Highly efficient simulated solar-light photocatalytic oxidation of gaseous NO with porous carbon nitride from copolymerization with thymine and mechanistic analysis

Abstract

We synthesized novel and efficient porous carbon nitride (CN) photocatalysts by facial supramolecular approach using cyanuric acid (C), melamine (M) and thymine (T) as starting material. The T-modified CNs display excellent photophysical and photochemical properties: high specific surface area, strong light adsorption as well as low recombination rate of photoinduced electron–hole pairs. They exhibit tremendous enhanced photocatalytic activity on photocatalytic oxidation (PCO) of NO (∼400 ppm) under simulated solar-light irradiation, wherein the CM + 2.5 mol%-T possesses the highest photoactivity (93.3% in 40 min). The enhanced photocatalytic performance is ascribed to the synergic effect of large specific surface area and high separation and transfer efficiency of photoinduced electron–hole pairs. In the PCO of NO process, the main reaction product is NO₃⁻, which was confirmed by Ion Chromatography. In addition, the mechanism of PCO is also intuitively analyzed by trapping experiment. The results indicate that ˙O₂⁻ plays a leading role in the PCO of NO process.

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1 Introduction

Nitrogen oxide (NO), mainly generated from power plants and vehicle exhausts, has aroused worldwide concerns owing to its alarming releasing rate in the surrounding environment.^1,2 It not only leads to atmospheric environmental problems such as smog, acid rain and ozone depletion, but also poses a serious threat to health conditions, including heart disease, stroke and lung cancer.^3,4 Therefore, numerous conventional denitrification technologies, such as NO_x storage and reduction (NSR),^5,6 catalytic ozonation,^7,8 selective catalytic reduction (SCR)^9–11 and so on, have been developed for NO removal. But these technologies need high operating cost and large energy consumption. Recently, photocatalytic oxidation (PCO) of NO has been reported in succession and their excellent experimental results indicated that PCO could be regarded as a promising denitrification technology.^12,13 Additionally, triggering by cheap solar light, PCO technology possesses economical, efficient and eco-friendly features.¹⁴ Consequently, PCO might provide a desirable pathway for NO removal in terms of conventional technologies. The efficient photocatalyst is crucial to acquire high photocatalytic efficiency in the PCO of NO system.

Recently, carbon nitride (CN) has attracted much attention in that it possesses an appropriate band gap ranging from 2.4 to 2.8 eV,¹⁵ and has been applied in photosynthesis, such as water splitting to hydrogen or oxygen, organic pollution degradation and NO oxidation under light irradiation.^16–18 In general, CN is prepared by direct thermal condensation from appropriate molecular precursors such as urea, melamine, dicyandiamide,^19–21 but the bulk CN exhibits small specific surface area and low photocatalytic performance. The photoactivity of photocatalysts usually depends on their specific surface areas, light absorption edges, and separation rate of photogenerated electron–hole pairs under light irradiation.^22,23 To ameliorate the electronic and photocatalytic performance of CN, the synthesis of modified CNs has been researched intensively.

Many promising methods have been developed to enhance the specific surface area and photocatalytic performance of CN, including hard-templating approaches,^24,25 doping with nonmetal or metal elements,^26–28 treatment with acid or alkali,^29,30 coupling with other semiconductors,^31–33 and so forth. Among these methods, the hard-templating approaches are the common pathways to create porous modified CN by using mesoporous silica or silica nanoparticles as template, but these methods are time consuming and high cost because of the use of sacrificial material and hazardous chemical reagents to remove template.³⁴ Hence, it's desirable to explore more economic and facile procedures to prepare modified CN with large specific surface area and high efficient separation of photoinduced electron–hole pairs, which are conductive to the improvement of photocatalytic activity. Currently, copolymerization using appropriate two or more monomers as starting material, such as melamine and 2,4,6-triaminopyrimidine, melamine and cyanuric acid, dicyandiamide and barbituric acid,^35–37 and so on, is regarded as a facial and efficient synthesis method to prepare modified CNs. The kind of modified CN possess large specific surface area, wide light adsorption and low recombination rate of photoinduced carrier. They have been used on water splitting to hydrogen, degradation of rhodamine B and NO oxidation at ppb levels under light irradiation. But little attention has been given to PCO of NO in flue gas of power plant on modified CN photocatalyst synthesized by using three monomers as starting material.

In this paper, we prepared novel and efficient porous T-modified CN photocatalysts from copolymerization with thymine by self-templating method with simple, safe, cheap and environment-friendly features,^38,39 which were synthesized from the pyrolysis of the supramolecular complex of cyanuric acid (C), melamine (M), and thymine (T) as starting material. Then the T-modified CNs were characterized by several techniques. The results indicate that they have larger specific surface areas, wider solar-light adsorption edges and higher separation and transfer efficiency of photoinduced electron–hole pairs in comparison with bulk CN. And then a concentration of NO (∼400 ppm) was used to investigate the PCO activity of the modified CNs. The enhanced ability of them on the PCO of NO was remarkable and the reasons contributing to this phenomenon were discussed. Additionally, the reaction products and possible mechanism of the PCO of NO were also analyzed.

2 Experiment

2.1 Photocatalyst preparation

The chemicals were directly used as received. Here, the T-modified CNs were prepared by supramolecular approach. Typically, 8 mmol C, 8 mmol M and x mol%-T, with molar ratios of T of x = 2.5 mol%, 5 mol% and 10 mol% were mixed in 100 mL deionized water, and vigorously stirred for 12 h to make sure the formation of crystalline supramolecular complex. Then the formed milky suspension was centrifuged and washed with deionized water for three times. The resulting powders were dried at 80 °C for 4 h under vacuum conditions and then calcined at 550 °C for 4 h at a heating rate of 2 K min⁻¹ under N₂ atmosphere (Fig. 1). The final products were designated as CM + x mol%-T (x = 0, 2.5, 5.0, 10 respectively).

Fig. 1 Synthesis scheme of the T-modified CNs from hydrogen bonded supramolecular complex.

2.2 Photocatalyst characterization

The material structure and chemical information of as-synthesized photocatalysts were characterized by several techniques, including X-ray diffraction (XRD, Cu Kα, Purkinjie XD-3, 35 kV, 20 mA), FTIR (Nicolet-iS10), field-emission scanning electron microscopy (SEM, FEI Quanta 250F), the nitrogen adsorption apparatus (BET-BJH, Quantachrome, autosorb IQ), UV-vis diffuse reflection spectra (DRS, Shimadzu UV-2600), photoluminescence spectra (PL, He–Cd laser, Labram-HR800). The ions in the solution was analyzed by Ion Chromatography (IC, DionexICS90).

2.3 Photocatalytic activity test

The photocatalytic activity test for NO removal was performed in a continuous flow reactor at environmental temperature and atmospheric pressure. The cylindrical reactor was made up of stainless steel and covered with quartz glass, the volume of which was 200 mL. One Teflon omentum containing the photocatalysts was placed in the middle of the reactor. A Xe lamp (350 W) was vertically placed outside the reactor above the sample, which was used as the simulated solar light source.

Photocatalyst (0.1 g) was uniformly dispersed in a small piece of clean cotton, which was placed on the Teflon omentum of the reactor. The premixed reactant gases (400 ppm NO, 7% O₂/N₂ balance) were fed into the reactor at a total flow rate of 100 mL min⁻¹. To provide more active species for the PCO of NO and reduce the accumulation of reaction products on the surface of photocatalysts, 30% H₂O₂ solution was injected into the reactor via a peristaltic pump at a flow rate of 0.044 mL min⁻¹. After reaching adsorption–desorption equilibrium between gases and photocatalysts, the lamp and peristaltic pump were turned on simultaneously and the experiment started. The gas products (every 10 min reaction) were analyzed by using SERVOMEX flue gas analyzer (Britain). The reaction of NO with O₂ was ignorable. The removal efficiency of NO is defined as:

NO conversion% = (NO_in − NO_out)/NO_in × 100%

2.4 Detection of hydroxyl radicals

The hydroxyl radicals formed in the light/T-modified CN/H₂O₂ experimental system were detected by using fluorescence technique, and terephthalic acid (TA) was used as a probe molecular. The detailed procedures were as follows: two quartz tubes (A and B) were filled with 40 mL aqueous solution containing 0.3 mmol TA and 50 μL 30% H₂O₂ aqueous solution at environmental temperature. Then 10 mg CM + 2.5 mol%-T was dispersed in solution of group (B). Subsequently, two tubes were irradiated by Xe lamp (350 W) under stirring condition for 80 min. After reaction, ˙OH in the clear supernatant was measured by using FL3-TCSPC fluorescence spectrophotometer.

2.5 Trapping experiment of active species

As shown in Fig. 2, five quartz tubes divided into four experimental groups (a–d) and control group (e) were filled with 40 mL deionized water and 4 mL 30% H₂O₂ solution. Then 10 mg CM + 2.5 mol%-T was added into each experimental group. Subsequently, 1 mL isopropyl alcohol (IPA) was injected into group (b) so that ˙OH could be quenched, 0.1 mmol p-benzoquinone (BQ) was added into group (c) as the scavenger of ˙O₂⁻, 1 mL ethanol (EtOH) was put into group (d) in order that h⁺ could be captured. Finally, NO was inputted in five quartz tubes, which were exposed to the simulated solar light source under stirring condition for 30 min. After reaction, 1 mL of clear supernatant obtained was taken out and injected into IC to reveal effect of active species.

Fig. 2 Schematic diagram of experimental groups (a–d) and the control group (e) in the trapping experiment.

2.6 Photoelectrochemical measurement

Electrochemical impedance spectra (EIS) measurement were carried out on the CHI 660B electrochemical workstation with a conventional three-electrode system using 0.5 mol L⁻¹ Na₂SO₄ as electrolyte solution. The work electrode was prepared by the following process: 2 mg photocatalyst was dispersed in mixed solution composed of 850 μL deionized water, 150 μL isopropyl alcohol and 26 μL naphthol to make homogeneous suspension. Then the suspension was dip-coated on the exposed area of the conductive side of the FTO glass (1 cm × 2 cm) and then dried at 180 °C for 12 h under N₂ atmosphere.

3 Results and discussion

3.1 Structural characterization

Power X-ray diffraction and FTIR spectra were performed to study the crystal structure of bulk CN and T-modified CNs (Fig. 3a). The patterns of the T-modified CNs are similar to that of bulk CN. Also, two distinct diffraction peaks can be found. The small angle peaks (indexed as 001) at ca. 13.32° are the in-plane structural packing motif,⁴⁰ which aren't observed remarkable variations. The strong peaks (indexed as 002) at ca. 27.2° are ascribed to the interlayer-stacking of aromatic system. However, the (002) peaks become weaker and broader than that of bulk CN. Therefore, the graphitic structure is disturbed by introduction to the T. Furthermore, the strong peaks for bulk CN display a shift to lower angles, indicating an increasing interplanar distance of graphitic layered structures. The increase of the interplanar distance promotes the reactant molecule's approach to the interplanar space to ameliorate the photocatalytic reaction.

Fig. 3 (a) XRD patterns and (b) FTIR spectra of bulk CN and T-modified CNs.

FTIR spectra was conducted to investigate the effect of T on the heptazine units of the as-synthesized samples, as shown in Fig. 3b. In terms of bulk CN, the multiple bands in the 1200–1700 cm⁻¹ region are corresponded to the stretching modes of typical aromatic CN heterocycles. The characteristic absorption peak at ca. 1571 and 1640 cm⁻¹ are C=N bonds, whereas the peaks at ca. 1243, 1323 and 1412 cm⁻¹ are attributed to C–N bonds.^41,42 In addition, another typical characteristic peak of the heptazine unit is found at ca. 808 cm⁻¹. It is noted that this peak for the T-modified CNs is slightly shifted from 808 to 811 cm⁻¹ when introduce the fraction of T in the supramolecular complex. The blue shift is probably attributed to the morphology change of the as-synthesized CNs, indicating that the T-modified CN has a looser packing structure compared with bulk CN. Similar results have been reported by Ho group.^30,35 The following SEM images also exhibit porous structure of the T-modified CN intuitively.

The microstructures and morphologies of the as-synthesized CNs were investigated via SEM and TEM (Fig. 4). As displayed in Fig. 4, all modified CNs present small fragment with abundant irregular porous texture on their surfaces, whereas the bulk CN, as shown in Fig. S1a,† exhibits layered, stacked texture and the smooth surface morphology and no porosity is occurred on the surface. Additionally, the T-modified CNs possess looser stacked structure and more porous texture than CM, which was synthesized from direct pyrolysis of supramolecular complexes without T. Meanwhile, with the increase of T content, some tubular structure can be found clearly for CM + 2.5 mol%-T and CM + 5 mol%-T from the SEM images, and the tubular morphology of CM + 10 mol%-T change into tubular sheet structure. The results of TEM test are consistent with that of SEM. The T-modified CNs, taking CM + 2.5 mol%-T for an instance, present small loose fragment structure and abundant irregular porous texture along with tubular morphology (Fig. 4f₁ and f₂), while the bulk CN displays layered, stacked structure and no obvious pore structure (Fig. S1g₁ and g₂†). The differences of shape and morphology demonstrate that hole-shaped defects on the surface can be easily created through supramolecular approach and copolymerization with T can further change structures and morphologies, which might contribute to the increase of BET specific surface areas and pore volume to provide more restive sites. Moreover, the presence of small loose fragment instead of large bulk ones accords well with the result from XRD analysis that the graphitic structure is disturbed along with the introduction to T.

Fig. 4 SEM images of as-prepared CNs: CM (a₁ and a₂); CM + 2.5 mol%-T (b₁ and b₂); CM + 5 mol%-T (c₁ and c₂); CM + 10 mol%-T (d₁ and d₂) and high resolution TEM images of CM (e₁ and e₂) and CM + 2.5 mol%-T CM (f₁ and f₂).

3.2 Photocatalytic activity

The as-prepared bulk CN and T-modified CNs were applied in PCO of NO under simulated solar light irradiation so that their potential of air purification could be evaluated. Initially, four control experiments were performed in absence of CM + 2.5 mol%-T, the simulated solar light, H₂O₂ solution, both CM + 2.5 mol%-T and H₂O₂ solution. As displayed in Fig. S2,† the corresponding NO removal ratios are low, which indicates that it is difficult for NO to be fully oxidized or photolyzed under corresponding experimental conditions. Subsequently, the simulated solar-light-induced PCO performances of CNs on NO oxidation were carried out. The gas streams were introduced into a continuous reactor after being premixed completely. When the absorption–desorption equilibrium between gases and photocatalysts was reached, the Xe-lamp and the peristaltic pump delivering H₂O₂ solution were turned on simultaneously. Fig. 5 is the curves of NO removal efficiency for as-prepared bulk CN and T-modified CNs. It can be seen that the efficiency of PCO of NO increases with the irradiation time and then reaches chemical equilibrium. What's more, the photocatalytic activity for modified CNs exhibits tremendous enhancement with the introduction to T in comparison with bulk CN. More importantly, the conversion rate of CM + 2.5 mol%-T reaches 93.3% and exhibits the best photocatalytic activity on NO removal. Also, the NO conversion rate in the presence of CM + 2.5 mol%-T is far higher than that of control experiment without CM + 2.5 mol%-T or light.

Fig. 5 Curves of the photocatalytic removal of NO for as-prepared bulk CN and T-modified CNs.

3.3 Photocatalytic activity enhancement mechanism

Photocatalytic activity of photocatalyst is determined by two aspects, including specific surface areas, separation and transfer efficiency of photoinduced electron–hole pairs. The fronter is considered to be a significant contributor for the photoactivity of photocatalyst. Large specific surface areas can provide more surface sites for absorption of reactant molecules to make the photocatalytic process more efficient. Hence, the nitrogen adsorption apparatus was used to measure specific surface areas of all samples. As shown in Fig. 6, the nitrogen adsorption–desorption isotherms with typical Type IV behavior indicate the presence of porous structure,⁴³ which is in agreement with results of the SEM images. Furthermore, in terms of the T-modified CN, the adsorption branch of the nitrogen isotherms display an increase at P/P₀ up till the closing point, which suggests the property of mesopores and macropores.⁴⁴ It is believed that the hierarchical porous structure not only could possess larger light-harvesting capacity, but also could provide more restive sites for photocatalytic reactions.⁴⁵ The calculated Brunauer–Emmett–Teller (BET) specific surface areas for bulk CN, CM, CM + 2.5 mol%-T, CM + 5 mol%-T, CM + 10 mol%-T are 8.5, 56.04, 67.71, 63.30, 62.76 m² g⁻¹, respectively. The BET specific surface areas for the modified CNs is far larger than bulk CN. It is worth noting that the CM + 2.5 mol%-T possesses the highest specific surface areas, which is consistent with the highest activity of CM + 2.5 mol%-T on the PCO of NO. The result also implies that CNs prepared by co-pyrolysis of the supramolecular complex of C, M and T could further enhance their photoactivity. Through Barrett–Joyner–Halenda (BJH) method, the total pore volume for bulk CN, CM, CM + 2.5 mol%-T, CM + 5 mol%-T, CM + 10 mol%-T are also summarized and they are 0.03, 0.32, 0.50, 0.44, 0.35 cm³ g⁻¹, respectively. It can be found that the experimental data of total pore volume exhibits a similar pattern to that of the BET and CM + 2.5 mol%-T possesses the largest total pore volume.

Fig. 6 Nitrogen adsorption–desorption isotherms of as-prepared bulk CN, CM, CM + 2.5 mol%-T, CM + 5 mol%-T, CM + 10 mol%-T.

The ability to have efficient separation of photoinduced electron–hole pairs under irradiation is another determined factor. It is conventionally related to photocatalyst photoexcitation, recombination and transfer efficiency of photoinduced electron–hole pairs. Photocatalyst photoexcitation often depends on light absorption edge and the band gap. Fig. 7a is UV-vis diffuse reflection spectra (DRS) of the synthesized samples. It can be seen that all T-modified CNs have broad and strong adsorption under the simulated solar light condition and their light absorption edges are obviously red-shifted to lower energies along with an increase of optical densities at longer wavelengths. The phenomena demonstrate that it's an effective approach to further enhance solar-light adsorption through the introduction to T. More importantly, the CM + 2.5 mol%-T does have a wide light absorption edge to make itself possess the highest photocatalytic activity, while its light absorption edge is not the largest. Also, the band gap (E_g) values for CM, CM + 2.5 mol%-T, CM + 5 mol%-T, CM + 10 mol%-T are calculated to be 2.75, 2.45, 2.24, 1.88 eV, respectively, according to the onset of the absorption edge. They show a tendency to decrease gradually with the increase of T, while the E_g of CM + 2.5 mol%-T that has the best photocatalytic activity isn't the smallest. The result indicates there is no clear correlation between light harvesting and photocatalytic activity, which corresponds to with literature.⁴⁶ Therefore, separation and transfer efficiency of photoinduced electron–hole pairs is a effective determined factor, while the enhanced photocatalytic activity may stem from better light harvesting. To test separation and transfer efficiency of photoinduced electron–hole pairs, the photoluminescence (PL) spectra were measured. The fluorescence intensity with lower values indicates higher separation rate of photoinduced electron–hole pairs and higher photocatalytic activity. As observed in Fig. 7b, bulk CN exhibits a strong and wide emission peak centered at ca. 470 nm, which can be ascribed to the band–band PL phenomenon originating from the n–π* electronic transitions in bulk CN.⁴⁷ Obviously, the emission intensity of CM + 2.5 mol%-T is strongly reduced, which evidently demonstrates high separation and transfer efficiency of photogenerated electron–hole pairs.

Fig. 7 (a) UV-vis diffuse reflection spectra; (b) photoluminescence spectra of as synthesized bulk CN and CM + 2.5 mol%-T.

To further confirm the improvement of photoinduced electron–hole migration and separation, electrochemical impedance spectra (EIS) measurements were carried out to investigate the charge transfer resistance and the separation rate of the photoinduced electron–hole pairs. Fig. 8 is the EIS Nyquist plots of the representative bulk CN, CM + 2.5 mol%-T. In general, a small radius indicates low overall charge transfer resistance and high separation efficiency of photogenerated electron–hole pairs.^48,49 It can be seen that the arc radius of the CM + 2.5 mol%-T electrode is smaller than that of bulk CN. The experimental result indicates that the as-synthesized CNs from copolymerization with T can decrease the charge-transfer resistance on the electrode surface, thus leading to an efficient separation of electron–hole pairs and further enhance the photocatalytic activity.

Fig. 8 EIS Nyquist plots of bulk CN, CM + 2.5 mol%-T.

3.4 Products and mechanism analysis of photocatalytic oxidation of NO

Ion Chromatography (IC) is employed to analyze the composition of the reaction products. When one experiment was completely conducted, the photocatalyst and the small piece of cotton containing ion were transferred to a quartz tube. And 8 mL deionized water was injected into the mixture. Then the quartz tube was placed overnight so that ions on the surface of the photocatalyst could dissolve into the solution completely. Subsequently, 1 mL of the clear supernatant obtained by centrifugation was diluted 50 times with deionized water and injected into IC. The qualitative analytical result, taking CM + 2.5 mol%-T for an instance, is shown in Fig. 9, which indicates the existence of NO₃⁻ in the solution. Additionally, there was no increase of NO₂ during the experimental process. And NO₂⁻ wasn't detected in the IC. These results could be ascribed to the introduction to H₂O₂, which had produced considerable amount of active species in the presence of CM + 2.5 mol%-T and made the oxidative ability of the system enhanced. Consequently, NO₃⁻ can be regarded as the main photocatalytic oxidation reaction product. The standard solution was prepared and corresponding IC was conducted. And the fitting line is shown in Fig. S3.† Consequently, the concentration value of NO₃⁻ in solution could be obtained through external standard method. Also, nitrogen balance calculation is carried out to investigate the possible byproducts, and the corresponding calculate process are in Table S1.† The results not only further confirm that NO₃⁻ is the primary product in the PCO of NO experiment, but also well corresponds with that of simulated solar-light-induced PCO performance of CNs on NO removal, which demonstrates the accuracy and reliability of the PCO experimental results.

Fig. 9 Ion chromatography analysis of ions in the solution after reaction in the presence of CM + 2.5 mol%-T.

NO₂ wasn't detected from the outlet gases during the PCO of NO experiment. Combined with the quantitative analysis results of IC, it could be found that NO₃⁻ is the domain reaction product in our PCO of NO system. It's well known that the main active species include hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻) and holes (h⁺). To investigate the involvement of active radical species and confirm the possible mechanism of photocatalytic oxidation, a series of experiments and characterizations were carried out. At first, the fluorescence spectra using terephthalic acid as a probe molecule were conducted to detect ˙OH.^50,51 Fig. 10 is fluorescence spectra of the control group and the experimental group with CM + 2.5 mol%-T. The fluorescence emission peak at ca. 440 nm indicates that 2-hydroxy terephthalic acid generated from the reaction between ˙OH and TA was formed under simulated solar-light irradiation. However, the fluorescence intensity of experimental group is much lower than that of the control group, which infers that less amount of ˙OH is generated in the presence of CNs in the PCO of NO under simulated solar-light irradiation. In other words, ˙OH isn't the main active species and other active species might play a leading role in the PCO of NO.

Fig. 10 Fluorescence spectra of TAOH solutions generated by the experimental group with CM + 2.5 mol%-T and the control group under simulated solar-light irradiation.

To further throw light upon the role of active species introduced in our PCO of NO system, the trapping experiment, taking CM + 2.5 mol%-T for an instance, was performed to investigate the involvement of active radical species intuitively. The qualitative analytical result displays that the NO₃⁻ concentrations of the experimental group (a)–(d) and control group (e) are 20.13, 11.54, 6.13, 15.07 and 2.78 mg L⁻¹, respectively. The NO₃⁻ concentration value of group (c) unaffected by ˙O₂⁻ is the lowest than any other experimental group and only a little higher than that of control group (e), which indicates that ˙O₂⁻ plays the leading role in the PCO of NO. Additionally, the NO₃⁻ concentration value of group (b) excluding the impact of ˙OH is much higher than that of group (c), but still in a low level in comparison with that of group (a). The results demonstrate that ˙OH could be inhibited, but are also of importance and efficiency in the PCO of NO. It's worth mentioning that the result of group (d) implies that h⁺ has weak oxidation ability and could directly oxidize NO to a certain extent in the PCO of NO system. From what has been discussed and analyzed above, it can be concluded that ˙O₂⁻ is the main active species and plays a leading role in our PCO of NO system.

4 Conclusions

In this work, porous efficient CN photocatalyst was successfully synthesized by supramolecular approach. The T-modified CN exhibits outstanding enhancement in photoactivity compared with that of bulk CN. More importantly, the CM + 2.5 mol%-T, displays the best efficiency in the PCO of NO experiment and its conversion rate of NO can reach 93.3% in 40 min. The outstanding photocatalytic performance of T-modified CNs could be ascribed to not only the large BET specific surface areas which could provide large numbers of active sites to improve photocatalytic performance, but also wide simulated solar light absorption edge as well as the enhanced separation rate of electrons and holes, as confirmed by DRS and PL. Further investigation on the mechanism of PCO of NO indicates that ˙O₂⁻ is the primary active species by using an intuitive trapping experiment and fluorescence spectra.

Acknowledgements

This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800). Assembly Foundation of the Industry and Information Ministry of the People's Republic of China 2012 (543), the National Natural Science Foundation of China (51408309 and 51578288), Science and Technology Support Program of Jiangsu Province (BE2014713), Natural Science Foundation of Jiangsu Province (BK20140777), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (BY2014004-10), Science and technology project of Nanjing (201306012), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062), Top-notch Academic Programs of Jiangsu Higher Education Institutions, A Project by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23251e

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