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

Abstract

Photocatalysis is a promising method for methylene blue (MB) degradation due to its effectiveness and environmental compatibility. Among the photocatalysts, titanium dioxide (TiO₂) has been widely used for MB degradation due to its exceptional photocatalytic activity. However, the wide bandgap limits the degradation efficiency of TiO₂ under visible light. Here, an interstitial nitrogen-doped TiO₂ (5%N_T/TiO₂) used thiourea as the N source was fabricated for visible light-derived MB degradation. The 5%N_T/TiO₂ exhibited an extended absorption range of visible light. Moreover, photoelectrochemical measurements showed an improvement in the photocurrent response and charge transfer behavior on N/TiO₂. Thus, 5%N_T/TiO₂ had enhanced photocatalytic activity compared with pristine TiO₂ and substitutive N-doped TiO₂ (5%N_AB/TiO₂). The accelerated photocatalytic MB degradation process on N/TiO₂ could be mainly attributed to the interstitial N doping, which caused the appearance of new energy states and extended optical properties. Through comparing the impact of interstitial and substitutive in TiO₂ activity, our work proposes a suitable form of element doping to enhance the optical properties and photocatalytic activity of TiO₂ and even other semiconductors, providing guidance for future work.

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 AOP 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 it determines the whole reaction’s 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 to the ultraviolet light region, constituting only about 5% of the solar spectrum. In order to improve the photoconversion efficiency and photocatalytic degradation properties of TiO₂, it is necessary to develop a reliable strategy for extending the photo-response range of TiO₂ photocatalysts. Nitrogen doping is a promising approach to modify the electronic structure of TiO₂ and enhance its capabilities for visible light absorption [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 on 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 x% N_T/TiO₂ and x% N_AB/TiO₂, with x being the wt% of N element) were prepared from two different N sources (i.e., thiourea and ammonium bicarbonate) and used for photocatalytic MB degradation under visible light irradiation. Compared with TiO₂ and N_AB/TiO₂, N_T/TiO₂ showed better photocatalytic degradation performance. The content of N elements was optimized, showing that 5% N_T/TiO₂ possessed the highest MB degradation efficiency, with more than 60% of the MB removed within 150 min. In order to elucidate the effect of N element doping and clarify the advantages of thiourea as the N source, a series of characterizations were conducted, which confirmed the interstitial N doping in N_T/TiO₂. N atoms from thiourea were mainly incorporated into the lattice of N_T/TiO₂. On the other hand, N atoms from ammonium bicarbonate were mainly incorporated into N_AB/TiO₂ in the form of oxygen substitution. Moreover, N_T/TiO₂ possessed an improved capacity for visible light absorption. The photoelectrochemical experiments indicated that the interstitial N doping increased the number of photoexcited electrons from N_T/TiO₂ under visible light illumination. Therefore, with enhanced optical properties and photoelectric response, N_T/TiO₂ with interstitial N atoms displayed the optimal MB degradation efficiency. This work proved the advantages of interstitial N doping in improving the photocatalytic activity of semiconductors represented by TiO₂.

2. Results and Discussion

2.1. Optimization of the Amount of N Doping

The concentration of nitrogen doping was optimized by gradually increasing the amount of nitrogen elements in 5%N_T/TiO₂ in terms of the degradation efficiency of MB. Figure 1 shows the efficiencies of photocatalytic MB degradation with different amounts of N doping. Under visible light irradiation, undoped TiO₂ exhibited ~15% MB degradation after 150 min (Figure 1). The 5%N_T/TiO₂ had the highest MB photodegradation efficiency. After 150 min, approximately 56% of the MB was degraded. The decline in the activity of 10%N_T/TiO₂ and 15%N_T/TiO₂ could be attributed to the excessive N atoms, which may be recombination sites of photoexcited charges, as found by previous reports [16,17,18]. After determining the optimal amount of N doping, namely 5%, in the subsequent research and analysis, the amount of doping remained consistent, and only the N source was changed.

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

X-ray diffraction (XRD) analysis was performed to determine the crystalline phases and crystallographic structure of 5%N_T/TiO₂, 5%N_T/TiO₂, and TiO₂ catalysts (Figure 2a). The undoped TiO₂ mainly consisted of the rutile crystalline phase. The peaks centered at 27.5°, 36.1°, 41.3°, and 54.3° corresponded to the (110), (101), (111), and (211) crystal planes of the rutile phase, respectively [19]. When TiO₂ was doped with thiourea as the nitrogen source, 5%N_T/TiO₂ mainly showed the anatase phase. The peaks centered at 25.2°, 37.8°, 48.0°, and 55.0° corresponded to the (101), (004), (200), and (211) crystal planes of the anatase phase, respectively [19]. However, the 5%N_AB/TiO₂ with ammonium bicarbonate as the nitrogen source mainly showed a mixed crystal phase of rutile and anatase. This transformation in the crystal form may be attributed to the fact that when N is doped into the TiO₂ lattice, it will replace some of the O atoms or enter the lattice’s interstitial positions. This will cause a distortion in the lattice structure of TiO₂ and generate stress. When the stress accumulates to a certain extent, in order to reduce the system’s energy, the crystal structure will change, thus leading to a change in the crystal form. Moreover, the (101) crystal plane of 5%N_T/TiO₂ shifted from 25.2° to 26.1° (Figure 2b). It indicated that the interstitial doping, which can cause lattice distortion, had been achieved in 5%N_T/TiO₂. However, 5%N_AB/TiO₂ maintained two TiO₂ phases without a peak shift, meaning that the doping mode of N in 5%N_AB/TiO₂ may be substitutive doping.

2.3. SEM Images and Corresponding Element Mappings of 5%N_T/TiO₂

The elemental distribution of the 5%N_T/TiO₂ catalyst was observed by scanning electron microscopy (SEM). As can be seen from Figure 3b–d, the N element showed a very uniform distribution state on the sample and had the same distribution as the Ti and O elements. This phenomenon strongly indicated that the N element had been successfully doped into the lattice of TiO₂.

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

X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the surface elemental composition of the TiO₂, 5%N_T/TiO₂, and 5%N_AB/TiO₂ catalysts. The results revealed that the XPS results of the catalysts showed the presence of Ti 2p, O 1s, and N 1s peaks (Figure 4). 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 4a 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 appeared at 459.1 and 464.9 eV, respectively, which were contributed by the O–Ti–O in TiO₂ [20,21,22]. Compared with TiO₂, in 5%N_T/TiO₂, the Ti 2p3/2 and 2p1/2 core energy level peaks appeared at 458.8 and 464.5 eV, respectively, which were the Ti 2p peaks of the N–Ti–N or O–Ti–N in N–TiO₂ [21,22,23]. For 5%N_AB/TiO₂, the Ti 2p peaks of 5%N_AB/TiO₂ shifted to the lower binding energy with changes of ~0.4 eV. The peak shifts of Ti 2p indicated the successful incorporation of N in both 5%N_T/TiO₂ and 5%N_AB/TiO₂ [20,21].

In Figure 4b, TiO₂ exhibits an O 1s peak at 529.9 eV, which was attributed to the lattice oxygen (Ti–O–Ti). And the peak at 531.8 eV was surface-adsorbed oxygen. In 5%N_AB/TiO₂ and 5%N_T/TiO₂, the peaks had an obvious shift owing to the interstitial doping of N into the TiO₂ lattice to form hyponitrite (N₂O₂)^2–, which was further confirmed in the FTIR analyses, as discussed below.

Figure 4c 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 previous literature [24], the peak at 401.9 eV indicated 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 could be attributed to the Ti–N–Ti bonding, which was due to the substitution of oxygen in the 5%N_T/TiO₂ lattice by N and thus was consistent with the XRD results.

Through a comparison of the XPS spectra, the difference between using thiourea and ammonium bicarbonate as N source could be attributed to the different chemical bonds between N and TiO₂. In N_T/TiO₂, more N atoms existed in the form of interstitial N in the lattice gap of TiO₂ and formed Ti–O–N bonds, indicating that the O atoms had not been replaced. The following Fourier transform infrared spectroscopy (FTIR) spectra also confirmed this idea due to the existence of N⁺–O types such as (N₂O₂)^2–. However, in N_AB/TiO₂, more N atoms tended to form Ti–N–Ti bonds, suggesting that many O atoms were replaced by N atoms.

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

Transmission electron microscopy (TEM) characterization was used to observe the morphology of the TiO₂ and N/TiO₂ catalysts. Undoped TiO₂ typically showed mainly rutile phases with particle sizes ranging from 25 to 50 nm (Figure 5a). After N element doping, the crystal phase of 5%N_T/TiO₂ transformed into the anatase phase, while 5%N_AB/TiO₂ became mainly a mixture of rutile and anatase phases (Figure 5b,c). The particle sizes of 5%N_T/TiO₂ and 5%N_AB/TiO₂ were slightly smaller than that of TiO₂. Among them, 5%N_T/TiO₂ had the smallest particle size (5.5–8 nm). High-resolution transmission eectron microscopy (HRTEM) images showed clear lattice fringes with an interplanar spacing of 3.20 Å and 2.48 Å (Figure 5d,f), corresponding to the (110) and (101) facets of rutile TiO₂, respectively. The lattice spacings of 3.48 Å and 3.51 Å (Figure 5e,f), respectively, corresponded to those of the (101) plane of anatase TiO₂ in 5%N_T/TiO₂ and 5%N_AB/TiO₂. It can be observed that due to the difference in the doping methods, significant differences in the lattice spacings of 5%N_T/TiO₂ and 5%N_AB/TiO₂ were induced. This was because 5%N_T/TiO₂ involved interstitial doping, and the N doping was in the lattice gap of TiO₂, which led to lattice distortion.

2.6. N₂ Adsorption–Desorption Isotherm Analysis of TiO₂, 5%N_T/TiO₂, and 5%N_AB/TiO₂ Catalysts

N₂ adsorption–desorption isotherms analysis was used to determine the specific surface area and porosity of the 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 displayed IV-type isotherms (Figure 6a). This indicated that all the catalysts had mesoporous structures with pore size distributions of 2–40 nm (Figure 6b). The 5%N_T/TiO₂ had a lower pore size distribution. Figure 6b and

Table 1. BET surface area and average pore size of the catalysts.

Catalysts	Surface Area (m2·g−1)	Average Pore Size (nm)
TiO2	37.0	9.5
5%NT/TiO2	107.7	3.5
5%NAB/TiO2	90.0	9.6

show that the N doping, which can create additional surface defects and porosity, led to higher specific surface area and pore size. The larger specific surface area is beneficial for mass transfer during photocatalytic reactions. According to previous reports, the N atoms can affect the properties of porosity, including the average pore size and distribution, and the specific surface area [25,26]. In detail, under high-temperature processing, NH₃ gas was produced via in-situ breakdown of ammonia precursors (e.g., thiourea) so that the porous structure was formed during synthesis [27,28]. On the other hand, the decomposition of ammonium bicarbonate produced CO₂, further leading to the formation of interconnected mesopores in the TiO₂ lattice [29].

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

FTIR spectroscopy was employed to identify the presence of nitrogen-related species. (Figure 7). The characteristic peaks in the low wavenumber range (500 cm⁻¹) were attributed to TiO₂. The spectra showed characteristic peaks of TiO₂ at 3400 cm⁻¹, 1630 cm⁻¹, and 700–500 cm⁻¹, which were attributed to the Ti–OH bond, the OH bending vibration of water molecules, and Ti–O–Ti bond stretching vibrations, respectively. Notably, the 5%N_T/TiO₂ presented an additional peak at 1388 cm⁻¹, which was also attributed to the stretching vibration of N–O. Several bands in the low-frequency region observed at 1050 and 1250 cm⁻¹ belonged to the N⁺–O type substances embedded in the TiO₂ network. The peak at 1150 cm⁻¹ was the vibration peak of (N₂O₂)^2– formed by interstitial N and O.

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

The light-harvesting capacities of the TiO₂, 5%N_T/TiO₂, and 5%N_AB/TiO₂ catalysts were evaluated via Ultraviolet-visible (UV–vis) spectroscopy, as shown in Figure 8a. TiO₂ showed a typical absorption edge at 420 nm (corresponding to the 3.05 eV bandgaps in Figure 8b), suggesting the lack of capacity for a visible light response in TiO₂. On the contrary, N doping enhanced the visible light absorption of 5%N_T/TiO₂ and 5%N_AB/TiO₂, for which the light absorption edge was ~520 nm. Therefore, the incorporation of nitrogen into the TiO₂ lattice led to the formation of new energy states, as the N 2p position was 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.

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

The photoelectrochemical (PEC) measurement was conducted to study the photocurrent responses and charge transfer behavior (Figure 9a). TiO₂ exhibited a lower photocurrent density under visible light irradiation due to its narrow light absorption range. However, 5%N_T/TiO₂ and 5%N_AB/TiO₂ showed an enhanced photocurrent density under visible light, indicating that nitrogen doping allowed the 5%N_T/TiO₂ and 5%N_AB/TiO₂ to convert visible light into photoexcited charges. In the results of electrochemical impedance spectroscopy (EIS) (Figure 9b), the 5%N_T/TiO₂ and 5%N_AB/TiO₂ catalysts showed lower semicircles compared with TiO₂, indicating lower charge transfer resistance and better charge separation [30,31]. Among them, 5%N_T/TiO₂ had the lowest resistance and the strongest photocurrent because of the smaller lattice spacing of 5%N_AB/TiO₂, as shown in the HRTEM image. In general, a smaller lattice spacing means that the atoms or ions are closer to each other and the electrons move more easily in the lattice, resulting in lower resistance and stronger photogenerated carriers [32,33].

Mott–Schottky (MS) plots were used for estimating the band structure features of the catalysts. TiO₂, 5%N_T/TiO₂ and 5%N_T/TiO₂ can be considered as an n-type semiconductors according to the positive slope of the MS curve. The estimated flat band potential for 5%N_T/TiO₂ was –0.6 V vs. an Ag/AgCl electrode, which can be converted to –0.36 V vs. a normal hydrogen electrode (NHE). In general, the conduction band (E_CB) edge of typical n-type semiconductors is 0.20 V below its flat band potential. Then, the E_CB value of 5%N_T/TiO₂ was determined to be −0.56 V vs. NHE. The valence band (E_VB) potential of the 5%N_T/TiO₂ can be expressed by using the equation

$${\mathrm{E}}_{\mathrm{C}\mathrm{B}}{=\mathrm{E}}_{\mathrm{V}\mathrm{B}}{-\mathrm{E}}_{\mathrm{g}}$$

(1)

In this way, the E_VB position of 5%N_T/TiO₂ was calculated to be +2.54 V vs. NHE, while the E_VB position of TiO₂ and 5%N_AB/TiO₂ were calculated to be +3.09 and +2.86 V vs. NHE, respectively. The 5%N_T/TiO₂ showed the most negative E_CB position compared with the potential of TiO₂ (+0.09 V vs. NHE) and 5%N_AB/TiO₂ (–0.19 V vs. NHE). Moreover, the 5%N_T/TiO₂ showed a more negative E_CB position (–0.56 V vs. NHE) compared with the potential of O₂/·O₂^– (–0.33 V vs. NHE), suggesting that the photogenerated electrons of 5%N_T/TiO₂ could capture dissolved O₂ to generate ·O₂⁻. Moreover, the E_VB value of 5%N_T/TiO₂ was +2.54 V vs. NHE, which was higher than the potentials of OH/·OH (+2.40 V vs. NHE) and H₂O/·OH (+2.38 V vs. NHE), indicating that photogenerated holes of 5%N_T/TiO₂ could directly yield ·OH radicals.

2.10. Photocatalytic MB Degradation Activity

It should be noted that the activity of 5%N_AB/TiO₂ was lower than that of 5%N_T/TiO₂ (Figure 10a), suggesting that interstitial doping was more effective than substitutive doping. This is because the interstitial doped N atoms were located in the lattice gaps of TiO₂. XRD and HRTEM showed that this doping method reduced the lattice spacing of TiO₂, meaning that the atoms were arranged more closely. This is beneficial for electron transfer, and this fact could be proved by the EIS curves. TEM and BET showed that interstitial doping made the grain size of TiO₂ smaller and the specific surface area larger. In addition, the UV–vis and Mott–Schottky curves indicated that interstitial doped TiO₂ had a stronger visible light absorption ability and adjusted its energy band structure to obtain stronger photogenerated carriers. This was confirmed by the photoinduced i–t curves.

At present, many studies have carried out various modifications of TiO₂ for the degradation of methylene blue dye. For comparison, a summary of very recent studies on the photodegradation of MB by different TiO₂-based photocatalysts is listed in

Table 2. Photocatalytic activity of recently studied TiO₂-based photocatalysts for MB degradation.

	Irradiation Time (min)	Degradation Efficiency (%)	Ref.
5%NT/TiO2	150	56.5	This work
TiO2–Fe2O3	60	62.9	[34]
P25	120	20.0	[35]
TiO2/CS	180	33.9	[36]
TiO2-GOx	60	50.0	[37]
AlHF-TiO2	60	30.0	[38]
Ag-TiO2	50	40.0	[39]
Hg-doped TiO2	120	56.7	[40]

. The percentages of degradation obtained by doped TiO₂ prepared under our conditions were very near to those reported in the literature for TiO₂ catalysts. Moreover, in our work, 5%N_T/TiO₂ still had good degradation performance for other organic dyes. As shown in Figure 10b, under the same reaction conditions, 5%N_T/TiO₂ could decolorize 59.0% of rhodamine B in 150 min or 50.0% of methyl orange. It was demonstrated that 5%N_T/TiO₂ has good general applicability. In addition, to explore the stability of the catalyst, we carried out SEM and XRD characterization analyses on the catalyst after 150 min of reaction with methylene blue. As shown in Figure 11a–e, the morphology, element distribution, and crystal form of the catalyst after the reaction were almost the same as those before the reaction. This indicated that the catalyst had good stability.

Previous studies demonstrated that the photocatalytic reaction pathway is believed to involve the reaction of MB with the generated ·OH radicals, producing a range of intermediate products to reach complete mineralization with the formation of CO₂ and H₂O [41]. To study the main active components in the degradation process of MB on the surface of TiO₂ and to understand the degradation mechanism in more detail, trapping experiments were carried out. Silver nitrate, methanol, and isopropanol were used as scavengers to capture photogenerated electrons (e^–), photogenerated holes (h⁺), and hydroxyl radicals (·OH), respectively. To demonstrate the involvement of these radicals, a mixture of MB and silver nitrate (2% v/v) or methanol (2% v/v) or isopropanol (2% v/v) was irradiated under the same conditions. The results thus obtained are shown in Figure 12a. As can be observed, the addition of a photogenerated electron scavenger inhibited the degradation of MB. These results indicated that holes were the primary active species in the degradation of MB, while ·OH and photogenerated holes (h⁺) radicals were likely of secondary importance in photodegradation.

According to the results above and literature reports [42], the possible photocatalytic mechanism of the 5%N_T/TiO₂ photocatalyst was plotted as shown in Figure 12b. The photodegradation process depends on the generation and separation of carriers; under light conditions, once the semiconductor absorbs energy that is higher than the energy of its energy band, electrons will be excited from the valence band across the forbidden band to the conduction band. The doping of interstitial N reduced the width of the forbidden band of TiO₂, which greatly increased the density of photogenerated carriers. O₂ adsorbed on the surface of TiO₂ formed active substances (·O₂^–) after accepting electrons. At the same time, holes remained on the former, and the formed holes reacted directly with MB molecules, and H₂O was adsorbed on the TiO₂ surface to form the active species (·OH).

3. Materials and Methods

3.1. Preparation of the Catalysts

3.1.1. Preparation of 5%N_T/TiO₂

First, 40 mL of tetrabutyl titanate (Aladdin, Shanghai, China) was uniformly dispersed in 50 mL of an ethanol solution. It was vigorously stirred at 60 °C for 2 h and was named Solution A. Subsequently, 3 mL of nitric acid was added to 30 mL of distilled water and vigorously stirred for 5 min, and this was named Solution B. Immediately after that, thiourea (Aladdin, Shanghai, China) was added to 20 mL of ethanol and thoroughly stirred for 5 min. Subsequently, 3 mL of nitric acid was added and stirred at 60 °C for 1 h, and this was named Solution C. Then, under stirring, 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 °C for 24 h. After centrifugal washing with ethanol, the sample was heated to 150 °C at a rate of 2 °C/min and maintained at this temperature for 1 h to remove the residual ethanol. After this stage was complete, the sample continued to be heated at the same rate until it reached 450 °C and was maintained at this temperature for 2 h.

3.1.2. Preparation TiO₂ and 5%N_AB/TiO₂

The preparation methods of 5%N_AB/TiO₂ and TiO₂ were the same as that for 5%N_T/TiO₂. The only difference was that the nitrogen precursor doped in 5%N_AB/TiO₂ was ammonium bicarbonate (Aladdin, Shanghai, China), and for TiO₂, no nitrogen precursor was added.

3.2. Characterization of the Catalysts

The crystalline structure and phase composition of the TiO₂, 5%N_T/TiO₂, and 5%N_T/TiO₂ catalysts were determined by X-ray diffraction (XRD) on an X-Pert diffractometer (Rigaku, Kyoto, Japan) 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, Agilent Technologies, Dallastown, PA, USA), where the tube’s voltage was 15 kV, the tube’s current was 10 mA, and an ultra-high vacuum chamber with a mu-metal magnetic shield was used. The specific surface areas were determined using a surface area analyzer (Micromeritics, Norcross, GA, USA) by 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, Thermo Fisher Scientific, Waltham, MA, USA). 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 the TiO₂, 5%N_T/TiO₂, and 5%N_T/TiO₂ catalysts. Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 70 infrared spectrometer, Bruker Corporation, Billerica, MA, USA) was used to analyze the chemical bonds and functional groups present on the surface of the 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 4000 cm⁻¹ using the KBr tabbing technique. The light absorption properties of the 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 (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China).

3.3. Evaluation of the Catalysts

The performance of different catalysts in the photocatalytic degradation of MB was evaluated in a photocatalytic reactor under visible light irradiation (300 W xenon lamp). Firstly, 0.02 g of each catalyst was added to 20 mL MB. Adsorption and desorption equilibrium were attained by agitating the suspensions with a magnetic stirrer for 1 h. Afterward, the condensate water was turned on to keep the reaction temperature at room temperature. Then the solution was irradiated with a 300 W xenon lamp without a light intensity meter as the source of visible light. An aliquot was collected periodically with a syringe from the reactor every 30 min during a 150 min 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 [43,44]

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

(2)

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 confirmed the presence of nitrogen in TiO₂ crystals. UV–vis spectra showed the extended light absorption range on 5%N_T/TiO₂. Thus, 5%N_T/TiO₂ had a better photocurrent response during PEC measurement. Through optimizing the nitrogen concentration, 5%N_T/TiO₂ showed higher MB degradation activity than TiO₂ and 5%N_AB/TiO₂ under visible light illumination. Our study clarified the advantages of interstitial N doping in improving photocatalytic activity under visible light conditions. More importantly, this work demonstrated that the introduction of nitrogen facilitates the utilization of solar light energy and makes TiO₂ a promising material for environmental remediation and beyond.