Surface area control of nanocomposites Mg(OH)₂/graphene using a cathodic electrodeposition process: high adsorption capability of methyl orange

Abstract

The nanocomposites Mg(OH)₂/graphene (nano-MG) were controllably prepared by a facile cathodic electrodeposition. The samples were characterized by field emission scanning electron microscopy with energy dispersive spectroscopy (FSEM-EDS), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), N₂ adsorption–desorption analysis and UV-vis spectrophotometry. Characterization results suggested that Mg(OH)₂ and graphene were combined successfully. Furthermore, the effects of the current density on the specific surface area of nano-MG have been investigated systematically. The specific surface area of nano-MG varied from 110 m² g⁻¹ to 525 m² g⁻¹, indicating that a suitable current density (0.07 A cm⁻²) is favorable for the uniform growth of Mg(OH)₂ on the surface of graphene. In addition, the nano-MG (0.425 wt% graphene) with a specific surface area of 525 m² g⁻¹ was used as an adsorbent to remove Methyl Orange (MO) from water. The results showed that the adsorption of MO onto nano-MG exhibited a maximum adsorption capacity of 1.074 g g⁻¹. Desorption experiments were carried out to explore the feasibility of adsorbent regeneration. And the possible mechanism responsible for electrodeposition and adsorption of MO on nano-MG were also elucidated.

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Introduction

With the rapid development of industrialization, the pollution caused by the discharge of dye wastewater has become a global environmental problem.¹ Removal of dyes is conventionally done by several physicochemical and biological methods, such as chemical oxidation,² photocatalytic degradation,³ membrane separation,⁴ adsorption⁵ and biodegradation,⁶ etc. Among these various treatments of dyes, adsorption is an effective and economic method.⁷ Many adsorbents, including activated carbon, alumina, silica gel, zeolites, clay, biomass, and nanomaterials, have been fabricated to remove dyes from wastewater.⁸ However, some factors, such as production cost, removal efficiency and preparation process, restrict their applications in a real system. Thus, finding a new adsorbent with high efficiency, low cost and environmental friendliness is highly essential.

As an important chemical product and intermediate, Mg(OH)₂ has the advantages of excellent cushioning property, large activity and strong adsorption capacity (surface area can reach to 500 m² g⁻¹).^9,10 So Mg(OH)₂ has been widely used in environmental protection fields.^11,12 Mg(OH)₂ can be synthesized by various approaches (solution precipitation, hydrothermal, sol–gel method, ultrasound-assisted and electrodeposition, etc.),¹³ among which electrodeposition is expected to produce Mg(OH)₂ in bulk scale, because of its prominent advantages, such as less investment, high purity, structural control and the process information easily obtained.^14,15 Mg(OH)₂, a positively charged surface, has strong electrostatic attractions with anionic dyes, which made it as a good candidate adsorbent for dyes.¹⁶

Graphene, a two-dimensional single layer of sp²-bonded carbon atoms, has attracted increasing interest for its potential application in dyeing wastewater treatment field because of its superior specific surface area (theoretically, 2630 m² g⁻¹).^17,18 It has been reported that GO can provide a workplace for other inorganic materials to improve the specific surface area. Lee et al. controllably synthesized Mg(OH)₂/GO (MG) with specific surface area (from 75.2 m² g⁻¹ to 465 m² g⁻¹) varying the amount of Mg(NO₃)₂ precursor precipitated on the surface of GO.¹⁹ Wang et al. submerged magnesium target into aqueous solution of GO to prepare MG (surface area is 310.8 m² g⁻¹) during the laser ablation in liquid process.²⁰ Liu et al. employed a hydrothermal method to synthesize MG (surface area is 40 m² g⁻¹) in an autoclave at 180 °C for 24 h using Mg(NO₃)₂ and GO as raw materials, NaNO₃ and Na₂C₂O₄ as additives, and NaOH and NH₃·H₂O as precipitants.²¹ They have proposed that, the network structure of GO can provide nucleation and growth sites for Mg(OH)₂, which prevent Mg(OH)₂ from agglomerating and increase the specific surface area of adsorbent. However, these composite methods have major shortcomings, including complex process, tedious procedures, harsh conditions, poor controllable morphology and higher requirements of equipments. A simple and efficient fabrication method of uniform MG is still a challenge.

Therefore, in this paper, nanocomposites Mg(OH)₂/graphene (nano-MG) were synthesized through a novel cathodic electrodeposition with the use of MgCl₂·6H₂O as precursor, a small amount of graphene as a substrate for growth. This method has the advantages of low cost, easy operation, fast, mild conditions, controllable morphology, uniform distribution and relatively higher specific surface area (525 m² g⁻¹). Meanwhile, we utilized the fabricated nano-MG to adsorb Methyl Orange (MO) in aqueous solution, which showed that the maximum adsorption capacity of MO can reach as high as 1.074 g g⁻¹. In addition, the adsorption–desorption experiments were conducted.

Experimental

Materials

Flexible graphite sheets were provided by Haimen Shuguang Carbon Industry Co., Ltd. Na₂SO₄, MgCl₂, HCl, NaOH and MO were purchased from Sinopharm Chemical Reagent Co., Ltd. All the reagents are analytical grade. Deionized water was employed for all experiments.

Synthesis of graphene

Flexible graphite sheets (4 × 8 cm²) was used as anode in a 250 mL aqueous solution containing 0.1 M Na₂SO₄, and Pt was used as cathode. The electrochemical exfoliation was carried out by a voltage of 10 V at room temperature.²² After exfoliation, the mixture was collected and washed several times with deionized water by vacuum filtration, and the resultant product was sonicated at low power for 30 min. In order to remove unexfoliated and large graphite sheets, the suspension was centrifuged at 3000 rpm for 20 min, and then the supernatant was poured into containers and completely frozen in 24 h.

Synthesis of nano-MG

Synthesis of nano-MG was performed in a single conventional compartment cell (capacity 1.5 L) exposed in the air. The titanium plate covered by RuO₂ catalyst was used as an anode and the titanium plate without coating was used as a cathode. The distance between anode and cathode was 4 cm, and the electrolysis area was 80 cm². In a typical run, 100 g of MgCl₂·6H₂O was dissolved in 1 L deionized water to make a stock solution, and then 100 mg graphene were dispersed in it by sonication for 1 h at room temperature. When electrolysis started, the deposits were obtained in cathode bottom of the electrolysis cell, which would be taken out every half an hour and then added suitable amount of MgCl₂·6H₂O and graphene to maintain the same solution concentration. After electrolysis, the deposits were filtered on a porous polymer membrane, washed with deionized water three times and dried in a vacuum drying oven at 70 °C for 12 h to produce nano-MG. In order to investigate the effect of current density on the specific surface area of nano-MG, the current density varied from 0.01 to 0.1 A cm⁻² (0.01 A cm⁻², 0.04 A cm⁻², 0.07 A cm⁻² and 1.0 A cm⁻²).

Characterization methods

The crystal phase of the samples were characterized by X-ray diffraction (XRD, X'Pert Pro, panalytical, Holland) with Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 5° min⁻¹ (2θ) in the range of 10–70°. The morphology and chemical composition of the particles were characterized by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan, the sample was sputtered with Au before the test) equipped with an energy dispersive spectroscopy (EDS, HORIBA, Hitachi, Japan) spectrometer. Thermal behavior of the composites was carried out by thermal gravimetric analysis (TGA, STA449 F3, Netzsch, Germany) under an Ar atmosphere, with a heating rate of 10 °C min⁻¹. The specific surface area of nano-MG was calculated by BET (Tristar II 3020M, Micromeritics, USA), with nitrogen adsorption desorption isotherms at 77 K and the sample conducted outgassing process in a vacuum system at 70 °C for 1 h before the test. The concentrations of MO were examined using a UV-vis spectrophotometer (UV-2600, Shimadzu, Japan) at a wavelength of 464 nm. Raman spectra were collected at a Raman Spectrometer (LabRAM HR800, Horiba Jobin Yvon, France) with a laser of 488 nm and power of 20 mW. The laser beam was focused on the surface of the sample with a ×50 objective. Analyzed region was from 500 cm⁻¹ to 2500 cm⁻¹, with a resolution of 0.6 cm⁻¹.

Adsorption experiments

For this study, 10 mg nano-MG was put into a beaker containing 20 mL MO solution (600 mg L⁻¹ concentration) and then stirred for 1 h in the dark. The pure Mg(OH)₂ and pure graphene were also be measured for comparison. The concentration of MO was detected at a wavelength of 464 nm by using UV-vis spectrophotometer. The amounts of MO adsorbed onto the MG were determined using the following equation:where q (mg g⁻¹) is the adsorption capacity, C₀ (mg L⁻¹) and C_e (mg L⁻¹) are the initial and final concentrations of MO, respectively. V (mL) is the solution volume and m (g) is the adsorbent mass.

Desorption experiments

Desorption study was aimed to obtain the possibility of reusability of the nano-MG materials. For the desorption study, the nano-MG (10 mg) and MO (20 mL, 200 mg L⁻¹) were mixed in 150 mL conical flasks and stirred for 1 h at 303 K. After filtration, the MO-adsorbed adsorbent was stirred with NaOH (1 mol L⁻¹, 50 mL) for 2 h and repeatedly washed by deionized water of 50 mL for 3 times.²³ Then MG, the regenerated adsorbents, was collected by filtration, and both two steps above built up an adsorption–desorption cycle. The experiments were successively operated for 5 times.

Results and discussion

Mechanism study

Mechanism of electrodeposition process is depicted in Fig. 1, first, the mixture of graphene and MgCl₂ were dispersed in deionized water by sonication for 10 min. There are two possible reasons for the effect of sonication. For one thing, large flake graphene became smaller using sonication, which would improve the dispersibility of graphene in aqueous solution. For another, the graphene present electronegativity owing to the oxygen-containing groups on surface,²⁴ so an electrostatic attraction can promote the interaction between the graphene and Mg²⁺. Second, the dispersion was added into the self-made electrolytic cell. When the electrolysis started, the Mg²⁺-adsorbed graphene moved towards cathode, and combined with the produced OH⁻ during electrolysis, and then generated Mg(OH)₂. Moreover, the Mg(OH)₂ adhered well onto the surface of graphene. And the composites deposited to the bottom of the electrolytic tank with the increasing of electrolysis process. Third, the product was filtered and dried at 70 °C for 2 h. Finally, the gray black powder product was obtained.

Fig. 1 Schematic illustration of electrodeposition process.

Characterizations of nano-MG

SEM analysis. The morphology of nano-MG was investigated using SEM. From the Fig. 2, it can be observed that a lamellate structure with ripples, which is the characteristic of few layer graphene. And Mg(OH)₂, representing a hexagonal structure, grows on the surface of graphene. In addition, it can be seen that the amount of Mg(OH)₂ covered on the graphene is quite few at the lower current density. And the graphene shows some heavy aggregation. With the increase of current density, Mg(OH)₂ uniformly grow on the surface of graphene in large-area and may be considered as a binder to prevent graphene from aggregation. Therefore, the specific surface area of nano-MG tends to increase, as shown in Fig. 3a. When the current density is 0.07 A cm⁻², the sample has a maximum specific surface area of 525 m² g⁻¹. However, as the current density further increases, the amount of Mg(OH)₂ synthesized on the graphene surface also increases, resulting in the aggregation of Mg(OH)₂ and lower specific surface area of nano-MG. So we can controllably synthesize nano-MG with different specific surface area by varying the current density, which could be used for various applications.

Fig. 2 SEM images for nano-MG preparing by different current density 0.01 A cm⁻² (a and b), 0.04 A cm⁻² (c and d), 0.07 A cm⁻² (e and f), 0.1 A cm⁻² (g and h).

Fig. 3 Variations in the BET surface area upon variations in the current density.

BET analysis. The specific surface area calculates was carried out through a multi-point Brunauer–Emmett–Teller (BET) method. The typical nitrogen adsorption–desorption isotherms curves for the pure Mg(OH)₂, pure graphene and nano-MG are shown in Fig. 4. From adsorption branch of the isotherm curves, the specific surface area of pure Mg(OH)₂, pure graphene and nano-MG are calculate to be 104.7 m² g⁻¹, 1022.9 m² g⁻¹ and 525.2 m² g⁻¹, respectively. According to the IUPAC classification, the sample of nano-MG exhibits IV isotherm with a typical H₃ hysteresis loop, indicating the formation of slit-shaped pores between hexagonal Mg(OH)₂ and lamellate graphene.²⁵ This result is consistent with SEM images.

Fig. 4 N₂ adsorption–desorption isotherm curves for nano-MG, graphene and Mg(OH)₂.

EDS analysis. Element distribution of nano-MG (Fig. 5a) indicates that the nano-MG is composed of Mg, C and O, and the distribution of Mg elements is consistent with O. The EDS pattern of nano-MG is presented in Fig. 5b. The sample shows that a stronger intensity in the carbon peak, suggesting that Mg(OH)₂ and graphene are combined successfully.

Fig. 5 EDS images of nano-MG.

XRD analysis. The crystal phase structure of Mg(OH)₂, graphene and nano-MG were characterized by XRD pattern (Fig. 6a). The diffraction peaks at 2θ value of (001), (100), (101), (102), (110), (111) and (103) match with the standard XRD values for Mg(OH)₂ (JCPDS no. 7-239), which shows that Mg(OH)₂ is a hexagonal phase system.²⁶ In the case of the nano-MG, a weak additive peak at (002) is corresponding to graphene,²⁷ implying successful synthesis of nano-MG.

Fig. 6 (a) XRD images for Mg(OH)₂, graphene and nano-MG. (b) Raman spectra of the graphene and nano-MG.

Raman analysis. Raman spectroscopy is sensitive methods to use identify the presence of sp² carbon. In this work, it was used to characterize the presence of graphene in nano-MG. Fig. 6b shows a comparison between the Raman spectra of a pure graphene and a nano-MG. Both of the spectra show typical spectral characteristics of graphitic carbon: the D-band (1368 cm⁻¹, commonly indicating the structural defects and partially disordered structures of sp² carbon) and the G-band (1593 cm⁻¹, ascribed to the vibration of sp²-bonded carbon atoms in a graphite layer),^28,29 which confirms existence of graphene in the nano-MG. However, the intensity ratio (0.821) of D band over G band (I_D/I_G) of the nano-MG is higher than that of graphene (0.728), which is regarded as the interaction between the Mg(OH)₂ and the graphene sheets.³⁰

TG-DSC analysis. Thermal analysis profiles of Mg(OH)₂ and nano-MG are shown in Fig. 7. The first stage started at 50 °C and ended at 330 °C with 2.97% and 4.2% weight loss, which may be attributed to the removal of water adsorbed onto the surface of Mg(OH)₂ and nano-MG. The second stage, one remarkable weight loss appeared from 330 °C to 450 °C with a corresponding well-defined endothermic peak observed near 370 °C and 359 °C, respectively. The different endothermic peak temperatures may be caused by the different microstructures of Mg(OH)₂ and nano-MG. This is because the addition of graphene prevent Mg(OH)₂ nanosheets from aggregation, the endothermic peak temperature of nano-MG is lower. The weight loss is attributed to the decomposition of Mg(OH)₂. However, the weight loss value of nano-MG is 28.66 wt%, which is less than that of Mg(OH)₂ (30.23 wt%). The result is due to the graphene included in nano-MG cannot be decomposed under an Ar atmosphere.

Fig. 7 TG-DSC images of Mg(OH)₂ and nano-MG.

Adsorption study

Nano-MG prepared under the current density of 0.07 A cm⁻² was considered as an adsorbent to remove MO in water. The content of graphene in the adsorbent is 0.425 wt%. The result of graphene content came from following experiment process:

10 g adsorbent was dissolved into 500 mL HCl (2 mol L⁻¹) to remove Mg(OH)₂, and then the product was filtered, washed, and dried, at last the obtained graphene was weighed with electronic analysis balance.

Nano-MG tends to adsorb MO may be attributed to three reasons. Firstly, it can be explained on the basis of the structure of MO molecule and the Mg(OH)₂, the surface charge of Mg(OH)₂ was positive and the negative charge at MO surface as –SO₃⁻ anion, it will facilitate electrostatic attraction between them. Secondly, a hydrogen bond may be exist between the –SO₃⁻ anion of MO and hydroxy contained in nano-MG.³¹ Thirdly, the surface area increases from 104.7 m² g⁻¹ (Mg(OH)₂) to 525.2 m² g⁻¹ (nano-MG), which will increase the chance of collision, facilitating adsorption process. The adsorption mechanism of MO on nano-MG is shown in Fig. 8.

Fig. 8 Adsorption mechanism of MO on nano-MG.

In order to investigate the effect of adsorbent dosage on the adsorption process of MO, the mass of the adsorbent were carried out from 5 mg to 25 mg with 20 mL MO solution whose initial dye concentration is 600 mg L⁻¹ at pH 6, under the temperature of 30 °C and 1 h adsorption time. As shown in Fig. 9a, the adsorption capacity of MO increases with increasing nano-MG dosage at first. This trend is due to the increasing number of active adsorption sites and surface area before the amount of adsorbent reaches the maximum point. And then the curve falls, which may arise from three reasons. The first reason is the overlap of the active sites. The second reason is that less mass transfer would take place when the interfacial tension between MO dye and nano-MG increases. The last reason is that the adsorption reaches equilibrium when the adsorbent dosage increases. As a consequence, the amount of adsorbate per unit mass decreases when the total amount of MO is constant, so the adsorption capacity reduces. From Fig. 9a, it also can observed that the dosage of nano-MG is 10 mg, the adsorbed amount of MO reaches maximum (1.074 g g⁻¹), which is higher than Mg(OH)₂ (0.468 g g⁻¹) and graphene (0.336 g g⁻¹). The previous literatures also use different adsorbing material to removal of MO. For example, Liu et al. synthesized a MnO₂–graphene–carbon nanotube hybrid material by a chemical method and the maximum adsorption capacity of MO was 0.476 g g⁻¹,³² Wang et al. prepared a porous chitosan aerogels doped with small amount of graphene oxide and used as adsorbents for MO, the adsorption capacity is 0.687 g g⁻¹.³³ Therefore, nano-MG has more advantages than other adsorbents for removal MO.

Fig. 9 (a) The effect of adsorbent dosage on the adsorption capacity for nano-MG, graphene and Mg(OH)₂ and (b) the adsorption–desorption experiments in various cycles.

Desorption study

It is generally known that the regeneration of adsorbent is considerably important in practical applications. As adsorption–desorption experiments shown in Fig. 9b, it can be seen that the adsorption capacity nearly keep constant after 5 repetitions, which indicated that MO could be easily released from adsorbent in each cycle. Therefore, nano-MG is a potential adsorbent for removing dye due to high recycling efficiency.

Conclusions

In summary, we innovatively synthesized nano-MG via a cathodic electrodeposition and controlled the specific surface area of nano-MG by varying the current density. The specific surface area of nano-MG increased with the increasing of current density at first, and then decreased. The Mg(OH)₂ grew uniformly on the surface of graphene at the current density of 0.07 A cm⁻². We have successfully explored the application of the nano-MG as an adsorbent, which has a maximum specific surface area of 525 m² g⁻¹. The nano-MG displays good performance in MO adsorption (adsorption capacity: 1.074 g g⁻¹) and excellent recycling performance, making it has a widely application prospect in the wastewater treatment.

Acknowledgements

This study was supported by the State Key Development Program for Basic Research of China (grant number 2013CB632606-1), the National Natural Science Foundation of China (grant number 51404054 and 51304044) and the Fundamental Research Funds for the Central Universities (grant number N150204020).

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