Growth Inhibition of Hexagonal Silver Nanoplates by Localized

Article pubs.acs.org/JPCC

Growth Inhibition of Hexagonal Silver Nanoplates by Localized Surface Plasmon Resonance Hisanori Tanimoto,* Kazuhiro Hashiguchi, and Satoru Ohmura Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 6, 2015 | doi: 10.1021/acs.jpcc.5b04664

S Supporting Information *

ABSTRACT: Characteristic light absorption occurred in silver citrate solution irradiated by monochromatic visible light above a threshold fluence (Φthes) due to the localized surface plasmon resonance (LSPR) of hexagonal silver nanoplates. The peak energy of the predominant absorption (Ehex) at Φthes was nearly the same as the incident energy (Eirrad) and showed a slow decrease with the fluence (Φ). Transmission electron microscopy indicated that the edge length of the hexagons (Dhex) increased from 25 to 58 nm with a decrease in Ehex, but the thickness was constant at about 8 nm despite the difference in Dhex. The slow decrease in Ehex with Φ suggests gentle planar growth of the hexagonal nanoplates. When Eirrad switched successively from 2.34 to 2.46 eV during light irradiation, the slow decrease in Ehex with Φ showed a temporal stagnation. Thus, the LSPR acted as an inhibitory agent for the planar growth of hexagonal nanoplates. Some precursors formed below Φthes begin to transform into critical seeds, and the growth of critical seeds is inhibited when the size is sufficient for LSPR excitation by Eirrad. Hence, hexagonal nanoplates are size-selectively formed by monochromatic light irradiation.

1. INTRODUCTION Free electrons in metallic nanoparticles can be coupled to the light of a specific energy through the excitation of their collective motion (localized surface plasmon resonance (LSPR)).1−3 A characteristic coloring due to strong light absorption by LSPR is suitable for vivid and long-life dyes. Metallic nanoparticles are used not only in colored glasses or car body paint4 but also in biosensing systems.5,6 Since the electromagnetic field around the nanoparticles is greatly enhanced by the LSPR excitation, metallic nanoparticles are also attracting interest in the field of “plasmonics”,7 e.g., surfaceenhanced Raman scattering8,9 and photoinduced charge injection into semiconductors or chemical reaction enhancement by hot electron−hole formation.10−12 The light energy for LSPR excitation (ELSPR) of spherical metal nanoparticles is mainly determined by the free-electron density,13−15 while the ELSPR of nonspherical nanoparticles also depends on the size and shape. Tuning of ELSPR in the visible light range can be realized by controlling the size and shape of nonspherical metal nanoparticles. Furthermore, a greater enhancement of electromagnetic fields at the corners and edges is expected in the LSPR excitation of polyhedral nanoparticles, such as nanorods or nanoplates, than nanospheres.16,17 The control of the size and shape of nonspherical nanoparticles is a key problem for their application in plasmonics, and an understanding of the formation process is very important. The preparation of metallic nanorods or © 2015 American Chemical Society

nanoplates has been reported by many researchers; however, the formation process is not well understood yet. To obtain nonspherical silver nanoparticles, a solution containing spherical silver nanoparticles as seeds was irradiated by visible light, and silver triangular nanoplates were prepared (referred to herein as the seed-mediated phototransformation method).18 The growth of spherical nanoparticles to platelets by the consumption of other nanoparticles18−20 or an aggregation of spherical nanoparticles into a triangular shape21,22 was suggested as the formation process in the seed-mediated phototransformation method. Recently, we found that hexagonal silver nanoplates could be formed in silver citrate solution by monochromatic visible light irradiation without prior seed preparation.23 Yang et al. also reported that silver nanodecahedrons were formed by light irradiation of a solution without preparing seed nanoparticles.24 In our previous study,23 hexagonal silver nanoplates began to form via light irradiation above a threshold fluence. It was suggested that embryos were formed below the threshold and converted into hexagonal nanoplates above the threshold. Furthermore, hexagonal nanoplates with smaller edge lengths were formed by light irradiation with higher energy, and the size-selective formation by monochromatic visible light irradiation was attributed to a resonance process relevant to Received: May 15, 2015 Revised: July 21, 2015 Published: July 27, 2015 19318

DOI: 10.1021/acs.jpcc.5b04664 J. Phys. Chem. C 2015, 119, 19318−19325

Article

The Journal of Physical Chemistry C


LSPR. In the present study, we demonstrate that growth inhibition is triggered when the size of the hexagons becomes large enough for the excitation of LSPR by the irradiated light, and hence, hexagonal nanoplates are size-selectively formed by monochromatic visible light irradiation.

2. EXPERIMENTAL METHODS Silver citrate prepared from silver nitrate and trisodium citrate dehydrate was dissolved in ultrapure water (resistivity >18 MΩ cm) containing ammonia solution. The concentration of silver citrate was 6.6 mM, and that of ammonia was 0.13 M. The solution was prepared in a dark room. Silver nitrate (>99.8%), trisodium citrate dihydrate (>99%), ammonia solution (25% w/ w), and ethanol (>99.5%, used for purification and dewatering of the silver citrate) were purchased from Wako Pure Chemical Industries, Ltd., Japan, and used as received. The silver citrate solution was irradiated with high-power monochromatic visible light emitting diodes (LUXEON Rebel color emitters, Philips Lumileds Lighting Co., USA). The nominal peak energy given in the data sheet was used for the energy of irradiated light (Eirrad). The silver citrate solution (22 mL) in a Petri dish was set at the center of a cubical box and irradiated by light from a square window at the center of the upper face of the box. The light flux was measured with a laser power meter. For details on the solution preparation and light irradiation, refer to our previous paper.23 In the present study, the box and the light source were set in an incubator in order to control the temperature during light irradiation. The ultraviolet−visible (UV−vis) absorption spectrum was measured using a V-650 iRM spectrophotometer (Jasco, Japan). The morphology of the silver nanoparticles formed by light irradiation was investigated with a JEM-2010 transmission electron microscope (JEOL, Japan; operating voltage 200 keV).

Figure 1. TEM images of nanoparticles formed in silver citrate solution after monochromatic light irradiation with (a) photon energy (Eirrad) of 1.98 eV (red) and fluence (Φ) of 290 J/cm2, (b) photon energy of 2.10 eV and fluence of 184 J/cm2 (amber), (c) photon energy of 2.34 eV and fluence of 59 J/cm2 (green), and (d) photon energy of 2.46 eV and fluence of 38 J/cm2 (cyan). (e) UV−vis absorption spectra of the corresponding solutions.

3. RESULTS 3.1. Size of the Hexagonal Silver Nanoplates. In our previous study,23 the peak energy of predominant UV−vis absorption due to LSPR of the hexagonal silver nanoplates (Ehex) just above the threshold fluence was close to Eirrad. Transmission electron microscopy (TEM) observations suggested that the edge length of the hexagonal silver nanoplates formed at Eirrad = 2.10 eV (amber light) was larger than at 2.46 eV (cyan). The exact values were slightly different, however; it was reported that the predominant peak of the LSPR for triangular (or hexagonal) silver nanoplates showed a higher energy shift in parallel with Eirrad, and the edge length decreased with Eirrad in the seed-mediated phototransformation method.18,19,25,26 In contrast to the edge length, thicknesses of 8−10 nm were reported for the triangular (or hexagonal) silver nanoplates when citrate was used as the stabilizing agent.18,19,25,27,28 These observations indicate that the change in the edge length of the hexagonal silver nanoplates in the solution can be monitored by Ehex. To specify a relationship between the size of the hexagonal nanoplates and Ehex more quantitatively, we conducted TEM observations for nanoparticles formed by light irradiation with different Eirrad values. Figure 1a−d shows typical TEM images of hexagonal nanoparticles in silver citrate solutions after light irradiation with Eirrad = 1.98 (red), 2.10 (amber), 2.34 (green), and 2.46 eV (cyan), and Figure 1e shows the UV−vis absorption spectra of the corresponding solution. An example large-area-TEM image

is also presented in Figure S1. As shown in Figure 1a−d, the edge length of the hexagonal nanoplates increased with decreasing Eirrad. The temperature also affected the formation of the hexagonal silver nanoplates (see section 3.2). All the results in this section were obtained for light irradiation at 309 K, the temperature at which the hexagonal silver nanoplates were formed most efficiently. The mean edge length (Dhex) of the hexagonal nanoplates was estimated from the TEM images, and its change with Ehex is plotted in Figure 2a. In the estimation, hexagons for which the length ratio between neighboring two edges was less than 2 were counted, and an apparent edge length was calculated from the basal plane area of the hexagon as if it were a regular hexagon. In Figure 2a, Dhex decreases monotonously with increasing Ehex in the range investigated here. Since Ehex showed a slow decrease with fluence (Φ), Ehex in Figure 2a became smaller than Eirrad by about 0.2 eV. The mean thickness of the hexagonal silver nanoplates (thex) was evaluated for several hexagons that appeared to stand up in the TEM image; thex was estimated from the apparent thicknesses after taking the geometrical factors expected from the projected height and width into account. The change in thex with Ehex is shown in Figure 2b. The ambiguity in the estimation for thex might be considerable, but thex appears to be almost constant at about 8 nm, even though Dhex varies by almost a factor of 2 from 25 to 58 nm with a change in Ehex from 2.25 to 1.7 eV. As mentioned above, similar values of 8−10 nm were reported for the mean 19319


Article



Figure 2. (a) Correlation between the mean edge length (Dhex) of hexagonal silver nanoplates and photon energy where the UV−vis absorption spectrum shows a predominant peak (Ehex). The vertical bars show the standard variation of Dhex. The numbers of hexagons used for the estimation and Eirrad are given in square brackets and parentheses, respectively. (b) Similar to (a) but showing the relationship between the thickness (thex) and Ehex. The filled symbols indicate the mean value for several observed data that are shown by the open symbols.

Figure 3. Temperature dependence in the development of the UV−vis spectrum for silver citrate solution irradiated with Eirrad = 2.46 eV at Φ = 10.8, 21.6, and 32.4 J/cm2. Light irradiation was conducted in an incubator kept at (a) 290, (b) 299, (c) 309, and (d) 318 K. The dashed lines in (c) show the fitted result for the spectrum with Φ = 32.4 J/cm2 by four Gaussians.

thickness of triangular silver nanoplates prepared with citrate as a surface-protecting group.18,19,25,27,28 The platelet facets with a thickness of 8−10 nm may originate from the strong surface protecting effect of the citrate group for silver (111) planes.20,29 3.2. Effect of Temperature on the Formation of Hexagonal Silver Nanoplates. Citrate acts not only as a stabilizer but also as a reducing agent.30 The reducing power at room temperature was not sufficient, and the reduction of silver ions by citrate was reported at elevated temperatures above 373 K.31,32 In our previous study,23 light irradiation was performed at room temperature without controlling the temperature. In order to determine the effect of temperature on the formation process, light irradiation was conducted in an incubator. Figure 3 shows changes in the UV−vis absorption spectrum of silver citrate solution after light irradiation of Eirrad = 2.46 eV (cyan) with Φ = 10.8, 20.6, and 32.4 J/cm2 in the incubator at 290, 299, 309, and 318 K. At each temperature, the absorption peak due to the LSPR of the hexagonal silver nanoplates generally grew with Φ, but the detailed behavior differed by temperature. When the spectra at the same Φ were compared, the growth rate of the predominant peak was largest at 309 K, and the peak energy shifted to higher energies with increasing temperature from 290 to 318 K. However, the absorption peak at around 3.1 eV, which is indicative of the formation of spherical silver nanoparticles with diameters of about 8 nm,18,23,25,33,34 monotonically increased with temperature and was largest at 318 K for the same Φ. To investigate the growth behavior more quantitatively, the absorption spectrum below 3.8 eV was fitted with four Gaussians. An example for the fitting is shown for the spectrum with Φ = 32.4 J/cm2 at 309 K in Figure 3c. The peak energy and peak intensity of the predominant Gaussian peak are denoted as Ehex and Abshex, and they are plotted against Φ in parts a and b, respectively, of Figure 4. In Figure 4c, the peak intensity at around 3.1 eV (Abssph) is also plotted as the relative change in the amount of spherical silver nanoparticles formed. As reported in our previous study of light irradiation at room temperature,23 the LSPR due to the hexagonal silver nanoplates appeared with light irradiation above a threshold fluence (Φthes), and the value

Figure 4. Fluence dependences of (a) peak energy (Ehex) and (b) peak intensity (Abshex) of predominant absorption due to hexagonal silver nanoplates in Figure 3. (c) Similar to (b) but for the absorption peak due to spherical silver nanoparticles at around 3.1 eV (Abssph). The dashed line in (a) shows the peak energy of the irradiated light (Eirrad = 2.46 eV).

of Ehex just above Φthes was almost the same as or slightly lower than Eirrad. Furthermore, Ehex showed a slow, lower-energy shift with Φ. The results in Figure 4a indicate that Ehex at Φ = 10.8 J/cm2, just above Φthes (see below), was close to Eirrad, and it showed an increase with temperature but saturated with no further increase above 309 K. However, a similar, slow, lowerenergy shift of Ehex with Φ was observed at every temperature with the same rate. In Figure 4b, Abshex increases linearly with Φ above Φthes, but the growth rate depends on the temperature; the rate at 309 K was the largest, and that at 19320


Article


The Journal of Physical Chemistry C 318 K was almost the same or slightly smaller. In Figure 4c, Abssph increases linearly with Φ above Φthes, and the growth rate monotonically increases with temperature; the rate at 318 K showed a rapid increase and became about twice that at 309 K. The predominant peak at 318 K was broad and asymmetric compared to that at 309 K. These results show that hexagonal silver nanoplates can be most effectively and selectively formed at around 309 K under these experimental conditions. The estimated values of Φthes for Abshex at 290, 299, 309, and 318 K are about 8, 4, 6, and 7 J/cm2, respectively. The threshold values estimated for Abssph at 290, 299, 309, and 318 K are about 7, 3, 7, and 6 J/cm2, respectively, and are almost the same as those of Abshex when they are compared at the same temperature. Since no systematic change in Φthes due to solution temperature was observed, we tentatively surmise that Φthes is independent of temperature in the temperature range investigated. However, the results in Figure 4a,b show that Ehex just above Φthes and the formation rate of hexagonal nanoplates are functions of the temperature. It is suggested that several reactions with different temperature dependences are involved in the size-selective formation of the hexagonal nanoparticles. 3.3. Growth Inhibition by LSPR. The result in Figure 2a indicates that Dhex was principally determined by Eirrad and showed an increase dependent on Eirrad. In contrast, thex appeared to be constant and independent of Dhex or Eirrad. As mentioned in reference to Figure 4a, Dhex showed a slow increase with Φ. It has been pointed out that the strong capping effect of citrate on the silver (111) plane plays an important role in the formation of triangular nanoplates.20,27,29,35,36 The following process can be expected for the size-selective formation of the hexagonal nanoplates by monochromatic light irradiation. Critical seeds start to appear at Φthes, and the seeds develop into hexagonal nanoplates by preferential planar growth along the silver (111) plane with a constant thickness of thex = 8 nm. When Dhex increases to the size at which the LSPR can be excited by Eirrad, the planar growth becomes inhibited and the increase in Dhex stagnates. If this is the case, the growth of hexagonal nanoplates formed by light irradiation at Eirrad1 can be stopped by switching the light energy to Eirrad2, which is slightly higher than Eirrad1 but still effective for the LSPR excitation of the nanoplates that are already formed. In contrast, a rapid increase in Dhex (or a rapid low-energy shift in Ehex) is induced by switching the light energy to Eirrad2, which is slightly lower than Eirrad1, because the growth inhibition by Eirrad1 becomes temporarily inactive. We carried out successive light irradiation by switching Eirrad from 2.34 eV (green) to 2.46 eV (cyan) and vice versa. The light irradiation in this section was performed at 309 K, the temperature at which the hexagonal silver nanoplates were formed most efficiently. As shown in Figure 5a, the high-energy side of the spectrum for Eirrad = 2.34 eV overlaps the low-energy side of the spectrum for Eirrad = 2.46 eV. The changes in the UV−vis absorption spectrum by switching Eirrad are shown in Figure 5b (from 2.34 to 2.46 eV) and Figure 5c (from 2.46 to 2.34 eV). The spectra in Figure 5b,c were fitted by Gaussians similar to the case of Figure 3. The changes in Ehex and Abshex with Φ are plotted in Figure 6a,b with switching from 2.34 to 2.46 eV and in Figure 6c,d with switching from 2.46 to 2.34 eV. As shown in Figure 6a, Ehex for successive irradiation of Eirrad2 = 2.46 eV with Φ = 4 J/cm2 after irradiation of Eirrad1 = 2.34 eV with 64 J/cm2 was slightly higher than the value expected from linear extrapolation of the data for the prior irradiation of Eirrad1 = 2.34 eV. In Figure 5b, the peak top at the successive

Figure 5. (a) Nominal spectra of monochromatic light emitting diode with Eirrad = 2.34 (green) and 2.46 eV (cyan). (b) Changes in UV−vis absorption spectrum by switching Eirrad from 2.34 to 2.46 eV after irradiation at Eirrad = 2.34 eV with Φ = 64 J/cm2 and (c) from 2.46 to 2.34 eV after irradiation at Eirrad = 2.46 eV with Φ = 32 J/cm2.

Figure 6. Effect of switching Eirrad on fluence evolution of (a) peak energy (Ehex) and (b) peak intensity (Abshex) of UV−vis absorption due to hexagonal silver nanoplates. Eirrad was switched to 2.46 eV (open circles) after irradiation at Eirrad = 2.34 eV up to Φ = 64 J/cm2 (filled circles). (c) and (d) are similar to (a) and (b), but Eirrad was switched to 2.46 eV (open circles) after irradiation at Eirrad = 2.34 eV up to Φ = 32 J/cm2 (filled circles).

irradiation of Eirrad2 = 2.46 eV with Φ = 4 J/cm2 appears to shift to a slightly higher energy than that at the prior irradiation of Eirrad1 = 2.34 eV with 64 J/cm2. The stagnation of the slow, lower-energy shift of Ehex with Φ observed in Figures 5b and 6a reflects the temporal inhibition of the gradual planar growth of the hexagonal nanoplates. To examine the change in the morphology of the nanoparticles, TEM observations were conducted before and after switching Eirrad from 2.34 to 2.46 eV. The changes in the UV−vis 19321


Article


irradiation of Eirrad1 = 2.46 eV with 32 J/cm2 showed a lowerenergy shift compared to that expected for the case where Eirrad = Eirrad1. This stepwise decrease in Ehex indicates that the planar growth of the hexagonal nanoplates was temporarily accelerated by switching Eirrad to a slightly lower value. A stepwise development of Abshex was also observed just after switching Eirrad in Figure 5b and Figure 6b. We reported that the formation rate of the hexagonal silver nanoplates (Δ(Abshex)/ ΔΦ) decreased with decreasing Eirrad and became zero at around Eirrad = 1.9 eV.23 The stepwise growth of Abshex after switching Eirrad from 2.34 to 2.46 eV in Figure 5b or Figure 6b reflects the acceleration of the formation rate of the hexagonal nanoplates. However, the growth stagnation of Abshex in Figure 5c or Figure 6d after switching Eirrad from 2.46 to 2.34 eV indicates the reduction of the formation rate.


absorption spectrum used for the TEM observation are shown in Figure 7b. Figure 7c presents the TEM image for the

4. DISCUSSION In Figures 5b and 6a, the stagnation of the slow, lower-energy shift of Ehex with Φ was observed just after switching Eirrad from 2.34 to 2.46 eV, which indicates that the gradual growth of the hexagonal nanoplates becomes inhibited by light irradiation with Eirrad slightly higher than the energy favorable for LSPR excitation. As already mentioned, Dhex was principally determined by Eirrad and showed an increase in parallel with Eirrad but the thickness was almost constant at about 8 nm (Figure 2). In addition, the result in Figure 7e indicates that the fraction of the hexagons for which the edge lengths are larger than Dhex decreased and those similar to Dhex increased upon switching Eirrad from 2.34 to 2.46 eV. These observations suggest that planar growth of the critical seeds along the (111) plane began just above Φthers and growth was inhibited when the size was sufficient for the excitation of the LSPR by Eirrad. The decrease in Ehex by switching Eirrad from 2.46 to 2.34 eV in Figure 5c shows that the preferential growth of the hexagonal nanoplates along the (111) plane was accelerated until the energy of the LSPR of the hexagonal nanoplates matched the lower Eirrad. The preferential growth along the (111) plane was attributed to the strong capping effect of citrate on the (111) plane.20,27,29,35,36 Strong charge localization in the nanoparticle and hence enhancement of the electromagnetic fields around the nanoparticle accompany excitation of the LSPR. The strong charge or electromagnetic field localizations can activate various chemical reactions such as photoreduction of metallic ions or dissolution of metal atoms from the nanoparticles. The photoinduced growth of the silver nanoparticles was explained by the negative potential shift of the nanoparticle induced by plasmon excitation.32 Lee et al. reported that triangular or hexagonal nanoplates turned into nanodisks with further light irradiation (photoablation).22,37 The photoablation of silver atoms started at the prism corners because citrate, which was the stabilizer, was depleted by prolonged light irradiation. Size reduction by light irradiation (photoetching) was reported not for metallic nanoparticles but for semiconductor quantum nanodots.38−40 The photoetching was induced by electron− hole pairs, which were created by light irradiation at an energy beyond the band gap. Since the band gap decreased inversely with the size of the dots owing to quantum effects, the photoetching was self-stopped when the size decreased because the band gap became larger than the monochromatic light energy irradiated.

Figure 7. (a) Nominal spectra of the monochromatic light-emitting diode with Eirrad = 2.34 eV (green) and 2.46 eV (cyan). (b) Change in the UV−vis spectrum after switching Eirrad from 2.34 to 2.46 eV after irradiation at Eirrad = 2.34 eV with Φ = 42 J/cm2. (c) TEM image of nanoparticles formed in silver citrate solution irradiated at Eirrad = 2.34 eV with Φ = 42 J/cm2 and (d) successively irradiated at Eirrad = 2.46 eV with Φ = 5 J/cm2. (e) Distributions of the edge length of the hexagonal nanoparticles estimated from the TEM images for irradiation at Eirrad = 2.34 eV with Φ = 42 J/cm2 (left-side bars (solid green)) and for successive irradiation at Eirrad = 2.46 eV with Φ = 5 J/cm2 (right-side bars (cyan-lined boxes)).

solution irradiated by Eirrad1 = 2.34 eV with Φ = 42 J/cm2, and Figure 7d presents that for the successive irradiation of Eirrad2 = 2.46 eV with Φ = 5 J/cm2. The TEM images in Figure 7c,d indicate that the predominant particles were hexagonal nanoplates in both cases and that the apparent change in morphology was not induced by switching Eirrad from 2.34 to 2.46 eV. The value of Dhex after irradiation of Eirrad1 = 2.34 eV with Φ = 42 J/cm2 was 35 ± 6 nm, and that after successive irradiation of Eirrad2 = 2.46 eV with Φ = 5 J/cm2 was 34 ± 6 nm; no significant difference was observed for Dhex. However, a comparison of the distribution of the edge length (Figure 7e) indicates that, after successive irradiation of Eirrad2 = 2.46 eV, the fraction of the hexagons with edge lengths larger than Dhex decreased, whereas those similar to Dhex increased. These observations indicate that the planar growth of the hexagonal nanoplates was temporarily stagnated by switching Eirrad to a slightly higher value. In contrast, in Figure 5c or Figure 6c, Ehex at successive irradiation with Eirrad2 = 2.34 eV with Φ = 8 J/cm2 after 19322


Article



The hexagonal platelets should reflect the shape of the seeds; however, details on the seeds and their formation process are still unknown at the moment. The formation of triangular or hexagonal platelet morphology from twinned crystals was reported for silver halide46,47 and germanium.48 Given the 6fold symmetry of the (111) plane of the fcc lattice, a hexagonal or triangular platelet morphology is preferable for a tabular twinned crystal with the (111) plane normal to the largest surface. If the tabular crystal contains one (111) twin plane, reentrant grooves are formed at the alternating three edges of the (111) twin plane and ridges at the other three edges. Since the growth rate at reentrant edges is faster than that at ridged edges, a seed containing one twin plane (or an odd number of twin planes) grows into a triangular plate. However, a seed containing two (or even-numbered) twin planes grows into a hexagonal plate because the six edges, with pairs of reentrant grooves and ridges, have the same growth rate. The formation of triangular and hexagonal noble metal nanoplates has been explained by the reentrant edge effect.20,49,50 Our previous study23 showed that the hexagonal nanoplates were formed at Eirrad between 1.9 and 2.77 eV. In the seedmediated phototransformation method, it was reported that triangular or hexagonal platelets were formed at around Eirrad = 2.48 eV but not at Eirrad = 2.61−2.92 eV.51 As mentioned in section 3.3, the lower bound indicates the threshold energy for photoassisted reduction of silver ions. For the higher bound, the data in Figure 2a are fitted by a linear function, and the extrapolation to Ehex = 2.77 eV leads to Dhex = 4 nm, which is close to the radius of the spherical nanoparticles formed by UV irradiation. It might be the case that further growth of the seeds with a size of 8 nm is inhibited, and the spherical shape becomes stabilized by LSPR excitation when Ehex ≥ 2.77 eV.

The following three-stage process is proposed for the sizeselective formation of hexagonal nanoplates by monochromatic visible light irradiation presented here. Stage I. Precursor Formation (Φ ≤ Φthers). During light irradiation below Φthers, precursors (“embryos” in our previous paper23) are formed. Except for a small trace of the peak at around 3.1 eV, which could be attributed to the spherical silver nanoparticles, no specific absorption was observed in the UV− vis spectrum in this stage. At Φ = Φthers, the precursors start to transform to critical seeds. Stage II. Planar Growth of Critical Seeds along the (111) Plane (Φ > Φthers). The seeds grow into hexagonal nanoplates in the (111) planar direction. The silver atoms are supplied through photoassisted reduction of silver ions with the assistance of citrate.32,33 In our previous study,23 a lower bound of Eirrad > 1.9 eV was found for hexagonal silver nanoplate formation, and it was attributed to the photoreduction of silver ions. In the seed-mediated phototransformation method, Eirrad > 1.68 eV was reported for triangular nanoplate formation.41 These observations suggest that reduction of silver with citrate can be photoassisted when Eirrad is above a certain value around 1.68 eV. The planar growth along the (111) direction appears to be much faster than the formation of the critical seeds. The incubation time for Eirrad = 2.46 eV is about 1 h (Φthes = ∼6 J/cm2 at a flux of 1.5 mW/ cm2) in the present study. It was reported that silver nanodisks changed their shape to truncated nanoplates by light irradiation within a few minutes.37 Stage III. Growth Inhibition by LSPR. When the hexagonal nanoplates grow to the size at which the LSPR can be excited by the irradiated monochromatic light, growth inhibition becomes activated. The results of the light energy switching experiments in Figures 5, 6, and 7 indicate that the LSPR plays an important role in growth inhibition. Two candidates are considered for the inhibition process by LSPR: photoinduced quenching of the reduction of silver ions and photoetching of the silver atoms from the nanoplates. H2O dissociation or H2 decomposition by plasmon-induced hot electron ejection has been reported.42,43 It has also been reported that gold44 or silver32,45 nanoparticles exhibit a negative charge shift due to electron donation from citrate decomposition resulting from plasmon-induced hot electron creation. In this case, when the size of the silver nanoparticle exceeds that necessary for LSPR excitation under monochromatic light irradiation, a rapid decrease in the reduction rate of silver ions is expected. However, it was reported that depletion of the stabilizing agent of citrate induced by prolonged light irradiation led to the shape change of triangular or hexagonal nanoplates into nanodisks via photoablation.22,37 The shrinkage of the hexagonal nanoplates along the planar direction might be induced by depletion of the stabilizing agent of citrate through the LSPR excitation, and it acts as a photoetching process. A clear, discontinuous change in Ehex is not found for the fitted results in Figure 6a; however, the peak position in Figure 5b appears to show a slightly higher-energy shift just after switching Eirrad from 2.34 to 2.46 eV. No significant decrease in Dhex was observed; however, the result in Figure 7e indicates that the hexagons with edge lengths larger than Dhex decreased after switching Eirrad from 2.34 to 2.46 eV. These observations might be an indication of the photoetching of the metallic nanoparticles due to the LSPR. The present results demonstrate that the size of the hexagonal silver nanoplates is controlled by LSPR excitation.

5. CONCLUSIONS Our previous study showed that hexagonal silver nanoplates were size-selectively formed in silver citrate solution by monochromatic visible light irradiation. Formation of the hexagonal silver nanoplates started above a threshold fluence, and the size was a function of the light energy irradiated (Eirrad). The solution containing hexagonal silver nanoplates showed a characteristic strong absorption peak at the energy (Ehex) where the localized surface plasmon resonance (LSPR) was excited. The value of Ehex observed just above the threshold fluence was close to Eirrad and showed a slow, lower-energy shift with fluence. In the present study, the role of the LSPR on sizeselective formation was investigated using the change in size due to switching Eirrad during irradiation. The relationship between the mean edge length (Dhex) and Ehex was investigated with transmission electron microscopy in order to monitor the change in Dhex by switching Eirrad indirectly from the variation of Ehex. It was found that Dhex increased from 25 to 58 nm linearly with decreasing Ehex from 2.25 to 1.7 eV, but the thickness was almost constant at about 8 nm. Stagnation in the slow, lowerenergy shift of Ehex with an increase in the fluence was observed by switching Eirrad from 2.34 to 2.46 eV, which indicates that the gradual growth of Dhex became inhibited by light irradiation with a value of Eirrad slightly higher than the energy for LSPR excitation. The TEM observation indicated that apparent changes in the hexagonal morphology and size were not induced by switching Eirrad from 2.34 to 2.46 eV. However, the decrease in the fractional amount of hexagons with edge lengths larger than the mean value and increase of those with edge lengths similar to the mean value were observed in the 19323


Article



(6) Fan, Z.; Huang, X.; Tan, C.; Zhang, H. Thin Metal Nanostructures: Synthesis, Properties and Applications. Chem. Sci. 2015, 6, 95−111. (7) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer: Heidelberg, Germany, 2007. (8) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. SurfaceEnhanced Raman Scattering. J. Phys.: Condens. Matter 1992, 4, 1143− 1212. (9) Moskovits, M.; Kneipp, H. Surface-Enhanced Raman Scattering; Kneipp, K., Ed.; Springer: Heidelberg, Germany, 2010. (10) Thrall, E. S.; Preska Steinberg, A.; Wu, X.; Brus, L. E. The Role of Photon Energy and Semiconductor Substrate in the PlasmonMediated Photooxidation of Citrate by Silver Nanoparticles. J. Phys. Chem. C 2013, 117, 26238−26247. (11) Manjavacas, A.; Liu, J. G.; Kulkarni, V.; Nordlander, P. PlasmonInduced Hot Carriers in Metallic Nanoparticles. ACS Nano 2014, 8, 7630−7638. (12) Clavero, C. Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photonics 2014, 8, 95−103. (13) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668− 677. (14) Maillard, M.; Giorgio, S.; Pileni, M. P. Tuning the Size of Silver Nanodisks with Similar Aspect Ratios: Synthesis and Optical Properties. J. Phys. Chem. B 2003, 107, 2466−2470. (15) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410− 8426. (16) Haes, A. J.; Haynes, C. L.; McFarland, A. D.; Schatz, G. C.; Van Duyne, R. P.; Zou, S. Plasmonic Materials for Surface-Enhanced Sensing and Spectroscopy. MRS Bull. 2005, 30, 368−375. (17) Hao, E.; Schatz, G. C. Electromagnetic Fields around Silver Nanoparticles and Dimers. J. Chem. Phys. 2004, 120, 357−366. (18) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced Conversion of Silver Nanospheres to Nanoprisms. Science 2001, 294, 1901−1903. (19) Callegari, A.; Tonti, D.; Chergui, M. Photochemically Grown Silver Nanoparticles with Wavelength-Controlled Size and Shape. Nano Lett. 2003, 3, 1565−1568. (20) Xue, C.; Métraux, G. S.; Millstone, J. E.; Mirkin, C. A. Mechanistic Study of Photomediated Triangular Silver Nanoprism Growth. J. Am. Chem. Soc. 2008, 130, 8337−8344. (21) Tsuji, T.; Okazaki, Y.; Higuchi, T.; Tsuji, M. Laser-Induced Morphology Changes of Silver Colloids Prepared by Laser Ablation in Water Enhancement of Anisotropic Shape Conversions by Chloride Ions. J. Photochem. Photobiol., A 2006, 183, 297−303. (22) Lee, G. P.; Shi, Y.; Lavoie, E.; Daeneke, T.; Reineck, P.; Cappel, U. B.; Huang, D. M.; Bach, U. Light-Driven Transformation Processes of Anisotropic Silver Nanoparticles. ACS Nano 2013, 7, 5911−5921. (23) Tanimoto, H.; Ohmura, S.; Maeda, Y. Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver Citrate Solution by Monochromatic-Visible-Light Irradiation. J. Phys. Chem. C 2012, 116, 15819−15825. (24) Yang, L.-C.; Lai, Y.-S.; Tsai, C.-M.; Kong, Y.-T.; Lee, C.-I; Huang, C.-L. One-Pot Synthesis of Monodispersed Silver Nanodecahedra with Optimal SERS Activities Using Seedless PhotoAssisted Citrate Reduction Method. J. Phys. Chem. C 2012, 116, 24292−24300. (25) Bastys, V.; Pastoriza-Santos, I.; Rodríguez-González, B.; Vaisnoras, R.; Liz-Marzán, L. M. Formation of Silver Nanoprisms with Surface Plasmons at Communication Wavelengths. Adv. Funct. Mater. 2006, 16, 766−773. (26) Huang, T.; Xu, X.-H. N. Synthesis and Characterization of Tunable Rainbow Colored Colloidal Silver Nanoparticles using SingleNanoparticle Plasmonic Microscopy and Spectroscopy. J. Mater. Chem. 2010, 20, 9867−9876.

histogram analysis. In contrast, a stepwise decrease in Ehex was observed by switching Eirrad from 2.46 to 2.34 eV, which shows that the growth of Dhex was accelerated until the energy of the LSPR of the hexagonal nanoplates matched the slightly lower Eirrad. These observations suggest that critical seeds form at the threshold fluence, and planar growth of the seeds along the (111) plane begins above the threshold fluence. Then, the planar growth is inhibited when the size becomes sufficient for excitation of the LSPR by Eirrad. In other words, the size of the hexagonal nanoplates can be tuned by the monochromatic light irradiation through LSPR. The hexagonal platelet shape appears to result from morphology of the critical seeds. Details about the seeds are unknown at the moment, but a formation model proposed for triangular and hexagonal noble metal nanoplates from twinned seeds is probably applicable to this case. The growth inhibition of the metallic nanoparticles induced by the LSPR found in the present study opens the way for the application of plasmonics to size-tuning of nanostructured metallic materials.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04664. UV−vis spectrum and TEM image of nanoparticles formed in silver citrate solution after successive light irradiation at Eirrad = 2.46 eV with Φ = 5 J/cm2 after irradiation at Eirrad = 2.34 eV with Φ = 42 J/cm2 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

S.O.: Suzuki Motor Corp., 300 Takatsuka-cho, Minami-ku, Hamamatsu, Shizuoka 432-8611, Japan. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The present study was partly supported by JSPS KAKENHI Grant 25390027 from Japan Society for the Promotion of Science (JSPS). The authors thank Prof. Hiroshi Mizubayashi (University of Tsukuba) for valuable discussions, Prof. Michio Ikezawa (University of Tsukuba) for the TEM observations, and Prof. Kikuo Yamabe (University of Tsukuba) for supplying the ultrapure water.

■

REFERENCES

(1) Schaefer, H.-E. Nanoscience; Springer: Berlin, Germany, 2010. (2) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, Germany, 1995. (4) Iwakoshi, A. Application of Metal Nanoparticles to Paint Colorants. Techno-Cosmos 2008, 21, 32−38 (in Japanese). (5) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. 19324


Article



(48) Wagner, R. S. On the Growth of Germanium Dendrites. Acta Metall. 1960, 8, 57−60. (49) Lofton, C.; Sigmund, W. Mechanisms Controlling Crystal Habits of Gold and Silver Colloids. Adv. Funct. Mater. 2005, 15, 1197− 1208. (50) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, M. J. The role of twinning in shape evolution of anisotropic noble metal nanostructures. J. Mater. Chem. 2006, 16, 3906−3919. (51) Murshid, N.; Keogh, D.; Kitaev, V. Optimized Synthetic Protocols for Preparation of Versatile Plasmonic Platform Based on Silver Nanoparticles with Pentagonal Symmetries. Part. Part. Syst. Charact. 2014, 31, 178−189.

(27) Sun, Y.; Xia, Y. Triangular Nanoplates of Silver: Synthesis, Characterization, and Use as Sacrificial Templates for Generating Triangular Nanorings of Gold. Adv. Mater. 2003, 15, 695−699. (28) Kim, B.-H.; Lee, J.-S. One-Pot Photochemical Synthesis of Silver Nanodisks using a Conventional Metal-Halide Lamp. Mater. Chem. Phys. 2015, 149−150, 678−685. (29) Wu, X.; Redmond, P. L.; Liu, H.; Chen, Y.; Steigerwald, M.; Brus, L. J. Photovoltage Mechanism for Room Light Conversion of Citrate Stabilized Silver Nanocrystal Seeds to Large Nanoprisms. J. Am. Chem. Soc. 2008, 130, 9500−9506. (30) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nature, Phys. Sci. 1973, 241, 20−22. (31) Lee, P. C.; Meisel, D. Adsorption and Surface-Enhanced Raman of Dyes on Silver and Gold Sols. J. Phys. Chem. 1982, 86, 3391−3395. (32) Redmond, P. L.; Wu, X.; Brus, L. Photovoltage and Photocatalyzed Growth in Citrate-Stabilized Colloidal Silver Nanocrystals. J. Phys. Chem. C 2007, 111, 8942−8947. (33) Munro, C. H.; Smith, W. E.; Garner; Clarkson, J.; White, P. C. Characterization of the Surface of a Citrate-Reduced Colloid Optimized for Use as a Substrate for Surface-Enhanced Resonance Raman Scattering. Langmuir 1995, 11, 3712−3720. (34) Maillard, M.; Giorgio, S.; Pileni, M.-P. Silver Nanodisks. Adv. Mater. 2002, 14, 1084−1086. (35) Yang, J.; Zhang, Q.; Lee, J. Y.; Too, H.-P. Dissolution− Recrystallization Mechanism for the Conversion of Silver Nanospheres to Triangular Nanoplates. J. Colloid Interface Sci. 2007, 308, 157−161. (36) Meátraux, G. S.; Millstone, J. E.; Mirkin, C. A. Rapid Thermal Synthesis of Silver Nanoprisms with Chemically Tailorable Thickness. Adv. Mater. 2005, 17, 412−415. (37) Lee, G. P.; Bignell, L. J.; Romeo, T. C.; Razal, J. M.; Shepherd, R. L.; Chen, J.; Minett, A. I.; Innis, P. C.; Wallace, G. G. The CitrateMediated Shape Evolution of Transforming Photomorphic Silver Nanoparticles. Chem. Commun. 2010, 46, 7807−7809. (38) Matsumoto, H.; Sakata, T.; Mori, H.; Yoneyama, H. Preparation of Monodisperse CdS Nanocrystals by Size Selective Photocorrosion. J. Phys. Chem. 1996, 100, 13781−13785. (39) Torimoto, T.; Tada, M.; Dai, M.; Kameyama, T.; Suzuki, S.; Kuwabata, S. Tunable Photoelectrochemical Properties of Chalcopyrite AgInS2 Nanoparticles Size-Controlled with a Photoetching Technique. J. Phys. Chem. C 2012, 116, 21895−21902. (40) Xiao, X.; Fischer, A. J.; Wang, G. T.; Lu, P.; Koleske, D. D.; Coltrin, M. E.; Wright, J. B.; Liu, S.; Brener, I.; Subramania, G. S.; Tsao, J. Y. Quantum-Size-Controlled Photoelectrochemical Fabrication of Epitaxial InGaN Quantum Dots. Nano Lett. 2014, 14, 5616− 5620. (41) Jin, R.; Cao, Y. C.; Hao, E.; Métraux, G. S.; Schatz, G. C.; Mirkin, C. A. Controlling Anisotropic Nanoparticle Growth through Plasmon Excitation. Nature 2003, 425, 487−490. (42) Chen, H. M.; Chen, C. K.; Chen, C.-J.; Cheng, L.-C.; Wu, P. C.; Cheng, B. H.; Ho, Y. Z.; Tseng, M. L.; Hsu, Y.-Y.; Chan, T.-S.; et al. Plasmon Inducing Effects for Enhanced Photoelectrochemical Water Splitting: X-ray Absorption Approach to Electronic Structures. ACS Nano 2012, 6, 7362−7372. (43) Mukherjee, S.; Zhou, L.; Goodman, A. M.; Large, N.; AyalaOrozco, C.; Zhang, Y.; Nordlander, P.; Halas, N. J. Hot-ElectronInduced Dissociation of H2 on Gold Nanoparticles Supported on SiO2. J. Am. Chem. Soc. 2014, 136, 64−67. (44) Wu, X.; Thrall, E. S.; Liu, H.; Steigerwald, M.; Brus, L. Plasmon Induced Photovoltage and Charge Separation in Citrate-Stabilized Gold Nanoparticles. J. Phys. Chem. C 2010, 114, 12896−12899. (45) Redmond, P. L.; Brus, L. E. Hot Electron” Photo-Charging and Electrochemical Discharge Kinetics of Silver Nanocrystals. J. Phys. Chem. C 2007, 111, 14849−14854. (46) Berriman, R. W.; Herz, R. H. Twinning and the Tabular Growth of Silver Bromide Crystals. Nature 1957, 180, 293−294. (47) Hamilton, J. F.; Brady, L. E. Twinning and Growth of Silver Bromide Microcrystals. J. Appl. Phys. 1964, 35, 414−421. 19325


Growth Inhibition of Hexagonal Silver Nanoplates by Localized

Recommend Documents