Chemistry
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Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy
Chapters
Summary July 28th, 2020
Dynamic, tensile strain is applied on TiO2 thin films to study the effects of strain on electrocatalysis, specifically proton reduction and water oxidation. TiO2 films are prepared by thermal treatment of the pseudo-elastic NiTi alloy (Nitinol).
Transcript
The ability to apply viable external forces to materials allow us to alter their surface properties at will, as well as find optimal catalytic activities and demonstrate new properties. This technique allows us to apply strains and study their effect on electro-catalytic activities without needing to prepare multiple materials for each discreet degree of strain. These methods can also be used to study a range of fin films and their electro-chemical properties such as electro-chemical activity and corrosion.
For a chemical and mechanical polishing of nickel-titanium substrates, first, cut a 0.05 millimeter thick piece of super-elastic nickel-titanium into one-by-five centimeter strips, and sequentially polish the resulting samples with 320, 600 and 1200-grit sand paper. Rinse the sample with ultra-pure water between each polishing. After the last rinse, polish the sample with one micron diamond, 0.3 micron diamond, and 0.05 micron-alumina polish.
After polishing, sonicate the samples with sequential five-minute baths in ultra-pure water, isopropanol, ethanol and ultra-pure water before drying the samples under nitrogen. To prepare 50-nanometer thick rutile-titanium dioxide films, after drying, place the polished nickel-titanium foils in a 500 degree celsius oven under aerobic conditions for 30 minutes. Heating will cause the surface color to change from gray to blue-purple.
To apply tensile stress to the heated film samples, gently clamp one foil in a mechanical tester, leaving one centimeter of foil exposed at each end. Then, strain the nickel-titanium, titanium dioxide sample at a rate of two millimeters per minute, keeping the strain at zero to three percent. Before starting the electro-chemical measurements, pre-strain the foil to five Newtons.
To conduct electro-chemical experiments under applied strain, assemble a custom-made electro-chemical cell loosely around the nickel-titanium, titanium dioxide foil. Carefully positioning the cell in the middle to ensure that the center of the foil is exposed. Tighten the cell gently onto the sample to create a solution-tight cell for the electro-chemical measurements and fill the cell with an electrolyte.
After gently purging the solution with nitrogen, increase the strain from zero to 0.5 percent and conduct cyclic photometry or linear sweep photometry measurements. To perform a hydrogen evolution reaction experiment using 0.5 molar sulfuric acid as the electrolyte, silver, silver chloride as the reference electrode, and a ten centimeter length, 0.5 millimeter diameter coiled platinum wire as the counter-electrode. Scan the potentials between the open circuit voltage to 0.8 folds versus RHE, which is the reversible hydrogen electrode, starting with the highest potential value and a scan rate of five to fifty milli-volts per second.
To perform an oxygen evolution reaction experiment using one molar sodium hydroxide as the electrolyte, mercury, mercuric oxygen as the reference electrode, and a coiled platinum wire as the counter electrode, scan the potential between the open circuit voltage to two volts versus RHE, starting with the lowest potential value and a scan rate of five to fifty milli-volts per second. After completing the measurements, loosen the electro-chemical cell around the nickel-titanium, titanium dioxide foil so that the sample can move freely and gently tighten the cell back onto the sample to realign the assembly around the foil. Then, refill and purge the electrolyte before increasing the strain from 0.5 to one percent, and repeating the electro-chemical experiments.
To determine if increases in hydrogen evolution reaction activities are due to increases in the electro-active surface, run cyclic photometry at different scan rates at a potential range at which faradaic currents are negligible, so that the currents represent only the charge-discharge of the electric double layer and plot the scan rates versus the currents. For the characterization of cracked films, keep a fifty nanometer titanium oxide foil strained at seven percent for thirty minutes or longer, before analyzing the surface for cracking by scanning electro-chemical microscopy. Then, conduct any desired measurements with a proper sample holder for scanning electron microscopy or electro-chemical cell for electro-chemical measurement with pristine and purposely cracked titanium dioxide films at different incrementally, increased and decreased strain values.
For surface characterization of a sample, after electro-chemical measurements, wash the sample with water to remove any residual solved and assemble the rinsed foils in the tensile stretcher. Secure the custom-made sample holders around the strained sample. The sample surfaces can then be assessed by scanning electron microscopy according to standard protocols.
Oxidization of nickel-titanium foils at 500 degrees Celsius results in calcination and a surface layer of rutile titanium dioxide. The thickness of the layer and the degree of N-type doping are affected by the annealing time and the temperature, as indicated by a color change from gray to uniform blue-purple after thirty minutes of heating. Longer heating times, result in thicker titanium dioxide films, and are accompanied by a gradual loss of the blue-purple color.
Nitinol behavior under thermal and mechanical stress, reflects a reversible solid state phase transformation between two different martensite crystal phases, making it a pseudo-elastic rather than an elastic material. Cyclic photometry and linear sweep photometry experiments are important for understanding the electro-chemical system such as faradaic versus non-faradaic ranges. Further electro-chemical characterization, can include electro-chemical impedance to study changes in electrode surface reactivities with strain.
To determine if increases in hydrogen evolution reaction and oxygen evolution reaction activities are simply due to increases in electro-active surface. Capacitance measurements can be performed at different strain values. To further determine if the changes in electro activities with strain are due to elastic or inelastic deformation under applied tensile stress, experiments can be conducted with pristine and purposely cracked titanium dioxide films.
The sample must be mounted properly to obtain and to produce more results. This stencil stretcher can be incorporated in many different characterizations and techniques, including spectroscopy, trans-reabsorption, confocal Raman or probe microscopy.
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