Compressible and Recyclable Monolithic g-C3N4/Melamine Sponge: A Facile Ultrasonic-Coating Approach and Enhanced Visible-Light Photocatalytic Activity

Introduction

Semiconductor photocatalysis is one of the promising strategies for pollutants abatement (Maggos et al., 2007; Huang et al., 2013, 2016; Zhao et al., 2015) and has attracted intense investigation in the past decades. Up to now, hundreds of semiconductors have been explored and applied in the field of environmental remediation (Liu et al., 2008; Hossain and Mukherjee, 2013; Huang et al., 2015a, 2017a; Zhou et al., 2016). However, only a few of them have been considered as potential candidates for practical application in view of their nontoxic, suitable band gap, band edge energy, good stability and earth-abundant source. g-C₃N₄ is one of those semiconductors, which possesses graphene-like structure and constituted mainly by carbon and nitrogen. Since g-C₃N₄ was first reported to photocatalytic water splitting by Wang et.al (Wang et al., 2009), it quickly becomes a hot material in photocatalysis. Afterwards, g-C₃N₄ has already been applied in various reactions such as CO₂ photoreduction, NO removal, and dye degradation (Yan et al., 2009; Dong et al., 2014a; Sun H. et al., 2017). However, extensive studies revealed that g-C₃N₄ suffers from fast photo-generated carriers recombination, limited visible-light absorption, and low surface area. Various strategies have been carried out to overcome these intrinsic drawbacks of g-C₃N₄, such as elemental doping, composite with other materials and morphology control synthesis, etc. (Liu et al., 2010; Zhao et al., 2012; Hou et al., 2014; Cheng et al., 2015; Han Q. et al., 2015; Li et al., 2017; Yang et al., 2017). Besides above disadvantages, as a potential photocatalyst for practical application, g-C₃N₄ is also hindered by difficult recycle originating from particle heavy loss during its complicated recovery process, inefficient utilization of active sites and light energy resulting from particle aggregation. Comparison with the intrinsic drawbacks of g-C₃N₄, these problems are vital to achieve successful application of photocatalyst on actual environmental issues, but they are rarely studied and generally beyond the aforementioned strategies to overcome.

Recently, monolithic or integrated photocatalysts are found to be a plausible way to solve the practical application problem of photocatalyst (Cheng et al., 2016; Wan et al., 2018). The so-called monolithic photocatalyst usually consists of two parts, one is a macroscopical support with porous three-dimensional (3D) skeleton, and the other is the loaded photocatalyst particles. After integrating powdery catalysts on its support, their recycling becomes easy to achieve by a tweezer (Liu W. J. et al., 2015; Tang et al., 2017). Meanwhile, the 3D porous structure of support gives the powdery catalysts a high dispersion and exposes more active sites by avoiding particles agglomeration. Moreover, this structure also benefits light energy harvest and transportation of liquid or gas pollutants. Until now, graphene aerogels are the most studied monolithic photocatalyst supports because of their inherent large surface area, high porosity and low density. We have fabricated monolithic C₃N₄/graphene oxide aerogel (GOA) in our previous work and found obvious activity enhancement (Wan et al., 2016), which is in line with the results from other monolithic photocatalyst/graphene aerogels (Fan et al., 2015; Cui et al., 2017; Wang et al., 2017). However, the intrinsic brittleness and weak firmness of aerogels make it easy to break into pieces during mechanical deformation, which seriously restrict its potential in practical application. Other firm supports, such as carbon foam and Al₂O₃ ceramic foam with hard 3D framework, are also selected to fabricate monolithic photocatalyst (Dong et al., 2014b; Lin et al., 2016). Nevertheless, it is very difficult to achieve uniform loading by directly mixing photocatalyst with these hard supports. To overcome this problem, special strategies with high cost and energy consumption, like in-situ immobilizing approach and laser ablating deposition (Liang et al., 2015; Lin et al., 2016), are used to achieve the good catalyst dispersion. The sophisticated preparation method severely limits the hard supports to be extensively utilized in fabrication of monolithic photocatalyst. Based on the above considerations, the proper support remains an obstacle for the practical application of monolithic photocatalyst. Lately, melamine sponge (MS), a cheap commercial polymer foam which is widely used as kitchen and construction materials, is successfully used for oil-water separation by integrating with graphene (Liu T. T. et al., 2015; Zhao et al., 2016). The fabricated graphene/MS exhibits low-density, high porosity and high elasticity inherited from MS, which exactly match the support characteristics of the monolithic photocatalyst. Importantly, the good elasticity makes MS more ductile and avoids the drawbacks of brittle and hard materials. Therefore, MS is a potential alternative and selected as the support for monolithic photocatalyst fabrication.

Herein, we prepare a monolithic g-C₃N₄/MS by a facile ultrasonic-coating method at room temperature, which is very easy to achieve mass production. The monolithic structure endows g-C₃N₄/MS with enhanced light harvest and more exposed active sites, ensuring its good photocatalytic activity. Importantly, the as-prepared monolithic photocatalyst exhibits low-density, high porosity, high elasticity and good firmness, which not only make it flexible in various environmental applications including NO removal, and CO₂ photoreduction, but also make it easy to recycle and suitable for practical application. Overall, our results provide a novel strategy to develop monolithic photocatlyst for practical application with large-scale production.

Experimental

Synthesis of g-C₃N₄

The polymeric g-C₃N₄ was prepared by pyrolysis of urea (Liu et al., 2011). In a typical process, 15 g urea was added into an alumina crucible with a cover and then heated to 550°C in a muffle furnace for 1 h with a heating rate of about 55°C min⁻¹. After cool down to room temperature, the final yellow agglomerates were the pristine g-C₃N₄ and subsequently ground into powder for further use.

Preparation of Monolithic g-C₃N₄/MS

For the preparation of monolithic g-C₃N₄/MS, 2.5 g g-C₃N₄ powder was dispersed in 500 mL water and sonicated for 1 h to form a g-C₃N₄ suspension. MS was cut into suitable size and washed with deionized water and alcohol in order, then dried at room temperature. Next, the clean MS was immersed into g-C₃N₄ suspension for 30 min, and then squeezed out the excess solution. After that, the sample was transferred into a culture dish and freeze-dried (−70°C pre-freezing) for 48 h to obtain g-C₃N₄/MS. For comparison, powdery g-C₃N₄ without MS was processed through the same procedures and denoted as sonicated g-C₃N₄. To obtain the best photocatalytic activity of monolithic g-C₃N₄/MS, we finely investigated the effect of g-C₃N₄ suspension concentration (3 mg mL⁻¹-40 mg mL⁻¹) and MS thickness (0.5–2.5 cm). The sizes of monolithic g-C₃N₄/MS varied with different experiments and were stated at their first appearance in the text.

Characterization

Powder X-ray diffraction (PXRD) was performed on a PANalytical X'pert diffractometer with a Cu Ka radiation. Transmission electron microscopy (TEM) was performed on a FEI tecnai G2 F30 microscope operated at 200 kV. The morphology of g-C₃N₄/MS was observed through scanning electron microscopy (SEM) on a ZEISS EVO MA15 microscopy. The Fourier transform infrared (FT-IR) spectra were measured using a Nicolet 6700 spectrometer on samples embedded in KBr pellets. UV-vis diffuse reflectance spectrum (DRS) data were recorded on a Shimadzu UV-2600 spectrophotometer. Photoluminescence spectra were recorded on F-7000 FL spectrofluorometer with an excitation wavelength at 320 nm. X-ray photoelectron spectroscopy (XPS) was performed by using a Thermo Scientific Escalab 250Xi spectrometer. The specific surface area (SSA) was determined via using methylene blue (MB) adsorption method on a UV-vis spectrophotometer (UV-5100, Anhui Wanyi; Tran et al., 2015), the SSA of g-C₃N₄ and g-C₃N₄/MS were calculated by the following equation:

$\begin{array}{l}SSA=\frac{N_A{A}_{MB}(C_0-C_e)V}{M_{MB}{m}_s}\end{array}$

Where N_A represents Avogadro's constant (6.02 × 10²³ mol⁻¹), A_MB represents the covered area of per MB molecule (typically assumed to be 1.35 nm²), C_o and C_e are the initial and equilibrium concentrations of MB, V is the volume of MB solution, M_MB is the relative molecular mass of MB, and m_s is the mass of the sample.

Evaluation of Photocatalytic Activity

The photocatalytic activity of g-C₃N₄/MS was evaluated in both gaseous systems. Photocatalytic removal of NO at ppb level was previously reported in details (Zhang et al., 2014). Typically, A 150 W metal halide lamp with a visible light filter (>420 nm) was employed to operate the experiment. A piece of g-C₃N₄/MS was put into the reactor for photocatalytic activity test. The light intensity is 35.88 mW cm⁻² measured by a light intensity meter. The initial concentration of NO was diluted to 500 ppb by drying air. The flow rates of dry air and NO are set at 2 L min⁻¹ and 9.5 mL min⁻¹, respectively. The formula of degradation rate of NO was counted by the following equation:

$\omega (\%)=\frac{[C\left(N{O}_X\right)-C\left(NO\right)-C_0\left(N{O}_2\right)}{C\left(N{O}_X\right)}\times 100\%$

Where C (NO_x) represents the concentration of total nitric oxide (NO₂ and NO), while C₀ is the initial concentration of NO₂ when reaching the adsorption-desorption equilibrium.

The photocatalytic reduction of CO₂ was performed in a 380 mL home-made reactor at ambient temperature and pressure. A 300 W Xe lamp was used as a light source and positioned 8 cm above the photocatalytic reactor. In a typical test, a plastic beaker with 20 mL deionized water was deposited at the bottom of the reactor, and a culture dish with 100 mg g-C₃N₄ powder or g-C₃N₄/MS (38.5 cm²) was placed on the plastic beaker. Before irradiation, the reactor was sealed and vacuumed by a pump, then removed air by blowing argon for 15 min. Subsequently, 1 mL CO₂ was injected into the reactor. After 4 h irradiation, 1 mL of gas was taken out from the reactor and analyzed by using a gas chromatograph (Techcomp GC7900) equipped with a flame ionized detector (FID) and a thermal conductivity detector (TCD). CO and CH₄ were analyzed by the FID, and H₂ was analyzed by TCD.

Results and Discussion

Fabrication and Physical Properties of g-C₃N₄/MS

The general preparation approach of g-C₃N₄/MS was illustrated in Figure 1A. Direct mixture of powdery g-C₃N₄ and MS is hard to gain a monolithic g-C₃N₄/MS with good particle dispersion and well contact between particle and MS skeleton. Therefore, the g-C₃N₄ powder was firstly added into water and sonicated for 30 min to form uniform suspension, which make the particle diffusion easy and fast in the porous structure of MS. Then, the pretreated MS was immersed into this suspension through dipping and squeezing procedures until MS was fully covered by g-C₃N₄ particles. Finally, followed by a conventional freezing-drying process, the monolithic g-C₃N₄/MS was obtained. Obviously, no large particles were observed on the cross-section photo in Figure 1A, indicating that the g-C₃N₄ powder was uniformly coated on MS skeleton. For comparison, we also dipped MS in a saturated solution of urea, which is the g-C₃N₄ precursor. After removing the water by freeze-drying, we attempted to fabricate the monolithic g-C₃N₄/MS by in-situ immobilizing approach at 550°C. However, the obtained monolithic composite became very fragile due to the carbonization of MS skeleton. Moreover, the g-C₃N₄ powder gathered on the composite surface with an inhomogeneous dispersion (Figure S1). The failed trial illustrated the mild ultrasonic-coating approach is superior to other methods for monolithic g-C₃N₄/MS fabrication. Importantly, the facile ultrasonic-coating approach developed here is also adapted to other porous supports, such as nickel foam, Al₂O₃ ceramic foam, glass fiber and polyester fiber, and the corresponding monolithic products of g-C₃N₄ are shown in Figure S2.

FIGURE 1

Figure 1. (A) Preparation process of g-C₃N₄/MS photocatalyst; (B) Ultra-light g-C₃N₄/MS resting on dog tail grass; (C) g-C₃N₄/MS with various shapes; (D) g-C₃N₄/MS with large area (12.0 × 12.0 × 0.5 cm³); (E) Mechanical property of g-C₃N₄/MS.

Physical properties of materials are pivotal to their practical application. As shown in Figure 1B, the monolithic g-C₃N₄/MS has an ultra-low density (11.5 mg cm⁻³), which can stay on dog tail grass and remarkably lighter than that of reported BiOBr/reduced GOA (50 mg cm⁻³) (Liu W. J. et al., 2015), TiO₂/graphene aerogel (19 mg cm⁻³) (Qiu et al., 2014), and MoS₂/reduced GOA (56.1 mg cm⁻³) (Zhang R. Y. et al., 2017). Moreover, SSA of g-C₃N₄/MS (7.6 m² g⁻¹) is much more larger than that of the pristine g-C₃N₄ (0.9 m² g⁻¹), which not only offer more active sites but also have larger absorption capacity than its powdery counterpart. In addition, facile modification of shapes and sizes endows g-C₃N₄/MS with a good flexibility to handle different situations in practical application (Figures 1C,D). More importantly, the g-C₃N₄/MS presents excellent elasticity. As revealed in Figure 1E, the g-C₃N₄/MS can instantaneously recover and maintain its integrity after removing of the heavy loading (410 g counterweight), suggesting that g-C₃N₄/MS possesses enough mechanical strength to deal with intricate operation in environmental abatement. To further ensure the firmness of g-C₃N₄ on MS skeletons, a test is carried out by blowing the g-C₃N₄/MS with a strong airflow for 12 h, the detailed simulation device diagram is shown in Figure S3a. The dropping g-C₃N₄ powder from g-C₃N₄/MS is collected and weighted up. The final weight loss is less than 7.60 mg, which account for 1.15% of g-C₃N₄ loaded on MS skeletons (Figure S3b). The above result demonstrates that g-C₃N₄ is firmly distributed on MS even under extreme work condition, which is pivotal to the recycle in practical application.

Photocatalytic Activity

Benefit from the characteristics of low-density, high porosity and good elasticity, the monolithic g-C₃N₄/MS can be used to removal gaseous pollutants. Therefore, two different photocatalysis applications including NO removal, dye degradation, and CO₂ photoreduction, are selected to test the photocatalytic activity of the as-prepared monolithic g-C₃N₄/MS. NO, a typical air contaminants, mostly produced from the combustion of fossil fuels and the emission of vehicle exhaust, can cause a series of atmosphere pollution problems such as acid rain, photochemical smog and haze (Wang et al., 2016). Photo-oxidation technique is considered an alternative to remove NO at low concentration (Zhou et al., 2014). Therefore, the g-C₃N₄/MS samples are firstly investigated by NO removal at the indoor ppb level. Figure 2A shows the effect of g-C₃N₄ concentration on NO photo-removal ratio occurring on g-C₃N₄/MS. Notably, the MS coated with 5 mg mL⁻¹ suspension achieves the highest removal ratio of 45% within 30 min. As the concentration of g-C₃N₄ suspension less than 5 mg mL⁻¹, the photocatalytic activity of g-C₃N₄/MS gradually enhanced with the increased concentration of g-C₃N₄ suspension. With a concentration higher than 5 mg mL⁻¹, the photocatalytic activity of the g-C₃N₄/MS slightly decreased, attributing to agglomeration of the excessive g-C₃N₄ which not only hinder the NO transport by blocking pore channels in MS, but also cause a reduced light transmittance. Moreover, the thicknesses of MS have also been investigated, which is closely associated with the light utilization. Benefit from the good transparence of MS, the activity of g-C₃N₄/MS enhanced along with increased MS thickness in Figure 2B, but the corresponding best unit mass rate constant (0.868 min⁻¹ g⁻¹) of NO removal is belong to g-C₃N₄/MS with 0.5 cm thickness (As shown in Figure S4). Based on above results, the optimal concentration and thickness were fixed at 5 mg mL⁻¹ and 0.5 cm, respectively.

FIGURE 2

Figure 2. Photocatalytic NO removal ratios in presence of g-C₃N₄/MS fabricated with (A) different concentrations and (B) different thicknesses under visible light irradiation.

The fabrication and utilization of monolithic photocatalysts with large area on macro-scale is significant to their practical application. Therefore, a monolithic g-C₃N₄/MS with area of 12.0 × 12.0 cm² is prepared for further test of NO removal under visible-light illumination (Figure 3A). For comparison, the NO removal on powdery g-C₃N₄ and pristine MS are also performed under the same conditions, respectively. Surprisingly, monolithic g-C₃N₄/MS presents the highest NO removal ratio of 78.6% in initial 5 min which is about 4.5 times higher than that of powdery g-C₃N₄ (17.6%), while no NO removal occurs on the pristine MS. After initial 5 min, NO removal ratio of g-C₃N₄/MS tends to be steady, which could be interpreted as partial active sites replaced by adsorptive ${\text{NO}}_3^{-}$ or NO (Ai et al., 2009; Huang et al., 2010; Liu et al., 2017) and finally reached an adsorption and desorption equilibrium of these oxynitrides. Moreover, comparison with other photocatalysts in our previous work, large area g-C₃N₄/MS exhibits optimum activity (78.6%), which is approximate 4.25, 2.25 and 2.32 times higher than that of Bi₂WO₆/graphene (Zhou et al., 2014), C₃N₄/GOA (GOA) (Wan et al., 2016), and N-Bi₂O₂CO₃/graphene quantum dots (Liu et al., 2017), respectively. Notably, the fraction of generated NO₂ is lower than 5.4% over all our samples (as shown in Figure S5a), indicating those samples selectively oxidize NO to ${\text{NO}}_3^{-}$ rather than NO₂. Based on above results, the g-C₃N₄/MS with large area shows obviously enhanced photocatalytic activity than its powdery counterpart, indicating the monolithic photocatalyst is an effective strategy to solve the problems encountered by powdery photocatalysts in large-scale application.

FIGURE 3

Figure 3. Photocatalytic activity of g-C₃N₄/MS under visible-light irradiation: (A) NO removal ratios of different samples; (B) Photocatalytic recycling test on large area g-C₃N₄/MS; (C) Production rate of CO, CH₄ and H₂ on g-C₃N₄, g-C₃N₄/MS respectively in photocatalytic CO₂ reduction under UV-Vis light irradiation.

The stability and recyclability of photocatalysts are also important to their practical applications (Hu et al., 2017). Here, we carried out cycle and long-time tests to further evaluate the performance of the monolithic g-C₃N₄/MS. The result of cycle test is listed in Figure 3B. It can be seen that the NO removal ratio of g-C₃N₄/MS dropped quickly during the first two cycles. After that, the NO removal ratio approached to stabilization within third cycle and slightly decreased with incremental cycle-index, which attributed to temporary absorption equilibrium of oxynitrides (NO, ${\text{NO}}_3^{-}$) in third cycle and their continuous accumulation on g-C₃N₄/MS. Fortunately, after UV lamp irradiation for 5 min, the adsorbed oxynitrides are desorbed and results in an immediate recovery of sample activity (Figure 3B). Furthermore, result of long-time test is displayed in Figure S5b. The g-C₃N₄/MS can achieve a stable catalytic performance after 1 h and keep a relatively high activity in the 2 h test interval. All above results confirm that the large area g-C₃N₄/MS has a superior recyclability and stability for NO removal, confirming its potential in practical application.

Apart from the photo-oxidation availability, the monolithic g-C₃N₄/MS also exhibits good ability of CO₂ photoreduction. CO₂ is the main greenhouse gas that generation from human activity and the combustion of fossil fuels which is responsible for global warming (Norby and Luo, 2004). As displayed in Figure 3C, the g-C₃N₄/MS shows higher photocatalytic activity (7.48 μmol g⁻¹ h⁻¹ CO, 3.93 μmol g⁻¹ h⁻¹ CH₄ and 0.26 μmol g⁻¹ h⁻¹ H₂) than that of powdery g-C₃N₄ (6.27 μmol g⁻¹ h⁻¹ CO, 0.52 μmol g⁻¹ h⁻¹ CH₄ and 0 μmol g⁻¹ h⁻¹ H₂). Notably, yield of CO is higher than that of CH₄ on both two samples, because the conversion from CO₂ to CO is 4-electron process, whereas that from CO₂ to CH₄ is 8-electrons process, obviously, the former is easier than the latter which can account for higher yield of CO. No H₂ generated on g-C₃N₄ and traced H₂ appeared on g-C₃N₄/MS indicate that all samples have high selectivity for CO₂ reduction rather than H₂ reduction.

In view of above results of NO removal, and CO₂ photoreduction, the monolithic g-C₃N₄/MS does show practical potential in various applications and enhanced photocatalytic activity than its powdery counterpart. The large SAA must be responsible for this activity enhancement. However, to get a deep insight into the reasons of the improved performance, more investigations further carried out on the monolithic g-C₃N₄/MS.

Structure and Morphology

The PXRD patterns of MS, g-C₃N₄/MS, sonicated g-C₃N₄ and g-C₃N₄ are displayed in Figure 4. The peaks at ca. 13.1 and 27.3° can be assigned to (100) and (002) crystal planes of g-C₃N₄, respectively (Gholipour et al., 2016). Obviously, the PXRD patterns of g-C₃N₄ are consistent before and after ultrasonic process, confirming the crystal structure of g-C₃N₄ is very stable. Moreover, no difference is found in the PXRD patterns of g-C₃N₄/MS and MS, indicating the g-C₃N₄ particles are uniformly dispersed on the porous framework of MS rather than aggregated on its surface.

FIGURE 4

Figure 4. PXRD patterns of g-C₃N₄, sonicated g-C₃N₄, g-C₃N₄/MS and MS.

To reveal the microstructure and morphology of samples, TEM and SEM were conducted as shown in Figure 5. The TEM images of g-C₃N₄ and sonicated g-C₃N₄ show similar thin nano-flake structure with some mesopores (Figures 5A,B), which derived from gases releasing such as NH₃ and CO₂ during the pyrolysis of urea (Mao et al., 2013). Combination with PXRD results, it is sure no noticeable change appeared on the structure and morphology of g-C₃N₄ before and after the ultrasonic. In Figure 5C, the SEM image of MS reveals an interconnected 3D network structure with abundant open-cell pores, which not only offer sufficient channels for reactant transport, but also offer enough locations for photocatalyst particle dispersion. It is notable that the g-C₃N₄ was successfully coated on the smooth skeleton of MS according to the g-C₃N₄/MS image in Figure 5D. Based on the analysis of structure and morphology, the uniform dispersion of g-C₃N₄ achieves in monolithic g-C₃N₄/MS, confirming the feasibility of our coating strategy.

FIGURE 5

Figure 5. TEM images of samples: (A) g-C₃N₄; (B) sonicated g-C₃N₄. SEM images of samples: (C) MS; (D) g-C₃N₄/MS.

Figure 6 shows the FT-IR spectra of g-C₃N₄, g-C₃N₄/MS and MS. The spectrum of g-C₃N₄ displays typical peaks at 3000-3600 cm⁻¹, 1200-1700 cm⁻¹, and 811 cm⁻¹, which are ascribed to the vibrational absorption of N-H and O-H, CN heterocycles and triazine unit (Kang et al., 2015; Wei et al., 2016; Sun Z. X. et al., 2017). Moreover, in the spectrum of MS, the prominent peaks located at 808, 1154, 1545, and 3422 cm⁻¹, attributing to triazine ring bending, C-O stretching, C = N stretching and N-H stretching, while peaks centered at 988, 1329, and 1466 cm⁻¹ corresponding to C-H bending vibrations (Pham and Dickerson, 2014; Zhang W. B. et al., 2017). Particularly, a new peak and an intensive peak appeared at 1334 cm⁻¹ and 813 cm⁻¹ in g-C₃N₄/MS spectrum, indicating a weak chemical interaction exists between g-C₃N₄ and MS skeletons. The above results reveal both van der waals and chemical interactions between g-C₃N₄ and MS, which explain the good firmness of monolithic g-C₃N₄/MS.

FIGURE 6

Figure 6. FTIR spectra of g-C3N4, g-C3N4/MS and MS.

Band Structure and Photocatalytic Mechanism

The band gap of MS, g-C₃N₄ and g-C₃N₄/MS were determined by the results of UV-vis DRS in Figure 7A. Obviously, g-C₃N₄ absorption is located in visible region with a calculated band gap of 2.99 eV, while MS shows only UV light absorption with a wide band gap of 4.31 eV. Moreover, the DRS profile of the g-C₃N₄/MS exhibits a mechanical combination of the absorption features of the g-C₃N₄ and MS alone. Importantly, visible-light absorption of g-C₃N₄/MS gets a slight enhancement (2.79 eV), which may be attributed to the light multistage refraction and reflection on the MS framework (Dong et al., 2014b). In addition, the g-C₃N₄/MS not only enhances the light absorption, but also significantly suppresses the recombination of photo-generated carriers according to photoluminescence (PL) spectra in Figure 7B, which accounts for the enhancement of photocatalytic activity.

FIGURE 7

Figure 7. (A) UV-vis DRS of g-C3N4, g-C3N4/MS and MS; (B) PL spectra of g-C3N4, g-C3N4/MS.

According to the above UV-vis DRS analysis (Figure 7A) and the XPS valence band spectrum (Figure S6), the band structure of g-C₃N₄/MS is proposed in Figure 8 with VB edge and CB edge located at 2.01 and −0.98 eV, respectively. As shown in Figure 8, the potential of VB holes (h⁺) is slight positive than OH⁻/OH (1.99 eV), while the potential of CB electron (e⁻) is much negative than that of O₂/${\text{O}}_2^{-}$ (−0.28 eV). Therefore, the photogenerated holes can directly oxidize OH⁻ to OH, and the photogenerated electrons can reduce easily O₂ to ${\text{O}}_2^{-}$. It is well known that OH and ${\text{O}}_2^{-}$ usually have strong oxidative ability and play the key role in photocatalytic oxidation reaction (Huang et al., 2015b,c, 2017b). Moreover, in consideration of the oxidation potentials of NO₂/NO (1.03 eV), HNO₂/NO (0.99 eV), HNO₃/NO (0.94 eV) (Wan et al., 2016), all OH, ${\text{O}}_2^{-}$ and hole generated on g-C₃N₄/MS are able to remove NO. In addition, the reduction potentials of E (CO₂/CH₄), E (CO₂/CO) and E (H₂O/H₂) were located at −0.24, −0.52, and −0.41 eV, respectively (Yu et al., 2014; Han B. et al., 2015). Comparison with CB potential (−0.98 eV), the photogenerated electron is capable of reducing CO₂ on g-C₃N₄/MS. Based on above analyses and the results in photocatalytic activity part, the possible photocatalytic mechanism are simply illustrated in Figure 8. In short, there are three main pathways for NO removal, involving three different active oxidation species (OH, ${\text{O}}_2^{-}$ and h⁺), while there is one pathway for CO₂ photoreduction, involving only one active species (e⁻).

FIGURE 8

Figure 8. Schematic illustration of the photocatalytic process over the g-C3N4/MS.

Conclusions

In summary, a novel monolithic g-C₃N₄/MS was fabricated by a facile ultrasonic-coating method. This monolithic g-C₃N₄/MS possesses a uniform dispersion of g-C₃N₄ and large SSA, which not only facilitate exposing more active sites of g-C₃N₄ but also enhancing the visible-light absorption. Consequently, monolithic g-C₃N₄/MS shows obviously improved visible-light photocatalytic activity. The PL detection further reveals that enhanced separation of photogenerated carriers is also responsible for the activity enhancement. More importantly, the characteristics of low-density and high porosity allow the monolithic g-C₃N₄/MS to be applied in various environmental issues, while the high elasticity, good firmness, and mechanical strength give it noticeable recyclability and stability, confirming its feasibility for practical application. All in all, in this work, we fabricated a monolithic g-C₃N₄/MS with enhanced photocatalytic activity which can be easy to achieve large-scale production for practical photocatalysis application. Our results provide a low-cost and mild method for mass production of new monolithic photocatalysts.

Author Contributions

YY: Partly designed the experiments and wrote the manuscript; RZ, TR, and WW: Assisted in the analysis and interpretation of the data; YZ and QZ: Proposed the project, designed the experiment, and wrote the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We gratefully acknowledge financial support from the Sichuan Provincial International Cooperation Project (2017HH0030), National Natural Science Foundation of China (21403172) and the Innovative Research Team of Sichuan Province (2016TD0011).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00156/full#supplementary-material

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