Chromium‑Doped Titanium Dioxide: A Novel Colored Cool Pigment with High Near‑Infrared Reflectance

Abstract

Chromium‑doped TiO₂ pigments were prepared by a solid‑state reaction and characterized with X‑ray diffraction, SEM, XPS, and UV‑VIS‑NIR reflectance spectroscopy. Incorporating Cr³⁺ accelerates the anatase‑to‑rutile phase transition and compresses the lattice. The crystal structure and Cr content jointly influence particle morphology, band gap, and reflectance. In rutile samples, a fraction of Cr³⁺ oxidizes to Cr⁴⁺ during high‑temperature sintering, generating a pronounced near‑infrared absorption band associated with the ³A₂ → ³T₁ electric‑dipole transition of Cr⁴⁺. This oxidation reduces the band gap, shifting the absorption edge to longer wavelengths as the doping level rises. Consequently, the visible and near‑infrared average reflectance of Ti_1−xCr_xO₂ rutile decreases by 60.2 % and 58 % respectively when x = 0.0375, while the pigment turns black‑brown. In contrast, anatase Ti_1−xCr_xO₂ shows only selective visible‑light absorption peaks from Cr³⁺, leaving the band gap and NIR reflectance largely intact. The resulting Cr‑doped anatase pigment exhibits a brownish‑yellow hue and retains ~90 % NIR reflectance.

Background

TiO₂ is widely used as a cool pigment in energy‑efficient buildings, thanks to its >85 % visible (VIS) and near‑infrared (NIR) reflectance. Because sunlight in the VIS and NIR ranges drives most heat gain, TiO₂‑based paints can reduce building heat buildup and cut air‑conditioning energy use by over 20 %.

However, the high VIS reflectance makes TiO₂ paint appear starkly white, which is often considered unattractive, offers limited stain resistance, and shortens the paint’s service life. Developing non‑white cool pigments that lower VIS reflectance while preserving high NIR reflectance is therefore a key challenge.

Elemental doping is a proven strategy to tailor VIS absorption: dopant ions introduce impurity levels, narrow the band gap, and enhance low‑energy photon absorption. Cr, Mn, Ni, and V dopants yield orange, tan, yellow, and gray pigments, respectively. Dopants also affect free‑carrier concentrations, which govern NIR absorption; controlling these carriers can further improve NIR reflectance.

This study investigates Cr‑doped TiO₂ as a colored cool pigment. Samples with varying Cr content and sintering temperatures were synthesized via solid‑state reaction, and their crystal phase, morphology, chemical composition, color, and VIS–NIR reflectance were systematically examined.

Experimental

Synthesis of Ti_1−xCr_xO₂ Pigment

Stoichiometric TiO₂ (99.9 %) and Cr₂O₃ (99.9 %) were ball‑milled in ethanol for 4 h at 450 rpm using an agate jar and balls (ball‑to‑powder ratio 10:1). The mixed powder (50 g) was dried at ~80 °C, then calcined in air at 800–1000 °C for 4 h (heating rate 5 °C min⁻¹). Final pigments were ground in an agate mortar.

Characterization

X‑ray diffraction (Bruker D2 PHASER, Cu Kα) and FE‑SEM (FEI QUANTA 250) were used to assess phase and morphology. Lattice constants were calculated with MDI Jade. XPS (Thermo Escalab 250Xi, Al Kα, 150 W) probed surface chemistry; C 1s at 284.8 eV served as internal reference. UV‑VIS‑NIR diffuse reflectance (250–2500 nm) was recorded with a Lambda 750 (Perkin‑Elmer). CIE LAB color coordinates were derived from the 400–700 nm reflectance spectrum (CIE D65, 10°). The band gap (E_g) was extracted from Kubelka–Munk analysis:

\[\{\begin{array}{c}{\left[F(R) h\nu\right]}^2=C\left(h\nu- E_g\right)\\{}F(R)=\frac{(1-R)^2}{2R}\end{array}\}\]

where F(R) is the Kubelka–Munk function, R the diffuse reflectance, hν the photon energy, and C a proportionality constant.

Result and Discussion

Phase Structure of the Samples

XRD patterns for Ti_1−xCr_xO₂ powders calcined at 800–1000 °C are shown in Fig. 1. At 800 °C only anatase peaks appear; traces of rutile emerge only when x = 0.0375. Raising the sintering temperature to 900 °C causes undoped TiO₂ to remain anatase, but Cr addition promotes rapid rutile formation, with the rutile fraction increasing with x. At 1000 °C both phases coexist in undoped TiO₂, yet rutile dominates all Cr‑doped samples, indicating that Cr³⁺ lowers the anatase–rutile transition temperature by ~100 °C. This acceleration is attributed to charge‑compensation oxygen vacancies that facilitate atom diffusion.

All samples show no separate chromium oxide peaks, confirming uniform Cr dispersion. Lattice constants (Table 1) decrease with rising Cr content in both anatase and rutile, despite Cr³⁺ being slightly larger than Ti⁴⁺. The contraction likely arises from oxygen vacancy formation and the smaller Cr⁴⁺ species that form during high‑temperature sintering.

Sample Morphology

SEM images (Fig. 2) reveal that undoped TiO₂ sintered at 800 °C forms nearly spherical particles (<100 nm). Low Cr levels (x = 0.00625) preserve this morphology. At higher Cr (x = 0.0375) particle size modestly increases and uniformity diminishes. Raising the temperature to 1000 °C produces a mixture of spherical and cubic particles in undoped TiO₂ (anatase/rutile coexistence). Adding Cr transforms particles into elongated columns; increasing x reduces the aspect ratio, and at the highest doping the morphology reverts toward spherical with a mean size of 2 µm.

XPS Analysis

Ti 2p spectra (Fig. 3a) show peaks at 458.9 and 464.2 eV, confirming Ti⁴⁺. Cr 2p spectra (Fig. 3b) display peaks at 577 and 586.4 eV (Cr³⁺) and additional peaks at 580.6 and 591 eV (Cr⁴⁺). The Cr⁴⁺ fraction rises from 29.6 % to 35.8 % as sintering temperature increases from 800 to 1000 °C, consistent with Cr evaporation‑induced oxidation.

O 1s spectra (Fig. 3c) contain lattice oxygen (529.8 eV) and surface‑adsorbed oxygen (530.8 eV); at 1000 °C a shoulder at 532.3 eV appears, attributed to hydroxyl or adsorbed water. The overall shift toward lower binding energy (~0.2 eV) with higher temperature aligns with increased Cr⁴⁺ content.

The Optical Property of the Samples

Colorimetric analysis (Fig. 4) shows negligible change in luminosity (L*) for 800 °C samples as x increases, but a rise then fall in a* and b* values, yielding a brownish‑yellow pigment. At 1000 °C, L* and b* decrease markedly with x; the color shifts from pale yellow to black‑brown in rutile samples, while anatase samples remain relatively light. This contrast reflects the differing VIS reflectance spectra: rutile dopants introduce strong absorption, darkening the pigment.

UV‑VIS‑NIR diffuse reflectance (Fig. 5) and averaged spectral reflectivity (Fig. 6) demonstrate that undoped TiO₂ (anatase or rutile) maintains ~90 % NIR reflectance. Cr doping in anatase introduces a visible‑light absorption peak near 710 nm (Cr³⁺ d–d transition), reducing VIS reflectance from 90.3 % to 68.2 % at x = 0.0375 while preserving NIR performance.

In rutile, Cr doping produces additional shoulders at 450 and 600 nm and a broad NIR absorption band (≈1150–1500 nm) due to the Cr⁴⁺ ³A₂ → ³T₁ transition. VIS reflectance drops dramatically, and NIR reflectance falls by ~58 %. The absorption edge redshifts, indicating a band‑gap reduction to 1.56 eV at x = 0.0375.

In summary, Cr doping affects anatase and rutile TiO₂ differently: anatase retains high NIR reflectance with modest VIS absorption, whereas rutile exhibits pronounced VIS and NIR absorption and a reduced band gap, leading to a black‑brown pigment.

Conclusions

Cr incorporation markedly influences the phase, morphology, and optical behavior of Ti_1−xCr_xO₂ pigments. Cr³⁺ expedites the anatase‑to‑rutile transition, compresses the lattice, and lowers the transition temperature by ~100 °C. In rutile, Cr doping increases particle size and transforms the morphology from columnar to near‑spherical at high concentrations.

Cr oxidation to Cr⁴⁺ during high‑temperature sintering generates a strong NIR absorption band in rutile, reduces the band gap, and redshifts the absorption edge. Consequently, the VIS and NIR average reflectance of rutile Ti_1−xCr_xO₂ drop by 60.2 % and 58 % respectively at x = 0.0375, while the pigment becomes black‑brown. In anatase, only selective VIS absorption peaks appear, leaving the band gap and NIR reflectance largely intact. Thus, a brownish‑yellow Cr‑doped anatase pigment with ~90 % NIR reflectance is achievable, offering a promising, aesthetically pleasing cool pigment.