Nature | Flatband titanium-based photothermal evaporator

Research Background
With the growth of the world's population, environmental pollution and climate change, many countries are facing a serious shortage of fresh water resources. Seawater desalination is seen as an ideal solution to this problem. Photothermal interfacial evaporation has become a promising process for fresh water preparation because of its ability to thermally localize the heat generated by solar energy. However, obtaining photothermal conversion materials with a wide absorption spectrum and high absorbance is the primary problem that needs to be solved for efficient photothermal conversion .

To this end, the Northeastern University team proposed a method to significantly increase the density of states (JDOS) by introducing a flat band electronic structure to improve the interfacial evaporation performance . The article is currently published in Nature (JCR Zone 1 , Top, IF ₂₀₂₂ = 64.8 ) with the title " Flatband λ-Ti3O5 towards extraordinary solar steam generation ". Associate Professor Yang Bo from the School of Materials Science and Engineering of Northeastern University is the first author of the article, and the corresponding author of the article is Professor Zuo Liang from Northeastern University.

【Article Interpretation】

The team prepared λ-Ti₃O₅ powder and mixed it with porous polyvinyl alcohol (PVA) hydrogel to make a photothermal interface evaporator. The evaporation performance was optimized by adjusting the effective height of the evaporator and the percentage of λ-Ti3O5 powder.

Starting from the nature of the interaction between light and matter, through DFT calculations and experiments, it is found that the Ti-Ti dimer structure existing in titanium dioxide (TinO₂n-1) leads to the localization of Ti-3d electrons in real space and introduces flat-band electronic states near the Fermi level, thereby enhancing the joint state density of electronic transitions .

The metallic λ-Ti₃O₅ has multiple flat-band electronic states originating from the Ti-3d orbital in a wide energy range near the Fermi level, which makes it present an absorbance of 96.4% in the full solar spectrum . The unique Ti-Ti dimer and U-shaped groove structure of λ-Ti₃O₅ powder help dissociate the adsorbed water molecules and promote the evaporation of interfacial water.

Using DFT simulation, it was found that the Ti-Ti dimer on the most stable surface of λ-Ti₃O₅ can decompose some of the water molecules initially chemically adsorbed into hydroxyl (-OH) and hydrogen (H), and the two are combined with Ti atoms and O atoms on the surface of λ-Ti₃O₅ respectively, resulting in hydroxylation of the λ-Ti₃O₅ surface; in addition, the special U-shaped groove structure of the surface can promote rapid proton exchange , making it easy to form metastable H₃O* units in the physically adsorbed water molecule layer on the hydroxylated surface, which can weaken the hydrogen bonding between the water molecule clusters containing H₃O* and the surrounding water molecules. This mechanism essentially reveals the mechanism of evaporation of water molecules on the surface of λ-Ti3O5 in the form of clusters under light, which revolutionizes the traditional understanding of the evaporation of single water molecules .

The 2D photothermal evaporator (2D-SSE) was prepared by dispersing λ-Ti₃O₅ (light absorber) on a cellulose film and supported by an insulating foam board. The evaporation rate of the 2D-SSE was 1.64 kg m⁻² h⁻¹ at 1.0 sun . By mixing λ-Ti₃O₅ with polyvinyl alcohol (PVA), a three-dimensional porous interconnected structure evaporator was produced, which achieved a water evaporation rate of up to 6.09 kg m^-2 h^-1 under 1 sun illumination (1 kW m^-2) , setting a new record for the evaporation rate of solar-driven photothermal water with long-term operation and no salt deposition. 

In terms of practical applications, by designing an outdoor water-cooled seawater desalination device, it was found that the average daily amount of fresh water collected under outdoor natural light reached 23 L m^-2 day^-1 , showing good application prospects.

【Article Summary】

This paper first proposed a new concept of flat-band photothermal conversion materials . Starting from the nature of the interaction between light and matter, using DFT calculations and experimental verification, it was found that the titanium dimer structure in titanium dioxide leads to the localization of Ti-3d electrons in real space, and introduces flat-band electronic states near the Fermi level, thereby enhancing the joint state density of electronic transitions . The multiple flat-band electronic state structure introduced in this way has developed a λ-Ti3O5 photothermal material with high absorption and low reflection in the entire solar spectrum; combined with the 3D evaporator structure design, the evaporation rate is as high as 6.09 kg m-2 h-1 .

This is the first article that topped Nature last year and is extremely relevant to the field of photothermal interface evaporation , which has brought great encouragement to scholars in the field of interface evaporation. It is a good idea to top a high-quality journal by innovating the structure of the material itself and proposing new concepts and insights, and then verifying them with experimental means . However, we still need to pay attention to the fact that the economic and technical analysis of the materials mentioned in this article is relatively small, and there is more room for improvement in the horizontal extension of the field of photothermal interface evaporation.

【Source】

Yang, B., Zhang, Z., Liu, P. et al. Flatband λ-Ti ₃ O ₅  towards extraordinary solar steam generation.  Nature 622 , 499–506 (2023).  

https://doi.org/10.1038/s41586-023-06509-3