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Preparation of Bi2WO6– and BiOI–allophane composites for efficient photodegradation of gaseous acetaldehyde under visible light
Applied Clay Science 101 (2014) 38–43
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Preparation of Bi2WO6– and BiOI–allophane composites for efficient photodegradation of gaseous acetaldehyde under visible light Mirabbos Hojamberdiev a,b,⁎, Ken-ichi Katsumata a, Nobuhiro Matsushita a, Kiyoshi Okada a a b
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380–-8553, Japan
a r t i c l e
i n f o
Article history: Received 9 September 2013 Received in revised form 5 July 2014 Accepted 9 July 2014 Available online 7 August 2014 Keywords: Clays Aluminosilicates Bismuth tungstate Bismuth oxyiodide Allophane Photodegradation
a b s t r a c t Clays have been used in combination with various photocatalysts to enhance the removal of organic pollutants. The aim of the present work was to study the impact of allophane as the support material on photocatalytic activity of Bi2WO6 and BiOI. In this work, we report on the preparation of highly adsorptive mechanicallymixed and as-synthesized Bi2WO6– and BiOI–allophane composites by mechanical mixing and hydrothermal synthesis, respectively. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (Raman), ultraviolet–visible spectroscopy (UV–vis), and BET method. The allophane powders showed higher specific surface area, SSA, (SBET = 263 m2/g) in comparison with the Bi2WO6 (SBET = 21.3 m2/g) and BiOI (SBET = 23.1 m2/g) powders. The mechanically-mixed composites possessed higher SSA compared to the as-synthesized composites probably due to the pore collapse or pore filling during the hydrothermal synthesis of the composites. The light absorption by both composites started from 700 nm; however, the Bi2WO6–allophane composites only absorbed the photons N 325 nm, whereas the BiOI–allophane composites absorbed the photons N 500 nm. The samples with higher SSA showed higher acetaldehyde adsorption. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI–allophane composites decomposed acetaldehyde completely within 5–7 h, whereas the Bi2WO6, mechanically-mixed and as-synthesized Bi2WO6–allophane composites decomposed 75.5%, 100%, and 85.6% within 8 h, respectively. With its significant adsorption of acetaldehyde, allophane contributed to the efficient photodegradation of acetaldehyde by allophane-containing composites. The obtained results suggested that both mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites can be useful material for efficient photodegradation of acetaldehyde under visible light. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since the Honda–Fujishima effect was first reported in 1972 (Fujishima and Honda, 1972), semiconductor-based photocatalysis has been acknowledged as a potential method for renewable energy generation and environmental remediation. Because of negligible activity of TiO2 under visible light, there have been intense scientific efforts to develop novel highly efficient visible light-responsive photocatalysts because visible light accounts for ca. 43% of the incoming solar energy (Chatterjee and Dasgupta, 2005). Moreover, for the purification of indoor air, the photocatalysts should induce the oxidative decomposition of organic pollutants under visible light
⁎ Corresponding author at: Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan. Tel.: +81 45 924 5369; fax: +81 45 924 5358. E-mail addresses: [email protected], [email protected] (M. Hojamberdiev).
http://dx.doi.org/10.1016/j.clay.2014.07.007 0169-1317/© 2014 Elsevier B.V. All rights reserved.
emitted from indoor lighting devices, such as fluorescent light tubes (Amano et al., 2008). Bismuth tungstate (Bi2WO6) is one of the simplest members of the Aurivillius oxide family of layered perovskites, which are structurally composed of alternating perovskite-like and fluorite-like blocks. It has several important physical properties, including piezoelectricity, pyroelectricity, oxide anion conductivity, nonlinear dielectric susceptibility, and luminescent property. As an n-type semiconductor with the band gap of 2.80 eV, Bi2WO6 has shown excellent photocatalytic activity for O2 evolution and for mineralizing CHCl3 and CH3CHO into CO2 under visible light (Kudo and Hijii, 1999; Amano et al., 2010). Bismuth oxyiodide (BiOI), another member of the Aurivillius oxide family, crystallizes in the tetragonal structure, and its layer structure is characterized by the [Bi2O2] slabs interleaved by double slabs of iodine atoms. BiOI is a p-type semiconductor with the band gap of 1.7 − 1.83 eV, and it can absorb the most visible light (λ b 700 nm) (Wang et al., 2011). Since the photodegradation reaction takes place on a photocatalyst surface, adsorption becomes one of the important criteria for an efficient photodegradation of organic pollutants. A feasible approach to achieve
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this is the use of adsorbents with large SSA as supporting materials for loading photocatalyst particles. So far, a wide variety of porous materials, such as silica gels, zeolites, activated carbons, and clay minerals, exhibiting large SSA have been regarded to be excellent supports for photocatalyst nanoparticles. Because of their outstanding adsorption capacity for organic pollutants and tailorable surface properties, natural and synthetic clays have been combined with photocatalysts to improve the removal of organic pollutants in contaminated water and air. According to the experimentally determined adsorption rates, significant amounts of volatile organic compounds (VOCs) were adsorbed onto internal clay surfaces, with inter-particle and intra-particle diffusional time constants spanning two orders of magnitude (Morrissey and Grismer, 1999). Embedding TiO2 particles in montmorillonite (Kameshima et al., 2009), imogolite (Katsumata et al., 2013), kaolinite and hectorite (Kibanova et al., 2009), etc. favored the adsorption and enhanced the photodegradation of organic pollutants. Allophane, a natural clay mineral, is a hydrated aluminosilicate (1–2SiO2 · Al2O3 · 5–6H2O) having a 3.5–5.0 nm-sized hollow spherical structure with 0.3–0.5 nm-sized defects on its surface. The walls of the hollow spheres consist of inner silica and outer alumina layers with hydroxylated or hydrated surfaces. These surfaces have the ability to adsorb ionic or polar pollutants due to their amphoteric ion-exchange activity and large specific surface area (Wada, 1967; Kitagawa, 1971; Okada, et al., 1975). In this work, allophane was selected as a clay support for loading Bi2WO6 and BiOI photocatalyst particles. The mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing and hydrothermal synthesis, respectively. Adsorption and photocatalytic activity of the prepared samples were evaluated by adsorption and photodegradation of gaseous acetaldehyde (AcH) in the dark and under visible light irradiation, respectively. 2. Experimental 2.1. Materials Bi(NO3)3 · 5H2O, KI, Na4SiO4, AlCl3 · 6H2O, NH4OH, and ethylene glycol were obtained from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). Na2WO4 was obtained from Strem Chemicals, Inc. (Newburyport, MA, USA). All the chemical reagents were of analytical grade and used as received without further purification. Deionized water from Millipore Milli-Q Plus purification system (18.2 MΩ cm) was used throughout the experimental process.
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mechanically-mixed Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing of allophane and Bi2WO6 (1:1 mass ratio) and allophane and BiOI (1:1 mass ratio) using an agate mortar and pestle, respectively. 2.3. Characterization The formation of allophane, Bi2WO6, and BiOI in the composites was confirmed by X-ray diffraction (XRD) using an RINT-2100 diffractometer (Rigaku, Japan) with monochromated Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 40 mA. The powder samples were scanned at a scanning rate of 2°/min in the 2θ range of 5–70°. The crystalline phases in the composites were also identified using a T64000 Raman spectrometer (Horiba Jobin Yvon S.A.S., France) with an Ar laser (514.5 nm) operated at 50 mW. The particle size and morphology of the composites were examined using an S-4500 field emission-scanning electron microscope (Hitachi, Japan) and a JEM-2010UHR transmission electron microscope (JEOL, Japan) operating at an acceleration voltage of 200 kV. The UV– vis diffuse reflectance spectra of the composites were recorded on a Lambda 950 UV/VIS/NIR spectrophotometer (Perkin-Elmer, USA). The SSA (SBET), total pore volume (Vt), and pore size distribution (PSD) were obtained from N2 adsorption–desorption isotherms at 77 K (Autosorb-3B, Quantachrome, USA) on the samples degassed at 120 °C for 6 h in vacuum. The SBET values were obtained by using the Brunauer, Emmett and Teller (BET) method, and the PSDs were determined by the Barrett, Joyner and Halenda (BJH) method using the N2 adsorption– desorption isotherms. 2.4. Photodegradation experiments The degradation of gaseous acetaldehyde (CH3CHO) was carried out at room temperature in a batch-type reactor using the powder samples. The powder sample (50 mg) was placed in the reaction vessel made of Pyrex® glass with the volume of 500 mL. Pure air (Taiyo Nippon Sanso Corp., Japan) was blown through the reaction vessel to remove any air contaminants. Then, 4.5 μmol (200 ppm) of acetaldehyde was introduced into the firmly closed reaction vessel using a Pressure-Lok® precision analytical syringe. After adsorption equilibration in the dark for 12 h, the reactor was irradiated by a LA-251Xe lamp (Hayashi Watch Works, Japan) with the wavelength range of 520–600 nm. The decrease in acetaldehyde concentration and increase in CO2 concentration were monitored using a GC-2014 gas chromatograph (Shimadzu, Japan), equipped with a 2 m Porapak-Q column, methanizer and a flame ionization detector.
2.2. Preparation 3. Results and discussion To synthesize allophane, Na4SiO4 (0.1 mol/L) and AlCl3 · 6H2O (0.1 mol/L) aqueous solutions were mixed together under stirring (the pH measured was 3.5) for 2 h, and then the solution was transferred into a 40 mL Teflon-lined stainless steel autoclave. After the hydrothermal treatment at 95 °C for 90 h, the resulting precipitate was collected by centrifugation, washed with deionized water and dried at 80 °C for 8 h. To obtain Bi2WO6 or BiOI powders, 5 mmol of Bi(NO3)3 · 5H2O and 2.5 mmol of Na2WO4 (or 5 mmol KI) were separately dissolved in 15 mL of ethylene glycol and 15 mL of deionized water, respectively, and then mixed under vigorous stirring. The pH of the mixed solution was adjusted to 7 with the addition of ammonium hydroxide (NH4OH). After being stirred for 30 min, the solution was transferred into a 40 mL Teflon-lined stainless steel autoclave. After the hydrothermal treatment at 180 °C for 12 h, the resulting precipitate was collected by centrifugation, washed with deionized water to remove unreacted ions and dried at 80 °C for 8 h. Finally, to synthesize the Bi2WO6– and BiOI–allophane composites, the identical synthesis procedure used for the synthesis of Bi2WO6 or BiOI powders was applied with the addition of allophane powders (0.3489 g in 10 mL deionized water, sonicated for 15 min) into the precursor solutions. The
The XRD patterns of the mechanically-mixed and as-synthesized Bi2WO6–allophane (denoted as BW:Allo) and BiOI–allophane (denoted as BI:Allo) composites are shown in Fig. 1. All the reflections in the XRD patterns of the mechanically-mixed and as-synthesized Bi2WO6– allophane composites can be assigned to orthorhombic Bi2WO6 with the space group of Pbca(61) (JCPDS card no. 39-0256). No traces of other phases were detected in the composite. The XRD patterns of the mechanically-mixed and as-synthesized BiOI–allophane composites show that all the reflections, without any other characteristic reflections of impurity phases, match with the previously reported data for tetragonal BiOI with the space group of P4/nmm(129) (JCPDS card no. 10-0445). No reflections assignable for other phases were detected. The XRD patterns of the mechanically-mixed composites show sharper and narrower reflections, compared to that of the as-synthesized composites because of slightly higher crystallinity or larger crystal size. Slightly broader reflections with lower intensity suggest that the as-synthesized composites may have smaller crystal size. As the intensity of the reflections of the Bi2WO6 and BiOI phases is high, it is difficult to note the main halos of amorphous allophane (JCPDS card
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BI:Allo mech.mix.
BI:Allo as-synthesized
Intensity [a.u.]
BW:Allo mech.mix.
BW:Allo as-synthesized
BiOI Bi2WO6 Allophane 10
20
30
40
50
60
70
2θ [deg.] Fig. 1. XRD patterns of the as-synthesized and mechanically mixed Bi2WO6– and BiOI– allophane composites.
no. 38-0449) at 2θ = 27° and 40°, corresponding to (002) and (040), in the XRD patterns. Nevertheless, we consider that the prepared composites consisted of allophane, Bi2WO6, and BiOI. Figs. 2 and 3 show the SEM and TEM images of the mechanicallymixed and as-synthesized Bi2WO6–allophane and BiOI–allophane composites. It is clear from Figs. 2 and 3 that the as-synthesized Bi2WO6–allophane composite powders have two different morphologies, that is, sphere-like Bi2WO6 particles with the diameter of less than 1 μm are homogenously distributed along with large allophane particles with layered structures (flakes). As can be noted, some sphere-like Bi2WO6 particles are attached onto the surface of allophane particles. Although the as-synthesized BiOI–allophane composite powders have no well-marked morphology, one can easily see that
BiOI and allophane particles (possibly quasi-spherical) with less than 1 μm are uniformly distributed in the composite (Fig. 3). Compared to that of the as-synthesized composites, the mechanicallymixed Bi2WO6– and BiOI–allophane composites have different morphologies, that is, a homogenous distribution of agglomerated and unagglomerated small particles of allophane, Bi2WO6, and BiOI. It is believed that the mechanical mixing of Bi2WO6 and BiOI with allophane using an agate mortar and pestle caused the reduction in particle size (Fig. 3). The agglomeration of small particles might have been induced by heat generated during the mechanical mixing. Although the SEM images show that some Bi2WO6 and BiOI particles are partially separated from allophane, it is believed that most of the Bi2 WO 6 and BiOI particles are still in contact with allophane particles. Fig. 4a shows the Raman spectra of the mechanically-mixed and assynthesized Bi2WO6– and BiOI–allophane composites. Since allophane is amorphous, we only can see the Raman spectra of Bi2WO6 and BiOI in the composites. The Raman spectra of Bi2WO6 with the orthorhombic Pbca(61) space group shown in Fig. 4a are similar to that of the orthorhombic Pca21 space group (Maczka et al., 2008a); therefore, the Raman peaks of the Pbca(61) phase can be readily assigned to those modes of the Pca21 phase. The number of optic modes for the Pca21 phase is 26A1 + 27A2 + 26B1 + 26B2. The A2 modes are Raman active and the A1, B1, and B2 modes are both IR and Raman active (Maczka et al., 2008b). The Raman peaks at 788 cm−1 and 703 cm−1 can be assigned to the symmetric and asymmetric stretching modes of the WO6 octahedra involving motions of the apical and equatorial oxygen atoms perpendicular to and within layers, respectively. The Raman peaks at 412, 300, 278, 256, 215, and 187 cm−1 can be attributed to the bending modes of the WO6 octahedra and the stretching and bending modes of the BiO6 polyhedra. The Raman peaks at 140 cm−1 and 147 cm−1 may be assigned to the translations of the tungsten and bismuth ions (Maczka et al., 2008a). BiOI with the space group of D74h has two molecular formulae per unit cell with the Raman active modes of A1g, B1g, and Eg. The Raman peaks at ~ 85 cm− 1, not shown here, are assigned to the A1g internal Bi–I stretching mode. The other Raman
BW:Allo
BW:Allo
(as-synth.)
(mech.mix.) thesized)
Allophane Bi2WO6
3 µm
3 µm
BI:Allo
BI:Allo
(as-synth.)
(mech.mix.)
Allophane BiOI
3 µm
3 µm
Fig. 2. SEM micrographs of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites.
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(b)
(a)
Allophane
Bi2WO6
Allophane
40 nm
500 nm (d)
(c)
BiOI Allophane
500 nm
500 nm
Fig. 3. TEM images of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites.
peaks observed at 145 cm− 1 are assigned to the Eg internal Bi–I stretching mode (Davies, 1973). Similar to the XRD data, low intensity in the Raman spectra was noticed for the as-synthesized composites, confirming slightly higher crystallinity or larger crystal size of the mechanically-mixed composites. The mode-assignable Raman peaks were not clearly observed for allophane due to the defects in the structure (Creton et al., 2008). The nitrogen adsorption–desorption isotherms of Bi2WO6, BiOI, allophane, Bi2 WO 6:allophane, and BiOI:allophane powders are shown in Fig. 4b. Nitrogen adsorption measurements of allophane, Bi2WO6:allophane and BiOI:allophane show high N2 adsorption and a typical type IV isotherm with a single hysteresis loop in IUPAC classification, with a step at about P/P0 = 0.7, which is associated with capillary condensation in mesopores. According to the IUPAC classification, the N2 adsorption–desorption isotherms of the Bi2WO6 and BiOI powders are type II, describing adsorption on non-porous or macroporous adsorbent with strong adsorbate–adsorbent interactions. The porous properties of the samples are given in Table 1. As expected, the allophane powders possess higher SSA (SBET = 263 m2/g) in comparison to the Bi2WO6 (SBET = 21.3 m2/g) and BiOI (SBET = 23.1 m2/g) powders. As can be seen in Table 1, the mechanically-mixed composites have four times higher SSA compared to the as-synthesized composites. The reason for the higher SBET values of the mechanically-mixed composites may be that the freshly synthesized allophane powders with high SBET value were mechanically mixed directly with Bi2WO6 or BiOI powders without being subjected to an additional hydrothermal treatment, which may also cause the collapse of pores, with the precursor solutions of Bi2WO6 or BiOI, of which the precipitated tiny particles might have also blocked the pores. The considerable reduction in the SBET values of the as-synthesized composites is unclear but we may
still relate it to the pore collapse or pore filling during the hydrothermal synthesis. Fig. 5a shows the UV–vis diffuse reflectance spectra of the mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites. As can be seen, the Bi2WO6–allophane composites have a milky white color, evidencing that they can partially absorb the wavelengths of visible light. The color of the BiOI–allophane composites is reddish orange, and the composites can absorb most of the wavelengths of visible light. The light absorption in both composites starts from ca. 700 nm. However, the Bi2WO6–allophane composites only absorb the photons N 325 nm, and the absorption edge is about 495 – 503 nm. In contrast, the BiOI–allophane composites absorb the photons N500 nm, and the absorption edge is about 675 – 685 nm. The optical band gaps of the prepared composites were estimated by the general procedure using the absorption spectra from the following equation: n=2 ahv ¼ A hv−Eg
ð1Þ
where α, hν, Eg, and A are the absorption coefficient, the photon energy, the optical band gap, and a constant, respectively (Butler, 1977). As known, n decides the characteristics of the transition in a semiconductor. According to the equation, the value of n for Bi2WO6 and BiOI is 1 (direct transition) and 4 (indirect transition). The optical band gaps of the mechanically-mixed and as-synthesized Bi2WO6–allophane composites are 2.81 eV and 2.77 eV, respectively, whereas the mechanically-mixed and as-synthesized BiOI–allophane composites have 1.89 eV and 1.84 eV, respectively. A slight difference in absorption edges and optical band gaps between the mechanically-mixed and assynthesized composites can be noted. We believe that this difference
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100
(a)
BI:Allo mech.mix.
(a)
90
Intensity [a.u.]
80
BI:Allo Reflectance [%]
70
as-synth.
BW:Allo mech.mix.
BW:Allo
BW:Allo
60
(mech.mix.)
BI:Allo
50
(mech.mix.)
40 BW:Allo
30
(as-synth.) (as-synthesized)
20
BI:Allo
10
as-synth.
(as-synth.) (as-synthesized)
0 300
200
400
600
800
500
CH3CHO concentration [µmol]
400
Allophane BiOI Bi2WO6 BiOI:Allophane (mech.mix.) Bi2WO6:Allophane (mech.mix.)
(b)
300 200 100 0 0.0
0.2
0.4
0.6
0.8
700
800
Visible light on
4.5
500
600
Wavelength [nm]
Raman shift [cm-1]
N2 adsorption at STP [cm3 g-1]
400
1000
1.0
(b)
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -12
0
2
4
6
8
Irradiation time [h]
P/P0 Fig. 4. (a) Raman spectra of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites and (b) N2 adsorption–desorption isotherms of Bi2WO6, BiOI, and allophane.
might have resulted from the differences in the local structure, packing, and defects formed during the synthesis processes. Adsorption and photocatalytic activity of the prepared samples are evaluated for the adsorption and photodegradation of gaseous acetaldehyde (CH3CHO), a typical indoor air contaminant, in the dark and under visible light, respectively. The results are shown in Fig. 5b. The concentration of the injected gaseous acetaldehyde decreased monotonously by adsorption mainly according to the SSA of the samples. That is, the higher SSA the higher adsorption of acetaldehyde. Particularly, allophane and mechanically-mixed allophane-containing samples showed higher adsorption capacity for acetaldehyde. This evidences the particular role of allophane, as part of composite, in the adsorption of acetaldehyde. However, it should be noted that the surface of allophane is hydrophilic, and therefore the already adsorbed water molecules might hamper the adsorption of acetaldehyde. As shown in Fig. 5b, the acetaldehyde concentration started to decrease after visible light irradiation. In the case of allophane, this decrease was negligible because Table 1 The compositions and porous properties of samples. Sample
SBET [m2/g]
Vp [ml/g]
Pore size [nm]
Bi2WO6 (BW) BiOI (BI) Allophane (Allo) BW:Allo (mech.mix.) BW:Allo (as-synth.) BI:Allo (mech.mix.) BI:Allo (as-synth.)
21.3 23.1 263.0 224.0 55.1 186.0 21.7
0.113 0.159 0.789 0.689 0.185 0.561 0.126
14.1 25.8 7.98 4.52 26.5 10.0 25.8
Fig. 5. (a) UV–vis diffuse reflectance spectra of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites and (b) change of CH3CHO concentration (C0 = 4.5 μmol) by reaction with the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites. Keys: □ — allophane; ○ — mechanically-mixed BiOI–allophane composite; △ — as-synthesized BiOI–allophane composite; ▽ — BiOI; ⋄ — mechanically-mixed Bi2WO6–allophane composite; E — as-synthesized BiOI–allophane composite; ✯ — Bi2WO6.
allophane cannot exhibit photocatalytic activity under visible light despite its adsorption of 37.8% of acetaldehyde. Although the total photodegradation of acetaldehyde was somehow favored by the presence of allophane in the allophane-containing composites, it mainly depended on the band structures of Bi 2 WO 6 and BiOI. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI-based composites completely mineralize acetaldehyde within 5–7 h, whereas the Bi 2WO6 and mechanically-mixed and as-synthesized Bi2WO6-based composites mineralize 75.5%, 100%, and 85.6% within 8 h, respectively. The profound photocatalytic activity of the BiOI and BiOI-based composites is related to the band structure of BiOI, allowing the samples to absorb a greater number of photons from visible light (N 500 nm). Bi 2WO 6 and Bi2 WO 6-based composites only absorb the limited number of photons from visible light (N 325 nm). As noted, the as-synthesized composites showed slightly lower photocatalytic activity compared to the mechanically-mixed composites despite a better contact of allophane particles with photocatalyst. This is associated with the crystallinity of the composites. The as-synthesized composites have lower crystallinity, according to the XRD data which might have facilitated the fast recombination of charge carriers (Amano et al., 2008). The impact of the presence of allophane in the composites can be seen in Fig. 5b as allophane-containing samples exhibited higher photocatalytic activity for the photodegradation
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of acetaldehyde under visible light. We believe that the active oxygen species generated on the surfaces of Bi2WO6 and BiOI are released, diffusing through the gas phase and oxidizing the acetaldehyde (Katsumata et al., 2013). Some water molecules adsorbed on the surfaces of allophane and photocatalysts can be transformed into the hydroxyl radicals (•OH) by reaction with the photogenerated holes (h+) or superoxide radicals (O−• 2 ) at the photocatalyst surface, and the acetaldehyde adsorbed on the surfaces of allophane and photocatalyst is decomposed into CO2 by either Bi2WO6 or BiOI according to the carbonyl-mediated chain reactions as follows: CH3 CHO þ H2 O þ 2h→CH3 COOH þ 2H
þ
ð2Þ þ
CH3 CHO þ 3H2 O þ 5‐7hðor8hÞ→2CO2 þ 10H :
ð3Þ
The obtained results suggest that both mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites can be useful environmental material for the photodegradation of acetaldehyde.
4. Conclusions In summary, highly adsorptive mechanically-mixed and assynthesized Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing and hydrothermal synthesis, respectively. The mechanically-mixed composites possessed higher SSA compared to the as-synthesized composites probably due to the pore collapse or pore filling during the hydrothermal synthesis of the composites. Adsorption and photocatalytic activity of the prepared samples were evaluated for the adsorption and photodegradation of gaseous acetaldehyde in the dark and under visible light, respectively. The adsorption of acetaldehyde was strongly dependent on the SSA of the samples. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI–allophane composites completely decomposed acetaldehyde within 5–7 h, whereas the Bi2WO6 and mechanically-mixed and as-synthesized Bi 2WO6– allophane composites decomposed only 75.5%, 100%, and 85.6% within 8 h, respectively. With its significant adsorption of acetaldehyde, a allophane presented a distinctive contribution to the efficient photodegradation of acetaldehyde by allophane-containing composites. Both the mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites could be useful environmental material for the efficient photodegradation of acetaldehyde under visible light.
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Acknowledgements MH would like to thank the Japan Society for the Promotion of Science (JSPS) for the postdoctoral fellowship under which the present study was carried out. The authors are grateful to Dr. Toshihiro Isobe and Mr. Yuki Makinose from Tokyo Institute of Technology for their kind assistance in performing N2-BET measurements and TEM observations. References Amano, F., Yamakata, A., Nogami, K., Osawa, O., Ohtani, B., 2008. Visible light responsive pristine metal oxide photocatalyst: enhancement of activity by crystallization under hydrothermal treatment. J. Am. Chem. Soc. 130, 17650–17651. Amano, F., Nogami, K., Tanaka, M., Ohtani, B., 2010. Correlation between surface area and photocatalytic activity for acetaldehyde decomposition over bismuth tungstate particles with a hierarchical structure. Langmuir 26, 7174–7180. Butler, M.A., 1977. Photoelectrolysis and physical properties of the semiconducting electrode WO2. J. Appl. Phys. 48, 1914–1920. Chatterjee, D., Dasgupta, S., 2005. Visible light induced photocatalytic degradation of organic pollutants. J. Photochem. Photobiol. C 6, 186–205. Creton, B.,Bougeard, D.,Smirnov, K.S.,Guilment, J.,Poncelet, O., 2008. Structural model and computer modeling study of allophane. J. Phys. Chem. C 112, 358–364. Davies, J.E.D., 1973. Solid state vibrational spectroscopy — III[1] The infrared and Raman spectra of the bismuth(III) oxide halides. J. Inorg. Nucl. Chem. 35, 1531–1534. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Kameshima, Y., Tamura, Y., Nakajima, A., Okada, K., 2009. Preparation and properties of TiO2/montmorillonite composites. Appl. Clay Sci. 45, 20–23. Katsumata, K.-i,Hou, X.,Sakai, M.,Nakajima, A.,Fujishima, A.,Matsushita, N.,MacKenzie, K.J.D., Okada, K., 2013. Visible-light-driven photodegradation of acetaldehyde gas catalyzed by aluminosilicate nanotubes and Cu(II)-grafted TiO2 composites. Appl. Catal. B 138–139 (243–252). Kibanova, D., Trejo, M., Destaillats, H., Cervini-Silva, J., 2009. Synthesis of hectorite–TiO2 and kaolinite–TiO2 nanocomposites with photocatalytic activity for the degradation of model air pollutants. Appl. Clay Sci. 42, 563–568. Kitagawa, Y., 1971. The “unit particle” of allophane. Am. Mineral. 56, 465–475. Kudo, A., Hijii, S., 1999. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3+ with 6 s2 configuration and d0 transition metal ions. Chem. Lett. 28, 1103–1104. Maczka, M., Hanuza, J., Paraguassu, W., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., 2008a. Phonons in ferroelectric Bi2WO6: Raman and infrared spectra and lattice dynamics. Appl. Phys. Lett. 92, 112911. Maczka, M., Paraguassu, W., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., Hanuza, J., 2008b. Phonon-instability-driven phase transitions in ferroelectric Bi2WO6:Eu3+: high-pressure Raman and photoluminescence studies. Phys. Rev. B 77, 094137. Morrissey, F.A., Grismer, M.E., 1999. Kinetics of volatile organic compound sorption/ desorption on clay minerals. J. Contam. Hydrol. 36, 291–312. Okada, K.,Morikawa, H.,Iwai, S.,Ohira, Y.,Ossaka, J., 1975. A structure model of allophane. Clay Sci. 4, 291–303. Wada, K., 1967. A structural scheme of soil allophane. Am. Mineral. 52, 690–708. Wang, Y.,Deng, K.,Zhang, L., 2011. Visible light photocatalysis of BiOI and its photocatalytic activity enhancement by in situ ionic liquid modification. J. Phys. Chem. C 115, 14300–14308.
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Note
Preparation of Bi2WO6– and BiOI–allophane composites for efficient photodegradation of gaseous acetaldehyde under visible light Mirabbos Hojamberdiev a,b,⁎, Ken-ichi Katsumata a, Nobuhiro Matsushita a, Kiyoshi Okada a a b
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380–-8553, Japan
a r t i c l e
i n f o
Article history: Received 9 September 2013 Received in revised form 5 July 2014 Accepted 9 July 2014 Available online 7 August 2014 Keywords: Clays Aluminosilicates Bismuth tungstate Bismuth oxyiodide Allophane Photodegradation
a b s t r a c t Clays have been used in combination with various photocatalysts to enhance the removal of organic pollutants. The aim of the present work was to study the impact of allophane as the support material on photocatalytic activity of Bi2WO6 and BiOI. In this work, we report on the preparation of highly adsorptive mechanicallymixed and as-synthesized Bi2WO6– and BiOI–allophane composites by mechanical mixing and hydrothermal synthesis, respectively. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (Raman), ultraviolet–visible spectroscopy (UV–vis), and BET method. The allophane powders showed higher specific surface area, SSA, (SBET = 263 m2/g) in comparison with the Bi2WO6 (SBET = 21.3 m2/g) and BiOI (SBET = 23.1 m2/g) powders. The mechanically-mixed composites possessed higher SSA compared to the as-synthesized composites probably due to the pore collapse or pore filling during the hydrothermal synthesis of the composites. The light absorption by both composites started from 700 nm; however, the Bi2WO6–allophane composites only absorbed the photons N 325 nm, whereas the BiOI–allophane composites absorbed the photons N 500 nm. The samples with higher SSA showed higher acetaldehyde adsorption. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI–allophane composites decomposed acetaldehyde completely within 5–7 h, whereas the Bi2WO6, mechanically-mixed and as-synthesized Bi2WO6–allophane composites decomposed 75.5%, 100%, and 85.6% within 8 h, respectively. With its significant adsorption of acetaldehyde, allophane contributed to the efficient photodegradation of acetaldehyde by allophane-containing composites. The obtained results suggested that both mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites can be useful material for efficient photodegradation of acetaldehyde under visible light. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since the Honda–Fujishima effect was first reported in 1972 (Fujishima and Honda, 1972), semiconductor-based photocatalysis has been acknowledged as a potential method for renewable energy generation and environmental remediation. Because of negligible activity of TiO2 under visible light, there have been intense scientific efforts to develop novel highly efficient visible light-responsive photocatalysts because visible light accounts for ca. 43% of the incoming solar energy (Chatterjee and Dasgupta, 2005). Moreover, for the purification of indoor air, the photocatalysts should induce the oxidative decomposition of organic pollutants under visible light
⁎ Corresponding author at: Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan. Tel.: +81 45 924 5369; fax: +81 45 924 5358. E-mail addresses: [email protected], [email protected] (M. Hojamberdiev).
http://dx.doi.org/10.1016/j.clay.2014.07.007 0169-1317/© 2014 Elsevier B.V. All rights reserved.
emitted from indoor lighting devices, such as fluorescent light tubes (Amano et al., 2008). Bismuth tungstate (Bi2WO6) is one of the simplest members of the Aurivillius oxide family of layered perovskites, which are structurally composed of alternating perovskite-like and fluorite-like blocks. It has several important physical properties, including piezoelectricity, pyroelectricity, oxide anion conductivity, nonlinear dielectric susceptibility, and luminescent property. As an n-type semiconductor with the band gap of 2.80 eV, Bi2WO6 has shown excellent photocatalytic activity for O2 evolution and for mineralizing CHCl3 and CH3CHO into CO2 under visible light (Kudo and Hijii, 1999; Amano et al., 2010). Bismuth oxyiodide (BiOI), another member of the Aurivillius oxide family, crystallizes in the tetragonal structure, and its layer structure is characterized by the [Bi2O2] slabs interleaved by double slabs of iodine atoms. BiOI is a p-type semiconductor with the band gap of 1.7 − 1.83 eV, and it can absorb the most visible light (λ b 700 nm) (Wang et al., 2011). Since the photodegradation reaction takes place on a photocatalyst surface, adsorption becomes one of the important criteria for an efficient photodegradation of organic pollutants. A feasible approach to achieve
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this is the use of adsorbents with large SSA as supporting materials for loading photocatalyst particles. So far, a wide variety of porous materials, such as silica gels, zeolites, activated carbons, and clay minerals, exhibiting large SSA have been regarded to be excellent supports for photocatalyst nanoparticles. Because of their outstanding adsorption capacity for organic pollutants and tailorable surface properties, natural and synthetic clays have been combined with photocatalysts to improve the removal of organic pollutants in contaminated water and air. According to the experimentally determined adsorption rates, significant amounts of volatile organic compounds (VOCs) were adsorbed onto internal clay surfaces, with inter-particle and intra-particle diffusional time constants spanning two orders of magnitude (Morrissey and Grismer, 1999). Embedding TiO2 particles in montmorillonite (Kameshima et al., 2009), imogolite (Katsumata et al., 2013), kaolinite and hectorite (Kibanova et al., 2009), etc. favored the adsorption and enhanced the photodegradation of organic pollutants. Allophane, a natural clay mineral, is a hydrated aluminosilicate (1–2SiO2 · Al2O3 · 5–6H2O) having a 3.5–5.0 nm-sized hollow spherical structure with 0.3–0.5 nm-sized defects on its surface. The walls of the hollow spheres consist of inner silica and outer alumina layers with hydroxylated or hydrated surfaces. These surfaces have the ability to adsorb ionic or polar pollutants due to their amphoteric ion-exchange activity and large specific surface area (Wada, 1967; Kitagawa, 1971; Okada, et al., 1975). In this work, allophane was selected as a clay support for loading Bi2WO6 and BiOI photocatalyst particles. The mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing and hydrothermal synthesis, respectively. Adsorption and photocatalytic activity of the prepared samples were evaluated by adsorption and photodegradation of gaseous acetaldehyde (AcH) in the dark and under visible light irradiation, respectively. 2. Experimental 2.1. Materials Bi(NO3)3 · 5H2O, KI, Na4SiO4, AlCl3 · 6H2O, NH4OH, and ethylene glycol were obtained from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). Na2WO4 was obtained from Strem Chemicals, Inc. (Newburyport, MA, USA). All the chemical reagents were of analytical grade and used as received without further purification. Deionized water from Millipore Milli-Q Plus purification system (18.2 MΩ cm) was used throughout the experimental process.
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mechanically-mixed Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing of allophane and Bi2WO6 (1:1 mass ratio) and allophane and BiOI (1:1 mass ratio) using an agate mortar and pestle, respectively. 2.3. Characterization The formation of allophane, Bi2WO6, and BiOI in the composites was confirmed by X-ray diffraction (XRD) using an RINT-2100 diffractometer (Rigaku, Japan) with monochromated Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 40 mA. The powder samples were scanned at a scanning rate of 2°/min in the 2θ range of 5–70°. The crystalline phases in the composites were also identified using a T64000 Raman spectrometer (Horiba Jobin Yvon S.A.S., France) with an Ar laser (514.5 nm) operated at 50 mW. The particle size and morphology of the composites were examined using an S-4500 field emission-scanning electron microscope (Hitachi, Japan) and a JEM-2010UHR transmission electron microscope (JEOL, Japan) operating at an acceleration voltage of 200 kV. The UV– vis diffuse reflectance spectra of the composites were recorded on a Lambda 950 UV/VIS/NIR spectrophotometer (Perkin-Elmer, USA). The SSA (SBET), total pore volume (Vt), and pore size distribution (PSD) were obtained from N2 adsorption–desorption isotherms at 77 K (Autosorb-3B, Quantachrome, USA) on the samples degassed at 120 °C for 6 h in vacuum. The SBET values were obtained by using the Brunauer, Emmett and Teller (BET) method, and the PSDs were determined by the Barrett, Joyner and Halenda (BJH) method using the N2 adsorption– desorption isotherms. 2.4. Photodegradation experiments The degradation of gaseous acetaldehyde (CH3CHO) was carried out at room temperature in a batch-type reactor using the powder samples. The powder sample (50 mg) was placed in the reaction vessel made of Pyrex® glass with the volume of 500 mL. Pure air (Taiyo Nippon Sanso Corp., Japan) was blown through the reaction vessel to remove any air contaminants. Then, 4.5 μmol (200 ppm) of acetaldehyde was introduced into the firmly closed reaction vessel using a Pressure-Lok® precision analytical syringe. After adsorption equilibration in the dark for 12 h, the reactor was irradiated by a LA-251Xe lamp (Hayashi Watch Works, Japan) with the wavelength range of 520–600 nm. The decrease in acetaldehyde concentration and increase in CO2 concentration were monitored using a GC-2014 gas chromatograph (Shimadzu, Japan), equipped with a 2 m Porapak-Q column, methanizer and a flame ionization detector.
2.2. Preparation 3. Results and discussion To synthesize allophane, Na4SiO4 (0.1 mol/L) and AlCl3 · 6H2O (0.1 mol/L) aqueous solutions were mixed together under stirring (the pH measured was 3.5) for 2 h, and then the solution was transferred into a 40 mL Teflon-lined stainless steel autoclave. After the hydrothermal treatment at 95 °C for 90 h, the resulting precipitate was collected by centrifugation, washed with deionized water and dried at 80 °C for 8 h. To obtain Bi2WO6 or BiOI powders, 5 mmol of Bi(NO3)3 · 5H2O and 2.5 mmol of Na2WO4 (or 5 mmol KI) were separately dissolved in 15 mL of ethylene glycol and 15 mL of deionized water, respectively, and then mixed under vigorous stirring. The pH of the mixed solution was adjusted to 7 with the addition of ammonium hydroxide (NH4OH). After being stirred for 30 min, the solution was transferred into a 40 mL Teflon-lined stainless steel autoclave. After the hydrothermal treatment at 180 °C for 12 h, the resulting precipitate was collected by centrifugation, washed with deionized water to remove unreacted ions and dried at 80 °C for 8 h. Finally, to synthesize the Bi2WO6– and BiOI–allophane composites, the identical synthesis procedure used for the synthesis of Bi2WO6 or BiOI powders was applied with the addition of allophane powders (0.3489 g in 10 mL deionized water, sonicated for 15 min) into the precursor solutions. The
The XRD patterns of the mechanically-mixed and as-synthesized Bi2WO6–allophane (denoted as BW:Allo) and BiOI–allophane (denoted as BI:Allo) composites are shown in Fig. 1. All the reflections in the XRD patterns of the mechanically-mixed and as-synthesized Bi2WO6– allophane composites can be assigned to orthorhombic Bi2WO6 with the space group of Pbca(61) (JCPDS card no. 39-0256). No traces of other phases were detected in the composite. The XRD patterns of the mechanically-mixed and as-synthesized BiOI–allophane composites show that all the reflections, without any other characteristic reflections of impurity phases, match with the previously reported data for tetragonal BiOI with the space group of P4/nmm(129) (JCPDS card no. 10-0445). No reflections assignable for other phases were detected. The XRD patterns of the mechanically-mixed composites show sharper and narrower reflections, compared to that of the as-synthesized composites because of slightly higher crystallinity or larger crystal size. Slightly broader reflections with lower intensity suggest that the as-synthesized composites may have smaller crystal size. As the intensity of the reflections of the Bi2WO6 and BiOI phases is high, it is difficult to note the main halos of amorphous allophane (JCPDS card
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BI:Allo mech.mix.
BI:Allo as-synthesized
Intensity [a.u.]
BW:Allo mech.mix.
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BiOI Bi2WO6 Allophane 10
20
30
40
50
60
70
2θ [deg.] Fig. 1. XRD patterns of the as-synthesized and mechanically mixed Bi2WO6– and BiOI– allophane composites.
no. 38-0449) at 2θ = 27° and 40°, corresponding to (002) and (040), in the XRD patterns. Nevertheless, we consider that the prepared composites consisted of allophane, Bi2WO6, and BiOI. Figs. 2 and 3 show the SEM and TEM images of the mechanicallymixed and as-synthesized Bi2WO6–allophane and BiOI–allophane composites. It is clear from Figs. 2 and 3 that the as-synthesized Bi2WO6–allophane composite powders have two different morphologies, that is, sphere-like Bi2WO6 particles with the diameter of less than 1 μm are homogenously distributed along with large allophane particles with layered structures (flakes). As can be noted, some sphere-like Bi2WO6 particles are attached onto the surface of allophane particles. Although the as-synthesized BiOI–allophane composite powders have no well-marked morphology, one can easily see that
BiOI and allophane particles (possibly quasi-spherical) with less than 1 μm are uniformly distributed in the composite (Fig. 3). Compared to that of the as-synthesized composites, the mechanicallymixed Bi2WO6– and BiOI–allophane composites have different morphologies, that is, a homogenous distribution of agglomerated and unagglomerated small particles of allophane, Bi2WO6, and BiOI. It is believed that the mechanical mixing of Bi2WO6 and BiOI with allophane using an agate mortar and pestle caused the reduction in particle size (Fig. 3). The agglomeration of small particles might have been induced by heat generated during the mechanical mixing. Although the SEM images show that some Bi2WO6 and BiOI particles are partially separated from allophane, it is believed that most of the Bi2 WO 6 and BiOI particles are still in contact with allophane particles. Fig. 4a shows the Raman spectra of the mechanically-mixed and assynthesized Bi2WO6– and BiOI–allophane composites. Since allophane is amorphous, we only can see the Raman spectra of Bi2WO6 and BiOI in the composites. The Raman spectra of Bi2WO6 with the orthorhombic Pbca(61) space group shown in Fig. 4a are similar to that of the orthorhombic Pca21 space group (Maczka et al., 2008a); therefore, the Raman peaks of the Pbca(61) phase can be readily assigned to those modes of the Pca21 phase. The number of optic modes for the Pca21 phase is 26A1 + 27A2 + 26B1 + 26B2. The A2 modes are Raman active and the A1, B1, and B2 modes are both IR and Raman active (Maczka et al., 2008b). The Raman peaks at 788 cm−1 and 703 cm−1 can be assigned to the symmetric and asymmetric stretching modes of the WO6 octahedra involving motions of the apical and equatorial oxygen atoms perpendicular to and within layers, respectively. The Raman peaks at 412, 300, 278, 256, 215, and 187 cm−1 can be attributed to the bending modes of the WO6 octahedra and the stretching and bending modes of the BiO6 polyhedra. The Raman peaks at 140 cm−1 and 147 cm−1 may be assigned to the translations of the tungsten and bismuth ions (Maczka et al., 2008a). BiOI with the space group of D74h has two molecular formulae per unit cell with the Raman active modes of A1g, B1g, and Eg. The Raman peaks at ~ 85 cm− 1, not shown here, are assigned to the A1g internal Bi–I stretching mode. The other Raman
BW:Allo
BW:Allo
(as-synth.)
(mech.mix.) thesized)
Allophane Bi2WO6
3 µm
3 µm
BI:Allo
BI:Allo
(as-synth.)
(mech.mix.)
Allophane BiOI
3 µm
3 µm
Fig. 2. SEM micrographs of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites.
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(b)
(a)
Allophane
Bi2WO6
Allophane
40 nm
500 nm (d)
(c)
BiOI Allophane
500 nm
500 nm
Fig. 3. TEM images of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites.
peaks observed at 145 cm− 1 are assigned to the Eg internal Bi–I stretching mode (Davies, 1973). Similar to the XRD data, low intensity in the Raman spectra was noticed for the as-synthesized composites, confirming slightly higher crystallinity or larger crystal size of the mechanically-mixed composites. The mode-assignable Raman peaks were not clearly observed for allophane due to the defects in the structure (Creton et al., 2008). The nitrogen adsorption–desorption isotherms of Bi2WO6, BiOI, allophane, Bi2 WO 6:allophane, and BiOI:allophane powders are shown in Fig. 4b. Nitrogen adsorption measurements of allophane, Bi2WO6:allophane and BiOI:allophane show high N2 adsorption and a typical type IV isotherm with a single hysteresis loop in IUPAC classification, with a step at about P/P0 = 0.7, which is associated with capillary condensation in mesopores. According to the IUPAC classification, the N2 adsorption–desorption isotherms of the Bi2WO6 and BiOI powders are type II, describing adsorption on non-porous or macroporous adsorbent with strong adsorbate–adsorbent interactions. The porous properties of the samples are given in Table 1. As expected, the allophane powders possess higher SSA (SBET = 263 m2/g) in comparison to the Bi2WO6 (SBET = 21.3 m2/g) and BiOI (SBET = 23.1 m2/g) powders. As can be seen in Table 1, the mechanically-mixed composites have four times higher SSA compared to the as-synthesized composites. The reason for the higher SBET values of the mechanically-mixed composites may be that the freshly synthesized allophane powders with high SBET value were mechanically mixed directly with Bi2WO6 or BiOI powders without being subjected to an additional hydrothermal treatment, which may also cause the collapse of pores, with the precursor solutions of Bi2WO6 or BiOI, of which the precipitated tiny particles might have also blocked the pores. The considerable reduction in the SBET values of the as-synthesized composites is unclear but we may
still relate it to the pore collapse or pore filling during the hydrothermal synthesis. Fig. 5a shows the UV–vis diffuse reflectance spectra of the mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites. As can be seen, the Bi2WO6–allophane composites have a milky white color, evidencing that they can partially absorb the wavelengths of visible light. The color of the BiOI–allophane composites is reddish orange, and the composites can absorb most of the wavelengths of visible light. The light absorption in both composites starts from ca. 700 nm. However, the Bi2WO6–allophane composites only absorb the photons N 325 nm, and the absorption edge is about 495 – 503 nm. In contrast, the BiOI–allophane composites absorb the photons N500 nm, and the absorption edge is about 675 – 685 nm. The optical band gaps of the prepared composites were estimated by the general procedure using the absorption spectra from the following equation: n=2 ahv ¼ A hv−Eg
ð1Þ
where α, hν, Eg, and A are the absorption coefficient, the photon energy, the optical band gap, and a constant, respectively (Butler, 1977). As known, n decides the characteristics of the transition in a semiconductor. According to the equation, the value of n for Bi2WO6 and BiOI is 1 (direct transition) and 4 (indirect transition). The optical band gaps of the mechanically-mixed and as-synthesized Bi2WO6–allophane composites are 2.81 eV and 2.77 eV, respectively, whereas the mechanically-mixed and as-synthesized BiOI–allophane composites have 1.89 eV and 1.84 eV, respectively. A slight difference in absorption edges and optical band gaps between the mechanically-mixed and assynthesized composites can be noted. We believe that this difference
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100
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Allophane BiOI Bi2WO6 BiOI:Allophane (mech.mix.) Bi2WO6:Allophane (mech.mix.)
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4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -12
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Irradiation time [h]
P/P0 Fig. 4. (a) Raman spectra of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites and (b) N2 adsorption–desorption isotherms of Bi2WO6, BiOI, and allophane.
might have resulted from the differences in the local structure, packing, and defects formed during the synthesis processes. Adsorption and photocatalytic activity of the prepared samples are evaluated for the adsorption and photodegradation of gaseous acetaldehyde (CH3CHO), a typical indoor air contaminant, in the dark and under visible light, respectively. The results are shown in Fig. 5b. The concentration of the injected gaseous acetaldehyde decreased monotonously by adsorption mainly according to the SSA of the samples. That is, the higher SSA the higher adsorption of acetaldehyde. Particularly, allophane and mechanically-mixed allophane-containing samples showed higher adsorption capacity for acetaldehyde. This evidences the particular role of allophane, as part of composite, in the adsorption of acetaldehyde. However, it should be noted that the surface of allophane is hydrophilic, and therefore the already adsorbed water molecules might hamper the adsorption of acetaldehyde. As shown in Fig. 5b, the acetaldehyde concentration started to decrease after visible light irradiation. In the case of allophane, this decrease was negligible because Table 1 The compositions and porous properties of samples. Sample
SBET [m2/g]
Vp [ml/g]
Pore size [nm]
Bi2WO6 (BW) BiOI (BI) Allophane (Allo) BW:Allo (mech.mix.) BW:Allo (as-synth.) BI:Allo (mech.mix.) BI:Allo (as-synth.)
21.3 23.1 263.0 224.0 55.1 186.0 21.7
0.113 0.159 0.789 0.689 0.185 0.561 0.126
14.1 25.8 7.98 4.52 26.5 10.0 25.8
Fig. 5. (a) UV–vis diffuse reflectance spectra of the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites and (b) change of CH3CHO concentration (C0 = 4.5 μmol) by reaction with the as-synthesized and mechanically mixed Bi2WO6– and BiOI–allophane composites. Keys: □ — allophane; ○ — mechanically-mixed BiOI–allophane composite; △ — as-synthesized BiOI–allophane composite; ▽ — BiOI; ⋄ — mechanically-mixed Bi2WO6–allophane composite; E — as-synthesized BiOI–allophane composite; ✯ — Bi2WO6.
allophane cannot exhibit photocatalytic activity under visible light despite its adsorption of 37.8% of acetaldehyde. Although the total photodegradation of acetaldehyde was somehow favored by the presence of allophane in the allophane-containing composites, it mainly depended on the band structures of Bi 2 WO 6 and BiOI. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI-based composites completely mineralize acetaldehyde within 5–7 h, whereas the Bi 2WO6 and mechanically-mixed and as-synthesized Bi2WO6-based composites mineralize 75.5%, 100%, and 85.6% within 8 h, respectively. The profound photocatalytic activity of the BiOI and BiOI-based composites is related to the band structure of BiOI, allowing the samples to absorb a greater number of photons from visible light (N 500 nm). Bi 2WO 6 and Bi2 WO 6-based composites only absorb the limited number of photons from visible light (N 325 nm). As noted, the as-synthesized composites showed slightly lower photocatalytic activity compared to the mechanically-mixed composites despite a better contact of allophane particles with photocatalyst. This is associated with the crystallinity of the composites. The as-synthesized composites have lower crystallinity, according to the XRD data which might have facilitated the fast recombination of charge carriers (Amano et al., 2008). The impact of the presence of allophane in the composites can be seen in Fig. 5b as allophane-containing samples exhibited higher photocatalytic activity for the photodegradation
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of acetaldehyde under visible light. We believe that the active oxygen species generated on the surfaces of Bi2WO6 and BiOI are released, diffusing through the gas phase and oxidizing the acetaldehyde (Katsumata et al., 2013). Some water molecules adsorbed on the surfaces of allophane and photocatalysts can be transformed into the hydroxyl radicals (•OH) by reaction with the photogenerated holes (h+) or superoxide radicals (O−• 2 ) at the photocatalyst surface, and the acetaldehyde adsorbed on the surfaces of allophane and photocatalyst is decomposed into CO2 by either Bi2WO6 or BiOI according to the carbonyl-mediated chain reactions as follows: CH3 CHO þ H2 O þ 2h→CH3 COOH þ 2H
þ
ð2Þ þ
CH3 CHO þ 3H2 O þ 5‐7hðor8hÞ→2CO2 þ 10H :
ð3Þ
The obtained results suggest that both mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites can be useful environmental material for the photodegradation of acetaldehyde.
4. Conclusions In summary, highly adsorptive mechanically-mixed and assynthesized Bi2WO6– and BiOI–allophane composites were prepared by mechanical mixing and hydrothermal synthesis, respectively. The mechanically-mixed composites possessed higher SSA compared to the as-synthesized composites probably due to the pore collapse or pore filling during the hydrothermal synthesis of the composites. Adsorption and photocatalytic activity of the prepared samples were evaluated for the adsorption and photodegradation of gaseous acetaldehyde in the dark and under visible light, respectively. The adsorption of acetaldehyde was strongly dependent on the SSA of the samples. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI–allophane composites completely decomposed acetaldehyde within 5–7 h, whereas the Bi2WO6 and mechanically-mixed and as-synthesized Bi 2WO6– allophane composites decomposed only 75.5%, 100%, and 85.6% within 8 h, respectively. With its significant adsorption of acetaldehyde, a allophane presented a distinctive contribution to the efficient photodegradation of acetaldehyde by allophane-containing composites. Both the mechanically-mixed and as-synthesized Bi2WO6– and BiOI–allophane composites could be useful environmental material for the efficient photodegradation of acetaldehyde under visible light.
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Acknowledgements MH would like to thank the Japan Society for the Promotion of Science (JSPS) for the postdoctoral fellowship under which the present study was carried out. The authors are grateful to Dr. Toshihiro Isobe and Mr. Yuki Makinose from Tokyo Institute of Technology for their kind assistance in performing N2-BET measurements and TEM observations. References Amano, F., Yamakata, A., Nogami, K., Osawa, O., Ohtani, B., 2008. Visible light responsive pristine metal oxide photocatalyst: enhancement of activity by crystallization under hydrothermal treatment. J. Am. Chem. Soc. 130, 17650–17651. Amano, F., Nogami, K., Tanaka, M., Ohtani, B., 2010. Correlation between surface area and photocatalytic activity for acetaldehyde decomposition over bismuth tungstate particles with a hierarchical structure. Langmuir 26, 7174–7180. Butler, M.A., 1977. Photoelectrolysis and physical properties of the semiconducting electrode WO2. J. Appl. Phys. 48, 1914–1920. Chatterjee, D., Dasgupta, S., 2005. Visible light induced photocatalytic degradation of organic pollutants. J. Photochem. Photobiol. C 6, 186–205. Creton, B.,Bougeard, D.,Smirnov, K.S.,Guilment, J.,Poncelet, O., 2008. Structural model and computer modeling study of allophane. J. Phys. Chem. C 112, 358–364. Davies, J.E.D., 1973. Solid state vibrational spectroscopy — III[1] The infrared and Raman spectra of the bismuth(III) oxide halides. J. Inorg. Nucl. Chem. 35, 1531–1534. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Kameshima, Y., Tamura, Y., Nakajima, A., Okada, K., 2009. Preparation and properties of TiO2/montmorillonite composites. Appl. Clay Sci. 45, 20–23. Katsumata, K.-i,Hou, X.,Sakai, M.,Nakajima, A.,Fujishima, A.,Matsushita, N.,MacKenzie, K.J.D., Okada, K., 2013. Visible-light-driven photodegradation of acetaldehyde gas catalyzed by aluminosilicate nanotubes and Cu(II)-grafted TiO2 composites. Appl. Catal. B 138–139 (243–252). Kibanova, D., Trejo, M., Destaillats, H., Cervini-Silva, J., 2009. Synthesis of hectorite–TiO2 and kaolinite–TiO2 nanocomposites with photocatalytic activity for the degradation of model air pollutants. Appl. Clay Sci. 42, 563–568. Kitagawa, Y., 1971. The “unit particle” of allophane. Am. Mineral. 56, 465–475. Kudo, A., Hijii, S., 1999. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3+ with 6 s2 configuration and d0 transition metal ions. Chem. Lett. 28, 1103–1104. Maczka, M., Hanuza, J., Paraguassu, W., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., 2008a. Phonons in ferroelectric Bi2WO6: Raman and infrared spectra and lattice dynamics. Appl. Phys. Lett. 92, 112911. Maczka, M., Paraguassu, W., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., Hanuza, J., 2008b. Phonon-instability-driven phase transitions in ferroelectric Bi2WO6:Eu3+: high-pressure Raman and photoluminescence studies. Phys. Rev. B 77, 094137. Morrissey, F.A., Grismer, M.E., 1999. Kinetics of volatile organic compound sorption/ desorption on clay minerals. J. Contam. Hydrol. 36, 291–312. Okada, K.,Morikawa, H.,Iwai, S.,Ohira, Y.,Ossaka, J., 1975. A structure model of allophane. Clay Sci. 4, 291–303. Wada, K., 1967. A structural scheme of soil allophane. Am. Mineral. 52, 690–708. Wang, Y.,Deng, K.,Zhang, L., 2011. Visible light photocatalysis of BiOI and its photocatalytic activity enhancement by in situ ionic liquid modification. J. Phys. Chem. C 115, 14300–14308.