WO₃ nanoparticles, MTMS, ethanol, and nitric acid were procured from Sigma Aldrich and used without further purification. The size of the WO₃ nanoparticles was confirmed using Transmission Electron Microscopy (TEM) (HT7700, Hitachi High-Tech Corporation, Tokyo, Japan), and their crystallinity was studied by X-ray Diffraction (XRD) (Empyrean, PANalytical, Almelo, The Netherlands). No further characterization was carried out because the WO₃ was commercially produced.

MTMS was diluted by mixing with an ethanol solution at a 1:1 weight ratio. To prepare the nanocomposites, WO₃ nanoparticles were added as a powder to a fixed volume of the solution. 25 wt% WO₃ nanoparticles were added to the mixture and were stirred for 2 hours. The resulting mixture was labeled as MTMS-WO₃-25%, indicating it contained 25 wt% WO₃ nanoparticles by weight. Mixtures containing 12.5 and 6.25 wt% WO₃ nanoparticles were also prepared by adjusting the weight accordingly.

To prepare the sol-gel from the mixtures, nitric acid (pH 1) was added as a catalyst. The acid was 10% by weight in the mixtures, and the mixtures were stirred for 2 minutes. The sol-gel was then applied to glass slides using a sponge and allowed them to dry at room temperature. In all, 5 samples were prepared for evaluation in this experiment (Table 1).

X-ray Diffraction was used to confirm the crystallinity of WO₃ does not change. Ultraviolet-visible (UV-Vis) spectroscopy (200–900 nm) (UV-1900i, Shimadzu Corporation, Kyoto, Japan) was used to measure the transparency of the coating. Fourier Transform Infrared (FTIR) Spectroscopy (Spectrum 400, PerkinElmer, Waltham, MA, USA) was utilized to identify the functional groups, hydrolysis and condensation reactions of the coating. A pencil scratch hardness test was conducted to evaluate the mechanical strength of the developed coatings. The contact angle was measured to evaluate the wettability of the coatings.

Fig. 1 illustrates the X-ray diffraction (XRD) patterns of the as-received WO₃ nanoparticles. The diffraction profiles show agreement with the standard monoclinic phase of WO₃ (JCPDS Card No. 43-1035). The high intensity and narrow full-width at half-maximum (FWHM) of the diffraction peaks indicate a highly crystalline structure. The XRD patterns for the composite coating (MTMS-WO₃-25%) similarly exhibit the characteristic crystalline peaks of WO₃. This preservation of crystallinity was expected, as the interaction between the MTMS and WO₃ is a surface-limited phenomenon. Since the chemical bonding is restricted to the particle-matrix interface, the bulk crystalline lattice of the WO₃ nanoparticles remains unaltered during the coating process.

Fig. 1.

X-ray diffraction (XRD) for WO₃ nanoparticles and the coating (MTMS-WO₃-25%). MTMS, methyltrimethoxysilane; WO₃, tungsten oxide.

Fig. 2 presents a TEM micrograph of the WO₃ nanoparticles. The particles exhibit a predominantly spherical and ellipsoidal morphology. Analysis of the image reveals a particle diameter of 5.0–15.0 nm.

Fig. 2.

TEM image of WO₃ nanoparticles. Scale bar = 50 nm.

The FESEM (Fig. 3) depicts the MTMS-WO₃ coating surface with a characteristic of granular morphology. The WO₃ nanoparticles were observed to be well-embedded within the MTMS binder. In some locations, WO₃ nanoparticles were observed to form clusters and agglomerates.

Fig. 3.

Field emission scanning electron microscopy (FESEM) of the MTMS-WO₃ coating (MTMS-WO₃-25%). Scale bar = 1.00 μm

Fourier-transform infrared (FTIR) spectroscopy was performed to investigate the chemical interactions between MTMS and the glass substrates during coating formation. To understand the reaction mechanisms, spectra were acquired for MTMS coating, WO₃ nanoparticles, and the MTMS-WO₃ coating (Fig. 4).

Fig. 4.

Fourier transform infrared (FTIR) spectra of the polymer coating.

The FTIR spectrum of MTMS exhibits characteristic peaks in the high-frequency region at 2945 cm^-1 and 2841 cm^-1 corresponding to the asymmetric and symmetric C-H stretching vibrations of the methyl (-CH₃) and methoxy (-OCH₃) groups, respectively. In the fingerprint region (600–1500 cm^-1), a distinct peak at 1268 cm^-1 is attributed to the symmetric deformation (scissoring) of the Si-CH₃ bond. The presence of these non-polar methyl groups is significant, as their low surface energy typically increases the coating’s hydrophobicity. A dominant absorption band between 1000 and 1200 cm^-1 represents the asymmetric stretching vibrations of the Si-O-Si siloxane network. This provides direct evidence of covalent bonding between the MTMS layer and the glass substrate. Furthermore, the absence of significant methoxy signals in the final coating suggests that the -OCH₃ groups underwent complete hydrolysis. Supporting this, the peaks at 837 cm^-1 and 790 cm^-1 attributed to a combination of Si-O stretching and CH₃ rocking have further confirmed the integration of methyl groups within the silicon-oxygen framework.

WO₃ nanoparticles display a broad absorption band in the 3100–3500 cm^-1 range, which is characteristic of surface hydroxyl (-OH) groups. This broad band was slightly reduced in the MTMS-WO₃ coating spectrum, confirming that the MTMS precursors consumed some of the surface hydroxyls to facilitate chemical bonding. Additionally, the Si-O-Si stretching band (1000–1200 cm^-1) exhibits a slight shift in the composite sample, indicating that the silicon network has successfully anchored to the heavier tungsten atoms.

The pencil scratch hardness results (Fig. 5) illustrate the mechanical resistance of the MTMS coating (MTMS-WO₃-0%) compared to composite coatings with varying WO₃ nanoparticle concentrations (6.25, 12.5, and 25 wt%). To evaluate hydrolytic stability, the samples were subjected to a 48 hours water-immersion test, after which their hardness was re-evaluated to determine the impact of moisture exposure on the coating integrity.

Fig. 5.

Pencil scratch hardness test for coatings with varying concentrations of WO₃.

All coatings tested exhibited a consistent hardness value of 9H before and after the immersion test. This shows that the MTMS binder forms a very hard and durable film, indicating excellent cross-linking and film formation properties of the silane precursor under the applied curing conditions.

Increasing WO₃ nanoparticle concentrations to 6.25, 12.5, and 25 wt% did not result in any observable decrease in hardness. This is an important finding as particulate fillers, especially at higher concentrations, can sometimes compromise the mechanical properties of a polymer matrix due to poor dispersion or agglomeration [14, 15]. The consistent hardness suggests that the WO₃ nanoparticles are well-integrated into the MTMS matrix.

Tack-free time is an important evaluation parameter for curing kinetics of a coating. It defines the transition point at which the surface reaches a dry state. The shortest tack-free time was approximately 23 min observed for MTMS-WO₃-25%. It was also observed (Fig. 6) that the tack-free time changes with respect to the concentration of WO₃ used in the samples. Samples with a higher concentration of WO₃ in the mixture tend to have shorter tack-free time and vice versa. The accelerated drying observed at higher WO₃ concentrations is attributed to the increased solids content and the high specific surface area of the nanoparticles. The high surface area of the WO₃ nanoparticles facilitates faster solvent evaporation.

Fig. 6.

Tack-free time test for coatings with varying concentrations of WO₃.

Fig. 7 illustrates the transmittance spectra of the various coatings. The uncoated glass substrate exhibits a high optical transparency, transmitting approximately 90% of incident light across the 400–900 nm range. In comparison, the MTMS coating (MTMS-WO₃-0%) resulted in a slight reduction in transmittance while maintaining a value above 85%.

Fig. 7.

Transmittance spectra for uncoated glass slide and coated glass slides with varying concentrations.

The incorporation of WO₃ nanoparticles significantly enhanced the ultraviolet (UV) reduction of the coatings. It was observed that the degree of UV reduction was proportional to the WO₃ filler loading. The sample with the highest WO₃ concentration exhibited the most substantial reduction in UV transmission. This trend of concentration-dependent transmittance continued into the visible light region (380–700 nm), where increasing the nanoparticle content led to a decrease in optical transparency.

The reduction in the degree of transmittance of the UV for coatings loaded with WO₃ can be attributed to the ability of the WO₃ to reflect and absorb UV light. According to absorbance and reflectance spectra (Figs. 8,9), the UV that did not pass through the coating was mostly absorbed by the WO₃, with a small percentage being reflected. The same mechanism explains the reduced transmittance of visible light through the coatings.

Fig. 8.

Absorbance spectra for uncoated glass slides and coated glass slides with varying concentrations.

Fig. 9.

Reflectance spectra for uncoated glass slides and coated glass slides with varying concentrations.

Table 2 summarizes the UV-shielding performance and transparency of the developed coatings. The uncoated glass substrate exhibited UV-shielding of 33.22% while maintaining high transparency (91.88%). The MTMS coating (MTMS-WO₃-0%) caused negligible optical variation, confirming the high quality of the silane binder. Increasing the WO₃ concentration revealed a clear trade-off between shielding and transparency:

(a) At 6.25 wt%: the shielding efficiency nearly doubled (63.23%) compared to the MTMS-WO₃-0% (35.49%) while the transparency remained high at 76.32%. This indicates that the nanoparticles were functioning effectively as UV absorbers without causing significant scattering in the visible range.

(b) At 12.5 wt%: the shielding slightly increased to 68.30%, but the transparency dropped to 69.25%. While effective, this formulation falls slightly below the 70% visible light transmission (VLT) standard often required for automotive and architectural windows.

(c) At 25 wt%: a shielding efficiency of 81.08% was achieved but this caused a reduction in clarity (T_Vis = 50.44%). The sharp drop in transparency suggests the onset of particle agglomeration and Mie scattering, rendering this concentration unsuitable for transparent applications.

Based on this, the 6.25 wt% formulation was considered as the optimal coating as it resulted in the highest UV protection while strictly satisfying the transparency criteria (T_Vis $>$70%) for practical applications.

Fig. 10 displays five glass substrates with an uncoated slide, MTMS coating (MTMS-WO₃-0%), and glass slides coated with varying concentrations of 6.25, 12.5, and 25 wt% WO₃ content, respectively. A distinct trend can be observed in which, as the concentration of WO₃ increases, the transparency of the coating decreases.

Fig. 10.

Images of (a) uncoated glass slides and glass slides coated with varying WO₃ content (b) MTMS-WO₃-0%, (c) MTMS-WO₃-6.25%, (d) MTMS-WO₃-12.5%, and (e) MTMS-WO₃-25%.

While the MTMS coating (MTMS-WO₃-0%) exhibits high transparency, the introduction of 6.25 wt% WO₃ results in a noticeable greyish-blue hue and reduces transparency. This effect intensifies at 12.5 wt% WO₃, where the coating appears darker and more opaque. At the maximum concentration of 25 wt%, the glass slide is significantly darkened, allowing very limited visible light transmission. A visual comparison of these samples is provided in Fig. 11.

Fig. 11.

Visual comparison of (a) glass slides and coatings with varying WO₃ content (b) MTMS-WO₃-0%, (c) MTMS-WO₃-6.25%, (d) MTMS-WO₃-12.5%, and (e) MTMS-WO₃-25%.

The observed decrease in transparency is directly related to the increasing concentration of WO₃ nanoparticles in the coatings. WO₃ is known for its strong absorption in the UV region due to its bandgap nature (E_g $\approx$ 2.6–3.0 eV) and its ability to absorb and scatter visible light [16]. As more WO₃ nanoparticles are embedded within the coating matrix, the density of absorbing and scattering centers increases, leading to a more effective blockage of incident photons across the visible spectrum. This demonstrates a simple method for tuning the visible light transmission by simply adjusting the filler concentration.

The observed decrease in transparency is directly correlated with the increasing concentration of WO₃ nanoparticles within the coatings. This behavior is attributed to the inherent optical properties of WO₃, which is characterized by strong UV absorption due to its bandgap (E_g $\approx$ 2.6–3.0 eV) and its capacity to both absorb and scatter visible light [16].

As a higher volume of WO₃ nanoparticles is embedded within the coating matrix, the density of these absorbing and scattering centers increases, resulting in more effective attenuation of incident photons across the visible spectrum. This relationship demonstrates a practical approach for tuning visible light transmission by simply modulating the filler concentration.

Fig. 12 presents the water contact angle ($\theta$_ω) measurements for the various coatings. The uncoated glass substrate exhibits a contact angle of 92°. Upon application of the neat MTMS coating (MTMS-WO₃-0%), the contact angle slightly decreases to 88°.

Fig. 12.

Contact angle of water droplets on (a) glass slides and coatings with varying WO₃ content (b) MTMS-WO₃-0%, (c) MTMS-WO₃-6.25%, (d) MTMS-WO₃-12.5%, and (e) MTMS-WO₃-25%.

The incorporation of 6.25, 12.5, and 25 wt% of WO₃ (Fig. 12c–e) significantly enhances the wettability of the surfaces, as evidenced by a sharp reduction in $\theta$_ω values. This downward trend in the contact angle confirms the transition toward more hydrophilic surface properties as the WO₃ concentration increases.

While the MTMS matrix provides hydrophobic methyl groups (-CH3), the addition of WO₃ introduces hydroxyl groups (W-OH) as WO₃ is a hydrophilic material [17]. The surface of the WO₃ particles is rich in hydroxyl (-OH) groups. These groups actively attract polar water molecules through hydrogen bonding, causing the water droplet to spread out and hence significantly decreasing the water contact angle ($\theta$_ω). As the concentration of this hydrophilic filler increases from 6.25 to 25 wt%, more of these water-attracting sites become present on the coating’s surface, resulting in a higher degree of hydrophilicity. Hence, at higher concentrations, the hydrophilic nature of the WO₃ dominates the surface energy, overcoming the effect of the MTMS methyl groups.

Despite promising results, this study has several limitations. The manual sponge-coating technique is simple and scalable but lacks the precise thickness control of automated methods like spin-coating or dip-coating. Secondly, while WO₃ nanoparticles successfully enhance UV-blocking, they make the surface more hydrophilic, hence preventing the achievement of a self-cleaning surface, which could potentially broaden the coating’s application. Lastly, haze at high concentrations (25 wt%) caused by particle agglomeration has put a limit to applications that demand strict color neutrality.