Influence Of Compostion On Visualand Near Infrared Spectroscopic Properties Of M-doped Cr₂O₃（M=Ti,V） Green Pigments

Chromium oxide(Cr2O3) is one of the most commonly used green pigments on civilian energy and military camouflage applications. Depending on the purpose, the visible-near infrared(Vis-NIR) reflectance spectra of doped Cr2O3 could be tuned by introducing dopant ions in Cr2O3. The amount of Ti and V introduced in the Cr2O3 matrix has been systematically varied respectively, in order to evaluate the effects of Ti and V contents on Vis-NIR reflectance spectra properties of the doped Cr2O3. Vis-NIR reflectance spectrum of M-doped Cr2O3(M=Ti,V) has many similar characteristics.Comparing that of undoped Cr2O3, all the reflectance of M-doped Cr2O3(M = Ti, V)samples decrease ranging from 400 nm to 650 nm, and the two characteristic absorption peaks around 460 nm and 590 nm are also broadened, producing color differences. This is because the metal ion Ti4+, V5+ in Cr2O3 lattice will change the range of d electrons’ activities of Cr3+. Compared undoped Cr2O3, the results reveal significant variations in the NIR region as a function of nominal composition for both Ti-doped Cr2O3 and V-doped Cr2O3 in the NIR light wavelength range from 780 to 2000 nm. For comparison,experimental reflectance measurements and mean-NIR reflectance(780-2000nm) of samples in the same doping concentration range(0-0.3 molar ratio) are also given. All Ti-doped Cr2O3 samples present higher average NIR reflectance than that of undoped Cr2O3 as two V-doped Cr2O3 samples present lower average NIR reflectance than that of undoped Cr2O3 in a smaller doping concentration range(0-0.02 mole ratio). Therefore,the NIR reflectance properties of Ti-doped Cr2O3 are more stable than that of V-doped Cr2O3. For the Ti-doped Cr2O3 samples, the decrease of free carriers and the electronic transitions between defect energy levels within the band gap and the conduction band,could be responsible for the variable NIR reflectance. However, for V-doped Cr2O3,apart from light absorptions of free carriers, there are new light absorption peaks range from 1190 to 2000 nm, resulting in the variations of the NIR reflectance. In preparing functional Cr2O3 related pigment, these two dopants could be selected according to the actual needs to tune Vis-NIR reflectance spectrum of the doped Cr2O3. For example, as the NIR reflectance properties of Ti-doped Cr2O3 are more stable and V2O5 has some toxicity, it is recommended that applying Ti-doped Cr2O3 to prepare Cr2O3 green pigment with high NIR reflectance for energy saving. On the other hand, it isrecommended that incorporating V ions in the Ti-doped Cr2O3 to reduce the brightness of the doped Cr2O3 pigment for the military use. Because the pigment is required to have not only a high NIR reflectance, but also called for a low brightness value for preparing military camouflage Cr2O3 green pigment.