(Invited) Harvesting Sunlight Using Titanium Nitride Nanostructures for Enhanced Visible Photocatalytic Activity and Solar Heating

Abstract

Harvesting sunlight enables to excite hot carrier and heat. Among different classes of materials, metals have unique properties in light harvesting. Since metals are highly conductive and do not have bandgaps, metals can generate hot carriers even with low energy photons in contrast to semiconductors. Subsequently, the hot carriers generated in metals can to be injected into an adjacent semiconductor. The excited hot carriers eventually become heat and heat the metals themselves and their surroundings. These photoelectric and photothermal effects can be enhanced by surface plasmon resonances. Hence, exciting plasmon resonances is a key in photoelectric and photothermal conversions using metals. In terms of plasmonic materials gold and silver are the two best metals, such that nanostructures made of gold and silver have been widely used in the recent studies. In contrast, we have been working with titanium nitride (TiN) nanostructure to show that it can also be used in photoelectric and photothermal conversions [1]. Titanium nitride is chemically stable and much cost-effective than gold or silver, making it a practical choice of material. In addition, TiN is plasmonic in visible and near infrared and superior to gold and silver in absorbing broad spectrum. In the first part, we present that TiN nanostructures can generate hot carriers by the irradiation of visible light. When TiN nanoparticles are in contact carbon nitride or titanium dioxide, the visible photocatalytic activities of carbon nitride and titanium dioxide are enhanced near one order of magnitude [2]. In the second part, we show that TiN nanoparticles are efficient sunlight absorbers to generate solar heat around the TiN nanoparticles. When TiN nanoparticles are dispersed in water, water evaporates efficiently under the illumination of sunlight [3]. Solar water evaporation can be further improved by fabricated TiN nanoparticle decorated ceramic wool [4] and TiN coated porous alumina [5]. In both cases, capillary actions helps to bring water to the surface of TiN. Solar heat generated at TiN nanoparticles can be also applied for chemical reactions such as oxidation of carbon monoxide [6]. Our results demonstrate that TiN nanostructures have the potential to harvest sunlight for photoelectric and photothermal conversions with higher than efficiencies than using gold and silver nanostructures.