Method of making nanoparticle colloid and nanoporous layer

A method of making a nanoporous layer, the method comprising:

• providing a liquid composition comprising a surfactant and metal ions, wherein the surfactant is in a reverse micelle phase defining hydrophilic spaces;
• adding a reducing agent to the liquid composition to cause reduction of at least part of the metal ions to form nanoparticles, which provides a first colloid, wherein at least part of the nanoparticles are inside at least some of the hydrophilic spaces;
• and
• removing the surfactant from the first colloid to form a second colloid, wherein removing the surfactant causes at least part of the nanoparticles get together and form a number of clusters, wherein each cluster has an irregularly shaped body comprising a number of nanoparticles having a generally oval or spherical shape with a length ranging between about 2 nm and about 5 nm,
• subsequently, adjusting a concentration of the nanoparticles in the second colloid to provide a colloid composition such that the nanoparticles contained in the colloid composition are in an amount between about 0.01 wt % and about 2 wt % with reference to the total weight of the colloid composition;
• dispensing the colloid composition over a substrate;
• and
• subjecting the dispensed colloid composition to drying to form a nanoporous layer.