The incorporation of copper nanoparticles into the glass matrix has attracted great interest in many fields due to their significant physical and chemical properties and their applications (e.g. opto-electronic and bio-medical tools). Several methodologies, such as the doping method and subsequent annealing of the glass in oxidizing and reducing atmospheres to synthesize copper nanoparticles in a glass are widely used. In this work, the doping method and subsequent annealing of the glass in a Ar/H2 atmosphere has been used. This treatment caused a martensitic-type transformation which, to our knowledge, has not been mentioned in the literature. The result was the transformation from cuprite to tenorite about 300°C, and the presence of dispersed rounded and polyhedral copper nanoparticles and dendritic shapes of nanoparticles. The reaction of CuO and H2, between 250 and 300°C, originated metallic copper particles. The mechanism of the martensitic-type transformation has been derived from the characterization results of copper-doped glasses annealed in reduced atmosphere by optical microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The martensitic-like type structure could have imparted shape memory characteristics to the copper-doped glasses annealed in reduced atmosphere, opening a window for future research.
La incorporación de nanopartículas de cobre en vidrio es de gran interés en muchos campos debido a sus importantes propiedades físicas y químicas y a sus aplicaciones (por ejemplo, optoelectrónica e instrumentos biomédicos). Metodologías para sintetizar nanopartículas de cobre en vidrio, como el dopaje y posterior recocido del vidrio en atmósferas oxidantes y reductoras, son ampliamente utilizadas. En este trabajo se ha utilizado este método en atmósfera Ar/H2. Este tratamiento provocó una transformación de tipo martensítico que, hasta donde sabemos, no se ha mencionado en la bibliografía. El resultado fue la transformación de cuprita a tenorita alrededor de 300°C, y la presencia de nanopartículas de cobre dispersas redondeadas y poliédricas y de nanopartículas con formas dendríticas. La reacción de CuO y H2, entre 250-300°C, originó partículas metálicas de cobre. El mecanismo de la transformación se ha establecido a partir de los resultados de la caracterización de los vidrios dopados con cobre recocidos en atmósfera reducida mediante microscopia óptica, espectroscopia de fotoelectrones de rayos X, microscopia electrónica de barrido y microscopia electrónica de transmisión. La estructura de tipo martensítico podría haber conferido características de memoria de forma a los vidrios dopados con cobre recocidos en atmósfera reducida, abriendo una ventana para futuras investigaciones.
The characteristics of glass, such as its optical transparency, chemical and thermal stability, make it a very suitable material for growing metallic nanoparticles. Metal-doped glasses have been around for a long time. In the fourth century A.D. Roman glassmakers made glass with metal nanoparticles and the stained glass windows of medieval cathedrals in several European countries contain metallic aggregates. Glasses containing metallic nanoparticles, such as gold, silver, copper, etc., are materials that can be used in many applications: optical waveguides, optical storage media and optical switches, in the fabrication of photonic and optoelectronic devices, as gas sensors, heterogeneous catalysts and in bio-medical tools [1–8]. Several methodologies are used to synthesize noble metal nanoparticles in a glass, such as the melting method, ion injection method, sol–gel method, laser irradiation method, co-sputtering method and doping method [9–13]. The glass doping synthesis method has been known for a long time [14–18]. In recent years, doping technology has reached a great interest mainly due to three reasons. First, doping makes it possible to obtain ultrathin glass films (∼50–100μm) with high mechanical strength [19–21]. Secondly, it is important to develop sensors whose functioning is based on the effect of surface plasmon resonance, from glasses doped with silver, gold and copper nanoparticles. Thirdly, obtaining metal doped glasses for the above mentioned applications. In doping technology, copper occupies a special place. A very important aspect of copper is that its ions can be present in the glass in the form of Cu+ and Cu2+, which differ substantially in their properties, for example, they have different polarizabilities and the mobility of the Cu+ ion in glasses with sodium is greater than that of Cu2+. The doping mechanism in glass is based on the inherent ionic conductivity of glass. The ionic diffusion can be stimulated by heating so that the metal ions propagate from the surface of the glass under its surface, forming a thin layer.
Copper nanoparticles have been the focus of attention for a wide variety of potential applications. Copper nanoparticles show a distinct optical response in the 560–570nm wavelength range, which happens when the frequency of the incident photon is in resonance with the collective oscillation of the conduction band electrons and is called surface plasmon resonance (SPR) [22,23]. SPR is extremely important for the usage in different linear and non-linear applications [24–27]. Consequently, the optical response of copper nanoparticles is an important aspect for use as surface plasmon-based components [28–30]. Copper nanoparticles are synthesized using different methods such as the polyol method, copper salt reduction, electro-reduction in an ionic liquid, thermal decomposition, laser ablation, electron beam irradiation, microwave heating and the inert gas condensation (IGC) method [31–39].
To understand the physical properties and chemical behavior of the surface layer of the copper compounds of the glass and the morphology of the copper particles, the identification of the surface state of the copper particles, as well as the structural characteristics of the surface layer and the morphology of the particles, are fundamental. With these considerations in mind, the aim of the present paper was to investigate the martensitic-like type transformation from cuprite (CuO2) to tenorite (Cu) with the formation of metallic copper particles on copper-doped microscope slides and annealed in a reduced atmosphere, which has not been reported yet, to our knowledge. Moreover, if the martensitic process is fully reversible, the martensitic structure could confer shape memory characteristics to a material, which are very important in different applications of materials possessing them [40], being possible to investigate in the case of glasses studied in the present paper. The characterization of the samples was carried out with optical microscopy (OM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM and HRTEM), to determine the mechanism of the martensitic-like type transformation.
Experimental partThe raw material consisted of microscope slides (supplied by Fisher Scientific) which were doped and annealed in reduced atmosphere. The chemical composition of the glass slides in wt.% is: 71.8±3 SiO2, 14.0±3 Na2O, 8.8 CaO, 4.2±0.2 MgO, 0.9±0.3 Al2O3, 0.3±0.2 K2O.
The doping of slides was performed by introducing five samples into a molten mixture of CuSO4:Na2SO4 (50:50mol.%) at 600°C for 1h. After, the doped samples were washed with distilled water and dried at room temperature. Subsequently, four samples were annealed with a silica tubular furnace, in Ar/H2 atmosphere (3vol.% H2) for 2h. Each sample was annealed at a different temperature (250°C, 350°C, 450°C and 550°C, respectively). After the indicated time, the furnace was turned off and the samples were cooled under hydrogen atmosphere and after one hour the samples were taken out. These experimental conditions are based on a previous work in which several samples with different doping times were used and on the existing literature on the method used in this manuscript. The reproducibility of the experiments is 100%.
The samples were labeled as follows:
The characterization of the samples was carried out with several techniques (optical and electron microscopy, XPS spectroscopy and X-ray diffraction) to determine the elemental composition and its variations, texture at different scales, mineral phases and crystalline structure. All of the characterizations were performed at room temperature.
Microscope Zeiss and loupe LEICA were used to image the textural characteristics of the samples to millimeter scale.
JEOL-6610LV scanning electron microscope at 20kV and 10mA, with a vacuum of 2.0×10−4Pa, and built-in energy-dispersive X-ray spectroscopy (EDX-INCA energy 350) microanalysis detector was used to view the superficial morphology and elemental composition of the samples. The samples were subjected to sputtering with a gold layer of ≈100Å thickness to improve its electron conductivity for SEM analysis.
X-ray photoelectron spectroscopy (XPS) was used for determining the binding states of copper. XPS SPECS equipment with Phoibos 100 MCD electron analyzer, Kα Al 1486.74eV monochromatic X-ray source and electron gun, to compensate the surface charge on the sample, was used. The overall spectrum (low resolution survey spectra) was taken with 90eV step energy every 1eV. The high resolution spectra (Cu 2p, etc.) were taken with a step energy of 30eV every 0.1eV. The measurement pressure was kept below 1×10−8mbar during data acquisition. The samples were placed in the sample holders using a double-sided carbon tape and stripping was performed with 2×10−4mbar Ar pressure, ion energy 3000eV, emission current 1mA.
Transmission electron microscopy with a JEOL JEM 1011 microscope operated at 100kV was used to obtain micrographs at low resolution. High-Resolution Transmission Electron Microscopy (HRTEM) with JEOL-JEM-2100F operated 200kV, with a vacuum of 1.0×10−5Pa. The crystallographic structure of the samples was analyzed using transmission mode and the Selected Area Electron Diffraction (SAED) mode of that electron microscope. For the analysis, a few drops of the suspension of each powdered sample (in ethanol) was dispersed on a carbon-coated copper grid. Powdered samples were obtained by grinding a portion of each glass slide with an agate mortar.
X-ray diffraction (XRD) was used with grazing incidence (ω=5°) and parallel sheet optics (PPC) in the diffracted beam, in the range 2θ 34° and 52°, so that that diffraction can be made surface sensitive to the investigated samples and detect the crystalline phases. An X-ray diffractometer PHILIPS X’PERT PRO was used. Setting conditions were 40mA and 45kV (Cu Kα radiation; λ=1.5418Å), 2θ range of 34–52°, sweeps per 2θ step of 0.04° and a counting time of 40s per step.
ResultsOptical microscopyOptical observations under optical microscope and magnifying glass of samples GD, GDA250, GDA350, GDA450, GDA550, showed: color and texture changes in sample GD indicating copper doping in the glass; formation of copper oxides with subsequent annealing, which varied in color with temperature, and martensitic-like type structure. Sample GD showed circles with different sizes, surrounded by yellowish green zones which would correspond to copper oxides on glass (Fig. 1a). The circles could be due to the glass immiscibility and consequent two-phase separation, probably formed via nucleation and crystal growth [41] that usually occurs in molten glass. Phase separation occurs in molten glass and may persist when the glass cools to a solid; however, such separation is not actually observed at this scale of magnification. Phase separation in glasses produces a higher nucleation density when such glasses are subsequently subjected to a reduced atmosphere. This is attributed to the increase in the number of copper atoms surrounding the nucleation sites in phase-separated glasses [42]. GDA250 (Fig. 1b) shows similar texture to Fig. 1a (GD sample). GDA350 (Fig. 1c) shows the coexistence of two textures: Fig. 1c1 shows a texture similar to the previous ones, with color change from greenish to reddish; Fig. 1c2 shows an incipient martensitic-like type structure and red color. This red color indicates changes in the oxidation state of copper. In addition, crescent shaped and darker colored borders are observed surrounding the circles which could correspond to a secondary phase separation. Sample GDA450 shows the crescent shaped circles and a consolidated martensitic-like structure (Fig. 1d1). The arc-shaped borders surrounding the circular features show different shades of red. The martensitic-like structure surrounds the circles but does not pass through them, and it is composed of several plates with different orientations [43], typical of cooling [44]. These plates form part of the so called annealing twins, which can be straight and curved [45] and requires the existence of axial stresses; twinned structure that can be observed inside the martensitic plates (Fig. 1d2). When the annealing temperature was increased up to 550°C, the martensitic-like structure was further developed, with the formation of more plates in sample GDA550 (Fig. 1e1). In addition, the red color of the glass became more intense. Small particles on the plates of the martensitic-like structures and on the crescent shaped borders surrounding the circles-like features were observed (see scheme Fig. 1e2).
Micrographs of the investigated samples: (a) GD showing the circles with different sizes, surrounded by yellowish green zones. (b) GDA250 with similar texture to GD sample. (c1) GDA350 showing a texture similar to the previous ones, but the color changed from greenish to reddish. (c2) GDA350 showing red color with martensitic-like type structure. (d1) GDA450 sample with the crescent shaped circles and martensitic-like structure. (d2) Details of the twins of the martensitic-like structure in GDA450 sample. (e1) Martensitic-like structure and the crescent shaped circles in GDA550 sample. (e2) Scheme showing the small rounded particles on the crescent shaped borders and on the plates of the martensitic-like structures of GDA550 sample.
XPS spectra of Cu 2p and their deconvolution for the samples GD, GDA250, GDA350, GDA450 and GDA550 are shown in Fig. 2(a–e). Their binding energy and the compounds to which they are related are shown in Table 1. Tenorite could not be detected in the GDA450 and GDA550 probably because increase in the metallic copper particles population that practically covered the surface of the martensitic plates on which they were formed. Atomic concentration (%) of Cu 2p for the investigated samples is present in Table 2.
Binding energy of the samples GD, GDA250, GDA350, GDA450, GDA550 and the compounds to which they are related.
| Sample | Binding energy (eV) | Compound | References |
|---|---|---|---|
| GD | 933.9 | CuO | [46] |
| 935.2 | |||
| GDA250 | 932.3 | Cu2O | |
| 933.1 | Cu | ||
| GDA350 | 933.1 | Cu2O and Cu | |
| 934.4 | CuO | ||
| GDA450 | 932.0 | Cu2O | |
| 933.1 | Cu | ||
| GDA550 | 932.0 | Cu2O | |
| 932.5 | Cu |
Scanning electron microscopy has made it possible to corroborate the observations under the optical microscope through the images taken with secondary and backscattered electrons and the analysis of the oxides of the elements obtained with SEM+EDX.
The raw glass, G, shows that its initial structure is homogeneous (Fig. 3). EDX analysis performed on some zones of sample G (Table 3) shows a Na average weight of 7.67%. The sample GD shows circular patterns with various sizes varying from 110 to 590μm and darker than the surrounded regions (Fig. 4a). SEM–EDX analysis performed on a circle (spectrum 1) and on the surrounding area (spectrum 2) on sample GD (Fig. 4a) is presented in inset table (Table 4). Spectrum 2 shows a much higher copper content and a much lower sodium content than spectrum 1, indicating that the exchange of copper for sodium was much higher in the area surrounding the circles than in the circles. On the other hand, the sodium content inside the circle and the other elements (Table 4) are similar to that obtained in the G sample (Table 3), differing the content of the corresponding elements of the surrounding zones; this would demonstrate the presence of different phases in the exchanged glass. In addition, sample GD shows dendritic-like structure (Fig. 4b), which shows twice as much copper as sodium content (Table 5). SEM images of the annealed samples in Ar/H2 atmosphere GD250, GD350, GD450 and GD550 are presented in Fig. 5 (a–d). In sample GD250 (Fig. 5a), the annealing treatment resulted in the formation of a structure consisting of dark circles with crescent-shaped edges (like the circles observed with the optical microscope) and incipient martensitic and dendritic features. These circular patterns have different diameter size, ranging from 0.03mm to 0.16mm, and very low copper content according to SEM–EDX analysis shown in Table 6 opposite the light region which is richer in copper and poor in sodium. In the sample GDA350, annealed at 350°C in Ar/H2, the elemental composition analyzed in some points with SEM–EDX (Fig. 5b) is detailed in Table 7, showing that lighter areas have less copper compared to sodium. In sample GDA450, annealed at 450°C in Ar/H2, some of the dark regions were surrounded by crescent-shaped edges, one in a light shade and one in a darker shade, probably because they evolved differently with temperature. According to SEM–EDX analysis, these boundaries contain the highest copper content (Fig. 5c and Table 8). In the sample GDA550, annealed at 550°C in Ar/H2 two different features are well distinguished (Fig. 5d). On one hand, circles with crescent-shaped edges surrounded by possible rounded copper particles; the axes of these borders have different orientations, varying from one to another at an angle of 10°. On the other hand, appearance of martensitic-like structure similar to that observed previously with the optical microscope, with possible copper particles distributed in the plates forming this structure and following three different directions, at 120° from each other. The martensitic-like structure surrounds the circles but does not pass through them. The angles between the axes of the crescent-like shaped borders and the martensitic plates vary by 5° or 10°. The dark martensitic plates size ranges between 0.4mm and 0.15mm and the size of the light plates ranges from 0.7mm and 0.19mm, at this scale. The elemental composition analyzed with SEM–EDX on sample GDA550 is presented in Table 9 and the spectra 2–4 corroborate the presence of copper particles. The copper particles size on the crescent-like shaped borders range from 0.13mm to 0.29mm, at this scale; the copper particles on the alternating martensitic plates have sizes between 0.01mm and 0.17mm, slightly lower than the previous ones.
SEM–EDX analysis (wt.%) of the elements of the sample G (Fig. 2).
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca |
|---|---|---|---|---|---|---|---|---|
| 1 | 10.33 | 54.10 | 7.63 | 1.68 | 0.47 | 22.47 | 0.34 | 2.98 |
| 2 | 9.66 | 54.62 | 7.74 | 1.68 | 0.46 | 22.58 | 0.28 | 2.99 |
| 3 | 9.65 | 54.73 | 7.68 | 1.69 | 0.50 | 22.43 | 0.28 | 3.05 |
| 4 | 10.84 | 53.71 | 7.62 | 1.68 | 0.47 | 22.34 | 0.32 | 3.01 |
(a) SEM image of the GD sample showing circles with various sizes and darker than the surrounded regions and the table in the box shows the difference in copper wt.% content, as per EDX analysis, among these two distinct areas (Table 4). (b) A magnified SEM image of the SEM micrograph in the inset for GD sample showing dendritic-like structure. The table in the box shows the copper wt.% as per EDX analysis (detailed in Table 5).
SEM–EDX analysis (wt.%) of the elements of the GD sample displayed in Fig. 4a.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 8.64 | 54.93 | 6.81 | 1.76 | 0.53 | 23.52 | 0.28 | 2.92 | 0.60 |
| 2 | 10.10 | 52.96 | 1.59 | 1.57 | 0.43 | 21.99 | 2.81 | 8.55 |
SEM–EDX analysis (wt.%) of the elements of the GD sample displayed in Fig. 4b.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 27.22 | 45.19 | 2.35 | 1.15 | 0.42 | 16.60 | 2.15 | 4.91 | |
| 2 | 28.53 | 46.58 | 2.17 | 1.12 | 0.36 | 14.95 | 1.98 | 4.32 | |
| 3 | 35.14 | 43.06 | 1.83 | 0.96 | 0.26 | 13.11 | 0.09 | 1.76 | 3.79 |
(a) SEM image of sample GD250 showing dark circles and incipient martensitic-like and dendritic-like structures. (b) SEM image of sample GDA350. The box table shows the SEM–EDX point analysis in wt.% for copper and sodium for the points indicated in the SEM micrographs. (c) SEM image of sample GDA450 showing crescent-like shapes and stacking faults. According to SEM–EDX point analysis, the spectra recorded at positions 2 and 3 exhibit the highest copper content. (d) SEM image of sample GDA550 showing circles surrounded by possible copper particles in shape of crescent-moonlike and the martensitic-like structure with possible metallic copper particles on their plates. Note: The box table shows the SEM–EDX point analysis in wt.% for copper and sodium for the points indicated in the SEM micrographs. Detailed values are displayed in Tables 6–9.
SEM–EDX analysis (wt.%) of the elements of the sample GD250, displayed in Fig. 5a.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 6.01 | 55.06 | 3.05 | 1.62 | 0.55 | 23.06 | 2.94 | 7.71 | |
| 2 | 11.76 | 54.34 | 2.36 | 1.58 | 0.46 | 21.41 | 0.33 | 2.77 | 4.99 |
| 3 | 14.53 | 52.65 | 3.05 | 1.34 | 0.38 | 19.66 | 2.57 | 5.83 |
SEM–EDX point analysis (wt.%) of the elements of the sample GDA350, displayed in Fig. 5b.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 13.90 | 53.28 | 4.49 | 1.40 | 0.46 | 20.89 | 0.37 | 2.63 | 2.58 |
| 2 | 18.92 | 51.16 | 4.10 | 1.29 | 0.43 | 18.88 | 0.34 | 2.43 | 2.44 |
| 3 | 14.12 | 53.00 | 5.62 | 1.54 | 0.45 | 21.20 | 0.32 | 2.65 | 1.10 |
| 4 | 18.30 | 51.87 | 4.61 | 1.37 | 0.38 | 19.04 | 0.31 | 2.37 | 1.74 |
| 5 | 6.08 | 55.61 | 7.23 | 1.79 | 0.51 | 25.31 | 0.29 | 3.20 |
SEM–EDX point analysis (wt.%) of the elements of the sample GDA450, displayed in Fig. 5c.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 13.63 | 53.31 | 5.89 | 1.59 | 0.41 | 21.66 | 0.23 | 2.77 | 0.50 |
| 2 | 15.05 | 52.00 | 3.92 | 1.55 | 0.41 | 21.04 | 0.21 | 2.74 | 3.10 |
| 3 | 15.62 | 50.23 | 1.98 | 1.37 | 0.51 | 20.39 | 2.68 | 7.22 | |
| 4 | 19.32 | 49.56 | 0.95 | 1.34 | 0.37 | 19.20 | 2.54 | 6.70 | |
| 5 | 18.06 | 51.08 | 5.34 | 1.43 | 0.44 | 19.54 | 2.49 | 1.61 |
SEM–EDX analysis (wt.%) of the elements of the sample GDA550 displayed in Fig. 5d.
| Spectrum | C | O | Na | Mg | Al | Si | K | Ca | Cu |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 17.23 | 52.35 | 5.43 | 1.48 | 0.52 | 20.16 | 0.23 | 2.60 | |
| 2 | 23.18 | 47.88 | 2.69 | 1.21 | 0.40 | 17.64 | 0.14 | 2.51 | 4.35 |
| 3 | 16.27 | 49.65 | 2.87 | 1.33 | 0.55 | 20.30 | 2.83 | 6.21 | |
| 4 | 13.10 | 48.28 | 1.90 | 1.35 | 0.44 | 20.92 | 2.98 | 11.02 |
The homogeneous morphology of the untreated glass sample (G) does not completely exclude the possible presence of predominant phase separation, which was confirmed through the presence of circles with crescent shaped borders with differential density in the doped sample (GD) using SEM inspection [42]. The elemental analysis using SEM+EDX proved that ion-exchange happened mostly in the areas surrounding the circles with crescent shaped borders of the GD.
Transmission electron microscopy (TEM and HRTEM)Transmission electron microscopy at low magnification, TEM, has provided nano-scale images on the textural aspects of the investigated samples, previously seen under optical microscope, and evidenced the presence of rounded phases in shape and also with crystalline habit corresponding to cuprite, tenorite and metallic copper.
The G sample shows the typical texture of an amorphous material (Fig. 6). In the doped glass GD, circle forms are observed at low resolution and high-contrast mode of imaging (Fig. 7). The annealed samples GDA250, GDA350, GDA450 and GD550, showed rounded nanoparticles and others particles with pseudo-hexagonal habit (Fig. 8a–d). The sample GDA550 revealed a well-developed martensitic structure (Fig. 8d). The size of rounded particles ranges from 2nm to 5nm, while the size of particles with polyhedral habit is between 6nm and 50nm. The size of martensitic plates at this scale ranges from 5nm to 6nm.
TEM images showing rounded nanoparticles and others particles with pseudo-hexagonal habit for GDA250 (a), GDA350 (b), GDA350 (c) and GDA550 (d). In addition, in Fig. 7d a well-developed martensitic structure was observed for sample GDA550.
The crystallographic analysis of the samples GDA250, GDA350, GDA450 and GDA550 carried out with HRTEM and SAED revealed the presence of cuprite and tenorite. Due to the polycrystallinity detected in GDA250 only SAED could be performed to multiple crystals (Fig. 9a). Interplanar spacing corresponding to (101), (011) and (022) of cuprite (JCPDS file 05-0667) and to (112), (310) and (221) of tenorite (JCPDS file 41-0254) were found. Since more prominent crystals were detected after annealing at 350°C, single-crystal electron diffraction patterns could be recorded from GDA350 (Fig. 9b).
![(a) SAED for a polycrystalline site in the GDA250 with concentric ring diffractions with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in yellow and dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single-crystal electron diffraction for a nanocrystal in GDA350 revealing dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in zone axis [1 01¯].](https://static.elsevier.es/multimedia/03663175/unassign/S0366317523000407/v1_202308240225/en/main.assets/gr9.jpeg?xkr=ue/ImdikoIMrsJoerZ+w997EogCnBdOOD93cPFbanNd3TCEZuNxhudow/+3Hrh9WJc+mtnYSSPL9K6mfMWVj1h2ECPUEE5aA7eng5pcAeBjPBwJYK3Ge1OsqmIdZkwCCW2EfcsbsitpMziXf6D8lh+/nD/1i/25YqWV3XalYLDAXrRhg9+9VqJvc/0E2nr0RUgQY3qtEDowKawzbsdM5V69ChYceafAOOcfMakryBeKUHo5/EGfBxT9UEqJmLXahuZj6PfppnsnzeNfrOEZ1z/VDJO4h3d1C7mOLcpA6HC1jIvFO03mzqjs63jBCVlHO "(a) SAED for a polycrystalline site in the GDA250 with concentric ring diffractions with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in yellow and dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single-crystal electron diffraction for a nanocrystal in GDA350 revealing dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in zone axis [1 01¯].")
(a) SAED for a polycrystalline site in the GDA250 with concentric ring diffractions with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in yellow and dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single-crystal electron diffraction for a nanocrystal in GDA350 revealing dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in zone axis [1 01¯].
Electron diffraction (Fig. 10a, b) and HRTEM (Fig. 10c) made for the sample GDA450 revealed the cuprite and tenorite phases and metallic copper nanoparticles.
![(a, b) Single crystal electron diffraction patterns for nanocrystals in GDA450 with d-spacing dhkl corresponding to cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [0 3 1¯] (a), and d-spacing dhkl corresponding to tenorite (CuO with crystal symmetry C2/c) in zone axis [110] (b); (c) HRTEM image of lattice fringes from GDA450 showing three different interplanar distances (colored squares) and their analyzed Fourier transforms that confirmed the presence of tenorite in zone axis [11¯ 0] (yellow) and copper nanoparticles in zone axes [100] (red) and [1 01¯] (blue); tweens also can be observed.](https://static.elsevier.es/multimedia/03663175/unassign/S0366317523000407/v1_202308240225/en/main.assets/gr10.jpeg?xkr=ue/ImdikoIMrsJoerZ+w997EogCnBdOOD93cPFbanNd3TCEZuNxhudow/+3Hrh9WJc+mtnYSSPL9K6mfMWVj1h2ECPUEE5aA7eng5pcAeBjPBwJYK3Ge1OsqmIdZkwCCW2EfcsbsitpMziXf6D8lh+/nD/1i/25YqWV3XalYLDAXrRhg9+9VqJvc/0E2nr0RUgQY3qtEDowKawzbsdM5V69ChYceafAOOcfMakryBeKUHo5/EGfBxT9UEqJmLXahuZj6PfppnsnzeNfrOEZ1z/VDJO4h3d1C7mOLcpA6HC1jIvFO03mzqjs63jBCVlHO "(a, b) Single crystal electron diffraction patterns for nanocrystals in GDA450 with d-spacing dhkl corresponding to cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [0 3 1¯] (a), and d-spacing dhkl corresponding to tenorite (CuO with crystal symmetry C2/c) in zone axis [110] (b); (c) HRTEM image of lattice fringes from GDA450 showing three different interplanar distances (colored squares) and their analyzed Fourier transforms that confirmed the presence of tenorite in zone axis [11¯ 0] (yellow) and copper nanoparticles in zone axes [100] (red) and [1 01¯] (blue); tweens also can be observed.")
(a, b) Single crystal electron diffraction patterns for nanocrystals in GDA450 with d-spacing dhkl corresponding to cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [0 3 1¯] (a), and d-spacing dhkl corresponding to tenorite (CuO with crystal symmetry C2/c) in zone axis [110] (b); (c) HRTEM image of lattice fringes from GDA450 showing three different interplanar distances (colored squares) and their analyzed Fourier transforms that confirmed the presence of tenorite in zone axis [11¯ 0] (yellow) and copper nanoparticles in zone axes [100] (red) and [1 01¯] (blue); tweens also can be observed.
The GDA550 sample showed a lot of polycrystallinity with dhkl attributable principally to tenorite (Fig. 11a) beside some single crystals whose SAED pattern could be recorded as cuprite (Fig. 11b).
![(a) SAED for a polycrystalline site in the GDA550 with concentric ring diffractions with d-spacing corresponding to dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single crystal electron diffraction pattern for nanocrystal in GDA550 with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [100].](https://static.elsevier.es/multimedia/03663175/unassign/S0366317523000407/v1_202308240225/en/main.assets/gr11.jpeg?xkr=ue/ImdikoIMrsJoerZ+w997EogCnBdOOD93cPFbanNd3TCEZuNxhudow/+3Hrh9WJc+mtnYSSPL9K6mfMWVj1h2ECPUEE5aA7eng5pcAeBjPBwJYK3Ge1OsqmIdZkwCCW2EfcsbsitpMziXf6D8lh+/nD/1i/25YqWV3XalYLDAXrRhg9+9VqJvc/0E2nr0RUgQY3qtEDowKawzbsdM5V69ChYceafAOOcfMakryBeKUHo5/EGfBxT9UEqJmLXahuZj6PfppnsnzeNfrOEZ1z/VDJO4h3d1C7mOLcpA6HC1jIvFO03mzqjs63jBCVlHO "(a) SAED for a polycrystalline site in the GDA550 with concentric ring diffractions with d-spacing corresponding to dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single crystal electron diffraction pattern for nanocrystal in GDA550 with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [100].")
(a) SAED for a polycrystalline site in the GDA550 with concentric ring diffractions with d-spacing corresponding to dhkl of tenorite (CuO with crystal symmetry C2/c) in pink. (b) Single crystal electron diffraction pattern for nanocrystal in GDA550 with d-spacing corresponding to dhkl of cuprite (Cu2O with crystal symmetry Pn3¯m) in the zone axis [100].
The low-angle XRD patterns of samples GD and GDA550 are shown in Fig. 12. The XRD spectra of GD show the incipient reflection (200) of cuprite (JCPDS file 05-0667). The XRD spectra of GDA550 sample provided the characteristic reflections (111) and (200) of copper (JCPDS 04-0836) which could correspond to the particles of copper on the glass surface. The spectra of samples GD and GDA250 show the cuprite phase in a very incipient form. In the spectrum of GDA350 sample the coexistence of cuprite and tenorite phases can be observed. In the spectrum of the GDA450 sample, similar to GDA550, the tenorite phase is less visible because at the temperature of 450°C and at surface level the copper is more prominently manifested. In conclusion, the X-ray diffraction of the investigated samples corroborates the observations made with TEM and XPS techniques.
Mechanism of martensitic-like structure and metallic copper particles formation. Discussion
From the data provided by the techniques used in the characterization of the Cu-doped glass and annealed at different temperatures in a reducing atmosphere, it is possible to describe the formation of the martensitic-like structure and metallic copper particles. The martensitic-like structure results due to a martensitic-like transformation, a non-diffusive mechanism of phase transformation in the solid state [47–49], in which the chemical composition of the original and transformed phases is maintained, but the reorganization of atoms into a new crystal structure and the appearance of twins arise to minimize energy and adaptation to the new environment.
After the doping in the CuSO4–Na2SO4 melt, copper in glass occurs in the Cu+ and Cu2+ states, the latter to a lower proportion [50].
In fact, during the ion exchange of Cu+–Na+, copper penetrates into the glass principally in the form of Cu+ ions, replacing the Na+ ions of the matrix [22].
The formation of the copper oxide Cu2O would have occurred according to Eq. (1):
Subsequently, Cu2O transforms into a cuprite structure and then it was transformed to tenorite, which has a more stable structure due to its mechanical strength. The transformation took place through a martensitic-like type process.
Finally, rounded particles of metallic copper were formed by the hydrogen reduction of tenorite [51]. Several authors [52–56] indicated that the reduction of copper oxide with H2 can occur in one, two or three steps; they also stated that the reduction of CuO is easier than the reduction of Cu2O, being able to originate metastable phases instead of originating Cu2O. In addition, the reaction would take place on the surface of the martensitic plates as this is the most favorable zone of interaction of tenorite with hydrogen.
At the temperatures used in the experiments of the present study and according to Rodriguez et al. [56] the reduction mechanism would consist of the two steps, which seem to occur simultaneously. The first would consist of a stepped reduction mechanism:
and the second would be a direct reduction mechanism:These equations would suggest that not all the tenorite transformed into cuprite or copper nanoparticles and not all cuprite transformed into copper nanoparticles. Therefore, the three phases would coexist, as was corroborated by TEM and HRTEM analyses and XPS spectra of the annealed samples.
The appearance of metallic copper particles by reaction of CuO and H2 at 250–300°C, as detected by XPS and observed under optical and electron microscope, would coincide with those cited in the literature for other experiments [53,56,57].
The formation of tenorite and copper nanoparticles through martensitic-like type transformation under reduced atmosphere were not mentioned in any previous publication to date. Certainly, the formation of tenorite was previously published [58], as cluster of spherical like particles and little rod-shaped particles, by the SILAR method (Successive Ionic Layer Adsorption and Reaction) from Cu2O film deposited on a glass substrate at 70°C and annealed in air atmosphere at 500°C. The authors of this method mentioned that the transformation temperature from cuprite to tenorite took place at 300°C, a temperature similar to that observed in the present work; but these authors did not mention the formation of martensitic-type structures such as those described here, under a reducing atmosphere.
Furthermore, the presence of dispersed rounded copper nanoparticles, dendritic growth of nanoparticles, particles with single crystal character and diffraction spots well defined and some twinned particles, was also reported by Diaz-Droguett et al. [59], although the procedure used to obtain nanoparticles under hydrogen atmosphere was different from the one used in this work and these authors also reported no mention of martensitic-like transformations in their samples.
ConclusionsCopper doping of the glass slides and subsequent annealing in Ar/H2 atmosphere for two hours produced cupric phases rounded in shape and also with crystalline habit. The appearance of tenorite and later metallic copper nanoparticles occurred through a martensitic type transformation from cuprite formed prior to annealing. The appearance of the martensitic-like type structure, not mentioned in any previous publication to date, could have provided it with shape memory characteristics, which opens a possibility for future research on this attribute in glass.
FundingThis work was supported by the University of Oviedo (Spain) (RSCT-PAPI-18-EMERG-13) and two work-study grants for the doctoral student Safa Toumi in Trento (Italy) and Oviedo (Spain), from the Tunisian Ministry of Higher Education and Research. An acknowledge the funds received from Ministry of Science and Research (Spain): (MCI-21-PID2020-113558RB-C41) and GRUPIN-IDI/2018/170 to pursue her work at the University of Oviedo, Spain.
The authors wish to acknowledge the operators of the Scientific-Technical Services of University of Oviedo (Spain) for facilitating the acquisition of X-ray, XPS, SEM, TEM and HRTEM characterization methodologies.
