Synthesis and Analysis of Metal Oxide-Graphene Composites

Home , Magnesium oxide

A project completed in partial fulfillment of the requirements for the Honors Program

McKenna Carey

May 8th, 2021

Chemistry Ohio Dominican University

Approved by Honors Project Review Committee:

Dan Little, Ph.D., Project Advisor Blake Mathys, Ph.D., Reviewer Kristall Day, Ph.D., Reviewer, Honors Committee

Accepted by

Director, ODU Honors Program

Abstract

There is not much research done regarding metal oxide and graphene composites.

However, there have been some studies done regarding zinc oxide and graphene, and this paper delves into research done regarding tin (IV) oxide, magnesium oxide, and iron (III) oxide to add to the ever-growing body of scientific research. These metal oxides were synthesized from precursors in situ directly onto graphene. Each metal oxide and graphene composite was then analyzed using XPS, while tin (IV) oxide was analyzed using XRD as well. Due to the presence of metal to oxygen to carbon bridges, it has been shown that these composites were successfully synthesized. 3

Equipment Needed Chemicals Needed: Graphene platelets, Tin (II) Chloride, Oxalic Acid, Magnesium Carbonate, Ferrocene, Ammonium Iron (II) Sulfate, Sulfuric Acid, Magnesium Nitrate Lab Equipment Needed:

Equipment Quantity

Beakers (250mL) 10

Crucibles 10

Petri Dish 3

Weigh Paper Box of 100

Balance 1

Scoopula 2

Centrifuge Tubes 5

Mortar and Pestle 1

Hot Plate 1

Stir Bar 2

Instruments/Machines Needed:

Instrument/Machine Name Location

X-Ray Photoelectron Spectrometer (XPS) Wisconsin

Powder X-Ray Diffractometer (XRD) Wisconsin

Ultrasonicator Ohio Dominican University

Drying Oven Ohio Dominican University

Muffle Furnace Ohio Dominican University

Introduction

Metal oxides are widely studied in the field of chemistry. More specifically the properties of these metal oxides are studied and analyzed to see if they can be applied to other fields of science. These studies often have to combine the metal oxides with another compound. The area of research that is lacking however, is when metal oxides are combined with graphene and analyzing the characteristics of this composite. There are a wide range of studies that metal oxides have been used in. There is evidence that the use of metal oxides, in the nano form, can be used to remove heavy metals from wastewater (1). Some of the metal oxides included iron (III) oxide (Fe2O3) and magnesium oxide (MgO), and these metal oxides, among others, were able to remove other metals from the water due to their high surface area and high affinity for other metals (1). Research like this could be a breakthrough in the field of renewable energy and may lead to a way to purify wastewater without causing harm to the environment, as well as other areas of science that could lead to a cleaner way to perform things necessary for our survival. Fe2O3 and MgO will be used in this research due to the research that has already shown that the physical characteristics of each have high surface areas, which could prove to be important when analyzing the data taken from this research. Fe2O3 is also a common photocatalyst due to it having a relatively narrow bandgap.

In a study where tin (IV) oxide (SnO2) was combined with graphene, it was found that the composite could be used to detect gas in a room at normal temperature (2). The study also led researchers to believe that this ability to detect gas in a room was due to high graphene conductivity, a great amount of surface area, and observed interactions between the graphene and

SnO2 (2). This is another example of an application of how metal oxides can impact other fields of science. SnO2 is the other metal oxide that is going to be analyzed in this study. Knowing that there are specific characteristics between graphene and SnO2 could be helpful when analyzing data of other metal oxides and graphene. There are two main chemical components in this research: metal oxides and graphene. Both compounds have chemical and physical characteristics that contribute and alter how they interact with one another: conductivity, band gap, and the physical structure. There are many types of semiconductors including metal oxides. Semiconductors are materials that have a low resistance to the flow of an electrical current so only some of the current can be conducted (3). When discussing semiconductors, there are bands that are energy 5 states where electrons can occur. The valence band would be considered the populated band with the highest energy level and the conduction band is the unpopulated band with the lowest energy level (4). However only electrons in the conduction band can move through the material. The difference in energy between the valence band and conduction band in known as the band gap (4). Band gap is considered to be the most important characteristic of a semiconductor because it has a strong influence over all of the semiconductor’s electrical properties (4). It is also important to note that the range for band gap in a semiconductor is between 0.3-3.5 eV (4). These band gaps vary between different metal oxides, as well as different semiconductors in general. When discussing band gap, semiconductors typically have a wider band gap than the band gaps of compounds that would be considered insulators, and their electrical properties can be analyzed and then applied to the field of electronics (5).

Figure 1: An explanation of band gap and the energy differences (4). Ecb represents the voltage of the conduction bad, while Evb represents the voltage of the valence band. E represents the g energy difference, or the band gap. The hv represents a photon that can be absorbed by the semiconductor. Graphene is a known conductor that is widely studied due to it having many interesting properties in regard to electricity (6). Graphene has the ability to conduct electricity because it has a lattice structure that is perfectly flat made up of carbon atoms. The electron charge is able to delocalize throughout the lattice due to the pi system which is the presence of alternating pi bonds, or double bonds, that can delocalize through the entire contiguous material (6). Graphene’s physical structure is two dimensional and has carbon rings in the form of hexagons, 6 as seen in figure 2 (7). Graphene also has a band gap with which is essentially zero, which is outside the range for a typical semiconductor (6).

Figure 2: This is the structure of what graphene looks like.

When discussing metal oxides there has been very little, if any, research of their properties when combined with graphene. In the research that has been done, ZnO has been the most commonly studied metal oxide and graphene combination. In many of these studies with ZnO, it has been found that there are useful electrical characteristics due to the high surface area and high conductivity when combined with graphene (8). The graphene increases the electrical activity or photocatalytic activity on the surface of the ZnO, so the electrical characteristics are even more present with the addition of graphene (8). Since the majority of the research revolves around ZnO, there is a need for information regarding the electrical characteristics of SnO2,

MgO, and Fe2O3 when combined with graphene. Studies about electrical characteristics involving these metal oxides can be recorded and then used for future use in studies involving these metal oxides. This research regarding ZnO has shown that it is an effective photocatalyst that can be used in the field of environmental conservation (8). In another study where ZnO was composited with graphene, it was shown that this composition can be an effective catalyst and can be then used in photo electronics (9). This is because ZnO, like many metal oxides, is an efficient electron donor, where graphene is a perfect match for metal oxides because it is a great electron acceptor (9). This study also showed that ZnO composited with graphene has a greater electrical conductivity when compared to just the conductivity of the graphene (9).

Aside from ZnO, SnO2 would be the most widely studied metal oxide that is combined with graphene. However, there is a great disparity between the research done regarding SnO2 with graphene when compared to ZnO. In studies that have a focus on SnO2, it has been shown that SnO2 by itself has efficient photocatalytic activity that can be useful in the field of photo 7

electronics (10). SnO2 has been shown to have a band gap of 0.1-0.8 eV, which is also known as a band gap that has been narrowing (11). SnO2 has also been used in various studies that look for a more environmentally friendly way to store energy, and has been shown to have highly efficient electrochemical properties (12).

Fe2O3 is found abundantly in nature and is a very stable compound. This makes it slightly more difficult to work with since the graphene is combined with a precursor so, an appropriate

Fe2O3 precursor needed to be found (13). Fe2O3 has electrical characteristics that can be utilized in the oxidation of water, which could cause great progress in the field of environmental protection and is being studied as an application of clean energy (14). There have also been studies that have shown that Fe2O3 can be used to oxidize organic products which can be useful in the purification of wastewater, and this oxidation can be amplified when composited with graphene (15). MgO can be synthesized in various ways using various precursors. MgO is much different than the other two metal oxides that will be studied within this research due to it having a very different band gap. The band gap of MgO is around 6 eV which is much greater than the band gap of SnO2 and Fe2O3 (16). MgO is still considered a semiconductor but is closer to an insulator rather than a conductor when compared to SnO2 and Fe2O3. MgO also has a wide range of other physical properties, in addition to its band gap, which makes MgO a very useful chemical compound in many fields of science including electronics, cosmetics, and many more (17). It has also been shown that the characteristics of MgO can be amplified, especially the surface properties, by a catalyst, which could be considered graphene in the case of this research (17). The method used to synthesize these composites is a great deal of the purpose of this research because of the novelty of this method. All three of these metal oxides have been synthesized from the precursors that were used in this research, however the metal oxide graphene composite has not been done in this way. It is also important to note that using oxalates as precursors for all three meatal oxides has not been done before. There are two points of novelty in this research: synthesizing metal oxide graphene composites, as well as generalizing a pathway to synthesizing these composites by using oxalate precursors.

SnO2 is a very simple compound to synthesize. The precursor is tin (II) oxalate, which can be synthesized with tin (II) chloride and oxalic acid, is then heated in air to become SnO2. 8

When combined with graphene a composite can be made and then heated at a high temperature of around 400ºC. SnO2 is an optimal metal oxide to research the electrical characteristics of because it is also a semiconductor and has the potential to higher conductivity when combined with graphene since graphene itself is a conductor (10). MgO is also fairly simple to synthesize and can be done very similarly as the synthesis of

SnO2. Magnesium carbonate (MgCO3) can be used as a precursor to MgO due to its ability to decompose right to MgO with heating (17). Magnesium oxide can also be synthesized using magnesium oxalate (MgC2O4), very similarly to the use of tin (II) oxalate. There is a wide range of temperatures, between 230-680ºC, where MgCO3 can decompose into MgO, but the desired temperature of decomposition is about 530ºC because it is the exothermic peak (18).

For this research, Fe2O3 will be synthesized using ammonium iron (II) sulfate and oxalic acid to synthesize the iron (II) oxalate (FeC2O4) precursor (19). The FeC2O4 will then be mixed with graphene and then heated to 500ºC to form the composite. When the synthesis of the composites is complete, they can be characterized using two separate instruments that are known as an x-ray photoelectron spectroscopy (XPS) and powder x-ray reaction instrument (XRD). An XPS instrument uses x-rays to measure how strongly atoms are bonded in a material, as well as the potential energy of core electrons which can tell us how easily the compound will give up electrons to another compound (20). An XRD instrument is used to look at the structure of material by diffracting x-rays off of the sample and then using the pattern they created to determine the arrangement of atoms in a material (21). This instrument can also be used to confirm the formation of these metal oxides (22).

Methods Each synthesis was done in a very similar fashion even though the chemicals that were used varied depending on which metal oxide was being synthesized. For each metal oxide, an oxalate was used as at least one of the precursors so that continuity was created throughout the research. Another tool that was used during the methodology of this research was an ultrasonicator. The metal oxide precursor was first synthesized, and then graphene was added which was mixed by hand. Water was then added to the precursor in a beaker and the slurry was exfoliated using the ultrasonicator. The ultrasonicator was used as a means to intimately mix the graphene platelets with the metal oxide precursor by breaking apart the particles of graphene so 9 that the metal oxides and graphene could create a composite after heating (23). The mixture was then annealed in the furnace at a high temperature.

The precursor to SnO2 was tin (II) oxalate. This precursor was synthesized from tin (II) chloride and oxalic acid. The chemical equation for this precipitation reaction was:

SnCl2 + H2C2O4 SnC2O4 + 2HCl. These two compounds were mixed together and then dissolved in water. This solution was then decanted, and the precipitate was collected. This precipitate was then dried in the drying oven so the remainder of the water could be evaporated out of the precipitate. After the precipitate was dried, it was then put into a mortar and pestle along with graphene and then mixed. This mixture was then put in the ultrasonicator along with 5mL of D.I. water. After 30 minutes, this mixture was put back into the drying oven so that the water could evaporate out. After the water had evaporated, the product was then put into a crucible and put into the furnace and was heated at around 400ºC for about three hours.

Since MgCO3 can decompose to MgO by being heated at a high temperature, the graphene can be added directly to the MgCO3. MgCO3 and graphene were both added and mixed into the mortar and pestle. After the mixing, 5mL of D.I. water was added so that the mixture could be put into the ultrasonicator for 30 minutes. After, the mixture is put into the drying oven overnight so that the water could evaporate. Then the mixture was transferred to a crucible and then put into the furnace and heated at around 530ºC for about three hours.

When MgC2O4 was used as the precursor, it first had to be synthesized by using magnesium nitrate and oxalic acid. The chemical equation for this reaction was:

Mg(NO3)2 + H2C2O4 MgC2O4 + 2HNO3 Once this was placed on the hotplate, urea was added so that the acidity would be raised so that the precipitate could form. After the compounds are mixed together, water was added and put on a hot plate for heating and continuous stirring. A precipitate formed and was vacuum filtered to remove all of the water. After the precipitate was placed in the drying oven until all of the water evaporated. Once dry, graphene was added to the precipitate and then mixed using a mortar and pestle. Water was then added to the mixture and placed in the ultrasonicator for 30 minutes to ensure the graphene is properly combined with the magnesium oxalate. The compound was then placed in the drying oven so that the water evaporated out. After the compound was finished drying, it was then placed in a crucible and then put in the furnace for three hours at 500ºC. 10

Iron (II) oxalate, FeC2O4, was used as the precursor for Fe2O3. Ammonium iron (II) sulfate and oxalic acid were mixed together and then dissolved with water and sulfuric acid. The chemical equation for this reaction was:

Fe(NH4)2(SO4)2 + H2C2O4 FeC2O4 + (NH4)2SO4 + H2SO4 This dissolution occurs with heating on a hot plate and continuous stirring. A precipitate is then formed and then decanted by hand before it was vacuumed. The mixture was washed three times with water. The solution was then checked with silver nitrate to see if there was any sulfate left behind because silver nitrate leaves behind a visible precipitate when in the presence of sulfate. When there was no sulfate left behind the precipitate was placed in the drying oven. After the precipitate was dried, it was combined with graphene in the mortar and pestle and then 5mL water was added so it could be placed in the ultrasonicator for 30 minutes. After, this compound was dried in the drying oven, and then put into furnace in a crucible at about 500ºC for three hours. Results

Results obtained using the XRD were only available for the samples regarding SnO2, along with analysis from XPS. Data for MgO and Fe2O3 will only come from the XPS analysis of the samples. The majority of the values are evaluated using the NIST XPS Database, while a few of the values are evaluated using previously published articles.

11 ArbitraryUnits

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Tin Oxide (Brown) Tin Oxide (yellow) Tin Oxide (white) Pure Tin Oxide Tin Oxide Graphene

Figure 1: This graph depicts the XRD data of multiple tin oxide samples with the samples that are considered the most homologous being at the top and working down to the bottom are the samples that are the least homologous.

In Figure 1, the samples of SnO2 were arranged with the SnO2 composited with graphene at the top as that was the only sample that contained graphene during analysis. Since the SnO2 was synthesized, some samples were more homologous than others and could be determined even through something as simple as observation. Right below the sample that contained graphene was the most homologous SnO2 sample that was synthesized Right below was another tin oxide sample that looked white in appearance but was less homologous than the sample above it (see Figure 1). The sample below that contained more impurities within the sample causing it to look more yellow than white, and the sample at the very bottom of the graph looked brown in appearance because it contained the most impurities of all the samples that were synthesized (see Figure 1). The samples that did not have a white color were typically heated at a temperature that was much higher than 530ºC due to error with setting the temperature of the furnace. 12

The XPS graphs are used to determine which elements in a compound are bonded to which. This is imperative because in order to determine if the metal oxide graphene composites have formed properly, a there needs to be a metal to oxygen to carbon bridge being formed (M- O-C). This can be explicitly determined by looking at the 1s orbitals of oxygen and carbon in each of the three metal oxide composites.

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Figure 2: These graphs represent the XPS scan s of the C 1s regions of the three metal oxides being studied. The dark blue lines on each represent the raw intensity, while gray lines represent a baseline fit. The orange lines are the sums of all the peaks, with the other colors representing individual peaks.

Figure 2 is a collection of graphs of the first carbon orbital (1s) from samples of SnO2, the MgO, and the Fe2O3. There are two main types of carbon peaks, one represents a single bonded (C-O) and the other represents a double bonded (C=O). On all three graphs, there is a peak that is around 285 eV which indicates a (C-O) which is indicative of a bond found within the metal-oxygen-carbon bridge (24). Also, each graph has a second peak that is located around 287 eV which is a (C=O) which indicates that this bond is not found in the bridge but is a 13 terminal oxygen and carbon bond (24). This peak can also be indicative of adventitious carbon which is expected and unavoidable. For peak 2 on the SnO2 graph and peak 3 of the MgO graphs, those represent (C-C) bonds, which are obsolete to this study because the XPS was analyzing pure graphene to create those peaks due to the crystal lattice structure of graphene (24).

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Figure 3: These graphs represent the XPS scans of the O 1s regions of the three metal oxides being studied. The dark blue lines are the raw intensity, and the gray lines are the background lines, similarly to Figure 2. The peak sum is once again orange with the individual peaks in the remaining colors.

Figure 3 shows graphs of the oxygen 1s orbital from samples of SnO2 as Figure 2 and also contains the oxygen 1s orbital from the MgO and Fe2O3. In the SnO2 graphene graph, the peak at about 530.9 indicates that the oxygen is bonded to tin. The other oxygen peak at 532.7 eV indicates some oxygen to carbon bond (24). This is indicative of the metal oxide forming properly. With oxygen being bonded to both tin and carbon at different peaks, this indicates that the bridge has been formed. Additionally, when looking at the MgO graphene graph, it is shown that peak 1 and peak 2, at 536.4 and 533.4 eV respectively, have been deemed that this is where oxygen is bonded to carbon (24). However, it has been determined that peak 3 is where the 14 oxygen is bonded to magnesium at 530.2 eV (24). This is indicative of the metal oxide formation. Since the oxygen is bonded to both the carbon and magnesium, this indicates that the bridge was formed when synthesizing the MgO graphene composite. Finally, when looking at the Fe2O3 graphene graph, the first peak at 532.8 eV is indicated to be oxygen being bonded to carbon (24). While the second peak at 530.3 eV has be determined that that is where oxygen is bonded to iron. This shows that the metal oxide was formed. Once again, since it is being shown that oxygen is bonded to both carbon and iron, the bridge has been formed to create the iron oxide graphene composite (24). There are also carbons that are known as adventitious carbon which is dust that gets mixed in with the signal which are part of the (C=O). It is also important to note that peaks in the oxygen and carbon graphs should be complementary of each other due to the formation of a (C-O) bond, which is indirect evidence of bridging due to the similar size of the peaks.

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Figure 4: These graphs represent the XPS scans of the metal regions of the three metal oxide composites

that were studied. Once again, the raw intensity is in dark blue, the background line is gray, and the sum of the peaks is orange. The other peaks are shown in the other colors.

Figure 4 are graphs of the specific metal oxide orbitals for the samples of SnO2, MgO,

Fe2O3. The sample of SnO2 displays the tin 3d orbital (see Figure 4). There are two peaks on the

SnO2 graphene graph and they both indicate that tin is bonded to oxygen. These two peaks are actually the same orbital because p orbitals will give off two peaks. Peak 1 is found at 495.4 eV, and indicates a bond made with oxygen, while Peak 2 is found at 487.1 eV which also indicates a bond made with oxygen (24). The MgO sample displays the 1s orbital (see Figure 4). There are two peaks found on the MgO graphene graph, where Peak 1 is found at 1309 eV and Peak 2 is found at 1306 eV. Peak 2 is indicative of a magnesium bonded to a hydrate which is a water molecule covalently bonded to the magnesium (25). This could occur if the water was not completely evaporated from the sample. Peak 1 is a peak that is formed in the presence of high amounts of heat which could be due to the high temperatures that these samples were heated at

(25). There are three peaks in the Fe2O3 graphene graph. Peak 1 is found at 724.7 eV and indicates that iron is bonded to oxygen, while Peak 2 is found at 711.5 eV also indicates an oxygen and iron bond (24). More research is needed before Peak 3 can be assigned.

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Figure 5: These graphs represent the XPS scans of the valence orbitals of the three metal oxides composites that were studied. The raw intensity is in the dark blue, and the background line is gray. The sum of the peaks is once again orange, while the individual peaks are the remaining colors.

Figure 5 are graphs of the valence bands of each metal oxide sample. The valence graphs are important because it represent the valence band of each element. The differences in these 16 valence bands are what gives different metals different photoelectronic properties. There are only the two which comes from the tin 3d orbital of the SnO2 sample and the magnesium 2p orbital as that is the valence orbital in MgO. There are three peaks found within the valence shell of the

SnO2 graphene. Peak 1 is at 26.6 eV, Peak 2 is at 22.1 eV, and Peak 3 is located at 16.4 eV. The MgO sample displays the first electron orbital, 2p. (see Figure 5). There are two peaks found on the MgO graphene graph and Peak 1 is found at 54.7 while Peak 2 is found at 53.3. Peak 1 is indicative of a magnesium and oxygen bond (26). This indicates that there is a bridge formation between magnesium, oxygen and carbon. Peak 2 is indicative of a magnesium-to-magnesium bond (26). Discussion

The synthesis of a SnO2 and graphene composite by utilizing tin (II) oxalate has been shown to be possible. The presence of the bridges from tin to oxygen to carbon are crucial to the structure of this composite. Furthermore, using MgC2O4 as the precursor to synthesize MgO to create a composite with graphene has been shown to work. Having the presence of a bridge that spans from magnesium to oxygen to carbon is a structural component that is needed to be present to show that this composite can be synthesized. Finally, using FeC2O4 as the precursor to synthesize Fe2O3 to create a composite with graphene has been shown to work. It has also been shown that there is a presence of bridges that span from iron to oxygen to carbon must be present to show that the composite has formed properly. There is also a notable point when looking at the valence band XPS data. The values for the MgO graphs are two to three times larger when compared to the values on the SnO2 graph. The higher MgO values indicate that the electrons present in MgO, if excited, would contain more energy because it would take more energy to excite them from the valence band. The ability for these three metal oxides to be synthesized and then become a composite with graphene allows for the possibility of there being more metal oxide graphene composites to be synthesized and studied.

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