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Enhancing the photocatalytic activity of bulk g-C3N4 by introducing mesoporous structure and hybridizing with graphene
Journal of Colloid and Interface Science 436 (2014) 29–36
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Enhancing the photocatalytic activity of bulk g-C3N4 by introducing mesoporous structure and hybridizing with graphene Yuhan Li a,b, Yanjuan Sun a, Fan Dong a,⇑, Wing-Kei Ho b,⇑ a b
Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, China Department of Science and Environmental Studies, The Centre for Education in Environmental Sustainability, The Hong Kong Institute of Education, Hong Kong
a r t i c l e
i n f o
Article history: Received 20 June 2014 Accepted 2 September 2014 Available online 16 September 2014 Keywords: Mesoporous g-C3N4/graphene Mesoporous g-C3N4/graphene oxide Visible light photocatalytic Activity enhancement NO removal
a b s t r a c t Bulk graphitic carbon nitride (CN) suffers from small surface area and high recombination of charge carriers, which result in low photocatalytic activity. To enhance the activity of g-C3N4, the surface area should be enlarged and charge carrier separation should be promoted. In this work, a combined strategy was employed to dramatically enhance the activity of bulk g-C3N4 by simultaneously introducing mesoporous structure and hybridizing with graphene/graphene oxide. The mesoporous g-C3N4/graphene (MCN-G) and mesoporous g-C3N4/graphene oxide (MCN-GO) nanocomposites with enhanced photocatalytic activity (NO removal ratio of 64.9% and 60.7%) were fabricated via a facile sonochemical method. The visible light-harvesting ability of MCN-G and MCN-GO hybrids was enhanced and the conduction band was negatively shifted when 1.0 wt% graphene/graphene oxide was incorporated into the matrix of MCN. As electronic conductive channels, the G/GO sheets could efficiently facilitate the separation of chare carriers. MCN-G and MCN-GO exhibited drastically enhanced visible light photocatalytic activity toward NO removal. The NO removal ratio increased from 16.8% for CN to 64.9% for MCN-G and 60.7% for MCN-GO. This enhanced photocatalytic activity could be attributed to the increased surface area and pore volume, improved visible light utilization, enhanced reduction power of electrons, and promoted separation of charge carriers. This work demonstrates that a combined strategy is extremely effective for the development of active photocatalysts in environmental and energetic applications. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Visible light-driven semiconductor photocatalytic technology has been the focus of considerable worldwide attention during the past decades because of its great potential in solving current environmental pollution and energy problems [1–5]. To date, the majority of research on photocatalysts is focused on photocatalysts containing metals such as metal oxide, metal sulfide, metal halides, tungstates, niobates, tantalates, and vandates [6–8]. However, the development of efficient, sustainable, and environmental-friendly photocatalysts remains a significant challenge. Recently, Wang et al. reported that a new kind of conjugated polymer semiconductor, graphitic carbon nitride (g-C3N4), can be used as an attractive metal-free organic photocatalyst that can work in visible light [9]. g-C3N4 possesses a high thermal and chemical stability as well as appealing electronic and optical properties. As a multifunctional catalyst, g-C3N4 has been applied in photosynthesis, energy conversion and storage, contaminants degradation, ⇑ Corresponding authors. Fax: +86 23 62769785 605. E-mail addresses: [email protected] (F. Dong), [email protected] (W.-K. Ho). http://dx.doi.org/10.1016/j.jcis.2014.09.004 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
carbon dioxide storage and reduction, solar cells, and sensing [10,11]. Nevertheless, the photocatalytic efficiency of bulk g-C3N4 is limited because of its low surface area and the fast recombination rate of photogenerated electron–hole pairs. To resolve these problems, numerous strategies have been employed to modify the bulk g-C3N4, such as texture tuning by templates, band gap modification by heteroatoms doping, post-functionalization, and semiconductor coupling [12–18]. The use of carbonaceous materials, such as fullerenes (C60) [19], carbon nanotubes (CNTs) [20], graphene (G) [21], and graphene oxide (GO) [22], for the enhancements of conductivity and photocatalytic performances of semiconductors has attracted wide research interest because of their special structures and unique electronic properties. Several research groups have developed novel heterojunction photocatalysts by combining g-C3N4 with carbonaceous materials. In particular, great interest is focused on combining g-C3N4 with graphene-based materials to improve its conductivity and catalytic performance. Graphene, a single twodimensional p-conjugation nanosheet of sp2 hybridized carbon, is a zero band gap semiconductor with outstanding mechanical, thermal, and optical properties, massless fermions, ballistic electronic
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transport, and ultrahigh electron mobility [23,24]. Graphene oxide is the oxidative mode of graphene, which possesses two-dimensional p-conjugation structure and superior electric property. Graphene oxide was also employed as an effective electron acceptor, which could efficiently facilitate charge separation [22]. Graphene and graphene oxide have been coupled with g-C3N4. Xiang et al. reported that graphene/C3N4 composite materials can be synthesized with enhanced visible light photocatalytic H2 production activity via a combined impregnation-chemical reduction strategy [21]. Liao et al. fabricated graphene oxide-modified g-C3N4 with efficient photocatalytic capability under visible light irradiation via a sonochemical approach [22]. These studies showed that graphene/graphene oxide could effectively improve the photocatalytic performance of g-C3N4 photocatalysts. Previously, mesoporous C3N4 has been fabricated from the pyrolysis of thiourea by introducing mesoporous structure in gC3N4 via SiO2 template [18]. Mesoporous C3N4 possesses controllable morphology, tunable pore diameter, higher surface areas, and larger number of active sites on the surface compared with the bulk g-C3N4. However, the separation of photogenerated electron–hole pairs and the efficiency of charge immigration need further improvement to significantly promote the photocatalytic activity. In this work, to address the drawbacks of bulk g-C3N4 with low surface area and high charge carrier recombination rate, we need to develop a combined strategy which is based on the simultaneous creation of mesoporous structure and hybridization with graphene or graphene oxide. We prepared mesoporous g-C3N4/ graphene (MCN-G) and mesoporous g-C3N4/graphene oxide (MCN-GO) nanocomposites via a facile sonochemical method. The as-prepared MCN-G and MCN-GO were applied to remove NO at 600 ppb level in air under visible-light irradiation. These nanocomposites photocatalysts exhibited highly enhanced performance, which can be ascribed to the increased surface areas, extended visible light absorption range, adjusted band structure, and enhanced separation of charge carriers. 2. Experimental section 2.1. Synthesis of mesoporous carbon nitride All reagents used in this research were of analytical grade and utilized without further purification. The typical procedure for the synthesis of mesoporous carbon nitride followed the hardtemplating method, in which 10 g of dicyandiamide was dispersed into 150 mL of distilled water. About 5 g of commercial SiO2 with a high surface area (380 m2/g, see Fig. S1) was then gradually added into the dicyandiamide solution with vigorous stirring for 12 h. The aqueous mixture was dried in an oven at 120 °C for 12 h. The obtained white powder was put into a 50 mL alumina crucible with a cover and heated to 550 °C for 2 h with a heating rate of 15 °C min 1. The released air products during thermal treatment were absorbed by dilute NaOH solution of 0.05 M. After cooling to room temperature, the obtained bright yellow powders were treated in 100 mL of 4 M of ammonium hydrogen difluoride (NH4HF2) for 24 h to remove the SiO2 template. The powders were then subjected to centrifugation and washed with distilled water and ethanol three times. Finally, the product was dried at 80 °C in an oven overnight. The resulting mesoporous sample was denoted as MCN. The bulk C3N4 (for simplicity: CN) was also prepared by heating dicyandiamide in a muffle furnace at the same thermal condition without the addition of the SiO2 template. 2.2. Synthesis of MCN-G and MCN-GO composite photocatalysts Graphene (G) and graphene oxide (GO) were purchased from Nanjing Xianfeng Chemical Factory. The MCN-G and MCN-GO
composite photocatalysts were prepared via a facile sonochemical method. A specific amount of G/GO was added into 50 mL of ethanol followed by consecutive sonication for 60 min at 40 °C. Subsequently, 0.4 g of MCN was added into the G/GO suspension and stirred for 60 min. After which, the dispersion was dried in an oven at 150 °C for 4 h. The weight ratio of G/GO to MCN was controlled at 1.0 wt%. The resultant products were labeled as MCN-G and MCN-GO. 2.3. Characterization The crystal structures of the samples were investigated using an X-ray diffractometer (XRD: model D/max RA, Japan). FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets (FT-IR: Nicolet, USA). The morphology and structure of the samples were characterized via transmission electron microscopy (TEM: JEM-2010, Japan) and scanning electron microscopy (SEM: JSM-6490, Japan). The optical absorption properties for the samples were obtained using a scan UV–vis spectrophotometer (UV–vis DRS: Shimadzu UV-2450, Japan) equipped with an integrating sphere assembly, and 100% BaSO4 was used as the reflectance sample. X-ray photoelectron spectroscopy with Al Ka X-rays (hm = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties and probe the total density of the state (DOS) distribution in the valence band. The shift of the binding energy caused by relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. Nitrogen adsorption–desorption isotherms were obtained using a nitrogen adsorption apparatus (BET-BJH: Micromeritics ASAP 2020, USA) with all samples degassed at 150 °C prior to measurements. The photoluminescence spectra (PL) for the samples were obtained using a fluorescence spectrophotometer (PL: FS-2500, Japan) with an Xe lamp with optical filter as the excitation source. Electrochemical impedance spectroscopy (EIS) was conducted using a CHI 660 B electrochemical system (CHI: Chenhua CHI660, China) with a standard three-electrode cell. The working electrode was prepared according to the following process. First, 20 mg of the sample was suspended in 0.5 mL of DMF, which was then dip-coated on a 10 mm 20 mm indium-tin oxide (ITO) glass electrode. The electrode was then annealed at 350 °C for 1 h at a heating rate 6 °C min 1. EIS was carried out at the open-circuit potential. A sinusoidal ac perturbation of 5 mV was applied to the electrode over the frequency range of 0.05–1 105 Hz. 2.4. Visible light photocatalytic activity The photocatalytic activities for the obtained products were evaluated by the oxidation of NO at ppb levels in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, which was 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 above the reactor. A UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. 0.1 g of the as-prepared photocatalysts was added into 30 mL of H2O and sonicated for 10 min, and then the resultant suspension was coated onto two dishes with a diameter of 12.0 cm, respectively. The coated dishes were then pretreated at 70 °C to remove water in the suspension. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance). 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 using a mass flow controller. After achieving
Y. Li et al. / Journal of Colloid and Interface Science 436 (2014) 29–36
the adsorption–desorption equilibrium, the lamp was turned on. The concentration of NO was continuously measured using a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., 42i-TL, USA), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 1.0 L/min. The removal ratio (g) of NO was calculated as g(%) = (1 C/ C0) 100%, where C and C0 are NO concentrations in the outlet steam and the feeding stream, respectively. 3. Results and discussion 3.1. Scheme for the formation of MCN-G and MCN-GO nanocomposites Fig. 1 is a schematic diagram illustrating the fabrication process, which shows that homogeneous dispersions of the components can be achieved when powders of MCN and G or GO in ethanol are subjected to sonication. After mixing and stirring the aqueous dispersions, the MCN particles are introduced onto the platform of dispersed G or GO sheets. The MCN functions as a ‘‘spacer’’ to increase the distance between the G or GO sheets, thereby making both the faces of G or GO accessible [25]. Consequently, the MCN-G and MCN-GO nanocomposites are formed, and more surfaces are exposed to come into contact with target pollutants. 3.2. Structure and morphology Fig. 2 shows the XRD patterns of the as-prepared samples. All the samples feature two distinct diffraction peaks. The peak observed at around 13.0°, which corresponds to a distance of d = 0.682 nm, is indexed as (1 0 0) and represents an in-plane structural packing motif [9]. The strongest XRD peak at around 27.5°, which corresponds to an interlayer stacking of aromatic segments with a distance of d = 0.324 nm, is indexed as (0 0 2) peak of graphitic materials [9]. Further observation on the enlarged view of (0 0 2) peak (Fig. 2b) for the MCN, MCN-G, and MCN-GO samples reveals that the peaks are weaker and broader than those of CN. This observation should be attributed to the conversion of the original graphitic-like structure to the new mesoporous structure [26], which generates smaller particle size because of the templating effects of SiO2. Notably, no diffraction peaks of G in the MCN-G and GO in the MCN-GO composites are observed, which can be attributed to the small amount of and very few G or GO layers in the composites as well the high dispersion on the hybrid surfaces. Moreover, the XRD patterns of MCN-G and MCN-GO do not differ from those of MCN and CN. Therefore, the modification with G or GO do not disturb the lattice structure of mesoporous g-C3N4, which is advantageous for the photocatalytic properties of the as-prepared nanocomposites [22]. SEM measurements were conducted to characterize the morphologies and structures of G, GO, MCN-G, and MCN-GO. As shown in Fig. 3a and c, graphene has a two-dimensional nanosheet structure with many thin wrinkles on the surface. The graphene oxide
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sheet surface also shows a few thin wrinkles. Previous studies have reported that these wrinkles can induce the formation of nanosized channels or pores on the surface of graphene materials [22]. After the addition of MCN onto the surface of G/GO, a large amount of MCN nanoparticles are uniformly dispersed on the G and GO sheet surfaces, as shown in Fig. 3b and d, respectively. This phenomenon indicates that MCN nanoparticles and G/GO sheets have high dispersion after sonication, which is beneficial for obtaining more contact areas between MCN nanoparticles and G/GO sheets. Moreover, the MCN-G and MCN-GO composites have a wrinkled structure because of the flexible G/GO template, which suggests that graphene materials are not incorporated into the lattice of MCN. This result is in agreement with the XRD patterns. TEM was performed to investigate the local morphological structures of the as-synthesized samples. As shown in Fig. 4a, larger aggregated particles are observed in the CN. After treatment with the high surface area SiO2 template (see Fig. S1), the mesoporous structure and smaller particles than CN can be observed in the MCN sample (Fig. 4b). After modification with G/GO, the TEM images (Fig. 4c and e) show a clear distinction between MCN layers and G/GO nanosheets with wrinkles. The MCN nanoparticles are uniformly dispersed on the graphene material sheets. The assembly of MCN and G/GO can be ascribed to the p–p stacking and hydrogen bonding interactions between them [27]. We further observed the TEM images of the MCN-G and MCN-GO hybrids (Fig. 4d and f). Considering the interfacial interactions and preferential heterogeneous nucleation, the MCN nanoparticles are densely attached to the G and GO sheets [28]. Although the content of graphene materials in the composites is only 1.0 wt%, their crumpled edges are still observed. This result is consistent with the SEM analysis. The N2 adsorption–desorption isotherms and Barrett–Joyner– Halenda pore size distributions of the CN, MCN, MCN-G, and MCN-GO samples are shown in Fig. 5. As shown in Fig. 5a, the adsorption–desorption isotherms of all the samples are characteristic of Type IV Brunauer–Deming–Deming–Teller classification, which indicates their porous property [29]. This result is in agreement with the SEM and TEM results (Figs. 3 and 4). Additionally, for the MCN, MCN-G, and MCN-GO samples, the adsorption branch of the nitrogen isotherms shows an increase at P/P0 approaching unity, which indicates that large mesopores and small macropores are formed. As shown in Fig. 5b, the pore size distributions of the MCN, MCN-G, and MCN-GO samples are broad with existing mesopores and macropores. The surface areas and pore volumes are summarized in Table 1. The surface areas and pore volumes of the MCN (104 m2/g and 0.43 cm3/g, respectively), MCN-G (103 m2/g and 0.57 cm3/g, respectively), and MCN-GO (104 m2/g and 0.59 cm3/g, respectively) samples are much higher than those of CN (30 m2/g and 0.19 cm3/g, respectively). Although the surface areas of MCN, MCN-G, and MCN-GO are much higher than that of CN, the surface areas of MCN-G and MCN-GO do not change compared with that of MCN because the quantity of G/GO in the
Fig. 1. Schematic diagram to illustrate the fabrication process of MCN-G and MCN-GO.
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Fig. 2. XRD patterns of CN, MCN, MCN-G, and MCN-GO samples (a), enlarged view of (0 0 2) diffraction region (b).
(a)
(b)
(c)
(d)
Fig. 3. SEM images of G (a), MCN-G (b), GO (c), and MCN-GO (d).
composites is as low as 1 wt%. In comparison with MCN, a significant increase in pore volumes of the MCN-G and MCN-GO samples can be observed, which can be ascribed to the fact that treatment with sonication and stirring induces the particles to redisperse, thereby increasing the pore volumes. We expect that the open porous structure and high surface area are beneficial for reactant adsorption and mass transfer. Moreover, mesopores can allow light to reflect and penetrate deep into the pores of the photocatalyst and lead to high mobility of charges, which result in enhanced photocatalytic activity [30,31]. 3.3. Chemical composition The FT-IR spectra of the as-prepared samples are shown in Fig. S2. For CN, MCN, MCN-G, and MCN-GO, the broad peak at 3000 cm 1–3700 cm 1 is assigned to the residual N–H component from uncondensed amine groups and surface-adsorbed H2O molecules [32]. Several strong bands in the range of 1200 cm 1 to 1700 cm 1 dominate the spectra, and the peaks at approximately 1251, 1334, 1415, 1562, and 1641 cm 1 correspond to the characteristic stretching mode of C–N heterocycles [32]. Additionally, the
typical breathing mode of triazine units at 810 cm 1 was observed [32]. The typical bands of G/GO cannot be observed in the MCN-G and MCN-GO samples because the content of G/GO is low. To further investigate the surface chemical compositions and VB states of the as-synthesized samples, XPS measurements were conducted. As shown in Fig. S3a, signals of N, C, and O elements are detected and no peaks for other elements can be observed. Higher-resolution spectra were further taken on the N1s, C1s, and O1s regions. Fig. S3b shows the high-resolution N1s spectra of the MCN-G and MCN-GO samples in comparison with that of CN. As shown in the figure, the N1s spectra can be fitted into three peaks. The main peak centered at 398.8 eV can be assigned to the sp2 hybridized nitrogen (C–N–C) in triazine rings. The two peaks at approximately 400.5 and 404.6 eV can be ascribed to the bridging nitrogen atoms (C)3–N and the p excitations, respectively [33]. The C1s spectra shown in Fig. S3c can be fitted into three peaks. The peak at 284.8 eV can be assigned to adventitious carbon species obtained via XPS. The other two peaks located at 286.2 and 288.4 eV originate from (C)3–N and C–N–C, respectively [33]. Table 2 shows that the C/N atomic ratio increases from 0.65 for
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(a)
(b)
MCN
(c)
(d)
G
(f)
(e) GO
MCN
Fig. 4. TEM images of CN (a), MCN (b), MCN-G (c, d), and MCN-GO (e, f).
Fig. 5. N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of CN, MCN, MCN-G, and MCN-GO. Inset in (b) shows the enlarged view of corresponding pore size distribution curve of CN.
Table 1 SBET, total pore volume, peak pore size, and NO removal ratio for CN, MCN, MCN-G and MCN-GO. Sample name
SBET (m2/g)
Total pore volume (cm3/g)
Peak pore size (nm)
NO removal ratio g (%)
CN MCN MCN-G MCN-GO
30 104 103 104
0.19 0.43 0.57 0.59
3.8/32.3 9.1/50.9 9.0/30.6 8.9/45.8
16.8 50.3 64.9 60.7
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the CN sample to 0.83 for the MCN-G sample and 0.86 for the MCNGO sample could be ascribed to the incorporation of G/GO into the nanocomposites or carbon contamination in the testing process. The O1s spectra shown in Fig. S3d can be fitted into three peaks at binding energies of 531.5, 532.6, and 533.6 eV. The major peak at 532.6 eV is assigned to C–O–N, which originates from the partial oxidation of CN during the pyrolysis of dicyandiamide in air. The other two peaks at 531.5 and 533.6 eV can be attributed to surface –OH groups and adsorbed H2O [33]. No signals of O1s belong to GO were detected because of the small amount of GO (1.0 wt%) in MCN-GO. Fig. S3e shows the DOS of the VB. The VB maximum of CN, MCN-G, and MCN-GO are 1.73, 1.47, and 1.32 eV, respectively. These results indicate that G or GO has a significant effect on band structure. 3.4. Optical properties To understand the optical properties of the as-prepared samples, UV–Vis DRS was conducted to study the effect of G/GO on the optical properties of MCN. As shown in Fig. 6a, all the samples have an absorption edge located in the visible light region. The MCN-G and MCN-GO samples show a broad background absorption in the visible light region because of the existence of G/GO. The band gap values (Eg) of all the samples based on the UV–Vis DRS data are calculated, as shown in Fig. 6b. The band gaps of CN, MCN, MCN-G, and MCN-GO are 2.55, 2.53, 2.44, and 2.40 eV, respectively. The band edges show evident redshift, which indicates that the visible light-responsive optical property of MCN can be successfully improved by modifying the G/GO sheets. Based on the band gap of the samples, the CB position can be determined, as shown in Table 2. These results indicate that an appropriate G/GO can significantly adjust the band structure of the nanocomposites and, thus, the photocatalytic efficiency. Table 2 shows the negative shift in VB and CB potential of MCN-G and MCN-GO. The potential level of OH/H2O and O2/O2 are 2.38 and 0.33 eV, respectively [34]. The photogenerated holes cannot oxidize H2O or –OH because the VB potential level of the as-prepared samples is more negative than that of OH/H2O. However, the VB potential level of the as-synthesized samples induces the reaction of the photogenerated electrons with O2 to produce O2 radicals. Evidently, the O2 radicals are the dominant active species responsible for NO oxidation. When the CB level of MCN-G and MCN-GO shifts to a more negative position, the photogenerated electrons have a higher reduction power in comparison with other samples, which is favorable for the production of O2 radicals. Fig. 7 shows the room temperature PL spectra of the CN, MCN, MCN-G, and MCN-GO samples. The PL spectra have been extensively used to determine the efficiency of charge carrier trapping, migration, and separation and to investigate the fate of photogenerated electron–hole pairs. In general, a lower PL intensity indicates a decrease in the recombination rate of photogenerated charge carriers. As shown in Fig. 7, the main emission peaks centered at approximately 470 nm for all the samples originate from the band-band transitions. The PL intensity of MCN decreased compared with that of CN. The results indicate that the recombination of electron–hole pairs is moderately inhibited (or the charge separation is significantly accelerated) on MCN possibly because of the
reduced density of charge carrier traps for electron–hole recombination in mesoporous samples [35]. In comparison with the intensity of the PL signal for the MCN samples, those for the MCN-G and MCN-GO composites are much lower. This finding implies that the nanocomposites have lower recombination rates of electrons and holes under visible light irradiation, which can be assigned to the transportation of photogenerated electrons from MCN to G/GO sheets, thereby preventing the direct recombination of electron– hole pairs. G and GO are proven to be one of the most effective electronic conductive channels to facilitate charge carrier separation [36]. EIS Nyquist plots were further used to characterize charge carrier migration. Fig. 8 shows the EIS Nyquist plots of CN, MCN, MCNG, and MCN-GO. A smaller arc radius implies a higher efficiency of charge transfer. Evidently, the diameters of the arc radius of the MCN-G and MCN-GO composites are smaller than those of the MCN and CN composites, which suggests that MCN-G and MCNGO show significantly enhanced interfacial charge transfer and effective separation of photogenerated electron–hole pairs when adding the G/GO nanosheets to MCN. This result is in accordance with the PL analysis. 3.5. Visible light photocatalytic activity The as-prepared samples were applied in the photocatalytic removal of gaseous NO under visible light irradiation in a continuous reactor to evaluate their potential ability for air purification. Fig. 9 shows the variation of NO concentration (C/C0, %) with irradiation time over the as-synthesized samples. Here, C0 is the initial concentration of NO and C is the concentration of NO after photocatalytic reaction at time t. A previous investigation indicated that NO could not be photolyzed under light irradiation without a photocatalyst and that the concentration of NO could not decrease in the presence of a photocatalyst without irradiation [37]. In addition, G and GO have a band gap of approximately zero [38]. As such, the direct catalytic influence of G and GO can be excluded. The bulk CN has a suitable band gap of approximately 2.5 eV and can be directly excited by visible light. Fig. 9 shows that the NO removal ratio of CN is only 16.8% after 30 min of irradiation (Table 1). Recently, Taizo Sano et al. developed an activation strategy to enhance the photocatalytic activity of g-C3N4 in gas phase NO oxidation via alkaline hydrothermal treatment [39]. In the present work, we developed another approach to improve the NO removal ratio of g-C3N4 by simultaneously adding the mesoporous structure and a small amount of G/GO. As shown in Fig. 9, the NO removal ratio of MCN (50.3%) exceeds that of CN because of the increase in surface area and pore volume. Importantly, the nanocomposite samples show a significant improvement of visible light photocatalytic activity over the mesoporous samples. The NO removal ratios of MCN-G and MCN-GO reach 64.9% and 60.7% after 30 min of irradiation, which is more efficient than other types of visible light photocatalysts, such as C-doped TiO2, N-doped TiO2, Bi2WO6, BiOBr, and N-doped (BiO)2CO3 [40– 43]. The significantly enhanced photocatalytic activities of the MCN sample compared with those of the CN sample can be ascribed to the large surface area and pore volume, enhanced light-harvesting ability, adjusted band structure, and suppressed
Table 2 Atomic ratio of C/N, O percentage, band gap, VB position, and CB position of CN, MCN, MCN-G, and MCN-GO (The data of MCN was cited from Ref. [18]). Sample
Ratio of C/N
O (at.%)
Band gap (eV)
VB position (eV)
CN MCN MCN-G MCN-GO
0.65 0.67 0.83 0.86
0.68 0.94 1.93 3.12
2.55 2.60 2.44 2.40
1.73 1.75 1.47 1.32
CB position (eV) 0.82 0.85 0.97 1.08
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Fig. 6. UV–Vis DRS (a) and plots of (ahm)1/2 vs. photon energy (b) of the CN, MCN, MCN-G, and MCN-GO samples.
Fig. 7. Room temperature PL spectra of CN, MCN, MCN-G, and MCN-GO samples.
Fig. 8. Nyquist plots for CN, MCN, MCN-G, and MCN-GO electrodes under visible light irradiation (k > 420 nm, [Na2SO4] = 0.5 M).
recombination of electron–hole pairs. The synergistic effects of these favorable factors are directly responsible for the highly enhanced visible light photocatalytic activities of MCN-G and MCN-GO. The photocatalytic activity of bulk g-C3N4 is low because of the low surface area and the fast recombination of charge carriers. These drawbacks can be successfully solved via a combined
Fig. 9. Photocatalytic removal of NO in a single-pass flow of air over the CN, MCN, MCN-G, and MCN-GO samples under visible light irradiation (continuous reactor, NO concentration = 600 ppb).
strategy by generating a mesoporous structure and hybridizing with G/GO. First, the enlarged surface area and improved porous structure by adding mesopores are favorable for pollutant adsorption and mass transfer [44,45]. The increased pore volume could provide more surface active sites for photocatalytic reaction and quick reactant diffusion [45,46]. Second, the reduced band gap caused by the addition of graphene materials improves the visible light-harvesting ability. Third, the negative shift in CB potential results in a high reduction power of photogenerated electrons, which is beneficial for the production of O2 radicals. Finally, the increased charge transport rate and enhanced charge separation are due to the excellent electron mobility of graphene (Fig. 10). All these favorable factors contributed to the significantly enhanced visible light photocatalytic activity. As the synthesis of MCN-G and MCN-GO nanocomposites mainly involves the physical processes of sonication and stirring, the scale up of these physical processes may have little effect on the photocatalytic efficiency of such nanocomposites. The present synthesis method could be feasible for potential large-scale applications. 4. Conclusion In summary, the mesoporous g-C3N4/graphene and mesoporous g-C3N4/graphene oxide nanocomposite photocatalysts were
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Fig. 10. Schematic drawings of the photocatalytic induction process in the MCN-G (a) and MCN-GO (b) hybrids under visible light irradiation.
prepared via a facile sonochemical method. After the simultaneous addition of mesoporous structure and G/GO sheets in bulk g-C3N4, the visible light photocatalytic activity of MCN-G and MCN-GO toward gaseous NO removal was significantly enhanced. The enhanced activity of MCN-G and MCN-GO can be ascribed to the enlarged surface area and pore volume, increased light-harvesting ability, enhanced reduction power of photogenerated electrons, and improved charge carrier separation. This work shows that a combined strategy is effective for the modification of photocatalysts for large-scale applications. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (51478070, 51108487), the Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA20018), the Science and Technology Project from Chongqing Education Commission (KJ1400617, KJ130725), and the Innovative Research Team Development Program in University of Chongqing (KJTD201314). The research was also supported by a research Grant of Early Career Scheme (ECS 809813) from the Research Grant Council. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.09.004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Enhancing the photocatalytic activity of bulk g-C3N4 by introducing mesoporous structure and hybridizing with graphene Yuhan Li a,b, Yanjuan Sun a, Fan Dong a,⇑, Wing-Kei Ho b,⇑ a b
Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, China Department of Science and Environmental Studies, The Centre for Education in Environmental Sustainability, The Hong Kong Institute of Education, Hong Kong
a r t i c l e
i n f o
Article history: Received 20 June 2014 Accepted 2 September 2014 Available online 16 September 2014 Keywords: Mesoporous g-C3N4/graphene Mesoporous g-C3N4/graphene oxide Visible light photocatalytic Activity enhancement NO removal
a b s t r a c t Bulk graphitic carbon nitride (CN) suffers from small surface area and high recombination of charge carriers, which result in low photocatalytic activity. To enhance the activity of g-C3N4, the surface area should be enlarged and charge carrier separation should be promoted. In this work, a combined strategy was employed to dramatically enhance the activity of bulk g-C3N4 by simultaneously introducing mesoporous structure and hybridizing with graphene/graphene oxide. The mesoporous g-C3N4/graphene (MCN-G) and mesoporous g-C3N4/graphene oxide (MCN-GO) nanocomposites with enhanced photocatalytic activity (NO removal ratio of 64.9% and 60.7%) were fabricated via a facile sonochemical method. The visible light-harvesting ability of MCN-G and MCN-GO hybrids was enhanced and the conduction band was negatively shifted when 1.0 wt% graphene/graphene oxide was incorporated into the matrix of MCN. As electronic conductive channels, the G/GO sheets could efficiently facilitate the separation of chare carriers. MCN-G and MCN-GO exhibited drastically enhanced visible light photocatalytic activity toward NO removal. The NO removal ratio increased from 16.8% for CN to 64.9% for MCN-G and 60.7% for MCN-GO. This enhanced photocatalytic activity could be attributed to the increased surface area and pore volume, improved visible light utilization, enhanced reduction power of electrons, and promoted separation of charge carriers. This work demonstrates that a combined strategy is extremely effective for the development of active photocatalysts in environmental and energetic applications. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Visible light-driven semiconductor photocatalytic technology has been the focus of considerable worldwide attention during the past decades because of its great potential in solving current environmental pollution and energy problems [1–5]. To date, the majority of research on photocatalysts is focused on photocatalysts containing metals such as metal oxide, metal sulfide, metal halides, tungstates, niobates, tantalates, and vandates [6–8]. However, the development of efficient, sustainable, and environmental-friendly photocatalysts remains a significant challenge. Recently, Wang et al. reported that a new kind of conjugated polymer semiconductor, graphitic carbon nitride (g-C3N4), can be used as an attractive metal-free organic photocatalyst that can work in visible light [9]. g-C3N4 possesses a high thermal and chemical stability as well as appealing electronic and optical properties. As a multifunctional catalyst, g-C3N4 has been applied in photosynthesis, energy conversion and storage, contaminants degradation, ⇑ Corresponding authors. Fax: +86 23 62769785 605. E-mail addresses: [email protected] (F. Dong), [email protected] (W.-K. Ho). http://dx.doi.org/10.1016/j.jcis.2014.09.004 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
carbon dioxide storage and reduction, solar cells, and sensing [10,11]. Nevertheless, the photocatalytic efficiency of bulk g-C3N4 is limited because of its low surface area and the fast recombination rate of photogenerated electron–hole pairs. To resolve these problems, numerous strategies have been employed to modify the bulk g-C3N4, such as texture tuning by templates, band gap modification by heteroatoms doping, post-functionalization, and semiconductor coupling [12–18]. The use of carbonaceous materials, such as fullerenes (C60) [19], carbon nanotubes (CNTs) [20], graphene (G) [21], and graphene oxide (GO) [22], for the enhancements of conductivity and photocatalytic performances of semiconductors has attracted wide research interest because of their special structures and unique electronic properties. Several research groups have developed novel heterojunction photocatalysts by combining g-C3N4 with carbonaceous materials. In particular, great interest is focused on combining g-C3N4 with graphene-based materials to improve its conductivity and catalytic performance. Graphene, a single twodimensional p-conjugation nanosheet of sp2 hybridized carbon, is a zero band gap semiconductor with outstanding mechanical, thermal, and optical properties, massless fermions, ballistic electronic
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transport, and ultrahigh electron mobility [23,24]. Graphene oxide is the oxidative mode of graphene, which possesses two-dimensional p-conjugation structure and superior electric property. Graphene oxide was also employed as an effective electron acceptor, which could efficiently facilitate charge separation [22]. Graphene and graphene oxide have been coupled with g-C3N4. Xiang et al. reported that graphene/C3N4 composite materials can be synthesized with enhanced visible light photocatalytic H2 production activity via a combined impregnation-chemical reduction strategy [21]. Liao et al. fabricated graphene oxide-modified g-C3N4 with efficient photocatalytic capability under visible light irradiation via a sonochemical approach [22]. These studies showed that graphene/graphene oxide could effectively improve the photocatalytic performance of g-C3N4 photocatalysts. Previously, mesoporous C3N4 has been fabricated from the pyrolysis of thiourea by introducing mesoporous structure in gC3N4 via SiO2 template [18]. Mesoporous C3N4 possesses controllable morphology, tunable pore diameter, higher surface areas, and larger number of active sites on the surface compared with the bulk g-C3N4. However, the separation of photogenerated electron–hole pairs and the efficiency of charge immigration need further improvement to significantly promote the photocatalytic activity. In this work, to address the drawbacks of bulk g-C3N4 with low surface area and high charge carrier recombination rate, we need to develop a combined strategy which is based on the simultaneous creation of mesoporous structure and hybridization with graphene or graphene oxide. We prepared mesoporous g-C3N4/ graphene (MCN-G) and mesoporous g-C3N4/graphene oxide (MCN-GO) nanocomposites via a facile sonochemical method. The as-prepared MCN-G and MCN-GO were applied to remove NO at 600 ppb level in air under visible-light irradiation. These nanocomposites photocatalysts exhibited highly enhanced performance, which can be ascribed to the increased surface areas, extended visible light absorption range, adjusted band structure, and enhanced separation of charge carriers. 2. Experimental section 2.1. Synthesis of mesoporous carbon nitride All reagents used in this research were of analytical grade and utilized without further purification. The typical procedure for the synthesis of mesoporous carbon nitride followed the hardtemplating method, in which 10 g of dicyandiamide was dispersed into 150 mL of distilled water. About 5 g of commercial SiO2 with a high surface area (380 m2/g, see Fig. S1) was then gradually added into the dicyandiamide solution with vigorous stirring for 12 h. The aqueous mixture was dried in an oven at 120 °C for 12 h. The obtained white powder was put into a 50 mL alumina crucible with a cover and heated to 550 °C for 2 h with a heating rate of 15 °C min 1. The released air products during thermal treatment were absorbed by dilute NaOH solution of 0.05 M. After cooling to room temperature, the obtained bright yellow powders were treated in 100 mL of 4 M of ammonium hydrogen difluoride (NH4HF2) for 24 h to remove the SiO2 template. The powders were then subjected to centrifugation and washed with distilled water and ethanol three times. Finally, the product was dried at 80 °C in an oven overnight. The resulting mesoporous sample was denoted as MCN. The bulk C3N4 (for simplicity: CN) was also prepared by heating dicyandiamide in a muffle furnace at the same thermal condition without the addition of the SiO2 template. 2.2. Synthesis of MCN-G and MCN-GO composite photocatalysts Graphene (G) and graphene oxide (GO) were purchased from Nanjing Xianfeng Chemical Factory. The MCN-G and MCN-GO
composite photocatalysts were prepared via a facile sonochemical method. A specific amount of G/GO was added into 50 mL of ethanol followed by consecutive sonication for 60 min at 40 °C. Subsequently, 0.4 g of MCN was added into the G/GO suspension and stirred for 60 min. After which, the dispersion was dried in an oven at 150 °C for 4 h. The weight ratio of G/GO to MCN was controlled at 1.0 wt%. The resultant products were labeled as MCN-G and MCN-GO. 2.3. Characterization The crystal structures of the samples were investigated using an X-ray diffractometer (XRD: model D/max RA, Japan). FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets (FT-IR: Nicolet, USA). The morphology and structure of the samples were characterized via transmission electron microscopy (TEM: JEM-2010, Japan) and scanning electron microscopy (SEM: JSM-6490, Japan). The optical absorption properties for the samples were obtained using a scan UV–vis spectrophotometer (UV–vis DRS: Shimadzu UV-2450, Japan) equipped with an integrating sphere assembly, and 100% BaSO4 was used as the reflectance sample. X-ray photoelectron spectroscopy with Al Ka X-rays (hm = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties and probe the total density of the state (DOS) distribution in the valence band. The shift of the binding energy caused by relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. Nitrogen adsorption–desorption isotherms were obtained using a nitrogen adsorption apparatus (BET-BJH: Micromeritics ASAP 2020, USA) with all samples degassed at 150 °C prior to measurements. The photoluminescence spectra (PL) for the samples were obtained using a fluorescence spectrophotometer (PL: FS-2500, Japan) with an Xe lamp with optical filter as the excitation source. Electrochemical impedance spectroscopy (EIS) was conducted using a CHI 660 B electrochemical system (CHI: Chenhua CHI660, China) with a standard three-electrode cell. The working electrode was prepared according to the following process. First, 20 mg of the sample was suspended in 0.5 mL of DMF, which was then dip-coated on a 10 mm 20 mm indium-tin oxide (ITO) glass electrode. The electrode was then annealed at 350 °C for 1 h at a heating rate 6 °C min 1. EIS was carried out at the open-circuit potential. A sinusoidal ac perturbation of 5 mV was applied to the electrode over the frequency range of 0.05–1 105 Hz. 2.4. Visible light photocatalytic activity The photocatalytic activities for the obtained products were evaluated by the oxidation of NO at ppb levels in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, which was 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 above the reactor. A UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. 0.1 g of the as-prepared photocatalysts was added into 30 mL of H2O and sonicated for 10 min, and then the resultant suspension was coated onto two dishes with a diameter of 12.0 cm, respectively. The coated dishes were then pretreated at 70 °C to remove water in the suspension. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance). 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 using a mass flow controller. After achieving
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the adsorption–desorption equilibrium, the lamp was turned on. The concentration of NO was continuously measured using a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., 42i-TL, USA), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 1.0 L/min. The removal ratio (g) of NO was calculated as g(%) = (1 C/ C0) 100%, where C and C0 are NO concentrations in the outlet steam and the feeding stream, respectively. 3. Results and discussion 3.1. Scheme for the formation of MCN-G and MCN-GO nanocomposites Fig. 1 is a schematic diagram illustrating the fabrication process, which shows that homogeneous dispersions of the components can be achieved when powders of MCN and G or GO in ethanol are subjected to sonication. After mixing and stirring the aqueous dispersions, the MCN particles are introduced onto the platform of dispersed G or GO sheets. The MCN functions as a ‘‘spacer’’ to increase the distance between the G or GO sheets, thereby making both the faces of G or GO accessible [25]. Consequently, the MCN-G and MCN-GO nanocomposites are formed, and more surfaces are exposed to come into contact with target pollutants. 3.2. Structure and morphology Fig. 2 shows the XRD patterns of the as-prepared samples. All the samples feature two distinct diffraction peaks. The peak observed at around 13.0°, which corresponds to a distance of d = 0.682 nm, is indexed as (1 0 0) and represents an in-plane structural packing motif [9]. The strongest XRD peak at around 27.5°, which corresponds to an interlayer stacking of aromatic segments with a distance of d = 0.324 nm, is indexed as (0 0 2) peak of graphitic materials [9]. Further observation on the enlarged view of (0 0 2) peak (Fig. 2b) for the MCN, MCN-G, and MCN-GO samples reveals that the peaks are weaker and broader than those of CN. This observation should be attributed to the conversion of the original graphitic-like structure to the new mesoporous structure [26], which generates smaller particle size because of the templating effects of SiO2. Notably, no diffraction peaks of G in the MCN-G and GO in the MCN-GO composites are observed, which can be attributed to the small amount of and very few G or GO layers in the composites as well the high dispersion on the hybrid surfaces. Moreover, the XRD patterns of MCN-G and MCN-GO do not differ from those of MCN and CN. Therefore, the modification with G or GO do not disturb the lattice structure of mesoporous g-C3N4, which is advantageous for the photocatalytic properties of the as-prepared nanocomposites [22]. SEM measurements were conducted to characterize the morphologies and structures of G, GO, MCN-G, and MCN-GO. As shown in Fig. 3a and c, graphene has a two-dimensional nanosheet structure with many thin wrinkles on the surface. The graphene oxide
31
sheet surface also shows a few thin wrinkles. Previous studies have reported that these wrinkles can induce the formation of nanosized channels or pores on the surface of graphene materials [22]. After the addition of MCN onto the surface of G/GO, a large amount of MCN nanoparticles are uniformly dispersed on the G and GO sheet surfaces, as shown in Fig. 3b and d, respectively. This phenomenon indicates that MCN nanoparticles and G/GO sheets have high dispersion after sonication, which is beneficial for obtaining more contact areas between MCN nanoparticles and G/GO sheets. Moreover, the MCN-G and MCN-GO composites have a wrinkled structure because of the flexible G/GO template, which suggests that graphene materials are not incorporated into the lattice of MCN. This result is in agreement with the XRD patterns. TEM was performed to investigate the local morphological structures of the as-synthesized samples. As shown in Fig. 4a, larger aggregated particles are observed in the CN. After treatment with the high surface area SiO2 template (see Fig. S1), the mesoporous structure and smaller particles than CN can be observed in the MCN sample (Fig. 4b). After modification with G/GO, the TEM images (Fig. 4c and e) show a clear distinction between MCN layers and G/GO nanosheets with wrinkles. The MCN nanoparticles are uniformly dispersed on the graphene material sheets. The assembly of MCN and G/GO can be ascribed to the p–p stacking and hydrogen bonding interactions between them [27]. We further observed the TEM images of the MCN-G and MCN-GO hybrids (Fig. 4d and f). Considering the interfacial interactions and preferential heterogeneous nucleation, the MCN nanoparticles are densely attached to the G and GO sheets [28]. Although the content of graphene materials in the composites is only 1.0 wt%, their crumpled edges are still observed. This result is consistent with the SEM analysis. The N2 adsorption–desorption isotherms and Barrett–Joyner– Halenda pore size distributions of the CN, MCN, MCN-G, and MCN-GO samples are shown in Fig. 5. As shown in Fig. 5a, the adsorption–desorption isotherms of all the samples are characteristic of Type IV Brunauer–Deming–Deming–Teller classification, which indicates their porous property [29]. This result is in agreement with the SEM and TEM results (Figs. 3 and 4). Additionally, for the MCN, MCN-G, and MCN-GO samples, the adsorption branch of the nitrogen isotherms shows an increase at P/P0 approaching unity, which indicates that large mesopores and small macropores are formed. As shown in Fig. 5b, the pore size distributions of the MCN, MCN-G, and MCN-GO samples are broad with existing mesopores and macropores. The surface areas and pore volumes are summarized in Table 1. The surface areas and pore volumes of the MCN (104 m2/g and 0.43 cm3/g, respectively), MCN-G (103 m2/g and 0.57 cm3/g, respectively), and MCN-GO (104 m2/g and 0.59 cm3/g, respectively) samples are much higher than those of CN (30 m2/g and 0.19 cm3/g, respectively). Although the surface areas of MCN, MCN-G, and MCN-GO are much higher than that of CN, the surface areas of MCN-G and MCN-GO do not change compared with that of MCN because the quantity of G/GO in the
Fig. 1. Schematic diagram to illustrate the fabrication process of MCN-G and MCN-GO.
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Fig. 2. XRD patterns of CN, MCN, MCN-G, and MCN-GO samples (a), enlarged view of (0 0 2) diffraction region (b).
(a)
(b)
(c)
(d)
Fig. 3. SEM images of G (a), MCN-G (b), GO (c), and MCN-GO (d).
composites is as low as 1 wt%. In comparison with MCN, a significant increase in pore volumes of the MCN-G and MCN-GO samples can be observed, which can be ascribed to the fact that treatment with sonication and stirring induces the particles to redisperse, thereby increasing the pore volumes. We expect that the open porous structure and high surface area are beneficial for reactant adsorption and mass transfer. Moreover, mesopores can allow light to reflect and penetrate deep into the pores of the photocatalyst and lead to high mobility of charges, which result in enhanced photocatalytic activity [30,31]. 3.3. Chemical composition The FT-IR spectra of the as-prepared samples are shown in Fig. S2. For CN, MCN, MCN-G, and MCN-GO, the broad peak at 3000 cm 1–3700 cm 1 is assigned to the residual N–H component from uncondensed amine groups and surface-adsorbed H2O molecules [32]. Several strong bands in the range of 1200 cm 1 to 1700 cm 1 dominate the spectra, and the peaks at approximately 1251, 1334, 1415, 1562, and 1641 cm 1 correspond to the characteristic stretching mode of C–N heterocycles [32]. Additionally, the
typical breathing mode of triazine units at 810 cm 1 was observed [32]. The typical bands of G/GO cannot be observed in the MCN-G and MCN-GO samples because the content of G/GO is low. To further investigate the surface chemical compositions and VB states of the as-synthesized samples, XPS measurements were conducted. As shown in Fig. S3a, signals of N, C, and O elements are detected and no peaks for other elements can be observed. Higher-resolution spectra were further taken on the N1s, C1s, and O1s regions. Fig. S3b shows the high-resolution N1s spectra of the MCN-G and MCN-GO samples in comparison with that of CN. As shown in the figure, the N1s spectra can be fitted into three peaks. The main peak centered at 398.8 eV can be assigned to the sp2 hybridized nitrogen (C–N–C) in triazine rings. The two peaks at approximately 400.5 and 404.6 eV can be ascribed to the bridging nitrogen atoms (C)3–N and the p excitations, respectively [33]. The C1s spectra shown in Fig. S3c can be fitted into three peaks. The peak at 284.8 eV can be assigned to adventitious carbon species obtained via XPS. The other two peaks located at 286.2 and 288.4 eV originate from (C)3–N and C–N–C, respectively [33]. Table 2 shows that the C/N atomic ratio increases from 0.65 for
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(a)
(b)
MCN
(c)
(d)
G
(f)
(e) GO
MCN
Fig. 4. TEM images of CN (a), MCN (b), MCN-G (c, d), and MCN-GO (e, f).
Fig. 5. N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of CN, MCN, MCN-G, and MCN-GO. Inset in (b) shows the enlarged view of corresponding pore size distribution curve of CN.
Table 1 SBET, total pore volume, peak pore size, and NO removal ratio for CN, MCN, MCN-G and MCN-GO. Sample name
SBET (m2/g)
Total pore volume (cm3/g)
Peak pore size (nm)
NO removal ratio g (%)
CN MCN MCN-G MCN-GO
30 104 103 104
0.19 0.43 0.57 0.59
3.8/32.3 9.1/50.9 9.0/30.6 8.9/45.8
16.8 50.3 64.9 60.7
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the CN sample to 0.83 for the MCN-G sample and 0.86 for the MCNGO sample could be ascribed to the incorporation of G/GO into the nanocomposites or carbon contamination in the testing process. The O1s spectra shown in Fig. S3d can be fitted into three peaks at binding energies of 531.5, 532.6, and 533.6 eV. The major peak at 532.6 eV is assigned to C–O–N, which originates from the partial oxidation of CN during the pyrolysis of dicyandiamide in air. The other two peaks at 531.5 and 533.6 eV can be attributed to surface –OH groups and adsorbed H2O [33]. No signals of O1s belong to GO were detected because of the small amount of GO (1.0 wt%) in MCN-GO. Fig. S3e shows the DOS of the VB. The VB maximum of CN, MCN-G, and MCN-GO are 1.73, 1.47, and 1.32 eV, respectively. These results indicate that G or GO has a significant effect on band structure. 3.4. Optical properties To understand the optical properties of the as-prepared samples, UV–Vis DRS was conducted to study the effect of G/GO on the optical properties of MCN. As shown in Fig. 6a, all the samples have an absorption edge located in the visible light region. The MCN-G and MCN-GO samples show a broad background absorption in the visible light region because of the existence of G/GO. The band gap values (Eg) of all the samples based on the UV–Vis DRS data are calculated, as shown in Fig. 6b. The band gaps of CN, MCN, MCN-G, and MCN-GO are 2.55, 2.53, 2.44, and 2.40 eV, respectively. The band edges show evident redshift, which indicates that the visible light-responsive optical property of MCN can be successfully improved by modifying the G/GO sheets. Based on the band gap of the samples, the CB position can be determined, as shown in Table 2. These results indicate that an appropriate G/GO can significantly adjust the band structure of the nanocomposites and, thus, the photocatalytic efficiency. Table 2 shows the negative shift in VB and CB potential of MCN-G and MCN-GO. The potential level of OH/H2O and O2/O2 are 2.38 and 0.33 eV, respectively [34]. The photogenerated holes cannot oxidize H2O or –OH because the VB potential level of the as-prepared samples is more negative than that of OH/H2O. However, the VB potential level of the as-synthesized samples induces the reaction of the photogenerated electrons with O2 to produce O2 radicals. Evidently, the O2 radicals are the dominant active species responsible for NO oxidation. When the CB level of MCN-G and MCN-GO shifts to a more negative position, the photogenerated electrons have a higher reduction power in comparison with other samples, which is favorable for the production of O2 radicals. Fig. 7 shows the room temperature PL spectra of the CN, MCN, MCN-G, and MCN-GO samples. The PL spectra have been extensively used to determine the efficiency of charge carrier trapping, migration, and separation and to investigate the fate of photogenerated electron–hole pairs. In general, a lower PL intensity indicates a decrease in the recombination rate of photogenerated charge carriers. As shown in Fig. 7, the main emission peaks centered at approximately 470 nm for all the samples originate from the band-band transitions. The PL intensity of MCN decreased compared with that of CN. The results indicate that the recombination of electron–hole pairs is moderately inhibited (or the charge separation is significantly accelerated) on MCN possibly because of the
reduced density of charge carrier traps for electron–hole recombination in mesoporous samples [35]. In comparison with the intensity of the PL signal for the MCN samples, those for the MCN-G and MCN-GO composites are much lower. This finding implies that the nanocomposites have lower recombination rates of electrons and holes under visible light irradiation, which can be assigned to the transportation of photogenerated electrons from MCN to G/GO sheets, thereby preventing the direct recombination of electron– hole pairs. G and GO are proven to be one of the most effective electronic conductive channels to facilitate charge carrier separation [36]. EIS Nyquist plots were further used to characterize charge carrier migration. Fig. 8 shows the EIS Nyquist plots of CN, MCN, MCNG, and MCN-GO. A smaller arc radius implies a higher efficiency of charge transfer. Evidently, the diameters of the arc radius of the MCN-G and MCN-GO composites are smaller than those of the MCN and CN composites, which suggests that MCN-G and MCNGO show significantly enhanced interfacial charge transfer and effective separation of photogenerated electron–hole pairs when adding the G/GO nanosheets to MCN. This result is in accordance with the PL analysis. 3.5. Visible light photocatalytic activity The as-prepared samples were applied in the photocatalytic removal of gaseous NO under visible light irradiation in a continuous reactor to evaluate their potential ability for air purification. Fig. 9 shows the variation of NO concentration (C/C0, %) with irradiation time over the as-synthesized samples. Here, C0 is the initial concentration of NO and C is the concentration of NO after photocatalytic reaction at time t. A previous investigation indicated that NO could not be photolyzed under light irradiation without a photocatalyst and that the concentration of NO could not decrease in the presence of a photocatalyst without irradiation [37]. In addition, G and GO have a band gap of approximately zero [38]. As such, the direct catalytic influence of G and GO can be excluded. The bulk CN has a suitable band gap of approximately 2.5 eV and can be directly excited by visible light. Fig. 9 shows that the NO removal ratio of CN is only 16.8% after 30 min of irradiation (Table 1). Recently, Taizo Sano et al. developed an activation strategy to enhance the photocatalytic activity of g-C3N4 in gas phase NO oxidation via alkaline hydrothermal treatment [39]. In the present work, we developed another approach to improve the NO removal ratio of g-C3N4 by simultaneously adding the mesoporous structure and a small amount of G/GO. As shown in Fig. 9, the NO removal ratio of MCN (50.3%) exceeds that of CN because of the increase in surface area and pore volume. Importantly, the nanocomposite samples show a significant improvement of visible light photocatalytic activity over the mesoporous samples. The NO removal ratios of MCN-G and MCN-GO reach 64.9% and 60.7% after 30 min of irradiation, which is more efficient than other types of visible light photocatalysts, such as C-doped TiO2, N-doped TiO2, Bi2WO6, BiOBr, and N-doped (BiO)2CO3 [40– 43]. The significantly enhanced photocatalytic activities of the MCN sample compared with those of the CN sample can be ascribed to the large surface area and pore volume, enhanced light-harvesting ability, adjusted band structure, and suppressed
Table 2 Atomic ratio of C/N, O percentage, band gap, VB position, and CB position of CN, MCN, MCN-G, and MCN-GO (The data of MCN was cited from Ref. [18]). Sample
Ratio of C/N
O (at.%)
Band gap (eV)
VB position (eV)
CN MCN MCN-G MCN-GO
0.65 0.67 0.83 0.86
0.68 0.94 1.93 3.12
2.55 2.60 2.44 2.40
1.73 1.75 1.47 1.32
CB position (eV) 0.82 0.85 0.97 1.08
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Fig. 6. UV–Vis DRS (a) and plots of (ahm)1/2 vs. photon energy (b) of the CN, MCN, MCN-G, and MCN-GO samples.
Fig. 7. Room temperature PL spectra of CN, MCN, MCN-G, and MCN-GO samples.
Fig. 8. Nyquist plots for CN, MCN, MCN-G, and MCN-GO electrodes under visible light irradiation (k > 420 nm, [Na2SO4] = 0.5 M).
recombination of electron–hole pairs. The synergistic effects of these favorable factors are directly responsible for the highly enhanced visible light photocatalytic activities of MCN-G and MCN-GO. The photocatalytic activity of bulk g-C3N4 is low because of the low surface area and the fast recombination of charge carriers. These drawbacks can be successfully solved via a combined
Fig. 9. Photocatalytic removal of NO in a single-pass flow of air over the CN, MCN, MCN-G, and MCN-GO samples under visible light irradiation (continuous reactor, NO concentration = 600 ppb).
strategy by generating a mesoporous structure and hybridizing with G/GO. First, the enlarged surface area and improved porous structure by adding mesopores are favorable for pollutant adsorption and mass transfer [44,45]. The increased pore volume could provide more surface active sites for photocatalytic reaction and quick reactant diffusion [45,46]. Second, the reduced band gap caused by the addition of graphene materials improves the visible light-harvesting ability. Third, the negative shift in CB potential results in a high reduction power of photogenerated electrons, which is beneficial for the production of O2 radicals. Finally, the increased charge transport rate and enhanced charge separation are due to the excellent electron mobility of graphene (Fig. 10). All these favorable factors contributed to the significantly enhanced visible light photocatalytic activity. As the synthesis of MCN-G and MCN-GO nanocomposites mainly involves the physical processes of sonication and stirring, the scale up of these physical processes may have little effect on the photocatalytic efficiency of such nanocomposites. The present synthesis method could be feasible for potential large-scale applications. 4. Conclusion In summary, the mesoporous g-C3N4/graphene and mesoporous g-C3N4/graphene oxide nanocomposite photocatalysts were
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Fig. 10. Schematic drawings of the photocatalytic induction process in the MCN-G (a) and MCN-GO (b) hybrids under visible light irradiation.
prepared via a facile sonochemical method. After the simultaneous addition of mesoporous structure and G/GO sheets in bulk g-C3N4, the visible light photocatalytic activity of MCN-G and MCN-GO toward gaseous NO removal was significantly enhanced. The enhanced activity of MCN-G and MCN-GO can be ascribed to the enlarged surface area and pore volume, increased light-harvesting ability, enhanced reduction power of photogenerated electrons, and improved charge carrier separation. This work shows that a combined strategy is effective for the modification of photocatalysts for large-scale applications. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (51478070, 51108487), the Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA20018), the Science and Technology Project from Chongqing Education Commission (KJ1400617, KJ130725), and the Innovative Research Team Development Program in University of Chongqing (KJTD201314). The research was also supported by a research Grant of Early Career Scheme (ECS 809813) from the Research Grant Council. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.09.004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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