FIREPROOF INTERIOR WALL COATING WITH VISIBLE-LIGHT PHOTOCATALYSIS, AND PREPARATION METHOD THEREOF TECHNICAL FIELD The present disclosure relates to the technical field of coatings, and in particular to a fireproof interior wall coating with visible-light photocatalysis, and a preparation method thereof. BACKGROUND The transition of electrons from the valence band to the conduction band under the action of UV and near-UV radiation makes the semiconductor material of titanium dioxide exhibit photocatalytic activity. Generated reactive electron-hole pairs migrate to the surface of titanium dioxide particles. In this case, the holes oxidize the adsorbed water to generate reactive hydroxyl radicals and the electrons reduce the adsorbed oxygen to generate superoxide radicals. These two types of radicals are both capable of degrading NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide and the like have been used in coatings or the like to remove pollutants from the air. Nano titanium dioxide has a band gap of 3.2 eV, which is 387.5 nm when converted into wavelength. When the wavelength of light is less than or equal to 387.5 nm, an electron is excited from the valence band to the conduction band through the band gap after a photon in the light is absorbed by nano titanium dioxide. At this time, a highly-active free electron ecb- is formed, and a positive hole hvb+ is formed on the valence band due to escape of the electron. As nano titanium dioxide has a discontinuous electronic band structure with a band gap, the hole-electron pair has an extremely-short lifetime, only at the picosecond level. The free electron ecb- is separated from the hole hvb+ under the action of the electric field of the substance itself, and migrates to different positions on the surface of nano titanium dioxide. At this time, the free electron may undergo catalytic oxidation with an organic substance adsorbed on the surface of nano titanium dioxide; may also be trapped by a defect site caused by the distortion of nano titanium dioxide; and may also be directly recombined with a hole, so that light energy is consumed in the form of heat. Moreover, the positive hole that loses an electron has oxidizability, and can oxidize the 02 and OH- adsorbed on the surface of nano titanium dioxide to form the hydroxyl radical -OH and superoxide radical -02 with a stronger oxidizing ability, which are powerful enough to completely oxidize an organic substance into water and carbon dioxide, without intermediate products. However, unmodified titanium dioxide is not suitable for interior wall systems, because it is impossible to stimulate the activity of a coating in a room where there is basically no ultraviolet light. Therefore, it is critical to independently study and develop modified nano titanium dioxide with visible-light photocatalysis. The energy band of a photocatalyst can be red-shifted, so that the photocatalyst can still exhibit catalytic activity under visible light and thus lead to the generation of superoxide radicals, thereby purifying indoor air. At present, most of the photocatalytic coatings on the market are coatings only with the single photocatalysis function, and the photocatalytic coatings that can be used for other special functions (such as fire prevention) are rare. This is because the addition of a photocatalyst will break the balance of the original coating and will also affect the surface tension, oil absorption, PVC, stability, etc. of the coating. Therefore, there are many problems to be overcome when a photocatalyst is used in a coating with other special functions. Therefore, it is of significant practical significance for both human health and safety to develop a novel composite functional coating with visible-light photocatalysis, broaden the application scope of photocatalytic technology, and finally realize the industrial application of photocatalytic technology. SUMMARY Therefore, in view of the above problems, the present disclosure provides a fireproof interior wall coating with visible-light photocatalysis and a preparation method thereof, and the fireproof interior wall coating with visible-light photocatalysis is environmentally-friendly, has both visible-light photocatalysis and fire prevention functions and a stable formulation system, and can improve air quality and home safety. To solve this technical problem, the present disclosure adopts the following solutions: The present disclosure provides a fireproof interior wall coating with visible-light photocatalysis, including the following raw materials and components, in parts by weight: visible-light-driven photocatalyst 2 to 6 parts emulsion 15 to 18 parts intumescent flame retardant (IFR) 25 to 40 parts inorganic pigment/filler 25 to 28 parts film-forming additive 0.5 to 1.5 parts defoaming agent 0.35 to 0.5 part thickening agent 0.2 to 0.3 part biocide mildewcide 0.4 to 0.8 part dispersing agent 0.1 to 0.5 part water 25 to 27 parts anti-settling agent 0.3 to 0.5 part where, the visible-light-driven photocatalyst is modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%; the IFR is a mixture of an acid source, a carbon source and a gas source with a ratio of 3:1:1; the acid source is any one of polyphosphate, (NH 4) 2 SO 4 ,
NH 4 Cl, and amine/amide phosphate; the carbon source is any one of starch, dextrin, sorbitol, and pentaerythritol; and the gas source is melamine. Further, the modified nano-TiO2 may be prepared by the following method: adding 40 mL of tetrabutyl titanate (TBT) dropwise to 300 mL of absolute ethanol, and subjecting a resulting mixture to ultrasonic stirring for 25 min to 35 min to obtain a solution 1; mixing 100 mL of absolute ethanol, 200 mL of deionized water, and 20 mL of a 6 mol/L nitric acid solution thoroughly to obtain a solution 2; slowly adding the solution 1 dropwise to the solution 2, and adding 100 g of diatomite after the dropwise addition is completed; stirring a resulting mixture with a magnetic stirrer for 3 h, and aging for 12 h; subjecting a resulting mixture to drying at 100°C to 110C for 1 h and to calcination at 500°C to 550°C for 3 h to obtain the modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%. Further, the dispersing agent may be an ammonium salt dispersing agent. Further, the emulsion may be a composite of styrene/acrylate polymer and elastic emulsion; the biocide mildewcide may include 5-chloro-2-methyl-4-isothiazolin-3-one (MIT) and 1,2-benzisothiazolin-3-one (BIT); and the film-forming additive may be plasticizer OE-400. Further, the 0.35 to 0.5 part of defoaming agent may consist of 0.15 to 0.2 part of a mineral oil defoaming agent and 0.2 to 0.3 part of a silicone defoaming agent. Further, the inorganic pigment/filler may be one of sericite powder, titanium dioxide, ultrafine calcined kaolin or washed kaolin, talc powder, and flake wollastonite, or a mixture of two or more thereof. The present disclosure also provides a method for preparing the fireproof interior wall coating with visible-light photocatalysis, including the following steps: (1) adding 0.2 to 0.3 part of thickening agent, 0.3 to 0.5 part of anti-settling agent, 0.1 to 0.5 part of dispersing agent, 0.15 to 0.2 part of defoaming agent, and 25 to 27 parts of water into a disperser, and conducting dispersion at 300 rpm to 500 rpm for 3 min to 5 min to make the solutes completely dissolved in water to form a dispersion system; and (2) adding 25 to 28 parts of inorganic pigment/filler and 30 to 40 parts of IFR to the dispersion system in step (1), and conducting dispersion at 900 rpm to 1,500 rpm for 2 min to 5 min to achieve complete dispersion to form a slurry; adjusting the rotational speed to 1,800 rpm to 2,000 rpm, slowly adding 2 to 5 parts of visible-light-driven photocatalyst to the slurry, and conducting dispersion at the rotational speed for 20 min to 40 min; and then adjusting the rotational speed to 800 rpm to 1,000 rpm, successively adding 15 to 18 parts of emulsion, 0.2 to 0.3 part of defoaming agent, and 0.4 to 0.8 part of biocide mildewcide, and conducting dispersion at the rotational speed for 5 min to 10 min to obtain the fireproof interior wall coating with visible-light photocatalysis; where, the visible-light-driven photocatalyst is modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%; the IFR is a mixture of an acid source, a carbon source and a gas source with a ratio of 3:1:1; the acid source is any one of polyphosphate, (NH 4) 2 SO 4
, NH 4 Cl, and amine/amide phosphate; the carbon source is any one of starch, dextrin, sorbitol, and pentaerythritol; and the gas source is melamine. Further, the defoaming agent in step (1) may be a mineral oil defoaming agent, and the defoaming agent in step (2) may be a silicone defoaming agent. Further, the modified nano-TiO2 may be prepared by the following method: adding 40 mL of TBT dropwise to 300 mL of absolute ethanol, and subjecting a resulting mixture to ultrasonic stirring for 25 min to 35 min to obtain a solution 1; mixing 100 mL of absolute ethanol, 200 mL of deionized water, and 20 mL of a 6 mol/L nitric acid solution thoroughly to obtain a solution 2; slowly adding the solution 1 dropwise to the solution 2, and adding 100 g of diatomite after the dropwise addition is completed; stirring a resulting mixture with a magnetic stirrer for 3 h, and aging for 12 h; subjecting a resulting mixture to drying at 100°C to 110°C for 1 h and to calcination at 500°C to 550°C for 3 h to obtain the modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%. Further, the dispersing agent may be an ammonium salt dispersing agent; the emulsion may be a composite of styrene/acrylate polymer and elastic emulsion; the defoaming agent may consist of 0.15 to 0.2 part of a mineral oil defoaming agent and 0.2 to 0.3 part of a silicone defoaming agent; the biocide mildewcide may be MIT and BIT; the inorganic pigment/filler may be one of sericite powder, titanium dioxide, ultrafine calcined kaolin or washed kaolin, talc powder, and flake wollastonite, or a mixture of two or more thereof; and the film-forming additive may be plasticizer OE-400. With the above technical solutions, the present disclosure achieves the following beneficial effects: The following advantages are achieved by adopting a mixture of ammonium polyphosphate (APP), melamine, and pentaerythritol with a ratio of 3:1:1 as an IFR, selecting APP with a degree of polymerization (DP) (n) > 20 and a particle size of 20 m to 30 m, and preparing modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%: (1) the interior wall coating has excellent fireproofness, and can achieve a fire resistance time of more than 60 min; (2) the interior wall coating can achieve an extremely-prominent layout effect, and the nanomaterial improves the fineness and fullness of a coating film; (3) with visible-light photocatalysis, the interior wall coating can achieve the stable and efficient degradation of harmful gases such as formaldehyde under visible light; (4) the interior wall coating has excellent stability, and can be stored for a time period as long as 1 to 2 years, which is much longer than the storage period of a steel structure fireproof coating
(only 3 months); and (5) the coating of the present disclosure is multifunctional, with functions of both visible-light photocatalysis and fireproofness, which broadens the application scope of photocatalytic technology and enables both health and safety for consumers. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic flow chart of the expansion process in an embodiment of the present disclosure. DETAILED DESCRIPTION The present disclosure will be further described below with reference to the accompanying drawing and specific embodiments. In the following Embodiments 1 to 3, the dispersing agent may be an ammonium salt dispersing agent; the emulsion may be a composite of styrene/acrylate polymer and elastic emulsion; the defoaming agent may consist of a mineral oil defoaming agent and a silicone defoaming agent; the biocide mildewcide may be MIT and BIT; the inorganic pigment/filler may be one of sericite powder, titanium dioxide, ultrafine calcined kaolin or washed kaolin, talc powder, and flake wollastonite, or a mixture of two or more thereof; the anti-settling agent may be a special modified ammonium polycarboxylate; and the film-forming additive may be plasticizer OE-400. In the following Embodiments 1 to 3 of the present disclosure, the visible-light-driven photocatalyst may be modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%; and the modified nano-TiO2 may be prepared by the following method: adding 40 mL of TBT dropwise to 300 mL of absolute ethanol, and subjecting a resulting mixture to ultrasonic stirring for 25 min to 35 min to obtain a solution 1; mixing 100 mL of absolute ethanol, 200 mL of deionized water, and 20 mL of a 6 mol/L nitric acid solution thoroughly to obtain a solution 2; slowly adding the solution 1 dropwise to the solution 2, and adding 100 g of diatomite after the dropwise addition is completed; stirring a resulting mixture with a magnetic stirrer for 3 h, and aging for 12 h; subjecting a resulting mixture to drying at 100°C to 110°C for 1 h and to calcination at 500°C to 550°C for 3 h to obtain the modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%. Embodiment 1 A fireproof interior wall coating with visible-light photocatalysis may be prepared by the following raw materials, in parts by weight: diatomite/nano-TiO2 composite material 2 parts styrene/acrylate polymer 16 parts crystalline phase II APP 15 parts melamine 5 parts pentaerythritol 5 parts titanium dioxide 14 parts mica powder 5 parts OE-400 1.3 parts silicone defoaming agent 0.2 part mineral oil defoaming agent 0.15 part hydroxyethylcellulose (HEC) 0.3 part MIT 0.2 part BIT 0.2 part special modified ammonium polycarboxylate 0.5 part bentonite 0.35 part water 26 parts 0.3 part of the HEC, 0.35 part of the bentonite, 0.5 part of the special modified ammonium polycarboxylate, 0.15 part of the mineral oil defoaming agent, and 26 parts of the water were added into a disperser and subjected to dispersion at 500 rpm for 5 min to make the solutes completely dissolved in water to form a dispersion system. 14 parts of titanium dioxide, 5 parts of mica powder and 25 parts of the IFR were added to the above dispersion system, and a resulting mixture was subjected to dispersion for 3 min at 1,500 rpm for complete dispersion to form a slurry. The rotational speed was adjusted to 1,900 rpm, 2 parts of the diatomite/nano-TiO2 composite material was slowly added to the above slurry, and a resulting mixture was subjected to dispersion at the rotational speed for 30 min. The rotational speed was then adjusted to 900 rpm, 16 parts of the emulsion, 0.2 part of the silicone defoaming agent, 0.2 part of the MIT, and 0.2 part of the BIT were added successively, and a resulting mixture was subjected to dispersion at the rotational speed for 5 min to obtain the fireproof coating with visible-light photocatalysis. Embodiment 2 diatomite/nano-TiO2 composite material 4 parts styrene/acrylate polymer 16 parts crystalline phase II APP 15 parts melamine 5 parts pentaerythritol 5 parts titanium dioxide 14 parts mica powder 4 parts 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (CS-12) 1.3 parts silicone defoaming agent 0.2 part mineral oil defoaming agent 0.15 part HEC 0.2 part MIT 0.2 part BIT 0.2 part special modified ammonium polycarboxylate 0.5 part bentonite 0.35 part water 26 parts 0.2 part of the HEC, 0.35 part of the bentonite, 0.5 part of the special modified ammonium polycarboxylate, 0.15 part of the mineral oil defoaming agent, and 26 parts of the water were added into a disperser and subjected to dispersion at 500 rpm for 5 min to make the solutes completely dissolved in water to form a dispersion system. 14 parts of titanium dioxide, 4 parts of mica powder and 25 parts of the IFR were added to the above dispersion system, and a resulting mixture was subjected to dispersion for 3 min at 1,500 rpm for complete dispersion to form a slurry. The rotational speed was adjusted to 1,900 rpm, 4 parts of the diatomite/nano-TiO2 composite material was slowly added to the above slurry, and a resulting mixture was subjected to dispersion at the rotational speed for 30 min. The rotational speed was then adjusted to 900 rpm, 16 parts of the emulsion, 0.2 part of the silicone defoaming agent, 0.2 part of the MIT, and 0.2 part of the BIT were added successively, and a resulting mixture was subjected to dispersion at the rotational speed for 5 min to obtain the fireproof coating with visible-light photocatalysis. Embodiment 3 diatomite/nano-TiO2 composite material 6 parts styrene/acrylate polymer 17 parts crystalline phase II APP 15 parts melamine 5 parts pentaerythritol 5 parts titanium dioxide 14 parts mica powder 4 parts CS-12 1.3 parts silicone defoaming agent 0.2 part mineral oil defoaming agent 0.15 part MIT 0.2 part BIT 0.2 part special modified ammonium polycarboxylate 0.5 part bentonite 0.35 part water 27 parts 0.35 part of the bentonite, 0.5 part of the special modified ammonium polycarboxylate, 0.15 part of the mineral oil defoaming agent, and 27 parts of the water were added into a disperser and subjected to dispersion at 500 rpm for 5 min to make the solutes completely dissolved in water to form a dispersion system. 14 parts of titanium dioxide, 4 parts of mica powder and 25 parts of the IFR were added to the above dispersion system, and a resulting mixture was subjected to dispersion for 3 min at 1,500 rpm for complete dispersion to form a slurry. The rotational speed was adjusted to 1,900 rpm, 6 parts of the diatomite/nano-TiO2 composite material was slowly added to the above slurry, and a resulting mixture was subjected to dispersion at the rotational speed for 30 min. The rotational speed was then adjusted to 900 rpm, 17 parts of the emulsion, 0.2 part of the silicone defoaming agent, 0.2 part of the MIT, and 0.2 part of the BIT were added successively, and a resulting mixture was subjected to dispersion at the rotational speed for 5 min to obtain the fireproof coating with visible-light photocatalysis. Performance tests: A fireproofness test was conducted for the fireproof interior wall coating with visible-light photocatalysis provided in the present disclosure as follows: Test method: each of the coatings prepared in Embodiments 1 to 3 was coated on a steel plate with a specification of 15 mm x 10 mm x 0.5 mm for three times using a 500 um wet-film applicator; resulting steel plates were burned at 450°C for 30 min in a muffle furnace after being completely dried; and the expansion ratio was tested. Fireproofness test results Blank group (without Embodiment 1 Embodiment 2 Embodiment 3 photocatalyst) Carbon layer Dense honeycomb Dense Dense Dense structure honeycomb honeycomb honeycomb Expansion 12 11 14 10 ratio/fold It can be seen from the above table that the coatings in Embodiments 1 to 3 all exhibit excellent fireproofness, and the photocatalyst poses no negative impact on the fireproofness of the coating. An air purification test was conducted for the fireproof interior wall coating with visible-light photocatalysis provided in the present disclosure as follows: Test method: A glass plate was coated with the coating all over the plate and then put into a 50 cm x 50 cm x 100 cm glass test box; 5 mL of formaldehyde was injected as a simulated catalytic substrate, and the box stood for 24 h; then samples were taken; and the formaldehyde degradation rate was determined for the samples using an MBTH spectrophotometer. Formaldehyde degradation rate Embodiment 1 Embodiment 2 Embodiment 3 Degradation rate 63.7% 78.9% 85.2% It can be seen from the above table that the coatings in Embodiments 1 to 3 all exhibit a formaldehyde removal rate > 63.7%, indicating an excellent photocatalytic air-purification ability. A stability test was conducted for the fireproof interior wall coating with visible-light photocatalysis provided in the present disclosure as follows: Test method: After the coatings of the embodiments were prepared, the samples were stored in a 55°C oven for 1 month and then stored at room temperature under shade for 1 year; and the samples were taken out and tested.
Sample stored in a Embodiment 1 Embodiment 2 Embodiment 3 55°C oven for 1 month Stratification None None None Precipitation None None None Layout effect Excellent Excellent Excellent Discoloration None None None Coating sample state Great Great Great
Sample stored at room Embodiment 1 Embodiment 2 Embodiment 3 temperature under shade for 1 year Stratification None None None Precipitation None None None Layout effect Excellent Excellent Excellent Discoloration None None None Coating sample state Great Great Great It can be seen from the above table that, after being placed for one year, the coatings in Embodiments 1 to 3 experience no stratification, no precipitation and no discoloration, have smooth touch, and remain in normal state. 1. A fireproof expansion process of the fireproof interior wall coating with visible-light photocatalysis provided in the present disclosure has the following reaction mechanism: (1) Charring mechanism In a charring flame retardant, inorganic acids react with polyhydric alcohols at a high temperature. The charring reaction of APP with pentaerythritol is adopted as an example, and the steps are as follows: a) The APP chain breaks at 210°C to generate phosphoester bonds; and pentaerythritol and APP are subjected to intramolecular dehydration to form ether bonds. b) As the temperature rises, the charring reaction continues, phosphoester bonds are completely broken, and an unsaturated carbon-rich structure is formed, which improves the fire resistance of the system. (2) Expansion reaction There are many closed cell structures in fireproof intumescent carbon layer that play an important role in fire prevention, and these structures are affected by the molar mass of gas generated from charring and the viscosity of the system. Therefore, the gas source must allow the gas release to match with the system charring. For the flame-retardant polymer system, the common mechanism of melamine is more complicated, where, melamine is progressively decomposed at high temperatures, that is, decomposition products at different temperatures are different. The reaction can lead to the production of ammonia gas. Melamine can also react with APP. The reaction is complete at about 650°C, and products can withstand a high temperature up to 950°C, exhibit a prominent comprehensive flame-retardant effect, and serve as both gas source and carbon source. (3) Influence of carbon layer structure The intumescent fireproof coating achieves the flame-retardant effect by forming an intumescent carbon layer, and the key is to reduce the thermal conductivity of a heat conduction system and to isolate the base material from oxygen. It can be concluded from above that the fireproof ability of the coating can be effectively improved only when honeycomb cells with an appropriate size are formed in the flame-retardant system. If the size is too large, the heat conduction ability of the flame-retardant system will be increased, the decomposition of the system will be accelerated, and the prevention of gas diffusion by the system is compromised, resulting in reduction of the flame-retardant performance. If the size is too small, the thickness of the intumescent carbon layer will be affected, resulting in reduction of the heat conduction distance and the flame-retardant ability of the material. A large number of studies show that the cell preferably has a diameter of 10 m to 50 m and a thickness of 1 m to 3 m. The intumescent carbon layer can prevent the combustible gas generated during the coating combustion from diffusing to the outside, and can also prevent the external combustible gas from contacting the surface of the substrate, so that the material will be extinguished because being unable to contact with combustibles, thereby reducing the combustion time. 2. Reaction mechanism of visible-light photocatalysis (with formaldehyde degradation as an example): Under visible-light irradiation, when a photon with an energy more than the band gap of nano-TiO2 irradiates the surface of the material, an electron in the valence band of nano-TiO2 will be excited to the conduction band, and a highly-active electron and hole are produced on the valence band and conduction band, respectively. The hole can oxidize the hydroxyl and water adhered to the surface of the material into •OH, the electron on the conduction band can reduce the oxygen adsorbed on the surface of the material into02-, and the •OH and •02-, with high oxidative activity, can oxidize formaldehyde into formic acid, which is eventually decomposed into water and carbon dioxide. HCHO +•OH -- CHO + H20 •CHO+•OH-* HCOOH •CHO +•02--- HCO3
HCO3-+ H+- HCOOOH
HCOOOH + HCHO -> HCOOH HCOOH + -H+ -HCOO HCOO- + -OH- H20 + C02 HCOO- + h+ --* H ++ C02 C02- + •OH + h+--*C02 The components in the present disclosure may preferably have the following parts by weight: visible-light-driven photocatalyst 2 to 6 parts emulsion 15 to 18 parts IFR 25 to 40 parts inorganic pigment/filler 25 to 28 parts film-forming additive 0.5 to 1.5 parts defoaming agent 0.35 to 0.5 part thickening agent 0.2 to 0.3 part biocide mildeweide 0.4 to 0.8 part dispersing agent 0.1 to 0.5 part water 25 to 27 parts anti-settling agent 0.3 to 0.5 part where, the visible-light-driven photocatalyst may be modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%; the IFR may be a mixture of an acid source, a carbon source and a gas source with a ratio of 3:1:1; the acid source may be any one of polyphosphate, (NH 4 ) 2 SO 4 , NH4Cl, and amine/amide phosphate; the carbon source may be any one of starch, dextrin, sorbitol, and pentaerythritol; and the gas source may be melamine. In the preparation method of the fireproof interior wall coating with visible-light photocatalysis of the present disclosure, the dispersion of materials in the disperser in step (1) may preferably be conducted for 3 min to 5 min at 300 rpm to 500 rpm; the dispersion after the IFR is added in step (2) may preferably be conducted for 2 min to 5 min at 900 rpm to 1,500 rpm; the dispersion after the visible-light-driven photocatalyst is added may preferably be conducted for 20 min to 40 min at 1,800 rpm to 2,000 rpm; and the dispersion after the emulsion, defoaming agent and biocide mildewcide are added may preferably be conducted for 5 min to 10 min at 800 rpm to 1,000 rpm. The present disclosure achieves the following advantages by adopting a mixture of APP, melamine, and pentaerythritol with a ratio of 3:1:1 as an IFR, selecting APP with a DP (n) > 20 and a particle size of 20 m to 30 m, and preparing modified nano-TiO2 with a grain size of 11.3 nm and a titanium dioxide mass load of 25.5%: (1) the interior wall coating has excellent fireproofness, and can achieve a fire resistance time of more than 60 min; (2) the interior wall coating can achieve an extremely-prominent layout effect, and the nanomaterial improves the fineness and fullness of a coating film; (3) with visible-light photocatalysis, the interior wall coating can achieve the stable and efficient degradation of harmful gases such as formaldehyde under visible light; (4) the interior wall coating has excellent stability, and can be stored for a time period as long as 1 to 2 years, which is much longer than the storage period of a steel structure fireproof coating (only 3 months); and (5) the coating of the present disclosure is multifunctional, with functions of both visible-light photocatalysis and fireproofness, which broadens the application scope of photocatalytic technology and enables both health and safety for consumers. Although the present disclosure is specifically illustrated and introduced in combination with preferred embodiments, those skilled in the art should understand that various changes may be made to the present disclosure in terms of forms and details without departing from the spirit and scope of the present disclosure defined in the appended claims, which shall fall within the protection scope of the present disclosure.