Study on the Preparation and Photocatalytic Performance of TiO2 Nanotube Films on the Surface of Nickel-Doped Porous Titanium

1. Introduction

With the advancement of urbanization and industrialization, environmental pollution is increasing day by day. In the process of sewage discharge, industrial wastewater usually needs to be treated; however, pollutants in industrial wastewater continue to affect our environment due to their complex molecular structure, stability, and refractory characteristics. As a traditional photocatalytic material, TiO₂, when irradiated by ultraviolet (UV) light with a wavelength less than 387 nm, undergoes valence band electron transition to the conduction band, forming photogenerated electron-hole pairs. The holes react with surface-adsorbed water to generate hydroxyl radicals, while the electrons combine with oxygen to form superoxide radicals¹. These two highly oxidizing species can decompose over 90% of common organic pollutants.

TiO₂ has a wide band gap, which allows it to absorb only UV light with a wavelength below 400 nm, limiting its activity to the UV region². In addition to the wide band gap, photogenerated electron-hole pairs are prone to recombination. However, nickel doping can precisely solve these two key issues, while synergizing with the porous titanium-based nanotube structure to achieve a performance leap. Nickel doping can narrow the band gap of TiO₂: Ni²⁺, as an impurity atom, introduces new impurity energy levels in the band gap of TiO₂, reducing the energy required for photogenerated electron transition². This expands the material's applicable range from the UV region to the visible light region, significantly improving the solar energy capture efficiency. Nickel doping can inhibit the recombination of photogenerated carriers: Ni²⁺ can act as a "carrier trapping center" to selectively capture photogenerated electrons on the TiO₂ surface, preventing rapid combination of electrons and holes². Meanwhile, the energy level difference between Ni²⁺ and TiO₂ can guide carriers to migrate to the material surface, enabling more active carriers to participate in catalytic reactions such as pollutant degradation and water splitting, directly improving the reaction efficiency².

Therefore, in this work, nickel-doped porous titanium was used as the substrate, and Ni_x-TiO₂ nanotubes were in-situ prepared on its surface via a hydrothermal method. Composite catalysts with different compositions were obtained by controlling the mass fraction of nickel powder. The effects of nickel content on the photocatalytic activity of the catalyst for methylene blue (MB) degradation and the stability of the catalyst were investigated. The influence of calcination temperature on its crystal phase was explored, and its structure, morphology, and optical properties were characterized. Finally, the photocatalytic reaction mechanism was proposed.

2. Experiments

2.1. Preparation of Porous Titanium Doped with Different Nickel Contents

First, irregular pure titanium powder (purity > 99.5%, particle size < 50 μm) and nickel powder (purity > 99.5%, particle size < 50 μm) were weighed in a certain proportion. The mass fractions of nickel in this experiment were 2.5 wt.%, 5 wt.%, 7.5 wt.% and 10 wt.%. Subsequently, the weighed titanium powder and nickel powder were placed into a omnidirectional planetary ball mill PM-QW4 for uniform mixing. The ball milling parameters were as follows: stainless steel balls were used as the milling medium, with a ball-to-powder ratio of 10:1 and a milling speed of 150 r/min. The program was set to "mill for 0.5 h and stop for 0.5 h, with a forward-reverse rotation switching frequency of once per hour", and the total milling time was 6 h. Since titanium is a highly active and easily oxidizable metal material, the stainless steel milling jar was filled with high-purity argon before the start of ball milling.

After obtaining the uniformly mixed titanium-nickel powder, spark plasma sintering (SPS) was adopted². Then, the mixed powder was blended with 60 wt.% NaCl (used as a pore-forming agent) in proportion to prepare porous titanium with a unique pore structure. Next, the mixed powder was loaded into a graphite mold and placed into a spark plasma sintering apparatus for sintering. The sintering parameters were set as follows: the pressure was 30 MPa, the sintering temperature was 600 ℃, and the heating rate was controlled at 100 ℃ per minute. When the temperature reached the specified value, it was held for 10 minutes, followed by furnace cooling, and finally the sample was obtained.

2.2. Preparation of TiO₂ Nanotube Thin Films

First, the samples were subjected to pretreatment. After sintering, the graphite paper adhering to the sintered billet was removed with sandpaper, and the surface of the sintered billet was polished to a depth of 1 mm to 2 mm. Subsequently, the billet was placed in a beaker filled with water, and the beaker was put into an oven at 80℃ for 2 hours to allow the moisture in the sample to evaporate thoroughly². Then, the mass of the sample was measured, and the above operation was repeated several times to remove the sodium chloride (NaCl), finally obtaining the porous titanium sample².

After pretreatment, a hydrothermal method was used to prepare a three-dimensional (3D) networked TiO₂ nanotube film on the surface of porous titanium. 10 mol/L sodium hydroxide (NaOH) aqueous solution and the nickel-doped porous titanium sheet were placed into a polytetrafluoroethylene (PTFE)-lined autoclave². The autoclave was then sealed and heated in an oven at 180 ℃ for 8 hours. After hydrothermal treatment, the sample was taken out and washed repeatedly with deionized water until its pH reached approximately 7. Afterwards, the sample was immersed in 0.1 M hydrochloric acid (HCl) aqueous solution for 12 hours, and then rinsed with deionized water until the pH was around 7. The washed sample was then placed in a drying oven for drying. Finally, the dried sample was calcined at 450 ℃ for 2 hours, with the heating and cooling rates controlled at 2 ℃/min.

2.3. Investigation on the Effect of Heat Treatment Temperature on the Crystal Structure and Photocatalytic Performance of Catalysts

Both the crystal phase structure and crystallinity of titanium dioxide (TiO₂) have a significant impact on its photocatalytic performance, while calcination temperature is regarded as a key factor affecting the phase structure, crystallinity, grain size, and specific surface area². Therefore, to investigate the effect of calcination temperature on the prepared catalyst, Ni_7.5-TiO₂—the catalyst with the best photocatalytic performance—was selected as the sample. It was calcined at 350 ℃, 450 ℃, 550 ℃, 650 ℃, and 750 ℃ respectively, followed by characterization and performance analysis.

2.4. Characterization of Materials

X-ray diffraction (XRD) technology², as one of the important methods for studying the structure of polycrystalline materials, is widely used in the structural characterization of various compounds and their derivatives. In this study, a DX-2700B X-ray diffractometer was used to analyze the phase composition of the prepared porous material samples, samples during the hydrothermal process, and samples after calcination. The XRD test conditions were as follows: monochromatic Cu Kα radiation (λ=1.541 Å) was adopted, with a scanning range of 20°–90°, a step scanning angle of 0.02°, an operating voltage of 40 kV, an operating current of 40 mA, and a sampling time interval of 0.5 s.

Scanning electron microscopy (SEM) can effectively display surface information such as the micromorphology and particle size of materials, with a large depth of field and a wide range of magnification². In this experiment, a ZEISS Sigma 300 was used to characterize and analyze the micromorphologies of raw titanium powder, titanium powder morphology in bulk samples, pore structure, and nanotubes².

In this study, methylene blue (MB) with a concentration of 20 mg/L was used as the pollutant model to evaluate the photocatalytic performance of the photocatalysts. A CEL-S500 500W xenon lamp was used as the light source. First, 50 mL of MB solution was placed in the reaction device, and the porous titanium sample was placed at the bottom of the device. Before irradiation, the MB solution containing the porous titanium sample was stirred in the dark for 1 hour to reach adsorption equilibrium². Then, it was placed under the xenon lamp for photocatalytic reaction, with the distance between the porous titanium sample and the light source maintained at 15 cm, and the irradiation time was 3 hours. During the experiment, the solution was sampled every 30 minutes, and its absorbance was measured using an ultraviolet-visible (UV-Vis) spectrophotometer with a test wavelength range of 500 nm–750 nm. Subsequently, the degradation rate was calculated using the formula. The formula for calculating the degradation rate P is²:

P=(C₀-C_t)/C₀×100%

(2.1)

Among them, P is the degradation rate; C₀ is the initial concentration of the solution; C_t is the concentration of the solution at time t.

To analyze the effect of Ni content on the phase composition of TiO₂ nanotubes, a series of prepared materials were characterized by X-ray diffraction (XRD). The morphology of Ni-doped samples was observed using a scanning electron microscope (SEM), and nitrogen adsorption-desorption tests were employed to investigate the pore size distribution and specific surface area of the photocatalysts doped with different Ni contents².

3. Results

3.1. Characterization and Analysis of Titanium Dioxide Nanotubes on Porous Titanium with Different Nickel Contents

3.1.1. XRD Analysis

To analyze the effect of Ni content on the phase of TiO₂ nanotubes, a series of prepared materials were characterized by XRD, and the results are shown in Figure 3.1. It can be observed from the figure that all samples exhibit sharp and distinct diffraction peaks at 2θ = 25.3°, 37.9°, 48.15°, 54.05°, 55.2°, 62.8°, and 75.27°. By comparing with the standard diffraction pattern (PDF No. 78-2486), these peaks are identified as the characteristic peaks of anatase TiO₂, corresponding to the (101), (004), (200), (105), (211), (204), and (215) crystal planes, respectively.When the Ni doping content is 0 wt.%, in addition to the diffraction peaks of anatase TiO₂, diffraction peaks of Ti are also present at 2θ = 35.09°, 38.42°, 40.17°, 53°, 70.66°, 72.21°, and 77.36°. With the addition of Ni, new diffraction peaks appear at 2θ = 37.24°, 43.27°, 44.49°, and 51.84°. According to the standard diffraction pattern (PDF No. 87-0712), the peaks at 2θ = 44.49° and 51.84° are attributed to Ni. The peaks at 2θ = 37.24° and 43.27° correspond to the standard diffraction pattern of NiO (PDF No. 47-1049), while the (220) peak of NiO at 62.8° overlaps with that of anatase TiO2².This indicates that after hydrothermal treatment and calcination of the Ni-doped porous titanium substrate, Ni does not enter the lattice of TiO₂ but forms NiO, which is a typical composite of two semiconductors².

3.1.2. SEM Analysis

The morphologies of the samples after Ni doping are shown in Figure 3.2. It can be observed from the figure that the surfaces of the catalysts prepared with different Ni contents (2.5 wt.%, 5 wt.%, 7.5 wt.%, 10 wt.%) are completely covered by interconnected nanotube networks. The nanotubes cross each other to form numerous pores with diameters less than 100 nm². This structure is beneficial for incident light absorption, as the incident light can scatter inside the networked nanotube film, thereby improving the incident light absorption rate². When comparing the catalysts prepared with different Ni contents, there is no significant difference in their surface morphologies, indicating that the catalyst has a relatively stable crystal structure and high reusability.

3.1.3. XPS Analysis

Figure 3.3 shows the X-ray photoelectron spectroscopy (XPS) spectra of the Ni_7.5-TiO₂ nanotube networks. Compared with the XPS spectra of undoped Ni samples in Chapter 3, there is no significant difference in the XPS spectra of O 1s and Ti 2p. From the O 1s XPS spectrum, it can be observed that the main existing form of oxygen in the catalyst is still TiO₂. The peak at the binding energy of 529.8 eV corresponds to the Ti-O bond, i.e., the binding energy of lattice oxygen, while the shoulder peak at 531.54 eV is related to surface hydroxyl groups. Compared with the XPS spectrum of undoped Ni, the O 1s peak of Ni_7.5-TiO₂ shows a slight shift toward the higher binding energy direction, which may be attributed to the introduction of Ni element. In the Ti 2p spectrum, two main peaks are observed at 464.2 eV and 458.5 eV, corresponding to Ti⁴⁺ (TiO₂) 2p1/2 and Ti⁴⁺ (TiO₂) 2p3/2, respectively. The energy difference of 5.7 eV between the two peaks indicates that Ti element mainly exists in the form of Ti⁴⁺. Meanwhile, a shoulder peak at 457.7 eV corresponds to the binding energy of Ti³⁺. Figure 3.3 confirms that the TiO₂ nanotube networks contain Ni element, which is consistent with the EDS data. Several impurity peaks appear in the spectrum due to the low content of Ni and weak signal. The peaks at binding energies of 855.6 eV and 873.3 eV correspond to Ni 2p3/2 and Ni 2p2/1, respectively, indicating that Ni element mainly exists in the +2 valence state².

3.1.4. BET Analysis

N₂ adsorption-desorption measurements were conducted to investigate the pore size distribution and specific surface area of photocatalysts doped with different Ni contents², and the results are shown in Figure 3.4. From the N₂ adsorption-desorption isotherms of the materials, it can be seen that within the relative pressure (P/P0) range of 0.0–1.0, the isotherms of the samples are type IV with distinct H3 hysteresis loops, indicating that the photocatalysts have a mesoporous structure. This may be attributed to the numerous pore structures of porous titanium and the micropores existing between the interconnected TiO₂ nanotubes, which is consistent with the SEM results.Compared with the undoped sample, the specific surface areas of Ni_X-TiO₂ are all improved to a certain extent². The specific surface areas of Ni_2.5-TiO₂, Ni₅-TiO₂, Ni_7.5-TiO₂, and Ni₁₀-TiO₂ are 24.36 m²/g, 26.78 m²/g, 31.87 m²/g, and 26.3 m²/g, respectively. The results show that Ni doping can effectively improve the specific surface area of TiO₂, which may be because the doping of a small amount of Ni can inhibit the growth of TiO₂ lattice, leading to an increase in the specific surface area of the photocatalyst. Figure 3.4(b) shows the pore size distribution of the prepared samples². It can be observed from the figure that the pore size distribution of the catalysts is mainly below 50 nm, further confirming that the materials have a mesoporous structure².

3.1.5. Study on the Performance of Photocatalytic Degradation of Dyes

To investigate the catalytic activity of the prepared nickel-doped photocatalyst under visible light, methylene blue (MB) solution with a concentration of 20 mg/L was used as the degradation substrate. First, the reaction was conducted at a hydrothermal temperature of 200 ℃ for a hydrothermal duration of 2 h². After the reaction, the product was subjected to acid washing with 0.1 M hydrochloric acid (HCl) for 12 h; subsequent to drying, it was annealed in a muffle furnace at 450 ℃ for 2 h. Once cooled, the porous titanium sample was placed at the bottom of the reaction vessel, and 50 mL of the methylene blue solution was added to the reaction device for photocatalytic degradation. Prior to light irradiation, the methylene blue solution containing the porous titanium sample was stirred in the dark for 1 h to achieve adsorption equilibrium, followed by a 3-hour visible light irradiation period for the photocatalytic reaction.Figure 3.5 shows the photocatalytic performance of TiO₂ nanotube array catalysts prepared with different nickel doping amounts. Figure 3.5a presents the absorbance spectrum of methylene blue solution with Ni_7.5/TiO₂ catalyst as a function of irradiation time. The results indicate that the peak intensity of absorbance of the methylene blue solution containing the catalyst gradually decreases with the increase of irradiation time. The absorbance of the methylene blue solution gradually decreases to nearly zero, meaning that the methylene blue dye is almost completely removed². Figure 3.5b intuitively displays the degradation of methylene blue by Ni_2.5-TiO₂, Ni₅-TiO₂, Ni_7.5-TiO₂, and Ni₁₀-TiO₂ within 3 hours. All four catalysts exhibit excellent degradation effects on methylene blue under simulated sunlight, with degradation rates all above 90%². Among them, Ni_7.5-TiO₂ shows the best degradation performance, degrading 85.33% of methylene blue within 90 minutes and achieving the highest degradation rate of 98.35% at 180 minutes, almost completely degrading methylene blue. As shown in Figure 3.5c, the degradation reactions of methylene blue by the four catalysts all conform to the first-order kinetics, with correlation coefficients all above 98%. From the first-order kinetic curves, it can be obtained that Ni_7.5-TiO₂ has the highest rate constant of 0.02368 min⁻¹, which is 1.56 times, 1.31 times, and 1.42 times that of Ni2.5-TiO2 (0.01515 min⁻¹), Ni₅-TiO₂ (0.01812 min⁻¹), and Ni₁₀-TiO₂ (0.01666 min⁻¹), respectively². Figure 3.5d is a line chart of the degradation rates of methylene blue by the four catalysts after 3 hours of irradiation under simulated sunlight. It can be intuitively seen from the figure that Ni_7.5-TiO₂ has the best degradation effect on methylene blue, reaching 98.35%, while the degradation rates of Ni_2.5-TiO₂, Ni₅-TiO₂, and Ni₁₀-TiO₂ are 92.8%, 96.11%, and 94.99%, respectively.

3.1.6. Adsorption Performance of Photocatalysts

Experiments revealed a significant change in the absorbance of the solution following dark treatment. Accordingly, photocatalysts based on pure titanium sheets and porous titanium doped with varying nickel mass fractions were selected as samples, and the concentration of methylene blue solution after dark treatment was determined². The results are presented in Figure 3.6.6. As indicated in the figure, the photocatalyst with pure titanium sheet as the substrate shows the weakest adsorption capacity, whereas all samples using porous titanium as the substrate exhibit excellent adsorption performance². This is attributed to the high porosity of porous titanium, which endows it with a larger specific surface area compared to pure titanium sheets. During dark treatment, materials with a high specific surface area can adsorb more molecules, thereby enhancing the adsorption efficiency. In addition, due to the diverse pore structures of porous titanium substrates², the mesoporous structure is favorable for catalyst loading. Medium-sized mesopores not only ensure a high specific surface area but also reduce mass transfer resistance. In contrast, the macroporous structure is more suitable as a catalyst carrier for macromolecules, since its larger pore size allows macromolecules to enter and diffuse easily, thus improving the efficiency of adsorption and catalytic reactions². Furthermore, it can be observed from the figure that nickel doping slightly enhances the adsorption performance, with Ni_7.5-TiO₂ demonstrating the optimal adsorption capacity.

3.2. Effect of Heat Treatment Temperature on the Crystal Phase and Photocatalytic Performance of Catalysts

3.2.1. XRD Analysis

Figure 3.7 presents the XRD patterns of Ni_x-TiO₂ nanotube arrays fabricated at different calcination temperatures. As can be seen from the figure, after calcination at 350 ℃, distinct characteristic diffraction peaks emerge at 2θ = 25.3° and 2θ = 48.1°, corresponding to the (101) and (200) crystal planes of the anatase phase of TiO₂. The diffraction peaks at 2θ = 44.49° and 2θ = 51.84° are attributed to nickel (Ni). When the heat treatment temperature is increased to 450 ℃, TiO₂ remains entirely composed of the anatase phase; meanwhile, the diffraction peak intensity of the (101) crystal plane is significantly enhanced, indicating improved crystallinity². In addition, the diffraction peaks at 2θ = 37.24° and 2θ = 43.27° match the standard XRD pattern of NiO (PDF No. 47-1049), confirming the formation of NiO.

Furthermore, the diffraction peaks of the titanium substrate and Ni are still relatively intense at this temperature. When the heat treatment temperature is further elevated to 550 ℃, the intensity of the anatase diffraction peak corresponding to the (101) crystal plane is further strengthened, demonstrating enhanced crystallinity². Simultaneously, a new weak diffraction peak is detected at 2θ = 27.4°, which corresponds to the (110) crystal plane of the rutile phase. However, the weak intensity of this diffraction peak indicates that although the rutile phase transformation temperature is reached, only a small portion of larger anatase grains are preferentially converted into the rutile phase. At this point, the two phases coexist, forming a mixed crystal phase of anatase and rutile². It is generally accepted that the photocatalytic performance of anatase-phase TiO₂ is superior to that of rutile-phase TiO₂. Nevertheless, studies have shown that anatase and rutile phases can exert a synergistic effect when present in an appropriate proportion². Owing to the difference in Fermi levels between the two TiO₂ polymorphs, a Schottky barrier is formed at the phase interface, which facilitates the transfer and separation of electrons and holes between the phases as well as their migration to the catalyst surface. Meanwhile, due to the intimate contact between the two phases, electrons transfer from the rutile phase to the anatase phase, reducing the electron-hole recombination rate and thus enhancing the photocatalytic activity of the material². However, in this study, as observed in Figure 3.7, the TiO₂ nanotubes exhibit slight damage at 550 ℃. According to the photocatalytic results, the degradation efficiency of the sample at a heat treatment temperature of 550 ℃ is slightly lower than that at 450 ℃, indicating that the factors affecting the photocatalytic activity of the sample are multifaceted, and judgments should be made based on practical conditions. When the heat treatment temperature rises to 650 ℃, the diffraction peak of the TiO₂ rutile phase corresponding to 2θ = 27.4° is significantly enhanced and surpasses that of the anatase phase, indicating increased crystallinity of the rutile phase². When the heat treatment temperature reaches 750 ℃, the intensity of the rutile phase diffraction peak (110) increases sharply, while the intensity of the anatase characteristic diffraction peak corresponding to the (101) crystal plane decreases significantly, indicating that most of the anatase phase has transformed into the rutile phase at this temperature.

In summary, at a calcination temperature of 450 ℃, the TiO₂ nanotubes are entirely composed of the anatase phase. When the temperature reaches 550 ℃, a small amount of anatase transforms into rutile, leading to the formation of a mixed anatase-rutile phase in the TiO₂ nanotubes. With the further increase of calcination temperature, the proportion of anatase transforming into rutile increases². At 750 ℃, the TiO₂ nanotubes consist predominantly of the rutile phase. Besides the above phenomena, characteristic peaks corresponding to Ni and NiO are observed in all samples under different calcination temperatures.

The content of crystal phases in samples obtained at different heat treatment temperatures can be calculated according to Formula 3.1:

$$\mathsf{\chi }={\displaystyle \frac{1}{1+0.8\frac{{\mathrm{I}}_{\mathrm{A}}}{{\mathrm{I}}_{\mathrm{R}}}}}$$

(3.1)

where χ is the mass fraction of the rutile phase; IA is the diffraction peak intensity of the anatase phase (101); and IR is the diffraction peak intensity of the rutile phase (110). In addition, the grain size of TiO₂ nanotubes can be calculated using the Scherrer equation, which is expressed as follows:

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathrm{Bcos}\mathsf{\theta }}}$$

(3.2)

Among them, D represents the average grain size; K is a constant, usually taken as 0.89; B is the full width at half maximum of the diffraction peak; λ is the wavelength of the characteristic X-ray. A Cu target was used in this experiment, so λ is 0.154056 nm. The calculated results of the relative contents of anatase and rutile phases, as well as the average grain size of TiO₂ nanotubes prepared at different heat treatment temperatures, are shown in the following table²:

As shown in Table 3.1, with the increase of heat treatment temperature, the average grain sizes of both the anatase phase and rutile phase increase correspondingly². This is because the crystallinity of titanium dioxide (TiO₂) gradually improves as the heat treatment temperature rises. At lower temperatures, TiO₂ may be in an amorphous state or a state with low crystallinity, and the grain size is small at this time. When the temperature increases, the amorphous structure gradually transforms into a crystalline structure, leading to an increase in grain size. In addition, the anatase phase will gradually transform into the rutile phase at higher temperatures². During this process, grains continue to grow by absorbing surrounding atoms or small grains. Compared with the anatase phase, the rutile phase has higher thermal stability, so it is easier to form larger-sized grains at high temperatures.

3.2.2. SEM Analysis

Figure 3.8 shows the SEM images of Ni_7.5-TiO₂ nanotubes treated at different calcination temperatures. As can be seen from the images, a uniform thin film is grown on the surface of porous titanium, with distinct differences in micromorphology under varying calcination temperatures. No obvious differences are observed between the samples calcined at 350 ℃ and 450 ℃, both of which exhibit intact nanotubular structures with uniform sizes. When the temperature reaches 550 ℃, a small amount of nanoparticles can be observed (see Figure 3.8c), indicating that the nanotubes are damaged—they split into multiple segments, resulting in shortened tube lengths. However, when the temperature exceeds 550 ℃, the morphology changes significantly. As shown in Figure 3.8d, most nanotubes collapse and gradually transform into nanoparticles with diameters ranging from 50 nm to 100 nm. This is because calcination induces microcracks in the nanotube film, leading to its detachment. When the temperature is further increased to 750 ℃, the nanotubes are completely fractured, forming nanoparticles with diameters larger than 100 nm. It can be concluded that a higher temperature provides more energy to dissolve the nanotubes². Combined with the XRD results , it is found that Ni_7.5-TiO₂ nanotubes undergo a phase transformation from anatase to rutile during the calcination process. This phase transformation may cause volume changes, which in turn may result in the breakage of the nanotubes. In particular, when the calcination temperature exceeds 550 ℃, the transformation from anatase to rutile becomes more pronounced². This transformation is accompanied by volume expansion, which is likely to cause the destruction of the nanotube structure. Therefore, intact nanotubes can be obtained by maintaining the calcination temperature at 450 ℃.

3.2.3. Study on Photocatalytic Degradation Performance of Dyes

Figure 3.9 shows the MB degradation efficiency of Ni_7.5-TiO₂ nanotubes within 3 hours under different calcination conditions. As presented in Figure 3.9a, the sample calcined at 350 ℃ exhibits relatively low photocatalytic activity, with MB degradation rates of 84.886% and 91.916% within 2 hours and 3 hours, respectively. Combined with the analysis of Figure 3.7, this is presumably attributed to the low crystallinity of anatase TiO₂². With the increase of calcination temperature, the photocatalytic activity of the samples increases and reaches the maximum at 450 ℃. The sample calcined at 450 ℃ achieves MB degradation rates of 94.311% (2 h) and 98.353% (3 h). Based on the XRD and SEM results, the sample treated at 450 ℃ possesses high crystallinity, along with well-defined and intact nanotube structures; thus, 450 ℃ is determined as the optimal calcination temperature. As the calcination temperature continues to rise, the photocatalytic activity gradually decreases, hitting the lowest at 750 ℃—with MB degradation rates of 80.828% (2 h) and 92.899% (3 h) for the sample treated at this temperature. The reduced photocatalytic activity of samples calcined above 450 ℃ may result from the following factors. First, according to the XRD results, a phase transformation from anatase to rutile occurs in the temperature range of 550 ℃ to 750 ℃. Compared with the anatase phase, the rutile phase exhibits lower photocatalytic activity. Second, as indicated by the SEM results, the nanotube structure gradually deteriorates with increasing temperature, leading to a corresponding decrease in specific surface area, which further reduces the photocatalytic activity². Figure 3.9b demonstrates that all samples fit the first-order kinetic curve. Through calculation, the rate constants (K values) of the samples under different calcination temperatures are obtained, among which the sample calcined at 450 ℃ has the highest rate constant of 0.02368 min-1.

3.2.4. Adsorption Performance of Photocatalysts

Figure 3.10 shows the adsorption performance of each catalyst toward dye molecules after calcination at different temperatures. As can be seen from the figure, the catalyst exhibits the maximum adsorption performance at a calcination temperature of 350 ℃. With the increase of calcination temperature, the adsorption performance decreases gradually². This phenomenon is attributed to the damage of the nanotube structure caused by elevated temperatures—when the calcination temperature reaches 750 ℃, the nanotubes have been transformed into nanoparticles with diameters larger than 100 nm. Consequently, the specific surface area of the catalyst is reduced. A smaller specific surface area implies a decreased contact area between the catalyst and reactants, thereby lowering the adsorption capacity for reactants. Additionally, the reduced specific surface area may result in a decrease in the number of pores or a shrinkage of pore size, which in turn hinders the mass transfer of reactants².

4. Conclusions

This chapter investigates the photocatalytic degradation performance of nickel-doped three-dimensional (3D) networked TiO₂ nanotube films prepared on the surface of porous titanium, and discusses various influencing factors. Based on the characterization results of the material’s phase structure, micromorphology, and chemical composition, it is confirmed that nickel-doped 3D networked TiO₂ nanotube films were successfully fabricated, with well-formed nanotube structures.

Compared with pure TiO₂, Ni_x-TiO₂ has an increased specific surface area, which can provide more active sites for photocatalytic reactions, thereby enhancing the efficiency of photocatalytic degradation of organic pollutants. Under simulated sunlight irradiation, Ni_x-TiO₂ exhibits excellent photocatalytic performance in MB degradation. Among them, Ni_7.5-TiO₂ shows particularly remarkable photocatalytic performance, achieving a high MB degradation rate of 98.353% within 3 hours under simulated sunlight. After 5 cycles of MB photocatalytic degradation tests, no obvious changes were observed in the photocatalytic activity or crystal structure of Ni_7.5-TiO₂, indicating its excellent cyclic performance and stability. Compared with the pure titanium sheet substrate, the catalyst prepared using porous titanium as the substrate shows a significant improvement in adsorption capacity. In addition, nickel doping also partially enhances the adsorption capacity of the catalyst. The calcination temperature has a significant impact on photocatalytic performance, and its influence is mainly reflected in two aspects. First, it affects the micromorphology of the catalyst: as the temperature increases, the nanotube structure is gradually damaged and eventually transformed into nanoparticles, resulting in a decrease in specific surface area. Second, it affects the crystal phase: above 550 ℃, anatase TiO₂ transforms into rutile TiO₂, and anatase TiO₂ possesses superior photocatalytic activity.