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

Summary

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).

Abstract

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.

Introduction

The ability to alter the surface reactivity of catalytic materials by introducing strain has been widely recognized^1,2,3. 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-alloying^5,6, annealing⁷, epitaxial growth on a mismatched crystal lattice

Protocol

1. Preparation of NiTi/TiO₂ electrodes

1. Chemical and mechanical polishing of NiTi substrates
   1. Cut the superelastic NiTi foil (0.05-mm thickness) into 1 cm x 5 cm strips.
   2. Polish sample using 320-, 600- and 1200-grit sandpaper, and then rinse with ultrapure water (18.2 MΩ).
   3. Polish sample with 1 μm diamond, 0.25 μm diamond, and 0.05 μm alumina polish.
   4. After polishing, sonicate for 5 min in sequential baths of ultrapure water (18.2 MΩ), isopropanol, ethanol, ultrapure water (18.2 MΩ), and then dry under nitrogen (used organic solvents were reagent grade).
      CAUTION: Organ....

Results

Pre-treated NiTi foils are oxidized at 500 °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 results .......

Discussion

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.......

Materials

List of materials used in this article

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