Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver

Article pubs.acs.org/JPCC

Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver Citrate Solution by Monochromatic-Visible-Light Irradiation Hisanori Tanimoto,* Satoru Ohmura, and Yoshitaka Maeda† Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan ABSTRACT: Silver citrate solution was irradiated by monochromatic light from light-emitting diodes (LEDs). Under visible-light irradiation, a characteristic peak (Abshex) was observed in the absorption spectrum above a certain value of fluence (Φthres). The formation of hexagonal silver nanoprisms was confirmed by transmission electron microscopy observations; the size of these hexagons in the in-plane direction decreased with increasing ELED (nominal peak energy). The formation process was investigated from the fluence evolution of Abshex, which was due to the localized surface plasmon resonance of the nanoprisms. With decreasing ELED, Φthres showed an exponential increase and the growth rate of Abshex beyond Φthres (ΔAbshex/ΔΦ*, Φ* = Φ − Φthres) showed a linear decrease. The ΔAbshex/ΔΦ* vs ELED data indicated that ΔAbshex/ΔΦ* became zero at ELED = 1.9 eV. Generally, the peak energy of Abshex expected at Φ* = 0 was close to ELED. These results indicated that embryos were formed by visible-light irradiation below Φthres, and some of them were converted into hexagonal silver nanoprisms. The conversion of embryos into nanoprisms is governed by a resonant process induced by the incident light. Further, the formation of nanoprisms is induced by visible-light irradiation with photon energy greater than 1.9 eV.

1. INTRODUCTION Metallic nanoparticles show characteristic optical properties that differ from those in the bulk state. For example, metallic nanoparticles dispersed in a dielectric material exhibit strong visible-light absorption owing to the excitation of the localized surface plasmon resonance (LSPR).1−5 In the case of spherical metal nanoparticles with sizes smaller than about one-tenth of the incident-light wavelength, collective oscillation of the free electrons (dipole mode oscillation) is triggered by the incident light, and the photon energy absorbed by the LSPR is mainly determined by the free-electron density.4,6,7 In the case of silver, light absorption due to LSPR by the above-mentioned mode (Abssphere, hereafter) occurs when the photon energy is around 3.1 eV (or when the wavelength is around 410 nm). The energy of Abssphere decreases slightly with an increase in the diameter of the spherical nanoparticles. It was reported that the energy (wavelength) of Abssphere shows a slight redshift from 2.91 eV (426 nm) to 2.77 eV (448 nm) with an increase in the mean diameter (d) from 25.2 to 46.7 nm,8 or from 3.15 (393 nm) to 2.69 eV (461 nm) with an increase in d from 2.6 to 10 nm.5 In the case of nonspherical nanoparticles such as nanorods or plate-like nanoprisms, nonuniform distribution of the free electrons is induced by light, as the electromagnetic field is enhanced at the corners and edges of the particles. As a result, shape-dependent LSPR modes are excited and characteristic light absorption is observed at energies lower than that of Abssphere.6,9 Pioneering research on the preparation of plate-like silver nanoparticles was reported by Jin et al.10 In their report, triangular silver nanoprisms were prepared by visible-light irradiation of a solution containing spherical silver nanoparticles as seeds (referred to herein as seed-mediated phototransforma© 2012 American Chemical Society

tion). It was reported that the size of the plate-like silver nanoparticles increased in the in-plane direction with a decrease in the irradiated photon energy. Since the LSPR energy for plate-like silver nanoparticles decreases notably with increased particle size, tuning of light absorption from visible-light absorption to infrared absorption can be realized by controlling the size of the nonspherical nanoparticles.5,11−13 Citrate acts not only as a stabilizer but also as a reducing agent.14 However, its reducing power is insufficient for the reduction of metal ions at room temperature. Reduction of metal ions by citrate is normally conducted with thermal agitation15,16 or in the presence of radicals induced by highenergy deposition, for example, by γ-ray irradiation.17 In some cases, citrate is mainly utilized to stabilize silver nanoparticles, in combination with a strong reducing agent such as sodium borohydride.10,13 In seed-mediated phototransformation, citrate-stabilized spherical silver nanoparticles are normally used as seeds. No nanoprism formation is observed when the citrate concentration is lower than 0.09 mM in this process.18 The capping effect of citrate for the silver (111) plane plays an important role in the shape transformation of the nanoparticles from spherical to triangular or hexagonal.16,19,20 Recently, we found that nonspherical silver nanoparticles were formed in a silver citrate solution by visible-light irradiation, even when no pretreatment for the production of spherical nanoparticles was adopted. For example, Figure 1 shows the change in the color of the silver citrate solution after visible-light irradiation. The almost transparent silver citrate Received: May 9, 2012 Revised: June 28, 2012 Published: July 2, 2012 15819

dx.doi.org/10.1021/jp304504c | J. Phys. Chem. C 2012, 116, 15819−15825

The Journal of Physical Chemistry C

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Table 1. Characteristics of LEDs Used for Irradiation of the Silver Citrate Solutiona UV λLED (nm) ELED (eV) Δλ1/2 (nm) dΦ/dt (mW/cm2) at case 1 dΦ/dt (mW/cm2) at case 2

Figure 1. (a) Silver citrate solution (3.3 mM) freshly prepared in a dark room and after visible-light irradiation by (b) blue (fluence Φ = 1273 J/cm2), (c) green (Φ = 1096 J/cm2), and (d) red (Φ = 1536 J/ cm2) LEDs. Solutions b and c were diluted 10-fold by ultrapure water to view the intrinsic color.

royal blue

blue

cyan

green

amber

red

365 3.40 8 9.2

448 2.77 20

470 2.64 20 3.4

505 2.46 20

530 2.40 30 2.9

590 2.10 30

627 1.98 20 5.4

1

1.5

1.5

1.5

1.5

2

1.5

a λLED: nominal peak wavelength; ELED: photon energy at nominal peak wavelength; Δλ1/2: spectral width at one-half of the peak intensity; dΦ/dt for case 1: light flux measured for the silver citrate solution under irradiation in case 1; dΦ/dt for case 2: similar to dΦ/dt in case 1 but measured under the conditions for case 2.

solution that was freshly prepared in a dark room turned green, blue, and purple in color upon blue-, green-, and red-light irradiation. In order to understand the nanoparticle formation process, we investigated the fluence evolution of the light absorption spectrum of the silver citrate solution by using lightemitting diodes (LEDs) as monochromatic light sources in the present study.

2. EXPERIMENTAL PROCEDURES Silver citrate was prepared from silver nitrate and trisodium citrate dehydrate.21 Silver nitrate (>99.8%), trisodium citrate dihydrate (>99%), ammonia solution (25% w/w), and ethanol (>99.5%) were purchased from Wako Pure Chemical Industries, Ltd., Japan, and used as received. Solutions of silver nitrate (100 mL, 0.3 M) and trisodium citrate dihydrate (100 mL, 0.1 M) were prepared by using ultrapure water (resistivity >18 MΩ cm). The trisodium citrate dehydrate solution was added dropwise to the silver nitrate solution in a beaker under stirring, at the rate of about two or three drops per second, using a Pasteur pipet. After the addition, the mixture was stirred for half an hour and then left to rest for about one hour to allow for the precipitation of silver citrate; then, the supernatant was decanted. To purify the precipitates, ultrapure water (100 mL) was added to the beaker, and the same stirring−precipitation− separation sequence was repeated. Finally, the procedure was repeated with ethanol instead of ultrapure water. The precipitates were dried in air, and the resulting lumps of silver citrate were pounded with use of a mortar and pestle. The silver citrate powder was stored in an opaque container. Since silver citrate is poorly soluble in neutral and acidic water, the silver citrate powder (169 mg) was dissolved in ultrapure water (50 mL) containing ammonia solution (0.5 mL) to obtain silver citrate solution (6.6 mM). The entire solution preparation process was carried out in a dark room. The prepared silver citrate solution was transferred to the equipment used for the LED irradiation experiment (see below) as soon as possible, in order to suppress unnecessary irradiation from the lamps in the room. Irradiation by monochromatic light from high-power LEDs was performed in a dark room. A set of eight LUXEON Rebel color emitters (Philips Lumileds Lighting Company, USA) was used for visible-light irradiation, whereas a UV curing system (Aicure UJ20, Panasonic, Japan) was used for UV-light irradiation. The nominal specifications of the LEDs used are summarized in Table 1, and the energy distributions of the LEDs are depicted in parts b and c of Figure 2. In the following text, the photon

Figure 2. (a) UV−vis absorption spectra of the silver citrate solution after irradiation by UV (fluence Φ = 7419 J/cm2), blue (Φ = 2766 J/ cm2), green (Φ = 2297 J/cm2), and red (Φ = 4296 J/cm2) LEDs for case 1. (b) Nominal energy distribution of UV, blue, green, and red LEDs used for irradiation. (c) TEM image of the silver nanoparticles collected from the silver citrate solution after UV irradiation at Φ = 7419 J/cm2. (d) Similar to panel c but after blue-light irradiation at Φ = 2766 J/cm2.

energy at the nominal peak wavelength of the LED is denoted as ELED. Irradiation of the silver citrate solution was conducted for the following two cases. The silver citrate solution (50 mL) was poured into a cylindrical bottle (inner diameter: 32 mm; solution height: about 60 mm). In order to ensure that the solution was shielded from unnecessary irradiation, the bottle was placed at the center of a cubical box (about 12 cm on each side, the inner surface of which was covered with aluminum foil) in the dark room. A square window with an area of 9 cm2 was cut at the center of one side of the box; the LED light source was attached to the window and light from the LEDs and was incident on the solution from the sidewall of the bottle (case 1). In this setup, the progress of the photochemical reaction was nonuniform because the path length varied along 15820



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the path of the incident light. In order to investigate the inherent evolution of the reaction by fluence, a Petri dish (inner diameter: 56 mm) containing the silver citrate solution (22 mL, solution height: about 9 mm) was set at the center of the cubical box. A square window whose dimensions were similar to those in case 1 was cut at the center of the upper face of the box, and light was made incident from the top surface of the solution (case 2). In both cases, the light flux was measured by using a laser power meter. The UV−visible (UV−vis) absorption spectrum was measured with a V-650 iRM spectrophotometer (Jasco, Japan). For some of the solutions, the sizes and morphologies of the silver nanoparticles after light irradiation were investigated by transmission electron microscopy (TEM; JEM-2010, JEOL, Japan; operating voltage: 200 keV, or Technai 20, FEI, USA; operating voltage: 200 keV).

Figure 3. (a) UV−vis absorption spectra of the silver citrate solution after irradiation by UV (Φ = 77.4 J/cm2), royal blue (Φ = 21.6 J/cm2), blue (Φ = 48.6 J/cm2), cyan (Φ = 48.6 J/cm2), green (Φ = 43.2 J/ cm2), amber (Φ = 173 J/cm2), and red (Φ = 297 J/cm2) LEDs at case 2. The characteristic absorption peak due to hexagonal silver nanoprisms appeared upon irradiation above the threshold fluence (Φthres), and the peak absorbance increased linearly with Φ beyond Φthres, as shown in Figure 6. As seen in panel a, the absorbance spectrum is depicted as the ratio of the observed absorbance to the effective fluence after the appearance of the characteristic absorption peak (Φ* = Φ − Φthres) (see text). (b) Nominal energy distribution of UV, royal blue, blue, cyan, green, amber, and red LEDs used for irradiation.

3. RESULTS AND DISCUSSION 3.1. Silver Nanoparticle Formation in Silver Citrate Solution by Light Irradiation (Case 1). Figure 2a shows the UV−vis absorption spectra of the silver citrate solutions after irradiation by UV, blue, green, and red LEDs from the sidewall of each respective bottle (case 1). After UV irradiation at a fluence (Φ) of 7419 J/cm2, an absorption peak was observed at around 2.9 eV. The TEM image shown in Figure 2c indicates that spherical silver nanoparticles with a mean diameter of 7.7 nm were formed in the silver citrate solution upon UV irradiation. The absorption peak at around 2.9 eV, as seen in Figure 2a, was identified to be Abssphere. After blue-light irradiation at Φ = 2766 J/cm2, an absorption peak was observed at around 2.2 eV, in addition to Abssphere. The TEM image shown in Figure 2d suggests that the absorption peak at around 2.2 eV was due to the truncated-triangular silver nanoprisms. After green-light irradiation at Φ = 2297 J/cm2, a predominant absorption peak was observed at around 2.0 eV, and Abssphere was the minor peak. After red-light irradiation at Φ = 4296 J/ cm2, a predominant absorption peak was observed at around 1.6 eV, and distributed broad absorptions were observed above ∼2 eV. The results in panels a and d of Figure 2 showed that plate-like silver nanoprisms were formed in the silver citrate solution upon visible-light irradiation. As mentioned above, light absorption due to LSPR of the triangular (or truncatedtriangular) silver nanoprisms shifted to the lower energy side when the nanoprism size increased in the in-plane direction.5,11−13 The lower energy shift of the characteristic absorption peaks with a decrease in ELED (2.2 eV for blue, 2.0 eV for green, and 1.6 eV for red) indicated that the size of the plate-like silver nanoprisms increased in the in-plane direction with a decrease in ELED. As evident from the TEM image in Figure 2d, the plate-like silver nanoprisms had a broad size distribution and nonuniform shapes. Although not depicted in the figure, the characteristic absorption peak showed broadening as well as a redshift with an increase in Φ. These observations indicated that in addition to the formation of plate-like silver nanoprisms, some secondary reaction of the silver nanoprisms started at Φ > ∼2000 J/cm2. Furthermore, the inhomogeneity in the path length in case 1 made the reaction complex. Following this, the upper surface of the silver citrate solution in the Petri dish was irradiated at a constant path length (case 2), and the formation of the plate-like silver nanoprisms was investigated at a smaller Φ. 3.2. Irradiated-Light-Energy Dependence of the UV− Vis Absorption Spectrum (Case 2). Figure 3a shows the

UV−vis absorption spectra for the silver citrate solution in the Petri dish after irradiation with UV, royal blue, blue, cyan, green, amber, and red LEDs for case 2. The characteristic absorption peak appeared at around 2.2, 2.1, 1.8, and 1.6 eV after cyan-, green-, amber-, and red-light irradiation, respectively. The evolution of the UV−vis absorption spectrum with an increase in Φ differed with ELED. As depicted in Figures 6 and 7a, the characteristic absorption peaks began to appear when Φ exceeded the threshold (Φthres), and the peak intensity increased linearly with Φ − Φthres (effective fluence Φ* = Φ − Φthres, hereafter). The values obtained by dividing the observed absorbance by Φ* were used to plot the absorption spectrum (Figure 3a), because the effective growth rate of the characteristic absorption peaks and Φthres varied with ELED. After UV irradiation, only Abssphere was observed, similar to case 1 (Figure 2a). Panels a and b of Figure 4 show the TEM images of the silver nanoparticles collected from the solutions after cyan-light irradiation at Φ = 54 J/cm2 and amber-light irradiation at Φ = 151 J/cm2, respectively. The UV−vis absorption spectra for these solutions are shown in Figure 4c. After cyan-light irradiation (Figure 4a), most of the particles were hexagonal, and the distance between two parallel sides of the hexagonal faces (Dhex) was in the range 40 to 60 nm. After amber-light irradiation (Figure 4b), hexagonal silver nanoprisms with Dhex = 80 to 120 nm were mainly observed, in addition to spherical silver nanoparticles. As shown in Figure 3a, a small absorption peak was observed at around 3.7 eV after royal blue-, blue-, cyan-, green-, amber-, and red-light irradiation. On the basis of LSPR simulation, it has been reported that the out-of-plane mode of the LSPR for truncated-triangular (or hexagonal) silver nanoprisms is excited at around 3.7 eV.4 These observations indicated that the characteristic absorption peaks at 2.2, 2.1, 1.8, and 1.6 eV for cyan, green, amber, and red in Figure 3a, respectively, were due to the LSPR of the hexagonal silver nanoprisms. The characteristic absorption peak is denoted as 15821



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Figure 5. Fluence evolution of the UV−vis spectrum of the silver citrate solution after cyan-light irradiation for case 2. The lower figure shows the spectra in the early stage. The spectrum labeled “0” indicates the result for the silver citrate solution before cyan-light irradiation, and the vertical broken line indicates the nominal peak energy of the cyan LED. For the spectrum after irradiation at Φ = 27 J/cm2, an example of the fitting result with two Gaussian functions is indicated by the dotted and broken lines.

Figure 4. TEM images of the silver nanoparticles collected from the silver citrate solution after (a) cyan-light irradiation at Φ = 54 J/cm2 and (b) amber-light irradiation at Φ = 151 J/cm2 for case 2. (c) UV− vis spectra of the silver citrate solution shown in TEM images a and b.

at Φ = 2.7 J/cm2, Abssphere showed a slight increase, but no other absorption peak was observed. After irradiation at Φ = 5.4 J/cm2, a shoulder appeared at around 2.4 eV, whereas Abssphere showed a further increase. Above Φ = 5.4 J/cm2, Abshex at around 2.4 eV became obvious and grew more rapidly than did Abssphere. The peak energy of Abssphere showed a slight redshift with an increase in Φ. To evaluate the changes in Abshex with Φ, the spectral peaks between 1.5 and 3 eV were fitted with two Gaussian functions. An example of the fitting result is shown for the spectrum after irradiation at Φ = 27 J/cm2 in Figure 5. A similar fitting procedure was conducted for the other absorption spectra after blue-, green-, amber-, and red-light irradiation. Figure 6 shows the Φ dependence of the peak absorbance of Abshex estimated from the fitting. Abshex appeared above Φthres,

Abshex hereafter. As shown in panels a and b of Figure 4, the lower energy shift of Abshex with decreasing ELED indicated that Dhex of the hexagonal silver nanoprisms increased with decreasing ELED. As already mentioned, an increase in the size of the triangular (or truncated triangular) silver nanoprisms with decreasing irradiated photon energy occurred during seedmediated phototransformation.5,11−13 The UV−vis spectra shown in Figure 3a and the TEM images shown in Figure 4a,b suggest that hexagonal silver nanoprisms were formed in the silver citrate solution by visiblelight irradiation with photon energy of less than 2.77 eV. A similar result was reported for the formation of silver nanoprisms by seed-mediated phototransformation; triangular silver nanoprisms were effectively formed when solutions containing spherical silver nanoparticles were irradiated by light with photon energy (wavelength) between 2.80 (450) and 1.68 eV (750 nm).11 In seed-mediated phototransformation, the major products were triangular (or truncated-triangular) silver nanoprisms, while hexagonal silver nanoprisms were the minor products. In the case of palladium, the shape of the nanoprisms depended on the reduction rate, i.e., the major products were converted from hexagonal nanoprisms to triangular ones with an increase in the reduction rate.22 In case 1, both truncated-triangular and hexagonal nanoprisms were observed in the TEM images (Figure 2d). On the other hand, the hexagonal nanoprisms were predominant in case 2 (Figure 4a,b), wherein the reaction was expected to be slow because of the lower light intensity. 3.3. Fluence Evolution of Light Absorption Peaks Due to Hexagonal Silver Nanoprisms. To investigate the process of formation of the hexagonal silver nanoprisms, the fluence evolution of the UV−vis absorption spectrum was investigated for case 2. Figure 5 shows the change in the UV− vis absorption spectrum upon cyan-light irradiation with increasing Φ. On the basis of the facts mentioned above, the absorption peaks at around 3.1 and 2.4 eV shown in Figure 5 were identified as Abssphere and Abshex, respectively. After irradiation

Figure 6. Fluence dependence of the peak intensity of the absorption peak due to the hexagonal silver nanoprisms (Abshex), estimated by fitting the absorption spectrum with two Gaussian functions. The definition of the threshold fluence (Φthres) is shown for the case of redlight irradiation.

and Φthres increased with a decrease in ELED. As mentioned above, Dhex increased with decreasing ELED (Figure 4a,b). These observations suggested that Φthres increases owing to the formation of hexagonal silver nanoprisms at larger Dhex. The existence of the threshold was indicative of a process whereby 15822



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embryos of hexagonal silver nanoprisms are formed during visible-light irradiation below Φthres. Panels a and b of Figure 7

Figure 7. Effective fluence (Φ* = Φ − Φthres) evolutions of the (a) peak intensity and (b) peak energy of the absorption peak due to Abshex for case 2. In panel a, the definition of the growth rate (ΔAbshex/ΔΦ*) is shown for the case of red-light irradiation. In panel b, the dashed lines along the ordinate axis indicate the nominal peak energies of the LEDs used.

Figure 8. Irradiated photon energy (ELED) dependence of (a) Φthres and (b) ΔAbshex/ΔΦ* (for Φthres and ΔAbshex/ΔΦ*, see Figures 6 and 7a).

the hexagonal silver nanoprisms were formed by visible-light irradiation. Assuming that the formation of embryos is proportional to the light fluence and that a fraction of the embryos is converted into stable hexagonal silver nanoprisms, the following scheme can describe the process,

depict changes in the peak absorbance and peak energy of Abshex against Φ*, respectively. Above Φthres, Abshex showed a linear increase with Φ*. Furthermore, the apparent growth rate of Abshex (ΔAbshex/ΔΦ*) increased with the increase in ELED. As seen in Figure 7b, the peak energy of Abshex at Φ* = 0 was close to ELED. This suggested that the conversion of an embryo into a hexagonal silver nanoprism is governed by a resonant process induced by the irradiated light. 3.4. Phenomenological Considerations for the Formation of Size-Selective Hexagonal Silver Nanoprisms through Monochromatic-Visible-Light Irradiation. Figure 8 shows the ELED dependences of Φthres and ΔAbshex/ΔΦ*. As seen in Figure 8b, ΔAbshex/ΔΦ* increases linearly with ELED. The relationship between ΔAbshex/ΔΦ* and ELED can be explained by the following linear equation

αΦ

β Nem

silver ions ⎯→ ⎯ embryos ⎯⎯⎯⎯→ hexagonal silver nanoprisms Nem

Nhex

where α and β are proportionality constants, and Nem and Nhex are the number densities of the embryos and hexagonal silver nanoprisms, respectively. In this case, the fluence evolution of Nem can be expressed by the differential equation, as denoted below: dNem = αΦ − βNem (2) dΦ From this, the fluence evolution of Nhex can be expressed as α Nhex = βNem dΦ = [βΦ + (exp(−βΦ) − 1)] β (3)

ΔAbs hex = a(E LED − Ep,0) (1) ΔΦ* where a is a proportionality constant and Ep,0 is the threshold energy. Least-squares fitting of the data shown in Figure 8b suggests that Ep,0 = 1.9 (eV). As seen in Figure 3a, Abshex was not predominant after visible-light irradiation at ELED ≥ 2.77 eV (royal blue light) and not observed after UV irradiation (3.4 eV). These observations indicated that hexagonal silver nanoprisms are formed when the silver citrate solution is irradiated by visible light with photon energy between 1.9 and 2.77 eV. As already mentioned, in the case of seed-mediated phototransformation, triangular silver nanoprisms are effectively formed by light irradiation with photon energy between 1.68 and 2.80 eV.11 As seen in Figure 7a, ΔAbshex/ΔΦ* is constant after the appearance of the absorption peak upon irradiation above Φthres at each ELED. In spite of the redshift of the peak energy of Abshex with increasing Φ*, the peak intensity of Abshex would correspond to the number density of the hexagonal silver nanoprisms. The existence of Φthres suggested that embryos of

∫

The asymptotic form of the function in eq 3 at smaller Φ values is

αβ 2 Φ 2 and that at larger Φ values is Nhex ≈

(4a)

Nhex ≈ αΦ

(4b)

Comparison of eqs 1 and 4b reveals that α corresponds to ΔAbshex/ΔΦ* and that it should be proportional to ELED − Ep,0. The phenomenological consideration detailed above shows that there exists an energy threshold of Ep,0 = 1.9 eV for the formation of embryos. When Ep,0 reflects the potential difference of a redox couple, the standard redox potential of Ag+/Ag is 0.7991 V vs NHE, and the standard redox potential for the counter reaction is expected to be −1.1 V. The lowest standard redox potential of ammonia in solution is −0.092 V. 15823



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be consistent with the process in which, similar to seedmediated phototransformation, the spherical silver nanoparticles act as seeds and contribute to the formation of hexagonal silver nanoprisms.

While the standard redox potential of citrate has been reported to be less than −0.01 V at pH >8,19 the typical value would be around 0 V, although the same has not been evaluated in the present study. On the other hand, the standard redox potential of Ag2+/Ag+ is 1.980 V, and the potential difference of the redox couple with ammonia or citrate is close to Ep,0. On the basis of the present phenomenological consideration, Nhex in the initial stage is stated to become a quadratic function of Φ, as expressed by eq 4a. If Abshex becomes observable above a certain threshold number of hexagonal silver nanoprisms, then Φthres is expected to be inversely proportional to (αβ)1/2. Since α is proportional to ELED − Ep,0, as mentioned above, β should be a strongly increasing function of ELED in order to account for the exponential decrease in Φthres with the increase in ELED (displayed in Figure 8a). As mentioned for panels a and b of Figure 4, Dhex of the hexagonal silver nanoparticles decreases with increasing ELED. This suggests that the driving force for the light-induced resonant process for conversion from an embryo into a hexagonal silver nanoprism becomes weak with decreasing ELED. The morphology of the embryos and the exact embryo-to-hexagonal-nanoprism conversion process are unknown at the moment. For the formation of calcium phosphate crystals in solution,23 it has been reported that amorphous-like clusters are formed and that their agglomerates turn to crystallites. In the present case, the appropriately shaped amorphous-like silver clusters that formed below Φthres might have been converted into crystalline hexagonal silver nanoprisms with the assistance of LSPR. The redshift of Abshex with an increase in Φ* (Figure 7b) indicated that the hexagonal silver nanoprisms grew in the inplane direction. It was reported that the electromagnetic field at the corners of the nanoprisms is enhanced by LSPR.4 Further growth may be induced by the reduction of the silver ions at the corners of the nanoprisms under the influence of an enhanced electromagnetic field.16 In seed-mediated phototransformation, spherical silver nanoparticles were reported to grow into hexagonal silver nanoprisms through visible-light irradiation.18,19,24 As shown in Figure 5, a trace of Abssphere was seen at around 3.1 eV even before LED irradiation. Although preparation of the silver citrate solution and LED irradiation was conducted in a dark room, the silver citrate solution was irradiated to a small extent by the room lights during the setup for LED irradiation. The UV−vis absorption of the silver citrate solution kept in the dark showed no further increase in Abssphere from that of the freshly prepared solution. In the case of UV irradiation, Abssphere was in proportion to Φ, and no threshold was detected. A small amount of spherical silver nanoparticles was probably formed in the silver citrate solution by the unavoidable UV irradiation during the experimental setup. Abssphere for the freshly prepared solution was so weak that the pale-yellow color corresponding to LSPR of the spherical silver nanoparticles was almost invisible to the human eye (see Figure 1a). In the case of redor amber-light irradiation, Abshex appeared above Φthres, despite the preexistence of spherical silver nanoparticles in the freshly prepared solution. Furthermore, as seen in Figure 3a, the increase in Abshex under royal blue-light irradiation was smaller than that under blue-light irradiation at the same fluence, whereas Abssphere showed the opposite behavior. At the moment, we cannot exclude the possibility that the small amount of spherical silver nanoparticles existing before visiblelight irradiation acted as seeds for the hexagonal silver nanoprisms. However, the observed results do not seem to

4. CONCLUSIONS Silver citrate solutions were irradiated by monochromatic visible light from LEDs (nominal peak energy ELED = 3.4 (UV), 2.77 (royal blue), 2.64 (blue), 2.46 (cyan), 2.40 (green), 2.10 (amber), and 1.98 eV (red)). When the solution was irradiated by UV light, an absorption peak appeared at around 3 eV in the UV−vis absorption spectrum. TEM observations and comparison with previously reported results showed that spherical silver nanoparticles formed in the silver citrate solution through UV irradiation. When the solution was irradiated by visible light with ELED less than 2.64 eV, a characteristic absorption peak was predominant above a certain threshold (Φthres). TEM observations showed that the characteristic absorption peak (Abshex) was due to the LSPR of hexagonal silver nanoprisms and that the nanoprism size in the in-plane direction increased with decreasing ELED. The peak energy of Abshex expected at Φthres was close to ELED. After the appearance of Abshex, the peak absorbance increased in proportion to the effective fluence Φ* = Φ − Φthres. The growth rate of Abshex (ΔAbshex/ΔΦ*) decreased with decreasing ELED. ΔAbshex/ΔΦ* = 0 at ELED = 1.9 (eV) was deduced from the least-squares fitting of the data. Furthermore, Φthres showed an exponential decrease with increasing ELED. These observations suggested that embryos are formed in the silver citrate solution by visible-light irradiation below Φthres, and their conversion into hexagonal silver nanoprisms becomes obvious above Φthres. Furthermore, the conversion is governed by a resonant process induced by the irradiated light. The rates of embryo formation and embryoto-hexagonal-nanoprism conversion are decreasing functions of ELED. In addition, a threshold photon energy of 1.9 eV exists for this process. Further investigation on the morphology of the embryos and the conversion of embryos to hexagonal nanoprisms is in progress.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81 29-853-5360 .Fax: +81 29-853-4490. E-mail: [email protected]. Present Address †

Project Planning Department, Surface Finishing Engineering Division, Toyota Motor Corporation, 1 Motomachi, Toyota, Aichi 471-8573, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Hiroshi Mizubayashi (University of Tsukuba) for his valuable discussions, Prof. Tokuji Kizuka and Prof. Kazuhiro Hono (University of Tsukuba) for their assistance in performing the TEM observations, and Prof. Kikuo Yamabe for supplying ultrapure water.

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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.

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Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver

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