Control of the Size of Silver Nanoparticles and Release of Silver in Heat Treated SiO2-Ag Composite Powders

The growth of silver nanoparticles, the activation energy for silver particle growth, and the release of silver species in heat treated SiO2-Ag composite powders are investigated. The silver particle growth is controlled by heat treatment for 75 min of the as-synthesized SiO2-Ag composite powder at 300–800 °C. During heat treatment the mean size of the Ag particles increases from 10 nm up to 61 nm with increasing temperature, however, the particle size distribution widens and the mean size increases with increasing heat treatment temperature. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, silver particles are crystalline and in a metallic state after annealing in all SiO2-Ag composite powders. The growth of Ag particles is suggested to take place via diffusion and Ostwald ripening. The activation energy for particle growth was determined as 0.14 eV. The dissolution of silver in aqueous solutions from the SiO2-Ag composites heat treated, at 300 °C, 600 °C, and 700 °C, was investigated by varying pH and temperature. The dissolution was reduced in all conditions with increasing silver particle size, i.e., when the total surface area of Ag particles is reduced. It is suggested that the dissolution of silver from the composite powders can conveniently be adjusted by controlling the Ag particle size by the heat treatment of the composite powder.


Introduction
Silver nanoparticles have attracted much attention during the past decade due to their versatile applications as antibacterial agents, in drug delivery, catalysis, optoelectronics, thermally conductive nanofluids and sensors [1][2][3][4][5]. Silver nanoparticles exhibit localized surface plasmon resonance (LSPR) when they are in interaction with electromagnetic radiation [2]. The unique optical properties are dependent on the size and shape of the nanoparticles [4]. Amongst other noble metals, silver has strongest surface plasmon band, giving superior performance in surface-enhanced Raman spectroscopy (SERS) sensing applications [2,6]. Silver nanoparticles have a large specific surface area, making them suitable for enhanced catalytic applications [5]. The antibacterial effects of Ag have also been known since ancient times and silver is being used in various medical devices, such as catheters [7] and in dental filling materials (glass ionomer cement) [8]. The antibacterial properties of silver are based on different forms of silver: silver, silver ions, and silver radicals [9,10]. Silver ions are prone to react with

Morphology and Structure
The size of the SiO 2 particles in the SiO 2 powder was determined by measuring the diameter of fifty SiO 2 particles from a scanning electron microscopy (SEM) image. An SEM image of the pure SiO 2 powder sample with a mean size of 670 nm is shown in Figure A1. SEM images of all heat treated SiO 2 -Ag composite powders at the same magnification are shown in Figure 1. The Ag particles appear as light spots and silica in grey. The Ag particles are observed on top of the SiO 2 matrix. The modified Stöber process and the subsequent heat treatment produces irregularly shaped SiO 2 -Ag clusters in various sizes, contrary to the reference SiO 2 particles, which are spherical without agglomeration. The reaction mechanism for silver nitrate in the water-ethanol-ammonia solution is described in Equations (1)-(3) [31]. AgNO 3 reacts with NH 4 OH forming the unstable AgOH (Equation (1)). AgOH then reacts further to form Ag 2 O (Equation (2)). Ag 2 O dissolves in the excess NH 4 OH and reacts forming the [Ag(NH 3 ) 2 ]OH complex (Equation (3)). The [Ag(NH 3 ) 2 ]OH complex is the source for the silver particles throughout the silica matrix. The as-synthesized SiO 2 -Ag composite powder is white in color, however, the color of the powder changes to brownish-yellow upon heat treatment at 300-800 • C, indicating growth of silver particles.  Figure 2 shows the X-ray diffraction patterns of the heat treated SiO 2 -Ag powders. The hump at about 2θ 22 • is attributed to the amorphous SiO 2 matrix. The amorphous hump is sharpened upon the increased heat treatment temperature and the peak position moves slightly to smaller 2θ values, suggesting gradual ordering in the amorphous SiO 2 structure. No silver peaks are detected after heat treatment below 600 • C, however, at or above 600 • C the silver peaks appear. The silver peaks (2θ) are found at 37.9 • , 44.1 • , 64.2 • , 77.1 • and 81.2 • , corresponding to (111), (200), (220), (311) and (222) crystalline planes (pattern No. 01-087-0597). The absence of diffraction peaks for silver at lower heat treatment temperatures is attributed to the small size of silver, which is consistent with previous research [32]. The silver peaks become more prominent with increasing temperature, indicating increased silver particle size.
Silver can also be observed by probing the surface plasmon resonance (SPR) peak around 410 nm by UV-vis spectroscopy. The optical absorption spectra of the powders are shown in Figure 3. The intensity of the broad absorption band starting at 300 nm decreases with heat treatment temperature. The silver plasmon peak is visible in SiO 2 -Ag powder samples heat treated at 600 • C, 700 • C and 800 • C, which is consistent with the XRD results ( Figure 2). The silver plasmon peak position blue-shifts slightly from 410 nm for SiO 2 -Ag 600 to 406 nm for both SiO 2 -Ag 700 and SiO 2 -Ag 800. The plasmon peak intensity increases with increasing average size of the Ag NPs. The plasmon peak is sustained due to Ag NPs within SiO 2 , even when the silver particles from the surface are dissolved (Supporting information, Figure A2). The plasmon peak positions are slightly blue shifted; SiO 2 -Ag 600 from 410 to 407 nm, SiO 2 -Ag 700 from 406 nm to 390 nm, and SiO 2 -Ag 800 from 406 nm to 403 nm. The SiO 2 -Ag 700 sample displayed a blue shift of 16 nm, suggesting that smaller sized Ag particles are present after dissolving Ag particles from the surface than before dissolution [33]. If the spherical shape of the Ag NPs is retained, this means that it takes longer for the Ag particles inside the silica matrix to grow compared to those on the surface. This is justified by the probable differences in diffusion rate of Ag in the bulk and the surface as well as the higher constrains of particle growth in the bulk, as opposed to on the free surface. On the other hand, it is known that the thickness of the silica on the silver particles also affects the plasmon peak position due to scattering [33][34][35].  The growth of Ag NPs and the detailed representation of the SiO 2 -Ag materials are depicted in the transmission electron microscopy (TEM) images in Figure 4. The Ag NPs appear as darker spots in various sizes in and on the SiO 2 matrix. The electron diffraction pattern in the insert of Figure 4b illustrates the crystalline nature of silver particles heat treated at 300 • C. The growth of the Ag particles is studied by measuring the Ag NP size distribution from several TEM and SEM micrographs and is given in Table 1. The size of Ag particle is represented by its diameter. Altogether, 60-495 Ag particles per sample were measured. There is a clear growth in mean Ag NP size after heat treatment, however, small Ag NPs are present after all heat treatment temperatures. They are evenly distributed on the surface of the SiO 2 particle surface up to 600 • C. At 700 • C and 800 • C, the Ag particles have grown into larger clusters and have started to desquamate from the surface. Nevertheless, small Ag NPs are still present in the samples. According to X-ray photoelectron spectroscopy (XPS) analysis, the composite powder surfaces consisted of silica and silver only, see Figure 5. The only significant changes observed at the surface were related to silver. Firstly, the measured amount of silver at the surface was 1.8 at. % in the non-heat treated reference, and it decreased monotonously from 1.3 at. % to 0.4 at. % when heat treatment temperature rose from 300 • C to 800 • C. Increase in the Ag particle size due to heat treatments, combined with the XPS analysis depth of only a few nanometers explains this result, which is in good accordance with the observed Ag particle sizes in TEM. As for the surface chemistry, we found no signs of silver oxidation, but in the non-heat treated reference the Ag 3d 5/2 signal was observed at 268.9 eV, while it appeared at a slightly lower binding energy, 268.5 eV, in all heat treated samples, see Figure 5b. A similar shift has been reported on some silver alloys [36,37] and on some organic complexes containing oxygen, so we believe that the higher binding energy in the non-heat treated sample is due to the interaction between colloidal silver and the SiO 2 matrix. The interaction between the Ag NPs and the SiO 2 matrix and functional groups in the composite samples were investigated by Fourier Transform Infrared spectroscopy (FTIR), as observed in Figure A3.
The two strong bands between 1000-1200 cm −1 and 750-850 cm −1 are ascribed to the asymmetric and symmetric Si-O-Si stretching vibrations, respectively [38][39][40]. The band around 950 cm −1 originates from the Si-OH bonds [39,40]. The peaks get weaker upon heat treatment till 400 • C and the peak disappeared for the samples heat treated at 500 • C, indicating -OH group removal with heat treatment. Figure A3b shows a small band around 554 cm −1 ascribed to the Si-O-Ag linkages [40]. The band appears in the SiO 2 -Ag composites suggesting bonding between the Ag NPs and the oxygen bonded to silicon, which supports the interpretation of the XPS spectra.
The surface area of the SiO 2 -Ag powders was studied by BET analysis (Table 1). A specific surface area of 5.2 m 2 g −1 was obtained for a pure SiO 2 sample prepared without the addition of AgNO 3 . The results show that the specific surface area of the SiO 2 -Ag powders decrease when the heat treatment temperature is increased, with the exception of the SiO 2 -Ag 800 powder. The surface area decrease from 5.2 m 2 g −1 for SiO 2 to smaller values for the composite powders may be due to agglomeration of the SiO 2 matrix. The change in specific surface area is investigated after the powders were HNO 3 treated to remove the Ag on the surface of the composites. The surface area decreases for all SiO 2 -Ag samples, except for the as-prepared SiO 2 -Ag powder and SiO 2 -Ag 700 powder. The decrease in specific surface area is probably due to the removal of Ag NPs from the surface, together with possible densification and sintering of the powders, however, it should be noted that the accuracy of BET measurements are typically ±5%. The slight increase in the specific surface area of the two samples after dissolution may result from the surface area of dents remaining after removal of Ag NPs from the surface. The largest difference in surface area is found for the SiO 2 -Ag 300 powder, as the removal of the smallest Ag particles from the SiO 2 surface provides the largest surface area. Table 1. Statistics of silver nanoparticle sizes, amount of measured particles (N total) and their standard error (SE). The specific surface area for the SiO 2 -Ag powders heat treated at different temperatures (measurement accuracy ±5%). The specific surface area determined by nitrogen adsorption-desorption is also presented for the SiO 2 -Ag samples with the surface Ag dissolved by HNO 3 (marked with "-dis").

T N Total Mean ± SE Min Max Surface Area Surface Area-Dis
RT The formation and growth of particles can be divided into three stages: nucleation phase, diffusion-based growth and Ostwald ripening [41,42]. Diffusion-based growth takes place at large supersaturation values, whereas Ostwald ripening proceeds when smaller particles dissolve to form larger particles [42]. The nucleation step occurs during the synthesis or drying step of the as-prepared SiO 2 -Ag sample, considering the as-prepared powder contains Ag particles after drying as confirmed by XPS and TEM. The Ag particle size (diameter) increases during heat treatment of the SiO 2 -Ag powder in air.
Let us assume that the growth of Ag NPs follows a typical kinetic law of type where D is the Ag particle size, t is time, and k and n are constants at constant temperature. The temperature dependence of the growth is hidden in term k. If the temperature dependence follows an Arrhenius type of relation, we can write the equation as: where k B is the Boltzmann constant, Q A the activation energy, T is the temperature and A is a constant. By insertion of (5) into (4), integrating and rearranging, we obtain the following equation, assuming a constant heat treatment time: where D is the particle size, D 0 is the particle size at t = 0, and A t is a constant. The activation energy Q A is obtained from the slope of the graph in Figure 6b, i.e., by plotting ln(D − D 0 ) as a function of 1/T. Based on the linear fit in Figure 6b, an activation energy of 0.14 eV is obtained for the Ag NP growth. Activation energies for Ag particle nucleation and growth in silica implanted at room temperature with energetic Ag ions have previously been found as 0.064 eV below 800 • C and 0.40 eV above 800 • C [43]. The study was carried out by Ag ion-implantation into silica glass substrates, where subsequent heat treatment leads to diffusion of the implanted atoms, nucleation and growth of Ag crystallites. Other studies [44][45][46][47][48] have found significantly higher activation energies of 1.8-5.2 eV for Ag particle growth in Ag-doped SiO 2 films on soda-lime glass. However, there are substantial differences between these studies: (i) the nucleation and growth of Ag are investigated in SiO 2 films, i.e., matrix affects the nucleation [48]; (ii) some studies [48] deal with narrow temperature ranges (e.g., 570-600 • C); (iii) holding times are varying; and (iv) materials are made with different preparation methods.
Furthermore, Ag is prone to oxidation. In another report, oxidation of Ag was described during heat treatment in air in a 55 nm SiO 2 nanofilm [42]. The oxidation occurred by oxygen uptake by the silica nanofilm restricting Ag growth. However, oxidation of Ag was prevented by shortening the heat treatment times down to 10 min. and keeping the heat treatment temperature above 550 • C [42,48]. The SiO 2 matrix seems to protect Ag from oxidation at lower temperatures even for the heat treatment time of 75 min. used in this work, as the XPS results shown in Figure 5 affirm. During Ostwald ripening, particles grow larger in size due to the dissolution of smaller particles. The Ostwald ripening stage can be distinguished in TEM images, which display larger Ag particles with a low concentration of small particles. A close study of Figure 4 and other TEM images not shown here, suggests that a large number of smaller Ag particles are sustained up to 600 • C, but are more scarce in SiO 2 -Ag 700 and SiO 2 -Ag 800. However, in each case, there are still small Ag particles present. Ostwald ripening can also be detected spectroscopically by observing the SPR peak of Ag. The Ag particle size increase should present higher SPR peak intensities, whereas a decrease in particle concentration should lower the SPR peak intensity. The SPR peak in Figure 3 provides this information, as the concentration of powder to solvent is kept constant at 1 mg · mL −1 . The Ag SPR peak intensity increases as the temperature is increased from 600 • C to 800 • C, suggesting the presence of larger particles at elevated temperatures. The increase in SPR peak intensity thus supports the conclusion that Ostwald ripening takes place during heat treatment. Additional experiments are needed in order to identify detailed growth mechanisms and the role of different forms of diffusion in the process.

Release of Silver
The release of silver (Ag 0 and Ag + ) from the SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 powder samples were examined to obtain the concentration of silver from the surface of the SiO 2 matrix. Control samples were measured to obtain a maximum silver concentration leached from the SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 powder samples. The theoretical maximum for the control samples with 3.3 mg · L −1 SiO 2 -Ag loading was 285 mg · L −1 Ag, corresponding to 431 mg · L −1 for a 5.0 mg · L −1 SiO 2 -Ag loading. The obtained release from the control samples with 3.3 mg · L −1 SiO 2 -Ag loading was 212 mg · L −1 , 19.1 mg · L −1 , and 192 mg · L −1 from the SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 samples, respectively. The calculated values were 321 mg · L −1 , 29 mg · L −1 , 290 mg · L −1 for the SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 samples, respectively, for a 5.0 mg · L −1 SiO 2 -Ag loading. The silver concentration was unexpectedly ca 10 times lower for the SiO 2 -Ag 600 control sample than SiO 2 -Ag 300 and SiO 2 -Ag 700 control samples. Comparing the obtained release and the theoretical release of the control samples, there is an indication that the silver species in SiO 2 -Ag 300 and SiO 2 -Ag 700 is mostly on the surface since up to 74% of silver was released during acid digestion. However, the case of SiO 2 -Ag 600 is a bit peculiar with a release of only 6.7% of the theoretical maximum. The control samples were prepared in triplicate, so it is unlikely to be a systematic error. This would suggest that the silver has diffused into the silica matrix, as previously observed by Babapour et al. in the case of Ag in a SiO 2 film [49].
The SiO 2 -Ag 600 was imaged by TEM after HNO 3 digestion as shown in Figure 7. Figure 7a,b show the SiO 2 -Ag 600 sample before acid digestion and Ag particles are observed both on the surface and inside the silica matrix. Ag is dissolved from the surface after acid digestion in Figure 7c,d, however, Ag particles are still observed inside the silica matrix. Hence, only a fraction of the theoretical maximum of Ag in the sample is on the surface of the silica matrix.
The first leaching test was performed in de-ionized water at room temperature for one, three and seven days, which is marked in Figure 8 as DI, RT. The results are depicted in Figure 8 and Figure A4, and listed in Table A1. Figure 8a shows the highest release of silver species for the SiO 2 -Ag 300 sample. The SiO 2 -Ag 300 shows a release of 6.0 mg · L −1 already after one day, increasing to 44.1 mg · L −1 for dissolution at seven days. The corresponding silver concentrations for the SiO 2 -Ag 600 sample is from 1.2 mg · L −1 to 2.1 mg · L −1 and for the SiO 2 -Ag 700 sample from 1.0 mg · L −1 to 1.6 mg · L −1 for 1 day to seven days dissolutions. This means that the SiO 2 -Ag 300 displays from six times up to 27 times larger silver release as compared to the SiO 2 -Ag 700 sample. The SiO 2 -Ag 300 sample has the smallest average silver particle size of the studied SiO 2 -Ag samples on the SiO 2 surface, indicating that smaller Ag NPs (10 nm) in the SiO 2 -Ag 300 dissolve or desquamate readily from the composite compared to larger NPs in SiO 2 -Ag 600 and SiO 2 -Ag 700. The release of Ag from SiO 2 -Ag was also investigated at elevated temperatures of 40 • C and 60 • C. The SiO 2 -Ag 300 and SiO 2 -Ag 600 composites displayed a trend of increasing Ag concentration with longer dissolution time. The concentration of Ag in the SiO 2 -Ag 300 sample increased with increasing temperature. The SiO 2 -Ag 600 sample released similar amounts of Ag in both 40 • C and 60 • C. The Ag concentration increased at 40 • C for the SiO 2 -Ag 700 sample, however, at 60 • C the concentration is unchanged. The temperature change appears to have a larger effect on smaller Ag particles leading them to readily dissolve, whereas a temperature change does not have the same effect for SiO 2 -Ag 600 and SiO 2 -Ag 700 samples.
The Ag concentration is expected to be larger at pH 4, as Ag dissolves in acidic environments. All samples displayed increasing Ag release with increased dissolution time, SiO 2 -Ag 300 having the most extensive release of Ag with 74.3 mg · L −1 at seven days. The SiO 2 -Ag 600 sample measured 17.9 mg · L −1 and SiO 2 -Ag 700 12.6 mg · L −1 at seven days, which correlates with the increasing Ag size. At pH 10, SiO 2 -Ag 300 displayed the largest Ag release of 28% of the theoretical maximum release of Ag and 38% of the Ag concentration in the control sample. The SiO 2 -Ag 600 and SiO 2 -Ag 700 samples did not follow this trend, i.e., they exhibited the largest release at pH 4.

Preparation of SiO 2 -Ag Composites
The SiO 2 -Ag composite powders were prepared by a modified Stöber method [50]. In brief, The product was then centrifuged and dried at room temperature. The dried product was heat treated in air at temperatures between 300 and 800 • C for 75 min. The dried powder was white in color and turned into deeper brownish-yellow color with increasing heat treatment temperature. A sample of SiO 2 powder was prepared with the same recipe excluding the AgNO 3 addition. The powder samples are named according to the heat treatment temperature, e.g., "SiO 2 -Ag 300" is heat treated at 300 • C for 75 min.

Dissolution of Silver
Tests for the dissolution of silver from the SiO 2 -Ag composite powders were performed on SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 samples with a sample loading of 5.0 g · L −1 . The release studies were made in aqueous solutions. All tests were made in triplicates for one, three and seven days. Test temperatures were room temperature, 40 • C and 60 • C. The media was de-ionized water and de-ionized water with the pH adjusted to 4 with HNO 3 or to 10 with NaOH. The mixtures were then filtered through 0.45 µm nylon syringe filters, acidified with HNO 3 and silver concentration analyzed. The control samples (sample loading of 3.3 g · L −1 ) were prepared by dissolving the silver from the SiO 2 -Ag samples in concentrated HNO 3 .

Characterization
The powders were characterized utilizing X-ray Diffraction (XRD), which was carried out on a PANalytical X'pert Powder Pro diffractometer (PANalytical, EA Almelo, The Netherlands) using Cu κ(α 1/2 ) λ(α 1 ) = 1.54060 Å radiation in the range of 20-90 • (2`). Hitachi S-4700 cold field emission gun scanning electron microscope (FEG-SEM, Tokyo, Japan) was used to study particle sizes, structure and appearance of the powders, while a Tecnai F20 G2 200 kV FEG (Tecnai, Hillsboro, OR, USA) transmission electron microscope (TEM) was used to analyze Ag particle size, morphology and the composition of selected powders. The Ag particle sizes were determined by measuring the diameter of the Ag particles with the ImageJ measurement tool from a number of TEM and SEM images. The TEM samples were prepared by dispersing the SiO 2 -Ag powder sample in ethanol by ultrasonication for 3 min and placing a drop of the dispersion on the TEM grid. The UV-vis absorbance spectra for the silver plasmon peak measurements were carried out on a Hitachi U-5100 UV-vis spectrophotometer. Surface chemical compositions of composite powders were determined with X-ray photoelectron spectroscopy (XPS), using AXIS Ultra electron spectrometer by Kratos Analytical (Manchester, UK) and 285.0 eV as the reference value for the CC component in C 1 s [51]. The FTIR measurements were carried out on a Nicolet 380 FT-IR (Thermo Fisher Scientific, Waltham, MA, USA). The specific surface area and porosity of the SiO 2 -Ag samples were examined by nitrogen adsorption-desorption isotherms at 77 K using a TriStar II 3020 equipment and the SiO 2 were analyzed using Bel Belmaster Mini II equipment (BEL, Tuggerah, Australia). Brunauer-Emmet-Teller (BET) analysis was used for the specific surface area. The silver concentrations were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) utilizing a Perkin Elmer ICP-AES Optima 7100 DV (Perkin Elmer, Waltham, MA, USA), the error for the measurements were estimated as ±5 %. The silver emission spectrum was probed at 328.068 nm and 338.289 nm.

Conclusions
The Ag NP size in SiO 2 -Ag composite powders was successfully controlled by heat treatment for 75 min at temperatures between 300 • C and 800 • C. The silver is shown to be in metallic form without utilization of reducing agent before heat treatment. The activation energy for growth of Ag particles was determined to be 0.14 eV, which is well below the activation energy for Ag NP growth in SiO 2 films. The release of Ag species from three samples was investigated; SiO 2 -Ag 300, SiO 2 -Ag 600 and SiO 2 -Ag 700 in de-ionized water at room temperature, at 40 • C and 60 • C, and at pH 4 and pH 10 for one to seven days time. The silver concentration increased or plateaued after three days, depending on the release conditions. The results suggest that the silver release can be controlled by varying the loading of the powders and dissolution conditions. The Ag release was clearly Ag size (surface area) dependent. participated in writing the XPS part. Saima Ali performed the nitrogen adsorption-desorption experiments, the FTIR measurements and participated in writing the introduction.; Henrika Granbohm performed SEM, XRD and dissolution tests; Henrika Granbohm wrote the paper under the supervision of Simo-Pekka Hannula. All authors participated in the revision of the article.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: