Microwave-assisted synthesis of oxygen vacancy associated TiO<sub>2</sub> for efficient photocatalytic nitrate reduction

Microwave-assisted synthesis of oxygen vacancy associated TiO₂ for efficient photocatalytic nitrate reduction

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Qian Li^a^,¹, Yunni Liu^b^,¹, Zhe Wan^a, Haiyan Cao^a, Shao Zhang^a, Yue Zhou^a, Xingyu Ye^a, Xiaoyan Liu^a^,^*, Dieqing Zhang^a^,^*

Chinese Chemical Letters | 2022, 33(8) : 3835 - 3841

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Chinese Chemical Letters | 2022, 33(8): 3835-3841

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Microwave-assisted synthesis of oxygen vacancy associated TiO₂ for efficient photocatalytic nitrate reduction

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Qian Li^a^,¹, Yunni Liu^b^,¹, Zhe Wan^a, Haiyan Cao^a, Shao Zhang^a, Yue Zhou^a, Xingyu Ye^a, Xiaoyan Liu^a^,^*, Dieqing Zhang^a^,^*

Affiliations

^a The Education Ministry Key Lab of Resource Chemistry, Joint International Research Laboratory of Resource Chemistry, Ministry of Education, and Shanghai Key Laboratory of Rare Earth Functional Materials, College of Chemistry and Materials Science, Shanghai Normal University, Shanghai 200234, China

^b School of Environmental and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China

Published: 2022-08-15 doi: 10.1016/j.cclet.2021.12.025

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Abstract

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The solar-driven photocatalytic technology has shown great potential in nitrate (NO₃⁻) pollutants reduction, however, it has been greatly hindered by the complex preparation and high cost of photocatalysts. Herein, a relatively low-cost photocatalyst, rutile and anatase mixed phase TiO₂ was synthesized by a facile microwave-hydrothermal method. Meanwhile, oxygen vacancy is successfully generated, leading to an acidic surface for strong adsorption towards NO₃⁻, which further improved the reduction activity. Compared with the commercial P25, a higher NO₃⁻ conversion of ca. 100% and nitrogen (N₂) selectivity of 87% were achieved under UV (365 nm) irradiation within 2 h. This research provides a promising strategy for designing efficient noble metal free photocatalyst in the NO₃⁻ reduction.

Key words

Photocatalysis / TiO₂ / Oxygen vacancy / Nitrate reduction

Cite this Article

Qian Li, Yunni Liu, Zhe Wan, Haiyan Cao, Shao Zhang, Yue Zhou, Xingyu Ye, Xiaoyan Liu, Dieqing Zhang. Microwave-assisted synthesis of oxygen vacancy associated TiO₂ for efficient photocatalytic nitrate reduction[J]. Chinese Chemical Letters, 2022 , 33 (8) : 3835 -3841 . DOI: 10.1016/j.cclet.2021.12.025

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Nitrogen (N) is an essential nutrient, but when its concentration accumulated to some threshold value, it could be a source of pollution in water or the atmosphere [1]. Nitrates (NO₃⁻) as one of the most common N species contaminants in the world, was mainly resulting from the use of nitrogen fertilizers and the dung from large animal farms [2]. However, high intake of nitrate would appear a serious threat to human health, such as an increased risk of natural preterm birth and central nervous system cancers (CNC) in children [2-4]. Furthermore, the nitrate can be reduced to dangerous chemicals, including nitrite (NO₂⁻), which caused blue baby syndrome [5]. Thus, many strategies such as reverse osmosis, electrodialysis and ion exchange have been widely studied for removing nitrate from ground water [6]. However, these works highly concentrated on the nitrate conversion into brines instead of removing it to harmless nitrogen (N₂) [7, 8].

Photocatalysis as an environment-friendly technology shows great potentials in pollutant removal by directly using solar energy [9-13]. As a semiconductor, titanium dioxide (TiO₂) has been widely used in photocatalytic nitrogen oxides (NO_x) oxidation [10, 11, 14-17], carbon dioxide (CO₂) reduction [18], hydrogen (H₂) production [19] and removal of volatile organic pollutants (VOCs), etc. [20], due to its low-cost, nontoxicity and good stability [21, 22]. However, its application in NO₃⁻ conversion has been limited because of the low efficiency of traditional TiO₂. Noble metals (e.g., Pd, Au, and Ag) loaded TiO₂ have been developed for promoted denitrification performance (Table S1 in Supporting information). However, this noble metal involving strategy greatly increased the cost, which is not practical for large-scale application [8, 23, 24]. TiO₂ with oxygen vacancies has attracted intensive attention in photocatalytic NO_x removal, H₂ evolution and CO₂ reduction [25-27] owing to the improved charge separation and reactant molecules adsorption. TiO₂ materials with oxygen vacancies are traditionally produced by hydrogen reduction or NaBH₄ reduction, both of which are time and energy consuming [28].

Herein, the mixed anatase and rutile phase TiO₂ (TiO₂-A-R) with oxygen vacancies and proper acid sites was successfully prepared by a simple microwave hydrothermal method. The obtained TiO₂-A-R showed outstanding photocatalytic NO₃⁻ conversion of ca. 100% and high N₂ selectivity of 89% under ultraviolet irradiation. This study will provide a novel approach for efficient and low-cost nitrate removal from water.

The synthetic procedure of TiO₂-A-R was illustrated in Scheme 1. In a typical process, potassium titanium oxide oxalate dihydrate and sodium chloride were dispersed in the mixed solution of ethanol and water, keeping stirring for 15 min. Then the as-prepared solution was transferred to the microwave reaction chamber for further microwave treatment under 200 ℃ for 30 min. As comparison, pure rutile (TiO₂-R) and pure anatase (TiO₂-A) were prepared. As shown in Fig. 1a, the TiO₂-A-R shows the typical X-ray diffraction (XRD) peaks of rutile and anatase phases, which are similar to the commercial P25. Besides, according to the peak intensity, the weight fraction of the rutile in TiO₂-A-R, W_R can be calculated from the formula (Eq. 1) [29, 30]. And the weight fraction of the anatase in TiO₂-A-R, W_A can be calculated from the formula as follows (Eq. 2) [31]:

(1)

(2)

In formula, A_ana and A_rut represented the diffraction peaks intensity of anatase (101) and rutile (110), respectively. Based on the XRD results, the anatase content of TiO₂-A-R was estimated to be 84 wt%. The ultraviolet-visible diffuse reflectance spectra (UV–vis DRS) in Fig. 1b demonstrated that all samples exhibited spectral absorption at 365 nm, which ensured the photocatalyst could be effectively excited under 365 nm irradiation. In addition, based on the UV–vis results, the band gap energy (E_g) of the TiO₂-A-R was calculated as followed (Eq. 3):

(3)

where α, h, v, A and E_g represented the absorption coefficient, the Planck constant, the light frequency, the constant and band gap, respectively [32]. Furthermore, n was equal to 1/2 or 2 for an indirect or direct band gap semiconductors, respectively. Thus, the estimated E_g of TiO₂-A-R is 3.18 eV (Fig. S1 in Supporting information). The calculated flat band potential (E_FB) value of TiO₂-A-R as shown in Fig. S2 (Supporting information) was –0.95 V vs. SCE, which is corresponding to −0.29 V vs. NHE. Besides, TiO₂-A-R as an n-type semiconductor, the conduction band (E_CB) was 0.2 V below E_FB [33]. Thus, E_CB level of TiO₂-A-R was −0.49 eV. And the valence band (VB) was 2.69 eV, which was obtained according to the formula Eq. 4 [32, 34]:

(4)

The nitrogen adsorption-desorption isotherms and pore size distributions were displayed in Figs. 1c and d. All the samples showed typical Ⅳ isotherms and a pore size distribution ranging from 2 nm to 50 nm, indicating the mesoporous structures. The detailed surfaces areas, pore size and pore volume were listed in Table 1. Compared with other photocatalysts [35-37], TiO₂-A-R had the distinguishing features of larger Brunauer-Emmett-Teller (BET) surface area (97.6 m²/g), the greater pore volume (0.3 cm³/g) and the pore diameter (11.4 nm), which might significantly enhance its adsorption ability of reactants and therefore facilitate the targeted reaction [38].

TEM images in Figs. 2a and b suggested that TiO₂-A-R was composed of flaky petal-like structure. Moreover, obvious diffraction rings could be observed in Fig. 2c (the selected area electron diffraction, SAED), indicating the TiO₂-A-R had good crystallinity [39]. Meanwhile, the lattice spacing of 0.351 nm and 0.325 nm were also clearly detected in Fig. 2d, which corresponded to (101) and (110) plane of anatase and rutile, respectively [40]. The element mappings demonstrated the homogeneous distribution of Ti and O in TiO₂-A-R (Fig. 2e). These results further confirmed the successful synthesis of mixed-phase titanium dioxide.

In order to probe the surface chemical compositions and the binding configuration of all the samples, X-ray photoelectron spectroscopy (XPS) measurement was performed. P25 and TiO₂-R show the peaks located at 458.9 and 464.6 eV corresponding to Ti 2p_3/2 and Ti 2p_1/2 (Fig. 3a). These peaks of TiO₂-A slightly shifted to lower binding energies. Notably, a clearly negative shift was also observed in the TiO₂-A-R, indicating the existence of Ti³⁺ [41]. Meanwhile, the O 1s XPS spectra of TiO₂-A-R presented two peaks centered at ~529.8 and ~531.6 eV (Fig. 3b), representing for the lattice oxygen and oxygen vacancy, respectively [7]. And the area ratio of oxygen vacancy peak (named O2) to the sum area of the O1 and O2 peaks (named Os) is shown in Table S2 (Supporting information). The O2/Os of TiO₂-A-R had the largest percentage (22.7%), which demonstrated that oxygen vacancy rooted more in the TiO₂-A-R sample [28].

Moreover, the strong electron paramagnetic resonance (EPR) signal (Fig. 4a) in TiO₂-A-R with a g-value of 2.001 further verified the existence of oxygen vacancy [42], which might play a vital role in promoting the rapid conversion of nitrate as previously reported [28]. Furthermore, the catalysts' activities are closely related to their surface properties such as alkaline and acidity properties [43]. Moreover, the NO₃⁻ presents Lewis base due to its electronegativity, which implies that it is easier to combine with the Lewis acid catalyst surface [44, 45]. Thus temperature-programmed desorption of ammonia (NH₃-TPD) of P25 and TiO₂-A-R were performed from 50 ℃ to 800 ℃ to find out their surface properties, and the curves were illustrated in Fig. 4b [46, 47]. The desorption peaks of NH₃ located below 200 ℃, 200–400 ℃ and above 400 ℃ are considered as indicators of the weak, medium and strong acid sites, respectively [48]. P25 exhibited three NH₃ desorption peaks at 200, 487 and 611 ℃, respectively. The former one is assigned to the weak acid sites and the other two peaks are indexed to the strong acid sites. As comparison, there are only medium (suggested by the peaks at 277 and 384 ℃) and strong acid sites (suggested by the peaks at 540, 606 and 706 ℃) observed, demonstrating its more acidic surface. As displayed in Table S3 (Supporting information), TiO₂-A-R shows a larger peak area than that of P25, confirming there are more active sites to possibly absorb and reduce the NO₃⁻ [44, 49].

Normally, NO₃⁻ could be reduced to N₂, NO₂⁻ and ammonium (NH₄⁺), but both NO₂⁻ and NH₄⁺ are hazardous to the environment. An ideal photocatalyst should have high NO₃⁻ conversion and good N₂ selectivity [24, 50]. In order to inhibit the rapid recombination of electron-hole pairs, formic acid (FA) was selected as a hole scavenger in this reaction [51]. To exclude the catalytic effect of FA on NO₃⁻ reduction, we conducted a control experiment (in FA without photocatalyst added) and the results were shown in Fig. S3a (Supporting information). No catalytic activity was observed in the absence of the photocatalyst, and thus we can conclude that FA itself will not react with NO₃⁻ and promote NO₃⁻ reduction. Then, TiO₂-A-R (0.060 g) and various amounts of FA were dispersed into 60 mL nitrate solution (50 mg/L) to evaluate the optimal photocatalytic performance. As shown in Figs. 5a-c, the activity sequence involving different amounts of FA were summarized as 5 mL FA ≈ 4 mL FA > 3 mL FA. Meanwhile, almost no NO₂⁻ was detected in the reaction process of the three controlled trials. When the reaction progressed to 120 min, the average conversion of NO₃⁻ involving 3 mL FA was 98%, the average selectivity of N₂ and NH₄⁺ were 88% and 12%, respectively. The experiments with 4 mL or 5 mL FA showed similar nitrate conversion (almost 100%), N₂ selectivity (89%) and NH₄⁺ selectivity (11%). Therefore, we determined adding 4 mL FA as the optimal hole scavenger amount for this reaction.

According to the literature, different hole scavengers such as oxalic acid (OC) and methanol may also be favorable for the photocatalytic nitrate reduction [50]. Thus, the 4 mL 0.1 mol/L methanol and OC solution was introduced for photocatalytic NO₃⁻ (50 mg/L) reduction experiment as shown in Fig. 5d. The OC involving system demonstrated the NO₃⁻ conversion of 11% and the 78% N₂ selectivity, which were much lower than that of FA. No NO₃⁻ conversions were observed in methanol involving system or hole scavengers absence system, indicating that FA significantly improved the photocatalytic activity [24]. Based on the previous study [51], we confirm that carbon dioxide anion radical (CO₂^•−) generated by reacting FA with photogenerated holes of photocatalyst has strong reductive ability for NO₃⁻ conversion to N₂. Then, EPR test was carried out to probe the production of CO₂^•− in the TiO₂-A-R system. As shown in Fig. 6, no signals were detected under dark conditions. While under light irradiation, a six-line DMPO-CO₂^•− spin adduct signal was formed with hyperfine parameter of magnetic factor g = 2.0059 (Fig. S3b in Supporting information), which can be assigned to reductive CO₂^•− species for further promoting NO₃⁻ degradation [52-54]. These results clearly indicated the important promoted effect of FA in the photocatalytic NO₃⁻ reduction reaction using noble-metal free TiO₂ as photocatalysts.

In order to compare the contribution of TiO₂-A-R, the performance of TiO₂-A, TiO₂-R and commercial P25 were evaluated in 4 mL FA and NO₃⁻ (50 mg/L) mixture solution system (Figs. 7a-c.). The photocatalytic removal rate of these samples followed the order of TiO₂-A-R (100%) > TiO₂-A (82%) > P25 (61%) > TiO₂-R (36%). And the selectivity of N₂ presented the trend of TiO₂-A-R (89%) > TiO₂-A (88%) > P25 (87%) > TiO₂-R (80%). Obviously, TiO₂-A-R showed the enhanced ability of NO₃⁻ reduction, which may be attributed to the existence of Ov and acid sites [28]. In addition, the cycling durability of TiO₂-A-R was conducted and displayed in Fig. 7d. After five cycles, the photocatalyst still had 98% NO₃⁻ conversion, demonstrating its great stability and big potential for practical application. Moreover, when the concentration of the initial NO₃⁻ solution was diluted to 30 mg/L (Fig. 8a), the NO₃⁻ conversion was achieved 100% after 90 min reaction and the selectivity of N₂ and NH₄⁺ reached 86% and 14%, respectively. Even when the NO₃⁻ concentration was increased to 100 mg/L, a high NO₃⁻ removal of 77% and N₂ selectivity of 91% were achieved after 120 min reaction (Fig. 8b), implying the excellent activity of TiO₂-A-R in a wide NO₃⁻ concentrations range.

The photocurrent response (Fig. 9a) was carried out to evaluate the charge transport properties [10]. Compared with commercial P25, TiO₂-A-R has a higher photocurrent density, which demonstrates the improved light source usage rate and effective separation of e⁻ and h⁺ excited by photons [55]. In addition, the steady-state photoluminescence (PL) spectrum was measured to investigate the electrons and holes recombination (Fig. 9b). Notably, a lower emission peak of TiO₂-A-R can be obtained, demonstrating the improved charge carrier separation efficiency [56]. And TiO₂-A-R showed the smaller radius under dark and 365 nm UV-LED irradiation (Figs. 9c and d), indicating a better conductivity [9]. These factors co-contributed the excellent NO₃⁻ conversion and good N₂ selectivity.

Based on the above discussion, the possible NO₃⁻ degradation mechanism is proposed as shown in Scheme 2. Firstly, TiO₂-A-R is excited to produce photo-generated electron-hole pairs under the UV-LED irradiation (Eq. 5). Then the electrons are consumed by NO₃⁻ to generate N₂ or NH₄⁺ (Eqs. 6 and 7) [24]. Meanwhile, the photo-generated holes are scavenged by FA to produce CO₂^•− species, which further reduces NO₃⁻ to N₂ (Eqs. 8 and 9) [8, 44, 57-59].

(5)

(6)

(7)

(8)

(9)

In conclusion, the mixed TiO₂ photocatalyst with oxygen vacancy was successfully synthesized via a facile microwave-assisted method. It has a NO₃⁻ conversion up to ca. 100% under 2 h ultraviolet radiation, which is much high than that of commercial P25 (61%). Moreover, the N₂ selectivity is as high as 89%. This work provides a novel strategy to design noble metal free photocatalysts for cheap, safe and efficient nitrate removal.

Declaration of competing interest

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The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the National Key Research and Development Program of China (No. 2020YFA0211004), and National Natural Science Foundation of China (Nos. 21876112, 21876113, 22022608, 92034301), "111" Innovation and Talent Recruitment Base on Photochemical and Energy Materials (No. D18020), Ministry of Education, and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Engineering Research Center of Green Energy Chemical Engineering (Nos. 18DZ2254200) and Shanghai government (Nos. 18SG41, 309-AC9103–21–413002, 19YF1436600).

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.12.025

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Year 2022 volume 33 Issue 8

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doi: 10.1016/j.cclet.2021.12.025

Receive Date：2021-09-27
Online Date：2025-12-19
Published：2022-08-15

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Received：2021-09-27
Revised：2021-11-06
Accepted：2021-12-11

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^b School of Environmental and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Scheme 1 Schematic illustration of the synthesis processes of TiO₂-A-R.

Fig. 1 (a) The XRD pattern, (b) UV–vis DRS spectra, (c) N₂ adsorption-desorption isotherms and (d) pore size distributions of TiO₂-A-R, TiO_2-A, TiO₂-R and P25 samples.

Fig. 2 (a, b) TEM images, (c) SAED pattern, (d) HRTEM image, (e) SEM image and corresponding elemental mappings of TiO₂-A-R sample.

Fig. 3 XPS spectra of (a) Ti 2p and (b) O 1s over TiO₂-A-R, TiO₂-A, TiO₂-R and P25 samples.

Fig. 4 (a) EPR spectra of TiO₂-A-R, TiO₂-R, TiO₂-A and P25 samples. (b) NH₃-TPD analysis over TiO₂-A-R and P25 sample.

Fig. 5 Photocatalytic nitrate reduction activity of TiO₂-A-R (a-c) with 3–5 mL formic acid and (d) with different hole scavengers.

Fig. 6 DMPO spin-trapping EPR spectra of TiO₂-A-R.

Fig. 7 Photocatalytic nitrate reduction activity of (a) P25, (b) TiO₂-A, and (c) TiO₂-R samples. (d) Cycling stability test of TiO₂-A-R.

Fig. 8 Photocatalytic nitrate reduction activity over the TiO₂-A-R sample involving different NO₃⁻ initial concentrations of (a) 30 and (b) 100 mg/L.

Fig. 9 (a) Photocurrent density, (b) PL spectra (e_x = 290 nm), electrochemical impedance spectra (c) in dark and (d) light irradiation of TiO₂-A-R and P25 samples.

Scheme 2 Schematic illustration of photocatalytic reduction mechanism of NO₃⁻.

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