Synthesis of nano-octahedral MgO via a solvothermal-solid-decomposition method for the removal of methyl orange from aqueous solutions

Nano magnesium oxide has wide applications, and MgO with (111) facets has wider potential applications than MgO with (100) facets (e.g., in catalysis and adsorption). However, nano MgO with (111) polar faces has not been studied throughly, so the preparation of nano-octahedral MgO (N-O-MgO) with eight exposed (111) facets remains a great challenge. Herein, we successfully synthesised N-O-MgO via an effective solvothermal-solid-decomposition method and studied its adsorption performance. The obtained N-O-MgO showed excellent performance (229.36 mg g−1) for methyl orange (MO). The adsorption follows the pseudo-second-order kinetic equation and the Langmuir isotherm model. The dimensionless parameter RL (0.042) and Gibbs free energy ΔG (−6.538 kJ mol−1) revealed that the adsorption of MO on N-O-MgO was a spontaneous and feasible process. The adsorption of MO and methyl blue (MB) on N-O-MgO were studied to determine the adsorption sites. Based on these experiments and analysis, it was determined that the adsorption sites were magnesium ions and the adsorption mechanism was proposed to describe the adsorption process.


Introduction
Nano metal oxide materials have attracted numerous researchers due to their unique applications in the elds of energy and environment. As an important nano metal oxide, nano magnesium oxide (nano MgO) shows a wide range of applications, for e.g., as a solid-base catalyst and carrier (for Claisen-Schmidt reaction, 1 the preparation of Schiff bases, 2 and the loading of nickel atoms 3 for CO 2 reforming of methane), as an adsorbent (for the removal of organic dyes such as methyl orange, 4 heavy metal ions such as cadmium ions and lead ions, 5 and uoride ions 6 ), and as a template and additive (for the preparation of graphene-like materials 7 and the additive of CO 2 adsorbent 8 ). Nano MgO also shows optical properties 9 and bactericidal properties. 10 There are some methods used for the synthesis of nano MgO, which include precipitation, 11,12 solid state reaction, 13 microwave radiation method, 14 and sol-gel route,. 15 However, in these synthetic procedures, 9,10,15 researchers have paid less attention to the crystal facet of nano MgO; MgO usually grows into a cubic structure with (100) facets. 16 In the past few years, numerous studies have focused on MgO with the (111) polar facet due to its higher reactivity in catalysis 17,18 and antibacterial 19 applications. However, the reports about nano MgO with (111) polar facets and its properties are rare. Although a large MgO crystal can be mechanically cut and polished to obtain micro-or nano-scale materials, it would be difficult to control the facet that is exposed. 20 It is an effective strategy to obtain MgO with the polar (111) facet by wet chemistry methods, which is oen necessary to use additives in the reaction system to create a polar environment and to inhibit the rapid disappearance of the polar surface during the reaction. Ryan Richard et al. 17 synthesised MgO (111) nanoplates by adding 4-methoxybenzyl alcohol in the reaction system, which has a strong interaction with the inorganic intermediate Mg(OH)-(OCH 3 ). Jupille et al. 21 obtained (111) facets via etching of cubic MgO smoke crystals using neutral water in wet conditions, which had an average particle size of 170 nm and a specic surface area of 5.5 m 2 g À1 . Hao et al. 19 obtained MgO with (111) facets exposed via the solution combustion process of the mixture of magnesium nitrate and urea. Xie et al. 16,20 prepared micro-scale octahedral MgO with 2.4 m 2 g À1 specic surface area by decomposing magnesium nitrate in molten lithium nitrate at 400 C. Further studies by Jeffrey D. Rimer 22 discussed the inuence of different factors on the morphology of MgO and demonstrated that the molten salt systems contributed to the formation of (111) facets in the decomposition of magnesium salts. For the molten salt systems, the crystal formed by the free movement of ions must grow to the microscale to reduce surface energy and maintain self-stability. Thus, the synthesis of nano-octahedral MgO with eight (111) facets exposed remains a huge challenge.
In this work, we rst successfully prepared nano-scale octahedral MgO with eight (111) facets exposed via an effective solvothermal-solid-decomposition method and evaluated its adsorption performance using MO. The adsorption characteristics and mechanism were studied by a series of experiments and calculations, which included the simulation of different kinetic and isotherm model, and the determination of relevant thermodynamic parameters. Based on these analyses, the adsorption sites were determined and an adsorption mechanism for MO adsorption on N-O-MgO was proposed. These studies should be a useful reference for the preparation of materials and the exploration of adsorption mechanisms as well as for the preparation of nano materials with polar facets, the characteristics of nano materials with polar facet, and the exploration of adsorption mechanisms.
N-O-MgO was prepared via a solvothermal-soliddecomposition method. In a typical route, 2.56 g of Mg(NO 3 ) 2 -$6H 2 O was dissolved in 50 mL of ethanol and the homogeneous solution was transferred into a 100 mL Teon bottle. Then, the mixture was thoroughly stirred aer adding 6.00 g of urea into the solution. Subsequently, the Teon bottle was placed in a stainless-steel vessel that was sealed and put into a temperature-controlled oven for solvothermal treatment at 200 C for 24 h. Aer the heat treatment, fresh precipitates were collected, washed, and dried at 80 C in air. N-O-MgO was produced by calcining the precipitates at 500 C and continuing for 5 h.

Characterisation
The crystal structures of the obtained samples were examined by X-ray diffraction (XRD) on a D8 Advance X-ray diffractometer (Germany) using Cu Ka radiation.
The microscopic morphology of MgO was analysed by scanning electron microscopy (SEM) on a Hitachi S-4100 FE-SEM instrument (Japan) at 10 kV and transmission electron microscopy (TEM) on a Tecnai G2 F20 eld emission transmission electron microscope (USA) at 200 kV. N 2 adsorption/desorption isotherm was recorded on a Micromeritics ASAP 2020 apparatus (USA) at 77 K for calculating the specic surface area and pore size distribution by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.
The vibrational characteristics of the obtained samples were recorded by a Fourier transform spectrometer (FT-IR; Agilent Cary 630, Agilent, America) in pressed KBr pellets.
The X-ray photoelectron spectrum (XPS) of N-O-MgO was obtained on a Kratos AMICUS spectrometer (SHIMADZU, JP) using Al Ka radiation. The binding energy of O element was calibrated relative to the carbon impurity with C 1s at 285 eV.
The concentrations of MO and MB in the aqueous solutions were measured using a Shimadzu UV-2550 UV-visible spectrometer (Japan).

Adsorption experiments
The batch adsorption experiments were carried out by mixing N-O-MgO particles with 20 mL aqueous MO solution under stirring at pH 6.0 and 303 K temperature at the rate of 150 rpm min À1 . The three parameters were changed during the experiments: adsorbent amount (0.5-1.75 g L À1 ), contact time (5-150 min), and MO concentration (20-200 mg L À1 ). Aer the experiments, the mixture was separated and the ultimate concentration of MO was determined.
The adsorption capacity of MO on N-O-MgO and the removal ratio of MO were calculated using eqn (1) and (2).
In which q e (mg g À1 ) is the adsorption capacity of MO on N-O-MgO, h (%) represents the MO removal efficiency, C 0 (mg L À1 ) is the MO initial concentration, C e (mg L À1 ) is the adsorption equilibrium concentration of MO, V (L) is the MO solution volume, and m (g) is the weight of N-O-MgO particles.
The pseudo-rst-order and pseudo-second-order (eqn (3) and (4)) kinetic models were simulated to describe the adsorption process of MO on N-O-MgO.
where q e and q t (mg g À1 ) are the adsorption capacities of MO adsorbed on N-O-MgO at the equilibrium time and at a specic time t (min), K 1 (min À1 ) is the pseudo-rst-order model rate constant, and K 2 (g mg À1 min À1 ) is the pseudo-second-order model rate constant.
The adsorption process of MO on the N-O-MgO particles was also simulated based on the Langmuir isotherm and Freundlich isotherm (eqn (5) and (6)). The dimensionless separation factor constant (R L , eqn (7)) was calculated to indicate the type of adsorption.
where K L (L mg À1 ) is the Langmuir constant, C e (mg L À1 ) is the adsorption equilibrium concentration of MO, q e (mg g À1 ) is the adsorbed amount of MO on N-O-MgO at adsorption equilibrium, q m (mg g À1 ) represents the maximum theoretical adsorption capacity, K F (mg g À1 ) (L mg À1 ) 1/n is the Freundlich constant and n is the adsorption intensity. R L > 1, R L ¼ 1, 0 < R L < 1, and R L ¼ 0 indicate that the adsorption process is unfavourable, linear, favourable, and irreversible, respectively.

Characterization of N-O-MgO
In this ethanol solvent system, urea molecules were a more advantageous ligand than water molecules for magnesium ions, which may inhibit the growth rate of MgO (111) facet and lead to octahedral growth under the decomposition process. Fig. 1(A) shows the XRD pattern of the products, where all the diffraction peaks agreed with the rock-salt structure MgO (JCPDS no. 45-0946), so the products were MgO. As seen in Fig. 1(B), the morphology of the products (MgO) was octahedral with an average particle size of 79.83 nm. This corresponded to the schematic model of rock-salt MgO with eight polar (111) facets exposed (inset a) and the histogram of particle size distribution is shown (inset b). TEM was carried out to characterize the exposed facets of the MgO octahedral. Fig. 1(C) presents the TEM image along the [110] zone axis, in which the octahedron MgO appeared to be a diamond. The diamond model (inset) was drawn by projecting the octahedron along the [110] direction, which was in good agreement with the TEM image. These analyses proved that the as-prepared MgO was nano-octahedral with eight exposed (111) polar facets. The N 2 adsorption/ desorption isotherm and pore size distribution curve (inset) of N-O-MgO are shown in Fig. 1(D). The N-O-MgO isotherm showed type-IV characteristics with a signicant H3 loop. The formation of pores in N-O-MgO was a result of the aggregation of the particles. The BET specic surface area was 36.84 m 2 g À1 , which was higher than that of micro-scale octahedral MgO (2.4 m 2 g À1 ). 16 Fig. 1(E) shows the FT-IR pattern of N-O-MgO. The broad bands located at 3420 cm À1 and 1637 cm À1 correspond to water molecules or hydroxyl groups. 23 The peak located at 3132 cm À1 may result from the hydrogen bonds. The peaks at 1400 cm À1 and 859 cm À1 were assigned to magnesium-oxygen (Mg-O) group or magnesium-hydroxyl (Mg-OH) group. 24 The

Adsorption experiment
3.2.1 Effect of adsorbent amount. As an important parameter, adsorbent amount was investigated for the removal of MO under equilibrium conditions, as shown in Fig. 2(A). As N-O-MgO concentration increased from 0.5 to 1.75 g L À1 , its adsorption capacity gradually decreased from 176.30 to 56.28 mg g À1 , likely due to decrease in the utilisation rate of the adsorption sites on increasing the amount of N-O-MgO. 26 The initial removal percentage was over 81.85% and reached 99.89% when the adsorbent amount was 0.75 g L À1 ; then, the removal rate increased slowly. This was explained by the increase in adsorption sites for higher N-O-MgO amounts, leading to the increase in removal rate. 27 3.2.2 Effect of contact time and adsorption kinetics. The contact time remarkably inuenced the adsorption of MO on N-O-MgO, as shown in Fig. 2(B). For the adsorption of MO on N-O-MgO, the adsorption was fast in the rst 5 min, implying that N-O-MgO could rapidly adsorb MO molecules from the aqueous solution; the adsorption equilibrium was reached aer 60 min, where the adsorption amount was 119.42 mg g À1 .
The adsorption kinetic behaviour of MO on N-O-MgO was simulated using pseudo-rst-order and pseudo-second-order kinetic models. The tting results and relevant parameters are shown in Fig. 2(C), (D) and Table 1, respectively. Compared to the simulation by pseudo-rst-order model, the calculation results using the pseudo-second-order model were closer to the experimental data and its correlation coefficient R 2 was higher than 0.999. This indicated that the adsorption of MO on N-O-MgO can be more adequately described by the pseudo-secondorder kinetic model. For this adsorption process, the overall adsorption rate of MO was controlled by the chemical process through ion exchange between MO and N-O-MgO. 28 A detailed explanation of the adsorption process is provided in Section 3.3.

Effect of pH.
The effect of pH in the range from 2 to 12 on the adsorption of MO on N-O-MgO is shown in Fig. 3(A). The pH was adjusted with 0.1 M HCl and 0.1 M NaOH solutions, and the pH of each solution was measured by using a laboratory scale pH meter (PHS-25). As the pH increased from 2 to 6, the adsorption capacity of N-O-MgO slowly increased from 120 to 124, and the highest adsorption capacity was obtained at pH ¼ 6. As the pH increased from 6 to 12, the adsorption capacity of N-O-MgO evidently decreased. With the increase in the concentration of H + , the inhibition of electrostatic interaction between the -SO 3 À groups of MO and the positive charge of the N-O-MgO surface was weak. With the increase in the concentration of OH À , it competed with the -SO 3 À groups of MO for the adsorption sites on the adsorbent surface. 29,30 The analysis indicated that the electrostatic interaction might be the adsorption mechanism for MO adsorption on N-O-MgO. Fig. 3(B). As the concentration increased from 20 to 200 mg L À1 , the adsorption capacity of N-O-MgO gradually increased. This indicated that N-O-MgO had good adsorption performance for MO molecules in aqueous solution.

Effect of MO concentration and adsorption isotherms. The effect of MO concentration on its adsorption on N-O-MgO was investigated, as shown in
The tting result and relevant parameters for MO adsorption using the Langmuir and Freundlich isotherm models are presented in Fig. 3(C), (D) and Table 2, respectively. According to the tting result, the adsorption parameters tted well with both the Langmuir isotherm (R 2 ¼ 0.9520) and Freundlich isotherm (R 2 ¼ 0.9594). Here, we used the Langmuir isotherm model to describe the adsorption process for MO. For the Langmuir isotherm model, the adsorption of MO on N-O-MgO was monolayer. The R L value was 0.042 by calculation, implying favourable adsorption of MO on N-O-MgO.
3.2.5 Recyclability. The reusability of an adsorbent is essential for its practical application. The results of N-O-MgO regeneration are shown in Fig. 4(A). The removal rate of the adsorbent aer two cycles was still over 94%. The results implied that N-O-MgO was an efficient adsorbent with excellent reusability. Moreover, the regeneration of N-O-MgO was achieved through simple calcination for 3 h at 500 C aer separation.

Comparison of synthetic MgO and N-O-MgO.
For the comparative experiment, our purpose was to study the adsorption performance of (100) and (111) facets of MgO. As shown in Fig. 4(B), the adsorption capacity and adsorption capacity per unit specic surface area for MO was examined for both synthetic MgO (N-C-MgO, MP-MgO) and N-O-MgO. N-C-MgO with the (100) facet exposed was prepared via a sol-gel method, which has a particle size distribution in the range of 20-40 nm. In the typical route, 1.00 g P123 was completely dissolved in 60 mL of ethanol and then, the mixture was thoroughly stirred aer adding 0.76 g magnesium ethanol and 1.5 mL HNO 3 for 5 h. The mixture solution was dried at 60 C and calcined at 500 C for 5 h. MP-MgO was prepared via a hydrothermal method previously reported by our research group, 31 which has a particle size distribution in the range of 20-80 mm. As MO is a long chain molecule and MP-MgO is a porous material, its low adsorption capacity might be attributed to diffusion restriction.

Adsorption mechanism
3.3.1 Determination of the adsorption sites. Adsorption experiments were carried out for two typical dyes (anionic MO and cationic MB) using N-O-MgO as the adsorbent to determine its adsorption sites. The results are presented in Fig. 4(C). The adsorption capacity and removal rate of MO on N-O-MgO were much higher than those of MB. The differences in adsorption were caused by the characteristics of N-O-MgO. In the outermost layer of N-O-MgO, whether it was positively or negatively charged, a hydroxyl layer was formed in the aqueous solution, which acted as the new outermost layer. 32,33 Compared to having only hydrogen atoms at the outermost, the hydroxyl dropped much more easily, forming adsorption sites with positive charge (Mg 2+ ). Thus, the -SO   ① The distance (d, eqn (8)) between two oxygen ions and the area (S 2 , eqn (9)) of a diamond composed by four adjacent oxygen ions on the surface of N-O-MgO were calculated based on rock-salt contracture.
The amount of outmost layer oxygen ions on the N-O-MgO (111) facet: The number of MO molecules adsorbed on N-O-MgO: The utilisation ratio of adsorption sites: In which a (0.4211 nm) is the cell constant, d (nm) is the distance between two magnesium on the (111)  The calculations indicate that the utilisation ratio of the adsorption sites on N-O-MgO was 91.70%, indicating that MO molecules effectively formed a monolayer on N-O-MgO.

Adsorption mechanism.
An equation for MO adsorption on N-O-MgO was proposed, which is eqn (14). To evaluate such a process, the thermodynamic parameters Gibbs free energy was calculated at 303 K. The distribution coefficient (K) and Gibbs free energy (DG) were determined using eqn (15) and (16). K In which C 0 (mg L À1 ), C e (mg L À1 ), R (8.314 J mol À1 K À1 ), and T (303 K) are the initial concentration of MO, equilibrium concentration of MO, gas constant, and absolute temperature, respectively.
DG was calculated as À6.538 kJ mol À1 , demonstrating that the adsorption process of MO on N-O-MgO is spontaneous and feasible, which is in accordance with the analysis of the Langmuir isotherm model.
In order to further describe the adsorption process, MO and N-O-MgO aer MO adsorption were characterized by FT-IR spectroscopy. Fig. 5(A) shows the FTIR spectra of MO and N-O-MgO aer MO adsorption. The FT-IR spectra peaks located at 1605 cm À1 , 1120 cm À1 , 1692 cm À1 and 1037 cm À1 , and 1006 cm À1 were attributed to phenyl groups, 30 C-N group, 34 and sulfonic acid group (-SO 3 À ), 35  Based on these analyses above, the adsorption mechanism was proposed in Scheme 2. As shown in Scheme 1(A), the

Conclusions
In summary, we have obtained N-O-MgO particles via an effective solvothermal-solid-decomposition method and used it as an adsorbent for MO adsorption to study its adsorption performance. The adsorption process of MO on N-O-MgO in aqueous solution was systematically studied under different experimental conditions (adsorbent amount, contact time, MO concentration, and pH). The results revealed that the pseudosecond-order kinetic model and monolayer adsorption precisely described the spontaneous and feasible adsorption process, which was analysed by the R L and thermodynamic analyses. N-O-MgO had excellent recyclability and higher activity than N-C-MgO and MP-MgO with (100) facets exposed. The adsorption sites were identied as magnesium ions that were effectively utilised on the adsorption process. A possible adsorption mechanism was proposed based on the analysis of experiments, calculation, and characterization. The results of this study may be a reference for the preparation of nanomaterials with polar facets and adsorption mechanism with, for example, different MgO morphologies or even different materials.

Conflicts of interest
There are no conicts to declare.