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J. Phys. Chem. B 1999, 103, 7064-7067
Remarkable Influence of Silver Islands on the Enhancement of Fluorescence from Eu3+ Ion-Doped Silica Gels S. Tamil Selvan,* Tomokatsu Hayakawa, and Masayuki Nogami Department of Materials Science & Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan ReceiVed: January 22, 1999; In Final Form: May 4, 1999
A simple methodology has been developed to enable a greater enhancement of fluorescence from europium (Eu3+)-doped silica gels containing adsorbed silver (Ag) islands. The procedure involves the preparation of Eu3+-doped silica gels by a sol-gel method and then immersion in a Ag sol for ca. 48 h to allow the adsorption of Ag islands on the pore surfaces of silica gels. UV-vis spectra show the characteristic surface plasmon resonance of the Ag at around 390 nm, and transmission electron microscopy (TEM) presents Ag islandlike particles on the surface of silica gels. The photoluminescence studies reveal the fact that the enhanced fluorescence originates from the local field enhancement around Eu3+ ions, caused by the electronic plasmon resonance of the Ag islands.
Introduction Nanoparticles of noble metals and semiconductor particles, in a more general sense, ascribe a variety of useful optical, electrical, and catalytic properties.1 Colloidal chemistry has been thoroughly enjoying the recent developments in the preparative strategies, viz., the use of surfactants,1b metal ligands,2 block copolymers,3 and thiol derivatization,4 to achieve control over the particle size and shape by providing kinetically controlled monodisperse colloids. Metal/semiconductor or rare-earth ion doped gels and glasses are another important class of materials, owing to their nonlinear optical properties and very recently, europium (Eu) and samarium (Sm) doped glasses have attracted great attention and are proposed as potential novel class of materials for use in optical memory devices, lasers, and fiber amplifiers.5 The optical properties play a crucial role in such applications, and fluorescence can particularly be served as the basis for spectroscopic applications.6 The incorporation of Ag particles using glass melting methods7 and sol-gel methods8 has been well documented in the literature. Levy et al demonstrated the fluorescence of Eu3+ as a sensitive probe for the gel-glass transformation.9 The total emission intensity was found to be increased as a function of time and temperature of dehydration of the gel. The normalized intensity for the glass sample at 873 K was ca. 6 times greater than that for the gel sample at 298 K. Here we report a greater enhancement (ca. 8 to 10 times) of fluorescence from Eu3+doped SiO2 gels at 298 K containing adsorbed Ag islands. This facile approach enables the increase of luminescence from Eu3+doped silica containing adsorbed Ag islands, even in the gel state, obviating the need for heated glasses at high temperatures, by hitherto reported procedures. Experimental Section Our methodology employs a two-step procedure involving both sol-gel and metal cluster chemistry. First, Eu3+-doped * Corresponding author. Fax: +81-52-735-5285. E-mail: selvan@ mse.nitech.ac.jp.
SiO2 gels were prepared and then an immersion process was followed to adsorb Ag islands onto the pore surface of the gel. Silica gels containing 0.5 wt % Eu2O3 were prepared by the sol-gel process using Si(OC2H5)4 (TEOS) and Eu(NO3)3 6H2O as starting materials. TEOS was first partially hydrolyzed with a mixed solution of H2O, C2H5OH, and HNO3 in a molar ratio of 1:1:0.0027 per mol of Si(OC2H5)4. After the solution had been stirred for 1 h, a solution of Eu(NO3)3 6H2O (0.127 g) dissolved in C2H5OH (5 g) was added and stirred for another 1 h. The resulting homogeneous solution was further hydrolyzed with the mixed solution, and the molar ratio of H2O, C2H5OH, and HNO3 was maintained at 4:4:0.011 per mol of metal alkoxide. After being stirred for 15 min, the solution was poured into polystyrene containers and left covered for about 2-3 weeks to form a stiff gel. A silver sol was prepared by mixing X cm3 (X ) 2, 5, and 10) of a 0.43 wt % AgNO3 aqueous solution with partially hydrolyzed tetrakis(hydroxymethyl) phosphonium chloride (THPC) as the reducing agent,1c having prepared the latter by adding 1 mL of a fresh 50 mM solution of THPC in water to 47 mL of 6.38 mM NaOH solution. The samples (Eu3+-doped SiO2 gels before and after Ag adsorption during immersion) were designated as Eu/SiO2, Eu/SiO2-Ag2, Eu/SiO2-Ag5 and Eu/SiO2-Ag10, respectively, for X ) 0, 2, 5, and 10 cm3. After immersion for ca. 48 h, the gels were washed with water and dried at 50 °C for ca. 24 h. To elucidate the difference in fluorescence between the samples with and without Ag and the structural changes that the sol-gel may undergo upon immersion in the Ag sol, followed by washing and drying, the same treatment was done for the Eu3+-doped sol-gel without Ag and designated as control sample. The thicknesses of the samples were ca. 0.4 mm. After immersion, the surface of the silica gel contained Ag particles, corroborated by TEM images. The Eu3+ ions are presumably randomly dispersed in the silica, but only those at the surface will interact with the surface Ag. The final loading of silica for a typical sample (Eu/SiO2-Ag2) is the following: 0.5 g Eu2O3 and 22 µg Ag per 100 g SiO2. The Ag concentration in the gels was estimated from the inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Prior
10.1021/jp9902755 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/03/1999
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J. Phys. Chem. B, Vol. 103, No. 34, 1999 7065
Figure 1. Absorption spectra of (a) Ag sol, (b) Eu3+/SiO2 gel without Ag and (c) Ag adsorbed Eu3+/SiO2 gel. λmax are noted at 390, 285, and 382 nm, respectively, for curves a-c.
to the measurement, Eu3+-doped silica gel containing Ag was vacuum dried at 100 °C and subsequently immersed in a solution of concentrated HNO3. Upon stirring for ca. 1 day, Ago particles dissolved out and the resulting solution was examined. Fluorescence spectra and decay curves were acquired with an INSPEC V ICCD system (Oriel Instruments). A N2 laser (λe ) 337.1 nm, pulse width < 1 ns) was used as the excitation source. The emitted light was led to a monochromator through an optical fiber and detected by an ICCD camera. After the excitation, the fluorescence was measured by adjusting the gate pulse widths (Tw) to 1 µs, 1 ms, and 10 ms. To overcome the pulse-to-pulse intensity fluctuations, the exposure time (gate pulse width) of the ICCD camera after the pulsed excitation was adjusted to be larger than the lifetime of Eu3+ fluorescence, and the obtained signals were averaged out for 100 times. Thus the resulting fluorescence enhancement could be comparable to that under steady state excitation. Particular care was taken to avoid optics geometry changes, inhomogeneities in concentration of the silver or europium, and laser pulse energy variations. The reported intensities were calibrated using the sample thicknesses and referred to relative intensities. The enhancement of luminescence was reproducible for all of the samples. The optical absorption spectra of gels were obtained by JASCO, U-best 50 spectrometer, in the range of 200 to 900 nm. Transmission electron micrographs were acquired with a JEM-2000FX transmission electron microscope (TEM) operating at 160 kV. The carbon coated Cu mesh was used as the support for the samples. Results and Discussion The UV-vis absorption spectra of the Ag sol (curve a) and Eu3+/SiO2 gel before (curve b-control) and after (curve c) immersion in the Ag sol are shown in Figure 1. Curve (a) displays a well-defined plasmon band (λmax) at around 390 nm, typical for Ag. The absorption spectrum of the gel doped with Ag, which has a broad maximum at around 380 nm (curve c) due to the excitation of the electronic plasma resonances, serves as a direct measure of the strength of the electromagnetic interactions and can be used to characterize the optical behavior of the film.10 As the concentration of adsorbed Ag in Eu3+/ SiO2 gel is much smaller (22 µg Ag per 100 g of silica containing 0.5 g Eu3+ from ICP-AES), the absorption is also smaller than Ag alone. Curve (c) is very broad which may indicate that a wide range of particle sizes exist. According to
Figure 2. Fluorescence spectra of Eu3+-doped silica gels without Ag (curves a,c: Eu/SiO2) and with Ag (curves b,d: Eu/SiO2-Ag2) at two different exposure times. The remarkable influence of Ag on the blue shift (curve b) and significant enhancement of fluorescence (curve d) are noticed. The enhancement of fluorescence in curve (d) is ca. 10 times greater than that in curve (c).
Mie theory, the absorption maxima of metallic spherical particles is shifted to the red when particle size is increased. Taking into account the electron mean free path effects, the intensity of the absorption band should be higher for bigger particles below the critical size.8c In our case, the particle sizes of Ag particles in the Ag sol (curve a) and Ag adsorbed Eu3+/SiO2 gel (curve c) are estimated to be ca. 3.5 and 2 nm, respectively, using the relation11
r ) VF/∆ω1/2 where r is the average crystal radius, VF is the Fermi velocity of an electron and is 1.39 × 106m/s for Ag, and ∆ω1/2 is the full width at half maximum of the plasmon absorption in angular frequency. The principal result of the present study is that the fluorescence enhances dramatically from Eu3+-doped SiO2 gels containing adsorbed Ag islandlike particles. This behavior is illustrated in Figure 2 (curve d). Figure 2 depicts the fluorescence spectra of the control samples, Eu/SiO2 (curves a and c) and Eu/SiO2-Ag2 (curves b and d), at two different gate pulse widths (Tw). A weak broad fluorescence band is observed at 475 nm (curve a) for the gate pulse width of 1 µs. The influence of the Ag particles on the fluorescence band shift is illustrated in curve (b). This broad fluorescence band blue shifts toward the lower wavelength (λmax at ca. 405 nm), which is caused by the plasmon vibration of low dimensional silver. Upon increasing the exposure time to 1 ms, a greater enhancement of fluorescence (ca. 10 times) at around 615 nm (curve d), in comparison with the control sample (curve c), is evidenced. It should be mentioned that the fluorescence between 350 and 550 nm can also be observed in undoped sol-gel glass. Yoldas12
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Figure 3. Fluorescence spectra of Eu3+-doped silica gels without Ag (curve a: Eu/SiO2) and with Ag (curve b: Eu/SiO2-Ag5, curve c: Eu/SiO2-Ag10). The enhancement of fluorescence in curve (c) is ca. 8 times greater than that in curve (a).
reported that the photosensitivity commenced by heating the gels without doping, e.g., Al2O3-SiO2, to ∼ 300-350 °C, which was related to chemical bond cleavage and resultant carbon formation at high temperatures. We also compare the relative fluorescence intensities of Eu3+doped silica gels (control-curve a) and Ag-doped gels at two different concentrations (samples Eu/SiO2-Ag5, Eu/SiO2Ag10) in Figure 3. An increase in fluorescence is noted for Agadsorbed gels (curves b and c). The fact that the relative fluorescence is small (curve b) may serve as an indication to the dilution of the silver ion concentration in the silica gel matrix. On the other hand, at a higher concentration, the increase in fluorescence (curve c) is ca. 8 times greater than that of the control sample (curve a). Shown in Figure 4 are representative TEM images of thin films of Ag-adsorbed Eu/SiO2 gels. Clearly, Ag islandlike particles are seen on the pore surfaces of SiO2 gels. We have also observed an enhancement of fluorescence from Eu3+-doped SiO2 glass in the presence of small Ag particles.13 Now the question arises as to the cause of the enhancement of fluorescence in the presence of Ag islands. The structural features could play a critical role, as the size, shape, and concentration of the metallic particles have a direct influence on the fluorescence enhancement.7c This is because the complex dielectric function of the composite medium depends directly on the structural features of the particles. The evolution of fluorescence from organic molecules has been documented in the literature.14 Furthermore, the fluorescence was enhanced from the laser dye Rhodamine 6G adsorbed onto a rough Ag surface.6 This enhancement arose from the electromagnetic interaction between the molecules and the electronic plasma resonance of the Ag islands. Most of the work in the literature detailed the effects of either fluorescent organic molecules adsorbed on rough Ag surfaces or Eu3+ entrapment in sol-gel glasses as probes for surface enhanced Raman spectroscopy
Figure 4. Typical transmission electron micrographs of the samples (a) Eu/SiO2-Ag2, (b) Eu/SiO2-Ag5. Ag islands are marked by the arrows. The dark area represents the SiO2 matrix.
(SERS) studies. Recently, Zink et al.15 delineated the encapsulation of organometallic gold precursor compounds such as dimethyl (trifluoroacetylacetonato) gold into the silica monoliths and irradiation with UV light to produce Au particles in the interior of the monoliths which were used as matrixes for SERS. It is noteworthy that the silver and gold surfaces and also colloids have been employed for most of the SERS studies.16 The intensity of fluorescence from Eu3+ in sol-gel glasses is highly dependent on the preparation and drying procedures. Levy et al.9 described the fluorescence enhancement from Eu3+ ions trapped in silica glasses as a function of time and temperature of dehydration of the gel. The low fluorescence intensity of the gels was due to electron-phonon coupling with C-H and O-H groups. The 5D0 f 7F2 transition of Eu3+ is hypersensitive. The fluorescence intensity ratio of 5D0 f 7F2 to 5D0 f 7F1 transitions indicated the degree of asymmetry in the vicinity of europium ions and Eu-O covalency.17 A convenient quantity for monitoring such intensity changes is the so-called asymmetry ratio, AS,
AS )
∫I0f2dγ ∫I0f1dγ
where I0fJ denotes the intensity of the 5D0 f 7FJ transition. Table 1 summarizes the quantitative behavior of the total emission intensities of our samples with a comparison to the Levy et al work.9 The intensity of the 5D0 f 7F2 emission relative to the 5D0 f 7F1 emission is increased with an increase in the Ag concentration. A similar effect was noticed in Eu3+trapped silica glasses heated at elevated temperatures, whereas the relative intensity of the 5D0 f 7F1 emission was greater than that of the 5D0 f 7F2 emission in the gel state.9 The normalized total emission intensity of Eu/SiO2-Ag10 is ca. 8 times greater than that of the control sample without Ag. When the emission spectrum of this sample (curve c in Figure 3) is compared with that of Levy et al.’s Eu3+-doped silica glass at
TABLE 1: Total Emission Intensities of Eu3+ in Gels (from Figure 3)a and in Gel/Glass Samples sample asymmetry ratio, AS emission intensity of all 5D f 7F transitions 0 J I ) ∑J∫I0fJ dγ (normalized intensity) a
This work. b Reference 9.
Eu/SiO2 gel (curve a) 3.12 407 (1)
Eu/SiO2-Ag5 gel (curve b) 3.23 1071 (2.63)
Eu/SiO2-Ag10 gel (curve c) 3.52 3348 (8.22)
b
Eu/SiO2 gel at 298 Kb 0.43 64.43 (1)
Eu/SiO2 glass at 873 Kb 5.33 406.2 (6.30)
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J. Phys. Chem. B, Vol. 103, No. 34, 1999 7067 Acknowledgment. S.T.S. gratefully acknowledges the Japan Society for the Promotion of Science (JSPS), Tokyo, for financial support. The authors thank Prof. A. Nakamura, Department of Applied Physics, Nagoya University for fruitful discussions. We thank Mr. K. Yamamoto for his help in ICPAES measurements. References and Notes
Figure 5. Decay curves of the 5D0 f 7F2 fluorescence (615 nm) from Eu3+-doped gels without Ag (b) and with Ag (s Eu/SiO2-Ag5; - - Eu/SiO2-Ag10).
873 K,9 a relative increase by a factor of about 1.3 in the normalized total emission intensity is observed. Shown in Figure 5 are the decay curves of the 5D0 level of the Eu3+ ion-doped silica gels with and without Ag. A surprising feature here is that, despite a great difference in the fluorescence intensity between the control and Ag adsorbed gels, the Eu3+ fluorescence decay curves are almost exponential in both the cases and the lifetime is found to be ca. 150 µs. It is obvious that there is no energy transfer from Ag to Eu3+ ions, since the lifetime of plasma oscillation of Ag particle (10-14 s) is much smaller than that of the Eu3+ ions (10-3 s). Importantly, there should be interaction between Eu3+ and Ag on the pore surface of silica, and in this process of interaction Ag particles must strongly influence Eu3+ ions owing to their electronic plasmon resonances, causing an intensified electromagnetic field around Eu3+ ions, resulting in an enhancement of fluorescence from Eu3+ ions. Summary A facile approach to a greater enhancement of fluorescence from Eu3+-doped silica gels adsorbed with Ag has been reported. An important feature of the immersion process is its ability to enhance the fluorescence from Eu3+-doped silica, even in the gel state. Conspicuously, the adsorption of Ag islands onto the pore surfaces of SiO2 gels would lead to an enhancement in fluorescence, which is attributed to the local field enhancement around Eu3+ ions. A significant enhanced fluorescence has high potential for application in optical displays, lasers, and memory devices. Further work is being carried out to exploit the existing colloidal chemistry techniques for the optical properties of rareearth ion-doped gels and glasses.
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