Controlled Deposition of Silver Nanoparticles in ...

Controlled Deposition of Silver Nanoparticles in Mesoporous Thin Films: Towards New Metallic–oxide Nanocomposites

María Cecilia Fuertes,1 Martín Marchena,1 Alejandro Wolosiuk,1,2 Galo Juan de Avila Arturo Soler-Illia1,2

1

Gerencia de Química, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica,

Av. Gral Paz 1499 (B1650KNA) San Martín, Buenos Aires, Argentina 2

Departamento de Química Inorgánica, Analítica y Química Física – DQIAQF,

Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pab. II, C1428EHA Buenos Aires, Argentina

Metal–oxide nanocomposites made up of Ag nanoparticle (NP) assemblies embedded within mesoporous oxide thin films were produced by mild reduction of Ag+ adsorbed onto the pore surface. The nanocomposites were characterized by Small Angle X–ray Scattering (2D SAXS, D03A SAXS2) and X–ray Reflectometry (XRR, D10A XRD2). A quantitative method based in XRR was developed in order to assess pore filling. Inclusion of Ag NP assemblies in mesoporous SiO2 or TiO2 requires different processing conditions. The difference of reactivity of both oxide matrices towards Ag+ reduction is exploited to selectively synthesize NPs in a pre–determined layer of a multilayered mesoporous stack. This leads to highly controlled 1D ordered multilayers with precise spatial location of nanometric objects.

Facility: XRD2, SAXS2, C2nano Publication: Small, 5: 272–280 (2009) Funding: CONICET, ANPCyT, MCT

3

Controlled Deposition of Silver Nanoparticles in Mesoporous Thin Films: Towards New Metallic-oxide Nanocomposites

The synthesis of controlled film–

MOTF were deposited by dip–coating

supported nanocomposite systems

glass substrates using water/ethanol

constitutes one of the most active fields

acidic solutions containing an inorganic

in nanotechnology. The rational design

precursor (TiCl4 or Si(OEt)4) and a pore

of these materials holds a promise for

template (cationic surfactant CTAB or

obtaining integrated devices for a diversity

block–copolymers: Brij58 and Pluronics

of applications. Mesoporous Oxide Thin

F127). Each surfactant provides a different

Films (MOTF) represent attractive template

pore size and pore ordering. 3 After

matrices for the inclusion of isolated

deposition, films were stabilized and

or interacting metal nanoparticles (NP),

calcined at 350 °C for 2 hours. Ag NPs

permitting to fully exploit their optical,

were deposited within single MOTF or

electronic or catalytic properties 1.The

multilayered stacks through an electroless

nano–derived properties are due to the metal

deposition reaction, using a 1:1 (mass ratio)

NP dimensions, confinement, interfacial

water:ethanol mixture of AgNO3 0.05 M

effects and the possibility to combine the

and a formaldehyde (HCHO) solution.

accessibility of the mesopore system and

Film thickness and electronic density

the protective properties of the matrix. A

were obtained from XRR measurements

variety of metallic or semiconductor NP

(λ = 1.5498 Å); film mesostructure was

have been embedded in mesoporous

analyzed using Small Angle X–ray Scattering

powders or thin films.1,2 An accurate control

with 2D detection (λ = 1.608 Å). Both

of NP size and pore filling is needed in

techniques were performed at LNLS.

order to control size–derived effects such

Transmission Electron Microscopy (TEM)

as electron transfer, surface plasmon

images were obtained with a Philips EM

resonance, fluorescence enhancement

301 microscope operating at 60 kV.

or surface–enhanced Raman scattering.

Figure 1a shows a TEM image of an

Consequently, non–destructive methods

F127–templated TiO 2 film where both

that permit to evaluate the actual metal

the mesostructure and uniform pore

loading inside the film are needed to

size can be clearly observed. The inset

better characterize these systems, and

shows the characteristic Im3m 2D–SAXS

understand their formation paths.

pattern from the same sample evidencing

In this work, we present a straightforward

the remarkable long range pore order.

approach for quantifying the filling of

Figure 1b shows a TEM micrograph of

mesoporous films with metallic NP using

Ag NPs synthesized within the TiO2 layer

X–ray reflectometry (XRR). This method

using HCHO as the reducing agent. The

permits to monitor the formation of Ag NP

NPs are 8–10 nm diameter, which is in

within a mesopore network, and hence to

good agreement with the known pore

control the pore fraction occupied by the NP.

dimensions of the F127 templated films

In addition, we demonstrate the possibility to

along the xy plane. The mesoporous matrix

determine the spatial localization of metallic

permits an homogeneous distribution of

NP inside a multilayer structure composed

NP with controlled maximum size inside

of different mesoporous oxides.

the MOTF.

24 | Activity Report 2009

Science Highlights

a

Figure 1.

b

Scheme of NP production within mesopores. a) TEM before and b) after Ag NP production inside mesoporous TiO2. Inset in (a) shows 2D SAXS (3° incidence) data typical of an Im3m cubic mesophase.

Despite the ample literature regarding metallic infiltration in mesostructured

volume fraction of mesopores, Fp, can be estimated as:

ordered films, no rigorous quantification of the metal loading in these systems has been informed. Analytical techniques

FP = 1−

ρfilm ρframework

(2)

such as EDS give an overall idea of film

where ρfilm is the electronic density of the

composition, but no details about NPs

mesoporous film and ρframework the electronic

location. In this context, XRR allows the

density of a non–mesoporous film with

simultaneous and direct determination of

the same composition.

density, thickness and interfacial roughness

Figure 2 shows the evolution of XRR

of thin films from the reflectivity intensity

data along reaction time for TiO2 or SiO2

variations with the X–ray incident angle.4

mesoporous films in contact with aqueous

The reflectivity critical angle θc measured

Ag+/HCHO solutions. Figure 2a shows a

allows determining the electronic density of the film, ρel, using:

ρel =

π 2 θc λ2re

significant and continuous increase in the measured θc for Brij 58–templated TiO2 films along reaction time. This implies that

(1)

the electronic density of the TiO2 porous film increases as Ag NPs are formed

where λ is the X–ray wavelength and re

inside the film. The θc values are close to

is the classical radius of the electron.

the porous titania values, and significantly

From the calculation of the ρel for both a

lower than those corresponding to pure

mesoporous and a non–porous film, the

Ag (θ c ~ 0.44°). In addition, no change Activity Report 2009 | 25


in the Kiessig interference spacing is a

b

observed, albeit the fringe amplitude decreases. These features imply that silver NP are continuously incorporated

c

within the mesopores, rather than forming a continuous film on top of the TiO2 film. The incorporation of Ag can be tracked by assessing the evolution of film density by following the changes in the θc with the Ag infiltration time. Figure 2d shows the time evolution of the reflectivity (θc region) for a CTAB–templated

d

e

SiO 2 mesoporous film in contact with the Ag+/HCHO solution. As opposed to TiO2 films, no significant changes in the SiO2–θc are observed, showing that the

f

film density remains almost unaltered within the reduction reaction time. Figure 2b shows a 2D–SAXS image at 90° incidence of TiO 2 mesoporous film before the silver infiltration process. The ring observed corresponds to the [110] in–plane pores of an Im3m cubic structure. After reaction in the Ag+/HCHO solution, the signal diminishes (Figure 2c) and, as the Ag+ concentration and time reduction increases, finally disappears. This effect is due to the random NP formation within the TiO2 pores, disrupting the periodic electronic density long–order structure, which is the origin of the diffraction signal. In addition, the signal intensity increases at low q values, probably due to

Figure 2.

Low–angle XRR patterns of mesoporous a) titania, and b) silica thin films exposed to Ag reduction for increasing periods. The arrows indicate the change in the critical angle θc. 2D SAXS patterns (90° incidence) of mesoporous TiO2 b) before and c) after silver infiltration. 2D SAXS patterns of mesoporous SiO2 e) before and f) after silver infiltration. g) Volume fraction of the pore system occupied by silver for each oxide, calculated from Equation 4. Dotted lines were added for eye guiding. Please note that the θc observed at 2θ =0.468° in (b) corresponds to the glass substrate.


the low–angle X–ray scattering produced by the Ag NPs. On the other hand, the signal intensity for the SiO2 layer does not change after the reduction reaction (Figures 2e,f), confirming that the Ag NP formation is strongly dependent of the inorganic framework.

Science Highlights

XRR can also be used to fully quantify

deposit Ag NP preferentially in the TiO2

the amount of silver present inside the

mesopores of a bilayered or multilayered

pores. Considering that the increase in

system composed of TiO 2 and SiO 2

film density is only due to the presence

layers. Figure 3a shows the XRR data

of metal, the silver volume fraction FAg

of a substrate/TiO2/SiO 2 mesoporous

within the film can be determined from

bilayer at different Ag reduction times.

the measured electronic densities:

No changes are observed for the θc of the silica layer along reduction; however,

FAg =

ρfilm + Ag − ρfilm ρAg

(3)

already observed in TiO2 single layer

where ρfilm+Ag is the electronic density of Ag–filled film, ρfilm is the electronic density of the empty mesoporous film and ρAg is the silver electronic density. The mesopore filling fraction at each infiltration time can be defined as the ratio between the silver volume fraction and the pore volume fraction:

R Ag (t) =

the TiO2–θc increases in a similar way as films. The preferential deposition of Ag NP within the titania mesopores alters dramatically the electronic density of the TiO2 film inside the bilayered stack (scheme in Figure 3e). Additional XRR experiments performed in the presence of water vapors confirm accessibility of the pore systems after Ag infiltration. The 2D SAXS signal from the bilayer

FAg (t) FP

(4)

before the infiltration is shown in Figure 3b. Two distinctive rings are observed, corresponding to the [110] in–plane pores

Figure 2g shows the evolution of the

of two different Im3m cubic structures.

mesopore filling fraction with Ag+ reduction

The lower–angle diffraction signal reflects

time calculated using Equation 4 for

the F127–SiO 2 mesostructure (cubic

the systems described above. It can be

parameter a = 18.5 nm) and the outer circle

observed that a plateau is reached by

corresponds to the Brij58–TiO2 characteristic

50–55% relative volume filling for TiO2

mesoporous distance (a = 9.1 nm). After

films, while SiO2 shows filling fractions

reaction in the Ag+/HCHO solution (Figure

well below 5%. This limit might be due to

3c), the TiO2 mesoporous signal disappears

partial pore blocking by silver NP. The

but the intensity for the SiO2 layer does

silver–loaded titania films present remaining

not change, confirming the clear different

accessible porosity, as confirmed with

behaviors of both oxide layers with

additional XRR experiments (not shown) in

respect to Ag NPs synthesis. The same

which the Ag–filled films were submitted

effect was observed by transmission 2D

to water vapor, and an increase in the

SAXS in photonic multilayered structures

critical angle was observed due to water

built using alternating TiO 2 and SiO 2

condensation.

mesoporous films.

The different reactivity of silica and

Figure 3d shows the time evolution

titania systems towards the Ag+/HCHO

of the silver filling in both oxides of the

systems can be exploited in order to

mesoporous bilayer. No production of Ag

Activity Report 2009 | 27


a

b

nanoparticles occurs in the SiO2 layer, but the TiO2 layer presents a plateau again, at approximately the same values obtained for the TiO2 single layer. Therefore, we

c

can conclude that each layer presents an independent chemical behavior in this particular bilayer system. The topmost SiO2 non reacting film is fully accessible and allows the diffusion of reactants to the TiO2 layer underneath, suggesting the possibility to design a “smart” coating for

d

an integrated nanodevice5,6. In conclusion, this work describes a chemical method to control the production of Ag/MOTF nanocomposites presenting spatially defined arrays of homogeneously distributed monodisperse nanoparticles. XRR and SAXS methods afford quantitative information about the Ag NP formation process in single and multilayered stacks. Formaldehyde oxidation proceeds at different rates in different MOTF matrices; this different reactivity on different oxide surfaces can be exploited in order to control the selective deposition of Ag NP within the titania layers in bilayered

Figure 3.

a) Low–angle XRR patterns of mesoporous TiO 2/SiO 2 bilayer exposed to Ag reduction for increasing periods.

or multilayered MOTF stacks in a single

The arrows indicate the change in the critical angle θc. 2D

and mild chemical step. We envision the

SAXS patterns (90° incidence) of mesoporous TiO2/SiO2

design of tailored multilayers with varying

bilayer b) before and c) after silver infiltration. d) Volume fraction of the pore system occupied by silver for each oxide, calculated from Equation 4. The dotted lines were added for eye guiding. e) Scheme of NP production only within the TiO2 mesoporous film.


and controlled Ag NP fractions and empty responsive layers for advanced optical devices and sensors.

Science Highlights

References 1. (a) Zhang, S. B. et al. One-pot synthesis of Ni-nanoparticle-embedded mesoporous titania/silica catalyst and its application for CO2-reforming of methane. Catalalysis Communications, v. 9, n. 6, p. 995-1000, 2008.

(b) Pérez, M. D. et al. Growth of gold nanoparticle arrays in TiO2 mesoporous matrixes. Langmuir, v. 20, n. 16, 6879-6886, 2004.

2. (a) Besson, S. et al. 3D quantum dot lattice inside mesoporous silica films. Nano Let ters, v. 2, n. 4, p. 409-414, 2002.

(b) Buso, D. et al. PbS-doped mesostructured silica films with high optical nonlinearity. Chemistr y of Materia ls, v. 17, n. 20, p. 4965-4970, 2005.

(c) Fukuoka, A. et al. Preferential oxidation of carbon monoxide catalyzed by platinum nanoparticles in mesoporous silica. Journal of the A merican Chemical Societ y, v.12, n. 33, p. 10120‑10125, 2007.

(d) Wang, H. W. et al. Synthesis and photocatalysis of mesoporous anatase TiO2 powders incorporated Ag nanoparticles. Jou rna l of Physics and Chemistr y of Solids, v. 6, n. 2-3, p. 633-636, 2008.

(e) Wang, L. C. et al. Formation of Pd nanoparticles in surfactant-mesoporous silica composites and surfactant solutions. Microporous and Mesoporous Materia ls, v. 110, n. 2-3, p. 451‑460, 2008.

3.

Fuertes, M. C. et al. Photonic crystals from ordered mesoporous thin-film functional building blocks. Advanced F unc tiona l Materia ls, v. 17, n. 8, p. 1247-1254, 2007.

4.

Daillant, J.; Gibaud, A. (Eds.). X- ray and neutron ref lec tivit y: principles and applications. Berlin: Springer, 2009. 348p.

6.

Angelome, P. C.; Fuertes; M. C.; Soler-Illia, G. J. A. A. Multifunctional, multilayer, multiscale: integrative synthesis of complex macroporous and mesoporous thin films with spatial separation of porosity and function. Advanced Materia ls, v. 18, n. 18, p. 2397-2402, 2006.

6.

Martínez, E. D.; Bellino, M. G.; Soler-Illia, G. J. A. A. Patterned production of silver−mesoporous titania nanocomposite thin films using lithography-assisted metal reduction. ACS Applied Materia ls & Interfaces, v. 1, n. 4, p.746-749, 2009.

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