured voltage, while Ag+ continues to be added to the reaction mixture. An 84 mV drop from the plot maximum shows that there is 90% less Ag+ in the reaction at that moment compared to the peak amount, but the actual amount of total reduction (from the start of the reaction until that moment) is even higher because additional Ag+ was reduced prior to Phase 3. Phase 4. Transition from a rapid Ag+ reducing environment into a steady-state phase is rather remarkable, particularly considering that the Ag+ ion continues to be added during this phase. Surprisingly, the steady-state phase is largely independent of the Ag+ ion addition rate. For example, a five-fold drop in the Ag+ addition rate simply lowers the steady-state [Ag+] position by 30-40 mV, but a steady-state condition is maintained as long as the Ag+ addition continues (Figure 7).


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Figure 7. Stepped Ag addition rates at 0.25 to 0.05 mL/min (potential vs. time of reaction).

Large numbers of silver nanowires have already been formed in this phase, which is consistent with previously published TEM images taken to monitor the reaction process.24 Addition of Ag+ to the reaction mixture maintains the steady-state phase indefinitely and it will continue until the Ag+ addition ceases. Also in Phase 4, NO gas (initially confirmed with a Griess reagent25) in the headspace of this reaction begins to evolve, which can be quantitatively monitored (Figure 8).


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Figure 8. Measured potential and NO concentration vs. time for a standard polyol reaction. The AgNO3 addition ends at approximately 59 minutes, and the heat is removed near 86 minutes. Circles indicate the measured potential and the solid lines show the NO concentration. (See Phase 5 discussion for the end of the Ag+ addition point.)

Surprisingly, little attention has been given in the literature to NOx evolution from the polyol reaction,26,27 which is the result of nitrate reduction, as observed in both copper and silver nitrate reduction reactions.28 In the formation of silver nanowires, significant quantities of NO are generated (we have observed up to 50% of the total nitrate added converted to NO) during the reaction. Throughout Phases 1-3, NO is barely detected in the headspace gas, but upon entering Phase 4, NO is clearly observed, which increases as long as the Ag+ addition continues. Once the Ag+ addition ceases, the NO headspace rapidly increases over ~15 minutes, then begins a steady decline. The post-silver ion addition NO spike (seen to begin at ~60 minutes in Figure 8) is routinely observed but it is difficult at this time to formulate a reasonable explanation to account


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for it. One possibility is that the NO is chemisorbed to a silver surface-silver ion intermediate, which is then released as the silver ion is reduced to metallic silver and there is no further silver ion to reform the intermediate. The steady-state reactions proceeding in Phase 4 are no longer steady-state once the silver addition ceases, which allows the release of coordinated NO. In addition, Ostwald ripening may continue even as the Ag+ addition stops, which reduces the overall surface area and releases chemisorbed NO. In the example in Figure 8 above, a total of 3.3 mmol of NO was detected, corresponding to 11% of the total NO3- added in that reaction. To our knowledge, this is the first time that NO has been definitively, and quantitatively, established as a byproduct of the Ag+ reduction in the polyol reaction. From the Ag+ and NO results, we propose two simultaneous reactions that produce the steadystate observed in Phase 4. First, Ag+ ions, both already present in the reaction mixture and those being added, are reduced to produce a mixture of Ag0 NP and growing silver nanowires. Scheme 2. The formation of Ag0 and H+ during polyol reduction of Ag+. 2Ag+ + RC(OH)–CH(OH)R → RC(OH)–C(O)R + 2Ag0 (as Ag0 NP and 1D-Ag0) + 2H+ Polyols are known reducing agents for metals,29 and they are suggested here as the preliminary reducing agent, while aldehydes may be involved at some point in the reaction.5 It should be noted, however, that even polyol solvents incapable of producing aldehydes have been reported as suitable solvents in the formation of silver nanowires.30 The precise role of the polyol as the electron source for this reaction is complicated because PVP has also been demonstrated to function as a reducing agent for Ag+.31,32 The second portion of the steady-state reaction involves oxidation of the smallest particles, which by virtue of their nanoscale diameters are those particles most subject to oxidation by the HNO3 (and available NO3–) being formed during the reduction of the Ag+.


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Scheme 3. Oxidation of Ag0 NP from HNO3 formed during Phase 4. 3 Ag0 NP + 4 HNO3 → 3 AgNO3 + 2 H2O + NO Thus, the small Ag0 particles formed under these circumstances are part of an electrochemical Ostwald ripening33,34 that occurs to recycle Ag0 into available Ag+ that can be reduced again, onto the growing silver nanowires, which is a direct function of the particle size. The silver nanowires are sufficiently large to be relatively invulnerable to subsequent re-oxidation under these conditions. Thus, the two reactions in Schemes 2 and 3 account for both the steady-state reaction incorporating re-oxidation of nano-size particles to provide Ag+ in a reaction feedback loop and the evolution of NO detected in the headspace gas. The mechanism by which the NO is formed in the reaction is not yet clear, although it appears, in general, that the formation of silver nanowires seems to require NO3-, implying Ag0 catalyzed reduction of nitrate to NO, which has been reported in the presence of alcohols.35 In addition, silver nitrate has been reported to form stable complexes with pyrrolidone,36 which suggests that PVP-AgNO3 solution complexes might be expected in addition to the reported PVP-Ag0 surface complexation.24,37 Presumably, the silver ion reduction process is similar to photographic image development: once metallic nuclei have formed (the latent image on the AgBr, in the photographic analogy), most of the remaining reduction occurs at the surface of the silver crystals located at the photographic latent image (seed) sites. Given that Cl- must be present in the reaction in order for silver nanowires to form, a silver nanowire surface complex that includes all three potential ligands, PVP/Cl-/NO3-, along the lines proposed recently by Kuo and Hwang,38 may be involved. Phase 5. The transition from Phase 4 to Phase 5 is rapid, demonstrating how quickly Ag+ can be reduced under these conditions. In the example illustrated in Figure 2, the voltage response at


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the end of Phase 5 shows that over 99% of the Ag+ added to the system has been reduced to Ag0. This transition is rapid, taking only a few minutes, and it is clearly detected potentiometrically. Another interesting feature of this reaction becomes apparent when the Ag+ addition during Phase 4 is interrupted. That is, while stopping the Ag+ addition immediately leads to Phase 5, restarting the Ag+ addition re-starts Phase 4. As long as the Ag+ addition continues, the steady-state reactions involved with Phase 4 are maintained, but as soon as the Ag+ addition ceases, all of the remaining Ag+ is rapidly reduced. It is unclear why the initial [Ag+] level is nearly completely re-

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Figure 9. Effect of multiple 10-minute interruptions of Ag+ addition on [Ag+], at 150 °C (84 mV = 10-fold change in [Ag+]) vs. time.

The corresponding NO headspace profile, Figure 9 (blue), follows the measured voltage: halting the Ag+ addition releases NO, and restarting the Ag+ addition drops the NO release close to the original levels. The resumption of the Ag+ addition immediately returns the system to the


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steady-state condition of Phase 4. Most of the silver nanowire production appears to have occurred in Phase 3 and as it proceeds into Phase 4. Once the reaction has moved into the steadystate condition of Phase 4, only small changes in the overall silver nanowire morphology are observed. Based on the total injected amount of AgNO3 and the associated electrode response, the overall reduction of Ag+ to Ag0 can be estimated. For example, from Figure 2, it follows that 99.9% of the injected Ag+ was reduced. The reduction rate can be estimated as a function of time using a simple mass balance (assuming a well-mixed system exists and no other sources or sinks of Ag+ occur):

dN Ag + dt where

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concentration of Ag+ in the feed, and rred is the reduction rate. The volumetric injection rate and feed concentration are known, and the accumulation of Ag+ is determined by calculating the number of moles of Ag+ in the reactor (based on the measured concentration and the known volume) and estimating its derivative with time. A centered finite difference provided a reasonable estimate of the accumulation term. This provided a reduction rate at any time point for which there is a measured concentration to be determined, shown in Figure 10.


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Figure 10. Calculated Ag+ reduction rate (open circles) and molar addition rate (solid line) vs. time. The latter shows the instantaneous Ag+ injection rate throughout the reaction.

There are two discontinuities in the reduction rate calculation (and plot) occurring at the start and end of AgNO3 injection into the reactor. This is explicitly considered in the mass balance equation where the molar injection term assumes a constant positive value (in this experiment, 0.5 mmol/min) or zero, depending on the point in time. For times when there is a constant flow of AgNO3 into the reactor, the reduction rate is estimated as the molar feed flow rate minus the measured accumulation rate. At the end of the addition, the reduction rate is simply the opposite sign of the measured accumulation rate. Figure 10 shows several interesting features of the reduction rate. Early in the reaction, there is little reduction of Ag+, which is consistent with visual observation (little to no solution color change), which define Phases 1 and 2, and the expectation is that the reduction rate is


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proportional to the Ag+ concentration. This is also a good consistency check for our fitting procedure. If the fit were poor, the measured reduction rate would be well above zero early in the reaction or even negative. As the concentration builds, due to steady addition of Ag+ into the reactor, the reaction rate increases roughly exponentially in about the first 30 minutes. The maximum reaction rate seen at 35 minutes corresponds to the inflection point in the concentration plot. Interestingly, there is a long induction period that is required to reach peak reduction in this system. As the reduction rate decreases (Phase 4), it begins to match the molar addition rate almost exactly. Indeed, every silver nanowire reaction (using a steady addition of AgNO3) we have monitored potentiometrically has shown this rate-matching behavior. The moment the addition ceases, the Ag+ concentration rapidly drops, and there is a concomitant drop in the reduction rate, tending toward zero. A consistent feature of this reaction is this steady-state region in Phase 4, where the accumulation is effectively zero, suggesting Ag+ is reduced at the same rate at which it is fed into the reactor. The reduction rate and Ag+ concentration are both observed to collapse once the injection of AgNO3 stops. Holding the reactor at the initial temperature results in further reduction of residual Ag+, however, the rate is exceedingly low due to the low Ag+ concentration. Since E o is temperature dependent, measured potentials observed while the temperature is dropping from 150 °C to room temperature cannot be interpreted without recalibration at multiple temperatures.

CONCLUSIONS We have shown how potentiometric, high-temperature monitoring of [Ag+] in organic solvents can be used to follow the [Ag+] in the “polyol” process used to prepare silver nanoparticles. These results illustrate the complexity of Ag+ reduction reactions that occur in the polyol process, particularly when combined with additional probes such as gas phase monitoring of NO


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production. In general, the high temperature reduction of silver ions in the polyol reaction in the formation of silver nanowires can be analyzed as a series of discrete phases, from the initial simple precipitation of AgCl to the steady-state electrochemical Ostwald ripening processes.

Supporting Information Available: Details for the electrode construction, potentiometric titration of halides, and the conversion of electrode potentials to concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT We gratefully acknowledge the technical discussions and suggestions from W. D. Ramsden and D. C. Lynch, D. Deibel (Rochester Institute of Technology, Rochester, NY); and LabVIEWTM programming assistance from C. Choudek, R. Brearey, W. Fernholz, and S. Riehm.


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For Table of Contents Use Only New perspectives on silver nanowire formation from dynamic silver ion concentration monitoring and NO production in the polyol process David R. Whitcomb, Aaron R. Clapp, Philippe Bühlmann, Jeffrey C. Blinn, and Junping Zhang

TABLE OF CONTENTS GRAPHIC AND SYNOPSIS Potentiometric measurements to monitor [Ag+] provide new insight into the complexity of Ag+ reduction reactions occurring in the polyol process. The high temperature reduction of Ag+ can be visualized as occurring through individual phases: precipitation of AgCl (Phase 1), silver ion buildup (Phase 2), induction (Phase 3), steady-state electrochemical Ostwald ripening (Phase 4) and final complete Ag+ reduction (Phase 5). 600

500

potential (mV)

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400

300

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3

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0 0

20

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New Perspectives on Silver Nanowire Formation from Dynamic Silver

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