Spectrum Selective Interfaces and Materials towards Non-photothermal Saltwater Evaporation: Demonstration with a White Ceramic Wick

TL;DR

This work addresses the energy intensity of solar desalination by challenging the photothermal paradigm with a spectrum-selective, non-photothermal approach using a white aluminum nitride (AlN) wick. By fabricating and characterizing AlN wicks and conducting controlled, self-referencing evaporation experiments under violet, orange, and infrared illumination, the authors demonstrate that violet light can drastically enhance brine evaporation beyond the photothermal limit, with efficiencies up to $\sim$150% (and $\sim$270% when including wick evaporation) under steady-state conditions. The results point to a non-photothermal mechanism, potentially driven by deep-UV upconversion that targets salt-water bonds at interfaces, aided by surface -OH groups, and offer a pathway toward low-energy, spectrum-aware desalination and evaporative cooling. The study also highlights the importance of spectral design for solar desalination materials and surfaces and suggests directions for mechanistic spectroscopy to validate the proposed upconversion pathway.

Abstract

Most solar desalination efforts are photothermal: they evaporate water with ``black'' materials that absorb as much sunlight as possible. Such ``brine-boiling'' methods are limited by the high thermal mass of water, i.e., its capacity to store and release heat. Here, we study the light-enhanced evaporation by a hard, white, aluminum nitride wick, and propose a route to selectively target salt-water bonds instead of bulk heating via deep-UV interactions. Through experiments and analyses that isolate the effects of light absorption and heating in aluminum nitride, we provide experimental evidence of a light-driven, spectrum-selective path to non-photothermal saltwater evaporation. Leverage of these light-matter interactions in white ceramic wicks may achieve low-cost, low-energy desalination, reduce the heat island effects of traditional solar technologies, and contribute to future cooling technologies where drought is also a concern.

Figures (5)

• Figure 1: (a) Extinction spectra of saltwater brine and AlN. (AlN spectra is adapted with permission from Ref. [Nagai2010] . Copyright Optical Society of America, 2010). Inset: Schematic showing blue light absorption and upconversion in AlN-OH (left) and red light transmission and absorption in Al (right). (b) Illustration of a ceramic wick in an outdoor saltwater swamp cooler.
• Figure 2: AlN wick characterization. (a) FTIR and (b) XRD spectra of the sample pre- and post-fabrication, with inset SEM photos showing porosity and thickness of AlN slab. The fabricated AlN sample (red) shows similar chemical and structural composition to the as-purchased AlN (black) with the exception of significant -OH as seen in the FTIR.
• Figure 3: Experimental setup and representative raw data. (a) Schematic of the dual-balance self-referencing experimental setup. A glass divider with gaps reduces air flow between the balances. Inset sample photos: during water uptake showing light spot size (top) and salt nucleation (bottom). (b) Representative experimental data showing the relative mass lost due to the open reservoir, wick, and light. (c) Data showing cross-talk between samples with no glass divider.
• Figure 4: Spectral dependence of the light-enhanced evaporation rate (EER) with a 25-wt% salinity reservoir. (a) EER vs. time for experiments with 385-nm (violet), 625-nm (orange) and 940-nm (IR) light. (b) Schematic showing energy paths in the porous AlN sample. Incident light energy ($Q_{\rm in}$) is reflected, waveguided, and transferred to the reservoir and associated with thermal loss ($Q_{\rm res}$). (c) Percent salinity change in concentration $\Delta S/S_{\rm 0}$ in the reservoir. Inset plot: temperature increase $\Delta T$ vs. time. The illumination intensity is 130 mW cm$^{-2}$.
• Figure 5: Evaporation trends and mechanism with violet light. (a)-(c) EER as the salinity increases from zero to 25 wt% and the light intensity increases from 0 to 130 mW cm$^{-2}$. (d) EER for high-salinity violet light experiments plot as a function of light intensity showing a quadratic dependence. (e) Schematic showing propose upconversion in bulk AlN and at -OH interface that targets brine evaporation and facilitates salt nucleation.