This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. http://pubs.acs.org/journal/acsodf Article From Silver Plates to Spherical Nanoparticles: Snapshots of Microwave-Assisted Polyol Synthesis Miquel Torras and Anna Roig* Cite This: ACS Omega 2020, 5, 5731−5738 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The fabrication of silver nanoparticles (Ag NPs) with different sizes by microwave (MW)-assisted synthesis is presented. The fast heating of the MW technique, combined with the possibility to thermally quench the reactions, enabled us to capture snapshots of nucleation and growth processes difficult to accomplish in other techniques. The Ag NPs were synthesized using poly(vinylpyrrolidone) (PVP) through a polyol approach. The effects of the reaction time, the reaction temperatures, and the silver precursor concentration were investigated. The influence of agitation, the PVP concentration, and the initial conditions of the silver precursor was also studied. It is found that at very short reaction times and at low temperatures, polyhedral plates are formed with sizes ca. 300 nm and large polydispersity. However, by increasing the time or the temperature, a size and shape refinement is observed resulting in 10 nm spherical NPs with low polydispersity. Mechanistic insights are provided based on the observations extracted from transmission electron microscopy (TEM) and ultraviolet−visible spectros- copy (UV−vis). A formation mechanism starting from kinetically favored silver polyhedral plates to thermodynamically favored spherical nanoparticles is proposed. Understanding these effects allowed us to control the particle size and the tuning of Ag NPs on- demand. Moreover, the reproducibility and scalability of the process and the long-term stability of the NPs in aqueous solutions are demonstrated. Finally, we provide a recommendation regarding the use of fresh PVP as a capping and stabilizing agent. 1. INTRODUCTION Ag NPs can be synthesized using various methods, namely Every day, nanotechnology is prevailing more in many society chemical reduction,13 electrochemistry,14 photochemistry,15 domains and industrial sectors such as health, pharmaceutics, laser ablation,16 sonochemistry,17 and sputtering.18 Among food, information technologies, electronics, energy, or the these, the most popular method for the preparation of silver environment.1 New applications, devices, and products derived colloids is based on the chemical reduction of the metal ions from advances in nanotechnology demand new nanomaterials, by a reducing agent, usually the solvent, and generally in the specifically, functional nanoparticles (NPs). The migration of presence of a stabilizing agent. Typical reducing agents include these NPs from the laboratory to the market is proceeding at a polyols.19 The polyol process is commonly used for the rather slow pace and needs to be accelerated. Thus, well- preparation of easily reducible metals.20 Polyols such as understood, reproducible, and scalable methods for efficient ethylene glycol (EG) can act as a reducing agent and as a NP production are needed. solvent in which the metal salts are dissolved. The most Metal nanoparticles are of particular industrial interest due commonly used stabilizing agents are polymers and surfac- to their unique properties and applications both in technology tants.21 Poly(vinylpyrrolidone) (PVP) is a widely used and in biomedicine.2−4 Their optic, catalytic, electronic, and polymer to passivate the surface of silver nanoparticles and spectroscopic properties make them interesting in many protect the nanoparticles from sintering.22 Polyol-mediated domains. They are characterized by a localized surface plasmon synthesis can be carried out at room temperature, but higher resonance (LSPR) absorption band. The position of this band temperatures are generally used for higher reaction rates.20 To strongly depends not only on the type of metal but also on its increase the reaction media temperature, conventional thermal size, shape, and to a lesser extent, the dielectric environment of heating23 or ultrasonic irradiation24 can be employed. Many of the particles.5−7 Silver nanoparticles (Ag NPs) are especially interesting due to their antimicrobial properties.8 They have been widely used in water and air filtration to eliminate Received: November 5, 2019 microorganisms9 and in the biomedical field because of their Accepted: February 21, 2020 intrinsic therapeutic properties.10 Moreover, their high Published: March 10, 2020 electrical conductivity and chemical stability make them the perfect material for inkjets in electronics, photonics, bio- markers, and chemical/biological sensors.11,12 © 2020 American Chemical Society https://dx.doi.org/10.1021/acsomega.9b03748 5731 ACS Omega 2020, 5, 5731−5738 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 1. Effect of the reaction time (at 120 °C and 1 mM AgNO3). (a−d) Representative TEM images of the different reaction times analyzed (10 s, 2 min, 5 min, and 10 min, respectively). (e) UV−vis measurements of the same samples. The inset contains digital images of the samples. (f) Mean size evolution with time (the error bars represent the standard deviation over the population mean). Inset: scheme of the Ag NP evolution with the reaction time. these preparations are limited due to long reaction times, the 2. RESULTS AND DISCUSSION consumption of large volumes of solvent, and poor stability of Fast synthesis of metal (Au) and metal oxide (Fe2O3, TiO2) the products. Microwave (MW)-assisted chemistry is an nanoparticles by MW-assisted chemistry, as well as their advantageous heating source that provides volumetric uniform combination in hybrid nanostructures, was previously reported heating of the reaction solution (leading to narrow size by our group.29−31 Here, Ag nanoplates and nanoparticles are distributions), shorter reaction times, lower energy consump- synthesized by a polyol approach using AgNO3 as the tion, and high product yield.25−27 Microwave ovens permit the precursor, ethylene glycol (EG) as the solvent and the synthesis of many types of inorganic nanoparticles, from single reducing agent, and polyvinylpyrrolidone (PVP) as the component (metals and metal oxides)28 to multicomponent nanoparticles or metastable alloys.29−31 reducing and capping agent. One of the most powerful advantages of the MW technique 2.1. Effect of the Reaction Time. The effect of the is that it is rapid.25−27 Its fast heating velocity, combined with reaction time on the resulting Ag NPs was investigated using 1 the possibility to thermally quench the reactions, enabled us to mM AgNO3, 120 °C reaction temperature, and synthesis times grasp snapshots of nucleation and growth processes difficult to of 10 s, 1, 2, 5, and 10 min; the results are shown in Figure 1. accomplish in other techniques. Consequently, some mecha- Transmission electron microscopy (TEM) micrographs nistic insights can be extracted as well as information on confirmed the successful formation of nanoparticles in all competing kinetics and thermodynamics processes, permitting conditions and depict the differences in terms of particle size, the stabilization of metastable phases. These advantages can shape, and polydispersity. For all particles, selected area lead to a greater understanding of the key parameters to obtain electron diffraction (SAED) patterns confirmed a face-centered a wide range of desired products, as well as to control the size cubic metallic silver as the sole crystalline phase (Figure S1). and shape of NPs to yield desirable chemical and physical At 10 s, an extremely short reaction time, particles display properties. various shapes, commonly polyhedral plates such as triangles Here, we report a systematic and detailed study of the and hexagons and mean sizes larger than 300 nm with a large diverse scenarios of Ag NP synthesis mapping several synthetic polydispersity of sizes, as can be observed in Figure 1a. Similar conditions such as the reaction time, the reaction temperature, results are obtained at 1 min reaction time, with a size and the silver precursor concentration. Platelet-like kinetically reduction (∼200 nm). The particle size is further reduced at favored Ag structures could be obtained for the first time using longer reaction times. At 2 min reaction time, the majority of highly reducing conditions benefitting from the very short the population consisted of small, spherical, and monodisperse reaction times of the MW-assisted synthesis as opposed to NPs (PDI < 30%) (Figure 1b−d). Particles have a mean size of thermodynamically favored morphologies such as spheres 9 ± 3, 12 ± 3, and 13 ± 4 nm at 2, 5, and 10 min, respectively, obtained at longer reaction times and higher temper- resulting from the fitting of the size histogram to a Gaussian atures.32−35 A formation mechanism from silver polyhedral distribution. A minority population of larger particles was also plates to spherical nanoparticles is proposed. Understanding observed with mean sizes of ∼150 nm for 2 and 5 min reaction these effects allowed us to control the particle size and the times. Thus, by increasing the reaction time from seconds to tuning of Ag NPs on-demand. Moreover, the reproducibility 10 min, particles experience a size refinement where the initial and scalability of the process were demonstrated. Finally, this nanoplates steadily disappear in favor of spherical mono- study provides a recommendation regarding the use of PVP as dispersed nanoparticles. Moreover, above 2 min of the a capping and stabilizing agent through a polyol approach. reaction, the longer the reaction time, the bigger the particles. 5732 https://dx.doi.org/10.1021/acsomega.9b03748 ACS Omega 2020, 5, 5731−5738 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 2. Effect of the reaction temperature (at 10 min and 1 mM AgNO3). (a−d) Representative TEM images of the different reaction temperatures analyzed (90, 120, 150, and 180 °C, respectively). (e) UV−vis measurements of the same samples. The inset contains digital images of the samples. (f) Mean size evolution with temperature (the error bars represent the standard deviation over the population mean). Inset: scheme of the Ag NP evolution with the reaction temperature. Figure 3. Effect of the reaction concentration (at 10 min and 120 °C). (a−d) Representative TEM images of the different reaction concentrations analyzed (1, 10, 15, and 20 mM, respectively). (e) UV−vis measurements of the same samples. The inset contains digital images of the samples. (f) Mean size evolution with concentration (the error bars represent the standard deviation over the population mean). Inset: scheme of the Ag NP evolution with the reaction concentration. TEM observations were confirmed by UV−vis spectroscopy. shapes. Figure 1f shows the mean sizes vs reaction times of Ag Figure 1e shows the absorbance spectra for these materials. In NPs. all cases, one single peak corresponding to the localized surface 2.2. Effect of the Reaction Temperature. The effect of plasmon resonance (LSPR) of Ag NPs is observed. It is well the reaction temperature was investigated by using 1 mM established that the shape, size, and composition of metal AgNO3, 10 min reaction time, and synthesis temperatures of nanoparticles are crucial in determining the LSPR spectral 60, 90, 120, 150, and 180 °C, and the results are shown in position.5−7 According to the previous size distributions, Figure 2. At 60 °C, a slight color change on the reaction tube polydisperse samples of 10 s and 1 min have a broad LSPR was observed, but no precipitate was collected. This indicates peak, while monodisperse samples from 2, 5, and 10 min that at this low temperature, no nucleation occurs. However, reaction times have a narrow one. Moreover, the wavelength of the tube changed its color to the expected orange-yellow after the maximum of the UV−vis bands shifted from 438 nm at 10 being stored one night, due to the room temperature reduction s to 403 nm at 10 min, due to the smaller size and isotropic of silver cations for a longer time by EG and PVP acting as the 5733 https://dx.doi.org/10.1021/acsomega.9b03748 ACS Omega 2020, 5, 5731−5738 ACS Omega http://pubs.acs.org/journal/acsodf Article reducing agents. Indeed, the chemical reduction reaction the silver precursor, additional synthesis was also undertaken. involved in this system is kinetically controlled. The synthesis conditions were always 10 min reaction time, When the temperature is increased to 90 °C, NPs were 120 °C reaction temperature, and 1 mM AgNO3. The formed after 10 min of the reaction. Figure 2a shows a influence of agitation was studied by heating the above representative TEM image of the sample, in which two solution in the MW reactor without agitation, the PVP populations are observed: a minoritybigger, polydisperse, concentration control was studied by changing the PVP with polyhedral shapes and with an average size of around 150 amount from 25 to 5 mg, and the silver precursor solvent nm; and a majoritysmaller, monodisperse and spherical with control was studied using EG instead of MQ-H2O to prepare 9 ± 3 nm size. These results are almost the same as those the initial AgNO3 solution. found for the reaction time sample of 2 min (Figure 1b), When the reaction solution is not stirred during heating, the heated at 120 °C. It shows how the system heated at higher obtained Ag NPs are polydisperse, as shown in the TEM image temperatures displays an accelerated reduction; therefore, in 2 (Figure 4a). Indeed, the corresponding UV−vis spectrum min at 120 °C, the same particles are formed as in 10 min at 90 °C. At 120 °C (Figure 2b), the Ag NPs were identical to the sample reported in Figure 1d. Thus, the reproducibility of MW-assisted syntheses is demonstrated. On increasing the temperature further, the most significant observation was the gradual increase in the mean particle size and the loss of monodispersity. Specifically, particles have an average size of 14 ± 5 and 15 ± 6 nm at 150 and 180 °C, respectively. The loss of monodispersity is attributed to the fact that at higher temperatures, particles start to fuse; aggregates are clearly observed in Figure 2c,d. UV−vis spectra for the samples of this series were recorded (Figure 2e). The sample at 90 °C has a broad LSPR peak at 405 nm wavelength, while the most monodisperse sample (120 °C) has a narrower band with a maximum at 403 nm. Otherwise, samples at higher temperatures (150 and 180 °C) show two broad peaks at 398 and 540 nm and at 396 and 560 nm, respectively. These secondary and red-shifted bands correspond to the bigger and nonisotropic particles present Figure 4. Control experiments (at 10 min, 120 °C, and 1 mM of in these samples in accordance with TEM results. Figure 2f AgNO3). (a−c) Representative TEM images of the different control shows the mean sizes vs reaction temperatures of Ag NPs. assays analyzed (nonagitated reaction, five times less PVP and initial 2.3. Effect of the Silver Precursor Concentration. The AgNO3 solution in EG, respectively). (d) UV−vis of the same effect of the silver precursor concentration was also samples. The standard sample (10 min, 120 °C, 1 mM AgNO3 from investigated by using 10 min reaction time, 120 °C reaction an aqueous solution, agitation, and 25 mg of PVP) is also represented temperature, and the synthesis concentrations of 0.5, 1, 10, 15, for comparison. and 20 mM AgNO3; the results are shown in Figure 3. On increasing the concentration, the most significant observations clearly defines two LSPR peaks at 394 and 560 nm wavelengths were the gradual increase in the average size and the gradual (Figure 4d). Agitation results in increased homogeneity of the increase in the number of nanoparticles (more particles were final spherical Ag NPs because it avoids diffusion control of the collected). Both phenomena are likely to occur simultaneously growth process after new nucleation of Ag(0) elements. When because of the increase of the number of silver nuclei in the the PVP concentration is reduced five times, Ag NP aggregates initial solution (coming from the Ag(I) reduction) with higher are observed, implying that there is not enough stabilizer both concentrations. Particularly, particles have an average size sterically and electrostatically (Figure 4b). Accordingly, the determined by TEM of 12 ± 4, 11 ± 3, 15 ± 3, 17 ± 3, and 20 UV−vis spectrum of this sample shows a broad peak at 407 nm ± 4 nm at 0.5, 1, 10, 15, and 20 mM AgNO3, respectively. wavelength (Figure 4d). Finally, when EG is used to prepare Then, in the range between 0.5 and 1 mM AgNO3, particles the initial AgNO3 solution (instead of a small amount of water are spherical, monodisperse, and have a similar size of around used in the described synthesis before), no significant 13 nm; when the concentration is increased by 1 order of differences were found. Ag NPs are spherical, monodisperse, magnitude (10, 15, and 20 mM AgNO3), particles can grow up and well stabilized by PVP (Figure 4c) and the corresponding to 15, 17, or 20 nm. UV−vis spectrum supports this (narrow peak at 401 nm) as UV−vis spectra for the samples of this series were recorded also for the standard sample represented in Figure 4d (10 min, (Figure 3e). The silver LSPR band maxima were located at 120 °C, 1 mM AgNO3 from an aqueous solution, agitation, 398, 396, 396, 400, and 403 nm wavelengths at 0.5, 1, 10, 15, and 25 mg of PVP). and 20 mM AgNO3, respectively. Samples experience a red 2.5. Mechanistic Insights. After all of the experiments shift with concentration according to their bigger size, as was described above, some mechanistic insights can be hypothe- seen in the TEM images. Note that samples with more sized. Figure 5 schematizes a simplified mechanism proposed polydispersity have broader LSPR bands. Figure 3f shows for the formation of the Ag NPs. mean sizes vs silver precursor concentrations of Ag NPs. We have observed that the reaction starts with anisotropic 2.4. Additional Experiments. To study the influence of platelet-like structures (triangles, hexagons, and truncated agitation, the concentration of PVP, and the initial solvent of shapes), as shown for the samples at 10 s, 120 °C and 10 min, 5734 https://dx.doi.org/10.1021/acsomega.9b03748 ACS Omega 2020, 5, 5731−5738 ACS Omega http://pubs.acs.org/journal/acsodf Article stabilizes with Ag nanospheres. Moreover, the PVP present during the reaction can help the formation of Ag nanoplates. When Ag(0) nucleates, free PVP adsorbs to all silver surfaces due to the strong affinity of the Ag surface to the nitrogen of the PVP, resulting in small, spherically shaped Ag NPs. However, decreasing the amount of PVP could break the initial symmetry to avoid spherical growth. During crystal growth, free PVP will preferentially adsorb to the faces with the lowest energy. Then, selective adsorption occurs on the {111} faces (top and bottom faces of the Ag nanocrystals), while {100} faces (three-sided faces) continued to grow at a faster rate. Thus, we expect that reducing further the PVP amount in the reaction media (without compromising the NP final stability and dispersion) could help to drive the nanoplate formation. Moreover, less PVP could lead to the large edge length of the observed anisotropic Ag nanostructure-like platelets (Figures Figure 5. Schematic representation of the formation of Ag NPs. The 1a and 2a). proposed mechanism for the formation of Ag, big, anisotropic Finally, increasing the reaction time and/or the reaction nanostructure-like platelets and Ag, small, spherical NPs depending temperature drives the system to thermodynamically favored on the synthetic conditions. Notation: AgNO3, silver nitrate; t, time; products, such as the spherical structures observed in Figures T, temperature; PVP, polyvinylpyrrolidone. 1−4. Indeed, Ag nanoplates are thermodynamically unstable in comparison with Ag spherical particles. As a metal inside a 90 °C (Figures 1a and 2a, respectively). When the reduction conventional MW oven, the big silver nanoplates can act as an starts, Ag atoms are formed in the solution and tend to antenna25,45 and absorb the MW irradiation, promoting its aggregate into small clusters or nuclei, which then aggregate fracture to small particles. Then, particles undergo a size into nanoparticles.36 If the reduction is fast (high temperature refinement, decreasing their size and changing their shape. In for a long time), there are enough Ag atoms to attach to the addition, at longer times, higher temperatures, and higher silver seeds’ surface for continuous growth, tending to thermody- precursor concentration, particles increase their size (and namically favored products. However, if the reduction is slow number) due to the increased reaction that contributes to new (low temperature or short times at high temperature and fast nucleation and growth. quenching), the concentration of Ag atoms is small, and the 2.6. Ag NPs On-Demand, Scalability of the Protocol, nanocrystals will take shapes deviating from the thermody- and Reaction Yield. Comprehending all of these effects namically favored ones.37 This synthesis is kinetically allowed us to control the particle size and the tuning of Ag NPs controlled. In this case, stacking faults can develop and induce on demand (Figure 6). For instance, if bigger monodisperse Ag the formation of thin nanoplates despite their high total NPs are desired, time, concentration, and reaction temperature energy.19,38−40 Each Ag nanoplate is enclosed by two {111} (<150 °C) can be increased. A successful test was prepared planes as the top and bottom faces, and a mix of {100} and with the synthesis conditions fixed to 20 min, 130 °C, and 20 {111} planes as the side faces.41 Moreover, these structures have {111} twin planes parallel to their flat faces38 and stacking faults.7,42 Then, due to a large surface area and the lattice strain caused by stacking faults, the formation of nanoplates is not favored in terms of surface energy. For this reason, Ag nanoplates are kinetically favored products. There are different methods of achieving kinetically controlled synthesis of metal nanoplates: the coupling of reduction with oxidative etching,43 the use of an extremely mild reducing agent,44 and the use of ligands to form complexes with Ag(I) ions.38 In this work, note that MW heating of AgNO3 in EG in the presence of PVP causes the reduction of Ag(I) to Ag(0), where both EG and PVP act as reducing reagents. This together with the fast heating of the solution with the MW-assisted synthesis method allows the rapid formation of many Ag atoms that are able to grow into thermodynamically favored morphologies such as spheres (as it happens at long reaction times). However, thanks to the fast heating and cooling speeds of the MW method, we were able to capture and freeze kinetically favored products in conditions (extremely short reaction time or very low temperature) where a smaller number of silver nuclei were allowed to grow. Note that the heating and the cooling velocities are high (around 1 Figure 6. Summary of Ag NP mean sizes depending on the synthetic °C/s). Indeed, the energy barrier to achieving kinetically conditions. The single green dot corresponds to the planned synthesis favored products is lower than the one for thermodynamic to demonstrate the capability of producing Ag NP with on-demand ones. Thus, the reaction starts with Ag nanoplates and it sizes, ∼30 nm in this case. 5735 https://dx.doi.org/10.1021/acsomega.9b03748 ACS Omega 2020, 5, 5731−5738 ACS Omega http://pubs.acs.org/journal/acsodf Article mM AgNO3. The obtained Ag NPs have a mean size of 33 ± spherical particles. Then, particles undergo a size refinement, 14 nm (Figure S2a and the single green dot in Figure 6). These decreasing their size and changing their shape. In addition, at particles are approx. 20 nm larger than the ones of the standard longer times, higher temperatures, and higher silver precursor sample (Figure S2b). Consequently, a red shift was observed in concentrations, particles increase their size (and number) due their UV−vis spectrum (Figure S2c). to the increased reaction that contributes to new nucleation It is well known that MW-assisted synthesis can be easily and growth. scaled up, accelerating the migration of nanoparticles from the Understanding these effects allowed us to control the laboratory to the market. We show that the synthesis presented particle size and the tuning of Ag NPs on-demand using cross here can be scaled up at least by a factor of 4. To study the experiments. Moreover, the reproducibility and scalability of scalability of the protocol, both, the same MW reactor (CEM) the process and the long-term stability -up to 9 months- of the and a Microwave Advanced Flexible Synthesis Platform NPs in aqueous solutions was demonstrated. Finally, we (flexiwave) from Milestone, were used. According to TEM provide a recommendation regarding the use of fresh PVP as a images (Figure S3a,b), Ag NPs were successfully fabricated by capping and stabilizing agent to reproduce the nanoparticles’ scaling up the synthesis by a factor of 4 with both MW synthetic results (see the Experimental Section). reactors, having an average size of 16 ± 4 and 24 ± 4 nm when CEM and Flexiwave MW reactors were used, respectively. In 4. EXPERIMENTAL SECTION comparison with the standard sample (Figure S3c), these 4.1. Materials. Silver nitrate for analysis, ACS, ISO spherical and always monodisperse Ag NPs are bigger. (AgNO3, 99.8%), ethylene glycol (EG ≥ 99%), and acetone Therefore, a red shift was observed in their UV−vis spectrum were purchased from Panreac. Poly(vinylpyrrolidone) (PVP, (Figure S3d). Such slight increase of the particle size may average molecular weight: 10000 g/mol) was purchased from come from the non-linear phenomena likely to occur when one Sigma-Aldrich. All materials were used as received without synthesis is scaled up, which should be readjusted on demand further purification. Milli-Q water (MQ-H2O) was used in all (outside the scope of this study). experiments. Finally, a chemical analysis of Ag was done with inductively 4.2. Synthesis of PVP-Coated Ag NPs. Ag NPs with a coupled plasma optical emission spectroscopy (ICP-OES). PVP surface coating are synthesized using a microwave (MW)- The results showed a chemical reaction yield of 57% for the assisted polyol synthesis protocol in a CEM Discover SP (with standard sample at CEM MW reactor, corresponding to 0.25 the autosampler Explorer 12 Hybrid) at a frequency of 2.45 mg of Ag. On the other hand, when the same CEM MW GHz and 300 W of power. This MW reactor is equipped with reactor was used for the four-fold scale-up, the yield was 61% pressurized vessels to which an active cooling with compressed corresponding to 1.05 mg of Ag per batch. Then, the defined air permits to control the set reaction temperature as well as to MW-protocol and the cleaning steps of this work for the rapidly quench the reaction at the end of the set time preparation of Ag NPs with controlled size result, on average, preventing Ostwald ripening. Note that since the reaction in a 60% yield and the production of silver colloids up to 1 mg. vessel remains at room temperature during the reaction, this Higher quantities should be expected when using the quenching mechanism is very effective. Flexiwave MW reactor due to its greater scalability potential, Briefly, 25 mg of PVP (2.5 μmol) is dissolved in 4 ml of EG since up to 15 reaction-vases of 100 ml can be used in one by continuous sonication (JP Selecta 3000683, frequency, 40 batch. kHz; sonication time, 5 min). Then, 4 μmol of AgNO3 (from an aqueous solution, 250 mM) is mixed with the above- 3. CONCLUSIONS prepared solution to give a homogeneous solution with no We have presented a fast, simple, reproducible, and scalable color of 1 mM AgNO3. The tubes (CEM, borosilicate glass, microwave-assisted polyol synthesis of Ag NPs with different and silicon with septum caps) are then placed in the MW shapes and controlled sizes by a judicious choice of the reactor and heated under continuous agitation (Labbox, 3 × 6 synthetic parameters such as reaction time, reaction temper- mm2 cylindrical, fully encapsulated with high-grade poly- ature, and silver precursor concentration. We have hypothe- (tetrafluoroethylene) [PTFE-Teflon], and a maximum stirring sized mechanistic insights for the formation of the obtained speed of the CEM MW reactor). Heating and cooling nanoplates and nanoparticles underpinned by the observations velocities of the MW reactor were the same in all samples, extracted from transmission electron microscopy (TEM) and with 1.1 and 0.8 °C/s, respectively. The effect of MW time (t = ultraviolet−visible spectroscopy (UV−vis). 10 s, 1, 2, 5, and 10 min) and temperature (T = 60, 90, 120, Very short reaction times and low temperatures lead to a 150, and 180 °C) on the resulting silver structures was small number of available nuclei yielding to kinetically investigated. The effect of silver nitrate concentration (0.5, 1, favorable anisotropic platelet-like structures (triangles, hex- 10, 15, and 20 mM) was also studied. In this synthesis, PVP is agons, and truncated shapes). Moreover, the low amounts of used as the capping agent to shape and stabilize the Ag NPs PVP used facilitate breaking the initial spherical symmetry by and as a mild reducing agent, while EG acts as a solvent and a preferential adsorption to the {111} faces (top and bottom reducing agent. The final solution is orange-yellow, and no faces of Ag nanocrystals), while {100} faces (three-sided faces) precipitate is observed, indicating that the nanoparticles are continue to grow at a faster rate, leading to the anisotropic Ag dispersed in the solution. Ag NPs are collected by adding 30 platelet-like structures. Note that platelet structures were ml of acetone in the solution and centrifuging at 6000 rpm for observed here for the first time in synthetic conditions (EG 30 min to precipitate the nanoparticles (Hettich EBA 21, type and PVP allow high reduction rates), where for longer reaction 1004, 4025 RCF). The supernatant with excess reactants times and higher temperatures, there is a rapid formation of a (PVP, EG, NO3−, and Ag+ ions not reduced) was discarded. large number of Ag nuclei that grow into thermodynamically The same procedure was repeated twice and the final solid NP favored morphologies such as spheres. Indeed, Ag nanoplates precipitate was dispersed in 1 mL of MQ-H2O and kept for are thermodynamically unstable in comparison with Ag further characterization. All of the above experiments were 5736 https://dx.doi.org/10.1021/acsomega.9b03748 ACS Omega 2020, 5, 5731−5738 ACS Omega ■ http://pubs.acs.org/journal/acsodf Article carried out in duplicate. PVP-capped Ag NPs are stable in ASSOCIATED CONTENT water. Sample colloidal stability in water was demonstrated up * sı Supporting Information to 9 months. Aggregates or precipitates were not significantly The Supporting Information is available free of charge at observed (Figure S4a). In addition, TEM analysis demon- https://pubs.acs.org/doi/10.1021/acsomega.9b03748. strates their shape stability (Figure S4b−g). Experiments to study the influence of agitation, PVP Diffraction data; size-control synthesis; high-resolution concentration, and initial conditions of silver nitrate were TEM images; scalability; colloidal and shape stability of also prepared. Comprehending all these effects allowed us to Ag solution with time; effect of aging PVP on NP control the particle size and tuning of Ag NPs on-demand. morphology (PDF) ■ This synthetic route can be scaled up by at least a factor of 4. To study the scalability of the protocol, both the same MW AUTHOR INFORMATION reactor and a Microwave Advanced Flexible Synthesis Platform (flexiwave) from Milestone at a frequency of 2.45 GHz and Corresponding Author 500 W power were used. Anna Roig − Institut de Ciència de Materials de Barcelona We provide a recommendation regarding the use of fresh (ICMAB), CSIC 08193 Bellaterra, Spain; orcid.org/0000- PVP as a capping and stabilizing agent. As mentioned above, 0001-6464-7573; Email: roig@icmab.es all of the experiments were carried out in duplicate. The Author replicates were synthesized 4 months apart. In these samples, the behaviors found for both the controls and different effects Miquel Torras − Institut de Ciència de Materials de Barcelona were reproduced, demonstrating the robustness of the MW- (ICMAB), CSIC 08193 Bellaterra, Spain; orcid.org/0000- assisted synthesis. However, the resulting Ag NPs changed the 0001-7132-6217 morphology and a slight decrease in the average size was Complete contact information is available at: observed (Figure S5). These differences were attributed to https://pubs.acs.org/10.1021/acsomega.9b03748 PVP aging since, when new PVP was bought and used, the initial shapes were recovered. PVP polymer powder, according Notes to the Safety Data Sheet (SDS), is stable and can be stored The authors declare no competing financial interest. under ordinary conditions without undergoing decomposition or degradation. However, the powder is hygroscopic, forms molecular adducts with many other substances, and cross-links ■ ACKNOWLEDGMENTS This research received funding from the Spanish Ministry of at high temperatures. This can result in a solubilizing action in Science, Innovation and Universities through the PCIN-2017- some cases or in precipitation in others (in water or alcohol). 090 and RTI2018-096273-B-I00 projects, the “Severo Ochoa” Thus, all these potential uncontrolled changes of the PVP can Programme for Centers of Excellence in R&D grants (SEV- affect the nanoparticles’ synthetic results. Our final suggestion 2015-0496), and the Generalitat de Catalunya (2017SGR765 is to buy small batches of PVP and use them for no longer than grant). The Spanish Ministry of Education funded the FPU 4 months. Fellow of MT (FPU16/05452). The authors would also like to 4.3. Material Characterization. The morphologies and thank Judith Oró from the TEM service at ICMAB and Marcos the crystalline phase of the Ag NPs were analyzed in a JEOL Rosado from the HRTEM service at ICN2 (Catalan Institute JEM-1210 transmission electron microscope (TEM) operating of Nanoscience and Nanotechnology). ■ at 120 kV and in a FEI Tecnai G2 F20 high-resolution transmission electron microscope (HRTEM) operating at 200 REFERENCES kV. One drop of the NP dispersion was placed in a TEM grid (1) Whitman, L. J.; Henderson, L. A.; Meador, M. A.; Friedersdorf, (Micro to Nano, EMR Carbon support film on copper 200 L. E.; Standridge, S.; Thomas, T.; Howard, J.; Biaggi-Labiosa, A. M.; square mesh). Typically, at least 500 different nanoparticles Madsen, L.; Cannizzaro, C. et al. 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