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Nanotechnology is known to be a research field that is related to the making of such devices which are based upon either atoms or molecules. The nanoparticles or materials that are involved in nanotechnology are so small that they cannot be seen with a naked eye and rather microscopes are used to observe them. Nanomaterials are now taking place of conventional materials for treatment of water.
Despite their size being so small, they possess outstanding properties as a result of which they exhibit efficient applications which are highly beneficial for the industries. One of the major applications of nanotechnology is the purification of water. The properties of nanomaterials involved enable the purification of certain means which are explained in this article.
A research field that’s concerned with the building of devices, materials, and things on an atomic to molecular scale is known as nanotechnology. A meter’s one-billionth is a nanometer and it is 10 times a hydrogen atom’s diameter. Human hair has an average diameter of 80,000 nanometres. The ordinary rules of chemistry and physics don’t apply at such scales anymore. For example, the characteristics of the material like their reactivity, conductivity, strength, and color, can be substantially different between the macroscale and the nano. As compared to steel, carbon nanotubes are 6 times lighter but 100 times stronger.
Nanotechnology is assumed to have the potential for solving significant health problems, helping in environmental cleaning, and increasing energy consumption efficiency. It is capable of increasing the manufacturing production massively at very lessened costs. According to the advocates of nanotech, nanotechnology products will be more functional, lighter, cheaper, smaller, needing less energy, and less raw materials for manufacturing.
The definition of the National Nanotechnology Initiative reflected that at this quantum-realm scale, quantum mechanical effects have a lot of significance, shifting the definition to a research category from a particular technological goal. Such research category is inclusive of technology and research of all types that deal with the matter’s special characteristics if the matter occurs below the given size threshold. Thus, when it comes to referring to a wide range of applications and research having size as the common trait, the plural form “nanotechnologies” is as good as the term “nanoscale technologies”.
Nanotechnology’s future implications are currently being debated over by scientists. Nanotechnology can produce various new devices and materials having applications in a broad range, for instance in consumer products, production of biomaterials energy, nano-electronics, and nano-medicine. However, many similar issues were raised by nanotechnology comparatively, which includes the concerns about the nanomaterial’s environmental impact, toxicity, and potential effects on global economics. Due to these concerns, there has been a debate among the governments and advocates about the warranty of nanotechnology’s special regulations.
Properties of Nanoparticles
When it comes to respecting the safety of nanoparticles, the main parameters of interest are;
- Structure, for instance, defect structure and crystallinity.
- Topography/morphology of the surface.
- Size distribution.
- Aggregation/Agglomeration state.
- Aspect ratio, specific surface area, shape, and size.
- Chemistry of the surface (zeta potential, photocatalytic characteristics, physical structure, reactive sites, tension, charge, and composition)
- Phase identity.
- Nano material’s composition (for instance degree of impurity, known additives, or impurities)
- Molecular structure/Structural formula.
In order to use nanomaterials in test systems, one should know that some of the significant characteristics are determined by the nano-materials temporal evolution and the surrounding media. Therefore, assessing the nanomaterials should be our major aim but we should assess them in their manufactured composition/form, and in the formulation in which they were delivered to the environment or the end-user if free nanoparticles are involved in the formulation.
The nanomaterials can exist in the form of nanopowders. They are incorporated in solids, suspended in liquids (colloids), and suspended in air (aerosols, nanoparticles, ultrafine particles). The dispersion of manufactured nanomaterials should be done in a suitable media for biological safety evaluation. Suspension’s behavior can be profoundly influenced by the interaction between the nanomaterials and these media.
Potential dissolution kinetics:
The significance of potential dissolution kinetics should be emphasized more as the amount of recently developing nanomaterials increases. Nanomaterials are capable of dissolving faster as compared to materials of larger size because of the dissolution kinetics being normally proportional to the surface area. This also implements on the silver nanoparticles as they are being more and more utilized for the release of their silver ions as the anti-bactericidal agents. There are no proper studies on dissolution kinetics yet. It is not necessary, neither is it possible to determine all of the characteristics in every situation.
Being a mythical substance, water’s material existence is secondary to the symbolic value as in our lives, it is manifested as life’s symbol. We need sustainable supplies of clean water as it is very important to the health of the economy, environment, and the world. Right now, the freshwater’s available supplies are reducing, which is making human society face a huge problem in fulfilling the increasing requirements of potable water. The reason for a decrease in freshwater’s available supplies is an increase in demands from numerous competitor users, unabated flooding, reduction in the quality of water specifically groundwater because of an increase in surface water and groundwater pollution, growth of population, and extended droughts.
The usage of water should require suitable management, development, and planning as it is a precious natural asset, a basic need of every human, and a prime natural resource. Water scarcity has been caused in the world’s numerous parts because of the increasing population and groundwater and surface water’s overexploitation over the past few decades. There has been a major increase in the wastewater and due to no proper measures of managing and treating this problem, it is also polluting the current freshwater reserves. Water consumption has been increased in cities and towns because of the increased urbanization. Therefore, it’s time to know how to manage the existing water reserves so that we can avoid having a water strain in the future.
Having safe drinking water available is a concern nowadays. By far, the most significant water resource is groundwater, for approximately all of the country’s water requirements. According to the study of UNEP (United Nations Environment Programme), aquifers have over 2 billion people dependent on them for their drinking water. Irrigated agriculture produces the world’s 40% of food and largely depends on the groundwater. About 95% of the planet’s freshwater makes up the groundwater, resulting in the groundwater being important for the development of the economy and human life. Although when groundwater’s increasing scarcity coupled with the reducing quality of water, it posed a serious problem for the rural population, therefore forcing all of them to look at groundwater’s treatment since the clean safe water is now turning into an endangered commodity.
Due to the unabated usage of groundwater resources, the world is now looking at a major problem in the shape of the reduced availability of groundwater. Thus, there’s no option other than to move to the management of groundwater from the development of groundwater, meaning that we should move toward groundwater’s optimal usage. Now, it’s everybody’s responsibility to offer water that is safe to drink, and to achieve that, there should be some processes of water treatment that should be sustainable, cost-effective, and easy to be implemented in the longer run.
Role of nanomaterials in water purification and treatment
Defeating the conventional technologies because of them being time-consuming and expensive, Nanomaterials are now emerging very rapidly as the potential candidates for the treatment of water. Developing countries like Bangladesh and India would specifically benefit from this as the cost of implementation of any new removal process there could result in a significant criterion in predicting its success. If we speak qualitatively, nanomaterials can be substituted for conventional materials which are harmful to the environment, takes more intensity of energy to be produced, or needs more raw materials.
Producing nanoparticles by using green chemistry principles may result in a lessening in hazardous chemical synthesis, a huge amount of reduction in generation of waste, and generally safer inherent chemistry. Although, more quantitative data is needed for substantiating these claims and whether the replacement of the traditional materials with nanoparticles leads to lower consumption of material and energy, whereas preventing unanticipated or unwanted side effects is open for debate. Nanoparticles’ safety and their potential influence on the environment have caused a broad debate. A major role is played by nanotechnology in offering clean water in a sustainable, cheap, and efficient way to developing countries.
One can’t overlook nanoparticles’ potential side effects. For example, when utilized for pollutant degradation, a nanoparticle’s catalytic activity can be beneficial but when a cell takes it up, it can also cause a toxic response. In broad-spread adoption of nanoparticles, this nanotechnology’s Janus face can be a problem. Although, the cost of nanotechnology can be lowered, thus making it more effective and efficient as compared to the current methods to remove the contaminants in a long run from the water. Nanoparticles can be utilized as catalysts for contaminants’ photochemical destruction, as separation media, and as potent sorbents. In order to remove organic compounds and metals from nanofiltration membranes and water, nanosized zerovalent iron is utilized.
Mechanisms of Removing Pollutants from Wastewater by Nanomaterials
Nanoparticles are made highly lubricative as solvents due to two vital characteristics. Nanoparticles have way larger surface areas than macroparticles, on a mass basis. Various reactor groups can be used to enhance them for increasing their chemical affinity towards the target compounds. Workers are exploiting these characteristics for the development of highly efficient and highly selective absorbents to remove inorganic and organic pollutants from the contaminated water. The properties of many materials are determined by their size. With 7 nm diameter hematite particles, for instance, adsorbed Cu ions at lower pH values as compared to the particles which have a diameter of 25-88nm, referring to the improved surface reactivity for iron oxide particles with a decrease in diameter.
A novel sorbent has been developed by Peng et al. in 2005. It has a high surface area of 189 m2/g and it consists of cerium oxide supported on the carbon nanotubes. It was seen that the efficient sorbents for As(V) are the CeO2-CNT particles. All of this demonstrates that how the traditional substance’s adsorption capacity can be enhanced by chemically modified nanomaterials. A novel As(V) sorbent was synthesized and characterized in 2003 by Deliyanni et al. and it consists of the nanocrystals of akaganeite [β-FeOOH], to remove inorganic ions and metals, for instance, nanosized metal oxides.
Within 4 hours, As (V) and As (III) equilibrium adsorption by nanocrystalline TiO2 takes place and the adsorption followed pseudo-second-order kinetics. In 2005, the equilibrium was obtained by Bang et al. in 63 minutes. As (III) was removed by utilizing a synthesized crystalline hydrous titanium dioxide in 2004 by Manna et al. Within contact time’s first 30 minutes, 70% of As (III) adsorption takes place. There have been successful synthesis and employment of the mixed oxides’ nano-agglomerates, for instance, cerium manganese, iron-chromium, iron-titanium, iron-zirconium, iron-manganese, and iron-cerium, etc., for removing the pollutant (fluoride, arsenic, etc.) from the aqueous solutions.
Indulgence of zinc and tin
Reduction capabilities like those of iron are possessed by other metals too for instance tin and zinc. In the process of decontamination, these metals are converted like iron into metal oxides. Similar results can be produced on the combination of other metals with iron. There have been demonstrations on the degrading of trichloro-ethene and trichloro-ethane by both the iron-copper and iron-nickel bimetallic particles. Iron-platinum particles is another example as it has the same capabilities when it comes to degrading chlorinated benzene.
Carbon as an adsorbent
Carbon is utilized in a huge amount to remove numerous pollutants including the removal of heavy metals from the aqueous solutions as it is a versatile adsorbent. In research, the carbon family’s latest member is graphene and it is one of the most potential materials for the processes of water treatment. With the thickness of a single carbon atom, graphene is an sp-2 hybridized, flat, with carbon atoms’ 2-D honeycomb arrangement. The utility is offered by graphene and its composites in various applications because of its associated band structure and remarkable 2-D nature. Graphene is made an attractive absorbent candidate for processes of water purification due to characteristics like the presence of the surface functional groups and large surface area. Arsenic was removed from water by using the RGO-magnetite and GO-ferric hydroxide composites.
Effectiveness of iron-based oxides
When it comes to eliminating arsenic from drinking water, hydroxides and iron-based oxides have already been proved as efficient materials. The materials supported by GO and RGO have higher binding capacity than the free nanoparticles. It is a matter of interest that the antibacterial characteristic is possessed by the reduced graphene oxide and that characteristic aids in preventing the biofilm’s development on the surface of the filter because of the growth of bacteria. The growth of bacteria can result in prematurely clogging of filters or unwanted odors and tastes.
Nanofiltration (NF) and other membrane processes are having an uprise as the key contributors to water purification. NF membranes (nanofiltration membranes) are utilized broadly for the treatment of wastewater or drinking water. Nanofiltration is a low-pressure membrane process, separating materials in the size of 0.001-0.1 micrometer. The pore sizes of Nanofiltration membranes are between 0.2-4 nm. They are pressure-driven membranes and they have characteristics between the characteristics of ultrafiltration membranes and reverse osmosis membranes. Nanofiltration membranes eliminate inorganic ions (Na and Ca), microorganisms, and turbidity.
Groundwater softeners are utilized for pretreatment in seawater desalination, for wastewater treatment (elimination of organic carbon and inorganic and organic pollutants), for the elimination of the dissolved trace pollutants and organic matter from the surface water, and to soften the groundwater by reducing the hardness of the water. Nanofiltration’s usage has been studied by Bruggen & Vandercasteele in 2003 for removing arsenic, nitrates, organic pollutants, biological contaminants, natural organic matter, and cations from surface water and groundwater.
Removal of minute materials
Nanofiltration is capable of being utilized for removing U(VI) minute quantities from seawater according to Favre-Reguillon et al. in 2003. Nanofiltration’s usage for desalinating water was evaluated by Mohsen et al. in 2003 too. It was observed that when in combination with reverse osmosis, nanofiltration can efficiently render the brackish water potable. Peltier et al. showed an enhancement in the quality of water in 2003 for a large water distribution system by utilizing nanofiltration. Moreover, huge prominence is being gained by the carbon nanotubes filters in the water treatment processes. Recently in 2004, the successful fabrication of carbon nanotube filters was reported by Srivastava et al..
New filtered membranes
They contain hollow cylinders with carbon nanotube walls radially aligned with them. The filters were efficient at eliminating bacteria like S. Aureus and E. Coli from the contaminated water. Autoclaving and ultrasonication readily clean the carbon nanotube filters.
A mixture of micro glass with a high positive charge and nano alumina fiber is known as nanoceramic filters and they can retain the negatively charged particles. High efficiency is possessed by the nanoceramic filters when it comes to eliminating bacteria and viruses. Nanoceramic filters have a high capacity for less clogging and for particulates. Dissolved heavy metals can be chemisorbed by the nanoceramic fillers.
Removal of Nanoparticles After Water Treatment
In environmental applications, the nanoparticles’ usage will invariably result in the release of nanoparticles into the environment. Their persistence, toxicity, bioavailability, and mobility, should be understood in order to assess their potential risks in the environment. There is not much information yet on what will happen on the terrestrial and aquatic life’s exposure to the nanoparticles in the soil and water. Engineered nanoparticles’ rapidly growing usage in numerous industrial scenarios and their potential for the treatment of drinking water and wastewater purification leads to the unavoidable question that how such nanoparticles can be eliminated in the urban water cycle.
Filtration and sedimentation are the two traditional methods to remove particulate matter during wastewater treatment. Although, the sedimentation velocities are comparatively low and there won’t be any major sedimentation until the production of larger aggregates because of the nanoparticles’ small sizes. Usual technologies like flocculation are not suitable for removing nanoparticles from the water, which results in the need of discovering the problem’s new solutions. Membrane filtration (reverse osmosis and nanofiltration) has been used already to remove pathogens from the water. Therefore, to eliminate the nanoparticles, this method can be utilized too.
Today, most of the nanoparticles in technical applications are functionalized in nature and thus virgin nanoparticles are being used in the studies, making those studies irrelevant to assess the behavior of the particles that were actually used. Functionalization is utilized for increasing the particle’s mobility by decreasing the agglomeration. However, until now, not enough is known about how functionalization influences the nanoparticles’ behavior in the environment.
There are certain mechanisms that are responsible for the purification of water through nanotechnology. It is indeed a great technological way to purify the contaminated water as it hazardous and can cause serious health problems. However, nanotechnology is one such great approach that is paving the way for industries to avail the maximum number of health benefits that it provides.
Wastewater treatment is an important topic concerning the sustainability of clean water resources and human health. Research shows that graphene and GO are promising adsorbent materials for the removal of organic and inorganic water pollutants. Graphene-based wastewater treatment systems should be developed considering the properties of adsorbent, contaminant, background solution, and adsorbent regeneration.
Water is the most fundamental material on Earth essential for survival, any industrial process, or day to day activity. Up until recently, water was considered to be an abundant material in nature. However, long gone the days we could carelessly waste the precious water sources of Earth. As industrialization and consumer based economy became prominent in modern times, the natural water cycle has become inefficient and failed to keep up with the pace of human activities. Water pollution needs to be controlled to avoid further effects on the entire biodiversity leading to the destruction of living and non-living things on Earth. For this purpose, contaminating sources should be identified and controlled. Proper wastewater treatment processes need to be installed at the output of these contaminating sources in order to remove toxic and hazardous components from the sludge. Cleaning the discharge water of industrial plants, sewage lines, and mining activities ensures suitability for household purposes, natural usages, groundwater recycling, and many other purposes. This is why water pollution and wastewater treatment have become one of the most important topics of science and engineering. Even though some innovative solutions are suggested and utilized in wastewater treatment plants, the current treatment methods are still not efficient enough and often require a lot of space. In order to obtain desirable solutions to these problems in wastewater treatment processes, scientists have recently turned to nanotechnology and the use of nanomaterials.
Nanomaterials are advantageous in wastewater treatment processes due to their wide surface area, better chemical properties, lower cost, and high reusability. Attributing to their desirable properties several different nanomaterials and nanostructures have been suggested to be used in wastewater treatment systems. In the search for better wastewater treatment methods and materials, graphene and graphene-based materials have recently attracted attention. In particular, graphene oxide (GO) holds great potential for effective wastewater treatment methods.
Graphene is a 2D material purely composed of carbon atoms arranged in a hexagonal structure. The structure and bond type of graphene endows unique properties such as high mechanical strength, porous structure, electrical and thermal conductivity, chemical resilience, optical activity, and high surface area. Graphene-based materials include graphene oxide, and reduced graphene oxide (rGO). These materials are the products obtained during graphene production trials. Graphene oxide is obtained through the oxidative treatment of graphite while reduced graphene oxide is obtained through the reduction of GO sheets. As much as they seem like undesired results of a specific process, th
ey are proven to be equally useful in several different application areas. GO is defined as a graphene layer with various oxygen containing functional groups attached to its surface. Such functionalities can include epoxide, carbonyl, carboxyl, and hydroxyl groups. Reduced graphene oxide is obtained by removing most of these functional groups on the GO surface. GO and rGO show different properties than pristine graphene sheets. The chemical, optical and electrical properties of GO and rGO are considered to be significantly different. Rather than decreasing their value, these differences give GO and rGO distinct benefits.
What is Wastewater Treatment?
Manufacturing processes and most of the day to day activities involve the use of highly toxic and hazardous materials. Types of pollutants to be removed from wastewater are as important as the material used for the treatment process. Water contaminants can be categorized as organic and inorganic pollutants. Organic pollutants include dyes, polycyclic aromatic hydrocarbons (PAHs), pesticides, fertilizers, herbicides, phenols, hydrocarbons, biphenyls, greases, oils, proteins, carbohydrates, and pharmaceuticals. Organic pollutants can be divided into two categories owing to their biotic degradation ability. The contaminants with modest structure and hydrophilic properties can break down in the water and show severe toxicity at high concentrations. Some components such as methanol and polysaccharides are degraded by fungal or algal bacteria. Other types of organic contaminants are referred to as persistent organic pollutants (POPs). This group includes polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dichloro-diphenyl-trichloroethane (DDT). These pollutants degrade slowly and cause great anxiety owing to their persistence, toxicity, long-distance transport, and bioaccumulation ability. Most of the POPs are considered teratogenic, carcinogenic, and neurotoxic. On the other hand, inorganic materials mainly include heavy metal ions and rare earth elements. Heavy metals such as mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) are released into the water through natural or anthropogenic acti
vities. Fuel combustion, mining activities, mineral processing, manufacturing activities, and sewage discharge release a significant amount of heavy metal ions to water resources which create a variety of health and environmental problems. Rare earth elements are chemically alike elements including La, Pr Ce, Pm, Nd, Dy, Sm, Tb, Gd, Er, Tm, Ho, Yb, Sc, Lu, Eu, and Y. these elements are commonly used as a catalyst or in fertilizers. Their active chemistry and resistivity make these contaminants highly toxic to the human body and the environment.
Wastewater treatment processes aim to remove or reduce the contaminants to provide the required safe water supply. These processes are usually composed of three stages. The first stage is the mechanical stage removing solids through physical methods such as filtration and coagulation. The second stage gets rid of remaining microorganisms while the third stage removes any impurities before the water can be accepted for everyday usage. The wastewater treatment methods for the removal of organic and inorganic impurities include membrane filtration, chemical precipitation, solvent extraction, and adsorption. While all of these methods have their advantages and disadvantages, adsorption is considered to be relatively easy, low-cost, and effective. Graphene and graphene-based materials are utilized as adsorbents for the removal of organic and inorganic pollutants from wastewater.
Removal of Organic Contaminants with Graphene-Based Adsorbents
The bioaccumulation of toxic organic materials we have mentioned before cause severe problems for human health and the environment due to their toxic nature. The removal of these contaminants is affected by a number of factors including the adsorption behavior of the system and the properties of materials involved. This is why it is crucial to understand these effects. To understand the adsorption behavior, the adsorption affinities, and the interaction between graphene and organic contaminants should be assessed. Hydrophobic, π-π electron donor-acceptor, van der Waals, electrostatic, Lewis acid-base, hydrogen bonding interactions hold an important place in graphene wastewater treatment methods. Furthermore, the properties of graphene-based materials, organic contaminants, and the background solution are also extremely important for these systems.
The important physical properties of graphene-based materials include surface area, pore volume, and pore size distribution. On the other hand, the important chemical properties of these materials are surface charge and polarity, surface chemistry, and purity. Understandably, the higher surface area is advantageous for adsorptio
n systems. On the other hand, pore volume and pore size distribution of the graphene sheets should comply with the properties of the contaminant. If the pore size is too small for the particular contaminant, the size exclusion will inhibit the effective adsorption of the toxic molecules. In various studies, graphene is reported to have meso- or macro-pores while GO is reported to have meso- and micro-pores. It is important to take these differences into consideration for obtaining better wastewater treatment methods based on graphene-like materials. The surface chemistry of graphene-based materials plays a crucial role in adsorption behavior. The oxygen containing functional groups on the surface of GO show two opposing effects on the adsorption capacity. While increased surface charge increases adsorption due to improved water solubility, the water clusters formed on the material surface decrease the active adsorption sites. On the other hand, the oxygen content on GO assists the adsorption of amino and hydroxyl containing contaminants due to strong hydrogen or Lewis interactions. The purity of graphene-based materials is crucial for accurate estimation of treatment performance.
The most important organic pollutant properties include size, geometry, hydrophobicity, functional groups, and aromaticity. The hydrophobicity of organic contaminants is one of the major factors in adsorption systems. Hydrophobic or polar organic pollutants are compatible with graphene while ionic or acidic/basic pollutants are compatible with the charged surface of GO. Additionally, the number of aromatic rings and their spatial arrangement influences the adsorption of the organic pollutant. Studies show that with the increased number of aromatic rings in the molecule structure, the adsorption affinity of the pollutant increases.
Besides the properties of graphene-based materials and the organic contaminants, the properties of the background solution are also important for wastewater treatment. The most important factors affecting the adsorption are reported as pH, temperature, ionic strength, and presence of NOM. These factors affect the chemical and physical interactions between graphene-based adsorbents and organic pollutants.
GO is found to be considerably effective in the removal of dye pollutants such as methyl orange (MO), methyl green (MG), methyl blue (MB), rhodamine B, basic red 12, etc. An endothermic adsorption process is observed during the removal of dye molecules. The degree of oxidation of GO is one of the important factors affecting the adsorption. As the degree of oxidation increases the adsorption of dye molecules from the aqueous phase also increases. Graphene oxide composites are also utilized for dye adsorption to obtain suitable wastewater treatment systems. These composites might be hydrogel/GO, polymer/GO, or magnetic graphene/GO systems.
Graphene and GO are also found to be effective for the removal of pharmaceutical contaminants such as antibiotics and endocrine disrupting chemicals. The adsorption process involving GO is mainly dominated by π-π electron donor/acceptor interactions and hydrophobic interactions. Oxygen containing groups of GO assists the effective adsorption of endocrine disruptive chemicals because of the strong hydrogen bonding between these two materials. Hydrogen/GO composites are also advantageous for the removal of pharmaceutical contaminants.
Polyaromatic hydrocarbons (PAHs) are a group of organic chemicals including naphthalene, pyrene, phenanthrene, etc. The removal of these contaminants is crucial because of their toxic nature and human health. Graphene is found to be more effective in removing PAHs than GO due to its slightly larger pores. Derivatives of naphthalene such as naphthol are also suitable for adsorption processes utilizing graphene and GO.
Overall, graphene and graphene-based products are great candidates for the removal of toxic organic compounds and the improvement of these adsorption systems is the key to obtaining better wastewater solutions.
Removal of Inorganic Contaminants with Graphene-Based Adsorbents
Heavy metals and rare earth materials raise great concerns in terms of clean water supply. The persistence of toxic inorganic materials leads to accumulation in the human body and environment causing various diseases. Hence, the removal of these materials holds an important place in wastewater treatment processes. The pH of water, temperature, presence of background ions, and NOM greatly affects the elimination of these contaminants. Adsorption of inorganic contaminants is highly sensitive to changes in the pH of the environment. Since each material has a different response in varying pH conditions, wastewater treatment systems should be analyzed accordingly. In general, lower pH values are favorable for anionic contaminants while higher pH values are favorable for cationic contaminants. The presence of background ions can interfere with the activity and mobility of inorganic contaminants. Furthermore, an increase in the ionic strength of the water reduces the amount of available binding sites on graphene and GO surface. NOM molecules affect the adsorption process by changing the electrostatic properties of the wastewater. The adsorption process of inorganic contaminants is endothermic and spontaneous which makes graphene-based wastewater treatment systems desirable.
Studies show that graphene is a promising adsorbent for the removal of cobalt (Co(II)), fluoride, and iron (Fe(II)). GO is another effective adsorbent for cationic pollutants such as copper, nickel, zinc, palladium, etc. The oxygen groups on GO sheets act as anchors in the adsorption process. While some studies focused on graphene and GO sheets solely, the others show that graphene and GO-based composites and rGO also have great adsorption capacities. For the removal of inorganic contaminants the surface of graphene and GO is often decorated with different nanoparticles or oxygen containing groups to increase the adsorption affinity. Decorating graphene and GO with magnetic nanoparticles such as magnetite is suggested as an effective method for the separation of inorganic contaminants.
Rare earth metal ions are noxious and known to create severe water pollution. Various studies incorporate graphene-based adsorption systems for the removal of these contaminants. Most of these studies focus on GO due to its charged surface and oxygen containing groups. GO is found to be effective for the elimination of La, Gd, Y, and Nd from wastewater. GO nanocomposites with polyaniline (PANI), GO functionalize with magnetite and titanium phosphate are also suggested as strong candidates for the job.
Regeneration and Toxicology of Graphene-Based Adsorbents
The same properties that make graphene-based materials great players in various applications also make these materials possible threats. High chemical and biological activity of graphene-based materials may cause toxic effects on the human body and eco-system. The negative effects of graphene on the human body include lung injury, kidney failure, cell membrane, and DNA damage, etc. At the moment there is no definitive guide to handle toxicity induced by graphene-based materials. This is why the control of graphene wastewater systems and regeneration of graphene-based materials are important topics.
Regeneration of graphene-based adsorbers is not only important for avoiding toxic effects but also the sustainability of wastewater treatment systems. Any successful adsorber should show great desorption behavior in addition to adsorption properties. Regeneration is an essential part of effective wastewater treatment, reduces the cost of the treatment system, and enables the recycling of valuable industrial materials such as organic and inorganic molecules. Furthermore, Spent and regenerated adsorbents may be employed in the manufacturing of steel, cement, brick, and other construction materials. Establishing a successful adsorption-desorption cycle is a tedious process requiring fine adjustment of process parameters. Several different methods are suggested for the effective regeneration of graphene-based materials. These methods include a mixture of physical and chemical procedures including centrifugation, cross-flow filtration, field-flow fractionation, solvent treatments, and electric field. Due to the different nature of each contaminant different strategies should be considered for each wastewater treatment process. Alcohols such as ethanol, basic materials such as NaOH, and acidic materials such as HCl and HNO3 are the commonly used ingredients for the regeneration of graphene-based adsorbents.
Water is arguably the most important material on Earth crucial for the survival of living species and the continuum of daily life. Preserving scarce water resources is an important issue as the water polluting activities gain acceleration and water consumption increases with the increasing population. One of the most important aspects of water sustainability is wastewater treatment systems. Waste treatment ensures that effluent water from industrial plants and sewage lines is cleaned from toxic and harmful materials. Preventing the bioaccumulation of these materials is crucial for human health and environmental protection. Such materials are divided into two categories as organic and inorganic. Toxic organic materials include pharmaceutical contaminants, dyes, PAHs, pesticides, fertilizers, herbicides, phenols, hydrocarbons, biphenyls, greases, oils, proteins, and carbohydrates. Organic contaminants interfere with the environmental systems and functions of the human body. On the other hand, toxic inorganic materials are heavy metal ions and rare earth metal ions. These inorganic materials are highly resistant and can accumulate in plants and the human body causing various diseases. Current wastewater treatment methods attempt to eliminate these contaminants; however, they are not efficient enough for today’s requirements and require a lot of space. Thus, researchers have turned to nanotechnology for more satisfying treatment methods. In this context, graphene and graphene-based materials offer promising wastewater treatment solutions. Graphene and GO are especially at the center of attention of adsorption based water treatment systems. Their high surface area, lower cost, reusability, and chemical properties are highly desirable. Furthermore, the oxygen containing groups on the GO surface provide anchoring sites for charged materials. For the removal of both organic and inorganic contaminants, the physical and chemical properties of adsorbent material, properties of contaminants, and properties of background solution are all important. Size, geometry, hydrophobicity, functional groups, and aromaticity of toxic organic materials greatly affect the adsorption efficiency. On the other hand, the charge of heavy metal and rare earth ions affect the adsorption efficiency. The regeneration of graphene-based adsorbents is as important as the adsorption of contaminants from wastewater. Regeneration is important for the system efficiency, reusability, and recycling of valuable materials. Common regeneration methods suggested for graphene-based wastewater treatment systems are centrifugation, cross-flow filtration, field-flow fractionation, solvent treatments, and electric field. Regeneration is also important in terms of controlling the toxic effects of graphene on the environment and human health. Graphene and graphene-based materials are reported to show toxicity because of their chemical and biological activity. Hence, proper separation of these adsorbents is crucial.
Overall, graphene and graphene-based materials show great potential as adsorbents and can be applied to develop better wastewater treatment systems. It is important to note that, these systems still require further improvement and optimization for widespread applications.
The Lotus effect means the self-cleaning properties of the lotus flower that come out as a result of ultra-hydrophobicity. In this method, all the dirt particles are extracted out by water droplets because of the miniature work done on the surface as a result of which the adhesion of droplets is minimized.
However, these properties are not only found in this flower but a few other flowers as well. They have a huge impact on the factors in which they play a role despite being at a certain level. The importance and applications of the lotus effect in the field of nanotechnology are quite huge and are drastically increasing over time. Their basic function is to cleanse the surface and make it water free so that it can be prevented from any wetness in the future too. The entire lotus effect is created by the layer of wax that is present on the lotus leaf as that layer of the wax exhibits the water-repelling properties and further activates the whole process of continuing it to a larger scale. This is the origin of the lotus effect. When researchers saw the adaptability of this effect, they further classified it so that it can be utilized in a better way and on a larger scale.
he lotus flower or leaves of Nelumbo gives an effect known as the lotus effect, which has self-cleaning characteristics due to the ultra-hydrophobicity, which the leaves display. Water droplets pick up the particles of dirt because of the surface’s nano and microscopic architecture. That architecture helps in reducing the adhesion of the droplet to that surface. In certain insect’s wings and some plants like cane, Alchemilla, Opuntia (prickly pear), and Tropaeolum (nasturtium) also have characteristics of self-cleaning and ultra-hydrophobicity. Wilhelm Barthlott and Ehler studied ultra-hydrophobic micro-nano structured surface’s self-cleaning characteristics in 1977 and named those characteristics as the ‘lotus effect’. In 1986, Brown developed perfluoropolyether and perfluoroalkyl ultra hydrophobic materials to handle biological and chemical fluids.
In 1964, Dettre and Johnson used rough hydrophobic surfaces to study the ultra hydrophobicity phenomenon. A theoretical model was developed because of their work in which PTFE telomere or paraffin coated the glass beads, which were used in different procedures. Since 1990, many different biotechnical applications have risen.
Water’s high surface tension results in droplets turn into an almost spherical shape. As minimal surface area is contained by the sphere, this shape reduces the solid-liquid surface energy. When liquid contacts with the surface, the surface gets wet due to the adhesion forces. The droplet’s fluid tension and surface’s structure determines whether the wetting is incomplete or complete. The surface’s hydrophobic water-repellent double structure is the reason for self-cleaning characteristics. The process of self-cleaning results when the adhesion force and contact area between the droplet and surface are majorly lessened. The covering waxes and characteristic epidermis (cuticle as the outermost layer) forms the hierarchical double structure. Papillar of 10 to 15 μm width and 10-20 μm height is possessed by the lotus plant’s epidermis. Epicuticular waxes are imposed on the width of papillae. Being hydrophobic, these superimposed waxes created the double structure’s second layer. Regeneration of this system is common. For the surface’s water repellency functioning, the responsible party is this biochemical characteristic.
Measurement of hydrophobicity
The contact angle of the surface can measure the surface’s hydrophobicity. If the contact angle is high, the hydrophobicity will be high too, and vice versa. If the surface has a contact angle greater than 90°, it is hydrophobic and if the contact angle of the surface is less than 90°, it is hydrophilic. The surface of the plants, having a contact angle of 160°, is known as ultra hydrophobic. Ultra hydrophobic means that only 2-3 percent of the droplet’s surface is in contact, which is the typical size. If the plant has a double structure surface, for instance, the lotus, then a 170° contact angle can be reached, however then in that case, 0.6 percent is the contact area of the droplet. All of this results in a self-cleaning effect.
Picking up of dirt particles
Water droplets pick up the particles of dirt, which have very less contact area. The particles are therefore removed from the surface easily. If the surface is contaminated, then the adhesion between the rolling water droplet and the dirt particle (doesn’t matter what chemistry) is much high as compared to the adhesion between the surface and the particle. On general materials like stainless steel, this cleaning effect has been displayed when a superhydrophobic surface is formed. The self-cleaning effect is not for the organic solvents as it depends on the water’s high surface tension. Thus, against graffiti, the surface’s hydrophobicity provides no protection. Acting as a defense against pathogens like the growth of algae or fungi, this effect holds major importance for animals, insects, and plants which can’t clean themselves properly. Self-cleaning has other positive effects too, like preventing the plant surface’s area from contamination which is exposed to light and leads to lessened photosynthesis.
The ultra hydrophobic surfaces having a property of self-cleaning is due to the chemical-physical characteristics at the nanoscopic to microscopic scale instead of just the leaf surface’s particular chemical characteristics. The usage of this effect on the surfaces that are made by men arose this possibility. Usage was supposed to be done by mimicking nature in a way that would be general instead of specific.
Fabrics, roof tiles, paints, coatings, treatments, and other surfaces are developed by some nanotechnologists which can stay clean and dry themselves by replicating like the lotus plant does as it has the self-cleaning characteristics. If the composition contains micro-scale particulates, then by using them with special silicone or fluorochemical treatments on the structured surfaces, this can be attained. For producing the lotus effect, other than chemical surface treatments, femtosecond pulse lasers sculpt the metals as the other treatments remove as time passes. At any angle, the black color of the material remains, so combining this with the self-cleaning characteristics will result in a low maintenance solar thermal energy collector. Metal’s high durability is capable of being utilized in the self-cleaning latrines for lessening the transmission of disease.
Ice and snow buildup and rain fade can be majorly reduced by the super-hydrophobic coatings when they are implemented to microwave antennas. Patterned ultra hydrophobic surfaces can significantly enhance bioanalysis that’s based on the surface. For water’s funneling to a basin to be used in irrigation and in drew harvesting, the hydrophobic and superhydrophobic characteristics have been utilized.
In graphic mode, the lotus effect is seen as a drop that instead of being absorbed, stays on the surface. The Lotus effect refers to self-cleaning and water repellency. Its origin is from a ‘lotus flower’. It is a mystery for scientists as no one has explained why that flower doesn’t get wet. The lotus flower’s hydrophobic property has been copied for many beneficial effects on human life. It is nanotechnology research’s main task.
Reproduction of lotus effect
On different surfaces like metal, glass, plastic, wood, and stones, the lotus effect has been reproduced through technological innovation’s chemical products. In personal, domestic, and business sectors, these achievements have had a very good effect as they contributed to enhancing the appearance of various materials, extending their lifetime, and helping in avoiding the continuous investment in cleaning and maintenance.
For instance, in windshield and body, a car can be very well protected, durability can be extended, and on rainy days, fog can be avoided. In textiles, liquid repellency is very beneficial as it lightens incidents like wine spillage and prevents stains. Because of its various advantages, a great impact is made by Lotus effects. Now the product’s commercialization has been globally spread, leading to nanotechnology becoming one of the most powerful markets. A broad quantity of products is offered by Nanografi that are related to nanotechnology in this nanotechnology’s preponderant rise.
Importance of the Lotus Effect
Lotus Effect has a lot of importance when it comes to creating self-cleaning materials. So much importance that the biologists, Barthlott and Neinhuis, gave the idea under the name ‘Lotus-Effect’. From that time, many physicists and botanists studied Lotus Effect to understand it better and to find technological applications that are possible. Superhydrophobicity is a physical characteristic whereas hydrophobicity (water repulsion) is a chemical characteristic. Both characteristics can be differentiated by the contact angle between the water and the surface. Hydrophobic characteristics are obtained if the contact angle is between 150 and 90 degrees whereas superhydrophobic characteristics are obtained when the angle of contact is more than 150 degrees, as it causes the effect to amplify and results in being impossible to wet surface.
Nanotechnology and lotus effect
Nanotechnology is the theory that hypothesizes the option of controlling the matter’s structure by working on the types of connections that is between the atoms. Therefore, when looking for solutions by studying the structure of matter, nanotechnology can be used.For measuring atoms and molecules, the nanometer scale is used. -1 nanometer is related to a meter’s billionth.
Application in nanotechnology
Nanotechnology is used in different fields, ranging from the field of textiles to optics, from the field of electronics to construction, and from robotics to medicine. By observing nature and its remarkable microscopic inventions, the idea came of making mirrors based on the refractive index’s periodic modulation for optical telecommunications applications through the study of all pigments is born, or the idea of replicating Gecko leg tissue’s morphology to create materials which permit perfect adherence to the surfaces, or the idea to create materials that self-clean and copy the Lotus plant surface’s microscopic morphology.
Nanotechnology can be brought into our homes through the products of Nanotechnology. For instance, nanotechnology products, that are based on silicon, utilizes nanoparticles of silicon that forms bonds of molecular-level in the silicon-based substrates like stone, ceramics, and glass. This modifies the substrate’s surface structure for forming a breathable and uniform active barrier, therefore making it Hydro-Oil Repellent. As a specific structure of the molecule is possessed by each product, there are also specific nanotechnological products for each surface which binds to the support’s structure at the level of the molecule, forming an effect that preserves it from the ease and wear of cleaning and protects it against the action of atmospheric agents, oil, and water. That effect is an invisible barrier effect.
Products with molecular activity
With solutions with high technological value and molecular activity, nanotechnology products are proposed by Lotus Effect, as lubricants and degreasers for cars, motorcycles, bicycles, and professional technical cloths, and in the sector of the maintenance, cleaning, and protection of external and internal supports like cement, stone, tiles, bricks, fabric, wood, ceramic, stainless steel, and glass. Therefore, because of the material’s high repellency for water, like the flower plant’s leaves, the lotus effect is a precise capacity for self-cleaning.
Researchers and scientists were inspired by the research on Lotus leaf, it resulted in the production and design of some very useful and beneficial substance in various technological applications, ranging from the field of transport to the field of biomedicine. Reproducing the lotus effect in floors, fabrics, paints, tiles, and other surfaces have been naturally tried. In conclusion, like Albert Einstein, we have to be more observant about our surroundings and environment and allow ourselves to be amazed and inspired to understand it.
Lotus effect via an example of SiO2 is explained further and a few other hydrophobic materials as well. These examples explain the lotus effect in-depth and give us an overview of the importance of the lotus effect.
Example of SiO2
In academia and industry, an attractive interest is created by hydrophobic coatings because of their remarkable self-cleaning characteristics due to their property of repelling water. By roughening the low surface energy materials, the highly hydrophobic surfaces have been made, based on the lotus leaf effect and other natural phenomena. Its surface free energy is determined by the coating’s chemical composition. One of the strong organic bonds is the C-F bond. One of the effective ways of lessening surface free energy on the molecular structure’s architecture and enhancing water impermeability is introducing the fluorinated component in the coating. This addition will improve the coating’s hydrophobic characteristics due to the C-F bond’s low polarizability and small dipole, together with the large free volume. For improving the roughness of the surface, including nanoparticles deposited, chemical vapor deposition, electrochemical deposition, plasma etching, etc., numerous methods have been implemented. Also, corrosion resistance is increased when the nanoparticles are added, for instance, nanoparticles of ZnO, nanoparticles of SiO2, and nanoparticles of Ti.
Effect of the SiO2 particle sizes on the water contact angles of coatings
The SiO2 particle sizes influence on the coating’s water contact angles are explained, with a constant of 1 wt % SiO2 nanoparticles. The coating’s water contact angles are affected when SiO2 is added with various sizes of particles. 62.8 is the minimum coating angle whereas 85.6 is the largest contact angle. Mainly, two factors determine the paint film’s water resistance: the paint film’s solid surface energy and structure. Different structural scales are made by SiO2 of different sizes.
Effect of the contents of SiO2 nanoparticles on the water contact angles of coatings
Scientists investigated the effect of 18 nm SiO2 (S103583) contents on different coatings’ water contact angles. When SiO2 content increases from 0.5 to 2.o wt%, the coating’s water contact angles also increases gradually. Particularly, when SiO2 nanoparticles of 1,5 wt% and 2.0 wt% are introduced, they advance the composite coating’s hydrophobicity significantly, therefore, inducing the coating’s water contact angles higher than 90°.
Although, a lot of addition of SiO2 results in the decrease of the water contact angles as some of the paint film’s outer edge was exposed to some of SiO2. The paint film’s hydrophobicity decreases because of the exposed hydrophilic SiO2. When 1.5 wt% is the amount of SiO2 content, the emulsion’s viscosity increases, and a taste is presented. When SiO2 weight content is 2.0 wt%, the hydrophobic effect’s benefits are not that obvious in comparison to when the weight content is 1.5 wt%. 1.5 wt% is used as appropriate content for taking the hydrophobic effect and coating process into consideration.
Waterborne fluorine-containing epoxy coatings
SiO2 nanoparticles-modified waterborne fluorine-containing epoxy coatings exhibited a higher contact angle. Coating’s corrosion resistance was enhanced on SiO2 addition and it also made coating display great thermodynamic stability.
Super-hydrophobic diatomaceous earth
Diatom’s microscopic skeletal remains are contained by the Diatomaceous earth. The skeleton of diatoms is made up of hydrated silicon dioxide. It has both micro and nano-porosity and a nano-roughness that greatly increase either its water absorption or its water repellency, based on its surface chemistry. Naturally, it is super hydrophilic because the chemistry of its surface is naturally hydrophilic. But it can become superhydrophobic if the DE is treated with a hydrophobic silane due to its texture’s amplification effect and nano-porosity and its hydrophobic surface chemistry. This superhydrophobic nano-porous powder is called superhydrophobic diatomaceous earth, or simply SHDE. A 160 to 170-degree contact angle can be possessed by the surface which consists of SHDE powder. The angle can be as high as 175° too.
When hydrophobic silane completely functionalizes the particles, they become superhydrophobic water doesn’t wet them. But if only partially functionalization occurs, then each of the particles can attain both of the behaviors, super hydrophilic and superhydrophobic. Water marbles are made when the drops of water interact with these particles.
At Oak Ridge National Laboratory, the researchers said that resin marbles can be formed when the superhydrophobic nano-textured silica (for instance, SHDE) interacts with the molten powder-coat resins. The electrostatic process is used for coating surfaces with the dry polymer resins in the powder-coat process. Once the charged resin particles sufficiently cover an electrically grounded surface, the coated surface is placed in an oven where the particles of resin become molten and make a cured and uniformly coated powder resin surface.
The process of the electrostatic application won’t be adversely affected by a blend of powder-coat dry resins and SHDE, since the charging of SHDE happens the same as the charging of dry resin particles does. The SHDE interaction with molten resin functions like the water marble forming process during the curing process. Resin is wet partially by SHDE while the molten resin is repelled by most of the SHDE. Resin marbles are completely formed if the SHDE proportion to resin is huge enough (for instance, 25% SHDE to resin), then without joining other molten resins, the molten resin cures.
Result of the resin marbles
As a result, a surface is formed which contains such cured resin marbles that are completely unbound to the substrate or the other resin marbles. If SHDE proportion to resin decreases to 5% then most of the molten resins will flow but a uniform coating won’t be formed because of the SHDE’s resin-phobic/philic nature.
An extremely porous interconnected surface with micro-porosity can be formed by the blended powder-coat resins with SHDE because of the resin marble’s restricted interaction. The coating’s entire volume got micro-porosity extended all over it. Moreover, the outer surfaces of the micro-pore are coated with SHDE, forming a surface nano-porosity and super hydrophobic effect all over the coating’s entire volume.
Thus, it is evident that the literal meaning of something can be more clearly defined via certain examples. So is the case in this article, the lotus effect has been further well explained by the example of SiO2 and some other hydrophobic materials to put a little more emphasis on the importance of the lotus effect and how it is certainly helping build the industry. As evident through certain examples and experiments, the results and outcomes of the lotus effect are quite striking. That is why researchers took this opportunity and formulated advancements so that the industries can take benefit from this whole phenomenon.
raphene particles are by far the best-known particles that are serving as one of the best catalyzers in the industry. When boron is doped with graphene by going through different processes it becomes and forms boron-doped graphene particles which serve various purposes in the industry.
The characteristics and properties which are exhibited by boron-doped graphene particles are highly rich in their content and are capable of exhibiting a lot of valuable processes. Technological applications have only increased over time and enhanced the entire purpose of forming these boron-doped graphene particles.
Graphene-based nanomaterial, boron-doped graphene (BG), is a carbon atom’s single sheet organized in a hexagonal lattice. When the boron atom impurities are added into pure graphene, the bandgap opens, redox reactions accelerate, catalytic ability enhances, and the activation region on its surface increases. All of these alterations give various applications to it in sensors, ultracapacitors, semiconductor devices, fuel cell chemistry, and other technologies.
Using electron-withdrawing (boron) or electron-donating groups (phosphorus, nitrogen), to dope graphene is significant to change graphene’s electrical characteristics. Graphene sheet’s electron density changes when they are doped with electron-withdrawing (electron acceptors, p-type) or electron-donating (n-type, electron donors) groups and thus also affects graphene sheet’s electrochemical characteristic. Doping level plays a very important part as carrier density is very significant to tune the material’s performance.
One of the other major problems is manufacturing doped materials for synthesizing scalable methods to doped material’s large quantities. In this article, a technique is showed for the scalable formation and tunable doping levels by graphene oxide’s exfoliation at various temperatures in the BF3 atmosphere. There are also investigations on p-doped material’s electrochemical characteristics and their comparison with the ones that the literature presents.
Nitrogen and boron-doped graphene
Unusual electrocatalytic effects are displayed by nitrogen-doped graphenes toward oxygen and H2O2 reduction which have extremely significant usages for applications in fuel cells and biosensing. In comparison with graphene, large capacitance is possessed by nitrogen-doped graphenes. Similar effects were displayed by boron-doped graphene, being increased capacitance and electrocatalytic toward oxygen reduction. According to observations, it doesn’t matter which atoms are doped in graphene as these characteristics are improved. Compound oxidation is easy with increased doping of materials with electron-deficient components, whereas reduction turns difficult.
These observations are contrary to the old reports on the boron-doped graphene that catalyzes the oxygen’s electrochemical reduction. Cyclic voltammetry, prompt γ-ray analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy, are used to characterize it in detail.
Graphene oxide’s thermal exfoliation was done to manufacture boron-doped graphenes in an atmosphere at 1000 °C in N2, 800 °C in N2, and with boron trifluoride diethyl etherate in N2/H2 at 1000 °C. Graphene oxide goes through many stages, exfoliation, deoxygenation (manufacturing CO2, CO, H2O, organic molecules), and then its simultaneous doping with boron. Cyclic voltammetry (CV), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and prompt γ-ray analysis (PGAA), are used to characterize resulting boron-doped graphenes for studying the electrocatalytic characteristics of it towards oxygen reduction and its capacitance. Here, there are all abbreviations of all boron-doped graphenes as B-G.
Thermal shock treatment
There were investigations on the Graphene oxide’s thermal shock treatment in the BF3 atmosphere and how it results in Graphene oxide’s exfoliation. SEM studied boron-doped graphene’s (B-G) morphology. SEM images are shown of boron-doped graphenes and those graphenes are made by exfoliation. A typical exfoliated structure is shown by all of these materials just like how it was shown in previous studies and this structure also assures graphite oxide’s successful thermal exfoliation in the BF3 atmosphere. Exfoliation temperature’s effect was consequently studied upon the amount of graphene used to dope boron through PGAA. (prompt γ-ray analysis)
Trace levels of elements
One of the best methods to determine the element’s trace level is PGAA. Boron’s absolute content in samples was determined comparatively to the liquid standard of H3BO3 with the known concentration of boron. In N2/H2 atmosphere, graphite oxide’s exfoliation at 1000 °C resulted in graphene’s doping at 23 ppm boron levels, whereas boron’s larger amount was introduced on graphite oxide’s exfoliation in the atmosphere of nitrogen, 590 ppm at 1000 °C, and 140 ppm boron at 800 °C.
According to the result, graphene can be doped with boron atoms of a higher amount if the exfoliation temperature was higher. A major decrease was observed in the concentration of boron for GO exfoliated in the atmosphere with hydrogen. Thus, boron’s successful doping into the graphene sheets is confirmed by this result.
Characterization of density
Raman spectroscopy is used by us for characterizing the defect density in boron-doped graphene by determining the doping level that boron did on Graphene. Defects are present and indicated by the D band at almost 1350 cm−1 because of the sp3 -hybridized carbon atoms. A G band is shown by the graphene sheet’s pristine sp2 lattice carbon atoms at almost 1560 cm−1. The carbon structure’s degree of the disorder can be indicated through the usage of the ratio between D and G band intensities (ID/IG). 0.732, 0.632, and 0.903 are the ID/IG ratio for 590 ppm B-G, 140 ppm B-G, and 23 ppm B-G, respectively.
Sizes of crystallites
D- and G- band intensities can be used to estimate various material’s average crystallite sizes (La) by applying this equation, 1.20 =× × × λ − L II 2.4 10 / a 10 laser 4 G D. In this equation, ID and IG are Raman D and G band’s intensities, and the excitation laser’s wavelength is denoted by λlaser in nanometers. The crystallite sizes in graphenes that are doped with boron are then calculated. La is 23.0 nm of 590 ppm B-G, 26.6 nm of 140 ppm B-G, and 18.6 nm of 23 ppm B-G. At 2700 and 1620 cm-1, both the 2D band and D’ band are displayed by all three boron-doped graphenes indicating disorder of low degree in their carbon structure.
XPS was used to consequently determine boron-doped graphene’s elemental compositions as it is a chemical analysis method and is beneficial for determining the bonding arrangement and the elemental composition. The obtained high-resolution XPS of C 1s and wide scan of C 1s for three graphenes referred to the fact that there was a removal of most of the oxygen-containing groups and the preparation of the thermally reduced graphenes was successfully done.
According to wide scan XPS, for 590 ppm B-G, the C/O ratio is 16.1, for 140 ppm B-G, the C/O ratio is 8.9, and for 23 ppm B-G, 14.2 is the C/O ratio. Huge insight is given of the residual oxygen-containing group’s chemical composition by high-resolution XPS of the C 1s signal. They exhibit different and various energy levels. When made at 800 °C in N2, 1000 °C in N2/H2, and 1000 °C in N2, boron-doped graphenes possess C-C bonds, C−C bonds, C−O bonds, CO bonds, and O−CO bonds, respectively. XPS can determine the residual oxygen-containing groups in the reduced graphene. However, trace amounts of boron can’t be measured by XPS as it is not sensitive enough.
Combustible elemental analysis
Boron-doped graphene’s elemental composition can be determined by using combustible elemental analysis as it is very beneficial for measuring the accurate content of combustible elements present in the bulk material. Further, more insight regarding the bonding character and composition of graphene sheets is provided when it is combined with XPS. 5.54 atom % of O, 9.63 atom % of H, and 84.83 atom % of C, was contained by 590 ppm B-G. 7.69 atom % of O, 10.03 atom % of H, and 82.28 atom % of C were contained by 140 ppm B-G. 7.48 atom % of O, 10.04 atom % of H, and 82.48 atom % of C were contained by 23 ppm B-G. Moreover, there was no nitrogen in any of the samples.
Four-point measurements of compressed tablets studied resulting boron-doped material’s electrical resistivity. 13 ppm B-G, 140 ppm B-G, and 590 ppm B-G, has the electrical resistivity of 9.5 × 10−5 Ωcm, 2.1 × 10−5 Ω cm, and 2.3 × 10−4 Ωcm. According to these results, the electrical resistivity is increased by boron’s higher content which is in accordance with boron’s electron acceptor characteristics. There was an investigation on the boron-doped graphene’s electrocatalytic activity toward oxygen reduction reaction (ORR). In both N2 saturated 0.1 M KOH aqueous solution and air, cyclic voltammograms were recorded at 3 of the boron-doped surfaces. A major reduction peak is observed. There was no electrochemical reduction in N2- saturated KOH solution, and the reduction peak for air-saturated KOH solution was ∼−460 mV.
Oxygen’s reduction originates ∼−460 mV reduction peak. Reduction of oxygen with different levels of doping was recorded by Cyclic voltammograms. For Graphenes with boron dopant’s increasing concentrations, reduction peak shifts to increasingly negative potentials. Thus, for boron-doped graphene, the electrocatalytic activity towards the reduction of oxygen would be lower if the doped boron atoms are more, and vice versa. This is in agreement with hole-doping electrochemistry’s general concept but also in contrast to the past observations.
Weight specific capacitance
Cyclic voltammetry capacitive charge-discharge current was used to determine the boron-doped graphene’s weight-specific capacitance. ν C = I (2), is used to express the capacitance. In this equation, ν is the scan rate (mV/s, can be expressed as dE/dt), I is current (A), and C is capacitance (F). There were investigations on the capacitances of boron-doped graphenes. The investigations were being done by measuring the current With scan rates of 400, 200, 100, 75, 50, and 25 mV/s and at 0.15 V of applied potential. Boron-doped graphene’s weight-specific capacitances are 10.3, 10.7, and 11.7 at 1000 °C in N2, 800 °C in N2, and at 1000 °C in N2/H2. These weight-specific capacitances refer to the fact that when the amount of doped boron atoms increases, the weight-specific capacitance decreases.
In comparison to non-doped parts
As compared to its non-doped counterparts, N-doped graphenes and B-doped graphenes exhibit higher capacitance contrary to the previous claims. No theoretical background explanation is present here whereas, in both hole and electron doping cases, improved capacitance should be expected. In Han et al.’s paper, the utilized method was based on a famous hydroboration reaction from organic chemistry. With the BH2 group being covalently bonded to the backbone of graphene, the production of sp3-hybridized carbon can be imagined from such a reaction’s mechanism. Attaining carbon’s direct substitution with boron through the methods that Han et al. presented, is impossible.
Surface plasmon resonance
Optical sensors, surface plasmon resonance (SPR) biosensors, gained a lot of attention because of their benefits like non-invasive measurements, room temperature real-time measurements, and high sensitivity. At the metal-dielectric interface, SPR is very sensitive to refractive index alteration. Because of the analyte’s presence, sensitivity in detection is exploited if there is any kind of change at the interface. Typically in SPR biosensors, gold is utilized as a resonant layer because of its chemical stability, where a higher shift of resonant angle with a change in refractive index is exhibited by gold. Although, biomolecule’s adsorption or detection at low concentrations is extremely difficult or even impossible at times because of gold’s inertness.
Single crystalline gold surface
At increased temperatures and pressures or under high vacuumed (UHV) conditions, the single-crystalline gold surface doesn’t specifically measure small molecules adsorption. Such molecules are NH3, NO, O2, H2, and CO. There have been investigations of the physically and chemically modified gold layer in various studies regarding improved detection and adsorption mechanisms, like CNTs, gold nanocrystal, kinked/stepped gold surface gold nanorods, and graphene oxide coupled with gold nanoparticles.
There have been investigations on some of the novel materials like MOS2, Au/Ag/Au/chitosan-graphene oxide (RGO), and graphene, for improving SPR sensor’s selectivity and sensitivity. In all of these novel materials, the most promising material is graphene because of its excellent and remarkable characteristics like high surface-to-volume ratio and high mobility that are advantageous for biomolecule’s effective adsorption in comparison with gold. An improved SPR response is displayed by the doped graphene oxide (GO) that is enriched on the gold layer’s top with abundant defect sites. Although, chemical doping causes the modulation of the optical bandgap and GO’s Fermi energy which leads to alteration in the optical characteristics like dielectric constants and refractive index. SPR sensing techniques can monitor these variations.
Enhancements in Graphene’s characteristics
According to recent literature, huge enhancements have been seen in the characteristics of graphene by doping graphene with various heteroatoms like Boron (B), Nitrogen (N), Oxygen (O)< and Fluorine (F). In numerous applications like improved photogenerated catalysis, solar cells, Li-ion batteries, and supercapacitors, there have been reports of the boron-doped graphene oxide. Also, numerous methods like chemical vapor deposition (CVD), arc-discharge process, and hydrothermal method have been used to dope graphene-based materials, where for tuning graphene’s electronic and optical characteristics, stable atomic substitution with carbon is extremely required. At high temperatures, GO is mostly lessened thermally among the reduction methods. Although, high-density defects are produced by reduction at high temperatures on the basal planes and edge, deteriorating graphene’s optical and electrical characteristics.
In post-treatments, there were decorations of each of the as-synthesized materials with platinum catalyst nanoparticles of 60 wt % that were made in the already reported procedure by utilizing a metal-organic Pt (acac) 2 precursor. The powdered support materials were briefly mixed uniformly in a glass vial of 4.5 mL with Pt(acac)2 in suitable ratios for yielding 60 wt% Pt. The mixture was heated under a low vacuum of 600 Torr for 16 hours at 210 C in an N2-H2O atmosphere, resulting in Pt metal nanoparticle’s deposition onto the support.
An anode-supported configuration was used to fabricate solid acid fuel cells. Mesh discs of 2.85 cm2 of Sintered 304 stainless steel functioned as the anode current collectors. There have been some previous reports on the architecture of the cell and after this, it was confirmed as the architecture of this cell was completely identical to the one in previous reports. CDP’s mechanical mixture formed identical anodes with 1.0 mg/cm2 (Pt) Pt loading. For all of the tested cells, those identical anodes with a Pt loading of 1.0 mg/cm2 (Pt), inhouse synthesized 60 wt% Pt supported on naphthalene (a fugitive binder), and carbon black (Cabot Crop., Vulcan XC-72R), by weight, in a 3:1:1 ratio.
A CDP membrane was possessed by each cell. The membrane’s thickness was 50 μm, and it was laminated at 125 MPa to the anode. Experimental cathodes of 25 mg of a 3:1 by CDP’s weight mixture and 60 wt% of Pt supported on CB9Ar, CH5Ar, or SWCNHs were implemented at 30 MPa pressure to the dense CDP membrane. 1.3 mg/cm2 was the experimental cathode’s areal Pt loading.
At 250 C temperature, ultrahigh purity H2 and the air was used to conduct cell testing by utilizing stainless steel test rigs, each at 75 C dew point. Bio-Logic VSP potentiostat was used to record polarization curves from open circuit potential to 0 V by scanning the cell potential at 10 mV s-1. At 0.8 V, each cell faced a multi-step cyclic testing protocol which included potentiostatic electrochemical impedance spectroscopy (EIS) spectrum for attaining the average high-frequency area-specific resistance (ASR), polarization curve’s recording, at 0.6 V a 30-minute potentiostatic hold, and then this protocol repeated. At 0.6 V, the cells were held after 30 cycles.
The polarization curves were corrected by using ASR for effects of membrane ohmic resistance, yielding iR-free polarization curves. For the formulation of each cathode, the results were averaged across multiple cells comprising two replicates at least.
There were performances of electron energy-loss spectroscopy (EELS) and transmission electron microscopy (TEM) imaging in a Zeiss Libra 200MC (0.1 nm is the information limit). ImageJ software was used to make an estimate of the dimensions of the nanoparticle from the TEM micrographs for calculating their cross-sectional areas and for tracing their parameters. There were derivations of the same area’s effective nanoparticle diameters. There have been many reports on the standard deviations and mean dimensions.
Sizes of the flake were measured by using a similar procedure, but by utilizing the square approximation. The concentration of the boron was determined by using EELS, and the background spectra were fit by utilizing the combination of a polynomial model and a power law. A polynomial model is, E−r + aE2 + bE + c, where E is energy, whereas r, c, b, and a, are fitting parameters. An In-lens detector was used to perform scanning electron microscopy (SEM) imaging at 3kV with a Zeiss Auriga 40. Pore size analyzer and Quantachrome Micromeritics surface area were used to perform BET surface area analyses. At 10 C/min ramp rate in air, Q50 from TA instruments was used to perform thermogravimetric Analysis (TGA).
Moreover, a continuous scan was done to carry out X-ray diffraction (XRD) measurements on a two-circle high-resolution X-ray powder diffractometer through the usage of Cu-Kα1 radiation (1.54059Å) with ½° fixed divergence slit.
A custom microscope-based system was used to perform Raman spectroscopy at 532 nm of excitation wavelength. The laser beam was focused by using a 100X objective with ~1 μm spot size on the sample, and ~0.5 mW was the measured power of the laser at the sample location. Spectral peak fitting was applied along with Gaussian/Lorentzian line shapes.
A lot of detailed processes are carried out to form these boron-doped graphene particles which are not only serving in the technological industries but are also paving way for the upcoming technologies which can be buckled up by the use of boron-doped graphene particles. Hence, these are excellent means of promoting technology and technology-based industries.