Solar steam generation on scalable ultrathin thermoplasmonic TiN nanocavity arrays

Highlights

• An ultrathin solar absorber based on TiN nanocavity arrays is introduced.

• The material exhibits 90% broadband solar absorption within 250 nm.

• The evaporation rate and the efficiency non-linearly increase with light intensity.

• An extensive multi-physics modeling supports the experimental findings.

• A quasi-two dimensional heat dissipation regime is theoretically discussed.

Abstract

Plasmonic-based solar absorbers exhibit complete light absorption in a sub-µm thickness, representing an alternative to mm-thick carbon-based materials most typically employed for solar-driven steam generation. In this work, we present the scalable fabrication of ultrathin plasmonic titanium nitride (TiN) nanocavity arrays that exhibit 90% broadband solar light absorption within ~ 250 nm from the illuminated surface and show a fast non-linear increase of performance with light intensity. At 14 Suns TiN nanocavities reach ~ 15 kg h^–1 m^–2 evaporation rate and ~ 76% thermal efficiency, a steep increase from ~ 0.4 kg h⁻¹ m⁻² and ~ 20% under 1.4 Suns. Electromagnetic, thermal and diffusion modeling of our system reveals the contribution of each material and reactor component to heat dissipation and shows that a quasi-two-dimensional heat dissipation regime significantly accelerates water evaporation. Our approach to ultrathin plasmonic absorbers can boost the performance of devices for evaporation/desalination and holds promise for a broader range of phase separation processes.

Introduction

The widespread availability and abundance of solar radiation as a renewable energy source have led to its application in diverse fields such as photovoltaics [1], [2], hydrogen production [3], photocatalysis [4], [5], solar-thermal conversion [6], and steam generation [7], [8], [9]. The latter, in particular, has recently emerged as a cost-effective solution for off-grid water desalination [10], [11], [12], distillation [13], [14] and purification [13], [15], with its potential implementation in remote locations thanks to simpler reactor designs compared to large-scale reverse osmosis desalination plants [7], [16].

Solar steam generation involves light-to-heat conversion within a broadband absorber that, by increasing water temperature in its proximity, promotes the evaporation process. The so-obtained steam can be then collected, upon condensation, as clean water [7], [8]. Ideally, solar absorbers feature large absorption coefficients to dissipate solar radiation in relatively thin layers, in contrast to mm-thick commercial black paints [17], thus minimizing the amount of required material and maximizing heat power density within the absorbers themselves. In the search of improved solar absorbers, various materials have been considered, such as plasmonic metal nanoparticles dispersed in liquid [14], [18], [19], or floating three-dimensional (3D) carbon- [11], [12], [13], [20], [21], [22], [23] and/or plasmonic-based [10], [24], [25], [26] structures. The great interest in plasmonic materials is motivated by the efficient light-to-heat conversion upon the dissipation of plasmonic modes [27] (i.e., collective oscillations of conduction electrons in response to the incident electromagnetic field) that can be easily tuned by controlling the size and the chemical environment of plasmonic nanostructures [28], [29], [30], [31], [32]. Such a high efficiency of heat generation by plasmonic materials has indeed given rise to the sub-field of the so-called thermoplasmonics [33].

Plasmonic materials offer even more unique advantages in the form of ultrathin layers based on periodic arrays, i.e. the so-called quasi two-dimensional (2D) metamaterials or metasurfaces [34], [35]. By carefully controlling the dimensional parameters of arrays of nanostructures in the sub-wavelength regime, the electromagnetic radiation can be manipulated in unusual ways, a technique that has led to scientific advancements in many optical devices [35], [36], [37]. Moreover, a 2D array of dissipative plasmonic nanostructures can effectively promote a substantial heating thanks to collective photothermal effects [38], [39]. Ultrathin broadband solar absorbers with a thickness limited to ~ 100–200 nm have been realized employing plasmonic metamaterials [40], [41], [42]. Although solar absorbers based on periodic structures have been reported, such as anodic alumina oxide (AAO) coated by Au or TiN [10], [24], [26], a further reduction of the thickness down to the ~ 100 nm scale may lead to faster evaporation dynamics because of the increased power densities in the plasmonic metasurface layer. For example, by absorbing unfocused solar radiation (~ 1 kW m^–2) in a ~ 100 nm thick dissipating layer would yield to an average heat power density of ~ 10 GW m^–3. Such large power densities can trigger fast thermal processes in ultrathin devices, opening the possibility for exploiting nanoscale photothermal effects in large area systems. Here we experimentally demonstrate and numerically validate the efficacy of plasmonic metasurfaces in vaporizing water and we theoretically show how the process can be accelerated through nanoscale heat transfer.