Name: applying dynamic strain on thin oxide films immobilized on a pseudoelastic nickel-titanium alloy
Uploaded: 2020-07-28T14:30:00
Description: Watch this Scientific Journal Video about Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy at JoVE.com

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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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 ...

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