Efficient and Durable Visible Light Photocatalytic Performance of

Article pubs.acs.org/IECR

Efficient and Durable Visible Light Photocatalytic Performance of Porous Carbon Nitride Nanosheets for Air Purification Fan Dong,*,† Meiya Ou,† Yanke Jiang,† Sen Guo,‡ and Zhongbiao Wu‡ †

Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China ‡ Department of Environmental Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China S Supporting Information *

ABSTRACT: Graphitic carbon nitride (g-C3N4) is an intriguing metal-free photocatalyst for pollution control. This research represents an efficient visible light photocatalytic removal of gaseous NO at 600 ppb level with porous g-C3N4 nanostructures synthesized by pyrolysis of thiourea. TG-DSC was employed to simulate the pyrolysis of thiourea, and the mechanistic formation process of g-C3N4 was revealed. The crystallinity, morphology, surface area, pore structures, band structure, and photocatalytic activity of g-C3N4 can be engineered by variation of pyrolysis temperature and time. A layer-by-layer coupled with layer-splitting process was proposed for the gradual reduction of layer thickness and size of g-C3N4 obtained at elevated temperature and prolonged time. The visible light photocatalytic activity of g-C3N4 nanosheets toward NO purification was significantly enhanced due to the enhanced crystallinity, nanosheet structure, large surface areas and pore volume and enlarged band gap as the pyrolysis temperature was increased and the pyrolysis time was prolonged. The optimized g-C3N4 nanosheets (CN-600 °C and CN-240 min) exhibited higher photocatalytic activity of 32.7% and 32.3% than C-doped TiO2 (21.8%) and BiOI (14.9%), which are also highly stable and can be used repeatedly without obvious deactivation under repeated irradiation, demonstrating their great potential for practical applications.

1. INTRODUCTION The development of visible light driven photocatalysts has been the focus of considerable worldwide attention as photocatalysis technology is intensively applied in several important areas, including especially environmental pollution control and solar energy conversion.1−5 In general, most of the photocatalysts are metal-containing, such as metal oxide, metal sulfide, tungstates, niobates, tantalates, and vandates.6−8 Until recently, a new kind of conjugated polymer semiconductor (graphitic carbon nitride, g-C3N4) has been discovered as a fascinating metal-free organic photocatalyst working under visible light.9−13 Graphite-like covalent g-C3N4 is constructed by poly(heptazine) heterocyclic planes packed closely in a way similar to graphite.9 The g-C3N4 is multifunctional with broad applications (energy conversion and storage, contaminants degradation, carbon dioxide storage and reduction, catalysis, solar cells, and sensing) owing to its high stability, appealing electronic structure, and medium band gap.14,15 The g-C3N4 can be facilely prepared by pyrolysis of nitrogenrich precursors via polycondensation.9−15 The texture, electronic structure, and performance of g-C3N4 are largely depended on the condensation conditions and the types of precursor.14,15 The precursors employed for synthesis of gC3N4 include cyanamide, dicyandiamide, trithiocyanuric acid, melamine, triazine, heptazine derivatives, and more recently discovered urea and thiourea.16−23 The texture and band structure of g-C3N4 can also be tuned by templating, doping, heterostrucutre design, and postfunctionalization in order to enhance the reactivity in photocatalysis, selective synthesis, and CO2 reduction.24−29 © 2014 American Chemical Society

Generally speaking, the enhancement of crystallinity and the increase of surface areas could improve the photocatalytic activity of materials.30,31 The former factor is favorable for the reduction of defects and inhibiting charge carriers recombination, while the later one could provide more active sites for adsorption and reaction.30,31 However, high crystallinity and large surface areas are contradictory in most of the cases. In another word, the synthesis of catalytic materials with high crystallinity can be normally realized at the expense of large surface areas. Thermal treatment is the most common way to enhance crystallinity of the catalytic materials. For example, by increasing the annealing temperature and prolonging the annealing time during synthesis of TiO2 and other inorganic photocatalysts, the crystallinity could be enhanced, which however inevitably resulted in the decrease of surface areas.30,31 It is highly desirable that high crystallinity and large surface areas for a catalyst can be achieved simultaneously. In spite of the advances made on the synthesis of g-C3N4 as a photocatalyst for hydrogen evolution and aqueous pollutant degradation, the micro/nanostructures of g-C3N4 need to be improved for better photocatalysis.16−23 Moreover, the photocatalytic treatment efficiency of g-C3N4 for gaseous air pollutants has seldom been reported. Previously, we have synthesized g-C3N4 by pyrolysis of urea and found that the pyrolysis conditions have significant effects on the microstructure and photocatalytic activity of g-C3N4.16,22 Received: Revised: Accepted: Published: 2318

November 11, 2013 January 21, 2014 January 23, 2014 January 23, 2014 dx.doi.org/10.1021/ie4038104 | Ind. Eng. Chem. Res. 2014, 53, 2318−2330

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic flow diagram of the reactor system.

perform the thermogravimetric-differential scanning calorimetry analysis (TG-DSC: NETZSCH STA 409 PC/PG, German), 20 mg of dry sample was sealed in an Al2O3 crucible with a lid and scanned at a rate of 20 °C/min. A scanning electron microscope (SEM, JEOL model JSM-6490, Japan) was used to characterize the morphology of the samples. The morphology and structure were examined by transmission electron microscopy (TEM: JEM-2010, Japan). The UV−vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV−vis spectrophotometer (UV−vis DRS: UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly, using BaSO4 as reflectance sample. Nitrogen adsorption−desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP 2020, USA) with all samples degassed at 150 °C prior to measurements. 2.3. Visible Light Photocatalytic Performance for NO Purification. The photocatalytic activity was investigated by removal of NO at ppb levels in a continuous flow reactor as shown in Figure 1 (Figure S1 shows the photo of the reactor system). The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). A 150 W commercial tungsten halogen lamp was vertically placed outside the reactor. A UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. Photocatalyst (0.2 g) was coated onto a dish with a diameter of 12.0 cm. The coated dish was then pretreated at 70 °C to remove water in the suspension. The catalyst adhesion on the dish was firm enough to avoid the erosion (or removal) of the catalyst during air flowing. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance, BOC gas). The initial concentration of NO was diluted to about 600 ppb by the air stream. The desired relative humidity (RH) level of the NO flow was controlled at 50% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 2.4 L/min by a mass flow controller. After the adsorption−desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., 42i-TL), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 1.0 L/min. The removal ratio (η) of NO was calculated as η (%) = (1−C/C0) × 100%, where C and C0 are concentrations of NO in the outlet stream and the feeding stream, respectively.

In the present work, we develop a simple method to engineer the micro/nanostructures of g-C3N4 from pyrolysis of thiourea and apply the as-prepared g-C3N4 in visible light photocatalytic air purification. The easily available thiourea is a superior precursor because it is nontoxic, low-cost, and earth-abundant. A layer-by-layer coupled with layer-splitting process was proposed for the gradual reduction of layer thickness and size of g-C3N4 obtained at elevated temperature and prolonged time. The formation mechanism of g-C3N4 from thiourea was also revealed. Interestingly and importantly, we find that both the crystallinity and the surface areas of g-C3N4 increase spontaneously with elevated pyrolysis temperature and prolonged pyrolysis time, which is very important to enhance the activity of g-C3N4. The morphology and band structure of g-C3N4 can also be simply engineered by variation of pyrolysis conditions. The optimized g-C3N4 nanosheets exhibit efficient and durable visible light photocatalytic performance in NO removal. This unique finding will shed new light on synthesis and engineering of organic photocatalysts for large-scale environmental applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis of g-C3N4 from Thiourea. All chemicals used in this study were analytical grade and were used without further purification. In a typical synthesis, 10 g of thiourea powder was put into an alumina crucible with a cover. The crucible was heated to 550 °C at a heating rate of 15 °C/min in a tube furnace in air and maintained for 120 min. The released air products during thermal treatment were absorbed by dilute NaOH solution of 0.05 M. The resulted final yellow powder was ground and collected for use without further treatment. In order to investigate the effects of pyrolysis temperature, g-C3N4 was synthesized at 500, 525, 550, 575, and 600 °C for 120 min, respectively. The resulted samples were labeled as CN-500 °C, CN-525 °C, CN-550 °C, CN-575 °C, and CN-600 °C. In order to investigate the effects of pyrolysis time, g-C3N4 was synthesized at 550 °C for 0, 15, 30, 60, 120, and 240 min, respectively. The resulted samples were labeled as CN-0 min, CN-15 min, CN-30 min, CN-60 min, CN-120 min, and CN240 min. Note that the pyrolysis time does not include the time the furnace spent to raise the temperature to 550 °C. 2.2. Characterization Methods. The crystal phase was analyzed by X-ray diffraction with Cu Kα radiation (XRD: model D/max RA, Japan). The scan rate was 0.02 deg/s. The accelerating voltage and the emission current were 40 kV and 40 mA, respectively. FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. To 2319

dx.doi.org/10.1021/ie4038104 | Ind. Eng. Chem. Res. 2014, 53, 2318−2330


Article

Figure 2. XRD pattern of g-C3N4 treated under different temperatures (a) and enlarged view of (002) peak (b), XRD pattern of g-C3N4 treated for different times (c) and enlarged view of (002) peak (d).

3. RESULTS AND DISCUSSION 3.1. Phase Structure and Transformation. Figure 2a shows the XRD patterns of the prepared g-C3N4 samples treated under different temperatures in the range of 500−600 °C. All of the g-C3N4 samples in Figure 2a have similar diffraction patterns, suggesting that all samples are similar in crystal structure. A typical (002) peak around 27.5° is observed, which indicates the graphite-like stacking of the conjugated aromatic units of CN with an interlayer distance of 0.33 nm.9 A typical (100) diffraction peak around 13.0° corresponding to a distance of 0.68 nm could be assigned to the in-plane repeated units.9 Further observation on an enlarged view of (002) peak in Figure 2b shows that the diffraction angle 2θ of (002) peak increases from 27.31° for the CN-500 °C sample to 27.73° for the CN-600 °C sample when the pyrolysis temperature increases from 500 to 600 °C. This result implies that gC3N4 becomes more compact when thiourea is treated at a higher pyrolysis temperature. Figure 2c shows the XRD patterns of the prepared g-C3N4 treated at 550 °C for different times in the range of 0−240 min. The two peaks at around 27.5° and 13° can be observed for all the as-prepared samples in Figure 2c. From Figure 2d, we can see that the diffraction angle 2θ of (002) peak increases from 27.24° for CN-0 min to 27.66° for CN-240 min when the pyrolysis time increases from 0 to 240 min (Figure 2d). This result suggests that the

interlayer distance of g-C3N4 decreases with prolonged pyrolysis time, which is similar to the effects of pyrolysis temperature on the crystal structure. Figure 2 also illustrates that the diffraction peak intensity become stronger when the pyrolysis temperature is increased and pyrolysis time is prolonged. This fact implies that the crystallinity of g-C3N4 is improved with the elevated pyrolysis temperature and prolonged pyrolysis time. In order to understand the phase transformation during pyrolysis of thiourea, TG-DSC was carried out. The range of temperature is from room temperature to 800 °C at a heating rate of 20 °C/min. An alumina crucible with a cover was used during thermal analysis to simulate the actual thermal environment of thiourea pyrolysis. The DSC and TG thermograms for thiourea (Figure 3) clearly show that several phase transformations can be observed in the semiclosed system. An endothermic peak at 190 °C is the melting point of thiourea. The strongest endothermic peak appears in the temperature range 210−295 °C, and the weight of the sample decreased rapidly by 70.5%. The peak at 236 °C (overlapped by the strong peak) indicates the reaction of thiourea into cyanamide. Cyanamide is a common precursor to synthesize g-C3N4. The sharp peak at 266 °C implies that the thermal condensation of cyanamide into melamine occurred in this temperature range. The weak endothermic peak at 312 °C corresponds to the further condensation process where 2320



Article

samples are illustrated in Figure 5. Figure 5a shows that the CN-500 °C sample is composed of thick layers attached with some agglomerated particles. Figure 5b demonstrates that the CN-550 °C sample is mainly composed of interconnected thin layers with some pores that may result from the gas bubbles during pyrolysis of thiourea. In the case of the CN-600 °C sample, as shown in Figure 5c, a large number of small thin layers with abundant pores can be observed. The gas bubbles play a key role in the formation of porous structure. Figure 5d demonstrates that the CN-0 min sample is composed of large irregular particles with some layered structure. For the CN-30 min sample as shown in Figure 5e, thick plates with some particles can be observed. Increasing the pyrolysis time to 240 min, the thickness of the sample is significantly reduced, and porous structure is generated at the same time (Figure 5f). By summarizing the above observations, we can conclude that elevating the pyrolysis temperature and prolonging the pyrolysis time could make the resulted g-C3N4 samples possess small size, thin layers, and porous structure. This is a facile way to tune the microstructures of g-C3N4. The EDX elemental mapping of the typical CN-120 min sample (Figure 5g) is shown in Figures 5h, 5i, and 5j. It can be seen that the C3N4 sample prepared from thiourea was composed of C, N, and O elements, indicating S was released from the pyrolysis. The microstructure was further investigated by TEM. Figure 6a shows that the CN-500 °C sample has a bulk structure composing of large particles with a layer structure. When the pyrolysis temperature was increased to 550 °C, the resulted gC3N4 sample was of a sheetlike structure with reduced thickness (Figure 6b). When the pyrolysis temperature was further raised up to 600 °C, the resulted g-C3N4 sample was composed of a thinner sheetlike porous structure due to the successful introduction of mesopores of several tens of nanometers in the CN-600 °C sample (Figure 6c). Further observation in Figure 6a and Figure 6c implies that the average size of the sheets are decreased with increasing temperature probably because the large layers are split into smaller ones under higher temperature. For the g-C3N4 samples prepared at 550 °C for different times, Figure 6d shows that the CN-0 min sample consists of large particles with irregular shape. A thick and smooth sheetlike structure is clearly observed for the CN-60 min in Figure 6e. The morphology of the CN-240 min sample was

Figure 3. TG-DSC thermograms for heating thiourea.

melamine is transformed to melem. The weight loss in this temperature range is about 10.4%. The further weight loss (about 6.4%) with an endothermic peak at 422 °C can be ascribed to the phase formation from melem to graphitic carbon nitride. Finally, the endothermic peak at 707 °C with a weight loss of 12.7% can be attributed to the sublimation of carbon nitride. The TG-DSC results imply the mechanistic transformation process of carbon nitride from pyrolysis of thiourea.9 3.2. Chemical Composition. The FT-IR spectra of all the samples are shown in Figure 4. We can observe the absorption band at 801 cm−1 corresponding to a breathing mode of triazine, the absorption bands in the range of 1200−1600 cm−1 attributing to stretching mode of C−N heterocycles, and the broad bands in the range of the 3000−3700 cm−1 region attributing to the adsorbed H2O molecules and N−H vibration.22 For the samples treated under lower temperature and for shorter time, the incomplete condensation of thiourea results in the weak vibration of the C6N7 units. This poor condensation can be improved by increasing the pyrolysis temperature and time to promote the formation process of gC3N4. 3.3. Morphology and Nanostructure Formation Mechanism. The typical SEM images of the as-prepared

Figure 4. FT-IR spectra of g-C3N4 treated under different temperatures (a) and g-C3N4 treated for different times (b). 2321



Article

Figure 5. SEM images of CN-500 °C (a), CN-550 °C or CN-120 min (b), CN-600 °C (c), CN-0 min (d), CN-30 min (e), CN-240 min (f), SEM image (g), and EDX elemental maping of C, N, and O (h, i, j) in image (g).

quite different, and many thin flat sheets and some mesopores can be clearly seen in Figure 6f. This typical sheetlike morphology imparts CN-240 min with a large specific surface area. Combining the SEM and TEM results, we can find that the thickness and the size of the g-C3N4 sheetlike nanostructures were reduced simultaneously when the pyrolysis temperature

was increased and the pyrolysis time was prolonged. Such variation in structure would lead to the formation of g-C3N4 with high surface areas and large pore volumes, which is beneficial for enhancing the photocatalytic activity. The mass of g-C3N4 products obtained under different temperatures and times with the same amount of thiourea was measured. The weight of g-C3N4 products was decreased with 2322



Article

Figure 6. TEM images for CN-500 °C (a), CN-550 °C or CN-120 min (b), CN-600 °C (c), CN-0 min (d), CN-60 min (e), and CN-240 min (f).

elevated pyrolysis temperature and prolonged pyrolysis time, resulting from gradual decomposition of solid g-C3N4 due to thermal oxidation in air. The conjugated layered g-C3N4 is constructed by the hydrogen bonding between aromatic CN units. The energy of the hydrogen bond is weak and can be destroyed by thermal oxidation. As a result, the layer of CN units would be gradually oxidized and removed in a layer-bylayer way during thermal treatment.16 Subsequently, the thickness of g-C3N4 samples would be decreased with elevated pyrolysis temperature and prolonged pyrolysis time (Figures 5 and 6). Meanwhile, large g-C3N4 layers were split into smaller layers to reduce surface energy (Figures 5 and 6).16 On this basis, a layer-by-layer coupled with layer-splitting process can be proposed for the explanation of reduction of layer thickness and size of g-C3N4 samples obtained at elevated temperature and prolonged time. 3.4. Texture Property. The nitrogen adsorption−desorption isotherms and Barrett−Joyner−Halenda (BJH) pore-size distribution of selected samples are displayed in Figure 7. Figures 7a and 7b show that the CN-500 °C sample exhibits

nonporous structure. When the pyrolysis temperature exceeds 550 °C, significant enlargement of surface areas and the generation of nanopores (mesopores) can be observed (Figure 7b and Table 1). The CN-600 °C sample is type IV (Brunauer, Deming, Deming, and Teller, BDDT classification) with a hysteresis loop at high relative pressure between 0.5 and 1.0, suggesting the presence of mesopores (2−50 nm) and macropores (>50 nm).32 There are type H3 hysteresis loops at 0.45 < P/P0 < 1.00 in the isotherms of the optimized samples (CN-600 °C and CN-240 min), which are often observed on the aggregates of platelike particles giving rise to slit-shaped pores which agrees well with the nanosheet-like morphology (Figures 5c and 5f).32 It can be seen from Figure 7a and Table 1 that increasing the condensation temperature from 500 to 600 °C causes a great enhancement of surface area and pore volume from 5 m2/g and 0.029 cm3/g for the CN-500 °C sample to 36 m2/g and 0.25 cm3/g for the CN-600 °C sample. The creation of a porous structure can also be observed directly from SEM images (Figures 5a-5c). The effects of pyrolysis time on the texture property of the as-prepared g-C3N4 samples are 2323



Article

Figure 7. N2 adsorption−desorption isotherms of CN-500 °C, CN-550 °C, and CN-600 °C (a) and the corresponding pore-size distribution curves (b), N2 adsorption−desorption isotherms of CN-0 min, CN-30 min, CN-60 min, CN-120 min, and CN-240 min (c) and corresponding pore-size distribution curves (d).

Table 1. SBET, Pore Volume, Peak Pore Size, and NO Removal Ratio for Selected g-C3N4 Samplesa sample name CN-500 °C CN-550 °C CN-600 °C CN-0 min CN-30 min CN-60 min CN-120 min CN-240 min C-doped TiO245 BiOI50 a

SBET (m2/g)

total pore volume (cm3/g)

peak pore size (nm)

η(NO) (%)

5 27 36 6 10 12 27 71 123

0.029 0.142 0.25 0.036 0.060 0.073 0.142 0.35 0.25

nonporous 2.6/4.1 2.6/3.8/32.6 3.8 3.8 3.8 2.6/4.1 2.8/3.8/31.1 3.5

10.2 22.0 32.7 7.7 14.1 17.6 22.0 32.3 21.8

6

0.027

3.7/18.3

14.9

confirms the introduction of mesopores in the CN-240 min sample treated for a longer time (Table 1). The high surface area and large pore volume of CN-600 °C and CN-240 min samples can be attributed to the reduced layer thickness and size. This interesting result is consistent with SEM and TEM observations (Figures 5 and 6). The crystallinity and the surface areas of g-C3N4 organic photocatalyst can be enhanced with elevated pyrolysis temperature and prolonged pyrolysis time (Figures 2 and 8). This thermal behavior of g-C3N4 is contrary to most porous inorganic photocatalysts, which typically undergo structure deformation/pore collapse with decreased surface area upon increasing the heating temperature in order to improve the crystallinity, as it is known that the creation of porous structures with high surface area in g-C3N4 relied largely on templates (for example SiO2, zeolite, and Triton X-100) followed by etching of the templates.33−36 Such a process is relatively tedious and thus prevents the large scale applications. This drawback can be overcome by our remarkable observation in this research. The porous nanostructure of g-C3N4 can be self-generated by a facilely optimized thermal treatment. Porous g-C3N4 with high surface area has been readily synthesized by a template-free method though treating thiourea at higher temperature for a longer time. The creation of porous nanostructure could facilitate catalytic sorption and promote the localization of light-induced electrons in the conjugated

The data for C-doped TiO2 and BiOI were collected from references.

similar. With increasing thermal treating time, the hysteresis loops shift to the region of lower relative pressure, and the areas of the hysteresis loops gradually become large. Prolonging the pyrolysis time from 0 to 240 min leads to significant enlargement of surface area from 6 m2/g for the CN-0 min sample to 71 m2/g for the CN-240 min sample, together with pore volume from 0.036 to 0.35 cm3/g (Figures 8a and 8b and Table 1). The change of peak pore size with pyrolysis time also 2324



Article

Figure 8. The correlation between SBET and the pyrolysis temperature and time for selected samples (a) and the correlation between pore volume and the pyrolysis temperature and time for selected samples (b).

Figure 9. UV−vis DRS (a, c) and plots of (αhν)1/2 vs photon energy (b, d) of g-C3N4 samples treated under different temperatures and treated for different times.

change with the variation of pyrolysis temperature and time. The band gap energy can be estimated from the intercept of the tangents to the plots of (αhν)1/2 vs photon energy, as shown in Figures 9b and 9d. Figures 9a and 9b indicate that when the temperature increases from 400 to 550 °C, slight reduction band gap energy from 2.49 to 2.42 eV can be detected. This bathochromic shift in band gap is ascribed to the enhanced structural connections with enhanced van der Waals interaction

systems, which are beneficial for photocatalysis by carbon nitride.11 3.5. Variation of Band Gap. The relationship between optical property and pyrolysis conditions is investigated by UV−vis DRS, as shown in Figure 9. An absorption edge located in a visible light region is observed for all the samples, which originates from band gap transitions from valence band to conduction band. The absorption edges of g-C3N4 samples 2325



Article

Figure 10. Visible light photocatalytic activities of g-C3N4 samples treated under different temperatures (a) and g-C3N4 samples treated for different times (b) for removal NO in air (continuous reactor, NO concentration: 600 ppb). Monitoring of the fraction of NO2 intermediate over g-C3N4 samples treated under different temperatures (c) and g-C3N4 samples treated for different times (d) during photocatalytic reaction.

C3N4 samples were applied for gaseous NO degradation under visible light irradiation in a continuous reactor in order to demonstrate their potential ability for air purification. Figures 10a and 10b show the variation of NO concentration (C/C0%) with irradiation time over g-C3N4 samples treated under different temperatures. Here, C0 is the initial concentration of NO, and C is the concentration of NO after photocatalytic reaction at time t. Previous investigation indicated that NO could not be photolyzed under light irradiation.40 It can be found in Figure 10a that NO could not be degraded without photocatalyst under light irradiation or with photocatalyst (CN600 °C) for lack of light irradiation. In the presence of photocatalyst, the NO reacted with the photogenerated reactive radicals to produce the final product of HNO3. Because g-C3N4 has a suitable band gap that can be directly excited by visible light, all g-C3N4 samples treated under different temperatures and for different times show decent visible light photocatalytic activity toward NO removal, as shown in Figure 10. Figure 10a indicates that the NO removal ratio of g-C3N4 samples increases from 10.2% to 32.7% when the pyrolysis temperatures increase from 500 to 600 °C after 45 min irradiation. Figure 10b implies that the NO removal ratio of g-C3N4 samples increases from 7.7% to as high as 32.3% when the pyrolysis time increases from 0 to 240 min (Table 1). The visible light activity of CN-600 °C and CN-240 min samples exceeds that of C-doped TiO2 (21.8%) and BiOI (14.9%), suggesting that

between the tri-s-triazine cores as higher pyrolysis temperature results in a higher degree of polymerization and a denser packing of the tri-s-triazine units (Figure 2).37 This, in turn, leads to a stronger overlapping of molecular orbitals of the aromatic sheet stacks. Further increasing the temperature from 550 to 600 °C leads to the hypsochromic shift of the absorption edges from 2.42 eV for CN-550 °C to 2.57 eV for CN-600 °C due to the quantum confinement effects induced by nanozised particles as high temperatures could significantly reduce the size of g-C3N4 through layer-by-layer oxidation coupled with layer splitting (Figures 5 and 6).38 Figures 9c and 9d imply that prolonging the pyrolysis time from 0 to 240 min causes the band gap energy of g-C3N4 samples to increase from 2.37 to 2.90 eV obviously. The relationship between band gap energy of g-C3N4 and pyrolysis conditions can be found in Figure 9. Recently, Wang et al. developed a novel comonomer strategy to tentatively modify the texture and band structure of g-C3N4 by chemical incorporation of monomer building blocks into the conjugated polymeric network of g-C3N4.39 In this research, we can find a simple approach to control the microstructure and band gap of g-C3N4 by tuning the pyrolysis temperature and time, being a potentially valuable way to alter the physical and chemical properties of polymeric semiconductors. 3.6. Visible Light Photocatalytic Activity and Stability for NO Removal. 3.6.1. Photocatalytic Removal of NO and Monitoring of Reaction Intermediates. The as-prepared g2326



Article

Figure 11. Multiple photocatalytic reaction over the CN-600 °C sample (a) and the CN-240 min sample (b) for removal of NO in air.

for boron-doped TiO2 with the highest activity was 400 °C in the range of 300−600 °C. However, in our case, the activity of the g-C3N4 sample is enhanced progressively with continuous elevated temperature and prolonged pyrolysis time (Table 1). This unique variation of the activity should be related to the unusual change of texture property and band gap of g-C3N4 with different thermal treatment conditions (Figures 5, 6, 8, and 9). The remarkably improved photocatalytic activities of the gC3N4 samples with respect to elevated temperature and prolonged pyrolysis time demonstrated above can be explained as the synergistic effects of enhanced crystallinity, nanosheetlike morphology, large surface area, large pore volume, and increased band gap. First, for the g-C3N4 sample treated at high temperature and for a long time, the enhancement of crystallization (Figure 2) is advantageous to reduce the recombination rate of photogenerated electrons and holes due to a decrease in the number of the defects.43 Second, the nanosheet-like structure (Figures 5 and 6) enhances the transport of photogenerated electrons along the nanosheet, thus lowering the hole−electron recombination.44−47 Third, the thin thickness and porous character result in a large surface area for pollutant adsorption.48,49 Fourthly, large pore volume (Figure 8) provides more active site for quick reactant diffusion.31,49,50 Lastly and importantly, the increase in the band gap increases the redox ability of charge carriers generated under irradiation (Figure 9).46 All these favorable factors cocontribute to the significantly improved photocatalytic activities of g-C3N4 samples synthesized at elevated temperature (600 °C) and treated for a long time (240 min). 3.6.2. Photochemical Stability with Multiple Runs. To further test the stability of the optimized CN-600 °C and CN240 min samples for practical application, repeated reaction tests were carried out. The sample after one run was used directly without further treatment for the next photocatalytic reaction run. As shown in Figure 11, the NO removal ratios of CN-600 °C and CN-240 min samples could be well maintained after five cycles under visible light irradiation. Except for a slight drop in the activity during the third running, no further decrease in activity in the following runs can be observed. These results clearly demonstrate that nanostructured porous g-C3N4 photocatalysts with enhanced and durable activity can be successfully synthesized and applied for efficient air purification.

variation of thermal treatment conditions is an effective approach to enhance the activity of g-C3N4. Under the optimized thermal conditions, the photocatalytic activity of gC3N4 from thiourea is higher than that of the sample from urea, demonstrating the advantage of thiourea as precursor.16 The reaction intermediate of NO2 during photocatalytic oxidation of NO is monitored online as shown in Figures 10c and 10d. The fraction of NO2 generated over g-C3N4 samples during irradiation decreases with increased pyrolysis temperature and prolonged pyrolysis time, which can be ascribed to the fact that the surface areas and pore volumes are increased accordingly. The diffusion rate of reaction intermediate over gC3N4 samples with high surface areas and large pore volume is faster, thus promoting the oxidation of intermediate NO2 to final NO3−, as shown in the following reactions.40 The final oxidation products (nitric acid or nitrate ions) can be simply washed away by water wash. Note that as the photocatalytic reaction was going on, the NO concentration in the outlet was decreased gradually due to the conversion of NO to NO3−. The NO concentration would reach minima until the photocatalytic reaction reached equilibrium. The slight rising of NO concentration was due to the accumulation of NO3− product on the catalyst surface.40,44 After long-term irradiation, the NO concentration in the outlet would reach a steady state. NO + 2 • OH → NO2 + H 2O

(1)

NO2 + •OH → NO3− + H+

(2)

NO + NO2 + H 2O → 2HNO2

(3)

NO + •O2− → NO3−

(4)

Thermal treatment is a general process employed to crystallize catalytic materials. The effects of thermal treatment conditions on the microstructure and photocatalytic activity of different types of photocatalysts have been widely investigated.23,30,31,41−43 Yu et al. studied the effects of calcination temperature on the photocatalytic activity of TiO2 from titanate and found that activity of TiO2 deceased with an increase in calcination temperature in the range of 400 to 900 °C due to the sintering and crystallite growth and decrease of surface areas and pore volume.41 In most cases, there was a medium calcination temperature (not too high and not too low) to make a balance between the surface areas and crystallinity in order to optimize the activity of photocatalysts. For example, Zaleska et al.42 found that the optimal preparation temperature 2327



Article

Figure 12. XRD pattern (a) and FT-IR spectra (b) of CN-600 °C and CN-240 min samples after multiple photocatalytic reactions.

volume, and enlarged band gap. The optimized CN-600 °C and CN-240 min nanosheets samples can be used multiplely without obvious deactivation, demonstrating their high stability under repeated light irradiation. The present work demonstrated that the nanostructures of conjugated carbon nitride derived from thiourea can be facilely engineered and improved by facile thermal treatment for efficient visible light photocatalytic air purification.

The stability of CN-600 °C and CN-240 min samples is further confirmed by XRD and FT-IR spectra after repeated reaction runs, as shown in Figure 12. It can be seen in Figure 12a that the crystal structure of CN-600 °C and CN-240 min samples were almost identical to the fresh samples (Figure 2), indicating their good phase stability. All the absorption bands for both samples are identical to the FT-IR spectra of fresh samples (Figure 4). The reaction intermediates and reaction products during photocatalytic oxidation of NO (such as NO2 and HNO2) cannot be observed. These results indicate that the reaction intermediates and products could diffuse rapidly owing to the beneficial porous nanostructures (Figures 6c and 6f).42 For the first time, we have discovered the unique effects of pyrolysis temperature and time on the texture property, band gap structure, and visible light photocatalytic activity of g-C3N4 derived from thiourea. By facile variation of the pyrolysis conditions, we can engineer the nanostructures of g-C3N4 and make them possess efficient and durable visible light photocatalytic performance for air pollutant purification. The asprepared g-C3N4 nanostructures can also be applied in other related areas such as solar energy conversion, photosynthesis, and catalyst support.

■

ASSOCIATED CONTENT

S Supporting Information *

The photo of the reactor system (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 23 62769785 605. Fax: +86 23 62769785 605. Email: [email protected]. Author Contributions

All authors have seen and given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

4. CONCLUSION Conjugated g-C3N4 nanostructures were synthesized by direct pyrolysis of thiourea in air. The formation of g-C3N4 from thiourea involved multiple processes under thermal treatment. The unique effects of pyrolysis temperature and time on the microstructure and photocatalytic activity of g-C3N4 were investigated and revealed. The crystallinity, morphology, surface area, pore structures, band gap structure, and photocatalytic activity of g-C3N4 was strongly dependent on the pyrolysis temperature and time. For the g-C3N4 samples obtained at elevated temperature and prolonged time, the layer thickness and size of g-C3N4 were reduced through a layer-bylayer coupled with layer-splitting process. When the pyrolysis temperature was increased from 500 to 600 °C and the pyrolysis time was prolonged from 0 to 240 min, the visible light photocatalytic activity of porous g-C3N4 nanosheets toward gaseous NO purification was significantly enhanced, exceeding that of C-doped TiO2 and BiOI. The activity enhancement of porous g-C3N4 nanostructures can be ascribed to the synergistic contributions of enhanced crystallinity, nanosheet-like morphology, large surface area, large pore

ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China (51108487), the Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA20018, cstc2013yykfB50008), the Science and Technology Project from Chongqing Education Commission (KJ130725), the Innovative Research Team Development Program in University of Chongqing (KJTD201314), and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (LZJ1204).

■

REFERENCES

(1) Chen, C. C.; Ma, W. H.; Zhao, J. C. Semiconductor-Mediated Photodegradation of Pollutants under Visible-Light Irradiation. Chem. Soc. Rev. 2010, 39, 4206. (2) Liu, G.; Wang, L.; Yang, H.; Cheng, H.; Lu, G. Q. Titania-Based Photocatalysts: Crystal Growth, Doping and Heterostructuring. J. Mater. Chem. 2010, 20, 831. (3) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Graphene-Based Semiconductor Photocatalysts. Chem. Soc. Rev. 2012, 41, 782.

2328



Article

(4) Rehman, S.; Ullah, R.; Butt, A. M.; Gohar, N. D. Strategies of Making TiO2 and ZnO Visible Light Active. J. Hazard. Mater. 2009, 170, 560. (5) Di Paola, A.; García-López, E.; Marcì, G.; Palmisano, L. A Survey of Photocatalytic Materials for Environmental Remediation. J. Hazard. Mater. 2012, 211−212, 3. (6) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. H. Nano-Photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229−251. (7) Liu, G.; Yu, J. C.; Lu, G. Q.; Cheng, H. M. Crystal Facet Engineering of Semiconductor Photocatalysts-Motivations, Advances and Unique Properties. Chem.Commun. 2011, 47, 6763. (8) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253. (9) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76. (10) Li, X. H.; Zhang, J. S.; Chen, X. F.; Fischer, A.; Thomas, A.; Antonietti, M.; Wang, X. C. Condensed Graphitic Carbon Nitride Nanorods by Nanoconfinement: Promotion of Crystallinity on Photocatalytic Conversion. Chem. Mater. 2011, 23, 4344. (11) Zhang, G.; Zhang, J.; Zhang, M.; Wang, X. C Polycondensation of Thiourea into Carbon Nitride Semiconductors As Visible Light Photocatalysts. J. Mater. Chem. 2012, 22, 8083. (12) Ge, L.; Han, C. Synthesis of MWNTs/g-C3N4 Composite Photocatalysts with Efficient Visible Light Photocatalytic Hydrogen Evolution Activity. Appl. Catal., B 2012, 117−118, 268. (13) Zhang, W. D.; Zhang, Q.; Dong, F.; Zhao, Z. W. The Multiple Effects of Precursors on the Physicochemical and Photocatalytic Properties of Polymeric Carbon Nitride. Int. J. Photoenergy 2013, 2013, 685038. (14) Wang, Y.; Wang, X.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogenous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem., Int. Ed. 2012, 51, 68−89. (15) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Graphitic Carbon Nitride Materials-Controllable Synthesis and Applications in Fuel Cells and Photocatalysis. Energy Environ. Sci. 2012, 5, 6717. (16) Dong, F.; Wang, Z. Y.; Sun, Y. J.; Ho, W. K.; Zhang, H. D. Engineering the Nanoarchitecture and Texture of Polymeric Carbon Nitride Semiconductor for Enhanced Visible Light Photocatalytic Activity. J. Colloid Interface Sci. 2013, 401, 70. (17) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Preparation and Enhanced Visible-Light Photocatalytic H2-Production activity of Graphene/C3N4 Composites. J. Phys. Chem. C 2011, 115, 7355. (18) Zhang, J.; Sun, J.; Maeda, K.; Domen, K.; Liu, P.; Antonietti, M.; Fu, X. Z.; Wang, X. C. Sulfur-Mediated Synthesis of Carbon Nitride: Band-Gap Engineering and Improved Functions for Photocatalysis. Energy Environ. Sci. 2011, 4, 675. (19) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397. (20) Zhang, J. S.; Chen, X. F.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem., Int. Ed. 2010, 49, 441. (21) Bojdys, M. J.; Mller, J.; Antonietti, M.; Thomas, A. Ionothermal Synthesis of Crystalline, Condensed, Graphitic Carbon Nitride. Chem.Eur. J. 2008, 14, 8177. (22) Dong, F.; Wu, L. W.; Sun, Y. J.; Fu, M.; Wu, Z. B.; Lee, S. C. Efficient Synthesis of Polymeric g-C3N4 Layered Materials as Novel Efficient Visible Light Driven Photocatalyst. J. Mater. Chem. 2011, 21, 15171. (23) Dong, F.; Sun, Y. J.; Wu, L. W.; Fu, M.; Wu, Z. B. Facile Transformation of Low Cost Thiourea into Nitrogen-Rich Graphitic Carbon Nitride Nanocatalyst with High Visible Light Photocatalytic Performance. Catal. Sci. Technol. 2012, 2, 1332.

(24) Long, B. H.; Ding, Z. X.; Wang, X. C. Carbon Nitride for the Selective Oxidation of Aromatic Alcohols in Water under Visible Light. ChemSusChem. 2013, 6, 2024. (25) Zhang, J. S.; Guo, F. S.; Wang, X. C. An Optimized and General Synthetic Strategy for Fabrication of Polymeric Carbon Nitride Nanoarchitectures. Adv. Funct. Mater. 2013, 23, 3008. (26) Chen, Y.; Zhang, J. S.; Zhang, M. W. Molecular and Textural Engineering of Conjugated Carbon Nitride Catalysts for Selective Oxidation of Alcohols with Visible Light. Chem. Sci. 2013, 4, 3244. (27) Lin, Z. Z.; Wang, X. C. Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis. Angew. Chem., Int. Ed. 2013, 52, 1735. (28) Zhang, J. S.; Zhang, M. W.; Sun, R. Q.; Wang, X. C. A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions. Angew. Chem., Int. Ed. 2012, 51, 10145. (29) Dong, F.; Zhao, Z. W; Xiong, T.; Ni, Z. L.; Zhang, W. D.; Sun, Y. J.; Ho, W. K. In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11392. (30) Zhou, W.; Sun, F.; Pan, K.; Tian, G.; Jiang, B.; Ren, Z.; Tian, C.; Fu, H. Well-Ordered Large-Pore Mesoporous Anatase TiO2 with Remarkably High Thermal Stability and Improved Crystallinity: Preparation, Characterization and Photocatalytic Performance. Adv. Funct. Mater. 2011, 21, 1922. (31) Yu, J. G.; Su, Y.; Cheng, B. Template-Free Fabrication and Enhanced Photocatalytic Activity of Hierarchical Macro/Mesoporous Titania. Adv. Funct. Mater. 2007, 17, 1984. (32) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data for Gas/Solid Systems. Pure Appl. Chem. 1985, 57, 603. (33) Lee, S.; Lintang, H. O.; Yuliati, L. A Urea Precursor to Synthesize Carbon Nitride with Mesoporosity for Enhanced Activity in the Photocatalytic Removal of Phenol. Chem. Asian J. 2012, 7, 2139. (34) Kwon, K.; Sa, Y.; Cheon, J.; Joo, S. Ordered Mesoporous Carbon Nitrides with Graphitic Frameworks as Metal-Free, Highly Durable, Methanol-Tolerant Oxygen Reduction Catalysts in an Acidic Medium. Langmuir 2012, 28, 991. (35) Jin, X.; Balasubramanian, V. V.; Selvan, S. T.; Sawant, D. P.; Chari, M. A.; Lu, G. Q.; Vinu, A. Highly Ordered Mesoporous Carbon Nitride Nanoparticles with High Nitrogen Content: a Metal-Free Basic Catalyst. Angew. Chem., Int. Ed. 2009, 48, 7884. (36) Shen, W.; Ren, L.; Zhou, H.; Zhang, S.; Fan, W. Facile One-Pot Synthesis of Bimodal Mesoporous Carbon Nitride and its Function as a Lipase Immobilization Support. J. Mater. Chem. 2011, 21, 3890. (37) Cui, Y.; Zhang, J.; Zhang, G.; Huang, J.; Liu, P.; Antonietti, M.; Wang, X. C. Synthesis of Bulk and Nanoporous Carbon Nitride Polymers from Ammonium Thiocyanate for Photocatalytic Hydrogen Evolution. J. Mater. Chem. 2011, 21, 13032. (38) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933. (39) Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Möhlmann, L.; Dołęga, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. C. Co-Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light. Angew. Chem., Int. Ed. 2012, 51, 3183. (40) Ai, Z. H.; Ho, W. K.; Lee, S. C.; Zhang, L. Z. Efficient Photocatalytic Removal of NO in Indoor Air with Hierarchical Bismuth Oxybromide Nanoplate Microspheres under Visible Light. Environ. Sci. Technol. 2009, 43, 4143. (41) Yu, J. G.; Yu, H. G.; Cheng, B.; Trapalis, C. Effects of Calcination Temperature on the Microstructures and Photocatalytic Activity of Titanate Nanotubes. J. Mol. Catal. A: Chem. 2006, 249, 135. (42) Zaleska, A.; Grabowska, E.; Sobczak, J. W.; Gazda, M.; Hupka, J. Photocatalytic Activity of Boron-Modified TiO2 under Visible Light: The Effect of Boron Content, Calcination Temperature and TiO2 Matrix. Appl. Catal., B 2009, 89, 469. 2329



Article

(43) Yu, J. G.; Zhang, L. J.; Cheng, B.; Su, Y. Y. Hydrothermal Preparation and Photocatalytic Activity of Hierarchically Sponge-like Macro-/Mesoporous Titania. J. Phys. Chem. C 2007, 111, 10582. (44) Dong, F.; Sun, Y. J.; Fu, M.; Ho, W. K.; Lee, S. C; Wu, Z. B. Novel in Situ N-Doped (BiO)2CO3 Hierarchical Microspheres SelfAssembled by Nanosheets as Efficient and Durable Visible Light Driven Photocatalyst. Langmuir 2012, 28, 766. (45) Dong, F.; Sun, Y. J.; Ho, W. K.; Wu, Z. B. Controlled Synthesis, Growth Mechanism and Highly Efficient Solar Photocatalysis of Nitrogen-Doped Bismuth Subcarbonate Hierarchical Nanosheets Architectures. Dalton Trans. 2012, 41, 8270. (46) Niu, P.; Zhang, L.; Liu, G.; Cheng, H. M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Adv. Funct. Mater. 2012, 22, 4763. (47) Dong, F.; Liu, H. T.; Ho, W. K.; Fu, M.; Wu, Z. B. (NH4)2CO3 Mediated Hydrothermal Synthesis of N-doped (BiO)2CO3 Hollow Nanoplates Microspheres as High-Performance and Durable Visible Light Photocatalyst for Air Cleaning. Chem. Eng. J. 2013, 214, 198. (48) Yan, H. J. Soft-Templating Synthesis of Mesoporous Graphitic Carbon Nitride with Enhanced Photocatalytic H2 Evolution under Visible Light. Chem. Commun. 2012, 48, 3430. (49) Li, Z.; Zhou, Y.; Xue, G.; Yu, T.; Liu, J.; Zou, Z. Fabrication of Hierarchically Assembled Microspheres Consisting of Nanoporous ZnO Nanosheets for High-Efficiency Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 14341. (50) Dong, F.; Sun, Y. J.; Fu, M.; Wu, Z. B.; Lee, S. C. Room Temperature Synthesis and Highly Enhanced Visible Light Photocatalytic Activity of Porous BiOI/BiOCl Composites Nanoplates Microflowers. J. Hazard. Mater. 2012, 219−220, 26.

2330


Efficient and Durable Visible Light Photocatalytic Performance of

Recommend Documents