Soluble Supercapacitors: Large and Reversible Charge Storage in Colloidal Iron-Doped ZnO Nanocrystals

Transcription

1 Supporting Information for Soluble Supercapacitors: Large and Reversible Charge Storage in Colloidal Iron-Doped ZnO Nanocrystals Carl K. Brozek, a Dongming Zhou, b Hongbin Liu, a Xiaosong Li, a Kevin R. Kittilstved, b and Daniel R. Gamelin a,* a Department of Chemistry, University of Washington, Seattle, WA b Department of Chemistry, University of Massachusetts, Amherst, MA gamelin@chem.washington.edu Experimental Methods General Considerations. All measurements and synthetic procedures were conducted under a dinitrogen atmosphere using standard Schlenk techniques or a glovebox. Chemicals. Anhydrous THF was purified through an alumina column pressurized with Ar. Tetrabutylammonium hexafluorophosphate (TBAPF6, 98%, Sigma Aldrich) was recrystallized from ethanol prior to use, as described elsewhere. 1 All other chemicals were used as received. Dimethylsulfoxide (DMSO, 99.9%), ethyl acetate (99.9%), heptane (99.3%), trioctylphosphine oxide (TOPO, 90%), and toluene (99.96%) were purchased from Fisher. Zn(OAc)2 2H2O (OAc = acetate, 98.0% [Fe] = 5 ppm) was purchased from Ricca Chemical. Fe(OAc)2 (94.4%), tetramethylammonium hydroxide pentahydrate (TMAOH, 99%), dodecylamine (DDA, 98%) were purchased from Acros Organics. Pure ethanol was purchased from Pharmco-Aaper. HBF4 (OCH2CH3)2 and ferrocene (Fc) were purchased from Sigma Aldrich. FcBF4 was prepared by following previously reported methods. 2 Silver conductive epoxy was purchased from MG Chemicals and solvent-resistant epoxy was acquired from Masterbond. Synthesis of Zn0.99Fe0.01O Nanocrystals. Zn(OAc)2 2H2O (0.55 g, 2.5 mmol) and Fe(OAc)2 (22 mg, 125 µmol) were dissolved in 25 ml of DMSO in a 125-mL Erlenmeyer flask equipped with a stir bar. After complete dissolution of the reagents, 1.7 equivalents of TMAOH (0.77 g, 4.25 mmol) dissolved in 7.7 ml of ethanol was added dropwise over 90 s under vigorous stirring. Ethyl acetate was added to the reaction mixture until precipitation of the colloid nanocrystals was observed. The resulting pellet was re-suspended in ethanol and then precipitated again by addition of heptane. Surface ligands were exchanged by heating the resulting pellet in neat DDA at 180 C for 20 min. Precipitation by ethanol was followed by ligand exchange with TOPO, achieved by heating the nanocrystals in neat TOPO at 120 C for 30 min. The nanocrystals were precipitated with ethanol, centrifuged, and re-suspended in heptane. Thin-Film Fabrication. 1 cm 0.5 cm FTO slides were sonicated in a 1-M HCl solution, followed by sonication in dionized water and then acetone. Half of the electrode surface was covered with adhesive tape to confine the NC film to a known area. Films were prepared by drop-casting colloidal suspensions of either ZnO or Zn0.99Fe0.01O NCs onto the FTO slides and allowed to dry in air. Chemical annealing was achieved by immersing the films in a 0.1-M MeCN solution of triethyloxonium tetrafluoroborate (Meerwein s Reagent) for 3 min., washing with MeCN, and air-drying. A freshly polished copper wire was adhered to the un-functionalized S-1

2 half of the FTO slide with silver epoxy. An additional layer of solvent-resistant epoxy was applied to protect the copper wire contact and silver epoxy from the electrolyte solution. Potentiometretic Titrations. Measurements were performed using a custom threeelectrode electrochemical cell, as described previously. 3 The cell included a septum-capped quartz cuvette containing two platinum wires as the working and counter electrodes and a 1-mm Ag/AgCl leakless reference electrode (Edaq). Open-circuit potentials (VOC) were recorded using a Gamry potentiostat under galvanostatic control at I = 0 A. Potentials were referenced versus the Fc + /Fc reduction potential by collecting cyclic voltammograms of the solutions after completion of potentiometric experiments. Zn0.99Fe0.01O NCs (0.4 µm) were suspended in a 0.1-M TBAPF6 THF solution with ethanol (0.285 M) as the hole-quencher, according to previous reports. 4 6 Photodoping was accomplished by irradiating the solution with a 340 nm photodiode (10 mw) until VOC stabilized. A 0.75-µM solution of FcBF4 was used as the titrant. Thin-Film Voltammetry. A three-electrode cell was assembled using a freshly polished Pt wire counter electrode, a Ag/AgCl leakless reference electrode (Edaq), and the functionalized FTO slide as the working electrode. All electrodes were immersed in a 0.1-M TBAPF6 MeCN solution. Integrated average gravimetric capacitance (Cg) was determined using the following relationship 1 V C g = f IdV (S1) 2mv(V f V i ) V i where m is the mass of the NC material (g), v is the scan rate (Vs 1 ), Vi is the initial potential (V), Vf is the final potential (V), and I is the current density (A). The mass loading was approximated by assuming complete removal of surface ligands and spherical symmetry of NC particles added from a stock solution with a known concentration. Physical Characterization. Powder X-ray diffraction patterns were collected on a PANalytical X'Pert Material Research Diffractometer. Nanocrystal radii for all samples were determined by power X-ray diffraction using a PAN-alytical X'Pert Material Research Diffractometer and the Scherrer relation. Transmission electron microscopy images were collected on carbon coated (3 nm) Cu TEM grids (JEOL JEM 2000FX). Fe and Zn concentrations were determined by inductively coupled plasma optical emission spectroscopy with a Perkin Elmer Optima 8300 spectrophotometer. Computational Methods Hybrid Quantum-Classical "Charged Sphere" Model. All calculations employ a modified version of the hybrid quantum-classical Charged Sphere model described previously. 7 In short, the energetics of CB electrons within spherical Zn1-xFexO QDs are computed quantum mechanically by treating CB electrons as particles in a finite spherical well potential that is classically determined. We first determine the potential V(r) acting on a single-particle wavefunction ψ(r). Using Gauss s law, inter-electronic repulsion and electron-cation stabilization are approximated as a mean-field potential. Only Coulomb interactions, not exchange, are considered. As a cross-check, we performed an analytical integration of s-p repulsion and s-p exchange on a d=8.7 nm sphere using an infinite potential well (analytical integration is not feasible for a finite well) and found that exchange energy is an order of magnitude smaller than repulsion. Therefore, neglecting exchange slightly increases electronic energies, but retains the qualitative result. As a starting point, CB electrons are initially treated as uniformly distributed, whereas surface charge-compensating protons are always treated as an evenly distributed charge shell. S-2

3 Localized electrons are compensated by an equal number of surface protons, whereas the surface anions compensating the aliovalent iron dopants exactly cancel with the surface protons accompanying the reductive formation of Fe 2+. For a specific configuration of spatially distributed positively and negatively charged particles, we use Gauss s law to calculate V(r) in the Hamiltonian as shown in Eq. S2 from the corresponding electric field, d 2 Ĥ = 1 l(l + 1) + 2 dr2 2r 2 + V(r) (S2) For each type of charged particles (e.g., electrons or holes) Gauss s law can be written as D ds = ρdv (S3) The left-hand side integrates the electric displacement field over the surface area of an arbitrary Gauss s sphere, and the right-hand side gives the total number of particles included in the sphere. Assuming we only draw Gauss s sphere inside the QD, Eq. S3 can be expanded as: ε 0 εe(r)4πr 2 = ρdv (S4) Initially, particle density ρ on the right-hand side of Eq. S4 is treated as a uniform distribution across the QD sphere independent of r. Therefore, E(r) becomes: E(r) = 4 Q 4 3 πr3 3 πr3 4πε 0 εr 2 The potential at a distance r is easily determined by integration: r V(r) = E(r )dr Substituting V(r) back into Eq. S2 gives the Hamiltonian for calculating the orbital energy (eigenvalue) and wavefunction ψ(r) (eigenvector) of the Fermi electron in the QD through simple numerical methods. To improve accuracy, iterative calculations can be performed by using the computed ψ(r) 2 to define ρ(r) as a starting point for ρ in Eq. S4. For iterative calculations, the right-hand side needs to be integrated from 0 r before moving the 4πε 0 εr 2 term to its denominator: (S5) (S6) E(r) = r 4πr 2 ρ(r )dr 0 4πε 0 εr 2 (S7) Improved E(r) can be calculated through repeated iterations, but to balance accuracy and computational cost we employ only one additional iteration. S-3

4 In the model, the electron affinity (EA) and dielectric constant of the material and the dielectric constant of the solvent are required as input parameters. For all calculations, the dielectric constant of the solvent (THF) is The dielectric constant and EA of Fe 2+ :ZnO is chosen to be the same as pure ZnO, i.e., 0 =10 and EA = 4.2 ev. More details of the improved model will be elaborated in a forthcoming publication. Fermi-Level Calculations. The Fermi-level of an electronically doped ZnO nanocrystal is obtained by solving its 1D radial Schrodinger equation. In ZnO QDs containing 20 e CB, the 20th e CB occupies the f orbital in keeping with Hund s rule and the Pauli exclusion principle, giving an angular momentum l=3. The potential energy of the Fermi-level electron is calculated using the single-iteration scheme described above. Reproducing Vmax. For maximally photodoped Zn1 xfexo QDs, the reduction of Fe 3+ dopants to Fe 2+ causes several parameters to cancel exactly: the electrons localized on Fe cancel the additional positive charges of the original Fe 3+, and the accompanying surface protons cancel the surface anions associated with the original Fe 3+ sites. We uniformly distribute the additional 95 localized electrons within the NC and compensate them with protons at the NC surface. This configuration produces a V(r) that destabilizes 20 CB electrons by 270 mv. Titration Curve Simulation. To simulate the titration curve, anions are added to the NC surface to compensate the removal of CB electrons. The Fermi-level energy of each titration data point is solved following the procedure above. The resulting values are sensitive to the thickness of the electrical double layer. Its value can be approximated using the Debye-Huckel equation for the given experimental electrolyte concentration, but the corresponding value of 0.03 nm is less than the additive van der Waals radii of solvated electrolyte cations and anions. Therefore, the latter value of 0.7 nm was used. S-4

5 Figure S1. Powder X-ray diffraction pattern of d = 8.7 nm Zn0.99Fe0.01O nanocrystals. Figure S2. Transmission electron micrograph of d = 8.7 nm Zn0.99Fe0.01O nanocrystals. References (1) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals; 6th ed.; (2) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, (3) Brozek, C. K.; Hartstein, K. H.; Gamelin, D. R. J. Am. Chem. Soc. 2016, 138, (4) Schimpf, A. M.; Gunthardt, C. E.; Rinehart, J. D.; Mayer, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2013, 135, (5) Wood, A.; Giersig, M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, (6) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, (7) Liu, H.; Brozek, C. K.; Sun, S.; Lingerfelt, D. B.; Gamelin, D. R.; Li, X. J. Phys. Chem. C 2017, 121, S-5