In situ thermal, optical, and structural investigations on reversible thermochromism in bismuth molybdate

Reversible thermochromic inorganic materials show stable and perceptible color change with changing environmental temperatures. This property makes them excellent materials for fabricating temperature indicators, military camouflage materials, and thermal warning devices. However, the mechanism of thermochromism has not been fully understood. Here, we prepared novel bismuth molybdate materials by calcination of Bi 2 O 3 and MoO 3 mixtures. The materials exhibit reversible thermochromism, gradually changing color from milky white at 25 °C to bright yellow at 500 °C. The phase composition of the product is tunable by adjusting the ratio of Bi 2 O 3 and MoO 3 in the starting materials, producing either single phase of α -Bi 2 Mo 3 O 12 or mixed phases of α -Bi 2 Mo 3 O 12, β -Bi 2 Mo 2 O 9 and γ -Bi 2 MoO 6 . The mechanism of reversible thermochromism was studied by in situ UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), in situ synchrotron radiation powder X-ray diffraction (SRPXRD), and differential scanning calorimetry (DSC). In situ SRPXRD revealed no phase transition during heating and cooling, which agrees with DSC analysis with no thermal event detected until the melting temperature of α -Bi 2 Mo 3 O 12 at 661.6 °C. With increasing temperature, in situ SRPXRD also revealed anisotropic expansion of the α -Bi 2 Mo 3 O 12 lattice parameters , while in situ UV/Vis DRS showed a continuous red shift in the absorption edge and gradual decrease of band gap, suggesting that lattice expansion and shrinking upon heating and cooling is the main reason for the thermochromic behavior in bismuth molybdate materials.