**1. Introduction**

The shortage of safe drinking water is one of the most severe global challenges facing human society. Desalination is one of the most cost-effective ways to increase drinking water supply. In recent years, the development of seawater desalination industry has made grea<sup>t</sup> progress. For example, as one of the most widely used desalination technologies, reverse osmosis (RO) has a recovery rate of about 50% and a consumption of only 2 kw·h·m<sup>−</sup>3. However, due to the need for complex infrastructure and high energy consumption, this technology is still unavailable in most underdeveloped countries. In addition, with the increase of feed water salinity, the working pressure, scaling probability and fuel consumption of the reverse osmosis system increase significantly [1–4]. Therefore, the development of new green seawater desalination technology is particularly important. At present, solar powered desalination is becoming one of the most promising technologies to increase the supply of clean water, because of the abundance of solar energy and seawater and their negligible carbon footprint, especially in remote areas lacking electricity or infrastructure [5–8].

To promote the further development of direct solar desalination, grea<sup>t</sup> efforts have been made to improve the efficiency of solar thermal conversion, prolong the service life of equipment and reduce the cost of solar desalination [9–13]. Some studies show that the evaporator with optimized structure design can produce fresh water and collect the by-product salt in salt water at the same time, to achieve zero liquid discharge. To

**Citation:** Huang, J.; Chu, Z.; Xing, C.;Li, W.; Liu, Z.; Chen, W. Luminescence Reduced Graphene Oxide Based Photothermal Purification of Seawater for Drinkable Purpose. *Nanomaterials* **2022**, *12*, 1622. https://doi.org/ 10.3390/nano12101622

Academic Editor: Sherif A. El-Safty

Received: 24 March 2022 Accepted: 6 May 2022 Published: 10 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

strengthen the relationship between water and energy and alleviate the energy shortage, various solar powered hydropower hydrogen production systems have also attracted people's attention. This system converts waste heat into electric energy, and then drives electrochemical decomposition of water, which not only improves the overall energy conversion efficiency, but also promotes the production of green fuel [14,15]. According to the mechanism of photothermal conversion, photothermal materials are mainly divided into three categories, such as plasma metal nanoparticles, semiconductors and carbonbased materials. Among them, carbon-based materials are ideal for solar evaporative power generation because of their excellent thermal properties, low cost and abundant source materials [16,17]. As an important carbon-based material, graphene oxide (Go) or reduced graphene oxide (rGO) has thin nanostructures, large surface area, low molar specific heat, stable mechanical strength and excellent light absorption covering the whole solar spectral range (250–2500 nm). Therefore, they are directly used as light absorbers or photothermal layers for various substrates (such as wood, sponge, polymer membrane and natural fiber) [18–20].

To better transfer heat to the water body that needs evaporation, good heat management performance and excellent light absorbing materials are two key factors. For the heat managemen<sup>t</sup> strategy, the traditional self-floating evaporation configuration generally has large heat loss due to the direct contact of the light absorbing material with the water body. In contrast, the use of thermal insulator assisted evaporation device can minimize the heat and conduction loss of photothermal materials [21,22]. At the same time, interface heating can limit the absorbed heat energy to a small amount of interface water in the upper layer, to shorten the start-up time of steam generation and increase the water evaporation rate. Although some transport auxiliary evaporators have been reported, their evaporation performance is still hindered due to the lack of good design structure. Therefore, the evaporation material must have a highly porous structure and hydrophilic channels to promote the transmission of water and the escape of water vapor. Non-woven fabric is made of polyester fiber and polyester fiber (PET for short). It has the characteristics of antibacterial, alkali corrosion and strong hydrophilicity [23,24]. In addition, Yu and others think that the hydrogel polymer network can restrict the water mass in the molecular grid, effectively reduce the enthalpy of vaporization, which is beneficial to improve the evaporation performance of the self-floating gel evaporator [25–27]. However, the combination of nanofiber hydrogel and rGO as a feasible strategy to improve the evaporative performance of the transport assisted evaporator and the mechanism of controlling the interfacial evaporation need further study.

Here, we report an approach based on reduced graphene oxide (rGO)-loaded nonwoven membrane for localizing evaporation, which is cost-effective and environmentally friendly. The hydrophilic non-woven transports water to the hot region by capillary forces. The rGO non-woven composite membrane has a superior mechanical stability. This method can be used repeatedly for many times. Under 1 sun intensity (1 kW m<sup>−</sup>2), the evaporation rate of photothermal membrane reached 1.33 kg·m<sup>−</sup>2·h−1**.** In addition, we tested the cell viability on the original seawater and the purified water. The results show the purified water has no toxicity, while the seawater is toxic to cells at a certain concentration. This indicates this is an excellent method for purification of drinkable water from seawater. The simple design of the system, along with the low costs, makes it possible for practical applications as an outstanding solution to the long-term challenge of drinking water issues.
