▎ 摘　　要

NOVELTY - Cobalt, N-graphene quantum dots (GQDs)-carboxyl/tin oxide nano composite material comprises secondary particle agglomerated by primary particles, primary particle takes porous tin oxide nano cubic material as substrate, cobalt, nitrogen-co-doped graphene quantum dot with carboxyl functional group is doped in the substrate, cobalt. The average diameter of the N co-doped carboxyl functionalized graphene quantum dot is 1.5-5 nanometer, the average thickness is 0.3-1 nanometer, the average side length of the porous tin oxide nano cubic material is 150-450 nanometer. The mol percentage of carbon, oxygen, hydrogen, nitrogen and cobalt five elements in cobalt, nitrogen-co-doped carboxyl functionalized graphene quantum dot is 100%, the mol percentage respectively, 70-82 wt.% oxygen, 8-16 wt.% hydrogen, 4-8 wt.% nitrogen and 1-3 wt.% cobalt and nitrogen-co-doped carboxyl functionalized graphene quantum dot is 0.1-0.5 % of porous tin oxide nano cubic material mass. USE - Cobalt, nitrogen-graphene quantum dots (GQDs)-carboxyl/tin oxide nano composite material used as gas sensitive material for formaldehyde sensor (claimed). ADVANTAGE - The composite material can shorten the response time and recovery time when used in formaldehyde sensor, and improve the sensitivity of the formaldehyde gas. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following: 1. Method for preparing a cobalt, N-graphene quantum dots (GQDs)-carboxyl/tin oxide nano composite material, involving: a. dissolving citric acid, graphene quantum dot doped with nitrogen source and divalent soluble cobalt salt in the water with more than de-ionized water purity, stirring and mixing uniformly to obtain the mixed solution, adding the mixed solution into the reaction kettle for hydrothermal reaction, the reaction temperature is 140-180degrees Celsius, the reaction time is 6-10 hours; b. cooling, collecting the solution in the reaction kettle, adding ethanol to the solution, the adding amount of the ethanol is 20 times of the volume of the solution, centrifuging and collecting the precipitate, drying to obtain a cobalt, nitrogen-co-doped carboxyl functionalized graphene quantum dot powder, the mol ratio of the citric acid, graphene quantum dot doped nitrogen source and divalent soluble cobalt salt is 1:2-4:2-4; c. dissolving the tetravalent soluble tin salt, divalent soluble cobalt salt and alkali metal hydroxide in the water with the purity of the de-ionized water, stirring and mixing uniformly, standing to precipitate completely, centrifuging and collecting precipitate, washing, after drying, annealing for 2-3 hours at 600-700degrees Celsius to obtain the intermediate product I, dispersing the intermediate product I in nitric acid solution with concentration of 3-5 mole/L, transferring to the reaction kettle, reacting for 3-6 hours at 130-150degrees Celsius, obtaining the intermediate product II, washing the intermediate product II to neutral, drying to obtain a porous tin oxide nano cubic material, where the alkali metal hydroxide is sodium hydroxide or potassium hydroxide and tetravalent soluble tin salt, the mol ratio of divalent soluble cobalt salt and alkali metal hydroxide is 1:1:8-12; d. adding the cobalt, nitrogen-co-doped carboxyl functional graphene quantum dot powder and porous tin oxide nano cubic material into the water with more than deionized water purity, ultrasonically mixing uniformly and drying to obtain cobalt, N-graphene quantum dots-COOH/tin oxide nano composite material where the cobalt, nitrogen-co-doped carboxyl functional graphene quantum dot powder and porous tin oxide nano cubic material with mass ratio of 1:200-1000; and 2. Method for preparing formaldehyde sensor, involving: a. grinding the composite material to obtain powder with particle diameter of 0.5-5 micrometer, dissolving the powder in the mixed solvent of organic solvent and water with the purity of de-ionized water; b. mixing uniformly to obtain slurry; coating the slurry on the test electrode area of the micro-heat plate chip, after drying, annealing for 2-4 hours at 200-400degrees Celsius to obtain the gas-sensitive element; c. placing the gas-sensitive element in the sealed air chamber, electrifying and aging the heating electrode of the micro-heat plate chip to obtain product.