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Al2O3 NANOPARTİCLES AND IT’S PROPERTİES AND APPLİCATİONS

Aluminum is one of the major elements from the periodic table and when combined with oxygen becomes aluminum oxide which is considered as one of the most used and beneficial products. Aluminum oxide nanoparticles are present as a white powder in the form of spheres. However, these are present in both solid and liquid forms.

They have a series of manufacturing processes that enable them for excellent characteristics that are excessively beneficial and serve as a key to bring forth all the excellent applications that this product offers. All these applications are serving industries all over the world and are being generated at a much greater rate to fulfill their needs and purposes.

Introduction

There has been a great deal of focus recently on the usage of nanoparticles in the industry and research because of the improved characteristics of the nanoparticles now as in comparison with the bulk materials. Scientists fabricated nanoparticles from ultrafine particles having less than 100 nm diameter.

The first signs of surface effects and other unusual characteristics appear in this size range first. Their small size is what makes these effects possible as when nanoparticles create materials, there is an exposure of a significant amount of atoms on the surface. When the materials are made from a nanoscale, their behavior and performance significantly improve. Some of the improvements that come naturally when the material is made from the nanoparticles are enhanced thermal and electrical conductivity, and enhanced strength and hardness.

Aluminum is an element of Period 3 and Block P, whereas oxygen is the element of Period 2 and Block P. Aluminum oxide nanoparticles have a spherical morphology and their appearance is like a white powder. In the form of both solid and liquid, aluminum oxide nanoparticles act as an irritant and are extremely flammable, resulting in respiratory and serious eye irritation.

Manufacturing Process

Laser ablation, hydrothermal, sputtering, pyrolysis, sol-gel, and ball milling are among the many techniques that can be used to synthesize aluminum oxide nanoparticles. One technique that’s most commonly utilized in the production of nanoparticles, is laser ablation as they can be made in liquid, vacuum, or gas. High purity and rapid processes are some of the numerous benefits that this technique provides in comparison to the other methods. However, if laser ablation makes nanoparticles in liquid, they will be collected easily as compared to their collection in the gas atmosphere. In Mülheim an der Ruhr, chemists have recently discovered a way of producing alumina’s extremely stable variant, corundum (alpha-alumina), in nanoparticle’s form through the usage of a simple mechanical method in a ball mill.

We are talking here about the nanosized spherical particles from a bulk of alumina. Laser ablation is very beneficial as collecting the nanoparticles is difficult in the gaseous environment whereas it is easier in a liquid environment. In the laser ablation method, we can easily set the specifications of the kind of nanoparticles that we want to get.

States of aluminum particles

Oriented fibers and spherical-shaped individual particles are the two states in which the aluminum nanoparticles are normally found. As compared to the other nanoparticles, the characteristics of the bulk alumina are very different as it is the industry’s versatile element. The surface of interaction won’t be larger with its surroundings due to the large diameter of bulk aluminum.

Therefore, the applications of its usages can’t be expanded further efficiently to other areas like adsorption or catalysis. If we branch into the nanoparticle’s field, it will create a new image for the nanoparticle’s possible usages in aluminum oxide’s already abundant and useful compound. There are numerous benefits of the decrease in their size but the main reason is the change in the nanoparticle’s chemistry in comparison with their larger counterparts, fascinating these nanoparticle’s incorporation into many engineering and scientific fields.

Physiochemical properties

The dimensions and size of the nanoparticles heavily influence the physiochemical characteristics of the nanoparticles. Whenever the size changes, atoms in the nanoparticles arrange themselves differently due to the structures characterizing the nanoparticles.

Nanoparticles have 3 distinctive layers and they are not pointed objects. The surface layer is the outermost and first layer which comprises the metal ions or molecules that gives function to the nanoparticle’s surface. The shell layer is the second layer and it is very noticeably different as compared to the other 2 layers. The nanoparticle’s core is the last and third layer, and more than most of the time, it is a representation of the nanoparticle’s basic chemical formula itself.

Right now, the structures of the nanoparticles are being determined as the methods implemented on the same system in bulk are not able to describe the major properties that are presented when the system is at the nanoscale. Thermal, magnetic, mechanical, optical, and electronic characteristics come in the general physiochemical characteristics.

Optical and Electronic properties

Due to the dependence of electric conductance, emission, and absorption on the energy levels of electrons and electrons, both optical and electronic characteristics are symbiotic. A spectrum of absorbance and emission is displayed by those nanoparticles that are not found in the bulk. Nanoparticle’s capillary force, friction, adhesion, elastic modulus, hardness, and other mechanical characteristics are studied in depth for facilitating the usage of these nanoparticles in the industry. When it comes to synthesizing wear-resistant products, improved hardness is used. Low friction is used to design longer-lasting lubricants and the particle removal processes use the adhesion characteristic.

Moreover, nanoparticle’s thermal characteristics are rooted in the types of metals that the nanoparticles are made of. Higher thermal conductivities are possessed by aluminum’s oxides and metals like copper as compared to most of the other fluids or solids of non-metallic compounds. The thermal conductivity of the fluid can be improved through the addition of the nanoparticles of these metals and metal oxides as before, the fluid was a bad thermal conductor.

Characterization of Al2O3 nanostructures

Thermogravimetric analysis (TGA) was done for determining the Al foil’s oxidizing properties. Pure aluminum foil was oxidated in the air at an increased temperature of the range of 28-650 C. Due to the Al foil having a melting point of 655 C, the highest temperature was restricted to 650 °C. 20 C/min was the set heating rate. 8-9% was the percentage weight loss because of the removal of moisture in the 28-120 C temperature range. When there comes an increase of 400 C in the temperature, the weight loss linearly increases for reaching 16 % further. The sample’s weight almost didn’t change from the range of temperature of 400 to 650 °C.

 

Preparation methods

Optimum conditions are needed to prepare nanostructures. A bulk material will be made on complete oxidation of the foil if there is an extremely high rate of oxidation. However, an oxidation layer will be formed on the surface and only surface oxidation will occur if there is an extremely low rate of oxidation. It was observed from the data that the weight loss stays the same in the 400-650 C temperature range which confirms thermal stability’s accomplishment. So, 400-650 C was the set experimental temperature range.

Size and Morphology

After annealing at 400 C, FESEM examined the size and morphology of the Al2O3 nanostructures and they only showed surface oxidation with no production of the nanostructures. The oxidation layer covers the surface which can be the onset of the initialization of the formation of the nanoparticle with the oxidation layer’s symmetry being the round structures joined together.

Nanostructures are formed if the aluminum foil is treated at 450 C without the oxidizing agent. 1200 nm is the average chain length whereas the size of the nanoparticles ranges from 10 to 80 nm. All of the aluminum foil along with the surface was oxidized at 550 and 500 C because at these temperatures, nano-chain-like structures, and bulk structures are visible with 100 nm as average particle size.

Rate of oxidation

Oxidizing agents like K2Cr2O7, KMnO4, and H2SO4 are added to adjust the oxidation rate. Numerous types of nanoparticles are formed according to the results showed by FESEM when different oxidizing agents (K2Cr2O7, KMnO4, and H2SO4) were used to treat with the Al foil at different annealing temperatures of 450-550 C. The best oxidizing agent is KMnO4 according to the KMnO4, K2Cr2O7, and H2SO4 standard potential values of 1.51 V, 1.33 V, and 0.45 V. Thus, the nanoparticles will be smaller if the oxidizing agents are stronger and vice versa. Large structures are formed for annealing the aluminum foil at 450 C with H2SO4 and the arrangement is a flower-like structure along with the formation of the nano-chain.

Size ranges

50-150 nm is the range of the size of the nanoparticle and 1800 nm is the average chain length. There was the formation of nanorods on the surface with nano-chains spreading all over the surface. 200 nm is the length of the nanochain whereas the width was about 20 nm. The aluminum foil was oxidized at 550 C to show traces of chains and nanoparticles. Rod-like nanoparticles of 600 nm of average length and 10-120 nm of size were formed for the treatment of aluminum foil at 450 C with K2Cr2O7.

Nanoparticles have an average size of 15-180 nm at 550 C however plate-like chains have 2000 nm of an average length. At 550 and 450 C, aluminum foils were treated with KMnO4. Nanoparticles with 25-130 nm size occur on annealing at 450 C and the nanoparticles were uniformly formed instead of chains as a layer throughout the surface. KMnO4 formed smaller nanoparticles at 550 C with 10-110 nm sizes and 600 nm average chain length. KMnO4 is a stronger oxidizing agent as compared to K2Cr2O7. Here, there was a formation of nanoparticles throughout the surface and almost the whole structure was round.

Effect of the contact time and the initial fluoride concentration

They utilized Al2O3 nanostructures made with both, oxidizing agents KMnO4, K2Cr2O7, and H2SO4 without an oxidizing agent at 450 °C, as adsorbents at 550 °C. 8 mg/I, 4 mg/I, and 2 mg/I initial fluoride concentrations were utilized with a 4 g/l adsorbent dose. Within 20 minutes, fluoride’s fast adsorption started, and after 20 minutes, the adsorption process turned slow and within 90 minutes, the equilibrium was reached. Fluoride removal is increased only by less than 1% on a further increase in the contact time for 24 hours. More than 90% of removal is offered by the nanostructures that are made at 450 C without an oxidizing agent with 2 mg/l initial fluoride concentration, whereas with the highest fluoride concentration of 8 mg/l, 74.87% removal was attained.

Percentage fluoride removal

The percentage of the fluoride removal through the adsorption of the nanoparticles made at 550 C utilizing oxidizing agents like KMnO4, K2Cr2O7, and H2SO4, at the initial fluoride concentrations of 2 g/l as 92%, 91%, and 88.5%, is displayed as the contacting time’s function. The fluoride removals are found as 88.12 %, 87.87 %, and 82.75 % with 8 g/l initial concentration. According to observations, for lesser initial fluoride concentrations, the adsorption was fast.

Effect of the adsorbent mass

When the dose of aluminum oxide nanoparticles increases at 500 C from 1 g/l to 4 g/l with no oxidizing agent, then the permissible limit of 1 mg/l was attained as there was a decrease in the concentration of residual fluoride. According to observations, alumina in 4g/l was needed for maintaining the permissible limit for initial fluoride concentration of almost 8 mg/l, 4 mg/l, and 2 mg/l. For doses of 4 mg/l and 2 mg/l, the residual fluoride concentration was almost constant but it decreased sharply on the adsorbent dose of 1 g/l.

Availability of more sites of adsorption and greater surface area and is the reason for this severe decrease in the concentration of fluoride. There was a decrease in the concentration of bulk fluoride and the number of active sites and with the increasing time, the equilibrium was reached. Thus, with any more increase in the adsorbent dose, there will be no change in the amount of residual fluoride concentration.

Effect of pH

Fluoride’s equilibrium sorption was studied at 25 C of operating temperature and 2-67-11.28 pH with 8 mg/I initial fluoride concentration for determining the optimum level of pH to remove the maximum amount of fluoride. It is crystal clear that the value of pH strongly determines the absorption of fluoride on alumina.

At 4.7 pH, 88.12% fluoride removal took place. There has been an initial increase in fluoride adsorption with pH, reached the maximum at 4.7 pH and when pH values increase to 9.31 then it slowly decreased. There was a sharp decrease in the percentage removal beyond pH of 9.31. There will be an increase in OH- ions amounts in the solution as the pH increases. The repulsion between OH- and F- ions will increase on the increase in pH as the ion that is concerned for removal is F- and it all leads to a decrease in the removal of fluoride. At a pH of less than 3, aluminum oxide starts to dissolve whereas, at a pH of 11, it stays stable.

Effect of the stirring speed

In adsorption phenomena, stirring is a very major parameter as it promotes and provides a proper contact between the solution and the adsorbent. Stirring helps in the solute distribution in the bulk solution, aiding in the production of an external boundary film. A shaking incubator was utilized by the lab tech for studying the stirring speed’s effect in our samples. Stirring speeds of 300, 200, and 100 rpm were utilized at 25 C with 90 minutes of contact time on KMnO4 treated nanoparticles. Fluoride removal percentage shifted from 85% to 87% with an increase in the stirring speed from 100 to 300 rpm. Now, why does the fluoride removal is increased by the increased stirring speed and it is because the film boundary layer that surrounds the adsorbent is reduced by the increase in stirring speed, therefore increasing the external film transfer coefficient and thus helps in stimulation of the better adsorption of fluoride?

Effect of other ions

Various ions were included in the natural fluoride contaminated water. 300 mg/l of NaCl solution was individually added to the fluoride solution for studying the effects of Na ions. The adsorption experiments were performed at 25 °C. Sulfate, nitrate, and chloride ions had a negligible effect and fluoride removal was 85.32, 86, and 87.62 %, respectively.

Decrease in fluoride removal

It is because of these anions and their competition for fluoride’s adsorption that there is a decrease in fluoride removal. Because of a major increase in the solution’s pH values, there was a decrease in the percentage of fluoride removal to 32.5 and 26.12%. Improved F- ions repulsion will take place because of the increase in the number of OH- ions with the increase in pH, thus lessening the percentage of fluoride removal.

Thermodynamical considerations

Operating temperatures of 55, 45, 35, and 25 C were used to conduct experiments for studying temperature’s effect on the percentage fluoride removal. 8 mg/l of initial fluoride concentration was maintained while the experiment was being conducted along with the maintenance of an Al203’s adsorbent dose of 4 g/l made with KMnO4. According to observations, due to adsorption’s endothermic behavior, the percentage removal increased to 89.2 % from 88.12 % with an increase to 55 C in the operating temperature from 25 °C.

Kinetic aspects

Pseudo-second-order and pseudo-first-order ones and other different models were used to study fluoride adsorption’s kinetics on Al2O3 nanostructure’s surface. According to the pseudo-first-order model, the changing rate of the solute uptake with time is directly proportional to the difference in equilibrium concentration and the number of solutes adsorbed with time. dqt/dt=K1qe−qt is used to give the first-order absorption rate constant.

The plot of In (qe-qt) versus t can be used to calculate qe (equilibrium adsorption capacity) and k1 (adsorption rate constant). It is clear from the calculated values that the fluoride adsorption’s kinetics on the aluminum oxide nanoparticles doesn’t follow the diffusion-controlled phenomena or the pseudo-first-order kinetics.

Applications of Al2O3

Following are the major and significant applications of aluminum oxide nanoparticles.

Pharmaceuticals

In different medicine branches, people use nanoparticles for drug delivery. Dosage’s accuracy determines the power that the nanoparticles possess in delivering the drugs into the body to release the drugs in the body to the particular locations at a planned time, leading to the drug’s improved therapeutic efficiency and a decrease in the number of side effects that the current existing method causes while delivering the drugs.

The reason for them being a good option for delivering the intravenous drug into the body parts is the alumina nanoparticle’s durability. However, the delivery is to only those body parts that have extremely low pH and that already have caused degradation of the other nanomaterials that have tried to reach their destination. In addition, most people recently are concerned with aluminum toxicity as there have been many rumored connections recently between cancer and aluminum, making it a concern. Although there has been no evidence of aluminum’s carcinogenic effect yet that doesn’t mean that it is not carcinogenic therefore when aluminum nanoparticles are being implemented for drug delivery, you should take caution.

Materials Manufacturing

Deviating nanoparticle’s physiochemical characteristics from the bulk gives an exciting opportunity to produce novel nanomaterials. These alumina’s novel nanomaterials will have particular physiochemical and optoelectronic characteristics for numerous applications currently in the industry. One can find the alumina nanomaterials in cosmetics like fillers, formed as single crystals for catalysis, high strength ceramics, YAG laser applications, and various other areas in the industry.

Mechanical industry

A lot of significant usages are given by the synthesized nanomaterials in the mechanical industry, specifically as the cutting tools in the case of the alumina nanoparticles. Some of its other usages are, packaging materials, vapor de-positioning substance, polishing material, integrated circuit’s baseboards, etc., and it is also integrated into plastics to increase the plastic’s hardness.

Conclusion

The two major applications of aluminum oxide nanoparticles are in the fields of pharmaceuticals and materials manufacturing industries. Both of these are an excellent source of trade that is why aluminum oxide nanoparticles are considered as one the best products that our industries have. Their use and application have excessively increased over the years.

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