Sol–gel synthesis and properties of europium–strontium copper silicates blue pigments with high near-infrared reflectance

• High NIR reflective blue pigments were synthesized via sol–gel method.
• The pigments have completely crystallized after calcined at 900 °C for 2 h.
• The color of the Sr1−xEuxCuSi4O10+δ pigments changed from sky-blue to dark blue.
• The developed pigments reached an optimal NIR solar reflectance of 72.31%.

Environmentally benign near-infrared reflective blue pigments Sr1−xEuxCuSi4O10+δ (x ranges from 0 to 0.4) were synthesized via sol–gel method. The pigment samples were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction technique, X-ray energy dispersive spectroscopy, scanning electron microscopy, UV–vis–NIR diffuse reflectance spectroscopy and the Commission Internationale de l’Eclairage 1976 L*a*b* colorimetric method. The prepared powders exhibited single-phase tetragonal crystalline structure. The color of the pigments changed from sky-blue to dark blue due to the substitution of Eu for Sr in SrCuSi4O10. With increasing Eu content, the near-infrared solar reflectance increased first and then declined. Adding specific amount of Eu can enhance the near-infrared reflectance and solar reflectance. The prepared pigments presented an optimum near-infrared solar reflectance of 72.31% when the doping content of Eu is 20 mol%.