Formation of Au Particles in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) Oxide Matrix by Sol-Gel Growth

Research in composite materials¹⁻³ is one of the thrust areas in materials chemistry and physics because of their wide application potentials. Sol-gel growth^2,4,5 of composite materials via various precursor routes is one of the most efficient and widely used processes for preparing the composite materials. A class of composite materials comprises dispersion of metal nanoparticles/clusters in host metal oxide matrices¹. Considering the wide application potentials of dispersed particles or clusters of Au or alike precious metals¹⁻³ in this report, we discuss our salient results on the formation of gold nano-micro size particles in Cu₄O₃ oxides from copper acetate, chloroauric acid, aniline and hydrochloric acid precursors by sol-gel^2,4,5 growth using a variety of direct structure - sensitive solid state techniques as described in the experimental section.

Reagent grade chemicals are used to prepare the samples. Three mixtures each containing 15 ml of toluene and 1:1 ratios by volume of aniline (C₆H₇N) and HCl are mixed with a fixed weight of chloroauric acid, HAuCl₄, (0.002 M) in the ratios 30:1; 60:1; 90:1 by volume to prepare three samples S1-S3, respectively. To each of the resulting solution 0.01 mol of copper acetate, Cu(OCOCH₃)₂.H₂O, and 30 ml of 0.002 M citric acid solutions are added after stirring for 30 min. The resulting sol is air dried by stirring continuously for 15−25 h at 300 K until gel is obtained in each case. The gel thus formed are decomposed at ~400 K, and finally sintered in the range 673−773 K to obtain the finely powder samples.

Powder XRD patterns of the samples are recorded on a X-ray diffractometer (Model X’pert powder XRD) using Cu Kα₁ radiation (λ=1.5406 Å) at 30 kV/40 mA power. Scanning electron micrographs (SEM) are recorded using ‘Hitachi-S3400’ scanning electron microscope and energy dispersive analysis of X-rays (EDX) by ‘Super dryer-II’ model analyzer attached to the scanning electron microscope. Chemical analysis is done by wet chemistry method⁶ and the density values of the samples are determined by liquid displacement method using carbon tetrachloride as the immersion liquid (density 1.596 g/cc at 300 K). Magnetic moments at 300 K are measured using a vibrating sample magnetometer (VSM) (Model 7404 Series) in the magnetic field range ±10 kG. X-band EPR data at 300 K are recorded on a JEOL-JES-TE100-ESR spectrometer system with 100 kHz magnetic field modulation. The g-values of the EPR lineshapes at 300 K are calibrated with respect to the resonance line of tempol(g_Tempol = 2.0032) which is used as a ‘field marker’. EPR data at 77 and 8 K are recorded at X-band on a Varian EPR spectrometer system using an Air Products Helitran cryostat. The magnetic field is calibrated using a Varian NMR Gaussmeter. Ultra-violet-visible diffuse reflectance spectra of the pressed powder samples recorded over the range 200−800 nm using a Varian model 5000 UV-VIS-NIR spectrophotometer fitted with external diffuse reflectance accessory (model External DRA 2500) consisting of a 150 mm diameter integrating sphere.

Fig. 1 shows the observed powder XRD patterns of the samples S1-S3 prepared by sol-gel method as described in previous section. The peaks at the diffraction angles 38.2354°, 44.4493°, 64.6542° and 77.8077° in 2θ of S1-S3 are ascertained from earlier reported data as due to Au crystallites in the samples.⁷⁻⁹ The unit cells corresponding to the above peaks in the diffraction patterns determined using Fullprof (version 3.90) software package¹⁰ are found to be cubic having space group Fm-3m, and the corresponding unit cell parameters are found to be: a = 4.0742, 4.0764 and 4.0772 Å, respectively in S1-S3. This result is in agreement with the Au crystals reported earlier.⁷⁻⁹ Indexed lattice planes of Au crystals corresponding to the above diffraction peaks in S1-S3 are shown in Fig. 1. Rest of the peaks of the diffraction patterns of the samples S1-S3 are found to be akin to tetragonal dicopper(I) dicopper(II) oxide, Cu₂^ICu₂^IIO₃, phase in the samples.¹¹ The lattice parameters determined from the diffraction peaks are shown in Table 1.

Figure1.

Observed powder XRD patterns of samples S1-S3. Indexed lattice planes are of dispersed Au particles diffraction peaks.

Average crystallites sizes of the dispersed Au crystallites determined by Scherrer equation¹² are found to be in the ranges ~85−140 Å, ~85−150 Å and ~80−150Å in S1-S3, respectively. These results indicate the formation of dispersed Au particles in nano-micro size ranges in the samples. However, it may be mentioned that Scherrer relation probes only the approximate average crystallite sizes of the particles. Moreover, the fullwidth at half of the maximum heights of the diffraction peaks have contribution from the instrumental broadening. Hence the above ranges of the average crystallite sizes of dispersed Au particles are presented with less precession. Chemical analysis result shows the concentrations of Cu¹⁺(3d¹⁰) and Cu²⁺(3d⁹) ions are of the order of ~10²² ions/g in S1-S3 having concentrations of Cu¹⁺(3d¹⁰) ions marginally less than those of Cu²⁺(3d⁹) ions in each sample. It may be mentioned that the Cu⁺ ions are formed by partial reduction of Cu²⁺ ions in the oxide matrices during the formation of the final product. From the above chemical analysis result we conclude that the Au crystallites are dispersed in nonstoichiometric dicopper(I) dicopper(II) oxide, Cu_2−x^ICu₂^IIO_3−δ, (nonstoichiometry parameters: x ≈ 0.20; δ ≈ 0.10) phase in the samples. Slight variation is lattice parameters in S3 as compared with those of S1 and S2 could be attributed to the bulkiness of the precursor materials as well as the time of processing of the samples. Observed values of the densities(Table 1) show marginal variation in S1 to S3 which could also be attributed to the role of the bulkiness of the intermediate solvents during the solid state reaction on the formation of Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) and the dispersion of Au crystallites in Cu₂^ICu₂^IIO_3+δ (x ≈ 0.20; δ ≈ 0.10).

Fig. 2 shows the SEM micrographs of samples S1-S3. The micrographs show somewhat porous surface structures with distributed dense phases of dispersed Au particles as irregular shiny structures. Representative EDX profile of sample S2 is shown in Fig. 3. The EDX profiles of the samples are fairly in conformity with the formation of Au particles in nonstoichiometric dicopper(I) dicopper(II) oxides. However, trace amounts of K and Cl are also detected in the samples as impurities. Presence of K and Cl appear to be the impurities in the precursor chloroauric acid, HAuCl₄.

Figure2.

SEM micrographs of the samples, (a) S1, (b) S2 and (c) S3.

Figure3.

EDX profile of the representative sample S2.

Fig. 4 shows the magnetic moments versus magnetic fields plots in the range ±10 kG at 300 K of the samples S1-S3. Hysteresis behaviour of the samples having very low loop areas shows weakly ferromagnetic nature of the samples at 300 K. Calculated values of the fairly low average magnetic susceptibilities: 9.889×10⁻⁶ emu/gG, 9.745×10⁻⁶ emu/gG and 5.835×10⁻⁶ emu/gG of the samples S1-S3, respectively are in agreement with weakly ferromagnetic nature of the samples at 300 K also. However, it may be mentioned here that these composite metal-metal oxide samples contain Cu²⁺(3d⁹) ions and Au(6s¹) atoms as the magnetic sites, and the contribution of the individual ions/atoms to the magnetic behavoiur of the samples could be interpreted from the EPR response of the samples described in the next section.

Figure4.

Magnetic moment versus magnetic field plots of S1-S3 at 300 K.

Fig. 5 shows the observed EPR lineshapes of the samples S1-S3 at 300, 77 and 8 K. All the EPR lineshapes of S1-S3 at 300, 77 and 8 K are broad and isotropic in nature without any hyperfine structures. The observed g_iso-values of the EPR lineshapes are presented in Table 2. The magnitudes of the g_iso-values show significant positive g-shift with respect to the free electron g_e-value = 2.002319¹³ which indicates that the lineshapes are predominantly due to the Cu²⁺(3d⁹) sites¹⁴⁻¹⁶ of the Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxides. The broad unresolved lineshapes are attributed mainly due to the rapid spin-lattice relaxation time of the Cu²⁺(3d⁹) sites in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxides and also partly due to the highly delocalized Au(6s¹) electrons in the nano-micro size Au metal particles containing cluster of atoms. Indication of very poor hyperfine features in some of the samples at 77 K and 8 K are due to ^{63 or 65}Cu nuclei(nuclear spin I=3/2) hyperfine interactions. Fairly similar g_iso-values^14-16 in the samples in the overall range ~2.053± 0.008−2.333±0.008 indicates the similar magnetic site symmetry distribution in the samples over the range 300-8 K.

Figure5.

Observed X-band EPR lineshapes at (a) 300 K, (b) 77 K and (c) 8 K of the samples S1-S3.

Fig. 6 shows the observed Ultraviolet-visible diffuse reflectance absorption spectra of the samples S1-S3 at 300 K. The spectra show metal-ligand charge-transfer bands ~240−350 nm caused by O²⁻ ligands on UV exposure.¹⁷ Cu²⁺, being a d⁹ ion, experiences a strong Jahn-Teller distortion which leads to the splitting of the d-orbital energy levels¹⁸ and causes predominantly an elongated octahedral coordination with four short in-plane and two longer axial σ-bonds(D_4h symmetry). Accordingly, the following three transitions, viz. ²B_1g→²A_1g, ²B_1g→²B_2g and ²B_1g→²E_g are expected from the ground state¹⁸ B_1g(|d_x²_-y²>). But normally only a single optical absorption maximum is observed in most of the cases.¹⁹ This single optical band is interpreted as the overlap of all the three transitions and is assigned to ²B_1g→²B_2g transitions.²⁰ However, in the present case, the observed weak band ~470 nm is due to the ²B_1g→²E_g transitions,²¹ and the very broad and the weak absorption band ~725 nm is assigned to ²B_1g→²A_1g transitions¹⁹ in Cu²⁺(3d⁹) ions in octahedral coordination having tetragonal distortion.

Figure6.

Observed UV-Vis diffuse reflectance optical absorption spectra of samples S1-S3 at 300 K.

Sol-gel method is used to disperse Au particles in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxide matrices. Powder XRD results and chemical analysis of the samples show rather nano-micro size Au particles are dispersed in tetragonal nonstoichiometric dicopper(I) dicopper(II) oxides, Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10). Presence of hysteresis loops in the magnetic moments versus magnetic field plots having low loop areas show weakly ferromagnetic nature of the samples at 300 K. Observed EPR lineshapes of the samples are broad and isotropic having g_iso-values in the range ~2.053±0.008−2.333±0.008 at 8, 77 and 300 K. The broad EPR lineshapes are due to the rapid spin-lattice relaxation process of the excited Cu²⁺(3d⁹) sites in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxide and also due to the delocalization of the Au(6s¹) electrons in the dispersed Au nano-micro particles in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxides. Ultra-violet-visible diffuse reflectance optical absorption spectra of samples S1-S3 show metal-ligand charge-transfer bands ~240−350 nm caused by O²⁻ ligands on UV exposure, and d-d transitions ~470 nm and ~725 nm in Cu²⁺(3d⁹) ions in octahedral coordination with tetragonal distortion in Cu_2−x^ICu₂^IIO_3−δ (x ≈ 0.20; δ ≈ 0.10) oxides.