CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional application entitled, "Slow-Release Fertilizer with Graphene Oxide Films" having serial number 61/899,350, filed on November 4, 2013, which is entirely incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No. CHE-1213333 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

In order to sustain crop yields, growers apply fertilizers to soils to provide plants with essential nutrients. Conservative estimates attribute 30 to 50% of crop yields to natural or synthetic commercial fertilizers. As modern agriculture relies increasingly on non-renewable fertilizer resources, future related minerals are likely to yield lower quality at higher prices. Some of the nutrients in non-renewable fertilizers are not absorbed by plants and, therefore, leach into groundwater or surface water, lead to eutrophication, and can impose great risk to the ecosystem. To increase the efficiency of fertilizer use, mounting research has been directed towards developing new technologies for delivering plant nutrients in a slow- or controlled-manner in the water or soil.

Past efforts for developing slow- or controlled-released fertilizers focused on employing polymers that were already used in coating various fertilizers. However, these coating techniques typically require either organic solvents or toxic polymerization initiators, or time-consuming complicated processes, which not only increase the costs of production, but also lead to environmental and health issues.

SUMMARY

Briefly described, embodiments of the present disclosure provide timed-release fertilizer compositions and methods of making timed-release fertilizer compositions.

In embodiments, the present disclosure provides a slow-release fertilizer composition including fertilizer particles coated with graphene/graphene oxide/reduced-graphene oxide thin films. In embodiments, the slow-release fertilizer composition includes fertilizer particles and a reduced-graphene oxide layer disposed on the surface of the particles. The present disclosure also provides methods for making slow-release fertilizer compositions of the present disclosure. In embodiments, methods of making slow-release fertilizer compositions of the present disclosure include providing a plurality of fertilizer particles including at least one nutrient in salt form, where the salt is capable of reducing graphene oxide; forming one or more layers of graphene oxide on the fertilizer particle such that the fertilizer particle is at least partially coated with graphene oxide; and heating the graphene oxide-coated fertilizer particles to form a coating of reduced-graphene oxide on the particles.

The present disclosure further provides a slow-release fertilizer composition made by the methods of the present disclosure. In embodiments, the slow-release fertilizer is made by the following steps: providing a plurality of fertilizer particles including at least one nutrient in salt form, where the salt is capable of reducing graphene oxide; forming one or more layers of graphene oxide on the fertilizer particle such that the fertilizer particle is at least partially coated with graphene oxide; and heating the graphene oxide-coated fertilizer particles to form a coating of reduced-graphene oxide on the particles.

Other methods, compositions, plants, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional compositions, methods, features, and advantages be included within this description, and be within the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a schematic illustration of preparation of an embodiment of a fertilizer composition of the present disclosure including re-GO-coated KN0 ₃ fertilizer particles. The figure includes images of various stages of preparation.

FIGS. 2A and 2B illustrate AFM analysis of GO sheets on a mica substrate. FIG. 2A is an AFM image of the GO sheets, and FIG. 2B is a sectional analysis of the AFM image in FIG. 2A along the white line (AFM channel). The cross in the figure helps to show the height of the graphene sheet.

FIGS. 3A-3C illustrate GO before and after heat treatment with KN0 ₃ . FIG. 3A is a graph of the Raman spectra of GO before and after heat treatment with KN0 ₃ ; FIG. 3B is a C1 s XPS spectra of GO before heat treatment with KN0 ₃ ; and FIG. 3C is a graph of C1 s XPS spectra of GO after heat treatment with KN0 ₃ . FIGS. 4A-4D illustrate SEM-EDX analysis of : re-GO-coated KN0 ₃ (FIG. 4A (15.0kV, X20, WD 12.4 mm, scale bar 1 mm)), observation of shell section (FIGS. 4B (15.0kV, X1 1 ,000, WD12.3 mm, scale bar 1 μπι) and 4C(15.0kV, X300, WD 12.9 mm, scale bar Ι Ομηι), and the EDX spectrum of the re-GO-coated KN0 ₃ (FIG. 4D).

FIGS. 5A and 5B illustrate a TEM image of a re-GO sheet taken from re-GO-coated KN0 ₃ (FIG. 5A), and a selected area electron diffraction (SAED) pattern of the re-GO sheet (FIG. 5B). The insert in FIG. 5A is an HR-lattice image of re-GO.

FIG. 6A is a graph illustrating the slow-release of potassium ions from re-GO-coated KNO ₃ and pure KN0 ₃ particles of the present disclosure. FIG. 6B is a digital image showing the re-GO-coated KN0 ₃ particles before (left) and after (right) soaking in water for about 8 hours.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, and as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

Publications and patents cited in this specification are incorporated by reference where specifically indicated. If incorporated by reference, such publications and patents so incorporated are incorporated to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present disclosure is not entitled to antedate such publication by virtue of prior disclosure. Further, the dates of publication provided could be different from the actual publication dates that may need to be independently confirmed.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of molecular biology, microbiology, nanotechnology, organic chemistry, biochemistry, botany and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

It must be noted that, as used in the specification and the appended embodiments, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a support" includes a plurality of supports. In this specification and in the embodiments that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

As used herein, the following terms have the meanings ascribed to them unless specified otherwise. In this disclosure, "consisting essentially of" or "consists essentially" or the like, when applied to methods and compositions encompassed by the present disclosure refers to compositions like those disclosed herein, but which may contain additional structural groups, composition components or method steps (or analogs or derivatives thereof as discussed above). Such additional structural groups, composition components or method steps, etc., however, do not materially affect the basic and novel characteristic(s) of the compositions or methods, compared to those of the corresponding compositions or methods disclosed herein. "Consisting essentially of" or "consists essentially" or the like, when applied to methods and compositions encompassed by the present disclosure have the meaning ascribed in U.S. Patent law and the term is open-ended, allowing for the presence of more than that which is recited so long as basic or novel characteristics of that which is recited is not changed by the presence of more than that which is recited, but excludes prior art embodiments.

Prior to describing the various embodiments, the following definitions are provided and should be used unless otherwise indicated. Definitions

In describing the disclosed subject matter, the following terminology will be used in accordance with the definitions set forth below.

"Graphene" refers to a thin sheet of carbon atoms (e.g., usually one-atom thick) arranged in a hexagonal format or a flat monolayer of carbon atoms that are tightly packed into a 2D honeycomb lattice (e.g., sp ₂ -bonded carbon atoms).

"Graphene oxide" (GO) refers to oxidized graphene, which is often made by reacting graphite powders with strong oxidizing agents.

"Reduced-graphene oxide" (re-GO) refers to graphene oxide's reduced form, which can be produced via chemical and/or physical reactions. However, some oxidized regions (e.g., less than 20%, less than about 10%, less than about 5%, or less than about 1 %) may remain on the reduced-graphene oxide

As used herein "reduced-graphene oxide shell" or "re-GO shell" refers to a substantially continuous coating (e.g., one or more layers of GO that overlap or otherwise contact each other to form into a continuous coating) around a particle of fertilizer.

As used herein, the term "fertilizer" refers to any additive containing organic and/or inorganic nutrients (synthetic and/or natural) that is added to soil to supply nutrients needed for plant growth and/or development. In embodiments, fertilizer may include one or more nutrients (macro- and/or micro-nutrients), such as, but not limited to: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), boron (B), chlorine (CI), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and nickel (Ni). In fertilizer, the nutrients, such as those listed above, do not have to be in elemental form, but may be in the form of a salt or other compound.

The term "fertilizer particle(s)" refers to a particulate material (e.g., granules, powders, etc.), where the particles include one or more fertilizer nutrients, such as, but not limited to, those listed above.

Discussion

The embodiments of the present disclosure encompass reduced graphene oxide (re- GO)-coated slow-release fertilizers and methods to prepare and use the re-GO-coated fertilizers.

Slow- or controlled-released fertilizers exist that employ polymers already used in coating various fertilizers. For instance, Jarosiewicz et. al reported that coating fertilizer nutrients with polymers such as polysulfone, polyacrylonitrile, and cellulose acetate tends to decrease the nutrient release rate [9]. Jia et al found that a polydopamine film coated on double copper potassium pyrophosphate trihydrate undergoes spontaneous oxidative polymerization of dopamine when reacted with the three essential nutrients (Cu, K, and P). Some of these polymer-coated fertilizers had good slow-release properties when incubated in either water or soil [10]. However, as mentioned above, these conventional polymer coatings and the techniques used to apply them often utilize organic solvents and/or toxic polymerization initiators, or time-consuming complicated processes, which increase production costs as well as raising environmental and health risks.

Graphene, an ultra-thin and ultra-light layered carbon material with high mechanical strength, super conductivity, and high surface area finds utility in various applications, including field effect transistors, sensors, transparent electrodes, batteries, supercapacitors, and composited materials. Also, recent advances in technologies make it possible to prepare graphene oxides with green methods, requiring no toxic starting materials or

oxidization/reduction agents. For example, graphene oxides can be produced in large scale via electrochemical exfoliation of pencil cores in aqueous electrolytes without a requirement for toxic chemical agents [33]. In addition, Guo et al reported a facile approach that can produce high quality graphene nanosheets in large scale through electrochemical reduction of exfoliated graphite oxide precursor at catholic potentials [34]. Because of its unique morphological structure and related properties, graphene has been considered as a possible carrier for various chemical compounds, thus holding potential opportunities for developing new controlled-release delivery systems [21-24]. For example, Yang et al developed a method to chemically deposit Fe ₃ 0 ₄ nanoparticles onto Graphene oxide (GO). This hybrid can be loaded with the anti-cancer drug DXR with a high loading capacity [23]. However, outside of these medical applications, little research has explored graphene-based slow- and controlled release systems for agricultural applications such as fertilizers, pesticides and so forth.

More recently, a simple and cost effective approach for producing graphene composites by reducing GO sheets was developed [25-29, incorporated by reference for the process of reducing GO sheets]. During a heating process, a variety of metal cations with different valences can not only reduce GO, but can also cross-link adjacent GO sheets to form reduced GO ("re-GO") films. The present disclosure includes methods using an ion- mediated thermal reduction method to provide a new route for coating fertilizer without the use of organic solvents and toxic initiators. As described in the examples below, a procedure was tested to prepare re-GO coated fertilizer particles including a nutrient salt (e.g., KN0 ₃ ) by encapsulating the nutrient-salt fertilizer pellets with a GO film and then baking GO-coated fertilizer pellets under heat for an amount of time. According to the analysis from TEM images, XPS and Raman spectra, the potassium ions are not only able to act as a "glue", soldering adjacent graphene sheets but also reduce GO to re-GO. This procedure allows GO films to form a shell around KN0 ₃ pellets inhibiting KN0 ₃ from fast release. This new method is different from the conventional polymer coating methods, which need organic solvents and toxic initiators. The re-GO-coated fertilizer pellets as prepared in the Example below took on improved slow-release properties. Because of its simplicity, feasibility and environmental friendliness, the fertilizer compositions and methods of making such fertilizer compositions of the present disclosure possess great potential as controlled-release fertilizers that provide plants with nutrients and ensure soil quality and crop productivity

In embodiments of the slow-release fertilizer compositions of the present disclosure, the compositions include a controlled/slow-release fertilizer composition including fertilizer particle sand a reduced-graphene oxide layer deposited on the surface of the particles. In embodiments the fertilizer is in the form of a fertilizer particle (e.g., grain, granule, pellet, etc.). The fertilizer particles include one or more nutrient. In embodiments, each particle can include the same combination of one or more nutrients, and in other embodiments, some particles can include different nutrients or different combinations of nutrients. In each particle, at least one such nutrient can act as a reducing agent.

In embodiments, at least one nutrient in the particles is in a salt form suitable for acting as a reducing agent of the graphene oxide. Not all of the nutrients in the fertilizer particle have to be in the form of a reducing agent (e.g., metal salt or ionic salt), so long as at least one nutrient is a nutrient salt, such as a metal salt, ionic salt, or other form capable of acting as a reducing agent to reduce GO. In embodiments of the fertilizer composition of the present disclosure, the fertilizer particle includes one or more nutrients (with at least one nutrient in salt form) including, but not limited to, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), boron (B), chlorine (CI), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and nickel (Ni). In embodiments, the fertilizer particle includes a combination of (e.g., two or more, three or more, etc.) such nutrients, with one or more of such nutrients in salt form.

In embodiments of the fertilizer compositions of the present disclosure, the one or more nutrient salts include, but are not limited to: aluminum sulfate, amino acid salt, ammonium chloride, ammonium molybdate, ammonium nitrate, ammonium phosphate, ammonium phosphate-sulfate, ammonium sulfate, borax, boric acid, calcium ammonium nitrate, calcium silicate, calcium chloride, calcium cyanamide, calcium nitrate, copper acetate, copper nitrate, copper oxalate, copper oxide, copper sulfate, diammonium phosphate, iron- ethylenediamine-N,N'-bis (EDDHA-Fe), iron-ethylenediaminetetraacetic acid (EDTA-Fe), elemental sulfur, ferric sulfate, ferrous ammonium phosphate, ferrous ammonium sulfate, ferrous sulfate, gypsium, humic acid, iron ammonium polyphosphate, iron chelates, iron sulfate, lime, magnesium sulfate, manganese chloride, manganese oxide, manganese sulfate, monoammonium phosphate, monopotassium phosphate, polyhalite, potassium bromide, potassium chloride (MOP), potassium nitrate, potassium polyphosphate, potassium sulfate, sodium chloride, sodium metasilicate, sodium molybdate, sodium nitrate, sulfate of potash (SOP), sulfate of potash-magnesia (SOP-M), superphosphate, triple superphosphate, urea, urea formaldehyde, zinc oxide, zinc sulfate, zinc carbonate, zinc phosphate, and zinc chelate. In embodiments, the fertilizer composition can include combinations of these salts and/or non-salt forms of the above-listed nutrients, among others. In some embodiments, at least one nutrient salt includes potassium nitrate (KN0 ₃ ).

In embodiments, the nutrient salt is capable of acting as a reducing agent under heat to reduce graphene oxide on the fertilizer particle. In embodiments, the fertilizer composition also includes cations (e.g., from the nutrient salt) capable of connecting adjacent reduced- graphene oxide sheets to form a substantially continuous coating of reduced-graphene oxide on the particle.

In an embodiment, the reduced-graphene oxide coating on the fertilizer particle can be single-layered, double-layered, few-layered, and multiple-layered re-GO. In

embodiments, the thickness of the re-GO shell can be in the rage of about 0.34 nm to 30 μηη. In embodiments, the thickness of a single-layer of re-GO can be about 0.34 to about 0.9 nm, with an average thickness of 0.7±0.2 nm. Two or more layers of re-GO can increase the thickness of the re-GO shell/coating on the fertilizer of the present disclosure. In

embodiments, the properties of the re-GO layers/coating can be controlled by the methods used to produce the fertilizer composition, such as by varying the parameters and conditions (such as ingredients, time, heat, and the like), as explained in greater detail below. For instance, in an embodiment, the thickness of the reduced-graphene oxide coating can be controlled by how much graphene oxide is combined with the fertilizer particles, the nutrient/reducing agent, the shape of the particles, the heating temperature, and the like. In an embodiment, the chemical and physical properties of reduced-graphene oxide coating can be changed by addition of the chemical reagents, reaction temperature, reaction time, pressure, gas atmosphere, gravity, graphene oxide, and/or pre/post-treatment

Methods of the present disclosure include methods of making slow-release fertilizer compositions of the present disclosure. In embodiments, the methods include providing a fertilizer particle including at least one nutrient in salt form (such as described above) that is capable of reducing graphene oxide, forming one or more layers of graphene oxide on the fertilizer particle such that the fertilizer particle is at least partially coated with graphene oxide, and then heating the graphene oxide-coated fertilizer particles to form a coating of reduced- graphene oxide on the particles.

In embodiments of the methods, the fertilizer particles are combined with GO to form GO-coated particles, having fertilizer particles at least partially covered with GO (e.g., GO films, GO suspensions, GO slurry, etc.). In embodiments the GO coatings are applied by physically wrapping prepared GO sheets on particles. In other embodiments, the GO coatings are applied to the particles by spray or dip coating with a GO composition. The GO coatings may be continuous or made of partial layers/coatings. As described below, in embodiments, cations in the fertilizer can act to fuse multiple CO layers or sections together to form a continuous coating.

In embodiments, the GO-coated particles are heated to enable the nutrient/reducing agent in the fertilizer to reduce the GO to form reduced GO coatings on the fertilizer particles. In embodiments, the GO-coated particles are heated (e.g., in an oven, microwave, or with another heat source) at a temperature of about 25 to about 500 °C. In embodiments, the GO-coated particles are heated at about 90 °C, or more. In embodiments, the particles are heated for a period of time sufficient to reduce the GO coating to form a reduced GO coating. In embodiments, the particles are heated from about 1 second to about 6 hours or longer. In embodiments the particles are heated for about 6 hours. The re-GO coating acts as a controlled-release coating for the fertilizer, releasing the fertilizer nutrients to the environment (e.g., soil, water, etc.) in a controlled manner.

In embodiments of these methods, the reducing agent can be a metal salt or an organic molecule or any other ionic compound or a mixture of one or more types of reducing agents. For instance, the metal in the metal salt can be magnesium, sodium, silver, iron, copper, silver, nickel, and the like. In an embodiment, the metal salt can include MgCI ₂ , NaCI, AgN0 ₃ , FeS0 ₄ , CuCI ₂ , AICI ₃ , NiCI ₂ , KN0 _3, and the like. At 90 °C, ionic solutions, such as MgCI ₂ , NaCI, AgN0 _3, FeS0 ₄ , CuCI ₂ , KN0 ₃ and AICI ₃ , not only reduce GO but also crosslink adjacent graphene sheets to form reduced GO (Re-GO) films on various substrates.

In embodiments of methods of producing slow-release fertilizer, the nutrient(s) to be used as fertilizer (e.g., in salt form, or other ionic compound) can be used as the reducing agent. Thus, fertilizer components such as, but not limited to, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), boron (B), chlorine (CI), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and nickel (Ni) can be formulated into metal salts and/or ionic compounds to serve a double role as the reducing agent in forming reduced graphene oxide films to coat the fertilizer as well as the nutrient component of the fertilizer itself. In embodiments, the nutrient/reducing agent is a salt ("nutrient salt") selected from metal salts and other ionic salts such as, but not limited to, aluminum sulfate amino acid, ammonium chloride, ammonium molybdate, ammonium nitrate, ammonium phosphate, ammonium phosphate-sulfate, ammonium sulfate, borax, boric acid, calcium ammonium nitrate, calcium silicate, calcium chloride, calcium cyanamide, calcium nitrate, copper acetate, copper nitrate, copper oxalate, copper oxide, copper sulfate, diammonium phosphate, EDDHA-Fe, EDTA-Fe, elemental sulfur, ferric sulfate, ferrous ammonium phosphate, ferrous ammonium sulfate, ferrous sulfate, gypsium, humic acid, iron ammonium polyphosphate, iron chelates, iron sulfate, lime, magnesium sulfate, manganese chloride, manganese oxide, manganese sulfate, monoammonium phosphate, monopotassium phosphate, polyhalite, potassium bromide, potassium chloride (MOP), potassium nitrate, potassium polyphosphate, potassium sulfate, sodium chloride, sodium metasilicate, sodium molybdate, sodium nitrate, sulfate of potash (SOP), sulfate of potash- magnesia (SOP-M), superphosphate, triple superphosphate, urea, urea formaldehyde, zinc oxide, zinc sulfate, zinc carbonate, zinc phosphate, and zinc chelates as well as

combinations of one or more of these salts.

In an embodiment, the reduced-graphene oxide layer is formed by the reduction of the graphene oxide by the nutrient/reducing agent. In an embodiment, the nutrient/reducing agent can reduce the graphene oxide and act as a bridge between or among adjacent reduced-graphene oxide sheets to form a continuous reduced-graphene oxide layer/shell. In a specific embodiment, when the reducing agent includes a divalent cation, the cation forms a cation bridge between reduced-graphene oxide sheets.

The present disclosure also includes slow-release fertilizer compositions having reduced-graphene oxide coatings made by the methods of the present disclosure described above. The disclosure further provides products including the slow-release fertilizer compositions of the present disclosure. The present disclosure further includes methods of using the slow-release fertilizer compositions of the present disclosures to treat soil and/or water for growing plants by adding the fertilizer compositions of the present disclosure to the solid and/or water before or during planting and/or cultivation/growth of the plants.

Embodiments of the present disclosure, such as described generally above and in the example below, include slow-release re-GO coated fertilizer and methods to prepare re- GO-coated fertilizer. In an embodiment described in greater detail in the example below, the nutrient is potassium, and the fertilizer includes KN0 ₃ particles. Additional details regarding the methods and compositions of the present disclosure are provided in the Examples below. The specific examples below are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present disclosure to its fullest extent.

It should be emphasized that the embodiments of the present disclosure, particularly, any "preferred" embodiments, are merely possible examples of the implementations, merely set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure, and protected by the following claims.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, eic), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1 % to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1 %, 2.2%, 3.3%, and 4.4%) within the indicated range. In an embodiment, the term "about" can include traditional rounding according to significant figures of the numerical value. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about 'y "'.

EXAMPLES

Now having described the embodiments of the present disclosure, in general, the following Examples describe some additional embodiments of the present disclosure. While embodiments of present disclosure are described in connection with the following examples and the corresponding text and figures, there is no intent to limit embodiments of the present disclosure to this description. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure

Example

The present example describes the development of a slow-release fertilizer prepared by encapsulating KN0 ₃ pellets with graphene oxide (GO) films. The material was then subjected to heat treatment, where adjacent GO sheets were soldered and reduced to reduced graphene oxide (re-GO) sheets by potassium. After the re-GO shell formed on KN0 ₃ pellets, the slow-release characteristics of the fertilizer dramatically improved. This new coating technology could hold great promise for environmentally-benign controlled- release fertilizer for crop production. Materials and methods

Materials

Graphene oxide (GO) and potassium nitrate (KN0 ₃ ) were obtained from ACS

Material and Fisher Scientific, respectively. All the chemicals are analytical grade and their solutions were prepared using deionized water (18.2 ΜΩ) (Nanopure water, Barnstead). Preparation of re-GO-coated KN0 ₃ pellets

GO solutions at the concentration of 2 mg/mL were prepared via 2 hours of ultrasound in 20 ml batches. To produce GO film, 10 mL of the resulting GO solutions was filtered through an Anodisc membrane filter (47 mm in diameter, 0.2 μηη pore size;

Whatman), air dried, and lastly peeled from the filter. The resulted GO film was mechanically coated on KN0 ₃ pellets surfaces with a little amount of water. After the slightly moist KN0 ₃ pellets were encapsulated with GO films, the GO-coated KN0 ₃ pellets were baked at 90 °C in an oven for about 6 hours (FIG. 1 ). Slow-release behavior of the as-prepared re-GO- coated KN0 ₃ pellets was characterized after being cooled to room temperature.

Characterizations

The microscopic features of re-GO-coated KN0 ₃ pellets were characterized with a field emission gun scanning electron microscopy (FEG-SEM, JEOL 6335F), transmission electron microscopy (JEOL 200CX TEM), and atomic force microscopy (SPM/AFM

Dimension 3100). X-Ray photoelectron spectra (XPS) of the samples were obtained with a Perkin Elmer 5100 XPS System. Raman spectra were recorded using a Renishaw Invia Bio Raman with excitation from a 785 nm diode laser.

Slow-Release Behavior of re-GO-coated KN0 ₃ pellets

To study the slow-release behavior of re-GO-coated KN0 ₃ pellets, the following experiment was carried out: 0.2 g samples were added into conical bottles containing 100 mL of distilled water. Then, the bottles were kept at 25 °C in an incubator for the duration of the experiment. At certain time intervals (0.5 or 1 hour), 2 mL of solution were sampled for potassium determination and an additional 2 mL of water was carefully injected into the bottles to maintain a constant amount of solvent. Only the potassium concentration was monitored during the experiment because the molar ratio between the dissolved K ⁺ and N0 ₃ ^{" 1} is 1 :1 in the solution. The potassium concentrations in the solution were analyzed by using an inductively coupled plasma atomic emission spectroscopy (ICP-AES). The release experiments were carried out in triplicate, and the average value was taken as the result.

Results and discussions

Preparation of re-GO-coated KN0 ₃ pellets

FIG. 1 gives the schematic illustration of an embodiment of a method of the present disclosure developed to encapsulate KN0 ₃ pellets with GO films. The color of GO film was matte brown, and the resultant re-GO-coated KN0 ₃ pellet is metallic grey. After the heat treatment, the color of the films changed from matte brown (GO color) to metallic grey, probably due to the recovery of ττ-conjugated system from GO sheets upon hydrothermal reduction in the presence of cations (K ⁺ )[25]. Meanwhile, GO's exposure to cations might have led to ring-opening of the epoxide [29], which further reduced GO. Atomic force microscopy (AFM) analysis of the dispersal state of GO individuals showed the presence of GO sheets on the mica surface and that the size of GO patches was in the micrometer range (FIG. 2A). Although a graphene sheet is thin, the AFM could easily characterize the morphological features of the graphene patches. Cross-sectional images of the AFM revealed that the thickness of a single-layer graphene on the mica surface ranged from 0.5 to 0.9 nm with an average of 0.7±0.2 nm (FIG. 2B), which is in agreement with the typical thickness (<1 nm) observed elsewhere for monolayer graphene sheets ¹⁶ .

Characterizations of re-GO-coated KN0 ₃ pellets

Raman spectroscopy was used to characterize GO on the surface of KN0 ₃ after heat treatment. The typical features of the G band at 1585 cm ^"1 and the D band at 1335 cm ^"1 are shown in the Raman spectra (FIG. 3A), which agrees with the literature by direct exfoliation approaches [30]. The G band is usually assigned to the E _2g phonon of C sp ² atoms, while the D band is a breathing mode of / -point phonons of A _1g symmetry. The A _1g mode is attributed to particle-size effects due to the existence of specific vibrations at the edges of graphene sheets. The appearance of a prominent D band in the spectrum is also an indication of disorder in graphene originating from the defects associated with vacancies and grain boundaries. It has been well documented that the size of the defect-free sp ² cluster regions is the inverse of the ratio of the D and the G band integrated intensities (/ _d //G)- This correlation has been used to determine the size of sp ² domains in various carbon materials including graphene. Comparisons of the Raman spectra of GO before and after coated on GO-coated N0 ₃ by thermal treatment showed a significant decrease of ratios of D and G band intensity from 1.00 to 0.51 (FIG. 3A), suggesting the simple approach successfully reduced the initial GO to re-GO with much fewer defects.

C1 s XPS spectra of GO and the GO coating on re-GO-coated KN0 ₃ were used to further determine the quality of GO and identified the presence of three functional groups, the non-oxygenated C-ring (C-C and C-H, at a binding energy of 284.6 eV), the C in the C-0 bond (C-OH, 286.2 eV), and the carbonyl C (C=0, 289.0 eV), for both samples (FIGS. 3B and 3C). The sp ² C-C component of the GO film from re-GO-coated KN0 ₃ sample (71.8%), however, was much higher than that of the GO film (58.6%), indicating the removal of oxygen-containing groups from GO and improved quality of GO sheets after being heated with KN0 ₃ . The results further confirmed that the metal cations can efficiently reduce the GO by thermal treatment, which is consistent with the results of Raman analysis. The analysis of cross-sectional re-GO-coated KN0 ₃ pellets by SEM shows the presence of both re-GO shell and KN0 ₃ core (FIG. 4A). High magnification SEM imaging of the surface on re-GO-coated KN0 ₃ pellet revealed that it had a dense re-GO shell without apparent apertures outside (FIG. 4B). With a closer look at a cross-sectional re-GO shell, it showed that the thickness of the shell was about 20-30 μηη (FIG. 4C). The energy-disperse X-ray (EDX) analysis confirmed that those pellets were made of carbon, oxygen, potassium and nitrate, a chemical composition that agreed well with re-GO-coated KN0 ₃ .

FIG. 5A shows TEM images of re-GO film of re-GO-coated KN0 ₃ , which clearly depict wrinkles and folding that indicates formation of a thin re-GO film. The TEM images also showed large pieces of re-GO sheets with size of at least 10 μηη. As reported by the manufacturer (ACS materials), the sizes of the original GO sheets should be around 1 -5 μηη, which is confirmed by the AFM analysis. The increase in size of re-GO sheets confirmed the ion-cross-linking mechanisms that the reducing ion reagent may act as a "cation bridge" or "glue", soldering adjacent graphene sheets. In addition, high-resolution TEM image of the graphene sheets showed that the carbon atoms were densely packed in a honeycomb crystal lattice (FIG. 5A, insert) The corresponding selected area electron diffraction (SAED) pattern of the graphene film is shown in FIG. 5B, where the ring patterns along with point patterns of hexagonal symmetry are clearly seen. The ring patterns indicate various orientations of re-GO sheets due to wrinkling and folding of a re-GO layer or overlapping with different re-GO layers, while the point patterns reflect the presence of a main single crystalline domain composed of sp ² -hybridized carbons arranged in a hexagonal lattice [32]. The SAED results confirm that well-reduced GO films formed on KN0 ₃ could be few- or multi-layer graphene sheets.

Slow-release behavior of re-GO-coated KN0 ₃ pellets

The slow-release behavior of as-produced re-GO-coated KN0 ₃ was examined to demonstrate its potential application as an agent for fertilizer delivery. The release characteristics of both the re-GO-coated KN0 ₃ and the pure KN0 ₃ were investigated. After soaking for 10 hours, the re-GO-coated KN0 ₃ pellets maintained substantially the same shape as prior to soaking, and some pellets appeared to drift on the solution without structural collapse. These observations indicate that, after thermal treatment, the GO film is capable of coating KN0 ₃ pellets. The concentrations of potassium that are released over time from the samples are shown in FIG. 6A, where C _K + denotes the concentration of potassium ions in the elutriant. It shows that the release rate of potassium took place in different stages when re-GO sheets are used as coating for delaying the overall release process. In the initial stage from the 0 to the 7 hours, the release rate was relatively slow compared with other stages. During this stage, only about 34.5% of potassium ions were released in the water. This could be attributed to the slow diffusion of water through the shell and into the core of re-GO-coated KN0 ₃ to establish 'channels' for the release of the potassium ions encapsulated in GO sheets. The burst release of potassium ions took place in the stage from 7 to 8 hours. As shown in FIG. 6B, the re-GO film on the fertilizer pellet cracked after immersion in water for about 8 hours. The cracks on the film likely lead to the burst release of potassium ions. After that, the release is restored to a slow rate, similar to that of the first 2 hours, with about 93.8% of the potassium ions released from the fertilizer composition. The data also reveal that the release of potassium out of the re-GO shell to water reaches its equilibrium after about 8 hours, indicating that the shell has excellent controlled-release ability. On the other hand, the release of potassium from the pure KN0 ₃ was rapid and reached equilibrium after only 1 hour. The results clearly demonstrate the reduction of GO films on the fertilizers provides a promising coating technique for the slow release characteristic.

Conclusions

The present example demonstrated a new method for developing fertilizer (KN0 ₃ ) that releases nutrients in a slow-release manner. The exemplary fertilizer was developed by encapsulating KN0 ₃ pellets with graphene oxide (GO) films at 90 °C for 6 hours in air. This new method is different from the conventional polymer coating methods, which uses organic solvents and toxic initiators. The results of this example show that with the aid of potassium ions, separated GO sheets not only fuse together to form a shell on KN0 ₃ , but also reduce to re-GO sheets during the heat treatment. The as-prepared re-GO-coated KN0 ₃ pellets exhibited slow-release behavior. Because of the unique characteristics of graphene, this newly developed method can be used for fertilizers that have controlled-release profiles, providing plants with nutrients, enhancing plant productivity, and minimizing nutrient loss.

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