NanoTin Catalysts for Electrochemical Reduction of Carbon Dioxide to Formate
High surface area tin oxide nanoparticles prepared by a facile hydrothermal method followed by electroreduction to tin act as electrocatalysts toward CO2 reduction to formate, in some embodiments. At certain of these nano-structured tin catalysts, CO2 reduction occurs selectively to formate at low overpotentials and with high Faradaic efficiencies, with high stability and significant current densities.
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This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/983,581, filed Apr. 24, 2014, having the title, “NanoTin Catalysts for Electrochemical Reduction of Carbon Dioxide to Formate,” the disclosure of which is incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTThis invention was made with government support under Grant No. DE-SC0001011 awarded by the Department of Energy. The U.S. Government has certain rights in the invention.
FIELD OF INVENTIONThis invention relates to nanoparticles comprising tin that function as catalysts, such as for example, in the reduction of carbon dioxide to formate.
BACKGROUND OF THE INVENTIONThe increasing carbon dioxide (CO2) content in the atmosphere has been cited as a major contributor to the greenhouse effect and global warming. Electrochemical or photoelectrochemical reduction of CO2 could provide an attractive solution to this climate issue with CO2 converted into useful fuels and utilized as a chemical feedstock. It is also an energy storage strategy with solar or electrical energy stored as chemical energy in a reduced carbon product. Formic acid, produced from CO2 reduction to the formate, is a good candidate as an anode fuel for fuel cells and a promising material as a hydrogen carrier. A number of homogeneous and heterogeneous catalysts have been evaluated for electrochemical CO2 reduction including metal electrodes. Electrodes of Pb, Hg, In, Cd, and TI are able to promote reduction of CO2 to formate but are highly toxic, expensive, or both.
SUMMARY OF THE INVENTIONWe report here the results of an electrochemical study on the reduction of CO2 at ultrafine nano tin oxide with controlled particle sizes reduced to contain metallic tin. These nano tin oxide particles were synthesized by utilizing a hydrothermal synthesis method. To maximize surface area, they were loaded onto high surface area carbon supports (carbon black and graphene) with the added advantage of utilizing their 3D porous structures to facilitate CO2 transport and reduction. The as-prepared nano tin oxide surfaces were characterized as tin(IV) oxide (SnO2) and shown to be quickly and efficiently reduced to the metal at an onset potential of about −1 V vs. SCE in aqueous 0.1 M NaHCO3. We find a notable size dependence on CO2 reduction efficiency to formate at the reduced nano tin oxide (i.e., metallic tin) catalyst surfaces. In one embodiment, efficiencies were maximized on 5 nm particles reaching a maximum Faradaic efficiency for formate production of >93%. These catalysts can be very stable during electrolysis and can continue producing formate for at least 18 hours. Electrocatalytic activity toward CO2 reduction can be tuned by morphology and electronic structure of the Sn catalyst.
From our study, we have invented a technology that comprises several embodiments.
In one embodiment of the present invention, nanoparticles of tin oxide, associated with high surface area carbon appear. Other embodiments provide nanoparticles comprising tin, associated with high surface area carbon. Still other embodiments relate to an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon. Further embodiments involve that electrically-conductive surface in the context of an electrocatalytic electrode. Still further embodiments place that electrocatalytic electrode in an electrochemical cell.
Additional embodiments relate to methods of making nanoparticles of tin oxide, associated with high surface area carbon, comprising reacting a tin halide precursor such as SnCl2, for example, in the presence of high surface area carbon, to form the nanoparticles of tin oxide, associated with high surface area carbon. Certain embodiments involve methods of making nanoparticles comprising tin, associated with high surface area carbon, comprising reducing nanoparticles of tin oxide, associated with high surface area carbon, thereby making the nanoparticles comprising tin, associated with high surface area carbon. Still other embodiments of the present invention relate to methods for making an electrocatalytic electrode, comprising: depositing, on an electrically-conductive surface, nanoparticles of tin oxide associated with high surface area carbon; reducing at least some of the tin oxide to form nanoparticles comprising tin, associated with high surface area carbon, thereby making the electrocatalytic electrode.
Yet additional embodiments relate to methods for reducing carbon dioxide to formate, comprising: providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises:
nanoparticles comprising tin, associated with high surface area carbon;
exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition;
applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react;
thereby reducing the carbon dioxide to formate.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. The figures are not necessarily to scale, and some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. In the event that there is a plurality of definitions for a term herein, those in this section prevail unless stated otherwise.
Where ever the phrase “for example,” “such as,” “including” and the like are used herein, the phrase “and without limitation” is understood to follow unless explicitly stated otherwise. Similarly “an example,” “exemplary” and the like are understood to be non-limiting.
The term “substantially” allows for deviations from the descriptor that don't negatively impact the intended purpose. Descriptive terms are understood to be modified by the term “substantially” even if the word “substantially” is not explicitly recited.
The term “about” when used in connection with a numerical value refers to the actual given value, and to the approximation to such given value that would reasonably be inferred by one of ordinary skill in the art, including approximations due to the experimental and or measurement conditions for such given value.
The terms “comprising” and “including” and “having” and “involving” (and similarly “comprises”, “includes,” “has,” and “involves”) and the like are used interchangeably and have the same meaning. Specifically, each of the terms is defined consistent with the common United States patent law definition of “comprising” and is therefore interpreted to be an open term meaning “at least the following,” and is also interpreted not to exclude additional features, limitations, aspects, etc. Thus, for example, “a device having components a, b, and c” means that the device includes at least components a, b and c. Similarly, the phrase: “a method involving steps a, b, and c” means that the method includes at least steps a, b, and c.
“Nanoparticles of tin oxide” relate to particles, dispersed or agglomerated, comprising tin oxide. In some cases, a majority of the mass of the particle is tin oxide. In other cases, the particle is substantially tin oxide. The nanoparticles can be any suitable size. For example, the particles can have an average size of about 1 μm or less, about 200 nm or less, about 10 nm or less, or range in size from about 3 nm to about 10 nm. In some cases, the nanoparticles have an average size of about 5 nm. Certain instances provide amorphous, semi-crystalline, or nanocrystalline tin oxide. Certain other instances provide rutile tin oxide.
“Nanoparticles comprising tin” relate to particles, dispersed or agglomerated, comprising tin. In some cases, a majority of the mass of the particle is tin. In other cases, the particle is substantially tin. In still other cases, the particle contains tin and tin oxide. The nanoparticles can be any suitable size. For example, the particles can have an average size of about 1 μm or less, about 200 nm or less, about 10 nm or less, or range in size from about 3 nm to about 10 nm. In some cases, the nanoparticles have an average size of about 5 nm.
When nanoparticles of tin oxide or nanoparticles comprising tin are “associated with high surface area carbon,” the nanoparticles are in contact with the carbon. The nature of that contact may involve any suitable interaction. Covalent bonding, ionic bonding, van der Waals interactions, electrical communication, or any combination thereof is possible. In some cases, the association happens because the particle is synthesized in the presence of the high surface area carbon.
“Fluid compositions” include any suitable form of matter that can flow. Gasses, liquids, sols, slurries, gels with a mobile fluid phase, and combinations thereof are possible.
Some embodiments relate to an electrocatalytic electrode, comprising an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon. In some cases, the high surface area carbon comprises carbon black, graphene, or a combination thereof. In other cases, the high surface area carbon is chosen from carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, and combinations of two or more thereof. Certain instances provide that the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof. Further instances involve the electrically-conductive surface comprising a gas diffusion electrode. Additional instances place the electrocatalytic electrode in an electrochemical cell adapted for the reduction of CO2, such as, for example, a flow cell.
Further embodiments relate to making nanoparticles of tin oxide associated with high surface area carbon, comprising reacting tin(II) chloride in the presence of high surface area carbon and water for a time. In some instances, the reacting takes place at an increased temperature. Any suitable temperature can be used. In some cases, the temperature is greater than about 50° C., greater than about 100° C., greater than about 150° C., greater than about 200° C., or greater than about 250° C. In certain cases, the temperature is about 196° C. The time for reacting is any suitable time. That time can be at least 30 minutes, at least 3 hours, or no more than about 6 hours.
Reducing tin oxide to form tin can occur according to any suitable method. Contact with a reducing agent, application of a reducing potential, or combinations thereof are possible. In some cases, the reducing comprises applying a reducing potential to the tin oxide no more positive than about −1 V versus SCE, or no more positive than about −1.1 V versus SCE.
Certain embodiments of the present invention involve methods for making an electrocatalytic electrode. In some cases, nanoparticles of tin oxide associated with high surface area carbon are deposited according to any suitable method onto an electrically-conductive surface. Then, at least some of the tin oxide is reduced according to any suitable method to form nanoparticles comprising tin, associated with high surface area carbon, thereby making the electrocatalytic electrode.
Further embodiments relate to methods for reducing carbon dioxide to formate. In some cases, those methods involve providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: nanoparticles comprising tin, associated with high surface area carbon; exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition; applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate. Any suitable electrocatalytic cell can be used. In certain instances, such a cell provides a three-electrode configuration with the electrocatalytic electrode functioning as the working electrode, a counter electrode comprising any suitable material such as, for example, platinum, and a reference electrode such as SCE. In other instances, a two-electrode cell is possible. The electrocatalytic electrode is exposed to any suitable fluid composition sufficient to allow carbon dioxide to react at the electrode. In some cases, the fluid composition is aqueous liquid. In certain cases, the fluid composition comprises one or more carbonate salts, one or more bicarbonate salts, one or more phosphate salts, one or more biphosphate salts, or a combination thereof. Still other cases provide a fluid composition comprising potassium bicarbonate, sodium bicarbonate, or a combination thereof. The fluid composition can contain any suitable concentration of carbon dioxide. In some cases, the fluid composition is saturated with carbon dioxide. Any suitable catalyzing potential can be applied to allow the carbon dioxide to react. For example, the catalyzing potential can be more negative than about −1 V versus SCE, more negative than about −1.2 V versus SCE, no more negative than about −1.8 V versus SCE, or no more negative than about −2.0 V versus SCE. In select instances, the catalyzing potential is about −1.8 V versus SCE.
Without wishing to be bound by theory, the practice of various embodiments of the present invention will be illustrated by a description of the experiments performed in our electrochemical study and an analysis of our observations.
Synthesis and Characterization of Nano Tin Oxide.Tin oxide nanoparticles were synthesized by a modification of a facile hydrothermal method.
The high resolution Sn3d XPS spectrum in
The three intense diffraction peaks in the XRD pattern shown in
In this equation, L is the mean particle size, λKα1 is the X-ray wavelength (λ=1.5418 Å), θmax is the angle for the (110) plane, and B2θ is the band width at half height for the Pt(110) plane. The XRD patterns are noisy in appearance due to the presence of carbon support. To get a more accurate value of nano-SnO2 particle size, these patterns had been smoothed using Gaussian Fitting before the average particle sizes of nano SnO2 were calculated based on the Scherrer equation. Based on this analysis, the SnO2 volume averaged diameter from the XRD data was ˜4.8 nm which is slightly less than the calculated volume area average diameters from TEM. The discrepancy probably arises from the fact that SnO2 nanoparticles that are smaller than 1 nm are poorly shown in TEM images. If included, they would decrease the averaged diameters from TEM images.
Electrocatalytic Reduction of CO2.Electrocatalytic carbon dioxide reduction by nano-SnO2 was evaluated in 0.1 M NaHCO3 aqueous solutions. In the linear sweep voltammetric (LSV) scans in
By comparison, the current density in
Controlled potential electrolyses were performed to investigate the effect of applied potentials on Faradaic efficiencies for formate production at reduced nano-SnO2/carbon black electrodes.
The results in
As shown in
Electrolysis at SnO2/graphene was also conducted at −1.8 V for 18 hours in 0.1 M NaHCO3,
CO2 reduction efficiencies on Sn catalysts depend on particle size. As shown in
To explore a possible role for adsorption of the reduced intermediate CO2.− during CO2 catalytic reduction cycles, and how it varies with particle size, we examined adsorption of OH− as a surrogate for CO2.− as a function of particle size. In these experiments, the current response was monitored following single oxidative LSV scans beginning at −1.6 V vs. SCE through the surface wave for Sn(0) oxidation to Sn(II). Based on the data in
In comparing the data in
A possible CO2 reduction mechanism on Sn is illustrated in eqs 1-5. In the reaction sequence, eq 3 is presumably the rate-determining step (RDS). This assignment is supported by the results in
Additional evidence is provided by the Tafel slope (
In these mechanisms, surface adsorption plays a role in electrocatalytic CO2 reduction. Surface stabilization by adsorption would increase the barrier to further reduction and protonation of CO2.− with weak adsorption decreasing the activating effect of surface binding. Experimentally, the interplay between the two leads to maximized efficiencies for 5 nm particles, in some embodiments.
CO2(solution)→CO2(ads) (1)
CO2(ads)+e−→CO2.−(ads) (2)
CO2.−(ads)+HCO3−+e−→HCO2−(ads)+CO32− (3)
HCO2−(ads)→HCO2−(solution) (4)
CO32−+CO2+H2O→2HCO3− (5)
The electronic structure of the nano tin catalyst may also play an role with catalysis maximized on graphene as the support. Graphene is a two dimensional, one atom thick sp2-bonded delocalized carbon substrate. Strong electronic interactions between graphene and added metal nanoparticles can modify the electronic structure of the latter and influence surface molecular adsorption and reactivity, in some embodiments of the present invention.
The TEM images in
The difference in catalytic activity may be a consequence of differences in the influence of electronic perturbations on Sn particle electronic structure on the two supports. From the high resolution Sn3d XPS spectrum in
Further embodiments of the present invention provide tin oxide nanoparticles of varying size. By controlling the size of tin oxide nanoparticles on carbon supports, overpotentials as low as ˜340 mV can be achieved for CO2 reduction to HCOO− with significant enhancements in both current density and efficiency. In aqueous NaHCO3 solutions saturated in CO2, maximum Faradaic efficiencies for formate production of >93% have been reached with current densities of over 10 mA cm−2 on high surface area graphene supports. The reduced nano tin oxide (i.e., metallic tin) catalysts are highly stable during controlled potential electrolysis and it is contemplated to achieve current density enhancements of 1-2 orders of magnitude by using flow cell or gas diffusion electrodes (GDE).
The reactivity toward CO2 reduction achieved here is notable. It may arise from a compromise between the strength of the interaction between CO2.− with the nano tin surface and its subsequent kinetic activation toward protonation and further reduction. With the graphene support there may also be a role for electronic interactions with the underlying graphene substrate. In any case, our results demonstrate important roles for catalyst morphology and electronic structure in the electrocatalytic reduction of CO2.
Examples 1 Materials PreparationPreparation of Nano Tin Oxide Catalysts.
Tin oxide nanoparticles were synthesized by a modification of a facile hydrothermal method. 100 mg of Tin(II) chloride (SnCl2, 98%, Acros Organics) precursor was dissolved in ethylene glycol (EG, Fisher Scientific) with trace amount of water, mixed with 160 mg of carbon support (carbon black, VULCAN® XC72, purchased from Cabot Corporation; or graphene prepared by a thermal expansion method reported in the literature), and then ultrasonicated for ˜30 min. The resulting mixture was heated up to 196° C. and then refluxed with vigorous stirring. The product was collected by ultrafiltration and washed with deionized water, and then dried for 3 h at 90° C. in vacuum. Particle size of the resultant tin oxide can be tuned by reaction time. When reaction time proceeded in 30 min, about 3 nm tin oxide nanoparticles were formed. Around 5 nm tin oxide nanoparticles were obtained when the reaction time was increased to 3 hours. The largest tin oxide nanoparticles prepared here were about 10 nm after 6 hours.
Electrodeposited Tin Particles on Pre-Polished Glassy Carbon Electrode.
A glassy carbon electrode was first well polished in 0.3 and 0.05 micro Alumina powder to obtain a mirror surface and then sonicated and thoroughly rinsed with Milli-Q water and acetone. The polished glassy carbon electrode as the working electrode was immersed into 10 mM SnCl2 dissolved in acetonitrile with 0.1 M TBAPF6 electrolyte, and then was scanned for 5 cycles from −1 to −2 V versus SCE with a Pt wire counter electrode. Based on CV curves described above, about 0.2 C of charge was consumed during the reduction of SnCl2 to Sn(0) giving the loading of electrodeposited Sn on glassy carbon to be about 123 μg.
Tin Foil.
Commercial tin foil (1.7 g/50×50 mm), 0.25 mm (0.01 in) thick, 99.998% (metals basis) was purchased from Alfa Aesar. Before electrochemical measurements, the as-received tin foil (about 3400 μg) was pretreated in 1 M HNO3 for 10 second to remove surface oxide and contaminants.
2 Electrochemical MeasurementsThe electrochemical measurements were carried out in a gas-tight two compartment electrochemical cell system controlled with a CHI601 D station (CH Instruments, Inc., USA) with Pt wire and Saturated Calomel Reference Electrode (SCE) as the counter electrode and reference electrode, respectively. The working electrodes were prepared by loading sample suspension onto the prepolished glassy carbon electrodes. In brief, as prepared nano-SnO2/carbon black or nano-SnO2/graphene sample was dispersed in ethanol and ultrasonicated for 15 min to form a uniform catalyst suspension (2 mg mL−1). A total of 7.5 μL of well dispersed catalyst ink was applied onto the pre-polished glassy carbon electrode (3 mm in diameter). This gives the loading of carbon supported nano-SnO2 sample to be about 15 μg, and the amount of Sn is calculated to be about 3.45 μg based on the nano-SnO2 content of ˜30% in catalyst. After drying at room temperature, 5 mL of 0.05 wt % Nafion solution was applied onto the surface of the catalyst layer to form a thin protective film. The well-prepared electrodes were dried at room temperature overnight before the electrochemical tests. Linear sweep voltammetric (LSV) scans were recorded in N2 or CO2 saturated 0.1 M NaHCO3, while controlled potential electrolysis was performed in 0.1 M NaHCO3 electrolyte with CO2 flow.
3 Physical CharacterizationsRaman spectra were collected by Raman measurements (Renishaw) with a 514 nm laser. X-ray data were collected on Bruker Smart APEX II CCD diffractometer equipped with Cu-target X-ray tube and operated at 1600 watts. High-resolution TEM (HRTEM) was obtained on a JEOL 2010F-FasTEM. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) were obtained on a FEI Helios 600 Nanolab Dual Beam System focused ion beam (FIB) equipped with an Oxford Instruments, INCA PentaFET-x3 X-ray detector with the electron beam set to 20 keV and a beam current of 0.69 nA. In addition, the ion beam was used to mill into the film and determine its permeability to the deposited metal particles. EDS measurements were made of film surfaces, and with a 1 μm2, 100 nm deep square milled into the film, another EDS spectrum was recorded.
NMR analysis was used to quantify the yield of formate during controlled potential electrolysis. NMR spectra were recorded on Bruker NMR spectrometers (AVANCE-400). 1H NMR spectra were referenced to residual solvent signals. At the end of electrolysis periods, gaseous samples (0.8 ml) were drawn from the headspace by a gas-tight syringe (Vici) and injected into the GC (Varian 450-GC, pulsed discharge helium ionization detector, PDHID). Calibration curves for H2 and CO were determined separately.
X-ray photoelectron spectra (XPS) were obtained at the Chapel Hill Analytical and Nanofabrication Lab (CHANL) at UNC. A Kratos Analytical Axis UltraDLD spectrometer with monochromatized X-ray Al Kα radiation (1486.6 eV) with an analysis area of 1 mm2 was used. A survey scan was first performed with a step size of 1 eV, a pass energy of 80 eV, and a dwell time of 200 ms. High resolution scans were then taken for each element present with a step size of 0.1 eV and a pass energy of 20 eV. The binding energy for all peaks was referenced to the C 1s peak at 284.6 eV XRD.
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As previously stated, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. It will be appreciated that many modifications and other variations stand within the intended scope of this invention as claimed below. Furthermore, the foregoing description of various embodiments does not necessarily imply exclusion. For example, “some” embodiments may include all or part of “other” and “further” embodiments within the scope of this invention. In addition, “a” does not mean “one and only one;” “a” can mean “one and more than one.”
Claims
1. A method for reducing carbon dioxide to formate, comprising:
- providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: nanoparticles comprising tin, associated with high surface area carbon;
- exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition;
- applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react;
- thereby reducing the carbon dioxide to formate.
2. The method of claim 1, wherein the fluid composition comprises potassium bicarbonate, sodium bicarbonate, or a combination thereof.
3. The method of claim 1, wherein the catalyzing potential is about −1.8 V versus SCE.
4. The method of claim 1, wherein the nanoparticles comprising tin, associated with high surface area carbon exhibit a specific current density greater than about 10 A g−1.
5. The method of claim 1, wherein the nanoparticles comprising tin, associated with high surface are carbon exhibit a specific current density greater than about 100 A g−1.
6. An electrocatalytic electrode, comprising:
- an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon.
7. The electrocatalytic electrode of claim 6, wherein the high surface area carbon is chosen from carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, and combinations of two or more thereof.
8. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 200 nm or less.
9. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 10 nm or less.
10. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size ranging from about 3 nm to about 10 nm.
11. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 5 nm.
12. The electrocatalytic electrode of claim 6, wherein the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof.
13. The electrocatalytic electrode of claim 6, wherein the electrically-conductive surface forms part of a gas diffusion electrode.
14. A method for making an electrocatalytic electrode, comprising:
- depositing, on an electrically-conductive surface, nanoparticles of tin oxide associated with high surface area carbon;
- reducing at least some of the tin oxide to form nanoparticles comprising tin, associated with high surface area carbon,
- thereby making the electrocatalytic electrode.
15. The method of claim 14, further comprising:
- reacting tin(II) chloride in the presence of high surface area carbon and water for a time, thereby forming the nanoparticles of tin oxide associated with high surface area carbon.
16. The method of claim 15, wherein the time is at least 30 minutes.
17. The method of claim 15, wherein the time is at least 3 hours.
18. The method of claim 15, wherein the time is no more than about 6 hours.
19. The method of claim 14, wherein the reducing comprises applying a reducing potential to the tin oxide no more positive than about −1 V versus SCE.
20. The method of claim 14, wherein the nanoparticles of tin oxide comprise rutile tin oxide.
Type: Application
Filed: Jan 12, 2015
Publication Date: Apr 7, 2016
Applicant: THE UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL (Chapel Hill, NC)
Inventors: Sheng Zhang (Chapel Hill, NC), Peng Kang (Durham, NC), Thomas J. Meyer (Chapel Hill, NC)
Application Number: 14/594,269