Rapid Transformation from Spherical Nanoparticles, Nanorods, Cubes

Apr 16, 2012 - Department of Biological and Environmental Chemistry, School of ... Rapid sphere-to-prism (STP) transformation of silver was studied in...
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Rapid Transformation from Spherical Nanoparticles, Nanorods, Cubes, or Bipyramids to Triangular Prisms of Silver with PVP, Citrate, and H2O2 Masaharu Tsuji,*,†,‡ Satoshi Gomi,§ Yoshinori Maeda,‡ Mika Matsunaga,† Sachie Hikino,† Keiko Uto,† Takeshi Tsuji,† and Hirofumi Kawazumi§ †

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan § Department of Biological and Environmental Chemistry, School of Humanity-oriented Science and Technology, Kinki University, Iizuka 820-8555, Japan ‡

S Supporting Information *

ABSTRACT: Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO3/NaBH4/polyvinylpyrrolidone (PVP)/ trisodium citrate (Na3CA)/H2O2 solutions by monitoring timedependent surface plasmon resonance (SPR) bands in the UV−vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na3CA, and H2O2 were studied without addition of a reagent, with different timing of each reagent’s addition, and with addition of H2O2 to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H2O2. The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H2O2 addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H2O2. Oxidative etching of spherical particles by H2O2 is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cubeto-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH4/PVP/Na3CA/H2O2 solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength.

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INTRODUCTION Silver nanoparticles have attracted considerable attention because of their promising applications in various fields such as catalysis,1 detection,2 sterilization,3 electronics,4 and optics.5 Among the properties of silver nanoparticles, surface plasmon resonance (SPR) properties of silver nanoparticles are of particular importance for applications related to biolabeling,6 surface-enhanced Raman scattering (SERS),7 surface-enhanced fluorescence (SEF),8 sensing,9 and fabrication of nanophotonic devices and circuits.10 The SPR of silver nanostructures depends strongly on the nanoparticle size and shape. It is also sensitive to their composition, dielectric environment, and interparticle spacing. Particularly, the nanoparticle size and shape are important for several aspects that determine the SPR of the metallic nanoparticles. It is therefore important to develop a simple technique for the shape-selective and sizeselective preparation of silver nanostructures. © 2012 American Chemical Society

Among the various shapes of silver nanostructures, triangular silver prisms or disks have recently attracted great attention because their SPR bands can be widely tuned by controlling the particle aspect ratio.11 Mainly, two methods have been used for Ag prism synthesis: chemical reduction of metal salts12−17 and photochemical growth.18−21 In general, the former chemical reduction method is better than the photochemical method for mass production of silver prisms, which is necessary for industrial applications. In 2005, Métraux and Mirkin12 proposed a unique simple chemical reduction method for the preparation of silver nanoprisms. They used a mixture of Special Issue: Colloidal Nanoplasmonics Received: January 7, 2012 Revised: April 12, 2012 Published: April 16, 2012 8845

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Zhang et al.23 reported that it remained an interesting assumption for future exploration that H2O2 can remove the relatively unstable Ag nanoparticles, leaving only the most stable ones. The shape selective oxidative etching of Ag nanostructures protected by a citrate ion is a key process for prism formation. To obtain direct information related to this mechanism, the relative etching rate of prisms to that of spherical particles was examined by monitoring the timedependent SPR band of mixtures of spheres and prisms after the addition of etching agent H2O2. To obtain more information related to the relative etching rates of triangular prisms and rods by H2O2, we used a flag-type of Ag nanostructure, which consists of a large triangular plate and a side pentagonal rod.24 Zhang et al.23 used not only silver salt but also metallic Ag particles such as wires and spheres as sources of prisms. However, in their experiments, metallic Ag particles were once completely dissolved to Ag+ by the addition of a sufficient amount H2O2. Then NaBH4 was added to reduce Ag+ back to Ag0. When all metallic Ag particles are oxidized completely to Ag+, the memory of initial Ag nanostructures is lost. It is quite natural that the same Ag nanoplates are formed from the same Ag+ ions. We demonstrate here that Ag prisms can be prepared from various Ag nanostructures such as spherical nanoparticles, nanorods,24 cubes,25 and bipyramids25 using a simpler technique without complete dissolution of metallic Ag particles to Ag+. In our technique, prisms can be prepared from Ag nanostructures described above by the simultaneous addition of H2O2 and NaBH4, where oxidative etching of metallic Ag nanostructures and rereduction of Ag+ to Ag0 occur at the same time. No reports of the relevant literature have described the measurement of Ag+ ion concentration during the formation of prisms, which is important to obtain information related to the crystal growth and oxidative etching of Ag nanostructures. We have measured the Ag+ ion concentration throughout the nanoprism reaction for the first time. Results show that the assumption of Zhang et al.23 for the partial reduction of Ag+ by the addition of NaBH4 in the initial stage is invalid. Although Zhang et al.23 reported that some gas bubbles, probably O2, were emitted during fast oxidative etching by H2O2, neither definite gas analysis nor their role has been examined. In this study, we found that not only O2 but also a small amount of H2 was emitted during reaction. Their concentrations were measured quantitatively throughout the reaction and their effects on reactions were examined. Based on the many experimental data, we discuss in greater detail the roles of the respective reagents and gas in various reaction stages and the formation mechanisms of triangular prisms from AgNO3, spherical Ag nanoparticles, Ag nanorods, Ag flag, Ag cubes, and Ag bipyramids in the presence of NaBH4/PVP/Na3CA/H2O2.

AgNO3/NaBH4/polyvinylpyrrolidone (PVP)/trisodium citrate (Na3CA)/H2O2 in an aqueous solution as reagents to prepare silver nanoprisms at room temperature. An advantage of this method is that the reaction is highly controllable and efficient under the appropriate conditions, with a high yield of prisms of the desired size (ca. 100%). Thereby, the wavelength of SPR band of prisms can be well controlled in the visible region. They reported that, after mixing AgNO3/NaBH4/PVP/ Na3CA/H2O2 in an aqueous solution, the color of the solution was pale yellow for about 20−30 min. After 30 min, however, the colloids were dark yellow, but the solution remained clear. After only 2−5 s, the color changed again, ranging from pink to turquoise depending on the NaBH4 concentration. They reported that it remains unclear why this latent period between reduction of the colloid and silver-nanoprism formation exists. Recently, Cathcart and Kitaev16a studied the formation of silver nanoprisms of a predominantly hexagonal shape using a ligand combination of a strongly binding thiol, captopril, and citrate together with hydrogen peroxide and a strong base that triggered nanoprism formation. They also studied effects of halides for the precise tuning of plasmon resonance maxima from 400 to >900 nm using a combination of borohydride and hydrogen peroxide.16b In their experiment, PVP was excluded from the preparation to shorten reaction times and to improve the reliability of halide effects on plasmon absorption wavelength without broadening the size distribution or compromising the Ag nanoparticles’ stability. They discussed the role of each reagent and the interplay of reagents during nanoprism synthesis. However, their conditions differed from those used by Métraux and Mirkin,12 who did not use thiol, a strong base (KOH), or halides (Cl− or Br−). Crystal structures and their growth processes of silver nanoparticles are wellknown to depend strongly on the presence or absence of a reagent.22 Very recently, Zhang et al.23 studied the role of each reagent in the formation of nanoprisms and the crystal growth mechanism in the aqueous AgNO3/NaBH4/PVP/Na3CA/ H2O2 solution, which was used in a pioneering work of Métraux and Mirkin.12 They studied and identified the critical role of hydrogen peroxide instead of the generally believed citrate in the chemical reduction route to silver nanoplates. By harnessing the oxidative power of H2O2, various silver sources, including silver salts and metallic silver, can be converted directly to nanoplates with the assistance of an appropriate capping ligand. Although they discussed the roles of H2O2 and citric acid and the formation mechanism of prisms, they made many assumptions about the relative etching rates of various shapes of Ag nanostructures, the concentration of Ag+ ion after NaBH4 addition, and emitted gas during reaction without definite experimental evidence, as described in the next paragraph. Consequently, more experimental studies must be undertaken to confirm the validity of their assumptions. For this study, we prepared silver nanoprisms using an aqueous AgNO3/NaBH4/PVP/Na3CA/H2O2 solution at room temperature. We studied the roles of each reagent, such as PVP, Na3CA, and H2O2, in the formation of clusters, spherical seeds, and triangular prisms, and the rapid shape evolution mechanism of prisms by monitoring time-dependent ultraviolet (UV)− visible (vis) extinction spectra, transmission electron microscopic (TEM) data, and gas analysis at each reaction stage. Effects of the addition of each reagent were also studied without the addition of a reagent and with different timing of the addition of Na3CA and H2O2. Regarding the role of H2O2,

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EXPERIMENTAL SECTION

For use in this study, AgNO3 (purity > 99.8%), PVP (Mw = 40 k in terms of monomeric units) (>99.5%), trisodium citrate (Na3CA > 99.5%), aqueous H2O2 solution (content 30 wt %), and high-purity H2O (HPLC grade) were purchased from Kishida Chemical Co. Ltd. O2 (>99.9%) gas was obtained from Taiyo Nippon Sanso Corp. All reagents were used without further purification. An aqueous mixture of AgNO3 (0.1 mM, 25 mL), PVP (0.7 mM, 1.5 mL), and Na3CA (30 mM, 1.5 mL) was prepared. Then H2O2 solution (30 wt %, 0.06 mL) was added to the solution described above. Subsequently, NaBH4 (100 mM, 0.14 mL) was injected to the reagent solution and the reaction was initiated. Final concentrations of 8846

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Figure 1. (a−c) Time evolution of SPR band of spherical particles and STP transformation from AgNO3/NaBH4/PVP/Na3CA/H2O2 solution. (d) Dependence of the absorbance of S and P bands on the reaction time. AgNO3, PVP, Na3CA, H2O2, and NaBH4 were, respectively, 0.089, 0.037, 1.60, 21, and 0.5 mM. This mixture is designated as the standard solution. Using a similar method to that reported previously,24 Ag nanorods were prepared. First, 30 mM AgNO3 and 265 mM PVP (Mw = 40 000) were dissolved in 19.4 mL of ethylene glycol (EG) solution, and the mixture was heated from room temperature to 140 °C. Then NaCl (10 mM, 0.6 mL) was injected drop-by-drop into the solution at a flow rate of 0.6 mL/min. The final concentrations of AgNO3, NaCl, and PVP were, respectively, 29.1, 0.30, and 257 mM. Then the solution was heated to 160 °C. This temperature was maintained for 10 min. After naturally cooling to room temperature, the Ag nanorods were separated and obtained from EG/H2O solution by centrifuging the colloidal solution at 12 000 rpm for 10 min and 10 000 rpm for 10 min three times. Flag type Ag nanoparticles were synthesized using nanorods prepared using the method described above as seeds.24 Then 1 mM of the Ag seed solution at atomic concentration was added to N,Ndimethylformamide (DMF) solution (15 mL) containing 400 mM of AgNO3 and 100 mM of PVP (Mw = 40 000). Then the solution was heated in an oil bath at 140 °C for 2 h. Product solutions were centrifuged at 10 000 rpm for 20 min. The yield of flag particles after centrifugal separation was >80%. A mixture of cubes and bipyramids was prepared using a similar method to that reported previously.25 In the process, 0.3 mM AgNO3, 0.3 mM NaCl, and 262 mM PVP (Mw = 40 000) were first dissolved in 19 mL of EG solvent. The obtained solution was heated from room temperature to 185 °C in an oil bath for 20 min and kept at 185 °C for 10 min. Then, 1 mL of AgNO3 (0.93 M) was injected into the solution drop-by-drop at a flow rate of 1 mL/min and heated at 197 °C for 1 h. The final concentrations of AgNO3, NaCl, and PVP were, respectively, 47, 0.29, and 249 mM. After naturally cooling to room temperature, Ag particles were separated and obtained from EG/C2H5OH solution by centrifuging the colloidal solution at 6000 rpm for 8 min and 15 000 rpm for 30 min three times.

Extinction spectra of the product solutions were measured in the UV−vis-near-infrared (NIR) region using a spectrometer (UV-3600; Shimadzu Corp.) and in the 300−800 nm region using a spectrometer with CCD (SEC2000-UV/vis; BAS Inc., maximum time resolution 0.1 s). Most experiments were conducted in a beaker, where the solution was stirred continuously using a magnetic stirrer. However, when timedependent SPR bands were measured, spectra in a standard quartz cell were measured without stirring. Although the timing of color changes in the standard solution depends on stirring, color changes and the final color of solution were fundamentally independent of stirring. As described below, some gas bubbles appeared during the reaction. The appearance of gas bubbles in a quartz cell creates some fluctuation of the peak intensity of extinction spectra. Gas bubbles emitted during the reaction were monitored using handy H2 and O2 gas meters (XP3118; New Cosmos Electric Co., Ltd.). For TEM (200 kV, JEM2000XS and 2100F; JEOL) observations, Ag particles were obtained from C2H5OH solution by centrifuging the colloidal solution three times at 15 000 rpm for 30 min. The Ag+ ion concentration was monitored using a silver ion electrode (8011-10C; Horiba Ltd.).

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RESULTS AND DISCUSSION Time Evolution of Triangular Prisms in Aqueous AgNO3/NaBH4/PVP/Na3CA/H2O2 Solution. Under standard conditions, the initial solution was colorless. It then changed to a weak yellow. When the fast reaction started to occur, the solution color changed rapidly from yellow to dark yellow. Then it became dark blue. At the same time, many gas bubbles appeared after the occurrence of fast reaction. The peak positions of SPR bands of Ag nanostructures have been known to depend strongly on the nanostructure shape and size.6 A symmetrical SPR band of spherical nanoparticles with a peak at approximately 400 nm and an in-plane dipole SPR band of prisms with a peak in the vis-NIR region are, respectively, good indicators of general spherical and platelike Ag nanostructures. 8847

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Therefore, direct information related to the rapid sphere-toprism (STP) and prism-to-sphere (PTS) transformation is obtainable by monitoring the UV−vis spectral changes of Ag nanoparticles in their characteristic wavelength region. Figure 1a portrays a series of extinction spectra of products monitored at 1 s and 60-s interval in the 60−540 s region after addition of NaBH4. The standard AgNO3/NaBH4/PVP/ Na3CA/H2O2 solution was prepared in a beaker with continuous stirring for 10 min. Then it was injected into a quartz cell for the measurement of the time evolution of SPR bands without stirring. The reaction time in Figure 1 is measured after injection of the reagent solution to the UV−vis spectrometer. Therefore, the actual reaction time after addition of NaBH4 is the usual period plus 10 min. It is noteworthy that rapid spectral change occurs in the 240−300 s range. Figure 1b and c shows extinction spectra measured at 6 s intervals in the 240−312 s region and 2 s intervals in the 255−271 s region, respectively, where rapid color changes occur because of the fast reaction. Although a broad tail band of reagent solution in the 250−700 nm region arising from small Ag seeds23 is observed below ca. 240 s, the 400 nm band of the spherical Ag seeds (denoted as S band) appears at ca. 257 s. It increases rapidly within the 261−271 s region. Although the S band decreases after ca. 271 s, a long tail band of prisms appears above ca. 500 nm after ca. 258 s. The in-plane dipole SPR band of prism (denoted as P band) becomes strong and shows a peak above ca. 560 nm after ca. 271 s. It shifts to red with increasing reaction time. Aside from these bands, a weak sharp band appeared at ca. 342 nm after ca. 294 s. Based on reported spectroscopic data for Ag triangular prisms,26 it was ascribed to an out-of-plane quadrupole SPR transition of triangular prisms. Figure 1d shows the dependence of peak intensities of the S band at 400 nm and P bands having maximum intensities at the 550−800 nm range measured at the 1 s interval in the 0−600 s region. It is noteworthy that after the intensity of S band increases rapidly in the 250−270 s range, it decreases rapidly in the 270−290 s range, and becomes rather flat thereafter because of the appearance of out-of-plane dipole SPR mode of prisms below 400 nm. The P band at >550 nm appears at ca. 260 s, rapidly increases in the 260−400 s range, and becomes constant after ca. 500 s. Figure S1 (Supporting Information) shows dependence of the concentration of Ag+ ions on the reaction time. The initial concentration of Ag+ ions in a AgNO3/PVP/Na3CA solution was measured to be 89 μM. After addition of NaBH4 and H2O2, it becomes zero within 10 s. It increases rapidly to 10.8 μM when spherical particles are formed, decreases to 1.6 μM at 400 s, and decreases more slowly from 1.6 to 0.96 μM in the 400− 1000 s range. Figure 2a1 and b1 respectively portrays TEM images of products obtained after the color changes from weak yellow to orange at ca. 265 s (Figure S2a, Supporting Information) and after a color change to a dark blue solution at ca. 600 s (Figure S2b, Supporting Information). Results show that spherical Ag seeds with average diameter of 4−6 nm are formed after ca. 265 s, whereas monodispersed triangular prisms with average edge length of 44 ± 8 nm with thickness of ca. 10 nm (inserted in Figure 2b1) are produced after ca. 10 min. The out-of-plane quadrupole SPR band at ca. 342 nm is known to be related to the thickness of prisms, and it shifts to red with increasing thickness.26 In the present case, no significant peak shift is observed after the formation of prisms, indicating that the thickness is nearly constant. When high-resolution TEM images

Figure 2. TEM images of particles obtained from (a-1) and (a-2) yellow color solution, (b-1) dark blue solution, and (b-2) SAED pattern of triangular prism shown in (b-3). These nanoparticles were prepared from the AgNO3/NaBH4/PVP/Na3CA/H2O2 solution with reaction time of 30 min.

of small seeds were measured, the interval of layers was 0.25 nm; a twin line was observed for some particles, as shown by a red arrow in Figure 2a2. Consequently, some Ag seeds of prisms have twin defects before oxidative etching and shape transformation from spherical particles to plate-like particles. The selected area electron diffraction (SAED) pattern of a single Ag nanoprism was measured to obtain structural information of prisms. Figure 2b2 portrays typical SAED patterns measured by focusing the electron beam perpendicularly on the flat surface of a typical triangular prism (red circle in Figure 2b3). The SAED spots exhibit a set of hexagonally symmetric patterns in which six heavy black spots are observed, corresponding to the {220} reflections of the faced-centeredcubic (fcc) Ag orientated in the [111] direction. These results strongly suggest that the flat surfaces of the nanoplates are parallel to the {111} plane. Furthermore, a set of spots at the positions of 1/3{422} has been found. The 1/3{422} reflections are forbidden for a perfect fcc single-crystalline structure.27−30 The presence of formally forbidden 1/3{422} spots in the SAED is consistent with the presence of structural defects parallel to the flat {111} plane, as reported by Aherne et al.14 and Zeng et al.17b Based on the time evolution of these SPR bands and TEM images, triangular prisms are formed via the shape transformation of spherical nanoparticles during rapid reaction after some latent time. Details of the formation mechanism of Ag prisms will be discussed later along with the results of many other experiments. Effects of PVP, Na3CA, and H2O2 for Triangular Prism Preparation. To examine the roles of PVP, Na3CA, and H2O2, Ag nanoparticles were prepared without addition of PVP, Na3CA, or H2O2. Figure 3a−c and d shows TEM images and UV−vis-NIR spectra of products obtained in each case. For comparison, UV−vis-NIR spectra obtained from the standard solution are also shown in Figure 3d. In the standard condition, triangular plates with average edge length of ca. 45 nm are prepared (Figure 2b1). Triangular prisms were also prepared without addition of PVP, although their average edge length 8848

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injection of NaBH4. They increase rapidly with a sharp peak at 35 s (Figure 4a3), which was much earlier than that observed in the standard condition. The P band appears immediately after NaBH4 injection. It increases rapidly during 20−40 s and increases gradually in the 40−200 s range. The lack of latent time for the rapid color change in the absence of PVP implies that PVP suppresses the crystal growth from clusters to seeds by protecting the aggregation of Ag clusters. Without addition of Na3CA, after a latent time of ca. 500 s, the S band increases rapidly and becomes nearly constant thereafter (Figure 4b3). However, no prism formation via STP transformation was observed. Without the addition of H2O2, the S band starts to increase immediately after injection of NaBH4 (Figure 5a2); no prism formation was observed as in the case without Na3CA addition. The S band increases rapidly until ca. 180 s and then increases gradually until ca. 300 s. These results show that the S band increases rapidly in all three cases, as observed in the standard condition, although little latent time was found in the cases of without PVP or H2O2 addition. The important finding inferred from these experimental results conducted in the absence of one reagent is that the reaction stops after the formation of spherical nanoparticles with an average diameter of 14−24 nm and that STP transformation does not occur when either H2O2 or Na3CA was absent from the initial reagent solution. The time evolution of SPR bands described above was measured in a standard quartz cell for UV−vis measurements without stirring. Consequently, we measured the latent time necessary for the rapid increase in the S band in reagent solutions in a beaker under continuous stirring by observing rapid color changes. When all four reagents are involved, the color changes caused by the increase in the S band start at 19 min 40 s. In contrast, the color changes immediately after NaBH4 addition without PVP or H2O2 addition, and after 17 min 10 s without Na3CA addition. These results show that PVP and H2O2 suppress the rapid formation of spherical Ag nanoparticles, whereas Na3CA has little effect on the start of the fast reaction to form spherical particles. In the absence of H2O2 in the initial reagent solution, the reaction stopped after the formation of spherical nanoparticles. When H2O2 was added to the solution of spherical particles prepared without addition of H2O2 30 min after preparation of Ag particles, STP transformation was observed as shown in Figure 5b1 and 5b2. Immediately after addition of H2O2, the S band weakens, although the P band strengthens and its peak shifts to red with increasing reaction time. These results imply that similar prisms can be prepared by the addition of H2O2 after preparation of spherical Ag nanoparticles from an aqueous AgNO3/NaBH4/PVP/Na3CA solution. Effects of Timing of Addition of H2O2 and Na3CA on Prism Preparation. Results show that Ag prisms can form only when both Na3CA and H2O2 are present in an aqueous reagent solution. In this section, effects of timing of addition of Na3CA and H2O2 were studied by changing the timing of addition of the two reagents. We examined the order of the addition of Na3CA and H2O2 for the prism formation using the experimental procedure shown in Figure 7. Initially, spherical Ag nanoparticles were prepared from an aqueous AgNO3/ NaBH4/PVP solution. Figure 6a1 and 6b1 shows that it took ca. 100 s until the intensity of the S band became constant. When Na3CA was added to this solution, no significant spectral change in the S band was observed after 30 min (Figure 6a2 and case (a) in Figure 7). After further addition of H2O2 to the

Figure 3. TEM images of products obtained (a) without PVP addition, (b) without Na3CA addition, and (c) without addition of H2O2 obtained after a reaction time of 30 min. (d) UV−vis-NIR spectra of Ag nanostructures obtained under standard conditions, and without PVP, Na3CA, or H2O2 after reaction time of 30 min.

and size distribution (20−120 nm) were larger than those in the case of standard condition. The red shift of the in-plane dipole SPR band of prisms without PVP addition relative to that observed in the standard condition is consistent with the formation of larger prisms in the TEM observation (Figure 3a). It is noteworthy that only spherical particles with an average diameter of 14 ± 4 nm were obtained without addition of Na3CA (Figure 3b). On the other hand, in addition to large spherical particles with average diameter of 23 ± 7 nm, small spherical nanoparticles with average diameter of 3 ± 1 nm were prepared without the addition of H2O2 (Figure 3c). These results are consistent with the fact that only the S band was observed without addition of Na3CA or H2O2 (Figure 3d). Based on the findings presented above, both Na3CA and H2O2 are necessary for the formation of triangular Ag prisms, although PVP is unnecessary for their preparation. The size distribution of Ag prisms was narrower in the presence of PVP, indicating that PVP is useful to prepare monodispersed prisms in the present system. Figures 4a1-a2, 4b1-b2, and 5a1, respectively, show the time evolution of SPR bands obtained without addition of PVP, Na3CA, or H2O2. The reaction times shown in Figures 4a1−a2 and 5a1 are the actual reaction times after the addition of NaBH4 to start the reaction in the quartz cell. In contrast, the actual reaction times in Figure 4b1−4b2 are each 10 min longer because the reagent solution was stirred continuously in a beaker for 10 min before its injection to the quartz cell. The dependence of absorbance of S and P bands on the reaction time in respective conditions is shown in Figures 4a3, 4b3, and 5a2. Prism formation via STP transformation is observed without PVP addition, as in the case of standard condition. However, spherical Ag seeds start to grow immediately after 8849

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Figure 4. (a-1), (a-2) Time evolution of Ag nanostructures obtained without PVP addition. (a-3) Dependence of absorbance of the S and P bands on the reaction time. (b-1), (b-2) Time evolution of spherical Ag nanoparticles obtained without Na3CA addition. (b-3) Dependence of absorbance of the S band on the reaction time.

dissolution of spherical Ag particles by oxidative etching (Figure S3a, Supporting Information). In this case, no prisms were prepared. NaBH4 is not a stable reducing reagent, but it decomposes gradually, accompanied by H2 emission. Therefore, the complete dissolution of spherical Ag particles results from a loss of activity of the reducing agent after leaving the reagent solution for 1 day. When we added NaBH4 to the dissolved solution of Ag prisms again, the S band appeared again with subsequent STP transformation (Figure S3b, Supporting Information). In this case, a broader S band starts to appear after a shorter induction time than that observed in a standard condition. The P band was more red-shifted than that in a standard condition, indicating that larger prisms are formed in this procedure. Very recently, Zhang et al.23 prepared Ag prisms from the same spherical Ag nanoparticles/NaBH4/PVP/Na3CA/H2O2 solution. However, in their experiment, spherical Ag nanoparticles were converted initially to Ag+ by the addition of a sufficient amount of H2O2 to the spherical Ag nanoparticles/ PVP/Na3CA solution. Then NaBH4 was added to reduce Ag+ to Ag0. Our present results indicate that Ag prisms can be prepared directly by the addition of H2O2 to the spherical Ag nanoparticles/NaBH4/PVP/Na3CA solution without complete conversion of spherical Ag nanoparticles to Ag+. Oxidative Etching of a Spherical Particle and Prism Mixture. In the preceding section, we explained that oxidative etching occurs by the addition of H2O2 to the spherical Ag nanoparticles/NaBH4/PVP/Na3CA solution. This section presents our examination of whether prisms are also etched

solution described above, STP transformation occurs immediately (Figure 6a3). In this case, the S peak decreases and the P peak increases rapidly in the 0−20 s and either gradually decreases or increases thereafter (Figure 6a4). Next we changed the order of addition of two reagents (case (b) in Figure 7). When H2O2 was added to the solution involving spherical Ag nanoparticles, little spectral change of the S band was observed for 30 min (Figure 6b2). By the further addition of Na3CA to the solution described above, little change in the S band was observed for 30 min (Figure 6b3−6b4). These results indicate that Na3CA should be added before the addition of H2O2 to the spherical Ag nanoparticles/NaBH4/PVP solution for the formation of prisms via STP transformation. In the experiments described above, spherical particles were initially prepared in the AgNO3/NaBH4/PVP solution. When H2O2 was added to this solution after Na3CA addition, STP transformation started to occur. Our results suggest that initial addition of H2O2 to the reagent AgNO3/NaBH4/PVP/Na3CA solution is unnecessary for prism preparation, and that it might be added after the formation of spherical Ag particles in the presence of Na3CA. However, results show that whether prisms can be prepared or not via STP transformation depends also on the timing of H2O2 addition after preparation of the spherical Ag nanoparticles. When H2O2 was added 30 min after preparation of Ag nanoparticles from the AgNO3/NaBH4/ PVP/Na3CA solution, STP transformation occurred (Figure 5b). In contrast, when H2O2 was added 1 day after preparation of Ag nanoparticles from the same solution, the intensity of the S band decreased, and disappeared completely because of 8850

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Figure 5. (a-1) Time evolution of spherical Ag nanoparticles obtained without H2O2 addition and (a-2) the dependence of absorbance of the S band on the reaction time. (b-1) Time evolution of Ag nanostructures obtained after H2O2 addition. (b-2) Dependence of absorbance of the S band on the reaction time. H2O2 was added 30 min after preparation of spherical Ag nanoparticles.

160 s, all SPR peaks disappear because of complete oxidative etching. The absorption ratio of S band/P band decreases from 0.72 to zero with increasing reaction time from zero to 160 s. These results give a definite experimental evidence that the oxidative etching rate of prisms by H2O2 is slower than that of spherical particles. Preparation of Ag Prisms from Ag Nanorods, Flags, Cubes, and Bipyramids in Aqueous NaBH4/PVP/Na3CA/ H2O2 Solution. We attempted to use Ag nanorods, cubes, and bipyramids instead of spherical Ag nanoparticles as new sources of prisms. Figure S5a (Supporting Information) shows the time evolution of Ag nanorods after the addition of H2O2 to the Ag nanorods/PVP/Na3CA solution. In this case, NaBH4 is absent, so that only etching of Ag nanorods by H2O2 is possible. The SPR band of Ag nanorods consists of an out-of-plane dipole SPR band with a peak at ca. 380 nm (R band) and out-of-plane quadrupole SPR band with a weak shoulder peak at ca. 350 nm.24,31−33 The long tail band observed above ca. 450 nm can be ascribed to overlapping of the in-plane quadrupole and dipole resonance modes of the Ag nanowires.33 The SPR band of Ag nanorods decreases concomitantly with increasing reaction time and no peak shift is observed. These results imply that Ag nanorods are broken or shortened by etching but that one-dimensional structures are maintained during etching. Figure 9a and b shows the time-evolution of Ag nanostructures after the addition of H2O2 to the Ag nanorods/NaBH4/PVP/Na3CA solution. Figure 9c shows UV−vis-NIR spectra of nanorods and products obtained after H2O2 addition for 30 min. Before H2O2 addition, typical SPR

by the addition of H2O2. If they are etched, then we examine their relative oxidative etching rate to that of spherical particles by observing time-dependent SPR spectra. Figure S4 (Supporting Information) shows time-dependent SPR band of prisms after addition of H2O2. In this case, H2O2 was added 1 day after the preparation of prisms under the standard condition, where NaBH4 does not work further as a reducing agent. Results show that prisms are also etched by H2O2, so that peak intensity decreases, shifts to blue with a weak peak at ca. 650 nm, and finally disappears after ca. 300 s. If both spherical and prism particles are etched by H2O2, then why can STP transformation occur ? To examine this important question, we measured the time evolution of SPR bands of the mixture of spherical particles and prisms after H2O2 addition (Figure 8a). Figure 8b shows that the peak of the P band for the initial prisms is located at ca. 800 nm. Figure 8c depicts the dependence of absorbance of the S and P bands on the reaction time. The dependence of the absorption ratio of S band/P band on the reaction time is also shown. For this experiment, prisms were prepared from the standard condition, whereas the spherical particles were obtained from the standard solution without addition of H2O2. The solutions were left for 1 day to remove activity of NaBH4. After the addition of H2O2 diluted 10 times in comparison with that used in the standard experiment to slow oxidative etching, both the S and P bands decrease concomitantly with increasing reaction time. Because the etching rate of prisms is slower than that of spherical particles, after disappearance of the S band, a weak S band with a peak at ca. 600 nm remains in the 120−150 s range. After ca. 8851

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Figure 6. Time evolution of SPR band of (a-1) spherical Ag nanoparticles from AgNO3/NaBH4/PVP solution (a-2) after addition of Na3CA to (a-1) solution, (a-3) and after addition of H2O2 to (a-2) solution. (a-4) Dependence of the absorbance of the S and P bands on the reaction time in (a-3). Time evolution of SPR band of (b-1) spherical Ag nanoparticles from AgNO3/NaBH4/PVP solution: (b-2) after addition of H2O2 to (b-1) solution and (b-3) after addition of Na3CA to (b-2) solution. (d-4) Dependence of the absorbance of the S band on the reaction time in (b-3).

Figure 7. Experimental procedure used in cases (a) and (b) of Figure 6 and growth mechanism of Ag nanostructures in each case.

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Figure 8. (a) Time evolution of SPR band of Ag nanostructures from a 1:1 mixture of spherical particles and prisms in the presence of NaBH4/PVP/ Na3CA after addition of H2O2. (b) Dependence of the absorbance of the S band of spherical particles and P band of prisms and the ratio of the S band/P band on the reaction time. (c) UV−vis-NIR spectra of rods, prisms, and 1: 1 mixture of rods and prisms.

Figure 9. (a, b) Time evolution of SPR band of Ag nanostructures from Ag nanorod/NaBH4/PVP/Na3CA solution after addition of H2O2. (c) UV− vis-NIR spectra of Ag nanorods and prisms before and after RTP shape transformation. (d) TEM image of nanorods. (e) TEM image of prism obtained after reaction time of 30 min.

bands of Ag nanorods were observed in the 300−600 nm region. After addition of H2O2 diluted 10 times in comparison with that in the standard solution, the R band becomes weak

and red shifts until ca. 80 s. The S bands were observed in the 80−140 s range. Based on UV−vis data shown in Figure S5a (Supporting Information), the red shifts and the appearance of 8853

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10c shows TEM images of products by the addition of H2O2 and NaBH4 to the Ag nanoflags/PVP/Na3CA solution. Results show that large triangular plates in flags are also partially etched and converted to smaller prisms (red circle) and spherical particles (blue circle) through oxidative etching of flags and rereduction Ag+ ions to Ag0. These results provide direct evidence of the formation of prisms through RTP transformation mechanism. Zhang et al.23 prepared Ag prisms from Ag nanowires after complete dissolution of Ag nanowires to Ag+ by H2O2 and then re-reduction of Ag+ to Ag0 by NaBH4. Our results show that Ag prisms can be prepared from a Ag nanorods/NaBH4/PVP/ Na3CA/H2O2 solution via RTP process without complete dissolution of nanorods to Ag+ by H2O2. We also attempted to prepare prisms from a mixture of Ag cubes and bipyramids as new sources. Figure S5b (Supporting Information) shows the time-evolution of a mixture of Ag cubes and bipyramids after the addition of H2O2 to the Ag nanocubes and bipyramids/PVP/Na3CA solution. The SPR band of cubes and bipyramids decreases concomitantly with increasing reaction time, and no S band appeared. These results imply that Ag cubes and bipyramids are broken by etching, but no quasi-spherical particles were formed during etching. Figure 11a shows the time-evolution of Ag nanostructures after the addition of H2O2 to the Ag cubes and bipyramids/ NaBH4/PVP/Na3CA solution and Figure 11b shows UV−visNIR spectra of cubes and bipyramids and products obtained before and after H2O2 addition for 30 min. Before H2O2 addition, a typical broad SPR band of Ag cubes and bipyramids with a peak at 420 nm was observed in the 250−800 nm region.25,34 After addition of H2O2, the SPR band becomes weak because of oxidative etching of cubes and bipyramids to until ca. 40 s. The S band with a peak at 400 nm appeared in the 40−120 s range. The appearance of the S band does not arise from shape change of cubes and bipyramids to quasispherical particles by etching but from the formation of spherical particles by etching of cubes and bipyramids to Ag+ and rereduction of Ag+ to Ag0 based on results shown in Figure S5b (Supporting Information). After 120 s, the P band appears and gradually increases concomitantly with increasing reaction time, whereas the S band becomes weak with increasing the reaction time and disappeared after 240 s. These spectral changes suggest that the CTP and BTP transformations via spherical particles occur. Figure 11c and d, respectively, portrays TEM images of Ag nanocubes and bipyramids and prisms obtained before and after H2O2 addition for 30 min. Results show that all cubes and bipyramids, with average edge sizes of 120 ± 20 and 190 ± 20 nm, respectively, can be converted to prisms with average edge length of 50 ± 5 nm via CTP and BTP transformation processes. Effects of O2 Gas Dissolved in the Solution for the Preparation of Prisms. Figure 12a shows some gas bubbles that appeared during the rapid growth of triangular prisms. The possibility exists that this gas also participates in the formation of triangular Ag prisms. Possible emitted gases were O2 originating from decomposition of H2O2 and H2 originating from decomposition of NaBH4 and H2O2. Under standard conditions, amounts of gases were too low to be detected quantitatively using our gas detector. Therefore, we measured outgases using five-times-higher concentrations of reagents than the standard solution. When concentrations of H2 and O2 gases were monitored using H2 and O2 gas meters, 0.6−5.8 vol % of

S bands below 140 s do not arise from oxidative etching of rods to quasi-spherical particles; they come from the formation of spherical Ag particles by oxidative etching of rods to Ag+ and re-reduction of Ag+ to Ag0. After 140 s, the P band appears and gradually increases concomitantly with increasing reaction time, whereas the S band becomes weak with increasing the reaction time. These spectral changes suggest that the rod-to-prism (RTP) transformation via spherical particles occurs when spherical nanoparticles are replaced by Ag nanorods. The maximum peak of prisms is located at ca. 900 nm, which is longer than ca. 800 nm in the standard condition. Figure 9d shows TEM images of Ag nanorods with average diameter and length of 33 ± 5 nm and 1210 ± 240 nm, respectively. After H2O2 addition for 30 min, all nanorods are converted to prisms with average edge length of 50 ± 5 nm (Figure 8e). A slightly greater edge length of the prism than that in the standard condition (44 ± 8 nm) is consistent with a slight red shift of the prism peak at ca. 900 nm (Figure 9d) in comparison with ca. 800 nm for prisms obtained in the standard condition. This result shows that prism formation occurs not only from spherical nanoparticles but also from pentagonal Ag nanorods. To obtain more information related to the RTP transformation, the shape change of flag-type of Ag nanostructure was studied. Ag flags consist of a large triangular plate having {111} major planes and a side pentagonal rod having {100} side planes, as shown in TEM images (Figure 10a). Figure 10b

Figure 10. TEM images of (a) Ag flags, (b) those from Ag flags/PVP/ Na3CA solution after partial etching by H2O2, and (c) products obtained from Ag flags/NaBH4/PVP/Na3CA solution after addition of H2O2.

portrays TEM images of products obtained using the addition of H2O2 to the Ag nanoflags/PVP/Na3CA solution. It is noteworthy that initially the rod part is partially broken by the oxidative etching of H2O2 (red arrows), although little change occurs for the major plate part. However, as the experiments of oxidative etching of a mixture of spherical particles and prisms show, the etching rate depends on the shape of Ag nanostructure and the etching rates of triangular plates with {111} facets are slower than those of other structures. Figure 8854

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Figure 11. (a) Time evolution of SPR band of Ag nanostructures from Ag cubes and bipyramids/NaBH4/PVP/Na3CA solution after addition of H2O2. (b) UV−vis-NIR spectra of Ag cubes and bipyramids and prisms before and after CTP and BTP shape transformations. (c) TEM image of cubes and bipyramids. (d) TEM image of prism obtained after reaction time of 30 min.

Figure 12. (a) Gas bubbles emitted during rapid color change from dark yellow to blue in an aqueous AgNO3/NaBH4/PVP/Na3CA/H2O2 solution and (b) dependence of H2 and O2 gas concentrations on the reaction time at five higher concentrations than those used in the standard condition.

transformation, in which Ag+ is rereduced to Ag0 by a reducing agent NaBH4. Although the concentration of H2 in the latter stage (maximum 0.9 vol %) is higher than in the first stage (0.5 vol %), the emission duration (30 s) is shorter than that in the first stage (90 s). The possibility exists that O2 gas arising from decomposition takes part in the formation of Ag prisms. To examine the effects of O2, pure O2 gas was bubbled into the AgNO3/NaBH4/PVP/ Na3CA solution in various reaction stages. When O2 was introduced before the color changes to yellow, just as color change to dark yellow started to occur, or after the color change occurs, no further color change was observed. It was therefore

O2 gas was detected by subtracting underground 21% O2 during the rapid growth of Ag nanostructures in the 850− 1000 s range and a low concentration (0.3−0.9 vol %) of H2 was detected at reaction times of 90−180 and 870−900 s (Figure 12b). The O2 gas emissions were observed only when both Na3CA and H2O2 were included in the reagents. Consequently, it is reasonable to assume that O2 arises from decomposition of H2O2 during the rapid decrease of the S band and increase in the P band with the assistance of Na3CA and Ag nanoparticles, where rapid STP transformation occurs. In contrast, H2 arises from in the first stage, in which Ag seeds are formed. It also comes out in the early stage of the rapid STP 8855

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Scheme 1. Growth Mechanisms of Ag Nanostructures Prepared under Various Combinations of Reagents

Scheme 2. Growth Mechanisms of Ag Nanostructures Prepared with Various Timing of Addition of Na3CA and H2O2

concluded that O2 gas does not take part in the formation of

decomposition. The amount of O2 gas suggests that H2O2

prisms and that it was outgassed as a result of H 2O 2

works well as an etching agent when O2 emission was observed. 8856

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Scheme 3. Growth Mechanisms of Ag Nanostructures Prepared from Mixtures of Spherical Nanoparticles and Prisms and Nanorods, Cubes, and Bipyramids via STP, RTP, CTP, and BTP Transformations

Growth Mechanisms of Triangular Silver Prisms from AgNO3 and Metallic Ag Nanostructures in the Presence of PVP/Na3CA/H2O2. We have conducted various experiments by changing the combination of reagents and the timing of the addition of each reagent. Based on time-evolution of SPR bands and TEM images, we obtained detailed information related to growth mechanisms of Ag nanostructures and the roles of each reagent in various reaction stages. Schemes 1−3 and Figure 7 present summaries of reaction processes observed in our various experiments. In the standard solution, the Ag prism proceeds through Scheme 1a. After reduction of Ag+ ions to Ag0 (stage A → B in Scheme 1a), Ag clusters are formed. Subsequently, they grow to spherical seeds (B → C). Small spherical seeds grow to larger ones (C → D); then seeds of prisms are produced by plane-selective oxidative etching (D → E). Seeds of the prisms grow to small prisms (E → F) and grow further to larger final triangular prisms (F → G). An outstanding feature in this system is the rate-determining step for the formation of prisms: C → D. Once this step starts, process C → G takes place without latent time. Based on the measurements of Ag+ ion concentration (Figure S1, Supporting Information), the reduction of Ag+ to Ag0 or stabilization of Ag+ by PVP and Na3CA occurs quickly, so that little free Ag+ ions remain after addition of NaBH4 to the AgNO3/PVP/ Na3CA/H2O2 solution. Results show that the Ag+ ion concentration increases again during occurrence of fast reaction for the formation of spherical particles and a small amount of Ag+ is present during the growth of prisms in the last stage.

Without PVP addition, crystal growth proceeds through a similar process, except for the formation of large triangular prisms having large size distribution (Scheme 1b). Observations indicate that no latent time exists for process C → D in this case. Therefore, the formation of small spherical particles begins immediately after NaBH4 addition. Based on these data, PVP is unimportant for the formation of the prism in our system. The role of PVP is suppression of crystal growth of Ag seeds to spherical Ag particles (C → D) by strongly stabilizing the Ag seeds by PVP. Another role of PVP is the formation of small monodispersed prisms by protecting surfaces of prisms in stages F and G. Without Na3CA addition in the initial solution, although crystal growth from A to D occurs after some latent time, it stops at step D (Scheme 1c). Based on this finding, citrate is inferred to be unnecessary for the formation of spherical Ag nanoparticles. Although STP transformation to the prism occurred by the addition of Na3CA to the initial solution (Scheme 2a1), it did not take place by the addition of Na3CA after formation of spherical Ag particles in step D (Scheme 2a2), which indicates that oxidative etching of spherical particles by H2O2 does not occur without the premixing of Na3CA before H2O2 addition (see also Figure 7). Results imply that citrate is necessary to initiate oxidative etching of spherical particles by H2O2 in stage D. Consequently, an important role of Na3CA, identified in this study, is as an assistant reagent for the selective oxidative etching of spherical Ag particles by H2O2. Mass spectrometry studies have shown that both citric acid 8857

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(CAH) and citrate ion (CA) can coordinate strongly with Ag+ ions, possibly through the carboxylate group.17b In our conditions, citrate acts as a charge stabilizing agent giving (Ag0)n−(CAH)x(CA)y (x, y ≥ 0) complexes, which assist the oxidative etching of Ag nanoparticles by H2O2. When H2O2 was premixed before Na3CA addition, adsorption of CAH and CA on Ag nanoparticles was blocked by H2O2 (case (b) in Figure 7). Under such a condition, no oxidative etching of spherical particles by H2O2 occurs. Another role of citrate is selective protection of {111} facets, which reduces the crystal growth over {111} facets. Consequently, the top and bottom {111} facets finally remain in the prisms. This conclusion is consistent with that expressed in a previous report on the selective adsorption of citrate to Ag nanostructures.16a Without H2O2 addition in the initial solution, the crystal growth process resembled that without Na3CA addition (Scheme 1d). Crystal growth from A to D occurs without a latent period; it stopped in stage D, as in the case of without Na3CA addition, which means that H2O2 suppresses crystal growth from A to D by oxidative etching of clusters and seeds. It was also confirmed by changing the H2O2 concentration. When the H2O2 concentration was increased by a factor of 2− 5, only small amounts of prisms were formed, and the dominant products were spherical particles. Under a high concentration of H2O2, the STP transformation is suppressed by the etching of prisms. At even higher concentrations of H2O2 (factor >7), no color change occurs because the oxidative etching of clusters occurs more rapidly than the formation of clusters by reduction of Ag+. Although the timing of Na3CA addition is important for prism formation, timing of H2O2 addition to the solution involving Na3CA is unimportant. Prisms can be prepared via rapid STP process during D → E by the addition of H2O2 either in the initial reagent solution (Scheme 2b1) or in stage D (Scheme 2b2), which indicates that oxidative etching of spherical particles by H2O2 occurs even when H2O2 was added after formation of spherical Ag particles. Based on results of our experiment of oxidative etching of a 1:1 mixture of spherical particles and prisms, both particles are etched, although the etching rates of spherical particles and prisms mutually differ (Scheme 3a). Consequently, a major role of H2O2 identified in this study is the shape-selective oxidative etching of Ag nanoparticles. The oxidative etching rate of prisms is slower than that of spherical nanoparticles. Therefore, seeds of prisms can survive after dissolution of spherical particles to Ag+ and grow again as larger prisms by re-reduction of Ag+ on the side facets of prisms. This growth is possible because of the protection of the top and bottom {111} facets by citrate. This process is crucial for the formation of prisms from an aqueous AgNO3/NaBH4/Na3CA/PVP/H2O2 solution. For the occurrence of STP transformation, proper concentration ratios are necessary for each reagent. In addition, proper timing of the addition of each reagent is important. For example, by the addition of H2O2 1 day after preparation of spherical Ag nanoparticles, they are completely dissolved by oxidative etching because the activity of NaBH4 is lost (black arrows in Scheme 2b3). However, readdition of NaBH4 to this solution again produced larger prisms via processes, as shown by the blue arrows in Scheme 2b3. Although five reagents were used (AgNO3, NaBH4, PVP, Na3CA, and H2O2), the major operating timing of each reagent differs, as shown by red arrows in Scheme 1a. AgNO3, a precursor of Ag particles and NaBH4, is a reducing agent of

Ag+. Reduction of Ag+ occurs in all times except for rapid oxidative etching of spherical particles (D → E). Although PVP acts as a stabilizer of Ag nanostructures in every stage, it is especially important as a stabilizer of Ag seeds and triangle prisms. When PVP was absent, it took little time to induce the formation of seeds (start of color change) and triangular prisms having much a wider size distribution were formed. Consequently, PVP plays an important role in stages C → D and E → G. For the formation of spherical particles from seeds (C → D), aggregation and crystal growth of Ag seeds by further reduction of Ag+ must occur. In this stage, PVP acts as a strong stabilizer of Ag seeds so that crystal growth from seeds to spherical Ag particles is greatly suppressed. From stage C to D, crystal growth occurs very rapidly, where reduction of Ag+ occurs over small Ag seeds by NaBH4 and where aggregation of small particles occurs efficiently. Consequently, AgNO3 and NaBH4 operate in this stage as major reagents. When small particles grow larger with stabilization by citrate, plane-selective oxidative etching of spherical particles by H2O2 occurs (D → E). The oxidative etching rate for triangular plates is much lower than that for spherical particles. Therefore, crystal growth E → F → G proceeds because of rereduction of Ag+ by NaBH4. H2O2 is decomposed efficiently in stage D → E. Therefore, the concentration of H2O2 is expected to be reduced greatly after stage E. This is another reason why the prism can grow after stage E. Na3CA plays a major role as a stabilizer of spherical particles that can be etched by H2O2 in stage D → E, and as a protective agent of the {111} facet of prisms. In our conditions, (Ag0)n(CAH)x(CA)y particles will be formed during fast reaction and they will be etched by H2O2 through the following reactions to give Ag+.35 H 2O2 → HO2− + H+

(1)

(Ag 0)n ‐(CAH)x (CA)y + H 2O2 → (Ag 0)n − 2 ‐(CAH)x (CA)y + 2Ag + + 2OH−

(2)

(Ag 0)n ‐(CAH)x (CA)y + HO2− + H 2O → (Ag 0)n − 2 ‐(CAH)x (CA)y + 2Ag + + 3OH−

(3)

Zhang et al.23 found that Ag prisms can be prepared using not only citrate having three carboxylate groups, but also some other reagents having at least single carboxylate group. Thus, combing our results with theirs, covering of Ag nanoparticles with carboxylate groups in citrate is necessary to initiate the shape-selectively oxidative etching of Ag nanoparticles by H2O2. The formation mechanism of prism from Ag nanorods is presented in Scheme 3b. Pentagonal Ag nanorods comprise five {100} side facets and two top five-{111} facets. Although citrate protects {111} facets, the {100} facets are not protected. Therefore, nanorods are etched more easily than prisms and are transformed to prisms via spherical particles. The shapeselective etching of rods in the coexistence of rod and triangular plate was confirmed by the observation of a faster etching rate of rod parts than that of plate in flag-type nanostructures. Results show that RTP transformation proceeds via simultaneous occurrence of dissolution of rods by oxidative etching followed by rereduction of Ag+ to Ag0 to form spherical particles and STP transformation process. Under such a condition, there are no times where nanorods are completely etched and dissolved as Ag+ by H2O2. Scheme 3c shows that 8858

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The new information obtained in this study is expected to spur development of a new simple preparation technique of Ag nanoprisms from various Ag sources. Silver nanoprisms are promising nanoparticles for use as plasmonics materials having an SPR band with a wide wavelength tuning range extending from the visible to the near-infrared region.

cubes and bipyramids are composed also of {100} facets. Therefore, they are transformed to prisms as in the case of nanorods via CTP and BTP transformations, respectively. Based on the time evolution of the SPR bands of Ag nanostructures, the CTP and BTP transformations proceed via spherical particles formed by oxidative etching of these Ag nanocrystals and rereduction of Ag+. Shape-selective oxidative etching of Ag nanostructures by O2/Cl− is known to give Ag nanocubes and Ag nanorods, depending on the experimental conditions.31d,36 In the present case, H2O2 in the presence of citrate also acts as another reagent of shape-selective oxidative etching, which selectively yields nanoprisms having {111} facets as major planes.

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ASSOCIATED CONTENT

S Supporting Information *

Dependence of concentration of Ag+ on the reaction time, solution colors, and time evolution of SPR bands in various conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION Formation of silver triangular prisms in aqueous AgNO3/ NaBH 4 /PVP/Na 3 CA/H 2O 2, spherical Ag nanoparticles/ NaBH4/PVP/Na3CA/H2O2, and a mixture of spherical and plate-like Ag nanoparticles/NaBH4/PVP/Na3CA/H2O2 solutions has been studied by monitoring time-dependent SPR bands in the UV−vis region, by analyzing TEM images, and by analyzing gases emitted during fast reaction. Triangular prisms are formed in all cases. Effects of the addition of each reagent show that citrate and H2O2 are more important than PVP for the preparation of triangular prisms. Moreover, the order of addition of the two reagents described above is important for prism formation. The premixing of citrate before H2O2 addition is necessary to initiate rapid STP transformation. When H2O2 was premixed before citrate addition, adsorption of citric acid and citrate ion on Ag nanoparticles is blocked by H2O2. Under such a condition, no oxidative etching of Ag nanoparticles by H2O2 occurred. Thus, a new important role of citrate found in this study is an assistive agent for shape-selective oxidative etching of Ag nanoparticles covered with carboxyl groups by H2O2. Strong stabilization of Ag seeds by PVP and oxidative etching of Ag seeds by H2O2 dictate a latent time before starting the fast reaction. The formation of prisms proceeds through repeated oxidative etching of spherical and small plates with subsequent re-reduction of Ag+ on side facets of plates because the top and bottom {111} facets are protected by citrate. We found that a key process for the STP transformation is shape-selective oxidative etching of spherical particles in a mixture of spheres and prisms. The etching rate of spherical particles by H2O2 is faster than that of prisms. Therefore, spherical particles are etched selectively and dissolved, leaving only seeds of prisms to grow into triangular prisms. The O2 gas emitted from the decomposition of H2O2 was found to be unimportant for prism formation. Zhang et al.23 recently prepared Ag prisms using Ag metallic precursors such as wires and spherical particles by the following steps: metallic Ag nanoparticles → Ag+ → Ag prisms. In their process, H2O2 was added to completely convert metallic Ag nanoparticles to Ag+. Then NaBH4 was added to rereduce Ag+ to Ag0. We show here for the first time that prisms can also be formed more easily through a simple single step, where H2O2 and NaBH4 were added simultaneously to PVP/Na3CA solution involving nanowires, cubes, and bipyramids. The time evolution of the SPR bands of Ag nanostructures implied that the RTP, CTP, and BTP transformations proceed via spherical particles formed by oxidative etching of these Ag nanocrystals and rereduction of Ag+. The slower etching rate of plate to that of other shapes leads us to prepare prisms in high yield.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT, No. 22310060) and by Management Expenses Grants for National University Corporations from MEXT.

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REFERENCES

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