Modified hydrothermal fabrication of a CoS2–graphene hybrid with improved photocatalytic performance

Materials Science in Semiconductor Processing 27 (2014) 173–180

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Modified hydrothermal fabrication of a CoS2–graphene hybrid with improved photocatalytic performance Ze-Da Meng a,b, Kefayat Ullah b, Lei Zhu b, Shu Ye b, Won-Chun Oh b,n a Jiangsu Key Laboratory of Environmental Functional Materials, College of Chemistry and Bioengineering, Suzhou University of Science and Technology, Suzhou 215009, China b Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam-do 356-706, Korea

a r t i c l e i n f o

Keywords: CoS2–graphene Raman Visible light FT-IR Methylene blue

abstract CoS2 and a CoS2–graphene composite were synthesized via a facile sonochemical and hydrothermal method. As-prepared samples were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy. The CoS2–graphene composites exhibited high dye adsorption, extended light absorption, and efficient charge separation properties. Hence, for photodegradation of methylene blue (MB), significant enhancement of the reaction rate was observed for CoS2–graphene compared to pure CoS2. The high activity can be attributed to positive synergetic effects between CoS2 and graphene. The excellent photoinduced charge separation and observed red shift make these hybrid materials potential candidates for the development of high-performance next-generation photocatalyst materials. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction As a low-cost, environmentally friendly, and sustainable treatment technology, semiconductor photocatalysis is effective in wastewater treatment, hydrogen generation, and air purification, and has received much attention. Photocatalysis is based on the reactive properties of photogenerated electron–hole pairs; thus, photocatalyst performance is the most important factor influencing catalytic efficiency [1]. Carbon-based materials such as graphite, carbon nanotubes, graphene sheets, and fullerene have been widely studied. Two-dimensional (2D) graphene has emerged as a material with high potential and is attracting increasing attention because of its fascinating physical properties including quantum electronic

n

Corresponding author. Tel.: þ 82 41 660 1337; fax: þ82 41 688 3352. E-mail address: [email protected] (W.-C. Oh).

http://dx.doi.org/10.1016/j.mssp.2014.06.016 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

transport, extremely high mobility, high elasticity, and electromechanical modulation [2,20]. This 2D material exhibits exceptional electrical, thermal, mechanical, and optical properties, which has led to huge interest in designing new graphene-based materials for various technological applications such as biosensing, polymer composites, nanoelectronics, hydrogen production and storage, drug delivery, supercapacitors, and photocatalysis [3,4]. Two strategies have been used to improve the efficiency of sunlight utilization. One is the use of modification techniques to shift the light absorption capacity towards the visible region, such as ion doping, noble metal loading, metal ion implantation, dye sensitization, and conjugated polymer modification. The other is the development of new oxide photocatalysts with visible light-driven activity [5,6]. Transition metal dichalcogenide compounds with a pyrite structure (MX2; M¼ Mn, Fe, Co, Ni; X¼S, Se, Te) showed various magnetic and electronic properties [7,19]. Among various biological, physical, and chemical techniques, use of

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graphene-based hybrid materials has been considered as a cost-effective alternative method for wastewater treatment. The advantage of photocatalytic techniques over traditional approaches are the rapid oxidation rate, high efficiency, lack of polycyclic product formation, and oxidation of pollutants present at low levels [8,9]. Semiconductors have attracted considerable interest in the past two decades because of their applications in single-electron transistors [10], lasers [11], light-emitting diodes [12], and infrared photodetectors [13] operating at lower currents and higher temperatures compared to other materials. Semiconductors have unique advantages: their bandgap can be modified by varying the size to alter the optical properties and using photoelectrons to generate multiple charge carriers via incident photons [14]. Various semiconductors, including PtSe2 [15], PbS [8], CdSe [14], and NiS2 [16], have been combined with graphene to increase the catalytic efficiency by varying the response towards the visible region of the electromagnetic spectrum. The synthesis of nanomaterials with a uniform size and shape and high crystallinity is an important challenge. Various synthesis approaches are available for the preparation of nanomaterials, including hydrothermal, microwave, sol–gel, microemulsion, and polyol techniques. The most challenging problem is fabrication of nanomaterials with a uniform shape and size and better crystalline phases. Every synthesis method has its own limitations, such as long reaction time, nonuniform shape and size, particle agglomeration, and high cost of the solvent medium [21,22]. Sonochemical synthesis can be used to prepare nanomaterials with a uniform size and shape in a shorter reaction time. In this process, temperatures up to 5000 K and pressures greater than 20 MPa are created, and the material is subsequently cooled at high cooling rates of up to 1010 K/s [17]. These conditions induce reactions such as oxidation, reduction, hydrolysis, dissolution, and decomposition [18]. In the present study, CoS2–graphene was prepared via a sonochemical and hydrothermal method using cobalt chloride and anhydrous purified sodium thiosulfate. Interesting phenomena were found and the enhanced photoactivity was attributed to photosensitization of graphene and enhanced interfacial charge separation between graphene and CoS2 particles. 2. Experimental 2.1. Materials Graphene oxide (GO) was prepared via the Hummers– Offeman method as previously reported [4]. For oxidization of carbon material, 3-chloroperoxybenzoic acid (TCPBA; Acros Organics, Morris Plains, NJ, USA) was chosen as the oxidizing agent. Benzene (99.5%; Samchun Pure Chemical Co., Seoul, Korea) was used as the organic solvent. Cobalt chloride (CoCl2) was purchased from DaeJung Chemicals & Metal Co. (Korea). Anhydrous purified sodium thiosulfate (Na2S2O3, 95%) was purchased from Duksan Pharmaceutical Co. (Korea). Methylene blue (MB, 4C16H18N3S  Cl) was purchased from Samchun Pure Chemical Co. (Korea). All chemicals were used without further

purification, and all experiments were carried out using distilled water. 2.2. CoS2–graphene synthesis In a typical experiment, 100 mg of GO was mixed in 200 ml of distilled water under vigorous stirring. In the second step, 1 mmol of CoCl2 and 1 mmol of Na2S2O3 were added to the GO aqueous solution and ultrasonicated for 30 min for homogeneous mixing. The solution was transferred to a Teflon-lined autoclave and maintained at 423 K for 12 h. The solution was then filtered and the product was washed three times with distilled water and dried in an oven for 6 h at 90 1C to yield CoS2–graphene powder. 2.3. Characterization X-Ray diffraction (XRD; Shimadzu XD-D1, Kumamoto, Japan) was used to identify the crystallinity of the composite with monochromatic, high-intensity Cu Kα radiation (λ ¼ 1.5406 Å). Scanning electron microscopy (SEM; JSM5600, Jeol, Tokyo, Japan) was used to observe the surface state and structure. Transmission electron microscopy (TEM; Jeol, JEM-2010, Japan) was used to determine the state and particle size. TEM at an acceleration voltage of 200 kV was used to investigate the number and stacking state of graphene layers in various samples. TEM specimens were prepared by placing a few drops of sample solution on a carbon grid. Elemental mapping over selected sample areas was performed using an energydispersive X-ray (EDX) analyzer attached to the SEM instrument. Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Renishaw InVia) in backscattering geometry with a 514.5-nm argon laser as the excitation source. UV-vis diffuse reflectance spectra were recorded on a UV-vis spectrophotometer (Neosys-2000, Scinco, Seoul, Korea) using BaSO4 as a reference at room temperature. Spectra were converted from reflection to absorbance mode according to the Kubelka-Munk method. 2.4. Photocatalytic degradation of MB Photocatalytic activity was evaluated in terms of dye degradation in aqueous solution under visible light irradiation. A glass reaction beaker (diameter 4 cm, height 6 cm) was held axially in a visible lamp box (8 W, halogen lamp, KLD-08L/P/N, Korea) at a distance of 100 mm from the lamp over a magnetic stirrer. The luminous efficacy of the lamp was 80 lm/W over the wavelength range 400– 790 nm. The initial dye concentration was 1  10  5 mol/L in all experiments. The amount of photocatalytic composite used was 0.05 g in 50 ml of solution. The reactor was held in the dark for 2 h to reach adsorption–desorption equilibrium. Then visible light irradiation was initiated to allow the degradation reaction to proceed for 90 min. Samples were withdrawn regularly from the reactor and the dispersed powder was removed by centrifugation. The solution was analyzed using a UV-vis spectrophotometer (Optizen POP; Mecasys, Korea). The dye concentration in

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the solution was determined as a function of the irradiation time. 2.5. Photocatalyst stability To demonstrate the photostability of CoS2–graphene photocatalysts, cyclic MB degradation experiments were carried out. Used CoS2–graphene photocatalysts were immersed in ethanol for 6 h, rinsed with deionized water, and dried at 353 K. Cleaned photocatalyst samples were then reused for MB removal, and such cyclic experiments were performed several times.

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in stacked graphene sheets. Characteristic CoS2 peaks at 27.851, 32.301, 36.321, 39.831, 46.361, and 54.971 were assigned to the (111), (200), (210), (211), (220), and (311) planes of crystalline CoS2 (JCPDS Card No. 41-1445) [8]. From the XRD patterns in Fig. 2 it is possible to calculate the percentage crystallinity and crystallite size. The amorphous content of a sample can be determined as the ratio of the amorphous area (area not under peaks) in an X-ray diffractogram to the total area [23]. Prasad et al. reported a method for estimating the amorphous content from XRD patterns [24]. The peaks for different crystal planes in the CoS2–graphene nanocomposite exactly match those for

3. Results and discussion

(111)

3.1. Elemental analysis

Co Co Intensity (a.u)

The elemental composition of samples was analyzed and characteristic elements were identified. Fig. 1 shows EDX spectra for CoS2 and CoS2–graphene samples. Strong Co and S peaks are evident, as well as C peaks for CoS2– graphene. Quantitative microanalyses confirmed that C, Co, and S were the major elements in CoS2–graphene.

Co

(220)

Co Co (211) (210)

(200) G

3.2. XRD analysis

(222) (311) Co Co

Co

The structural properties of the CoS2–graphene nanocomposite were characterized and compared to CoS2 using XRD (Fig. 2). The graphene diffraction peaks observed at 12.51 and 23.291 (2θ) were attributed to short-range order

10

20

30

40

50

60

70

2θ (Degree) Fig. 2. XRD patterns for CoS2 and CoS2–graphene.

Fig. 1. EDX elemental microanalysis for (a) CoS2 and (b) CoS2–graphene.

80

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Fig. 3. SEM images of (a) CoS2, (b) graphene, and (c) CoS2–graphene.

CoS2, indicating no difference with respect to the type of crystalline phase in the two samples. We found that CoS2 showed greater crystallinity compared to CoS2–graphene. This is attributed to the adverse environment created during sonication, which facilitates faster reactions and inhibits nucleation and full crystal growth. The XRD peak intensity is greater for CoS2–graphene than for CoS2, which is an indication of the presence of larger CoS2 particles on graphene nanosheets in the nanocomposite.

yielded nanoscale CoS2, as observed in Fig. 3c, with a favorable morphology but slight tendency to agglomerate. Typical TEM images of graphene and CoS2–graphene are displayed in Fig. 4. The morphology of graphene, consisting of thin stacked flakes with a well-defined fewlayer structure at the edge, is clear in Fig. 4a. Nanoscale CoS2 exhibited well-dispersed nanoparticles with an average size of  30–45 nm, as observed in Fig. 5b. The mechanism of CoS2 particle formation and the exact role of graphene sheets in this process await further studies.

3.3. SEM and TEM analyses

3.4. FT-IR spectroscopy

The typical microsurface structure and morphology of the samples were characterized by SEM and TEM. In Fig. 3a, CoS2 particles appear as subspheroidal particles with good dispersion. A flake-like morphology was observed for graphene, reflecting its layered microstructure (Fig. 3b). The large interlayer spaces and thin layer edges of graphene can be clearly observed. It is noteworthy that sonochemical synthesis of CoS2 and CoS2–graphene

The simple FT-IR spectrum for graphene suggests extensive oxidation. The spectra of crystalline materials show well-distinguished and sharp bands, whereas spectra for amorphous materials are less well resolved. The hydration results establish the importance of defined conditions for FT-IR (Fig. 5). They also suggest that this type of study can be performed for oxidized graphene. Fig. 5 shows FT-IR spectra for oxidized graphene and

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Absorbance(a.u)

CoS2-Graphene

200

CoS2

400

600

Wavelength (nm) Fig. 6. UV-vis absorption spectra for CoS2 and CoS2–graphene.

Fig. 4. TEM images of (a) graphene and (b) CoS2–graphene.

1685

1538

(a) Oxidation graphene

3.5. UV-vis diffuse reflectance spectroscopy

1407

(b) CoS2-graphene

1712

1427

C-OH 1594

1091

995

Transmitance

C=O C=C

-CH3

C=C C=O

1052

-CH3 C-OH

1000

1500

1091 cm  1. An -OH peak is also observed at approximately 1407 cm  1. The results confirm that artificial ageing occurred on the graphene surface. The types of structural changes inferred from the spectra are consistent with the mechanism proposed in the literature: formation of O-H bonds resulting from oxidation of hydrocarbon molecules via direct binding of O and O-O radicals, followed by further oxidation to carbonyl functional groups [9,10]. In Fig. 5b, the weak peak at 1052 cm  1 was assigned to C-OH groups. Peaks for C-O, C¼C, and C ¼O functional groups were observed at approximately 1427, 1549, and 1712 cm  1, respectively. Comparison of the spectra reveals a lower peak intensity for functional groups in oxidized graphene. This is because some of the functional groups combined with CoS2 particles. The CoS2 particles were bound to graphene via different functional groups [11].

2000

2500

3000

Wavenumber (cm-1)

Fig. 6 shows UV-vis absorption spectra for the samples. A large absorption edge for CoS2–graphene compared to CoS2 is evident. This means that the composite has high photocatalytic activity under visible light irradiation. Because CoS2 has a smaller bandgap, it can be used to induce photocatalysis. When CoS2 is coupled to graphene, graphene acts as a photosensitizer for excitation and injection of electrons into the conduction band of CoS2. This suggests that absorption in the visible region is due to the well-dispersed graphene nanosheets and not to any modification of the bandgap of CoS2 [25,26].

Fig. 5. FT-IR spectra for oxidized graphene and the CoS2–graphene composite.

3.6. Raman spectroscopy

CoS2–graphene. In Fig. 5a, the peak at 995 cm  1 was assigned to alkane bending vibrations, which occur between 650 and 1000 cm  1, and the peak at 806 cm  1 was assigned to aromatic symmetric stretching, which occurs in the range 690–900 cm  1. Strong C–O bands at approximately 1102 cm  1 and a strong C¼ C band at 1538 cm  1 are evident. C ¼O and C-OH functional groups are indicated by the peaks at approximately 1685 and

Fig. 7 is the Raman spectra (taken with the 532 nm Nd: YAG laser) of both the typical D band and G band for graphene appeared. There are two typical bands for graphitic materials: the so-called G band located at 1593 cm  1and the D band located at  1350 cm  1. The D band corresponds to sp3 bonds due to graphene defects such as pentagons and heptagons. The G band corresponds to sp2 bonds between crystalline graphite sheets. The D

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D band

1.0

G band

0 min Adsorption 30 min Irradiation 30 min Irradiation 60 min Irradiation 90 min Irradiation 120 min

Abs(a.u)

Intensity (cts)

0.8

CoS2

0.6

Adsorption

Degradation

0.4

0.2

0.0

200 250 300 350 400 450

1500

2000

2500

300

3000

400

Raman Shift (cm-1) Fig. 7. Raman spectrum for CoS2–graphene.

600

700

800

1.0 0 min Adsorption 30 min Irradiation 30 min Irradiation 60 min Irradiation 90 min

0.8

Abs(a.u)

band correlates with structural disorder in graphene, which originates from defects including disordered materials, poor graphitization, functionalized carbon, and amorphous carbon on the side wall of nanotubes [27,28]. The intensity ratio of the D band to the G band (ID/IG) is a measure of the crystallinity of graphite layers [29]. According to Fig. 7, CoS2–graphene exhibited a large ID/IG ratio. This suggests that the graphite layers were inclined slightly with respect to the fiber axis, exposing the graphite edges, which serve as anchors for metal particles. Defects on the graphene surface, as suggested by the large ID/IG ratio, could also mediate the deposition of CoS2 particles. Raman measurements for commercial CoS2 powder showed characteristic peaks at 389 and 410 cm  1, in close agreement with data for single-crystal CoS2 [30].

500

Wavelength (nm)

0.6

Adsorption

0.4 Degradation

0.2

0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 8. Degradation of methylene blue under visible light using (a) CoS2 and (b) CoS2–graphene.

3.7. Photocatalytic activity 1.0

1st run

2st run

3st run

4st run

0.8

C/C0

The photocatalytic activity of CoS2 and CoS2–graphene was investigated by degradation of MB dye as model standard dye in aqueous solution to demonstrate the degradation ability of organic dyes. All CoS2 and CoS2–graphene samples were processed using the same procedure. Photodegradation was measured as temporal evolution of the MB UV-vis spectrum (Fig. 8). During photodegradation under visible light irradiation, the MB peak intensity gradually decreased over time. Besides the maximum MB absorption at 660 nm, the spectral intensity decreased over time interval, reflecting the catalytic behavior of the nanoscale samples. Two steps are involved in photocatalytic decomposition of dyes: adsorption of dye molecules and degradation. After physical adsorption in the dark for 30 min with magnetic stirring, the samples reached adsorption–desorption equilibrium [31]. In the first step, CoS2–graphene exhibited better physical adsorption in comparison to CoS2. MB solution was degraded within 120 min in the presence of CoS2 (Fig. 8a), and within 90 min in the presence of CoS2–graphene (Fig. 8b), which is a significantly shorter photodegradation time. The adsorption capacity of CoS2–graphene was better than that of CoS2 because graphene addition enhances the surface area [32]. CoS2–graphene has a larger BET surface

0.6

0.4

0.2

0.0 0

90

180

270

360

Time (min)

Fig. 9. Cyclic runs for photocatalytic degradation of methylene blue under visible light irradiation.

area, which enhances the adsorption effect. CoS2–graphene also showed good degradation capacity, as shown by the UV-vis absorption spectra. Comparison of decolorization for the different catalysts showed that degradation can be increased by increasing the adsorption capacity. To demonstrate the photostability and cyclic performance of the CoS2–graphene photocatalyst, consecutive

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Fig. 10. Schematic diagram of the catalytic mechanism involving separation of photogenerated electrons and holes at the CoS2–graphene interface.

photocatalytic MB degradation reactions were conducted in the presence of CoS2–graphene under visible light. As shown in Fig. 9, CoS2–graphene did not exhibit any significant loss of photocatalytic activity after four runs of MB degradation. These results indicate that the CoS2– graphene photocatalyst has high stability and is not photocorroded during photocatalytic oxidation of MB. Thus, the CoS2–graphene composite is promising for practical photocatalyst applications in environmental purification. Graphene modification improves not only the photocatalytic performance but also the long-term stability of CoS2 nanocrystals. This result is significant from a practical viewpoint, as the enhanced photocatalytic activity and stability will lead to more cost-effective operation. CoS2 has a relativity small bandgap and can be used for photocatalysis with visible light irradiation. We propose that hydroxyl radicals are easily generated on the surface of CoS2 nanoparticles. These hydroxyl radicals can degrade organic pollutants adsorbed on the nanoparticle surface, resulting in good photodegradation performance [33]. In graphene-modified CoS2 systems, incident photons during light irradiation are absorbed by graphene, resulting in emission of photoelectrons and transfer to the conduction band of CoS2 [34]. Simultaneously, excited electrons in the conduction band of CoS2 transfer to the graphene sheet and prolong the recombination time. These electrons from CoS2 and graphene react with absorbed molecules on the graphene surface and prevent recombination, thus improving the catalytic effect. This synergistic effect is the main reason for the improved catalytic behavior of CoS2–graphene nanocomposites. It is considered that photoinduced charge transfer occurs in electronic interactions between the carbon layers of graphene and CoS2. Electrons on the surface of graphene migrate to the surface of CoS2, which leads to a higher rate of reduction in e  /h þ pair recombination. Thus, graphenemodified CoS2 exhibits higher photon efficiency, which reduces the quantum yield of the CoS2 catalyst. Graphene

can also enhance the adsorption effect during decolorization processes [35,36]. Fig. 10 shows a schematic diagram of the catalytic mechanism involving separation of photogenerated electrons and holes at the CoS2–graphene interface. 4. Conclusion A simple and efficient modified hydrothermal method was used to fabricate CoS2 and a CoS2–graphene nanocomposite. CoS2 and CoS2–graphene were used as photocatalyst materials for removal of MB as an organic dye. High catalytic activity was observed for the CoS2–graphene composite. This high activity is attributed to a positive synergistic effect between CoS2 and graphene that provides a large number of reaction sites, suppresses charge recombination, and improves interfacial charge transfer. The DRS result gives a large absorption edge for CoS2– graphene, confirming that our nanocomposite can be used as a photocatalyst in the visible light range. References [1] O.K. Dalrymple, E. Stefanakos, M.A. Trotz, D.Y. Goswami, Appl. Catal. B Environ. 98 (2010) 27–38. [2] Z.D. Meng, W.C. Oh, Chin. J. Catal. 33 (2012) 1495–1501. [3] P. Xu, T. Xu, J. Lu, S.M. Gao, N.S. Hosmane, B.B. Huang, Y. Dai, Y.B. Wang, Energy Environ. Sci. 3 (2010) 1128–1134. [4] K. Ullah, L. Zhu, Z.-D. Meng, S. Ye, Q. Sun, W.-C. Oh, Chem. Eng. J. 231 (2013) 76–83. [5] H. Tada, T. Kiyonaga, S.I. Naya, Chem. Soc. Rev. 38 (2009) 1849–1858. [6] W. Kim, T. Tachikawa, T. Majima, C. Li, H.J. Kim, W. Choi, Energy Environ. Sci. 3 (2010) 1789–1795. [7] S.K. Kwon, S.J. Youn, B.I. Min, Phys. Rev. B 62 (2000) 357. [8] K. Ullah, Z.-D. Meng, S. Ye, L. Zhu, W.-C. Oh, J. Ind. Eng. Chem. 20 (2014) 1035–1042. [9] K. Ullah, S. Ye, L. Zhu, Z.-D. Meng, S. Sarkar, W.-C. Oh, Mater. Sci. Eng. B 180 (2014) 20–26. [10] K. Kawasaki, D. Yamazaki, A. Kinoshita, H. Hirayama, K. Tsutsui, Y. Aoyagi, Appl. Phys. Lett. 79 (2001) 2243–2248. [11] T.R. Nielsen, P. Gartner, F. Jahnke, Phys. Rev. B 69 (2004) 235314–235319.

180

Z.-D. Meng et al. / Materials Science in Semiconductor Processing 27 (2014) 173–180

[12] K. Dai, L.H. Lu, C.H. Liang, J.M. Dai, G.P. Zhu, Z.L. Liu, Q.Z. Liu, Y.X. Zhang, Mater. Chem. Phys. 143 (2014) 1410–1416. [13] Z. Ye, J.C. Campbell, Z. Chen, E.-T. Kim, A. Madhukar, J. Appl. Phys. 92 (2002) 7462–7468. [14] T. Ghosh, K. Ullah, V. Nikam, C.-Y. Park, Z.-D. Meng, W.-C. Oh, Ultrasonics Sonochem. 20 (2013) 768–776. [15] K. Ullah, L. Zhu, Z.-D. Meng, S. Ye, S. Sarkar, W.-C. Oh, J. Mater. Sci. 49 (2014) 4139–4147. [16] K. Ullah, S. Ye, L. Zhu, S.B. Jo, W.K. Jang, K.-Y. Cho, W.-C. Oh, Solid State Sci. 31 (2014) 91–98. [17] K.S. Suslick, S.B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414–416. [18] C.C. Yu, M. Yu, C.X. Li, C.M. Zhang, P.P. Yang, J. Lin, Cryst. Growth Des. 9 (2009) 783–791. [19] A.E. Bocquet, K. Mamiya, T. Mizokawa, A. Fujimori, T. Miyadai, H. Takahashi, M. Môri, S. Suga, J. Phys. Condens. Matter 8 (1996) 2389–2400. [20] G. Chai, H. Heinrich, L. Chow, T. Schenkel, Appl. Phys. Lett. 91 (2007) 103101. [21] H. Khallaf, C.-T. Chen, L.-B. Chang, O. Lupan, A. Dutta, H. Heinrich, A. Shenouda, L. Chow, Appl. Surf. Sci. 257 (2011) 9237–9242. [22] N. Wu, Y.B. Losovyj, D. Wisbey, K. Belashchenko, M. Manno, L. Wang, C. Leighton, P.A. Dowben, J. Phys. Condens. Matter 19 (2007) 156224–156234. [23] L. Zhao, M. Nelson, H. Aldridge, et al., J. Appl. Phys. 115 (2014) 034104.

[24] S. Kumar, L.K. Sahay, A.K. Jha, K. Prasad, Adv. Nano Res. 1 (2013) 211–218. [25] Q. Folkerts, G.A. Sawatzky, C. Haas, R.A. Groot, F.U. Hillebrecht, J. Phys. C Solid State Phys. 20 (1987) 4135–4144. [26] J.R. Guimarães, C.R. Turato Farah, M.G. Maniero, P.S. Fadini, J. Environ. Manage. 107 (2012) 96–101. [27] C. Hou, Q. Zhang, Y. Li, H. Wang, J. Hazard. Mater. 205–206 (2012) 229–235. [28] S.J. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [29] B.J. Li, H.Q. Cao, J. Mater. Chem. 21 (2011) 3346–3349. [30] N.R. Khalid, Z. Hong, E. Ahmed, Y. Zhang, H. Chan, M. Ahmad, Appl. Surf. Sci. 258 (2012) 5827–5834. [31] L. Zhu, D. Susac, M. Teo, K.C. Wong, P.C. Wong, R.R. Parsons, D. Bizzotto, K.A.R. Mitchell, S.A. Campbell, J. Catal. 258 (2008) 235–242. [32] D. Kibanova, M. Sleiman, J. Cervini-Silva, H. Destaillats, J. Hazard. Mater. 211–212 (2012) 233–239. [33] Q. Xiang, J. Yu, Phys. Chem. Lett. 4 (2013) 753–759. [34] Y.H. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, ACS Nano 6 (2012) 9777–9789. [35] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Adv. Mater. 22 (2010) 3906–3924. [36] K. Ullah, S. Ye, S.-B. Jo, L. Zhu, K.-Y. Cho, W.-C. Oh, Ultrasonics Sonochem. 21 (2014) 1849–1857.