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1 Binding Of Earth Abundant Metal Coordination Complexes As Molecular Spacers To Single-Walled Carbon Nanotubes Chauncey McNeill University of North Carolina at Charlotte Faculty Mentor: Jordan Poler University of North Carolina at Charlotte ABSTRACT Nanostructured carbon has generated a lot of attention as electrical energy storage materials because of their extreme surface area. To enhance the storage capabilities of these materials, interparticle spacer strategies are needed to preserve the surface area between particles. In this article a comparison of different divalent, dinuclear coordination complexes with strong adsorption to single-walled carbon nanotubes (SWCNTs) will be presented along with the spacer properties that maintain SWCNTs s electrochemical properties. Dinuclear zinc hydrazone coordination complexes bind with a higher loading than dinuclear ruthenium coordination complexes. Aggregation of SWCNTs happens at a higher ionic concentration when bound to dinuclear zinc hydrazone than dinuclear ruthenium coordination complexes. This is supported by dispersion stability, binding kinetics, and equilibrium data. Membrane resistance data is also presented supporting the complexes acting as molecular spacers in SWCNTs dispersions. Nanomaterials are paramount to the performance of many technologies because of their extremely high specific surface area (SSA) and surface characteristics. Current technologies are reaching the threshold of their improvement and performance. 1 For example, nanomaterials combined with other molecules have been used in aptasensor designs. 2 A major challenge associated with most nanomaterials is their aggregation, which radically decreases their SSA. In earlier work, long range interactions (LRI) between SWCNTs were explored by investigating tube interactions with various solvated ionic species. 3 The stability of dispersed SWCNTs is dominated by Derjaguin, Landau, Verwey and Overbeek (DLVO) interactions. 4 It was found that the species most effective at collapsing the electrical double layer (EDL) of the nanomaterials were dinuclear coordination complexes with strong binding to the SWCNTs, acting as a molecular spacer between tubes. Below, we present results on coordination complexes that bind to SWCNTs by acting as molecular spacers; however, the coordination complexes do not collapse the EDL as adequately as coordination complexes with bridging ligands. The binding and less effective collapsing of these species leads to higher loading onto the SWCNTs which may increase their dispersion stability. This allows us to utilize SWCNTs SSA for supercapacitor materials. Adsorption kinetics and equilibrium of two divalent, dinuclear metal coordination complexes were compared. The adsorption of the coordination complexes, +2Zn2 and +2Ru2, can be understood through LRI such as van der Waals forces, electrostatics, and π-π interactions. Electrostatic interactions occur between the partial negative surface charge of the SWCNTs and the positive charge of the molecular spacer. Further, π-π interactions exist between the aromatic ligands of the molecular spacers and the π- electrons on the SWCNTs. 5 SWCNT s adsorption capacity 136

2 Chauncey McNeill depends on the number of active binding sites available to interact with the adsorbant. 6 +2Ru2 bind more strongly to SWCNTs than +2Zn2, but the loading is reversed. METHODS Sample Preparation The SWCNT were HiPco (Grade P CNT from Unidym, nm diameter, nm length). SWCNT dispersions were made by adding N,N- Dimethylformamide (DMF) (30 ml) to powder (0.5 mg) then tip ultrasonicated for 30 min at 10 W RMS. Following ultrasonication the dispersions were ultracentrifuged at g for 20 min to deposit undispersed SWCNTs. The supernatant was carefully removed, with the stable dispersion stored for later use. Samples were ultrasonicated for 20 min before use in experiments. All experiments were conducted at room temperature C. Molecular Spacers Two metal coordination complexes were adsorbed to the SWCNTs as possible molecular spacers. A dinuclear ruthenium coordination complex, [Cl(2,2 ;6,2 terpyridine)ru(tetrapyrido[3,2-α:2,3 - c:3,2 -h:2,3 -j]phenazine)ru(2,2 ;6,2 - terpyridine)cl]-(pf 6 ) 2 (+2Ru2) 7, and a dinuclear zinc coordination complex, 4-(tertbutyl)-2 6-bis((2-(phthalazin-1-yl)hydrazono)methyl)phenol(m 2 -methoxo) dizinc(ii)- acetate (+2Zn2) 8 were dissolved in dry DMF for binding studies. Binding Kinetics and Adsorption Studies Adsorption of molecular spacers to the surface of SWCNTs was performed by adding different concentrations of adsorbent to the dispersion. Samples were ultracentrifuged at g for an hour to guarantee sediment did not interfere with optical adsorption measurements. The supernatant was pipetted off, and the equilibrium concentration of unbound species was measured using UV-Vis spectroscopy Measured concentrations were normalized to a pristine SWCNTs control dispersion. The concentration of molecular adsorbate was compared to initial controls to determine the amount bound to SWCNTs. These studies were performed at various incubation periods from 10 to 90 min increasing in 5 min intervals to determine binding kinetics. Membrane Resistance Membrane resistance studies were conducted using vacuum filtration across a 9.55x10-4 m mg thin film of SWCNTs. The flux of DMF through the membrane was measured with successive 50.0 ml rinses. +2Zn2-SWCNTs films were produced by incubating dispersed tubes with 9.04 μm of +2Zn2 for 90 min. The film was formed by pouring the +2Zn2-SWCNTs dispersion onto a 0.45 mm pore polypropylene membrane and vacuum filtered, leaving behind a film of +2Zn2-SWCNTs. The filtrate was collected with the amount of coordination complex remaining after each rinse being used to determine q. RESULTS AND DISCUSSION Binding Kinetics and Adsorption Results The amount of +2Zn2 bound to SWCNTs, qe the mg of +2Zn2 adsorbate per g of SWCNTs adsorbent, was measured versus time. Adsorption kinetics studies were used to find the binding mechanism. The kinetics of +2Zn2 bound to SWCNTs were measured as a function of initial concentration, C o, of +2Zn2. The concentration of SWCNTs and +2Zn2 were measured using UV-Vis spectroscopy. Because they do not absorb in the same region, a correction was applied for any remaining SWCNTs in the dispersion as shown in Figure 1. At low C o, q t nears the theoretical maximum packing, q max. The initial rates of adsorption were computed from the regression of q t versus time. 9 The binding rate was not dependent on the number of binding sites, supporting an independent binding site model. This pseudo-zeroth-order kinetics 137

3 also supports an independent binding model of the +2Zn2 complex onto the surface sites of the SWCNTs where [+2Zn2] = C 0 - kt. The rate constants k = 7.3 ± 0.5 (mg g -1 ) min -1 and k = 7.1 ± 1 (mg g -1 ) min -1 are experimentally the same for different initial concentration C 0 = 5.71 mg L -1 (b) and C 0 = 19.0 mg L -1 (a), respectively. 9 The kinetics data are not in agreement with a pseudo-first or pseudosecond-order kinetics models. 10 An adsorption isotherm of +2Zn2 intercalating with SWCNTs was made by incubating samples for 2 h before removing the SWCNTs via ultracentrifugation. The equilibrium concentration, C eq, was computed using UV-Vis spectroscopy. Adsorption data was analyzed using a modified Brunauer-Emmett-Teller (BET) 11 isotherm, where q m is the adsorbate loading of one monolayer, K s is the equilibrium constant for the adsorbate SWCNTs surface interaction, and K L is the equilibrium constant for the subsequent adsorbate layer bound to the first monolayer. 9 A non-linear least squares fit on these parameters to the data (see Figure 2) determined that q m = 704 ± 29 mg g -1, K s = 1.1 ± 0.15 mg L -1, and K L = ± mg L q m * is the theoretical maximum packing +2Zn2 could have when π-π stacked onto a flat surface, represented as the highest dashed line in Figure 2. The actual q m = 704 ± 29 mg g -1 is relatively close to q m *= 1057 mg g -1, which is surprising as SWCNTs surfaces are curved. Indicated by the middle dashed line in Figure 2, as the adsorption of coordination complex onto the SWCNTs surface ends, the SWCNTs become saturated. At higher concentrations, i.e., above the onset of aggregation X o = 14.2 ± 0.3 μm, 9 some of the SWCNTs will aggregate before adsorption of +2Zn2 can happen. After aggregation, the surface is not accessible to the coordination complex. The high ionic concentration of coordination complex destabilizes the dispersions causing aggregation. The same adsorption studies were carried out for +2Ru2 onto SWCNTs surfaces. This coordination complex fully dissociates and is a strong electrolyte in DMF, whereas +2Zn2 forms an ion pair in DMF. Both coordination complexes are held together with the same tetrapyridophenazine bridging ligand that is able to π-π stack with SWCNTs surfaces. The heteroligands on the +2Ru2 coordination complex hinder the adsorption of the complex onto the SWCNTs surface; this problem is not present in the planar +2Zn2 coordination complex. The samples were incubated for 2-3 days at low concentrations because of the slow binding kinetics. The +2Ru2 binds stronger to SWCNTs than +2Zn2, and the onset of aggregation is much lower at X 0 = 1.22 ± 0.01 μm. 9 SWCNTs begin to aggregate at much lower concentrations, reducing their surface area. Figure 3 shows the Langmuir isotherm model of the +2Ru2 bound to SWCNTs. K L is essentially zero making the models equivalent with qm = 87 ± 4 mg g -1 and K s = 3.5 ± 0.6 mg L Ru2 binding constant, K s, is larger than +2Zn2 binding constant at K s = 1.1 ± 0.15 mg L -1. This is expected as the ruthenium fully dissociates in DMF and has more delocalization on its ligand. Membrane Resistance Results We have theorized that our coordination complex acts as a molecular spacer, intercalating with SWCNTs and increasing the distance between tubes. These molecular spacers increase the accessible SSA and the flow rate through condensed films. The membrane resistance (MR) of these thin films can be determined using Darcy s law 12, where ΔP is the differential pressure across the membrane, a is the area of the film, μ is the dynamic viscosity, and Q is the flux across the membrane. Figure 4 presents the normalized MR of pristine (diamonds) and +2Zn2- SWCNTs thin films. The MR of pristine films is consistently higher until the fourth rinse, 138

4 Chauncey McNeill where the MR of both film types converge. For the +2Zn2-SWCNTs thin films, the MR increased after successive rinses until the MR stabilized. The MR of the first rinse of the +2Zn2-SWCNTs thin films is almost half that of the pristine thin film. There is an increase in MR of the +2Zn2-SWCNTs thin films as the +2Zn2 is washed off after each rinse. CONCLUSIONS Using the large SSA of SWCNTs, electrical energy from renewable sources can be stored using a double layer capacitance mechanism. We have used molecular spacers to prevent the aggregation of the SWCNTs such that their SSA can be used for energy storage applications. We studied the binding kinetics and equilibrium of two different coordination complexes that intercalate between the SWCNTs in dispersions and in thin films. The +2Ru2 complex binds more strongly to SWCNT surfaces then does the +2Zn2 complex; however, the loading onto the SWCNTs is much lower for the ruthenium complex. In addition, ruthenium is too expensive of a metal to make ruthenium-based molecular spacers a viable option. Improving molecular spacer design and application can help ensure a future with less non-renewable energy sources. Energy storage devices based on our molecular spacer results have shown promise. Other metal coordination complexes are being investigated that are more earth abundant and cost effective. In future studies, coordination complexes with enhanced oxidation-reduction activity will be tested and characterized. We have shown that molecular spacers with high π conjugation and steric bulk on the ligands, along with low charge on the metal center, perform the best for electrical energy storage applications. Figure 1. Adsorption kinetics of +2Zn2 bound to SWCNT dispersed in DMF. As the initial concentration of adsorbate increased so does the amount of complex bound to the SWCNT, until saturation. Initial adsorption rates are constant with time, making them quasi-independent of adsorbate concentration. 139

5 Figure 2. Adsorption isotherm of +2Zn2 onto a SWCNT. The theoretical max packing, q m * is shown as the highest dashed line. The inset is the molecular spacer intercalating with a SWCNT. Figure 3. Adsorption isotherm of +2Ru2 onto a SWCNT surface. The C eq of adsorbate is measured after the SWCNT have been centrifuged out of the dispersion. 140

6 Chauncey McNeill Figure 4. Membrane resistance of pristine SWCNT (diamond) and +2Zn2-SWCNT thin films. The membrane resistance of the +2Zn2-SWCNT thin films increases with successive DMF rinses. 141

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