Nadeesh M.
Adassooriya
a,
Dilek
Ozgit
a,
Sai G.
Shivareddy
a,
Pritesh
Hiralal
a,
Damayanthi
Dahanayake
b,
Rachel A.
Oliver
c and
Gehan A. J.
Amaratunga
*a
aElectrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK. E-mail: gaja1@cam.ac.uk; Fax: +44 (0)1223748348; Tel: +44 (0)1223 748325
bSri Lanka Institute of Nanotechnology (SLINTEC), Mahenawatta, Pitipana, Homagama, CO 10206, Sri Lanka
cDepartment of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK
First published on 19th December 2022
The interface between the polymer and nanoparticle has a vital role in determining the overall dielectric properties of a dielectric polymer nanocomposite. In this study, a novel dielectric nanocomposite containing a high permittivity polymer, cyanoethylated cellulose (CRS) and TiO2 nanoparticles surface modified by hydrogen plasma treatments was successfully prepared with different weight percentages (10%, 20% and 30%) of hydrogenated TiO2. Internal structure of H plasma treated TiO2 nanoparticles (H-TiO2) and the intermolecular interactions and morphology within the polymer nanocomposites were analysed. H-TiO2/CRS thin films on SiO2/Si wafers were used to form metal–insulator–metal (MIM) type capacitors. Capacitances and loss factors in the frequency range of 1 kHz to 1 MHz were measured. At 1 kHz H-TiO2/CRS nanocomposites exhibited ultra-high dielectric constants of 80, 118 and 131 for nanocomposites with 10%, 20% and 30% weight of hydrogenated TiO2 respectively, significantly higher than values of pure CRS (21) and TiO2 (41). Furthermore, all three H-TiO2 /CRS nanocomposites show a loss factor <0.3 at 1 kHz and low leakage current densities (10−6 A cm−2–10−7 A cm−2). Leakage was studied using conductive atomic force microscopy (C-AFM) and it was observed that the leakage is associated with H-TiO2 nanoparticles embedded in the CRS polymer matrix. Although, modified interface slightly reduces energy densities compared to pristine TiO2/CRS system, the capacitance values for H-TiO2/CRS-in the voltage range of −2 V to 2 V are very stable. Whilst H-TiO2/CRS possesses ultra-high dielectric constants (>100), this study reveals that the polymer nanoparticle interface has a potential influence on dielectric behaviour of the composite.
One route being explored to achieve high effective permittivity is to form a composite of two (or more in principle) dielectrics. However, the challenge is to achieve an increase in the effective permittivity of the dielectric without increasing its effective thickness, which would reduce capacitance, beyond that required for sustaining the electric field without breakdown at the specified operating voltage. One possible way of achieving this is to introduce one of the materials in nanoparticle form within the other i.e. a nanocomposite.4 This has the advantage of maximising surface (interface) area to volume ratio of the material included in nanoparticle form. In recent years, it has been shown that with the introduction of nanosized fillers into polymer matrices significantly higher dielectric constants compared the dielectric constant of the filler could be achieved. This increase in effective dielectric constant has been ascribed to polarisation phenomena at the nanoparticle–polymer interfaces.5
It is in this vein that nanosized fillers have been introduced into dielectric polymer matrix to form dielectric polymer nanocomposites. Enhanced dielectric properties with low leakage currents while preserving the facile processability, flexibility and lightness of the polymer matrix have been reported.6–8 TiO2 is a versatile material used in a range of applications including nanodielectrics where composites of nanoscale TiO2 with different polymers form the dielectric.9–18 In 1988, Bandyopadhyay et al. reported the first dielectric nanocomposite based on TiO2 nanoparticle and polystyrene.9 Afterwards, in 1994 Lewis10 reported a landmark theoretical study on dielectric nanocomposites the significance which was recognised after publication of the experimental work on TiO2-epoxy resin dielectric nanocomposite (k = 8.5 at 1 kHz for 500–750 μm thick films) published by Nelson et al. in 2002.11 In 2004, Cheng et al. reported a cross linked poly-4-vinylphenol-TiO2 dielectric nanocomposite with dielectric constant of 7.5 at 50 kHz. Maliakal et al. achieved over 3 times enhancement of dielectric constant compared to that of bulk polystyrene for a 1.25 μm thick core shell TiO2–polystyrene nanocomposite films at 1 kHz.12 Nanocomposites of polypyrrole/TiO2 (k = 140 at 1 kHz for 0.5 mm thick films),13 PMMA/TiO2 (k = 25 at 1 kHz for 5 mm thick films),14 poly(vinylidenefluoride-ter-trifluoroethylene)-TiO2 (k = 12 at 1 kHz for 25–50 μm thick films)15 and poly (vinylidenefluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene)-TiO2 (k = 42 at 1 kHz for 25–50 μm thick films),16 PEOX–PVP–TiO2 nanocomposite (k = 18.56 at 1 kHz 0.40 mm thick films)17 have been reported subsequently in the literature. In our previous work we explored the use of TiO2 nanoparticles within a high dielectric polymer called cyanoethylated cellulose (CRS) and reported effective relative permittivities in excess of 200 ((k = 207 at 1 kHz, 25 nm TiO2 particles, 120 nm thin films); 10 times higher than that of the pristine CRS polymer).18 CRS is a cellulose derivative with very high dielectric constant (k = 21 at 1 kHz) compared to other dielectric polymers, but has received comparatively less attention in the literature. Dipoles of CO and CN present in CRS are responsible for dielectric behaviour and due to the polar nature of the polymer matrix, it also shows a strong affinity for inorganic oxides.19 In addition, dielectric behaviour of CRS/BaTiO3 (k = 133 at 1 kHz, 2-micron BaTiO3 particles and film thickness 100–350 μm)20 and MMT/CRS (k = 71 at 1 kHz, 180 nm thin films)21 have been reported in the literature. Here we further explore the TiO2 nanoparticle system by subjecting the surface to a plasma treatment. The aim is to see whether the effective interface charge density can be enhanced by introducing additional surface states through mild plasma etching.
In the hydrogenation process TiO2 nanoparticles are treated in hydrogen containing environment or hydrogen plasma at defined temperatures for certain period of time. During this process the surface of the TiO2 nanoparticles completely change as it forms a highly disordered surface layer containing large number of oxygen vacancies, Ti3+ though self-doping, surface hydroxyl groups and T–H bonds. Hydrogenated TiO2 or black TiO2 is now one of the most attractive materials in the areas of photocatalysis, electrochemical solar cells, supercapacitors and batteries.22–25
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Fig. 1 Stepwise preparation of MIM capacitors with H plasma treated TiO2–CRS nanocomposite thin films. |
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Fig. 2 PXRD patterns of (a) FTIR spectra (b) of TiO2 and H plasma treated TiO2 and (c) UV-visible spectra and (d) corresponding photographs of H plasma treated for different exposure time. |
FTIR spectra for pristine TiO2 and H-TiO2 are shown in Fig. 2(b). Both pristine TiO2 and H-TiO2 show same IR activity; absorption bands at 3350 cm−1, 1630 cm−1 and 734 cm−1 are due to O–H stretching, O–H bending of surface adsorbed water and Ti–O stretching mode of TiO2 respectively.
Fig. 2(c) shows the UV-Visible absorption spectra for pristine TiO2 and H-TiO2 with different reaction times. It was observed that a significant change in the absorption intensity in the UV region and the absorption in the visible light region gradually increased with extended hydrogenation time. This can be attributed to the colour change in the material, where the white colour of the pristine TiO2 turns into black. The optical absorption characteristics suggest that H plasma treatment introduces surface defect states within the band gap of the TiO2.26–28 Fully black H plasma treated TiO2 sample for 10 min at 500 °C was used for all the other characterisations and nanocomposite formation from here on.
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Fig. 3 SEM (a) and STEM (b) images of TiO2 nanoparticles and SEM (c) and STEM (d) images of H plasma treated TiO2 nanoparticles. |
HR TEM images and electron diffraction patterns for pristine TiO2 and H-TiO2 are shown in Fig. 4.
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Fig. 4 HR TEM (a) and FFT (b) images of TiO2 nanoparticles and HR TEM (c) and FFT (d) images of H plasma treated TiO2 nanoparticles. |
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Fig. 5 (a) EEL spectra (b) H-NMR spectra (c) XPS Ti 2p spectra and (d) UV absorption spectra arranged according to the Kubelka–Munk function for H-TiO2 NPs. |
H-NMR spectra of pristine TiO2 and H-TiO2 nanoparticles show broad peaks at chemical shifts of 9.3 ppm and 7.6 ppm respectively. However, H-TiO2 nanoparticles shows two small additional peaks at 4.0 ppm 2.4 ppm which could be ascribed the presence of weakly bound hydrogen atoms as a result of the H plasma treatment. The multiplicity of the NMR peaks can be used to identify the number of H environments within the sample. The peaks at 4.0 ppm and 2.4 ppm can be attributed to bridging and terminal titanol groups.29
XPS analysis of Ti 2p region reveals a slight shift of binding energies as shown in Fig. 5(c). The Ti 2p XPS spectra shows two main peaks at 458.4 eV and 463.9 eV due to the spin–orbit pairs of 2p3/2 and 2p1/2. In addition, a satellite peak at 471 eV was observed.31 A similar trend was observed for the Ti 2p in H-TiO2 (2p3/2 peak at 458.6 eV, 2p1/2 peak at 464.3 eV and satellite peak at 472 eV). These slight changes in the binding energies can be attributed to the oxygen vacancy formation in the TiO2 surface. A notable change in the binding energies of O 1s was not observed in agreement with previous studies. HOMO and LUMO energy levels of H plasma treated TiO2 NPs were extracted using UV/Vis Fig. 5(d) and UPS data (Fig. S1†). It was observed that band gap of pristine TiO2 NPs (3.20 eV) has reduced to 3.1 eV following the hydrogenation process which is in agreement with the previous studies.26 HOMO level of energy of pristine TiO2 measured in our previous work was −7.5 eV. H-TiO2 gives a HOMO value of 5.90 eV during the hydrogenation process. This is a very significant shift of 1.6 eV. UPS probes the first 2–3 nm of the surface of the nanoparticles. The NMR shows that the surface of the H-TiO2 has titanol groups. The surface modification through the titanol groups may contribute to the shift in the HOMO energy measured. In addition to above characterisation techniques, elemental analysis for hydrogen was performed on hydrogenated TiO2 samples and it shows that 0.2% of average weight percentage of hydrogen is present. It suggests that TiO2:
H molecular ratio is roughly 5
:
1. Pristine TiO2 did not give any significant reading.
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Fig. 6 (a) PXRD traces, (b) SEM images cross section of a H-TiO2/CRS (10%) (c) XPS N 1s spectra for CRS and H-TiO2/CRS (d) XPS Ti 2p spectra for H-TiO2 and H-TiO2/CRS (10%) nanocomposite. |
X-ray photoelectron spectroscopy of the N 1s peak from the bare CRS polymer and H-TiO2/CRS and Ti 2p peak from bare TiO2 and H-TiO2/CRS are shown in Fig. 6(c). The N 1s spectra for the CRS polymer showed two major peaks with binding energies of 397.4 eV and 397.9 eV whereas, N 1s spectra for the H-TiO2/CRS resolved into three peaks at 400.3 eV, 401.5 eV and 402.5 eV. The shifting of the binding energy of N 1s in the CN group of the polymer confirms the interaction of the CRS polymer with H-TiO2 nanoparticles. In Fig. 6(d) the Ti 2p XPS spectra for H-TiO2 shows two main peaks at 457.2 eV and 462.8 eV due to the spin–orbit pairs of 2p3/2 and 2p1/2. In addition, a satellite peak at 470.4 eV was observed. For H-TiO2/CRS; the Ti 2p3/2 and 2p1/2 peaks are shifted to 461.3 eV and 466.8 eV respectively. However, satellite peak was not observed. A notable change in the binding energies of C 1s and O 1s were not observed. Furthermore, N 1s spectral line of CRS-H-TiO2 has shifted to higher values compared to CRS-TiO2 nanocomposite. This can be ascribed as to the changes that arise in the electronic structure of H-TiO2 during hydrogenation which facilitate the bonding interactions of H-TiO2 with the CRS polymer matrix. The XPS data suggest that the interaction of the CRS with H-TiO2 occurs via interactions of Ti with N to TiN type complexes.30
The capacitance values for the bare polymer and 10% weight CRS–H-TiO2 composite remain constant up to 500 kHz and then gradually decreases with increasing frequency. For 20% and 30% weight H-TiO2/CRS composite, capacitance values sharply decrease after 10 kHz with increasing frequency. This behaviour is due to the relaxation process in H-TiO2/CRS interface.31
However, capacitance values for all TiO2/CRS nanocomposites reported in our previous work are stable up to 500 Hz compared to H-TiO2/CRS nanocomposites. This suggests that the addition of H plasma treated TiO2 initiates the relaxation process at lower frequencies compared to pristine TiO2, in the CRS matrix. Which is consistent with the H plasma treatment creating surface defect states deep in the band gap which take longer to fill/empty. Dielectric constants for all three composites were calculated using the average film thickness measured by AFM (Fig. S2†). At 1 kHz CRS–H-TiO2 nanocomposites exhibited high dielectric constants of 80, 118 and 131 with 10%, 20% and 30% weight of H-TiO2 respectively which is significantly higher than of pure CRS. Fig. 7(c) shows capacitance–voltage measurements for all three nanocomposite films and bare CRS polymer and it was observed that capacitance values for all three nanocomposites and bare CRS polymer in the voltage range of −2 V to 2 V are very stable. The leakage current densities are relatively low, in the range of 10−6 A cm−2–10−8 A cm−−2. However, as seen in TiO2/CRS nanocomposites,5 the H-TiO2/CRS current voltage characteristics also possess a distinct asymmetry and a shift in the zero current up to 1 V is seen for composites with different TiO2 loading. This shift in the minimum current from 0 V could be due to the charging of the H-TiO2/CRS interfaces. There is likely to be charging at the contacts when there is a high concentration of NPs >10% in the nanocomposite. Furthermore, all three H-TiO2/CRS nanocomposites show a loss factor <0.3 at 1 kHz (Fig. S3†).
In the C-AFM configuration the biasing conditions negative currents are measured, corresponding to dark spots in the C-AFM image. The dark spots in the current image are in the vicinity of H-TiO2 particles and leakage paths through the nanocomposite are associated with the H-TiO2 nanoparticles. For a negative potential at the tip, I–V curve shows typical diode type behaviour between the Pt coated AFM tip and the H-TiO2 nanoparticle embedded in CRS polymer.
Though the CRS is an insulator, at the interface with the H-TiO2 it is shown as having the capability to charge through interaction with defect and polarised bonding states. The interface at equilibrium therefore conforms to a semiconductor heterojunction model. However, when a potential is applied at the terminals and there is a shift from the equilibrium, only states on the H-TiO2 side of the interface are available for further filling or emptying. The equilibrium situation is taken as frozen-in charge at the interface. Therefore, the CRS acts as an insulator (no charge) away from the interface, while the charge on the H-TiO2 side of the interface can change. It is this additional charge which has to be balanced at the terminals of the composite dielectric which leads to the increase in the overall relative permittivity of the nanocomposite film.
Surface modification of NPs was used to improve the chemical interactions at the interfaces between nanoparticles and polymer matrix and thereby influence the charge transfer at the interface to tailor the dielectric properties of the nanocomposites. Hydrogenated TiO2 nanoparticles obtained by H plasma treatments of pristine confirmed the presence titanol type functionality on TiO2 NPs and the surface amorphous nature H-TiO2. The pristine TiO2 NPs with CRS polymer show higher energy densities (At 1 kHz, 80, 118 and 131 dielectric constants extracted were for H-TiO2/CRS nanocomposites with 10%, 20% and 30% weight of H-TiO2 respectively. In the case of TiO2/CRS nanocomposites dielectric constants were 118, 176 and 207 with 10%, 20% and 30% weight of TiO2 respectively). In the case of H-TiO2, introduction of titanol type functionality on the TiO2 surface improves the bonding interactions at the polymer–nanoparticles interface resulting reduced structural mobility. Thus, charging of the interface due to defects from unpaired bond is inhibited. However, capacitance values for H-TiO2/CRS nanocomposites in the voltage range of −2 V to 2 V are very stable compared to pristine TiO2 and CRS nanocomposite. In traditional dielectric composites the highest dielectric constant that can be achieved is always found to be below the dielectric constant of the filler as explained by the logarithmic mixing rule. However, it has been shown that with the introduction of nanosized fillers into polymer matrices significantly higher dielectric constants compared to that of the filler could be achieved. Two models have been proposed to explain this behaviour, one termed the interface model10 and the other the multi core model.32 According to the interface model, the interface zone between the nanoparticle and the polymer has a vital influence on the properties of the dielectric as whole. In the multicore model, dielectric nanoparticles dispersed in a polymer matrix are assumed to result in the formation of different layers termed the bound layer, intermediate layer and the loose layer. The bound layer (1st layer) corresponds to the nanoparticle and polymer bonding through functional groups to form an interface which is responsible for the formation of the diffuse electrical double layer. The intermediate layer (2nd layer) consists of polymer chains strongly bonded to the bound layer. The loose layer (3rd layer) is a region of polymer loosely bound to the second layer. The polarisation phenomena occurring in the interface zones of nanoparticles are proposed as being responsible for ultra-high dielectric constants measured in these dielectric nanocomposites. We propose an extension of this model based on the nanoparticle being a semiconductor which can be internally charge depleted/accumulated to give an increase in the effective dielectric permittivity (constant).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr04680f |
This journal is © The Royal Society of Chemistry 2023 |