- Email: [email protected]
reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants
Separation and Purification Technology 142 (2015) 25–32
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Visible light-driven Bi2Sn2O7/reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants Hong Liu ⇑, Zhitong Jin, Yun Su, Yong Wang ⇑ Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 26 December 2014 Accepted 26 December 2014 Available online 8 January 2015 Keywords: Bi2Sn2O7 Graphene Photocatalysis Visible light Degradation
a b s t r a c t In this work, a novel Bi2Sn2O7/reduced graphene oxide (RGO) nanocomposite was synthesized by a onestep hydrothermal method. The prepared composite was characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrometry (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) emission spectroscopy and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the Bi2Sn2O7/RGO composite was investigated by the degradation of rhodamine B (RhB) and phenol. An increase in photocatalytic activity was observed for Bi2Sn2O7/RGO composite compared with pure Bi2Sn2O7 under visible light. The enhanced photocatalytic performance of the composite was mainly ascribed to the more effective charge separations and the excellent adsorption capacity of RGO. The composite maintained its ability to degrade pollutants efficiently, even after 4 cycles of photocatalysis. Further study proved that both the holes and hydroxyl radicals were the active species in the degradation process. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis has attracted worldwide attention because of its potential applications in water splitting and the degradation of organic compounds, and TiO2 is the most studied semiconductor photocatalyst because of its high photocatalytic activity, low cost, non-toxicity, and chemical stability [1–4]. However, its large band gap (3.2 eV for anatase) determined that it was only active under ultraviolet (UV) light irradiation. To take sufficient advantage of solar energy, the development of photocatalysts capable of photoinduced charge separation upon excitation in the visible spectral region is emerging as an important research direction in this field [5–9]. Bi2Sn2O7, a novel semiconducting material with a typical pyrochlore structure and a band gap of 2.5–2.8 eV, has received considerable attention in catalysis and gas sensors in recent years [10–12]. The crystal structure of Bi2Sn2O7 is constructed of octahedral SnO6, and these octahedra connect to each other by sharing vertexes. A network of corner-shared octahedra can facilitate the mobility of the charged carriers, which may make Bi2Sn2O7 an efficient visible light-driven photocatalytic material [13–15]. Very recently, Wu
⇑ Corresponding authors. Tel.: +86 21 66137487; fax: +86 21 66137725. E-mail addresses: [email protected] (H. Liu), [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.seppur.2014.12.027 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
et al. [13] synthesized nano-Bi2Sn2O7 by a hydrothermal method and studied its photocatalytic activity in the degradation of methyl orange. Tian et al. [14] reported the synthesis of Bi2Sn2O7 and its application in photocatalytic removal of As (III). Xu et al. [15] reported the photocatalytic activity of Bi2Sn2O7 in the degradation of rhodamine B (RhB) under visible light irradiation. However, the photocatalytic efficiency of individually Bi2Sn2O7 was relatively very low. For practical use, the photocatalytic performance of Bi2Sn2O7 should be further improved. Combination of different types of carbon with semiconductor has been suggested as a promising method for an enhanced photocatalytic performance [16–19]. In the past few years, graphene as a novel carbonaceous nanomaterial has attracted more and more interests due to its large specific surface area, remarkable electrical conductivity, excellent adsorptivity, and high chemical and thermal stability [17–19]. Furthermore, graphene can be easily produced at a low cost from natural graphite through chemical oxidation– dispersion–reduction procedures. Efforts have been made to combine semiconductor and graphene or reduced graphene oxide (RGO) to obtain hybrid materials with superior photocatalytic performance [20–29]. For example, significant improvement has been reported for metal oxides [22–26], metal sulfides [27,28], and nonmetal g-C3N4 [29] by using graphene (or RGO) as a synergistic catalyst material. If Bi2Sn2O7 is coupled with graphene, it is possible to improve the efficiency of photoinduced charge separation, leading
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
to high photocatalytic performance. However, to the best of our knowledge, there is no report on the design and fabrication of coupled Bi2Sn2O7/graphene heterostructure so far. In this paper, a novel Bi2Sn2O7/RGO composite has been synthesized by a one-step hydrothermal method for the first time. The prepared sample was characterized, and the photocatalytic activity under visible light irradiation (k > 420 nm) was evaluated by the degradation of RhB and phenol. The result revealed that the Bi2Sn2O7/RGO composite exhibited an enchenced photocatalytic activity in comparison with bare Bi2Sn2O7. The obtained Bi2Sn2O7/RGO photocatalyst displayed good cycling stability under visible light. Furthermore, the mechanism of enhanced photocatalytic activity as well as the main active oxygen species in the photocatalytic reaction process was also investigated.
2. Materials and methods 2.1. Preparation of Bi2Sn2O7/RGO composite All the chemicals were of analytical grade from the Sinopharm Chemical Reagent Co., Ltd. Graphene oxide (GO) was prepared by a modified Hummers method [30]. The Bi2Sn2O7/RGO composite was prepared by a one-pot hydrothermal method. In a typical synthesis, 0.73 g Bi(NO3)35H2O and 0.45 g K2SnO33H2O were added into 50 mL of deionized water. Under vigorous stirring, the pH value of the mixture was adjusted to 12 by using NaOH solution. Meanwhile, 0.1 g GO was dispersed in 20 mL ethanol by sonication for 1 h to get a homogenous suspension of exfoliated graphene oxide. Then, the obtained GO solution was added to the reactedmixture gradually. After stirring continuously for 2 h at room temperature, the mixture was transferred into a Teflon-lined autoclave with a volume of 100 mL. Then, the autoclave was sealed, heated under autogenous pressure at 180 °C for 24 h, and then cooled down to room temperature naturally. The resulting precipitate was filtrated, washed thoroughly, and vacuum dried at 60 °C overnight. For comparison, pure Bi2Sn2O7 was prepared by using the same method without the addition of GO.
2.2. Characterization The crystalline phases of prepared samples were identified by X-ray diffraction (XRD) on a D/MAX-2550 at 40 kV and 40 mA with Cu Ka radiation (k = 0.15418 nm). Structure and morphology of as prepared samples were evaluated by TEM (200CX) and HRTEM (JEM-2010F). Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw, INVIA) with an excitation of 514.5 nm laser light. UV–vis diffuse reflectance spectra (DRS) of the samples were obtained on an UV–vis spectrophotometer (Hitachi U-3010) using BaSO4 as a reference. The photoluminescence (PL) spectra were measured using a Hitachi F-7000 fluorescence spectrophotometer at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660E electrochemical workstation in a standard three-electrode system using the prepared samples as the working electrodes, a platinum wire as the counter electrode, and an Ag/AgCl (saturated KCl) electrode as the reference electrode over the frequency range from 0.01 Hz to 106 Hz. Na2SO4 (1 M) aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 0.08 g of photocatalyst was grinded with 0.01 g of polytetrafluoroethylene (PTFE), 0.01 g of carbon black and 2 mL of isopropanol to produce a slurry. The slurry was then coated onto a 1 cm 1.5 cm titanium mesh electrode by the doctor blade method. Next, the resulting electrodes were dried in vacuum at 60 °C for 24 h. All electrodes studied had a similar film thickness.
2.3. Photocatalytic activity test The photocatalytic activity of the as-prepared photocatalyst was estimated by monitoring the degradation of RhB and phenol in an aqueous solution under visible light irradiation. The reaction was performed in a photochemical reactor (BL-GHX-V, Shanghai Bilon Instruments Co., Ltd., China), equipped with a 500 W Xe lamp combined with a 420 nm cut-off filter as a light source. All experiments were conducted at room temperature in air. In a typical photocatalytic experiment, 0.05 g of the photocatalyst was added into 50 ml of 10 mg/L pollutant solution in a reaction cell with a Pyrex jacket. Prior to irradiation, the suspension was magnetically stirred in the dark for 1 h to reach an adsorption–desorption equilibrium. Then, the suspension was exposed to visible light irradiation under magnetic stirring. At given time intervals, about 5 ml suspensions were collected and centrifuged (12,000 rpm, 6 min) to remove the photocatalyst particles. Then, the pollutant concentration of the obtained solution was analyzed by a UV–vis spectrophotometer (Hitachi, U-3310) by checking the absorbance at 553 nm and 270 nm for RhB and phenol, respectively. To probe the oxidizing species in the photocatalytic process, the disodium ethylenediamine tetraacetate (EDTA-2Na), tert-butyl alcohol (tBuOH), or benzoquinone (BQ), was added in the solution. The concentrations of the quenching chemicals were at 1 mM. 3. Results and discussion 3.1. Structure and properties characterization The XRD patterns of the pristine Bi2Sn2O7 and Bi2Sn2O7/RGO composite are shown in Fig. 1. The XRD of Bi2Sn2O7 shows several strong peaks at 2h = 28.8°, 33.4°, 47.9°, and 56.9°, which represent the formation of a pure cubic phase of Bi2Sn2O7 (JCPDS No. 87-0284) [13,15]. It is found that the main diffraction peaks of Bi2Sn2O7/RGO composite are similar to that of pristine Bi2Sn2O7, and no apparent peaks of RGO are observed in the Bi2Sn2O7/RGO composite, which might be due to the low amount of RGO and relatively low diffraction intensity of RGO in the composite. No apparent peaks of graphene oxide are observed in the Bi2Sn2O7/RGO composite, indicating the successful reduction of GO to RGO through the hydrothermal process. The morphologies of the pristine Bi2Sn2O7 and Bi2Sn2O7/RGO composite were investigated by TEM as shown in Fig. 2. Fig. 2a shows that pure Bi2Sn2O7 is comprised of many irregular nano-
GO Intensity (a. u.)
26
Bi2Sn2O7/RGO
Bi2Sn2O7
10
20
30
40
50
60
70
2 Theta (degree) Fig. 1. XRD patterns of GO, Bi2Sn2O7 and Bi2Sn2O7/RGO.
80
27
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
particles with diameters ranging from 5 to 15 nm. From Fig. 2b, it can be clearly observed that the RGO sheets are decorated by irregular Bi2Sn2O7 nanoparticles, which confirmed the successful attachment of Bi2Sn2O7 nanoparticles to RGO sheets. The HRTEM image shows the characteristic lattice fringes of Bi2Sn2O7 in the RGO nanosheets matrix (Fig. 2c). The lattice spacing is about 0.310 nm, which can be assigned to the (2 2 2) plane of cubic Bi2Sn2O7 [13]. Fig. 2c also reveals the intimate contact between Bi2Sn2O7 nanoparticles and RGO. This intimate contact is in favour of the electronic interaction between Bi2Sn2O7 and RGO and improves the charge separation and the photocatalytic activity. The EDS spectrum of Bi2Sn2O7/RGO (Fig. 2d) exhibits the presence of Bi, Sn, O and C elements. The X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface elemental compositions of Bi2Sn2O7 and Bi2Sn2O7/ RGO. The survey XPS spectra (Fig. 3a) reveal that both Bi2Sn2O7 and Bi2Sn2O7/RGO are composed of Bi, Sn, O and C, which is in good agreement with the EDS results. The Bi 4f XPS spectra in Fig. 3b shows that the binding energies of Bi 4f7/2 and Bi 4f5/2 for Bi2Sn2O7 occur at 158.8 and 164.2 eV, respectively, which are indicative of Bi3+ in Bi2Sn2O7 [8,9,20]. The Sn 3d spectrum of Bi2Sn2O7 (Fig. 3c) shows two peaks at 486.7 and 495.0 eV, indicating the presence of Sn4+ [31]. The peaks corresponding to Bi 4f and Sn 3d in the XPS spectra of the as-prepared Bi2Sn2O7/RGO composite slightly shift toward higher binding energies as compared to pure Bi2Sn2O7. This kind of shift in XPS measurement indicates that there is a strong interaction between Bi2Sn2O7 and RGO sheets in the nanohybrid [32,33]. The intense interaction may result in the formation of electron transfer channel, which is beneficial to the improvement of
(a)
(c)
photoinduced charge separation rate during the photocatalytic process [33]. In addition, the high-resolution spectra in Fig. 3d shows that the peak binding energies of 530.4 eV is assigned to O 1s [34]. The XPS spectrum of C1s from GO (Fig. 3e) can be deconvoluted into three smaller peaks, which are ascribed to the following functional groups: sp2 bonded carbon (CAC, 284.8 eV), epoxy/hydroxyls (CAO, 286.8 eV), and carboxyl (OAC@O, 288.8 eV) [35], indicating the high percentage of oxygen-containing functional groups. In Fig. 3f, although the three types of carbon species can still be seen in the Bi2Sn2O7/RGO composite, the XPS peaks corresponding to CAO and OAC@O are severely weakened in comparison to Fig. 3e, indicating that GO has been highly reduced during the hydrothermal synthesis. Besides, no any peak corresponding to the BiAC bond (at 281.2 eV in C 1s spectrum [36]) is observed, suggesting no carbon doping in the lattice of Bi2Sn2O7. The Raman spectra of GO and Bi2Sn2O7/RGO are presented in Fig. 4. In Raman spectra, both samples exhibit two strong peaks, denoted as the disorder peak (D, centered at 1359 cm 1) and the graphitic peak (G, at 1606 cm 1) [37,38]. Compared with GO, Bi2Sn2O7/RGO displays an increased D/G intensity ratio, indicating the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated GO [39]. The UV–vis diffuse reflectance spectra of the synthesized samples are shown in Fig. 5a. Compared to pure Bi2Sn2O7, Bi2Sn2O7/ RGO composite shows an increased absorption in both UV and visible range and an obvious red-shift in the absorption edge, which can be attributed to the presence of carbon in the composites, reducing reflection of light [22,40]. Similar phenomenon has been noted in previous studies [3,9,20,27,40,41]. The band gap energy
(b)
(d)
Itensity (a.u.)
Cu
C
Bi Sn Sn
O
0
2
4
Cu
6
8
Bi
10
Bi
12
14
Energy (keV) Fig. 2. TEM images of Bi2Sn2O7 (a) and Bi2Sn2O7/RGO (b), HRTEM image of Bi2Sn2O7/RGO (c) and EDS spectrum of Bi2Sn2O7/RGO (d).
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
O 1s Sn 3d Bi 4d Bi 4d
Bi 4f
(b) Bi 4f
Intensity (a. u.)
Bi2Sn2O7
Bi 4f7/2
Bi 4f5/2 Bi 5d O 2s
Sn 3p Sn 3p Bi 4p
O Sn 3s
Sn
Intensity (a. u.)
(a) Survey
C 1s
28
Bi2Sn2O7
Bi2Sn2O7/RGO
Bi2Sn2O7/RGO
1000
800
600
400
200
0
168
166
Binding energy (eV)
164
162
160
158
156
154
Binding energy (eV)
(d) O 1s
(c) Sn 3d Sn 3d5/2
Intensity (a. u.)
Intensity (a. u.)
Sn 3d3/2
Bi2Sn2O 7
Bi2Sn2O7
Bi2Sn2O7/RGO
Bi2Sn2O /RGO 7
500 498 496 494 492 490 488 486 484 482
540 538 536 534 532 530 528 526 524
Binding energy (eV)
Intensity (a. u.)
(e) C 1s
Binding energy (eV)
(f) C 1s
C-O
Intensity (a. u.)
C-C
O-C=O
292
29 0
28 8
28 6
28 4
282
Binding energy (eV)
C-C
292
O-C=O
O-C
290
288
286
284
282
Binding energy (eV)
Fig. 3. XPS spectra of Bi2Sn2O7 and Bi2Sn2O7/RGO: (a) survey, (b) Bi 4f, (c) Sn 3d, (d) O 1s, (e) C 1s of GO, and (f) C 1s of Bi2Sn2O7/RGO.
can be estimated by the following formula: a h m = A(h m–Eg)n/2, where a, h, m, Eg, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively. And n is the electron transitions depending on the semiconductor characteristics (direct transition n = 1 and indirect transition n = 4). For Bi2Sn2O7, the value of n is 4 for the indirect transition [13]. Fig. 5b plots the relationship of (a h m)1/2 versus photon energy (h m), and shows that the bandgap energy of pure Bi2Sn2O7 is 2.48 eV, whereas this value is reduced to 1.85 eV by decorating with RGO. This phenomenon is possibly ascribed to the interaction between Bi2Sn2O7 and RGO [3,20,22,29], which has been evidenced by XPS analysis as described above.
3.2. Photocatalytic activity The photoactivity of the Bi2Sn2O7/RGO composite was evaluated by photocatalytic degradation of RhB and phenol under visible light irradiation, and the results are shown in Fig. 6. It can be seen that both RhB and phenol are very stable under visible light irradiation without the catalyst. Fig. 6a shows that the RhB removal rate is about 95.8% after irradiation for 125 min in the presence of Bi2Sn2O7/RGO composite, while the value of pristine Bi2Sn2O7 is only 83.7%. Meanwhile, the degradation rate of phenol on Bi2Sn2O7/RGO composite reaches 81.1% after irradiation for 200 min, whereas only 64.4% of phenol is removed by pristine Bi2Sn2O7 after same
29
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
Table 1 Pseudo first-order rate constants of the photocatalytic decomposition of different pollutants.
G
D
Catalyst
Intensity (a. u.)
GO
ID/IG= 0.667
Bi2Sn2O7 Bi2Sn2O7/RGO
Bi2Sn2O7/RGO
ID/IG= 0.924
800
1200
Raman
1600
2000
Shift (cm-1)
Fig. 4. Raman spectra of the Bi2Sn2O7/RGO composite and GO.
irradiation time (Fig. 6b). These results indicate clearly that the introduction of RGO can enhance the photocatalytic performance of Bi2Sn2O7. Notably, the photocatalytic activity of the obtained Bi2Sn2O7/RGO composite on the degradation of organic pollutant RhB and phenol is higher than the Bi2Sn2O7-based materials reported previously [42,43]. These photocatalytic properties are also better than various graphene-based composites [20,28,44–52]. For detailed analysis the photocatalysis kinetics of the pollutants degradation, the pseudo-first order model was applied. This model is generally used for photocatalytic degradation process if the initial
(a)
(αhν)1/2
Adsorbance (a.u.)
1.2
Bi2Sn2O7/RGO
0.8382 1.5350
R2
k (h
1
0.9777 0.9872
0.3093 0.5146
)
R2 0.9976 0.9907
Bi2Sn2O7/RGO
2.0 1.5 Bi2Sn2O7
1.0
Bi2Sn2O7
0.0 200
)
3.0 2.5
0.8
Phenol 1
concentration of pollutant is low [53]. The rate constants were evaluated from the data plotted in Fig. 6a and b and summarized in Table 1. The rate constant for Bi2Sn2O7/RGO composite to remove RhB is 1.54 h 1, which is 1.83 times as large as that of pristine Bi2Sn2O7 (0.8382 h 1). This value for the composite is also 1.66 times larger than pristine Bi2Sn2O7 for the degradation of phenol. In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications. To confirm the stability of the Bi2Sn2O7/RGO photocatalyst, the circulating runs in the photocatalytic degradation of RhB and phenol in the presence of Bi2Sn2O7/RGO under visible-light was checked. In the work, Bi2Sn2O7/RGO was recycled for four times in the same photocatalytic reactions. After each reuse cycle which lasted for 125 min (for degradation of RhB) or 200 min (for degradation of phenol), the photocatalyst was separated from the aqueous suspension by filtration, washed with deionized water and ethanol, dried, and weighed for the next reuse cycle. It can be seen from Fig. 7, after four recycles for the photodegradation of RhB and phenol, the catalyst do not exhibit any significant loss of activity, indicating the good stability of the Bi2Sn2O7/RGO composite during photocatalytic degradation of model pollutant.
(b)
1.6
0.4
RhB k (h
0.5 0.0
300
400
500
600
700
800
1.5
2.0
Wavelength (nm)
2.5
3.0
3.5
4.0
4.5
5.0
Photo energy (eV)
Fig. 5. (a) UV–vis spectra of Bi2Sn2O7 and Bi2Sn2O7/RGO composite and (b) plots of the (a h m)1/2 versus photon energy (h m) for as-synthesized samples.
(a) 1.0
(b) 1.0
0.8
C/C0
C/C0
Bi2Sn2O7/RGO
0.6
Bi2Sn2O7 Bi2Sn2O7/RGO
0.8
Bi2Sn2O7 blank 0.4
blank
0.6 0.4
0.2 0.2 0.0 0
25
50
75
Time (min)
100
125
0
40
80
120
160
200
Time (min)
Fig. 6. Photocatalytic activities of as-prepared samples on the degradation of RhB (a) and phenol (b) under visible-light irradiation.
30
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
1.00
(a) 1.0 2 nd run
1 st run
4 th run
3 rd run
Bi2Sn2O7 Bi2Sn2O7/RGO
0.8
C/C0
0.96 0.6 0.92
C/C0
0.4 0.2
0.88
0.0 0
125
125
125
125
0.84
Time (min) 0
(b) 1.0
2 nd run
1 st run
20
4 th run
3 rd run
30
40
50
60
Time (min)
0.8
Fig. 9. Absorption activities of Bi2Sn2O7 and Bi2Sn2O7/RGO.
0.6 0.4 0.2 0.0 0
200
200
200
200
Time (min) Fig. 7. The cyclic test of as-prepared Bi2Sn2O7/RGO composite on the degradation of RhB (a) and phenol (b).
3.3. Mechanism on enhancement of photocatalytic activity
Relative intensity (a. u.)
The photocatalytic activity of semiconductors is closely related with the recombination rate of the photoinduced electrons and holes. To investigate the efficiency of charge carrier trapping, migration, and transfer in the as-prepared Bi2Sn2O7 and Bi2Sn2O7/ RGO, the photoluminescence (PL) emission spectra of these samples were measured. Fig. 8a presents PL emission spectra of pure Bi2Sn2O7 and Bi2Sn2O7/RGO photocatalyst monitored at an excitation wavelength of 350 nm. It can be seen that the pure Bi2Sn2O7 shows a broad PL emission band, which is ascribed to luminescence from localized surface states due to recombination of photogenerated electron–hole pairs. The Bi2Sn2O7/RGO composite exhibits a much lower emission intensity than pure Bi2Sn2O7,
implying that the recombination of photogenerated electrons and holes is inhibited greatly in the Bi2Sn2O7/RGO system. The improved efficiency in inhibiting the recombination of photogenerated charge carriers of Bi2Sn2O7 by the use of RGO can be further supported by the electrochemical impedance technique which is a reliable way to characterize electrical conductivity. Fig. 8b shows the electrochemical impedance spectra (EIS) of Bi2Sn2O7 and Bi2Sn2O7/RGO. It is clear that the Bi2Sn2O7/RGO photocatalyst has smaller arc as compared to the pristine Bi2Sn2O7, suggesting the more efficient interfacial charge transfer over Bi2Sn2O7/RGO. Graphene or RGO as a two-dimensional sheet of carbon atoms has superior electrical conductivity, which would make it an excellent electron-transport material in the process of photocatalysis. The photogenerated electrons of excited Bi2Sn2O7 were transferred instantly from the conduction band of Bi2Sn2O7 to RGO, resulting in a minimized charge recombination and offering an enhanced photocatalytic activity. In addition, adsorption performance of the targeted substance is also very important for the surface reaction. To evaluate the adsorption ability of Bi2Sn2O7 and Bi2Sn2O7/RGO, the residue rate of RhB after 60 min stirring in the dark was determined. As shown in Fig. 9, the residue rate of RhB in the solution with Bi2Sn2O7 as the adsorbent is 87.4%, whereas that of Bi2Sn2O7/RGO as the adsorbent is 84.6%. It is obviously that the adsorption ability of Bi2Sn2O7 was enhanced by introduction of RGO, which is also a prequisite for good photocatalytic activity. To understand the photocatalytic mechanism, the main active oxidant in the photocatalytic reaction process should be identified.
50
(a)
(b) Bi2Sn2O7 Bi2Sn2O7/RGO
40 Bi2Sn2O7
Bi2Sn2O7/RGO
-Z''(ohm)
C/C0
10
30 20 10 0
380 400 420 440 460 480 500 520 540 560 580
Wavelength (nm)
10
15
20
25
30
35
40
Z'(ohm)
Fig. 8. (a) PL spectra and (b) Nyquist impedance plots of Bi2Sn2O7 and Bi2Sn2O7/RGO composite.
45
50
31
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
(a) 1.0
(b) 1.0 Bi2Sn2O7/RGO + EDTA-2Na + t-BuOH + BQ
C/C0
0.6 0.4 0.2
0.8
C/C0
0.8
0.6 0.4
Bi2Sn2O7/RGO + EDTA-2Na + t-BuOH + BQ
0.2
0.0
0.0 0
25
50
75
100
125
0
40
Time (min)
80
120
160
200
Time (min)
Fig. 10. Photocatalytic degradation of RhB (a) and phenol (b) with the addition of hole and radical scavenger.
4. Conclusion
O2
e-
· O2
-
Pollutants
ee-
e-
CB
Bi2Sn2O7 h+
h+
h+ VB
h+
Oxidized products Pollutants
A novel Bi2Sn2O7/RGO photocatalyst has been synthesized via a one-step hydrothermal method in this study. Under visible irradiation, the Bi2Sn2O7/RGO photocatalyst was found to exhibit higher photocatalytic activities than pure Bi2Sn2O7 in the degradation of RhB and phenol. The photocatalytic activity of Bi2Sn2O7/RGO in degradation of RhB and phenol was 1.83 and 1.66 times that of pure Bi2Sn2O7, respectively. The enhanced photocatalytic performance of the composite was mainly ascribed to the more effective charge separations and the excellent adsorption capacity of RGO. Both the holes and hydroxyl radicals were the active species in the degradation process. In addition, Bi2Sn2O7/RGO displayed an excellent stability during four reaction cycles, indicating Bi2Sn2O7/ RGO has a potential application in water purification. Acknowledgments
Scheme 1. Photocatalytic mechanism of the Bi2Sn2O7/RGO composite under visible light irradiation.
It is well-known that the oxidants generated in the photocatalytic process can be measured through trapping by t-BuOH (a scavenger of hydroxyl radicals), EDTA-2Na (a scavenger of holes) or BQ (a scavenger of O2 ). It can be clearly seen from Fig. 10 that the addition of EDTA-2Na greatly reduces the photodegradation rate of RhB and phenol in the Bi2Sn2O7/RGO suspension, whereas the addition of BQ has little effect on the photodegradation rate of RhB and phenol. In addition, the addition of t-BuOH as hydroxyl radicals’ scavenger causes a minor change in the photocatalytic degradation of the above pollutants. It indicates that both the holes and hydroxyl radicals are the active species in the photocatalytic degradation of RhB and phenol, while O2 can be negligible in the reaction. Based on the experimental results, a schematic mechanism for the degradation of RhB and phenol with Bi2Sn2O7/RGO as the photocatalyst is illustrated in Scheme 1. As irradiated, the Bi2Sn2O7 particles were excited and generated holes and electrons. Due to the higher work function of RGO than that of Bi2Sn2O7, the excited electrons quickly transported to RGO and then reacted with the adsorbed O2 on the RGO surfaces to form superoxide radical anions (O2 ), which could further react with H+ or H2O to produce OH radicals responsible for the degradation of the pollutants [5,54]. Meanwhile, holes left on Bi2Sn2O7 surfaces participated in the degradation of pollutants either by direct oxidation or by OH radicals produced during the reaction of holes with the adsorbed H2O or OH [5,54].
The authors gratefully acknowledge Natural Science Foundation of Shanghai (12ZR1410300), the follow-up Program for Professor of Special Appointment in Shanghai (Eastern Scholar), the National Natural Science Foundation of China (51271105), Shanghai Municipal Government (11JC1403900, 11SG38, IRT13078) for financial support. The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations. References [1] S.D. Perera, R.G. Mariano, K. Vu, N. Nour, O. Seitz, Y. Chabal, K.J. Balkus Jr., ACS Catal. 2 (2012) 949–956. [2] L.W. Zhang, H.B. Fu, Y.F. Zhu, Adv. Funct. Mater. 18 (2008) 2180–2189. [3] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [4] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026–3033. [5] X.F. Zhang, X. Quan, S. Chen, H.T. Yu, Appl. Catal. B: Environ. 105 (2011) 237– 242. [6] Y.S. Fu, X. Wang, Ind. Eng. Chem. Res. 50 (2011) 7210–7218. [7] A. Mukherji, B. Seger, G.Q. Lu, L.Z. Wang, ACS Nano 5 (2011) 3483–3492. [8] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Appl. Catal. B: Environ. 111–112 (2012) 271–279. [9] H. Liu, Y. Su, Z. Chen, Z.T. Jin, Y. Wang, J. Hazard. Mater. 266 (2014) 75–83. [10] G.S.V. Coles, S.E. Bond, G. Williams, J. Mater. Chem. 4 (1994) 23–27. [11] G.S. Devi, S.V. Manorama, V.J. Rao, J. Electrochem. Soc. 145 (1998) 1039–1044. [12] L. Mones, P. Ruiz, B. Delmon, M. Devillers, Appl. Catal. A: Gen. 171 (1998) 131– 143. [13] J.J. Wu, F.Q. Huang, X.J. Lu, P. Chen, D.Y. Wan, F.F. Xu, J. Mater. Chem. 21 (2011) 3872–3876. [14] Q.F. Tian, J.D. Zhuang, J.X. Wang, L.Y. Xie, P. Liu, Appl. Catal. A: Gen. 425–426 (2012) 74–78. [15] W.C. Xu, G.Y. Zhou, J.Z. Fang, Z. Liu, Y.F. Chen, C.P. Cen, Int. J. Photoenergy (2013) 1–9. [16] R. Leary, A. Westwood, Carbon 49 (2011) 741–772. [17] B.F. Machado, P. Serp, Catal. Sci. Technol. 2 (2012) 54–75.
32
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
[18] X.Q. An, J.C. Yu, RSC Adv. 1 (2011) 1426–1434. [19] Q.J. Xiang, J.G. Yu, M. Jaroniec, Chem. Soc. Rev. 41 (2012) 782–796. [20] H. Liu, W.R. Cao, Y. Su, Z. Chen, Y. Wang, J. Colloid Interface Sci. 398 (2013) 161–167. [21] E.P. Gao, W.Z. Wang, M. Shang, J.H. Xu, Phys. Chem. Chem. Phys. 13 (2011) 2887–2893. [22] X.Y. Zhang, H.P. Li, X.L. Cui, Y.H. Lin, J. Mater. Chem. 20 (2010) 2801–2806. [23] P.D. Tran, S.K. Batabyal, S.S. Pramana, J. Barber, L.H. Wong, S.C.J. Loo, Nanoscale 4 (2012) 3875–3878. [24] H. Seema, K.C. Kemp, V. Chandra, K.S. Kim, Nanotechnology 23 (2012) 355705. [25] Y. Hou, X.Y. Li, Q.D. Zhao, G.H. Chen, Appl. Catal. B: Environ. 142 (2013) 80–88. [26] H. Li, Q.D. Zhao, X.Y. Li, Z.R. Zhu, M. Tade, S.M. Liu, J. Nanopart. Res. 15 (2013) 1670. [27] X.J. Liu, L.K. Pan, T. Lv, G. Zhu, Z. Sun, C.Q. Sun, Chem. Commun. 47 (2011) (1986) 11984–11986. [28] W.G. Tao, J.L. Chang, D.P. Wu, Z.Y. Gao, X.L. Duan, F. Xu, K. Jiang, Mater. Res. Bull. 48 (2013) 538–543. [29] Q.J. Xiang, J.G. Yu, M. Jaroniec, J. Phys. Chem. C 115 (2011) 7355–7363. [30] Y.Q. Zou, Y. Wang, Nanoscale 3 (2011) 2615–2620. [31] C.D. Wagner, Handbook of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, Minnesota, 1979. [32] S.X. Min, G.X. Lu, J. Phys. Chem. C 116 (2012) 25415–25424. [33] B.J. Jiang, C.G. Tian, Q.J. Pan, Z. Jiang, J.Q. Wang, W.S. Yan, H.G. Fu, J. Phys. Chem. C 115 (2011) 23718–23725. [34] K.L. Zhang, C.M. Liu, F.Q. Huang, C. Zheng, W.D. Wang, Appl. Catal. B: Environ. 68 (2006) 125–129. [35] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214–20220. [36] X.M. Tu, S.L. Luo, G.X. Chen, J.H. Li, Chem. Eur. J. 18 (2012) 14359–14366. [37] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (2007) 238–242.
[38] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095–14107. [39] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Appl. Catal. B: Environ. 100 (2010) 179–183. [40] W.S. Wang, D.H. Wang, W.G. Qu, L.Q. Lu, A.W. Xu, J. Phys. Chem. C 116 (2012) 19893–19901. [41] X.J. Liu, L.K. Pan, T. Lv, Z. Sun, C.Q. Sun, J. Colloid Interface Sci. 408 (2013) 145– 150. [42] W.C. Xu, G.Y. Zhou, J.Z. Fang, Z. Liu, Y.F. Chen, C.P. Cen, Int. J. Photoenergy (2013) 1–9. [43] Y.L. Xing, W.X. Que, X.B. Liu, H.M.A. Javed, Z.L. He, Y.C. He, T. Zhou, RSC Adv. 4 (2014) 49900–49907. [44] S. Chandra, P. Das, S. Bag, R. Bhar, P. Pramanik, Mater. Sci. Eng., B 177 (2012) 855–861. [45] K.X. Zhu, L.W. Guo, J.J. Lin, W.C. Hao, J. Shang, Y.P. Jia, L.L. Chen, S.F. Jin, W.J. Wang, X.L. Chen, Appl. Phys. Lett. 100 (2012) 023113. [46] Y.L. Min, F.J. Zhang, W. Zhao, F.C. Zheng, Y.C. Chen, Y.G. Zhang, Chem. Eng. J. 209 (2012) 215–222. [47] G.K. Pradhan, D.K. Padhi, K.M. Parida, ACS Appl. Mater. Interfaces 5 (2013) 9101–9110. [48] G.D. Chen, M. Sun, Q. Wei, Y.F. Zhang, B.C. Zhu, B. Du, J. Hazard. Mater. 244– 245 (2013) 86–93. [49] H. Liu, T.T. Liu, X.N. Dong, Y.W. Lv, Z.F. Zhu, Mater. Lett. 126 (2014) 36–38. [50] L.Z. Dong, Y.M. He, H.J. Lin, S.S. Wang, W.D. Hu, M.F. Luo, L.H. Zhao, Mater. Lett. 122 (2014) 17–20. [51] H.W. Huang, K. Liu, Y.L. Zhang, K. Chen, Y.H. Zhang, N. Tian, RSC Adv. 4 (2014) 49386–49394. [52] M. Lan, G.L. Fan, L. Yang, F. Li, Ind. Eng. Chem. Res. 53 (2014) 12943–12952. [53] D. Mahanta, U. Manna, G. Madras, S. Patil, ACS Appl. Mater. Interface 3 (2011) 84–92. [54] W.J. Li, D.Z. Li, Y.M. Lin, P.X. Wang, W. Chen, X.Z. Fu, Y. Shao, J. Phys. Chem. C 116 (2012) 3552–3560.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Visible light-driven Bi2Sn2O7/reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants Hong Liu ⇑, Zhitong Jin, Yun Su, Yong Wang ⇑ Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 26 December 2014 Accepted 26 December 2014 Available online 8 January 2015 Keywords: Bi2Sn2O7 Graphene Photocatalysis Visible light Degradation
a b s t r a c t In this work, a novel Bi2Sn2O7/reduced graphene oxide (RGO) nanocomposite was synthesized by a onestep hydrothermal method. The prepared composite was characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrometry (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) emission spectroscopy and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the Bi2Sn2O7/RGO composite was investigated by the degradation of rhodamine B (RhB) and phenol. An increase in photocatalytic activity was observed for Bi2Sn2O7/RGO composite compared with pure Bi2Sn2O7 under visible light. The enhanced photocatalytic performance of the composite was mainly ascribed to the more effective charge separations and the excellent adsorption capacity of RGO. The composite maintained its ability to degrade pollutants efficiently, even after 4 cycles of photocatalysis. Further study proved that both the holes and hydroxyl radicals were the active species in the degradation process. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis has attracted worldwide attention because of its potential applications in water splitting and the degradation of organic compounds, and TiO2 is the most studied semiconductor photocatalyst because of its high photocatalytic activity, low cost, non-toxicity, and chemical stability [1–4]. However, its large band gap (3.2 eV for anatase) determined that it was only active under ultraviolet (UV) light irradiation. To take sufficient advantage of solar energy, the development of photocatalysts capable of photoinduced charge separation upon excitation in the visible spectral region is emerging as an important research direction in this field [5–9]. Bi2Sn2O7, a novel semiconducting material with a typical pyrochlore structure and a band gap of 2.5–2.8 eV, has received considerable attention in catalysis and gas sensors in recent years [10–12]. The crystal structure of Bi2Sn2O7 is constructed of octahedral SnO6, and these octahedra connect to each other by sharing vertexes. A network of corner-shared octahedra can facilitate the mobility of the charged carriers, which may make Bi2Sn2O7 an efficient visible light-driven photocatalytic material [13–15]. Very recently, Wu
⇑ Corresponding authors. Tel.: +86 21 66137487; fax: +86 21 66137725. E-mail addresses: [email protected] (H. Liu), [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.seppur.2014.12.027 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
et al. [13] synthesized nano-Bi2Sn2O7 by a hydrothermal method and studied its photocatalytic activity in the degradation of methyl orange. Tian et al. [14] reported the synthesis of Bi2Sn2O7 and its application in photocatalytic removal of As (III). Xu et al. [15] reported the photocatalytic activity of Bi2Sn2O7 in the degradation of rhodamine B (RhB) under visible light irradiation. However, the photocatalytic efficiency of individually Bi2Sn2O7 was relatively very low. For practical use, the photocatalytic performance of Bi2Sn2O7 should be further improved. Combination of different types of carbon with semiconductor has been suggested as a promising method for an enhanced photocatalytic performance [16–19]. In the past few years, graphene as a novel carbonaceous nanomaterial has attracted more and more interests due to its large specific surface area, remarkable electrical conductivity, excellent adsorptivity, and high chemical and thermal stability [17–19]. Furthermore, graphene can be easily produced at a low cost from natural graphite through chemical oxidation– dispersion–reduction procedures. Efforts have been made to combine semiconductor and graphene or reduced graphene oxide (RGO) to obtain hybrid materials with superior photocatalytic performance [20–29]. For example, significant improvement has been reported for metal oxides [22–26], metal sulfides [27,28], and nonmetal g-C3N4 [29] by using graphene (or RGO) as a synergistic catalyst material. If Bi2Sn2O7 is coupled with graphene, it is possible to improve the efficiency of photoinduced charge separation, leading
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
to high photocatalytic performance. However, to the best of our knowledge, there is no report on the design and fabrication of coupled Bi2Sn2O7/graphene heterostructure so far. In this paper, a novel Bi2Sn2O7/RGO composite has been synthesized by a one-step hydrothermal method for the first time. The prepared sample was characterized, and the photocatalytic activity under visible light irradiation (k > 420 nm) was evaluated by the degradation of RhB and phenol. The result revealed that the Bi2Sn2O7/RGO composite exhibited an enchenced photocatalytic activity in comparison with bare Bi2Sn2O7. The obtained Bi2Sn2O7/RGO photocatalyst displayed good cycling stability under visible light. Furthermore, the mechanism of enhanced photocatalytic activity as well as the main active oxygen species in the photocatalytic reaction process was also investigated.
2. Materials and methods 2.1. Preparation of Bi2Sn2O7/RGO composite All the chemicals were of analytical grade from the Sinopharm Chemical Reagent Co., Ltd. Graphene oxide (GO) was prepared by a modified Hummers method [30]. The Bi2Sn2O7/RGO composite was prepared by a one-pot hydrothermal method. In a typical synthesis, 0.73 g Bi(NO3)35H2O and 0.45 g K2SnO33H2O were added into 50 mL of deionized water. Under vigorous stirring, the pH value of the mixture was adjusted to 12 by using NaOH solution. Meanwhile, 0.1 g GO was dispersed in 20 mL ethanol by sonication for 1 h to get a homogenous suspension of exfoliated graphene oxide. Then, the obtained GO solution was added to the reactedmixture gradually. After stirring continuously for 2 h at room temperature, the mixture was transferred into a Teflon-lined autoclave with a volume of 100 mL. Then, the autoclave was sealed, heated under autogenous pressure at 180 °C for 24 h, and then cooled down to room temperature naturally. The resulting precipitate was filtrated, washed thoroughly, and vacuum dried at 60 °C overnight. For comparison, pure Bi2Sn2O7 was prepared by using the same method without the addition of GO.
2.2. Characterization The crystalline phases of prepared samples were identified by X-ray diffraction (XRD) on a D/MAX-2550 at 40 kV and 40 mA with Cu Ka radiation (k = 0.15418 nm). Structure and morphology of as prepared samples were evaluated by TEM (200CX) and HRTEM (JEM-2010F). Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw, INVIA) with an excitation of 514.5 nm laser light. UV–vis diffuse reflectance spectra (DRS) of the samples were obtained on an UV–vis spectrophotometer (Hitachi U-3010) using BaSO4 as a reference. The photoluminescence (PL) spectra were measured using a Hitachi F-7000 fluorescence spectrophotometer at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660E electrochemical workstation in a standard three-electrode system using the prepared samples as the working electrodes, a platinum wire as the counter electrode, and an Ag/AgCl (saturated KCl) electrode as the reference electrode over the frequency range from 0.01 Hz to 106 Hz. Na2SO4 (1 M) aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 0.08 g of photocatalyst was grinded with 0.01 g of polytetrafluoroethylene (PTFE), 0.01 g of carbon black and 2 mL of isopropanol to produce a slurry. The slurry was then coated onto a 1 cm 1.5 cm titanium mesh electrode by the doctor blade method. Next, the resulting electrodes were dried in vacuum at 60 °C for 24 h. All electrodes studied had a similar film thickness.
2.3. Photocatalytic activity test The photocatalytic activity of the as-prepared photocatalyst was estimated by monitoring the degradation of RhB and phenol in an aqueous solution under visible light irradiation. The reaction was performed in a photochemical reactor (BL-GHX-V, Shanghai Bilon Instruments Co., Ltd., China), equipped with a 500 W Xe lamp combined with a 420 nm cut-off filter as a light source. All experiments were conducted at room temperature in air. In a typical photocatalytic experiment, 0.05 g of the photocatalyst was added into 50 ml of 10 mg/L pollutant solution in a reaction cell with a Pyrex jacket. Prior to irradiation, the suspension was magnetically stirred in the dark for 1 h to reach an adsorption–desorption equilibrium. Then, the suspension was exposed to visible light irradiation under magnetic stirring. At given time intervals, about 5 ml suspensions were collected and centrifuged (12,000 rpm, 6 min) to remove the photocatalyst particles. Then, the pollutant concentration of the obtained solution was analyzed by a UV–vis spectrophotometer (Hitachi, U-3310) by checking the absorbance at 553 nm and 270 nm for RhB and phenol, respectively. To probe the oxidizing species in the photocatalytic process, the disodium ethylenediamine tetraacetate (EDTA-2Na), tert-butyl alcohol (tBuOH), or benzoquinone (BQ), was added in the solution. The concentrations of the quenching chemicals were at 1 mM. 3. Results and discussion 3.1. Structure and properties characterization The XRD patterns of the pristine Bi2Sn2O7 and Bi2Sn2O7/RGO composite are shown in Fig. 1. The XRD of Bi2Sn2O7 shows several strong peaks at 2h = 28.8°, 33.4°, 47.9°, and 56.9°, which represent the formation of a pure cubic phase of Bi2Sn2O7 (JCPDS No. 87-0284) [13,15]. It is found that the main diffraction peaks of Bi2Sn2O7/RGO composite are similar to that of pristine Bi2Sn2O7, and no apparent peaks of RGO are observed in the Bi2Sn2O7/RGO composite, which might be due to the low amount of RGO and relatively low diffraction intensity of RGO in the composite. No apparent peaks of graphene oxide are observed in the Bi2Sn2O7/RGO composite, indicating the successful reduction of GO to RGO through the hydrothermal process. The morphologies of the pristine Bi2Sn2O7 and Bi2Sn2O7/RGO composite were investigated by TEM as shown in Fig. 2. Fig. 2a shows that pure Bi2Sn2O7 is comprised of many irregular nano-
GO Intensity (a. u.)
26
Bi2Sn2O7/RGO
Bi2Sn2O7
10
20
30
40
50
60
70
2 Theta (degree) Fig. 1. XRD patterns of GO, Bi2Sn2O7 and Bi2Sn2O7/RGO.
80
27
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
particles with diameters ranging from 5 to 15 nm. From Fig. 2b, it can be clearly observed that the RGO sheets are decorated by irregular Bi2Sn2O7 nanoparticles, which confirmed the successful attachment of Bi2Sn2O7 nanoparticles to RGO sheets. The HRTEM image shows the characteristic lattice fringes of Bi2Sn2O7 in the RGO nanosheets matrix (Fig. 2c). The lattice spacing is about 0.310 nm, which can be assigned to the (2 2 2) plane of cubic Bi2Sn2O7 [13]. Fig. 2c also reveals the intimate contact between Bi2Sn2O7 nanoparticles and RGO. This intimate contact is in favour of the electronic interaction between Bi2Sn2O7 and RGO and improves the charge separation and the photocatalytic activity. The EDS spectrum of Bi2Sn2O7/RGO (Fig. 2d) exhibits the presence of Bi, Sn, O and C elements. The X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface elemental compositions of Bi2Sn2O7 and Bi2Sn2O7/ RGO. The survey XPS spectra (Fig. 3a) reveal that both Bi2Sn2O7 and Bi2Sn2O7/RGO are composed of Bi, Sn, O and C, which is in good agreement with the EDS results. The Bi 4f XPS spectra in Fig. 3b shows that the binding energies of Bi 4f7/2 and Bi 4f5/2 for Bi2Sn2O7 occur at 158.8 and 164.2 eV, respectively, which are indicative of Bi3+ in Bi2Sn2O7 [8,9,20]. The Sn 3d spectrum of Bi2Sn2O7 (Fig. 3c) shows two peaks at 486.7 and 495.0 eV, indicating the presence of Sn4+ [31]. The peaks corresponding to Bi 4f and Sn 3d in the XPS spectra of the as-prepared Bi2Sn2O7/RGO composite slightly shift toward higher binding energies as compared to pure Bi2Sn2O7. This kind of shift in XPS measurement indicates that there is a strong interaction between Bi2Sn2O7 and RGO sheets in the nanohybrid [32,33]. The intense interaction may result in the formation of electron transfer channel, which is beneficial to the improvement of
(a)
(c)
photoinduced charge separation rate during the photocatalytic process [33]. In addition, the high-resolution spectra in Fig. 3d shows that the peak binding energies of 530.4 eV is assigned to O 1s [34]. The XPS spectrum of C1s from GO (Fig. 3e) can be deconvoluted into three smaller peaks, which are ascribed to the following functional groups: sp2 bonded carbon (CAC, 284.8 eV), epoxy/hydroxyls (CAO, 286.8 eV), and carboxyl (OAC@O, 288.8 eV) [35], indicating the high percentage of oxygen-containing functional groups. In Fig. 3f, although the three types of carbon species can still be seen in the Bi2Sn2O7/RGO composite, the XPS peaks corresponding to CAO and OAC@O are severely weakened in comparison to Fig. 3e, indicating that GO has been highly reduced during the hydrothermal synthesis. Besides, no any peak corresponding to the BiAC bond (at 281.2 eV in C 1s spectrum [36]) is observed, suggesting no carbon doping in the lattice of Bi2Sn2O7. The Raman spectra of GO and Bi2Sn2O7/RGO are presented in Fig. 4. In Raman spectra, both samples exhibit two strong peaks, denoted as the disorder peak (D, centered at 1359 cm 1) and the graphitic peak (G, at 1606 cm 1) [37,38]. Compared with GO, Bi2Sn2O7/RGO displays an increased D/G intensity ratio, indicating the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated GO [39]. The UV–vis diffuse reflectance spectra of the synthesized samples are shown in Fig. 5a. Compared to pure Bi2Sn2O7, Bi2Sn2O7/ RGO composite shows an increased absorption in both UV and visible range and an obvious red-shift in the absorption edge, which can be attributed to the presence of carbon in the composites, reducing reflection of light [22,40]. Similar phenomenon has been noted in previous studies [3,9,20,27,40,41]. The band gap energy
(b)
(d)
Itensity (a.u.)
Cu
C
Bi Sn Sn
O
0
2
4
Cu
6
8
Bi
10
Bi
12
14
Energy (keV) Fig. 2. TEM images of Bi2Sn2O7 (a) and Bi2Sn2O7/RGO (b), HRTEM image of Bi2Sn2O7/RGO (c) and EDS spectrum of Bi2Sn2O7/RGO (d).
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
O 1s Sn 3d Bi 4d Bi 4d
Bi 4f
(b) Bi 4f
Intensity (a. u.)
Bi2Sn2O7
Bi 4f7/2
Bi 4f5/2 Bi 5d O 2s
Sn 3p Sn 3p Bi 4p
O Sn 3s
Sn
Intensity (a. u.)
(a) Survey
C 1s
28
Bi2Sn2O7
Bi2Sn2O7/RGO
Bi2Sn2O7/RGO
1000
800
600
400
200
0
168
166
Binding energy (eV)
164
162
160
158
156
154
Binding energy (eV)
(d) O 1s
(c) Sn 3d Sn 3d5/2
Intensity (a. u.)
Intensity (a. u.)
Sn 3d3/2
Bi2Sn2O 7
Bi2Sn2O7
Bi2Sn2O7/RGO
Bi2Sn2O /RGO 7
500 498 496 494 492 490 488 486 484 482
540 538 536 534 532 530 528 526 524
Binding energy (eV)
Intensity (a. u.)
(e) C 1s
Binding energy (eV)
(f) C 1s
C-O
Intensity (a. u.)
C-C
O-C=O
292
29 0
28 8
28 6
28 4
282
Binding energy (eV)
C-C
292
O-C=O
O-C
290
288
286
284
282
Binding energy (eV)
Fig. 3. XPS spectra of Bi2Sn2O7 and Bi2Sn2O7/RGO: (a) survey, (b) Bi 4f, (c) Sn 3d, (d) O 1s, (e) C 1s of GO, and (f) C 1s of Bi2Sn2O7/RGO.
can be estimated by the following formula: a h m = A(h m–Eg)n/2, where a, h, m, Eg, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively. And n is the electron transitions depending on the semiconductor characteristics (direct transition n = 1 and indirect transition n = 4). For Bi2Sn2O7, the value of n is 4 for the indirect transition [13]. Fig. 5b plots the relationship of (a h m)1/2 versus photon energy (h m), and shows that the bandgap energy of pure Bi2Sn2O7 is 2.48 eV, whereas this value is reduced to 1.85 eV by decorating with RGO. This phenomenon is possibly ascribed to the interaction between Bi2Sn2O7 and RGO [3,20,22,29], which has been evidenced by XPS analysis as described above.
3.2. Photocatalytic activity The photoactivity of the Bi2Sn2O7/RGO composite was evaluated by photocatalytic degradation of RhB and phenol under visible light irradiation, and the results are shown in Fig. 6. It can be seen that both RhB and phenol are very stable under visible light irradiation without the catalyst. Fig. 6a shows that the RhB removal rate is about 95.8% after irradiation for 125 min in the presence of Bi2Sn2O7/RGO composite, while the value of pristine Bi2Sn2O7 is only 83.7%. Meanwhile, the degradation rate of phenol on Bi2Sn2O7/RGO composite reaches 81.1% after irradiation for 200 min, whereas only 64.4% of phenol is removed by pristine Bi2Sn2O7 after same
29
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
Table 1 Pseudo first-order rate constants of the photocatalytic decomposition of different pollutants.
G
D
Catalyst
Intensity (a. u.)
GO
ID/IG= 0.667
Bi2Sn2O7 Bi2Sn2O7/RGO
Bi2Sn2O7/RGO
ID/IG= 0.924
800
1200
Raman
1600
2000
Shift (cm-1)
Fig. 4. Raman spectra of the Bi2Sn2O7/RGO composite and GO.
irradiation time (Fig. 6b). These results indicate clearly that the introduction of RGO can enhance the photocatalytic performance of Bi2Sn2O7. Notably, the photocatalytic activity of the obtained Bi2Sn2O7/RGO composite on the degradation of organic pollutant RhB and phenol is higher than the Bi2Sn2O7-based materials reported previously [42,43]. These photocatalytic properties are also better than various graphene-based composites [20,28,44–52]. For detailed analysis the photocatalysis kinetics of the pollutants degradation, the pseudo-first order model was applied. This model is generally used for photocatalytic degradation process if the initial
(a)
(αhν)1/2
Adsorbance (a.u.)
1.2
Bi2Sn2O7/RGO
0.8382 1.5350
R2
k (h
1
0.9777 0.9872
0.3093 0.5146
)
R2 0.9976 0.9907
Bi2Sn2O7/RGO
2.0 1.5 Bi2Sn2O7
1.0
Bi2Sn2O7
0.0 200
)
3.0 2.5
0.8
Phenol 1
concentration of pollutant is low [53]. The rate constants were evaluated from the data plotted in Fig. 6a and b and summarized in Table 1. The rate constant for Bi2Sn2O7/RGO composite to remove RhB is 1.54 h 1, which is 1.83 times as large as that of pristine Bi2Sn2O7 (0.8382 h 1). This value for the composite is also 1.66 times larger than pristine Bi2Sn2O7 for the degradation of phenol. In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications. To confirm the stability of the Bi2Sn2O7/RGO photocatalyst, the circulating runs in the photocatalytic degradation of RhB and phenol in the presence of Bi2Sn2O7/RGO under visible-light was checked. In the work, Bi2Sn2O7/RGO was recycled for four times in the same photocatalytic reactions. After each reuse cycle which lasted for 125 min (for degradation of RhB) or 200 min (for degradation of phenol), the photocatalyst was separated from the aqueous suspension by filtration, washed with deionized water and ethanol, dried, and weighed for the next reuse cycle. It can be seen from Fig. 7, after four recycles for the photodegradation of RhB and phenol, the catalyst do not exhibit any significant loss of activity, indicating the good stability of the Bi2Sn2O7/RGO composite during photocatalytic degradation of model pollutant.
(b)
1.6
0.4
RhB k (h
0.5 0.0
300
400
500
600
700
800
1.5
2.0
Wavelength (nm)
2.5
3.0
3.5
4.0
4.5
5.0
Photo energy (eV)
Fig. 5. (a) UV–vis spectra of Bi2Sn2O7 and Bi2Sn2O7/RGO composite and (b) plots of the (a h m)1/2 versus photon energy (h m) for as-synthesized samples.
(a) 1.0
(b) 1.0
0.8
C/C0
C/C0
Bi2Sn2O7/RGO
0.6
Bi2Sn2O7 Bi2Sn2O7/RGO
0.8
Bi2Sn2O7 blank 0.4
blank
0.6 0.4
0.2 0.2 0.0 0
25
50
75
Time (min)
100
125
0
40
80
120
160
200
Time (min)
Fig. 6. Photocatalytic activities of as-prepared samples on the degradation of RhB (a) and phenol (b) under visible-light irradiation.
30
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
1.00
(a) 1.0 2 nd run
1 st run
4 th run
3 rd run
Bi2Sn2O7 Bi2Sn2O7/RGO
0.8
C/C0
0.96 0.6 0.92
C/C0
0.4 0.2
0.88
0.0 0
125
125
125
125
0.84
Time (min) 0
(b) 1.0
2 nd run
1 st run
20
4 th run
3 rd run
30
40
50
60
Time (min)
0.8
Fig. 9. Absorption activities of Bi2Sn2O7 and Bi2Sn2O7/RGO.
0.6 0.4 0.2 0.0 0
200
200
200
200
Time (min) Fig. 7. The cyclic test of as-prepared Bi2Sn2O7/RGO composite on the degradation of RhB (a) and phenol (b).
3.3. Mechanism on enhancement of photocatalytic activity
Relative intensity (a. u.)
The photocatalytic activity of semiconductors is closely related with the recombination rate of the photoinduced electrons and holes. To investigate the efficiency of charge carrier trapping, migration, and transfer in the as-prepared Bi2Sn2O7 and Bi2Sn2O7/ RGO, the photoluminescence (PL) emission spectra of these samples were measured. Fig. 8a presents PL emission spectra of pure Bi2Sn2O7 and Bi2Sn2O7/RGO photocatalyst monitored at an excitation wavelength of 350 nm. It can be seen that the pure Bi2Sn2O7 shows a broad PL emission band, which is ascribed to luminescence from localized surface states due to recombination of photogenerated electron–hole pairs. The Bi2Sn2O7/RGO composite exhibits a much lower emission intensity than pure Bi2Sn2O7,
implying that the recombination of photogenerated electrons and holes is inhibited greatly in the Bi2Sn2O7/RGO system. The improved efficiency in inhibiting the recombination of photogenerated charge carriers of Bi2Sn2O7 by the use of RGO can be further supported by the electrochemical impedance technique which is a reliable way to characterize electrical conductivity. Fig. 8b shows the electrochemical impedance spectra (EIS) of Bi2Sn2O7 and Bi2Sn2O7/RGO. It is clear that the Bi2Sn2O7/RGO photocatalyst has smaller arc as compared to the pristine Bi2Sn2O7, suggesting the more efficient interfacial charge transfer over Bi2Sn2O7/RGO. Graphene or RGO as a two-dimensional sheet of carbon atoms has superior electrical conductivity, which would make it an excellent electron-transport material in the process of photocatalysis. The photogenerated electrons of excited Bi2Sn2O7 were transferred instantly from the conduction band of Bi2Sn2O7 to RGO, resulting in a minimized charge recombination and offering an enhanced photocatalytic activity. In addition, adsorption performance of the targeted substance is also very important for the surface reaction. To evaluate the adsorption ability of Bi2Sn2O7 and Bi2Sn2O7/RGO, the residue rate of RhB after 60 min stirring in the dark was determined. As shown in Fig. 9, the residue rate of RhB in the solution with Bi2Sn2O7 as the adsorbent is 87.4%, whereas that of Bi2Sn2O7/RGO as the adsorbent is 84.6%. It is obviously that the adsorption ability of Bi2Sn2O7 was enhanced by introduction of RGO, which is also a prequisite for good photocatalytic activity. To understand the photocatalytic mechanism, the main active oxidant in the photocatalytic reaction process should be identified.
50
(a)
(b) Bi2Sn2O7 Bi2Sn2O7/RGO
40 Bi2Sn2O7
Bi2Sn2O7/RGO
-Z''(ohm)
C/C0
10
30 20 10 0
380 400 420 440 460 480 500 520 540 560 580
Wavelength (nm)
10
15
20
25
30
35
40
Z'(ohm)
Fig. 8. (a) PL spectra and (b) Nyquist impedance plots of Bi2Sn2O7 and Bi2Sn2O7/RGO composite.
45
50
31
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
(a) 1.0
(b) 1.0 Bi2Sn2O7/RGO + EDTA-2Na + t-BuOH + BQ
C/C0
0.6 0.4 0.2
0.8
C/C0
0.8
0.6 0.4
Bi2Sn2O7/RGO + EDTA-2Na + t-BuOH + BQ
0.2
0.0
0.0 0
25
50
75
100
125
0
40
Time (min)
80
120
160
200
Time (min)
Fig. 10. Photocatalytic degradation of RhB (a) and phenol (b) with the addition of hole and radical scavenger.
4. Conclusion
O2
e-
· O2
-
Pollutants
ee-
e-
CB
Bi2Sn2O7 h+
h+
h+ VB
h+
Oxidized products Pollutants
A novel Bi2Sn2O7/RGO photocatalyst has been synthesized via a one-step hydrothermal method in this study. Under visible irradiation, the Bi2Sn2O7/RGO photocatalyst was found to exhibit higher photocatalytic activities than pure Bi2Sn2O7 in the degradation of RhB and phenol. The photocatalytic activity of Bi2Sn2O7/RGO in degradation of RhB and phenol was 1.83 and 1.66 times that of pure Bi2Sn2O7, respectively. The enhanced photocatalytic performance of the composite was mainly ascribed to the more effective charge separations and the excellent adsorption capacity of RGO. Both the holes and hydroxyl radicals were the active species in the degradation process. In addition, Bi2Sn2O7/RGO displayed an excellent stability during four reaction cycles, indicating Bi2Sn2O7/ RGO has a potential application in water purification. Acknowledgments
Scheme 1. Photocatalytic mechanism of the Bi2Sn2O7/RGO composite under visible light irradiation.
It is well-known that the oxidants generated in the photocatalytic process can be measured through trapping by t-BuOH (a scavenger of hydroxyl radicals), EDTA-2Na (a scavenger of holes) or BQ (a scavenger of O2 ). It can be clearly seen from Fig. 10 that the addition of EDTA-2Na greatly reduces the photodegradation rate of RhB and phenol in the Bi2Sn2O7/RGO suspension, whereas the addition of BQ has little effect on the photodegradation rate of RhB and phenol. In addition, the addition of t-BuOH as hydroxyl radicals’ scavenger causes a minor change in the photocatalytic degradation of the above pollutants. It indicates that both the holes and hydroxyl radicals are the active species in the photocatalytic degradation of RhB and phenol, while O2 can be negligible in the reaction. Based on the experimental results, a schematic mechanism for the degradation of RhB and phenol with Bi2Sn2O7/RGO as the photocatalyst is illustrated in Scheme 1. As irradiated, the Bi2Sn2O7 particles were excited and generated holes and electrons. Due to the higher work function of RGO than that of Bi2Sn2O7, the excited electrons quickly transported to RGO and then reacted with the adsorbed O2 on the RGO surfaces to form superoxide radical anions (O2 ), which could further react with H+ or H2O to produce OH radicals responsible for the degradation of the pollutants [5,54]. Meanwhile, holes left on Bi2Sn2O7 surfaces participated in the degradation of pollutants either by direct oxidation or by OH radicals produced during the reaction of holes with the adsorbed H2O or OH [5,54].
The authors gratefully acknowledge Natural Science Foundation of Shanghai (12ZR1410300), the follow-up Program for Professor of Special Appointment in Shanghai (Eastern Scholar), the National Natural Science Foundation of China (51271105), Shanghai Municipal Government (11JC1403900, 11SG38, IRT13078) for financial support. The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations. References [1] S.D. Perera, R.G. Mariano, K. Vu, N. Nour, O. Seitz, Y. Chabal, K.J. Balkus Jr., ACS Catal. 2 (2012) 949–956. [2] L.W. Zhang, H.B. Fu, Y.F. Zhu, Adv. Funct. Mater. 18 (2008) 2180–2189. [3] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [4] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026–3033. [5] X.F. Zhang, X. Quan, S. Chen, H.T. Yu, Appl. Catal. B: Environ. 105 (2011) 237– 242. [6] Y.S. Fu, X. Wang, Ind. Eng. Chem. Res. 50 (2011) 7210–7218. [7] A. Mukherji, B. Seger, G.Q. Lu, L.Z. Wang, ACS Nano 5 (2011) 3483–3492. [8] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Appl. Catal. B: Environ. 111–112 (2012) 271–279. [9] H. Liu, Y. Su, Z. Chen, Z.T. Jin, Y. Wang, J. Hazard. Mater. 266 (2014) 75–83. [10] G.S.V. Coles, S.E. Bond, G. Williams, J. Mater. Chem. 4 (1994) 23–27. [11] G.S. Devi, S.V. Manorama, V.J. Rao, J. Electrochem. Soc. 145 (1998) 1039–1044. [12] L. Mones, P. Ruiz, B. Delmon, M. Devillers, Appl. Catal. A: Gen. 171 (1998) 131– 143. [13] J.J. Wu, F.Q. Huang, X.J. Lu, P. Chen, D.Y. Wan, F.F. Xu, J. Mater. Chem. 21 (2011) 3872–3876. [14] Q.F. Tian, J.D. Zhuang, J.X. Wang, L.Y. Xie, P. Liu, Appl. Catal. A: Gen. 425–426 (2012) 74–78. [15] W.C. Xu, G.Y. Zhou, J.Z. Fang, Z. Liu, Y.F. Chen, C.P. Cen, Int. J. Photoenergy (2013) 1–9. [16] R. Leary, A. Westwood, Carbon 49 (2011) 741–772. [17] B.F. Machado, P. Serp, Catal. Sci. Technol. 2 (2012) 54–75.
32
H. Liu et al. / Separation and Purification Technology 142 (2015) 25–32
[18] X.Q. An, J.C. Yu, RSC Adv. 1 (2011) 1426–1434. [19] Q.J. Xiang, J.G. Yu, M. Jaroniec, Chem. Soc. Rev. 41 (2012) 782–796. [20] H. Liu, W.R. Cao, Y. Su, Z. Chen, Y. Wang, J. Colloid Interface Sci. 398 (2013) 161–167. [21] E.P. Gao, W.Z. Wang, M. Shang, J.H. Xu, Phys. Chem. Chem. Phys. 13 (2011) 2887–2893. [22] X.Y. Zhang, H.P. Li, X.L. Cui, Y.H. Lin, J. Mater. Chem. 20 (2010) 2801–2806. [23] P.D. Tran, S.K. Batabyal, S.S. Pramana, J. Barber, L.H. Wong, S.C.J. Loo, Nanoscale 4 (2012) 3875–3878. [24] H. Seema, K.C. Kemp, V. Chandra, K.S. Kim, Nanotechnology 23 (2012) 355705. [25] Y. Hou, X.Y. Li, Q.D. Zhao, G.H. Chen, Appl. Catal. B: Environ. 142 (2013) 80–88. [26] H. Li, Q.D. Zhao, X.Y. Li, Z.R. Zhu, M. Tade, S.M. Liu, J. Nanopart. Res. 15 (2013) 1670. [27] X.J. Liu, L.K. Pan, T. Lv, G. Zhu, Z. Sun, C.Q. Sun, Chem. Commun. 47 (2011) (1986) 11984–11986. [28] W.G. Tao, J.L. Chang, D.P. Wu, Z.Y. Gao, X.L. Duan, F. Xu, K. Jiang, Mater. Res. Bull. 48 (2013) 538–543. [29] Q.J. Xiang, J.G. Yu, M. Jaroniec, J. Phys. Chem. C 115 (2011) 7355–7363. [30] Y.Q. Zou, Y. Wang, Nanoscale 3 (2011) 2615–2620. [31] C.D. Wagner, Handbook of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, Minnesota, 1979. [32] S.X. Min, G.X. Lu, J. Phys. Chem. C 116 (2012) 25415–25424. [33] B.J. Jiang, C.G. Tian, Q.J. Pan, Z. Jiang, J.Q. Wang, W.S. Yan, H.G. Fu, J. Phys. Chem. C 115 (2011) 23718–23725. [34] K.L. Zhang, C.M. Liu, F.Q. Huang, C. Zheng, W.D. Wang, Appl. Catal. B: Environ. 68 (2006) 125–129. [35] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214–20220. [36] X.M. Tu, S.L. Luo, G.X. Chen, J.H. Li, Chem. Eur. J. 18 (2012) 14359–14366. [37] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (2007) 238–242.
[38] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095–14107. [39] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Appl. Catal. B: Environ. 100 (2010) 179–183. [40] W.S. Wang, D.H. Wang, W.G. Qu, L.Q. Lu, A.W. Xu, J. Phys. Chem. C 116 (2012) 19893–19901. [41] X.J. Liu, L.K. Pan, T. Lv, Z. Sun, C.Q. Sun, J. Colloid Interface Sci. 408 (2013) 145– 150. [42] W.C. Xu, G.Y. Zhou, J.Z. Fang, Z. Liu, Y.F. Chen, C.P. Cen, Int. J. Photoenergy (2013) 1–9. [43] Y.L. Xing, W.X. Que, X.B. Liu, H.M.A. Javed, Z.L. He, Y.C. He, T. Zhou, RSC Adv. 4 (2014) 49900–49907. [44] S. Chandra, P. Das, S. Bag, R. Bhar, P. Pramanik, Mater. Sci. Eng., B 177 (2012) 855–861. [45] K.X. Zhu, L.W. Guo, J.J. Lin, W.C. Hao, J. Shang, Y.P. Jia, L.L. Chen, S.F. Jin, W.J. Wang, X.L. Chen, Appl. Phys. Lett. 100 (2012) 023113. [46] Y.L. Min, F.J. Zhang, W. Zhao, F.C. Zheng, Y.C. Chen, Y.G. Zhang, Chem. Eng. J. 209 (2012) 215–222. [47] G.K. Pradhan, D.K. Padhi, K.M. Parida, ACS Appl. Mater. Interfaces 5 (2013) 9101–9110. [48] G.D. Chen, M. Sun, Q. Wei, Y.F. Zhang, B.C. Zhu, B. Du, J. Hazard. Mater. 244– 245 (2013) 86–93. [49] H. Liu, T.T. Liu, X.N. Dong, Y.W. Lv, Z.F. Zhu, Mater. Lett. 126 (2014) 36–38. [50] L.Z. Dong, Y.M. He, H.J. Lin, S.S. Wang, W.D. Hu, M.F. Luo, L.H. Zhao, Mater. Lett. 122 (2014) 17–20. [51] H.W. Huang, K. Liu, Y.L. Zhang, K. Chen, Y.H. Zhang, N. Tian, RSC Adv. 4 (2014) 49386–49394. [52] M. Lan, G.L. Fan, L. Yang, F. Li, Ind. Eng. Chem. Res. 53 (2014) 12943–12952. [53] D. Mahanta, U. Manna, G. Madras, S. Patil, ACS Appl. Mater. Interface 3 (2011) 84–92. [54] W.J. Li, D.Z. Li, Y.M. Lin, P.X. Wang, W. Chen, X.Z. Fu, Y. Shao, J. Phys. Chem. C 116 (2012) 3552–3560.