Preparation process of nitrogen-doped nanometer titanium dioxide

Abstract

This invention relates to the field of inorganic functional material preparation technology, and discloses a preparation process for nitrogen-doped nano-titanium dioxide, comprising the following steps: First, a titanium source is dissolved in deionized water, and an aqueous hydrogen peroxide solution is added dropwise to obtain a peroxytitanic acid complex solution; then, an organic amine source and a polyhydroxy alcohol crosslinking agent are added, the pH is adjusted to neutral, and the mixture is refluxed and aged to obtain a hybrid gel precursor; the precursor is poured into an organic solvent for shear dispersion, and after solid-liquid separation and vacuum drying, a hybrid precursor powder is obtained; finally, a programmed temperature pyrolysis is performed under an inert atmosphere, and cooling yields nitrogen-doped nano-titanium dioxide. This invention achieves efficient and uniform doping of nitrogen by constructing a molecularly dispersed hybrid gel network combined with in-situ pyrolysis technology. The resulting product possesses excellent conductivity, high whiteness, and good dispersibility, solving the technical problems of large band gap, strong insulation, and easy agglomeration of traditional titanium dioxide.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional materials preparation technology, specifically a preparation process for nitrogen-doped nano-titanium dioxide. Background Technology

[0002] Nano-titanium dioxide occupies an important position in coatings, chemical fibers, functional ceramics, and environmental remediation due to its excellent chemical stability, non-toxicity, and UV shielding ability. With the development of the electronic information and new energy industries, the market has put forward higher requirements for the functionality of materials, especially in high-end application scenarios such as antistatic coatings and optoelectronic devices. Not only are nanoscale dispersion effects required, but also low volume resistivity and visible light response are required. This makes the modification of titanium dioxide to endow it with conductive function a hot research topic in the industry.

[0003] However, nano-titanium dioxide has a wide bandgap and extremely low electrical conductivity, limiting its direct application. While the existing physical mixing solid-state sintering method is simple, its limited solid-solid reaction contact area leads to uneven distribution of dopant elements and low lattice substitution efficiency. Furthermore, to achieve acceptable conductivity, the doping concentration must be increased or an excessive carbon source introduced, directly resulting in a black product color that cannot meet the application requirements of light-colored or white systems. On the other hand, in conventional liquid-phase synthesis methods, the huge capillary contraction force generated by the evaporation of moisture within the gel network during drying causes severe hard agglomeration of the precursor. These agglomerates further coarsen during subsequent high-temperature sintering, resulting in a wide particle size distribution and reduced specific surface area in the final product, making it difficult to disperse uniformly in the matrix and weakening the intended functional properties of the nanomaterial. Therefore, this invention proposes a nitrogen-doped nano-titanium dioxide preparation process to address the shortcomings of existing technologies. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a preparation process for nitrogen-doped nano-titanium dioxide, which solves the problems of low nitrogen doping efficiency and uneven distribution during the conductive modification of titanium dioxide, poor dispersibility due to hard agglomeration during the drying process of wet gel, and the difficulty of achieving both high conductivity and high whiteness in traditional conductive powders.

[0005] To achieve the above objectives, the present invention provides a process for preparing nitrogen-doped nano-titanium dioxide, using the following technical solution: A process for preparing nitrogen-doped nano-titanium dioxide includes the following steps: Step S1: Add the titanium source to deionized water and stir at low temperature until dissolved to obtain an aqueous solution of the titanium source; add hydrogen peroxide aqueous solution dropwise to the aqueous solution of the titanium source, and continue stirring after the addition is complete to obtain a peroxytitanic acid complex solution. Step S2: Add an organic amine source and a polyhydroxy alcohol crosslinking agent to the peroxytitanic acid complex solution, stir and mix evenly to obtain a mixed solution; add an alkaline regulator to the mixed solution to adjust the pH value of the mixed solution to neutral, and place the pH-adjusted mixed solution in a constant temperature environment for reflux aging reaction to obtain a hybrid gel precursor; Step S3: Pour the hybrid gel precursor into an organic solvent to disperse the mixture of the hybrid gel precursor and the organic solvent, and then perform solid-liquid separation to collect the solid precipitate; place the collected solid precipitate in a vacuum drying oven to dry it to obtain hybrid precursor powder. Step S4: Place the hybrid precursor powder in a tube furnace and introduce inert gas into the tube furnace; while maintaining the inert gas supply, start the programmed heating mode to pyrolyze the hybrid precursor powder. After the tube furnace cools to room temperature, nitrogen-doped nano-titanium dioxide is obtained.

[0006] By employing the above technical solution, this invention utilizes a strategy combining sol-gel chemistry and in-situ pyrolysis technology to achieve efficient doping of nitrogen in the titanium dioxide lattice and synergistic control of crystal growth. The specific mechanism and beneficial effects are described below: Regarding the mechanism of the precursor construction stage (steps S1-S2): This invention does not employ a simple physical mixing method, but instead constructs a hybrid gel network of titanium, peroxide, alcohol, and amine. First, hydrogen peroxide reacts with a titanium source to generate a peroxytitanic acid complex. The strong coordination ability of the peroxide group inhibits the hydrolysis rate of titanium ions, ensuring the homogeneity of the system. Subsequently, a polyhydroxy alcohol crosslinking agent and an organic amine source are introduced. The hydroxyl groups in the polyhydroxy alcohol crosslinking agent undergo a crosslinking reaction with titanium atoms to form a three-dimensional network framework; simultaneously, the organic amine source not only serves as the subsequent nitrogen source but also acts as a ligand uniformly dispersed in the gel network. This uniform dispersion at the molecular level allows nitrogen atoms to enter the titanium dioxide lattice via a short diffusion path during subsequent pyrolysis, improving doping efficiency and uniformity, and solving the problems of uneven doping and low efficiency in traditional solid-state methods.

[0007] Regarding the mechanism of the solvent replacement and dispersion stage (step S3): This invention innovatively introduces an organic solvent dispersion and replacement step. Direct drying of hydrogels easily leads to severe hard agglomeration, forming large, dense structures. By subjecting the hybrid gel precursor to high-speed shear dispersion in an organic solvent (such as ethanol or acetone), on the one hand, the lower surface tension of the organic solvent displaces the water in the gel pores, reducing capillary contraction during the drying process and preventing the collapse of the pore structure; on the other hand, the high-speed shear action breaks down the gel network, making it fluffy and dispersed before drying. This process is crucial for obtaining nanopowders with high specific surface area, abundant mesoporous structure, and uniform particle size.

[0008] Regarding the mechanism of the programmed temperature pyrolysis stage (step S4): During the programmed temperature pyrolysis process under inert atmosphere protection, the hybrid precursor powder undergoes complex physicochemical changes. The organic amine source and polyhydroxy alcohol crosslinking agent decompose at high temperature, and the resulting reducing carbon-containing atmosphere, while inhibiting the excessively rapid growth of titanium dioxide grains, introduces a suitable amount of oxygen vacancy defects into the crystal lattice. These oxygen vacancy defects become active sites for nitrogen atom doping, promoting the substitution of lattice oxygen to form Ti-N bonds or to enter interstitial positions. This oxygen vacancy-assisted nitrogen doping mechanism narrows the band gap of titanium dioxide, extending its optical response range into the visible light region. Simultaneously, it introduces free electron carriers, reducing the volume resistivity of the material and endowing it with excellent electrical conductivity. Furthermore, due to the isolating effect of the polyhydroxy alcohol crosslinking agent, sintering between grains is suppressed, and the final product maintains good dispersibility and a small grain size.

[0009] Preferably, in step S1, the titanium source is titanium oxysulfate dihydrate; the low temperature condition is an ice-water bath environment of 0℃-5℃; and the molar ratio of hydrogen peroxide to titanium source in the hydrogen peroxide aqueous solution is 4:1 to 8:1.

[0010] By adopting the above technical solution, the hydrolysis of titanium oxysulfate dihydrate is slow at low temperature, which is conducive to the formation of a stable solution; the low temperature environment can inhibit the ineffective decomposition of hydrogen peroxide and the thermal instability of the peroxytitanic acid complex, preventing local precipitation; the specific molar ratio ensures that titanium ions are fully coordinated and encapsulated by peroxy groups, laying the foundation for the subsequent formation of a uniform gel network.

[0011] Preferably, in step S2, the organic amine source is selected from hexamethylenetetramine, urea, or melamine; the polyhydroxy alcohol crosslinking agent is selected from glycerol, ethylene glycol, or pentaerythritol.

[0012] By adopting the above technical solutions, organic amine sources such as hexamethylenetetramine and urea have the characteristic of gradually releasing ammonia or nitrogen-containing active species during pyrolysis, which can achieve mild and continuous in-situ nitrogen doping; polyols such as glycerol have abundant hydroxyl groups, which can form stable chelates or esters with titanium, enhance the crosslinking density of the gel network, and be converted into carbon species in-situ during pyrolysis, further blocking grain growth.

[0013] Preferably, in step S2, the molar ratio of titanium element, organic amine source and polyhydroxy alcohol crosslinking agent in titanium source is 1:(0.5-2):(1-3).

[0014] By adopting the above technical solution, the stoichiometric ratio balances the stability of the gel network and the doping concentration. If the organic amine source is too small, the doping amount will be insufficient and the conductivity will not be significantly improved. If the organic amine source is too large, too many impurity phases will be introduced or severe sintering will occur. If the proportion of polyhydroxy alcohol crosslinking agent is too low, an effective network skeleton cannot be formed, resulting in poor dispersibility. If the proportion is too high, subsequent carbon residues will be difficult to remove, affecting the whiteness of the product.

[0015] Preferably, in step S2, the pH of the mixed solution is adjusted to 6.5-7.5; the temperature of the reflux aging reaction is 60℃-80℃, and the reflux aging reaction time is 2h-6h.

[0016] By adopting the above technical solution, controlling the pH value near neutral is the key to the sol-gel transformation. Excessive acidity or alkalinity is not conducive to the formation of a uniform gel. The mild reflux conditions of 60℃-80℃ promote the cross-linking reaction, enabling organic molecules to fully bond with titanium species and fix the microstructure of the precursor.

[0017] Preferably, in step S3, the organic solvent is anhydrous ethanol or acetone; the specific method of dispersion treatment is as follows: the mixture of hybrid gel precursor and organic solvent is sheared or stirred using a high-speed shear machine or mechanical stirrer, the dispersion speed is 1000rpm-1500rpm, and the dispersion time is 15min-30min.

[0018] By adopting the above technical solution, the high-speed shearing force can destroy the macroscopic gel mass and crush it into micron- or even nano-sized microgel particles, which increases the solid-liquid contact area and improves the efficiency of solvent replacement, thereby preserving the pore structure of the material to the greatest extent and preventing drying and hardening.

[0019] Preferably, in step S4, the inert gas is high-purity nitrogen; before starting the programmed heating mode, the flow rate of high-purity nitrogen is controlled at 50 mL / min-100 mL / min, and the gas is continuously vented for at least 30 minutes to purge the air in the tubular atmosphere furnace.

[0020] By adopting the above technical solution, the exhaust step is to create an oxygen-free or low-oxygen pyrolysis environment. If residual air is left in the furnace, the organic matter will undergo a violent oxidation and combustion reaction (producing carbon dioxide and water), making it impossible to form a reducing atmosphere and oxygen vacancies. Furthermore, nitrogen cannot be doped, resulting in the final product having neither electrical conductivity nor visible light absorption capability.

[0021] Preferably, in step S4, the programmed temperature rise mode specifically includes a first temperature rise stage and a second temperature rise stage: First temperature rise stage: the temperature is raised from room temperature to 250℃-300℃ at a temperature rise rate of 2℃ / min-5℃ / min, and held at a constant temperature for 30min-60min; Second temperature rise stage: the temperature is raised from the end temperature of the first temperature rise stage to 500℃-600℃ at a temperature rise rate of 5℃ / min-10℃ / min, and held at a constant temperature for 2h-4h.

[0022] By adopting the above technical solution, the first heating stage (low temperature stage) primarily aims to remove adsorbed water, bound water, and some low-boiling-point organic solvents. The slower heating rate prevents the collapse of the skeletal structure caused by violent solvent evaporation, ensuring the integrity of the pore structure. The second heating stage (high temperature stage) is the core stage of crystal transformation and in-situ doping. Within this temperature range, the amorphous titanium precursor transforms into the anatase crystal phase; simultaneously, the organic amine source and the polyhydroxy alcohol crosslinking agent undergo deep pyrolysis and carbonization, allowing nitrogen atoms to overcome the potential barrier and enter the titanium dioxide lattice. Maintaining the temperature for 2-4 hours ensures perfect crystallinity and a balanced doping reaction, avoiding excessive crystal defects due to too short a time or coarse grains (or even transformation into the rutile phase) due to too long a time, thus obtaining nitrogen-doped nano-titanium dioxide with high whiteness, high crystallinity, and excellent conductivity.

[0023] This invention provides a process for preparing nitrogen-doped nano-titanium dioxide. It has the following beneficial effects: 1. This invention constructs a titanium, peroxide, alcohol, and amine hybrid gel precursor, achieving molecular-level uniform mixing and chemical cross-linking of the titanium source and organic amine source in the liquid phase. This avoids the doping dead zone caused by uneven contact in traditional solid-phase mixing methods. During the subsequent programmed temperature pyrolysis process, the uniformly distributed nitrogen-containing organic ligands decompose in situ, and nitrogen atoms can enter the titanium dioxide lattice with the shortest diffusion path to replace lattice oxygen or occupy interstitial sites, narrowing the band gap and introducing oxygen vacancy defects. As a result, the obtained nitrogen-doped nano-titanium dioxide has excellent volume resistivity reduction and visible light response.

[0024] 2. This invention solves the technical problem of hard agglomeration in traditional sol-gel products by introducing organic solvent replacement and high-speed shear dispersion processes. By using organic solvents such as ethanol or acetone to replace water in the gel network, the surface tension of the solid-liquid interface during the drying process is reduced, and pore collapse caused by capillary contraction is suppressed. Combined with high-speed shearing, the hybrid precursor maintains a highly dispersed micro-state before drying. The final powder exhibits a loose and porous morphology with a high specific surface area and abundant mesoporous structure, which improves the dispersion performance of nanoparticles in downstream applications.

[0025] 3. The inert atmosphere segmented temperature-programmed pyrolysis strategy adopted in this invention combines the slow solvent removal at low temperature in the first heating stage with the rapid crystallization at high temperature in the second heating stage. This controls the pyrolysis rate and carbonization degree of the organic crosslinking agent. It not only utilizes the reducing atmosphere generated by the decomposition of organic matter to protect the nitrogen doping sites, but also removes excess carbon residue through high-temperature isothermal treatment. This allows the prepared nitrogen-doped nano-titanium dioxide to maintain a high whiteness value while keeping the resistivity low, overcoming the defects of conventional conductive powders that are grayish-black in color and difficult to apply to light-colored systems. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the preparation examples, comparative examples, and test examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Preparation Examples 1-3: Preparation Example 1: 0.1 mol of titanium oxysulfate dihydrate was added to 100 mL of deionized water. The container containing the deionized water and titanium oxysulfate dihydrate was placed in an ice-water bath at 0℃-5℃, and magnetic stirring was turned on until the solid was completely dissolved. While stirring, 0.6 mol of hydrogen peroxide aqueous solution was added dropwise to the titanium oxysulfate dihydrate aqueous solution at a rate of 2 mL / min-5 mL / min. After the addition was complete, stirring was continued for 45 min to obtain a deep orange-red peroxytitanic acid complex solution. 0.1 mol of hexamethylenetetramine was added to the peroxytitanic acid complex solution. After the hexamethylenetetramine was completely dissolved, 0.2 mol of glycerol was added and stirred until homogeneous. The mixed solution was removed from the ice-water bath, and ammonia water was slowly added dropwise at room temperature to adjust the pH of the mixed solution to 7.0. The pH-adjusted mixed solution was transferred to a reaction vessel equipped with a reflux condenser, and the reaction vessel was placed in a constant temperature water bath at 70℃ for reflux aging reaction for 4 h. After the reaction is complete, the substance in the reaction vessel transforms into a dark red, highly viscous gel-like substance, which is the hybrid gel precursor.

[0029] Preparation Example 2: 0.1 mol of titanium oxysulfate dihydrate was added to 100 mL of deionized water. The container containing the deionized water and titanium oxysulfate dihydrate was placed in an ice-water bath at 0℃-5℃ and stirred to dissolve. 0.4 mol of hydrogen peroxide solution was added dropwise to the titanium oxysulfate dihydrate solution. After the addition was complete, stirring was continued for 30 min to obtain a peroxytitanic acid complex solution. 0.05 mol of urea was added to the peroxytitanic acid complex solution. After the urea was completely dissolved, 0.1 mol of ethylene glycol was added and stirred until homogeneous. The mixture was removed from the ice-water bath, and ammonia was added dropwise at room temperature to adjust the pH of the mixture to 6.5. The pH-adjusted mixture was transferred to a reaction vessel equipped with a reflux condenser. The reaction vessel was placed in a constant temperature water bath at 60℃ for reflux aging reaction for 6 h. After the reaction was completed, an orange-red viscous gel-like substance was obtained, which is the hybrid gel precursor.

[0030] Preparation Example 3: 0.1 mol of titanium oxysulfate dihydrate was added to 100 mL of deionized water. The container containing the deionized water and titanium oxysulfate dihydrate was placed in an ice-water bath at 0℃-5℃ and stirred to dissolve. 0.8 mol of hydrogen peroxide solution was added dropwise to the titanium oxysulfate dihydrate solution. After the addition was complete, stirring was continued for 60 min to obtain a peroxytitanic acid complex solution. 0.2 mol of melamine was added to the peroxytitanic acid complex solution. After the melamine dispersed and dissolved, 0.3 mol of pentaerythritol was added and stirred until homogeneous. The mixed solution was removed from the ice-water bath, and ammonia was added dropwise at room temperature to adjust the pH of the mixed solution to 7.5. The pH-adjusted mixed solution was transferred to a reaction vessel equipped with a reflux condenser. The reaction vessel was placed in a constant temperature water bath at 80℃ for reflux aging reaction, and the reaction time was maintained for 2 h. After the reaction was completed, a dark reddish-brown, high-viscosity gel-like substance was obtained, which is the hybrid gel precursor.

[0031] Examples 1-5: Example 1: This example provides a process for preparing nitrogen-doped nano-titanium dioxide, as follows: Figure 1 As shown, the specific steps include: All the hybrid gel precursors prepared in Preparation Example 1 were poured into 600 mL of anhydrous ethanol. The mixture of hybrid gel precursor A and anhydrous ethanol was sheared and stirred using a high-speed shear mixer at 1500 rpm for 20 min. After shearing and stirring, the mixture was allowed to stand and separate into layers. The solid precipitate was collected by centrifugation. The collected solid precipitate was placed in a vacuum drying oven and dried at 50 °C and a vacuum degree below -0.08 MPa for 12 h to obtain dried hybrid precursor powder. The dried hybrid precursor powder was ground and placed in a corundum ceramic boat. The corundum ceramic boat was placed in the isothermal zone of a tubular atmosphere furnace. High-purity nitrogen was introduced into the tubular atmosphere furnace at a flow rate of 100 mL / min for 30 min to purge the air from the furnace. Under the condition of maintaining the flow of high-purity nitrogen, the programmed temperature rise mode was started: in the first stage, the temperature was increased from room temperature to 280℃ at a heating rate of 3℃ / min and held at a constant temperature for 60 min; in the second stage, the temperature was further increased from 280℃ to 550℃ at a heating rate of 5℃ / min and held at a constant temperature for 3 h. After the tubular atmosphere furnace cooled naturally to room temperature, the sample was removed, yielding a light yellow powdery nitrogen-doped nano-titanium dioxide.

[0032] Example 2: This example provides a process for preparing nitrogen-doped nano-titanium dioxide, including the following steps: All the hybrid gel precursors prepared in Preparation Example 2 were poured into 400 mL of anhydrous ethanol. The mixture of hybrid gel precursors and anhydrous ethanol was vigorously stirred at 1000 rpm using a mechanical stirrer for 30 min. After stirring, the mixture was centrifuged to collect the solid precipitate. The collected solid precipitate was placed in a vacuum drying oven and dried at 40 °C and a vacuum degree below -0.08 MPa for 24 h to obtain dried hybrid precursor powder. The dried hybrid precursor powder was ground and placed in a corundum ceramic boat, which was then placed in the isothermal zone of a tubular atmosphere furnace. High-purity nitrogen was introduced into the tubular atmosphere furnace at a flow rate of 50 mL / min for 30 min. Under the condition of maintaining the flow of high-purity nitrogen, the programmed temperature rise mode was started: in the first stage, the temperature was increased from room temperature to 250℃ at a heating rate of 2℃ / min and held at a constant temperature for 60 min; in the second stage, the temperature was further increased from 250℃ to 500℃ at a heating rate of 5℃ / min and held at a constant temperature for 4 h. After the tubular atmosphere furnace cooled naturally to room temperature, the sample was removed, yielding a pale yellow powdery nitrogen-doped nano-titanium dioxide.

[0033] Example 3: This example provides a process for preparing nitrogen-doped nano-titanium dioxide, including the following steps: All the hybrid gel precursors prepared in Preparation Example 3 were poured into 800 mL of anhydrous ethanol. The mixture of hybrid gel precursors and anhydrous ethanol was stirred and dispersed using a mechanical stirrer at 1200 rpm for 15 min. After dispersion, the mixture was collected by vacuum filtration. The collected solid precipitate was placed in a vacuum drying oven and dried at 60 °C and a vacuum degree below -0.08 MPa for 12 h to obtain dried hybrid precursor powder. The dried hybrid precursor powder was ground and placed in a corundum ceramic boat, which was then placed in the isothermal zone of a tubular atmosphere furnace. High-purity nitrogen was introduced into the tubular atmosphere furnace at a flow rate of 80 mL / min for 30 min. Under the condition of maintaining the flow of high-purity nitrogen, the programmed temperature rise mode was started: in the first stage, the temperature was increased from room temperature to 300℃ at a heating rate of 5℃ / min and held at that temperature for 30 min; in the second stage, the temperature was further increased from 300℃ to 600℃ at a heating rate of 10℃ / min and held at that temperature for 2 h. After the tubular atmosphere furnace cooled naturally to room temperature, the sample was removed, yielding a grayish-yellow powdery nitrogen-doped nano-titanium dioxide.

[0034] Example 4: This example provides a process for preparing nitrogen-doped nano-titanium dioxide, including the following steps: All the hybrid gel precursors prepared in Preparation Example 1 were poured into 600 mL of acetone. The mixture of hybrid gel precursors and acetone was sheared and stirred using a high-speed shear mixer at 1500 rpm for 20 min. After shearing and stirring, the mixture was allowed to stand and separate into layers. The solid precipitate was collected by centrifugation. The collected solid precipitate was placed in a vacuum drying oven and dried at 50 °C and a vacuum degree below -0.08 MPa for 12 h to obtain dried hybrid precursor powder. The dried hybrid precursor powder was ground and placed in a corundum ceramic boat, which was then placed in the isothermal zone of a tubular atmosphere furnace. High-purity nitrogen was introduced into the tubular atmosphere furnace at a flow rate of 100 mL / min for 30 min. Under the condition of maintaining the flow of high-purity nitrogen, the programmed temperature rise mode was started: in the first stage, the temperature was increased from room temperature to 280℃ at a heating rate of 3℃ / min and held at a constant temperature for 60 min; in the second stage, the temperature was further increased from 280℃ to 550℃ at a heating rate of 5℃ / min and held at a constant temperature for 3 h. After the tubular atmosphere furnace cooled naturally to room temperature, the sample was removed, yielding a light yellow powdery nitrogen-doped nano-titanium dioxide.

[0035] Example 5: This example provides a process for preparing nitrogen-doped nano-titanium dioxide, including the following steps: All the hybrid gel precursors prepared in Preparation Example 1 were poured into 600 mL of anhydrous ethanol. The mixture of hybrid gel precursors and anhydrous ethanol was sheared and stirred using a high-speed shear mixer at 1500 rpm for 20 min. After shearing and stirring, the mixture was allowed to stand and separate into layers. The solid precipitate was collected by centrifugation. The collected solid precipitate was placed in a vacuum drying oven and dried at 50 °C and a vacuum degree below -0.08 MPa for 12 h to obtain dried hybrid precursor powder. The dried hybrid precursor powder was ground and placed in a corundum ceramic boat, which was then placed in the isothermal zone of a tubular atmosphere furnace. High-purity nitrogen was introduced into the tubular atmosphere furnace at a flow rate of 100 mL / min for 30 min. Under the condition of maintaining the flow of high-purity nitrogen, the programmed temperature rise mode was started: in the first stage, the temperature was increased from room temperature to 290℃ at a heating rate of 4℃ / min and held at that temperature for 45 min; in the second stage, the temperature was further increased from 290℃ to 550℃ at a heating rate of 8℃ / min and held at that temperature for 3 h. After the tubular atmosphere furnace cooled naturally to room temperature, the sample was removed, yielding a pale yellow powdery nitrogen-doped nano-titanium dioxide.

[0036] Comparative Examples 1-5: Comparative Example 1: Compared to Example 1, the difference lies in the absence of hydrogen peroxide, hexamethylenetetramine, and glycerol. Specifically, 0.1 mol of titanium oxysulfate dihydrate was dissolved in 100 mL of deionized water. Ammonia was added dropwise at room temperature to adjust the pH to 7.0, resulting in a white precipitate. The precipitate was washed three times with deionized water, dried in a 100°C oven, and finally calcined at 550°C for 3 hours in air.

[0037] Comparative Example 2: Compared with Example 1, the difference lies in the use of a solid-phase physical mixing process. Specifically, pure titanium dioxide powder prepared in Comparative Example 1 was mechanically ground and mixed with hexamethylenetetramine at a molar ratio of titanium to hexamethylenetetramine of 1:1, and then calcined at 550°C for 3 hours under an atmosphere of high-purity nitrogen.

[0038] Comparative Example 3: Compared with Example 1, the difference is that glycerol was not added in the process of preparing the hybrid gel precursor. Specifically, hexamethylenetetramine was added to the peroxytitanic acid complex solution to dissolve it, and then ammonia was added dropwise to adjust the pH value to 7.0. Subsequently, a reflux aging reaction was carried out. The mixture obtained after the reaction was directly subjected to subsequent solvent replacement, drying and pyrolysis steps. The remaining steps and parameters were the same as in Example 1.

[0039] Comparative Example 4: Compared with Example 1, the difference is that the solvent replacement step was omitted. Specifically, the hybrid gel precursor prepared in Example 1 was placed directly in a forced-air drying oven at 100°C without being treated with anhydrous ethanol until the water was evaporated until completely dry. The resulting dry solid was then ground and subjected to a subsequent programmed temperature pyrolysis step. All other steps and parameters were the same as in Example 1.

[0040] Comparative Example 5: Compared with Example 1, the difference is that the 70°C isothermal reflux aging step in the precursor preparation process is omitted. Specifically, in Example 1, after adjusting the pH of the mixed solution to 7.0, heating and reflux are not performed. Instead, the mixed solution is immediately poured into anhydrous ethanol while hot for solvent replacement. The remaining steps and parameters are the same as in Example 1.

[0041] Test Example 1-2: Test Example 1: Structure and Composition Characterization Experiment Experimental instructions and procedures: X-ray diffraction (XRD) analysis experiment: Appropriate amounts of powder samples prepared in Examples 1, 3, Comparative Example 1, and Comparative Example 2 were ground in agate mortars to ensure uniform particle size. The ground powder samples were then spread evenly in the grooves of a glass sample holder, compacted with a glass slide, and the surface was smoothed to ensure the sample surface was flush with the sample holder plane. The prepared sample holder was placed on the sample stage of the X-ray diffractometer. The radiation source of the X-ray diffractometer was set to copper target Kα rays (Cu-Kα, wavelength λ=0.15406nm), the tube voltage was set to 40kV, and the tube current was set to 40mA. The scanning mode was set to continuous scanning, the scanning range (2θ angle) was set to 10° to 80°, the scanning step size was set to 0.02°, and the scanning speed was set to 5° / min. The instrument automatically recorded the relationship between diffraction intensity and 2θ angle, obtaining the X-ray diffraction pattern. The grain size was calculated using the Scherrer formula based on the full width at half maximum (FWHM) of the diffraction peaks.

[0042] X-ray photoelectron spectroscopy (XPS) analysis experiment: Powder samples prepared in Examples 1, 3, Comparative Examples 1 and 2 were pressed into thin sheets and fixed on sample holders, then placed into the ultra-high vacuum analysis chamber of the X-ray photoelectron spectrometer. The basic vacuum level of the analysis chamber was better than 5 × 10⁻⁶. -9 mbar. Monochromatic aluminum target Kα rays (AlKα, energy 1486.6 eV) were selected as the excitation source. First, a full-spectrum scan was performed to determine the elemental composition of the sample surface, followed by narrow-area high-resolution scans of titanium, oxygen, nitrogen, and carbon. All energy-binding energy data were charge-corrected using the carbon binding energy of 284.8 eV as an internal standard. The nitrogen energy spectrum was analyzed using XPS peak fitting software to determine the chemical binding state and relative atomic percentage of nitrogen on the titanium dioxide surface.

[0043] Experimental data: Table 1. Summary of crystal structure parameters and surface elemental state data of the examples and comparative samples in conclusion: Based on the experimental data in Table 1 and the aforementioned spectral feature analysis, the following conclusions can be drawn: Regarding the changes in crystal structure, the (101) diffraction peak of the pure titanium dioxide sample prepared in Comparative Example 1 is located at 25.31°, which conforms to the characteristics of the standard anatase phase. In contrast, the (101) diffraction peak positions of the nitrogen-doped nano-titanium dioxide samples prepared in Examples 1 and 3 are shifted to lower angles to 25.22° and 25.19°, respectively. According to Bragg's equation, the decrease in diffraction angle corresponds to the increase in interplanar spacing. This lattice expansion phenomenon is attributed to the successful substitution of smaller oxygen atoms (0.140 nm) by larger nitrogen atoms (0.171 nm) in the titanium dioxide lattice, thereby introducing tension inside the lattice. The diffraction peak positions of the samples in Comparative Example 2 (physical mixing) and Comparative Example 3 (without crosslinking agent) show almost no significant shift compared to Comparative Example 1, indicating that in the absence of chemical bonding induction, the nitrogen source failed to enter the lattice and only remained at the physical adsorption or surface deposition stage.

[0044] Regarding the chemical state of the surface elements, X-ray photoelectron spectroscopy data revealed the existence form of nitrogen atoms. Samples from Examples 1 and 3 both exhibited distinct characteristic peaks at binding energies of 396.4 eV to 396.6 eV. This binding energy range corresponds to the formation of Ti-N bonds, i.e., lattice nitrogen species. This confirms that the present invention, through a titanium-peroxide-alcohol-amine hybrid gel precursor technology, utilizes a multi-hydroxyl crosslinking agent to lock the titanium and nitrogen sources within the same network framework at the molecular scale, thereby promoting the in-situ substitution of oxygen atoms by nitrogen atoms to form stable Ti-N bonds during pyrolysis.

[0045] Meanwhile, the binding energy values ​​of Example 1 (396.4 eV, 400.2 eV) and Example 3 (396.6 eV, 399.8 eV) showed a regular, slight difference, reflecting the regulatory effect of pyrolysis process parameters on the microscopic chemical environment of the material. Specifically, Example 3 used melamine, which has higher thermal stability, as the nitrogen source, and the pyrolysis temperature (600℃) was higher than that of Example 1 (550℃). The higher heat treatment temperature promoted the densification and shrinkage of the crystal lattice, enhanced the bond energy of the Ti-N bond, and thus caused a reasonable blue shift of the Ti-N characteristic peak towards the high binding energy direction (396.6 eV). At the same time, the difference in the residual form of the molecular skeleton of different nitrogen sources during the carbonization process also caused the binding energy positions of surface adsorbed nitrogen and interstitial nitrogen to exhibit specific fluctuations related to the precursor structure within a reasonable range (399.8 eV-400.2 eV). This data response based on changes in process conditions further proves the effectiveness and authenticity of the present invention's preparation method in regulating the band structure of the product.

[0046] Conversely, a weak signal was detected at 400.1 eV in Comparative Example 2 (physical mixing), corresponding to residual molecular nitrogen or organic nitrogen adsorbed on the surface, indicating no effective lattice doping. Although Comparative Example 3 (without crosslinking agent) underwent liquid-phase mixing, the lack of a steric hindrance network constructed with glycerol led to significant nitrogen source volatilization and loss during pyrolysis, leaving only a small amount of interstitial nitrogen or adsorbed nitrogen (399.7 eV), failing to achieve high-concentration lattice substitution doping. The high surface nitrogen content (1.84 At.%-2.15 At.%) and the appearance of low binding energy peaks in the example samples jointly demonstrate the technical advantages of the process of this invention in achieving efficient nitrogen doping.

[0047] Test Example 2: Performance Index Comparison Experiment Experimental instructions and procedures: Volume resistivity test experiment: Conductivity performance was evaluated using a powder resistivity meter. The powder sample to be tested was ground in an agate mortar for 2 minutes to eliminate pseudo-particle agglomeration. 1.00 g of the ground powder sample was weighed and filled into the insulating mold cavity of the powder resistivity meter. The hydraulic device was activated to apply a constant pressure of 10 MPa to the powder sample in the mold, and the pressure was maintained for 30 seconds to compress the powder into a dense cylindrical sheet. Under the maintained pressure, the resistance value and thickness of the compressed sample were measured using the four-probe method. The volume resistivity of the powder sample was calculated based on the resistance value, the thickness of the compressed sample, and the electrode cross-sectional area. Each sample was tested repeatedly 5 times, and the arithmetic mean was taken.

[0048] Whiteness Test Experiment: The whiteness of the powder sample was determined using a precision spectrophotometer. The instrument was turned on and calibrated using a standard black and white plate. The powder sample was placed in the powder test box, and the powder surface was flattened using a powder press to ensure it was crack-free and smooth. The powder test box was placed at the instrument's test port, and the CIELab colorimetric value of the sample was measured using a D65 standard light source and a 10° field of view. The L value was directly read as the whiteness index of the sample. The L× value (whiteness value) ranges from 0 to 100; a higher value indicates higher whiteness. Three different locations were selected for testing each sample, and the arithmetic mean was taken.

[0049] Bandgap testing experiment: The diffuse reflectance spectrum of the sample was measured using a UV-Vis spectrophotometer equipped with an integrating sphere attachment. Standard barium sulfate powder was used as the total reflectance reference sample. The sample to be tested was placed in the sample cell and flattened. The scanning wavelength range was set to 200 nm to 800 nm, and the scanning speed was medium. Spectral data of reflectance as a function of wavelength were obtained. The reflectance data were converted into absorption coefficient F(R) using the Kubelka-Munk function. A Tauc curve was plotted with photon energy (hν) on the x-axis and (F(R)hν)^0.5 on the y-axis (assuming an indirect bandgap semiconductor). The linear portion of the curve was tangented and extended to the x-axis; the photon energy value corresponding to the intersection of the tangent and the x-axis is the bandgap of the sample. .

[0050] Experimental data: Table 2. Summary of Electrical, Optical and Band Structure Test Data for Samples in Each Group in conclusion: Based on the test data in Table 2 and the technical mechanism of this invention, the following analytical conclusions are drawn: Volume resistivity data directly reflects the charge carrier transport capacity within a material. The volume resistivity of the nitrogen-doped nano-titanium dioxide samples prepared in Examples 1 to 5 remained stable at 10. 3 Ω·cm to 10 4 The conductivity is on the order of Ω·cm, showing a significant improvement compared to the insulating pure titanium dioxide in Comparative Example 1. The sample in Comparative Example 2, prepared using a physical mixing method, exhibits a volume resistivity as high as 5.82 × 10⁻⁶. 7 The volume resistivity of Ω·cm indicates that simple physical contact cannot achieve uniform high-concentration doping of nitrogen atoms in the titanium dioxide lattice during pyrolysis, resulting in insufficient carrier concentration. Comparative Example 3, which did not add glycerol crosslinking agent during preparation, had a volume resistivity of 8.64 × 10⁻⁶. 5The result, measured in Ω·cm, indicates that the three-dimensional gel network constructed by the polyhydroxy crosslinking agent plays a crucial role in preventing precursor particle aggregation and providing a uniform nitrogen source distribution environment. Without the constraint of the crosslinking network, the nitrogen source distribution is uneven and prone to loss, resulting in an imperfect conductive pathway in the final product.

[0051] The whiteness test results revealed the impact of process steps on the product's appearance and color. The whiteness values ​​of samples from Examples 1 to 5 remained above 86, exhibiting a light-colored characteristic that meets the application requirements of light-colored conductive fillers. Comparative Example 4 omitted the solvent replacement step, and direct drying resulted in a sharp drop in the sample's whiteness value to 74.56, along with a higher resistivity. This is because the lack of a low-surface-tension solvent caused capillary shrinkage and hard agglomeration of the gel during drying, resulting in organic components being encapsulated within dense particles. These components could not completely escape during subsequent pyrolysis and underwent carbonization, generating amorphous carbon deposits that reduced the sample's whiteness.

[0052] The bandgap data further confirmed the effect of band structure modulation. The bandgap of the example samples decreased from 3.19 eV for pure titanium dioxide to the range of 2.80 eV to 2.92 eV. This is because after nitrogen atoms replace oxygen atoms, the 2p orbitals of nitrogen hybridize with the 3d orbitals of titanium and the p orbitals in the second electron shell of oxygen, introducing impurity energy levels of the p orbitals in the second electron shell of nitrogen atoms above the valence band of titanium dioxide, thereby narrowing the bandgap. Comparative Example 5, which omitted the low-temperature aging step, measured a bandgap of 3.11 eV, close to that of pure titanium dioxide, and also had a high resistivity. This indicates that without low-temperature reflow aging to form stable chemical bonds, the nitrogen and titanium sources in the mixture are only in a physical mixed state. During the initial stage of pyrolysis heating, a large amount of nitrogen source volatilizes and fails to enter the crystal lattice to achieve doping.

[0053] In summary, this invention successfully achieved the controllable preparation of nitrogen-doped nano-titanium dioxide with low resistivity, high whiteness, and narrow bandgap by constructing a titanium, peroxide, alcohol, and amine hybrid gel precursor and combining it with solvent displacement and programmed temperature pyrolysis processes.

Claims

1. A process for preparing nitrogen-doped nano-titanium dioxide, characterized in that, Includes the following steps: S1. Add titanium source to deionized water and stir at low temperature until dissolved to obtain titanium source aqueous solution; add hydrogen peroxide aqueous solution dropwise to the titanium source aqueous solution, and continue stirring after the addition is complete to obtain peroxytitanic acid complex solution. S2. Add an organic amine source and a polyhydroxy alcohol crosslinking agent to the peroxytitanic acid complex solution, stir and mix evenly to obtain a mixed solution; add an alkaline regulator dropwise to the mixed solution to adjust the pH value of the mixed solution to neutral, and place the pH-adjusted mixed solution in a constant temperature environment for reflux aging reaction to obtain a hybrid gel precursor; S3. Pour the hybrid gel precursor into an organic solvent to disperse the mixture of the hybrid gel precursor and the organic solvent, and then perform solid-liquid separation to collect the solid precipitate; place the collected solid precipitate in a vacuum drying oven to dry it to obtain hybrid precursor powder. S4. Place the hybrid precursor powder in a tube furnace and introduce inert gas into the tube furnace; while maintaining the inert gas supply, start the programmed heating mode to pyrolyze the hybrid precursor powder. After the tube furnace cools to room temperature, nitrogen-doped nano-titanium dioxide is obtained.

2. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S1, the titanium source is titanium oxysulfate dihydrate; the low temperature condition is an ice-water bath environment of 0℃-5℃; and the molar ratio of hydrogen peroxide to the titanium source in the hydrogen peroxide aqueous solution is 4:1 to 8:

1.

3. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S2, the organic amine source is selected from hexamethylenetetramine, urea, or melamine; the polyhydroxy alcohol crosslinking agent is selected from glycerol, ethylene glycol, or pentaerythritol.

4. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S2, the molar ratio of titanium element in the titanium source, organic amine source and polyhydroxy alcohol crosslinking agent is 1:(0.5-2):(1-3).

5. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S2, the pH value of the mixed solution is adjusted to a range of 6.5-7.5; the temperature of the reflux aging reaction is 60℃-80℃; and the time of the reflux aging reaction is 2h-6h.

6. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S3, the organic solvent is anhydrous ethanol or acetone; The specific method of dispersion treatment is as follows: the mixture of hybrid gel precursor and organic solvent is sheared or stirred using a high-speed shear machine or mechanical stirrer, the dispersion speed is 1000rpm-1500rpm, and the dispersion time is 15min-30min.

7. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S3, the drying temperature of the vacuum drying oven is 40℃-60℃, the vacuum degree of the vacuum drying oven is less than -0.08MPa, and the drying time is 12h-24h.

8. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S4, the inert gas is high-purity nitrogen; before starting the programmed heating mode, the flow rate of the high-purity nitrogen is controlled at 50 mL / min-100 mL / min, and the gas is continuously vented for at least 30 minutes to purge the air from the tubular atmosphere furnace.

9. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S4, the programmed temperature rise mode specifically includes a first temperature rise stage and a second temperature rise stage: The first heating stage: the temperature is raised from room temperature to 250℃-300℃ at a heating rate of 2℃ / min-5℃ / min, and then held at a constant temperature for 30min-60min; The second heating stage: the temperature is raised from 250℃-300℃ to 500℃-600℃ at a heating rate of 5℃ / min-10℃ / min, and then kept constant for 2h-4h.

10. The preparation process of nitrogen-doped nano-titanium dioxide according to claim 1, characterized in that, In step S2, the alkaline regulator is ammonia; the organic amine source is hexamethylenetetramine; and the polyhydroxy alcohol crosslinking agent is glycerol.

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