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WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas
Journal of Hazardous Materials 260 (2013) 475–482
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Preparation of graphitic carbon nitride (g-C3 N4 )/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas Ken-ichi Katsumata ∗ , Ryosuke Motoyoshi, Nobuhiro Matsushita, Kiyoshi Okada Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• g-C3 N4 /WO3 composite was prepared by simplifying mechanical mixing method. • Composite exhibited higher visible light activity than bare g-C3 N4 and WO3 . • Probable photocatalytic mechanism under visible light was proposed. • Composite is effective visible-lightdriven photocatalyst decompose or remove VOCs.
a r t i c l e
i n f o
Article history: Received 12 February 2013 Received in revised form 28 May 2013 Accepted 30 May 2013 Available online 7 June 2013 Keywords: Graphitic carbon nitride (g-C3 N4 ) WO3 Photocatalyst Acetaldehyde Visible light
a b s t r a c t Novel visible-light-driven graphitic carbon nitride (g-C3 N4 )/WO3 composite photocatalysts were prepared, and the acetaldehyde (CH3 CHO) degradation activity of these composites was evaluated. The prepared g-C3 N4 /WO3 composites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflection spectroscopy (UV–vis), and the N2 gas adsorption Brunauer–Emmett–Teller (BET) method (N2 -BET). The WO3 particles, which were 100–300 nm in size, were in direct contact with the g-C3 N4 sheet surface. The optical band gap and specific surface area of the g-C3 N4 /WO3 composites were in the range of 2.65–2.75 eV and 4–7 m2 /g, respectively. The g-C3 N4 /WO3 composites exhibited higher activity for the photodegradation of CH3 CHO under visible light irradiation compared to g-C3 N4 . The optimal WO3 content for the CH3 CHO photodegradation activity of the heterojunction structures was determined. The synergistic effect of g-C3 N4 and WO3 was considered to lead to improved photogenerated carrier separation. A possible degradation mechanism of CH3 CHO over the g-C3 N4 /WO3 composite photocatalyst under visible light irradiation was proposed. These results should usefully expand applications of g-C3 N4 as a visible-light-driven photocatalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are typical air pollutants, mainly emitted from industrial processes and transport vehicles. These cause various environmental problems and have adverse effects on the health of humans. Removal of VOCs from air has been attempted using physical, chemical, and biological techniques [1–3], of which adsorption by activated carbon is the most widely
∗ Corresponding author. Tel.: +81 45 924 5323; fax: +81 45 924 5358. E-mail address: [email protected] (K.-i. Katsumata). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.058
studied and used [4–10]. However, this method simply transfers organics from the gas to the solid phase, and increases the environmental load because of the need to dispose of the activated carbon containing the adsorbed VOCs. A method that can completely decompose or remove VOCs is crucially needed. Polymeric graphitic carbon nitride (g-C3 N4 ), which has an optical band gap of 2.7 eV, is the most stable allotrope of carbon nitride [11], and its potential applications for energy conversion [12,13], hydrogen and carbon dioxide storage [14–17], gas sensors [18,19], and solar cells [20–22] have been reported. Recently, g-C3 N4 has been actively studied as a novel metal-free photocatalyst for the splitting of water into hydrogen or oxygen gas using solar energy
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[23–29]. As a photocatalyst, it also exhibits activity for the photodegradation of organic pollutants in solution [30,31] or in air [32] under visible light irradiation. Since the g-C3 N4 photocatalyst possesses high thermal and chemical stability, it is a valuable and useful material for applications. However, the photocatalytic efficiency of bare g-C3 N4 is still limited due to the high recombination rate of photogenerated electron–hole pairs. One of the techniques for increasing the separation efficiency of photogenerated electron–hole pairs is to form a composite photocatalyst using two kinds of semiconductors. Suitable matching of the band levels of the conduction and valence bands in the two semiconductors offers appropriate driving forces to separate and transfer photogenerated electron–hole pairs [33]. Various g-C3 N4 /semiconductor composite photocatalysts, including g-C3 N4 /TaON [34], g-C3 N4 /Bi2 WO6 [35], g-C3 N4 /N-doped NaNbO3 [36], g-C3 N4 /ZnO [37], and g-C3 N4 /TiO2 [38], have been prepared and used for the photodegradation of organic dyes in solution. However, the photodegradation of air pollutants using g-C3 N4 /semiconductor composite photocatalysts has not yet been reported. In the present study, we focused on the novel g-C3 N4 /WO3 composite photocatalyst. WO3 is a semiconductor material with an optical band gap of 2.7 eV, and it exhibits photocatalytic activity under visible light [39–41]. Since the optical band gap of WO3 is almost the same as that of g-C3 N4 , WO3 and g-C3 N4 are simultaneously excited. Therefore, it is expected that the composite structure may improve the photocatalytic activity of g-C3 N4 by increasing the number of photogenerated electron–hole pairs. In this work, the g-C3 N4 /WO3 composite was prepared by a physical mixing method, and the photodegradation activity of acetaldehyde gas using the g-C3 N4 /WO3 composites was evaluated under visible light irradiation. Acetaldehyde is one of the VOCs of interest, and it is used in conformity with JIS R1751-2, which is a test method for air purification performance of photocatalytic materials under an indoor lighting environment as specified in the Japanese Industrial Standards (JIS). 2. Experimental 2.1. Catalyst preparation The chemicals used in this work, melamine (C3 H6 N6 ), dicyanodiamide (C2 H4 N4 ), and tungstic anhydride powder (WO3 ), were reagent grade and were purchased from Wako Pure Chemical Industries, Tokyo, Japan. Metal-free g-C3 N4 powders were synthesized by heating mixed melamine and dicyanodiamide powder in a muffle furnace. In a typical synthesis, 2.0 g of melamine and 2.0 g of dicyanodiamide were well mixed using a mortar. After mixing, the powder was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 550 ◦ C and held for 4 h at a heating rate of 2.2 ◦ C min−1 . After the reaction, the alumina crucible was cooled to room temperature. The product was then collected and ground into powder. The synthesized g-C3 N4 and WO3 powders were mixed using an agate mortar. The mixed mass ratios in this work were as follows: g-C3 N4 :WO3 = 8:2, 6:4, 4:6, 2:8. Samples with these different mass ratios are denoted here as G8W2, G6W4, G4W6, and G2W8, respectively. 2.2. Characterization The crystalline phases in the samples were identified using a powder X-ray diffractometer (XRD, RINT2100; Rigaku, Japan) with monochromated CuK˛ radiation. The applied voltage and current to the Cu target were 40 kV and 40 mA, respectively. The interlayer
distance of the synthesized g-C3 N4 was calculated using the Bragg equation. IR spectroscopy was performed using a Fourier transform infrared spectrometer (FT-IR, JIR-7000; JEOL, Japan). UV–visible absorption properties of the samples were measured using a UV–visible scanning spectrometer (UV–vis, UV-2450; Shimadzu, Japan). Photoluminescence (PL) spectra of the samples were measured using a luminescence spectrometer (LS 55; PerkinElmer Inc., USA) with a Xe lamp. The surface microstructures of the samples were examined by field emission-scanning electron microscopy (FE-SEM, S-4500; Hitachi, Japan) using an acceleration voltage of 15 kV. The morphology of the samples was investigated using TEM (H-8100; Hitachi, Japan) operating at 200 kV. TEM samples were prepared by dispersing one drop of the sample in water and depositing this on an amorphous carbon grid. The specific surface area of the samples was measured by the N2 gas adsorption BET method (GeminiV, Shimadzu, Japan). The samples were preheated in vacuo at 200 ◦ C for 1 h. 2.3. Evaluation of photocatalytic activity The degradation of gaseous acetaldehyde (CH3 CHO) was carried out at room temperature in a batch-type reactor using the powder samples. This reaction temperature is higher than the boiling point of pure acetaldehyde (approximately 20 ◦ C). The reactor vessel was made of Pyrex glass, with a volume of 500 mL. Before photodegradation, the samples were irradiated in air using UV light (1.0 mW/cm2 ) for 1 day to remove the ubiquitous organic pollutants from the surface. The powder samples (0.05 g) were then placed in the reaction vessel. Commercially available pure air was humidified to 50%RH by flowing it through a water/ice mixture, and then introduced into the reaction vessel at room temperature. A measured quantity of acetaldehyde gas (5.6 mol (250 ppm)) was then introduced into the reaction vessel using a PressureLok® precision analytical syringe. After adsorption equilibration in dark for 24 h, the reactor was placed below a fluorescent light lamp (FL10, Toshiba Lighting & Technology Corp., Japan). A filter (<420 nm, SC42; Fujifilm, Japan) was used to cut the irradiated wavelengths in the UV range. The fluorescent light intensity was 6000 l×. The decrease in acetaldehyde concentration and increase in CO2 concentration were monitored using gas chromatography with nitrogen as the carrier gas (GC-2014; Shimadzu, equipped with a 2 m Porapak-Q column ZP-17 and a flame ionization detector). 3. Results and discussion 3.1. Characterization of g-C3 N4 , WO3 , and g-C3 N4 /WO3 composites The XRD patterns of the samples are shown Fig. 1. The XRD pattern of the synthesized g-C3 N4 shows two broad peaks at 2 = 13.2 and 27.3◦ . It is well known that g-C3 N4 is based on tri-s-triazine building blocks [42]. The strongest peak at 27.3◦ is close to the peak position of graphite (0 0 2), and is due to the interlayer stacking of aromatic systems. The calculated interplanar distance of the aromatic units (d = 0.327 nm) is smaller than that for crystalline gC3 N4 (d = 0.34 nm) [43]. This is attributed to electron localization and stronger binding between the layers in the former case [30]. The small angle peak at 13.2◦ , corresponding to 0.671 nm, is associated with interlayer stacking. This distance is smaller than the size of one tris-s-triazine unit (0.713 nm), which can be attributed to the presence of a small tilt angularity in the structure [23,30]. With increasing WO3 content of the composite samples, the peaks assigned to g-C3 N4 became weaker. It is difficult to confirm the presence of the peak at 27.3◦ in the XRD patterns of the G4W6 and
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Fig. 2. FT-IR spectra for the synthesized g-C3 N4 , WO3 and prepared composite samples.
Fig. 1. XRD patterns for the synthesized g-C3 N4 and prepared composite samples. The standard pattern for WO3 (monoclinic; ICDD#00-043-1035) is shown at the bottom for reference.
G2W8 samples. If the interlayer stacking is not preserved in the G4W6 and G2W8 samples, the specific surface area of the samples becomes high due to exfoliation of the g-C3 N4 sheets. However, the specific surface area of the samples is almost the same, as shown in Table 1. Therefore, we consider that the interlayer stacking is preserved in the G4W6 and G2W8 samples. The XRD patterns for WO3 can be assigned to a monoclinic structure. Fig. 2 shows IR spectra of g-C3 N4 , WO3 , and the composite samples. Several bands in the 1200–1650 cm−1 region were found, which correspond to the typical stretching modes of CN heterocycles [44,45]. The sharp band at 810 cm−1 and the broad bands at around 3000 cm−1 were indicative of the breathing mode of the triazine units and the NH stretching vibration modes, respectively [30,43]. This indicates that amino groups exist in the samples. Zhao et al. [46] reported that residual hydrogen atoms bind to the edges of the graphene-like C N sheet in the form of C NH2 and 2C NH bonds. It is considered that a small amount of the amino groups remains in the samples synthesized by direct heating of the melamine and dicyanodiamide mixture. The absorption assigned to CN heterocycles became weaker with increasing WO3 content in the composite samples. Additionally, a luminescence peak at
Table 1 Specific surface area, optical band gap, CH3 CHO degradation ratio, and CO2 generation ratio for the synthesized g-C3 N4 , WO3 , prepared composite samples, and Cu(II)-grafted TiO2 . Sample
SSAa (m2 /g) Band gapb (eV) CH3 CHO degradation C/C0 c (%)
CO2 generation C/Ctheory d (%)
g-C3 N4 G8W2 G6W4 G4W6 G2W8 WO3 Cu(II)-grafted TiO2
7 6 5 5 4 4 6
16 20 24 29 45 28 26
a
2.70 2.73 2.73 2.75 2.73 2.65 3.02
72 69 72 65 100 100 87
Specific surface area. Optical band gap estimated by UV–vis spectra. c (CH3 CHO con. after 24 h)/(initial CH3 CHO con.). d (Generated CO2 con. after 24 h)/(theoretical CO2 con. after completely CH3 CHO decomposition). b
470 nm was observed for the composite samples, and its intensity also became weaker with increasing WO3 content (Fig. S1). Figs. 3 and 4 show SEM and TEM images of the g-C3 N4 , WO3 , and composite samples. The surface morphology of the synthesized gC3 N4 seemed to be smooth, consisting not of particles but of sheets. Indeed, the sheet-like morphology was confirmed by TEM observations of the g-C3 N4 (Fig. 4). The morphology of the synthesized g-C3 N4 was due to aggregation and gathering of the g-C3 N4 sheets. The WO3 sample had a particle size of 100–300 nm, and a grainlike morphology with polygonal grain shapes. The morphology of the WO3 sample was similar to that of monoclinic WO3 particles prepared by a sol–gel method [47]. In the composite samples, WO3 particles were found to cover the g-C3 N4 surface. Various WO3 particles, which did not agglomerate, were sparsely observed on the g-C3 N4 surface in the G8W2 sample, and almost all particles were in direct contact with the g-C3 N4 surface. With increasing WO3 content in the composite samples, the area covered by the WO3 particles increased on the g-C3 N4 surface in the G6W4 and G4W6 samples. In the G2W8 sample, WO3 particles almost entirely covered the g-C3 N4 surface. Although the WO3 particles were partly agglomerated, they were in direct contact with the g-C3 N4 surface in the G4W6, G6W4, and G8W2 samples (Fig. 4). The optical properties of the g-C3 N4 , WO3 , and composite samples were examined by UV–vis diffuse reflectance spectroscopy (Fig. 5). For all samples, the optical absorption edge was estimated to be at around 480 nm. This is consistent with the pale yellow color of the samples. In the case of the g-C3 N4 sample, slight absorption was observed at wavelengths of more than 480 nm. Conventional carbon nitride shows the typical absorption pattern of an organic semiconductor with very strong adsorption at about 420 nm [11]. Yan et al. [30] reported that the synthesis method has a slight effect on the absorption edge of carbon nitride, and this is attributed to differences in the local structure, packing, and defects formed during the synthesis processes. It is considered likely that the synthesized g-C3 N4 can absorb visible light since it contains very few impurities (amino groups). The optical band gap can be estimated using the general procedure from the absorption spectra from the following equation [48]: ˛h = A(h − Eg )
n/2
(1)
where A, ˛, h and Eg are, respectively, a constant, the absorption coefficient, the photon energy and the optical band gap. In this equation, n is determined by the transition type; in the case of direct and indirect transitions, the n value is 1 and 4, respectively. The n and Eg values were estimated by the following steps: first,
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Fig. 3. SEM images for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
a plot of ln(˛h) vs. ln(h − Eg ) was created using an approximate value of Eg , and then the value of n was estimated from the slope of the straight line near the band edge; second, a plot of (˛h)2/n vs. h was created and a tangential line was plotted near the band edge, with the x-intercept of the tangential line corresponding to the optical band gap. The estimated band gaps for g-C3 N4 , G8W2, G6W4, G4W6, G2W8, and WO3 were 2.70, 2.73, 2.73, 2.75, 2.73, and 2.65 eV, respectively (Table 1 and Fig. S2). These values were in good agreement with the reported values for g-C3 N4 and WO3 [11,39]. Since the estimated band gaps for all samples were almost the same, the photon energies used in the photocatalysis experiments were almost identical.
3.2. Photocatalytic activity of g-C3 N4 , WO3 , and g-C3 N4 /WO3 composites Fig. 6 shows the results of acetaldehyde (CH3 CHO) degradation under visible light irradiation in the presence of the synthesized gC3 N4 , WO3 and prepared composite samples. Prior to visible light
irradiation, the initial CH3 CHO concentration for all the samples decreased below the injected CH3 CHO concentration (5.6 mol) introduced into the reaction vessel; this effect become more marked with increasing g-C3 N4 content. This indicates that the gC3 N4 plays an important role as an adsorbent. After visible light irradiation, the CH3 CHO concentration was drastically reduced for the WO3 and G2W8 samples due to photodegradation by the photocatalyst, and after irradiation for 24 h, the CH3 CHO was completely degraded. On the other hand, the CH3 CHO concentration for the G4W6, G6W4, G8W2, and g-C3 N4 samples gradually decreased with increasing visible light irradiation time. The degree of degradation of CH3 CHO depended on the catalyst. Upon exposure to visible light, the CH3 CHO concentration decreased. In the presence of the photocatalyst, the oxidative photodegradation of CH3 CHO proceeds as follows [49]: CH3 CHO(ads.) + 3H2 O + 10 h+ → 2CO2 + 10H+
(2)
CH3 CHO(ads.) + H2 O + 2 h+ → CH3 COOH(ads.) + 2H+
(3)
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Fig. 5. UV–vis diffuse reflectance spectra for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
CH3 COOH(ads.) + 2H2 O + 8 h+ → 2CO2 + 8H+
(4)
where h+ is a hole generated by photoinduced charge separation in the photocatalyst. One CH3 CHO molecule decomposes into two CO2 molecules, which is the final product. Although the g-C3 N4 sample adsorbed the most CH3 CHO, the photodegradation rate of CH3 CHO was lower than that for the other samples. This is attributed to the fact that the photocatalytic efficiency of bare g-C3 N4 is limited due to the high recombination rate of photogenerated electron–hole pairs [50]. With increasing WO3 content, the CH3 CHO photodegradation rate became higher, which indicates that WO3 is strongly associated with photodegradation of CH3 CHO. The photoluminescence peak at 470 nm formed by electron–hole recombination in g-C3 N4 , and the peak intensity of the G2W8 sample was less than a quarter of that for the G8W2 sample (Fig. S1). It is considered that the electron–hole recombination might be reduced by the increase in interfaces between g-C3 N4 and WO3 .
Fig. 4. TEM images for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
Fig. 6. Change in CH3 CHO concentration as a function of visible light irradiation time in the presence of the synthesized g-C3 N4 , WO3 , and prepared composite samples.
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vapor in air. The water molecules can be transformed into • OH by reaction with photogenerated holes (h+ ) or superoxide radicals (O2 •− ) at the photocatalyst surface via the following reactions [51]: h+ + H2 O → • OH + H+
(5)
2O2 •− + 2H2 O → 2• OH + 2OH− + O2
(6)
In addition, hydrogen peroxide (H2 O2 ) is also known to be a strong oxidant. Oxygen molecules can be transformed into H2 O2 by reaction with photoexcited electrons at the WO3 surface via the following reaction [52]: O2 + 2H+ + 2e− → H2 O2
Fig. 7. Change in concentration of CO2 generated by the degradation of CH3 CHO as a function of visible light irradiation time in the presence of the synthesized g-C3 N4 , WO3 , and prepared composite samples.
Fig. 7 shows the concentration of CO2 generated from the degradation of CH3 CHO under visible light irradiation in the presence of the composite, synthesized g-C3 N4 , and WO3 samples. When the samples were irradiated with visible light, the CO2 concentration in all cases began to increase, indicating that CH3 CHO was decomposed into CO2 , the final product. After visible light irradiation for 24 h, the CO2 concentration of the g-C3 N4 sample was the lowest among all samples. With increasing WO3 content, the CO2 concentration became higher, in agreement with the results for the photodegradation of CH3 CHO (Fig. 6). The G2W8 sample exhibited higher activity for photodegradation and photodecomposition of CH3 CHO into CO2 , indicating that the G2W8 sample is a photocatalyst with excellent photocatalytic performance under visible light irradiation. On the other hand, the CO2 concentration for the WO3 sample was lower than that for the G2W8 and G4W6 samples, whereas it exhibited the highest CH3 CHO photodegradation rate among all samples. These results indicate that the photodegradation activity of CH3 CHO was not necessarily correlated with the photodecomposition of CH3 CHO into CO2 , and WO3 produces larger intermediate products before photodecomposition to CO2 compared to G2W8. In some cases, the intermediate products also have adverse effects on the health of humans. It is thus important that acetaldehyde be decomposed into CO2 under visible light. Since the photocatalytic reaction occurred on the crystal surface, the photocatalytic activity is dependent on the specific surface area. The specific surface areas of the samples are shown in Table 1, and they are all in the range of 4–7 m2 /g. Therefore, it is considered that the specific surface area of the samples did not significantly affect the photocatalytic activity. In addition, the band gap energies of the samples were almost the same (Table 1), indicating that the number of absorbed photons was almost the same for all samples. 3.3. Enhanced visible-light-driven mechanism for g-C3 N4 /WO3 composites Hydroxyl radicals (• OH) are known to be strong oxidants, contributing to the degradation of CH3 CHO in the presence of water
(7)
When the g-C3 N4 /WO3 composite samples were irradiated with visible light, electrons were excited from the valence band (VB) of g-C3 N4 and WO3 to the conduction band (CB) of g-C3 N4 and WO3 , respectively, which left holes in the VB in both materials, as shown in Scheme 1. Some of the photoinduced electrons in the g-C3 N4 diffuse to the g-C3 N4 surfaces, and they react with O2 to produce O2 •− . As described by reaction (6), hydroxyl radicals (• OH) are then formed by the reaction of O2 •− with H2 O. Other photoinduced electrons in g-C3 N4 are transferred to the CB of WO3 because the CB bottom of g-C3 N4 (−1.13 V vs. SHE) is more negative than that of WO3 (+0.5 V vs. SHE) [23,35,52]. Since the potential for multi-electron reduction of O2 , as described by reaction (7), is more positive (O2 + 2H+ + 2e− = H2 O2 (aq), +0.682 V) than the CB bottom of WO3 , electrons transferred from the CB of g-C3 N4 to the CB of WO3 , and photoinduced electrons in the WO3 , react with O2 to produce H2 O2 . Some of the photogenerated holes in WO3 , which has a high oxidation potential (+3.15 V vs. SHE), diffuse to the WO3 surfaces, and they directly degrade the CH3 CHO absorbed on the WO3 particles or react with surface-bound H2 O to produce hydroxyl radical species, • OH. Other photogenerated holes in WO3 are transferred from the VB of WO3 (+3.15 V vs. SHE) to the VB of g-C3 N4 (+1.57 V vs. SHE), and these transferred holes and photogenerated holes in g-C3 N4 directly degrade the CH3 CHO absorbed on the g-C3 N4 particles. Active oxygen species such as • OH, O •− , and H O generated on the g-C N and WO surface 2 2 2 3 4 3 are released, diffusing through the gas phase and oxidizing the CH3 CHO. The activities for the photodegradation of CH3 CHO for g-C3 N4 , WO3 , the composite samples, and Cu(II)-grafted TiO2 are summarized in Table 1. Cu(II)-grafted TiO2 is a photocatalyst which exhibits high photocatalytic activity under visible light irradiation [53,54]. In this case, Cu(II)-grafted TiO2 was prepared so that the specific surface area was almost the same value as that of the composite samples (see Supplementary data for preparation detail). As the band gap of Cu(II)-grafted TiO2 was larger than those of the composite samples, visible light intensity was increased so that the number of absorbed photons for Cu(II)-grafted TiO2 was almost the same as that for the composite samples. The G2W8 sample exhibited the highest activity for photodegradation of CH3 CHO and photodecomposition of CH3 CHO into CO2 among all samples containing Cu(II)-grafted TiO2 . This result indicated that G2W8 is a very effective visible-light-driven photocatalyst, compared to other visible-light-driven photocatalysts. In the g-C3 N4 /WO3 composite, the WO3 content was thus pivotal for achieving high photocatalytic activity. An appropriate WO3 content caused good dispersion and contact on the g-C3 N4 surface, which favored the transfer and separation of charge carriers. However, for WO3 content lower than 80 mass%, the number of interfaces and heterojunction structures between the g-C3 N4 and WO3 particles decreased. As a result, interfacial charge transfer was suppressed and photocatalytic activity was reduced. Thus, the G2W8 sample showed the best performance. We believe this to be a
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Scheme 1. Possible degradation mechanism of CH3 CHO over a g-C3 N4 /WO3 composite photocatalyst under visible light irradiation.
very important result because this material exhibits photocatalytic activity under a variety of environments, in particular, under visible (fluorescent) light. In addition, the present g-C3 N4 /WO3 composites can be prepared by a very simple technique: physical mixing using a mortar. The composites exhibit very efficient CH3 CHO photodecomposition under visible light irradiation. This suggests that these results should usefully expand the application of g-C3 N4 as a visible-light-driven photocatalyst. 4. Conclusion Graphitic carbon nitride (g-C3 N4 ) with a nanosheet structure was synthesized and used to prepare g-C3 N4 /WO3 composites. The photocatalytic degradation of CH3 CHO using these composites was investigated. The optical band gap and specific surface area of the composites were almost the same. Under visible light irradiation, the CH3 CHO photodegradation by the g-C3 N4 /WO3 composite was greater than that for g-C3 N4 alone. With increasing WO3 content of the composites, the photodegradation effect became larger, indicating that WO3 is strongly associated with the photodegradation of CH3 CHO. The optimum mixture of g-C3 N4 and WO3 for photocatalysis corresponded to the composition G2W8. A CH3 CHO photodegradation mechanism was proposed and discussed in terms of energy band positions. The present g-C3 N4 /WO3 composite is thought to be a very effective visible-light-driven photocatalyst and could be used to completely decompose or remove VOCs from air. Acknowledgements The authors are grateful to Prof. Y. Kitamoto (Tokyo Institute of Technology, Japan) for the TEM observations, to Prof. F. Wakai and Prof. Y. Shinoda (Tokyo Institute of Technology, Japan) for the FE-SEM observations, to Prof. T. Akatsu (Tokyo Institute of Technology, Japan) for the XRD, to Mr. Y. Makinose (Tokyo Institute of Technology, Japan) for the TEM observations, and to Ms. H. Takei (Kanagawa Academy of Science and Technology, Japan) for helpful support for using the N2 -BET and UV–vis instruments. This work was supported in part by the Nippon Sheet Glass Foundation for Materials Science and Engineering, and by the project “Development Base of Advanced Materials Development and Integration of Novel Structured Metallic and Inorganic” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 05.058.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Preparation of graphitic carbon nitride (g-C3 N4 )/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas Ken-ichi Katsumata ∗ , Ryosuke Motoyoshi, Nobuhiro Matsushita, Kiyoshi Okada Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• g-C3 N4 /WO3 composite was prepared by simplifying mechanical mixing method. • Composite exhibited higher visible light activity than bare g-C3 N4 and WO3 . • Probable photocatalytic mechanism under visible light was proposed. • Composite is effective visible-lightdriven photocatalyst decompose or remove VOCs.
a r t i c l e
i n f o
Article history: Received 12 February 2013 Received in revised form 28 May 2013 Accepted 30 May 2013 Available online 7 June 2013 Keywords: Graphitic carbon nitride (g-C3 N4 ) WO3 Photocatalyst Acetaldehyde Visible light
a b s t r a c t Novel visible-light-driven graphitic carbon nitride (g-C3 N4 )/WO3 composite photocatalysts were prepared, and the acetaldehyde (CH3 CHO) degradation activity of these composites was evaluated. The prepared g-C3 N4 /WO3 composites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflection spectroscopy (UV–vis), and the N2 gas adsorption Brunauer–Emmett–Teller (BET) method (N2 -BET). The WO3 particles, which were 100–300 nm in size, were in direct contact with the g-C3 N4 sheet surface. The optical band gap and specific surface area of the g-C3 N4 /WO3 composites were in the range of 2.65–2.75 eV and 4–7 m2 /g, respectively. The g-C3 N4 /WO3 composites exhibited higher activity for the photodegradation of CH3 CHO under visible light irradiation compared to g-C3 N4 . The optimal WO3 content for the CH3 CHO photodegradation activity of the heterojunction structures was determined. The synergistic effect of g-C3 N4 and WO3 was considered to lead to improved photogenerated carrier separation. A possible degradation mechanism of CH3 CHO over the g-C3 N4 /WO3 composite photocatalyst under visible light irradiation was proposed. These results should usefully expand applications of g-C3 N4 as a visible-light-driven photocatalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are typical air pollutants, mainly emitted from industrial processes and transport vehicles. These cause various environmental problems and have adverse effects on the health of humans. Removal of VOCs from air has been attempted using physical, chemical, and biological techniques [1–3], of which adsorption by activated carbon is the most widely
∗ Corresponding author. Tel.: +81 45 924 5323; fax: +81 45 924 5358. E-mail address: [email protected] (K.-i. Katsumata). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.058
studied and used [4–10]. However, this method simply transfers organics from the gas to the solid phase, and increases the environmental load because of the need to dispose of the activated carbon containing the adsorbed VOCs. A method that can completely decompose or remove VOCs is crucially needed. Polymeric graphitic carbon nitride (g-C3 N4 ), which has an optical band gap of 2.7 eV, is the most stable allotrope of carbon nitride [11], and its potential applications for energy conversion [12,13], hydrogen and carbon dioxide storage [14–17], gas sensors [18,19], and solar cells [20–22] have been reported. Recently, g-C3 N4 has been actively studied as a novel metal-free photocatalyst for the splitting of water into hydrogen or oxygen gas using solar energy
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[23–29]. As a photocatalyst, it also exhibits activity for the photodegradation of organic pollutants in solution [30,31] or in air [32] under visible light irradiation. Since the g-C3 N4 photocatalyst possesses high thermal and chemical stability, it is a valuable and useful material for applications. However, the photocatalytic efficiency of bare g-C3 N4 is still limited due to the high recombination rate of photogenerated electron–hole pairs. One of the techniques for increasing the separation efficiency of photogenerated electron–hole pairs is to form a composite photocatalyst using two kinds of semiconductors. Suitable matching of the band levels of the conduction and valence bands in the two semiconductors offers appropriate driving forces to separate and transfer photogenerated electron–hole pairs [33]. Various g-C3 N4 /semiconductor composite photocatalysts, including g-C3 N4 /TaON [34], g-C3 N4 /Bi2 WO6 [35], g-C3 N4 /N-doped NaNbO3 [36], g-C3 N4 /ZnO [37], and g-C3 N4 /TiO2 [38], have been prepared and used for the photodegradation of organic dyes in solution. However, the photodegradation of air pollutants using g-C3 N4 /semiconductor composite photocatalysts has not yet been reported. In the present study, we focused on the novel g-C3 N4 /WO3 composite photocatalyst. WO3 is a semiconductor material with an optical band gap of 2.7 eV, and it exhibits photocatalytic activity under visible light [39–41]. Since the optical band gap of WO3 is almost the same as that of g-C3 N4 , WO3 and g-C3 N4 are simultaneously excited. Therefore, it is expected that the composite structure may improve the photocatalytic activity of g-C3 N4 by increasing the number of photogenerated electron–hole pairs. In this work, the g-C3 N4 /WO3 composite was prepared by a physical mixing method, and the photodegradation activity of acetaldehyde gas using the g-C3 N4 /WO3 composites was evaluated under visible light irradiation. Acetaldehyde is one of the VOCs of interest, and it is used in conformity with JIS R1751-2, which is a test method for air purification performance of photocatalytic materials under an indoor lighting environment as specified in the Japanese Industrial Standards (JIS). 2. Experimental 2.1. Catalyst preparation The chemicals used in this work, melamine (C3 H6 N6 ), dicyanodiamide (C2 H4 N4 ), and tungstic anhydride powder (WO3 ), were reagent grade and were purchased from Wako Pure Chemical Industries, Tokyo, Japan. Metal-free g-C3 N4 powders were synthesized by heating mixed melamine and dicyanodiamide powder in a muffle furnace. In a typical synthesis, 2.0 g of melamine and 2.0 g of dicyanodiamide were well mixed using a mortar. After mixing, the powder was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 550 ◦ C and held for 4 h at a heating rate of 2.2 ◦ C min−1 . After the reaction, the alumina crucible was cooled to room temperature. The product was then collected and ground into powder. The synthesized g-C3 N4 and WO3 powders were mixed using an agate mortar. The mixed mass ratios in this work were as follows: g-C3 N4 :WO3 = 8:2, 6:4, 4:6, 2:8. Samples with these different mass ratios are denoted here as G8W2, G6W4, G4W6, and G2W8, respectively. 2.2. Characterization The crystalline phases in the samples were identified using a powder X-ray diffractometer (XRD, RINT2100; Rigaku, Japan) with monochromated CuK˛ radiation. The applied voltage and current to the Cu target were 40 kV and 40 mA, respectively. The interlayer
distance of the synthesized g-C3 N4 was calculated using the Bragg equation. IR spectroscopy was performed using a Fourier transform infrared spectrometer (FT-IR, JIR-7000; JEOL, Japan). UV–visible absorption properties of the samples were measured using a UV–visible scanning spectrometer (UV–vis, UV-2450; Shimadzu, Japan). Photoluminescence (PL) spectra of the samples were measured using a luminescence spectrometer (LS 55; PerkinElmer Inc., USA) with a Xe lamp. The surface microstructures of the samples were examined by field emission-scanning electron microscopy (FE-SEM, S-4500; Hitachi, Japan) using an acceleration voltage of 15 kV. The morphology of the samples was investigated using TEM (H-8100; Hitachi, Japan) operating at 200 kV. TEM samples were prepared by dispersing one drop of the sample in water and depositing this on an amorphous carbon grid. The specific surface area of the samples was measured by the N2 gas adsorption BET method (GeminiV, Shimadzu, Japan). The samples were preheated in vacuo at 200 ◦ C for 1 h. 2.3. Evaluation of photocatalytic activity The degradation of gaseous acetaldehyde (CH3 CHO) was carried out at room temperature in a batch-type reactor using the powder samples. This reaction temperature is higher than the boiling point of pure acetaldehyde (approximately 20 ◦ C). The reactor vessel was made of Pyrex glass, with a volume of 500 mL. Before photodegradation, the samples were irradiated in air using UV light (1.0 mW/cm2 ) for 1 day to remove the ubiquitous organic pollutants from the surface. The powder samples (0.05 g) were then placed in the reaction vessel. Commercially available pure air was humidified to 50%RH by flowing it through a water/ice mixture, and then introduced into the reaction vessel at room temperature. A measured quantity of acetaldehyde gas (5.6 mol (250 ppm)) was then introduced into the reaction vessel using a PressureLok® precision analytical syringe. After adsorption equilibration in dark for 24 h, the reactor was placed below a fluorescent light lamp (FL10, Toshiba Lighting & Technology Corp., Japan). A filter (<420 nm, SC42; Fujifilm, Japan) was used to cut the irradiated wavelengths in the UV range. The fluorescent light intensity was 6000 l×. The decrease in acetaldehyde concentration and increase in CO2 concentration were monitored using gas chromatography with nitrogen as the carrier gas (GC-2014; Shimadzu, equipped with a 2 m Porapak-Q column ZP-17 and a flame ionization detector). 3. Results and discussion 3.1. Characterization of g-C3 N4 , WO3 , and g-C3 N4 /WO3 composites The XRD patterns of the samples are shown Fig. 1. The XRD pattern of the synthesized g-C3 N4 shows two broad peaks at 2 = 13.2 and 27.3◦ . It is well known that g-C3 N4 is based on tri-s-triazine building blocks [42]. The strongest peak at 27.3◦ is close to the peak position of graphite (0 0 2), and is due to the interlayer stacking of aromatic systems. The calculated interplanar distance of the aromatic units (d = 0.327 nm) is smaller than that for crystalline gC3 N4 (d = 0.34 nm) [43]. This is attributed to electron localization and stronger binding between the layers in the former case [30]. The small angle peak at 13.2◦ , corresponding to 0.671 nm, is associated with interlayer stacking. This distance is smaller than the size of one tris-s-triazine unit (0.713 nm), which can be attributed to the presence of a small tilt angularity in the structure [23,30]. With increasing WO3 content of the composite samples, the peaks assigned to g-C3 N4 became weaker. It is difficult to confirm the presence of the peak at 27.3◦ in the XRD patterns of the G4W6 and
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Fig. 2. FT-IR spectra for the synthesized g-C3 N4 , WO3 and prepared composite samples.
Fig. 1. XRD patterns for the synthesized g-C3 N4 and prepared composite samples. The standard pattern for WO3 (monoclinic; ICDD#00-043-1035) is shown at the bottom for reference.
G2W8 samples. If the interlayer stacking is not preserved in the G4W6 and G2W8 samples, the specific surface area of the samples becomes high due to exfoliation of the g-C3 N4 sheets. However, the specific surface area of the samples is almost the same, as shown in Table 1. Therefore, we consider that the interlayer stacking is preserved in the G4W6 and G2W8 samples. The XRD patterns for WO3 can be assigned to a monoclinic structure. Fig. 2 shows IR spectra of g-C3 N4 , WO3 , and the composite samples. Several bands in the 1200–1650 cm−1 region were found, which correspond to the typical stretching modes of CN heterocycles [44,45]. The sharp band at 810 cm−1 and the broad bands at around 3000 cm−1 were indicative of the breathing mode of the triazine units and the NH stretching vibration modes, respectively [30,43]. This indicates that amino groups exist in the samples. Zhao et al. [46] reported that residual hydrogen atoms bind to the edges of the graphene-like C N sheet in the form of C NH2 and 2C NH bonds. It is considered that a small amount of the amino groups remains in the samples synthesized by direct heating of the melamine and dicyanodiamide mixture. The absorption assigned to CN heterocycles became weaker with increasing WO3 content in the composite samples. Additionally, a luminescence peak at
Table 1 Specific surface area, optical band gap, CH3 CHO degradation ratio, and CO2 generation ratio for the synthesized g-C3 N4 , WO3 , prepared composite samples, and Cu(II)-grafted TiO2 . Sample
SSAa (m2 /g) Band gapb (eV) CH3 CHO degradation C/C0 c (%)
CO2 generation C/Ctheory d (%)
g-C3 N4 G8W2 G6W4 G4W6 G2W8 WO3 Cu(II)-grafted TiO2
7 6 5 5 4 4 6
16 20 24 29 45 28 26
a
2.70 2.73 2.73 2.75 2.73 2.65 3.02
72 69 72 65 100 100 87
Specific surface area. Optical band gap estimated by UV–vis spectra. c (CH3 CHO con. after 24 h)/(initial CH3 CHO con.). d (Generated CO2 con. after 24 h)/(theoretical CO2 con. after completely CH3 CHO decomposition). b
470 nm was observed for the composite samples, and its intensity also became weaker with increasing WO3 content (Fig. S1). Figs. 3 and 4 show SEM and TEM images of the g-C3 N4 , WO3 , and composite samples. The surface morphology of the synthesized gC3 N4 seemed to be smooth, consisting not of particles but of sheets. Indeed, the sheet-like morphology was confirmed by TEM observations of the g-C3 N4 (Fig. 4). The morphology of the synthesized g-C3 N4 was due to aggregation and gathering of the g-C3 N4 sheets. The WO3 sample had a particle size of 100–300 nm, and a grainlike morphology with polygonal grain shapes. The morphology of the WO3 sample was similar to that of monoclinic WO3 particles prepared by a sol–gel method [47]. In the composite samples, WO3 particles were found to cover the g-C3 N4 surface. Various WO3 particles, which did not agglomerate, were sparsely observed on the g-C3 N4 surface in the G8W2 sample, and almost all particles were in direct contact with the g-C3 N4 surface. With increasing WO3 content in the composite samples, the area covered by the WO3 particles increased on the g-C3 N4 surface in the G6W4 and G4W6 samples. In the G2W8 sample, WO3 particles almost entirely covered the g-C3 N4 surface. Although the WO3 particles were partly agglomerated, they were in direct contact with the g-C3 N4 surface in the G4W6, G6W4, and G8W2 samples (Fig. 4). The optical properties of the g-C3 N4 , WO3 , and composite samples were examined by UV–vis diffuse reflectance spectroscopy (Fig. 5). For all samples, the optical absorption edge was estimated to be at around 480 nm. This is consistent with the pale yellow color of the samples. In the case of the g-C3 N4 sample, slight absorption was observed at wavelengths of more than 480 nm. Conventional carbon nitride shows the typical absorption pattern of an organic semiconductor with very strong adsorption at about 420 nm [11]. Yan et al. [30] reported that the synthesis method has a slight effect on the absorption edge of carbon nitride, and this is attributed to differences in the local structure, packing, and defects formed during the synthesis processes. It is considered likely that the synthesized g-C3 N4 can absorb visible light since it contains very few impurities (amino groups). The optical band gap can be estimated using the general procedure from the absorption spectra from the following equation [48]: ˛h = A(h − Eg )
n/2
(1)
where A, ˛, h and Eg are, respectively, a constant, the absorption coefficient, the photon energy and the optical band gap. In this equation, n is determined by the transition type; in the case of direct and indirect transitions, the n value is 1 and 4, respectively. The n and Eg values were estimated by the following steps: first,
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Fig. 3. SEM images for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
a plot of ln(˛h) vs. ln(h − Eg ) was created using an approximate value of Eg , and then the value of n was estimated from the slope of the straight line near the band edge; second, a plot of (˛h)2/n vs. h was created and a tangential line was plotted near the band edge, with the x-intercept of the tangential line corresponding to the optical band gap. The estimated band gaps for g-C3 N4 , G8W2, G6W4, G4W6, G2W8, and WO3 were 2.70, 2.73, 2.73, 2.75, 2.73, and 2.65 eV, respectively (Table 1 and Fig. S2). These values were in good agreement with the reported values for g-C3 N4 and WO3 [11,39]. Since the estimated band gaps for all samples were almost the same, the photon energies used in the photocatalysis experiments were almost identical.
3.2. Photocatalytic activity of g-C3 N4 , WO3 , and g-C3 N4 /WO3 composites Fig. 6 shows the results of acetaldehyde (CH3 CHO) degradation under visible light irradiation in the presence of the synthesized gC3 N4 , WO3 and prepared composite samples. Prior to visible light
irradiation, the initial CH3 CHO concentration for all the samples decreased below the injected CH3 CHO concentration (5.6 mol) introduced into the reaction vessel; this effect become more marked with increasing g-C3 N4 content. This indicates that the gC3 N4 plays an important role as an adsorbent. After visible light irradiation, the CH3 CHO concentration was drastically reduced for the WO3 and G2W8 samples due to photodegradation by the photocatalyst, and after irradiation for 24 h, the CH3 CHO was completely degraded. On the other hand, the CH3 CHO concentration for the G4W6, G6W4, G8W2, and g-C3 N4 samples gradually decreased with increasing visible light irradiation time. The degree of degradation of CH3 CHO depended on the catalyst. Upon exposure to visible light, the CH3 CHO concentration decreased. In the presence of the photocatalyst, the oxidative photodegradation of CH3 CHO proceeds as follows [49]: CH3 CHO(ads.) + 3H2 O + 10 h+ → 2CO2 + 10H+
(2)
CH3 CHO(ads.) + H2 O + 2 h+ → CH3 COOH(ads.) + 2H+
(3)
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Fig. 5. UV–vis diffuse reflectance spectra for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
CH3 COOH(ads.) + 2H2 O + 8 h+ → 2CO2 + 8H+
(4)
where h+ is a hole generated by photoinduced charge separation in the photocatalyst. One CH3 CHO molecule decomposes into two CO2 molecules, which is the final product. Although the g-C3 N4 sample adsorbed the most CH3 CHO, the photodegradation rate of CH3 CHO was lower than that for the other samples. This is attributed to the fact that the photocatalytic efficiency of bare g-C3 N4 is limited due to the high recombination rate of photogenerated electron–hole pairs [50]. With increasing WO3 content, the CH3 CHO photodegradation rate became higher, which indicates that WO3 is strongly associated with photodegradation of CH3 CHO. The photoluminescence peak at 470 nm formed by electron–hole recombination in g-C3 N4 , and the peak intensity of the G2W8 sample was less than a quarter of that for the G8W2 sample (Fig. S1). It is considered that the electron–hole recombination might be reduced by the increase in interfaces between g-C3 N4 and WO3 .
Fig. 4. TEM images for the synthesized g-C3 N4 , WO3 , and prepared composite samples.
Fig. 6. Change in CH3 CHO concentration as a function of visible light irradiation time in the presence of the synthesized g-C3 N4 , WO3 , and prepared composite samples.
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vapor in air. The water molecules can be transformed into • OH by reaction with photogenerated holes (h+ ) or superoxide radicals (O2 •− ) at the photocatalyst surface via the following reactions [51]: h+ + H2 O → • OH + H+
(5)
2O2 •− + 2H2 O → 2• OH + 2OH− + O2
(6)
In addition, hydrogen peroxide (H2 O2 ) is also known to be a strong oxidant. Oxygen molecules can be transformed into H2 O2 by reaction with photoexcited electrons at the WO3 surface via the following reaction [52]: O2 + 2H+ + 2e− → H2 O2
Fig. 7. Change in concentration of CO2 generated by the degradation of CH3 CHO as a function of visible light irradiation time in the presence of the synthesized g-C3 N4 , WO3 , and prepared composite samples.
Fig. 7 shows the concentration of CO2 generated from the degradation of CH3 CHO under visible light irradiation in the presence of the composite, synthesized g-C3 N4 , and WO3 samples. When the samples were irradiated with visible light, the CO2 concentration in all cases began to increase, indicating that CH3 CHO was decomposed into CO2 , the final product. After visible light irradiation for 24 h, the CO2 concentration of the g-C3 N4 sample was the lowest among all samples. With increasing WO3 content, the CO2 concentration became higher, in agreement with the results for the photodegradation of CH3 CHO (Fig. 6). The G2W8 sample exhibited higher activity for photodegradation and photodecomposition of CH3 CHO into CO2 , indicating that the G2W8 sample is a photocatalyst with excellent photocatalytic performance under visible light irradiation. On the other hand, the CO2 concentration for the WO3 sample was lower than that for the G2W8 and G4W6 samples, whereas it exhibited the highest CH3 CHO photodegradation rate among all samples. These results indicate that the photodegradation activity of CH3 CHO was not necessarily correlated with the photodecomposition of CH3 CHO into CO2 , and WO3 produces larger intermediate products before photodecomposition to CO2 compared to G2W8. In some cases, the intermediate products also have adverse effects on the health of humans. It is thus important that acetaldehyde be decomposed into CO2 under visible light. Since the photocatalytic reaction occurred on the crystal surface, the photocatalytic activity is dependent on the specific surface area. The specific surface areas of the samples are shown in Table 1, and they are all in the range of 4–7 m2 /g. Therefore, it is considered that the specific surface area of the samples did not significantly affect the photocatalytic activity. In addition, the band gap energies of the samples were almost the same (Table 1), indicating that the number of absorbed photons was almost the same for all samples. 3.3. Enhanced visible-light-driven mechanism for g-C3 N4 /WO3 composites Hydroxyl radicals (• OH) are known to be strong oxidants, contributing to the degradation of CH3 CHO in the presence of water
(7)
When the g-C3 N4 /WO3 composite samples were irradiated with visible light, electrons were excited from the valence band (VB) of g-C3 N4 and WO3 to the conduction band (CB) of g-C3 N4 and WO3 , respectively, which left holes in the VB in both materials, as shown in Scheme 1. Some of the photoinduced electrons in the g-C3 N4 diffuse to the g-C3 N4 surfaces, and they react with O2 to produce O2 •− . As described by reaction (6), hydroxyl radicals (• OH) are then formed by the reaction of O2 •− with H2 O. Other photoinduced electrons in g-C3 N4 are transferred to the CB of WO3 because the CB bottom of g-C3 N4 (−1.13 V vs. SHE) is more negative than that of WO3 (+0.5 V vs. SHE) [23,35,52]. Since the potential for multi-electron reduction of O2 , as described by reaction (7), is more positive (O2 + 2H+ + 2e− = H2 O2 (aq), +0.682 V) than the CB bottom of WO3 , electrons transferred from the CB of g-C3 N4 to the CB of WO3 , and photoinduced electrons in the WO3 , react with O2 to produce H2 O2 . Some of the photogenerated holes in WO3 , which has a high oxidation potential (+3.15 V vs. SHE), diffuse to the WO3 surfaces, and they directly degrade the CH3 CHO absorbed on the WO3 particles or react with surface-bound H2 O to produce hydroxyl radical species, • OH. Other photogenerated holes in WO3 are transferred from the VB of WO3 (+3.15 V vs. SHE) to the VB of g-C3 N4 (+1.57 V vs. SHE), and these transferred holes and photogenerated holes in g-C3 N4 directly degrade the CH3 CHO absorbed on the g-C3 N4 particles. Active oxygen species such as • OH, O •− , and H O generated on the g-C N and WO surface 2 2 2 3 4 3 are released, diffusing through the gas phase and oxidizing the CH3 CHO. The activities for the photodegradation of CH3 CHO for g-C3 N4 , WO3 , the composite samples, and Cu(II)-grafted TiO2 are summarized in Table 1. Cu(II)-grafted TiO2 is a photocatalyst which exhibits high photocatalytic activity under visible light irradiation [53,54]. In this case, Cu(II)-grafted TiO2 was prepared so that the specific surface area was almost the same value as that of the composite samples (see Supplementary data for preparation detail). As the band gap of Cu(II)-grafted TiO2 was larger than those of the composite samples, visible light intensity was increased so that the number of absorbed photons for Cu(II)-grafted TiO2 was almost the same as that for the composite samples. The G2W8 sample exhibited the highest activity for photodegradation of CH3 CHO and photodecomposition of CH3 CHO into CO2 among all samples containing Cu(II)-grafted TiO2 . This result indicated that G2W8 is a very effective visible-light-driven photocatalyst, compared to other visible-light-driven photocatalysts. In the g-C3 N4 /WO3 composite, the WO3 content was thus pivotal for achieving high photocatalytic activity. An appropriate WO3 content caused good dispersion and contact on the g-C3 N4 surface, which favored the transfer and separation of charge carriers. However, for WO3 content lower than 80 mass%, the number of interfaces and heterojunction structures between the g-C3 N4 and WO3 particles decreased. As a result, interfacial charge transfer was suppressed and photocatalytic activity was reduced. Thus, the G2W8 sample showed the best performance. We believe this to be a
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Scheme 1. Possible degradation mechanism of CH3 CHO over a g-C3 N4 /WO3 composite photocatalyst under visible light irradiation.
very important result because this material exhibits photocatalytic activity under a variety of environments, in particular, under visible (fluorescent) light. In addition, the present g-C3 N4 /WO3 composites can be prepared by a very simple technique: physical mixing using a mortar. The composites exhibit very efficient CH3 CHO photodecomposition under visible light irradiation. This suggests that these results should usefully expand the application of g-C3 N4 as a visible-light-driven photocatalyst. 4. Conclusion Graphitic carbon nitride (g-C3 N4 ) with a nanosheet structure was synthesized and used to prepare g-C3 N4 /WO3 composites. The photocatalytic degradation of CH3 CHO using these composites was investigated. The optical band gap and specific surface area of the composites were almost the same. Under visible light irradiation, the CH3 CHO photodegradation by the g-C3 N4 /WO3 composite was greater than that for g-C3 N4 alone. With increasing WO3 content of the composites, the photodegradation effect became larger, indicating that WO3 is strongly associated with the photodegradation of CH3 CHO. The optimum mixture of g-C3 N4 and WO3 for photocatalysis corresponded to the composition G2W8. A CH3 CHO photodegradation mechanism was proposed and discussed in terms of energy band positions. The present g-C3 N4 /WO3 composite is thought to be a very effective visible-light-driven photocatalyst and could be used to completely decompose or remove VOCs from air. Acknowledgements The authors are grateful to Prof. Y. Kitamoto (Tokyo Institute of Technology, Japan) for the TEM observations, to Prof. F. Wakai and Prof. Y. Shinoda (Tokyo Institute of Technology, Japan) for the FE-SEM observations, to Prof. T. Akatsu (Tokyo Institute of Technology, Japan) for the XRD, to Mr. Y. Makinose (Tokyo Institute of Technology, Japan) for the TEM observations, and to Ms. H. Takei (Kanagawa Academy of Science and Technology, Japan) for helpful support for using the N2 -BET and UV–vis instruments. This work was supported in part by the Nippon Sheet Glass Foundation for Materials Science and Engineering, and by the project “Development Base of Advanced Materials Development and Integration of Novel Structured Metallic and Inorganic” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013. 05.058.
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