polymers Article
Polymeric Nanocomposites Membranes with High Permittivity Based on PVA-ZnO Nanoparticles for Potential Applications in Flexible Electronics Roberto Ambrosio 1, * , Amanda Carrillo 2 , Maria L. Mota 2,3 , Karla de la Torre 2 , Richard Torrealba 1 , Mario Moreno 4 , Hector Vazquez 5 , Javier Flores 1,6 and Israel Vivaldo 1 1 2 3 4 5 6
*
Electronic Department, Meritorious Autonomous University of Puebla, 72590 Puebla, Mexico;
[email protected] (R.T.);
[email protected] (J.F.);
[email protected] (I.V.) Electrical Department, Ciudad Juarez Autonomous University, 32310 Chihuahua, Mexico;
[email protected] (A.C.);
[email protected] (M.L.M.);
[email protected] (K.d.l.T.) CONACYT, Ciudad Juarez Autonomous University 32310 Chihuahua, Mexico Electronic Department, National Institute for Astrophysics Optics and Electronics, 72000 Puebla, Mexico;
[email protected] Electronic Instrumentation Faculty, Universidad Veracruzana, 91000 Xalapa, Mexico;
[email protected] National Institute of Technology of Mexico -I.T. Puebla, 72220 Puebla, Mexico Correspondence:
[email protected]; Tel.: +52-(222)-229-55-00
Received: 24 October 2018; Accepted: 7 December 2018; Published: 11 December 2018
Abstract: This study reports the optical, structural, electrical and dielectric properties of Poly (vinyl alcohol) thin films membranes with embedded ZnO nanoparticles (PVA/ZnO) obtained by the solution casting method at low temperature of deposition. Fourier Transform Infrared spectra showed the characteristics peaks, which correspond to O–H and Zn–O bonds present in the hybrid material. The X-ray diffraction patterns indicated the presence of ZnO structure into the films. The composite material showed low absorbance and a wide band of gap energy from 5.5 to 5.83 eV. The surface morphology for the thin films of PVA/ZnO was studied by Atomic Force Microscopy and Scanning Electron Microscopy. The electrical properties of the membranes were also characterized by current-voltage characteristics and the DC conductivity showed Arrhenius behavior with values of activation energy from 0.62 to 0.78 eV and maximum conductivity at 2.4 × 10−12 S/cm. The dielectric properties of the nanocomposites were measured from low to high frequencies, and the results showed a high dielectric constant (ε) in the order of 104 at low frequency and values from ε ≈ 2000 to 100 in the range of 1 KHz–1 MHz respectively. The properties of PVA/ZnO such as the high permittivity and the low temperature of processing make it a suitable material for potential applications in the development of flexible electronic devices. Keywords: solution process; thin films; composite material; dielectric constant
1. Introduction Composite materials based on a polymeric matrix with embedded nanoparticles have gained attention due to their electrical, mechanical, optical and chemical properties that can be used in the development of biomedical devices, solar cells, sensors, capacitors, and absorbers for electromagnetic (EM) attenuation, as well as other devices [1–4]. Polymeric materials meet the requirements for flexible electronics with high breakdown strength, low temperature of deposition and easy steps for processing such as like spin coating and drop, however they have low dielectric permittivity and poor heat tolerance. By contrast, high-k ceramic materials have high permittivity and strong thermal resistance, however they present low breakdown strength and high mechanical brittle [5]. Polymers 2018, 10, 1370; doi:10.3390/polym10121370
www.mdpi.com/journal/polymers
Polymers 2018, 10, 1370
2 of 17
Therefore, the composite dielectric materials prepared via blending polymer and high-k nanoparticles like ceramics have been extensively researched. A hybrid material consists of soluble polymers with inorganic component with excellent mechanical, optoelectronics and dielectric properties due to the combination of the organic and inorganic components, and it can be deposited as a thin film in different substrates. Therefore, the number of contributions in the development of hybrid composites based on polymers and nanoparticles with high permittivity, low cost, and easily tunable properties, has become a major topic in the research of materials [6]. Recently, some works have reported the integration of thin films based on polymeric materials such as Polyvinyl alcohol (PVA) as a gate dielectrics for the development of organic Thin Film Transistors (TFT). This integration has a high dielectric constant which enhances the gate capacitance, with the advantages of solution processable material, low cost, non-toxic, with flexible hydrophilic network and a low temperature of deposition [7,8]. Recently polymer nanocomposite (PNC) films based on the blend matrix of poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) incorporating zinc oxide (ZnO) nanoparticles have been reported for flexible nano-electronics with a dielectric constant value of 8 at 20 Hz [9]. Other works have reported nanocomposite systems using Silicon carbide SiC/PVA and SiC/PVC with Polyvinyl chloride prepared by solution cast method, obtaining values of dielectric constant around 239 at 1 GHz, these composites dielectrics could be used at high frequencies (such as 1 MHz and 1 GHz) [5]. PVA is a poor electric conductor, is water soluble, has carbon chain backbone with OH groups and is eco-friendly, and its physical properties may be adapted to a specific requirement in conjunction with inorganic materials [10]. On the other hand, nanoparticles of Zinc oxide (ZnO Nps) have been used in memory devices, gas sensors, thin film devices, and flexible electronic devices [11–13]. Furthermore, the utilization of ZnO as semiconductive filler to prepare high dielectric constant polymer composites has been reported [14]. In relation to hybrid materials, few studies have carried out research about the dielectric properties of PVA with embedded ZnO nanoparticles into the polymeric matrix. J.J. Mathen et al. synthesized membranes of PVA/ZnO for development of an UV-A sensor on an ITO substrate that showed the interfacial interaction between the filler and the matrix, resulting in a large improvement in the dielectric, optical and mechanical properties [2]. P. I. Devi et al. studied the dielectric properties of a hybrid composite based on Polyvinylidene fluoride PVDF-ZnO for microwave frequencies, it showed a decrease in dielectric constant and dielectric loss with the frequency, and the ZnO composition has a great influence on the trend and magnitude of dielectric properties [15]. Sugumaran et al. obtained a hybrid poly (vinyl alcohol)-indium zinc oxide (PVA-InZnO) thin films by a simple dip coating method with dielectric constant values of around 6 to 20 [16]. Recently, a nanocomposite polymer films based on PVA and TiO2 nanoparticles have been reported with relative high permittivity [17]. Therefore, there is interest in obtaining a composite material with a high dielectric constant which to meet the requirements for flexible electronics such as a low temperature of deposition, stability, flexibility and low cost. To the best of our knowledge, very few works have been done to obtain high dielectric polymer composites based on semiconductor nanoparticles for a broad range of frequencies, in addition to reporting the electrical properties with a good ohmic behavior in the interface metal-PVA/ZnO Nps. This work reports the synthesis and characterization of Poly (Vinyl Alcohol) thin films with embedded ZnO nanoparticles (ZnO Nps) by solution casting method, incorporating the advantage that the electrical and optical properties can be tuned by adding ZnO nanoparticles into the polymeric matrix. The nanocomposites have been characterized using Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), UV-vis spectroscopy to determine band gap, and Atomic Force Microscopy (AFM) for surface roughness of the membranes. Current-Voltage (I-V) characteristics for DC conductivity. The dielectric properties of PVA–ZnO nanocomposites were measured from low to high frequencies. The results show high dielectric constant (ε) at low frequencies, even at high frequencies ε is higher than other related composites materials, thus the hybrid PVA-ZnO Nps make it a suitable for potential applications in electronic devices.
Polymers 2018, 10, 1370
3 of 17
2. Materials and Methods Polymers 2018, 10, x FOR PEER REVIEW
3 of 17
2.1. Materials
For For the the synthesis synthesis was was used used Polyvinyl Polyvinyl Alcohol Alcohol (PVA) (PVA) from from sigma sigma Aldrich Aldrich (CDMX, (CDMX, MX) MX) with with an an average 130,000and and99% 99%hydrolyzed hydrolyzedtotoobtain obtainthe themembranes. membranes. solution average molecular weight Mww==130,000 AA solution of ◦ C in order to of PVA wasprepared preparedusing using2.5g 2.5 g powderinin5050mL mLofofdistilled distilledwater waterand andstirred stirred at at 90 °C PVA was powder to obtain obtain aa homogenous homogenous solution. solution. For For ZnO ZnO Nps, Nps, Sodium Sodium Dodecyl Dodecyl Sulfate Sulfate (SDS), (SDS), Zinc Zinc chloride chloride (ZnCl (ZnCl22), Acid Potassium Hydroxide Hydroxide (KOH) (KOH) and Ammonia (HN44)) were used to form Acid Citric Citric (C (C66H H88O77),), Potassium form ZnO ZnO nanoparticles. nanoparticles. The The general general process process is is depicted depicted in in Figure Figure 1. 1.
Figure 1. General sequence of synthesis of PVA/ZnO membranes. Figure 1. General sequence of synthesis of PVA/ZnO membranes.
Preparation of PVA/ZnO Membranes Preparation of PVA/ZnO Membranes The membranes as thin films were prepared by solution casting method, following the next steps: as thin films were was prepared bywith solution casting method, following firstly,The themembranes solution of ZnO nanoparticles cleaned distilled water. The next step wasthe to next add steps: the solution(TEA) of ZnO nanoparticles was cleaned with distilled The was 0.3 mLfirstly, of trietanolamyne (1 M) in 3 mL of nanoparticles solution; thenwater. 19.2 mL of next PVA step that was to add 0.3 mL of trietanolamyne M)solution in 3 mL was of nanoparticles then 19.2 mL previously prepared was added. (TEA) Finally,(1the deposited onsolution; glass petri dishes andofit PVA was that wasatpreviously was added. Finally, the solution wasindeposited glassmass petri(g/mol) dishes heating 80 ◦ C for 40prepared min to obtain the membranes. The quantities liters andon molar andthe it was heatinginatthe 80synthesis °C for 40ofmin to Nps obtain membranes. The for precursors ZnO arethe listed and labeled in quantities Table 1. in liters and molar mass (g/mol) for the precursors in the synthesis of ZnO Nps are listed and labeled in Table 1. Table 1. Details of the solutions to synthetize ZnO nanoparticles. Table 1. Details of the solutions to synthetize ZnO nanoparticles. Labeled Sample SDS ZnCl2 C6 H8 O7 Solution Labeled Sample Z1
Z1
Y1
Y1
Z4
Z4
Y4
Y4
SDS 5 mL 5 mL 288.3 (g/mol) 288.3 (g/mol) 0.00005 mol 0.00005 mol 5 mL 5 mL 288.3 (g/mol) 288.3 (g/mol) 0.000050.00005 mol mol 5 mL 5 mL 288.3 (g/mol) 288.3 (g/mol) 0.00005 (mol) 0.00005 (mol) 5 mL 5 mL 288.3 (g/mol) 288.3 0.00005 (mol)(g/mol)
0.00005 (mol)
ZnCl2 1 mL 1 mL 136.28 (g/mol) 136.28 (g/mol) 0.0002 mol 0.0002 mol mL 1 1mL 136.28(g/mol) (g/mol) 136.28 0.0002mol mol 0.0002 2.5 mL 2.5 mL 136.28 (g/mol) 136.28 (g/mol) 0.0005 (mol) 0.0005 (mol) 2.5 mL 2.5 mL 136.28 (g/mol) 136.28 (g/mol) 0.00005 (mol)
0.00005 (mol)
C6 H 8 O 7
1 mL 1 mL 192.15 (g/mol) 192.15 (g/mol) 0.00002 (mol)
0.00002 (mol) 1 1mL mL 192.15 (g/mol) 192.15 (g/mol) 0.00002 0.00002(mol) (mol) 1.2 mL 1.2 mL 192.15 (g/mol) 192.15 (g/mol) 0.000025 (mol) 0.000025 (mol) 1.2 mL 1.2 mL 192.15 (g/mol) 192.15 (g/mol) 0.000025 (mol)
Solution
NH4
NH4
KOHKOH
NH4 NH4 KOH
KOH
0.000025 (mol)
2.2. Characterization 2.2. Characterization The ZnO Nps were chemical, optical and structural characterized using the methods of Fourier The ZnO Nps were chemical, optical structural characterized the methods of Fourier Transform Infrared Spectroscopy (FTIR), and Ultra Violet-Visible spectra using measured by UV-Vis, model Transform Infrared Spectroscopy (FTIR), Ultra Violet-Visible spectra measured by UV-Vis, model 6850 jenway spectrometer, and morphology by Scanning Electron Microscopy (SEM) as well as the 6850 jenway spectrometer, and morphology Scanning Microscopy (SEM) as well as the surface topography of PVA/ZnO membranesby were carriedElectron out by atomic force microscopy (AFM) in surface topography of PVA/ZnO membranes were carried out by atomic force microscopy (AFM) in a a scan area of 4 µm × 4 µm and the structural analysis was determined by X-ray diffraction (XRD 2θ scan area of 4 µm × 4 µm and the structural analysis was determined by X-ray diffraction (XRD 2θ to 80θ). to 80θ). The electrical properties were performed through measurements of current-voltage (I-V) characteristics from −10 to 10 V and the temperature dependence of conductivity in a vacuum chamber at 70 m Torr, using an electrometer (Model 6517A, Keithley, OH, USA) on samples deposited on corning glass containing two aluminum stripes electrodes; the temperature of the substrate was varied in a range from 300–350 K.
Polymers 2018, 10, 1370
4 of 17
The electrical properties were performed through measurements of current-voltage (I-V) characteristics from −10 to 10 V and the temperature dependence of conductivity in a vacuum chamber at 70 m Torr, using an electrometer (Model 6517A, Keithley, OH, USA) on samples deposited on corning glass containing two aluminum stripes electrodes; the temperature of the substrate was varied in a 4range Polymers 2018, 10, x FOR PEER REVIEW of 17 from 300–350 K. For the properties of the material, firstly, the solutions of PVA/ZnO Nps were Nps measured Fordielectric the dielectric properties of the material, firstly, the solutions of PVA/ZnO were usingmeasured the open-ended probecoaxial technique [18]. This technique implemented using a Vector using thecoaxial open-ended probe technique [18]. This was technique was implemented using a Vector Network Analyzer and the open-ended coaxial performance probe (N1501A Dielectric Probe Network Analyzer and the open-ended coaxial performance probe (N1501A Dielectric Probe Kit, Kit, Keysight, CA, The USA). The Figure 2 shows the setup of the open-endedcoaxial coaxialprobe probe technique. technique. Keysight, CA, USA). Figure 2 shows the setup of the open-ended The measurements were performanceininaafrequency frequency range 2020 GHz andand the system was The measurements were performance range from from0.5 0.5toto GHz the system calibrated using air and distilled water. was calibrated using air and distilled water.
Figure 2. Setup of the open-ended coaxial probe technique for dielectric measurements in the solution of PVA/ZnO Figure 2. Nps. Setup of the open-ended coaxial probe technique for dielectric measurements in the solution of PVA/ZnO Nps.
Using the Equation (1) where ε0 is the dielectric constant and ε” is the loss factor is possible to obtain theUsing conductivity and the which is determined (2) and (3) the Equation (1) tangential where ε’ isloss the (tanδ) dielectric constant and ε’’ is by thethe lossEquations factor is possible to obtain the conductivity and the tangential loss (tanδ) which is determined by the Equations (2) and ε = ε0 − jε00 (1) (3) (1) = 𝜀f ε00 ε𝑗𝜀 σ =𝜀 2π (2) 0 (2) 00 𝜎 = 2𝜋𝑓𝜀 𝜀 ε tanδ = 0 (3) ε 𝜀 (3) 𝑡𝑎𝑛𝛿 = where f is the frequency in Hz and ε0 is the permittivity 𝜀in the vacuum. Later af Metal-Insulator-Metal structure was fabricatedinonthe thevacuum. membranes using aluminum as top permittivity where is the frequency in Hz and ε0 is the and bottom electrodes. Aluminum circular contacts with a diameter of 2000 µm using and aaluminum thickness as of Later a Metal-Insulator-Metal structure was fabricated on the membranes topwere and bottom electrodes. Aluminum circular contacts with a diameter of dielectric 2000 µm and a thickness 300 nm deposited by e-beam evaporation through a shadow mask. The constant of the of 300 nm were deposited by e-beam evaporation through a shadow mask. The dielectric constant of PVA/ZnO membranes was carried out using the MIM structure and LCR impedance analyzer (LCR the3536, PVA/ZnO carried out using and the aMIM structure andofLCR impedance analyzer model Hioki, membranes Nagano, JP)was at room temperature constant voltage 1 Vrms in the frequency 3536, JP) at room temperature and aatconstant voltage Vrms in the the range(LCR frommodel 500 Hz to 1Hioki, MHz.Nagano, C-V measurement was conducted 1 KHz in order of to1compare frequency range from 500 Hz to 1 MHz. C-V measurement was conducted at 1 KHz in order to values of the permittivity on the samples with the LCR method and also to verify the capacitive compare the values of the permittivity on the samples with the LCR method and also to verify the behavior on the samples. capacitive behavior on the samples. 3. Results and Discussion 3. Results and Discussion 3.1. FTIR Analysis 3.1. FTIR Analysis In order to determine the chemical bonds in the PVA and ZnO nanoparticles, the FTIR spectrum In order the chemical in the PVA and ZnOpeak nanoparticles, the spectrum −1FTIR was measured in to thedetermine membranes, which isbonds shown in Figure 3. The at 3267 cm is due to OH −1 is due to OH was measured in the membranes, which is shown in Figure 3. The peak at 3267 cm groups in the polymer backbone, the peaks at 2906 cm−1 and 918 cm−1 are due to CH2 asymmetric groups in the polymer backbone, the peaks at 2906 cm−1 and 918 cm−1 are due to CH2 asymmetric and and symmetric stretching, respectively. The peak observed around 1420 cm−1 is due to C–C stretching symmetric stretching, respectively. The peak observed around 1420 cm−1 is due to C–C stretching which is in accordance to reference [19]. Furthermore, the band observed at 420–417 cm−1 is due to Zn-O stretching, this suggests the presence of ZnO in the membranes. The interaction of PVA bonds with the addition of ZnO nanoparticles is attributed to the intermolecular interaction between OH groups by PVA and the surface of the nanoparticles [20]. The hydroxyl groups of PVA have a strong tendency to form a charge transfer complex with ZnO
nanoparticles through chelation [21]. The addition of TEA in the solution allows complete interaction between the nanoparticles and the polymer and also provides a dispersion of the nanoparticles avoiding agglomerates. The bands of PVA are more or less pronounced depending on if KOH or NH4 is employed in the synthesis, and also depending on the nanoparticles concentration in the polymer (PVA) matrix, thus the intensity of the OH bonds are decreasing as the amount of ZnO increases. Polymers 2018, 10, 1370 5 of 17 Hydroxyls groups are the most representative to analyze the chemical changes due to evident reduction intermolecular interactions that provide the composites formation. The chemical reaction −1 is due of PVA ZnO and its precursors shown in thethe inset of Figure 3. PVA can be cm chemically orto which is in with accordance to reference [19].isFurthermore, band observed at 420–417 thermally cross-linked, as their hydroxyl groups generate water as a by-product [22]. Zn-O stretching, this suggests the presence of ZnO in the membranes.
Figure 3. FTIR spectra of PVA matrix with ZnO nanoparticles, the inset shows the reaction of PVA Figure 3. FTIR spectra of PVA matrix with ZnO nanoparticles, the inset shows the reaction of PVA and ZnO. and ZnO.
The interaction of PVA bonds with the addition of ZnO nanoparticles is attributed to the 3.2. Absorbance Characteristics and Optical Band Gap intermolecular interaction between OH groups by PVA and the surface of the nanoparticles [20]. The UV-Vis absorbance spectra for thetendency ZnO Nps to is shown Figuretransfer 4, in thecomplex samples there a The hydroxyl groups of PVA have a strong form ain charge with is ZnO peak at 339 nm, close to 385 nm, which corresponds to the bulk ZnO [23]. The spectra is identified nanoparticles through chelation [21]. The addition of TEA in the solution allows complete interaction due tothe thenanoparticles addition of ZnO consequently, this isa an indicationofofthe thenanoparticles interaction between and nanoparticles, the polymer and also provides dispersion between PVA and ZnO. The inset of Figure 4 is a SEM image for the ZnO Nps corroborating the avoiding agglomerates. The bands of PVA are more or less pronounced depending on if KOH or NH4 aspheric morphology and the completely formation of the nanoparticles. Prior to embedding the ZnO is employed in the synthesis, and also depending on the nanoparticles concentration in the polymer into the PVA, the Tauc’s method was used to calculate the ZnO band gap (Eg), obtaining a mean (PVA) matrix, thus the intensity of the OH bonds are decreasing as the amount of ZnO increases. value of 3.9 eV, that is a high value for ZnO obtained from a chemical method. Hydroxyls groups are the most representative to analyze the chemical changes due to evident reduction Figure 5a shows the UV–Vis absorbance spectra in the region from 200 to 1100 nm for PVA/ZnO intermolecular interactions that 5a provide the composites The chemical reaction of PVA Nps membranes. From Figure it is possible to observe formation. an absorption band at 225 nm attributed to withPVA ZnO and its this precursors is shown in the inset of 3.containing PVA can be chemically or thermally polymer, band arises due to the presence of Figure carbonyl structures connected to the cross-linked, as theirchains hydroxyl generate water as a by-product [22].in high optical band gap PVA polymeric as isgroups reported in references [20,24,25], resulting around 5.8 eV. On the other hand, if the nanoparticles with a spherical shape and a proper size are 3.2. Absorbance Characteristics and Optical Band Gap embedded into the polymer matrix and are dispersed at the nanoscale, it minimizes light scattering which results absorbance in high transmittance the caseinfor the samples and Z1,there which The UV-Vis spectra for[26]. the This ZnOmay Npsbe is shown Figure 4, in theY1 samples is a presented a spherical shape forwhich the ZnO nps correlated (see inset in Figure due 4) peak at 339 nm, close to 385 nm, corresponds to thewith bulkSEM ZnOmicrographs [23]. The spectra is identified
to the addition of ZnO nanoparticles, consequently, this is an indication of the interaction between PVA and ZnO. The inset of Figure 4 is a SEM image for the ZnO Nps corroborating the aspheric morphology and the completely formation of the nanoparticles. Prior to embedding the ZnO into the PVA, the Tauc’s method was used to calculate the ZnO band gap (Eg), obtaining a mean value of 3.9 eV, that is a high value for ZnO obtained from a chemical method. Figure 5a shows the UV–Vis absorbance spectra in the region from 200 to 1100 nm for PVA/ZnO Nps membranes. From Figure 5a it is possible to observe an absorption band at 225 nm attributed to
proportional to the number of absorbing molecules and it increases with the increasing weight % of ZnO. Therefore a higher ZnO concentration promotes aggregation to affect UV absorption and scatter visible light. Further studies are necessary to study the contribution of the shape and size of the ZnO nps to the polymer matrix and the light scattering. The nanocomposites membranes containing ZnO nanoparticles absorb UV light starting at Polymers 2018, 10, 1370 6 of 17 around 340 nm (λonset), down to 225–240 nm. The UV–vis spectra also show red-shifting of λonset and the absorption peak as the ZnO concentration in PVA increases, which suggests the formation of PVA polymer, this in band due to the presence of carbonyl containing structures connected the larger aggregates thearises nanocomposite membrane containing more ZnO nanoparticles in to PVA PVA polymeric chains as is reported in references [20,24,25], resulting in high optical band gap around matrix, increasing the UV light absorbed. The absorbance in the UV region is enhanced with the 5.8 eV. Onofthe other hand, if the due nanoparticles with a spherical shape and a proper size with are embedded addition ZnO nanoparticles to the high energy gap [27]. This finding agrees the SEM into the polymer and areand dispersed at the nanoscale, it minimizes light scattering results observation in thematrix membranes these results demonstrated the interaction between which the polymer in high transmittance [26]. This may be the case for the samples Y1 and Z1, which presented a spherical and the nanoparticles. The spectral redshifting due to ZnO aggregates has also been reported for shaperelated for the works ZnO nps correlated withPMMA/ZnO, SEM micrographs (see ZnO inset and in Figure and these samples other using PEO/ZnO, PVA/PVP PVA 4) CaF 2 nanocomposite showed high transmittance films, respectively [9,27–29].from 225 to 800 nm. This could be attributed to the dispersed nanoparticles and aFigure smaller of the ZnO plot NPsfor in the samples. absorbance Z4 andmembranes. Y4 is higher 5bpercent shows the Tauc’s obtaining theThe band gap (Eg) in ofsamples the PVA/ZnO compared to other which could high concentration of larger ZnO agglomerates, this is in in The Eg values are samples, in the range from 5.5be todue 5.8 to eV, which are slightly than the reported agreement with the report by reference [21], where the absorption is proportional to the number of references [9,30], the samples prepared from the NH4 (sample Z4) showed high values due to the absorbing molecules and it increases with the the Eg increasing weight of ZnO. a higher ZnO agglomeration of nanoparticles, however values are very%closed to Therefore each another, which is concentration to affect UVin absorption and of scatter visible material light. Further indicative thatpromotes there are aggregation no significant changes the structure the hybrid and astudies good are necessary to study the contribution of the shape and size ofmatrix. the ZnO nps to the polymer dispersion of the ZnO Nps being presented into the polymeric The inset in Figure 5bmatrix show andvariations the light scattering. the of Eg for the different samples.
Figure 4. 4. UV-Vis UV-Vis absorbance absorbance spectra spectra of of ZnO ZnO nanoparticles nanoparticles synthesized synthesized at at different different contents, contents, the the inset inset Figure shows a micrograph image for one of the ZnO agglomerates. shows a micrograph image for one of the ZnO agglomerates.
The nanocomposites membranes containing ZnO nanoparticles absorb UV light starting at around 340 nm (λonset), down to 225–240 nm. The UV–vis spectra also show red-shifting of λonset and the absorption peak as the ZnO concentration in PVA increases, which suggests the formation of larger aggregates in the nanocomposite membrane containing more ZnO nanoparticles in PVA matrix, increasing the UV light absorbed. The absorbance in the UV region is enhanced with the addition of ZnO nanoparticles due to the high energy gap [27]. This finding agrees with the SEM observation in the membranes and these results demonstrated the interaction between the polymer and the nanoparticles. The spectral redshifting due to ZnO aggregates has also been reported for other related works using PEO/ZnO, PMMA/ZnO, PVA/PVP ZnO and PVA CaF2 nanocomposite films, respectively [9,27–29]. Figure 5b shows the Tauc’s plot for obtaining the band gap (Eg) of the PVA/ZnO membranes. The Eg values are in the range from 5.5 to 5.8 eV, which are slightly larger than the reported in references [9,30], the samples prepared from the NH4 (sample Z4) showed high values due to the agglomeration of nanoparticles, however the Eg values are very closed to each another, which is
Polymers 2018, 10, 1370
7 of 17
indicative that there are no significant changes in the structure of the hybrid material and a good dispersion of the ZnO Nps being presented into the polymeric matrix. The inset in Figure 5b show the Polymers 2018, 10, x FOR PEER REVIEW 7 of 17 Polymers 2018,of10,Eg x FOR PEER REVIEW samples. 7 of 17 variations for the different
Figure 5.5. (a) (a) Absorbancespectra spectra of PVA-ZnO Nps; (b) Tauc’s plotdetermination for determination Eg, the inset Figure PVA-ZnO Nps; (b) (b) Tauc’s plot plot for Eg, theEg, inset Figure 5. (a) Absorbance Absorbance spectraofof PVA-ZnO Nps; Tauc’s for determination theshows inset shows the band gap obtained for the samples. the band gap obtained for the samples. shows the band gap obtained for the samples.
3.3. 3.3. Surface Morphology 3.3. Surface Morphology Figure 6a 6a shows shows the the surface surface morphology morphology for for the the samples samples with with different different concentration concentration of of ZnO ZnO Figure 6a into shows the surface morphology for the samples with different concentration of ZnO nanoparticles the the membranes. membranes. Average Average roughness roughness for for PVA/ZnO PVA/ZnO membranes is in a range from from nanoparticles into the higher membranes. Average roughness for PVA/ZnO membranes is in a range some from 1.9 1.9 nm nm to to 30 30 nm. nm. The The higher surface surface roughness roughness for for PVA/ZnO PVA/ZnO is attributed to the presence of of some 1.9 nm to 30 nm. The higher surface roughness for PVA/ZnO is attributed to the presence of some agglomerates of ZnO nanoparticles into the PVA. Figure Figure 7b 7b shows shows the the variation variation of of the the average average agglomerates of ZnO nanoparticles into the PVA. Figure 7b shows the variation of the average roughness of the film as a function of the precursors NH or KOH for ZnO Nps. The membranes the film as a function of the precursors NH44 or KOH for ZnO Nps. The membranes roughness of the film as a function of concentration the precursors NH4 or KOH for ZnO Nps. The membranes prepared showed lowlow roughness (samples Z1 and Y1). Y1). It is preparedwith withaalow lowZnO ZnOnanoparticles nanoparticles concentration showed roughness (samples Z1 and prepared with a low ZnO nanoparticles concentration showed low roughness (samples Z1 and Y1). therefore possible to observe that these have a homogeneous topography and low roughness, It is therefore possible to observe thatsamples these samples have a homogeneous topography and low It is therefore possible to observe that these samples have a homogeneous topography and low which is in agreement SEM analysis. roughness, which is inwith agreement with SEM analysis. roughness, which is in agreement with SEM analysis.
(a) (a)
(b) (b)
Figure 6. (a) 3D-AFM surface topography for the composite membranes, (b) Average Average roughness. roughness. Figure 6. (a) 3D-AFM surface topography for the composite membranes, (b) Average roughness.
The The Scanning Scanning electron electron micrographs micrographs of of PVA/ZnO PVA/ZnO Nps Nps membranes membranes are are shown shown in in Figure Figure 7. 7. SEM SEM Theshowed Scanning electron micrographs of PVA/ZnO Nps membranes are shown in Figure Y1 7. SEM images ZnO nanoparticles distribution in the polymer membrane. The samples images showed ZnO nanoparticles distribution in the polymer membrane. The samples Y1 andand Z1 images showedfiner ZnOmorphology nanoparticles distribution insurface, the polymer membrane. The samples Y1analysis. and Z1 Z1 presented a smooth which is corroborated by AFM presented finer morphology withwith a smooth surface, which is corroborated by AFM analysis. The presented finer morphology with a smooth surface, which is corroborated by AFM analysis. The formation of agglomerates is observed when the ZnO concentration increases for samples Y4 and Z4, formation of agglomerates is observed when the ZnO concentration increases for samples Y4 and Z4, however homogeneous agglomeration distribution is obtained in all samples. This distribution is however homogeneous agglomeration distribution is obtained in all samples. This distribution is attributed to the wet chemistry process used to obtain ZnO nanoparticles in solution where the ZnO attributed to the wet chemistry process used to obtain ZnO nanoparticles in solution where the ZnO Nps are compatible with the process to obtain the polymer. The ZnO nanoparticles synthetized by Nps are compatible with the process to obtain the polymer. The ZnO nanoparticles synthetized by
Polymers 2018, 10, 1370
8 of 17
Polymers 2018, 10, x FOR PEER REVIEW
8 of 17
The formation of agglomerates is observed when the ZnO concentration increases for samples Y4 and sol showed an agglomeration average size ofdistribution around 100isnm from samples obtained the use of Z4, gel-method however homogeneous obtained in all samples. Thiswith distribution is KOH or NH 4 precursors. attributed to the wet chemistry process used to obtain ZnO nanoparticles in solution where the ZnO Polymers 10, x FOR with PEER REVIEW 17 Nps are2018, compatible the process to obtain the polymer. The ZnO nanoparticles synthetized 8byofsol gel-method showed an average size of around 100 nm from samples obtained with the use of KOH or sol showed an average size of around 100 nm from samples obtained with the use of NHgel-method 4 precursors. KOH or NH4 precursors.
(a)
(b)
(a)
(b)
(c)
(d)
Figure 7. SEM images for PVA-ZnO membranes: (a) Y1; (b) Z1; (c) Y4; (d) Z4.
The surface morphology of ZnO nanoparticles is studied using SEM as is shown in Figure 8. The (c)shapes and the morphologies of the ZnO nanoparticles (d) particles formed irregular changed from the spherical structure into a hemispherical structure and for Z4 sample in an irregular Figure 7. SEM SEM images images for for PVA-ZnO PVA-ZnO membranes: (a) Y1; (b) Z1; (c) Y4; (d) Z4.shape. The sizes Figure of the agglomerates of ZnO nanoparticles are in the range of 100–160 nm. The surface surface morphology ZnO nanoparticles is studied using SEM isthe shown in Figure 8. particlemorphology size distribution for ZnO nps isisplotted in Figure 9.asFrom histograms, the The ofofZnO nanoparticles studied using SEM isasshown in Figure 8. The The particles formed irregular shapes and the morphologies of ZnO the ZnO nanoparticles changed from diameter of the nanoparticles can be in the range from 100 to 160 nm, thechanged large value is for particles formed irregular shapes andestimated the morphologies of the nanoparticles from the the spherical structure into a hemispherical structure and for Z4 sample in an irregular shape. The sizes Y4, wherestructure when theinto content of ZnCl is higher, the and distribution for Z4 in cannot be measured the spherical a hemispherical structure for Z4 sample an irregular shape.due Theto sizes of the the agglomerates agglomerates ofagglomerates. ZnO irregular shape of the of of ZnO nanoparticles nanoparticles are are in in the the range range of of 100–160 100–160 nm. nm. The particle size distribution for ZnO nps is plotted in Figure 9. From the histograms, the diameter of the nanoparticles can be estimated in the range from 100 to 160 nm, the large value is for Y4, where when the content of ZnCl is higher, the distribution for Z4 cannot be measured due to the irregular shape of the agglomerates.
Figure 8. Cont.
Polymers 2018, 10, 1370 Polymers 2018, 10, x FOR PEER REVIEW
9 of 17 9 of 17
Polymers 2018, 10, x FOR PEER REVIEW
9 of 17
Figure 8. 8. SEM images and morphology for the ZnO nanoparticles used for the composite polymer, the insets show show less less magnification magnification with with some some bright bright spots spots related relatedto toZnO ZnOnanoparticles. nanoparticles.
The particle size distribution for ZnO nps is plotted in Figure 9. From the histograms, the diameter of theFigure nanoparticles can beand estimated in theforrange from 100 to 160 nm, value is for Y4, where 8. SEM images morphology the ZnO nanoparticles usedthe forlarge the composite polymer, whenthe theinsets content of ZnCl is higher, the distribution for Z4 cannot be measured due to the irregular show less magnification with some bright spots related to ZnO nanoparticles. shape of the agglomerates.
Figure 9. Size distribution for the ZnO prepared under different conditions.
3.4. Structural Analysis The X-ray diffraction pattern (XRD) for PVA/ZnO membranes is showed in Figure 10. The broad Figure 9. Size distribution distribution for the prepared different conditions. 9. Size fordue the ZnO ZnO prepared under under conditions. diffraction peakFigure located at 2θ = 21.53° is to amorphous PVAdifferent [16]. Peaks located at 2θ = 29.5°, 36.1°, 39.5°, 43.2°, 47.6° and 48.6° corresponding to (100) (100) (102) (110) and (103) respectively, the 3.4. Structural Analysis 3.4. Structural Analysis reflection plane of ZnO, showing the presence and hexagonal structure of the nanoparticles, and cThe X-ray diffraction pattern (XRD) for PVA/ZnO membranes is showed in Figure 10. The broad The X-ray diffraction pattern (XRD) for PVA/ZnO is showed in Figure 10. The broad axis orientation, in addition to well-defined diffractionmembranes peaks indicating complete crystal formation diffraction peak located at 2θ = 21.53◦ is due to amorphous PVA [16]. Peaks located at 2θ = 29.5◦ , diffraction peak located at 2θ = 21.53° is due to amorphous PVA [16]. Peaks located at 2θ = 29.5°, [2,31]. The values of interplanar spacing (d), the average of lattice parameters and the unit cell (u) in 36.1◦ , 39.5◦ , 43.2◦ , 47.6◦ and 48.6◦ corresponding to (100) (100) (102) (110) and (103) respectively, the 36.1°, 39.5°, 43.2°, 47.6° and 48.6° (100) (100)d= (102) (110) and (103) the the membranes were obtained by corresponding Bragg law, withtothe results (3.0201Å, 3.0206 Å)respectively, and u = (76 and reflection plane of ZnO, showing the presence and hexagonal structure of the nanoparticles, and c-axis reflection plane of ZnO, showing the presence and hexagonal structure of the nanoparticles, and c69 nm) respectively. orientation, in addition to well-defined diffraction peaks indicating complete crystal formation [2,31]. axis orientation, in addition to well-defined diffraction peaks indicating complete crystal formation The values of interplanar spacing (d), the average of lattice parameters and the unit cell (u) in the [2,31]. The values of interplanar spacing (d), the average of lattice parameters and the unit cell (u) in membranes were obtained by Bragg law, with the results d= (3.0201Å, 3.0206 Å) and u = (76 and the membranes were obtained by Bragg law, with the results d= (3.0201Å, 3.0206 Å) and u = (76 and 69 nm) respectively. 69 nm) respectively.
Figure 10. XRD pattern of PVA/ZnO Nps membranes.
36.1°, 39.5°, 43.2°, 47.6° and 48.6° corresponding to (100) (100) (102) (110) and (103) respectively, the reflection plane of ZnO, showing the presence and hexagonal structure of the nanoparticles,10and cPolymers 2018, 10, x FOR PEER REVIEW of 17 axis orientation, in addition to well-defined diffraction peaks indicating complete crystal formation [2,31]. The values of interplanar spacing (d), the average of lattice parameters and the unit cell (u) in 3.5. Electrical Properties the membranes were obtained by Bragg law, with the results d= (3.0201Å, 3.0206 Å) and u = (76 and Polymers 2018, 10, 1370 of 17 Current–voltage (I–V) characteristics of PVA/ZnO membranes were measured at 10 room 69 nm) respectively. temperature and are given in Figure 11a. The I–V characteristics showed an ohmic behavior, which is an indication of a better charge transport through the polymer matrix and the metal electrode contact. This linear behavior in the interface metal/insulator is very important in the development of electronic devices with good performance and reliability. The resistivity of the polymer composite decreases as the concentration of ZnO increases, this is due to the continuous conductive network formation by the agglomerates within the polymeric matrix. Figure 11b shows the Arrhenius plots of the temperature dependence conductivity (σRT) for the membranes. From these graphs, Ea was obtained as the curve slope using the Equation (4) 𝜎 = 𝜎 exp
𝐸 𝑘𝑇
(4)
where σ0 is the pre-exponential factor, Ea is the activation energy, k is Boltzmann constant, and T is temperature in Kelvin. The values of activation energies are in the range from 0.62 to 0.78 eV, in samples Y1 and Z1 it is possible to identify to two regions of conductivity and the Ea for these samples was evaluated in region at 330 K from the slope of the fitting lines. These values are compared in Table 2. The activation energy values obtained are in theNps range for polymers using PVA reported Figure 10. pattern of Nps membranes. Figure 10. XRD XRD patternhere of PVA/ZnO PVA/ZnO membranes. in the literature, for example on PVA-PVP blend films the reported values for Ea = 0.6, and 0.9 eV [9], 3.5. Electrical Properties and for PVA-PVP blend film with different concentration Ea was in the range from 0.64 to 0.78 eV [32]. As noticed in Figure the DC conductivity increases as temperature increases. Thetemperature maximum Current–voltage (I–V)11b, characteristics of PVA/ZnO membranes were measured at room conductivity temperature forcharacteristics the PVA/ZnOshowed nps wasan 2.44 × 10−12 (S/cm). In addition, in sample and are givenatinroom Figure 11a. The I–V ohmic behavior, which is an indication Y4 is observed that the activation energy increases in ZnOcontact. content.This From the of aitbetter charge transport through the polymer matrixwith andthe the increase metal electrode linear Arrhenius the electrical conduction inimportant the PVA/ZnO composite is similar to a behavior inbehavior the interface metal/insulator is very in thepolymer development of electronic devices semiconductor and this isand duereliability. to a hopping the particles, this trend is similar to with good performance Themechanism resistivity between of the polymer composite decreases as the the reported works forincreases, PVA-Glycogen filmsto[33], and for PVA with Succinic acidformation compositeby films concentration of ZnO this is due the continuous conductive network the [34]. agglomerates within the polymeric matrix.
Figure 11. characteristic for the membranes at room temperature, (b) DC conductivity Figure 11.(a) (a)I-VI-V characteristic forPVA/ZnO the PVA/ZnO membranes at room temperature, (b) DC as a function of temperature. conductivity as a function of temperature.
Figure 11b shows Arrhenius plots offor the temperature dependence conductivity (σRT ) for the Tablethe 2. Electrical properties the PVA/ZnO composites membranes. membranes. From these graphs, Ea was obtained as the curve slope using the Equation (4) Sample Activation Energy, Ea, (eV) Conductivity, (300 K), (S/cm)) Band Gap, Eg, (eV) Ea × 10−12 Y1 0.72 1.18 5.8 σ = σ0 exp (4) −12 kT Z1 0.62 1.20 × 10 5.6 Y4 0.68 1.71 × 10−12 5.5 where σ0 is the pre-exponential factor, Ea is the activation energy, k is Boltzmann constant, and T is Z4 0.78 2.44 × 10−12 5.83 temperature in Kelvin. The values of activation energies are in the range from 0.62 to 0.78 eV, in samples Y1 and Z1 it is possible to identify to two regions of conductivity and the Ea for these samples was 3.6. Dielectric Properties evaluated in region at 330 K from the slope of the fitting lines. These values are compared in Table 2. The activation energy values obtained here are in the range for polymers using PVA reported in the literature, for example on PVA-PVP blend films the reported values for Ea = 0.6, and 0.9 eV [9], and
Polymers 2018, 10, 1370
11 of 17
for PVA-PVP blend film with different concentration Ea was in the range from 0.64 to 0.78 eV [32]. As noticed in Figure 11b, the DC conductivity increases as temperature increases. The maximum conductivity at room temperature for the PVA/ZnO nps was 2.44 × 10−12 (S/cm). In addition, in sample Y4 it is observed that the activation energy increases with the increase in ZnO content. From the Arrhenius behavior the electrical conduction in the PVA/ZnO polymer composite is similar to a semiconductor and this is due to a hopping mechanism between the particles, this trend is similar to the reported works for PVA-Glycogen films [33], and for PVA with Succinic acid composite films [34]. Table 2. Electrical properties for the PVA/ZnO composites membranes. Sample
Activation Energy, Ea, (eV)
Conductivity, (300 K), (S/cm)
Band Gap, Eg , (eV)
Y1 Z1 Y4 Z4
0.72 0.62 0.68 0.78
1.18 × 10−12 1.20 × 10−12 1.71 × 10−12 2.44 × 10−12
5.8 5.6 5.5 5.83
3.6. Dielectric Properties In order to know the properties of the material for future applications in the development of flexible antennas and EMI absorbers, an analysis of dielectric properties in the range of Giga-Hertz was performed. Firstly, the solution of PVA/ZnO Nps was analyzed with the technique that was described in the experimental section. For this study distilled water was used as dielectric reference and also for the calibration, while the measurements were performed in the range from 0.5–20 GHz at room temperature. In Figure 12a,b the variation in the dielectric constant (ε) and loss factor (ε”) is shown as function of frequency for different content of ZnO Nps. The dielectric constant (ε) decreased with respect to the frequency for all samples as is shown in Figure 12a, being in the range from 77.4 to 78.6 for 0.5 GHz and falling from 33.2–35 at 20 GHz. In the case of the loss factor (ε”) it increases as the frequency reached its maximum value at about 14 GHz. The dielectric properties of the samples are closer to the dielectric properties of water. The relative permittivity of water is ε0 ≈ 80 at 500 MHz, this is due to the water component in the PVA solution. The dielectric constant for the solution of the PVA with ZnO nanoparticles decreases as frequency increases, this is in accordance with the reported by Yeow et al., and could be explained by the dipoles which are not able to follow the variation field at higher frequencies [31]. At relatively low frequencies in the range for microwave applications, the solution for the membranes showed a high dielectric constant, and this is related to electrode polarization of the polymer [35]. On the other hand, with the obtained values of the real and imaginary part of the complex permittivity, we compute the AC conductivity and the tangential loss (tanδ) as a function of the frequency using the Equations (2) and (3), respectively. AC conductivity (σac ) of the samples is shown in Figure 12c. The conductivity of the PVA increases with the presence of the nanoparticles being in the range from 1.2 to 1.5 S/m for 0.5 GHz and 37–40 S/m for 20 GHz. Furthermore, the tangential loss increases with the frequency. From the Figure 12d we appreciate that it (tanδ) presents values close to 0.06 for all samples at 0.5 GHz, these values and the high value of permittivity at 0.5GHz could be employed to design small antennas devices in the range of sub-gigahertz. Figure 13 shows the dielectric constant as a function of frequency for the PVA membranes with variations on the content of ZnO Nps. The measured capacitance with the LCR equipment was used to calculate the dielectric constant in the films, the inset in the Figure 13 shows one of the samples used for the characterization. From Figure 13, it is possible to observe the influence at low frequency on the dielectric constant, which is high for all samples, and the value decreases as frequency increases. This trend is similar to the reported by other authors, however the maximum value of ε = 11,468 obtained for the sample Y4 at 500 Hz is higher than that reported by other works. Table 3 shows a comparison with materials, methods and permittivity values. At lower frequencies, all the free
Polymers 2018, 10, 1370
Polymers 2018, 10, x FOR PEER REVIEW
12 of 17
11 of 17
dipolar functional groups present in of PVA chain canapplications align themselves, higher In order to know the properties thepolymeric material for future in the resulting development of permittivity values [35]. For frequencies up to 1 KHz, the bigger dipolar groups find it difficult to flexible antennas and EMI absorbers, an analysis of dielectric properties in the range of Giga-Hertz orient at the same pace as alternating field, so the ofwith this dipolar group that decrease was performed. Firstly, thethe solution of PVA/ ZnO Npscontributions was analyzed the technique was along with the permittivity [21]. The permittivity in ZnO nanoparticles also decreases when increases described in the experimental section. For this study distilled water was used as dielectric reference frequency ofthe thecalibration, applied field, thisthe is due to ZnO being a polar ceramicinmaterial with relatively high and also for while measurements were performed the range from 0.5–20 GHz permittivity, therefore In theFigure ε values of the also found to be(ε) higher [36].factor In addition, at room temperature. 12a,b thenanocomposites variation in the are dielectric constant and loss (ε’’) is the ZnO Nps embedded in the PVA matrix enhances the dielectric permittivity of the shown as function of frequency for different content of ZnO Nps. The dielectric constant (ε)composite, decreased 2+ and O2− ions and has a high value of because ZnOtoexhibits a strongfor ionic polarization to Zn with respect the frequency all samples as is due shown in Figure 12a, being in the range from 77.4 static permittivity [37]. Therefore, the samples Z4 and Y4 presented value. Furthermore, to 78.6 for 0.5 GHz and falling from 33.2–35 at 20 GHz. In the case ofthe thehigher loss factor (ε’’) it increases the increasing of ε values in the composites obtained at low frequencies can be attributed to the as the frequency reached its maximum value at about 14 GHz. The dielectric properties of the samples interfacial exhibits due toThe therelative difference in the permittivity values theMHz, ZnO are closer topolarization, the dielectricwhich properties of water. permittivity of water is ε’ ≈ 80 atof500 and the PVA matrix. This can be explained by the Maxwell-Wagner-Sillars (MWS) effect, which is this is due to the water component in the PVA solution. The dielectric constant for the solution of the a common characteristic of the polymer nanocomposite dielectric materials. It affects attributes to PVA with ZnO nanoparticles decreases as frequency increases, this is in accordance with the reported theYeow accumulation charges at the interfaces different permittivity and constituents by et al., andofcould be explained by the of dipoles which are not able toconductivity follow the variation field of a composite dielectric material, which results in the formation of micro capacitors over the entire at higher frequencies [31]. At relatively low frequencies in the range for microwave applications, the volume offor thethe material. Such micro capacitors contributed theisdielectric solution membranes showed a high significantly dielectric constant, and to this related polarization, to electrode and thereforeofthe of[35]. ε0 values at low frequencies [9]. polarization theincrease polymer
Figure 12. (a) The dielectric constant (ε); (b) loss factor (ε”); and (d) (d) tanδ; tanδ; (ε’’); (c) (c) conductivity conductivity (σ (σ in in S/m); S/m); and as a function of the frequency for various ZnO nanoparticles content content in in PVA PVAsolution. solution.
On the other hand, with the obtained values of the real and imaginary part of the complex permittivity, we compute the AC conductivity and the tangential loss (tanδ) as a function of the frequency using the Equations (2) and (3), respectively. AC conductivity (σac) of the samples is shown in Figure 12c. The conductivity of the PVA increases with the presence of the nanoparticles being in the range from 1.2 to 1.5 S/m for 0.5 GHz and 37–40 S/m for 20 GHz. Furthermore, the tangential loss
Polymers 2018, 10, 1370 Polymers 2018, 10, x FOR PEER REVIEW
13 of 17 13 of 17
Figure of PVA PVA with Figure 13. 13. Variation Variation of of dielectric dielectric constant constant with with frequency frequency for for the the membranes membranes of with different different content of ZnO Nps. The inset is a picture of one of the fabricated capacitors over glass content of ZnO Nps. The inset is a picture of one of the fabricated capacitors over glass substrates. substrates. Table 3. Comparison of permittivity values with related works of Polymer matrix/ZnO. Material
Deposition Method and Temperature
Permittivity (ε), Frequency
Reference
PVA/ZnO membranes
Solution casting/50 ◦ C
17.8 at 100 Hz
[2]
PVDF/xGnPs
Solution mixing process
2.080 at 103 Hz (4.1 vol%)
[3]
PVDF/ZnO composites
combination of solution blend, sequential precipitation, and hot-press processes/60 ◦ C
102 at 500 HZ
[14]
PVA/ZnO nanocomposites
Solution casting/40 ◦ C
502 at 102 Hz 102 at 106 Hz
[19]
PVDF/ZnO nanowires clusters
Microemulsion/80 ◦ C
113 at 102 Hz
[38]
(PVA)/(PVP)/silver-doped solution blended process and 804 at 103 Hz zinc oxide (Ag-doped ZnO) calcined at 500 ◦ C Figure 14. Frequency dependence of dielectric◦ loss ε” for PVA/ZnO composites. p-type PVA/CuI Solution casting at 24 C 103 at 102 Hz ◦C
≈104
[39] [40]
PVA/ZnO castingmeasured method at as 80 an additional This in work at characterization 500 Hz Capacitance-Voltage (C-V) Solution curves were order to corroborate the high dielectric constant of the composites, the data were obtained using a 4200 A Keithley semiconductor analyzer KHz voltage sweeping frommeasurements −1 V to 1 V. The dielectric loss partparameter ε” in the range from at 5001Hz to 1with MHzawas determined through Figure shows(δ) theand C-Vusing plot for samples and Z1 the three regions: of loss 15 tangent thethe equation 3. Y1 Figure 14 identifying shows the imaginary part ofaccumulation, the dielectric depletion and inversion. From accumulation region, the dielectric constant wasZ4 extracted withthe a value properties with nonlinear behavior as frequency increases. Samples Y4 and presented high ε ~ 1079, is close to the obtained within thethe impedance analyzer thefor value here is values ofwhich ε” at low frequencies, beginning range from 9300 toand 1000 500reported Hz and falling one of1 the highest usingthus ZnO nanoparticle for the composite. from to 75 at 1 MHz, is semiconductor due to the increase of ZnO nps, similar behavior has been reported in The introduction references [9,32,40]. of conductive or semiconductive inorganic particles into the polymer matrix has resulted in dielectric composites with anmeasured effective permittivity that is much higher than that to of Capacitance-Voltage (C-V) curves were as an additional characterization in order the matrix. Relative (εr) values up to 105 have beenwere reported in some corroborate the high permittivity dielectric constant of theofcomposites, the data obtained usingcomposite a 4200 A systems [41]. A large dielectric constant can be in composites; the physical Keithley semiconductor parameter analyzer at achieved 1 KHz with a voltage sweeping fromreason −1 V for to 1the V. critical 15 behavior of C-V the plot dielectric near is the of microcapacitor Figure shows the for theconstant samples Y1 andpercolation Z1 identifying the existence three regions: accumulation, networks.and Each microcapacitor is formed byregion, the neighboring conductive fillerextracted particleswith andaavalue very depletion inversion. From accumulation the dielectric constant was layerwhich of dielectric and contributes an abnormally largeand capacitance duehere to the εthin ~ 1079, is closeintobetween the obtained with the impedance analyzer the valuemainly reported is introduction of nanofiller [42].semiconductor Further studiesnanoparticle are necessary investigate the effect of the filler shape one of the highest using ZnO fortothe composite. on dielectric constant and breakdown strength of the PVA/ZnO composites, however the higher dielectric values in these membranes are very attractive for applications as an insulator in TFT for flexible electronics due to the compatible process for the semiconductor materials, as well as its low
Figure 13. Variation of dielectric constant with frequency for the membranes of PVA with different Polymers 2018, 10, 1370 Nps. The inset is a picture of one of the fabricated capacitors over glass substrates. 14 of 17 content of ZnO
Polymers 2018, 10, x FOR PEER REVIEW
14 of 17
temperature of processing. In addition, the composite material provides the possibility to develop devices and absorbers for high frequencies. For future work, we are taking in account fabricating electronic devices such as thin film transistor to obtain some basic digital gates using a complete process with the integration of the dielectric material and semiconducting materials obtained in solution process in flexible substrates. Figure 14. Frequency dependence of dielectric loss ε” for PVA/ZnO composites. Figure 14. Frequency dependence of dielectric loss ε” for PVA/ZnO composites.
Capacitance-Voltage (C-V) curves were measured as an additional characterization in order to corroborate the high dielectric constant of the composites, the data were obtained using a 4200 A Keithley semiconductor parameter analyzer at 1 KHz with a voltage sweeping from −1 V to 1 V. Figure 15 shows the C-V plot for the samples Y1 and Z1 identifying the three regions: accumulation, depletion and inversion. From accumulation region, the dielectric constant was extracted with a value ε ~ 1079, which is close to the obtained with the impedance analyzer and the value reported here is one of the highest using ZnO semiconductor nanoparticle for the composite. The introduction of conductive or semiconductive inorganic particles into the polymer matrix has resulted in dielectric composites with an effective permittivity that is much higher than that of the matrix. Relative permittivity (εr) values of up to 105 have been reported in some composite systems [41]. A large dielectric constant can be achieved in composites; the physical reason for the critical behavior of the dielectric constant near percolation is the existence of microcapacitor networks. Each microcapacitor is formed by the neighboring conductive filler particles and a very thin layer of dielectric in between and contributes an abnormally large capacitance mainly due to the introduction of nanofiller [42]. Further studies are necessary the effect of the filler shape Figure 15. C-V measurements for the samplesto Y1investigate and Z1 (PVA/ZnO). Figure 15. C-V measurements for the samples Y1 and Z1 (PVA/ZnO). on dielectric constant and breakdown strength of the PVA/ZnO composites, however the higher The introduction of conductive semiconductive particles the polymer matrix dielectric values in these membranesorare very attractiveinorganic for applications asinto an insulator in TFT for 4. Conclusions has resulted in dielectric composites with an effective permittivity that is much higher than that flexible electronics due to the compatible process for the semiconductor materials, as well as its low of theComposites matrix. Relative permittivity ) values of up 105 haveNPs beenwere reported in some polymers based in(εrmembranes of to PVA/ZnO prepared by composite a solution systems [41]. A large can beusing achieved in composites; the electrical physical reason for the casting process. The dielectric thin filmsconstant were studied the structural, optical, and dielectric critical behavior of the dielectric constant near percolation is the existence of microcapacitor networks. characterization. The FTIR analysis demonstrated a good interaction between PVA matrix and the Each formed byZnO the neighboring conductive filler particles and very thinprepared layer of ZnO microcapacitor Nps. In samplesiswith more Nps the O–H groups decrease, while in thea samples dielectric between and contributes an abnormally capacitance mainly due to the introduction using NHin 4 as the precursor, a reduction in the O–H large groups is observed. The UV-vis analysis showed of nanofiller [42]. Further studies are necessary to investigate the effect of the filler shape on dielectric that the addition of ZnO nanoparticles affect the absorbance close to the UV region and the maximum constant and breakdown strength of the composites, however the higher dielectric in band gap is 5.83 eV for the sample withPVA/ZnO the higher ZnO content. The XRD analysis showedvalues that the these membranes areZnO veryisattractive forinapplications as an insulator in TFT for flexibleofelectronics due crystal structure of presented the PVA matrix. The surface morphology the PVA/ZnO to the compatible forathe semiconductor as well as its lowfrom temperature processing. obtained by AFMprocess showed smoother surface materials, with average roughness 1.9 to 30ofnm, and the In addition, composite materialdispersion provides the to develop and for high SEM imagesthe presented a uniform of possibility ZnO nanoparticles in devices the PVA. In absorbers general, structural frequencies. futureconfirmed work, we that are taking in nanoparticles account fabricating electronic devices thin and chemicalFor analysis the ZnO were embedded into the such PVA as matrix. film transistor to obtain some basic digital gates using a complete process with the integration of the The current-voltage characteristic showed ohmic behavior. The maximum conductivity has been dielectric semiconducting obtained solution properties process in flexible found to material be 2.4 ×and 10−12 S/cm at roommaterials temperature. The in dielectric of PVAsubstrates. depend on frequency and also on the ZnO content, thus this work obtained a hybrid material using 4. Conclusions semiconductive nanoparticles with the highest dielectric constant due to the interaction of nanoparticles. These nanocomposites thin films areofa PVA/ZnO very promising for applications in the Composites polymers based in membranes NPs material were prepared by a solution developprocess. of transistors for flexible electronics radio devices in electrical the rangeand of sub-GHz. casting The thin films were studiedand using thefrequency structural, optical, dielectric Author Contributions: R.A., A.C. and M.L.M. prepared and designed the experiments. K.d.l.T. performed the experiments. R.T. determined the dielectric properties at High Frequencies. J.F. and H.V. analyzed the data. M.M. and I.V. measured the structural properties and morphology of the materials. R.A writing and preparation of draft version. All of the authors contributed to the writing of this article.
Polymers 2018, 10, 1370
15 of 17
characterization. The FTIR analysis demonstrated a good interaction between PVA matrix and the ZnO Nps. In samples with more ZnO Nps the O–H groups decrease, while in the samples prepared using NH4 as the precursor, a reduction in the O–H groups is observed. The UV-vis analysis showed that the addition of ZnO nanoparticles affect the absorbance close to the UV region and the maximum band gap is 5.83 eV for the sample with the higher ZnO content. The XRD analysis showed that the crystal structure of ZnO is presented in the PVA matrix. The surface morphology of the PVA/ZnO obtained by AFM showed a smoother surface with average roughness from 1.9 to 30 nm, and the SEM images presented a uniform dispersion of ZnO nanoparticles in the PVA. In general, structural and chemical analysis confirmed that the ZnO nanoparticles were embedded into the PVA matrix. The current-voltage characteristic showed ohmic behavior. The maximum conductivity has been found to be 2.4 × 10−12 S/cm at room temperature. The dielectric properties of PVA depend on frequency and also on the ZnO content, thus this work obtained a hybrid material using semiconductive nanoparticles with the highest dielectric constant due to the interaction of nanoparticles. These nanocomposites thin films are a very promising material for applications in the develop of transistors for flexible electronics and radio frequency devices in the range of sub-GHz. Author Contributions: R.A., A.C. and M.L.M. prepared and designed the experiments. K.d.l.T. performed the experiments. R.T. determined the dielectric properties at High Frequencies. J.F. and H.V. analyzed the data. M.M. and I.V. measured the structural properties and morphology of the materials. R.A writing and preparation of draft version. All of the authors contributed to the writing of this article. Funding: This research received no external funding. Acknowledgments: J. Flores thanks to National Institute of Technology of Mexico for the support in this research. The authors Maria de la Luz Mota and A. Carrillo want to thank to the program catedras-CONACYT. R. Ambrosio and R. Torrealba would like to acknowledge the VIEP-BUAP department for the support of the research. Conflicts of Interest: The authors declare no conflict of interest.
References 1.
2.
3.
4. 5. 6. 7. 8. 9.
10.
Martin, M.; Prasad, N.; Sivalingam, M.M.; Sastikumar, D.; Karthikeyan, B. Optical, phonon properties of ZnO–PVA, ZnO–GO–PVA nanocomposite free standing polymer films for UV sensing. J. Mater. Sci. Mater. Electron. 2018, 29, 365–373. [CrossRef] Mathen, J.J.; Madhavan, J.; Thomas, A.; Edakkara, A.J.; Sebastian, J.; Joseph, G.P. Transparent ZnO–PVA binary composite for UV-A photo detector: Optical, electrical and thermal properties followed by laser induced fluorescence. J. Mater. Sci. Mater. Electron. 2017, 28, 7190–7203. [CrossRef] Wang, Z.; Han, N.M.; Wu, Y.; Liu, X.; Shen, X.; Zheng, Q.; Kim, J.K. Ultrahigh dielectric constant and low loss of highly-aligned graphene aerogel/poly(vinyl alcohol) composites with insulating barriers. Carbon N. Y. 2017, 123, 385–394. [CrossRef] Pawar, S.P.; Biswas, S.; Kar, G.P.; Bose, S. High frequency millimetre wave absorbers derived from polymeric nanocomposites. Polymer 2016, 84, 398–419. [CrossRef] Peng, C.; Feng, Y.; Hu, J. Enhancing High-Frequency Dielectric Properties of Beta-SiC Filled Nanocomposites from Synergy between Percolation and Polarization. Materials 2018, 11, 1699. [CrossRef] [PubMed] Dang, Z.-M.; Yuan, J.-K.; Yao, S.-H.; Liao, R.-J. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25, 6334–6365. [CrossRef] MacHado, W.S.; Hummelgen, I.A. Low-voltage poly(3-Hexylthiophene)/Poly(Vinyl Alcohol) field-effect transistor and inverter. IEEE Trans. Electron. Devices 2012, 59, 1529–1533. [CrossRef] Van Etten, E.A.; Ximenes, E.S.; Tarasconi, L.T.; Garcia, I.T.S.; Forte, M.M.C.; Boudinov, H. Insulating characteristics of polyvinyl alcohol for integrated electronics. Thin Solid Films 2014, 568, 111–116. [CrossRef] Choudhary, S.; Sengwa, R.J. ZnO nanoparticles dispersed PVA–PVP blend matrix based high performance flexible nanodielectrics for multifunctional microelectronic devices. Curr. Appl. Phys. 2018, 18, 1041–1058. [CrossRef] Mahendia, S.; Tomar, A.K.; Kumar, S. Electrical conductivity and dielectric spectroscopic studies of PVA-Ag nanocomposite films. J. Alloys Compd. 2010, 508, 406–411. [CrossRef]
Polymers 2018, 10, 1370
11.
12. 13.
14.
15. 16.
17. 18.
19. 20.
21.
22. 23. 24. 25. 26. 27.
28. 29.
30. 31.
16 of 17
Son, D.I.; Park, D.H.; Choi, W.K.; Cho, S.H.; Kim, W.T.; Kim, T.W. Carrier transport in flexible organic bistable devices of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) polymer layer. Nanotechnology 2009, 20. [CrossRef] [PubMed] Kazemi, A.S.; Afzalzadeh, R.; Abadyan, M. ZnO Nanoparticles as Ethanol Gas Sensors and the Effective Parameters on Their Performance. J. Mater. Sci. Technol. 2013, 29, 393–400. [CrossRef] Hmar, J.J.L. Flexible resistive switching bistable memory devices using ZnO nanoparticles embedded in polyvinyl alcohol (PVA) matrix and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). RSC Adv. 2018, 8, 20423–20433. [CrossRef] Wu, W.; Huang, X.; Li, S.; Jiang, P.; Toshikatsu, T. Novel three-dimensional zinc oxide superstructures for high dielectric constant polymer composites capable of withstanding high electric field. J. Phys. Chem. C 2012, 116, 24887–24895. [CrossRef] Devi, P.I.; Ramachandran, K. Dielectric studies on hybridised PVDF-ZNO nanocomposites. J. Exp. Nanosci. 2011, 6, 281–293. [CrossRef] Sugumaran, S.; Bellan, C.S.; Muthu, D.; Raja, S.; Bheeman, D.; Rajamani, R. Novel hybrid PVA–InZnO transparent thin films and sandwich capacitor structure by dip coating method: Preparation and characterizations. RSC Adv. 2015, 5, 10599–10610. [CrossRef] Hdidar, M.; Chouikhi, S.; Fattoum, A.; Arous, M.; Kallel, A. Influence of TiO2 rutile doping on the thermal and dielectric properties of nanocomposite films based PVA. J. Alloys Compd. 2018, 750, 375–383. [CrossRef] Komarov, S.A.; Komarov, A.S.; Barber, D.G.; Lemes, M.J.L.; Rysgaard, S. Open-Ended Coaxial Probe Technique for Dielectric Spectroscopy of Artificially Grown Sea Ice. IEEE Trans. Geosci. Remote Sens. 2016, 54, 4941–4951. [CrossRef] Roy, A.S.; Gupta, S.; Sindhu, S.; Parveen, A.; Ramamurthy, P.C. Dielectric properties of novel PVA/ZnO hybrid nanocomposite films. Compos. Part B Eng. 2013, 47, 314–319. [CrossRef] Karthikeyan, B.; Pandiyarajan, T.; Mangalaraja, R.V. Enhanced blue light emission in transparent ZnO:PVA nanocomposite free standing polymer films. Spectrochim. Acta—Part A Mol. Biomol. Spectrosc. 2016, 152, 485–490. [CrossRef] Rashmi, S.H.; Raizada, A.; Madhu, G.M.; Kittur, A.A.; Suresh, R.; Sudhina, H.K. Influence of zinc oxide nanoparticles on structural and electrical properties of polyvinyl alcohol films. Plast. Rubber Compos. 2015, 44, 33–39. [CrossRef] Schroeder, R.; Majewski, L.A.; Grell, M. High-performance organic transistors using solution-processed nanoparticle-filled high-k polymer gate insulators. Adv. Mater. 2005, 17, 1535–1539. [CrossRef] Bouropoulos, N.; Psarras, G.C.; Moustakas, N.; Chrissanthopoulos, A.; Baskoutas, S. Optical and dielectric properties of ZnO-PVA nanocomposites. Phys. Status Solidi Appl. Mater. Sci. 2008, 205, 2033–2037. [CrossRef] Guirguis, O.W.; Moselhey, M.T.H. Optical study of poly(vinyl alcohol)/hydroxypropyl methylcellulose blends. J. Mater. Sci. 2011, 46, 5775–5789. [CrossRef] El-Khodary, A. Vibrational, thermal, optical and magnetic investigations of PVA films filled with FeCl3 and CoCl2 . Phys. B Condens. Matter 2009, 404, 1287–1294. [CrossRef] Jeon, I.Y.; Baek, J.B. Nanocomposites derived from polymers and inorganic nanoparticles. Materials 2010, 3, 3654–3674. [CrossRef] Dazhi Sun, H.-J.S. Multifunctional polymer/ZnO nanocomposites: Controlled dispersion and physical properties. In Multifunctionality of Polymer Composites; Friedrich Klaus, U.B., Ed.; Elsevier: Amsterdam, The Netherlands, 2015; pp. 858–874. ISBN 9780323264341. Xiong, H.M.; Zhao, X.; Chen, J.S. New polymer-inorganic nanocomposites: PEO-ZnO and PEO-ZnO-LiClO4 films. J. Phys. Chem. B 2001, 105, 10169–10174. [CrossRef] Nakhaei, O.; Shahtahmassebi, N.; Rezaeeroknabadi, M.; Bagheri Mohagheghi, M.M. Synthesis, characterization and study of optical properties of polyvinyl alcohol/CaF2 nanocomposite films. Sci. Iran. 2012, 19, 1979–1983. [CrossRef] Abdelrazek, E.M.; Elashmawi, I.S.; El-khodary, A.; Yassin, A. Structural, optical, thermal and electrical studies on PVA/PVP blends filled with lithium bromide. Curr. Appl. Phys. 2010, 10, 607–613. [CrossRef] Yeow, Y.K.; Abbas, Z.; Khalid, K.; Rahman, M.Z.A. Improved dielectric model for polyvinyl alcohol-water hydrogel at microwave frequencies. Am. J. Appl. Sci. 2010, 7, 270–276. [CrossRef]
Polymers 2018, 10, 1370
32.
33. 34.
35.
36.
37. 38. 39.
40. 41. 42.
17 of 17
Rajeswari, N.; Selvasekarapandian, S.; Karthikeyan, S.; Prabu, M.; Hirankumar, G.; Nithya, H.; Sanjeeviraja, C. Conductivity and dielectric properties of polyvinyl alcohol- polyvinylpyrrolidone poly blend film using non-aqueous medium. J. Non. Cryst. Solids 2011, 357, 3751–3756. [CrossRef] Abd El-kader, F.H.; Gaafer, S.A.; Mahmoud, K.H.; Mohamed, S.I.; Abd El-kader, M.F.H. Electrical Conduction in (Polyvinyl Alcohol/Glycogen) Blend Films. Polym. Compos. 2008, 30, 214–220. [CrossRef] Alakanandana, A.; Subrahmanyam, A.R.; Siva Kumar, J. Structural and Electrical Conductivity studies of pure PVA and PVA doped with Succinic acid polymer electrolyte system. Mater. Today Proc. 2016, 3, 3680–3688. [CrossRef] Latif, I.E.; AL-Abodi, E.H.; Badri, D.; Al Khafagi, J. Preparation, Characterization and Electrical Study of (Carboxymethylated Polyvinyl Alcohol/ZnO) Nanocomposites. Am. J. Polym. Sci. 2013, 2, 135–140. [CrossRef] Chandrakala, H.N.; Ramaraj, B.; Shivakumaraiah; Madhu, G.M.; Siddaramaiah. The influence of zinc oxide-cerium oxide nanoparticles on the structural characteristics and electrical properties of polyvinyl alcohol films. J. Mater. Sci. 2012, 47, 8076–8084. [CrossRef] Levinson, L.M.; Philipp, H.R. Low-temperature ac properties of metal-oxide varistors. J. Appl. Phys. 1978, 49, 6142–6146. [CrossRef] Wang, G.; Deng, Y.; Xiang, Y.; Guo, L. Fabrication of radial ZnO nanowire clusters and radial ZnO/PVDF composites with enhanced dielectric properties. Adv. Funct. Mater. 2008, 18, 2584–2592. [CrossRef] Ramesan, M.T.; Varghese, M.; Jayakrishnan, P.; Periyat, P. Silver-Doped Zinc Oxide as a Nanofiller for Development of Poly(vinyl alcohol)/Poly(vinyl pyrrolidone) Blend Nanocomposites. Adv. Polym. Technol. 2018, 37, 137–143. [CrossRef] Makled, M.H.; Sheha, E.; Shanap, T.S.; El-Mansy, M.K. Electrical conduction and dielectric relaxation in p-type PVA/CuI polymer composite. J. Adv. Res. 2013, 4, 531–538. [CrossRef] Roscow, J.I.; Bowen, C.R.; Almond, D.P. Breakdown in the Case for Materials with Giant Permittivity? ACS Energy Lett. 2017, 2, 2264–2269. [CrossRef] Nan, C.-W.; Shen, Y.; Ma, J. Physical Properties of Composites Near Percolation. Annu. Rev. Mater. Res. 2010, 40, 131–151. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
