Supporting Information
Turning Indium Oxide into a Superior Electrocatalyst: Deterministic Heteroatoms
Bo Zhang†, Nan Nan Zhang†, Jian Fu Chen†, Yu Hou, Shuang Yang, Jian Wei Guo, Xiao Hua Yang, Ju Hua Zhong, Hai Feng Wang*, P. Hu, Hui Jun Zhao and Hua Gui Yang*
† These authors contributed equally to this work
Part I: Experimental section
S1 Chemicals.
1-octadecene (90%), oleic acid (90%) and 1-octadecylamine (97+%) were peruchased from Sigma-Aldrich. Indium (III) nitrate hydrate (99.5+%, analytical reagent grade) was obtained from Sinopharm Chemical Reagent Co. Ltd. Indium (III) acetate (99.99%) was purchased from Alfa. All synthetic experiments were conducted under an inert argon atmosphere.
S2 Synthesis of undoped In2O3 nanooctahedrons.
Undoped In2O3 nanooctahedrons were prepared according to the literature.1 And the details are as follows: Under magnetic stirring and heating to 90 ℃, 1.0 mmol indium (III) acetatewas dissolved in mixed 1.0 mL oleic acid and 4.0 mL 1-octadecene to form 1.0 mmol indium oleate. The resulting colorlesssolution was then degassed under vacuum at 90 ℃ for15 min followed by quickly heating to 320 ℃. By maintainingat this temperature for 30 min, a milky white solution wasobtained. The product was collected by centrifugation, washed with hexane and alcohol several times.
S3 Characterizations.
Figure S1. SEM-EDX spectra of interstitial N-In2O3 nanocrystals.
Figure S2. X-ray photoelectron survey spectra ofIn2O3 nanooctahedrons
Figure S3. High-resolution In 3d XPS. The open circles are the raw data of the X-ray photoelectron spectra, and the blue and red/purple/green/brown lines represent the base line and fitted lines, respectively.
Figure S4. XRD of interstitial N-In2O3 nanocrystals.
Figure S5. SEM of undoped In2O3 nanooctahedrons
S4 Preparation of dye-covered nanocrystalline-TiO2 electrodes.
To prepare the DSC working electrodes, the FTO glass was used as current collector (8 Ω/square, Nippon Sheet Glass, Japan). The commercially-available TiO2 powders, P25(Degussa, Germany), was used as raw material. A 12 μm thick layer of P25 was loaded on FTO by screen printer technique.2 After sintering in the air at 125 °C, a 4 μm thick scattering layer of 200nm-sized TiO2 particles (HEPTACHROMA,DHS-NanoT200) was coated on the up layer. After sintering at 500 °C for 30 min, the TiO2-loaded FTO were immersed in a 5×10-4 M solution of N719 dye (Solaronix SA, Switzerland) in acetonitrile/tert-butyl alcohol (V/V=1/1) for 20-24 h to complete the sensitiser.
S5 Preparation of counter electrodes.
The N-In2O3 nanocrystals and undoped In2O3 nanooctahedronswererespectively mixed with 2% Hydroxypropylmethylcellulose (HPMC) in deionized water and absolute ethanol (V/V=5/3) solution to form a homogeneous paste. A film was then made by the doctor-blading method on a FTO conductive glass.The Pt-electrode was prepared by drop-casting 0.5 mM H2PtCl6/ethanol solution on the clean FTO conductive glass.Subsequently, the FTO glass coated with N-In2O3, undoped In2O3 or Pt was then sintered in a muffle furnace at 450 °C for 30 min and the CEs were obtained.
S6 DSCs assembling.
The counter electrode and dye-covered TiO2 electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 25 μm (Surlyn1702, DuPont). A drop of the electrolyte, which is prepared via dissolving 0.60 M 1-butyl-3-methylimidazolium iodide,0.03 M I2,0.50 M 4-tert-butyl pyridine, and 0.10 M guanidinium thiocyanate in acetonitrile, was put on the hole in the painted counter electrode. Then the electrolyte was introduced into the cell via vacuum backfilling. The dummy cells for electrochemical impedance spectra and Tafel polarization measurements were assembled with two identicalN-In2O3, undoped In2O3 or Pt loaded FTO, and the electrolyte was the same as the above.
S7 Electrochemical impedance spectra of undoped In2O3 nanooctahedrons based DSC.
Figure S6. Electrochemical impedance spectra of the symmetrical cells fabricated with two identical undoped In2O3 nanooctahedrons electrodes, and the insert gives the equivalent circuit.
S8 Cyclic voltammetryand Tafel polarization measurements of N-In2O3 and Pt
Figure S7. Cyclic voltammograms of Pt and N-In2O3 deposited on FTO in an acetonitrile solution of 0.1M LiClO4, 10mM LiI, and 1 mM I2 at a scanning rate of 20 mV s-1
Figure S8. Tafel curves of the symmetrical cells fabricated with two identical counter electrodes
Part II: Calculation section
S9 Free energy calculation.
Here we would show how to calculate the free energy change () of reaction (1).
(a1)
It is hard to directly calculate the energy of the charged periodic system accurately. However, it iswell known that the Gibbs free energy change of the standard hydrogen electrode (SHE) reaction is zero (), that is:
(a2)
By combining reaction (a1) and (a2), we can get reaction (a3), in which the eU term represents the electron free energy shift in the counter electrodes at the voltage U relative to the SHE. It is clear that the Gibbs free energy change of reaction (a3) is equivalent to, i.e. =.
(a3)
As shown below,we can design a thermodynamic cycle based on Hess’s Law to calculateindirectly:
(a4)
For the above cycle, it is obvious that . We used Gaussian 03 software to calculate in reaction cycle (a4), and the salvation energies of I- in acetonitrile solvent, and H+ in water were taken from experimental value(), giving -2.86, -11.53 eV, respectively.3-4For the value of , that is chemical potential difference of I2 molecule in between gas phase and CH3CN solvent, we showed our calculation method in details below.
S10 Estimation of chemical potential difference of I2 molecule in the gas phase and CH3CN solvent.5-7
Considering the phase equilibrium of I2 dissolvation at the gas/liquid interface, we have . By expanding it we can get the following equation (b1):
(b1)
in which and are the vapor pressure of I2 molecule above the I2/CH3CN solution and the concentration of molar percent of I2 molecule in the solution. It could be reasonable to consider I2 dissolvation in acetonitrile solvent as ideal solution, and we have
(b2)
where is the saturated vapor pressure of I2 molecule. Therefore, we can easily get equation (b3)
(b3)
The saturated vapor pressure () of I2 was calculated according to the Antonie Equation (b4)
(b4)
in which the values of A, B and C are taken from the NIST WebBook.
S11 Effect of the coverage of the adsorbed solvent molecule
Based on our calculation results, it was found that solvent molecule CH3CN has a relatively strong adsorption energy (-0.57, -0.55 eV on pure and N doped In2O3(110) respectively), indicating that the solvent molecule possibly has a considerable adsorption coverage on the electrode surface. When I atom was introduced on the electrode surface, there should exist a competing adsorption effect between I atom and CH3CN molecule. We explored various co-adsorption configurations of I and CH3CN on the electrode surface by adjusting the number of CH3CN molecule adsorbed around I*. We found that it is the most favored thermodynamically when two CH3CN molecules adsorb nearby I*. As a result, this configuration was selected as the model to calculate the binding strength of I atom on the electrode surface, which gives an adsorption energy of -0.94 eV. Compared to the case of I adsorption in the absence of CH3CN solvent, it is clear that the solvent effect plays a positive role on I adsorption. To be systematic, we did a series of tests about the influence of solvent adsorption coverage on EadI. From Table S1, it is obvious that as the number of CH3CN molecules adsorbed around I*, the strengthen on I adsorption is more evident, and the bond length (dIn-I ) of In-I increases accordingly.
Table S1 Effect of solvent adsorption coverage on.
Number of adsorbed CH3CN nearby I atom / Energy/ eV / / eV / dI-In / ÅNo solvent (gas phase) / -0.54 / 2.79
0 / 0.44 / -0.78 / 2.86
1 / 0.27 / -0.88 / 2.89
2 / 0.00 / -0.94 / 2.93
S12 Derivation of exchange current
With respect to the I3-reduction reaction (I3-(sol) +2e-→3I-(sol)) occurring on the CE, the general consensus of the reaction mechanism can be described as follows:
I3-(sol) ↔I2(sol) + I-(sol) (1)
I2(sol) + 2* → 2I* (2)
I* + e- → I-(sol) (3)
where * represents the free site on the electrode surface and sol indicates the acetonitrile solution. The solution reaction, I3-(sol) ↔ I2(sol) + I-(sol), has been verified to be usually fast and considered to be in equilibrium.8The I2 reduction reaction (IRR) occurring at the liquid-solid interface, would thus determine the overall electrocatalytic activity. Hence we focused our studies on these two elementary reaction steps at the CH3CN/electrode interface to explore the activity.
The total IRR at the cathode electrode can be simplified as:
I2(sol) +2e-→2I-(sol) (4)
During IRR a current I will be running
i = – e·r
where r = r+ – r– is the net rate of step (4).
Within micro-kinetic framework, considering the reaction rate of Tafel mechanism, the reaction rate of step (2) and (3) can be written as:
where θ is the coverage of surface free site, CI2(sol) and CI-(sol)is the relative concentration of I2 and I- in the solution, respectively. Z1 and Z2 are the reaction reversibility of step (2) and (3), expressed as:
where Keq1 and Keq2 are the reaction equilibrium constant of step (2) and (3), determined by the adsorption energy of I EadI and ΔG0 and can be expressed as:
where kB and h are constants, T is reaction temperature. k1 and k2 are the rate constants of reaction step (2) and (3), respectively, which can be determined by the transition state theory:
The exchange current is the forward (or backward) rate when step (1) is in equilibrium (Z1 = 1, Z2 =1), then i0 = i+|eq = |i–|eq = – e·r0
Then the coverage of I can be calculated from Eq(c3) and we have
Thus the exchange current of the reaction is expressed as:
Under the condition of T =300 K and,when < –0.12 eV (correspond to EadI = (ΔG1 – TΔS + I2)/2 < –0.46 eV), then, Eq(c9) can be simplified as
From the Eq(c10), we can see that when EadI < –0.46 eV, there is a direct correlation between the exchange current i0and desorption barrier ΔEadesof I*.
S13Relative Bader charge analysis
Table S2.Relative Bader charge analysis (with the pure In2O3 as the reference) of In2O3 (110) with or without interstitial N doping, as well as the corresponding I adsorption state.
Bader charge / O / In / sum / N / I / / eVIn2O3(110) / 0.00 / 0.00 / 0.00 / / / / / /
I*/In2O3(110) / -0.28 / -0.01 / -0.29 / / / -0.29 / 0.45
N-In2O3(110) / -1.04 / 1.14 / 0.10 / 0.10 / / / /
I*/N-In2O3(110) / -1.12 / 0.86 / -0.26 / 0.21 / -0.47 / -0.54
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