Interstitial N-Doped TiO2 for Photocatalytic Methylene Blue Degradation under Visible Light Irradiation

1. Introduction

Methylene blue (MB) is a common dye used in various industries.[1,2] However, with its widespread utilization, MB leads to significant environmental challenges including water pollution, toxicity to aquatic life, bioaccumulation, and potential human health risks.[3,4] Thus, effective removal of MB is crucial for ecosystems and human health. Several strategies have been developed for MB remediation, such as advanced oxidation processes (AOPs), adsorption techniques, bioremediation, and membrane filtration.[5,6] Among them, based on AOPs technology, photocatalytic degradation of organic compounds is an ideal method for the removal of industrial pollutants with solar energy as the only input energy source. Improving photocatalytic degradation efficiency has been attracting a great deal of research interest.

The design of the photocatalyst is so important for photocatalytic degradation that determines the whole reaction efficiency [7,8,9,10]. Titanium dioxide (TiO₂) is a widely studied semiconductor material known for its excellent photocatalytic properties, chemical stability, and non-toxicity. It has garnered significant attention for applications in environmental remediation, particularly in the degradation of organic pollutants. However, the practical application of TiO₂ is limited by its wide bandgap (~3.2 eV for anatase), which restricts its photo-response ability to the ultraviolet light region, constituting only about 5% of the solar spectrum. In order to improve the photoconversion efficiency and photocatalytic degradation property of TiO₂, it is necessary to develop a reliable strategy for extending the photo-response range of TiO₂ photocatalyst. Nitrogen doping is a promising approach to modify the electronic structure of TiO₂ and enhance its visible light absorption capabilities[11,12]. By incorporating nitrogen atoms into the TiO₂ lattice, the bandgap can be narrowed, allowing it to absorb visible light and thus improving its photocatalytic efficiency under solar irradiation[13,14,15]. Although numerous studies have proved the positive influences of N element doping in the photocatalytic activity of TiO₂, the effects of different N precursors achieving N doping should be further explored to elucidate the impact of N atoms on the structure and activity of TiO₂.

In this study, nitrogen-doped TiO₂ catalysts (denoted as 5%N_T/TiO₂ and 5%N_AB/TiO₂ with 5 wt% of N element content) are prepared via two different N sources (i.e. thiourea and ammonium bicarbonate) and used for photocatalytic MB degradation under visible light irradiation. A series of characterizations confirm the successful N element doping into TiO₂. When thiourea is used as the N source, N is mainly incorporated into the lattice of 5%N_T/TiO₂ in the form of interstitial N. On the other hand, N atoms from ammonium bicarbonate are mainly incorporated into 5%N_AB/TiO₂ in the form of oxygen substitution. Moreover, 5%N_T/TiO₂ possesses an improved visible light absorption capacity. The photoelectrochemical experiments indicate that the N doping increased the number of photoexcited electrons from 5%N_T/TiO₂ under visible light illumination. Therefore, compared to TiO₂ and 5%N_AB/TiO₂, 5%N_T/TiO₂ shows the optimal photocatalytic degradation performance with more than 60% of the MB removal rate within 150 min.

2. Results and Discussion

2.1. XRD Patterns of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

XRD analysis is performed to determine the crystalline phases and crystallographic structure of 5%N_T/TiO₂, 5%N_T/TiO₂ and TiO₂ catalysts (Figure 1a). The undoped TiO₂ mainly consists of the rutile crystalline phase. The peaks centered at 27.5°, 36.1°, 41.3°, and 54.3° correspond to the (110), (101), (111), and (211) crystal planes of the rutile phase, respectively[16]. When TiO₂ is doped with thiourea as the nitrogen source, 5%N_T/TiO₂ mainly shows anatase phase. The peaks centered at 25.2°, 37.8°, 48.0°, and 55.0° correspond to the (101), (004), (200), and (211) crystal planes of the anatase phase, respectively[16]. While the 5%N_AB/TiO₂ with ammonium bicarbonate as the nitrogen source mainly shows a mixed crystal phase of rutile and anatase. Besides, the (101) crystal plane of 5%N_T/TiO₂ shifts from 25.2° to 26.1°. It indicates that the interstitial doping, which can cause lattice distortion, is achieved in 5%N_T/TiO₂. However, 5%N_AB/TiO₂ maintains two TiO₂ phases without peak shift, meaning that the doping mode of N in 5%N_AB/TiO₂ may be substitutive doping.

2.2. XPS Spectra of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

XPS analysis is conducted to investigate the surface elemental composition of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ catalysts. The results revealed that the XPS results of catalysts showed the presence of Ti 2p, O 1s, and N 1s peaks (Figure 2). The XPS spectra of 5%N_T/TiO₂ and 5%N_AB/TiO₂ revealed the presence of nitrogen, indicating that nitrogen was doped into TiO₂.

Figure 2 (a) illustrates the HR-XPS spectra of Ti 2p in TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂. For TiO₂, the Ti 2p3/2 and 2p1/2 core energy level peaks appear at 459.1 and 464.9 eV respectively, which are contributed by O-Ti-O in TiO₂[17,18,19]. Compared with TiO₂, in 5%N_T/TiO₂, the Ti 2p3/2 and 2p1/2 core energy level peaks appear at 458.8 and 464.5 eV, respectively, which are the Ti 2p peaks of N-Ti-N or O-Ti-N in N-TiO₂[18,19,20]. For 5%N_AB/TiO₂, the Ti 2p peaks of 5%N_AB/TiO₂ shift to the lower binding energy with ~ 0.4 eV changes. The peak shifts of Ti 2p indicate the successful N incorporation in both 5%N_T/TiO₂ and 5%N_AB/TiO₂ [17,18].

In Figure 2(b), TiO₂ exhibits the O 1s peak at 529.9 eV, which is attributed to the lattice oxygen (Ti-O-Ti). And the peak at 531.8 eV is surface-adsorbed oxygen. In 5%N_AB/TiO₂ and 5%N_T/TiO₂, the peaks have an obvious shift owing to the interstitial doping of N into the TiO₂ lattice to form hyponitrite ((N₂O₂)^2-, which will be further confirmed in the FTIR analyses as discussed below.

Figure 2(c) clearly shows the binding energies of N 1s at 399.8 and 401.9 eV for 5%N_T/TiO₂ and 5%N_AB/TiO₂, respectively. For the 5%N_T/TiO₂, according to the previous literature[21], the peak at 401.9 eV means the formation of Ti-O-N bonds based on the form of interstitial N in the lattice gap of 5%N_T/TiO₂. In the XPS spectrum of 5%N_AB/TiO₂ the peak of N 1s at 399.8 eV can be attributed to the Ti-N-Ti bonding, which is due to the substitution of oxygen in the 5%N_T/TiO₂ lattice by N and thus consists of the XRD results.

2.3. TEM Images of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

TEM characterization is used to observe the morphology of TiO₂ and N/TiO₂ catalysts. Undoped TiO₂ typically mainly shows rutile phases with particle sizes ranging from 25 to 50 nm (Figure 3a). After N element doping, the crystal phase of 5%N_T/TiO₂ transformed into the anatase phase, while 5%N_AB/TiO₂ mainly a mixture of rutile and anatase phases(Figure 3b and Figure 3c). The particle sizes of 5%N_T/TiO₂ and 5%N_AB/TiO₂ are slightly smaller than that of TiO₂. Among them, 5%N_T/TiO₂ has the smallest particle size (5.5-8 nm). HR-TEM images show clear lattice fringes with interplanar spacing of 3.20 Å and 2.48 Å (Figure 3d and Figure 3f), corresponding to the (110) and (101) facet of rutile TiO₂ respectively. The lattice spacings of 3.48 Å and 3.51 Å (Figure 3e and 3f), respectively, are corresponding to those of the (101) plane of anatase TiO₂ in 5%NT/TiO₂ and 5%N_AB/TiO₂. It can be observed that due to the difference in doping methods, significant differences in the lattice spacings of 5%N_T/TiO₂ and 5%N_AB/TiO₂ were induced. This is because 5%N_T/TiO₂ is interstitial doping, and N doping is in the lattice gap of TiO₂, which lead to lattice distortion.

2.4. N₂ Adsorption-Desorption Isotherms Analysis of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

N₂ adsorption-desorption isotherms analysis is used to determine the specific surface area and porosity of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ catalysts. The N₂ isotherms of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ display IV-type isotherms (Figure 4a). It indicates that all the catalysts have mesoporous structures with 2–40 nm of pore size distributions (Figure 4b). 5%N_T/TiO₂ has a lower pore size distribution. Figure 4b and Table 1 show that the N doping, which can create additional surface defects and porosity, leads to higher specific surface area and pore size. The larger specific surface area is beneficial for mass transfer during photocatalytic reactions.

2.5. FTIR of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂. Catalysts

FTIR spectroscopy is employed to identify the presence of nitrogen-related species. (Figure 5). The characteristic peaks in the low wavenumber range (400–4000 cm^-1) are attributed to TiO₂. The spectra show characteristic peaks of TiO₂ at 3400 cm^-1, 1630 cm^-1, and 700–500 cm^-1, which are attributed to the Ti-OH bond, OH bending vibration of water molecules, and Ti-O-Ti bond stretching vibrations, respectively. The spectra of 5%N_T/TiO₂ and 5%N_AB/TiO₂ exhibit additional peaks at 1427–1430 cm^-1 and 1280–1295 cm^-1. These peaks are attributed to the vibrations of the N-Ti bond, indicating that N atoms have been incorporated into the TiO₂ lattice. Notably, the 5%N_T/TiO₂ presents additional peaks at 1388 cm⁻¹ is also attributed to the stretching vibration of N-O. Several bands in the low-frequency region observed at 1050 and 1250 cm⁻¹ belong to the N⁺-O type substances embedded in the TiO₂ network. The peak at 1150 cm⁻¹ is the vibration peak of (N₂O₂)^2- formed by interstitial N and O.

2.6. Optical Properties of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

The light-harvesting capacities of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ catalysts were evaluated via UV-vis spectroscopy as shown in Figure 6. TiO₂ showed a typical absorption edge at 420 nm, suggesting the lack of visible light response ability in TiO₂. On the contrary, N doping enhanced the visible light absorption of 5%N_T/TiO₂ and 5%N_AB/TiO₂. of which the light absorption edge is ~520 nm. Therefore, the incorporation of nitrogen into the TiO₂ lattice leads to the formation of new energy states, as the N 2p position is more negative than the O 2p state, leading to a decrease in the energy bandgap and a shift in the optical absorption towards the visible light region.

2.7. Photoelectrochemical Measurements of TiO₂, 5%N_T/TiO₂ and 5%N_AB/TiO₂ Catalysts

The photoelectrochemical (PEC) measurement is conducted to study the photocurrent responses and charge transfer behavior (Figure 7a). TiO₂ exhibits a lower photocurrent density under visible light irradiation due to its narrow light absorption range. 5%N_T/TiO₂ and 5%N_AB/TiO₂ show enhanced photocurrent density under visible light, indicating nitrogen doping makes 5%N_T/TiO₂ and 5%N_AB/TiO₂ can convert visible light into photoexcited charges. In the electrochemical impedance spectroscopy (EIS) (Figure 7b), 5%N_T/TiO₂ and 5%N_AB/TiO₂ catalysts show lower semicircles compared to TiO₂, indicating lower charge transfer resistance and better charge separation. [22,23] Among them, 5%N_T/TiO₂ has the lowest resistance and the strongest photocurrent because of the smaller lattice spacing of 5%N_AB/TiO₂ as shown in the HR-TEM image. In general, the smaller lattice spacing means that the atoms or ions are closer to each other and electrons move more easily in the lattice, resulting in lower resistance and stronger photogenerated carriers.[24,25]

2.8. Photocatalytic MB Degradation Activity

The concentration of nitrogen doping is optimized by gradually increasing the amount of nitrogen elements in 5%N_AB/TiO₂ and 5%N_T/TiO₂ in terms of the degradation efficiency of MB. Figure 8 shows the efficiencies of photocatalytic MB degradation with different N doping amounts. Under visible light irradiation, undoped TiO₂ exhibits ~15 % MB degradation after 150 min (Figure 8a). For the 5%N_AB/TiO₂ catalysts, 5%N_T/TiO₂ has the highest MB photodegradation efficiency. After 150 min, approximately 55% of MB was degraded. The activity decline of 10%N_T/TiO₂ and 15%N_T/TiO₂ should be attributed to the excessive N atoms, which can be recombination sites of photoexcited charges as previous reports[26,27,28]. In addition, the above results displayed that N doping significantly enhances the catalytic performance of TiO₂ in the degradation of MB under visible light irradiation. It should be noted that the activity of 5%N_AB/TiO₂ is lower than that of 5%N_T/TiO₂ (Figure 8b), suggesting that interstitial doping is more effective than substitutive doping.

3. Materials and Methods

3.1. Catalyst Preparation

3.1.1. 5%N_T/TiO₂ Preparation

First, 40 min of tetrabutyl titanate was uniformly dispersed in 50 min of ethanol solution. It was vigorously stirred at 60 ℃ for 2 hours and was named Solution A. Subsequently, 3 min of nitric acid was added to 30 min of distilled water and vigorously stirred for 5 min, and it was named Solution B. Immediately after that, thiourea was added to 20 min of ethanol and thoroughly stirred for 5 min. Subsequently, 3 min of nitric acid was added and stirred at 60 ℃ for 1 hour, and it was named Solution C. Then, under the stirring state, Solution B and Solution C were poured into Solution A at intervals of 5 min successively. Finally, the obtained mixture was poured into the polytetrafluoroethylene liner in the autoclave and maintained at 100 ℃ for 24 hours. After centrifugal washing with ethanol, the sample was heated to 150 ℃ at a rate of 2 ℃/min and maintained at this temperature for 1 hour to remove the residual ethanol. After this stage was completed, the sample continued to be heated at the same rate until it reached 450 ℃ and was maintained at this temperature for 2 hours.

3.1.2. TiO₂ and 5%N_AB/TiO₂ Preparation

The preparation methods of 5%N_AB/TiO₂ and TiO₂ are the same as that of 5%N_T/TiO₂. The only difference is that the nitrogen precursor doped in 5%N_AB/TiO₂ is ammonium bicarbonate. And for TiO₂, no nitrogen precursor is added.

3.2. Catalyst Characterization

The crystalline structure and phase composition of TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ catalysts were determined by X-ray diffraction (XRD) method on an X-Pert diffractometer equipped with graphite monochromatized Cu-Kα radiation. The elemental composition, chemical states, and surface chemistry of TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ were analyzed using Agilent 5100 X-ray Photoelectron Spectroscopy (XPS), where tube voltage was 15kV, tube current was 10 mA and an ultra-high vacuum chamber with mu-metal magnetic shield was used. The specific surface areas were determined using a surface area analyzer (BEL Sorp-II mini, BEL Japan Co., Japan) with the Brunauer-Emmett-Teller (BET) method. The morphology and size of the TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ catalysts were observed using a Transmission Electron Microscope (TEM-16-TS-008). The acceleration voltage was 200 kV, the point resolution was 0.248 nm, and the maximum magnification was 1.05 million times to visualize the morphology, size, and surface features of TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ catalysts. The Fourier-Transform Infrared Spectroscopy (FTIR, Bruker Vertex 70 infrared spectrometer) was used to analyze chemical bonds and functional groups present on the surface of TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ catalysts. FTIR spectra in transmission mode were recorded at a resolution of 4 cm^-1 in the range of 400 to 1400 cm^-1 using the KBr tabbing technique. The light absorption properties of TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ catalysts were determined by UV-Vis Spectroscopy with a wavelength range from 200 to 800 nm, on a Japanese Shimadzu UV-3600i Plus Spectrometer. The photocurrent and electrochemical impedance spectroscopy were measured on an electrochemical workstation.

3.3. Catalyst Evaluation

The performance of different catalysts in the photocatalytic degradation of MB is evaluated in a photocatalytic reactor under visible light irradiation. Firstly, 0.02 g catalyst was added to 20 mL MB. Adsorption and desorption equilibrium were attained by agitating the suspensions with a magnetic for 1 h. Afterward, the condensate water was turned on to keep the reaction temperature at room temperature. Then the solution was irradiated with an Xenon lamp as the source of visible light. An aliquot was collected periodically with a syringe from the reactor every 30 min during a 3 h interval. Finally, the catalyst was filtered and recycled. Changes in the concentration of MB were observed from its characteristic absorption at 664 nm using a UV-Vis spectrometer. The degradation efficiency of MB at each time point was calculated using the following formula[29,30]:

$$\mathrm{Degradation}\mathrm{efficiency}(\%)={\displaystyle \frac{\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{t}}{\mathrm{C}\mathrm{o}}}\times 100\%$$

Where C_o is the initial concentration of MB and C_t is the concentration of MB at a time.

4. Conclusions

In conclusion, this work provides a straightforward method to fabricate nitrogen-doped TiO₂ photocatalysts based on thiourea as the N source. The systematic characterizations confirm the presence of nitrogen in TiO₂ crystalline. UV-vis spectra show the extended light absorption range on 5%N_T/TiO₂. Thus, 5%N_T/TiO₂ possesses better photocurrent response during PEC measurement. Through optimizing nitrogen concentration, 5%N_T/TiO₂ shows the highest MB degradation activity than TiO₂ and 5%N_AB/TiO₂ under visible light illumination. Our study clarifies the advantages of interstitial N doping in improving photocatalytic activity under visible light conditions. More importantly, this work demonstrates the introduction of nitrogen facilitates the utilization of solar light energy and leads TiO₂ to be a promising material for environmental remediation and beyond.