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High photocatalytic activity of C-ZnSn(OH)6 catalysts prepared by hydrothermal method
Journal of Molecular Catalysis A: Chemical 378 (2013) 164–173
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High photocatalytic activity of C-ZnSn(OH)6 catalysts prepared by hydrothermal method Huiquan Li, Wenshan Hong, Yumin Cui ∗ , Qingfeng Jia, Suhua Fan School of Chemistry and Chemical Engineering, Fuyang Normal College, Fuyang 236037, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 5 February 2013 Received in revised form 9 June 2013 Accepted 17 June 2013 Available online 25 June 2013 Keywords: Hydrothermal method Carbon-doped ZnSn(OH)6 Photocatalysis
a b s t r a c t Carbon-doped nano-ZnSn(OH)6 (C-ZnSn(OH)6 ) photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, UV–vis diffuse reflection spectroscopy (UV–vis DRS) and nitrogen physisorption studies. The photocatalytic activity was evaluated on the degradation of organic pollutants under ultraviolet (UV) light illumination. The results showed that compared to commercial P25 and pure ZnSn(OH)6 , the photocatalytic performance of 0.58% C-ZnSn(OH)6 was remarkably improved. The photocatalytic conversion ratio of benzene and cyclohexane by 0.58% C-ZnSn(OH)6 was up to 77% and 50.5%, which is about 16.0 and 1.39, 12.3 and 6.39 times higher than that of P25 and pure ZnSn(OH)6 , respectively. The 0.58% CZnSn(OH)6 catalyst also exhibited high photocatalytic activity toward methyl orange (MO) and phenol in suspended solution. Based on the characterization results and the detection of active species, the enhanced photocatalytic activities of C-ZnSn(OH)6 catalysts were discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The green photocatalytic technology has been a very attractive research topic in the treatment of persistent organic pollutants (POPs) [1–6]. According to previous studies, many POPs could be efficiently degraded into CO2 and H2 O by the photocatalysts under ultraviolet (UV) light irradiation, however, in the degradation process of POPs, the accumulation of less-reactive by-products on the photocatalyst surface could result in the deactivation of photocatalyst [7]. Therefore, it becomes a tremendous challenge on the further research of efficient treatment of POPs that persisted in the environment. As one of the best photocatalysts, TiO2 has been widely used in photocatalytic degradation of POPs [4,6], but it suffers from the high recombination of photogenerated electron–hole pairs and the low conversion efficiency of solar energy. The two main drawbacks limit its industrial application. In recent years, some new non-titania photocatalysts have been developed, such as Bi2 WO6 [8–10], BiOCl [11], BiOI [12], ZnSn(OH)6 [13]. ZnSn(OH)6 has been widely applied in high effective flame retardants due to its environment-friendly and safety [13–15]. In the last two years, ZnSn(OH)6 has been
∗ Corresponding author. Tel.: +86 558 2596249; fax: +86 558 2596703. E-mail address: [email protected] (Y. Cui). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.06.012
reported to exhibit certain photocatalytic activity for the degradation of POPs under UV light irradiation [13,16], but the low quantum efficiency of photocatalytic reactions impair its future applications to great extent. Previous studies have shown that carbon can effectively transfer the photogenerated electrons in carbon-doped TiO2 [17,18], reducing the recombination of electron–hole pairs and increasing photon efficiency, and even modify the band-gap structure of the intrinsic absorption [19,20]. In addition, the photocatalytic activity of TiO2 could be also enhanced by using activated carbon in the photocatalytic degradation of cytarabine [21], and the presence of ozonated activated carbons with a high carboxyl groups content enhanced 2,4-dichlorophenoxyacetic acid photodegradation by the UV/TiO2 system [22]. Thus, carbon-doped ZnSn(OH)6 may be an ideal system to improve the separation efficiency of photogenerated charge carriers, and then achieve a high photocatalytic activity toward POPs. However, to the best of our knowledge, there was no report on the synthesis and photocatalytic performance in the C-ZnSn(OH)6 system. In this work, carbon-doped nano-ZnSn(OH)6 photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K. The obtained samples were characterized by XRD, TEM, HR-TEM, XPS, PL, UV–vis DRS and N2 physisorption studies. The photocatalytic performance on the photodegradation of benzene and cyclohexane in O2 gas stream, methyl orange (MO) and phenol in suspended
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solution under UV light irradiation was studied. The enhanced photocatalytic performance of C-ZnSn(OH)6 was discussed. 2. Experimental 2.1. Catalyst preparation The carbon doped ZnSn(OH)6 samples with different carbon content were prepared by hydrothermal method. In a common preparation, ZnCl2 (2.5 mmol, 25 mL) and NaOH (15.0 mmol, 25 mL) was added to a solution of SnCl4 (2.5 mmol, 25 mL) and an amount [C/(C + ZnSn(OH)6 ) = 0.12 wt%] of 0.01 g glucose was added was directly added into the mixture solution under vigorously stirring at room temperature. Subsequently, the above mixed aqueous solution was put into a 100 mL stainless steel autoclave. The autoclave was sealed and maintained at 433 K for 24 h. Finally, the white product was centrifuged, washed and dried at 373 K in air. A series of C-ZnSn(OH)6 samples were prepared in this way with different weight percents of carbon. The final C-ZnSn(OH)6 samples with various weight percents of carbon of 0.00, 0.12, 0.58 and 1.17 wt%, respectively, were denoted as 0.00%, 0.12%, 0.58% and 1.17% CZnSn(OH)6 , respectively. 2.2. Catalyst characterization X-ray diffraction (XRD) were performed on a Philips X’Pert diffractometer equipped with Ni-filtered Cu K␣ radiation source ( = 0.15418 nm). X-ray photoelectron spectra (XPS) measurements were carried out using Multilab 2000 XPS system with a monochromatic Mg K␣ source and a charge neutralizer (Multilab 2000 XPS, Thermo Scientific, America). All the binding energies were referenced to the C 1s peak at 284.6 eV of the surface carbon. The Brunauer–Emmett–Teller (BET) surface areas of samples were determined from N2 adsorption isotherms at 77 K using a Micromeritics ASAP 2020 instrument with a computercontrolled measurement system. Resolution transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM) images using a JEM-2100 electron microscope. UV–vis diffuse reflection spectroscopy (UV–vis DRS) of the samples was determined with a Shimadzu UV-3600 spectrophotometer (Japan) using BaSO4 as a reference. The photoluminescence (PL) spectroscopy, obtained at room temperature with an excitation wavelength of 280 nm, was recorded on a CARY Eclipse (America) fluorescence spectrophotometer. The actual C content of the obtained C-ZnSn(OH)6 samples was detected by IRIS (INTREPID 2) inductively coupled plasma atomic emission spectrometry (ICPAES, ICP 710, Varian, America), and the results were listed in Table 1. 2.3. Photocatalytic reaction Photocatalytic conversion of gas pollutants was performed in a tubular quartz microreactor with a continuous flow mode. The quartz tube reactor loading 0.3 g catalyst samples was surrounded by four UV bulbs (TUV 4W/G4 T5, Philips, max = 254 nm, intensity = 17 mW cm−2 ). The reaction temperature was maintained around 308 K by an air cooling system. Diluted benzene or cyclohexane gas (273 K, 280 ± 20 mg L−1 ) was introduced into the reactor along with a continuous O2 gas stream at a total flow rate of 20 cm3 min−1 . Prior to irradiation, the adsorption of benzene or cyclohexane on the catalysts reached an equilibrium. The concentration of organic gas benzene or cyclohexane pollutant was analyzed by an online gas chromatograph (GC, model DS 6200; Donam Instruments Inc., Gyeonngi-do, Korea). Photoacatalytic degradation of methyl orange (MO) and phenol was performed in an aqueous solution under UV light irradiation. For each UV light test, 150 mL MO or phenol aqueous solution
Fig. 1. XRD patterns of C-ZnSn(OH)6 samples with different carbon contents (wt%): (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
(20 mg L−1 ) and 0.06 g catalyst samples were used. A general procedure was carried out as follows. First, MO or phenol aqueous solution was placed into a water-jacketed reactor maintained at 298 K, and then the catalyst samples were suspended in the solution. The suspension was stirred vigorously for 1.0 h in the dark to establish the adsorption–desorption equilibrium of MO or phenol, then irradiated under UV light. About 3.0 mL solution was withdrawn from the reactor periodically and centrifuged and analyzed for the degradation of MO and phenol by using a TU-1901 spectrophotometer. In order to study the effect of relevant reactive species, a quantity of different appropriate species quenchers were introduced into the photocatalytic degradation process of MO and phenol in a manner similar to the photodegradation experiment. The dosages of these species quenchers were referred to the literatures [23,24]. 2.4. Reuse of catalyst The 0.58% C-ZnSn(OH)6 catalyst was immersed in ethanol for 3.0 h and rinsed with deionized water, and then dried at 373 K. After that, the 0.58% C-ZnSn(OH)6 catalyst was reused for the degradation of MO and phenol, and the reuse experiment has been done several times. 3. Results and discussion 3.1. Catalyst structure Fig. 1 shows the XRD patterns of C-ZnSn(OH)6 samples with different carbon contents. The diffraction patterns of all samples were in line with the standard spectrum (JCPDS No. 20-1455). The nine distinctive peaks at 22.89◦ , 32.76◦ , 36.72◦ , 38.53◦ , 40.28◦ , 46.83◦ , 52.79◦ , 58.27◦ and 68.11◦ were matched with the (2 0 0), (2 2 0), (3 1 0), (3 1 1), (2 2 2), (3 3 1), (4 2 0), (4 2 2) and (4 4 0) crystal planes of ZnSn(OH)6 , respectively. No diffraction peaks assigned to carbon were found, which could be attributed to the small amount of carbon dopant (max 1.17 wt%) and high dispersion in samples. With increasing the carbon content, the intensities of the diffraction peaks of ZnSn(OH)6 are decreased, implying a decline in crystallinity and a decrease of crystallites. The average crystallite sizes of ZnSn(OH)6 in the C-ZnSn(OH)6 samples were approximately estimated from the (2 0 0) peaks according to the Scherrer formula [25]: L = K/ˇ cos , where is the wavelength of the X-ray radiation ( = 0.15418 nm), K is the Scherrer constant (K = 0.9), is the angle of characteristic X-ray diffraction peak and ˇ is the full-width-at-half-maximum, and the results were listed in Table 1. As shown in Table 1, the average
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Table 1 Actual C content, relative crystallinity, average crystallite sizes (L), BET surface areas (SBET ), pore volume and the pseudo-first order rate constants (kapp ) of C-ZnSn(OH)6 samples with different carbon contents (wt%). Sample
ZnSn(OH)6 0.12% C-ZnSn(OH)6 0.58% C-ZnSn(OH)6 1.17% C-ZnSn(OH)6
Actual C content (%)
0.00 0.11 0.56 1.16
Relative crystallinity
1.000 0.955 0.925 0.910
crystallite size of ZnSn(OH)6 in the C-ZnSn(OH)6 sample calculated from Scherrer’s equation decreases with the increase of carbon content, indicating that the doped carbon species could inhibit the growth of ZnSn(OH)6 crystallites. Fig. 2 shows the nitrogen adsorption–desorption isotherms of C-ZnSn(OH)6 samples with different carbon contents. The presence of carbone in the synthetic system exerted a significant influence on the pore structures of the obtained products. With increasing carbon, the isotherms showed higher adsorption at high relative pressures, indicating an increasing pore volume. The data related to BET surface areas and pore volumes obtained from N2 adsorption–desorption analysis were summarized in Table 1. It can be seen that with carbon content increasing, the BET surface areas and pore volumes of C-ZnSn(OH)6 samples increase, respectively. The BET surface areas and pore volumes of C-ZnSn(OH)6 samples are higher than those of pure ZnSn(OH)6 . The microstructure and the particle sizes of ZnSn(OH)6 , 0.12% C-ZnSn(OH)6 , 0.58% C-ZnSn(OH)6 and 1.17% C-ZnSn(OH)6 samples detected by TEM were shown in Fig. 3A–D. It can be seen that ZnSn(OH)6 , 0.12% C-ZnSn(OH)6 , 0.58% C-ZnSn(OH)6 and 1.17% CZnSn(OH)6 nanoparticles with mean diameter of about 91, 79, 68 and 50 nm, respectively, were cubic and some small nanoparticles were attached to the edge of the nanocubes. The representative HRTEM images of ZnSn(OH)6 and 0.58%% C-ZnSn(OH)6 samples with the lattice fringes were shown in Fig. 3E and F. The interlayer spacing of 0.278 nm and 0.263 nm corresponded to the (2 2 0) and (3 1 0) plane of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 , respectively. The chemical state of the C-ZnSn(OH)6 samples was investigated by using X-ray photoelectron spectroscopy (XPS), and the results were shown in Fig. 4. The typical XPS survey spectrum in Fig. 4A shows that 0.58% C-ZnSn(OH)6 was composed of C, Sn, Zn and O elements. As displayed in Fig. 4B–D, the binding energies of Zn (Zn 2p3/2 1021.7 eV and Zn 2p1/2 1044.7 eV), Sn (Sn 3d5/2 486.6 eV and Sn 3d3/2 495.1 eV) and O (O 1s 531.4 eV) in 0.58% C-ZnSn(OH)6
L (nm)
38.6 34.6 33.7 32.5
37.2 40.7 42.3 45.1
Pore volume (cm3 g−1 )
0.238 0.255 0.301 0.381
kapp (min−1 ) MO
Phenol
0.02056 0.02653 0.03882 0.01743
0.01618 0.01880 0.02733 0.01211
were consistent with those in pure ZnSn(OH)6 . This implies that low doping amount of carbon in the catalyst did not affect the binding energies significantly. However, the binding energy of Zn (Zn 2p3/2 1022.1 eV and Zn 2p1/2 1045.1 eV), Sn (Sn 3d5/2 487.0 eV and Sn 3d3/2 495.5 eV) and O (O 1s 531.5 eV) in 1.17% C-ZnSn(OH)6 shifts to a higher value compared with that of pure ZnSn(OH)6 [26,27]. These results probably suggest that a shift toward higher binding energy upon carbon addition indicates the successful incorporation of carbon into ZnSn(OH)6 lattice [28,29] and C atoms may substitute for some of the lattice oxygen atoms [30–32]. The optical properties of the C-ZnSn(OH)6 samples with different carbon contents were investigated by UV–vis diffuse reflectances spectroscopy. As revealed from Fig. 5A, with increasing carbon contents, the absorption intensity of C-ZnSn(OH)6 samples increased in the 200–600 nm light region and the absorption edge shifted significantly to longer wavelengths as compared to pure ZnSn(OH)6 sample, clearly revealing that the absorption edges of C-ZnSn(OH)6 samples shifted to the lower energy region. It is well known that the optical absorption near the band edge of a crystalline semiconductor follows the formula: ˛hv = A(hv − Eg )n/2 , where ˛, A, v and Eg are the absorption coefficient, a constant, light frequency and band-gap energy, respectively [33]. Among them, n depends on the characteristics of the transition in a semiconductor (direct transition: n = 1; indirect transition: n = 4). For ZnSn(OH)6 , the value of n is 1 [16]. The band-gap energies (Eg values) of C-ZnSn(OH)6 samples can be thus estimated from a plot of (˛hv)2 versus photon energy (hv), as shown in Fig. 5B. The intercept of the tangent to the x-axis will give a good approximation of the band-gap energies for the C-ZnSn(OH)6 samples, respectively. 3.2. Adsorption studies The activity of photocatalyst is related to its adsorbability [34,35]. Therefore, a research of MO and phenol adsorption was performed on ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts by using 10 mg of catalyst at room temperature in the dark. Fig. 6 depicts the adsorption isotherms of MO and phenol on the ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples. The experimental data were interpreted with the Langmuir adsorption isotherm, which is mathematically represented as follows [21,22]: q=
Fig. 2. Nitrogen adsorption–desorption isotherms of C-ZnSn(OH)6 samples with different carbon contents (wt%): (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
SBET (m2 g−1 )
qm KCe 1 + KCe
(1)
where Ce is MO or phenol concentration at equilibrium (mg L−1 ), q is the mass of MO or phenol adsorbed per mass unit of catalyst (mg g−1 ), qm is the maximum amount of MO or phenol adsorbed per adsorbent mass unit (mg g−1 ), and K is the Langmuir constant (L mg−1 ). It can be seen that MO and phenol uptake capacity moderately increases in the presence of 0.58% C-ZnSn(OH)6 compared with that of ZnSn(OH)6 . Herein, the qm of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 was 9.25 and 10.3 mg gcatalyst −1 for MO and 5.10 and 6.40 mg gcatalyst −1 for phenol, respectively. Obviously, the absorbance ability of the pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples did not have obvious difference, demonstrating that the
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Fig. 3. TEM and HR-TEM images of ZnSn(OH)6 (A and E), 0.12% C-ZnSn(OH)6 (B), 0.58% C-ZnSn(OH)6 (C and F) and 1.17% C-ZnSn(OH)6 (D) samples.
absorbance ability was not the main influencing factor for the different photocatalytic activities of C-ZnSn(OH)6 samples. 3.3. Photocatalytic activities of C-ZnSn(OH)6 The photocatalytic activities of C-ZnSn(OH)6 photocatalysts were evaluated by the degradation of benzene, cyclohexane, MO and phenol under UV light irradiation. The gas-phase photocatalytic degradation of benzene or cyclohexane in a dry O2 gas stream was carried out under UV light irradiation for 300 min, as shown in Fig. 7A and B. It can be seen that under 300 min UV light irradiation, the conversion ratio of benzene and cyclohexane on 0.58% C-ZnSn(OH)6 was about 77% and 50.5%, respectively. As to pure ZnSn(OH)6 and P25, the conversion ratio of benzene and cyclohexane was only 55.3% and 7.9%, 4.8% and 4.1%, respectively. The experimental data of XPS on the 0.58% C-ZnSn(OH)6 before and after reaction of benzene demonstrated that the as-prepared photocatalyst was stable (as shown in Fig. 4). It can be seen that the binding energy of Zn, Sn and O in 0.58% C-ZnSn(OH)6 before
and after reaction of benzene was nearly the same. These results suggested that the surface valence states of C-ZnSn(OH)6 were all stable during the photocatalytic reaction of benzene. For photocatalytic reaction in liquid-phase, prior to illumination, an adsorption–desorption equilibrium between C-ZnSn(OH)6 and MO (phenol) was established in the dark for 1.0 h. The effect of carbon contents on the photocatalytic activity of C-ZnSn(OH)6 photocatalysts has been investigated by MO and phenol degradation in an aqueous solution under UV irradiation. As was shown in Fig. 7C and D, under UV light irradiation the self-degradation of MO and phenol is negligible in the absence of photocatalyst, while in the presence of photocatalysts, the irradiation of UV light can result in the obvious degradation of MO and phenol, and the carbon content in the ZnSn(OH)6 exerts great influences on the photocatalytic activity of C-ZnSn(OH)6 photocatalysts. With carbon content increasing, the photocatalytic activities of CZnSn(OH)6 photocatalysts first increase, reaching the maximums at carbon content of 0.58%, and then decrease with further increasing carbon content. Under 60 min UV light irradiation, the 0.58%
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Fig. 4. XPS spectra of C-ZnSn(OH)6 samples: (A) a survey spectrum, (B) Zn 2p, (C) Sn 3d and (D) O 1s.
C-ZnSn(OH)6 catalyst exhibits obviously higher UV light photocatalytic activity than commercial Degussa P25 (TiO2 ) and pure ZnSn(OH)6 . The Langmuir–Hinshelwood equation was widely used to model the photocatalytic degradation. The general reaction rate equation is represented as follows [36]: −r0 =
K1 C −dC = dt 1 + K2 C + Ki Ci
is the adsorption coefficient of the reactant onto the C-ZnSn(OH)6 particles (L mg−1 ) and Ki Ci is absorption term for all organic intermediate products. If we consider t = 0, C = C0 and the term for the intermediate organic products Ki Ci = 0, the equation reduces to the expression [36,37]: −r0 =
(2)
where −r0 is the oxidation rate of the reactant; C is the concentration of the reactant (mg L−1 ); K1 is the reaction rate (min−1 ); K2
K1 C −dC = 1 + K2 C dt
(3)
This equation has been used to represent the kinetic behavior of the photocatalytic degradation of MO or phenol. When the initial concentration (C0 ) is very low (C0 = 20 mg L−1 for MO or phenol in
Fig. 5. UV–vis diffuse reflectances spectra of C-ZnSn(OH)6 samples with different carbon contents (wt%) (A): (a) 0.00, (b) 0.12, (c) 0.58, (d) 1.17, and the plotting of (˛hv)2 versus photon energy (B).
H. Li et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 164–173
Fig. 6. Adsorption isotherms of MO and phenol on ZnSn(OH)6 (a and c) and 0.58% C-ZnSn(OH)6 (b and d) samples.
the present experiment), this equation could be simplified to an apparent first-order mode [37–39]: ln
C 0
C
= kKt = kapp t
(4)
where kapp is the apparent pseudo-first-order rate constant (min−1 ), C is MO or phenol concentration in aqueous solution at time t (mg L−1 ), C0 is initial MO or phenol concentration (mg L−1 ). Thus, the linear relationship between ln(C0 /C) and t shown in Fig. 8 confirmed that the photocatalytic degradation process of MO and phenol followed the expressed as Eq. (4), and the kapp of
169
as-prepared samples was calculated and shown in Table 1, respectively. The UV–vis spectra of MO aqueous solution as a function of UV light irradiation time in the presence of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts are illustrated in Fig. 9. The progressive decrease of 274 and 464 nm bands can be seen in Fig. 9. The absorbance at 274 nm represented the aromatic content of MO and the decrease of the band at this wavelength could be attributed to the degradation of MO’s aromatic moiety [40]. It can be also seen that the visible region peak intensities in the photo-degradation of MO by the 0.58% C-ZnSn(OH)6 catalyst decrease more obviously than those of pure ZnSn(OH)6 catalyst after 60 min UV light irradiation, which is in agreement with the results of Fig. 7C. Since no new peak appears, the loss of absorbance can be mainly attributed to the degradation reaction [41,42]. In order to test the stability of the high photocatalytic performance of the 0.58% C-ZnSn(OH)6 photocatalyst, Fig. 10 shows the circulating runs of the photocatalytic degradation of MO and phenol in the presence of the 0.58% C-ZnSn(OH)6 photocatalyst under UV light irradiation. It can be seen that the photocatalytic activity does not exhibit any significant loss after six recycles for the photodegradation of MO and phenol. The result indicates that the 0.58% C-ZnSn(OH)6 photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of MO and phenol molecules, which is very important for the application of C-ZnSn(OH)6 photocatalysts in the future. 3.4. Discussion of photocatalytic mechanism 3.4.1. Reactive species involved in the photocatalytic process The effect of various radical scavengers on the degradation of MO and phenol over pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 under
Fig. 7. Effects of carbon contents (wt%) in the C-ZnSn(OH)6 catalysts on the degradation of benzene (A), cyclohexane (B), MO (C) and phenol (D) under UV (max = 254 nm) light irradiation: (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
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Fig. 8. Linear transform ln(C0 /C) = f(t) of the kinetic curves of MO (A) and phenol (B) degradation over C-ZnSn(OH)6 catalysts with different carbon contents (wt%) under UV (A) (max = 254 nm) light irradiation: (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
Fig. 9. UV–vis spectra of the MO aqueous solution under UV light irradiation in the presence of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts.
UV light irradiation was performed to investigate the underlying photo-degradation mechanism. Radical species generated in C-ZnSn(OH)6 activation by UV light may give rise to oxidation or reduction reactions [21,22]. In order to study which of these species are involved in phenol and MO degradation, we conducted experiments with the radical scavengers isopropanol (IPA), benzoquinone (BQ), ammonium oxalate (AO), catalase (CAT), and NO3 − ions. IPA is a scavenger of • OH radical species [23,43]; BQ acts as • O2 − radical
scavenger [23,44]; AO is a scavenger of h+ radical species [45]; CAT acts as H2 O2 radical scavenger [43] and NO3 − ions mainly act as ecb − scavengers [46]. The results were shown in Fig. 11. Once the radical species played a major role in the degradation of MO and phenol, the degradation ratio was expected to be decreased greatly. In Fig. 11, it can be seen that MO and phenol degradation were reduced by the addition of each radical scavenger, more markedly with BQ, followed by CAT, NO3 − ions, AO and IPA. The
Fig. 10. Cycling runs in the photocatalytic degradation of MO (A) and phenol (B) in the presence of 0.58% C-ZnSn(OH)6 catalyst under UV (max = 254 nm) light irradiation.
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Fig. 11. The effects of different scavengers on the degradation of MO and phenol over ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 under UV light irradiation.
degradation ratios of MO and phenol over pure ZnSn(OH)6 were 16.3% and 12.9% (BQ), 33.1% and 25.5% (CAT), 46.3% and 30.2% (NO3 − ), 57.5% and 39.3% (AO), 63.7% and 43.1% (IPA), respectively. These degradation ratios were on average 4.4 and 4.8, 2.1 and 2.4, 1.5 and 2.0, 1.2 and 1.6, 1.1 and 1.4-fold smaller than those obtained with the ZnSn(OH)6 system with no scavenger under otherwise identical conditions, respectively. In addition, the degradation ratios of MO and phenol over 0.58% C-ZnSn(OH)6 were 23.5% and 12.5% (BQ), 47.6% and 40.7% (CAT), 58.4% and 46.8% (NO3 − ), 70.5% and 50.9% (AO), 77.7% and 54.3% (IPA), respectively. These degradation ratios were on average 4.0 and 6.6, 1.9 and 2.0, 1.6 and 1.7, 1.3 and 1.6, 1.2 and 1.5-fold smaller than those obtained with the 0.58% C-ZnSn(OH)6 system with no scavenger under otherwise identical conditions, respectively. Eqs. (12)–(16) indicate that IPA, BQ, AO, CAT, and NO3 − mainly inhibit • OH, • O2 − , h+ , H2 O2 , and ecb − radicals, respectively. Hence, we conclude that: (i) MO and phenol degradation takes place by oxidation and reduction reactions (Fig. 11); (ii) MO and phenol degradation by oxidative pathway is the main degradation mechanism, as shown by the larger decrease in degradation ratio with the addition of benzoquinone versus nitrate ions. 3.4.2. Origin of reactive species for MO and phenol degradation Taking the kinds of reactive species involved in the MO (phenol) degradation into account, the photocatalytic process could be described as the following Eqs. (5)–(16): Catalyst + hv → ecb − + hvb +
(5)
ecb − + O2 → • O2 −
(6)
ecb − + • O2 − + 2H+ → H2 O2
(7)
H2 O2 + ecb − → • OH + OH−
(8)
H2 O2 + hv → 2• OH
(9)
H2 O + hvb + → • OH + H+
(10)
OH− + hvb + → • OH
(11)
MO(phenol) + • OH → products
(12)
MO(phenol) + • O2 − → products
(13)
MO(phenol) + hvb + → products
(14)
MO(phenol) + H2 O2 → products
(15)
MO(phenol) + ecb − → products
(16)
In the above process, electron–hole pairs were directly produced by catalyst af ter UV light illumination. Then, the photogenerated electrons transfered to conduction band bottom of the photocatalyst and reacted with the adsorbed O2 on the surface of catalyst to form • O2 − , H2 O2 , and • OH. Meanwhile, the holes were left on the valence band top, reacting with the adsorbed H2 O or OH− on the surface of catalyst to form • OH. After that, • OH, • O2 − and H2 O2 that oxidized MO (phenol), or the hvb + and ecb − reacted directly with MO (phenol). 3.4.3. Photocatalytic activity enhancement mechanism of C-ZnSn(OH)6 When irradiation with energy (hv) greater than the bandgap energy of C-ZnSn(OH)6 , electron–hole pairs (ecb − /hvb + ) are generated in C-ZnSn(OH)6 (Eq. (5)). Then, the ecb − /hvb + pairs migrated to the surface of the photocatalysts and reacted with the species adsorbed on the surface (e.g. Eqs. (6)–(11)). These reactions prevented the ecb − /hvb + pairs from combining. However, if the ecb − /hvb + pairs were not consumed by the locally absorbed species, they would be recombined, resulting in the decrease of reaction efficiency. The degradation efficiency of MO and phenol over the 0.58% C-ZnSn(OH)6 was greatly enhanced as compared to pure ZnSn(OH)6 , indicating that a proper carbon content in the ZnSn(OH)6 improved the separation efficiency of photogenerated electron–hole pairs of C-ZnSn(OH)6 photocatalysts. The enhancement of separation efficiency of electron–hole pairs in the C-ZnSn(OH)6 photocatalysts was also confirmed by photoluminescence (PL) emission spectra of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 photocatalysts in Fig. 12. It is well known that PL emission spectra have been a useful technique to investigate the separation efficiency of photogenerated charge carriers in a semiconductor photocatalyst [47–49]. The comparison of PL emission spectra (excited at 280 nm) of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 photocatalysts at room temperature was shown in Fig. 12. It can be seen that the two samples have a broad emission peak in the wavelength range of 350–690 nm. The PL emitting peaks are similar while PL intensity of 0.58% CZnSn(OH)6 is dramatically weakened compared with that of pure ZnSn(OH)6 , indicating that the recombination of photogenerated charge carriers was greatly inhibited by an appropriate carbon incorporation. In generally, the photocatalytic activity of catalyst is related to its BET surface areas, particle sizes, crystallinity, light absorption ability, surface states, etc. The increase of photocatalytic activities
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preparation route not only reduced the energy consumption for the fabrication of C-ZnSn(OH)6 photocatalysts but also might extend the utilization of C-ZnSn(OH)6 photocatalysts at low temperature in the future. Acknowledgments This work was supported by the National Natural Science Foundation of China (21201037) and Natural Science Foundation of Higher Education Institutions in Anhui Province (KJ2012A217) and school-level item (2012HJJC01ZD) of Anhui Provincial Key Laboratory for Degradation and Monitoring of Pollution of the Environment. References
Fig. 12. PL spectra of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples recorded at room temperature with the excitation wavelength of 280 nm.
of C-ZnSn(OH)6 samples with increase of carbon content (Fig. 7) can be understood by the two facts. Firstly, as shown in Fig. 5A, with increasing the carbon content, the absorption intensity of C-ZnSn(OH)6 samples increases in the 200–600 nm regions, which results in the increases of photocatalytic activities. Secondly, with increasing carbon content, the BET surface area (Table 1) of C-ZnSn(OH)6 catalysts increases and the particle size (Fig. 3A–D) of C-ZnSn(OH)6 catalysts decreases, which enhanced the interfacial charge transfer of inter-particles and improved the separation efficiency of photo-generated electron–hole pairs [50,51], leading to the increase of photocatalytic activities. When the carbon content is higher than 0.58%, the light absorption intensity (Fig. 5A) of C-ZnSn(OH)6 particles further increases in the 200–600 nm regions, but the photocatalytic activities (Fig. 7) of C-ZnSn(OH)6 catalysts decreased remarkably, indicating that the excessive carbon doped in the ZnSn(OH)6 decreases the photocatalytic activity of C-ZnSn(OH)6 catalysts. This may be attributed to the two facts. Firstly, an excess of carbon decreased the crystallinity (Table 1) of the C-ZnSn(OH)6 catalyst, thereby inhibiting the transport of electrons and increasing the probability of electrons and holes being trapped by both the crystal defects and the recombination process [50,51]. Secondly, an excess carbon maybe result in the introduction of new defect sites or recombination centers that enhance the recombination of photogenerated electrons and holes [52–55], leading to the decrease of photocatalytic activities. Thus, 0.58% C-ZnSn(OH)6 with an appropriate carbon content exhibited the highest photocatalytic activity in our studies. 4. Conclusions In summary, C-ZnSn(OH)6 photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K. The C-ZnSn(OH)6 samples exhibited a cubic perovskite ZnSn(OH)6 phase. With increasing carbon content, the absorption intensity of C-ZnSn(OH)6 increased in the 200–600 nm light region and the absorption edge shifted significantly to longer wavelengths compared with that of pure ZnSn(OH)6 . The photocatalytic conversion ratio of benzene and cyclohexane by 0.58% C-ZnSn(OH)6 was up to 77% and 50.5%, which is about 16.0 and 1.39, 12.3 and 6.39 times higher than that of P25 and pure ZnSn(OH)6 , respectively. The 0.58% C-ZnSn(OH)6 sample also exhibited high photocatalytic activity toward MO and phenol in suspended solution. Based on the characterization results and the detection of active species, the enhanced photocatalytic activities of C-ZnSn(OH)6 were discussed. In this study, the simple and low-cost
[1] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746–750. [2] G. Palmisano, E. García-López, G. Marcí, V. Loddo, S. Yurdakal, V. Augugliaro, L. Palmisano, Chem. Commun. 46 (2010) 7074–7089. [3] Y.Z. Li, H. Zhang, Z.M. Guo, J.J. Han, X.J. Zhao, Q.N. Zhao, S.J. Kim, Langmuir 24 (2008) 8351–8357. [4] Y.J. Hua, C.T. Wang, J.Y. Liu, B. Wang, X.L. Liu, C.Y. Wu, X.Y. Liu, J. Mol. Catal. A: Chem. 365 (2012) 8–14. [5] J. Velásquez, S. Valencia, L. Rios, G. Restrepo, J. Marín, Chem. Eng. J. 203 (2012) 398–405. [6] Y.Y. Lv, L.S. Yu, X.L. Zhang, J.Y. Yao, R.Y. Zou, Z. Dai, Appl. Surf. Sci. 257 (2011) 5715–5719. [7] G.S. Martra, L. Coluccia, V. Marchese, V. Augugliaro, L. Loddo, M. Palmisano, Catal. Today 53 (1999) 695–702. [8] A.K.P. Mann, S.E. Skrabalak, Chem. Mater. 23 (2011) 1017–1022. [9] D.X. Wu, H.T. Zhu, C.Y. Zhang, L. Chen, Chem. Commun. 46 (2010) 7250–7252. [10] F. Amano, K. Nogami, M. Tanaka, B. Ohtani, Langmuir 26 (2010) 7174–7180. [11] F. Dong, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, J. Hazard. Mater. 219-220 (2012) 26–34. [12] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Appl. Catal. B: Environ. 111–112 (2012) 271–279. [13] Y.B. Chen, D.Z. Li, M. He, Y. Hu, H. Ruan, Y.M. Lin, J.H. Hu, Y. Zheng, Y. Shao, Appl. Catal. B: Environ. 113–114 (2012) 134–140. [14] J.Z. Xu, C.Y. Zhang, H.Q. Qu, C.M. Tian, J. Appl. Polym. Sci. 98 (2005) 1469–1475. [15] F. Andre, P.A. Cusack, A.W. Monk, R. Seangprasertkij, Polym. Degrad. Stab. 40 (1993) 267–273. [16] L.L. Wang, K.B. Tang, Z.P. Liu, D. Wang, J. Sheng, W. Cheng, J. Mater. Chem. 21 (2011) 4352–4357. [17] H.T. Gao, C.H. Ding, D.M. Dai, J. Mol. Struct.: Theochem. 944 (2010) 156–162. [18] H.Y. Li, D.J. Wang, H.M. Fan, P. Wang, T.F. Jiang, T.F. Xie, J. Colloid Interface Sci. 354 (2011) 175–180. [19] S. Mozia, M. Toyoda, M. Inagaki, B. Tryba, A.W. Morawski, J. Hazard. Mater. 140 (2007) 369–375. [20] F. Dong, H.Q. Wang, Z.B. Wu, J. Phys. Chem. C 113 (2009) 16717–16723. [21] R. Ocampo-Péreza, M. Sánchez-Polo, J. Rivera-Utrilla, R. Leyva-Ramos, Appl. Catal. B: Environ. 104 (2011) 177–184. [22] J. Rivera-Utrilla, M. Sánchez-Polo, M.M. Abdel daiem, R. Ocampo-Pérez, Appl. Catal. B: Environ. 126 (2011) 100–107. [23] G.T. Li, K.H. Wong, X. Zhang, C. Hu, J.C. Yu, R.C. Chan, P.K. Wong, Chemosphere 76 (2009) 1185–1191. [24] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Catal. Commun. 13 (2011) 63–68. [25] M. Galceran, M.C. Pujol, C. Zaldo, F. Díaz, M. Aguiló, J. Phys. Chem. C 113 (2009) 15497–15506. [26] L.X. Han, J. Liu, Z.J. Wang, K. Zhang, H. Luo, B. Xu, X. Zou, X. Zheng, B. Ye, X.B. Yu, Cryst. Eng. Commun. 14 (2012) 3380–3386. [27] H. Fan, S.Y. Ai, P. Ju, Cryst. Eng. Commun. 13 (2011) 113–117. [28] H.Y. Li, D.J. Wang, H.M. Fan, P. Wang, T.F. Jiang, T.F. Xie, J. Colloid Interf. Sci. 354 (2011) 175–180. [29] L.X. Mai, C.M. Huang, D.W. Wang, Z.G. Zhang, Y.Y. Wang, Appl. Surf. Sci. 255 (2009) 9285–9289. [30] Y. Choi, T. Umebayashi, M. Yoshikawa, J. Mater. Sci. 39 (2004) 1837–1839. [31] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243–2245. [32] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, W.F. Maier, Appl. Catal. B: Environ. 32 (2001) 215–223. [33] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747–753. [34] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [35] S.F. Chen, Y.Z. Liu, Chemosphere 67 (2007) 1010–1017. [36] C.A. Aguilar, C. Montalvo, J.G. Ceron, E. Moctezuma, Int. J. Environ. Res. 5 (4) (2011) 1071–1078. [37] X.F. Chang, J. Huang, Q.Y. Tan, M. Wang, G.B. Ji, S.B. Deng, G. Yu, Catal. Commun. 10 (2009) 1957–1961. [38] J.J. Xu, Y.H. Ao, D.G. Fu, Appl. Surf. Sci. 256 (2009) 884–888. [39] C.H. Wu, H.W. Chang, J.M. Chen, J. Hazard. Mater. 137 (2006) 336–343. [40] A.N. Ökte, Ö. Yılmaz, Appl. Catal. A: Gen. 354 (2009) 132–142. [41] P. Kumar, M. Govindaraju, S. Senthamilselvi, K. Premkumar, Colloids Surf. B 103 (2013) 658–661. [42] C.S. Guo, J. Xu, Y. He, Y. Zhang, Y.Q. Wang, Appl. Surf. Sci. 257 (2011) 3798–3803.
H. Li et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 164–173 [43] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Environ. Sci. Technol. 44 (2010) 1392. [44] M.C. Yin, Z.S. Li, J.H. Kou, Z.G. Zou, Environ. Sci. Technol. 43 (2009) 8361–8366. [45] N. Zhang, S.Q. Liu, X.Z. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 9136–9145. [46] U.F. Alkaram, A.A. Mukhlis, A.H. Al-Dujaili, J. Hazard. Mater. 169 (2009) 324–332. [47] L.Q. Jing, Y.C. Qu, B.Q. Wang, S.D. Li, B.J. Jiang, L.B. Yang, W. Fu, H.G. Fu, J.Z. Sun, Sol. Energy. Mater. Sol. Cells 90 (2006) 1773–1787. [48] Y.M. Wu, J.L. Zhang, L. Xiao, F. Chen, Appl. Catal. B: Environ. 88 (2009) 525. [49] C. Borgohain, K.K. Senapati, K.C. Sarma, P. Phukan, J. Mol. Catal. A: Chem. 363–364 (2012) 495–500.
173
[50] L.C. Chen, Y.C. Ho, W.S. Guo, C.M. Huang, T.C. Pan, Electrochim. Acta 54 (2009) 3884–3891. [51] L.C. Chen, F.R. Tsai, S.H. Fang, Y.C. Ho, Electrochim. Acta 54 (2009) 1304–1311. [52] F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607–365617. [53] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808–3816. [54] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483–5486. [55] E.B. Gracien, J.N. Shen, X.R. Sun, D. Liu, M.C. Li, S.D. Yao, J. Sun, Thin Solid Films 515 (2007) 5287–5297.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
High photocatalytic activity of C-ZnSn(OH)6 catalysts prepared by hydrothermal method Huiquan Li, Wenshan Hong, Yumin Cui ∗ , Qingfeng Jia, Suhua Fan School of Chemistry and Chemical Engineering, Fuyang Normal College, Fuyang 236037, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 5 February 2013 Received in revised form 9 June 2013 Accepted 17 June 2013 Available online 25 June 2013 Keywords: Hydrothermal method Carbon-doped ZnSn(OH)6 Photocatalysis
a b s t r a c t Carbon-doped nano-ZnSn(OH)6 (C-ZnSn(OH)6 ) photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, UV–vis diffuse reflection spectroscopy (UV–vis DRS) and nitrogen physisorption studies. The photocatalytic activity was evaluated on the degradation of organic pollutants under ultraviolet (UV) light illumination. The results showed that compared to commercial P25 and pure ZnSn(OH)6 , the photocatalytic performance of 0.58% C-ZnSn(OH)6 was remarkably improved. The photocatalytic conversion ratio of benzene and cyclohexane by 0.58% C-ZnSn(OH)6 was up to 77% and 50.5%, which is about 16.0 and 1.39, 12.3 and 6.39 times higher than that of P25 and pure ZnSn(OH)6 , respectively. The 0.58% CZnSn(OH)6 catalyst also exhibited high photocatalytic activity toward methyl orange (MO) and phenol in suspended solution. Based on the characterization results and the detection of active species, the enhanced photocatalytic activities of C-ZnSn(OH)6 catalysts were discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The green photocatalytic technology has been a very attractive research topic in the treatment of persistent organic pollutants (POPs) [1–6]. According to previous studies, many POPs could be efficiently degraded into CO2 and H2 O by the photocatalysts under ultraviolet (UV) light irradiation, however, in the degradation process of POPs, the accumulation of less-reactive by-products on the photocatalyst surface could result in the deactivation of photocatalyst [7]. Therefore, it becomes a tremendous challenge on the further research of efficient treatment of POPs that persisted in the environment. As one of the best photocatalysts, TiO2 has been widely used in photocatalytic degradation of POPs [4,6], but it suffers from the high recombination of photogenerated electron–hole pairs and the low conversion efficiency of solar energy. The two main drawbacks limit its industrial application. In recent years, some new non-titania photocatalysts have been developed, such as Bi2 WO6 [8–10], BiOCl [11], BiOI [12], ZnSn(OH)6 [13]. ZnSn(OH)6 has been widely applied in high effective flame retardants due to its environment-friendly and safety [13–15]. In the last two years, ZnSn(OH)6 has been
∗ Corresponding author. Tel.: +86 558 2596249; fax: +86 558 2596703. E-mail address: [email protected] (Y. Cui). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.06.012
reported to exhibit certain photocatalytic activity for the degradation of POPs under UV light irradiation [13,16], but the low quantum efficiency of photocatalytic reactions impair its future applications to great extent. Previous studies have shown that carbon can effectively transfer the photogenerated electrons in carbon-doped TiO2 [17,18], reducing the recombination of electron–hole pairs and increasing photon efficiency, and even modify the band-gap structure of the intrinsic absorption [19,20]. In addition, the photocatalytic activity of TiO2 could be also enhanced by using activated carbon in the photocatalytic degradation of cytarabine [21], and the presence of ozonated activated carbons with a high carboxyl groups content enhanced 2,4-dichlorophenoxyacetic acid photodegradation by the UV/TiO2 system [22]. Thus, carbon-doped ZnSn(OH)6 may be an ideal system to improve the separation efficiency of photogenerated charge carriers, and then achieve a high photocatalytic activity toward POPs. However, to the best of our knowledge, there was no report on the synthesis and photocatalytic performance in the C-ZnSn(OH)6 system. In this work, carbon-doped nano-ZnSn(OH)6 photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K. The obtained samples were characterized by XRD, TEM, HR-TEM, XPS, PL, UV–vis DRS and N2 physisorption studies. The photocatalytic performance on the photodegradation of benzene and cyclohexane in O2 gas stream, methyl orange (MO) and phenol in suspended
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solution under UV light irradiation was studied. The enhanced photocatalytic performance of C-ZnSn(OH)6 was discussed. 2. Experimental 2.1. Catalyst preparation The carbon doped ZnSn(OH)6 samples with different carbon content were prepared by hydrothermal method. In a common preparation, ZnCl2 (2.5 mmol, 25 mL) and NaOH (15.0 mmol, 25 mL) was added to a solution of SnCl4 (2.5 mmol, 25 mL) and an amount [C/(C + ZnSn(OH)6 ) = 0.12 wt%] of 0.01 g glucose was added was directly added into the mixture solution under vigorously stirring at room temperature. Subsequently, the above mixed aqueous solution was put into a 100 mL stainless steel autoclave. The autoclave was sealed and maintained at 433 K for 24 h. Finally, the white product was centrifuged, washed and dried at 373 K in air. A series of C-ZnSn(OH)6 samples were prepared in this way with different weight percents of carbon. The final C-ZnSn(OH)6 samples with various weight percents of carbon of 0.00, 0.12, 0.58 and 1.17 wt%, respectively, were denoted as 0.00%, 0.12%, 0.58% and 1.17% CZnSn(OH)6 , respectively. 2.2. Catalyst characterization X-ray diffraction (XRD) were performed on a Philips X’Pert diffractometer equipped with Ni-filtered Cu K␣ radiation source ( = 0.15418 nm). X-ray photoelectron spectra (XPS) measurements were carried out using Multilab 2000 XPS system with a monochromatic Mg K␣ source and a charge neutralizer (Multilab 2000 XPS, Thermo Scientific, America). All the binding energies were referenced to the C 1s peak at 284.6 eV of the surface carbon. The Brunauer–Emmett–Teller (BET) surface areas of samples were determined from N2 adsorption isotherms at 77 K using a Micromeritics ASAP 2020 instrument with a computercontrolled measurement system. Resolution transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM) images using a JEM-2100 electron microscope. UV–vis diffuse reflection spectroscopy (UV–vis DRS) of the samples was determined with a Shimadzu UV-3600 spectrophotometer (Japan) using BaSO4 as a reference. The photoluminescence (PL) spectroscopy, obtained at room temperature with an excitation wavelength of 280 nm, was recorded on a CARY Eclipse (America) fluorescence spectrophotometer. The actual C content of the obtained C-ZnSn(OH)6 samples was detected by IRIS (INTREPID 2) inductively coupled plasma atomic emission spectrometry (ICPAES, ICP 710, Varian, America), and the results were listed in Table 1. 2.3. Photocatalytic reaction Photocatalytic conversion of gas pollutants was performed in a tubular quartz microreactor with a continuous flow mode. The quartz tube reactor loading 0.3 g catalyst samples was surrounded by four UV bulbs (TUV 4W/G4 T5, Philips, max = 254 nm, intensity = 17 mW cm−2 ). The reaction temperature was maintained around 308 K by an air cooling system. Diluted benzene or cyclohexane gas (273 K, 280 ± 20 mg L−1 ) was introduced into the reactor along with a continuous O2 gas stream at a total flow rate of 20 cm3 min−1 . Prior to irradiation, the adsorption of benzene or cyclohexane on the catalysts reached an equilibrium. The concentration of organic gas benzene or cyclohexane pollutant was analyzed by an online gas chromatograph (GC, model DS 6200; Donam Instruments Inc., Gyeonngi-do, Korea). Photoacatalytic degradation of methyl orange (MO) and phenol was performed in an aqueous solution under UV light irradiation. For each UV light test, 150 mL MO or phenol aqueous solution
Fig. 1. XRD patterns of C-ZnSn(OH)6 samples with different carbon contents (wt%): (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
(20 mg L−1 ) and 0.06 g catalyst samples were used. A general procedure was carried out as follows. First, MO or phenol aqueous solution was placed into a water-jacketed reactor maintained at 298 K, and then the catalyst samples were suspended in the solution. The suspension was stirred vigorously for 1.0 h in the dark to establish the adsorption–desorption equilibrium of MO or phenol, then irradiated under UV light. About 3.0 mL solution was withdrawn from the reactor periodically and centrifuged and analyzed for the degradation of MO and phenol by using a TU-1901 spectrophotometer. In order to study the effect of relevant reactive species, a quantity of different appropriate species quenchers were introduced into the photocatalytic degradation process of MO and phenol in a manner similar to the photodegradation experiment. The dosages of these species quenchers were referred to the literatures [23,24]. 2.4. Reuse of catalyst The 0.58% C-ZnSn(OH)6 catalyst was immersed in ethanol for 3.0 h and rinsed with deionized water, and then dried at 373 K. After that, the 0.58% C-ZnSn(OH)6 catalyst was reused for the degradation of MO and phenol, and the reuse experiment has been done several times. 3. Results and discussion 3.1. Catalyst structure Fig. 1 shows the XRD patterns of C-ZnSn(OH)6 samples with different carbon contents. The diffraction patterns of all samples were in line with the standard spectrum (JCPDS No. 20-1455). The nine distinctive peaks at 22.89◦ , 32.76◦ , 36.72◦ , 38.53◦ , 40.28◦ , 46.83◦ , 52.79◦ , 58.27◦ and 68.11◦ were matched with the (2 0 0), (2 2 0), (3 1 0), (3 1 1), (2 2 2), (3 3 1), (4 2 0), (4 2 2) and (4 4 0) crystal planes of ZnSn(OH)6 , respectively. No diffraction peaks assigned to carbon were found, which could be attributed to the small amount of carbon dopant (max 1.17 wt%) and high dispersion in samples. With increasing the carbon content, the intensities of the diffraction peaks of ZnSn(OH)6 are decreased, implying a decline in crystallinity and a decrease of crystallites. The average crystallite sizes of ZnSn(OH)6 in the C-ZnSn(OH)6 samples were approximately estimated from the (2 0 0) peaks according to the Scherrer formula [25]: L = K/ˇ cos , where is the wavelength of the X-ray radiation ( = 0.15418 nm), K is the Scherrer constant (K = 0.9), is the angle of characteristic X-ray diffraction peak and ˇ is the full-width-at-half-maximum, and the results were listed in Table 1. As shown in Table 1, the average
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Table 1 Actual C content, relative crystallinity, average crystallite sizes (L), BET surface areas (SBET ), pore volume and the pseudo-first order rate constants (kapp ) of C-ZnSn(OH)6 samples with different carbon contents (wt%). Sample
ZnSn(OH)6 0.12% C-ZnSn(OH)6 0.58% C-ZnSn(OH)6 1.17% C-ZnSn(OH)6
Actual C content (%)
0.00 0.11 0.56 1.16
Relative crystallinity
1.000 0.955 0.925 0.910
crystallite size of ZnSn(OH)6 in the C-ZnSn(OH)6 sample calculated from Scherrer’s equation decreases with the increase of carbon content, indicating that the doped carbon species could inhibit the growth of ZnSn(OH)6 crystallites. Fig. 2 shows the nitrogen adsorption–desorption isotherms of C-ZnSn(OH)6 samples with different carbon contents. The presence of carbone in the synthetic system exerted a significant influence on the pore structures of the obtained products. With increasing carbon, the isotherms showed higher adsorption at high relative pressures, indicating an increasing pore volume. The data related to BET surface areas and pore volumes obtained from N2 adsorption–desorption analysis were summarized in Table 1. It can be seen that with carbon content increasing, the BET surface areas and pore volumes of C-ZnSn(OH)6 samples increase, respectively. The BET surface areas and pore volumes of C-ZnSn(OH)6 samples are higher than those of pure ZnSn(OH)6 . The microstructure and the particle sizes of ZnSn(OH)6 , 0.12% C-ZnSn(OH)6 , 0.58% C-ZnSn(OH)6 and 1.17% C-ZnSn(OH)6 samples detected by TEM were shown in Fig. 3A–D. It can be seen that ZnSn(OH)6 , 0.12% C-ZnSn(OH)6 , 0.58% C-ZnSn(OH)6 and 1.17% CZnSn(OH)6 nanoparticles with mean diameter of about 91, 79, 68 and 50 nm, respectively, were cubic and some small nanoparticles were attached to the edge of the nanocubes. The representative HRTEM images of ZnSn(OH)6 and 0.58%% C-ZnSn(OH)6 samples with the lattice fringes were shown in Fig. 3E and F. The interlayer spacing of 0.278 nm and 0.263 nm corresponded to the (2 2 0) and (3 1 0) plane of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 , respectively. The chemical state of the C-ZnSn(OH)6 samples was investigated by using X-ray photoelectron spectroscopy (XPS), and the results were shown in Fig. 4. The typical XPS survey spectrum in Fig. 4A shows that 0.58% C-ZnSn(OH)6 was composed of C, Sn, Zn and O elements. As displayed in Fig. 4B–D, the binding energies of Zn (Zn 2p3/2 1021.7 eV and Zn 2p1/2 1044.7 eV), Sn (Sn 3d5/2 486.6 eV and Sn 3d3/2 495.1 eV) and O (O 1s 531.4 eV) in 0.58% C-ZnSn(OH)6
L (nm)
38.6 34.6 33.7 32.5
37.2 40.7 42.3 45.1
Pore volume (cm3 g−1 )
0.238 0.255 0.301 0.381
kapp (min−1 ) MO
Phenol
0.02056 0.02653 0.03882 0.01743
0.01618 0.01880 0.02733 0.01211
were consistent with those in pure ZnSn(OH)6 . This implies that low doping amount of carbon in the catalyst did not affect the binding energies significantly. However, the binding energy of Zn (Zn 2p3/2 1022.1 eV and Zn 2p1/2 1045.1 eV), Sn (Sn 3d5/2 487.0 eV and Sn 3d3/2 495.5 eV) and O (O 1s 531.5 eV) in 1.17% C-ZnSn(OH)6 shifts to a higher value compared with that of pure ZnSn(OH)6 [26,27]. These results probably suggest that a shift toward higher binding energy upon carbon addition indicates the successful incorporation of carbon into ZnSn(OH)6 lattice [28,29] and C atoms may substitute for some of the lattice oxygen atoms [30–32]. The optical properties of the C-ZnSn(OH)6 samples with different carbon contents were investigated by UV–vis diffuse reflectances spectroscopy. As revealed from Fig. 5A, with increasing carbon contents, the absorption intensity of C-ZnSn(OH)6 samples increased in the 200–600 nm light region and the absorption edge shifted significantly to longer wavelengths as compared to pure ZnSn(OH)6 sample, clearly revealing that the absorption edges of C-ZnSn(OH)6 samples shifted to the lower energy region. It is well known that the optical absorption near the band edge of a crystalline semiconductor follows the formula: ˛hv = A(hv − Eg )n/2 , where ˛, A, v and Eg are the absorption coefficient, a constant, light frequency and band-gap energy, respectively [33]. Among them, n depends on the characteristics of the transition in a semiconductor (direct transition: n = 1; indirect transition: n = 4). For ZnSn(OH)6 , the value of n is 1 [16]. The band-gap energies (Eg values) of C-ZnSn(OH)6 samples can be thus estimated from a plot of (˛hv)2 versus photon energy (hv), as shown in Fig. 5B. The intercept of the tangent to the x-axis will give a good approximation of the band-gap energies for the C-ZnSn(OH)6 samples, respectively. 3.2. Adsorption studies The activity of photocatalyst is related to its adsorbability [34,35]. Therefore, a research of MO and phenol adsorption was performed on ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts by using 10 mg of catalyst at room temperature in the dark. Fig. 6 depicts the adsorption isotherms of MO and phenol on the ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples. The experimental data were interpreted with the Langmuir adsorption isotherm, which is mathematically represented as follows [21,22]: q=
Fig. 2. Nitrogen adsorption–desorption isotherms of C-ZnSn(OH)6 samples with different carbon contents (wt%): (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
SBET (m2 g−1 )
qm KCe 1 + KCe
(1)
where Ce is MO or phenol concentration at equilibrium (mg L−1 ), q is the mass of MO or phenol adsorbed per mass unit of catalyst (mg g−1 ), qm is the maximum amount of MO or phenol adsorbed per adsorbent mass unit (mg g−1 ), and K is the Langmuir constant (L mg−1 ). It can be seen that MO and phenol uptake capacity moderately increases in the presence of 0.58% C-ZnSn(OH)6 compared with that of ZnSn(OH)6 . Herein, the qm of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 was 9.25 and 10.3 mg gcatalyst −1 for MO and 5.10 and 6.40 mg gcatalyst −1 for phenol, respectively. Obviously, the absorbance ability of the pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples did not have obvious difference, demonstrating that the
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Fig. 3. TEM and HR-TEM images of ZnSn(OH)6 (A and E), 0.12% C-ZnSn(OH)6 (B), 0.58% C-ZnSn(OH)6 (C and F) and 1.17% C-ZnSn(OH)6 (D) samples.
absorbance ability was not the main influencing factor for the different photocatalytic activities of C-ZnSn(OH)6 samples. 3.3. Photocatalytic activities of C-ZnSn(OH)6 The photocatalytic activities of C-ZnSn(OH)6 photocatalysts were evaluated by the degradation of benzene, cyclohexane, MO and phenol under UV light irradiation. The gas-phase photocatalytic degradation of benzene or cyclohexane in a dry O2 gas stream was carried out under UV light irradiation for 300 min, as shown in Fig. 7A and B. It can be seen that under 300 min UV light irradiation, the conversion ratio of benzene and cyclohexane on 0.58% C-ZnSn(OH)6 was about 77% and 50.5%, respectively. As to pure ZnSn(OH)6 and P25, the conversion ratio of benzene and cyclohexane was only 55.3% and 7.9%, 4.8% and 4.1%, respectively. The experimental data of XPS on the 0.58% C-ZnSn(OH)6 before and after reaction of benzene demonstrated that the as-prepared photocatalyst was stable (as shown in Fig. 4). It can be seen that the binding energy of Zn, Sn and O in 0.58% C-ZnSn(OH)6 before
and after reaction of benzene was nearly the same. These results suggested that the surface valence states of C-ZnSn(OH)6 were all stable during the photocatalytic reaction of benzene. For photocatalytic reaction in liquid-phase, prior to illumination, an adsorption–desorption equilibrium between C-ZnSn(OH)6 and MO (phenol) was established in the dark for 1.0 h. The effect of carbon contents on the photocatalytic activity of C-ZnSn(OH)6 photocatalysts has been investigated by MO and phenol degradation in an aqueous solution under UV irradiation. As was shown in Fig. 7C and D, under UV light irradiation the self-degradation of MO and phenol is negligible in the absence of photocatalyst, while in the presence of photocatalysts, the irradiation of UV light can result in the obvious degradation of MO and phenol, and the carbon content in the ZnSn(OH)6 exerts great influences on the photocatalytic activity of C-ZnSn(OH)6 photocatalysts. With carbon content increasing, the photocatalytic activities of CZnSn(OH)6 photocatalysts first increase, reaching the maximums at carbon content of 0.58%, and then decrease with further increasing carbon content. Under 60 min UV light irradiation, the 0.58%
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Fig. 4. XPS spectra of C-ZnSn(OH)6 samples: (A) a survey spectrum, (B) Zn 2p, (C) Sn 3d and (D) O 1s.
C-ZnSn(OH)6 catalyst exhibits obviously higher UV light photocatalytic activity than commercial Degussa P25 (TiO2 ) and pure ZnSn(OH)6 . The Langmuir–Hinshelwood equation was widely used to model the photocatalytic degradation. The general reaction rate equation is represented as follows [36]: −r0 =
K1 C −dC = dt 1 + K2 C + Ki Ci
is the adsorption coefficient of the reactant onto the C-ZnSn(OH)6 particles (L mg−1 ) and Ki Ci is absorption term for all organic intermediate products. If we consider t = 0, C = C0 and the term for the intermediate organic products Ki Ci = 0, the equation reduces to the expression [36,37]: −r0 =
(2)
where −r0 is the oxidation rate of the reactant; C is the concentration of the reactant (mg L−1 ); K1 is the reaction rate (min−1 ); K2
K1 C −dC = 1 + K2 C dt
(3)
This equation has been used to represent the kinetic behavior of the photocatalytic degradation of MO or phenol. When the initial concentration (C0 ) is very low (C0 = 20 mg L−1 for MO or phenol in
Fig. 5. UV–vis diffuse reflectances spectra of C-ZnSn(OH)6 samples with different carbon contents (wt%) (A): (a) 0.00, (b) 0.12, (c) 0.58, (d) 1.17, and the plotting of (˛hv)2 versus photon energy (B).
H. Li et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 164–173
Fig. 6. Adsorption isotherms of MO and phenol on ZnSn(OH)6 (a and c) and 0.58% C-ZnSn(OH)6 (b and d) samples.
the present experiment), this equation could be simplified to an apparent first-order mode [37–39]: ln
C 0
C
= kKt = kapp t
(4)
where kapp is the apparent pseudo-first-order rate constant (min−1 ), C is MO or phenol concentration in aqueous solution at time t (mg L−1 ), C0 is initial MO or phenol concentration (mg L−1 ). Thus, the linear relationship between ln(C0 /C) and t shown in Fig. 8 confirmed that the photocatalytic degradation process of MO and phenol followed the expressed as Eq. (4), and the kapp of
169
as-prepared samples was calculated and shown in Table 1, respectively. The UV–vis spectra of MO aqueous solution as a function of UV light irradiation time in the presence of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts are illustrated in Fig. 9. The progressive decrease of 274 and 464 nm bands can be seen in Fig. 9. The absorbance at 274 nm represented the aromatic content of MO and the decrease of the band at this wavelength could be attributed to the degradation of MO’s aromatic moiety [40]. It can be also seen that the visible region peak intensities in the photo-degradation of MO by the 0.58% C-ZnSn(OH)6 catalyst decrease more obviously than those of pure ZnSn(OH)6 catalyst after 60 min UV light irradiation, which is in agreement with the results of Fig. 7C. Since no new peak appears, the loss of absorbance can be mainly attributed to the degradation reaction [41,42]. In order to test the stability of the high photocatalytic performance of the 0.58% C-ZnSn(OH)6 photocatalyst, Fig. 10 shows the circulating runs of the photocatalytic degradation of MO and phenol in the presence of the 0.58% C-ZnSn(OH)6 photocatalyst under UV light irradiation. It can be seen that the photocatalytic activity does not exhibit any significant loss after six recycles for the photodegradation of MO and phenol. The result indicates that the 0.58% C-ZnSn(OH)6 photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of MO and phenol molecules, which is very important for the application of C-ZnSn(OH)6 photocatalysts in the future. 3.4. Discussion of photocatalytic mechanism 3.4.1. Reactive species involved in the photocatalytic process The effect of various radical scavengers on the degradation of MO and phenol over pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 under
Fig. 7. Effects of carbon contents (wt%) in the C-ZnSn(OH)6 catalysts on the degradation of benzene (A), cyclohexane (B), MO (C) and phenol (D) under UV (max = 254 nm) light irradiation: (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
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Fig. 8. Linear transform ln(C0 /C) = f(t) of the kinetic curves of MO (A) and phenol (B) degradation over C-ZnSn(OH)6 catalysts with different carbon contents (wt%) under UV (A) (max = 254 nm) light irradiation: (a) 0.00, (b) 0.12, (c) 0.58, and (d) 1.17.
Fig. 9. UV–vis spectra of the MO aqueous solution under UV light irradiation in the presence of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 catalysts.
UV light irradiation was performed to investigate the underlying photo-degradation mechanism. Radical species generated in C-ZnSn(OH)6 activation by UV light may give rise to oxidation or reduction reactions [21,22]. In order to study which of these species are involved in phenol and MO degradation, we conducted experiments with the radical scavengers isopropanol (IPA), benzoquinone (BQ), ammonium oxalate (AO), catalase (CAT), and NO3 − ions. IPA is a scavenger of • OH radical species [23,43]; BQ acts as • O2 − radical
scavenger [23,44]; AO is a scavenger of h+ radical species [45]; CAT acts as H2 O2 radical scavenger [43] and NO3 − ions mainly act as ecb − scavengers [46]. The results were shown in Fig. 11. Once the radical species played a major role in the degradation of MO and phenol, the degradation ratio was expected to be decreased greatly. In Fig. 11, it can be seen that MO and phenol degradation were reduced by the addition of each radical scavenger, more markedly with BQ, followed by CAT, NO3 − ions, AO and IPA. The
Fig. 10. Cycling runs in the photocatalytic degradation of MO (A) and phenol (B) in the presence of 0.58% C-ZnSn(OH)6 catalyst under UV (max = 254 nm) light irradiation.
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Fig. 11. The effects of different scavengers on the degradation of MO and phenol over ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 under UV light irradiation.
degradation ratios of MO and phenol over pure ZnSn(OH)6 were 16.3% and 12.9% (BQ), 33.1% and 25.5% (CAT), 46.3% and 30.2% (NO3 − ), 57.5% and 39.3% (AO), 63.7% and 43.1% (IPA), respectively. These degradation ratios were on average 4.4 and 4.8, 2.1 and 2.4, 1.5 and 2.0, 1.2 and 1.6, 1.1 and 1.4-fold smaller than those obtained with the ZnSn(OH)6 system with no scavenger under otherwise identical conditions, respectively. In addition, the degradation ratios of MO and phenol over 0.58% C-ZnSn(OH)6 were 23.5% and 12.5% (BQ), 47.6% and 40.7% (CAT), 58.4% and 46.8% (NO3 − ), 70.5% and 50.9% (AO), 77.7% and 54.3% (IPA), respectively. These degradation ratios were on average 4.0 and 6.6, 1.9 and 2.0, 1.6 and 1.7, 1.3 and 1.6, 1.2 and 1.5-fold smaller than those obtained with the 0.58% C-ZnSn(OH)6 system with no scavenger under otherwise identical conditions, respectively. Eqs. (12)–(16) indicate that IPA, BQ, AO, CAT, and NO3 − mainly inhibit • OH, • O2 − , h+ , H2 O2 , and ecb − radicals, respectively. Hence, we conclude that: (i) MO and phenol degradation takes place by oxidation and reduction reactions (Fig. 11); (ii) MO and phenol degradation by oxidative pathway is the main degradation mechanism, as shown by the larger decrease in degradation ratio with the addition of benzoquinone versus nitrate ions. 3.4.2. Origin of reactive species for MO and phenol degradation Taking the kinds of reactive species involved in the MO (phenol) degradation into account, the photocatalytic process could be described as the following Eqs. (5)–(16): Catalyst + hv → ecb − + hvb +
(5)
ecb − + O2 → • O2 −
(6)
ecb − + • O2 − + 2H+ → H2 O2
(7)
H2 O2 + ecb − → • OH + OH−
(8)
H2 O2 + hv → 2• OH
(9)
H2 O + hvb + → • OH + H+
(10)
OH− + hvb + → • OH
(11)
MO(phenol) + • OH → products
(12)
MO(phenol) + • O2 − → products
(13)
MO(phenol) + hvb + → products
(14)
MO(phenol) + H2 O2 → products
(15)
MO(phenol) + ecb − → products
(16)
In the above process, electron–hole pairs were directly produced by catalyst af ter UV light illumination. Then, the photogenerated electrons transfered to conduction band bottom of the photocatalyst and reacted with the adsorbed O2 on the surface of catalyst to form • O2 − , H2 O2 , and • OH. Meanwhile, the holes were left on the valence band top, reacting with the adsorbed H2 O or OH− on the surface of catalyst to form • OH. After that, • OH, • O2 − and H2 O2 that oxidized MO (phenol), or the hvb + and ecb − reacted directly with MO (phenol). 3.4.3. Photocatalytic activity enhancement mechanism of C-ZnSn(OH)6 When irradiation with energy (hv) greater than the bandgap energy of C-ZnSn(OH)6 , electron–hole pairs (ecb − /hvb + ) are generated in C-ZnSn(OH)6 (Eq. (5)). Then, the ecb − /hvb + pairs migrated to the surface of the photocatalysts and reacted with the species adsorbed on the surface (e.g. Eqs. (6)–(11)). These reactions prevented the ecb − /hvb + pairs from combining. However, if the ecb − /hvb + pairs were not consumed by the locally absorbed species, they would be recombined, resulting in the decrease of reaction efficiency. The degradation efficiency of MO and phenol over the 0.58% C-ZnSn(OH)6 was greatly enhanced as compared to pure ZnSn(OH)6 , indicating that a proper carbon content in the ZnSn(OH)6 improved the separation efficiency of photogenerated electron–hole pairs of C-ZnSn(OH)6 photocatalysts. The enhancement of separation efficiency of electron–hole pairs in the C-ZnSn(OH)6 photocatalysts was also confirmed by photoluminescence (PL) emission spectra of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 photocatalysts in Fig. 12. It is well known that PL emission spectra have been a useful technique to investigate the separation efficiency of photogenerated charge carriers in a semiconductor photocatalyst [47–49]. The comparison of PL emission spectra (excited at 280 nm) of pure ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 photocatalysts at room temperature was shown in Fig. 12. It can be seen that the two samples have a broad emission peak in the wavelength range of 350–690 nm. The PL emitting peaks are similar while PL intensity of 0.58% CZnSn(OH)6 is dramatically weakened compared with that of pure ZnSn(OH)6 , indicating that the recombination of photogenerated charge carriers was greatly inhibited by an appropriate carbon incorporation. In generally, the photocatalytic activity of catalyst is related to its BET surface areas, particle sizes, crystallinity, light absorption ability, surface states, etc. The increase of photocatalytic activities
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preparation route not only reduced the energy consumption for the fabrication of C-ZnSn(OH)6 photocatalysts but also might extend the utilization of C-ZnSn(OH)6 photocatalysts at low temperature in the future. Acknowledgments This work was supported by the National Natural Science Foundation of China (21201037) and Natural Science Foundation of Higher Education Institutions in Anhui Province (KJ2012A217) and school-level item (2012HJJC01ZD) of Anhui Provincial Key Laboratory for Degradation and Monitoring of Pollution of the Environment. References
Fig. 12. PL spectra of ZnSn(OH)6 and 0.58% C-ZnSn(OH)6 samples recorded at room temperature with the excitation wavelength of 280 nm.
of C-ZnSn(OH)6 samples with increase of carbon content (Fig. 7) can be understood by the two facts. Firstly, as shown in Fig. 5A, with increasing the carbon content, the absorption intensity of C-ZnSn(OH)6 samples increases in the 200–600 nm regions, which results in the increases of photocatalytic activities. Secondly, with increasing carbon content, the BET surface area (Table 1) of C-ZnSn(OH)6 catalysts increases and the particle size (Fig. 3A–D) of C-ZnSn(OH)6 catalysts decreases, which enhanced the interfacial charge transfer of inter-particles and improved the separation efficiency of photo-generated electron–hole pairs [50,51], leading to the increase of photocatalytic activities. When the carbon content is higher than 0.58%, the light absorption intensity (Fig. 5A) of C-ZnSn(OH)6 particles further increases in the 200–600 nm regions, but the photocatalytic activities (Fig. 7) of C-ZnSn(OH)6 catalysts decreased remarkably, indicating that the excessive carbon doped in the ZnSn(OH)6 decreases the photocatalytic activity of C-ZnSn(OH)6 catalysts. This may be attributed to the two facts. Firstly, an excess of carbon decreased the crystallinity (Table 1) of the C-ZnSn(OH)6 catalyst, thereby inhibiting the transport of electrons and increasing the probability of electrons and holes being trapped by both the crystal defects and the recombination process [50,51]. Secondly, an excess carbon maybe result in the introduction of new defect sites or recombination centers that enhance the recombination of photogenerated electrons and holes [52–55], leading to the decrease of photocatalytic activities. Thus, 0.58% C-ZnSn(OH)6 with an appropriate carbon content exhibited the highest photocatalytic activity in our studies. 4. Conclusions In summary, C-ZnSn(OH)6 photocatalysts with different weight percents of carbon were successfully synthesized by a facile and economical hydrothermal process at 433 K. The C-ZnSn(OH)6 samples exhibited a cubic perovskite ZnSn(OH)6 phase. With increasing carbon content, the absorption intensity of C-ZnSn(OH)6 increased in the 200–600 nm light region and the absorption edge shifted significantly to longer wavelengths compared with that of pure ZnSn(OH)6 . The photocatalytic conversion ratio of benzene and cyclohexane by 0.58% C-ZnSn(OH)6 was up to 77% and 50.5%, which is about 16.0 and 1.39, 12.3 and 6.39 times higher than that of P25 and pure ZnSn(OH)6 , respectively. The 0.58% C-ZnSn(OH)6 sample also exhibited high photocatalytic activity toward MO and phenol in suspended solution. Based on the characterization results and the detection of active species, the enhanced photocatalytic activities of C-ZnSn(OH)6 were discussed. In this study, the simple and low-cost
[1] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746–750. [2] G. Palmisano, E. García-López, G. Marcí, V. Loddo, S. Yurdakal, V. Augugliaro, L. Palmisano, Chem. Commun. 46 (2010) 7074–7089. [3] Y.Z. Li, H. Zhang, Z.M. Guo, J.J. Han, X.J. Zhao, Q.N. Zhao, S.J. Kim, Langmuir 24 (2008) 8351–8357. [4] Y.J. Hua, C.T. Wang, J.Y. Liu, B. Wang, X.L. Liu, C.Y. Wu, X.Y. Liu, J. Mol. Catal. A: Chem. 365 (2012) 8–14. [5] J. Velásquez, S. Valencia, L. Rios, G. Restrepo, J. Marín, Chem. Eng. J. 203 (2012) 398–405. [6] Y.Y. Lv, L.S. Yu, X.L. Zhang, J.Y. Yao, R.Y. Zou, Z. Dai, Appl. Surf. Sci. 257 (2011) 5715–5719. [7] G.S. Martra, L. Coluccia, V. Marchese, V. Augugliaro, L. Loddo, M. Palmisano, Catal. Today 53 (1999) 695–702. [8] A.K.P. Mann, S.E. Skrabalak, Chem. Mater. 23 (2011) 1017–1022. [9] D.X. Wu, H.T. Zhu, C.Y. Zhang, L. Chen, Chem. Commun. 46 (2010) 7250–7252. [10] F. Amano, K. Nogami, M. Tanaka, B. Ohtani, Langmuir 26 (2010) 7174–7180. [11] F. Dong, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, J. Hazard. Mater. 219-220 (2012) 26–34. [12] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Appl. Catal. B: Environ. 111–112 (2012) 271–279. [13] Y.B. Chen, D.Z. Li, M. He, Y. Hu, H. Ruan, Y.M. Lin, J.H. Hu, Y. Zheng, Y. Shao, Appl. Catal. B: Environ. 113–114 (2012) 134–140. [14] J.Z. Xu, C.Y. Zhang, H.Q. Qu, C.M. Tian, J. Appl. Polym. Sci. 98 (2005) 1469–1475. [15] F. Andre, P.A. Cusack, A.W. Monk, R. Seangprasertkij, Polym. Degrad. Stab. 40 (1993) 267–273. [16] L.L. Wang, K.B. Tang, Z.P. Liu, D. Wang, J. Sheng, W. Cheng, J. Mater. Chem. 21 (2011) 4352–4357. [17] H.T. Gao, C.H. Ding, D.M. Dai, J. Mol. Struct.: Theochem. 944 (2010) 156–162. [18] H.Y. Li, D.J. Wang, H.M. Fan, P. Wang, T.F. Jiang, T.F. Xie, J. Colloid Interface Sci. 354 (2011) 175–180. [19] S. Mozia, M. Toyoda, M. Inagaki, B. Tryba, A.W. Morawski, J. Hazard. Mater. 140 (2007) 369–375. [20] F. Dong, H.Q. Wang, Z.B. Wu, J. Phys. Chem. C 113 (2009) 16717–16723. [21] R. Ocampo-Péreza, M. Sánchez-Polo, J. Rivera-Utrilla, R. Leyva-Ramos, Appl. Catal. B: Environ. 104 (2011) 177–184. [22] J. Rivera-Utrilla, M. Sánchez-Polo, M.M. Abdel daiem, R. Ocampo-Pérez, Appl. Catal. B: Environ. 126 (2011) 100–107. [23] G.T. Li, K.H. Wong, X. Zhang, C. Hu, J.C. Yu, R.C. Chan, P.K. Wong, Chemosphere 76 (2009) 1185–1191. [24] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Catal. Commun. 13 (2011) 63–68. [25] M. Galceran, M.C. Pujol, C. Zaldo, F. Díaz, M. Aguiló, J. Phys. Chem. C 113 (2009) 15497–15506. [26] L.X. Han, J. Liu, Z.J. Wang, K. Zhang, H. Luo, B. Xu, X. Zou, X. Zheng, B. Ye, X.B. Yu, Cryst. Eng. Commun. 14 (2012) 3380–3386. [27] H. Fan, S.Y. Ai, P. Ju, Cryst. Eng. Commun. 13 (2011) 113–117. [28] H.Y. Li, D.J. Wang, H.M. Fan, P. Wang, T.F. Jiang, T.F. Xie, J. Colloid Interf. Sci. 354 (2011) 175–180. [29] L.X. Mai, C.M. Huang, D.W. Wang, Z.G. Zhang, Y.Y. Wang, Appl. Surf. Sci. 255 (2009) 9285–9289. [30] Y. Choi, T. Umebayashi, M. Yoshikawa, J. Mater. Sci. 39 (2004) 1837–1839. [31] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243–2245. [32] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, W.F. Maier, Appl. Catal. B: Environ. 32 (2001) 215–223. [33] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747–753. [34] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [35] S.F. Chen, Y.Z. Liu, Chemosphere 67 (2007) 1010–1017. [36] C.A. Aguilar, C. Montalvo, J.G. Ceron, E. Moctezuma, Int. J. Environ. Res. 5 (4) (2011) 1071–1078. [37] X.F. Chang, J. Huang, Q.Y. Tan, M. Wang, G.B. Ji, S.B. Deng, G. Yu, Catal. Commun. 10 (2009) 1957–1961. [38] J.J. Xu, Y.H. Ao, D.G. Fu, Appl. Surf. Sci. 256 (2009) 884–888. [39] C.H. Wu, H.W. Chang, J.M. Chen, J. Hazard. Mater. 137 (2006) 336–343. [40] A.N. Ökte, Ö. Yılmaz, Appl. Catal. A: Gen. 354 (2009) 132–142. [41] P. Kumar, M. Govindaraju, S. Senthamilselvi, K. Premkumar, Colloids Surf. B 103 (2013) 658–661. [42] C.S. Guo, J. Xu, Y. He, Y. Zhang, Y.Q. Wang, Appl. Surf. Sci. 257 (2011) 3798–3803.
H. Li et al. / Journal of Molecular Catalysis A: Chemical 378 (2013) 164–173 [43] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Environ. Sci. Technol. 44 (2010) 1392. [44] M.C. Yin, Z.S. Li, J.H. Kou, Z.G. Zou, Environ. Sci. Technol. 43 (2009) 8361–8366. [45] N. Zhang, S.Q. Liu, X.Z. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 9136–9145. [46] U.F. Alkaram, A.A. Mukhlis, A.H. Al-Dujaili, J. Hazard. Mater. 169 (2009) 324–332. [47] L.Q. Jing, Y.C. Qu, B.Q. Wang, S.D. Li, B.J. Jiang, L.B. Yang, W. Fu, H.G. Fu, J.Z. Sun, Sol. Energy. Mater. Sol. Cells 90 (2006) 1773–1787. [48] Y.M. Wu, J.L. Zhang, L. Xiao, F. Chen, Appl. Catal. B: Environ. 88 (2009) 525. [49] C. Borgohain, K.K. Senapati, K.C. Sarma, P. Phukan, J. Mol. Catal. A: Chem. 363–364 (2012) 495–500.
173
[50] L.C. Chen, Y.C. Ho, W.S. Guo, C.M. Huang, T.C. Pan, Electrochim. Acta 54 (2009) 3884–3891. [51] L.C. Chen, F.R. Tsai, S.H. Fang, Y.C. Ho, Electrochim. Acta 54 (2009) 1304–1311. [52] F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607–365617. [53] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808–3816. [54] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483–5486. [55] E.B. Gracien, J.N. Shen, X.R. Sun, D. Liu, M.C. Li, S.D. Yao, J. Sun, Thin Solid Films 515 (2007) 5287–5297.