Preparation of porous nano-magnesium oxide using a corn starch template method

Nano-porous magnesium oxide was prepared using a corn starch template and a magnesium nitrate hexahydrate precursor, and characterized by TGA, XRD, FTIR, EDS, TEM, and BET surface area analysis. The results showed that porous magnesium oxide composed of nano-particles could be obtained using this method. The final magnesium oxide product was in the form of large flakes under low magnification, and the flakes were composed of small nano-scale particles under high magnification. The particle size of nano-porous magnesium oxide was 20–28 nm, the specific surface area was 29.37 m2/g, and the average pore diameter was 19 nm.


Introduction
Magnesium oxide is an inexpensive catalyst with a high catalytic potential for degrading pollutants such as dyes or drugs [1][2][3].Numerous studies have shown that magnesium oxide possesses high surface reactivity and strong surface alkalinity [4].To improve the surface reactivity of magnesium oxide, its specific surface area can be increased by the preparation of nano-magnesium oxide or porous magnesium oxide.Recently, research on the surface structure and porous structure of nano-magnesium oxide has increased.Commonly used methodologies for preparing magnesium oxide nanoparticles include sol-gel, hydrothermal, microemulsion, microwave hydrothermal, and microwave solid-phase methods [5][6][7][8][9], and the number of available methods for preparing nano-metal oxides using biomacromolecular templates has gradually increased.Compared with commonly used methodologies, the biological template method uses a single raw material, has a simple process route, and consumes less energy during the synthesis process.It also produces relatively few by-products.Moreover, because many biological macromolecules have many hydroxyl groups [10], nano-sized cavities are formed among the macromolecules with multiple hydroxyl groups [11], and these cavities can be used to limit nanoparticle size.Starch is a natural, renewable, and degradable biological macromolecule compound with a single spiral cavity structure, which allows for the absorption of numerous guest molecules [12].Many studies have shown that starch is a suitable template and dispersing agent for nanoparticle preparation.Furthermore, compared with other biological templates, starch is considerably cheaper [13].Duan et al. [14] used the embedding effect of starch to prepare a porous composite oxide metal material with a unique morphology, and the product exhibited good catalytic properties.Zhao et al. [15] also successfully prepared sponge-like porous nano-cerium oxide using a starch template, and the chemical oxygen demand (COD) removal rate was 82.5%.In 2019, Manikanda and Sangeetha [16] used chemically denatured soluble starch to prepare nano-magnesium oxide for transesterification reactions.However, a method for preparing porous nano-magnesium oxide using a corn starch template under mild conditions has not yet been reported.
Therefore, this work proposes a method for preparing porous nano-magnesium oxide using a natural corn starch template.First, under mild conditions, the template was mixed with a magnesium nitrate hexahydrate precursor in a solvent, and then the solvent was evaporated.Afterward, the precursor was converted into magnesium oxide by calcination, and the template was simultaneously removed to obtain highly pure porous nano-magnesium oxide.This method avoided the cumbersome steps of washing the product during the preparation process and used general chemical methods to remove the numerous by-products.This preparation process thus combined a single precursor with an environmentally friendly template for nanoparticle preparation.

Materials and equipment
The raw materials used in this study consisted of magnesium nitrate hexahydrate (AR), edible corn starch, methylene blue dye (AR), absolute ethanol (AR), and deionized water.
The following equipment was used in the preparation experiment: a constant-temperature blast drying oven, a 75 ml stainless steel reactor, a homogeneous reactor, and a box-type resistance furnace.
The characterization equipment consisted of a high-resolution scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), a thermogravimetric analyzer (TG), an X-ray diffraction (XRD) analyzer, a Fourier transformed infrared spectrometer (FTIR), a specific surface area and pore size analyzer (ASAP), a transmission electron microscope (TEM), and an ultraviolet-visible spectrophotometer (UV).

Experimental method for preparing porous nano-magnesium oxide
For pre-treatment, the cornstarch was washed two to three times with absolute ethanol to remove the protein and fat from the cornstarch, and the material was then dried at 60°C.For the experimental methods, 3 g of Mg(NO 3 ) 2 •6H 2 O was dissolved in a 100 ml mixture of deionized H 2 O and ethanol (volume ratio of 1:1), then 7.5 g of pre-processed corn starch was added in batches (small amounts, multiple times) and the mixture was stirred to obtain a white suspension.Subsequently, the suspension was moved to a stainless-steel reactor with a polytetrafluoroethylene lining, and the reactor was placed in a homogeneous reactor with a rotation speed of 200 r/min.The reaction then proceeded for three hours at 70°C and cooled to room temperature.Next, the reaction mixture was transferred to a beaker, aged overnight, and dried in a thermostatic blast dryer at 80°C to obtain a bulky white solid.The bulky solid white was then calcined in a box-type resistance furnace at 750°C for one hour to obtain a white product, which was directly subjected to characterization measurements without grinding or sieving.

Thermogravimetric analysis
The thermo-gravimetric measurement results of Mg(NO 3 ) 2 •6H 2 O and corn starch from room temperature to 800°C at a heating rate of 10°C/min in a nitrogen atmosphere are shown in Figure 1.The TGA curves show that When the temperature exceeded the melting point of 95°C, Mg(NO 3 ) 2 •6H 2 O began to dehydrate and formed basic magnesium nitrate.It started to decompose at 300°C, and as the temperature exceeded 467°C, the residual weight no longer changed.At this temperature, the basic magnesium nitrate completely decomposed into magnesium oxide and nitrogen oxide gas.For the corn starch, there were only small reductions in weight in the range of 25-256°C, which may be induced by the desorption of small quantities of H 2 O in air absorbed on the cornstarch.Between 256-328°C, most of the weight of the corn starch was lost (about 68%), and when the temperature reached 677°C, the corn starch completely decomposed.Therefore, the calcination temperature in the experiment was selected to be a minimum of 677°C.This was verified by many decomposition experiments, which showed that the actual calcination temperature is 750°C.This temperature difference was possibly caused by the differences in temperature control during the TGA and decomposition experiments.

XRD analysis
The XRD results of the calcined product are illustrated in Figure 2, which indicated that the strong absorption peaks of magnesium oxide were mainly found at 2θ=42.8° and 62.0°, which corresponded to the (200) and (220) crystal planes of MgO (PDF#45-0946), respectively.The remaining absorption peaks at 2θ=36.6°, 74.4°, and 78.4° corresponded to the (110), (311), and (222) crystal planes of MgO, respectively.No diffraction peak for the Mg(NO 3 ) 2 •6H 2 O precursor was observed in the XRD patterns of calcined magnesium oxide, indicating that the precursor completely decomposed and produced MgO.Meanwhile, the particle size of the magnesium oxide product was calculated by the Scherrer Equation to be about 22.3 nm.

FTIR analysis
The infrared absorption results of the product are displayed in Figure 3, which indicated that the products had absorption peaks at 1438, 1044, and 605 cm −1 .The absorption spectra at 1438 and 1044 cm −1 were caused by C=O stretching vibrations and the bending vibrations of H-O, respectively.This was due to the product absorbing small amounts of H 2 O and CO 2 from the air.The absorption at 605 cm −1 was caused by the stretching, bending, and crystal lattice vibrations of Mg-O.Additionally, many inorganic metal oxide group absorptions were observed in the mid-infrared region.The standard infrared absorption of magnesium oxide occurs at 500 cm −1 ; however, in this study, the absorption peak of magnesium oxide shifted in the short-wave direction by 105 cm −1 and a blue shift occurred.This may have been due to the quantum size effect, as when the magnesia crystal grains were reduced, the Fermi vibrational energy level gap became larger, which resulted in differences in molecular vibration frequency, resulting in a broadening of the absorption bandwidth and blue shift of the magnesium oxide absorption peak.In addition, the absorption peaks of other organic groups were not observed in the infrared absorption spectrum, which confirmed that the template was eliminated by calcination at this temperature and the precursor was completely decomposed, generating pure magnesium oxide.Thus, the above results were consistent with the XRD characterization results.

Elemental analysis
After the obtained product was calcinated at 750°C for one hour, an EDS energy spectrum analysis was conducted (Figure 4) which showed that the product contained 8.66% of C. The C was possibly derived from the incomplete calcination of the template or the attached conductive adhesive during sample measurement.When the XRD and infrared measurement results were compared, the product did not show any impurity diffraction peaks other than magnesium oxide, and C=C absorption vibrations were also not observed in the infrared measurements.This confirmed that the 8.66% C element came from the conductive adhesive.Furthermore, the calculated molar mass ratio of Mg:O was approximately 1:1, which proved that the C template was eliminated.Also, there were no C elements in the material, and pure magnesium oxide could be produced under this condition.

TEM measurements
The magnesium oxide prepared at a solvent ratio of 1:1 was characterized by TEM, and the results are depicted in Figure 5. Figure 5(a) shows the transmission morphology of the product.Electron density differences were visible and the magnesium oxide product had a rounded and granular shape with numerous pore structures.The particle size was the same, with an average particle size of 25 nm.IOP Publishing doi:10.1088/1742-6596/2713/1/0120385 PDF#45-0946 magnesium oxide, respectively.Figure 5(c) displays the selected area electron diffraction pattern of the product, which consisted of concentric circles, indicating that the product possessed a polycrystalline structure.The particle sizes calculated by the Scherrer Equation differed from those obtained by electron microscope characterization.This is because the particle size calculated by the Scherrer equation is more accurate for single crystals, while for polycrystalline structures, direct measurement of particle size using TEM is more intuitive and accurate.Therefore, the characterization of particle size in this study was mainly based on direct TEM observations.The diffraction patterns were calibrated, and the results corresponded to the (111), ( 200), ( 220), (311), and (222) crystal planes of PDF#45-0946 magnesium oxide, which were consistent with the XRD characterization results.

BET characterization
To further determine the specific surface area and pore size of the product, Brunauer-Emmett-Teller (BET) characterization was conducted and the results are presented in Figure 6.The adsorptiondesorption curve was type IV of Langmuir's six adsorption types, and a typical H3 hysteresis loop appeared at a relative pressure greater than 0.6, indicating that the material was flaky.This type of hysteresis loop mostly appears in materials with pore sizes of 2-50 nm, this indicated that the material contained a mesoporous structure, the pores were in the form of cracks, and the pores were interconnected.There was no adsorption plateau at relatively high pressure, suggesting that the formation of these pores was due to the loose accumulation of lamellar particles.Using the Barrett-Joyner-Halenda (BJH) method, the calculated pore sizes were mostly between 5 and 50 nm, pore size was not uniform, and some of the pore sizes were caused by gas that escaped from the product during calcination.Thus, the average pore diameter was 19 nm and the measured specific surface area was 29.37 m 2 /g.The specific surface area of the magnesium oxide product prepared without the template was only 4.343 m 2 /g.The above findings, therefore, indicated that a corn starch template can be used to increase the specific surface area of the product and create pores.Additionally, the product met the requirements for commercial magnesium oxide catalysts.When dispersed in a solvent system, starch particles contain many active groups on the surface, such as hydroxyl groups, causing the particle surfaces to be negatively charged.During the immersion process, the magnesium nitrate hexahydrate precursor first ionizes Mg 2+ in the solution.The ionized Mg 2+ and the hydroxyl groups are then adsorbed on the surface of the corn starch particles due to electrostatic effects and coordinate bonds.Corn starch particles can enter the precursor in an embedded manner, as shown in Figure 7.As the temperature rises during the drying process, the hydrolysis reactions of Mg 2+ occur, producing basic magnesium nitrate solids that crystallize on the corn starch surface.After calcination at high temperatures, the basic magnesium nitrate crystalline nucleus gradually increases and decomposes to produce magnesium oxide.Meanwhile, the embedded template decomposes, producing CO 2 and H 2 O, and creating the macroporous structure as shown in the TEM images.At low magnification, a net-like structure of magnesium oxide was observed, while at higher magnification, irregular flakes appeared locally and there were gaps between the magnesium oxide particles.There are two causes of pores formation.First, the thermal expansion of the template during template removal increased the spacing between the magnesium oxide particles, and second, the gas escaped from the product during the decomposition process.In the growth process of nanomagnesium oxide particles, the degree of dispersion between the corn starch particles can control the particle size and morphology of magnesium oxide products.

Conclusions
Porous nano-magnesium oxide was successfully prepared using an environmentally friendly and economical corn starch template, and the resulting product was in the form of flakes.High magnification TEM observations showed that the flakes were composed of small nano-scale particles with clear particle boundaries.The average particle size of nano-magnesium oxide was 25 nm and pores were observed between the particles.This finding improves the preparation of magnesium oxide nanoparticles and offers new thought for the fabricating of other nanomaterials.

Figure 1 .
Figure 1.TGA curves of the magnesium nitrate hexahydrate precursor and corn starch.

1 Figure 3 .
Figure 3. Infrared absorption spectrum of the product.

Figure 4 .
Figure 4. EDS spectrum of the MgO product.

Figure 5 (
b) shows a high-resolution TEM image of the product.The calculated lattice fringe spacings of the product were 0.241 and 0.207 nm, which correspond to the (200) and (111) crystal planes of AMCE-2023 Journal of Physics: Conference Series 2713 (2024) 012038

Figure 5 .
Figure 5. TEM images of the magnesium oxide product at a solvent ratio of 1:1: (a) TEM morphology, (b) HRTEM image and (c) SAED image.

Figure 6 .
Figure 6.Nitrogen adsorption-desorption curve and pore size distribution graph of the magnesium oxide product (solvent ratio 1:1).

4 .Figure 7 .
Figure 7. Schematic of the growth mechanism of the magnesium oxide product.