Preferred embodiments provide a method for forming at least one catalyst nanoparticle on at least one sidewall of a three-dimensional structure on a main surface of a substrate, the main surface lying in a plane and the sidewall of the three-dimensional structure lying in a plane substantially perpendicular to the plane of the main surface of the substrate. The method comprises obtaining a three-dimensional structure on the main surface, the three-dimensional structure comprising catalyst nanoparticles embedded in a non-catalytic matrix and selectively removing at least part of the non-catalytic matrix at the sidewalls of the three-dimensional structure to thereby expose at least one catalyst nanoparticle.