Evolution of silver in a eutectic-based Bi₂O₃–Ag metamaterial

Plasmonics [1, 2] is currently one of the rapidly developing fields due to its role in enhancing optical properties, which makes it useful for application in solar cell efficiency enhancement [3, 4], cancer treatment [5], improved hard disks [6], lasers [7] and home-used diagnostics [8]. To achieve plasmonic effects on local field enhancement, localized surface plasmon resonance (LSPR) or surface plasmon propagation, an interface between two media—plasmonic (metallic-like) and dielectric—is needed [9]. Due to the collective oscillations of free charges, which are responsible for negative real permittivity, metals like silver and gold are currently the most used materials in plasmonics [10]. However, though other plasmonic materials have been considered [11, 12], it is still silver in the visible wavelengths, which is most widely used due to its lowest optical losses [13] and highest electrical conductivity at room temperature [14].

Recently, eutectic solidification [15, 16] has been proposed as one of the most promising bottom-up methods for manufacturing of metamaterials [17‐20], plasmonic materials [21, 22] and photonic crystals [23, 24]. Eutectic solidification enables crystallization of a two-phase solid, often with an interesting self-organized micro-/nanostructure (typical are rod-like and lamellar structures) from a miscible liquid phase at a certain temperature [25]. Eutectic composites have been investigated for various applications such as solar energy conversion [26‐28], power generation gas turbines [29], scintillators [30, 31] or second-harmonic generators [32]. The plasmonic effect was presented in a eutectic composite for the first time with a Bi₂O₃–Ag composite [21, 22]. After annealing the Bi₂O₃–Ag eutectic material, metal nanoparticles (silver and bismuth) are formed, which are responsible for the occurrence of plasmonic resonance in the visible wavelength range, at ~595 nm. Using different annealing conditions such as the atmosphere, time and temperature, it is possible to control the peak frequency of the LSPR [22]. However, the development of metallic silver in this material is not yet well understood. Here, we demonstrate the evolution of silver in a Bi₂O₃–Ag eutectic starting from the crystal growth, and formation of the microstructure, through the influence of the post-growth annealing of the samples on its micro-/nanostructure and thus the optical properties. The analysis is based on high-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD).

The Bi₂O₃–Ag material was obtained by the micro-pulling down method [33, 34] in a nitrogen atmosphere from pure powders of bismuth oxide (Alfa Aesar, 99.99% purity) and silver (Alfa Aesar, 99.95% purity). Detailed growth and preparation methods have been described elsewhere [21]. After growth, the samples were annealed in vacuum at 600 °C for 60 min [22]. The obtained Bi₂O₃–Ag composites were characterized by the following methods: high-resolution transmission electron microscopy (HRTEM) connected with selected area electron diffraction and with scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDX). Samples for transmission electron microscope analysis were prepared by a focused ion beam (FIB) lift-out technique using a 30-keV Ga+ ion beam in an AURIGA CrossBeam Workstation (Carl Zeiss) equipped with a Canion FIB column (Orsay Physics). TEM investigations were conducted with the use of a Tecnai F20ST (FEI) TEM-STEM microscope with field emission gun and electron beam energy of 200 keV and coupled to a high-angle annular detector (HAADF) and a EDAX X-ray energy-dispersive spectrometer (EDX). The software “TE Imaging & Analysis” (EI) was used to provide standardless semiquantitative analyses of the EDX spectra. These analyses take into account the thickness and the chemical elements of the thin foil (ZAF corrections) and use preregistered K factors. Differential scanning calorimetry/thermogravimetry (DSC/TG) measurements were taken on an STA 449 (NETZSCH) with a platinum furnace under argon flow with different amounts of purge oxygen. Stoichiometric portions of powders were measured out and mixed manually with minor addition of isopropyl alcohol until a homogeneous color was achieved. The mixture was subsequently dried at 80 °C to remove the alcohol. The signals were measured with a Pt–Rh thermocouple using platinum crucibles in a temperature range from room temperature to 1173 K with a heating/cooling rate of 10 K min⁻¹. Bi₂O₃–Ag powder mixtures were heated to 1173 K and cooled to 923 K. The process was repeated three times in each atmosphere to ensure homogeneity of the melt. The solidus temperatures were extracted from the heating curves as onset values, whereas liquidus temperatures were extracted from the cooling curves as the end temperatures. The influence of oxygen content on the phase diagram was investigated using powder samples of mixed bismuth oxide (99.9% purity) with silver (99.95% purity) in the range of 0–10 mol% in steps of 1 mol%.

From the Ag–½(Bi₂O₃) phase diagram [35] in the mentioned system, several transformations at an oxygen partial pressure of 1.01 × 10⁵ Pa are observed (1) allotropic of Bi₂O₃, α-Bi₂O₃ → δ-Bi₂O₃), (2) monotectic (Ag) + Liq2 ↔ Liq1 at 939 °C and eutectic reaction (Ag) + α-Bi₂O₃ ↔ Liq2 at 687 °C with a composition of 18.54 mol% Ag and 81.46 mol% ¹/₂Bi₂O₃ (~8.6 vol% Ag). With the decrease in oxygen content in the atmosphere, the composition of the eutectic point shifts toward lower content of silver (12.3 mol% Ag for pO₂ = 2.1 × 10⁴ Pa, ~5.5 vol% Ag). In our growth experiments, the successful composition that led to good-quality Bi₂O₃–Ag rods was 15.4 mol% Ag and 84.5 mol% ¹/₂Bi₂O₃ (~7.8 vol% Ag). This composition is between the eutectic points determined by Assal et al. [35] for different oxygen partial pressures, as shown in Fig. 1. We have confirmed part of the phase diagrams investigated by Assal et al. with DSC/TG measurements for different partial pressures of oxygen: pO₂ = 0.32 × 10⁴, 0.77 × 10⁴, 1.43 × 10⁴ and 2 × 10⁴ Pa. It can be noticed that the liquidus line is in a very good agreement; however, the solidus line yielded a slightly lower temperature.

The as-grown Bi₂O₃–Ag composite is characterized by a three-dimensional micro-/nanostructure of silver-containing phase in a Bi₂O₃ matrix. Silver is located in a second phase wetting the Bi₂O₃ grain boundaries and at triple points where it adopts a triangular shape. Between two Bi₂O₃ grains, silver is in the form of plates with lengths of several tens of microns and thicknesses of a few nanometers.

To investigate the phase in which silver is placed in the Bi₂O₃–Ag composite during growth and to understand the processes which lead to the formation of silver nanoparticles after the annealing of the material, high-resolution transmission electron microscopy has been performed on the as-grown and annealed-in-vacuum samples. Both the selected area electron diffraction and the energy-dispersive X-ray spectroscopy confirmed that the as-grown material consists of two phases. In the region analyzed, the dominant phase is γ-Bi₂O₃ (bcc, a = 1.027 nm), which was detected in the larger grains. These grains enclosed smaller grains, which were probably the α-Bi₂O₃ phase. However, the second phase previously not recognized by other techniques is an amorphous (halo in the SAD pattern) phase containing Bi, Ag and O, with an atomic ratio Bi/Ag ≈ 1, as shown in Fig. 2. The shape of this second phase, forming triangles at Bi₂O₃ triple points and wetting the Bi₂O₃ grain boundaries, is consistent with the solidification of a liquid after the crystallization of the major primary phase, in this case Bi₂O₃. The radius of the halo in Fig. 2e provides the distance of the first neighbors in the amorphous phase, which is 0.32 nm (superimposed with the d₃₁₀ = 0.325 nm of γ-Bi₂O₃). This distance does not match with the strongest reflections of silver or silver oxides but with the strongest reflections of γ-Bi₂O₃ as well as with those of an ilmenite-type, AgBiO₃ phase [36]. The amorphous phase might enclose clusters of atoms prefiguring an ilmenite crystal. Ilmenite-type oxides have two kinds of combinations of metal cations: A²⁺M⁴⁺O₃ and A⁺M⁵⁺O₃ [37], where AgBiO₃ corresponds to the second one.

Due to a strong dependence of the eutectic point on the content of oxygen in the atmosphere, it is not clear whether the composition of the Bi₂O₃–Ag composite we have grown is shifted to a higher abundance of Bi₂O₃ or Ag, as shown in Fig. 1. From the phase diagram of Bi₂O₃–Ag [35], if the composition is shifted from the eutectic point to a higher abundance of Bi₂O₃, it is the Bi₂O₃ phase that should crystallize first in contact with a liquid phase. While, if the composition is shifted toward a higher abundance of Ag, the Ag phase should crystallize first, and then, the eutectic will be made of Bi₂O₃ and Ag. The geometry of the Ag–Bi–O phase placed in between the grains of Bi₂O₃ is typical of a liquid phase that solidified after the main grains, though potentially it could be a Ag-containing phase in which the grains were squeezed by growing Bi₂O₃ grains. The AgBiO₃ phase is probably formed in the following reactions [38]: 4AgO + Bi₂O₃ → 2AgBiO₃ + Ag₂O, and Ag₂O + Bi₂O₃ → 2AgBiO₂.

The amorphous Ag–Bi–O phase is not stable when irradiated by the electron beam and partially decomposes into ca. 50-nm-diameter γ-Bi₂O₃ nanocrystallites and metallic silver particles a few nanometers in size. Nanocrystals of bismuth oxide up to 50 nm in size are clearly seen in the dark-field image (Fig. 3a) and as bright spots at the SAD image (Fig. 3b). The first internal ring is from the amorphous Ag–Bi–O phase. A second finer halo at 0.23 nm is clearly observed, which may correspond to metallic nanosilver (strongest reflection 111 Ag fcc) exsoluted from the Ag–Bi–O phase, with sizes of a few nm, as shown in Fig. 3b.

Concerning the optical properties of the Bi₂O₃–Ag composite, it has been previously confirmed that only the annealed samples demonstrate LSPR, due to the formation of Ag nanoparticles after the post-growth annealing [21]. The best of the investigated annealing conditions, which led to the most intensive LSPR peak, was annealing at 600 °C for 60 min in vacuum [22]. That is why such samples were investigated here further, as shown in Fig. 4. In the vacuum-annealed samples in comparison with the as-grown (un-annealed) samples, it can be seen that the large Bi₂O₃ grains did not change, while the amorphous phase transformed during the annealing into γ-Bi₂O₃ grains several hundreds of nm in size, as shown in Fig. 4a–c, and intergranular precipitates of Ag a few nm in size (Fig. 4d, red circle), as confirmed by EDX.

Both the annealing and irradiation with the electron beam during the TEM experiments resulted in phase and structural change of the material, which can be the result of a few processes. Thermal stability studies of AgBiO₃ [39] performed in argon show weight loss from AgBiO₃, which corresponds to two transitions such as:

As a result of the final decomposition of AgBiO₃, a mixture of metallic silver and bismuth can be obtained. The formation of both Ag and Bi nanoparticles was already previously observed by us in this material [22]. This suggests that it is the decomposition of AgBiO₃ into Ag and Bi nanoparticles in between the big Bi₂O₃ grains, which probably causes the formation of the smaller (ca. few hundred nanometers in size) Bi₂O₃ grains. The few-hundred-nm-in-size Bi₂O₃ grains have grown from the nano-Bi₂O₃ grains seen in Fig. 3a, with decomposition of the amorphous phase. In Fig. 5, we illustrate the proposed mechanism of silver evolution in the Bi₂O₃–Ag system.

In summary, an explanation of the evolution of silver in a Bi₂O₃–Ag eutectic composite from the crystal growth through the post-growth annealing processes has been presented. In the as-grown samples, silver is inserted in an amorphous phase enclosing AgBiO₃ clusters, which is formed as intergranular films and triple points wetting the grains of Bi₂O₃ primary phase. After the post-growth annealing, the AgBiO₃ phase decomposes into Bi₂O₃ nanocrystals and intergranular Ag. These studies give us an explanation for the origin of silver nanoparticles in the Bi₂O₃–Ag eutectic-based material after the annealing procedure and thus the localized surface plasmon resonance at yellow light wavelengths.