Understanding how hydration affects color-changing windows can boost their efficiency

Nov 20, 2013
Materials: Getting smarter with water
‘Smart’ films that change color on demand can be fabricated more efficiently, thanks to a new mechanistic study. Credit: A*STAR Institute of Materials Research and Engineering

Electrochromic materials dynamically alter how they transmit light in response to an applied electrical signal. Engineers are currently working to turn these compounds into 'smart windows' for buildings that change from transparent to colored states at the flick of a switch. Such devices can help to prevent heat loss and maximize the amount of natural light passing through windows. Sing Yang Chiam from the A*STAR Institute of Materials Research and Engineering in Singapore and co-workers have discovered how to improve the manufacturing and performance of smart windows by elucidating the critical roles that water molecules play during coloration.

Nickel oxide (NiO) is a low-cost, inorganic compound widely used as an anode material inside because it is a reversible color-changer. Unlike other electrochromic substances, however, researchers have struggled to comprehend how coloration occurs in NiO in the presence of common aqueous electrolytes. Part of the problem is that NiO can form different crystal structures in its bleached and colored states, depending on how much water becomes incorporated into the material.

Chiam and his team set out to unravel this puzzle with a 'chemical bath deposition' technique that allowed rapid fabrication of NiO thin films simply by dipping a conductive glass slide into a nickel precursor solution (see image). The researchers annealed the films at increasingly elevated temperatures to gradually drive water out of NiO, checking its structure with X-ray diffraction and infrared spectroscopy along the way. They also investigated how these structures had changed after multiple electrochromic color-change cycles.

The team's experiments revealed a complex coloration mechanism involving water and NiO particles. Initially, two intertwined reactions hydrated the thin film by turning NiO into nickel hydroxide. This process enhanced the material's optical response to electrical signals by allowing more of the thin film to contribute to coloration reactions. However, repeated cycling caused 'over-hydration' that trapped inside the thin film structure—a development that degrades electrochromic activity by generating irreversibly colored hydroxide grains.

The researchers found that a simple high-temperature annealing process could mitigate the effects of over-hydration in the NiO thin film. This improved mechanistic knowledge—in combination with their simple and scalable chemical dip coating technique—helped them to achieve one of the best optical modulations reported for NiO films.

Currently, the team is investigating how to extend their work to flexible substrates. "Fabricating electrochromic thin films on rolls of plastic could make retrofitting onto existing windows affordable and easy," explains Chiam.

Explore further: Go ahead, dunk your cell phone in salt water: Barrier films by atomic layer deposition

More information: Ren, Y., Chim, W. K., Guo, L., Tanoto, H., Pan, J. & Chiam, S. Y. The coloration and degradation mechanisms of electrochromic nickel oxide. Solar Energy Materials and Solar Cells 116, 83–88 (2013). dx.doi.org/10.1016/j.solmat.2013.03.042%20

add to favorites email to friend print save as pdf

Related Stories

Size matters when reducing NiO nanoparticles

Nov 27, 2012

(Phys.org)—New research finds that size plays a major role in how nanoscale nickel oxide (NiO) shells behave when being reduced to solid nickel nanoparticles.

'Holey' Nanosheets for Wastewater Dye Removal

Jul 01, 2009

(PhysOrg.com) -- Researchers have discovered that extremely thin sheets of nickel oxide with hexagonally shaped holes can absorb hazardous dyes from wastewater nearly as well as the best traditional methods, but are recyclable. ...

Recommended for you

Jindal: Obama hasn't done enough to harness energy

9 hours ago

The governor of the state of Louisiana, a possible Republican presidential candidate, said Tuesday that President Barack Obama's administration has become "science deniers," failing to do enough to harness the nation's energy ...

User comments : 0