Preparation of Silver-Montmorillonite Nanocomposites by Reduction with Formaldehyde and Borohydride

Montmorilonita rica em sódio (MMT) foi intercalada com cátions prata, os quais foram subsequentemente reduzidos com formaldeído ou boroidreto de sódio. Os nanocompostos prata-montmorilonita foram estudados por espectrometria UV-Vis, microscopia eletrônica de transmissão (TEM) e difratometria de raios-X (XRD). A influência de diferentes agentes redutores na dispersão e no tamanho das partículas de prata depositadas na superfície da montmorilonita foi investigada. A redução com boroidreto produziu partículas de prata dispersas uniformemente, com tamanhos limitados entre 3 e 13 nm. A redução com formaldeído gerou partículas dispersas irregularmente com uma distribuição de tamanho muito maior, variando entre 3 e 100 nm. Além disso, entre as partículas de prata preparadas por redução com formaldeído, algumas partículas de Ag 2 O foram obtidas como resultado de redução incompleta. Os compósitos de prata-montmorilonita preparados por redução com boroidreto e formaldeído contêm 2,4% e 5,3% em massa de Ag, respectivamente.


Introduction
Silver nanocomposites are used for many industrial applications, such as resonant and nonlinear optical elements, high-dielectric strength media for capacitors, enhanced electrical conductivity of ceramic media, pattern etching using HF, augmentation of electrode response, magnetic spin glasses, etc.A variety of conventional solgel preparation methods exists for Ag-nanocomposites fabrication, either as bulk powder or thin films, using organically modified gels or gel precursors. 1,2Another type of Ag-nacomposites are silver nanoparticles deposited on inorganic platforms, such as Al 2 O 3 , SiO 2 , SiO 2 /B 2 O 3 , SiO 2 /TiO 2 , TiO 2 , BaTiO 3 , etc. 1 Phyllosilicates, such as montmorillonite [2][3][4] and kaolinite, 5 have been also used as carriers for silver particles.
For the preparation of Ag-nanocomposites, silver nitrate is often used as the primary source of silver ions.There are several ways of Ag + reduction: use of gamma, 6 UV, 7 or femtosecond laser irradiations, 8 heating, 9 electrochemical reduction, 10 application of reducing chemicals, such as hydrazine, 11 sodium borohydride, 5 glycerol, 3,12 N,Ndimethylformamide, 13 glucose, 14 ethylene glycol, 4 folmadehyde, 15,16 sodium in liquid ammonia, 17 etc.In addition, reduction of [Ag(NH 3 ) 2 ] + by various saccharides was described by Paná ek et al. 18 The aim of this work was to prepare a temperature-stable silver nanocomposite consisting of silver nanoparticles deposited on an inorganic carrier.For this purpose, silver ions were adsorbed on monmorillonite (MMT) and reduced by borohydride or formaldehyde.The size and dispersity of silver particles located on the phyllosilicate surface were studied.The development of methods for the preparation of silver nanoparticles of required diameter ranges is desired.

Adsorption procedure
Na + -rich montmorillonite SWy 2 (Crook County, Wyoming) with exchange capacity of 1.2 mequiv g -1 , determined by saturation with NH 4 + 19 and by analysis of released metals (Na + , K + , Ca 2+ , Mg 2+ ), was used for the adsorption of silver cations.The MMT formula is Na 0.38 K 0.04 (Ca 0.12 Mg 0.50 Fe 3+ 0.41 Al 2.90 Ti 0.01 Mn 0.01 )Si 8 O 20 (OH) 4 , as calculated from the results of X-ray fluorescence analysis.A fraction of MMT (particle size < 5 µm) separated by sedimentation was used.
The adsorption procedure was adopted from our recent work. 2 A portion of MMT (about 0.1 g) was added into 100 mL of 0.010 mol L -1 AgNO 3 solution and this suspension was shaken vigorously for 24 h.The suspension was then centrifuged (4500 min -1 ) for 20 min, filtered through 1.2 µm glass fibre filters (Whatman), and dried gently at 50 o C for 24 h.All experiments were performed at room temperature varied from 20 o C to 24 o C. Montmorillonite containing silver ions is labelled Ag + -MMT.

Reduction procedure
Two procedures for silver reduction were tested: the reduction with formaldehyde and the reduction with NaBH 4 .Formaldehyde reduction consisted of the following steps: (i) 10 mg of Ag + -MMT were placed into a test tube, (ii) 5 mL of 0.37% formaldehyde solution and (iii) 0.5 mL of alkaline reagent (NaOH, NaHCO 3 , TEA) were added, and the suspension was mixed and shaken.For the reduction with NaBH 4 , 10 mg of Ag + -MMT were placed into a test tube and 5 mL of NaBH 4 solution (0.010 mol L -1 ) were added and shaken. 2 This reaction is vigorous and takes only several min.Ag-MMT suspensions were analysed by UV-visible spectrometry.
After reduction, Ag-MMT was filtered out through 1.2 µm glass fibre filters and washed several times with water to remove residual Ag + ions, which were then analysed in the filtrates by silver electrode (Theta 90, Czech Republic).Silver was also analysed in the filtrates, after dissolution with HNO 3 , by atomic absorption spectrometry (see below).Silver particles adsorbed on MMT were further studied by transmission electron microscopy and X-ray powder diffraction.
Ag-montmorillonite composites prepared with borohydride were labelled as Ag-MMT/NaBH 4 , while those prepared with formaldehyde were labelled as Ag-MMT/F.The products of reduction with formaldehyde were also labelled as F-NaOH, F-NaHCO 3 and F-TEA, depending on the alkaline reagent used.

UV-visible absorption spectrometry
UV-visible absorption spectra were measured with a Lambda 25 double-beam spectrometer (Perkin Elmer, USA).All spectra were recorded using 1 cm quartz cuvettes within the range of 200 nm to 800 nm.The cuvettes were filled with shaken suspensions and the spectra of MMT, Ag + -MMT and Ag-MMT were recorded immediately against distilled water.

Atomic absorption spectrometry
An atomic absorption spectrometer Spectra AA30 (Varian Inc., USA) was used for the determination of silver in the filtrates using a standardised method. 20Air-acetylene flame was employed as the atomisation technique.

Transmission electron microscopy
Transmission electron microscopy analysis was carried out on a JEOL JEM 3010 microscope operated at 300 kV (LaB 6 cathode, point resolution 0.17 nm) with an EDX (Energy Dispersive X-ray) detector attached.Images were recorded on a CCD camera with the resolution of 1024 × 1024 pixels using the Digital Micrograph software package.
Powder samples were dispersed in ethanol and the suspension was treated with ultrasound for 10 min.A drop of a very dilute suspension was placed on a holey-carboncoated copper grid and allowed to dry by evaporation at room temperature.Selected-area electron diffraction Vol. 20, No. 7, 2009   (SAED) patterns were analyzed using the Process Diffraction software package. 21

X-ray powder diffraction
The X-ray powder diffraction study was performed using a powder diffractometer INEL CPS 120 (INEL, France) equipped with a curved position-sensitive detector PSD 120 MB/11 (reflection mode, Ge-monochromatized, Cu-K α1 radiation), which allows simultaneous data collection over 0° to 120° with steps of 0.031° in 2-theta.The diffraction patterns were acquired in ambient atmosphere under constant conditions (2000 s, 35 kV, 20 mA).

Statistical software
All statistical calculations were made at α = 0.05 significance level by the program QC.Expert 2.5 (Trilobyte Statistical Software, Ltd., Czech Republic).

Results and Discussion
In the first step of nanocomposite preparation, silver cations were adsorbed on montmorillonite (Ag + -MMT) by shaking in AgNO 3 solution for 24 h.Thereafter, Ag + -MMT was filtered out and dried, and silver cations were reduced by addition of aqueous solutions of formaldehyde or sodium borohydride.

Reduction of silver ions
The reduction with formaldehyde is described by the following reactions: 14,22 Alkaline conditions, which are essential for reaction 1, were provided by addition of sodium hydroxide, sodium bicarbonate or triethanolamine (TEA).
The reduction with borohydride consists of (i) in-situ generation of hydrogen by the decomposition of NaBH 4 and (ii) reaction of hydrogen and silver ions: No other additives are necessary.Ag-MMT composites were separated from the dispersions by filtration (see Experimental).The efficiency of the reduction was determined by electrochemical analysis of residual silver cations in the filtrates with a silver electrode.The limit of quantification (LOQ) of the method was estimated at 1×10 -6 mol L -1 of Ag + , which corresponds to 0.02 wt% of silver in Ag-MMT.The concentrations of residual Ag + were lower than LOQ in all filtrates.It corresponds to a reduction efficiency ≥ 99.98%.
The content of silver in each Ag-MMT composite was determined by AAS after its dissolution in a mixture of HF, HNO 3 and HClO 4 (5:4:4). 23The reproducibility of the Ag-MMT preparation was estimated at 2.6%.The amount of Ag in Ag-MMT significantly depends on the reducing reagent used.While using borohydride, 2.4 wt% Ag (0.22 mmol g -1 ) was found.The content of Ag formed by formaldehyde was similar in all Ag-MMT samples regardless of the employed alkaline agent, i.e., 5.3 wt% (0.49 mmol g -1 ).This difference is given by the size of silver particles.Small Ag particles of Ag-MMT/NaBH 4 probably passed through the filter pores into the filtrates in higher amounts than those of Ag-MMT/F.This was also confirmed by TEM (see below) and by analysis of Ag in the filtrates.A higher content of Ag in Ag-MMT should be achieved by filtration through denser filters.

UV-visible absorption study
It is well known that the surface plasmon of silver nanosized particles exhibits a characteristic adsorption band around 410 nm.Therefore, the optimal reaction time and concentrations of all reagents could be tuned by recording the UV-visible absorption spectra of Ag + -MMT suspensions.
Graphs showing the changes of absorbance versus reaction time are shown in Figures 1a-c.In the case of formaldehyde, the absorbance increases during the first two hours and then stays nearly constant.Therefore, two hours were selected as the sufficient reaction time for the quantitative reduction.The optimum concentrations of the bases were chosen so that the highest reduction efficiency, expressed by the highest absorbance around 410 nm, was reached: 0.10 mol L -1 for both NaOH and NaHCO 3 , and 0.50 mol L -1 for TEA.At the higher concentrations, the absorption band becomes lower and broader.The optimum conditions for the reduction with borohydride (0.01 mol L -1 ) have been studied recently. 2 max ca.400 nm.The absorption maxima related to formaldehyde reduction are shifted to longer wavelengths depending on the base employed: λ max ca.412 nm for NaOH and TEA, λ max ca.425 nm for NaHCO 3 .The shifts in the λ max of the silver plasmon to longer (red shift) and shorter wavelengths (blue shift) have already been described in the literature. 22,24,25There are several reasons for these effects: electron density on the surface of nanoparticles, irregular shape and size of nanoparticles, media constants of solutions, refraction coefficients, etc.
Besides the shifts, broadening of the plasmon peak was also observed, which is in agreement with the findings of other authors. 11,16Such broadening might be caused by the aggregation of silver particles.The narrowest peak belongs to silver particles reduced by NaBH 4 .The other silver peaks become broader in accordance with the alkaline agents used in combination with formaldehyde: NaOH < TEA < NaHCO 3 .
In addition, there is a band at about λ max = 250 nm in the absorption spectra (Figure 2), which most likely originates from light scattering on montmorillonite particles.The absorption band at around 300 nm, which has been attributed to residual Ag + ions 14 or to the existence of Ag nanoparticles smaller than 2 nm, 25 was not found at all.
Based on the data mentioned above, the following can be said.Borohydride reduction is much faster than that by formaldehyde, and thus silver particles are expected to be smaller, as indicated by the narrower absorption peak and the shorter λ max .As the concentration of formaldehyde was identical in all experiments, the concentration of hydroxide ions should be a key parameter for the formaldehyde reduction rate (see reactions 1 and 2).Taking into account the strength of the bases (TEA with pK b = 6.24 and NaHCO 3 Figure 1.(a) Time dependence of silver plasmon absorbance after the reduction with formaldehyde using NaOH: ( ) 0.010 mol L -1 , ( ) 0.050 mol L -1 , ( ) 0.10 mol L -1 , ( ) 0.20 mol L -1 , ( ) 0.30 mol L -1 , and ( ) 0.40 mol L -1 ; (b) time dependence of silver plasmon absorbance after the reduction with formaldehyde using NaHCO 3 : ( ) 0.010 mol L -1 , ( ) 0.050 mol L -1 , ( ) 0.10 mol L -1 , ( ) 0.20 mol L -1 , ( ) 0.50 mol L -1 ; (c) time dependence of silver plasmon absorbance after the reduction with formaldehyde using TEA: ( ) 0.05 mol L -1 , ( ) 0.10 mol L -1 , ( ) 0.50 mol L -1 .

UV-visible absorption spectra
The absorption spectra of silver particles are presented in Figure 2. The absorption maximum corresponding to borohydride reduction occurs at wavelength with pK b = 7.65), the concentration of hydroxide ions in the aqueous solutions of the weak bases can be estimated from the well-known equation: pOH ≈ 0.5 (pK b -log c B ), where c B is the concentration of the weak base.The calculated concentration of OH -ions, c OH , decreases in the order: NaOH (c OH ≈ 10 -1 mol L -1 ) > TEA (c OH ca. 10 -3.3 mol L -1 ) > NaHCO 3 (c OH ≈ 10 -4.3 mol L -1 ), which is consistent with the order of absorption band broadening.For comparison, λ max values increase in the order: NaBH 4 < NaOH ≈ TEA < NaHCO 3. Therefore, it can be assumed that the size of silver particles increases in the order NaBH 4 < NaOH < TEA < NaHCO 3 .The particles diameter and their dispersity were further investigated by TEM.

TEM study
TEM micrographs of silver particles deposited on montmorillonite surface are shown in Figure 3, together with the corresponding SAED patterns as insets.In the SAED patterns, which are compared to the XRD diffraction patterns in Figure 4, crystalline cubic Ag and montmorillonite were detected.The apparent miss of nonbasal peaks of montmorillonite in SAED against XRD is due to the strongly preferred orientation of montmorillonite in TEM samples, which were prepared by sedimentation from suspension.The diffraction peaks of Ag are not well defined, which suggests a number of structural defects.
In addition, a diffraction peak corresponding to AgO/ Ag 2 O was observed in the sample Ag-MMT/F-NaHCO 3 .
This peak very likely indicates the presence of Ag 2 O as a result of incomplete reduction according to reaction 2. The fact that the oxide is not found in the SAED patterns of Ag-MMT/NaBH 4 (Figure 4) suggests that silvermontmorillonite composites are stable against oxidation.The composites were repeatedly analyzed by TEM during five months after their preparation.They were stored in common Eppendorf tubes, in the dark, at room temperature.
The size distribution of silver nanoparticles was quantified using images in the 100 nm scale.About 150 particles were analysed for each sample.As the particles are more or less isometric (Figure 3), their size can be represented by the diameter of a circle with the same area.The sizes were plotted into a histogram with a 1 nm interval to characterize the size distribution of silver nanoparticles (Figure 5).Statistics was calculated on the diameters as well as their logarithms.Generally, if the distribution is plotted against a logarithmic scale, the maxima are closer to a Gaussian curve and the statistical results (Table 1) appear more meaningful than those based on a linear scale.
The reduction with formaldehyde leads to a wide particle size distribution (3-102 nm) and a lower dispersion on the MMT surface (Figure 3a).The reduction with    NaBH 4 produces silver nanoparticles with sizes from 3 nm to 13 nm (the mean size is 5.4 nm), highly concentrated and evenly dispersed on MMT (Figure 3b).The statistical data are summarised in Table 2.In comparison with other authors, 4,14,18 the particle size distribution is quite narrow.For the production of particles with a diameter range of several nanometers, 11 the method of NaBH 4 has to be further optimised.
The particle size distributions correspond well with the spectral band broadening (Figure 2), except of the sample prepared by reduction with formaldehyde and NaOH.According to the absorption spectra of this sample, the formation of much smaller particles than those observed by TEM was expected.This discrepancy could be explained by the defect microstructure of silver particles, such as twining (Figure 3c), likely originated by their fast formation due to the high hydroxide concentration.The existence of particles composed from Ag and Ag 2 O should be also expected.These big defective particles absorb light as the aggregates of individual smaller ones welded together; therefore, the absorption band most probably represents these individuals rather than the big particles.

Figure 2 .
Figure 2. UV-visible absorption spectra of silver particles produced under different conditions.

Table 2 .
Statistics of silver particle size distribution based on logarithms of particle diameter