III. RESULTS AND DISCUSSION

The process for obtaining silica-gel involves the hydrolysis of TEOS [Eq. (1)] and the condensation of hydrolysis products with the elimination of water or alcohol as indicated in reaction [Eq. (2) or (3)], depending on the H₂O/TEOS ratio.^{Reference Stefanescu, Stoia and Stefanescu27} No matter if the hydrolysis process is carried out in a basic or acid condition, only providing high amount of acid and water as occurs in the present case, the fully hydrolyzed molecule obtained in the process is Si(OH)₄.^{Reference Monsivais-Gámez, Ruiz and Martinez28} When the fully hydrolyzed silica sol is mixed with a high polarizable ion, such as Ag⁺ or Au⁺, the reaction [Eq. (4)] should be expected with the formation of nonbridging oxygen by a cation exchange process.^{Reference De, Licciulli, Massaro, Tapfer, Catalano, Battagin, Meneghini and Mazzoldi29–Reference Bansal31} Finally, the Ag/SiO₂ or Au/SiO₂ films were obtained by decomposition of Si(OH)₄ and SiO(OH)₃Ag or SiO(OH)₃Au [Eq. (5)], eliminating several water molecules. The possible formation process is proposed as follows:

(1)$${\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}} \right)_4} + \left( {4 - n} \right){{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}} \right)_n}{\left( {{\rm{OH}}} \right)_{4 - n}} + \left( {4 - n} \right){{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}\quad \left( {0 \,\le \,n \,\le \,3} \right)\quad ,$$

(2)$${\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}} \right)_n}{\left( {{\rm{OH}}} \right)_{4 - n}} + {\left( {{\rm{HO}}} \right)_{4 - n}}{\left( {{{\rm{C}}_2}{{\rm{H}}_{\rm{5}}}{\rm{O}}} \right)_n}{\rm{Si}} \to {\left( {{\rm{HO}}} \right)_{3 - n}}{\left( {{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{O}}} \right)_n}\,{\rm{Si}}-{\rm{O}}-{\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}} \right)_n}{\left( {{\rm{OH}}} \right)_{3 - n}} + {{\rm{H}}_2}{\rm{O}}\quad {\rm{,}}$$

(3)$${\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_5}} \right)_4} + {\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_5}} \right)_n}{\left( {{\rm{OH}}} \right)_{4 - n}} \to {\left( {{{\rm{C}}_2}{{\rm{H}}_5}{\rm{O}}} \right)_n}{\left( {{\rm{OH}}} \right)_{3 - n}}\,{\rm{Si}}-{\rm{O}}-{\rm{Si}}{\left( {{\rm{O}}{{\rm{C}}_{\rm{2}}}{{\rm{H}}_5}} \right)_3} + {{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}\quad {\rm{,}}$$

(4)$${\rm{Si}}{\left( {{\rm{OH}}} \right)_4} + {\rm{A}}{{\rm{g}}^ + }\,\,{\rm{or}}\,\,{\rm{A}}{{\rm{u}}^ + } \to {\left( {{\rm{HO}}} \right)_3}{\rm{Si}}-{\rm{O}}-{\rm{Ag}}\,{\rm{or}}\,\,{\left( {{\rm{HO}}} \right)_3}{\rm{Si}}-{\rm{O}}-{\rm{Au}} + {{\rm{H}}^ + }\quad {\rm{,}}$$

$${\rm{Si}}{\left( {{\rm{OH}}} \right)_4} \to {\rm{Si}}{{\rm{O}}_{\rm{2}}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}}$$

or

(5)$$2{\rm{SiO}}{\left( {{\rm{OH}}} \right)_3}{\rm{Ag}}\;\,{\rm{or}}\,\;{\rm{2SiO}}{\left( {{\rm{OH}}} \right)_3}{\rm{Au}} \to {\rm{2Si}}{{\rm{O}}_{\rm{2}}} + 3{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_{\rm{2}}} + 2{\rm{Ag}}\,\,{\rm{or}}\,\,{\rm{2Au}}\quad {\rm{.}}$$

To determine the formation temperature of Au and Ag NPs in SiO₂ matrix and to select proper annealing temperature, the DTA analysis was performed on Au_0.1Ag_0.1/(SiO₂)_0.8 gel powder. As can be seen from Fig. 2, there are three endothermic peaks at about 98, 188, 216 °C and one exothermic peak at 468 °C in the curve. The first broad endothermic peak at 98 °C is due to the volatilization of water and ethanol that come from the added solvent in the colloid preparation process or give off by hydrolysis and condensation reaction of Si(OC₂H₅)₄^{Reference Stefanescu, Stoia and Stefanescu27} as indicated in the reactions of (1) and (2) or (3). Based on the mechanism analysis of AuAg/SiO₂ film, it can be inferred that Si–O–Ag and Si–O–Au exist in the formation process of Au_0.1Ag_0.1/(SiO₂)_0.8 film. In addition, it has been reported earlier that the temperature formation of silver metal clusters by breaking Si–O–Ag is at about 200 °C^{Reference De, Licciulli, Massaro, Tapfer, Catalano, Battagin, Meneghini and Mazzoldi29,Reference Khan, Kumar, Ahamed, Alrokayan, Alsalhi, Alhoshan and Aldwayyan32} ; our previous work^{Reference Li, Zhang, Zhao and Zhao33} also confirmed the appearance of SPR peak resulting from Ag NPs when the Ag/SiO₂ film annealed above 200 °C. Taking into account the fact that the ionic radius of Au⁺ is larger than the radius of Ag⁺, it can be deduced that the endothermic peaks at 188 and 216 °C are attributed to the formation of Au and Ag NPs by breaking Si–O–Au and Si–O–Ag. The exothermic peak at 468 °C may be attributed to the combustion of residual carbon and organics.^{Reference De, Licciulli, Massaro, Tapfer, Catalano, Battagin, Meneghini and Mazzoldi29,Reference Que, Zhou, Lam, Chan, Tan and Kam34} Evidently, the crystallization temperature of Au and Ag NPs is different. Therefore, we choose 190 °C as the pyrogenation temperature of Au/SiO₂ film for the formation of Au NPs and 500 °C (>216 °C) as the annealing temperature of the composite film for the formation of Ag NPs.

FIG. 2.

DTA curve of the gel powders for Au_0.1Ag_0.1/(SiO₂)_0.8 sample. The sample is prepared by mixing Au_0.1/(SiO₂)_0.4 and Ag_0.1/(SiO₂)_0.4 gel powders before testing.

Figure 3 shows the TEM image of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film. It can be seen that the considered Ag or Au NPs are almost in a spherical structure with large size distributions. The SAED pattern of the composite film in Fig. 3(b) shows the characteristic diffraction rings of metal Ag or Au, which from inner to outer correspond to the (111), (200), (220), (311), (222), (400), and (331) crystal planes of the cubic phases of Ag or Au. However, at this stage, it is hard to determine the existence form of the two metals in the composite film, whether it is in their alloy or in a separate NP mixed state, since Ag and Au have the same face-centered cubic structure with almost the same bulk lattice constant (Au: 4.0786 Å; Ag: 4.0862 Å).^{Reference Sun and Xia35}Figure 3(c) shows the particle size distribution histogram corresponding to Fig. 3(a), which is mainly in the range of 10–30 nm, and the mean particle size is about 23 nm.

FIG. 3.

TEM micrograph and (a) the selected area electron diffraction pattern (b) of Au_0.1Ag_0.1/(SiO₂)_0.8 thin film, (c) the nanoparticle size distribution histogram of the sample corresponding to (a). The sample is prepared by spin-coating one Au_0.1/(SiO₂)_0.4 film and one Ag_0.1/(SiO₂)_0.4 film.

Figure 4 shows the XPS survey spectrum of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film. The appearance of Au 4f, Au 4p, Ag 3d, Si 2p, Si 2s, O 1s, and O KLL photoelectron peaks confirm the existence of gold, silver, silicon, and oxygen species in the film. In addition, Na KLL, C KLL, and C 1s peaks are also observed in the spectrum, which originate from the diffusion of Na atoms that existed in the glass substrate into the sample surface during the annealing process and the air pollution or CO/CO₂ adsorption on the sample surface, respectively. These are unavoidable for XPS analysis.

FIG. 4.

XPS survey spectrum of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film. The sample is prepared by spin-coating four layers of Ag_0.1/(SiO₂)_0.4 and four layers of Au_0.1/(SiO₂)_0.4 film.

Figure 5 shows the high-resolution XPS spectra of (i) Si-2p, (ii) O-1s, (iii) Ag-3d, and (iv) Au-4f core level regions of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film. In Figs. 5(a) and 5(b), the peaks centered at 103.3 and 532.9 eV belong to Si 2p and O 1s in SiO₂.^{Reference Moulder, William and Peter36} The peaks located at 367.9 and 373.9 eV in Fig. 4(c) with a peak separation of 6.0 eV are attributed to Ag⁰ 3d _3/2 and Ag⁰ 3d _5/2, in spite of a little deviation from the values reported in the literature, i.e., the standard binding energies of Ag⁰ 3d _3/2 and Ag⁰ 3d _5/2 are 368.2 and 374.2 eV, respectively.^{Reference Moulder, William and Peter36} Similarly, two peaks at 83.8 and 87.4 eV in Fig. 5(d) can be referred to Au⁰ 4f _7/2 and Au⁰ 4f _5/2, respectively, in spite of a little deviation from the corresponding values of pure metallic Au (Au⁰ 4f _7/2, 84.1 eV and Au⁰ 4f _5/2, 87.7 eV^{Reference Moulder, William and Peter36}). The lower binding energies of Ag 3d and Au 4f may be attributed to a charge transfer between SiO₂ and metal particles because the combination of Ag, Au, and SiO₂ adjusts their Fermi level, similar to the case of the other metal-oxide composites.^{Reference Wang, Zhang, Yan and Deng37,Reference Sun, Qiao, Tan, Wang and Qiu38} However, the absence of the peaks located at 84.9 and 88.1 eV^{Reference Sudipto and Goutam39} corresponding to the oxidized Au species implies that Au also remains in a metal state after coating the Ag_0.1/(SiO₂)_0.4 layer. The above results indicate that Ag and Au NPs are formed in SiO₂ film, and they exist in each metal state.

FIG. 5.

High-resolution XPS spectra of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film: (a) Si 2p, (b) O 1s, (c) Ag 3d, and (d) Au 4f. The sample is prepared by spin-coating four layers of Ag_0.1/(SiO₂)_0.4 and four layers of Au_0.1/(SiO₂)_0.4 film.

The NPs formed in the composite film in its pure metal or alloy form can be exactly determined from its optical absorption spectra. In single metal dispersed system, the absorption spectra can be predicted by the well-known Mie scattering theory,^{Reference Mie40} which can be explained by Bohren and Huffman^{Reference Bohren and Huffman41} and have been calculated by various algorithms.^{Reference Du42–Reference Hosoya, Suga, Yanagawa and Kurokawa45} The absorbance (α) is proportional to the extinction cross section (σ), which can be obtained by the Mie scattering theory in the form as^{Reference Hosoya, Suga, Yanagawa and Kurokawa45}:

$${\rm{\sigma }}\left( {\rm{\omega }} \right) = 9{{\rm{\omega }} \over c}{\rm{\varepsilon }}_{\rm{0}}^{3/2}{V_0}{{{\varepsilon _{{{\rm{m}}_{\rm{2}}}}}\left( {\rm{\omega }} \right)} \over {{{\left[ {{{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{1}}}}}\left( {\rm{\omega }} \right) + 2{{\rm{\varepsilon }}_0}\left( {\rm{\omega }} \right)} \right]}^2} + {\rm{\varepsilon }}_{{{\rm{m}}_{\rm{2}}}}^{\rm{2}}}}\quad ,$$

where c, ${\rm{\omega }}\left( {{\rm{\omega }} = {{2{\rm{\pi }}} \over {\rm{\lambda }}}} \right)$, and ${V_0}\left( {{V_0} = {4 \over 3}{\rm{\pi }}{R^3}} \right)$ are the wave vector, angular frequency, and particle volume; λ and R are the wave length and the metal NP's radii; ε₀ and ${{\rm{\varepsilon }}_{\rm{m}}}\left( {{{\rm{\varepsilon }}_{\rm{m}}} = {{\rm{\varepsilon }}_{{{\rm{m}}_1}}} + i{{\rm{\varepsilon }}_{{{\rm{m}}_2}}}} \right)$ are the dielectric functions of the matrix and metal particle, respectively. ${{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{1}}}}} = {n_{\rm{m}}} - {k_{\rm{m}}}$, ${{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{2}}}}} = 2{n_{\rm{m}}}{k_{\rm{m}}}$, where n _m and k _m are the refraction index and the extinction coefficient of the metal, respectively. For inorganic materials ${{\rm{\varepsilon }}_0} = n_{\rm{s}}^2$, n _s is the refractive index of SiO₂. Therefore, α can be expressed as^{Reference Belotelov, Carotenuto, Nicolais, Longo, Pepe, Perlo and Zvezdin9}:

$${\rm{\alpha }}\left( {\rm{\lambda }} \right) = 24{{\rm{\pi }}^2}n_{\rm{s}}^3{{{{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{2}}}}}} \over {{{\left[ {{{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{1}}}}}\left( {\rm{\omega }} \right) + 2n_{\rm{s}}^2} \right]}^2} + {\rm{\varepsilon }}_{{{\rm{m}}_{\rm{2}}}}^{\rm{2}}}}{R^3}\quad .$$

For the bimetallic system, considering, the system is obtained by layer-by-layer coating and step-by-step annealing, Au and Ag NPs distribute in different layers; hence, the absorption coefficient of A_xB_y/(SiO₂)_z composite film can be expressed as:

$${\rm{\alpha }}\left( {\rm{\lambda }} \right) = {{\rm{\alpha }}_{\rm{A}}}\left( {\rm{\lambda }} \right) + {{\rm{\alpha }}_{\rm{B}}}\left( {\rm{\lambda }} \right)\quad ,$$

$${{\rm{\alpha }}_{\rm{A}}}\left( {\rm{\lambda }} \right) = {{24{{\rm{\pi }}^2}xn_{\rm{s}}^3} \over {{\rm{\lambda }}\left( {x + y + z} \right)}}{{{\rm{\varepsilon }}_{{{\rm{m}}_2}}^{\rm{A}}} \over {{{\left[ {{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{1}}}}^{\rm{A}} + 2n_{\rm{s}}^2} \right]}^2} + {\rm{\varepsilon }}_{{{\rm{m}}_2}}^{{{\rm{A}}^2}}}}R_{\rm{A}}^3\quad ,$$

$${{\rm{\alpha }}_{\rm{B}}}\left( {\rm{\lambda }} \right) = {{24{{\rm{\pi }}^2}yn_{\rm{s}}^3} \over {{\rm{\lambda }}\left( {x + y + z} \right)}}{{{\rm{\varepsilon }}_{{{\rm{m}}_{\rm{2}}}}^{\rm{B}}} \over {{{\left[ {{\rm{\varepsilon }}_{{{\rm{m}}_1}}^{\rm{B}} + 2n_{\rm{s}}^2} \right]}^2} + {\rm{\varepsilon }}_{{{\rm{m}}_2}}^{{{\rm{B}}^2}}}}R_{\rm{B}}^3\quad ,$$

where α_A, α_B, and α are the optical absorption coefficients of A metal, B metal, and their total, respectively. The x, y, and z denote the molar fractions of the constituent materials, respectively, and x + y + z = 1. The R _A and R _B represent metal particle's radii. Here, the n _m and k _m of Au and Ag are cited from Ref. Reference Palik46. The particle radii used here is 20 nm, according to the TEM results.

Figure 6 shows the theoretical and experimental optical spectra of Ag_0.1/(SiO₂)_0.4, Au_0.1/(SiO₂)_0.4, and Au_0.1Ag_0.1/(SiO₂)_0.8 composite films. In theory, as shown in Fig. 6(a), the peaks related to the SPR of Ag and Au particles are revealed at about 410 and 515 nm, respectively. If bimetal NPs dispersed system is formed in Au_0.1Ag_0.1/(SiO₂)_0.8 composite film, two SPR peaks at 410 and 515 nm resulting from Ag and Au NPs are expected to be observed.^{Reference Sanchez-ramirez, Pal, Nolasco-Hernandez, Mendoza-Alvarez and Pescador-Rojas47} While, if Ag–Au alloy is formed in the composite film, there will be only one SPR peak and its position is revealed in-between the SPR peak positions of monometal Ag (410 nm) and monometal Au (515 nm) dispersed systems.^{Reference Sangpour, Akhavan and Moshfegh48}Figure 6(b) shows the optical absorption spectra in experiment of Ag_0.1/(SiO₂)_0.4, Au_0.1/(SiO₂)_0.4, and Au_0.1Ag_0.1/(SiO₂)_0.8 composite films. For Ag_0.1/(SiO₂)_0.4 and Au_0.1/(SiO₂)_0.4 films, the SPR peaks appeared around 410 and 540 nm, respectively. For Au_0.1Ag_0.1/(SiO₂)_0.8 composite film, both peaks clearly appeared and confirmed the formation of bimetal NPs rather than alloy NPs. Notably, the two peaks have nearly equal intensity, which is rarely obtained by a chemical approach. In literature reports based on Au–Ag bimetal dispersed system, one SPR peak (usually Ag SPR peak or the peak location between Au and Ag) or two peaks with one in the form of a shoulder peak are shown. In our present work, two peaks with almost the same intensity are obtained. This result indicates that layer-by-layer coating and step-by-step annealing method affords a new opportunity for the preparation of bimetal NPs dispersed composite films.

FIG. 6.

Optical absorption spectra of Au_0.1Ag_0.1/(SiO₂)_0.8 composite film: (a) in theoretical and (b) in experimental. The sample is prepared by spin-coating one Au_0.1/(SiO₂)_0.4 film and one Ag_0.1/(SiO₂)_0.4 film.

Furthermore, the SPR peaks originating from Au NPs in Au_0.1/(SiO₂)_0.4 and Au_0.1Ag_0.1/(SiO₂)_0.8 films show a red shift compared with the calculated one accordingly, maybe caused by large size distributions of Au NPs. It is known that the SPR frequency depends on the size, content, shape of the metal particles, and the dielectric properties of metal particles along with the surrounding matrix material.^{Reference Wang, Zhang, Yan and Deng37} Since Mie theory considers a case of perfect sphere metal particles with the same size and uniform distribution, it does not take into account the effects of the particle sizes, their distribution and the state of surrounding dielectrics.^{Reference Wang, Zhang, Yan and Deng37} However, it is reported that the size of Ag particles would induce the shift of the SPR peak when the size of particles becomes larger than 15 nm.^{Reference Kreibig and Frragstein49} The position shift and even the appearance of a shoulder at higher wave length were also observed when the silver nanocluster had two very different sizes.^{Reference Martinez-Castanon, Martinez, Zarzosa and Ruiz50} Likewise, as the size of Au NP increases, a red shift of SPR peak can be observed in the Au/SiO₂ films, which is attributed to the enhancement of the screening of the d-electrons resulting from the decreasing surface-to-volume ratio of particles.^{Reference Lamarre, Yu, Harkati, Roorda and Martinu51} In fact, as shown in Fig. 3(a), the metal NPs have analogous special structure with large size distributions, but it is difficult to distinguish Au and Ag particles between them. But considering the Au/SiO₂ film was annealed twice, at 190 and 500 °C, this annealing process may be caused by the larger particle size, which may induce a red shift of the experimental optical absorption spectra.

In addition, the relative intensity of Ag SPR peak for Au_0.1Ag_0.1/(SiO₂)_0.8 composite film is weaker than the calculated one, which may be caused by the experimental factors. In our present work, Au_0.1Ag_0.1/(SiO₂)_0.8 composite film is obtained by spin-coating Au_0.1/(SiO₂)_0.4 and Ag_0.1/(SiO₂)_0.4 layers one after another, so Au_0.1/(SiO₂)_0.4 film is on the surface of glass substrate and primarily absorbs the incident light. This is because the incident light is illuminated from the glass substrate side. The intensity of incident light can be reduced due to the scattering and absorption phenomenon of Au_0.1/(SiO₂)_0.4 film, which in turn gives rise to relative weaker intensity of incident light used for Ag_0.1/(SiO₂)_0.4 film. Indeed, the calculated optical absorption does not consider these experimental factors; thus, the intensity of Ag SPR peak for Au_0.1Ag_0.1/(SiO₂)_0.8 composite film weakens to the calculated one. Just because of these experimental factors, it is possible to control the experiment condition and obtain two SPR peaks with almost the same intensity. However, it is reported that the particle size not only affects the position of SPR peak, but also the intensity and width.^{Reference Zhao, Zhang and Shang52,Reference Haruta53} When the particle size is larger to some extent, the SPR peak will be weak and broad.^{Reference Zhao, Zhang and Shang52} Taking into the account the large size distributions of metal NPs in the film, therefore, in addition to the experimental factors, the large particle size may be another considering factor contributing to the weaker intensity. Moreover, accompanied with the relative intensity change is also the broadening of the SPR peaks of Au_0.1/(SiO₂)_0.4, Ag_0.1/(SiO₂)_0.4, and Au_0.1Ag_0.1/(SiO₂)_0.8 composite films, which can be attributed to the broader distribution of their NPs.^{Reference Martinez-Castanon, Martinez, Zarzosa and Ruiz50}

Figure 7(a) shows the optical absorption spectra of the Au_xAg_0.2−x/(SiO₂)_0.8 composite films, with x values from 0.02 to 0.1. All samples exhibit two absorption bands around 410 and 540 nm due to the SPR effect of Ag and Au NPs. The intensities of both peaks depend on their initial content. The intensity of the dominant peak located at 410 nm intensifies with increasing Ag content. A second peak at 540 nm develops with increasing Au content. The second peak appears as a shoulder in the relative lower content Au sample but is clearly discernible in the highest Au content sample. The variational trend of absorption spectra as the contents of two metal NPs is in accordance with the calculated one by the modified Mie theory [as shown in Fig. 7(a)]. Besides, the SPR peaks manifest a red shift trend with increasing Au content, given by the quantum size effect.^{Reference Wang, Zhang, Yan and Deng37,Reference Wang, Lau and Li54}

FIG. 7.

Optical spectra of Au_xAg_0.2−x/(SiO₂)_0.8 composite films with different x values: (a) in theoretical and (b) in experimental. The sample is prepared by spin-coating one Au_x/(SiO₂)_0.5−x film and one Ag_y/(SiO₂)_0.5−y film.