Article pubs.acs.org/JPCC
X‑ray Photoelectron Spectroscopic Characterization of Ag Nanoparticles Embedded Bioglasses T. Radu,†,§ D. Benea,† R. Ciceo-Lucacel,†,§ L. Barbu-Tudoran,‡ and S. Simon*,†,§ †
Faculty of Physics and ‡Faculty of Biology and Geology, Babes Bolyai University, 400084 Cluj-Napoca, Romania § Institute of Interdisciplinary Research in Bio-Nano-Sciences, Babes Bolyai University, 400271, Cluj-Napoca, Romania ABSTRACT: This article reports on synthesis and X-ray photoemission spectroscopy (XPS) results regarding the development of silver nanoparticles (AgNPs) in CaO-SiO2−B2O3 bioactive glasses with three different Ag2O contents (0.7, 1, 3%). The analysis of the core level and valence band photoemission spectra is discussed in terms of AgNPs shape, size, and distribution at the sample surface for all investigated samples. A comparison with a theoretical valence band photoemission spectrum (VB-XPS) is presented to check the correlation between clusters size and electronic structure. Our data provide complete experimental evidence of the correlation between AgNPs size and electronic structure that has been predicted theoretically.
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INTRODUCTION There is an increasing interest in Ag nanoparticles (AgNPs) embedded in glassy matrix in the last three decades that stems from the peculiar electronic properties of AgNPs that have applications in various fields such as medicine and pharmaceutical research as active drug in targeted drug delivery, gene delivery, artificial implants, and in imaging as a diagnostic agent and sensors.1 This generates a growing need to understand their properties and behaviors as they are synthesized, applied, and developed in a particular environment. Moreover, the characterization of the AgNPs cluster geometry and their electronic properties is an essential step to understand why these particles have a size selective bactericide activity. It was shown that Ag ions exhibit bactericide activity toward a broad spectrum of bacteria and fungi and, in contrast with antibiotics, do not easily provoke microbial resistance.2 The real bactericide mechanism of AgNPs is not completely understood, although there are already in literature few scenarios for the AgNPs− bacteria interaction mechanism.2−6 However, it was shown that AgNPs interaction with bacteria is dependent on the size and shape of the AgNPs.6−8 Despite the numerous investigations in this direction, the synthesis of AgNPs and the correlation of their chemical and physical properties with their size are unsolved challenges. It is worth mentioning that the synthesis of well-defined AgNPs is difficult due to aggregation of nanoclusters. Moreover, the analysis of the effects of silver doping in glasses is complicated because single ions, clusters of a few atoms, and nanoparticles of various sizes can coexist.9 Therefore, analyses examining the sizes, morphologies, elemental compositions, degrees of crystallinity and atomic structures, coatings, and aggregation states of AgNPs in the environment are scarce.10 It is also difficult to control the size of the metal NPs that is often weak reproducible, being determined by their preparation method. In previous studies, clusters of nanoparticles supported © 2012 American Chemical Society
on well-characterized substrate were generated by cluster sources (e.g., laser vaporization source, arc discharge source, magnetron sputter source) in the gas phase and subsequently mass-selected (via mass-spectrometer).11,12 However, additional information can be obtained by preparing AgNPs with relative narrow size distributions by nucleation and growth on the support. This can be obtained via conventional methods as metal vapor deposition13 and chemical methods such as synthesis of colloidal particles surrounded by organic stabilizers. 14 To predict the environmental impact of engineered AgNPs, their characterization from environmental matrices should be pursued.10 In a recent study,9 X-ray photoelectron spectroscopy (XPS) was performed on three Ag-exchanged soda lime glasses produced for photonic applications. It was shown that XPS spectral changes correspond to well-defined chemical and structural organization of Ag in the glass. Surface chemical analysis methods play an important role in the characterization of NPs due to their ability to detect the oxidation state or states of an element and the chemical compositions of sample surface.15 This investigation method is very useful because the process of photoelectron emission from solids is strictly dependent on the electronic configuration of the analyzed atoms; therefore, we expect to observe sensible modifications of the XPS spectral features as the size of the AgNPs changes. In this article, XPS was used to investigate high-resolution Ag core level and valence band (VB) region on a CaO−SiO2− B2O3 glass matrix with different Ag2O contents (0.7, 1, 3 mol %) obtained by conventional melt quenching method, with the aim to correlate changes of line shape and binding energies Received: June 19, 2012 Revised: July 18, 2012 Published: July 30, 2012 17975
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XPS) theoretical approach is based on a one-step model of photoemission20,21 and allows the calculations in cluster approximation.
(BEs) to the chemical state and size of AgNPs. In our study, the scenario and interpretation of the XPS Ag 3d core level and VB spectra with respect to silver content are built in the framework of reduced coordinartion atoms number, and this approach brings new insight from both theoretical and experimental points of view. Currently, CaO−SiO2−B2O3-based biomaterials attracted much interest due to their large biomedical applications.16 It was shown that bioactive glasses are able to react with physiological fluids to form stable bonds to bone through the formation of bone-like hydroxyapatite layers. Here we present the experimentally obtained XPS results with a special emphasis on Ag core-level and VB spectra. Our goal was to check whether there is a systematic development of AgNPs in the sample with increasing the Ag concentration. We aimed to correlate the theoretical XPS spectra calculated for Ag clusters of increasing size with the experimental spectra recorded from bioactive glasses with 0.7, 1, and 3 mol % Ag2O to identify the size effects on electronic structure of AgNPs. Transmission electron microscopy (TEM) investigations have been employed to get an image of the AgNPs size distribution in the bioactive glasses and to relate them with the theoretical and experimental XPS spectra.
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RESULTS The microstructure of the prepared bioactive glasses has been investigated by TEM. In Figure 1a,b, TEM micrographs are
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EXPERIMENTAL SECTION The studied glasses have the composition expressed by the formula xAg2O(100 − x)[1.5B2O3•SiO2•CaO] with x = 0, 0.7, 1, and 3 mol %. They were prepared using the conventional melt quenching method. Appropriate quantities of reagent grade AgNO3, H3BO3, SiO2, and CaCO3 were mixed in an agate mortar. The batches were melted in air in sintered corundum crucibles in an electric furnace at 1350 °C for 25 min. The melts were quickly cooled to room temperature by pouring and stamped between two copper plates (initially cooled with liquid nitrogen). XPS measurements were used to investigate the core-level and VB spectra of our samples. The measurements were performed by using a SPECS XPS system with a monochromatic Al Kα source (1486.69 eV) operated with a power of 280 W. The background pressure during measurements was 7 × 10−10 mbar. A low-energy charge neutralizer was used to remove the charge shifts during photoemission. The BE scale was calibrated to that of the C 1s photoelectron peak (284.6 eV). Survey scans were recorded at a pass energy Epass = 100 eV, in steps of 1 eV, and high-resolution spectra with 0.05 eV step and Epass = 30 eV. Analysis of the data was carried out with Casa XPS software.17 The AgNP size in the samples with x = 1 and 3 mol % Ag2O was determined by means of TEM using a Jeol JEM1010 TEM (Jeol) equipped with Mega View III CCD Camera (Soft Imaging Systems).
Figure 1. TEM images and the corresponding size distribution of the (a) x = 1 and (b) x = 3 samples. Isolated small NPs are visible in the glass matrix. In the upper TEM images the bar is 200 nm, whereas in the lower images the bar is 100 nm.
shown. As one can see here, isolated circular AgNPs have been formed in the glassy matrix. The size of the individual particle has been taken as the diameter of the observed particles, which is on average 7.4 and 9 nm (±0.2 nm) for x = 1 and x = 3 samples, respectively. Table 1 summarizes average values obtained from the XPS spectra. Figure 2 shows the evolution of Ag 3d core level spectra with increasing x, measured by XPS. The Ag 3d spectrum consists of a doublet for the Ag 3d5/2 and Ag 3d3/2 components at BEs slightly higher than the characteristic values of metallic silver.4 The spin−orbit splitting is 5 eV and the area ratio is ∼0.6.
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THEORETICAL BASIS The electronic structure was calculated self-consistently by means of the spin-polarized relativistic Korringa−Kohn− Rostocker (SPR-KKR) method in the atomic sphere approximation (ASA) mode. The details of the KKR Green’s function formalism, which makes use of multiple scattering theories, have been described in detail elsewhere.18 The local spin density approximation (LSDA) for the exchange-correlation energy using the Vosko, Wilk, and Nusair (VWN) parametrization was used.19 The VB photoemission spectra (VB-
Table 1. Binding Energy (BE) of Ag 3d Core Level Spectra, The Effective Spin-Orbit Splitting, Δ, and Atomic Concentration of Ag for xAg2O(100-x)[1.5B2O3•SiO2•CaO] Samples
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x (mol %)
BE Ag 3d5/2 (eV)
BE Ag 3d3/2 (eV)
Δ (eV)
Ag concentration (at %)
0.7 1 3
368.5 368.4 368.2
373.8 374.2 374.1
5.3 5.8 5.9
0.2 0.2 0.4
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Figure 3. VB-XPS spectra of Ag bulk and Ag clusters with radius between 1.8a and 2.3a (where a is the lattice parameter of Ag) calculated with the Munich SPR-KKR package, as described in text. The experimental VB spectrum of the investigated sample (x = 3) is obtained after subtraction of the glass matrix contribution.
direct comparison between experiment and theory for the sample with x = 3. The obtained spectra for samples with x = 0.7 and 1 (not shown) have very low intensity and high signalto-noise ratio, and thus it was difficult to analyze their shape. XPS VB of Ag bulk and Ag clusters with radius between 1.8a and 2.3a (where a is the lattice parameter of Ag) have been calculated with the Munich SPR-KKR package to describe the size variation of the XPS spectra for small clusters (with radius up to 1 nm).
Figure 2. (a) Ag 3d high-resolution photoelectron spectra for xAg2O(100 − x)[1.5B2O3•SiO2•CaO] samples. (b) VB spectra for the same samples; the deconvolution of the obtained VB spectra for each sample yields two components: I and II representing the matrix and the Ag 4d, respectively.
It is known that peak area is proportional to the number of corresponding atoms.5 Our data show that the area of the Ag 3d peaks is increasing with the increase in Ag concentration, indicating the generation of a larger number of AgNPs. The full width at half-maximum (fwhm) of the 3d lines of the Ag 3d spectra for the sample with x = 3 is narrower than that of the samples with lower Ag content. The larger fwhm values indicate a reduced coordination of atoms due to reduced cluster surface, whereas the narrowing of the Ag 3d line indicates an increase in the cluster size.22 For the sample with x = 3, the peak position shifts toward a lower BE by ∼0.3 eV compared with those of the x = 1 and 0.7. This shift is connected to the dimensions of Ag NPs. The increase in the Ag content also caused significant changes in the VB spectra, as shown in Figure 2b. The VB spectra were decomposed using two Gaussian− Lorentzian (GL = 30) components: one attributed to matrix contribution (I) and the second attributed to Ag 4d (II). Maintaining the line width of component I almost unchanged, the deconvolution allowed us to describe changes in the VB width due to changes in the Ag cluster dimension. The fwhm of component II increases from 3.1 to 3.4 (see Table 2) with
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DISCUSSION The BE of AgNPs were measured by XPS to investigate the shift of the spectra and correlate with the dimensions of Ag clusters. The Ag particles size can be estimated using the Ag 3d core level shifts, which were shown to be sensitive to the particle size.1,23−25 The observed small positive shift (∼0.3 eV) of Ag 3d states with respect to the corresponding Ag bulk value (368.3 eV) and the broadening of the Ag 3d core level spectra (see Figure 2) for lower silver content can be attributed to the decrease in particle size by decreasing x. Similar behavior was observed in early studies for Ag NPs deposited on different substrates1,9,22,23,25 as a consequence of the competition between two phenomena: initial state effects (partial electron transmission to the clusters) with a tendency to shift the peak to lower BE and final state effect, which reflects the relaxation energy of the system after photoemission, having a tendency to shift the peak to higher BE, which may both result in a net XPS shift with increasing Ag cluster size but not necessarily at the same ratio. However, for our samples, TEM images clearly show that the mean cluster size is small (d < 10 nm), and thus no significant dependence of the final state relaxation on size can be expected. According to previously reported results,22 the spectral changes observed and the broadening of the Ag 3d core level spectra with decreasing x may indicate a reduced coordination of atoms due to the decrease in the cluster size. The changes of the Ag core-level spectra related to size reduction clearly suggest that there should be size-dependent changes in the VB density of states of AgNPs. It is worth noting that the mean coordination number of the cluster atoms directly affects the spin−orbit splitting energy of Ag 4d electrons.1,22,23,26 With decreasing coordination number, narrowing of the VB is expected because of a reduction of the atomic orbital overlap. Figure 2b shows the VB-XPS spectra obtained for different silver contents. Each spectrum was decomposed by using a combination of two Gaussian−
Table 2. Data Obtained from the Deconvolution of VB Spectra of xAg2O(100 − x)[1.5B2O3•SiO2•CaO] Samples as show in Figure 2b matrix component (I)
Ag component (II)
x (mol %)
fwhm (eV)
%
fwhm (eV)
%
BE (eV)
0.7 1 3
7 7 6.9
69.7 66.7 61.5
3.1 3.4 3.4
30.3 33.3 38.5
5.5 5.3 5.3
increasing x and is slightly shifted to lower BE values (by 0.2 eV). It is worth mentioning that other components present in the matrix may give contribution to the VB spectra, which give rise to larger VB line. Therefore, for a more accurate comparison between experiment and theory, subtraction of the pure matrix contribution was performed after normalization of the spectra to matrix-integrated intensities. Figure 3 shows a 17977
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respect to the corresponding bulk value when x decreases and narrowing of the VB component attributed to Ag 4d photoelectrons accompanied by a BE shift toward higher values with decreasing x. On the basis of the above experimentally and theoretically obtained VB-XPS spectra, we showed that the reduction of coordination number is dominant in describing the sizedependent VBs of small Ag clusters. Our evidence of how AgNPs develop in CaO-SiO2−B2O3 bioglass with increasing Ag concentration and how the chemical state and size of AgNPs correlate with changes of line shape and BEs from both experimentally and theoretically spectra are giving more insight into an extremely important scientific and technological research topic.
Lorentzian functions (GL30). The observed narrowing of the VB component attributed to Ag 4d photoelectrons accompanied by a BE shift toward higher values with decreasing x can be linked to the size-dependent XPS spectra of Ag nanoparticles. Moreover, this represents a common feature for systems containing nanoparticles of noble metals.1,22,23 To minimize the contribution of glass matrix elements (Si, Ca, B, O) to the VB spectra even more, and to clarify better the correlation with changes induced by increasing Ag content on the AgNPs, we subtracted experimentally obtained spectra for samples without AgNPs from those obtained for samples with AgNPs. The VB spectra obtained for x = 3 sample after subtraction of the matrix contribution (Figure 3) show good agreement with the computed spectra of Ag clusters. In the VBXPS spectra, the main contribution comes from Ag 4d band located between 2 and 8 eV BE. The 4d band is superposed over a wide s−p band formed mainly by 5s and 5p electrons (partially hybridized with 4d electrons). The shift to higher BEs and the narrowing of the VB is obtained by the XPS calculations as the clusters size decreases. A gradual reduction of the lowest BE peak in the VB is observed by decreasing the cluster size. The tendency observed in the calculated spectra is consistent with the XPS and UPS VB photoemission spectra of Paszti et al.22 An increase in the BE of the Ag 4d states is attributed to the modified electronic structure due to initial and final states effects.23,27 In the case of the final state effects, in the final state of the photoemission process the nanoparticle remains with a unit positive charge. The final-state energy is then increased by the Coulomb energy e2/2R, where R is the particle size and e is the electron charge. The size-induced changes of the electronic structure of NPs (initial states effects) due to the quantum confinement of the electrons also result in an increase in the BE of the electrons. The size-induced changes in NPs (contraction of surface bonds and increase of the surface to volume ratio) are explained by the quantum size effects supported by a model known as surface bond contraction model of Sun et al.27 In our calculations, a finite cluster centered on the atom of interest is cut out of the infinite system without considering the surface effects and the length bond contraction. The calculated VB-XPS spectra of Ag NPs show not only the shift of BE dependent on the cluster size but also the diminishing of the lower BE peak and VB narrowing by decreasing the cluster size, in agreement with the experimental spectra. As a consequence, we conclude that the reduction of coordination number is dominant in describing of the size-dependent VBs of small Ag clusters.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.B. acknowledges financial support of the Sectoral Operational Programme for Human Resources Development 20072013, cofinanced by the European Social Fund, under the project number POSDRU 89/1.5/S/60189 with the title “Postdoctoral Programs for Sustainable Development in a Knowledge Based Society”. This work was supported by grant PCCE_ID_76/2008 by the Romanian National Council for Scientific Research in Higher Education.
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CONCLUSIONS Three compositions of xAg2O(100 − x)[1.5B2O3•SiO2•CaO] bioactive glasses containing AgNPs were synthesized by conventional melt quenching method. The samples were characterized by XPS and TEM to correlate changes of line shape and BEs to the chemical state and size of AgNPs. TEM images indicate the presence of dispersed roughly spherical AgNPs at the samples surface. The AgNP size increase with increasing x was related to changes in the electronic structure of samples as it was measured by XPS and confirmed by theoretical calculation. Moreover, our observed spectral changes are in accordance with the reported trends characteristic for nanoparticles with decreasing size and can be linked to AgNP size-dependent XPS spectra: small positive shift (∼0.3 eV) and broadening for the Ag 3d core level spectra with 17978
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