Nb/N Co-Doped Layered Perovskite Sr₂TiO₄: Preparation and Enhanced Photocatalytic Degradation Tetracycline under Visible Light

Abstract

Sr₂TiO₄ is a promising photocatalyst for antibiotic degradation in wastewater. The photocatalytic performance of pristine Sr₂TiO₄ is limited to its wide bandgap, especially under visible light. Doping is an effective strategy to enhance photocatalytic performance. In this work, Nb/N co-doped layered perovskite Sr₂TiO₄ (Sr₂TiO₄:N,Nb) with varying percentages (0–5 at%) of Nb were synthesized by sol-gel and calcination. Nb/N co-doping slightly expanded the unit cell of Sr₂TiO₄. Their photocatalytic performance towards antibiotic (tetracycline) was studied under visible light (λ > 420 nm). When Nb/(Nb + Ti) was 2 at%, Sr₂TiO₄:N,Nb(2%) shows optimal photocatalytic performance with the 99% degradation after 60 min visible light irradiation, which is higher than pristine Sr₂TiO₄ (40%). The enhancement in photocatalytic performance is attributed to improving light absorption, and photo-generated charges separation derived from Nb/N co-doping. Sr₂TiO₄:N,Nb(2%) shows good stability after five cycles photocatalytic degradation reaction. The capture experiments confirm that superoxide radical is the leading active species during the photocatalytic degradation process. Therefore, the Nb/N co-doping in this work could be used as an efficient strategy for perovskite-type semiconductor to realize visible light driving for wastewater treatment.

1. Introduction

Tetracycline (TC) is a widely used broad-spectrum antibiotic in medical treatment, animal husbandry, and aquaculture [1]. The accumulation of antibiotics in the environment leads to drug-resistant bacteria, which could bring serious threats to the ecological environment and human health [2,3,4,5]. Tetracycline is not only hard to self-degrade in the natural environment [3], but also difficult to be eliminated by conventional techniques.

Photocatalytic technology [6,7,8,9] utilizes a photocatalyst to generate light-generated holes and electrons under irradiation. Holes or electrons react with H₂O or O₂ to generate superoxide radicals and hydroxyl radicals with strong oxidation capacity, thereby thoroughly oxidizing and degrading organic pollutants [10] in wastewater. The core of photocatalytic oxidation technology are photocatalysts. Among many photocatalytic materials, strontium titanate has attracted wide attention because of its advantages, such as being inexpensive, pollution-free, and light corrosion resistant [11]. However, strontium titanate also has the disadvantages of having a large bandgap and high photo-generated carrier recombination rate. Layered perovskite Sr₂TiO₄ exhibits higher photocatalytic performance than perovskite SrTiO₃, due to the particularly layered crystal structure and typical 2D charge transportation properties [12,13].

The wide bandgap of Sr₂TiO₄ exhibits photocatalytic activity only under ultraviolet light, which gravely affects further enhancement of photocatalytic performance. Ion doping can reduce the bandgap [14] of the perovskite material [11], including single metal doping, nonmetal doping, and co-doping. For instance, by introducing Cr [15], Ag [16], F [17], chalcogens [18], La/N [13], Cr/F [19], La/Rh [20], and La/Fe [12], the light response range of Sr₂TiO₄ is extended. Properties of Sr₂TiO₄ doping with different ions was shown in

Table 1. Properties of Sr₂TiO₄ doping with differentiation.

Doping Elements	A-Site/B-Site or O-Site Doping	Doping Content	Eg (eV)	References
Cr	B	5 at%	~1.5	[15]
Ag	A	2.5 at%	3.05	[16]
F	O	3 at%	3.20	[17]
Chalcogens (S, Se, Te)	O	--	0~2.99	[18]
La/N	A/O	10 at%/-	2.2	[13]
Cr/F	B/O	5 at%/40 at %	~2.5	[19]
La/Rh	A/B	1.5 at%/3 at %	2.43	[20]
La/Fe	A/B	1.5 at%/3 at %	~2.75	[12]

. Among them, co-doping has gained more interest, because co-doping is expected to maintain the charge-balance without forming oxygen vacancies.

Among various doping elements for oxide semiconductors, doping the nitrogen at the O site can narrow the band gap through the hybridization N 2p with the O 2p [21]. Meanwhile, N-doped oxide semiconductors are often accompanied by oxygen vacancies [22] due to charge compensation. Too high of a concentration of oxygen vacancies will act as a charge carrier recombination center, which is detrimental to photocatalytic activity [16,23,24]. The preparation methods of Sr₂TiO₄ mainly includes conventional solid-state reactions, the molten-salt method, and sol-gel method. Among them, the sol-gel synthesis process can achieve molecular-level doping.

This study chooses N-doped Sr₂TiO₄, where N³⁻ replaces O²⁻ to regulate the band structure. In order to control the concentration of oxygen vacancies, we replace Ti⁴⁺ (r = 0.061 nm) ions with Nb⁵⁺ (r = 0.064 nm) to balance the negative charge introduced by the unequal substitution of N³⁻ and O²⁻ Nb/N co-doped Sr₂TiO₄, which perhaps obtains charge-balanced Sr₂TiO₄:N,Nb and avoids too high of a concentration of oxygen vacancies. The amount of the dopant should also be optimized. Hence, we perform an investigation on Nb/N co-doped layered perovskite Sr₂TiO₄ for photocatalytic degradation tetracycline by varying Nb⁵⁺ doping content. To the authors’ knowledge, although there have been several studies on the doping modification of Sr₂TiO₄ [12,13,15,16,17,19,20] for photocatalytic hydrogen production, there is no literature report on the preparation and photocatalytic degradation performance of Nb/N co-doped Sr₂TiO₄. In this work, Sr₂TiO₄:N,Nb(2%) shows the best photocatalytic degradation performance towards tetracycline under visible light. The superior photocatalytic performance can be attributed to N-doping, narrowing the bandgap and Nb-doping compensating charge imbalance. This work affords a new insight to realizing visible-light-driven perovskite-type semiconductors.

2. Results and Discussion

2.1. Materials Characterization

As shown in Figure 1, the crystal structure of Sr₂TiO₄, Sr₂TiO₄:N and Sr₂TiO₄:N,Nb with different Nb-doping content were investigated by X-ray diffraction (XRD). There was only one phase of Sr₂TiO₄ (JCPDS NO:39-1471) without preferred growth orientation in the XRD patterns. Characteristic peaks of 2θ at 23.9, 28.3, 31.4, 32.6, 43.0, 43.7, 46.7, 55.0, 57.3, 65.4 and 68.2° were indexed to (101), (004), (103), (110), (006), (114), (200), (116), (213), (206) and (220) planes of Sr₂TiO₄ (JCPDS NO:39-1471), respectively.

After N-doping, the peak of (110) shown by Sr₂TiO₄:N in XRD shifted to a lower angle, meaning larger d-spacing than that of Sr₂TiO₄, and this larger spacing proved that N-doped Sr₂TiO₄ successfully. This is due to the N³⁻ radius (0.146 nm) being slightly larger than the O²⁻ radius (0.140 nm); when N³⁻ enters the Sr₂TiO₄ crystal lattice, the crystal lattice expands. According to Bragg’s equation 2dsinθ = λ, with the increase of d-spacing, the diffraction angle 2θ shifts to a lower angle. Similar to Sr₂TiO₄:N, 2θ of Sr₂TiO₄:N,N shifted to a lower direction gradually with the increase of Nb-doping content, as shown in Figure 1. Because the Nb⁵⁺ radius (0.064 nm) and N³⁻ radius (0.146 nm) are slightly larger than the Ti⁴⁺ radius (0.061 nm) and O²⁻ radius (0.140 nm), respectively, the more the Nb-doping content, the more the lattice expansion [25,26], and the more diffraction angle offset. According to the XRD patterns, it can be concluded that both Nb⁵⁺ and N³⁻ were doped into the lattice of Sr₂TiO₄ successfully. The average crystallite sizes were calculated by Scherrer’s equation [14] of the Sr₂TiO₄ (103) XRD reflection and were shown in

Table 2. Average crystallite size of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb.

	Sr2TiO4	Sr2TiO4:N	Sr2TiO4:N,Nb(1%)	Sr2TiO4:N,Nb(2%)	Sr2TiO4:N,Nb(3%)	Sr2TiO4:N,Nb(4%)	Sr2TiO4:N,Nb(5%)
Average crystallite size	43 nm	65 nm	64 nm	55 nm	68 nm	71 nm	61 nm

.

The morphological characterization of Sr₂TiO₄ and Sr₂TiO₄:N,Nb(2%) were shown in Figure 2. After being calcined at 1000 °C for 4 h, both pristine Sr₂TiO₄ and Sr₂TiO₄:N,Nb(2%) were composed of irregular particles ranging from tens of nanometers to 1 micron.

TEM elemental mapping of Sr₂TiO₄ was performed and the results were shown in Figure 3a–d. It verified the existence of Sr, Ti, O, and these elements were distributed evenly. TEM elemental mapping of Sr₂TiO₄:N,Nb(2%) was also performed and the results were shown in Figure 3f–j. It confirmed the existence of Sr, Ti, O, Nb, and N, and these elements were evenly distributed as well.

Figure 4a shows the absorbance of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb. The UV-Vis absorption spectra of the pristine and doped Sr₂TiO₄ showed a significant difference. The absorption edge of pristine Sr₂TiO₄ is located in the ultraviolet region. However, the absorption edge of N doping and Nb/N co-doping Sr₂TiO₄ were significantly red and shifted into the visible light region, and Sr₂TiO₄:N,Nb(2%) showed the strongest absorption capacity of visible light. According to the previous literature, N doping can reduce the band gap through hybridization of N 2p with the O 2p, raising the valence band maximum. The energy level of Nb⁵⁺ is similar to titanium 3d orbital energy, so Nb can replace Ti and mix Nb 4d with Ti 3d orbitals without lowering the conduction band minimum [27]. The same observation has been seen in other metal oxides doped with nitrogen, such as N-doped SrTiO₃ [28], N-doped NaLaTiO₄ [29], N-doped Sr₂TiO₄ [29], Nb/N co-doped TiO₂ [27], and so on. The bandgap E_g deduced from the Tauc plots in Figure 4b were 3.48 eV, 3.02 eV, and 2.95 eV for pristine Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%), respectively. Pristine Sr₂TiO₄ had a wide bandgap (3.48 eV), which was consistent with the previous literature [13]. N doping and Nb/N co-doping can reduce the bandgap of Sr₂TiO₄. Similar phenomena have also been observed in other materials involving N doping [13,24,30].

The chemical state and chemical composition on the surfaces of the pristine and doped Sr₂TiO₄ were investigated by XPS. The complete measurement scan in Figure 5a showed the presence of Sr, Ti, O, N (except pristine Sr₂TiO₄), and Nb (only for Sr₂TiO₄:N,Nb(2%)). Figure 5b shows the fine spectra of Ti. The presence of Ti⁴⁺ was validated by peaks at 458 and 464 eV, which were attributable to Ti 2p_3/2 and Ti 2p_1/2 [15,20]. The Ti 2p signal was gradually weakened by simultaneous Nb/N doping, confirming the substitution of Ti with Nb. A similar phenomenon was also observed in La/Fe co-doped Sr₂TiO₄ [12]. The peaks located around 529 eV and 531 eV belonged to the lattice oxygen and surface OH⁻ groups [31], respectively (Figure 5c). The signals of lattice oxygen gradually diminished along with N doping, meaning the substitution of lattice oxygen with N³⁻ for Sr₂TiO₄:N and Sr₂TiO₄:N,Nb(2%), which was consistent with XRD results(Figure 1). Due to the strong signal of surface OH⁻, it can be inferred that all samples are hydrophilic [13]. The peak around 398 eV was assigned to lattice N³⁻ as shown in Figure 5d. Along with Nb/N co-doping, this signal of N 1s was enhanced, which demonstrated the introduction of Nb promoting N doping. The 3d_5/2 and 3d_5/2 of Nb 3d electrons were detected at binding energies of 207 eV and 209 eV [32], indicating the existence of Nb⁵⁺. It was worth noticing that binding-energy shifted towards lower binding energy after Nb/N co-doping, which suggests Nb⁵⁺ doping based on N³⁻ doping was hole-doping. A similar shift was observed in SrCuO₂ [33].

The charge carrier behaviors of the pristine Sr₂TiO₄ and Sr₂TiO₄:N,Nb with different Nb-doping content were examined by PL with an excitation wavelength of 325 nm at room temperature. As shown in Figure 6, the emission intensity of Nb/N co-doped Sr₂TiO₄ with different Nb doping content were lower than that of pristine Sr₂TiO₄, implying that the charge separation capability was enhanced significantly, especially for Sr₂TiO₄:N,Nb(2%). The negative charge (O vacancy) introduced by the unequal replacement of N³⁻ and O²⁻ can be balanced by Nb-doping. Therefore, Nb/N co-doping Sr₂TiO₄ can reduce the photoelectron-hole recombination probability, which was beneficial to the photocatalytic reaction. Nb⁵⁺, as a charge balancing agent, can compensate for a charge imbalance caused by N substitution of O, ${\mathrm{Nb}}_{\mathrm{Ti}}^{•}+{\mathrm{N}}_{\mathrm{O}}^{\prime }$, just as La played a charge balancing role in La/N co-doped Sr₂TiO₄ [13], ${\mathrm{La}}_{\mathrm{Sr}}^{•}+{\mathrm{N}}_{\mathrm{O}}^{\prime }$.

2.2. Photoelectrochemical Properties of Sr₂TiO₄:N,Nb

To further confirm the effect of Nb doping on photo-carriers separation and migration of Sr₂TiO₄:N,Nb(2%), photochemical measurements were performed. As shown in Figure 7a, the transient-state photocurrent densities of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) remains stable under chopped light conditions for AM 1.5 illumination. An anodic photocurrent is clearly seen for all investigated samples, indicating that they are n-type semiconductors. The photocurrent density of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) are 0.034 μA·cm⁻², 0.199 μA·cm⁻², and 0.274·cm⁻² at 1.23 V_RHE, respectively. The photocurrent density of Sr₂TiO₄:N,Nb(2%) was 8 times that of Sr₂TiO₄ and 1.37 times that of Sr₂TiO₄:N, implying the charge-separated significantly more efficiently by the introduction of Nb⁵⁺ based on N³⁻.

Figure 7b shows the electrochemical impedance spectroscopy (EIS) of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) and the equivalent circuit, which can further verify the charge transport and transfer. It is well known that the smaller resistance radius indicates the higher separation and migration of electrons and holes. Compared with Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) had a smaller resistance radius, indicating more efficient separation of photocarriers. The result was consistent with the PL spectra and transient photocurrent response. The enhancement of the photocurrent density was attributed to the sufficient light absorption and effective photocarriers separation by introducing Nb⁵⁺ based on N³⁻ doping.

The specific surface area was tested by an N₂ adsorption-desorption isotherm at 77 K. As shown in Figure 8, both Sr₂TiO₄ and Sr₂TiO₄:N,Nb(2%) had typical IV isotherms and H3 hysteresis loops. H3 type hysteresis loops can be observed virtually on adsorbents with a lamellar structure. At high specific pressure, the curves showed an obvious hysteresis loop, which indicated that there was a mesoporous structure in the samples. The average pore diameter was determined using the Barrett–Joyner–Halenda (BJH) methods. BJT desorption average pore diameter (4 V/A) of Sr₂TiO₄ and Sr₂TiO₄:N,Nb(2%) were 38.24 nm and 23.22 nm, respectively. The specific surface areas of Sr₂TiO₄ and Sr₂TiO₄:N,Nb(2%) were 1.213 m²/g and 6.718 m²/g, respectively, which were very low. Specific surface area was not the main reason for the improvement of the photocatalytic degradation performance of Sr₂TiO₄:N,Nb(2%).

2.3. Photocatalytic Degradation of TC

As shown in Figure 9a, the adsorption capacity of photocatalysts towards TC after stirring for 30 min in the dark shows that Sr₂TiO₄:N,Nb(2%) exhibited the best adsorption property among them. However, the adsorption capacity was low, which was consistent with BET results. The photocatalytic properties of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) were investigated by photocatalytic degradation of TC with a 1 g·L⁻¹ photocatalyst under visible light. A blank experiment without a photocatalyst was also carried out to exclude the impact of photolysis. The self-degradation rate was 3.6% after 60 min due to negligible photooxygenation reactions in the presence of dissolved oxygen (Figure 9b) [34,35]. As shown in Figure 9b, the photocatalytic performance of all Nb/N co-doping Sr₂TiO₄ exhibited a higher degradation rate than prime Sr₂TiO₄ under the same conditions. The photocatalytic degradation rate of Sr₂TiO₄:N,Nb increased as Nb-doping content increased from 0% to 2%. When the content of Nb exceeded 2%, the degradation rate of Sr₂TiO₄:N,Nb decreased, which may be due to the introduction of new defects caused by excessive Nb, ig. ${\mathrm{Nb}}_{\mathrm{Ti}}^{•}+{\mathrm{Ti}}_{\mathrm{Ti}}^{\prime }$. Sr₂TiO₄:N,Nb(2%) exhibited the best photocatalytic degradation rate (99%), which was higher than that of prime Sr₂TiO₄ (40%) and Sr₂TiO₄:N (94%). On the one hand, Nb/N co-doping improved the light absorption in the visible-light region. On the other hand, the recombination probability of Sr₂TiO₄:N,Nb decreased, especially Sr₂TiO₄:N,Nb(2%). For comparison, the degradation rates of Sr₂TiO₄ towards different organic pollutants are shown in

Table 3. Photocatalytic properties of Sr₂TiO₄.

Samples	Doping Elements	Organic Pollutants	Degradation Rates	Irradiation System	References
Sr2TiO4	——	methylene orange	32% (120 min)	UV lamp (400 W Osram lamps)	[36]
Sr2TiO4	——	methylene orange	3.2% (180 min)	Xenon lamp (300 W without filter)	[37]
Sr2TiO4	——	methylene orange	78% (120 min)	UV lamp (λ = 253.7 nm, 2200 mW·cm−2)	[38]
Sr2TiO4	La	methylene orange	90.5% (120 min)	UV lamp (λ = 253.7 nm, 2200 mW·cm−2)	[38]
Sr2TiO4	——	tetracycline	40% (60 min)	LED lamp (λ > 420 nm)	This work
Sr2TiO4	Nb/N	tetracycline	99% (60 min)	LED lamp (λ > 420 nm)	This work

. To the authors’ knowledge, the photocatalytic degradation performance of N-doped Sr₂TiO₄ and Nb/N co-doped Sr₂TiO₄ has not been reported.

The photocatalytic degradation and fitting results of the TC kinetics followed pseudo-first-order kinetics, as shown in Figure 9c. Sr₂TiO₄:N,Nb(2%) showed a maximum apparent reaction rate constant of 0.0655 min⁻¹ which was nearly 24.9 times that of Sr₂TiO₄ (0.00263 min⁻¹) and just 1.25 times that of Sr₂TiO₄:N (0.0522 min⁻¹). This indicated that N doping had the main effect whereas adding Nb yielded only a slight increase to photocatalytic degradation rates. The maximum apparent reaction rate constant obtained by Sr₂TiO₄:N,Nb(2%) was attributed to an appropriate Nb/N co-doping concentration.

To examine the stability and reusability of Sr₂TiO₄:N,Nb(2%), five cycles of photodegradation experiments were carried out. After each cycle, the photocatalyst was centrifuged (at 9000 rpm) and washed several times with deionized water for several times and re-placed in a deionized water solution containing fresh TC. As shown in Figure 9d, the degradation rate of Sr₂TiO₄:N,Nb(2%) from 99% to 90% after five repeated cycles without an obvious decrease, indicating that Sr₂TiO₄:N,Nb(2%) exhibited relatively good stability and repeatability. The decrease of degradation performance could be ascribed to the mass loss in the centrifugation and washing process. Sr₂TiO₄:N,Nb(2%) shows good potential in photocatalytic degradation of antibiotic pollutants.

To further understand the key active species in the photocatalytic process of TC degradation, active species trapping experiments were performed, as shown in Figure 10. Isopropyl alcohol (IPA, 0.01 M), triethanolamine (TEOA, 0.01 M), p-benzoquinone (BQ, 0.005 M), and AgNO₃ (0.01M) were added as scavengers for hydroxyl radical ($\cdot \mathrm{OH}$), hole (h⁺), superoxide radical ($\cdot {\mathrm{O}}_2^{-}$), and photogenerated electrons (e⁻) to TC solution at the presence of Sr₂TiO₄:N,Nb(2%), respectively. When the IPA, TEOA, and AgNO₃ was added, the photodegradation rate of TC only slightly decreased. However, when BQ was added, the degradation rate of TC reduced greatly, indicating that superoxide radical ($\cdot {\mathrm{O}}_2^{-}$) was the main active species during the photocatalytic degradation of TC.

2.4. Improvement Mechanism of Photocatalytic Performance

The effect of N doping and Nb/N co-doping on the valence bands of Sr₂TiO₄ was evaluated by XPS valence band (VB) spectra. As shown in Figure 11, valence band potential (E_VB, _XPS) measured by XPS valence band spectra of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) were 2.86, 2.66, and 2.03 eV, respectively. E_VB vs. SHE can be deduced according to the formula: E_{VB, NHE} = φ + E_{VB, XPS} − 4.5 (φ is the work function of the instrument: 4.59 eV, 4.5 eV vs. vacuum level is 0 V vs. SHE) [39,40,41,42]. Thus, E_{VB, NHE} of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) were calculated to be 2.95 eV, 2.75 eV, and 2.12 eV, respectively. The conduction band (VB) minimum of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) are calculated to be −0.53 eV, −0.27 eV, and −0.83 eV(vs. SHE) according to these valance band and bandgap values (Figure 4b) mentioned above. The conduction band minimum of Sr₂TiO₄ reported in the previous literature varies widely and ranges from −0.254 eV to −0.87 eV [16,17,20,41], and the conduction band minimum of N-doped Sr₂TiO₄ and Nb/N co-doped Sr₂TiO₄ have not been reported.

According to our experimental results of XPS valence band spectra (Figure 11) and UV-Vis spectra (Figure 4b), schematic band structures of Sr₂TiO₄, Sr₂TiO₄:N, and Sr₂TiO₄:N,Nb(2%) were illustrated in Figure 12. The pure Sr₂TiO₄ had a wide bandgap. In contrast, Sr₂TiO₄:N displayed visible light response derived from N orbital. The state of the oxygen vacancies (accompanied by N-doping) was located below the conduction band minimum, which was consistent with previous literature about SrTiO₃ co-doped with N and La [21]. Regarding Sr₂TiO₄:N,Nb(2%), Nb/N co-doping uplifted the valence band furthermore. As we know, the more negative the valance band, the higher reduction ability of photo-generated electrons to generate superoxide radicals ($\blacksquare {\mathrm{O}}_2^{-}$). The conduction band potential of Sr₂TiO₄:N,Nb(2%) was more negative than Sr₂TiO₄, Sr₂TiO₄:N, and ${\mathrm{O}}_2/\blacksquare {\mathrm{O}}_2^{-}$ (−0.28 eV), so electrons can more easily transfer to the oxygen molecules, producing $\blacksquare {\mathrm{O}}_2^{-}$. The uplift of conduction band after cationic doping had also been observed in Ag doping Sr₂TiO₄ [16].

Hydroxyl radical ($\mathrm{OH}$) may be difficult to produce by the photogenerated holes, directly, because the valence band position (+2.12 eV) in Sr₂TiO₄:N,Nb(2%) was higher than that of $\mathrm{OH}/{\mathrm{H}}_2\mathrm{O}$ (+2.27 eV) and slightly more positive than that of $\mathrm{OH}/{\mathrm{OH}}^{-}$(1.99 eV). Therefore, $\blacksquare {\mathrm{O}}_2^{-}$ is the predominant active species in the degradation of TC, which coincided with the previous result (Figure 10). A similar situation was also discovered in Bi₂WO₆ [43,44].

3. Materials and Methods

3.1. Chemicals

The chemicals in this work were obtained from Shanghai Maclin Biochemical Technology Co., Ltd. (Shanghai, China) and Tianjin Yongda Chemical Reagent Co., Ltd. (Tianjin, China), without any further purification.

3.2. Materials Synthesis

3.2.1. Synthesis of Sr₂TiO₄

Firstly, 0.01 mol tetrabutyl titanate was dissolved in 50 mL of absolute ethanol, and 0.07 mol citric acid was injected as a complex agent (solution A). Secondly, 0.02 mol Sr(NO₃)₂ was dissolved in deionized water (solution B). Solution B was added dropwise to solution A to gain a transparent mixed solution. The mixed solution was heated and stirred in a water bath at 60 °C to form a homogeneous transparent sol. After aging for 24 h, the sol was dried at 120 °C for several hours to form a dry gel. Finally, the obtained gel was ground and calcined at 1000 °C for 4 h (heating rate 10 °C·min⁻¹) to obtain Sr₂TiO₄ powder.

3.2.2. Synthesis of N-Doped Sr₂TiO₄

The preparation process of N-doped Sr₂TiO₄ was as follows: After mixing urea and Sr₂TiO₄ uniformly (m(urea):m(Sr₂TiO₄) = 1.5:1), the mixture was calcinated at 400 °C for 2 h, cooled naturally, and grinded. N-doped Sr₂TiO₄ was labeled as Sr₂TiO₄:N.

3.2.3. Synthesis of Nb/N Co-Doped Sr₂TiO₄

The preparation process of Nb/N co-doped Sr₂TiO₄ was as follows: Nb-doped Sr₂TiO₄ was prepared first, which was the same as that of Sr₂TiO₄, except for the addition of NbCl₅ in solution B (Nb/(Nb + Ti), which was 1 at%, 2 at%, 3 at%, 4 at%, 5 at%. Nb/N co-doped Sr₂TiO₄ was prepared the same as that of N-doped Sr₂TiO₄, except that urea was mixed with Nb-doped Sr₂TiO₄. Nb/N co-doped Sr₂TiO₄ with different Nb doping content were denoted as Sr₂TiO₄:N,Nb(1%), Sr₂TiO₄:N,Nb(2%), Sr₂TiO₄:N,Nb(3%), Sr₂TiO₄:N,Nb(4%), and Sr₂TiO₄:N,Nb(5%), respectively.

3.3. Characterization

The crystal structure analysis was carried out by a Japanese D/MAX2500PC X-ray diffractometer employing Cu Kα radiation; the surface morphology was investigated by a Japan JEM-2010 transmission electron microscope (TEM); a specific surface and pore size analyzer (3H-2000PM1, BeiShiDe Instrument Technology Co., Ltd., Beijing, China) was used to calculate the specific surface area (BET) and pore size of the sample; the optical properties were investigated by an Ultraviolet-Visible spectrophotometer (Lambda750, PerkinElmer, Norwalk, CT, USA); the photoluminescence (PL, SR830) with the excitation at 325 nm was characterized using a fluorescence spectrometer; the chemical composition and valence band spectrum were detected by X-ray photoelectron spectroscopy (Thermo Scientific, Waltham, MA, USA, ESCALAB 250XI).

3.4. Photoelectrochemical Measurements

Photocurrent density-time (I-t) and electrochemical impedance spectroscopy (EIS) were characterized on an electrochemical workstation (CHI660D, Shanghai Chenhua Instrument, Shanghai, China) through a three-electrode system. Three-electrode system was set up using 0.5 M Na₂SO₄ solution, Ag/AgCl as reference electrode, Pt foil as counter electrode, and photoelectrodes fabricated by these sample powders on FTO glass as working electrodes. During the measurements of photocurrent density, simulated sunlight was irradiated with an intensity of 100 mW·cm⁻² (AM1.5G filter) under chopped light at a bias of 1.23 V vs. RHE. The EIS was measured with a frequency from 100 kHz to 0.01 Hz with an AC amplitude of 5 mV.

3.5. Photocatalytic Degradation of TC

The photocatalytic performances of the samples were assessed by photocatalytic degradation towards TC. In total, 50 mg Sr₂TiO₄ or 50 mg Sr₂TiO₄:N,Nb was dispersed in 50 mL of TC solution at a concentration of 20 mg·L⁻¹. Before irradiation, the mixtures were continuously stirred in the dark for 30 min to reach adsorption-desorption equilibrium. Then, a photocatalytic reaction was performed under visible light using a 5 W LED lamp (white light, λ > 420 nm, 250 mW/cm²). Extract 3 mL of TC solution every 10 min and centrifuge at 9000 rpm for 5 min. The TC degradation rate was calculated as follows:

$$\mathrm{Degradation}\mathrm{rate}=\frac{C_0-C_t}{C_0}\times 100\%=\frac{A_0-A_t}{A_0}\times 100\%$$

(1)

where C₀ (A₀) and C_t (A_t) represent the TC’s concentration (absorbance at 357 nm) of initial and after min irradiation, respectively.

In order to ascertain the active species during the photocatalytic degradation process, the active species capture experiment was carried out, similar to the photocatalytic degradation of TC experiments, except that TC solution was replaced by TC and scavenger.

4. Conclusions

In summary, Nb/N co-doped layered perovskite Sr₂TiO₄ (Sr₂TiO₄:N,Nb) with varying percentages (0–5 at%) of Nb were successfully synthesized by sol-gel and calcination. Nb/N could co-dope into Sr₂TiO₄ with a slight unit cell expansion, which was confirmed by X-ray diffraction. Nb/N co-doped Sr₂TiO₄ showed better photocatalytic performance for tetracycline photocatalytic degradation than pristine Sr₂TiO₄ under visible light, especially Sr₂TiO₄:N,Nb(2%). Sr₂TiO₄:N,Nb(2%) showed optimal photocatalytic performance with the 99% degradation after 60 min visible light irradiation, which was higher than pristine Sr₂TiO₄ (40%). Nb/N co-doping enhanced the photocatalytic activity by broadening light response to the visible region and reducing the photogenerated carrier recombination. Nb/N co-doping had a slight effect on morphology and the average grain size of Sr₂TiO₄. Sr₂TiO₄:N,Nb(2%) showed good stability and recyclability after five cycles photocatalytic degradation reaction. The superoxide radical ($\cdot {\mathrm{O}}_2^{-}$) was the leading contributor to tetracycline degradation. Nb/N co-doping strategy in this work affords insight to realizing visible-light-driven perovskite-type semiconductors for wastewater treatment.