Nanometer-scale particles (nanoparticles) and fine structures (nanostructures) exhibit unique optical properties that differ significantly from those of materials at the macroscopic scale familiar to us in everyday life. For example, when we think of gold (Au), we typically imagine gold medals or bullion. However, when gold is structured into nanoparticles, it strongly absorbs light in the visible to near-infrared region due to a phenomenon known as localized surface plasmon resonance (LSPR).

　The optical properties of such nanomaterials are governed not only by the size and shape of individual particles, but also by their arrangement, such as interparticle distance and periodicity, as well as by combinations of different nanostructures. Therefore, the ability to “create nanostructures as designed” is key to unlocking their functions and developing new applications.

　Guided by our motto, “Creating designed nanostructures through the power of chemistry and light to unlock their functions,” our laboratory aims to develop novel fabrication methods and discover new optical functionalities by integrating chemical synthesis with photoelectrochemical approaches.

　Redox reactions driven by light are known as photoelectrochemical reactions, in which materials can be deposited or dissolved simply by light irradiation. In our research, we aim to fabricate well-controlled nanostructures by precisely controlling where these photoelectrochemical reactions occur. This is achieved by designing the shape of the substrate and optimizing the way light is irradiated.

　For example, when linearly polarized light is applied to a transparent electrode to drive a photoinduced oxidative deposition process, periodic nanostructures are formed along the polarization direction, as shown below (ACS Appl. Nano Mater., 7, 5426-5433 (2024)). Because this approach enables the simple and large-area fabrication of controlled nanostructures using only light, we are working to expand the range of applicable materials and nanostructures.

　Because light cannot generally be focused to dimensions smaller than its wavelength, it is difficult to achieve nanometer-scale processing using light alone. In contrast, nanoparticles exhibiting plasmon resonance can confine light within a region of only a few tens of nanometers near their surface. Taking advantage of this property, we investigate methods for processing nanoparticles by driving nanoscale photoelectrochemical reactions at specific sites on their surfaces.

　For example, as shown below, when green light is irradiated onto cubic gold nanoparticles (gold nanocubes) to induce oxidative deposition or dissolution, the reaction occurs only on the top face of the nanocubes. In contrast, when red light is used to drive the same reactions, they occur selectively near the interface between the nanocubes and the electrode. In this way, different nanostructures can be selectively fabricated by simply changing the irradiation wavelength (Nanoscale, 11, 19455-19461 (2019)). Currently, we are working to expand the range of applicable reactions, improve site selectivity, and enhance the functionality of nanoparticles through such processing techniques.