Carbon dioxide gas sensor

A method for aging a carbon dioxide gas sensor, comprising:

• continuously operating a gas sensing layer of the carbon dioxide gas sensor for 48 hours or more at a temperature of 30020 C. to 400° C. in a gas atmosphere containing 300 ppm to 3000 ppm of carbon dioxide and having a relative humidity of 20% to 90%, wherein the gas sensing layer comprises lanthanum dioxycarbonate represented by La2O2CO3.

A method for production of lanthanum dioxycarbonate represented by La2O2CO3, the method comprising:

• heating a carboxylate, a carbonate, or a hydrate of lanthanum at 425° C. to 575° C. for 2 hours to 80 hours.

A method for selectively producing crystal polymorphism of lanthanum dioxycarbonate represented by La2O2CO3, the method comprising:

• heating lanthanum acetate hydrate represented by La[CH3COO]3·nH2O or lanthanum oxalate hydrate represented by La2[C2O4]3·nH2O at a heating temperature of 425° C. to 575° C. for a heating time of 2 hours to 80 hours in a gas atmosphere containing 350 ppm to 500 ppm of carbon dioxide;
• and
• adjusting the heating temperature and the heating time to selectively produce monoclinic lanthanum oxycarbonate, hexagonal lanthanum oxycarbonate, and lanthanum oxycarbonate containing both monoclinic lanthanum oxycarbonate and hexagonal lanthanum oxycarbonate.