Fabrication and Characterization of Cosmetic-grade TiO₂@SiO₂ for UV-shielding Ingredient

Materials

Sodium silicate (Na₂SiO₂·9H₂O) was procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Chengdu Ke Long Reagent Co., Ltd. (Chengdu, China) supplied sulfuric acid (H₂SO₂), oxalic acid, and sodium hydroxide (NaOH). The Pangang Group Research Institute Co., Ltd. provided rutile-phase nano-TiO₂. All reagents were utilized as received without further purification. Double-distilled water was employed throughout the experiments.

Synthesis

Fifty grams of TiO₂ nanoparticles were dispersed in 100 mL of deionized water and subjected to ultrasonic treatment for 20 minutes to achieve a uniform suspension. The prepared TiO₂ dispersion was then transferred into a flask and continuously stirred at 300 rpm within a water bath set to 90 °C. Concurrently, solutions of 0.15 mol·L⁻¹ sodium silicate and 10% sulfuric acid were introduced into the suspension via two precision-controlled pumps over 3 hours, maintaining a constant flow rate for the sodium silicate solution. The mass ratio of SiOM₂ to TiO₂ was fixed at 1:10 throughout the coating process. The pH was actively maintained between 9.5 and 10 by adjusting the addition rate of the sulfuric acid solution. After completing the reagent addition, the suspension was aged under stirring at 90 °C for 1 hour. Thereafter, the pH was fine-tuned to approximately 7.5–7.8 by incremental addition of either 10% sulfuric acid or 10% sodium hydroxide solution, followed by a further aging step lasting 2 hours. The resulting material was collected by filtration, rinsed thoroughly with deionized water, and dried at 120 °C for 12 hours. Samples prepared at different pH levels during the sodium silicate addition were designated TiO₂@SiO₂-X, where X corresponds to the pH value maintained during coating.

Characterization

The crystal structures of the samples were examined using X-ray diffraction (XRD, EMPYREAN, Panalytical) with Cu Kα radiation as the X-ray source. Morphological features and elemental compositions were investigated by transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS) (Talos F200S, Thermo Fisher). Specific surface areas were determined via nitrogen adsorption–desorption isotherms measured on a 3S-2000PSI analyzer (BeiShiDe), with the Brunauer–Emmett–Teller (BET) method employed to calculate the SBET values. Chemical states and elemental compositions were further characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher) using an Al Kα radiation source. Ultraviolet–visible (UV-vis) diffuse reflectance spectra were recorded with a UV-2700i spectrophotometer (SHIMADZU), employing BaSO₄ as the reflectance standard. Whiteness values were quantified based on the CIE-Lab* 1976 color space using a Datacolor 800V spectrophotometer. Oil absorption measurements were conducted following a spatula rub-out method analogous to ASTM Standard D281-95.

Figure 1 shows the comparison of the transmission electron microscope (TEM) image of uncoated titanium dioxide nanoparticles and silicon dioxide-coated titanium dioxide samples prepared under different pH conditions. As shown in Figure 1(a), the uncoated titanium dioxide nanoparticles are elliptical, with a short axis size of about 15-20 nm and a long axis range of 45-60 nm. It was observed that the morphology of the silicon dioxide coating layer strongly depends on the pH of the reaction medium. Under alkaline conditions (pH ≥ 9), titanium dioxide nanoparticles are evenly covered by a continuous silicon dioxide shell. As shown in Figures 1(e) and 1(f), when the pH value rises to about 10, the silicon dioxide layer becomes denser and more uniform, with an average thickness of nearly 3 nanometers. Under the condition of slightly lower pH (about 9), a thicker silicon dioxide shell (about 5 nanometers) was formed, but the porosity of the coating layer increased. In contrast, as shown in Figures 1(g) and 1(h), the near-neutral pH conditions cause the surface of titanium dioxide to form a discontinuous island-like silica morphology, and the transverse size of these silicon dioxide islands is about 10-13 nanometers. Interestingly, when the pH value is further reduced to 4, the size of the silicon dioxide area decreases (5-8 nanometers), and the coating continuity is enhanced. Based on these observations, it can be concluded that increasing the pH of the reaction solution is conducive to the formation of a dense, continuous, and uniform silicon dioxide shell on the surface of titanium dioxide nanoparticles.

Figure 1: TEM images of (a,b) the nanoTiO₂ and (c,d) TiO₂@SiO₂ -10, (e) TiO₂@SiO₂-9, (f) TiO₂@SiO₂ -7, (g) TiO₂@SiO₂ -5, (h) TiO₂@SiO₂ -4.

In order to further verify the existence of the silicon dioxide coating layer and analyze it quantitatively, the energy dispersion spectrum (EDS) analysis of TiO₂@SiO₂ samples was carried out (Figure S1 and Table S1). The results clearly confirm the presence of titanium and silicon elements in all samples, which provides direct chemical evidence in support of the morphological observation of TEM. Figure 2 (b-d) shows the surface distribution images corresponding to the elements of titanium, oxygen, and silicon, respectively. The analysis of elemental distribution and relative strength shows that titanium and oxygen are the main elements, which is in line with the expectations of titanium dioxide core materials; the silicon element content is small but clearly measurable, which is consistent with the structural characteristics of the thin shell. Most importantly, as shown in Figure 2, silicon elements are observed uniformly dispersed on the entire surface of titanium dioxide nanoparticles. This uniform distribution provides key evidence for real shell formation (not a simple physical mixture of titanium dioxide and silicon dioxide particles), confirming that the synthesis method successfully prepared a uniform core-shell structure in the whole sample. The complete surface coverage shown by the silicon element surface distribution map is crucial to ensure consistent ultraviolet protection and surface passivation performance in actual sun protection applications.

Figure 2: EDS mapping of the TiO₂@SiO₂-10 sample.

XRD analysis was performed to verify the crystalline phases and determine whether the SiOM₂ coating affects the TiO₂ core structure. As shown in Figure 3, the diffraction spectrum of uncoated titanium dioxide and all silicon-coated titanium dioxide samples corresponds to rutile-type titanium dioxide, showing sharp and strong diffraction peaks unique to highly crystalline materials. The diffraction peaks at 2θ angles 27.4°, 36.2°, 41.3°, 44.1°, 54.3°, 56.6°, 62.7°, and 68.1° belong to (110), (101), (111), (210), (211) of rutile titanium dioxide, respectively. (220), (002) and (301) crystal surface (JCPDS No. 21-1276) [26,27]. These well-preserved rutile characteristic peaks do not shift the peak position, and there is no additional heterophase, which proves that silicon dioxide coating does not change the crystal structure of titanium dioxide, which is crucial to maintaining the light stability and ultraviolet barrier performance of the material. It is worth noting that amorphous silica characteristic peaks, which usually appear around 22.3°, were not detected in silicon dioxide-covered titanium dioxide samples. The reason for the absence of this peak is that: first, the silicon dioxide content is low, which is lower than the detection limit of XRD for amorphous materials; second, it forms an ultra-thin homogeneous amorphous layer instead of crystalline silicon dioxide particles [14,28].

Figure 3: XRD patterns of nanoTiO₂ and TiO₂@SiO₂ samples.

XPS was employed to investigate the surface composition and electronic environment of the nanoparticles. The full survey spectra are presented in Figure 4(a), where TiO₂@SiO₂-10 exhibits two prominent peaks at 152.06 eV and 103.53 eV, corresponding to Si 2s and Si 2p orbitals, respectively, providing direct evidence of silicon incorporation. As shown in Figure 4(b), the Ti 2p₃/₂ and Ti 2p₁/₂ peaks of pristine TiO₂ appear at 458.35 eV and 464.05 eV. Upon SiOM₂ coating, these peaks in TiO₂@SiO₂-10 shift to higher binding energies of 458.67 eV and 464.40 eV, respectively [28,29]. This positive shift is particularly significant as it demonstrates strong electronic interaction between the coating and core materials. The O 1s spectrum of bare TiO₂ displays two peaks at 529.63 eV and 530.37 eV, assigned to Ti–O bonds and surface hydroxyl groups. After SiOM₂ deposition, the O 1s spectrum of TiO₂@SiO₂-10 reveals three distinct peaks at 529.95 eV, 531.23 eV, and 532.85 eV, corresponding to Ti–O–Ti, Ti–O–Si, and Si–O–Si bonds, respectively [30,31]. Critically, the emergence of the Ti–O–Si peak at 531.23 eV serves as definitive evidence of chemical bonding at the core-shell interface rather than simple physical deposition. The systematic shift toward higher binding energies is attributed to silicon's higher electronegativity, which withdraws electron density from titanium atoms, thereby reducing the shielding effect [14]. Additional XPS spectra of other samples are provided in Figures S2–S4. These findings conclusively demonstrate successful SiOM₂ coating with strong interfacial Ti–O–Si bonding on TiO₂ nanoparticle surfaces.

Figure 4: XPS of nanoTiO₂ and TiO₂@SiO₂-10 sample. (a) full range XPS spectra, (b) Ti 2p, (c) O 1s, (d) Si 2p spectra.

FT-IR spectroscopy was employed to identify chemical functional groups and verify interfacial bonding in the TiO₂@SiO₂ composites. The FTIR spectra of nano-TiO₂ and TiO₂@SiO₂-10 are presented in Figure 5. Broad absorption bands at 3438 cm⁻¹ and 1619 cm⁻¹ correspond to stretching and bending vibrations of hydroxyl groups (–OH) from adsorbed water [14]. Critically, the SiOM₂-coated spectrum exhibits distinct new bands at 1226 cm⁻¹, 1077 cm⁻¹, and 955 cm⁻¹, assigned to asymmetric stretching of Si–O–Si linkages and Ti–O–Si bonds [31,32]. The appearance of these Si-related peaks provides direct spectroscopic evidence of successful silica coating. Vibrations in the 600–900 cm⁻¹ region, present in both samples, are attributed to Ti–O bond stretching [8]. Significantly, after SiOM₂ coating, the absorption band near 781 cm⁻¹ shows a substantial decrease in intensity accompanied by a pronounced red shift of approximately 27 cm⁻¹ compared to uncoated nano-TiO₂. This dual change—reduced intensity and frequency shift—is particularly diagnostic, as it results from lattice distortion and Ti–O–Si bond formation at the core-shell interface [13]. The magnitude of this shift (27 cm⁻¹) unambiguously confirms chemical bonding between the SiOM₂ shell and TiO₂ core through partial substitution of Si for Ti, rather than mere physical adsorption. These complementary spectroscopic features collectively provide robust evidence for strong interfacial coupling in the TiO₂@SiO₂ structure.

Figure 5: FT-IR spectra of nanoTiO₂ and TiO₂@SiO₂-10 sample.

The UV–vis transmittance spectra of TiO₂@SiO₂ samples synthesized at varying pH values were recorded to assess their ultraviolet shielding capabilities, with the results depicted in Figure 6. All samples demonstrated excellent transparency within the visible light range (400–800 nm), which can be attributed to the small size of TiO₂ nanoparticles (15–20 nm), less than half the wavelength of visible light, thereby allowing efficient passage of visible radiation. TiO₂ nanoparticles are commonly employed in sunscreen formulations, where maintaining visible-light transparency is essential for cosmetic acceptability. Conversely, a pronounced decrease in transmittance was observed in the UV region (200–400 nm) for the SiOM₂-coated samples, corresponding to the strong absorption band of TiO₂ nanoparticles [31,32]. Notably, samples prepared under neutral to alkaline conditions exhibited UV transmittance values below 5%, indicative of substantial UV blocking ability [33,34]. This enhanced UV attenuation is likely due to the formation of a dense SiOM₂ shell at these pH levels, which improves the ultraviolet light scattering efficiency of the TiO₂@SiO₂ composites.

Figure 6: UV-vis transmittance spectra of nanoTiO₂ and TiO₂@SiO₂ samples.

The specific surface area and oil absorption capacity of TiO₂ are critical determinants of sunscreen tactile properties [35]. Figure 7 demonstrates how coating pH directly controls the Brunauer–Emmett–Teller (BET) surface area and oil absorption values of TiO₂@SiO₂ composites. Bare nano-TiO₂ exhibited the highest values—265.98 m²/g BET surface area and 90.19 g/100 g oil absorption. Significantly, SiOM₂ encapsulation dramatically reduced both parameters, with the reduction magnitude being strictly pH-dependent. Most notably, TiO₂@SiO₂-10 (synthesized at pH 10) achieved the lowest BET surface area and oil absorption among all samples, providing compelling evidence for the formation of a dense, minimally porous SiOM₂ coating. This conclusion is strongly corroborated by electron microscopy observations, which independently confirmed the enhanced morphological compactness and density of the coating layer at this pH. Table 1 reveals critical differences in brightness (L) and Hunter Whiteness (Wh) values between bare nano-TiO₂ and SiOM₂-coated TiO₂ samples prepared at various pH levels. Uncoated TiO₂ nanoparticles measured 96.86 for L and 92.02% for Wh. Significantly, SiOM₂ encapsulation consistently enhanced these optical properties, elevating brightness to 97.96 and whiteness to 94.03%. The coating pH exerted minimal influence on these parameters across the pH range tested. Most importantly, TiO₂@SiO₂-7 (synthesized at pH 7) achieved the highest brightness and whiteness values of all samples examined. This superior optical performance directly results from the enhanced dispersibility conferred by the SiOM₂ coating, which demonstrably increases the brightness and whiteness of the coated powders by preventing particle agglomeration.

Figure 7: (a) SBET and (b) oil absorption of nanoTiO₂ and TiO₂@SiO₂ samples.

In order to make the TiO₂@SiO₂ sample useful in sunscreen, it is necessary to evaluate its fluidity, skin sensation, and sun protection factor (SPF). Table 2 presents the critical performance characteristics of sunscreens incorporating different TiO₂@SiO₂ samples. With TiO₂ content fixed at 15.6%, pH values remained consistent across all formulations. Crucially, the sunscreen containing TiO₂@SiO₂-10 exhibited markedly lower viscosity than the pristine nano-TiO₂ formulation—a direct consequence of the enhanced dispersibility imparted by the SiOM₂ shell, which fundamentally improved fluidity by preventing TiO₂ aggregation. More significantly, SiOM₂ coating dramatically elevated the SPF value from 29 to 41, representing a 41% enhancement in UV protection efficacy [36,37]. These compelling performance data unequivocally establish TiO₂@SiO₂-10 (synthesized at pH 10) as the superior formulation, demonstrating optimal properties that position it as the ideal candidate for ultraviolet filtration in commercial sunscreen products.

In this work, TiO₂@SiO₂ core-shell nanoparticles were fabricated successfully using a chemical deposition technique. The study focused on tuning the morphology of the SiOM₂ coating by manipulating the reaction pH and SiOM₂ content. Optimal coating was achieved at pH 10 with a SiOM₂ loading of 10%, producing a uniform and continuous silica shell approximately 3 nm thick on TiO₂ surfaces. This configuration markedly improved the material’s ultraviolet blocking capability. The TiO₂@SiO₂ sample prepared under these conditions demonstrated an impressive whiteness index of nearly 98% and an SPF value reaching 41. Such enhancements translate to better whitening effects, smoother skin feel, and increased compatibility for cosmetic formulations. Overall, this work introduces a straightforward and scalable method to produce densely coated TiO₂ particles, underscoring their promising applicability for industrial-scale cosmetic manufacturing.

Supplementary-File