Using Polyvinylpyrrolidone and Citrate Ions to Modify the Stability of

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C: Physical Processes in Nanomaterials and Nanostructures

Using Polyvinylpyrrolidone and Citrate Ions to Modify the Stability of Ag NPs in Ethylene Glycol Suyue Chen, Hong Bok Lee, and R. Lee Penn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00339 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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The Journal of Physical Chemistry

Using Polyvinylpyrrolidone and Citrate Ions to Modify the Stability of Ag NPs in Ethylene Glycol Suyue Chen†, Hong Bok Lee‡, and R. Lee Penn* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States

ABSTRACT

Silver nanoparticles (Ag NPs) can serve as seeds in the polyol synthesis of silver nanostructures. However, when heating suspensions of Ag NP seeds up to commonly employed reaction temperatures (e.g., 90 ºC or higher for reactions in ethylene glycol), the size distribution of the product Ag NPs can broaden dramatically through a combination of oxidative dissolution, aggregation, and ripening. Results demonstrate that judicious selection of capping agent and pH can protect Ag NP seeds from these reactions, which prevents broadening of the size distribution. Further, polycrystalline seeds can be selectively dissolved, leaving single crystal seeds largely unaltered.

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INTRODUCTION Silver nanostructures have drawn significant research interest due to their potential use in a wide range of applications.1,2 Their outstanding surface plasmonic resonance (SPR) properties3 and high conductivity4,5 make them desirable materials for sensors and transparent electronic devices. These applications typically require silver nanostructures with controlled size and high shape purity. For example, the diameter of silver nanowires (Ag NWs) impacts the performance of transparent conductive films containing them, with nanowires that are too wide causing substantial light scattering6 and too narrow providing insufficient conductivity.7,8 The polyol synthesis is a versatile method to produce silver nanostructures in a variety of shapes and sizes.9–11 These reactions are usually performed at elevated temperatures with the polyol serving as both solvent and reducing agent and the dissolved silver salt as the source of silver.10,12,13 Our previous study presented a facile synthesis of monodisperse silver nanoparticles (Ag NPs) in a polyol solvent at room-temperature using UV light as the driving force.14 The Ag NPs resulting from that synthesis may be used as seeds for growth into larger silver nanostructures by heating a suspension of the particles and adding silver salt. Ag NPs produced using other synthetic methods, such as aqueous synthesis,15 can also serve as seeds. These seeded polyol synthesis methods have the potential to improve the control over the size and size distribution of silver nanoparticles and other nanostructures in the final product.12,13,16–20 A potential complication of seeded reactions is that the Ag NP seeds in the heated polyol solvent can undergo substantial dissolution,11,21 aggregation,22 and even ripening,23 which can compromise the controlled growth of silver seeds. Moreover, since polyol solvents usually need relatively high reaction temperatures to exhibit sufficient reducing power to reduce silver cations (e.g., the


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minimum temperature for ethylene glycol (EG) is 90 ºC),10,24,25 dissolution, aggregation, and ripening can be the main processes occurring during the heating stage. Options for overcoming this complication could include the injection of seeds into a hot solution of precursors or the use of capping agents. The latter is applied in this study due to its potential simplicity. In the former case, silver salt present in the hot polyol solvent may undergo homogeneous nucleation in addition to growth onto injected Ag NP seeds.26 In addition, the use of capping agents can be employed when the seed preparation and growth are performed in a simple one-pot reaction design, which can avoid a necessary separation of Ag NP seeds from their reaction medium. Capping agents may enable maintenance of the monodispersity of the Ag NPs by inhibiting oxidative dissolution, aggregation, and ripening during the heating stage. Here, we employed polyvinylpyrrolidone (PVP) and citrate as capping agents. Previous work have demonstrated that both capping agents can promote the production of stable dispersions of Ag NPs in polyol solvents.27,28 In addition, PVP and citrate have been reported to preferentially adsorb onto (100) and (111) silver crystal facets, respectively, 28–31 which might increase the stability of Ag NPs with (100) surfaces as compared to those with (111) surfaces when PVP is used and vice versa when citrate is used. Herein, we report results from the kinetic and structural analyses of the evolution of PVP and citrate protected Ag NPs in EG at temperatures of 60 ºC to 75 ºC. Results elucidate the impact of capping agents on Ag NP stability against oxidative dissolution, aggregation, and ripening: PVP can effectively stabilize Ag NPs with a stronger protection of single crystal than twinned Ag NPs and the effect of citrate can be controlled by pH.


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EXPERIMENTAL SECTION Chemicals and Instruments Chemicals used in this study included MilliQ Water (using Millipore, 18.2M Ohms), ethylene glycol (ACS certified, Fisher Scientific), ethanol (200 proof, ACS reagent, Pharmco-AAPR), acetone (ACS certified, Fisher Scientific), AgNO3 (ACS reagent, 99.0%, Sigma-Aldrich), PVP (BASF Kollidon 30, MW 55k), sodium citrate dihydrate (Na3Cit, ACS reagent, Macron), NH4Cl (ACS certified, Fisher Scientific), NaCl (ACS reagent, 99.0%, Mallinckrodt Chemicals), and hydrochloric acid (HCl, ACS reagent, BDH). All chemicals were used as received unless specifically noted. Instruments used in this study included a magnetic stir plate, a 100 mL three-neck Pyrex glass flask, a 500 mL Pyrex glass reaction vessel, an Allihn water cooled condenser, a heating mantle, a UVP Pen-Ray mercury lamp, a quartz tube with one end sealed, and a temperature control unit composed of an Omega PFA coated 12′′ Type J thermocouple and NI USB-TC01 thermocouple measurement device. All glassware was cleaned by submerging in 4 M HCl solution for at least 8 h, rinsed using MilliQ water, and then dried before use. PVP-coated Ag NP Preparation PVP-coated Ag NPs were prepared using a scaled up version of the UV-driven polyol reaction at room temperature described in our previous work.14 A 300 mL EG solution mixture containing 0.05 mM AgNO3, 0.05 mM NH4Cl, and 30.6 mM PVP (by repeat units) in a 500 mL Pyrex glass reaction vessel was stirred using a mechanical mixer. UV light was applied beneath the liquid surface for 24 h using a UV lamp contained in a quartz tube that was submerged in the mixture.


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The resulting Ag NPs were purified using a 3-step procedure described later in this section and stored in a refrigerated ethanol suspension. In addition, a small scale (30 mL) synthesis of PVP-coated Ag NPs was carried out, and the product suspension was directly used in a Ag NP growth reaction. The synthesis reaction conditions were similar to the aforementioned larger scale (300 mL) synthesis with two differences: the reaction vessel was a 100 mL three-neck Pyrex glass flask, and the UV lamp was placed above liquid surface. Citrate-coated Ag NP Preparation A suspension of citrate-coated Ag NPs was prepared in an aqueous reaction using synthesis method reported by Bastús et al.,15 in which the total reaction volume was 30 mL, and the concentration of tannic acid was 0.25 mM. Ag NPs were separated by centrifugation from the product mixture (16500 g for 10 min), washed with 5 mL MilliQ water, and settled by centrifugation for another 10 min at 16500 g. After removing the washing water, the particles were resuspended in 5 mL MilliQ water and kept refrigerated before the seed growth reaction. Monitoring Ag NP number concentration, size, and microstructure during heating UV-vis spectroscopy was employed to monitor reaction the kinetics and evolution of particle size and morphology. The UV-vis absorbance peak position and shape are sensitive to the distribution of Ag NP sizes and shapes, while the height is sensitive to the concentration of Ag NPs in a suspension.32–34 To monitor the dissolution of PVP-coated Ag NPs, 1.5 mL of stock particle suspension in ethanol was first dried under nitrogen gas flow, and the particles were then resuspended in 6.5 mL of 25 mM PVP (by repeat units) in EG. In each reaction, 18 mL of the 25 mM PVP and 13.3 mM NaCl


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in EG was preheated to 60 ºC under air, followed by the addition of 2 mL of the PVP-coated Ag NPs suspended in EG. To examine growth upon injection of silver salt after pre-heating the suspension of PVP-coated Ag NPs under argon, 9 mL of as synthesized reaction mixture was taken from the small-scale (30 mL) UV-driven polyol reaction. It was mixed with EG, 100 mM NaCl, and 50 mM PVP (by repeating units) to make a 22 mL mixture containing 12 mM NaCl, 25 mM PVP, and 0.02 mM Ag NPs (by atoms). The mixture was purged via argon sparging for 3 h and then heated to 60 °C under argon according to the process described in Scheme 1. Four samples (1 mL each) were taken between 0 min and 30 min before heating to 120 ºC and the injection of silver precursor (30 µL of 25 mM AgNO3) at 60 min. A sample was taken immediately after the injection of silver precursor.

Scheme 1. Reaction procedure for the experiment tracking dissolution and subsequent growth upon injection of silver nitrate under argon. The time zero for the growth reaction is 60 min after heating to 60 ºC. To examine the dissolution of citrate-coated Ag NPs, particles were first settled from 2.4 mL of stock suspension by centrifugation in a 15 mL centrifuge tube at 16500 g for 5 min. The supernatant was discarded, and the precipitate was resuspended in 650 µL EG. In each reaction, 19.8 mL of 5 mM Na3Cit, x mM HCl, and 12-x mM NaCl (x = 0, 3, 6, or 12) in EG was preheated to 75 ºC under air, followed by the addition of 200 µL of the citrate-coated Ag NPs suspended in


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EG. Different temperatures were chosen for the dissolutions of PVP and citrated coated Ag NPs. Both temperatures are significantly lower than 90 ºC so that Ag+ reduction would not be significant, and the two temperatures used (60 ºC for PVP coated Ag NPs and 75 ºC for citrate coated Ag NPs at x = 3) resulted in comparable reaction rates. In all experiments, reaction mixtures were sampled at various times after Ag NP addition (sample size was 1 mL each) and cooled in an ice bath. UV-vis spectra were obtained from the sample directly after cooling using an Agilent 8453 UV–vis system, and transmission electron microscopy (TEM) images were obtained using an FEI Tecnai T12 TEM after the sample preparation processes described below. TEM sample preparation TEM imaging was used to characterize the size, shape, and microstructure of Ag NPs remaining after various degrees of dissolution. For PVP-coated Ag NPs, each sample mixture was mixed with 7 mL acetone and then separated by centrifugation at 15000 g for 5 min. The supernatant (colorless) was discarded, and 50 µL of 1:1 (v/v) ethanol/water mixture was used to resuspend the precipitate. The TEM grid of each sample was prepared by drop casting 5 µL of the suspension onto a holey carbon-film-coated copper grid and allowing the solvent to evaporate in air. For citrate-coated Ag NPs, each sample mixture was mixed with 4 mL water and then separated by centrifugation at 16500 g for 10 min. The supernatant (colorless) was discarded. The precipitate was washed by adding 1 mL water followed by centrifugation at 16500 g for 10 min, with the colorless supernatant discarded. The final precipitate was resuspended in 20-60 µL water, of which 20 µL was drop casted onto a holey carbon-film-coated copper TEM grid and allowed to dry in air.


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RESULTS AND DISCUSSION In our previous work, PVP-coated Ag NPs were prepared in EG at room temperature using UV irradiation as driving force for Ag+ reduction.14 In that study, the product nanoparticles were either single crystals, polycrystals, or twinned particles with a range of twinning conditions observed. Since some particles are expected to develop into high aspect ratio nanowires (e.g., the five-fold twins),10,35 assessing the stability of the particles during the heating stage is desirable. If, for example, the preferential dissolution of five-fold twins during the heating stage would compromise their potential use as seeds for nanowire production. In this study, Ag NPs were suspended in EG with added chloride and heated under air at 60 ºC in order to quantify how susceptible the PVPcapped particles were to dissolution, aggregation, and ripening. Results from experiments tracking particle size as a function of the extent of dissolution in solutions containing both PVP and chloride demonstrate that the suspended Ag NPs are protected from aggregation and ripening. The UV-vis spectra of reaction mixtures sampled at various times all showed a single peak with slight blue shift over time (Figure 1), and TEM images (Figure S1) also show that PVP-coated Ag NPs retained their near spherical shape and relatively stable size distribution through the reaction. Results in the literature have demonstrate that the aggregation of Ag NPs in a suspension can lead to red-shift of peak position and/or an additional shoulder or peak at longer wavelength in the UV-vis spectrum.

32–34,36

The absence of aggregated Ag NPs or

particles larger than the initial sizes in the TEM is images also consistent with minimal aggregation and ripening. The statistical analysis of Ag NP sizes measured from calibrated TEM images (Figure S1) showed that the average nanoparticle diameter decreased gradually from 14.5 nm before dissolution to 11.3 nm, when less than 20% of the particles remained (Figure 2 and Figure S2 and Table S1), which is consistent with the slight blue shift of UV-vis absorbance peak from


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413 nm to 409 nm over time (Figure 1). Based on these results, we conclude that PVP was effective in slowing the oxidative dissolution of the Ag NPs and effective in protecting the particles from aggregation and coarsening.

Figure 1. (a) UV-vis spectra of dissolution reaction mixture at different time and (b) the fraction of Ag NPs dissolved as a function of time. The fractions in (b) are calculated using the peak heights in (a) and the calibration curve in Figure S3, the initial Ag NPs were PVP coated, and the dissolution reaction conditions were 25 mM PVP, 12 mM NaCl, at 60 ºC, and under air.


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Figure 2. Size distribution as determined from TEM image analysis of Ag NPs as a function of dissolution. The initial Ag NPs were PVP coated, and the dissolution reaction conditions were 25 mM PVP, 12 mM NaCl, at 60 ºC, and under air.


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The dissolution of PVP-coated Ag NPs followed pseudo first order kinetics, and examination of the TEM images reveals, interestingly but not necessarily surprisingly, that the single crystal particles exhibited higher stability than the twinned ones. The number concentration of Ag NPs remaining in suspension was calculated using the peak height in the UV-vis spectra (Figure 1a) and a calibration curve (Figure S3), and kinetics of the decrease in Ag NP number concentration over time was pseudo first order (Figure 1b). Similar kinetics were also reported for PVP-coated Ag NPs suspended in water.32 The rapid decrease in number (Figure 1) as compared to size of PVP-coated Ag NPs (Figure 2) also suggested that some particles might preferentially dissolve as compared to the others. To examine which particles were most susceptible to dissolution, the twinning conditions of Ag NPs remaining as a function of the extent of dissolution were characterized using TEM images.14 The resulting data shows that single crystalline Ag NPs comprised an increasing number percentage amongst the remaining particles as the dissolution proceeded (Figure 3), indicating their higher resistance to dissolution.


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Figure 3. The evolution of particle microstructure distribution during the dissolution reaction of PVP-coated Ag NPs. The dissolution reaction conditions were 25 mM PVP, 12 mM NaCl, at 60 ºC, and under air. We hypothesize that oxidative dissolution is the primary mechanism responsible for the dissolution of PVP-coated Ag NPs at intermediate temperatures, and a one-pot seeded polyol reaction using Ag NPs produced in situ as seeds could be viable under argon. For example, when the as-synthesized Ag NP suspension using our previous UV-driven method14 was mixed with PVP and NaCl in EG and then subjected to the dissolution conditions (Scheme 1), results demonstrate slowed dissolution when under argon as opposed to under air. Figure S4a shows that under argon, 85% of PVP-coated Ag NPs remained after 30 min at 60 ºC under argon as compared to only 70% under air. The remaining suspension under argon could be then used for seeded silver growth after a silver salt injection at 120 ºC (Scheme 1 with results shown in Figure S4b). More of the in situ Ag NP seeds survived the heating stage under argon, even at a relatively high Clconcentration, which would promote oxidative dissolution. By optimizing reaction conditions, a


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one-pot Ag NP growth with high yield using in situ seeds prepared at room temperature seems possible. Such an approach would also eliminate the need to separate seed particles from the initial reaction mixture, thus reducing both waste and time required to produce the desired product. A plausible reason for preferential dissolution of twinned particles under oxidative conditions is their lower stability due to the facets exposed combined with their smaller crystal domain size. Ho et al. suggested that the rate-determining step during the oxidative dissolution of Ag NPs is the oxidation of silver atoms on nanoparticle surfaces.32 Since twinned Ag NPs have higher overall surface energy,11 they are more favorable to be oxidized. It is worth noting that PVP preferentially adsorbs to (100) silver crystal surfaces over the (111).27,37 However, it is also reported that PVP can passivate all silver crystal facets at PVP/Ag molar ratios (repeat units of PVP to atoms for Ag) higher than 18.24 Since the PVP/Ag molar ratio in our reaction system was over 1000, the difference in adsorption of PVP on Ag NPs with different surfaces exposed was deemed an insignificant factor for their stability. The higher stability of single crystalline PVP-coated Ag NPs was also reported in the literature at a higher temperature (148 ºC) in EG, which was successfully used to dissolve twinned Ag NPs and promote single crystalline seed formation in polyol syntheses.10,11 Similarly, a heating process that promotes oxidative dissolution could be used to selectively remove twinned particles when single crystalline PVP-coated Ag NP seeds are desired. On the other hand, if twinned PVP-coated Ag NP seeds are desired, it could be beneficial to employ a shorter heating process under an inert gas atmosphere in order to preserve the twinned particles present. Citrate is another a widely used capping agent in the synthesis of Ag NPs,28,30 and the stability of the citrate-coated Ag NPs during the heating stage will be similarly important for their use as seeds in polyol synthesis. However, in contrast to PVP, citrate is an anionic small molecule that


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preferentially adsorbs onto (111) silver surfaces,28,31 and the dissolution behavior of Ag NPs suspended in solutions containing citrate is expected to differ as compared to the above. The effect of citrate on Ag NP stability, which was investigated using a similar experimental approach as with PVP, showed a strong and unsurprising dependence on pH. The pH of the reaction mixture was altered using HCl, and the concentration of NaCl was adjusted accordingly to maintain a constant Cl- concentration. The degree of Ag NPs conversion was again monitored by UV-vis spectroscopy, and Figure 4 shows the fraction of Ag NPs converted over time. As explained in the following paragraph, the decrease in UV-vis peak height is the convoluted result from the dissolution, aggregation, and ripening of Ag NPs, and the fraction of Ag NPs converted in Figure 4a provides an estimate of the contribution from all three reactions. The kinetics of Ag NP conversion followed pseudo first order kinetics at all pHs, but the half life of Ag NPs shortened from 30 h to 5 min when HCl concentration was increased from 0 mM to 12 mM (a H+/citrate molar ratio of 2.4). Higher H+ concentrations can lead to even faster reaction rates and quicker change of particle morphologies, making the reactions hard to track using methods employed in this study. Such a trend could be the combined result of increased O2 etching ability with higher acidity38–40 as well as decreased citrate adsorption and zeta potential on the Ag NPs due to increasing citrate protonation with increasing HCl concentration.41 These data suggest that pH is a powerful handle to adjust the stability of citrate-coated Ag NPs. For example, higher pH can be used to prevent seed loss during heating, and acid can be added at the higher reaction temperature to increase seed reactivity. After the desired growth of seeds, reactions could be quenched by the addition of base, leaving behind a soluble salt that could be removed with washing.


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Figure 4. (a) The fraction of Ag NPs converted as a function of time, in which the initial Ag NPs were citrate coated, and the reaction conditions were 5 mM Na3Cit, x mM HCl, 12-x mM NaCl, at 75 ºC, and under air; and (b) UV-vis spectra of reaction mixtures collected at different times, in which HCl concentration was 3 mM. For each reaction, the fractions of Ag NPs converted were calculated using the equation 1 – At/A0, in which At and A0 are the main UV-vis peak heights at time t and 0 min, respectively, for the corresponding reaction. The UV-vis spectroscopy and TEM results showed that the conversion of the citrate-coated Ag NPs was more complicated than observed with the PVP-coated particles: significant oxidative dissolution, aggregation, and ripening were all observed. In the UV-vis spectra (Figure 4 and


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Figure S5), the main peak at ~410 nm decreased in intensity while also shifting to shorter wavelengths. This suggests a substantial decrease in both the number of particles and their average size, which correlates to oxidative dissolution. In addition, a broad peak around 580 nm appeared, indicating the formation of larger Ag NPs as a result of particle aggregation and/or ripening. TEM images (Figure S6) showed that monodisperse near-spherical Ag NPs were the predominant species at time zero. However, as the reaction proceeded, the products evolved into mixtures of near-spherical NPs with size similar to or smaller than that before dissolution (presumably due to oxidative dissolution), aggregates of multiple near-spherical particles, and irregularly shaped silver nanostructures over 100 nm in size (from aggregation and/or ripening). Moreover, the three reactions can interfere with each other and further complicate the reaction kinetics. For example, the dramatic decrease of particle surface area caused by aggregation and ripening can also affect the rate of dissolution.42 TEM analysis (Figure 5) also demonstrated that oxidative dissolution was the main reaction occurring during the early stages while significant aggregation and ripening took place later in the reaction. This is consistent with the UV-vis spectra shown in Figure 4b. The spectra of the original and 18 min samples are similar in shape, and the isolated near-spherical Ag NPs were the predominate species observed. These particles were relatively monodisperse, and little aggregation was observed in the TEM images. With continued reaction, the number concentration of this population decreases gradually, and the additional peaks at 580 nm in UV-vis spectra observed for samples collected after 30 min correlates to the appearance of larger silver nanoparticles in the TEM images. In addition, the particle size distribution of the near-spherical Ag NPs collected after 45 minutes is bimodal. The source of the particles with sizes of ca. 10 nm could be either


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homogeneous nucleation of new Ag NPs or substantial dissolution of existing Ag NPs. Finally, an increasing proportion of the Ag NPs were aggregated after 45 min.

Figure 5. Size distributions as determined from TEM image analysis of Ag NPs remaining as a function of dissolution time. The initial Ag NPs were citrate coated, and the dissolution reaction conditions were 5 mM Na3Cit, 3 mM HCl, 9 mM NaCl, at 75 ºC, and under air. Percentage of Ag NPs converted were calculated same as in Figure 4. a INS/Agg ratio refers to the number ratio between isolated near-spherical Ag NPs to aggregated Ag NPs. These results demonstrate that citrate-coated Ag NPs at relatively low pH are much more reactive than PVP-coated particles. Since citrate ions favor the (111) facets of Ag crystals and PVP the (100) facets,28 using a mixture of PVP and citrate ions as capping agents may facilitate the production of silver nanostructures that consist of citrate-coated (111) surfaces with pH-tunable


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reactivity and PVP-coated (100) surfaces with low reactivity regardless of pH. This could be useful for size- and shape-controlled polyol syntheses of silver nanowires and nanobars, in which the elongation of these 1-D structures can be controlled by tuning the reactivity of the tips of wires that exposed (111) surfaces using pH, and the width of wires and bars can be controlled by using PVP to stabilize (100) surfaces on the side of wires.43

CONCLUSIONS In this work, we demonstrated the effect of PVP and citrate on the stability of Ag NPs against oxidative dissolution, aggregation, and ripening at intermediate temperatures. PVP provided strong protection of Ag NPs against aggregation and ripening and moderate protection against oxidative dissolution. Oxidative dissolution resulted in the preferential dissolution of twinned particles, with the single crystalline PVP-coated Ag NPs comprising a growing fraction of particles remaining as a function of continued dissolution. Citrate-coated Ag NPs were very stable at high pH, but aggregation, ripening, and oxidative dissolution were dramatically accelerated by increasing acidity. Insights gained in this study can provide guidance when choosing reaction conditions, such as capping agent and pH, in seed-mediated Ag NP synthesis in polyol.


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ASSOCIATED CONTENT Supporting Information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses † Complete Genomics Inc., 2904 Orchard Pkwy, San Jose, California 95134, United States ‡ Iowa Advanced Technology Laboratories, 205 N Madison St, Iowa City, Iowa 52245, United States

ACKNOWLEDGEMENT AND FUNDING SOURCES We thank the Nanostructural Materials and Processes (NMP) program at the University of Minnesota for financial support. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.


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Using Polyvinylpyrrolidone and Citrate Ions to Modify the Stability of

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