ZnO nanostructures for efficient visible-light-driven photocatalysis

Journal of Molecular Catalysis A: Chemical 401 (2015) 81–89

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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Microwave-assisted synthesis of Ag/Ag2 SO4 /ZnO nanostructures for efficient visible-light-driven photocatalysis Wenrong Cao, Lifang Chen ∗ , Zhiwen Qi ∗ State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 3 November 2014 Received in revised form 27 February 2015 Accepted 28 February 2015 Available online 3 March 2015 Keywords: Ag/Ag2 SO4 /ZnO nanostructures Visible-light Degradation Photocatalysis

a b s t r a c t Novel Ag/Ag2 SO4 /ZnO nanostructures were successfully fabricated by a facile microwave-assisted template-free hydrothermal method. The results reveal that Ag/Ag2 SO4 nanoparticles are uniformly dispersed on the surfaces of ZnO plates, which forms the hierarchical Ag/Ag2 SO4 /ZnO (AZ) nanostructures. The AZ-2 photocatalyst obtained by adjusting the amounts of thiourea and AgNO3 with the molar ratio of 1:1 exhibits the highest photocatalytic efficiency, and rhodamine B can be decolorized within 35 min and 20 min under visible-light ( > 420 nm) and natural sunlight irradiation, respectively. The enhanced photocatalytic performance of AZ catalysts may result from the synergistic effects of Ag/Ag2 SO4 nanoparticles loaded on ZnO surfaces, which can enhance visible-light absorption capability of ZnO and promote photoinduced electron–hole separation. Therefore, the Ag/Ag2 SO4 nanoparticles doped on semiconductors can be used as effective visible-light photocatalysts and show a great potential for practical applications in the treatment of dye-containing wastewater. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Since the water photolysis on TiO2 electrode under UV light irradiation was discovered by Fujishima and Honda in 1972, many researches have been carried out on the development of semiconductor materials for highly efficient photocatalytic applications [1–3]. These photocatalysts provide great potentials in solving current environment and energy issues by the utilization of abundant solar energy [4,5]. Among numerous semiconductor materials, ZnO as one of excellent photocatalysts has been extensively studied owing to its unique physical and chemical properties, such as adjustable morphologies, low cost, high reaction activity and nontoxicity [6,7]. However, ZnO with a wide band gap energy of 3.2 eV can only absorb UV light ( < 387 nm) accounting for around 5–7% of solar irradiance, which heavily hampers the large-scale applications of the effective utilization of solar energy [8]. On the other hand, ZnO-based photocatalysts are extremely vulnerable to light corrosion and have the rapid recombination rate of photogenerated electron–hole pairs, resulting in the low quantum efficiency and strongly inhibiting the improvement of photocatalytic activity [9]. Therefore, it is a major challenge to improve photocatalytic efficiency of visible-light-responsive ZnO-based photocatalysts.

∗ Corresponding authors. E-mail addresses: [email protected] (L. Chen), [email protected] (Z. Qi). http://dx.doi.org/10.1016/j.molcata.2015.02.023 1381-1169/© 2015 Elsevier B.V. All rights reserved.

To address these obstacles, the design of advanced photocatalytic materials must not only increase the visible-light absorption capability, but also speed up the separation rate of photoinduced charge carriers and prolong the lifetime of the carriers [10]. Many methods, such as metal and non-metal doping, coupling with semiconductors and photosensitization, have been exploited to make photocatalysts active under visible-light irradiation for wastewater treatment [11–14]. Ag, as an excellent candidate for surface modification, has attracted widespread attention with enhanced charges separation and surface plasmon resonance (SPR) [15,16]. The plasmonic Ag nanoparticles can expand the absorption range of a semiconductor to the visible-light region and transfer the SPR electron from noble metal to the conduction band (CB) of the semiconductor, which can greatly enhance the efficiency of photocatalysis for organic pollution decomposition. Currently, The typically plasmonic photocatalysts Ag/AgX (X = Cl, Br, I) have been developed and arouse extensive interests due to their outstanding photocatalytic performance under visible-light irradiation. The enhanced photocatalytic activity of Ag/AgX system is mainly derived from the SPR of Ag nanoparticles and the interaction between Ag and AgX. Therefore, Ag/AgX is widely used as the dopant to improve the photocatalytic performance of TiO2 , Al2 O3 and BiVO4 , etc. [17–19]. To date, the doping of Ag/Ag2 SO4 on semiconductor materials has not yet been investigated to improve their photocatalytic performance.

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In comparison with conventional convective heating, microwave-assisted heating recently has been recognized as a promising method to produce nanomaterials for the rapid volumetric heating with controllable thermal activation. It results in higher reaction rate and selectivity, reduction in chemical reaction time, and generating smaller and more uniformed particles [20]. Therefore, this technique can be considered as a “simpler” and “greener” method to synthesize photocatalysts. We previously reported that Ag2 SO4 /ZnO catalyst can completely degrade rhodamine B (RhB) solution in only six minutes under UV–vis light irradiation [21]. In this work, we report a microwave-assisted heating method to prepare highly dispersed Ag/Ag2 SO4 nanoparticles deposited on the surfaces of ZnO plates (AZ catalysts) under template-free hydrothermal condition. The photocatalytically active region of the catalysts is expanded to the visible-light region. The effect of thiourea amount on the photocatalytic performance of as-prepared catalysts has been investigated for RhB photodegradation under visible-light and solar light irradiation. In comparison with pure Ag2 SO4 , ZnO, Ag/ZnO and Ag2 SO4 /ZnO, Ag/Ag2 SO4 /ZnO nanostructures as a new type of plasmonic photocatalysts exhibit significantly enhanced photocatalytic performance owing to the synergistic effects of three components. Furthermore, the possible electron transfer mechanism has also been explored for visible-light induced RhB degradation based on Ag/Ag2 SO4 /ZnO nanostructures. 2. Experimental 2.1. Materials All chemicals used in this work, i.e., Zn(CH3 COO)2 ·2H2 O, urea, thiourea, AgNO3 , rhodamine B (RhB), isopropyl alcohol, ethylene diamine tetraacetic acid and benzoquinone, were analytical grade and supplied by Aladdin Inc. They were used without further purification. 2.2. Synthesis of Ag/Ag2 SO4 /ZnO In a typical procedure, 0.41 g of Zn(CH3 COO)2 ·2H2 O, 0.56 g of urea and a specific amount of thiourea were dissolved in 30 ml of deionized water with constant stirring and a transparent solution was formed. Then, 300 ul of AgNO3 solution (1 M in deionized water) were added dropwise into the precursory solution with vigorous stirring, and the transparent solution immediately turned into black suspension. The molar ratio of thiourea and Ag+ was kept at 1:2, 1:1, 2:1, respectively. The above mixture was heated by microwave (CEM Mars, USA) to 170 ◦ C under magnetic stirring with a power setting of 400 W for 30 min. After naturally cooling to room temperature, black precipitates were collected by centrifugation, washed with deionized water and ethanol for several times, and dried at 60 ◦ C for 6 h. The obtained powders were subsequently calcined at 500 ◦ C for 4 h. The final products were denoted as AZ-1, AZ-2, AZ-3 according to the above molar ratio of thiourea and Ag+ , respectively. For comparison, pure ZnO, Ag/ZnO and Ag2 SO4 /ZnO were synthesized as described in Supporting information (SI).

with a JEM-2100 microscopy operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with Al Ka X-ray as the excitation source. All binding energies were referred to the C 1s peak at 284.8 eV of the surface adventitious carbon. Solid-state UV–vis diffuse reflectance spectra (DRS) were obtained by a PerkinElmer Lamda 35 UV–vis spectrometer in the wavelength range of 200–800 nm, using BaSO4 as a background. The room-temperature photoluminescence (PL) spectra were measured at an excitation wavelength of 325 nm using a Shimadzu RF-5301PC fluorescent spectrophotometer. The nanosecond-level time-resolved PL spectra were recorded on a FLS 980 fluorescence spectrometer (Edinburgh Instruments) monitored at 470 nm under 325 nm excitation. 2.4. Photocatalytic activity test The photocatalytic properties of the as-prepared photocatalysts were evaluated by the degradation of RhB under visible-light and natural sunlight irradiation. The visible-light was obtained from a 350 W Xenon lamp equipped with a 420 nm ultraviolet cutoff filter. The average light intensity was 20 mW/cm2 . The distance between the lamp and the reactor was kept at a constant height of 15 cm. In order to obtain photocatalytic performance of the catalysts under solar light irradiation, all experiments were conducted on a sunny day in July between 12:30 pm and 1:00 pm in Shanghai, China. The outside temperature was measured and maintained at around 36 ◦ C. Regardless of visible-light or solar light irradiation, 30 mg of photocatalyst powder was added into 30 ml of 30 mg/l RhB aqueous solution and then ultrasonicated for 15 min. The suspensions were magnetically stirred for 1 h in the dark to establish an adsorption–desorption equilibrium between photocatalysts and RhB molecules prior to irradiation, and the concentration of the dye maintained fairly constant in all experiments. Afterward, these suspensions were exposed to visible-light or solar light illumination under magnetic stirring. At regular time intervals, approximately 2.0 ml of samples were taken out and centrifuged to remove the catalyst particles. The concentration of the organic dye was analyzed by recording the maximum absorption wavelength at 554 nm via a PerkinElmer Lamda 35 UV–vis spectrophotometer. The degradation efficiency of dye was calculated by the following equation: Degradation (%) =



1−

C C0



× 100%

(1)

where C0 was regarded as the initial RhB concentration after adsorption equilibrium, and C was the residue concentration of RhB after a certain irradiation time. In order to detect the active species and further clarify the possible photocatalytic pathway, isopropanol (IPA, 1 mM), ethylene diamine tetraacetic acid (EDTA, 1 mM), and benzoquinone (BQ, 1 mM) were added into RhB solution together with AZ-2 photocatalyst and used to capture hydroxyl radicals (• OH), holes (h+ ), and superoxide radicals (• O2 − ), respectively. Then, the photocatalytic test was performed under visible-light irradiation and the RhB concentration was also analyzed by measuring the maximum absorption band of RhB solution at 554 nm.

2.3. Characterization 3. Results and discussion The crystalline phases of the prepared samples were determined by power X-ray diffraction analysis (XRD) using a Rigaku D/MAX2550 diffractometer with Cu-K␣ radiation ( = 1.5406 Å) in the range of 2 = 20–70◦ at room temperature. The surface morphologies and microstructures of the products were observed by field emission electron scanning microscopy (FE-FEM,NOVA NANOSEM 450) equipped with an energy X-ray dispersive spectrometer (EDS), and high-resolution transmission electron microscopy (HR-TEM)

3.1. Photocatalyst characterization Fig. 1 shows the normalized XRD patterns of the as-prepared photocatalysts. Before calcination, the AZ-2 catalyst prepared by the microwave-assisted method at 170 ◦ C consists of Ag2 S, ZnO and hydrolyzed zinc carbonate (Zn5 (CO3 )2 (OH)6 ) phases (Fig. 1a). The characteristic diffraction peak at 2 = 13.1◦ can be indexed as

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83

the precursor solution. In comparison with the catalysts before calcination, the disappearance of Zn5 (CO3 )2 (OH)6 and Ag2 S diffraction peaks demonstrates that the as-synthesized samples can be progressively converted into Ag/Ag2 SO4 /ZnO composite during high-temperature calcination in air. The corresponding oxidation processes are proceeded by the Eqs. (7), (10) and (11) [25]: Ag2 S + O2 → 2Ag + SO2

(10)

Ag2 S + 2O2 → Ag2 SO4 H2 S + Zn

Fig. 1. Normalized XRD patterns of (a) as-synthesized AZ-2 before calcination, (b) pure ZnO, (c) AZ-1, (d) AZ-2 and (e) AZ-3 catalysts after calcination ( Ag2 SO4 ,

ZnO,

Ag, + Ag2 S, 䊉 Zn5 (CO3 )2 (OH)6 ).

the monoclinic phase of Zn5 (CO3 )2 (OH)6 (JCPDS card No. 99-0062). In this case, urea as a homogeneous precipitation agent can be decomposed into OH− and CO3 2− anions in aqueous solution under temperature above 60 ◦ C [22]. These anions can rapidly react with Zn2+ cations to form Zn5 (CO3 )2 (OH)6 [23]. However, the microwave irradiation displays much higher heating efficiency than conventional thermal conduction. It can greatly facilitate the fabrication of ZnO nanostructures from the reaction between Zn2+ and OH− , and the pyrolysis of Zn5 (CO3 )2 (OH)6 . The chemical reactions from urea to zinc precursors can be formulated as: CO(NH2 )2 + H2 O → CO2 + 2NH3 +

NH3 + H2 O → NH4 + OH −

CO2 + 2OH → CO3

2−

−

(2) (3)

+ H2 O

(4)

5Zn2+ + 2CO3 2− + 6OH− → Zn5 (CO3 )2 (OH)6 ↓

(5)

Zn2+ + 2OH− → ZnO ↓ + H2 O

(6)

Zn5 (CO3 )2 (OH)6 → 5ZnO + 2CO2 + 3H2 O

(7)

In addition, owing to the decomposition of urea, the reaction solution can be adjusted into weak alkaline conditions. In this alkaline medium, thiourea can be hydrolyzed to yield amine and H2 S intermediates, resulting in forming Ag2 S phase by the following reactions [24]: (NH2 )2 CS + 2H2 O → 2NH3 + H2 S + CO2 +

H2 S + 2Ag → Ag2 S ↓ + 2H

+

(8) (9)

As mentioned above, ZnO, Ag2 S and Zn5 (CO3 )2 (OH)6 coexist in the AZ-2 catalyst before calcination. Fig. 1b–e shows the normalized XRD patterns of the calcined samples at 500 ◦ C for 4 h. The diffraction peaks appear at 2 = 31.7◦ , 34.4◦ , 36.3◦ , 47.5◦ and 56.6◦ for the calcined products without thiourea and AgNO3 in the prepared process (Fig. 1b). These peaks can be readily indexed to (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) planes of standard wurtzite ZnO (JCPDS card No. 36-1451). The sharp and intense diffraction peaks demonstrate high crystallinity of ZnO. The calcined AZ-1 and AZ-2 composites (Fig. 1c and d) exhibit the co-existence of orthorhombic Ag2 SO4 (JCPDS card No. 27-1403), cubic Ag (JCPDS card No. 87-0597) and wurtzite ZnO phases by adding both thiourea and AgNO3 into

2+

→ ZnS ↓ + 2H

(11) +

(12)

However, with further increase of thiourea dosage, AZ-3 catalyst can be indexed to wurtzite ZnO, ZnS (28.6◦ ) and monoclinic Ag2 S (JCPDS card No. 14-0072) (Fig. 1e). No significant diffraction peaks corresponding to Ag and Ag2 SO4 can be detected, owing to large amount of Ag2 S formed under high concentration of thiourea in the preparation. The formed Ag2 S is difficult to be completely converted to metal Ag and Ag2 SO4 during high-temperature calcination in air. Moreover, the amount of amine and H2 S intermediates can increase by the hydrolysis of thiourea with increased thiourea concentration, which also results in ZnS nanocrystals formation (Eq. (12)). In addition, in contrast with pure ZnO, the ZnO diffraction peaks for AZ-1, AZ-2 and AZ-3 catalysts have no remarkable shift, suggesting that Ag, Ag2 SO4 and Ag2 S are only deposited on the ZnO surface and not incorporated into the ZnO lattice. The chemical composition and surface structure of AZ-2 catalyst were further analyzed by X-ray photoelectron spectroscopy (XPS). A typical XPS survey spectrum and high-resolution XPS spectra of different elements are displayed in Fig. 2. All peaks in Fig. 2a can be ascribed to Zn, O, Ag, S and C elements, and no peaks of unlabeled elements are detected. The presence of C 1s peak mainly comes from carbon dioxide absorbed on the surface of the catalyst from air and hydrocarbon contaminant from XPS instrument. It is confirmed that AZ-2 catalyst is composed of Zn, O, Ag, and S elements, which is consistent with the previous XRD results. As shown in Fig. 2b, the high-resolution Ag 3d spectra are divided into two sets of symmetric peaks. The higher peaks at approximately 368.2 and 374.2 eV are attributed to the binding energies of Ag 3d5/2 and Ag 3d3/2 of Ag0 , respectively [26]. The weaker peaks with the binding energies at 367.8 and 373.8 eV are assigned to Ag 3d5/2 and Ag 3d3/2 of Ag+ in Ag2 SO4 [27]. The obviously asymmetrical O 1s XPS peaks in Fig. 2c demonstrate the presence of different O species in AZ-2 catalyst. The O 1s profile can be fitted to three symmetrical peaks. The dominant peak contributes to lattice oxygen in ZnO with a lower binding energy of 531.2 eV [28]. The other two peaks correspond to O species in sulphate with the binding energy of 531.5 eV and chemisorbed oxygen of surface hydroxyl ( OH) at a higher binding energy of 532.5 eV [27,29]. Fig. 2d shows the XPS spectrum of S 2p with a high binding energy at the range of 167–172 eV. Since the sample was calcined at high temperature in air, the peak can be assigned to S6+ oxidation state of Ag2 SO4 [27,30]. No signals of organic S2− for C S bond and inorganic S2− for Zn S replacing Zn O in the ZnO lattice are found at relatively low binding energies of 166.1 and 162.3 eV, respectively [31,32]. They further confirm that Ag2 SO4 are not incorporated into the ZnO lattice and residue organic chemicals are removed from the catalyst. The XPS results also prove that the combination of Ag, Ag2 SO4 and ZnO forms the hierarchical AZ-2 nanostrucure. The detailed morphologies of representative Ag/Ag2 SO4 /ZnO composite (AZ-2 catalyst) were examined by FESEM and TEM images, as shown in Fig. 3. The obtained plate-like structure consists of numerous small nanoparticles, and possesses very rough and irregularly porous surfaces without any surfactant in the preparation. Owing to the hydrolysis reactions of urea and thiourea, Ag2 S/Zn5 (CO3 )2 (OH)6 /ZnO nanoparticles are obtained at

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(b)

(a)

Ag 3d 368.2

Intensity (a.u.)

Zn LMM O 1s

0

C 1s Ag 3d

Zn 3d Zn 3p Zn 3s S 2p

Intensity (a.u.)

Zn 2p

200

400

600

800

1000 1200

374.2

367.8

364

366

Binding Energy (eV)

Intensity (a.u.)

(c)

530

532

372

374

376

378

S 2p

Intensity (a.u)

532.5

534

370

(d)

531.2

528

368

Binding Energy (eV) O 1s

531.5

373.8

536

538

Binding Energy (eV)

162

164

166

168

170

172

174

Binding Energy (eV)

Fig. 2. (a) The XPS full spectrum of AZ-2, high-resolution XPS spectra for (b) Ag 3d, (c) O 1s and (d) S 2p of AZ-2 catalyst.

the initial stage of microwave heating. Then, these nanoparticles quickly congregate plate-like aggregates in order to decrease their surface energies under microwave-assisted process [33]. The diameters of these plates are about 1.5–2 ␮m with thickness of about 100–200 nm. Detailed structural analysis of AZ-2 was investigated by TEM image. The TEM image in Fig. 3b clearly shows the monodis-

persed Ag/Ag2 SO4 nanoparticles (NPs) with diameters in the range of 4–7 nm are uniformly anchored on the surfaces of ZnO plates. A high-resolution TEM (HR-TEM) image was also shown in Fig. 3c. Unfortunately, only two different kinds of the lattice fringes with 0.24 nm and 0.26 nm are found, corresponding to the distance of Ag (1 1 1) and ZnO (0 0 2) planes, respectively. Ag2 SO4 nanoparticles

Fig. 3. (a) Typical FE-SEM image, (b) TEM image, (c) HR-TEM image and (d) EDS spectrum of AZ-2 catalyst.

W. Cao et al. / Journal of Molecular Catalysis A: Chemical 401 (2015) 81–89

(a)

1.0 0.8

C/C 0

0.6 0.4 0.2 0.0 0

Blank ZnO P25 AZ-1 AZ-2 AZ-3 AZ-2-hydro

10

20

30

40

Time (min) (b)

1.0 0.8

C/C0

0.6 0.4 0.2 0.0

0

Blank ZnO P25 AZ-1 AZ-2 AZ-3

5

10

15

20

Time (min) Fig. 4. Photocatalytic degradation curves of the RhB aqueous solution over different photocatalysts under (a) visible-light and (b) solar light irradiation.

are irradiation-sensitive to the high-energy (e.g., 200 kV) electron beam during HRTEM observation [34]. It is not suitable to observe the lattice fringes of Ag2 SO4 because Ag2 SO4 nanoparticles are reduced in situ to metal Ag species. In addition, the EDS spectrum of AZ-2 catalyst is shown in Fig. 3d in order to confirm the chemical compositions. The result demonstrates the co-existence of Zn, Ag, O and S elements, and the lack of other elements in AZ-2 catalyst. Therefore, the hierarchical Ag/Ag2 SO4 /ZnO nanostructures are formed by the microwave-assisted hydrothermal method.

mainly result from the photocatalytic reaction in the presence of catalysts under visible-light (Fig. 4a) and natural sunlight (Fig. 4b) irradiation. As shown in Fig. 4a, 40% of RhB solution undergoes degradation in the presence of commercial TiO2 (Degussa P25), while no degradation of RhB occurs in the presence of pure ZnO under visible-light irradiation. As wide band-gap semiconductors, TiO2 and ZnO can only absorb ultraviolet light. However, the visible-light photocatalytic degradation of RhB can occur by photosensitization. Therefore, the photocatalytic degradation of RhB over P25 catalyst under visible-light irradiation is attributed to photosensitization process, while photosensitization of ZnO by the dye molecule can be ruled out in this study [35,36]. However, the composites display superior photocatalytic activities by doping silver-based compounds on the surfaces of ZnO. Under visible-light irradiation, the degradation efficiency of RhB is as high as 97% over AZ-2 catalyst after 35 min. AZ-1 catalyst can also effectively degrade 60% of RhB. By contrast, only 34% of RhB is decomposed over AZ-3 catalyst under visible-light irradiation. It indicates that the doping of Ag/Ag2 SO4 nanoparticles on the ZnO surfaces is more effective in improving its visible-light photocatalytic activity for the decomposition of organic substances. In order to illustrate the effect of microwave, the controlled AZ-2-hydro catalyst was also prepared by the hydrothermal method (see SI for details). It can remove 84% of RhB solution after 35 min irradiation, exerting a lower photocatalytic activity than AZ-2 prepared by microwave-assisted method. Compared with conventional heating, microwave irradiation as a non-classical energy source can generate smaller and more uniformed particles by promoting nucleation, which contributes to the enhanced photocatalytic performance [20,37]. To explore the utilization efficiency of solar energy over the as-prepared catalysts, a series of photocatalytic experiments were conducted under natural sunlight irradiation. As seen in Fig. 4b, for the RhB degradation, their photoreactivity order is consistent with the above results under visible-light irradiation. The decolorization efficiency is only 23% within 20 min over pure ZnO. Approximately 85% of RhB is degraded after 20 min in the presence of P25, AZ-1 and AZ-3 catalysts. In the photocatalytic process of AZ-2 catalyst, 73% of RhB can be degraded in the first 10 min, and complete photodegradation of the dye is achieved after exposure to the sunlight for 20 min. Obviously, AZ-2 catalyst displays higher photoactivity than AZ-1, AZ-3 and P25. Moreover, it is interesting to note that all photocatalytic reaction rates increase remarkably under solar light compared with visible-light irradiation. The main reason is that the existence of UV light in solar light can promote the electron excitation from ZnO and generate more charge carriers. The kinetic analysis of RhB decolorization can provide insights into the photodegraded mechanism. The linear relations of ln (C/C0 ) versus the irradiation time are observed for all catalysts under visible-light and natural sunlight irradiation in Fig. 5. Therefore, the photocatalytic reaction follows Langmuir–Hinshelwood pseudofirst-order kinetic model, which can be expressed as follow [38,39]:

3.2. Photocatalytic activity ln The plasmonic photocatalysts have wide applications in degradation of pollutants and water purification [15]. To evaluate the photooxidative capabilities of catalysts, the photodegradation of RhB under visible-light ( > 420 nm) and natural sunlight irradiation was chosen as a model reaction in the presence of ZnO, commercial P25 (TiO2 ), AZ-1, AZ-2 and AZ-3, respectively. To the best of our knowledge, Ag/Ag2 SO4 /ZnO composites have never been used as visible-light and solar-light driven photocatalysts. Fig. 4 shows the variations of RhB concentration versus irradiation time over different catalysts. The blank experiments reveal that negligible RhB was degradated in the absence of photocatalyst. It can be deduced that the degradations of dye

85

C C0

= −kapp t

(13)

where kapp is the apparent reaction rate constant. The photocatalytic performances of catalysts under visible-light and natural sunlight irradiation can be evaluated by the corresponding kapp as listed in Table 1. It is obvious that photocatalytic activities of all photocatalysts under natural sunlight are much higher than those under visible-light irradiation. This is due to the existence of UV light in solar light, which increases photocatalytic activities of catalysts. Moreover, it can be seen that kapp of ZnO is negligible, indicating the stabilization of RhB under visible-light irradiation. After Ag/Ag2 SO4 nanoparticles doped on the surfaces of ZnO plates, these Ag/Ag2 SO4 nanoparticles expand the absorption range of ZnO

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W. Cao et al. / Journal of Molecular Catalysis A: Chemical 401 (2015) 81–89

(a) 0.0

Absorbance (a.u.)

-0.5

ln(C/C0)

-1.0 -1.5

ZnO P25 AZ-1 AZ-2 AZ-3

-2.0 -2.5

AZ-3

AZ-1

AZ-2

ZnO

0

10

20

30

40

Time (min)

200

(b) 0.0

300

400

500

600

700

Wavelength (nm)

-0.5 Fig. 6. UV–vis diffuse reflectance spectra of as-prepared catalysts. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

ln (C/C0)

-1.0 -1.5 ZnO P25 AZ-1 AZ-2 AZ-3

-2.0 -2.5 -3.0 0

5

10

15

20

25

Time (min) Fig. 5. ln (C/C0 ) versus irradiation time for the RhB degradation over different photocatalysts under (a) visible-light and (b) natural sunlight irradiation.

to visible-light region and transfer the SPR electrons from Ag to the CB of ZnO, which can greatly enhance the efficiency of photocatalysis. As shown in Table 1, the kapp of RhB degradation in the presence of AZ-2 catalyst is 0.0747 min−1 under visible-light irradiation, which is up to 2.9 and 6.3 times faster than those of AZ-1 and AZ-3 catalysts, respectively. Moreover, under natural sunlight irradiation, AZ-2 catalyst still displays the highest photocatalytic activity with a kapp of 0.1450 min−1 , compared with 0.0997 min−1 for pure TiO2 (Degussa, P25). It is clear that the optimal content of thiourea plays a vital role in preparing the high-efficiently visiblelight and sunlight driven Ag/Ag2 SO4 /ZnO photocatalysts. 3.3. Photocatalytic mechanism In order to explore the reasons of the superior visible and solar light photocatalytic activities of the prepared Ag/Ag2 SO4 /ZnO catalysts, the relevant characterizations and experiments were performed as follows.

Table 1 Apparent rate constants (kapp ) and linear regression coefficients of RhB degradation under visible-light and natural sunlight irradiation.a Catalyst

Visible-light −1

ZnO P25 AZ-1 AZ-2 AZ-3 a

Sunlight

kapp (min)

R

kapp (min)−1

R2

– 0.0141 0.0257 0.0747 0.0118

– 0.9968 0.9991 0.9883 0.9976

0.0125 0.0997 0.1027 0.1450 0.0898

0.9932 0.9882 0.9830 0.9872 0.9948

2

30 mg catalyst in 30 ml RhB solution (30 mg/l).

Generally, the visible-light absorption abilities of catalysts have a significant impact on the photodegradation efficiency of organic pollutants. The UV–vis diffuse reflectance spectra of AZ catalysts and neat ZnO are shown in Fig. 6. Compared with pure ZnO, AZ catalysts exhibit significantly increased photoabsorption capacities and shifts of the absorption edge to longer wavelengths in the visiblelight region, indicating that the catalysts have great potentials for photocatalytic decomposition of contaminants under visible-light irradiation. It is in good agreement with the above photocatalytic results of AZ catalysts under visible-light and sunlight irradiation. The enhanced visible-light absorptions for AZ-1 and AZ-2 catalysts are attributed to the remarkable surface plasmon resonance of Ag nanoparticles [39]. By comparison, AZ-3 catalyst shows the strongest absorbance in the visible-light region. It may be ascribed to the introduction of Ag2 S with very low band gap energy of about 1.0 eV [40]. In addition, the red-shifted absorption spectrum may derive from the interfacial electronic transfer between ZnO and Ag-based substances [41]. However, AZ-2 catalyst possesses the highest photocatalytic performance, although the absorbance of AZ-2 catalyst is much lower than those of AZ-1 and AZ-3catalysts in the visible-light region. It demonstrates that the photocatalytic activity is related to not only the visible-light absorption ability, but also other factors, such as the Ag/Ag2 SO4 components and the separation efficiency of photogenerated carriers. During the synthetic process, the appropriate thiourea dosage is conductive to the formation of the optimized Ag/Ag2 SO4 component in AZ-2 catalyst, which induces the highest photocatalytic activity. On the other hand, excessive addition of thiourea would result in the generation of Ag2 S instead of Ag/Ag2 SO4 in AZ-3 catalyst. The separation efficiencies of charge carriers in AZ-3 catalyst is inferior to those in AZ-1 and AZ-2 catalysts, owing to the unfortunate mismatch of band gap positions between Ag2 S and ZnO [40]. Therefore, AZ-3 catalyst has very poor photocatalytic performance, although its visible-light absorption ability is significantly enhanced in the presence of Ag2 S. It is reported that the doping of noble metal on the semiconductor surfaces is an effective approach to increase the separation rate of electrons and holes, thus improving the photocatalytic performance of doped semiconductor [11]. To investigate the effect of Ag/Ag2 SO4 nanoparticles deposited on ZnO surfaces for the separation and recombination rate of photoinduced electron–hole pairs, the photoluminescence (PL) spectra of ZnO and AZ-2 catalysts excited at 325 nm are presented in Fig. 7. Since the photoluminescence derives from the recombination of electron–hole pairs,

W. Cao et al. / Journal of Molecular Catalysis A: Chemical 401 (2015) 81–89

(a)

87

1.0

Intensity (a.u.)

0.8

C/C0

0.6

ZnO

0.2

AZ-2

450

500

550

Wavelength (nm)

its intensity is directly proportional to the recombination rate of electron–hole pairs. That is the lower recombination rate the weaker PL signals. It can be seen that the PL spectra show similar shapes with the same main peak at 470 nm. However, the intensities of emission peaks for ZnO and AZ-2 catalysts are distinctly different. Bare ZnO gives strong and broad-band emission peaks from 400 to 500 nm, which is normally attributed to the recombination of the photoexcited holes and electrons occupied by the ionized oxygen vacancies in ZnO [42]. It is noteworthy that AZ-2 catalyst has a negligible visible-light range emission after the deposition of Ag/Ag2 SO4 nanoparticles on ZnO surfaces. It indicates the minimum recombination of charge carriers in the case of AZ-2 catalyst [43]. Therefore, the photocatalytic performance of AZ-2 catalyst can be effectively improved resulting from improved visible-light harvesting and the rapid charge separation. To probe the photophysical properties of photoexcited charge carriers, the time-resolved photoluminescence spectra of ZnO and AZ-2 catalysts were recorded, as shown in Fig. 8. Two radiative lifetimes ( 1 ,  2 ) with different percentages were obtained by fitting the decay spectra using a biexponential decay function. The shorter lifetime ( 1 ) of ZnO is 0.09 ns. After deposition of Ag/Ag2 SO4 nanoparticles, the shorter lifetime of AZ-2 increases up to 0.13 ns. As the dominant process, the shorter time component increases from 89.49% for ZnO to 91.36% for AZ-2. In addition, the longer lifetime ( 2 ) of the charge carriers increases from 2.07 ns for ZnO to 2.13 ns for AZ-2. These results suggest that the radiative lifetimes of all charge carriers are increased by the formation of AZ-2 composite. The prolonged fluorescence lifetime can be attributed to

1000 (a)

100

Counts

10 0

2

4

6

8

10

12

1000 (b)

100 10 1

0

2

4

6 8 Time (ns)

Ag2SO4 ZnO Ag/ZnO Ag2SO4/ZnO AZ-2

10

20

30

40

Time (min)

Fig. 7. Photoluminescence (PL) spectra of ZnO and AZ-2 catalyst.

1

0.0 0

10

12

Fig. 8. The nanosecond-level time-resolved PL spectra of (a) ZnO and (b) AZ-2 catalyst.

(b) 1.0 0.8 0.6 C/C0

400

0.4

0.4 0.2 0.0 0

AZ-2 AZ-2 + 1mM IPA AZ-2 + 1mM EDTA AZ-2 + 1mM BQ

10

20

30

40

Time (min) Fig. 9. (a) Controlled photocatalytic experiments of different catalysts and (b) reactive species trapping experiments of AZ-2 catalyst under visible-light irradiation.

the improved electron transfer between ZnO and Ag/Ag2 SO4 particles, and increase the probability of charge carriers captured by the reactive substrates to initiate the photocatalytic reactions [44,45]. In order to illustrate the cooperative effect of Ag, Ag2 SO4 and ZnO on the photocatalytic activity of Ag/Ag2 SO4 /ZnO nanostructures, the controlled photocatalytic experiments of Ag2 SO4 , ZnO, Ag/ZnO, Ag2 SO4 /ZnO and AZ-2 catalysts were performed under visible-light irradiation. As shown in Fig. 9a, the photodegradation of RhB in the presence of bare ZnO is negligible while pure Ag2 SO4 can decompose 36% of RhB after exposure to 35 min. When Ag nanoparticles are loaded on ZnO surfaces using microwave-assisted method, the decolorization of RhB dye molecules increases slightly to 11% at the same irradiation time. In contrast, Ag2 SO4 as a more efficient dopant can greatly contribute to the improvement of ZnO photocatalytic activity. The RhB removal efficiency reaches 84% in the presence of Ag2 SO4 /ZnO after 35 min. Furthermore, Ag/Ag2 SO4 nanoparticles co-doped on ZnO surfaces exhibit the highest photocatalytic performance and 97% of RhB can be decomposed over AZ-2 catalyst. The experimental results for RhB degradation indicate that the loading of Ag/Ag2 SO4 on the ZnO surfaces would be a much more efficient approach to improve the photocatalytic performance of ZnO. In a photocatalytic process, the photoexcited electron–hole pairs can migrate to the surface of catalysts and then generate active species involved in the photoreaction process. To further elucidate the photocatalytic degradation mechanism, a series of the reactive species trapping experiments were performed to capture the primary reactive oxygen species in the photocatalytic oxidation

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process. Fig. 9b shows the photocatalytic performance of AZ-2 catalyst on the photodegradation of RhB in the presence of different scavengers. Isopropyl alcohol (IPA), ethylene diamine tetraacetic acid (EDTA), and benzoquinone (BQ) were employed as a • OH radical scavenger, a hole scavenger, and a • O2 − radical scavenger, respectively [46]. The results show that the addition of IPA into the RhB solution has no apparent effect on the photocatalytic activity of AZ-2, suggesting that • OH radicals could not be the active species in the photocatalytic process. However, the introduction of EDTA and BQ into the photocatalytic system could greatly inhibit the degradation efficiencies of RhB from 97% to 16% and 27% in 35 min, respectively. It indicates that the additions of EDTA and BQ to the AZ-2 photocatalytic system can capture irradiated h+ and • O − radicals, and the fast decreases in the amount of h+ and • O − 2 2 active radicals slow down the photocatalytic reactions. EDTA can be adsorbed on the surface of the photocatalyst and act as a hole trap, resulting in the photocatalytic decrease of AZ-2 catalyst. As for BQ, it can capture • O2 − radicals and reduce the amount of active • O2 − radicals in the photocatalytic reaction. Therefore, it can be deduced by our quenching experiments that h+ and • O2 − radicals are the main active species in the photocatalytic reaction. Based on the experimental results, AZ-2 catalyst exhibits prominently photocatalytic performance under visible-light and natural sunlight irradiation for the following reasons. The Ag/Ag2 SO4 nanoparticles with very small particle sizes are highly dispersed on the surface of ZnO. Thus, the hierarchical structure with profuse interfaces of Ag/Ag2 SO4 /ZnO facilitates the transfer of photogenerated electrons. Moreover, Ag nanoparticles anchored on ZnO surfaces remarkably expand the absorption to the visible-light range and enhance its absorption capability, which contribute to enhancing the photocatalytic efficiency. In addition, Ag2 SO4 can play the most important role on the effective separation of e− /h+ for the purpose of increasing the photooxidative ability of ZnO. Therefore, the synergistic effect of Ag, Ag2 SO4 and ZnO expands the visible-light absorption capability and promotes the charge migration and separation, resulting in the improvement of photocatalysis. It is well known that the photocatalytic reaction depends on the separation and recombination rates of electron–hole pairs. The migration direction of the photoinduced carriers is determined by the band edge positions of individual components. In order to estimate the optical band gap energy (Eg ), the UV–vis diffuse reflectance spectra of pure ZnO and Ag2 SO4 are obtained and shown in Fig. S1. As a crystalline semiconductor, the band gap energy can be calculated by Mulliken electronegativity theory using the following formula [47]:



˛hv = A hv − Eg

n/2

(14)

where ˛, , A and Eg are the absorption coefficient, light frequency, absorption constants and band gap, respectively. Among them, n is 1 for direct transition and 4 for indirect transition. As for ZnO and Ag2 SO4 , the value of n is 1 for the direct transition. Consequently, Eg of ZnO and Ag2 SO4 can be estimated from a plot of (˛h)2 versus photon energy (h). The X-intercept of the tangent line gives an approximation of the band gap energy of the samples as shown in Fig. S2. The band gap energies of ZnO and Ag2 SO4 are estimated to be 3.06 and 3.8 eV, respectively. The Eg of ZnO is slightly lower

Fig. 10. Schematic diagram of the electron–hole pairs separation over AZ-2 photocatalyst under visible-light irradiation.

than the value reported in the literature because of the quantum confinement effect [48]. The band edge positions with reasonable results can be estimated by Mulliken electronegativity theory for many photocatalysts. Therefore, the band edge positions of the conduction band (CB) and valence band (VB) of ZnO and Ag2 SO4 can be predicted according to the empirical equation [49]: EVB = X − E c + 0.5Eg

(15)

ECB = EVB − Eg

(16)

where X is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, and Ec is the energy of free electrons on the hydrogen scale (4.5 eV). The X values for ZnO and Ag2 SO4 are determined to be 5.95 and 6.31 eV, respectively. Based on the given equation above, the EVB values of ZnO and Ag2 SO4 are calculated to be 2.98 and 3.71 eV, respectively (Table 2). These calculated values are similar to a recent report on ZnO/BiOI heterostructures [50]. Although these calculated values are slightly more anodic than the measured, the comparison of their relative positions is not affected. Therefore, the possible charge separation processes is proposed in Fig. 10 on the basis of the above experimental results and band structure analyses. Under visible-light irradiation, ZnO and Ag2 SO4 cannot be excited owing to their wide band gap energy of 3.06 and 3.8 eV, respectively. In contrast, the photoinduced electron–hole pairs are easily generated from Ag nanoparticles due to surface plasmon resonance effect. Consequently, the excited electrons in Ag nanoparticles with strong reduction ability will be easily injected from Ag nanoparticles to the CB of ZnO and Ag2 SO4 , respectively [52]. The photosensitive Ag2 SO4 is reduced into Ag0 deposited on the ZnO surface, and Ag0 is subsequently excited to generate the electron–hole pairs under visible-light illumination due to the SPR effect. These electrons further react with oxygen molecules in water, generating superoxide radical anion (• O2 − ) for the

Table 2 Absolute electronegativity, estimated band gap energy, calculated conduction band (CB) and valence band (VB) edge for ZnO and Ag2 SO4 . Semicondutor

Absolute electronegativity X (eV)

Estimated energy band gap Eg (eV)

Calculated ECB (eV)

Calculated EVB (eV)

ZnO Ag2 SO4

5.95 6.31

3.06a /3.2b 3.80

−0.08/−0.25 −0.09

2.98/2.93 3.71

a b

Estimated from our results. From references [51].

W. Cao et al. / Journal of Molecular Catalysis A: Chemical 401 (2015) 81–89

degradation of dye molecules. Simultaneously, SO4 2− ions having negative charges in Ag2 SO4 prefer to attract the holes on the surface of metallic Ag nanoparticles, which is in favor of the formation of • SO4 − . • SO4 − as a strong oxidizing agent is able to oxidize dye molecules, according to the previous report [53,54]. The leftover holes can also participate in the photocatalytic reactions and directly mineralize organic pollutants as a result of their strong oxidation abilities. The continual migration process of the photogenerated charge carriers in this way can promote the effective separation of photoinduced electron–hole pairs and consequently inhibit the unfavorable recombination of counterparts. Moreover, the photocatalytically produced h+ , • O2 − and • SO4 − possess strong oxidation power to decompose the organic pollutants. 4. Conclusions Highly efficient visible-light and sunlight driven Ag/Ag2 SO4 /ZnO nanostructures were successfully prepared via a microwaveassisted hydrothermal reaction under template-free conditions. The Ag/Ag2 SO4 nanoparticles are uniformly dispersed on the surfaces of ZnO plates. The optimized composition was investigated by adjusting the amount of thiourea. The results show that AZ-2 catalyst exhibits the highest photocatalytic activity toward RhB degradation under visible-light and natural sunlight irradiation. The enhanced photocatalytic performance of AZ-2 catalyst is attributed to (i) the hierarchical microstructures with large amounts of interfacial active sites, (ii) the excellent visiblelight absorption ability owing to the surface plasmon resonance of Ag nanoparticles, and (iii) the positively synergistic effect of three components (Ag, Ag2 SO4 and ZnO) in favor of the efficient separation of photogenerated electron–hole pairs and prolonged lifetime of the charge carriers. Therefore, the novel semiconductor composites provide an effective strategy for the development of heterostructured materials with potential applications in wastewater treatment. Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (NSFC 21476084), the National High Technology Research and Development Program of China (8632012AA0161601), the Fundamental Research Funds for the Central Universities, Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 20120074110008), the Open Project of State Key Laboratory of Chemical Engineering (SKL-Che12C05), 111 Project (B08021). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2015.02.023. References [1] A. Fujishima, K. Honda, Nature 57 (1972) 238–245. [2] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24 (2012) 229–251. [3] T. Hirakawa, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 3928–3934. [4] G. Miao, L.F. Chen, Z.W. Qi, Eur. J. Inorg. Chem. 35 (2012) 5864–5871. [5] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [6] A. Mclaren, T. Valdes-Solis, G.Q. Li, S.C. Tsang, J. Am. Chem. Soc. 131 (2009) 12540–12541. [7] L.P. Xu, Y.L. Hu, C. Pelligra, C.H. Chen, L. Jin, H. Huang, S. Sithambaram, M. Aindow, R. Joesten, S.L. Suib, Chem. Mater. 21 (2009) 2875–2885.

89

[8] K.J. Kim, P.B. Kreider, C. Choi, C.H. Chang, H.G. Ahn, RSC Adv. 3 (2013) 12702–12710. [9] A. Hamrouni, N. Moussa, F. Parrino, A. Di Paola, A. Houas, L. Palmisano, J. Mol. Catal. A: Chem. 390 (2014) 133–141. [10] Y.P. Bi, S.X. Ouyang, J.Y. Cao, J.H. Ye, Phys. Chem. Chem. Phys. 13 (2011) 10071–10075. [11] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570. [12] M. Pelaez, P. Falaras, V. Likodimos, A.G. Kontos, A.A. de la Cruz, K. O’shea, D.D. Dionysiou, Appl. Catal. B: Environ. 99 (2010) 378–387. [13] A.D. Liyanage, S.D. Perera, K. Tan, Y. Chabal, K.J. Balkus, ACS Catal. 4 (2014) 577–584. [14] F. Theil, A. Dellith, J. Dellith, A. Undisz, A. Csáki, W. Fritzsche, J. Popp, M. Rettenmayr, B. Dietzek, J. Colloid Interface Sci. 421 (2014) 114–121. [15] P. Wang, B.B. Huang, X.Y. Qin, X.Y. Zhang, Y. Dai, J.Y. Wei, M.H. Whangbo, Angew. Chem. Int. Ed. 47 (2008) 7931–7933. [16] M. Rycenga, C.M. Cobley, J. Zeng, W.Y. Li, C.H. Moran, Q. Zhang, D. Qin, Y.N. Xia, Chem. Rev. 111 (2011) 3669–3712. [17] O. Mehraj, N.A. Mir, B.M. Pirzada, S. Sabir, M. Muneer, J. Mol. Catal. A: Chem. 395 (2014) 16–24. [18] C. Hu, T.W. Peng, X.X. Hu, Y.L. Nie, X.F. Zhou, J.H. Qu, H. He, J. Am. Chem. Soc. 132 (2010) 857–862. [19] Z.J. Zhou, M.C. Long, W.M. Cai, J. Cai, J. Mol. Catal. A: Chem. 353–354 (2012) 22–28. [20] M. Baghbanzadeh, L. Carbone, P.D. Cozzoli, C.O. Kappe, Angew. Chem. Int. Ed. 50 (2011) 11312–11359. [21] W.R. Cao, L.F. Chen, Z.W. Qi, Catal. Lett. 144 (2014) 598–606. [22] L.F. Chen, Z. Song, X. Wang, S.V. Prikhodko, J.C. Hu, S. Kodambaka, R. Richards, ACS Appl. Mater. Interfaces 1 (2009) 1931–1937. [23] E. Hosono, S. Fujihara, I. Honma, H.S. Zhou, Adv. Mater. 17 (2005) 2091–2094. [24] S. Banerjee, S. Bhattacharya, D. Chakravorty, J. Phys. Chem. C 111 (2007) 13410–13413. [25] L. Armelao, P. Colombo, M. Fabrizio, S. Gross, E. Tondello, J. Mater. Chem. 9 (1999) 2893–2898. [26] M.L. Pang, J.Y. Hu, H.C. Zeng, J. Am. Chem. Soc. 132 (2010) 10771–10785. [27] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273–277. [28] C. Battistoni, J.L. Dormann, D. Fiorani, E. Paparazzo, S. Viticoli, Solid State Commun. 39 (1981) 581–585. [29] Y. Guo, X.B. Cao, X.M. Lan, C. Zhao, X.D. Xue, Y.Y. Song, J. Phys. Chem. C 112 (2008) 8832–8838. [30] W. Zhao, Q. Zhong, Y.X. Pan, R. Zhang, Chem. Eng. J. 228 (2013) 815–823. [31] K.L. Lv, J.C. Hu, X.H. Li, M. Li, J. Mol. Catal. A: Chem. 356 (2012) 78–84. [32] Y. Liu, J.C. Hu, C. Ngo, S. Prikhodko, S. Kodambaka, J.L. Li, R. Richards, Appl. Catal. B: Environ. 106 (2011) 212–219. [33] B. Viswanath, S. Patra, N. Munichandraiah, N. Ravishankar, Langmuir 25 (2009) 3115–3121. [34] D.L. Chen, S.H. Yoo, Q.S. Huang, G. Ali, S.O. Cho, Chem. Eur. J. 18 (2012) 5192–5200. [35] M. Logar, I. Braˇcko, A. Potoˇcnik, B. Janˇcar, Langmuir 30 (2014) 4852–4862. [36] G. Begum, J. Manna, R.K. Rana, Chem. Eur. J. 18 (2012) 6847–6853. [37] Y. Hu, H.H. Qian, Y. Liu, G.H. Du, F.M. Zhang, L.B. Wang, X. Hu, CrystEngComm 13 (2011) 3438–3443. [38] M.T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M.M. Müller, H.J. Kleebe, K. Rachut, J. Ziegler, A. Klein, W. Jaegermann, J. Phys. Chem. C 117 (2013) 22098–22110. [39] D.L. Chen, T. Li, Q.Q. Chen, J.B. Gao, B.B. Fan, J. Li, X.J. Li, R. Zhang, J. Sun, L. Gao, Nanoscale 4 (2012) 5431–5439. [40] B. Subash, B. Krishnakumar, V. Pandiyan, M. Swaminathan, M. Shanthi, Sep. Purif. Technol. 96 (2012) 204–213. [41] B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Langmuir 29 (2013) 939–949. [42] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983–7990. [43] M.K. Lee, T.G. Kim, W. Kim, Y.M. Sung, J. Phys. Chem. C 112 (2008) 10079–10082. [44] F. Dong, Q.Y. Li, Y. Zhou, Y.J. Sun, H.D. Zhang, Z.B. Wu, Dalton Trans. 43 (2014) 9468–9480. [45] P. Niu, L.L. Zhang, G. Liu, H.M. Cheng, Adv. Funct. Mater. 22 (2012) 4763–4770. [46] M. Ge, N. Zhu, Y.P. Zhao, J. Li, L. Liu, Ind. Eng. Chem. Res. 51 (2012) 5167–5173. [47] R.S. Mane, W.J. Lee, H.M. Pathan, S.H. Han, J. Phys. Chem. B 109 (2005) 24254–24259. [48] S. Bhattacharyya, A. Gedanken, J. Phys. Chem. C 112 (2008) 13156–13162. [49] M.C. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, J. Phys. Chem. B 110 (2006) 20211–20216. [50] J. Jiang, X. Zhang, P.B. Sun, L.Z. Zhang, J. Phys. Chem. C 115 (2011) 20555–20564. [51] X.M. Zhang, Y.L. Chen, R.S. Liu, D.P. Tsai, Rep. Prog. Phys. 76 (2013) 046401–046442. [52] S.A. Ansari, M.M. Khan, M.O. Ansari, J. Lee, M.H. Cho, J. Phys. Chem. C 117 (2013) 27023–27030. [53] W.J. Liao, Y.R. Zhang, M. Zhang, M. Murugananthan, S. Yoshihara, Chem. Eng. J. 231 (2013) 455–463. [54] U.I. Gaya, A.H. Abdullah, Z. Zainal, M.Z. Hussein, J. Hazard. Mater. 168 (2009) 57–63.