An in situ fabrication of chitosan–TiO₂ transparent nanocomposite films from an acid stabilized TiO₂ sol, and study on their photocatalytic efficiency

Abstract

Chitosan–TiO₂ nanocomposites in powder form (10 to 60 wt% of chitosan) and in transparent film form (70 to 90 wt% of chitosan) have been synthesized through an in situ method by using an acid stabilized TiO₂ sol. The transparency of these films was evaluated from their optical transmittance measurements. A chitosan–TiO₂ nanocomposite opaque film (with 70 wt% of chitosan) has also been cast through an ex situ method by using TiO₂ nanoparticles, precipitated from the acid stabilized TiO₂ sol. Powder X-ray diffraction (PXRD) analysis confirmed the presence of anatase TiO₂ in both the powder and film forms of the chitosan–TiO₂ nanocomposites. FT-IR analysis revealed the presence of characteristic chitosan peaks in all the synthesized composite samples. SEM-EDX analysis of the composite films substantiated the presence of a TiO₂ coating on the film surface, with a thicker coating observed in the ex situ prepared composite film sample, TC70FP. TEM analysis of the representative transparent composite film (TC70F) and the ex situ cast composite film (TC70FP) further confirmed the presence of TiO₂ in these composite samples. The in situ cast films exhibited superior mechanical properties and enhanced thermal stability, as demonstrated by tensile stress–strain and TGA analyses, respectively. They also showed lower swelling indices, while ICP-MS analysis confirmed negligible TiO₂ leaching from the representative in situ cast film TC70F. The in situ cast representative transparent film, TC-70F, was able to degrade methyl orange (MO) solution under UV light irradiation, whereas the pristine chitosan film (ChF) showed no degradation activity.

Transparent chitosan–TiO₂ nanocomposite films with improved mechanical strength and stability have been synthesized through an in situ method using an acid-stabilized TiO₂ sol.

1. Introduction

Over the past few decades, non-biodegradable plastics have been extensively utilized due to their exceptional durability, flexibility, and performance in a variety of applications. Their resilience to natural degrading processes, however, is a significant disadvantage, since it allows them to remain in the environment for many years after their productive lifespan has ended. As a result, they generate a lot of waste that presents ecological and environmental problems, greatly adding to the growing issue of plastic pollution.^1,2 The environmental and health risks associated with plastic pollution have driven increased interest in developing biodegradable polymers as sustainable alternatives. The key advantage of biodegradable polymers lies in their ability to maintain stability under environmental conditions during use, while undergoing degradation in the presence of bioactive agents under appropriate disposal conditions.^3–5

Chitosan, a deacetylated derivative of Chitin, and its composites have emerged as promising biopolymers owing to their wide-ranging applications in bioplastics, biomedical engineering, food packaging, wastewater treatment, textile industries, fuel cells, nanocomposites, and other advanced technological fields.^6–11 Most of these studies utilize chitosan predominantly in film form, owing to the specific requirements of their intended applications. Chitosan films can be easily cast from its aqueous solution with 1–2.5 wt% acetic acid.^12,13 This is achieved since chitosan dissolves in aqueous acidic media by protonation of the –NH₂ functional group on the C-2 position of the repeating d-glucosamine unit.^14,15 One of the key parameters which determines the extend of chitosan solubility, in aqueous acidic media, is the degree of deacetylation in it. Approximately, a minimum of 50–60% deacetylation in chitosan makes it soluble in aqueous acidic medium.^12,14,16 Chitosan composite films incorporated with various materials have demonstrated enhanced physicochemical and functional properties, thereby expanding their range of potential applications.^17,18 Especially, nano crystalline TiO₂–chitosan composite films are promising in waste water treatment, food packaging, and other allied technologies.^6,19–22 Titanium dioxide (TiO₂) is a chemically stable and multifunctional material widely utilized across diverse sectors, including food technology, pharmaceuticals, biomedicine, antimicrobial applications, environmental remediation, and sustainable energy systems.^7,23–26 The incorporation of TiO₂ into chitosan enhances its structural properties by achieving improved dispersion and uniform distribution on the surface of the resulting composite films.^6,22,27 Most reported methods for preparing chitosan–TiO₂ composite films involve incorporating pre-synthesized TiO₂ nanoparticles into a chitosan–acetic acid solution, followed by film casting.^6,28–30 Here, we demonstrate an in situ TiO₂ incorporation technique for the casting of transparent chitosan–TiO₂ composite film, based on our previously reported low temperature sol–gel TiO₂ synthesis.³¹ The in situ casting method reported here enabled the production of transparent chitosan–TiO₂ films with up to 30 wt% TiO₂ loading, exhibiting excellent mechanical properties, enhanced thermal stability, and reduced swelling indices.

2. Experimental

2.1. Materials

Commercially available high purity chemicals were used without further purification in the entire study. Chitosan (Ch), (85% deacetylated, molecular weight ≈350–550 kDa) was procured from Marine hydrocolloids, Kochi, Kerala, India. Titanium(iv) tetra iso-propoxide (97%) was purchased from Sigma-Aldrich, India. Acetic acid, hydrochloric acid, and Na₂CO₃ were obtained from MERK India Ltd; Mumbai. Deionised distilled water was used throughout the study.

2.2. Synthesis of chitosan–TiO₂ composites

Chitosan–TiO₂ composites with varying compositions were synthesized from an acid stabilised TiO₂ sol. TiO₂ sol was synthesized by the hydrolysis and condensation reactions of 5% titanium tetra iso-propoxide, to form an acidic aqueous sol containing 5% acetic acid and 1.4% hydrochloric acid.³¹ The mixture was heated at 60 °C under vigorous stirring for 2 h. TiO₂–chitosan composites with varying compositions (weight%) viz. 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% of chitosan were formulated by adding required amount of chitosan into the acid stabilized TiO₂ sol. The amount of the TiO₂ sol taken in each case was calculated based on the desired TiO₂–chitosan composition, and the whole mixture was diluted to a final volume of 100 mL by adding distilled water. These samples were then kept for overnight stirring. TiO₂–chitosan composite samples TC-10, TC-20, TC-30, TC-40, and TC-50 (where the numeral represents the weight% of chitosan in the TiO₂–chitosan composite) could not cast as films and were extracted in the powder form by adding Na₂CO₃ solution into each of them. These extracted powder samples were separated by centrifugation and washed several times with distilled water and dried in an oven at 60 °C for 12 hours. TC-60 (60 wt% of chitosan) sample could neither cast as film nor could extracted in powder form, instead appeared in a plastic texture.

Transparent TiO₂–chitosan composite films were successfully fabricated for the TC-70F, TC-80F, and TC-90F samples, containing 70%, 80%, and 90 wt% chitosan, respectively. TC-70F, TC-80F, and TC-90F films were fabricated by casting the respective TiO₂ sol–chitosan mixtures, following overnight stirring, into Petri dishes and allowing them to dry under ambient conditions for 4 days. Sodium carbonate solution (2%) was added to the Petri dishes and maintained for 3 h, followed by washing with distilled water and subsequent air drying for 24 h. Final drying of the films was carried out in an oven at 40 °C for one hour. Another TiO₂–chitosan composite film (70% chitosan), designated as TC-70FP, was fabricated under similar experimental conditions using a mixture of chitosan and TiO₂ powder, instead of TiO₂ sol, in 5% acetic acid and 1.4% HCl solution. A pristine chitosan film (ChF; blank chitosan film without TiO₂ loading) was also fabricated in a similar manner using a chitosan solution prepared in 5% acetic acid and 1.4% HCl.

2.3. Characterizations

2.3.1. Powder X-ray diffraction

Powder X-ray diffraction (XRD) patterns of all samples were recorded using an Aeris XRD diffractometer. The diffractograms were obtained using Cu Kα radiation over a 2θ range of 5–80° at a scanning rate of 3.5° min⁻¹. For XRD analysis of the films, small sections of the films were cut and mounted on the sample holder of the diffractometer. The average crystallite size of the TiO₂ nanoparticles was determined using the Scherrer equation.

2.3.2. FT-IR spectroscopy

Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) analysis of the samples was performed using a PerkinElmer Spectrum Two (L160000A) spectrophotometer equipped with an optimized KBr beam splitter and operated at a spectral resolution of 0.5 cm⁻¹. The spectra were recorded over the wavenumber range of 450–4000 cm⁻¹.

2.3.3. Scanning electron microscopy (SEM)

The morphology of the samples was examined using field-emission scanning electron microscopy (FESEM, Sigma VP, Carl Zeiss, Jena, Germany). Powder samples were dispersed directly onto the specimen stub, while film samples were mounted using carbon tape. Energy-dispersive X-ray spectroscopy (EDS) of the film samples was performed using the same instrument to determine their elemental composition.

2.3.4. Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) analysis of the representative in situ-cast transparent film (TC-70F) and the ex situ-cast opaque film (TC-70FP) was performed using a JEOL/JEM 2100 electron microscope. The films were treated with acetic acid to separate the TiO₂ nanoparticles from the polymer matrix, followed by drop-casting of the resulting dispersion onto TEM grids.

2.3.5. Optical transmittance analysis of the films

The optical transmittance of the chitosan–TiO₂ composite films and pristine chitosan film was measured in the wavelength range of 200–800 nm using a Shimadzu UV-1800 spectrophotometer. Baseline correction was performed using air prior to the measurement of each film.

2.3.6. Mechanical characterization of the films

Mechanical properties of the cast films were evaluated using an Instron Universal Testing Machine following ASTM D-412 standards for tensile testing. Tear strength was measured according to ASTM D-624.

2.3.7. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) of the film samples was performed using a PerkinElmer TGA 6000 instrument at a heating rate of 20 °C min⁻¹. Samples weighing approximately 3–5 mg were heated from 30 to 600 °C under a nitrogen atmosphere to evaluate their thermal stability.

2.3.8. Water absorption and TiO₂ leaching studies

Water absorption of the chitosan and chitosan–TiO₂ films was evaluated by immersing pre-weighed film samples in water and recording their weights at predetermined time intervals. The swelling index of the films was then calculated using the following formula:w₀ = weight of the film before swelling, w₁ = weight of the film after swelling.

A representative chitosan – in situ TiO₂ film sample, TC-70F (0.15 g), was immersed in 20 mL of water and agitated on a magnetic shaker for 3 h. The resulting water sample was then analysed for the presence of leached Ti using a Thermo Scientific iCAP RQ ICP-MS analyser.

2.4. Photocatalytic efficiency of TiO₂–chitosan composite films

The photocatalytic degradation efficiency of the transparent TiO₂–chitosan composite film (TC-70F, selected as a representative sample) was evaluated and compared with that of the pristine chitosan film (ChF). Film pieces of TC-70F (0.15 g) were immersed in 20 mL of a 2 ppm methyl orange (MO) solution in a 100 mL beaker. The reaction mixture was agitated on a mechanical shaker for 30 min in the dark to establish adsorption–desorption equilibrium, followed by irradiation under UV light. Photocatalytic degradation was monitored by measuring the decrease in the absorbance at λ_max of MO in aliquots (≈4 mL) withdrawn at different time intervals. The collected samples were centrifuged prior to UV-visible spectroscopic analysis.

3. Results and discussion

3.1. Chitosan–TiO₂ composite films

Here demonstrated in situ method for the synthesis of chitosan–TiO₂ composite materials can be used to derive them in either powder form or to be cast as films. Chitosan–TiO₂ composites could be extracted only in powder form with samples containing 10% to 60% (w/w) of chitosan content, and casting of the films was unsuccessful in these samples. The percentage of chitosan in these samples was not enough to be cast as a free-standing film. However, the texture of the powder chitosan–TiO₂ samples was found to be change to amorphous in nature as the chitosan content increases. Especially, the TC-60 sample (60% w/w of chitosan) was extremely soft in nature so that it could not be ground in to fine powder. Chitosan–TiO₂ composite film casting was achieved in 70%, 80% and 90% (w/w) chitosan containing samples. The pristine chitosan film (ChF) and the other in situ cast composite films, TC-70F, TC-80F and TC-90F, were transparent in appearance. These films (except ChF) were cast from the mixture of chitosan and TiO₂ sol. The transparency of the in situ cast chitosan–TiO₂ films could be attributed to the homogeneously dispersed TiO₂ nano crystals with smaller crystallite size in the chitosan matrix. Interestingly, the composite film cast by adding TiO₂ powder into chitosan (TC-70FP) was almost opaque in nature (Fig. 1a). In TC-70FP, pre-synthesized TiO₂ nanoparticles were incorporated into chitosan; consequently, aggregated TiO₂ particles are dispersed within the chitosan matrix, contributing to its increased opacity. This demonstrates the importance of TiO₂ sol-based synthesis to achieve chitosan–TiO₂ composite transparent films.

Fig. 1. (a) Photographs of the cast films placed on black chart paper labelled “SJC CRCS” and (b) optical transmittance spectra of the cast films of chitosan (ChF), chitosan – in situ TiO₂ (TC-70F, TC-80F, and TC-90F), and chitosan–TiO₂ powder (TC-70FP).

Transparency of the cast films was further evaluated by measuring their optical transmittance using a UV-visible spectrophotometer. All the chitosan – in situ TiO₂ composite films (TC-70F, TC-80F, and TC-90F) exhibited high optical transmittance, ranging from 75% to 85% (Fig. 1b), whereas the pristine chitosan film (ChF) exhibited an optical transmittance of 51%. In contrast, the chitosan–TiO₂ film prepared via the ex situ method was almost opaque, exhibiting negligible optical transmittance.

3.2. Powder X-ray diffraction

When the degree of deacetylation of chitin reaches about 50% (depending on the origin of the polymer), it becomes soluble in aqueous acidic media and is called chitosan.¹² Chitosan has three characteristic diffraction peaks at 2θ = 10.22°, 19.9° and 20.5° ³² whereas anatase TiO₂ has the characteristic diffraction of its (101) plane at 2θ = 25.3°.³³ The powder X-ray diffractogram of the purchased chitosan sample (Ch) has recorded diffraction peaks at 2θ = 10.1°, 19.7° and 21.3°, and the synthesised TiO₂ powder has exhibited the characteristic anatase diffraction at 2θ = 25.3° along with the other attributed diffraction peaks of anatase phase (Fig. 2a). The X-ray diffractogram of chitosan–TiO₂ powder composite samples, TC-10 to TC-40, (Fig. 2a) were dominated by the diffraction peaks of TiO₂, and the intensity of these peaks was found to decrease with the increase of chitosan weight percentage from TC-10 to TC-40. However, the diffraction peaks of chitosan were not significantly observed in chitosan–TiO₂ powder composite samples, and neither TiO₂ nor chitosan diffraction peaks were visible in TC-50 and TC-60 samples. This could be attributed to the highly amorphous nature of TC-50 and TC-60 powder samples as the chitosan content increases. The amorphous nature of these samples was clearly observed as a plastic behaviour while handling these samples, especially in TC-60. The average crystallite size of TiO₂ in the powder composite samples, calculated using the Scherrer equation, was ≈5.6 nm.

Fig. 2. Powder X-ray diffractograms of TiO₂–chitosan composite (a) powder samples and (b) films.

On the other hand, the diffraction peaks of chitosan have found to be present in all the cast film samples (Fig. 2b). The diffraction peaks at 2θ = 10.1° and 19.7° of the purchased Ch sample has shifted to 9.5° and 16°, respectively, in the pristine chitosan film, ChF, which was cast from Ch. W. M. Facchinatto et al. has demonstrated the 2θ peak shift of chitosan samples from 10.1° to lower scattering angles as the crystallinity of the sample increases.³⁴ We believe that the observed 2θ peak shift to lower scattering angles in cast ChF film is due to the increase in its crystallinity as compared to the Ch precursor. The 2θ peak shift to lower scattering angles observed in the pristine chitosan film ChF was the same in all the chitosan–TiO₂ composite films, TC-70FP, TC-70F, TC-80F and TC-90F. In addition, the diffraction peaks of anatase TiO₂ were present in these composite films which confirms the incorporation of TiO₂ in chitosan films (Fig. 2b). Another notable observation is that the average crystallite size of TiO₂ in the in situ cast composite film samples, calculated using the Scherrer equation, was ≈3.1 nm, whereas that of the ex situ cast film was ≈5.6 nm.

3.3. FT-IR spectroscopy

The FT-IR analysis of chitosan has well reported in the literature.^{12,14,35–37} Most of these reports discuss a broad peak in the 3000–3600 cm⁻¹ region which corresponds to the combined stretching of –OH and –NH₂ groups, symmetric stretching of –CH₃ and –CH₂ -groups at around 2930 cm⁻¹ and 2856 cm⁻¹ respectively, stretching and bending vibrations of amide moieties at 1580–1655 cm⁻¹ region, and the >C O stretching vibrations at 1031–1080 cm⁻¹ range. Fig. 3 represents the FT-IR spectra of Ch, TCs, ChF and TC-Fs. The characteristic FT-IR peaks of chitosan have observed in the FT-IR analysis of all these samples. However, it is difficult to distinguish the IR absorption peaks of TiO₂ in TC composite samples because of the strong absorption peaks of chitosan, which is anticipated.

Fig. 3. FT-IR spectra of TiO₂–chitosan composite (a) powder samples and (b) films.

3.4. Scanning electron microscopy analysis (SEM)

Fig. 4 shows the SEM images of representative TiO₂–chitosan composite powder samples TC-10 and TC-40. It is obvious from the SEM images that the increase in the chitosan content in the powder samples, from TC-10 to TC-40, has resulted in bigger agglomerated composite particles in TC-40 than in TC-10. The SEM images of the pristine chitosan film (ChF) and the chitosan – in situ TiO₂ composite films (TC-70F, TC-80F and TC-90F) are shown in Fig. 5. Compared to ChF, the texture of the chitosan – in situ TiO₂ composite films have changed as the result of TiO₂ coating on the surface. However, a thick deposition of TiO₂ was not observed in these composite films which in turn might have resulted in the formation of transparent TC-70F, TC-80F and TC-90F films (Fig. 1). The TC-70F, TC-80F and TC-90F films were prepared by casting the corresponding chitosan–TiO₂ sol mixture in which TiO₂ is in sol state. When these cast films were treated with Na₂CO₃ solution TiO₂ particles were precipitated on the surface of the film thus forming transparent chitosan–TiO₂ films. Conversely, a thick deposition of TiO₂ particles was observed in the SEM images of TC-70FP (Fig. 6).

Fig. 4. SEM images of representative TiO₂–chitosan composite powder samples (a) TC-10 and (b) TC-40.

Fig. 5. SEM images of ChF (a & b), TC-70F (c & d), TC-80F (e & f) and TC-90F (g & h).

Fig. 6. SEM images of TC-70FP.

Energy-dispersive X-ray spectra (EDX) of TC-70F, TC-80F, TC-90F, TC-70FP and pristine ChF film samples are provided in Fig. 7. The presence of Ti peak was observed in all the samples except in the pristine chitosan film, ChF. This result further confirms the successful incorporation of TiO₂ in the composite films.

Fig. 7. Energy dispersive spectra of (a) ChF, (b)TC-70F, (c) TC-80F, (d) TC-90F and (e) TC-70FP.

In the preparation of TC-70FP, the required amount of TiO₂ powder was mixed with chitosan in 5% acetic acid solution followed by casting the film. The film was then treated with Na₂CO₃ solution as described in the preparation of chitosan – in situ TiO₂ composite films. Since TiO₂ is added in the powder form in TC-70FP, rather than in the sol state, a thick deposition of the agglomerated TiO₂ particles were formed on the film surface. This thick deposition of TiO₂ on the film is the reason for the opacity of TC-70FP (Fig. 1) in comparison with the in situ cast chitosan–TiO₂ composite films.

3.5. Transmission electron microscopy analysis (TEM)

TEM analysis was performed on a representative chitosan – in situ TiO₂ film sample, TC-70F, and on the chitosan-powder TiO₂ sample (TC-70FP). These films were pretreated with aqueous acetic acid solution to dissolve the chitosan part of the film, and were spread on the grid for the TEM analysis. Fig. 8 represents the TEM images of TC-70F and TC-70FP samples. TiO₂ crystals were spotted in these samples which is obvious from the images (Fig. 8). The TiO₂ crystallite size in chitosan-powder TiO₂ (TC-70FP) was found to be larger than in chitosan-sol TiO₂ (TC-70F). This could be attributed to the in situ synthesis method adopted for TC-70F where TiO₂ is in the sol state. TEM analysis of the pristine chitosan film (ChF) was also performed in a similar method but nothing could detect in the sample.

Fig. 8. TEM images of representative chitosan–TiO₂ film samples (a) TC-70F and (b) TC-70FP, recorded after dissolving the chitosan part of the films in aqueous acetic acid solution.

3.6. Mechanical properties

Mechanical properties of the chitosan–TiO₂ composite films were evaluated by analysing their tensile stress–strain behaviour and compared with that of the pristine chitosan film (ChF).

Fig. 9 presents the tensile stress–strain curves of all the cast films, with each curve representing the best-performing sample from the respective film composition. All the in situ-cast chitosan–TiO₂ films (TC-70F, TC-80F, and TC-90F) exhibited superior mechanical properties compared with the pristine chitosan film. In particular, TC-70F showed approximately 67% and 35% increases in tensile strength (109 MPa) and elongation at break (42%), respectively, compared with the pristine chitosan film (65 MPa and 18%, respectively). In contrast, the ex situ-synthesized chitosan–TiO₂ powder composite film exhibited the lowest mechanical performance, with a tensile strength of 19.7 MPa and an elongation at break of 15.5%. These results demonstrate that the in situ casting method employed in this study can produce chitosan–TiO₂ films with superior mechanical properties compared with both pristine chitosan films and conventionally synthesized chitosan–TiO₂ powder composite films. The enhanced mechanical performance of TC-70F, TC-80F, and TC-90F may be attributed to strong intermolecular interactions between the in situ-generated TiO₂ particles and the chitosan polymer chains facilitating efficient stress transfer through the matrix.

Fig. 9. Tensile stress–strain curves of all the cast films.

3.7. Thermogravimetric analysis (TGA)

Thermogravimetric analysis results for all the cast films are shown in Fig. 10. The thermogram of chitosan films typically exhibits three distinct regions of weight loss.^38,39 The first region involves a gradual weight loss, mainly attributed to the removal of adsorbed water molecules from the film, and extends up to approximately 200 °C. The second region, observed between 200 and 350 °C, corresponds to a pronounced weight loss associated with the onset of thermal decomposition of chitosan. This is followed by a third, comparatively slower weight-loss region, which can be attributed to charring and the subsequent decomposition of the carbonaceous residue.

Fig. 10. Thermal decomposition curves of all the cast films obtained from TGA analysis.

Among the cast films, the in situ-cast TC-70F, TC-80F, and TC-90F films exhibited slightly higher decomposition onset temperatures (255–262 °C) than the pristine chitosan film (ChF, 245 °C), indicating their enhanced thermal stability. In contrast, the ex situ-cast TC-70FP film began to decompose at approximately 197 °C, indicating its comparatively lower thermal stability. These findings are consistent with the mechanical strength results presented in Fig. 9. At 600 °C, the pristine chitosan film exhibited the highest weight loss (66%), whereas TC-70F and TC-70FP exhibited the lowest weight loss. This trend is consistent with the TiO₂ loading in the respective films, as the thermally stable inorganic TiO₂ component contributes to the higher residual mass at elevated temperatures.

3.8. Water absorption and TiO₂ leaching studies

Swelling of the cast films was evaluated by measuring the change in film weight before and after immersion in water. The film samples were weighed at 15 min intervals following immersion to monitor water uptake over time. All three in situ cast chitosan–TiO₂ films (TC-70F, TC-80F, and TC-90F) reached saturation within the first 15 min, whereas the pristine chitosan film (ChF) required approximately 30 min to reach saturation. The swelling indices calculated for TC-70F, TC-80F, TC-90F, and ChF were 69%, 82%, 96%, and 107%, respectively. These results indicate that water absorption decreased with increasing TiO₂ loading in the chitosan–TiO₂ films, suggesting improved water stability of the composite films. In contrast to the in situ cast films, the ex situ cast chitosan–TiO₂ powder film (TC-70FP) was completely torn apart within the first 15 min of immersion in water. This observation indicates the comparatively fragile nature and poor water stability of the ex situ cast composite film and is consistent with the results obtained from the mechanical strength and TGA analyses.

The possibility of TiO₂ leaching from the composite film was evaluated using a representative TC-70F film sample by ICP-MS analysis. A 150 mg portion of the TC-70F film, containing 30 wt% TiO₂, was immersed in 20 mL of water for 3 h under mechanical agitation. The water was subsequently analysed by ICP-MS, which revealed a Ti concentration of 35 ppb. This corresponds to an estimated TiO₂ leaching of only approximately 0.0026% of the initial TiO₂ content, indicating negligible leaching of TiO₂ from the composite film. The very low level of TiO₂ release suggests strong interactions between the chitosan matrix and the in situ incorporated TiO₂ nanoparticles.

3.9. Photocatalytic efficiency of transparent chitosan–TiO₂ composite films

The photocatalytic degradation efficiency of the representative transparent chitosan–TiO₂ composite film, TC-70F was studied for the degradation of aqueous methyl orange (MO) solution under the UV light irradiance. In a typical experiment, 50 mL of 2 ppm MO solution containing TC-70F film pieces (total weight 0.15 g), was irradiated with UV light for one hour. The same was tested with pristine ChF film as well. The UV-visible absorption spectrum of the reaction mixture collected at different time intervals of UV light irradiance is given in the Fig. 11. Photocatalytic degradation of MO was not observed with pristine ChF film during one hour of UV light exposure, which is evident from the unchanged MO absorbance (Fig. 11a). On the other hand, UV-visible absorption of MO was found to decrease in the reaction mixtures with TC-70F film pieces indicating the photocatalytic degradation of MO (Fig. 11b) by TC-70F. Here demonstrated sol-based in situ synthesis is promising to achieve transparent chitosan–TiO₂ composite films with self-cleaning efficiency.

Fig. 11. UV-visible absorption spectra of the 2 ppm aqueous methyl orange solution (OS) and the MO solution after photocatalysis using (a) ChF and (b) TC-70F, and (c) the corresponding C/C₀versus time plot for the photocatalytic degradation.

4. Conclusion

An in situ method for the synthesis of chitosan–TiO₂ nanocomposites is demonstrated by using an acetic acid based TiO₂ sol. When the chitosan content was varied from 10 to 60 wt%, these composites could be extracted in powder form but not able to cast as films. While transparent films could be cast from chitosan–TiO₂ nanocomposite samples containing 70–90% chitosan using the in situ casting method, the transparency of these films was confirmed by optical transmittance measurements. On the other hand, chitosan–TiO₂ nanocomposite film cast through an ex situ method, by mixing chitosan (70 wt%) with TiO₂ nanoparticles instead of TiO₂ sol, was almost opaque. Powder X-ray diffraction (XRD) analysis confirmed the successful formation of chitosan–anatase TiO₂ nanocomposites, with average crystallite sizes of ≈3.1 nm in the in situ cast films and ≈5.6 nm in the ex situ cast film and powder composite samples. The FT-IR analysis has revealed the presence of characteristic chitosan peaks in all the synthesized composite samples. SEM-EDX analysis of the composite films has substantiated the presence of TiO₂ coating on the film surface with a thicker coating in ex situ made composite film sample, TC70FP. TEM analysis of the representative transparent composite film (TC70F), and of the ex situ cast composite film (TC70FP) has further confirmed the presence of TiO₂ in these composite samples. The in situ cast films exhibited superior mechanical properties and enhanced thermal stability compared to the pristine chitosan film and the ex situ cast film, as evidenced by tensile stress–strain and thermogravimetric analyses. Water absorption decreased in the in situ cast films with increasing TiO₂ loading, as evidenced by their swelling indices of 69–96%, compared with 107% for the pristine chitosan film. In contrast, the ex situ cast film, TC-70FP, exhibited the lowest stability in water and was completely torn apart upon immersion. The photocatalytic efficiency of the chitosan–TiO₂ nanocomposite films were successfully experimented with the representative transparent composite film sample TC-70F. This transparent film was able to degrade methyl orange solution (2 ppm) under UV light irradiation whereas the pristine chitosan film could not.

Conflicts of interest

All authors declare that they have no conflict of interest.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request. Additional supporting data are included within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra04655j.