Near-infrared reflecting inorganic pigments based on Fe3⁺-doped TiO2 prepared via the sol-gel method: characterization and chromatic properties

2.1. Synthesis of Ti_1-xFe_xO₂ pigments

Synthesis of Ti_1-xFe_xO₂pigments ($x=$0-0.20) by sol-gel method.

Step 1: Prepare two solutions: Solution A: Take 50 mL tetrabutyl orthotitanate (TBOT, analytically pure) as the titanium source and add it to 100 mL anhydrous ethanol. Then, weigh iron nitrate according to the atomic ratio Ti:Fe (0.97:0.03, 0.095:0.05, 0.92:0.08, 0.9:0.1, 0.85:0.15, 0.8:0.20) and add it to the solution, stir vigorously for 30 minutes. Solution B: Weigh 50 mL of distilled water (Double-distilled water) and 20 mL of glacial acetic acid (analytically pure), add them to 50 mL of anhydrous ethanol, and adjust the pH to 1 with a small amount of nitric acid (analytically pure). Then, stir vigorously for 30 minutes using a magnetic stirrer.

Step 2: Slowly add Solution A to Solution B with stirring, then stir 30 min to form Fe-TiO₂ sol via hydrolysis/condensation.

Step 3: Dry gel at 90 ℃ for 24 h, grind, then calcine in air at varied temperatures for 3 h. Sieve to obtain final pigment powders.

Reference: Commercial iron oxide pigments were used for color comparison.

2.2. Characterization methods

Powder XRD (Bruker D8 ADVANCE) was performed using Cu K$\alpha$ radiation ($\lambda =$ 0.15418 nm) at 40 kV/40 mA. Scans covered 10°-70° 2$\theta$ with a step size of 0.02° and 0.2 s per step. Surface composition and chemical states were analyzed with a Shimadzu Axis Ultra DLD spectrometer (Al K$\alpha$, 1486.6 eV). The C 1s peak (284.6 eV) served as the energy reference. Analysis chamber pressure was kept at 10⁻⁹ Pa. Diffuse reflectance spectra (200-2200 nm) were recorded on a Lambda-950 spectrophotometer with an integrating sphere, using PTFE as a reference. NIR solar reflectance ($Rₛ$, 700-2500 nm) was calculated according to ASTM G173-03, using the Eq. (1):

where $R\left(\lambda \right)$ is the measured reflectance and $E\left(\lambda \right)$ is the solar spectral irradiance from ASTM G173-03.

Color coordinates $\left(L,a,b\right)$ were obtained with an X-Rite 8200 colorimeter in CIE Labspace. Chroma ($C^{*}$) was calculated as $C^{*}=\sqrt{{\left(a^{*}\right)}^2+{\left(b^{*}\right)}^2}$. Color change ($\mathrm{\Delta }{E}^{*}$) was calculated as $\mathrm{\Delta }{E}^{*}=\sqrt{{\left(\mathrm{\Delta }{L}^{*}\right)}^2+{\left(\mathrm{\Delta }{a}^{*}\right)}^2+{\left(\mathrm{\Delta }{b}^{*}\right)}^2}$.Alkali/acid immersion: Ti_0.95Fe_0.05O₂ was immersed in 10 % (w/v) NaOH and 10 % (v/v) HCl separately, stirred 30 min, washed, dried, and weighed to determine mass loss. TG-DSC was performed on a NETZSCH STA449C under N₂ from 50 ℃ to 1000 ℃ at 10 ℃/min, using 9.40 mg of sample.

3.1. XPS analysis

XPS analysis was performed on Ti_0.95Fe_0.05O₂ and undoped TiO₂ samples. The characteristic peaks of the main elements are clearly visible in the wide-scan spectrum (Fig. 3(a)) and the fitted Fe 2p spectrum (Fig. 3(d)), confirming the presence of Fe in the material. The highest peak appears at 532 eV, attributed to the signal of hydroxyl groups in trace organic residues remaining on the sample surface. This peak remains unchanged after doping. Fig. 3(b) compares the high-resolution O 1s spectra of undoped TiO₂ and Ti_0.95Fe_0.05O₂. The stronger peak at 529.5 eV is attributed to metal oxide (O²⁻) in the TiO₂lattice. The metal oxide peak of Ti_0.95Fe_0.05O₂ is observed at 530.2eV, which is higher than that of undoped TiO₂ (529.5 eV). This increase in binding energy is attributed to the formation of Fe-O bonds in the lattice [12]. Fig. 3(c) shows the Ti 2p doublet XPS spectra. The Ti 2p_1/2 and Ti 2p_3/2 binding energies of Ti_0.95Fe_0.05O₂ are located at about 465.2 eV and 458.9 eV, respectively, shifting to higher binding energies compared to undoped TiO₂ (464.2 eV and 458.1 eV). The Ti 2p spectra mainly correspond to the Ti⁴⁺ oxidation state [13]. The observed shifts are attributed to the reduced coordination number of Ti and the shortening of Ti-O bonds, indicating the formation of Fe-O-Ti bonds in the doped sample. In summary, the XPS results confirm the successful incorporation of Fe³⁺ cations into the TiO₂ lattice.

Fig. 3XPS spectra of the undoped TiO2 and Ti0.95Fe0.05O2 samples calcined at 800 ℃: a) wide spectra b) O 1s, c) Ti 2p, d) Fe 2p

3.2. Chromatic properties analysis

The CIE 1976 color coordinate values $\left(L^{*},a^{*},b^{*},C^{*}\right)$ of Ti_1-xFe_xO₂ samples ($x=$0-0.20) calcined at 700 ℃ are presented in Table 3. The chromatic parameters are defined as follows: $L^{*}$ for lightness, $a^{*}$ for redness, $b^{*}$ for yellowness, and $C^{*}$ for chroma.

It can be clearly seen from Table 3 that with the systematic increase in Fe³⁺ doping content from $x=$0.05 to 0.20, the lightness value ($L^{*}$) and yellowness value ($b^{*}$) of the pigments show a continuous decreasing trend. The redness value ($a^{*}$) increases first to the maximum of 25.70 at $x=$0.10 and then decreases to 20.38 at $x=$ 0.20, while the chroma value ($C^{*}$) follows the same variation trend as $a^{*}$, peaking at 34.89 at $x=$ 0.10 and then dropping to 24.77 at $x=$0.20. As a result, the color tone of the Ti_1-xFe_xO₂ pigments changes gradually from light red to deep red and then to brownish red with the increase in Fe³⁺ doping content. The coloring mechanism of the prepared Ti_1-xFe_xO₂ pigments is mainly based on the O2p→Ti3d charge transfer transition of the TiO₂ lattice, and this electron transition is further enhanced by the substitution of Ti⁴⁺ with Fe³⁺ in the TiO₂ crystal structure [14].

3.3. NIR reflectance analysis

Ti_0.9Fe_0.1O₂ exhibits the highest color coordinate $a^{*}$ value. To clarify the correlation between the color and reflectivity of Ti_1-xFe_xO₂, further optimization of Fe doping levels was conducted. Table 4 present the color coordinate $a^{*}$ and NIR solar reflectance values of Ti_1-xFe_xO₂ pigments ($x=$0.03-0.10) calcined at 700 ℃, together with the data of the commercial iron oxide pigment for comparison.

As shown in Table 4 and Figs.4-5, when the Fe³⁺ doping content increasing from $x=$ 0.03 to $x=$ 0.10, the NIR solar reflectance of Ti_1-xFe_xO₂ pigments shows a continuous decreasing trend from 74.06 % to 62.21 %, and the color coordinate a* value increasing from 15.03 to 25.7. The fundamental reason for this phenomenon is that the valence state of Fe³⁺ is lower than that of Ti⁴⁺ (the main metal cation in the lattice), and the substitution of Ti⁴⁺ with Fe³⁺ induces charge imbalance in the TiO₂ lattice, thus promoting the formation of crystallographic point defects (e.g., oxygen vacancies) [15]. With the increase in Fe³⁺ doping content, the number of lattice defects accumulates continuously, which significantly strengthens the absorption of visible light and NIR light and weakens its reflection. Notably, all the prepared Ti_1-xFe_xO₂ samples ($x=$0.03, 0.05, 0.08, 0.10) exhibit a much higher NIR solar reflectance than the commercial iron oxide pigment with similar color (44.25 %).

Fig. 4The pigments photographs of Ti1-xFexO2(x= 0.03-0.10) (left)

Fig. 5The relationship between NIR reflectance values and a* with Fe doping content x (right)

3.4. Chemical and thermal stability analysis

Table 5 presents the color properties of the pigments after acid/alkali tests, compared with untreated samples, the values of $L^{*}$, $a^{*}$, and $b^{*}$ showed minimal differences after washing with 10 % HCl and 10 % NaOH. The total color difference ($\mathrm{\Delta }{E}^{*}$) was calculated using $\mathrm{\Delta }{E}^{*}={\left[{\left(\mathrm{\Delta }{L}^{*}\right)}^2+{\left(\mathrm{\Delta }{a}^{*}\right)}^2+{\left(\mathrm{\Delta }{b}^{*}\right)}^2\right]}^{1/2}$. The $\mathrm{\Delta }{E}^{*}$ values was less than 2 % (the qualification requirement $\mathrm{\Delta }{E}^{*}$ for the coating or exterior wall pigment industry is less than 2 %), indicating good chemical stability against the tested acid/alkali. And TG-DSC test results (Fig. 6) showed that the negligible weight change and phase transition indicate that the pigment has good thermal stability.

Fig. 6TG-DSC curves of Ti0.95Fe0.05O2 pigment