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g-C3N4 heterojunction: Enhanced efficiency, mechanism and reaction pathway
Applied Surface Science 458 (2018) 77–85
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Full Length Article
Photocatalytic NO oxidation on N-doped TiO2/g-C3N4 heterojunction: Enhanced efficiency, mechanism and reaction pathway ⁎
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Guangming Jiang , Jiwu Cao, Min Chen, Xianming Zhang , Fan Dong Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of Education, Chongqing Technology and Business University, Chongqing 400067, China
A R T I C LE I N FO
A B S T R A C T
Keywords: N-TiO2 g-C3N4 Heterojunction Photocatalysis In situ DRIFTS
Heterojunction photocatalysts with a high activity have been pursuit for years. In this work, we report a facile three-step approach to the synthesis of N-doped TiO2/g-C3N4 composite (denoted as N-TiO2/g-C3N4), in which N-TiO2 nanoplates are uniformly assemblied over g-C3N4 layers, leading to a maximization of the heterojunction effect. This resultant composite is highly efficient and durable in photocatalytic removal of ppb-level NO from a continuous air flow under visible light illumination. The photocatalytic NO removal efficiency is N-TiO2 contentdependent, and an optimal content lies at around 25 mg on 0.25 g of g-C3N4, under which the NO removal efficiency reaches 46.1%. The mechanism study reveals that the enhanced photocatalytic performance originates from the N-doping in TiO2 and the N-TiO2/g-C3N4 heterojunction construction, which promote the utilization of visible light and the separation of photoexcited electrons and holes. The active radicals of %O2− and %OH are both identified to play important roles in photocatalytic NO oxidation. The reaction pathway study of photocatalytic NO oxidation over N-TiO2/g-C3N4 by in-situ DRIFTS demonstrates that NO can be completely mineralized into nitrate. Overall, the presented work may contribute to a deeper understanding of the photocatalytic mechanism and the design of robust catalysts for air purification.
1. Introduction In recent years, the rapid industrialization and civilization process have raised severe environmental issues [1,2] especially the air pollution by the over emission of SOx, NOx, VOCs and PM2.5 [3]. To improve the situations, enormous efforts have been devoted to developing effective solutions, of which photocatalysis is receiving an ever-increased attention due to its high efficiency and environmental-friendly feature (this technology can effectively convert solar light to highly active oxidative free radicals for pollutant detoxification) [4]. In the development of photocatalysis technology, the core part is the design/ synthesis of highly effective and durable photocatalyst [5,6]. By looking through the history, we find that the semiconductor graphitic carbon nitride (g-C3N4) and titanium oxide (TiO2) are the two most popular materials. g-C3N4 is preferred by its facile synthesis, layered structure, large surface area, and narrow band gap allowing for visible light excitation, while TiO2 is attracted due to its robust performance and stable structure [7–9]. Though great advancements have been achieved in the development of g-C3N4 and TiO2-based products, several challenges still exist to limit their application. Firstly, due to its large band gap of 3.20 eV, pure TiO2 is only sensitive to the ultraviolet light [10],
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which only constitutes 7% of the solar energy. To improve visible light adsorption, various strategies have been proposed, such as the element doping and morphology/structure regulation to narrow band gap or form a new energy level in band gap [11,12]. The fast electron-hole recombination is another general issue for both the TiO2 and g-C3N4 [13,14], which usually lower the utilization efficiency of the absorbed light, and reduce the yield of active radicals for pollutant removal. To hinder the electron-hole recombination, one well-accepted strategy is to decorate semiconductor with metal particles or another semiconductor to form heterojuntions [15–17]. The difference in the location of Fermi level [18] between the semiconductor and metal or between the two semiconductors will generate a drive force for electron-hole separation. For example, our group ever constructed a heterojunction by assembling bismuth nanoparticle (Bi NP) over g-C3N4 [19]. Due to the lower Fermi level of Bi NP than the conduction band edge of g-C3N4, the photoexcited hot electrons on g-C3N4 will spontaneously flow to Bi NP, which effectively hinders their recombination with holes, and enhances NOx oxidation. Inspired by above achievements, we attempted to construct a novel N-doped TiO2/g-C3N4 heterojunction. With N doping and heterojunction construction, it is expected to achieve a robust visible light–driven photocatalytic NO oxidation.
Corresponding authors. E-mail addresses: [email protected] (G. Jiang), [email protected] (X. Zhang).
https://doi.org/10.1016/j.apsusc.2018.07.087 Received 24 May 2018; Received in revised form 7 July 2018; Accepted 11 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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spectroscopy (XPS, Thermo ESCALAB 250, USA). The optical property of the sample was examined on UV–vis diffuse-reflectance spectrometry (UV–vis DRS, Shimadzu, Japan) and Photoluminescence spectrometry (PL, F-7000, Hitachi, Japan). One spectrometer (JES FA200) was used to record the Electron Spin Resonance (ESR) signals of the radical %OH and %O2−, and the sample was dispersed into a 40 mM DMPO solution tank before the testing (aqueous dispersion for ·OH trapping and methanol dispersion for %O2− trapping). In- situ DRIFTS measurements (VERTEX70 FTIR spectrometer, Bruker) equipped with an in-situ diffuse-reflectance cell (Harrick) were conducted to probe into the photocatalytic oxidation pathway over the catalyst.
Due to the merits of g-C3N4 and TiO2, and their appropriate band structure, many groups have worked for years to construct effective gC3N4/TiO2 heterojunctions [20]. Wang [21] ever reported the fabrication of TiO2/g-C3N4 composite by calcining the mixed precursors of titanic acid nanotubes and melamine at 550 °C for photocatalytic degradation of methyl orange. Zhu [22] constructed the TiO2/g-C3N4 composite by ball milling of pre-synthesized TiO2 and g-C3N4, while Fu [23] employed a solid-state method to synthesize TiO2/g-C3N4 core/ shell composite. Though all the reported approaches are easy to achieve mass production, uniform mixing and close interface interaction between TiO2 and g-C3N4 are usually unsatisfactory, which reversely limits their synergistic effect. In addition, the reaction pathway of photocatalytic NO oxidation on N-TiO2/g-C3N4 composite has never been revealed [24]. Herein, we made improvements to synthetic procedure, and one novel N-TiO2/g-C3N4 photocatalyst was successfully prepared with the N-TiO2 nanoplates uniformly assembled on g-C3N4 layers. Photocatalytic performance of the resultant sample was evaluated by its efficiency in removing ppb-level NO from a continuous air flow under visible light illumination. The results show that N-TiO2/g-C3N4 composites present a much higher NO removal efficiency than the single gC3N4 and N-TiO2, as well as the mechanically mixed N-TiO2/g-C3N4. The involved mechanism for the enhanced photocatalytic activity was studied by the combined investigation on the microstructure, optical property and band structure of the sample, as well as the electron/hole separation efficiency and the species of produced active radicals in sample under visible light illumination. Additionally, the reaction pathway of photocatalytic NO oxidation over N-TiO2/g-C3N4 was examined by in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).
2.3. Photocatalytic activity evaluation The photocatalytic activity of samples was evaluated by their ability to remove ppb-level NO from a continuous reactor under a visible light illumination. Firstly, 0.2 g of the sample was dispersed in ethanol with the assistant of sonication, and then evenly pasted onto the bottom of two culture dishes (made of glass, 12 cm in diameter). The dishes were vacuum dried and placed in the middle of a rectangular reaction vessel (30 × 15 × 10 cm) for photocatalysis. The feeding NO concentration is set to 600 ppb (diluted by air) with a flow rate of 2.4 mL min−1. Incandescent lamps were placed over the reactor. When the air flow and the NO concentration were steady, light was turned on and the analyzer started to record the real-time concentration of NO. The NO removal efficiency (η) wan calculated as
η (%) = (1−C/C0) × 100% where C0 and C denote the NO concentration in inlet and outlet stream, respectively.
2. Experimental section
2.4. In-situ DRIFTS experiment
2.1. Material syntheses
In-situ DRIFTS equipment is composed of a Tensor II FTIR spectrometer (Bruker) and a reaction chamber (Harrick). As showed in Fig. 1, sample was placed in the reaction chamber, treated at a high temperature under a He flow to remove surface impurities, including water, and carbon dioxide, and then scanned to produce a baseline. After that, the mixed reaction gas (50 mL/min NO, 50 mL/min O2) was pumped into the chamber for a 20-min adsorption. The spectrum evolving along with the adsorption was recorded every two minutes. Once the adsorption was completed, light was turned on, and the photocatalytic reaction was carried out for another 60 min. The scanned wavelength range was set from 600 to 4000 cm−1.
All the chemical reagents are of analytical grade purity and used without further purification. Synthesis of N-TiO2. 3.0 g of TiN powder was added to the alumina crucible (100 mL), which was then put in the middle of a muffle furnace without a lid cover and calcined at 450 °C for 2 h under a static air atmosphere [25]. After cooling down to room temperature, pale yellow products were collected for use. Synthesis of g-C3N4. g-C3N4 was synthesized following a reported method [26]. Typically, 10 g of urea were dissolved in 20 mL of water in an alumina crucible (50 mL). The solution was dried at 60 °C overnight to drive the recrystallization of urea. The recrystallized urea in alumina crucible with a cover was then heated to 550 °C with a ramping rate of 15 °C min−1 in a muffle furnace and maintained for 2 h. After cooling down to room temperature, pale yellow powders were collected for use. Preparation of N-TiO2/g-C3N4 composite. 0.25 g of the as-prepared g-C3N4 and different weight (15, 20, 25, 30 and 35 mg) of N-TiO2 were dispersed in 30 mL of water by mild sonication, respectively. The N-TiO2 dispersion was then dropwise added to the g-C3N4 dispersion. The mixture was sonicated for another 20 min and then filtrated. The collected solid phase was dried at 60 °C before further calcination at 450 °C for 2 h. Eventually, pale yellow product was collected.
3. Results and discussion 3.1. Synthesis and characterization of the N-TiO2/g-C3N4 composite Fig. 2 schematically demonstrates the synthetic procedure for NTiO2/g-C3N4 composite [27]. In our method, N-TiO2 NPs and g-C3N4 are both pre-synthesized by calcination of the precursor TiN NP and
2.2. Characterization The crystal phase of the sample was analyzed by X-ray diffraction (XRD) (model D/max RA, Rigaku Co., Japan). Its particle morphology and structure were examined on Scanning electron microscopy (SEM, model JSM-6490, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan), while its chemical composition and electronic structure were investigated on X-ray photoelectron
Fig. 1. In-situ DRIFTS system for signal recording. 78
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Fig. 2. Schematic illustration of the synthesis process of N-TiO2/g-C3N4 heterojunctions.
layers can be assigned to N-TiO2 and g-C3N4 [30], respectively. The uniform distribution of black particles on gray layers indicates the successful construction of N-TiO2 nanoparticles/g-C3N4 heterojunction, and more importantly, the uniform assembly of N-TiO2 over g-C3N4, both of which will raise the number of heterojunction and are beneficial for the photocatalytic reaction. The HRTEM image shows two lattice fringes of 0.234 and 0.353 nm (Fig. 4d), which can be calibrated as (1 1 2) and (1 0 1) planes of anatase TiO2. Due to the semi-crystalline property of g-C3N4, its lattice spacing is hardly observed. However, the interface between these two materials is very clear, which further demonstrates the formation of heterojunction. XPS was performed to investigate the chemical composition in sample and the electronic state of each component element. The XPS survey spectrum of N-TiO2 sample in Fig. 5a clearly displays the elements of N, O and Ti, which well confirms that the N is doped into TiO2. The high-resolution XPS spectrum of N 1s in Fig. 5b shows a weak peak at 399.5 eV, which can be assigned to the N-O coordination [31]. The O 1s spectrum in Fig. 5c can be fitted with two peaks. The peak at 529.4 eV is related to O2− in N-TiO2, and the other at 531.2 eV is derived from the N-O coordination [32]. Fig. 5d presents the peaks of Ti 2p 3/2 and Ti 2p 1/2 at binding energies of 458.2 and 463.9 eV, which can be assigned to Ti4+ in N-TiO2 [33]. Nitrogen doping results in the formation of new electronic state above valence band, and the valence edge of N-TiO2 is determined to be around 2.17 eV (See Valence Band (VB) XPS result in Fig. 5e). We ever tried the one-step synthesis of N-TiO2/g-C3N4 by calcination of the mixed precursors of TiN and urea in one crucible. Fig. 6a and b compare the digital images of the as-synthesized composites by one step (denoted as N-TiO2/g-C3N4-1) and three-step (N-TiO2/g-C3N4-3), respectively. Different from N-TiO2/g-C3N4-3 with a uniform pale yellow color, N-TiO2/g-C3N4-1 appears partially dark and yellow. The dark and yellow part should correspond to the unconverted TiN (pure TiN powder is black) and g-C3N4. The uneven color indicates that the TiO2 and g-C3N4 phase are phase separated, and the TiO2 is actually not well assembled on the g-C3N4. As a result, the number and the quality of the heterojunction should both be low, and correspondingly, the photocatalytic activity is unsatisfactory (See below). Fig. 6c compares the XRD patterns of g-C3N4, N-TiO2 and N-TiO2/g-C3N4-1. Both TiO2 and TiN phases were detected in N-TiO2/g-C3N4-1, confirming that TiN is not fully converted. Fig. 6d presents the TEM image of N-TiO2/g-C3N41, which clearly show that N-TiO2 NPs are aggregated over g-C3N4 layers.
Fig. 3. XRD Patterns of the g-C3N4, N-TiO2 and N-TiO2/g-C3N4.
urea, respectively. The N-TiO2 NPs are then uniformly assembled on the layer structured g-C3N4 via a solution-mediated method, leading to the heterojunction formation. An additional calcination step at a relatively low temperature of 450 °C is employed to intensify the interface interaction between g-C3N4 and N-TiO2. Fig. 3 presents the XRD patterns of the as-synthesized g-C3N4, N-TiO2 and N-TiO2/g-C3N4 composite. gC3N4 shows two characteristic diffraction peaks at 2θ = 13.5° and 27.5°, corresponding to the (1 0 0) and (0 0 2) crystal plane of g-C3N4 respectively. The stronger peak can be attributed to the characteristic inter-layer stacking of the conjugated aro-matic C-N units, while the other is derived from a stacked structure of in-plane repeating structural units [28]. The N-TiO2 sample shows the typical diffraction peaks of TiO2 at 2θ = 25.3°, and no other phase is discerned, indicating the successful synthesis of anatase TiO2 phase with a high purity, and that the N doping will not change the crystalline structure [29]. As for composite sample, several new peaks of the rutile TiO2 phase can be observed, and all the diffraction peaks are narrowed, indicating the crystallinity of N-TiO2 is improved in the secondary step of calcination. g-C3N4 is evidenced by the typical diffraction peak at 2θ = 27.5°. Fig. 4 presents the TEM images of g-C3N4, N-TiO2 and N-TiO2/g-C3N4. g-C3N4 in Fig. 4a displays a layered and porous structure, which provides sufficient sites for heterojunction formation. Fig. 4b shows that the NTiO2 owns a lamellar shape with an irregular bilateral size of around 15–20 nm. Such a structure is very helpful for it to keep a sufficient contact with g-C3N4, and enhance the heterojunction effect. Fig. 4c shows the TEM image of one representative sample (25N-TiO2/g-C3N4). By their electron density differences, the black particles and the grey
3.2. Optical properties analyses The UV–vis diffuse reflection spectra of different samples are displayed in Fig. 7a. A sharp fundamental absorption edge of N-TiO2 and
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Fig. 4. TEM images of (a) g-C3N4, (b) N-TiO2, (c) 25N-TiO2/g-C3N4 composite and (d) HRTEM images of 25N-TiO2/g-C3N4 composite.
TiO2 and g-C3N4 can both generate the electrons and holes, and PL spectrum actually reflects the overall electron/hole separation efficiency in both the N-TiO2 and g-C3N4. With an increased N-TiO2 content, the increased production of electrons and holes in TiO2 will in some extent hinder the flow in of the electrons from g-C3N4, and as a result, weaken the function of heterojunction in the electron/hole separation.
g-C3N4 is 400 and 440 nm. At wavelengths less than 400 nm, a significant increase in absorption results from the intrinsic band gap absorption of TiO2. The extended absorption spectra in visible light region can be ascribed to the formation of one new electronic state above valence band caused by nitrogen doping. This is also reflected by the narrowed band gap of 2.84 eV compared to the pure TiO2 of 3.2 eV (See Fig. 7b). PL is one of the effective methods to detect the optical property of solid-state semiconductor, which can reflect the recombination rate of the photogenerated electron and hole pairs. The stronger the signal of molecular fluorescence spectrum, the higher the recombination rate of photogenerated electron-hole pairs. Fig. 8a shows the room-temperature PL spectra of different samples under an excitation light of 360 nm. The peak positions of all the N-TiO2/g-C3N4 samples in PL spectra have a slight shift compared to that of pure g-C3N4 and this is possibly derived from the contribution of N-TiO2 to the PL spectrum. The peak intensity as a function of TiO2 content is summarized in Fig. 8b. It is shown that all N-TiO2/g-C3N4 samples present lower fluorescence intensities compared to pure g-C3N4, indicating that the introduction of N-TiO2 on g-C3N4 can suppress the recombination of photogenerated electron-hole pairs. An optimal TiO2 content is found to be 25%, with which the peak intensity of the PL spectrum reaches lowest, and thus the electron/hole separation efficiency reaches highest. The existence of the optimal TiO2 content for an prohibited electron/hole recombination originates from the fact that both the N-
3.3. Photocatalytic performance The photocatalytic performances of N-TiO2, g-C3N4 and N-TiO2/gC3N4 composites were evaluated by their efficiencies in removing NO from a continuous air flow with visible light irradiation [34–35]. Fig. 9a shows that all the samples are active to remove NO under visible light illumination, of which N-TiO2/g-C3N4 are all more active than both the g-C3N4 and N-TiO2, demonstrating that the heterojunction construction can promote the photocatalytic activity of g-C3N4. The photocatalytic performance of N-TiO2/g-C3N4 is N-TiO2 content dependent. With an increase in N-TiO2 content, the NO removal efficiency of N-TiO2/gC3N4 firstly increases, reaches the peak of 46.1% at 25N-TiO2/g-C3N4 and then decreases (Fig. 9b). The presence of the optimal N-TiO2 content is well consistent with the PL result, which demonstrates that the construction of heterojunctions with a high quality and an appropriate number is required to achieve a maximum photocatalytic activity. The
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Fig. 5. High resolution XPS spectra of N-TiO2: (a) survey spectrum, (b) N 1s, (c) O 1s, (d) Ti 2p. (e) Valence band XPS of N-TiO2.
durability test result for the 25N-TiO2/g-C3N4 heterojunction in Fig. 9c suggests that the composite can maintain a high NO removal efficiency under a long-term run. To further emphasize the importance of uniform interface interactions between TiO2 and g-C3N4, the photocatalytic performances of 25N-TiO2/g-C3N4-1 and -3 were compared. The results in Fig. 9d show that 25N-TiO2/g-C3N4-1 prepared by one step calcination of TiN and urea is much less active to oxidize NO when compared to 25N-TiO2/gC3N4-3. As demonstrated above, in 25N-TiO2/g-C3N4-1, the N-TiO2 is not well assembled on g-C3N4, and thus less heterojunctions are actually constructed, which, we believe, is key reason for its poor photocatalytic performance.
and g-C3N4, an effective heterojunction is formed, by which the photogenerated electrons (e−) produced in g-C3N4 and the holes (h+) in NTiO2 will spontaneously flow to the CB of N-TiO2 and the VB of g-C3N4 respectively. In this way, the photoexcited e− and h+ can be utilized in a larger extent rather than be wasted by electron-hole recombination. The hot e− on CB of N-TiO2 with a potential above −0.67 eV can reduce O2 to %O2− (the redox potential for O2 into %O2− is −0.33 eV), while the h+ on VB of g-C3N4 with a potential of 1.57 eV can directly oxidize NO, but not trigger the formation of %OH from the oxidation of water molecules (the redox potential for H2O to %OH is 1.99 eV). The generation of %OH in system of N-TiO2/g-C3N4 should be evolved from % O2− with two additional hot e− (%O2− + 2 e− + 2H2O → 2%OH + 2OH−) [19]. In order to intuitively reveal the reaction pathways, In-situ DRIFTS study was further performed to monitor the related molecular or bond evolution over our catalyst during the photocatalytic NO oxidation under visible light irradiation [39]. Fig. 10c shows the spectra varying with the reaction time before and after the light turned on. After turning on the light, an absorption peak appears in 1037 cm−1, which can be assigned to NO [40]. The increased peak intensity with the time indicates that the surface of N-TiO2/g-C3N4 composite is adsorbing NO. The adsorbed NO will react with O2 to form NO2 (Eq. (1)). The characteristic peaks at 961 and 1143 cm−1 belong to the oxidized N2O3 [39] and N2O [41], respectively, which are formed via the coupling reaction between NO2 and NO (Eq. (2)) or NO and the N atom in N-TiO2/g-C3N4 (Eq. (3)). The typical adsorption peak of NO2− can be discerned easily at 813 cm−1 [41], which illustrates that NO can be oxidized to NO2− (Eq. (4)). At the same time, there is a sharp rise of monodentate NO3− at 1043 cm−1 [42], indicating that NO and NO2− can be directly
3.4. Reaction mechanism and pathway DMPO-ESR analyses are conducted to identify the active species produced during the photocatalytic reaction, and explore more on the role of g-C3N4, N-TiO2 and 25N-TiO2/g-C3N4. Fig. 10a reveals that the DMPO-%O2− signal intensity in the three samples follows an increasing order of N-TiO2 < g-C3N4 < 25N-TiO2/g-C3N4. Fig. 10b shows that gC3N4 has no signal of DMPO-·OH, while 25N-TiO2/g-C3N4 has the strongest signal intensity. It is suggested that the construction of NTiO2/g-C3N4 heterojunction can promote the formation of both the % O2− and %OH. The Conduction Band (CB) and VB edge energy of TiO2 were reported to lie at −0.29 and 2.91 eV, while those of g-C3N4 at −1.13 and 1.57 eV, respectively [36,37]. However, the band gap of TiO2 is narrowed by N doping [38], and the new locations of CB and VB for N-TiO2 are thus −0.67 and 2.17 eV. According to the band structure of N-TiO2
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Fig. 6. The digital images of (a) N-TiO2/g-C3N4-3 and (b) N-TiO2/g-C3N4-1; (c) XRD Patterns of the g-C3N4, N-TiO2 and N-TiO2/g-C3N4-1 (d) TEM image of N-TiO2/gC3N4-1 composite.
peak can be attributed to NO− (Eq. (8)) [44]. The NO2 will get another two electron and transform to N2O22− (1096 and 1124 cm−1) (Eq. (9)) [46]. The characteristic peaks of these two main intermediate products can be observed apparently. The final product (NO3−) during the photocatalytic reaction process is formed with a variety of chemical reaction pathways (Eqs. (10)–(13)). We can observe the intensity of the adsorption peak is obviously enhanced over irradiation time. [47] The
oxidized to NO3− (Eqs. (5)–(7)). Nitrate will remain on the surface of photocatalyst after the reaction as the absorption peaks (1193 cm−1) belonging to the NO3− is kept during the reaction [43], which is the leading origin of the gradual decrease in photocatalyst activity after a long-term run. However, monodentate NO3− is not stable enough, which can transform into the more stable bidentate NO3− (1110 cm−1) [44] and bridging NO3− (1002 cm−1) [45]. 1174 cm−1 absorption
Fig. 7. (a) UV–vis spectra of N-TiO2, g-C3N4 and N-TiO2/g-C3N4 composites with different TiO2 contents. (b) Plots of (αhν)2 versus photon energy (hν) for the band gap determination. 82
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Fig. 8. (a) Photoluminescence spectra and (b) the corresponding peak intensities of the g-C3N4 and N-TiO2/g-C3N4 composites with different contents of N-TiO2.
Fig. 9. (a) Photocatalysis activity and (b) the 30-min NO removal efficiency of g-C3N4, N-TiO2 and N-TiO2/g-C3N4 composites with different TiO2 contents. (c) Durable test of 25N-TiO2/g-C3N4 composite. (e) Photocatalytic activity of the g-C3N4, N-TiO2/g-C3N4-1 and N-TiO2/g-C3N4-3.
NO2− + 2%OH → NO3− + H2O
observed IR bands of the adsorbed species and their chemical assignments are listed in Table 1. According to the active species identification and reaction pathway study, a possible reaction mechanism is schematically proposed in Fig. 10d. 2NO + O2 → 2NO2 NO2 + NO → N2O3 NO + N(CN) → N2O %
−
−
2NO + O2 + e → 2NO2 %
−
NO + O2 → NO3
−
−
−
2NO2 +
%
O2−
−
NO + e → NO −
→ 2NO3
−
−
(6) (7) (8)
→ N2O22−
(1)
NO2 + e
(2)
2N2O22−
(3)
NO− + %O2− → NO3−
(11)
(4)
N2O22− + %OH → NO3− + H2O
(12)
−
%
%
+3
O2− →
−
(9) 4NO3−
NO + OH → NO3 + H2O
(5)
83
(10)
(13)
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Fig. 10. DMPO spin-trapping ESR spectra of pure g-C3N4, N-TiO2 and 25N-TiO2/g-C3N4: (a) methanol dispersion for DMPO−%O2− and (b) aqueous dispersion for DMPO−%OH. (c) In situ DRIFTS spectra of photocatalytic NO oxidation over 25N-TiO2/g-C3N4 under visible light irradiation. (d) Possible photocatalytic mechanism over N-TiO2/g-C3N4 heterojunction.
over N-TiO2/g-C3N4 by in-situ DRIFTS reveals that the NO can be completely converted into nitrate. Overall, the present work could provide new insights into heterojunction photocatalysis mechanism for air purification.
Table 1 Assignments of the IR bands observed during NO adsorption processes over 25N-TiO2/g-C3N4 under visible light irradiation. Wavenumber (cm−1)
Band assignment
References
1037 924, 813, 840, 867 1174 1096, 1124 961 1143 1043 1110 1002 1193
NO NO2− NO− N2O22− N2O3 N2O monodentate NO3− bidentate NO3− bridging NO3− NO3−
[32] [35] [38] [33] [33] [34] [36] [38] [39] [37]
Acknowledgements This research is financially supported by Chongqing Postdoctoral Science Foundation (Xm2016020), and China Postdoctoral Science Foundation (2016M602660), the Natural Science Foundation of Chongqing Science & Technology Commission (cstc2016jcyjA0154), and Research Startup Foundation of Chongqing Technology and Business University (2016-56-01 and 2016-56-02), National Natural Science Foundation of China (21676037).
4. Conclusions
References
In summary, we reported an updated approach to the synthesis of NTiO2/g-C3N4 heterojunction photocatalyst, in which N-TiO2 nanoplates are uniformly assemblied over g-C3N4 layers. This composite is highly efficient and durable in photocatalytic removal of ppb-level NO from a continuous air flow under visible light illumination. The mechanism study reveals that the enhanced photocatalytic performance originates from the N-doping in TiO2 and the heterojunction construction between N-TiO2 and g-C3N4, which intensify both the utilization of the visible light and the separation of the photoexcited electron and hole. %O2− and %OH are the two key active radicals for photocatalytic NO oxidation. The reaction pathway study of the photocatalytic NO oxidation
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Full Length Article
Photocatalytic NO oxidation on N-doped TiO2/g-C3N4 heterojunction: Enhanced efficiency, mechanism and reaction pathway ⁎
T
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Guangming Jiang , Jiwu Cao, Min Chen, Xianming Zhang , Fan Dong Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of Education, Chongqing Technology and Business University, Chongqing 400067, China
A R T I C LE I N FO
A B S T R A C T
Keywords: N-TiO2 g-C3N4 Heterojunction Photocatalysis In situ DRIFTS
Heterojunction photocatalysts with a high activity have been pursuit for years. In this work, we report a facile three-step approach to the synthesis of N-doped TiO2/g-C3N4 composite (denoted as N-TiO2/g-C3N4), in which N-TiO2 nanoplates are uniformly assemblied over g-C3N4 layers, leading to a maximization of the heterojunction effect. This resultant composite is highly efficient and durable in photocatalytic removal of ppb-level NO from a continuous air flow under visible light illumination. The photocatalytic NO removal efficiency is N-TiO2 contentdependent, and an optimal content lies at around 25 mg on 0.25 g of g-C3N4, under which the NO removal efficiency reaches 46.1%. The mechanism study reveals that the enhanced photocatalytic performance originates from the N-doping in TiO2 and the N-TiO2/g-C3N4 heterojunction construction, which promote the utilization of visible light and the separation of photoexcited electrons and holes. The active radicals of %O2− and %OH are both identified to play important roles in photocatalytic NO oxidation. The reaction pathway study of photocatalytic NO oxidation over N-TiO2/g-C3N4 by in-situ DRIFTS demonstrates that NO can be completely mineralized into nitrate. Overall, the presented work may contribute to a deeper understanding of the photocatalytic mechanism and the design of robust catalysts for air purification.
1. Introduction In recent years, the rapid industrialization and civilization process have raised severe environmental issues [1,2] especially the air pollution by the over emission of SOx, NOx, VOCs and PM2.5 [3]. To improve the situations, enormous efforts have been devoted to developing effective solutions, of which photocatalysis is receiving an ever-increased attention due to its high efficiency and environmental-friendly feature (this technology can effectively convert solar light to highly active oxidative free radicals for pollutant detoxification) [4]. In the development of photocatalysis technology, the core part is the design/ synthesis of highly effective and durable photocatalyst [5,6]. By looking through the history, we find that the semiconductor graphitic carbon nitride (g-C3N4) and titanium oxide (TiO2) are the two most popular materials. g-C3N4 is preferred by its facile synthesis, layered structure, large surface area, and narrow band gap allowing for visible light excitation, while TiO2 is attracted due to its robust performance and stable structure [7–9]. Though great advancements have been achieved in the development of g-C3N4 and TiO2-based products, several challenges still exist to limit their application. Firstly, due to its large band gap of 3.20 eV, pure TiO2 is only sensitive to the ultraviolet light [10],
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which only constitutes 7% of the solar energy. To improve visible light adsorption, various strategies have been proposed, such as the element doping and morphology/structure regulation to narrow band gap or form a new energy level in band gap [11,12]. The fast electron-hole recombination is another general issue for both the TiO2 and g-C3N4 [13,14], which usually lower the utilization efficiency of the absorbed light, and reduce the yield of active radicals for pollutant removal. To hinder the electron-hole recombination, one well-accepted strategy is to decorate semiconductor with metal particles or another semiconductor to form heterojuntions [15–17]. The difference in the location of Fermi level [18] between the semiconductor and metal or between the two semiconductors will generate a drive force for electron-hole separation. For example, our group ever constructed a heterojunction by assembling bismuth nanoparticle (Bi NP) over g-C3N4 [19]. Due to the lower Fermi level of Bi NP than the conduction band edge of g-C3N4, the photoexcited hot electrons on g-C3N4 will spontaneously flow to Bi NP, which effectively hinders their recombination with holes, and enhances NOx oxidation. Inspired by above achievements, we attempted to construct a novel N-doped TiO2/g-C3N4 heterojunction. With N doping and heterojunction construction, it is expected to achieve a robust visible light–driven photocatalytic NO oxidation.
Corresponding authors. E-mail addresses: [email protected] (G. Jiang), [email protected] (X. Zhang).
https://doi.org/10.1016/j.apsusc.2018.07.087 Received 24 May 2018; Received in revised form 7 July 2018; Accepted 11 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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spectroscopy (XPS, Thermo ESCALAB 250, USA). The optical property of the sample was examined on UV–vis diffuse-reflectance spectrometry (UV–vis DRS, Shimadzu, Japan) and Photoluminescence spectrometry (PL, F-7000, Hitachi, Japan). One spectrometer (JES FA200) was used to record the Electron Spin Resonance (ESR) signals of the radical %OH and %O2−, and the sample was dispersed into a 40 mM DMPO solution tank before the testing (aqueous dispersion for ·OH trapping and methanol dispersion for %O2− trapping). In- situ DRIFTS measurements (VERTEX70 FTIR spectrometer, Bruker) equipped with an in-situ diffuse-reflectance cell (Harrick) were conducted to probe into the photocatalytic oxidation pathway over the catalyst.
Due to the merits of g-C3N4 and TiO2, and their appropriate band structure, many groups have worked for years to construct effective gC3N4/TiO2 heterojunctions [20]. Wang [21] ever reported the fabrication of TiO2/g-C3N4 composite by calcining the mixed precursors of titanic acid nanotubes and melamine at 550 °C for photocatalytic degradation of methyl orange. Zhu [22] constructed the TiO2/g-C3N4 composite by ball milling of pre-synthesized TiO2 and g-C3N4, while Fu [23] employed a solid-state method to synthesize TiO2/g-C3N4 core/ shell composite. Though all the reported approaches are easy to achieve mass production, uniform mixing and close interface interaction between TiO2 and g-C3N4 are usually unsatisfactory, which reversely limits their synergistic effect. In addition, the reaction pathway of photocatalytic NO oxidation on N-TiO2/g-C3N4 composite has never been revealed [24]. Herein, we made improvements to synthetic procedure, and one novel N-TiO2/g-C3N4 photocatalyst was successfully prepared with the N-TiO2 nanoplates uniformly assembled on g-C3N4 layers. Photocatalytic performance of the resultant sample was evaluated by its efficiency in removing ppb-level NO from a continuous air flow under visible light illumination. The results show that N-TiO2/g-C3N4 composites present a much higher NO removal efficiency than the single gC3N4 and N-TiO2, as well as the mechanically mixed N-TiO2/g-C3N4. The involved mechanism for the enhanced photocatalytic activity was studied by the combined investigation on the microstructure, optical property and band structure of the sample, as well as the electron/hole separation efficiency and the species of produced active radicals in sample under visible light illumination. Additionally, the reaction pathway of photocatalytic NO oxidation over N-TiO2/g-C3N4 was examined by in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).
2.3. Photocatalytic activity evaluation The photocatalytic activity of samples was evaluated by their ability to remove ppb-level NO from a continuous reactor under a visible light illumination. Firstly, 0.2 g of the sample was dispersed in ethanol with the assistant of sonication, and then evenly pasted onto the bottom of two culture dishes (made of glass, 12 cm in diameter). The dishes were vacuum dried and placed in the middle of a rectangular reaction vessel (30 × 15 × 10 cm) for photocatalysis. The feeding NO concentration is set to 600 ppb (diluted by air) with a flow rate of 2.4 mL min−1. Incandescent lamps were placed over the reactor. When the air flow and the NO concentration were steady, light was turned on and the analyzer started to record the real-time concentration of NO. The NO removal efficiency (η) wan calculated as
η (%) = (1−C/C0) × 100% where C0 and C denote the NO concentration in inlet and outlet stream, respectively.
2. Experimental section
2.4. In-situ DRIFTS experiment
2.1. Material syntheses
In-situ DRIFTS equipment is composed of a Tensor II FTIR spectrometer (Bruker) and a reaction chamber (Harrick). As showed in Fig. 1, sample was placed in the reaction chamber, treated at a high temperature under a He flow to remove surface impurities, including water, and carbon dioxide, and then scanned to produce a baseline. After that, the mixed reaction gas (50 mL/min NO, 50 mL/min O2) was pumped into the chamber for a 20-min adsorption. The spectrum evolving along with the adsorption was recorded every two minutes. Once the adsorption was completed, light was turned on, and the photocatalytic reaction was carried out for another 60 min. The scanned wavelength range was set from 600 to 4000 cm−1.
All the chemical reagents are of analytical grade purity and used without further purification. Synthesis of N-TiO2. 3.0 g of TiN powder was added to the alumina crucible (100 mL), which was then put in the middle of a muffle furnace without a lid cover and calcined at 450 °C for 2 h under a static air atmosphere [25]. After cooling down to room temperature, pale yellow products were collected for use. Synthesis of g-C3N4. g-C3N4 was synthesized following a reported method [26]. Typically, 10 g of urea were dissolved in 20 mL of water in an alumina crucible (50 mL). The solution was dried at 60 °C overnight to drive the recrystallization of urea. The recrystallized urea in alumina crucible with a cover was then heated to 550 °C with a ramping rate of 15 °C min−1 in a muffle furnace and maintained for 2 h. After cooling down to room temperature, pale yellow powders were collected for use. Preparation of N-TiO2/g-C3N4 composite. 0.25 g of the as-prepared g-C3N4 and different weight (15, 20, 25, 30 and 35 mg) of N-TiO2 were dispersed in 30 mL of water by mild sonication, respectively. The N-TiO2 dispersion was then dropwise added to the g-C3N4 dispersion. The mixture was sonicated for another 20 min and then filtrated. The collected solid phase was dried at 60 °C before further calcination at 450 °C for 2 h. Eventually, pale yellow product was collected.
3. Results and discussion 3.1. Synthesis and characterization of the N-TiO2/g-C3N4 composite Fig. 2 schematically demonstrates the synthetic procedure for NTiO2/g-C3N4 composite [27]. In our method, N-TiO2 NPs and g-C3N4 are both pre-synthesized by calcination of the precursor TiN NP and
2.2. Characterization The crystal phase of the sample was analyzed by X-ray diffraction (XRD) (model D/max RA, Rigaku Co., Japan). Its particle morphology and structure were examined on Scanning electron microscopy (SEM, model JSM-6490, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan), while its chemical composition and electronic structure were investigated on X-ray photoelectron
Fig. 1. In-situ DRIFTS system for signal recording. 78
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Fig. 2. Schematic illustration of the synthesis process of N-TiO2/g-C3N4 heterojunctions.
layers can be assigned to N-TiO2 and g-C3N4 [30], respectively. The uniform distribution of black particles on gray layers indicates the successful construction of N-TiO2 nanoparticles/g-C3N4 heterojunction, and more importantly, the uniform assembly of N-TiO2 over g-C3N4, both of which will raise the number of heterojunction and are beneficial for the photocatalytic reaction. The HRTEM image shows two lattice fringes of 0.234 and 0.353 nm (Fig. 4d), which can be calibrated as (1 1 2) and (1 0 1) planes of anatase TiO2. Due to the semi-crystalline property of g-C3N4, its lattice spacing is hardly observed. However, the interface between these two materials is very clear, which further demonstrates the formation of heterojunction. XPS was performed to investigate the chemical composition in sample and the electronic state of each component element. The XPS survey spectrum of N-TiO2 sample in Fig. 5a clearly displays the elements of N, O and Ti, which well confirms that the N is doped into TiO2. The high-resolution XPS spectrum of N 1s in Fig. 5b shows a weak peak at 399.5 eV, which can be assigned to the N-O coordination [31]. The O 1s spectrum in Fig. 5c can be fitted with two peaks. The peak at 529.4 eV is related to O2− in N-TiO2, and the other at 531.2 eV is derived from the N-O coordination [32]. Fig. 5d presents the peaks of Ti 2p 3/2 and Ti 2p 1/2 at binding energies of 458.2 and 463.9 eV, which can be assigned to Ti4+ in N-TiO2 [33]. Nitrogen doping results in the formation of new electronic state above valence band, and the valence edge of N-TiO2 is determined to be around 2.17 eV (See Valence Band (VB) XPS result in Fig. 5e). We ever tried the one-step synthesis of N-TiO2/g-C3N4 by calcination of the mixed precursors of TiN and urea in one crucible. Fig. 6a and b compare the digital images of the as-synthesized composites by one step (denoted as N-TiO2/g-C3N4-1) and three-step (N-TiO2/g-C3N4-3), respectively. Different from N-TiO2/g-C3N4-3 with a uniform pale yellow color, N-TiO2/g-C3N4-1 appears partially dark and yellow. The dark and yellow part should correspond to the unconverted TiN (pure TiN powder is black) and g-C3N4. The uneven color indicates that the TiO2 and g-C3N4 phase are phase separated, and the TiO2 is actually not well assembled on the g-C3N4. As a result, the number and the quality of the heterojunction should both be low, and correspondingly, the photocatalytic activity is unsatisfactory (See below). Fig. 6c compares the XRD patterns of g-C3N4, N-TiO2 and N-TiO2/g-C3N4-1. Both TiO2 and TiN phases were detected in N-TiO2/g-C3N4-1, confirming that TiN is not fully converted. Fig. 6d presents the TEM image of N-TiO2/g-C3N41, which clearly show that N-TiO2 NPs are aggregated over g-C3N4 layers.
Fig. 3. XRD Patterns of the g-C3N4, N-TiO2 and N-TiO2/g-C3N4.
urea, respectively. The N-TiO2 NPs are then uniformly assembled on the layer structured g-C3N4 via a solution-mediated method, leading to the heterojunction formation. An additional calcination step at a relatively low temperature of 450 °C is employed to intensify the interface interaction between g-C3N4 and N-TiO2. Fig. 3 presents the XRD patterns of the as-synthesized g-C3N4, N-TiO2 and N-TiO2/g-C3N4 composite. gC3N4 shows two characteristic diffraction peaks at 2θ = 13.5° and 27.5°, corresponding to the (1 0 0) and (0 0 2) crystal plane of g-C3N4 respectively. The stronger peak can be attributed to the characteristic inter-layer stacking of the conjugated aro-matic C-N units, while the other is derived from a stacked structure of in-plane repeating structural units [28]. The N-TiO2 sample shows the typical diffraction peaks of TiO2 at 2θ = 25.3°, and no other phase is discerned, indicating the successful synthesis of anatase TiO2 phase with a high purity, and that the N doping will not change the crystalline structure [29]. As for composite sample, several new peaks of the rutile TiO2 phase can be observed, and all the diffraction peaks are narrowed, indicating the crystallinity of N-TiO2 is improved in the secondary step of calcination. g-C3N4 is evidenced by the typical diffraction peak at 2θ = 27.5°. Fig. 4 presents the TEM images of g-C3N4, N-TiO2 and N-TiO2/g-C3N4. g-C3N4 in Fig. 4a displays a layered and porous structure, which provides sufficient sites for heterojunction formation. Fig. 4b shows that the NTiO2 owns a lamellar shape with an irregular bilateral size of around 15–20 nm. Such a structure is very helpful for it to keep a sufficient contact with g-C3N4, and enhance the heterojunction effect. Fig. 4c shows the TEM image of one representative sample (25N-TiO2/g-C3N4). By their electron density differences, the black particles and the grey
3.2. Optical properties analyses The UV–vis diffuse reflection spectra of different samples are displayed in Fig. 7a. A sharp fundamental absorption edge of N-TiO2 and
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Fig. 4. TEM images of (a) g-C3N4, (b) N-TiO2, (c) 25N-TiO2/g-C3N4 composite and (d) HRTEM images of 25N-TiO2/g-C3N4 composite.
TiO2 and g-C3N4 can both generate the electrons and holes, and PL spectrum actually reflects the overall electron/hole separation efficiency in both the N-TiO2 and g-C3N4. With an increased N-TiO2 content, the increased production of electrons and holes in TiO2 will in some extent hinder the flow in of the electrons from g-C3N4, and as a result, weaken the function of heterojunction in the electron/hole separation.
g-C3N4 is 400 and 440 nm. At wavelengths less than 400 nm, a significant increase in absorption results from the intrinsic band gap absorption of TiO2. The extended absorption spectra in visible light region can be ascribed to the formation of one new electronic state above valence band caused by nitrogen doping. This is also reflected by the narrowed band gap of 2.84 eV compared to the pure TiO2 of 3.2 eV (See Fig. 7b). PL is one of the effective methods to detect the optical property of solid-state semiconductor, which can reflect the recombination rate of the photogenerated electron and hole pairs. The stronger the signal of molecular fluorescence spectrum, the higher the recombination rate of photogenerated electron-hole pairs. Fig. 8a shows the room-temperature PL spectra of different samples under an excitation light of 360 nm. The peak positions of all the N-TiO2/g-C3N4 samples in PL spectra have a slight shift compared to that of pure g-C3N4 and this is possibly derived from the contribution of N-TiO2 to the PL spectrum. The peak intensity as a function of TiO2 content is summarized in Fig. 8b. It is shown that all N-TiO2/g-C3N4 samples present lower fluorescence intensities compared to pure g-C3N4, indicating that the introduction of N-TiO2 on g-C3N4 can suppress the recombination of photogenerated electron-hole pairs. An optimal TiO2 content is found to be 25%, with which the peak intensity of the PL spectrum reaches lowest, and thus the electron/hole separation efficiency reaches highest. The existence of the optimal TiO2 content for an prohibited electron/hole recombination originates from the fact that both the N-
3.3. Photocatalytic performance The photocatalytic performances of N-TiO2, g-C3N4 and N-TiO2/gC3N4 composites were evaluated by their efficiencies in removing NO from a continuous air flow with visible light irradiation [34–35]. Fig. 9a shows that all the samples are active to remove NO under visible light illumination, of which N-TiO2/g-C3N4 are all more active than both the g-C3N4 and N-TiO2, demonstrating that the heterojunction construction can promote the photocatalytic activity of g-C3N4. The photocatalytic performance of N-TiO2/g-C3N4 is N-TiO2 content dependent. With an increase in N-TiO2 content, the NO removal efficiency of N-TiO2/gC3N4 firstly increases, reaches the peak of 46.1% at 25N-TiO2/g-C3N4 and then decreases (Fig. 9b). The presence of the optimal N-TiO2 content is well consistent with the PL result, which demonstrates that the construction of heterojunctions with a high quality and an appropriate number is required to achieve a maximum photocatalytic activity. The
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Fig. 5. High resolution XPS spectra of N-TiO2: (a) survey spectrum, (b) N 1s, (c) O 1s, (d) Ti 2p. (e) Valence band XPS of N-TiO2.
durability test result for the 25N-TiO2/g-C3N4 heterojunction in Fig. 9c suggests that the composite can maintain a high NO removal efficiency under a long-term run. To further emphasize the importance of uniform interface interactions between TiO2 and g-C3N4, the photocatalytic performances of 25N-TiO2/g-C3N4-1 and -3 were compared. The results in Fig. 9d show that 25N-TiO2/g-C3N4-1 prepared by one step calcination of TiN and urea is much less active to oxidize NO when compared to 25N-TiO2/gC3N4-3. As demonstrated above, in 25N-TiO2/g-C3N4-1, the N-TiO2 is not well assembled on g-C3N4, and thus less heterojunctions are actually constructed, which, we believe, is key reason for its poor photocatalytic performance.
and g-C3N4, an effective heterojunction is formed, by which the photogenerated electrons (e−) produced in g-C3N4 and the holes (h+) in NTiO2 will spontaneously flow to the CB of N-TiO2 and the VB of g-C3N4 respectively. In this way, the photoexcited e− and h+ can be utilized in a larger extent rather than be wasted by electron-hole recombination. The hot e− on CB of N-TiO2 with a potential above −0.67 eV can reduce O2 to %O2− (the redox potential for O2 into %O2− is −0.33 eV), while the h+ on VB of g-C3N4 with a potential of 1.57 eV can directly oxidize NO, but not trigger the formation of %OH from the oxidation of water molecules (the redox potential for H2O to %OH is 1.99 eV). The generation of %OH in system of N-TiO2/g-C3N4 should be evolved from % O2− with two additional hot e− (%O2− + 2 e− + 2H2O → 2%OH + 2OH−) [19]. In order to intuitively reveal the reaction pathways, In-situ DRIFTS study was further performed to monitor the related molecular or bond evolution over our catalyst during the photocatalytic NO oxidation under visible light irradiation [39]. Fig. 10c shows the spectra varying with the reaction time before and after the light turned on. After turning on the light, an absorption peak appears in 1037 cm−1, which can be assigned to NO [40]. The increased peak intensity with the time indicates that the surface of N-TiO2/g-C3N4 composite is adsorbing NO. The adsorbed NO will react with O2 to form NO2 (Eq. (1)). The characteristic peaks at 961 and 1143 cm−1 belong to the oxidized N2O3 [39] and N2O [41], respectively, which are formed via the coupling reaction between NO2 and NO (Eq. (2)) or NO and the N atom in N-TiO2/g-C3N4 (Eq. (3)). The typical adsorption peak of NO2− can be discerned easily at 813 cm−1 [41], which illustrates that NO can be oxidized to NO2− (Eq. (4)). At the same time, there is a sharp rise of monodentate NO3− at 1043 cm−1 [42], indicating that NO and NO2− can be directly
3.4. Reaction mechanism and pathway DMPO-ESR analyses are conducted to identify the active species produced during the photocatalytic reaction, and explore more on the role of g-C3N4, N-TiO2 and 25N-TiO2/g-C3N4. Fig. 10a reveals that the DMPO-%O2− signal intensity in the three samples follows an increasing order of N-TiO2 < g-C3N4 < 25N-TiO2/g-C3N4. Fig. 10b shows that gC3N4 has no signal of DMPO-·OH, while 25N-TiO2/g-C3N4 has the strongest signal intensity. It is suggested that the construction of NTiO2/g-C3N4 heterojunction can promote the formation of both the % O2− and %OH. The Conduction Band (CB) and VB edge energy of TiO2 were reported to lie at −0.29 and 2.91 eV, while those of g-C3N4 at −1.13 and 1.57 eV, respectively [36,37]. However, the band gap of TiO2 is narrowed by N doping [38], and the new locations of CB and VB for N-TiO2 are thus −0.67 and 2.17 eV. According to the band structure of N-TiO2
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Fig. 6. The digital images of (a) N-TiO2/g-C3N4-3 and (b) N-TiO2/g-C3N4-1; (c) XRD Patterns of the g-C3N4, N-TiO2 and N-TiO2/g-C3N4-1 (d) TEM image of N-TiO2/gC3N4-1 composite.
peak can be attributed to NO− (Eq. (8)) [44]. The NO2 will get another two electron and transform to N2O22− (1096 and 1124 cm−1) (Eq. (9)) [46]. The characteristic peaks of these two main intermediate products can be observed apparently. The final product (NO3−) during the photocatalytic reaction process is formed with a variety of chemical reaction pathways (Eqs. (10)–(13)). We can observe the intensity of the adsorption peak is obviously enhanced over irradiation time. [47] The
oxidized to NO3− (Eqs. (5)–(7)). Nitrate will remain on the surface of photocatalyst after the reaction as the absorption peaks (1193 cm−1) belonging to the NO3− is kept during the reaction [43], which is the leading origin of the gradual decrease in photocatalyst activity after a long-term run. However, monodentate NO3− is not stable enough, which can transform into the more stable bidentate NO3− (1110 cm−1) [44] and bridging NO3− (1002 cm−1) [45]. 1174 cm−1 absorption
Fig. 7. (a) UV–vis spectra of N-TiO2, g-C3N4 and N-TiO2/g-C3N4 composites with different TiO2 contents. (b) Plots of (αhν)2 versus photon energy (hν) for the band gap determination. 82
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Fig. 8. (a) Photoluminescence spectra and (b) the corresponding peak intensities of the g-C3N4 and N-TiO2/g-C3N4 composites with different contents of N-TiO2.
Fig. 9. (a) Photocatalysis activity and (b) the 30-min NO removal efficiency of g-C3N4, N-TiO2 and N-TiO2/g-C3N4 composites with different TiO2 contents. (c) Durable test of 25N-TiO2/g-C3N4 composite. (e) Photocatalytic activity of the g-C3N4, N-TiO2/g-C3N4-1 and N-TiO2/g-C3N4-3.
NO2− + 2%OH → NO3− + H2O
observed IR bands of the adsorbed species and their chemical assignments are listed in Table 1. According to the active species identification and reaction pathway study, a possible reaction mechanism is schematically proposed in Fig. 10d. 2NO + O2 → 2NO2 NO2 + NO → N2O3 NO + N(CN) → N2O %
−
−
2NO + O2 + e → 2NO2 %
−
NO + O2 → NO3
−
−
−
2NO2 +
%
O2−
−
NO + e → NO −
→ 2NO3
−
−
(6) (7) (8)
→ N2O22−
(1)
NO2 + e
(2)
2N2O22−
(3)
NO− + %O2− → NO3−
(11)
(4)
N2O22− + %OH → NO3− + H2O
(12)
−
%
%
+3
O2− →
−
(9) 4NO3−
NO + OH → NO3 + H2O
(5)
83
(10)
(13)
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Fig. 10. DMPO spin-trapping ESR spectra of pure g-C3N4, N-TiO2 and 25N-TiO2/g-C3N4: (a) methanol dispersion for DMPO−%O2− and (b) aqueous dispersion for DMPO−%OH. (c) In situ DRIFTS spectra of photocatalytic NO oxidation over 25N-TiO2/g-C3N4 under visible light irradiation. (d) Possible photocatalytic mechanism over N-TiO2/g-C3N4 heterojunction.
over N-TiO2/g-C3N4 by in-situ DRIFTS reveals that the NO can be completely converted into nitrate. Overall, the present work could provide new insights into heterojunction photocatalysis mechanism for air purification.
Table 1 Assignments of the IR bands observed during NO adsorption processes over 25N-TiO2/g-C3N4 under visible light irradiation. Wavenumber (cm−1)
Band assignment
References
1037 924, 813, 840, 867 1174 1096, 1124 961 1143 1043 1110 1002 1193
NO NO2− NO− N2O22− N2O3 N2O monodentate NO3− bidentate NO3− bridging NO3− NO3−
[32] [35] [38] [33] [33] [34] [36] [38] [39] [37]
Acknowledgements This research is financially supported by Chongqing Postdoctoral Science Foundation (Xm2016020), and China Postdoctoral Science Foundation (2016M602660), the Natural Science Foundation of Chongqing Science & Technology Commission (cstc2016jcyjA0154), and Research Startup Foundation of Chongqing Technology and Business University (2016-56-01 and 2016-56-02), National Natural Science Foundation of China (21676037).
4. Conclusions
References
In summary, we reported an updated approach to the synthesis of NTiO2/g-C3N4 heterojunction photocatalyst, in which N-TiO2 nanoplates are uniformly assemblied over g-C3N4 layers. This composite is highly efficient and durable in photocatalytic removal of ppb-level NO from a continuous air flow under visible light illumination. The mechanism study reveals that the enhanced photocatalytic performance originates from the N-doping in TiO2 and the heterojunction construction between N-TiO2 and g-C3N4, which intensify both the utilization of the visible light and the separation of the photoexcited electron and hole. %O2− and %OH are the two key active radicals for photocatalytic NO oxidation. The reaction pathway study of the photocatalytic NO oxidation
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