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Honors Theses AY 16/17 Undergraduate Honors Theses

Spring 5-12-2017

Synthesis and Properties of Transition-MetalCarbides from Structured Carbon PrecursorsKenneth E. MadsenUniversity of Wyoming, kmadsen2@uwyo.edu

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Recommended CitationMadsen, Kenneth E., "Synthesis and Properties of Transition-Metal Carbides from Structured Carbon Precursors" (2017). HonorsTheses AY 16/17. 31.http://repository.uwyo.edu/honors_theses_16-17/31

University of Wyoming, Honors Program Thesis

Synthesis and Properties of Transition-MetalCarbides from Structured Carbon Precursors

Kenneth MadsenUnder the guidance of Dr. Brian Leonard

A thesis presented for the completion of the University of Wyoming Honors Program

Department of ChemistryUniversity of Wyoming1000 E University Ave,

Laramie, WY 82071

Contents

1 Introduction 2

2 Background 22.1 Solid State Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.1.2 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.3 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.4 General Solid State Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.5 Salt Flux Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Material Properties of Transition Metal Carbides . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Electrocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Experimental Methodology 83.1 Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2 previous attempts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.1 Initial Synthetic method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.2 Initial Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Carbide Buckypaper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.3.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Carbide Buckypaper From Graphitic Nano-Platelets . . . . . . . . . . . . . . . . . . . . . . . 133.4.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.5 Carbide Micro Spheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.5.1 Hollow Carbon Sphere Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.5.2 Hollow Carbide Sphere Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.5.3 Hollow Carbide Sphere Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4 Discussion 18

5 Conclusion 18

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1 Introduction

As humanities technological and scientific needs continue to grow, so too does our reliance on novel materials.Material science describes a collaborative effort in many areas of research, physics, chemistry, and biology,to better understand and develop new materials to meet the demands of our society. These new materialsrepresent a vast frontier of untapped potential, if we can learn to utilize their properties. Materials chemistryhas developed methodologies for producing extraordinary compounds in response to some of the pressingneeds of a modern society. The variety of these materials ranges from developing efficient and renewablepower generation to synthesizing mechanically strong building components and even utilizing biologicallycompatible materials for medical applications. This report will review some of these fields as they pertainto a specific class of compounds, metal carbides.

A metal carbide is any compound in which carbon is bound to a less electronegative metal. Metal car-bides have demonstrated some extremely interesting and useful mechanical and chemical properties, which,if we can harness them, could prove enormously beneficial. Researchers have observed extreme materialhardness, electrocatalytic activity, corrosion resistance, and even superconductivity with these compoundsmaking them a material of interest for many applications. One key hurdle in the efficient production ofmetal carbides is the synthetic route to obtain them. Typically a high temperature solid-state synthesis isemployed, which requires extremely robust reaction vessels, large amounts of energy, and usually results inmarginal control over the final product. Since material properties are intimately related to the compositionand structure of the material, this lack of control is clearly problematic. This report will investigate a newmethodology for synthesizing transition metal carbides in a more controllable and robust way while stillusing the familiar framework of solid-state chemistry.

The specific aim of this investigation is two fold. The first goal is to determine if there exists asynthetic methodology for changing the morphology of metal carbides by variation of the morphologies ofthe reactants. More specifically we sought to maintain the morphology of structured carbon precursorsto form structured carbide products. The second goal was to observe any novel material properties thatmay emerge after the development of new carbide architectures. A synthetic method was developed to thisend, which demonstrates a high degree of flexibility and versatility across a variety of carbon precursorsand metal systems. The breadth of the project extended to producing carbides from large (2 cm) discsof carbon, composed of either graphitic carbon platelets or carbon nano-tubes. The same technique wasalso used to synthesize carbides with a spherical morphology. This demonstrates that, despite the hightemperature synthetic route, We are able to maintain some control over the final products, both in terms oftheir composition and their morphology. These materials were subsequently investigated to determine theircrystal structure, composition, and material hardness.

2 Background

2.1 Solid State Synthesis

2.1.1 History

When most people think about chemistry they imagine beakers and test tubes full of brightly colored solu-tions cluttering a workbench. While this may be true of inorganic chemistry in general it is not the case forchemistry in general. Indeed solution phase chemistry provides many limitations on the types of compoundsthat are accessible. For example, with traditional solution-phase chemistry there is usually no way to achievereaction temperatures above the solvents boiling point. Similarly solvent-solute interactions play a decisiverole on the nature of the reactions at play, often making it difficult to choose the correct solvent to minimizepesky side reactions. These issues among a number of others can be subverted by removing the solventaltogether and reacting solid-phase components. This is the approach taken by solid-state chemistry

Historically, solid-state chemistry has been around, in one form or another, for thousands of years,although it typically isn’t referred to as such. Most metallurgical processes, industrial purifications, and

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ore processing involves solid-phase reactions. A prime example is the alloying of carbon and molten iron toproduce steel. It may not seem as though chemistry is going on, however this is one of the most basic typesof solid-state syntheses. While organic chemistry evolved out of curiosity, solid-state chemistry has alwayshad a close relationship to industry and commercialization. This link has grown stronger with the discoveryof noble metal catalysis in the early 20th century among other useful reactions involving solid components.Around this same time the renowned physical chemist Carl Wagner began investigating the physical prop-erties of solids, including oxidation rates and diffusion behaviors of various solid solutions. This work pavedthe way for our understanding of atomic behavior in solids.

With the ongoing push to develop new and improved energy technologies, including oil refinement, fuelsynthesis, and renewable energy applications, this field of chemistry has seen a vast increase in popularity.The efficient synthesis of highly active catalysts has become an extremely lucrative business, as catalysis isa key process of almost every industrial process. A few key catalysts include an iron oxide/chromium oxideamalgam used to catalyze the conversion of carbon monoxide to carbon dioxide and hydrogen gas in thewater gas shift reaction, zinc/chromium oxide used in the synthesis of methanol, and a variety of coppercatalysts used in steam reformation[1]. Each of these catalysts are present in the solid state, and a syntheticroute which demonstrates high yields and good control over product composition and structure is ideal. Sofar we have briefly mentioned some of the potential benefits of solid state chemistry over solution phasereactions, however it is pertinent to examine what makes these types of reactions particularly viable for usein industry and application.

2.1.2 Advantages

As mentioned above it is very important for any industrial process to have an appropriately pure and ac-tive catalyst to carry out the reaction of interest. Catalysts must also be durable enough to withstand theconditions under which the reactions are carried out, namely high temperatures and pressures. With theseconditions in mind it is easy to see how a heterogeneous, solid catalyst is the preferred method; as molecu-lar catalysts are typically insufficiently robust to withstand industrial processing. This presents a problemsynthetically for typical liquid phase chemistry, as compounds that are stable at very high temperaturestypically require equally high temperatures to achieve reaction. Typical solutions have boiling points muchbelow this temperature, making a traditional synthesis impossible. As we have already mentioned we cancircumvent this problem by simply removing the solvent and reacting the solid components of our catalystat high temperatures.

Figure 1: Color coded periodic table demonstrating the abundance of solid compounds capable of reactionin solid state chemistry[2].

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Another advantage of solid-state chemistry is the sheer breadth of elements available to play with.Solution phase chemistry is typically limited by the solubility of components in the solvent of choice, thisprecludes pure metals, and elemental components from being used in most solvent reactions. In a moltenflux this is less of a concern. Similarly solution chemistry relies on the availability of molecular precursors ofthe elements of choice, while any solid element, which fortunately comprises most of the periodic table, canbe used in a solid state synthesis. The periodic table in figure 1 makes the point quite well, with solids rep-resented in light blue, liquids in green, and gas phase elements in yellow. The huge number of solid elementsprovides great variability and a diverse toolbox to create some remarkable materials typically inaccessible totraditional solution chemistry.

2.1.3 Disadvantages

While there are a number of advantages there are just as many drawbacks to solid state synthetic methods.The primary difficulty arises from the necessity for extreme temperatures or pressures, making the logisticsof containing a reaction somewhat challenging. There are two properties of solids which necessitates theseextreme reaction conditions. The first being the nature of solid materials. Solid materials form with a char-acteristic lattice structure governing the spatial orientation of atoms in the solid. This lattice has associatedwith it an energy which must be overcome to break the solid apart. As the liberation of individual atoms isoften necessary for the chemical reaction to occur, it becomes necessary to break the lattice which requiresextreme amounts of energy. The lattice energy of a given ionic solid can be approximated by the Kapustinskiiequation shown below.

∆HL =Nion|zAzb|(1− d∗

d )k

d

k = 1.21× 105 kJ pm mol−1

(1)

The resultant energy is a function of the amount of crystal which must be broken apart and the energynecessary to accomplish the task. The larger the crystal being heated the more heat it will require to achievefull decomposition.

This ties in to the second criterion. In a solid system, a reaction can only occur at the interface betweenthe reactants. This is based on the diffusivity of atoms in the solid phase. Again we see a departure fromthe typical considerations of solution phase chemistry where diffusion is a non-issue. In solids, atoms arecapable of moving around and displacing one another, As seen in figure 2. This atomic diffusion is necessaryfor a reaction to occur beyond the surface of the reactants. The diffusion of an atom in the solid phase isgiven by the following relationship

< x >=√DT

T = Temperature

D = Diffusion Coefficient

(2)

In solids the diffusion coefficients are extremely small, somewhere on the order of 10−16 yet to obtainreasonable rates of reaction diffusivity should be on the order of 10−12. This requirement leads to ”TammansRule,” which states that to obtain a reasonable reaction, constituent solid state reactants must be heatedto at least 2/3 the melting point of the highest melting species. An example of this would be the synthesisof tungsten carbide. Elemental tungsten doesn’t melt until roughly 3500oC making the necessary reactionconditions almost inaccessible.

Figure 2 demonstrates the process by which solids can diffuse through a material, the speed with whichdiffusion occurs increases dramatically as the size of the particles decrease, and as the temperature increases.Decreasing particle size also has the added benefit of increasing the ratio of surface are to volume, thus agreater portion of the material is available for reaction as the precursor dimensions are decreased. There is adrawback to decreasing particle size, in that as size goes down the reactivity of the particle increases, oftenresulting in unstable reactants and copious side reactions.

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Figure 2: Mechanisms by which solid state diffusion can occur in a bulk solid[3].

2.1.4 General Solid State Syntheses

We have thusfar discussed some of the advantages and disadvantages of solid state chemistry in general,however we have not looked at any specific mehtods for achieving these reactions. The most common, andsimplest, synthetic method is the so called ”heat and beat,” approach where solid starting materials arethoroughly ground together, often in a mortar and pestle, and heated to reactive temperatures. This processis repeated until the desired purity is achieved, hence the name. While this is often effective, it is also a verycrude method for producing the final material, and it typically removes any control over the compositionof the product, aside from the thermodynamically most stable compound. Other methods exist, includingspark plasma synthesis (SPS) and arc melting, which operate under a similar principle, using a huge electricalarc to heat up the reactants to form products. Again this is a very brute force approach to synthesis. Othermethods operate at lower temperatures, such as epitaxial methods, atomic layer deposition, vapor phaseepitaxy, and chemical vapor deposition, by applying thin layers of material to a substrate, which as we haveseen removes the diffusion issue, and also lowers reaction temperatures. While these techniques give goodcontrol over the composition of the product, they take a long time, and are very expensive to set up[4].

2.1.5 Salt Flux Synthesis

In an attempt to achieve good control over reactions, without the drawbacks of layer by layer techniquesour lab has employed a technique known as salt flux synthesis. By introducing a salt eutectic (a mixtureof two or more salts) which melts at a lower temperature than the reactive temperature of the precursorsof interest a high temperature solution is generated. The specific temperature at which the salt mixturemelts is highly variable based on the composition of the mixture as we can see in figure 4. This gives goodcontrol over the temperature range for a given reaction. Salt flux reactions can be thought of analogouslyas a typical solution phase reaction, however where a typical solution-phase reaction uses solutions whichare liquid at room temperature, the salt flux reaction uses a molten salt at high temperatures. This moltenflux allows the solid precursors to achieve much faster diffusion through the solution effectively reducing thediffusive barrier drastically. Similarly including the salt can affect the formation of reactive intermediateswhich can lower the temperature of the reaction further. This salt flux technique is the primary syntheticmethod being discussed in this investigation.

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Figure 3: Melting point dependence on the composition of a mixture of halide salts. The eutectic point isthe lowest melting point achieved by the composition[5].

Figure 4: A general schematic for a solid state salt flux synthesis. In this example V8C7 can be synthesizeda much lower temperatures than traditional methods[6].

2.2 Material Properties of Transition Metal Carbides

As mentioned in the introduction transition-metal carbides posses a number of very interesting materialproperties which make them particularly interesting in a number of applications. A major application ofinterest for many of these materials is the development of extremely durable and hard materials. Two car-bides in particular, WC and TiC, possess extraordinary material hardness comparable to that of diamond.Other carbides including HfC and ZrC demonstrate similar hardnesses making them potentially useful asultra-hard coatings for industrial applications. In fact WC is often used to coat drill bits for industry as itis extremely durable and much cheaper than diamond coated tools.

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Figure 5: Comparative hardness of materials, titanium carbide (TiC) demonstrates material hardness neardiamond, as does boron carbide[7].

Another convenient property of these materials is that they are electrically conductive. Conductivitymakes carbides of interest in electronics and electrocatalysis, which we will discuss in a moment. Along withnominal conductivity, carbides can be doped with magnetic metals including iron to generate inherentlymagnetic carbides. Some carbides even demonstrate superconductivity at sufficiently low temperatures,although this property is more of a curiosity than a utility.

2.3 Electrocatalysis

Perhaps the greatest potential use for carbides is their ability to catalyze chemical reactions. Specificallyseveral transition metal carbides can catalyze two important reactions for renewable energy, the oxygen re-duction reaction (ORR) and the Hydrogen evolution reaction (HER)[8][9]. These two reactions in conjunctioncan be used to convert hydrogen gas directly to H2O. This reaction is of particular interest as it generatesan electrical current if the two reactions are appropriately separated. It should be noted that this reactionis precisely the same as what occurs when hydrogen is burned in the presence of oxygen. Interestingly thedirect electrochemical conversion of hydrogen to water is actually much more efficient in theory, as it doesn’tgenerate as much wasted heat and light as a combustion reaction does.

As mentioned above the reactions must be appropriately separated to make use of the electrochemicalreaction. This separation is often accomplished by introducing an exchange membrane, in what is appropri-ately refereed to as a proton exchange membrane (PEM) fuel cell. Fuel cells have been of increasing interestas a potential replacement for fossil fuels, however these fuel cells can only operate efficiently by making useof electrochemical catalysts.

Currently the only sufficiently efficient catalysts are precious metal catalysts. Pt, Pd, and Ir are com-monly used as catalysts in fuel cells making them extremely expensive. The expense is the primary reasonfuel cells are still not feasible as a replacement for fossil fuels. The development of better catalysts fromless expensive materials is of paramount importance and is being looked into extensively. As previouslymentioned transition metal carbides are capable of acting as catalysts to replace these precious metals.Specifically Mo2C demonstrates high activity for both HER and ORR, although it is still much lower thanplatinum.

Another way to improve catalytic efficiency and activity is to mount platinum on a substrate whichimproves its inherent activity. Currently platinum is mounted on carbon black, a porous carbon network.

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Figure 6: Schematic view of a proton exchange membrane (PEM) fuel cell. The cell operates by takinghydrogen, produced during HER, oxidizing it to liberate an electron. The protons travel accross a membraneand reduce oxygen to water (ORR)[10]

This network stops the small particles of platinum from aggregating but does nothing to enhance the activityof the active catalyst. Research has shown that mounting platinum on Mo2C actually improves the catalyticactivity of the platinum[9]. Similarly due to carbides great material properties they are far more durablethan the traditional carbon supports. These factors make carbides an interesting modification to existingcatalyst systems.

3 Experimental Methodology

3.1 Goal

As mentioned in the introduction the goal of this project was to investigate the feasibility of producing metalcarbides from structured carbon precursors as opposed to fine powdered precursor. The original aim wasto use carbon buckypaper, comprised of compressed carbon nanotubes, as the carbon precursor in a hightemperature salt flux reaction. We have seen excellent results using powdered carbon precursors howeverdiffusive limitations (section 2.1.3) presented a major hurdle towards this synthetic goal.

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3.2 previous attempts

The initial investigation centered on reacting the structured carbon paper with a well ground mixture of saltand metal surrounding the disc. The initial hope was to rely on the natural porosity of the carbon paperand the molten salt to provide sufficient diffusion to produce a fully reacted product. Unfortunately thesereactions did not produce the said product, rather they produced a disc of mostly carbon with a thin carbideshell. Despite the limited success of these reactions we saw that carbide will form at least to some degreewith a surface of carbon nanotubes.

3.2.1 Initial Synthetic method

At this point It is necessary to delve into the synthetic methods specific to this system. As mentionedabove we used carbon buckypaper as a precursor. The buckypaper was produced by sonicating a solution ofmulti-walled carbon nanotubes (.03g) in isopropanol (20 mL) for 30 minutes. This well mixed solution wasthen transferred into a 20 mL plastic syringe and filtered through a 0.2 µm nitrocellulose filter paper. Oncethe entirety of the solution had been forced out of the syringe, two or three syringe-fulls of air were passedthrough the filter housing to press as much isopropanol out of the disc as possible. Finally, the disc wasdried between two watch glasses for 15 minutes at 60oC to ensure most of the solvent had been removed.The resulting material was a flexible dark black disc with a diameter of approximately 2 cm.

To create the carbide material we produced a salt flux solution based on research conducted earlier inour group, which provided the appropriate eutectic point (section 2.1.3) for the desired reaction. The saltflux of choice was a mixture of KCl, LiCl, and KF in a 40 : 39 : 1 mixture. This salt mixture was thenground together with the metal of choice for the reaction. We initially focused on Nb, Ti and V to produceTiC, NbC, and V8C7. We then placed the carbon buckypaper into an alumina boat and surrounded it withthe salt-metal mixture. The boat was then placed into a Lindberg Blue tube furnace and heated to 950oCfor 12 hours under argon flow. The entire synthetic procedure is detailed in figure 7.

Figure 7: Original synthetic attempts at forming carbide from carbon buckypaper. Steps 1 and 2 depictthe synthesis of buckypaper which will act as the carbon precursor. In this example MWCNT’s act as thecarbon material forming the disc. Once the disc has been synthesized it is covered in a well ground mixtureof metal and salt (3) and furnaced (4) to achieve reaction.

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3.2.2 Initial Results

The initial results of this investigation demonstrate the importance of diffusion is solid-state chemistry.While the desired carbide product was produced along the contact between the buckpaper disc and themetal powder, there was no reaction throughout the majority of the disc. This results in the formation ofa thin carbide shell on the surface of the material, leaving the rest of the disc unreacted. We attempted tochange the orientation of the disc within the salt metal mixture with no change in the result. The disc wasplaced on top of, fully surrounded by, or buried from the top by the metal mixture, all of which demon-strated the same behavior; surface reactivity with little penetration into the disc. These results are depictedin figure 8, where it is evident both from X-ray diffraction (XRD) and scanning electron microscopy (SEM)that huge amounts of residual carbon remain in the material. The labeled peak at approximately 25o2θis indicative of this leftover carbon and, looking at the SEM image on the left, we see the top of the discas a bright white material corresponding to carbide formation, whereas the inner layer of material is darkblack, clearly showing the leftover carbon which was inaccessible to the metal reactant. From this initial in-vestigation it was evident that a new synthetic method must be developed to account for the diffusive barrier.

Figure 8: Results of our initial synthetic approach to forming carbide buckypaper. It is evident form thelabeled peak in the XRD plot that a significant portion of the product is unreacted carbon. This is backedup by the SEM image which shows the carbide material (white) on the top of the disc, while the bottom ofthe disc remains carbon (black).

3.3 Carbide Buckypaper

3.3.1 Synthesis

Considering the limited diffusion of metal through the carbon material we needed to develop an improvedmethodology for introducing metal to the middle of the material. To accomplish this a subtle change to thesynthetic method mentioned above was made. As opposed to adding the metal after the synthesis of thebuckypaper, we tried to add the metal during the synthesis of the buckypaper an idea inspiered by the workof M. Ding and A. Star[11]. This was accomplished by simply adding the fine mesh metal powder to thesuspension of carbon nanotubes during sonication. The remainder of the synthesis was the same as alreadymentioned, with the notable exception that rather than a mixture of metal and salt surrounding the disc,the disc was surrounded by just the salt eutectic. While this introduced the metal to the center of the discefficiently it posed an additional problem. The fine metal mesh is highly susceptible to oxidative processes,i.e. rusting. Using isopropanol as the solvent, as was the case for the buckypaper synthesis, proved to be tooreactive for the metal powder to withstand. As a result, the products of the reaction were riddled with metaloxide impurities. To alleviate oxidative concerns, hexanes were substituted for isopropanol as the solventof choice. A few other subtle augmentations were made to the procedure, all with the goal of minimizing

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potential oxidation the metal mesh. One change was the removal of the extensive drying step after thesynthesis of the buckypaper. Discs were dried at 60oC for approximately 5 minutes to drive off as muchsolvent as possible without exposing the metal to oxygen for extended periods of time. Heating times andtemperatures remained the same as with the previous efforts and can be found in table 1.

3.3.2 Results

We initially looked at Vanadium, Titanium, and Niobium as the metals of choice to produce metal carbidesfrom carbon buckypaper. Following the aforementioned synthetic route we produced a brittle hard material,very different from the flexible buckypaper starting material. The greyish black discs maintained roughly thesame macroscopic dimensions as they had before annealing (diameter≈2cm). The materials were analyzedusing powder X-Ray diffraction and Scanning electron microscope as before, and a very different patternwas observed. Through XRD we observed a massive reduction in the amorphous carbon peak (2θ ≈ 20-30o)indicating that much more of the initial carbonaceous material reacted to form the carbide. Under SEM,using a back-scattered electron detector (BSED), we were able to image the edges of some of the as syn-thesized carbide discs. We see that the material is fully reacted through the entire disc. After successfullysynthesizing NbC, TiC, and V8C7 we looked to synthesize all metals from groups IV, V, and VI of theperiodic table. The figure below demonstrates that we were able to successfully synthesize each of thesecarbides with little to no residual carbon. We have also optimized the heating times and temperatures forthis synthetic method as tabulated below.

Figure 9: XRD patterns of TMCs synthesized via salt flux method using buckypaper as the carbon precursor.The reference PDF numbers are WC (00-025-1047), TaC (00-035-0801), HfC (03-065-8749), Mo2C (00-011-0680), NbC (01-070-8416), ZrC (00-035-0784), Cr3C2 (01-074-7137), V8C7 (01-089-2608), TiC (01-089-3828).

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Figure 10: Scanning electron microscope (SEM) images of transition metals carbides synthesized usingbuckypaper as the carbon precursor. Discs hold together and maintaint the macroscopic dimensions of theprecursor.

Carbide Metal : Carbon Ratio Temperature (oC) Dwell time (hrs.) Salt FluxTiC 1 : 1 950 12 KCl, KF, LiClZrC 1 : 1 950 12 KCl, KF, LiClHfC 1 : 1 950 12 KCl, KF, LiClV8C7 8 : 7 950 12 KCl, KF, LiClNbC 1 : 1 950 12 KCl, KF, LiClTaC 1 : 1 950 12 KCl, KF, LiClCr3C2 3 : 2 950 12 KCl, KF, LiClMo2C 1 : 4 1050 24 NaCl, NaFWC 1 : 5 1200 12 NaCl NaF

Table 1: Optimized reaction parameters for synthesizing early-transition metal carbides using buckypaperas the carbon precursor

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Figure 11: Successive magnifications of the carbide disc (top left) demonstrate that the solid disc is composedof a network of carbide nanotubes with a very similar morphology to the buckypaper starting material.

3.4 Carbide Buckypaper From Graphitic Nano-Platelets

3.4.1 Synthesis

In the previous sections we looked at the synthesis of carbide discs from multi-walled carbon nanotubes. Thisinitial success led us to consider other possible carbon sources, in this case graphitic carbon nanoplatelets.The synthetic procedure for these materials doesn’t differ too much from the previous procedure with thenotable exception that rather than creating a suspension of carbon nanotubes, we created a suspension ofnanoplatelets. We also changed the solvent slightly to 10% mixture of isopropanol in hexanes, which alloweda better suspension of the carbon source, while still preventing excessive metal oxidation. When filteredand dried we found that the disc was much less pliable, and far more likely to fall apart. To alleviate thisconcern we placed the resulting disc into a pellet press and pressed it under 3 tonnes of pressure to enhancethe structural integrity of the disc. These discs were furnaced using similar heating patterns to the carnonnanotube derived buckypaper, although in general time and temperature had to be increased slightly. Thenew heating profiles are tabulated below.

3.4.2 Results

As with the previous investigations we characterized the products with XRD. Via XRD we observe cleanproduct formation, although there is often residual graphitic carbon as evidenced by the peak at approx-imately 28o. We were able to synthesize carbides of all the early transition metals of interest with theexception of tungsten and chromium, which are still under investigation. XRD results are shown shown infigure 11.

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Figure 12: XRD patterns of TMCs synthesized via salt flux method using graphitic nano-platelets as thecarbon precursor. The reference PDF numbers are TaC (00-035-0801), HfC (03-065-8749), Mo2C (00-011-0680), NbC (01-070-8416), ZrC (00-035-0784), V8C7 (01-089-2608), TiC (01-089-3828).

Carbide Metal : Carbon Ratio Temperature (oC) Dwell time (hrs.) Salt FluxTiC 1.5 : 1 950 24 KCl, KF, LiClZrC 1.5 : 1 950 24 KCl, KF, LiClHfC 1.5 : 1 950 24 KCl, KF, LiClV8C7 12 : 7 950 24 KCl, KF, LiClNbC 1.5 : 1 950 24 KCl, KF, LiClTaC 1.5 : 1 950 24 KCl, KF, LiClMo2C 1 : 4 1200 24 NaCl, NaF

Table 2: Optimized reaction parameters for synthesizing early-transition metal carbides using graphiticnano-platelets as the carbon precursor

The materials were then taken to SEM to characterize the morphology. We were hoping to observea similar result to the carbon nanotubes, such that the materials maintain the morphology of the startingmaterial. We do observe a conservation of the starting morphology with a fair number of carbides derivedusing this method, however those carbides that didn’t keep their morphology with MWCNT’s likewisedeviated from the plate-like structure of the starting material. The results of this investigation are shown infigure 13.

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Figure 13: Scanning electron microscope (SEM) images of transition metals carbides synthesized usinggraphitic nano-platelets as the carbon precursor.

3.5 Carbide Micro Spheres

3.5.1 Hollow Carbon Sphere Synthesis

A simple synthesis was adopted to produce hollow carbon spheres with an approximate diameter of 5µm asdescribed by X. Sun and Y. Li[12]. We began by dissolving 0.08g sodium lauryl sulfate, and 3.5g Glucose inwater. When the solution was completely clear we placed the solution into a teflon lined Parr bomb. Thetemperature of the bomb was raised to 160oC for 16 hours. The resulting black solution was washed bycentrifugation, 3 times with water, and 3 times with absolute ethanol pouring off the supernatant liquid eachtime. The final solution was dried on a watch glass at 60oC for an hour to drive off any remaining ethanol.Finally the red-brown powder was annealed at 800oC, with a ramp rate of 5oC per minute. The final blackpowder was confirmed as hollow carbon spheres by SEM as seen below.

3.5.2 Hollow Carbide Sphere Synthesis

Hollow carbide spheres were synthesized using the same methodology as with the previous two investigations.One notable exception to the synthetic procedure is that the discs obtained after filtration of the carbon-metalsuspension had essentially no structural integrity. It would have been possible, as was the procedure for thegraphitic nanoplatelets, to peletize the resultant powder to obtain a solid disc, however this was discountedas that much pressure would most likely collapse the hollow sphere structure of the carbon precursor. Ratherthan maintaining a disc the precursor material was transfered to a mortar and pestle and ground togetherwith the salt eutectic before being placed into the furnace. Annealing temperature remained the same aswith the previous two projects, however the heating time was increased to 24 hours to ensure a completereaction.

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Figure 14: A) Primary reaction used to produce hollow carbon spheres. B) Synthetic route including ahydrothermal reaction to form carbon spheres, washing the crude product with with water and ethanol usingcentrifucgation, and annealing at 800oC to form hollow carbon spheres.

3.5.3 Hollow Carbide Sphere Results

The powder product was analyzed using XRD as with the previous two investigations to determine thecomposition of the material. Figure 15 demonstrates that the carbon spheres were fully reacted with themetal to form carbide. We see a small oxide impurity in the NbC sample, which is, at present, attributedto a small leak of air into the furnace during reaction, as opposed to a systematic presence of oxygen in theprecursor.

The product was then analyzed using SEM to determine the morphology. As shown in figure 16, weobserve reasonable yields of spherical carbide particles, with roughly the same dimensions as the carbonprecursor (diameter≈ 5µm). While this is promising we also observe a significant quantity of material ex-hibiting other morphology, specifically large chunks of unstructured material. It is unclear to what extentthis is the result of the carbon precursor or the result of the high temperature reaction. It is possible thatboth are contributing to the lack of homogeneity in the product. While we observe fairly high yields with thesynthesis of the precursor material, however we do see a small amount of non-spherical material. Similarly itis entirely possible that the high temperatures required for these reactions result in particle decompositionand agglomeration, which to some extent is unavoidable.

Carbide Metal : Carbon Ratio Temperature (oC) Dwell time (hrs.) Salt FluxTiC 1.5 : 1 950 24 KCl, KF, LiClV8C7 12 : 7 950 24 KCl, KF, LiClNbC 1.5 : 1 950 24 KCl, KF, LiCl

Table 3: Optimized reaction parameters for synthesizing early-transition metal carbides using hollow carbonspheres as the carbon precursor

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Figure 15: XRD spectra of NbC and TiC hollow carbide spheres. PDF numbers NbC (01-070-8416), TiC(01-089-3828).

Figure 16: SEM images of hollow carbide spheres, demonstrating that the morphology of the carbonprecursor is maintained through annealing. 17

4 Discussion

It is evident from the results above that the current synthetic methodology is a versatile route for the synthe-sis of carbide materials. It presents a means by which we can achieve high levels of precursor mixing, requisitefor a homogeneous solid state reaction. Similarly it subverts the issue of limited diffusion in solids by placingmetal throughout the carbon material. Finally it allows us to obtain reactions with carbon materials whichwould ordinarily require extraordinarily high temperatures. We have demonstrated that the carbide discsproduced as the products of the first two investigations have roughly the same dimensions as the precursormaterial as shown in figures 11 and 13. We have also shown shown that we can maintain some control overthe product morphology with variation of the reactant morphology. This is not ubiquitous however, as manyof the carbides we synthesize have a preferred morphology regardless of the starting material. Examplesof these products are seen in figures 10 and 13 with ZrC, HfC, Mo2C, V8C7, and WC, which despite thevariation of carbon precursor tend to form alternative structures. This characteristic can be attributed, tosome extent, to the high temperatures required for the reaction to proceed. In general we have seen that thehigher the reaction temperature, the less like the morphology of the precursor will be maintained. In future,it would be interesting to see if a lower synthetic temperature would produce carbides which rigorously keepto the structure of the starting material.

We have seen good reactions using multi-walled carbon nanotubes, graphitic nanoplateslets, and hollowcarbon wheres as precursor materials, and we suspect this methodology may be applicable to many othersmall scale carbon sources. We have seen that the time and temperature of the reaction must, to some extent,be manipulated to fascilitate complete reaction, however the synthesis is easily modified and fairly versatile.Research is ongoing to produce WC and Cr3C2 with graphitic nanoplatelets, WC has been synthesized us-ing graphitic nanoplatelets however it was not sufficiently pure to merit mention in this report. Similarlyresearch is continuing to synthesize hollow carbide spheres with the remaining early-transition metals.

5 Conclusion

This report has demonstrated the modern importance of carbide materials, as both materials towards renew-able energy applications, as well as a material of great structural and physical interest. We have seen someof the methods by which these carbides are synthesized, namely by high temperature solid-state methods.We then looked towards alternative methods for reducing the temperature to achieve greater control over theresulting products. We jumped into a series of three projects investigating a novel synthetic methodologyfor producing carbide materials in high yields with interesting morphological characteristics. By a slightmodification to an already well studied molten salt flux synthesis we were able to increase structural controlover these systems. Through XRD and SEM the we have demonstrated that with certain systems we maycontrol the morphology of the product with a simple variation of the carbon source. Finally we have seen asystematic trend suggesting that by increasing the dimension of our carbon source we make it more difficultto achieve a complete reaction, however this difficulty can be overcome by simply increasing reaction timesand metal loadings. While this research has characterized the morphology of various metal carbides, furtherresearch is necessary into these materials to determine their viability as catalytic supports or structuralmaterials.

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