1 Introduction

For power engineering, organic–inorganic material composites represent not only a new and exciting field of basic research, but also offers perspectives for several new applications in different technological fields due to their low cost, good processability, high transparency in the visible range, broad and interdisciplinary area of fundamental research and development activity [1, 2]. In general, nanoparticles were introduced into polymer matrix using various techniques. The various dispersion processes are necessary in order to transfer the nanoparticles from the agglomerated state into a homogeneously dispersed state. The direct incorporation using chemical methods and by application of high shear forces during mechanical powder dispersion process are the popular one [3,4,5]. Ultrasound vibration was used to disperse nanoparticles, which also helps to improve the dispersion state of nanoparticles. Chemical methods are able to generate individual and non-agglomerated nanoparticles “in situ” within a thermosetting/thermoplastic polymer. However, an additional chemical treatment of the nanoparticle surface may further enhance the composite’s properties by improving the filler-matrix coupling quality [3,4,5].

TiO2 is an amorphous white powder and it is a very stable material. The TiO2 material is produced and used in the workplace in varying particle size contents, including fine and ultrafine sizes. Therefore, it is used extensively and finding applications in several commercial products such as cosmetics, plastics, paper and food. Therefore, TiO2 nanospheres have been considered biologically inactive in experimental animals [6] and humans [7]. The TiO2 nanoparticles have been proposed as one of the promising candidates to achieve high refractive index and maintain high transparency, because it have a high refractive index (n = 2.45 and 2.7 for anatase and rutile phase, respectively) and a very low absorption coefficient in the visible range [8]. In the frame of environmental protection, TiO2 becomes an interesting component in electronics. TiO2 exists in three different stable modifications and has good electrical, catalytic and electrochemical properties [9]. Literature has also shown that nanoscale TiO2 reinforcement brings new optical, electrical, physiochemical properties attained at very low TiO2 content, which make polymer TiO2 nanocomposites a promising new class of materials. Among different inorganic nanofillers, TiO2 was used to enhance some properties of polymer matrix [10]. Various methods have been used to produce titanium dioxide powders in the past two decades, such as the classic sulfate process, the chloride route, the sol–gel method [11, 12], the hydrothermal method [13,14,15] and the gas condensation method [16, 17]. The sol–gel method has generated a large scope of interest in the preparation of inorganic ceramic and glass materials. This simple, cost effective and low temperature synthesis procedure has also been favored and largely applied in catalyst preparation due to its potential to fabricate catalysts with high purity, homogeneity, fine-scale and controllable morphology.

Epoxy resins are thermosetting materials that require a cure treatment to attain suitable physical, optical, electrical and mechanical properties for industrial applications. Neat epoxy and epoxy reinforced nanoparticles are preferred chemicals, electrically insulating and adhesive materials for several applications, such as printed circuit boards, bushings, generator ground wall insulation systems and cast resin transformers. Excellent adhesive properties, resistance to heat and chemicals and very good electrical insulating properties make epoxy a favoured insulating material [18]. Bauer et al. [19] showed that a high content of nanosized silica, alumina, and Titania was embedded in epoxy adhesives. Yang et al. [20] proved a UV/IR blocking in the neat epoxy via the addition of inorganic nanoparticles. Chau et al. [21] show that the obtained refractive index of adding 30 wt% TiO2 nanoparticles into the epoxy matrix via sol–gel method was found to be 1.668. Recently, compared to neat epoxy and epoxy with nanoparticles content, epoxy nanocomposites systems have displayed enhanced electric and optical properties [22]. Fujishima et al. [23] found that the TiO2 possesses strong photocatalytic properties and excellent ultraviolet absorption properties which are effective for aging-resistant properties of polymers. Morselli et al. [24] were synthesized by means of TiO2 generated nonhydrolytic sol–gel process mixed with a cycloaliphatic epoxy resin. It found that the presence of Titania nanoparticles resulted in a reinforcing and stiffening effect due to both their hydrodynamic effect and, most important, a significantly higher cross-linking density of the composite material with respect to the unfilled epoxy resin. The solid content of Titania in the epoxy matrix can be more than 70 wt% without affecting the optical transparency of the hybrid film [25]. Sowntharya et al. [26] synthesized hybrid nanocomposite coatings from titanium tetraisopropoxide, and epoxy or acrylic modified silanes were deposited on polycarbonate.

This article focuses to study the TiO2 filler effect on microstructure and opto-electrical properties of Epoxy–TiO2 composites (Ep–TiO2–Cs). The band gap energy of Ep–TiO2–Cs was determined and its comparison with the other data published in the literature was also studied. In addition, the important optical parameters of this study such as real and imaginary dielectric constants, optical conductivity, electric conductivity and VELF and SELF were also evaluated.

2 Experimental Techniques

2.1 Synthesis of the As-Prepared TiO2 Particles

The TiO2 powder is prepared by using sol–gel method [27]. We dissolved 5.5 ml of Titanium (IV) isopropoxide in 18 ml of methanol under magnetic stirring for 1.5 h and then we added 1.3 ml of acetic acid under magnetic stirring for 1 h. Finally, TiO2 particles were obtained after calcination in an oven at 300 °C for 3 h.

2.2 Synthesis of Epoxy–TiO2-Composites

The 2,2-bis (4-glycidyloxyphenyl) propane epoxy resin (molar mass 340 g/mol), and difunctional amine hardener provided by Maestria, are used to prepare the polymeric matrix. The epoxy resin and hardener are used in a molar ratio (75:25). Epoxy–TiO2-composites (Ep–TiO2–Cs) were synthesized using the ultrasonication mixing method to disperse TiO2 fillers in epoxy matrix as described in previous work [22]. The obtained Ep–TiO2–Cs samples, with 0.1; 0.2; 0.5; 1.0; 3.0 and 5.0 wt% TiO2 contents, have thicknesses about 1.19 mm.

2.3 Characterization Techniques

Scanning electron microscopy (SEM) operating at 15 kV was used to get the morphological information of TiO2 fillers and Ep–TiO2–Cs systems.

The synthesized as prepared TiO2 particles, neat epoxy and Ep–TiO2–Cs samples were studied using X-Ray diffractometer (XRD) (Shimadzu LabX XRD-6000) of monochromatic Cu Kα radiation source operating at 40 kV voltage and 30 mA current. We used a step of 0.02° per second in the angular range of 10°–70° to confirm the “crystallinity” of the composite samples. Afterthought, crystallite size, dislocation density and lattice strain were determined.

The absorbance A(λ), transmittance T(λ) of the Ep–TiO2–Cs samples, over the wavelength range of 200–1000 nm, were investigated using UV–Vis–NIR spectrophotometer (Shimadzu UV-3101). The absorption coefficient (α), the band gap energy (E g ), the extinction coefficient (k), the real and imaginary permittivity parts (ε 1 , ε 2 ), the refractive index (n), the surface and volume energy loss functions (SELF, VELF) are calculated from the recorded data.

3 Results and Discussions

3.1 Surface Morphology of Ep–TiO2–Cs Samples

The SEM was also done for the neat resin and its composite samples. The typical microstructure of the as prepared TiO2 particles, neat epoxy and 5.0 wt% TiO2 contents into epoxy resin were shown in Fig. 1. As shown in Fig. 1a, a separate particle can be distinguished. It exhibits that TiO2 particles are spherical in shape. The obtained medium diameter of the synthesized as prepared TiO2 particles is 4.23 µm. The fracture surface of the neat epoxy reveals a brittle behavior characterized by large smooth area and flat surface due to any reinforcing action which corresponds to the low growth of crack like defects (see Fig. 1b). As it can be seen in From Fig. 1c, it is found that a suitable adhesion between TiO2 spheres content and the resin matrix have been attained at high level of loading 5.0 wt% TiO2 composite samples. Furthermore, the micrograph exhibits a rough surface morphology and TiO2 spheres can be identified on the surface of epoxy with 5.0 wt% TiO2 composite sample. The overall size distribution of TiO2 spheres is observed to be narrow and the overall dispersion of particles within the epoxy matrix is quite uniform. It seems that the cohesion of the TiO2 spheres fillers and the viscosity of the resin might prevent the uniform dispersion. Furthermore, TiO2 spheres seem to be well embedded into the matrix showing excellent particles–matrix adhesive, which may be attributed to weak physical interactions of TiO2 spheres and epoxy matrix. The morphologies of fractured surface show brittle fracture with dispersed stresses in more than one crack propagation direction and less flat area with existence of TiO2 sphere particles.

Fig. 1
figure 1

SEM representative images of a the TiO2 nanospheres, b the neat epoxy and c 5.0 wt% TiO2 in epoxy resin

Full size image

3.2 XRD Analysis of Ep–TiO2–Cs Samples

XRD is the best tool to characterize the structure of the as prepared TiO2 particles, neat epoxy and Ep–TiO2–Cs to investigate the effect of TiO2 spheres on the epoxy matrix. Figure 2 presents the typical XRD patterns of as prepared TiO2 particles, neat epoxy, and Ep–TiO2–Cs. The as prepared TiO2 particles were confirmed to be a tetragonal structure with space group I41/amd. The deduced lattice parameters are found to be a = 3.7852 Å and c = 9.5139 Å, matching with data from JCPDS card No: 21-1272. The as prepared TiO2 particles synthesized by sol gel method acquired enhanced structure. The XRD patterns of the Ep–TiO2–Cs (in the contents of 1.0, 3.0 and 5.0 wt% TiO2 contents) show dominant peak with values of 2θ located at 25.28°, 37.8°, 48.04°, 53.5°, 53.89° and 62.68°, corresponding to the (101), (004), (200), (105), (211) and (204) indexed planes, respectively. They indicate that the added TiO2 fillers have the crystal phase of anatase. The strong and sharp diffraction peaks showed highly crystalline TiO2 contents. From Fig. 2, it is evident that neat epoxy and the composites systems exhibit a broad diffraction peak at approximately 2θ = 19° which reveals the amorphous nature of the Epoxy matrix [8]. The small diffraction peak in the (0.1, 0.2 and 0.5 wt% TiO2 samples) indicates that the low TiO2 content was uniformly dispersed into epoxy matrix and it is also a strong evidence for the formation of an exfoliated composite structure. Incorporated and confined TiO2 particles into epoxy matrix (at 3.0 and 5.0 wt% TiO2) through the curing process of composites showed a significant diffraction peaks relative to TiO2 anatase.

Fig. 2
figure 2

XRD patterns of TiO2 nanospheres and Ep–TiO2–NCs with different content of TiO2

Full size image

To estimate the better orientation of crystal planes of TiO2 confined structure in Epoxy matrix, the texture coefficient TC(hkl) was determined using the following equation [28].

$${T_C}_{{(hkl)}}=\frac{{{I_{\left( {hkl} \right)}}}}{{{I_{0\left( {hkl} \right)}}}}/\frac{1}{n}\mathop \sum \limits_{i} \frac{{{I_{\left( {hkl} \right)i}}}}{{{I_{0\left( {hkl} \right)i}}}},$$
(1)

where I (hkl) is the measured intensity of the (hkl) X-ray reflection, I 0(hkl) is the corresponding intensity for randomly oriented sample taken from the JCPDS card No: 21-1272 and ‘n’ is the number of diffraction peaks observed in the XRD patterns. The variation of T C with the crystal planes peaks for TiO2 in all Ep–TiO2–Cs samples is shown in Fig. 3. As shown in this figure, the similar deviation among the TC(hkl) of various facets (hkl) is observed for different added TiO2 fillers in an epoxy matrix. It can be easily remarked that no preferential orientation of TiO2 particles is observed. Using Scherer’s equation, the particle size of the added TiO2 contents is evaluated [29]:

Fig. 3
figure 3

Texture coefficient upon (hkl) crystal planes for Ep–TiO2–NCs

Full size image
$$D=\frac{{0.9\lambda }}{{\beta cos\theta }},$$
(2)

where D is the particle size of the added fillers in the epoxy matrix, λ is the wavelength (λ = 1.5418 Å), β is the full width at half maximum and θ is the scattering Bragg angle. The obtained values of 2θ and FWHM are gathered and added in Table 1.

Table 1 The values of peak position 2θ and FWHM for TiO2/epoxy composites
Full size table

The internal strain of TiO2 in the composite systems is calculated using the formula [30]:

$$\varepsilon =\frac{{\beta cos\theta }}{4},$$
(3)

Also, the dislocation density (δ) is established using the relation [30]:

$$\delta =\frac{1}{{{D^2}}},$$
(4)

The particle size (D), dislocation (δ) and internal strain (ε) analysis results of TiO2 nanoparticles embedded in the Ep–TiO2–Cs are given in Fig. 4. From Fig. 4, we can easily note that the average size of TiO2 particles embedded in the epoxy matrix decreases with increasing the added TiO2 fillers. The average size decreases from about 50 nm in 1.0 and 3.0 wt% TiO2 to 30 nm in 5.0 wt% TiO2 sample, indicating the better dispersion of TiO2 particles in highly filled epoxy matrix. Also, the dislocation and the internal strain increase with increasing added TiO2 contents, in agreement with the particle size decrease, which can provide the information about the good dispersion of TiO2 particles in the epoxy matrix. This confirmed the observed SEM representative micrograph of Ep–TiO2–Cs with 5.0 wt% TiO2 contents (Fig. 1).

Fig. 4
figure 4

Evolution of crystallite size (D), dislocation (δ) and internal strain (ε) analysis for different incorporating TiO2 nanospheres

Full size image

3.3 Linear Optical Properties of Ep–TiO2–Cs Samples

The effect of TiO2 spheres fillers on the linear optical properties of Ep–TiO2–Cs has been probed using UV–Vis–NIR spectrophotometer. Optical characterization like transmittance, absorbance, absorption coefficient, refractive index and energy band gap provide basic information about the quality and gives electrical properties of the prepared materials. Figure 5 shows the variation of transmittance and absorbance spectra of the Ep–TiO2–Cs with different TiO2 contents, recorded in the wavelength range of 200–1000 nm. From As it can be seen in Fig. 5a, the transmittance of the neat epoxy is high in the visible and NIR range. The neat epoxy system exhibits an excellent transparency of more than 60% of the visible light, and reaches 80% in the NIR range. Whereas Ep–TiO2–Cs transmittance falls to less than 10% when the TiO2 contents into epoxy matrix increases up to 5.0 wt% TiO2. This transmittance shrinkage is more important in the NIR range for the highly-filled composites (see the inset of Fig. 5a). Therefore, adding TiO2 contents in epoxy matrix obviously improves the UV-shielding efficiency, with an enhancement of about 65–80% up to 5.0 wt% TiO2.

Fig. 5
figure 5

Variation of a transmittance and b absorbance upon wavelength of Ep–TiO2–NCs

Full size image

An alternative cause of light scattering is the mismatch of refractive index between the polymer matrix and the particles (the refractive index of epoxy polymer is 1.54–1.55 [31], and that of TiO2 is 2.6–2.9 [32, 33]) which leads to intense light scattering [34]. In the same way, the intensity losses of incident light can be described by [35, 36]:

$$\frac{{\text{I}}}{{{{\text{I}}_0}}}=\exp [\frac{{ - 3{{\text{V}}_{\text{p}}}{{{\upchi}~}}{{\text{r}}^3}}}{{4{{{{\uplambda}}}^4}}}(\frac{{{{\text{n}}_{\text{p}}}}}{{{{\text{n}}_{\text{m}}}}} - 1)],$$
(5)

where r is the radius of valued spherical particles, n p and n m are the refractive indices of the particles and the matrix respectively, I is the intensity of the scattered light, I o the intensity of incoming light for non-absorbing materials, V p is the volume fraction of the particles, λ is the light wavelength and is the optical path length. The above equation indicates that in the presence of mismatching of refractive index, the scattered light intensity decreases with increasing the particle radius r. Thus, the RI incompatibility may be compensated by the reduction of the particle size under visible light wavelength, and therefore transparency can be kept at a level relatively high. But if the particle size grows, the particle agglomeration becomes important. These agglomerations persist in the composites and scatter visible light.

Besides, in the UV-light region, the transmittance drops sharply due to the onset of fundamental absorption. Furthermore, the UV-light absorption was extended up to about 400 nm for the 5.0 wt% TiO2 filled composites (Fig. 5b). Figure 5b shows the changes in the absorbance spectra of neat epoxy and Ep–TiO2–Cs. The Ep–TiO2–Cs samples were found to have higher visible absorbance in comparison to neat epoxy. It is important to mention that the absorption edge of composite samples, instead of being abrupt, occurs quite smoothly. The transmittance of sample 5.0 wt% TiO2 nearly reached zero at the range of 200–280 nm (see Fig. 5a), indicating that the TiO2 spheres fillers in epoxy matrix possessed a strong UV absorbance, which was also confirmed in other papers [37]. In the UV-light range, the neat epoxy polymer only blocks UV-light in the range of 200–280 nm, however it becomes high UV-light blocker (up to 400 nm) via the addition of TiO2 powder from 5.0 wt% filling concentration. These results prove that the Ep–TiO2–Cs can be potential candidates to be used in the UV-shielding coatings.

Using Tauc’s model, the optical band gap energy (E g ) of synthesized Ep–TiO2–Cs was calculated from the absorption data [38, 39]:

$${(\alpha h\upsilon )^m}=A(h\upsilon - {E_g})$$
(6)

where A is the pre-factor constant independent of photon energy for this transition, in this case, m = 2 is a parameter if the band transition is direct, and hν is the incident photon energy. α is the absorption coefficient evaluated from absorbance data. The calculated values of (αhν)2 versus (hν) are plotted in Fig. 6a. The band gap energies are obtained by extrapolating the linear portion of the graph on the hν axis at (αhν)2 = 0, as shown in Fig. 6a.

Fig. 6
figure 6

a Tauc’s plots of variation of optical band gap and b variation of direct band gaps and constant A for our samples

Full size image

The obtained values of direct gap energy Eg are plotted in Fig. 6b as a function of TiO2 content. It is clear that the band gap decreases with increasing TiO2 contents in epoxy matrix, which reflects the change in the electronic structure of the epoxy matrix. This decrease is due to the interactions between TiO2 particles and epoxy matrix. In addition, the decrease in optical band gap can be related to the increasing in the density of defect states [40, 41]. Indeed, polymer composites contain a high concentration of defect states and these defects are responsible for the presence of localized states in the band gap. So, it can be explained by the broadening of the valance band inside the forbidden gap and leads to band tailing and hence band gap shrinkage.

Extinction coefficient (k) and refractive index (n) are often referred to as the optical constants of the material. Together, n and k have a key role to determine several optical properties of the composite. The refractive index (n) was determined from the transmittance spectrum as function of the wavelength in the range of 200–1000 nm. The value of n can be determined by using Fresnel relation based on the reflectance R and the extinction coefficient k is as follows [22]:

$$n=\frac{{1+R}}{{1 - R}}\sqrt {\left( {\frac{{4R}}{{{{(1 - R)}^2}}} - {k^2}} \right)} ,$$
(7)

where, \(k=\alpha \lambda /4\pi\) is the extinction coefficient. Those optical parameters were shown in Fig. 7 for Ep–TiO2–Cs systems. The extinction coefficient is inversely related to the transmittance spectra. From Fig. 7a, it is found that the value of extinction coefficient k is larger which proved that there is a strong absorption at 5.0 wt% TiO2 sample compared to the neat epoxy. The value of k generally varying with added TiO2 contents and shows that the neat epoxy is highly transparent. The extinction coefficient was found to be high. The values indicate the loss of electromagnetic radiation energy through medium of the material. It is clear that the k values are slighter at the lower wavelengths and exceedingly important at high frequencies. At the high added TiO2 contents, it is highly transparent compared to the neat epoxy sample. The variation in the k values is may attribute to the light scattering resulting from the interface between the increasing added nanoparticles into the epoxy matrix.

Fig. 7
figure 7

a Variation of extinction coefficient and b dependence of the refractive index upon to wavelength for different TiO2 nanospheres content

Full size image

The refractive index n is an important physical parameter related to microscopic atomic interactions. Figure 7b represented the variation of the refractive index n for Ep–TiO2–Cs upon the wavelength. As shown in this figure, the increase in added TiO2 fillers results in overall increasing of the refractive index which due to the largely decreasing of transmission as TiO2 content in epoxy matrix increases. It is used to achieve both electronic polarization and the applied electric local field inside the optical materials. In UV–Vis region, the refractive index becomes constant in the higher added TiO2 contents at the content 3.0 and 5.0 wt%. Also, the refractive index values were remaining almost unchanged with added TiO2 contents. It is found to be 2.448 at a high content of TiO2 concentrations, which resulting in higher reflection; the highest value of refractive index is suitable for optical devices.

The dielectric function is a complex quantity and a fundamental intrinsic property of the composite material which consists of both real and imaginary parts. The real part indicates how the speed of light in the material can be slowed down due to the scattering effect at the material grains and boundaries and it is related to n value. While, the imaginary part deals with the energy absorption from electric field due to dipole motion and it is related to k value. Using the complex dielectric permittivity ε*, we can be determined other optical parameters such as the volume and surface energy loss functions (VELF) and (SELF) of the solid material, respectively and relaxation time τ. It can be calculated from the relation:

$${\varepsilon ^*}={\varepsilon _1}+i{\varepsilon _2},$$
(8)

With values of refractive index n and extinction coefficient k, we can be deduced the real and imaginary parts of the dielectric permittivities ε 1 and ε 2 for Ep–TiO2–Cs using the following equations [42]:

$${\varepsilon _1}={n^2} - {k^2},$$
(9)
$${\varepsilon _2}=2nk,$$
(10)

From Eqs. (7 & 8), several optical constants can be determined such as the dielectric permittivity at high frequencies ε∞, the plasma frequency ωp and relaxation time τ, about the relations [43]:

$${\varepsilon _1} \approx {\varepsilon _\infty } - \frac{{{\varepsilon _\infty }\omega _{p}^{2}}}{{4{\pi ^2}{c^2}}}{\lambda ^2},$$
(11)
$${\varepsilon _2}=\frac{{{\varepsilon _\infty }\omega _{p}^{2}}}{{8{\pi ^3}{c^3}\tau }}{\lambda ^3},$$
(12)

The intrinsic carrier’s concentration related to the effective mass ratio N opt /m* is then calculated using:

$$\omega _{p}^{2}=\frac{{4\pi {N_{opt}}{e^2}}}{{{\varepsilon _\infty }m_{e}^{*}}},$$
(13)

At high electromagnetic waves, it is considered that the real part ε1 of the dielectric permittivity was related to the dispersion energy of the propagation speed through the material. However, the imaginary part ε2 of the dielectric permittivity was responsible for the energy absorption from the applied electric field due to dipole motion [44]. For all samples and in the IR range, it is found that the real part of the dielectric permittivity has a linear portion in the representative square of the wavelength (see Fig. 8a), meanwhile the imaginary part is linear with λ 3, as shown in Fig. 8b. The variation of the dielectric permittivity with the wavelength was occurred some molecular interactions between the TiO2 contents and epoxy matrix through curing process. The calculated values of these dielectric permittivities have been gathered in Table 2. Using the Table 2, it is noted that all optical parameters were decreased linearly in composites system compared to neat epoxy when increasing the added TiO2 fillers quantities in the epoxy matrix. To confirm the red shift of the band gap energy in Ep–TiO2–Cs, the optical resistivity ρopt of the investigated Ep–TiO2–Cs can be determined by using the values of Nopt/m* and τ, from the equation [45]:

Fig. 8
figure 8

a Real part of dielectric permittivity versus λ2 and b Imaginary part of dielectric permittivity versus λ3

Full size image
Table 2 Calculated values of optical constants
Full size table
$${\rho _{opt}}=\frac{{{m^*}}}{{{e^2}\tau {N_{opt}}}}$$
(14)

The estimated values of optical resistivity for Ep–TiO2–Cs were summarized in Table 1. The intrinsic carrier concentration N opt /m* in composite systems was calculated and then its variation with TiO2 content was presented in Fig. 6b. It can be obviously noted that the red shift of band gap energy is related to the increase of carrier concentration in Ep–TiO2–Cs. The variation of optical resistivity for differently prepared samples was illustrated in Fig. 9. From this figure, the optical resistivity decreased with increasing TiO2 contents compared to neat epoxy. It is close to the good insulator system of the epoxy polymer.

Fig. 9
figure 9

Optical resistivity us function the added wt% TiO2 nanospheres in epoxy matrix

Full size image

The optical properties originated from movement of the electron inside the lattice optical conductivity. The optical conductivity directly depends on the absorption coefficient and refractive index of the material and follows the same trend as the absorption coefficient and refractive index with increasing wavelength. We used the absorption coefficient α to evaluate the optical and electrical conductivity σ opt and σ e respectively, and is shown in Fig. 10 as following relationships [46]:

Fig. 10
figure 10

a Optical conductivity and b electrical conductivity versus incident energy for different TiO2 nanospheres content (the inset represents the evolution of σ e us function of wt% TiO2 nanospheres for λ = 700 nm)

Full size image
$${\sigma _{opt}}=\frac{{\alpha nc}}{{4\pi }},$$
(15)
$${\sigma _e}=\frac{{2\lambda {\sigma _{opt}}}}{\alpha },$$
(16)

where c is the velocity of light. As can be shown in Fig. 10, the added TiO2 fillers in epoxy matrix greatly increased the optical conductivity and the electric conductivity was increased at λ = 700 nm in the Ep–TiO2–Cs samples (see inset Fig. 10b). The electrical conductivity of these composites has been affected by the presence of TiO2 fillers content. The variation in optical conductivity with photon energy is related to the transmission for all prepared composite materials. The increase of optical conductivity in the high photon energy region is due to the high absorbance of the composite samples compared to neat epoxy and also may be due to the electron excited by photon energy, and with increasing adding TiO2 content. This increase because of creation of new levels in the band gap, lead to facilitate the crossing of electrons from the valence band to these local levels to the conduction band, consequently the band gap decreases and the conductivity increase. As the results are agreements with other work [47].

The energy loss is related to the optical properties of the material through the dielectric function. The volume and the surface energy loss functions (VELF and SELF) can define the probability that the fast electrons will lose energy when migrant within the material and on its surface. These two functions are related to real and imaginary parts ε 1 and ε 2 of the complex dielectric constant by the following relationships [48]:

$$VELF=\frac{{{\varepsilon _2}}}{{\varepsilon _{1}^{2}+\varepsilon _{2}^{2}}},$$
(17)
$$SELF=\frac{{{\varepsilon _2}}}{{{{({\varepsilon _1}+1)}^2}+\varepsilon _{2}^{2}}},$$
(18)

The variation of the volume and the surface energy losses as function wavelength of Ep–TiO2–Cs were shown in Fig. 11. In the curves shown in Fig. 11a, it is revealed that the VELF curves increased with increasing TiO2 contents and with increasing wavelength in visible and mainly in NIR region compared to neat epoxy. Up to 400 nm, the VELF values were strongly changed. This increased VELF with wavelength can be enabling these materials for electro-optical applications. In the other hand, the SELF-values decreased with added TiO2 fillers quantities into the epoxy matrix (see Fig. 11b). The decreasing values of SELF lead to deduce that the materials improve the UV-shielding devices.

Fig. 11
figure 11

a Evolution of the volume energy loss functions (VELF) and b surface energy loss functions (SELF) as function of wavelength of the Ep–TiO2–NCs samples

Full size image

4 Conclusion

The TiO2/epoxy composites (Ep–TiO2–Cs) were prepared and characterized to investigate some advantageous optoelectronic behaviors at different TiO2 contents compared to the neat epoxy. The obtained data from SEM analysis enhanced the homogenous and the uniform distribution of TiO2 on the epoxy matrix. Furthermore, TiO2 particles seem to be well embedded into the epoxy matrix. The data obtained from XRD analysis shows that grain size, dislocation, and strain were affected by the addition of TiO2 contents. The experimental absorbance and transmittance data were investigated using the Ultra-Violet (UV–Vis) spectrophotometer. In the UV-light range, the neat epoxy polymer only blocks UV-light in the range of 200–280 nm, however it becomes high UV-light blocker (up to 400 nm) via the addition of TiO2 nanopowder from 5.0 wt% filling concentration. The neat epoxy system exhibits an excellent transparency of more than 60% of the visible light, and reaches 80% in the NIR range. Whereas, Ep–TiO2–Cs transmittance falls to less than 10% when the TiO2 contents into epoxy matrix increases up to 5.0 wt% TiO2. This transmittance shrinkage is more important in the NIR range for the highly-filled composites. The band gap decreases when the adding TiO2 increases. This decreases due to the decrease of particle size. This property is called as red shift in the prepared composites. Optical parameters of composite samples are determined. Up to 400 nm, the VELF values were strongly changed. The SELF values decreased with added TiO2 fillers quantities into the epoxy matrix. All optical parameters and dielectric studies of the investigated Ep–TiO2–Cs samples prove that these composites are more suitable in their great potential of optoelectronic devices and UV-shielding coatings.