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The photocatalytic ·OH production activity of g-C3N4 improved by the introduction of NO
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Jun Jinga, Kai Qia, Guohui Dong*, a, Mengmeng Wanga, Wingkei Ho*, b
Chinese Chemical Letters | 2022, 33(10) : 4715 - 4718
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Chinese Chemical Letters | 2022, 33(10): 4715-4718
• Communication •
The photocatalytic ·OH production activity of g-C3N4 improved by the introduction of NO
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Jun Jinga, Kai Qia, Guohui Dong*, a, Mengmeng Wanga, Wingkei Ho*, b
Affiliations
  • a School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
  • b Department of Science and Environmental Studies and Centre for Education in Environmental Sustainability, The Education University of Hong Kong, Hong Kong, China
Published: 2022-10-15 doi: 10.1016/j.cclet.2021.12.071
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The efficiency of photocatalytic pollutant removal largely depends on the ability of the photocatalytic system to produce hydroxyl radicals (·OH). However, the capability of photocatalyst to produce ·OH is not strong at present. Advancing the capacity of photocatalytic system to produce ·OH has always been a tough problem and challenge in the field of environmental science. In this research, it was found that introducing nitric oxide (NO) into the graphitic carbon nitride (g-C3N4) photocatalytic system could memorably enhance the ability of producing ·OH group. This study provides a new idea for improving the capacity of photocatalytic ·OH production.

Active oxygen species  /  Nitrogen oxide  /  Photocatalysis  /  Organic pollution  /  g-C3N4
Jun Jing, Kai Qi, Guohui Dong, Mengmeng Wang, Wingkei Ho. The photocatalytic ·OH production activity of g-C3N4 improved by the introduction of NO[J]. Chinese Chemical Letters, 2022 , 33 (10) : 4715 -4718 . DOI: 10.1016/j.cclet.2021.12.071
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Active oxygen species (AOS) are effective green oxygenants for environmental-pollution control. AOS have many types, including superoxide radical (·O2–), hydrogen peroxide (H2O2), and hydroxyl radical (·OH) [1-8]. To remove environmental pollution, many methods such as electrochemistry, Fenton reaction, and photocatalysis have been developed to produce ·OH. Electrochemistry method requires substantial electric energy, and Fenton reaction requires extra H2O2 [9-11]. Moreover, these two methods may harm the environment. For example, electrochemistry method may release metal ions and harmful gas, and Fenton reaction may produce iron sludge [12, 13]. Photocatalytic technology has sufficient energy source (sunlight) and is environmentally friendly [14]. Meanwhile, photocatalytic technology could produce ·OH under mild conditions. Thus, photocatalytic ·OH production has the potential for practical applications [15]. However, the ·OH production efficiencies of photocatalysts remain at a low level because the recombination rate of photogenerated electron and hole is relatively high. Many methods have been developed to solve this problem. For example, the photocatalyst can be modified by various modification methods (e.g., doping, heterojunction formation, and noble-metal deposition) to suppress the recombination of photogenerated electron and hole, or a sacrificial agent may be added to improve the redox half-reaction [12, 16-18]. However, photocatalyst modification is a complicated process [19-21], and adding a sacrificial agent may cause environmental pollution. For example, Xiao's team attempted to improve the photocatalyst with ozone (O3) as a sacrificial agent [5, 7, 8, 15]. O3 is known to be harmful to human health and the environment because of the lung damage it inflicts. Thus, developing a new method of improving the photocatalytic production of ·OH is very important.
Nitrogen oxide (NOx), a major pollutant in the atmosphere, originates from the combustion of fossil fuel in boilers and motor vehicles (exhaust gas) [22]. The N atom of NO has an unpaired electron, so NO possesses strong reducing property (Fig. S1 in Supporting information) [23, 24]. Therefore, we believe that NO can be used as a sacrificial agent for photogenerated hole, meaning that photocatalytic AOS production can be improved by NO bubbling. Meanwhile, NO can be oxidized to NO2 by using a photogenerated hole, and then the generated NO2 is absorbed by water (became NO3−) [25]. In this case, the NO in the exhaust gas can be used and further removed. To verify the above analysis, the visible-light photocatalyst g-C3N4 was used to produce ·OH through NO bubbling. We found that the introduction of NO could improve the photocatalytic ·OH generation activity of g-C3N4. In this process, the structure and activity of g-C3N4 are very stable [22, 26]. The improvement mechanism was studied systematically. The details can be found as below.
We detected the generation of active oxygen species (AOS) with or without NO gas (500 ppb, diluted by air stream) bubbling. Figs. 1a and b show the EPR signal of ·O2–. No clear ·O2– signal was observed in darkness. After the light was turned on, ·O2– can be produced in g-C3N4 suspension and g-C3N4/NO suspension. However, the bubbling of NO did not significantly affect the intensity of generated ·O2–. Figs. 1c and d display the generated H2O2 under dark and light-irradiation conditions. H2O2 can be produced only under light irradiation in the presence of g-C3N4 sample. Similarly, with ·O2–, NO bubbling did not significantly affect the intensity of generated H2O2. As for ·OH, the effect of NO bubbling was highly significant. Fig. 1e shows that ·OH was undetected in darkness. After the light was turned on, ·OH can be produced in g-C3N4 suspension and g-C3N4/NO suspension (Fig. 1f). Surprisingly, the concentration of generated ·OH in the g-C3N4/NO system was nearly twice as high as that in the g-C3N4 suspension. Therefore, NO bubbling helped g-C3N4 produce more ·OH under light irradiation (see Supporting information for details).
To understand the mechanism of NO in inducing g-C3N4 to produce more ·OH under light irradiation, we analyzed the converted products of NO during photocatalysis. In the photocatalytic reaction of NO in the gas-solid phase (Eq. 1 and Fig. 2a), most reaction products were NO2 [3]. However, the NO2 signal disappeared when the hole scavenger was added, suggesting that NO can be oxidized to NO2 by hole. This can be further proved by the hole capture experiment (Fig. S3a in Supporting information). Interestingly, NO2 cannot be detected when the photocatalytic reaction proceeded in the liquid-solid phase (Fig. 2b). This result suggested that the generated NO2 decomposed (Eq. 2) or absorbed (Eq. 3) in water. If NO2 decomposes, the generated oxygen free radicals (·O) would react with water to produce ·OH [27]. To verify this mechanism, NOx was bubbled in water (without the presence of photocatalysts) under light irradiation (λ = 420 nm). The results are displayed in Fig. 2c. ·OH can be detected in the NO2-bubbled water but not in the NO-bubbled one. These results confirmed Eqs. 2 and 4. Accordingly, NO2 photolysis can contribute to the increase in ·OH yield. Fig. 2d shows that NO2 photolysis (70 ppb) could produce only 3.33% of the ·OH increment. Thus, other reasons for the ·OH increment besides NO2 photolysis must exist. The photolysis-mechanism diagram of NO2 is shown in Fig. 2e. Extra NO2 would be absorbed by water and became NO3−. Besides, the conversion rate of NO to NO3− is 47%. (Fig. S4 in Supporting information).
Since NO could consume hole and inhibit the recombination of photogenerated carriers. Interestingly, NO bubbling obviously improved the photocurrent signal of g-C3N4 under the same condition. And the further confirmation could be found in Fig. S3b (Supporting information). This phenomenon ensured that NO could inhibit the recombination of photogenerated carriers. Consequently, the valence band of g-C3N4 can provide more excitation electrons for the higher fluorescent signal (Fig. S3c in Supporting information). For a visual expression, the reaction-mechanism diagram is shown in Fig. S3d (Supporting information). The more consumption of photogenerated hole could be created, the more adsorbed O2 molecules could be reduced.
Based on the redox potential of H2O2/O2 (0.659 V) [28] and the valence-band potential of g-C3N4 (1.4 eV) [29], H2O2 may be oxidized to O2 by the photogenerated holes (Eq. 5). This speculation can be proven by H2O2 catalytic-decomposition experiment. (Figs. 3a and b and S2.3 in Supporting information). To investigate the effect of NO on H2O2 catalytic decomposition, NO was introduced in the above experiments. Fig. 3c shows that the amount of produced O2 significantly decreased after NO introduction, indicating that NO can inhibit the oxidation reaction of H2O2 caused by photogenerated holes consumed. Meanwhile, the redox potential of NO/NO2 (0.428 V) was smaller than that of H2O2/O2 (0.659 V), which can be proven by the cyclic voltammetry curve in Fig. 3d. In this case, H2O2 can be protected by NO from the oxidation of photogenerated holes, just like the drawing in Fig. 3e. Therefore, more H2O2 can be used to produce ·OH (Eq. 6).
After ·OH generation, it diffused in water. However, NO is insoluble in water, so NO cannot consume the ·OH generated in our system. When NO was oxidized to NO2 by photogenerated holes, NO2 was adsorbed by water and became NO3– and NO. However, NO3– cannot be further oxidized, so it cannot consume the generated ·OH. In this case, NO2 did not consume the generated ·OH in our system. Based on the above experimental results and discussion, although NO was used as the hole sacrificial agent, and it did not consume ·OH. For this reason, our system could accumulate more ·OH for the degradation of organic pollutants [30] (Fig. S5 and S2.4 in Supporting information). Herein, Fig. 4 can be well matched with this theme.
Stability is an important evaluation indicator for the application value of one technology or sample [31]. Accordingly, cycling experiments were conducted to investigate the stabilities of this technology and g-C3N4 sample [32, 33]. Fig. S6a (Supporting information) shows that the ·OH yield did not decrease after five cycles of use. Meanwhile, the degradation rate of tetracycline performed using our technology was very stable throughout the five cycles of repeat experiments (Fig. S6b in Supporting information). These results suggested that our technology was stable and recyclable. The stability can be further confirmed by the XRD, FT-IR, UV-DRS, and SEM characterizations. A detailed analysis can be found in Figs. S6c, S6d, S7 and S8 (Supporting information).
In conclusion, the introducing NO could significantly enhance the photocatalytic ·OH production ability of g-C3N4. After a series of mechanism studies, we found that there were four reasons for this improvement: (1) The ·O produced by the NO2 photolysis can react with H2O to form ·OH; (2) NO can be used as a hole-capture agent, which makes g-C3N4 generate more photo-generated electrons to reduce O2; (3) H2O2 can be protected by NO from being oxidized by holes; (4) ·OH cannot be consumed by NO or NO2. Since NO could enhance the ·OH production, NO was introduced in the photocatalytic tetracycline removal process. It was found that NO could enhance the mineralization rate of tetracycline from 59% to 86%. Moreover, the presence of NO in the photocatalytic process does not affect the stability of g-C3N4.
Declaration of competing interest
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The authors report no declaration of competing interest.
Acknowledgments
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Financial support by the National Nature Science Foundation of China (Nos. 21876104 and 21603271) is gratefully appreciated. The work described in this paper was also partially supported by the General Research Fund (Nos. 18300920 and 18301117) of Research Grants Council, the Department Collaborative Fund (No. 04490), Dean's Research Fund (No. FLASS/DRF 04554) of the Faculty of Liberal Arts and Social Sciences, The Education University of Hong Kong, Hong Kong Special Administrative Region, China.
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.2021.12.071
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Year 2022 volume 33 Issue 10
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Article Info
doi: 10.1016/j.cclet.2021.12.071
  • Receive Date：2021-10-28
  • Online Date：2025-12-24
  • Published：2022-10-15
Article Data
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  • Received：2021-10-28
  • Revised：2021-11-18
  • Accepted：2021-12-26
Affiliations
    a School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
    b Department of Science and Environmental Studies and Centre for Education in Environmental Sustainability, The Education University of Hong Kong, Hong Kong, 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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