Length control of Ag nanorods in mesoporous SiO₂–TiO₂ by light irradiation

Abstract

The length of Ag nanorods is uniquely controlled by irradiation with visible light using anatase TiO₂-containing tubular SiO₂ mesoporous templates. Spherical Ag nanoparticles and Ag nanorods of controlled length were fabricated by irradiation with visible light of selected wavelengths.

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Noble metal nanoparticles are attracting a great deal of attention, in particular because of their surface plasmon resonance (SPR)-related optoelectronic properties.^1–5Nanoparticles (NPs) with one-dimensional (1-D) structures, such as rods and wires, possess interesting properties. For example, noble metal nanorods (NRs) exhibit anisotropic SPR, where the wavelength may be controlled by changing the length of the NRs.^6–8 Among a variety of noble metal NPs, Ag NPs show SPR with the strongest field enhancement factor, which is favorable for their application in surface enhanced Raman scattering (SERS), surface enhanced fluorescence (SEF), and molecular labeling (ML).^9–15 However, the preparation of Ag NRs of controlled size, which should be relatively small (5–10 nm in diameter, and 10–100 nm in length) to effectively exhibit SPR, has seldom been reported, in contrast to the large quantity of research on the preparation and application of Au NRs of controlled length.^16–18 This is because of the stronger oxidation and ionization tendencies of Ag compared with Au and some other noble metals, which make it more difficult to work with.¹⁹

The preparation of Ag NRs using a SiO₂ template with tubular mesopores has been reported.^20–22 In this procedure, Ag was chemically or thermally deposited in the tubular mesopores, so continuous loading of Ag led to the formation of Ag NRs in the mesopores. However, the length of the Ag NRs was not controlled, which caused wide SPR bands. This means that the obtained samples do not possess wavelength selectivity, which is very important to obtain a high quantum efficiency in SERS, SEF, and ML applications. On the other hand, it was recently reported that Ag NPs deposited on TiO₂ crystals exhibited multicolor photochromism.^23–26 Irradiation with monochromatic light caused the size distribution of Ag NPs to change, which led to color changes. The change in size distribution was attributed to the partial dissociation of Ag NPs caused by electron transfer from Ag NPs exhibiting SPR to TiO₂ crystals. These results suggest the possibility of fabricating monodisperse Ag NPs by irradiation with light of a specific wavelength, which would induce SPR only on Ag NPs of unintended size and shape, thus removing undesired NPs from the sample.

In this work, the work of ref. 24 and 25 was extended. Namely, the length of Ag NRs was controlled by light irradiation. Ag NRs were chemically deposited in the tubular mesopores of SiO₂ templates that contained TiO₂ nanocrystals. During or after the deposition of Ag NRs, the samples were exposed to light with a wavelength that induced SPR on Ag NRs of certain length. This caused length-selective dissociation of Ag NRs, which allowed precise control of the length distribution of Ag NRs in the mesoporous templates. The SPR band of the resulting Ag NRs was also varied as intended.

Experimentally, a mixture of the surfactant Pluronic P123, HCl, and H₂O was added to tetraethoxysilane (TEOS) and stirred at 40 °C for 20 h. Titanium tetra-n-butoxide (TTB) was added, and the solution was stirred for a further 6 h. The stirred solution was transferred into an autoclave vessel and kept at 80 °C for 24 h. The resulting precipitate was collected and then suspended in a mixture of toluene and hexamethyldisilazane (HMDS) at 30 °C for 18 h to allow silylation of the outer surface of the powder. The powder was then added to a mixture of diethyl ether and ethanol and stirred at 60 °C for 5 h to remove the surfactant from the mesopores. Prior to the deposition of Ag, the inner surface of the mesopores in the powder was functionalized with hemiaminal groups (–NH–CH₂–OH) using 3-aminopropyltriethoxysilane (APTES) and formaldehyde to effectively adsorb and reduce Ag ions in the mesopores. A powder with a silylated outer surface and hemiaminal-functionalized inner wall was added to a solution of AgNO₃ and stirred at 40 °C for 0.5 h. During or after this process, the sample was irradiated with light using a 300 W Xe lamp with bandpass filters (490–550, 570–690, or 650–810 nm, ∼5 mW cm⁻²). This induced excitation of SPR on Ag NRs of specific length, allowing the aspect ratios of the Ag NRs in the mesoporous templates to be precisely controlled.

The XRD pattern of mesoporous 100SiO₂ showed a broad peak at around 2θ = 23°, which is attributed to amorphous SiO₂ (Fig. 1A). In the XRD pattern of mesoporous 80SiO₂·20TiO₂, several peaks consistent with anatase TiO₂ were observed in addition to the halo from amorphous SiO₂ (Fig. 1B). The size of anatase TiO₂ was estimated to be ca. 11 nm using the Scherrer equation. The corresponding TEM images show that both samples possessed tubular mesopores with a caliber of ca. 8 nm and a wall thickness of ca. 5 nm (2-D hexagonal mesoporous structure, Fig. 1C and D). Nonporous particles were also observed in the 80SiO₂·20TiO₂ sample (indicated by circles in Fig. 1D). A high-resolution TEM image of a nonporous particle showed fringes with spacings of 3.52 Å, which is consistent with the d value of the anatase {011} plane (Fig. 1D-1). The mechanism of deposition of anatase TiO₂ in this study (stirring at 80 °C in aqueous solution) is known to cause dissolution and deposition of TiO₂.^27,28 Therefore, the observed nonporous particles of anatase TiO₂ were presumably formed by deposition of an excess of Ti and O on preformed anatase nanocrystals on the mesoporous silica wall. The presence of preformed anatase TiO₂ nanocrystals on the mesoporous wall is confirmed in the TEM image in Fig. 1D-2, where only the region with a mesoporous structure was observed. The observed fringes with spacings of 3.52 and 1.85 Å are attributed to the {011} and {200} planes of anatase TiO₂, respectively. Anatase nanocrystals with diameters of less than 5 nm were well dispersed throughout the mesoporous structures. These nanocrystals can contact and interact with the Ag NRs deposited in the mesopores, and thus contribute to the length control of the Ag NRs induced by light irradiation. The crystalline size estimated by the Scherrer equation (ca. 11 nm) is consistent with the results that large (tens of nm) and small anatase TiO₂ (less than 5 nm) are coexistent.

Fig. 1 XRD patterns of (A) 100SiO₂ and (B) 80SiO₂·20TiO₂ mesoporous templates before Ag deposition. TEM images of (C) 100SiO₂ and (D) 80SiO₂·20TiO₂ templates before Ag deposition (scale bars: 50 nm). HRTEM images of a (D-1) nonporous particle and (D-2) mesoporous region observed in panel D.

Fig. 2 TEM images of 80SiO₂·20TiO₂ samples after Ag deposition (A) without, and with light of (B) 490−550 and (C) 650−810 nm (scale bars: 100 nm). (D, E, and F) are histograms showing the distribution of aspect ratios of the Ag NRs observed in the samples shown in panels A, B, and C, respectively. (G, H, and I) are the corresponding UV-Vis-NIR DR spectra of the samples shown in panels A, B, and C, respectively (the wavelengths of the incident light are indicated by squares).

After the deposition of Ag in the mesoporous 80SiO₂·20TiO₂ template, TEM images revealed Ag was deposited along the inner wall of the tubular mesopores, forming Ag NRs with various aspect ratios (Fig. 2A and 2D).

During deposition, the solution containing the template, Ag precursor and growing Ag NRs was continuously irradiated with visible light to control the aspect ratio of the resultant Ag NRs (the diameter of the mesopore is fixed, so it is only the length of the NRs that varies). Irradiation with a light of 490–550 nm caused predominantly spherical Ag NPs (Fig. 2B and 2E) to form, whereas light of 650–810 nm caused Ag NRs with an aspect ratio of 1–4 (Fig. 2C and 2F) to form. This implies that irradiation with light of a specific wavelength is a promising tool to control the length of Ag NRs in a tubular mesoporous template containing well-dispersed anatase nanocrystals. The importance of the well-dispersed anatase nanocrystals was revealed by the finding that the mesoporous template containing less than 10 mol% TiO₂ did not show any photoactivity to allow length control of the Ag NRs deposited in the mesopores. This is probably caused by the lack of interface between Ag and anatase TiO₂, which is where dissociation of the Ag NRs occurs. Light irradiation excites free electrons in the Ag NRs, so they exhibit SPR. The excited electrons in Ag are transferred into the conduction band of the anatase nanocrystals where they contact each other.^23,24 Therefore, the Ag NRs which released electrons are partially dissociated and become Ag⁺ ions and spherical or shorter Ag NRs, that are not SPR-active under the wavelength of the incident light. On the other hand, use of a template containing 30–50 mol% TiO₂ resulted in the predominant formation of spherical Ag NPs regardless of irradiation with light, and increasing the TiO₂ content to greater than 50 mol% prevented the formation of tubular mesopores in the template under the reaction conditions used. Therefore, templates containing more than 30 mol% TiO₂ were disregarded in this work.

The DR spectrum of the sample prepared without light irradiation showed a peak at 395 nm that is attributed to both the SPR of spherical Ag NPs and transverse SPR mode of Ag NRs, and a peak at 600 nm that is consistent with the longitudinal SPR mode of Ag NRs (Fig. 2G). In comparison, the DR spectrum of the sample prepared under irradiation with light of 490–550 nm showed only one peak at 398 nm, which is attributed to spherical Ag NPs (Fig. 2H). Irradiation with light of 650–810 nm caused a blue shift of the peak at 600 nm to 580 nm (Fig. 2I), which indirectly suggests that the length of Ag NRs is shortened by irradiation with light of this wavelength range. Alternatively, it can be said that exposure to light of a certain wavelength causes a decrease in absorbance at longer wavelength than that of the incident light. This suggests that when the continuously growing Ag NRs reach a critical length, the incident light causes SPR excitation, which induces dissociation. As a result, the formation of Ag NRs longer than this critical length is prevented.

A schematic illustration of the results of the length control of Ag NRs in tubular mesopores by light irradiation is shown in Fig. 3. Without light irradiation, various lengths of Ag NRs are obtained from deposition of Ag in the tubular mesoporous template (Fig. 3B). Irradiation with light of 490–550 nm excites SPR on the relatively short Ag NRs (P2 and P3 in Fig. 3) and causes them to dissociate, resulting in the predominant formation of spherical Ag NPs (Fig. 3C). Excitation of the SPR of relatively long Ag NRs (P4 and P5) by light of 650−810 nm leads to their partial dissociation and conversion into shorter Ag NRs, so mostly relatively short Ag NRs are formed (Fig. 3D). Irradiation of the template with light during Ag deposition caused the growing Ag NRs to dissociate and resulted in the predominant formation of spherical or relatively short Ag NRs (Fig. 2 and 3). On the other hand, irradiation of the sample with light after Ag deposition gave a different result. As a comprehensive example, irradiation of the sample with light of 570–690 nm after Ag deposition resulted in decreased absorbance in the same wavelength region as that of the incident light, and increased absorbance at longer wavelength in the DR spectrum (Fig. 4).

Fig. 3 Schematic illustration of the results of deposition of Ag NRs in tubular mesoporous SiO₂–TiO₂ templates with/without light irradiation.

Fig. 4 UV-Vis-NIR DR spectra of a sample prepared with/without irradiation with light of 570–690 nm after deposition of Ag NRs in 80SiO₂·20TiO₂ template (the wavelength of the incident light is indicated by a square). The inset shows a schematic illustration of the changes in the length of Ag NRs upon irradiation.

This is presumably caused by the formation of Ag NRs that are too long to exhibit SPR. Namely, some of the Ag⁺ ions generated by the dissociation of SPR-active Ag NRs are deposited on the long Ag NRs that are unaffected by light irradiation (see the inset of Fig. 4). This increases the amount of Ag atoms present in long Ag NRs, so the absorbance at longer wavelength than that of the incident light increases. Overall, this means that light irradiation can cause length-selective dissociation of Ag NRs in TiO₂-containing tubular mesoporous templates. Thus, precise control of the length of Ag NRs would become possible by irradiation of the sample with light of carefully selected wavelength.

In conclusion, we have demonstrated that irradiation of Ag NRs deposited in mesoporous SiO₂–TiO₂ template with light of a specific wavelength can be used to control their length. Irradiation during the deposition of Ag to excite SPR on relatively short Ag NRs in the tubular mesopore resulted in the formation of predominantly spherical Ag NPs. Irradiation with wavelengths that excited SPR on relatively long Ag NRs led mostly to the formation of relatively short Ag NRs. These results indicate that light irradiation during Ag deposition can be used to prevent the formation of excessively long Ag NRs. In addition, light irradiation after the deposition of Ag NRs of various length caused length-selective dissociation of the NRs, because only Ag NRs of certain length are SPR-active when exposed to light of a specific wavelength. Exposure to light of a specific wavelength allowed the growth of Ag NRs that were not SPR-active in that wavelength range. Our results show that Ag NRs of a required length can be fabricated in high yield by selecting appropriate wavelengths of incident light and timing of the irradiation. Moreover, this photoinduced dissociation of Ag can be used to control not only the length of Ag NRs, but also the shapes of Ag NPs by choosing the optimum mesoporous template structure. Hence, this study provides a valuable and practical method for fabricating novel Ag NP-based optoelectronic devices that possess precisely customized SPR characteristics.

Acknowledgements

This work was supported by Grants-in-Aid for Young Scientists (Start-up) 21860045 and Young Scientists (B) 22760539 from the Japan Society for the Promotion of Science (JSPS).

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Footnote

† Electronic Supplementary Information (ESI) available: Detailed experimental procedures included. See DOI: 10.1039/c1ra00317h/

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