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Article

A Facile Strategy for the Preparation of N-Doped TiO2 with Oxygen Vacancy via the Annealing Treatment with Urea

by
Zhe Zhang
1,
Zhenpeng Cui
1,2,*,
Yinghao Xu
1,
Mohamed Nawfal Ghazzal
3,
Christophe Colbeau-Justin
3,
Duoqiang Pan
1,2 and
Wangsuo Wu
1,2
1
School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
2
Frontiers Science Center for Rare Isotopes, Lanzhou University, Lanzhou 730000, China
3
Institute of Physical Chemistry, Paris-Saclay University, 91405 Orsay, France
*
Author to whom correspondence should be addressed.
Nanomaterials 2024, 14(10), 818; https://doi.org/10.3390/nano14100818
Submission received: 3 April 2024 / Revised: 29 April 2024 / Accepted: 2 May 2024 / Published: 7 May 2024
(This article belongs to the Special Issue Heterogeneous Photocatalysts Based on Nanocomposites)
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Abstract

:
Although titanium dioxide (TiO2) has a wide range of potential applications, the photocatalytic performance of TiO2 is limited by both its limited photoresponse range and fast recombination of the photogenerated charge carriers. In this work, the preparation of nitrogen (N)-doped TiO2 accompanied by the introduction of oxygen vacancy (Vo) has been achieved via a facile annealing treatment with urea as the N source. During the annealing treatment, the presence of urea not only realizes the N-doping of TiO2 but also creates Vo in N-doped TiO2 (N-TiO2), which is also suitable for commercial TiO2 (P25). Unexpectedly, the annealing treatment-induced decrease in the specific surface area of N-TiO2 is inhibited by the N-doping and, thus, more active sites are maintained. Therefore, both the N-doping and formation of Vo as well as the increased active sites contribute to the excellent photocatalytic performance of N-TiO2 under visible light irradiation. Our work offers a facile strategy for the preparation of N-TiO2 with Vo via the annealing treatment with urea.

1. Introduction

Titanium dioxide (TiO2) has been widely investigated because of its excellent chemical stability, nontoxicity, and low cost [1,2]. Although TiO2 shows great potential in many fields [3,4,5], it can only absorb ultraviolet (UV) light [6], which limits its sufficient absorption of solar light because visible light (43%) takes up the majority of the solar spectrum [7]. In addition, the fast recombination of photogenerated charge carriers in TiO2 impedes its efficient use of solar energy [8,9]. Thus, in order to improve the photocatalytic performance of TiO2, not only does the photoresponse range need to be extended but also the recombination of photogenerated charge carriers needs to be inhibited [10,11].
To extend the photoresponse range of TiO2 from UV light to visible light, the doping strategy is commonly applied to adjust the intrinsic wide bandgap of TiO2 [12,13]. Among various doping strategies, it is reported that nitrogen (N) doping is an effective approach to reduce the bandgap of TiO2 and enables its absorption of visible light [14,15,16]. Generally, the preparation of N-doped TiO2 can be realized by the annealing treatment with the presence of additional N sources such as urea and ammonia [17,18,19]. To reduce the recombination of photogenerated charge carriers, the formation of defective structure is reported to be a useful strategy [6,20]. The introduction of point defects into TiO2, such as oxygen vacancy (Vo), could trap the photogenerated electron inhibiting the recombination of the photogenerated charge carriers [10,21]. Although many methods have been reported for the creation of Vo in TiO2 [22,23,24], the annealing treatment of TiO2 with an organic additive is reported to be an easy approach for the creation of Vo [25,26]. Thus, it is of great interest to realize both the N-doping and creation of Vo to enhance the photocatalytic performance of TiO2.
To realize both the N-doping and introduction of Vo in one step, in this work, a facile strategy has been developed to prepare N-doped TiO2 accompanied by the formation of Vo via the annealing treatment with urea. The effects of the annealing treatment on the photoresponse property and crystal structure of TiO2 have been investigated as a function of the amount of urea. The photocatalytic performance of as-synthesized TiO2 photocatalysts has been evaluated by the photocatalytic degradation of organic pollutants under visible light irradiation.

2. Materials and Methods

2.1. Materials

Hydrothermally prepared TiO2 was obtained according to the reported synthetic procedures [27]. Commercially available TiO2 (P25, Degussa, Evonik, Resource Effiency GmbH, Essen, Germany) was purchased and used without further treatment. Custom-built high-borosilicate glass tubes were used for the annealing treatment. Methyl orange (MO, AR grade, Beijing Chemical Plant Co., Beijing City, China) was chosen as the model organic pollutant for evaluating the photocatalytic performance of all photocatalysts.

2.2. Annealing Treatment

To prepare the N-doped TiO2 via the annealing treatment, both hydrothermally prepared TiO2 and urea were sealed in high-borosilicate glass tubes under vacuum and annealed at 500 °C for 2 h. The amount of TiO2 (100 mg) was kept the same and the weight ratios of TiO2 to urea were set as 1:0, 2:1, 1:1, and 1:2 (Figure S1). The obtained samples were named as A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2), respectively. Similarly, the annealing treatment of P25 with urea was conducted under the same conditions (Figure S2). The obtained samples were abbreviated as A-P25, N-P25 (2:1), N-P25 (1:1), and N-P25 (1:2), respectively (Figure S3). For comparison purposes, the annealing treatment of urea (100 mg) was also performed under the same conditions (Figures S4 and S5). All the as-synthesized samples were collected for further characterizations and tests.

2.3. Characterizations

The diffuse reflectance spectra (DRS) were measured by the UV-vis diffuse reflectance spectroscopy (UV-2600, Shimadzu, Kyoto, Japan). The diffraction patterns were recorded by X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) with a Cu-Kα radiation source (λ = 1.5406 Å). Brunauer–Emmett–Teller (BET) surface areas were determined with an accelerated surface area and porosity analyzer (ASAP 2460, Micrometrics, Norcross, GA, USA). Electron paramagnetic resonance (EPR, ER200DSRC, Bruker, Mannheim, Germany) spectra were taken by applying an X-band (9.44 GHz, 2.47 mW) microwave and sweeping magnetic field at room temperature. The ultraviolet-visible (UV-vis) spectra were obtained with a UV-vis spectrophotometer (Lambda 35, PerkinElmer, Waltham, MA, USA).

2.4. Photocatalytic Performance

Photocatalytic degradation of MO was carried out under visible light irradiation (Xenon lamp, 300 W, PerfectLight, Beijing, China) with a long-wavelength pass filter (>420 nm). TiO2 photocatalysts (25 mg, 1 mg/mL) were added to aqueous solutions of MO (25 ppm, 25 mL) and stirred in dark for 60 min. The concentration variation of MO as a function of irradiation time was monitored by measuring its characteristic peak centered at 464 nm. The photocatalytic degradation ratio of MO was estimated by the expression: (C0 − C)/C0 × 100%, where C0 is the initial concentration of MO and C corresponds to the concentration of MO at different time intervals.

3. Results and Discussion

3.1. Characterizations of TiO2 Photocatalysts

As shown in Figure 1, hydrothermally prepared TiO2 appears as a white powder before the annealing treatment (Figure 1a and Figure S1a). In the absence of urea, TiO2 turns to a dark grey powder after the annealing treatment (Figure 1b and Figure S1e) due to the pyrolysis of the organic solvent [27,28]. With the presence of urea (Figure S1b–d), TiO2 is transformed to a brown powder after the annealing treatment (Figure S1f–h). This obvious color change indicates that N-doping of TiO2 may happen during the annealing treatment with urea as the N source [29,30]. Although the amount of urea increases, the colors of all N-TiO2 samples are similar, implying that a low amount of urea is enough for N-doping (Figure 1c–e). This facile strategy is also suitable for the preparation of N-doped P25 (Figure S2) and white P25 changes to a brown powder after the annealing treatment with urea (Figure S3). Since the annealing treatment results in no clear color change of urea (Figures S4 and S5), it is reasonable to propose that the pyrolysis of urea leads to the N-doping of TiO2 during the annealing treatment.
To investigate the effects of the annealing treatment on the photoresponse property of TiO2, the DRS spectra of all samples were recorded. According to Figure 2a, both TiO2 and A-TiO2 show clear UV absorption and a negligible visible light response. This result is in agreement with the color of TiO2 and the reported phenomenon [27]. Conversely, all N-TiO2 samples present obvious visible light absorption with almost identical absorbance, which further proves that a low amount of urea is enough for N-doping. The annealing treatment of TiO2 with urea definitely expands the photoresponse range of TiO2 to the visible light range suggesting that N-doping occurs [13,31]. To further confirm the N-doping of TiO2, N1s XPS fine spectra of all samples were collected. Compared with TiO2 and A-TiO2, all N-TiO2 samples show a clear N1s peak which undoubtedly proves that the N-doping of TiO2 occurs [32,33]. In addition, the peak intensity increases as the amount of urea goes up, implying an increase in the N-doping level [18,34]. Thus, the annealing treatment of urea offers a facile approach for the preparation of N-doped TiO2 to extend the photoresponse range of TiO2.
To further study the influences of the annealing treatment on the crystal structures of TiO2, XRD measurements were conducted and the results are shown in Figure 3. All the characteristic diffraction patterns of TiO2 photocatalysts are in accordance with the diffraction peaks of anatase, proving that the crystal phase of all TiO2 samples is anatase (JCPDS-21-1272) [25,35]. In the absence of urea, the crystallinity of TiO2 is improved after the annealing treatment as indicated by the increased intensity of the diffraction patterns which correspond to the (101) and (200) crystal planes. However, the crystallinity of all N-TiO2 samples is similar to that of TiO2 after the annealing treatment with urea. This unexpected phenomenon suggests that N-doping hampers the further crystallization of N-TiO2 which may enlarge its specific surface area [3,36]. In addition, new diffraction patterns appear close to the (101) crystal plane of N-TiO2 (1:2), suggesting that a low amount of urea is enough for the N-doping.
The BET surface areas of all samples were measured to further analyze the effect of the annealing treatment on TiO2. From Table 1, it is clear that the BET surface area of hydrothermally prepared TiO2 (134.67 m2/g) is the largest whereas that of A-TiO2 is the smallest (40.21 m2/g). Compared with TiO2, the specific surface areas of N-TiO2 samples decrease and a decreasing tendency is observed as the amount of urea increases. Clearly, the annealing treatment of TiO2 with urea inhibits its crystallization which is in agreement with the results of XRD (Figure 3). For the moment, the reason for this phenomenon has been reported and it may result from the N-doping induced by the annealing treatment [3]. This phenomenon is also observed in the P25 samples obtained after the annealing treatment with urea (Table S1). As a result, the annealing treatment with urea offers a facile approach for the preparation of N-doped TiO2 with more active sites reserved.
To further investigate the influences of the annealing treatment on the crystal structure of TiO2, EPR spectra of all samples were recorded and are shown in Figure 4. As shown in Figure 4, the characteristic peak with a g value of 2.002 corresponds to Vo [20,37]. Compared with TiO2, A-TiO2 contains a certain amount of Vo which may result from the both the crystallization of TiO2 and the pyrolysis of the organic solvents [25,26]. With the presence of urea, more Vo is introduced into the N-doped TiO2 and its amount increases first and then decreases as the amount of urea increases. This result may be due to the N-doping process which not only induces the formation of Vo but can also occupy the Vo. The formation of Vo in N-doped TiO2 may contribute to the separation of photogenerated charge carriers [38,39]. Thus, the annealing treatment with urea not only induces the N-doping to extend the photoresponse range of TiO2 but also creates Vo in N-doped TiO2 which contributes to the separation of photogenerated charge carriers, which could both favor the enhancement of the photocatalytic performance of TiO2.

3.2. Photocatalytic Performance

The photocatalytic performance of all TiO2 photocatalysts was evaluated by the photocatalytic degradation of MO under visible light irradiation. From Figure 5, all TiO2 photocatalysts are capable of adsorbing a certain amount of MO and N-TiO2 (1:2) shows (1:1) the highest adsorption capacity for MO (12.9%). Compared with TiO2, the specific surface area of N-TiO2 (1:2) is smaller and its improved adsorption capacity of MO may be due to the formation of functional groups, which is in agreement with the XRD analysis (Figure 3). During visible light irradiation, both TiO2 and A-TiO2 are not able to degrade MO because they do not absorb the visible light (Figure 2a). With N-TiO2 as the photocatalyst, the photocatalytic degradation of MO is feasible and among the samples N-TiO2 (2:1) presents the best photocatalytic performance. The photocatalytic performance of N-TiO2 proves that the annealing treatment with urea extends the photoresponse range of TiO2 to the visible light range [18,40]. The photocatalytic performance of N-TiO2 is comparable to the reported results (Table S2) which may be ascribed to the formation of Vo and the increased specific surface area [33,39,41,42]. Based on the photocatalytic performance, the optimal weight ratio of TiO2 to urea for the preparation of N-TiO2 via the annealing treatment is found to be 2:1.

4. Conclusions

In summary, a facile strategy was developed for the preparation of N-TiO2 via the annealing treatment with urea. On the one hand, the photoresponse of TiO2 is extended by N-doping via the annealing treatment with urea as the N source. On the other hand, Vo is introduced into N-TiO2 which may contribute to the separation of photogenerated charge carriers. In addition, the specific surface area of N-TiO2 is enlarged with the presence of urea during the annealing treatment by inhibiting the crystallization of TiO2. Thus, more active sites could be reserved for photocatalytic reactions. All the above favorable aspects induced by the annealing treatment with urea contribute to the excellent photocatalytic performance of N-TiO2. This facile strategy is also suitable for other TiO2 photocatalysts such as P25 and, thus, our work offers a universal approach for the preparation of N-doped TiO2 via the annealing treatment with urea. The annealing treatment with other additives for different elements doping, not merely N doping, may be also possible and further work is underway.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano14100818/s1, Figure S1: Digital photographs of TiO2 and urea (a–d) before and (e–h) after the annealing treatment (500 °C, 2 h). The weight ratios of TiO2 to urea are (a,e) 1:0, (b,f) 2:1, (c,g) 1:1 and (d,h) 1:2, respectively. Figure S2: Digital photographs of P25 and urea (a–d) before and (e–h) after the annealing treatment (500 °C, 2 h). The weight ratios of P25 to urea are (a,e) 1:0, (b,f) 2:1, (c,g) 1:1 and (d,h) 1:2, respectively. Figure S3: Digital photographs of (a) P25, (b) A-P25, (c) N-P25 (2:1), (d) N-P25 (1:1), and (e) N-P25 (1:2). Figure S4: Digital photographs of urea (a) before and (b) after the annealing treatment (500 °C, 2 h). Figure S5: Digital photographs of the urea (a) before and (b) after the annealing treatment (500 °C, 2 h). Table S1: BET surface area of P25, A-P25, N-P25 (2:1), N-P25 (1:1), and N-P25 (1:2). Table S2: Comparison of photocatalytic activity of N-TiO2 photocatalyst for degradation of MO [43,44,45,46,47,48,49].

Author Contributions

Z.Z.: investigation, visualization, formal analysis, and writing—original draft. Y.X.: validation, formal analysis, and investigation. M.N.G.: validation and data curation. C.C.-J.: validation, formal analysis and data curation. D.P.: validation and formal analysis. W.W.: validation and formal analysis. Z.C.: conceptualization, project administration, supervision, resources, writing—review and editing and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 12005086).

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors acknowledge the China Scholarship Council (CSC) for the financial support.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Digital photographs of (a) TiO2, (b) A-TiO2, (c) N-TiO2 (2:1), (d) N-TiO2 (1:1), and (e) N-TiO2 (1:2).
Figure 1. Digital photographs of (a) TiO2, (b) A-TiO2, (c) N-TiO2 (2:1), (d) N-TiO2 (1:1), and (e) N-TiO2 (1:2).
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Figure 2. (a) DRS spectra and (b) N1s XPS fine spectra of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
Figure 2. (a) DRS spectra and (b) N1s XPS fine spectra of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
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Figure 3. XRD patterns of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2). The arrow shows the (200) diffraction peak of A-TiO2 [25].
Figure 3. XRD patterns of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2). The arrow shows the (200) diffraction peak of A-TiO2 [25].
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Figure 4. EPR spectra of TiO2, ATiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
Figure 4. EPR spectra of TiO2, ATiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
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Figure 5. Photocatalytic degradation curves of MO by TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2) under visible light irradiation.
Figure 5. Photocatalytic degradation curves of MO by TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2) under visible light irradiation.
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Table 1. BET surface area of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
Table 1. BET surface area of TiO2, A-TiO2, N-TiO2 (2:1), N-TiO2 (1:1), and N-TiO2 (1:2).
SampleBET (m2/g)
TiO2134.67
A-TiO240.21
N-TiO2 (2:1)107.59
N-TiO2 (1:1)73.36
N-TiO2 (1:1)51.87
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Zhang, Z.; Cui, Z.; Xu, Y.; Ghazzal, M.N.; Colbeau-Justin, C.; Pan, D.; Wu, W. A Facile Strategy for the Preparation of N-Doped TiO2 with Oxygen Vacancy via the Annealing Treatment with Urea. Nanomaterials 2024, 14, 818. https://doi.org/10.3390/nano14100818

AMA Style

Zhang Z, Cui Z, Xu Y, Ghazzal MN, Colbeau-Justin C, Pan D, Wu W. A Facile Strategy for the Preparation of N-Doped TiO2 with Oxygen Vacancy via the Annealing Treatment with Urea. Nanomaterials. 2024; 14(10):818. https://doi.org/10.3390/nano14100818

Chicago/Turabian Style

Zhang, Zhe, Zhenpeng Cui, Yinghao Xu, Mohamed Nawfal Ghazzal, Christophe Colbeau-Justin, Duoqiang Pan, and Wangsuo Wu. 2024. "A Facile Strategy for the Preparation of N-Doped TiO2 with Oxygen Vacancy via the Annealing Treatment with Urea" Nanomaterials 14, no. 10: 818. https://doi.org/10.3390/nano14100818

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