Sensing applications of hydrovoltaic effect in low-dimensional carbon materials

Abstract

A variety of phenomena observed in low-dimensional carbon materials, such as waving potential, moving droplet induced drawing potential, flow-induced voltage, water-evaporation-induced electricity, moisture-enabled electricity, have exhibited that low-dimensional carbon materials can generate electrical signals by interaction with dynamic water. This series of phenomena, which is termed as hydrovoltaic effect, has the potential of developing advanced technologies for water energy harvesting and enables the design of flexible sensing systems. This review focuses on the sensing ability of low-dimensional carbon materials to transform those physical/chemical parameters such as flow velocity, ion concentration, humidity, etc. into electrical signals through the hydrovoltaic effect. Generally speaking, as long as dynamic water (including water solutions, water vapor or water molecules) interacts with low-dimensional carbon materials and simultaneously produce a measurable electrical signal, the signal then can be utilized to monitor various information of the solid-liquid interface. In the first section, we describe the fundamental properties of water-solid interfaces, look back the history of widely used electrical double layer model and four classical electrokinetic effects: electro-osmosis, electrophoresis, streaming potential and sedimentation potential. Then we discuss the recently proposed water-carbon interaction mechanisms, including phonon dragging, electrical friction, pseudocapacitance and dynamic boundary at the liquid-gas interface. Among these mechanisms, the latter two, pseudocapacitance and dynamic boundary, can be considered as an expandedness of streaming potential model under confinement low-dimensional space. The second section reviews the phenomena of moving water induced voltage and its applications for dynamic water detecting. Diverse kinds of bulk water movement such as flowing, waving and drops sliding, can partially separate the positive ions from negative ions to produce an electrical potential difference along a nano-carbon sample under proper interface conditions. This electrical potential difference is usually proportional to the velocity and mass of the moving water near the solid-liquid interface and thus can be used as an indicator to measure these physical quantities. The third section introduces several examples of detecting ingredients and concentration of water solution using hydrovoltaic effect. For drawing potential of moving droplets on graphene, the induced voltage increases with increasing ion concentration over a low concentration range (10–7–10–2 mol/L). However, for the electrical potential produced by water evaporation on carbon black film, the output voltage decreases rapidly as the ion concentration increases. The next section presents the humidity sensing performance of different hydrovoltaic devices of low-dimensional carbon materials. Graphene and graphene oxide devices can be utilized as humidity and temperature sensors with convenient features such as flexibility and transparency. Single-walled carbon nanotubes can output a millivolt level of voltage in response to humidity variation at parts per million level. Finally, this review makes suggestions for developments in this field by highlighting the potential of hydrovoltaic sensors to be applied in the fields of physiological nanodevices, wearable electronics and self-powered intelligent systems.