Introduction

In recent years, preparation of semiconductor nanocrystalline thin films has gained a great deal of attention for their unique size-dependent properties and potential applications in solar cells [1, 2], light emitting diodes (LED) [3], nonlinear optical devices [4], semiconductor lasers [5] and fiber filter [6]. Cadmium sulfide (CdS) and titanium dioxide (TiO2) are two of the most important semiconductors which are used as a window layer in solar cells. The former is an II–VI semiconductor due to having direct band gap of 2.42 eV. This material together with narrow band gap materials such as CdTe, Cu2S and InP has been used as a window layer [7, 8] in solar cell heterostructures [9, 10]. The latter is an important semiconductor among various potential metal oxides which are used as photocatalyst due to its chemical stability and commercial availability [11].These properties make TiO2 as a potential n-type semiconductor. It is usually used as a powder in solution [6].

There are several deposition techniques to produce CdS thin films such as thermal evaporation [12], spray pyrolysis [13], laser evaporation [14], solid state reaction [10] and chemical bath deposition (CBD) [15, 16]. Among them, chemically deposited CdS thin film appears to be most suitable for solar cells application. However, this technique requires using ammonia, which is difficult to handle due to its volatility and relative toxicity, as a complexing agent for cadmium ions [17]. To overcome the problems, many researches have been conducted to ammonia-free synthesis of CdS [18–26]. In these researches, sodium citrate, ethylene diamine, etc. were used as an alternative to ammonia. Another approach to ammonia-free synthesis of CdS is the use of polymer for synthesizing semiconductor nanoparticles in thin film which provide the advantages such as (i) higher band gap, (ii) good adherence to the substrate and (iii) better coverage of the surface minimizing pin-holes and thereby permitting the use of very thin film of semiconductor (<100 nm) as a window layer in solar cell [27]. Life of the composites and their brightness of light emission are enhanced by the existence of semiconductor nanoparticles in polymeric matrices [28]. Nanocomposites impart several advantages such as greater chemical and thermal stability, increased strength, enhanced electrical conductivity, and improved flammability properties to polymeric matrix [29].There are many reports on the synthesis of semiconductor nanoparticles embedded into polymeric matrices [27, 28, 30–36].

Poly(vinyl alcohol) (PVA) is a semi-crystalline, hydrophilic polymer with good chemical and thermal stability [37]. PVA is highly biocompatible, non-toxic and water-soluble polymer that readily reacts with different cross-linking agents to form a gel [38, 39]. In addition, semi-crystalline PVA is greatly used as a polymeric matrix for preparation of nanocomposites by economically and environmentally friendly methods due to its easy processability and optical transparency [40–43].

The alternative method for preparing nanocomposite thin films is electrospinning. Immobilization of photocatalyst on nanofiber webs with the aim of increasing surface area for photocatalytic activity has been performed via electrospinning of photocatalyst contained polymer solution [6, 35]. Nanofibers obtained from electrospinning are very small in diameters which allow more chance for nanoparticles to appear on fiber surface and as a result better exposure to radiation is accessible. This could provide enhanced photocatalytic activity.

In this work, we report the preparation of PVA/CdS and PVA/TiO2 nanocomposite thin films at different ratio of semiconductor nanoparticles by employing electrospinning method. CdS nanoparticles are first synthesized using cadmium nitrate (Cd (NO3)2) and sodium sulphide (Na2S) in a simple and economical way for preparation of PVA/CdS nanocomposites. In the case of PVA/TiO2 nanocomposites, TiO2 nanoparticles were purchased. Morphology and structure of nanocomposites are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Optical properties of samples are characterized by diffuse reflectance spectroscopy (DRS). Finally, it is postulated that this method can yield pinhole free, homogeneous and transparent films which may be suitable for application as a window layer in solar cells.

Experimental part

Materials, instrumental and analysis methods

The chemicals used in the synthesis of all nanocomposites were obtained from Aldrich Chemicals and Merck Company and were used without further purification. Cd (NO3)2 (99 %) and Na2S (35 %) were used as a source of cadmium ion and sulfide ion, respectively. TiO2 nanoparticles (<15 nm) were purchased for preparation of PVA/TiO2 thin layers. PVA with a weight average molecular weight (M w) of 72,000 g/mol (97.5–99.5 mol% hydrolysis)was used as polymeric matrix for nanocomposites. SEM measurements were performed on a Philips XL30i. XRD measurements were taken using a Philips PW1840. DRS was measured using a Shimadzu UV–Vis 2100 spectrometer. Preparation of solution was carried out with Sonopuls ultrasonic homogenizer (Bandline HD 3100). Electrospinning of solutions was performed using a New era pump systems NE-1000.

Synthesis of CdS nanoparticles

0.3 M Cd (NO3)2 was stirred vigorously at ambient temperature for 30 min. 0.3 M Na2S was added to adjust the pH of the solution in the range of 8–9. After 24 h, the sol was filtered and washed to remove all NaNO3 and the water in the sol was replaced with absolute ethanol. The resulting alcogel was dried under vacuum at ambient temperature. The CdS in the form of gel was finally obtained.

Preparation of PVA/CdS and PVA/TiO2 solutions

Aqueous PVA solution (10 wt%) was prepared by dissolving PVA in deionized (DI) water at 80 °C with constant stirring for at least 2 h. Aqueous solutions of CdS (10 wt%) and TiO2 (10 wt%) were then prepared by dissolving CdS and TiO2 nanoparticles powder in DI water, respectively, using ultrasonic homogenizer at room temperature. CdS solution at the following compositions; (v/v between CdS and PVA ratio in DI) 20 (PVA/20CdS), 30 (PVA/30CdS), 40 (PVA/40CdS), and 50 (PVA/50CdS) was poured into a PVA solutions (pH = 6–7) to obtain PVA/CdS solutions. Also, PVA/TiO2 solutions were prepared at different ratio of TiO2 (20, 30, 40 and 50) as mentioned for PVA/CdS solutions.

Electrospinning of PVA/CdS and PVA/TiO2 solutions

PVA/CdS and PVA/TiO2 nanocomposite thin films were prepared using electrospinning method. The electrospinning setup used in this research was composed of a high voltage DC power supply, a syringe with a flat-end needle (with inner diameter of 0.49 mm and length of 25 mm), a syringe pump and an electrically grounded aluminum plate. The solutions were electrospun with a constant feed rate of 0.2 µl min−1 at positive voltage of 10 kV at collection distance of 10 cm. The fibers were collected on the grounded aluminum plate. The electrospinning process was carried out at 30 °C and relative humidity of 60–70 %.

Results and discussion

Structural and surface morphology

The SEM images of PVA/CdS and PVA/TiO2 nanocomposite thin films are shown in Fig. 1. The appearance of semiconductor nanoparticles in PVA fibers clearly was increased by increasing the ratio of semiconductor. CdS and TiO2 nanoparticles were well distributed in the PVA nano-fibrous matrix in both nanocomposites. No coagulation of the semiconductor nano-particles was observed in the matrix. This could be most likely attributed to the intermolecular interaction between the PVA solution and semiconductor nanoparticles and the strong electric field during electro-spinning. The average diameter of the both nanocomposites fibers was determined from SEM and is shown in Table 1. The diameter of the electrospun PVA/CdS and PVA/TiO2 fibers was found to decrease from 203 to 168 nm and 329 to 162 nm, respectively, as the ratio of CdS and TiO2 increased from 20 to 50. As the fiber diameter decreases, the surface area increases. From SEM images, it was found that the average diameter of both PVA/CdS and PVA/TiO2 nanocomposite with 50:50 ratio is smaller than those of the other ratios, which can be interpreted as follows: thin layers with 50:50 ratio had a higher surface area than the others. Therefore, high concentration of semiconductor nanoparticles in the nanofiber resulted in a high active surface area which actually enhanced the photocatalytic activity [6]. Also, the particles’ size was determined by XRD analysis and absorption spectra which were in agreement with each other.

Fig. 1
figure 1

SEM images of distribution of nanoparticles in PVA matrix for a PVA/CdS, 50:50; b PVA/CdS, 80:20; c PVA/TiO2, 50:50; d PVA/TiO2, 80:20 ratio

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Table 1 Diameter of PVA/CdS and PVA/TiO2 nano fibers from SEM
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XRD results indicated the cubic and anatase phase of CdS and TiO2 nanoparticles formed within the pores of PVA matrix, respectively. The results show the influence of CdS and TiO2 nanoparticles upon the crystalline structure of PVA. Figure 2 shows the X-ray diffraction spectra of PVA/CdS nanocomposite thin films prepared at different ratios. The X-ray diffractogram of PVA/CdS nanocomposites shows broadened diffraction profiles. The peaks were detected at around 2θ = 26.5° and 43.3° which can be ascribed due to the cubic CdS structure [27, 44, 45]. These peaks can be indexed to the planes (111) and (220) [46].

Fig. 2
figure 2

XRD patterns of PVA/CdS at different CdS ratio: a 50; b 40; c 30 and d 20

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The X-ray diffraction spectra of PVA/TiO2 nanocomposite thin films are shown in Fig. 3. The peaks at around 2θ = 25.47°, 38° and 47.9° show the anatase structure of TiO2 nanoparticles in PVA matrix. It is known that the crystalline nature of PVA results from the strong intermolecular interaction between PVA chains through the intermolecular hydrogen bonding. By comparing the X-ray diffraction patterns for PVA/CdS and PVA/TiO2 nanocomposite, it can be noticed that there is a noticeable change in the intensity of XRD peaks which become weaker and broader with decreasing the volume ratio of semiconductors nanoparticles. Also, the weak intensity and broad diffraction peak at around (2θ = 20°) indicates the absence of crystalline phase of the PVA. This behavior can be correlated to the intermolecular interactions between PVA and CdS and TiO2 nanoparticles, respectively. Thus, it is possible that in our study the interactions between PVA chains and semiconductor particles led to the decrease of intermolecular interaction between the PVA chains and thus the crystalline degree decreases. The particle size of the semiconductors in thin films is estimated using the Scherrer formula, Eq. (1) [46–49].

$$D \, = \, K\lambda /\beta_{2\theta } \cos \theta$$
(1)

where K is a constant taken to be 0.94, λ the wavelength of X-ray used (λ = 1.54 Å) and β 2θ the full width at half maximum of diffraction peak of XRD pattern, Bragg angle 2θ, is around 26.5° and 25.47° for CdS and TiO2, respectively. The values of particle sizes of the films are shown in Table 2. The particle sizes of CdS and TiO2 nanoparticles were found to be in agreement with the results obtained from EMA method and absorption spectra. The prominent peaks seen in Figs. 2a and 3a for the case of the film synthesized at 50:50 ratio are indication of good crystallinity of the film for both PVA/CdS and PVA/TiO2 nanocomposites.

Fig. 3
figure 3

XRD patterns of PVA/TiO2 at different TiO2 ratio: a 50; b 40; c 30 and d 20

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Table 2 Particle sizes calculated from Scherrer fomula (XRD)
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Absorption spectra of the PVA/CdS and PVA/TiO2 nanocomposites

Optical properties which are directly related to the size of nanoparticles can be studied using the absorption spectra of nanocomposites. The UV–Vis absorption spectra of the PVA/CdS and PVA/TiO2 nanocomposite thin films prepared at various semiconductor ratios (20, 30, 40 and 50) are shown in Fig. 4. It is evident from the spectra that absorbance edges of all the films are blue shifted relative to the bulk CdS (520 nm) and bulk TiO2 (388 nm) indicating quantum confinement effect in nanoparticles [27, 50]. Optical properties of semiconductor nanoparticles are dependent on the size of particles which render them attractive from the viewpoint of integrated photonic devices. Optical properties are changed considerably by the dimensions of the nanocrystallites when they are comparable or less than Bohr radius of exciton wave function. In molecular terminology, this refers to the broadening of the energy gap as size decreases [51]. In Fig. 4, the sharp increase in absorbance near the absorption edge for the films prepared at 50:50 ratio, for both nanocomposites, is an indication of good crystallinity of the nanostructures inside the matrix. A little redshift in absorption edges among the spectra was observed as the nanoparticle ratio was increased from 20 to 50. In contrast, when the proportion of nanoparticle is low, a well-defined blue shift was observed indicating good quantum confinement. However, although the crystallinity of sample with the proportion of 50:50 was high according to XRD characterization, the absorption edge was not sharp compared with the absorption edge of the other samples and this is in agreement with the finding of other researchers [27]. The decrease in particle sizes of the films with the reduction in CdS ratio in PVA/CdS nanocomposites, calculated by effective mass approximation (EMA) method, could be the reason of observed blue shift. A summary of the results of the absorption spectroscopy measurements is shown in Table 3.

Fig. 4
figure 4

UV–Vis absorption spectra of a PVA/CdS; b PVA/TiO2 nanocomposite thin films

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Table 3 Band gap, shift in band gap and particle sizes calculated from absorption spectra
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According to Thambidurai et al. definition [51], there are two situations, called the weak confinement and the strong confinement regime. In the weak regime, the particle radius is larger than the radius of the electron–hole pair, but the range of motion of the exciton is limited, which causes a blue shift of the absorption spectrum. When the radius of the particle is smaller than the orbital radius of the electron–hole pair, the motion of electron and hole become independent and the exciton does not exist. The hole and the electron have their own set of energy levels. Here also there is a blue shift. As the particle size obtained from optical absorption studies is larger than the Bohr radius of 3 nm for both CdS and TiO2, the weak confinement effect can be assumed to be present in the mentioned nanoparticles.

Transmittance spectra of the PVA/CdS and PVA/TiO2 nanocomposites

Figure 5 shows the transmission versus wavelength plot of PVA/CdS and PVA/TiO2 nanocomposite thin films prepared at different ratio. It is evident from the spectra that all the films show high transmittance for wavelength greater than 500 nm. However, a little higher transmittance (~90 % for PVA/CdS and ~80 % for PVA/TiO2) was observed for the film, which was prepared at 50:50 ratio for both nanocomposites. The sharp fall in transmission near the fundamental absorption edge shows good crystallinity of the film [28].

Fig. 5
figure 5

Transmittance spectra of a PVA/CdS; b PVA/TiO2 nanocomposite thin films

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Optical band gap of the PVA/CdS and PVA/TiO2 nanocomposites

The optical band gaps of the films were obtained using Eq. (2) [52] for a semiconductor

$$A = \frac{{K(hv - E_{\text{g}} )^{m/2} }}{hv}$$
(2)

where ‘A’, ‘K’, ‘E g’ and ‘hv’ are the absorbance, constant, band gap of the semiconductor and incident photon energy, respectively, and ‘m’ is equal to ‘1’ for direct transition and ‘2’ for indirect transition. According to linear plot of (Ahv)2-hv for nanocomposite thin films, ‘m’ is equal to ‘1’. Band gap of the film material is obtained by extrapolation of the straight line to (Ahv)2 = 0 axis (Fig. 6). The band gap of the PVA/CdS films was found to gradually increase from 2.53 to 2.62 eV as the CdS ratio was reduced from 50 to 20 and the band gap of the PVA/TiO2 films increased from 3.31 to 3.51 eV with the reduction of TiO2 ratio, too (Table 3). The size of the CdS nanoparticles was calculated by the use of EMA method and Eq. (3) for a semiconductor [44, 52].

Fig. 6
figure 6

Band gap spectra of a PVA/CdS, 50:50 and 60:40; b PVA/CdS, 70:30 and 80:20; c PVA/TiO2, 50:50 and 60:40; d PVA/TiO2, 70:30 and 80:20

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$$E_{\text{gn}} - \, E_{\text{gb}} = \frac{{(h^{2} \pi^{2} /2R^{2 } )1}}{{m^{*} }}$$
(3)

where m* is the effective mass of the specimen, R is radius of the particle, E gb is the bulk band gap and E gn is the band gap of the sample. From the calculations, it is found that the particle size decreases with the reduction in CdS ratio. The observations are shown in Table 3 and CdS particle sizes were in the range of 6.1–8.22 nm.

Conclusion

PVA/CdS and PVA/TiO2 nanocomposite thin films were prepared by means of electrospinning method. This production method is simple, low cost and helps increasing semiconductor nanoparticles photocatalytic activity. XRD results indicated the cubic and anatase phase of CdS and TiO2 nanoparticles formed within the pores of PVA matrix, respectively. The blue shift in the absorbance spectra was attributed to the quantum confinement effect in nanoparticles. The film prepared at 50:50 ratio showed better optical and structural properties in comparison to the films prepared at lower semiconductor ratio for both nanocomposites. SEM analysis showed that semiconductor nanoparticles were successfully coated by polyvinyl alcohol molecules. The particle sizes obtained from XRD analysis were in good agreement with the results obtained from absorption spectra. The film prepared at 50:50 ratio for both PVA/CdS and PVA/TiO2 is more suitable for application as a window layer in solar cell because of its favorable optical band gap (2.53, 3.31 eV), high transmittance and good crystallinity.