Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents
© Mastuli et al.; licensee Springer. 2014
Received: 29 November 2013
Accepted: 4 March 2014
Published: 21 March 2014
In the preparation of nanostructured materials, it is important to optimize synthesis parameters in order to obtain the desired material. This work investigates the role of complexing agents, oxalic acid and tartaric acid, in the production of MgO nanocrystals. Results from simultaneous thermogravimetric analysis (STA) show that the two different synthesis routes yield precursors with different thermal profiles. It is found that the thermal profiles of the precursors can reveal the effects of crystal growth during thermal annealing. X-ray diffraction confirms that the final products are pure, single phase and of cubic shape. It is also found that complexing agents can affect the rate of crystal growth. The structures of the oxalic acid and tartaric acid as well as the complexation sites play very important roles in the formation of the nanocrystals. The complexing agents influence the rate of growth which affects the final crystallite size of the materials. Surprisingly, it is also found that oxalic acid and tartaric acid act as surfactants inhibiting crystal growth even at a high temperature of 950°C and a long annealing time of 36 h. The crystallite formation routes are proposed to be via linear and branched polymer networks due to the different structures of the complexing agents.
KeywordsMgO Nanostructured materials Crystal growth Sol-gel process Complexing agent
Magnesium oxide (MgO) is a versatile metal oxide having numerous applications in many fields. It has been used as a catalyst and catalyst support for various organic reactions [1, 2], as an adsorbent for removing dyes and heavy metals from wastewater [3, 4], as an antimicrobial material , as an electrochemical biosensor  and many other applications. Conventionally, MgO is obtained via thermal decomposition of various magnesium salts [7–9]. The drawback with this method of obtaining MgO is the large crystallite size with low surface area-to-volume ratio that limits its applications for nanotechnology. Some properties of MgO, such as catalytic behaviour, can be further improved if it is used as nanosized particles compared to micron-sized particles. Therefore, the formation of MgO nanostructures with a small crystallite size of less than 100 nm and homogeneous morphology has attracted much attention due to their unique physicochemical properties including high surface area-to-volume ratio. It is widely accepted that the properties of MgO nanostructures depend strongly on the synthesis methods and the processing conditions. Much effort has been devoted to synthesize MgO nanostructures using various methods such as precipitation , solvothermal , chemical vapour deposition , electrochemical , sonochemical , microwave , electron spinning , combustion , template  and carbothermic reduction . Each method has its own advantages and disadvantages. An important issue regarding synthesis and preparation of nanostructured MgO is controlling the parameters in order to obtain a more uniform size as well as morphology of the nanoparticles.
Over the past decades, various starting materials were used in the synthesis methods producing nanosized MgO that may give multiple morphologies. Precursors that may be obtained from the synthesis methods may take many forms such as magnesium hydroxide [10, 15], magnesium carbonate [20, 21] and basic magnesium carbonate [22, 23]. Each precursor is annealed at a different temperature to produce highly crystalline and pure MgO. Another precursor, magnesium oxalate dihydrate (MgC2O4 · 2H2O), has also received considerable interest among researchers [24, 25]. A sol-gel method is a promising technique for the formation of magnesium oxalate dihydrate followed by annealing at a suitable temperature to form MgO. The advantages are its simplicity, cost-effectiveness, low reaction temperature, high surface area-to-volume ratio, narrow particle size distribution and high purity of the final product. Early attempts to prepare magnesium oxalate dihydrate were by using either magnesium methoxide or magnesium ethoxide that was reacted with oxalic acid in ethanol to form a precursor based on the sol-gel reaction [26–28]. Later on, inorganic salts like magnesium nitrate hexahydrate [29–31], magnesium chloride hexahydrate  and magnesium acetate tetrahydrate  are preferred. The sol-gel reaction of magnesium oxalate dihydrate and annealing of the obtained precursors give various morphologies of MgO nanostructures [29–32]. However, the controlled synthesis of MgO nanostructures with homogeneous morphology, small crystallite size and narrow size distribution is a challenging aspect to be investigated. Understanding the growth mechanism is an important part of controlling the size of nanostructures. The synthetic strategies of tailoring the size and shape of the nanostructures are key issues to be addressed in nanomaterials research.
To the best of our knowledge, there is no report on the effect of the molecular structure of complexing agents on MgO nanostructures even though the control of nanostructures presents an important part of nanotechnology work. Our work is focused on the effect of complexing agents on the MgO nanostructures finally obtained after synthesis. The study is done by using two different types of complexing agents, namely oxalic acid and tartaric acid. The molecular structures of these complexing agents are taken into account, and chemical reactions involving the complexing agents and site attachments of the Mg2+ and O2− ions in the process of the formation of MgO nanostructures are considered. Results give some insights into the mechanisms of size and shape formation of MgO nanostructures.
All the chemicals used were analytical grade and directly used as received without further purification. Magnesium acetate tetrahydrate, Mg(CH3COO)2 · 4H2O (Merck, 99.5% purity); oxalic acid dihydrate, C2H2O4 · 2H2O (Merck, >98% purity); tartaric acid, C4H6O6 (Merck, 99.5% purity); and absolute ethanol, C2H5OH (J. Kollin Chemical, 99.9% purity) were used for the formation of MgO nanostructures. These chemicals were manufactured by Merck KGaA Company at Darmstadt, Germany. The MgO samples were synthesized using the sol-gel method with two different types of complexing agents, namely oxalic acid and tartaric acid. Magnesium acetate tetrahydrate of mass 53.2075 g was initially dissolved in 150 ml of absolute ethanol under constant stirring. The pH of the solution was then adjusted to pH 5 using 1 M oxalic acid. The mixture was continuously stirred until a thick white gel was formed. The gel formed was left overnight for further gelation process before being dried in an oven at 100°C for 24 h. The dried materials were grounded using mortar and pestle to produce fine powder precursors. Subsequently, the precursors were annealed at 950°C for 36 h to form MgO nanostructures. The samples were identified as MgO-OA and MgO-TA for complexing agents oxalic acid and tartaric acid, respectively.
All the MgO samples were systematically characterized using various instruments. The thermal profiles of the precursors were studied using simultaneous thermogravimetric analysis (STA; SETARAM SETSYS Evolution 1750, Caluire, France). This thermal analysis method has the advantage of giving very accurate calorimetric data that is simultaneously measured and calculated with weight loss. It gives more accurate insight into the processes occurring while the precursor is heated. The obtained precursors were heated from room temperature to 800°C at a heating rate of 10°C min−1. The X-ray diffraction (XRD) patterns of MgO-OA and MgO-TA were obtained by XRD PANalytical X'Pert Pro MPD (Almelo, Netherlands) with CuKα radiation. The Bragg-Brentano optical configuration was used during the data collection. The size and morphology of the MgO crystallites were determined using a field emission scanning electron microscope (FESEM; JEOL JSM-7600 F, Tokyo, Japan) and a transmission electron microscope (TEM; JEOL JEM-2100 F, Tokyo, Japan).
Results and discussions
The use of oxalic acid and tartaric acid has been demonstrated to be very useful in producing thermally stable MgO nanostructures with a relatively uniform particle size. The growth mechanisms of the MgO nanostructures have been attributed to the very different molecular structures of the complexing agents which affected the crystal growth rate of MgO giving different crystallite sizes of the final products. The molecular structures and complexation site density play an important role in the fixing of the metal cation, Mg2+, and the formation of MgO nanoparticles. It is also clear that MgO-OA is able to produce nanocrystals not only of narrower size distribution but also of uniform morphology.
field emission scanning electron microscope
simultaneous thermogravimetric analysis
transmission electron microscope
The authors would like to thank the Ministry of Higher Education, Malaysia, for supporting this work through the Fundamental Research Grant Scheme (600-RMI/ST/FRGS 5/3/Fst(200/2010)). The authors were also grateful for the international grant, 100-RMI/INT 16/6/2(9/2011), from the Organisation for the Prohibition of Chemical Weapons (OPCW), Netherlands, for the financial support of this research work.
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