Matrix Effect of Properties of Au, ZnO and Eu₂O₃: Silica, Titania and Alumina Matrices

Abstract

The composites Au/SiO₂, Au/TiO₂, Au/Al₂O₃, ZnO/TiO₂, ZnO/TiO₂, ZnO/Al₂O₃ and Eu₂O₃/SiO₂, Eu₂O₃/TiO₂ and Eu₂O₃/Al₂O₃ were prepared using a solid-state method. The effect of the polymer precursors was investigated using two precursor polymers, Chitosan and Poly(styrene-co-4vinylpyridine), (PS-co-4-PVP) in the M/M_xL_y•Chitosan//M’_xO’_y as well as M/M_xL_y•PS-co-4-PVP//M’_xO’_y with M’_xO’_y = SiO₂, TiO₂ and Al₂O₃. The effects on the particle size and morphology were observed. The new composites were characterized using X-ray powder diffraction, SEM-EDS mapping and HRTEM analysis. The distribution of the metallic nanoparticles as well as the metal oxide nanoparticles inside the matrices depend on the matrix. Marked optical and photocatalytic effects of the Au, ZnO and Eu₂O₃ inside the SiO₂, TiO₂ and Al₂O₃ matrices are expected. An experiment is in course.

1. Introduction

Some of the typical properties of nanostructured nanoparticles such as the plasmon, their size, their morphology and band gap depend on the media that surround them [1,2]. For nanoparticles in a solution, their stabilizer and solvents control and/or affect these properties [1,2]. In a solid state, when these nanoparticles are inside of a solid matrix, those properties are affected by the characteristic of the matrix. Scarce studies of the medium on the optical effects and on other properties have been reported, with almost all of them using metallic nanoparticles [2,3,4], and no studies with metal oxide nanoparticles have been reported. For solid states, the optical properties of metal nanoparticles (Au, Ag, and Cu) are greatly influenced by the interface between the nanoparticles and the matrix [1,3,4]. The influence of the structure’s environment on the optical properties of some metal noble nanoparticles were recently revised by Tondello and Parkin [3,4]. Matsuoka [5] studied the influences of the dielectric constant of TiO₂, ZrO₂ and Al₂O₃ on the optical properties of Au nanoparticles. A direct linear relation between the maxima absorption of the Au nanoparticles with the dielectric constant was found. On the other hand, a solution study of the external dielectric medium (of the solvent) on the surface plasmon resonance spectrum of silver nanoparticles, from an experimental approach as well as a theoretical approach, was reported by Schatz et al. [1,2]. A theoretical study of the influences of the size, shape and dielectric environment on the optical properties of metal nanoparticles in a solution was reported by Schatz et al. [1]. Saviot [6] studied the optical properties of gold and silver inside an alumina matrix. Goutam De [7] studied the optical properties of the nanoclusters of Cu inside silica. Additionally, Torrell studied the functional and optical properties of Au:TiO₂ nanocomposite films [8]. A direct relation between the size of the Au particles and the annealing temperature was found.

Finally, Mattei et al. [9] reported the effect of the ZrO₂•SiO₂ matrix on the optical properties of silver-doped ZrO₂. Although the composites M/M’_xO’ have been studied the most, the composites M_xO_y/M’_xO’_y have been scarcely investigated [10,11,12].

Previously, we studied the matrix effect of the medium effect of NiO on the optical properties and catalytic degradation of methylene blue inside the matrices SiO₂, TiO₂ and Al₂O₃ [13]. The efficiency of the photocatalytic activity depends on the formation of a p-n junction between NiO acting as p-NiO and the metal oxide matrix acting as an n-metal oxide. TiO₂ presents the most effective p-NiO//n-TiO₂ junction. Moreover, the optical parameters Eg and λ_max depend on the dielectric constant and the refractive index of the matrix medium, in a manner of which depends on the preparation procedure [13].

Now, we report the synthesis and characterization of the nanocomposites Au/SiO₂ [14], Au/TiO₂, Au/Al₂O₃, ZnO/SiO₂ [15], ZnO/TiO₂, ZnO/Al₂O₃, Eu₂O₃/SiO₂, Eu₂O₃/TiO₂ and Eu₂O₃/Al₂O₃ [16] for a future study of the optical as well as photocatalytic properties of Au, ZnO and Eu₂O₃ inside SiO₂, TiO₂ and Al₂O₃ matrices. We selected Au as a typical noble metal, ZnO as a typical metal oxide and Eu₂O₃ as a typical representative of the lanthanide series. As a preparative general method for these composites, we used the solid-state method from the pyrolysis of the precursors M_xL_y•Chitosan//M’_xO’_y, where M_xL_y = AuCl₃, ZnCl₂ and Eu(NO₃)₃ and M’_xO’_y = SiO₂, TiO₂ and Al₂O₃, using a general previously reported method [13,14,15,16,17,18]. Some preliminary reports of Au/SiO₂ [14], ZnO/SiO₂ [15], Eu₂O₃/SiO₂ [16], ReO₃/SiO₂ [17], Rh/RhO₂/SiO₂ [18], Rh₂O₃/SiO₂ [18], ThO₂/SiO₂ and ThO₂/TiO₂ [19] have recently been reported. A general schematic diagram of the method is shown in Figure 1.

2. Materials and Methods

The composites were prepared according to a previously reported method [13,14,15,16,17,18]. Some experimental details used for the preparation of the composites Au//M’_xO’_y, ZnO//M’_xO’_y and Eu₂O₃//M’_xO’_y are shown in Supplementary Information S1.

2.1. Materials and Common Procedures

AuCl₃, ZnCl₂ and Eu(NO₃)₃ from Aldrich (Sigma-Aldrich Quimica Ltda, Chile) were used as received. Chitosan (Aldrich) of low molecular weight was used as received. An estimation of the molecular weight was performed via viscosimetry. The average molecular weight was determined using the Mark–Houwink equation, and values of [h] were obtained using the parameter previously reported by Rinaudi et al. [20]. The solvent used was an aqueous solution of acetic acid, NaCl and urea. The value was Mw = 61.000. All the reactions were made in CH₂Cl₂ as solvent. Poly(styrene-co-4vinylpyridine) (Aldrich) with 90% of pyridine groups was used as received.

2.2. General Procedure

Metal macromolecular complexes (1)–(18) were prepared according to published procedures [13,14,15,16,17,18]. In a typical synthesis, the respective metallic salt was added in a Schlenk tube over a CH₂Cl₂ solvent under magnetic stirring and then the respective polymer PSP-co-4-PVP or Chitosan was added according to a 1:1 molar ratio. The reaction time and other details for each metallic salt’s reaction are given in Table S1 of the Supplementary Information. After this, the supernatant solution (if the solid decanted) was extracted with a syringe, and the solid was dried under a reduced pressure. Further experimental details for the reactions are given in Table S1 of the Supplementary Information. Owing to their insolubility, the characterization of the precursors was made only via IR spectroscopy.

2.3. Pyrolysis of the Precursors

The pyrolysis experiments were made by pouring a weighed portion (0.05–0.15 g) of the metal polymer precursor 1–18 into aluminum oxide boats that were placed in a furnace (Daihan oven model Wise Therm FHP-12) under a flow of air, heating from 25 °C to upper temperature limits of 300 °C, and then to 800 °C, followed by annealing for 2–4 h in each case. The heating rate was consistently maintained at 10 °C min⁻¹ for all experiments. Solid pyrolytic samples were characterized via X-ray diffraction of powders (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infra-red (FTIR) spectroscopy. SEM images were acquired with a Philips EM 300 scanning electron microscope. Energy dispersive X-ray analysis (EDAX) was performed on a NORAN Instrument micro-probe attached to a JEOL 5410 scanning electron microscope. TEM data were acquired using a JEOL SX100 and a JEOL 2011 transmission electron microscope. High-resolution transmission electron microscopy (HR-TEM) was performed using a JEOL 2000FX TEM microscope at 200 kV to determine the average particle size, distribution, elemental composition and crystallinity of the samples. The average particle size was calculated using the Digital Micrograph software. The TEM samples were prepared by dispersing pyrolyzed material onto copper grids and dried at room temperature. X-ray diffraction (XRD) was conducted at room temperature on a Siemens D-5000 diffractometer with θ–2θ geometry. The XRD data were collected using Cu-Kα radiation (40 kV, 30 mA). FTIR measurements were performed on a Perkin Elmer FTIR spectrophotometer, Spectrum BXII model.

3. Results and Discussions

The composites Au/SiO₂, Au/TiO₂, Au/Al₂O₃, ZnO/TiO₂, ZnO/TiO₂, ZnO/Al₂O₃ and Eu₂O₃/SiO₂, Eu₂O₃/TiO₂ and Eu₂O₃/Al₂O₃ were identified via power XRD diffraction. The composite Au/SiO₂ was previously studied in [14]. For Au/TiO₂ (see Supplementary Information S2), the main diffraction peaks of the Au of planes (111), (200), (2201) and (311) were clearly observed [14], as well as the main diffraction peaks of the matrix TiO₂ in the anatase of planes (101), (004), (220), (105) and (211) [13]. For the Au/TiO₂ obtained from the PS-co-4-PVP precursors, a similar XRD diffraction pattern was observed (see Supplementary Materials). For Au/Al₂O₃, the diffraction pattern shows the main diffraction peaks of the Au of planes (111), (200), (2201) and (311) [14], and shows less intense main diffraction peaks of the matrix Al₂O₃ for planes (014), (110), (300), (105) and (119) [13] (see Supplementary Information S2). A similar XRD diffraction pattern was observed for Au in the Au/Al₂O₃ composite in addition to the diffraction pattern of Al₂O₃ (indicated in the XRD pattern) from the Chitosan as well as the PS-co-4-PVP container precursors (see also Supplementary Information S2). For ZnO/TiO₂ from the Chitosan, weak peaks of ZnO from planes (100), (002), (103) and (201) [15] were observed. On the other hand, strong diffraction peaks from planes (101), (103), (200), (105), (211) and (215) of the TiO₂ anatase were observed. A similar XRD diffraction pattern was observed for the ZnO/TiO₂ composite from the PS-co-4-PVP precursors. For the ZnO/Al₂O₃ composite from the Chitosan precursors, strong diffraction peaks from planes (100), (002), (102) and (103) assigned to ZnO are present in their XRD patterns [15]. On the other hand, weak diffraction peaks assigned to planes (014), (110), (300), (105) and (116) of Al₂O₃ were also observed [13]. A similar XRD pattern from the ZnO/Al₂O₃ composite from the PS-co-4-PVP polymer was observed. For the Eu₂O₃/TiO₂ composite from Chitosan, an intense plane (211), as well as weak peaks corresponding to planes (123), (411), (600), (543), and (642) of Eu₂O₃, were observed [16]. On the other hand, for Eu₂O₃/TiO₂ from PS-co-4-PVP, a similar XRD pattern was observed. For the Eu₂O₃/Al₂O₃ from Chitosan, owing to the luminescence of the sample, the base line increased as the value of 2θ increased. Despite this, the typical peaks of Eu₂O₃ and Al₂O₃ were clearly observed (see Supplementary Information S2).

The HRTEM measurements were made for some representative M_xL_y•Chitosan//M_xO_y composites. According to the HRTEM analysis, for the Au/TiO₂ composite, the TiO₂ matrix induced the nucleation of uniform nanoparticles smaller than the Au/Al₂O₃ composite, and where the Au nanoparticles were bigger and more irregular, they were induced by the Al₂O₃ matrix (see Figure 2).

On the other hand, for the ZnO/Al₂O₃ composite obtained from the Chitosan precursor, some polymorphological shapes were observed (see Figure 3). In addition, some squared and octahedron-shaped nanoparticles (enclosed in red circles) as well as some lamellar areas were observed.

For Eu₂O₃/SiO₂ (see Figure 4), big shapes were observed, as shown in Figure 4.

As for the Eu₂O₃/TiO₂ nanoparticles (see Figure 5), big shapes of TiO₂ coated with Eu₂O₃ were observed. The EDS analysis shown in Figure 5b,c, confirms these features.

Also, in some areas, the nanoparticles of the TiO₂ anatase showing the typical interplanar 0.35 nm corresponding to the (101) plane were observed, as shown in Figure 6, which is in agreement with the findings from the XRD analysis.

For the Eu₂O₃/Al₂O₃ composite, small “type worms” were observed (see Figure 7).

For some of the composites, the distributions of the respective Au, ZnO and Eu₂O₃ inside the SiO₂, TiO₂ and Al₂O₃ matrices were investigated via SEM-EDS mapping. Well-dispersed and small Au nanoparticles were observed.

The SEM EDS mapping for the composite Au/SiO₂ was previously reported in [14] (see Supplementary Information S3).

For the composite Au/TiO₂, as shown in Figure 8, a homogeneous distribution of Au nanoparticles inside TiO₂ was observed.

For the composite Au/Al₂O₃, the distribution of the Au nanoparticles inside the Al₂O₃ matrix was not very homogeneous, and some agglomerations of the Au nanoparticles in some zones of the aluminum oxide matrix were observed (see Figure 9). Additionally, there appears to be a matrix effect of the dispersion of the Au nanoparticles inside the Al₂O₃ matrix.

On the other hand, for the composite ZnO/Al₂O₃, a near homogeneous distribution of the ZnO nanoparticles inside the Al₂O₃ matrix was observed (see Figure 10).

For the Eu₂O₃/TiO₂ composite, a distribution of very abundant and small Eu₂O₃ nanoparticles inside TiO₂ was observed (see Figure 11).

On the other hand, for the Eu₂O₃/Al₂O₃ composite, a linear sweep study per element was performed. As shown in Figure 12, there are zones where the contents of oxygen and aluminum are high, and the Eu content is lower, suggesting the presence of a minor content of Eu₂O₃ inside the matrix Al₂O₃, but it would need to have a higher content to be in accordance with the behavior of Al₂O as a matrix.

3.1. Effect of the SiO₂, TiO₂ and Al₂O₃ Matrices on the Distribution of Au, ZnO and Eu₂O₃ inside Them

The SiO₂, TiO₂ and Al₂O₃ matrices influence the distribution of the Au, ZnO and Eu₂O₃ inside them. For Au included in SiO₂, a uniform distribution was observed (see Supporting Information S3). On the other hand, for the Au inside TiO₂, non-uniform nanoparticles and some agglomerate nanoparticles were observed (see Figure 8). For the Au nanoparticles inside Al₂O₃ and similar situations, see Figure 9.

For ZnO inside Al₂O₃, nanoparticles that were not well distributed and some agglomerate nanoparticles of the oxide inside alumina were observed (see Figure 10).

On the other hand, for Eu₂O₃ inside TiO₂, a uniform distribution but very close lanthanide oxide inside the matrix was observed (see Figure 11).

3.2. Morphology Effect of the Au, ZnO and Eu₂O₃ Nanoparticle by the Matrices SiO₂, TiO₂ and Al₂O₃

The SEM image of the Au nanoparticles included inside the SiO₂, TiO₂ and Al₂O₃ matrices shows some minor effect on the morphology, as shown in the Supporting Information (S4). For the Au inside SiO₂ and TiO₂, dense agglomerate grains with variated forms were observed, while for the Au inside Al₂O₃, a “some foam” 3D form was observed.

For the Eu₂O₃ nanoparticles, the inclusion inside the matrices TiO₂ and Al₂O₃ produces significant changes in their morphology, as shown in the Supporting Information (S5). For Eu₂O₃ inside TiO₂, dense grains of several shapes that were not distributed uniformly were observed. On the other hand, for Eu₂O₃ inside TiO₂, a “foam like” morphology was observed.

For the composite ZnO/Al₂O₃, a “foam type” morphology was observed, as shown in Supporting Information S6, which is formed by “worms”, as shown in Supporting Information S6b, in the most enlarged image. A comparison with the morphology of the ZnO/SiO₂ was not possible because ZnO reacts with SiO₂ to give Zn₂SiO₄ and SiO₂ [15]. On the other hand, the study for the ZnO/TiO₂ composite is not available.

3.3. Morphology Comparation

3.3.1. Au

The morphology of the Au/SiO₂, Au/TiO₂ and Au/Al₂O₃ composites depends a lot on the preparation method used. For Au/SiO₂, rather large grains and agglomerates of various shapes in some areas were observed (see Supplementary Materials S4a), while for the Au/SiO₂ composite obtained from a PE-CVD/RF sputtering route, low-sized particles with a pseudo-spherical shape (cluster-like systems) were observed on the growth surface [21]. On the other hand, SiO₂/Au hybrid microspheres with a diameter of 200 nm were obtained via the galvanic replacement of SiO₂/Ag hybrid microspheres and chlorauric acid (HAuCl₄) solution [22]. Also, monodispersed nanospheres were obtained via a straightforward one-step method, which was developed to synthesize Au–SiO₂ composite nanospheres [23]. Monodisperse hybrid silica nanospheres that had anilino-methyl on the surfaces were prepared first. The as-prepared hybrid silica nanospheres were then used to obtain Au nanocrystal through the redox reaction of HAuCl₄ and anilino-methyl.

3.3.2. For Au/TiO₂ Composites

Using our current method in the solid state, Au/TiO₂ nanocomposites were obtained that present dense, separated grains of various sizes and shapes (see Supplementary Material S4b). On the other hand, for the Au/TiO₂ composites prepared using a spray hydrolytic method and using a photoreduction technique at 90 °C, non-uniform growth of particle size distribution in spherical morphology were observed. The particle size is in the range of 6.0–6.7 nm [24]. Using electrospinning combined with a calcination treatment, the Au/TiO₂ nanofiber composites were prepared [25].

3.3.3. Au/Al₂O₃

Using our solid-state method, a “foam” type morphology was observed (see Supplementary Material S4c), and Au@Al₂O₃ core–shell particles that were mostly spherical in shape with a conical cavity were obtained using a one-step method with continuous-wave fiber laser ablation on an aluminum (Al) plate coated with a gold (Au) monolayer immersed in ethanol [26]. The shape of the nanoparticles was roughly spherical with an average size of approximately 20 nm.

Spherical nanoporous forms of the Au/Al₂O₃ composite with sizes in the range of 54–293 nm were obtained via a combination of solid-state dewetting of Ag/Au bi-layers and subsequent dealloying [27]. The morphology seen by the SEM shows nanoporous spheres with pore sizes in the range of 11.6–50.4 nm.

3.3.4. ZnO/Al₂O₃

As can be seen from Figure S6 of the Supplementary Material, the SEM image shows a morphology of a mixture of “worm-like” shapes and continuous zones of metallic foams. This morphology contrasts with that obtained using a method in which the ZnO and Al₂O₃ sols are mixed to give the mixture of two colloidal sols, which was subjected to a multi-spin-coating process, which exhibits a morphology of dense grains and is united by grain boundaries [28]. On the other hand, a continuous porous morphology was obtained from zinc nitrate (Zn(NO₃)₂·6H₂O, Sigma-Aldrich), aluminum nitrate (Al(NO₃)₃x9H₂O) and NH₃-H₂O, followed by the addition of a 1:1 (v/v) solution of NH₃, precipitation to pH 7 and drying at 100 °C and then at 800 °C [29].

A different ZnO/Al₂O₃ morphology with a “metallic foam” type morphology was obtained from a mixed suspension of Al₂O₃ and ZnO powders. Then, the stable mixed suspension spray dried, and the resulting powder was first burned out at 500 °C for 4 h in air, and then sintered in flowing argon at 1400 °C for 2 h [30].

A morphology of the ZnO/Al₂O₃ composite similar to that obtained by our present method was observed using a solution method starting from the precursors (Zn(CH₃COO)₂·2H₂O, Merck, 99.5%), urea, NH₂CONH₂ and bayerite powder, Al(OH)₃; in the presence of urea, the collar induced a precipitate, which was calcined at 400 °C [31].

The morphology of the Eu₂O₃/TiO₂ composite (see Figure S5a of Supplementary Materials) shows fractured and multiform cubohedrons joined in a disorderly manner. No SEM studies of this type of composite for the comparison of morphologies were found in the literature.

The morphology of the Eu₂O₃/Al₂O₃ composite (see Figure S5b of Supplementary Materials) is of the “metallic foam” type. No SEM studies of similar composites prepared using other methods for the comparison of morphologies were found in the literature.

4. Conclusions

The composites Au/SiO₂, Au/TiO₂, Au/Al₂O₃, ZnO/TiO₂, ZnO/TiO₂, ZnO/Al₂O₃ and Eu₂O₃/SiO₂, Eu₂O₃/TiO₂ and Eu₂O₃/Al₂O₃ were prepared in a single step, unlike another in a solution that involves at least two stages. The effect of the polymer precursors was investigated using the polymer Chitosan and Poly(styrene-co-4vinylpyridine), (PS-co-4-PVP) in the M/M_xL_y•Chitosan//M’_xO’_y as well as M/M_xL_y•PS-co-4-PVP//M’_xO’_y with M’_xO’_y = SiO₂, TiO₂ and Al₂O₃. The nanocomposites were obtained in a single step, unlike another in solution that involves at least two stages. SiO₂ induces small and well-distributed Au nanoparticles inside the matrix, while TiO₂ and Al₂O₃ induce nanoparticles that are not well distributed rather than agglomerate Au nanoparticles inside both matrices.

In relation to morphology, SiO₂ and TiO₂ induce dense agglomerate grains of Au inside with varied forms, while for the Au inside Al₂O₃, a “some foam” 3D form was observed. Marked optical and photocatalytic effects of the Au, ZnO and Eu₂O₃ inside the SiO₂, TiO₂ and Al₂O₃ matrices are expected. Those experiments are in course.