Preparation of Graphene-BiOCl/Fe₃O₄ Nanocomposites and Their Use as Photocatalysts for Organic Dyes Degradation

Preparation of Graphene-BiOCl/Fe ₃ O ₄ Nanocomposites and Their Use as Photocatalysts for Organic Dyes Degradation

Fuyong Zhang and Weon Bae Ko ^†

Department of Chemistry, Sahmyook University, Seoul, 139-742, South Korea (Received February 2, 2017, Revised February 9, 2017, Accepted February 14, 2017)

Abstract: Graphene-BiOCl/Fe

O

nanocomposites were synthesized from BiOCl/Fe

O

and graphene in an electric furnace operating at 700°C for 12 h. The nanocomposite surface morphology and crystal structure were characterized by scanning electron microscopy and X-ray diffraction. The produced graphene-BiOCl/Fe

O

nanocomposites acted as efficient het- erogeneous photocatalysts for the degradation of organic dyes, as confirmed by UV-vis spectrophotometry.

Keywords: graphene, BiOCl/Fe

O

, photocatalyst, degradation, organic dyes

Introduction

The growth of the chemical, textile, and dye industries over recent years has led to increasing levels of water pollution, with organic dyes contributing significantly to this pollution.

Hence, various photocatalysts have been synthesized and studied for application in the degradation of organic dyes. ^1,2 For example, a number of semiconductor photocatalysts have been extensively investigated for this purpose, with a few being successfully applied in the treatment of organic pol- lutants. ^3-6 However, photocatalysts with large band gaps require an input of energy to function, and the treatment of organic pollutants in wastewater is limited to the use of nat- ural solar light. The development of novel visible light pho- tocatalysts is therefore of particular importance. ^7,8

More recently, bismuth-containing photocatalysts have received growing attention due to their unique crystalline structure. ⁹ For example, the bismuth oxyhalide BiOCl has a layered structure, which can efficiently separate electron-hole pairs and enhance photocatalytic activity. However, BiOCl has a large band gap of 3.19~3.44 eV, and as such, it is unable to degrade organic dyes under visible light. ⁹ To address this issue, significant efforts have been made to develop BiOCl-based visible light photocatalysts through coupling with other semiconductors, with examples including BiOCl/

Fe ₂ O ₃ and BiOCl/Bi ₂ O ₃ . ^10-16

Moreover, graphene as a two-dimensional carbon material, has also been studied in great detail due to its unique struc- ture, large specific surface area, and excellent charge carrier mobility. ^17-19 Thus, we herein report a facile synthetic method for the preparation of graphene-BiOCl/Fe ₃ O ₄ nanocomposites, and examine their photocatalytic performance under UV light irradiation at 254 nm.

Experimental

1. Materials

Ferrous sulfate hexahydrate (FeSO ₄ ·6H ₂ O), ferric chloride hexahydrate (FeCl ₃ ·6H ₂ O) and bismuth nitrate pentahydrate (Bi(NO ₃ ) ₃ ·5H ₂ O) were purchased from Shinyo pure chemi- cals co. (Japan). Hydrochloric acid (HCl), ammonium hydr- oxide (NH ₄ OH), potassium chloride (KCl), ethylene glycol (C ₂ H ₆ O ₂ ), methylene blue (MB), rhodamine B (RhB), bril- liant green (BG), methyl orange (MO) and tetrahydrafuran (THF) were obtained from Samchun chemicals co. (Korea).

2. Synthesis of the Fe ₃ O ₄ nanoparticles

FeSO ₄ ·6H ₂ O (0.02 mol) and FeCl ₃ ·6H ₂ O (0.02 mol) were dissolved in a 0.01 M aqueous solution of HCl (100 mL) under magnetic stirring. The resulting homogeneous yellow solution was then added to a 3 M aqueous solution of NH ₃ ·H ₂ O (80 mL), and the reaction mixture was heated at 80°C for 3 h.

†

Corresponding author E-mail: [email protected]

10 Fuyong Zhang et al. / Elastomers and Composites Vol. 52, No. 1, pp. 9-16 (March 2017)

After this time, the obtained products were washed with distilled water, separated using an external magnet, and dried under air at 60°C for 12 h.

3. Synthesis of the BiOCl/Fe ₃ O ₄ nanocomposites

Following the dispersion of the as-prepared Fe ₃ O ₄ nano- particles (0.1 g) in ethylene glycol (40 mL), Bi(NO ₃ ) ₃ ·5H ₂ O (3.5 mmol) was added with stirring over 20 min. After this time, KCl (3.5 mmol) was added, and the mixture was allow- ed to react under magnetic stirring for 20 min. Subsequently, the mixture was annealed at 700°C for 12 h, and the electric furnace was allowed to cool to room temperature. The resulting products were washed several times with distilled water and dried at 60°C.

4. Synthesis of the graphene-BiOCl/Fe ₃ O ₄ nanocomposites

A 1:1 mixture of the synthesized BiOCl/Fe ₃ O ₄ nanocom- posites with graphene was added to tetrahydrofuran (THF) then annealed at 700°C for 12 h to give the desired graphene- BiOCl/Fe ₃ O ₄ nanocomposites.

5. Characterization

The structural and compositional characterization of the graphene-BiOCl/Fe ₃ O ₄ nanocomposites was carried out using X-ray diffraction (XRD, D8 ADVANCE, Bruker) and scan- ning electron microscopy (SEM, JSM-6510, JEOL Ltd.). A UV lamp was employed as the light irradiation source (8 W, 254 nm, CEDEX 1, 77202 Marne La Vallee, France). Finally, the UV-vis spectra of various liquid samples were recorded on a UV-1601 PC UV-vis spectrometer (Shimadzu, Japan).

6. Photocatalytic activity of the nanocomposites for the degradation of organic dyes

All photocatalytic tests were performed at room temperature.

Methylene blue (MB), brilliant green (BG), methyl orange (MO) and rhodamine B (RhB) were used as model pollutants.

Samples of the graphene-BiOCl/Fe ₃ O ₄ nanocomposites (0.01 g each) were added to fixed concentration solutions of the various organic dyes (10 mL). The resulting mixtures were stored in the dark for 0.5 h to allow the adsorption/desorption equilibrium of the organic dyes to be established. After this time, the solution was irradiated using a UV lamp at 254 nm,

and UV-vis spectrophotometry was employed to determine the degree of photocatalytic degradation of the organic dyes. ²⁰ More specifically, the concentrations of the organic dyes in the aqueous solutions were monitored during the photocatalytic degradation by measuring the absorbance of each sample at λ _max values of 664, 625, 554, and 464 nm, for MB, BG, RhB, and MO, respectively. The degradation efficiency (η%) was calculated according to Equation (1) below:

η(%) = (1 − c/c ₀ ) × 100 (1)

where c ₀ is the initial concentration of the organic dye, and c is the concentration of the organic dye at time t.

Results and Discussion

1. Characterization

The XRD patterns of the graphene-BiOCl/Fe ₃ O ₄ nano- composites are shown in Figure 1. Reflection peaks were observed at 2 θ values of 30.19, 35.46, 43.11, 46.90, 57.01, 62.18, 70.78, and 86.64° corresponding to the (220), (311), (400), (331), (511), (440), (620), and (642) due to Fe ₃ O ₄ planes, respectively (JCPDS Card No. 88-0315). In addition, signals originating from the BiOCl component were observed at 23.79, 25.69, 32.68, 41.28, 46.21, 48.68, 55.49, 74.48, and 77.50°, which corresponded to the (002), (101), (110), (112), (200), (201), (104), (301), and (310) planes, respectively (JCPDS Card No. 73-2060). Furthermore, the diffraction sig- nals at 2θ values of 26.54 and 54.24° correspond to the (002) and (004) planes of graphene, respectively (JCPDS Card No.

01-0646). The Scherrer Equation, Eq. (2) ^9,21 was then em- ployed to calculate the average crystal sizes of the samples:

(2)

where d is the average crystal size, k is the Scherrer constant (0.89), λ is the X-ray wavelength (0.154 nm), β is the full width at half maximum (FWHM) in radians measured on the 2θ scale, and θ (°) is the value of the Bragg diffraction peak.

The crystallite size of the graphene-BiOCl/Fe ₃ O ₄ nanocom- posites was therefore estimated to be ~30.71 nm.

SEM was then employed to examine the surface morpho- logies of graphene-BiOCl/Fe ₃ O ₄ nanocomposite samples.

Upon examination of Figure 2, it is clear that the BiOCl/

Fe ₃ O ₄ nanoparticles are distributed between the layers of graphene sheets, thus leading to the formation of porous

d = kλ

β cosθ

---

graphene-BiOCl/Fe ₃ O ₄ nanocomposites. In addition, the pre- sence of Fe ₃ O ₄ particles on both the BiOCl and the graphene surfaces was also observed. Through comparison of the composites prepared herein to previously reported BiOCl/

Fe ₃ O ₄ composites, it was apparent that the deposition of Fe ₃ O ₄ as a precipitate on the BiOCl surface via liquid phase deposition produced a similar structure and distribution pattern as that observed herein.

Figure 1. XRD patterns of (a) the BiOCl/Fe

O

nanocomposites, and (b) the graphene-BiOCl/Fe

O

nanocomposites.

12 Fuyong Zhang et al. / Elastomers and Composites Vol. 52, No. 1, pp. 9-16 (March 2017)

2. Photocatalytic activity of the graphene-BiOCl/Fe ₃ O ₄ nanocomposites for degradation of organic dyes

The photocatalytic activity of the graphene-BiOCl/Fe ₃ O ₄ nanocomposites was evaluated by examining the degradation of four different organic dye molecules (i.e., MO, BG, RhB, and MB) under UV irradiation. Figure 3 shows the decrease in concentration of the organic dyes under UV irradiation at 254 nm. The adsorption of the four organic dyes on the graphene-BiOCl/Fe ₃ O ₄ samples in the absence of light over

30 min was also examined, with the concentration of the four dyes decreasing during this time, thus indicating that the dyes were partially adsorbed by the photocatalysts. As shown in Figure 3(a-d), the nanocomposite sample exhibited higher photocatalytic activity towards the MB dye than to the other three dyes (i.e., BG, RhB, and MO). The highest activity of photocatalytic degradation of MB may be explained by it’s unstable chemical structure effect as compared to other organic dyes such as MO, BG and RhB. Also, the lowest photocatalytic degradability of MO could be explained due to it has a stable chemical structure.

In terms of the degradation of organic dyes under UV irra- diation, the decomposition efficiencies of MB, BG, RhB, and MO were 80.53, 61.52, 45.35, and 29.24%, respectively after 110 min (see Figure 4). These results confirmed that the synthesized photocatalyst exhibited photocatalytic activity and promoted the degradation of the four organic dyes examined herein. Moreover, it was found that degradation of the dyes followed pseudo-first order kinetics according to the linear transformation outlined in Equation (3) below:

ln(c/c 0 ) = −kt (3)

where c is the concentration of organic the dye at time t, c 0

is the initial concentration of the dye, and k is the kinetic Figure 2. SEM image of the graphene-BiOCl/Fe

O

nano-

composites.

Figure 3. Photocatalytic degradation of four organic dyes by graphene-BiOCl/Fe

O

nanocomposites under UV irradiation: (a) MB, (b)

BG, (c) RhB, and (d) MO.

constant. The corresponding kinetic plots are provided in Figure 5. We could therefore conclude from the above data that our photocatalyst exhibited good photocatalytic activity in the degradation of select organic dyes under UV irradiation at 254 nm.

3. Photocatalytic degradation mechanism of organic dyes using graphene- BiOCl/Fe ₃ O ₄ nanocomposites

Upon irradiation with UV light at 254 nm, excitation and

charge transfer took place between the Fe ₃ O ₄ particles and

Figure 3. Continued.

14 Fuyong Zhang et al. / Elastomers and Composites Vol. 52, No. 1, pp. 9-16 (March 2017)

the graphene attached to the BiOCl particles. In this process, the graphene sheets acted as good electron acceptors, accepting the electrons produced by UV light irradiation at 254 nm.

Furthermore, the excitation of Fe ₃ O ₄ can also take place,

resulting in the production of electrons and holes in its conduction band (CB) and valence band (VB), respectively.

As such, electrons derived from the graphene were transferred to the CB of the Fe ₃ O ₄ particles prior to subsequent transfer Figure 3. Continued.

Figure 4. Efficiency of photocatalytic degradation of organic dyes using graphene-BiOCl/Fe

O

nanocomposites.

to the BiOCl particles. This mechanism accounts for the enhancement in the photodegradative ability of BiOCl toward the organic dyes in the presence of both Fe ₃ O ₄ and graphene.

The generated electrons (e ⁻ ) react with dissolved oxygen molecules in the aqueous solution to produce superoxide anion radicals (O ₂ ^•− ), while the positively charged holes (h ⁺ ) can react with hydroxide anions (OH ⁻ ) derived from water to make hydroxyl radicals (OH ^• ). Degradation of the MB, BG, RhB, and MO dye molecules to make degradation products (CO ₂ , H ₂ O) took place through their reaction with the superoxide anion and hydroxyl radicals. Based on the above results, a possible photocatalytic pathway for the graphene-BiOCl/Fe ₃ O ₄ nanocomposites was proposed, as outlined in Equation (4)~(9) below ^22-26 :

Fe ₃ O ₄ /BiOCl + hν → Fe ₃ O ₄ (h ⁺ ) + BiOCl(e ⁻ ) (4)

O ₂ + e ⁻ → O ₂ ^•− (5)

BiOCl(e ⁻ ) + O ₂ → BiOCl + O ₂ ^•− (6)

h ⁺ + OH ⁻ → OH ^• (7)

h ⁺ + H ₂ O → OH ^• + H ⁺ (8)

O ₂ ^•− or OH ^• + organic dye → degradation products (9)

Conclusion

In summary, graphene-BiOCl/Fe ₃ O ₄ nanocomposites were synthesized via a facile and economical method. UV-vis analysis indicated that the photodegradative ability of graph- ene-BiOCl/Fe ₃ O ₄ nanocomposites under UV irradiation was superior to that of traditional photocatalysts. Furthermore, O ₂ ^•− and OH ^• were found to be the dominant reactive species in the photodegradation of organic dyes by graphene-BiOCl/

Fe ₃ O ₄ nanocomposites as photocatalyst. The order of pho- tocatalytic activity for graphene-BiOCl/Fe ₃ O ₄ nanocomposites as photocatalyst in the degradation of organic dyes was MB

> BG > RhB > MO.

Acknowledgement

This work was supported by Research Foundation of Sah- myook University in 2016. The authors are grateful to the university for financial support.

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Figure 5. Kinetic studies on photocatalytic degradation of various organic dyes using graphene-BiOCl/Fe

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