This work was conducted by all co-authors, employees of the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program.

1. Preparation of NiTi/TiO2 electrodes
<ol>
	<li>Chemical and mechanical polishing of NiTi substrates
	<ol>
		<li>Cut the superelastic NiTi foil (0.05-mm thickness) into 1 cm x 5 cm strips.</li>
		<li>Polish sample using 320-, 600- and 1200-grit sandpaper, and then rinse with ultrapure water (18.2 M&#937;).</li>
		<li>Polish sample with 1 &#956;m diamond, 0.25 &#956;m diamond, and 0.05 &#956;m alumina polish.</li>
		<li>After polishing, sonicate for 5 min in sequential baths of ultrapure water (18.2 M&#937;)...

<ol>
	<li>Li, J., Shan, Z., Ma, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elastic+strain+engineering+for+unprecedented+materials+properties.">Elastic strain engineering for unprecedented materials properties.</a> MRS Bulletin. 39, 108-114 (2014).</li><li>Luo, M., Guo, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strain-controlled+electrocatalysis+on+multimetallic+nanomaterials.">Strain-controlled electrocatalysis on multimetallic nanomaterials.</a> Nature Reviews Materials. 2, 17059 (2017).</li><li>Yang, S., Liu, F., Wu, C., Yang, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tuning+Surface+Properties+of+Low+Dimensional+Materials+via+Strain+Engineering.">Tuning Surface Properties of Low Dimensional Materials via Strain Engineering.</a> Small. 2016, 4028-4047 (2016).</li><li>Clark, E. 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A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+Elastic+Strain+on+Electrocatalysis+of+Oxygen+Reduction+Reaction+on+Pt.">Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt.</a> The Journal of Physical Chemistry C. 119, 19042-19052 (2015).</li><li>Gu, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+graded+catalytic-protective+layer+for+an+efficient+and+stable+water-splitting+photocathode.">A graded catalytic-protective layer for an efficient and stable water-splitting photocathode.</a> Nature Energy. 2, 16192 (2017).</li><li>Mariano, R. G., McKelvey, K., White, H. S., Kanan, M. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+increase+in+CO2+electroreduction+activity+at+grain-boundary+surface+terminations.">Selective increase in CO2 electroreduction activity at grain-boundary surface terminations.</a> Science. 358, 1187-1192 (2017).</li><li>Liu, F., Wu, C., Yang, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strain+and+Ligand+Effects+on+CO2+Reduction+Reactions+over+Cu-Metal+Heterostructure+Catalysts.">Strain and Ligand Effects on CO2 Reduction Reactions over Cu-Metal Heterostructure Catalysts.</a> The Journal of Physical Chemistry C. 121, 22139-22146 (2017).</li><li>Wang, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strain+Effect+in+Bimetallic+Electrocatalysts+in+the+Hydrogen+Evolution+Reaction.">Strain Effect in Bimetallic Electrocatalysts in the Hydrogen Evolution Reaction.</a> ACS Energy Letters. 3, 1198-1204 (2018).</li><li>Deng, Q., Smetanin, M., Weissm&#252;ller, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+modulation+of+reaction+rates+in+electrocatalysis.">Mechanical modulation of reaction rates in electrocatalysis.</a> Journal of Catalysis. 309, 351-361 (2014).</li><li>Yang, Y., Kumar, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Elastic+Strain+Effects+on+the+Catalytic+Response+of+Pt+and+Pd+Thin+Films+Deposited+on+Pd-Zr+Metallic+Glass.">Elastic Strain Effects on the Catalytic Response of Pt and Pd Thin Films Deposited on Pd-Zr Metallic Glass.</a> Journal of Materials Research. 32, 2690-2699 (2017).</li><li>Yan, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Influence+of+Elastic+Strain+on+Catalytic+Activity+in+the+Hydrogen+Evolution+Reaction.">The Influence of Elastic Strain on Catalytic Activity in the Hydrogen Evolution Reaction.</a> Angewandte Chemie International Edition. 55, 6175-6181 (2016).</li><li>Lee, J. 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The authors declare no competing interests.

Nitinol is a suitable elastic substrate for applying mechanical stress on thin films. It is commercially available, highly conductive and can be easily functionalized. Preparation of rutile TiO2 thin films by thermal treatment of nitinol, results in highly n-type doped TiO2. It is important to emphasize that NiTi/TiO2 is a unique system where TiO2 films are prepared by thermal treatment of NiTi rather than a deposition method. Our previous publications have shown that strain ap...

The ability to alter the surface reactivity of catalytic materials by introducing strain has been widely recognized1,2,3. Effects of strain in crystalline materials can be introduced either by adjusting material architecture (static strain) or by applying a variable external force (dynamic strain). In crystalline materials, static strain can be introduced by doping4, de-alloying5,6, annealing7, epitaxial growth on a mismatched crystal lattice2 or size confinement2,3. In polycrystalline materials, strain can occur within grain boundaries due to crystal twinning8. Determining the optimal degree of static strain with material architectures requires designing a new sample for each discrete level of strain, which can be time consuming and expensive. Furthermore, introducing static strain often introduces chemical or ligand effects9,10, making it difficult to isolate the strain contribution. Applying a dynamic strain precisely controlled by an external force allows systematic tuning of a material&#8217;s structure/function relationship in order to explore a dynamic range over the strain space without introducing other effects.
To study the effects of dynamic strain on electrocatalysis, metals or metal oxides are deposited on elastic shape or volume tunable substrates, such as organic polymers11,12,13,14,15 or alloys16,17. Applications of mechanical, thermal or electrical loading results in bending, compression, elongation or expansion of an elastic substrate, further inducing a stress-strain response on the deposited catalytic material. So far, catalyst engineering through dynamic strain has been exploited to tune electrocatalytic activities of various metallic and semiconducting materials. Examples include i) the hydrogen evolution reaction (HER) on MoS2, Au, Pt, Ni, Cu, WC11,12,13,14, ii) the oxygen evolution reaction (OER) on NiOx16, nickel-iron alloys18 and iii) the oxygen reduction reaction (ORR) on Pt, Pd12,15,19,20. In most of these reports, organic polymers, such as polymethyl methacrylate (PMMA), were used as elastic substrates. We previously demonstrated the application of elastic metallic substrates, such as stainless steel16 and a superelastic/shape-memory NiTi alloy (Nitinol17,21) for strain studies. Nitinol has also been used as an elastic substrate for deposition of platinum films for ORR19 and deposition of battery cathode materials for energy storage22,23. Due to its shape memory and pseudoelastic properties, NiTi alloys can be deformed by applying moderate heat19 or mechanical strain17, respectively. In contrast to organic elastic substrates, metallic substrates typically do not require deposition of adhesion promoters, are highly conductive and can easily be functionalized. Nitinol is used as a more elastic alternative to stainless steel (SS). While SS can be reversibly strained up to 0.2%, nitinol can be reversibly strained up to 7%. Nitinol owes its unique properties to a martensitic solid state crystal transformation that allows for large elastic deformations24,25. Both materials are commercially available in different geometries (e.g., foils, wires, and springs). When shaped into elastic springs, metallic substrates can be used to study effects of dynamic strain on electrocatalysis without the need for expensive instrumentation16; however, defining the stress-strain response is more challenging than for other geometries.
In previous experimental studies with transition metal catalysts, changes in activities of catalytic surfaces under strain have been attributed to changes in the energetics of the d orbitals colloquially known as d-band theory26. In contrast, the effects of strain on metal oxides is significantly more complex, as it can effect bandgap, carrier mobility, diffusion and distribution of defects and even direct/indirect transitions21,27,28,29,30,31. Herein we provide detailed protocols for the preparation and characterization of n-type doped TiO2 thin films, as well as protocols to study electrocatalytic activities of these films under tunable, tensile strain. The equivalent system can be applied to study electrocatalytic activities of different materials as a function of dynamic strain.

<table>
	<tbody>
		<tr>
			<td>2-Propanol</td>
			<td>Sigma Aldrich</td>
			<td>109634</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag/AgCl (3M NaCl) Reference Electrode</td>
			<td>BASi</td>
			<td>MF-2052</td>
			<td></td>
		</tr>
		<tr>
			<td>Alkaline Reference Electrode</td>
			<td>Basi</td>
			<td>EF-1369</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethyl alcohol, Pure, 200 proof, anhydrous, =99.5%</td>
			<td>Sigma Aldrich</td>
			<td>459836</td>
			<td></td>
		</tr>
		<tr>
			<td>MT I I / F u l l am SEMTester Series</td>
			<td>MTI Instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Nitinol foil, 0.05mm (0.002in) thick, superelastic, flat annealed, pickled surface</td>
			<td>Alfa Aesar</td>
			<td>45492</td>
			<td></td>
		</tr>
		<tr>
			<td>PK-4 Electrode Polishing Kit</td>
			<td>BASi</td>
			<td>MF-2060</td>
			<td></td>
		</tr>
		<tr>
			<td>Potentiostat 600D</td>
			<td>CHI instruments</td>
			<td>600D</td>
			<td></td>
		</tr>
		<tr>
			<td>Pt wire</td>
			<td>Sigma Aldrich</td>
			<td>267228-1G</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium hydroxide</td>
			<td>Sigma Aldrich</td>
			<td>221465</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfuric acid</td>
			<td>Sigma Aldrich</td>
			<td>30743</td>
			<td></td>
		</tr>
	</tbody>
</table>

applying dynamic strain on thin oxide films immobilized on a pseudoelastic nickel-titanium alloy

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to emerge. Tuning material structure can be achieved by controlling an external force imposed on materials and inducing stress-strain responses (i.e., applying dynamic strain). Electroactive thin films are typically deposited on shape or volume tunable elastic substrates, where mechanical loading (i.e., compression or tension) can affect film structure and function through imposed strain. Here, we summarize methods for straining n-type doped titanium dioxide (TiO2) films prepared by a thermal treatment of a pseudo-elastic nickel-titanium alloy (Nitinol). The main purpose of the described methods is to study how strain affects electrocatalytic activities of metal oxide, specifically hydrogen evolution and oxygen evolution reactions. The same system can be adapted to study the effect of strain more broadly. Strain engineering can be applied for optimization of a material function, as well as for design of adjustable, multifunctional (photo)electrocatalytic materials under external stress control.

Pre-treated NiTi foils are oxidized at 500 &#176;C under aerobic conditions (Figure 1). Due to the oxophilic nature of titanium, calcination at elevated temperatures results in a surface layer of rutile TiO2. The thickness of the layer and degree of n-type doping are affected by annealing time and temperature, which is reflected in color change from gray (untreated sample) to uniform blue/purple after 20 min heating (Figure 2). Longer heating time res...

Watch this Scientific Journal Video about Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy at JoVE.com

Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy

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).

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to ...

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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Research

JoVE Journal

Chemistry

4.9K 조회수.  National Renewable Energy Laboratory. 재료에 실행 가능한 외부 힘을 적용하는 기능을 통해 표면 특성을 유의하게 변경하고 최적의 촉매 활동을 찾고 새로운 특성을 입증할 수 있습니다. 이 기술을 통해 각 신중한 변형 정도에 대해 여러 재료를 준비할 필요 없이 균주를 적용하고 전기 촉매 활동에 미치는 영향을 연구할 수 있습니다. 이러한 방법은 또한 다양한 지느러미 필름과 전기 화학 활동 및 부식과 같은 전?...