Describe the structure and properties of near-infrared reflective pigments
image: In a new study, researchers from the Nagoya Institute of Technology, Japan used a combination of experimental and theoretical approaches to understand the optical, electronic, and magnetic properties of the complex solids of layered perovskite compounds, providing valuable insights. This approach can be extended to various functionalized crystalline ceramic compounds.
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Credit: Ryohei Oka from Nagoya Institute of Technology, Japan
Urban areas without sufficient tree cover are significantly warmer than their surroundings. This “urban heat island” effect results mainly from the absorption of near-infrared radiation (NIR) in sunlight. Therefore, NIR reflective pigments that can reduce such heating effects are highly desirable.
In particular, functional inorganic pigments are attractive candidates on this front. In fact, Dr. Ryohei Oka and colleagues from the Nagoya Institute of Technology, Japan, have demonstrated that type A coated perovskite ceramic compounds2BO4 ideal for reflecting NIR. In previous studies, it was found that new perovskites such as calcium manganese oxide (Ca.)2(Mn,Ti)O4) ceramics are much better at reflecting NIR radiation than commercially available black pigments. However, the mechanism by which Ca2(Mn,Ti)O4 achieved this remarkable feat is still unknown.
In a recent study published in Inorganic ChemistryDr. Oka and his colleague, Dr. Tomokatsu Hayakawa, analyzing the structure and composition of Ca2(Mn,Ti)O4 used a combination of standard theoretical and experimental techniques to investigate factors contributing to the increase in NIR reflectivity. This paper was available online on April 19, 2022, and was published in Volume 61 Issue 17 of the journal on May 2, 2022.
In their work, the duo used X-ray diffraction (XRD) and Raman spectroscopy in combination with a computational method called “density functional theory” (DFT) to successfully extract missing details about the crystal structure and electronic state of Ca.2(Mn,Ti)O4. “Several studies so far have performed Raman spectroscopy of Ca.2(Mn,Ti)O4. In addition, they have not provided details of the vibration mode. However, information about their electronic state and vibrational modes is critical to understanding how these perovskites turn into powerful NIR reflectors.” said Dr. Oka, explains the motivation behind their approach.
The duo analyzed the crystal structure of calcium manganese oxide (Ca2MnO4) and track the structural changes that occur with the addition of Ti impurities. Next, they identified how the chemical bonds within the perovskite were modified after introducing a Ti impurity. They found that, compared to Ca2MnO4Ca2(Mn,Ti)O4 shows additional Raman peaks likely due to the activation of the “silent mode” induced by the Ti impurity. However, the XRD pattern of Ca2MnO4 and Ca2(Mn,Ti)O4 identical. The duo attributed this to the Ti-Ti correlation at some distance.
Another highlight of their study is the striking agreement between the computational results from the DFT and the experimental data. The energy gap obtained from the three models for Ca2(Mn,Ti)O4 used by the duo in their calculations agree with each other as well as the experimental value. Moreover, the result is independent of the Ti substitution or its position in the crystal. In addition, calculations reveal that the increase in NIR reflectivity upon addition of Ti ions results from a decrease in the “density state” (number of electronic states per unit volume per unit energy) near the Fermi level (the highest energy level that an electron can occupy at absolute zero).
This finding brings us one step closer to uncovering the thermal protective properties of perovskite ceramics. The perfect combination of experimental and theoretical approaches developed in this study provides a general recipe for understanding the structure and properties of not only A2BO4 types of ceramics but various complex perovskite ceramics. As Dr. ok, “This combinational approach is applicable to a wide range of functionalized crystalline ceramics to understand their optical, electronic and magnetic properties in a much better way with more reliable structural models obtained computationally.”
Indeed, a detailed understanding of the enhanced NIR reflection mechanism will be of great benefit as inorganic pigments find more applications as superior thermal coatings for urban buildings.
Journal
Inorganic Chemistry
Research methods
Experimental study
Research Subject
Cannot be applied
Article title
Raman Spectroscopy Investigation and Electronic State Calculation for Black Pigment Ca2(Mn,Ti)O4 with High Near Infrared Reflectivity (NIR)
Article Publication Date
2-May-2022
COI statement
The authors declare no competing financial interests.
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