Anomalous electrical properties of poly vinyl alcohol films with Tb³⁺ ions and copper nanoparticles in different solvents

Abstract

Laser ablation was used to fabricate copper nanoparticles (NPs) in different solvents from sodium lauryl sulphate, acetone and ethanol by applying 1064 nm radiation from a Nd:YAG laser. The sizes and shapes of the colloidal Cu NPs were investigated by transmission electron microscopy (TEM). The electrical conductivities of Tb³⁺ in polymer films with Cu NPs were measured in the frequency range of 20 Hz to 1 MHz and the temperature range of 308–343 K. It was found that the electrical conductivity of Tb³⁺ was greater for larger Cu NPs with sizes ranging from 20–40 nm in ethanol as compared to Cu NPs with smaller dimensions formed in the other two solvents. It was concluded that using ethanol as the solvent enhanced the electrical conductivity of Tb³⁺ in the polymer film. The activation energy as well as the variation in the loss tangent with temperature also explains the enhanced electrical conductivity.

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I. Introduction

Nanoparticle technology has wide applications in various fields in our day to day life.¹ Nanoparticles (NPs) exhibit unique electrical properties different from those of bulk metals due to quantum size effects. These properties can be further tuned by controlling the shapes, sizes, interparticle spacings and dielectric environments of the NPs. NPs of various elements such as Ag, Au, Pt, Si, C and Co have been prepared to study their properties in nano-regimes and observe their influences on neighbouring particles.^2–6 Copper (Cu) NPs are the focus of current research due to their interesting roll in, electrical, thermal and conducting properties.^7,8 The fabrication process plays an important role in the properties of nanoparticles such as crystallite size, shape and particle size distribution. These factors strongly affect the electrical properties of the material. Various methods are used for the preparation of NPs.⁹ Among these, the pulsed laser ablation of a metal target has attracted great attention due to its simplicity and purity.^10–12 This top-down method provides contaminant-free nanostructural particles in the desired solvent and facilitates the production of nanoparticles in vacuum or in a desired atmosphere. Although the copper NPs produced by this method are highly unstable, they are easily oxidised and precipitate immediately after being exposed to an oxidising medium. The synthesis of Cu nanoparticles by the pulsed laser ablation of a metal target in ambient gas and in non-oxidising liquid media plays a role in preventing the oxidation of NPs.^9,13

Polyvinyl alcohol is a semi-crystalline polymer. Its crystallinity depends on the synthetic procedure and physical aging.^14,15 Its thin film is especially suitable as a host matrix for mixing the colloidal solution of NPs prepared by laser ablation.^16,17 The doping of lanthanides into vinyl multiphase polymer systems can induce changes in various complex properties. These complexes can be used as sensors, electrochromic displays and solid state lasers.^18,19

The electrical behaviour of doped copper nanoparticles in polyurethane has been studied by Valodkar et al.²⁰ Frequency-dependent conductivity and its variation with temperature was the focus of their study. Bose et al.²¹ prepared copper oxide nanoparticles by a chemical route and measured both the dc and ac electrical properties. To the best of our knowledge, few reports exist about the electrical properties of rare earth metals doped with copper nanoparticles in polymers. Rare earth metals exist in the ionic state, and it is interesting to study the electrical properties of polymer films with Tb³⁺ containing Cu NPs.

The focus of the present work is the preparation of colloidal Cu NPs in different solvents from sodium lauryl sulphate (SDS) solution in distilled water, acetone and ethanol with the help of a pulsed Nd-YAG laser. The ablation of bulk copper targets in different solvents was conducted using a 1064 nm laser line, and structural characterisation was performed. Different solvents were adopted because the dielectric constant and dipole moment of the solvent may affect the stability, size, composition and electrical properties of the metal nanoparticles. Furthermore, we have studied the electrical conductivity of Tb³⁺ in PVA films with and without Cu NPs prepared in different solvents.

II. Experimental details

A. Materials

A copper plate (99.99%), acetone, ethanol, sodium lauryl sulphate (C₁₂H₂₅NaO₄S, molecular weight 288.38), terbium oxide (99.9%) and polyvinyl alcohol (MW 14 000) were used as starting materials. All the chemicals were purchased from Sigma-Aldrich.

B. Methodology

Preparation of copper nanoparticles. Fig. 1 shows the experimental setup for laser ablation used to produce the nanoparticles. The Cu NPs were prepared by laser ablation in SDS solution, acetone and ethanol using 1064 nm laser radiation with a pulse duration of 7 ns, a repetition rate of 10 Hz and a fluence of 64 J cm⁻². Using a converging lens with a focal length of 10 cm, the laser beam was focused on the copper target (thickness: 0.5 mm, size: 8 × 5 mm²) containing 10 ml of different solvents. The diameter of the focused laser beam on the copper target was 0.5 mm. The copper plate was ablated for 45 min with constant stirring every time.

Fig. 1 Experimental setup for laser ablation used to produce the Cu nanoparticles with Nd:YAG laser.

Fig. 1(a–c) shows the basic principle of the formation of copper nanoparticles. When the laser beam is allowed to fall on Cu NPs the copper plate, a high temperature and pressure copper plasma is produced at the solid–liquid interface (Fig. 1a). The adiabatic expansion of the hot plasma results in quick cooling, producing copper clusters (Fig. 1b). The copper clusters interact with the surrounding liquid solution to produce copper nanoparticles (Fig. 1c). The copper plate was taken out of the solution, and the prepared colloidal solution of Cu NPs was ultrasonicated for up to four hours. The colloidal NPs were used for structural characterisation and the preparation of polymeric films with Tb³⁺.

Preparation of polymeric films. Polyvinyl alcohol (PVA), terbium oxide (Tb₄O₇), hydrochloric acid (HCl) and colloidal Cu NPs prepared in different solvents by laser ablation were used for the fabrication of four different polymeric films:
1. Film containing PVA + Tb³⁺. A TbCl₃ solution was prepared by heating 0.1 g of Tb₄O₇ in 10 ml HCl at 150 °C. Separately, a solution containing 0.15 g of transparent PVA solution in 5 ml distilled water was prepared. Both the solutions were mixed and stirred continuously with a magnetic stirrer at 800 rpm for 30 min. The resulting mixture was then poured in a petri dish and kept in an oven under slow evaporation at 50 °C, resulting in the PVA + Tb³⁺ polymer film.
2. Film with PVA + Tb³⁺ + Cu NPs in SDS. To the same composition of TbCl₃ and PVA as used in the preparation of sample 1, 10 ml of a colloidal solution of Cu NPs prepared in SDS was added, and the mixture was continuously stirred for 30 min. The mixture was then poured into a petri dish and put in an oven under slow evaporation at 50 °C, yielding the polymer films of PVA + Tb³⁺ + Cu NPs in SDS. A similar procedure was followed to prepare the corresponding thin films in acetone (3) and ethanol (4).

C. Characterisation of the samples

Transmission electron micrographs were taken using a Technai 20G² (Philips) transmission electron microscope (TEM) equipped with a charge coupled device (CCD). The electrical properties of the thin films were measured with a Wayne Kerr precision impedance analyser (6500 P) in the frequency range of 20 Hz to 1 MHz. For these measurements, circular polymer films were cut and sandwiched between two circular silver discs and then placed between the two electrodes of the measuring cell. The contact area of the polymer films was 140.534 mm², and the thickness was about 0.266 mm. The conductance G and the loss tangent D were measured with the help of the impedance values. The impedance analyser settings automatically controlled the temperature of the sample holder and monitored the data through the software. All measurements were carried out in the temperature range of 308–343 K, well below the glass transition temperature of the polymeric film.

III. Results and discussion

A. Structural characterisation using transmission electron microscopy (TEM)

Cu-NPs prepared in different solvents were adhered onto a coated mesh, and the mesh was examined after being dried in a vacuum. Fig. 2A shows the microstructures of the copper NPs in SDS solution (a), acetone (b) and ethanol (c), and Fig. 2B shows the size distribution and particle size. The TEM images show that the copper NPs in different solvents have different shapes (shown in the respective insets). In the cases of SDS and acetone, the Cu NPs are hexagonal in shape with average particle sizes ranging from 12 to 25 nm and 15 to 30 nm, respectively. However, the Cu NPs prepared in ethanol are spherical with particle size varying between 20 and 40 nm. This results from medium effects; SDS may stabilise copper hydroxide and delay its conversion to copper oxide, whereas the nanoparticles may be complexed with the carbonyl group at their surfaces when prepared in acetone.²² In addition, the dipole moment of acetone is higher than that of ethanol, resulting in stronger bonds to the surface of NPs. This may also be due to the fact that the dielectric constant of acetone is less than that of ethanol.

Fig. 2 (A) The microstructures of copper NPs in SDS solution (a), acetone (b) and ethanol (c); insets show the individual nanoparticle shapes in different solvents. (B) Graphs of size distributions and particle sizes for NPs in different solvents.

B. Electrical measurements

The frequency and temperature-dependent conductance G were measured using a impedance analyser. The conductivity was calculated by the following relation:where σ is the real part of the conductivity, L is the thickness of the sample film and A is the area of the films. Fig. 3 shows the frequency dependence of the real part of the conductivity of PVA with Tb³⁺ and PVA with Tb³⁺ and Cu-nanoparticles in different solvents at different temperatures. In Fig. 3(a–d), the conductivity value in the low frequency range (below 100 Hz) changes very slowly. Fig. 3(a–c) indicate an almost frequency independent part below 100 Hz. Nevertheless, Fig. 3d shows a frequency independent part in the low frequency regime (below 100 Hz). Due to the limitations of the instrument, conductivity spectra could not be measured below 20 Hz. It appears that for the samples shown in Fig. 3a–c, the frequency independent part will be clearly visible below 20 Hz. In summary, the frequency independent part could not be observed in all the samples; however, a clear indication of the frequency independent part in the low frequency range is observed in all the samples. In any material, conduction occurs due to the hopping motion of the charged particles. In the low frequency range when the time window is large, the ions and the surroundings have enough time to rearrange themselves, causing random hopping of the charged particles, which gives rise to DC conduction. At the same time, at higher frequencies, the time window is small; thus, the ion surroundings do not have enough time to rearrange themselves, resulting in non-random correlated forward–backward hopping motions. This leads to an increase in conductivity with increasing frequency, causing dispersion in the frequency spectra.²³ Thus, the real part of the conductivity spectra of these systems can be approximated as the sum of the dc and ac conductivities. Fig. 3 also shows that the conductivity increases with increasing frequency in the dispersion regimes. The conductivity is maximised in the case of the PVA film containing Tb³⁺ ions with Cu nanoparticles in ethanol.

Fig. 3 Variation in electrical conductivity versus frequency of polymeric films in the presence of Tb³⁺ ion and Tb³⁺ ion plus Cu NPs in different solvents in the temperature range of 308–343 K.

The combined impedance of a sample is expressed as Z = Z′ − iZ′′, where Z′ and Z′′ are the real and imaginary parts of impedance, respectively. These values are calculated from the following relation:

where D is the loss tangent. The typical impedance plots of Z′ and Z′′ for all the polymeric samples at a temperature of 323 K are shown in Fig. 4. Fig. 4(a–d) show the Cole–Cole plots of PVA with Tb³⁺ (sample 1), PVA with Tb³⁺ and Cu NPs in SDS solution (sample 2), PVA with Tb³⁺ plus Cu NPs in acetone (sample 3) and PVA with Tb³⁺ and Cu NPs in ethanol (sample 4), respectively. The high frequency semicircle corresponds to the bulk response of the polymer film. This arises due to the migration of terbium ions in the polymer matrix. It is also controlled by the bulk resistance of the sample. The immobile polymer chain is polarised in the field of the applied electric field. Thus, the migration of ions and bulk polarisation is parallel in the semicircle region.²⁴ In the low frequency region, the spike characteristic behaves as blocking double layer capacitance.²⁵ This characteristic at low frequency is due to the metal electrode interface and can be regarded as a capacitance. When the capacitance is ideal, it should show a vertical spike. However, in our case, the spikes in the Cole–Cole plots are inclined at angles of less than 90°, which may be attributed to the roughness of the electrode interface.^26,27

Fig. 4 Cole–Cole plots of polymeric films in the presence of Tb³⁺ ion and Tb³⁺ ion with Cu NPs in different solvents at a temperature of 323 K (50 °C).

The bulk resistances R_b of the polymer films were calculated from the intercepts of the semicircle in Fig. 4 with the x-axis. The resistances of the polymer films at 323 K were found to be 2.7 × 10⁶ Ω, 1.8 × 10⁶ Ω, 4.5 × 10⁵ Ω and 5.6 × 10⁴ Ω for samples 1, 2, 3 and 4, respectively. Thus, the maximum resistance is found for the film with Tb³⁺ in PVA, and the minimum resistance corresponds to the film with Tb³⁺ and Cu NPs in ethanol. The bulk conductivities of the samples were calculated using the relation:

The bulk conductivities σ_b for all four samples are plotted in Arrhenius form in Fig. 5(a). It is clear from the figure that the systems exhibit linear behaviour in the temperature region of 308–343 K. The activation energy for conduction was calculated using the Arrhenius relation:

where σ₀ is the pre-exponential factor, E_a is the activation energy for conduction, k is the Boltzmann constant and T is the absolute temperature. The activation energy of a polymer sample in the temperature range of 308–343 K was calculated by the linear fit of Fig. 5(a) between log σ_bT [(ohm cm)⁻¹ K] and 1000/T (K⁻¹); the values thus obtained for the four samples are tabulated in Table 1. A decreasing trend in activation energy is clearly evident from sample 1 to sample 4. This corresponds to increasing electrical conductivity moving from the polymer film with Tb³⁺ alone to that with PVA, Tb³⁺ and Cu NPs in ethanol.

Fig. 5 (a) Arrhenius representation of bulk conductivity and (b) variation in the bulk conductivity of polymeric films in the presence of Tb³⁺ ion and Tb³⁺ ion plus Cu NPs in different solvents.

Table 1 Activation energy for polymeric films in the presence of Tb³⁺ ion and Cu NPs in different solvents

Sample	Activation energy
	308–343 K
PVA + Tb ion (sample 1)	0.26 eV
PVA + Tb ion + Cu NPs in SDS with water (sample 2)	0.11 eV
PVA + Tb ion + Cu NPs in acetone (sample 3)	0.10 eV
PVA + Tb ion + Cu NPs in ethanol (sample 4)	0.02 eV

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The variation in the conductivity of the polymeric samples in different solvents at 323 K is shown in Fig. 5(b). The increase in conductivity from sample 1 to sample 4 can be understood as follows. In sample 1, when only Tb³⁺ ions are dispersed in PVA, the conduction is due to Tb³⁺ ions moving through the cross-linked PVA chains. In sample 2, which contains PVA with Tb³⁺ and Cu NPs in SDS solution, the conduction is due to Cu NPs moving through matrix consisting of PVA + Tb³⁺ cross-linked chains. Individual Cu NPs and Tb³⁺ ions migrate among the vacant sites of the cross-linked chains. The Cu-NPs increase in size and are agglomerated in acetone. Due to the larger particle sizes, small islands are formed in sample 3, which contains PVA with Tb³⁺ and laser-ablated Cu NPs in acetone. This provides more conducting channels for the system, enhancing the conductivity. In sample 4, which contains PVA with Tb³⁺ and Cu NPs in ethanol, the further improvement in particle size provides more conducting channels for the free ions. This causes a further increase in conductivity in sample 4. The observed conductivity phenomenon for all four samples explained above is diagrammed in Fig. 6. The TEM micrographs and corresponding particle size graphs (Fig. 2) also confirm the increase in particle size moving from sample 2 to sample 4. This confirms our hypothesis explaining the increases in conductivity.

Fig. 6 Pictorial representation of the conductivity phenomenon of Tb³⁺ ion in polymer films with copper NPs in different solvents.

To further confirm the enhanced electrical conductivity, we show the variation in loss tangent with temperature at constant frequency in Fig. 7(a–c) for all four samples at 10 kHz (Fig. 7a), 100 kHz (Fig. 7b) and 1 MHz (Fig. 7c). These figures clearly show that the loss tangents are low for all samples, but they show an increasing trend moving from sample 1 to sample 4 (with the exceptions of the first two points of sample 4 and the last two points of sample 2 in Fig. 7a). For any frequency, the loss tangent is greatest for sample 4 (i.e., PVA with Tb³⁺ and Cu nanoparticles in ethanol). The loss tangent D is actually defined as:

where ε′ and ε′′ are the real and imaginary parts of the permittivity, respectively, and σ′ and σ′′ are the real and imaginary parts of the electrical conductivity, respectively. The above equation clearly reveals a higher real part of the conductivity results in a larger loss tangent, which also explains the electrical behaviours of the four samples. This can also be explained in terms of ohmic loss. Higher conductivity will result in larger ohmic loss and vice versa. The loss tangent basically represents the ohmic loss (conduction loss) in any sample. Hence, the sample with the maximum loss tangent (sample 4) will possess the maximum conductivity.

Fig. 7 Variation in loss tangent with temperature at constant frequency for the polymeric films.

IV. Conclusions

Cu-NPs have been prepared in different solvents by laser ablation using a 1064 nm pulsed Nd-YAG laser. The TEM micrographs show hexagonally-shaped Cu NPs in the case of SDS solution and acetone and larger spherical NPs in the case of ethanol. The electrical properties of Tb³⁺ ions in PVA polymer films with Cu NPs in different solvents have been studied. It is concluded that the electrical conductivity of Tb³⁺ in polymer films is the greatest in the presence of larger spherical Cu NPs prepared in ethanol. It is proposed that the bigger size of the nanoparticles in ethanol forms more islands in the polymer matrix, enhancing the electrical conductivity of the film. The activation energy and variation in loss tangent with temperature also confirm the enhanced electrical behaviours of the samples.

Acknowledgements

B. Kumar would like to acknowledge UGC for the Meritorious Fellowship in Science, and G. Kaur would like to acknowledge DST for a project grant.

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